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etween first and third world countries thus regardless of its location in the world any operation needs higher productivity and continuous improvements to remain competitive this chapter will discuss the most common methods to move store and recover ore used in mineral operations including newer developments and developments foreseen for the near future the intent is to give a general overview in the limited space available keeping in mind that a comprehensive and in depth discussion goes beyond the intent of this handbook ore movement this section reviews transport systems in which the payload is motionless in relation to the transporting media truck haulage rail marine and multimodal transportation and belt conveyors and stationary systems in which the material is in motion during transport truck haulage the loading and transportation of mined products by truck within the mine and to the processing plant are discussed in detail in other sections of this handbook that deal with surface mining and underground mining mine operations use underground or off road type trucks for which design and size evolve continuously large off road trucks have reached capacities of up to 364 t 400 st with gross machine weights around 625 t 688 st caterpillar n d in the past large off road trucks were mostly diesel electric with direct current dc wheel drives however the newer larger trucks have mechanical transmissions or alternate current ac wheel drives recent developments include autonomous trucks for both underground and surface operations komatsu 2008 that are aimed at increasing reliability and safety while reducing labor requirements trolley assisted trucks have been in operation in southern africa since the 1980s a pantograph is used to connect an overhead trolley line which feeds energy directly to the truck s electric wheel motors figure 10 7 1 while the diesel engine runs idle used on inclined ramps the system allows savings in fuel consumption while the truck can climb faster and thus operate with higher efficiency alvarado 2009 the advantages of a trolley line are linked to the availability of cheap electrical energy and to high fuel costs initial designs had dc electrical feed the advent of ac trucks has allowed the use of simpler and lower cost ac distribution lines off road trucks are intended not for long distance haulage but for operations with complex deposits and multiple loading points it is not unusual to find them operating on routes several kilometers long long hauls are also common for overburden handling where more effective transportation methods such as conveyors require the sizing of the mined material as opposed to the trucks than can handle run of mine material without any further size reduction beyond blasting as pits grow and become deeper overburden has to be lifted more and more and deposited at ever increasing distances as waste dumps are filled transportation of mined

products to market by road is less significant on a volume and distance basis than other methods e g rail or marine transportation however trucking is often involved at some stage of the transport chain between producer and final customer the flexibility of trucks is diminished by the high cost per metric ton per kilometer or per ton per mile over longer distances trucks designed for the movement of bulk minerals on standard roads are divided into three general classifications the first and most common is the end dump design a variation of the standard design is the frameless trailer in which the dump body is self supporting the objective with this custom design is to gain more payload while staying within the load limitations imposed by traffic authorities another alternative to increase payload is the use of aluminum construction in the trailer normally employing extruded sections the second classification is that of bottom dump design a special unloading facility is required normally a long grated hopper over feeders and conveyors the truck drives over the hopper to discharge the load this design offers a fast unloading cycle but at the expense of custom designed truck bodies and significant civil works associated with underground structures the third classification is the side dump design figure 10 7 2 the advantage here is a faster unloading cycle although larger receiving hoppers are required than for the end dump design the typical bulk handling truck is composed of a tractor unit and a trailer or semitrailer attached to it on long distances it is quite common to use longer combination vehicles that allow trucks to haul more on a single trip they usually have a tractor unit that pulls two or three trailers this type of truck is allowed in some states in the united states mostly in the western part of it they are also used in canada argentina and other countries mostly in remote areas and where access roads are relatively flat australia allows the use of even longer vehicles some australian states allow the use of triple and ab quad road trains that are up to 53 m 174 ft long and weigh up to 170 t 187 st the use of powered trailers allows for even larger combinations with longdistance haulage systems reaching payloads up to 350 t 386 st the transport of small size and dusty bulk materials is normally done using pneumatic trailers air pressure is used to discharge and transport the material through hose or piping directly into the storage container the truck body is divided internally so the bulk cargo can be gravity fed to the discharge ports airflow allows the material to move through the discharge pipe to its destination typical trailer capacity is 28 m3 1 000 ft3 these trailers normally are used to transport fine and free flowing material e g cement and limestone used as a reactive in mineral concentrators dump trucks are loaded most of the time using front end loaders or simil

ar equipment however on high capacity applications loading bins are used at times figure 10 7 3 enabling 300 t 331 ton trucks to be loaded in 60 seconds crushed material is loaded onto trucks to be placed on a heap and leached rail transportation in smaller operations common carriers generally handle rail transport which is therefore outside the scope of the mining company however the mine retains control of loading and unloading the trains and can influence the decision of the type of trucks to be used in a small to medium size operation partial train load can be used the normal procedure is for the common carrier to drop off the required empty cars since the loading time is not a critical factor the plant operator moves the empty cars to position them under a loading point with a car puller or a shifting locomotive most carriers charge per car not per ton of material transported however they do penalize the plant if the cars are overloaded therefore controlling the loading accurately makes economic sense as it minimizes transport cost this is accomplished by using weigh belt conveyors or track scales for throughputs ranging up to several million metric tons per year it is normal to use unit trains a unit train is a freight train composed of cars carrying a single type of commodity to the same destination running nonstop between the loading and unloading points this results in reduced transit time and lower transport cost american railway companies introduced the unit train concept in the 1950s for the transportation of coal from mines to power plants by the late 20th century about 50 of the coal shipped in the united states was carried by these types of trains typically in convoys of 100 cars other bulk cargoes transported by unit trains are cement and minerals such as iron ore the largest train ever run in the world was a unit train carrying iron ore in western australia the train consisted of 8 locomotives and 682 loaded wagons carrying more than 80 000 t 88 185 st of ore over a distance of 275 km 171 mi to the port the train was more than 7 km 4 mi long today trains with more than 200 cars and over 25 000 t 27 558 st capacity transport iron ore daily in northwestern australia with more than one train per hour per line some of these trains are fully autonomous trains of comparable size are operated for iron ore transport in south africa and brazil as well as other countries it is quite common in larger operations to have a dedicated railway and port owned and operated by the mine owner or a consortium of mine owners that handle the same commodity the need to load larger trains in an efficient and accurate way led to the development of specialized loading stations fletcher 1997 to load a train efficiently it is necessary to ensure that the full unit train can be loaded that is that sufficient storage is required a reliable accurate and fast loading system and a device to move the

train through the load station are also required the most common type of high capacity train loading plant is the flood loading system in which the height of the material in the wagon is controlled by a profile chute while the discharge gate is controlled by photocells that monitor the position of the wagon two in motion scales are installed one before the loading system and one after so the amount of material loaded in the wagon can be measured material loaded into the wagon is measured after the operation the accuracy of the amount loaded depends on the dimensional and suspension consistency of the wagons the need to maximize the loading of each wagon without exceeding the maximum gross weight allowed on the tracks led to the development of the flask loading system in the 1970s in south africa in this system the material is loaded onto a weigh flask first when the required load is achieved the gate from the storage bin is closed and the gate to the profile chute is opened and the wagon is loaded with a predetermined load train load can be controlled accurately up to capacities of 8 000 t h 8 818 stph in some installations a silo with a capacity comparable to the train capacity is used lower capacity systems fill the silo before the train arrives so that train loading time is minimized typical silos have a capacity up to 12 000 t 13 228 st another option would be to use a smaller silo linked to a large storage system such as a stockpile in this instance a larger loading system operating at a capacity equal to the load out rate is required this option is preferred when loading large unit trains having more than 100 wagons losses of material due to wind can be reduced by applying crusting agents to the mineral surface on fine and environmentally sensitive materials such as mineral concentrates railcars are covered when high tonnage rates are not required and capital expenditure needs to be minimized the use of bottom dump hopper cars is the logical choice normally car movers are used to position the car over a hopper and the bottom gates are opened manually to discharge the contents to the hopper feeders and conveyors or elevators are used to take the material to its final destination one way to obtain higher discharge rates is to use a trestle structure over a storage area the loaded cars are tripped open by a device alongside the truck and are emptied while in motion the disadvantages of this method are the additional capital and maintenance costs of the cars with special quickopening doors and potential environmental dust problems when the material is dumped from the top of the trestle to the ground at an installation recently built in germany the railcars are discharged over a 6 4 m 21 ft wide belt that transfers the material at a rate of 17 000 t h 18 739 stph of course in this case a higher discharge rate justifies the high capital and operational costs extreme cold weather can cause the

material to freeze if the cars are left to sit idle or the traveling time is long this can cause serious operating problems when attempting to unload the cars if this is a normal situation a car thawing system is required even then large frozen lumps might form and will have to be broken up before going into the receiving system it can also be hard to unload fine materials after a long trip this is particularly true with some mineral concentrates having a high moisture content that are loaded into the trains vibration and heat during the trip causes the material to consolidate making car unloading very difficult on relatively short distances and low throughputs round pots are placed over flat cars and the cars are overturned with the help of a crane during unloading vibrators or beaters are used to loosen up material stuck to the pot when transporting mineral concentrates over long distances several hundred kilometers it becomes harder to unload the cars one method that is used successfully is to place a backhoe excavator on a platform over the cars and excavate the material out throughputs up to 700 t h 772 stph are achievable in this fashion unit trains are normally discharged using a rotary car dumper to achieve a high unloading rate each car is turned upside down and the contents discharged into a large hopper that feeds a conveying system leading to the storage location traditionally it was necessary to uncouple each car so that it entered the dumper unattached newer higher capacity systems use rotary couplers that permit dumping without separating the cars also high capacity systems have twin dumpers that allow the discharge of two cars at the same time a high unloading rate is linked to the ability to move the cars in and out of the dumper quickly to this effect it is common to use a shifting locomotive with local or remote control these units can handle a group of cars or an entire unit train an alternative is the employment of a remotely controlled train positioner that can handle a full unit train the device uses a pickup arm or arms to push the train one or two car lengths depending on whether it is a single or twin dumper arrangement while the cars are unloaded the arm moves to the next car or cars this type of operation requires the use of rotary couplers to prevent uncoupling of the cars environmental regulations now require effective dust control by means of dust suppression or dust extraction systems in most cases today the installation for car dumping is entirely enclosed marine transportation the vast majority of long distance transportation of bulk minerals is done using marine transportation coupled in most cases with some degree of overland transport the capital and operational costs of loading and unloading equipment are outweighed by the low cost of water transport the continuous growth of international trade has resulted in the transport of increasingly large amounts of bu

lk minerals down rivers and across the oceans the largest transport volumes are associated with coal steam and metallurgical and iron ore each representing about 800 mt 882 million st transported by sea every year vk 2005 wsa 2009 inland marine transportation allows large amounts of lowcost minerals to be moved to final destinations at a cost per metric ton per kilometer or per ton per mile that is among the lowest for this process in the united states the mississippi river system is used to transport about 50 mt 55 million st per year of coal approximately 10 of the coal transported within the country barges are also used extensively in europe for the inland transport of iron ore and coal barges can be self propelled or towed by tugboats or they can be pushed by towboats in a series of several barges at the same time for example a 10 000 hp towboat can push up to 30 barges down the open river section of the mississippi river to the gulf coast barge dimensions are limited by lock sizes in the area in which they operate a typical barge is about 60 m 197 ft long and 10 m 33 ft wide with a loading capacity of approximately 1 350 t 1 488 st ship loading typical barge loading systems include a fixed loading conveyor that discharges directly onto the barge the barge is moved as necessary by a barge puller that is basically a system of winches and cables various methods are employed to unload barges the traditional method is the clamshell type unloader other designs include continuous unloading systems with bucketchain unloaders or bucket wheel unloaders in coal and iron ore applications these systems can achieve capacities up to 5 000 t h 5 511 stph for fine materials pneumatic unloaders are used they can be stationary mobile within the dock area barge mounted or fully mobile mounted on a truck materials such as cement are normally handled using pressure conveying coarse and abrasive materials such as lime alumina and petroleum coke are handled using vacuum systems in locations without proper infrastructure load from barges is transferred directly to ocean vessels in relatively low tonnage operations this is done using the ship s cranes larger operations use custom made ships to transfer the load using clamshell or continuous systems ocean transport of minerals involves four main types of ships namely handy handymax panamax and capesize handy and handymax sizes are the most common with deadweight dwt ranging from 10 000 to 39 000 t 11 023 to 42 990 st for the handy type and between 40 000 and 59 000 t 44 092 and 65 036 st for the handymax type they are used for multiple cargoes either broken or bulk in bulk materials they are limited to either short distance transportation or the transport of low volume high value cargoes such as copper concentrate panamax size vessels are the largest ships that can go through the panama canal and their deadweight fluctuates between 60 000 and 8

0 000 t 66 139 and 88 185 st their maximum draft is 14 m 45 ft and they have less than 36 m 118 ft beam they are the most common type of vessel used for moderate and large volumes of bulk solids transported over long distances capesize vessels are not limited in size other than by port capacities the most common sizes vary between 130 000 and 180 000 dwt some of the largest bulk carriers exceed 300 000 dwt being as long as 340 m 1 115 ft with a draft of 28 m 92 ft and a beam of 60 m 197 ft as volumes transported grow steadily the tendency is toward using larger ships in this way both operating costs and carbon emissions are reduced there are several ways of loading bulk materials onto a ship on low capacity operations fixed single point loaders are used and the ship is moved to load the different hatches this is by far the simplest and cheapest way to load a ship telescopic heads tilting telescopic chutes and or loading spouts are used to distribute the material inside the ship hull on berths used for multiple purposes mobile conveyors and loaders are also used when loading is completed the conveyors and loaders are removed to leave the area open for other uses as transported volumes increase so do ship size and the number of hatches to be loaded in these cases the low efficiency of a single loading point doesn t make economic sense since the loading operation has to be stopped frequently to reposition the ship moving a large ship generally involves a tugboat or a large crew to operate the ship s winches it is possible to reduce ship movement and loading interruptions by using a radial stacker that can rotate from the hatch a traveling ship loader moves along a wharf normally on rails so it can move from one hatch to another making it unnecessary to move the ship the ship loader is normally fed by a conveyor parallel to the wharf and a tripper car in most installations the feed conveyor is installed inside an enclosed gallery figure 10 7 4 in this case the boom conveyor can lift its boom so it can move easily from hatch to hatch this type of loader is relatively inexpensive most operational costs are related to the civil cost of the continuous wharf and the structure of the elevated gallery the loader depicted has a tilting telescopic chute and a rotating loading spout that allow for better trimming of the load in the ship another type of traveling ship loader has a slewing boom in some cases the boom is of fixed length in others it is telescopic which allows for an optimum distribution of the load on the ship this factor is quite relevant with low density materials such as coal for which trimming the material is more complex than with high density materials such as copper concentrate and iron ore in the case of a slewing boom the machine needs to have an intermediate feed conveyor that connects the feed and the boom conveyors another variation uses a ground level conveyor with a l

arge tripper car that feeds a luffing slewing and telescopic loader that is parallel to the ship this type of unit can load ships on either side of a dedicated wharf figure 10 7 5 shows a 14 000 t h 15 432 stph ship loader being transported fully assembled to its final destination it is a common practice to assemble large port equipment on a barge while traveling and then move the equipment directly into its working position when the port is dedicated exclusively to the loading of bulk materials the cost of the wharf can be minimized by using mooring dolphins to support the ship loading system a design that allows the loading of multiple hatches with a minimal civil cost is the so called quadrant ship loader it is formed by a fixed span bridge that turns around a fixed point on the land side while traveling over a curved runway on the water side a shuttle boom assembly travels over the bridge the radial movement of the boom and the slewing bridge combine to provide access to the ship hatches this type of loader can load ships from handy to panamax sizes without requiring movement of the ship during loading operations the cost saving in civil structures is offset by the higher cost of the machine a variation of the quadrant ship loader is the linear loader in which the runway is straight and the bridge moves back and forth over an inverted carriage mounted at the pivot point thereby changing its span a further development is the dual linear ship loader in which a slew bearing and an inverted carriage are mounted on the carriage traveling on a runway parallel to the ship the loading point is mounted over a fixed pivot and a shuttle boom is mounted over the bridge for larger ships e g capesize vessels dual arrangements of quadrant or linear ship loaders are sometimes used dibben 2006 ship unloading unloading ships is a significant part of the transport cost of minerals the most common system used to unload ships is the clamshell bucket in handy and panamax type vessels it is quite common to use the ship s own deck crane to unload the material at smaller rates up to 300 t h 331 stph this type of ship is referred to as a geared vessel dock mounted clamshell unloaders can be classified into two main categories crane and portal normally the unloaders are used in a fixed position and operate at capacities up to 700 t h 772 stph some units are capable of moving along the wharf but it is more common to use two cranes simultaneously to unload a handy or panamax ship for larger capacities and ships e g capesize vessels with capacities up to 2 500 3 000 t h 2 756 3 307 stph portal clamshell unloaders are frequently used again using two unloaders simultaneously is quite common the efficiency or the actual average unloading rate achieved from start to finish divided by the free digging rate is normally 50 or less the ship s hold is cleaned by small earthmoving equipment that is lowered into the hold area

the need for extensive cleaning associated with clamshells is larger for smaller ships that have smaller holds reducing the efficiency accordingly the need to improve efficiency and reduce ship turnaround times has prompted the development of continuous ship unloaders a relatively lightweight continuous unloading solution is the vertical screw system which draws the material from the ship s hold and transfers it to a transfer conveyor belt fine materials can be handled at relatively low capacities in this fashion higher capacities can be achieved using a bucket chain continuous unloader the lower demurrage cost and higher utilization of this type of unloader balance the higher cost when compared with the clamshell unloader there are several variations to this design that include steel buckets on steel chains and steel buckets on rubber belting rubber pocket belts or sandwich belts loaded by bucket wheels are also used once the material is elevated above the ship s deck it is transferred to a belt conveyor which transfers the material to a dock conveyor and port storage when fine and dusty materials are involved pneumatic unloaders either stationary or mobile are normally used gantry type unloaders can unload vessels up to 50 000 dwt great lakes coastal and gulf of mexico transport comprise a significant part of the mineral transportation market in north america the basic differences between the ships used in these environments and in oceangoing applications are size and draft although some self unloading ships are used in ocean bulk transport the majority of these vessels are found in the great lakes and the gulf of mexico however they are also used in northern europe and east asia osc 2002 for transport distances up to 5 000 km 3 107 mi in most cases the ship cargo holds have a w shaped bottom with several hydraulically operated gates that discharge the material onto a belt conveyor arrangement the material is carried to deck level using sandwich type belts or rubber pocket belts a boom conveyor transfers the material to a hopper and conveying system on the dock or in some cases directly onto the ground high unloading rates can be achieved with free flowing materials the relatively short transport time and shorter unloading time offset the machinery cost and lost payload capacity on lake and coastal applications short travel time also reduces the probability of material consolidation and arching caused by long marine trips another alternative is the use of hybrid vessels that add a system of hoppers and conveyors to a conventionally outfitted ship most of the advantages of the selfunloading ship are obtained for a limited cost but discharge rates are limited because of crane capacity multimodal transportation conventional transportation involves the use of a separate contractor or operator for each type of transport i e trucks railway and marine multimodal transportation covers doorto door deliv

ery by a single transport operator even when transport through several countries is involved this method of delivery was developed during the container revolution of the 1960s and 1970s today most multimodal cargo is containerized however it is different than typical container transport most minerals are transported in bulk rather than in packaged or break bulk shipments however some sophisticated or friable bulk minerals can be loaded in standardized marine containers or packed in flexible intermediate bulk containers fibcs also known as big bags bulk bags or super sacks the fibcs have capacities from 500 kg 1 102 lb up to 2 t 2 2 st with volumes up to 3 m3 106 ft3 fibcs are normally made out of polypropylene or canvas fabric regardless of transport method container bulk or fibc it can be handled in a multimodal fashion belt conveyors since their introduction in the first half of the 20th century belt conveyors due to their simplicity universality and reliability have become indispensable in the layout of facilities that store or process bulk solids belt conveyors allow optimal use of floor space and maximum flexibility in the design of the material flow scheme while capital cost comparisons between conveyor and truck haulage are dependent on the application operating costs of a conveyor system are far lower in general the larger the throughput of the system the greater the operational savings when compared with an equivalent truck operation current technology allows single flight belts with lengths up to 20 to 30 km 12 4 to 18 6 mi the longest system in use is 100 km 62 mi long and has been in operation since the 1970s although most conveyors use 35 degree troughing idlers and three equal rolls different troughing angles and idler configurations are employed in some operations speeds vary from less than 1 m s 3 3 ft s on some short conveyors up to 8 5 m s 28 ft s on selected overland conveyor applications and even up to 15 m s 49 ft s on spreading conveyors that handle up to 40 000 t h 44 092 stph maximum belt width is limited to 3 2 m 10 5 ft using conventional manufacturing techniques although belts can be spliced along their axes to produce even wider belts belt width is normally determined by belt capacity operating speed and maximum lump size conventional wisdom says that short conveyors handling large lumps are operated at low speed whereas longer conveyors can be operated faster without compromising belt life and system availability belt width must be more than three to five times the maximum lump size of material to be handled the higher the proportion of large lumps the closer to five the factor should be lump size is defined as the maximum dimension of one side of a piece of material it should not be confused with crusher opening or dimension of the screen mesh that the material passes through on slab like materials the longest dimension can be up to five times the

other dimensions or the opening that the lump has passed through the advent of bulk flow simulation techniques such as dem discrete element modeling has made it possible to design transfers that reduce belt wear and tear from the beginning of a project and therefore limit the trial and error used in the past that said it is important to note that properties of bulk solids can vary widely so proper testing is required even then modeling can have limitations poor transfer design can lead to premature belt wear poor tracking material degradation and dust generation there are several published guides for conveyor design the conveyor equipment manufacturers association cema handbook is widely used both in north america and abroad to design conveyors cema 2007 the cema book contains recommended practices from the north american conveyor equipment manufacturers and considers a simple calculation method for regular installations that has changed very little since the 1960s the sixth edition published in 2007 incorporated a universal calculation method in line with modern research and experience in large systems other publications such as din deutsche institut f r normung standards contain regulations about belt rating component testing and other aspects of conveyor design in long conveying systems the aim is to reduce the number of transfers to the minimum possible of course this is limited by available technology and economic factors with longer conveyors vertical and horizontal curves are used to follow the terrain and bypass natural obstacles along the desired route figure 10 7 6 currently most long conveyors use steel cord belts in shorter conveyors polyester nylon fabrics known as ep fabric have replaced nylon nylon and cotton fabrics used in the past downhill conveyor applications offer the advantage of regeneration the potential energy of the material is converted to usable energy as the conveyor drives generate electricity during operation in many cases the value of the energy generated is greater than the operating cost of the conveyor resulting in an operating profit an alternative technology to conventional conveyor belts is the cable belt which was developed more or less simultaneously with steel cord belting during the 1970s in this type of system the belt is supported between two steel cables that transfer the driving forces and carry the belt and material however a limited number of installations use cable belts whereas literally thousands use steel cord belts most likely the reason for this is that cable belts are available through one source only and their capital and operating costs appear higher than costs for conventional belts maton 2009 another variation of the conventional conveyor design is the pipe conveyor in which the belt is forced into a pipe shape and encloses the material completely since the material is enclosed spillage is eliminated pipe conveyors therefor

e have a lower environmental impact than conventional systems the belt configuration allows for tighter vertical and horizontal curves in the design as compared with conventional belts this reduces the civil works necessary with overland conveyors and makes the implementation of new conveyors easier in existing plants also since the material is contained by the belt pipe conveyors can operate on inclines approximately 50 greater than trough belt conveyors in addition the return side can easily be adapted to transport material simultaneously in the opposite direction of the main feed another advantage of the pipe conveyor is its reduced space requirements as compared with a conventional belt a dramatic example is given in figure 10 7 7 which shows part of an 8 5 km 5 3 mi pipe conveyor that transports cement on the carry side and clinker and coal on the return six kilometers 3 7 mi of this conveyor is located in a tunnel beneath the city a conventional conveyor would have required a wider tunnel and would have had serious difficulty following the complicated conveyor path which involved dozens of tight vertical and horizontal curves conveyors can negotiate steeper inclines than haul trucks so the required ramps to go up or downhill are shorter a pipe conveyor requires an even shorter distance to negotiate a slope since they can operate on steeper ramps high steep conveyors such as the flexible sidewall belts can go up to 90 flexible wall conveyors are quite popular and handle mostly free flowing low abrasion materials they are used sometimes on difficult applications mainly when layout constraints do not allow the use of more conventional alternatives however their maintenance cost tends to be quite high system cost is directly related to capacity in an exponential sense with the belt being the driving factor most of these systems that are in operation have capacities less than 1 000 t h 1 102 stph another type of steep angle system is the sandwich belt in this design a second belt rides above the carry belt in the steep section holding the material in place with the assistance of inverted pressing idlers in a system that combines a flexible wall belt with cableway technology a belt with sidewalls and integrated sets of wheels travels over carrier cables guided over supports long unsupported spans are feasible and environmental impact is reduced since civil works are minimal this type of conveyor is appropriate in irregular terrain with great differences in elevation along the conveyor traveling trippers are used to discharge material from any point along a conveyor belt they offer great flexibility for storage layouts trippers can be mounted on tires crawlers or rails at ground level and they can feed a traveling stacker to form a wedge shaped longitudinal pile trippers can also be mounted on elevated structures and discharge the material to a pile below shiftable conveyors can be used to fo

llow an excavator or to feed a stacker that moves along the length of the conveyor when the excavator or stacker needs to move to a new position the shiftable belt is relocated the individual conveyor tables are not connected to one another and are mounted on skids rails connect the skids longitudinally when the time comes to shift the conveyor belt tension is slackened and a bulldozer with a roller hook travels along the conveyor pulling it sideways the belt tables move snakelike in the direction of the pull 50 100 cm 1 6 3 3 ft with each completed pass passes are repeated until the conveyor is shifted to its new location the mobile conveyor is also used in combination with excavators or stackers or with its own hopper or cross conveyor depending on whether the machine is used in a stacking or excavating reclaiming operation the conveyor is composed of a series of steel truss sections mounted on crawlers a single conveyor belt moves along the top chord of the truss sections the whole system moves as a unit because of a control system that monitors the movement and alignment of each section figure 10 7 8 it is important that belt conveyors are designed so that material is not blown from the belt during operation covers eliminate this concern and protect both the material and the belt and idlers from the elements vertical sideboards are often sufficient for wind problems complete full covers usually made from corrugated metal enclose the entire troughed belt and components variations include three quarter and doghouse covers that leave one side open so the belt can be inspected without having to remove the cover fully enclosed galleries are used when handling environmentally sensitive materials a variation being large culvert style steel tubes in which the conveyor is located inside the tube functionally this is quite similar to the pipe conveyor discussed previously choosing one over the other depends on operational and cost factors pipe transport unlike the systems discussed to this point pipe transport systems are stationary and the material is in motion in truck rail marine and conveying systems the hardware is in motion and the payload is motionless relative to the transporting media conveying solids in a pipe involves transporting solids using the drag resistance and pressure of a flowing medium such as gas or liquid to move the material pneumatic and hydraulic systems must be considered separately as the characteristics of the carrier fluids are completely different drag resistance is proportional to the fluid density the crosssectional area of the solid particle the square of the relative flow speed of the solid and the reynolds number pneumatic conveying systems use a gas as the carrier medium the low density of the gas in the range of 1 kg m3 0 062 lb ft3 makes buoyancy nonexistent for practical purposes the difference in density between solids and media must be compensated with high

speeds to obtain the required carrying force which results in high power consumption high speeds create wear and attrition degradation so speed is limited in this regard since gases are compressible the friction loss causes pressure loss in other words the gas expands along the conveying length and unless the diameter is increased along the line the speed energy consumption and wear increase therefore pneumatic transport is generally restricted to distances less than 4 km 2 5 mi and is used mainly in plant applications for materials that are dry free flowing and relatively small in particle size when the ratio between the mass of the transported solids and the mass of the gas is lower than 30 the system is said to be a dilute phase system in this case the transport velocity is high the mix is homogeneous and the risk of blockage is minimal when the mass ratio is much higher than 30 the system is called a dense phase system plugs of material are transported at low speed but with high pressure gradient calculating the design parameters of pneumatic systems is dependent on empirical information and varies depending on what materials are to be transported although published information about most common materials is available it is recommended that appropriate laboratory tests be performed when designing a new system hydraulic conveying as its name implies uses a liquid such as water as the carrier medium the density of the mixture of liquid and solids results in significant buoyancy for the solids particles transport can be achieved at relatively low speeds far less energy is required for hydraulic conveying than for pneumatic conveying and other comparable transport methods such as trucks as the liquid is nearly incompressible long transport distances up to hundreds of kilometers or miles are achievable typically depending on the material handled solids concentrations vary between 40 and 65 by weight in systems in which the solids particles are small enough 30 m and can therefore be maintained in suspension at very low speeds one can consider homogeneous conveying the homogeneous suspension appears as a liquid with a higher density and viscosity than the carrier fluid and in this case the system can be designed in a similar fashion as for a pure liquid system if turbulence is required to keep the particles in suspension the solids are no longer distributed homogeneously however up to a certain point normally defined as a reynolds number value of 2 weber 1981 it can still be considered pseudohomogeneous conveying and conventional fluid formulas can be used for calculations also a mixture with less than 20 variation in density between the pipe axis and the vicinity of the pipe wall can be considered homogeneous for practical purposes shou 1990 with reynolds numbers above 2 conveying takes place in heterogeneous suspensions in which the velocity of the solids and the liquid are diff

erent the method used to design these types of systems was first proposed by durand 1953 and is still used today with improvements and variations the use of empirical data is fundamental to the design of systems having heterogeneous suspensions increasing the density of the mixture will result in lower liquid requirements but the flow properties of high density mixtures are non newtonian with a nonlinear relationship between shear forces and shear rate once again empirical determination of the rheological behavior of the mixture is required before design is carried out slurry transportation lines are used to transport diverse materials such as coal iron ore and copper concentrate bauxite and phosphate mineral they have a high capital cost but operating costs are low in most cases slurry lines are used successfully in south america to transport copper concentrate from the andes to sea level using the natural gradient and dissipation stations instead of pumps the energy requirement is nil they also offer the advantage of having a smaller footprint and lower environmental impact than other transport methods a buried pipeline has almost zero environmental impact after it is built another application of hydraulic transport is the disposal of tailings from concentrating plants because of the low grades involved in most concentrators the volumes of tailings are several times larger than those of concentrates tailings lines involve large pumping systems and pipes up to 1 350 mm 53 in in diameter as with any other hydraulic transport operation the decision of proper solids to liquid ratio is driven by economic factors a high concentration requires smaller pipes and less water but most likely more energy to operate the tendency is to produce tailings with higher solids concentrations to the point at which the mixture is a paste not slurry in downhill operations open troughs are sometimes used in lieu of closed pipes a hydraulic transport system requires a slurry preparation station pumping stations or dissipating stations in downhill applications and a slurry receiving station it might also include a dewatering plant to treat or reuse the water contained in the solids liquid mixture ore storage since transport from the mine is not necessarily in sync with the processing production and consumption of materials there is a need to store the ore at various stages for trucking and marine shipping ore is typically handled in batches crushing and milling are continuous operations that do not lend themselves to discontinuity in the flow of material storage sizing traditionally rules of thumb are used to size a stockpile typically stockpiles are based on storage for one day or one shift the size of a ship or the duration of a shutdown in coarse ore stockpiles with gravity recuperation a typical value for live capacity is one shift of production and for total capacity the duration of a long shutdown a more accur

ate way to determine sizing of a stockpile is through a dynamic simulation which includes factors such as the availability of feed the process plant and the output system for a given live capacity it is possible to estimate the probable number of hours that auxiliary equipment would be required to reclaim dead material or the number of hours a plant would stand because of lack of process material the net present value of the operating cost for the auxiliary equipment or production losses throughout the plant life can be compared with the extra capital cost of a system having larger live capacity operation below the live capacity line requires the use of mobile equipment the savings in operating costs and or lost production might be enough to pay for the higher capital cost of a larger stockpile dead capacity should be enough to accommodate probable shutdowns such as primary crusher liner replacements mobile crusher relocations or conveyor splices this sort of shutdown usually lasts a few days if space is available additional dead capacity can be created using a bulldozer to spread the material this kind of storage usually known as a strategic stockpile is often used on a coal fired power station feed so the power supply is not interrupted due to lack of feed in ship loading and unloading operations the high cost of ship demurrage creates the need for large stockpiles the key word in sizing a stockpile is probable as opposed to possible a stockpile should be large enough that continuity of operations can be maintained under reasonably probable conditions no matter how big the stockpile it cannot accommodate all the possible situations so when sizing stockpiles it is important to balance the certain costs of a larger stockpile against the probable cost of an operational shutdown storage configurations the conical shape is probably the most common configuration for stockpiles a conveyor feeds material at a single discharge point creating a cone although the natural instinct is to move the discharge point higher to achieve larger capacities issues such as permissible ground loading degradation of material when falling from the conveyor energy cost to lift the material cost of elevated structures and reclaim considerations limit the maximum practical height it is quite common also to use a fixed tripper or a second conveyor to create a dual cone when higher capacities are required longitudinal stockpiles can be created if the feed conveyor is elevated and has a tripper with a moving head or it feeds a shuttle conveyor to distribute the material along the axis the elevated conveyor is supported by steel or concrete columns or by an a frame structure radial stockpiles can be created using a radial stacker or a slewing type stacker various stockpile configurations are discussed by zamorano 2006 another option for forming a longitudinal stockpile is to use a wheel or track mounted traveling stacker if the stack

ing boom can be raised and lowered luffed degradation of material when falling to the ground can be minimized a boom that can rotate slew allows the formation of two parallel piles with one machine multiple stackers can be used to form multiple stockpiles in a stockyard operation covered storage there are several reasons covered storage may be required the most common is environmental regulation however protecting the material and preventing loss of fines due to wind are also reasons for small tonnages steel bins are satisfactory whereas for larger volumes up to 10 000 12 000 t 11 023 13 228 st concrete silos are commonly used in the case of conical or circular stockpiles the covers can be conical or dome shaped the conical shape is suitable for conventional steel structures constructed from steel trusses arranged radially similar to a tepee domes are mostly designed as shell membranes with a variety of alternative designs conventional steel structures can be used to create a dome shaped membrane type structure that is covered with cladding another option is a space truss type membrane formed with tubes and mechanical connections and covered with cladding this type of dome is called geodesic and can be manufactured in steel or aluminum other types of domes are made of thin concrete membranes inflatable membranes that require a blower in permanent operation and covers using fabric cladding instead of metal or plastic covers for longitudinal stockpiles are usually a variation of the a frame design closely spaced frames are used to support the cladding and the feed conveyor geodesictype frames are suitable for stockpiles with rail mounted traveling stackers or with a small light feed conveyor supported by the cover ore recovery ore recovery comprises both reclaiming and blending the material reclaiming systems in enclosed storage facilities material is recovered via a gravity trough at the bottom of the bin or silo if the material properties are known a proper design can be developed and problems like arching and rat holing can be avoided material can be recovered from stockpiles by means of mobile equipment such as front end loaders and bulldozers that feed a reclaim hopper although this option presents the lowest capital cost the operational cost is quite high so this option can be justified only in small throughput operations material can also be recovered by gravity using feeders and conveyors located underneath a stockpile in this case only a portion of the material called live capacity flows unassisted the rest of the material called dead capacity is reclaimed with mobile equipment in conical stockpiles or where free flowing materials are involved 20 to 25 of the storage is considered live however in large stockpiles or depending on material properties rat holing or arching may appear and the live capacity of the stockpile can be dramatically reduced cabrejos and goodwill 1996 to a poi

nt that mobile equipment is required at all times a higher percentage of live storage can be attained using longitudinal stockpiles and several withdrawal points or rotary plow feeders however in this instance live capacity is 50 at best bucket wheel reclaimers or other types of reclaimers such as a portal scraper can put live capacities in reach nearly all the time these machines remove material placed by stackers when required i e stockpiles leach pads etc in some cases these machines are designed to operate as a stacker and a reclaimer they are called combined machines figure 10 7 9 the additional capital expense of mechanized reclaiming systems is generally outweighed by the operational cost savings realized another approach is the use of large squat silos with assisted feed and reclaim to obtain a 100 live capacity within a small footprint nijhof and ruijgrok 1986 blending variation of material properties in different areas of a specific mine or from different mines in a single processing facility necessitates blending the material to obtain a homogeneous product this might involve a chemical component or even particle distribution there is a certain amount of natural mixing whenever a bulk material is handled a crude level of mixing is achieved on stockpiles with the use of front end loaders or clamshell buckets the same can be said regarding boom mounted bucketwheel reclaimers and scraper reclaimers another approach is to use different bins to feed a common conveyor the blending effect is defined as the ratio of the standard variation of the control property at the store input and the standard deviation of the same property at the store output theoretically the blending ratio is proportional to the square root of the number of layers reclaimed simultaneously in reality it is somewhat lower particularly in coarse materials in longitudinal stockpiles when blending is not required the preferred stacking method is the cone shell method in which the stacker discharges onto a single point once the maximum height is reached the stacker moves a discrete distance and continues to discharge forming contiguous cones the chevron stacking method consists of depositing the material while the stacker moves to and fro over the center of the stockpile to ensure proper blending the material must be reclaimed from the face of the pile working across the entire cross section this can be achieved with a bridge scraper reclaimer that employs a harrow to guide the material into the scraper chain at ground level the reclaimer recovers material from previously stacked material while more material is stacked in the area that has been reclaimed the chevron method can also be used on a circular blending stockpile as shown in figure 10 7 10 where work is done on a continuous basis a slewing stacker is mounted at a center pile and a bridge scraper reclaimer feeds a discharge point also on the center according to the

windrow method material is deposited over several positions across the full width of the stockpile this method prevents segregation and the reclaimer can operate on one part of the cross section at a time a bridge bucketwheel reclaimer can be used in this type of arrangement environmental considerations environmental factors become more relevant every day in connection with ore transport and storage operations obtaining the relevant local environmental permits has become a key element for most mining operations and compliance with environmental management standards such as iso 14000 is becoming a requirement for international trade this section addresses concerns directly related to the environment the handling of bulk solids generates unwanted airborne dust during transport by truck or train it is quite common to use canvas covers to prevent contamination along the transport route as has been discussed covers are used along belt conveyors for the same purpose when fine and troublesome materials are an issue the entire conveyor is completely enclosed or closed systems such as pipe conveyors are used loading and unloading activities and transfer points generate large amounts of dust which is controlled by enclosure suppression extraction control of fall speed or by a combination of these coal storage presents the risk of spontaneous combustion and can produce toxic fumes management systems on open stockpiles include the use of large spraying cannons to maintain surface moisture levels this type of system requires a water management plan to control the acid water percolated from the storage area stormwater treatment facilities are required in open storage facilities for coal or other types of contaminating materials enclosures can reduce the fugitive emissions however sometimes additional suppression or extraction systems are required at times negative pressure systems are employed on enclosed stockpiles to capture dust dust suppression involves the addition of water at a transfer or drop point to capture airborne particles chemical surfactants can also be used to increase the efficiency of a dust suppression system as they facilitate the coalescence of the water droplets and dust particles another approach is the use of dry fog in which a special nozzle produces very small droplets these droplets are approximately the same size as the dust particles and improve the chances of capture at the same time this minimizes water consumption and the addition of water to the ore in open stockpiles large spraying cannons are used to maintain superficial moisture and reduce dust emissions dust extraction systems use negative pressure to collect the dust from a transfer point or other dust source the dust is then removed using a filter cyclone or scrubber depending on the particular application cascade chutes are used to reduce the speed of the material on large drops the lower the speed the less dust is gen

erated any water used for dust control or hydraulic transport is treated prior to discharge however in dry areas conflicts can arise between the mine and local communities because water is a scarce resource and residents don t want it to be diverted for mining use in addition local communities are concerned that water contamination will result from the mining process the issue of noise including its impact on endangered species can be another area of conflict between a mine and local communities if this is an issue conveyors and pipelines have a clear advantage over trucks an aspect of mining that became an issue only recently is carbon emissions compared with truck haulage continuous transport methods such as conveyors and pipelines can reduce the amount of carbon released into the atmosphere energy efficiency and the method by which power is generated also greatly influence carbon emissions impacts on the landscape and surrounding communities are also relevant factors to be considered when making decisions about handling systems for bulk materials again continuous methods tend to produce the lowest impact planners often neglect to consider the disruption caused by a great many trucks moving through communities for minerals that lie within economic reach by surface mining and when the deposit s specific geometry either tabular or bedded allows or dictates an approach to extraction called open cast or strip mining as it is more commonly referred to can be employed although a range of commodities such as phosphate bauxite tar sands manganese and even industrial materials from quarries have been recovered in this manner the most common deposits worked by strip mining are coal deposits and for this reason this chapter will refer predominantly to coal the economic depth for strip mining is dependent on realizing a margin between its unit revenue value and the unit cost to recover driven by depth and deposit complexity the lesser value bulk commodities will typically only be economic to shallower depths whereas high value coking coal such as that found in australia s bowen basin and where the stripping ratio allows shows some strip mines currently operating at up to 150 m of depth with potential to progress to more than 300 m strip mining is a bulk earth moving operation making use of large scale mechanized equipment an attractive geometry for strip mining is a tabular deposit with coal subcropping at economic depth and extending laterally either flat or gently dipping and constrained vertically within a number of seams deposit continuity relatively free of geological structure or intrusions is also attractive but not a prerequisite although a heavily disturbed and or intruded deposit and the resulting economies may limit the selection of equipment and operating method strip mining is characterized by its method of waste material or overburden movement which is placed almost entirely in pit an initial

cut is made on the coal subcrop called the boxcut and the overburden is placed on a natural surface updip of the subcrop line the exposed coal is mined out and successive cuts or strips are taken to progress the mining downdip with the overburden from each strip placed inside the previous mining void individual strip geometry is typically from 30 to 100 m wide and to the economically recoverable basal coal seam a strip will extend along the strike until it is constrained by a mining property boundary limiting surface feature geological discontinuity or economic limit with the maximum length of some strips being in the kilometers where conditions allow a form of strip mining where the strips are aligned downdip instead of along the strike is applied in some instances but is rarer as economics generally favor strike mining each strip working area is denoted as a pit with a strip mine consisting of a number of pits strip mining encompasses a number of different mining strategies each a unique combination of pit configuration equipment selection and operating methodology this chapter begins by outlining a generic strip mining process it then goes on to discuss alternative pit configurations equipment selections and operating methodologies for each of four mining strategies the strategies have been developed in some detail with a major case study example used to provide comparison and contrast additional examples are referred to for variants of note the chapter closes with some discussion on what drives the selection of one strip mining strategy over another as well as a brief commentary on future trends in strip mining generic strip mining the strip mining process from first workings to final closure can be described generically in six steps 1 clearing and topsoil removal 2 fragmentation 3 waste removal 4 waste placement including soil restoration and initial revegetation 5 coal mining 6 mine restoration maintenance and eventual closure figure 10 8 1 provides a stylized schematic to show these processes each of these process steps will be discussed briefly as a prelude to discussing the four strip mining strategies some additional operational activities that support the mining process have been covered briefly due to their influence these include rejects and tailings storage or disposal river and creek diversions and water management clearing and topsoil removal prior to first workings a mining area is typically covered with vegetation and topsoil the vegetation must be cleared and the topsoil must be recovered and stockpiled for later use in postmining rehabilitation vegetation is cleared by track dozers and pushed into piles and disposed of by burning shredding for mulch burying or a variety of methods dependent on local customs and laws an examination is made as to the quantity and quality of topsoil and a target thickness to recover is identified high quality soils may need to be separately iden

tified and stored topsoil is removed with earthmoving equipment typically scrapers however where thickness allows dozers assisting excavators to load trucks may also be an efficient operating method the topsoil is stockpiled in the vicinity of the future mining operation so that it may be recovered for postmining rehabilitation for a mature mine with ongoing clearing and topsoil removal occurring in parallel to final rehabilitation it may be possible and is certainly desirable to place topsoil directly in its final position without stockpiling it is important that topsoil stockpiles are not compacted by machinery or excessive thickness as biological activity and thus their value will be adversely affected by the exclusion of oxygen and moisture from the topsoil mass fragmentation following topsoil removal there is a relatively small thickness of weathered and or unconsolidated material overlying much thicker units of competent overburden however where there is a substantial thickness of weak material it may be possible or even necessary to progress directly to a wasteremoval process this is the exception and so fracturing or fragmentation of the overburden in advance of mining operations is required so that it may be handled safely and productively two main methods of achieving fragmentation are dozer ripping and drill and blast with method selection dependent on the strength of the overburden its thickness and the volumetric demand of the mine dozer ripping for relatively thin overburden of less than 4 m and where rock strength allows a dozer ripping process may be used for fragmentation use is made of the track dozer tines and the machine s mass to break the rock into a manageable size operational effectiveness is highly sensitive to rock strength with high strength rock leading to poor productivity as well as maintenance downtime due to adverse wear on the machine drilling and blasting drilling involves the creation of holes in the overburden within which explosives are placed drill holes are laid out in a regular pattern either square or offset the spacing of which is determined by the diameter of holes being drilled and the desired explosives density per volume of rock to be broken or powder factor drill holes are drilled to either the roof of the target coal seam or to an operational depth to match the waste removal operating methodology where the valuable material i e coal is variable in depth the driller may drill into the coal to ensure correct hole depth and then put a short length of stemming into the bottom of the hole to protect the relatively soft coal from damage when the overburden is blasted otherwise holes are measured to ensure that the correct depth has been reached with wet holes being marked and pumped out if the depth of water warrants it poor depth control will lead to uneven pit floors and inefficient waste removal a presplit may also be used where a line of holes at a t

ighter spacing is drilled along the line of the next strip s highwall and is blasted separately from the main pattern the intention is to deliver a smooth wall that is productive to excavate the waste back to and safe for personnel and equipment to work under during coal mining a blast design is prepared based on the overburden strength characteristics the wetness of the holes and the desired direction of movement of the blasted material controlled by placement and duration of hole delays bulk explosives are placed in the drill holes by mechanical means with ammonium nitrate and fuel oil anfo being the most common explosive used specialist explosives may be used for stronger overburden with water resistant emulsions used in wet applications as each hole is loaded with explosives an explosive booster tied to a detonating cord is placed within the explosives near the bottom of the hole the top 5 m or so of the hole is filled with stemming to prevent the blast energy from escaping at the top of the hole and instead direct the blast energy into the rock mass to fracture it and in some applications to actually move it sideways the cuttings at the top of the drill hole are often used as stemming material but some mines import size graded rock to deliver a superior blast containment mechanism after the area to be blasted has been loaded with explosives the shot firer will connect each hole with detonating cord and time delays a lead in detonating cord is run out to a position of safety and with guards in place and the area confirmed as safe to blast the shot firer will initiate the blast electrically initiated detonators with millisecond delays may be preferred where sensitive vibration control is required a good blast outcome is typified by an absence of flyrock no excessive sound and or vibration no misfired holes and good visible fragmentation with the rock mass traveling in the desired direction strip mining is unique in that the drill and blast process itself can be employed as an overburden removal process as the overburden is to be placed into the mined out void immediately adjacent certain pit configurations and operating methodologies lend themselves to cast blasting cast blasting is where a powder factor and delay design is selected to purposely cause the fractured rock mass to heave in the direction of the mined out void with large quantities of overburden up to 30 resting in final position it therefore requires no further handling by mining equipment this is a particularly economical method of overburden removal waste removal after blasting the overburden may then be removed each end of the strip is called an endwall the wall of the strip that is to progress successively downdip and is cut in in situ material or prime is called the highwall the in pit overburden or spoil side wall is called the low wall because of its flatter slope compared to the highwall refer to figure 10 8 1 cast doze excav

ating at shallow depths of 30 m a combination of cast blasting and pushing with track dozers will provide the most costeffective method of waste removal cast blasting can place up to 30 or more of the prime overburden into a final position track dozers can then push the remaining overburden into final position if dozer pushes become excessively long or uphill and therefore less productive it may be more economical to supplement the operation with an excavator and a shorthaul truck operation the highwall will typically be cut to an angle of 65 with excavator assistance in cutting the batters back to hard material the low wall will typically be placed at between 37 the angle of natural repose for fragmented rock and 45 effective rock fragmentation is critical to the success of this method as blocky oversize overburden has a significant negative impact on the productivity of dozer operations poorly cemented overburden may be moved by scrapers dragline stripping where scale and deposit geometries allow up to 85 m or so in overburden depth depending on the class of machine being applied a dragline can provide a more cost effective method of waste removal this requires sufficient annual demand for overburden stripping and mine life in excess of 10 years to pay back a high capital low operating cost piece of equipment simplistically the dragline operates by moving overburden or prime from the current strip and placing it in the previous mined out strip as spoil figure 10 8 2 shows a diagram of a dragline identifying its main physical features the dragline needs to be placed at a level from which it can reach the top of the deposit to be exposed this will either be its dig depth or operating level if less than the machine s maximum dig depth although the dragline can handle overburden above its operating level via overhand methods the total thickness of overburden to be allocated to the dragline therefore may consist of a number of passes made up of a combination of overhand and underhand operations and perhaps even repeated for multiple seams figure 10 8 3 shows a typical walking dragline operation highwall angles range from 45 to 65 and depend on the competency of the material to be excavated as well as joint orientations offset benches in the highwall may be established for reasons of overall wall safety in terms of stability to reduce risk to subsequent mining operations below the wall or for operational reasons relating to the maximum reach of the overburden drills low wall angles are typically cut at 45 up to the dragline operating level and then lay back at a 37 angle of repose to the peak of the dragline spoil the dragline will advance along the strip moving by dragging itself on a revolving tub by 1 2 m steps delivered by two large feet on an eccentric cam in regular blocks typically 30 m in length but they can be longer or shorter depending on the dragline s operating level and thus its abil

ity to reach the toe of its excavated block for a narrow strip width the dragline is often placed directly into a final position whereas for a wider strip as is more commonly the case the dragline builds temporary operating positions out of overburden from which it places waste from the previous block into final position this is discussed in more detail later any overburden that the dragline has to handle a second time is termed rehandle an efficient dragline operation will minimize rehandle which is achieved by setting the dragline s operating level as low as possible unfortunately the lower the dragline operating level the lower its direct overall spoiling volume capacity which will then have to be compensated for either by employing spoil side rehandle passes or removing material in advance by truck methods both of these are expensive so rehandle is often an outcome of balancing desired coal exposure rates with installed dragline capacity and overburden removal economics dragline operating geometries provide a relatively narrow operating envelope for the larger class of draglines their operating parameters are a dig depth of 65 m a dump height of 55 m and a dump radius of 95 m with an effective thickness of overburden allocated to the dragline system of around 85 m bucyrus 2008 overburden outside of this envelope will need to be rehandled if the dragline is to deal with it if it needs to be rehandled twice it will generally be cheaper to remove by truck methods errors in design and unexpected geological impacts such as faults or discovered areas of geotechnical weakness may result in overburden being locally over allocated to the dragline somewhere along the strip this may result in the dragline becoming spoil bound where it can no longer place all of the overburden it has been allocated to a final position using its planned operating methodology this needs to be rectified either by ramping its bench higher to provide additional spoil room or if spoil fit is already maximized by walking the dragline into the spoil and rehandling overburden even further away via an elevated bench or pullback operation to create room for the current strip s waste this is an expensive exercise and can cause significant unplanned delays in coal exposure rates and so is to be avoided mobile track mounted draglines are still popular in variable terrain where overburden depths do not exceed the digging capacity of these smaller machines the annual operating capacity of a dragline is a product of its rostered time mechanical and electrical availability operating use and productivity as they are capital intensive in excess of us 200 million per unit draglines are typically rostered to operate 24 hours per day 7 days per week their relative simplicity as a predominantly electrical machine leads to high availabilities typically running in excess of 90 apart from geotechnical failures excessive rainfall or mine planning

failures lack of blasted inventory operating use is also high running typically in excess of 90 of available time productivity will vary depending on the operation with large swing angles from dig to dump high spoiling poor fragmentation and rework of the excavated face or bench resulting in lower productivities total volumes moved per annum will range from 15 mbm3 yr million bank cubic meters per year for the most common smaller sized machines to 30 mbm3 yr for the larger machines currently deployed a dragline depending on its size will consume the equivalent electricity per annum to supply a small town of 3 000 to 5 000 people concentrating so much productive capacity in a single machine and in a pit configuration that requires vertical stripping however is a significant risk exposure and there have been instances of catastrophic failure via machines falling into the pit because of bench failures basic mechanical failure of key structural components materials failure or operator error or flooding for mines relying on a single or a small number of draglines this results in a significant business impact that is not quickly or inexpensively recovered from small efficiencies in dragline use lead to large cost savings so most large draglines will be fitted with operating monitors these assist the machine operator to select optimum swing angles bucket fill factors and casting radii dozer assisted dragline stripping the dragline can be supplemented by track dozers pushing prime material into the void where depending on strip width it can be rehandled to its final position by the dragline this operation is called production dozing and has the effect of increasing the effective prime waste movement rate and thus coal exposure rate of the dragline system but at the expense of increasing dragline rehandles nonetheless where there is sufficient spoil side room due to the dragline being underallocated waste in terms of spoil fit for coal exposure rate reasons it is a low cost low risk way to increase the total thickness of overburden allocated to the dragline system truck and loader stripping for more restrictive deposit geometries shorter mine lives and where a more variable scale of operation is planned the superior flexibility of truck based waste removal methods may be suitable truck methods involve the use of trucks and loading equipment to dig overburden and dump it loading equipment is selected to match the trucks whereas the truck and loader package is selected to match the task highwall batter angles are 60 for each bench in competent material with flatter angles for poorer material but often with 5 m berms between benches leading to much flatter overall highwall angles large bulk thicknesses of overburden typically 15 m will be removed by electric rope shovel and ultra class 300 t payload trucks as the most cost effective operation provided there is a sufficient annual requirement for

waste stripping and mine life 10 years to pay back the higher capital loweroperating cost position offered by larger electric shovels compared to smaller hydraulic excavators for thicknesses 20 m and up to 25 m overburden can be pushed down with dozers to create a safe working face height beyond 25 m of total thickness multiple benches are often created these might be separately blasted or through blasted depending on material competency to allow them to be run over with the trucks where ready supplies of electric power are not available or at contract operations this overburden task will alternatively be handled by large hydraulic tracked excavators either in shovel or backhoe configuration electric rope shovels work most productively on a relatively flat bench so where the dip of the coal seams exceeds the effective operating grade for the electric shovel a secondary wedge operation using an excavator and trucks may be needed truck shovel operations can be used in an advancedbench mode on upper layers of overburden to prepare a working bench for a large dragline the truck shovel operation removes variable topography leaving a horizontal bench for the dragline the dragline then is able to operate uniformly and efficiently at its optimum digging depth meanwhile the truck haulage routes are arranged so that a natural looking final topography is created without the need for spoil rehandle for overburden thicknesses 15 m it will usually be more cost effective to run with smaller hydraulic tracked excavators loading appropriately sized trucks as overburden thicknesses reduce smaller excavators may be selected below 2 m of thickness it may become more effective to have a dozer ripping and pushing overburden up to the excavator whereas sometimes a pure dozer method is employed where a void is available within a short push distance wheel loaders are rarely used to load waste often due to their inferior economics in overburden operations a typical truck and loader fleet will consist of a loading tool sufficient trucks to match the total cycle time load haul dump and return to keep the loading tool running continuously and various earthworks support gear including dozers graders and water carts when used to remove overburden above the dragline the truck operation is often termed a prestrip operation when used to remove waste between coal seams below the dragline operation the truck operation will often be termed an interburden operation whereas for thin waste bands it may be called a parting operation where the truck operation uncovers coal in its own right either above a dragline or in a nondragline mine it is more commonly termed a truck and shovel operation the width of strip for the truck and loader operation will typically be set to match the dragline strips so that a regular release of dragline strips occurs this can limit the effectiveness of the loading operation in narrower strips by reducing t

he operating room increasing the influence of edge effects and limiting the opportunity to deploy doublesided loading of the trucks thereby reducing productivity wider strips however result in longer truck cycle times and increased work in progress in the pit that manifests as a larger invested working capital scheduling bottlenecks are also introduced by larger batch sizes via wider truck and loader strips these effects need to be carefully evaluated depending on local conditions figure 10 8 4 shows a typical truck and electric rope shovel operation the annual operating capacity of an electric rope shovel is a product of its rostered time mechanical and electrical availability operating use and productivity as they are capital intensive in excess of us 20 million per unit electric rope shovels are typically rostered to operate 24 hours per day 7 days per week their relative simplicity as a predominantly electrical machine leads to high availabilities typically running in excess of 90 apart from geotechnical failures excessive rainfall mine planning failures lack of blasted inventory or sufficient trucks in the cycle to match the haul duty required operating use is also high running typically in excess of 90 of available time productivity will generally vary less than a dragline because of the relatively generic nature of electric rope shovel operation most of the time productivity will be affected when digging less than optimal bench height ramping in or out of an area or when poor fragmentation and difficult materials such as clays result in carry back within the bucket leading to increased truck loading times total volumes for electric rope shovels typically range from 15 mbm3 yr to 20 mbm3 yr for the larger machines currently deployed with the variability driven by the specific application three electric rope shovels will consume approximately the same electricity per annum as a large dragline electric rope shovels are inherently flexible but can present some constraints in terms of minimum required working areas maximum operating cross grade and the requirement to manage a cable interacting with blasting operations and a fleet of trucks continuous system stripping for pits that require large annual quantities of waste movement 15 mm3 yr to be placed via long haul cycles 30 minute return it may be cost effective to deploy a continuous or conveyor waste system either by itself or in conjunction with other waste removal methods relatively few of these operate globally and their successful implementation is highly dependent on developing a specific and robust mine plan that marries overall pit configuration equipment selection and operating methodology to the fixed nature of these systems compared to other mobile overburden moving equipment options these systems comprise a crusher station at the dig end a long conveyor 5 km and a spreader at the dump end the crusher may be fed by trucks

or directly by track dozer excavator shovel bucket wheel or even dragline these systems are capital intensive being comparable in capital cost to a dragline with similar annual prime capacity but with an operating cost that sits between a dragline and truck and loader systems for these reasons only the longest equivalent truck hauls provide an effective cost offset economic payback can take a number of years and depends on achieving large annual volumes which means minimizing the magnitude and frequency of partial or full system relocations this is a developing mining strategy that will see more potential for economic application as pits deepen but it is also under competition from emerging technology step change improvements in truck and loader systems such as partial or full automation the environmental advantage of continuous system stripping is the ability to restore spoil in an approximation of the original strata profile waste placement the movement of waste is a pure cost to the mining operation and has no direct economic benefit so the placement of waste will generally be driven by a least effort approach for equipment other than trucks and conveyors this dictates the overburden only be moved a short distance within the practical and economical operating envelope of the equipment in question this will typically be 200 m in the horizontal from the point of origin with changes in elevation generally 50 m cast doze excavation dragline stripping and dozerassisted dragline stripping methods generally place the overburden into the void from the previous strip either directly or with some rehandle minor truck stripping operations engaged in either parting or thin interburden operations and generally at or near the bottom of the strip will seek to minimize the haul overburden is thus placed inside the strip being mined or nearby perhaps being used to regrade an access ramp that may be running at a flatter grade than that required by the coal mining trucks bulk truck stripping operations not supporting a dragline stripping method also place the overburden in the previous void least cost operation is generally achieved when material is hauled on grade or as flat as possible and it is desirable to establish the dumping strategy to sustain an average haul this will ensure that short hauls do not idle spare trucks unduly and long hauls do not create instances where the loading equipment is undertrucked and production is lost it is vital to have an established road system that allows access to multiple dump levels and this road system will grow in complexity with pit depth and thus the number of dump lifts being developed for bulk truck stripping operations supporting a dragline stripping method otherwise known as prestrip the overburden is placed on top of the dragline spoil the proximity of the truck dump to the active spoil area is determined by the geotechnical competence of the waste material itself and

the strip floor however a good general rule is two spoil peaks or prior strips back dump geometry is dictated again by geotechnical competence but also the location of roads to access the dump and the desired intensity of dumping operations that need to be designed for a typical overall operating angle of 19 for the active dump face is typical for truck dumping and this profile is the result of a series of dump lifts that batter down at angle of repose interrupted by catch berms and roads the economic cost of elevating overburden with trucks relative to horizontal haul is approximately 20 1 which is to say a dump will preferentially develop horizontally until economics dictate that an additional dump lift be added for this reason the dumps for mature pits will adopt a typical profile with the greatest height in proximity to the active dig area with a trailing back of the dump at this point it is worth considering the geometric drivers of dump development as well as economics figure 10 8 5 shows how the dumping strategy changes over time as a strip mine deepens as the pit progresses downdip batter effects demand greater and greater volumes of overburden to be moved and this generally reports directly to the truck operation as the dragline is allocated a fixed thickness of overburden for each strip on the dump side the dump end batters play a similar role but have the inverse effect as the dump gets higher it has a correspondingly lower volumetric capacity due to less pit length available for dumping the system dynamic is simplistically one of an inverted cone shaped pit getting deeper filling a conical stockpile that has a finite capacity for a fixed basal area after the dump reaches capacity only three responses are available first any perpendicular ramps to the base of the strips that create large valleys reducing available dump room can be filled with alternative clearance of the mineral to be mined to occur by parallel low wall ramps or via a highwall ramp system second the dump can be extended in the direction of the original commencement of mining this can have grave consequences for any progressive rehabilitation that was undertaken without a final landform design and can lead to redisturbance of previously rehabilitated areas the same issue might also apply to any infrastructure that was also placed without due consideration of long term dump designs e g coal haul roads power lines stockpile areas third overburden can be placed out of pit or in other words beyond the limits of the excavated to date pit shell in some regions this out of pit material is called excess spoil this can be expensive involving long hauls or placing overburden on the highwall side of the pit and on top of future resource areas this may be undesirable unless there are sterile areas due to geological disturbance intrusions or underground operations or other factors that may preclude strip mining in that area or unless

the subsequent rehandle of this waste at a future date is still economical an additional downside to out of pit dumping is that it increases the total area of mining disturbance and creates an additional net area to be rehabilitated and maintained after mining ceases excess spoil dumps have created environmental challenges whenever they intrude into established natural drainage channels the placement of overburden for deep strip mines is an emerging problem and generally starts to manifest at depths of around 150 m a further uncertainty and risk is the geotechnical stability of large in pit dumps and is the subject of ongoing research for an integrated truck and dragline method an additional operational complexity to be resolved is the interaction between the trucks and the dragline with both equipment types often having to operate in close proximity at the same time the truck operation will seek to minimize its average cycle time by crossing the open strip when it is excavating near the middle of the strip to get to the dump but the dragline operation seeks to operate long continuous strips for maximum efficiency one compromise is the use of cross pit bridges that the trucks travel across temporary bridges are constructed from blasted overburden and built to the height of the dragline operating level when the dragline is a suitable distance away from the temporary bridge the trucks use it to provide a short haul access across the pit they are rolled by the dragline on each strip to recover the coal that lies beneath them the downside of temporary bridges is that the bridge is not always available for use by the trucks or its use by the trucks becomes a constraint on the mine schedule and the rolling of bridges has a productivity impact on the dragline an alternative approach is the use of permanent bridges these are constructed from in situ material that is left in place in each successive strip the coal beneath them is effectively sterilized as the successive mined out strips are filled with overburden that either buttresses or buries the remnant mineral material the bridges may be built to any height and may incorporate multiple crosspit roadways at different levels to further optimize trucking operations the downsides of permanent bridges are the sterilization of resources which is magnified for higher bridges due to larger bases from the batter effects as well as their role in limiting the length of strip the dragline operates with the corresponding productivity impact from end effects and more frequent machine relocations the rest of the trucked overburden either travels around the end of the pit or up a highwall ramp system to a natural surface and then around to the dump a major advantage for endwall road systems is the provision of cost effective on grade highwalls with a detraction being additional stripping demand from access roads cut into sterile ground which can be substantial at depth especially for sh

ort strips a major advantage for highwall ramp systems is that they are excavated in material that is part of the ongoing mining operation and generally already exist if coal seams are present within the prestrip working horizon a detraction is that productivity impacts the excavation operation from removing and re creating ramps if not already in place as well as the potential need for loaded trucks to haul up to a natural surface and then haul downhill on the dump side which is inefficient placement of overburden by conveyor systems is most similar to truck operations a spreader constructs the overburden dump depending on the equipment configuration the overburden dump may be built in benches radially or along the strike an upper stacked overburden bench of 15 m height may be built simultaneously with a lower filled overburden bench of up to 45 m thick before the whole system is advanced because of the immobile nature of continuous waste systems the conveyor will either travel around the end of the strip or across a central permanent bridge figure 10 8 6 shows a diagram of a typical spreader dump landform strategy all placement methods the waste dump or landform strategy is dependent on a number of key factors including the long term swell factor operating pit and progressive and final landform geometries any mining or dumping constraints the total static volume balance and the dynamics of optimal dump construction the influence of the final pit limit and final pit treatment should also not be underestimated with impacts for deep pits likely to affect the entire postmining landform constructed early in the mine life but not manifested until the latter years of the mine life variations in the elevation of spoil dumps due to swell factor and the inherent displacement by draglines of contour highs and lows may lead to substantial drainage mismatch between disturbed and undisturbed lands surface contouring software can generate plans for final restoration that respect geomorphological principles coal mining although the overburden removal process accounts for the bulk of the operating expenditure for a strip mine the coal itself is the reason for the stripping and provides the revenue to support overburden removal the overburden and coal seams are physically coincident at their boundary so it is inevitable that imperfect mining processes will result in some unplanned removal of coal with the overburden or loss and some unplanned addition of overburden to the coal or dilution typical losses and dilutions by mass can range from 5 to 10 with higher losses incurred for thinner seams and dependent on local practices and conditions loss is directly incurred through blasting that can cast some of the coal into the final spoil position to be lost forever but also blast enabled through the disturbance of the overburden coal interface by using large scale earthmoving equipment driven by the paradigm and key performance

indicator kpi of minimizing unit operating cost removing the overburden near the coal can easily be biased toward coal loss an effective remedy can be to set coal recovery as a kpi or deploy a more selective equipment configuration when approaching the overburden coal interface in the last 1 2 m this incurs a higher unit cost for overburden removal but the affected volume is quite small and the total cost impact minimal similarly the coal mining operation is often biased toward loss over dilution due to the high visibility of overburden within the mined coal for high value coking coal where a processing plant is available to remove dilution and the cost of replacing lost coal is high it will generally be economical to favor dilution over coal loss for lower value coal such as thermal coal which generally has a lower cost to produce and is often crushed and sold without washing it is usually more economical to favor coal loss over dilution some mines reduce dilution by use of a commercial road sweeper on the coal surface to windrow rocky material away from the coal loading operation coal mining is commonly by loader and truck systems for thin seams or soft coal it is dug unblasted or may be ripped and pushed by dozers or even graders as part of the mining process high value thin seam coals have been excavated by tarmac recovery machines called fine graders or special continuous surface miners whose operation combines the fragmentation and mining process for thick and or hard seams a square drill and blast pattern is employed to fragment the coal prior to mining loading equipment used is either a frontend wheel loader a diesel excavator in shovel or backhoe configuration or more rarely a smaller electric rope shovel for some mines that have particularly thick coal seams the choice of loading equipment will be driven by the required mining rate and the thickness of the coal seam and its continuity with excavators in backhoe configuration being the digging tool of choice for highly faulted or banded coal seams where selective mining is required trucks will either be rear dumps electric or diesel or belly dumps rear dumps have the advantages of being more maneuverable and faster up the coal access ramps compared to belly dumps as well as being able to handle ramp grades of up to 10 whereas they have the disadvantage of being slower on the flat with top speeds of 65 km h compared to 75 km h for belly dumps for very long flat hauls 20 km and where ramp grades allow highwayadapted road trains consisting of multiple self tipping trailers with up to 300 t capacity are used figure 10 8 7 illustrates a typical coal mining operation depending on blending requirements coal may be directmined and fed to the wash plant continuously from a number of strips in different pits at once alternatively it may be mined to stockpile for reclaiming later either by a mechanical stacker reclaimer system or by manned equipment su

ch as a batch fed process mine restoration maintenance and eventual closure the objective of mine closure is to return to the wider community a postmining or final landform that is safe stable and sustainable safety dictates that the final landform does not present a risk to humans or wildlife stability dictates that the final landform is both erosionally and geotechnically stable with erosional stability more challenging to achieve sustainability dictates that the final landform reduces and prevents pollution enhances biodiversity and delivers a beneficial use with no ongoing liability to the company that owned and operated the mine strip mining by definition disturbs large areas of land so the greatest impact and ultimately closure cost and potential trailing liability relates to the creation and maintenance of the final landform from the open voids consisting of final strips and ramps other aspects of mine closure include the demolition and rehabilitation of mine and fixed plant infrastructure but these are small contributors compared to the landform in some jurisdictions the final landform is highly prescribed whereas in others legislation provides broad guidance within a set of principles final landforms can range from voids whose batters have been regraded to voids that have been fully backfilled to the original topographic levels in a strip mine it is worth noting that by virtue of the operating methodology mine closure in terms of the delivery of a final landform should be seen as a progressive activity that commences the day the mine opens and occurs more or less continuously until the site is fully rehabilitated for this reason a strategic landform study should be undertaken as part of the initial development study or soon after so that the construction of progressive landforms can be undertaken in a cost effective manner maximizing the design of truck dumps to emulate as near as possible the final landform design this will involve the bulk placement of material to progressively deliver the final landform fill preferential placement of adverse material types such as acid forming or highly erosive spoils within the spoil mass preferential storage with progressive use of materials suitable for controlling erosion of external surfaces and topsoil to aid in revegetation of the regraded spoil mass to deliver the final landform finish the other aspect of the final landform is the treatment of the final voids consisting of access ramps and the last mined strip significant costs and liabilities can be incurred if closure is not approached in a planned way however there are also significant operating benefits to be accessed from developing a comprehensive operating to close strategy and implementing it in the last 5 to 10 years of the mine s life for deep pits it is likely that any perpendicular coal access ramps will have long been closed for dump room reasons but for shallower pits approaching closure and

where still in use they can represent a low cost dump opportunity final strip voids become more valuable as a low cost dumping opportunity for adjacent pits than as a source of marginally economic coal a mitigated closure strategy will generally see all ramp voids and most final strip voids filled with waste from the remaining operating pits in time these pits themselves will self fill as a way to reduce unit operating costs and still yield a positive cash margin albeit with ever reducing total mine output where sufficient area exists and economics allow some mines have wheeled their pits around in a large set of 90 offsets so that the last pit is adjacent to the first pit and thereby fills it regulating authorities in some countries demand a final landform design and progressive delivery as a condition for operating a mine whereas in other countries the approach is less prescriptive nonetheless the final landform is an integral part of operating a strip mine and it is critical that a strategic landform plan is developed early and followed throughout the mine life mining companies are able to operate by virtue of the license to operate lto they hold a company s lto is a product of its adherence to statutory requirements and the literal permits this provides in combination with figurative permissions provided by meeting the expectations placed on it by the wider community at its broadest definition this community can consist of local regional national and international stakeholder groups comprised of individuals institutions and organizations the strength of an open cast mining company s lto is directly related to how it manages progressive and final closure and specific mine planning and design aspects such as landform treatment of final voids water drainage and quality and how they ultimately impact postmining land use relative economics by business process and changes with depth having discussed the key operational tasks that define the strip mining process it is worthwhile contrasting the relative economics of each how these relativities change with depth and the operational implications of these movements four scenarios were developed based on a single 10 m coal seam increasing waste depths and differential waste allocation by waste stripping process costs include both capital and operating components figure 10 8 8 highlights the contribution waste removal makes to total mine gate cost for the four scenarios dragline operating productively at shallow depth 35 m dragline operating at its effective limit 70 m 50 m of truck and shovel prestrip introduced with relatively short haul 6 trucks and 100 m of truck and shovel prestrip introduced with relatively long haul 12 trucks some key points become clear for shallow deposits 35 m coal related processes make up a significant proportion of total cost and the operation will be oriented around minimizing coal mining cost this suggests frequent c

oal access ramps with coal haul roads optimized for haulage to the processing plant with management focus on the coal mining process as the seams progress deeper 70 m drill and blast makes up a more significant proportion of cost and focus will shift to optimizing drill and blast operating practices dragline operations are now the majority of total cost and there will be extreme focus on productivity and equipment uptime beyond 70 m as truck and shovel prestrip is introduced it will work around the dragline process and its role will be to ensure costeffective dragline operation at extreme depths 100 m of truck and shovel and 70 m of dragline waste stripping dominates the cost structure with truck and shovel operations now contributing as much cost as all of the other operations together the coal mining and dragline operations will work around the truck and shovel operation to ensure its cost effectiveness the mine design will be modified to optimize truck and shovel at the expense of all else the scenario also shows the additional cost loading due to task the additional depth has a linear impact providing that the same haul cycles can be maintained but simple dump geometry dictates that with depth not only is there more waste but it also has to be hauled farther and higher the additional cost from this increased task is shown in figure 10 8 8 as the 6 truck to 12 truck cost increment with the result being an exponential cost increase at this point attention will be given to seeking out optimized dumping practices new dump locations potentially on top of high cost coal or future underground areas and innovative waste movement methods conveyors being one example consideration at this point should be given to the relative cost of underground operations as an alternative means of generating raw coal for processing a 4 m thick seam at around 70 m depth will be exposed by dragline for approximately the same operating cost per metric ton as a longwall operation and for around the same mining capital investment the choice of approach around this approximate opencut stripping ratio of 12 1 prime to raw coal 9 1 for a pure truck and shovel operation will be governed by practical considerations opencut mining is inherently lower risk and has a wider envelope of application given that underground operations can be severely impacted or even excluded by factors such as geological faulting intrusions weak immediate roof seam gas seam water and so on other operational aspects three other operational aspects are worth discussing in respect to their unique impact on strip mining these are the storage or disposal of washery rejects and tailings river and creek diversions and water management rejects and tailings storage or disposal typical product yields range from 50 to 90 by mass for all material fed into the coal washery coal washery by products the remaining 10 to 50 of the mined coal mass include coarse re

ject and wet tailings with the bulk by volume being the coarse reject material coarse reject is often stored in close proximity to the wash plant either in a stockpile on a natural surface or if available a completed pit void and it is placed there either by conveyors or trucks tailings are either placed in a conventional wet tailings facility constructed as a dam or codisposed with reject in either a dam or completed pit void wet tailings facilities have large footprints are high in capital to construct are expensive to rehabilitate and carry significant environmental risks both during their operation and postrehabilitation for this reason a completed pit can often be seen as an ideal storage and disposal solution deposit geometries however generally do not offer up completed pits early in the mine life as a strip mine will work the entire strike on a cost equivalence basis for the entire mine life little economic delineation exists between pits that are often arbitrary subdivisions of what is usually a single deposit broken up by operational needs such as coal access ramps or creek corridors more commonly available are final ramp voids left behind as the deep operation has moved to fill the mouth of a ramp as a short haul dump option unfortunately a competing alternative use for any final voids either ramps or completed pits is a potential low cost dump and given the cost ratio between horizontal and vertical haul it is surprising how far away a truck system can reach economically when its alternative is to elevate overburden for this reason the best economic use of voids may not be to fill them with wet tailings especially when water itself for many mines is becoming a scarce and valuable commodity in these cases a better alternative may be a partially or fully dewatered tailings paste or cake mixed with reject and disposed directly into the dump mass where it is contained and the long term liability is minimized no burial of washery wastes into the reclaimed overburden should occur without a thorough analysis of the wastes potential for harm to the restored groundwater regime this may be among other things an acid base accounting or a column test for long term solubilities river and creek diversions most strip mines are tens of kilometers long with few reasons to break a continuous strip into separate pits other than arbitrary subdivisions based on the needs of the mining operation infrastructure corridors for road rail power or water pipelines and so on it is highly likely that in this long strike there will be a number of water courses traversing planned mining areas it then becomes an engineering and strategic analysis to weigh up the positive value of the coal that lies below a creek or river and the dump space created by removing a valley through the future maximum dump envelope against the negative value from the initial capital cost of a diversion the ongoing operating cost to maintain the di

version a final capital cost to reinstate the water course in or near its original course if applicable and an estimate of the risk of exposure to a trailing liability to maintain the diversion into perpetuity the nature of a strip mine means that it is likely that any permanent diversion will be quite long whereas a temporary diversion can just shift the problem to another pit with the commensurate risk of reestablishing a water course through what is now effectively a postmining landform that is more susceptible to damage from water erosion water management because of their long strike and progressive development over time mature strip mines can carry large internal water catchments environmental licensing conditions relating mainly to salt levels and turbidity can often mean that poor quality mine water is not easily discharged off site for this reason mine water is stored and either evaporated or partially consumed in the water based coal processing operation the large volumes that can accumulate over time or from single intense rain events often mean that a sacrificial pit is selected as the only economically viable water storage solution the pit may be used for a number of years until it becomes the most attractive pit remaining on site at which point another pit is selected for water storage and the first pit is pumped out after the water is removed there is often a thick layer of mud in the bottom of the previously dormant pit that also needs to be removed mud removal is a slow and expensive process and if not fully completed before stripping is recommenced in the affected pit can lead to ongoing spoil stability issues massive spoil failure in a deep pit is prohibitively expensive to remediate it is conceivable that for a very deep pit subjected to a massive failure it would not be economical to recover the pit because of the large capital cost involved thus significant resource sterilization could occur if extraction by underground means is not deemed viable alternative strip mining strategies the four most common strip mining strategies will be examined as follows 1 cast doze excavate operation 2 single seam dragline operation with prestrip 3 multiple seam dragline operation with prestrip 4 truck and shovel operation a strip mining strategy may be typified by a unique combination of pit configuration equipment selection and operating methodology with the combination essentially a response to the deposit being worked for this reason a methodical approach making use of the following discussion framework will be applied to examine each of the four major strip mining strategies deployed in mines today a brief generic discussion of the mining strategy a specific mine case study used to examine pit configuration mine location and layout and the deposit s geological orientation equipment selection the physical and volumetric scale of extraction and operating methodology an operational description

in terms of overburden movement and coal mining reference made to variants of note as applied at other mining operations other variants within the presented strip mining strategies are deployed where the specific circumstances warrant it the combination of final and interseam waste removal and coal mining in thin seam mines may be achieved by the application of continuous surface miners or graders steepdip mining as in colombian thermal coal mines or the united states anthracite district gives rise to operational variations of the truck and shovel strategy whereas mountain top removal in the united states appalachia region is unique and can be considered to be a multiseam modified area mining method cast doze excavate operation cast doze excavate refers to a strip mining technique whereby the overburden is cast blast followed by significant removal by large bulldozers and the remainder of the overburden is removed using an excavator and trucks or sometimes by draglines this method is most applicable to relatively shallow deposits in these operations the overburden blast is designed to not only fragment the overburden to enable removal but also for the blast to significantly move the overburden into the adjacent open void reducing the amount to be removed with mining equipment this style of overburden blasting is referred to as cast blasting because of the increased blasting vibrations and noise the method should not be used near dwellings and residential districts without first determining potential dust noise and vibration levels bulldozers are more productive and hence more economically attractive for combinations of relatively short push distances and fairly flat grades where bulldozers can be used in this manner they are usually lower total cost than truck and loader systems in an ideal situation the bulldozers are used to remove the overburden until the push distance and grade combination is no longer lower cost than the next alternative either truck and loader systems or draglines in some situations cast doze is used to remove all of the overburden material case study groote eylandt mine groote eylandt is located off the coast of east arnhem land in the gulf of carpentaria about 640 km from darwin australia the mining leases are located on the western plains and cover 84 5 km2 or 3 74 of the island s area pit configuration geological orientation the groote eylandt ore body occurs as a subhorizontal sedimentary layer gently undulating and dipping to the west it is a continuous horizon ranging in thickness from a few centimeters to 9 m consisting of hard high grade cemented pisolitic and massive manganese oxides the overburden ranges in thickness from 0 5 to 35 m and consists of lateritic overlying clays and gravels the ore body is vertically zoned and can be mined as two distinct layers 1 a middle mining horizon consisting of massive mangite cemented mangan pisolite or loose mangan piso

lite 2 a bottom mining horizon consisting of siliceous mangite the beneficiation plant is located in the middle of the mining leases with 90 of the resource within 8 km of the processing plant equipment selection physical and volumetric scale of extraction the mine typically removes 18 mbm3 of overburden each year comprising 0 4 mbm3 of topsoil 17 mbm3 of dozer production and 0 6 mbm3 truck and excavator operation this total overburden removal exposes 6 9 million t of run of mine rom ore to be delivered to the primary crushing station to yield 3 6 mt of manganese product current mining equipment is a mix of stripping dozers excavators and rear dump trucks the mining blocks or cuts are 40 m wide and 200 m long with strips arranged to follow ore body contours and to control groundwater operating methodology the groote eylandt mining company pty ltd gemco mine is a conventional shallow opencut strip mining operation involving the removal of manganese ore the mining sequence is a continuous cycle areas from which ore has been extracted are backfilled with overburden from the next strip to be mined before then being rehabilitated this method results in the mining site moving across the ore body disturbing only a small section of land surface at any given time to meet gemco s customer product quality requirements ore can be mined from a variety of pit sources the first step is to clear trees on the planned mining blocks using a bulldozer with a tree rake the topsoil is then removed and returned to prepared backfill areas or stockpiled for future rehabilitation overburden is primarily removed using a fleet of global positioning system equipped track dozers that push the material into the adjacent mined out cut strip mining technique the dozers are used to remove all overburden exposing the ore typically the dozers work in two separate teams each team working a strike length of 250 m to 500 m and a pit width of 40 m at times they are used as one fleet to provide peak capacity for short term ore requirements historically scrapers were also used to remove topsoil and overburden when the overburden was significantly thinner to reduce costs and handle a wider range of material types production dozing was introduced a small amount of overburden is removed using a truck and excavator method and this method is applied in thicker overburden areas and where digging from above with excavators is used to better handle wet conditions a rotary percussion rig is used to drill the ore horizon and blasting is conducted using emulsions for both wet and dry holes the blasted ore is removed using a hydraulic excavator the excavator is also used for overburden stripping for initial or box cuts ore is trucked from the pits to the primary crushing stockpiles in 85 t capacity rear dump trucks some direct haulage also goes to the primary crusher provided the ore type coincides with the ore treatment campaign through the plant

approximately 22 stockpiles are maintained near the primary crusher and categorized according to grade and lump tofines ratio the haulage distance from the pit to the stockpiles varies from 2 to 10 km in total there are about 30 km of haulage roads that are serviced by ancillary equipment rehabilitation of mined areas started in 1970 seeds from approximately 25 native forest species are collected from within the mine lease this seed is stored until the wet season when it is broadcast over areas that have been backfilled and covered with topsoil backfill areas are designed to maintain the general premining slopes single seam dragline operation with prestrip for single coal seams that occur deeper than 30 m and where economies of scale allow a single seam dragline method is often applied although multiple seams may be recovered for the purposes of terminology where only one seam is directly exposed by dragline operations it is called a single seam dragline operation because of their specific geometric limitations efficient dragline operations are dictated by the interaction of the particular deposit geometries and the machine s key working dimensions a dragline moves overburden by a simple repetitive cycle involving filling the bucket through a combination of pulling in the drag rope and letting out the hoist rope raising the bucket by the reverse rope movements swinging to face the spoiling area emptying the bucket by releasing the tension on the drag rope and swinging back to return the bucket to the dig face or bank strip width is typically 50 70 m wide with the strip being placed into the adjacent mining void in discrete excavation blocks of 25 30 m in length depending on the dragline operating level a number of different dragline methods are available to place the overburden in the previous void selection of method is driven by a number of factors deposit geology the number of seams to be uncovered by the dragline and the overburden and interburden thicknesses geotechnical stability certain materials in the strata may be either unsuitable to place the machine on or unsuitable to place in certain locations within the spoil spoil fit the machine will need to sit low enough to reach the top of the coal but it will also need to sit high enough to ensure it can place the spoil into its final position productivity will be maximized with lower average swing angles but swing angle may be less of an issue if there are a large number of hoist dependent swings where the swing speed is less than maximum due to the machine waiting on the bucket to be hoisted to height scheduling at times it may be necessary to adopt a dragline method that may be less productive overall due to increased relocation times but that releases coal sooner and more consistently dragline methods are analyzed in two and threedimensional 2 d and 3 d volumetric models as well as simulations two dimensional analyses called range diagrams are

used to check for theoretical spoil fit and calculate sectional rehandles figure 10 8 9 shows an example of a range diagram sequence the section is perpendicular to the strip and shows where material is dug from a dumped to a and the operational sequence steps and fill type where topography is variable or the influence of certain design features are such that a 2 d analysis is unsuitable a 3 d check for spoil fit and rehandle rates can be undertaken in addition to the simple dig to dump balance more sophisticated software models can be used to simulate the dragline machine itself to yield theoretical production rates in cubic meters per hour based on a dynamic analysis to determine the average swing angle and relative frequency of hoist and swing dependent cycles a brief overview of the main dragline methods are given in the following paragraphs a completed range diagram is shown in figure 10 8 10 in the side cast method the overburden is dug from in front of the machine to expose coal and dumped directly in the void from the previous strip figure 10 8 10a it is most often used for shallow overburden thicknesses where the dip is relatively flat and there is no requirement for selective placement of overburden in the spoil it is also used for multiseam operations for exposing the top coal seam as there is a large void to spoil the overburden into the key bridge method figure 10 8 10b is suitable for deeper deposits up to the dragline s ideal operating envelope sequentially it involves excavating a block of overburden 30 m long to create the new highwall the key and using this overburden to build a bench out into the previous strip s void the dragline then progressively repositions across the bench excavating overburden and exposing coal as it goes the block after it has uncovered the full strip width of coal it repositions diagonally down the strip into position to commence digging the next key the method has higher rehandle but is productive because of the short swings the previous strip floor can be cleaned up prior to overburden being placed in it the highwall is dug in line giving a safe wall the coal edge is also excavated in line and it is a simple method to execute a variant of this called the extended key with inpit bench method figure 10 8 10c involves the key being excavated for 10 or 20 blocks and an in pit bench built on the low wall side at the end of the extended key the dragline then repositions to the in pit bench and pulls the remaining blocks for the length of the extended key exposing coal an advantage of this method is lower rehandle than the key bridge as the in pit bench does not have to be filled in completely from the highwall to the low wall side no bridge required in the chop cut with in pit bench method figure10 8 10d use is made of throw or cast blasting to move as much of the prime material across to the low wall as possible the dragline excavates the key are

a along the strip by digging perpendicular to the new highwall and creates an in pit bench toward the low wall side of the strip the dragline then retreats along the in pit bench and excavates the block that it was previously sitting on and places it in final position the method has low rehandle and high prime productivity but is a more complicated two pass operation that gives poor control of highwall the double in pit bench method figure 10 8 10e is used when the depth of material is such that the dragline operating envelope is not sufficient to place all of the overburden into a final position from a single working level for this reason it may also be known as an elevated bench method an extended key or chop cut with in pit bench method is used but instead of the bench being fully excavated to final position a partial excavation or trim is undertaken and the overburden placed to create another bench at a higher working level when a suitable length of upper bench is created the dragline moves onto the upper bench and excavates the remaining waste to expose the full strip width of coal the higher second bench increases the volumetric capacity of the dragline and so allows the exposure of deeper coal walking delays increase overall productivity is lower the method is more complex and coal exposure is more intermittent although not a specific method the pullback technique figure 10 8 10f involves walking the dragline up into the spoil to pull existing overburden back prior to placing overburden from the current strip it is time consuming to prepare access for the dragline and the pullback material does not expose coal so it is all rehandle but productivity can be reasonable depending on the volume to be excavated it is rarely used as part of an ongoing operation because of its high cost but it can be used in areas where spoil room is particularly tight such as around ramps or deep boxcuts or to recover from design errors or geotechnical failures operationally this method can be delivered by two separate draglines one on spoil in the pullback mode and one on the bench to be mined as the seams progress to greater depths typically at around 85 m a truck and loader prestrip operation must be introduced to assist the dragline to continue to reach the top of the coal by creating a working level for the dragline that is lower than the natural topography prestrip may have been introduced sooner than this however depending on the deposit and dragline geometry and general economics prestrip may be directed to relieve the dragline in tight spoiling areas such as when crossing ramps or endwalls or where the natural topography is variable or prestrip may be applied more generally across the whole strip case study norwich park mine the norwich park hard coking coal opencut mine is located on the western fringe of the bowen basin coalfield in central queensland australia the economic seams are contained in the mid to la

te permian german creek formation which are overlain by up to 55 m of poorly consolidated cemented sediments consisting predominantly of sand and clay with irregular gravel beds and weathered basalt flows the depth of weathering varies from 15 to 25 m north of leichhardt pit to 25 50 m in the leichhardt pit and south four coal seam groups are present over the mining area with the dip of the sequence to the east and varying from 2 to 10 in some areas equipment selection physical and volumetric scale of extraction initial mining operations commenced on the subcrop of the dysart seam s in 1979 to uncover coal in strips oriented along the strike of seams mining has progressed along the strike and downdip over the intervening period surface features are generally flat with a few ephemeral creek systems running across the deposit the mining area has been divided into a number of pits with these features incorporated into the layout design the mine processing plant and other facilities are located on the western side of the deposit in a generally southern location currently the mine employs a fleet of major mining equipment consisting of six electric walking draglines supported by excavators and rear dump trucks for waste removal and front end loaders and bottom dump trucks for coal mining and haulage the coal haulers haul rom coal to the crushing and processing plant along a haul road network total annual product metric tons are typically 6 mt from a plant feed of 8 5 mt running at an average yield of approximately 71 total annual prime overburden moved is typically 100 mbm3 giving a prime to product strip ratio of approximately 17 1 allocation of this prime is mainly to dragline 60 mbm3 yr followed by prestripping 27 mbm3 yr and the remainder to production dozing and minor waste operations operating methodology figure 10 8 11 provides a schematic of the mining process representing the typical dragline and truck and shovel activities truck and shovel stripping is used to complement and expedite dragline productivity with waste hauled around the dragline operation and dumped on top of dragline spoil typically the truck dump is two spoil peaks behind the current dragline strip single seam dragline methods are used in the price and leichhardt pits where successive seams are uncovered using trucks and excavators as well as areas of campbell and roper where the upper seams are coked or weathered the two methods used in the single seam areas can be broadly categorized as the jensen and curtis off line key bridge method conventional or extended key and the extended key and elevated bench method the conventional jensen and curtis method progressively uncovers the full seam width whereas the extended key does not the extended key method is used where the height of the elevated bench above the in pit bench is too great to use the conventional jensen and curtis method this might be the result of an area that has a very low i

n pit bench because of the blasting and dozing techniques or an area where the elevated bench has to be built high to enable the dragline to exit the pit the latter is often used from the endwall back and is known as a reverse key little difference exists in the planning requirements in regard to range diagram analysis for spoil fit for these two methods the main difference relates to the timing of the coal exposure the conventional jensen and curtis method has continuous coal exposure whereas the extended key method has sporadic coal exposure due to the need to return to the front of the strip to pull blocks to expose the full seam width jensen and curtis off line key bridge method leichhardt pit example after blasting and any dragline prestrip a dragline access road is formed to the ramp mouth if time permits dozers will be used to bulk push waste across the pit and lower the in pit bench level the dragline will then be walked to the ramp mouth and the dozer key will be set up to break away from the ramp mouth this initial waste is spoiled behind the dragline to form the in pit or chop bench the dragline will then be positioned on the chop line to start working away from the ramp the dragline digs key material and dumps it into the in pit bench the dozer will be cleaning coal and pushing up spoil on the low wall side at this stage the dragline will widen or trim the key and throw the material to final spoil the dozer will continue preparing the in pit bench the dragline works off the in pit bench and throws the remaining low wall side material to final spoil at this stage the dozer will be working against the highwall cutting down the key material and cleaning the highwall for the rest of the strip dozer push will be used to excavate the highwall portion of the key and expose down to the top of the coal the progress of the method in sectional view is identical to the example range diagram sequence provided in figure 10 8 12 operationally the method makes extensive use of dozer operations to cost effectively supplement the dragline operation extended key and elevated bench method roper pit example using this technique the floor of the previous strip is cleaned before blasting any mud and water is dammed up on the low wall side the area is not presplit before blasting the overburden material is cast blast onto the pit floor with the blastholes drilled at 75 after blasting the dragline takes a highwall trim from the surface figure 10 8 12a down to 20 m below the surface and forms a safety bench the dragline or dozers then prepare an access into the pit and will prepare the chop bench the dragline will walk into the pit and commence digging off line extended keys from the ramp mouth figure 10 8 12b the spoil will be used to build an elevated bench when coal needs to be exposed to suit coal mining requirements the dragline will form a ramp up onto the elevated bench and will walk back to the front of t

he strip to pull blocks exposing the full width of the dysart seam figure 10 8 12c additional low wall trim will be taken to ensure a stable lowwall angle and a defined low wall edge offset truck and shovel prestrip prestrip is usually conducted by contract planned prestrip is the difference between the waste removal requirement to uncover the annual coal production and the installed dragline capacity shovel truck priorities include geotechnical recommendations spoil relief and poststrip activities free dig prestrip areas comprise horizons that cannot be drilled because blastholes will not stay open i e they are too sandy variants at other mining operations a number of possible variants of single seam dragline operations exist track dozers may be deployed to assist the dragline operation and speed up coal exposure rates as at the nearby gregory mine this mine has also used excavator and truck fleets to remove the dragline key material to accelerate coal exposure the advantage is that this material is placed outside of the dragline operating area and does not contribute to rehandle unlike production dozing shallow mines in the united states have employed a direct cast or side cast method the operating strip width is narrower and the wasteto coal ratio is such that the dragline is able to directly place all material into the final position without building working pads and thus incurring rehandle for steeply dipping seams or highly faulted deposits the dragline may expose a false floor and leave the remaining wedge of material overlying coal to be removed by smaller truck and excavator methods multiple seam dragline operation with prestrip for multiple coal seams that occur deeper than 30 m and where economies of scale allow a multiple seam pit configuration is selected within this broad configuration a combination of a number of the single seam dragline methods is applied to uncover each successive seam strip and block geometry is similar to single seam dragline methods but the geometry dictates significant time and effort to move the dragline up and down between what can be quite different working horizons a brief overview of the two broad categories of multiple seam pit configurations namely stacked and offset follows stacked configuration in a typical stacked configuration the dragline removes the overburden in a sequential operation working from the top of the sequence to the bottom multiple dragline passes are required with one coal seam being exposed on each pass generally the first pass is a simple direct cast operation into the void from the previous strip subsequent passes will generally involve key or chop operations and extended benching to expose the coal and place the spoil into its final position the stacked configuration can result in low rehandle 25 or less and high productivity especially for narrow strips one disadvantage is that the lower pass burden blasts are buffered by upper pass

overburden and so less cast blast is achieved and fragmentation can be poorer leading to reduced digging productivity a second disadvantage is that the material from the upper pass ends up on the floor of the whole spoil pile may not be suitable to use as a spoil base and can frequently remain in the base of the pit for extended time between passes where it undergoes further degradation a third disadvantage is that if lower passes are relatively thin compared to the upper pass high rehandles can result offset configuration in a typical offset configuration the upper seam is offset at least one complete pit width from the lower seam this allows the lower pass to be blasted first followed by the upper pass which is generally cast as much as possible to lower the dragline working level the overall sequence is then very similar to an extended bench method where the extended bench covers the entire lower seam it is important that the bench level is as low as possible as the bench is built entirely from rehandle advantages include the exposure of two coal seams simultaneously which allows them to be mined at the same time enabling blending blast performance is improved for the lower pass as it is not buffered and scheduling is simplified as the dragline does not require an alternative working area while a seam is mined out and the next pass of overburden is drilled and blasted disadvantages are that overall rehandle is generally higher and can increase dramatically with increased total waste allocated to the dragline and in deeply prestripped mines the large open void requires a significant investment in working capital to establish and maintain as well as placing further stress on the dump volume balance the selection of one configuration over the other is dependent on an analysis of many factors seam dip relative geometry of the upper and lower pass overburden and seam thicknesses geotechnical characteristics of the overburden scheduling and blending considerations strip mining is a form of open pit mining that uses strip cuts to mine generally shallow flat lying deposits strip mines are typically layered deposits a classic example is surface coal mining this chapter will concentrate on considerations in the mine planning and design process of strip mines the primer for mining engineers is surface mining pfleider 1972 see kennedy 1990 for the updated second edition of that text mine planning and design there are two general approaches to mine planning the first is the development of a greenfield or new property capital investments have not been committed the property may already have been acquired or is in the process of evaluation the prefeasibility study establishes the value of the property and potential capital commitment prefeasibility may require a general mine plan to establish if mining is practical and may go as far as suggesting what equipment would be used the feasibility study then details t

he capital commitment and life of the property and includes a detailed economic evaluation to develop economics in terms of cash flow expected net present value npv and internal rate of return irr the feasibility study requires a detailed mine plan equipment selection schedules identifying the timing and amount of capital expenditures and estimates of projected revenue and costs information on mining economics can be found in stermole and stermole 1987 table 10 9 1 is a list of items to consider in a mine plan feasibility study an existing property or brownfield expansion considers an operation where equipment and facilities are already in place and future development is planned mine planning for existing operations may require a different approach to address market conditions and the need to expand scale back or maintain a steady state production level in an existing property where equipment and facilities are established the two stages of mine planning are the shortterm mine plan and the long term or life of mine plan these stages are significant in that mines often assign designated staff and instigate significant mine planning efforts to best meet the differing requirements and level of detail for these two functions see table 10 9 2 for a list of items to consider in the mine planning process short term mine planning the short term mine plan or operating plan generally relates to a period of less than 5 years and is typically broken down into stages of monthly quarterly and annual operating plans each stage reflects a level of detail designed to guide the mining toward fulfilling annual production and economic budgets short term mine plans consist of details and sequences for drilling and blasting quality control and material excavation and placement to be used by operations to direct equipment operations and meet production goals long term mine planning long term mine plans usually cover a period greater than a year beyond the current active operations up to and including the point when the life of mine plan starts typically these long term mine plans are broken down into levels of detail that include quarterly plans for up to 5 years annual plans out to a minimum of 10 years and general plans out to the life of mine planning horizon the defined planning periods can vary depending on the size and life of the reserves along with the general purpose and objectives of the operation the objectives of the long term mine plan include identifying and scheduling labor levels equipment production levels and capital requirements including major equipment purchases infrastructure expansion reserve expansion acquisition of new reserves and end of life reclamation other applications of long term planning are to determine production sensitivity to revenues and costs which would affect production rates capital projects and the life of the reserve both short and long term mine plans are decision t

ools to help manage the risk and reward opportunities that confront mine management risk and reward the successful implementation of a correctly formulated mine plan will yield the optimal recovery of the resource as measured by optimizing the resulting revenues reward and effectively controlling costs risk costs for a new property can vary substantially because of unknown conditions it is the mining engineer s task to identify and characterize key conditions and factors and to assign costs to them that realistically represent the risks involved in developing and safely mining the reserve it is also the mining engineer s function to get the most value from the reserve by using proven methods to mine and produce the resource in a timely manner determining the time value of money or the npv of a project is the most common method of measuring the economic balance between risk costs and reward revenues for a mine development project a base case mine plan is typically developed to identify and quantify the costs and revenues associated with developing the property successive planning scenarios may then be developed to evaluate the cost revenue effects of changing mine plan layout sequencing production schedules equipment or other parameters to minimize cost impacts and maximize the resource recovery and revenues to optimize npv outlining reserves mapping project mapping often begins with a general location map that shows geopolitical boundaries towns roads general topographic features the mine location and any nearby residences or structures planning starts with more detailed mapping of topography surface drainage features surface and mineral ownership existing facilities and infrastructure and potential mine area and disturbance boundaries aerial flights provide an up to date surface contour of the property and help to identify existing structures facilities and other surface features ground surveys using global positioning system equipment provide additional detail and will become the basis for the ongoing process of mapping and documenting mining and related activities infrastructure includes the location of existing features roads rail lines pipelines power lines communication lines wells buildings and structures on and adjacent to the mining property that may be used for or affected by mining activities it is important to identify not only features related to the mining activities but also features and resources that may require special environmental considerations such as streams forest land wetlands wildlife habitat and other protected or environmentally sensitive areas initial geological maps should include surficial geology outcrops surface geologic exposures structural features and indications of strike and dip of structures maps prepared by governmental research organizations such as the u s geological survey are a typical starting point for new operations conversion of gps survey

s to a geographic information system format will allow the ongoing collection and integration of ownership environmental geologic and mine and reclamation planning and operational information since modern mapping is computerized map information is developed and saved as individual layers that can then be selected and combined to produce a range of specialized maps although it is important to define and establish a common mapping datum for compatibility determining a map scale is not critical because scale can be adjusted later to meet map presentation requirements after a standard mapping datum is established all mapping should be completed and input to that datum the mapping datum and coordinate system should be compatible with the maximum area expected to be affected by mining and should accommodate both short and long term planning it is important to document the basis for the mapping information metadata and to preserve original data in a digital format with appropriate backups drill hole coverage sufficient drill hole information is critical for the development of geologic and mine planning models if a company has a history of drilling in the area then all known drilling information should be reviewed and evaluated for reliability and content information is then consolidated into a geologic computer model in a new operation or one that is expanding into a new reserve area a drilling program will be required to collect enough information to adequately evaluate the property a resource is typically defined by the level of confidence in the occurrence and extent of the mineable seams in open pit mining the terms measured indicated and inferred convey both the extent of drilling and the level of confidence in the reserve characterization in surface strip mines the deposits tend to have consistent thickness characteristics therefore drill spacing for exploration often involves a grid spacing of 0 4 to 0 8 km 0 25 to 0 50 mi areas that require more extensive drilling or seismic studies are seam boundaries structural features and surface disconformities such as valleys surface displacements indicating faulting and depositional unconformities the greater the variation in reserve characteristics the greater is the need for increased drilling density or supplemental data collection data quality can be evaluated and supplemented using geostatistics techniques which take into consideration variances in the ore body when drill data are being correlated in operating mines drilling information can be supplemented by structure and quality information obtained from drilling and blasting activities in active operating areas the engineer should become familiar with the reserve and should identify where structure thickness and quality issues may exist in planned mining areas geology to mine planning using the drill hole information and surface structural features a geological model is developed this model shoul

d include seam identification structure thickness quality and characteristics that may affect mining efficiency such as overburden interburden thickness and characteristics fracturing variations in thickness washouts and water table the engineer should be looking for inconsistencies in quality and thickness to determine the extent of mineable seams and the mining characteristics that would affect equipment selection and productivity it is also important to understand overburden interburden characteristics and structural constraints as indicators of potential stability problems information on highwall angle and spoil angle can be estimated by material type and stability characteristics thickness strike dip strip ratio and quality maps should be developed for all potentially mineable seams this can either be a fairly straightforward process or more complex and timeconsuming if seams combine and split transitioning from a geologic model to a mining model requires the understanding of reserve characteristics potential structure hazards equipment capabilities and strip mining methods geological cross sections history and knowledge of the area research on similar deposits and knowledge of mining methods are all resources that the engineer can tap into in order to optimize mine planning the basis for a good mine plan is a good understanding of the topography and geology of the property to facilitate this an accurate topographical map must be developed with a minimum 6 1 m 20 ft contour interval and a grid base must be used that has a minimum spacing of 61 m 200 ft and covers all of the projected mining area along with potential facilities and any significant surface structures in addition it is necessary to become familiar with the drilling information including seam characteristics structure and the quality information for each mineable seam referred to as target seams contour thickness maps also known as isopach maps should be generated for each of the targeted seams and should include information on overburden interburden waste material between targeted seams and the undiluted thickness of the targeted seams quality maps should also be created for each targeted seam geologic structure maps need to be developed to identify any structures e g faulting and offsets that would affect the mining method or decrease recovery and increase dilution and waste coal loss due to ribs barriers top bottom of coal seam dilution and overblasting can be as high as 10 in eastern operations anon 1977 reserve evaluation modern mine planning starts with a computer model of the resource the extent structure quantity and quality of the resource and the associated burden material should be well defined these data will allow the engineer to develop a mining model the mining model will incorporate and consolidate data on the resource into a model of mineable seams waste and soil materials sometimes referred

to as suitable plant growth material for this purpose a reserve is identified as a resource that is delineated by its economic strip ratio strip ratio the economic stripping limit is usually the first factor to be determined in establishing the mine plan the economic stripping ratio is defined as the cubic meters yards of waste material to be removed to uncover one metric ton short ton of product for illustration purposes in this chapter coal will be used as the resource developing maps that show the ratio of overburden thickness to a mineable coal thickness is a good place to begin this ratio can be converted to a strip ratio map by mapping the thickness of the overburden and interburden converted to cubic meters and divided by the thickness of the coal converted to metric tons short tons this ratio is calculated using the cumulative thickness of both overburden interburden and coal seams down to and including the lowest mineable seam the ratio becomes the economic mining limit and is the first step in establishing the economically mineable reserve the formula can become more complex as issues of quality affect the economic value of the coal it is important for the engineer to understand what the economics are and what contract restrictions including penalties and bonus provisions may affect revenues other adjustments to coal recovery and actual mineable reserves would be dilution and in seam losses which tend to downgrade the stripping ratio by increasing the cubic meters cubic yards of waste and decreasing the number of metric tons short tons of resource depending on mining conditions and on thickness and quality of the seam losses can amount to several percent and have a significant impact on the reserve resource control land and mineral ownership ownership of land and minerals is a key consideration that should be determined and addressed early in the planning process as it can affect development time frames permitting requirements development cost structure and project profitability land ownership typically consists of two components the surface estate and the mineral estate either or both may be held by private fee estate or public public estate entities in some cases the surface and mineral estates may be held separately severed estate the rights to access and utilize the surface and to develop and produce the mineral resource can be secured through direct ownership purchase of the surface and or mineral estate lease agreement or a combination of these legal vehicles in many cases the right to develop and produce the mineral resource particularly where public estate minerals are involved carries with it certain rights of surface access and use with the exception of direct ownership other access and development rights typically involve structured payments to secure and exercise these rights these typically take the form of advance payments to secure the rights flat rate annual

fees for use of the lands royalty payments based on mineral production rates fee payments keyed to specific activities or combinations of these payment mechanisms secure well defined access and development rights are an important element of a stable cost structure and are key to project cost control and profitability it should also be emphasized that ownership rights for the project area and an adjacent buffer zone can be important in minimizing and successfully addressing potential conflicts with adjacent landowners and uses other resources in wyoming united states gas collected from coal seams i e coal bed methane has become a significant consideration relative to permitting and the timing for development of coal reserves where both independently recoverable resources exist existing oil and gas or other mineral leases must be a consideration in the extent of and timing for development and production of reserves permitting mine permitting is the process of preparing and submitting relevant project information for review and approval by jurisdictional government authorities to verify project plan compliance with applicable laws and regulations in general applicable laws and regulations as they relate to mining are designed to prevent control minimize or effectively mitigate potential adverse mining related impacts on the environment and on human health and welfare mine permitting may involve submittal of individual permit applications for approval of specific mining related activities mining and reclamation air emissions water discharge facility construction and so on alternately it may involve environmental analysis and plan approvals for the project as a whole or a combination of all of these approval mechanisms may be involved an outline of typical mine permitting requirements is provided in table 10 9 3 typically the process involves review by national or state provincial agencies that have approval authority over land uses or mining and reclamation plans it may also involve review by agencies with authority over specific environmental resources such as air water wildlife and other specific project aspects normally some provision is provided in the permitting process for input by affected parties and nongovernmental organizations environmental baseline at the start of the permitting process it is important to effectively characterize environmental resources and values as they exist in the project area baseline prior to mining disturbance generally baseline characterization involves field studies by qualified professionals of all resources and values that may be affected the following resource values are typically characterized land use cultural archaeological and paleontological resources geology meteorology and air quality surface and groundwater hydrology soils vegetation and wetlands fish wildlife and related habitat values aesthetics and noise socioeconomic condi

tions baseline characterization then forms the basis for development of specific measures to prevent control minimize or mitigate the potential impacts and for evaluation of potential mining related impacts with consideration of planned control and mitigation measures mitigation plans at the point in a project at which permitting activities are initiated mining and reclamation plans are generally well defined including the locations extent and nature of surfacedisturbing activities comparison of the extent and nature of mining activities with information from environmental baseline characterization provides the basis for development of project mitigation plans in many cases required mitigation measures are defined to some extent by specific regulatory requirements a common example is the requirement for the collection and treatment of runoff from mine disturbance areas to comply with effluent standards before water is discharged to natural drainages to address site conditions and constraints or enhance postmining land use the mine operator may have an opportunity to develop site specific mitigation plans or to modify mitigation plans within the limits of regulatory constraints impact analysis and monitoring environmental impact analysis is an integral part of the permitting process whether it is the evaluation of regulatory compliance as part of a permit review or of the significance of potential impacts in the context of a permit review impact analysis focuses on whether or not the proposed mining and related activities and planned mitigation measure meet specific regulatory requirements and performance standards for environmental factors impact analysis typically includes the evaluation of direct indirect and cumulative impacts and assesses whether or not potential impacts meet an objective or indeed the subjective measure of significance prepared and evaluated as part of the permitting process monitoring plans provide a mechanism for the direct measurement of impacts on specific environmental resources if properly designed and administered monitoring can identify significant changes in resource conditions it can be used to assess the effectiveness of mitigation measures and the accuracy of impact assessments and to modify operating and mitigation practices timing it is critical to allow sufficient time in the project schedule for project permitting it normally takes between 12 and 18 months to collect adequate information for environmental baseline characterization depending on project complexity permit preparation may require between 6 and 24 months required agency reviews and approval may extend the overall permitting schedule by another 6 to 24 months important factors in minimizing permitting time frames include ensuring adequate and timely baseline characterization conducting effective ongoing communication with stakeholders and coordinating closely with jurisdictional agencies infrastructure existi

ng infrastructure existing infrastructure i e roads water wells gas pipeline or power lines that run through the property must be identified and mapped and the landowners must be contacted and told the intent and potential impacts of mining where existing infrastructure may be affected by mining mitigation action may be necessary either in the form of compensation or relocation replacement of the structures mine infrastructure the necessary infrastructure to support mining and related operations will need to be planned developed and accounted for in the economic evaluation process this infrastructure will include roads and utilities office and changehouse facilities warehouse and maintenance facilities material handling processing and product transportation facilities drainage and sediment control systems and so forth infrastructure mapping maps showing both existing and planned mine infrastructure are typically developed as part of the mine planning process features that will be mined around or that are not within the mining area are not shown on the reserve map some structures require economic analysis to determine whether mining around or compensation relocation replacement is the better economic approach these areas may be included in the reserve but until the status of the areas is resolved they should be excluded in the base case mine plan as nonmineable reserves pit design mining methods for surface strip mines the choice of mining method is dictated by the terrain geology and depth of the resource the terms contour mining and area mining are used to describe mining methods that are suited to specific geologic and topographic conditions where the terrain is variable and multiple seams are present contour mining may be the best option in the appalachian mines of the united states thin seams undulating topography and sometimes steeply dipping seams require equipment that is highly adaptive and mobile and can move material relatively long distances these conditions favor contour mining using large tracked dozers rubber tired scrapers shovel truck or loader truck equipment fleets and occasionally small draglines 23 35 m3 30 45 yd3 as the economic stripping ratio increases contour mining may be coupled with follow up to auger or with conventional underground methods where the terrain is relatively uniform and the seam or seams are flat lying area mining is often the preferred approach relatively large flat lying deposits such as the coal reserves of wyoming s powder river basin prb phosphate deposits in florida united states and the middle east and the canadian tar sands are well suited to area mining methods using large draglines 46 120 m3 60 160 yd3 fleets of large scale shovel trucks shovel capacity 46 76 m3 60 100 yd3 truck capacity 140 230 m3 180 300 yd3 or even bucket wheel excavators bwes and cross pit conveyor systems using large stripping equipment and moving l

arge quantities of material keeps unit operating costs to a minimum the prb mines were typically mined at a 1 1 stripping ratio in the 1980s however by the 1990s the stripping ratio had increased to more than 2 1 as mining progresses to the west the stripping ratio increases as the coal seams dip between 1 and 2 and the overburden gets thicker this increase in strip ratio is also reflected where split seams occur coal quality also increases from north to south which can have a significant effect on the esr changes in geology must be taken into consideration in pit design and equipment selection bise 1986 gives several examples of equipment selection bucyrus erie company 1979 is a good source of information on shovel truck selection and operation large electric shovels are used in both contour and area mining they offer good flexibility reasonable mobility and moderate to high loading capacity and they can be used both for stripping overburden and for ore loading however electric shovels do require a truck fleet along with associated haulageroad requirements which can increase production costs draglines are high production machines used to strip and move overburden over short distances draglines have limited mobility and are generally not suited for loading product it can take up to a year to construct a dragline and its cost can range from 50 to 100 million nevertheless draglines are dominant in the large surface coal mines in the united states and in other large flat lying deposits throughout the world cassidy 1973 in the lignite mines of europe and the u s gulf coast large bwes and conveyor belt systems are used to excavate and move large volumes of overburden and product economically the bwe is a continuous excavation machine capable of removing up to 240 000 m3 314 000 yd3 of material per day bwes are found mostly in coal mining in europe australia and india a bwe can cost more than 100 million and take 5 years to construct a compilation of equipment application and various mining scenarios as applied at operating properties can be found in chironis 1978 and kirk 1989 support equipment including dozers scrapers loaders graders water trucks and depending on individual mine requirements a multitude of other equipment required to complete the mining cycle is a significant capital and operating cost sources for accounting for this equipment include vendor publications such as the caterpillar performance handbook caterpillar 1995 and vendor web sites cast blasting is a technique that is used in combination with dragline operations to increase overburden production capacity and reduce costs cast blasting utilizes the blast energy and gravity to cast overburden from the highwall side of the pit both into the pit and onto the spoils the portion of the overburden cast into the pit is then moved onto the spoils using the dragline effectively thereby reducing overall dragline production r

equirements and rehandling however effective control of blasting is important to prevent loss of the underlying resource particularly in multiseam pits highwall mining is a method of increasing coal recovery in a pit where the stripping ratio has reached its economic limit and surface operations are no longer cost effective two types of highwall mining are commonly used auger mining in which a large diameter auger bores parallel holes into the exposed coal seam and conventional mining in which a remote controlled continuous miner coupled with an extensible conveyor extends parallel entries into the exposed coal seam for highwall mining to be effective the key criterion is that the coal and surrounding rock must be competent enough to be self supporting when a portion of the coal seam is removed geotechnical calculations determine the size and allowable depth of the auger or miner entries recoveries above 50 are possible with augers as large as 2 4 m 8 ft in diameter and conventional methods can result in slightly higher recoveries experience has shown that highwall mining is not well adapted to steeply dipping seams pit geometry after the boundary of the mineable reserves is established the geometry and structure of the reserve dictate pit orientation and configuration when the pit orientation is laid out the mapped dip and strike of the seams and any geological structures such as faulting or discontinuities should be taken into consideration geotechnical evaluations provide additional valuable information for determining highwall and spoil slope angles slope angles for waste and soil stockpiles pit end and inter bench slope angles and the overall highwall angle for multiple seam operations for both planning and operations tracking reporting a standard naming convention should be established for designating and referring to specific mining areas pits within each mining area and cut sequencing within each pit this naming convention should designate and explain mining units to be used in the scheduling process the width and length of the pits are typically constrained by the pit geometry physical limitations of the equipment e g dragline reach and the targeted production rate required pits are broken down into mining cuts based on both production scheduling considerations and the accuracy required for reporting volumes short term planning often requires smaller cuts depending on drill spacing and quality variations but it provides increased accuracy relative to production volumes and quality long term planning does not require the same accuracy so larger cuts or mining blocks may be acceptable thereby reducing data requirements and the amount of time required to input and analyze the data for mine scheduling points to consider when developing the design of mine pits include the following orientation of the pit with respect to the strike and dip of the seam and site topography pits with inside curve

s result in insufficient spoil room and additional rehandle outside curves create additional spoil room and provide for spoil side access to the coal seam similarly mining updip creates more spoil room and reduces the potential for spoil failure if the terrain is relatively flat straight pits usually give better control over the pit and tend to result in better productivity prominent structural features such as faulting along with intersecting drainages and groundwater dewatering may be required which would have an impact on highwall stability length and width of the pit generally a maximum pit length of 1 6 km 1 mi is a good rule of thumb where ramp access to the pit floor is sufficient most ramps are spoil side but there are occasions when highwall ramps and drilling and blasting ramps are required minimum pit width is driven by the depth of the pit and equipment clearances needed in the pit bottom there generally needs to be sufficient width in the bottom of the pit to operate coal loading equipment with truck and drill access keeping the pit width to a minimum is necessary to minimize rehandling haul access this consideration of pit design should provide sufficient room for drilling blasting and coal extraction intermediate spoil ramps usually at least one per pit are required to access the pit bottom from either side of the main haulage ramp pit end ramps are also a consideration so that coal extraction drilling and blasting are not cut off by overburden operations haulage access design becomes more complex for multiple seam operations highwall and pit end ramps these are used when spoil material is unstable the disadvantage of highwall ramps is that the ramp must be reestablished after each cut and the main haul road requires relocation when mining advance encroaches on the existing haul road rehandle rehandle is defined as material that is moved more than once to uncover the same amount of resource a normal amount of rehandle for a dragline is 10 some circumstances require increased rehandle such as when overburden depth is increasing above the average design thickness for the equipment being used in such cases rehandling material may be more cost effective than purchasing additional equipment or using equipment that is more costly on a unit production basis when rehandle costs and volumes for major production equipment increase beyond normal limits it reduces the availability and effective productivity of the equipment for stripping utilizing shovel truck prestripping on a dragline pit may decrease rehandle significantly resulting in additional stripping capacity managing rehandle becomes a trade off between production capacity and lowest cost utilization of the equipment unnecessary rehandle can occur when stockpiles are placed too close to the active pit or when production scheduling requires rehandle of stockpiled material spoil and highwall failures which can also result in un

necessary rehandle may result from poor maintenance of the pit bottom stacking spoils too high or highwall spoil angles that exceed design parameters an elevated water table water in the pit and poor surface water drainage all can greatly influence highwall and spoil stability unplanned rehandle is an unnecessary cost that can have a direct impact on production cost mining sequence for a new property the mine planning process comprises setting a target production selecting the equipment necessary to meet that production rate and running a projected production schedule to determine the economics of the plan production sensitivities are then run to determine if the plan can be improved by changing the production level equipment production sequence or other preselected parameters it is not uncommon to run several iterations to generate an economically optimized mine plan for an operating mine production targets and equipment fleet may be preset in such situations meeting the target production rate becomes the focus of planning runs to best fit adjustments to pit configuration and sequencing to the equipment for a given production rate in this case planning options may include supplementing replacing or idling existing equipment to meet the required production rate the mining sequence is considered in the pit design phase a typical example of mine sequencing is a dragline operation which is initiated with a box cut in a dragline operation the box cut is typically the initial excavation which creates sufficient spoil room for the first dragline pit figure 10 9 1 illustrates three box cut scenarios a typical mining sequence would involve topsoil removal overburden removal coal removal backfilling of the resulting pit with overburden spoils and grading of the spoils to the final reclamation configuration followed by reclamation grading replacement of soil materials and revegetation for most regulated operations unless there is a variance in the permit soil materials and in some cases underlying weathered materials are stripped and stockpiled for future replacement on regraded spoil materials in some cases these materials can be directly replaced on existing regraded spoil areas avoiding the need for stockpiling and often resulting in better reclamation figures 10 9 2a g illustrate the steps in a typical dragline sequence figure 10 9 2a shows the pit as it would look at the end of the previous cycle the coal is extracted and the pit is ready for the next cut which is shown by the vertical dashed line figure 10 9 2b and figure 10 9 2c illustrate the results of cast blasting on both the upper and lower seams figure 10 9 2d shows the upper bench being graded in preparation for the dragline bench figure 10 9 2e illustrates the several phases of stripping that a dragline would perform the first is extending the bench to allow the dragline to walk out far enough to cast the next cut of spoil to build a sp

oil side bench for the dragline to sit on in figure 10 9 2f the dragline is moved to the spoil side where the bench has been graded to an elevation that allows the dragline to uncover the coal seam finally figure 10 9 2g shows the section immediately before removing the coal which is the last step in the cycle when the coal has been extracted the cycle is repeated the purpose of generating range diagrams is to determine the amount of material moved by the production equipment in each sequential step these volumes are then used to determine the productivity of the equipment for that specific pit geometry overburden thickness and changes in the coal seam geometry can affect the production levels and a different configuration may be required the trade off with the cast blasting technique is the loss of coal and dilution due to fracturing of the coal seam at the exposed highwall of the pit other configurations that could be compared to cast blasting would be a pre bench shovel truck or scraper fleet depending on the depth of the material above the first seam computer programs that simulate dragline scenarios are used to generate the equipment volumes needed for scheduling by adjusting the equipment configuration for different geometries a database of scheduling volumes can be generated by equipment type with rehandle quantities and expected coal production volumes being taken into account each schedule is run through an economic model and scenarios are compared to evaluate the impacts of each on npv this process is used in both proposed and ongoing mine projects to provide justification for equipment purchases and modifications to the mine plan specialized blasting techniques for strip open cast mining overburden must often be fragmented by blasting so it can be efficiently and economically excavated for many years mining engineers have considered cast blasting explosives casting of overburden both to take advantage of the explosive energy used to fragment the overburden and to reduce material excavation and handling costs brealey and atkinson 1968 mcdonald et al 1982 the low cost and high gas pressures of ammonium nitrate and fuel oil anfo explosives make cast blasting more attractive as a production tool the advantages of cast blasting become increasingly significant for thick overburden with resistant layers requiring high powder factors kilograms of explosives per cubic meter pounds per cubic yard of overburden blasted 0 65 kg m3 1 1 lb yd3 is typical for some very strong sandstones for example in south africa and australia applications have shown that in certain circumstances cast blasting in deep overburden can be more economic than conventional stripping this method which is based on reducing the primary overburden cast by the dragline figure 10 9 3 has the added advantage of promoting gravity segregation of the cast material large rock fragments come to rest near the pit floor to form the

base of the spoils thereby improving spoil drainage and stability figure 10 9 4 illustrates a typical blast cast profile presplit blasting may be used in conjunction with cast blasting to dewater permeable overburden additionally in strong ground a vertical face can be created in conjunction with cast blasting rather than the irregular sloping face produced by conventional blasting figure 10 9 5 it is obvious from figure 10 9 5b that the vertical face with greatly reduced distance from the front row of blastholes to the toe of the highwall will result in far more efficient cast blasting nonvertical highwalls with an inclined presplit line can be incorporated into the blast design along with modification of the blasthole pattern where clay rich and similar materials are absent lowcost bulk placed anfo has been used in place of more expensive water resistant explosives e g water gels heavy anfo the geometry of the spoil after cast blasting must be controlled to provide a section suitable for dragline operations while moving as much spoil as possible across the pit conventional blasting patterns in strip mining are usually square or rectangular rows of holes perpendicular to the highwall are detonated sequentially so that the spoil is thrown parallel to the highwall toward the last open cut in cast blasting rows of blastholes parallel to the highwall are detonated sequentially initiated from the highwall progressively from front to back resulting in maximum spoil cast across the pit experience shows that marginally more spoil is cast using this design but a section of overburden between the presplit line and the last row of blastholes may remain in place figure 10 9 6 the best results appear to occur with about a 30 offset using a staggered v1 pattern with relatively long inter row delays figure 10 9 7 illustrates a typical section and the desired section the throw depression can be greater than desirable and some minor rehandle of the thrown spoil is necessary to establish a bench for dragline operations the spacing burden ratio of the blasting pattern should be determined using site specific factors and requires detailed consideration to control the trajectory of the thrown spoil a means of evaluating the feasibility and effectiveness of cast blasting is the use of the depth width d w ratio this is a ratio varying from 0 4 to 0 9 where d is the pit depth from the highwall crest to the bottom of the lowermost seam and w is the width of the exposed pit bottom toe of highwall to toe of spoils blasthole diameters of 254 to 311 mm 10 to 12 5 in are typical it is doubtful whether larger diameter holes will provide additional advantages in strong ground unless explosive density energy can be increased in the blasthole closer spacing rather than larger blasthole diameters may be preferable cast blasting walton and atkinson 1978 can facilitate spoil side dragline operations which offer several operat

ing advantages access ramp roads can be located on the highwall side of the pit eliminating problems caused by lack of spoil room at the junction of the active pit with the access ramps the stripping sequence is uninterrupted since access ramps do not have to be reestablished on the spoil side resulting in increased dragline productivity equipment scheduling is simplified access to individual seams is simplified in multiseam operations surface reclamation is simplified since spoil grading does not have to address reclamation of access ramps disadvantages of spoil side operations are limited but include the following the dragline may operate in the less efficient chop down mode separate power distribution systems are required on both sides of the pit on the spoil side for the dragline and on the highwall side for blasthole drills single seam cross pit chop down operation a dragline used in the chop down mode is about 60 efficient compared with the conventional drag mode bucket maintenance costs are also higher although this is considerably alleviated by the better fragmentation achieved in cast blasting the depth width ratio of the pit should exceed 0 4 for cast blasting to be considered i e this method is best suited to deeper pits figure 10 9 8 shows the method of operation the dragline bench height in figure 10 9 8c can be fixed so that the spoil crests are essentially level virtually eliminating the need to grade the spoil peaks during reclamation this advantage can only be fully realized however if the height of the dragline bench above the top of the seam does not exceed the optimum digging depth where this height exceeds the optimum digging depth the bucket must be cast and dragged prior to the swing cycle thereby increasing cycle times in these circumstances it is usually more economic to level the spoil peaks with either the dragline rehandle or conventional mobile equipment the single seam chop down method may be used to strip seam partings in multiseam operations this operation results in a reduction of dragline productivity of up to 50 but if the ratio of parting to lower seam thickness is low this approach can be an economical option two seam method the steps utilizing cast blasting are illustrated in the following sequence refer to figure 10 9 2a c and as described previously in the mining sequence section 1 the pit section prior to blasting figure 10 9 2a 2 interburden blasted into the void left after extracting the mineral from the previous strip large rocks will form the base of the spoil heap and are not handled by the dragline figure 10 9 2b 3 blasted overburden the throw depression of the interburden blast is filled by overburden spoil figure 10 9 2c this method results in an increase in dragline productivity above that of conventional dragline stripping but insufficient experience is available in comparable conditions to quantify this increase there

are however other marked advantages both seams are exposed allowing simultaneous seam recovery the upper seam can be dozed over the side of the bench or by other suitable means onto the lower seam thereby concentrating and simplifying mineral loading operations and improving blending the volume of in pit exposed reserves is increased allowing greater flexibility in operation spoil stability is enhanced by gravity segregation of the cast blast materials with larger durable rock forming a base for the spoils with both seams exposed simultaneously additional dragline moves and dead heading can be eliminated or reduced reduced need to handle durable rock materials with the dragline can result in reduced bucket maintenance costs this method can also be used for single seam thick overburden applications to eliminate chop down operations for upper overburden benches where weathered material exists near surface the upper bench can be presplit with more closely spaced holes than for the lower more competent overburden provided that weathering is not too deep the upper bench slope can be increased up to a maximum of 90 vertical since presplitting will limit blasting effects and reduce drainage concerns stripping previously worked deposits in some situations thick coal seams that have been previously partially extracted by underground methods e g room andpillar mining can be economically recovered by strip mining in these situations stability and the safe operation of the dragline can be in question where conventional dragline stripping is proposed cast blasting provides an alternative stripping method for suspect areas where the dragline is not located over the pillared coal thereby minimizing stability and dragline safety concerns equipment selection the following criteria are among those that should be considered when selecting equipment for a new or expanding operation the life of the project certain capital and operating cost considerations should be evaluated smaller equipment e g scrapers has lower capital costs but much higher operating costs depending on the haul distance and the unit volume moved in comparison draglines have much higher capital costs and much lower unit operating costs for example a scraper may have a capital purchase cost of 2 million with a recurring replacement cost every 5 years a unit capacity of 23 m3 30 yd3 a life of 35 000 hours and an operating cost of 0 98 m3 0 75 yd3 varying with the haulage cycle a 115 m3 150 yd3 dragline might have a capital purchase price of 50 million an operating cost of 0 20 m3 0 15 yd3 and a life of 150 000 hours it typically will last for the life of the project which could be 30 years with a single major rebuild costing 10 million depth and thickness of the seam s overburden and coal with total depth to last mineable seam typically pits up to 150 m 500 ft in depth can be surface mined strike and di

p of the property is the deposit type more suited to area mining or contour mining comparison with similar operations consider similar operations but do not restrict the decision by existing convention in the 1980s there were only truck shovel operations in the prb the first dragline was introduced in 1982 and since then draglines have dominated united states surface mines in wyoming montana colorado texas and north dakota combinations of mining methods equipment type and size for draglines evaluate the equipment type and size by generating range diagrams from cross sections across the length of the reserve figure 10 9 3 illustrates a typical range diagram and the volume distribution choose typical cross sections that represent variations in the reserve for shovel truck operations bench height and passes per cycle help to determine equipment matching three to five bucket passes by a shovel to fill a truck is considered a good match it will be important to select equipment that can handle changes in pit geometry dragline capacity multiple seam operations require handling of intermittent coal and interburden which will impact the productivity of a dragline and require loader truck equipment to handle the smaller seams dragline capacity must take into account such production interruptions haulage scenarios evaluate haulage scenarios for truckshovel and scraper operations cast blasting look at the possibility of augmenting production with cast blasting used draglines evaluate the market for used draglines that are currently idle finding a dragline that can be relocated for half the cost of a new dragline may fit the mine plan needs all options should be evaluated for the highest npv and irr ground control and pit slope stability effective ground control and planning for pit slope stability comprise a two step process first geological structures and groundwater conditions that may affect stability need to be identified and characterized the planning process then focuses on the proper layout and design of the pit geometry and mine structures to address site specific structural and groundwater conditions in addition to adequate exploration drilling sampling and logging to characterize geology lithology general structure water levels and reserve characteristics seismic studies can prove invaluable in delineating structural features and characteristics groundwater studies can also provide supplemental information on groundwater occurrence and flows proper pit layout minimizes exposure of structural unconformities in the pit highwall provides for effective drainage of both surface water and groundwater and avoids geometries that tend to adversely concentrate ground forces i e noses or notches similar considerations are appropriate for mine structures including mine spoils overburden and waste piles tailings facilities water impoundments and mine buildings for pit slopes and spoil p

iles as well as engineered structures such as tailings facilities and water impoundments geotechnical analysis by qualified professionals is appropriate these analyses assess both static and dynamic seismic stability and are often required by applicable regulations in the case of engineered structures foundation testing is often a component of the geotechnical analysis and specific recommendations for foundation preparation and drainage measures are incorporated into project construction plans mine safety as a critical element of modern mining operations safety must be a key consideration in planning all aspects of mine operations layout and design of pit geometry and mine structures and facilities road design equipment selection and operations planning and scheduling applicable regulatory requirements often factor in safety as well as environmental considerations however effective protection of worker safety and health requires thought and effort well beyond the scope of regulations for this reason the planning phase should address both engineered safety considerations i e pit design and equipment safety features and safety systems i e worker training safety awareness equipment inspection and maintenance and health monitoring land rehabilitation on completion of active mining operations whether for an individual mining area or for the mine as a whole mine disturbance areas should be returned to a stable condition the potential for any short or long term adverse environmental effects or hazards to human health and safety should be minimized and productive postmining land use s should be supported generally this involves backfilling and grading mine pit areas to blend with the surrounding terrain and promote effective drainage replacing stockpiled soil materials and reseeding with compatible native vegetation species as part of the planning process it is sometimes possible to enhance the natural conditions and land uses that existed prior to mining or to make land modifications that may facilitate higher and better land uses effective land rehabilitation can minimize the potential for any short or long term liabilities and maintain or increase the value of mined lands land rehabilitation is important in promoting a positive perception of the operator and the mining industry as a whole as being environmentally aware and responsible the concept of highwall mining refers to recovering coal by boring openings beyond the highwall limits produced by strip mining after the economic limit is reached due to increasingly high strip ratios in general the term encompasses traditional auger mining as well as the more recent advances using a relatively new class of equipment under the heading of highwall miners auger mining is in essence the practice of using a large drill turned horizontally to bore into the coal seam and is generally limited to thicknesses of 0 6 to 4 8 m 2 to 16 ft it began in west v

irginia united states in the mid 1940s but the machines at that time were limited in the penetration they could achieve advancements in technology were made intermittently over the ensuing decades and peaked in the 1970s and 1980s with higher horsepower units that could achieve a deeper penetration into the seam highwall miners developed primarily in the 1990s use continuous miners to advance into the coal seam and are able to achieve greater penetration depths and increased coal recovery highwall mining is most commonly practiced when contour strip mining has been exhausted such as in the appalachia region of the united states mainly kentucky and west virginia it has also found some application in other parts of the united states as well as a number of coal regions around the world according to newman and zipf 2005 the concept of highwall mining has been used effectively to mine abandoned pre reclamation law highwalls points or ridges that are considered uneconomic to mine by traditional underground or other surface methods outcrop barriers left at the entrance to underground mines previously augered areas containing otherwise inaccessible reserves and multiple coal seams in another paper zipf 2005 further reports that highwall mining may account for approximately 4 of total u s coal production or upward of some 60 mt 65 million tons of raw coal annually there are reported to be approximately 60 highwall mining systems and as many as 150 auger systems in use across the united states site requirements and conditions in order for a site to be considered for highwall mining a number of site conditions must be taken into account important details include the overburden thickness including maximum expected thickness in mountainous areas the pit floor condition the possibility of intersecting abandoned or current underground mine workings any intersection of previously drilled auger holes and any fractures or jointing in the highwall highwall stability is a major ground control related safety concern because of the proximity of the highwall mining equipment to the highwall in fact some states have certain regulations for reclaiming areas mined by highwall mining as well as for working below a highwall including the requirement for benching of the highwall for example ohio and kentucky united states mining regulations include special requirements for reclaiming mining pits that have been auger mined mccarter and smolnikar 1992 these include special sealing of the auger holes and time requirements for backfilling the area when the augering has been completed other geologic condition requirements include continuity of the coal seam a relatively uniform seam thickness and a near horizontal orientation less than 10 pitch these are considered essential because of the potential adverse effects encountered particularly related to contamination dilution of the coal any highwall mining plan sh

ould consider the hole width or diameter the web pillar width coal left in place between holes barrier pillar width a wider block of coal left between two series of holes and the number of holes between barrier pillars additional care must be taken regarding the design if operations are recovering multiple seams in close proximity to one another the major equipment manufacturers often offer their experience and expertise to users to design the web and barrier pillars and for hole pattern planning in addition several modeling programs are available to assist with highwall mine design lamodel is a nonlinear boundary element method used to examine in seam pillar behavior udec is a distinctelement code used to examine the stability and interaction of the floor seam and roof additional details can be found in the paper by vandergrift et al 2004 preparation in advance of using a highwall miner is relatively straightforward the highwall should be groomed to remove any loose materials that could fall the pit floor needs to be leveled and cleaned and a roadway parallel to the highwall should be constructed for access to the equipment and for beltways and or truck transportation to move the mined coal the area should be at least 15 to 25 m 50 to 70 ft in width although some design engineering is under way to make the equipment more compact for working on narrower benches advantages and limitations highwall mining methods are generally a low cost highproduction application operation is generally considered to be safer than traditional surface or underground mining but equipment size is somewhat limited it can often result in less ash from dilution by the surrounding rock than surface mining maximum recovery is achieved with straight highwalls while the inside and outside of curves require fanning of the holes and loss of the reserves located between holes because of this careful mine planning is required figure 10 10 1 shows a typical range of costs per metric ton of coal recovered by highwall mining as shown the primary factors influencing costs are seam height and roof and floor conditions seam height is defined as low 1 m 3 2 ft medium 1 2 m 3 2 6 6 ft and high 2 m 6 6 ft roof and floor conditions include flatness smoothness and hardness of the floor material as well as competency of the roof material for example a floor that has less than a 1 incline and is free of undulations will have a significant cost advantage over an area where the floor is at a higher angle or has undulations that are cut with the coal causing contamination of the coal or a roof that has material that will fall once the coal is removed these have been classified as easy average and difficult augers are also a highly productive method of coal recovery with relatively low capital and labor requirements a three to four person crew can average 90 to 2 200 t 100 to 2 500 tons per shift however because a web p

illar generally of 0 3 m 1 ft for every 0 6 m 2 ft in hole diameter is left in place recovery averages only 40 to 60 additionally productivity decreases as the depth increases because of the greater torque requirements from the power unit after the auger is in the seam the operator cannot see the moving machinery so operating an auger unit is a highly specialized skill requiring an experienced operator further guidance is difficult as holes tend to drift downward and in the direction of rotation most auger mining is limited to a penetration distance into the coal seam of 90 to 150 m 300 to 500 ft or less because of these limitations more advanced highwall mining systems were developed in the 1990s these units also allow operation with three to four person crews but they can produce at a considerably higher rate than traditional auger miners up to 3 600 t 4 000 tons per shift the coal recovery is also improved up to 70 of the coal being recovered penetration into the coal seam can be up to nearly 500 m 1 640 ft technological advances also allow better ability for the operator to keep the unit operating in a straight line and in the coal equipment and operation augers are essentially large drills turned and used in a horizontal direction they can range from 0 6 to 2 4 m 2 to 8 ft in diameter and 18 to 61 m 60 to 200 ft in length a typical auger consists of the cutterhead auger flights to add increased penetration depth and move the coal to the surface and a prime mover to provide the power to drive the auger augers achieve production by exploiting the low tensile strength of the coal rather than overcoming its high compressive strength as such the key parameters in achieving production are the cutterhead auger diameter the available power the penetration depth and the coal type and hardness auger performance is directly related to the machine power and the cutterhead diameter highwall mining systems were developed in the 1990s to combat the deficiencies experienced with traditional auger mining dominated principally by two original equipment manufacturers oems these systems consist of four basic parts 1 the cutterhead module consisting of a continuous miner head and gathering arms 2 the powerhead assembly which pushes the cutterhead forward 3 some type of loading and conveying system and 4 the base unit which contains the electrical and hydraulic systems these units are used to mine parallel entries rectangular in cross section into the coal the cutterhead modules range from 0 75 to 5 m 2 5 to 16 ft in diameter geology and location quarrying is the extraction of rock from the ground usually through open pit methods as such the geology of a country or region determines where a quarry is located the type of rock also affects the end use of the aggregates produced for example granite makes good rail ballast limestone is essential for cement and graywacke is excellent for r

oad surfacing products the other key factor when considering geology is the distance from the marketplace because aggregates are often a high bulk low value product that is expensive to transport many construction materials can be produced from local materials such as sand and gravel which are abundant in many countries the high value of cement makes it more worthwhile to transport this product farther aggregates can be naturally occurring and extracted from a quarry recycled from old construction materials products of other processes or artificially made some rock types are suitable for many uses others are restricted to specific needs the main general categories of rock are igneous sedimentary and metamorphic igneous the most common igneous rocks are granite or basalt usually formed by the cooling and solidification of molten rock or magma they can be extrusive from volcanic lava or intrusive as a result of slow crystallization in the earth s crust igneous rocks are high in silica which make good construction materials particularly rail ballast and aggregates for concrete and asphalt however they are more expensive to produce because of high rates of wear on processing equipment sedimentary the most common sedimentary rocks are limestone sandstone or graywacke which are formed by the consolidation of sediment over millions of years fragments of rock sand and soil are deposited by water action wind or ice and then compressed by increasing layers of material when laid down in large quantities material organically deposited from the shells of marine creatures can form limestone a widely used rock quarried for cement industrial powders lime flue gas desulfurization and construction aggregates consolidation of chemical precipitates in multiple layers can also form sedimentary rocks metamorphic the most common metamorphic rocks are slate quartzite or schist which originate from igneous or sedimentary rocks but have undergone a change due to exposure to extremely high temperatures and or pressures quartzite is used in construction and slate is extracted for specialist roof tiles and roofing felt quarries can be split into two main types rock operations or sand gravel sites each consisting of any of the main rock types rock quarries extract material from a solid mass from the side of hills or from the surface working down into a pit sand gravel is usually the result of weathering and consists of unconsolidated material that has been broken down and deposited some distance from the source rock either as glacial moraine in river floodplains or in a marine environment sands and gravels usually exist together as a blend of different particle sizes and proportions in addition these materials will also be either rounded in shape or angular depending on the extent of weathering by water wind or ice the location of any potential quarry should be considered vis vis the distance to the market and

numerous local factors when undertaking site investigations for a new quarry table 10 11 1 many countries have detailed geological records that have been built up over many years and should give an indication of the type of material and geological structure in any given area several methods are available to give a better indication of the type of material in a given piece of land for rock deposits a large drill rig is required and the objective is to extract a solid core to allow further physical and chemical tests to be carried out in sand and gravel deposits samples of the unconsolidated material are brought up from much shallower boreholes by an auger also geophysical surveys can be carried out consisting of seismic testing ground radar and magnetic analysis to determine the size and extent of the mineral deposit when the geologist is satisfied that a particular area of land contains a large enough source of rock suitable for use as a construction aggregate or industrial mineral the mineral surveyor either secures the purchase of the land or agrees to a lease with the landowner such negotiations usually include an upfront payment and a royalty paid on each ton of aggregate extracted this usually time consuming process can take many months or even years to achieve a satisfactory outcome the next stage is to obtain a permit or planning consent from the local regulatory authority to start operating the quarry this process will vary from country to country permit to operate in most countries a permit is required before any organization can open a quarry in many instances during the last 50 years this process has evolved significantly and is now one of the most expensive and time consuming aspects of setting up a quarry us 3 million and a 5 to 10 year process span is not unusual local residents have a right not to be adversely affected by the quarry and most governments have established a process to control all aspects of land development quarries are no exception and permitting legislation helps to determine suitable locations and satisfy a number of conditions to ensure that the site is operated in an environmentally acceptable manner in addition a site restoration plan may be needed to implement when quarrying has been completed to meet the demand of construction and other needs the local regulatory authorities usually have a duty to grant sufficient permits to extract aggregates in the united kingdom the planning application is usually a set of documents and drawings submitted to the local mineral planning authority the application is reviewed by professional officers and numerous third parties such as the environment agency and a decision is made by elected representatives for example the study work and drafting required to obtain the appropriate permits for the lafarge aggregates brooksby sand and gravel quarry established in leicestershire in 2006 consumed an estimated three full time equivalent yea

rs the final submission consisted of 30 separate documents which came to approximately 750 pages and 25 detailed drawings or maps these documents and drawings contain a report on all potential areas of concern with a detailed description of how the quarry will be set up and operated the landscaping is shown as are the site entrance roads weighbridge offices wheel cleaners and so forth plans detail how the overburden will be stripped and stored as well as how the mineral will be extracted details and drawings of the proposed processing plant and associated infrastructure are also included the rehabilitation design is a key section of any application the planning consent or permit will usually include up to 100 separate conditions that must be complied with failure to do so by the operator can result in fines or in extreme circumstances cancellation of the permit smith and watkins 2007 the following are examples of the type of conditions issued with a permit to open a new quarry or to extend an existing operation an environmental management system is established to monitor compliance with the permit conditions and implement remedial action if necessary visual impacts and landscaping requirements include details of the type of soils mounds required around the perimeter of the site a tree planting plan and the site entrance design vegetation management requirements ensure that landscaped features are maintained in good condition by watering weeding and mowing trees or hedges should be replaced if they die biodiversity and ecological management may apply to animal habitats in and around the operation for example badger sets must be left undisturbed or relocated ponds or lakes must be protected from pollution and where possible quarry faces must be left for bird nesting fuel and chemical storage must be stored in wellmaintained double skinned tanks to prevent the risk of leakage water management plans must clearly show the location of water bodies on the site and how water is to be used onsite for mineral washing or dust suppression dewatering abstraction and discharge must be licensed and managed to ensure there is no risk of pollution by suspended solids or hydrocarbons solid waste management includes surplus materials from maintenance activities such as scrap metal conveyor belting worn crusher parts and office waste that must be stored in segregated areas and disposed at a licensed facility not just buried on site dust from plant and machinery is not acceptable it is detrimental to the health of the quarry work force is a visual hazard to passing traffic and can cause damage to nearby properties dust emissions must be minimized by the use of sprayed water or approved chemical additives during periods of dry weather enclosed crusher buildings can also help solve this problem noise from plant and machinery must not exceed statutory limits this condition is imposed to protect e

mployees hearing and prevent operations from being a nuisance to local residents this can be achieved by fitting suitable and well maintained silencers to mobile equipment putting crushers and screens inside insulated buildings installing rubber lined chutes and fitting synthetic screen media vibration from blasting must be controlled most local authorities apply a vibration limit well below the level that may damage nearby properties the operator is expected to design quarry blasts to achieve vibration levels within these limits monitoring the vibration level of each blast and reporting the results to the local authority is often a requirement of the permit to minimize the effect of heavy trucks used for the distribution of aggregates on the local road network various measures will be necessary these include sheeting the loads driving through a wheel cleaning system ensuring trucks are not overloaded maintaining legal speed limits and ensuring that trucks are well maintained in some circumstances trucks will be prevented from using certain roads based on weight limits archaeology must be investigated during soil stripping and time must be allowed to ensure that historical remains can be investigated recorded and removed offsite if necessary this work is usually at the expense of the operator emergency procedures to protect the environment must be in place to be implemented if any installed system fails for example if a fuel tank leaks it is also essential to train employees in this activity including rehearsals exercises soil handling requirements during stripping and restoration are necessary soil can only be handled when the moisture content is low enough not to damage its structure and must be stored in separate mounds and retained on site for use in rehabilitation time limits and hours of operation may be required if operations are located a long distance from residential property it may be acceptable to operate 24 hours a day however if not it is common to restrict running the site between 7 am and 5 pm five days a week a rehabilitation plan is required when quarries were exhausted of mineral reserves in the past they were often simply left however all modern operations have a rehabilitation plan built into the conditions of the permit options for rehabilitation include low level agriculture or nature conservation created by placing soils on the quarry floor a lake formed by letting the void fill with water or sometimes filling the site with domestic or industrial nonhazardous waste and returning it to agriculture if the plan is established in advance the mineral operator can plan the material handling to ensure that the final design is achieved economically and to the required standard these mineral extraction permit conditions have often evolved over time some will be the result of national guidelines or good practices applied to all mineral operators others will be based

on the needs of local residents or are the result of negotiation between the mineral operator and the local authority when the conditions for the mineral permit have been agreed it is not unusual for them to be enforced by a legal contract inevitably the whole process takes months or even years to complete the mineral operator must be prepared for a planning application for a quarry to take a long time and to make a submission well before on site work needs to start quarry infrastructure when commencing quarry work at a greenfield site the location will have been chosen by the operator based on the quality and quantity of the mineral and its location relative to the market and competitors after the necessary permits are in place the first task will be to secure the site with fencing gates and tree planting around the boundary and by the main entrance from the nearest highway at many sites obstacles will need to be removed or diverted such as public rights of way overhead power lines trees and hedges to be cut down only if absolutely necessary water courses and pipelines if the product processing system includes a washing plant then a water handling system will need to be constructed this usually consists of the licensed abstraction point from groundwater or from a nearby water course and the necessary pumps and pipelines water for cleaning aggregates will also need to be stored in a large capacity pond the residue from any washing operation is usually fine silt which must be separated from the water before the water is reused or discharged off site the most economic method of handling silt is gravity settlement through a series of lagoons thickeners and mechanical presses are more complex and can cost up to 10 times more than settling ponds but in certain circumstances they may be the only option some quarries extract from below the water table in which case dewatering pumps and if the waste is not then reused for mineral processing settlement and discharge facilities will be needed the next task is to strip the soils from the first phase of the area to be quarried most quarries are worked in a series of phases and the land to be used for plant roads and buildings soils are often up to 1 m thick and can consist of topsoil and subsoil this material is often used to rehabilitate the site in the future and should be stored carefully including being handled only when it is dry and friable soil storage mounds are often placed around the boundary of the site to act as a noise barrier and minimize the visual impact for local residents during the planning permitting process a walkover and or desktop study into the potential for finding archaeological remains on the site is conducted the subsequent permit will instruct the quarry operator how to proceed and whom to inform if a discovery is made and what steps will likely follow during soil stripping an archaeologist usually observes the work in person if any

thing of interest is located the area will be cordoned off to allow further investigation often with hand tools this work will usually only lead to minor delays but sometimes projects can be held up for months if the find is deemed to be archaeologically significant overburden stripping is then carried out this involves the removal of the material below the soils and above the mineral to be extracted overburden can consist of poor quality rock clay sand and peat and can vary in depth from 1 to 20 m or more depending on the local geology the initial cut is often placed in storage for rehabilitation use at a later date however most of the remaining overburden should wherever possible be directly placed for rehabilitation in order to avoid the extra cost of multiple handling of these materials the most common equipment used for stripping soils and or overburden is a combination of hydraulic backhoe excavators articulated or rigid haul trucks and tracked bulldozers in the past motor scrapers were commonly used for this work but they are slowed by wet conditions and can cause excessive compaction of sensitive soils however they may still have a place in drier climates as the mineral is being exposed it is usual to construct roads and foundations install the electrical supply erect the processing plant construct the necessary administration buildings such as an office scale house employee welfare facilities and build the maintenance shop in the past quarries have tended to extract the most easily accessible material often with little regard for a proper plan and geotechnical design this resulted in many accidents due to high faces collapsing or rockfall also it was common for some reserves to be left in the ground because the haul road was quarried away prematurely and there was no access when needed every quarry should have a design that includes the following local geological details hydrogeology direction of working or phasing and timetable safe access roads bench heights and widths stability of the geology when material is removed details of chemistry for cement or milling plant quarries geotechnical surveys the level of detail and complexity of the quarry design will reflect the scale of the operation for example the plan for a 100 000 t a sand and gravel operation will contain significantly less detail than one for a 5 mt a blasted limestone quarry although some governments have legislated in favor of strict geotechnical management of quarries many quarries around the world still lack appropriate quarry working plans and likely operate at higher levels of risk and greater inefficiency than is necessary or indeed acceptable the purpose of a managed geotechnical plan is to ensure the safety of the site employees and local residents and to secure the long term economic viability of the site quarry faces must be designed to ensure they are safe and the risks of rockfall are minimized

the geotechnical design must also include safe access roads the overall slope stability of the operation and a method to economically extract the mineral being quarried without sterilizing any reserves a risk assessment should be carried out to help determine how high the rock face should be factors include the geology results of blasting and the size and type of the machine working the face historically face heights were often limited only by drilling capacity and 30 to 40 m was quite common as a result of rockfall incidents many jurisdictions have placed limits on the height of quarry faces for example in the united kingdom it is typically 12 to 15 m the face should be inspected daily and loose rocks should be removed by scaling with a hydraulic excavator all edges at the top of quarry benches must have edge protection strong enough to prevent a haul truck from going over the quarry face this is usually an embankment of quarried material placed to a height of 1 5 to 2 m along the open edge of the haul road unworked faces should have rock traps at the base to ensure that falling rocks do not roll into a position of danger haul roads should be designed with a slope of no more than one in ten and should be at least three times the width of the haul trucks darlow 2007 stockpiles of soils overburden and quarry products must be designed to ensure they are stable the underlying ground must be assessed to ensure it is able to take the weight of the stockpile which may be more than 100 000 t and taking into account the geology and the groundwater that may be present the stockpile should be constructed by building up from the base and compacted in 1 m thick layers to ensure it is stable the type of material being stocked will often determine the angle of the slopes of the stockpile s perimeter the maximum safe height of the stockpile will be determined by the surface area available and the design of the slope the location of the stocked material will also affect the factor of safety to be used in the calculations if it is close to the site boundary the processing plant or water storage lagoons the risk of damage would be greater if there was a failure a drainage system should be included in the design to ensure that heavy rain does not erode the surface of the stockpile or cause a major slip of the slope geotechnical design should also be applied to water storage and silt lagoons which must be constructed on a suitable base with the necessary drainage systems the sides must be built using compacted material that will form an impermeable seal to prevent leakage from the lagoon sufficient freeboard 1 to 2 m should be included to protect against wave action and an overflow system should be in place to cope with heavy rainfall all geotechnical structures must be designed by a competent engineer and constructed to that design when in operation these structures must be regularly inspected and maintained to ens

ure they remain stable and fit for purpose drilling and blasting drilling and blasting is an essential part of the quarrying activity at a rock quarry and enables the operator to break the solid rock into suitable sizes for loading hauling and primary crushing five important factors should be considered 1 safety there should be no premature initiation or fly rock darlow 2007 2 ground vibration and air overpressure these must be minimized to avoid disturbing local residents 3 fragmentation the rock is broken to a size suitable for further processing 4 efficiency the operation should not take too long or be expensive 5 optimization it is often cheaper to break rock chemically with explosives than mechanically with crushers the blasted rock must be neither too small nor too large for the available processing equipment since the 1950s when technology rapidly developed large capacity drilling rigs have become the equipment of choice for creating blastholes in quarries these machines are usually mounted on tracks or sometimes on wheels and are self propelled with their own onboard diesel engines and compressors compressed air provides the power to operate the hammer drill and flush the holes clean of the rock fines generated during drilling operations the drill bits are often made from a wear resistant material called tungsten carbide however different parts of the world prefer driving the bit in different ways in scandinavia where many of these machines are manufactured a hydraulically powered vibrating hammer is used to drill at the top of the drill rig mast in the united kingdom the compressed airdriven down the hole hammer is used to provide the vibrating power in softer material it may be possible to drill by using a hydraulic drive to rotate the drill tubes which in turn rotate a tungsten carbide bit whichever type of drill rig is employed the objective is always the same to create 75 mm to 150 mm diameter holes at the quarry face ready to receive explosives most quarry blasting is carried out using one or more rows of holes located parallel to the open quarry face typically 12 to 15 m high the spacing between holes and the burden distance from the open face is related to a number of factors and varies for almost every quarry the blast design should start with a laser survey of the quarry face to be blasted which assists the competent blaster in establishing the position of the face bottom so that the burdens of the holes are within acceptable parameters if the burden is too large the blasted rock will contain oversize or if too small the risk of fly rock increases the detailed design is usually calculated using proprietary computer software and taking account of local conditions relating to that particular face and the experience and knowledge of the blaster the cheapest and most commonly used explosive is anfo which is a mixture of approximately 95 prilled ammonium nitrate and 5 fuel

oil or diesel this material can be mixed by hand on a small scale or more often by a truck mounted mechanical mixer however anfo needs to be initiated by a detonator sensitive explosive and is not suitable in wet holes as it dissolves in water although nitroglycerine explosives were commonly used in quarries up to about 1990 in many parts of the world their use has been replaced by packaged slurry explosives or bulk emulsion which are mixed from various ingredients mostly ammonium nitrate and are pumped into the blasthole from a truck initiation of these explosives was usually achieved with detonating cord but this method was phased out and replaced by electric delay detonators in the 1970s the microseconds delay between each hole being fired minimized the accumulated ground vibration without affecting fragmentation a decade later nonelectric shock tube known as nonel became the most common means of initiating quarry explosives this was due to the reduced risk of premature initiation from radios thunderstorms or other interference nonel systems were also cheaper to manufacture in recent years the world of electronics has been applied to quarry blast initiation it is reliable has almost infinite settings for accurate delays and is safer to use although vibration control and or fragmentation is often improved the cost of programmable initiation is currently too high for its common usage in aggregate quarries face loading sand and gravel quarries are often to be found in valley floors or floodplains that is under the water table the most effective way to deal with this is to dewater the working area and excavate dry because it is often safer and cheaper if pumping is uneconomic and working underwater is the only option this can be done with a dragline or alternatively due to significant improvements in technology with a long reach hydraulic backhoe excavator alternatively when working sand gravel from underwater a dredger can be used these machines are built on pontoons or attached to ships and can operate as suction pumps grabs or bucket elevators their operation is not dissimilar to dredgers used in mining of placer deposits at the top end of the scale these dredgers can operate in large freshwater lakes or at sea because sand and gravel is usually unconsolidated its extraction from the ground is achieved without the need for in situ breaking although some deposits may need some additional ripping with a tooth attached to the back of a large bulldozer or a heavy duty excavator since the post world war ii reconstruction boom of the 1950s and 1960s diesel driven draglines were commonly used in sand and gravel quarries to excavate and load haul trucks or field conveyors draglines were able to sit on top of the seam of sand and gravel and reach depths of up to 10 m and it was not necessary to construct good quarry floors they were also used on some sites to strip and cast the overburden currently

many quarries use wheel loaders to excavate and load sand and gravel as long as the floor is competent enough to carry heavy wheeled traffic in all types of weather this method is commonly utilized in north america where in many areas these machines work safely and efficiently as long as a number of core factors are present competent operator suitable sized machine dry level working area and a working face height that is less than the maximum reach of the machine in many parts of the world sand and gravel quarries do not have the right geological or weather conditions to operate using wheel loaders so the machine of choice is the hydraulic backhoe excavator set up correctly such machines will work the sand and gravel face in a safe and cost effective manner loading at the face in a rock quarry has closer links to open pit hard rock mining for metal ore than to a sand and gravel operation but the type of machines available are similar wheel loaders face shovels and hydraulic backhoe excavators however the equipment in quarries and mines is usually larger than that employed with sand and gravel in north america the blasted rock is usually loaded into haul trucks by wheel loaders the trend on other continents is to excavate and load shot rock using hydraulic backhoes or face shovels figure 10 11 2 because both methods have advantages and disadvantages each quarry operator should consider its own application carefully before deciding on which method and equipment to employ table 10 11 2 hauling to the plant after the sand and gravel or blasted rock has been excavated it is often transported to a fixed processing plant which is usually on the same site and within 1 km of the working area since world war ii the diesel driven haul truck in ever increasing sizes has monopolized this work the haul truck is simple to operate flexible because it can drive anywhere within the quarry reliable and effective two main types of haul truck operate in modern quarries the rigid haul truck rht is a two axle machine with rear wheel drive and easily recognizable by the large body canopy extending over an offset cab its capacity ranges from 30 to 300 t and it is more commonly found in a rock quarry where its higher payload provides economies of scale some large sand and gravel quarries in north america also use rigid haul trucks where the geology and weather conditions combine to allow good haul roads to be utilized the other category of haul truck is the articulated haul truck aht this is a smaller machine capacity 20 to 50 t which has three axles and an articulated hitch between the centrally mounted cab and the material carrying tipping body usually a six wheel drive machine it is designed to cope with wet sticky conditions and is commonly found in european sand and gravel quarries ahts are often used for hauling stripped overburden to stockpiles and tips usually because allwheel drive traction is required

for difficult ground conditions several factors are important in optimizing the safe and efficient use of haul trucks both rhts and ahts a good haul road the right sized machine and the size of the haul truck fleet good haul road the road should have as hard a surface as possible an incline no steeper than 10 and a camber to allow drainage into ditches or runoff areas the road should also be at least three haul truck widths wide and have edge protection made from quarried granular material at least 1 5 times as high as the largest haul truck tire using the route the haul road must be maintained to ensure it remains safe and effective to use for this work the best machine is a road grader in dry conditions a water bowser will also be required for dust suppression right sized machine a truck that is too big to feed the processing plant or too small for the quarry excavator has few benefits to recommend it the truck should be loaded in four to five passes of the excavator or wheel loader to enable it to tip into the primary crusher the capacity needs to be of an appropriate size the tipping body should be suitable for the material being quarried if the rock being carried by the haul truck is abrasive additional protection will be carried to minimize the cost of excessive wear hardened steel or rubber liners can be effective in particularly difficult conditions a suspended rubber body could be fitted to the haul truck similar to a steel haul truck body except the floor is made of industrial rubber and suspended from the steel frame by special ropes in cold conditions it may be necessary to heat the tipping body to prevent wet material from sticking heating is achieved by directing the engine s exhaust through channeling in the tipping body size of haul truck fleet the factors that determine the size of the haul truck fleet are the size of the individual trucks the tons per hour required to feed the plant the distance and gradient from the loading point to the hopper expected truck availability and productivity having too many trucks will be expensive and lead to delays in the haul cycle too few trucks will produce inefficiency and leave the excavator and or the plant running empty in addition maintenance of a single machine should not affect the production schedule for some quarries particularly in sand and gravel the most cost effective method of material haulage is a field conveyor that runs from the excavator directly to a surge pile at the processing plant this type of system is usually feasible only where it is unnecessary to blend material from different benches and where the gradient is shallow usually the conveyor system is made up of a number of straight sections changing direction at transfer points these transfer points often cause material spillage during the last few years some quarry operators have installed shallow radius curved conveyors which can be a cost effective way of avoiding expen

sive conveyor transfer points as quarries are extended often the most cost effective means of connecting the new extraction area with an existing plant is by a long field conveyor from a hopper at the edge of this new area which is fed by a smaller team of haul trucks a number of disadvantages are associated with conveyors they are not very flexible i e they cannot be moved from day to day the installation cost is often high and difficulties often exist in providing power supplies however they do have many advantages they require little labor are reliable productive and quiet reduce on site heavy vehicle movements and only need small amounts of dust suppression at transfer points they do not require large haul roads and can be a safe and low cost method of transferring material in pit processing the traditional quarry consists of the working face a haul road and a fixed processing plant however in the second half of the 20th century during the huge surge in demand for construction aggregates many north american operators set up quarries with portable equipment so they could supply projects in remote areas such as dams bridges and power stations and also optimize the use of their equipment during seasonal construction campaigns equipment suppliers rose to the challenge by designing and manufacturing portable quarry equipment which has been further developed and improved over the years a typical crushing spread consists of a number of wheel mounted chassis for crushers and screens with wheeled hoppers and stacking conveyors for stockpiling material the machines are linked together and usually powered by a diesel generator in europe since about 1990 there has been a revolution in the track mounted aggregate processing plant scandinavian manufacturers such as metso eloranta 2009 and sandvik were the pioneers of this technology and are still the global leaders the machines consist of a set of caterpillar type tracks used on backhoes and dozers and a strong chassis fitted at one end is a feed hopper and grizzly screen in the middle are the crusher jaw impactor or cone and the diesel hydraulic power source at the other end is a discharge conveyor figure 10 11 3 over the years these primary units have been modified to include a sizing screen with more discharge conveyors and a closed circuit return belt back to the crusher this type of unit is now available from numerous manufacturers in a variety of configurations including the installation of different types of screen secondary and tertiary crushers hoppers stacking conveyors and other attachments this type of track mounted processing plant is usually located at the quarry face and can consist of one unit producing a crusher run material or can be linked by conveyors to a fixed secondary processing plant alternatively it can be linked to two or three other track mounted machines to form a complete plant on the quarry floor in pit proc

essing has an advantage in the following situations if the quarry face has moved a long distance from the processing plant expensive haulage with off highway trucks can be eliminated by crushing at the face the material can be transferred to the fixed secondary plant by a conveyor system small reserves of aggregate that do not justify the cost of a fixed plant can be operated using tracked mobile machines which then move to the next reserve on completion seasonal campaign processing is common in parts of north america a single portable crushing plant can travel between groups of quarries in long established quarries the processing plant is commonly located on good aggregate reserves as the original quarry becomes exhausted it is often cost effective to dismantle the old plant and extract the reserves beneath it with a tracked mobile unit materials handling aggregates manufactured from rock both blasted and sand and gravel consist of graded particles usually less than 100 mm in dimension which are heavy and often abrasive for quarry plant and equipment the crushing and screening of aggregates is a continuous process and the handling of material between the plant s major components is as important as the crushing and screening operations themselves a modern quarry plant will consist of hoppers feeders conveyors storage bins surge piles chutes and transfer points the most commonly used material handling equipment is the belt conveyor it has numerous applications and if constructed and maintained in the correct manner will be reliable and effective table 10 11 3 numerous types of conveyor are available for almost every conceivable application in a quarry field conveyor long distance material transport figure 10 11 4 curved conveyor wide radius curves to eliminate transfer points high angle conveyor very steep or vertical sandwich belt plant conveyor typical structural steel conveyor within a plant truss conveyor for heavy duty applications bridging large gaps stacking conveyor inclined to discharge on a stockpile radial conveyor stacking conveyor that moves sideways in an arc shuttle conveyor moves horizontally to feed hoppers or storage bins tripper conveyor horizontal with a moving discharge point pipe conveyor where the belt wraps around to enclose the material conveyors move material around the quarry but the material needs to be fed between conveyors or to from processing equipment by feeders there are several types belt feeders belt feeders are usually very short conveyors 1 to 3 m vibrating pan feeders usually electromechanical the vibrating motion is provided by conversion of the rotary motion of an electric motor and an eccentric weight into an inclined stroke apron feeders these feeders are heavy duty chaindriven steel conveyors grizzly feeder often used to feed a primary crusher its vibrating bars have a dual purpose of removing undersize mater

ial and feeding the oversize one of the most frustrating problems experienced when operating conveyors is potential material spillage around some transfer points and in particular underneath the conveyor from the return rollers many types of belt scraper are available for minimizing this problem one method that can almost eliminate spillage under the conveyor is the belt turner a device that turns the empty belt over at the conveyor s tail this allows the belt to return right side up with the clean side in contact with the rollers as a result any material that remains in the belt stays on it rather than being knocked off when the belt reaches the head of the conveyor another belt turner turns the belt over again to carry its normal load crushing the use of quarry explosives is the first application of energy to the virgin rock in its journey to being converted into a marketable product blasting is an important and effective form of crushing a well executed blast transforms a solid rock formation into fragments small enough to be accepted by a processing plant it is generally cheaper to effect size reduction chemically explosives than mechanically crushers albeit with far less control rothery and mellor 2007 the manufacture of aggregates requires larger rocks to be broken down into smaller rocks for example blasted rock may be up to 1 m across whereas rail ballast is a 50 mmdiameter product and concrete aggregate is usually 20 mm in size various stages of mechanical crushing are necessary in the production of aggregates with the following key objectives maximize product yield minimize wastage optimize the particle shape remove deleterious material minimize energy consumption optimize crusher wear characteristics a typical rock quarry will usually have three or four stages of crushing 1 primary crushing blasted rock is reduced from up to 1 m diameter to less than 300 mm 2 secondary crushing the product from the primary crusher is further reduced in size to 50 60 mm which may form some of the final products 3 tertiary crushing taking the secondary product produces the final aggregate sizes usually 20 mm 4 quaternary crushing this may be used to produce manufactured sand or recrush surplus oversize several types of crushers are used in numerous applications gyratory jaw cone impact and roll gyratory crusher a gyratory crusher eloranta 2009 consists of a long spindle carrying a hard steel conical grinding element known as the head seated in an eccentric sleeve the spindle is suspended from a spider and as it rotates about 80 to 150 rpm it sweeps a conical path within a fixed crushing chamber maximum movement of the head occurs near the discharge at the bottom during the crushing process large rocks are compressed between the rotating head and the top shell segments of the crushing chamber becoming smaller as the material drops through the machine gyratory crushers are large c

apacity machines up to 10 000 t h and are only used in primary applications a gyratory crusher is suitable for most rock types figure 10 11 5 jaw crusher a jaw crusher consists of two steeply inclined heavy duty metal plates known as jaws within a cast steel frame a wide opening at the top receives blasted rock and the product is discharged through a smaller opening at the bottom one jaw is fixed while the other is powered to swing back and forth jaw crushers can be used as primary crushers for many different rock types unless very large capacity is required they are usually designed according to the size of their feed opening the first dimension is the width of the jaw and the second is the gape distance between the fixed and the swing jaws at the entry to the crushing chamber scalping off or removal of 50 mm diameter material usually takes place before the crusher and feeder as this is essential for efficient operation all jaw crushers have a large flywheel attached to the drive which stores energy on the idling half of the stroke and delivers it on the crushing half there are two main types of jaw crusher 1 double toggle jaw crusher the swing jaw is suspended from a shaft and its crushing cycle comes from the rotation of the eccentric shaft causing the pitman to rise and fall which in turn rises and lowers the twin toggle plates moving the jaw stock in and out although doubletoggle jaw crushers are not commonly used they are suitable for hard abrasive rock 2 single toggle jaw crusher a driven eccentric shaft passes through the top of the swing jaw creating an elliptical motion at the bottom of the jaw this is a simpler design than the double toggle machine with fewer moving parts and is suitable for many primary crushing applications some older quarry plants may use small jaw crushers as secondary or tertiary machines cone crusher a cone crusher is a smaller and modified version of a gyratory crusher the shorter spindle of the cone crusher is suspended from a spider in some models but not in others the capacity of a cone crusher is directly proportional to the diameter of the head which can be quite high given sufficient motor power also the steeper the head angle the larger the capacity the throw of a cone crusher is many times greater than a gyratory crusher and the cone crusher operates at higher speeds older machines were designed to operate at a slower fixed speed whereas newer models are faster and adjustable within a speed range cone crusher speed is inversely proportional to capacity but directly proportional to size reduction for example as the speed of a cone crusher increases the capacity decreases and the product becomes finer the wide displacement of the head at each stroke is at a speed that allows each piece of rock to fall under gravity and be caught farther down by the rising head on its return stroke in this way the material passing through the crusher is su

bject to a series of hammer like blows rather than being gradually compressed as is the case in a gyratory crusher at the bottom of the crushing chamber the faces of the upper and lower mantles are parallel so that all rock will be crushed and at least one dimension is equal to or less than this closed side setting the cone crusher is a well established machine in the production of aggregates and is suitable for many applications except primary crushing and most rock types however older cone crushers have been criticized for sometimes producing material with a poor product shape the newer generation of high speed cone crushers virtually eliminates this characteristic general principles for good cone crusher performance follow ensure that the motor is big enough for the duty and consider the benefits of variable speed inverters and direct drive from the motor to the crusher a controlled and continuous unsegregated feed distributed evenly to the full periphery of the crushing cavity is essential for good performance avoid feeding sticky wet or contaminated rock to a cone crusher because this will reduce its throughput capacity the closed side setting i e the smallest gap between the upper and lower mantle should be set in accordance with the products required and once set should be monitored regularly to ensure it does not deviate magnets and metal detectors should be fitted within crushing circuits to pick off the numerous pieces of metal that wear away from the plant such devices protect the crushers from damage and unnecessary downtime impact crusher an impact crusher eloranta 2009 consists of a fabricated steel chamber with breaker plates mounted at certain points in the middle of the chamber sits a horizontal rotor which electrically driven at high speed carries fixed blow bars or swing hammers large rocks are fed in at the top and after a series of high speed collisions between the rock and the rotor as well as between the rock and the chamber linings or even between rock particles much smaller rocks are discharged from the bottom of the machine although most impact crushers have a horizontal shaft figure 10 11 6 some specialist applications such as shaping aggregate or manufacturing sand employ a vertical shaft impact crusher impact crushers can be used in fixed or portable applications and at primary secondary or tertiary stages but only with less abrasive rock the impact crushing process causes immediate fracturing of the rock which can be important in the production of high quality construction aggregates impact crushers are also well known for their high reduction ratio good aggregate shape and the generation of significant quantities of fines when considering using impact crushers the desired product type needs to be evaluated methods of adjustment product grading can be changed by altering the following rotor speed feed rate position of breaker bars metallurgical

content of blow bars spacing of grid bars another type of impact crusher is the hammer mill where high strength metal hammers are pivoted on the rotor rather than as fixed blow bars the hammers can pivot out of the way of oversize or tramp metal and are usually best suited for a smaller throughput and for softer material otherwise the way rate would be too high note the way rate is that percentage of material that passes uncrushed through the mill the base of the crusher chamber is perforated so that only products of a certain size can pass through any oversize passes through the crushing process again the hammer mill will achieve much of its crushing by stone on stone attrition in the crushing chamber which makes it difficult to control particle size although the product will contain a large proportion of fines the aggregate products will have a good cubical shape roll crusher roll crushers are still used in some quarries although in many cases they have been replaced with modern cone or impact crushers however they have a use for crushing friable frozen or sticky materials such as chalk gypsum and soft limestone the most common type of roll crusher consists of two horizontally mounted steel cylinders which are free to revolve toward each other one of the rollers usually rotates around a fixed point axle while the other has its axle set at a variable distance away this mechanism provides the adjustment for different sizes of feed material and products diameters and speeds of the rollers can be varied to produce different sized products some roll crushers have a single cylinder rotating to a fixed plate whereas others have multiple rollers but these machines are less common the feed to a roll crusher must be evenly spread across the full width of the roller otherwise wear rates will be differential the rollers are often smooth and lined with abrasionresistant manganese steel to make them last longer for some particularly friable or sticky materials the rolls are made with protruding intermeshing teeth that dig into the material through a process of ripping and compression which helps to pull the material into the machine for crushing screens screening is the separation of aggregate particles into various size ranges and can vary from the separation of 50 mm scalpings from blasted rock to the separation of coarse dust from fine dust a typical screen consists of a rectangular steel frame mounted on springs to which the screen media is attached and an exciter unit to vibrate the frame and create the screening action the exciter unit usually consists of out of balance weights mounted on a shaft driven by an electric motor which may be attached to the screen or mounted on the support structure and connected through v belts the weights can be adjusted to give the required amplitude and stroke of the screen which in turn determines its performance in screening different types of materials into a wide var

iety of products the following screen types are used dry screens for blasted rock processing wet screens for sand and gravel washing inclined screens for general sizing horizontal screens for final product sizing dewatering screens for removing surplus moisture from sand multideck screens for making more than one product high energy screens for separating sticky material the following are guidelines for good screen performance screens should be sized at least to 133 of the required capacity springs must be regularly maintained material bed depth should not exceed four times the aperture of the screen media material should be fed evenly across the full width of the screen discharge chutes should be large enough to avoid material flow restrictions screen media three main types of material are used steel rubber and polyurethane with numerous designs for each of these media the selection of a particular medium depends on many factors and should be analyzed carefully for example polyurethane lasts much longer than steel but is also more expensive rubber will also last longer than steel but does not have as much open area typical screen media used in quarries are punched steel plate for scalping blasted rock heavy duty woven wire for medium sized limestone products fine piano wires for removing oversize from dry sand tensioned rubber for wet screening and abrasive materials tensioned polyurethane for sticky or fine materials and modular polyurethane for dewatering screens and abrasive material sand and gravel washing the majority of sand and gravel produced around the world needs to be washed to remove deleterious material such as clay and silt sand and gravel deposits contain these materials because they have been laid down in relatively recent geological times by rivers or glaciers most sand and gravel quarries produce less than 500 000 t a therefore the processing plant figure 10 11 7 is relatively small compared with many hard rock quarries sand and gravel brought from the quarry face by haul trucks or field conveyors is usually fed to the plant from a hopper or a surge pile onto a washing screen fitted with numerous spray bars delivering 20 to 40 l of water per minute per ton of feed this screen simply removes any large lumps of clay or other oversized material which is retained on the top deck gravel retained on the bottom deck is sent for further processing and fine material passing the bottom deck with most of the water is sent for separate processing and dewatering some quarries where the sand and gravel is contaminated with lumps of clay have fitted ingenious devices to remove this material part way along the conveyor to the first screen the troughing rollers are sometimes replaced with flat rollers for a couple of meters to enable a rotating spiked shaft to be employed which knocks large lumps from the belt this is simple but effective although the gravel from

the washing screen may be clean enough to be screened into its product sizes stockpiled and sold further washing is often required washing trommels or scrubbing barrels are large horizontally mounted cylinders perhaps 2 m in diameter and up to 10 m long traditionally a trommel was driven by a chain on a toothed sprocket encircling the unit and the whole unit rests on two or more rings on solid rubber or polyurethane trunnions today a trommel is usually driven through pneumatic tires rotating on a shaft trommels are almost autogenous i e they rely on gravel sand and clay tumbling in slurry and rubbing and rolling against each other to loosen and bring into suspension any silt or clay the transport of material through these machines usually relies on the discharge aperture being greater than the feed aperture as the fall between the two is usually only 5 the dwell time for materials in the trommel is around 2 minutes which is usually sufficient to clean the material to the required standard for particularly dirty material a longer dwell time would be set trommels are often lined with wear resistant rubber liners with rubber lifter bars installed longitudinally to assist in cascading the material in order to improve the cleaning action the peripheral speed ranges from 16 m min in washers to twice this speed in scrubbers note scrubbers operate at about 60 of critical speed that is the speed at which the contents would cling to the sides by centrifugal force trommels use approximately 25 l of water per minute per ton log washers are used for particularly dirty material they consist of two counter rotating steel shafts logs fitted with many steel blades which agitate and scrub the material pushing the coarse fraction up the inclined tank with the fines overflowing the weir at the back the two shafts rotate inward toward each other so the material is forced into scrubbing contact blade mills work in a similar manner to log washers except they contain an archimedean screw instead of the bladed logs the discharge from these machines is usually fed to another screen which gives an opportunity by using further spray bars to rinse any remaining fines from the cleaned gravel the clay sand silt and water are screened out to a collecting sump from where they join the fines handling process cleaned gravel is then fed by conveyor to a product sizing screen or crusher if necessary during sand production clay and high levels of silt in sand and aggregates used for concrete cause weakness and therefore have to be removed usually by washing particle size definitions used to describe sand production are as follows sand 75 m to 4 mm silt 2 m to 75 m clay 2 m methods of processing sand from the collecting sump a number of methods are available for separating the sand from the silt and clay and at the same time classifying the sand into different grades based on particle size littler and millbur

n 2007 free settling classifiers 30 to 70 efficient free settling classifiers operate on the principle that sand settles out of slurry into a sump by gravity and is then mechanically separated and partly dewatered by rakes bucket wheels or screws from there it is often carried by a radial stacking conveyor for further dewatering by gravity the silt and surplus water overflows a weir adjacent to the bucket wheel or screw and is then pumped to settling lagoons or mechanical thickeners horizontal current classifiers 50 efficient horizontal current classifier machines are commonplace in north american sand and gravel quarries but rarely seen in europe and other parts of the world these classifiers provide consistently graded products from a varying feed and allow enough flexibility to achieve the required specification the long horizontal tank typically consists of 11 compartments known as stations slurry enters at the feed end where the coarser i e larger particles begin to settle out settling continues along the length of the tank with the particles that settle out in each station being smaller than from the previous because horizontal classifiers rely on free settling a large amount of fines unfortunately are entrained in the early coarse sand stations this can be overcome by introducing through slatted plates a rising current of water which pushes particles farther down the tank the required grade of sand can be obtained by blending the material from a number of different stations each of which has three outlet pipes discharging at 60 to 70 solids by weight into three separate flumes each flume runs the length of the tank picking up material from all stations and transferring it to a dewatering system which often involves a fine material screw classifier hindered settling classifiers 80 to 90 efficient hindered settling occurs when the settling rate of a particle within the sand slurry is increased the transition from free settling to hindered settling occurs as the concentration of the solids in the slurry increases this reduces the distance between particles sufficiently that the drag force created by the settling particles affects the movement of nearby particles however if the concentration of solids in the slurry is too high then entrapment and misplacement of particles will dominate one of the most common hindered settling machines currently in use is the hydrosizer which works in the following way 1 the slurry is fed through the top opening of the hydrosizer with additional feedwater if required 2 the upward current of water is created by introducing more water through the teeter plate at the bottom 3 the upward flow of water counters the settling velocities of the solids 4 the remaining solids build a high density fluidized bed until a desired height or bed thickness is achieved 5 after the desired bed thickness is reached an automatic controller is used to discharge

the product at the bottom 6 surplus water and silt are discharged from a weir at the top elutriators 50 efficient an example of this group of equipment is the s type classifier which is designed to take a continuous feed of up to 25 solids by weight feed is passed down a central tube until it meets a deflector plate and is turned radially outward at this point the flow is split with fine sand and silt tending to move upward to the overflow while the coarser particles move downward to the underflow which particles move in which direction is determined by the velocity of the upward current with inevitably some fine particles becoming entrained with the coarser particles and passing downward a second classifier is often required to further separate the overflow into fine sand and silt hydrocyclones or hydroclones 60 to 80 efficient the hydrocyclone is a simple machine with few if any moving parts it consists of an upper cylindrical section over a lower downward tapering conical section the bottom of which is known as the spigot and is 20 of the upper section s diameter a hollow pipe the vortex finder extends through the top of the cyclone and is usually around 30 of the upper section s diameter hydrocyclones are usually constructed in sections for ease of maintenance and are lined with polyurethane or rubber to minimize wear the feed of slurry sand and water enters the unit tangentially via a pipe and onto an involuted feed well which in effect wraps around the top of the cyclone this process pressurizes the feed which is forced onto the cyclone walls under centrifugal force the relatively density of sand is 2 5 times that of water and as such forces its way to the walls of the cyclone pressing out the less dense water which moves toward the central axis of the cyclone the partially dewatered sand spirals down the inside of the cyclone and discharges through the spigot dirty water exits through the vortex finder into a lower pressure zone the purpose of the vortex finder is to stop the sand in the feed shortcutting to the overflow after sand exits any of these machines it should be dewatered prior to stockpiling the most efficient machine for this task is the dewatering screen which usually accepts sand at 60 to 80 solids by weight and discharges it at greater than 90 solids dewatering screens are often horizontal or inclined slightly up at the discharge end the decks can be stainless steel wedge wire or slotted natural rubber however by far the most common screen media is modular polyurethane the underflow water is usually fed back to the sand plant in order to recover any remaining fines management and disposal of silt the waste from sand and gravel washing is a suspension of silt and clay in water by far the most economic way of dealing with this material is to allow it to flow by gravity to settling ponds which are usually excavated in the floor of the quarry where the sand and gravel h

as been extracted the ponds should be geotechnically engineered to ensure they are stable and do not leak the silt and clay settle out by gravity usually in the first of a series of ponds the water in each pond becoming successively cleaner this cleaned water is often pumped back to the washing plant for further use ideally the silt ponds remain where they are and when they are full form part of the quarry restoration however in a number of cases settling ponds are not practical because of for example lack of space severe shortage of water no planning consent high silt content environmental issues or proximity to local amenities or the airspace quarry void has been allocated for landfill if any or a combination of these factors apply then the silt laden water has to be mechanically and chemically with flocculent thickened prior to discharging to the silt pond this will provide recycled water for immediate reuse in washing thereby reducing the size of the silt pond if no space exists for a silt pond the thickened silt has to be made into a semidry cake using a filter press the conventional silt thickener consists of a large tank containing a slowly revolving rake mechanism the dirty water is fed in at the top of the tank where a measured amount of flocculent is added this causes the silt particles to coagulate together which makes them heavier and they sink toward the bottom of the tank the clean water overflows at the top of the tank usually to another adjacent tank for immediate use in the washing process the thickened silt is drawn to the center of the tank by the revolving rake from where it is pumped to a silt pond or filter press the filter belt press is one of a number of systems available that will create a semidry filter cake the thickened silt is treated with more flocculent and is forced between two filter cloths that are pressed together with mechanically driven rollers to remove the surplus water the water is reused and the semidry cake is usually stockpiled by the conveyor to be disposed of by haul trucks a more efficient method of filtering is the plate press made up of a number of recessed plates forming chambers into which the thickened silt is pumped the plates are supported in a fabricated steel frame and are pushed together by the force of a hydraulic ram each plate is covered by a filter cloth which retains the solid particles but allows the water to pass through the solid particles gradually build up on the surface of the filter cloth to form a solid cake after a predetermined period of time the pressure is released the filter plates separate and the cake falls into a stocking bay to be disposed of later removing lignite from sand some sand and gravel deposits contain small amounts of organic matter and or lignite which will cause damage in concrete if not removed several ways to separate these deleterious materials rely on the principle that these materials are less dense

than sand one of the more common methods of lignite removal uses an upward current of water in a tall cylindrical tank which collects the much lighter lignite particles that overflow from a weir at the top of the tank this mixture of lignite fine sand and water often flows over a sieve bend or curved steel fixed screen the water and fine sand return to the product while the lignite is removed by a separate conveyor or chute if lignite is still present in the sand the process is repeated in a second adjacent set of equipment dry screening in many cases natural sand can simply be dry screened straight from the quarry face without the need for complicated and expensive washing equipment this is typically done with a mobile screen fed from the face by a wheel loader into a hopper sand then passes up an inclined conveyor to a double deck screen the top deck of which having 20 mm apertures is used simply to protect the bottom deck which is often a series of longitudinal piano wires the oversize and the product are usually stockpiled separately with conveyors the productivity of a dry screen will depend on the feed size and moisture and clay content and in some cases has to be processed twice manufactured sand in certain parts of the world no local deposits of sand are available or if present they are unsuitable for use in construction materials in such areas the only solution is to manufacture the sand from a normal crushing and screening process making sand for producing concrete is quite difficult often too many coarse or gritty particles or too many fines exist and sometimes particle shape is a problem especially if the fines have been scalped without crushing manufactured sand can be made from surplus 20 mm or 10 mm aggregate by reprocessing it through a vertical shaft impactor with a high proportion of stone on stone crushing in these machines and with high rotor speeds well graded sand with good particle shape is achievable however as always this is often dependent on the geology of the feed material further fine screening may be needed to fine tune the particle size grading of the manufactured sand the mogensen sizer is a multideck dry screen that is capable of making a cut between different sizes of fines if manufactured sand is made dry it should be stored in a building to avoid being windblown causing an environmental problem an air classifier can be used to remove surplus fines from sand made from crushed rock this operates in a similar way to the cyclones used except that pressurized air is used instead of water specialist quarrying operations super quarries typically aggregate quarries are relatively small especially when compared to mines this is primarily due to the relative abundance of suitable geology for the raw materials and the need for the operation to be close to market because of high transport costs as aggregates are heavy and of relatively low value on average sand and gra

vel quarries are smaller than rock quarries as the deposits are fairly shallow i e between 3 and 10 m below the ground level and consume huge areas of land most quarries in europe produce less than 1 mt a and many generate much less than half this amount however a number of large super quarries 3 mt a have significant reserves large processing plants automated plant control systems and fewer but larger items of mobile equipment and can take advantage of economies of scale when distributing by rail or water the united kingdom for example has eight super quarries out of a total of about 1 500 taking advantage of these economies of scale they produce approximately 15 of the nation s requirements for primary aggregates the typical quarry in north america is usually larger than those found in other parts of the world because land is relatively cheap aggregate consumption per head is higher and fuel costs are lower a number of super quarries in the united states and canada have been established for various reasons over the years remote location with massive reserves special properties for cement or lime production close proximity to the rail network for aggregate distribution to rapidly growing urban areas innovative plant designs using large scale equipment borrowed from the mining industry large ships 30 000 t or more that have proved costeffective for transporting aggregates on the great lakes and along the west coast underground quarries the overwhelming majority of quarries operate as open pit mines however sometimes a rare combination of circumstances makes it cost effective for aggregates to be mined from underground these include favorable geology high strength rock structure to allow room and pillar operation relatively scarce high specification material extension to an existing quarry so a quarry face to tunnel into and other infrastructure are in place significant environmental constraints that prevent further surface quarrying and large depths of overburden marine aggregates the european continental shelf is relatively shallow and contains similar sand and gravel deposits to those found on land the reserves are considerable but a complex system of permitting is required to ensure that the rules governing a country s territorial waters and international waters are respected additionally possible damage to the marine habitat which could affect fish stocks and the potential of an increased risk of coastal erosion are concerns although no evidence currently exists to substantiate either of these claims marine sand and gravel is dredged from the seabed by large specialist ships which operate 24 hours per day 7 days per week the dredging areas are clearly defined and the ship s efficiency and compliance with the permit are monitored closely by sophisticated global positioning systems suction pumps attached to steel pipes are lowered from the ship during dredging up to

10 000 t of sand and gravel may be extracted in one sailing most ships have the facilities to screen either surplus sand or gravel from the as dredged cargo when the ship is full it returns to port i e amsterdam london or antwerp where the cargo is discharged at a marine aggregate wharf which is just like any other wharf except it has a sand and gravel processing plant usually with a high capacity 250 to 500 t h there is a ship to shore conveyor and a stocking area capable of holding at least two ships worth of material the sand and gravel is washed in fresh water and screened in much the same way as land won material the main differences are as follows the salt content will corrode the plant quickly unless galvanized and well maintained fresh water must be used in the washing process the aggregate products have to undergo extra testing to ensure no deleterious effect from the extra seashell content or chlorides the risk of dredging unexploded munitions left from world war ii exists and suitable precautions need to be in place marine dredged aggregate has a low silt content which minimizes waste however a wharf has no place to put traditional settling lagoons and therefore silt thickeners and filter presses will be necessary to handle this difficult material dimension stone prior to the invention of concrete many buildings were made of natural stone quarried for that purpose many castles mosques stately homes banks and government buildings are built with dimension or building stone historically every town needed its local building stone quarry which were often small and labor intensive since concrete has become one of the world s most heavily consumed materials dimension stone production has fallen dramatically and is used now only for the cladding of some prestige buildings quarries supplying these materials are few and as such support a niche market as ever the local geology is critical a dimension stone quarry must have massive unfractured rock strata that are strong enough for a building but not too strong to extract in blocks overburden will be stripped by conventional methods but a variety of different systems will be used to extract the blocks of stone slow burn explosives in small diameter holes diamond toothed saws ultra high pressure water cutters traditional wedge and feather system hydraulic version of wedge and feather large hydraulic backhoe excavator with a ripping tooth and or bucket wire ropes impregnated with sand or industrial diamonds which are looped through drilled holes and cut the rock as they travel at high speed against the rock when the large blocks have been removed from the quarry face they are moved to a plant for further processing by various combinations of wheeled loaders cranes and haul trucks processing of the blocks into paving slabs or walling stone is carried out either by automated diamond toothed saws or workers with hammers a

nd chisels rehabilitation in old quarries all the effort was put into the task of winning material from the ground converting it to a product and selling it no plans were made to rehabilitate the land either during the quarrying process or at the end when the reserves were exhausted nevertheless nature has done a reasonable job in rehabilitating some of these old quarries over many years and indeed some are now protected areas for flora and fauna smith and watkins 2007 modern quarries in many parts of the world will not receive an operating permit without a rehabilitation plan this must be included as part of the mine plan in some circumstances the rehabilitation result is designed first and the quarry is planned to achieve it many quarry rehabilitation projects have been carried out in the united kingdom during the last 20 years or so and it is difficult to identify where the quarry was located because of the continuously improving techniques of the quarry industry vis vis postextraction planning sand and gravel quarries are usually easier to rehabilitate than rock quarries because they are shallower and often operate quickly over a large area of land in which case the rehabilitation can be progressive the overburden stripped from one part of the quarry is directly placed as backfill onto another part of the quarry that has already been worked this is good rehabilitation practice and economic because the material is only excavated and transported once rock quarries are much more difficult to rehabilitate progressively as they are often worked to a greater depth than sand and gravel sites it is not uncommon for rock quarries to be exhausted before they undergo any rehabilitation process also the number of options is limited in designing rehabilitation schemes for rock quarries especially if they are deep examples of types of rehabilitation for worked out quarries are discussed in the following sections lakes to create a lake the margin of the worked out quarry is graded to a shallow slope trees and aquatic plants are planted and the water table is allowed to rise the lake could be used as a nature reserve with shallows for wading birds or reed beds for other species and blinds for bird watchers there are many spectacular examples of former quarries that have been converted into some excellent wildlife habitats goodquarry 2009a lakes can also be used for fishing sailing rowing diving and other water sports these activities will usually require a means to enable public access such as roads and parking lots as well as buildings for cafes and washrooms or the storing of equipment the rowing lakes in sydney for the 2000 olympic games and in london for 2012 were both constructed following sand and gravel quarrying agriculture many quarries are rehabilitated back to agriculture which is often what the land was used for prior to quarrying in some operations it is a simple matter of leveling the qua

rry s base and replacing the soils at a low level usually with a pumpoperated drainage system goodquarry 2009b in other circumstances the quarry void is backfilled with construction and demolition waste to the original level and the soils put back on top some quarry voids are backfilled with household waste although this requires the construction of an impermeable seal to prevent the leaking of unwanted pollutants into the water table this type of quarry landfill usually needs a large dome of material to allow for settlement but after this process the land can be converted back to agriculture other uses quarries either rock or sand and gravel can be valuable as landfill sites if the waste is from households it will emit methane as the waste degrades which can be harnessed through pumps and pipelines and used to generate electricity some quarries are converted back to open land or forestry for wildlife and later the harvesting of timber other rehabilitation projects include residential housing industrial and commercial premises golf courses and shooting ranges marketing the production of quarried aggregates can be a complex time consuming and expensive business the sales and marketing of quarry products is a specialist field but essential to ensure a good return for the operator unlike metals and coal no international market exists for such commodities nor are aggregates in the normal course of events worth extracting and storing until they can be sold and used a quarry s location is determined primarily by the local geological conditions and the numerous environmental conditions necessary to secure a permit the distance to the market is important because the cost of transport whether by road rail barge or ship ranges from high to very high in comparison to the value of the product most quarries usually serve a market within a radius of approximately 50 km many quarries are operated by vertically integrated companies which sell the aggregates internally for the production of ready mixed concrete and asphalt and precast concrete products such as blocks roof tiles curb stones and paving blocks for many years these products were seen as basic with little or nothing to choose between them however the 21st century has seen changes in concrete and asphalt technology the major producers now heavily promote their brand of self leveling colored or crack resistant concrete similarly asphalt producers focus their marketing on skid resistance low traffic noise and high durability materials the most significant user of aggregates and aggregate products is the construction industry most of the materials used in civil engineering are seen as commodities because price is usually the most important factor the nearest quarry to the construction project has an advantage in terms of transport costs sometimes other factors will give an operator that is farther away from the site the opportunity to supply a pro

ject with superior or more consistent quality materials or a better service by delivering the correct quantity of material on time every time because the market for aggregates can be quite variable operators have to be able to react to significant changes in sales volumes in the northern hemisphere for example extreme winter weather can reduce the production season to 8 or 9 months a year if material has to be washed only a 5 month window of operation may be available quarries adapt to these climatic conditions by operating a night shift in the summer and building up stocks for the winter concrete production can continue at low temperatures by using hot water and or special additives thereby giving the operator a competitive advantage quarries located close to major railway lines navigable rivers or the sea enables them to compete in markets a considerable distance from the site by using low cost bulk transport by rail or water local distribution is then carried out by short haul road deliveries quarries with rail access are also in a good position to supply ballast for maintaining the railway infrastructure a large construction project in a particular area can also affect the sales and marketing strategy of local quarrying companies new roads railways airport runways dams power stations or ports consume huge quantities of materials usually over a 2 to 3 year time period sometimes luck will give a quarry an advantage simply by being the closest to the project the structure of the quarrying industry in a country or a region can also influence how aggregates are sold and distributed to the construction sites in the united kingdom 80 of the quarries are operated by five global companies peaks in demand give a national operator the ability to back up material supplies from other sites in italy most quarries are operated by different companies competing in different local markets as in every other aspect of quarrying geology plays a part in marketing as well some quarries produce aggregates with special qualities simply due to the rock type this means the product has a higher value often due to scarcity and can be distributed effectively over a much larger area for example the ideal geology for the manufacture of cement is a deposit that is near the market and contains limestone and shale high purity calcium carbonate is required for flue gas desulfurization in large volumes and distribution by rail is essential the aggregate for high friction road surfacing can be quite rare and often located long distances from the market so the costs will be much higher hydraulic mining today today after the factual features i e grade size structure depth and location of a deposit are taken into account the energy costs associated with hydraulic mining are probably the most important factor influencing the viability of this mining method to minimize the water pumping requirement the liberation of ore fro

m country rock using water must be carried out at the highest fluid density possible low fluid density liberation means that a great deal of energy is required for pumping the water to the production face pumping the resulting ore streams away from the production face recycling the water and separating the ore and setting aside a great deal of space for large settling ponds or investing in high cost dewatering systems water jet cutting water jet cutting is normally achieved by using a water canon or hydraulic monitor figure 11 1 1 water jet cutting technology has been developed regionally to match the specific mining requirements of the ore bodies alluvial gold mining for instance and china clay mining use quite different designs the basic principles remain consistent however and there is starting to be a convergence of design based on significant step changes introduced by the fire suppression industry design parameters conditions that will maximize the efficiency of hydraulic mining include the following a reliable source of water high above the washing faces that minimizes pumping energy pumps that are energy efficient and in the correct location a ready supply of large diameter pipe to feed water to the monitors monitors that do not need to be moved very often to maintain their optimum position at the mine face a mine face drifting into a hill with a gravity runoff that will direct the ore stream toward the processing plant no need to recycle the water in today s operations however the mine designer may have to cope with many suboptimal parameters probably the most crucial of which will be the requirement to recycle the water this reflects on the energy use most of all and has been the reason that many hydraulic mining efforts have been replaced with mechanical make down systems such as washer barrels and water sprayed vibrating screens whatever these parameters may be the task is exactly the same to maximize the efficient use of the energy available system design water jet mining projects require a pumped water supply it is essential that the pump and pipe work system be designed to make the most efficient use of the electricity supply more energy efficiency gains are possible from pump and pipe work optimization than from monitor optimization pumps in an ideal project a single pump would be preferred however most pressure pumps have a fairly narrow efficiency curve if there is a requirement to move monitors between benches thus varying the head requirement of the pump or if there is a need to run varying numbers of monitors then a multiple pump setup should be considered a single pump can be used for varying loads if the pump has a very flat efficiency curve and if it can be driven by an inverter controlled motor large inverters above 200kw are also very expensive and in certain circumstances they can cause unwanted harmonic effects back to the grid for this reason some electric

ity utilities place limits on their use a complete cost analysis should be carried out therefore to determine which system is more appropriate the ideal pump arrangement is based on one pump per monitor where the head and volume requirements are matched if the water is free of suspended solids and has a neutral ph consideration should be given to hydrophobic pump coatings which will definitely improve pumping efficiency sometimes by up to 5 if suspended solids are present they should be minimized by the use of a large settling pond which can also be used as a water storage lagoon the presence of even small amounts of suspended solids or adverse ph conditions will significantly alter the choices regarding pump construction materials in these cases it is recommended that duplex stainless steel be used modern pressure pumps need a net positive suction head in order to run efficiently and therefore the pumps should be gravity fed with a minimum of least 1 m 3 2 ft of head consideration needs to be given to stone traps isolation valves servicing room crane access and most importantly pipe friction it is always good practice to oversize the pump pipe work on the suction side on start up a lot of damage to the pumps can occur through cavitation especially if they are required to pump into an empty and unpressurized pipe work system it is therefore advisable that the pressure side isolation valve be designed so that it can be partially closed during the start up sequence allowing the pump to be throttled during the start up phase many installations have a timer sequence built into this valve for automatic throttling during start up and shutdown it is advisable to have some facility for pump health checks that does not necessitate pump dismantling many modern diagnostic systems need pressure tappings before and after the pump as well as on the pump casing and also a facility to measure the bearing temperatures on high power installations it is always advisable even with flexible couplings to use laser alignment techniques between the motor and pump misalignment will mean heat buildup and bearing damage in the long term finally pressure pumps can have very high starting currents it is therefore prudent to consider this when designing the power supply and also when designing the control panels it is wise to have these control panels linked in some way to prevent operators from turning pumps on simultaneously pipelines multiple pump systems should be fed via a well designed manifold for efficient distribution to the pipe work system if the manifold has not been well designed and properly aligned a great deal of energy can be needlessly wasted at this stage ideally there should be a swept bend off each pump into the manifold and the manifold itself should be at least large enough and long enough to reduce the turbulence of the water before it is fed into the pipe work system manifolds without swept bends ca

n be used if they are considerably oversized the manifold will need drain and inspection points figure 11 1 2 shows a four pump duplex stainless steel arrangement with 355 kw motors of particular note are the oversized suction and delivery manifolds the suction isolation valves and the motorized delivery valves for throttling the start up the stone traps are out of view to the right it is all too common to build a system based on a christmas tree approach to pipeline design where sections are added over time with little thought for either isolation or friction losses where possible the entire pipeline system should be designed to have a pipe friction of less than 1 many free on line calculators for pipe friction and other pumping parameters are available on the internet because collisions between mobile plant and the pipeline network are inevitable having the ability to isolate a section without losing production is a necessity the use of ring mains with multiple pathways and isolation points will allow the friction losses to be kept to a minimum multiple pathways enable smaller diameter pipes to be used thus saving money and making pipe handling maintenance and movement considerably easier the location and inclusion of both drain points and air bleed off points should be considered during the design process if there are a variety of pipe sizes in the system it is obviously wise to put the biggest diameters where the highest flows occur any large pipe however wherever it is located in the main feed system before a branch occurs will always reduce the friction losses it is normal to use steel pipes with rubber lined steel wraps for pipe jointing this allows a slight give for ease of fitting movement under varying pressure and some flexibility in alignment some industries have experimented successfully with high density polyethylene pipes which can be constructed in convenient lengths for dragging around the mine typically 100 m 328 ft these longer lengths of pipe suffer proportionately less from the friction associated with the pipe joints and short bends monitor design historically the major centers of hydraulic mining have tended to develop their own style of monitors with little concern about what was happening in other industries or areas this has resulted in specialized designs that have given rise to an evolutionary fit for the hydraulic mining industry the basic principles are universal and the overriding principle of jet cutting is to maximize the energy per unit area exerted by the water jet impacting the face therefore the water jet should hold its shape and coherence for as long as possible which means that the main efficiency driver for a monitor is the nozzle to perform efficiently the nozzle must be provided with nonturbulent water to steer the water jet in two dimensions it is necessary to have two axes of maneuver and therefore at least two sets of bearings in the monitor to

balance the forces involved a loop arrangement is needed where the bearings cross over figure 11 1 3 shows a monitor in operation in the cornish clay industry the thrust is in excess of half a metric ton 5 000 n and therefore the forces must be balanced other approaches to this balanced forces issue are shown in figure 11 1 4 a monitor is built of the following components a base to counteract any overturning moment a stone trap a loop for balancing the forces as much as possible a multiple bearing assembly for two dimensional movement hydraulic rams hydraulic ring motors or electrical actuators for steering the jet a barrel that contains a streamformer a stilling zone and a nozzle the nozzle needs to have a quick change facility normally using steel pipe wraps for varying the volume and pressure in order to keep the pressure pump s on the peak efficiency point all of these elements must work together to minimize any turbulence before the water hits the nozzle to facilitate this there must be minimum friction all the way through the unit which calls for large diameter pipes swept bends constant diameter and a low friction streamformer there is a tradeoff among all these elements as for instance large diameter pipes will require much tighter bends while an efficient streamformer will cause friction current design cues coming from the fire suppression industry however indicate that large diameters are more important than tight bends as shown in figure 11 1 4 the latest thinking suggests that the bends be approximately two pipe diameters with the pipe indexed downward off the top flange to keep the turning moment as low as possible the streamformer is to some extent dependent on the operating conditions since even the most efficient streamformers can be blocked or damaged by foreign bodies in the water a degree of pragmatism has to be used in this case the cornish china clay industry for instance uses a streamformer that has no center at all figure 11 1 5 the fins are mounted on the circumference of the barrel and point inward the fire industry uses a much more delicate honeycomb design because their water tends to be much cleaner the nozzle construction material is dependent on the water quality and ranges from aluminium backed polyurethane to brass and gunmetal figure 11 1 6 the industry is converging however on a fire hose type design of a simple cone with some variations in angle many other designs have been tried but ultimately the original fire hose type has tended to be the most efficient of major importance is the surface roughness of the nozzle it is essential that the nozzle be free of nicks and gouges as a general rule no roughness should be felt when a fingernail is passed over the surface the water emerging from a good nozzle setup should appear as a straight bar for at least 50 nozzle orifice diameters and should degrade slowly but steadily on its path to the mining fac