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ng the desired productivity and thus directly impacts the economics of the project the cavability may be established through empirical comparisons of the rock mass to other known caving operations numerical modeling or both a key aspect of the cave design is the hydraulic radius hr which is a calculation of the area of the caved zone divided by the length of the perimeter geotechnical studies will determine the required hr to initiate the cave and also the hr required to sustain the cave this ties directly into the mine design as the production blocks must honor the required hr for that deposit in general caves tend to grow laterally in a circular fashion trying to reach equilibrium in contrast long laterals with narrow leads extended rectangles may satisfy the hr requirement but may not cave until more area has been opened fragmentation the fragmentation characteristics of the deposit are perhaps the single most important geotechnical feature fragmentation impacts most aspects of mine design and operation including drawpoint size and spacing equipment selection production rates draw control requirements dilution hang ups and mill throughput thus an accurate prediction of fragmentation is a key to a successful mine design and yet this prediction is one of the more difficult to produce with confidence the fragmentation of a deposit changes over time and all of the stages below must be considered in the mine design in situ fragmentation refers to the natural unaltered blocks before any caving takes place primary fragmentation is the distribution of block sizes during initiation of the cave secondary fragmentation is the fragmentation that occurs as the blocks move through the draw column during mining monitoring monitoring of the progress of the cave development is important for many purposes including safety by maintaining a reasonable air gap between the cave back and the cave muck pile initiation and development of caving in order to validate the conceptual models but also to allow modifications to the design as required by that information stability of the production and infrastructure areas in order to react to issues such as areas of excessive convergence and surface subsidence monitoring block cave mine description following is a discussion of block cave mine planning that offers additional details about the layout of a block caving mine mine layout a block caving mine consists of several horizontal operating levels the undercut level extraction level service ventilation level and haulage level an additional level for drainage may be required depending on water conditions in the mine undercut level undercut level designs depend on the method of undercutting chosen undercut methods are described in more detail later in this chapter in the cases of pre or advance undercutting the undercut level is much closer to the extraction level and the major apex pillar is formed during the

undercutting process post undercutting usually places the undercut level drifts directly over the extraction level drifts and at the top of the major apex long hole drilling and blasting is required for the formation of the drawbell in all cases but is sequenced differently according to method pre and advance undercutting start with the extraction of the rings across the drawbell and create the angles of the major and minor apexes this is followed by a final blast from the drawpoint to create the drawbell connection between the undercut and extraction levels post undercutting has the opposite sequence with the initial blast from the drawpoint followed by a series of bell formation shots from the undercut level to create the drawbell the final sequence of shot or shots will be the undercut rings which will remove the remaining pillar material at the undercut inducing caving extraction level although there are a variety of possible extraction level layouts there are two main drawpoint layouts employed in the majority of current and planned caving mines the offset herringbone and the el teniente or straight through both methods have advantages and disadvantages that must be considered when selecting the layout herringbone layout figure 13 10 2 shows an example of an offset herringbone extraction level layout herringbone designs are best suited when electric load haul dumps lhds are to be employed because the mining direction is the same on both sides of a production drift electric lhds also help minimize the ventilation requirements this option tends to be used in smaller footprint operations such as northparkes australia and palabora south africa that do not require the ore body to be split into multiple production blocks the drawbell of the herringbone layout is rectangular and lends itself well to single shot drawbell blasting el teniente or straight through layout in the el teniente layout the drawpoint drifts are developed in straight lines oriented at about 56 60 to the production drifts and the major apexes figure 13 10 3 is an example of the straightthrough layout at the henderson colorado united states mine the minor apexes are short and inclined to the major apexes the el teniente layout is generally considered to be more robust than comparable herringbone layouts because it has a better pillar shape from the development perspective the el teniente layout has advantages over the herringbone layout especially in a panel caving operation the el teniente straight through design allows for the drawpoints to act as temporary crosscuts and accesses or pseudo fringes well before final boundary drifts must be completed the straight line drifts in the el teniente layout make development more efficient and costeffective and potentially create less overbreak especially around the nose pillar temporary fringes can be achieved with the herringbone layout but this is more difficult as the drawpoi

nts are staggered and therefore the temporary fringe drifts are not straight in addition the boundaries of the panels in a large deposit become problematic such problems occur because if a fringe drift is established between panels the width of the pillar between the herringbone layouts on either side of the fringe drift is excessive and merging the caves across that pillar is very difficult and likely to cause excessive weight problems from a scheduling perspective the el teniente layout also has advantages over the herringbone layout this is because a line of drawpoints outside of the planned production block can be used as a temporary or pseudo fringe and in this way development meters can be deferred this also postpones construction of drawpoints grizzlies and chutes overall this would delay capital spending as well as reduce the risk of slipping on the planned schedule a significant disadvantage of the el teniente layout is that it is impractical for electric lhds using trailing cables with a herringbone layout the lhd can produce from all drawpoints in a production drift while facing the same direction but with an el teniente layout the lhd will have to change direction for either side of the production drift this will require additional lost productive time to disconnect and reconnect the trailing cable at the other end of the production drift the flexibility of having diesel equipment that can easily move to other areas or function in the mine is an advantage for el teniente layouts of course diesel equipment requires a higher ventilation airflow rate a safety concern in the el teniente layout is with tramming the loaded lhd bucket first this restricts the operator s visibility 50 of the time because he or she has to operate the lhd in both bucket first and engine first positions in order to pull from all drawpoints secondary breaking units and pedestrians who access operating panels are examples of an increased risk that has to be managed in this case procedures are typically in place to protect pedestrians in a working panel and include lights reflective personal protective equipment and worker machine proximity detection systems a positive aspect of the el teniente design is that the lhd operator always enters the drawpoints from his side of the lhd giving him better visibility for safety and operational efficiency another safety issue is the concern over the impact of a wet muck spill in an el teniente layout during a significant wet muck spill the straight through configuration of the el teniente layout can result in the lhd being pushed back into the adjacent drawpoint by the wet muck and being completely covered this results in greater damage to remote controlled lhds and an extended time required for recovery of the gear from the muck spill with the herringbone layout the lhd is typically pushed down the drift and remains exposed in either case areas of wet muck would need to be worked by re

motecontrolled or automated lhds thereby eliminating any risk to the operators service ventilation level the service ventilation level is typical of a larger footprint mine and is primarily a way to provide intake and exhaust ventilation across the production footprint for the undercut extraction and haulage levels it also provides an access to these levels that is independent of production accesses and activities smaller footprint mines will typically not require a separate ventilation level and will simply use the existing levels extraction and haulage to provide the required ventilation a large caving footprint is a very porous system from a ventilation point of view with many accesses and cross connections in the preproduction and production areas in order to provide reliable fresh air splits and exhaust connections a series of ventilation plenums are required this is usually via two ventilation levels intake and exhaust located between the extraction level horizon and the haulage level this series of plenums are connected to vent raises that intersect the production levels in key points the ventilation levels cover the extent of the footprint and are usually quite large diameter drifts which makes them good candidates for a service level the service level can be used to access the haulage level in key areas as required for development as well as for construction and repair of haulage level infrastructure this is especially important in a rail haulage system where independent rubbertired access to the track is required for cleanup construction or maintenance work the service level can also be used to assist with level drainage and waste haulage and act as a conduit for services such as water compressed air and power haulage level the haulage level design will be determined by the type of transportation method required rail haulage trucks conveyors or a combination of these each method has its advantages and disadvantages which need to be carefully weighed in the context of that particular deposit before development begins because conversion after the fact is almost impossible the greatest advantage of rail haulage is its ability to move large tonnages in a single movement this is especially important if the material requires long distance travel to either crusher systems or to a surface milling facility some of the drawbacks to this system include large capacity bins to accommodate train sizes grade and curve radii restrictions increased power consumption for electric locomotives track maintenance electrical capital and maintenance and potential production shortfalls as a result of disabled trains in the loading drifts or main haulage lines the greatest advantage of truck haulage is its flexibility trucks have the ability to maneuver in smaller areas and can be reversed in traffic flow if the need arises road maintenance is continual but not labor intensive and generally a single disabled truck ca

n be removed from the traffic flow fairly easily thereby reducing production shortfalls another advantage of a truck haulage level is the ability to modify designs easily over the course of time if new reserves or discoveries come into play the main disadvantages of truck haulage include significantly smaller payloads which require larger fleet sizing larger ventilation requirements increased maintenance costs more labor and the inability to efficiently move large daily tonnages long distances in the case of the henderson mine for example a fairly confined truck haulage route to a centralized crusher feeds a 24 km conveyor system to the mill and has proven to be very reliable and productive several small footprint caving operations e g northparkes and palabora use a system of conveyors to gather the production material from multiple crushing stations onto a central conveyor that feeds production shaft hoisting mining methods the selection of the methods used to undercut and extract the ore from a caving operation are fundamental to the success and efficiency of the operation selecting the appropriate methods at the design phase is key as making major changes in methodology after the mine is in production can be difficult and costly general mining systems the key to designing a suitable mining system is to have a good understanding of the geotechnical characteristics of the deposit in particular the fragmentation of the deposit will control the width of the draw cone or area of influence of the drawpoint as the material is mined the area of influence then determines the required spacing of the drawpoints the three principal methods of block caving are the gravity slusher and lhd systems historically the gravity system was commonly selected for ore bodies consisting of finer material essentially free flowing to the drawpoint for somewhat coarser material the slusher system was employed while the lhd system was employed for relatively coarsely fragmented deposits today mines are moving toward higher production rates with greater reliance on mechanization and automation and moving away from the labor intensive gravity and slusher mucking systems the majority of existing and planned block caving operations now use some variant of the lhd rubber tired system of mucking of course this assumes that the ground conditions allow for development of stable production levels that can handle the size of the equipment the majority of this section is therefore devoted to the lhd mining system full gravity grizzly system the grizzly system consists of the haulage level transfer raises grizzly level finger raises and undercut level the haulage and grizzly levels are driven across the ore block to be mined this work can be done simultaneously because they are on different levels finger raises are driven from the grizzly level to the undercut elevation and then short horizontal connections are driven between the top

s of the finger raises forming pillars that will later be long holed and blasted to initiate the caving action transfer raises are driven between the haulage and the grizzly level with a loading chute at the haulage level and grizzly rails for sizing at the grizzly level after the undercut has been blasted broken ore flows down the finger raises and is sized by the grizzly rail spacing passes into the transfer chutes and is loaded into rail cars sledgehammers or semi mobile rock breakers are used to break up oversized pieces at the grizzly some large pieces may hang up in the finger raises and these are usually broken up by secondary blasting with packaged explosives placed strategically against the oversized rocks the andina copper mine in chile is an excellent example of the grizzly system lhd rubber tired system the use of lhd equipment usually requires that the drawpoints be spaced at greater intervals to allow room for the equipment to operate the larger equipment requires larger drift drawpoint excavations so larger spacing is also needed for pillar strength the wider spacing is suitable for coarser rock finer fragmented deposits may require tighter drawpoint spacing and therefore smaller equipment this system consists of the haulage level ore transfer raises the production level drawpoint entries draw cones to the undercut level and undercut drifts long holing and blasting is used to form the undercut that promotes caving the large draw cone will allow larger pieces of ore to move down near the drawpoint where small drills and explosives can be used to break up the larger pieces high hang ups practically never occur with this system the haulage production and undercut levels can be developed simultaneously using separate accesses the haulage and production levels should be separated by a substantial distance to provide adequate storage in the transfer raises so that the loading of the trains is not delayed waiting for the lhds when the haulage and production drifts are complete the transfer raises and drawpoint entries can be driven drilling of the draw cones can be done from the drawpoint entries or from the undercut level freeport mcmoran s henderson molybdenum mine in colorado the doz copper gold mine in indonesia and codelco s el teniente copper mine in chile are examples of mines that use the lhd system undercutting methods and issues the undercutting strategy design and management are critical elements to a successful block cave mine some of the aspects of undercutting to consider include the following undercutting strategy post pre or advance undercutting methods undercut design initiation direction and shape of the undercut face rate of undercut advance and undercut heights shape of the undercut and extraction method flat versus crinkle undercutting impact of the undercut design on stresses in the undercut and extraction levels drill and blast designs for the underc

ut and drawbell post undercutting post or conventional undercutting refers to undercut drillingand blasting after development of the underlying extraction level has been completed figure 13 10 4 drawbells are also prepared ahead of the undercut blasting production blocks can be brought into production more quickly than with other undercutting methods and this method has been most successfully used on smaller mining footprints other advantages are that this system requires about half of the development on the undercutting level as the other undercutting systems in addition no separate ore handling facility is required on the undercut level because the ore is transferred through the already developed drawbells the primary disadvantage is that the mine openings are all subjected to the abutment stresses when the cave front passes through during undercutting the resulting damage to the major and minor apex can be substantial and reduces the effective life of the drawpoints and production drifts or at least often requires repair work to maintain drawpoint integrity selection of this method requires careful consideration of ground conditions and stress issues drawbell blasting can be easier to recover from because the undercut is not yet blasted but conversely undercut remnant pillars stumps can be difficult to deal with the drawbell can be mucked clean and inspected prior to undercutting pre undercutting in pre undercutting the undercut is mined ahead of the extraction level development figure 13 10 5 this allows the development of the extraction level in a destressed environment this method is typically seen in small footprint mines and in mines where ground compaction is not deemed an issue the pre undercut can occur quite some distance ahead of the last drawbell with a considerable amount of development and construction work carried out in the cave shadow area the footprint has to be small enough that the pre undercut does not sit idle too long allowing the abutment stress to refocus on the undercut level or the extraction level through compaction in addition facilities must be developed to handle the swell muck from the undercut level advance undercutting advance undercutting is increasingly popular and many future caving mines are planning to use this technique in this method the undercutting is completed above a partially developed extraction level figure 13 10 6 the extraction drifts and in some cases all or part of the drawbell drifts are completed before the undercut passes when the undercut has passed the drawbelling is completed the crucial drawbell blast is thus completed in a destressed condition because significantly less of the extraction level excavation is completed in this method damage to the pillars can be greatly reduced the cave is brought into production more quickly than for the pre undercut method and the probability of damage from compacted undercut material is reduced typicall

y an advance undercut only extends ahead by the length of one drawbell thereby keeping the active cave close to the leading edge of the cave front if the caved undercut is left to sit for too long before taking the drawbell beneath it can begin to compact and accept the weight of the ground above the abutment stress is redirected to the undercut level and far less damage is seen on the extraction level a separate level for undercutting is required and typically has twice the development as the other undercutting methods because development and construction occur ahead of the undercutting and drawbelling this method requires relatively greater up front capital and use of development resources it can be difficult to recover a drawbell if it is not blasted properly if this becomes necessary a hung drawbell needs to be drilled from the adjacent drawbell to the unblasted drawbell through the minor apex preproduction development and construction when initial mine development is complete and depending on the geotechnical requirements for the area a significant amount of preproduction ground support may be required in the extraction level to maintain panel and drawpoint integrity for the duration of the caving and production periods the production period can last from 3 to 5 or more years depending on the drawdown rate and column heights the ground support may for example consist of grouted threadbar or cable bolts wire mesh and shotcrete throughout the extraction area and will vary depending on ground conditions preproduction work is followed by drawpoint construction which typically consists of two lintel steel sets placed at the mucking point of the drawpoint to protect the brow and pillar from erosion by the caving material the goal is to maintain the opening for the life of the ore column with a minimum of repair figures 13 10 7 and 13 10 8 show typical drawpoint construction the lintel sets are often concreted and bolted in place with the posts being set into concrete in the floor the drawpoint and panel floors are typically concreted some operations choose to add surface hardening products and steel wear bars often this is simply used rail to prevent early floor wear from both mucking and tire wear by the lhds if the area has particularly poor ground a full series of steel sets and concrete may be applied to the drawpoint and panel section in addition to the two lintel sets a typical drawpoint and corresponding section of panel may cost in the range of us 250 000 to us 500 000 depending on the level of support required designing for adequate drainage in the panels is key as water can play a major role in early floor cracking and wear a good general rule is to keep panel roadways at a minimum 3 gradient to ensure adequate drainage some mines are small enough that a separate ore handling level is not required e g palabora and northparkes where the footprint is larger and the haul distance to the crusher

is prohibitively high a separate haulage level is employed as the panel and drawpoints are being constructed the relevant orepasses ore handling systems and ventilation infrastructure must keep pace orepasses can be drop raised raise climbed mechanically or manually or raise bored down to a handling level the top of the orepass is usually fitted with a grizzly and often a rock breaker for oversize material grizzly dimensions vary depending on the orepass size and handling method and can range from 400 400 mm up to 1 000 1 000 mm openings for mines with the larger equipment that can handle material of that size grizzlies should be sized to the estimated fragmentation parameters otherwise excessive choking can occur and clearing the obstructing material will decrease the lhd efficiencies ideally as much of the rock reduction or breaking as possible should be done at the primary crusher to reduce secondary breaking requirements on the extraction level this requires the largest sized grizzly openings that can be delivered to by the lhd without causing hang ups in the orepass or other downstream systems with a 1 000 1 000 mm grizzly the orepasses would need to be at least 4 000 mm in diameter with chutes and transport systems sized accordingly the doz mine in indonesia initially used 1 000 1 000 mm grizzlies on a 4 100 mm diameter orepass these fed into large chutes and 55 t capacity haul trucks that dumped at a 1 370 1 955 mm gyratory crusher figure 13 10 9 shows a typical grizzly installation from the doz mine the grizzly openings are 800 mm feeding into a 4 1 m diameter bored orepass the bumper is 300 mm high the orepasses can vary in length and angle depending on the haulage level elevation orepass diameter is a function of grizzly size and expected material types a good general rule is to keep the orepass diameter greater than three to four times the size of a grizzly opening to minimize the potential for hang ups the base of the orepass is fitted with a typical lip and gate chute arrangement that is used to load either trucks or rail cars with ore where the broken ore is fine enough a feeder may be used to load onto coarse ore belts the haulage level delivers the ore to a primary crusher from which it is transferred to the surface by means of conveyor hoists or rail as surface topography permits production the production rate of a caving mine plays a major role in the required infrastructure the time to ramp up to full production and the drawpoint opening requirements selecting the production rate is one of the primary mine design variables required early in the process capacity a primary benefit of block cave mining is the high production capacity relative to other underground mining methods some of the larger caving mines in the world are producing up to 75 000 t d from single mining blocks the doz mine in indonesia and mines operating in multiple mining blocks are producing greater than

130 000 t d el teniente chile those upper limits will continue to grow as more and more of the large open pit mines in the world transition to underground mining and the technology for large scale underground production improves mines are being planned today that will exceed 150 000 t d when at full production setting the sustained maximum production rate for a caving operation is a key variable in the mine design process the production rate is driven by several criteria the rate of drawpoint opening the rate of draw t d per drawpoint the height of draw hod the life of each drawpoint calculated from the previous two variables and ventilation and orehandling constraints the latter two variables typically act as a step function in production capacity as the addition of either ventilation capacity or ore handling infrastructure results in a significant step up in production capacity for a mine the goal is to ramp up production to achieve a production rate that is sustainable over a period of time in the order of 7 to 10 years that may be less than a potential maximum production rate because it would not make economic sense to ramp up to some maximum rate only to have production immediately fall back as the mining block begins to become depleted commonly the construction of drawpoints proves to be a primary limiting factor in the production ramp up the maximum possible production may not be the economically best option and an analysis of the economics of the mine plan needs to be considered for example should an inordinate amount of incremental capital be required to achieve a modestly higher production rate cash flow analyses would indicate that the optimal plan would be for the lower production rate daily draw rates commonly range from about 150 to 600 mm the allowable safe rate of draw depends on the rate at which the ore will cave drawing faster than the cave rate can result in an air gap that can produce conditions for a dangerous air blast should the cave fail suddenly coarsely fragmented material typically caves more slowly than weak more finely fragmented material in addition coarsely fragmented ore will require extra effort in secondary blasting of oversize material and hang ups affecting the average rate of draw for those drawpoints drawpoints at the leading edge of the cave are commonly mined at significantly lower rates to allow for new cave growth and to minimize opening of an air gap at the leading edge in addition draw rates can change over time for a given drawpoint as the draw column is pulled the material commonly becomes finer fragmented through comminution in the column typically early in the life of the drawpoint very slow average draw is achieved due to coarse fragmentation and the resulting oversize and hang ups that need to be dealt with most block cave planning tools use some sort of production rate curve that takes the change over time of the draw rate into account so that

the production capacity of the mine is not overestimated another major consideration in determining the optimal production rate for a caving operation is the manner in which the cave is advanced in the ore body large deposits cannot be caved in a single pass because the advance of the cave front is slowed by the limits of the drawpoint opening rate and the cave fronts themselves get excessively long long slowmoving cave fronts generate a range of geotechnical issues that can be very damaging to the mine araneda and sougarret 2007 thus the larger footprints must be divided into smaller mining blocks that can be caved and managed effectively which limits the risk of excessive geotechnical issues larger deposits will require multiple caves that at times will be adjacent to or merged with other blocks the issue then becomes one of production block design and drawpoint opening sequences that allow for an overall sustained production rate and at the same time are geotechnically achievable the maximum hod varies from deposit to deposit and commonly ranges from 200 to 400 m although column heights of 500 m have been achieved the limit depends largely on the ability to maintain the drawpoint and production drifts in good and safe operating condition if a drawpoint is well constructed excessive stress conditions avoided and an even draw maintained the life of the drawpoint can be maximized it may be necessary to repair or rebuild a drawpoint for higher draw columns more than once which is a common occurrence in mines with high column heights with a good estimate of the maximum sustained production rate and the life of mine production profile the size and quantity of the major ore handling and ventilation infrastructure can be designed drawpoint opening rates the rate at which an operation can open new drawpoints controls the production ramp up as well as the full production rate that can be sustained if the number of new drawpoints does not equal or exceed the number of drawpoints closed for depletion or due to damage then production cannot be sustained commonly the development and construction of the drawpoint is the critical path to drawpoint opening rates undercutting and drawbelling can generally keep up with the pace of drawpoint construction for example a typical maximum sustained rate of drawpoint opening for the doz mine is 10 drawpoints per month usually referred to as five drawbells the henderson mine typically opens four to six drawbells a month although the mine has peaked at higher rates drawpoint opening sequences few aspects of the design of the block cave mine are more critical than developing a workable drawpoint opening sequence the drawpoint opening sequence determines the order in which the ore is extracted where possible preference is commonly given to the higher value zones of the deposit however equally importantly the sequence determines the shape and direction of the advancing cave and fo

r larger mines determines how the production blocks interact with each other as the mine life progresses it is critical to devise an opening sequence that is geotechnically sound otherwise the production goals and perhaps even the mine itself can be put at risk the mining sequence on a large footprint cave will also drive the development and construction schedule with a smaller footprint e g northparkes the whole mine can be developed and constructed prior to caving on larger footprints e g doz el teniente or henderson the development and construction of the levels takes place on an as needed basis ahead of the caving front this reduces early capital expenditures and smoothes development and construction activities overextending development in a large active cave can often cause ground control problems in the open areas especially if they sit for extended periods of time in stressed ground ahead of the cave front production issues cave fragmentation is a key issue as the amount of ore that requires secondary breaking is a major factor in the ability of the mine to reach and sustain its maximum production level coarse fragmentation can give rise to high medium and low hang ups in the drawpoints as well as excessive secondary breaking at the grizzly figure 13 10 10 shows the types of hang ups that may occur low hang ups are dealt with using smaller drills and can be easily reached by the operators often nonexplosive breaking techniques are used to prevent damage to the drawpoint and lintel set area the same mechanism can be used to break oversize material in the drawpoint or material too large to go through the grizzly mobile rock breakers and fixed rock breakers at the grizzly areas are also employed for secondary breaking when a drawpoint has been caved accessing the drawpoint past the lintel set is too dangerous as it exposes personnel to falling rock longer reach equipment is required to deal with intermediate and high hang ups past the lintel set an intermediate step is the use of a water cannon to sound the hang up and often bring it down by dislodging key sections these units consist of mobile water tanks with a high pressure water nozzle fitted to a robotic boom medium height hang ups are dealt with using singleboom jumbos and harness remotes to drill holes in the key rocks in some cases high bombing will be used without drilling a bundle of explosives is tied to the end of a series of blasting sticks and placed at key intersections of the hangup to dislodge and bring the arch down this process is high risk however as personnel spend considerable time around the front of the drawpoint and are exposed to potential falling and bouncing rocks in a high hang up the coarser fragmentation jumbles together inside the drawbell at a height above about 8 m these are very rare occurrences and are handled using remote long boomed drilling and charging rigs or by drilling through the major apex and bla

sting the larger rocks ventilation there are several components to ventilating a caving operation that bear particular attention planning these components into a general scheme early on will reduce operational trouble at a later stage because multiple lhds will be operating in parallel galleries sufficient air must be provided to support this regulation and control of the airflows becomes critical especially with diesel equipment in order to maintain appropriate levels for emissions and heat additional volume is also required in panel caving operations to support the development drawpoint construction and shotcrete concrete operations that will be in the same panels albeit upstream from the production depending on the form of undercutting more parallel paths on the undercut level will need to be planned for this needs to be done years ahead of the actual caving henderson mine designs for example take several paths across the panel at set intervals to allow for the changing of the main intake as the caving advances the intake level uses loop connections so that the airflow is equalized across the width of the panel and will not starve the last intakes to the production level at the end of a drive likewise as the cave progresses over a long length proper design can allow for the conversion of the intake paths to an exhaust path at the back end thereby reducing the overall requirements to a boundary drift another aspect that deserves attention is the orepass connections to the exhaust for dust mitigation every time a bucket of muck is dumped dust will congregate at the orepass and potentially move downstream in the main drift this not only causes visibility issues but also compromises the filtering and air conditioning systems on the lhds by connecting to the exhaust level either at intercepts or through other channels regulation of air can be accommodated to cause a slight drawdown pulling the dust out of the production drift without compromising the overall airflow downstream of the orepass in small footprint caves orepasses can be designed in the middle of the production drifts and if connected to the main exhaust will allow two lhds to operate in the same panel from opposite ends without interference a final prudent design consideration is to have direct exhaust connections around any and all underground crusher stations often multiple accesses for high production rates are designed at these stations without proper assessment of the ventilation requirements to support them automation caving mines lend themselves to automation due to the regular drawpoint layouts and fairly consistent nature of the production operations several caving mines employ automated lhds on the extraction level e g the doz and el teniente mines and in some cases automated trucks e g the finsch mine in south africa and rail e g el teniente on the haulage level many aspects of rail haulage systems are automated and can typi

cally be managed from a remote control center the automated equipment operates in isolated areas to prevent worker machine interface issues automation is employed to improve productivities and reduce exposure of operators to injury in certain mines the risk of rock bursting or wet muck rushes is great enough to warrant the removal of the operator from the panel area these areas have been typically operated using remote control stations but automated technology provides better guidance for tramming than manual remote control the automation uses scanning lasers mounted fore and aft on the lhd to scan and remember the tunnel profile on a single horizontal section this allows for greater lhd speeds and less damage from side wall collisions the buckets still require manual remote loading using cameras as the muck type and size have been shown to be too variable to allow automation of the bucket loading process the isolation of areas due to remote and automated loading can cause productivity issues as all automated equipment is shut down when personnel are present for drawpoint sampling inspection secondary breaking and so on typical equipment production in rubber tired operations today is fairly standardized around the lhd equipment these may be electric or diesel driven both having their advantages and disadvantages small footprints and herringbone drawpoint designs favor the electric lhd which also requires significantly less ventilation support diesel driven lhds have more versatility to move anywhere and can be used for more than production mucking however they do require significantly more ventilation newer regulations for dust noise and diesel exposure are also driving operators to use enclosed air conditioned cabs which reduce some visibility but definitely give the operator a muchimproved working environment capacities will vary depending on ore characteristics and drift sizing but generally range from 8 to 14 t in load capacity for most caving operations some of the super caves plan to use 17 t lhds production haulage is typically by truck or by train in the case of trucks most are end dump designs in the 40 60 t range these smaller trucks are designed more for development haulage but are being used in a production mode in many mines pt freeport indonesia s doz operation employs 55 t capacity trucks the henderson and andina mines operate 80 t side dump trucks which have good haulage characteristics but only on level ground they are designed for production haulage only and cannot easily negotiate ramps when loaded trains will be sized according to production requirements the larger block cave mines using rail will typically use full size rail equipment with a rail gauge of 1 4 m for example the el teniente operation produced more than 130 000 t d in 2008 using electric locomotives ranging from 72 to 130 t for ore haulage each of the five coarse ore trains hauls 18 bottom dump railcars at 100 t per c

ar another large caving operation plans for trains with two 40 t locomotives and 24 cars at 30 t per car rail systems lend themselves very well to automation and in the right environment can have much lower operating costs than haul trucks several varieties of long hole drills exist and newer computerized versions are available as with most other equipment new regulations for operator exposures are pushing operators to use enclosed cab drills undercut drilling requires these drills to have the capability of 360 rotation some drills have the enhanced ability to drill at forward angles from the perpendicular so that the same machine is able to drill the v cuts as well as undercut rings depending on the method of undercutting explained in previous sections other drilling equipment such as blind bore machines or raise machines will be required for the bell relief slot raise in addition to the primary production equipment a wide range of support equipment is used to develop and support production needs in direct support are the secondary drills and rock breakers used to deal with hang ups and oversize although these types of equipment may seem of lesser importance at first glance they are probably among the most valuable tools for maintaining high production the ability to efficiently clear choked or hung drawpoints has a huge impact on the ability of lhds to meet draw control parameters and production requirements storage of boulders for secondary blasting not only detracts from a lhd s primary function of production but requires removal of the lhd from mucking operations to load and blast this oversize a variety of smaller drills and breakers are on the market but no standardized methodology has arisen as yet to combat these issues some particular examples of this style of equipment were described in a previous section also to be considered is the variety of equipment that supports the development and general operation of the mine in particular the concrete shotcrete delivery systems are an integral part of maintaining appropriate development leads and ground support shotcrete placers concrete pumps drawpoint forming equipment and concrete transporters are critical components of the mobile fleet concrete shotcrete batching systems whether surface or underground and the support needed for material storage are major considerations in infrastructure design to ensure a smooth operation miscellaneous support equipment for moving workers materials and utilities throughout the mine should also be given careful thought forklifts lube fuel trucks high lift trucks for hanging utilities and ventilation fans ventlines water trucks for dust suppression cable trucks for power runs and service trucks for localized construction should all be understood and planned for these types of units are as critical as production equipment for keeping the operation working efficiently and minimizing disruption to production suppor

t equipment for rail haulage operations typically includes smaller 12 t locomotives often diesel powered so as to be independent from issues with the electrical power supply functions handled include shift change track repair cleanup under ore chutes switching and marshalling electrical and control maintenance and development support in addition to the locomotives a variety of flat cars man haul cars and muck cars are used cave management the ultimate goal of the cave management functions is to safely maximize the recovery of the ore reserves an effective cave management system is critical to the block cave mine operation and has an active role in virtually all phases of the mine from the calculation of mining reserves and development of the production forecast the drawpoint opening schedule is developed the development drifting construction and undercutting plan are then based on that schedule as caving proceeds draw orders are developed that will pull the cave evenly to avoid early dilution while managing issues such as drift convergence rock bursts areas of high stress excessive dilution water inflows and the like the cave must be monitored and plans altered to account for any issues that arise reserves reporting and forecasts the initial ore reserves are established during the feasibility and detailed design period which sets the target production rate and guides the mine layout design and development and production schedules typically mine planning and scheduling software is used production forecasting is a critical process in cave management the metal to be produced from the mine is the ultimate goal for the caving operation and so as accurate a prediction as possible is required forecasting is an everevolving process as mining proceeds and more is learned about how the cave is progressing it is necessary to modify forecasting assumptions to match predicted to actual metal recovery as closely as possible as the ore body is mined production data is tabulated and reported this information must be reconciled back to the predicted tons and grade as well as to the actual tons and grade received by the concentrator facility these reconciliations provide key data on the modeling of the draw behavior of the cave which in turn feeds into both the forecasting and reserve models tracking the progress of the cave how the cave progresses has a major impact on what can be expected from the drawpoints and so provides a constantly changing input into the forecasting process geotechnical monitoring provides input on the overall geometry of the cave geological observations at the drawpoints are reconciled back to the in situ geological models that determine key parameters for modeling the draw such as the dilution rates and draw cone interaction managing the cave when issues such as excessive weight problems or wet muck develop modification of the draw in the problem areas can often alleviate or eliminate

those issues it is therefore important for cave management to understand and react to those issues special draw orders for specific drawpoints might be issued to address those issues for example a hung up drawpoint might be causing excessive weight in a production drift so orders are implemented to prioritize blasting of that hangup extra tons might be pulled from that drawpoint and surrounding drawpoints for a period of time until the condition is stabilized for a drawpoint with wet muck a priority will be placed on pulling ore from that drawpoint which helps to pull the water from the cave and prevents the water from spreading to additional drawpoints draw control draw control is an essential component in the caving process controlling the shape of the cave back and keeping the drawdown of the cave even and controlled will minimize cave issues such as early dilution accelerated breakthrough to older workings and excessive air gaps proper cave draw will improve the prediction of ore grades for the diluted block model draw control is managed by setting daily draw orders for each drawpoint in the mine a record of draw is held in a database and regularly compared to the plan corrections in the short term draw order are made to keep the draw control in line with the long term plan systems to monitor draw from each drawpoint can range from a manual form filled out by the operator on an honor system or an electronic tagging system where every drawpoint has an individual tag that registers with a reader on the lhd every time a bucket is taken drawpoint sampling is performed based on tonnage drawn and the hod frequency of sampling is typically increased as a drawpoint nears the end of its life so the drawpoint can be closed as required to prevent overdilution sampling involves personnel manually sampling each drawpoint using some form of a sampling grid although this statistically tends to be quite a poor sample it is a good indicator of drawpoint grades and can be compared back to the model to ensure a reasonable correlation capital cost considerations caving operations typically require a significant commitment in up front capital expenditures and capital development for a caving mine requires a long lead time the mine accesses and infrastructure as well as sufficient portions of the operating levels have to be in place before caving begins this is particularly true for the current and planned super caves exceeding 100 000 t d some of the deep deposits may take several years just to establish access and ventilation to the deposit followed by multiple additional years developing the mine infrastructure once production is initiated the time required to ramp up to full production is significant as an example a mine with a planned full production of 100 000 t d may require 5 years after caving is initiated to reach that objective thus for a large caving operation the lead time from initial access development to full

production can be 10 15 years capital estimates are typically divided into the following capital development which includes all preproduction and production buildup development and infrastructure required to bring the mine to its sustainable full production rate and sustaining capital work required on long term development facilities or systems to sustain full production operations but not direct production costs a typical breakdown of capital development and sustaining capital components follows a number of the categories are the same as for the operating cost breakdown these costs are classified as capital or operating based on whether or not full production has been achieved drifting and excavating rail haulage conveyor drift material handling excavation ramps and accesses fixed facilities shops lunchroom etc extraction and undercut fringe drift production drift drawpoint drift drawbell drift development production and drilling drift on extraction and undercut levels ventilation drift and accesses mass excavation dewatering drifts shafts raises ventilation raises orepasses and waste passes preproduction ground support drawbelling and caving ground repair drawpoint rebuild and repair and production drift repair indirects costs not directly allocated to production such as ventilation electrical engineering water control and maintenance equipment and infrastructure rail or truck haulage or other ore flow infrastructure drawpoint construction mobile equipment power generation fixed facilities ventilation pumping batch plant shops offices warehouses compressors etc table 13 10 1 presents a generic example of development capital costs for an 80 000 t d block caving operation to illustrate the distribution of costs by major category these expenditures occur over about a 10 year period from initial access development through to full production operating cost considerations block caving is typically the lowest cost underground mining method this is principally because the amount of drilling and blasting per metric ton of ore is much less than with any other system once the cave is progressing blasting is limited to secondary breakage of oversize material within a drawpoint in addition because permanent production levels are established from which all ore tonnage is mined total development drifting per metric ton of ore is substantially less than for other methods block caving lends itself to a high degree of mechanization both in the production areas and in the ore flow infrastructure the overall result is lower costs due to reduced requirements for labor mobile equipment and consumables table 13 10 2 is an example from an 80 000 t d block cave mine operating in 2009 to illustrate the general distribution of costs by category of course costs can vary widely from mine to mine because of many factors such as country of operation ground conditio

ns infrastructure requirements maturity of the mine depth and so on labor block caving mines can be highly mechanized and efficient operations commonly achieving more tonnage per workershift than any other underground method and potentially producing many times more tonnage per worker shift the initial labor required to develop a caving mine can be relatively high and is a factor in the early high capital expenditures typically required to bring a block cave into production as major infrastructure items such as crushers fixed facilities ventilation and ore handling systems are completed labor levels will reduce down to a set of relatively specialized crews that focus on the repetitive activities required to achieve and maintain full production these crews typically consist of a level development b preproduction support c construction d caving and e production in addition required support groups include management engineering geology geotechnical mine services logistics maintenance and electrical crews most of these skill sets are common to most underground mining methods in a caving operation the control and interaction of these different crews in a fairly constrained area can be a challenging task requiring clear management good engineering and seasoned supervision each of those key activities listed above requires careful scheduling to prevent the various crews from obstructing each other in addition the crews must work around interruptions such as scheduled and unscheduled blasting and shifting priorities to ensure that the cave is propagating well and being drawn correctly as the mine life progresses activities such as development construction and undercutting will be completed before the mine is closed thus the number of different crews begins to decrease until finally the mine will be left with only production activities in the case of small footprint caves all of the preproduction activities are managed by contractors and production crews are quickly the only set of crews that remain northparkes lift 1 was a good example of this with only 63 employees involved in production at 22 000 t d as the mine had been fully constructed in the case of the much largerfootprint doz mine where development and preproduction activities continue on an actively producing and expanding cave front labor levels remain higher for longer periods with approximately 1 800 personnel planned to produce at 80 000 t d with only about one half of the ore body developed figure 13 10 11 shows the labor distribution by category for the doz mine in 2005 when the mine was ramping up to a production rate of 50 000 t d risk considerations this section presents a review of typical risks in the block caving process the first part discusses risks that present major operational hazards and the second part discusses risks to consider when designing or operating a caving mine a detailed discussion of the risk assessmen

t process is beyond the scope of this chapter but an informative section on operational hazards and risk assessment for block caving can be found in chapters 10 and 11 of block caving geomechanics brown 2003 as with any mining project a detailed risk assessment is required typically a qualitative risk analysis is initially completed to identify potential risks to the project assess the likelihood and consequences of those risks and develop an action plan to mitigate or eliminate the identified risks a quantitative risk assessment where a cost is calculated for each risk should its consequences be realized could then be completed major operational hazards major operational hazards that could result in loss of life or premature mine closure and are associated with caving methods include 1 issues with inrushes such as air blasts mud rushes or water and 2 issues with stability of the underground workings such as rock bursts or major excavation collapses inrushes an air blast may occur when the draw rates exceed the cave propagation rate so that an air gap develops in the cave prior to propagation of the cave to surface should the cave back then experience a large enough failure into the air gap the compression of the air in that confined space can produce a rapid flow of air through exposed underground openings and can produce a violent and damaging air shock wave a mud rush is a sudden inflow of saturated fines from the drawpoint or other underground openings a mud rush can occur when water builds up within caved material that contains sufficient potentially mud producing materials the inrush is then triggered by a mechanism such as production from the drawpoint blasting or seismicity the sudden inflow puts at risk the personnel in the path of the mud rush mud rushes are a fact of life in caves with the appropriate conditions and must be managed to mitigate the risk to the people and to the operation for example automated or remote controlled loaders can be used to produce from the wet areas a key is to attempt to keep the material moving in the wet areas to minimize the buildup of the wet material and prevent the event from spreading further into the cave an external mud rush might occur when the cave breaks up into a saturated zone of material such as a tailings or slimes retention area or perhaps a backfilled zone from an upper lift of the mine tailings or slimes ponds should never be sited over caving areas as catastrophic failures and inrushes can and have occurred that can claim hundreds of lives e g west driefontein in south africa and mufulira in zambia underground stability issues a rock burst is an underground seismic event that depending on its scale can cause violent and significant damage to excavations in the mine rock bursts are the result of stored strain energy that is released suddenly into the rock mass the induced stresses are high enough to overcome the strength of the rock mass

and the released energy cannot be absorbed in slips or fractures rock bursts will usually occur only in strong brittle rocks such as quartzite or granite effects include violent fracture and expulsion of rock from and near the boundaries of excavations and the full or partial closure of mining excavations damage can range from localized spalling in a drift to complete destruction and closure of a part of the mine current and planned caving mines are tending toward deeper higher stress conditions and in stronger and more brittle rock thus it is expected that the risk of rock bursts in future caving mines is an issue that will have to be addressed an additional type of collapse is stress induced but unlike a traditional collapse it is generally progressive rather than sudden and violent collapses of this type are typically caused by the stress abutment produced ahead of the undercut this is sometimes referred to as weight problems this type of collapse involves failure of the pillars left on and above the extraction level it may take weeks or months to develop and once initiated is difficult to arrest although some mines have had success by overpulling from drawpoints within the stressed areas such collapses are exacerbated by the presence of faults or other discontinuities in the rock mass by a change in rock type or by incomplete blasting in the undercut level that leaves a stump or remnant that puts excessive stress point loading on the major or minor apex major risk categories the following subsections address major risk categories that must be considered during the planning and operation phases of a caving mine geotechnical issues most block caving operations which tend to be large in size and occur at considerable depths below the surface are difficult to explore completely in advance of having to make a development decision virtually all aspects of the mine design from level designs ground support criteria ore flow systems equipment fleets and so on depend on a design that correctly incorporates the geological and geotechnical aspects of that deposit therefore a considerable number of risks are present in the category of geotechnical information a few of the potential risks include cavability an assumption that the ore will cave completely and in a predictable manner fragmentation underestimation of the fineness of the fragmentation that causes the drawpoint spacing to be too large leading to isolated draw and loss of reserves underestimating the coarseness of the fragmentation can cause excessive hang ups in the drawpoint leading to excessive secondary blasting and creating a bottleneck in an ore flow system that was designed for fine fragmentation caving performance in situ stress underestimation of the stress levels encountered leads to inadequacies in ground support mass excavation designs and undercut sequencing undercut sequence design the geometry of the panels and the seque

nce of the undercutting are key components to the caving performance should the chosen design prove to be unstable large portions of the production area could be lost and ground repair would be extensive leading to lost or delayed production caving rate if lower than expected ramp up will be delayed cave shape if the developing cave fails to propagate as expected excessive dilution could be introduced earlier than anticipated excavation stability panel excavations may be designed too large in areas of unanticipated poor ground conditions resulting in too large a drawcone spacing and loss of reserves the poor ground also leads to weaker pillars and excessive ground repair costs location of the major underground fixed facilities relies on identifying adequate areas of good ground conditions in which to locate those facilities orepasses are at risk as the cave front passes and must be designed to handle the abutment stresses other issues the following are additional risk categories to be considered during planning and operation of a caving mine fixed facilities design infrastructure required to deliver workers and materials to the mine must be adequate for the production rates required the ore flow system must be capable of adequate ore delivery to the mill environmental impacts and permitting if surface impacts exceed those anticipated it could have an impact on the mine life or production capacity for example larger than expected subsidence areas that affect the surrounding area or development of acid rock drainage exceeding that permitted and so on procurement lead times for procurement of major components can be lengthy leading to delays in mine development and production if not managed properly labor many future major block cave mines are scheduled to be developed and operated in similar time frames in the next few decades experienced labor both in engineering and operations may be limited when needed safety for new mines various aspects of the operation will comprise new types of facilities unfamiliar to the personnel and will require training familiarization and supervision to minimize hazard exposure the term mineral processing is used in a broad sense throughout this chapter to analyze and describe the unit operations involved in upgrading and recovering minerals or metals from ores the field of mineral processing is based on many fields of science and engineering in addition environmental science and engineering have become inseparable components the steps involved in mineral processing have to be founded not only on sound scientific and technological bases but on environmentally acceptable grounds as well goals and basics of mineral processing in the traditional sense mineral processing is regarded as the processing of ores or other materials to yield concentrated products most of the processes involve physical concentration procedures during which the chemical nature

of the mineral s in question does not change in hydrometallurgical processing however chemical reactions invariably occur these systems are operated at ambient or elevated temperatures depending on the kinetics of the processes the ultimate goal in the production of metals is to yield metals in their purest form mineral processing plays an integral part in achieving this objective figure 14 1 1 shows a generalized flow diagram for metals extraction from mining step 1 through chemical processing steps 2 and 3 involve physical processing and steps 5 and 7 involve low temperature chemical processing hydrometallurgy all four steps are considered part of mineral processing high temperature smelting and refining pyrometallurgy steps 4 and 6 are not included under the heading of mineral processing table 14 1 1 specifies processing routes from ore to pure metal for a number of metals processing routes can be quite different and more than one route may be possible for many of these metals for example in the extraction of copper or gold from low grade ores dump or heap leaching is commonly practiced the choice of this leaching practice is frequently driven by the overall economics of the operation because crushing and grinding of ores are quite expensive leaching of ores in large sizes is attractive compared to the leaching of finely ground ores even though the overall recovery of metals from the leaching of fine particles is in general much greater than that obtained with large particles the introduction of this innovative leaching process has made feasible the mining of many mineral deposits that could not be processed economically through conventional technologies metallurgical efficiency one of the most important and basic concepts in mineral processing is metallurgical efficiency two terms are commonly used to describe the efficiency of metallurgical processes recovery and grade these phenomena are illustrated in the generalized process presented in figure 14 1 2 in this example 100 t h metric tons per hour of ore are being fed into a concentration operation that produces 4 5 t h of concentrate and 95 5 t h of tailings in upgrading this process then 1 0 t h of the desired material a is introduced into the unit operation and 0 9 t h 4 5 0 2 of this material reports to the concentrate resulting in 90 recovery 0 9 1 0 100 the grade of the mineral a has been improved from 1 to 20 the term percent recovery refers to the percentage of the valuable material reporting to the concentrate with reference to the amount of this material in the feed sometimes obtaining the highest possible recovery is not necessarily the best approach in a concentration process high recovery without acceptable grade will lead to an unsalable product and is therefore unsatisfactory mineral processing engineers are responsible for optimizing processes to yield the highest possible recovery with acceptable purity grade

for the buyers or engineers who will treat this concentrate further to extract the metal values to achieve this goal economic assessments of all possible technological alternatives must be conducted economic concerns table 14 1 2 summarizes the total u s supply and recycled supply of selected metals in 1996 although the data shown here are historic reprinted from an earlier publication the trends are still current the total supply of iron and steel includes supply from primary and secondary sources as well as imports these two metals represent by far the largest of commodities produced and consumed followed by aluminum copper and lead the recycled supply of these metals from processing scrap is strikingly high in addition the tonnage of precious metals consumed is rather small however because of the high prices of precious metals their monetary value is substantial for example the monetary value of 516 t metric tons of gold was 12 8 billion in 1996 compared to 10 7 billion for 5 3 million t of copper and lead table 14 1 3 lists the relative abundance of various metals in the earth s crust most metals are present in extremely small concentrations in nature and none of these metals can be recovered economically at these concentrations rock that contains metals at these concentrations is not ore ore is rock that can be processed at a profit an average copper ore for example may contain 0 3 to 0 5 copper even this material cannot be treated economically at high temperature without prior concentration there is no way that rock containing 4 5 kg 10 lb of copper and 903 kg 1 990 lb of valueless material can be heated to 1 300 c 2 372 f and treated to recover this quantity of metal economically concentrating the ore by froth flotation to approximately 25 or more copper results in a product that can be smelted and refined profitably unit operations numerous steps called unit operations are involved in achieving the goal of extracting minerals and metals from ores in their purest possible form these steps include size reduction size separation concentration froth flotation gravity concentration and magnetic and electrostatic concentration dewatering and aqueous dissolution size reduction the process of crushing and grinding ores is known as comminution the purpose of the comminution process is threefold to liberate valuable minerals from the ore matrix to increase surface area for high reactivity and to facilitate the transport of ore particles between unit operations 2 size separation crushed and ground products generally require classification by particle size sizing can be accomplished by using classifiers screens or water elutriators screens are used for coarse particulate sizing cyclones are used with fine particulates 3 concentration physicochemical properties of minerals and other solids are used in concentration operations froth flotation gravity concentration and magne

tic and electrostatic concentration are used extensively in the industry froth flotation the surface properties of minerals composition and electrical charge are used in combination with collectors which are heterogeneous compounds containing a polar component and a nonpolar component for selective separations of minerals the nonpolar hydrocarbon chain provides hydrophobicity to the mineral after adsorption of the polar portion of the collector on the surface gravity concentration differences in the density of minerals are used to effect separations of one mineral from another equipment available includes jigs shaking tables and spirals heavy medium is also used to facilitate separation of heavy minerals from light minerals magnetic and electrostatic concentration differences in magnetic susceptibility and electrical conductivity of minerals are utilized in processing operations when applicable 4 dewatering most mineral processing operations are conducted in the presence of water solids must be separated from water for metal production this is accomplished with thickeners and filters 5 aqueous dissolution many metals are recovered from ores by dissolving the desired metal s in a process termed leaching with various lixiviants in the presence of oxygen following leaching the dissolved metals can be concentrated by carbon adsorption ion exchange or solvent extraction purified and concentrated metals may be recovered from solution with a number of reduction techniques including cementation and electrowinning examples of mineral processing operations figure 14 1 3 shows a typical flow sheet for crushing and sizing rock in a quarrying operation run of mine ore can be present as lumps as large as 1 5 m 5 ft in diameter in this figure s example 1 m 3 ft lumps of rock are fed to a crusher that reduces the material to 200 mm 8 in or less in diameter after screening to remove rock that is less than 57 mm 2 in in size rock between the sizes of 57 mm 2 in and 200 mm 8 in is further reduced in size by a gyratory crusher the product from this step is then classified by screening to the desired product for sale figure 14 1 4 shows an integrated circuit demonstrating crushing grinding size separation and gravity concentration of a tin ore initial size separation is effected with a grizzly set at 38 mm 1 in oversize material is fed to a jaw crusher set at 38 mm 1 in and the crushed product is then further reduced in size to 20 mesh by ball milling the minus 20 mesh material is classified by hydrocyclones set at 150 mesh and the minus 150 mesh material is sent to shaking tables to concentrate the heavy tin mineral cassiterite the middlings in this process receive additional treatment the concentrate from this operation is reground and sized at 200 mesh twostage vanning is used to produce a fine tin concentrate the flow sheet describing the flotation processing of a copper ore contai

ning chalcopyrite and molybdenite is shown in figure 14 1 5 after grinding and classification pulp is fed to rougher flotation the rougher tailings are thickened and sent to a tailings dam the rougher concentrate is classified and the oversize is reground cyclone overflow is fed to cleaner flotation and the cleaner concentrate is recleaned cleaner tailings are recycled back to rougher flotation and the recleaner concentrate is thickened and sent to the molybdenum recovery plant for further processing in this operation the feed contains 0 32 cu and 0 03 mo rougher concentrate cleaner concentrate and recleaner concentrate contain 7 to 9 cu 18 cu and 25 cu respectively recleaner concentrate also contains 2 to 3 mo figure 14 1 6 depicts a flow sheet for processing freemilling oxidized gold ore the kinetics of gold leaching is slow and gold ores are frequently ground to less than about 76 mm 3 in before leaching even then one day is usually required in the leaching step in this process run of mine ore is crushed and ground the ball mill discharge is subjected to gravity concentration to recover the larger particles of free gold the tailings from this operation are thickened and the underflow from the thickeners is then subjected to cyanide leaching in some instances ores may contain oxygenconsuming minerals such as pyrrhotite and marcasite and a preaeration step may be conducted ahead of cyanide leaching heap leaching has revolutionized the gold mining industry low grade oxidized ores containing approximately 0 933 g gold per metric ton 0 03 oz gold per short ton of ore can be processed with this technology whereas they could not be processed by the higher cost of the grinding agitation leaching milling process figure 14 1 7 presents a simplified flow sheet of heap leaching as the figure shows run of mine ore may or may not be crushed if crushing is done the ore is generally crushed to 51 mm 2 in in diameter comminution is the collective term used to describe the progressive reduction in size of run of mine rom ore size reduction is required either to prepare the ore for market for iron ore and coal for example or as in the case of base and precious metals to allow liberation and separation of the valuable constituents for ore requiring beneficiation liberation and separation are the essential steps in mineral processing the degree of liberation attained by comminution defines the grade recovery curve for a given beneficiation process as well as typically representing the largest category of mineral processing capital and operating expenses as such the comminution process has a large impact on an operation s bottom line proper circuit design and operation are critical to overall project success crushing and grinding have been regarded as separate processes with each step performed on a certain size range of material this concept is rooted in the historical conventional cir

cuit that entailed staged crushing followed by open circuit rod milling and closed circuit ball milling modern equipment has blurred this concept with autogenous grinding semiautogenous grinding ag sag mills and high pressure grinding rolls hpgrs in particular spanning a broad range of comminution applications and particle size ranges this chapter addresses in general terms comminution circuit configuration equipment types and operation stages of comminution are discussed from rom through fine grinding with sections devoted to special grinding applications ore characterization and circuit design and mill liners and media no attempt is made to cover all comminution machines or flow sheets however methods processing the largest tonnages of ore in the metals mining segment are summarized selected unit operations in broad use for each stage of comminution are presented in table 14 2 1 with size ranges for the most commonly used unit operations depicted in figure 14 2 1 unit operations are discussed in more detail in relevant sections examples of selected circuit configurations are presented in table 14 2 2 this selection is not exhaustive the summary presents both a range of grinding applications and demonstrates examples of selected circuit configurations throughout this chapter commonly used references to describe feed and product sizes are employed f and p represent feed and product respectively with the subscript indicating the percentage of the material passing a given size for example a p80 of 13 mm describes a size distribution for a material that has 80 of the material finer than 13 mm individual comminution unit operations are well described in contemporary mineral processing textbooks wills 2006 kelly and spottiswood 1995 classic works such as taggart 1945 are still useful and provide perspective and a number of comminution monographs provide detailed references including mular and jergensen 1982 napier munn et al 1996 to keep up with changing practices various forums addressing academic engineering and operational aspects of comminution were developed perhaps the most focused of these forums have been the international autogenous and semiautogenous grinding technology conferences hosted by the university of british columbia proceedings from these conferences 1989 1996 2001 and 2006 are the broadest reference body for case studies on contemporary milling practices despite the name a broad spectrum of mineral processing comminution circuits is addressed mining and primary crushing because the process of comminution starts with mining the degree of comminution achieved should be used as a performance indicator for the mining process correspondingly the comminution circuit should also be developed in consideration of the selected mining process for surface mines that produce broken ore or muck via drilling and blasting there are numerous case studies of optimizing drilling and bl

asting in conjunction with the overall economics of shovel productivity primary crusher throughput and downstream comminution efficiency efficient blasting is effective at both decreasing top size as well as improved fines generation in general the following conclusions can be drawn staggered patterns versus square patterns reduce the maximum distance between drill holes and typically reduces the rom top size boulders quality of drilling in terms of the accuracy of holes in x y and z axes improves fragmentation as well as shovel productivity customized blast designs by material type burden and spacing explosive type and amount and initiation timings can improve fragmentation and offer cost benefits the effect of blast sequence including presplitting production and trim blasting where required for wall control blast size and pattern geometry should be considered for the effect on comminution additional considerations for underground mines apply as a consequence of mining through areas previously subjected to ground control many underground mines produce extraneous metal in the mill feed additionally block cave mines typically produce coarser material at drawpoints early in their development with higher draw columns greater ag breakage occurs and drawn material becomes finer block cave mines usually employ secondary breakage to control material hang ups in drawpoints as well as mechanical rock breakers and grizzlies to ensure top size control prior to truck loading before primary crushing is undertaken typically rom material must undergo a stage of size reduction to facilitate conveyor transport questions to be addressed in circuit development include the size number and type of crusher used to produce suitable mill feed where to place the crushers and how to feed them the choice of primary crushing equipment is typically between jaw and gyratory crushers jaw crushers can be selected for applications with low throughput while throughputs greater than 1 000 t h 1 100 stph favor gyratory crushers gyratory crushers of up to 1 9 3 2 m 75 124 in and installed power of 1 mw or more are currently being marketed a modern gyratory crusher is depicted in figure 14 2 2 the location of primary crushing stations is a critical question primary crushers can be permanently located semimobile or mobile even permanently located crushers can be disassembled and relocated during a mine s life so the question as to which of the three options to select is one of tradeoffs between the capital and operating cost of the incremental truck haulage fleet required versus mobile or semimobile crushing stations fixed plants simply rely on the haulage fleet to bring ore to the crusher mobile and semimobile plants often used tracked crawlers which have capacities of more than 1 000 t 1 100 st to periodically relocate the crushing unit in conjunction with movable conveyors such systems are placed to shi

ft the haulage burden from a mobile equipment fleet to conveyors and more economically transport material mined to either ore or overburden stockpiles the evolution of larger truck sizes along with a general trend of steeper and deeper pits have tended to disfavor in pit crushing and conveying however these can offer advantages in terms of reduced operating costs increased availability and reduced labor requirements compared to a truck fleet a thorough review of the topic is provided by boyd and utley 2002 aside from selection of crusher and crusher location and mobility design considerations include methods to feed the crusher and convey crusher product to the mill stockpile crusher feeding considerations include the number of truck dump locations and the size of the crusher feed hopper and appropriate design for dead beds primary crushers can either be fed with direct truck dumping as in figure 14 2 3 or by using ore pockets and apron feeders direct dumping is the simplest but it can be more difficult to sustain peak crusher utilization and truck wait times can be incurred apron feeders offer the ability to partially disconnect the crusher from truck dumping with the idea of both reducing truck queues and improving crusher performance but introduce additional capital costs as well as the substantial maintenance costs of operating apron feeders in the demanding duty crusher installations should also consider the requirement for mobile equipment to clear dump pockets in case of blockages rock breaker installations to handle oversize inadvertently fed to the crusher overhead craneage fixed or mobile for maintenance and overall elevation required for an installation controlled by crusher dump pocket and product bin dimensions crusher product handling using a bin and apron feeders to place the material on a conveyor belt is effective and allows matching the operation of the semicontinuous variable capacity crusher depending on material type with a conveying system of fixed capacity product bin sizing should consider peak flow through rates observed when crushing fine ore to avoid bin overfilling in cases with sufficient belt capacity relative to crusher throughput product can be fed directly to belts via appropriate chutes alternatives to conventional crushing prior to further processing include mineral sizers whose manufacturers include krupp and mmd these machines operate dry employ horizontal counter rotating shafts equipped with wear tips and are alternatives for size reduction of low to moderate strength materials with low head height devices a somewhat similar device the mclanahan log washer operates wet and serves to deagglomerate coarse from fines rather than affect true comminution these devices are well proven and work effectively in appropriate applications there can be substantial capital cost savings with reduced civil and structural work for these units all of these machines are vul

nerable to damage through introduction of incompressible foreign objects notably ground engagement tools and tramp metal in cases where direct dumping is used protection is limited to tools such as cameras and image analysis software useful for detection of the loss of shovel teeth spotters and the installation of safety clutches to limit damage for unavoidable events where material is reclaimed or dumped to feeders metal detectors or magnets can be used in some applications primary milling a critical linkage point between the mining and milling operations is the mill stockpile most comminution plants reclaim from a stockpile of primary crushed material and stockpile management is essential to maintain consistent throughput there is generally some degree of stockpile segregation with coarse material preferentially accumulating toward the outside of stockpiles maintaining a live stockpile and balancing multiple reclaim feeders will result in the highest average and most stable throughput feed size to a primary milling circuit can have a substantial impact on throughput particularly for ag and sag circuits this underscores the importance of evaluating mining technique and practice primary crushing and the downstream comminution circuit it is also important in terms of nonsteady state operation such as stockpile drawdown conditions as the proportion of material drawn down from the stockpile increases the feed to the comminution circuit typically becomes coarser correspondingly requiring a higher unit energy input for comminution the often observed effect was well quantified by morrell and valery 2001 that paper also discusses broadly the effect of sag feed on circuit performance maintaining reclaim stockpiles at reasonable levels minimizes the effect of load and haul equipment shift changes on downstream operations and decouples the mine and mill from maintenance activities associated with shovel and mill maintenance periods stockpile size should be based on anticipated interruptions due to primary crusher maintenance load andhaul asset maintenance mill maintenance downtime requirements and normal fluctuations due to mine sequencing depending on climate ore moisture surrounding facilities dusting characteristics and regulatory requirements covering stockpiles may be desired or necessary the following sections outline three circuit configurations in broad use as the first stage in a comminution plant primary milling 1 standard crushing plants 2 ag sag mills and 3 the combined application of cone crushers and hpgrs the latter circuit is now broadly viewed as an alternative to sag circuits crushing circuits the classic crushing circuit prepares primary crushed ore for secondary milling through two stages of crushing most typically stockpiled primary crusher product is reclaimed and fed to a secondary crusher in turn the product of a secondary crusher operating in open circuit is screened wit

h oversize fed to a tertiary crushing circuit operating in closed circuit with screens the product of a standard tertiary crushing plant is nominally 100 passing 13 mm the basic design of a tertiary crushing plant secondary crushers in open circuit with tertiary crushers in closed circuit with screens along with variations has been widely used such a circuit inclusive of hpgrs in a quaternary role and follow on single stage ball milling is depicted in figure 14 2 4 the crushing plant and concentrator depicted has a nominal capacity of 75 000 t d 83 000 stpd the plant depicted shows wet primary screening followed by dry tertiary screening in some applications this is reversed and some circuits particularly for heap leaching are fully dry selection of wet or dry screening is a function of material type rom moisture efficiency of classification required and the type of follow on material handling or processing the product of such a typical crushing plant can be directly fed to a fine crush heap leach operation in milling operations the product of a crushing plant was historically further reduced in an open circuit rod mill prior to comminution in a ball mill however rod mills are limited to nominally less than 5 m 16 4 ft in diameter and 6 m 19 7 ft in length due to practical constraints on mill size before rod tangling occurs such a mill has a power draw of less than 2 0 mw as such economies of scale have largely led mill circuit designers to send crushing plant product directly to ball mill circuits doing so may introduce a degree of grinding inefficiency based on the coarseness of feed to a ball mill or a high reduction ratio in one stage of grinding but this has been widely accepted as being favorable to a requirement for a large number of rod mill lines and an additional unit operation as will be discussed in a following section hpgrs in a quaternary role perform top size reduction admirably as well as doing an excellent job of fines generation depending on circuit configuration large single line secondary tertiary crushing lines have a nominal installed power of 3 mw and the capacity to crush 25 to 40 kt d 28 000 to 44 000 st crushers are generally of two types of mechanical design in the symons design figure 14 2 5 a threaded bowl liner is adjusted to modify the crusher gap the bowl is threaded to the adjustment ring which is secured flush against the mainframe through hydraulic accumulators that release in the event of a tramp metal event in the allis design figure 14 2 6 the crusher mantle is shaft supported against a fixed bowl liner analogous to a gyratory crusher s concaves to maintain either a fixed gap or a fixed pressure setting in the event of tramp metal or other uncrushable item entering the crushing chamber the hydraulically supported shaft drops allowing the material to pass because crushing plants are well understood the skill sets required for plant operation and

maintenance can be readily sourced or developed throughput for the machines used is largely volumetrically controlled as such plant throughput is relatively insensitive to changes in feed hardness or feed size distribution though the amount of comminution work and power drawn by the machines is clearly affected power input is a direct result of comminution effort required to effect breakage unlike tumbling mills where it is the outcome of the potential energy transmitted to the mill charge crushing plants have some drawbacks relative to other plant configurations first particularly for larger plants they normally require many more units of equipment bins crushers screens conveyors chutes etc for an equivalent throughput than other major circuit types this is due to the fact that the current upper limit for installed power for a conventional crusher is 750 to 1 000 kw smaller than the average unit installed power for mills partially as a result of the number of units in series and partially due to liner change outs crushing plant run time is lower than that of rotating mills crushing plants do not produce fines to the same degree as other primary milling options which increases the amount of comminution effort required in a second stage of milling the mode of breakage is dominated by compressive fracture which produces fewer fines and screening wet or dry at finer sizes is not practical at large tonnages screen area required escalates nonlinearly with decreasing cut size finally crushing plants have material handling difficulties with sticky ores significant downtime due to metal detector trips and potentially a resulting loss in production with ores that contain a high degree of tramp metal or magnetite ag sag circuits since their early development ag and sag mills have become a mainstay of comminution in mineral processing applications although crushing rod mill ball mill circuits were referred to as conventional plants until relatively recently ag sag mills are now the foundation unit operation for most large grinding circuits the variety of ag sag circuits is substantial the following duties are all well represented single stage autogenous semiautogenous milling ag sag mills as one stage in a larger comminution circuit inclusion of pebble crushing circuits in the ag sag circuit employment of a crushing stage after primary crushing such that sag feed has been through two stages of crushing single stage ag circuits have been installed in relatively high tonnage coarse grind applications such as iron ore company of canada s ioc s carol concentrator and in lower tonnage finer grind applications closed with hydrocyclones like noranda s brunswick operation in quebec canada very high single line capacities have been demonstrated with some circuits having up to 70 mw of installed grinding power in a single plant pt freeport indonesia s 11 6 m 38 ft diameter sag mill in concentrator

4 papua indonesia presented in figure 14 2 7 has registered monthly tonnages averaging more than 125 000 t d 138 000 stpd with single day performances in excess of 150 000 t d 165 000 stpd although common convention generally refers to highaspect ratio mills as sag mills with diameter to effective grinding length ratios of 3 1 to 1 1 low aspect ratio mills generally a mill with a significantly longer length than diameter can also grind autogenously or semiautogenously such mills are common in south african operations and are sometimes referred to as tube mills or rom ball mills many of these mills operate at higher mill speeds nominally 90 of critical speed and often use grid liners to form an ag liner surface these mills typically grind rom ore in a single stage either autogenously or semiautogenously the broad application of ag sag mills lies in the large single line capacity of such plants the ability of circuits to mill a broad range of feed types and the variety of circuit configurations the application of gearless drives has allowed mills to grow beyond sizes allowed by twin pinion drives a nominal limitation of 12 16 mw based on a limit of 6 8 mw per pinion on a twin pinion drive results in a restriction of nominally an 11 m 36 ft diameter sag mill based on typical diameter length ratios if using geared drives many large mills around the world including cadia collahuasi new south wales australia with 12 2 m 40 ft mills and antamina peru escondida ph iv antofagasta chile pt freeport indonesia and others with 11 6 m 38 ft mills have installed sag mills of nominally 20 mw with gearless drives a number of mills with 25 mw or higher gearless drives are either under construction or in engineering some of these grinding lines have design capacities exceeding 100 000 t d 110 000 stpd the process flow sheet for the large sag installation with pebble crusher product combining with sag discharge and feeding screens depicted in figure 14 2 7 is presented in figure 14 2 8 ag sag mills comminute ore through impact breakage attrition breakage and abrasion of ore serving as media ag circuits require an ore of suitable competency or fractions within the ore of suitable competency to serve as media sag circuits may employ low to relatively high ball charges ranging from 2 to 22 expressed as volumetric mill filling to augment ag media higher ball charges shift the breakage mode away from attrition and abrasion breakage toward impact breakage as a result autogenous milling produces a finer grind than semiautogenous milling for a given ore and otherwise equal operating conditions with a higher density mill charge sag mills have a higher installed power density for a given plant footprint relative to ag mills with the combination of finer grind and a lower installed power density based on the lower density of the mill charge a typical ag mill has a lower throughput a lower powe

r draw produces a finer grind and has a higher unit power input kilowatt hours per ton than a sag circuit milling the same ore in the presence of suitable ore an ag circuit can provide substantial operating cost savings due to a reduction in grinding media expenditure and liner wear in broad terms this makes sag mills less expensive to build in terms of unit capital cost per metric ton of throughput than ag mills but more expensive to operate as a result of increased grinding media and liner costs sag circuits tend to be less susceptible to substantial fluctuations because of feed variation than ag mills and are more stable to operate ag circuits are more frequently but not exclusively installed in circuits with high ore density with iron ore being the classic example a small steel charge addition to an ag mill can boost throughput and result in more stable operations typically at the consequence of a coarser grind and higher operating costs ag or sag mills with low ball charges are often used in single stage grinding applications conversely sag mills produce coarser grinds than ag mills in equivalent applications based on their higher throughput and coarser grind relative to ag mills sag mills are more likely to be used as the primary stage of grinding followed by a second stage of milling ag sag circuits producing a fine grind particularly single stage grinding applications are often closed with hydrocyclones circuits producing coarser grinds often classify mill discharge with screens for circuits classifying mill discharge at a coarse size coarser than approximately 10 mm trommels can also be considered to classify mill discharge trommels however are less favorable in applications requiring high classification efficiencies and can be constrained by available surface area for high throughput mills regardless of classification hydrocyclone screen or trommel oversize material can be returned to the mill or directed to a separate comminution stage the addition of pebble crushing is the most common variant to closed circuit autogenous semiautogenous milling instead of direct recycle of oversize material the potential efficiency benefits both in terms of grinding efficiency and in capital efficiency through incremental throughput has long been recognized but the challenges of metal removal primarily grinding media passing mill grates were perceived to be a substantial hurdle with time and after successful pioneer operations operators and circuit designers became comfortable with the required magnetic separation and metal detectors bypass chutes required to protect pebble crushers from tramp steel even after steel ball removal had proved to be reliable pebble crushers installations were still thoroughly scrutinized at the design stage because of the additional cost and circuit complexity instead of foregoing pebble crushing completely though many operations chose instead to leave provisions

in the design for future pebble crushing expansions today every ag sag flow sheet conceived is likely to seriously consider the inclusion of a pebble crusher circuit for certain ore types particularly those with a chert andesite or other hard component that develops a critically sized material that constrains milling rates a pebble crushing circuit is almost an imperative for efficient circuit operation for other cases it is simply a technique to improve circuit capacity construction of an ag sag circuit without provision for later pebble crusher installation should have a persuasive economic justification important aspects of pebble crusher circuit design include preparation of a clean well sized and dry feed metal removal with additional protection via metal detectors and bypass surge capacity through bins or more costly a pebble stockpile sufficient capacity primarily a concern of large circuits where multiple pebble crushers are required to serve one grinding line design for bypassing crusher s during maintenance and evaluation of where to reintroduce the crushed pebbles back to the grinding circuit the standard destination for crushed pebbles has been to return them to sag feed doing so is often convenient based on conventional plant layouts however open circuiting the sag mill by feeding crushed pebbles directly to a ball mill circuit is often considered a technique to increase sag throughput pt freeport indonesia s operation has the ability to return crushed pebbles to sag feed per a conventional flow sheet or to sag discharge by combining with sag discharge and screening on the sag discharge screens top size control to the ball mill circuit feed is maintained while still unloading the sag circuit mosher et al 2006 aside from parameters fixed at design mill dimensions installed power and circuit type the major variables affecting ag sag mill circuit performance throughput and grind attained include feed characteristics in terms of ore hardness or competency feed size distribution selection of circuit configuration in terms of liner and grate selection and closing size screen apertures or hydrocyclone operating conditions ball charge fraction of volumetric loading and ball size and mill operating conditions including mill speed for circuits with variable speed drives density and total mill load the effect of feed hardness is the most significant driver for ag sag performance which is the case to a much greater degree than for compressive crushing plants variations in ore hardness result directly in variations in circuit throughput with ag circuits the variations can be substantial and occur suddenly the effect of feed size is also marked with both larger and finer feed sizes having a significant effect on throughput a number of case histories of ag mills have failed to consistently meet throughput targets because of a lack of coarse media compoundin

g the challenge of feed size is that for many ores the overall coarseness of the primary crusher product is correlated to feed hardness larger more competent material consumes mill volume and limits throughput to counteract this effect many circuits including troilus quebec kidston queensland australia ray arizona united states porgera papua new guinea granny smith western australia geita gold tanzania st ives western australia and kcgm western australia currently use or have used a secondary sag precrush circuit to further comminute primary crusher product prior to feeding to a sag mill the objective of sag precrush is to increase sag throughput occasionally secondary crushing is included in the original design but more often is introduced as an additional circuit to account for harder ore that proves to be harder than expected or becomes harder as the deposit is developed or as a capital efficient mechanism to boost throughput in an existing circuit such a flow sheet is not without drawbacks not surprisingly some of the advantages of semiautogenous milling are reduced in terms of increased liner wear and increased maintenance costs also precrushing can lead to an increase in midsize material thereby overloading pebble circuits and in challenges controlling recycle loads in certain circuits the loss of top size material can lead to decreased throughput it is now widespread enough to be considered a standard circuit variant and is often considered an option in trade off studies the concept of feeding ag mills with as coarse a primary crusher product as possible in order to maximize ag mill throughput is obviously at the other end of the spectrum to a more significant degree than in other comminution devices liner design and configuration can have a substantial effect on mill performance in general terms lifter spacing and angle grate open area and aperture size and pulp lifter design and capacity must be considered mill liner designs have moved toward more open shell lifter spacing increased pulp lifter volumetric capacity and tailored grate design to facilitate maximizing both pebble crushing circuit utilization and sag mill capacity mosher 2005 ag sag circuits can handle a broad range of feed sizes as well as sticky clayey ores which challenge crushing plants a range of circuit configurations have been developed and large single line plants can be designed with significant economies of scale and streamlined maintenance shutdowns and practices compared to crushing plants wear media usage is reduced and plants run at higher availabilities circuits however are more sensitive to variations in circuit feed characteristics of hardness and size distribution unlike crushing plants for which throughput is largely volumetrically controlled ag sag throughput is defined by the unit power required to comminute the ore to the closing size desired a higher degree of operator skill

is typically required of ag sag circuit operation as is more advanced process control to maintain steady state operation with different operator advanced process control regimes based on different ore types additionally very hard ores can severely constrain ag sag mill throughput in such cases the circuits can become capital inefficient in terms of the size and number of primary milling units required and in some cases require more total power input compared to alternative comminution flow sheets crusher hpgr circuits recently hpgrs have made broad advances into the nonferrous metals mining long established in the cement industry hpgrs first made inroads into diamond processing where rock fracture along grain lines favored a reduction in diamond breakage during comminution and in the iron ore industry penetration to the hard rock mining industry was slow and hampered by high maintenance requirements both for wear surfaces in general and high wear on the edge of rolls the intensity of the maintenance effort relative to other comminution machines required for a trial unit at the then phelps dodge sierrita operation arizona in 1996 was high significant product improvements since then however have increased wear life improved availability and decreased the overall maintenance effort required incorporation of studs on the surface of rolls to allow formation of ag wear surfaces as well as implementing edge blocks of a long wearing material for edge protection have allowed hpgrs to break into the mainstream of mineral processing these wearretarding innovations were the focal point of a full scale trial at lone tree in nevada united states seidel et al 2006 successful completion of this trial marked somewhat of a turning point for interest in hpgrs for hard rock applications manufacturers have also paid special attention to the bearings wear surfaces and the handling of tramp metal through the rolls to improve operational reliability reduce maintenance and obtain longer service lives availabilities are now such that aside from rolls change outs akin to a mill liner change the unit rarely controls circuit availability the wear on a roll s surface is a function of the ore s abrasiveness increasing roll speed or pressure increases wear with a given material studs allowing the formation of an ag wear layer edge blocks and cheek plates are depicted in figure 14 2 9 development in each of these areas continues including profiling of stud hardness to minimize the bathtub effect wear of the center of the rolls more rapidly than the outer areas low profile edge blocks for installation on worn tires and improvements in both design and wear materials for cheek plates in hard rock metals mining applications hpgrs are currently used in tertiary and quaternary crushing applications as well as in secondary pebble crushing in some respects hpgrs replace crushers as a unit operation however from a proce

ss standpoint hpgrs produce a product with substantially more fines for a given p80 than a crushing circuit in this regard the size distribution of an hpgr circuit is much more similar to the product of a sag circuit than a conventional crushing circuit reducing the required amount of power in the ball mill circuit relative to a crushing circuit also an hpgr represents a much larger installation of power in a given footprint relative to tertiary crushers an hpgr has up to a 5 mw power installation per unit compared to 750 kw for a single large cone crusher hpgrs have also been cited as crushing more efficiently than rotating mills as well as generating residual cracks that improve efficiency in subsequent milling operations this effect is material and operatingcondition specific as a result the number of material handling units feeders conveyors screens chutes can be sharply reduced compared to a conventional crushing plant and plant layouts are also much more straightforward freeport mcmoran s phelps dodge at the time of construction cerro verde operation in peru was a groundbreaking installation because the combination of secondary crushing using mp1000s tertiary crushing using hpgrs and screens replaced what would have been more typically a large sag mill feeding a multiple ball mill circuit vanderbeek et al 2006 this was a significant step as it presented an alternative to conventional crushing plants or autogenous semiautogenous milling for primary milling applications see figure 14 2 10 newmont mining corporation s consideration and ultimate selection of hpgr comminution for its boddington project in western australia widely documented in the literature has now been commissioned with a similar comminution flow sheet to the cerro verde flow sheet and also employing four units measuring 2 4 1 7 m although the primary focus of this section is to discuss hpgrs as they have been combined with crushers and screens in primary milling roles the versatility of the hpgrs in grinding applications must be noted though limited in top size they can accept based on nip angle and resultant stud breakage leading to selection in tertiary rather than secondary roles the machines are quite versatile because they have been installed in tertiary quaternary second stage pebble crushing roles and a number of finer grinding applications in the fine grinding role hpgrs are widely used in pelletizing operations one area not currently applicable to hpgrs is secondary milling in a wet process in the cement industry it is common practice to use hpgrs in closed circuits with a dry air classifier for finish grinding s kirsch personal communication however wet classification and subsequent dewatering does not compare favorably to the use of ball mills for wet processes most commonly used in mineral processing a summary of the broad range of comminution applications of hpgrs is presented in table 14 2 3 se

condary milling for those circuits requiring secondary grinding most circuits aside from heap leach operations and single stage ag sag circuits the use of wet ball mills has remained the standard unit operation most such circuits use ball mills in closed circuit with hydrocyclones however closing circuits with fine screens is becoming more common particularly in the iron ore industry valine et al 2009 the majority of applications grind from a typical range of f80 values of 1 13 mm to p80 values of 300 37 m pebble mills are also used in a number of applications ball mills as large as 8 5 m 28 ft in diameter are now being constructed as with sag mills twin pinion and girth gears can be considered for mills with power input up to at least 15 mw generally mills with a diameter of 7 3 m 24 ft or less wraparound drives are the standard for larger mills in general given a mill of suitable power and operated with an appropriate feed size reduction ratio media size selection and the use of efficient classifiers accurately predicting grinding circuit response is relatively straightforward these principles apply for either wet or dry milling aside from the widely used bond equation discussed later in the circuit design section bond used eight factors to correct for the power required for grinding in different operating scenarios rowland 2002 bond s work to develop criteria for use in designing grinding circuits has proved to be enduring and useful even with the contemporary availability of powerful simulation tools although the utility of correction factors for quantitative contemporary use in formal design is debatable using the factors for qualitative evaluation of aspects affecting grinding efficiency can have value two of the eight correction factors apply strictly to rod milling with the remaining six to ball mill grinding of greatest interest for current applications are corrections for coarse feed with the definition of what is excessively coarse being a function of ore hardness open circuit milling vs closed circuit grinding to a p80 finer than 75 m and excessively low reduction ratios one correction factor was developed to correct for inefficiency of large ball mills many mill designers today believe that this factor had more to do with inefficiencies of classification in large hydrocyclones in early mill installations than in any actual milling inefficiency the last factor for ball milling applies to dry grinding in summary although using these factors as originally developed for design may not be relevant using them qualitatively to assess efficient operating conditions can be a useful tool by far the largest tonnages processed through secondary milling today are classified using hydrocyclones specialized low tonnage operations may use screens selected contemporary operations still employ screw classifiers and dry operations generally use cyclones a detailed discussion of the appr

opriate design for recirculating load is beyond the scope of this chapter milling efficiency typically increases with closed circuit operation and will often increase to a certain level with increasing circulating load after a certain point the grinding efficiency declines with increasing recirculating load also at a certain point the power required for pumping for increased recirculation is not offset by improved grinding efficiency some circuits perform grinding in one step others in multiple stages though rarely exceeding two and occasionally with beneficiation integrated into the grinding circuit the use of gravity concentration integrated into the mill recirculating load is common for the recovery of gold flash flotation is often seen in sulfide mills and recovery efforts after each stage of grinding are common for recovering platinum group metals and certain iron ore operations relying on magnetic separation tertiary milling in the context of this paper the term tertiary milling is used to describe concentrate regrind grinding conducted for materials handling iron ore pelletizing and pumping of mineral concentrates for example and fine grinding for specialized metallurgical applications the border between a typical tertiary milling application and fine grinding is not distinct however as the methodologies for sizing change substantially at nominally 37 m 400 mesh and switching from conventional sieving practice toward subsieve analysis and alternate techniques at finer sizes designating grinding to a p80 37 m as fine grinding is a useful designation most grinding circuits are developed with the concept that the mill feed will be ground to an economic optimum such that further grinding expense in terms of energy and media wear is not justified based on the additional liberation and recovery of additional value units in many cases grinding 100 of the bulk mill feed to this size represents the optimum in terms of overall project economics and simplicity in some circuits it is desirable to have multiple stages of liberation and beneficiation since physical mineral separation processes have an optimum size range for efficiency and essentially all lose efficiency at very fine particle sizes this sequence can allow recovery of value minerals as they are liberated and before they are overground particularly in flotation value minerals are often recovered to rougher concentrates when only partially liberated further grinding of such concentrates is employed to increase liberation and facilitate production of concentrate at a salable grade aside from liberation and recovery considerations material handling of concentrates may dictate further size reduction examples include grinding of concentrates to allow pumping without pipeline wear or grinding of iron ore concentrates prior to pelletizing operations the field of options for fine grinding is diverse with many originally developed to fulfill a specif

ic need and subsequently marketed although not all operations require this stage of grinding for those that do it is often a critical unit operation although the mills are smaller in size than mills for primary and secondary grinding unit energy input ranging from 5 to 40 kw h t for tertiary mill feed often exceeds the total unit energy requirement for the bulk feed in broad terms the equipment field can be classified into rotating mills stirred mills and a general category of specialized mills in general the stirred mill category has the ability to grind finer with a higher efficiency than rotating mills specialized mills can cover a broad range of sizes but tend to be more focused to specific applications options for fine grinding are summarized conventional rotating mills ball mills stirred mills vertical shaft mill screw type also referred to generically as tower mills after an early manufacturer the japan tower mill company similar mills are now made by several manufacturers with perhaps the most common being the descendant of the original mill design by nippon eirich co ltd by the trade name of vertimill marketed by metso lynch and rowland 2005 vertical shaft mill pin type one trade name is the metprotech mill marketed by bradken another with a higher tip speed is the stirred media detritor smd marketed by metso horizontal shaft mill the isamill has made broad inroads into fine grinding other specialized mills jet mills vibratory ball mills roller mills centrifugal mills these machines are classified primarily by their mechanical configuration lichter and davy 2002 provide another useful distinguishing feature for stirred mills the charge motion they cite a distinct difference between mills in which the charge is stirred tower mills and some pin mills and those in which the charge is fluidized some pin mills and isamills indicating that stirred mills are more efficient with coarse hard feeds and that fluidized mills are more efficient with relatively finer softer feeds conventional rotating tumbling ball mills are often used to grind concentrates down to sizes of nominally 37 m common applications include tertiary milling with multiple stages of grinding in regrind applications and in preparation of iron ore concentrates for pelletizing stirred mills are often used for fine grinding mills in this category are sometimes more commonly referred to by their trade names stirred mills offer a higher power density kilowatts per cubic meter which combined with the fact that they are filled with media as opposed to tumbling mills where the charge volume is nominally 35 of the total mill volume means that these mills are compact in terms of installed power tower mills are occasionally used as way to retrofit additional grinding power with a small footprint and low capital for civil works in this capacity they are often used for additional secondary or f

or tertiary grinding they are widely used in lime slaking and in concentrate regrind applications pin mills of either nonfluidizing or fluidizing tip speed are generally the next mill considered at progressively finer grinds to a nominal grind size with a p80 of 5 m isamills are established as a versatile fine grinding mill having been commercialized after being developed at mount isa mines queensland australia the isamill has demonstrated an ability to grind sizes with a nominal p80 of 5 m isamill models with up to 3 mw of installed power have broadened the mill s usage to regrind applications where higher tonnages dictate larger mill sizes burford and niva 2008 grinding energies required sometimes judged by the calculation of an operating work index wio at coarser sizes and at finer sizes by the specific energy input required to attain the target grind often increase substantially as grinds get progressively finer this can be due to a combination of factors including higher energy required as the mineral grain size is approached the number of micro imperfections in the particle structure to exploit decreases and not insignificantly the efficiency of classification is reduced the required energy inputs can vary substantially based on mill type media type and the efficiency of classification employed tests should be conducted with a variety of machines and with a variety of media types to measure the specific energy input required to attain the target grind media types and sizes are summarized in a subsequent section media and liners but it is worth noting that the energy efficiency of fine grinding is strongly influenced by the media type selected gao et al 2001 additionally based on the high unit power input the integrity of media is also critically important at very fine grind sizes and depending on the material ground the energy input can modify the reactivity of the material and this can be a consideration in the chosen grinding methodology there are number of specialized fine grinding applications particularly in the fields of industrial minerals powders and the like fine grinding in specialized applications is often conducted dry and using either ceramic media or autogenously to avoid iron contamination examples include jet milling centrifugal mills including the nutating mill vibratory ball mills and roller mills the reader is directed toward manufacturers and the body of literature for detailed coverage of these applications no discussion of ultrafine grinding would be complete without touching on the method of particle size analysis at extremely fine sizes particle size determination is not always straightforward the measurements used are often indirect measurements from which particle size is estimated with different techniques resulting in a different size being inferred additionally the intense energy input can change the surface chemistry of the ground material which

can have effects including agglomeration or increased galvanic activity for example the use of specialized electroformed nickel subsieves which offer precisely sized apertures for sieves at sizes less than 20 m is restricted for copper concentrates because of rapid galvanic action with the sieve itself techniques often used include conventional sieves specialized subsieves generally available at 15 10 and 5 m made of textile or electroformed and photo etched nickel and indirect methods which employ material properties techniques rely on measurements of settling velocity by means of elutriation sedimentation or centrifugal sedimentation light dispersion characteristics or both examples include the warman cyclosizer the malvern mastersizer laser analysis microtrac s laser unit quantachrome s microscan units as well as others including the coulter counter and surface area techniques regardless of technique it is important to use the same one throughout a study it can also be worthwhile to confirm sizing calculated via a certain technique using microscopy either direct or in combination with image analysis software though this is unlikely to have any benefit for a circuit in operation it can be useful in comparing size distributions of different machines comparing numbers derived using different techniques should be performed only with great caution special applications this chapter has focused largely on the comminution means employed by the majority of mineral processing operations as judged by the tonnages processed a number of special considerations apply either to specific industries minerals or focused applications comminution to develop a specific size distribution rather than the simpler approach of assessing the size distribution through the use of a single data point and a certain shape of particle rather than using liberation as the goal along mineral boundaries and beneficiation steps integrated in the comminution process are all often considerations beneficiation during comminution is often employed the concentration of heavier specific gravity minerals in hydrocyclone feed or more often in hydrocyclone underflows is well documented and is used as a preconcentration step in flash flotation and gravity concentration units in a number of grinding circuits the addition of reagents and lixiviants to grinding mills is commonly practiced either for conditioning commonly in flotation or to increase leach residence time in certain applications typically aggregate or industrial minerals the particle shape resulting from comminution can be important and is a criterion in the selection of certain types of comminution machines examples include vertical shaft impact crushers or gyradisc marketed by metso crushers which are examples of machines sometimes selected for comminution to a specific particle shape similarly in certain applications minimizing fines generation while attaining a gr

ind below a maximum particle size is important when this requirement is overriding certain grinding equipment listed in order of coarser grinds and lower fines generation can be used center discharge rod mills end peripheral discharge rod mills overflow rod mills and grate discharge ball mills in quarrying and certain other applications maximizing yield in certain size fractions preferentially over other size fractions can add significant value for specialized applications such as this close consultation with vendors and thorough testing are recommended dry grinding is an important albeit niche application for any pyrometallurgical operation dry grinding is an obvious consideration newmont s carlin mill and barrick s goldstrike mill in nevada both have roaster facilities fed by krupp double rotator dry mills the double rotator mills consist of a twin chamber mill with sides for coarse and fine milling with both sides discharging to a common air classification system operation of goldstrike s system is well described by thomas et al 2001 newmont s minahasa gold operation in indonesia employed an aerofall sag mill followed by dry ball milling prior to decommissioning the ioc s carol concentrator in labrador canada formerly milled iron ore for gravity concentration using dry aerofall ag mills chung et al 2001 demonstrated substantially better energy efficiency in grinding using dry milling energy costs for drying particularly for a wet downstream process contributed ultimately to the full conversion of the carol concentrator to wet ag richards bay minerals uses an aerofall mill for dry grinding of titanium slags dry grinding with air classification should be considered in applications where the followon process is pyrometallurgical and drying would be required anyway minimization of fines during comminution is critical with air sweep and classification assisting in this regard or an ore s aggressive soluble components make dry grinding more cost efficient than construction of a wet milling circuit made from corrosion resistant materials a number of other specialized grinding units are referenced in this chapter based on their comminution range primary secondary or tertiary grinding media selection to avoid contamination is also a consideration referenced later in the media and liners section finally certain applications find that media selection can impact metallurgical performance through the interaction of redox potential and dissolved oxygen for certain applications ag high chrome or inert media can be preferred ore characterization circuit design and circuit selection many ore bodies can be successfully processed using a range of comminution circuit configurations in some cases however certain types of circuits are favored over others ore characteristics competency density work index abrasivity chemical composition plant capacity downstream processing requirements mine l

ife capital costs operating costs delivery time for major components and plant location are all factors in process selection given ore characterization data alternative circuit designs to accomplish the required comminution can be developed capital and operating cost estimates for each circuit alternative allow for trade off studies ore characterization circuit design and circuit selection are summarized briefly with each addressed in depth in the broader literature before an effective study culminating in a circuit design can be commissioned a sufficient knowledge of the deposit and the mining method and sequence must be defined to allow development of a sampling design sampling of a greenfield deposit via drilling can be an expensive undertaking and collection of a bulk sample for pilot testing if desired or required often exceeds the cost of the testing itself knowledge of the overall range of comminution characteristics within a deposit can be useful but as a guideline the testing effort based on ore type should be proportional to the representation in the overall reserves if ore types have divergent characteristics and or will be mined at different periods in the mine life it may be prudent to consider construction of a modular plant in some cases blending of samples may also be considered if so blending should be done in a fashion consistent with the planned mining sequence for instance blending two samples that will be milled together may make sense however compositing of an entire drill core consisting of different horizons almost never makes sense and typically results in misleading data many designers prefer to blend the results of testing mathematically of individual ore types rather than blending a sample for testing both approaches have advantages and disadvantages obviously testing for circuit design should be performed on ore and not waste too many projects have had initial characterization done on waste with waste samples being more readily available efforts in reasonable ore body definition and the planned mining sequence to allow development of a rational sampling program is almost always a good investment ore characterization bench scale breakage characterization tests are the basis for circuit design such tests fall into three broad categories 1 grindability 2 single particle breakage and 3 standard materials pilot scale testing has long been considered the most reliable way to perform test work however the cost of testing the difficulty in getting representative samples advances in design methodologies and greater experience with newer comminution circuits have tended to decrease the importance of pilot testing typically grindability tests rely on 1 a known size distribution of feed 2 the application of a known or measured amount of energy to 3 a measured feed rate and 4 a measured product size distribution based on these four items an index is then calcu

lated the traditional bond work index is perhaps the best known but similar indices are calculated by other tests grindability tests that reach a locked cycle equilibrium or steady state continuous operation are a better indicator of actual mill performance than batch tests the design methodology for hpgrs using a bench scale test such as polysius labwal test is largely analogous to this approach except that primary index is a specific throughput with measurements for specific energy input and the comminution outcome of that energy input also measured single particle breakage tests typically endeavor to quantify breakage as a function of energy input regardless of the test using a breakage energy regime as close to the actual milling conditions as possible is preferred single particle breakage tests provide fundamental data critical for a simulation effort traditional grindability tests are unable to generate this data unfortunately tests of this type cannot reliably predict the steady state mill load for autogenous semiautogenous milling the composition of the mill load defines mill performance some comminution machines use conventional civil engineering measurements of strength as a direct basis for design such as unconfined compressive strength this is particularly true for crushers aside from the comminution tests presented other indicative data can be used to map the overall comminution profile of an ore body particularly rock quality designation and point load testing such tests though not directly usable for design can prove beneficial in determining the distribution of ore regimes in some cases correlations to comminution tests using these inexpensive and quick test results can prove constructive in addition to breakage tests because media and liner costs represent significant operating costs tests to determine wear rates are often conducted the pennsylvania abrasion test often called the bond abrasion test is the most widespread however plant correlation with test results is often not particularly good because the test is conducted dry it does not measure corrosion a significant source of media consumption in wet grinding circuits abrasion data can also be obtained using specialized equipment such as polysius atwal test for hpgrs or by direct measurements of wear in pilot testing a good approach is to use these tests to determine the relative wear of an ore type to others then to benchmark with similar circuits and similar ores in operation pilot testing involves larger scale steady state testing at a range of operating conditions including a feed size similar to the anticipated design to evaluate potential design and operating scenarios the response is then either directly scaled from pilot sized to full scale equipment using either a volumetric or a power basis pilot testing offers a high degree of confidence but can be inadequate in actual utility because of the limited number of v

ariables that can be reasonably tested the amount of sample required and the logistics and expense of sample acquisition a number of tests have been developed over the years with various strengths and weaknesses this is partly a reflection of proprietary design approaches by various companies and designers and perhaps also a reflection of the never ending quest for a test that can fully define the comminution response of an ore type quickly with little sample and at low cost a summary of selected ore characterization tests are presented in table 14 2 4 most of the tests summarized are conventional comminution tests relevant for primary and secondary grinding operations bench scale and pilot scale hpgr tests are included with direct consultation with manufacturers recommended for tertiary grinding tests most of which rely on direct scaling based on unit power input all testing to include the benchmark of pilot testing presents trade offs in terms of data developed with the time effort and cost of sample collection and testing in many cases factoring how representative the samples tested are of the ore body presents its own challenges fundamentally a large number of poorly selected samples cannot overcome a lack of sample design data developed from testing samples that poorly represent the intended population are at best useless and at worst damaging in that they can contribute to incorrect designs or performance projections ore testing should also consider the methodology of the subsequent circuit design traditional power based design methods are generally based on work indices measured during grindability tests then these data are typically scaled based on power input simulation circuit design generally relies on the results of single particle tests a combination of computational mathematical modeling circuit simulation and use of traditional work indices are generally used in current practice additionally to broaden the applicability of certain tests some designers have developed correlations some with defined bands of variance that allow prediction of singleparticle test parameters from grindability tests particularly the smc test morrell 2006 and vice versa a sound approach for ore body comminution is an integrated test program using both well designed steady state tests and single particle breakage tests these tests may be complemented with other tests to map a deposit and with equipment specific tests i e bench scale tests for specific comminution machines supplanting or replacing portions of the test program simulation of bench and pilot scale mill tests via discrete element modeling or other high definition models is well within the scope of today s technology perhaps single particle ore characterization in conjunction with highdefinition modeling and scaling of steady state bench scale tests will become the norm for future design work circuit design given a design throughput grind size ta

rget and ore breakage characteristics for an ore body most appropriately evaluated as a range and ideally based on a preliminary mine plan a number of circuit designs can be prepared for evaluation based on circuit specifications ore characteristics downstream processes location operator preferences and scoping and prefeasibility studies a short list of comminution circuit types can generally be developed that merit development of a process design fundamental to achieving size reduction is the application of power applying power at peak efficiency reduces unit operating costs and ensures efficient utilization of capital the machines most widely used to apply comminution energy have been discussed in previous sections the machines described apply energy via two mechanisms 1 directly through devices that break particles through compressive force crushers for example 2 initially to impart motion to a grinding charge a portion of which energy is then translated into particle breakage for the first case the machines are generally controlled by volumetric throughput the amount of power drawn to impart compressive forces is a direct function of the hardness of the material being broken by compression as a result the unit power can vary based on material type at a fixed throughput for a given material type and circuit machine configuration the size distribution of the product tends to be fairly consistent in contrast application of power in tumbling mills is somewhat more complex once power is drawn in a rotating mill a portion of the power is applied to the charge resulting in breakage through the mechanisms of impact abrasion and attrition with a steady state charge the power imparted is constant regardless of material hardness or throughput in overflow mills the throughput through the mill can be readily varied and there is a degree of control over the unit power input a sag mill s throughput though is volumetrically controlled based on the mill s discharge grates as throughput decreases in a tumbling mill with a constant power draw the unit power increases and a finer grind results in both cases though the fundamental driver of the amount of breakage achieved is the amount of power delivered to each unit mass of material therefore in designing for volumetric throughput machines it is essential to have a firm understanding of throughput and how much power will be drawn under the operating conditions used in tumbling mills it is essential to understand the power draw that the mill will achieve and the range of throughput that can be expected under such conditions historical articles on comminution circuit design have presented tabulations of power draw estimates for mills and machines of a given size indeed substantial effort by mill designers went toward understanding how the power installed on a mill would be drawn in numerous case studies the installed motor power was inadequate for operati

on at the desired operating conditions conversely motors with rated power outputs larger than could ever be drawn have been installed leading to confusion on the part of designers and operators who believed that this power should be available for grinding much of the art of power estimation was converted to direct computation through the development of a mathematical model by morrell 1996 his model is an outcome of modeling the physics of the power required to impart charge motion and lift fundamentally a mill s power draw is the sum of the power applied over time required to impart kinetic and potential energy to the mill charge the power draw of a mill is a function of 1 the charge shape including the volumetric filling of the mill the charge density amount of slurry pooling etc 2 charge position particularly top or shoulder and bottom or toe position of the charge and 3 the mill speed it is evident to practitioners that these three fundamental variables of mill power draw are interrelated for example charge shape and position are impacted by changes in mill speed the critical question in design is the efficient application of power as with ore characterization there are a number of design techniques the use of power based simulationbased and direct scaling benchmarking with existing plants or preferably a combination of techniques can be used to develop circuit designs for evaluation regardless of methodology a key metric of the design process is the design power input kilowatt hours per ton for each ore type designers often evaluate and sometimes rank circuits in terms of unit power input mill operators more commonly reference circuit performance based simply on throughput at target operating conditions gnizing it is essential in design mcken et al 2001 power based designs rely on circuit configurations that will deliver the targeted energy input as such sound use of computational methods tables or benchmarking to ensure that the designed mill motor configurations will draw and apply the target power is essential aside from proper equipment sizing the most critical pitfall of a power based design is ensuring that the design is achievable in terms of the assigned power split for each grinding stage many designs have relied on arbitrary power allocation between grinding stages when commissioned it was found that throughput constraints between units would either prevent the design throughput from being achieved most often while attaining a finer grind than designed or vice versa simulation based designs use a set of mathematical models to calculate the anticipated response of a circuit under a set of operating conditions a number of commercial and proprietary simulation packages are available major equipment manufacturers often use in house simulation models many of these packages are based on a combination of empirical and fundamental models and correlations with observed pl

ant performance simulation is a powerful tool that offers the ability to evaluate circuit response for a range of operating conditions and of a given circuit for a range of feed types and readily compare a number of circuit options it can also provide a great deal of useful process information in terms of the volumes and size distributions of intermediate flows that is unavailable from power based designs notably simulation offers the ability to evaluate the impact of classification on the comminution circuit something that cannot be done with power based techniques in addition to computer simulation to model steady state comminution circuit performance discrete event simulation can be useful in evaluating overall circuit response with respect to linkages with discontinuous truck dumping or semicontinuous such as a crushing plant with periodic stops due to metal detects processes when conducting a design exercise it is critical to validate that the circuit being modeled is within the boundaries of the underlying model although this is generally the case for typical processing plants the use of models outside of the operating window for which the model was developed can be misleading calculating the operating work index of circuits developed using circuit simulation can provide a crossreference reality check and can be useful for evaluating the relative efficiency of circuits direct scaling is employed for a number of comminution machines and depending on the design approach can be based on achieving a target unit energy input kilowatt hours per ton or on a specific throughput determined in laboratory testing for single unit operations and when used in conjunction with a good database of existing plant operations robust designs can be produced however examples frequently emerge of direct scaling and commissioning of plants that could not achieve the design throughput but are capable of grinding finer than target grind or vice versa that is they exceed nameplate throughput but do not achieve the target grind regardless of the design technique benchmarking is a useful tool frequently comparison of ore breakage characteristics and circuit design parameters can yield a list of relevant similar operations in fact although this approach may not be bankable it is widely used for low risk self financed expansion projects or when scaling the anticipated throughput of a new deposit in an existing mill using it as the sole basis for design however can introduce a higher degree of uncertainty in the design outcome an additional consideration is the question of how to integrate the stages in a comminution circuit primary grinding units whether they are crushers ag sag mills or hpgrs arrive at their unit power input largely based on the volumetric throughput at which they are operated for ag sag mills at a given operating condition of speed and load the power input is fixed so the only variable is feed rate

which will change based on ore hardness and size distribution for crushers or hpgrs though the feed rate is fixed based on volumetric throughput at a given aperture and the power drawn varies as a function of the ore characteristics regardless a design methodology that considers designing the primary milling stage based on the required throughput with the secondary stage based on the amount of power required to finish grind the design throughput is more successful than a methodology that counts on a set amount of power input in the primary grinding stage for power based designs determining the appropriate and efficient way to input the design grinding power is the core question for simulation based designs determining the amount of power drawn under a given set of simulated conditions is required cross checking this value with calculated power requirements or benchmarking with operating circuits is wise for scaled designs it is essential to ensure that power input scales similarly to volumetric throughput or the unit power inputs will not match and consequently the desired size reduction may not be attained at the target throughput most modern circuit designers consider the various design techniques to be complementary although various designers may have preferences a robust design will consider several different approaches a higher degree of confidence can be assigned to a design that is confirmed based on traditional power based approaches with various operating conditions investigated and evaluated by simulation and which benchmarks and scales favorably with similar operations although any of the techniques can be used independently the most robust design will eventuate through the combination of these techniques circuit selection staged crushing plants are favored in cases of low tonnage operations where little fines generation is desired and a dry coarse product is required heap leaches or in moderate tonnage operations with hard ore plants that will be fed sticky ores with large amounts of tramp metal or high tonnage operations are generally less favorable for a traditional crushing plant a benchmark for annual run time including mechanical availability and all other factors for a well maintained and run modern crushing plant is 85 lower availabilities are typical in more demanding applications semiautogenous milling is favored for high capacity plants soft to moderately hard ore types sticky or clay bearing ores and ores containing significant amounts of tramp metal or magnetite ores with a competent fraction particularly higher density ores can be amenable to autogenous grinding this is particularly true for small to medium throughputs with a long mine life because ag mills tend to have a lower unit power input for a given mill size they can be more expensive to construct but much less expensive to run when compared on unit cost basis compared to a sag mill circuits designed for autogeno

us grinding are often designed to accommodate semiautogenous grinding either for operational flexibility or with the anticipation of milling future ore bodies a case study of an operation with a versatile single stage ag sag circuit is brunswick larsen et al 2001 when using the largest mills available a single sag mill often produces feed for secondary grinding two or more ball mills in a typical sag ball mill circuit the sag circuits should generally be designed for the target throughput with the ball mill circuit designed to achieve the target grind based on ore breakage characteristics feed size grate aperture and circuit closing size the ability to shift the transfer size substantially between the sag and ball mill circuits is often limited though exposure to significant downtime for a failure in a single line is often considered a drawback sag plants have demonstrated annual run times of 95 hpgrs are considered favorably as a circuit alternate to autogenous semiautogenous milling such a circuit must be designed in conjunction with secondary crushing to reduce rom ore to a size suitable for hpgr feed hpgrs in closed circuit with screens then produce ball mill feed hpgrs were selected for the cerro verde cu mo project commissioned in 2006 and have also been specified for the boddington project particularly in the case of hard and abrasive ores hpgrs can offer operating cost benefits relative to sag depending on the delivery time for large sag mills and often its accompanying wraparound electric motor delivery and construction of hpgrs may be executed more quickly than for a sag mill circuit the largest hpgrs can be reasonably sized with a single large crusher to feed one ball mill for typical circuits a crusher and closed circuit hpgr grinding line feeding a ball mill will typically have lower availability and more unit operations than a similar sag ball mill line substantial progress has been made in terms of availability and reliability preventive maintenance periods are required for cheek plates and inspections but hpgr maintenance is rarely the controlling factor in overall circuit maintenance aside from rolls changes akin to a mill reline and typically occurring between 2 000 and 15 000 running hours depending on the material and application more often circuit availability is controlled by belt chute screen and conventional crusher maintenance circuit availabilities between a traditional crushing plant and a sag mill should be expected for circuits where a secondary crusher screen hpgr replaces a sag mill with 90 run time a nominal benchmark aside from the fact that the degree of ancillary equipment belts transfer chutes additional pumps etc can increase the maintenance demand relative to a sag plant the additional unit operations also drive up capital costs circuit selection is typically based on the calculation of the net present value of various circuit options implicitly incorpora

ting both differences in capital and operating costs and in performance between plant types qualitative considerations include location operator preference equipment lead times and the like selection of motor and drive types for tumbling mills is a specialized topic in itself for larger mills nominally more than 7 2 m 23 6 ft in diameter for ball mills and more than 9 8 m 32 2 ft for sag mills gearless wraparound motor and geared pinion bull gear combinations drives are normally compared gear manufacturers indicate that geared drives with up to 10 mw per pinion are feasible hankes 2001 failures with both large wraparound motors and large gears have occurred in the industry and engineering trade off studies will likely continue for those mills where either geared or wraparound motor drives could be used media and liners table 14 2 5 summarizes the typical media sizes and types in the broadest use for mills discussed in this chapter contemporary practice in tumbling mills is to add a singlesize makeup ball mass to maintain constant power draw ball charging should be done on a daily basis or more ideally on a shift wise basis some plants with a mechanized ball charging system typically for sag mills have adopted continuous charging at a rate matching the mill s steel consumption to facilitate operating at peak conditions as steel media most often have a hardness profile with a softer center once balls reach a certain size they tend to wear very quickly the use of a mixed size makeup ball charge has been repeatedly demonstrated in laboratory studies to offer incremental efficiency benefits but the benefits have been difficult to document at plant scale selected operations have adopted mixed sized makeup ball charging for sag mills but it remains uncommon for ball mills there is little reason to believe that the steady state media size distribution resulting from the wear rate of the makeup ball size corresponds to the optimum ball size based on the mill s feed and target grind the concept of selecting a makeup ball size based on the coarseness of feed has long been recognized taggart 1945 in general a mixed makeup ball charging regime improves grinding efficiency with greatest potential benefit for single stage milling applications with large size reductions mcivor and weldum 2004 suggests that the benefits from graded charges extend to pebble mills and hypothesizes that this is a fundamental reason for improved efficiency relative to laboratorymeasured work indices at the empire mill nonetheless most operations tend to use a single sized makeup ball size for reasons of convenience in practical terms for grate discharge mills there is essentially zero grinding media smaller than the grate aperture at any given time for overflow mills the lower media size is constrained by the size at which media flow out of the mill which in turn is effected by volumetric throughput pulp density and med

ia size and density the use of ore as ag media was reviewed previously in the ag sag circuits section pebble milling a form of secondary milling most often uses oversize material extracted from the primary milling circuit as media this is most often done in applications with higher ore densities and can represent a substantial savings in milling costs with ore densities lower than steel densities larger mills are often required and some degree of flexibility may be needed when the generation and consumption rates of ag media do not match the use of recycled sag scats worn and broken media recovered via magnets from sag mill grate discharge in ball mills has grown in application the balls are direct charged to ball mills as a replacement for new grinding balls the larger soft core of sag media as a result of larger size and requirements for ball durability means that the recycled media wear more quickly than a typical like sized new ball mill ball discussion of the use of alloys cast forged or other types of media or whether to effect improved wear life through surface hardness hardness profile or improved resistance to corrosion is outside the scope of this chapter liner innovation has contributed substantially to comminution performance nowhere has this been more evident than in semiautogenous milling after early work that tended to focus more on maximizing life current best practice is focused on maximizing total productivity over the life of a liner set generally this means development of a shell lifter set that facilitates within the constraints of original mill shell drilling a quick startup period with little to no lifter packing and for which full power draw can be maintained over the planned life of the liner set critical aspects of shell lifter design include lifter spacing height and face angle in general common practice has evolved toward operation with liner sets that are more open have greater spacing between lifter bars relative to lifter heights in general terms this philosophy leads to less liner packing the shortening or elimination of a break in period with new liners and higher mill performance over the liner set going too far can lead to short liner life offsetting performance benefits and in extreme cases insufficient charge lift to achieve baseline mill performance design of pulp lifters and discharge grates has been an area of intense effort by mill operators particularly large high volumetric throughput mills because of the hpgr s role in replacing duties performed by mills hpgr liner wear is often relevant in comparison with liner and media cost cost savings relative to the overall costs of liner and media consumption in a rotating mill can often be realized relative to mill liners and grinding balls hpgr wear surfaces are much lower volume and a more highly engineered product the unit cost of media wear generally in terms of dollars per metric ton at equivalent kil

owatt hours per ton is a good metric for comparison the use of liner handlers to reline mills is now standard practice and the use of larger machines has facilitated consolidation of liner parts the world s current largest liner handler is in use at pt freeport indonesia the machine s 7 250 kg capacity has allowed consolidation of parts and reduction in the number of units requiring handling during a reline progressive refinements with each pushing the then existing technology s capacity in terms of casting integrity and liner handlers now facilitates the use of 40 fewer liner pieces than the original design in pt freeport indonesia s 10 4 m 34 ft and 11 6 m 38 ft sag mills the use of a sound integrated and well resourced maintenance approach can lead to annual run times in the 95 96 range in even the most challenging environments such techniques can also be applied to ball mill liners or to primary crusher concave relines e g with larger castings reducing the number of rows from a typical five row installation to three row looking forward this chapter has summarized a range of comminution options for mineral processing applications although machine design performance maintenance and other aspects have been touched upon the focus has been on the process since the last comminution chapter in the previous edition the range of equipment options for mineral processing comminution plants has broadened ag sag circuits as well as circuits incorporating hpgrs are in wide use in particular ag sag mills and hpgrs have blurred the distinctions of using a certain machine for a certain feed size the low fraction of total power input in the crushing and grinding process that actually goes toward size reduction with the balance being dissipated largely as heat and noise has long been cited kelly and spottiswood 1995 despite intense research efforts with both public and private sponsorship no viable commercial processes with markedly better efficiency are on the horizon with increasing focus on energy input from both high energy cost aspects but also considering the embodied energy and greenhouse gas impacts of various products these efforts will continue interestingly in the case of energy input in mineral processing applications the effects of increased energy input for liberation are offset by the benefits of improved resource recovery to the extent that the marketplace can accurately assign a value to the material produced and a cost to the negative impacts assessing the benefits of increased comminution to affect increased recovery versus the costs of higher energy consumption should be clear the focus on comminution as often the highest cost category of mineral processing both in terms of capital and operating is likely to increase given continuing demand for resources and increasing awareness and attempts to mitigate the impact of the human carbon footprint at first glance decreasing the comminution e

ffort may seem to be an easy target to achieve both cost and carbon footprint benefits in many cases though the incremental comminution effort required for the incremental value recovery is less than the energy requirement to recover a like quantity of values from a freshly mined ton of ore from an energy perspective increasing comminution will in many cases be favorable to mining and milling new ore tons thorough study and cost quantification are required for a true optimization of the overall process gravity concentration is one of the oldest processes of separating minerals and is aimed at separating heavy minerals from lighter gangue gravity concentrators have wide applications in the heavy minerals industry for the recovery of heavy minerals such as ilmenite rutile leucoxene and zircon from the lighter gangue they are used to separate metal sulfides from ores where the sulfides are coarse grained they have been used to exploit placer gold deposits over many centuries and as an adjunct in modern mills to recover coarse gold early in the process when the feed material is comprised of particles with a wider size distribution gravity concentrators do not produce a clean concentrate and have been used mainly for preconcentration or as a rougher stage gravity concentrators are simple devices that are inexpensive to operate and cause comparatively much less environmental pollution than using harmful chemical reagents for the same purpose gravity separation may be achieved in either of two ways relative gravity separation and absolute gravity separation in relative gravity separation all particles settle in the same direction within the device and large differences in specific gravity density between the minerals are needed for effective separation as settling velocity of particles depends not only on density but also on size it is important to prepare narrowly sized feeds to these devices however as particles of finer sizes settle under gravity at much slower rates the effectiveness of these devices decreases considerably below about 50 mm the heavy and light minerals that settle at different rates are made to report to different exit streams by applying a transverse force there are many devices of this nature such as jigs spiral concentrators wilfley tables shaking tables air tables gemini tables pinched sluices riffled sluices reichert cones and so on to improve the efficiency of separation of finer material centrifugal concentrating devices have been introduced within the past two decades including knelson concentrators falcon concentrators kelsey jigs and wateronly cyclones these devices use centrifugal acceleration to enhance the movement of finer particles and have been extensively used in the gold processing plants to recover fine gold in absolute gravity separation a fluid or a nonsettling slurry comprised of fine solid particles with a specific gravity sg intermediate to those of the t

wo minerals to be separated is used as the medium of separation particles of higher and lower specific gravities than the medium move in opposite directions this technique requires only a small difference in specific gravities of the minerals for effective separation heavy or dense medium separation methods are absolute separation methods devices of this sort include dense medium vessels and dense medium cyclones gravity concentrators gravity concentrators may be classified according to the way particles are made to move relative to each other so that the denser particles will segregate from the lighter ones under the influence of the gravitational force a gravity concentrator uses essentially one of three mechanisms to provide movement of particles within it jigging shaking and flowing film devices these gravity separation devices rely on a fluid medium such as water to transport the material through the device and also bring about a separation based on settling characteristics of heavy and light particles the process requires the feed material to be uniformly sized and fed as a slurry if the feed has a wide size distribution the smaller heavy mineral particles will report to the same exit as larger light particles because their settling velocities are similar as the particle size decreases the differences in gravitational forces acting on heavy and light particles which bring about differences in their movement also diminish recent advances in the design of gravity separation devices have addressed this problem by introducing multi g devices that impart higher accelerations on smaller particles through centrifugal forces than those due to gravity these devices are known as centrifugal concentrators thus each gravity concentrator has an optimal feed particle size range for effective separation the magnitude of the applied acceleration is usually quoted in terms of multiples of the gravitational acceleration g figure 14 4 1 shows the classification of gravity separation devices table 14 4 2 shows the range of feed sizes suitable for various gravity separators jigging devices jigging is one of the oldest methods of gravity concentration it is commonly used in coal cassiterite gold and iron ore industries generally jigs are used for separating coarser material from about 2 to 10 mm in diameter although efficient separation of particles down to about 75 mm is possible with modified devices they can handle large tonnages and operate on a continuous basis jigging essentially uses alternating expansion and contraction of a bed of particles the bed is generally formed over a screen through which the fluid is allowed to flow ragging comprised of heavy particles is generally placed over the screen to retain the bed and release the segregated heavies to the hutch during the suction stroke the bed movement is activated by a pulsating current of fluid usually water or by the movement of the screen that holds the bed

the pulsation of the bed may be achieved by means of a plunger or by admitting compressed air at regular short intervals into a limb of a u shaped tank light material is generally discarded through a gate positioned opposite to the feed end gaudin 1939 suggested that three segregation mechanisms may take place within a jig differential initial acceleration hindered settling and consolidation trickling the extent of prevalence of these mechanisms depends on the properties of the material being processed such as size and density and the manner in which the force is applied to dilate and contract the bed the differential initial acceleration between particles in a pulsating bed is a function of their density and is independent of size that is the initial acceleration conveyed to heavy mineral particles by the pulsating current of fluid is larger than that for lighter mineral particles by applying a higher frequency of pulsation this effect may be accentuated this mechanism will segregate heavy minerals beneath the lighter gangue also during the pulsation stroke heavy particles within the expanded bed will settle faster than the lighter ones however because of the presence of a large number of particles within the bed hindered settling of particles takes place the result of particle particle interactions as opposed to free settling the differences in hindered settling rates of larger particles are more pronounced than those of smaller particles hence jigs perform better on coarser particles when the dilated bed collapses during the suction stroke larger particles tend to interlock leaving voids through which the finer particles may segregate this process is known as interstitial or consolidation trickling coal jig baum jigs are commonly used for processing coarse coal whereas batac jigs are used mainly for fine coal both these units use compressed air to pulsate the bed in these units water is typically pulsed at 60 to 80 pulses per minute and the units handle feed sizes up to 130 mm at capacities of up to 40 t m2 h mineral jig jigs that treat placer deposits generally have a diaphragm in the hutch area which is placed underneath the screen that holds the bed typical jigs of this type are cleveland and panamerican placer jigs which have a smaller footprint harz and denver jigs have a wall that separates the hutch area and use an eccentrically driven plunger in one section to pulsate the bed schematic diagrams of a few jigs are shown in figure 14 4 2 the in line pressure jig ipj combines a circular bed with a moveable sieve action the screen is pulsed vertically by a hydraulically driven shaft the length and speed of the upand down stroke can be varied to suit the application screen aperture ragging dimension and ragging material can also be altered for the application which varies the recovery of solids to the concentrate stream it uses less hutch water compared to traditional jigs the engineered ce

ramic ragging may be selected from a range of specific gravities from 1 6 to 5 0 ipjs have been used across many mineral types including gold sulfides tin coal and diamonds figure 14 4 3 shows a schematic diagram of an ipj ipjs are available in a range of sizes handling 2 to 100 t h of feed material the solids recovery to concentrate may be varied from about 5 to about 50 by selecting the appropriate ragging density ragging size and screen aperture for a given feed size lower yields require tighter ragging sizing the units are supplied with automatic controllers to control the feed rate pulse frequency stroke length and air bleed rate for optimal performance kelsey centrifugal jig the kelsey centrifugal jig kcj imparts a centrifugal acceleration on the feed and ragging particles while a jigging action is being applied it enables the effective separation of particles below about 500 mm down to fine sizes and has found applications in tin tantalum gold nickel tungsten chromite base metals iron ore and mineral sands industries the centrifugal acceleration is conveyed by a spinning rotor the speed of which may be varied and controlled inside the rotor a screen of cylindrical shape is spun coaxially with the rotor the ragging material is spread evenly on the inside of the screen because of the centrifugal acceleration the feed enters through a fixed central pipe then accelerates toward and continually rises up the ragging bed pressurized water is introduced into a series of hutches behind the screen which is pulsed through the ragging to fluidize the bed the pulsing of the bed is achieved via pulse arms connected to pulse pads which push against flexible diaphragms at the back of each hutch thereby pushing the water in the hutch through the screen and ragging bed kcjs can handle capacities up to 50 t h solids and typically use about 30 to 50 m3 h of hutch water the rotor spins at speeds up to 200 rpm generating accelerations of 40 g the pulse rate is between 1 800 and 2 200 pulses per minute at a stroke length of 2 to 3 mm a schematic diagram is shown in figure 14 4 4 the size of ragging used depends on the size distribution of the valuable mineral particles in the feed and should be chosen to avoid excessive blinding of the internal screen other operating variables include pulsation frequency hutch water addition rate ragging type ragging sg depth of bed and so on these units also have a number of design features such as automatic screen cleaners and vibration detection programmable logic controller based interlocks and sacrificial shear pins are provided to avoid major damage in the event of a malfunction air jig air jigs have been used in both coal and gold industries in these devices a low pressure high volume air source is used to fluidize the particulate bed while a high pressure lowvolume air source is used to impose the pulsation stroke for stratification recently the air jig has

found wider application in the coal industry as this method does not use process water with the advantage of eliminating the need to dewater fines or manage slurry confinement particulate emissions have been minimized by the inclusion of fabric dust collectors the new units use a punch plate instead of a fine wire mesh cloth to support a deeper particle bed however their application has limitations due to particle size particle shape moisture and near gravity content alderman 2002 flowing film separators flowing film separators have been used for centuries to accumulate heavy minerals such as gold and tin these devices are cheap to make and operate in flowing film concentrators the feed slurry is made to flow over an inclined surface and segregation of the heavy minerals beneath the lighter minerals takes place during the passage at low flow rates the thickness of the film is small and the particles generally settle out and roll over the surface heavier particles tend to accumulate in the lower regions closer to the surface and travel at lower velocities while the lighter materials travel closer to the surface and faster at high flow rates the particles will be suspended in the flow due to turbulent suspension resulting in a density gradient across the depth of the flow the particles also experience shearing because of bagnold forces that give rise to a dispersive pressure on the particles commonly used flowing film separators are pinched sluices reichert cones riffled sluices and spiral concentrators pinched sluice a pinched sluice is a device comprised of an inclined plane bottom surface and tapering sides the feed slurry is fed at the higher end and is made to flow down the inclined surface the segregated heavy minerals flowing closer to the bottom of the surface are removed through slots placed across the bottom of the channel at the discharge end while the lighter minerals flow over the slot and report to tailings the depth of flow increases toward the lower end because of the lowering of the area of cross section these sluices are used by smallscale operators treating alluvial gold deposits and in tin and heavy minerals industries figure 14 4 5 shows a schematic diagram of a pinched sluice these units are operated at inclinations of between 15 and 20 to the horizontal subasinghe and kelly 1984 demonstrated that at higher inclinations the recovery of coarser particles decreases because of the presence of bagnold forces acting on the particles at lower slopes coarse and dense particles settle out on the deck resulting in poor segregation of heavies in the bed reichert cone reichert cones operate in a similar way to that of pinched sluices the cone is designed as a series of pinched sluices joined in a circular pattern with their sides removed an inverted cone is placed above the concentrating cone to distribute the feed slurry evenly heavy and light materials are removed through annular slots placed

near the center column several cone pairs may be stacked on top of each other to increase the quality of the separation cones are operated continuously figure 14 4 6 shows a schematic diagram of a reichert cone riffled sluice riffled sluices are commonly used by artisan miners to recover gold the segregated gold or heavy minerals are recovered behind the riffles placed across the sluice the feed flow rate height and spacing of the riffles determine the size of the recovered particles if the feed contains fine gold then a lower flow rate and smaller riffles are used sometimes instead of the riffles corduroy cloth or coir mats are used to trap the fine gold particles sluices have been used to process fine cassiterite ores spiral concentrator spiral concentrators have found wide applications in the beach sand industry in a spiral concentrator the feed slurry is made to flow down a helical conduit of semicircular cross section the slurry experiences a centrifugal force and has a tendency to move particles toward the outer rim because of the shape of the trough s cross section these particles then travel inward closer to the bottom of the trough the heavy particles settle out of suspension and flow down the spiral closer to the trough s inner edge there they are subjected to a smaller centrifugal acceleration and take up a trajectory closer to the axis of the spiral the lighter particles move closer to the free surface and are carried toward the outer rim as a result of the centrifugal acceleration when the slurry reaches the bottom of the spiral separation of the heavies has occurred and the heavies may be collected through the inner ports while the lighter material is collected from the outer port to save floor space two or more spirals may be incorporated into one unit known as a twin triple or quad start thereby increasing the throughput rate the shape of the trough varies with the type of mineral being separated coal spirals generally have a flatter profile than mineral spirals the pulp density may vary between 20 and 40 solids the feed sizes range from about 2 mm to about 75 mm the capacity of a single start spiral may be up to 7 0 t h depending on the material being processed the cut specific gravity depends on the pulp density feed rate and size distribution of the feed in the beach sand industry spirals are commonly used as a preconcentration stage figure 14 4 7 shows a schematic diagram of a concentrating spiral shaking devices the most common shaking device is the concentrating table or the wilfley table concentrating tables are very efficient gravity separators generally used in cleaning concentrates obtained from other devices concentrating tables or shaking tables are comprised of a rectangular flat table inclined horizontally by about 15 to 20 riffles are placed along the length of the table and the table is made to vibrate in the longitudinal direction in a reciprocating manner with a

slow forward stroke and a rapid return the feed slurry enters through a port at the top corner and is distributed over the flat surface because of the vibratory motion wash water is introduced along the top edge of the table surface washing the particles down the slope heavy mineral particles get trapped between the riffles and travel across the table in a longitudinal direction the lighter minerals flow over the riffles and are carried down the slope and report to the tailings stream nearer to the feed end of the table the heavy mineral concentrate is collected at the opposite end of the table with middling particles collected in between the feed to tables may vary from about 1 mm down to about 50 mm the feed pulp density is between 25 and 35 solids figure 14 4 8 shows a schematic diagram of a wilfley table many designs and riffle geometries exist depending on the type of material being processed tables have been in use for processing of minerals such as coal barites beach sands chromite garnet iron manganese tantalum tin tungsten and zircon generally the variables that may be manipulated to increase the efficiency of separation are deck slope wash water flow rate and the frequency and amplitude of the vibratory motion with larger particles longer strokes and lower shaking frequencies are used the separation is judged mainly by visual inspection and modifications to operating variables may be made accordingly these are low capacity devices with about 2 and 0 5 t h m2 for 1 5 mm and 100 mm particles respectively as they take up much floor space multiple decks may be stacked up one above the other to save space gemini tables have found wide application in the gold industry as a device for cleaning concentrates of centrifugal devices such as knelson and falcon concentrators for finer feeds bartles mozley tables are generally used in which the shaking mechanism is replaced by a smoother oscillator movement to avoid mixing of the bed due to turbulence air tables have also been used in the mineral industry to recover fine particles in which the bed is fluidized using air as an aside air tables were originally developed for use in arid regions but have also been used in the separation of flat fibrous materials of lower bulk densities such as asbestos and vermiculite however these devices demand that the feed be very narrowly sized for their efficient operation as they also have only limited capacities their use is limited only to special applications centrifugal or enhanced gravity concentrators the movement of fine particles that settle under gravitational force within a fluid medium is slow even if there is a significant density difference between heavy and light minerals the separations achieved are inadequate however by allowing the particles to traverse a circulatory motion at high speed a centrifugal acceleration may be transmitted to them that may be several hundred times that of gravity de

vices that use this technique are called centrifugal concentrators two devices that have found wide applications in recovering fine gold and other minerals are the knelson and falcon concentrators knelson concentrator knelson concentrators as shown in figure 14 4 9 are comprised of a slightly conical shaped rotating bowl with a series of grooves on the outer wall to capture the segregated heavy mineral particles the feed material enters through a central feed inlet as a slurry the slurry descends onto the base plate at the bottom of the rotating bowl and is thrown outward because of centrifugal acceleration as a result some of the particles get trapped within the grooves of the bowl forming a bed while the excess get carried upward into the tailings stream by the rising current of water injection of water through fluidization ports located in the grooves of the bowl prevents compaction of the concentrate bed this creates a fluidized bed that acts as a concentrating chamber for heavy mineral particles under an enhanced separating force upon conclusion of the concentration cycle concentrates are collected manually in batch units or flushed from the bowl into a concentrate launder through a multiport hub in continuous units as heavy particles experience a higher centrifugal acceleration they tend to move through the fluidized bed to displace the light materials and produce a heavy mineral concentrate within the grooves of the bowl the lighter particles are displaced into the tailings stream by the fluidizing water fluidizing water flow rate bowl rotational speed and particle characteristics of the feed such as the size and size distribution determine the optimal conditions for separation and the cycle time falcon concentrator falcon concentrators rely on a rotating bowl to generate high centrifugal accelerations similar to knelson concentrators the feed slurry rises upward over a slightly inclined smooth wall stratification of higher density particles occurs toward the bowl wall and lower density particles move toward the center of the bowl different models vary in the way they collect and produce the concentrate in the semibatch unit the upgraded product that has been forced against the bowl wall is made to move into a vertical fluidized collection zone in the upper portion of the bowl the collection zone consists of a number of rings that have pressurized water injected from the back effectively elutriating and cleaning the concentrate that sits in these collection rings until a rinse cycle begins these machines are known as semibatch devices because they continually accept feed during the run cycle but only produce concentrate during periodic rinse cycles run times range from 30 minutes to several hours depending on the application while the rinse times are generally less than a minute the throughput rates range from 1 to 400 t h the centrifugal accelerations range from 50 to 200 g these units are mainly used

in precious metal recovery plants in continuous devices at the top of the bowl is a ring of specially designed concentrate flow hoppers followed by pneumatically controlled variable orifice valves these valves allow for a constant stream of concentrate to be produced the continuous concentrator is able to vary mass pull to concentrate by adjusting valve orifice size through changes in air pressure because of their high mass pull rates of 5 to 40 these units are used in the recovery and upgrade of tin tantalum tungsten chrome cobalt iron fine oxidized coal and uranium ores the ultrafine units are designed to concentrate material below 50 mm at less than 20 solids and may be typically used in deslime cyclone overflow streams figure 14 4 10 shows a schematic diagram of a falcon continuous concentrator heavy medium separation heavy medium separation hms is a very effective gravity separation technique in which a nonsettling heavy medium is used in place of a fluid the heavy medium is generally comprised of a nonsettling suspension of fine heavy particles the density of which is much higher than that of water thus the concentration criterion described in equation 14 4 1 increases considerably in this case which indicates a more efficient separation also since the medium density is between those of the heavy and light minerals to be separated it gives rise to an absolute separation where the two products are separated into a sinks product and a floats product this technique requires a much lower density difference between the minerals to be separated compared to the relative separation techniques described previously with hms it is possible to a produce a finished concentrate and a final waste product in one operation b reject a waste product at a coarser size thereby saving grinding costs c achieve separation at a low operating cost with low maintenance costs d make relatively sharp separations e operate continuously f tolerate feed with wide size distributions and g produce a consistent product for further processing the main industrial applications of hms with coarse material feeds are in coal and iron ore industries hms has also been used in base metal industries such as copper lead and zinc the most commonly used medium in coal separation is magnetite although silica barite and galena had been used in the past as the medium solid for ore separations in recent times ferrosilicon fesi has been used extensively with finer feed materials the viscosity of the slurry increases and the separation efficiency decreases as a result this situation has been rectified by the use of centrifugal separators such as heavy medium cyclones hmcs the water only cyclones work similarly to an hmc that uses a loess medium instead of an external medium see figure 14 4 11 for a schematic diagram of a heavy medium separator heavy medium solids the solids used to make up the heavy medium depend on the

type of material to be separated the heavy medium suspension should be nonsettling which requires the solid particles to be sufficiently small for coal the most commonly used are magnetite and barite as the medium density should be less than the ash density for ore separations more dense solid materials such as ferrosilicon and galena have been used aplan 2003 ferrosilicon contains about 15 si ground ferrosilicon is used for separations at sgs up to 3 2 for higher sgs up to 3 8 atomized fesi may be used the main requirement in selecting a medium for a given operation is its ability to be separated from the products generally the products are washed and screened to remove the medium wet magnetic separators are also used extensively to recover magnetite and ferrosilicon the medium recovered from the product streams needs to be thickened to the required consistency prior to recycling it back to the separating vessel feed preparation it is important to prepare the feed prior to hms usually by wet screening the main purpose is to size the ore into fractions that may be treated in various separating units more importantly screening is used to remove the slimes that would otherwise increase the viscosity of the medium giving rise to inefficiencies medium recovery medium is generally removed from the products in drain and wash screens sieve bends are the preferred screen type for medium recovery prior to reusing the medium needs to be cleaned using wet magnetic separators magnetic separators are fed at about 30 to 35 solids and the rate is determined by the type of separator and magnetic susceptibility of the medium heavy medium separators hms equipment must ensure the feed particles have sufficient time to report to the relevant discharge streams they generally differ in the way the float and sink products are discharged the float products will often discharge over a weir with or without the need for paddles or scrapers the wemco drum discharges the heavy mineral using lifters while rotating the drewboy washer uses paddles to elevate the heavies while the norwalt bath uses a bucket elevator the dutch state mines bath uses a drag conveyor with a paddle to deliver the heavies and lights to different discharge ports the separations of finer sizes are carried out effectively in units that use centrifugal acceleration to aid in the separation and discharge of particles hmcs and dyna whirlpool separators dwps are the widely used equipment of this type heavy medium cyclones the hmcs are essentially a modified version of hydrocyclones used with a heavy medium as the separating fluid instead of water in an hmc the heavy mineral migrates to the outside wall and is discharged through the vortex finder the shape of the hmc however differs from that of a classifying cyclone in that the height of the cyclone is comparatively shorter hmcs treating coal operate at inlet pressures with a minimum head of about nine times

the diameter of the cyclone the diameters of these hmcs range from about 0 5 m to 1 4 m and have capacities up to 500 t h the relative density of the overflow stream should be between 3 and 12 lower than that of the feed stream dyna whirlpool separator the dwp also imparts a centrifugal acceleration to the feed particles a dwp is essentially a sloping cylinder with cover plates and central openings to which the raw feed is fed centrally at the top end there are also tangential orifices at either end the medium enters tangentially at the lower end and migrates toward the top end leaving the cylinder tangentially along with the heavy mineral the lights gravitate to the bottom end and leave through a central discharge port generally centrifugal devices such as the hmc and dwp experience higher media losses than static type devices performance evaluation of gravity separators gravity separators aim to separate minerals at a desired separation density however because of the presence of middling particles which have densities between those of heavy and light minerals a sharp separation is not possible also since these devices rely on differences between settling rates of the two minerals fine feed material inherently gives rise to inefficiencies the efficiency of a gravity concentrator is determined by two factors material characteristics and machine characteristics the degree of liberation of the feed material that affects the separation efficiency is represented by separability curves whereas machine characteristics are evaluated in terms of performance curves kelly and spottiswood 1982 the performance curve of an operating machine is represented by a plot of the fractional mass of particles reporting to the concentrate stream against the extent of the property being exploited for the separation for gravity concentration traditionally density or sg of the particles has been taken as the property being exploited such a plot is commonly known as the performance curve partition curve or the tromp curve which may be construed as a probability plot that indicates the probability of a particle of given sg reporting to the concentrate stream a flatter curve represents a poor separation whereas a steeper slope represents a sharp separation the specific gravity of particles that have equal chances of reporting to either exit is known as the cut specific gravity sg50 or separation density for static heavy medium separators the separation density is close to the medium density whereas for centrifugal separators it is slightly higher selection of gravity separation equipment the selection of gravity separation equipment is based on the response of the ore to sink and float tests using heavy liquids the commonly used heavy liquids are methylene iodide tetrabromoethane methylene bromide and lithium sodium tungstates whose densities are 3 3 2 96 2 48 and 2 85 respectively for higher density separations clerici

solution is used generally an organic solvent may be used to dilute the reagents to a required specific gravity caution should be exercised in handling these reagents as most of them are toxic the data from these tests are presented as washability curves that essentially indicate the grade and recovery of the concentrate that can be achieved at various separation densities the selection of gravity equipment is based on the amount of material present in the ore that is near the separation density the amount of ore within 0 1 sg units of the separation density is known as the tolerance value which is a reflection of the degree of difficulty of separation table 14 4 4 shows the recommended equipment at various tolerance values the selection also depends on the feed size of the ore as each equipment type can handle material only within a specified size range for the selection of centrifugal separators for fine gold recovery a gravity recoverable gold test has been proposed laplante et al 2000 this test is a material characterization test that indicates the amenability of the ore to centrifugal gravity concentration for the purpose of determining optimal operating conditions of a knelson concentrator a model based on a mechanistic approach has been proposed subasinghe 2007 froth flotation is a highly versatile method for physically separating particles based on differences in the water repellency characteristics of the surfaces of various rock and mineral species contained in aqueous slurry the air bubbles generated within a separation vessel collect hydrophobic particles and ascend to the surface with the particles attached to be removed while the particles that remain completely wetted stay in the liquid phase froth flotation can be adapted to a broad range of mineral separations as it is possible to use chemical treatments to selectively alter mineral surface characteristics so that they have the necessary hydrophobicity for the separation to occur many diverse applications both mineral and nonmineral are currently in use examples of mineral applications include separating sulfide minerals from silica gangue and selectively from other sulfide minerals separating potassium chloride sylvite from sodium chloride halite separating coal from ash forming minerals removing silicate minerals from iron ores and separating phosphate minerals from silicates an example of a nonmineral application is the de inking of recycled newsprint flotation is particularly useful for processing fine grained ores that are not amenable to conventional gravity concentration because of their low density differentials that render gravity separation unacceptably ineffective the flotation system includes many interrelated components and changes in one area will produce compensating effects in other areas as shown in figure 14 5 1 klimpel 1995 therefore it is important to take all of these factors into account when developing specific

froth flotation applications changes in the settings of one factor such as feed rate will automatically cause changes in other parts of the system such as flotation rate particle size recovery airflow rate and pulp density as a result it is difficult to study the effects of any single factor in isolation and compensation effects within the system can sometimes result in an unexpected and undesirable outcome klimpel 1995 this makes it difficult to develop predictive models for froth flotation although work is being done to develop models that utilize readily measurable parameters such as solids recovery and tailings solid content rao et al 1995 such models incorporate fundamental physical and chemical reactions as well as empirical data that relate to the mineral components and they are usually validated by various levels of test work i e bench scale pilot plant and commercial plant trials grade recovery curves although these calculated values are useful for comparing flotation performance for different conditions it is most useful to consider both the grade and the recovery simultaneously using a grade recovery curve this is a graph of the recovery of the valuable metal versus the product grade at that recovery and it is particularly useful for comparing separations where both the grade and the recovery are varying a set of grade recovery curves is shown in figure 14 5 3 if 100 of the feed is recovered to the product then the product will obviously have the same composition as the feed and so the curve starts at the feed composition with 100 recovery similarly if the purest mineral grain that contains the metal of interest is removed this will be the maximum grade that can be produced by a physical separation and so the zero percent recovery end of the curve terminates at an assay that is less than or equal to the assay of the purest grains available in the ore in the curves shown in figure 14 5 3 the points that are higher and to the right show better performance than the points that are lower and to the left hydrophobicity hydrophilicity the basis of froth flotation is the difference in wettabilities of different minerals particles range from those that are readily wettable by water hydrophilic to those that are waterrepellent hydrophobic if a mixture of hydrophobic and hydrophilic particles are suspended in water and air is bubbled through the suspension then the hydrophobic particles will tend to attach to the air bubbles and float to the surface as shown in figure 14 5 4 the froth layer that forms on the surface will then be heavily loaded with the hydrophobic mineral and it can be removed as a separated product the hydrophilic particles will have limited or no tendency to attach to air bubbles and so they will remain in suspension and be transported away in the slurry whelan and brown 1956 particles can be naturally hydrophobic or hydrophobicity can be induced by chemical treatme

nts naturally hydrophobic materials include hydrocarbons and nonpolar solids such as elemental sulfur coal is a good example of a material that is normally naturally hydrophobic because it is mostly composed of hydrocarbons chemical treatments to render a surface hydrophobic are essentially methods for selectively coating a particle surface with a monolayer of nonpolar oil the attachment of the bubbles to the surface is determined by the interfacial energies between the solid liquid and gas phases article bubble contact when particles are rendered hydrophobic contact with gas bubbles must occur so that attachment to the surface can take place contact between particles and bubbles can be accomplished in a flotation cell such as the agitated cell shown in figure 14 5 6 in the agitated cell the rotor draws slurry through the stator and expels it to the sides creating a suction that draws air down the shaft of the stator the air is then dispersed as bubbles through the slurry and comes in contact with particles in the slurry that is circulated through the stator particle bubble collision is affected by the relative sizes of the particles if the bubbles are large relative to the particles then fluid flowing around the bubbles can sweep the particles past without them coming in contact therefore it is best if the bubble diameter is comparable to the particle diameter to ensure good particle bubble contact collection in the froth layer when a particle and bubble have come in contact the bubble must be large enough for its buoyancy to lift the particle to the surface this is obviously easier if there are low density particles as is the case for coal than if there are high density particles such as lead sulfide the particle and bubble must remain attached while they move up into the froth layer at the top of the cell the froth layer must persist long enough to either flow over the discharge lip of the cell by gravity or be removed by mechanical froth scrapers if the froth is insufficiently stable the bubbles will break and drop the hydrophobic particles back into the slurry prematurely however the froth should not be so stable as to become persistent foam as foam is difficult to convey and pump through the plant the surface area of the bubbles in the froth is also important because particles are carried into the froth by attachment to bubble surfaces increasing amounts of bubble surface area allows a more rapid flotation rate of particles at the same time increased surface area also carries more water into the froth as the film between the bubbles because fine particles that are not attached to air bubbles will be unselectively carried into the froth along with the water entrainment excessive amounts of water in the froth can result in significant contamination of the product with gangue minerals reagents the properties of raw mineral mixtures suspended in normal plant circulating water are rarely suitable for fr

oth flotation chemicals are needed both to control the relative hydrophobicities of the particles and to maintain the proper froth characteristics therefore many different reagents are involved in the froth flotation process with the selection of reagents depending on the specific mineral mixtures being treated klimpel 1980 thompson 2002 collectors collectors are used to selectively adsorb onto the surfaces of particles they form a monolayer on the particle surface that essentially makes a thin film of nonpolar hydrophobic hydrocarbons the collectors greatly increase the contact angle so that bubbles will adhere to the surface selection of the correct collector is critical for an effective separation by froth flotation collectors can be generally classed depending on their ionic charge they can be nonionic anionic or cationic as shown in figure 14 5 7 glembotskii et al 1963 nonionic collectors are simple hydrocarbon oils whereas the anionic and cationic collectors consist of a polar part that selectively attaches to the mineral surfaces and a nonpolar part that projects out into the solution rendering the surface hydrophobic collectors can either chemically bond to the mineral surface chemisorption or they can be held on the surface by physical forces physical adsorption chemisorption in chemisorption ions or molecules from solution undergo a chemical reaction with the surface becoming irreversibly bonded this permanently changes the nature of the surface chemisorption of collectors is highly selective as the chemical bonds are specific to particular atoms physisorption in physisorption ions or molecules from solution become reversibly associated with the surface attaching due to electrostatic attraction or van der waals bonding the physisorbed substances can be desorbed from the surface if conditions such as the ph or the composition of the solution changes physisorption is much less selective than chemisorption as collectors will adsorb on any surface that has the correct electrical charge or degree of natural hydrophobicity nonionic collectors hydrocarbon oils and similar compounds have an affinity for surfaces that are already partially hydrophobic they selectively adsorb on these surfaces and increase their hydrophobicity the most commonly floated naturally hydrophobic material is coal the addition of collectors such as no 2 fuel oil and kerosene significantly enhances the hydrophobicity of the coal particles without affecting the surfaces of the associated ash forming minerals this improves the recovery of the coal and increases the selectivity between coal particles and mineral matter fuel oil and kerosene have the following advantages over specialized collectors for froth flotation they have a low enough viscosity to disperse in the slurry and spread over the coal particles easily they are very low cost compared to other compounds that can be used as coal collectors in addition to coa

l it is also possible to float naturally hydrophobic minerals such as molybdenite elemental sulfur and talc with nonionic collectors nonionic collectors can also be used as extenders for other collectors if another more expensive collector makes a surface partially hydrophobic adding a nonpolar oil will often increase the hydrophobicity further at low cost anionic collectors anionic collectors are usually weak acids or acid salts that ionize in water producing a collector that has a negatively charged end that will attach to the mineral surfaces and a hydrocarbon chain that extends out into the liquid as shown in figure 14 5 8 the anionic portion is responsible for the attachment of the collector molecule to the surface while the hydrophobic part alters the surface hydrophobicity anionic collectors for sulfide minerals the most common collectors for sulfide minerals are the sulfhydryl collectors such as the various xanthates and dithiophosphates xanthates are most widely studied and commonly used and they have structures similar to what is shown in figure 14 5 9 xanthates are highly selective collectors for sulfide minerals as they chemically react with the sulfide surfaces and do not have any affinity for the common nonsulfide gangue minerals crozier 1992 the ocss group attaches irreversibly to the sulfide mineral surface using xanthates with longer hydrocarbon chains tends to increase the degree of hydrophobicity when they adsorb on the surface there are a wide variety of other sulfhydryl collectors thompson 2002 bulatovic 2007 which have somewhat different adsorption behaviors so they can be used for some separations that are difficult using xanthates these include the following dithiophosphates have greater selectivity against pyrite than do xanthates and some of these can collect liberated grains of gold they are also useful in lead sulfide zinc sulfide and silver sulfide flotation thionocarbamates selectively f