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mining methods vs underground crusher location

In Section 2 we dismissed those big-room applications not needing fundamentally new equipment, and lumped all the rest together. Now, as we examine specific mine types, it is appropriate to differentiate within these categories the manner in which portable crushers might be used, and the consequent effect on desired machine parameters. The common yardstick by which we will measure all these mining methods is the basic decentralized crusher concept, beyond which portability, or the degree thereof, will be the special and distinguishing feature. Accordingly, underground hard-rock mines are divided into three major groups, listed below with their distinguishing characteristics.

Low Head Room Room and Pillar:Most similar to coal mines in their mining methods, this group exhibits more rapidly moving mining fronts and the greatest need for portability. Machine height is critical, but other dimensions may be more flexible due to room size and single level mine plans. Great portability also demands minimal site preparation.

High Head Room Room and Pillar:No particular height problems are encountered here. Slow moving mining fronts (on single or multi-levels) allow less frequent crusher moves and generally create greater throughputs from a single mining unit.

Non Room and Pillar:This large group includes many different mining plans, but most are characterized by multi-level mining and vertical, as well as horizontal, ore haulage. In addition, access and haulage drifts are frequently quite small both in height and width, which makes crusher size the critical parameters. Crusher move-ups may be less frequent, allowing more extensive site preparation.

With 75% of all U.S. hard rock mines over 1200 tpd using the room and pillar method, this group becomes the most important for the purposes of this study. Supporting this conclusion is the fact that equipment useable in low headroom situations is also useable in high headroom mines. The reverse is obviously not true, and high headroom operations might even prefer low profile equipment, as we shall discuss in Section 4. 2.

Low headroom causes these mines to rely on LHDs, loaders, shuttle cars, and low ore trucks for face haulage. While these machines provide great flexibility and, in the case of LHDs, multi-function in a single vehicle, they all have one thing in common: Costs escalate and productivity drops with increasing haul distances, at a greater rate than with larger, taller equipment. The mining industry is experiencing increasing acceptance of LHD type equipment because one man, operating one machine, is not disrupted by uncoordinated equipment cycles. The resulting greater productivity (and reduced labor) more than offsets higher per-hour operating costs (perhaps 50% higher) for vehicle proper, as compared to haul-only or load-only machines. Clearly then, the haul portion of the load-haul-dump cycle is better left to specialized equipment when distances become excessive. In order of increasing haul distance, the typical equipment selection for ROM ore is as follows:

From the pocket, long hauls are best handled by modern rail (ROM ore), but if a crusher can be installed, belt conveyors are by far the lowest ton-mile method. Clearly then, there is incentive to get the crusher and conveyor as close to the face as possible, thereby eliminating as many costly intermediate pieces of equipment (and operators), and as much rehandling, as possible. Inevitably, we must, reach a compromise, but the optimum would surely be represented by one type of face haulage equipment (FEL or LHD) operating over as short a distance as possible.

The trade off of course, is haul distance versus crusher and belt move-up costs. Haul distance can be kept down only by frequent moveups, and move-up costs can be kept down only quick,easy, installations. We have thus zeroed in on the key characteristic of decentralized crushers for this mine group: Portability.

Expanding further, let us examine the typical mining cycle in greater detail, in an attempt to quantify portability. The concensus among mine operators is that face haulage costs begin to escalate at a disproportionate rate when average distances exceed 400-500 feet, depending primarily on the payload of a given vehicle (three to seven yard buckets predominate in low headroom). Traffic and blasting considerations limit minimum distances to around 250 feet, so the maximum desired distance becomes about 650 feet, and the moveup distance is 400 feet. Very wide panels may require two crushers, thus minimizing haulage distance to the flanks, and allowing one crusher to operate while the other is moved. Variations in panel width (and crusher location) will obviously influence not only these numbers, but advance rate as well. For the example cited, a mining advance rate of 100 feet per month would require a (single) crusher moveup every four months.

In addition, site preparation should not interfere with normal operations prior to the move, a fact which strongly influences the design of the conveyor system. Moveups should be accomplished in one shift or less to allow minimum disruption to production. This achievement can be facilitated by leapfrogging spare crushers from the maintenance department if production is sufficient to justify spares. Quick moves (and servicing) are also enhanced by modular components, such as hopper-feeder and crusher, or hopper-feeder and crusher and discharge conveyor. Wheels (perhaps removeable) and/or skids are entirely satisfactory for moveups.

The crusher hopper should be able to handle at least two full loads (10-20 yards) from the face haulage equipment. Due to minimal site preparation, the dump must essentially be at floor level, and the hopper feeder must elevate the muck if the crusher so requires. Excavations for the panel (section or feeder) belt are to be avoided, so the crusher must discharge material, elevating it if necessary, to a typical height of about three feel.

This mine group does, however, offer the possibility of handling oversize at any location in the mining section, including (if absolutely necessary) at the crusher, with a mobile, impactor-equipped, inexpensive vehicle.

By its very name, this class of mines would appear to be outside the scope of this study. While this is strictly true, and the needs of industry are most acute in lower headrooms, we will learn by studying the methods of this group just as we learned by studying potash and other soft mineral mines.

The availability of generous headroom greatly simplifies the crushing task per se, but may not allow improved ore handling unless used wisely. Historically, operators have used high backs to employ larger and more efficient haulage vehicles, still tramming to a central crusher, with a recent emphasis on trackless rather than rail methods for main haulage. Due to the high tire cost associated with truck haulage, a new concept is beginning to appear in an attempt to control truck haul distances. Some open pit mines (the ultimate in high head room) are foregoing ever larger trucks over even longer hauls in favor of portable crushers. These mines have found that large primary crushers can be made portable for little more than the cost of one permanent installation ( 12, 13). Hoppers with elevating feed conveyors are used in conjunction with portable discharge conveyors, and at least, one non U.S. underground mine is using this portable primary approach. A variety of crusher types have been made portable, but all are basically standard, very tall machines like those presented in Section 3.

Portability of a large open pit primary crusher has merits because production sites, though moving, are not so numerous and widespread that trucks become economical. The situation underground is a little different, however, because even high head room mines have some height restrictions particularly on entry), truck payloads are smaller, and traffic and tramming speed limitations are greater. As a result, portability and decentralization of crushing becomes imperative if haul distances and costs are to be reduced.

Decentralized portable crushing, even of hard rock, could be achieved in high head room room and pillar mines by custom-designed combinations of standard or near standard, crushers, hoppers, feeders, conveyors, grizzlies, etc. . However, if suitable hard-rock, low head room crushers are forthcoming as a result of this study, these, as well as open pit applications, may represent an additional market base. Three quite different application are worth noting.

Many underground limestone mines need crusher types better suited to hard abrasive materials then are feeder-breakers. While many would have to be considered of low or medium headroom, some high room operations can consider using near standard crushers in a decentralized arrangement. One new mine in Kentucky has done just that, and is singled out in Sections 5 and 7 as being worthy of special note.

Room and pillar mining in these mines usually calls for a heading and bench plan. This provides an opportunity to situate a fairly tall portable crusher at the bench, which, in turn, allows adequate dumping conditions for the face haulage equipment.

Horizontally fed (i. e. , low profile) crushers, perhaps used on both levels, would allow the bench and ramp to be divorced from the crusher, further reducing haulage distances, and adding more flexibility to the mining and traffic patterns. If headroom is generous, standard talllcrushers could be used if provided with feed-elevating means at floor-level dumps. However, more effective decentralization and portability would appear to be provided by low-profile crushers meeting the requirements set forth in Section 2.

Due to their geological setting and ore body shape, the Missouri lead mines are both high and medium headroom operations. Simple room and pillar, heading and bench, and shrinkage methods are used simultaneously in reaching the ore boundaries. Main haulage is by LHD, truck, or by rail, with the recent trend being toward trackless methods. Jaw and gyratory type primary crushers are used.

These mines are unique among room and pillar mines because the common practice of leaving irregular pillar patterns is not compatible with belt conveyors. Conversion to belts for main haulage would have to be a case by case decision, and might be feasible only for new developments in a given mine. Decentralized portable crushers are, however, a potentially attractive alternative to present methods, and most of the comments from the preceeding section on limestone mines would apply here as well. Since the very high rooms in these mines are created by connecting two separately developed not-so-high room and pillar levels, low profile and floor-level dumping characteristics would be desirable in a portable crusher. Consequently, these mines, though potentially served by custom arrangements of standard (tall hard rock crushers, are also potential candidates for the subject low head room portable units.

It has been determined by others (4, 15, 16, 17) that oil shale can be effectively sized for belt haulage by feeder-breakers, and by sledging type (toothed) roll crushers, as well as by traditional primaries, the gyratory and jaw types. In a study of mining methods for deep, thick, oil shale deposits, Cameron Engineers investigated six candidate mining plans for a 30 million tpy operation. Two of these plans were modified room and pillar geometries, each with drift and room heights sufficient to allow a standard sledging roll crusher to be moved around underground. Both plans were based on ten 1000 tph portable crushers, fed by LHD face haulage equipment, and discharging eight inch material onto a 36 and/or 60 inch belt system.

At 1000 tph per crusher, these decentralized crushers are really portable multiple primaries when compared to most mines, and resemble the open pit portables already discussed, Oil shale, though tough, is really neither hard nor highly abrasive, but the results of the Cameron study do represent another indication of the potential of the decentralized crusher concept for hard rock mines. Both decentralized systems were strong contenders, if not outright winners, in the economic analysis section of the study, and more will be presented on this subject in Section 7.

Some of the similarities among the many mine plans in this group have already been listed. In addition to multi-level mining and vertical/horizontal ore handling, these mines also tend, as a group, to have the strongest ore and country rock. Some form of rail haulage is another oft-common denominator. In order to intelligently discuss decentralized crusher concepts for this group, it is appropriate to take the major mine plans one at a time and investigate the potential for improved ore handling methods.

This is the large scale, high development, low cost mining method used (where applicable) in many enormous or low grade ore bodies. Gravity fragmentation allows this mine plan the least control of any over muck size, and muck size distribution tends to vary widely from mine to mine as Table I of Section 2 indicated. Production sites tend to be very numerous, and gravity is freqently employed to gather the ore into fewer loading stations for transfer to the main haulage system. Besides being numerous (and therefore of low average throughput) production sites historically have been served by rather small extraction drifts. The trend, however, is away from slushers toward mechanized loading and hauling, and this is causing larger drifts to come into use. The use of LHD equipment in extraction drifts also reduces somewhat the need for secondary breakage, but the general problem of interrupted ore handling due to oversize remains, causing one chief engineer to write of block caving in general : small portable crushers that could be installed at various points throughout the mine and conveyors that could handle fair-size rock fragments would considerably simplify the problem of ore transport in the mine.

In the limiting case, one could consider portable crushers at all active production sites and a fully mechanized downstream ore handling system. However, this extreme degree of decentralization is probably not justified and some form of primary haulage (i. e. , ore gathering) would be more adaptable to portable crushers.

A three-shift production would likely be loaded by LHD equipment at many dozens of drawpoints and trammed to perhaps as many as 40 active pockets, depending on efficient haul distances, block size, etc. Portable crushers placed at each pocket would see only 50 tph based on five operating hour per shift, and would have to function on the crowded extraction level. These forty dump pockets would typically converge, via a series of connecting ore passes and transfer raises, to only ten or so loading points (chutes) where the main haulage system (presumably rail) would take over. At 200 tons per hour, these ten chutes are a more likely and uncongested location for portable crushers, and transfer to a belt system could be made at this or a lower level.

The life of a typical chute is many months, perhaps 2-3 years, and new chutes might be created and retired at the rate of several per year, depending on block dimensions and other factors. Site preparation could then be amortized over significant time periods, but the need for crusher portability is probably not diminished by this rather extended installation life because rapid change out would be needed for major overhaul of the crushers.

It would appear that we have defined a 200-plus ton per hour ROM crusher capable of moving around on a haulage level. There are, with this approach, two major problems particularly acute in block cave mines, but common to most non-room and pillar mines as well. First, conversion of main haulagewavs from rail to conveyor precludes the use of those drifts for significant movement of men, materials, and certainly of crushers. In addition, typically curved rail systems are difficult if not impossible to convert to straight-line belt conveyors. The answer would seem to be separate drifts or, more likely, separate levels for crushers and main belt haulage. However, to economically justify an entirely new haulage level (and system) in a mine with a working rail system is improbable, and indeed, most operators interviewed expressed this opinion. Consequently, new mines and new levels or developments in existing mines, are probably the only legitimate candidates for radically new ore handling schemes based on decentralized crushers. One new block cave mine did in fact consider conveyorized ore handling in the planning stage before choosing a modern rail system. Among the reasons given for its rejection were the inability to belt ROM ore, and the cost and extent of an underground crushing system based on available equipment. The decision might well have been reversed, and haulage costs reduced, had suitable portable hardrock crushers been available.

The point to be made is that high development plans like block caving are the least flexible and represent perhaps the most difficult situation in which to attempt conversion to portable crushers, regardless of their state of development. This does not imply that decentralized crushing is inherently unattractive for block cave methods; it simply means that early successful application of decentralized crushing will likely occur in mines employing more flexible plans.

The second major problem with decentralized crushing in block cave mines is old and well known: It is oversize muck, not in the beltable context, but oversize to the extent that serious disruptions occur in the ore handling system resulting in lost production and increased accidents. Dravo and Theodore Barry both reported the widespread existance of this problem in all mines and recommended several solutions, with most utilizing some arrangement of hydraulic impact breakers.

Successful implementation of any decentralized crusher concept in a block cave mine must be accompanied by a workable solution to the oversize problem, a problem which will be accentuated by efforts to reduce crusher size. An obvious solution is to make the feed opening of the crusher very large, but this is incompatible with portability and does nothing to relieve existing hangups ahead of the crusher site. A better solution would be to eliminate oversize as early as is practicable, thus improving flow through ore passes and the like, and allowing a reasonably sized portable crusher to efficiently recieve conditioned feed. In the example cited, oversize breakers could be built into the LHD dump pockets, perhaps allowing use of smaller ore passes, or into the extraction drifts, after Barry.

Many secondary breakage means have been developed into prototypes (8, 9, 10), but of those methods applicable to a variety of hard rock types, hydraulic impact breakers are the only devices achieving commerical-scale application with any significant success. The secondary or oversize problem is outside the scope of this study, but its solution, about which more is said in subsequent sections, holds a key to the successful application of decentralized crushing in many block cave mines. Another key is, of course, the crushers themselves, but even very-fine ore mines may need oversize breakers.

We have not intentionally omitted those block cave-mines still predominately slusher or gravity oriented, but they too are difficult to convert for economic reasons. New developments within such mines should consider decentralized crushing when suitable proven equipment is available.

Although a caving method, fragmentation control for this plan is intended to be by blasting. Occasional muck still tends to be quite large, as in many block caves, but production sites are far fewer, they are multi-level, and continually moving, hence the popularity of versatile LHD equipment for face haulage. Ore is typically passed from sublevels to haulage levels where it is trammed by rail to either the shaft, or to a large crusher from which it may be belted to the shaft. This latter arrangement would be more likely in a mine having more than one underground crusher, or a laterally extensive ore body with workings far from the shaft. It is significant that, where crushers are located in the ore body, conveyors are the predominant choice for haulage to the shaft.

Ore handling on a given sublevel or at a given ore pass is, by the very nature of this mining method, a part-time activity, perhaps precluding efficient utilization of crushers at numerous, multi-level LHD dump pockets. However, less numerous chutes at the lower (rail) haulageways are usually supplied with ore from somewhere above, and these would be more suitable sites for crusher placement. Average throughputs would be modest (as opposed to high but intermittent rates of rail car loading), in the 60 tph range for each of 3-6 LHDs delivering ore. Lateral movement of crushed material could take place on the crusher level, but in order to minimize congestion, crushed ore might better be passed directly through surges to a lower main belt. This approach is more applicable to the narrower ore bodies of sublevel caves (as opposed to block caves), and allows main belt levels to become crusher levels as mining goes deeper. Due to the use of trackless equipment on sublevels and main levels, drift size is reasonable (1013 ft. minimum) and ramps are frequently employed between levels.

The increased use of large crushers away from the shaft is a beginning step in this direction, but only portable crushers will eliminate costly small scale rail tramming and retramming. However, to justify an attempt at decentralized crushing, the mine plan must be based on and designed around proven equipment. Conversion (at new lower levels) is more feasible than with block caves, but that first step is a big one and probably cannot be an experiment.

In this respect, sublevel and block caves are very similar; that is, portable crushers (like large central crushers) should not, and probably cannot, be made to take all ROM ore. There will always be muck too big to handle at the face, and some of that which can be handled is still big enough to cause problems ahead of and independent of the crusher. In short, the oversize problem will always exist, but some mines will solve it more easily than others.

We have suggested the possiblity of oversize breakers at dump pockets for block caves. The same approach could be used for sublevel cave, influenced somewhat by the aforementioned part-time and multi-level aspects of those pockets. Oversize control could also be accomplished at the crusher level, perhaps incorporated into a portable feeder or feeder-scalper module ahead of the crusher. Oversize breakage, scalping, and modularization in general are all excellent and perhaps necessary means to provide portability and respectable throughput in a hard rock, underground crusher. Obviously the feed size capability of any successful new crusher will determine, for a given mine, the extent and degree of oversize breakage required. If we assume a critical input dimension of 36 inches (the larger range should apply to non-room and pillar) then the tonnage falling in the oversize category may be only a few percent of the total.

From an ore handling point of view, sublevel stoping represents a mixture of block and sublevel cave procedures. As in a block cave, ore is drawn from stationary cone drawpoints into a nearby rail haulage system by a variety of slusher, grizzly, and boxhole arrangements. As in a sublevel cave, fragmentation control is by blasting, production (hauling) may be taking place on multiple levels simultaneously, and in newer mechanized developments LHDs may be used, loading into mine cars or tramming to ore passes.

Sublevel stoping is adaptable (as opposed to convertible), to decentralized crushing. Since main belt galleries must be straight and cannot be shared by large mobile equipment, they would have to be located on separate levels or in separate drifts. Crushers and LHD equipment could conceivably share the extraction level, but a more flexible and higher surge capacity arrangement would involve passing to a portable crusher on a lower (future) main or extraction level, followed by another pass to a similarly situated main belt. Economical conversion of existing stopes is very unlikely, but new developments should be able to consider decentralized crushing. Comments relating to oversize breakers apply as they would to sublevel caves.

As a general mining method, cut and fill is singled out for attention because its usage is increasing, due to the need for subsidence control, and aided by improvements in productivity (mechanization).

There are many variations of the basic method too numerous to mention in detail, but some features are common and worth noting. Large room-like stopes, some with pillars, are common, lending a room and pillar aspect to face haulage. Dump pockets (ore passes) may be man-made in the fill, or driven in the waste, and LHD-type equipment seems to predominate in the larger operations.

Very good fragmentation control is achievable with this plan, greatly reducing the need for oversize breakage capability in conjunction with a portable crusher and improving (via scalping) the capacity of any given crusher. Crusher location is highly dependent on variations within the basic mine plan, but most comments applicable to sublevel cave will apply here as well, including those relating to conversion of existing rail systems and haulage levels to conveyorized ore handling. Decentralized crushing is potentially more economical for new developments, and the required machine parameters are within the limits already listed.

There are many non-room and pillar mining plans in use in addition to the four discussed. They include top slicing, breast stoping, overhand, underhand, and shrinkage stoping, horizontal and inclined cut and fill, undercut and fill, and many timbered arrangements. Most tend to be small, and possibly narrow-vien, operations, and would use few (or one) seldom-moved crushers. Any new or converting mine must plan on the basis of existing proven equipment, but all are considered potential candidates for portable crushers.

The best indication of the economic benefits that may be derived from the introduction of portable crushers can be found in operations experienced in their use. Hence this study included visits to mines handling soft materials for which present portable feeder-breaker equipment is entirely satisfactory. Not surprisingly, these visits were to low head room room and pillar mines, the type most adaptable to decentralized crushing and conveyorized ore handling. It is felt, however, that these case studies realistically reflect the potential of the decentralized low head room crusher concept for the hard rock industry.

Salt, potash, trona, and copper mines have successfully employed portable crushers. Much of the trona is machine mineable, so the drill and blast methods of the other mines are more appropriate for an economic analysis. The copper mine is a medium to hard rock mine and deserves separate scrutiny.

Specifically, one salt and four potash mines were selected, all employing the decentralized crusher concept. Operations were quite similar, and all believed their use of the equipment to be optimum hence their experience represents a true measure of the full potential of portable crushers.

One potash mine, facing expensive face haulage problems and long (as long as five miles), inefficient rail haulage, started a modernization program in 1967. An initial attempt at a 48-inch main belt, fed by rail and ROM ore resulted in severe alignment and spillage problems in fact the belt was abandoned for a period of about one year because of excessive haulage delays (roughly 2-3 hours lost time per shift). Alignment and spillage problems were alleviated with the introduction of feeder-breakers at shuttle car discharge points in 1970. Lost time related to haulage was completely eliminated, and a manpower reduction was possible. The mine went completely to feeder-breakers and belts in 1972.

Productivity, at roughly 100 tons per man shift with the original rail system, dropped to 84 tons/manshift with the introduction of ROM belting, and has since risen to 153 tons manshift with feeder-breakers and belts. Presently, belt life is considered to be unlimited, with minimal, largely routine, maintenance. Crushing and conveying cost is presently $. 15/ton ($ 1. 93/ton/total mining cost) as compared to $ 24/ton haulage cost alone prior to 1968. Feeder-breakers run untended, although a mechanic visits each one twice per shift. Crew size has been reduced from 9 or 10 to 7. In this time span, mining costs have been reduced 3. 5%, despite labor and material cost increases of about 40%. Safety has also been enhanced: three fatalities were associated with the rail haulage system prior to 1972; none have been experienced since.

As would be the case in any mine, this modernization program was not limited solely to the introduction of a better haulage system hence the improvements cannot be ascribed entirely to the use of portable crushers. The program also included new face mining equipment, improved maintenance training, improved blasting practices, and an improved safety program. Still, the use of portable crushers and the accompanying major haulage advantages played a dominant role in the striking results.

A second potash mine had been using portable standard roll crushers that required about four feet of installation excavation, complete with a concrete base, and required 50 man shifts per move. Introduction of truly portable feeder-breaker units reduced crushing costs by about 50% and significantly increased the availability of other equipment due to shorter face haulage distances. Thus, less than optimum portable equipment, though perhaps satisfactory in comparison to other alternatives, can fall far short of the full potential of the proper equipment.

A third potash mine was experiencing an oversize problem while conveying ROM ore. Introduction of feeder-breakers reduced conveying costs from the former $. 15/ton to $.07/ton, and reduced conveyor downtime to 10% of its former value. Furthermore, the ability to eliminate oversize material at the feeder-breaker allowed a reduction in face drilling from a 27 to a 16 hole pattern. Larger loaders were also used, and an increase in productivity, from 125 tons/manshift to 200 tons/ manshift was experienced, with major credit being given to the use of portable crushers.

The fourth potash mine has just changed to portable feeder-breakers. Previous ROM belts suffered excessive maintenance problems, with up to 2 hours lost time per shift. In line with experience in similar nearby mines, feeder-breakers are expected to provide a crushing and conveying cost of roughly $. 15/ton. a 30% increase in tons/manshift elimination of one laborer per crew, and unlimited belt life with minimal maintenance.

The salt mine was also using a roll crusher, a large unit requiring a 26 foot deep site excavation and a 200 foot long conveyor ramp. Every installation (moveup) cost $20,000 to $35,000 and required three months time, in spite of rather generous 18-20 foot high backs. A full time operator was required, and haul distances were excessive for much of the three year life of the installation.

A conversion from loaders and trucks to LHDs coincided with the introduction of feeder-breakers. With the new multi-crusher system, maximum haul distances were reduced from 4400 feet to 750 feet, and moveups were reduced from three months to two shifts. Equipment investment costs are 8. 75 cents per ton per year versus 17. 5 cents for the old system. Operating labor was reduced by 50.3%, maintenance labor by 39.1%, maintenance materials by 57.9%, and the crusher operator has been eliminated.

It is worth noting again that these examples have been selected as representative of optimum conditions, unlimited by machine shortcomings. All use short (150 to 300 ft) average haul distances, and short, rapid (1-2 shifts) crusher moves which require relatively little labor (10 man shifts) and little or no site preparation.

The situation at White Pine is not so clearly advantageous as the preceeding situations, largely because White Pines system is far from that optimum that would exist in the absence of machine shortcomings. This particular mine is, of course, of special interest to this study because it represents the state of the art in portable hard rock crushing.

At the present time, in comparison to present alternatives, portable crushers are the system of choice at White Pine, even though clear savings in crushing and conveying as startling as those in potash and salt mines, are not demonstrable. As discussed in Section 5.1, other advantages, believed to be substantial but certainly difficult to quantify, justify the system.

But what could be achieved in the absence of machine shortcomings? Present major shortcomings at White Pine include high crushing costthree to five times as high as conventional (albeit non-portable) hard rock crushers handling the same material, and, as used at White Pine, lark of portability. The latter results not only in excessive cost and downtime for moving; it also renders the benefits of optimum haul distances unattainable, as Table III in Section 5.1 indicated. The situation is remarkably similar to that mentioned above in the second potash mine: an initial system, which, though presumably better than other alternatives, was substantially short of its full potential because of the shortcomings of the key portable crusher element. Thus, with development of a truly satisfactory hard rock portable crusher, there is no reason to believe that the substantial economic benefits enjoyed by the soft rock industry cannot be enjoyed by this mine and the rest of the hard rock industry.

Projected potential economic benefits of portable crushers are difficult to obtain for this category because there are no existing operations with published data on which to base estimates. There are, however, two significant cases worthy of mention.

Operational details for this new 11,000 tpd mine were presented in Section 5.4, but a few items bear repeating. Thirty-five foot backs allow this mine to utilize modified (for portability) quarry equipment. The rock is sufficiently strong (22,000 psi) that jaw type crushers are preferred, making this a hard rock application.

Projected mining costs are not published, and, as the mine is still on development, cost histories do not yet exist. What is interesting to note is that of all available mining plans and equipment alternatives, this new mine decided on a decentralized, semi-portable crusher/belt arrangement utilizing proven hard rock crushing principles. This choice was made in spite of the fact that both vertically fed crushers are married to a bench because of their dump height requirements. Low profile crushers loaded at floor level might have made an even greater contribution toward reduction of ore handling costs.

Only two panels are being worked at Maysville, so greater decentralization of the crushing function is perhaps not justified. However, the lead mines, and other, perhaps not so high limestone mines, could well be future beneficiaries of the advances currently being attempted by Dravo.

In Section 4. 2. 3 we discussed a Cameron Engineers study of six large scale oil shale mining plans, two of which were high head room room and pillar methods based on portable crushers. The results of the economic analysis portion of that study were most interesting from the point of view of this study. Both portable crusher mining plans were lowest in per ton pre-production cost, and one (chamber and pillar) was the lowest in production cost as well. Capital costs for the ten decentralized crushers were lower than those for the large fixed crushers of the other four plans, and underground personnel requirements for the chamber and pillar method were the lowest of all plans.

Oil shale is not really hard rock, and the portable crushers selected in the Cameron study were not, and did not need to be, hard rock crushers. In spite of this, portable crushers, used in conjunction with belt conveyors, appear to be an impressive tool in the effort to control high head room room and pillar mining costs. With proper equipment, these benefits should be attainable in hard rock mines as well.

In spite of the overwhelming number of room and pillar mines, the non room and pillar group accounts for much of the total tons mined underground in this country. Consequently, if portable crushers for low head room room and pillar mines were adaptable to this group, the impact on the industry as a whole would indeed be tremendous. Since no mines in this group are presently using portable crushers, and none are known to be contemplating such use, we cannot cite case histories of savings so gained. In addition, since rather sweeping changes in traditional mine plans would be required in order to incorporate portable crushers, and since suitable crushers do not exist at this time, accurate prediction of the potential benefits is beyond the scope of this study.

A trend was found, however, toward decentralized, if not portable, crushing. In order to eliminate costly rehandling and retramming of ROM ore, some mines have located their central, or perhaps a second, primary crusher in the ore body away from the shaft. In virtually all cases, bolt conveyors were used between the crusher and the shaft, and all felt that this arrangement was preferrable to longer rail or truck hauls of ROM material. The portable crusher concept under study here is really nothing more than an extension of this trend, and with mining plans based on successful portable crushers, non room and pillar mines should be able to realize reductions in ore handling costs.

Portable crushers close to production sites would provide ore handling cost reductions in room and pillar mines, and decentralized crushers (i.e., small to fit within headings but not necessarily frequently moved) could provide savings in many non room and pillar mines. The earliest applications of portable crushers would be in room and pillar mines where the savings are greater and where the flexibility of the system permits a relatively easy switch to new, conveyorized methods. Early development work should concentrate on this application.

At the present time, portable crushers in use on medium to hard rock in this country are based upon feeder-breaker principles developed for coal and other soft minerals. In terms of machine size, throughput, and critical inlet dimensions, they are quite satisfactory. When viewed simply as hard rock crushers, however, crushing costs for these machines are well above those typical of conventional designs intended for hard rock. The latter, in turn, simply cannot meet the necessary dimensional requirements for portability. Thus there is a clear need for the development of a new, compact, portable crusher fundamentally suited to handling hard rock.

For hard rock, jaw crushers, or units using similar principles, are preferred, at least within, or nearly within, conventional means. Three novel low head room concepts will be presented, although only one, utilizing jaw crusher principles, is recommended for immediate development.

Economical belt conveyor haulage is not compatible with ROM ore. However, the use of crushers to obtain a beltable product at or near production sites should not be construed as a complete solution to the oversize problem:

Virtually any crusher, regardless of type or size, will occasionally be fed material too large for its critical inlet dimension. Present means for handling oversize range from the double jack (sledge hammer) to dynamite. As machine size is decreased in the interest of portability, it can be concluded that the handling of oversize feed will be of increasing importance. The development of truly portable hard rock crushers will be greatly enhanced by the parallel development of automated, reliable means to reduce occasional oversize in the feed to manageable dimensions.

The greatest oversize problem in non room and pillar mines occurs as large chunks right at the production site. Because of the number of such sites, the relatively low average throughput at each site, and the frequency of such chunks, portable crushers cannot be justified at each production site.

Once oversize problems are successfully handled, perhaps by mobile impactors in extraction drifts, or by impact or other means at dump pockets, decentralized crushers placed at collection points of adequate throughput could provide significant haulage savings in conjunction with a (new) mine development geared to economical belt haulage.

Portability and maintainability can both be enhanced by employing separate units such as hopper-feeder and crusher, or hopper-feeder-scalper and crusher. Separate units become mandatory as the number of features increases to include surge capacity, feeding, scalping, oversize breakage, and crushing.

Using the data of Section Six and the references cited therein the potential market for portable underground hard rock crushers can be estimated as follows: Total annual, underground, non-fuel mineral production was 153 million tons in 1971. The exclusion of mines producing less than 1200 tons per day reduces this figure to 127 million tons per year, produced by 95 mines.

If one assumes that room and pillar mines represent the only realistic near and medium term market, another 50 million tpy are excluded resulting in a 77 million tpy market. Longwall, feeder-breaker, and other coal-type hardware has already been found acceptable in salt, potash, and trona mines producing perhaps 35 million tpy. Exclusion of these mines leaves approximately 50 mines producing about 42 million tpy, or about 140 thousand tons per day, 27 percent of the total industry. The market breakdown is as follows:

Having narrowed the market so drastically to that 27% having a clear need, no hardware alternatives, and maximum adaptability to belt conveyors and decentralized crushing, a near term penetration of 50 percent of this market segment would seem conservative (70 thousand tons per day). With very rough estimates for availability, utilization, shifts per day, etc. this production could be handled by perhaps 50 machines rated at 200 tons per hour. At a guestimated price of $250,000 including related hoppers, feeders, discharge conveyors, and the like, the dollar market becomes $12. 5 million exclusive of replacements, rebuilds, and spare parts.

Half of the medium and hard rock room and pillar mines are considered to be the readily reachable near term market. They number about 25 mines producing 21 million tpy (70, 000 tpd) and represent 14 percent of the total industry production.

The cost of mine-wide conversion to decentralized crushing depends in large part on the existance, or absence, of a workable belt conveyor system within a given mine. Where conveyors are lacking, this cost far exceeds the cost of the portable crushers alone.

The preceding market estimations are deliberately conservative, and evidence exists to indicate that early success would eventually lead to much greater usage of the portable crusher/belt conveyor concept. The salt and potash mines are prime examples: Where suitable portable crushers are available (in this case feeder-breakers) they are the economic system of choice in room and pillar mines. Full acceptance by room and pillar mines would result in a doubling of the previous market estimates, particularly if some of the smaller mines (under 1200 tpd) are included.

Non room and pillar mines produce 40 percent of all production over 1200 tpd. Because of the costly and complex aspects of conversion to decentralized crushing and (probably) conveyorized haulage, penetration of this segment of the market will necessarily follow successful acceptance by the room and pillar mines. Some mines will undoubtedly find conversion too costly or impossible, but there is no fundamental reason why non room and pillar mining geometries cannot be planned to take advantage of the economies of decentralized crushing and low cost conveyorized haulage.

These would be oil shale mines, open pit mines, quarries, as well as Canadian and other non domestic mining industries. These markets, though potentially large, are beyond the scope of the present study.

As far as machine parameters are concerned, the market for portable hard rock crushers is broadly divided into those mining applications that require, or do not require, crusher concepts dictated entirely by the need for a low profile configuration:

Where installed crusher size is not a deterrent (high head room, room and pillar mines like limestone and, perhaps oil shale), significantly new crusher-concepts are not required, and adequate throughput, though large is already available. However, improvements in portability can be made by better design of modular assemblies, and new low-profile concepts would be applicable, perhaps preferred. Manufacturers have indicated a willingness to undertake development of such improvements to customer specifications.

Where machine dimensions are the principle deterrent to portability (low head room room and pillar and, later, most non room and pillar mines), a portable hard rock crusher should have the following design parameters:

Throughput, which might otherwise be of considerable concern, will not be difficult to provide for the selected input dimensions, although scalping will likely be required for the higher throughputs. Scalping of feed should be provided, where possible, to improve capacity and avoid unnecessary crushing of beltable material.

When one is desireous of portability, there is always a tradeoff between machine size and machine capacity. Within these dimensional limits, it is expected that most mine operators will desire crushers closer to the small end of the range; that is, seven feet high and eight feet wide, and perhaps even lower in height during tramming. A small machine can be used in a large-drift mine, but the reverse is obviously not true. If a seven foot high crusher can achieve 200-300 tons per hour, there may be no appreciable underground market for nine foot machines regardless of their throughput. In short, we need to know more about what is achievable within the seven foot height, from a designers point of view, before we can say there is an underground market for taller or larger machines.

The vast majority of underground applications in the near term will require a machine capable of achieving a seven foot height, at least during tramming. Modular construction for entry into shaft mines is a requirement.

Scalping, rather than a larger crusher, may be the least expensive method of achieving greater throughput. On the other hand, a larger crusher could presumably handle larger ore. This introduces the whole question of oversize ore, how and where it is handled, and by what means. These are obviously questions having different answers for different mines:

The middle of the throughput range, 200 tons per hour, is a capacity that can realistically be made portable in a low profile. It is also compatible with typical production rates from a single mining unit. Higher unit capacities are available through varied combinations of face haulage equipment, traffic patterns, surge capacity, and scalping.

rock crushers

The size requirement of the primary rock crusher is a function of grizzly openings, ore chute configuration, required throughput, ore moisture, and other factors. Usually, primary crushers are sized by the ability to accept the largest expected ore fragment. Jaw crushers are usually preferred as primary crushers in small installations due to the inherent mechanical simplicity and ease of operation of these machines. Additionally, jaw crushers wearing parts are relatively uncomplicated castings and tend to cost less per unit weight of metal than more complicated gyratory crusher castings. The primary crusher must be designed so that adequate surge capacity is present beneath the crusher. An ore stockpile after primary crushing is desirable but is not always possible to include in a compact design.

Many times the single heaviest equipment item in the entire plant is the primary crusher mainframe. The ability to transport the crusher main frame sometimes limits crusher size, particularly in remote locations having limited accessibility.

In a smaller installation, the crushing plant should be designed with the minimum number of required equipment items. Usually, a crushing plant that can process 1000s of metric tons per operating day will consist of a single primary crusher, a single screen, a single secondary cone crusher, and associated conveyor belts. The discharge from both primary and secondary crushers is directed to the screen. Screen oversize serves as feed to the secondary crusher while screen undersize is the finished product. For throughputs of 500 to 1,000 metric tons per operating day (usually 2 shifts), a closed circuit tertiary cone crusher is usually added to the crushing circuit outlined above. This approach, with the addition of a duplicate screen associated with the tertiary cone crusher, has proven to be effective even on ores having relatively high moisture contents. Provided screen decks are correctly selected, the moist fine material in the incoming ore tends to be removed in the screening stages and therefore does not enter into subsequent crushing units.

All crusher cavities and major ore transfer points should be equipped with a jib-type crane or hydraulic rock tongs to facilitate the removal of chokes. In addition, secondary crushers must be protected from tramp iron by suspended magnets or magnetic head pulleys. The location of these magnets should be such that recycling of magnetic material back into the system is not possible.

Crushing plants for the tonnages indicated may be considered to be standardized. It is not prudent to spend money researching crusher abrasion indices or determining operating kilowatt consumptions for the required particle size reduction in a proposed small crushing plant. Crushing installations usually are operated to produce the required mill tonnage at a specified size distribution under conditions of varying ore hardness by the variation of the number of operating hours per day. It is normal practice to generously size a small crushing plant so that the daily design crushing tonnage can be produced in one, or at most two, operating shifts per working day.