Table of Contents
- Portable Rock Crushers in Underground Mining
- Portable Rock Crusher Requirements
- Primary Crushers & Primary Crushing
- Crusher Types and Operating Characteristics
- Pressure Crushers
- Gyratory Crushers
- Jaw Crushers
- Horizontal Jaw Crushers
- Impact Crushers
- Combination (Sledging ) Crushers
- Single Roll Sledging Crushers
- Features of Sledging Roll Crushers
- Feeder Breakers
- Longwall Breakers
- Suitability of Present Crushers
- Portable Crusher Throughput
- Crushing Machine Height
- Portable Rock Crusher Cost
- Portable Crusher for Open Pit and Quarry Operations
As it relates to portable crushers, the basic portability concept under investigation here might better be described by the phrase ”decentralized crushing to allow automated ore haulage”. Clearly this means more and smaller crushers exhibiting some degree of mobility, and automated ore haulage usually means belt conveyors. The trade-off is a necessarily more costly crushing system against a more efficient and productive ore handling system. From the crusher manufacturer’s point of view the challenge is to achieve small size and portability without sacrificing too much in the important areas of feed opening, throughput, system availability, and capital and operation costs.
In order to better understand that which follows and to establish a common basis for discussion within the purposes of this study, a few definitions are in order:
“Portable” in Portable Rock Crushers simply means that the crusher is moved periodically in order to be close to production, thus minimizing costly haulage of run of mine material. Within this simplified definition however, portability has quite different meanings in mines of widely varying ore bodies and mining plans. We shall further assume that a portable crusher is one that can be moved through standard mine passageways with minimal dismantling, and can be set up with little or no site excavation.
“Underground” is obvious, and when taken with “portable” brings to mind such terms as low, narrow, horizontal, light, serviceable, and mobile. This study may define a machine that is also applicable to some above ground installations but no attempt will be made to enhance such applicability at the expense of underground performance.
“Hard-rock” is sometimes taken to mean “non-coal”, but this broad definition would include many weaker mineral mines not in need of the fundamentally new equipment that is the subject of this study. Many of these non-coal mines have, however, developed highly efficient and mechanized coal-like mining methods that would be applicable to hard-rock mines if suitable equipment (crushers) were available. We have therefore gained valuable information by studying these mines, but the intended beneficiary of this investigation is the underground “hard-rock” industry, defined as those mines that cannot economically make use of presently available portable underground crushers.
To begin, let us attempt to define approximate requirements in order to establish a background for further specification of performance parameters, and to form the basis for a critical examination of existing crusher designs. In fact, it seems clear that no single “optimum” set of parameters can ever be sharply defined. However, with adequate documentation and an appreciation of likely individual case variations, such an approximate set of parameters can serve as the basis for new concept generation and further development work.
Portable Rock Crushers in Underground Mining
Before defining what a portable rock crusher is, we need to know how it will be used. Fortunately for the purposes of this study, portable underground crusher applications may be divided into two rather distinct categories, and one of these, though worthy of further thoughts and development, does not require fundamentally new hardware development. The distinction, perhaps predictably, is primarily one of physical machine size, although, to a lesser degree, distinctions can also be made in the desired degree of portability within a given size category.
The first category, which we shall dismiss for the moment, is one in which machine size, per se, is not limiting. Applications in this category are high head-room room and pillar mines, such as large limestone mines having 35 foot backs , and, in the future, oil shale mines having even higher backs. While significant portability improvements can be made in assembly methods and general layout, as discussed in Section 9, this category of applications ran in general be satisfied by existing manufacturers through modification of essentially standard machine components.
The second category is that in which machine size is very much a limiting factor—so much so that today’s standard hard rock primaries are simply not applicable. The two general mine types falling in this category include, obviously, low head room room and pillar mines and, perhaps not so obviously, most mines with vertically oriented ore bodies. The latter include caving mines, whatever the caving mechanism (block caving, sub-level caving, etc.), and other generally vertical mine plans such as open stope, shrinkage stoping, cut and fill, etc. . For purposes of this study, these mines are collectively characterized by gravity delivery of ore to a stationary or nearly stationary, draw point or chute from which the ore is handled (and often rehandled) by a variety of means in both the horizontal and vertical directions. Even though massive ore bodies may be involved, typical drift dimensions in such mines are not large, on the order of 8 to 12 feet high by not much greater widths.
Both mine types in this category of “small” applications suggest maximum installed crusher sizes of 7 to 9 feet high, 8-10 feet wide, and any reasonable length (the latter determined by transport conditions rather than installed dimensions”. It is important to note that this height includes whatever overhead feed components (and dump space) may be required by vertical feed crushers—thus standard top fed jaw crushers, which would normally be selected for hard rock, are much too tall.
Portable crushers will receive run of mine material from the “face” regardless of the mining method or the primary haulage system used, and then crush this ore and feed it into a more continuous and efficient ore haulage system. Within these applications it appears that for a decentralised crusher arrangement a throughput of 100 to 800 tons per hour will suffice. Although there is no clear-cut limit, this throughput is obviously a function of the size of the mining unit it services, and the ability, within the stated drift dimensions, of the primary haulage system to deliver material to the crusher. Thus it is not surprising that a limited range of throughput’s will serve a wide variety of mining operations.
The Input to the System
Just like the very large central crusher located (probably) at the shaft, the proposed decentralised portable crusher system must handle ROM (run of mine) ore. This fact, when taken with the low headroom restrictions, will continually challenge the would be portable crusher designer.
A study by the U. S. Bureau of Mines in five underground mines, utilising five different mining methods, in extremely different types of rocks, showed a striking similarity of “over-size ore”, not only in mean size but in shape as well. Table I presents these results. The indicated size uniformity is considered misleading, particularly in view of the fact that the study did not attempt to
determine the percentage of ore exceeding the stated oversize. The shape trend of this data (3:2:1) is more interesting, indicating a condition somewhere between block and slabby. Larger variations in size of oversize are supported by another study which was concerned with block caving mines. Results of this study, also presented in Table I, characterize the block cave mine of the preceding study as having “fine ore”. There is clearly no single optimum crusher feed opening for these, let alone all, block caving mines, although it is probably safe to say that block caving permits the least control of fragment size and can thus be expected to present highly variable conditions.
Mining plans relying on drilling and blasting for fragmentation control will, no doubt, show greater uniformity in size of oversize, but great variations are to be expected in the size distribution of ROM ore from mine to mine. Assuming a successful crusher can avoid direct attack of the three-to-five font major fragment dimension indicated in Table I, and assuming some form of control over occasional abnormal oversize, it is likely that minimum or “critical” feed openings in the 30-36 inch range will satisfy a very large percentage of mines.
Output from the System
To establish approximate product size, let us assume that the product is to be belt conveyed. In most cases this will be true, and it is expected that maximum economic benefit will occur in this combination. The feeder-breaker, so successfully used on coal mine section belts, is generally set to produce nine inch maximum lumps for 36 inch belts. For first-cost and other reasons, this belt width appears to be very common for section and feeder applications, and for the denser-than-coal ores found in the hard rock industry, a maximum product size in the range of 6-8 inches is appropriate, it is interesting to note that even for very large oil shale installations (very wide belts) a six inch product is recommended.
Portable Rock Crusher Requirements
In summary, approximate parameters for the development of portable underground hard rock crushers are as follows:
It appears that there is relatively little need to simultaneously develop a range of machinery between these small units and the large central primaries now being used. Ultimately a range of intermediate sizes will be desirable, of course, but this can easily be developed from low head room equipment meeting the above specifications.
As will be illustrated in the following section, these requirements cannot be met by existing hard rock crushing equipment. In fact, noting that the desired dimensions include whatever overhead clearance is needed to load the crusher proper, and space underneath to deliver its product (assuming a typical vertical jaw or gyratory design), it is obvious that standard machines are far from satisfactory. It follows, then, that satisfactory new concepts cannot be found among minor variations of standard concepts: the sought after design will differ substantially from present designs. At the same time, it would be comforting if a new concept did not depart substantially from the basic comminution means of proven designs. Economical crushing of hard rock, day in and day out, through many millions of tons, is, after all, a rather difficult task, even without severe space limitations, and proven means should not be so quickly discarded.
The inventor’s task is not quite so formidable as the proceeding may suggest. In comparison to a typical aggregate production application for example, some aspects of the portable application actually ease the design problems: The crusher is needed only for oversize (unbeltable) material. Thus, while the crusher should avoid fines, it has no rigid product size requirement other than maximum size, and essentially no product shape requirement (a requirement that justifies some rather subtle variations of crusher geometry in many conventional applications). Furthermore, if the crusher is designed to pass undersize material freely, or if its feed mechanism provides scalping to bypass smaller material, much of the throughput will be “free”, a provision which will also reduce the production of fines, and, more importantly, dust.
Section 9 describes low head room, hard rock, portable crusher concepts which may meet the indicated requirements, and one of these, at least, uses the proven principles of conventional jaw crushers.
Primary Crushers & Primary Crushing
Many manufacturers were contacted in an extensive effort to include all available equipment and manufacturing capability in this study. Appendix A is a list containing the names and (if available) addresses of those manufacturers who were contacted. Although not all were responsive, many were quite helpful and the majority expresses the opinion that they would need the results of this study if the industry or any single manufacturer were to consider the development of portable, underground, hardrock crushers.
An extensive literature search was also conducted, and the pertinent work is included in the bibliography. References to this list are found throughout this report.
Definition of the Application
This study was neither intended, nor will it attempt, to instruct the reader in the complete art of primary rock crushing. There are many good references in this area; notable among these is McGrew. Our goal is to define the optimum parameters for the design of a portable, underground, hard rock crusher in order to insure that future development will lead to maximum utilization by the industry.
In summary then, we want to study present crusher types with an eye toward moving them around in hard-rock mines. Though small, these units will handle essentially as mined or ROM material, and should rightfully be called “primary crushers”.
Crusher Types and Operating Characteristics
There are three basic classes of crushers, which, when restricted to the task of underground primary (large feed) crushing can be listed as follows:
- Pressure Crushers —This category includes gyratory and three types of jaw crushers.
- Impact Crushers—The only “primaries” in this category are the hammermill and the impactor.
- Combination (Sledging) Crushers –Included here are single or double sledging roll crushers and the relatively new feeder-breaker.
This class of crusher historically has been used on the strongest ores. Crushing is accomplished by relatively slow moving members exerting very high force levels. Understandably, these crushers are typically very big, very strong, and heavy.
Figure 1 shows a simplified section of a typical gravity fed gyratory crusher. Clearly the typical portable underground crusher requirements presented in Section 2 cannot be met by a standard gyratory. However, because the crushing action of the gyratory works well on hard rock, the portable crusher designer should be aware of the favorable features exhibited by this important member of the primary field:
Benefits of Gyratory Crushers
- High capacity per dollar of investment.
- Annular discharge opening minimizes slabbing; makes for more cubical product.
- Shape of receiving openings favorable for slabby material. This, plus point 2, gives the machine a distinct advantage over the jaw for handling thinly stratified stone.
- Long receiving openings, plus large area, tend to minimize bridging.
- Double receiving openings permit dumping from two sides of crusher.
- Relatively low flywheel effect minimizes starting peaks; also allows the machine to stop quickly when power is cut off due to over-loads caused by tramp iron or choking.
- Machine can be serviced by simple overhead crawl-type hoist, whereas jaw crusher requires crane with travel in two directions to properly handle parts.
- Relatively high pinion shaft speeds permit use of higher speed motors, and lower-ratio drives.
- Lubrication is simpler and more economical than is possible to achieve in the jaw crusher.
- Cost of foundation will usually be lower than for the jaw crusher.
- Necessary safety guards are less extensive than for the jaw crusher.
There are two popular types of jaw crushers; the Blake or double toggle type as shown in Figure 2, and the overhead eccentric or single toggle type shown in Figure 3.
As a class, jaw crushers exhibit the following characteristics:
Benefits of Jaw Crushers
- Large receiving opening per dollar of investment.
- Shape of receiving opening favorable for blocky feed. This, plus point 1, gives the jaw a definite advantage over the gyratory, except in the very large sizes, for handling rock of massive formation.
- The jaw crusher is more easily adjusted, to compensate for wear of liners. This is particularly true in the larger sizes, and the range of adjustment is greater in all sizes.
- The jaw crusher will handle sticky or dirty feed better than the gyratory, there being no diaphragm below the crushing chamber on which such material can pack.
- Routine maintenance and repair jobs are generally more easily accomplished on the jaw crusher than on the gyratory. Major repairs are about a standoff.
- For crushing extremely hard, tough materials, extra strength can be built into the jaw crusher at less extra expense than in the gyratory. So-called “standard” designs, i. e. unreinforced, are usually more rugged than the standard design of gyratory.
Single and double toggle jaw crushers differ in the motion characteristics of the moving jaw, which results in somewhat different operating characteristics. Jaw action in the Blake (double toggle) type is a simple pivoting motion about a stationary bearing near the receiving opening. Displacement is thus a maximum at the discharge, tapering to zero at the pivot.
Because of its simplicity, the overhead eccentric (single toggle type) exhibits lighter weight, much lower cost, and a greater potential for portability, although it is not significantly shorter than the Blake (double toggle type). Due to the pronounced vertical components of motion from the overhead eccentric, it elliptical wiping motion provides good feeding action, and hence capacity. The price for this action is, of course, accelerated wear of the jaw plates in addition to increased shock loading on the eccentric and shaft bearings caused by the large jaw motion ‘relative to Blake type machines’ at the receiving opening. Consequently, Blake types, with their low scrubbing motion and great leverage on larger feed, tend to be favoured for highly abrasive or very hard, tough rock.
Horizontal Jaw Crushers
The basic overhead eccentric jaw motion has been built in a vertical double-eccentric version (both jaws moving in unison), with the intention of providing more capacity for a given feed opening and longer jaw life due to reduced scrubbing provided by lower relative jaw velocity. The Eimco Division of Knvirotech, and the Westfalia Company of Germany, have tipped this arrangement on edge (eccentrics vertical), thereby changing the feed direction from vertical to horizontal and greatly reducing machine height.
Little is known about the German machines, as none are in use in North America and none are believed to be handling predominately hard rock. Eimco, on the other hand, has built two prototypes which have been tested in medium and hard rock in low headroom conditions. The Eimco crusher, shown in Figure 4, utilizes a feeder-breaker style chain flite conveyor which pulls material from the bottom of the surge pile and stuffs it into the jaw region. Discharge occurs immediately after the choke region of the jaws, onto a customer supplied conveying means. The chain conveyor obviously must pass beneath the active region between the jaws, severely diminishing or eliminating its feeding ability, particularly during the crushing stroke. To achieve better feeding in the crushing zone, Eimco has modified the common overhead eccentric toggle geometry so that both jaws close every where at the same time, with the crushing stroke strongly oriented in the feed direction. These measures enable a second generation machine to achieve throughputs approaching (perhaps 80%) the capacity of a vertical, single overhead eccentric crusher of comparable inlet dimensions. The Eimco inlet is approximately 40 x 40 inches.
Both prototypes were tested at White Pine Copper in White Pine, Michigan. Problems were encountered and changes were made, as with most prototypes, but large blocks of 20-28,000 psi sandstone were successfully handled on a regular basis. Since Dial time, mining
at White Pine has been concentrated in medium strength shale, where the horizontal jaw is not sufficiently perfected to be competitive with heavy duty feeder-breakers, about which more is presented in subsequent sections. Very strong ores have not been tried on a significant scale in the horizontal jaw.
Though low in profile, this crusher design utilizes a feed means that tends to orient slabby material horizontally, hence the wide, square jaw opening. Slabs that do get fed on edge can be passed untouched through the jaws, a common problem with vertically fed jaw crushers as well. Dimensionally, horizontal jaw crushers are quite acceptable, though they could use elevating discharge means to reduce site excavation requirements, and with more development in hard rock applications, this concept may become an economical alternative candidate for the subject application.
“True impact crushers for primary crushing are limited to “hammer” types. They are included here only because there may be a specialized situation justifying their unique characteristics. Figure 5 shows a section of a typical hammermill; Figure 6 shows an Impactor.
Impact type crushers are high reduction machines (up to 40:1 vs. 8:1 for a jaw). In part because of this, they produce a considerably finer product than is necessary to achieve mechanized underground haulage. Very large feed, as is common with ROM material, is not easily handled by the hammer mill because of its impact principle of operation. Crushing is accomplished by the high velocity impact (5000 fpm) between the hammers (and liners) and individual pieces of rock in the feed, with the only means of support of rock fragments being the inertia of the rock itself. Under these conditions the rock fragments should not only be less massive than the hammer, but also quite friable. Abrasive feeds cannot be economically handled by hammermills or by impactors.
Impactors, as Figure 6 indicates, are better suited to large feeds than is the hammermill. This type uses fewer and stouter hammers, but, like the hammermill, relies on the inertia of the feed to “hold” the rock while it is chipped away. Primary crushing, even of non-abrasive and friable material, and particularly underground, is better handled by other machines unless very special conditions exist. An admittedly unlikely example of a situation in which an impact type crusher could be successfully employed as a portable underground primary crusher might be described by the following conditions:
(a) abnormally small ROM material suitable for impactor feed but too big to be conveyed.
(b) very friable, non-abrasive feed, material.
(c) fine product allows less expensive form of mechanized haulage and eliminates the need for secondary crushing equipment.
Due to the specialized and unlikely nature of this hypothetical application, pure impact crushers are excluded from further consideration for the purposes of this study.
Combination (Sledging ) Crushers
“Roll crushers” is a term sometimes used to describe the combination (impact & pressure) class of crushers. “Sledging roll crushers” is a more suitable name, since it is distinguishing from the impact and pressure terminology and, in fact, the rotor in a roll crusher is frequently called a sledging roll. Sledging roll crushers are characterized by a medium velocity impact (500 fpm or less) between a rotor protrusion and the feed material while the feed is supported in the crusher, hence the term sledging.
The term “roll” is used in a wide variety of non-sledging equipment types and needs clarification here. “Crushing rolls”, two-roll feed-pinching machines, are really a high speed continuous pressure class of crusher used for secondary and tertiary crushing. Sometimes they are confusingly called two-roll crushers, or double roll crushers, or four-roll crushers. The roll surfaces are usually smooth or nearly so and impact ‘or even sledging’ does not play a significant part in the comminution process. “Roll crusher” may also be used to describe a high speed machine in which the feed is neither supported by the crusher nor “nipped” by the roll protrusions. As described in the previous section, this is a high reduction pure impact class crusher sometimes used to avoid secondary crushing.
Single Roll Sledging Crushers
Sledging roll crushers may be of the single- or double-roll type, the latter being distinguishable from smooth pressure class crushing rolls by the characteristic protrusions (sledges) which work on the feed material. Double-roll sledging crushers usually employ more impact and less sledging by virtue of higher tip speeds, and are principally used for secondary crushing. Figure 7 shows a typical single-roll sledging crusher. There are several features of this type of crusher worthy of mention.
Features of Sledging Roll Crushers
- A dumped load of feed experiences vigorous agitation, immediately sifting smaller pieces down into the crushing chamber.
- This sorting action holds up larger pieces on top of the roll where they are worked on by slugger teeth until material is small enough to be nipped between the rotor and the anvil.
- The pure impact action which takes place on top of the roll on very large feed makes these machines unsuited for stone of massive or blocky structure unless it is friable.
- Maximum feed size for non-friable material should not exceed that which can be nipped, except that breakage of long slabs can and does take place on top of the roll by beam bending.
- Agitation in the chamber and hopper reduces bridging and improves feeding.
- The impact and moderate velocity aspects of this crusher class make it unsuited for abrasive silica bearing materials, and strength of feed is usually limited to about 15,000 psi compressive strength.
- Sledging roll crushers have been found to be generally well suited to crushing oil shale.
The feeder-breaker is an adaptation of the single roll-sledging crusher developed specifically for portability and use in low headroom coal mines. Since it has found successful use in a number of non-coal mines it is therefore worthy of mention. Figure 8 shows a typical feeder breaker.
To achieve low profile, this specialized machine passes material horizontally under the roll, or breaker shaft as it is usually called. The anvil (or bed in this configuration) is flat, and feed is accomplished by a chain-flite conveyor which pulls feed from under the pile of material in the attached surge hopper, and, after passing through the breaking zone, continues on to feed at a relatively controlled rate over the conveyor head pulley, hence the name feeder-breaker. Another characteristic of this single-roll sledging crusher is the shape of the breaker teeth, or picks, as they are generally called. They are relatively few in number (particularly for weak material), replaceable, and pointed, generally being carbide tipped.
Feeder breakers have greatly advanced the practice of conveyorized haulage in coal mines, and during recent years beefed-up versions, pioneered by the W. R. Stamler Corporation, have been successfully employed in a variety of non-coal mines. Among these are underground salt, potash, trona, iron, copper mines, and some open pit mines. These mines use a wide variety of primary short haulage means, but they all make use of low labor, high capacity conveyor systems made possible by the feeder-breaker.
When applied to stronger and/or more abrasive ores, feeder breaker crushing costs naturally escalate to levels well above those of conventional hard rock (i. e., jaw) crushers. In fact it appears that feeder-breakers are used, in some applications, solely because of their low headroom characteristics, and despite crushing costs from 3 to 5 times what could be expected of a jaw crusher in the same material. However, sufficient savings are achieved elsewhere in the haulage system, so that feeder-breakers are the economic choice in one copper mine where the ore is routinely between 12-20,000 psi compressive strength, and also abrasive. That mine also uses feeder-breakers in sandstone sections where ore strength runs to 28,000 psi. Maintenance and rebuild costs are higher in such areas, and this is considered by many to be about the hard rock limit of feeder breakers as a class of crusher.
A narrow version of the feeder-breaker has been developed by a German company for use on longwall systems. Various sledge configurations (not sharp picks) are used, and the unit is generally incorporated in a chain-flite bridge conveyor between the longwall system and a headgate conveyor. Two such units are in use on longwalls in U.S. trona mines (7000 psi max.), which accounts in part for their mention here. The concept (sizing of longwall discharge) is worth noting, in view of U.S. research efforts to apply new technology and longwall methods to hard rock mines.
Comminution Without Crushing
There are many other comminution processes that one could bring to mind. Among these would be all the primary and secondary breakage methods, grinding and milling methods, thermomechanical, and even ballistic and nuclear concepts. These are not considered here because there are no presently available machines using these processes. Other comminution methods in general will be considered in the concepts section (Section 9) after the problem statement has been fully developed and conclusions drawn.
Suitability of Present Crushers
Having discussed the various classes and types of hard rock primary crushers, we can examine their potential for meeting the general requirements previewed in Section 2. Those requirements call for a crusher of low height, large feed opening, and modest throughput. Since multiple small crushers will be less efficient to operate and more costly to purchase than one central crusher, we must also consider cost as a factor in suitability.
Critical Input Dimension (CID)
The one mining parameter that is least controllable in a given mine and has the greatest influence on crusher selection is size of feed. Although drift dimensions obviously cannot be specified by the crusher designer, machine height, to some extent, is in his hands. Accordingly, machine height, throughput, and cost will be examined with respect to the common parameter, feed opening. Since “feed opening” implies a two dimensional passageway for material, the smaller or Critical Input Dimension (CID) will be used where appropriate. The implication is that most any crusher can (and should) be fed so as to avoid direct attack of the largest dimension of the feed material. Also implied, but perhaps less obvious, is the desire and intention to feed material so as to attack the smallest dimension of the feed, not the middle dimension.
Portable Crusher Throughput
Figure 9 presents representative manufacturer’s throughput data as a function of CID for 3 classes of crushers totalling six different types. Capacities have been “normalized” on medium limestone and minus 6 inch product in most cases. Gyratories are clearly high capacity machines at any feed size, and they tend to he applied to very large material. The Blake type jaw crushers are considerably lower in capacity, reflecting to some extent their application to very hard and abrasive feeds. Also noticeable is the range of capacities available for a given CID, a favorable feature afforded by variable jaw or rotor width. The tremendous forces encountered in crushing very large feed tend to leave the stronger Blake as the only jaw type in this region.
Getting down into the throughputs of most concern (400 tph and less), both Blake and overhead eccentric types appear, with the edge in capacity going to the overhead eccentrics. Also appearing are the horizontal jaw crushers and the sledging class, both single roll and feeder-breaker types. Maximum feed size for a given CID will be somewhat less in the case of horizontal jaws because the feed mechanism for this type tends to cause attack of the middle, rather than the smallest dimension of the feed material.
This data seems to indicate the following with respect to the general requirements established in Section 2, that is; 30-36 inch CID and 100-300 tph:
- At the required CID, more than ample capacity (throughput) is available from a variety of machines.
- Gyratories are too much machine—in physical size and capacity.
- Low capacity—large feed is a unique combination not well satisfied by existing hard-rock machines.
Crushing Machine Height
Figure 10 is a plot of bare machine height as a function of CIB for the same six types of crushers. Keeping in mind that bare height is exclusive of any foundations if required) or feeding and discharge means, all conventional gyratory and vertical jaw types are clearly beyond our need for 7-9 foot installed height at 30-36 inch CID. Nor can these standard machines be significantly shortened, as an examination of earlier figures will reveal.
We are left, at present, with horizontal jaws and the sledging class of crusher. But sledging roll crushers and to a lesser extent, feeder breakers, reach their economic limit at medium strength ore, characterized by (among other things) compressive strengths
in the 12-20,000 psi range and, even then, only under specialized conditions. The horizontal jaw crusher would appear to be the lone contestant, but it is relatively new and little can be learned about its economic performance at this time. Westfalia, a German manufacturer of longwall and other mining equipment, developed the concept, and, although machines are in use in Europe, no information is available regarding hard or very strong ore applications, and none are in service in North America. Eimco Division of Envirotech is the U.S. pioneer of horizontal jaw crushers, having built two generations of machines. These machines were technically successful in crushing a regular diet of stronger ore (20-28,000 psi) but could not compete economically in the medium strength range against the then highly developed heavy duty feeder-breakers, a statement which most certainly would apply to weaker ores as well. Dimensionally, the horizontal jaw is virtually identical to the successful feeder-breaker (Eimco data is plotted) and with further experience this basic concept may prove to he one answer to low profile hard-rock crushing.
Portable Rock Crusher Cost
Figure 11 shows the bare cost (no drives, hoppers, feeders, etc. ) of the various crushers under discussion. Some of the data are approximations, but the plot is useful in several respects. It shows, for instance, that something must be sacrificed to get low profile. In the case of horizontal jaws, increased initial cost is the penalty. Feeder-breakers, the low profile member of the sledging class, cannot economically handle the stronger ores. To work on the very hard or abrasive ores, machine height aside, requires that one choose the more expensive Blake type vertical jaw instead of the lighter overhead eccentric. Gyratories having the required CID again are inherently much too much machine for this application.
Using the larger Blake type or gyratories as an example (they dominate as centralized crushers in hard-rock mines) we can get an idea of the capital investment against which a multiplicity of portable crushers must inevitably be judged. Suppose a 7000 tpd mine would need a 48×60 Blake type jaw crushing 500 tph of minus 6 inch product. Such a crusher would cost perhaps $350,000 including significant installation costs. An equivalent portable crusher system might involve five machines, four of which would be in service, with each capable of 250 tph. The greater total crushing capacity of the portable system is necessitated by its need to keep moving up, and by its vulnerability to downstream haulage interruptions. If these five portables cost in the vicinity of $200,000
each (a reasonable assumption for hard rock), the capital investment for portables becomes one million dollars versus $350,000 for a fixed installation. In addition, since the operating and maintenance costs of the two crusher systems are likely to be in about the same ratio, it is clear that the portable system must achieve great savings in other categories. These would likely include primary and secondary haulage costs (capital and labor) find productivity.
Portable Crusher for Open Pit and Quarry Operations
The primary use of a portable crusher, i.e., a crusher mounted on crawlers or tires, in the rock and mining industries is to reduce costs by permitting the substitution of conveyor belt haulage for truck or track haulage. The usual sequence of operations in surface mining is drilling, blasting, loading, haulage, and crushing. Haulage is normally accomplished by truck or track-mounted cars, the latter method being used for the longer distances.
In addition to potential cost savings in haulage procedures, a portable crusher would allow better utilization and performance of shovels. Loading operations would not be interrupted as often by the necessity of waiting for cars or trucks. Unfortunately, the application of belts in open pits for haul¬age from bench sites is generally not practical under existing conditions because a belt fed directly by a mechanical shovel can be torn, damaged, or worn out quickly by the large rock fragments falling on it during loading.
As previously noted, the use of a portable crusher would increase the performance of a loading shovel and thereby decrease the number of shovels required to maintain the same rate of production. However, there are quarries where rock must be taken from different parts of the pit and mixed together in order to get a desirable composition. This is usually done in cement quarries. For such cases, storage of material at the end of the stationary conveyor or along its route is suggested, where the desirable mixture of product could be achieved.
Quarries or open pits using track haulage often require a large number of workers to move the track after blasting as well as to operate the railroad switches. The use of a long-boom shovel would make it possible to increase the distance between the bench face and the track. It would also aid in reducing the amount of time now consumed in moving the track and the number of workers to do the job, but such a shovel is more expensive and slower.
Application of the portable crusher might encourage the use of higher benches with the commensurate less blasting that would be required. Domestic practice, however, does not favor the use of high bench faces, partly for safety reasons during loading and partly because higher benches usually require a large borehole diam, larger drill, etc. Inclined drilling might solve such blasting problems because it reduces the resistance of the rock to blasting at the toe of the bench.