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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 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 main frame. 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 which can process 300 to 500 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 (see Figure 1). 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 (see Figure 2).
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.
The screen or screens in a small crushing plant should be oversized and of the four-bearing type if possible. Installed screens should be standard sized and, if multiple screens are used, all screen panels should have the same dimensions. The screen layout should be such that screen decks are readily accessable. Screens should be located for optimum visibility and required chutes should be designed with the maximum open area possible. To minimize the height of the undersize chute, discharge conveyors should be oriented along the long dimension of the screen. The slope of the discharge conveyor should be as nearly parallel to the slope of the screen as is possible. The utilization of rubber top decks should be considered due to the longevity of this material. The use of tall chutes should be avoided or, if absolutely necessary, should be provided with dead beds and cleanout ports. Spare screen panels should be provided with the original equipment since considerable time can lapse between ordering panels and actual receipt, particularly for rubber covered decks.
Although design calculations may indicate overcapacity, no conveyor in any crushing plant should be less than 0.5 meters wide and, in actuality, it is best to standardize on a minimum belt width of 0.67 meters throughout the crushing system. Design should be such that conveyor troughing and return idlers can be lubricated from one side. Barely are small conveyors designed and manufactured with walk-ways on both sides of the belt. In smaller installations some designers prefer to specify permanently self-lubricated idlers. The design of tail pulleys should be oriented toward ease of cleanup. The lowest point of any belt should be at least 0.6 meters above the floor. Rock deflectors or plows located immediately in front of tail pulleys above the returning belt can serve to prevent rooks from cycling between the belt and pulley thereby increasing belt life. In tunnels, beneath stockpiles, and in other locations below grade, provisions must be made to include the cleanup sumps and pumps. In addition, these locations should be protected from flooding. Counterweights and/or mechanical takeups must be positioned in accessable locations for ease of adjustments. Usually gravity takeups are preferred since they tend to be self-adjusting and minimize sideways belt movement. Conveyor layout must take into account wind direction and, for elevated conveyors, covers should be considered. Naturally, all conveyors must be equipped with emergency shutdown switches and all drives, sprockets, and gear trains must be provided with adequate guards. The emergency stop cable should run the length of the belt on the operating side. Conveyor interlock systems must be carefully designed and must be understood by all operational personnel.
In the recent past, conveyor equipment suppliers have indicated an interest in quoting completely engineered and prefabricated conveyor systems, rather than components only. The design engineer must carefully consider these packaged systems since overall costs can sometimes be considerably reduced using these pre-fabricated conveyors rather than custom designed assemblies.
There is a tendency in small plants to overlook or undersize the dust collection system. This can prove to be a serious problem and will usually result in the handling of very wet ore in the fine ore bin since the usual “quick fix” for a dusting problem is the installation of numerous crudely fabricated water sprays. It is best to consider spray locations at all conveyor transfer points in the initial design. It is usually difficult to precisely estimate the dusting problem which will be encountered and, consequently, a selection of interchangeable spray nozzles having different orifices should be supplied as original equipment.
The Blake Crusher was patented by E.W. Blake in 1858. It was soon improved to the final form in which the entering feed received the least and the departing product the greatest crushing movement. Variations in detail on this basic form are embodied in the bulk of the jaw crushers offered by manufacturers today.
The functional drawing shows the essential details and gives a typical cross-section. Parts (2), (3), (4) and (5) form a loose linkage which oscillates with a compound eccentric movement in the fixed framework (1), (6), (7) and (8). The size of the largest escaping particle is governed by the set, which is the horizontal distance between (6) and the tip of (5) when at the widest opening. The back toggle plate (2) pivots loosely from a bearing in (1). It is oscillated radially by the pitman (3), which is driven by the eccentric (8). As this toggle rises it presses the lower end of the pitman forward, a movement transmitted via the front toggle (4) to the swing jaw (5). The horizontal displacement is greatest at the bottom of the stroke and diminishes steadily through the rising half of the pitman’s cycle. Thus, though the driving force applied through the eccentric does not vary, the horizontal travel of the swing jaw diminishes rapidly. Crushing force is least at the start of the rising half-cycle when the angle between the toggles is most acute, and is strongest at the top, when full power is being delivered over a reduced travel of the jaw. Since the jaw (5) is pivoted from above, it moves a minimum distance at the point where a large lump of ore has newly entered and a maximum distance at the discharge end.
Consider a large piece of ore falling into the feed end or “gape” of the jaw crusher. The swing jaw is moving to and fro at a rate depending on the size of the machine and of the material it must crush (see Table 4). The running speed should not be so high as to strain the moving parts, which must withstand reciprocal action, severe loading on the compression stroke and sudden release on the return. It must also give time for rock broken at each “bite” of the jaw to fall to a new position in the constricting space of the crusher throat. The piece of ore falls till it is arrested, either above other ore, or by being nipped between the fixed and swinging jaw.
Within a fraction of a second the moving jaw again closes on it, fast at first and then more slowly but with increasing power to the end of the stroke. Though the jaw only squeezes the ore for a short distance of its movement, this suffices to break the big lump. The fragments now fall to a new arrest point where they find themselves somewhat crowded, since the total cross-section is now less, while the overall volume has been swollen by newly created voids. At this arrest point another squeeze is delivered, this time with greater amplitude, since the radius of the swing jaw from its centre has increased. Crushing continues stroke after stroke until the crushed particles reach the lower end and fall clear. At each arrested fall the crowding together of the fragments would increase, owing to the combined effect of the increase in voids and the decrease in cross-section, were it not for the steady increase in amplitude of swing.
This accelerates the discharge of finished material, which works down and out at a rate sufficient to leave space for material arriving from above. Crushing under these conditions, in which particles are relatively free to fall between successive squeezes, is termed “arrested” in contradistinction to “choked” crushing, in which the volume of material arriving at a given cross-section would be greater than that leaving it if the rate of feeding was unrestricted. In arrested crushing the main force exerted upon a particle is directly applied by the jaws of the machine. In choked crushing a substantial amount of comminution results from impact of particle upon particle. The character of the product is different. Since in arrested crushing any particle small enough can escape at the discharge area, much of the crushed rock is finally delivered at a fairly coarse size. In choked crushing comminution continues even when particles are smaller than the “set”. The difference is analogous to that of the orderly departure of a theatre crowd and of a panic in which people are crushed by other bodies arriving at the exits at too great a rate. The “set” of the Blake is the maximum opening between the jaws at the bottom, measured with the “V” of the toggles at the steepest point, with the eccentric full down. It is adjusted by using toggle plates of the desired length. Wear is taken up when required by adjusting the back pillow on which the end of the toggle bears. Since the toggles are loose in their sockets, a tension rod is used to hold the system together and to aid the return stroke of the swing jaw. A vertical spring may also be used to preserve smooth contact of the eccentric by acting upon the bottom of the pitman.
Arrested crushing can only take place if the rock broken during each nipping stroke falls with reasonable freedom during the return half of the jaw’s swing. Since the overall volume swells with each stroke (owing to newly created voids), yet must drop into a decreasing horizontal cross-section, there would be congestion if the rate of fall was not steadily accelerated on the journey downward. This is made possible by a proper inter-relation of the following factors:
- Gape to set—the reduction ratio
- Rate of change of vertical cross-section of crusher throat in respect of fall of ore between strokes
- Speed and amplitude of swing-jaw strokes
- Sizing analysis of entering ore
- Crushing characteristics of ore
To minimise damage when uncrushable material enters with the feed, a weak point is built into the crusher. This can break and be quickly and cheaply replaced. In some crushers this is a weak belt-fastener on the drive; in others the driving pulley is weakly bolted to the very heavy flywheel of the Blake; the eccentric may be held down by weak bolts which break and allow the whole pitman to rise; or one toggle-plate may be scarf-jointed by a line of weak rivets. These protective devices should not be called upon. Tramp iron should be dealt with ahead of the crusher.
The heavy flywheel stores energy on the idling half of the stroke and delivers it on the crushing half, thus saving on driving power and smoothing the inevitable vibration of the machine. Since it works on half-cycle only, the reciprocating jaw crusher is somewhat limited in capacity for its weight and size. Owing to the alternate loading and release it must be very rugged, and requires strong foundations designed to avoid the transmission of vibration. The tough work the crusher must do calls for rugged mechanical details, good bearing lubrication, and generous cooling of the eccentric, possibly by circulating water. Forced-feed oiling is usual, with protection against the entry of abrasive dust from the ore into working parts. The lubricants must not be allowed to leak in such a way that they can contaminate the ore. Maintenance must be systematic but as the crusher plant usually works in rhythm with the underground hoisting program and not continuously as does the concentrating plant, this can easily be organised. The dust generated during dry crushing should be trapped, drawn off, and safely disposed of. The “throw” or moving-jaw displacement varies from a minimum of 3/8″ in small crushers to a minimum of an inch in big ones, the maximum possible being about three times the minimum in any case. With brittle material a minimum throw may be best. When the rock has pronounced elasticity so that it cracks, or deforms locally, as is the case with slabby and decomposed ore, far more movement is needed. Some adjustment is possible by varying the toggle-V, but the usual method of varying throw is to change the eccentric. The greater the throw, the better the evacuation of crushed material from the discharge end, and the less the danger of choking. Clay or “sticky” ore is liable to cling to the jaws. If it builds up in so doing, the set may be reduced to the point where arrested crushing is no longer certain, and increasing strain is thrown on the toggles and eccentric. Capacity is reduced and the quality of work suffers. If such conditions are serious enough to call for remedial action, the ore should be washed before crushing. Packing of the crushing throat by clay or fine material could lead to breakage.
The crushing jaws, which take heavy punishment in addition to abrasive wear from the passing rock, are protected by replaceable plates. This is standard in all machines which come in contact with ore, and with most conveying systems, gates and pulp launders. Since local crushing stress is severe, the wearing part must fit snugly on its supporting steel structure so that the load can be to some extent distributed. Heavy-duty machines frequently have an intermediary filling medium which is poured while liquid into the space between wearing part and support, so that it can set solidly and provide continuous backing. For lighter machines plastics or wood are used (e.g., behind ball mill liners). Jaw crushers lend themselves to close fitting by careful machining, but where jaw plates are reversible end for end, to take up wear, a backing medium may be used. The usual metals are zinc or babbitt alloy. One difficulty with molten metal, particularly when used between support and crushing head of a gyratory machine, is premature solidification, despite super-heating, so that non-continuous fingers or blobs form and the cavity is incompletely filled. Premature breakage of the wearing plate or cone may then occur, owing to “working” and deformation. Another arises when gold ore is crushed, if by mischance breakage leads to the entry of zinc into the cyanide process which follows. An organic chemical mixture can be used, which sets soon after its two constituent compounds have been mixed.
Variations on the Blake Jaw Crusher
Variations from the simple Blake crusher are numerous. They fall into two main divisions:
- Variations in application of toggle motion
- Variations in slope of crusher throat
In the Telsmith crusher the movement is transmitted directly from an eccentric to the moving jaw. There are no toggles and the need for imparting reciprocating movement to a heavy pitman is avoided.
In the single-toggle crusher the swing jaw is hung on an eccentric and the whole of its surface is in lateral and vertical motion. In another crusher both plates move.
The effect of using curved jaws instead of straight ones is shown below.
The wearing plates which line the crusher throat are frequently cast with vertical wedge-shaped corrugations, so as to impart a measure of beam-loading to the applied force. Of the alloys used for these plates, manganese steel is most favoured.
Dodge Jaw Crusher
This crusher reverses the jaw action of the Blake, in that it applies the maximum movement to the largest piece and the minimum to the smallest. The fulcrum is below, and only slight variation of set occurs as the moving jaw advances and recedes. Hence, for laboratory purposes where throughput is less important than close control of rock sizes, such a crusher, if lightly fed, can be made to do arrested-crushing work. If it is choke-fed, the choking becomes serious as the cross-section decreases. Rock crushes rock and there is not enough dilation as the moving jaw recedes to expedite departure of the finished material. This results in over-crushing, in which the crusher does work that would be better handled by grinding mills, and itself suffers