It is generally not possible to comminute blasted rock satisfactorily in one operation, so crushers are described as primary, secondary or tertiary depending on the size of the feed. The principal types are shown in
Figures 5.4 to 5.9. Most are available in either primary or secondary versions, but jaws and gyratories are usually primary crushers, while impactors and cones perform secondary duty. The maximum particle size usually processed is about 800mm for primaries and 200 mm for secondaries.
One means of classifying crushers is in terms of reduction ratio, the ratio of maximum feed size to maximum product size. Alternatively, this can be defined as the ratio of 80% of entry width (gape) to 80% of exit width (closed side setting). Compression crushers (jaws and gyratories) have maximum reduction ratios of 8:1 to 10:1 but normally operate at about half this, since high ratios result in excessive fines due to interparticle rubbing and generate platy particles through shearing. Impact crushers perform satisfactorily at much higher reduction ratios, in the range 10:1 to 20:1.
Most primary crushers have a feeder, which evens out the flow of broken rock and separates (scalps) undersize using an inclined bar screen. Spalls retained on the screen trickle into the primary crusher, while the undersize bypasses this stage and is directed either straight into the secondary crusher, or to waste if it is weathered. This arrangement increases the throughput or
capacity of the primary machine and prevents fines packing in its crushing
chamber. Coarse oversize is separated at the blast face prior to loading, or fractured by a hydraulicarm rock breaker when it jams in the primary crusher entry slot.
Jaw crushers
Jaw crushers (Figure 5.4) break rock by slow compression-release cycles between ribbed plates, one fixed and one moving, on opposite sides of a
wedge-shaped chamber. This narrows downwards, so that after blocks are split on the compression stroke the pieces slip further down on the release stroke. At some point they become jammed again and the cycle is repeated, until eventually the fragments fall out of the base of the crusher.
Jaw crushers can be classified into those with a single or double toggle action. Double-toggle machines are able to exert larger compressive loadings, and are therefore used for handling the strongest (UCS up to 500 MPa), most blocky (up to 3 m) and most-abrasive feed. However, in most quarrying applications, single-toggle machines perform satisfactorily and are less costly. They are well suited to small to medium-sized operations, including mobile plant, where intermittent production is normal. A number of jaw arrangements involving different pivot points, curved and planar plates are also offered. Jaws are cheaper in both capital and maintenance
Figure 5.4 Single-toggle jaw crusher, schematic. The left side plate (shaded) is fixed,
Figure 5.5 Gyratory crusher, schematic. The inner cone moves eccentrically, but the
costs than gyratories handling the same feed size, but their production rate is much less. They are economic for up to 500–1000 tph (tonnes per hour), above which gyratories are favoured.
Gyratory crushers
A gyratory crusher resembles a pestle in a narrow open-base mortar, or two cones, one inverted within the other (Figures 5.5 and 5.6). The inner pestle- like solid cone, known as the head, moves eccentrically around the fixed outer bowl, alternately closing and opening gaps around the lower rim. It functions as a multitude of jaw crushers, and can therefore be much smaller for the same capacity. Gyratory crushers tend to be chosen where high throughput (up to 5000 tph) is required and where a relatively fine and uniformly blasted feedstock is assured, since they are less tolerant of oversize than jaws. They are more compact than jaw crushers of the same capacity and more efficient in terms of energy consumption per unit of output. Because their construction is lighter, they require less elaborate foundations (but more headroom) than jaw crushers and their feeding arrangements are simpler. They are ‘choke fed’, meaning that the machine works best when
Figure 5.6 Mortar-and-pestle arrangement of a gyratory crusher at Prospect quarry,
near Sydney, New South Wales. When operating, this would be buried in blasted rock spalls (‘choke feeding’).
buried in rock fragments, as opposed to the ‘trickle feed’ arrangements needed for jaw crushers.
Both types of compression crushers turn out a fairly uniformly sized product, though the gyratory output has somewhat more fines. In hard rock the gyratory action is said to produce more cubic chips, although both types tend to generate a proportion of sheared and unsound particles. Product grading curves for both of these compression machines are similar: the particle size range is narrower than from impact crushing, but wider than from a rolls crusher.
Rolls crushers
A third primary type, the rolls crusher (Figure 5.7), is limited to weaker (UCS less than 100 MPa) and non-abrasive rock, such as limestone and brick shale. The machine may be equipped with single, double or multiple rollers, and these can be fitted with teeth, cleated or ribbed. Double rolls are most common, with one drum axis fixed and the other spring-loaded to accommodate oversize slabs.
Rolls crushers are relatively cheap in relation to their high capacity, economical in power consumption, easily transportable and low in profile
Figure 5.7 Double rolls crusher with interlocking teeth. Both rolls rotate towards the
centre, but one is fixed and the other sprung to allow oversize blocks to pass through.
when erected (which simplifies feeding). These characteristics make them a good choice for crushing demolition rubble, especially concrete slabs and brickwork, or weak excavated rock for select fill on construction sites. Rolls crushers are particularly suited to clay-rich feed, and to processing plastic and slabby bedded rocks, which would tend to block jaw or gyratory crushers.