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57 Figure 21. Schematic of Barmac autogenous vertical shaft impact crusher (156, originally from Barmac). and are often used for crushing fines. An example of an auto- ner as shown in Figure 22a. This occurs because the stresses genous vertical shaft impact crusher is shown in Figure 21. are essentially concentrated at two points, causing long cracks Crushers are often referred to by the order in which they in between the contact points. Multilayer crushing produces crush the aggregate. For instance, the "primary" crusher is the a greater number of contact points as shown in Figure 22b. first crusher into which the aggregate feed, either shot rock or Multilayer crushing produces more cubical aggregate shape. gravel, is introduced. Following the primary crusher are the Multilayer crushing is achieved by keeping the upper portion secondary, tertiary, and, in some cases, quaternary crushers-- of the crusher cavity full or chocked. This requires that the that is, the second, third, or fourth crusher in the crushing cir- crusher be operated as part of a close circuit. A closed circuit cuit. Generally each crusher is used to produce a progressively provides a recirculating load and surge piles to supply a rel- smaller product, although in some cases, a crusher may be atively constant feed rate to the crusher. The feed rate must used more for shape improvement and less for size reduction. be adjusted (increased) as the crusher liner wears open or more particles will pass through the open-side setting with- out being crushed. 2.10.2 Factors Affecting Aggregate Shape The crushing head of gyratory or cone-type crushers oscil- lates around the central axis of the crusher as shown in Fig- The geology of the aggregate is probably the most signif- ure 23. As discussed previously, the close-side setting is the icant factor affecting crushed aggregate shape. Fine-grained smallest distance between the crusher head and the bowl liner. (i.e., aphinitic) aggregates, such as limestone, tend to be more The open-side setting is the largest distance between the brittle and therefore fracture into more F&E (157). Slates, crusher head and the bowl liner. The open-side setting occurs injected quartzites, and basalts are examples of other aphinitic at a point opposite the close-side setting. Increasing crusher aggregates. Quartzites, basalts, and cherts also tend to frac- speed decreases the number of particles that pass through the ture in a concoidal manner--that is, they produce curved frac- open-side setting of a cone or gyratory crusher without being ture surfaces like glass (157, 158). crushed. Most older gyratory and cone crushers operate at a The following factors tend to improve the shape of parti- fixed speed. Newer high-pressure cone crushers tend to oper- cles crushed with compression crushers (159161): ate at a higher or variable speed. The crusher should be run with a full or choked feed cavity to promote interparticle crushing. Crushers should be operated in closed circuits where a recirculating feed can be used to fill the crusher cavity. The reduction ratio should be reduced. Reducing the feed size or increasing the circulating load can accom- plish this. The close-side setting should be approximately equal to the desired product size. When single layer (a single aggregate particle trapped between the crusher jaws or crushing head and bowl liner) occurs, the particles are more likely to split in a flat and elongated man- Figure 22a. Single-layer crushing.