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The character of product delivered by crushing rolls may vary quite widely on the same material. We have mentioned that a low reduction ratio is advisable if a minimum percentage of fines is desired. A light feed, i.e., a low rate of feed, will usually result in a cleaner product because the material does not become so closely packed in the choke-zone. Conversely, a choke-feed promotes production of fines; the rock is crowded into the choke-zone so rapidly that voids are eliminated, and the normal operating condition in this zone amounts to what in a jaw or gyratory crusher would be a full-choke. In the rolls it is relieved by the movement of the spring roll, which crowds back against the spring pressure when the unit pressure in the crushing zone exceeds the pre-set working pressure of the springs. Heavy-duty rolls can be operated in this way with good results, especially when run with high circulating loads in closed circuit with screens.
The tendency of rolls to create a packed condition in the choke-zone may sometimes have an unfavorable effect on the product If the material is both soft and adhesive it may be discharged in cakes, which are sometimes quite hard and difficult to disintegrate in other apparatus. Caking can be minimized by using a low ratio of reduction per stage, and a regulated feed to avoid excessive packing.
Some materials-notably, those of sedimentary origin-contain numerous parallel cleavage lines. Such materials are almost certain to "Flake" in crushing rolls; that is, the product will contain a sizable proportion of flat spalls. While this is of no particular moment in some products, it is a serious detriment in others, such as concrete sand for example. When a cubical product is essential, and rolls are in all other respects suitable for the proposed application, laboratory or field tests should be run on the material to determine if the rolls will turn out such a product. |
