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Most of the particles which have a one-way dimension smaller than the product size probably require only one crack to reduce them to under-size, because the break need only be made in one direction: through the small dimension. On the other hand, if the crusher is set to a plus-product size, the over-size "flats" must be shattered in two directions to reduce them to under-size. Theoretically, be amount of work that must be done on these plus-product-size flat spalls is at least triple that which is required to break down the minus-product-size spalls. When we consider that most of this over-size breaking must be accomplished by catching the pieces against another particle, or particles, we can see why our figures, which work out quite well for minus-product-size crusher settings, cannot safely be used for plus-product-size settings. The circulating loads, for such settings, are apt to be inordinately high, and there is nothing to be gained by running the crusher that way, except, perhaps, on extremely soft and easy-crushing material.
Whether or not it is advisable to set the crusher to a discharge opening equal to the product size depends upon the crushing characteristics of the material. If the material is friable, or if it cubes well, it is safe to assume that the crusher can be so set; on° the other hand, it is hard, or tough, or tends to slab in the crusher, the setting should be held to a maximum of 80 to 85 percent of product-size, regardless of what our figures indicate in the way of capacity for any coarser settings.
"Stage-crushing," as we use the term here, applies to open-circuit reduction in a flow line comprising two or more crushing stages, with scalping between each stage, but without close-circuiting any stage upon itself-except, perhaps, the final one.
Generally speaking, such an arrangement works out more efficiently than one in which each stage is run in closed-circuit. There is nothing to be gained by sending oversize rock back to the crusher that let it go through once, unless the stage is the final "clean-up" stage in the flow line.
The oversize should go on to a stage wherein the setting is smaller, and where these oversize pieces will be crushed by positive metal-to-metal contact. The scalping between stages in such a system should remove most of the material which is smaller than the setting- of the next following stage.
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