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We have mentioned the fact that, as the percentage of voids in the crushing chamber decreases, the production of fines by attrition increases. This is equivalent to saying that, as the ratio-of-reduction in any given crusher is increased, the percentage of fines in the product will increase, even though the discharge setting remains unchanged. Both of these statements are true, but the degree to which the product is affected depends to a much greater extent upon the ratio-of-volume-reduction in the crusher chamber than it does upon the actual degree of reduction performed on the material. For a given ratio-of-reduction, the type of crusher with a flared crushing chamber will usually deliver a cleaner product than any of the older types, conversely, more reduction can be performed in the machine without creating excessive fines.
The facts outlined in the foregoing paragraph have an important bearing on crushing plant design. Commercial crushing plant operators design. Commercial crushing plant operators are usually desirous of making as few fines as possible, and this is becoming increasingly important as the demand for small grades of screened material increases. To hold down the amount of dust or screenings in the combined plant product, it is essential that the amount of reduction per crushing stage be held within conservative limits; moreover, it is important that the work in each stage be apportioned with due regard to the characteristics of the crushers comprising these stages.
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