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This open-circuit, stage-crushing system is an excellent arrangement for the commercial crushing plant, because it tends to minimize production of fines, especially so if the ratio of reduction, per stage, is held within moderate limits. The system can be carried out to a point where relatively little tonnage need be handled in closed-circuit, as has been exemplified in a number of plants during recent years. As an example: a plant is running, let us say, with a final, close-circuited stage that is taking an original feed of about 100 tons per hour, with a circulating load of about 25 tons per hour through this stage. If a small "clean-up" crusher is installed to handle the 25 tons oversize-this machine being set somewhat finer, of course, than the larger machine-the circulating load through this smaller crusher can generally be held within 5 tons per hour, and the feed rate to the former final stage can be increased by the amount of the circulating load which has been removed from it. This practice of adding "clean-up" crushers to an existing flow-line is becoming increasingly popular, particularly where the demand for smaller sizes of product has been throwing a heavy strain on existing equipment.
Open-circuit, stage-crushing is equally well adapted to the preparation of feed for grinding mills. For such service the reduction per stage need not be limited with a view to minimizing fines; otherwise, the design factor are much the same as for the aggregates plant.
Nothing that we have said here should be construed as antagonistic to the closed-circuit, crushing-stage, which is a very necessary adjunct to most minerals-reduction plants. What we are trying to convey is the fact that material which has been through a pressure-type crusher once can be processed more efficiently, for further reduction, in another crusher, or crushers, with smaller discharge setting; and, that circulating loads should be held to a minimum by carrying the stage-crushing system through to its logical limit. |
