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Grinding may be classified as single-stage or multi-stage. Single-stage grinding involves grinding using one mill per circuit. Single stage grinding may be done open or closed circuit in a rod mill, ball mill, or autogenous mill. Multi-stage grinding involves any combination of rod, ball or augenous mills in open or closed circuits. The horsepower required to reduce material from ball mill feed to ball mill product is often about twice the power required to reduce material from rod mill feed to rod mill product. Thus the power ratio selected for a two stage rod/ball mil circuit is usually 1:2 This may involve one rod mill and two ball mills of equal horsepower, or one rod mill with roughly half the horsepower of the second stage ball mill. A regrind mill, often used to process flotation or gravity middlings* may be considered a tertiary stage of grinding or it may be considered a later, but separate, single stage.
* Middlings are particles of part valuable mineral and part non-valuable material. The are often reprocessed to free the valuable constituent.
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