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Closed circuit grinding is used where only product of a certain size can be treated efficiently. As shown in the diagram, material in a closed circuit is separated at the classifier into fine material and coarse material. Coarse material is returned to the mill and the fine material is sent on for further processing. The proper split is dependent on the classifier itself and on the many variables affecting the performance of the classifier.
A certain reduction in needed to obtain the liberation necessary to allow a satisfactory recovery or in the case of a regrind circuit, a satisfactory grade. While too coarse a grind hinders recovery and grade, too fine a grind is also undesirable. Grinding beyond that necessary to produce the desired.
Product often produces excessively fine materials called slimes. Slimes hinder flotation in two ways. The surface area of slimes is so great that large quantities of reagent are needed to produce the required surface chemistry. Also selectivity, or the ability of the flotation reagents to differentiate between different types of particles, is decreased when particles become too fine.
Overgrinding is also undesirable because the wasted energy put into overgrinding is an excessive cost represent by unwarranted power consumption and recycling of oversize material in closed circuit grinding is frequently the most efficient way. Closed circuit grinding creates a circulating load which is defined as the ratio of the weight of material to the weight of new feed.
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