In the past, mechanical classifiers were usually selected to operate in closed circuit with grinding mills. These devices require much floor space and have been essentially replaced in large plants, as well as small, by hydrocyclones. Since classification is a potential plant bottleneck, it is recommended that the cyclone feed system be provided with redundant pumps. It is relatively straightforward to design a split sump so that mill discharge may be deflected to one side or the other and thence to the appropriate feed pump. Care must be taken to insure that the cyclone feed sump has sufficient depth to satisfy NPSH requirements for the cyclone feed pumps under all conceivable operating conditions.
Cyclone circuits are relatively straightforward to operate in that these systems do not require extensive instrumentation. Usually, cyclone feed sump level control by either water addition or recirculation of cyclone overflow is the only requirement for smooth operation in a small installation. It is usual to pipe each cyclone feed pump individually and thereby eliminate the necessity of high cost gate valves around each cyclone feed pump. With proper design, it is usually possible to cluster cyclones so that the underflow product, consistant with housekeeping requirements, is visible. If head room allows, the cyclone cluster should be arranged so that the cyclone overflow product flows to subsequent unit operations by gravity.