RUBBER LINED HYDROCYCLONES

SHARPNESS OF CLASSIFICATION

A considerable amount of work has been done to improve the sharpness of classification. This work has concentrated on the fines returning to the mill with less attention paid to produce sizing and plant metallurgy. Although reducing the amount of fines returning to the mill can be beneficial, the main objective of the classifier should be to minimize the coarse tramp material and compact the size distribution of the grinding circuit product. Options to improve the sharpness of the classification include screens, two-stage cycloning, and improved cyclone design.

Screens, in combination with cyclones or as an alternative to cyclones, have been considered to improve classification efficiency. Davis (1925) developed his original circulating load relationships using a screen in a small pilot mill, however screens have not been popular in grinding circuits because of high capital and maintenance costs. An overlooked disadvantage of screens is that they classify particles strictly by size and do not produce the preferential classification which is metallurgically beneficial in many base metal grinding circuits. The classification by size concept can be beneficial in a circuit where the preferential classification of an extremely friable high specific gravity mineral could be detrimental. Screens could be used in place of cyclones in a phosphate grinding circuit where the separation is 35 mesh (420 microns) or coarser and the product density requirements are 60% solids and higher.

Since cyclones offer the best compromise between performance and ease of operation, considerable work has been done to improve classification efficiency by using two stages of cyclones. A secondary cyclone desliming a primary cyclone underflow will reduce, the amount of fines returning to the mill but this has not proven to increase circuit capacity. Adding a secondary cyclone to handle the primary cyclone overflow will sharpen the separation. These possible performance benefits must be balanced against increased capital costs, maintenance costs, and energy consumption, plus the complexity of a two-stage circuit.

Rather than making the circuit more complex, more attention should be paid to the design of the single stage cyclone classification circuit. Poor cyclone design and maintenance can result in poor classification efficiency and reduced circuit capacity. Numerous examples of increased circuit capacity have been documented as the result of improved cyclone performance.