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.
- Tangential fed cyclones were replaced with involute fed cyclones at gold and copper mills in Montana, Nevada, Indonesia, and the Philippines. The improved cyclone design increased circuit capacity and/or produced finer grinds. In all cases, the improvements minimized the amount of coarse topsize in the circuit product. See Table 10.
- Proper cyclone geometry reduces the amount of coarse topsize in the circuit product. Liner fit and condition are critical to maintain proper cyclone geometry. One copper concentrator reported a 3% reduction in the +65 mesh fraction in their flotation feed by placing more emphasis on their cyclone maintenance program.
- The objective of the classifier in the grinding circuit is to increase the capacity of the mill and produce a compact product size distribution.
- Cyclones are an improvement over mechanical classifiers and screens because of preferential classification.
- The cyclone and mill are interrelated and must be viewed as a single unit operation.
- Increasing the circulating load increases circuit capacity and produces a more compact product sizing. However, it can lead to bottlenecks because of volume constraints in the classification circuit.
- The amount of fines returned to the mill by the typical cyclone/mill circuit does not lead to over grinding or reduce circuit capacity.
- Horizontal cyclones offer advantages in coarse grinding circuits. However, the reduction of fines returning to the mill has not resulted in increased circuit capacity.
- Alternatives such as screens and two-stage cyclone circuits have been considered for improving sharpness of classification. However, for most applications single stage classification with well designed and maintained cyclones offers the best compromise between performance and ease of operation.
- The purpose of the comminution circuit is to grind the maximum amount of material to the desired particle size consistent with the metallurgical requirements at the minimum cost. The objective of perfect classification must be balanced against other practical requirements such as on-line reliability, maintenance costs, performance consistency and overall power consumption.