Variables associated with mill power and capacity

Grinding capacity (throughput) and power are terms often used to describe the capabilities of a grinding mill. The grind achieved and the capacity are directly related to the power applied. The power a mill draws can be increased by increasing its ball charge or speed. As the power applied to a mill is increased, the grind becomes finer or the capacity at a given fineness of grind increases.

Variables associated with mill power and capacity include diameter, length, speed, pulp level, media loading and dilution. Of these, diameter has the greatest. Power varies approximately as diameter to the 2.6 or 2.65 power in a grate mill and approximately as diameter to the 2.5 power in an overflow mill. The difference in the power relationship between grate and overflow mills lies in the power lost to the cushioning effect of the higher pulp level in an overflow mill. The fact that diameter has a greater effect on capacity than it does on power tends to make it economical to use larger diameter mills. There is a limitation to this effect. As can be seen from the tables on page 14, the decrease in kwh per ton with increasing mill diameter tapers off at the larger diameter mills.