Maximum power is drawn by a mill when the charge occupies approximately 50% by volume. The power curve becomes very flat in the range above 45%. As a result, mills are seldom run with charge levels greater than 45%. In rod mills, the charge is swollen by particles of feed which separate the rods. If the mill is shut down immediately after the feed is shut off, the charge level will be greater than if the mill had been “ground out” prior to shutdown. Rod mills are normally operated with a 32 to 40 percent charge by volume which during operation, becomes a 40 to 50% charge, with a bulk density considerably lower than that of stacked rods. This chart shows the impact a mill’s grinding ball load has on the mill’s motor power draw.
