A laboratory investigation of the effect of charge density on power draw in tumbling mills

Power draw is one of the key parameters to control mill performance. It is dependent on various factors such as charge density, ore characteristics, ball sizes and type of the mill (AG, SAG, ball mill).  . In calculation of mills power draw, voidage and the fraction filled by the voidage in the charge is essential. In estimation of power draw, for the voidage between the balls a value of 40% is assumed for all the ball sizes. In this study it was found that the voidage between balls largely depends on the balls shapes and size distribution. The voidage between balls for 8, 9, 10, 11 and 12 mm ball sizes in both single size and seasoned balls was measured. By decreasing the ball size from 12 mm to 8 mm, the voidage decreased from 39.0% to 37.8%. The voidages with Bond chain with 11.4 and 7.6 mm top size balls were measured 36.0 and 35.2%, respectively. The effect of charge density on power draw was studied through scale-downing the ball size distribution of the Sarcheshmeh ball mills at the start up and the crush stop in a laboratory container. The voids of the two distributions were measured to be 33.5 and 36.1%, respectively. The difference in the amount of calculated power draw for these values of voidage was equal to 164 kW, which is equivalent to the weight of 15 tons of balls. An equation was proposed to correct the effect of bulk density on calculation of ball mill power draw.