Ball size distribution for the maximum production of a narrowly-sized mill product

Ngonidzashe Chimwani*, François K. Mulenga, Diane Hildebrandt, David Glasser

University of South Africa (UNISA), Florida Campus, Private Bag X6, Johannesburg 1710, South Africa

Abstract

Ball size distribution in tumbling mills is commonly used to optimally control the mill product size distribution leaving the mill. This is owing to the fact that each ball size effectively break a particular size range in the mill. The optimum ball size distribution is determined by the amount and combination of ball sizes in the make-up charge. Grinding media wear caused by the abrasive environment in tumbling mills necessitates charging of new grinding balls periodically in order to increase the milling efficiency.
A grinding circuit simulator combined with a ball wear model was used to determine the best make-up ball charge. The objective function was to find the ball charge that will guarantee a maximum production of floatable size class at optimal values of ball filling, feed size distribution and feed flow rate (i.e. -75 +9 m) for a platinum ore.
It was found that the simulated product size distributions displayed a close match with the measured product from an operational mill. Maximum amount of floatable size class produced enabled the tracking and determination of the best combination of mass fraction of each ball size constituting the make-up ball charge in relation to feed flow rate, feed size distribution and ball filling.

Key words: Ball size distribution, Population balance model, Milling parameters, Floatable size class, Ball mass fraction