Tailoring ball mill feed size distribution for the production of a size-graded product

Abstract

Evidence that particle breakage during milling varies with material size is commonly cited in literature. This makes feed tailoring in ball milling an interesting parameter to investigate with the aim of maximising the desired mill product.To that end, batch tests were used to investigate the effect of feed tailoring in ball milling using a South African gold ore. Narrow-size feeds of the ore were prepared and milled with 20 and 30 mm sets of single-size balls using the one-size fraction method. From the experimental data obtained, milling parameters of the ore were determined, validated and then used in the population balance model (PBM) to simulate the breakage patterns of different feed size distributions for further analysis. Data from those simulations were analysed using a graphical analysis technique called Attainable Region (AR) theory with the purpose of finding the feed size distribution that produces the highest amount of the desired size range (–150 + 25 µm) for gold leaching.The results showed that feeds composed of higher fractions of finer sizes resulted in better milling rates and higher throughput of the desired product size than feeds with higher fraction of coarse particles. Further AR analysis also revealed that the milling process is efficient only to a point where the rate of producing the desired product size is greater than or equal to the rate of producing unwanted fines. This study identified that point as a switch point where grinding should be stopped followed by the classification of the material.