Ball mills are used to obtain a product size below 150 microns. With the rotation of the mill, collision and attrition forces grind down the material in the mill chamber. The balls act as the grinding media, where contact points are created with each other and the material is either crushed or ground.
In Grinding, selecting (calculate) the correct or optimum ball size that allows for the best and optimum/ideal or target grind size to be achieved by your ball mill is an important thing for a Mineral Processing Engineer AKA Metallurgist to do. Often, the ball used in ball mills is oversize "just in case". Well, this safety factor can cost you much in recovery and/or mill liner …