A laboratory stirred ball mill was used in a batch manner, with the milling time being varied so as to produce a number of different product size distributions diameter ratio of one, and has a jacket through which cooling water is passed. The vessel is mounted on a turntable which is located on a thrust bearing. The turntable is restrained from rotating by an extension arm which is attached to a load cell; with this arrangement the torque is measured, and hence the energy consumption can be determined. The vessel is filled with grinding medium which is agitated by a vertical shaft that has a number of arms protruding from it. The shaft is driven by a 1.5 Kw motor, through a variable speed gearbox, with the final drive being belt and pulleys; the speed is variable between 70 and 500 rpm.