Optimisation of autogenous grinding

Batch tests in a 0.6m diameter mill were used to simulate fully autogenous primary and secondary grinding. Substantial improvements in primary mill capacity were obtained by modifying the mill contents to simulate crushing of small pebbles and recycle of the crushed material. Secondary grinding efficiency (as measured by kWh per ton finer than 75 microns or Work Index), was improved substantially by reducing the pebble size to 20/28mm. The process as a whole was optimised when crushed rock coarser than 4 mm was recycled to primary mill, as the small pebbles were most efficient with “sand”. The benefit of using small grinding media has been known for some time, but the high cost and rapid consumption of small steel balls makes their use uneconomical. However a plentiful supply of small pebbles is available from a primary mill. A further and unexpected improvement in power efficiency was obtained when pebble milling was performed at 92% of critical speed.