Semi-autogenous grinding is used in place of autogenous grinding when the ore does not produce the necessary competent media. Forged steel balls are used to supplement the coarse ore. In the past, the same ill geometry has been used as in autogenous mills with modifications in the structural design to account for the increased weight of media and pulp loading. (Because supplementary ball charge is needed frequently for intended pure autogenous operations, most mill manufactures now include I their basic autogenous design the necessary strength for semi-autogenous operation.) Ball sizing depends on the top size of the ore fed to the mill. Ball sizes commonly used vary from 1 ½” to 5” diameter. The ball charge is generally in the range of 2% to 10% of the mill volume.

Circulating load is an indication of the adequacy of the adequacy of the grinding media. High circulating load indicates a lack of large diameter balls or an insufficient number of balls depending upon the size distribution in the circulating load.

Autogenous and semi-autogenous mills are typically operated at slightly higher speeds than ball mills of the same diameter.

These purpose of a grinding circuit is to reduce the size of feed material to a size sultable for either a final product or for further processing. Further processing may involve a leach. Flotation, gravity separation or magnetic separation. Of thee processes, flotation, gravity concentration and magnetic separation require grinding fin enough to liberate the valuable mineral from the gangue.