Operational work index for grinding plants

Abstract An global operational work index, W iog , is defined for a conventional grinding plant consisting of a rod mill followed by three inverse hydrocyclone-ball mill grinding circuits and its value is estimated online. This global operational work index is based on an equivalent unitary ball mill circuit. The foundation for W iog is Bond's equation for a grinding circuit, but here it is defined on the basis of measurements performed on the complete grinding plant: solids feed flow to the rod mill, f 80 of the feed ore, p 80 in the hydrocyclones overflow and the power draft of the mills. The on-line calculation of either the rod mill operational work index or the overall plant operational work index W iog requires the feed size f 80 , which is not measured on-line. A sensitivity analysis is made to assess the influence of f 80 and p 80 in the determination of the work index. It is found that only a small error is made if the average f 80 is used instead of f 80 to compute W iog . However, for the case where the particle size at the hydrocyclones overflow is not measured, it is shown that a soft-sensor is necessary, but using only mill power draft and solids feed flow as inputs. This procedure produces relatively small errors in the calculation of the operational work index. Data collected in a large grinding plant is used here, including laboratory measurements of f 80 and p 80 , so it is possible to asses the effects of the soft-sensor inference in the on-line calculation of the global work index. The results may be extended to other grinding plants.