Vertical stirred mills are more frequently being applied at coarser grinding duties as a replacement for ball milling. This trend is being driven by reductions in footprint and operating costs such as power and media consumption that counter the increased capital cost. Vertical stirred mills have been reported to use up to 40 per cent less energy (decreasing to 20 per cent for coarser applications P80>200 μm) and 20 per cent less media than equivalent ball mills. However, this reduction is difficult to validate in operations as there is a tendency for these circuits to create a sharper product size distribution, and the evaluation techniques typically use P80 figures. Although benefits are well established in regrind and tertiary duties, they have not been clearly proven in coarser applications following autogenous (AG) or semi-autogenous (SAG) mills and high-pressure grinding rolls (HPGR). This paper brings together data from three operating sites to better evaluate the performance of vertical stirred mills in coarse duties.
There are many applications where vertical stirred mills have been installed in coarse duties. Morenci conducted a long-term trial of a VertiMill™ (VTM) in secondary duty following an HPGR closed with an 8 mm wet screen. Boungou has recently commissioned a milling circuit with VTM following a SAG mill closed with a 6 mm scalping screen. Tambomayo has an open circuit VTM following a single stage SAG circuit closed with hydrocyclones. Cannington has employed a VTM in closed circuit following a single stage AG mill circuit. Finally, Cadia, New Afton, Chino and Raglan have employed VTMs in a tertiary application to debottleneck the ball mills and stabilise the float feed by providing a dampening effect to process variability. The efficiencies of these plants are evaluated to better define when and where VTMs are best employed.