In recent times, declining commodity prices (see Graph 1) and tightening financial markets have seen many operating mines reduce production or enter care and maintenance. This strategy allows mining companies to preserve liquidity and defer processing of high-grade reserves until market conditions stabilise and improve. For those mines currently operating, travel restrictions and supply chain interruptions pose additional threats to production.
Graph 1: Drop in price of industrial metals
Impact of a grinding media shortage
A key operating consumable for mineral processing is grinding media, which are used in large quantities to break down rocks in mineral concentrators to extract valuable minerals. In today’s climate, one could ask - what would be the impact on operating mines if grinding media supplies were limited due to upstream manufacturing and logistics interruptions (e.g. shutdowns in South Africa, China, and India – all major manufacturing zones)?
This predicament might force some sites to modify their grinding circuit operating strategies from semi-autogenous milling (SAG - with grinding media) to autogenous milling (AG - without grinding media). Other operations might reconfigure the grinding circuit entirely to operate without ball mills (i.e. in single-stage AG operation) to maintain production.
What to address at your operation?
Converting SAG mills to AG mills brings a new set of operating challenges to those lacking experience running AG mills. The most immediate impact to be addressed is the drop in throughput and unstable operation conditions. To reach a stable operation and maximise throughput, we recommend:
1) Feeding the right blend of coarse, fine, hard and soft rocks
2) Determining the optimal mill operating conditions to break the rocks effectively
3) Modifying control strategies where needed
Longer-term, mill internals can be adjusted to optimise performance under the new AG milling configuration.
Converting grinding circuits to single-stage operation (AG mill operating only, ball mill switched off) requires some modifications to the piping layout and circuit operating philosophy. Operating in single-stage requires the hydrocyclone underflow stream from the ball mill circuit to be redirected to the AG mill. Stability in the single-stage circuit is maintained by optimising the hydrocyclone (or screen) operation.
Is it worth it?
Despite the challenges of operating without grinding media, there are benefits, including higher energy efficiency, finer grind size and better flotation chemistry. Combined with optimized resource planning, this results in lower operating costs, higher recoveries and higher margins per tonne of ore.
A redefined mine plan can take advantage of the improved plant performance afforded by AG milling by targeting high-grade ore pockets to maximise revenue or pursuing low-grade pockets enabled by higher operating margins. (See paper An Assessment of the energy efficiency and operability of the world’s largest AG mills at the Boliden’s Aitik Mine, by McElroy et al., presented at SAG Conference 2019).
Maximise the value of your operations
Converting your mill operating strategy might be a consideration even in times when grinding media supply is strong, not just in times of uncertainty or need. Should a grinding media shortage arise, know that you have options and that you can tailor your comminution circuit operation.