Coarse particles, known as ‘pebbles’ or ‘scats’, discharge from any autogenous grinding (AG) or semi-autogenous grinding (SAG) mill and often end up recirculating back to the mill feed. Sometimes, these pebbles are crushed while other times they are returned intact and considered a natural consequence of primary grinding, due to the critical energy requirement for certain sized particles.
Increasingly, the impact of pebble recycle is being scrutinized by operations dealing with competent feed. This includes the negative effect on circuit efficiency, mill capacity and even pebble crushing capacity. Operations are moving to finer feed conditions more akin to primary ball milling circuits, so why are we not assessing the impact of pebble recirculation? Have the pebbles not already proven themselves to be competent and resilient, and in fact, worthy of a higher cut-off grade compared to the remainder of the ‘ore’?
The authors recently evaluated AG and SAG mill pebble samples from North American copper operations for their hardness, grade distribution and suitability for detection using x-ray transmissive (XRT) sensors. Following a laboratory protocol developed by the authors, the pebble samples showed a remarkable range of metal grades, which could be detected using the XRT sensor.
As pebble streams are prime candidates for particle sorting - due to their limited range of size and presentation on a recycle conveyor - these results support rejection of the low-grade portion of pebbles. This paper summarises these test results and estimates the increase in both mill capacity and metal grades that is possible with up to 50% pebble rejection of below cut-off grade material.
