This paper summarises some observations regarding the way SAG and AG mills work based on the last ten years of pilot plant operation, grinding circuit design and operational reviews for various projects in Australia and overseas. The main purpose is to promote discussion on key factors influencing breakage in a SAG mill._x000D_
The focus of the paper is on two questions: How do mill lifters and liners really work and have an effect on grinding? How does the mill charge composition impact on grinding performance? The focus of the paper is on observations of mill behaviour in both pilot plant trials and plant operation, particularly on Cadia ore. DEM, by others, and JKSimMet modelling of SAG mill performance have greatly assisted in forming the ideas expressed in the paper and both are valuable tools in developing our understanding of the comminution process._x000D_
Put in the extreme' form the major observations and conclusions expressed in this paper are that: the main purpose of mill lifters is to reduce mill liner wear and not to promote rock breakage; the often promoted theory that balls impacting the toe of the charge' is a dominant form of breakage is incorrect; and fines production in a SAG mill is primarily a function of the rock load mass, with a secondary dependence on ball load for any given circuit configuration.