Particle size reduction is a process carried out either on dry materials or on slurries where a material is gradually reduced in size to produce material of the desired size class. It is an energy intensive process concerned with the relationship between energy input and the particle size made from the given feed size. It is of paramount importance to mill to the right size while minimising the energy costs. In this study, the specific rate of breakage and primary breakage distribution functions for gold ore sample taken from Carletonville plant, Johannesburg, were determined in a laboratory size ball mill. Grinding at about U= 1 gave the maximum amount of size class for leaching (-150+75 µm), and is a good powder-ball loading ratio to give efficient breakage in a ball mill for the gold sample. The AR analysis is then used to optimise leaching and milling time in terms of costs.