The process of grinding is complex with many factors affecting the result. As the composition of the ore fed to the concentrator varies, implying changes in grindability, the optimal operation conditions for a pebble mill will also vary. In an attempt to increase the understanding of charge dynamics, a series of statistically planned experiments were done in a pilot-scale pebble mill with differing charge types. This pebble mill is equipped with an in-mill sensor, which measures the deflection of a single lifter as it passes through the mill charge. The experimental setup was a factorial design with two factors; two levels of magnetite pebbles content and three different size distributions. The experiments show that there is an advantage to keep the magnetite pebbles proportion as high as possible. This will increase the power consumption and maximum deflection of the lifters, but at the same time increase the production of <45 mu m material, the grindability and the pebbles consumption. A pebble size fraction 10-35 mm improves the grindability the most and the amount of <45 mu m material. It is strongly suggested that the 10-35 mm and 100% magnetite pebbles fraction should be tested in a larger scale pebble mill to confirm these findings. (C) 2012 Elsevier Ltd. All rights reserved.