Speed is one of the important parameters in tumbling ball mills because it affects the dynamic motion of the balls within the mill. In turn the motion of the balls affects the distribution of the forces and thus the power draft of the mill. The main objective of this work is to model the dynamics of mill startup using the discrete element method (DEM). To achieve the experimental objective speed and torque as a function of time during mill startup were measured experimentally. A 0.55 m diameter mill was used to perform the experimental analysis. Load volume, J of 0% and 20% and mill speed of 70%, 100%, 120% and 150% of the critical are analyzed in detail. The DEM software, MillSoft (by Rajamani) was used to model the experimental results. During the experiments, the mill was allowed to ramp from rest until it reached the setpoint; after this it continued at constant speed. We have developed good dynamic models of mill speed. The experiments show that torque is the same for all conditions up to the point where each attains steady state, because the ramp rate is constant. The final torque for speeds greater than 100% of the critical speed (experiments and simulations) is not zero because only one layer of balls centrifuges in the time used. The experimental and the simulated torque compare well. Overall the dynamics of mill startup can be modelled with semiquantitative accuracy using DEM. (C) 2002 Elsevier Science Ltd. All rights reserved.