Nowadays computer simulations have been shown to be powerful tools to understand the performance of systems where we have mathematical models that can capture the physics of the problem. In this paper we attempt to simulate the behavior of the particles that move inside a large- scale mine mill, specifically through the grates and the discharge ducts of the material, when small changes are made in the geometry of the grates. Basically, the continuity, energy and momentum conservation equations are the ones that can solve the behavior of the material in that zone. The discrete element method is used to carry out the simulation, under the hypothesis that the restitution coefficient can substitute for the presence of the fluid inside the mill, and that by changing the angle of inclination of the grate slots the performance can improve the classification that they make and in the total discharge flow. A corollary of this study is that the pulp-lifters have a greater impact than the grates in improving the discharge flow. It was possible to quantify phenomena like flow-back and carry-over, effects that are not evaluable experimentally, in this way showing the usefulness of this simulation. The application of the DOE method has allowed to back up statistically the results and indicate that the slot angle increases the mill’s outlet flow.