Milling circuits play an important role in achieving the desired product; tonnage, size distribution, and properties, since they are directly linked to the comminution energy consumption for the whole process. A grinding model with a mathematical simulation was developed for an open milling circuit with a scalped feed from a hydrocyclone. The circuit was optimized by determining the optimal conditions in terms of energy consumption which would produce a floatable product having a desired size range (i.e., -75 to +9 mu m) for a platinum-bearing ore. The simulation results showed an operating region comprising the best combination of the ball filling and mill speed to achieve the highest production of a floatable size class, with the minimum energy consumption. [GRAPHICS] .