Arafertil＇s regrinding circuit is responsible for the recovery of apatite with a low degree of liberation, representing approximately 11% of the overall apatite concentrate production. Low grinding capacity prevented an ideal size distribution of product for maximum recovery in the subsequent flotatian stage, as shown in a liberation study. This work was aimed at attaining the required increase in regrinding capacity, using process simulation by computer mathematical modelling, with the objective of implementing an optimised grinding and classification circuit. The ball mill and hydrocyclones models were calibrated, and their equations were defined, from industrial data acquired in the old concentrator configuration and results from laboratory grinding tests with power consumption measurements. The model was validated by means of comparison between the results predicted by simulation and those achieved in the plant after refurbishment. The apatite flotation recovery increased from 31.5% to 48.0%, in agreement with the previous apatite liberation study.