Effect of High Pressure Grinding Rolls (HPGR) pre-grinding and two-compartment ball mill intermediate diaphragm grate design on grinding capacity of an industrial scale conventional two-compartment ball mill cement grinding and classification circuit was investigated. For this purpose, cement clinker was crushed in an industrial scale HPGR in open circuit and fed to a Polysius two-compartment ball mill and air classifier closed circuit. Two-compartment ball mill intermediate diaphragm middle grate design was changed when HPGR crushed clinker was fed to the circuit to obtain optimum flow of material from the first compartment into the second compartment. Modifications on the diaphragm design were required due to the increased throughput when processing HPGR product. Two sampling campaigns were performed at the steady state conditions of the circuit. Raw clinker (uncrushed clinker) was fed to the conventional two-compartment ball mill air classifier circuit in sampling campaign-1 whereas HPGR crushed clinker was fed to the same circuit with modifications on the intermediate diaphragm middle grate design in sampling campaign-2. Mass balanced tonnage and particle size distributions were estimated by using JKSimMet Steady State Mineral Processing Simulator in both sampling campaigns. Polysius (R) two-compartment ball mill was modeled by using perfect mixing modeling approach (Whiten, 1974). Specific discharge and breakage rates in the grinding compartments were estimated by using the two-compartment ball mill model structure proposed by Genc and Benzer (2015). Specific discharge rates were found to increase at coarse size ranges when raw clinker was fed to the circuit in compartment-1. However, specific discharge rates of particles were increased slightly at coarse size ranges in the second compartment at higher mill capacity condition. Specific breakage rates were increased when the circuit was fed with HPGR crushed clinker. Modifications in the circuit and the ball mill intermediate diaphragm grate design enabled the optimization of the grinding capacity of the conventional two-compartment ball mill cement grinding and classification circuit. Ball mill grinding and classification circuit capacity was increased by 10% and specific energy consumption of the ball mill was decreased by 9.1%. (C) 2016 Elsevier Ltd. All rights reserved.