An industrial scale FLSmidth (R) three-compartment cement grinding ball mill operating in open circuit was sampled at the steady state conditions. The three-compartment ball mill was modelled by using the perfect mixing modelling approach [1]. Classification performance of the intermediate and discharge diaphragms of the ball mill were modelled by the efficiency curve approach [2]. Second and third compartments were combined and the mill was simulated as a two-compartment by using the JKSimMet simulation program. Grinding capacity increase when the three-compartment ball mill was converted to a two-compartment ball mill was evaluated by simulation. The same cylpebs size distribution and load were assumed to be applied in the second compartment of the new mill design. Simulation result showed that, 10% capacity increase can be achieved with 16% material retained on 0.045 mm in the mill discharge when the mill was converted to a two-compartment mill. (C) 2016 Elsevier B.V. All rights reserved.