A control strategy to increase the mineral feed tonnage in grinding operations was developed and evaluated in an industrial mill. The control strategy maximizes the solid concentration at the hydrocyclone underflow discharge while keeping the particle size constant. The manipulated variables are the set-points of the fresh solids rate controller and the sump water flowrate controller. The system was tested in two grinding circuits at El Teniente Division, Codelco-Chile. Results showed a 5-12.5% increase in treatment capacity while the product size at the hydrocyclone overflow was kept under control (25% + 150 mum). Alternatively. the extra capacity generated by the optimizing control strategy can be used to shutdown some of the grinding circuits without losing the actual plant capacity. Also. the dynamic response of the grinding circuit was characterized for open loop step changes in fresh mineral rate and sump water flowrate. (C) 2002 Elsevier Science Ltd. All rights reserved.