In hydrocyclone separation optimal results regarding solids recovery and solids content in the underflow can be expected at the transition point rope/spray discharge. It is calculated under which conditions this transition between the two discharge types appears. Furthermore, a control system has been developed, which enables a forced transition from rope to spray discharge even at high feed solids concentrations. Applying this control system, which is focused on the hydrocyclone operation at the transition point, remarkably good separation effects could be achieved. (C) 2004 Elsevier Ltd. All rights reserved.