In hydrocyclones, the classification efficiency is limited by the fines, which are discharged together with the water in the underflow. It is well known that the injection of water in the conical portion of the cyclone reduces the fines in the underflow. This paper presents an improved technique, which is done via an injection at the upper end of the apex or the conical end. This results in a greater washing effect and reduced consumption of wash water. The process is stabilized by controlled water injection specific to the underflow shape. This controlled wash water injection is applied to kaolin processing for the reduction of kaolin losses in the cyclone underflow. A series of tests using 50-mm cyclones was conducted in this work, which demonstrates the marked improvement of the partition curves using controlled water injection. (C) 2009 Elsevier Ltd. All rights reserved.