A model has been developed to describe the operation of a Knelson concentrator by considering the main forces that act on particles within the concentrating chamber. It is postulated that the deportment of particles is dependent on the dynamic equilibrium between the fluid drag, centripetal and Bagnold's forces, which are functions of the material properties as well as the key operating parameters such as fluidisation water flowrate and speed of rotation. Based on this concept a criterion has been developed that determines the recovery of minerals of different sizes and densities adequately. It has also been shown that this criterion determines the extent of competition between particles that exist within Knelson concentrators. (C) 2004 Elsevier Ltd. All rights reserved.