This paper presents radioisotope tracer experiments conducted in a hydrocyclone processing copper ore. Different ore grain sizes ware traced using a radioactive isotope Cu-64 and the experimental residence time distribution (RTD) functions have been obtained. These functions lead to the determination of the kinetic parameters (mean residence time, flow rate or real volume occupied by matter and degree of mixing) and relative separation of each class of grain in the hydrocyclone. Macromodelling of the hydrocyclone shows that the flow behaviour of the solid is not far from perfectly mixed which results in a low efficiency observed in the relative separation of the larger and smaller grains. (C) 2002 Elsevier Science B.V. All rights reserved.