As opposed to "normal" hydrocyclones, large-cone-angle hydrocyclones can be used to separate solids with equal free-fall velocities but with different densities. One of the applications of these cyclones is the recovery of heavy minerals from sand. In order to investigate the hydrodynamical behaviour of such a cyclone the following experiments have been carried out on two cyclones of diferent dimensions: determination of solids distribution at steady state operation; measurement of the residence time distribution of the liquid; pressure gradient and pressure drop measurements. From the experimental results it follows that at the bottom a bed of suspended solids rotates, which more or less seems to be fluidized. Similar to the derivation of an expression of the pressure gradient in a one-phase vortex flow from the Navier-Stokes equation, an expression of the pressure gradient in such a solid-liquid vortex flow has been obtained from two-phase momentum equations. From this theory it follows that in the rotating bed a secondary solids flow has to occur; solids are moving towards the centre at the bottom and away from the centre at the top.