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 different 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 fluid&d. Similar to the derivation of an expression of the pressure gradient in a onephase 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. 1. INTEODUCFION Marine deposits of sand sometimes contain a low concentration of so-called heavy minerals (t-utile, monazite, cassiterite, etc. density 4 -8 x lo3 kg/m3). During settling the free-fall velocities determined at
what place sand and minerals reached the sea or river bottom, hence, minerals and sand won from marine deposits, generally, wilt have about the same free-fall velocities. Therefore, in order to concentrate the minerals the required properties of the separating device are: separation according to density difference rather than to settling velocity difference; a high throughput of solids; a considerable effected concentration of minerals.
From capacity considerations normal hydrocyclones would be suitable, they separate solids, however, according to their difference in settling velocity. In literature hydrocyclones with a largecone-angle (45-180") are mentioned to separate solids with equal free-fall velocities. As opposed to normal hydrocyclones which are not very critical with regard to the direction of their axis, the axis of largeconcangle cyclones should always be vertical.
Applications of these cyclones in mineral dressing are, among others, published by Hoffman[l], Fontein[2], Visman [3], Man'kov and Sannikova [4]