Mechanics of a concentrated slurry feed system

Spherical ballotini particles, about 125 mu m in diameter, were combined wi th water to form a sediment in a tank. The volume fraction of particles in the sediment formed at the base was always 0.64, the normal level for the r andom packing of spheres of a single size. Slurry from below the sediment w as permitted to discharge through an orifice at the base connected to verti cal tubes of various lengths. A reproducible slurry discharge was achieved with the volume fraction of the slurry always about 0.58. Acrivos et al. (1 990, 1995) obtained the same solids volume fraction for a sediment layer sl iding down an incline. An estimate for this critical solids volume fraction was obtained by considering the proportion of vacant particle locations ne eded, firstly, for all particles to be near a vacancy and, secondly, for fr ee-flow to be possible. In this study, the slurry discharge rate increased with an increase in the tube length. It was concluded that the slurry moved through the tube as a plug flow, with a lubricating liquid film of about 1 3 mu m in thickness between the particles and the tube wall. (C) 1999 Elsev ier Science S.A. All rights reserved.