Spherical ballotini particles, about 125 mu m in diameter, were combined with 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 random packing of spheres of a single size. Slurry from below the sediment was permitted to discharge through an orifice at the base connected to vertical tubes of various lengths. A reproducible slurry discharge was achieved with the volume fraction of the slurry always about 0.58. Acrivos et al. (1990, 1995) obtained the same solids volume fraction for a sediment layer sliding down an incline. An estimate for this critical solids volume fraction was obtained by considering the proportion of vacant particle locations needed, firstly, for all particles to be near a vacancy and, secondly, for free-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 13 mu m in thickness between the particles and the tube wall.