An Experimental Investigation of Dilute Slurry Sedimentation

Abstract

<p>The velocity and concentration of slurries of closely-sized, spherical particles in water were measured at several locations in the dilute blanket of a laboratory-scale, continuous, thickener-clarifier. Particle velocities were measured by the laser Doppler technique; particle concentrations were measured by local scattered light intensity and by optical transmittance.</p> <p>Steady-state settling velocities as much as 70 percent higher than the average particle Stokes velocity occurred at slurry concentrations between 7.04 x 10‾² and 1.07 x 10ˉ¹ percent by volume. These findings contradicted all of the deterministic models commonly used to correlate slurry settling velocity with particle concentration. The failure of these models at dilute concentration was verified by flux measurements.</p> <p>The formation of clusters, whereby several particles settle as a group rather than individually, was used to explain the high velocities. Based on a binomial spatial distribution of particles, large clusters at concentrations slightly higher than the bulk solution concentration were predicted to occur with high probability.</p> <p>The few previous studies noting the formation of particle clusters were shown to have been dominated by wall effects at volumetric concentrations as low as 0.1 percent. The inverse variation of cluster size with slurry concentration accounted for the fact that the high velocities observed at low slurry concentrations have not been reported before.</p> <p>The results of this investigation have significant implications for optimizing the design and control of those unit operations and unit processes which depend on relative motion between particles and fluid.</p>