| When substantial amounts of rigid buoyant particles are mixed with a sedimenting suspension of heavy particles, it is found that the two types of solids undergo rapid lateral segregation from each other. This segregation takes the form of 3-5 mm diameter streams of the less populous species flowing through a concentrated continuum suspension of the other species. Convection is buoyancy driven and results in greatly enhanced sedimentation rates. The phenomenon has been investigated both theoretically and experimentally.;Parameters investigated experimentally include particle concentrations, densities, sizes and size distribution, fluid properties and container geometry. The most difficult-to-settle suspensions are found to benefit most from buoyant-particle addition. In general greatest enhancement (factors of up to 15 times in settling rate) is obtained at a total solids concentration of 35-40% v/v. Accelerated settling rates are unaffected by the width of particle size distributions and by vessel geometry for vertically aligned tubes deeper than 30 cm and larger than 16 mm diameter (the smallest size used).;The theory is based on the postulate that streams move in plug flow with all resistance to motion confined to a thin lubricating layer at the stream-continuum interface. The size of the two particulate species determines the layer thickness; the viscosity of the mixture within the layer is decided by particle size and concentrations, and by the viscosity of the supporting fluid. The model gives an a priori prediction of the density driving force and correctly accounts for the effect of each system parameter on the observed acceleration. |
