Continuous gravity separation of a bidisperse suspension in a vertical column

Abstract

Continuous separation of bidisperse suspensions containing particle species lighter and heavier than the suspending fluid in a vertical settler has been studied. Polystyrene (light) and polymethyl methacrylate (heavy) beads of respective uniform size and density suspended in a salt solution were used. The purity and recovery of both species in the overflow and underflow streams were evaluated as a function of the feed flow rate [up to 1(cm 3/s)/(cm 2)], feed total solids concentration (up to 18 vol.%) and the fraction of feed in the underflow (i.e. split ratios of 0.1 to 0.9). A theoretical model, based on mass balances and slip velocities, has been developed. The model predicts all the observed trends and agrees fairly well with the quantitative measurements. The degree of differential settling achieved in the column decreases with increasing feed flow rate or increasing feed total solids concentration. At a fixed feed flow rate, there is a threshold split ratio, beyond which the recovery drops linearly with increasing split ratio. The threshold split ratio itself decreases with increasing feed flow rate as well as increasing feed total solids concentration. Increasing feed flow rate or total solids concentration also results in increased impurities in the product streams (i.e. light in underflow and heavy in overflow).