Effects of aggregate densification upon thickening of Kynchian suspensions

Abstract

A model is presented for the thickening of a raked suspension. The model is based on Kynch theory (Kynch, 1952), i.e. it describes systems with solids fractions sufficiently low that the solids have not gelled into a weight-bearing network. However the model incorporates a modification to describe how raking the suspension causes flocs or aggregates within it to densify. This floc densification opens up channels between the flocs through which liquid escapes, making the suspension easier to dewater. The densification theory presented here predicts profiles of varying solids fraction vs height in the settling zone in a thickener, information which is not normally available when designing thickeners via the conventional Kynch theory. Performance enhancements for thickeners due to raking can be readily determined, either in terms of increased underflow solids fraction or increased solids flux. As underflow solids fraction is increased, thickeners operated at a specified aggregate densification rate (or equivalently at a fixed settling zone height) tend to approach a ‘fully densified’ suspension state (defined as a point at which the extent of aggregate densification ceases to change with time), with improved thickening performance.