Equilibrium-state Density Profiles of Centrifuged Cakes of Flocculated Suspensions

Many advanced ceramics are formed by colloidal consolidation using techniques such as sedimentation, centrifugation, or pressure filtration. In all these, the minimization of density variations of various colloidally-consolidated cakes has not been examined systematically until recently. Schilling et al.' used y-ray densitometry to study the density variations in a sedimented cake and showed that the sediment of a flocculated alumina suspension exhibited significant density variations within the cake whereas the sediment of a dispersed alumina suspension showed a constant density profile. Auzerais et al. 2 used a medical X-ray computer tomography (CT) to study the settling of dispersed and flocculated silica suspensions and obtained similar results: The sedimented cake of a dispersed silica suspension was fairly uniform whereas those of flocculated silica suspensions exhibited significant density variations. 2 However, Shih et al. examined pressure-filtered cakes of flocculated alumina suspensions with y-ray densitometry and showed that the density profiles of pressure-filtered cakes of flocculated alumina suspensions were uniform, in contrast to their sedimentation counterparts which showed significant density variations.' These studies illustrate that how the local density varies within a consolidated cake depends not only on the suspension conditions but also on how the suspensions are consolidated, e.g., the consolidation pressure. The goal of this paper is to examine the equilibrium-state density profiles of centrifuged cakes of flocculated suspensions both theoretically and experimentally. The emphasis is to take into account the effect of the consolidation pressure by relating the equilibrium-state density profiles to the pressure-cake-density relationship. By equilibrium state we mean the stage in which the cake height no longer changes with time. Theoretically, the equilibrium-state density profiles of centrifuged cakes were obtained by implementing the experimentally obtained pressure-density relationships into the general differential equations and solving the differential equations with appropriate boundary conditions. Experimentally, the density profiles were examined by y-ray densitometry.,