Centrifugal filtration of particulate systems

Analysis of centrifugal filtration is complicated by its formulation in rad ial-cylindrical coordinates and the inclusion of a significantly varying ce ntrifugal force across the filter cake. The process often commences, howeve r, with a filter cake of zero thickness so there will be a period of time d uring which one-dimensional, constant centrifugal force analysis of the pro cess will be exact. It is important to understand the nature of Bow during this time because it accurately represents the early stages of, and therefo re a base from which to analyse, the more complicated system. In addition, it may emerge that, for some materials, the simplified analysis may be suff icient for practical purposes. For materials where the permeability and the effective stress are well-defined functions of the water content, the earl y stages of centrifugal filtration are described by a one-dimensional non-l inear Fokker-Planck equation cast in a material coordinate based on the dis tribution of the solid phase. The analysis takes full account of the non-st eady nature of flow and the non-linearity of the equations that describe it . It does not require common inaccurate assumptions that the flux of solid can be neglected nor that quasi-steady flow takes place. The analysis predi cts, and experiments on a sodic bentonite confirm, that substantially non-u niform solid and liquid profiles are observed during 'early stages' of the process, which evolve in a manner which is effectively identical to those o bserved during constant pressure filtration. The generality of the approach is illustrated with material data for Red mud. Limits to the times during which flow might be regarded as one-dimensional rather than cylindrically a xi-symmetric, and during which some centrifugal effects might be neglected, are discussed and shown to be readily calculated. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.