Cross-flow filtration of bentonite suspensions, II

Abstract

Previous work on cross-flow microfiltration of swelling clay suspensions is summarized. We conjectured the existence of a shear-dependent critical filtration flux, above which uniform irreversible accretion to a filter cake proceeds as in batch filtration; below this flux continued accretion to the cake cannot occur. Filtration fluxes therefore stabilize at the critical flux. This flux is supposed to be dependent only on conditions at the filter cake surface, i.e., the physicochemical state of the suspension and the wall shear stress, but not on conditions within the cake or the properties of the underlying medium. Two testable predictions of this conjecture, not previously verified, are noted and examined experimentally. For a very low permeability medium and low differential pressures, filtrate fluxes are obtained which are below the critical flux. In the majority of these cases, no filter cake was formed, and the fluxes were controlled by the medium, in accordance with Darcy's law. For cases where a definite cake formed, the steady state fluxes reached are shown to be unaffected by medium resistance for two filter media whose permeabilities are in a 10 4 ratio.