Cake growth mechanism in cross-flow microfiltration of mineral suspensions

Abstract

This paper describes the fouling mechanism in cross-flow filtration of a mineral CaCO 3 suspension. Since the minimum particle size (1 μm for a mean of 6 μm) was larger than the pore size (0.2 μm) of the ceramic membrane, fouling is in this case exclusively superficial and Brownian diffusion effects are negligible. The transmembrane pressure was increased and decreased in steps at constant velocity to form successive cycles. The permeate flux variation presented a hysteresis which is interpreted as an irreversible compression of the cake deposited on the membrane. Cake resistance was measured as a function of the pressure in the permeation experiments. The porosity was calculated from Ergun's equation and found to be a function of the pressure gradient in the cake. The mean cake thickness in the microfiltration experiments was calculated as a function of the pressure drop across the cake using the porosity data from the permeation experiments. The thickness was found to be a significant fraction of the membrane radius (up to 38%). This result suggested that the cake growth was limited by the shear stress acting on its surface. The cake grew as long as the top particle layer obeyed the Coulomb friction equilibrium. When due to an increase in shear stress by lumen reduction the Coulomb equilibrium was not satisfied, incident particles were ripped off the surface and the cake ceased to grow. However, subsequent pressure reduction did not decrease the cake thickness because of its irreversible compressibility.