THE EFFECT OF PARTICLE SEDIMENTATION ON GRAVITY FILTRATION

Simulation of cake formation of mono-sized and dual-sized particles un der gravitational sedimentation and filtration is presented. The dynam ic analysis proposed by Lu and Hwang in 1993 is applied to examine the local cake properties formed under a falling head by considering the hindered settling effect of particles in the slurry and the variation of the pressure drop across the filter septum. Results of this study s how that, at a given position in a cake, the solid compressive pressur e reaches a maximum value and then decreases for a gravity filtration due to the decrease in the driving head. A cake constructed with dual- sized particles has a more compact structure than does one with mono-s ized particles, and larger particles will form looser packing than wil l smaller ones for mono-sized particles. A dual-dispersed suspension w ith a lower fraction of large particles will result in the lowest cake porosity and the highest specific filtration resistance of cake. Comp arison of the porosity distribution in filter cake formed by means of gravity filtration and constant head filtration shows that the porosit y near the filter septum of gravity filtration has a convex behavior w hile that of constant head filtration has a tendency toward concavity. This discrepancy is mainly due to the change in the driving head duri ng the filtration process. Both theoretical and experimental results s how that the uniformity of particle size distributions in the filter c ake will be much better when the relative settling velocity between la rge and fine particles is reduced.