Abstract
In order to investigate the properties of very thin filter cake formed during microfiltration of a dilute suspension, the microfiltration experiment of dilute polystyrene latex suspension was performed by using polycarbonate membrane under constant pressure. The average specific filtration resistance was calculated based on the cake filtration theory, and the cake porosity was estimated from the specific filtration resistance by applying Kozeny-Carman equation, on the assumption that the incompressible filter cake was formed. The estimated porosities of 0.070 µm and 0.196 µm PSL at latex concentration of above 0.5 ppm were the constant values. Especially, the value for 0.070 µm PSL was close to 0.391 which indicates a random packing of spherical particles. However, the cake porosity obtained in filtration of very dilute suspensions with the concentration of below 0.5 ppm had a tendency to be decreased with a decrease of concentration, and ultimately the value of porosity was approached to 0.260 implying a hexagonal closed packing. Thus, the filter cake has an orderly structure when the suspension is too dilute. This variation of the cake structure was also confirmed by a scanning electron microscope. Furthermore, microfiltration of saline polystyrene latex suspension was performed for various sodium chloride concentrations. As a result, it was shown that the addition of sodium chloride brought about the increase of cake porosity because the electric double-layer thickness was reduced with an increase of sodium chloride concentration and the adherence of particles prevented the particles from forming the filter cake with orderly structure. Thus, the structure of the filter cake in microfiltration of a dilute particulate suspension can be controlled by the salt concentration.
