Influence of interaction on transfer mechanisms occuring during colloid ultrafiltration
Résumé
Experimental ultrafiltration (UF) of clay suspensions having different salinity shows that the presence of salt affects the transfer mechanisms, such as deposition kinetics, hydraulic resistance of the deposit and salt retention by the deposit. Such changes are the results of the variation in surface repulsive interaction between particles with the salt concentration (DLVO theory). With respect to the deposition kinetics, it is observed that as salt concentration increases the critical flux decreases to zero: this coincides with the occurrence of an adhesion mechanism. These phenomena are modelled by considering transport by surface interaction of the particles near the surface. Variations in hydraulic resistance of the cake are explained through a modified Kozeny-Carman equation. A reduction in porosity due to a decrease in repulsive interaction and a particle size increase due to coagulation are involved. Salt retention is described by considering (i) electrostatic partition (ii) salt transport by convection and diffusion in the boundary layer and in the porous medium. Analysis of salt retention and of hydraulic resistance of the cake lead to the same conclusions about the deposit properties. An important result is that surface interactions (hence the suspension stability) are as important as operating conditions for the membrane processing of colloidal suspensions.