Electrokinetic and bouyancy effects in colloidal suspensions

Abstract

Dewatering of silica and alumina suspensions was accomplished using electrodecantation and electrocoagulation. Electrocoagulation was found to occur in high-conductivity alumina suspensions (250-1300 μS/cm), while electrodecantation was found to be the separation mechanism in low-conductivity suspensions of alumina and silica ( < 20 μS/cm). With these low-conductivity suspensions, a clear fluid layer developed on the surface of the suspension. A clear fluid layer did not develop in high conductivity silica suspensions, 250 μS/cm, even though electrodecantation was found to dominate the separation. Spatial variations in the pH and conductivity were measured at the completion of electrodecantation experiments. A boundary-layer model was developed to quantitatively establish the principles of electrodecantation. This model provides an explanation for the formation of the clear fluid layer on the surface of colloidally stable suspensions and provides an understanding of how buoyancy driven motion redistributes ions produced/consumed at the electrodes, which results in the formation of pH and conductivity gradients. The growth rate of the clear layer at the top of the chamber is initially slower than that predicted by the model; at later stages the theory and experiments are in agreement. Numerical simulations were performed to support the boundary-layer model and were used to incorporate important features such as electrode reactions, ion gradients, and cell geometry omitted from the model. Two-dimensional simulations were performed to study the effects of buoyancy driven motion in the absence of any ion gradients. Due to limited computer resources, one-dimensional simulations were used to show that a clear fluid layer would not necessarily be expected in high-conductivity suspensions and to study the effects of electrode reactions in the absence of any fluid motion. To characterize properties important to electrodecantation, two techniques were developed to measure particle diffusion coefficients, size, and electrophoretic mobility. Taylor-Aris dispersion measurements are shown to provide accurate diffusion coefficient measurements for colloidal particles up to about 0.3 μm in diameter and capillary electrophoresis is used to establish a novel method for measuring electrophoretic mobilities of colloids that compares favorably with existing methods.