Influence of aqueous solution chemistry on the surface charge, viscosity and stability of concentrated alumina dispersions in water

Abstract

This study examines the effects of pH, electrolyte concentration and solids concentration on the surface charge characteristics, viscosity and stability of concentrated alumina dispersions, and the relationships between these properties of the dispersions. The surface charge properties of various concentrated alumina dispersions were investigated using acid-base titrations. Suspension viscosity and stability were also investigated for selected dispersion concentrations, electrolyte concentrations, and pH values. The initial rate of mass removal from a sedimenting alumina suspension was used as a measure of suspension stability. Results indicated that an increase in solid concentration caused a decrease in the surface charge at pH values below the point of zero net proton charge (PZNPC), possibly reflecting diffuse layer overlap between particles. The dispersion viscosity was observed to depend greatly on the aggregated state of the dispersion; maximum viscosity was observed near the PZNPC where the stability was at a minimum. Viscosity was decreased and stability was increased at pH values away from the PZNPC, where an increased surface charge causes electrostatic repulsion between particles. Away from the PZNPC, higher electrolyte concentrations yielded slightly higher suspension viscosity than lower electrolyte concentrations due to increased aggregation. This study indicates that control of stability via solution chemistry is critical to the control of the viscosity of concentrated dispersions of alumina in water.