Detrimental Thixotropic Thinning of Filter Cake of SiO2–Al2O3 Composite Coated TiO2 Particles and Its Control

Abstract

A SiO2–Al2O3 double layer coating process is widely used for TiO2 particle to reduce its photocatalytic activity. The conventional coating process needs a long time and uses large amounts of acid and alkali solutions. We recently developed a new SiO2–Al2O3 composite coating process that needs less time and does not use acid and alkali solutions. This process produced coated TiO2 particles with excellent weather durability, but it had the problem that the filter cakes composed of these particles were thixotropically thin and were easily broken up by shear or disturbance on a conveyor belt; that is, the cakes thinned into slurries very quickly. A slurry is not a suitable physical form for the subsequent drying process in industrial production. To understand this problem better, the rheology of the filter cakes was measured. Experiments to investigate the mechanism of the thixotropic thinning behavior indicated that the Si–O–H groups on the surface of the coated particles formed, by hydrogen bonding, a network structure of the particles that entrapped water within the structure. This structure was easily destroyed by an applied shear or disturbance, leading to the release of the occluded water, and the presence of water caused the cake to become a slurry and its viscosity to decrease sharply. This thixotropic thinning behavior of the filter cake was eliminated by coating the particle surface with a thin film of aluminum oxide.