COPRECIPITATION OF HYDROUS ALUMINA AND SILICA WITH TIO2 PIGMENT AS SUBSTRATE

Hydrous alumina (Alu) and silica (Sil) were coprecipitated by adding solute acidic and basic precursors to an aqueous neutral suspension of TiO2 pigment particles in a stirred tank reactor, varying the rate of addition. The products were compared to those precipitated in H2O. By the combined use of physical and chemical bulk and surface methods, the mechanism of this reaction, common in the industrial production of TiO2 pigments, was elucidated. Even at the fastest rate of addition the whole precipitate material adheres to TiO2 in the products from the TiO2 suspension and forms a thin amorphous coat on the TiO2 particles. However, the precipitation rate influences the porosity of the coat and the degree of coverage of the TiO2 surface. Slow precipitation gives diffusion-controlled growth of Alu and Sil homogeneously mixed on TiO2. Increasing the precipitation rate shifts the mechanism to spontaneous nucleation of increasing fractions of Sil and subsequent growth of a mixture of Alu and the rest of Sil as alumosilicate on the nuclei, followed by coagulation on the TiO2 particles. (C) 1994 Academic Press, Inc.