EFFECTS OF THE STRUCTURE OF SILICA-ALUMINA GEL ON THE HYDROTHERMAL SYNTHESIS OF KAOLINITE

Kaolinite was hydrothermally synthesized from two kinds of silica-alum ina gels to examine the effect of the structure of the starting materi al. Two kinds of gels were prepared by precipitation at different pH c onditions (pH = 9.6 and 4.2) from solutions containing water glass and aluminum sulfate. Na ions in the gels were removed with a resin befor e the hydrothermal treatment, but a slight amount of sulfate ions was still present in the gels. The nuclear magnetic resonance spectra of t he starting gels suggested that the gel prepared at pH 9.6 consists of networks with alternating SiO4- and AlO4-tetrahedra (partially AlO4-o ctahedra), whereas the gel prepared at pH 4.2 consists of a sheet stru cture related to allophane. After the hydrothermal treatment at 220-de grees-C for 9 days, kaolinite particles with spherical shape were obta ined from the former gel, and platy kaolinite was crystallized from th e latter one. The difference in morphology of synthetic kaolinite was attributable to the structures of the starting gels, and the pH values in the hydrothermal reactions were not very significant to the morpho logy.