Effects of the structure of silica-alumina gel on the hydrothermal synthesis of kaolinite

Kaolinite was hydrothermally synthesized from two kinds of silica-alumina gels to examine the effect of the structure of the starting material. Two kinds of gels were prepared by precipitation at different pH conditions (pH = 9.6 and 4.2) from solutions containing water glass and aluminum sulfate. Na ions in the gels were removed with a resin before the hydrothermal treatment, but a slight amount of sulfate ions was still present in the gels. The nuclear magnetic resonance spectra of the starting gels suggested that the gel prepared at pH 9.6 consists of networks with alternating SiO₄- and AlO₄-tetrahedra (partially AlO₆-octahedra), whereas the gel prepared at pH 4.2 consists of a sheet structure related to allophane. After the hydrothermal treatment at 220°C for 9 days, kaolinite particles with spherical shape were obtained from the former gel, and platy kaolinite was crystallized from the 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 morphology.