Effects of the Structure of Silica-Alumina Gel on the Hydrothermal Synthesis of Kaolinite

Shigeo Satokawa1, *, Yasushi Osaki2, Soichiro Samejima2, Ritsuro Miyawaki3, Shinji Tomura3, Yasuo Shibasaki3 and Yoshiyuki Sugahara4
1 Engineering Research Association for Artificial Clay, National Industrial Research Institute of Nagoya, Kita, Nagoya 462, Japan
2 Chemical Research Laboratory, Tosoh Corporation, Shin-nanyo, Yamaguchi 746, Japan
3 Ceramic Technology Department, National Industrial Research Institute of Nagoya, Kita, Nagoya 462, Japan
4 Department of Applied Chemistry, Waseda University, Ohkubo, Shinjuku, Tokyo 169, Japan
* Present address: Fundamental Technology Research Laboratory, Tokyo Gas Co., Ltd., Shibaura, Minato, Tokyo 105, Japan.

Abstract: 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.

Key Words: Gel structure • Hydrothermal synthesis • Kaolinite • Nuclear magnetic resonance • Silica-alumina gel

Clays and Clay Minerals; June 1994 v. 42; no. 3; p. 288-297; DOI: 10.1346/CCMN.1994.0420307
© 1994, The Clay Minerals Society
Clay Minerals Society (www.clays.org)