EFFECTS OF ACIDITY ON THE HYDROTHERMAL SYNTHESIS OF KAOLINITE FROM SILICA-GEL AND GIBBSITE

A comparative study is reported in which kaolinite has been hydrotherm ally synthesized at several pH conditions. The syntheses were carried out at 220 degrees C for 3 to 10 d with distilled water or acidic solu tions using a mixture of silica-gel derived from alkoxide and gibbsite with a Si/Al ratio of 1:1 as the starting material. Use of acidic sol ution for the synthesis promotes the dissolution of the starting mater ials and leads to kaolinitization at an earlier stage of the reaction. However, the rate of kaolinitization is found to be rather slow, in c omparison to the reaction with distilled water. The synthetic kaolinit e was characterized by X-ray powder diffraction pattern. Kaolinite syn thesized with distilled water was poorly grown for direction of the st acking. For example, crystallite size along the c-axis = 155 Angstrom , whereas kaolinite synthesized with acidic solution gave a higher cry stallite size along the c-axis, such as 253 Angstrom in the case of t he synthesis with 0.1 N HCl. Hinckley index of the synthetic kaolinite was varied from 0.35 to 0.80 by the acidity of the reaction. Differen t kaolinitization processes are implied by differences observed in the rate of kaolinitization, which has an influence on the nature of the stacking faults of the kaolinite.