FORMATION PROCESS OF KAOLINITE FROM THE AMORPHOUS MIXTURE OF SILICA AND ALUMINA

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The formation process of kaolinite from the amorphous mixture of silica and alumina by hydrothermal reaction was studied by XRD, IR, DTA-TG, TEM, and high-resolution solid state ^<29>Si-and ^<27>Al-MAS/NMR. The amorphous mixture of the starting material prepared with commercially available silica-sol and alumina-sol was hydrothermally treated with distilled water in a Teflon pressure vessel at 220℃ for a reaction time varying from 1 to 144 hours. The Q^4 state component of the amorphous starting material changed into an intermediate amorphous Q^3 state. The intermediate amorphous phase converted into crystalline kaolinite with spherical shape (Q^3 state). The yield and crystallinity of kaolinite increased with an increase in the reaction time. On the other hand, the coordination number of aluminum converted from a mixture of six-and four-coordinations to six-coordination. In the earlier stages of crystallization of kaolinite, spherical kaolinite is more abundant than platy kaolinite, and the spherical kaolinite changes into platy one with increasing hydrothermal reaction time.
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