EFFECT OF GRINDING OF THE STARTING MATERIAL ON HYDROTHERMAL SYNTHESIS OF KAOLINITE

概要

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It has been found that initial grinding of starting materials is effective in promoting reactivity in the synthesis of kaolinite. A mixture of amorphous silica and aluminum hydroxide with a Si/Al atomic ration of 1.0 was ground and then used as the starting material for hydrothermal synthesis. The starting material was placed in a Teflon reaction vessel together with distilled water, and kept at 220℃. The characterization of the starting materials and products was carried out with XRD, IR, DTA-TG, ^<29>Si-and ^<27>Al-MAS/NMR, and TEM. The starting material which had been ground for 200 h was an amorphous aluminosilicate with four-and sixcoordinated Al. Kaolinite could be obtained from this well-ground starting material in a relatively short reaction time in comparison to mixtures not subjected to the pre-grinding process. In the latter case, bayerite was changed into bohmite, and was then converted to well crystallized kaolinite with euhedral morphology of the hexagonal plate. The differences in the local structures of the starting material around the Si, Al atoms, and OH groups, which had been brought about by the mechanochemical reaction during the grinding, are likely to have an influence on formation of kaolinite.
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