里川 重夫

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.

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 AlO4-octahedra), whereas the gel prepared at pH 4.2 consists of a sheet structure related to allophane. After the hydrothermal treatment at 220-degrees-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.

Water quenched blast furnace slag was examined as raw material for the hydrothermal synthesis of kaolinite at 220&deg;C. Kaolinite could not be obtained from the raw slag with a lower polycondensation state of unhydrated silica nor from the annealed slag with gehienite-structure. The HCl-treatment of the slag successively makes hydration and polycondensation of the silica so as to form a silica-gel consisting of hydrated silanol and unhydrated silica. The obtained silica-gel reacts with aluminum hydroxide under the hydrothermal condition to form kaolinite with b&ouml;hmite. The uses of acidic solutions as the hydrothermal liquids can improve the formation of kaolinite. Spherical kaolinite was dominantly formed with the reaction in the H₂SO₄-solution, whereas platy kaolinite was crystallized in the HNO₃-solution.