The Kaolinite-Mullite Reaction Sequence

Abstract

This thesis describes the investigation of the kaolinitemullite reaction sequence by kinetics, X-ray diffractometry, D.T.A., infra-red absorption spectrophotometry, and electron spin resonance. The kinetic aspect of this work concerned the rates of mullite formation from its preceding phases, as measured by a X-ray method, and the effects on the rates of selective cation doping, kaolinite particle size and morphology. Konetic analyses alone were found to be inadequate in describing the mechanism of mullite formation. The entire sequence of phases was then studied from the viewpoint of structural transformation using the techniques of X-ray, D.T.A., and infra-red absorption. Preceding the experimental study the infra-red spectral lines of kaolinte, metakaolinite, and mullite were assigned to vibrations of SiO4, AlO4, and AlO6 groups in their structures from a survey of the literature and the application of spectral comparisons with similar minerals. A normal coordinate analysis was also carried out for the infra-red spectrum of mullite. The experimental data, particularly the infra-red spectra, indicated that the reaction sequence was likely to be: i.e. as metakaolinite (probably containing residual OH groups) forms there is a partial decomposition to free alumina and silica, then a complete decomposition to Al-Si spinel and a crystallization of γ-Al2O3 at 900-1000°. Secondary mullite forms largely from a release of silica from Al-Si spinel and the combination of s-Al2O3 and free SiO2. The equation is left unbalanced since the relative proportions of u-Al2O3 and Al-Si spinel etc probably vary, perhaps determined by the crystallinity of the parent kaolinite. The variation in Al3+ ion coordination in the sequence was supported by the change in the esr pattern of impurity Fe3+ in Al3+ sites in kaolinite and its fired products.