High-Temperature Phase Changes in Kaolinitic-clays

Abstract

A large number of kaolinitic-clays of various structures and impurities were examined by differential thermal analysis and X-ray diffraction method. Effects of crystalliniety of kaolinite minerals and of impurities contained within the clays on the exothermic reactions above 1, 000°C were investigated.
The results obtained were as follows:
1) The second exothermic reacion at about 1, 200°-1, 250°C results from formation of mullite and shape of the thermal peak depends on the crystallinity of initial kaolinitic clays. In well-crystallized clays, mullite crystal develops at lower temperature than that of poorly crystallized ones and continues to develop with increasing temperature. On the contrary, rapid development of mullite is observed in poorly crystallized ones at about 1, 200°-1250°C. The second exothermic reaction for well-crystallized ones, therefore, show more broad peak than that of poorly crystallized ones.
2) Further, the third exothermic reaction occures in some clays in the temperature range of 1, 300°-1, 450°C at which cristobalite is more clearly defined. These thermal peak are greatly affected by alkali contained within the clays. The third exothermic peak of clays. containing a small amounts of alkali occures at 1, 400°-1, 450°C. When the amount of alkali in clays increases, the temperature of the thermal peak lowers and the intensity of the peak decreases. Consequently, the third exothermic reaction is not observed for some clays which contain alkali more than a limited amount.