Formation of Mullite from Kaolin Minerals at the Temperatures Lower than 900°C

Abstract

Kaolin minerals were mixed with 0.1, 0.2 and 0.4mol of minerallizers. The crystal phase of the heated samples was examined by X-ray and differential thermal analysis.It was found that the temperature of the endothermic reaction of kaolin minerals containing fluoride was higher and the temperature of the exothermic reaction was extremely lower than that of the thermal reaction of original kaolin minerals. The variation of the endothermic reaction was found to be related to the crystallinity of kaolin minerals, and was large in well-crystallized kaolinite in contrast with the variation in poorly crystallized kaolinite or halloysite. These thermal reactions may be due to the effect of fluorine ion in the minerallizers. Furthermore, the kaolin minerals containing fluoride was found to form a mullite type phase at much lower temperatures, so that a small amount of mullite type phase was recognized even at 550°C, when 0.4mol of LiF was added to kaolin minerals.The samples heated at 600°-900°C were then treated by 5-10% of hydrofluoric acid to separate mullite type phase. The cell dimension of these samples was measured and some indications of the composition of the mullite type phase were obtained from the relation between unit-cell volume and cell edge c. The results indicated that the mullite type phase formed at lower temperatures in the kaolin minerals containing LiF is composed of the crystals of normal mullite (3Al2O3⋅2SiO2).