High Temperature Phase Transformation in Kaolinite Reaction

A systematic series of experimental techniques were performed on a selected Sudanese kaolin clay. Its thermal behaviour was investigated within the range of temperature of 100-1300 o C. No change was observed when heating between 300-500 o C. Dehydroxylationtook  place above 500 o C, and the mineral kaolinite changed to metakaolinite. At 900 o C the mineral decomposed to γ-alumina and silica (SiO 2 ). Upon heating up to 1200 o C solid state reaction took place resulting into the formation of sillimanite AL 2 O 3 -SiO 2 . The presence of this phase was confirmed by both XRD and IR techniques. Due to its resemblence to mullite (3Al 2 O 3 2SiO 2 ) many investigators thought it was mullite but in this paper it was confirmed that mullite was formed above 1200 o C.. However, above 1200 o C, sillimanite was converted into mullite, and on further heating at 1300 o C, mullite was formed in addition to cristobalite.