Characterization of thermal behavior of two types of kaolin in China by ultrafast Joule heating combined with XRD, FT-IR, TG-DSC and SEM

Thermal behavior of sandy kaolin selected from Guangxi and hard kaolin from Inner Mongolia in China were characterised by ultrafast Joule heating combined with XRD, FT-IR, TG-DSC and SEM. The results indicated that the sandy kaolin was composed of kaolinite, muscovite and quartz, and hard kaolin was predominantly composed of kaolinite, with minor quantities of anatase. During ultrafast Joule heating, the layered structure of kaolinite of sand kaolin was destroyed, and all hydroxyl groups were lost at 700 °C, while dehydroxylation completion of the hard kaolin at higher temperature, accompanied by rupture of lamellar kaolinite. Muscovite disappeared completely and accompanied by the formation of mullite at 1100 °C, while quartz is relatively unaffected, resulting in the sandy kaolin consisting of quartz and mullite phases at 1200 °C. In contrast, the transformation rate and crystallinity of mullite phase of hard kaolin was significantly higher than that of sandy kaolin.