The influence of heating rate on the thermal behaviour and mullite formation from a kaolin raw material

The thermal behaviour of a kaolin raw material (Bio) containing mainly well-crystallised kaolinite and muscovite minerals was studied in relation to heating rate, from 3 to 20°C min−1. For comparison, reference raw materials, which were very rich in kaolinite or muscovite, were also analysed. The transformation heats during heating were quantified from DTA measurements and phase changes were followed by X-ray diffraction analyses. Results indicated that a fast heating rate increases the transformation heats associated with the endothermic dehydroxylation transformation of kaolinite and suggests the formation of a more disordered metakaolin. A similar trend was observed in the exothermic phenomenon. This suggests that Bio kaolin has a complex nature that changes the mechanisms involved in the structural transition. In Bio kaolin, mullite crystallises only from the kaolin-derived phase, which underlines the specific role of clay mineral interfaces in a natural mineral mix. In this case, the crystallisation of mullite occurs via a direct route without any spinel intermediate phase. The amount of mullite between 1050 and 1150°C increases with the heating rate and is comparatively higher than from a pure kaolinite phase.