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 h eating rate, from 3 to 20 degreesC min(-1). For comparison, reference raw m aterials, which were very rich in kaolinite or muscovite, were also analyse d. The transformation heats during heating were quantified from DTA measure ments and phase changes were followed by X-ray diffraction analyses. Result s indicated that a East heating rate increases the transformation heats ass ociated with the endothermic dehydroxylation transformation of kaolinite an d suggests the formation of a more disordered metakaolin. A similar trend w as observed in the exothermic phenomenon. This suggests that Bio kaolin has a complex nature that changes the mechanisms involved in the structural tr ansition. In Bio kaolin, mullite crystallises only from the kaolin-derived phase, which underlines the specific role of clay mineral interfaces in a n atural mineral mix. In this case, the crystallisation of mullite occurs via a direct route without any spinel intermediate phase. The amount of mullit e between 1050 and 1150 degreesC increases with the heating rate and is com paratively higher than from a pure kaolinite phase. (C) 2001 Elsevier Scien ce Ltd and Techna S.r.l. All rights reserved.