57Fe Mِssbauer study of iron distribution in a kaolin raw material: influence of the temperature and the heating rate

The thermal transformation of a kaolin raw material results from individual contributions of kaolinite and mica minerals, and from the complex nature of their mixture. In addition, it appears that the iron sites are subject to changes with the firing temperature and the heating rate, which influence the thermal transformations. Mِssbauer spectroscopy provides information about iron site geometry and iron valence, and the resolution of this technique is high. For the kaolin studied, we determined the relative quantity of structural FeIII and hematite as a function of temperature, up to 1100 °C and for two heating rates, 3 and 50 °C/min. Results indicate the loss of hematite above 900 °C and a subsequent increase of FeIII in the structure of silico-aluminate phases. This trend is accentuated for a higher heating rate.