Influence of iron on the occurrence of primary mullite in kaolin based materials: A semi-quantitative X-ray diffraction study

Iron-enriched reference kaolins (KGa-1b, KGa-2 and KF) were used to study the effect of iron on the development of mullite phases during the sintering of kaolin-based materials. Up to 1050 °C, primary mullite formation occurred at earlier temperature within iron-enriched kaolins than in the case of iron-free kaolins. At 1150 °C, the presence of ferric ions tended to promote the transformation of the spinel (γ-Al2O3-like) phase into primary mullite. This action was correlated with an enhancement of the diffusion mechanism of the main constitutive species of the samples (Al, Si). In the range 1300–1400 °C, iron-enriched kaolins exhibited an abnormal grain growth of secondary mullite crystals and a partial reduction of hematite (Fe2O3) into magnetite (Fe3O4). These two iron compounds reacted with mullite and cristobalite, leading to the occurrence of eutectic liquids as expected from phase equilibrium diagrams.