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 degrees C, primary mullite formation occurred at earlier temperature within iron-enriched kaolins than in the case of iron-free kaolins. At 1150 degrees C, the presence of ferric ions tended to promote the transformation of the spinel (gamma-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 degrees 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. (C) 2012 Elsevier Ltd. All rights reserved.