Investigation of iron oxide and kaolinite interactions during sintering under controlled atmosphere

This study investigates the impact of atmosphere conditions on the thermal transformations of iron-enriched kaolin. Natural kaolin was chemically enriched with iron at two concentrations. Fine characterisation was conducted using X-ray diffraction, Mössbauer spectroscopy, solid-state nuclear magnetic resonance and electron dispersive spectroscopy. The samples were heat-treated at 1050 °C and 1400 °C under synthetic air and argon to assess the influence of these conditions on their transformations. Analyses revealed that presence of iron significantly influenced reaction kinetics. Under oxidizing atmosphere, the formation of mullite was facilitated by an equilibrium with hematite. The presence of iron accelerated the formation of secondary mullite and cristobalite, though the effect was not proportional to iron concentration. Conversely, under oxygen-deficient atmosphere, the reaction pathway diverged, leading to the formation of hercynite, tridymite (and after cristobalite) and mullite. Iron similarly promoted formation of these phases. No significant differences were observed for the pure kaolin sample under either atmosphere.