Influence of the firing atmosphere onto the thermal transformation of iron-enriched kaolin

Understanding the transformations and interactions of kaolinite with secondary phases is a key point to control the physical and chemical properties of resulting materials. The production of ceramics involves multiple steps, among which, sintering is a critical step regarding the achievement of the target properties of use.

The sintering environment and the surrounding atmosphere can significantly affect the transformation kinetics by changing heat transfer patterns and phase stability. Therefore, the challenge of the present study was to understand the effects of such modifications, especially on the physical and chemical transformations of kaolin-based ceramics regarding the presence of iron-enriched compounds.

One typical kaolin was chosen as new material for this study: a kaolin denoted “CR” that was provided by Imerys company. The influence of chemically added iron oxide was studied according with reference to the Ellingham diagram. To this end, controlled additions of 5 and 10 wt% of added iron oxide were performed. The thermal behaviour of these samples was investigated from room temperature to 1400 °C under controlled atmosphere using air, argon, or nitrogen. DTA/TG, XRD and SEM analyses were performed to enhance the understanding of the phase transformations and interactions of kaolinite with iron oxide. The presence of iron in kaolin promoted the formation of secondary mullite at lower temperatures, followed by cristobalite formation under air. When the atmosphere was modified using argon or nitrogen (lower partial pressure of dioxygen) these effects were even more pronounced. In addition to decreasing the onset temperature of secondary mullite and cristobalite crystallisation, the reaction paths were modified.