Mechanistic study of solid-state reaction between kaolinite and ferrous oxide at high temperatures

Abstract

A mechanistic investigation of the solid-state reaction occurring in the mixture of kaolinite and ferric oxide heated in a mildly reducing atmosphere at 900-1300°C was performed, and the following reaction scheme was postulated. Above 950°C, FeO reacted with Al2O3 in both the spinel phase (SiO2·6Al2O3) and the amorphous phase (6SiO2 · Al2O3) which were formed as the result of phase separation of metakaolin to produce hercynite (FeAl2O4). This extraction of Al2O3 from the amorphous phase should decrease the Al content. The mineralizing action of FeO caused the crystallization of quartz solid solution from the SiO2-enriched amorphous phase. On the other hand, the spinel phase which lost a considerable amount of Al2O3, also due to the formation of hercynite, yielded mullite. Concentration analyses by electron spectroscopy for chemical analysis (ESCA) combined with the Ar etching treatment for Al, Si and Fe revealed that, during the above solid-state reaction, migration of Al toward FeO grains occurred.