Role of Iron in Mullite Formation from Kaolins by Mössbauer Spectroscopy and Rietveld Refinement

Abstract

This paper examines the role of iron in mullite nucleation and growth from kaolins. We chose two typical raw kaolins containing a reduced impurity level and characterized by very different degrees of crystallinity of the kaolinite phase. Both the structural iron in kaolinite and also some iron deposited onto phyllosilicate layers by a chemical route were considered. After firing in the 900–1100°C temperature range, the Fe environment was determined by Mössbauer spectroscopy. From X‐ray spectra of samples fired at 1250°C, mullite stoichiometries were obtained by Rietveld refinements. It was shown that iron contributes to the structural reorganization stage of the material, when mullite is nucleated. Fe atoms are essentially in octahedral sites, which favors an increase of the c parameter of the orthorhombic cell. The iron quantity attains a saturation level for an Fe‐to‐Al ratio between 0.3 and 0.4, depending on the raw kaolinite crystallinity. Besides mullite, the excess iron associates with titanium to form a pseudobrookite phase and hematite.

Microstructural evolution, phase transformation, and variations in physical properties of coal series kaolin powder compact during firing

“…10 and Table 6 reveals that, besides Al 3+ , Si 4+ , and O 2− , the mullite phase contains Ti 4+ in addition, which demonstrates that the mullite crystals yield from the liquid phase at firing temperature. Impurities as TiO 2 and Fe 2 O 3 have been proven to facilitate mullite formation from kaolin (Kong et al, 2003;Mitra et al, 2002;Sen and Aggarwal, 1994;Soro et al, 2003). Because the growth mechanism of mullite in kaolin, which is known as a dissolution-precipitation process (Ji et al, 2013), is generated by the appearance of a Si-rich liquid phase and promoted by the presence of TiO 2 and Fe 2 O 3 due to their assistance for lowering the formation temperature and viscosity of liquid phase during firing, the dissolution rate of Al 2 O 3 into liquid phase increases with the decrease of the viscosity of liquid phase.…”

Influence of iron onto the structural reorganization process during the sintering of kaolins

“…2). This ferrihydrite exhibits a typical exothermic transformation into hematite 25,26 as detected by DSC characterization near to 350 • C. The various samples used in this study were labelled as describe in Table 3.…”

Mullite formation in clays and clay-derived vitreous ceramics

“…Djemai et al 16 studied the effect of iron oxide addition <10 wt.% on mullite recrystallisation in iron-rich kaolin and found that at 1050 • C the quantity of mullite formed increases whereas >1250 • C the growth of mullite crystals was enhanced. Soro et al 17 used Mossbauer spectroscopy and X-ray diffraction (XRD) Rietveld refinement to show that Fe contributes to the structural reorganisation during mullite nucleation from iron-containing kaolinite fired 900-1250 • C. Fe substituted for octahedral Al increasing the orthorhombic c parameter. The quantity of structural Fe attains a saturation level with >5 wt.% Fe 2 O 3 for Fe/Al between 0.3 and 0.4 depending on the crystallinity of the kaolinite.…”