Kinetic study of the kaolinite-mullite reaction sequence. Part II: Mullite formation

Abstract

Abstract. The present work is a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction. In the present work we study the high temperature reaction of mullite synthesis from kaolinite, from the starting point of the results obtained in the first part.Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation in isothermal conditions. The apparent activation energy for mullite nucleation and growth is found to be related to the defective structure of the starting kaolinite, which thus must have an influence on the chemical homogeneity of the amorphous intermediate phase.The analysis of the kinetic data indicate that the initial reaction mechanism is controlled by mullite nucleation, while as the reaction proceeds it shifts towards a grain growth-limited process which is intermediate between phase boundary and diffusion controlled. The order of the reaction obtained from standard analysis of the isothermal kinetic data is lower in the case of the ordered kaolinite KGa-1, in agreement with a rate limiting process more strongly limited by diffusion.For each sample there is a small but significant decrease in the order of the reaction at higher temperature: we interpret the change as related to the variation of the diffusion process in the amorphous phase due to the growing grains of mullite and cristobalite.The values of the activation energies and induction times are comparable neither to a model of mullite formation from a monophasic gel, nor mullite formation from a diphasic gel, being intermediate between the two. We can infer that the amorphous precursors from natural kaolinites can be considered pseudo-monophasic geMike phases, approaching the monophasic gel-like behaviour as the defectivity of the initial kaolinite increases.

Mullitization kinetics from silica- and alumina-rich wastes

“…Therefore, the discrepancy in the energies involved in the reactions of the kaolin or slate-based systems do not depend only on the presence or absence of a-alumina in the raw materials, which is considered the critical factor in determining the reactions occurring in the Al 2 O 3 -SiO 2 system [31], but also on the aluminosilicates source employed and on the overall preparation methods and experimental conditions. Nevertheless, the non-isothermal activation energies for primary and secondary mullite formation in Valongo slate-aluminium sludge mixtures are in the range determined by Gualtieri et al [30] (kaolinite-to-mullite reaction sequence): 350-550 kJ/mol, as also verified for mullite crystallyzation for Valongo slates only (490 kJ/mol) [18], being close to that of the isothermal mullitization of single-phase aluminosilicate gels (362 AE 145 kJ/mol) [3].…”

“…Both primary and secondary mullite formation activation energies reported by these authors are significantly higher than those obtained in the present slate-sludge mixtures. Also, through the years values of the activation energy for mullite crystallization in kaolin ceramics as different as 350-550 kJ/mol [30] or 1182.3 kJ/mol [13] have been reported, thus emphasizing that different activation energy values are associated not only with different crystallization processes but also with differences in the experimental conditions employed for E a determination [13]. Therefore, the discrepancy in the energies involved in the reactions of the kaolin or slate-based systems do not depend only on the presence or absence of a-alumina in the raw materials, which is considered the critical factor in determining the reactions occurring in the Al 2 O 3 -SiO 2 system [31], but also on the aluminosilicates source employed and on the overall preparation methods and experimental conditions.…”

Dehydroxylation kinetics of lizardite

“…In a huge volume of literature on the kinetics of the dehydroxylation of phyllosilicates, data are typically treated by the Avrami or the TGF method and the activation energy and the ''reaction order'' is interpreted in a mechanistic way (e.g., Redfern, 1987;Bose & Ganguly, 1994;Bellotto et al, 1995;Gualtieri et al, 1995;Bray & Redfern, 1999;Mazzucato et al, 1999;Cattaneo et al, 2003;Tokiwai & Nakashima, 2010). This procedure, however, is only strictly valid for reactions for which the rate-determining step does not change with reaction progress.…”

Ceramic products obtained from rock wastes

“…The grey background is mainly glassy phase, the white areas were identified as mullite and the dark zones are phases with iron and aluminium. It should be noted that mullite is a phase also obtained by heat treatment of kaolinite [3], which is a main constituent of clays. However, the presence of glassy phase in products resulting from slate wastes induce higher performances than those obtained from the clays currently used for flooring tiles.…”