Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

Trittschack, Roy; Grobéty, Bernard; Brodard, Pierre

doi:10.1007/s00269-013-0638-9

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Journal article

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

Trittschack, Roy Department of Geosciences, University of Fribourg, Switzerland
Grobéty, Bernard Department of Geosciences, University of Fribourg, Switzerland
Brodard, Pierre College of Engineering and Architecture of Fribourg, Switzerland

01.03.2014

Published in:

Physics and Chemistry of Minerals. - 2014, vol. 41, no. 3, p. 197–214

English The dehydroxylation reactions of chrysotile Mg₃Si₂O₅(OH)₄ and brucite Mg(OH)₂ were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO₂ in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (‘time to a given fraction’ and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress- resolved (‘α’-resolved) course of the apparent activation energy E _a and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing E _a (α ≤ 15 %, 240–300 kJ/mol), before coming down and forming a plateau (15 % ≤ α ≤ 60 %, 300–260 kJ/mol). The reaction ends with an increasing E _a (α ≥ 60 %, 260–290 kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E _a versus α (160–110 kJ/mol). In contrast to that, the decomposition of brucite under CO₂ delivers a dynamic course with a much higher apparent E _a characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent E _a comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E _a versus α plots

Faculty

Faculté des sciences et de médecine

Department

Département de Géosciences

Language