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

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

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    Summary
    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%, 240-300kJ/mol), before coming down and forming a plateau (15%≤α≤60%, 300-260kJ/mol). The reaction ends with an increasing E a (α≥60%, 260-290kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E a versus α (160-110kJ/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 290kJ/mol. Afterwards, the apparent E a comes down to around 250kJ/mol at α~65% before rising up to around 400kJ/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.