Isoconversional Kinetic Modelling and In-Situ Synchrotron Powder X-Ray diffraction analysis for Dehydroxylation of Antigorite

Mineral carbonation offers permanent and safe disposal of anthropogenic CO₂. Well distributed and abundant resources of serpentine minerals and natural weathering of these mineral to stable and environmentally benign carbonates^{1, 2} favour the exploitation of these minerals as the most suitable raw material for mineral carbonation. However, slow dissolution kinetics are impeding the large scale implementation of mineral carbonation³. Heat treatment of serpentine minerals results in enhanced reactivity for subsequent carbonation processes at the expense of an additional energy penalty4. Heat treatment of these minerals results in the removal of structurally bound hydroxyl groups which leads to partial amorphisation of the structure and enhanced reactivity⁵. Therefore, understanding the role of the mineralogical changes during dehydroxylation and determination of activation energy (Ea) is crucial for providing an energy efficient solution for commercialisation of mineral carbonation.

In-situ synchrotron powder X-ray diffraction (S-PXRD) at the Australian Synchrotron was employed for detailed observation of mineralogical changes and estimation of kinetic parameters during the heat treatment from room temperature to 1000 oC under constant N₂ flow. The synchrotron beamline offers high signal to noise ratio necessary for an accurate identification of minor phases and onset temperature for phase transitions. Moreover, the fast data acquisition of S-PXRD enables acquisition of data with temporal resolution, which is crucial for accurate estimation of kinetic parameters. During dehydroxylation via heat treatment, antigorite remained stable up to 520 oC. Above 520 oC, antigorite started to decompose and forsterite formation occurred at around 700 oC. Enstatite formation was observed only after the complete dissociation of antigorite.

We performed prograde heating experiments at 2, 4, 6 and 8 oC/min under constant N₂ flow for the estimation of Ea via isoconversional kinetic modelling. The change in activation energy with reaction progress showed the multistep nature of dehydroxylation of antigorite. The variation of Ea can be divided into three stages a) nearly constant Ea of 130 kJ/mol (α ≤ 0.25) b) increase in Ea from 130-209 kJ/mol (0.25≤ α ≥0.4) which remained constant at around 204 kJ/mol till α = 0.8. Finally, the reaction ended with an increase in Ea from 204 kJ/mol to 236 kJ/mol. In this study we exploit the potential of in-situ SXRD for determination of isoconversional kinetic parameters in comparison to conventional kinetic analysis based on TGA-DSC methods. While S-XRD based kinetic analysis appears to be sensitive to phase quantification parameters (e.g. peak integration vs. full pattern fitting) it provides valuable structural information that is not available in conventional kinetic methods. S-XRD based kinetic analysis further has the ability to resolve the formation of individual mineral phases, including reaction

intermediates (talc-like phases) and products (olivine and enstatite). Consequently, this study will further advance the development of cost and energy-efficient dehydroxylation of serpentine minerals for large scale storage of CO₂ by mineral carbonation.

Keywords: serpentine minerals, heat treatment, isoconversional, kinetic modelling, mineral carbonation

References: [Note: removed because abstract exceeds word limit; can be included in full manuscript later - D.Wu]