Kinetics of antigorite dehydroxylation for CO2 sequestration

Optimisation of heat treatment of serpentine is critical for the development of large-scale CO2 sequestration by mineral carbonation. [Display omitted] •Full amorphisation of antigorite cannot be achieved before the onset of forsterite formation.•Aluminium-rich antigorite shows significantly higher thermal stability than lizardite.•The activation energy for dehydroxylation of antigorite varies between 290 and 515 kJ mol−1.•Forsterite begins to form at 700 °C when only 49% antigorite converted to amorphous phase.•High Al2O3 antigorite is more suitable for mineral carbonation at high pressure conditions. Heat-treatment of serpentine minerals generates structural amorphicity and increases reactivity during subsequent mineral carbonation, a strategy for large-scale sequestration of CO2. This study employs thermal analyses (TGA-DSC) in conjunction with in-situ synchrotron powder X-ray diffraction (PXRD) to record concurrent mass loss, heat flow, and mineralogical changes during thermal treatment of antigorite. Isoconversional kinetic modelling demonstrates that thermal decomposition of antigorite is a complex multi-step reaction, with activation energies (Eα) varying between 290 and 515 kJ mol−1. We identify three intermediate phases forming during antigorite dehydroxylation, a semi-crystalline chlorite-like phase (γ-metaserpentine) showing an additional reaction pathway for the decomposition of Al2O3-rich antigorite into pyrope, and two distinct amorphous components (α and β-metaserpentine) which convert into forsterite and enstatite at higher temperature, respectively. The combination of isoconversional kinetics with in-situ synchrotron PXRD illustrates, for the first time, that local crystal structure changes, related to intermediate phase and forsterite formation, are responsible for the steep increase in activation energy above 650 °C and only 49% dehydroxylation can be achieved prior to this increase. This suggests that the high thermal stability of Al2O3-rich antigorite would severely limit Mg extraction during application of mineral carbonation under flue gas conditions.