Kinetics of Thermal Dehydroxylation and Carbonation of Magnesium Hydroxide

The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH) 2 were studied using isothermal and nonisothermal thermogravimetric analyses. Specimens were analyzed using X‐ray diffraction, transmission electron microscopy, and through measurements of the volume of carbon dioxide evolved in a subsequent reaction with hydrochloric acid. From 275° to 475°C, the kinetics of isothermal dehydroxylation in helium were best fit to a contracting‐sphere model, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation under vacuum (53–126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously. The overall kinetics also could be fit to a contracting‐sphere model, yielding a net activation energy of 304 kJ/mol. The most rapid carbonation kinetics occurred near 375°C. At this temperature, Mg(OH) 2 underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO 3 precipitated on the surface of disrupted Mg(OH) 2 crystals acting as a kinetic barrier to both the outward diffusion of H 2 O and the inward diffusion of CO 2 .