Kinetics of thermal dehydroxylation and carbonation of magnesium hydroxide

The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH){sub 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){sub 2} underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO{sub 3} precipitated on the surface of disrupted Mg(OH){sub 2} crystals acting as a kinetic barrier to both the outward diffusion of H{sub 2}O and the inward diffusion of CO{sub 2}.

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