KINETICS OF THERMAL DEHYDROXYLATION AND CARBONATION OF MAGNESIUM-HYDROXIDE

The kinetics of simultaneous dehydroxylation and carbonation of precip itated Mg(OH)(2) were studied using isothermal and nonisothermal therm ogravimetric analyses, Specimens were analyzed using X-ray diffraction , transmission electron microscopy, and through measurements of the vo lume of carbon dioxide evolved in a subsequent reaction with hydrochlo ric acid, From 275 degrees to 475 degrees C, the kinetics of isotherma l dehydroxylation in helium were best fit to a contracting-sphere mode l, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation unde r vacuum (53-126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously, The overall ki netics 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 degrees C. At this temperature, Mg(OH)(2) underwent rapid dehydroxylation and subsequent phase transformation, whereas the rmodynamics favored the formation of carbonate, During carbonation, Mg CO3 precipitated on the surface of disrupted Mg(OH)(2) crystals acting as a kinetic barrier to both the outward diffusion of H2O and the inw ard diffusion of CO2.