THE THERMAL-DECOMPOSITION OF MAGNESITE IN NITROGEN

Numerical predictor-corrector analysis of non-isothermal TG curves with heating rates of 3, 10 and 30-degrees-C min-1 were made for different models for the decomposition of magnesite in nitrogen. A contracting volume (R3) model was found to give the best fit, and the values of the kinetic parameters were refined by non-linear least-squares methods using a minicomputer. Respective values of the activation energy of 170, 174 and 174 kJ mol-1, and of the pre-exponential constant of 4.2 x 10(11), 6.7 x 10(11) and 7.8 x 10(11) min-1, were found for heating rates of 3, 10 and 30-degrees-C min-1. The R3 model was also found to fit best the isothermal measurements of the decomposition of two particle sizes of magnesite in nitrogen over the temperature range 500-650-degrees-C. The reaction rates were fitted to the Arrhenius equation, and gave values of 161 kJ mol-1 for the activation energy and 2.9 x l0(8) min-1 for the pre-exponential constant. The good agreement between the results from the two experimental methods confirms that sensible values of the activation energies can be obtained from non-isothermal TG measurements.