Isothermal kinetic analysis of the thermal decomposition of magnesium hydroxide using thermogravimetric data

In the present study, the kinetics of the thermal decomposition of magnesium hydroxide is investigated, using isothermal methods of kinetic analysis. For this purpose, experiments in thermogravimetric analyser were carried out in standard values of temperature (350 degrees, 400 degrees, 450 degrees and 500 degrees C) which resulted in weight loss percent as a function of time. The data were further modified to give fraction reacted 'alpha' versus time to be tested in various forms of 'alpha' functions. In order to determine the mechanism of the magnesium hydroxide decomposition and the form of the conversion function which governs the dehydroxylation of Mg(OH)(2), four different methods of isothermal kinetic analysis were used. Applying each of these methods to the data, it was concluded that the nucleation mechanism predominates the Mg(OH)2, decomposition for all values of temperature tested; at 350 degrees C the kinetic model which represents the experimental data is that of reaction at phase boundaries (random nucleation), F-1: ln(1-alpha)=kt) while for the higher temperatures 400 degrees, 450 degrees and 500 degrees C the kinetic equation of nucleation and development in two dimensions, A(2): [-ln(1-alpha)](1/2)=kt was found to fit better the experimental results. The activation energy was evaluated applying two alternative methods; the Arrhenius plot, using maximum rates of reaction, from which the activation energy was evaluated to be 20.54 kcal/mol. An alternative method based on plots of ln t versus 1/T corresponding to the same value of 'alpha' gave values of 10.72, 13.82 and 16.31 kcal/mol for 'alpha' values of 0.25, 0.50 and 0.75, respectively. (C) 1998 Elsevier Science B.V.