Mechanism and Kinetic Parameters of the Thermal Decomposition of Gibbsite AI(OH)3 by Thermogravimetric Analysis

In this study, the mechanism and the kinetic parameters of the thermal decomposition of gibbsite Al(OH)(3) were studied by differential thermogravimetry technique under non-isothermal conditions, between room temperature and 1200 K at heating rates of 5, 10, 15 and 20 degrees C min(-1). The obtained differential thermogravimetry curves show clearly three distinct peaks. The first peak is due to the partial dehydroxylation of gibbsite. Among the 32 types of differential equations of non-isothermal kinetics, we have found that the most suitable mechanism is (A(3/2): g(x) = [-ln(1-X)] (2/3)) also called Avrami-Erofeev equation of order 2/3. The values of the activation energy E-A and of the pre-exponential factor K are 157 kJmol(-1) and 7.58 x 10(15) s(-1), respectively. The second peak corresponds to the decomposition of gibbsite to boehmite. Decomposition is controlled by the rate of second-order reaction (F-2 : g(x) = (1-x)-1-1), under the applied conditions. The activation energy E-A and pre-exponential factor K correspond to 243 kJ mol(-1) and 3.73 x 10(22) s(-1), respectively. The third peak is due to transformation of boehmite to alumina. However the mechanism for such transformation is better described by the 3/2 rate order reaction (F-3/2: g(x) = (1-x) (-1/2) -1). In addition, the values of E-A and K were determined to be around 296 kJ mol(-1) and 1.82 x 10(19) s(-1), respectively. The results of differential thermogravimetry were supplemented by the differential thermal analysis. X-ray powder diffraction analysis was carried out for samples of gibbsite treated at different temperatures between 200 and 1200 degrees C in 200 degrees C steps.