Effect of heating rate on the kinetics of gibbsite calcination

Abstract

The effect of heating rate on the thermodynamics and kinetics of gibbsite decomposition in an air is reported. Heating rates were assessed systematically over the range 10.38 to 25.43 °C/s, which is an order of magnitude higher than those typically used in thermo-gravimetric analysers (0.1 °C/s to 0.8 °C/s). It is found that conventional reaction kinetic models for the calcination of gibbsite yield poor prediction at these heating rates, while improved prediction is achieved with a new model that makes the reaction rate constant (K) a function of the heating rate. Furthermore, the activation energy, Ea, is reduced by a factor of 6.3, from 116.7 kJmol−1 to 18.4 kJmol−1, when the heating rate is increased by a factor of 10. Additionally, the average reaction rate constant K¯ for 10.38 °C/s and 0.8 °C/s are 14.4 times and 6.3 times higher than that for 0.1 °C/s, when approaching the same asymptotic temperature. A similar trend is found for the time needed to transform gibbsite to boehmite. Specific surface area measurements also revealed that the decrease in activation energy with heating rate is associated with increased surface area to volume ratio, which accelerates the conversion process. Approaching the same asymptotic temperature, the specific surface area was found to increase by a factor of 1.3 for a heating rate of 10 times higher, although the pore width was found to be unchanged.