The kinetics of non-isothermal decomposition of the Z/Al(OH)3 mixtures (Z = ZnO or Zn4CO3(OH)6·H2O)

Abstract

The kinetics of non-isothermal decomposition of ZnO/Al(OH)3 and Zn4CO3(OH)6·H2O/Al(OH)3 has been studied both for the starting mixtures and for mixtures after co-grinding. The process has been controlled by X-ray diffraction, scanning electron microscopy, thermogravimetry and differential scanning calorimetry under air atmosphere (the heating rates were 5, 10 and 15 °C min−1). The activation energy was calculated using the Friedman analysis and Ozawa–Flynn–Wall analysis. It was shown that both methods gave similar results. It was found that the dehydration activation energy of gibbsite to boehmite in the mixture with ZnO gradually is decreased from 150–170 to 100 kJ mol−1. At the final stage (conversion degree was more than 0.9), the dehydration process converted from kinetic mode into the diffusion mode (the activation energy was 50–70 kJ mol−1). It was established that the activation energy of Zn4CO3(OH)6·H2O/Al(OH)3 decomposition at a conversion degree <0.5 (decomposition of basic zinc carbonate to ZnO) has values 130–170 kJ mol−1. For a conversion degree 0.5–0.9 (dehydration of gibbsite to boehmite), the activation energy amounted to 90–130 kJ mol−1. At the final stage of decomposition (degree of conversion above 0.9), the process occurred in the diffusion mode in which the activation energy amounted to 40–70 kJ mol−1. It was revealed that after co-grinding of the ZnO/Al(OH)3 composition, the dehydration process of activation energy has decreased by 10–20 % compared with the original mixture. After the co-grinding of Zn4CO3(OH)6·H2O/Al(OH)3 mixture, the activation energy has increased by 10–15 %. These facts had been explained by changes in the size and defectiveness of the solid-phase particles.