Kinetics study of microwave enhanced reactions between diasporic bauxite and alkali solution

In this work, the kinetic reactions between NaOH and the Al-bearing minerals were investigated, when the slurry mixture of diasporic bauxite and alkaline solutions were roasted by microwave (MWH) and conventional heating (CH). The XRD patterns reveal that the MWH method effectively promotes the reactions between NaOH with Al-bearing minerals to form NaAlO2, and inhibits the dehydration of AlOOH. Compared to CH, MWH greatly decreases the roasting temperature nearly 400 °C and shortens the reaction time by 3 times to completely transform Al-bearing minerals from raw bauxite into NaAlO2. The model fitting results show that the kinetics of the reactions between NaOH and the Al-bearing minerals for both heating methods can be described by the Jander's model. The value of apparent activation energy for the reactions between NaOH and Al-bearing minerals by MWH is 31.31 kJ/mol, which is 23% less than that of the reactions in CH. In addition, the SEM-EDS analysis results show that the reacted layer gradually expands outward when the microwave roasting time prolonged. Moreover, the influence of MWH on the kinetics of the reaction between NaOH and Al-bearing minerals explained in detail by a specific activation mechanism of H2O vapor and the differences between dielectric loss of reactants and products.