Thermal analysis and reaction kinetics study of red mud and phosphogypsum mixed solid waste with added carbon and alkali

Red mud and gypsum are the main solid waste products of the aluminum and phosphate chemical industries. Due to their large-scale generation, storage, and complex chemical composition, they pose potential environmental risks, which have attracted widespread attention. Although there have been some advances in the utilization of red mud and gypsum in co-pyrolysis, there is still a relative lack of research on the reaction mechanisms, especially the high-temperature kinetics of their co-calcination or pyrolysis. Therefore, this study used differential thermal-thermogravimetric analysis to systematically investigate the high-temperature reaction kinetics of the mixed solid waste of red mud and gypsum in the presence of sodium carbonate and charcoal. The experimental results show that under the condition of separate mixing pyrolysis with decomposition temperature lower than 200 ℃, the reaction activation energy of red mud and gypsum is 61.72 kJ/mol. After adding sodium carbonate, the reaction activation energy is significantly reduced to 14.09 kJ/mol, and the reaction rate is accelerated. Further addition of charcoal reduces the reaction activation energy to 1.58 kJ/mol, and the reaction is significantly intensified, with a marked decrease in the decomposition temperature of CaSO₄.Under single-component calcination, the most intense reaction occurs in the 1 000-1 200 ℃ range. With the addition of sodium carbonate, the reaction window shifts markedly downward to 600-850 ℃, indicating that the alkali effectively lowers the required temperature and accelerates the reaction rate. Upon further addition of coke, the reaction window rises slightly to 800-1 000 ℃, suggesting that coke promotes the reaction process. Although the temperature increases somewhat, it remains within a relatively lower range and the reaction is still vigorous. In addition, this study reveals that the reaction mechanism of mixing pyrolysis is mainly composed of nucleation and growth and three-dimensional diffusion. These findings provide a new theoretical basis for optimizing the synergistic utilization of red mud and gypsum.