Carbonation of calcined Red Mud: Multifactor influences, reaction mechanism and environmental impact assessment

Red mud (RM) is a bulk alkaline solid waste generated by the alumina industry, carbonation can achieve alkalinity reduction and carbonation in red mud. However, its low reactivity limits further utilization. This study systematically investigates the effect of calcination modification on RM carbonation and its activation mechanism. Carbonation process parameters were further optimized, and the reaction mechanism was elucidated and also use Life Cycle Assessment (LCA) to evaluate the environmental impacts of various RM disposal methods. The results indicate that calcination induces the dehydroxylation and amorphization of primary minerals in RM, and create a honeycomb porous structure on the surface, thereby promoting its carbonation efficiency. The optimal carbonation parameters are as follows: liquid to solid (L/S) ratio of 0.3, CO₂ concentration of 20%, reaction pressure of 0.4 MPa, temperature of 30°C, and reaction time of 1 h, resulting in a 23.92% increase (carbon content). Mechanistic analysis reveals that RM carbonation is a multiphase coupled (gas, solid and liquid) process, include diffusion, migration and carbonation. Finally, LCA results demonstrate that the direct storage of RM has the most significant impact on the environment, and carbonation after calcination is the best disposal method.