Mechanistic insights into temperature-driven retention and speciation changes of heavy metals (HMs) in ash residues from Co-combustion of refuse-derived fuel (RDF) and red mud

Red mud is a promising candidate for promoting the incineration of Refuse Derived Fuel (RDF) and stabilizing the resulting incineration ash. The combustion conditions, notably temperature, significantly steers the migration and transformation of harmful metal components during combustion, and ultimately affect their retention and speciation in the ash residue. The study attempted to investigate the effect of co-combustion temperature on the enrichment and stability of Cr, Ni, Cu, Zn, Cd and Pb within bottom ashes, and to reveal the underlined promotion mechanism of red mud addition. As temperature increased, red mud's active components formed a robust matrix, helping the formation, melting, and vitrification of silicates and aluminosilicates in the bottom ashes. The process significantly contributed to the encapsulation and stabilization of heavy metals such as Ni, Cu, Zn, Cd, and Pb, with their residual fractions ascending to 71.37%, 55.75%, 74.78%, 84.24%, and 93.54%, respectively. Conversely, high temperatures led to an increase in the proportion of Cr in the extremely unstable acid-soluble fraction of the bottom ashes, reaching 31.52%, posing a heightened risk of environmental migration. Considering the stability of heavy metals in the bottom ashes and the combustion characteristics, 800 degrees C is identified as the optimal temperature for the co-combustion of RDF and red mud, balancing efficiency and environmental safety. The findings will provide valuable insights for the co-utilization strategy of RDF and red mud, contributing to more informed decision-making in waste-to-energy processes.