Study on the mechanisms of heavy metal immobilization and migration in the plasma co-melting process of iron-rich hazardous waste fly ash and bottom ash

A highly efficient and environmentally friendly plasma co-melting vitrification process was developed. • The mechanisms of heavy metal fixation and migration during the plasma melting process were investigated. • Changes in the valence states and forms of various elements during the melting process were studied. Hazardous fly ash (HFA) contains large amounts of toxic heavy metals and poses significant potential risks to the environment. In this study, HFA was co-melted with hazardous bottom ash (HBA) and waste glass (WG) in a plasma system to investigate its vitrification behavior, heavy metal migration, and solidification mechanisms. The results indicate high vitrification efficiency (99.5 %) and ash solidification rate (97.1 %). The original multi-phase crystalline structure transformed into a structure dominated by an amorphous silicate network following plasma melting and rapid cooling. The surface became smoother and denser, and heavy metals were distributed uniformly, indicating enhanced physical encapsulation capacity and a more homogeneous structure. Although a considerable amount of heavy metals remained in the melt product (GP), the leaching concentrations of all detected heavy metals were significantly reduced. BCR results further indicate improved long-term stability of heavy metals. Additionally, the solidification mechanism was elucidated, involving a synergistic process of glass network encapsulation, fixation in iron-rich crystalline phases, and volatilization-redistribution-residual stabilization. As, Pb, and Cu are primarily immobilized in the glass phase, while Cr and Ni are preferentially stabilized in iron-rich spinel/ferrite-like phases; Zn and Mn exhibit dual-host behavior between the glass and crystalline phases. The results demonstrate that plasma co-melting vitrification is an effective method for the detoxification, stabilization, and resource utilization of hazardous incineration residues.