Plasma vitrification and heavy metals solidification of MSW and sewage sludge incineration fly ash

Thermal plasma treatment has been considered as one of promising methods for fly ash disposal. In this study, firstly, the characteristics of municipal solid waste incineration (MSWI) fly ash and sewage sludge incineration (SSI) fly ash were analyzed, followed by the raw material formulations of low melting temperature determined by thermodynamic equilibrium calculation. Then verified experiments were carried out by thermal plasma system, focused on the formation condition of vitrified slags with various CaO-SiO₂-Al₂O₃ ratios and the influence factors of heavy metals (Cd, Cr, Ni, Pb, Zn, Cu) transference. According to the results: During the co-treatment process of fly ashes, a lower temperature of vitrified slag formation as low as 1230 ℃ was observed. Vitrification is determined by the molten phase content during melting process, the correlation coefficient between the glass phase content of slag and the molten phase content was 0.81 (P < 0.01). CaO content of raw materials was a major element for the volatilization of Cd and Pb. High Al₂O₃ content can remain more Cr, Cu and Ni in slags, but it is not conducive to the solidification of Zn. The synthetic toxicity index of heavy metals would greatly reduce from 23153.15 to 663.29-820.63 after thermal plasma treatment.