Novel and sustainable silica-assisted low-temperature roasting method for the solidification of lead in municipal solid waste incineration fly ash: Process and mechanism

During the high-temperature treatment process of municipal solid waste incineration (MSWI) fly ash (FA), heavy metals and chlorine are severely volatilized, leading to serious secondary pollution. Therefore, developing low-temperature solidification technology for heavy metals in FA is of great significance. This article proposes a process for silica-assisted low-temperature sintering to solidify Pb in FA. The effects of roasting temperature (100–800 °C), roasting time (30–180 min) and curing agent dosage (0–12.5 wt%) on the leaching toxicity and immobilization rate of Pb were investigated. The optimum conditions for Pb solidification were calcination temperature of 600 °C, roasting time of 180 min and silica proportion of 10 wt%. The Pb leaching concentration, Pb immobilization rate and comprehensive strength of cured FA were 0.086 mg/L, 99% and 1.65 MPa, respectively, which satisfied the requirements of hazardous waste landfill (GB 18598–2019). The solidification mechanism indicates that most of the Pb 2+ reacts with SiO 2 to form a stable mineral phase PbSiO 3 , which is the main reason for the reduction of Pb leachability. In addition, at high temperatures, the added silicon as well as the Al 2 O 3 in the FA reacted with CaO to form aluminosilicate skeletons, which can also solidify a small portion of the Pb 2+ into the crystal lattice . As the roasting temperature increases, the chemical speciation of Pb gradually changes to the oxidizable and residual states. When the temperature is higher than 700 °C, a large amount of CaO reacts with silica to form CaSiO 3 , which hinders the reaction between PbO and SiO 2 and reduces the solidification rate of Pb. Finally, the feasibility and economy of the process were analyzed. The findings can provide theoretical guidance for the development of low-temperature detoxification process of FA.