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 degrees 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 degrees 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 Pb2+ reacts with SiO2 to form a stable mineral phase PbSiO3, which is the main reason for the reduction of Pb leachability. In addition, at high temperatures, the added silicon as well as the Al2O3 in the FA reacted with CaO to form aluminosilicate skeletons, which can also solidify a small portion of the Pb2+ 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 degrees C, a large amount of CaO reacts with silica to form CaSiO3, which hinders the reaction between PbO and SiO2 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.