Mechanism of heavy metal solidification in ceramics by synergistic sintering of MSWI fly ash and silica-aluminum solid waste

For the sintering of ceramics from municipal solid waste incineration (MSWI) fly ash, it is a challenging task to effectively solidify heavy metals and reveal their solidification mechanism. To address this, the experiments designed a technical route for sintering fly ash leached by membrane concentrate into ceramics with the synergistic silica-aluminum solid wastes, constructed a CaO-SiO₂-Al₂O₃-NaF heavy metal solidification system, clarified the optimal ratio of ceramic billet preparation and the parameters of the sintering process, and analysed the mechanism of heavy metal solidification. The results showed that membrane concentrate leaching could remove 98.46% of soluble chlorine salts in fly ash, and the optimum billet ratio was: leach residue (50%), overhaul slag (15%) and waste glass (35%), which could be sintered at 1 050 ℃ for 20 min to produce high-strength 900-grade ceramics. The solidification rates of heavy metals Pb, Cd, Cr, Cu and Zn in ceramics were 40.52%, 70.22%, 88.45%, 76.12% and 90.19%, respectively. The mechanisms were as follows: Pb, Cu and Zn were solidified mainly by chemical bonding to form crystals such as (PbF₂)_12.7(PbO)_58.2(SiO₂)_28.4, CuO, Ca₂Zn₂SiO₇, etc., respectively. Cr and Cd were solidified mainly through ion substitution to form crystals such as Na₆Ca₂Cr₆Si₆O₂₄(SO₄)₂, CdMgSi₂O₇, etc. A small amount of CdS, PbCl₂, PbS and PbO were physically sequestered via ceramic pores and CaF₂-based solid matrix. The results of this research provide a new path for the ecologically sound and resourceful in-situ waste incineration of fly ash, synergized with silica-alumina solid waste.