A novel method of calcium dissolution-crystallization-polymerization for stabilization/solidification of MSWI fly ash

Municipal solid waste incineration fly ash (MSWI FA) stabilization/solidification using calcium carbonate (CaCO₃) oligomer is an efficient, low-carbon disposal method. The insoluble Ca in FA was converted to free-Ca, utilizing for CaCO₃ oligomer preparation, which was crystallized and polymerized by thermal induction to develop continuous cross-link or bulk structures for stabilization/solidification of potentially toxic elements (PTEs, e.g., lead (Pb) and zinc (Zn)). Experimental results showed that the weakly alkaline acid-leaching suspension provided an excellent condition for the generation of CaCO₃ oligomers, with Pb and Zn immobilization reaching over 99.4%. With the acid strengthening of the suspension, H⁺ took the lead in protonating with TEA and limiting the capping action of TEA, which was harmful to the synthesis of CaCO₃ oligomers. Ethanol with a low dielectric constant was considered an ideal solvent for oligomer production, and triethylamine (TEA) as a capping agent established hydrogen bonds (N⋯H) with protonated CaCO₃. H₂O molecules competed with the protonated CaCO₃ molecules for TEA with ethanol concentration decreasing, resulting in erratic precipitation of CaCO₃ molecules and significantly elevated leaching risk of Pb and Zn. The sequential extraction procedure, pH-dependent leaching, and geochemical analysis results revealed that the dissolution/precipitation of Ca, Pb, and Zn in treated FA was mostly controlled by the carbonate mineral phases. Moreover, the low boiling points of ethanol and TEA can be recovered for recycling. The gel-like, flexible combination of CaCO₃ oligomers and FA particles formed by FA offers great resource utilization potential via a controlled crystallization polymerization process.