Low-carbon mechanochemical stabilization of Hg-rich fly ash from hazardous waste incineration

Environmental sound disposal of ash with elevated mercury content (up to 21.0 mg/kg) and high soluble salt content (>50 wt%) is still a big challenge, because conventional methods often rely on large amounts of cement-based solidification-which incurs substantial CO₂ emissions-and the coexistence of mobile mercury species and abundant soluble chlorides undermines stabilization treatments and complicates waste management and resource recovery. This study develops a mechanochemical stabilization route that immobilizes Hg using a natural pyrite additive and integrates directly with downstream desalting. Replacing soluble sulfiding agents with pyrite maintains robust Hg immobilization while lowering cost and improving operational safety by avoiding additional reagents. Reaction and mechanistic investigations in Ca/Cl-rich matrices from semi-dry flue-gas treatment show that controlled milling with pyrite outperforms molecular sulfiding agents. Solid-phase pyrite supplies sulfur to form stable HgS and Fe-S-Hg complexes via chemical sulfidation and structural sequestration, overcoming matrix inhibition. The milled ash can be water-washed to remove > 90 % of soluble salts; leachate Hg falls to 7 μg/L, meeting national standards, and co-leached metals are reduced to trace levels. Evaporative crystallization of washing liquors yields salts dominated by NaCl with KCl and Na₂SO₄ as secondary phases; total metal content in the recovered crystals meets de-icing specifications. A cradle-to-gate life-cycle assessment indicates about a 60 % lower carbon footprint than conventional washing plus cement-based solidification, primarily by eliminating cement and simplifying wastewater treatment. The work establishes a practical, low-carbon ball-milling, washing and crystallization pathway for integrated treatment and resource recovery of high-risk incineration fly ash.