Innovative methodology for comprehensive utilization of arsenic-bearing neutralization sludge

Arsenic-bearing neutralization (ABN) sludge is a classical hazardous waste commonly found in nonferrous metallurgy. However, the current storage of these hazardous wastes not only has to pay costly hazardous waste taxes but also poses significant risks to both the environment and human health. To address these issues and achieve the comprehensive utilization and minimization of ABN sludge, this study proposes a new combined process. The process involves selective reduction roasting, leaching, and carbonation, through which, the arsenate and gypsum in the ABN sludge were recovered in the form of As(s), high-purity CaCO₃, and H₂S. The selective reduction behaviors of arsenate and gypsum were investigated through thermodynamic analysis and roasting experiments. The results indicated that the 95.35 % arsenate and 96.55 % gypsum in the sludge were selectively reduced to As₄(g) and CaS at 950 °C by carbothermic reduction. The As₄(g) was condensed to As(s) and enriched in the dust (As, 96.78 wt %). In the leaching process, H₂S gas was adopted to promote the leaching of CaS, and resulted in 97.41 % of CaS in the roasted product was selectively leached in the form of Ca(HS)₂, leading to a 74.11 % reduction in the weight of the ABN sludge. Then, the Ca(HS)₂ was subjected to capture CO₂ for the separation of Ca²⁺ and S^2-. The result depicted that 99.69 % of Ca²⁺ and 99.12 % of S^2- were separated as high-purity (99.12 wt %) CaCO₃ and H₂S (24.89 vol %) by controlling the terminal carbonation pH to below 6.55. The generated H₂S can be economically converted to sulfur by the Clause process. The whole process realized the comprehensive resource recovery and the minimization of the sludge, which provides an alternative solution for the clean treatment of hazardous ABN waste.