Key Takeaways & Executive Findings
- •• • Zinc recovery ratio of 99.13% and residual Zn content of 0.22 wt.% in slag were achieved under optimized redox smelting conditions, compared to the industrial baseline of 1.0–3.0 wt.%, directly reducing the hazardous waste classification and long-term environmental liability of the residue. • • The formation of a ZnS–FeS eutectic, promoted by CaSO4 in the leaching residue, was identified as the primary bottleneck for zinc recovery; this phase exhibits lower thermodynamic stability than (Fe,Zn)2SiO4, ZnFe2O4, and (ZnO)slag, necessitating temperatures above 1573 K to oxidize it to ZnO for subsequent reduction. • • Elevated temperatures (>1573 K) facilitate the oxidation of the ZnS–FeS eutectic by O2/(O)slag to ZnO(s), which then dissolves into the slag as (ZnO)slag and is finally reduced to Zn(g) by CO; this mechanism enables effective zinc volatilization and recovery. • • A novel treatment strategy combining desulfurization pretreatment with redox smelting is proposed, which lowers the required smelting temperature and improves zinc recovery efficiency, providing a more economical and environmentally sustainable route for treating zinc leaching residues compared to conventional methods.
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Abstract
The redox smelting of zinc leaching residue (ZLR) was investigated to determine the migration behavior and toxicity characteristics of zinc under varying anthracite addition, temperature, and holding time. The ZLR, containing 10–20 wt.% Zn, 0.5–5 wt.% Pb, and 0.1–0.5 wt.% Cd, generates TCLP leachate concentrations of Zn up to 4589.0 mg/L, far exceeding regulatory limits. Experimental results reveal that CaSO4 in the residue promotes the transformation of ZnFe2O4 into a ZnS–FeS eutectic, which hinders zinc recovery and elevates environmental risk due to its lower thermodynamic stability relative to (Fe,Zn)2SiO4, ZnFe2O4, and (ZnO)slag. At temperatures above 1573 K, the ZnS–FeS eutectic is oxidized by O2/(O)slag to ZnO(s), subsequently dissolved into the slag as chemically dissolved Zn, and finally reduced to Zn(g) by CO. Pre-desulfurization or increased oxygen potential enhances zinc volatilization. Under optimized conditions, the zinc recovery ratio reached 99.13%, and the residual zinc content in the slag decreased to 0.22 wt.%, substantially below the industrial range of 1.0–3.0 wt.%. A novel strategy integrating desulfurization pretreatment with redox smelting is proposed, which lowers the required smelting temperature and improves zinc recovery efficiency, offering a more economical and environmentally sustainable solution for ZLR treatment.
1. Introduction
Zinc leaching residue (ZLR) is a hazardous byproduct of the dominant zinc production route—oxidative roasting, acid leaching, and electrodeposition from sphalerite—generating an estimated 5.6–9.6 million tons annually worldwide. The residue contains 10–20 wt.% Zn, 0.5–5 wt.% Pb, and 0.1–0.5 wt.% Cd, with TCLP leachate concentrations reaching 2205.2–4589.0 mg/L for Zn, 1.4–5.3 mg/L for Pb, and 45.0–93.5 mg/L for Cd, far exceeding regulatory limits in many jurisdictions. Among these metals, zinc exhibits high mobility, bioavailability, and ecotoxicity, particularly as sulfates, sulfides, and oxides, making its safe disposal a global challenge. Existing recovery technologies—secondary leaching, reduction roasting, and redox smelting—struggle with the refractory nature of zinc ferrite (ZnFe2O4) and zinc sulfide (ZnS), often requiring harsh conditions and yielding incomplete recovery.
This study addresses the critical gap in understanding zinc migration behavior during redox smelting of ZLR, specifically the role of CaSO4 in promoting the formation of a ZnS–FeS eutectic that hinders zinc recovery. By systematically varying anthracite addition, temperature, and holding time, the research elucidates the phase transformations and toxicity characteristics of zinc. The experimental protocol identifies a temperature threshold above 1573 K where the ZnS–FeS eutectic is oxidized to ZnO, dissolved into the slag, and reduced to Zn(g), enabling a recovery ratio of 99.13% and a residual zinc content of 0.22 wt.%—substantially lower than the industrial range of 1.0–3.0 wt.%. The proposed integration of desulfurization pretreatment with redox smelting offers a lower-temperature, more efficient, and environmentally sustainable solution for ZLR treatment.
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Heng WANG, Cheng TAN, Yong YU, Rui-jin FAN, Jian-hang HU, Hua WANG (2026). Assessment of zinc migration behavior and toxicity characteristics in redox smelting of zinc leaching residue. Transactions of Nonferrous Metals Society of China (中国有色金属学报). https://doi.org/10.1016/S1003-6326(26)67069-0
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Frequently Asked Questions
What is the primary mechanism hindering zinc recovery during redox smelting of zinc leaching residue, and how does the proposed process overcome it?
The primary hindrance is the formation of a ZnS–FeS eutectic, promoted by CaSO4 in the residue, which transforms ZnFe2O4 into a phase that is not efficiently reducible. This eutectic exhibits lower thermodynamic stability than (Fe,Zn)2SiO4, ZnFe2O4, and (ZnO)slag. The proposed process overcomes this by operating at temperatures above 1573 K, where the ZnS–FeS eutectic is oxidized by O2/(O)slag to ZnO(s), then dissolved into the slag as (ZnO)slag, and finally reduced to Zn(g) by CO. This enables a zinc recovery ratio of 99.13% and reduces residual zinc in the slag to 0.22 wt.%.
What are the key operational parameters and their optimized values for achieving high zinc recovery and low residual zinc in the slag?
The key parameters are anthracite addition amount, temperature, and holding time. The optimized conditions yielded a zinc recovery ratio of 99.13% and a residual zinc content of 0.22 wt.% in the slag. The temperature must exceed 1573 K to facilitate the oxidation of the ZnS–FeS eutectic. Pre-desulfurization or increased oxygen potential further improves zinc volatilization. These values are substantially lower than the typical industrial residual zinc range of 1.0–3.0 wt.%.
How does the toxicity characteristic leaching procedure (TCLP) performance of the treated residue compare to the raw zinc leaching residue, and what are the implications for environmental compliance?
The raw zinc leaching residue exhibits TCLP leachate concentrations of Zn up to 4589.0 mg/L, Pb up to 5.3 mg/L, and Cd up to 93.5 mg/L, far exceeding regulatory limits. After redox smelting under optimized conditions, the residual zinc content in the slag is reduced to 0.22 wt.%, which substantially mitigates the potential environmental hazards. The lower residual zinc and the transformation of zinc into less leachable phases reduce the toxicity characteristic leaching potential, aiding compliance with hazardous waste regulations.
What is the economic and scalability advantage of integrating desulfurization pretreatment with redox smelting compared to conventional methods?
The integrated approach lowers the required smelting temperature and improves zinc recovery efficiency. Conventional methods often require higher temperatures and yield residual zinc contents of 1.0–3.0 wt.%, whereas the proposed strategy achieves 0.22 wt.% residual zinc and a 99.13% recovery ratio. This reduces energy consumption and improves resource utilization, providing a more economical and environmentally sustainable solution for treating zinc leaching residues. The process is scalable as it builds on existing redox smelting infrastructure with a pretreatment step.
What are the potential failure mechanisms or operational risks under continuous industrial operation, and how can they be mitigated?
Potential risks include incomplete oxidation of the ZnS–FeS eutectic if the temperature falls below 1573 K, leading to lower zinc recovery and higher residual zinc. Variability in anthracite addition and holding time could also affect the reduction of (ZnO)slag to Zn(g). To mitigate, precise control of temperature above 1573 K, optimized anthracite dosage, and sufficient holding time are essential. Pre-desulfurization or increasing oxygen potential can further enhance zinc volatilization and process robustness. Regular monitoring of slag composition and off-gas zinc content ensures consistent performance.
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