Key Takeaways & Executive Findings
- •• • The lead-zinc molten slag slurry achieved >90% SO2 removal efficiency sustained for approximately 70 hours, demonstrating its viability as a low-cost desulfurizing agent for smelting flue gas. • • NaClO2 was the most effective oxidant among tested (KMnO4, H2O2, K2Cr2O7, NaClO2), yielding up to 94.76% NOx removal when combined with the desulfurization slurry. • • Under optimized conditions (NaClO2 0.025 mol·L−1, 45 °C, 200 mL·min−1, 10% O2, 0.03% NOx, pH 6), NOx removal reached 97.24%, highlighting the process's high efficiency. • • Fe3+ ions leached from the slag significantly enhanced NOx removal by promoting NaClO2 decomposition into ClO2 and other reactive species, enabling a cyclic catalytic mechanism that boosts oxidation performance.
Abstract
The emission of sulfur dioxide (SO2) and nitrogen oxides (NOx) from fossil fuel combustion and metal smelting industries poses severe risks to environmental and human health. This study utilized depleted lead-zinc molten slag as a desulfurizer for wet flue gas desulfurization, and the resulting desulfurization slurry was further employed for NOx removal, achieving resource utilization. The desulfurization efficiency of the slag was determined, and NaClO2 was identified as the most effective oxidant when combined with the slag slurry for NOx removal. The effects of NaClO2 concentration, reaction temperature, flue gas flow rate, oxygen concentration, NOx concentration, and pH on removal efficiency were investigated. Optimal conditions were found at NaClO2 concentration of 2.5 mmol·L−1, temperature 45 °C, flue gas flow 200 mL·min−1, O2 volume fraction 10%, NOx volume fraction 0.03%, and pH 6, achieving a NOx removal efficiency of 97.24%. Metal ion experiments revealed that Fe3+, Zn2+, Mn2+, and K+ exhibited synergistic effects with NaClO2, with Fe3+ showing the most significant enhancement. Fe3+ promoted the decomposition of NaClO2 to generate stronger oxidants such as ClO2, thereby enhancing NOx oxidation and absorption. This approach offers a cost-effective and environmentally friendly alternative to traditional selective catalytic reduction, avoiding ammonia slip and secondary pollution.
1. Introduction
The management of lead-zinc smelting slag presents a critical environmental challenge, with annual production exceeding 3 million tonnes in China and historical stockpiles surpassing 100 million tonnes. Conventional disposal in construction materials risks heavy metal leaching (Pb, Zn, Cd) into soil and groundwater. Concurrently, smelting operations emit SO2 and NOx, major precursors to acid rain and haze. While wet flue gas desulfurization using limestone is common, it requires external reagents and generates gypsum by-products. The use of lead-zinc slag as a desulfurizer offers a dual benefit: waste valorization and cost reduction, but its efficacy for NOx removal is limited.
To overcome the low NOx conversion in slag-based systems, this study integrates NaClO2 as an oxidizing agent. Traditional selective catalytic reduction (SCR) requires ammonia or urea, posing risks of ammonia slip and equipment corrosion. The proposed method leverages the metal ions (Fe3+, Mn2+) naturally leached from the slag to catalyze NaClO2 decomposition, generating potent oxidants like ClO2. This synergistic approach achieves high NOx removal efficiency (up to 97.24%) without external reductants, offering a sustainable and economically attractive alternative for smelting flue gas treatment.
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LIU Jiawen, ZHU Han, SUN Lina, LI Kai, SUN Xin, MA Yixing (2026). Desulfurization of Lead-Zinc Molten Slag and Synergistic Oxidation of NOx with NaClO2. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202508101
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Frequently Asked Questions
What is the long-term stability of the slag slurry for SO2 removal under industrial flue gas conditions?
The slag slurry maintained >90% SO2 removal for approximately 70 hours under the tested conditions (SO2 0.2%, O2 10%, flow 500 mL·min−1, 45 °C). This suggests adequate durability for batch operations, but continuous operation would require periodic slurry replacement or regeneration.
How does the presence of other flue gas components (e.g., CO2, HCl) affect the NOx removal efficiency?
The study did not investigate the impact of CO2 or HCl. However, the presence of acidic gases could lower the slurry pH, potentially affecting NaClO2 decomposition and metal ion leaching. Further research is needed to assess real flue gas matrix effects.
What is the cost comparison between this method and conventional SCR for NOx removal?
This method eliminates the need for ammonia or urea, reducing reagent costs and avoiding secondary pollution. The use of waste slag as a desulfurizer also reduces waste disposal costs. However, NaClO2 is a relatively expensive oxidant; a detailed techno-economic analysis is required to quantify overall savings.
What are the potential mechanisms for Fe3+ enhancing NaClO2 oxidation, and are there any competing side reactions?
Fe3+ is proposed to catalyze NaClO2 decomposition into ClO2 and other reactive species. This involves redox cycling between Fe3+ and Fe2+, which can generate radicals. Side reactions may include the consumption of oxidants by other reducing species in the slurry, but the study indicates a net positive effect on NOx removal.
How does the slag's particle size and composition affect its desulfurization and NOx removal performance?
The slag was ground to ≤74 μm (200 mesh) to increase surface area. Its composition includes metal oxides (Fe, Mn, Zn) and alkaline earth oxides (CaO, MgO) that facilitate SO2 absorption and provide catalytic metal ions. Variations in slag source could alter performance, necessitating characterization for each batch.
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