Key Takeaways & Executive Findings
- •• • Zinc sulfidation rate of 90.39% achieved at 700 °C with 30% carbon powder, 10% Na2CO3, gypsum residue to secondary zinc oxide mass ratio of 1.4:1, and 2 h roasting time, enabling high-efficiency conversion of ZnO to ZnS for subsequent flotation recovery. • • Simultaneous removal of 76.32% F, 72.11% Cl, and 93.41% As under the same optimized conditions, mitigating impurity accumulation that typically degrades zinc electrowinning performance and reduces product quality. • • ZnS grain size increased from 5 to 10 μm via controlled cooling at 1 °C/min and sodium salt addition, which is critical for effective flotation separation, as fine grains are notoriously difficult to float and result in high losses. • • Thermodynamic enhancement of ZnO sulfidation by increasing CaSO4 and carbon dosage within 500−800 °C, with TG/DTG−DSC and 3D FTIR confirming conversion of CaSO4 to CaCO3 and suppression of CO2 and SO2 emissions, addressing environmental compliance and carbon footprint concerns.
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Abstract
Synergistic sulfidation roasting of heavy metal gypsum residue and secondary zinc oxide fume was proposed by using the research idea of 'waste to treat waste'. Thermodynamic studies indicated that the sulfidation of zinc oxide could be effectively enhanced by increasing the dosage of calcium sulfate and carbon powder in the range of 500−800 °C. The synergistic sulfidation experiments of heavy metal gypsum residue with secondary zinc oxide showed that the sulfidation rate of zinc reached 90.39% and the grain size of ZnS increased from 5 to 10 μm under the conditions of temperature 700 °C, carbon powder 30%, Na2CO3 10%, mass ratio of gypsum residue to secondary zinc oxide 1.4:1, roasting time 2 h and cooling rate 1 °C/min. Meanwhile, 76.32% F, 72.11% Cl and 93.41% As were removed. TG/DTG−DSC, 3D FTIR spectra and SEM analysis showed that the conversion of CaSO4 to CaCO3 and the avoidance of CO2 and SO2 production were achieved under optimized conditions. This study achieves efficient sulfidation of zinc as well as growth of ZnS grains, laying the theoretical and technological foundation for subsequent recovery of ZnS by flotation.
1. Introduction
Existing commercial approaches for treating heavy metal gypsum residue and secondary zinc oxide fume—landfill, hydrometallurgical, and pyrometallurgical processes—face critical limitations. Landfilling prevents secondary fume accumulation but causes severe soil and groundwater contamination and wastes valuable zinc resources. Pyrometallurgical routes, such as rotary kiln and multi-hearth furnace roasting, are mature but suffer from high energy consumption, serious environmental pollution, and high costs, and they are unsuitable for materials with high impurity content. Hydrometallurgical processes often struggle with fluorine and chlorine removal, which are detrimental to zinc electrowinning. These bottlenecks have stalled the efficient recovery of zinc from secondary resources, leaving a significant gap in sustainable resource utilization.
This study introduces a synergistic sulfidation roasting protocol that leverages the 'waste to treat waste' principle, using heavy metal gypsum residue as a sulfidizing agent for secondary zinc oxide fume. By optimizing temperature, carbon dosage, sodium salt addition, and cooling rate, the method achieves a 90.39% zinc sulfidation rate and promotes ZnS grain growth to over 10 μm, while simultaneously removing 76.32% F, 72.11% Cl, and 93.41% As. The process avoids CO2 and SO2 production under optimized conditions, offering a cleaner, more efficient pathway for zinc recovery and hazardous waste mitigation.
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Yong-wei WANG, Rui HUANG, Wen-qing QIN, Jun-wei HAN (2026). Efficient separation of heavy metals from gypsum residue and secondary zinc oxide fume based on synergistic sulfidation. Transactions of Nonferrous Metals Society of China (中国有色金属学报). https://doi.org/10.1016/S1003-6326(26)67068-9
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Frequently Asked Questions
What is the sulfidation rate of zinc under the optimized conditions, and how does it compare to conventional roasting?
The sulfidation rate of zinc reaches 90.39% at 700 °C, 30% carbon powder, 10% Na2CO3, mass ratio of gypsum residue to secondary zinc oxide 1.4:1, and 2 h roasting time. Conventional pyrometallurgical processes often yield lower sulfidation efficiencies and suffer from high energy consumption; this synergistic approach significantly enhances zinc conversion while suppressing SO2 and CO2 emissions.
How does the process address fluorine and chlorine removal, which are critical impurities for zinc electrowinning?
Under the optimized conditions, 76.32% F and 72.11% Cl are removed, along with 93.41% As. This simultaneous removal mitigates the detrimental effects of these impurities on downstream electrowinning, reducing the need for additional purification steps and improving overall process economics.
What is the mechanism behind ZnS grain growth, and why is it important for flotation recovery?
ZnS grain size increases from 5 to 10 μm by increasing roasting temperature to 700 °C, adding 10% Na2CO3, and decreasing the cooling rate to 1 °C/min. Larger grains are essential for efficient flotation, as fine particles are difficult to float and lead to high losses. This grain regulation directly enhances the subsequent recovery of ZnS by flotation.
Does the process produce hazardous gases such as SO2 or CO2, and how are emissions controlled?
TG/DTG−DSC, 3D FTIR spectra, and SEM analysis confirm that under optimized conditions, the conversion of CaSO4 to CaCO3 is achieved and the production of CO2 and SO2 is avoided. This is a significant environmental advantage over conventional roasting, which often generates these gases and requires costly scrubbing systems.
What are the scalability and cost implications of using sodium carbonate and controlled cooling in an industrial setting?
The addition of 10% Na2CO3 and a cooling rate of 1 °C/min are moderate requirements that can be integrated into existing roasting circuits. While sodium carbonate adds a reagent cost, it enhances grain growth and sulfidation, potentially reducing downstream flotation costs. The avoidance of SO2 and CO2 emissions also lowers environmental compliance costs, improving overall economic viability.
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