Transformation of arsenic-rich copper smelter flue dust in contrasting soils: A 2-year field experiment

Eco-friendly treatment of copper smelting flue dust for recovering multiple heavy metals with economic and environmental benefits

Handling flue dust in an environmentally friendly manner has become an urgent task for pollution prevention in the copper industry. Here, driven by the low-carbon notion, we report a process that enables the selective retrieval of multiple metals (As, Cu, Pb, Zn, and Bi) from copper smelting flue dust (CSFD). This process employed low-temperature roasting to separate arsenic from heavy metals, thereby eliminating the tedious separation steps required by existing processes. Subsequently, Zn and Cu were dissolved in water, while Pb and Bi were left as a solid residue. We achieved 98.23% extraction of Cu via Zn cementation at a micro-voltage of 0.50 V. Utilizing the difference in solubility, Bi was selectively dissolved from the residue using a NaCl-HCl medium, which enabled the subsequent production of metallic Bi through electrowinning. Finally, more than 99% of Pb in the solid was reduced to elemental Pb by mechanochemical reduction. Through optimized process conditions, high-purity As2O3 (99.04%), lead ingot (99.95%), metallic copper (94.16%), and bismuth (99.20%) were obtained. Our economic assessment revealed significant advantages, demonstrating the industrial feasibility of this process. Consequently, this study presents an effective and cost-efficient system for CSFD disposal while minimizing the environmental impact and fostering a circular economy.

Potential mobilization of cadmium and zinc in soils spiked with smithsonite and sphalerite under different water management regimes

Particulate cadmium (Cd) and zinc (Zn) are ubiquitous in agricultural soils of Pb-Zn mining regions. Water management serves as an important agronomic measure altering the bioavailability of Zn and Cd in soils, but how this affects particulate Cd and Zn and the underlying mechanisms remain largely unknown. Microcosm soil incubation combined with spectroscopic and microscopic characterization was conducted. During a two-year-long incubation period we observed that the concentrations of soil CaCl₂-extractable Zn and Cd increased 3-10 times in sphalerite-spiked soils and 1-2 times in smithsonite-spiked soils under periodic flooding conditions due to the long-term dissolution of sphalerite (SP) and smithsonite (SM). However, the increase in the concentration of CaCl₂-extractable metals (Zn: from 0.607 mg kg^-1 to 1.051 mg kg^-1 and Cd: from 0.047 mg kg^-1 to 0.119 mg kg^-1) was found only in SP-treatment under continuous flooding conditions, indicating the mobilization of metals. Ultrafiltration analysis shows that the nanoparticulate fraction of Zn and Cd in soil pore water increased 5 and 7 times in SP-treatments under continuous flooding conditions, suggesting the increment of metal pools in soil pore water. HRTEM-EDX-SAED further reveals that these nanoparticles were mainly crystalline ZnS and Zn-bearing sulfate nanoparticles in the SP-treatment and amorphous ZnCO₃ and ZnS nanoparticles in the SM-treatment. Therefore, the formation of the stable crystalline Zn-bearing nanoparticles in the SP-treatment may explain the elevation of the concentration of soil CaCl₂-extractable Zn and Cd under continuous flooding. The potential mobility of particulate metals should therefore be expected in scenarios of continuous flooding such as paddy soils and wetland systems.

Synthetic Sulfide Concentrate Dissolution Kinetics in HNO3 Media

The nature of tennantite (Cu12As4S13), chalcopyrite (CuFeS2) and sphalerite (ZnS) particles’ mixture dissolution in nitric acid (HNO3) media was investigated in this study. The effects of temperature (323−368 K), HNO3 (1−8 mol/L) and Fe3+ (0.009−0.036 mol/L) concentrations, reaction time (0−60 min) and pyrite (FeS2) additive (0.5/1−2/1; FeS2/sulf.conc.) on the conversion of the minerals were evaluated. It has been experimentally shown that the dissolution of the mixture under optimal conditions (>353 K; 6 mol/L HNO3; FeS2/synt. conc = 1/1) allows Cu12As4S13, CuFeS2 and ZnS conversion to exceed 90%. The shrinking core model (SCM) was applied for describing the kinetics of the conversion processes. The values of Ea were calculated as 28.8, 33.7 and 53.7 kJ/mol, respectively, for Cu12As4S13, CuFeS2 and ZnS. Orders of the reactions with respect to each reactant were calculated and the kinetic equations were derived to describe the dissolution rate of the minerals. It was found that the interaction between HNO3 solution and Cu12As4S13, CuFeS2 and ZnS under the conditions investigated in this are of a diffusion-controlled nature. Additionally, the roles of Fe(III) in the initial solution and FeS2 in the initial pulp as catalysts were studied. The results indicated that the increase in Fe3+ concentration significantly accelerates the dissolution of the mixture, while the addition of FeS2 forms a galvanic coupling between FeS2, and Cu12As4S13 and CuFeS2, which also accelerates the reaction rate. The results of the study are considered useful in developing a hydrometallurgical process for polymetallic sulfide raw materials treatment.

Interaction between arsenic metabolism genes and arsenic leads to a lose-lose situation

Microorganisms are essential for modifying arsenic morphology, mobility, and toxicity. Still, knowledge of the microorganisms responsible for arsenic metabolism in specific arsenic-contaminated fields, such as metallurgical plants is limited. We sampled on-field soils from three depths at 70 day intervals to explore the distribution and transformation of arsenic in the soil. Arsenic-metabolizing microorganisms were identified from the mapped gene sequences. Arsenic metabolism pathways were constructed with metagenomics and AsChip analysis (a high-throughput qPCR chip for arsenic metabolism genes). It has been shown in the result that 350 genera of arsenic-metabolizing microorganisms carrying 17 arsenic metabolism genes in field soils were identified, as relevant to arsenic reduction, arsenic methylation, arsenic respiration, and arsenic oxidation, respectively. Arsenic reduction genes were the only genes shared by the 10 high-ranking arsenic-metabolizing microorganisms. Arsenic reduction genes (arsABCDRT and acr3) accounted for 73.47%-78.11% of all arsenic metabolism genes. Such genes dominated arsenic metabolism, mediating the reduction of 14.11%-19.86% of As(V) to As(III) in 0-100 cm soils. Arsenic reduction disrupts microbial energy metabolism, DNA replication and repair and membrane transport. Arsenic reduction led to a significant decrease in the abundance of 17 arsenic metabolism genes (p < 0.0001). The critical role of arsenic-reducing microorganisms in the migration and transformation of arsenic in metallurgical field soils, was emphasized with such results. These results were of pronounced significance for understanding the transformation behavior of arsenic and the precise regulation of arsenic in field soil.

An experimental comparison: Horizontal evaluation of valuable metal extraction and arsenic emission characteristics of tailings from different copper smelting slag recovery processes

This study comprehensively investigated arsenic's enrichment, distribution, and characteristics in tailings. XRD and SEM-EDS characterized the phase and morphology of tailings from various smelting processes. At the same time, the embedding characteristics of arsenic in the ore phase were analyzed by EPMA. The differences between arsenic's leading ore phase carriers in different recovery processes were found. It was discussed that this phenomenon would be related to the element-binding ability and the precipitation priority of the ore phase. The occurrence state of arsenic was discussed by sequential chemical extraction experiments. The proportion of leachable arsenic is higher than the low-risk limit, whatever which smelting method is adopted, which leads to high environmental risk. In the experiment of comparing the leaching toxicity of tailings by different leaching methods, the arsenic concentration in the leaching solution of tailings recovered by the flotation method exceeds the specified safety range. Although the tailings after reduction smelting did not show high leaching toxicity, a large number of accumulations also would not represent absolute safety.

Nitric Acid Dissolution of Tennantite, Chalcopyrite and Sphalerite in the Presence of Fe (III) Ions and FeS2

This paper describes the nitric acid dissolution process of natural minerals such as tennantite, chalcopyrite and sphalerite, with the addition of Fe (III) ions and FeS₂. These minerals are typical for the ores of the Ural deposits. The effect of temperature, nitric acid concentration, time, additions of Fe (III) ions and FeS₂ was studied. The highest dissolution degree of sulfide minerals (more than 90%) was observed at a nitric acid concentration of 6 mol/dm³, an experiment time of 60 min, a temperature of 80 °C, a concentration of Fe (III) ions of 16.5 g/dm³, and an addition of FeS₂ to the total mass minerals at 1.2:1 ratio. The most significant factors in the break-down of minerals were the nitric acid concentration, the concentration of Fe (III) ions and the amount of FeS₂. Simultaneous addition of Fe (III) ions and FeS₂ had the greatest effect on the leaching process. It was also established that FeS₂ can be an alternative catalytic surface for copper sulfide minerals during nitric acid leaching. This helps to reduce the influence of the passivation layer of elemental sulfur due to the galvanic linkage formed between the minerals, which was confirmed by SEM-EDX.

Alkali circulating leaching of arsenic from copper smelter dust based on arsenic-alkali efficient separation

Leaching arsenic from solid waste selectively and removing arsenic from alkaline leachate efficiently are two key points in alkali treatment of copper smelter dust, and the latter is challenging. In this study, composite salt precipitation of magnesium ammonium arsenate (NH₄MgAsO₄·6H₂O), similar to magnesium ammonium phosphate (NH₄MgPO₄·6H₂O), was proposed to solve the difficult problem of separation arsenic from alkali. Based on the thermodynamic analysis, the selective leaching of arsenic from copper smelting dust was carried out in the NaOH-Na₂S system. In the alkali leaching system, more than 80% arsenic can be leached out from the dust with the diffusion-controlled type in the Avrami model, while the leaching rates of valuable metals are less than 0.5%. For the strong alkaline leachate containing arsenic obtained by alkali leaching, the selective removal of arsenic was achieved by adding magnesium salt and ammonium salt. With the change of the amount of magnesium salt and ammonium salt, the sedimentation performance and composition of the arsenic slag changed accordingly. At the mole ratio of NH₄⁺: As = 8:1 and Mg²⁺: As = 1.5:1, 96.38% of arsenic was removed, and the content of arsenic in the arsenic slag composed of MgNH₄AsO₄·6H₂O reached 28.96%. On this basis, the circulating alkali leaching of copper smelter dust based on arsenic-alkali separation was successfully carried out. The whole scheme is not only economical and safe, but also achieves the reuse of wastewater without secondary pollution, which provides an alternative solution for the treatment of arsenic containing solid waste.

Strategies for arsenic pollution control from copper pyrometallurgy based on the study of arsenic sources, emission pathways and speciation characterization in copper flash smelting systems

Arsenic in copper flash smelting (FS) systems not only affects the quality of products but also poses significant technological and environmental problems. Based on the assessment of arsenic mass partitioning in the FS system, arsenic elimination in off-gassing and tailings is 22%, and most of the arsenic output (69%) is recycled in the FS system. Circulating arsenic, especially arsenic in recycled dust and slag concentrate, is the key reason for high-arsenic-content feed. Dust-type recycled materials (RMs) contribute much more arsenic to the feed than slag-type RMs. Flash smelting furnace electrostatic precipitator (FSF ESP) dust contributes makese the largest contribution to arsenic among the dust-type RMs of mixed dust, especially trivalent arsenic, followed by FSF and flash converting furnace waste heat boiler (FCF WHB) dust, which contributes pentavalent arsenic. FCF WHB dust exhibits a relatively low arsenic content, consisting mainly of As(V)-O. Slag-type recycled materials contribute As(V)-O to the total feed, and As(III) originates from copper concentrates. Considering the arsenic contribution and environmental risk, reducing the recovery of FSF ESP dust can greatly decrease the arsenic grade of FSF feed and volatile As₂O₃. As one of the main arsenic sources in feed, FSF slag concentrate should be carefully disposed of if separated from feed materials because of its high arsenic-related environmental risk. In contrast, WHB dust and FCF slag are more suitable as RM due to their high copper content and low arsenic risk.

Mineralogical and morphological factors affecting the separation of copper and arsenic in flash copper smelting slag flotation beneficiation process

Separating copper and arsenic has always been a major problem in the copper slag flotation process, which influences copper slag utilization and the environmental safety. A comparative study of flash smelting furnace (FSF) slag and its flotation products (concentrate and tailing) reveals the factors affecting the separation of copper and arsenic in the beneficiation process from the perspective of mineralogy and morphology. The elemental fractionation in the process shows a positive correlation of As, Cu and Cd and an obvious correlation between speciation transformation of copper and arsenic was observed. The occurrence of arsenic and copper in FSF slag correlate the key phases of arsenic copper alloys, accounted for 88.91 % of total arsenic-bearing phases and 32.28% of copper-bearing phases. Closely-embeded matte and copper-arsenic alloys incerease the difficulty of the separation suggesting the finer grinding is needed for slag. Arsenic is liberated and oxidized into arsenate compounds while the recombination of As-O and Cu-S happened in the process affecting the selectivity of copper and arsenic. Arsenic fixed in silicate minerals is discharged into tailing which suggested to induce and fix arsenic into silicate minerals can facilitate arsenic removal from concentrate. FSF slag and its flotation concnetrate show risks of some of some of HMs which should be cautiously transported, disposed, and utilized.

Role of temperature, wind, and precipitation in heavy metal contamination at copper mines: a review

The increasing demand for minerals pressurizing the mining authorities to extract low-grade ore results in more mining waste and degradation of the environment. The main aim of review was to understand the role of climatic factors (temperature, wind, and precipitation) in dispersal and mobility of heavy metals in soil, water, and vegetation in Cu mining region. The major source of contamination in the mining sector is tailings, overburden rocks, and abandoned mines. The contaminates or fine particles of sulfide-rich mining waste follow two major pathways for the dispersal: aerial and leaching. Sulfides on exposure to oxygen and water generate acid mine drainage which results in leaching of heavy metals. The pit water of abandoned mines is also a cause of concern which contaminates the groundwater resources. Climatic factors such as temperature, precipitation, and wind significantly influence the paths of contaminate dispersal. In arid/semi-arid regions, high temperature forms fine-grained efflorescence salts on tailings or exposed surficial mines which are carried away by strong winds/water and contaminates the surroundings. In wet regions, the leaching of heavy metals from both tailings and overburden rocks sulfides results in environmental contamination. The application of impermeable layers is highly recommended. The climatic factors (temperature, wind, and precipitation) significantly control the dispersal and mobility of heavy metals in Cu mining region. The implementation of waste management policies and pollution control technologies is recommended after considering the climatic factors.

Selective Separation of Arsenic from Lead Smelter Flue Dust by Alkaline Pressure Oxidative Leaching

This study investigated the feasibility of using an alkaline pressure oxidative leaching process to treat lead smelter flue dust containing extremely high levels of arsenic with the aim of achieving the selective separation of arsenic. The effects of different parameters including NaOH concentration, oxygen partial pressure, liquid-to-solid ratio, temperature, and time for the extraction of arsenic were investigated based on thermodynamic calculation. The results indicated that the leaching efficiency of arsenic reached 95.6% under the optimized leaching conditions: 80 g/L of NaOH concentration, 1.0 MPa of oxygen partial pressure, 8 mL/g of liquid-to-solid ratio, 120 °C of temperature, 2.0 h of time. Meanwhile, the leaching efficiencies of antimony, cadmium, indium and lead were less than 4.0%, basically achieving the selective separation of arsenic from lead smelter flue dust. More than 99.0% of arsenic was converted into calcium arsenate product and thus separated from the leach solution by a causticization process with CaO after other metal impurities were removed from the solution with the addition of Na2S. The optimized causticization conditions were established as: 4.0 of the mole ratio of calcium to arsenic, temperature of 80 °C, reaction time of 2.0 h. The resulting product of calcium arsenate may be used for producing metallic arsenic.