Prospective (Bio)leaching of Historical Copper Slags as an Alternative to Their Disposal

A review on e-waste contamination, toxicity, and sustainable clean-up approaches for its management

Ecosystems and human health are being negatively impacted by the growing problem of electrical waste, especially in developing countries. E-waste poses a significant risk to ecological systems because it can release a variety of hazardous substances into the environment, containing polybrominated diphenyl ethers and heavy metals, brominated flame retardants, polychlorinated dibenzofurans and polycyclic aromatic hydrocarbons, and dioxins. This review article provides a critical assessment of the toxicological consequences of e-waste on ecosystems and human health and data analyses from scientific journals and grey literature on metals, BFRs, PBDEs, PCDFs, and PAHs in several environmental compartments of commercial significance in informal electronic trash recycling. The currently available techniques and tools employed for treating e-waste are sustainable techniques such as bioremediation, chemical leaching, biological leaching, and pyrometallurgy have been also discussed along with the necessity of implementing strong legislation to address the issue of unregulated exports of electronic trash in recycling practices. Despite the ongoing hurdles, implementing environmentally sustainable recycling methods have the potential to address the detrimental impacts of e-waste and foster positive economic development.

Pressure Leaching of Copper Slag Flotation Tailings in Oxygenated Sulfuric Acid Media

In this study, a hydrometallurgical method for the recovery of copper, cobalt, and zinc from copper slag flotation tailings (SFT) was investigated. SFT contains large amounts of valuable metallic compounds, such as copper, cobalt, and zinc. A representative SFT sample containing 0.50% Cu, 0.148% Co, 3.93% Zn, and 39.50% Fe was used in experimental studies. High-pressure oxidative acid leaching of SFT was carried out to assess the effects of sulfuric acid concentration, oxygen partial pressure, reaction time, solid/liquid ratio, and temperature on the extraction of copper, cobalt, zinc, and iron. The dissolution of metals from the SFT sample increased with temperature and sulfuric acid concentration. However, high acid concentrations and high solid/liquid (S/L) ratios led to gel formation that caused filtration problems and inhibited metal dissolution. The optimum leaching conditions were found to be a leaching time of 90 min, an acid concentration of 250 kg/t, a temperature of 220 °C, an S/L ratio of 1:5, and an oxygen partial pressure of 0.7 MPa. Under these conditions, 93.1 ± 1.1% Cu, 96.3 ± 1.8% Co, and 92.3 ± 1.7% Zn were extracted. Iron dissolution was only 0.5 ± 0.1%. This hydrometallurgical process almost completely recovers valuable metals. In particular, cobalt, which is of great importance in the production of lithium-ion batteries, has been declared a critical metal by the United States, Canada, and the EU and was taken into solution with very high extraction efficiency (>95%). Additionally, oxygen partial pressure enhanced copper, cobalt, and zinc dissolution. When O₂ was not introduced into the leaching system, the extraction efficiencies of Co, Cu, and Zn were approximately 24.5, 5.3, and 26.3%, respectively, after 2 h of leaching treatment.

The Effect of Zn Content and Granulation Temperature on Zn Leaching in an Fe-Saturated (FeXZn(1−X))2SiO4 System

The zinc in the fayalite slag of copper smelters, in which Zn-containing raw materials are used, is mainly found to be in oxidic phases, such as glassy iron silicate. During the slag water granulation process, the molten slag is heated, whereby the granulated slag achieves varying granulation temperatures. Therefore, in this study, we aimed to characterize and assess the leaching behavior of a synthesized Fe-saturated (FeX,Zn(1−X))2SiO4 system to understand the dependance of the zinc leaching behavior on the parameters of the ZnO content (1–10 wt.%) and granulation temperature (1300 or 1400 °C). It was found that the Zn leaching increased with the increasing Zn content and granulation temperature, using both batch and static pH leaching methods. Zn leaching was further increased at pH 5 using diluted nitric acid under oxidation conditions. Among the oxides in the samples—fayalite, spinel, and glass—glass was found to contribute to Zn leaching, owing to its weathering during pH-titration.

Synthetic biology approaches to copper remediation: bioleaching, accumulation and recycling

One of the current aims of synthetic biology is the development of novel microorganisms that can mine economically important elements from the environment or remediate toxic waste compounds. Copper, in particular, is a high-priority target for bioremediation owing to its extensive use in the food, metal and electronic industries and its resulting common presence as an environmental pollutant. Even though microbe-aided copper biomining is a mature technology, its application to waste treatment and remediation of contaminated sites still requires further research and development. Crucially, any engineered copper-remediating chassis must survive in copper-rich environments and adapt to copper toxicity; they also require bespoke adaptations to specifically extract copper and safely accumulate it as a human-recoverable deposit to enable biorecycling. Here, we review current strategies in copper bioremediation, biomining and biorecycling, as well as strategies that extant bacteria use to enhance copper tolerance, accumulation and mineralization in the native environment. By describing the existing toolbox of copper homeostasis proteins from naturally occurring bacteria, we show how these modular systems can be exploited through synthetic biology to enhance the properties of engineered microbes for biotechnological copper recovery applications.

Value-added strategies for the sustainable handling, disposal, or value-added use of copper smelter and refinery wastes

Metallurgical plants constituting of smelters and refineries recover metals (i.e., copper) from mineral deposits. Copper production generates several waste streams of which slag, sludge and dust are generated in the largest quantities. The need to eliminate or at least reduce their adverse effects on the environment call for developing methods for recovering valuable components such as copper, zinc and iron through their selective separation from toxic components present in the waste (mainly arsenic and lead). This can be achieved through hydrometallurgical methods (leaching with organic and inorganic media), techniques facilitating mobility of elements (roasting with leaching) and biological processes (bioleaching). The valorization of metallurgical waste as a source of fertilizer micronutrients can be a sustainable and value-added direction of its management. This review presents ways of useful-metals recovery from the copper smelter and refinery wastes, including selective separation of valuable metals. The novelty of this review is a demonstration of the application potential of recovered components from metallurgical waste in the agricultural sector.

Editorial for Special Issue “Metallurgical Slags”

Exploitation and further processing of mineral resources are of great importance for modern society [...]

Recovery of Iron from Copper Slag Using Coal-Based Direct Reduction: Reduction Characteristics and Kinetics

The Fe3O4 and Fe2SiO4 in copper slag were successfully reduced to metallic iron by coal-based direct reduction. Under the best reduction conditions of 1300 °C reduction temperature, 30 min reduction time, 35 wt.% coal dosage, and 20 wt.% CaO dosage (0.75 binary basicity), the Fe grade of obtained iron concentration achieved 91.55%, and the Fe recovery was 98.13%. The kinetic studies on reduction indicated that the reduction of copper slag was controlled by the interfacial reaction and carbon gasification at 1050 °C. When at a higher reduction temperature, the copper slag reduction was controlled by the diffusion of the gas. The integral kinetics model research illustrated that the reaction activation energy increased as the reduction of copper slag proceeded. The early reduction of Fe3O4 needed a low reaction activation energy. The subsequent reduction of Fe2SiO4 needed higher reaction activation energy compared with that of Fe3O4 reduction.

Efficiency of Chemical and Biological Leaching of Copper Slag for the Recovery of Metals and Valorisation of the Leach Residue as Raw Material in Cement Production

Copper slags produced in vast quantities in smelting operations could be considered as secondary material sources instead of stockpiling them in landfills. This study investigates the recovery of valuable metals from copper slag and the valorisation of the leach residue as construction material in line with the principles of a circular economy. By taking into account that the environmental characterization of the as-received copper slag did not allow its disposal in landfills without prior treatment, chemical and biological leaching were tested for the recovery of metals. Pre-treatment with acids, namely HNO3 and H2SO4, resulted in the extraction of several target metals and the production of an almost inert waste. Despite the clearly better oxidative conditions prevailing in the bioleaching reactors, chemical leaching resulted in the higher dissolution of Cu (71% vs. 51%), Co (70% vs. 36%), and Zn (65% vs. 44%). The acid consumption was much lower during the bioleaching experiments compared to the chemical leaching. The bioleach residue was suitable for its use as supplementary cementitious material, showing a better performance than the reference sample without causing any detrimental effects to the calcium aluminate cement (CAC) quality. The complete valorisation of copper slags is expected to improve the economics of the process, by avoiding landfill costs and producing saleable products with high added value.

Processing of Waste Copper Converter Slag Using Organic Acids for Extraction of Copper, Nickel, and Cobalt

An innovative, economical, and environmentally sound hydrometallurgical process has been proposed for recovering Cu, Ni, and Co from copper-rich converter slag by organic acids. In the leaching experiments, the effects of organic acid concentrations, pulp density, temperature, and time were investigated. Optimum recovery of 99.1% Cu, 89.2% Ni, 94% Co, and 99.2% Fe was achieved in 9–10 h at 308 K (35 °C) temperature and 15% pulp density with 2 N citric acid using <45 µm particles. Pourbaix diagrams of metal-water-citrate systems were supplemented to examine solubility of ligands at the desired conditions. Furthermore, the leaching mechanism was based on the SEM-EDS (energy-dispersive X-ray spectroscopy) and XRD characterization as well as the leaching results obtained.