Two-step biohydrometallurgical technology of copper-zinc concentrate processing as an opportunity to reduce negative impacts on the environment

Understanding positive contributions to sustainability. A systematic review

Current approaches for measuring and assessing contributions of companies and their products to sustainability largely focus on reducing negative impacts. However, becoming "less bad" still means having adverse impacts on the environment. Various authors have therefore called for investigating how positive contributions can be made to further sustainable development. This systematic literature review explores how positive contributions to sustainability have been discussed in the environmental management literature dealing with sustainability performance measurement and assessment. Our review of 328 publications reveals an understanding of positive contributions to sustainability that is a mostly implicit or vague use. Inductive analysis, however, reveals three distinct understandings - an operationalization, a stakeholder and a transformation perspective - each of which is embedded in a different theoretical frame, namely decision, stakeholder and transition theory. These perspectives have so far been discussed separately in the literature. By drawing on theoretical foundations of performance measurement, we propose an integrated understanding of positive contributions to sustainability: A positive sustainability contribution has the goal of bringing about a sustainability transformation, considers the environmental, economic and social context through stakeholder participation, and is operationalized to facilitate decision-making and the implementation of effective sustainability measures. A clear definition is of key importance for both research and practice to both reduce negative and increase positive contributions to sustainable development.

Bulk flotation followed by selective leaching with biogenic ferric iron is a promising solution for eco-friendly processing of complex sulfidic ores

The development of cost-effective environmentally friendly technologies is of current importance for the intensification of metal recovery. Here, we propose a new direction in the use of a two-step process for the treatment of complex sulfidic ores. In the first step, ore flotation allows the obtainment of a bulk copper-zinc concentrate and low-toxicity waste. In the second step, zinc is selectively extracted by chemical leaching with a biogenic ferric iron solution, while copper is accumulated in the leach residue. Importantly, the efficiency of this step depends on the chemical and mineralogical composition of polymetallic concentrates. Four samples of the copper-zinc sulfide concentrate with various contents of copper and zinc have been leached in multiple cycles at 80 °C, 10% of pulp density, and pH 1.3-1.5. The leaching of the concentrate that contained 14.7% of copper and 5.0% of zinc for three leaching cycles (total duration, 5.75 h) allowed the obtainment of a leach residue containing 15.3 and 0.14% of copper and zinc, respectively. At the same time, the leaching of another concentrate that contained 19.2% of copper and 2.64% of zinc after one leaching cycle (total duration, 0.67 h) led to copper and zinc contents of 19.1 and 0.72%, respectively, in the leach residue. Therefore, the treatment of only two concentrates allowed the obtainment of high-grade copper concentrates containing low amounts of zinc. Ferric iron, the oxidant of sulfide minerals, was fully regenerated in leachate within 30-35 h at 40 °C using an acidophilic microbial community dominated by Leptospirillum ferriphilum, Sulfobacillus spp., and Ferroplasma acidiphilum. The loss of copper and zinc in solid waste of the bioregeneration did not exceed 1.0% and 1.8%, respectively. The proposed flowsheet of bulk concentrate processing has several important advantages over conventional technologies, including short duration of leaching, formation of low-toxicity solid waste, and pyrometallurgical processing of only high-grade copper concentrates. This approach can be a promising solution for the efficient processing of polymetallic bulk concentrates.

A Case of Predominance of Alicyclobacillus tolerans in Microbial Community during Bioleaching of Pentlandite-Chalcopyrite Concentrate

Bacterial isolates assigned to the species Alicyclobacillus tolerans, which occupies an intermediate position between an organotrophic genus Alicyclobacillus and mixotrophic genus Sulfobacillus, were revealed as members of the acidophilic chemolithotrophic community during stirred-tank bioleaching of violarite–pentlandite–chalcopyrite concentrate at 40 °C. Surprisingly, this species succeeded more common iron-oxidizing community members after a series of bioleaching processes in bioreactors. The possibility of mixotrophic and organoheterotrophic growth of Al. tolerans, tolerance to low pH values (1.0–1.15), as well as preservation of cells via sporulation under unfavorable conditions, may explain its key role in the bioleaching of the copper–nickel bulk concentrate. Isolation of two other sulfur-oxidizing pure cultures dominating the microbial community, together with their phylogenetic characterization, allowed the assignment of these isolates to the species Acidithiobacillus caldus. This and other studies of acidophilic microbial communities are important for the development and intensification of the bioleaching processes, including a biobeneficiation approach previously proposed by us.

Effect of Temperature on Biobeneficiation of Bulk Copper-Nickel Concentrate with Thermoacidophilic Microbial Communities

Bioleaching of the bulk copper–nickel sulfide concentrate was proposed as a method to remove nickel from it and to obtain a concentrate containing copper as chalcopyrite. This approach is based on the different refractoriness of sulfide minerals in ferric sulfate solutions and oxidation by acidophilic microorganisms. The bulk concentrate contained 10.8% copper in the form of chalcopyrite (CuFeS2) and 7.2% nickel that occurred in pentlandite ((Ni,Fe)9S8) and violarite (FeNi2S4). Three microbial communities grown at 35, 40, and 50 °C were used for bioleaching. The microbial community at 40 °C was the most diverse in the genus and species composition. At all temperatures of the process, the key roles in bioleaching belonged to mixotrophic and heterotrophic acidophiles. The highest levels of nickel leaching of 97.2 and 96.3% were observed in the case of communities growing at 40 and 50 °C, respectively. At the same time, the bioleach residue, which could be characterized as a marketable high-grade copper (chalcopyrite) concentrate, was obtained only at 40 °C. This solid contained 15.6% copper and 0.54% nickel. Thus, the biobeneficiation of bulk sulfide concentrates can be a promising field of biohydrometallurgy.

Optimization of zinc bioleaching from paint sludge using Acidithiobacillus thiooxidans based on response surface methodology

The reduction of zinc metal in the paint sludge, a hazardous waste, was investigated using Acidithiobacillus thiooxidans by a two-stage bioleaching process. This process was performed using the response surface methodology (RSM) method based on the central composite design (CCD). Four variables, a temperature range of 32-34.5-37 °C, rotation speed of shaker 120-150-180 rpm, pH of 4.2-3.2-2.2, and particle sizes of 1-2-3 mm, were used to optimize the experiments. The results showed that with a constant pulp density of 10 g/L at 32 °C, shaker speed of 120 rpm, a particle size of 1 mm and pH of 4.2, the highest removal predicted by the software (Design Expert version 11) was 22.89%. Repeating the experiments confirmed a decrease in zinc to the nearest predicted point. According to the ANOVA result, the rotation speed of the shaker has the greatest effect on the bioleaching process, followed by the two variables of the rotation speed of shaker and pH together affects. After the bioleaching process, energy dispersive X-ray (EDX) and mapping analysis showed quantitative changes in the chemical composition of the paint sludge, and morphological changes of texture were confirmed by scanning electron microscopy (SEM).

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.

Sequential Bioleaching of Pyritic Tailings and Ferric Leaching of Nonferrous Slags as a Method for Metal Recovery from Mining and Metallurgical Wastes

In this work, we proposed a method for biohydrometallurgical processing of mining (old pyritic flotation tailings) and metallurgical (slag) wastes to recover gold and other nonferrous metals. Since this processing allows the removal of toxic metals or at least decreases their content in the solids, this approach may reduce the negative environmental impacts of such waste. The proposed process was based on pyritic tailings’ bioleaching to recover metals and produce leach liquor containing a strong oxidizing agent (ferric sulfate) to dissolve nonferrous metal from slag. This approach also allows us to increase concentrations of nonferrous metals in the pregnant leach solution after pyritic waste bioleaching to allow efficient extraction. The old pyritic tailings were previously leached with 0.25% sulfuric acid for 10 min to remove soluble metal sulfates. As a result, 36% of copper and 35% of zinc were extracted. After 12 days of bioleaching with a microbial consortium containing Leptospirillum spp., Sulfobacillus spp., Ferroplasma spp., and Acidithiobacillus spp. at 35 °C, the total recovery of metals from pyritic tailings reached 68% for copper and 77% for zinc; and subsequent cyanidation allowed 92% recovery of gold. Ferric leaching of two types of slag at 70 °C with the leachate obtained during bioleaching of the tailings and containing 15 g/L of Fe3+ allowed 88.9 and 43.4% recovery of copper and zinc, respectively, from copper slag within 150 min. Meanwhile, 91.5% of copper, 84.1% of nickel, and 70.2% of cobalt were extracted from copper–nickel slag within 120 min under the same conditions.

Understanding Stress Response to High-Arsenic Gold-Bearing Sulfide Concentrate in Extremely Metal-Resistant Acidophile Sulfobacillus thermotolerans

Biooxidation of gold-bearing arsenopyrite concentrates, using acidophilic microbial communities, is among the largest commercial biohydrometallurgical processes. However, molecular mechanisms of microbial responses to sulfide raw materials have not been widely studied. The goal of this research was to gain insight into the defense strategies of the acidophilic bacterium Sulfobacillus thermotolerans, which dominates microbial communities functioning in industrial biooxidation processes at >35 °C, against the toxic effect of the high-arsenic gold-bearing sulfide concentrate. In addition to extreme metal resistance, this acidophile proved to be one of the most As-tolerant microorganisms. Comparative proteomic analysis indicated that 30 out of 33 differentially expressed proteins were upregulated in response to the ore concentrate, while the synthesis level of the functional proteins required for cell survival was not negatively affected. Despite a high level of cellular metal(loid) accumulation, no specific metal(loid)-resistant systems were regulated. Instead, several proteins involved in the metabolic pathways and stress response, including MBL fold metallo-hydrolase, sulfide:quinone oxidoreductase, and GroEL chaperonin, may play crucial roles in resistance to the sulfide ore concentrate and arsenic, in particular. This study provides the first data on the microbial responses to sulfide ore concentrates and advances our understanding of defense mechanisms against toxic compounds in acidophiles.

Distinct Roles of Acidophiles in Complete Oxidation of High-Sulfur Ferric Leach Product of Zinc Sulfide Concentrate

A two-step process, which involved ferric leaching with biologically generated solution and subsequent biooxidation with the microbial community, has been previously proposed for the processing of low-grade zinc sulfide concentrates. In this study, we carried out the process of complete biological oxidation of the product of ferric leaching of the zinc concentrate, which contained 9% of sphalerite, 5% of chalcopyrite, and 29.7% of elemental sulfur. After 21 days of biooxidation at 40°C, sphalerite and chalcopyrite oxidation reached 99 and 69%, respectively, while the level of elemental sulfur oxidation was 97%. The biooxidation residue could be considered a waste product that is inert under aerobic conditions. The results of this study showed that zinc sulfide concentrate processing using a two-step treatment is efficient and promising. The microbial community, which developed during biooxidation, was dominated by Acidithiobacillus caldus, Leptospirillum ferriphilum, Ferroplasma acidiphilum, Sulfobacillus thermotolerans, S. thermosulfidooxidans, and Cuniculiplasma sp. At the same time, F. acidiphilum and A. caldus played crucial roles in the oxidation of sulfide minerals and elemental sulfur, respectively. The addition of L. ferriphilum to A. caldus during biooxidation of the ferric leach product proved to inhibit elemental sulfur oxidation.

Life cycle analysis of copper-gold-lead-silver-zinc beneficiation process

Gold, silver, lead, zinc, and copper are valuable non-ferrous metals that paved the way for modern civilisation. However, the environmental impacts from their beneficiation stage was always overlooked. This paper analysed the life cycle environmental impacts from the beneficiation process of gold-silver-lead-zinc-copper combined production. The analysis is conducted by utilising the SimaPro software version 8.5. The life cycle assessment methodologies followed are the International Reference Life Cycle Data System (ILCD) method, the IMPACT 2002+ method, and the Cumulative Energy Demand Method (CED). The most significant impact categories are ecotoxicity, climate change, human toxicity, eutrophication, acidification, and ozone depletion among nearly 15 impact categories which are assessed in this study. The analysis results from the ILCD method indicate that there is a noteworthy impact on ionising radiation caused by the beneficiation process. Out of the five metals considered, gold and silver beneficiation impacts the most while lead‑zinc beneficiation impacts the least. Gold beneficiation has most impacts on the category of climate change and ecosystems. Other major impact categories are ionising radiation, terrestrial eutrophication, photochemical ozone formation, human toxicity, and acidification. The IMPACT 2002+ method shows the overall impact is on ecosystem quality and human health from this combined beneficiation process, dominantly from gold‑silver beneficiation. The life-cycle inventory results show that the blasting process and the amount of electricity consumption in the beneficiation process contribute to cause significant amount of environmental impacts. The comparative impact results are presented and discussed in detail in this paper. Sensitivity analyses are presented based on various electricity grid-mix scenarios and energy-mix scenarios, and the results suggest that electricity grid mix has a dominant effect over the fossil-fuel mix. This paper also highlights the potential steps which could cut down the environmental effects by integrating renewable-energy technologies.

Environmentally friendly recovery of valuable metals from spent coin cells through two-step bioleaching using Acidithiobacillus thiooxidans

The technology for recycling the spent coin cells is pressing needed due to a large amount of generated spent coin cells. However, there is little information about the recycling technology of spent coin cells. In this work, a two-step bioleaching method for recovery of metals from spent coin cells by Acidithiobacillus thiooxidans is performed for the first time. In this regard, the growth characteristics of A. thiooxidans was investigated in pure culture and during the two-step bioleaching approach. The highest recovery of Li, Co and Mn was achieved at a pulp density of 30 g L^-1, in values of 99%, 60%, and 20%, respectively. The structural analyzes confirmed the progress of bioleaching process. In addition, the kinetics models showed that the chemical reaction was the rate-controlling step of the two-step bioleaching of spent coin cells. The comparative results between bioleaching and chemical leaching showed that Acidithiobacillus thiooxidans can enhance the leaching of metals. Toxicity characteristic leaching procedure of the spent coin cells powder demonstrated that the bioleached residue met the environmental limitations for safe disposal. In fact, bioleaching is an effective and promising route to reduce the environmental hazard of spent coin cells.