Reviving Riches: Unleashing Critical Minerals from Copper Smelter Slag Through Hybrid Bioleaching Approach

Abstract

Due to the emission of hazardous chemicals and heat, the traditional smelting method used to extract critical minerals from ore and mine slag/tailings is considered bad for the environment. An environmentally friendly procedure that can stabilize sulfur emissions from mine waste without endangering the environment is bioleaching. In the present study, sequential oxidative (Oxi) and reductive (Red) bioleaching of acid-pretreated copper smelter slag using iron-oxidizing/reducing Acidithiobacillus ferrooxidans was applied to investigate critical minerals’ recovery for the dissolution of copper smelter slag. In this batch flask experiment, up to 55% Cu was recovered on day 11 during the Oxi stage, which increased to 80% during the Red stage on day 20. A sequential oxidative and reductive bioleaching of an acid-pretreated copper smelter slag at pH (1.8) and 30 °C positively affects the extraction of Cu (80%), Zn (77.1%), and Al (65.3%). In contrast to the aerobic bioleaching experiment, the reduction of Fe³⁺ iron under anaerobic conditions resulted in a more significant release of Fe²⁺ and sulfate, limiting the development of jarosite, surface passivation, and the subsequent loss of metal recovery due to co-precipitation with Fe³⁺. Overall, the Oxi-Red bioleaching process combined with acid pretreatment showed promising results toward creating a method for recovering valuable metals from metallurgical waste that is economical and environmentally beneficial.

1. Introduction

Massive amounts of metallurgical slag, acid mine drainage (AMD), and sludge are produced by industrial metallurgical processes, representing long-term environmental risks. Mine tailings come in various forms depending on their mineralogical makeup, the type of mining, and the processing techniques used. Millions of tons of mineral-containing ores are processed each year by the mining industry, and approximately 95% of this waste is disposed of as mine waste (tailings and/or slag). This waste may contain precious metals like gold (Au) and silver (Ag), as well as critical minerals like copper (Cu), iron (Fe), nickel (Ni), and zinc (Zn), sometimes in relatively high concentrations [1]. The exposure of mineral waste to water and oxygen accelerates the microbially catalyzed oxidative decomposition of the sulfide crystals and minerals.

Because of the rising demand for minerals and the depletion of high-grade ore deposits, it is anticipated that leftovers from previous mining operations may be able to replace some of the metal that is now being extracted from ores. However, rare earth elements (REEs), typically not valued for extraction, may be present in minimal quantities in mine tailings, ultimately increasing process costs. Through secondary waste resources, the scientific community has worked for many decades to create a zero-waste strategy, focusing on the simultaneous separation of valuable metals from waste resources using eco-friendly technology to increase revenues for mining stakeholders. Therefore, it is now crucial for maintaining mineral reserves and safeguarding the environment to transform solid waste from one form to another within the same valorization unit or by using various technical apparatus. Due to their use in super-alloys and rechargeable batteries, essential minerals are in greater global demand. The recovery of copper (Cu), aluminum (Al), nickel (Ni), cobalt (Co), and zinc (Zn) from metallurgical slag can be accomplished utilizing hydrometallurgical and flotation technologies, since 2.2 tons of copper slag are produced for every ton of copper metal throughout the smelting and converting stages of copper processing [2].

Due to the complicated mineralogical characteristics of copper slag, processing copper slag by hydrometallurgy is ineffective [3]. For instance, silica gel produced by concentrated acid leaching causes crude accumulation, increases leach liquor viscosity, and hinders solid filtration during solvent-based metal extraction [3]. Currently, pyrometallurgical methods of reducing copper slag at high temperatures (e.g., carbothermal reduction [4] may be the most effective approach for extracting metals from copper slag, but at a significant environmental cost. The use of microbes to extract metals is gaining popularity in many fields due to the limitations of conventional technology [4,5]. In a classical way, bioleaching processes are based on the activity of extreme acidophiles such as Acidithiobacillus spp. that convert insoluble metal sulfides via biochemical oxidation reactions into water-soluble metal sulfates. It is thus mostly limited to reduced (sulfidic) materials. Potential microbial species, such as A. ferrooxidans, also catalyze the breakdown of sulfide minerals under reductive conditions [5,6,7].

Most of the metals of interest in metallurgical slag are in oxide mineral forms, making them more accessible for reductive rather than oxidative bioleaching [8]. Reductive bioleaching facilitates the reduction of metal ions, including those in refractory minerals, to their metallic or less oxidized states. The formation of jarosite layers on the mineral surface may also limit the rate of reaction by hindering the diffusion of soluble reactants or products through this layer. Reductive dissolution with A. ferrooxidans and A. thiooxidans was recently shown to hinder jarosite development and surface passivation due to the reverse gear of Fe³⁺ iron and was also shown to improve metal leaching efficiency [9,10].

In this study, a hybrid strategy was used that sequentially combined oxidative (Oxi) and reductive (Red) bioleaching, where the impact of each stage was evaluated to increase the extraction efficiency of crucial minerals from acid-pretreated slag at a 15% (w/v) pulp loading rate at a shake flask level. Elemental sulfur (S⁰), which serves as an electron contributor for acidophile A. ferrooxidans, was added during the study. Bacteria produce H₂SO₄ from S⁰ and Fe²⁺ under aerobic conditions and may convert S₀ and H₂SO₄ into H₂S under the anaerobic stage, where electrons (e⁻) are transferred to ferric iron (Fe³⁺), which subsequently undergoes reduction to ferrous iron (Fe²⁺). In order to ascertain the binding of metals to minerals like chalcopyrite, fayalite, and hematite prior to the leaching procedure, slag was examined using microwave plasma atomic emission spectroscopy (MP-AES). Slag was also analyzed with scanning electron microscopy (FEI QEMSCAN 650F) for elemental mapping and mineralogical quantification.

2. Materials and Methods

2.1. Metallurgical Slag Material

The metallurgical slag was obtained from a Quebec copper smelter. The slag was filtered to eliminate extra water before being dried at 60 °C for 48 h. An Agilent 4100 Microwave Plasma Atomic Emission Spectrometer (Agilent Technologies, Melbourne, Australia) equipped with a standard glass concentric nebulizer and cyclonic spray chamber (Agilent Technologies, Melbourne, Australia) was used to analyze the heavy metals. The sample’s mineralogical composition, shape, grain size, and fractures were further examined using a quantitative evaluation of the minerals by scanning electron microscopy (FEI QEMSCAN 650F). This method is quite sensitive and can find mineral phases even in small amounts. Finding the best metal recovery procedure requires combining these two techniques.

2.2. Pretreatment of Metallurgical Slag

In order to extract valuable metals from metallurgical slag, a pretreatment procedure was carried out utilizing a mixture of 2% (w/v) sulfuric acid, and dried slag was selected to work with a particle size under 1.75~75 μm in a ratio of 1:5. The complex mineral structure was successfully altered and to make it easier to release the desired metals, autoclaving the mixture at 134 °C and 18 psi for 1 h was performed. The metal composition of the resultant leachate was examined to assess the efficiency of the pretreatment procedure. The pretreated slag slurry was subjected to an Oxi-Red bioleaching process to remove more precious metals.

2.3. Bacterial Adaptation and Bioleaching Process

The acidophilic bacterium A. ferrooxidans (DSM 14882) was cultivated in a 9K medium (without FeSO₄) with 1.0% elemental sulfur and 2% slag to acclimatize to the heavy metal environment/shock. The resultant preculture was employed as the inoculum for bioleaching experiments after 14 days of incubation. On the acid-pretreated slurry, sequential oxidative (oxi)–reductive (Red) bioleaching was carried out in 500 mL shake flasks with 290 mL of bioleaching medium. The medium comprised 45 g of acid-treated slurry, a 9K medium enriched with trace elements, elemental sulfur (1.0% w/v), and 10% (v/v) sulfur-adapted inoculum. Incubation was conducted at 30 °C with continuous stirring at 170 rpm. Oxi bioleaching was carried out for 11 days until the cell number reached 1.09 × 10⁸ cells/mL followed by Red bioleaching with sparging of the gas mixture with 10% CO₂ and 90% N₂ for the rest of the process. Samples were withdrawn at regular intervals until day 28 to analyze the mineral slurry and leachate, which were separated by centrifugation. Abiotic controls were performed in parallel.

2.4. Analytical Methods

Total iron (Fe) and ferrous iron (Fe²⁺) concentrations were measured using the FerroVer and 1,10-phenanthroline colorimetric methods, respectively. The amount of ferric iron (Fe³⁺) was calculated by deducting the total Fe from Fe²⁺. The total sulfate and metal contents were determined using ion chromatography and MP-AES, respectively. Electrodes were used to measure physico-chemical characteristics such as pH, redox potentials, and conductivity. The FEI QEMSCAN 650F used line scan and particle mapping measurement modes with a 25 kV acceleration voltage to conduct the mineralogical investigation.

3. Results and Discussion

3.1. Chemical and Mineralogical Composition of the Slag

The mineral grain size distribution is a crucial variable in the bioleaching process. The slag grain size range investigated in the present study was roughly 1.75 µm to 75 µm. The slag was primarily dominated by Fe (28%), followed by Zn (2.1%), Al (0.6%), and Cu (0.4%). According to FEI QEMSCAN 650F findings, the slag contained a variety of minerals, including hematite (Fe₂O₃), fayalite (Fe₂(SiO₄)), Cu native, Cu sulfide (Cu₂S), chalcopyrite (CuFeS₂), Cu native, magnetite Cr, chromite, Zn sulfide/sulfate, ilmenite, feldspar, and amphibole (Figure 1). Other minerals were found in traces, such as Pb sulfide/sulfate, silver, and Si-Al glass. The QEMSCAN investigation also displayed the crystal structures of the minerals and confirmed their existence. Cu (native) was present in the free metallic state; chalcopyrite had a compact massive tetragonal crystal structure; hematite was an irregular crystal with sharp edges; and fayalite had a light gray strip orthorhombic crystal structure. The milky white-colored Pb sulfide/sulfate minerals attach to the surface of chalcopyrite and a dark gray-colored glassy mineral exists in the form of a strip (Figure 1).

3.2. Variation in Physico-Chemical Parameters During Bioleaching Process

The initial pH was 1.8 and raised to 4.58 on the following days during the Oxi bioleaching process, which was maintained by feeding 0.5 M of H₂SO₄ at regular intervals of 3 days until the pH became stable. The interaction of metal sulfides and oxides with H⁺ ions created by the dissociation of H₂SO₄ led to the dissolution of metals, which caused the pH to rise, indicating that the slag was consuming more acid (Figure 2a). The pH dropped to 2.8 in the presence of A. ferrooxidans after the third day of bioleaching, indicating bacterial proliferation and H₂SO₄ production compared to the abiotic control, where the pH was raised to ~4.5 due to the lack of microbial activity and did not show any significant pH drop until day 28. After day 11, the experiment shifted to a Red stage and the pH was maintained throughout the bioleaching procedure (Figure 2a). Redox potential was initially low (362 mV), which increased gradually to 725 mV until day 11 during the Oxi stage in the presence of A. ferrooxidans. After that, redox potential declined to 570 mV due to the lack of oxygen during the Red stage. The redox potential in the abiotic control was attained as less than 500 mV (Figure 2b). Mineral dissolution and metal release into the media were responsible for the redox potential shifts. Notably, the Fe³⁺/Fe²⁺ ratio was principally responsible for the considerable variations in redox potential, and the total oxidation of Fe²⁺ resulted in a high redox potential and a low Fe²⁺/Fe³⁺ ratio and vice versa. Additionally, active S⁰ use encouraged a high redox potential in the media [11,12]. The heterodisulfide reductase (Hdr), which is tethered to the cell’s inner membrane and contains an active site in the cytoplasm, aids in the oxidation of S⁰ [13,14]. An incremental rise in sulfate concentration, which over 11 days helped to lower the pH of the leaching solution, served as evidence of S⁰ oxidation by A. ferrooxidans (Figure 2c). There was also a correlation between viable bacterial cells and the dissolution of iron minerals. Acidophiles need time to cling to the mineral surface and acclimate to their new surroundings [15,16,17]. The bacterial size was higher (5.24 × 10⁹ cells/mL) during Oxi bioleaching due to favorable conditions, while bacterial count declined regressively during Red bioleaching due to the lack of O₂, i.e., ~3.21 × 10⁷ cells/mL.

3.3. Bioprocessing of Copper Smelter Slag and Recovery of Critical Minerals

Due to the dissolving of fayalite and hematite during acid pretreatment (APT), several metals, namely Zn (67 mg/L), Al (45 mg/L), and Cu (89.7 mg/L), were leached out from metallurgical slag, although in minimal concentrations, while the removal of Fe concentration was high (700 mg/L), as shown in Figure 3a,b. These metals were recovered initially, and the acid-treated dried slurry was then subjected to bioleaching with A. ferrooxidans.

During the Oxi-Red bioleaching process, different metals exhibited varying leaching behaviors. Within 28 days of bioleaching, the highest total iron concentration was reached at 2340 (mg/L). There was a significant decrease in ferrous iron (Fe²⁺) concentration under oxidative conditions. The reductive dissolution of Fe-containing minerals (e.g., hematite) was accompanied by increasing concentrations of soluble Fe²⁺ iron and other metals from iron minerals (Figure 3a).

The rate of dissolution of Zn sulfides with A. ferrooxidans was higher than in an abiotic control. In the slag, 77.1% of the total Zn sulfides were released at their maximum (2430 mg/L) during the Oxi stage; the Zn concentration in the leachate did not significantly change throughout the Red stage (Figure 3a,c). Zinc sulfides (e.g., sphalerite) are generally more reactive and soluble in acidic environments than zinc oxides (e.g., zincite); therefore, a higher extraction of Zn in the Oxi stage was expected. Acidithiobacillus spp. can generate sulfuric acid from sulfide oxidation and create an acidic environment. This acid promotes the dissolution of zinc sulfides, leading to the release of zinc ions into the solution. The reaction between zinc sulfides and acid generates sulfate ions, which also contribute to the leaching process. Greater concentrations of Al were also found in the copper smelter slag subjected to bacterially catalyzed Oxi-Red bioleaching as compared to the abiotic control. The concentration of soluble Al increased progressively as bioleaching progressed, reaching 450 mg/L (Oxi stage) and 587 mg/L (Red stage), as shown in Figure 3b,c. The total amount of Cu bioleached from copper slag by oxidative dissolution was 55%, which increased to 480 mg/L (corresponding to 80% extraction efficiency) under reductive dissolution after 20 days of bioleaching (Figure 3b,c).

High bioleaching extraction rates were attained for Cu (80%), Zn (77.1%), and Al (65.3%) after 28 days at a high 15% pulp density. In a study conducted by Kinnunen et al. [18], where oxidative bioleaching was performed in flasks for 21 days using a mixed culture of seven microbial strains, a low pulp density of 1% (w/v) led to leaching rates of Zn (41%), Cu (71%), and Co (49%), while at a higher pulp density of 5% (w/v), metal recovery decreased to Cu (61%), Co (21%), and Zn (17%). Schippers [19] conducted two separate experiments using a mixed culture of A. ferrooxidans, A. thiooxidans, and Acidiphilium spp. on 2% (w/v) slag material (<63 µm). The aerobic bioleaching experiment resulted in 91% Cu leaching over a 50-day period, while the anaerobic bioleaching experiment did not show significant copper release. In contrast, the current research used a higher pulp density of 15%, resulting in an enhanced leaching of metals under subsequent stages of oxidative and reductive dissolution. In a study by Stankovic et al. [20], Cu was extracted from the Bor mine flotation tailings using acidified Fe³⁺ iron-rich water at a 5% (w/v) solid loading rate with an extraction efficiency of ~80%. Similarly, Falagan et al. [12] achieved effective copper extraction (>90%) from Cobre Las Cruces (CLC) tailings at 5% (w/v) solid density under a remarkably low pH and an alternative Oxi-Red leaching process. In the present study, the Oxi-Red bioleaching process was found to be the most efficient method for the recovery of critical minerals. This process is cost-effective and yields significantly higher concentrations of critical minerals at high solid loading rates (15% w/v) than the previous studies discussed above. Moreover, the bio-processing of mining secondary resources and waste can help alleviate environmental risks associated with the discharge of metals and acid into water bodies, leading to overall economic benefits [12,21]. These results suggest that A. ferrooxidans can play a crucial role in improving the recovery efficiency of critical minerals. This finding has important implications for the bio-processing of mining secondary resources and waste, as it offers an effective method to recover valuable metals and mitigate environmental hazards.

3.4. Mineralogical Study on Slag After Bioleaching and Copper Recovered from Bioleach Liquor

The SEM analysis conducted on the metallurgical slag revealed that the Oxi-Red bioleaching process carried out by A. ferrooxidans caused mineral decomposition. This was evidenced by the roughness observed on the mineral surfaces and the distinct dissolution features. After the bioleaching process, the grains displayed etched zones on minerals like glass, fayalite, and sulfides, which were deeply perforated (Figure 4a,b). The removal of Cu droplets from various minerals was also observed. The SEM investigations showed that the slag contained highly disintegrated small-sized crystals that provided a larger surface area to the bacteria for leaching out metals during the bioleaching process (Figure 4a,b). While the bioleaching process caused dissolution features on fayalite and sulfides, the dissolution of the slag was restricted due to the encapsulation of metals in sulfides and silicate matrices such as glass and fayalite. The release of Fe and Cu was attributed to the dissolution of chalcopyrite and Cu sulfide. Furthermore, the dissolution of Zn sulfide and Al hydroxide was responsible for liberating metals like Zn and Al [22,23].

4. Conclusions

This research is an improvement over previous studies, as it proposes a sequential Oxi-Red bioleaching of acid-pretreated copper smelter slag. It offers a promising solution for efficiently recovering critical minerals from mine waste. Depending on the mineralogy of slag, such as acid labile iron minerals and Cu sulfates/sulfide and Zn sulfide, using biogenic acid generated under oxidative conditions can be sufficient to solubilize much of the Zn, Cu, and Al. In contrast, microbially mediated Fe³⁺ reduction is more important in promoting the dissolution of Fe³⁺ minerals and the solubilization of metals. Using the acidophile A. ferrooxidans at a high solid density of acid-pretreated slag resulted in a significantly higher level of critical minerals (Zn, Al, and Cu) than the abiotic control. The process was effective under highly acidic conditions (pH 1.8), moderately low temperatures (30°), and by coupling the oxidation of S⁰ to the reduction of Fe³⁺ iron. The reduction of Fe³⁺ iron in the Red stage led to a higher release of Fe²⁺ and sulfate than in the Oxi stage, preventing jarosite formation and surface passivation and the associated loss of metal recovery due to co-precipitation with Fe³⁺. This approach offers an environmentally friendly process for recovering critical minerals from metallurgical waste.