Non-Ferrous Metal Bioleaching from Pyrometallurgical Copper Slag Using Spent Medium of Different Fungal Species

Abstract

Copper slag, a by-product of copper ore and concentrate smelting, is rich in non-ferrous metals; therefore, it has been considered a valuable raw material in recent years. This study aimed to compare the extraction of zinc, copper, and cobalt from two types of copper slag from a dump located near the village of Eliseyna, Bulgaria, which differ in mineralogical composition and chemical content, using indirect bioleaching with a spent medium of Aspergillus niger and Penicillium ochrochloron. Chemical leaching with sulphuric acid revealed that zinc and cobalt existed mainly as an acidic-soluble phase in both types of copper slag. In contrast, it contained 50–75% of the total copper content. Each fungal species was cultivated for one week, and the biomass and the spent medium were separated a week later. Owing to the production of a higher concentration of citric acid, A. niger facilitated more efficient base metal recovery. However, their effective recovery from the acidic-soluble phase required leaching at a 5% pulp density and supplementing the spent medium with sulphuric acid. The temperature played a secondary role. Conclusions: Non-ferrous metal extraction from copper slag exposed to weathering using a spent medium supplemented with sulphuric acid was achieved under milder leaching conditions and with better selectivity. In contrast, slag unaffected by weathering behaved as a refractory due to the worsened results of base metal extraction under similar experimental conditions.

1. Introduction

The social evolution of human beings is closely tied to the discovery of new raw materials and the development of suitable methods for extraction and processing into usable final products. Copper is one of the primary base metals, whose classical method of processing involves ores rich in sulphide that are processed through flotation and smelting at higher temperatures into anodic copper with higher purity as a final product and slag, a solid waste, generated at a ratio of 1:2.2 [1]. Due to the growth of industrial sectors such as IT and technologies and their green transitions, the higher demand for base metals and some critical raw materials over the past few decades has resulted in a 2.6% annual growth of copper pyrometallurgy and the worldwide annual copper slag generation of 37.7 million tons [2]. Apart from copper, copper slags contain a significant amount of iron, other valuable metals, such as cobalt and nickel, as well as some toxic elements (arsenic and lead) in some cases [3,4,5]. Therefore, copper slag, as well as other industrial waste [6,7], is regarded as a raw material containing valuable chemical elements whose processing and extraction are a part of their use as a resource in the circular economy [2,8,9].

Base metals exist in several different phases in copper slag, such as sulphides, zero valency, and oxides, and are absorbed or encapsulated in the crystal lattice of fayalite, silicates, or magnetite. The distribution of each element depends on its specific geochemical properties, the characteristic of the raw materials being processed, and the conditions under which the smelting took place [10,11,12]. Therefore, classical methods of mineral processing, such as flotation, yield a final copper recovery of around 50% of the total copper content in the slag [13], and their inefficiency further increases due to the loss of cobalt and nickel in the generated flotation tailing [14]. Another approach to copper slag processing involves integrating pyrometallurgy and hydrometallurgy, followed by leaching the treated slag with water or alkaline solutions [15,16]. In this case, the recovery of non-ferrous metals is very high, regardless of their initial phase distribution in the slag. This could be achieved through smelting copper slag with a wide range of roasting agents, such as sulphuric acid [17,18,19], and slag disintegration at an alkaline pH with different reagents [16,20]) in the presence of some strong oxidants [21,22,23].

Although the efficiency and selectivity of base metal extraction from copper slag surpass those of iron and silicon extraction, the industrial application of these processes is limited by the higher operational costs for energy and reagents.

Therefore, a substantial number of studies are oriented towards the use of (bio)hydrometallurgy for the recovery of valuable metals from copper slags [24,25]. Their main advantages are that they operate at ambient temperature and pressure and that lixiviants participating in the leaching process are generated/recycled due to microbial growth. The process efficiency depends on several factors, including pH, temperature, the liquid-to-solid ratio at which the leaching is carried out, the duration of bioleaching, and the chemical and mineral contents in the raw material [26,27,28]. Several studies utilise chemolithotrophic bacteria with sulphur-oxidising/ferrous-oxidising abilities, which grow at highly acidic pH levels, thereby enhancing the efficient extraction of copper and other base metals [29,30,31]. The main drawback of this copper slag bioprocessing approach is the intensive co-leaching of silicon and iron from slag.

Apart from chemolithotrophic bacteria, heterotrophic microorganisms also participate in mineral transformations, especially at slightly acidic to alkaline pH levels. In that case, the bioleaching of metals from raw material is mainly due to the synthesis of exopolysaccharides with a higher surface area, metabolic products acting as specific ligands to the relevant valuable metal, or the presence of complex enzyme systems [6]. Several bioleaching mechanisms, including acidolysis, complexolysis, redoxolysis, biosorption, bioaccumulation, and alkalolysis, driven by heterotrophic microorganisms, participate in the bioleaching of base metals from raw materials [32]. Each microbial species with a heterotrophic metabolism utilises a narrow range of the aforementioned mechanisms. Accordingly, certain ecological groups, such as fungi producing citric acid, bacteria producing amino acids, ammonifying bacteria, silicate-solubilising bacteria, and cyanogenic bacteria, are distinguished [32].

Therefore, heterotrophic microorganisms have been extensively studied under laboratory conditions for the biorecovery of various types of raw materials (ores and industrial waste), especially those with higher oxide, carbonate, and silicate contents [32]. However, the interaction between heterotrophic microorganisms and raw materials is a complex process. The efficiency valuable metal biorecovery, in addition to the factors regarding aforementioned for chemolithotrophic bacteria, also depends on factors such as microbial adaptation to higher concentrations of heavy metals, the indirect control of microbial metabolism by the concentration of some microelements that are co-leached from the raw material, the shear stress of the solid particles on the microbial population in terms of logarithmic growth, etc. [33,34]. Therefore, three different bioleaching techniques, both direct and indirect, are widely studied to determine the optimal conditions for the biorecovery of valuable metals from raw materials, driven by the growth of heterotrophic microorganisms [6,34]. Direct bioleaching techniques rely on the intimate contact between microbial cells and raw materials. The indirect bioleaching technique utilises the spent medium, obtained from the growth of the relevant microbial strain, which contains the microbial products (types and concentrations) synthesised at the optimal cultivation conditions. Therefore, bioleaching is a flexible technique that can be adapted to the specific requirements of the solid sample and the raw materials it contains.

This study aimed to compare the recovery of copper, zinc, and cobalt from two copper slag samples (differing in base metal content, mineralogy, and properties) taken at different depths from tailings near the village of Eliseyna, Bulgaria, comparing pH-dependent chemical leaching with sulfuric acid and indirect bioleaching with the spent media of two fungal strains (Aspergillus niger and Penicillium ochrochloron) in batch mode, as well as determining the effect of pulp density, temperature, and the addition of different amounts of sulfuric acid to the spent medium on the selectivity of non-ferrous metal leaching versus iron extraction. The principal conclusion from the studies being carried out is that the acidity of the spent medium is the primary factor controlling the rate and selectivity of non-ferrous metal leaching from copper slag. However, a higher percentage of copper, zinc, and cobalt recovery requires supplementing the spent medium with sulphuric acid, resulting in a non-selective acidolysis process and the accumulation of iron in the pregnant leach solution.

2. Materials and Methods

2.1. Copper Slag Samples Collection and Characterisation

Two copper slag samples were collected from a copper slag dump near Eliseyna, a village in Northwestern Bulgaria. Copper slag sample F originated from the dump surface, where the seasonal amplitude of the temperature and humidity values affected the structure of the solid material. Therefore, the collected copper slag, already fragmented into small pieces, with a size ranging from 2.5 to 10 cm, was sampled. Copper slag sample III was collected at a depth of 1.0 m below the surface from the dump zone, where material fragmentation w in its initial stage. Therefore, the chemical content and mineralogy of that sample closely reflect the characteristics of copper slag before the onset of secondary weathering and leaching processes. Both samples were transported to a laboratory for grinding by a ball mill to a dominant particle size of 75 µm to 25 µm. The grounded samples were put in dust-free buckets and stored at a lower temperature for further use.

A representative portion of each copper slag sample was subjected to digestion with aqua regia. The content of chemical elements in the resulting liquid was analysed by atomic absorption spectrometry and inductively coupled plasma.

The copper slag pH was determined in distilled water at a pulp density of 10% after one hour of agitation with an overhead mixer at 150 rpm. A representative portion of each copper slag sample was sterilised at two different pressure atmospheres at 180 °C for 2 h. It was used in all leaching experiments in that form.

2.2. Sulphuric Acid Consumption of Copper Slag and pH-Dependent Leaching of Base Metals

The acid consumption of the slag was studied at 25 °C. The addition of sulphuric acid (30% H₂SO₄) to pulp containing 50 g of copper slag per 1 L of distilled water was performed via agitation using an overhead mixer at a rate of 300 rpm. The acid addition stopped when the pulp’s pH reached the target value (5.0, 4.0, 3.5, 2.5, and 1.5) and remained constant for 6 h.

The pH-dependent leaching of copper, zinc, and cobalt was determined in the transparent solutions obtained from pulp samples taken at the respective pH values and after solid particles were sedimented by centrifugation (10,000 rpm, 10 min).

All experiments were repeated in triplicate, and the results are reported as the mean; their statistical significance was assessed through calculation of standard deviation with statistical significance set as p ≤ α, α = 0.5.

2.3. Microorganisms and Cultivation

The abilities of fungal species Aspergillus niger and Penicillium ochrochloron to change their mineral composition and to leach the non-ferrous metal content of copper slag samples were studied and compared. Aspergillus niger originated from the Microbial Culture Collection maintained at the Department of General and Industrial Microbiology, University of Sofia. Penicillium ochrochloron CCM F-158 was obtained from the Czech Collection of Microorganisms at the Department of Experimental Biology at Masaryk University. The strains were maintained on Czapek Dox medium by sub-culturing at 25 °C and were preserved at 4 °C when needed. Each fungal species was cultivated in a medium consisting of sucrose (185 g/L), KH₂PO₄ (3.0 g/L), NH₄Cl (0.96 g/L), MgSO₄·7H₂O (1.2 g/L), and traces of FeSO₄·7H₂O, ZnSO₄·7H₂O, CuSO₄·7H₂O, pre-sterilised at 90 °C and 1.8 bar to prevent the growth of other microbiological strains. The medium was inoculated with 10⁶ spores/mL and incubated in batch mode for 7 days at 25 °C under continuous shaking (150 rpm). The spent medium for further experiments was obtained by separating the fungal biomass with vacuum filtration.

2.4. Non-Ferrous Metal Bioleaching from Copper Slag with Spent Medium

The leaching experiments were performed using the shake-flask technique with a 500 mL Erlenmeyer flask, containing 150 mL of spent medium and copper slag at pulp densities of 5%, 10%, or 15%, respectively. The pulp was homogenised by an orbital shaker (150 rpm) at temperatures of 25 °C or 55 °C, respectively. The effect of the spent medium supplementation with different dosages of sulphuric acid (5 g, 10 g, and 25 g/L) on the efficiency of non-ferrous metal extraction was also studied. Regular monitoring of pH and the concentration of non-ferrous metals in the pregnant leach solution was conducted during the experiments. The solid particles were separated by centrifugation at 5000 rpm for 10 min at room temperature. The resulting transparent supernatants were then acidified with hydrochloric acid and stored at 5 °C until analysis. The total duration of the leaching experiments was 72 h. At the end of the experiments, the copper slag residues from each Erlenmeyer flask were separated by filtration and washed with tap water until the pH reached 7.0. They were then dried to a constant weight at 250 °C for 2 h and ground in an agate mortar.

All experiments were repeated in triplicate, and the results are reported as the mean; their statistical significance was assessed by calculation of standard deviation and meeting the criterion of statistical significance of p ≤ α, α = 0.5.

2.5. Analyses

The concentrations of citric and oxalic acids in the spent medium of both fungal species were determined using titration and spectrophotometric methods [35,36]. The alkaline consumption (0.2 N NaOH) of the spent medium, up to a pH of 8.5, was determined using a Metrohm 718 STAT Titrino titrator (Herisau, Switzerland).

The concentration of non-ferrous metals and iron in the pregnant leach solution were determined by atomic absorption spectrophotometry and inductively coupled plasma.

The chemical content of the copper slag residue was determined after preliminary digestion at a strongly acidic/alkaline pH, followed by AAS-ICP methods, respectively.

The mineralogical composition of the raw copper slag samples and the residues obtained after bioleaching with spent medium at the relevant experimental conditions was determined using an EMPYREAN X-ray Diffraction Analyser (Malvern Panalytical, Almelo, The Netherlands), which operates with Cu-Kα radiation at 40 kV and 30 mA. The surface morphology of the copper slag was determined using an electron microscope (JEOL 6390, Tokyo, Japan) with an INCA Oxford EDS detector (Abingdon, UK).

Element recovery (ERl) in liquid samples was calculated according to Equation (1), as shown below:

$$ERl\left(\mathrm{\%}\right)=[\left({\displaystyle \frac{\left(\frac{CEl}{m}\right)}{\left(\frac{1000}{V}\right)}}\right)\times 1000)/ECm]\times 100$$

(1)

where ERl—element recovery, %;

• CEl—concentration of leached element in the pregnant leaching solution, mg/L;
• V—volume of the solution during the leaching test, mL;
• m—copper slag mass during the leaching test, kg;
• ECm—concentration of element in raw copper slag, mg/kg.

The efficiency of metal (non-ferrous metals and iron) extraction from the copper slag sample under the relevant experimental conditions (ER_s) was calculated according to the following formula:

$${ER}_s={\displaystyle \frac{C2}{C1}}\times 100,\mathrm{\%}$$

(2)

where ER_s—element recovery, %;

• C2—metal concentration in the copper slag residue, mg/kg;
• C1—metal concentration in the raw copper slag, mg/kg.

The selectivity coefficient (S) of the non-ferrous leaching from the copper slag sample under the relevant experimental conditions was calculated according to the following formula:

$$\mathrm{S}=\left[\mathsf{\Sigma }\right(\mathrm{C}\mathrm{c}\mathrm{o}\mathrm{p}\mathrm{p}\mathrm{e}\mathrm{r}+\mathrm{C}\mathrm{z}\mathrm{i}\mathrm{n}\mathrm{c}+\mathrm{C}\mathrm{c}\mathrm{o}\mathrm{b}\mathrm{a}\mathrm{l}\mathrm{t}\left)\right]/\left[\mathrm{C}\mathrm{i}\mathrm{r}\mathrm{o}\mathrm{n}\right]$$

(3)

where S—selectivity coefficient;

• $\mathsf{\Sigma }(\mathrm{C}\mathrm{c}\mathrm{o}\mathrm{p}\mathrm{p}\mathrm{e}\mathrm{r}+\mathrm{C}\mathrm{z}\mathrm{i}\mathrm{n}\mathrm{c}+\mathrm{C}\mathrm{c}\mathrm{o}\mathrm{b}\mathrm{a}\mathrm{l}\mathrm{t})$—sum of copper, zinc, and cobalt concentration in the pregnant leach solution, mg/L;
• Ciron—iron concentration in the pregnant leach solution, mg/L.

3. Results and Discussions

3.1. Chemical Content and Mineralogy

A copper slag dump near Eliseyna, a village in Northwestern Bulgaria, was formed over more than a century, resulting from the industrial smelting of copper ores and concentrates applied in the area. During that period, millions of tons of this industrial waste were deposited over a vast area, as its chemical content, mineralogy, and properties reflected the technologies applied in the processing of raw materials. The copper slag dump is situated at an altitude of 330 in an area with a typical humid continental climate. Therefore, the secondary process of the weathering and leaching of copper slag is readily observable, especially on the dump surface. That was the reason for collecting two kinds of copper slag samples from the dump. Sample F was collected from the surface of the dump. In contrast, sample III originated from a depth of 1.0 m below the surface where slag fragmentation and leaching were still in the initial stage. One of the aims of this study was to reveal the effect that storage conditions have on the mineralogy and leachability of the base metals contained in the copper slag samples.

The results of the chemical content analysis revealed that copper, zinc, and cobalt are the base metals that were present in higher amounts in the studied copper slag samples (

Table 1. Chemical content of the copper slag samples used in this study.

Chemical Element, %	Sample F	Sample III
Cu	0.36	0.47
Zn	1.93	1.34
Co	0.09	0.05
Fe	27.2	36.3
Si	15.9	16.0
Ca	6.9	6.9
Mn	0.8	0.3
Al	2.78	2.58
P	0.08	0.07
S	0.91	1.06
pH (H2O)	7.95	8.81

).

Zinc is a non-ferrous metal with a significantly higher content compared to other non-ferrous metals, reaching a value of 1.93% and 1.34% in copper slag samples F and III, respectively (Table 1). Copper is a non-ferrous metal, whose content in both samples determined it as the second most significant base metal. The cobalt content in both samples was below 0.1%. These results are in accordance with data from other studies and reviews, which reported the same order of base metal content in copper slags from different countries [2,4]. The copper content was significantly higher in the converter slag (reaching up to 15%), which is an intermediate product in terms of the modern technology used for smelting copper-containing raw materials [5].

Considering the formation of copper slag, iron and silicon are the chemical elements that dominate in terms of their chemical content and structure. The iron content in sample III, at 36.3%, is 25% higher than that of sample F. There is no significant difference in the content of silicon, other metals, phosphorus, and sulphur between the two samples.

Both copper slag samples have a slightly alkaline to alkaline pH, which reflects their chemical content and mineralogy.

An X-ray diffraction study revealed that fayalite, a member of the olivine group, comprised 33% and 56% in samples F and III, respectively (Figure 1;

Table 2. Mineral content of the copper slag samples used in this study.

Chemical Element, %	Sample F	Sample III
Fayalite (Fe2SiO4)	33	56
Pyroxene group (XY(Si,Al)2O6):
Unidentified type of clinopyroxene (Ca0.949Fe1Na0.051O6Si2)	10	-
Diopside (CaMgSi2O6)	-	44
Protomangano-ferro-anthophyllite ( )	56	-

).

This is in accordance with the published results about the mineralogy of copper slag, considering the flowsheet and operational conditions maintained during the smelting processes [2,3,25].

The second most common mineral in sample III was diopside, a member of the pyroxene group, at 44%. An unidentified type of clinopyroxene, also a member of the pyroxene group with the formula (Ca_0.949Fe₁Na_0.051O₆Si₂), was described in sample F, with a relative content of 10%.

In addition to the olivine and pyroxene groups of minerals, the aforementioned reviews of copper slag also described the following in their structure: other silicate minerals, members of the groups of melitite, feldspar, and quartz; free metal oxides presented by the spinel group with a standard formula AB₂O₄ (typically presenting a mixture of bivalent and trivalent metal ions, for example, magnetite (Fe₃O₄)) and wustite; and some glass phases. None of the mineral groups mentioned above were described in the studied copper slag samples from Eliseyna. They are probably present in the studied samples, but their content was below the detection limit of the analytical apparatus used in X-ray diffraction analyses. Instead, protomangano-ferro-anthophyllite (H₂Fe_4.48Mg_1.08Mn_1.44O₂₄Si₈), a member of the group of protoamphiboles, was described as the third constituent of copper slag sample F, with a relative content of 57%. This higher content in hydroxyl groups in comparison to the pyroxene group of minerals, enhances the appearance of a pH-dependent charge on the mineral surface, which plays a crucial role in the specific adsorption of already-leached ions. Its presence, as well as that of unidentified clinopyroxene (Ca_0.949Fe₁Na_0.051O₆Si₂), provided solid evidence of the secondary weathering and chemical processes (leaching, co-precipitation, and adsorption) that are carried out in copper slag after being deposited and stored for decades at the dump under changing environmental conditions.

X-ray diffraction analyses did not reveal the presence of metal in zero valency forms, such as Fe, Cu, and Zn, sulphides (especially sulphides of copper and sphalerite (ZnS)), or some intermetallic compounds.

3.2. Sulphuric Acid Consumption and pH-Dependent Leaching of Base Metals from Copper Slags

A study [14] has shown that the leaching of non-ferrous metals from copper slags is a strongly pH-dependent process and follows a U-shaped curve, with a minimum efficiency at a neutral to slightly alkaline pH, and maximum efficiency at a highly acidic and highly alkaline pH; in addition, its process efficiency is better at a highly acidic ph. Therefore, the sulphuric acid consumption of copper slag samples and related pH-dependent non-ferrous metal leaching was studied in accordance with steadily decreasing values of the target pH.

The chemical composition, mineralogy, and surface area of both finely ground copper slag samples determine their alkaline properties and significant acid consumption during the pH-dependent leaching test (Figure 2a). To reach a pH of 5.0, the acid consumption of both samples was similar and a minimal amount of sulfuric acid was added. The leaching of secondary minerals, including iron oxides and carbonates, determined the proton consumption around that pH. The acid requirement to reach the target pH of 4.0 increased significantly, to values higher than 70 g and 300 g H₂SO₄/kg copper slag for samples F and III, respectively (Figure 2a). The higher proton concentration at this pH caused mineral acidolysis, leading to the decomposition of the more reactive minerals, or the removal of impurities from the structures of other minerals and their conversion into a mineral form, in equilibrium with the environmental conditions. This process affected the minerals with higher contents in the respective copper slag samples, particularly protomangano-ferro-anthophyllite in sample F and fayalite in sample III. The difference in sulphuric acid consumption between the two samples steadily diminished in order to reach a highly acidic target pH value of 2.5 and values in the range of 568–648 g H₂SO₄/kg copper slag.

Noting the similarities in the chemical content and mineralogy of the studied copper slag samples and copper slag that were the objects of other studies, the reported acid consumption values needed to be in the range of 650–700 g H₂SO₄/kg copper slag to reach the target pH of 2.5 [8,27]. The higher proton concentration required to achieve a pH below 3.5 in the pH-dependent leaching test accelerated mineral acidolysis, dissolving the mineral structure at a higher rate and releasing non-ferrous metals locked in their crystal lattices into the pregnant solution. Therefore, the leaching of silicates, fayalite, and diopside mainly governs the primary zones of sulphuric acid consumption at highly acidic pH levels (3.5–1.5) [37]. For example, fayalite dissolution at acidic pH occurs according to the following reaction:

Fe₂SiO₄ + 2H₂SO₄ → 2FeSO₄ +H₄SiO₄

(4)

The pH-dependent leaching of non-ferrous metals from raw materials involves variety of phases and ratios regarding the presence of each. The graphs in Figure 2b–d reveal that the respective values of non-ferrous metals extracted from copper slag samples exhibited a direct correlation with increases in the amount of sulphuric acid. For example, zinc extraction at a pH of 5.0 was 12.6% and 18.5% for sample III and sample F. Process efficiency further increased to 68.9% and 74.9% at a pH of 2.5, respectively (Figure 2c). Regarding cobalt leaching, the process efficiency improved substantially at a target pH below 3.5, reaching values of 54.4% and 73.3% for samples F and III, respectively, at a target pH of 2.5. At a pH of 1.5, the recovery of both non-ferrous metals varied in a narrow range, 79.1–85.5% for zinc, and 87.6–92.2% for cobalt, which was in accordance with the results from other studies [10,12,26]. The chemical extraction of copper from copper slag samples is less efficient than that of zinc and cobalt at a pH of 2.5, reaching 61.7% and 37.7%, respectively, for samples F and III. When the studies ended at a target pH of 1.5, the process efficiency increased up to 49.8% and 76.2%. The results of this study support the observations made by other researchers [10,22], who found that, compared to copper, zinc and cobalt were more readily leached from copper slag during pH-dependent leaching at acidic pH levels. This resulted from the distribution of the base metals among different phases at the conditions under which their leaching occurred. For example, numerous studies have demonstrated that silicates and ferrites can entrap approximately 95% of the total cobalt in copper slags. Therefore, a highly acidic pH and the acidolysis process are sufficient for the substantial recovery of the base metal at a higher rate. Zinc follows a similar pattern of distribution among phases to cobalt, as it can be present as sulphides (e.g., sphalerite (ZnS), marmatite (Zn, Fe)S)) and also entrapped in glass [25]. Therefore, the classic chemical extraction of copper slag with a diluted sulfuric acid solution is still a widely used method for extracting non-ferrous metals from this type of raw material for several reasons. First, sulfuric acid, with a typical efficiency typically above 80%, can extract base metals at a higher rate than their acid-soluble phase [38]. Second, sulfuric acid is a by-product of the combustion and smelting of coal sulphide concentrate, and therefore its market price remains low. Third, the process is well known, both theoretically and practically, and is integrated into the overall technological scheme for processing mineral raw materials and the resulting rich solutions. The main disadvantages of chemical leaching with sulfuric acid are the significant co-leaching of silicon, iron, and magnesium, ranging from 70% to 80% [38], and the highly corrosive and toxic properties of the resulting rich solution.

In contrast, as a rule, an insignificant amount of the total copper content in copper slag is trapped in the structure of silicates, ferrites, and glass. A significant part (50–70%) of it is present as sulphides (covellite (CuS), chalcocite (Cu₂S), bornite (Cu₅FeS₄), and chalcopyrite (CuFeS₂)) and zero-valency copper [10,25,39]. Therefore, chemical leaching with strong oxidants such as H₂O₂ and K₂Cr₂O₇ [22,23] or bioleaching with chemolithotrophic bacteria [23,39] at an acidic pH has a strongly positive effect on copper leaching from slags [10,26,40]. The pH-dependent leaching test revealed that, at a target pH of 1.5, the recovery rates of copper were 76.2% and 49.8% for samples F and III, respectively. These results indirectly demonstrate that the remaining copper in the studied copper slag samples from Eliseyna existed in refractory forms that resist acidic leaching, such as sulphides and zero-valence copper.

3.3. Selectivity of Non-Ferrous Metal Chemical Leaching from Copper Slag Using Sulphuric Acid

Non-ferrous metal recovery from raw materials through acidic leaching is concomitant with the substantial leaching of some structural elements, such as calcium, magnesium, or aluminium. In that case, iron and silicon are co-leached from copper slag samples via treatment with sulphuric acid solutions. For instance, 68.9% and 48.5% of iron and silicon, respectively, were leached at a pH of 3.0 from sample F, while the respective values for sample III were 44.7% and 21.4%.

At a pH below 3.0, silicon co-leaching from both samples further increased, and then dropped significantly due to the rapid transformation of silicic acid (H₄SiO₄) into silica gel (SiO₂) according to the following reaction:

2H₄SiO₄ → 2(SiO₂) + 4H₂O

(5)

The larger surface area of silica gel and its surface charge increase the losses of non-ferrous metals through the adsorption process. In addition, silica gel formation increases operational costs due to the need for additional reagents and the inclusion of auxiliary steps in the flow sheet for pregnant leach processing.

In contrast to silicon, ferrous iron, liberated due to the degradation of copper slag minerals, was in a stable iron form at a pH below 3.00, enhancing its accumulation in the laden leach solution. Therefore, the selectivity of chemical leaching of non-ferrous metals from copper slag samples with sulphuric acid, as determined by the calculation of the selectivity coefficient, was assessed in terms of iron concentration in the pregnant leach solution.

Due to the higher total content of zinc and cobalt in copper slag sample F, its selectivity coefficient (S) values for base metals during iron recovery during pH-dependent leaching with sulphuric acid were higher over a wide pH interval (5.0–3.5) compared to the respective values of sample III (Figure 3).

Because of the identical mineralogy of both samples and the intensive leaching of silicate minerals, the contrast between the calculated values of S diminished at a target pH of 2.5. This indicated a higher iron concentration in the pregnant leach solutions and characterised sulphuric acid as a non-selective leaching agent. Other researchers observed a similar relationship between the higher acid concentration used in the leaching process and its positive effect on the recovery of non-ferrous metals from the respective raw material, as well as the higher iron leaching and lower selectivity of the leaching process [40,41].

The selectivity of acidic leaching with sulphuric acid improved when the studied raw material was blast furnace and converter slags, due to their higher content of base metals [39].

The values further improved in variants that combined acidic leaching with chemical oxidation (using H₂O₂) due to the oxidation of ferrous iron ions to ferric iron and their selective precipitation as ferric iron hydroxides. Numerous laboratory studies have been conducted on the physicochemical pre-treatment of copper slags as a preliminary step for the selective extraction of non-ferrous metals. These were based on a combination of higher-temperature treatment [14] in the presence of acid [15,19] or alkaline agents [20], as well as some strong oxidants, such as a higher pressure of oxygen [17,18], or potassium bichromate [23]. As a result, the recovery of base metals improved significantly. At the same time, iron and/or silicon leaching was inhibited, producing a pregnant leach solution with a higher concentration of non-ferrous metals. However, significant operating costs hinder the scaling up of the results.

3.4. Indirect Copper Slag Bioleaching with Spent Medium of Aspergillus niger and Penicillium ochrochloron

The selected strain of A. niger was included in this study due to its bioleaching potential, which has been extensively evaluated and proven for a broad spectrum of raw materials. The strain of P. ochrochloron was selected due to its ability to grow in biotopes with elevated copper ion concentrations.

A. niger and P. ochrochloron were cultivated on a nutrient medium optimal for citric acid synthesis, and both species exhibited a similar reaction, with branched microbial biomass formation, displaying a bulbous appearance and colour, and morphology that could be used to distinguish each species. Following a week of cultivation under shaking conditions at 25 °C, designed to enhance citric acid production, the colour of A. niger biomass and its spent medium was intense yellow, indicating the significant accumulation of citric acid. In the same period, the microbial biomass of P. ochrochloron and its spent medium showed a greyish-milky colour, indicating the accumulation of other organic acids as main products. The chemical analyses of the spent medium proved that A. niger exhibits a higher rate of sucrose oxidation to citric acid than P. ochrochloron and 24.8 g/L citric acid accumulated after a week of cultivation (

Table 3. Spent medium of the fungal strains used in the study after a week of cultivation.

Index	Penicillium  ochrochloron	Aspergillus  niger
pH	3.24	2.58
Citric acid, g/L	18.9	24.8
Oxalic acid, g/L	10.9	8.1
Acidity, g/L	0.65	0.94
Fungal biomass, g/L	7.92	9.75

).

This resulted in the synthesis of a higher amount of fungal biomass and a highly acidic pH value. Furthermore, the total amount of protons in the spent medium of A. niger available for acidolysis reached almost 1.0 g H⁺/L, which was equal to the presence of 46.1 g H₂SO₄. In contrast, P. ochrochloron grew slowly and produced oxalic acid as a primary metabolite of its growth and a significantly lower concentration of citric acid; therefore, the acidity value in its spent medium was lower than that of A. niger, relating to the presence of 31.9 g H₂SO₄/L in the spent medium. The differences in the spent medium content between the fungal strains were due to differences in metabolism and responses to the nutrient medium and the cultivation conditions. For example, citric acid is a product of the tricarboxylic acid cycle and is produced when fungal growth is carried out under limited nitrogen conditions. Fungi synthesis of oxalic acid via three different metabolic pathways—the tricarboxylic acid pathway and glyoxylate and cytoplasmic pathways—and the temperature and pH have a substantial effect on the oxalic acid yield [42].

Fungi produce dozens of organic acids, such as citric, oxalic, gluconic, malic, and tartaric acids. Citric and oxalic acids are usually found at higher concentrations, and some reviews have shown their significant role in the leaching of base and precious metals from various types of industrial waste [34]. The advantages of leaching raw materials with natural organic acids over leaching with inorganic acids include better selectivity in the leaching of non-ferrous metals and a lower environmental risk due to their biodegradability [43,44]. The trait shared by all of these metabolites is that they are polycarboxylic acids containing more than one carboxylic group, which donate hydrogen ions in a wide pH range (reaction 6). For example, oxalic acid is a diprotonic strong acid with lower pKa values in the carboxylic groups (1.25 and 4.14, respectively). In contrast, citric acid is a weak triprotonic acid with pKa values of 3.13, 4.76, and 6.39, respectively. The hydrogen ions are donated due to organic acid deprotonation and facilitate the proton attack of minerals (reaction 7) in the biotope, resulting in the release of ions needed for fungal growth.

The following set of reactions describe processes carried out during copper slag bioleaching with spent medium:

• Deprotonation of organic acids:

(COOH)₂ → (C₂O₄)²⁻ + 2H⁺

(6)

2.

Proton attack and mineral acidolysis:

Fe₂SiO₄ + 4H⁺ → 2Fe²⁺ + H₄SiO₄

(7)

3.

Formation of complexes between cations and organic anions, which possess different solubilities (complexolysis):

Zn²⁺ + 3C₆H₇0₇⁻ → Zn(C₆H₇O₇)₃⁻

(8)

Cu²⁺ + C₂O₄²⁻ + nH₂O → CuC₂O₄.nH₂O ↓

(9)

The spent media of A. niger and P. ochrochloron exhibited varying effects on the bioleaching of non-ferrous metals from both copper slag samples due to differences in the content of organic acids and the acidity of the relevant solution. First of all, the acidity of A. niger was near 1.0 mmol/L, and its value was higher than the respective value of P. ochrochloron. Therefore, indirect bioleaching using the spent medium of A. niger helped to maintain a lower pH during the leaching experiments of both copper slag samples, which accelerated mineral acidolysis to a greater extent, resulting in better extraction of non-ferrous metals (

Table 4. Effect of the spent medium on the non-ferrous metal leaching from the studied copper slag samples at 5% pulp density and 25 °C.

Index	Penicillium  ochrochloron		Aspergillus  niger
	Sample F	Sample III	Sample F	Sample III
pH	5.27	7.19	4.39	6.03
Cu leached, mg/kg	715	97	810	151
Cu leaching, %	19.9	2.1	22.5	3.2
Zn leached, mg/kg	4490	1407	4850	1568
Zn leaching, %	23.3	10.5	25.1	11.7
Co leached, mg/kg	345	61	400	69
Co leaching, %	38.3	12.2	44.4	13.8

). For example, the pH at the end of sample F’s leaching with the spent medium of A. niger was 4.39, whereas in the experiment with P. ochrochloron, it was 5.27. Therefore, the amount of leached copper, zinc, and cobalt determined at the end of the experiment with A. niger was in the range of 9.3–13.7% higher than the values obtained in the experiment with P. ochrochloron. Therefore, all subsequent experiments on the indirect bioleaching of copper slags were conducted using a spent medium of A. niger.

Secondly, the medium of A. niger, characterised by higher values of citric acid content and a higher citric–oxalic acid content than the medium of P. ochrochloron, enabled better extraction of copper, zinc, and cobalt from copper slag. Furthermore, the higher oxalic acid content in the spent medium of P. ochrochloron and the pulp pH at which indirect bioleaching occurred enabled the secondary precipitation of non-ferrous metals, mainly copper as highly insoluble copper oxalate, as shown in reaction 9. A similar phenomenon was observed in the experiments on the one-step direct bioleaching technique of sample F, where A. niger and P. ochrochloron reacted to the higher concentration of copper in the pregnant leach solution with the overproduction of oxalic acid due to the dominance of the acidic-soluble copper phase. Therefore, copper recovery in that case was in the range of 5.5% to 8.3% [45].

The indirect bioleaching of non-ferrous metals from sample III was a less efficient process compared to the results of the respective experiments with sample F. This was attributed to the higher acid-consuming potential of the sample, which was necessary to achieve the target pH. Consequently, the leaching process was carried out at a slightly acidic or neutral pH, depending on the spent medium used in the experiment. However, the amount of non-ferrous metals bioleached from copper slag samples F and III was higher than the respective values of base metals leached with sulphuric acid, considering the pH at which the leaching took place. For example, the leaching efficiency of copper and cobalt from sample F by the spent medium of A. niger was approximately two- and five-fold higher than the respective recovery values obtained by chemical leaching with sulphuric acid. The results of sample III revealed a similar trend, where copper and cobalt leaching with the spent medium of both fungal species was higher than the respective results of chemical leaching at a target pH of 5.0, even though the pH in one case (A. niger spent medium) was slightly acidic (6.03), and neutral (7.19) in another case (P. ochrochloron spent medium) (Table 3, Figure 2b,c). Therefore, the positive effect of organic acids, contained in the spent medium, on the leaching of base metals from the studied copper slag samples, even at slightly acidic and neutral pH levels, was revealed.

The selectivity of non-ferrous metal leaching from the studied copper slag samples using the spent medium of the respective fungal strains is presented in Figure 4. This reveals that non-ferrous metal leaching from copper slag samples using the spent medium of P. ochrochloron was combined with fewer base metals along with iron co-leaching, resulting in a higher selectivity coefficient compared to the respective experiment with the spent medium of A. niger, where a higher total content of organic acids enhanced base metal extraction but with lower process selectivity due to the improved iron co-leaching. For example, the indirect bioleaching of sample F at 5% pulp density yielded a selectivity value of 0.239 using the spent medium of P. ochrochloron and 0.221 using the spent medium of A. niger. This showed that acidolysis and the release of non-ferrous metals adsorbed on the surface of copper slag particles, rather than the accompanying iron co-leaching, depleted the limited hydrogen ion concentration in the spent medium of P. ochrochloron. In contrast, the spent medium of A. niger, containing a higher concentration of citric acid, demonstrated a positive effect of the combined action of acidolysis and complexolysis on base metal extraction, with a selectivity better than the respective results with chemical leaching using sulphuric acid at a target pH of 4.5. The technique of directly bioleaching sample F in the presence of active biomass from the respective fungal strains, employing a one-step approach, yielded higher selectivity for non-ferrous metal recovery due to a significant increase in the recovery of zinc and cobalt [41].

Due to the higher acid-consuming potential and lower total content of non-ferrous metals in sample III compared to the characteristics of sample F, the selectivity of base metals leaching was lower than the respective value of sample F, regardless of the origin of the spent medium being used.

Experiments conducted at a pulp density of copper slag exceeding 5% reveal the negative impact of this factor on the leaching of non-ferrous metals using the spent medium of A. niger. This impact was more pronounced with sample III, which exhibits stronger acid-consuming properties than those of sample F (Figure 5a,b). Consequently, the constant concentration of hydrogen ions in the spent medium limited the acidolysis of copper slag minerals and the recovery of base metals. For example, the equilibrium pH at the end of the indirect bioleaching test with sample F increased from 4.39 (at a 5% pulp density) to 6.85 (at a 15% pulp density). In contrast, the pH at the end of the leaching experiment for sample III under the same conditions was slightly alkaline (8.43). The efficiency of copper bioleaching from sample F dropped from 22.5% (at a 5% pulp density) to 18.7% (at a 15% pulp density), whereas for sample III, the respective values were 3.2% and 2.4%.

The value of the selectivity coefficient steadily increases in experiments conducted at a pulp density exceeding 5% for both types of copper slag. For the copper slag sample F, the value increases from 0.221 (at a 5% pulp density) to 0.253 (at a 15% pulp density) (Figure 5c), and from 0.102 to 0.165 for sample III, respectively. Two factors determine this trend. Firstly, the zinc concentration in both copper slag samples was higher compared to the content of copper and cobalt, and it is soluble even at a neutral to slightly alkaline pH, which enhances its accumulation in the pregnant leach solution after the leaching processes from copper slag particles take place. Secondly, apart from the lower leaching of ferrous iron from the solid particles, its content in the solution further decreased during the tests due to secondary processes, such as chemical oxidation and hydrolysis of the produced ferric iron ions. This revealed that the tested fungal strains were unable to produce a spent medium with suitable characteristics for the substantial bioleaching of non-ferrous metals from the studied copper slag samples.

A review of the results of more than twenty recently published studies, conducted by Weng et al. [28], revealed that the pulp density, acidity of leaching solution, and temperature at which the process occurs are the three most important factors which limit the recovery of non-ferrous metals from copper slag. For example, Meshram et al. [44] reported a significant improvement in cobalt and nickel extraction, to 88.3% and 95%, respectively, combined with the substantial co-leaching of iron from granulated copper slag when increasing the citric acid concentration to 2.0 N in the chemical leaching test at room temperature using 10% pulp density over 8–9 h. However, copper recovery under these conditions was 4.47%. Therefore, bioleaching with acidophilic chemolithotrophic bacteria is a still competitive approach for raw material processing due to the maintenance of higher non-ferrous metal extraction at higher pulp densities (10% and above) resulting from the biological oxidation of sulphides and S° to sulphuric acid. For example, Georgiev et al. [26] showed that mesophilic, moderately thermophilic, and thermophilic microbial cultures grown at 37 °C, 55 °C, and 75 °C, respectively, could be used for the extraction of non-ferrous metals. The microbial consortia consisting of different bacterial species showed better leaching activity, ranging from 65 to 88%, 62 to 91%, and 64 to 91% for copper, zinc, and cobalt, respectively, at pH 2.00 and a pulp density of 10% under batch bioleaching conditions for 48 h. This process is combined with substantial iron co-leaching, reaching up to 30%, and the partial loss of already-leached non-ferrous metals due to the oxidation of ferrous iron ions to the ferric state and subsequent jarosite precipitation. Therefore, some authors [46] studied an alternative approach for the bioleaching of non-ferrous metals from copper slag, combining oxidative and reductive leaching steps using Acidithiobacillus ferrooxidans at pH 1.8, thereby avoiding ferric iron accumulation and achieving 80% and 77% copper and zinc extraction over 20 days.

Therefore, another set of experiments was conducted to investigate the effect of sulphuric acid concentration supplementation of the A. niger spent medium and temperature on the bioleaching of non-ferrous metals from both copper slags and the corresponding values of the selective coefficient. These experiments were carried out at a 5% pulp density.

Indirect Copper Slags Bioleaching with Spent Medium of A. niger at 25 °C/55 °C, Supplemented with Different Dosages of Sulphuric Acid

This indirect technique of raw material bioleaching with the spent medium containing lixiviants due to the growth of specific microbial species has some advantages, in comparison to direct techniques, such as carrying out the leaching at a higher temperature than the optimal value of the microbial species and/or manipulation of the spent medium by adding a respective dosage of the compound at a limited concentration. Therefore, the effect of temperature (at 25 °C and 55 °C) and spent medium supplementation with different dosages of sulphuric acid (ranging from 5 to 25 g H₂SO₄/L) on the recovery of base metals from both types of copper slags was investigated (Figure 6 and Figure 7).

The leaching efficiency of copper, zinc, and cobalt from sample F was between three- and six-fold higher than the respective efficiency from sample III when the indirect bioleaching was carried out with the native spent medium of A. niger at a temperature of 25 °C, reaching values of 22.5, 25.1, and 44.4%, respectively (Figure 6c). The process started with a very high value for sample F (0.607), indicating preferential non-ferrous metal extraction over iron co-leaching four hours from the start of experiment (Figure 8a). This resulted in 12.2% of the total amount of base metal extraction and 1.7% iron co-leaching for that period. The respective values increased to 25.5% and 10.1% by the end of the experiment, resulting in a final selectivity value of 0.221.

In contrast, the selectivity coefficient of sample III was 0.093 for the same period, and it increased slightly to 0.102 by the end. This indicated that both processes, non-ferrous metal extraction and iron co-leaching, occurred at approximately constant rates throughout the entire experiment.

Comparative experiments focusing on bioleaching of both types of copper slag using a one-step direct technique, with the presence of hyphae of the same strain of A. niger at 25 °C, resulted in selectivity values for non-ferrous metal extraction of 0.061 and 0.109 for samples III and F, respectively. Base metals, especially copper, were extracted from both types of copper slag to varying extents because of the distinctive features of their mineralogy as well as the higher proportion of the acidic-soluble phase of copper in sample F, resulting in oxalic acid overproduction and secondary copper precipitation as oxalate [45], which reduced the selectivity of the bioleaching process.

The leaching experiments of both types of copper slags with the native spent medium of A. niger, carried out at a temperature of 55 °C, revealed a higher rate of both processes, non-ferrous metal extraction and iron co-leaching; however, different ratios were obtained depending on the mineralogy of the respective copper slag sample.

For example, the higher temperature enhanced the extraction of base metals, reaching 26.7% and 26.5% for the respective total content of copper and zinc, compared to iron co-leaching, at 7.1%, from sample F, with a selectivity value of 0.335. Under the same leaching conditions, native spent medium and 55 °C, iron co-leaching dominated in base metal extraction from sample III (Figure 7c), resulting in a selectivity value of 0.079 (Figure 8b). A significant improvement in zinc and cobalt extraction from sample III at 55 °C and a higher selective leaching value were observed after supplementing the spent medium with a minimal dosage of 5 g H₂SO₄/L.

When comparing the selectivity coefficients of both samples under similar experimental conditions, we must consider how the pH changes during the leaching processes (Figure 8). This could indicate a process of net iron loss due to enhanced acid consumption, resulting in an increase in solution pH to above 6.0. The pH of the leaching solution increased from 5.43 (at the fourth hour) to 6.22 (at the end) during sample III leaching, causing approximately 35% precipitation of already-dissolved iron and 30% net loss of the base metals due to their co-sorption (Figure 6a). The pH of the leaching solutions remained within a narrow range (5.07–5.16) during sample F leaching due to its lower acid-consuming potential (Figure 7a). This prevents the net losses of already leached iron and base metals, and their amounts increased during the experiment.

The significant difference in the selectivity of non-ferrous metal recovery from both types of copper slag revealed how the native spent medium composition, its supplementation with sulphuric acid, and the leaching temperature interacted with the mineralogical composition of the relevant type of copper slag during the experiments. It also revealed their cumulative effect on the extraction of non-ferrous metals and iron co-leaching. For example, protomangano-ferro-anthophylite, described in copper slag sample F, a product of the weathering process carried out at the dump surface, acts as a filter that adsorbs the base metals were already leached. Therefore, the recovery of base metals was carried out efficiently, even with the characteristics of the native spent medium of A. niger at a temperature of 25 °C. Moreover, their recovery steadily increased with the addition of sulphuric acid to the spent medium, which accelerated the acidolysis and weathering of the minerals. The iron content of protomangano-ferro-anthophyllite, according to its idealised formula (3H₂Fe_4.48Mg_1.08Mn_1.44Si₈O₂₄), is 25.8%, which explains the higher values obtained for the selectivity coefficient of base metal extraction compared to those obtained for the copper slag sample III under similar experimental conditions.

In contrast, fayalite and diopside, as typical products of the smelting process, were described in copper slag III following X-ray diffraction analyses. It is well known that diopside is less susceptible to acidolysis compared to fayalite’s reactivity (Hausrath et al. 2008 [37]). Therefore, the process of fayalite dissolution governed the acid consumption of the copper slag sample during the pH-dependent leaching test, as well as in the leaching test with the spent medium of A. niger at 25 °C and 55 °C, respectively. The iron content of fayalite, according to its idealised formula (Fe₂SiO₄), is 54.8%, resulting in substantially higher iron co-leaching, especially at a highly acidic pH, and lower selectivity values for the base metal leaching.

The reasonable explanation for the more than two-fold difference in the selectivity of base metal extraction from both types of copper slag, apart from the difference in the mineralogy of the samples, is probably the significantly lower content of zinc and cobalt in sample III, which reduced the probability of interaction between the base metals from copper slag and protons from the leaching solution, thus enhancing iron co-leaching.

Supplementation of the spent medium with different amounts of sulphuric acid aimed to enhance proton attack and mineral acidolysis by providing the processes with a higher concentration of hydrogen ions. Therefore, the leaching process was carried out at pH values lower than those measured in the variants with the native spent medium of A. niger (Figure 6a and Figure 7a). Apart from the enhanced mineral acidolysis, spent medium supplementation with sulphuric acid had other beneficial effects on the efficiency of base metal leaching, especially for copper, which maintained a pH lower than 6.0 (minimising copper loss due to the base metal hydrolysis in case of sample III leaching (as is visible in Figure 7c)) or a pH lower than 4.5 (minimising copper loss due to its co-precipitation/sorption by ferric iron oxides (as visible in Figure 9a,b)). For example, the final pH, measured at the end of the leaching experiments with samples F and III, carried out at 25 °C, was 5.16 and 6.22, respectively.

However, the efficiency of the base metal extraction was between two and four times higher than the respective efficiency calculated at the chemical leaching with sulphuric acid at a target pH of 5.00, revealing the positive effect of citric acid from the native spent medium, which acted both as a donor of hydrogen ions for proton attack and acidolysis as well as a ligand, particularly to copper ions, thereby enhancing their accumulation in the pregnant leach solution.

Supplementation of the spent medium with 5 g H₂SO₄/L corresponded to approximately 1040 g H₂SO₄/kg of copper slag, if expressed as the total available acidity as sulphuric acid, allowing for the final pH of the leaching experiments at 25 °C to drop to 4.19 and 5.95 for samples F and III, respectively. Therefore, the efficiency of copper extraction from both copper slags increased almost twofold, while zinc and cobalt extraction improved by 29–100% compared to the values obtained from the respective variants with the native spent medium, reaching values of 26.6% and 19.9% for sample III and 34.5% and 50.6% for sample F, respectively (Figure 6c and Figure 7c).

The combination of spent medium supplementation with 5 or 10 g H₂SO₄/L and 55 °C of indirect bioleaching had a clearly noticeable positive effect on the extraction of copper, zinc, and cobalt from sample F and only on zinc and cobalt extraction from sample III, increasing their results by 7 to 15% compared to the respective results obtained at 25 °C (Figure 6c and Figure 7c).

Any further supplementation of the spent medium with a higher concentration of sulphuric acid resulted in a lower final pH, as measured at the end of the experiments. Therefore, the spent medium supplementation with 25 g H₂SO₄/L, corresponding to 1440 g H₂SO₄/kg of copper slag (if expressed as the total available acidity as sulphuric acid), resulted in the final pH levels falling to a strongly acidic (3.42 for sample F) and a moderately acidic range (4.06 for sample III) (Figure 6a and Figure 7a). The copper, zinc, and cobalt extraction efficiencies further increased, reaching values of 27.6%, 78.1%, and 64.1% for sample III, and 67.6%, 52.8%, and 63.9% for sample F, respectively (Figure 6c and Figure 7c). The values mentioned above were at least equal to (regarding zinc and cobalt) or two times higher (regarding copper) than the respective results obtained from the chemical leaching with sulphuric acid at the respective target pH (3.5, sample F; 4.0, sample III). Moreover, the respective values for copper and zinc recovery were more than twice as high as those obtained with native spent medium leaching at the same temperature (for samples F and III).

Regarding copper, a nearly complete recovery of the acidic-soluble phase of copper from sample F, previously determined by pH-dependent chemical leaching with sulphuric acid to a pH of 1.5, was achieved under these conditions, regardless of the temperature. Therefore, the concentration of sulphuric acid had a more pronounced positive effect on base metal recovery compared to the effect of increasing the temperature from 25 °C to 55 °C. In contrast, under conditions of indirect bioleaching, only 55% of the above phase of copper in sample III was susceptible to leaching. Therefore, comparing copper leaching between the two types of copper slags, sample III was a refractory raw material due to its mineralogy, resulting in a higher requirement for sulphuric acid for the efficient recovery of copper.

The positive effect of the combination of 55 °C and sulphuric acid dosage was more pronounced for cobalt extraction compared to zinc from sample III, as cobalt recovery increased by approximately 30% with a dosage of 25 H₂SO₄ g/L, compared to the result obtained at 25 °C, reaching 94% of its total content (Figure 7c).

Supplementation of the spent medium of A. niger with 25 g H₂SO₄/L resulted in substantial iron co-leaching from both copper slags, reaching a value of up to 51.8% and 67.3% for sample F, and 53.9% and 67.7% for sample III, at temperatures of 25 °C and 55 °C, respectively. Therefore, the selectivity of non-ferrous metal extraction dropped significantly, reaching a value of 0.076 for sample F and 0.07 for sample III at 55 °C, respectively (Figure 8a,b).

The results regarding the selectivity of the non-ferrous metal extraction from both copper slag samples, determined at the used set of experimental conditions, we can sum them are summarised as follows:

• The acidity of the spent medium (native or supplemented with sulphuric acid), but not the temperature, was the main crucial factor that controlled the efficiency of base metal extraction from the studied copper slags;
• The insignificant difference in the selectivity values of base metal extraction at the tested temperatures indicated that copper slag III was more refractory to leaching, considering the extent of base metal extraction from sample F.

3.5. XRD Diffractograms of Copper Slag Leaching Residues

Fayalite (Fe₂SiO₄) has been identified as the most common mineral in both types of copper slags, with a relative content ranging between 33% (copper slag taken from the surface of the slag dump) to 56% (copper slag taken from a depth of 1 m below the surface) (

Table 5. The relative content of minerals identified by XRD analysis in the copper slag residues, obtained due to the indirect bioleaching of slag samples at a 5% pulp density under the relevant experimental conditions.

Copper Slag/Copper Slag Residue	Relative Content, %
	Fayalite	Pyroxene Group			Proto-Mangano-Ferro-Anthophyllite	Magnetite
		Clinopyroxene	Diopside	Augite
Sample F
Raw sample	33	10	-	-	56	-
Leaching with the spent medium at 55 °C	60	-	-	37	-	3
Leaching with the spent medium supplementation with 5 g H2SO4/L at 55 °C	40	-	-	28	-	32
Sample III
Raw sample	56	-	44	-	-	-
Leaching with the spent medium at 55 °C	40	-	-	32	-	25
Leaching with the spent medium supplementation with 5 g H2SO4/L at 55 °C	33	-	-	45	-	22

). The diffractograms of both samples revealed that fayalite formed peaks located at 15°, 20°, 25°, 29°, 32°, 35–37°, 52°, and 61° 2θ copper positions (marked as 1 in Figure 1a,b). Clinopyroxene and diopside, members of the pyroxene group, were identified as the second most abundant minerals in slag samples F and III, with relative contents of 10% and 44%, respectively. The XRD pattern indicated that the peaks of clinopyroxene were located at 11°, 20°, 30°, 35°, 40–45°, and 50°, and were distributed uniformly between 52° and 72° 2θ copper positions (marked as 2 in Figure 1a). The peaks of diopside, described in the copper slag unaffected by weathering, occupied the aforementioned positions as well positions at 15°, 18°, 24°, 28°, 36–37°, and 46–48° (marked as 2 in Figure 1b). Protomangano-ferro-anthophyllite (H₂Fe_4.48Mg_1.08Mn_1.44Si₈O₂₄) was the third mineral, described only in the copper slag sample taken from the surface of the dump, whose presence was an indication that weathering occurred there (marked as 3 in Figure 1a). Its peaks were located at 11°, 20°, 30°, 35°, 40–45°, and 50°, as well as being uniformly distributed between the 52° and 72° 2θ copper positions. Its relative content in the raw copper slag sample was 56%.

The diffractograms in Figure 9 compare the transformations of minerals, carried out due to the applied indirect bioleaching of both types of copper slag under the same experimental conditions. The graphs (Figure 9a) showed that protomangano-ferro-anthophyllite was the mineral most susceptible to bioleaching at 55 °C and in the presence of citric and oxalic acids contained in the native spent medium of A. niger, resulting in its accelerated acidolysis and the decomposition of its structure. Therefore, the mineral was not observed in the copper slag residue obtained after the end of the experiment. Moreover, the higher reactivity of protomangano-ferro-anthophyllite protected fayalite from proton attack and decomposition, increasing its relative content in the copper slag residue to 60% (Table 5). However, the higher temperature of indirect leaching and the presence of organic acids in the native spent medium initiated the decomposition of pyroxene minerals, clinopyroxene and diopside, resulting in the observation of augite, another member of the pyroxene group, in the copper slag residues of both raw samples. Its chemical content, determined by XRD analyses, varied within a narrow range, Al_0.3Ca_0.8Fe_{(0.27–0.38)}Mg_(0.65–0.8)O₆Si_(1.8–2.0), as well as minimal contents of Na, Mn, Cr, and Ti, and depended on the presence or absence of sulphuric acid in the spent medium.

Under these conditions, copper, zinc, and cobalt recovery was 26.7%, 26.5%, and 48.3%, respectively, with a process selectivity value of 0.335 (Figure 6c and Figure 8a). The dissolution of protomangano-ferro-anthophyllite to such an extent was not observed in the copper slag residue obtained after one month of direct, two-step bioleaching of the same copper slag in the presence of A. niger at a 5% pulp density at 25 °C [45].

In contrast, in the respective experiment under the same experimental conditions with the slag sample unaffected by the weathering process, the hydrogen ions attacked fayalite and diopside as members of the pyroxene group, degrading their structure, which resulted in lower content in the slag residue, 40% and 32% for augite, respectively (Table 5). The clearance of only the fayalite micro-peaks between 20° and 30°, 50° and 58°, and 60° and 68° positions in the diffractograms of the slag residue was an indication of the enhanced decomposition of the mineral (Figure 9c). This supported the observation of other researchers that diopside is a mineral more resistant to proton attack compared to fayalite [37]. The values of copper, zinc, and cobalt recovery from copper slag, as well as their overall selective extraction over iron co-leaching, were 5.0%, 12.3%, 14.3%, and 0.079, respectively (Figure 7c and Figure 8b).

It is interesting to note that magnetite (Fe₃O₄) was described in both copper slag residues, with a relative content of 3% and 25% in copper slag residues (marked as 4 in Figure 9a–c) of samples F and III, respectively. The combination of factors such as 55 °C, intensive pulp mixing, the duration of the experiment, a moderate acidic or almost neutral pH, a higher ferrous concentration in the pregnant leach solutions, and the enhanced chemical oxidation of ferrous iron to the ferric state all contribute to its formation under these experimental conditions. Moreover, the monitoring applied during the bioleaching of sample III confirmed the iron net loss in the solution between 4 and 24 h.

The combination of 55 °C and the spent medium, supplemented with a dosage of 5 g H₂SO₄/L during bioleaching, further accelerated the acidolysis process, affecting not only the recovery of non-ferrous metals but also the degradation of the structure of the copper slag. In that case, fayalite consumed a higher concentration of hydrogen ions in the solutions, resulting in lower content in the copper slag residues, with values of 40% and 33%, respectively (Table 5). The minerals from the pyroxene group, clinopyroxene and diopside, were refractory at the aforementioned conditions, and the augite content, the product of their transformation, increased to 28% and 45% in the copper slag residues of samples F and III, respectively (Table 5). These mineral transformations resulted in the significant extraction of copper, zinc, and cobalt from copper slag taken from a dump surface, with values up to 49.2%, 41.3%, and 62.2%, respectively (Figure 6c). The selectivity value of their overall extraction compared to iron co-leaching was 0.206 (Figure 8a). In contrast, the accelerated acidolysis under these experimental conditions resulted in a lower recovery of all non-ferrous metals from the copper slag unaffected by weathering and enhanced iron co-leaching, with a selectivity value of 0.08 (Figure 8b).

Magnetite was also observed in both copper slag residues obtained at 55 °C and spent medium supplemented with 5 g H₂SO₄/L, with relative contents of 32% and 22% (Table 5). The diffractograms revealed the more pronounced peaks of magnetite at the 18°, 30°, 35°, 37°, 44°, 53°, and 62° positions (Figure 9b,d). In contrast, magnetite formation was not observed in the residues obtained after the direct, two-step, one-month bioleaching of both types of copper slag in the presence of A. niger at a 5% pulp density and 25 °C [45].

3.6. Morphological Analysis of Copper Slag Leaching Residues

The morphological analysis of the copper slag, carried out by SEM, revealed that both samples consisted of small particles with a grainy structure, distributed almost evenly on a base on which medium- and large-sized particles were disposed. These particles had an almost regular form, flat surfaces, and sharp edges (Figure 10a,d).

The indirect leaching of copper slag with the spent medium of Aspergillus niger had a substantial effect on the surface of the solid particles, as evidenced by observations of furrows with different sizes, irregular forms, and orientations (Figure 10b,e). The furrows were visible only on the surface of medium- and large-sized particles of both copper slags. It is interesting to note that the marks of the acidolysis process, carried out due to the presence of citric and oxalic acids in the native spent medium, were easily observed at a 650× magnification level on the surface of particles of sample F, where they formed a net of tiny furrows oriented parallel to each other and with an average size of 20–30 μm. In contrast, the marks of bioleaching in sample III were observed at a magnification level of 750×, where furrows were larger in size and width, but with a lower density. That observation, as well as the results regarding the recovery of non-ferrous metals and the X-ray analysis, confirmed that sample F is the more chemically reactive of the studied copper slags.

The insignificant supplementation of the spent medium of A. niger with 5 g H₂SO₄/L had a significant effect on the surface of solid particles, with the formation of grainy surface structures divided from each other by furrows shorter in size but with a higher depth, in the case of copper slag sample F (Figure 10c), and tiny scratches, larger in size, in the case of copper slag sample III (Figure 10d).

The morphological analysis revealed a positive effect of supplementing the spent medium with sulphuric acid on acidolysis and the enhanced recovery of non-ferrous metals; however, it also led to intensive erosion of the flat surface and the edges of medium- and larger-sized copper slag particles.

The superficial architecture of both copper slag which underwent indirect bioleaching under these conditions (spent medium supplementation with a dosage of 5 g H₂SO₄/L and a temperature of 55 °C for 72 h) looked similar to the marks of Aspergillus niger hyphae due to the direct one-step bioleaching of copper slag at a temperature of 25 °C for a month [45].

The results of the experiments and analyses conducted allow us to propose the following flowsheet for the recovery of non-ferrous metals from copper slag via indirect bioleaching using a spent medium of Aspergillus niger (Figure 11). The flowsheet consists of three main steps: Aspergillus niger cultivation under optimal conditions for citric acid synthesis, spent medium separation, and the indirect bioleaching of non-ferrous metals from copper slag under relevant experimental conditions (temperature, sulphuric acid supplementation, and pulp density).

The economic viability of any raw material processing technology depends on several factors, including the efficiency of extracting valuable compounds, the associated operating costs, and the market prices of the products obtained. Copper slag is rich in non-ferrous metals such as copper, zinc, cobalt, and nickel, as well as iron and silicon, whose market prices are slightly volatile. The main advantage of hydrometallurgical methods for processing copper slag is the simultaneous extraction of non-ferrous metals contained in copper slag, as well as an established technological scheme for their selective separation, concentration, and conversion into marketable products through extraction and removal methods using solvents, selective precipitation, or electrolytic extraction [47]. Therefore, the economic assessment of copper slag processing by hydrometallurgical methods developed and studied in laboratory conditions is positive, despite the inclusion of some very expensive operations, such as grinding.

Guidelines for Future Scientific Research

The indirect bioleaching of copper slag using the spent medium of A. niger must meet several prerequisites to improve the method’s efficiency and enable experiments on a larger scale. Firstly, the cultivation of fungi should be replaced by a sucrose-based medium with a nutrient medium based on inexpensive organic waste matter rich in cellulose, such as straw and hay. Aspergillus niger strains are excellent producers not only of citric acid but also of cellulase enzymes that catalyse the depolymerisation of cellulose to cellobiose. Secondly, some strong oxidants, such as H₂O₂, should be incorporated in the flowsheet with the primary aim of recovering the oxidisable phase of copper in copper slag. Thirdly, a prerequisite for transitioning any mineral processing method from the laboratory at a larger scale is its adaptation from batch to continuous operation and confirmation of its economic viability. Fourthly, citric acid should be recovered and recycled from metal–citrate complexes in the pregnant solution and used in the next cycle of the indirect biological extraction of non-ferrous metals from copper slag. Fifthly, the mineral residue obtained as a result of the indirect biological leaching of copper slag using a spent medium of Aspergillus niger should be studied and incorporated into the production of concrete with suitable specific properties. When the method meets the conditions mentioned above, it can be defined as a resource-efficient and environmentally friendly method for the bioprocessing of copper slag characterised by low energy consumption and low emissions of greenhouse gases and other waste products.

4. Conclusions

Chemical leaching with sulphuric acid revealed that the acidic-soluble phase of zinc and cobalt comprised between 80% and 90% of their total content in the studied copper slags. In contrast, the copper content in that phase was 50%, increasing to 75% due to weathering that occurred at the dump surface, as the residual amount constituted the oxidisable phase of metal in the respective type of copper slag. The effective extraction of non-ferrous metals from the acidic-soluble phase required a substantial amount of sulphuric acid, and the process exhibited lower selectivity due to the significant co-leaching of iron.

Compared to Penicillium ochrochloron, Aspergillus niger, a fast-growing fungus, produced a spent medium with a lower pH and organic acid content, which facilitated the better extraction of non-ferrous metals from both types of copper slag. The main mechanisms for extracting base metals from copper slags were acidolysis and complexolysis, as the amount of copper, zinc, and cobalt extracted at an equilibrium pH of 5.0–5.5 was equal to the amount extracted with sulphuric acid at a target pH of 4.5. However, the extraction of base metals was inefficient, even at a pulp density of 5%, due to the limited acidity of the spent medium of A. niger.

The bioleaching of non-ferrous metals from both types of copper slag at 55 °C using a spent medium of A. niger supplemented with sulphuric acid resulted in almost complete extraction of non-ferrous metals from their acidic-soluble phase, as the suitable doses were 10 g H₂SO₄/L and 25 g H₂SO₄/L for copper slag affected by weathering and the slag sample unaffected by this process, respectively. However, due to the reduced extraction of base metals and intensive iron co-leaching under similar experimental conditions, copper slag unaffected by weathering process can be defined as a raw material refractory to bioleaching with the spent medium of A. niger.

The accelerated acidolysis and decomposition of the protomangano-ferro-anthophyllite (regarding copper slag affected by weathering) and fayalite, comprising the structure of both types of copper slags, resulting from supplementing the spent medium of A. niger with sulphuric acid, provided a foundation for the efficient recovery of non-ferrous metals.