**1. Introduction**

Sulfide minerals are an important chemical form of several metal resources in nature. For example, 90% of global copper resources are sulfide ores, where most of the copper is in the form of covellite (CuS), chalcocite (Cu2S), chalcopyrite (CuFeS2), and bornite (Cu5FeS4). Hydrometallurgical processes were used in the treatment of covellite and chalcocite deposits in a similar way to oxidized forms, due to their solubility in acid medium, but these resources are largely exhausted. On the other hand, chalcopyrite the most abundant copper-bearing resource, accounting for appropriately 70% of the known reserves in the world [1–3], and it is submitted to pyrometallurgical treatment. With the increase in the copper demand, the decline of high-grade ores has extended and, nowadays, deposits with grades around 0.4–0.5% of copper are mined. The exploitation of these reserves by traditional flotation methods followed by pyrometallurgical processes (smelting-converting-electrorefining route) is in the limit of economic viability. In addition, the presence of impurities, like As, the generation of large amount of wastes, like slags, and the control of atmospheric emissions, make the research of more eco-friendly processing technologies necessary [4]. The hydrometallurgical route has the advantages of being able to process low-grade ores, to allow better control of co-products, and to have a lower environmental impact [5]. Nevertheless, chalcopyrite is refractory in sulfuric media with slow dissolution rates [6,7].

**Citation:** Álvarez, M.L.; Fidalgo, J.M.; Gascó, G.; Méndez, A. Hydrometallurgical Recovery of Cu and Zn from a Complex Sulfide Mineral by Fe3+/H2SO4 Leaching in the Presence of Carbon-Based Materials. *Metals* **2021**, *11*, 286. https://doi.org/10.3390/met11020286

Academic Editor: Dariush Azizi Received: 31 December 2020 Accepted: 3 February 2021 Published: 6 February 2021

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The most commonly employed hydrometallurgical process for the oxidation of chalcopyrite and extraction of copper is the sulfuric acid ferric sulfate system [8,9]. It is well known that low dissolution rate of chalcopyrite and, thus, low metal recoveries, is mainly due to the formation of a passivation layer on the mineral surface [10–12]. Different authors proposed that the composition of this passive layer is elemental sulfur, copper-rich polysulfide, or iron salts with low electrical conductivities, which may prevent the electrons/ions transfer to chalcopyrite matrix and, thus, hinder its copper dissolution [8,13,14].

Different researches have been performed in acidic sulfuric media to accelerate the kinetics of chalcopyrite leaching. Recent researches have studied the chalcopyrite leaching in sulfuric acid with the presence of chloride and nitrate ions [15,16]. Copper extraction increases in the presence of both ions [15,16]. However, the use of chloride could be limited by environmental problems that are related to high solubility of metallic chlorides and high costs of solvent extraction and electrowinning processes. Castillo-Magallanes et al. [17] studied the addition of some organic compounds, like ethylene glycol (C2H6O2) and polysorbates. These authors found that the adsorption of organic agents limit the development of surface phases on chalcopyrite, allowing for a higher extraction of copper. Tehrani et al. [18] proposed that the addition of ethylene glycol increase the chalcopyrite dissolution rate by removing the elemental sulfur layer from the surface. Nevertheless, the addition of these two organic agents does not completely inhibit passivation and, although there are improvements in the leaching process, the effect is limited. Kartal et al. [19] investigated the improvement of chalcopyrite leaching by the addition of 20% vol of tetrachloroethylene (C2Cl4). These authors found that tetrachlorethylene-assisted leaching led to 80% of copper extraction before passivation.

Other researchers studied the catalytic effect of inorganic compounds, like pyrite [20], iron powder [21], nanosized silica [22], and, more recently, coal, carbon black, or activated carbon [14,23]. The use of activated carbon enhances the kinetics of sulfide minerals leaching in sulfate media, leading to a substantial increase in copper extraction rate [24]. Nakazawa [14] proposed that the enhanced kinetics of chalcopyrite leaching could be attributed to a decreased redox potential, as well as galvanic interaction between chalcopyrite and carbon matrix. The most extensive use of activated carbon in hydrometallurgy is the recovery of gold from cyanide solutions [25]. Activated carbon could show a great electron donor/acceptor due to its surface functional groups. In addition, they usually have an appropriate electrical conductivity that has been shown to promote a galvanic interaction between sulfide minerals and carbon matrix. Moreover, a well-developed porosity (micro, meso, and macroporous) in these carbon materials may facilitate the accessibility to the redox active sites through dissolved exogenous compounds [26].

Nevertheless, the majority of the carbon materials used in these previous studies were commercial activated carbons or carbon black powders, with low information being available on their physicochemical properties. Furthermore, they are relatively expensive and their use could significantly raise the cost of the industrial process. For this reason, the main objective of the present work is to study the use of cheaper carbon-based materials in the leaching of copper and zinc from a sulfide complex mineral. The addition effect of commercial charcoal and two magnetic biochars that were obtained by pyrolysis of biomass wastes was compared to that of commercial activated carbon. The effect of different mineral/carbon material ratios was studied to reduce the consumption of carbon material. pH, redox potential, copper, and zinc content of leaching solution were monitored during the leaching experiments of sulfide ore samples

#### **2. Materials and Methods**

#### *2.1. Materials Selection and Characterization*

The sulfide ore concentrate from the massive sulfide deposits of the Iberian Pyrite Belt (IPB) was collected after grinding and flotation processes (80% < 50 μm). The sample was air-dried, crushed, and sieved below 50 μm while using a mortar mill Retsch RM 100. Wavelength X-ray fluorescence (WDXRF) was performed in an ARL ADVANT'XP + sequential

model from THERMO (SCAI-Malaga University). Concentration data were obtained using the UNIQUANT Integrated Software. XRD was performed using a Bruker diffractometer model D8 Advance A25.

The four carbon-based materials that were used in this work (Table 1) were: a commercial activated carbon (AC) supplied by Panreac (Spain); a commercial charcoal (VC) supplied by Ibecosol (Spain); and, two magnetic biochar samples (BM and HM) obtained, respectively, from pyrolysis of pruning waste and pruning waste hydrochar. Ingelia (Náquera, Spain) supplied pruning waste and pruning waste hydrochar. The two magnetic biochar were prepared by impregnation with ferric sulfate salts of pruning waste (BM) or corresponding hydrochar (HM), followed by pyrolysis at 500 ◦C for 5 h [27].

**Sample <sup>C</sup> (%) H (%) N (%) S (%) O (%) H/C O/C Ash (%) pH Eh (mV) SBET (m2/g)** CA 85.72 0.88 0.00 0.00 13.41 0.12 0.12 1.00 8.03 453 1138.96 CV 80.21 3.12 0.92 0.00 15.74 0.47 0.15 14.94 8.31 355 2.18 BM 20.18 0.29 0.60 7.83 0.71 0.17 0.03 70.38 7.33 314 56.86 HM 43.81 0.81 1.14 5.12 8.14 0.22 0.14 40.98 8.22 298 183.63

**Table 1.** Main characteristics of carbon-based materials.

The four carbon-based materials (AC, VC, BM, and HM) were air-dried, crushed, and then sieved below 100 μm using a mortar mill Retsch RM 100 and characterized according to the following properties: pH, Eh, BET surface (SBET), cation exchange capacity (CEC), pore volume (cm3/g), ash content (%), and elemental C, H, N, O, and S content (dry basis).

The pH and Eh of samples were determined on aqueous solutions at a concentration of 4 g·L−<sup>1</sup> of carbon-based material sample in distilled water. pH was measured using a Crison micro pH 2000 and Eh with a pH 60 DHS equipment. C, H, N and S content were determined by dry combustion using a LECO CHNS 932 Analyzer. The ash content was calculated by combustion of samples at 850 ◦C in a Labsys Setaram TGA analyzer. 20 mg of each sample were heated at a rate of 15 ◦C min−<sup>1</sup> up to 850 ◦C using 30 mL min−<sup>1</sup> of air. O was obtained by difference as 100% − (%C + %H + %N + %S + %Ash). Atomic ratios H/C and O/C were also calculated.

#### *2.2. Leaching Experiments*

A thermostatic bath with stirring GFL 1083 (heating power of 1500 W and the voltage 230 V) was used for the leaching tests. The temperature (90 ◦C) and stirring speed (250 rpm) were controlled during the leaching process for 96 h. The leaching experiments were carried out in 250 mL ISO borosilicate glass jars.

Leaching agent used was prepared by 0.5 M H2SO4 solution with a concentration of 5 g·L−<sup>1</sup> of Fe (III). The pH and Eh of a 0.5 M H2SO4 solution were 0.983 and 423 mV, respectively. The purities of Sigma-Aldrich® sulfuric acid and Fe (III) sulfate hydrated Labkem were 97% and 99.5%, respectively. 2.5 g of IPB was weighed in every jar. 50 mL of leaching agent as added. Except for the control, the ratios IPB/carbon-based material (weight/weight) were 1/1, 1/0.5, and 1/0.25.

1 mL of the supernatant liquor of each leaching experiment was withdrawn at different reaction times (2, 4, 24, 48, 72, and 96 h). The sampling was carried out, as follows: first, the stirring was stopped to let the sample stand and favors its decantation. After this short period of time, 1 mL of the supernatant solution was removed and, then, filtered and transferred to a 25 mL graduated flask and make up to volume with distilled water. In order to compensate for this extracted mL and maintain the same conditions throughout the system, 1 mL of the leaching solution was added. Metal extraction in solution was calculated, as follows:

$$\text{Metal extraction in solution (\%)} = \frac{\text{metal content in leading solution (g)}}{\text{initial IPB metal content (g)}} \times 100 \quad \text{(1)}$$

Stirring was stopped after 96 h. The pulp was filtered and the solid waste was washed twice with 50 mL of H2SO4 solution pH = 2 in order to recover the adsorbent content of metals. The total Cu and Zn extraction degree was determined while taking their content in the final into account and washed solutions using the following formula:

$$\text{Total metal extraction degree } (\%) = \frac{\text{total metal extracted (g)}}{\text{initial IPB metal content (g)}} \times 100\tag{2}$$

where total metal extracted (*g*) was metal content in leaching solution at 96 h (*g*) plus metal content in washed solutions (*g*).

The pH and Eh of leaching solution were determined at different reaction times (0, 2, 4, 24, 48, 72, and 96 h) using a Crison micro pH 2000 and a pH 60 DHS, respectively. The concentration of Cu and Zn in the leaching and washed solutions was determined using a Perkin Elmer AAnalyst 400 Atomic Absorption Spectrophotometer.

#### **3. Results and Discussion**

#### *3.1. Characteristics of Samples*

Table 1 summarized the main properties of VC, AC, BM, and HM. In spite of the fact that several researches use carbon materials as catalysts in the leaching of metal sulfides, there are not studies comparing the effect of carbon materials with different properties [14,23,24].

The highest C and O content corresponded to AC and VC. AC followed by BM showed the lowest H/C ratios and, consequently, the highest aromaticity. BM showed the lowest O/C and, consequently, the lowest content on oxygen functional groups. BM and HM showed the lowest values of Eh, whereas CA showed the highest value (455 mV).

The XRD analysis of IPB (Figure 1) showed a 52.6% of chalcopyrite, 8.4% of sphalerite, 32.4% of pyrite, 1.4% of cristobalite, and 5.1% of nacrite (Table 2). Table 3 summarizes the chemical composition of IPB that was obtained by X-ray fluorescence. The main valuable metals were Cu (17.61%) and Zn (6.76%). For this reason, the present work is focused on the recovery of these two metals.

**Figure 1.** XRD pattern of IPB sample.


**Table 2.** XRD analysis of Iberian Pyrite Belt (IPB) sample.

**Table 3.** Chemical composition of IPB sample.


#### *3.2. Extraction Degree of Cu and Zn*

Figure 2 shows the extraction degree of Cu and Zn after 96 h of IPB leaching in sulfuric/Fe3+ solution at 90 ºC. The effect of carbon materials (VC, AC, MB, and HM) on the recovery of Cu and Zn was different, depending on metal, carbon-based material, and their ratio with respect to the IPB sample. The amount of Cu and Zn extracted from IPB without the addition of carbon-based material was 63 and 72%, respectively. The highest zinc extraction degree (>90%) was obtained with the addition of VC and AC in the IPB/carbon-based material ratio of 1/0.25. The highest Cu extraction degree (85%) was obtained after the addition of VC in a ratio of 1/0.25. In general, the addition of BM and HM significantly decreased the extraction degree of Cu from IPB sample. Only an increment in extraction of Zn was observed with a IPB/BM ratio of 1/1.

**Figure 2.** Extraction degree (wt%) of Zn and Cu.

The experimental results showed that the characteristics of a carbon-based material seem to play an important role in the recovery of Cu and Zn from complex sulfide mineral. This means that it is necessary to possess a better knowledge of the mechanism of reaction in the presence of carbon-based materials and the optimization of the properties of the carbon-based material for its potential application as catalyst in the leaching of metals from sulfide ores. Figure 3 shows the Cu and Zn extracted in the leaching solution at different reaction times (2, 4, 24, 48, 72, and 96 h). It is observed that the extraction of both metals fluctuates along time, indicating that an amount of the dissolved Cu or Zn could be adsorbed on the surface of mineral and, especially, on the carbon-based material. Adsorption is more important in the case of Zn. The comparison of Figures 2 and 3 shows that, at 2 h, the Zn concentration in the leaching solution was similar to the extraction degree of Zn. At longer reaction times, the concentration of Zn in the solution decreases. This effect could be due to its adsorption on the surface of the mineral or the carbon-based adsorbent.

**Figure 3.** Evolution of Cu (**a**) and Zn (**b**) dissolved at different reaction times (h)**.**

#### *3.3. Eh Evolution during Leaching Experiments*

Nakazawa [14] used carbon black in sulfuric acid media at 50 ◦C and proposed that the enhanced kinetics of chalcopyrite leaching could be attributed to dissolution reactions at the low redox potential in addition to the galvanic interaction between chalcopyrite and carbon black. Cordoba et al. [28] concluded that the redox potential is a key factor in the leaching of chalcopyrite and proposed a critical potential of approximately 450 mV. A high potential of the leaching favors the rapid precipitation of ferric as jarosite and the corresponding passivation of chalcopyrite. Hiroyoshi et al. [29] showed that the leaching rate of chalcopyrite by Fe3+/H2SO4 solutions greatly depends on the redox potential and that there is a maximum leaching rate at an optimum redox value. Previous works [30] pointed out that the redox potential has to be low enough for chalcocite formation and high enough for the subsequent chalcocite oxidation. Kamentani and Aaoki [31] investigated the effect of the redox potential in the range of 300 and 650 mV on chalcopyrite leaching at 90 ◦C. They found that the leaching rate increased with an increase in the suspension potential, until it reached a maximum at 400–430 mV. Thereafter, the leaching rate decreased. Córdoba et al. [28] found that chalcopyrite leaching was remarkably enhanced at low redox potential, which suggested that chalcopyrite dissolves through the intermediate formation of covellite. Figure 4 shows the Eh evolution in the different leaching systems along reaction time. The initial Eh (V) value varies between 641 for IPB and 612 mV for IPB+AC (ratio 1/1). An important decrease on the Eh (V) was observed after 2 h of leaching. The reduction was higher in samples that were treated with AC in spite of this carbon-based materials showing the highest Eh (V) among the materials used in this work (Table 1). The redox potential fell with decreasing Fe3+ concentrations, probably due to AC participating in the reaction of sulfide mineral leaching, decreasing the Fe3+/Fe2+ ratio according to Nakazawa [14]. Leaching systems with the three AC ratios (1/1, 1/0.5, and 1/0.25) showed similar Eh (V) values along reaction time; however, different leaching percentage of Cu were obtained. Therefore, the Eh (V) was not the only factor that controlled the effect of carbon-bases material on the leaching of metals from sulfide minerals.

**Figure 4.** Evolution of Eh (mV) at different reaction times (h).

#### *3.4. pH Evolution during Leaching Experiments*

Figure 5 shows the pH evolution in the different leaching systems along reaction time. In spite of carbon-based materials showing alkaline pH (Table 1), their addition to leaching systems significantly decreased the pH of the leaching solution. The highest pH reduction was observed at 2 h of leaching reaction and especially for BM and HM. After 2 h, the slightly increased pH remains, in general, lower than the pH without the addition of carbon-based material. This effect could be due to the reaction of surface functional groups of carbon-based adsorbents with the oxidizing agent (Fe3+/H2SO4). These can react with different oxygenated groups generating organic acids and releasing protons to the medium. In addition, the addition of carbon-based adsorbents decreased the Eh of the leaching system (Figure 4) that favors the generation of H2S, which then was oxidized to S by Fe3+ [9]. Córdoba et al. [32] studied the passivation of chalcopyrite in the presence of ferric sulfate solutions at different pH and Eh. They concluded that low pH values (especially <1) of the leaching solution have a negative effect on the chalcopyrite dissolution. The high reduction in the pH of leaching solution after the addition of BM and HM that was observed at 2 and 4 h (Figure 5) could be the reason for an important reduction of dissolved Cu (Figure 4).

In summary, it has been observed that carbon-based materials greatly influence the dissolution of Cu and Zn from chalcopyrite and sphalerite minerals in Fe3+/H2SO4 media at 90 ◦C. The amount of Cu and Zn extracted from IPB without the addition of carbon-based material was, 63 and 72%, respectively. The highest amount of extracted Zn (>90%) was obtained with the addition of VC and AC in a IPB/carbon-based material ratio of 1/0.25. It is possible to recover 85% of copper after the addition of VC in 1/0.25. In general, the addition of carbon-based adsorbents decreases the Eh and pH of the leaching solution. An optimization of the properties of the carbon-based material and experimental conditions used in the presence of carbon-based adsorbent is necessary for its potential application as catalyst in the leaching of metals from sulfide minerals.

**Figure 5.** Evolution of pH at different reaction times (h).

#### **4. Conclusions**

The main conclusions obtained from the present work were the following:

In general, the addition of carbon-based adsorbents reduces the Eh and pH of leaching systems and probably modifies the reaction mechanisms. The Cu and Zn extracted with Fe3+/H2SO4 vary in the presence of carbon-based material. The effect was different, depending on characteristics and the amount of carbon-based material. The addition of commercial charcoal in the ratio 1/0.25 of complex sulfide mineral/charcoal increases the Cu and Zn extracted to 92 and 85%, respectively. Future researches are necessary for knowing the reaction mechanism of chalcopyrite and sphalerite leaching in the presence of carbon-based materials and the properties of carbon-based materials that are involved in the reaction.

**Author Contributions:** Conceptualization, M.L.Á., J.M.F., G.G. and A.M.; methodology, M.L.Á., J.M.F., G.G. and A.M.; formal analysis, M.L.Á., J.M.F., G.G. and A.M.; M.L.Á., J.M.F., G.G. and A.M writing—original draft preparation; funding acquisition, A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ministerio de Ciencia, Innovación y Universidades (Spain), grant number RTI2018-096695-B-C31.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

