Hydrogen Generation in the Leaching of Chalcopyrite Concentrate in Acid Medium Assisted by Methanol

Abstract

Currently, chalcopyrite is the world’s largest copper reserve. Commonly, the copper contained in chalcopyrite is obtained by pyrometallurgical processes. Still, in recent years, due to the environmental problems generated by this route, more environmentally friendly techniques have been proposed, such as hydrometallurgy; but chalcopyrite has the drawback of passiveness. A promising alternative to minimize this phenomenon is using polar organic solvents in an acidic medium, obtaining copper extraction percentages of 90% in five h. A solvent that has significant functionality is methanol. Moreover, a topic barely studied in depth is the characterization of the gases emitted in the leaching of minerals such as chalcopyrite. In this sense, one gas generated through chalcopyrite leaching is molecular hydrogen, which would increase the economic viability of the process. In this work, the gases formed during the leaching of chalcopyrite concentrate are analyzed, and the formation of only molecular hydrogen by gas chromatography was detected. The hydrogen production was 0.24 µmol in 300 min, and the copper extraction was around 65%, using a concentration of 0.5 M of H₂SO₄, 60 mL of methanol, and 20 mL of H₂O₂. Thus, based on the detected chemicals in solid residues of the leaching of chalcopyrite concentrate, the thermodynamic analysis supports the spontaneous formation of hydrogen with a value of ΔG = −119.66 kJ/mol.

1. Introduction

Currently, access to metals is fundamental for humanity’s technological development [1]. In this way, the hydrometallurgical process has shown to be a viable alternative for metal recovery [2]. This process has been studied to extract copper from chalcopyrite (CuFeS₂); nevertheless, the refractory and passive characteristics of the mineral are a challenging task for the scientific community [3]. Alternative processes have been proposed to minimize these difficulties with interesting results, such as using pyrite as a catalyst and reductor agent [4]; however, pyrite must be pretreated with silver, which causes an increase in the operating process cost. Furthermore, to achieve high copper extraction, oxidizing agents are required [5]. The role of this agent is to avoid passivation caused by Fe³⁺, O₂, H₂O₂ and others such as ${Cr}_2{O}_7^{2-}$, ${ClO}_4^{-}$, O₃ and ${MnO}_4^{-}$ [6,7].

Additionally, most of the alternatives operate in an acidic medium, mainly sulfuric and hydrochloric acid, where the best results are obtained with hydrochloric acid, leading to a problematic corrosive medium. A less harmful alternative is employing ionic liquids, since refractory metal oxides can be leached with less corrosion, facilitating the extraction with solvents [8]. In this sense, organic solvents (methanol, ethanol, acetic acid, acetone, and ethylene glycol) are promising leaching systems in acidic mediums, achieving around 90% extraction of copper in five h [9]. These processes have lacked development beyond a pilot scale because they have not shown solid economic viability. On the other hand, the gases generated during chalcopyrite leaching have not been analyzed by qualitative and quantitative methods, regardless of the possibility of hydrogen formation with positive implications in economic profitability [10,11].

Czech and Troczynski [12] demonstrated hydrogen generation by aluminum leaching, achieving around 1200 cm³ in 60 min using the water-soluble inorganic salts KCl and NaCl [12]. The critical issue limiting the efficiency of hydrogen generation is the formation of a dense oxide film, limiting the access to the aluminum surface; also, the aluminum dissolution represents an increase in the cost of the process [13]. Furthermore, there are other options for hydrogen generation that do not depend on hydrocarbon, like the photo-split of water [14]. This process includes a photocatalyst, water, and solar radiation as an energy source since this process is thermodynamically non-spontaneous [15]. In the literature, values around 3.6 eV for the formation of H₂ using a photocatalyst of SiC have been reported [16]. Similarly, in the leaching process, there is a catalyst in an aqueous medium, with the concentrate acting as the catalyst, and the oxidizing agent as the energy source.

Wu and Lee [17] investigated the formation of hydrogen with Cu deposited in TiO₂ as a photocatalyst, finding that the oxidation of metallic copper occurs in the reaction with considerable hydrogen generation. The works of photocatalysts and chalcopyrite leaching have the presence of copper in common. During the formation of hydrogen by water splitting assisted with a photocatalyst, the Cu performed as a reducing agent. Xu and Sun [11] studied the hydrogen production with CuO deposited on TiO₂, finding that leaching of the copper in the catalyst occurs, diminishing the amount of hydrogen generation. This Cu leaching represents a problem since the catalyst cannot be used for extended periods, thereby losing economic viability. This phenomenon in the leaching of chalcopyrite is adequate since the dissolution of copper is desired to be recovered later and simultaneously be capable of generating hydrogen. The process is improved by using an oxidizing agent like hydrogen peroxide.

Likewise, adding methanol favors chalcopyrite dissolution and the leaching of chalcopyrite. Here, we investigate the leaching of chalcopyrite concentrate with methanol and sulfuric acid using hydrogen peroxide as an oxidizing agent. Simultaneously, the hydrogen contained in the gas phase was quantified. The project deals with two goals: extraction of copper and hydrogen generation.

2. Materials and Methods

The chalcopyrite concentrates employed in this study were provided by COZAMIN from the CAPSTONE COPPER unit mine localized in Zacatecas, México, with average size particle of 200 µm. The metallic content in percentage is Fe 23%, Cu 21%, Pb 0.001%, Ag 243 ppm, and Au 11.3 g/ton; the phases present in raw chalcopyrite concentrate were determined by analysis of X-ray diffraction (XRD). The leaching experiments were carried out in a 1 L hermetic reactor with a gas sampler and magnetic stirring. All chemicals were reagent grade (J. T. Baker), and deionized water was used to form the solutions. Figure 1 illustrates the experimental procedure of the leaching and quantification of gas by chromatography. Leaching tests were performed using sulfuric acid/methanol solutions and added hydrogen peroxide (30% v/v) as the oxidizing agent. The leaching base solution was prepared with 140 mL of a 2 M sulfuric acid solution, 60 mL of methanol, and the corresponding amount of H₂O₂ to achieve the desired concentration. The final volume solution was 260 mL by adding deionized water at desired temperatures.

First, the required amount of concentrate was added to the leaching mixture. Aliquots of the leaching liquor were taken at pre-set times to determine the amount of dissolved iron and copper; the quantity of iron and copper was determined by atomic absorption spectrophotometer brand BUNSEN. The raw concentrate and residue were analyzed by X-ray diffraction (Bruker, D8 Advance, Austria Vienna) to set the initial phase in raw concentrate and formed phases, in a range of 10° to 70° in the 2θ scale degree, with a step of 0.02°, equipped with Cu Kα radiation (λ = 0.154 nm). The gas phase was analyzed in an Agilent 7890A gas chromatograph with two capillary columns Agilent CP 7430. These columns were connected in parallel to a thermal conductivity detector (TCD) and to a flame ionization detector (FID). Samples of 1 mL were extracted manually with the gas syringe and injected into the gas chromatograph. Nitrogen was used as a carrier, and the following temperature sequence was used: (a) at 50 °C for 6.5 min, (b) using a 15 °C/min ramp until reaching 250 °C, and (c) at 250 °C for 6 min. The Cu (I) ion determinations were carried out in a Varian Cary5000 spectrophotometer. Diluted samples 1:100 of the pregnant solution were analyzed in a quartz cell in the 200 to 800 nm range.

3. Results

3.1. Oxidizing Agent Effect

One of the most relevant variables in leaching is whether an oxidizing agent is incorporated, because they are usually expensive; therefore, the amount of this reagent can define the economic viability of the process. In this sense, Figure 2 shows the amount of hydrogen generated versus time in the leaching of 10 g of chalcopyrite concentrate, with 140 mL of 1 M solution of H₂SO₄, and 60 mL of methanol at room temperature with different amounts of H₂O₂.

The results show hydrogen production through chalcopyrite leaching, supporting the generation of molecular hydrogen in chalcopyrite concentrate leaching. An increased hydrogen generation with time was observed; meanwhile, the largest hydrogen production (around 0.086 µmol) was achieved with the lowest quantity of hydrogen peroxide (20 mL). Increasing this H₂O₂ quantity, hydrogen production dropped. This behavior could be due to the dismutation properties of hydrogen peroxide, where hydroxyl radicals are generated and they increase with hydrogen peroxide, reacting with cuprous ions in a similar way to the Fenton reaction, which lead to quick oxidation of the cuprous ions formed in chalcopyrite leaching, limiting the hydrogen formation.

Analysis of the X-ray diffractograms (Figure 3) PDF 00-037-0471, of raw chalcopyrite concentrate (Figure 3A) showed CuFeS₂ and Fe₂O₃ as the main phases, but sulfur was not detected. For the solid leaching residues (Figure 3B), the corresponding peaks of sulfur, Fe₂O₃, and no reacted chalcopyrite were detected, but in Figure 3A they are not present, since they are products of the leaching.

Hence, with the species detected by XRD, the analysis of gas chromatography only found molecular hydrogen, and in atomic absorption spectrophotometry of the liquids, an overall reaction is proposed in Equation (1). The reaction establishes that copper in the chalcopyrite is like a cuprous ion [18] oxidized to a cupric ion and iron in ferric oxide, thereby providing selective leaching.

$${4CuFeS}_2+{5H}_2{O}_2+{14H}^{+}\Rightarrow {4Cu}^{2+}+{Fe}_2{O}_3+{5H}_2+{8S}^0+{7H}_{2}O+{2Fe}^{3+}$$

(1)

Thermodynamic Analysis

Gibbs free energy is a thermodynamic variable and determines whether a reaction is spontaneous under constant pressure and temperature conditions. One way to calculate Gibbs free energy is using the difference between the Gibbs free energy of formation for the products and reactants, as in Equation (2). Then, the Gibbs free energy was calculated considering the formation of the Gibbs energy (

Table 1. Gibbs free energy of formation.

Substance	ΔG°f/kJ mol−1
H+	0.00
H2	0.00
H2O2	−120.42
Cu2+	65.52
CuFeS2	−179.20
H2O	−273.14
Fe2O3	−742.68
S0	0.00

) of the species involved in Equation (1); if the result is negative, the reaction is spontaneous.

$${∆G°}_r=\sum n{∆G°}_f\left(products\right)-\sum m{∆G°}_f\left(reactants\right)$$

(2)

Substituting the Gibbs free energy values into Equation (2):

$${∆G°}_r=\left(-1438.56{\displaystyle \frac{kJ}{mol}}\right)-\left(-1318.9{\displaystyle \frac{kJ}{mol}}\right)$$

$${∆G°}_r=-119.66{\displaystyle \frac{kJ}{mol}}$$

The calculations yield a negative value, meaning the reaction will occur in experimental conditions.

The amount of dissolved copper can affect hydrogen formation in chalcopyrite leaching. In this sense, Bashiri et al. [14] found that the generation of hydrogen by the photocatalysis of water splitting with a TiO₂ containing copper catalysts improved the coexistence of Cu⁺ and Cu²⁺ ions due to the oxidation of the cuprous ions to cupric ions, as demonstrated in Equation (3). Similarly, it is proposed that the on-chalcopyrite dissolution of the copper first dissolved in cuprous form after the oxidation of copper (I) to copper (II) occurs by the reduction in the hydrogen ion to molecular hydrogen, as shown in Equation (3). This behavior suggests that the overall reaction performs in several steps.

$$H^{+}+{Cu}^{+}\to {Cu}^{2+}+0.5H_2$$

(3)

On the other hand, the cuprous ion also reacts with oxygen peroxide, producing hydroxyl radical and hydroxide ion (Equation (4)) [19]. In this reaction, the oxygen peroxide competes with hydrogen ions, limiting the generation of molecular hydrogen, but the chalcopyrite dissociation increases.

$${Cu}^{+}+H_2{O}_2\to {Cu}^{2+}+{OH}^{·}+{OH}^{-}$$

(4)

Then, the leached chalcopyrite concentrate amount can be related to hydrogen production. Figure 4 shows the percentage of copper extracted versus time of 10 g of chalcopyrite concentrate with 140 mL of 1M H₂SO₄ solution, 60 mL of methanol, and different amounts of H₂O₂ (20, 40, and 60 mL). An ascending behavior in the extraction of copper for all tests is noticeable, obtaining a higher extraction of around 65% with 60 mL of hydrogen peroxide, continuing with 40 mL of H₂O₂ with 61% extraction, and reaching 55% extraction with 20 mL of H₂O₂. Proportional behavior between copper extraction and the amount of hydrogen peroxide is appreciated. With 20 mL of hydrogen peroxide, the highest hydrogen formation and the lowest copper extraction are achieved, employing a different H₂O₂ quantity probably favors Equation (4).

On other hand, Solis-Marcial and Lapidus (2014) [20] used the UV-Vis spectrophotometry technique to detect that at a wavelength of 300 nm, a characteristic peak of the Cu (I) ion, is presented. Where the height is directly associated with the concentration of Cu (I). For the determination of Cu (I) ion, experiments were performed using different amounts of H₂O₂, samples were taken and analyzed with a UV-Vis spectrophotometer to determine where the maximum value of absorbance is located. The spectra of each sample are reported in Figure 5, observing a peak of around 300 nm, which is characteristic of the Cu (I) ion, this corroborates that Cu (I) is present in this system.

The quantity of Cu (I) ion is quantified using the Equation (5), obtained in the curve of Solís-Marcial and Lapidus (2014) [20], the results are shown in

Table 2. Concentration of Cu (I) with the amount of H₂O₂.

H2O2 (mL)	Absorbance	[Cu(I)] (ppm)
20	0.68	8256
40	0.48	5828
60	0.62	7528

.

$$\left[Cu\left(I\right)\right]=121.42\ast Absorbance$$

(5)

3.2. Effect of the Chalcopyrite Concentrate Amount

Another variable that can affect hydrogen generation is the amount of chalcopyrite concentrate. Figure 6 shows the generated H₂ versus time on the leaching of 5 g, 10 g, and 15 g of chalcopyrite concentrate in a medium of 140 mL of 1 M of a H₂SO₄ solution, 60 mL of methanol, and 20 mL of H₂O₂. Hydrogen production increases with the reduction in chalcopyrite concentrate. This behavior may be due to the fact that, the amounts of leaching reagents such as H₂O₂ and sulfuric acid are insufficient to dissolve a larger amount of chalcopyrite concentrate [21,22,23].

Figure 7 shows the percentage of copper extraction under the same conditions as the previous experiment. The copper extraction percentages are 51, 50, and 45% for 5, 10, and 15 g of chalcopyrite, respectively. With 5 and 10 g of concentrate, the leaching behavior is similar. Whereas with 5 g of concentrate, the collection of copper is slightly higher for shorter times. For 15 g of concentrate, the copper extraction was the lowest, caused by an insufficient amount of reagents. With 5 g of concentrate, the highest hydrogen generation and copper extraction is seen. Allegedly, with these conditions, Equation (4) is fulfilled. Conversely, with different amounts of hydrogen peroxide, the relationship does not exist, and the reaction proceeds in several steps.

3.3. Effect of H₂SO₄ Concentration

The study also investigated the influence of different initial sulfuric acid concentrations. Figure 8 shows the amount of hydrogen generated over time by leaching 5 g of chalcopyrite concentrate in 60 mL of methanol, 20 mL of H₂O₂, and 140 mL of H₂SO₄ solution with different concentrations. All the profiles show an ascending hydrogen production over time; the H₂SO₄ concentration of 0.5 M produced the highest amount of hydrogen, 0.24 µmoles in 300 min. The behavior of hydrogen production with the other two acid concentrations is very similar, achieving 0.05 µmoles of hydrogen. There is an optimal acid concentration for hydrogen generation. It was previously postulated that hydrogen generation is dependent on the stability of the cuprous ion. In this regard, Navon et al. (1997) [24] determined that cuprous ion stabilization depends on pH, since to low [H⁺] does not achieve the minimum amount of ligand to complex Cu (I), and when the [H⁺] increases, Cu (I) oxidates to Cu (II), thereby decreasing hydrogen generation. Due to this reason, an optimal production rate of hydrogen is observed in the concentration range.

The amount of copper extracted was monitored as well. Figure 9 shows the percentage of copper extraction vs. time of 5 g of chalcopyrite concentrate, 20 mL of H₂O₂, 60 mL of methanol, and 140 mL of H₂SO₄ solutions with different concentrations (0.25 M, 0.5 M, and 0.8 M). The extraction percentage increases throughout the experiment and is directly proportional to the acid concentration. The highest copper extraction was 90% when adding an acid concentration of 0.8 M. Around 50% copper extraction was reported with concentrations of 0.25 and 0.5 M for the acid solution.

4. Discussion

Because it is more environmentally friendly, leaching has been suggested as an alternative for obtaining metals; a factor that has not been studied much in this context is the gases generated. During the leaching of chalcopyrite concentrate with sulfuric acid, hydrogen peroxide, and methanol, molecular hydrogen is one of the gases produced. The solid residue contained iron oxide, elemental sulfur, and chalcopyrite, while ionic copper was present in the solution. Given the reagents and products detected in the experiments, the global leaching reaction is proposed (Equation (1)). With a ΔG of −119.66 kJ/mol, the thermodynamic analysis of this equation indicates that the reaction is spontaneous. The hydrogen generation depends on the initial amount of hydrogen peroxide, where the highest hydrogen generation is achieved with the lowest amount of hydrogen peroxide (20 mL), followed by 60 mL, and then 40 mL. There is no straightforward behavior between hydrogen generation and the initial amount of hydrogen peroxide during chalcopyrite leaching, possibly due to the nature of the steps involved. In the first step of the chalcopyrite dissolution, the cuprous ion is released from the chalcopyrite, reacting with the hydrogen ions, and reducing them to molecular hydrogen. In this sense, the hydrogen peroxide reacts directly with the cuprous ion forming hydroxyl radicals with high oxidizing power that favors the dissolution of chalcopyrite. The above would explain why hydrogen generation is limited with a higher amount of hydrogen peroxide, and the dissolution of chalcopyrite is favored. Hydrogen generation by water splitting by photocatalysis using methanol as a sacrificial agent is similar to the leaching system proposed in this work, where the chalcopyrite concentrate has the role of the photocatalyst. One of the disadvantages of photocatalysis is that the catalyst is leached, losing its catalytic activity and decreasing its economic viability. The more a mineral dissolves through leaching, the more economically viable the process becomes.

During the experiments for chalcopyrite concentrate, the highest hydrogen generation was achieved with the lowest amount of concentrate (5 g). Allegedly, with the higher amount of concentrate diminishing the copper extraction percent, the amount of solid is higher than solvents, resulting in insufficient amounts of necessary reagents in the solution [19], hindering leaching. Regarding the experiments conducted for the initial concentration of sulfuric acid (0.25 M, 0.5 M, and 0.8 M), an optimal hydrogen generation was produced with 0.5 M. Nonetheless, for the extraction of copper from chalcopyrite concentrate with a greater sulfuric acid concentration, more copper was extracted. This phenomenon could be because the dissociation of hydrogen peroxide is favored with a higher concentration of hydrogen ions, thereby causing a greater amount of hydroxyl radicals and accelerating the oxidation of the cuprous ion.

5. Conclusions

The gases generated in the leaching process of chalcopyrite concentrate have lacked chemical characterization and quantification; these gases can be harmful or beneficial. In this work, it was found that only one is formed, hydrogen, a value-added product that provides greater profitability to the leaching of chalcopyrite concentrate since two products, such as copper and hydrogen, can be obtained. The study of the economic viability and scaling of the process is part of a subsequent work.

Based on the reagents and products generated, the leaching reaction was proposed. The thermodynamic analysis agrees with a spontaneous chemical reaction (ΔG = −119.66 kJ/mol). From this perspective, the leaching process offers two benefits, industrial and environmental, given that in addition to copper extraction, it generates molecular hydrogen, one of the cleanest fuels. The highest amount of hydrogen produced was 0.24 μmol in 300 min with the initial conditions of 0.5 M of H₂SO₄ solution, 5 g of chalcopyrite concentrate, 20 mL of H₂O₂, and 60 mL of methanol. In turn, the leaching reaction was dependent on the initial concentrations of the reagents.

The leaching process involves several steps to produce cuprous ions, which then act as the reducing agent to generate molecular hydrogen from hydrogen ions. In this sense, during the leaching, two competitive reactions for the cuprous ion exist: with the hydrogen ion generating molecular hydrogen, and when it reacts with hydrogen peroxide to form hydroxyl radicals and favors the chalcopyrite dissolution.