*Review* **Leaching Chalcocite in Chloride Media—A Review**

**Norman Toro 1,\* , Carlos Moraga <sup>2</sup> , David Torres <sup>1</sup> , Manuel Saldaña <sup>1</sup> , Kevin Pérez <sup>3</sup> and Edelmira Gálvez <sup>4</sup>**


**Abstract:** Chalcocite is the most abundant secondary copper sulfide globally, with the highest copper content, and is easily treated by conventional hydrometallurgical processes, making it a very profitable mineral for extraction. Among the various leaching processes to treat chalcocite, chloride media show better results and have a greater industrial boom. Chalcocite dissolution is a two-stage process, the second being much slower than the first. During the second stage, in the first instance, it is possible to oxidize the covellite in a wide range of chloride concentrations or redox potentials (up to 75% extraction of Cu). Subsequently, CuS<sup>2</sup> is formed, which is to be oxidized. It is necessary to work at high concentrations of chloride (>2.5 mol/L) and/or increase the temperature to reach a redox potential of over 650 mV, which in turn decreases the thickness of the elemental sulfur layer on the mineral surface, facilitating chloride ions to generate a better porosity of this. Finally, it is concluded that the most optimal way to extract copper from chalcocite is, during the first stage, to work with high concentrations of chloride (50–100 g/L) and low concentrations of sulfuric acid (0.5 mol/L) at a temperature environment, as other variables become irrelevant during this stage if the concentration of chloride ions in the system is high. While in the second stage, it is necessary to increase the temperature of the system (moderate temperatures) or incorporate a high concentration of some oxidizing agent to avoid the passivation of the mineral.

**Keywords:** Cu2S; CuS; dissolution; chloride

## **1. Introduction**

Most of copper minerals correspond to sulfide minerals and a minor part to oxidized minerals. Chalcopyrite is the most abundant among the sulfide copper minerals [1–4]. However, this mineral is refractory to conventional leaching processes due to forming a passivating layer that prevents contact between the mineral and the leaching solution [5–7]. Positive results have only been achieved for this mineral when working at medium-high temperatures (over 60 ◦C); therefore, it has not been possible to implement it on a large scale in industrial heap leaching processes [8].

In the natural mineral, chalcopyrite is commonly associated with secondary sulfides that include chalcocite (Cu2S), digenite (Cu1.8S), and covellite (CuS) [9]. Chalcocite is the most abundant secondary copper sulfide, with the highest copper content, and is easily treated by hydrometallurgical processes, which makes it a very profitable mineral for its extraction [10–14]. Chalcocite has a dark gray color and belongs to the copper-rich mineral family ranging from CuS to Cu2S (see Table 1), commonly found in the enriched supergenic environment below the oxidized zone of copper porphyry deposits [15–19]. This is formed by oxidation, reduction, dissemination, and migration of primary sulfides

**Citation:** Toro, N.; Moraga, C.; Torres, D.; Saldaña, M.; Pérez, K.; Gálvez, E. Leaching Chalcocite in Chloride Media—A Review. *Minerals* **2021**, *11*, 1197. https://doi.org/ 10.3390/min11111197

Academic Editors: Kyoungkeun Yoo, Shuai Wang, Xingjie Wang and Jia Yang

Received: 26 September 2021 Accepted: 24 October 2021 Published: 28 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

such as chalcopyrite [20–22], being also the main component of the tufts of copper and white metal [23–25].


**Table 1.** Composition, structure, and stability of minerals CuxS (Data from [19,26]).

Due to the fact that hydrometallurgical processes are more economical and friendly to the environment than pyrometallurgical processes [27], and that it is relatively easy to dissolve copper from chalcocite, several investigations for the leaching of this mineral with the use of multiple additives and in different media have been carried out, such as those that include bioleaching [22,28–36], ferric sulfate solution [12,37–40], chloride medium [13,24,41–45], MnO<sup>2</sup> as an oxidizing agent [46], pressure leaching [47,48], cyanide medium [49,50], ethylenediaminetetraacetic acid (EDTA) [51,52], and synthetic chalcocite (white metal) [23,53,54].

Among the different leaching media to treat secondary copper sulfides, chloride media has had the greatest growth at an industrial level. This is not only due to the good results presented by chloride media in heap leaching processes but also due to freshwater shortage. This is due to three aspects. (i) Chlorinated media have a higher dissolution rate than traditional sulfated systems. This is due to the ability of the chloride ion to stabilize cuprous through the formation of CuC*l* 2− 3 [8,13,43,46]. Furthermore, the addition of chloride ions makes it possible to overcome passivation due to the formation of the sulfur layer. The chloride ions increase the redox potential, generating a thinner layer and making it easier for the chloride ions to cause porosity [55]. Figure 1 shows the effect of chloride ions in a heap leaching process at an industrial level (Chilean mine company). For an acid medium without the addition of chloride, extractions of 35% Cu are obtained, while when working at 20 and 50 g/L of chloride, extractions of 50% and 55% Cu are obtained, respectively (for 90 days) [56]. (ii) It is an economic system since the chloride present in seawater (20 g/L Cl−) or wastewater from desalination plants (~40 g/L Cl−) can be used. (iii) Government restrictions on the use of aquifer water in large-scale mining projects. Although mining consumes much less water in its processes than other industries such as agriculture, mining deposits are generally found in arid areas where freshwater is scarce. Therefore, the use of seawater becomes practically a necessity. For example, in Chile (the largest copper producer in the world), it is projected that by 2030 seawater will represent almost 50% of the consumption of water in mining [57]. *Minerals* **2021**, *11*, x FOR PEER REVIEW 3 of 16

**Figure 1.** Extraction of copper from sulfide ores in a heap leach using H2SO<sup>4</sup> and chloride (modified from: [56]). **Figure 1.** Extraction of copper from sulfide ores in a heap leach using H2SO<sup>4</sup> and chloride (modified from: [56]).

The oxidative dissolution of chalcocite by Fe3+

**2. Fundamentals**

**Cu2S**

**Cu2-XS**

A bibliographic review based on scientific publications in recent years on chalcocite

, O2, or Cu2+, either in a sulfate or chlo-

2+ + 2Fe2+ + CuS (1)

(2)

0

2+ + 2Fe2+ + S

**0**

**CuS<sup>n</sup>**

**CuS**

chalcocite, comparing the impact that each one has on the extraction of copper from it.

ride system, occurs in two stages (reactions 1 and 2), where the chalcocite dissolves, a progressive transformation occurs of this copper sulfide, passing through different stages called polysulfides (digenite Cu1.8S; geerite Cu1.6S; spionkopite Cu1.4S; yarrowite Cu1.1S), until reaching the covellite CuS [31,39,58]. The formation of said intermediate polysulfides in the transformation of chalcocite to covellite during the first stage of leaching generates a passivating layer in the mineral particle, which can be seen in Figure 2, where said layers formed are represented. Between 10% to 20% extraction, a thin layer of covellite covers the surface of the mineral, and in the same way, but toward the interior, an intermediate layer is formed with a decreasing proportion of Cu / S. In the second leaching stage, when 49% to 55% of copper has already been extracted, the chalcocite has already been converted to covellite, and a mixture of polysulfide and sulfur is generated on the surface, while in the interior, it remains covellite and as measured. As the leaching progresses, the covellite progressively converts into sulfur and polysulfide (CuSn) with a decreasing Cu / S ratio [42].

Cu2S + 2 Fe3+ = Cu

CuS + 2 Fe3+ = Cu

**CuS S**

A bibliographic review based on scientific publications in recent years on chalcocite leaching in chloride media is carried out in the present manuscript. The objective of this work is to evaluate the different operational parameters that influence the dissolution of chalcocite, comparing the impact that each one has on the extraction of copper from it. A bibliographic review based on scientific publications in recent years on chalcocite leaching in chloride media is carried out in the present manuscript. The objective of this work is to evaluate the different operational parameters that influence the dissolution of chalcocite, comparing the impact that each one has on the extraction of copper from it.

*Minerals* **2021**, *11*, x FOR PEER REVIEW 3 of 16

**Figure 1.** Extraction of copper from sulfide ores in a heap leach using H2SO<sup>4</sup> and chloride (modified from: [56]).

#### **2. Fundamentals 2. Fundamentals**

The oxidative dissolution of chalcocite by Fe3+, O2, or Cu2+, either in a sulfate or chloride system, occurs in two stages (reactions 1 and 2), where the chalcocite dissolves, a progressive transformation occurs of this copper sulfide, passing through different stages called polysulfides (digenite Cu1.8S; geerite Cu1.6S; spionkopite Cu1.4S; yarrowite Cu1.1S), until reaching the covellite CuS [31,39,58]. The formation of said intermediate polysulfides in the transformation of chalcocite to covellite during the first stage of leaching generates a passivating layer in the mineral particle, which can be seen in Figure 2, where said layers formed are represented. Between 10% to 20% extraction, a thin layer of covellite covers the surface of the mineral, and in the same way, but toward the interior, an intermediate layer is formed with a decreasing proportion of Cu/S. In the second leaching stage, when 49% to 55% of copper has already been extracted, the chalcocite has already been converted to covellite, and a mixture of polysulfide and sulfur is generated on the surface, while in the interior, it remains covellite and as measured. As the leaching progresses, the covellite progressively converts into sulfur and polysulfide (CuSn) with a decreasing Cu/S ratio [42]. The oxidative dissolution of chalcocite by Fe3+ , O2, or Cu2+, either in a sulfate or chloride system, occurs in two stages (reactions 1 and 2), where the chalcocite dissolves, a progressive transformation occurs of this copper sulfide, passing through different stages called polysulfides (digenite Cu1.8S; geerite Cu1.6S; spionkopite Cu1.4S; yarrowite Cu1.1S), until reaching the covellite CuS [31,39,58]. The formation of said intermediate polysulfides in the transformation of chalcocite to covellite during the first stage of leaching generates a passivating layer in the mineral particle, which can be seen in Figure 2, where said layers formed are represented. Between 10% to 20% extraction, a thin layer of covellite covers the surface of the mineral, and in the same way, but toward the interior, an intermediate layer is formed with a decreasing proportion of Cu / S. In the second leaching stage, when 49% to 55% of copper has already been extracted, the chalcocite has already been converted to covellite, and a mixture of polysulfide and sulfur is generated on the surface, while in the interior, it remains covellite and as measured. As the leaching progresses, the covellite progressively converts into sulfur and polysulfide (CuSn) with a decreasing Cu / S ratio [42].

$$\text{Cu}\_2\text{S} + 2\text{ Fe}^{3+} = \text{Cu}^{2+} + 2\text{Fe}^{2+} + \text{CuS} \tag{1}$$

$$\text{CuS} + 2\,\text{Fe}^{3+} = \text{Cu}^{2+} + 2\text{Fe}^{2+} + \text{S}^{0} \tag{2}$$

**Figure 2.** Graphic representation of the dissolution of the two stages of chalcocite leaching (modified from: [42]).

According to Niu et al. [39], in Equation (1) a rapid leaching occurs from chalcocite to covellite due to the low activation energy required (4–25 kJ/mol) in the kinetic model of the unreacted nucleus, being a reaction controlled by the diffusion of the oxidant on the surface of the mineral. Meanwhile, Equation (2) shows leaching is slower and can accelerate as a function of temperature [13]. The researchers Ruan et al. [59] and Miki et al. [38] have concluded that this is because this reaction is controlled by chemical and/or electrochemical reactions under the kinetic model of the unreacted nucleus, requiring an activation energy of around 71.5–72 kJ/mol for the transformation of covellite to dissolved copper. Nicol and Basson [60] suggest that in the oxidation of covellite, an intermediate stage occurs in which it is transformed to a polysulfide CuS2.

$$\text{Cu}\_2\text{S}\_2 = \text{CuS}\_2 + \text{Cu}^{2+} + 2\text{e}^- \tag{3}$$

$$\text{CuS}\_2 = \text{Cu}^{2+} + \text{2S}^0 + \text{2e}^- \tag{4}$$

Covellite can be oxidized over a wide range of chloride concentrations or potentials to CuS<sup>2</sup> polysulfide (Equation (4)). Still, the oxidation of CuS<sup>2</sup> (Equation (5)) can only occur under conditions of high chloride concentrations or high potentials (chloride concentrations greater than 2.5 mol/L or potentials greater than 650 mV) [60].

For the dissolution of chalcocite in a sulfated–chloride medium, various investigations have been carried out using different additives and operational conditions (Table 2). There is a consensus in all the investigations regarding the positive effect on the dissolution kinetics of Cu2S when adding chloride, either synthetic or using seawater. This is because chloride ions promote the formation of long crystals that allow the reagent to penetrate through the passivating layer [43].


**Table 2.** Comparison of previous investigations for the leaching of chalcocite in a chloride medium.

In a Cu2S leaching process, adding O<sup>2</sup> to the system at ambient pressure, with H2SO<sup>4</sup> being the leaching agent, the leaching agents generated during leaching in a Cu2+/Cl<sup>−</sup> system are Cu2+, CuCl<sup>+</sup> , CuCl− 2 , and CuCl3−. The general reaction being the following:

$$2\text{Cu}\_2\text{S} + \text{O}\_2 + 4\text{H}^+ + 8\text{Cl}^- = 4\text{CuCl}\_2^- + 2\text{H}\_2\text{O} + 2\text{S}^0 \tag{5}$$

Although chalcocite leaching reactions occur in two stages, guiding us to Equation (5), where the following occurs:

$$4\text{Cu}\_2\text{S} + \text{O}\_2 + 4\text{H}^+ + 8\text{Cl}^- = 4\text{CuCl}\_2^- + 2\text{H}\_2\text{O} + 4\text{CuS} \tag{6}$$

$$4\text{CuS} + \text{O}\_2 + 4\text{H}^+ + 8\text{Cl}^- = 4\text{CuCl}\_2^- + 2\text{H}\_2\text{O} + 4\text{S}^0 \tag{7}$$

By leaching Cu2S, the expected resulting products should be soluble copper such as CuCl − 2 and a solid residue of elemental sulfur (S<sup>0</sup> ) with covellite residues or copper polysulfides (CuS2) that still contain valuable metals.

CuCl − 2 is the predominant soluble species due to the complexation of Cu (I) with Cl− at room temperature in a system with high chloride concentrations (greater than 1 mol/L). This CuCl − 2 is stable in a potential range between 0–500 mV and pH < 6–7 (depending on the chloride concentration in the system).

For a sulfated–chloride system where MnO<sup>2</sup> is incorporated as an oxidizing agent, the following reactions are proposed:

$$2\text{Cu}\_2\text{S} + \text{MnO}\_2 + 4\text{H}^+ + 4\text{Cl}^- = 2\text{CuCl}\_2^- + \text{Mn}^{2+} + 2\text{CuS} + 2\text{H}\_2\text{O} \tag{8}$$

$$\text{2CuS} + \text{MnO}\_2 + 4\text{H}^+ + 4\text{Cl}^- = 2\text{CuCl}\_2^- + \text{Mn}^{2+} + 2\text{S}^0 + 2\text{H}\_2\text{O} \tag{9}$$

During the first leaching stage (Equation (8)), the chalcocite becomes covellite; this reaction being thermodynamically possible with a Gibbs free energy value of −138.59 kJ. The second reaction (Equation (9)) is slower and is also thermodynamically possible (∆G<sup>0</sup> <sup>=</sup> <sup>−</sup>84.512 kJ).

#### **3. Operational Variables**

#### *3.1. Effect on Chloride Concentration*

Several authors point out that chalcocite leaching in a chloride medium is the best way to dissolve this copper sulfide [8,13,24,38,41,63]. Even if chloride ions are added to a chalcocite leaching with H2SO<sup>4</sup> or HNO3, the kinetics increases considerably. As explained by Cheng and Lawson [24], this occurs because in leaching with only sulfate or nitrate ions, a layer of elemental sulfur is formed on the surface of the particles. In this way, an impermeable particle is generated, that is, contact between the particle with the leaching agent is prevented. This implies that the kinetics decrease in the first leaching stage and prevent the reaction in the second stage. However, when chloride ions are found, either alone or associated with sulfate or nitrate, dissolution kinetics increase along with copper extraction, as shown in Figure 3. *Minerals* **2021**, *11*, x FOR PEER REVIEW 6 of 16

**Figure 3.** Effect of chloride ions on the acid leaching of chalcocite (T = 85 °C, particle size 31 µm) (Modified from: [24]). **Figure 3.** Effect of chloride ions on the acid leaching of chalcocite (T = 85 ◦C, particle size 31 µm) (Modified from: [24]).

Several studies have shown that working at high chloride concentrations favors the leaching kinetics of secondary sulfides [24,38,44,64]. Chloride ions pass through the sulfur layer and generate a porous layer instead of an amorphous layer formed in the sulfate and nitrate system. The porous layer allows the entry of the leaching solution through said pores, thus allowing contact with the particle, thus accelerating the leaching kinetics in the first stage and making possible the dissolution reaction in the second stage of leaching Several studies have shown that working at high chloride concentrations favors the leaching kinetics of secondary sulfides [24,38,44,64]. Chloride ions pass through the sulfur layer and generate a porous layer instead of an amorphous layer formed in the sulfate and nitrate system. The porous layer allows the entry of the leaching solution through

wastewater is stable (such as elemental sulfur) and non-polluting.

easier for chloride ions to generate porosity [13].

of CuClଷ

[24,65,66]. In the study carried out by Toro et al. [13], leaching tests were carried out in

indicate that the highest Cu extractions are obtained when working at the highest chloride concentrations (see Figure 4). Furthermore, in other studies [43,46] involving the use of seawater (20 g/L Cl−) and wastewater from desalination plants (~40 g/L Cl−) for the dissolution of Cu2S in an acid medium, the researchers point out that better results are obtained when working with wastewater compared to seawater due to its higher concentration of chloride. Additionally, it is highlighted that the waste generated when working with

The high dissolution rate in the chloride system relative to the sulfated system is attributed to the ability of the chloride ion to stabilize the cuprous ion through the formation

ଶି. In the chloride system, copper can be extracted directly from the chalcocite without causing the oxidation of Cu+ to Cu2+. On the other hand, in the sulfated system, Cu+ must be oxidized to Cu2+ on the surface of the particles before copper is released into the solution [8,13,41,64]. The addition of chloride ions allows breaking the passivated sulfur layer since an increase in the concentration of chloride ions implies an increase in the redox potential [42], and a higher redox potential generates a thinner layer that makes it

said pores, thus allowing contact with the particle, thus accelerating the leaching kinetics in the first stage and making possible the dissolution reaction in the second stage of leaching [24,65,66]. In the study carried out by Toro et al. [13], leaching tests were carried out in stirred reactors for a pure mineral of chalcocite in an acid medium, comparing different concentrations of chloride in the system (20, 40, and 100 g/L). In their results, the authors indicate that the highest Cu extractions are obtained when working at the highest chloride concentrations (see Figure 4). Furthermore, in other studies [43,46] involving the use of seawater (20 g/L Cl−) and wastewater from desalination plants (~40 g/L Cl−) for the dissolution of Cu2S in an acid medium, the researchers point out that better results are obtained when working with wastewater compared to seawater due to its higher concentration of chloride. Additionally, it is highlighted that the waste generated when working with wastewater is stable (such as elemental sulfur) and non-polluting. *Minerals* **2021**, *11*, x FOR PEER REVIEW 7 of 16

**Figure 4.** Effect of the chloride concentration in Cu2S solution (T = 25 °C, H2SO<sup>4</sup> = 0.5 mol/L) (Modified from: [13]). **Figure 4.** Effect of the chloride concentration in Cu2S solution (T = 25 ◦C, H2SO<sup>4</sup> = 0.5 mol/L) (Modified from: [13]).

*3.2. Effect on Stirring Speed* The agitation speed in a reactor leaching system decreases the thickness of the boundary layer and maximizes the gas–liquid interface area [67]. This variable is not very significant in copper extraction for tests of the dissolution of Cu2S in an acid–chloride medium. There is a consensus on the part of different authors in previous research [24,38,61,62,68] where it is stated that it is only necessary to stir at a sufficient speed to keep all the chalcocite particles in suspension within the reactor. Additionally, it is important to note that of the various agitation systems used in these investigations it is advisable to work with mechanical agitation since with other systems anomalous results are obtained, for example, in the study carried out by Herreros and Viñals [62] the authors indicate that in their air agitation tests the results were superior under the same opera-The high dissolution rate in the chloride system relative to the sulfated system is attributed to the ability of the chloride ion to stabilize the cuprous ion through the formation of CuCl 2− 3 . In the chloride system, copper can be extracted directly from the chalcocite without causing the oxidation of Cu<sup>+</sup> to Cu2+. On the other hand, in the sulfated system, Cu+ must be oxidized to Cu2+ on the surface of the particles before copper is released into the solution [8,13,41,64]. The addition of chloride ions allows breaking the passivated sulfur layer since an increase in the concentration of chloride ions implies an increase in the redox potential [42], and a higher redox potential generates a thinner layer that makes it easier for chloride ions to generate porosity [13].

#### tional conditions compared to mechanical agitation tests. This occurred because the air increased the extraction of copper. After all, the oxygen reacted with the CuCl (solid) *3.2. Effect on Stirring Speed*

formed during the leaching process, favoring the formation of CuCl<sup>+</sup> . On the other hand, Velásquez-Yévenes [68], in his study, mentions that when working with the use of magnetic agitation the mineral is reduced in size due to the abrasion that is generated when it passes under the rotating magnet, generating an increase in the dissolution of chalcocite. *3.3. Effect on Acid Concentration* Regarding the acid concentration in a sulfate–chloride system, the findings presented by Dutrizac [69], Cheng and Lawson [24], Senanayake [45], Toro et al. [13], Saldaña et al. [44], and Torres et al. [46] confirm that the concentration of chloride ions in the system is the variable that most influences the kinetics of the dissolution of chalcocite at room temperature, making other operational variables, such as acid concentration, particle size, stirring speed, etc., less relevant. These same results were obtained for other copper sulfides such as covellite [64] and chalcopyrite [70]. The agitation speed in a reactor leaching system decreases the thickness of the boundary layer and maximizes the gas–liquid interface area [67]. This variable is not very significant in copper extraction for tests of the dissolution of Cu2S in an acid–chloride medium. There is a consensus on the part of different authors in previous research [24,38,61,62,68] where it is stated that it is only necessary to stir at a sufficient speed to keep all the chalcocite particles in suspension within the reactor. Additionally, it is important to note that of the various agitation systems used in these investigations it is advisable to work with mechanical agitation since with other systems anomalous results are obtained, for example, in the study carried out by Herreros and Viñals [62] the authors indicate that in their air agitation tests the results were superior under the same operational conditions compared to mechanical agitation tests. This occurred because the air increased the extraction of

Toro et al. [13] performed statistical analysis (ANOVA) for the dissolution of Cu2S in

method (See Table 3). In their results, the researchers indicate that, although sulfuric acid helps to improve the dissolution kinetics of the mineral, the chloride concentration in the

copper. After all, the oxygen reacted with the CuCl (solid) formed during the leaching process, favoring the formation of CuCl<sup>+</sup> . On the other hand, Velásquez-Yévenes [68], in his study, mentions that when working with the use of magnetic agitation the mineral is reduced in size due to the abrasion that is generated when it passes under the rotating magnet, generating an increase in the dissolution of chalcocite.
