*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].

Toro et al. [13] performed statistical analysis (ANOVA) for the dissolution of Cu2S in a chloride medium in stirred reactors. For this, the copper extraction was evaluated through the effect of the independent variables with the response surface optimization 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 system has much more impact on copper extraction, as shown in Figure 5. These results are consistent with those presented by Cheng and Lawson [24], where the researchers mention that a low concentration of H2SO<sup>4</sup> (0.02 mol/L) is sufficient to dissolve chalcocite and later phases of it such as djurleite and digenite. However, it is essential to maintain a high concentration of chloride ions since in its absence the dissolution kinetics considerably decrease (first stage) and later the covellite is not dissolved (second stage). On the other hand, a recent study by Torres et al. [46] worked with wastewater at different concentrations of sulfuric acid to dissolve a pure chalcocite mineral. The authors mention that the same results were obtained in their results, even in short periods of time at H2SO<sup>4</sup> concentration ranges between 0.1 and 1 mol/L (see Figure 6). *Minerals* **2021**, *11*, x FOR PEER REVIEW 8 of 16 system has much more impact on copper extraction, as shown in Figure 5. These results are consistent with those presented by Cheng and Lawson [24], where the researchers mention that a low concentration of H2SO<sup>4</sup> (0.02 mol/L) is sufficient to dissolve chalcocite and later phases of it such as djurleite and digenite. However, it is essential to maintain a high concentration of chloride ions since in its absence the dissolution kinetics considerably decrease (first stage) and later the covellite is not dissolved (second stage). On the other hand, a recent study by Torres et al. [46] worked with wastewater at different concentrations of sulfuric acid to dissolve a pure chalcocite mineral. The authors mention that the same results were obtained in their results, even in short periods of time at H2SO<sup>4</sup> concentration ranges between 0.1 and 1 mol/L (see Figure 6).

> **Table 3.** Experimental parameters used in statistical analysis [13]. **Table 3.** Experimental parameters used in statistical analysis [13].


**Figure 5.** Linear effect graph for the extraction of Cu from chalcocite in a chloride medium [13]. **Figure 5.** Linear effect graph for the extraction of Cu from chalcocite in a chloride medium [13].

0.1 mol/l 0.5 mol/l 1 mol/l

**Figure 6.** Effect on the H2SO<sup>4</sup> concentration in the Cu2S solution with the use of wastewater (~40 g/L

Time [h] 0 5 10 15

Copper disolution [%]

Cl<sup>−</sup> ) [46].

a)

**Figure 6.** Effect on the H2SO4 concentration in the Cu2S solution with the use of wastewater (~40 g/L Cl<sup>−</sup>) [46]. **Figure 6.** Effect on the H2SO<sup>4</sup> concentration in the Cu2S solution with the use of wastewater (~40 g/L Cl−) [46]. *Minerals* **2021**, *11*, x FOR PEER REVIEW 9 of 16

system has much more impact on copper extraction, as shown in Figure 5. These results are consistent with those presented by Cheng and Lawson [24], where the researchers mention that a low concentration of H2SO4 (0.02 mol/L) is sufficient to dissolve chalcocite and later phases of it such as djurleite and digenite. However, it is essential to maintain a high concentration of chloride ions since in its absence the dissolution kinetics considerably decrease (first stage) and later the covellite is not dissolved (second stage). On the other hand, a recent study by Torres et al. [46] worked with wastewater at different concentrations of sulfuric acid to dissolve a pure chalcocite mineral. The authors mention that the same results were obtained in their results, even in short periods of time at H2SO4

**Experimental Parameters Low Medium High**  Time (h) 4 8 12 Cl− concentration (g/L) 20 50 100 H2SO4 (mol/L) 0.5 1 2

**Figure 5.** Linear effect graph for the extraction of Cu from chalcocite in a chloride medium [13].

concentration ranges between 0.1 and 1 mol/L (see Figure 6).

**Table 3.** Experimental parameters used in statistical analysis [13].

#### *3.4. Particle Size Effect*

The effect of particle size on chalcocite leaching has been studied by different authors; however, these studies have been carried out with relatively small particle sizes: 25 to 4 mm [71]; 4 mm to 12 µm [62]; 4 to 0.054 mm [12]; 11 to 63 µm [24]; 150 to 75 µm [63]; 150 to 106 µm [72]. These authors agree that a smaller particle size implies an increase in the dissolution kinetics and the extraction rate in the first leaching stage. But the effect decreases significantly in the second stage. Naderi et al. [71] reported that for fine particle sizes the first stage is controlled by diffusion through the liquid film. In the second stage, the accumulation of the elemental sulfur layer in the solid product, accompanied by a jarosite precipitate, transformed the control mechanism into solid diffusion. Phyo et al. [12] studied the effect on the dissolution kinetics of Cu2S in stirred reactors using an acid medium. In their results, as can be seen in Figure 7a, a significant effect of the particle size is observed in the dissolution of copper, especially in the size of −0.074 + 0.054 mm, which in 2.5 h had already reached 45% recovery compared to the almost 17 hours it took to achieve the same recovery with −4 + 2 mm particles. In Figure 7b, the researchers observed a turning point of around 75% copper dissolution at different times depending on the granulometry and divided the second stage into two sub-stages, indicating that the first sub-stage has a dissolution speed 20 times faster than the second sub-stage. *3.4. Particle Size Effect* The effect of particle size on chalcocite leaching has been studied by different authors; however, these studies have been carried out with relatively small particle sizes: 25 to 4 mm [71]; 4 mm to 12 μm [62]; 4 to 0.054 mm [12]; 11 to 63 μm [24]; 150 to 75 μm [63]; 150 to 106 μm [72]. These authors agree that a smaller particle size implies an increase in the dissolution kinetics and the extraction rate in the first leaching stage. But the effect decreases significantly in the second stage. Naderi et al. [71] reported that for fine particle sizes the first stage is controlled by diffusion through the liquid film. In the second stage, the accumulation of the elemental sulfur layer in the solid product, accompanied by a jarosite precipitate, transformed the control mechanism into solid diffusion. Phyo et al. [12] studied the effect on the dissolution kinetics of Cu2S in stirred reactors using an acid medium. In their results, as can be seen in Figure 7a, a significant effect of the particle size is observed in the dissolution of copper, especially in the size of −0.074 + 0.054 mm, which in 2.5 h had already reached 45% recovery compared to the almost 17 hours it took to achieve the same recovery with −4 + 2 mm particles. In Figure 7b, the researchers observed a turning point of around 75% copper dissolution at different times depending on the granulometry and divided the second stage into two sub-stages, indicating that the first sub-stage has a dissolution speed 20 times faster than the second sub-stage.

**Figure 7.** Cu2S dissolution at different particle sizes in two different stages: (**a**) first stage, (**b**) second stage ([Fe3+] = 10 g/dm<sup>3</sup> , pH = 1.00–1.50, Eh = 750 mV, temperature = 45 °C) (Modified from: [12]). **Figure 7.** Cu2S dissolution at different particle sizes in two different stages: (**a**) first stage, (**b**) second stage ([Fe3+] = 10 g/dm<sup>3</sup> , pH = 1.00–1.50, Eh = 750 mV, temperature = 45 ◦C) (Modified from: [12]).

a critical parameter during the second stage, being much slower, and can be accelerated with temperature, which indicates that the process is controlled by chemical and/or electrochemical reactions [43,59]. Miki et al. [38] pointed out that the dissolution rate of CuS is largely independent of the concentration of chloride and HCl in the ranges of 0.2 to 2.5 mol/L and 0.1 to 1 mol/L, reporting activation energy values of 71.5 kJ/mol. Therefore, it can be concluded that the process is controlled by a chemical or electrochemical reaction

In the results presented by Pérez et al. [43] for the dissolution of a pure chalcocite mineral in a chloride medium in a stirred reactor at different temperatures, the authors point out that at temperatures above 65 °C extractions of copper were close to 40% in short periods of time (15 min) (see Figure 7), which, the researchers concluded, is due to the

*3.5. Effect of Temperature*

on the surface of the mineral.

#### *3.5. Effect of Temperature*

It is known that temperature is the operational variable that most influences the dissolution of copper sulfide minerals [43,73,74]. For the specific case of chalcocite, it becomes a critical parameter during the second stage, being much slower, and can be accelerated with temperature, which indicates that the process is controlled by chemical and/or electrochemical reactions [43,59]. Miki et al. [38] pointed out that the dissolution rate of CuS is largely independent of the concentration of chloride and HCl in the ranges of 0.2 to 2.5 mol/L and 0.1 to 1 mol/L, reporting activation energy values of 71.5 kJ/mol. Therefore, it can be concluded that the process is controlled by a chemical or electrochemical reaction on the surface of the mineral. *Minerals* **2021**, *11*, x FOR PEER REVIEW 10 of 16

> In the results presented by Pérez et al. [43] for the dissolution of a pure chalcocite mineral in a chloride medium in a stirred reactor at different temperatures, the authors point out that at temperatures above 65 ◦C extractions of copper were close to 40% in short periods of time (15 min) (see Figure 7), which, the researchers concluded, is due to the phase change that governs the first stage of leaching from chalcocite to covellite, which requires low activation energy. More energy is necessary for the second stage to become a copper polysulfide, which requires more demanding conditions to achieve its complete dissolution. In addition, Pérez et al. [43] mentioned that there is good synergy between the chloride concentration in the system and the temperature, since, in their research, they achieved copper extractions of 97% in 3 hours under the conditions operations that are presented in Figure 8. The research carried out by Ruiz et al. [54] investigated the dissolution of white metal (chalcocite and djurleite) working under similar operational conditions. Without chloride in the system, 55% extractions were obtained in a time of 5 hours at a temperature of 105 ◦C. phase change that governs the first stage of leaching from chalcocite to covellite, which requires low activation energy. More energy is necessary for the second stage to become a copper polysulfide, which requires more demanding conditions to achieve its complete dissolution. In addition, Pérez et al. [43] mentioned that there is good synergy between the chloride concentration in the system and the temperature, since, in their research, they achieved copper extractions of 97% in 3 hours under the conditions operations that are presented in Figure 8. The research carried out by Ruiz et al. [54] investigated the dissolution of white metal (chalcocite and djurleite) working under similar operational conditions. Without chloride in the system, 55% extractions were obtained in a time of 5 hours at a temperature of 105 °C.

**Figure 8.** Cu2S dissolution as a function of temperature (0.5 mol/L H2SO<sup>4</sup> and 100 g/L of Cl<sup>−</sup> ) [43]. **Figure 8.** Cu2S dissolution as a function of temperature (0.5 mol/L H2SO<sup>4</sup> and 100 g/L of Cl−) [43].

#### *3.6. Effect of Redox Potential 3.6. Effect of Redox Potential*

Miki et al. [38] studied the effect of the redox potential in a Cu2S solution (synthetic) with the use of a 0.2 M HCl solution, 0.2 g/L of Cu (II), and 2 g/L of Fe (III)/Fe (II) at a temperature of 35 °C. The researchers, in their findings, reported that the dissolution of Cu2S occurs rapidly at a potential of 500 mV but then stops when 45% copper is removed (end of the first stage). For potentials of 550 mV, there is then an increase in the dissolution until reaching 50% extraction of copper. Subsequently, the copper mineral present is mainly covering, which requires potentials of at least 600 mV to be able to dissolve. However, Niu et al. [39] point out that these results were not determined in a range of industrial redox potentials. In their experiments for the dissolution of Cu2S, Niu et al. [39] worked Miki et al. [38] studied the effect of the redox potential in a Cu2S solution (synthetic) with the use of a 0.2 M HCl solution, 0.2 g/L of Cu (II), and 2 g/L of Fe (III)/Fe (II) at a temperature of 35 ◦C. The researchers, in their findings, reported that the dissolution of Cu2S occurs rapidly at a potential of 500 mV but then stops when 45% copper is removed (end of the first stage). For potentials of 550 mV, there is then an increase in the dissolution until reaching 50% extraction of copper. Subsequently, the copper mineral present is mainly covering, which requires potentials of at least 600 mV to be able to dissolve. However, Niu et al. [39] point out that these results were not determined in a range of industrial redox potentials. In their experiments for the dissolution of Cu2S, Niu et al. [39] worked in mini

in mini glass columns (30 cm long and 6 cm in diameter), adding Fe2(SO4)<sup>3</sup> as a leaching

Cu2S leaching was insensitive to the redox potential at moderate temperatures (30–40 °C) in the industrial range of 650–800 mV. In the study conducted by Hashemzadeh et al. [63], the researchers modeled the dissolution kinetics of Cu2S in chloride media using leaching data obtained under fully controlled temperature, pH, and solution potential. In their results, the researchers mentioned that an increase in the chloride concentration and temperature generated an increase in the redox potential, increasing from 680 to 830 mV with the addition of 0.1 chlorides and 3 mol/L of NaCl, respectively, and consequently higher

The results obtained from the aforementioned studies are directly related to the formation of a layer of elemental sulfur on the surface of the covellite in the second stage,

dissolution kinetics, mainly in the second leaching stage.

glass columns (30 cm long and 6 cm in diameter), adding Fe2(SO4)<sup>3</sup> as a leaching agent. In their results, the researchers note that the dissolution rate of the second stage of Cu2S leaching was insensitive to the redox potential at moderate temperatures (30–40 ◦C) in the industrial range of 650–800 mV. In the study conducted by Hashemzadeh et al. [63], the researchers modeled the dissolution kinetics of Cu2S in chloride media using leaching data obtained under fully controlled temperature, pH, and solution potential. In their results, the researchers mentioned that an increase in the chloride concentration and temperature generated an increase in the redox potential, increasing from 680 to 830 mV with the addition of 0.1 chlorides and 3 mol/L of NaCl, respectively, and consequently higher dissolution kinetics, mainly in the second leaching stage.

The results obtained from the aforementioned studies are directly related to the formation of a layer of elemental sulfur on the surface of the covellite in the second stage, which decreases with the increase in the potential of the solution; however, the layer in the solid surface is a mixture of sulfur and polysulfides (CuSn) [42], where these could be responsible for a slow reaction during this stage.
