3.7.3. MnO<sup>2</sup>

The use of MnO<sup>2</sup> as an oxidizing agent in chloride media has recently been studied for secondary and primary sulfides [46,70,76–78] where positive results have been obtained in the dissolution of copper. For example, for a mineral refractory to conventional processes such as chalcopyrite, in the study carried out by Toro et al. [70] it was possible to extract 77% of copper at room temperature when working at high concentrations of MnO<sup>2</sup> (4/1 and 5/1) and chloride (~40 g/L), which allowed maintaining of the redox potential values between 580 and 650 mV. For the specific case of chalcocite, Torres et al. [46] worked under the same operational conditions as Toro et al. [13] (see Table 4). In their results, Torres et al. (2020a) showed that incorporating MnO<sup>2</sup> at low concentrations significantly

improves the dissolution of chalcocite in short periods, which is important in continuous leaching operations.

**Table 4.** Comparison between studies for the dissolution of chalcocite in chloride media, with and without the addition of MnO<sup>2</sup> .


#### **4. Conclusions**

Among the various leaching processes to treat chalcocite, chloride media show better results and have greater industrial relevance. This is because of the positive results in copper extraction, low cost, and the possibility of working with seawater. However, chalcocite leaching is a process that occurs in two stages, which must be evaluated individually according to the different operational parameters that can be tested in the process.

In general:

Working in chloride media favors the dissolution of Cu2S, accelerating the leaching kinetics in the first stage and making possible the dissolution reaction in the second stage. This is mainly due to two reasons: (i) the chloride ions in the system allow the cuprous ions to be stabilized through the formation of CuCl<sup>3</sup> <sup>2</sup>−, allowing the copper to be extracted directly from Cu2S without prior oxidation of Cu<sup>+</sup> to Cu2+; (ii) the chloride ions promote the formation of long crystals that allow the penetration of the reagent through the passivating layer. Furthermore, the concentration of chloride ions is the variable that most influences the dissolution kinetics of Cu2S at room temperature, making other operational variables, such as acid concentration, particle size, stirring rate, and addition of other oxidizing agents (air, ferric ions, etc.), less relevant.

Evaluating by stage:

The first stage of leaching occurs quickly, requiring low activation energy (4–25 kJ/mol) in the unreacted core model, via a controlled reaction by diffusion of the oxidant on the mineral surface, while reaction 2 is much slower and requires higher activation energy (71.5–72 kJ/mol), being a stage controlled by chemical reaction. 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 to be oxidized it is necessary to work at high concentrations of chloride (>2.5 mol/L) and/or increase the system's temperature. This is because to dissolve covellite it is necessary to increase the redox potential of the system (>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. Furthermore, it is important to note good synergy between the chloride concentration in the system and the temperature. The operational parameters impact differently in each of the stages, as can be seen below in Table 5:


**Table 5.** Impact of the different operational parameters on the dissolution of Cu2S.

Finally, it is concluded that the most optimal way to extract copper from chalcocite is, during the first stage, to work at high concentrations of chloride (50–100 g/L) and low concentrations of sulfuric acid (0.5 mol/L) at a temperature environment. Other variables become irrelevant during this stage if the concentration of chloride ions in the system is high. 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.

**Author Contributions:** N.T., D.T. and K.P. contributed in research and wrote paper, C.M., M.S. and E.G. contributed with research, review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Kevin Pérez acknowledges the infrastructure and support of Doctorado en Ingeniería de Procesos de Minerales of the Universidad de Antofagasta. Carlos Moraga acknowledges the support of Centro Tecnológico de Conversión de Energía of the University of Talca.

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

## **References**

