*3.2. The E*ff*ect of Temperature on CuFeS2 Dissolution*

Figure 3 shows the effect of temperature and MnO2/CuFeS2 ratios on CuFeS2 dissolution. Dutrizac [35] stated that it is difficult to precisely determine the effect of temperature on copper dissolution from chalcopyrite in chloride media, owing to the presence of small amounts of secondary copper mineralization that can affect data interpretation. However, this problem was avoided in this study by using pure chalcopyrite. It can be seen from Figure 3 that at high temperatures (80 ◦C), the extraction of copper in the system is greater, with similar results obtained with MnO2/CuFeS2 ratios of 2/1 and 5/1. It can also be seen that at ambient temperature there is a significant difference in Cu extraction (About 30%) at ambient temperature between MnO2/CuFeS2 ratios of 2/1 and 5/1. The potential values for the tests at room temperature were between 540 and 590 mV, which is within the potential range where the dissolution rate of the chalcopyrite is linear (550 and 620 mV), as Velásquez-Yévenes et al. [37] noted. The potential values in the tests at temperatures of 50 and 80 ◦C were between 610 and 660 mV, and yielded higher copper dissolution rates. This is because high concentrations of chloride can raise the range of potential values [34]. The pH levels ranged between −0.5 and 1.4 in all the tests.

**Figure 3.** *Cont*.

**Figure 3.** Effect of the temperature on the dissolution of Cu from chalcopyrite at different ratios of MnO2/CuFeS2 (particle size of −47 + 38 μm, ratio MnO2/CuFeS2 of 2/1 (**a**) and 5/1 (**b**), H2SO4 concentration to 1 mol/L and 39.19 g/L of chloride).

Manganese nodules are composed of 29.85% MnO2 and 26.02% Fe2O3, which are dissolved in the acidic environment. Figure 4a shows how the MnO2 used as an oxidising agent for the dissolution of copper, where the Mn (IV) is reduced to Mn (II). Manganese has a high extraction at a temperature of 80 ◦C. The manganese dissolved in the PLS can be present in two forms, such as MnSO4 or as MnCl2, due to the effect of sulfuric acid and/or chloride, respectively. In Figure 4b, the iron (II) present in the chalcopyrite oxidises and slowly dissolves in an environment of high concentration of sulfuric acid and high level of chloride forming ferric chloride, which is positive since it is a compound that helps the copper solution from the chalcopyrite, and the kinetics of the iron solution increases with temperature. Regarding the pH, it can be seen that lowering the acidity in the Mn solution it does not decrease the dissolution of this element. While in iron, more positive results are presented in a more acidic environment.

(**a**)

**Figure 4.** *Cont*.

**Figure 4.** Dissolution of Fe and Mn over time from CuFeS2 at room temperature (25 ◦C) and high temperature (80 ◦C) (**a**): Dissolution of Fe and its behavior at changes in pH; (**b**): Dissolution of Mn and its behavior at changes in pH).

In the big copper mining in Chile, the Fe and Mn present in the pregnant leaching solution (PLS) are considered as impurities, this because in the electro-obtaining process, the Fe reduces the efficiency of current. At the same time, the Mn generates corrosion in lead anodes. However, both impurities are controlled in metallurgical plants minimizing the problems that they could produce. Manganese and iron can be transferred to the electro-obtaining stage through physical drag generated by solvent extraction. To reduce the physical transfer of impurities, the plants must optimize the equipment to retain water trawlers in organic (A/O), in addition to adopting some operational practices like maintaining good organic quality through the treatment of organic with clay; avoid over agitation in the mixers; surfactant addition control; and maintain design parameters within the recommended range (linear speed, specific decantation flow, etc.). For the reasons stated, it is possible to apply this process at the industrial level through the conventional hydrometallurgical route (leaching, solvent extraction and electro-obtaining), since solvent extraction processes in Chile have solved this problem. Also, it works in several miners with the use of seawater or adding high concentrations of chloride in synthetic form.

#### **4. Conclusions**

This research presents the results of dissolving copper from chalcopyrite by adding MnO2 as an oxidizing agent (manganese nodules) in a chloride medium (wastewater). As previously concluded by Devi et al. [26]; Devi et al. [27]; Havlik et al. [28] and Toro et al. [29], the addition of MnO2 and chloride in high concentrations generate a positive effect on the chalcopyrite solution. The main findings of this study are:


**Author Contributions:** All of the authors contributed to analyzing the results and writing the paper. All authors have read and agreed to the published version of the manuscript.

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

**Acknowledgments:** The authors are grateful for the contribution of the Scientific Equipment Unit-MAINI of the Universidad Católica del Norte for facilitating the chemical analysis of the solutions. Pedro Robles thanks the Pontificia Universidad Católica de Valparaíso for the support provided. Also, we thanks Conicyt Fondecyt 11171036 and Centro CRHIAM Project Conicyt/Fondap/15130015.

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