*3.3. The E*ff*ect of the H2SO4 Concentration*

Figure 4 shows the effect of the acid concentration on dissolving Mn dissolution from the two black copper samples with the addition of high concentrations of iron oxides from tailings or ferrous ions. It can be seen from Figure 4a,c that for sample BCS-1, the sulfuric acid concentration was not significant in either case when working with high concentrations of the reducing agent. Differences in the effect of the acid concentration could only be noted with very low concentrations of iron oxide tailings (0.5 mol/L). The above concurs with findings of previous studies by Toro et al. [24,26] on extraction of MnO2 from manganese nodules. The Mn extraction rate from the BCS-2 sample increased with higher concentrations of H2SO4, possibly owing to the high consumption of acid generated by the presence of mineral impurities in this sample, mainly montmorillonite, kaolinite, and chlorite. This is consistent with what was previously found by Helle and Kelm [29], where the leaching of exotic Cu minerals (atacamite, chrysocolla, and malachite) required higher acid consumption by incorporating reactive bargains into the system. This was driven by smectites, mordenite bargain, and the presence of kaolinite, illite, and quartz.

**Figure 4.** *Cont*.

**Figure 4.** The effect of the sulfuric acid concentration on the system (**a**) BSCe-1; (**b**) BSC-2, MnO2/Fe2O3 ratio of 1/2; (**c**) BSC-1; (**d**) BSC-2, MnO2/Fe2<sup>+</sup> ratio of 1/2.

#### **4. Conclusions**

This study presents the results obtained for dissolving Mn from black copper using iron oxides (and specifically magnetite) from tailings and Fe2<sup>+</sup> as reducing agents in acid media. Both reducing agents yielded good results with the two samples studied. Similar behavior was observed with the two samples in relation to Mn extraction, with the best results obtained in all the experiments with the BCS-1 sample. These encouraging results give new options to extract the Cu present in these exotic minerals, which are considered as industrial waste today. The main findings are the following:


Despite the good results obtained with BCS-1, BCS-2 was more like the mineralogy found at the industrial scale. It should be noted that although lower Mn extraction rates are obtained using tailings instead of ferrous ions, tailings can be a more attractive additive for leaching black copper because they are an industrial waste with no economic value. Given the above results, future investigations should aim to optimize operational parameters for leaching black copper minerals with high gangue content using industrial waste or wastewater as reducing agents, with the aim of taking this process to the industrial scale.

**Author Contributions:** K.P. contributed in research and wrote paper, N.T. and R.I.J. contributed in project administration, E.C. and A.N. contributed resources, J.G. contributed in review and editing and M.H.R. contributed in data curing.

**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 aiding in generating data by automated electronic microscopy QEMSCAN® and for facilitating the chemical analysis of the solutions. We are also grateful to the Altonorte Mining Company for supporting this research and providing slag for this study, and we thank to Marina Vargas Aleuy and María Barraza Bustos of the Universidad Católica del Norte for supporting the experimental tests. Also, we Conicyt Fondecyt 11,171,036 and Centro CRHIAM Project Conicyt/Fondap/15130015.

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