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

Figure 6 shows the effect of sulfuric acid concentration when working at Mn/Fe ratios of 1/2 with the use of different Fe reducing agents. Figure 6b,c shows that the concentration of H2SO4 was irrelevant in the extraction of Mn when working at low ratios of Mn/Fe with the use of Fe2<sup>+</sup> and FeC. This is compatible with previous studies conducted by Zakeri et al. [24] and Bafghi et al. [22]. The researchers indicated that working at high concentrations of ferrous ions, variables like acid concentration and particle size were irrelevant. For the case shown in Figure 6d, it was observed that when working with the use of Fe2O3 there was a slight increase in Mn solutions when working above 0.1 mol/L, although it was observed that there were no differences between 0.5 and 1 mol/L, which reaffirms what was raised by Saldaña et al. [2], where they indicated that when working on

acid-reducing leaching of MnO2 using tailings, the acid concentration only influenced the extractions of Mn when it was not operated in high levels of Fe or no temperature increase. Finally, it can be seen in Figure 6a that when working with pyrite, the concentration of acid in the system was important. This was consistent with the results obtained by Kanungo et al. [21], which states that in an acid solution of marine nodules with the use of pyrite as the acidity of the medium decreases, the rate of reduction of MnO2 decreases.

**Figure 6.** Effect on the concentration of H2SO4 at room temperature of (25 ◦C), ratio of MnO2/reducing agent of 1/2, 600 rpm and particle size of <sup>−</sup><sup>75</sup> <sup>+</sup> <sup>53</sup> <sup>μ</sup>m (reducing agent: (**a**) FeS2, (**b**) Fe2<sup>+</sup>, (**c**) FeC and (**d**) Fe2O3).

#### **4. Conclusions**

The Fe presented in the different additives proved to be a good reducing agent, increasing the dissolution of MnO2. The main findings of this study were the following:

(1) Fe0 (FeC) proved to be the best reducing agent for the dissolution of Mn from marine nodules since the direct contact of Fe in the liquid solution kept the regeneration of ferrous ions, due to high levels of ferrous and ferric ions.

(2) When working with Fe2<sup>+</sup>, FeC and Fe2O3, and having high concentrations of reducing agent (MnO2 ratios/reducing agent 1/2 or lower), low potential values were maintained, which allowed working at low acid concentrations (0.1 mol/L). However, for FeS2, better results were achieved at higher ratios of MnO2/FeS2 (1/3) and acid levels of 1 mol/L, which was possibly due to the refractoriness of pyrite.

(3) For the tests carried out in this study with the different Fe reducing agents, the potential and pH ranges were from −0.4 to 1.4 V and −2 to 0.1, favoring the dissolution of Mn from marine nodules, and avoiding the formation of precipitates of the Fe.

(4) The best results of this research (97% of Mn) were obtained at MnO2/FeC ratios of 1/2, 0.1 mol/L of H2SO4, in a time of 20 min.

In future work, other industrial iron wastes, generated in large industries, should be evaluated to create novel acid-reducing processes of MnO2. Subsequently, to recover the manganese present in the solution, zero-valent iron (ZVl) is a good alternative. Zero valence iron can be reused, from scraps of the metal finishing industry.

**The authors wish to make the following corrections to this paper:**

**We worked at a temperature of 60** ◦**C for all the tests carried out in which FeS2 was added as a reducing agent, whereas in the other experiments, other added Fe reducing agents were worked at room temperature (25** ◦**C). This a**ff**ects the results presented in Table 5 (results when working with FeS2), Figures 4a and 6a. By accident and through carelessness, we did not indicate this important detail in the work methodology. For this reason, we must correct it for the readers, otherwise the reproduction of the results of our experiments will not be possible due to incorrect working parameters. However, we confirm that this error does not a**ff**ect the conclusions of the manuscript.**

**We must indicate that it is unlikely that the following series of reactions that were presented in the document could occur at room temperature:**

$$\rm 3FeS\_2 + 4H\_2SO\_4 = 3FeSO\_4 + 4H\_2O + 7S \tag{1}$$

$$\rm 6FeSO\_4 + 4H\_2SO\_4 = 3Fe\_2(SO\_4)\_3 + 4H\_2O + S \tag{2}$$

$$2\text{FeS}\_2 + 4\text{H}\_2\text{SO}\_4 = \text{Fe}\_2(\text{SO}\_4)\_3 + 4\text{H}\_2\text{O} + 5\text{S} \tag{3}$$

$$\rm 15MnO\_2 + 2FeS\_2 + 14H\_2SO\_4 = Fe\_2(SO\_4)\_3 + 15MnSO\_4 + 14H\_2O \tag{4}$$

#### **We will update the article and the original version will remain available on the article webpage.**

**Author Contributions:** N.T. and R.I.J. contributed in project administration, M.C., S.N., L.A. and J.C. contributed in investigation and D.T. and N.T wrote paper, M.S. contributed in the data curation and software, P.R. contributed in validation and supervision and review and editing.

**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 of the Universidad Católica del Norte for supporting the experimental tests. Pedro Robles thanks the Pontificia Universidad Católica de Valparaíso for the support provided.

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

#### **References**


© 2019 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 (http://creativecommons.org/licenses/by/4.0/).
