*3.2. E*ff*ect on MnO2*/*Reducing Agent Ratio*

In Figure 4, results are presented for the dissolution of Mn with the use of different Fe reducing agents at different ratios of Mn/Fe. For all the cases presented (Figure 4a–d), when working at low Mn/Fe ratios the highest recoveries of Mn were obtained. Ratios of 1/2 proved to be an optimum in Figure 4b–d. While for the Figure 4a in ratios of 1/3, the increase in the dissolution of Mn continued. The best results were obtained in Figure 4c when working with FeC because it allowed a high activity ratio through the regeneration of ferrous ions, favoring the dissolution of Mn and allowing better results to the use of Fe2<sup>+</sup> in a direct way that is presented in Figure 4b. Using Fe2O3 shows good results when working with MnO2/Fe2O3 ratios of 1/2, although it is lower than those presented when using Fe2<sup>+</sup> and FeC. However, this may be an attractive proposal due to the reuse of tailings that are an environmental responsibility. For the use of pyrite, the lowest Mn solutions could be observed in this study. In previous studies [2,22,23,33], it has been indicated that it is not necessary to work at high concentrations of H2SO4 in the system to obtain high Mn solutions from marine nodules, but that if it is important to have low Mn/Fe ratios. The results presented in Figure 4a show a progressive increase in the Mn dissolution when increasing the amounts of FeS2 in the system, however, it may be necessary to increase the acid concentration or temperature because of the kinetics of dissolution of ferrous ions from the pyrite ore.

For the performed tests, the values of potential and pH for the different reducing agents used for Mn/Fe ratios of 1/2 are presented in Figure 5. Senanayake [13] indicated that dissolving Mn from marine nodules requires to work in potential ranges between −0.4 and 1.4 V and pH between −2 and 0.1. With this, it is possible to avoid the precipitation of the Mn through the oxidation-reduction reaction, due to the presence of ferrous and ferric ions [34]. The outcomes met the operational condition mentioned above, which is due to the high concentrations of reducing agent. The lowest potential values were obtained with Fe2<sup>+</sup> and FeC, wherein the iron (FeC) favored the regeneration of ferrous ions, which allows maintaining low potential ranges [22].

**Figure 4.** Effect on the ratio of MnO2/reducing agent at room temperature (25 ◦C), 0.1 mol/L H2SO4, 600 rpm and particle size of –75 + 53 μm (reducing agent: (**a**) FeS2, (**b**) Fe2<sup>+</sup>, (**c**) FeC and (**d**) Fe2O3).

**Figure 5.** Effect of the potential and pH in the solution of Mn with different reducing agents (MnO2/Fe2O3 ratio of 1/2, 25 ◦C, 600 rpm, −75 + 53 μm, acid concentration to 0.1 mol/L).
