*3.1. The E*ff*ect of the Fe<sup>2</sup>*+/*MnO2 Ratio*

Figure 2a,b show the results for the dissolution of two black copper samples using Fe2<sup>+</sup> in acid media. As can be seen, better results were achieved with the sample BCS-1, which was due to the high presence of clay in sample BSC-2. It was observed that high Mn extraction rates can be achieved in short periods of time using MnO2/Fe2<sup>+</sup> ratios of 1/2 or less, achieving dissolution rates of over 78% in 5 min with sample BCS-1, and 65% in 5 min with sample BCS-2. The results shown in Figure 2a are similar to the 90% recovery obtained by Zakeri et al. [25] in 20 min leaching MnO2 from manganese nodules with an Fe2+/MnO2 ratio of 3, and an H2SO4/MnO2 molar ratio of 2/1. A 1/1 Fe<sup>2</sup>+/MnO2 ratio resulted in a lower MnO2 dissolution kinetics, with an extraction of 40% in 5 min with sample A and 31% in 5 min with sample B. In general, Mn dissolution rates were similar with a longer period (30 min). However, the dissolution kinetics were slower for the sample BCS-2.

**Figure 2.** The effect of the Fe2<sup>+</sup> concentration on MnO2 dissolution (**a**) Black copper sample-1 (BSC-1); (**b**) Black copper sample-2 (BCS-2), 25 ◦C, particle size range of −75–+53 μm, 1 mol/L H2SO4.
