*3.2. The E*ff*ect of the Fe2O3*/*MnO2 Ratio*

Figure 3a,b show Mn dissolution with two black copper samples using Fe2O3 in acid media. As in earlier investigations by Toro et al. [26,28], working with an Fe2O3/MnO2 ratio of 2/1 or higher significantly increased MnO2 dissolution kinetics. There was little difference in the Mn extraction rates working with Fe2O3/MnO2 ratios of either 2/1 or 3/1, while the Mn extraction fell significantly when the quantity of Fe2O3 was reduced. Potential and pH levels were respectively in the ranges of −0.5 to 1.3 V and −1.5 to 0.4 in all the tests in this study.

*Metals* **2019**, *9*, 1112

**Figure 3.** The effect of the iron oxide tailings concentration on MnO2 dissolution (**a**) BSC-1; (**b**) BSC-2, 25 ◦C, particle size range of −75–+53 μm, 1 mol/L H2SO4.
