Oxidation—MnO2 Recovery

The recovery of MnO2 was performed by manganese oxidation using sodium persulfate (Na2S2O8) according the following equation:

$$\text{MnSO}\_4 + \text{Na}\_2\text{S}\_2\text{O}\_8 + 2\text{H}\_2\text{O} \to \text{MnO}\_2 + \text{Na}\_2\text{SO}\_4 + 2\text{H}\_2\text{SO}\_4\tag{7}$$

The purified solution of ZnSO4 and MnSO4 from the previous step was used to recover MnO2. The kinetic reaction was carried out with 1 SMR Na2S2O8 at 60 ◦C (nearby 0.6 M of Mn), and the pH increased in the range of 2–4 by adding NaOH. The S/L separation was carried out by filtration and the resulting liquid was transferred to the next step for Zn recovery. The MnO2 precipitate was washed with an S/L ratio of 10% for 10 min (200 rpm) to obtain high-purity MnO2.

#### Electrowinning—Zn Recovery

An acrylic reactor, 8.5 cm (width) × 70 cm (length) × 15 cm (depth), was used for electrodeposition experiments. The electrode sets consisted of 14 aluminum cathodes and 15 Pb/Ag anodes, each with a surface area of 119 cm<sup>2</sup> (4.5 cm (width) <sup>×</sup> 10 cm (height) <sup>×</sup> 1 cm (thickness)). Electrodes were placed in parallel, 1 cm apart. The cathodes and anodes were connected to the negative and positive outlets of a DC power supply, respectively, Xantrex XFR (ACA TMetrix, Mississauga, ON, Canada). The working volume was fixed to 5 L at ambient temperature for all electrowinning experiments. Assays were conducted in batch mode with continuous mixing using a water recirculation system in the reactor. The purified solution of ZnSO4 (pH = 2) after MnO2 precipitate recovery was used in the electrowinning experiments, in which Zn was deposited on the cathode surface with a current density between 250 and 750 A/m2 and an electrowinning time of 180 min. In this condition, the potential was dropped from 5.6 to 5.2 V. Samples of electrolyte were taken after 15, 30, 45, 60, 90, and 180 min to analyze the residual metal concentration.
