Precipitation of MnO2

Across all experiments, 1 SMR of Na2S2O8 was used for the oxidation of Mn2<sup>+</sup> to MnO2. As suggested by Demopoulos et al. (2002), this reaction was performed at 60 ◦C and at a controlled pH to investigate the oxidation kinetics of Mn2<sup>+</sup> ions in MnO2 at a pH between 2 and 4 [31].

The residual concentration of Mn at pH 2, pH 3, and pH 4 was investigated. According to our results, the oxidation at pH = 3 gave a good oxidation yield (93%). After 120 min, Mn concentration in the solution decreased from 31.3 to 2.03 g/L (Table 8). An amount of 2 g Mn/L in the solution had a positive effect on the next step of Zn electro-deposition [32]. Notably, traces of Co co-precipitated with MnO2, whereas Ni did not precipitate with MnO2 (Table 8).


**Table 8.** Oxidation kinetics of Mn2<sup>+</sup> to MnO2 in different pHs.

#### 3.3.2. Electrowinning for Recovery of Zn Metal from PLS-1 Solution

The effect of current density on the electrowinning of Zn was evaluated by measuring the residual concentration of Zn between 250 and 750 A/m<sup>2</sup> for 180 min. The results clearly indicate that the deposition efficiency of Zn increased with an increase in current density. The high concentration of Mn (>5 g/L) in the solution decreased the faradic yield [32], while a lower concentration (between 1 and 5 g/L) had a positive effect on oxidation, thereby protecting the electrodes and increasing the purity of the deposited Zn [32]. At a high current density (750 A/m2), a decrease in Mn concentration was observed (from 2.03 to 0.22 g/L), which is consistent with the results obtained by Poinsignon et al. [33], where Mn oxidized at the anode and precipitated as MnO2. The largest Zn deposit was obtained with a current density of 500 A/m2. Under these conditions, Mn concentration remained stable (from 2.03 to 1.92 g/L).

Figure 4 shows the changes in Zn deposition as a function of time at a current density of 500 mA/m2. In the figure, two different zones can be distinguished, with the yield of Zn deposits increasing linearly with time until 90 min. After 90 min, the rate of Zn deposits decreased significantly. Notably, at the start of the electrowinning process, Zn concentration was relatively high (83.1 g/L) and, accordingly, the Zn deposit rate was subjected to current control. As the Zn deposit achieved a certain thickness and the Zn concentration was below a certain level (approximately 40 g/L), we conclude that the Zn deposit rate is limited by mass transfer control. This explains why the Zn deposit rate remained constant with time. Figure 4 also presents the change in energy consumption as a function of time, indicating that energy consumption increased linearly to 156 kWh/m<sup>3</sup> after 180 min. It has been established that Zn deposition efficiency is affected by reaction time and the cost of the electrowinning process. In view of reducing power use and increasing Zn deposits, a time of 90 min was selected for the process. Under these conditions, the residual concentration of Zn in the solution was 36.1 g/L. The solution was also recirculating for the first leaching (S/L ratio of 40%) and, consequently, an S/L ratio of 20% was used for the second cycle.

**Figure 4.** Variation in residual Zn concentration and effect of energy consumption as a function of time applying a current density of 500 A/m2.
