Zinc electrowinning uses an acidic zinc sulfate solution prepared from zinc ore, which contains some other metal ions such as Mn²⁺. The solution after zinc electrowinning goes back to the solution preparation process, in which the zinc concentration is recovered to the appropriate level, while that of Mn²⁺ is increased, if no Mn²⁺ is consumed by anodic deposition in electrowinning cell, which is possible with amorphous RuO₂-Ta₂O₅/Ti anodes [1]. Therefore, the situation that Mn²⁺ concentration becomes high enough to recover Mn²⁺ as manganese oxide is expected, and it would resemble the commercial production process of EMD (Electrolytic Manganese Dioxide) that is famous as the positive electrode material of some batteries. For such a novel manganese recovering system in zinc electrowinning, the anode for deposition of MnO₂ from zinc electrowinning solution is needed, with which MnO₂ is deposited on and harvested from the anode that should be durable for the deposition and harvesting cycle. In this study, we prepared the titanium electrode covered with manganese oxide as the catalytic layer for MnO₂ deposition from zinc electrowinning solution, and the obtained electrode was characterized by XRD, SEM, and EDX. The polarization measurement and continuous electrolysis of the anode in Mn²⁺ containing solutions were also performed to examine the obtained product on the anode and the current efficiency.

The manganese oxide coated titanium electrode was prepared by thermal decomposition of the precursor solution containing Mn²⁺ painted on a titanium substrate which had been degreased in acetone and etched in 10% oxalic acid. The obtained electrode was analyzed by XRD, SEM, and EDX. The anodic polarization of the anode in H₂SO₄ solution with and without Mn²⁺ was examined by cyclic voltammetry with a conventional three-electrode cell equipped with a platinum plate counter electrode and an Ag/AgCl reference electrode in saturated KCl solution. Constant current electrolysis was also performed to obtain MnO₂ on the anode, and the weight of the anode before and after the electrolysis was measured to know the amount of the product which was characterized by XRD. The electrolytic solution was 2 mol/L H₂SO₄ + MnSO₄ and used at a temperature from 40 ^oC to 75 ^oC.

The catalytic layer prepared by thermal decomposition was found to be Mn₂O₃ by XRD measurements. Constant current electrolysis with the anode was carried out under different conditions, in which the typical examples are 40 ^o C or 75 ^o C as the electrolyte temperature with electrolysis at 5 mA/cm² for 30 min. For these conditions, the product on the anode was obtained with the increase of the anode’s weight by electrolysis, and the XRD results showed the diffraction pattern corresponding to MnO₂, although the diffraction peak intensity was weak for the product obtained at 40 ^oC compared to that at 75 ^oC. The current efficiency for MnO₂ deposition was calculated with the assumption that no oxygen evolution occurs, and the results were 17.5% at 40 ^oC and 70.6% at 75 ^oC. It was also found that the electrolyte was transparent before constant current electrolysis, which was unchanged by the electrolysis at 75 ^oC, while that became dark red at 40 ^oC, implying that Mn³⁺ is generated at 40 ^oC and the oxidation of Mn²⁺ to MnO₂ is not completed. More detailed results on the effects of the electrolysis conditions on the obtained product and the current efficiency will be shown in this paper.

This work was financially supported by “Kyoto Super Cluster Program” of Japan Science and Technology Agency (JST).

Reference

[1] T. Zhang, Ph.D. Thesis, Doshisha University (2015).