Zinc electrowinning produces a high purity of zinc on the cathode from acidic zinc sulfate solutions containing Zn(II) ions and other metal ions, e.g., Mn(II), extracted from zinc ores. Lead alloy anodes are frequently used as the anode, at which oxygen evolution mainly occurs, but unwanted manganese oxide deposition also happens, since the onset potential of Mn(II) oxidation to manganese oxide is less noble than that of oxygen evolution due to a high overpotential of lead alloy to oxygen evolution. This unwanted reaction induces the increase in cell voltage and makes the anode’s lifetime short, so that the anodic deposition of manganese oxide is required to inhibit in zinc electrowinning. This paper presents a novel method to suppress manganese oxide deposition with using amorphous oxide coated titanium anodes which have significantly larger active surface area than the cathode of zinc electrowinning. The new method is constant current electrowinning as same as the present process, although a large difference in actual current densities of the anode and the cathode is effective to suppress the anodic deposition and cathodic dissolution of electrowon zinc at low current density less than 1 mA/cm².

The anode used in this work is amorphous IrO₂-Ta₂O₅ coated titanium electrodes prepared by thermal decomposition at 340 ^oC of the precursor solution containing Ir(IV) and Ta(V) at 80 mol% of Ir painted on titanium plates [1]. Constant current electrolysis was carried out in 2 mol/L H₂SO₄ + 0.1 mol/L MnSO₄ + 0.95 mol/L ZnSO₄ + 0.02 g/L glue, which were held at 40 ^oC without stirring, with a two-electrode cell using a zinc plate as the cathode. The geometric surface area of the anode and the cathode was 1 cm².

Constant current electrolysis was carried out at various current densities for 14 hours which is a typical electrolysis period of zinc electrowinning at low current density during daytime in Japan. The electrolysis at 0.40 mA/cm² or more resulted in the increase in anode’s weight, and the deposit formed on the anode was found to be δ-MnO₂ by XRD, while the current density less than 0.30 mA/cm² produced no deposit on the anode by electrolysis, even though the electrolyte became light pink after the electrolysis suggesting that Mn(II) was oxidized to Mn(III), but not to manganese oxide. On the other hand, no dissolution of electrowon zinc was observed at 0.15 mA/cm² or more. From the results, anodic δ-MnO₂ deposition can be suppressed at 0.15 to 0.30 mA/cm² with the amorphous oxide coated titanium anode even for 14 hours without the dissolution of zinc.

Reference

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