Development of Amorphous Oxide Coated Anode for Electrowinning of Zinc and Copper

        Zinc and copper are produced by electrowinning (EW) using sulfuric acid based solutions, in which oxygen evolution from water is the main anodic reaction, while it is known that the oxidation of Mn(II) or Pb(II) ions contained in the EW solutions also occur as unwanted side reactions on the anode, resulting in anodic deposition of MnOOH or PbO₂ [1, 2]. These reactions cause an increase in oxygen overpotential and a decrease in lifetime of the anode, because such metal oxides have a low conductivity and oxygen evolution is disturbed by the accumulation of the oxides. Although a commercially available IrO₂-Ta₂O₅/Ti anode comprising crystalline IrO₂ and amorphous Ta₂O₅ has a high catalytic activity and a long lifetime for oxygen evolution [3], it is impossible to suppress such unwanted side reactions even with this anode [4]. On the other hand, one of the authors of this paper has previously revealed that amorphous IrO₂-Ta₂O₅/Ti anodes prepared by a low temperature thermal decomposition, e.g., at 360 ^oC, have a higher catalytic activity for oxygen evolution than crystalline IrO₂-Ta₂O₅/Ti anodes and can suppress anodic depositions of MnOOH and PbO₂ [5,6]. Furthermore, we have been developing RuO₂-Ta₂O₅/Ti anode consisting of amorphous RuO₂ and amorphous Ta₂O₅ for oxygen evolution in EW. This paper reports the effect of the crystallographic structure and surface morphology of the RuO₂-Ta₂O₅ catalytic coatings on oxygen evolution, including suppression of MnOOH and PbO₂deposition in addition to a significant cell voltage reduction for zinc and copper EW.

        RuO₂-Ta₂O₅/Ti anodes were prepared by thermal decomposition of the precursor solution containing Ru (III) and Ta (V). The precursor solution was painted on a titanium substrate and calcined at a temperature between 260 ^oC to 500 ^oC. The characterization of the oxide coating was carried out with XRD, SEM and EDX. Electrochemical measurements were performed using a conventional three-electrode cell, in which the electrolytes were H₂SO₄ solutions with and without MnSO₄ 5H₂O or HNO₃ solutions with and without Pb(NO₃)₂. Constant current electrolysis was conducted using a two-electrode cell to determine the cell voltage during zinc or copper EW and to evaluate the anode’s durability for a long term operation.

        The crystallographic structure of RuO₂ changed from crystalline to amorphous when the thermal decomposition temperature decreased from 500 ^oC to 260 ^oC. The surface morphology of the amorphous coating showed that nano-RuO₂ particles, ca. 20 nm, were dispersed in amorphous Ta₂O₅ matrix. Such nano particles worked well to increase the active surface area for oxygen evolution, resulting in not only a low overpotential for oxygen evolution but also a high overpotential for PbO₂ deposition as shown Fig. 1. The onset potentials obtained with RuO₂-Ta₂O₅ coatings prepared at 500 ^oC, 280 ^oC and 260 ^oC indicates that the overpotential for oxygen evolution decreases, but that of PbO₂ deposition increases as thermal decomposition temperature becomes lower; therefore, the difference between the onset potentials for oxygen evolution and PbO₂ deposition becomes larger and especially reaches 500 mV with amorphous RuO₂-Ta₂O₅/Ti anode obtained at 260 ^oC. This means that the overpotentials of two difference reactions occurring on the same anode are individually controlled and the amorphization of RuO₂ works well to accelerate only to oxygen evolution, not to PbO₂ deposition. As similar to this, MnOOH deposition was also suppressed on the amorphous RuO₂-Ta₂O₅/Ti anode.

        The cell voltage during zinc or copper EW was measured using different anodes; amorphous RuO₂-Ta₂O₅/Ti, amorphous IrO₂-Ta₂O₅/Ti, and Pb alloy. Table 1 shows a comparison of the cell voltages at 500 A/m². The results revealed that the amorphous RuO₂-Ta₂O₅/Ti anode showed 700 mV lower voltage than the Pb alloy anode and 140-160 mV lower voltage than the amorphous IrO₂-Ta₂O₅/Ti anode. This significant voltage reduction is important because a large energy saving is expected for zinc and copper EW. This work further demonstrated the excellent durability of amorphous RuO₂-Ta₂O₅/Ti anode for oxygen evolution for more than 1000 hours.

This work was financially supported by Grant-in-Aid for "Kyoto Environmental Nanotechnology Cluster" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Table 1 Comparison of cell voltages during constant current electrolysis in zinc or copper EW at 500 A/m². 

	RuO2-Ta2O5/Ti	IrO2-Ta2O5/Ti	Pb alloy
Zn	2.32 V	2.46 V	3.02 V
Cu	1.22 V	1.38 V	1.92 V

References

1)     S. Nijjer, J. Thonstad, G. M. Haarberg, Electrochim. Acta, 46, 3503 (2001).

2)     Z. S. Msindo, V. Sibanda, J. H. Potgieter, J. Applied Electrochemistry, 40, 691 (2009).

3)     S. Trasatti, Electrochimica Acta, 45, 2377 (2000).

4)     M. S. Moats, JOM, 60, 46 (2008).

5)     M. Morimitsu, N. Oshiumi, T. Yamaguchi, Proc. Lead-Zinc 2010, 813 (2010).

6)     M. Morimitsu, N. Oshiumi, N. Wada, Proc. Copper 2010, 4, 1511 (2010).