Development of Amorphous Oxide Coated Anode for Electrowinning of Zinc and Copper
Authors: Zhang Tian , Ueda Masashi , Yamada Yuji , Morimitsu Masatsugu 
Published: February 2020

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
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