Passivation of Cu–Sb anodes in H2SO4−CuSO4 aqueous solution observed by the channel flow double electrode method and optical microscopy

In the electrorefining of Cu, the precipitation of copper sulfate on the anode surface causes passivation, which decreases current efficiency. Because the presence of Sb in a Cu anode is known to accelerate passivation, this study investigated the anodic behaviors of Cu–Sb alloys in a H2SO4−CuSO4 electrolyte by a direct observation method that combined optical microscopy with the channel flow double electrode technique. When a Cu−Sb (5 wt% Sb) anode (here denoted “Cu−5 wt%Sb”) with Sb segregation was dissolved, the Sb-rich region remained on the surface, inhibiting Cu dissolution and ion transfer and resulting in acceleration of passivation, which was observed as a drastic decrease in current density during voltammetry. However, a Cu−5 wt%Sb anode solutionized by heat treatment showed a moderate decrease in current density in some measurements, indicating prevention of passivation. It was observed that the dissolution of the solutionized Cu−5 wt%Sb anode generated an Sb-rich layer with complex sulfate compounds on its surface and accompanying pits. Copper sulfate precipitated selectively on the Sb-rich layer and grew into coarse grains, without covering the entire anode surface. These results suggest that passivation can be prevented by controlling the microstructure, even when an anode contains Sb. The results of this work are expected to contribute to the development of an efficient electrorefining process of Cu anodes containing high concentrations of Sb. [Display omitted] •Cu−5 wt%Sb anodes were studied with the aim of preventing passivation.•Anode surfaces were optically observed during voltammetry in H2SO4−CuSO4 electrolyte.•Segregated Sb in the Cu−5 wt%Sb remained on the surface, accelerating passivation.•Passivation of solutionized Cu−5 wt%Sb anode was prevented occasionally.•Microstructure control potentially prevents passivation of Cu with high Sb content.