Passivation Behaviour of Anode in Copper Electrorefining Using Recycled Low-grade Copper Anode

Abstract

Cu electrolysis was performed using low-grade copper anode with purity 78.7 mass% at a anode current density of 200 A ・ m -2 in an unagitated sulfate solution containing 0.596 mol ・ dm -3 of Ni 2+ ions as impurity at 60 ℃ , and the structure of anode slime before and after passivation was analyzed. The highly-concentrated Sb, Pb and Ag were dispersed in copper anode before electrolysis. The passivation occurred at short time of 17.7 hours in the case of low-grade copper anode, and it occurred at shorter time of 11.4 hours in the solution containing Ni 2+ ions as impurity. At normal dissolution of copper anode, copper parent phase dissolved in the form of dendrite and the framework of anode slime was composed of remained Cu-Ni-Sb-Sn-As compound. Cu 2 S and single phase of Ag and Pb were observed around this framework. At passivation, Cu, Ni and Sn dissolved from the slime framework composed of Cu-Ni-Sb-Sn-As compound, resulting in formation of Sb condensed phase. These Cu-Ni-Sb-Sn-As compound, Sb condensed phase, Cu 2 S compound around the slime framework, Ag and needle-shaped PbO dotted in the whole slime suppressed the diffusion of Cu 2+ ions, as a result, CuSO 4 was formed at interface between the slime and copper substrate. The passivation occurred at slime thickness of approximately 500 μ m in Ni 2+ -free solution, while it occurred at slime thickness of approximately 300 μ m in solution containing Ni 2+ ions. This indicates that the passivation do not depend on only the thickness of slime. The thickness of CuSO 4 formed at passivation in Ni 2+ -free solution was almost identical with that from the solution containing Ni 2+ ions. In solution containing Ni 2+ ions, the passivation occurred at less thickness of slime, showing that CuSO 4 is easy to be formed in the vicinity of copper substrate due to decrease in solubility of CuSO 4 and diffusion coefficient of Cu 2+ ions.