Under normal industrial electrorefining operating conditions, pure copper will not passivate. Therefore, the ultimate cause of passivation is the impurities contained within the anode and electrolyte. Impurities in the anode are the result of the original ore or scrap feed. As the anode dissolves, some impurities report to the slimes and others to the electrolyte. Impurities in the electrolyte become concentrated over time in the closed electrolyte system.

The studying of anode impurities has been limited to specially alloyed copper samples and a few commercial copper anodes. The alloyed copper usually contains one or two metallic impurity elements and oxygen. The passivation results from alloys and the electrochemical method used are listed in Table 1.2. Minotas et al. used cyclic voltammetry (CV) on alloys of Cu-Sb, Cu-As, Cu-O to conclude that arsenic and oxygen delay passivation and antimony increased the likelihood of passivation. Bounoughaz et al. studied alloys of Cu-O, Cu-Ag, Cu-Se, and Cu-Ag-Se using CV, chronopotentiometry (CP) and impedance spectroscopy (EIS). It was found that tendency to passivate was Cu < Cu-O < Cu-Ag < Cu-Se < Cu-Ag-Se. Noguchi et al. used copper samples alloyed individually with S, Se, Pb, Ag, Bi, Sn, Ni, and Sb in the range of 2000 to 50000 ppm. Using CP, anodes with sulfur (2000 ppm), arsenic (30000 ppm), bismuth (40000 ppm), and silver (30000 ppm) passivated (impurity level where passivation was first detected is within the parentheses) while samples with nickel, tin, lead, and selenium did not passivate. However, the impurity loading in the samples that did passivate is much higher than normally found in commercial copper anodes. Gumowska and Sedzimir investigated copper alloyed with lead and oxygen using CP. They concluded that lead contents up to 2 wt% (20000 ppm) did not influence passivation. Oxygen, on the other hand, was found to promote passivation.

Table 1.2

The Effect of Impurities in Specialty Alloys on Passivation

It is apparent the results using specialty alloys are contradictory. Alloys with single elements also eliminate the possibility of interactions between impurities and may not have a microstructure similar to commercial copper anodes. Therefore, commercial copper anode samples have been studied as well. A summary of findings on passivation using commercial copper anodes is given in Table 1.3. Abe et al. studied thirteen anodes using CP. They illustrated that time to passivation (t_p) decreased with increasing oxygen and slimes. Abe and Goto , also using CP, illustrated that the susceptibility to passivate increased when nickel, oxygen, or silver levels were high in the anode. Baltazar et al. investigated the effect of arsenic and antimony using twelve different anode samples with chronopotentiometry at near to industrial conditions. Anodes with a molar ratio of As to Sb that was greater than two resisted passivation. Cheng and Hiskey revealed that increasing arsenic within commercial copper anodes inhibited passivation in CP experiments. Increasing oxygen in anodes with low arsenic (<400 ppm) decreased t_p. Hiskey et al. studied thirteen commercial copper samples using CP and showed that t_p decreased with increasing oxygen and antimony and decreasing arsenic.

Table 1.3

Effects of Anode Impurities in Commercial Copper Anodes on Passivation