There are impurities in the anodes that are not usually involved with anode slime formation that can also affect passivation. These elements, even at low concentration, can affect the complex copper dissolution reaction. Stankovic, using pure copper, illustrated that Fe slightly accelerated, As significantly increased, and Sb slightly hindered the dissolution reaction of copper when present in the electrolyte at concentrations ranging from 10-5 to 10-1 M. In commercial anodes, the most significant impurities appear to be oxygen and the Group VB elements (As, Sb, and Bi). Minotas, Djellab, and Ghali , using cyclic voltammetry, concluded that oxygen was beneficial in delaying passivation. However, Abe, Burrows, and Ettel had previously indicated that oxygen content in commercial anodes was detrimental from chronopotentiometric results. This latter viewpoint of oxygen accelerating passivation was also concluded by Bounoughaz, Manzini, and Ghali and Hiskey et al. .
Arsenic, antimony, and bismuth are very troublesome elements in copper electrorefining. Their reduction occurs at nearly the same voltage as copper, which can cause cathode contamination during periods of passivation. Another problem caused by the Group VB elements is their accumulation in the electrolyte, which can lead to the formation of floating slimes. Floating slimes, which are SbAsO4 and BiAsO4 , form by precipitation typically away from the anode surface. In the electrolyte, they can float to the cathode where they can be entrapped; thus causing contamination. However, none of these reasons overtly affects anode passivation. Hiskey et al. demonstrated using chronopotentiometry on commercial anodes, that arsenic inhibits passivation, antimony accelerates it, and bismuth has a complex interaction. Finally, arsenic has been reported by Demaeral to cause a decrease in the adhesion of slimes, which would hinder passivation by the removal or sloughing of the diffusion barrier.
Hiskey et al. proposed that the effect of Group VB elements and oxygen could be explained by using a bi-layer passivation model. It is theorized that the slimes layer, because of its hindrance to diffusion, creates a substantial boundary layer adjacent to the anode surface. The concentrations of ions can be extremely different in this boundary layer than that in the bulk electrolyte. As oxides, particularly of As, Sb and Bi, dissolve, they either cause acid consumption or generation. Samples of possible reactions are as follows:Cu2O + 2H+ ® Cu2+ + Cu + H2O (Eq. 10) As2O3 + 3H2O ® H2AsO4 + 2H+ +2e (Eq. 11) Sb2O3 + 2H+ ® 2SbO+ + H2O (Eq. 12) Bi2O3 + 6H+ ® 2Bi3+ + 3H2O (Eq. 13)
The consumption of hydrogen ions can lead to a localized area of high pH at the anode surface. This can lead to the formation of cuprous oxide by several reactions including the reverse of (10) and cause passivation. It would appear that cuprous oxide would be unstable in the highly acidified electrolytes utilized, but due to the diffusion barrier caused by the slimes layer, localized pH conditions could be favorable for this phase to form. Arsenic, which is the only impurity to show an inhibiting effect on passivation, could generate acid by (11), which would hinder the formation of Cu2O.
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Anode Passivation Main Page
Passivation of Pure Copper
Passivation of Impure Copper Anodes
Secondary Phases Within the Anode
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