Fundamental studies of copper anode passivation during electrorefining: Part II. Surface morphology
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I.
INTRODUCTION
A N O D E passivation is an extremely important aspect of copper electrorefining and has been the subject of considerable study in recent years. It is of practical importance to establish the cause of copper anode passivation in order to maintain efficient operations. Previous investigations have shown that anode passivation depends to a large degree on the properties of anode slimes, since these films behave as a barrier toward copper diffusion.tq The precipitation of a nonconducting "bluevitriol" ( C u S O 4 - 5 H 2 0 ) layer on the anode surface, because of supersaturation in the electrolyte, has been considered as a direct reason for passivation.V-~ The formation of other insoluble surface films such as P b S O 4 , NiO, and Cu20 has also been suggested as a possible cause for passivation.V.2,4.5.~o] Some studies found that deposits of silvert3,7,9,151 and various slime constituentsu~ could block active surface sites and result in copper anodes becoming passivated. Although many mechanisms have been suggested, the precise cause of anode passivation still remains unclear. Noteworthy studies of the morphology of impure copper anodes have been carried out by Chen and Dutrizac.t2-5] They have reported that the major phases found in anode slimes consist of selenides (Cu:Se, AgCuSe, Ag2Se), arsenates ( S b A s O 4 , B i A s O 4 ) , oxides (As2Oa, Sb203, Bi203, Cu20, NiO, SnO2), and sulfates (CuSO4, PbSO4). The amounts of each phase associated with anode slimes depend largely on the composition of the original copper anodes. Although only Cu2Se is generally present in copper anodes, various Ag-Cu-Se phases are usually detected in anode slimes. The following reactions have been suggested to account for the formation of a series of Ag-Cu-Se compounds:[3,4,tSl
XUAN CHENG, Research Associate. and J. BRENT HISKEY, Professor, are with the Copper Research Center, Materials Science and Engineering Department, University of Arizona, Tucson, AZ 85721 Manuscript submitted June 20, 1995.
61~-VOLUME 27B, AUGUST 1996
Cu2Se + x A g + --4 (Cu2_~Ag~) Se + x C u § (Cu2_~Ag.,)Se + (1 - x)Ag* --4 AgCuSe + (1 - x)Cu + AgCuSe + (1 - x)A T ---> (Ag2_~Cux)Se + (I - x)Cu § (Ag2_~Cu~)Se + x A g + --4 AgzSe + x C w
[1]
[2] [3] [4]
The formation of cuprous oxide was proposed by Jin and Ghali [~~ from cathodic voltammetry measurements. The possible paths include the following: (a) reacting with dissolved oxygen, 2Cu + 1/202 ---) Cu20
[5]
(b) chemical reaction with water molecules, 2Cu + + H_,O --~ Cu:O + 2H +
[6]
(c) electrochemical oxidation, 2Cu
+
H2O
-'+ C u z O +
2H § + 2e-
[7]
(d) and releasing Cu20 directly from the copper matrix. They indicated that Cu~O could then remain intact in the electrode surface or react electrochemically according to the following reactions: Cu,O + 2H + --> 2Cu 2+ + H z O + 2 e Cu20 --4 Cu 2+ + CuO + 2e-
[8] [9]
It was concluded by Jin and Ghali [~~ that at high current density, the Cu20 formed through electrochemical Reaction [7] is a compact, adherent, and continuous layer whic
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