Fundamental studies of copper anode passivation during electrorefining: Part I. development of techniques
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I.
INTRODUCTION
COPPER electrorefining in acidified copper sulfate electrolytes has been widely used for over a century to produce high-purity copper. Knowledge of the basic principles and theories were well understood near the turn of the century; however, some operating problems still remain unsolved. Prominently among them is anode passivation, frequently encountered in commercial copper electrorefining operations. Passivation results in a reduction of refinery output and an increase of power consumption. A major cause of passivation is impurities in copper anodes which usually contain notable amounts of Se, Te, As, Bi, Sb, S, Ni, and precious metals such as Ag and Au. Although total impurity concentration in commercial copper anodes is less than 1 wt pct, the small amount of certain impurities can have a dominant effect on the anode behavior and the refining process. The effects of individual anode impurities such as arsenic, antimony, silver, nickel, and selenium on passivation have been previously recognized and extensively studied. [~-~~ Due to the electrochemical nature of the copper refining process, various electrochemical techniques (i.e., cyclic voltammetry, potentiodynamic polarization, and galvanostatic measurement) have been considered as complementary and powerful approaches for studying copper anode passivation. Kucharska-Giziewicz and MacKinnonttOl identified five distinct regions in the cyclic voltammograms of silver-rich (0.1 pct Ag) anodes. The five regions consist of (1) initial dissolution, (2) formation of the first peak, (3) development of the second peak, (4) current oscillations, and (5) regular dissolution which occurs after surface films have formed on the electrode. The formation of peaks corresponded to the accumulation of reaction products on the anode surface. The oscillations in region 4 were related to the detachment of surface product layers. The shape of the second peak was claimed to be related to the nature and composition of product layers which were associated with the accumulation of metallic silver on the electrode surface. Based on poten-
XUAN CHENG, Research Associate, and J. BRENT HISKEY, Professor, are with the Copper Research Center, University o f Arizona, Tucson, AZ 85721. Manuscript submitted May 17, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
tiodynamic polarization measurements with pure copper and three silver-rich anodes, Compare et al.t8] observed different current maxima depending on the silver content of anode samples. They observed two well-defined maxima at the highest silver level (0.87 pct Ag) and peak current densities considerably below that of pure copper. Passivation times of pure copper and impure copper anodes were obtained by Abe et al.tq and Compare et al.tS] through galvanostatic measurements. The previous studies have focused primarily on single element effects, notably, As, Sb, Ag, Ni, and Se. In most instances, experimental anodes were modified by adding large amounts of individual elements to amplify their effect on passivation. Unf
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