Fundamental studies of copper anode passivation during electrorefining: Part III. The effect of thiourea
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I.
INTRODUCTION
CATHODE deposits obtained from acidified copper sulfate solutions without additives are often soft and coarsely crystalline or nodular. In copper electrorefining, such deposits tend to entrain electrolyte and anode residues, resulting in cathodes of less-than-desirable purity. Organic additives are widely used in industrial operations to modify deposit structure and morphology and to impure cathode quality.[1–5] Thiourea and glue are common organic additives used in modern tank houses to produce smooth, dense, coherent deposits. In addition, the chloride ion is an essential member of the normal suite of addition agents. These additives influence the electrocrystallization of copper in various ways: glue (protein colloid) serves as a leveling agent, and thiourea and chloride ions serve as grain refiners. The need for these and possibly other additives (i.e., Avitone A, Tembind, Orzan, and Magnafloc) with regard to the cathodic deposition is clearly established. However, the role of these electrolyte constituents on the anodic dissolution of impure copper anodes is not so well understood. Anode passivation is an important problem in many commercial copper electrorefineries. In general, anode passivation is believed to be related to a number of chemical, physical, and operational factors (including the nature of adherent slime layers, composition of the anode and electrolyte, current density and temperature, and the chemical and electrochemical formation of anodic surface films). Very little is know about the influence of organic additives on the passivation behavior of impure copper anodes. Commercial tank houses commonly use glue in the range of 1 to 10 ppm, thiourea in the range of 0.5 to 3 ppm, and chloride in the range of 20 to 60 ppm. Jin and Ghali[6] examined the influence of glue, thiourea, Avitone (surfactant), and Percol 351 (nonionic flocculating agent) on anode passivation behavior. With an electrolyte containing 42 g/L Cu, 160 g/L H2SO4,
17 g/L Ni, and 12 ppm Cl at 65 7C, they found that the time to passivation of impure anodes was almost independent of thiourea in the range of 0 to 30 ppm. However, the effect of thiourea concentration and its degradation chemistry on anode passivation remains unclear. Galvanostatic (chronopotentiometry) potential-time curves are extremely useful in helping to identify the formation of a passive surface layer. In Part I of this article, it was demonstrated that chronopotentiograms generated at high current density (3820 A/m2) are a convenient method of producing potential-time (E-t) responses in about 1 hour.[7] Anode passivation phenomena as examined by this method yield a time to passivation (tp) parameter and four distinct electrochemical regions in the chronopotentiograms. A typical chronopotentiogram for a commercial copper anode is depicted in Figure 1. Indicated on this pattern are the passivation time (tp) and the four characteristic regions: active dissolution (region I), prepassivation (region II), passivation onset (region III), and passivati
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