Like anode secondary phases, the understanding of the phases present in the slimes layer has been greatly enhanced by Chen and Dutrizac . The important phases that were detected were (Cu,Ag)2Se rings, euhedral NiO crystals, PbSO4, silicates, kupferglimmer, and various complex oxides. Most of these phases were formed during prior pyrometallurgical processing. During dissolution they are released from the anode. Several phases, however, are formed or transformed during the electrorefining process. Selenium occurs predominantly as a metal deficient copper selenide (Cu2-xSe) along the grain boundaries of the copper matrix. During electrorefining, silver ions have been shown by Scott and Chen and Dutrizac to rapidly react with the selenides causing a transformation from Cu2Se to Ag2Se. Hiskey et al. illustrated that this reaction causes a 20% increase in the volume of the selenide phase. This would help to explain why Abe and Goto noted that an anode with high silver exhibited a thicker and denser slimes layer than other anodes. The presence of lead sulfate is another indication of a reaction occurring within the slimes layer. Lead typically occurs as a complex lead oxide associated with various elements. These inclusions usually dissolve upon exposure to the highly acidic electrolyte. The resulting lead ions precipitate with the sulfate contained within the electrolyte to form PbSO4 within the slimes barrier.
An understanding of the slimes chemistry and morphology is imperative because the slimes layer can inhibit diffusion, which leads to passivation. Abe and Gotoh illustrated that pure copper, which did not passivate, passivated when masked with an artificial slimes layer (Millipore filter paper). It was also demonstrated that the tendency to passivate increased with decreasing pore size. Thus, a slimes layer that is denser and thicker will increase the probability of passivation. This was perhaps shown more precisely by Abe, Burrows, and Ettel , who demonstrated that the time to passivation measured by chronopotentiometry decreased with increasing amounts of slime formers within commercial copper anodes. Petkova performed a sedimentary study to determine that the particle sizes varied from 0-30 microns in diameter with most being between 10-30 microns.
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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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