To fully realize the effect of each impurity within the electrorefining system, the behavior of that impurity during casting and electrorefining must be understood. With this in mind, the mineralogy and microstructure of anodes will be reviewed. The deportment of the impurities during electrorefining will then be discussed in terms of slimes, non-slime elements, and impurities in the electrolyte.
Chen and Dutrizac , because of their numerous investigations concerning phases in commercial copper anodes, have greatly expanded the available information in this area. They primarily used scanning electron microscopy with an electron microprobe for morphological and elemental analyses. Their findings are numerous. Some of the more significant ones will be listed here. Silver is mostly contained in solid solution (85% to 95%) with the remainder tied to oxidate and selenide phases. Arsenic is also detected in solid solution between 30-60% of its original concentration with the rest occurring as arsenic oxide associated with complex lead-copper-oxide structures. Nickel remains in solid solution up to 3000 pm; above which NiO and/or kupferglimmer forms. The amount of nickel oxide increases toward the set side of the anode due to increased oxygen concentration from the atmosphere. Kupferglimmer, which is Cu3Ni2-xSbO6-x with x = 0.1-0.2, is detected when Sb is above 200 ppm. Tin can also substitute for antimony in the kupferglimmer structure. Cuprous oxide was the most abundant inclusion phase. Some of the Cu2O particles are encased by Cu2(Se,Te) shells . These result in selenide spheroids and tubular shape phases. Most tellurium is found in solid solution within the selenide phases except when tellurium is high. In high tellurium anodes, Te can be found in solid solution up to 2-8% of its original concentration and as copper tellurides . Lead is usually found as complex oxides involving copper and Group VB elements associated with Cu2O and Cu2(Se,Te) . Some lead is also found in solid solution. Silicates of various compositions have been detected.
Research directed by Forsen has greatly expanded the understanding of the deportment of nickel within copper anodes. Forsen and Tikkanen indicate that NiO and kupferglimmer will form when nickel is greater than 2500 ppm . This is in disagreement with the value reported by Chen and Dutrizac. Recent work at the University of Arizona indicates that 2500 ppm is more correct than 3000 ppm . Forsen's group also discovered that both NiO morphology and the abundance of kupferglimmer depend on cooling rate. The abundance of kupferglimmer was also determined as a function of nickel and oxygen concentrations within copper containing 1000 ppm Sb .
Finally, Mitan using optical and scanning electron microscopy with energy dispersive spectroscopy studied ten of the same commercial anodes that were electrochemically investigated in this study. His findings were similar in many aspects to the work reported previously except for a focus on grain size. A correlation was made between the maximum average grain size of an anode and time to passivation. Also for a kupferglimmer containing anode, the appearance of kupferglimmer increased as the grain size increased within the anode.
Using this information, a list of the more common phases and elements within commercial copper anodes is given in Table 1.4 from Hiskey and Moats .
Table 1.4
Common Phases Present in Commercial Copper Anodes
|
Elements |
Cu, Ag, Au |
O, S, Se, Te |
As, Sb, Bi |
Ni, Zn, Fe, Pb |
|
Phases |
Cu0, Ag*, Au*
(Cu, Ag)Se, Ag2Se, Au2Te, Ag2Te |
Cu2O, CuO, Cu2S, CuS, Cu2Se, Cu2Te |
As*, As2O3, Cu3As, Sb*, Sb2O3, Bi*, Bi2O3 |
Ni*, NiO, kupferglimmer ZnO, Fe*, Fe2O3, Pb*, Pb0, PbO, Pb- (As, Sb, Bi)-oxides |
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* - denotes solid solution
It is important to understand for anode passivation that impurities are present and in what phases to predict which of these will not dissolve into the electrolyte. As the copper anode dissolves, refractory (i.e. do not dissolve) inclusions are released at the surface of the anode. After a time, the build up of these inclusions forms a porous barrier that inhibits diffusion of species to and from the surface. This porous barrier in industrial terminology is the anode slimes.
Passivation of Impure Copper Anodes