Passivation of copper in the refining electrolyte: influence of oxygen content and of periodic current reversal (PCR)

Influence of oxygen content (up to ca. 0.48 weight %, i.e. 4800 ppm) in copper anodes on their susceptibility to passivation in a refining electrolyte, has been investigated. The t p -value, i.e. time after which the anode passivation occurs, has been used as a measure of this susceptibility. Another applied factor is the permissible current density, i pr . The electrolysis has been carried out for constant current or when periodic current reversal (PCR) has been applied. The obtained results prove the complex character of the influence of oxygen presence in copper anodes on their passivation. The oxygen content of the order of hundreds ppm (hundredth of percent) promotes passivation. At first an increase in oxygen content diminishes and afterwards rises again the passivation susceptibility of copper anodes. In order to get some more data which could help to explain these phenomena, anodic current efficiency and the mass of copper slime remaining on the anode surface, have been determined. It has been stated that anodic current efficiency, ranging from 102 to 103% in the case of the oxygen free anodes, exceeds 115% if oxygen content reaches 4800 ppm. In the case of oxygen containing anodes, the periodic current reversal causes decrease in their passivation susceptibility and also in the anodic current efficiency, shifting the latter towards its theoretical value. The obtained results combined with data taken from literature on the system Cu-Cu 2 O [7] , enabled us to formulate a hypothesis which explains the observed relations. In the case of copper containing less than 0.4% of oxygen (hypo-eutectic structure), Cu 2 O is present solely as a component of the fine-grained Cu-Cu 2 O eutectic. At higher oxygen contents (hyper-eutectic alloys) a part of Cu 2 O is also present in a form of larger grains embedded in this eutectic mixture. The Cu 2 O grains exposing while metallic copper was dissolving anodically, react with the H + ions. It leads to the decrease of H + ions concentration in the near-electrode layer. Consequently it could promote passivation related to the precipitation of sparingly soluble basic copper sulfate. No distinct correlation between the mass of Cu slime, remaining on the surface of anodes, and oxygen content in these anodes, has been found. It indicates that the Cu grains, formed in the reaction (2), partially fall down from the anode surface. This effect is particularly distinct in the case of eutectic or hyper-eutectic anodes in which metallic copper is present solely as a component of the finegrained Cu-Cu 2 O eutectic mixture. As a result of anodic dissolution of copper grains the inter-granular bonds are weakening and both the eutectic components and also the embedded large Cu 2 O grains, could easily fall down. The effect of oxygen content in copper anodes on their susceptibility to passivation depends on the two processes set against each other: on one hand - decrease in the concentration of H + ions near the anode (following the reaction (2)) will promote passivation process and on the other hand - convection of the electrode layer caused by falling down slime particles will facilitate the transportation of Cu 2+ ions and consequently will counteract the passivation. The presented hypotheses related to the mechanism of the investigated process explain the observed influence of oxygen content as well as application of the PCR, on the passivation and current efficiency of the anodic copper dissolution.