The electrowinning process, EW, consists of the selective cathodic deposition of metals contained in a concentrated electrolytic solution to generate high quality and high purity products. This controlled precipitation is achieved by circulating a low intensity direct electric current between an insoluble anode, inert to the process, and a cathode (stainless steel plates). Oxidation-reduction reactions allow the incorporation of the cupric ion as metallic copper into a polycrystalline network free of impurities, which remain dissolved in the electrolyte or precipitate in anodic sludge.
As electrowinning is the final stage in the processing of copper ores via hydrometallurgy, its main objective is to produce high quality cathodes to maximize the economic results of selling the product in the market. Although the valuable metal recovery processes and their technologies have matured to the point of producing copper with similar or even higher purity than the copper obtained by electrorefining, sometimes certain dissolved contaminating species such as chlorine can be entrained in the process solutions and generate adverse effects for the system and the final product.
In EW operations, it has been shown that at concentrations lower than 0.03 g/L chlorine has a favorable effect on grain size reduction and inhibition of the co-deposition of impurities such as bismuth, which favors the mechanical properties of the cathode. However, since Cl- is an anion coming from a strong acid, a higher concentration in solution could cause an attack on the passivation layer of the permanent stainless steel cathode, deterioration of the electrode contacts and of the equipment involved in the process, finally inducing a localized corrosion process.
Pitting corrosion is greatly enhanced by the oxidizing-acidic environment of the solution, as chloride ions migrate towards the affected areas promoting the continuous propagation of pitting and avoiding repassivation. On the other hand, if the chloride ion concentration exceeds 0.05 g/L, it adsorbs on the cathode interacting with copper and favoring the precipitation of the species of interest in the form of cuprous chloride (CuCl(s)), which decreases the efficiency of the electrodeposition process.
Another collateral effect is the inevitable anodic oxidation of the chloride ion to dichlorine Cl2(g), a highly corrosive and pneumotoxic gas that attacks the respiratory system, damaging the health of operators.
Currently, an alternative to mitigate the precipitating effect of Cl- is to wash with water from reverse osmosis, which, although it reduces the concentration of chlorine, increases operational costs and the consumption of water resources, which today is very scarce, and also such an operation is not selective since it could drag certain additives such as cobalt and iron that would later have to be restored.
Another alternative is the use of sacrificial cells in the first line of electrowinning vessels, by connecting the system to a less noble metal and electronegative enough to favor the displacement of the voltage from the metal of interest to values below the thermodynamic potential, which minimizes the probability of corrosion.
However, preventive actions to purify the chloride in the leaching stages and the use of highly selective extractants against this species in solvent extraction operations could limit the possibility of the effects described above and ensure the physical and chemical quality of both the final cathode and the equipment and steels used in the process.
