Electrochemical Probing of Causes for Variation in Lifetime of Iridium-Tantalum Oxide Electrode Used in Copper Electrowinning

Abstract

In hydrometallurgical copper production plants, titanium-based electrodes coated with a conductive layer of IrO2-Ta2O5 are widely used as anodes in acidic copper electrowinning baths because of their long service life and low overpotential for oxygen evolution. The presence of trace amounts of ions such as fluoride, aluminum, and iron in sulfate-based electrowinning baths is believed to affect the stability of IrO2-Ta2O5 coated anodes. Hence, in this study, the effect of fluoride and metallic cations on the lifetime of IrO2-Ta2O5 coated Ti electrodes in sulfuric acid solutions has been investigated, and a degradation mechanism for IrO2-Ta2O5 coatings in the presence of fluoride has been proposed. Typical lifetime of the conductive ceramic coated anodes is 1 to 2 years. In order to predict electrode performance over this long period, an accelerated laboratory test that can be carried out in a few weeks is often used. This test, known as accelerated lifetime test (ALT), is conducted by electrically stressing the anodes at a current density that is much higher than the current density used for electrowinning while monitoring the change in the cell potential. The time required for the cell voltage to increase by 5 V is taken as the accelerated lifetime of the oxide electrode. In this research, titanium mesh samples coated with mixed iridium oxide-tantalum oxide layers were tested as anodes in 2 M sulfuric solution a constant current density of 0.54 A/cm2. A two-electrode cell with a bare titanium mesh serving as the cathode was used for experiments. In addition to ALTs, anodic polarization measurements were also carried out to study the changes in overpotential for oxygen evolution on electrodes before and after ALTs. Additionally, morphology and chemical composition analyses were performed on electrodes before and after ALTs using various techniques such as scanning electron microscopy (SEM) analysis, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Chemical species that are likely to be present in the electrowinning bath were predicted using the computer software STABCAL and presented in distribution-pH and Pourbaix diagrams. The results of multiple ALTs in the absence and presence of various levels of fluoride indicate that the anode lifetime was greatly reduced by the presence of fluoride in sulfuric acid solutions. The greater the amount of fluoride added, the shorter the anode lifetime. Additionally, both in the absence and presence of fluoride, the molar ratio of IrO2 to Ta2O5 in the coating did not change during dissolution. In studying strategies to prolong the lifetime of the electrode in a fluoride-containing solution, a method of complexing fluoride ions using metallic cations such as Al3+ and Fe3+ was developed and demonstrated. The anode lifetime was successfully prolonged from 200 to over 500 hours with the addition of aluminum ions to a fluoride-containing solution. Compared with ferric ions, aluminum ions are more efficient in complexing with fluoride to extend the lifetime of electrodes.