Effect of Copper Electrowinning Additives on Performance and Reliability of Iridium-Tantalum Oxide Electrodes

Abstract

Hydrometallurgical copper electrowinning (EW) tank houses employ IrO2 - Ta2O5 coated titanium anodes in sulfate-based EW baths due to their high durability in acidic conditions and low overpotential for oxygen evolution. EW baths mainly consist of a sulfuric acid electrolyte with copper ions; however, chemical additives are added to enhance the performance of the EW process. These additives include grain refiners that improve the copper plating conditions and acid mist suppressors that reduce acid mist generation originating from the oxygen evolution reaction that occurs at the anode. Despite the widespread use of these additives, little is known about the interaction of grain refiners and acid mist suppressors on IrO2-Ta2O5 anode surfaces within the context of anode lifetime and oxygen evolution reactions. Thus, this study investigates the effects of EW additives on the lifetime and electrochemical stability of IrO2-Ta2O5 coated Ti mesh electrodes through electrochemical and microstructure analysis. The EW additives used in this research were a modified cornstarch grain refiner named Hydrostar and a fluorocarbon-based acid mist suppressor known as FC-1100. In commercial practice, a current density of 0.18 A/cm2 is used, which yields anode lifetimes of two to three years. To study the effects of current density on the lifetime of IrO2-Ta2O5 anodes, two current densities of 0.54 and 0.72 A/cm2 were used for accelerated lifetime (ALT) tests to observe the lifetime of the anodes in a shorter timeframe. ALT test is a laboratory method that electrically stresses the anode at increased current densities, while simultaneously monitoring the cell potential. The time required for the cell potential to increase ~5 volts is defined as the accelerated lifetime of the anode. A two–electrode configuration was used for ALT tests with a titanium mesh substrate coated with composite iridium oxide–tantalum oxide layers set as the anode and a bare titanium mesh substrate set as the cathode. The ALT test solution was 2 M H2SO4 containing fluoride in the absence and presence of EW additives. Polarization measurements were conducted to investigate the electrochemical performance of the anodes before and after ALT testing. Furthermore, the chemical bonding information of the Ir-Ta-O electrodes were obtained by x-ray photoelectron spectroscopy (XPS). The study concluded that after multiple ALT tests in the absence and presence of EW additives, the anode lifetime significantly increased in the presence of the EW additives. To determine which of the two additives possessed life extending properties, additional ALT tests were conducted using test solutions containing FC-1100 in the absence and presence of the cornstarch grain refiner. However, the cornstarch had little influence on extending the lifetime of the anode, which revealed that the FC-1100 demonstrated life extending capabilities. Additionally, higher current densities significantly decreased anode lifetime, which stems from the increased rate of oxygen evolution that encouraged spalling and removal of the oxide coating.