Abstract
The main aim of this thesis is to investigate how specific anions like chloride and fluoride affect dimensionally stable anodes (DSA) during zinc electrowinning and how they affect the overpotential and the characteristics of the oxygen gas bubbles formed at the anode. Metallic zinc is typically produced through electrolysis, which uses electricity to reduce zinc ions in an electrolyte into metal. This process has a high energy consumption and is very susceptible to impurities.
Today zinc is primarily used to galvanize other materials as a form of corrosion protection. Metallic zinc has a relatively low reduction potential, and because of this, it will often corrode before other metals. The world is currently in a state of transitioning to cleaner, more environmentally friendly production, and zinc electrowinning is no exception. The large overpotential of the oxygen evolution reaction on the anodes results in large amounts of extra energy being needed to produce a set amount of metal.
To mimic the conditions found during the industrial production of zinc, a blank electrolyte of 1.7 M sulphuric acid was created to function as the base electrolyte with DSA acting as the working electrode. To this base electrolyte, varying concentrations of fluoride and chloride were added. Voltammograms for each electrolyte were created by performing cyclic voltammetry, while linear sweep voltammetry (LSV) and chronopotentiometry (CP) were used to determine the anodic overpotentials.
It was discovered that chloride resulted in an initial increase in overpotential, with minimal change observed with further additions. A similar increase in overpotential was observed with the introduction of fluoride, with a linear increase as more fluoride ions were added. When both ions were combined the total increase in overpotential was slightly higher than the sum of the individual ions.
To get a more accurate understanding of the adsorption of the impurities, a series of electrochemical quartz crystal microbalance (EQCM) measurements were conducted. These were done on a standard gold electrode and gold electrodes coated with a layer of iridium oxide through electrodeposition. The deposition of an iridium oxide layer on the electrodes was observed in the EQCM as a change in the respective voltammogram and massogram with the emergence of peaks corresponding to iridium oxide. The adsorption of sulfate, chloride, and fluoride was observed as a change in the massograms when performing cyclic voltammetry with the iridium-coated gold electrode. The adsorption of fluoride was further tested through Mott-Schottky impedance measurements.
The effect on the electrode surface after extended exposure to an acidic solution containing fluoride ions was also studied in detail. The surface tension of electrodes exposed to different electrolytes was tested with a drop shape analysis (DSA100) test. This gave insight into how the different solutions change the interaction between the electrode surface and water in the solution. Activated DSA electrodes that had not been exposed to any electrolyte prior to the drop-shape analysis were the most hydrophobic with the largest contact angle. As the electrodes were exposed to sulphuric acid, the surface became more hydrophilic. The most hydrophilic behaviour of the DSA was observed when the electrode had been exposed to an electrolyte containing fluoride ions. This resulted in an instant and complete wetting of the surface. No difference was observed between the electrodes only exposed to sulphuric acid and those exposed to sulphuric acid containing chloride ions.
It was found that electrodes exposed to a moderate concentration of fluoride ions over an extended period of time became far less hydrophobic than prior to exposure. This resulted in water getting easier access to the surface and displacing formed gas bubbles. In-situ imaging of gas formation with a high-speed camera revealed that exposure to fluoride ions drastically reduced the total coverage of gas on the anode. The diameter, shape and growth rate of the produced were also affected by electrolyte composition.