|dc.description.abstract||Industrial zinc electrowinning is carried out in an aqueous solution of zinc sulfate in sulfuric acid. The anodes are made of a lead-silver alloy and the cathodes are made of a relatively pure aluminium alloy.
A common issue in zinc electrowinning is the rapid and unexpected nodular growth of zinc from the cathode, leading to short circuiting between the anode and cathode. This is a source of current loss and may also cause irreversible damages to the electrodes. It is therefore important to better understand the mechanism with which zinc is deposited on aluminium in order to better control the electrochemical growth of zinc.
A thorough theoretical groundwork was established for zinc electrowinning. General theories for electrolysis, kinetics and electrocrystallization were presented and related to the aqueous zinc sulfate system. The theorized mechanisms of the cathodic reactions are also explained.
The effect of the temperature, zinc sulfate concentration and current density on the structure of the deposited zinc was studied using a Hull cell. Cyclic voltammetry was used to study the effect of the temperature on the system. To study the structure and morphology of the deposited zinc, scanning electron microscopy was used.
The current density was shown to be the most influential parameter determining the structure of the zinc deposits. Three main zinc structures were observed as the current density was increased: Small, single grains at low current densities, a dense dislocation-rich structure with random orientation at medium current densities, and finally columns of hexagonal zinc discs parallel to the substrate surface as the current density was increased further. The roughness of the zinc deposit also increased with increasing current density. An increase in the zinc concentration seemed to promote dislocation-driven growth, leading to a more compact deposit, while low concentrations promoted growth of clearly defined single crystals. No dendritic structures were observed at any current densities, and the structure was close to that expected from kinetically controlled growth at all current densities. It was suggested that dendrites are formed at a later stage, propagating from uneven growth created during the early stages of electrodeposition.
Increasing the temperature caused zinc to corrode from the cathode at small current densities. This was explained as a time-dependent effect based relationship between the Tafel slope of the cathodic hydrogen evolution and zinc deposition.
Hollow blisters of zinc were observed on the deposits, increasing in size with increased current density. These blisters were formed as a result of locally enhanced current density around hydrogen bubbles sticking to the surface.||en