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The electrocrystallization of nickel and its relationship to the physical properties of the metal

Kittelty, Daniel (2002) The electrocrystallization of nickel and its relationship to the physical properties of the metal. PhD thesis, Murdoch University.

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The industrial process of the electrowinning of nickel requires strict control of the quality of the electrolyte and the operating parameters in the tank house, as improper control can result in the formation of stressed deposits that may delaminate, resulting in the occurrence of short-circuits and damage to the cathode or anode bag. The primary aim of this thesis was the study of this electrocrystallization process to establish the causes of the stress, and also to determine the optimum conditions for deposition and the effects of common impurities on the quality of the electrodeposits. In order to accomplish this, several parallel activities have been carried out.

i) A fundamental study has been made of the nucleation and growth of nickel onto stainless steel and titanium substrates. It has been shown that an anodically oxidized titanium substrate produces a smooth and coherent deposit, compared to the stressed deposit produced on a freshly surface and this was correlated to the density and distribution of nuclei produced on the oxidised surface. Similarly, a stainless steel substrate was found to give an elevated nucleation rate. At a constant potential, the nucleation and both lateral and outward growth rates of the nickel crystallites increased with increasing concentration of nickel and also with higher temperatures. On the other hand, the opposite was found in the presence of additional sodium sulfate while no observable trends could be established for the effects of the concentration of boric acid and the pH of the electrolyte.

ii) A number of extended electrowinning runs were carried out under various conditions in a laboratory-scale cell designed to simulate the operation of a full-scale industrial cell. A method was developed which enabled the development of the stress in the growing deposit to be monitored continuously during each run. An experimental design was used to plan, execute and analyse the data from the experiments. The deposits generated after extended deposition periods exhibited variations in the size. shape and orientation of the crystals making up the deposit, as well as the presence of chemical and structural imperfections such as occluded metal hydroxides and hydrogen pits. These microscopic characteristics were correlated with the internal stress of the electrodeposits that were shown to reduce by the use of elevated concentrations of nickel as well as optimised levels of temperature, pH, and sodium sulphate and boric acid concentrations.

iii) The presence of impurities in the electrolyte, both organic and inorganic, affect the internal stress of the electrodeposits as a result of modifications to the crystal structure and incorporation into the electrodeposit. These structural defects were found to occur in the presence of manganese and the organic reagents L1X 841, Versatic 10 and Cyanex 272. Low concentrations of aluminium in the electrolyte resulted in the severe delamination of the electrodeposits, whilst higher concentrations produced smooth and ductile deposits. This unusual observation was shown to relate to the pH buffering capacity of the electrolyte, which increased in the presence of higher concentrations of aluminium due to the formation of soluble hydroxide complexes of aluminium.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Nicol, Michael
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