UBC Theses and Dissertations
Corrosion behaviour of austenitic steels in basic thiosulfate gold leaching environments Benyahya, Matlub Mohammed
Thiosulfate leaching of gold is being proposed as an environmentally friendly alternative to the dominant cyanide-leaching technology. Thiosulfate leaching is, to a large extent, dependent on the availability of materials of construction. There are indications that materials of construction could be a major issue if not addressed; particularly their corrosion resistance. Therefore, understanding the corrosion behaviour of materials of construction, such as stainless steels, in this medium is critical to the future development and acceleration of industrial implementation of this new technology. The debate surrounding the use of cyanide in the mining industry has fuelled considerable investigation into the development of some more environmentally benign alternatives. The thiosulfate process has been widely accepted by researchers around the world as a potential alternative lixiviant for the leaching and recovery of gold. Moreover, sodium thiosulfate is relatively nontoxic and, consequently, from an environmental standpoint, has a definite advantage over cyanide. Comparing reagent unit costs, sodium thiosulfate is far cheaper than sodium cyanide. Thus, with similar or even slightly higher lixiviant consumption, the application of thiosulfate for gold recovery can be economical and compete directly with cyanidation. ‘Rust never sleeps!’, so says a popular song. In a broad sense, corrosion appears to be the reverse process of extraction. In this process, most metals tend to presume a chemicallycombined state rather than a pure metallic form achieving, in the process, the most energy stable iii form possible. Thus, freshly-extracted metals, as well as finished metallic products, tend to deteriorate when exposed to certain environments. Thiosulfate has been identified as such an environment in mining and other industries, where thiosulfate-dominated processes, such as thiosulfate gold leaching, are strongly believed to highly promote localized corrosion. In this work, the pitting corrosion behaviour of 304- and 316-type stainless steels were studied using cyclic polarization techniques and electrochemical impedance spectroscopy (EIS) run on a thiosulfate system at several pH and temperature levels in the presence and absence of chloride ions. The stainless steels studied were found to show similar general corrosion behaviour with varying degrees of corrosion resistance to pitting as control variables were regularly perturbed. The results indicate that SS316 has a relatively lower corrosion rate and is more resistant to localized corrosion than SS304 under the conditions considered. The presence of thiosulfate seems to activate both anodic and cathodic current densities of steel in chloride solutions. The higher the thiosulfate concentration is, the greater the current densities. In addition, for both alloys, the corrosion rate generally increases as the magnitude of the variables is increased; except when the content of chloride ions is altered. Thiosulfate seems to promote passivation in the presence of chloride ions and deter interaction of those ions with the surface of the metal alloys by the formation of an adsorption layer of sulfur that seems to protect the metal surface.
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