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The dissolution of goethite in aqueous solutions of sulphur dioxide Hay, Malcolm Geoffrey

Abstract

The leaching of goethite in perchloric acid ("direct leaching") and in acidified sulphur dioxide solutions ("reductive leaching or dissolution") has been investigated. The effects of temperature, hydrogen ion concentration, sulphur dioxide partial pressure, stirring speed and sample weight on the reductive leach rate have been studied. The enhancement of the reductive leach rate as a consequence of adding cupric ions was also investigated. A leaching mechanism was proposed for the direct and uncatalyzed reductive dissolution. Protonation of the oxide surface sites, described by a Langmuir Adsorption Isotherm, resulted in the formation of ferric-hydroxy species. These either desorbed (the single solution step in direct leaching) or underwent the adsorption of a sulphur dioxide molecule possibly followed by another hydrogen ion. The overall reductive leach rate was a function of the desorption of the intermediate and the final surface species. Cupric ions did not appear to directly catalyze a step in the postulated reductive dissolution mechanism. Rather, the essential feature of their catalytic role was thought to be an electron exchange reaction between Cu[superscript]I and Fe[superscript]III on the oxide surface to form cupric and ferrous ions in solution. An anionic cuprous species capable of adsorbing on the oxide surface was formed by cupric ion reduction with aqueous sulphur dioxide and subsequent complexing with bisulphite and/or sulphite ions. A comparison of the reductive leach rates of magnetite and hematite with goethite was made both with and without the presence of cupric ions.

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