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Leaching of metal oxides Devuyst, Eric August Pierre

Abstract

The leaching of metal oxides in acids has been investigated. The experiments were focussed on the leaching of ferric oxides in perchloric, hydrochloric, sulphuric, oxalic and malonic acids. Additional studies were made of the leaching of aluminum, cuprous, cupric and manganous oxides in the above acids. In dilute solutions of the acids (0.2M), the order decreased progressively by one unit (i.e. from 1 to 0 or 2 to 1), with increasing acidity. The addition of ferrous salts to oxalic and malonic acids greatly enhanced the rates of leaching of ferric oxides. A general mechanism for the direct leaching of metal oxides in acids has been proposed. It is postulated that the oxide surface becomes rapidly hydroxylated, followed by the successive adsorptions of hydrogen ions, anions of the acid and again hydrogen ions at hydroxylated sites. With the exception of dehydrated aluminum oxides, the kinetics of the leaching of the oxides in acids were consistent with the assumption that the rate determining step was the desorption of metal species which are formed at the oxide surface during the above adsorption reactions. For the dissolution of dehydrated aluminum oxides, it appeared that the rate of surface hydroxylation was rate-controlling under some conditions. The adsorption pre-equilibria could be correlated to the pH of zero-point of charge (Z.P.C.) of the oxide surface. Oxide surfaces exhibiting a high pH of Z.P.C, e.g. Cu₂O, CuO and MnO, are suggested to become more rapidly saturated by ionic species from solution with increasing acidity than oxide surfaces having a lower pH of Z.P.C, e.g. α-Fe₂O₃. This saturation of the oxide surface is postulated to be the reason for the observed decreasing dependence of the rates of leaching on a[sub ±] with increasing acidity. It is also concluded that the complexing ability of the acids for metal ions is essentially correlated with the rates of desorption of the metal-anion species formed at the oxide surface. The catalytic effect of ferrous ion additions to oxalic and malonic acids is explained by an electrochemical mechanism involving the formation and rapid desorption of ferrous and ferric species at the ferric oxide surface.

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