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I.

INTRODUCTION

L E A C H I N G of oxides has long been investigated for the recovery of metal values, as there are appreciable amounts of oxidized minerals available as primary sources and large quantities of oxide materials as secondary sources. One of the most important instances to be mentioned is the oxidized minerals of copper, such as cuprite, malachite, chryosochola, azurite, etc., which accompany the sulfide deposits. Among these minerals, leaching of cuprite has been shown to be interesting, as its dissolution in acid solutions proceeds through a disproportionation reaction, resulting in the formation of metallic copper and cupric ion. However, in the presence of oxygen, the metallic copper slowly continues to dissolve. Therefore, formation of metallic copper on the surface may considerably slow down further dissolution. Sullivan and Oldright, I1,21 in their study on dissolution of crushed natural cuprite in sulfuric acid, obtained a parabolic reaction rate which was attributed to the resistance introduced by metallic copper formed on the surface of the sample. However, Wadsworth and Wadia, 131 in a similar study, using a synthetic cuprite sample in the form of plate, obtained a linear rate law which indicated that the dissolution of cuprite is not inhibited by the formation of metallic copper. It was found that the majority of metallic copper formed during the reaction was extremely porous and spongy. Their reaction rate study led them to propose a kinetic model based on adsorption of H 2 S O 4 o n surface sites. The observed rate was explained quantitatively by two simultaneous rate reactions, one of which was attributed to the thermal decomposition of a surface site containing adsorbed H 2 S O 4 and the second of which was due to the reaction between H + ion and a surface site containing adsorbed HzSO4. In another study by Majima et al. ,14j disproportionation dissolution of cuprite in sulfuric acid and perchloric acid solutions from which oxygen had been stripped was investigated. They observed that the metallic copper formed as a result of disproportionation reaction suppresses further dissolution of cuprite. From the values of activation energy, they concluded that mass transfer controls the rate at dilute acid solution, whereas chemical or mixed A.A. YOUZBASH1, Research Fellow, and S.G. DIXIT, Professor of Physical Chemistry, are with the Department of Chemical Technology, University of Bombay, Bombay 400 019, India. Manuscript submitted June 1, 1992. METALLURGICAL TRANSACTIONS B

kinetics control the rate in concentrated solutions. These authors also concluded that adsorption of H § ion onto the surface site is an important factor in determining the dissolution rate. Aqueous solution of sulfur dioxide is being increasingly investigated as a leaching agent due to its novel chemical properties. Its significance in dissolution of metal oxides lies in the fact that it behaves as an acid as well as reducing agent. However, in some instances, it forms complexes with the concerned metal and enhances di