The Kinetics and Mechanisms of the Non-Oxidative Dissolution of Metal Sulphides

Abstract

The base metals form a series of sulphides of widely varying composition, many of which occur abundantly in nature. Conventional processing of these minerals has, until rather recently, involved pyrometallurgical operations which convert the sulphides directly to metal in some cases (e.g. copper and lead) or to oxides in others (e.g. nickel and zinc). Alternative hydrometallurgical routes have received much attention in recent years as a result of many factors,the principal of which is the necessity of treating low-grade ores in a relatively pollution-free manner. The first step in such a process is the leaching of the sulphide mineral by a process which has, in most cases, involved the oxidation of the sulphide to the corresponding metal ion and elemental sulphur or, in some instances, to sulphate. Little attention has been given to an alternative dissolution reaction which one can write in the general form and which we have termed “non-oxidative” dissolution in that no change occurs in the formal oxidation states of the reactants. In the case of predominantly ionic sulphides such as those of the alkaline and alkaline-earth elements, the above reaction simply involves the transfer of ions across the double-layer from the lattice to the solution. However, the base metal sulphides have varying degrees of covalent character from the partially ionic ZnS to the almost metallic iron and nickel sulphides. In the case of these solids, ionic transfer would necessarily have to be preceded by other step(s) involving varying degrees of electron transfer.