Michael Nicol

This chapter examines the use of electrical power to reduce metal ions in solution to metals on a cathode. The power can also be used to oxidize metal to metallic ions or metal ions to metal oxides at an anode. This chapter has covered both the theoretical and practical aspects of the recovery of pure metals and metallic compounds by electro-refining and -winning processes in aqueous solutions. It has built on the fundamental electrochemistry presented in Chapters 3 and 5Chapter 3Chapter 5. In general terms, the important role played by mass transfer of electroactive species to electrode surfaces has been emphasized quantitatively with many examples. The reader should now be able to make realistic assessments of mass transport effects in electrochemical reactors. The application of the equilibrium and kinetic theory of electrochemical reactions has been demonstrated in the electrowinning of a number of metals and shown to predict and rationalize many important observations. The surface and chemical quality of cathodes is governed by the process of electrocrystallization, and this has been dealt with initially in a general way that enables the reader to appreciate the main factors that dictate cathode quality. The importance of good current distribution at both the tankhouse scale and in individual cells has been emphasized, and the reader is equipped with the theoretical tools to identify and rectify such problems. The processes involved in the electrorefining of copper, gold, silver, and lead have been described in detail. Similarly, the electrowinning of the important base and precious metals, copper, zinc, nickel, cobalt, manganese (including manganese dioxide), gold, and silver has been outlined in some detail with sections on current efficiency, purity of the products, and energy consumption.

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.

The dissolution of uranium dioxide in acid solution containing an oxidant is shown to occur through an electrochemical mechanism in which anodic oxidation of the uranium dioxide is coupled to cathodic reduction of the oxidant on the surface. A correlation between the rate of dissolution and the electrochemical properties of the oxidants is derived and verified by experimental results. Certain effects observed in uranium dioxide leaching and hitherto unexplained are clarified in terms of an electrochemical mechanism.