Gamini Senanayake

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.

Reducing agents can be applied to leach metal values from natural and secondary resources such as laterite ores and deep sea manganese nodules and recycled materials such as spent batteries/catalysts, which contain high-valent metal oxides of manganese, cobalt, and iron. Selective leaching of some metals can be achieved by selecting conditions to control the saturated solubility and dissolution rates of metals from oxides or mixed metal/oxide matrices. The leaching results are rationalized on the basis of chemical species, Eh–pH diagrams and heterogeneous kinetic models. The role of reducing agents, such as hydrogen peroxide, sulfur dioxide, and iron(II) sulfate, in acid, alkali, and in the presence of complexing ligands such as ammonia for the improvement of the leaching of metal oxides and metals is discussed. Previous studies on acid leaching of metals from nickel laterites, manganese nodules, and spent catalysts/batteries in the absence or presence of reducing agents show that the rate controlling step involves the diffusion of protons through a thickening product layer on the particle being leached. It is shown that the leaching of nickel and cobalt from manganese nodules and zinc–carbon batteries in ammoniacal sulfur dioxide solutions also obey shrinking core kinetics.