1. Introduction

Thermodynamics plays a critically important role in hydrometallurgy (as in any chemical process industry). It tells us what is possible and how. Thermodynamics tells us if a reaction is naturally inclined to occur under specified conditions; whether or not it is spontaneous or favourable. And, if not spontaneous, what has to be done to make it proceed. There are various ways of organizing thermodynamic information. Speciation diagrams and Eh-pH diagrams are just two possibilities commonly used in hydrometallurgy. An Eh-pH diagram is constructed by plotting electrochemical reduction potential versus pH under specified conditions. What it will tell us is domains of electrochemical potential and pH under which each stable chemical species is predominant, i.e. has the highest concentration (activity, actually) among all the possible stable species involved. These diagrams are thus also termed predominance area diagrams. Another name is Pourbaix diagrams after the man who pioneered their development. Eh-pH diagrams may be of particular use in leaching, but also in other aspects of hydrometallurgy. Corrosion and environmental chemistry also make use of these diagrams.

Note that thermodynamics tells us what is possible. It considers the differences between an initial state and a final state of matter and energy. It tells us nothing about how long it will take, or about the detailed steps by which the process occurs. Those considerations are the province of chemical kinetics. Since hydrometallurgy involves relatively low temperatures, kinetics must be carefully considered. In pyrometallurgy, where temperatures are very high, thermodynamics and is rapidly obeyed. In hydrometallurgy some spontaneous reactions are so slow that effectively they do not occur at all.