Pyrometallurgy involves high temperature processes where chemical reactions take place among gases, solids, and molten materials.  Solids containing valuable metals are treated to form intermediate compounds for further processing or converted into their elemental or metallic state.  Pyrometallurgical processes that involve gases and solids are typified by calcining and roasting operations.  Processes that produce molten products are collectively referred to as smelting operations.  The energy required to sustain the high temperature pyrometallurgical processes may derive from the exothermic nature of the chemical reactions taking place.  Typically, these reactions are oxidation, e.g. of sulfide to sulfur dioxide .  Often, however, energy must be added to the process by combustion of fuel or, in the case of some smelting processes, by the direct application of electrical energy.
Ellingham diagrams are a useful way of analysing the possible reactions, and so predicting their outcome.