Ellingham oxide reduction

When the metal can form a stable carbide, the product of the carbothermic reduction of its oxide may be a carbide instead of the metal itself. The question as to whether a carbide or the metal forms under standard conditions when the oxide is reduced by carbon is not answered by the Ellingham diagram. To obtain an answer to this question, a more detailed consideration of the thermodynamic properties of the system is necessary. 

Values for G° are known for various oxidation reactions and can be plotted on diagrams called Ellingham diagrams as shown in Fig. (3.4). Such diagrams are very useful to predict reactions associated with oxidation and reduction. 

The Ellingham diagram of the oxides also involves information pertaining to the equilibria of reduction of oxides by CO/CO2 mixtures and H2/H2O mixtures. 

Since many ceramics are oxides, the oxygen partial pressure, p02, is an important variable. There is a lot of information about many metal-oxygen systems. In part, this is due to interest in how to obtain metals by direct reduction from their oxides. A frequent way of representing free energies of formation of oxides as a function of pOi and T is the Ellingham diagram (Ellingham, 1944) that was popularized by Richardson and Jeffes (1948) for iron and steel production. Much less is known about nitrides and oxynitrides or even carbides. 

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