Harold Ellingham created the first of these diagrams in 1944. The illustrations are helpful in anticipating how an ore will be converted to its metal content.The use of Ellingham diagram in metallurgy is to estimate the temperature of equilibrium between a metal, its oxide and oxygen — and by extension, interactions between a metal and sulphur, nitrogen and other non-metals. The study is thermodynamic in nature and does not take into account reaction kinetics. As a result, even processes projected to be favourable by the Ellingham diagram can be delayed.

Diagram Features

• In Ellingham diagrams, curves for the synthesis of metallic oxides are often straight lines with a positive slope. S is proportional to the slope and is roughly constant with temperature.
• As the temperature rises, the stability of metallic oxides decreases. Very unstable oxides, such as Ag2O   and HgO, are easily thermally broken.
• The lower the line in the Ellingham diagram is, the more stable a metal’s oxide is. The Al (aluminium oxidation) line is found to be lower than the Fe (ferrous oxidation) line (formation of Fe2O3).
• Carbon dioxide (CO2) has a practically temperature-independent formation free energy but carbon monoxide (CO) has a negative slope and crosses the CO2 line near 700 °C. Carbon monoxide dominates carbon dioxide at higher temperatures (above roughly 700 °C), according to the Boudouard reaction, and the greater the temperature (above 700 °C), the more powerful the reducing agent carbon is.
• When two metals’ oxidation curves are compared at the same temperature, the metal with the lower Gibbs free energy of oxidation reduces the oxide with the higher Gibbs free energy of generation on the diagram. Metallic aluminium, for example, can transform iron oxide to metallic iron while oxidising aluminium oxide. (This reaction produces thermite.) 
• The reducing chemical associated with the bottom line is more effective the larger the distance between any two lines. 
• It’s an oxidation-reduction reaction. The intersection of two lines denotes balance.Reduction with that reductant is possible at temperatures above the junction point where the G line of the reductant is lower on the diagram than that of the metallic oxide to be reduced. At the point of intersection, the free energy change for the reaction is zero, positive below this temperature, and the metallic oxide is stable in the presence of the reductant, and negative above this temperature, and the oxide can be reduced. 

Uses of Ellingham Diagram 

• Ellingham diagrams are most commonly used in the extractive metallurgy industry, where they aid in the selection of the optimal reducing agent for various ores in the extraction process, as well as purification and grade setting for steel production.
• The Ellingham diagram aids in the selection of a suitable reducing agent and a reduction temperature range.
• If the metal oxide is more stable, oxygen stays with the metal; however, if the oxide of the element used for reduction is more stable, oxygen from the metal oxide mixes with the elements required for reduction.
• We can deduce the relative stability of different metal oxides at a given temperature using the Ellingham diagram. 
• The Ellingham diagram is used to calculate the thermodynamic feasibility of reducing one metal’s oxides by another metal. Any metal can diminish the oxides of other metals in the diagram that are above it.
• Ellingham diagrams illustrate that hydrogen may reduce iron oxides to metal by itself, which is how the direct reduction process for creating iron works.

Limitations of Ellingham Diagram

• Ellingham diagrams are created solely on the basis of thermodynamic principles. It provides information on a reaction’s thermodynamic feasibility. It provides no information regarding the reaction rate. 
• Furthermore, it provides no indication of the probability of other reactions taking place.
• Delta G interpretation is predicated on the premise that the reactants and products are in equilibrium, which is not always the case. 

Conclusion

The Ellingham diagram depicts a reaction’s standard free energy as a function of temperature. Initially, the values for oxidation and sulphidation reactions for a number of metals were shown, as they were relevant to the extraction of metals from their ores (extraction metallurgy). In most of these reactions, a gaseous phase (the oxidising gas) reacts with (nearly) pure condensed phases (the metal and oxidised compounds). The standard free energy change for every included reaction can be found using the diagram at any temperature. The standard free energy change for a reaction is an important quantity to understand since its value influences the equilibrium constant and determines the system’s composition.