Question

What is Ellingham diagram and how is it useful in metallurgical processes?

What is Ellingham diagram and how is it useful in the choice of a reducing agent in a particular metallurgical operation? Give an example.

Answer 1

Graphical representation consisting of the plots of Δ_fG° vs T for formatiion of oxides of elements is known as Ellingham Diagram. 

The Ellingham diagram is extremely important in metallurgy because:

1. Prediction of Feasibility of Reduction: It helps to predict whether a metal oxide can be reduced by a given reducing agent (C, CO, H₂, Al, etc.) at a particular temperature. The reaction is feasible if the line of the reducing agent lies below the line of the metal oxide (i.e., ΔG of the reducing agent is more negative).

2. Choice of Reducing Agent: For example, carbon reduces ZnO to Zn, but it cannot reduce Al₂O₃ because the Al₂O₃ line lies much lower than the C/CO line. Thus, Al is used instead of C in the thermite process.

3. Coupling of Reactions: A non-feasible reaction (ΔG > 0) can be made feasible by coupling it with another highly negative ΔG reaction (as shown in the diagram).