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Thermodynamic Principles of Metallurgy


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Thermodynamic Principles

Thermodynamic Principles are applied to the ore extraction process. Gibbs energy is the most important term used to explain the thermodynamic principle. Gibbs equation is given below:

ΔG = ΔH  -  TΔS

ΔG is the change in the Gibbs energy

ΔH is the change in the enthalpy.

T is the temperature

ΔS is the change in the entropy.

ΔG = -RT lnK

K is the equilibrium constant.

R is the universal gas constant

Ratio of equilibrium concentrations of reactants and products will give equilibrium constant K. When a reaction moves from reactants to products, products are present in excess and equilibrium constant will be positive. But when reaction moves from products to reactants, equilibrium constant will be negative.

Ellingham Diagram

It is a graphical representation of Gibbs energy. These diagrams are used for finding the choice of reducing agent in the reduction of the oxides. These are used to find out the feasibility of thermal reduction of an ore. 

Ellingham Diagram

Fig. 1. Ellingham Diagram

As we know, during reduction, the oxide of a metal decomposes,

MxO(s) → xM (solid or liq) + 1/2 O2 (g)

If reduction is being carried out then,

C(s) + 12 O2(g) → CO(g)

CO(g) + 12 O2(g) → CO2(g)

If carbon is taken, there may also be complete oxidation of the element to CO2:

½ C+1/2O2    1/2CO2

Limitations of Ellingham Diagram

  • It does not give any information regarding the kinetics of the reduction process.

  • These diagrams believe that reactants and products are in equilibrium, but this is not always the case.

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Thermodynamic Principles of Metallurgy

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