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Thermodynamic principles of Metallurgy

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Thermodynamic principles of Metallurgy - Lesson Summary

Thermodynamic principles can be applied to the ore extraction process.

The Gibbs energy equation that relates to the enthalpy and entropy of the system at a certain temperature.

                                    ΔG = ΔH  -  TΔS
                                               ΔG = Gibbs Free Energy Change
                                               ΔH = Change in Enthalpy
                                                  T = Temperature
                                                ΔS = Change in Entropy

        Spontaneous Process   → ΔG = -ve
 Non-Spontaneous Process   → ΔG = +ve

According to Gibbs energy free energy can be utilized to do useful work. The term ∆H represents the enthalpy change and ∆S is the change in entropy at temperature T.

Relation between free energy change and equilibrium constant,

  ΔG = -RTlnK

The equilibrium constant K is obtained by taking the ratio of equilibrium concentrations of reactants and products.

When a reaction proceeds from reactants to products, products are present in excess and equilibrium constant is positive. Alternatively, the equilibrium constant is negative for a reverse reaction.

For ∆G to be negative, the change in entropy should be positive and on increasing the temperature, the value T∆S should exceed the enthalpy change for the reaction.

A reaction with a positive ∆G can also occur, if it is coupled with another reaction that has a large negative ∆G value, so that the net value of ∆G would still be negative.

Ellingham diagrams are used as a tool in extraction of a metal in metallurgy to find the appropriate conditions for reduction of ores of important metals.

In the reaction between metal oxide and carbon, carbon is the reducing agent, itself undergoes oxidation to reduce the element.

Carbon may undergo a partial oxidation, it takes up only half mole of oxygen to form carbon monoxide (or) it may undergo complete oxidation, takes up 1mole of oxygen to form carbon dioxide.

       MxO + C → xM + CO
       MxO + C → xM + CO2
       MxO + ½C → xM + ½CO2

It is a redox reaction. Redox reaction is one where oxidation and reduction take place. In this reaction the metal oxide is reduced and carbon and carbon monoxide will oxidized.

Heating favors a negative value for ∆G. Hence the temperature is chosen such that the sum of ∆G in the two combined redox process is negative.

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