Thermodynamic Principles of Metallurgy
Thermodynamic considerations are very important in metallurgical processes. These help in deciding the temperature and the choice of reducing agents in the reduction processes.
The feasibility of a process can be predicted in terms of Gibbs energy change (ΔG) at any specified temperature. This is related to enthalpy change (ΔH) and entropy change (ΔS) as :
ΔG = ΔH – TΔS
where T is the absolute temperature at which process is carried out.
ΔH is a measure of energy changes while ΔS is a measure of randomness or disorder during the process.
For any reaction, the Gibbs energy change is also related to equilibrium constant (K) of the reactant – product system at the temperature T as
ΔGº =-RT In K
1) For a spontaneous process or reaction, the energy change (ΔG) must be negative. This also implies positive value of K and can happen only when reaction proceeds towards products.
On the basis of these basic concepts, we can make the following conclusions :
a) For any reaction to occur, the value of ΔG must be negative. This will be possible if ΔH is negative and AS is positive. On increasing temperature (T), the value of TΔS would increase and then ΔG will become more negative and the reaction will be feasible. It may be noted that even if ΔH is positive, the ΔG will become negative after certain temperature as T increases (ΔH < TΔS).
b) If reactants and products of two reactions are put together in a system and the net ΔG of the two possible reaction is negative, then the overall reaction will occur. This involves coupling of two reactions, getting the sum of their ΔG values taking into account their magnitude and sign.
The graphical representation showing the variation of Gibbs energy with increase of temperature for the formation of oxides (i.e., oxidation of metals to their oxides) was first used by H.J.T. Ellingham and is known as Ellingham diagram.
Electrochemical Principle of Metallurgy
The thermodynamic principles can also be applied for the reduction of metal ions in solution or molten state. The reduction occurs by electrolysis or by adding some reducing element.
a) The reduction of a molten metal salt is done by electrolysis. This method is based on electrochemical principles. The Gibbs energy change is related to electrode potential of the redox couple formed in the system as:
ΔG = -nFEº
where n is the number of electrons taking part in the redox reaction and F is the Faraday of electricity. More reactive metals have large negative values of the electrode potential and therefore, their reduction is difficult.
If the difference of two Eº values of a redox system corresponds to a positive E° value and consequently negative ΔG° in equation, then the less reactive metal will come out of the solution and more reactive metal will go into the solution.
For example :
The electrode potential of two electrodes are:
Fe2+ (aq) + 2e¯ ——-> Fe (s) E° = – 0.44V
Cu2+ + 2e¯ ——-> Cu (s) E° = + 0.34V
More active metal iron will go into the solution while less reactive metal copper will come out of the solution as:
Cu2+(aq) +Fe (s) ——> Cu(s) + Fe2+ (aq)
The Mn+ ions are discharged at negative electrode (cathode) and deposited there. During electrolysis, precautions are taken considering the reactivity of the metal produced and suitable materials are used as electrodes. A flux is also added for making the molten mass more conducting.
Copper from low grade ores and scraps
Copper is extracted by hydrometallurgy from low grade ores. It is leached out using acid or bacteria. The solution containing copper ions (Cu2+) is treated with scrap iron or H2 as:
Cu2+ (aq) + Fe (s) ——> Cu (s) + Fe2+ ( aq)
Cu2+ (aq) + H2 (g) —> Cu (s) + 2 H+ (aq)
Since Eº of Fe2+| Fe (-0.44V) and that of H+|H2(0.0V) redox couple is lower than that of Cu2+|Cu (+0.34 V) therefore, iron or hydrogen can displace copper from Cu2+ ions.
In this way, copper is obtained.