We have seen how principles of thermodynamics are applied to pyrometallurgy. Similar principles are effective in the reductions of metal ions in solution or molten state. Here they are reduced by electrolysis or by adding some reducing element.

In the reduction of a molten metal salt, electrolysis is done. Such methods are based on electrochemical principles which could be understood through the equation,

\(\Delta G^0 = –nE^0F\) ------------------(i)

here n is the number of electrons and E⁰ is the electrode potential of the redox couple formed in the system. More reactive metals have large negative values of the electrode potential. So their reduction is difficult. If the difference of two E⁰ values corresponds to a positive E0 and consequently negative \(\Delta G^0\) in equation (i), then the less reactive metal will come out of the solution and the more reactive metal will go into the solution, e.g.,

Cu²⁺ (aq) + Fe(s) → Cu(s) + Fe²⁺ (aq) --------------------(ii)

In simple electrolysis, the Mn+ ions are discharged at negative electrodes (cathodes) and deposited there. Precautions are taken considering the reactivity of the metal produced and suitable materials are used as electrodes. Sometimes a flux is added for making the molten mass more conducting.

What is the net reaction in Hall-Heroult process?

2Al₂O₃ + 3C → 4Al + 3CO₂

The reaction involved in the Hall-Heroult process are:-
Na₃AlF₆ → AlF₃ + 3NaF
AlF3 → Al⁺³ + 3F^–
Cathode: Al⁺³ + 3e^– → Al
Anode: 6F^– → 3F₂ + 6e^–
F₂ + Al₂O₃ → AlF₃ + O₂
C + O^2- → CO or CO₂ + 2e^–
The overall reaction may be written as 2Al₂O₃ + 3C → 4Al + 3CO₂.