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.

Which of the following metals are reduced from molten salt solutions by electrolysis?

Aluminium

The correct answer is option 1. Aluminium.

The metal that is reduced from molten salt solutions by electrolysis is Aluminum. Aluminum is commonly extracted from its ore, bauxite, through the Hall-Héroult process. In this process, aluminum oxide (alumina) is dissolved in molten cryolite \((Na_3AlF_6)\), forming a molten electrolyte. Electrolysis is then used to reduce the aluminum ions in the electrolyte, resulting in the production of aluminum metal at the cathode.

The extraction of aluminum from its ore, bauxite, involves a process called the Hall-Héroult process, which is a type of electrometallurgical extraction. Here is how it works:

1. Preparation of Aluminum Oxide (Alumina): Bauxite ore is first refined to extract aluminum oxide (Al2O3), also known as alumina. This alumina is typically obtained through the Bayer process, where bauxite ore is dissolved in hot sodium hydroxide solution, yielding sodium aluminate. This solution is then cooled and seeded to precipitate out pure alumina hydrate, which is then calcined to produce alumina.

2. Formation of Molten Electrolyte: The alumina obtained from the Bayer process is then dissolved in a molten mixture of cryolite \((Na_3AlF_6)\) and fluorspar \((CaF_2)\) to form a molten electrolyte. This mixture significantly lowers the melting point of alumina, making it suitable for electrolysis.

3. Electrolysis: The molten electrolyte is then placed in an electrolytic cell, where carbon electrodes are inserted. The carbon electrodes serve as the cathode (negative electrode) and anode (positive electrode).

4. Reduction at the Cathode: At the cathode, aluminum ions \((Al^3+)\) migrate towards it. At the cathode, each aluminum ion gains three electrons and is reduced to form aluminum metal \((Al)\).

\(Al^{3+} + 3e^- \rightarrow Al\)

5. Oxidation at the Anode: At the anode, oxygen ions \((O^{2-})\) from the alumina in the electrolyte are attracted and oxidized to form oxygen gas \((O_2)\).

\(2O^{2-} \rightarrow O_2 + 4e^-\)

6. Overall Reaction: The overall reaction of the Hall-Héroult process is the electrolytic reduction of alumina \((Al_2O_3)\) to produce aluminum metal \((Al)\) and oxygen gas \((O_2)\).

\(2Al_2O_3 \rightarrow 4Al + 3O_2\)

7. Collection of Aluminum: The molten aluminum produced at the cathode is collected and further processed into various aluminum products.

The Hall-Héroult process is highly energy-intensive due to the need for high temperatures to maintain the electrolyte in a molten state and the significant electrical energy required for the electrolysis. However, it is the primary method used worldwide for the industrial production of aluminum.