Extraction of Metals by Electrolytic Reduction

Electrolytic reduction (electro metallurgy)

This method is used for the extraction of metals that are very reactive and form very stable compounds such as the oxides, chlorides, hydroxides, etc., of metals like Na, K, Mg, Ca, Al. These compounds are very stable and hence cannot be reduced to free metals by carbon or aluminium. While oxides of these metals can be reduced by carbon at very high temperatures, the metal formed reacts with carbon to form metal carbide. Therefore, the metals are obtained by reducing their suitable molten (fused) salts like chlorides, hydroxides, etc., by electrolytic reduction. The metal is liberated at the cathode. An aqueous electrolyte is not used because the metal obtained at the cathode would react with water in the solution.

In order to make electrolytic reduction successful, the following conditions must be satisfied:

1. The liberated metal should not be miscible with the melt (not soluble in the melt). For example, potassium cannot be obtained by electrolysis of KCl and CaCl2 melt, as liquid potassium is soluble in CaCl2.
2. The products obtained from electrolysis may react and hence must be collected separately.
3. Some salts may be added to lower the melting point of compounds that melt at very high temperatures. The decrease in melting point makes the electrolysis process take place at a lower temperature.
4. The cell and electrodes must be made of materials that are not affected by the electrolyte or the products obtained after electrolysis.

Electrolytic reduction of aluminium from aluminium oxides (Al2O3)

In the electrolytic reduction of aluminum, pure Al2O3 is mixed with Cryolite (Na3AlF6) and the mixture is melted in an electric furnace. The fused mixture of Al2O3 and Na3AlF6 is placed in an iron tank lined with carbon on the inside. This carbon lining acts as the cathode, while carbon rods dipped into the molten Al2O3 and Na3AlF6 act as the anode. When electric current is passed, aluminum metal is liberated in the molten state at the cathode and sinks to the bottom of the cell, where it is removed periodically.

Oxygen from the electrolyte is liberated at the anode. Some of the oxygen combines with the carbon anodes to form carbon monoxide and carbon dioxide. As a result, the anodes are gradually worn out.

Note: Cryolite serves as a solvent and does not decompose during the process.

$$2O^{2-} - 4e^- \rightarrow O_{2(g)}$$ $$4C_{(s)} + 3O_{2(g)} \rightarrow 2CO_{(g)} + 2CO_{2(g)}$$

Purification of blister copper

Copper obtained from copper-iron pyrite (CuFeS2) is called blister copper. This copper contains many impurities, such as Fe, Ni, Zn, Ag, Au, etc.

In refining blister copper, a thick plate of impure copper is made the anode, and a thin plate of pure copper is made the cathode. The cathode is coated with graphite and oil, which helps in scraping the copper that gets deposited on the cathode. A 15% CuSO4 solution containing 5% H2SO4 is used as the electrolyte.

On passing electric current through the electrolyte, ionization of CuSO4 occurs:

1. Ionization: Copper ions (Cu2+) are generated and move towards the cathode, where they accept electrons and become neutral Cu atoms. As the electrolysis continues, the cathode (pure copper plate) becomes thicker.
2. Reduction: The ions move towards the anode, where they lose electrons and form sulphate radicals. These radicals combine with Cu atoms of the copper anode (impure copper plate) to form CuSO4, which dissolves into the solution. The CuSO4 is ionized again to give Cu2+ ions, which migrate to the cathode and are deposited as free Cu atoms.

$$\text{i)} \quad \text{SO}_4^{2-} \rightarrow \text{SO}_4 + 2e^-$$ $$\text{ii)} \quad \text{SO}_4 + \text{Cu} \ (\text{from anode}) \rightarrow \text{CuSO}_4 \ (\text{go into solution})$$ $$\text{iii)} \quad \text{CuSO}_4 \rightarrow \text{Cu}^{2+} + \text{SO}_4^{2-}$$

On adding equation (i), (ii) and (iii) we obtain:

$$\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$$

The equation above represents the anode reaction (oxidation), indicating that the anode becomes thinner as the reaction progresses. The pure copper is transferred from the impure copper plate to the pure cathode plates.

Soluble impurities like Fe2+, Ni2+, and Zn2+ pass into the solution as sulphates, while insoluble impurities like Ag and Au settle down below the anode as anode mud or sludge.