Industrial Electrolysis (Aluminium Extraction)

Raw Materials for Aluminium Extraction

Aluminium is extracted from the ore bauxite, which contains aluminium oxide (Al₂O₃) mixed with impurities. The aluminium oxide must be purified before extraction.

Pure aluminium oxide has a very high melting point (about 20506C), making it impractical to melt for electrolysis. To reduce the melting point and save energy, aluminium oxide is dissolved in cryolite, a mineral that acts as a solvent. This mixture melts at around 9506C, which is much lower and more manageable for industrial processes.

Electrolytic Process

The purified aluminium oxide dissolved in molten cryolite is heated to about 9506C to keep it liquid. An electric current is passed through this molten mixture to cause electrolysis, which separates aluminium from oxygen.

The high temperature is necessary to keep the mixture molten and allow ions to move freely, which is essential for the flow of electric current.

During electrolysis, aluminium ions (Al³⁺) and oxide ions (O^22) move towards the electrodes where they gain or lose electrons, producing aluminium metal and oxygen gas.

Electrode Reactions

At the cathode (negative electrode): Aluminium ions gain electrons (reduction) to form aluminium atoms, which collect at the bottom of the cell as molten aluminium.

This process can be summarised as aluminium ions gaining electrons:

Al³⁺ + 3e⁻ 1rarr; Al (molten aluminium)

At the anode (positive electrode): Oxide ions lose electrons (oxidation) to form oxygen gas:

2O^22 1rarr; O₂ + 4e⁻

The oxygen gas then reacts with the carbon anodes, which are made of graphite, producing carbon dioxide gas:

C (anode) + O₂ 1rarr; CO₂

Because the carbon anodes are consumed during this reaction, they must be replaced regularly in the industrial process.

Industrial Considerations

The aluminium extraction process is energy intensive because it requires maintaining the molten mixture at a very high temperature and passing a large electric current through it.

Carbon anodes are gradually worn away as they react with oxygen, so they need to be replaced continuously to keep the process running.

The aluminium produced is very pure and can be used in many industries, including aerospace, packaging, and construction.

Note: The carbon dioxide produced contributes to greenhouse gas emissions. Research is ongoing into inert anodes to reduce this environmental impact.

For example, if 4.5 moles of aluminium ions are reduced at the cathode, the mass of aluminium produced is:

Mass = 4.5 17 27 = 121.5 g