Using Electrolysis to Extract Metals (AQA GCSE Chemistry: Combined Science)

How is aluminium extracted?

• Aluminium is a reactive metal, above carbon in the reactivity series  
• Its main ore, is bauxite, which contains aluminium oxide
• Aluminium is higher in the reactivity series than carbon, so it cannot be extracted by reduction using carbon
• Instead, aluminium is extracted by electrolysis  

The electrolytic cell for extraction of aluminium

Diagram showing the extraction of aluminium by electrolysis

• Bauxite is first purified to produce aluminium oxide, Al₂O₃
• Aluminium oxide is then dissolved in molten cryolite 
  • This is because aluminium oxide has a melting point of over 2000°C which would use a lot of energy and be very expensive
  • The resulting mixture has a lower melting point without interfering with the reaction
• The mixture is placed in an electrolysis cell, made from steel, lined with graphite
• The graphite lining acts as the negative electrode, with several large graphite blocks as the positive electrodes
• Aluminium is produced at the cathode
• Oxygen is produced At the anode 

Why does the anode need to be replaced?

• The carbon in the graphite anodes reacts with the oxygen produced to produce CO₂

C (s) + O₂ (g)   →   CO₂ (g)

• As a result the anode wears away and has to be replaced regularly
• A lot of electricity is required for this process of extraction, this is a major expense