Aluminium is an attractive, lightweight, corrosion resistant metal. It is about one third the weight of steel. It is a good thermal and electrical conductor which makes it ideal for use in electrical appliances, where it is often used as a heat sink. Aluminium is a much better electrical conductor than copper which is why it is often used to make the power lines which carries our electricity across the country. It has many uses including in the aircraft industries, construction and building, motor manufacture and for many household items such as cooking foil, drinks cans, pots and pans and mobile phones. The image below shows a few of the uses of this versatile metal.

Aluminium is the third most abundant element in the Earth, its main ore is called bauxite. Bauxite contains a high proportion of aluminium compounds, chiefly aluminium oxide (Al₂O₃). The aluminium oxide contains postively charged aluminium ions (Al³⁺⁾ and negatively charged oxide ions (O^2-)ions. It is named after a village in southern France called Les Baux, where it was first discovered in 1821.

Despite the fact that aluminium is the third most common element on Earth in the late 19th century it was more valuable than silver or gold and it was considered a rare metal. The reason for this is that before the discovery of electrolysis aluminium was extracted by carrying out a displacement reaction using sodium or potassium on aluminium compounds, this made it a very expensive metal indeed.

However with the discovery of the electrolytic process the cost of aluminium fell dramatically. However the extraction of aluminium by electrolysis was not as straightforward as might have been expected at first. In order to extract a metal from its ore by electrolysis the metal ore needs to be molten (melted to form a liquid) or dissolved to form a solution. However bauxite is not soluble in many solvents and it also has a very high melting point, in excess of 2000⁰C. This made the extraction of aluminium from its ore difficult and expensive.

The solution to the problem was found by two scientists working independently from each other, Charles Hall an American scientist and Paul Heroult a French scientist found that aluminium oxide would dissolve in cryolite (Na₃AlF₆), a rare mineral found in Greenland.

If cryolite is heated to around 850⁰C then aluminium oxide will dissolve in it and form a solution which conducts electricity. This made it possible to extract aluminium by electrolysis from its ore cheaply and following this the cost of aluminium metal plummeted. Aluminium oxide (Al₂³⁺O₃^2-) dissolves in the molten cryolite inside the cell where temperatures are around 850⁰C. When it dissolves its giant lattice structure is broken down and the aluminium and oxide ions are free to move. This is exactly the same as what would happen if an ionic compound was dissolved in water, only in this case it is dissolved in molten cryolite. The image below shows a typical cell used to extract aluminium.

The cell is lined with a layer of graphite, this graphite is made the cathode. This negatively charged cathode will attract the positively charged aluminium ions and reduce them to aluminium atoms:

At the cathode

At the anode

O^2- → O + 2e

2O^2- → O₂ + 4e

graphite (carbon)_(s) + oxygen_(g)→ carbon dioxide_(g)

C + O₂ → CO₂

Overall equation:

cathode: Al³⁺ + 3e → Al

anode: 2O^2- → O₂ + 4e

cathode: 4Al³⁺ + 12e → 4Al

anode: 6O^2- → 3O₂ + 12e

4Al³⁺ + 6O^2- → 4Al + 3O₂

2Al₂O₃ → 4Al + 3O₂

Costs

One of the major costs in extracting aluminium is the cost and energy required to continually make new anodes as the old one burn away and turn into carbon dioxide gas. The other major consideration in aluminium extraction is the vast amount of electricity required to extract aluminium from its ore. Huge electrical currents in excess of 150 000 amps are used and so a cheap source of electricity nearby is required. A hydro-electric power station would be ideal!

Key points

• Aluminium ore or bauxite is mainly aluminium oxide.
• Bauxite has a high melting point and is insoluble in water but soluble in molten cryolite.
• Aluminium ions are reduced at the cathode, where liquid alumnium forms.
• Initially oxygen gas is produced at the anodes, but it reacts with the carbon anodes and slowly burns them away. The anodes burn and turn into carbon dioxide gas. Inside the cell the anodes need to be regularly replaced.
• Aluminium extraction uses vasts amounts of electricity and aluminium metal should be recycled as much as possible to protect the environment.

Check your understanding - Questions on aluminium extraction.

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