Extracting Aluminium by Electrolysis

The extraction of aluminium is one of the most important industrial applications of electrolysis on the Cambridge 0620 syllabus. It combines knowledge of the reactivity series, electrode reactions, and industrial economics.

Why electrolysis is needed

Aluminium is a very reactive metal — it sits above carbon in the reactivity series. This means:

• Carbon cannot reduce aluminium oxide (Al₂O₃) to aluminium
• The strong ionic bonds in Al₂O₃ require a large input of energy to break
• Electrolysis provides this energy through electrical current

Less reactive metals (iron, zinc, lead) can be extracted by reducing their oxides with carbon in a blast furnace. Aluminium cannot.

The raw material

Aluminium ore is called bauxite, which contains aluminium oxide (Al₂O₃) mixed with impurities (iron oxide, silicon dioxide). The bauxite is purified to give pure Al₂O₃ (alumina) before electrolysis.

The electrolysis cell

Setup

• The electrolyte is molten aluminium oxide (Al₂O₃) dissolved in cryolite (Na₃AlF₆)
• The cathode (negative electrode) is a carbon (graphite) lining on the steel tank
• The anode (positive electrode) consists of large carbon (graphite) blocks suspended in the electrolyte
• Temperature is maintained at about 950–1000 °C

Role of cryolite

Pure aluminium oxide melts at about 2050 °C — far too hot and expensive to maintain. Dissolving Al₂O₃ in molten cryolite lowers the melting point to about 950 °C, dramatically reducing the energy (and therefore cost) required.

Electrode reactions

At the cathode (negative electrode) — REDUCTION

Aluminium ions gain electrons and are reduced to aluminium metal:

Al³⁺ + 3e⁻ → Al

The molten aluminium sinks to the bottom of the cell (it is denser than the electrolyte) and is tapped off periodically.

At the anode (positive electrode) — OXIDATION

Oxide ions lose electrons and are oxidised to oxygen gas:

2O²⁻ → O₂ + 4e⁻

Overall equation

2Al₂O₃ → 4Al + 3O₂

This is the electrolytic decomposition of aluminium oxide.

Why the anodes must be replaced

The oxygen produced at the anode reacts with the hot carbon electrode:

C + O₂ → CO₂

The carbon anodes gradually burn away and must be replaced regularly. This is a significant running cost. The cathode (carbon lining) lasts much longer because it is not attacked by the products.

Energy costs

Electrolysis of aluminium is extremely energy-intensive. A single tonne of aluminium requires about 15,000 kWh of electricity. This is why aluminium smelters are located near cheap sources of electricity (hydroelectric power stations).

The high energy cost also explains why recycling aluminium is so important — recycling uses only about 5% of the energy needed to extract new aluminium from ore.

Properties and uses of aluminium

Property	Use
Low density (lightweight)	Aircraft, drink cans, foil
Good electrical conductor	Overhead power cables
Good thermal conductor	Saucepans, radiators
Resistant to corrosion (protective oxide layer)	Window frames, aircraft bodies
Malleable and ductile	Can be shaped for many applications

Aluminium resists corrosion because a thin, tough layer of aluminium oxide (Al₂O₃) forms immediately on its surface, preventing further reaction with oxygen or water.

Worked exam question

Describe how aluminium is extracted from purified aluminium oxide by electrolysis. Include the electrode reactions. [5]

Purified aluminium oxide is dissolved in molten cryolite to lower the melting point [1]. The electrolyte is electrolysed using carbon/graphite electrodes [1]. At the cathode, aluminium ions are reduced: Al³⁺ + 3e⁻ → Al; molten aluminium collects at the bottom of the cell [1]. At the anode, oxide ions are oxidised: 2O²⁻ → O₂ + 4e⁻ [1]. The carbon anodes burn away in the oxygen produced and must be replaced regularly [1].

Common exam mistakes

1. Writing “aluminium oxide is melted” without mentioning cryolite — the role of cryolite in lowering the melting point is almost always worth a mark.
2. Getting the electrode reactions the wrong way round — reduction (gaining electrons) happens at the cathode; oxidation (losing electrons) at the anode. Remember: RED CAT (reduction at cathode), AN OX (anode oxidation).
3. Saying the anode is “dissolved” — the anode is oxidised/burned away by reacting with the oxygen produced. It does not dissolve in the electrolyte.
4. Forgetting to state that molten aluminium sinks to the bottom — this is how the product is collected.