Aluminium extraction SA - CCEA (Higher tier only)

• Electrolysis breaks down molten or dissolved ionic compounds because their ions can move and carry charge.

• Cations in compounds go to the negative cathode and gain electrons.

• Anions go to the positive anode and lose electrons.

• Molten salts produce a metal at the cathode and a non‑metal at the anode (eg aluminium and oxygen from aluminium oxide).

• Extracting aluminium by electrolysis is energy‑intensive, so recycling aluminium is far cheaper and reduces waste.

Electrolytes are ionic compounds that are:

• in the molten state (heated so they become liquids), or

• dissolved in water.

Under these conditions, the ions in electrolytes are free to move within the liquid or solution and can carry a charge.

Electrolysis is a process in which electrical energy, from a direct current (dc) supply, breaks down electrolytes.

The free moving ions in electrolytes are attracted to the oppositely charged electrodes which connect to the dc supply.

The negatively charged electrode is called the cathode.

Positively charged ions move towards the cathode.

The positively charged electrode is called the anode.

Negatively charged ions move towards the anode.

Graphite electrodes are inert electrodes because they do not take part in the electrolysis reactions.

Instead, they provide a surface on which these reactions can happen.

When ions reach an electrode, they gain or lose electrons.

As a result, they form atoms or molecules of elements:

• positive ions gain electrons from the negatively charged cathode.

• negative ions lose electrons at the positively charged anode.

Worked example

Predict the products of the electrolysis of molten calcium chloride

Positively charged calcium ions move to the negative electrode.

Here, they gain electrons to form calcium atoms, so calcium is formed at the negative electrode.

Negatively charged chloride ions move to the positive electrode.

Here, they lose electrons to form chlorine atoms.

The atoms join up in pairs to form Cl₂ molecules, so chlorine gas is formed at the positive electrode.

Key fact

During the electrolysis of molten salts, a metal forms at the cathode and a non-metal forms at the anode.

Aluminium is more reactive than carbon so it must be extracted from its compounds using electrolysis.

Even though aluminium is more abundant than iron in the Earth's crust, aluminium is more expensive than iron.

This is mainly because of the large amounts of electrical energy used in the extraction process.

What electrolyte is used in aluminium extraction?

Aluminium ore (bauxite) is treated to produce pure aluminium oxide (alumina).

The electrolytes used in electrolysis are ionic compounds when:

• in the molten state, or

• dissolved in water.

Aluminium oxide is insoluble in water, so it must be molten to act as an electrolyte.

However, the melting point of aluminium oxide is high.

A lot of energy must be transferred to break its strong ionic bonds, and this is expensive.

To reduce costs, powdered aluminium oxide is dissolved in molten cryolite.

This mixture melts at a lower temperature than aluminium oxide, reducing costs.

Cryolite also increases conductivity.

What is the electrolysis process when extracting aluminium?

The diagram shows an electrolysis cell used to extract aluminium.

Both electrodes are made of graphite, a form of carbon with a high melting point and which conducts electricity.

What happens during electrolysis?

• At the cathode, aluminium ions gain electrons and form aluminium atoms.

• At the anode, oxide ions lose electrons and form oxygen gas.

Half-equation (Higher tier only)

At the cathode: Al³⁺ + 3e^- → Al

At the anode: 2O^2- → O₂ + 4e^-

The oxygen reacts with the carbon anodes, forming carbon dioxide (C + O₂ → CO₂).

The anodes are gradually oxidised.

They must be replaced frequently, adding to the cost of producing aluminium.

Worked example (Higher tier only)

Question

Explain, with the help of a half equation how oxide ions are oxidised during the electrolysis of aluminium oxide.

Answer

The half equation is 2O^2- → O₂ + 4e^-.

It shows that oxide ions lose electrons and oxidation is loss of electrons.

Extracting aluminium from its ore is expensive because its electrolysis requires a lot of energy.

Recycling aluminium uses far less energy and is therefore much cheaper than extracting fresh aluminium from bauxite.

Recycling aluminium also saves waste which is important because landfill sites are becoming full.