Role of electrolyte in the extraction of aluminium

Hi Firdaus,

Aluminium is extracted from it's ore, bauxite. Bauxite is actually aluminium oxide - Al2O3 - and it has to be melted so that the aluminium can be extracted - this is the electrolyte. The melting point of bauxite is really high (around 2000 'C), so it is mixed with cryolite, which means that the electrolyte can be melted at around 1200'C which is cheaper.

Once the electrolyte (bauxite / cryolite mixture) is molten, electricity is passed through it using carbon electrodes.

At the negative electrode (called the cathode), the positive aluminium ions gain electrons and form aluminium atoms.

At the positive electrode (the anode), oxygen ions give up their additional electrons and form oxygen gas.

The oxygen reacts with the carbon electrodes to form carbon dioxide, so the electrodes have to be replaced on a regular basis.

I hope this helps!

Catherine

It is used to extract the aluminium from its ore called bauxite.

Aluminium is the most abundant metal on Earth, but it is expensive, largely because of the amount of electricity used in the extraction process.

Aluminium ore is called bauxite (Al₂O₃). The bauxite is purified to yield a white powder – aluminium oxide (also known as alumina) – from which aluminium can be extracted.

The extraction is done by electrolysis, but first the aluminium oxide must be melted so that electricity can pass through it. However, aluminium oxide has a very high melting point (over 2,000°C) so it would be expensive to melt it.

Instead, it is dissolved in molten cryolite – an aluminium compound with a lower melting point than aluminium oxide. The use of molten cryolite as a solvent reduces some of the energy costs involved in extracting aluminium by allowing the ions in aluminium oxide to move freely at a lower temperature.

Both the negative electrode (cathode) and positive electrode (anode) are made of graphite, which is a form of carbon.

Aluminium ions receive electrons at the negative electrode and are reduced to aluminium atoms:

Al³⁺ + 3e^– → Al (reduction – gain electrons)

The molten aluminium sinks to the bottom of the cell, where it is tapped off.

Oxide ions lose electrons at the positive electrodes and are oxidised to oxygen gas:

2O^2– → O₂ + 4e^– (oxidation – lose electrons)

This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, so they gradually burn away. As a result, the positive electrodes have to be replaced frequently. This adds to the cost of the process.