An easy-to-understand guide to how aluminium is extracted from bauxite, including chemical equations of what goes on and an explanation for each of these chemical reactions.
Also includes analysis of the sustainability of this process and its impact on the environment.
Aluminium is the most common metal on Earth and is the third most common element on Earth. However, it is very reactive, meaning that it isn’t found in its pure form in the environment. The main source of aluminium is bauxite. Aluminium is present in bauxite as the compound aluminium hydroxide (Al(OH)3) where it is present in the form of an ion Al3+.
Description of Process/Explanation
There are two stages involved in extracting aluminium from bauxite. In the first stage, aluminium oxide / alumina (Al2O3) is extracted from bauxite. This is done using the Bayer Process. In the second stage, pure aluminium is extracted from the alumina using electrolysis in the Hall-Hérlout Process.
In the Bayer Process, the bauxite is mixed in a hot solution of sodium hydroxide (NaOH) at 175°C. The result is a solution of sodium aluminate. This is shown by the chemical equation:
Al(OH)3 (s) + OH- (aq) → Al(OH)4- (aq)
A hot sodium hydroxide solution is used because it is basic, while aluminium hydroxide and alumina are amphoteric, meaning that they can react with both acids and bases. The other major components of bauxite, iron (III) oxide (Fe2O3) and silicon dioxide (SiO2), don’t react with the sodium hydroxide solution because the iron oxide is basic, meaning that it only reacts with acids while silicon dioxide only reacts with very hot concentrated basic solutions. Therefore all substances originally in the bauxite remain solid except for aluminium hydroxide. These solid substances combine to form an insoluble substance called red mud. The solution of red mud and sodium aluminate is allowed to settle after which they are separated through filtration. The solution of sodium aluminate is supersaturated so by adding crystals of pure Al(OH)3 to the solution, the aluminium hydroxide precipitates.
Al(OH)4- (aq) → Al(OH)3 (s) + OH- (aq)
The aluminium hydroxide is separated is from the solution using filtration and heated to 1050 – 1200 °C, causing it to decompose and produce alumina.
2 Al(OH)3 (s) → Al2O3 (s) + 3 H2O (g)
The Hall-Hérlout Process
To reduce alumina to aluminium requires a lot of energy requires a lot of energy, making it inefficient.
2 Al2O3 (s) → 4 Al (s) + 3 O2 (g) ; △H = +3 352 kJ
Therefore it is more efficient to use electrolysis to do this. In the Hall-Hérlout Process, the alumina is dissolved in cryolite (Na3AlF6) at 950°C. this lowers the melting point of the alumina from 2 000 °C to 1 000 °C, thus saving energy. This mixture is then placed in the Hall-Hérlout cell, with a graphite cell lining acting as the cathode (attracts the + charged ion), as well as graphite anodes (attracts the – charged ions).