Aluminium is made from BAUXITE, which makes up approximately 8% of the earth’s crust.
BAUXITE is a soft mineral with varying hardness. It is made up of mostly Aluminium Oxide but it has unwanted companions of Iron Oxide and Silica Dioxide. Its colour varies from white to brown depending on how much Iron Oxide is present. The more Iron Oxide the darker the colour.
Click here for a picture of Bauxite
Before Aluminium can be made the Bauxite has to
be refined to form Alumina.
Refining the bauxite separates the Aluminium Oxide in the ore from its unwanted companions. This separation takes place in three steps and is known as the Bayer Process. It depends on the fact that Silica Dioxide is acidic, Iron Oxide is basic and Aluminium Oxide is Amphoteric (reacts with both acid and alkali).
STEP ONE
Finely crushed Bauxite is mixed with a solution of aqueous Sodium Hydroxide (NaOH) and heated. The amphoteric Aluminium Oxide (Al2O3) dissolves in the Sodium Hydroxide to form sodium Aluminate (Na3Al(OH)6). The Silica Dioxide although acidic needs a stronger alkali to dissolve in and remains a solid along with the Iron Oxide.
Al2O3 (s) + 6NaOH (aq) + 3H2O
(l) 2Na3Al(OH)6
STEP TWO
The solution of Sodium Aluminate is filtered from the unwanted residues. Carbon Dioxide gas (CO2) is blown through the solution. Since Carbon Dioxide is more acidic than Aluminium Oxide it forms Sodium Bicarbonate and a precipitate of Aluminium Hydroxide.
Na3Al(OH)6 + 3CO2 Al(OH)3
+ 3NaHCO3
STEP THREE
The solution is then filtered again and the Aluminium Hydroxide is heated. The heat drives out water leaving a fine white powder of Alumina (Aluminium Oxide)
Click here for a picture
of Alumina
The Alumina can then be used to produce Aluminium in the process of electrolysis