Metals - Extraction of Metals

Objective

State that the main ore of aluminium is bauxite and that aluminium is extracted by electrolysis.

Bauxite - The Ore of Aluminium

The primary ore of aluminium is bauxite. Bauxite is a rock that is rich in aluminium oxide (Al₂O₃), but it also contains impurities like iron oxides, silica, and titanium oxides.

Component	Percentage by mass (approximate)
Aluminium Oxide (Al2O3)	50-80%
Iron Oxides (Fe2O3, FeO)	5-15%
Silica (SiO2)	5-15%
Titanium Oxides (TiO2)	Less than 5%

Extraction of Aluminium by Electrolysis

Aluminium is not found in its pure form in nature. It is extracted from bauxite through a series of chemical processes. The main method of extracting aluminium is by electrolysis of molten aluminium oxide (Al₂O₃). This process requires a large amount of electricity.

The process involves the following steps:

1. Bauxite Treatment: Bauxite is treated with concentrated sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃) to dissolve the aluminium oxide and remove impurities. This process produces a solution of sodium aluminate (NaAl(OH)₄).
2. Precipitation of Aluminium Hydroxide: The sodium aluminate solution is then cooled and seeded with aluminium hydroxide (Al(OH)₃) to precipitate pure aluminium hydroxide.
3. Calcination: The aluminium hydroxide is heated to a high temperature (around 900-1000 °C) to decompose it into aluminium oxide (Al₂O₃).
4. Electrolysis: The molten aluminium oxide is electrolysed. The electrolyte is molten Al₂O₃, which is a poor conductor of electricity. Therefore, the electrolysis is carried out using a molten salt, typically cryolite (Na₃AlF₆), to increase its conductivity.

During electrolysis, aluminium ions (Al³⁺) are attracted to the cathode (negative electrode) and are reduced to molten aluminium metal:

$$Al^{3+}(l) + 3e^- \rightarrow Al(l)$$

Oxygen ions (O^2-) are attracted to the anode (positive electrode) and are oxidised to form oxygen gas:

$$2O^{2-}(l) \rightarrow O(g) + 4e^-$$

The molten aluminium metal collects at the bottom of the electrolytic cell and is tapped off.