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 source of aluminium is the mineral bauxite. Bauxite is an aluminium ore containing aluminium oxide ($Al_2O_3$) along with impurities such as iron oxides, silica, and titanium dioxide.

• Bauxite is typically found in tropical regions.
• The aluminium in bauxite is chemically bonded to oxygen, making it unsuitable for direct extraction using methods like heating with carbon.

Extraction of Aluminium by Electrolysis

Aluminium is extracted from bauxite using an electrolytic process. This process involves dissolving bauxite in a hot solution of sodium hydroxide (NaOH) to form sodium aluminate ($NaAl(OH)_4$). The impurities remain undissolved and are removed.

The solution is then filtered, and the sodium aluminate solution is cooled and evaporated to crystallise pure aluminium hydroxide ($Al(OH)_3$). This aluminium hydroxide is then heated to a very high temperature (around 950°C) in an electrolytic cell. During electrolysis, the aluminium hydroxide is broken down into aluminium metal, water, and oxygen.

Stage	Process	Chemical Equation
Bauxite Treatment	Bauxite is reacted with hot sodium hydroxide.	$Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2NaAl(OH)_4(aq) + H_2(g)$
Filtration	Undissolved impurities are removed by filtration.	N/A
Crystallisation	Sodium aluminate solution is cooled and evaporated to crystallise aluminium hydroxide.	$NaAl(OH)_4(aq) \rightarrow Al(OH)_3(s) + NaOH(aq)$
Electrolysis	Aluminium hydroxide is electrolysed in a molten state.	$2Al(OH)_3(l) \rightarrow Al_2O_3(s) + 3H_2O(g)$ $2Al_2O_3(l) \xrightarrow{\text{electrolysis}} 4Al(l) + 3O_2(g)$

The electrolysis is carried out in a molten state because aluminium hydroxide decomposes at high temperatures. The anode is typically made of graphite, and the cathode is also graphite. The molten aluminium is collected at the cathode.

The process of extracting aluminium is energy-intensive, which contributes to the relatively high cost of aluminium.