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Extraction of Aluminium

The extraction of aluminium

Aluminium is a commonly used metal in industry, where its properties of being both light and strong can be utilised. It is used in the manufacture of products such as aeroplanes and motor cars. The metal is present in deposits of bauxite. Bauxite is a mixture of silicas, iron oxides and hydrated alumina (\(\text{Al}_{2}\text{O}_{3}\).x\(\text{H}_{2}\text{O}\)).

Fact:

Bauxite is a rock that contains a large amount of aluminum oxide (\(\text{Al}_{2}\text{O}_{3}\)) and aluminium hydroxide (\(\text{Al}(\text{OH})_{3}\)) as well as many other aluminium containing minerals. Bauxite is the richest source of aluminum when compared with any other common rock, and is the best aluminum ore.

Extraction of Aluminium

Electrolysis can be used to extract aluminium from bauxite. The process described below produces 99% pure aluminium:

  1. Aluminium is melted along with cryolite (\(\text{Na}_{3}\text{AlF}_{6}\)) which acts as the electrolyte. Cryolite helps to lower the melting point and dissolve the ore.

  2. The \(\color{blue}{\text{carbon rod anode}}\) provides a site for the oxidation of \(\text{O}^{2-}\) and \(\text{F}^{-}\) ions. Oxygen and fluorine gas are given off at the anode and also result in anode consumption.

    \(\color{blue}{\text{2O}^{2-}\text{(aq)} \to \text{O}_{2}\text{(g) + 4e}^{-}}\)

    \(\color{blue}{\text{2F}^{-}\text{(aq)} \to \text{F}_{2}\text{(g) + 2e}^{-}}\)

  3. At the \(\color{red}{\text{cathode cell lining}}\), the \(\text{Al}^{3+}\) ions are reduced and metal aluminium deposits on the lining.

    \(\color{red}{\text{Al}^{3+}\text{(aq) + 3e}^{-} \to \text{Al(s)}}\) (99% purity)

  4. The \(\text{AlF}_{6}^{3-}\) electrolyte is stable and remains in its molten state.

The overall reaction is as follows:

\(2\text{Al}_{2}\text{O}_{3}(\text{s})\) \(\to\) \(4\text{Al}(\text{s}) + 3\text{O}_{2}(\text{g})\)

The only problem with this process is that the reaction is endothermic and large amounts of electricity are needed to drive the reaction. The process is therefore very expensive.

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