Chemology

Introduction:

Firstly, aluminium has to be removed from the Earth, by mining. This is relatively simple, given the abundance of Aluminium. However, Aluminium is always bounded to other elements, and exists as compounds, often containing oxygen and silicon. Thus, it is impossible to obtain a pure substance of aluminium through mining.

Aluminium's ore is bauxite. Bauxite is an impure aluminium oxide. The major impurities include:iron oxides, silicon dioxide and titanium dioxide. Aluminium is high in the reactivity series, thus it is impossible to extract it from its ore using carbon reduction, as the bonds in the aluminium oxide are very strong. Carbon is not reactive enough to break these bonds and reduce the oxides of these metals. Thus Aluminium is extracted from it's ore, Bauxite, by decomposing their compounds with electricity, or otherwise known as a process called electrolysis.

A brief summary:

After being dug up from Earth, the ore is first converted into pure aluminium oxide by the Bayer Process, and this is then electrolysed in solution in molten cryolite - another aluminium compound. (This is so as the aluminium oxide has too high a melting point to electrolyse on its own.)

Bayer process:

Aluminium can be obtained through purification, using the Bayer process:

Reaction with sodium hydroxide solution

1. The bauxite is first crushed into smaller pieces. The crushed bauxite is then treated with moderately concentrated sodium hydroxide solution.

Conditions for the reaction:
(The concentration, temperature and pressure used depend on the source of the bauxite and exactly what form of aluminium oxide it contains.)

Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres.

High pressures are necessary to keep the water in the sodium hydroxide solution liquid at temperatures above 100°C. The higher the temperature, the higher the pressure needed.

With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium tetrahydroxoaluminate.

2. The impurities (unwanted elements) in the bauxite will remain in the solid state. This is so as the other metal oxides present in the Bauxite will not react with the sodium hydroxide solution.

However, some of the silicon dioxide will react will the sodium hydroxide solution (which is removable, through precipitation)

3. All of these solids are separated from the sodium tetrahydroxoaluminate solution by filtration. They form a "red mud" which is just stored in huge lagoons.

Precipitation of aluminium hydroxide

4. The sodium tetrahydroxoaluminate solution is then allowed to cool. After which, it is "seeded" with some previously produced aluminium hydroxide. This provides a substance for the new aluminium hydroxide to precipitate around.

Formation of pure aluminium oxide

Aluminium oxide (or otherwise known as alumina) is made by heating the aluminium hydroxide to a temperature of about 1100 - 1200°C.

Conversion of the aluminium oxide into aluminium by electrolysis

The aluminium oxide is electrolysed in solution in molten cryolite, Na3AlF6. Cryolite is another aluminium ore, but is rare and expensive, and most is now made chemically.

The electrolysis cell

Although the carbon lining of the cell is labelled as the cathode, the effective cathode is mainly the molten aluminium that forms on the bottom of the cell.

Molten aluminium is syphoned out of the cell from time to time, and new aluminium oxide added at the top.

The cell operates at a low voltage of about 5 - 6 volts, but at huge currents of 100,000 amps or more. The heating effect of these large currents keeps the cell at a temperature of about 1000°C.

The electrode reactions

The simplified electrode reaction

Aluminium is released at the cathode. Aluminium ions are reduced by gaining 3 electrons.

Oxygen is produced initially at the anode.

At the temperature of the cell, the carbon anodes burn in this oxygen to give carbon dioxide and carbon monoxide.

Aluminium formed at the cathode gets collected at the bottom of the electrolytic cell from where it is removed periodically.
The metal obtained by this method is about 99% pure.

Further purification is carried out by Hoop's electrolytic method. The electrolytic refining of aluminum is carried out in Hoop's cell. The hoop's cell consist of an iron tank, lined with carbon. It has three layers of molten liquids with different densities. Molten impure aluminum in the bottom layer which along with the carbon lining acts as the cathode. The middle layer is a mixture of molten fluorides of sodium, barium and aluminum in the molten fluorides of sodium, barium and aluminum in the molten state. This act as the electrolyte. Top layer consist of molten pure aluminum in which a number of carbon electrodes are suspended. These carbon electrodes act as the cathode.
When electric current is passed, the aluminium ions from the middle layer move to the top layer and are discharged at the cathode as pure aluminium. At the same time, an equivalent amount of aluminum from the bottom layer migrates to the middle layer leaving behind the impurities. The pure aluminum is removed from the tapping hole from time to time.

Pure sample of Aluminium: sliver white, crystalline