ALUMINIUM

This page starts by looking at the extraction of aluminium from its ore, bauxite, including some economic and environmental issues. It finishes by looking at some uses of aluminium.

Extracting aluminium from bauxite

Introduction

Aluminium is too high in the electrochemical series (reactivity series) to extract it from its ore using carbon reduction. The temperatures needed are too high to be economic.

Instead, it is extracted by electrolysis. 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. The aluminium oxide has too high a melting point to electrolyse on its own.

Aluminium ore

The usual aluminium ore is bauxite. Bauxite is essentially an impure aluminium oxide. The major impurities include iron oxides, silicon dioxide and titanium dioxide.

Purifiying the aluminium oxide - the Bayer Process

Reaction with sodium hydroxide solution

Crushed bauxite is treated with moderately concentrated sodium hydroxide solution. 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.

What you actually get will depend on things like the temperature and the concentration of the sodium hydroxide solution. In any case, the truth is almost certainly a lot more complicated than any of these.

The impurities in the bauxite remain as solids. For example, the other metal oxides present tend not to react with the sodium hydroxide solution and so remain unchanged. Some of the silicon dioxide reacts, but goes on to form a sodium aluminosilicate which precipitates out.

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

The sodium tetrahydroxoaluminate solution is cooled, and "seeded" with some previously produced aluminium hydroxide. This provides something for the new aluminium hydroxide to precipitate around.

You may well find that the product is quoted as Al₂O₃,3H₂O. If you add everything up, that is the same as 2Al(OH)₃. So which is it? I suspect that it probably isn't exactly either of these. The aluminium hydroxide originally precipitated doesn't have that formula - it's a simplification. And it will rearrange to form some sort of hydrated oxide, but whether it is as simple as the other formula suggests, I doubt!

The second thing is to wonder why this reaction happens at all. If you have done any aluminium chemistry, you may recognise that what normally happens is exactly the reverse of this reaction. So why is it going the other way this time? I suspect (although I don't know for sure) that it is an effect of lowering the temperature on the position of equilibrium. If the formation of the NaAl(OH)₄ is endothermic, it would be favoured by high temperatures (the conditions under which the NaAl(OH)₄ is formed during the first stage of the reaction). If you lowered the temperature (by cooling the reaction mixture after filtration), it would favour the exothermic change - the reverse.

Formation of pure aluminium oxide

Aluminium oxide (sometimes 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

The diagram shows a very simplified version of an 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

These are very complicated - in fact one source I've looked at says that they aren't fully understood. For chemistry purposes at this level, they are always simplified (to the point of being wrong! - see comment below).

This is the simplification:

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

Oxygen is produced initially at the anode.

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

Continual replacement of the anodes is a major expense.

Some economic and environmental considerations

This section is designed to give you a brief idea of the sort of economic and environmental issues involved with the extraction of aluminium. I wouldn't claim that it covers everything!

Economic considerations

Think about:

• The high cost of the process because of the huge amounts of electricity it uses. This is so high because to produce 1 mole of aluminium which only weighs 27 g you need 3 moles of electrons. You are having to add a lot of electrons (because of the high charge on the ion) to produce a small mass of aluminium (because of its low relative atomic mass).

• Energy and material costs in constantly replacing the anodes.

• Energy and material costs in producing the cryolite, some of which gets lost during the electrolysis.

Environmental problems in mining and transporting the bauxite

Think about:

• Loss of landscape due to mining, processing and transporting the bauxite.

• Noise and air pollution (greenhouse effect, acid rain) involved in these operations.

Extracting aluminium from the bauxite

Think about:

• Loss of landscape due to the size of the chemical plant needed, and in the production and transport of the electricity.

• Noise.

• Atmospheric pollution from the various stages of extraction. For example: carbon dioxide from the burning of the anodes (greenhouse effect); carbon monoxide (poisonous); fluorine (and fluorine compounds) lost from the cryolite during the electrolysis process (poisonous).

• Pollution caused by power generation (varying depending on how the electricity is generated.)

• Disposal of red mud into unsightly lagoons.

• Transport of the finished aluminium.

Recycling

Think about:

• Saving of raw materials and particularly electrical energy by not having to extract the aluminium from the bauxite. Recycling aluminium uses only about 5% of the energy used to extract it from bauxite.

• Avoiding the environmental problems in the extraction of aluminium from the bauxite.

• Not having to find space to dump the unwanted aluminium if it wasn't recycled.

• (Offsetting these to a minor extent) Energy and pollution costs in collecting and transporting the recycled aluminium.

Uses of aluminium

Aluminium is usually alloyed with other elements such as silicon, copper or magnesium. Pure aluminium isn't very strong, and alloying it adds to it strength.

Aluminium is especially useful because it

• has a low density;

• is strong when alloyed;

• is a good conductor of electricity;

• has a good appearance;

• resists corrosion because of the strong thin layer of aluminium oxide on its surface. This layer can be strengthened further by anodising the aluminium.

  Anodising essentially involves etching the aluminium with sodium hydroxide solution to remove the existing oxide layer, and then making the aluminium article the anode in an electrolysis of dilute sulphuric acid. The oxygen given of at the anode reacts with the aluminium surface, to build up a film of oxide up to about 0.02 mm thick.

  As well as increasing the corrosion resistance of the aluminium, this film is porous at this stage and will also take up dyes. (It is further treated to make it completely non-porous afterwards.) That means that you can make aluminium articles with the colour built into the surface.

Some uses include: