10.09.2 Extracting Aluminium

Syllabus

- Metals can be extracted from molten compounds using electrolysis. Electrolysis is used if the metal is too reactive to be extracted by reduction with Carbon or if the metal reacts with Carbon.
- Large amounts of energy are used in the extraction process to melt the compounds and to produce the electrical current.
- Aluminium is manufactured by the electrolysis of a molten mixture of Aluminium Oxide and Cryolite using Carbon as the positive electrode (anode).
- Students should be able to:

1. explain why a mixture is used as the electrolyte
2. explain why the positive electrode must be continually replaced.

What does this mean?

Aluminium is more reactive than Carbon.

So it cannot be smelted like less reactive metals (extracted by heating with coke/charcoal).

It must be extracted by electricity through the process of electrolysis.

Unfortunately, Aluminium ore is mostly Aluminium Oxide (Alumina) which is insoluble in water.

So, it has to be melted before it becomes an electrolyte - before it can be split up by electricity.

But Ionic compounds have high melting points, which makes extracting Aluminium by electrolysis very expensive.

Cryolite is another Aluminium compound. It melts at 1000oC, rather than 2000oC - the melting point of Aluminium Oxide.

So Cryolite is melted and Aluminium Oxide dissolves into this liquid, allowing its ions to move.

This reduces the energy costs a little but the enormous amount of electrical power required to extract Aluminium means it will always be an expensive metal.