Getting Metals From Rocks

Some unreactive metals, like gold, are found in the earth just as themselves. The rest though you have to get out of rocks….and that can get pretty tricky.

Metal Ores

  • These are rocks that contain enough metal to make extraction worthwhile
  • In many cases they are just an oxide of the metal. For example the rock Bauxite, the main aluminium ore, is just aluminium oxide (Al2O3)
  • Mostly metals are extracted in a chemical reactions
  • Economics play a part in metal extraction and this can change over time. If the market value of a metal drops for example, it might no longer be worth extracting it and vice versa. Also as technology improves it might be possible to get more metal out of any given rock than before and so it becomes more profitable to extract the metal.

Extraction

There’s two ways a metal can be extracted from its ore – chemically (by reduction) or by electrolysis (using electricity). We’ll go into both of these below.

Some ores have to be concentrated first though – i.e. just getting rid of all the unwanted rocky material. Electrolysis may be used to purify any metal extracted too.

  • Reduction

This involves reacting the ore with carbon. In the reaction oxygen is removed.

e.g. iron oxide + carbon → iron + carbon dioxide

2Fe2O3 + 3C → 4Fe + 3CO2

Whether reduction can be done depends of how far down the reactivity series the metal is:

extraction-reactivity

Carbon can only take oxygen away from metals than are less reactive than itself.

Therefore metals below it in the reactivity series above can be extracted through reduction – e.g. iron oxide is reduced in a blast furnace

Metals above in in the reactivity series above have to be extracted via electrolysis – which can get expensive!

  • Electrolysis

This is just the breaking down of a substances using electricity.

It requires a liquid to conduct the electricity, called the electrolyte. These are often metal sea salt solutions made from ore (e.g. copper sulfate) or molten metal oxides – it requires a lot of energy to actually melt a metal oxide, which means it can get very expensive!

The electrolytes has free ions which conduct electricity.

Electrons are taken away by the positive anode and given away by the negative cathode. As ions gain or lose electrons they become atoms or molecules and are released.

This is all very confusing – and you might want to look at our notes of electrolysis if it’s getting a bit much – but let’s also take an example:

Copper can extracted using carbon. In a process called ‘smelting’ copper ore is put in a furnace with carbon, but the copper that comes out isn’t very pure. This is annoying because impure copper doesn’t conduct electricity and a major use of copper is as electrical wiring. Therefore electrolysis can be used to make the copper even purer.

Here’s what happens during the process:

copper-electrolysis

1) The anode is made of impure copper and the cathode of pure copper. There are suspended in a solution of copper (2+) sulphate.

2) Electrons are pulled off copper atoms at the anode, which means they go into the solution as Cu (2+) ions.

3) These ions gain electrons at the cathode and go back into pure copper atoms.

4) The impurities that dropped off the anode form a sludge at the bottom.

5) All this causes the anode of impure copper to shrink…and the cathode of pure copper to grow in size.