Occurrence and extraction of metals

Where do metals come from?

Most metals are found in the earth's crust, or in sea water as salts. Some are found free, such as gold, silver, platinum and sometimes copper, because they are very unreactive. Most are found as compounds.

• A mineral is a natural substance in the earth that contains a metal compound.
• An ore is a mineral from which a metal can be got profitably.
• Gangue is the unwanted sand, soil and rock that is mixed in the ore.

Examples: bauxite ($\text{Al}_2\text{O}_3$) for aluminium, haematite ($\text{Fe}_2\text{O}_3$) for iron, cinnabar (HgS) for mercury, zinc blende (ZnS) for zinc.

The steps

1. Concentrate the ore, to remove the gangue.
2. Extract the metal. The method depends on the place of the metal in the reactivity series.
3. Refine the metal, to remove impurities.

Extraction by reactivity

1. The most reactive metals (K, Na, Ca, Mg, Al): electrolysis. Their oxides are very stable, and carbon cannot reduce them. We melt the compound and pass an electric current.

Aluminium is got in this way from molten aluminium oxide: $2\,\text{Al}_2\text{O}_3 \rightarrow 4\,\text{Al} + 3\,\text{O}_2$.

2. Metals of medium reactivity (Zn, Fe, Pb): reduction with carbon. It is easier to reduce an oxide, so a sulphide or a carbonate ore is changed to an oxide first.

• Roasting a sulphide ore: heating it strongly in air. $2\,\text{ZnS} + 3\,\text{O}_2 \rightarrow 2\,\text{ZnO} + 2\,\text{SO}_2$.
• Calcination of a carbonate ore: heating it strongly in little air. $\text{ZnCO}_3 \rightarrow \text{ZnO} + \text{CO}_2$.
• Reduction with carbon: $\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$. In the blast furnace: $\text{Fe}_2\text{O}_3 + 3\,\text{CO} \rightarrow 2\,\text{Fe} + 3\,\text{CO}_2$.

Aluminium can also be used to reduce the oxide of a less reactive metal, and give out a lot of heat. This is the thermit reaction, used to join railway tracks: $\text{Fe}_2\text{O}_3 + 2\,\text{Al} \rightarrow 2\,\text{Fe} + \text{Al}_2\text{O}_3 + \text{heat}$. The iron is molten.

3. The least reactive metals (Cu, Hg, Ag, Au): heating. Mercury is got by heating cinnabar in air to give the oxide, and heating the oxide again: $2\,\text{HgS} + 3\,\text{O}_2 \rightarrow 2\,\text{HgO} + 2\,\text{SO}_2$, then $2\,\text{HgO} \rightarrow 2\,\text{Hg} + \text{O}_2$.

You should find that a poor ore gives much more gangue than iron, which is why ores are concentrated first.

Refining copper

Metals from the extraction contain impurities. Copper is refined by electrolysis.

• The anode (positive) is a thick block of impure copper.
• The cathode (negative) is a thin strip of pure copper.
• The solution is acidified copper sulphate.

Copper dissolves from the anode and is deposited on the cathode as pure copper. The insoluble impurities fall to the bottom as anode mud.

Practise with new numbers

Try it in Python

This program finds the mass of a metal that is got from its ore.

Remember these

• An ore is a mineral from which a metal can be got at a profit. Gangue is the unwanted material.
• K, Na, Ca, Mg, Al: electrolysis of the molten compound. Zn, Fe, Pb: reduce the oxide with carbon. Cu, Hg, Ag, Au: heat.
• Roasting changes a sulphide to an oxide. Calcination changes a carbonate to an oxide.
• Copper is refined by electrolysis, with pure copper at the cathode.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is gangue?

2. Why is a carbonate ore of zinc calcined before it is reduced?

3. Name a metal that is found free in nature.

4. What is the thermit reaction used for?