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Ores

Unreactivemetals, such as gold, are found in the Earth’s crust as the uncombined elements. However, most metals are found combined with other elements to form compounds.

An ore is a rock that contains enough of a metal or a metal compound to make extracting the metal worthwhile:

• low grade ores contain a small percentage of the metal or its compound
• high grade ores contain a larger percentage

Most metals are extracted from ores found in the Earth’s crust. It is more expensive and wasteful to extract a metal from a low grade ore, but a lot of the high grade ores have already been used.

Extraction methods

The extraction method used depends upon the metal's position in the reactivity series. In principle, any metal could be extracted from its compounds using electrolysis. However, large amounts of electrical energy are needed to do this, so electrolysis is expensive.

If a metal is less reactive than carbon, it can be extracted from its compounds by heating with carbon. Copper is an example of this. Copper mostly occurs as sulfide ores, which are heated in air to convert them to copper oxide. Molten copper can be produced from copper oxide by heating with carbon:

copper oxide + carbon → copper + carbon dioxide

2CuO(s) + C(s) → 2Cu(l) + CO₂(g)

Copper oxide is reduced as carbon is oxidised, so this is an example of a redox reaction. The impure copper is purified by electrolysis.

The table summarises the extraction methods used for different metals.

Although an unreactive metal is found as the uncombined element, chemical reactions are often needed to remove other elements that might contaminate it.

Extracting iron

Iron is extracted from iron ore in a large container called a blast furnace. Iron(III) oxide is reduced to molten iron when it reacts with carbon. For example:

iron(III) oxide + carbon → iron + carbon monoxide

Fe₂O₃(s) + 3C(s) → 2Fe(l) + 3CO(g)

In the high temperatures of a blast furnace, carbon monoxide also reduces iron(III) oxide:

iron(III) oxide + carbon monoxide → iron + carbon dioxide

Fe₂O₃(s) + 3CO(s) → 2Fe(l) + 3CO₂(g)

This method of extraction works because carbon is more reactive than iron, so it can displace iron from iron compounds.

Extracting a metal by heating with carbon is cheaper than using electrolysis.