Extracting metals

Ores

Unreactivemetals such as gold are found in the Earth's crust as 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, eg either:

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

It is more expensive and wasteful to extract a metal from a low grade ore, but most high grade ores have already been used.

Extraction methods

The extraction method used depends on the metal’s position in the reactivity series. In theory, 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 oxide by heating with carbon. The carbon displaces the metal from the compound, and removes the oxygen from the oxide. This leaves the metal.

The table summarises the extraction methods used for different metals.

Table of metals in order of ease of extraction and the methods used.Carbon is a non-metal but can be placed in the reactivity series

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

Extraction of zinc

Zinc mostly occurs as sulfide ores, which are heated in air to convert them to zinc oxide.

Molten zinc can then be made from zinc oxide by heating with carbon:

zinc oxide + carbon → zinc + carbon monoxide

ZnO(s) + C(s) → Zn(l) + CO(g)

Zinc oxide loses oxygen. It is reduced. Carbon gains oxygen. It is oxidised.

Carbon acts as a reducing agent. Carbon has displaced the metal from the oxide.

The carbon monoxide is oxidised further to produce the less harmful carbon dioxide, CO2.

Question

Copper may be extracted from copper oxide by reaction with carbon.

State which substance has been oxidised and which reduced.

Carbon has been oxidised. Copper oxide has been reduced.