It is more expensive and wasteful to extract a metal from a low grade ore, but most high grade ores have already been used.
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.
Although an unreactive metal is found as an uncombined element, chemical reactions are often needed to remove other elements that might contaminate it.
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)
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.
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.