Metal reactivity and ease of extraction

Metals are often found in ores as compounds rather than as uncombined metals. To obtain the metal, its compound must be converted into the metal. The more reactive the metal, the harder its compounds are to convert.

A metal’s position in the reactivity series helps predict which extraction method can be used. Carbon provides a useful reference point:

• Metals less reactive than carbon can generally be extracted by heating their compounds with carbon or carbon monoxide. For example, iron can be obtained by reacting iron oxide with carbon monoxide. Oxygen is removed from the iron oxide, leaving iron; this removal of oxygen is called reduction.
• Metals more reactive than carbon cannot be extracted by carbon reduction. Their compounds require a different method, such as electrolysis, which uses electricity to break down a compound. Aluminium, for example, is more reactive than carbon and is extracted using electrolysis.

Carbon’s position in the series therefore helps distinguish metals whose compounds can generally be reduced by carbon or carbon monoxide from those that need another extraction method.

Extraction of iron in the blast furnace

Iron is extracted from hematite, an ore containing iron(III) oxide, $$\ce{Fe2O3}$$. In a blast furnace, oxygen is removed from the iron(III) oxide to produce iron. This removal of oxygen is called reduction.

Producing carbon monoxide

Coke, a fuel containing carbon, burns in the hot air blown into the furnace. This releases heat and produces carbon dioxide:

The carbon dioxide then reacts with more hot carbon from the coke to form carbon monoxide:

Reduction of iron(III) oxide

The carbon monoxide removes oxygen from iron(III) oxide. Iron is produced, and the carbon monoxide becomes carbon dioxide:

Removal of impurities as slag

Limestone, which contains calcium carbonate, is also added to the furnace. The heat decomposes the calcium carbonate into calcium oxide and carbon dioxide:

The calcium oxide reacts with impurities in the ore to form slag. For example, it reacts with silicon dioxide to form calcium silicate:

The molten slag is less dense than the molten iron, so it floats on top and can be separated from the iron.

Extraction of aluminium by electrolysis

Aluminium is found in ores as compounds, rather than as the metal itself. Its main ore is bauxite, from which aluminium oxide, $$\ce{Al2O3}$$, is obtained.

To extract the metal, aluminium in its compound must be reduced. Carbon can reduce the compounds of some less reactive metals, but aluminium is too reactive for carbon to reduce its compounds.

Aluminium is therefore extracted by electrolysis: an electric current drives the conversion of an aluminium compound into aluminium metal. During this process, aluminium ions gain electrons to form aluminium atoms. Electrical energy supplies the energy needed for the conversion.