Metal Extraction Principles

Choosing how to extract a metal

Most metals occur in ores as compounds rather than as pure metals. To obtain a metal from a metal oxide, its oxygen must be removed: the metal oxide is reduced. The position of carbon in the reactivity series tells us when carbon can do this. Carbon can remove oxygen from the oxides of metals below it, such as iron and copper. It cannot be used in this way to extract metals above it, such as magnesium, because those metals hold on to oxygen more strongly. Their extraction needs a different method.

Extracting iron on an industrial scale

Iron is below carbon in the reactivity series, so iron oxide can be reduced using carbon. In a blast furnace, iron ore is heated with coke, a form of carbon, while hot air is blown in. Some coke burns to form carbon dioxide. This reacts with more hot coke to make carbon monoxide:

$\mathrm{CO_2 + C \rightarrow 2CO}$

Carbon monoxide then removes oxygen from iron(III) oxide:

$\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}$

Iron(III) oxide is reduced to iron; carbon monoxide is the reducing agent and is converted to carbon dioxide. The molten iron can be collected from the furnace. Thus, saying that iron is extracted ‘using carbon’ does not mean that solid carbon must directly touch every piece of iron oxide: carbon also produces the carbon monoxide that carries out much of the reduction.

A non-ferrous example: copper

Copper is non-ferrous because it is not iron. It lies below carbon in the reactivity series, so carbon can reduce copper(II) oxide. A useful simplified equation is:

$\mathrm{2CuO + C \rightarrow 2Cu + CO_2}$

The black copper(II) oxide loses oxygen and forms reddish-brown copper. This illustrates the principle of carbon reduction for a non-ferrous metal. It is not a claim that every copper ore is an oxide or that every industrial copper-extraction route uses carbon.

OCR suggests showing this reaction by heating copper oxide with carbon. In a school practical, mix a little carbon powder with copper(II) oxide in a hard-glass test tube, hold it with a test-tube holder and heat it strongly with a Bunsen burner while wearing eye protection. After cooling, look for reddish-brown copper among the dark solids. The colour change is evidence that copper oxide has been reduced. Comparing this result with an oxide of a metal above carbon, such as magnesium oxide, shows why position in the reactivity series matters: under comparable conditions, carbon does not reduce magnesium oxide.

The common industrial principle

Industrial extraction depends on both the ore and the metal’s reactivity. For an oxide of a metal below carbon, heating with carbon or with carbon monoxide made from it can remove oxygen and release the metal. Iron production in a blast furnace and the reduction of copper oxide are two examples of that principle. Metals above carbon require another extraction method rather than simply more carbon.

Carbon and the reactivity series

Carbon can reduce oxides of metals below it, including iron and copper; it cannot reduce oxides of metals above it, such as magnesium. Reduction here means removal of oxygen.

Iron and copper

In a blast furnace, coke helps produce carbon monoxide, which reduces iron oxide: $\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}$. Copper is a non-ferrous example: heating copper(II) oxide with carbon produces reddish-brown copper. $\mathrm{2CuO + C \rightarrow 2Cu + CO_2}$ is a simplified equation for that reduction.

Practical link

Strongly heat a mixture of black copper(II) oxide and carbon, then look for reddish-brown copper after cooling. The observation supports the conclusion that carbon has removed oxygen from the oxide.