Extraction Methods and Reactivity

A more reactive metal forms more stable compounds and is harder to extract. Carbon is therefore placed in the reactivity series as a useful reference point, even though carbon is not a metal:

$\mathrm{K,\ Na,\ Ca,\ Mg,\ Al,\ (C),\ Zn,\ Fe,\ (H),\ Cu,\ Ag,\ Au}$

The position of a metal relative to carbon indicates whether heating with carbon is an effective extraction method.

Metals below carbon: heating with carbon

A metal below carbon is less reactive than carbon. Carbon can therefore take oxygen from the metal oxide, reducing the oxide to the metal. Depending on the process, carbon itself or carbon monoxide made from carbon acts as the reducing agent.

Iron is below carbon, so iron can be extracted from iron oxide by heating in a carbon-based process. One important reduction is:

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

Iron(III) oxide loses oxygen and is reduced to iron. Carbon monoxide gains oxygen and is oxidised to carbon dioxide. Detailed knowledge of the blast furnace is not required here; the important connection is that iron lies below carbon, so carbon reduction is possible.

Heating with carbon requires fuel and high-temperature equipment, but it is generally cheaper than electrolysis. Consequently, a suitable metal oxide below carbon would not normally be electrolysed when carbon reduction can do the job more economically.

Metals above carbon: electrolysis

A metal above carbon is more reactive than carbon. Its oxide is too stable for carbon to remove the oxygen, so carbon reduction cannot extract the metal. Electrical energy must instead force the compound to decompose by electrolysis.

Aluminium lies above carbon, so aluminium cannot be extracted from aluminium oxide by heating it with carbon. In aluminium extraction, aluminium oxide is used in a molten ionic mixture so that its ions can move. Positively charged aluminium ions move to the negative cathode and gain electrons:

$\mathrm{Al^{3+} + 3e^- \rightarrow Al}$

This produces aluminium metal. Cryolite is used as a solvent because it allows electrolysis to occur at a lower temperature, reducing the energy needed.

Why electrolysis costs more

Industrial electrolysis needs a large, continuous supply of electricity and enough energy to keep the electrolyte molten. It also requires specialised equipment. These energy and equipment costs make electrolysis more expensive than carbon reduction.

The choice is therefore both chemical and economic:

• below carbon, reduction by heating with carbon is possible and is generally the cheaper choice;
• above carbon, carbon cannot extract the metal, so costly electrolysis is necessary;
• very unreactive metals may occur naturally as uncombined elements, so they may not need chemical reduction from an ore.

Thus iron illustrates the cheaper carbon-reduction route, whereas aluminium illustrates why a more reactive metal requires electrolysis despite its higher cost.