Extraction of metals
Francine Taylor Campbell, Contributor

A scene from a Manning Cup football match between St. George's College and Kingston High School on September 27. - Ian Allen Photo

THE REACTIVE metals are found in nature as compounds called ores. These compounds contain the metal in combination with oxygen, sulphur or chlorine, along with other impurities. Metals that are not very reactive are usually found in their free state, that is, uncombined, for example, gold.

To extract a metal Xx+ from its ore, the process of reduction is used, that is, the metal ions gain electrons to become atoms. Xx+ + xe === X

For the process of reduction, a reducing agent, such as carbon or hydrogen, is generally used. The reducing agent works by displacing the metal from its ore. However, for very reactive metals, for example, sodium and potassium, the process of electrolysis is used to supply electrons to the molten ore to extract the metal.

In the extraction of aluminium, the ore (bauxite) is purified and dissolved in molten cryolite (Na3AlF6) from which it produces the ions Al³+ and O²-. The aluminium oxide/cryolite solution is then electrolysed in a cell using graphite (carbon) electrodes. Aluminium ions (+ve) move towards the cathode where they gain electrons and form molten aluminium.

Al³+ (l) + 3e ==== Al (l) This molten aluminium is collected at the bottom of the cell. The oxide ions (-ve) move towards the anode where they lose electrons to form oxygen which is collected.

2O²-(l) ==== O2 (g) + 4e

The extraction of iron is different from that of aluminium. This is because iron is not as reactive, hence a reducing agent, such as carbon monoxide, can be used to extract it from its ore.

In this process, the ore (haematite) is mixed with limestone (CaCO3) and coke (carbon) and heated in a blast furnace. The reaction occurs in different stages:

STAGE 1

Carbon burns to form carbon dioxide

C (s) + O2 (g) === CO2 (g)

STAGE 2

The carbon dioxide reacts with more coke (carbon) to form carbon monoxide which is the reducing agent in this method.

CO2 (s) + C (s) === 2CO (g)

STAGE 3

The carbon monoxide reduces the haematite to iron which is collected at the bottom of the furnace as molten iron.

Fe2O3 (s) + 3CO (g) ========== 2Fe (s) + 3CO2 (g)

Impurities in the ore, mainly silica, are removed by reacting with calcium oxide formed from the decomposition of limestone.

CaCO3 (s) ========== CaO (s) + CO2 (g)

CaO (s) + SiO2 (s) ========= CaSiO3 (l) slag or calcium silicate

This slag is also collected at the bottom of the furnace.

This reaction is facilitated because the calcium oxide is basic, while silica is acidic.

The slag produced is used for road building and also as fertiliser. The iron produced is impure, containing carbon, which is removed by reacting it with oxygen. From this, steel and other types of iron are produced.

In our lessons to come, we will use our knowledge of metals and non-metals to answer some questions. Please ensure that you review the facts and practise on your own!

* Francine Taylor-Campbell is an independent contributor.