Extraction of metals
Francine Taylor-Campbell, Contributor

Ricardo Frater (right), of the school's Cadet Corps, shakes hands with Penwood High School's principal, Austin Burrell, during a training session at the school on Wednesday, November 21, 2007. The Cadet Corps came into being to instill discipline among troubled students. -Anthony Minott/Freelance Photographer

You should be able to:

• Relate the principles underlying the extraction of a metal to its position in the electro-chemical series
• Describe the extraction of aluminium and iron

Points to note

• 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 its free state, that is, uncombined, for example gold.
• To extract a metal X^x+, from its ore the process of reduction is used, that is the metal ions gain electrons to become atoms. X^x+ + 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:

1. the carbon burns to form carbon dioxide

C (s) + O₂ (g) = CO₂ (g)

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)

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 a 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.

These Greater Portmore High School students revise for a test. - Anthony Minott/Freelance Photographer

Francine Taylor-Campbell is an independent contributor.