Where Do Metals Come From?
- Some unreactive metals, such as gold or silver, are found as pure elements in the Earth's crust.
- Most metals are found in their ores as a mixture of metals and metal compounds.
- These metals need to be extracted from their ores to be useful.
Extraction of Metal
It is possible to extract all metals using electrolysis. However, electrolysis is a very expensive process since it consumes a lot of electricity, so a cheaper process is used if possible.
- For metals above carbon in the reactivity series, electrolysis is used for metal extraction.
- For metals below carbon in the reactivity series, carbon can displace metals from metal oxides, so metals are extracted by burning with carbon.
1. Aluminum Extraction by Electrolysis
- Aluminum ore (bauxite) is first purified to aluminum oxide (Al2O3).
- Aluminum oxide is insoluble in water, so it must be molten to become electrolytes for electrolysis.
- Melting point of aluminum oxide is high. To save energy, powdered aluminum oxide is dissolved in molten cryolite.
- Molten cryolite has a lower melting point than aluminum oxide and is also a better electricity conductor, so dissolving aluminum oxide in molten cryolite saves costs.
- During electrolysis:
- At the cathode: Al3+ + 3e- → Al
- At the anode: 2O2- → O2 + 4e-
- Some of the oxygen gas produced at the anode reacts with graphite electrodes and produces carbon dioxide. (C + O2 → CO2) The electrodes need to be replaced frequently.
2. Iron Extraction by Burning Carbon in a Blast Furnace
- Iron ore (haematite), coke (impure carbon) and limestone are fed to the top of the blast furnace. Iron ore contains iron(III) oxide (Fe2O3) and impurities such as silicon dioxide (SiO2).
- Hot air is blasted into the furnace at the base.
- Carbon undergoes combustion with hot air to form carbon dioxide.
- Carbon dioxide reacts with coke to form carbon monoxide.
- Carbon monoxide (reducing agent) reduces iron(III) oxide to form iron and carbon dioxide.
- Molten iron flows to the bottom and is collected at the bottom.
- Limestone (CaCO3) is decomposed into calcium oxide and carbon dioxide. Calcium oxide reacts with silicon dioxide to form calcium silicate, which is collected as molten slag.