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: 2O^2- → 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.

1. Carbon undergoes combustion with hot air to form carbon dioxide.
2. Carbon dioxide reacts with coke to form carbon monoxide.
3. Carbon monoxide (reducing agent) reduces iron(III) oxide to form iron and carbon dioxide.
4. Molten iron flows to the bottom and is collected at the bottom.
5. 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.

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