Extraction of Metals (Oxford AQA IGCSE Combined Science Double Award)

Unreactive Metals

• The Earth’s crust contains metals and metal compounds such as gold, copper, iron oxide and aluminium oxide

• The position of the metal on the reactivity series influences the method of extraction

Pie chart to show the abundance of different elements in the Earth's crust

Native metals

• Unreactive metals do not have to be extracted chemically as they are often found as the uncombined element

  • They are known as native metals

  • E.g. gold and silver 

• Most metals in the Earth's crust are chemically combined with other substances forming ores

  • A metal ore is a rock that contains enough of the metal to make it worthwhile extracting

Reduction of Metals with Carbon

• Metals that are less reactive than carbon can be extracted by reduction with carbon

• An example is the extraction of iron in the Blast Furnace

• The raw materials: iron ore (hematite), coke (an impure form of carbon), and limestone are added into the top of the blast furnace

• Hot air is blown into the bottom

The Blast Furnace

Zone 1

• Coke burns in the hot air forming carbon dioxide 

  • The reaction is exothermic so it gives off heat, heating the furnace

carbon + oxygen → carbon dioxide

C (s)  +  O₂ (g)  →  CO₂ (g)

Zone 2

• At the high temperatures in the furnace, more coke reacts with carbon dioxide forming carbon monoxide

• Carbon dioxide has been reduced to carbon monoxide

carbon dioxide + carbon → carbon monoxide

CO₂ (g)  +  C (s)  →  2CO (g)

Zone 3

• Carbon monoxide reduces the iron(III) oxide in the iron ore to form iron 

• This will melt and collect at the bottom of the furnace, where it is tapped off:

iron(III) oxide + carbon monoxide  →  iron + carbon dioxide

Fe₂O₃ (s)  +  3CO (g)  →  2Fe (I)  +  3CO₂ (g)

Removal of impurities 

• Limestone (calcium carbonate) is added to the furnace to remove acidic impurities in the ore

  • The calcium carbonate in the limestone thermally decomposes to form calcium oxide

calcium carbonate → calcium oxide + carbon dioxide

CaCO₃ (s)  →  CaO (s)  +  CO₂ (g)

• The calcium oxide formed reacts with the silicon dioxide, which is an impurity in the iron ore, to form calcium silicate by neutralisation

calcium oxide + silicon dioxide →  calcium silicate

CaO (s)  +  SiO₂ (s)  →  CaSiO₃ (l)

• This melts and collects as a molten slag floating on top of the molten iron, which is tapped off separately

Extracting Metals by Electrolysis

• Metals that are more reactive than carbon can be extracted using electrolysis

• Aluminium is higher in the reactivity series than carbon, so it cannot be extracted by reduction using carbon

The electrolytic cell for extraction of aluminium

• Bauxite is first purified to produce aluminium oxide, Al₂O₃

• Aluminium oxide is then dissolved in molten cryolite 

  • This is because aluminium oxide has a melting point of over 2000°C which would use a lot of energy and be very expensive

  • The resulting mixture has a lower melting point without interfering with the reaction

• At the cathode, molten aluminium is formed 

• The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell

Al³⁺ +  3e^–  → Al 

• At the anode oxygen gas is produced

2O^2– → O₂ + 4e^–

• The carbon in the graphite anodes reacts with the oxygen produced to produce CO₂

C (s) + O₂ (g)   →   CO₂ (g)

• As a result the anode wears away and has to be replaced regularly

• A lot of electricity is required for this process of extraction, this is a major expense