6.1.1 Extracting Metals

• The Earth’s crust contains metals and metal compounds such as gold, copper, iron oxide and aluminium oxide
• Useful metals are often chemically combined with other substances forming ores
• A metal ore is a rock that contains enough of the metal to make it worthwhile extracting
• Common examples of oxide ores are iron and aluminium ores which are called haematite and bauxite respectively
• Unreactive metals do not have to be extracted chemically as they are often found as the un-combined element
• Examples include gold and platinum which can both be mined directly from the Earth’s crust
  • They are known as native metals
• The position of the metal on the reactivity series determines the method of extraction
• Metals placed above carbon are extracted using electrolysis
• Lower placed metals can be extracted by heating with carbon which reduces them, two common examples being iron and carbon 

The extraction method depends on the position of a metal in the reactivity series

Extraction of Copper 

• Most copper ores exist as copper (II) sulfide 
• The copper can be extracted in two stages
• Stage 1: The copper sulfide is heated in the air to produce the oxide 
  • 2CuS (s) + 3O₂ (g)    →   2CuO (s) + 2SO₂ (g)

• Stage 2: The copper oxide is reduced by carbon
  • 2CuO (s) + C (s)  →   2Cu (s) + CO₂ (g) 
• This is an example of a redox reaction, whereby both reduction and oxidation have taken place 

Extraction of Iron

• Iron is extracted in a large container called a blast furnace from its ore, haematite 
• Modern blast furnaces produce approximately 10000 tonnes of iron per day 
• The process is demonstrated and explained below 

• The raw materials: iron ore (haematite), coke (an impure form of carbon), and limestone are added into the top of the blast furnace
• Hot air is blown in the bottom
• Zone 1:
  • Coke burns i the hot air forming 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

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:

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

• Limestone (calcium carbonate) is added to the furnace to remove impurities in the ore.
  • The calcium carbonate in the limestone decomposes to form calcium oxide:

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

CaO (s)  +  SiO₂ (s)  →  CaSiO₃ (l)

• Some metals are too reactive to be reduced by carbon 
• For these metals they are extracted using electrolysis, e.g. aluminium from aluminium oxide (bauxite) 
• To extract 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
  • The mixture is placed in an electrolysis cell, made from steel, lined with graphite. 
  • The graphite lining acts as the negative electrode, with several large graphite blocks as the positive electrodes.
  • Aluminium is produced at the cathode: 

Al³⁺ +  3e^-   →  Al 

• • Oxygen is produced at the anode:

2O^2- →   O₂ + 4e^-

• • The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell.
  • A lot of electricity is required for this process of extraction, this is a major expense
  • The carbon in the graphite anodes reacts with the oxygen produced at the anode to produce CO₂ , so the anodes have to be replaced regularly

Diagram showing the extraction of aluminium by electrolysis

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