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6.1.1 Extracting Metals

Extracting Metals with Carbon

  • 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 reactivity series and extraction of metals table, IGCSE & GCSE Chemistry revision notes

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 

Carbon Extraction of Iron, downloadable AS & A Level Chemistry revision notes

  • 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)

Extracting Metals using Electrolysis

  • 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:

2O2- →   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

Aluminium extraction, IGCSE & GCSE Chemistry revision notes

Diagram showing the extraction of aluminium by electrolysis

Exam Tip

Remember, there are two definitions for oxidation and reduction.

Oxidation is the gain of oxygen and loss of electrons.

Reduction is the loss of oxygen and gain of electrons.

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