Extraction of Metals

• • useful metals are often chemically combined with other substances to form ores
  • a metal ore is a rock that contains enough of the metal to make it worthwhile extracting
  • they have to be extracted from their ores through processes such as electrolysis, using a blast furnace or by reacting with more reactive material
  • in many cases, the ore is an oxide of the metal, therefore the extraction of these metals is a reduction process since oxygen is being removed

  common examples of oxide ores are...

  • iron (hematite)
  • aluminium (bauxite)
• • unreactive metals do not easily react with other substances due to their chemical stability and are known as native metals (ex: gold and platinum). They can both be mined directly from the Earth's crust
  • metals placed higher up on the series (above carbon) have to be extracted using electrolysis whereas metals lower down on the series can be extracted by heating with a reducing agent such as carbon or carbon monoxide in a blast furnace
• extraction of iron

  • iron is extracted in a blast furnace from its ore hematite

  Raw materials are

  • Iron Ore (hematite): Fe2O3
  • coke (impure form of carbon)
  • Limestone (CaCO3)

  1. Hot air is blown into the bottom of the blast furnace to provide oxygen
  2. the iron ore contains impurities so limestone is added to remove acidic impurities that will contaminate any iron that is formed
• 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
• Zone 2

  • At the high temperature in the furnace, more coke reacts with carbon dioxide forming carbon monoxide
  • carbon dioxide has been reduced to carbon monoxide since the carbon monoxide reacts as the reducing agent
  • carbon + carbon dioxide --> carbon monoxide
• Zone 3

  • carbon monoxide reduces the iron(III) oxide in the iron ore to form molten iron
  • the liquid molten iron is more dense than the impurities so it sinks to the bottom of the blast furnace and is tapped off
  • this will melt and collect at the bottom of the furnace, where it is tapped off:

  iron(III) oxide + carbon monoxide --> iron + carbon dioxide
• Removing impurities from hematite

  1. Impurities react with oxygen to form oxides

  2. Acidic oxides are neutralised by calcium oxide

  3. Calcium oxide reacts with silicon dioxide to form calcium silicate (slag)

  4. Slag is less dense than molten iron and floats on top, allowing it to be removed
• Impurities in hematite

  • Carbon
  • Sulfur
  • Silicon (forming silicon(IV) dioxide)
  • Phosphorus (forming phosphorus (III/V) oxide)
• If the oxide is a gas it will escape out of the top of the blast furnace
• Neutralisation reaction

  Removes acidic oxides
• Calcium carbonate

  Undergoes thermal decomposition to form calcium oxide and carbon dioxide

  • calcium carbonate --> calcium oxide + carbon dioxide
• Calcium oxide


  Reacts with acidic impurities to neutralise them
• Reaction with silicon dioxide

  Forms calcium silicate (common name is slag)
• Slag is less dense than molten iron so floats on top and can be removed
• Zone 1: the burning of carbon (coke) to provide heat and produce carbon dioxide:

  • C(s) + O2 (g) ---> CO2 (g)
• zone 2: the reduction of carbon dioxide to carbon monoxide

  • CO2 (g) + C(s) ---> 2CO (g)
• zone 3: the reduction of iron(III) oxide by carbon monoxide

  • Fe2O3 (s) + 3CO (g) ---> 2Fe(l) +3CO2 (g)

  the thermal decomposition of calcium carbonate to produce calcium oxide:

  • CaCO3 (s) ---> CaO (s) + CO2 (g)

  the formation of slag:

  • CaO (s) + SiO (s) ---> CaSiO3 (l)
• Aluminium's main ore is bauxite, which contains aluminium oxide and is extracted by electrolysis
• process of aluminium extraction by electrolysis

  1. bauxite is first purified to produce aluminium oxide, Al2O3
  2. aluminium oxide is then dissolved in molten cryolite (this is because Al2O3 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)
  3. 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
• At the cathode (negative electrode)

  • aluminium ions gain electrons (reduction)
  • molten aluminium forms at the bottom of the cell
  • 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 (positive electrode):

  • oxide ions lose electrons (oxidation)
  • oxygen is produced at the anode
  • 2O2- ---> O2 + 4e-
• The overall equation for the reaction is:

  • 2Al2O3 ---> 4Al + 3O2

  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, which is a major expense