Extraction

• The least reactive metals occur in nature as free elements e.g. copper, silver, gold
• The more reactive metals occur as compounds, together with impurities called ores
• The most important ores are oxides, sulphides, chlorides and carbonates
• Extraction of a metal from its ore is a reduction process because metal ions have to be changed to atoms by gaining electrons
• Very reactive metals form stable ions which are difficult to reduce and require electrolysis of their molten ores to reduce. The cathode provides electrons and therefor acts as the reducing agent
• Less reactive metals form less stable ions which are easier to reduce and require the heating of the oxide ore with the reducing agents carbon (in the form of coke) or carbon monoxide to be reduced
• Least reactive metals usually occur as free elements since their ions are very unstable. They are extracted from their ores by heating the ore in air at high temperatures, which is the least powerful method of reduction
• Aluminium ores - bauxite (impure aluminium oxide), cryolite
• Aluminium is extracted by electrolysis in three stages

  1. bauxite is purified
  2. purified bauxite is dissolved in molten cryolite at 900C to separate ions.
  3. The solution is electrolysed using graphite electrodes
• Electrolysis of molten aluminium oxide is not possible because its melting point is extremely high and the liquid is a poor conductor d
• Iron ores: (Fe2O3) haematite, (FeO4) magnetite, (FeCO3) siderite

  • magnetite and siderite are converted to haematite
• Iron extraction is carried out by reduction of haematite by carbon monoxide in a blast furnace
• Extraction of iron:

  1. mixture of haematite, limestone, and coke (charge) is placed at the top of the furnace
  2. Hot air blown into bottom of furnace
  3. stage 1 - coke burns in air forming carbon dioxide
  4. stage 2 - more coke reduces carbon dioxide to carbon monoxide
  5. stage 3 - carbon monoxide reduced haematite to iron
  6. impurities, mainly silica have to be removed between stages 2 and 3
  7. slag and iron are taped off separately
  8. waste gases are removed from top of furnace and burnt to heat air blow in at the borrom