Chemistry - Extraction of Metals II

  • Created by: Siana
  • Created on: 12-02-14 21:09
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  • Extraction of Metals
    • Electrolysis (ie. aluminium)
      • Expensive, reserved for metals which are too reactive to be reduced by chemical means.
      • Purification of bauxite
      • Al2O3 dissolved in molten cryolite.
      • Oxygen liberated at anode (graphite). reacts with graphite which burns slowly.
        • anodes must be regularly replaced
      • Aluminium liberated at cathode (graphite)
      • Molten aluminium sinks to the bottom of the cell and can be tapped off
      • Advantages
        • continuous, and therefore, efficient
        • pure metal produced
      • Disadvantages
        • cost of melting aluminium and electricity for electrolysis is high
        • only suitable for ionic oxides
    • Reduction of Metal Halides with more reactive metals
      • where carbon cannot be used to reduce a metal oxide because the reaction forms the carbide.
        • Tungsten and titanium
      • reactive metal (eg. magnesium) is reacted with metal oxide.
        • However cannot form pure titanium as magnesium forms alloy with titanium
      • For pure titanium, the oxide is converted to the chloride which is then reduced by Mg or Na.
        • Conversion of ore into chloride: titanium oxide is heated in a stream of chlorine in the presence of coke, forming titanium chloride.
          • Titanium chloride is a simple covalent molecule, and therefore volatile. Can be separated from other products by fractional distillation.
        • Reduction of chloride: Ti is extracted by reduction with Mg or Na. These are exothermic, keeping the reaction vessel at a high temperature.
          • An inert atmosphere of argon is required to prevent react between oxygen and magnesium/sodium
        • Advantages: very pure titanium
        • Disadvantages: batch process, Na/Mg is expensive, high energy costs
          • High cost results in titanium not being widely used.
    • Reduction of metal oxides with Hydrogen
      • Hydrogen - reducing agent
      • extraction of tungsten from its oxide - high temperature reaction
      • Advantages: very pure tungsten, cheap hydrogen
      • Disadvantages: high energy costs, use of flammable gas at high temperatures is very dangerous.


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