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    Extraction and purification of iron

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    Describe in detail the extraction and purification of iron.

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    Describe in detail the extraction and purification of iron(2 pages)

    Major iron ores are Iron pyrites (FeS2), Haematitie (Fe2O3), Magnatite (Fe3O4), Limonite (Fe2O3.3H2O), Chromite (FeO.Cr2O3) etc. The metallurgy of Iron involves three main operations:-
    1. Concentration or dressing
    2. Reduction
    3. Purification or refining
    First Operation - Concentration
    Ores usually contains soil, sand, stones and other useless silicates. These undesired impurities are called as matrix. Removal of these impurities is referred to as concentration. It is done either by physical process or chemical process. Before the ore is subjected to concentration it is crushed into small pieces in gyratory crushers.
    Physical method
    1. Gravity separation: The powdered ore is agitated with running stream of water. The heavy ore particle settles down while the lighter impurities are washed away. Usually haemetite ore is concentrated by gravity method
    2. Electromagnetic separation: The powdered ore is dropped on the belt from one end (non-magnetic) and at the other end (magnetic) the magnetic particles are attracted and fall nearer the roller. Iron ores are normally concentrated by this method
    Chemical Methods
    1. Calcination : It involves heating of the ore below its fusion temperature in the absence of air.It expels the organic matter and moisture from the ore. It also makes the ore porus and is performed in reverberatory furnace.
    2Fe2O3.3H2O = 2Fe2O3 + 6H2O
    Second Operation - Reduction
    1. Smelting : This involves the reduction of the ore to the molten metal at ahigh temperature. Carbon reduction process is usually employed for iron
    Fe2O3 + 3C = 2Fe + 3CO
    Fe2O3 + 3CO = 2Fe + 3CO2
    The ores even after concentration contains some matrix. To remove matrix, certain substances are mixed with concentrated ore which combine with matrix to form fusible material which is not soluble in molten metal. The substances used are called as flux and the fusible material formed during the process is called as slag. Slag is usually lighter and floats over the molten metal.
    Common fluxes include limestone (principally calcium carbonate) and dolomite (calcium-magnesium carbonate). Other fluxes may be used depending on the impurities that need to be removed from the ore. In the heat of the furnace the limestone flux decomposes to calcium oxide (quicklime):
    CaCO3 → CaO+ CO2
    Then calcium oxide combines with silicon dioxide to form a slag.
    CaO+ SiO2 → CaSiO3
    The slag melts in the heat of the furnace, which silicon dioxide would not have. In the bottom of the furnace, the molten slag floats on top of the more dense molten iron, and apertures in the side of the furnace are opened to run off the iron and the slag separately. The iron once cooled, is called pig iron.

    Third Operation - Refining or purification
    The metals obtained by the reduction process contain some impurities. Pyrometallurgical oxidation process is used when the impurities have affinities for the oxygen.It may be done by cupellation, Bessemerisation or poling.

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