METALLURGY - 1

Electromagnetic Separation : When one component either the ore or impurity is magnetic in nature, this method can be used for separation. A magnetic separator consisting a belt moving on two rollers is used. One of which is a strong magnet. 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 while non-magnetic particles fall away from the roller. Ferro-magnetic ores are concentrated by this method. For example, wolframite (FeWO₄) - a magnetic ore, is separated from the non-magnetic ore, cassiterite SnO₂, by this method.

Froth floatation process : This method is used for the concentration of sulphide ores. The method is based on the preferential wetting properties with the frothing agent and water. The powdered ore is added to water containing pine oil (frothing agent) and sodium ethyl xanthate (collecting agent). A vigorous stream of air is now passed through, which thoroughly agitates the mixture and disposes the oil into colloid sized particles. As a result of this, the sulphide particles of the ore stick to the oil droplets and rise to the surface in the form of froth supported by air bubbles. Water wets the gangue particles which sink to the bottom. With this method, it is possible to concentrate the dense ores such as galena and zinc blende.

Chemical methods

1. Calcination : It involves heating of the ore below its fusion temperature in absence of air. This step expels organic matter and moisture from the ores. It can remove moisture from hydrated oxides or carbon dioxide from carbonates. For example :
   
   
   
   Al₂O₃·2H₂OAl₂O₃ + 2H₂O
   2Fe₂O₃·3H₂O2Fe₂O₃ + 6H₂O
   CuCO₃·Cu(OH)₂2CuO + CO₂ + H₂O
   
   Calcination makes the ore porous. The step is generally performed in reverberatory furnace.
   
2. Roasting : It is also heating of the ore either alone or with some other material usually in presence of air below its fusion temperature. roasting is generally done in a reverberatory furnace or in a blast furnace. In roasting definite chemical changes like oxidation, chlorination, etc., take place while in calcination these occurs only expulsion of organic matter, water, carbon dioxide etc., i.e., it does not involve any major chemical change.
   
   The roasting process may be any one of the following types :
   1. Oxidising roasting : This is very common type of roasting in metallurgy and is carried out to remove sulphur and arsenic in the form of their volatile oxides such as SO₂ and As₂O₃ respectively. The ores are simultaneously converted into corresponding oxides. This type of roasting is generally applied in ores of lead, zinc, nickel, copper, etc.
      S + O₂ SO₂
      
      4As + 3O₂2As₂O₃
      
      2PbS + 3O₂2PbO + 2SO₂
      
      2CuS + 3O₂2Cu₂O + 2SO₂
      
      
   2. Partial oxidising or sulphating roasting : This type of roasting is carried out at a temperature below the melting point of the charge and air is admitted. Part of the sulphide ore is oxidised to sulphate and the rest is converted into oxide. For example, the roasting of galena leads to the formation of a mixture of lead oxide and lead sulphate.
      2PbS + 3O₂2PbO + 2SO₂
      
      PbS + 2O₂PbSO₄
      
      
   3. Chlorinating roasting : This type of roasting is done in the case of silver ore. The ore, argentite is mixed with common salt and the mixture is heated in the presence of air. The final product is the chloride of the metal.
      Ag₂S + 2NaCl2AgCl + Na₂S