General Extraction of Metals

The metals can be extracted from its ores by using different convenient ways. It involves the various processes given below.
1. Concentration: The removal of gangue particles from the powdered ores is called concentration. It is done by:

  • Gravity seperation: It is the methods based in densities of ore and impurities. It is usually applied for the non-sulphide ores like oxide and carbonate. Example: SnO2, Fe2O3 etc.
  • Froth Floatation process: It is the process of seperating hydrophobic (that does not gets attracted with water) materials from hydrophilic (that are attracted by water) materials. It is done for the sulphide ores. Example: ZnS, CuS etc.
  • Electromagnetic seperation method: Ores having magnetic impurities are concentrated by this process.
  • Leaching: Ores are chemically concentrated in leaching process. This is the modern method for concentration of minerals.

2. Calcination: It is the process of conversion of carbonate, hydrated ore into their respective oxide by heating either in the absence of air or in a limited supply of air. It is done in reverberatory furnace.
$$\ce{SnO2.XH20 ->SnO2 + XH2O}$$
$$\ce{ZnCO3 ->ZnO + CO2}$$

Note: In calcination: Moisture is removed, organic matter is destroyed and CO2 is removed.

3. Roasting: The conversion of ores into their metallic oxide in the presence of air is roasting. It is done in sulphide ore. Calcination are carried out for sulphide ore.
$$\ce{PbS ->PbSO4 + PbO}$$
$$\ce{PbS + 2PbO ->3Pb + SO2}$$
$$\ce{PbS + PbSO4-> 2Pb + 2SO2}$$

4. Reductions reactions: Metals are purified by using the suitable reduction reactions.

  •  Reduction with carbon:
    $$\ce{SnO2 + 2C->Sn + 2CO}$$
    $$\ce{Fe2O3 + 3C->2Fe + 3CO}$$
  •  Reduction with aluminium:
    $$\ce{Cr2O3 + 2Al->2Cr + Al2O3}$$
    $$\ce{3Mn3O4 + 8Al->9Mn + 4Al2O3}$$
  •  Reduction with electrolysis:
    $$\ce{NaCl ->Na^+ + Cl^-}$$
    At cathode: $$\ce{Na^+ + e^–>Na}$$
    At anode: $$\ce{2Cl^-2e ->Cl2}$$
  • Reduction with precipitation:
    $$\ce{2Na[Ag(CN)2] + Zn->Na^2[Zn(CN)4] + 2Ag}$$
  • Reduction by water gas:
    $$\ce{2NiO + CO + H2 -> 2Ni + CO2 + H2O}$$

5. Refining: The final method of concentration of metals is refining. Refining process includes:

  • Liquation: This process is used only when the impurities are less fusible than the metal to be purified.
  • Distillation: This process is applied for the purification of volatile metals such as zinc and mercury.
  • Fractional Crystallization: In this process, impure metal rod is heated with the help of circular heater at one end. The metal melts and on cooling the pure metal gets solidified while impurities pass on into the molten state. This process is repeated several times to get pure metal.
  • Pooling: In this method, the molten metal is stirred with green poles of wood, which liberates gas like methane. This reduces any oxides present in the metal.
  • Electrolytic refining: The blocks of impure metal from anode and pure metal form cathode and anode. Aqueous solution of appropriate salt is electrolyzed. The pure metal will be deposited at cathode.
  • Vapour -phase refining: Metal is removed as volatile compound which is then decomposed by heating to get pure substance.
  • Mond’s Process: Nickel is purified by this process.
  • Lian Arkes Process: Elements like zirconium, Hf, Ti, Si are refined by this process.
  • Cupellation: It is used to purify silver containing lead as impurity.

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