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General Principles And Processes Of Isolation Of Metals

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General Principles and Processes of Isolation of Metals, PDF Notes, Important Questions and Synopsis

SYNOPSIS



METALLURGY


The compound of a metal found in nature is called a mineral. The minerals from which metals can be economically and conveniently extracted are called ores. An ore is usually contaminated with earthy or undesired materials known as gangue.

  1. Native ores contain the metal in the free state. Silver, gold and platinum occur as native ores.
  2. Oxidised ores consist of oxides or oxysalts, such as carbonates, phosphates, sulphates and silicates of metals.
  3. Sulphurised ores consist of sulphides of metals like iron, lead, zinc and mercury.
  4. Halide ores consist of halides of metals.

    Metal

    Ore

    Composition

    Aluminium

    Bauxite

    AlOX(OH)3-2X [where 0< X<1] Al2O3

     

    Diaspore

    Al2O3.H2O

     

    Corundum

    Al2O3

     

    Kaolinite (a form of clay)

    [Al2(OH)4Si2O5]

    Iron

    Haematite

    Fe2O3

     

    Magnetite

    Fe3O4

     

    Siderite

    FeCO3

     

    Iron pyrite

    FeS­2

     

    Limonite

    Fe2O3.3H2O

    Copper

    Copper pyrite

    CuFeS2

     

    Cuprite

    Cu2S

     

    Malachite

    CuCO3.Cu(OH)2

     

    Azurite

    2CuCO3.Cu(OH)2

    Zinc

    Zinc blende or sphalerite

    ZnS

     

    Calamine

    ZnCO3

     

    Zincite

    ZnO

    Lead

    Galena

    PbS

     

    Anglesite

    PbSO4

     

    Cerussite

    PbCO3

    Magnesium

    Carnallite

    KCl.MgCl2.6H2O (K2MgCl4.6H2O)

     

    Magnesite

    MgCO3

     

    Dolomite

    MgCO3,CaCO3

     

    Epsom salt (Epsomite)

    MgSO4 7H2O

     

    Langbeinite

    K2Mg2(SO4)3

    Tin

    Cassiterite (Tin stone)

    SnO2

    Silver

    Silver glance (Argentite)

    Ag2S

     

    Chlorargyrite (Horn silver)

    AgCl



    Metallurgy:

    The scientific and technological process used for the extraction/isolation of the metal from its ore.
    Isolation and extraction of metals from their ores involve the following major steps:

  1. Crushing and grinding: The ore is first crushed by jaw crushers and ground to a powder.
  2. Concentration:
    1. Hydraulic washing or gravity separation or levigation method
    2. Electromagnetic separation
    3. Froth flotation process
    4. Leaching

  3. Extraction of crude metal from concentrated ore:
    The removal of unwanted useless impurities from the ore is called dressing, concentration or benefaction of ore.
    Isolation of metals from their concentrated ores involves two major steps:

    1. Conversion to oxide:
      Calcination: Process of strongly heating the concentrated ore in a limited supply of air or in the absence of air.
      Roasting: Process of strongly heating the concentrated ore (generally sulphide ore) in excess of air or O2 below its melting point.
      Smelting: In many extraction processes, an oxide is added deliberately to combine with other impurities and form a stable molten phase which is immiscible with the molten metal called slag. This process is termed smelting.
      Slag formation: 
      2CuFeS2 + 4O2 begin mathsize 12px style rightwards arrow with blank on top end styleCu2S + 2FeO + 3SO2 
      begin mathsize 12px style table attributes columnalign left end attributes row cell Cu subscript 2 straight S text   end text plus text   end text FeO text   end text plus text   end text SiO subscript 2 rightwards arrow FeSiO subscript 3 left parenthesis Fusible text   end text slag right parenthesis text   end text plus text   end text Cu subscript 2 straight S left parenthesis matte right parenthesis end cell row cell text             end text left parenthesis roasted text   end text pyrite right parenthesis text          end text left parenthesis upper text   end text layer right parenthesis text                     end text left parenthesis lower text   end text layer right parenthesis end cell end table end style

      CaCO3 begin mathsize 12px style rightwards arrow with blank on top end styleCaO+CO2
      CaO + SiO2 begin mathsize 12px style rightwards arrow with blank on top end styleCaSiO3 (fusible slag)
      6CaO + P4O10 begin mathsize 12px style rightwards arrow with blank on top end style2Ca3(PO4 )2 (fusible slag: Thomas slag)

    2. Reduction of a metal oxide:
      The free metal is obtained by reduction of a compound using either a chemical reducing agent or electrolysis.
      Chemical reduction method:
      Reduction with carbon:
      PbO + C begin mathsize 12px style rightwards arrow with blank on top end stylePb + CO (extraction of lead)
      Reduction with CO:
      Fe2O3 + 3CObegin mathsize 12px style rightwards arrow with blank on top end style 2Fe + 3CO2

      Reduction by other metals:
      The process is known as Goldschmidt or aluminothermic process and the reaction is known as thermite reaction.
      Cr2O3 + Albegin mathsize 12px style rightwards arrow with blank on top end style 2Cr + Al2O3

      Magnesium reduction method: 

      begin mathsize 12px style table attributes columnalign left end attributes row cell TiCl subscript 4 plus 2 Mg rightwards arrow from 1000 minus 1150 degree straight C to Krollprocess of Ti plus 2 MgCl subscript 2 end cell row cell TiCl subscript 4 plus 4 Na rightwards arrow with IMI text    end text Process on top Ti plus 4 NaCl end cell end table end style

      Self-reduction method:
      This method is also called auto-reduction method or air reduction method.
      Cu2S + 3O2 begin mathsize 12px style rightwards arrow with blank on top end style3Cu2O + 2SO2
      2Cu2O + Cu2Sbegin mathsize 12px style rightwards arrow with blank on top end style 6Cu + SO2

      Electrolytic reduction:
      1. In aqueous solution: Copper and zinc are obtained by electrolysis of aqueous solution of their sulphates.
      2. In fused melts: Aluminum is obtained by electrolysis of a fused mixture of AI2O3 and cryolite Na3[AIF6].

      Extraction of aluminium: It involves the following processes:

      1. Purification of bauxite:

        1.  Bayer’s method:
          (used for red bauxite containing Fe2O3 and silicates as impurities)
          Error converting from MathML to accessible text.
        2. Hall’s method:
          (used for red bauxite containing Fe2O3 and silicates as impurities)
          begin mathsize 12px style Al subscript 2 straight O subscript 3.2 straight H subscript 2 straight O text   end text plus text   end text Na subscript 2 CO subscript 3 text   end text rightwards arrow with Fuse on top text   end text 2 NaAlO subscript 2 left parenthesis soluble right parenthesis plus CO subscript 2 plus 2 straight H subscript 2 straight O end style
          begin mathsize 12px style 2 NaAIO subscript 2 plus 3 straight H subscript 2 straight O plus CO subscript 2 rightwards arrow with 60 degree straight C on top 2 AI open parentheses OH close parentheses subscript 3 downwards arrow plus Na subscript 2 CO subscript 3
2 AI open parentheses OH close parentheses subscript 3 rightwards arrow with 1473 KK on top AI subscript 2 straight O subscript 3 plus 3 straight H subscript 2 straight O end style
        3. Serpeck’s method:
          (used for white bauxite containing silica as impurities)
          begin mathsize 12px style table attributes columnalign left end attributes row cell Al subscript 2 straight O subscript 3.2 straight H subscript 2 straight O text   end text plus text   end text 3 straight C text   end text plus text   end text straight N subscript 2 rightwards arrow from Electric text   end text furnace to 1800 text   end text to the power of straight o text   end text straight C of text   end text 2 AlN text   end text plus text   end text 3 CO text   end text plus text   end text 2 straight H subscript 2 straight O end cell row cell 2 AlN plus text   end text 3 straight H subscript 2 straight O rightwards arrow Al open parentheses OH close parentheses subscript 3 downwards arrow text   end text plus text   end text NH subscript 3 end cell row cell SiO subscript 2 text   end text plus text   end text 2 CO rightwards arrow text   end text 2 CO subscript 2 text   end text plus text   end text Si end cell end table end style
        4. Silicon volatilises at this temperature
           begin mathsize 12px style 2 Al left parenthesis OH right parenthesis subscript 3 text   end text rightwards arrow with 1473 straight K on top text   end text Al subscript 2 straight O subscript 3 text   end text plus text   end text 3 straight H subscript 2 straight O end style
      2. Electrolytic reduction (Hall–Heroult process):
        2Al2O3 + 3C begin mathsize 12px style rightwards arrow with blank on top end style4Al + 3CO2
        Cathode: Al3+ (melt) + 3ebegin mathsize 12px style rightwards arrow with blank on top end style Al(l)
        Anode: C(s) + O2- (melt) begin mathsize 12px style rightwards arrow with blank on top end styleCO(g) + 2e-
        C(s) + 2O2-(melt) begin mathsize 12px style rightwards arrow with blank on top end styleCO2(g) + Ae-

      Metallurgy of some important metals:

      1. Extraction of iron from haematite:
        Reactions involved:
        At 500–800 K (lower temperature range in the blast furnace):
        3Fe2O3 + CObegin mathsize 12px style rightwards arrow with blank on top end style 2Fe3O4 + CO2 
        Fe3O4 + CO begin mathsize 12px style rightwards arrow with blank on top end style3Fe + 4CO2
        Fe2O3 + CO begin mathsize 12px style rightwards arrow with blank on top end style2FeO + CO2
        At 900–1500 K (higher temperature range in the blast furnace):
        C + CO2 begin mathsize 12px style rightwards arrow with blank on top end style2CO; FeO + CObegin mathsize 12px style rightwards arrow with blank on top end style Fe + CO2
        Limestone is also decomposed to CaO which removes the silicate impurity of the ore as slag. The slag is in the molten state and separates out from iron.
        CaCO3 begin mathsize 12px style rightwards arrow with blank on top end styleCaO + CO2; CaO + SiO2 begin mathsize 12px style rightwards arrow with blank on top end styleCaSiO2
      2. Extraction of copper:
        From copper glance/copper pyrite (self-reduction):
        2CuFeS2 + 4O2 begin mathsize 12px style rightwards arrow with blank on top end styleCu2S + 2FeO + 3SO2
        Cu2S + FeO + SiO2 begin mathsize 12px style rightwards arrow with blank on top end styleFeSiO3 (fusible slag) + Cu2S (matte)
        2FeS + 3O2 begin mathsize 12px style rightwards arrow with blank on top end style2FeO + 2SO2; FeO + SiO2 begin mathsize 12px style rightwards arrow with blank on top end styleFeSiO3
        2Cu2O + Cu2begin mathsize 12px style rightwards arrow with blank on top end style6Cu + SO2 (self-reduction)

      3. Extraction of lead:
        begin mathsize 12px style table attributes columnalign left end attributes row cell 2 PbS left parenthesis straight s right parenthesis plus 3 straight O subscript 2 text   end text end subscript open parentheses straight g close parentheses rightwards arrow with straight capital delta on top 2 PbO left parenthesis straight s right parenthesis rightwards arrow from straight capital delta to plus straight c of 2 Pb left parenthesis calligraphic l right parenthesis plus CO subscript 2 left parenthesis straight g right parenthesis end cell row cell 3 PbS left parenthesis straight s right parenthesis rightwards arrow from air to heat text   end text ing of PbS left parenthesis straight s right parenthesis plus 2 PbS left parenthesis straight s right parenthesis rightwards arrow from absence text     end text air to Heat text     end text in of 3 Pb left parenthesis straight l right parenthesis plus SO subscript 2 left parenthesis straight g right parenthesis end cell end table end style

      4. Extraction of zinc from zinc blende:
        The ore is roasted in the presence of excess of air at a temperature of 1200 K.
        2ZnS + 3O2 begin mathsize 12px style rightwards arrow with blank on top end style2ZnO + 2SO2
        Zinc oxide is reduced using coke.

        begin mathsize 12px style ZnO plus straight C rightwards arrow with Coke comma text   end text 1673 text   end text straight K on top Zn text   end text plus CO end style

      5. Extraction of tin from cassiterite: 

        SnO2 + 2Cbegin mathsize 12px style rightwards arrow with blank on top end style Sn + 2CO
        2Fe + O2 begin mathsize 12px style rightwards arrow with blank on top end style2FeO

      6. Extraction of magnesium:
        From sea water (Dow’s process): 
        MgCl2 ⇌ Mg2+ + 2Cl
        At cathode: Mg2+ + 2e begin mathsize 12px style rightwards arrow with blank on top end styleMg (99% pure)At anode: 2Cl⎺begin mathsize 12px style rightwards arrow with blank on top end style Cl2 + 2e

      7. Extraction of gold and silver (MacArthur–Forrest cyanide process):

      1. From native ores: Extraction of gold and silver involves leaching the metal with CN-.
        4Au/Ag(s) + 8CN(aq) + 2H2O(aq) + O2(g) 4[Au/Ag(CN)2 (aq) + 4OH (aq)
        2[Au/Ag(CN)2 (aq) + Zn(s) 2Au/Ag(s) + [Zn(CN)4]2⎺ (aq)
      2. From argentite ore: 
        begin mathsize 12px style Ag subscript 2 straight S text end text open parentheses conc. text end text ore close parentheses text end text plus text end text 2 NaCN rightwards harpoon over leftwards harpoon with Air on top text end text 2 AgCN text end text plus text end text Na subscript 2 straight S end style

        4Na2S + 5O2 + 2H2begin mathsize 12px style rightwards arrow with blank on top end style2Na2SO4 + 4NaOH + 2S
        AgCN + NaCN begin mathsize 12px style rightwards arrow with blank on top end styleNa[Ag(CN)2] (soluble complex)
        2Na[Ag(CN)2] + Zn (dust) begin mathsize 12px style rightwards arrow with blank on top end style2Ag↓ + Na2[Zn(CN)4]

  4. Purification or refining of metals:
    Physical methods:

  1. Liquation process: This process is used for the purification of the metal, which itself is readily fusible, but the impurities present in it are not. It is used for the purification of Sn and Zn, and for removing Pb from Zn–Ag alloy.
  1. Fractional distillation process: This process is used to purify metals which are volatile and the impurities in them are nonvolatile or vice versa. Zn, Cd and Hg are purified by this process.
  2. Zone refining method (Fractional crystallisation method): This process is used when metals are required to have very high purity for specific applications. For example, pure Si and Ge are used in semiconductors.

Chemical methods:

  1.  Oxidative refining:
    The molten impure metal is subjected to oxidation by various ways. This method is used for refining metals such as Pb, Ag, Cu and Fe.
  2. Poling process:
    This method is used for the purification of copper and tin which contain their own oxides as impurities.
    Green wood→ Hydrocarbons →CH4
    4CuO + CH4 → 4Cu (pure metal) + CO2 + 2H2O
  3. Electrolytic refining:
    Metals such as Cu, Ni and Al are refined electrolytically.
  4. Vapour phase refining:
  1. Extraction of nickel (Mond’s process): 
    H2O(g) + C → CO(g) + H2
  2. van Arkel–de Boer process:
    begin mathsize 12px style Impure text     end text Ti plus 2 straight l subscript 2 text   end text end subscript rightwards arrow with 50 minus 250 degree straight C on top Til subscript 4 text   end text end subscript rightwards arrow from Tungsten text    end text filament to 1400 degree straight C of Ti plus 2 straight l subscript 2 end style
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