Monday, 8 April 2013

Extraction of Aluminium

Extraction of Aluminium

Aluminium Atomic Number: 13
Aluminium Electronic configuration: [Ne]3s23p1

Aluminium being very reactive does not occur free in nature. Aluminium is obtained from bauxide ore.

The bauxide ore is generally contaminated with ferric oxide and silica. The extraction of aluminium from bauxide ore involves the following three steps.
  1. Concentration of the ore
  2. Electrolytic reduction of Al2O3
  3. Electrolytic purification of aluminium
  1. Concentration of bauxite ore:
    Following methods are used for concentrating the bauxide ore:

    1. Baeyer's process: This method is mainly applied when ferric oxide is present as chief impurity. The crushed ore is first heated at moderate temperature so as to convert ferrous oxide into ferric oxide. It is then digested with concentrated solution of sodium hydroxide in which aluminium oxide dissolves forming soluble sodium meta aluminate while ferric oxide and silica remains insoluble and settle down.

      Al2O3.2H2O + 2NaOH  2NaAlO2 + 3H2O

      The filtrate containing sodium meta aluminate solution is agitated with freshly precipitated Al(OH)3 when sodium meta aluminate undergoes  hydrolysis to precipitate Al(OH)3 when sodium meta aluminate undergoes hydrolysis to precipitate Al(OH)3.

      NaAlO2 + 2H2 NaOH + Al(OH)3

      The precipitate is washed, dried and heated to get Al2O3.

      2Al(OH)3   Al2O3 + 3H2O
    2. Hall's process: Bauxide ore is fused with sodium carbonate, Al2O3 combines with sodium carbonate forming sodium meta aluminate. The fused mass is extracted with water where Fe2Oand SiO2 remains insoluble in the residue.

      Al2O+ Na2CO 2NaAlO+ CO2

      The solution containing sodium meta aluminate is warmed to 50-60oC and CO2 is circulated through it. Al(OH)3separates out as precipitate.

      2NaAlO2 + CO2 + 3H2 2Al (OH)3 + Na2CO3

      The precipitate is filtered, washed, dried and ignited to get Al2O3.
    3. Serpeck's process: This process is used when silica is present in considerable amount of bauxide ore. The ore is mixed with coke and heated to 1800C in presence of nitrogen, when aluminium nitride is forms.

      Al2O3 + 3C +N 2AIN + 3CO

      Silica is reduced to silicon and volatilises off at this temperature

      SiO+2C  Si + 2CO

      AIN is hydrolysed with water into aluminium hydroxide.

      AIN + 3H2 Al (OH)3 + NH3

      The precipitate is filtered, washed dried and ignited to get Al2O3.
  2. Electrolytic Reduction of Pure Alumina
    The electrolysis of pure alumina involves two difficulties.

    1. Pure alumina is a pure conductor of electricity.
    2. The fusion of temperature of pure alumina is about 2000oC and the metal formed vaporises at this temperature. (boiling point of Al is 1800C).
    To remove this difficulties, cryolite (Na3AlF6) and Fluorspar (CaF2) is added which reduced the fusion temperature to 900C and makes ir good conductor of electricity. The electrolysis is carried out in an iron tank lined inside with carbon which acts as cathode. The anode consists of carbon rod dipped in the fused mixture of electrolyte. The fused electrolyte is covered with a layer of coke.

    On passing current, aluminium is discharged at cathode. Aluminium being heavier than electrolyte settles down in the bottom and is periodically tapped out. Oxygen is liberated at anode. It attacks carbon rods forming CO and CO2.

    AlF3 from cryolite ionised as

    AlF3   Al3+ + 3F-

    At cathode: Al3+ + 3e-  Al

    At anode: 2F-  F2 + 2e-

    The liberated fluorine reacts with alumina to form AlF3 and O2.

    Al2O3 + 3F2  2AlF3 + 3/2O2

    2C + O2  2CO

    C + O2  CO2
  3. Refining of Aluminium by Hoope's Electrolytic Method
    The electrolytic cells consists of an iron box lined inside with carbon. The cell consists of three layers which differ in specific gravity.

    1. The upper layer is of pure aluminium which acts as cathode.
    2. The middle layer consists of a mixture of the fluorides of Al, Ba and Na.
    3. The lowest layer consists of impure aluminium which acts as anode.

    On electrolysis, aluminium is deposited at cathode from the middle layer and an equivalent amount of aluminium is taken up by the middle layer from the bottom layer (impure aluminium). Therefore aluminium is transferred from bottom to the top layer through middle layer while the impurities are left behind.

Extraction of Lead

Extraction of Lead

Lead Atomic Number: 82
Lead Electronic configuration: [Xe]4f144d106s26p2

Lead is extracted from galena (PbS) involving the following steps:
  1. Concentration:
    The crushed, powdered ore is concentrated by the froth floatation process.
  2. Reduction:
    The concentrated ore is reduced in two steps:
    1. Roasting:
      The concentrated ore is roasted in reverberatory furnace, where the ore is partially oxidised.

      2PbS + 3O2  2PbO + 2SO2

      PbS + 2O PbSO4
    2. Smelting: After roasting, the temperature of furnace is raised and air supply is reduced. Some more concentrated galena is added. PbS reacts with PbO and PbSO4 forming lead metal.

      2PbO + PbS  3Pb + SO2

      PbSO+ PbS  2Pb = 2SO2

      Molten lead is drawn off from the lower part of the furnace.
    3. Purification:
      Lead obtained from furnace still contains Bi, Sb, Cu, Fe, Ag, etc. as impurity. These impurities can be removed by the following processes.

      1. Softening process: The impure metal is melted on the hearth reveberatory furnace in a current of air. The base metals are oxidised and float over the surface of the molten mass as scum, which is removed.
      2. Desilverisation: The removal of silver is done by Parke's process or Pattinson's process.
      3. Electrolytic refining: The desilverised lead is further purified by electrolytic method.
        On passing current, pure lead is deposited on the cathode. The less electropositive metals like Ag, Au, Sb etc. collect below anode as anodic mud while more electropositive impurities like Zn, Fe, Ni etc. go into the solution.

Extraction of Magnesium

Extraction of Magnesium

Magnesium Atomic Number: 12
Magnesium Electronic configuration: [Ne]3s2

Magnesium is highly electropositive and strong reducing agent. Therefore it is extracted by electrolysis of their fused metal halides containing alkali metal halides. The function of alkali metal halides is
  1. To lower the operating temperature.
  2. To increase the electrical conductivity.
  1. From Magnesite: (MgCO3)
    The magnesite ore is calcinated into magnesium oxide.

    MgCO3  MgO + CO2

    Magnesium from magnesium oxide is obtained either by thermal reduction or electrolytic reduction. The magnesium oxide is dissolved in a mixture of molten fluorides of magnesium, bairum and sodium. The electrolysis is done by using carbon rods as anodes suspended in molten mass and cast iron rods as cathodes at 650oC. The magnesium is obtained in molten state.
  2. By Electrolysis of anhydrous magnesium chloride:

    Step 1
    : Preparation of anhydrous MgCl2
    1. From carnallite: (KCl.MgCl2.6H2O)
      Carnallite can not be dehydrated by heating alone. Because the water of crystallization reacts chemically with magnesium chloride to form oxychloride and finally to magnesium oxide.

      MgCl2.6H2O MgCl2.2H2O Mg2OCl2 MgO

      So, carnallite is heated in a current of HCL gas and air.

      MgCl2.6H2O    MgCl2 + 6H2O
    2. From MgO:
      Chlorine is passed over red hot mixture of MgO and carbon.

      MgO + C + Cl2    MgCl2 + CO
    3. From Dow's process: Sea water containing magnesium compound is treated with calcium hydroxide when magnesium hydroxide get precipitated. It is dissolved in dil. HCL. The solution on concentration gives hydrated magnesium chloride, which is hydrated as

Electrolysis of MgCl2

Anhydrous magnesium chloride is fused with NaCl and anhydrous calcium chloride in the ratio of 35% (MgCl2); 50% (NaCl) and 15% (CaCl2) to lower the fusion temperature and to increase the conductivity. The mixture is electrolysed at 700oC in presence of an inert gas to protect it from reaction of N2, O2 and CO2 present in air.

Magnesium is discharged at cathode. Magnesium being lighter than the fused mass floats on the surface from where it is removed periodically.

Extraction of Silver

Extraction of Silver

Silver Atomic Number: 47
Silver Electronic configuration: [Kr]3d104s1

Silver occurs in native state as well as in combined state. Silver is extracted from Argentite ore (Ag2S).

Cyanide process:
This process is also called Mac Arthur Forest Process. The process involves the following steps:
  1. Concentration of ore:
    The crushed and fine powdered ore is concentrated by froth flotation process.
  2. Cyanidation:
    The finally powdered concentrated ore is treated with dilute solution (0.4 to 6%) of sodium cyanide in presence of a current of air to form sodium argentocyanide.

    Ag2S + 4NaCN  2Na[Ag(CN)2)] + Na2S

    The air blown in converts sodium sulphide to sodium sulphate thus shifting the equilibrium to right side.

    4Na2S + 502 +2H2 2Na2SO4 + 4NaOH + 2S

    The soluble sodium argentocyanide is separated out by filtration.
  3. Recovery of Silver:
    Zinc powder in finely divided state is added to clear filtrate to precipitate Ag as a dark amorphous mass.

    2Na[Ag(CN)2] + Zn  Na2[Zn(CN)4] + 2Ag
  4. Refining:
    Silver obtained by the above method is contaminated with impurities of lead, copper and gold. Impurities are removed by:

    Electrolytic Refining
    : Pure silver is obtained by the electrolysis of AgNO3 solution containing 10% HNO3 and using the impure silver as anode and pure silver plate as cathode. During electrolysis copper passes into solution as copper nitrate while gold collects as anodic mud. Silver is deposited at cathode.

    Anode : Ag  Ag+ + e-

    Cathode : Ag+ + e-  Ag

    Note: Same process can be applied for gold as well.

Extraction of Zinc

Extraction of Zinc

Ores:Zinc Blende, ZnS
 Calamine, ZnCO3
 Zincite, ZnO
  1. Concentration:
    Powdered Zinc Blende is concentrated by froth floatation process.
  2. Roasting:

    2ZnS + 3O2      2ZnO + 2SO2
  3. Reduction:

    ZnO + C  Zn + CO
  4. Purification:
    Impure Zinc obtained above contain impurities of Pb, Fe, Cd, As etc. This impure Zinc is known as spelter. Firstly, Cd is distilled off at 800C and then electrolytic purification is done using impure Zn as anode and cathode consists of sheets of pure Al. Pure Zn is scrapped off from Al-sheets.

Extraction of Tin

Extraction of Tin

Atomic Number: 50
Electronic configuration: [Kr]4d105s25p2)

Tin is extracted from cassiterite ore. The extraction involves the following steps:
  1. Concentration: The crushed and powdered ore is washed with running water, which removes lighter siliceous matter. Wolframite is removed by electromagnetic separator.
  2. Roasting: S and As are removed as their volatile oxides. Iron and copper pyrites are converted into their oxides and sulphates
  3. Washing: This ore is then washed with hot water to dissolve out copper sulphate and iron sulphate. The lighter ferric oxide is washed off while the concentrated tin stone sinks to the bottom. It is called black tin.
  4. Smelting: Black tin is mixed with carbon and smelted in a reverberatory furnace. The ore is reduced to the metallic state while SiOis removed as slag.

    SnO2 + 2C   Sn + 2CO

    CaCO3  CaO + CO2

    CaO + SiO2 CaSiO3 (Slag)

    Molten metal is cast into ingots or blocks. It is known as block tin and contains about 99.5% of metallic tin.
  5. Purification:
    The metal is purified by:

    1. Liquation: The easily fusible tin melts away and less fusible impurities (Fe, W, S, As) are left behind.
    2. Electrolytic refining: In this method, impure tin is made anode and a thin sheet of pure tin is made the cathode. Electrolyte consist of H2SiF6, tin sulphate and sulphuric acid. On passing current, tin is dissolved from anode and gets deposited on the cathode.

Blast Furnace

Blast Furnace
The smelted ore is introduced in the furnace by lowering the cup and cone arrangement and at the same time furnace is lit and a blast of hot air is sent upwards through the Tuyeres. On the basis of variation in the temperature, there are four zones where different chemical changes occur.
  1. Combustion zone: this is lowest part of furnace where the temperature is about 1500 - 1600 C.

    C + O2   CO2H = -97 kcals

    CO2 + C  2CO
  2. Reduction zone:

    2Fe2O3 + CO   2Fe3O4 + CO2

    Fe3O4 + CO   3FeO + CO2

    Fe + CO   Fe + CO2

    Iron formed is called sponge iron. In this reduction zone heat is also evolved due to which CO partially decomposes.

    2CO  CO2 +  C
  3. Slag formation zone: This is the central zone where the temperature varies from 800-1000 C.

    CaCO3  CaO + CO2

    Impurities in the ore are reduced to Si, P and Mn, respectively.

    P4O10 + 10C  4P + 10CO

    SiO2 + 2C  Si + 2CO

    MnO2 + 2C  Mn + 2CO

    These are partly absorbed by iron and partly by slag.
  4. Zone of fusion: This zone is above combustion zone. The temperature ranges between 1200-1500oC. The spongy iron melts at 1300 C and collects at the bottom of hearth. The slag being lighter floats over the molten iron and prevents oxidation of molten metal. The slag and molten metal are removed from their respective holes. The molten metals is run into moulds and is allowed to solidify.

    Iron obtained from the blast furnace is called pig-iron. It contains 93% iron, 5% carbon and rest silicon. The pig-iron is remelted and cast into moulds. This is known as cast iron.

There are three commercial varities of iron depending on their carbon content.
  1. Cast iron - most impure form, containing carbon from 2.5 to 5%.
  2. Wrought iron - purest form of iron, containing carbon from 0.25% to 0.3%.
  3. Steel - most important commercial variety of iron, containing carbon from 0.25 to 2%