Extraction of Aluminium

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.

    Mechanism
    :
    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.

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