Boron Family : Aluminium and its compounds

 

Aluminium and its compounds

(1)     Ores of Aluminium : Bauxite (Al2O3. 2H2O), Cryolite (Na3AlF6, Felspar (KAlSi3O8), Kaolinite (Al2O3. 2SiO2. 2H2O), , Mica (K2O. 3Al2O3. 6SiO2. 2H2O, Corundum (Al2O3), Diaspore (Al2O3. H2O), Alunite or alum stone [K2SO4. Al2(SO4)3. 4Al(OH3].

(2)     Extraction : Aluminium is obtained by the electrolysis of the oxide (alumina) dissolved in fused cryolite. This involves following steps,

Purification of ore

(i)      Baeyer’s process

\underset { Finely\quad powdered\quad (red) }{ Bauxite } \xrightarrow [ FeO\rightarrow F{ e }_{ 2 }O_{ 3 } ]{ Roasted\quad } \quad Roasted\quad ore\quad \quad

 

\xrightarrow [ High\quad pressure\quad (15{ 0 }^{ 0 }C,\quad 80\quad atm)\quad filtered,\quad F{ e }_{ 2 }O_{ 3 }\quad as\quad residue ]{ +\quad Caustic\quad soda\quad solution }

 

\underset { (Sod.\quad Aluminate) }{ Filtrate } \quad \xrightarrow [ C{ O }_{ 2 } ]{ Filtrate\quad } \quad Pure\quad Al_{ 2 }{ O }_{ 3 }\quad \underrightarrow { \quad Heat\quad \quad } Al(OH{ ) }_{ 3 }

 

(ii)     Hall’s process

\underset { Finely\quad powdered\quad (red) }{ Bauxite } \quad \xrightarrow [ Fused,\quad extracted\quad with\quad water.\quad Residue\quad F{ e }_{ 2 }O_{ 3 } ]{ +\quad N{ a }_{ 2 }C{ O }_{ 3 } } \quad \quad

 

Precipitate\quad Al(OH{ ) }_{ 3 }\quad \underrightarrow { \quad Heat\quad \quad } \quad Pure\quad Al_{ 2 }{ O }_{ 3 }\quad

 

(iii)    Serpek’s process

\underset { Finely\quad powdered\quad (white) }{ Bauxite } \quad \xrightarrow [ Heated\quad to\quad 180{ 0 }^{ 0 }C ]{ +Coke+{ N }_{ 2 }\quad } \quad

{ Silica\quad reduce\quad to\quad Si\quad \\ which\quad volatalises }\quad +\quad \quad { aluminium\quad form\\ aluminium\quad nitride }

AlN\quad \underrightarrow { \quad Hydrolysis\quad } \quad Pure\quad Al_{ 2 }{ O }_{ 3 }\quad \underrightarrow { \quad Heated\quad } Al(OH{ ) }_{ 3 }       

 

(iv)    Hall and Heroult process : It is used for extraction of aluminium. In this process a fused mixture of alumina (20%), cryolite (60%) and fluorspar (20%) is electrolysed using carbon electrodes. Whereas cryolite makes  conducting fluorspar decreases  the m.pt. of alumina.

Note  :    Aluminium is refined by Hoope’s electrolytic process.

 

(3)     Compounds of Aluminium

(i)      Aluminium oxide or Alumina (Al2O3) : It occurs in nature as colourless corundum and several coloured minerals like ruby (red), topaz (yellow), Sapphire (blue), amethyst (violet) and emerald (green). These minerals are used as precious stones (gems).

(ii)     Aluminium chloride (Al2Cl6) :  It is prepared by passing dry chlorine over aluminium powder.

                   Al2O3 + 3C + 3Cl2 → 2AlCl3 + 3CO(g)

                                                            (anhydrous)

It exists as dimer Al2Cl6, in inert organic solvents and in vapour state. It sublimes at 100oC under vacuum. Dimeric structure disappears when AlCl3 is dissolved in water.  It is hygroscopic in nature and absorbs moisture when exposed to air.

(iii)    Thermite : A mixture of aluminium powder and Fe2O3 in the ratio 1:3. It is used for welding of iron. The reaction between  and Fe2O3 is highly exothermic, Al + Fe2O3 → Al2O3 + Fe + Heat

(iv)    Aluminium sulphate [Al2(SO4)3] : It is used for the preparation of alums e.g., potash alum Al2(SO4)3. K2SO4. 24H2O. It is also used for making fire proof clothes.