Extraction of Aluminium :
(a) `text(Ores)` :
(i) Oxides : Bauxite `Al_2O_3*2H_2O` (chief); Diaspore `Al_2O_3*H_2O`; Corundum `Al_2O_3`
(ii) Silicates : Felspar `KAl Si_3O_8`; Mica `K_2O*3Al_2O_3*6SiO_2*2H_2O`; Kaolinite `Al_2O_3*2SiO_2*2H_2O`
(iii) Fluorides : Cryolite `Na_3 Al F _6`
(b) `text(Process)` :
(i) `text(Purification of Bauxite)` :
(x) `text(Baeyer's Method)`
If `Fe_2O_3` is major impurity- Red bauxite
`-`ore is roasted to convert ferrous oxide to ferric oxide
`-Al_2O_3*2H_2O + 2NaOH + H_2O -> 2Na[Al(OH)_4]` (`Fe_2O_3` undissolved)
`- NaAl(OH)_4 -> NaOH + undersettext(white ppt.)[Al(OH)_3]`
`- 2Al(OH)_3 -> Al_2O_3 + 3H_2O`
(y) `text(Halls' method)`
lf `Fe_2O_3` is major impurity - Red bauxite
- bauxite ore is fused with `Na_2CO_3`
`- Al_2O_3*2H_2O + Na_2CO_3 oversettext(Fuse)-> 2NaAlO_2 + CO_2 uparrow + 2H_2O`
`- 2NaAlO_2 + 3H_2O + CO_2 overset(50^oC )-> 2Al(OH)_3 downarrow + Na_2CO_3`
`- 2Al(OH)_3 overset(Delta)-> Al_2O_3 + 3H_2O`
(z) `text(Serpeck's method)` : See fig.1.
(i) lf `SiO_2` is major impurity - white bauxite
`- Al_2O_3 + 3C + N_2 overset(1000^oC)-> 2AlN + 3CO`
`- AlN + 3H_2O -> Al(OH)_3 + NH_3`
`- SiO_2 + 2CO -> 2CO_2 uparrow + Si`
`- 2Al(OH)_3 -> Al_2O_3 + 3H_2O`
(ii) Electrolysis of fused Alumina.
Cathode : Iron-tank lined with carbon bricks
Anode : carbon
`text(Electrolyte)` : Molten [`Al_2O_3` (`5%`) `+` `Na_3 AlF_6` (`85%`) `+ CaF_2` (5%) `+ AlF_3` (`5%`)]
`O_2` is liberated at anode and `Al` collects at the bottom.
(iii) Reactions : `underset(AlF_3)(Na_3AlF_6) -> underset[Al^(3+) + 3F^(-)](3NaF +AlF_3)`
At anode : `Al_2O_3 + 6F^(-) -> 2Al F_3 + 3//2 O_2 + 6e^(-)`
At cathode : `2Al^(3+) + 6e^(-) -> 2Al`
(iv) Electrolytic refining (Hoope's process), three layers process : See fig.2.
Cathode : carbon electrodes
Anode : `Fe` tank lined with carbon bricks
Electrolyte : `Na_3AlF_6 + CaF_2`
Bottom layer : Impure aluminium consists of `Cu`, `Si` etc. in molten state.
Middle layer : molten mixture of Fluorides of `Na`, `Ba`, `Al` and `Al_2O_3`
Top layer : pure molten aluminium.
On passing the current, `Al` is deposited at cathode from the middle layer and an equivalent amount of `Al` from the bottom layer moves into the middle layer leaving behind the impurities.
(i) Oxides : Bauxite `Al_2O_3*2H_2O` (chief); Diaspore `Al_2O_3*H_2O`; Corundum `Al_2O_3`
(ii) Silicates : Felspar `KAl Si_3O_8`; Mica `K_2O*3Al_2O_3*6SiO_2*2H_2O`; Kaolinite `Al_2O_3*2SiO_2*2H_2O`
(iii) Fluorides : Cryolite `Na_3 Al F _6`
(b) `text(Process)` :
(i) `text(Purification of Bauxite)` :
(x) `text(Baeyer's Method)`
If `Fe_2O_3` is major impurity- Red bauxite
`-`ore is roasted to convert ferrous oxide to ferric oxide
`-Al_2O_3*2H_2O + 2NaOH + H_2O -> 2Na[Al(OH)_4]` (`Fe_2O_3` undissolved)
`- NaAl(OH)_4 -> NaOH + undersettext(white ppt.)[Al(OH)_3]`
`- 2Al(OH)_3 -> Al_2O_3 + 3H_2O`
(y) `text(Halls' method)`
lf `Fe_2O_3` is major impurity - Red bauxite
- bauxite ore is fused with `Na_2CO_3`
`- Al_2O_3*2H_2O + Na_2CO_3 oversettext(Fuse)-> 2NaAlO_2 + CO_2 uparrow + 2H_2O`
`- 2NaAlO_2 + 3H_2O + CO_2 overset(50^oC )-> 2Al(OH)_3 downarrow + Na_2CO_3`
`- 2Al(OH)_3 overset(Delta)-> Al_2O_3 + 3H_2O`
(z) `text(Serpeck's method)` : See fig.1.
(i) lf `SiO_2` is major impurity - white bauxite
`- Al_2O_3 + 3C + N_2 overset(1000^oC)-> 2AlN + 3CO`
`- AlN + 3H_2O -> Al(OH)_3 + NH_3`
`- SiO_2 + 2CO -> 2CO_2 uparrow + Si`
`- 2Al(OH)_3 -> Al_2O_3 + 3H_2O`
(ii) Electrolysis of fused Alumina.
Cathode : Iron-tank lined with carbon bricks
Anode : carbon
`text(Electrolyte)` : Molten [`Al_2O_3` (`5%`) `+` `Na_3 AlF_6` (`85%`) `+ CaF_2` (5%) `+ AlF_3` (`5%`)]
`O_2` is liberated at anode and `Al` collects at the bottom.
(iii) Reactions : `underset(AlF_3)(Na_3AlF_6) -> underset[Al^(3+) + 3F^(-)](3NaF +AlF_3)`
At anode : `Al_2O_3 + 6F^(-) -> 2Al F_3 + 3//2 O_2 + 6e^(-)`
At cathode : `2Al^(3+) + 6e^(-) -> 2Al`
(iv) Electrolytic refining (Hoope's process), three layers process : See fig.2.
Cathode : carbon electrodes
Anode : `Fe` tank lined with carbon bricks
Electrolyte : `Na_3AlF_6 + CaF_2`
Bottom layer : Impure aluminium consists of `Cu`, `Si` etc. in molten state.
Middle layer : molten mixture of Fluorides of `Na`, `Ba`, `Al` and `Al_2O_3`
Top layer : pure molten aluminium.
On passing the current, `Al` is deposited at cathode from the middle layer and an equivalent amount of `Al` from the bottom layer moves into the middle layer leaving behind the impurities.