Question

explain the method of obtaining Pure aluminium from Alumina​

Answer 1

Answer:

HOPE IT  HELPS YOU :)

Explanation:

Alumina produced by concentration of bauxite is mixed with cryolite and subjected to electrolysis. Electricity is passed through this mixture, it gets heated, cryolite melts and alumina dissolves in it. Electricity acts as reducing agent here. The alumina gets dissociated into aluminium cations and oxygen anions.

Answer 2

Explanation:

Concentration of bauxite ore:

Bauxite is the main ore of aluminium. Silica (SiO2

), ferric oxide (Fe2

O3

) and titanium

oxide (TiO2

) are the impurities present in bauxite. Separation of these impurities is done

by leaching process using either Bayer’s method or Hall’s method. In both these methods

finally the concentrated alumina is obtained by calcination.

In the Bayer’s process the ore is first ground in a ball mill. Then it is leached by heating

with concentrated solution of caustic soda (NaOH) at 140 to 150 0

C under high pressure

for 2 to 8 hours in a digester. Aluminium oxide being amphoteric in nature, it reacts with

the aqueous solution of sodium hydroxide to form water soluble sodium aluminate. This

means that bauxite is leached by sodium hydroxide solution.

Al2

O3 2H2O (s) + 2 NaOH (aq) -----> 2NaAlO2(aq)+ 3H2O (l)

The iron oxide in the gangue does not dissolve in aqueous sodium hydroxide. It is

separated by filtration. However, silica in the gangue dissolves in aqueous sodium

hydroxide to form water soluble sodium silicate.

Aqueous sodium aluminate is diluted by putting in water and is cooled to 50 0

C. This

results in precipitation of aluminium hydroxide.

NaAlO2

+ 2H2

O NaOH + Al(OH)3

In the Hall’s process the ore is powdered and then leached by heating with aqueous

sodium carbonate in the digester to form water soluble sodium aluminate. Then the

insoluble impurities are filtered out. The filtrate is warmed and neutralised by passing

carbon dioxide gas through it. This results in the precipitation of aluminium hydroxide.

ii. Electrolytic reduction of alumina

a. In this method electrolysis

of molten mixture of

alumina (melting point > 2000 0C) is done in a steel tank. The tank has a graphite lining on theinner side. This lining does the work of acathode. A set of graphite rods dipped in the

molten electrolyte works as anode. Cryolite (Na3AlF6) and fluorspar (CaF2)are added in themixture to lower its melting point upto 1000 0C

b. Aluminium is deposited on the cathode on

passing electric current. Molten aluminium being

heavier than the electrolyte, is collected at the bottom

of the tank. It is taken out from there from time to time,

Oxygen gas is liberated at the anode.