Extraction of Aluminium

ChemistryGeneral Principles And Processes Of Isolation Of ElementsFor JEE aspirants

Important ores of Aluminium

Aluminium: $[_{13} A l^{27}, e l ec t r o n e g a t i v i t y 1.5, 1 s^{2}, 2 s^{2}, 2 p^{6}, 3 s^{2}, 3 p^{1}]$

Ores:

$∙$ Corundum $(A l_{2} O_{3})$

$∙$ Bauxite $(A l_{2} O_{3} .2 H_{2} O), G ibb s i t e (A l_{2} O_{3} .3 H_{2} O) D ia s p or e (A l_{2} O_{3} . H_{2} O)$

$∙$ Cryolite $(A l F_{3} .3 N a F)$ or Na₃AlF₆

$∙$ Alunite $[K_{2} S O_{4} . A l_{2} (S O_{4})_{3} .4 A l (O H)_{3}]$

$∙$ Spinel $(M g O . A l_{2} O_{3})$

$∙$ Felspar $(K_{2} O . A l_{2} O_{3} .6 S i O_{3})$ or KAlSi₃O₈

$∙$ China Clay or Kaoline $[A l_{2} O_{3} .2 S i O_{2} .2 H_{2} O]$

Extraction of aluminium is shown stepwise in the following flow sheet:

Purification of Bauxite:

By Bayer's process commercially it is being carried out (for red bauxite not for the white bauxite). In other words this process is applied to the bauxite ore containing ferric oxide as chief impurity.

Flow sheet of Bayer's process for the preparation of pure $A l_{2} O_{3}$ :

Diagram being restored — will be back shortly

Reaction of the above flow chart are as follows:

Crude bauxite is heated at $1100^{o} C w i t h N a_{2} C O_{3},$

$A l_{2} O_{3} + N a_{2} C O_{3} 2 N a A l O_{2} + C O_{2} ↑$

$F e_{2} O_{3} + N a_{2} C O_{3} 2 N a F e O_{2} + C O_{2} ↑$

$S i O_{2} + N a_{2} C O_{3} N a_{2} S i O_{3} + C O_{2} ↑$

$C a O + S i O_{2} C a S i O_{3}$

Then at $50^{o} - 60^{o} C C O_{2}$ is formed through $N a A l O_{2}$ solution and produces thereby $A l (O H)_{3}$

$2 N a A l O_{2} + C O_{2} + 2 H_{2} O 2 A l (O H)_{3} ↓ + N a_{2} C O_{3}$

$A l (O H)_{3} 100^{o} C A l_{2} O_{3} + 3 H_{2} O ↑$

Electrolytic reduction of $A l_{2} O_{3}$

Electrolysis of molten mixture:

Cathode : Carbon

Anode : Graphite rods

Electrolyte : 60 parts cryolite + 20 parts fluorspar + 20 parts pure $A l_{2} O_{3}$

Temperature : $900^{o} C$

Reactions:

According to the $1^{s t}$ theory the following reaction occurs, $A l_{2} O_{3} 2 A l^{+ 3} + 3 O^{2 -}$

At cathode: $2 A l^{+ 3} + 6 e 2 A l$ , At anode : $3 O^{- 2} - 6 e \frac{3}{2} O_{2} ↑$

As cryolite has greater electrochemical stability it does not dissociate. It only increases the dissociation of $A l_{2} O_{3} .$ But the second theory states that, cryolite undergoes electrolytic dissociation first then $A l^{+ 3}$ goes to the cathode, produced $F_{2}$ at anode then reacts with $A l_{2} O_{3} p r o d u ces A l F_{3} .$ $A l F_{3} .3 N a F A l^{+ 3} + 3 N a^{+} + 6 F^{-}$

At cathode: $A l^{+ 3} + 3 e A l$

At anode: $6 F^{-} - 6 e 3 F_{2}$ , $2 A l_{2} O_{3} + 6 F_{2} 4 A l F_{3} + 3 O_{2} ↑$

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