Question
Explain Bayer’s process.
✓
Answer
1. In Bayer's process, the bauxite ore is first crushed and then it is leached by heating with hot concentrated caustic soda $( NaOH )$ solution under high pressure for 2 to $8 hrs$ at 140 to $150^{\circ} C$ in a tank called a digester.
2. Aluminium oxide, being amphoteric in nature, dissolves in an aqueous $NaOH$ solution, forming water-soluble sodium aluminate.
$
\underset{\substack{\text { Aluminium } \\ \text { oxide }}}{ Al _2 O _3 \cdot 2 H _2 O _{( s )}}+\underset{\substack{\text { Sodium } \\ \text { hydroxide }}}{2 NaOH _{( aq )}} \longrightarrow \underset{\substack{\text { Sodium } \\ \text { aluminate }}}{2 NaAlO _{2( aq )}}+\underset{\text { Water }}{3 H _2 O _{( l )}}
$
3. The iron oxide present in the gangue does not dissolve in aqueous $NaOH$ solution and is separated by filtration.
4. However, silica from the gangue dissolves in aqueous $NaOH$ solution forming water-soluble sodium silicate.
5. The solution of sodium aluminate is diluted with water and then cooled to $50^{\circ} C$. This gives aluminium hydroxide as a precipitate.
$
\underset{\substack{\text { Sodium } \\ \text { aluminate }}}{ NaAlO _{2( aq )}}+\underset{\text { Water }}{2 H _2 O _{( l )}} \longrightarrow \underset{\substack{\text { Sodium } \\ \text { hydroxide }}}{ NaOH _{( aq )}}+\underset{\text { hydroxide }}{ Al ( OH )_3 \downarrow}
$
6. The aluminium hydroxide is then filtered, washed, dried and calcined by heating at $1000^{\circ} C$ to get pure aluminium oxide, called alumina.
$
\underset{\substack{\text { Aluminium } \\ \text { hydroxide }}}{2 Al ( OH )_{3( s )}} \stackrel{\text { heat }}{1000^{\circ} C } \underset{\substack{\text { Aluminium } \\ \text { oxide }}}{ Al _2 O _{3( s )}}+\underset{\substack{\text { Water } \\ \text { vapour }}}{3 H _2 O _{( g )}}
$
2. Aluminium oxide, being amphoteric in nature, dissolves in an aqueous $NaOH$ solution, forming water-soluble sodium aluminate.
$
\underset{\substack{\text { Aluminium } \\ \text { oxide }}}{ Al _2 O _3 \cdot 2 H _2 O _{( s )}}+\underset{\substack{\text { Sodium } \\ \text { hydroxide }}}{2 NaOH _{( aq )}} \longrightarrow \underset{\substack{\text { Sodium } \\ \text { aluminate }}}{2 NaAlO _{2( aq )}}+\underset{\text { Water }}{3 H _2 O _{( l )}}
$
3. The iron oxide present in the gangue does not dissolve in aqueous $NaOH$ solution and is separated by filtration.
4. However, silica from the gangue dissolves in aqueous $NaOH$ solution forming water-soluble sodium silicate.
5. The solution of sodium aluminate is diluted with water and then cooled to $50^{\circ} C$. This gives aluminium hydroxide as a precipitate.
$
\underset{\substack{\text { Sodium } \\ \text { aluminate }}}{ NaAlO _{2( aq )}}+\underset{\text { Water }}{2 H _2 O _{( l )}} \longrightarrow \underset{\substack{\text { Sodium } \\ \text { hydroxide }}}{ NaOH _{( aq )}}+\underset{\text { hydroxide }}{ Al ( OH )_3 \downarrow}
$
6. The aluminium hydroxide is then filtered, washed, dried and calcined by heating at $1000^{\circ} C$ to get pure aluminium oxide, called alumina.
$
\underset{\substack{\text { Aluminium } \\ \text { hydroxide }}}{2 Al ( OH )_{3( s )}} \stackrel{\text { heat }}{1000^{\circ} C } \underset{\substack{\text { Aluminium } \\ \text { oxide }}}{ Al _2 O _{3( s )}}+\underset{\substack{\text { Water } \\ \text { vapour }}}{3 H _2 O _{( g )}}
$
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