Question
Explain the difference between Bayer’s process and Hall’s process by explaining the Bayer’s process.
✓
Answer
Difference between Bayer's process and Hall's process:
In Bayer's process, aluminium ore is heated with concentrated caustic soda while in Hall's process, it is heated with aqueous sodium carbonate to obtain water-soluble sodium aluminate. In Bayer's process, the solution of sodium aluminate is diluted with water to obtain aluminium hydroxide while in Hall's process, sodium aluminate is converted to aluminium hydroxide by reaction with $CO _2$.
Bayer's process:
1. In this 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 an aqueous $NaOH$ solution and is separated by filtration. However, silica from the gangue dissolves in an aqueous $NaOH$ solution forming water-soluble sodium silicate.
4. 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{\substack{\text { Aluminium } \\ \text { hydroxide }}}{ Al ( OH )_3 \downarrow}
$
5. 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 )}} \underset{1000^{\circ} C }{\stackrel{\text { heat }}{\longrightarrow}} \underbrace{ Al _2 O _{3( s )}}_{\begin{array}{c}
\text { Aluminium } \\
\text { oxide }
\end{array}}+\underset{\begin{array}{c}
\text { Water } \\
\text { vapour }
\end{array}}{3 H _2 O _{( g )}}
$
In Bayer's process, aluminium ore is heated with concentrated caustic soda while in Hall's process, it is heated with aqueous sodium carbonate to obtain water-soluble sodium aluminate. In Bayer's process, the solution of sodium aluminate is diluted with water to obtain aluminium hydroxide while in Hall's process, sodium aluminate is converted to aluminium hydroxide by reaction with $CO _2$.
Bayer's process:
1. In this 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 an aqueous $NaOH$ solution and is separated by filtration. However, silica from the gangue dissolves in an aqueous $NaOH$ solution forming water-soluble sodium silicate.
4. 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{\substack{\text { Aluminium } \\ \text { hydroxide }}}{ Al ( OH )_3 \downarrow}
$
5. 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 )}} \underset{1000^{\circ} C }{\stackrel{\text { heat }}{\longrightarrow}} \underbrace{ Al _2 O _{3( s )}}_{\begin{array}{c}
\text { Aluminium } \\
\text { oxide }
\end{array}}+\underset{\begin{array}{c}
\text { Water } \\
\text { vapour }
\end{array}}{3 H _2 O _{( g )}}
$
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