Maharashtra BoardEnglish MediumSTD 10ScienceP-1 Metallurgy3 Marks
Question
Describe Bayer’s process for concentration of bauxite.
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Answer
(1) Bayer's process is used to obtain pure aluminium oxide from bauxite. (2) Bauxite is then concentrated by chemical separation. Bauxite contains impurities like iron oxide $\left( Fe _2 O _3\right)$ and silica $\left( SiO _2\right)$. (3) Bauxite ore is powdered and heated with sodium hydroxide under high pressure for 2 to 8 hours at $140^{\circ} C$ in the digester. The aluminium oxide being amphoteric in nature present in bauxite reacts with sodium hydroxide to form water soluble sodium aluminate. This means that bauxite leached by sodium hydroxide. Silica reacts with sodium hydroxide to form soluble sodium silicate. The basic iron oxide $\left( Fe _2 O _3\right)$ in the gangue remains unaffected. It is separated by filtration. $ \begin{aligned} & Al _2 O _{3( s )}+2 NaOH _{( aq )} \rightarrow 2 NaAlO _{2( aq )}+ H _2 O _{(l)} \\ & \text { (From impure Sodium } \\ & \text { bauxite ore) aluminate } \\ & SiO _2+2 NaOH _{\text {(aq) }} \rightarrow Na _2 SiO _3+ H _2 O \\ & \end{aligned} $ (4) The filtrate containing sodium aluminate and sodium silicate is stirred with water and then cooling to $50^{\circ} C$. It is hydrolysed to give precipitate of aluminium hydroxide. $NaAlO _{2( aq )}+2 H _2 O _{(l)} \longrightarrow Al ( OH )_{3( s )} \downarrow+ NaOH _{( aq )}$ Aluminium hydroxide (5) Aluminium hydroxide is then filtered, washed with water, dried and then calcinated by heating at $1000^{\circ} C$ to get pure aluminium oxide. $ 2 Al ( OH )_{3( s )} \underset{1000^{\circ} C }{ C } Al _2 O _{3( s )}+3 H _2 O _{( g )} $
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