Leaching Process MCQ - Practice Questions & Answers

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Leaching: Chemical Method

In this method, the powdered ore is treated with a suitable chemical reagent which dissolves the ore while impurities remain insoluble in that reagent.
For example:
Bauxite is separated from Fe₂O₃, SiO₂, TiO₂, with the help of NaOH in which Al₂O₃ gets dissolved while rest are insoluble.
For example:

$\mathrm{Al}_{2} \mathrm{O}_{3}(\mathrm{s})+2 \mathrm{NaOH}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{Na}\left[\mathrm{Al(OH)}_{4}\right](\mathrm{aq})$
The sodium aluminate present in solution is neutralised by passing CO₂ gas and hydrated Al₂O₃ is precipitated. At this stage, small amount of freshly prepared sample of hydrated Al₂O₃ is added to the solution. This is called seeding. It induces the precipitation.

$2 \mathrm{Na}\left[\mathrm{Al}(\mathrm{OH})_{4}\right](\mathrm{aq})+\mathrm{CO}_{2}(\mathrm{g}) \rightarrow \mathrm{Al}_{2} \mathrm{O}_{3}.\mathrm{xH}_{2} \mathrm{O}(\mathrm{s})+2 \mathrm{NaHCO}_{3}(\mathrm{aq})$

Sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al₂O₃.

$\mathrm{Al}_{2} \mathrm{O}_{3}.\mathrm{xH}_{2} \mathrm{O}(\mathrm{s}) \stackrel{1470 \mathrm{K}}{\longrightarrow} \mathrm{Al}_{2} \mathrm{O}_{3}(\mathrm{s})+\mathrm{xH}_{2} \mathrm{O}(\mathrm{g})$