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Leaching: Chemical Method - (Concept)

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})

Exam Chapter
MHT-CET General principles and processes of isolation of elements
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