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Question: Answered & Verified by Expert
In aluminium extraction by the Bayer process, alumina is extracted from bauxite by sodium hydroxide at high temperature and pressures :
$\mathrm{Al}_2 \mathrm{O}_3(\mathrm{~s})+2 \mathrm{OH}^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{Al}_2 \mathrm{O}_2(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}(1)$

Solid impurities such as $\mathrm{Fe}_2 \mathrm{O}_3$ and $\mathrm{SiO}_2$ are removed and then $\mathrm{Al}(\mathrm{OH})_4^{-}$is reprecipitated : $2 \mathrm{Al}(\mathrm{OH})_4^{-} \rightarrow \mathrm{Al}_2 \mathrm{O}_3 \cdot 3 \mathrm{H}_2 \mathrm{O}(\mathrm{s})+2 \mathrm{OH}^{-}$(aq) . In the industrial world :
ChemistryGeneral Principles and Processes of Isolation of MetalsWBJEEWBJEE 2009
Options:
  • A Carbon dioxide is added to precipitate the alumina
  • B Temperature and pressure are dropped and the supersaturated solution seeded
  • C Both (1) and (2) are practised
  • D The water is evaporated
Solution:
1329 Upvotes Verified Answer
The correct answer is: Temperature and pressure are dropped and the supersaturated solution seeded
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