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Metallurgy — Chemistry STD 12 — Question

Tamilnadu BoardEnglish MediumSTD 12ChemistryMetallurgy3 Marks
Question
Write a notes on alkali leaching process?
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Answer

In this method, the ore is treated with aqueous alkali to form a soluble complex. For example, bauxite, an important ore of aluminium is heated with a solution of sodium hydroxide or sodium carbonate in the temperature range $470-520 K$ at 35 atm to form soluble sodium meta-aluminate leaving behind the impurities, iron oxide and titanium oxide.
$
Al _2 O _3( S )+2 NaOH ( aq )+3 H _2 O ( l ) \rightarrow 2 Na \left[ Al ( OH )_4\right]( aq )
$

The hot solution is decanted, cooled, and diluted. This solution is neutralised by passing $CO _2$ gas, to the form hydrated $Al _2 O _2$ precipitate.
$
2 Na \left[ Al ( OH )_4\right]( aq )+ CO _2( g ) \rightarrow Al _2 O _3 \cdot xH _2 O ( s )+2 NaHCO _3( aq )
$
The precipitate is filtered off and heated around $1670 K$ to get pure alumina $Al _2 O _3$.

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