Extraction of Metals - Result Question 1
####1. The correct statement is
(a) leaching of bauxite using concentrated $NaOH$ solution gives sodium aluminate and sodium silicate. (2019 Main, 12 April II)
(b) the hall-heroult process is used for the production of aluminium and iron.
(c) pig iron is obtained from cast iron.
(d) the blistered appearance of copper during the metallurgical process is due to the evolution of $CO _2$.
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Solution:
- The correct statement is “leaching of bauxite using concentrated $NaOH$ solution gives sodium aluminate and sodium silicate”. Bauxite usually contains $SiO _2$, iron oxides and titanium oxide $\left(TiO _2\right)$ as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of $NaOH$ at $473-523 K$ and $35-36$ bar pressure. $Al _2 O _3$ is leached out as sodium aluminate (and $SiO _2$ too as sodium silicate) leaving the impurities behind.
$Al _2 O _3(s)+2 NaOH(a q)+3 H _2 O(l) \longrightarrow 2 Na\left[Al(OH) _4\right](a q)$
The aluminate in solution is neutralised by passing $CO _2$ gas and hydrated $Al _2 O _3$ is precipitated. Here, the solution is seeded with freshly prepared samples of hydrated $Al _2 O _3$ which induces precipitation.
$$ 2 Na\left[Al(OH) _4\right](a q)+CO _2(g) \longrightarrow $$
$$ Al _2 O _3 \cdot x H _2 O(s)+2 NaHCO _3(a q) $$
The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure $Al _2 O _3$.
$$ Al _2 O _3 \cdot x H _2 O(s) \stackrel{1470 K}{\longrightarrow} Al _2 O _3(s)+x H _2 O(g) $$
