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.
(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 $CO _2$.
(2019 Main, 12 April II)
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Answer:
Correct Answer: 1. (a)
Solution:
- The correct statement is “leaching of bauxite using concentrated NaOH solution gives sodium aluminate and sodium silicate”. Bauxite usually contains $\mathrm{SiO}_2$, iron oxides and titanium oxide $\left(\mathrm{TiO}_2\right)$ as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at $473-523 \mathrm{K}$ and $35-36$ bar pressure. $\mathrm{Al}_2 \mathrm{O}_3$ is leached out as sodium aluminate (and $\mathrm{SiO}_2$ too as sodium silicate) leaving the impurities behind.
$\mathrm{Al} _2 \mathrm{O} _3(s)+2 \mathrm{NaOH}(a q)+3 \mathrm{H} _2 \mathrm{O}(l) \longrightarrow 2 \mathrm{Na}[\mathrm{Al}(\mathrm{OH}) _4](a q)$
The aluminate in solution is neutralised by passing $\mathrm{CO}_2$ gas and hydrated $\mathrm{Al}_2 \mathrm{O}_3$ is precipitated. Here, the solution is seeded with freshly prepared samples of hydrated $\mathrm{Al}_2 \mathrm{O}_3$ which induces precipitation.
$ 2 \mathrm{Na}[\mathrm{Al}(\mathrm{OH})_4](a q)+\mathrm{CO}_2(g) \longrightarrow \mathrm{Al}_2 \mathrm{O}_3 \cdot x \mathrm{H}_2 \mathrm{O}(s)+2 \mathrm{NaHCO}_3(a q) $
The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure $\mathrm{Al}_2 \mathrm{O}_3$.
$ \mathrm{Al}_2 \mathrm{O}_3 \cdot x \mathrm{H}_2 \mathrm{O}(\mathrm{s}) \xrightarrow{1470 \mathrm{K}} \mathrm{Al}_2 \mathrm{O}_3(\mathrm{s})+x \mathrm{H}_2 \mathrm{O}(\mathrm{g}) $
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