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Question

Which of the following reactions does not take place during leaching for concentration of bauxite?


A
Al2O3+2NaOH+3H2O2Na[Al(OH)4]
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B
2Na[Al(OH)4]+CO2Al2O3.xH2O+2NaHCO3
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C
Al2O3.2H2OΔAl2O3+2H2O
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D
Al2O3.xH2OΔAl2O3+xH2O
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Solution

The correct option is C $$Al_2O_3.2H_2O\xrightarrow{{\Delta}}Al_2O_3+2H_2O$$
  • Bauxite is the principal ore of aluminium.  The impurities present in bauxite are $$SiO_2$$, iron oxides and titanium oxide ($$TiO_2$$). 
  • Concentration is carried out by heating the powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. This process is called digestion. 
  •  $$Al_2O_3$$ is extracted out as sodium aluminate. The impurity, $$SiO_2$$ too dissolves forming sodium silicate. Other impurities are left behind.
$$Al_2O_3+2NaOH+3H_2O \rightarrow 2Na[Al(OH)_4]$$
  • The sodium aluminate present in solution is neutralised by passing $$CO_2$$ gas and hydrated $$Al_2O_3$$ is precipitated. At this stage, small amount of freshly prepared sample of hydrated$$Al_2O_3$$ is added to the solution. This is called seeding. It induces the precipitation.
$$2Na[Al(OH)_4]+CO_2 \rightarrow Al_2O_3.xH_2O+2NaHCO_3$$
  • Sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure$$Al_2O_3$$.
$$Al_2O_3.xH_2O \rightarrow Al_2O_3 + xH_2O$$
  • Hence, option C is correct answer.

Chemistry

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