Question

What is the role of concentrated naoh in leaching of alumina from bauxite ?

Answer 1

The separation of the alumina from the impurities in the bauxite ore is usually accomplished by the Bayer Process. In this process the ground bauxite is leached with caustic soda (NaOH) to produce soluble sodium aluminate (NaAlO2), leaving the impurities in the insoluble residue.

Answer 2

NaOH dissolves the ore and leave the impurities .

Explanation:

Leaching is the process of treating powdered ore with a suitable reagent which can selectively dissolves the ore and leave the impurities .

Bauxite ore usually contains impurities of iron oxide , silica  and titanium oxide . The powdered ore is heated with a concentrated solution of $NaOH$ at 473-523 K. The alumina dissolves in

$Al_2O_3(s)+2NaOH(aq)+3H_2O(l)\rightarrow 2Na[Al(OH)_4](aq)$

$2Na[Al(OH)_4](aq)+2CO_2(g)\rightarrow Al_2O_3.xH_2O(s)+2NaHCO_3(aq)$

$Al_2O_3.xH_2O(s)\rightarrow Al_2O_3(s)+xH_2O(g)$

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