Ncert Exercises & Solutions

6.13 How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.

To separate alumina from silica in bauxite ore associated with silica, first the powdered ore is digested with a concentrated NaOH solution at 473 − 523 K and 35 − 36 bar pressure. This results in the leaching out of alumina (

A I_{2} O_{3}

) as sodium aluminate and silica (

S i O_{2}

) as sodium silicate leaving the impurities behind.

$A I_{2} O_{3 (s)} + 2 N a O H_{(a q)} + 3 H_{2} O H_{(a q)} \to 2 N a {[A I {(O H)}_{4}]}_{(a q)}$
$A l u m i n a S o d i u m a l u m i n a t e$

$S i O_{2} + 2 N a O H_{(a q)} \to N a_{2} S i O_{3 (a q)} + H_{2} O_{(I)}$
$S i l i c a S o d i u m s i l i c a t e$

Then, ${CO}_{2}$ gas is passed through the resulting solution to neutralize the aluminate in the solution, which results in the precipitation of hydrated alumina. To induce precipitation, the solution is seeded with freshly prepared samples of hydrated alumina.

$2 N a {[A I {(O H)}_{4}]}_{(a q)} + C O_{2 (g)} \to A I_{2} O_{3} x H_{2} O_{(s)} + 2 N a H C O_{3 (a q)}$
$S o d i u m a l u m i n a t e H y d r a t e d a l u m i n a S o d i u m h y d r o g e n$
$c a r b o n a t e$

During this process, sodium silicate remains in the solution. The obtained hydrated alumina is filtered, dried, and heated to get back pure alumina.
$A I_{2} O_{3} x H_{2} O_{(s)} \overset{1470 K}{\to} A I_{2} O_{3 (s)} + x H_{2} O_{(g)}$
$H y d r a t e d a l u m i n a A l u m i n a$

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