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Metallurgy — Chemistry STD 10 — Question

ICSE BoardEnglish MediumSTD 10ChemistryMetallurgy5 Marks
Question
How is ore purified (give equations also)
✓

Answer

Purification of aluminium ore is done by the Baeyers process.
Baeyers process involves the following steps:
1. Bauxite is crushed to a fine powder.
2. The powdered bauxite is then treated with a strong solution of sodium hydroxide.
3. The mixture is then heated under pressure to $150^{\circ} C$ to $250^{\circ} C$ for about 30 minutes. The heat and the pressure cause the alumina to dissolve in the sodium hydroxide to form sodium aluminate.
$
Al _2 O _3 \cdot 2 H _2 O +2 NaOH \longrightarrow 2 NaAlO _2+3 H _2 O
$
Alumina being amphoteric dissolves in sodium hydroxide.
4. The sodium aluminate solution is then filtered to separate the impurities.
5. The solution of sodium aluminate is then cooled lightly and sent into tanks called precipitators.
6. Crystals of aluminium hydroxide are then added when most of the sodium aluminate undergoes hydrolysis to precipitate insoluble aluminium hydroxide.
$
NaAIO _2+2 H _2 O \longrightarrow NaOH + Al ( OH )_3
$
7. The solid aluminium hydroxide is separated by filtration.
8. The solid aluminium hydroxide is then washed and again filtered and dried, and then heated to about $1100^{\circ} C$ to $1200^{\circ} C$, when aluminium hydroxide decomposes to form aluminium oxide.
$
2 Al ( OH )_3 \longrightarrow Al _2 O _3+3 H _2 O
$

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