What is the role of `CO_(2)` in the extractive metallurgy of aluminium

1.	What is the role of `CO_(2)` in the extractive metallurgy of aluminium from its ore ?
Answer» The solution which now contains the oxides of aluminum and silicon are next treated by bubbling carbon dioxide gas through the solution . Carbon dioxide forms a weak acid solution of carbonic acid which neutralizes the sodium hydroxide from the first treatment . This neutralization selectively precipitates the aluminum oxide, but leaves the silicates in solution. Again filtration is used for the separation . After this stage the the purified aluminum oxide is heated to evaporate the water.

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(i) Arrange the following compounds in an increasing order of basic strength : `C_(6)H_(5)NH_(2), C_(6)H_(5)N(CN_(3))_(2), (C_(2)H_(5))_(2)NH and CH_(3)NH_(2)` (ii) Arrange the following compounds in a decreasing order of `pK_(b)` values: `C_(2)H_(5)NH_(2), C_(6)H_(5)NHCH_(3), (C_(2)H_(5))_(2) NH and C_(6)H_(5)NH_(2)`
Rerrange the following in an increasing order of their basic strengths: `C_(6)H_(5)NH_(2),C_(6)H_(5)N(CH_(3))_(2),(C_(6)H_(5))_(2)` NH and `CH_(3)NH_(2)`.
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What is the role of `CO_(2)` in the extractive metallurgy of aluminium from its ore ?

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