Give the chemistry of Solvay process for the manufacture of `Na_(2)CO_(3)`.

Explanation

Sodium carbonate or soda ash `(Na_(2)CO_(3))`: The process used for the manufacture of sodium carbonate is called Solvay ammonia process.
Theory: When `CO_(2)` is passed through brine solution (aqueous solution of NaCl) saturated with ammonia, sodium bi-carbonate is formed.
`NH_(3)+CO_(2)+H_(2)O to underset("ammonium bicarbonate")(NH_(4)HCO_(3))`
`NH_(4)HCO_(3)+underset(("Brine"))(NaCl) to underset("ppt. of sodium bicarbonate")(NaHCO_(3)+NH_(4)Cl)`
These precipitates on heating give sodium carbonate.
`2NaHCO_(3) overset(Delta)toNa_(2)CO_(3)+CO_(2)+H_(2)O`
Process: The steps involved in solvay ammonia process are described below:
Step I: Ammoniation tower
A saturated solution of brine is introduced from the top of the tower which is made up of iron. brine solution gets saturated with ammonia which comes from the ammonia recovery tower. impurities of brine solution get precipitated as their corresponding insoluble carbonates.
`2NH_(3)+CO_(2)+H_(2)O to (NH_(4))_(2)CO_(3)`
`CaCl_(2)+(NH_(4))_(2)CO_(3) to underset(("white ppt."))(CaCO_(3))+2NH_(4)Cl`
Step II: Carbonation tower
It is made up of iron and is fitted with number of perforated horizontal plates. the ammoniated brine solution is introduced from the top of the tower and `CO_(2)` is introduced rom the lime kiln into the bottom of the tower.
`NaCl+NH_(3)+CO_(2)+H_(2)O to NaHCO_(3)(s)+NH_(4)Cl(aq)`
Temperature of 300-310 K is maintained in the carbonation tower.
Step III: Filtration : The solution coming out of the carbonation tower contains crystals of sodium bicarbonate. these are filtered by passing through vacuum filters.
Step IV: Calcination of sodium bicarbonate : Sodium bicarbonate `(NaHCO_(3))` is heated in the absence of air. sodium carbonate `(Na_(2)CO_(3))` is formed.
`2NaHCO_(3) overset(Delta)toNa_(2)CO_(3)+CO_(2)+H_(2)O`
Step V: Ammonia recovery tower: The filtrate obtained from carbonation tower also contains ammonium chloride and a little ammonium bicarbonate. a ammonium bicarbonate is decomposed by steam and ammonium chloride by slaked lime as follows:
`NH_(4)HCO_(3) overset(Delta)toNH_(3)+CO_(2)+H_(2)O`
`2NH_(4)Cl+Ca(OH)_(2) overset(Delta)to2NH_(3)+CaCl_(2)+2H_(2)O`
These two gases `NH_(3) and CO_(2)` are pumped up back into the ammoniation tower.