Discuss the preparation and properties of sodium carbonate.

Explanation

(i) Sodium carbonate is prepared by ammonia soda (or) Solvay process.
Raw materials : The raw materials required are (1) Brine solution (saturated) (2) Ammonia and (3) Lime stone.
Principle: Brine solution is saturated with Ammonia and `CO_(2)` gas is passed through it. Then sodium bicarbonate is formed.
`NH_(3) + H_(2)O + CO_(2) to NH_(4)HCO_(3)`
`NH_(4)HCO_(3) + NaCl to NaHCO_(3)+ NH_(4)Cl`
The sodium bicarbonate thus formed on heating decompose to give sodium carbonate.
`2NaHCO_(3) to Na_(2)CO_(3) + H_(2)O + CO_(2)`
Process:
(1) Saturation of Brine with ammonia : Ammonia absorber is filled with brine solution and saturated with ammonia gas containing a little amount of `CO_(2)`. Then the calcium and magnesium impurities present in the brine, precipitate as their corbonates and hydroxides.
`2NH_(3) + H_(2)O + CO_(2) to (NH_4)_(2) CO_(3)`
`MgCl_(2) + (NH_4)_(2)CO_(3) to MgCO_(3) + 2NH_(4)Cl`
The precipitates are filtered in "Filter press" and the filtrate is cooled and sent to Carbonation Tower.
(2) Carbonation of ammoniacal Brine : This process is carried out in Carbonation tower. It is a tall cylindrical tower containing perforated plates arranged one above the other. Ammonical brine solution is dropped from upper half and `CO_(2)` gas is passed from the lower part. Then both of them reacts together forming sodium bicarbonate.
`NH_(3) + H_(2)O + CO_(2) to NH_(4)HCO_(3)`
`NaCl + NH_(4)HCO_(3) to NaHCO_(3) + NH_(4)Cl`
(3) Filtration : The dense liquid from the carbonation tower is sent into rotary vacuum filter and filtered.
(4) Fusion of sodium bicarbonate : The `NaHCO_(3)` obtained in the Rotary filter is heated to high temperatures. Then `NaHCO_(3)` decomposes to give `Na_(2)CO_(3)`.
`2NaHCO_(3) overset(Delta)(rarr) Na_(2)CO_(3) + H_(2)O + CO_(2)`
(5) Recovery of Ammonia : The filtrate from the Vacuum filter is pumped into ammonia recovery tower, mixed with `Ca(OH)_(2)` and heated with steam. Then `NH_(3)` gas is liberated which is sent back to saturation tower.
`NH_(3)HCO_(3) overset(Delta)(rarr) NH_(3) + H_(2)O + CO_(2)`
`2NH_(4)Cl + Ca(OH)_(2) overset(Delta)(rarr) CaCl_(2) + 2NH_(3) + 2H_(2)O`
Note: The milk of lime required for the regeneration of `NH_(3)` is obtained from the Lime Kiln by heating lime stone.
(ii) Chemical properties of `Na_(2)CO_(3)`.