CHEMISTRY OF SODIUM CARBONATE ( Na2 CO3 ) SODA ASH

CHEMISTRY OF SODIUM CARBONATE ( Na2CO3 )
Soda Ash
INDUSTRIAL PREPARATION OF SODIUM CARBONATE
On industrial scale sodium carbonate is prepared by Ammonia Solvay Process.
Raw Materials
1. Sodium Chloride 2. Lime Stone (CaCO3) 3. Ammonia
Details of the process
At first stage a saturated solution of sodium chloride is prepared which is also known "BRINE". Composition of solution of NaCl (brine) is 28% m/m.
Steps of preparation
Ammoniation of Brine
In this stage saturated brine is allowed to flow down an ammoniating tower. This tower is fitted with mushroom shaped baffles. These baffles control the flow of brine and ensure the proper mixing and saturation of ammonia.
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Carbonation of ammoniated brine
In the second step, ammoniated brine is allowed to trickle down a carbonating tower known as solvay tower. This tower is also fitted with baffle plates. Here brine is mixed with carbon dioxide gas, produced by heating lime stone in a separate chamber called "kiln".
CaCO3 èCaO + CO2
the baffle plates ensure the flow of solution and breaks up carbon dioxide into small bubbles to produce good conditions for reaction.
Flow Chart
Chemistry of Solvay tower
CO2 reacts with ammonia to form ammonium carbonate.
2NH3 + CO2 + H2O è(NH4)2CO3
Ammonium carbonate further reacts with CO2 to form ammonium bicarbonate.
(NH4)2CO3 + CO2 + H2O è2NH4HCO3 (aq.)
Ammonium bicarbonate then react with NaCl to form sodium bicarbonate.
NH4HCO3 + NaCl èNaHCO3 + NH4Cl
Due to exothermic nature of above reactions, solubility of NaHCO3 increases. To counter this effect , lower part of Solvay tower is cooled , ppt of NaHCO3 are separated by vacuum filtration and washed to remove ammonium salts.
Conversion of NaHCO3 to Na2 CO3
Dry sodium bicarbonate is heated in rotary furnace called "CALCINER" to give anhydrous sodium carbonate or soda ash. Carbon dioxide is recirculated to carbonation tower.
2NaHCO3 èNa2 CO3 + CO2 + H2 O
Ammonia recovery process
When CaCO3 is heated, CaO is obtained along with CO2 . CaO is treated with water to form Ca(OH)2 .
CaO + H2 O èCa(OH)2
Quick lime is heated with NH4 Cl to form NH3 and calcium chloride (by product) . Ammonia is used again in this process.
2NH4 Cl + Ca(OH)2 èCaCl2 + 2NH3 + 2H2 O