Question

How is sodium carbonate prepared?

Answer 1

Sodium carbonate is prepared by 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
1. 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.
2.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".
$CaC{O}_3\to CaO+C{O}_2$
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
$C{O}_2$ reacts with ammonia to form ammonium carbonate.
$2N{H}_3+C{O}_2+H_{2}O\to (N{H}_4{)}_{2}C{O}_3$
Ammonium carbonate further reacts with CO_2 to form ammonium bicarbonate.
$(N{H}_4{)}_{2}C{O}_3+C{O}_2+H_{2}O\to 2N{H}_{4}HC{O}_3(aq.)$
Ammonium bicarbonate then react with NaCl to form sodium bicarbonate.
$N{H}_{4}HC{O}_3+NaCl\to NaHC{O}_3+N{H}_{4}Cl$
Due to exothermic nature of above reactions, solubility of $NaHC{O}_3$ increases. To counter this effect , lower part of Solvay tower is cooled , ppt of $NaHC{O}_3$ are separated by vacuum filtration and washed to remove ammonium salts.
Conversion of $NaHC{O}_{3}toN{a}_{2}C{O}_3$
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.
$2NaHC{O}_3\to N{a}_{2}C{O}_3+C{O}_2+H_{2}O$
Ammonia recovery process
When $CaC{O}_3$ is heated, CaO is obtained along with $C{O}_2$. CaO is treated with water to form $Ca(OH{)}_2$ .
$CaO+H_{2}O\to Ca(OH{)}_2$
Quick lime is heated with $N{H}_4$ Cl to form $N{H}_3$ and calcium chloride (by product) . Ammonia is used again in this process.
$2N{H}_{4}Cl+Ca(OH{)}_2\to Ca{C}_{l2}+2N{H}_3+2H_{2}O$