MAKING OF SODIUM CARBONATE BY SOLVARY PROCESS
Sodium carbonate is known as washing soda or soda ash. It is commonly occurred as a crystalline heptahydrate which readily effloresces to form a white powder, the monohydrate.
Pure sodium carbonate is a white, odorless powder that is hygroscopic means it absorbs moisture from air. It has an alkaline taste and forms a strongly alkaline water solution. It is produced by solvary process by using common salt (sodium chloride), ammonia and limestone.
The solvary process centered on a large hollow tower. At the bottom limestone was heated to release carbon dioxide.
The steps in solvary process:
- Brine purification:
Brine is concentrated by evaporation. Impurities such as calcium, magnesium and iron are removed by precipitation.
Ca+2(aq) + CO3-2(aq) ——–à CaCO3(s)
Mg+2(aq) + 2OH–(aq) ——àmg(OH)2(s)
Fe+3(aq) + 3OH–(aq) ——àFe(OH)3(s)
After removing the impurities Brine solution is then filtered and passed through an ammonium tower to dissolve ammonia.
- Important point to remember:
It is an exothermic process, so the ammonia is cooled.
- Formation of sodium bicarbonate:
Carbon dioxide is produced by the thermal decomposition of limestone.
Carbon dioxide is bubbled through the ammoniated Brine Solution in the carbonating tower.
The carbon dioxide dissolves to form a weak acid:
The ammonia in the brine reacts with H+ to form Ammonium ion.
The bicarbonate ion reacts with Na+ to form sodium Hydrogen carbonate (sodium bicarbonate).
Sodium bicarbonate precipitates
- Formation of sodium carbonate:
The suspended sodium hydrogen carbonate is removed from the carbonating tower and heated at 300°C to produce sodium carbonate.
This CO2 is recycled back into the carbonating tower.
- Ammonia recovery:
Calcium oxide which was formed as a byproduct of the thermal decomposition of limestone in the lime kiln, react with water to form calcium hydroxide
CaO(s)+ H2O(l)——–à> Ca(OH)2 (aq)
This calcium hydroxide reacts with ammonium chloride separated out of the carbonating tower by filtration.
Ca(OH)2+2NH4Cl—-à CaCl2+2H2O+ 2NH3
The ammonia is recycled back into the process to form ammoniated brime.
Calcium chloride is formed as a by-product of the solvey process.
Properties of sodium carbonate:
- At room temperature, sodium carbonate (Na2CO3) is an odorless, grayish white powder which is hygroscopic. This means when it is exposed to air, it can spontaneously absorb water molecules.
- Sodium carbonate has a melting point of 1,564°F (851°C), a density of 2.53 g/cm3, and is soluble in water.
- Anhydrous (without water) sodium carbonate can absorb various amounts of water and form hydrates which have slightly different characteristics.
Uses of sodium carbonate:
- It is use for glass making.
- It is used for paper making.
- It is used for production of baking soda.
- It acts as water softening agent.