Physical properties :
Sodium carbonate (Na2CO3) also known as soda ash, is a sodium salt of carbonic acid.
- Molecular weight :105.978 gm/mole
- Appearance :White crystalline solid
- Odour :Odourless
- Boiling point :1633ºC, Melting point :851ºC
- Solubility :Soluble in water
- Na2CO3.10H2O is known as washing soda
Solvay or ammonia soda process :
Raw materials :
- 1 ton sodium carbonate
- Salt →1550 kg (sea water)
- Limestone→1200 kg (mineral calcite or aragonite)
- Coke →90 kg
- Ammonia as a catalyst→1.5 kg
- CaCO3 → CaO + CO2 ΔH = +43.4kcals
- C(s) + O2 (g) → CO2 (g) ΔH = –96.5kcals
- CaO(s) + H2O (l) → Ca(OH)2 (aq) ΔH = –15.9kcals
- NH3(aq) + H2O(l) → NH4OH(aq) ΔH = –8.4kcals
- 2NH4OH + CO2 → (NH4)2CO3 + H2O ΔH = –22.1kcals
- (NH4)2CO3 + CO2 + H2O → 2NH4HCO3
- NH4HCO3 + NaCl → NH4Cl + NaHCO3
- 2NaHCO3 → Na2CO3 + CO2 + H2O ΔH = +30.7kcals
- 2NH4Cl + Ca(OH)2 → 2NH3 + CaCl2 + 2H2 ΔH = +10.7kcals
Overall reaction :
- CaCO3 + 2NaCl → Na2CO3 + CaCl2
Preparation and purification of brine :
- Saturated solution of NaCl is used.
- Brine contains impurities such as calcium, magnesium and iron compounds.
- To remove calcium sulfate, magnesium and iron salts sodium carbonate and sodium hydroxide are added.
- The precipitated carbonates and hydroxide are removed by filtration.
- Sometimes sulfate are removed with BaCl2 or the hot brine is treated with OH¯ and CO3−2 ions.
- The calcium, magnesium and iron salts from saturated brine may be precipitated by dilute ammonia and CO2 in a series of washing towers.
- The brine is purified by allowing it to settle in vats, as a result of which precipitated CaCO3, MgCO3, Mg(OH)2 and iron hydroxide settle down and pure brine solution is pumped to the ammonia absorber tower, where it dissolve NH3 with the liberation of heat.
Ammoniation of brine :
- The purified brine is allowed to percolate down the ammonia tower in which ammonia gas is passed through the bottom in a counter current fashion.
- The brine solution thus takes up the necessary amount of ammonia and liberates heat.
- The gas which escapes solution in the tank is absorbed by the brine falling down the tower.
- Some carbon dioxide is also absorbed by ammonia, as a result of which some insoluble carbonate is also precipitated.
- The ammoniated brine is allowed to settle, cooled to about 30°C and pumped to the carbonating tower.
Carbon dioxide formation :
- Limestone is calcined to get CO2 in a lime kiln filled with coke.
- As a result of burning of coke necessary heat required for the decomposition of lime stone is generated.
- CaO obtained from the lime kiln is converted into slaked lime and pumped to the ammonia recovery tower.
Carbonation of ammonium brine :
- CO2 from the lime kiln is compressed and passed through the bottom of carbonating tower down which ammoniated brine percolates.
- Carbonating towers operated in series with several precipitation towers are constructed of cast iron having 22–25 m height, 1.6–2.5 m in diameter.
- During the precipitation cycle, the temperature is maintained about 20-25°C at the both ends and 45-55°C at the middle by making use of cooling coils.
- The tower gradually becomes flooded as sodium bicarbonate cakes on the cooling coils and shelves.
- The cooling coils of the foulded tower are shut off.
- Then the fresh hot ammoniated brine is fed down the tower in which NaHCO3 are dissolved to form ammonium carbonate solution.
- The solution containing (NH4)2CO3, unconverted NaHCO3 is allowed to fall down a second tower, called making tower.
- The making, towers are constructed with a series of boxes and sloped baffles.
- Ammoniated brine and CO2 gas (90-95%) from the bicarbonate calciner is recompressed and pumped to the bottom of the making tower.
- The ammonium carbonate first reacts with CO2 to form ammonium bicarbonate and the latter reacting with salt, forms sodium bicarbonate.
- The heat of exothermic reaction is removed by cooling coils.
- NaHCO3 slurry is then filtered on a rotary vacuum filter which helps in drying of bicarbonate and in recovering ammonia.
- The filter cake after removal of salt and NH4Cl by washing with water,sent to a centrifugal filter to remove the moisture or calcined directly.
- During washing, about 10% NaHCO3 also passes into filtrate.
- The filtrate containing NaCl, NH4Cl, NaHCO3 and NH4HCO3 is treated with lime obtained from lime kiln to recover NH3 and CO2.
- NaHCO3 from the drum filter is calcined at about 200°C in a horizontal calciner, which is either fired at feed end by gas or steam heated unit.
- The heating being through the shell parallel to the product, which prevent the formation of bicarbonate lumps.
- The hot soda ash form the calciner is passed through a rotary cooler and packed in bags.
- The exit gases (CO2, NH3, steam etc.) are cooled and condensed to get liquid ammonia; the rich CO2 gas is cooled and returned to the carbonating tower.
- The product from the calciner is light soda ash.
- To produce dense soda ash, sufficient water is milled with it to form more mono hydrate Na2CO3.H2O and the mixture is recycled.
Recovery of ammonia :
- The NH3 is recovered in strong NH3 liquor still, consisting of two sections.
- The parts above and below the lime inlet is called as heater and lime still,respectively.
- The filtrate obtained from washing of NaHCO3 from the pressure type rotary filter is fed into the heater, where free NH3 and CO2 are driven off by distillation.
- Dry lime or milk of lime (slaked lime) obtained from lime kiln is fed through the lime inlet and mixed with the liquor from the heater.
- As the liquor flows down the column, calcium chloride and calcium sulfate are formed and NH3 gas is released.
- NH4Cl + Ca(OH)2 -> CaCl2 + 2NH3 + H2O
- (NH4)2SO4 + Ca(OH)2 -> CaSO4 + 2NH3 + 2H2O
- The liquor from the bottom of the lime still is free from ammonia and contains unreacted NaCl and largely CaCI2, which is disposed off.
- The liquor is, therefore allowed to settle in settling ponds and the clear liquid is evaporated till the salt separates out and is sold as such for calcium chloride or further evaporated.