- No.3. Write a detailed note on Ammonia Solvay process.
Answer. Principle of Solvay process lies in the low solubility of sodium bicarbonate at low temperature i.e. at 15°C. When CO2 is passed through an ammonical solution of NaCl called ammonical brine, only NaHCO3 precipitates.
Na+(aq) + HCO3–(aq) NaHCO3(s)
The raw materials needed for this process are cheap and easily available. They are in abundance, such as,
(i) Sodium chloride (NaCl) or brine.
(ii) Limestone (CaCOa3).
(iii) Ammonia gas (HN3)
The process consists of the following steps:
(i) Preparation of ammonical brine:
First -of all ammonical brine is prepared by dissolving ammonia gas in sodium chloride solution (brine),
(ii) Carbonation of ammonical brine:
Ammonical brine is fed into carbonating tower and carbon dioxide is passed through it. Following reactions take place in the carbonating tower.
CO2(g) + NH3(g) + H2O NH4HCO3(1)
NH4HCO3(1) + NaCl (brine) NaHCO3(1) + NH4Cl(s)
The temperature of the mixture is lowered to 15°C and precipitates of NaHCO3 are obtained.
(iii) Filtration of precipitates
The milky solution from the carbonating tower is filtered to get sodium bicarbonate. It is used as a baking soda.
(iv) Formation of sodium carbonate
Sodium carbonate is heated to get sodium carbonate.
2NaHCO3(1) ——— > Na2CO3(s) + CO2(g) + H2O(1)
CO2 is again used in tower. It is about half of CO2 needed in the process.
(v) Preparation of carbon dioxide and slaked lime:
CO2 is prepared by heating limestone in a lime kiln. Then it is carried to carbonating tower
CaO3(s) CO2(g) + CO2(g)
Quick lime (CaO) formed in lime kiln is slaked with water. Then, it is pumped to the ammonia recovery tower.
CaO(g) + H2O(1) ——— > Ca(OH)2
(vi) Ammonia recovery tower
Ammonia is recovered in this tower from ammonium chloride solution produced in the carbonated tower and calcium hydroxide formed in lime kiln.
2NH4Cl(s) + Ca(OH) 2(1) ——— 2NH3(g) + CaCl2(s) + 2H2O(1)
In fact, all ammonia is recovered in this tower and is reused in the process. There are minor losses of ammonia in the process which are compensated by using some fresh ammonia.
Flow sheet diagram of Solvay’s Process for the manufacturing of
Advantages of Solvay’s process
(i). It is a cheap process as raw materials are available at very low prices.
(ii). Carbon dioxide and ammonia are recovered and reused.
(iii). Process is pollution free, because the only waste is calcium chloride solution.
(iv). Sodium carbonate of very high purity is obtained.
(v). Consumption of fuel is very less since no solution is to be evaporated.
(vi). Pakistan is self-sufficient as far as demand of sodium carbonate is concerned.
Imperial Chemical Industries (ICI) Khewra (Distict Jhelum) is producing enough sodium carbonate. This unit was established in 1944 in Khewra because abundant raw material sodium chloride is available here. A Sindh alkali limited was established near Karachi in 1966. Sodium carbonate and sodium bicarbonate are important industrial chemicals and are used by many industries.