Caustic soda available in solid flakes and liquid solution(different concentrations) are produced by chlor-alkali chemical companies. Caustic soda production done by any one of the following manufacturing process:
1. Electrolytic processes:
- Electrolysis of brine by diaphragm electrolytic cell
- Electrolysis of brine by mercury electrolytic cell
- Membrane process
2. Chlorine processes:
- HCl – air oxidation process
- HCl –air- Cl2 process (oxychlorination process)
- HNO3- NaCl – air process
3. Lime soda process
Let’s consider caustic soda production in continuous causticising plant by lime soda process:
This process does not use any cell technology but by simple reaction between calcium hydroxide and sodium carbonate. Calcium hydroxide is called milk of lime and sodium carbonate as soda ash, hence the process is named as lime soda process.
Chemical reaction that takes place in caustic soda production:
Na2CO3 + Ca(OH2) ↔ 2NaOH + CaCO3
The heat of reaction of the above reaction is 2100 calories, soda ash with concentration 20% is added to the dissolving tank which is mixed with recycle stream of weak liquor obtained from the rotary filter. The solution is mixed with slaked lime in required proportions and for series of causticizers which act like CSTR equipped with agitators. Causticizers are heated to 80-900C with injection of steam and air is used for additional agitation; the operation is carried for 2-3 hours till the equilibrium is attained. This process is called causticising process and the bottom of the CSTR (continuous stirred tank reactor) is fed to first Dorr thickener which is tank equipped with less rpm thickener blade.
The stream is fed in the center of the thickener and slurry outlet from bottom is fed to washing and over flow of the thickener is fed back for recycle it contains about 10-11% NaOH which sent to triple effect vacuum evaporator for concentrating it to 50%. The sludge which settles at the bottom of the thickener is washed and again sent to second thickener and finally the sludge from the second thickener is sent to rotary drum vacuum filter to remove CaCO3 in solid form and weak liquor is sent to starting dissolving tank. Sludge is mostly contains only calcium carbonate due to regular wash. NaOH obtained from overflow of the thickener is concentrated to the required concentrations.
Consumption pattern and uses:
Caustic soda flakes and solution are used in production of consumer chemicals and products of organic chemicals, textile industry, paper and pulp, alumina, soap and detergents, inorganic chemicals, and dyes.
Drawback in caustic soda production by lime soda process: Due to high cost of sodium carbonate this process is not encouraged in large production scale. For instant preparation in small scale this process was practiced.