Dechlorination
The depleted brine discharged from the membrane electrolytic cells contains free chlorine, which exists in two forms: dissolved chlorine and ClO⁻. These two forms together are referred to as free chlorine, expressed as chlorine gas.
Free chlorine in the depleted brine can corrode equipment and piping, poison the resin in the secondary brine purification system, interfere with precipitate formation during primary brine purification, and damage the ion-exchange membranes in the electrolytic cells. Therefore, it is essential to install and strictly control the dechlorination process to ensure that free chlorine is thoroughly removed before the brine is returned to the system. This is a critical step to guarantee the long-term, safe, stable, and efficient operation of the chlor-alkali plant.
1. Process Principles
Cl₂, HClO, ClO⁻, and H+ coexist in the depleted brine, and the following chemical equilibrium exists among them:
Cl₂+H₂O=HClO+H++Cl-
HClO=ClO⁻+H+
The principle of dechlorination is to disrupt this chemical equilibrium and drive the above reaction in the direction of chlorine gas generation. For the small amount of remaining free chlorine, a reducing agent is added to completely remove it.
2. Process Schemes
(1) Vacuum Dechlorination Scheme
Under vacuum and relatively high-temperature, the depleted brine is brought to a boiling state, generating water vapor. The bubbles formed carry away the chlorine gas.
(2) Air Stripping Scheme
Compressed air is introduced into the dechlorination tower, where it contacts the depleted brine on the surface of the packing material, carrying away the chlorine gas.
(3) Chemical Removal Scheme
After the above treatments, a small amount of residual free chlorine remains in the depleted brine and must be removed using Na₂SO₃.
Under alkaline conditions, SO₃2⁻ is oxidized by ClO⁻ to SO₄2⁻.
ClO⁻+SO₃2⁻→SO₄2⁻+Cl⁻
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