Industry Description and Practices

There are three basics process for the manufacture of chlorine and caustic soda from brine: the mercury cell, the diaphragm cell, and the membrane cell. The membrane cell is the most modern and has economic and environmental advantages. The two other processes generated hazardous Wastes (containing mercury or asbestos).

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda. The depleted brine is dechlorinated and recycled to the input stage. The membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances if non-asbestos diaphragms are used. The energy consumption in a membrane cell process is of the order of 2,200-2,500 kiloWatt tons per metric ton (kWh/t) of chlorine for a diaphragm cell process.

Waste Characterisatics

The major waste stream from the process is the "brine muds", which are the sludges from the brine purification step and which may contain magnesium, calcium, iron, and other metal hydroxides, depending on the source and purity of the brines. The muds are normally filtered or settled, the supernatant is then recycled and the mud is dried and landfilled.

Chlorine is a highly toxic gas, and strict precautions are necessary to minimise risk to workers and possible released during its handling. Major sources of fugitive air emissions of chlorine and hydrogen are vents, seals, and transfer operations.

Acid and caustic wastewaters are generated in both the process and the materials recovery stages.

Pollutions Prevention and Control

The following pollution prevention measures should be considered:

a) Use metal rather than graphite anodes to reduce lead and chlorinated oganics.
b) Resaturate brine in closed vessels to reduce the generation of salt sprays.
c) Use noncontact condensers to reduce process wastewater quantities
d) Scrub chlorine tail gases to reduce chlorine discharge and to produce hypochlorite.
e) Recycle condensates and waste process water to the brine system, if possible.

For the chlor-alkali industry, an Emergency Preparedness and Response Plan is required for potential uncontrolled chlorine and other releases. Carbon tetrachloride is sometimes used to scrub nitrogen trichloride (formed in the process) and maintain its levels below 4 Percent to avoid explosion. However, substitute for carbon tetrachloride may have to be used as the use of carbon tetrachloride may be banned globally.

Treatment Technologies

Caustic scrubber systems should be installed to control chlorine emissions from condensers and at storage and transfer points for liquid chlorine. Sulfuric acid used for drying chlorine should be neturalized before discharge.

Brine muds should be discharged to lined settling ponds (or equivalent) to prevent contamination of soil and ground water. Effluents should be controlled for pH by neutralization. Settling and filtration are performed to control total suspended solids/ Dechlorination of wastewaters is performed using sulfur dioxide or bisulfite.

Key issues

The following box summarises the key production and control practices that will lead to compliance with emissions guidelines.

1. Give preference to the membrane process.
2. Adopt the following pollution prevention measures to minimize emissions.
3. Use metal instead of graphite anodes.
4. Resaturate brine in closed vessels.
5. Recycle brine wastes.
6. Scrub chlorine from tail gases to produce hypochlorite
7. Provide lined settling ponds for brine muds.