Chlor-Alkali Plants

Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998

Chlor-Alkali Plants

Industry Description and Practices Major sources of fugitive air emissions of chlorine and hydrogen are vents, seals, and transfer There are three basic processes for the manu- operations. Acid and caustic wastewaters are facture of chlorine and caustic soda from brine: generated in both the process and the materials the mercury cell, the diaphragm cell, and the recovery stages. membrane cell. The membrane cell is the most modern and has economic and environmen- Pollution Prevention and Control tal advantages. The other two processes gen- erate hazardous wastes containing mercury or The following pollution prevention measures asbestos. should be considered: In the membrane process, the chlorine (at the • Use metal rather than graphite anodes to re- anode) and the hydrogen (at the cathode) are kept duce lead and chlorinated organics. apart by a selective polymer membrane that al- • Resaturate brine in closed vessels to reduce the lows the sodium ions to pass into the cathodic generation of salt sprays. compartment and react with the hydroxyl ions • Use noncontact condensers to reduce the to form caustic soda. The depleted brine is amount of process wastewater. dechlorinated and recycled to the input stage. • Scrub chlorine tail gases to reduce chlorine The membrane cell process is the preferred pro- discharges and to produce hypochlorite. cess for new plants. Diaphragm processes may • Recycle condensates and waste process water be acceptable, in some circumstances, if to the brine system, if possible. nonasbestos diaphragms are used. The energy • Recycle brine wastes, if possible. consumption in a membrane cell process is of the order of 2,200–2,500 kilowatt-hours per metric For the chlor-alkali industry, an emergency ton (kWh/t), as against 2,400–2,700 kWh/t of preparedness and response plan is required for chlorine for a diaphragm cell process. The World potential uncontrolled chlorine and other re- Bank does not finance mercury cell technology. leases. Carbon tetrachloride is sometimes used to scrub nitrogen trichloride (formed in the pro- Waste Characteristics cess) and to maintain its levels below 4% to avoid explosion. Substitutes for carbon tetrachloride The major waste stream from the process con- may have to be used, as the use of carbon tetra- sists of brine muds—the sludges from the brine chloride may be banned in the near future. purification step—which may contain magne- sium, calcium, iron, and other metal hydroxides, Target Pollution Loads depending on the source and purity of the brines. The muds are normally filtered or settled, the Implementation of cleaner production pro- supernatant is recycled, and the mud is dried and cesses and pollution prevention measures can landfilled. yield both economic and environmental ben- Chlorine is a highly toxic gas, and strict pre- efits. The production-related targets presented cautions are necessary to minimize risk to work- in Table 1 can be achieved by measures such as ers and possible releases during its handling. those described above. The numbers relate to

279 280 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES

Table 1. Target Levels per Unit of Production, achieved by well-designed, well-operated, and Chlor-Alkali Industry well-maintained pollution control systems. (maximum load/ton chlorine) The guidelines are expressed as concentrations Diaphragm Membrane to facilitate monitoring. Dilution of air emissions Parameter process process or effluents to achieve these guidelines is un- acceptable. Lead (kg) 0.04 — All of the maximum levels should be achieved Wastewater for at least 95% of the time that the plant or unit (cubic meters) 1.6 0.1 is operating, to be calculated as a proportion of — Not applicable. annual operating hours.

Air Emissions the production processes before the addition of pollution control measures. Chlorine concentration should be less than 3 milligrams per normal cubic meter (mg/Nm3) for Treatment Technologies process areas, including chlorine liquefaction.

Caustic scrubber systems should be installed to Liquid Effluents control chlorine emissions from condensers and at storage and transfer points for liquid chlorine. For membrane cell effluents, pH levels should Sulfuric acid used for drying chlorine should be be in the range 6–9. neutralized before discharge. For nonasbestos diaphragm plants, the efflu- Brine muds should be discharged to lined set- ents levels presented in Table 2 should be tling ponds (or the equivalent) to prevent con- achieved. In some cases, bioassay testing of ef- tamination of soil and groundwater. Effluents fluents may be desirable to ensure that effluent should be controlled for pH by neutralization. toxicity is at acceptable levels, say, toxicity to fish Settling and filtration are performed to control at a dilution factor of 2. total suspended solids. Dechlorination of wastewaters is performed using sulfur dioxide or Ambient Noise bisulfite. Noise abatement measures should achieve either Emissions Guidelines the levels given below or a maximum increase in background levels of 3 decibels (measured on the Emissions levels for the design and operation of each project must be established through the environmental assessment (EA) process, on the ba- Table 2. Effluents from Nonasbestos sis of country legislation and the Pollution Prevention Diaphragm Plants, Chlor-Alkali Industry and Abatement Handbook as applied to local con- (milligrams per liter, except for pH) ditions. The emissions levels selected must be Parameter Maximum value justified in the EA and acceptable to the World Bank Group. pH 6–9 The guidelines given below present emissions TSS 20 levels normally acceptable to the World Bank COD 150 Group in making decisions regarding provision AOX 0.5 Sulfites 1 of World Bank Group assistance. Any deviations Chlorine 0.2 from these levels must be described in the World Bank Group project documentation. The emis- Note: Effluent requirements are for direct discharge to surface sions levels given here can be consistently waters. Chlor-Alkali Plants 281

A scale) [dB(A)]. Measurements are to be taken format. The results should be reported to the at noise receptors located outside the project responsible authorities and relevant parties, as property boundary. required.

Maximum allowable log Key Issues equivalent (hourly measurements), in dB(A) The key production and control practices that will Day Night lead to compliance with emissions guidelines can Receptor (07:00–22:00) (22:00–07:00) be summarized as follows. Residential, institutional, Give preference to the membrane process. educational 55 45 Adopt the following pollution prevention mea- Industrial, commercial 70 70 sures to minimize emissions: • Use metal instead of graphite anodes. Monitoring and Reporting • Resaturate brine in closed vessels. • Recycle brine wastes. Frequent sampling may be required during start- • Scrub chlorine from tail gases to produce hypo- up and upset conditions. Once a record of con- chlorite. sistent performance has been established, • Provide lined settling ponds for brine muds. sampling for the parameters listed in this document should be as described below. Sources Daily monitoring for parameters other than pH (for effluents from the diaphragm process) is Arthur D. Little, Inc. 1975. Assessment of Industrial Haz- recommended. The pH in the liquid effluent ardous Waste Practices, Inorganic Chemicals Industry. should be monitored continuously. Chlorine U.S. Environmental Protection Agency, Contract 68- monitors should be strategically located within 01-2246. Washington, D.C.: USEPA. the plant to detect chlorine releases or leaks on a Kirk, Raymond E., and Donald F. Othmer. 1980. Kirk- continuous basis. Othmer Encyclopedia of Chemical Technology. 3d ed. Monitoring data should be analyzed and re- New York: John Wiley and Sons. viewed at regular intervals and compared with World Bank. 1996. “Pollution Prevention and Abate- the operating standards so that any necessary ment: Chlor-Alkali Industry.” Draft Technical Back- corrective actions can be taken. Records of moni- ground Document. Environment Department, toring results should be kept in an acceptable Washington, D.C.