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6.5 Effluents Wastewater Is Treated To Remove Chlorine and Acids Prior to Discharge Wastewater from diaphragm and membrane cells originates from caustic evaporation, chlorine drying, washing of the ion-exchange resin, and from purification of salt recovered from evaporators. Caustic evaporation, where the sodium hydroxide solution is concentrated to a 50 or 70 percent solution, evaporates about 5 tons of water per ton of 50 percent caustic soda produced. The water vapor from the evaporators is condensed, and in the case of the diaphragm process, will contain about 15 percent caustic soda solution and a relatively high salt content of 15 to 17 percent. If sodium sulfate is not removed during the brine purification process, salt recovered from evaporators may be recrystallized to avoid buildup of sulfate in the brine. If the salt is recrystallized, the wastewater may contain sodium sulfates. Wastewater from membrane processes contains caustic soda solution, but is virtually free of salt or sodium sulfates. Wastewater from caustic soda processing is usually neutralized with hydrochloric acid, then lagooned and discharged to and or to receiving ponds. During chlorine gas processing, water vapor is removed by scrubbing with concentrated sulfuric acid. Between 6 kilograms and 35 kilograms of 79 percent sulfuric acid wastewater is generated per 1000 kilograms of chlorine produced (EPA 1995a). Most of this wastewater is shipped off-site for processing into concentrated sulfuric acid or for use in other processes. The remainder is used for pH control or discharged to water treatment facilities for disposal (EPA 1995a). Ion-exchange wash water from membrane cell processes usually contains dilute hydrochloric acid with small amounts of dissolved calcium, magnesium, and aluminum chloride. This wastewater is usually treated along with other acidic wastewaters by neutralization. Wastewater streams are generated from mercury cells during the chlorine drying process, brine purge, and from other sources. Mercury is present in the brine purge and other sources (floor sumps, cell wash water) in small amounts. This mercury is generally present in concentrations ranging from 0 to 20 parts per million, and is precipitated out using sodium hydrosulfide to form mercuric sulfide. The mercuric sulfide is removed through filtration before the water is discharged (EPA 1995a, Orica 1999). Sodium carbonate manufacture creates significant volumes of wastewater that must be treated prior to discharge or recycling to the process. These may contain both mineral (e.g., shale) and salt impurities (EPA 1997g). Limitations for toxic or hazardous compounds contained in these wastewaters are given by the U.S. Environmental Protection Agency in 40 CFR, Chapter 1, Part 415, which was originally promulgated in 1974 and has been revised several times since then. The chemicals in the chlor- alkali industry and sodium carbonate are covered under Subparts F and O. Specific limitations for restricted compounds and total suspended solids (TSS) are shown in Tables 6-8 through 6-10. “BPT Standards” refers to the use of the best practicable control technology currently available. “BAT” refers to the best available technology economically achievable. “NSPS” refers to new source performance standards that apply to new process water impoundment or treatment facilities. Table 6-10 provides BPT limitations only for the production of sodium chloride (the primary input to chlor-alkali manufacture) by solution brine-mining. In this process water is pumped into a salt deposit and a saturated salt solution is removed that is about one-third salt. Provisions are that no process wastewater pollutant may be returned to navigable waters. The exception is unused bitterns (saturated brine solution remaining after precipitation of sodium chloride), which may be returned to the body of water from which the process brine solution was originally withdrawn. 196PDF Image | The Chlor-Alkali Industry
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