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6.6 Hazardous Wastes and Byproducts Brine Mud Constitutes the Largest Solid Waste Stream from Chlor-Alkali Production Brine purification results in brine mud, one of the largest waste streams from the chlor-alkali industry. About 30 kilograms of brine mud are generated for every 1000 kilograms of chlorine produced, but this varies with the purity of the salt used to produce the brine. Pre-purified salts, for example, will generate only about 0.7 to 6.0 kilograms per 1000 kilograms of chlorine produced. The brine mud contains a variety of compounds, typically magnesium hydroxide and calcium carbonate formed during the addition of compounds to purify the brine. The sludge or brine mud containing these impurities must be disposed of in a landfill (EPA 1995a, Orica 1999). If a mercury process is being used, the brine mud may contain trace levels of mercury. In this case, the sludge is treated with sodium sulfide to create mercury sulfide, an insoluble compound. The sludge is further treated by casting it into concrete blocks, which are treated for leachability and sent to a controlled landfill. Both diaphragm and membrane cell processes generate solid wastes from scrapping of cell parts (cell covers, piping, used diaphragms, used membranes, cathodes, and anodes). Discarded cell parts are landfilled on- site or shipped off-site for disposal. Used cathodes and anodes are shipped off-site to a third party recovery facility. Depending on the cell technology, the cathodes may be refurbished and reused, particularly those made of nickel. The anodes for diaphragm cells are refurbished and rec-coated with RuO2/TiO2 and returned to service. Solid wastes from mercury cells include spent graphite from decomposer cells, spent caustic filtration cartridges from the filtration of caustic soda solution, spilled mercury from sumps, and mercury cell “butters.” (The latter are semi-solid amalgams of mercury with barium or iron, formed when an excess of barium is used during purification). Mercury is recovered from these wastes where possible, and the remainder is disposed of in secure landfills to prevent migration of mercury, which can cause significant damage to the environment. Mercury cell brine muds may also contain mercury in elemental form or as mercuric chloride. These muds are considered hazardous and must be disposed of in a RCRA Subtitle C landfill after treatment with sodium sulfide, which creates an insoluble sulfide compound (EPA 1995a). Other brine muds are segregated and stored in lagoons, which are periodically dredged or drained and covered over. The dredgings are usually landfilled off-site. The processing of hydrogen gas from mercury cells also creates hazardous waste. In this process, small amounts of mercury present in the hydrogen gas are extracted by cooling the gas. A large part of the condensed mercury is removed in this fashion and returned to the electrolytic cell. Some facilities use activated carbon treatment to further purify the hydrogen of mercury, and the spent carbon is shipped off-site for disposal as a hazardous waste. The effluents and solid wastes and hazardous wastes from chlor-alkali production are summarized in Table 6-11. Specific wastes from chlor-alkali manufacture that are listed by the EPA as hazardous as shown in Table 6-12. Most of the hazardous wastes are generated from the mercury cell process. The dissolving and clarification steps in sodium carbonate production create a waste sludge containing non-hazardous impurities, such as salts and minerals. This sludge is disposed of in landfills. 198PDF Image | The Chlor-Alkali Industry
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