Chlor alkali industry

Production of CI2 and NaOH by electrolysis of NaCl is a huge industry with annual production capacity in excess of 50 million tons of NaOH per year. Membrane cells are the state-of-the-art technology, but mercury and diaphragm cells are still used because the capital cost for their replacement is substantial. The mercury cell technology is more than a century old and stiU accounts for nearly half of the world s production capacity. Chlorine evolves from a DSA (dimensionally stable anode) situated above a pool of mercury with NaCl brine in between. Mercury reacts with sodium to form sodium amalgam, which is removed and hydrolyzed in a separate reactor. 

The reconstituted metaUic mercury is returned to the cell. The major advantage of the mercury cell is that it produces very pure NaOH at 50% concentration. But electrical energy consumption is high, and the threat of mercury pollution is a major concern. Traces of mercury appear in the NaOH and in air emissions from the cells. Because of their design it is impractical to modify mercury cells into membrane cells. 

The diaphragm cell has a membrane of sorts, with a sheet of asbestos felt separating the two compartments. The diaphragm keeps the chlorine gas out of the cathode compartment, but it offers no selectivity for diffusion and migration of ions. Therefore, the NaOH produced is dilute (12% maximum) and loaded with NaCl. Evaporation of the catholyte to concentrate the NaOH to 50% causes most of the NaCl to precipitate, but the chloride level of the product remains about 1%. In some cases the asbestos diaphragms have been replaced with other porous membranes (principally PVC) to improve membrane life and avoid environmental criticism. In other cases diaphragm cells have been converted to membrane cells. 

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Chlor-alkali cell Chlor-alkali industry... 

J. L. Hurst, ImplementingMembrane Cell Technology Within OuyChem Manufacturing International Symposium on Chlor—Alkali Industry, Tokyo, Japan, April, 1988. 

For the chlor-alkali industry, an emergency preparedness and response plan is mandatory for potential uncontrolled chlorine and other releases. Carbon tetrachloride is sometimes used to scrub nitrogen trichloride (formed in the process) and to maintain its levels below 4% to avoid fire and explosion. Substitutes for carbon tetrachloride may have to be used, as the use of carbon tetrachloride may be banned in the near future due to its carcinogenicity. 

This trade-off may not even occur in some cases. Membranes used in the PEMFC have been developed for the chlor-alkali industry and have 40,000-hour durability (shutdowns are prohibitively expensive in stationary applications), require only 5,000-hour durability (corresponding Co 100,000 miles) for automotive applications. Hence, it maybe possible to develop less expensive membranes that still meet automotive requirements. 

Hine, F. Chemistry and Chemical Engineering in the Chlor-Alkali Industry 18... 

Thanks to its high stability and permselectivity, Nation has been used as a Na+ conductor in membrane electrolysis of brine in the chlor-alkali industry. This application was introduced in the early 1980s and is by far the most important use of ionomer membranes. 

Summary of Contents of Waste Contents of the Three Cells Used in Chlor-Alkali Industry... 

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

Bosch C, Olivares A, Faria M, Navas JM, del Olmo I, Grimalt JO, Pina B, Barata C (2009) Identification of water soluble and particle bound compounds causing sublethal toxic effects. A field study on sediments affected by a chlor-alkali industry. Aquat Toxicol 94(1) 16-27... 

The Hg-electrolysis technology is one of the major point sources of Hg contamination, and its impact on the environment has been studied worldwide [23-26]. Although mercury cell chlor-alkali industry is obsolete in most of the European Union countries [27], in Spain it will be allowed until the end of 2010. 

OSPAR (2007) Mercury losses from the chlor-alkali industry (1982-2005). ISBN 978-1-905859-56-6... 

A second risk zone corresponds to Monzon (17,042 inhabitants), a highly industrialized city in the middle Cinca River. Its industrial activity has caused the historical release of organic and inorganic compounds to the river coming from chlor-alkali industry, from production and utilization of solvents and organochlo-rine pesticides, and from the use of brominated flame retardants in the production processes. Very high concentrations of mercury have been recorded for sediment and fish samples in the downstream of Monzon [2-4]. 

At one time, sodium hypochlorite was manufactured electrochemically on a substantial scale. Now it is regarded as a by-product of the chlor-alkali industry [10]. On the other hand, there are many situations where low volumes of hypochlorite may be required or the requirement is irregular. Aqueous solutions of hypochlorite are much safer than chlorine gas but contain < 15wt% of active chlorine. Hence, storage and transportation costs are relatively high. Often the most convenient and cost-effective solution is to eleetrolytically generate OC1- in situ [10]. 

Dr M Harris ICI Halo chemicals, PO Box 13, The Heath, Runcorn, Cheshire, WA7 4QP, UK. Phase-out Issues for Mercury Cell Technology in the Chlor-Alkali Industry. 

This chapter examines the business of the chlor-alkali industry. Without a need for chlorine there is no chlor-alkali business. This industry is now in its second century and having passed the millennium bug without any serious interruption, the future can now be examined. In this chapter there is an emphasis on chlorine rather than alkali as the alkali industry is thousands of years old and has been documented extensively since the time of the ancient Egyptians. 

This chapter examines an issue that is one of the key determinants of the future of the West European chlor-alkali industry. It examines the environmental, economic, financial and political aspects of a debate whose effects are likely to spill over into North America and Asia - and to affect the development of the chlor-alkali industry world-wide. 

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

We can t do much about the volcanoes, but we do need to address the anthropogenic emissions. To do so, we need to know where these come from. Historically, there is no doubt that the chlor-alkali industry was one of the biggest anthropogenic sources. However, the industry has reduced its emissions by an order of magnitude and it no longer represents a major source. Annual emissions of the West European... 

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