Chlor-alkali mercury cell process

TABLE 10.2.2. Mass balance for Chlor-Alkali Mercury Cell Process (All mass units are kg/kg CI2)... 

To strip mercuric ion from effluent resulting from the chlor-alkali mercury cell process, the concentration being reduced from 5 to 0.06 mg dm". ... 

Summary of Raw Waste Loadings Found in Verification Sampling of Unit Product of Chlor-Alkali (Mercury Cell and Diaphragm Cell Processes)... 

FIGURE 10.2.10. Process flowsheet for chlor-alkali mercury cell. (The numbers inside the process equipment refer to the enthalpy change in kWhrton" of chlorine.)... 

Historically, the Japanese chlor-alkali industry started in 1881, when the LeBlanc process was used to produce caustic soda. Osaka Soda and Hodogaya Chemical commercialized the mercury- and diaphragm-cell technologies in 1915. Asahi Glass started the Solvay process soon after. By 1973, 95% of the chlorine was produced by the mercury-cell process and 5% by diaphragm cells. In 1973, mercury pollution issues... 

It is essential for all technical chlor-alkali electrolysis processes - as subsequently discussed in Section 6.19.2.2 - that the transport of hydroxide ions formed at the cathode into the anode compartment is excluded (membrane process) or at least largely suppressed (diaphragm process). In the mercury cell process, OH ions are not formed in the entire process. 

In 1800, Cruickshank was the first to prepare chlorine electrochemically [38] however, the process was of little significance until the development of a suitable generator by Siemens and of synthetic graphite for anodes by Acheson and Castner in 1892. These two developments made possible the electrolytic production of chlorine, the chlor-alkali process, on an industrial scale. About the same time, both the diaphragm cell process (1885) and the mercury cell process (1892) were introduced. The membrane cell process was developed much mpre recently (1970). Currently, more than 95 % of world chlorine production is obtained by the chlor-alkali process. Since 1970 graphite anodes have been superseded by activated titanium anodes in the diaphragm and mercury cell processes. The newer membrane cell process uses only activated titanium anodes. 

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

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... 

FIGU RE 22.5 General wastewater treatment process flow diagram at a mercury cell plant for the production of chlor-alkali. 

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. 

Lindley, A.A. (1997) An Economic and Environmental Analysis of the Chlor-Alkali Production Process Mercury Cells and Alternative Technologies. Prepared for the European Commission (DG III C-4, Chemicals, Plastics, Rubber). See also OSPAR Document WOCAI 99/5/8 (Madrid, 1999). 

This chapter gives an overview of the chlor-alkali industry in Australia and examines the background to the decision to replace the mercury cell plants. It then describes the new plants, their technical and safety features and the process used to arrive at the selection of the technology supplier. 

Chlorine (from the Greek chloros for yellow-green ) is the most abundant halogen (0.19 w% of the earth s crust) and plays a key role in chemical processes. The chlor-alkali industry has been in operation since the 1890s and improvements in the technology are still important and noticeable, for example, the transition from the mercury-based technology to membrane cells [60]. Most chlorine produced today is used for the manufacture of polyvinyl chloride, chloroprene, chlorinated hydrocarbons, propylene oxide, in the pulp and paper industry, in water treatment, and in disinfection processes [61]. A summary of typical redox states of chlorine, standard potentials for acidic aqueous media, and applications is given in Scheme 2. 

Relatively pure solid NaOH is recovered from the aqueous solution, and the regenerated mercury is then pumped back to the electrolysis cell. This process, called the chlor-alkali process, has often resulted in significant mercury contamination of the environment the waste solutions from this process are now carefully treated to remove mercury. 

Chlor-alkali process the process for producing chlorine and sodium hydroxide by electrolyzing brine in a mercury cell. (11.8)... 

National Emission Standards for Hazardous Air Pollutants (NESHAPs)-list of pollutants and applicability standard for mercury ore processing facilities and mercury cell chlor-alkali plants (mercury) Yes < 2300 g per 24-hour period 40 CFR 61.01 EPA 1971a 40 CFR 61, Subpart E EPA 1975c... 

FIGURE 21.18 Mercury cell used in the chlor-alkali process. The cathode contains mercury. 

The choice of technology, the associated capital, and operating costs for a chlor—alkali plant are strongly dependent on local factors. Especially important are local energy and transportation costs, as are environmental constraints. The primary difference in operating costs between diaphragm, mercury, and membrane cell plants results from variations in electricity requirements for the three processes (Table 25) so that local eneigy and steam costs are most important. 

Chlor-alkali process the process for producing chlorine and sodium hydroxide by electrolyzing brine in a mercury cell. (18.9) Chromatography the general name for a series of methods for separating mixtures by employing a system with a mobile phase and a stationary phase. (1.9)... 

The unit operations in a commercial chlor-alkali plant can be generally classified as follows (1) brine purification, (2) electrolytic cells, (3) H2 and Cl2 collection, and (4) caustic concentration and salt removal. In this section, the general process flowsheets for diaphragm, membrane, and mercury cell technologies are discussed with emphasis on the need for brine purification and the manner in which it is carried out. 

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