Mercury chloralkali cell

A mercury chloralkali cell was found to require a current of 37.2 kA in order to produce 1 metric ton of caustic soda per day. 

TABLE 8.3 Typical Operating Characteristics of Small and Large Mercury Chloralkali Cells... 

Since the products of the electrolysis of aqueous NaCl will react if they come in contact with each other, an essential feature of any chloralkali cell is separation of the anode reaction (where chloride ion is oxidized to chlorine) from the cathode reaction (in which OH- and H2 are the end products). The principal types of chloralkali cells currently in use are the diaphragm (or membrane) cell and the mercury cell. 

Mercury is emitted from the mercury cell process from ventilation systems and by-product streams. Control techniques include (1) condensation, (2) mist elimination, (3) chemical scrubbing, (4) activated carbon adsorption, and (5) molecular sieve absorption. Several mercury cell (chloralkali) plants in Japan have been converted to diaphragm cells to eliminate the poisonous levels of methyl mercury found in fish (9). 

The ozone concentration in the atmosphere is only a few pphm. In certain chemical plants as in electrolytic mercury cell houses in the chloralkali industry, the ozone concentration is higher. Although the atmospheric ozone level is low, it reacts with rubber double bonds rapidly and causes cracking of rubber products. Especially when rubber is under stress (stretching and bending as in the case of flexible cell covers), the crack development is faster. Neoprene products resist thousands of parts per hundred million of ozone for hours without surface cracking. This nature of neoprene is quite suitable for cell house application in chlor-alkali industries. Natural rubber will crack within minutes when subjected to ozone concentration of only 50 pphm. 

Following acute mercury vapor intoxication of two humans, it was found that, despite chelation therapy with multiple chelators (2,3-dimercaptopropanol [BAL] followed by 2,3-dimercaptosuccoinic acid [DMSA]), relatively high concentrations of mercury remained in the plasma for a very long time (Houeto et al. 1994). The authors suggested that this could be explained by the progressive release of mercury from red blood cells and tissues after oxidation. In a group of chloralkali workers exposed to metallic mercury vapor for 1-24 years (median, 10 years), a decrease in the mercury concentration (following... 

Other remaining technical concerns with membrane cells relate to somewhat lower current efficiencies and to relatively short membrane lifetimes. At present, this is limited to 2-3 year of operation when coupled to much more careful brine pretreatment than is required for conventional asbestos diaphragm cells. A combination of mercury cell and membrane cell technologies has been recently tested for commercial feasibility [19]. The economics of the three primary chloralkali technologies have also been reviewed [20]. 

Any of the products of brine electrolysis, chlorine, sodium hydroxide, and hydrogen can be hazardous if released. When releases do occur, it is usually from process upsets or breakdowns, which may be minimized by the construction of fail-safe plants, proper maintenance, and by safe transport and storage practices. Probably of greater long-term concern is the mercury loss experienced through the process streams of a mercury cell chloralkali operation. These losses can also carry over to the products of the diaphragm cell, even though this does not use mercury, if a common brine well or common salt dissolver is used for both sets of cells. 

Mercury vaporization losses to the cell room air can amount to 1-5 g/tonne chlorine [30]. Therefore, ventilation is important to ensure safe working conditions, which require that the mercury vapor concentration be kept below 0.05 mg/m. This is achieved by tight cell construction, localized hoods and venting of critical cell areas, and cell room ventilation rates of six to eight air changes per hour (e.g., [31]). Mercury cell chloralkali plants in moderate climates are able to operate their cells outside, which avoids these ventilating problems, but does not control potential emissions. 

Complications of mercury containment, and awareness of its natural conversion in aqueous bioactive media into the much more toxic methylated forms [34], have convinced the North American chlorine and caustic producers to build new facilities, or at the time of a major overhaul to convert existing facilities to diaphragm cell technology [35, 36]. Nearly 60% of U.S.A. chloralkali production was by mercury cells in 1970, whereas by 1991 this had dropped to 18% [37]. In Japan, mercury cell chloralkali production has been phased out entirely. In Europe, these complications have not had such a dramatic effect on the cell choice since a large proportion of the total chlorine is still produced by mercury cells. 

M. Drabkin and E. Rissmann, Waste Minimization Audit Report Case Studies of Minimization of Mercury-bearing Wastes at a Mercury Cell Chloralkali Plant, Haz. Waste Eng. Res. Lab., Office of Res. and Deveh, U.S. Envir. Protection Agency, Cincinnati, Ohio, 1988. 

S.K. Dangwal,... Control of Mercury Vapor Exposure in the Cell House of Chloralkali Plants, Environ. Research, 60(2), 254—258, Feb. (1993). 

Perfluorinated polyethers have also gained importance as actively functional materials. Ionic polymer membranes (e.g. DuPont s Nafion ) based on sulfonic acid-derivatized perfluoropolyethers have been used for nearly 30 years as ion-con-ducting membranes in chloralkali electrolysis cells, replacing the large amounts of toxic mercury used until then in the classic Castner-Kellner cells (Scheme 4.8.). One of the earliest applications of Nafion was as a membrane in the hydrogen-oxygen fuel cells which powered the Apollo spacecraft carrying the first men to the moon. 

Mercury compounds continue to have numerous commercial uses. Besides its use as a preservative, mercury is used in the manufacture of many technical and medical instruments including blood pressure measurement devices, manometers, thermometers, and barometers. Mercury is also used in production of certain types of fluorescent lamps and in the chloralkali industry, where chlorine and caustic soda are produced using brine electrolysis in mercury cells. Metallic mercury is used in the production of precious metals such as gold and silver. As part of the production process, metallic mercury can be used to concentrate gold from... 

In 2000, 45 Mt of CI2 was manufactured by the chloralkali process this represents 95% of the global supply. The main producers are the US, Western Europe and Japan. Whereas the Japanese chloralkali industry operates almost entirely with the membrane cell, the US favours use of the diaphragm cell, and just over half of the Western European industry retains use of the mercury cell. On environmental grounds, the chloralkali industry is being pressured to replace mercury and diaphragm cells by the membrane cell. This is not the only environmental concern facing the industry demand for CI2 has fallen in the pulp and paper industry and in the production of chlorofluorocarbons, the latter being phased out as a result of the Montreal Protocol for the Protection... 

Chloralkali plants 31)] loss of mercury for each ton of chlorine (cell ventilation, hydrogen vent)... 

The chloralkali process, which involves the electrolysis of brine, is widely used for the production of sodium hydroxide and chlorine gas. During electrolysis it is necessary to keep the sodium hydroxide separate from the chlorine, to prevent the formation of sodium hypochlorite, NaOCl, and this determines cell design. In older processes, the cathode used was flowing mercury. At this electrode, sodium is formed, and this dissolves in the mercury to form a sodium amalgam. The sodium amalgam is removed continually from the cell and reacted with water to produce hydrogen gas and... 

The low melting point (234 K) of Hg results in its being a unique metal. Its high thermal expansion coefficient makes it a suitable liquid for use in thermometers, and it has widespread application in barometers, dilfusion pumps and in Hg switches in electrical apparatus. The use of mercury cells in the chloralkali process is gradually being phased out (see Box 11.4). Some other metals dissolve in mercury to give amalgams their uses are varied, for example ... 

For many years, cells with mercury cathodes were used in the chloralkali industry. Mercury is not very reactive or soluble and was thought to be harmless in the environment. Then some individuals who ate fish from mercury-contaminated waters became seriously ill. This event brought to light the fact that aquatic microorganisms convert metallic mercury to a toxic, water-soluble compound that enters the food chain (Section 15.6). 

Caustic soda synthesis via brine electrolysis was described in Eqn (15.3). The main alternate chloralkali manufacturing technologies are based on diaphragm, mercury, and membrane cells (Burney, 1993 Venkatesh Tilak, 1983). Of course, the NaCl and NaOH in Eqn (15.3) are fuUy dissociated in the aqueous solution, i.e., NaCl(aq) Na" - -Cl and NaOH(aq) Na - -OH , so that an alternate, and... 

Mercury Cell Process. The mereury cell process is not energy-efficient and is falling out of favor as a result of government regulations restricting the use of mercury. Less than 16% of Canadian chloralkali production currently uses mereury cells, and the process is gradually being phased out. 

Membrane Cell Process. Less than 5% of chloralkali production in Canada is done using the membrane cell process. In this process, a cation-permeable ion-exchange membrane separates the anode from the cathode and only sodium ions and a little water can pass through the membrane. The brine is dechlorinated and recirculated, thus requiring solid salt for resaturation as in the mercury process. The chloride content in the caustic soda is similar to that in the mercury process. The chlorine gas is purified either by liquefaction or evaporation because it contains some oxygen. 

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