Chloralkali mercury process

The chloralkali water electrolysis is the only large-scale technological method to be commercialized, where the H2 is actually a byproduct of the chlorine production and mostly used as the thermal energy source and substitute of natural gas. A solution of salt in water is electrolytically decomposed into hydrogen and soda lye (cathode) and chlorine (anode) as shown in Fig. 5-9 for the mercury process ... 

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. 

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). 

In the profiles of the core from the industrial area, mercury displays the highest accumulation. Mercury in this area, close to the industrial district, has probably derived from a large chloralkali plant which has employed mercury cathodes since the fifties. Whereas, at present, very severe measures are taken to prevent mercury spills into the Lagoon, in the past, polluted waters and solid materials were discharged almost untreated. In the most superficial strata a marked decrease in the accumulations is, in fact, recorded. Lead and Cd accumulations are lower here by a factor of 5-10. The presence of cadmium in the sediments of the Lagoon has been referred to sphalerite (ZnS) processing on the basis of a strict concomitant... 

A mixture of hydrogen and chlorine gas, eventually in combination with air, can be very explosive if one of the components exceeds certain limits. In chlorine production plants, based on the electrolysis of sodium chloride solutions, there is always a production of hydrogen. It is, therefore, essential to be aware of the actual hydrogen content of chlorine gas process streams at any time. There are several places in the chlorine production process where the hydrogen content in the chlorine gas can accumulate above the explosion limits. Within the chloralkali industry, mainly two types of processes are used for the production of chlorine—the mercury- and the membrane-based electrolysis of sodium chloride solutions (brine). 

Several industrial processes use mercury in large amounts, and the resulting potential for spills and loss to the environment is great. One of the largest is the chloralkali industry, in which mercury is used as an electrode for the electrolysis of brine to form chlorine gas and sodium hydroxide ... 

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 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... 

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... 

Based on samples from museum collections, it was demonstrated that mercury content in feathers from fish-eating birds were comparatively low in the years 1815 through 1940. However, since 1940, or the advent of the chlo-ralkali industry (wherein mercury is used as a catalyst in the process to produce sodium hydroxide and chlorine gas from sodium chloride and water, with significant loss of mercury to the biosphere), mercury concentrations in feathers were eight times higher on average. Mercury levels were also elevated in feathers and tissues of aquatic and fish-eating birds from the vicinity of chloralkali plants these increased levels of mercury were detectable up to 300 km from the chloralkali plant. 

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 ... 

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. 

Occupational exposure to mercury vapor occurs in a variety of industries such as chloralkali plants, thermometer factories, gold extraction process, and mercury mining. Dentists and dental assistants are also exposed to mercury vapor during insertion, polishing, and removal of amalgam fillings. 

There are three major processes used for chloralkali production. They are mercury, diaphragm, and membrane and differ in the type of cell used. In the mercury cell process, the cathode is a mercury film. This process was... 

Production of caustic soda solution. In 1998, the worldwide production capacity was about 54 million tons per year. Ca. 96-98% of this amount is produced by chloralkali electrolysis [313). The three processes are described in detail in chapter 5 (Mercury Cell Process), chapter 6 (Diaphragm Process) and chapter 7 (Membrane process), a comparison of the relative qualities is given in chapter 9. 

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