Chlorine mercury cell

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

In 1988 diaphragm cells accounted for 76% of all U.S. chlorine production, mercury cells for 17%, membrane cells for 5%, and all other production methods for 2%. Corresponding statistics for Canadian production are diaphragm cells, 81% mercury cells, 15% and membrane cells, 4% (5). for a number of reasons, including concerns over mercury pollution, recent trends are away from mercury cell production toward the more environmentally acceptable membrane cells, which also produce higher quality product and have favorable economics. 

Chlorine—hydrogen ha2ards associated with mercury cells result from mercury pump failures heavy-metal impurities, particularly those with very low hydrogen overvoltage, ie. Mo, Cr, W, Ni excessively low pH of feed brine low NaCl concentrations in feed brine and poor decomposer operation, which leads to high sodium amalgam concentrations in the cell. 

Mercury is used in the manufacture of thermometers, barometers and switchgear, and in the production of amalgams with copper, tin, silver and gold, and of solders. A major use in the chemical industry is in the production of a host of mercury compounds and in mercury cells for the generation of chlorine. Mercury has a significant vapour pressure at ambient temperature and is a cumulative poison. 

Recently it has been shown that the oxides of the platinum metals can have a higher corrosion resistance than the metals themselves , and have sufficient conductivity to be used as coatings for anodes, e.g. with titanium cores. Anodes with a coating of ruthenium dioxide are being developed for use in mercury cells for the electrolysis of brine to produce chlorine , since they are resistant to attack if in contact with the sodium-mercury amalgam. 

This yields a solution of highly pure alkali (free of chloride ions), which can be used in the manufacture of synthetic fibers. The mercury, which has been stripped of sodium, is returned to the electrolyzer. The cost of chlorine is higher in the mercurycell than in the diaphragm-cell process. In addition, the mercury-cell process is ecologically dangerous, owing to the possible escape of mercury into the environment hence, it has increasingly been discontinued in all countries. 

In the decomposer, deionized water reacts with the amalgam, which becomes the anode to a short-circuited cathode. The caustic soda produced is stored or evaporated, if higher concentration is required. The hydrogen gas is cooled by refrigeration to remove water vapor and traces of mercury. Some of these techniques are employed in different facilities to maximize the production of chlorine, minimize the consumption of NaCl, and also to prevent the buildup of impurities such as sulfate in the brine.26 The production of pure chlorine gas and pure 50% sodium hydroxide with no need for further concentration of the dilute solution is the advantage that the mercury cell possesses over other cells. However, the cell consumes more energy and requires a very pure brine solution with least metal contaminants and above all requires more concern about mercury releases into the environment.4... 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

There are different ways of producing chlorine from brine, for example, Dow cells, Hooker cells, and mercury cells. Which process is to be used must be known in order to make an accurate economic evaluation, since the capital costs and operating costs are different for each of these processes. The process engineer may have to investigate the different processes and economically evaluate each before deciding which process is best. 

In the results the emissions of mercury appear to have a very substantial contribution for the human toxicity impact score. These emissions are caused by the coproduction of chlorine and sodium hydroxide by electrolysis using a mercury cell. However, this technique is phased out. Therefore, the process descriptions in the Ecoinvent database do not represent up to date technology. In the Ecoinvent database the process for PVC production, in which chlorine is used as one of the compounds, is an aggregated processes based on, seemingly outdated, data from PlasticsEurope. These outdated data also influence the impacts related to waste treatment by incineration because sodium hydroxide is necessary for the waste incineration process. 

On the other hand, the industrial activity of the other two mercury cell chlor-alkali plants have caused important Hg pollution in Sabinanigo (capacity of 25 x 103 Mg chlorine/year) and Monzon (31 x 103 Mg chlorine/year) (see Fig. 1), two small and industrial cities located in the middle course of the Gallego and Cinca Rivers, respectively, two tributaries of the Ebro River [31]. 

De Nora An electrolytic process for making chlorine and sodium hydroxide solution from brine. The cell has a mercury cathode and graphite anodes. It was developed in the 1950s by the Italian company Oronzio De Nora, Impianti Elettrochimici, Milan, based on work by I. G. Farbenindustrie in Germany during World War II. In 1958 the Monsanto Chemical Company introduced it into the United States in its plant at Anniston, AL. See also Mercury cell. 

Griesheim (1) An early process for producing chlorine by electrolysis, developed by Chemische Fabrik Griesheim-Elektron, in Germany, and commercialized in 1890. The electrolyte was saturated potassium chloride solution, heated to 80 to 90°C. The byproduct potassium hydroxide was recovered. The process was superseded in the United States by several similar electrolytic processes before being ousted by the mercury cell, invented by H. Y. Castner and K. Kellner in 1892. See Castner-Kellner. 

The issues of mercury cell phase out and risk assessment of chlorine compounds. 

The chlorine industry really took off at the end of the 1890s with the installation of the first mercury cell units. Since then the industry has coped with many problems, the main one being the fixed amount of caustic soda per ton of chlorine out of the cells. There have always been imbalances on the demand for chlorine and the demand for caustic soda. The two products quite often go their own way and no reason is seen to change this situation. 

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. 

Ru losses can occur during electrolysis, as well as those due to shorting in mercury cell operations, from erosion, loss of Ru-based intermediates involved during the course of chlorine and oxygen evolution reactions and during shut-downs. 

Orica Australia Pty Ltd, formerly ICI Australia, is the largest manufacturer of chlorine and caustic soda in Australia. In December 1998, the Orica Board sanctioned a project to replace two mercury cell plants with new membrane plants. The plants are due to be commissioned sequentially at the end of 2000/mid-2001 at a total capital cost of US 100 million. 

Orica is the largest producer of chlorine in Australia and currently operates three chlor-alkali plants on the east coast. Two of these plants (in Melbourne and Sydney) are mercury cell plants dating back over 50 years while the third plant is a small, modern membrane cell plant in the central Queensland town of Gladstone. The mercury cell plants have both reached the end of their useful economic lives. 

Orica Sydney 50000 Liquid chlorine, hypo3, HCIb, ferric chloride Mercury cell... 

Melbourne 14000 Liquid chlorine, hypo, HCI, chlorinated paraffin Mercury cell... 

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