Mercury cell, chlor-alkali production

The typical mercury-cell chlor-alkali plant has elaborate facilities for containment of mercury within the process area and for removing it fix>m discharged streams. The amount of mercury released into the environment by chlor-alkali producers has been decreasing steadily, partly because of the emphasis on its control and partly because of the downward trend in mercury-cell production. During the five years ending in 2001, for example, the mercury-cell capacity of EC producers dropped from 64% of the... 

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). 

H. Y. Castner (US/UK) and C. Kellner (Vienna) independently developed commercial mercury-cathode cell for chlor-alkali production... 

The development of the membrane cell cut the energy consumption in chlor-alkali production. A good cell will produce a ton of caustic for around 2400 kWh. Membrane caustic can only be produced up to around 35%. Several cell designers have tried to develop a cell and membrane combination that would allow 50% caustic to be made, but this has proved to be commercially elusive so far. Membrane cells have probably reached the theoretical limit on energy consumption for a commercial plant. In Japan, power consumption has been cut by 30% over the last 20 years as the conversion from mercury cell progressed. 

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). 

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

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. 

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. 

The use of polyperfluorosulfonic acid membranes as the cell separator was first demonstrated about three decades ago. Yet it was not until the mid-1980s when the economic advantages of membrane cells over the traditional mercury- and diaphragm-cell technology were fully demonstrated—consequent to better membrane performance, higher caustic product concentrations, and lower power consumption. Retrofitting chlor-alkali facilities with membrane cells accounted for much of the growth and sustenance of this industry over the past two decades. 

Many of the world s major chlor-alkali companies have developed their own mercury cells and the designs will differ in the way they seek to obtain the maximum electrode area and in the arrangement of the auxiliary equipment. The development of the cells during almost a century of electrolytic chlorine and caustic soda production and the variation in the cells recently available are described in the texts at the end of the chapter. 

Mercury-cell production of chlorine and alkalis is gradually being phased out. There has been substantial progress in the reduction of mercury emissions of all kinds, but these have served to delay rather than prevent the abandonment of the technology. During the 5 years ending in 2001, the combined mercury-cell capacity of Euro Chlor producers dropped fix)m about 64% of the total to 54% [20]. Measured emissions of mercury were 1.25 g t chlorine. This includes mercury released to the air, to water, and in products. This number represents a 74% drop in 10 years. The trend has continued, and less than half of European production is now in mercury cells [21]. 

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