Chlorine Mercury cathode cells

Chlorine is produced industrially by electrolysis of brine using either mercury cathode cells or, preferably, various commercially available membrane cells. Chlorine gas is hberated at the anode while sodium hydroxide and hydrogen are liberated at the cathode ... 

Derivation (1) Electrolysis of sodium chloride brine in either diaphragm or mercury-cathode cells chlorine is released at the anode. (2) Fused-salt electrolysis of sodium or magnesium chloride. (3) Electrolysis of hydrochloric acid. (4) Oxidation of hydrogen chloride with nitrogen oxide as catalyst and absorption of steam with sulfuric acid ( KeloChlor process). No by-product caustic is produced. 

Nearly all sodium compounds are soluble in water. Sodium hydroxide is produced commercially by the electrolysis of brine using diaphragm cells or mercury-cathode cells chlorine is a coproduct. 

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. 

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). 

This process not only produces chlorine, it is also a way in which enormous quantities of sodium hydroxide are produced with hydrogen being the other product. Two types of cells are in use. The first and by far the most important employs a diaphragm to separate the anode and cathode compartments. A second type of cell utilizes a mercury cathode with which the sodium forms an amalgam. 

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. 

Of the chlorine production capacity installed in Germany, which totalled 4.4 million tonnes in 2003, 50% were from the membrane cell process, 27% from the mercury cell process and 23% from the diaphragm cell process. The mercury cell process has been the subject of environmental policy criticism for years because of its use of mercury cathodes and resulting pollutant emissions. Hence, no new mercury plants will be... 

Another somewhat more complicated cell for the production of chlorine and sodium hydroxide by the electrolysis of sodium chloride solution is the Castner-Kellner cell, which employs a liquid mercury cathode. 

Direct current flows through a cell of salt solution to give the chlorine at the anode. Cells of two different kinds, the mercury cathode and the diaphragm, are used. Figure 6.15 gives chlorine production figures in the United States between 1955 and 1995. 

Fig. 11.11 Chlorine manufacture. Schematic diagram of mercury cathode electrolytic cell.

The most common cell with horizontal electrodes is the mercury cell from the chlor-alkali industry (see Chapter 3). The mercury cathode flows down the slightly sloping base plate of the cell and many rectangular dimensionally stable anodes (DSA) (of gauze or expanded-metal-type structure to allow the chlorine gas to rise with only minimum restriction) are mounted from the top of the cell so that they cover the surface area of the mercury and give an inter-electrode gap of a few centimetres (see Fig. 2.13). The cell may be as big as 70 m. The brine is, however. 

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