Mercury cell process

The mercury cell process has developed extensively since its inception in 1892. A number of distinct phases can be identified, from the rocking cell to the 400 000 A modem mercury cell unit. The cell chemistry is identical in all cases 

Cl — e = Cr chloride ion loses an electron to form a chlorine atom 

2Cr = CI2 chlorine atoms combine to escape as chlorine molecules 

Decomposer reaction (essentially a short circuited cell) 

The first operating membrane cell was the rocking cell, largely developed by Baker, who was Castner s chief chemist, at Oldbury, and later at Runcorn. The history of the Castner Kellner plant at Runcorn gives the history of the development of the mercury cell, and indicates the way in which a technology has developed in the drive to increase production quantities and efficiencies while minimizing capital costs. 

In the mercury cell process, sodium amalgam is produced at the cathode  

The amalgam is subsequently decomposed with graphite as catalyst  

The equilibrium potential of oxygen formation (25 °C, 1.013 bar, neutral solution, 

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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). 

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. 

Figure 6.31 (A) Mercury cell concept (B) overall mercury cell process areas.

In this chapter brief information on the origin and cost of titanium will be discussed. A definition of the term unique properties will be given and how these properties are exploited in the membrane, diaphragm and in the mercury cell processes will be considered and miscellaneous applications touched upon. The chapter will conclude with a summary of the financial benefits which, after all, propel the use of this challenging material. 

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... 

In the mercury cell process chlorine is liberated from a brine solution at Ihe anodes which are. today, typically melal anodes (Dimensionally Stable Anodes or DSAl. Collection and processing of the chlorine is similar lo Ihe techniques employed when diaphragm cells are used. However. Ihe cathode is a flowing bed or mercury. When sodium is released by electrolysis it is immediately amalgamated with the mercury The inereury amalgam is then decomposed in a separate cell 10 form sodium hydroxide and Ihe mercury is returned for reuse. 

Mercury cell process -> alkali chloride electrolysis... 

Derivation Mercury is heated to about 200C and sodium, in small pieces, is added slowly. Also formed at one stage of the process for making chlorine and sodium hydroxide by the mercury cell process. 

FIGURE 8.4 Mercury cell process for the electrolytic production of chlorine and sodium hydroxide. Packing in the vertical decomposer is graphite lumps. For treatment of cell products, see Fig. 8.2. 

K07I. . Brine purification muds from the mercury cell process in chlorine production where separately prepurified brine is not used. 

The advent of membrane cells is largely dictated by environmental regulations related to the hazardous nature of mercury effluents (from mercury cell process) and asbestos (used in diaphragm cell technology).37 However, there are several advantages offered by the membrane cell technology as noted below. 

Membrane cell process for manufacture of caustic soda has mostly replaced the mercury cell process due to better efiftciency. The mercury cell process also had environmental pollution issues that have been removed in the membrane cell process. 

Figure 3.6 Complete mercury cell process for the conversion of salt deposits into chlorine and 50% caustic soda. 

Caustic from the mercury-cell process is called Rayon-grade caustic. Rayon-grade caustic and membrane-grade caustic are referred to as low salt caustic soda, which is a premium product. 

The depleted brine from the membrane and mercury cell processes carries dissolved chlorine. This brine is acidified to reduce the chlorine solubility, and then dechlorinated in a vacuum brine dechlorinator. The dechlorinated brine is returned to the brine wells for solution mining, or to the salt dissolver. If the membrane and diaphragm processes coexist at a given location, the dechlorinated brine is sent for re-saturation before being fed to the diaphragm cells. 

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