Chloralkali cells operational process

Figure 1. Basic operational process for membrane chloralkali cells.

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... 

Figure 7-29 Salt electrolysis to produce CI2 gas and NaOH solution in a chloralkali process. This is the process by which most alkali and chlorine are produced The reactor operates continuously with brine solution flowing into the cell and NaOH and Q2 gas flowing out.

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. 

CaCl2 is added to reduce the operating temperature to about 870 K, since pure NaCl melts at 1073 K (see Section 9.12). The design of the electrolysis cell (Fig. 11.2) is critical to prevent reformation of NaCl by recombination of Na and CI2. Although the Downs process is the major manufacturing process for Na, the CI2 produced contributes only 5% of the world s supply. The remaining 95% is produced by the chloralkali process which involves the electrolysis of aqueous NaCl (see Box 11.4). 

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