Chlor-alkali process

The chloralkali process (also chlor-alkali and chlor alkali) is an industrial process for the electrolysis of NaCl. It is the technology used to produce chlorine and sodium hydroxide (lye/caustic soda), which are commodity chemicals required by industry. 35 million tons of chlorine were prepared by this process in 1987. Industrial scale production began in 1892.

Usually the process is conducted on a brine (an aqueous solution of NaCl), in which case NaOH, hydrogen, and chlorine result. When using calcium chloride or potassium chloride, the products contain calcium or potassium instead of sodium. Related processes are known that use molten NaCl to give chlorine and sodium metal or condensed hydrogen chloride to give hydrogen and chlorine.

The process has a high energy consumption, for example over 4 billion kWh per year in West Germany in 1985. Because the process gives equivalent amounts of chlorine and sodium hydroxide (two moles of sodium hydroxide per mole of chlorine), it is necessary to find a use for these products in the same proportion. For every mole of chlorine produced, one mole of hydrogen is produced. Much of this hydrogen is used to produce hydrochloric acid or ammonia, or is used in the hydrogenation of organic compounds.

Three production methods are in use. While the mercury cell method produces chlorine-free sodium hydroxide, the use of several tonnes of mercury leads to serious environmental problems. In a normal production cycle a few hundred pounds of mercury per year are emitted, which accumulate in the environment. Additionally, the chlorine and sodium hydroxide produced via the mercury-cell chloralkali process are themselves contaminated with trace amounts of mercury. The membrane and diaphragm method use no mercury, but the sodium hydroxide contains chlorine, which must be removed.

The performance of these devices is governed by the considerations of electrochemical engineering.

The most common chloralkali process involves the electrolysis of aqueous sodium chloride (a brine) in a membrane cell.

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