The chloralkali industry

The chloralkali industry produces huge quantities of NaOH and C_l2 by the electrolysis of aqueous NaCl (brine).

Three types of electrolysis cell are available:

• the mercury cell, which employs a mercury cathode;
• the diaphragm cell, which uses an asbestos diaphragm separating the steel cathode and graphite or platinumcoated titanium anode;
• the membrane cell, in which a cation-exchange membrane, with high permeability to Na ions and low permeability to Cl^- and [OH]^- ions, is placed between the anode and cathode. In 2000, ≈45 Mt of Cl₂ 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 etains 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 Cl₂ 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 of the Ozone Layer. Nevertheless, overall demand for Cl₂ remains high, much being used in the production of chloroethene (polyvinylchloride, PVC).

 Aqueous NaOH from the electrolytic process is evaporated to give solid NaOH (caustic soda) as a white, translucent solid which is fused and cast into sticks, or made into flakes or pellets. Uses of NaOH are summarized.

The chloralkali industry illustrates an interesting market problem. While the electrolysis of brine produces NaOH and Cl₂ in a fixed molar ratio, the markets for the two chemicals are different and unrelated. Interestingly, prices of the two chemicals follow opposite trends; in times of recession, demand for Cl₂ falls more sharply than that of NaOH, with the result that the price of Cl₂ falls as stocks build up. Conversely, industrial demand for Cl₂ increases faster than that of NaOH when the economy is strong; consequently, the price of the alkali falls as stocks increase.    The net result is clearly important to the long-term stability of the chloralkali industry as a whole.