chloralkali

Chloralkali, or the chlor-alkali process, is an industrial electrolysis method for producing chlorine gas and sodium hydroxide (caustic soda) from sodium chloride (brine). The key overall reaction in aqueous solution is 2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH. Brine is purified to remove hardness ions and other impurities before electrolysis.

There are three major electrolysis technologies used in chlor-alkali plants:

- Mercury cell: chlorine is produced at the anode and sodium hydroxide forms in contact with liquid mercury at the cathode. This technology has declined due to environmental concerns over mercury emissions and is being phased out in favor of membrane or diaphragm cells.

- Diaphragm cell: a porous diaphragm separates the anode and cathode compartments. Chlorine gas is evolved at the anode, while sodium hydroxide forms in the cathode region but with some contamination from brine. This design avoids mercury but can produce lower-purity NaOH and historically used asbestos diaphragms, which have largely been replaced.

- Membrane cell: an ion-exchange membrane completely separates the anode and cathode sides, allowing selective transport of ions. This configuration yields chlorine gas at the anode and a relatively pure, concentrated sodium hydroxide solution at the cathode with minimal chloride contamination, and it is energy-efficient and widely adopted in modern plants.

Chlor-alkali products are fundamental to many industries: chlorine is used for disinfection and chemical synthesis (notably PVC), while sodium hydroxide serves as a versatile base in chemical processing, paper production, and water treatment. Safety considerations include the toxicity of chlorine and the caustic nature of NaOH; environmental concerns are most pronounced with mercury-based processes.