The chlor-alkali industry involves the production of chlorine and sodium or potassium hydroxide through the electrolysis of brine solutions. Brine refers to water saturated with sodium chloride, which is purified and used as the electrolyte in chlor-alkali plants. Three main electrolysis processes are used: mercury cell process, diaphragm cell process, and membrane cell process. Each process utilizes direct current electricity to facilitate the chemical reactions that produce chlorine gas, hydrogen gas, and sodium hydroxide or potassium hydroxide solutions.

Mercury Cell Process

The mercury cell process was historically one of the most commonly used methods for Chlor-Alkali production. It involves electrolyzing a solution of sodium chloride between a graphite anode and a mercury cathode. Chlorine gas forms at the anode while sodium collects as an amalgam with the mercury cathode. Water is then added to the mercury-sodium amalgam to separate sodium hydroxide. This process generates sodium hydroxide, chlorine, hydrogen gas, and liquid mercury as a byproduct. Mercury cells remained popular for their low energy requirements but have largely been phased out due to environmental concerns over mercury emissions.

Diaphragm Cell Process

The diaphragm cell process serves as an alternative to mercury cells. In this process, an asbestos diaphragm separates the anode and cathode compartments of the electrolysis cell. A solution of sodium chloride flows between the electrodes on either side of the diaphragm. Chlorine forms at the anode on one side while sodium hydroxide collects on the other cathode side without mercury use. Hydrogen gas is also liberated during the reaction. This process is energy efficient like mercury cells and eliminates liquid mercury emissions.

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