What is chloralkali

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Chloralkali electrolysis is the name given to the electrolysis of aqueous sodium chloride solutions, which is carried out according to various processes.

Diaphragm process

In the diaphragm process, the cathode and anode compartments are separated by a porous wall. The diaphragm is intended to prevent the solutions in the cathode and anode compartment from being mixed by convection. The cathode is made of iron, the anode of graphite.

The redox couple H2/H3O+ in a neutral NaCl solution has a higher potential than Na / Na+. In addition, since the overvoltage of the hydrogen at the iron is not too great, hydrogen is developed at the cathode. The oxonium ions present in the solution originate from the autoprotolysis equilibrium of the water. As a result of the discharge of oxonium ions at the cathode, the solution in the cathode compartment becomes basic. At the anode, because of the overvoltage of the oxygen on the carbon, Cl-Ions discharged.

The electrolysis thus supplies the products hydrogen, chlorine and caustic soda. In order to obtain the purest possible sodium hydroxide solution, mixing of the NaCl solution with the sodium hydroxide solution must be avoided. In addition, the chlorine formed must be kept away from the sodium hydroxide solution, otherwise the chlorine will be disproportionated:

Both are achieved through the use of the diaphragm.

Chlor-alkali electrolysis using the mercury process

The chlor-alkali electrolysis according to the mercury process (amalgam process) works with liquid mercury as the cathode.

Graphite is used as the anode. Here, too, the oxidation of chloride to chlorine takes place at the anode. Because hydrogen has a high overvoltage in relation to mercury, sodium ions are discharged here, whereby the sodium formed dissolves in the mercury as sodium amalgam. By pumping the liquid amalgam it gets into the water-filled decomposer, where the conversion takes place on graphite catalysts.

The addition of the partial equations results in the same overall equation as in the diaphragm process. The main advantage of the mercury process is the higher purity of the caustic soda. Disadvantages are the high purity requirements for the NaCl brine and the high costs for cleaning it in order to avoid environmental pollution from waste water containing mercury. Since chlorine is the main product in demand today, the less problematic diaphragm process is gaining importance again. (The situation could be reversed if the production of organochlorine compounds declines in the future).