The membrane cell process
The two electric connection points of each chlorine production cell, the anode and the cathode, are separated by an ion-exchange membrane. Only sodium ions and a little water pass through the membrane.
The brine is de-chlorinated and re-circulated. Solid salt is usually needed to re-saturate the brine. After purification by precipitation-filtration, the brine is further purified with an ion exchanger.
The caustic solution leaves the cell with about 30% concentration and, at a later stage in the process, is usually concentrated to 50%. The chlorine gas contains some oxygen and must often be purified by liquefaction and evaporation.
The consumption of electric energy is the lowest of the three processes and the amount of steam needed for concentration of the caustic is relatively small (less than one tonne per tonne of caustic soda).
Chlorine producers across Europe are progressively moving towards this method of making their product as the membrane cell process is the most environmentally sound way of manufacturing chlorine. In 2014, membrane cell capacity accounted for 60% of total installed chlorine production capacity in Europe.
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