Chlorine manufacture - Diaphragm Cell
The diaphragm separates the anode and cathode, but allows ions to pass in both directions. Anodes are titanium coated, and cathodes are made from steel.

In order to prevent hydroxyl ions passing through the diaphragm the level of liquid is kept higher in the anode compartment, maintaining a flow from anode to cathode. Unfortunately this results in some chloride ion migration into the cathode compartment, giving a sodium hydroxide/sodium chloride mixture.

One of the main disadvantages of the diaphragm cell is that it produces low quality sodium hydroxide (containing sodium chloride) at only 12% concentration, and so additional energy is needed to purify and concentrate it before it can be sold. In the US, where energy tends to be cheaper, the diaphragm cell is used more widely than in the EU.

There are also safety and disposal issues when the asbestos diaphragms reach the end of their life. The cell can, however, use low purity rock salt, saving the need for purification before electrolysis.
diagram: diaphragm cell

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