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In a process for chlorine-alkali electrolysis, use is made of an oxygen depletion cathode. The process is run with a high excess of oxygen. The oxygen needed for this is provided for a device of the gas separation, for example a VPSA plant or an air fractionation plant. The large quantities of oxyge

In a process for chlorine-alkali electrolysis, use is made of an oxygen depletion cathode. The process is run with a high excess of oxygen. The oxygen needed for this is provided for a device of the gas separation, for example a VPSA plant or an air fractionation plant. The large quantities of oxygen produced lead to considerable costs of the process. According to the invention, the oxygen-rich atmosphere remaining after passing through the process is fed back into the device for gas separation as input gas. The device the gas separation is therefore operated with an oxygen-rich input gas and therefore produces a larger quantity of oxygen-rich gas, which in turn is fed to the oxygen depletion cathode. As a result of the circulation of the gas, the economy of the overall process is increased considerably.

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1. A process for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride in an electrolysis cell, which comprises: providing an anode half element having an anode, a cathode half element having a gas diffusion electrode as a cathode and a cation-exchange membrane for el

1. A process for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride in an electrolysis cell, which comprises: providing an anode half element having an anode, a cathode half element having a gas diffusion electrode as a cathode and a cation-exchange membrane for electrolytically separating the anode half element and the cathode half element,feeding an oxygen-containing gas to the cathode half element and discharging gas containing excess oxygen from the cathode half element, andat least partly feeding the gas containing excess oxygen taken off from the cathode half element to a gas separation apparatus in which the gas containing excess oxygen is separated into an oxygen-rich fraction and a low-oxygen fraction and the oxygen-rich fraction is subsequently fed to the cathode half element. 2. The process as claimed in claim 1, wherein the gas separation apparatus operates according to a membrane process, an adsorption process or a cryogenic gas separation process. 3. The process as claimed in claim 1, which further comprises separating off trace gases such as hydrogen or argon from the oxygen rich fraction using the gas separation apparatus. 4. An apparatus for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride in an electrolysis cell, which comprises: an anode half element having an anode,a cathode half element having a gas diffusion electrode as a cathode,a cation-exchange membrane for electrolytically separating the anode half element and the cathode half element,an oxygen inlet for feeding an oxygen-rich gas to the cathode half element,an oxygen outlet for discharging gas containing excess oxygen from the cathode half element, anda gas separation apparatus,the oxygen outlet is connected so as to allow flow to the gas separation apparatus in which the oxygen-containing gas is separated into an oxygen-rich fraction and a low-oxygen fraction and the gas separation apparatus is equipped with an outlet line for the oxygen-rich fraction which is connected so as to allow flow to the oxygen inlet of the cathode half element. 5. The apparatus as claimed in claim 4, wherein a pressure swing adsorption plant, a vacuum pressure swing adsorption plant or a cryogenic air fractionation plant is used as gas separation apparatus. 6. The apparatus as claimed in claim 4, wherein the gas separation apparatus separates off trace gases such as hydrogen or argon from the oxygen rich fraction.