Process for electrolysis of alkali metal chlorides with oxygen-consuming electrodes having orifices

Abstract

An oxygen-consuming electrode for use in chloralkali electrolysis, having a novel coating, the production thereof, an electrolysis cell comprising the oxygen-consuming electrode and parameters for the startup and shutdown of the electrolysis apparatus, compliance with which prevents damage to the cell.

1 . Oxygen-consuming electrode for use in electrolysis cells with micro-gap configuration, having at least one carrier element in the form of a sheetlike structure and a coating with a gas diffusion layer and a catalytically active component, wherein the coating with the gas diffusion layer is provided with one or more apertures having a diameter or a height of 0.5 mm-20 mm. 
     
     
         2 . Oxygen-consuming electrode according to  claim 1 , wherein the total area of the apertures in relation to the side of the electrode which is to face the ion exchange membrane is 0.05 to 15% of the electrode area. 
     
     
         3 . Oxygen electrode according to  claim 1 , wherein only the coating of the carrier element with the gas diffusion layer and of the catalytically active component has apertures. 
     
     
         4 . Oxygen electrode according to  claim 1 , wherein the apertures are continuous through coating and carrier element. 
     
     
         5 . Oxygen-consuming electrode according to  claim 1 , wherein the apertures are formed from an oxygen-consuming electrode by punching, cutting, machining or drilling. 
     
     
         6 . Oxygen-consuming electrode according to  claim 1 , wherein corresponding orifices, in the process for producing the electrode, are kept clear in the course of coating of the carrier element by insertion of removable displacement bodies which are subsequently removed after completion of the coating. 
     
     
         7 . Electrolysis cell, comprising oxygen-consuming electrode according to  claim 1 , wherein the electrolysis cell has a distance of 0.01 mm to 2 mm between ion exchange membrane and oxygen-consuming electrode. 
     
     
         8 . Process for conducting a chloralkali electrolysis with an electrolysis cell comprising an oxygen-consuming electrode according to  claim 1 , in a micro-gap configuration, the cell having at least one anode space with anode and an anolyte comprising alkali metal chloride, an ion exchange membrane, a cathode space with an oxygen-consuming electrode comprising a silver-containing catalyst as the cathode, and a flat, porous element, which has a thickness of 0.01 mm to 2 mm, between OCE and membrane and through which catholyte flows, wherein application of the electrolysis voltage is preceded, in a first step, by wetting of the oxygen-consuming electrode on the gas side with an aqueous alkali metal hydroxide solution having a content of chloride ions of not more than 1000 ppm and, after subsequent introduction of the anolyte into the anode space and of an oxygenous gas into the cathode space, the electrolysis voltage is applied. 
     
     
         9 . Process for conducting a chloralkali electrolysis with an electrolysis cell comprising an oxygen-consuming electrode according to  claim 1  in a micro-gap configuration, the cell having at least one anode space comprising anolyte, an ion exchange membrane, a cathode space with an OCE having a silver-containing catalyst, and a flat porous element, having a thickness of 0.01 mm to 2 mm, between OCE and membrane wherein, at the end of the electrolysis operation, for shutdown, at least the following steps are conducted in the following sequence:
 switching off the electrolysis voltage 
 emptying the anode space 
 preferably refilling the anode space with one of the following liquids: dilute alkali metal chloride solution of not more than 4 mol/l or deionized water, with subsequent emptying of the anode space 
 filling the cathode space with one of the following liquids: dilute alkali metal hydroxide solution of not more than 4 mol/l or deionized water, with subsequent emptying of the cathode space. 
 
     
     
         10 . Process according to  claim 8 , wherein the alkali metal hydroxide solution used to wet the cathode space prior to application of the electrolysis voltage has a content of chlorate ions of not more than 20 ppm. 
     
     
         11 . Process according to  claim 8  wherein, after shutdown and emptying of the electrolysis cell, and optionally repeatedly every 1 to 12 weeks, the anode space is flushed with a dilute alkali metal chloride solution having a content of 1.7 to 3.4 mol/l, and the cathode space with a dilute alkali metal hydroxide solution having a content of 2 to 9 mol/l. 
     
     
         12 . Process according to  claim 8 , wherein the alkali metal chloride is sodium chloride or potassium chloride.