Membrane electrolysis processes for akaline chloride solutions, using a gas-diffusion electrode

Abstract

The invention relates to processes for the electrolysis of alkali chlorides by means of oxygen-depolarized electrodes, said processes having specific operating parameters for shut-down and restarting.

1 .- 8 . (canceled) 
     
     
         9 . A process for chloralkali electrolysis using an electrolysis cell in a gap arrangement, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell comprises at least one anode space with anode and an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode which comprises a silver-containing catalyst and an in particular from 0.01 mm to 3 mm thick sheet-like porous element through which catholyte flows between gas diffusion electrode and membrane, the electrolysis process comprises at least the following steps in this order:
 a) lowering of the electrolysis voltage and removal of chlorine from the anolyte so that less than 10 mg/l of active chlorine is present in the anolyte by maintaining an electrolysis voltage per element of from 0.1 to 1.4 V and a current density which is greater than zero,   b) and setting of the pH of the anolyte to a value in the range from pH 2 to pH 12 during step a),   c) residence under these conditions as long as electrolyte is present in the catholyte gap (or electrolyte flows through the latter),   and optionally for emptying of the electrolysis cell the further steps:   d) cooling of the anolyte to a temperature below 70° C. with maintenance of the electrolysis voltage in the range from 0.1 to 1.4 V,   e) switching off of the electrolysis voltage at an electrolyte temperature of <55° C.,   f) emptying of the cathode gap,   g) emptying of the anode space,   h) preferably renewed filling of the anode space with one of the following liquids: dilute alkali metal chloride solution having a maximum concentration of 4 mol/l or deionized water, and subsequent emptying of the anode space,   i) filling of the cathode space with one of the following liquids: dilute alkali metal hydroxide solution having a maximum concentration of 10 mol/l or deionized water, with subsequent emptying of the cathode space.   
     
     
         10 . The process as claimed in  claim 9 , wherein the alkali metal chloride is sodium chloride or potassium chloride. 
     
     
         11 . The process as claimed in  claim 9 , wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide. 
     
     
         12 . The process as claimed in  claim 9 , wherein the gas diffusion electrode is supplied with oxygen gas on its side facing away from the catholyte. 
     
     
         13 . The process as claimed in  claim 9 , wherein the oxygen gas flow to the gas diffusion electrode is maintained when the electrolysis is switched off. 
     
     
         14 . A process for chloralkali electrolysis using a membrane electrolysis cell in a gap arrangement between ion exchange membrane and gas diffusion electrode, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell has at least one anode space with anode for accommodating an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode, which comprises a silver-containing catalyst, and a sheet-like, porous element in the gap between ODE and membrane, which element has a thickness of, in particular, from 0.01 mm to 3 mm and through which catholyte flows during operation, wherein, for start-up of the electrolysis process, at least the following steps are carried out in this order:
 j) filling of the anode space with anolyte having a temperature of at least 50° C. and passage of the anolyte through it,   k) preheating of catholyte to a temperature of at least 50° C.,   l) filling of the cathode space and the porous element with preheated catholyte having a concentration of from 7.5 to 10.5 mol/l and passage of the catholyte through them,   m) setting of the electrolysis voltage to a value in the range from 0.1 to 1.4 V,   n) setting and maintenance of the temperature of the catholyte and anolyte leaving the cell independently of one another to a temperature in the range from 70 to 100° C.,   o) setting of the concentration of the catholyte in the feed to the cell so that an alkali metal hydroxide concentration in the range from 7.5 to 12 mol/l is obtained in the output,   p) setting of the concentration of the anolyte in the feed to the cell so that an alkali metal chloride concentration in the range from 2.9 to 4.3 mol/l is obtained in the output,   q) setting of the production current density to a value of at least 2 kA/m 2 , preferably at least 4 kA/m 2 .   
     
     
         15 . The process as claimed in  claim 14 , wherein the increase in the current density to the production current density in step q) is carried out at a rate of from 0.018 kA/(m 2 *min) to 0.4 kA/(m 2 *min) until the current density at the electrolysis element is at least 2 kA/m 2 . 
     
     
         16 . The process as claimed in  claim 14 , wherein the start-up is a restarting of an electrolysis cell which has been shut down according to a process comprising at least the following steps in this order:
 a) lowering of the electrolysis voltage and removal of chlorine from the anolyte so that less than 10 mg/l of active chlorine is present in the anolyte by maintaining an electrolysis voltage per element of from 0.1 to 1.4 V and a current density which is greater than zero,   b) and setting of the pH of the anolyte to a value in the range from pH 2 to pH 12 during step a),   c) residence under these conditions as long as electrolyte is present in the catholyte gap (or electrolyte flows through the latter),   and optionally for emptying of the electrolysis cell the further steps:   d) cooling of the anolyte to a temperature below 70° C. with maintenance of the electrolysis voltage in the range from 0.1 to 1.4 V,   e) switching off of the electrolysis voltage at an electrolyte temperature of <55° C.,   f) emptying of the cathode gap,   g) emptying of the anode space,   h) preferably renewed filling of the anode space with one of the following liquids: dilute alkali metal chloride solution having a maximum concentration of 4 mol/l or deionized water, and subsequent emptying of the anode space,   i) filling of the cathode space with one of the following liquids: dilute alkali metal hydroxide solution having a maximum concentration of 10 mol/l or deionized water, with subsequent emptying of the cathode space.