Method for improving the performance of nickel electrodes

Abstract

The invention relates to a method for improving the performance of coated nickel electrodes in chloralkali electrolysis by adding water-soluble platinum compounds to the catholyte during the electrolysis at low current density.

1 - 9 . (canceled) 
     
     
         10 . A method for improving the performance of nickel electrodes which are uncoated or have a coating based on platinum metals, platinum metal oxides or a mixture of platinum metals and platinum metal oxides and are used in sodium chloride electrolysis by the membrane process, where a platinum compound which is water-soluble or soluble in sodium hydroxide solution, in particular hexachloroplatinic acid or a sodium platinate, particularly preferably Na 2 PtCl 6  and/or Na 2 Pt(OH) 6  is metered into the catholyte during the electrolysis of sodium chloride, wherein the addition is carried out during electrolysis operation at a reduced current density of from 0.2 A/m 2  to 95 A/m 2 , preferably from 0.5 A/m 2  to 70 A/m 2 , particularly preferably from 1 A/m 2  to 50 A/m 2 , at a temperature of the catholyte in the range from 40° C. to 95° C., using an amount of platinum per m 2  of electrode area of from 0.3 g/m 2  to 10 g/m 2 , preferably from 0.35 g/m 2  to 8 g/m 2 , particularly preferably from 0.4 g/m 2  to 5 g/m 2 , with the decreased current density being maintained from the commencement of the metered addition for a total of from 2 to 360 minutes, preferably from 4 minutes to 300 minutes, particularly preferably from 5 minutes to 200 minutes. 
     
     
         11 . The method as claimed in  claim 10 , wherein not only the platinum compound but also further other water-soluble compounds of the noble metals of transition group  8  of the Periodic Table of the Elements, in particular compounds of the platinum group, particularly preferably of palladium, iridium, rhodium, osmium or ruthenium, preferably palladium or ruthenium, are added. 
     
     
         12 . The method as claimed in  claim 11 , wherein the proportion of noble metal of the further water-soluble compounds of the noble metals of transition group 8 is from 1 to 50% by weight, based on the platinum metal of the soluble platinum compound. 
     
     
         13 . The method as claimed in  claim 10 , wherein the temperature of the catholyte at which the metered addition of the platinum compound is carried out is in the range from 60 to 90° C. 
     
     
         14 . The method as claimed in  claim 10 , wherein the proportion of platinum of the platinum compound in the catholyte after the metered addition is from 0.01 to 310 mg/l. 
     
     
         15 . The method as claimed in  claim 10 , wherein the volume flow of the catholyte during the contact time of the electrode surface with the catholyte containing the platinum compound is from 0.1 to 10 l/min. 
     
     
         16 . The method as claimed in  claim 10 , wherein the concentration of platinum metal in the catholyte exiting from the electrolysis cell is continuously or discontinuously monitored. 
     
     
         17 . The method as claimed in  claim 10 , wherein the method is carried out on coated nickel electrodes, with the coating comprising platinum metal/platinum metal oxide based on one or more metals from the group consisting of: ruthenium, iridium, palladium, platinum, rhodium and osmium, preferably from the group consisting of: ruthenium, iridium and platinum. 
     
     
         18 . A process for producing chlorine, sodium hydroxide and hydrogen according to the principle of membrane electrolysis on a production scale using nickel electrodes or coated nickel electrodes as cathode, comprising the steps:
 introduction of an aqueous solution containing sodium chloride into an anode chamber having an anode and introduction of sodium hydroxide solution into a cathode chamber having a cathode, where anode chamber and cathode chamber are separated from one another by an ion-exchange membrane;   setting of a production current density of at least  1  kA/m 2  based on the electrode area;   discharge of the solution containing sodium chloride from the anode chamber together with the chlorine gas formed at the anode and separation of the chlorine from the liquid phase;   feeding of the chlorine which has been separated off to a suitable treatment, in particular comprising at least drying, purification and optionally compression of the chlorine gas;   feeding of the sodium chloride-containing solution discharged from the anode space to concentration and purification, in particular comprising at least the steps: destruction of chlorate by-products, dechlorination, increasing of the concentration by addition of sodium chloride, purification by means of precipitation reagents, filtration and ion exchange to remove undesirable cations,   subsequent reintroduction of the solution containing sodium chloride into the anode chamber;   discharge of the sodium chloride solution from the cathode chamber together with the hydrogen formed at the cathode and separation of the hydrogen from the liquid phase;   optionally feeding of the hydrogen which has been separated off to a suitable treatment and purification;   feeding of the sodium hydroxide solution discharged from the cathode chamber to a collection vessel and optionally to a further suitable treatment and purification;   dilution of a partial amount of the sodium hydroxide solution discharged from the cathode space with water and reintroduction into the cathode space;   
       wherein the current density is reduced to a value of less than 100 A/m 2  but at least 0.2 A/m 2  in order to lower the electrolysis voltage on attainment of a prescribed average maximum voltage value during electrolysis operation, the method as claimed in  claim 10  is carried out and the current density is subsequently increased again to the production current density and production is continued.