Electrode for electrochlorination

Abstract

The invention relates to an electrode for electrochemical generation of hypochlorite. The electrode comprises a valve metal substrate coated with a catalytic system consisting of two super-imposed layers of distinct composition and having a different activity towards hypochlorite anodic generation from chloride solutions. The electrode has a high duration in cathodic operation conditions, imparting self-cleaning characteristics thereto when used in combination with an equivalent one with periodic polarity reversal. Moreover, the deactivation of the electrode at the end of its life cycle occurs in two subsequent steps, allowing to schedule the substitution thereof with a significant notice period.

1 . Electrode for electrolytic cell comprising a valve metal substrate, an internal catalytic coating and an external catalytic coating of different composition and higher activity overlaid thereto, the roughness profile of said substrate having an R a  value of 4 to 8 μm and an R z  value of 20 to 50 μm, said internal catalytic coating containing oxides of iridium, ruthenium and a valve metal selected between tantalum and niobium with an overall specific loading of iridium and ruthenium expressed as metals of 2 to 5 g/m 2 , said external catalytic coating containing noble metal oxides with a specific loading not lower than 7 g/m 2 . 
     
     
         2 . The electrode according to  claim 1 , wherein said external catalytic coating comprises a mixture of oxides of iridium, ruthenium and titanium with a molar concentration of ruthenium of 12-18%, a molar concentration of iridium of 6-10% and a molar concentration of titanium of 72-82%. 
     
     
         3 . The electrode according to  claim 1 , wherein said internal catalytic coating contains a mixture of oxides of iridium, ruthenium and tantalum with a molar concentration of ruthenium of 42-52%, a molar concentration of iridium of 22-28% and a molar concentration of tantalum of 20-36%. 
     
     
         4 . The electrode according to  claim 1 , further comprising a protective layer consisting of valve metal oxides interposed between said substrate and said internal catalytic coating. 
     
     
         5 . Method for the production of an electrode comprising:
 thermally treating a valve metal substrate, the roughness profile of said substrate having an R a  value of 4 to 8 μm and an R z  value of 20 to 50 μm, for a time not lower than 4 hours at a temperature not lower than 560° C.,   acid etching,   applying an internal catalytic coating by thermal decomposition of a precursor solution, the internal catalytic coating comprising oxides of iridium, ruthenium and a valve metal selected between tantalum and niobium with an overall specific loading of iridium and ruthenium expressed as metals of 2 to 5 g/m 2 ;   applying an external catalytic coating by thermal decomposition of a precursor solution, external catalytic coating containing noble metal oxides with a specific loading not lower than 7 g/m 2 .   
     
     
         6 . The method according to  claim 5 , wherein said acid etching step is carried out in 25-30% by weight sulphuric acid containing 5 to 10 g/l of dissolved titanium, at a temperature of 80 to 90° C. until reaching a weight loss not lower than 180 g/m 2  of metal. 
     
     
         7 . The method according to  claim 5  comprising, before said internal catalytic coating application step, an additional step of application of a protective layer consisting of valve metal oxides by a technique selected between a thermal treatment of the substrate, a flame or plasma spray application and a thermal decomposition of a precursor solution. 
     
     
         8 . Electrochemical cell for alkali metal hypochlorite production comprising an aqueous electrolyte containing alkali metal chlorides, at least one pair of electrodes, the electrodes each comprising a valve metal substrate, an internal catalytic coating and an external catalytic coating of different composition and higher activity overlaid thereto, the roughness profile of said substrate having an R a  value of 4 to 8 μm and an R z  value of 20 to 50 μm, said internal catalytic coating containing oxides of iridium, ruthenium and a valve metal selected between tantalum and niobium with an overall specific loading of iridium and ruthenium expressed as metals of 2 to 5 g/m 2 , said external catalytic coating containing noble metal oxides with a specific loading not lower than 7 g/m 2 ; and
 a timed control system to alternately polarise said electrodes for a predetermined period of time determining the functioning thereof as anode and as cathode respectively. 
 
     
     
         9 . The cell according to  claim 8 , wherein said aqueous electrolyte has an ion chloride concentration of 2 to 20 g/l. 
     
     
         10 . The cell according to  claim 8 , wherein said predetermined period of time is 0.5 to 60 minutes.