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.

Description

FIELD OF THE INVENTION[0001]The invention relates to an electrode for electrochemical generation of hypochlorite.BACKGROUND OF THE INVENTION[0002]The electrolytic production of hypochlorite from diluted brines of alkali metal chlorides, e.g. of sodium hypochlorite by electrolysis of aqueous solution of sodium chloride or of sea-water, is one of the most common processes in the domain of industrial electrochemistry. The production of hypochlorite is always accompanied by the generation of various by-products deriving from the oxidation of chlorides (generally grouped under the name of “active chlorine”) and in some cases of oxygenated species such as peroxides, most of which have a very limited lifetime; for the sake of brevity, in the present text the whole of such products in aqueous solution, mostly consisting of alkali metal hypochlorite and hypochlorous acid in a ratio mainly depending on pH, is indicated as hypochlorite. Depending on the production volumes and concentrations, p...

AI Technical Summary

Benefits of technology

[0010]The thermal treatment of the substrate followed by etching according to the indicated parameters has likely the effect of segregating the impurities of the substrate in correspondence of the crystalline grain boundaries; in this way, grain boundaries become a zone of preferential attack for the subsequent etching. This can have the advantage of favouring the formation of a roughness profile consisting of deep and relatively spaced apart peaks and valleys, so as to efficaciously anchor the internal catalytic layer even at reduced specific loadings of noble metal. The treatment can be carried out in a common oven with forced air ventilation; at the end of the thermal treatment, the substrate can be allowed to cool down slowly in the oven and extracted when the temperature goes below 300° C. In one embodiment, particularly suited to titanium and titanium alloy substrates, the acid etching is carried out with 25-30% by weight sulphuric acid containing 5 to 10 g/l of dissolved titanium, at a temperature ranging from 80 to 90° C. until reaching a weight loss not lower than 180 g/m2 of metal. The inventors observed that these conditions are particularly favourable for the preferential attack of impurities segregated on the grain boundary during the previous thermal treatment, facilitating the achievement of the desired roughness profile. The etching bath can be put into service with a dissolved titanium concentration of about 5 g/l and used until the titanium concentration reaches about 10 g/l by effect of the dissolution of the substrate itself, then reintegrated with a fresh acid addition until the concentration of dissolved titanium is brought back to the original value of about 5 g/l. The titanium in the solution favours the kinetics of dissolution of the valve metal in the etching phase: concentrations below 5 g/l are associated with a dissolution rate which...

Problems solved by technology

The titanium in the solution favours the kinetics of dissolution of the valve metal in the etching phase: concentrations below 5 g/l are associ...