Classifications

• • C25B11/0405—
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
  • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
  • C02F1/46104—Devices therefor; Their operating or servicing
  • C02F1/46109—Electrodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/093—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds at least one noble metal or noble metal oxide and at least one non-noble metal oxide
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
  • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
  • C02F1/467—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis by electrochemical disinfection; by electrooxydation or by electroreduction
  • C02F1/4672—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis by electrochemical disinfection; by electrooxydation or by electroreduction by electrooxydation
  • C02F1/4674—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis by electrochemical disinfection; by electrooxydation or by electroreduction by electrooxydation with halogen or compound of halogens, e.g. chlorine, bromine
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
  • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
  • C02F1/46104—Devices therefor; Their operating or servicing
  • C02F1/46109—Electrodes
  • C02F2001/46119—Cleaning the electrodes
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
  • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
  • C02F1/46104—Devices therefor; Their operating or servicing
  • C02F1/46109—Electrodes
  • C02F2001/46133—Electrodes characterised by the material
  • C02F2001/46138—Electrodes comprising a substrate and a coating
  • C02F2001/46142—Catalytic coating
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2201/00—Apparatus for treatment of water, waste water or sewage
  • C02F2201/46—Apparatus for electrochemical processes
  • C02F2201/461—Electrolysis apparatus
  • C02F2201/46105—Details relating to the electrolytic devices
  • C02F2201/4612—Controlling or monitoring
  • C02F2201/46125—Electrical variables
  • C02F2201/4613—Inversing polarity
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2303/00—Specific treatment goals
  • C02F2303/14—Maintenance of water treatment installations

Definitions

• the invention relates to an electrode for electrochemical generation of hypochlorite.
• 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.
• 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.
• product hypochlorite may be used in several ways, for instance in paper and cloth bleaching, in the disinfection of drinking or pool water or for domestic uses.
• Potassium hypochlorite is also employed in preventive or therapeutic treatment of agricultural cultivations.
• Hypochlorite is generally produced in undivided electrolytic cells with electrodes of various shapes and geometry, for example with interleaved planar electrodes.
• hypochlorite production takes place by anodic oxidation of chloride, while hydrogen is evolved at the cathode; when the chloride solution to be electrolysed contains sensible amounts of calcium or magnesium ions, such as the case of civil water chlorination, the natural alkalinisation of the electrolyte in the proximity of the cathode surface causes the local precipitation of limestone, which tends to deactivate the cathodes and prevent their operability after some time.
• one of the most effective consists of submitting the electrodes to cyclic potential reversal, alternating their use as cathodes and as anodes.
• the carbonate deposit which settles on the surface of an electrode under cathodic operation is dissolved during the subsequent operation as anode, in which condition the surrounding environment tends to acidify.
• the electrodes of an electrochlorinator designed to work under alternate electrodic polarisation are activated with a catalyst aimed at maximising the efficiency of the hypochlorite generation anodic reaction, more critical both in terms of overpotential and of selectivity due to the concurrent, undesired oxygen evolution anodic reaction.
• a catalyst whose efficiency is well known in this process comprises a mixture of oxides of noble metals (typically ruthenium and optionally iridium or palladium) and of valve metal oxides, preferably titanium oxide.
• noble metals typically ruthenium and optionally iridium or palladium
• valve metal oxides preferably titanium oxide.
• the noble metal in the catalytic formulation has the main purpose of catalysing the anodic reaction and is bound to the valve metal in a solid solution which contributes to reduce the consumption thereof; other valve metals, such as tantalum and niobium, might be used to replace titanium, although they are considered a less valid alternative due to their tendency to decrease the anodic reaction selectivity thereby producing a higher amount of oxygen with a net loss of hypochlorite.
• the functioning of the electrodes in alternate polarisation conditions allows operating with good efficiency while keeping the electrode surface sufficiently clean from insoluble deposits; nevertheless, cathodic operation under hydrogen evolution of such type of electrode configurations entails a reduced operative lifetime, because the adhesion of the coating to the substrate tends to be hampered in these conditions.
• the deactivation mechanism of this type of electrodes mainly associated with the consumption of the catalyst layer or the detachment thereof from the substrate, brings about a sudden failure with no significant premonitory sign; in order to prevent serious inconveniences, an estimation of residual lifetime of the electrodes in a cell is usually carried out on a statistical basis and their replacement is scheduled before a quick and irreversible failure occurs. Since the deactivation of electrodes working under this kind of operative conditions is affected by several factors, the variability is rather high, so that keeping a sufficient margin of safety often implies replacing electrodes which might have been functioning for a significant residual time.
• an electrode for hypochlorite generation comprises a substrate made of a valve metal, typically titanium optionally alloyed, having a suitable roughness profile, an internal catalytic coating and an external catalytic coating of different composition and higher activity overlying the internal catalytic coating, the roughness profile being characterised by R a comprised between 4 and 8 ⁇ m and R z comprised between 20 and 50 ⁇ m, the internal catalytic coating containing oxides of iridium, ruthenium and a valve metal selected between tantalum and niobium with an overall specific loading of iridium plus ruthenium expressed as metals of 2 to 5 g/m 2 , the external catalytic coating containing noble metal oxides with an overall specific loading not lower than 7 g/m 2 .
• a valve metal typically titanium optionally alloyed
• the 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%.
• a roughness profile as indicated, characterised by rather deep cavities relatively spaced apart, allows overlying two distinct catalytic coatings so that the innermost coating, strongly anchored within the cavities, starts working only upon completion of the detachment of the outermost coating.
• the inventors surprisingly observed that the electrode as hereinbefore described is characterised by a two-step deactivation mechanism, with a first voltage increase with respect to the normal operative voltage up to sensibly higher values still suitable however to continue its operation (for instance a voltage increase of 500-800 mV) and a second, quicker voltage increase forcing its definitive shut-down.
• a configuration providing an internal coating not too efficient towards hypochlorite generation such as a combination of oxides of noble metals as iridium and ruthenium in admixture with an oxide of tantalum and/or niobium, and an external coating of higher performances, such as a mixture of oxides of iridium, ruthenium and titanium, allows the electrode to operate at excellent voltage levels until the external coating, conveniently provided at a higher specific loading, is present on the electrode surface; once the external coating is worn off, the internal coating gets uncovered.
• the internal coating which can have a limited specific loading but which is extremely well anchored to the surface due to the specially selected roughness profile, is capable of operating, although at lesser efficiency and higher cell voltage, for a sufficiently prolonged period of time that permits scheduling the substitution of the whole cell or of the electrodes preventing the risk of a sudden failure.
• the internal catalytic coating contains a mixtures of oxides of iridium, ruthenium and tantalum with a ruthenium molar concentration of 42-52%, an iridium molar concentration of 22-28% and a tantalum molar concentration of 20-36%.
• the electrode as hereinbefore described comprises a thin protective layer of valve metal oxides, for instance a mixture of oxides of titanium and tantalum, interposed between the substrate and the internal catalytic layer. This can have the advantage of protecting the substrate from passivation phenomena, improving its overall duration without fundamentally affecting the characteristic two-step deactivation mechanism.
• a method for manufacturing an electrode as hereinbefore described comprises:
• 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.
• 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/m 2 of metal.
• sulphuric acid containing 5 to 10 g/l of dissolved titanium
• 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 results too slow for practical purposes. On the other hand, an excessive concentration slows down the attack again.
• a thermally-treated titanium substrate subjected to an etching step as described reaches a weight loss of 180-220 grams per square metre of surface, a value considered to be suitable for the subsequent catalytic coating deposition, in a 2 to 3 hour time.
• the thermally-treated substrate can be subjected to sandblasting before the acid etching.
• the substrate subjected to the desired thermal treatment, the optional sandblasting and the etching is provided with a this protective layer consisting of valve metal oxides prior to the application of catalytic coatings.
• the protective layer can be applied by means of a second thermal treatment of the valve metal substrate in air, with growth of the corresponding oxide, or by an application, for instance of titanium and/or tantalum oxide, by flame or plasma spraying, or by thermal decomposition of a suitable precursor solution.
• the formation of the internal catalytic layer can be carried out by application and subsequent thermal decomposition, optionally in multiple coats, of a precursor solution containing salts or other compounds of iridium, ruthenium and at least one valve metal selected between tantalum and niobium, until reaching an overall loading of 2-5 g/m 2 of noble metal defined as sum of iridium and ruthenium expressed as metals.
• the formation of the external catalytic layer can be carried out by application and subsequent thermal decomposition, optionally in multiple coats, of a precursor solution containing salts or other compounds of noble metals, for instance iridium and ruthenium, and of at least one valve metal, for instance titanium, until reaching an overall loading of at least 7 g/m 2 of noble metal defined as sum of iridium and ruthenium expressed as metals.
• a precursor solution containing salts or other compounds of noble metals for instance iridium and ruthenium
• at least one valve metal for instance titanium
• an electrochemical cell for production of hypochlorite from a chloride-containing aqueous electrolyte comprises pairs of electrodes as hereinbefore described and a timed control used to polarise the electrodes alternatively so as to determine the operation of one electrode of the pair as cathode and of the other as anode and to cyclically reverse their polarity after a predetermined period of time, which in one embodiment is comprised between 30 seconds and 60 minutes.
• the aqueous electrolyte is an alkali metal chloride solution, for instance sodium chloride or potassium chloride or a mixture of the two, with a chloride ion concentration of 2 to 20 g/l.
• a first precursor solution containing 47% molar Ru, 24.7% molar Ir and 28.3% molar Ta was obtained starting from a 20% by weight commercial solution of RuCl 3 , a 23% by weight commercial solution of H 2 IrCl 6 and a solution of TaCI 5 at a concentration of 50 g/l obtained by dissolution of solid TaCl 5 in 37% by weight HCl by heating under stirring and subsequent dilution with water and commercial 2-propanol.
• the components were mixed under stirring, first adding a weighted amount of H 2 IrCl 6 solution, then the corresponding amount of RuCl 3 .
• the solution of TaCl 5 was added and after 30 more minutes the mixture was brought to volume with 2-propanol, protracting the stirring for 30 minutes more.
• the thus obtained precursor solution was applied to the titanium sheets, previously dried on air at 50° C., by brushing in 3 coats, with a subsequent decomposition cycle in forced air ventilation oven at 510° C. for a time of 10 minutes after each intermediate coat and of 30 minutes after the final coat.
• a second precursor solution containing 15% molar Ru, 7.9% molar Ir and 77.1% molar Ti was obtained starting from the same reagents used for the first one, with the addition of commercial TiOCl 2 at 160-180 g/l as a replacement for TaCl 5 . Also the preparation was carried out by mixing under stirring exactly as in the previous case, except that TiOCl 2 and soon after 18% HCl were added instead of the solution of TaCl 5 .
• the second precursor solution was applied to the titanium sheets, previously dried on air at 50° C., by brushing in 14 coats, with a subsequent decomposition cycle in forced air ventilation oven at 510° C. for a time of 10 minutes after each intermediate coat and of 30 minutes after the final coat.
• the thus obtained electrodes were characterised in an accelerated life-test under hypochlorite production with periodic polarity reversal.
• the accelerated test is carried out at a current density of 1 kA/m 2 in an electrolyte consisting of an aqueous solution containing 4 g/l of NaCl and 70 g/l of Na 2 SO 4 , adjusting the temperature at 25 ⁇ 1° C. and reversing the polarity of the two electrodes after every 60 seconds.
• Example 1 was repeated in identical conditions, save for the use of a first precursor solution containing 47% molar Ru, 24.7 molar Ir and 28.3 molar Nb, obtained by replacing the TaCl 5 solution with a 1M solution of NbCl 5 .
• the obtained electrodes were characterised in the accelerated life-test of Example 1, which gave substantially equivalent results than in the previous example, with a constant cell operation at about 3 V cell voltage for approximately 215 hours, followed by a progressive voltage increase stabilised after a total of 320 hours, at a new constant value, about 600 mV higher than the previous one. In this case the test was protracted for a total of 400 hours, with some shifting of the cell voltage to higher values in the course of the last 40 hours.
• a catalytic coating was applied by thermal decomposition of a precursor solution containing 15% molar Ru, 7.9% molar Ir and 77.1% molar Ti, equivalent to the second precursor solution of Example 1.
• the precursor solution was applied to the titanium sheets, previously dried on air at 50° C., by brushing in 17 coats, with a subsequent decomposition cycle in forced air ventilation oven at 510° C. for a time of 10 minutes after each intermediate coat and of 60 minutes after the final coat.
• a subsequent weight check evidenced the application of an external catalytic coating of 15 g/m 2 of noble metal, expressed as the sum of Ir and Ru.
• the thus obtained electrodes were characterised in the accelerated life-test of Example 1, allowing a constant cell operation at a cell voltage of about 3 V for approximately 230 hours, followed by a sudden voltage increase indicating the deactivation of the electrodes to an extent which forced the discontinuation of the test.

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