ES2337271T3 - ANODIC COATING OF HIGH EFFICIENCY HYPOCLORITE. - Google Patents

Abstract

The present invention relates to an electrocatalytic coating and an electrode having the coating thereon, wherein the coating is a mixed metal oxide coating, preferably platinum group metal oxides, with or without a valve metal oxide. The electrocatalytic coating can be used especially as an anode component of an electrolysis cell and in particular a cell for the electrolysis of aqueous hypochlorite solutions.

Description

Anodic hypochlorite coating high efficiency.

Background of the invention 1. Sector of the invention

The present invention relates to an electrode electrolytic and to a coating of mixed metal oxides arranged on it for the generation of hypochlorite.

2. Description of the related technique

In the art, the use of mixed metal oxide coatings for hypochlorite generation by electrolysis of solutions brine. However, conventionally, when it occurs hypochlorite by brine electrolysis, concentration of available chlorine from the hypochlorite product can be so low as 1 percent by weight (% by weight) or less. Besides, the current efficiency and electrode life decrease in cases where brine feed solutions are less concentrated (i.e. 10-30 g / l) and the Desired hypochlorite concentrations are greater than 8 g / l.

Various solutions have been proposed for obtain solutions with a high concentration of hypochlorite of sodium without negatively affecting current efficiency or life of the electrodes. For example, in the document of US Patent 4,495,048, an electrolytic cell of the filter press type, in which sodium hypochlorite is produced with reduced cell voltage and improved current efficiency.  The anode of the electrolytic cell consists of a substrate of titanium presenting a coating of a ternary mixture of 3 to 42% by weight of platinum oxide, from 3 to 34% by weight of palladium, 42% by weight of ruthenium dioxide and 20-40% by weight of titanium oxide.

In US Patent 4,517,068, a electrode, particularly for the production of chlorine e hypochlorite, comprises an electrocatalyst consisting of 22-44 mol% of ruthenium oxide, 0.2-22 mol% palladium oxide and 44-77.8 mol% titanium oxide. Saying electrocatalyst can form a coating on a substrate metal valve and can be coated with a porous oxide layer of titanium or tantalum.

In US Patent Document 5,622,613, describes a method to efficiently produce hypochlorite using an anode that has an oxide coating of palladium in 10 to 45% by weight, ruthenium oxide in 15 to 45% in weight, titanium dioxide by 10 to 40% by weight and platinum by 10 to 20% by weight, as well as an oxide of at least one metal selected from cobalt, lanthanum, cerium or yttrium, by 2 to 10% in weight.

Hirakata and others have described an anode for the diluted brine electrolysis that reaches an efficiency of 90% current and a hypochlorite concentration of 21,000 ppm. Said anode is constituted by a titanium substrate and a coating composition containing 3-42% in Pt weight, 3-34% by weight of PdO, 42-94% by weight of RuO2 and 20-40% by weight of TiO2, based on the total weight of the precious metal components (Hirakata K. and others, abstracts expanded, electrochemical society. Princeton, New Jersey, USA, vol. 89/1, May 7, 1989, p. 565-566).

EP 0 174 413 demonstrates the effect of iridium in a coating containing oxide of ruthenium-titanium and palladium oxide to increase the resistance of a titanium electrode.

It would be desirable to provide an electrode that present an electrocatalytic coating on it, which is able to provide an improved life of the electrodes and operating efficiencies in electrolytic environments used for the generation of hypochlorite from solutions of feed of 15-30 grams per liter (g / l) of NaCl or KCl, and in which the desired hypochlorite concentrations They are greater than 8 g / l. In addition, it would be desirable to provide a electrode of this type with a reduced cost compared to Platinum based formulations.

Brief Description of the Invention

Now, a coating of electrode that provides an improved lifespan while maintains high efficiencies in electrolytic solutions for the hypochlorite generation. Said coating is a coating of mixed metal oxides consisting of combinations of the oxides of palladium, iridium, ruthenium and titanium.

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In one embodiment, the present invention is refers to an electrode for use in electrolysis of an aqueous solution for the production of hypochlorite, presenting said electrode an electrocatalytic coating thereon, wherein said electrode comprises a metal electrode base valve; a coating layer of a coating electrochemically active on the basis of metal electrode valve, said coating comprising a coating of mixed metal oxides of platinum group metal oxides and a metal oxide titanium valve, said said consisting coating of mixed metal oxides essentially in oxides of Platinum group metals of ruthenium, palladium and iridium; at that

(a) the molar ratio of the metal oxide valve of titanium with respect to the metal oxides of the group of platinum is between approximately 90:10 and about 40:60;

(b) the molar ratio of ruthenium with respect to to iridium is between approximately 90:10 and about 50:50; Y

(c) the molar ratio of palladium oxide with regarding the sum of the ruthenium and iridium oxides is between about 5:95 and about 40:60, based on 100 mole percent of the metals present in the coating;

so the electrode works with a high current efficiency to produce concentrations of hypochlorite at least 8 grams per liter.

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In another embodiment, the present invention is refers to a procedure for the electrolysis of a solution aqueous in an electrolytic cell that has at least one anode, said anode having an electrocatalytic coating thereon, said process comprising the steps of provide a non-separated electrolytic cell, set to said cell an electrolyte containing chloride, contact the anode of the cell with the electrolyte, said anode presenting the electrocatalytic coating, which comprises a coating of mixed metal oxides of platinum group metal oxides and a metal oxide titanium valve, said said consisting coating of mixed metal oxides essentially in oxides of platinum group metals of ruthenium, palladium and iridium, in the that

(a) the molar ratio of the metal oxide valve of titanium with respect to the metal oxides of the group of platinum is between approximately 90:10 and about 40:60;

(b) the molar ratio of ruthenium with respect to to iridium is between approximately 90:10 and about 50:50; Y

(c) the molar ratio of palladium oxide with regarding the sum of the ruthenium and iridium oxides is between about 5:95 and about 40:60, based on 100 mole percent of the metals present in said coating;

apply an electric current on the anode; and oxidize the chloride in the anode to produce hypochlorite at concentrations of at least 8 grams per liter.

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Description of the invention

According to the present invention, a electrode presenting an electrocatalytic coating with a high current efficiency for high concentrations of hypochlorite, for example,> 8 gpl (grams per liter), and that It has a low electrode potential and an improved lifespan. In one embodiment, depending on the concentration of hypochlorite, the current efficiency will be between approximately 90% and approximately 100%, with a hypochlorite concentration between 16 and 0 grams per liter (g / l). The electrode that presents the electrocatalytic coating described herein Document will almost always serve as an anode. Thus, the term "anode" is frequently used in this document referring to the electrode, although said designation is used for a simple matter of comfort and should not be considered limiting of the invention.

The electrode used in the present invention comprises an electrocatalytically active film on a base conductive The conductive base can be a metal, such as nickel or manganese, or a sheet of any film-forming metal, such as titanium, tantalum, zirconium, niobium, tungsten and silicon, and alloys containing one or more of these metals, being Titanium is preferred for economic reasons. The term "metal film former "refers to a metal or alloy that presents the property that, when connected as an anode in the electrolyte in which the coated anode, quickly forms a passivating oxide film which protects the metal that is below it from the corrosion by the electrolyte, that is, those metals and alloys often referred to as "valve metals" as well as alloys containing a metal valve (for example, Ti-Ni, Ti-Co, Ti-Fe and Ti-Cu), but that under the same conditions form a non-passive anodic surface oxide film. As base electrode, plates, rods, tubes, wires or knitted wire meshes or expanded metal meshes of titanium or other film forming metals. It also can use a titanium plating or other metal film former on a conductive core. It is also possible to try superficially sintered porous titanium with solutions of paint diluted in the same way.

Titanium is particularly interesting for its roughness, corrosion resistance and availability. In addition to the normally available elemental metals themselves, Suitable metals for substrate include metal alloys and intermetallic mixtures, as well as ceramics and alloys metalloceramics, such as those containing one or more metals valve. For example, titanium can be alloyed with nickel, cobalt, iron, manganese or copper. More specifically, grade 5 titanium it can include up to 6.75 percent by weight of aluminum and 4.5 weight percent vanadium, grade 6 titanium up to 6 per percent aluminum and 3 percent tin, grade 7 titanium up to 0.25 percent by weight of palladium, grade 10 titanium, between 10 and 13 percent by weight plus 4.5 to 7.5 per weight percent zirconium, etc.

Through the use of elementary metals very particularly understand the use of metals in their state normally available, that is, with smaller amounts of impurities Thus, for metal with a particular interest, that is, titanium, various grades of said metal are available, including those in which other constituents may be alloys or alloys plus impurities. The titanium grades have been described more specifically in the standard specifications for titanium detailed in ASTM B standard 265-79. Since it is a metal with an interest particular, in this document and for comfort reasons, reference will be made to titanium when reference is made to metal for the electrode base.

Regardless of the selected metal and of the shape of the electrode base, before applying a coating composition, said electrode base is advantageously a clean surface. This can be obtained through any of the treatments used to achieve a Clean metal surface, including mechanical washing. Too washing procedures can be used advantageously usual degreasing, whether chemical or electrolytic, or Other chemical methods of washing. When preparing the base Includes tempered, and the metal is grade 1 titanium, titanium can be temper at a temperature of at least about 450 ° C for a period of at least approximately 15 minutes, although a tempering temperature is often advantageous higher, for example, from 600 ° C to 875 ° C.

For most applications, it turns out advantageous to obtain a base with a rough surface. This is achieved by means that may include intergranular attack of metal, plasma spray application, said application by Spraying can be from a metal particulate valve or from ceramic oxide particles, or both, chemical attack and treatment with sandblasting the metal surface, optionally followed  of a surface treatment in order to remove the sand embedded and / or clean the surface, or combinations of the same. In some cases, the base can simply be cleaned, obtaining a very smooth substrate surface. Alternatively, the conductive film forming base can present a previously applied oxide surface film metallic film former that, during the application of active coating, can be attacked by an agent of the coating solution (for example, HCl) and reconstituted as part of the integral surface film.

The chemical attack is carried out with a attack solution active enough to develop roughness  of surface and / or surface morphology, including a possible Edge attack by aggressive grain. They are typical solutions of Chemical attack acid solutions. These solutions may be constituted by hydrochloric, sulfuric, perchloric, nitric acid, oxalic, tartaric and phosphoric, as well as mixtures thereof, by example, royal water. Other chemical attackers that can be used include caustic attackers, such as a hydroxide solution of potassium / hydrogen peroxide, or a hydroxide melt of  potassium with potassium nitrate. After the chemical attack, the attacked metal surface can be subjected to washing steps and dried Proper surface preparation by attack chemical has been described in greater detail in the document of US Patent 5,167,788, which is incorporated herein as reference.

In plasma spraying to obtain a adequately rough metal surface, the material will be applied in particulate form, such as droplets of molten metal. In this plasma spray, as would be applied to the spray of a metal, said metal melts and is sprayed in a flow of plasma generated by heating with an electric arc at high atmosphere temperature of inert gas, such as argon or nitrogen, which optionally contains a smaller amount of hydrogen. At this document, for the use of the term "spray by plasma "it should be understood that, although spraying by plasma is preferred, the term generally includes the thermal spraying, such as magnetohydrodynamic spraying, flame spraying and arc spraying, thereby that the spray can simply be designated as "fusion spray" or "thermal spray".

The particulate material used can be a metal valve or oxides thereof, for example, titanium oxide, tantalum oxide and niobium oxide. The fusion spraying of titanates, spinels, magnetite, oxide of tin, lead oxide, manganese oxide and perovskites. Too it is contemplated that the oxide that is sprayed may be doped with various additives, including dopants in ionic form, such as Niobium or tin or Indian ions.

It is also contemplated that said application by plasma spraying can be used combined with the attack chemical of the metal surface of the substrate. Or the basis of electrode can be prepared first by jet treatment of sand, as described above, which may be followed or not by chemical attack.

It has also been discovered that it can be obtained a properly rough metal surface by treatment  special sandblasting with cutting sand, optionally followed by the removal of sand embedded in the surface. The sand, which will usually contain angled particles, will cut the metal surface instead of hitting it. The usable sand For this purpose it can include sand, aluminum oxide, steel and Silicium carbide. After the sandblasting attack can be used chemical, or other treatment, such as water jet, for the purpose of remove embedded sand and / or clean the surface.

From the above, you can deduce that the surface can then be subjected to various operations that provide a pretreatment before coating, for example, the plasma spray described formerly of a metal oxide coating valve. Too Other pretreatments may be useful. For example, it is contemplated that the surface undergoes a hydrurization treatment or nitrurization Before coating with a material electrochemically active, it has been proposed to provide a layer of oxide by heating the substrate in air or by anodic oxidation of said substrate, as described in the patent document USA 3,234,110. Various proposals have also been made in which an outer layer of material is deposited electrochemically active on a sublayer, which primarily serves as protective intermediary and driver. In the patent documents USA 4,272,354, 3,882,002 and 3,950,240 are disclosed various tin oxide based sub-layers. It is also contemplated that the surface can be prepared with an antipasivation layer.

After surface preparation, you can include providing a pretreatment layer, as has been described above, a coating layer is applied electrochemically active on the substrate. They are coatings electrochemically active frequently applied typically representative those provided by active coatings of oxide, such as platinum group metal oxides, magnetite, ferrite, cobalt spinel or oxide coatings mixed metal. Such coatings may be based on water, such as aqueous solutions, or in solvent, for example, using an alcoholic solvent. However, it has become manifest that, for the electrode according to the present invention, the Preferred coating composition solutions are typically those consisting of an oxide coating mixed metal oxides of platinum group metals and a metal oxide valve.

The metal oxides of the platinum group according to the present invention preferably comprise RuCl 3, PdCl2, IrCl3 and hydrochloric acid, all in solution alcoholic, in combination with a metal oxide valve. Must be understand that RuCl_ {3}, PdCl_ {2} and IrCl_ {3} can be used in a form such as RuCl 3 xH 2 O, PdCl 2 x H 2 O e IrCl_ {3} \ cdotxH2 {O}. For reasons of comfort, in the present document such forms will be designated in a general way simply RuCl 3, PdCl 2 and IrCl 3. Generally, the metal salts will dissolve in an alcohol, such as isopropanol or butanol, combined with or without small acid additions hydrochloric, with n-butanol being preferred. Must be understand that constituents are substantially present in form of its oxides in the final coating, and that the reference to metals is made for comfort reasons, particularly when reference is made to proportions.

A metal valve component is present in the coating composition for stabilization purposes additionally said coating and / or modify the efficiency of the anode. Various valve metals can be used, including titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum and tungsten, titanium being preferred. Metal component valve can be formed from a metal alkoxide valve in an alcoholic solvent, with or without the presence of an acid. Sayings metal alkoxides valve contemplated for use in the Present invention include methoxides, ethoxides, isopropoxides and butoxides For example, titanium ethoxide, the Titanium Proxide, Titanium Butoxide, Tantalum Ethoxide, tantalum isopropoxide or tantalum butoxide, being preferably titanium butoxide.

The coating of mixed metal oxides according to the present invention it contains a molar ratio of titanium with respect to metal oxides of the platinum group included between about 90:10 and about 40:60, a relationship ruthenium molar with respect to iridium between about 90:10 and about 50:50, and a molar ratio of Pd: (Ru + Ir) between approximately 5:95 and approximately 40:60. A particularly preferred composition. of the coating of mixed metal oxides according to the present invention contains a molar ratio of titanium with respect to precious metal oxides of approximately 70:30 and a ratio Pd molar: (Ru + Ir) of approximately 20:80.

Coating layers of metal oxides mixed used in the present invention are applied by any of the appropriate means to apply a composition liquid coating to a metal substrate. Such methods include immersion-centrifugation techniques and immersion-drained, application with brush, roller coating and spray application, such as electrostatic spraying In addition, you can use application by spraying and combined techniques, for example, immersion-drained together with application by spray. With the coating compositions mentioned previously to provide a coating electrochemically active, a roller coating operation.

Regardless of the method of application of coating, conventionally a coating procedure repeated in order to provide a coating weight uniform and higher than reached by a single covering. However, the amount of coating applied it will be sufficient within the range of approximately 0.05 g / m2 (gram per square meter) at approximately 6 g / m2, and preferably, from about 1 g / m2 to about 4 g / m2, based on the ruthenium content, as metal, on the side of the electrode base.

After application of the coating, the applied composition is heated in order to prepare the mixed oxide coating resulting from thermal decomposition of the precursors present in the coating composition. This operation prepares the mixed oxide coating it contains the mixture of oxides in molar proportions based on the oxides metals, as described above. Saying heating to reach thermal decomposition will take to conducted at a temperature of about 450 ° C to about 550 ° C for a period of approximately 3 minutes at approximately 15 minutes per coating. More typically, the applied coating is heated to a higher temperature of up to approximately 490-525 ° C for a period not greater than approximately 20 minutes per coating. The suitable conditions may include heating in air or oxygen. In general, the heating technique used can be any of those that can be used to cure a coating on a metal substrate. Thus, it can be used oven coating, including ribbon ovens. In addition, they can Infrared curing techniques are useful. After said heating, and before the additional coating in the case where  an additional application of the composition of coating, usually the heated and coated substrate is let cool, at least, at substantially room temperature. Particularly, after completing all the applications of the Coating composition, post-turning can be performed. Typical post-turning conditions for coatings they can include temperatures between approximately 450 ° C and approximately 550 ° C. Baking times may vary. from about 1 hour to about 6 hours.

The following examples, unless indicated as comparative examples, they generally demonstrate the hypochlorite production in high concentrations with increased efficiencies with an anode containing the coating according to the present invention:

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Example 1

A flat plate titanium titanium grade 1 no alloy, measuring approximately 0.15 cm thick and approximately 10 x 15 cm, was treated with sandblasting using alumina to obtain a rough surface. Then the sample was subjected to chemical attack at 90-95 ° C in a 18-20% hydrochloric acid solution for 25 minutes

Coating compositions were applied, such as indicated in table 1, to separate samples measuring 10 cm x 15 cm x 0.15 cm of grade 1 titanium, prepared by sandblasting treatment with alumina 54 grit. The A-D coating solutions were prepared dissolving a sufficient amount of metals, such as salts chloride, to obtain the concentrations indicated in the table, in a solution of n-butanol and HCl with a 4.2% concentration by volume. The compounds used were RuCl 3, IrCl 3 and PdCl 2 (all hydrated) and titanium orthobutyl titanate. After mixing to dissolve all the salts, the solutions were applied to individual samples of prepared titanium plates. The coatings were applied in layers by brush, applying each coating separately and letting it dry at 110 ° C for 3 minutes, followed by heating in air up to 500 ° C for 6 minutes. A total of 5 was applied coatings to each sample. Sample A is a sample of according to the present invention. Samples B, C, D, E are Consider comparative examples.

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TABLE I

one

The hypochlorite efficiency of the samples is measured in a beaker cell by dipping an area of 26 cm 2 in a solution of 28 gpl of NaCl with 1 gpl of Na 2 Cr 2 O 7 and applying an anodic current of 4.86 amps (0.186 A / cm2). A titanium cathode was used, separated 3 mm of the anode. A sample was taken every 8 minutes and assessed to determine hypochlorite. The current efficiencies for hypochlorite production based on concentrations of Hypochlorite are represented in Figure 1 and in Table II.

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2

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Next, the sample set A-E were used as anodes in one trial accelerated, like an anode of evolution of oxygen at a density of 10 kA / m2 current in an electrochemical cell containing 150 g / l of H 2 SO 4 at 65 ° C. Data were collected from cell voltage as a function of time every 30 minutes and it was taken The service life as the turning point at which the voltage It was beginning to increase rapidly. The results are indicated in the Figure 2 and Table II, standardized for the amount of metal in the platinum group. Normalization was done by measuring the count X-ray fluorescence for metallic peaks using Jordan Valley EX-300 spectrometer with an Rh tube and a 0.15 mm Sn filter. The applied voltage was 40 kV (kilovolts) and the current of 25 µA. The measured peaks were the Ru K-alpha, Pd K-alfa and Ir L-beta The total counts of Ru, Pd and / or Ir are They used to normalize useful lives.

Consequently, it is evident from the results of table II that the prepared samples of according to the present invention they have an efficiency of substantially increased current compared to the examples comparatives, while equaling or improving the service life, such as highlights the longest time that elapses before a significant increase in voltage (> 1 volt).

Although, according to the statutes of the patent documents, the best embodiment has been described and the preferred embodiment, the scope of the present invention does not is limited thereto, but within the scope of the claims attached.

Claims (14)

1. Electrode for use in the electrolysis of an aqueous solution for the production of hypochlorite, which comprises: - a metal valve electrode base; - an electrocatalytic coating on said metal electrode base valve, comprising a coating of mixed metal oxides of metal oxides of the group of Platinum and a metal oxide titanium valve, consisting essentially said metal oxides of the platinum group in oxides of ruthenium, palladium and iridium; in which (a) the molar ratio of said metal oxide titanium valve with respect to said metal oxides of the platinum group is between approximately 90:10 and about 40:60; (b) the molar relationship of said ruthenium with regarding this iridium is between approximately 90:10 and about 50:50; Y (c) the molar ratio of said palladium oxide with respect to the sum of the oxides of ruthenium and iridium is between about 5:95 and about 40:60, based on 100 mole percent of the metals present in said coating. 2. Electrode according to claim 1, in the that said metal valve electrode is a mesh, a sheet, a sheet, tube, perforated plate or metal wire valve. 3. Electrode according to claim 1 or 2, in which said metal valve electrode base is constituted by one or more metals from titanium, tantalum, aluminum, hafnium, niobium, zirconium, molybdenum or tungsten, their alloys and mixtures intermetallic thereof. 4. Electrode, according to any of the previous claims, wherein a surface of said base Metal electrode valve is a rough surface. 5. Electrode according to claim 4, in the that said rough surface is obtained by one or more stages of Intergranular chemical attack, sandblasting or spraying thermal 6. Electrode according to claim 4, in the that a ceramic oxide barrier layer is established as a layer of pretreatment on said rough surface. 7. Electrode, according to any of the previous claims, wherein the molar ratio of said metal oxide titanium valve with respect to said oxides of Platinum group metals is approximately 70:30, and the molar ratio of said palladium oxide with respect to the sum of the oxides of ruthenium and iridium is approximately 20:80. 8. Electrode, according to any of the previous claims, wherein the molar ratio of oxide of  Ruthenium with respect to iridium oxide is approximately 1: 1. 9. Use of an electrode, according to any of the preceding claims, as anode in electrolysis of sea water. 10. Use of an electrode, according to any one of claims 1 to 8, as an anode in a process of hypochlorite production at concentrations of at least 8 g / l, and with a current efficiency within the range between approximately 90% and approximately 100%. 11. Procedure for electrolysis of a aqueous solution in an electrolytic cell equipped at least with an anode according to any one of claims 1 to 8, said procedure comprising the steps of: - provide an electrolytic cell not separated; - establish in said cell an electrolyte that contains chloride; - contacting said anode in said cell with said electrolyte, - applying an electric current to said anode; Y - produce hypochlorite at concentrations, such as minimum of 8 g / l by oxidation of said chloride in said anode.

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12. Method according to claim 11, wherein said chloride electrolyte in said cell is one or more Chlorides between sodium chloride or potassium chloride. 13. Method according to claim 11 or 12, in which a surface of said anode is a surface rough prepared by one or more stages of intergranular attack, sandblasting or thermal spraying. 14. Method according to claim 13, wherein said rough anodic surface comprises titanium, and said electrocatalytic coating is disposed on said titanium by a procedure selected from the group constituted by application by electrostatic spraying, application with brush, roller coating, immersion application and combinations thereof.