DE2532553A1 - ANODE FOR ELECTROLYTIC PROCEDURES - Google Patents

Description

Patent Attorneys Di * l.- Ing.F. Y / eickmann,

Dipl.-Ing. H. Weickmann, Dipl.-Phys. Dr. K. Fincke H / WE / MY Dipl.-Ing. F. A. Weickmann, Dipl.-Chem. B. Huber

Case 3452 ^{8 MUNICH 86 DEN}

PO Box 860 820

MÖHLSTRASSE 22, CALL NUMBER 48 3921/22

<983921/22>

HOOKER CHEMICALS & PLASTICS CORPORATION Niagara Falls, N.Y. 14302 / USA

Anode for electrolytic processes

The invention relates to an electrode for electrolytic processes comprising a valve metal substrate such as titanium, a Contains a coating thereon of conductive tin oxide and an outer coating of a noble metal or noble metal oxide.

The invention relates to improved electrodes, particularly used as anodes in electrochemical processes in which salt solutions are electrolyzed.

A variety of materials have been tested and used as chlorine anodes in electrolytic cells. The material that Most commonly used for this purpose has been graphite in the past. The use of graphite anodes results however, several disadvantages. The chlorine overvoltage of graphite is relatively high compared to the noble metals. In the corrosive media of electrolytic cells, graphite wears out quickly and this results in a significant Loss of graphite and ultimately costs for replacement and also continuous maintenance difficulties, conditional by an adjustment of the distances between the anode and cathode, which is often necessary when the graphite is located

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wears out. The use of precious metals and precious metal oxides as anode materials has significant advantages compared to the use of graphite. the electrical conductivity of precious metals is much higher and the chlorine overvoltage is much lower than with graphite. Furthermore, the dimensional stability of the noble metals and noble metal oxides is much better than that of graphite. However, the use of precious metals as the main building materials for anodes results in economic disadvantages, due to the extremely high cost of these materials.

In order to avoid the use of the expensive precious metals, various other anode materials have been used as coatings for valve metal substrates suggested. In US-PS 3,627,669 it is described that a mixture of tin dioxide and oxides of the Antimony can be formed as an adhesive coating on valve metal substrates and that an anode is obtained in the process, useful in electrochemical processes. In the electrolytic production of chlorine, these have anodes the advantage of economy, and it is not necessary to use expensive precious metals or precious metal oxides. In addition, the tin oxide coating provides effective protection for the substrate. However, the tin oxide compositions have although useful as anode material, a chlorine overvoltage that is much higher than that of the noble metals or precious metal oxides. Despite the elimination of the expensive precious metals, the cost of making chlorine is at Processes using such anodes are relatively high.

Considerable effort has been devoted to improved anode materials and structures in recent years to develop in which the advantages of precious metals or precious metal oxides are exploited. Much effort has been put on that Development of anodes that have a high operating surface area made of precious metal or precious metal oxide.

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zen compared to the total amount of material used. This can be done, for example, by placing the precious metal as a thin film or coating on an electrically conductive one Substrate used. If one tries, however, the previously described economic disadvantages of precious metals are so small as possible by applying them in the form of a very thin film to a metal substrate has been found to that such very thin films are often porous. As a result, the substrate becomes through the pores of the outer layer of the Exposed to anode environment. Furthermore, in normal use in an electrolytic cell, there is little wear, a chipping or flaking of parts of the noble metal or the noble metal oxides and the substrate becomes further exposed to the above-mentioned effect. Many materials that are particularly suitable as substrates are sensitive to chemical attack and rapidly decompose when exposed to the anode environment. In order to keep the decomposition of the substrate to a minimum in these cases, the anode manufacturers usually use valve metals like titanium as a substrate material. Under the influence of an anodic environment, titanium, like other valve metals, forms a surface layer of oxide that serves to protect the substrate from chemical attack. That so educated However, oxide is not catalytically active and consequently the usable surface area is the anode reduced.

The present invention is based on the object of improved To create electrodes that can be used as anodes in electrolytic processes. Anodes with an effective surface made of precious metal or precious metal oxide and improved performance and better maintenance properties are to be created.

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The invention relates to a new electrode, in particular for use as an anode in chlor-alkali cells. The new electrode contains a valve metal substrate with a protective coating of conductive tin oxide on top Surface of it and an outer thin layer of precious metal or precious mexal oxide. Electrodes of this type show one high degree of durability in addition to the relatively low overvoltage properties of the precious metal or precious metal oxide, and this makes them particularly useful as anodes in the electrolytic production of chlorine are.

One of the advantages of such a design is that the metal substrate protected by the conductive tin oxide coating. The preferred substrate materials for the Anodes according to the invention are valve metals such as titanium, tantalum, niobium or zirconium, in particular titanium. However, if one is sufficient If a thick intermediate layer of tin oxide is used, other conductive metals can be considered as substrates to be pulled. The tin oxide coating, which have a coating weight of about 0.1 to 100 g / m or more can, depending on the desired level of protection, prevent contact of the substrate and the electrolyte, and thereby the corrosion or surface oxidation and the accompanying decomposition or passivation of the substrate prevented or kept to a minimum. At the same time, the outer layer has the advantageous catalytic properties of precious metals or precious metal oxides. In addition, the protective layer made of conductive tin oxide enables use a relatively thin layer of precious metal or precious metal oxide, and accordingly savings are possible, because the precious metal is only used in minimal quantities. Typically the layer is made of precious metal or Noble metal oxide has a coating weight in the range of about 0.1 to about 20 g / m or higher, and preferably about 3 to 10 g / m 2 and have the appropriate thickness.

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The disadvantages of pores or "pinholes" that are commonly found in precious metal layers that are particularly thin occur are avoided by the presence of the intermediate layer of conductive tin oxide. Pores or pinholes in the noble metal layer or wear and tear of the outer layer over long periods of time cause a gradual effect Exposing the tin oxide layer. The intermediate layer of tin oxide becomes a further catalytically active surface in the revealed areas. The catalytic properties of the tin oxide are essentially better than those of the tin oxide Valve metal oxides, although they are not as good as those of the precious metals or precious metal oxides. All of the deterioration the catalytic properties of the anode is slower and the maintenance problems are correspondingly lower.

In addition, it has been found that the intermediate layer of tin oxide has an increase in the surface area of the anode and corresponding Improvements in the overvoltage result. It was also found that the adhesion of the noble metal or the Noble metal oxide on the substrate is increased by the presence of the intermediate layer of tin oxide and that the difficulties the chipping of the surface layer can be reduced.

The valve metal substrate that is the interior or base component the electrode forms is an electroconductive metal with sufficient mechanical strength to be used as a carrier can serve for the coating and has a high degree of chemical resistance, especially to anodic Environment of electrolytic cells. Typical valve metals are e.g. Ti, Ta, Nb, Zr and alloys thereof. it is good It is known that valve metals have the property of forming an inert oxide film when exposed to an anodic environment form. The preferred valve metal is titanium because of cost and availability as well as electrical and chemical properties. The conductivity of the substrate can

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possibly improved by creating a central core from a highly conductive metal such as copper. In such a design, the core must be bonded to and fully protected by the valve metal substrate be.

Tin oxide can easily be used as an adhesive coating on a valve metal substrate be formed in the manner previously described, wherein a protective, electrically conductive layer which is particularly resistant to chemical attack in an anodic environment. However, pure tin oxide possesses and exhibits a relatively high electrical resistance as compared with metals undesirable changes as a function of temperature. It is well known that the electrical stability of tin oxide coatings can be significantly improved and that the electrical resistance can be lowered by having a low Incorporated portion of a suitable inorganic material, often referred to as "dopant", i.e. additive will. A variety of materials, particularly various metal oxides and other metal compounds and mixtures of which, it has been described in the literature, the suitable additives for stabilization and humiliation the electrical resistance of tin oxide compositions. Of the materials that are considered useful in the literature Additives for conductive tin oxide compositions are described and those in the tin oxide compositions for the anodes according to the invention can be used are, for example, fluorine compounds, in particular the metal salts of Fluorine such as sodium fluoride, potassium fluoride, lithium fluoride, Beryllium fluoride, aluminum fluoride, lead fluoride, chromium fluoride, Calcium fluoride and other metal fluorides; Hydrazines, Phenylhydrazines; Phosphorus compounds such as phosphorus chloride, phosphorus oxychloride, ammonium phosphate, organic phosphorus esters such as tricresyl phosphate; as well as tellurium, tungsten, antimony, molybdenum, arsenic and other compounds and mixtures of that. The conductive tin oxide according to the invention

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Coatings contain tin oxide and preferably a small amount of a suitable additive. The preferred additive is an antimony compound which can be added to the tin oxide coating composition either as an oxide or as a compound such as SbCl, and which can form an oxide when heated in an oxidizing atmosphere, although the exact nature of the antimony oxide in the final coating is not certain is assumed to _{exist as Sb 2} O-, and the values and proportions in the present invention and the appended claims are based on this assumption. The preferred compositions of the invention contain mixtures of tin oxide and a small amount of antimony oxide, the latter preferably being present in an amount between about 0.1 and 20% by weight (calculated on the basis of the total weight of SnOo and Sb ₂ O ^).

Conductive tin oxide coatings can be adhered to the surface of the valve metal substrate by various known methods are formed. Typically, such coatings are formed by first chemically cleaning the substrate, for example degreased, and the surface etched in or with a suitable acid, e.g. with oxalic acid, and then a solution of the appropriate, thermally decomposable salts applies, dries and warmed in an oxidizing atmosphere. The salts that can be used are generally any thermally decomposable, inorganic or organic Salts or esters of tin and the additive, e.g. antimony including, for example, their chlorides, Alcoholates, alkoxyhalides, resinates, amines, etc. Typical Salts are, for example, tin (IV) chloride, dubutyltin dichloride, Tin tetraethylate, antimony trichloride, antimony pentachloride Suitable solvents are, for example, ethyl alcohol, propyl alcohol, butyl alcohol, penty alcohol, Amyl alcohol, toluene, benzene and other organic solvents as well as water.

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The solution of thermally decomposable salts, which contains, for example, a tin salt and an antimony salt or another additive in the desired proportions, can be applied to the cleaned surface of the valve metal substrate by painting, brushing, dipping, rolling, spraying or some other method. The coating is then dried by for example, heating for several minutes at about 100 to 200 ° C to evaporate the solvent and then heated at a higher temperature, for example 250 to 800 ⁰ C, in an oxidizing atmosphere to the tin and To convert antimony compounds into their corresponding oxides. The process can be repeated as many times as necessary to obtain the desired coating weight or thickness. The final coating weight of this conductive tin oxide coating can vary widely, but is preferably in the range of about 3 to about 30 g / m ² .

Optionally, a small amount such as up to 3 wt.% Chlorine discharge catalyst such as at least one of the following compounds, namely difluorides of manganese, iron, cobalt or nickel, can be incorporated into the tin oxide coating to reduce the overpotential required for chlorine gas release in a Chlor-alkali cell is required to lower. Of the Chlorine discharge catalyst can be added to the tin oxide coating by adding a particulate, pre-formed Sintered from tin dioxide and the catalyst suspended in a solution of the thermally decomposable salts. Such a Chlorine discharge catalyst in the tin oxide coating is not essential in the anodes of the invention, but it can if desired, can be used in a manner known per se, as described in US Pat. No. 3,627,669.

The outer coating of the anode contains a noble metal or noble metal oxide such as platinum, iridium, rhodium, palladium, Ruthenium or osmium or mixtures or alloys of these

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Metals or oxides or mixtures of the oxides of these metals. An outer coating made of noble metal can be made using known methods can be applied such as by electroplating, chemical deposition from a platinum coating solution, spraying, or other procedures.

The outer coating of the anode preferably contains a noble metal oxide. The noble metal oxide coating can be applied by first depositing the noble metal in a metallic state and then oxidizing the noble metal coating, for example by galvanic oxidation or chemical oxidation, using an oxidizing salt melt as the oxidizing agent, or by using an oxidizing salt melt at elevated temperatures, e.g. 300 to 600 ° C or higher, heated in an oxidizing atmosphere such as atmospheric oxygen, at atmospheric or superatmospheric pressure, in order to convert the noble metal coating into a coating made of the corresponding noble metal oxide. Other suitable methods are, for example, the electrophoretic deposition of the noble metal oxide or application of a dispersion of noble metal oxide in a carrier such as an alcohol by spraying, brushing, rolling, dipping, brushing or other methods on the tin oxide surface and then heating at an elevated temperature in order to evaporate the carrier and sintering the oxide coating. A preferred method for producing the noble metal oxide coating is to coat the conductive tin oxide surface with a solution of a noble metal compound, evaporate the solvent, and convert the noble metal compound coating into the oxide by chemical or electrochemical reaction. For example, the conductive tin oxide surface can be coated with a solution of a thermally errable salt of a noble metal such as a solution of a noble metal halide in an alcohol, the solvent can be evaporated and then it can be used at an elevated temperature between approximately 300 and 800 ⁰ C in an oxidizing atmosphere such as air or oxygen during

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a time sufficient to remove the noble metal halide to be converted into a noble metal oxide. The process of forming a noble metal or noble metal oxide coating can be repeated as many times as necessary to obtain the desired thickness. The one previously described and others Methods for making coatings from precious metals and precious metal oxides are well known and are described, for example, in U.S. Patent 3,711,385.

The following examples illustrate the invention. In the examples, in the application and in the claims, all temperatures are in ⁰ ° C. and all parts and percentages, unless stated otherwise, are expressed by weight.

example 1

(IA) Preparation of a conductive tin oxide coating

A strip of titanium sheet is made by immersing it in hot oxalic acid for several hours to etch the surface, then wash and dry. The titanium is then made with a composition of tin oxide with antimony oxide coated as an additive, working as in Example 4 of US Pat. No. 3,627,669 in the following manner:

Tin dioxide is made by dissolving metallic tin (84 parts) in concentrated nitric acid and heating until tin dioxide has deposited. Antimony trioxide (18 parts) is heated to boiling in concentrated nitric acid until the development of nitrogen oxides ceases, and then mixed well with the precipitated tin oxide. The mixture is further treated with hot nitric acid, washed free of acid and dried in air at about 200 ⁰ C. About 3 weight percent manganese difluoride is added and mixed with the dried, mixed oxides. The mixture is then made into pellets, heated in air for 24 hours at approximately 800 ° C, then broken up and spread on a particle

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shredded to a size below 60 microns. The crushed, mixed Oxide composition is pelletized and heated again as previously described, and then crushed and in a ball mill to a particle size below 5 microns marry.

An antimony trichloride alkoxy tin solution is prepared, by refluxing a mixture of 15 parts of tin (IV) chloride and 55 parts for 24 hours n-amyla alcohol heated to boiling and then 2.13 parts in it Antimony trichloride dissolves.

A suspension of 0.17 parts of the mixed oxide composition and 3 »6 parts Anti-Mont Rich Lord Alkoxyzinn solution is prepared and applied to the cleaned titanium surface and then the coating is dried at 150 ⁰ C in the oven. Two additional coatings of the same composition are applied in a similar manner and dried and then the coated strip is heated for about 15 minutes in air at 450 ⁰ C, in order to convert the coating in substantially tin and antimony oxides with manganese fluoride. The application of the coating, including the final heating at 450 ^° C., is repeated three times in order to increase the thickness of the coating.

The theoretical composition of the conductive coating produced in this way is 85.6% SnO ₂ ; 13.7% antimony oxides (calculated as SbpO ^) and 0.7% MnF ₂ . The coating weight of the finished coating is 21.2 g / m 2.

(IB) Production of a RuO ₂ coating

The titanium coated with conductive tin oxide is further coated in the following way:

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A solution of 1 g of ruthenium trichloride in 0.4 ecm 36% hydrochloric acid and 6.2 ecm butyl alcohol is brushed several times onto the tin oxide surface and is then allowed to dry in air at room temperature. After drying, the samples in air at 56o ⁰ C for 25 minutes to be heated to the RuCl to decompose and to form RuOp. Another coating of RuCl ^ is applied, dried and thermally treated in a similar manner, giving a final _{coating of RuO 9} with a coating weight of approximately 6.0 g ruthenium / m.

In the previous example, a small amount of chlorine discharge agent, manganese difluoride, was added to the conductive Tin oxide coating incorporated. According to the invention, an anode can also be produced by, as in Example 1 works described, with the exception that no chlorine discharge agent is added.

Example 2

Chlorine cell experiment

The anode, manufactured as described in example (IB), is installed and tested as an anode in a chlorine cell, the chlorine cell including a steel cathode separated from the anode by a cationic membrane; the anode compartment is with preheated saline solution with a composition of about 310 g / l NaCl and a pH of about 4.5 loaded. The anolyte is held at approximately 95 ° C. The experiment is carried out at a constant current density of 310 mA / cnr (2.0 ASI) performed. The anode shows a potential of 1.19 volts (compared to a saturated calomel electrode), whereby the potential remains constant during a long test period.

For comparison purposes, a commercially available anode, which contains a titanium substrate, on its surface directly

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a ruthenium oxide coating was applied, installed and checked under identical conditions. The anode showed a potential of 1.26 volts (versus a saturated Calomel electrode). It can be seen that an improvement in overvoltage is obtained in anodes such as that in FIG Anode of example (IB) when the outer coating of noble metal oxide on a surface of conductive tin oxide is deposited and not directly on the surface of the valve metal substrate.

Example 3

An anode made as described in Example (IB), i.e. an anode having a titanium substrate, an outer coating of ruthenium oxide and an intermediate layer of conductive tin oxide is tested in comparison with an anode, which is produced according to example (IA), i.e. an anode which has a titanium substrate and a coating of conductive Contains tin oxide. The anodes are placed in a chlorine cell with a steel cathode under identical conditions separated from the anode by a cationic membrane, installed and tested. The anode compartment is preheated with Saline solution at a concentration of approximately 310 g NaCl / l and a pH value of approximately 4.5. The anolyte is kept at approximately 95 C and the experiment is carried out at at a constant current density of 310 mA / cm (2.0 ASI). The anode of example (IB) shows an initial potential of about 1.20 volts (versus a saturated calomel electrode), the potential being essentially remains constant during a test period of 127 hours. With identical test conditions, the anode from example shows (IA) an initial potential of approximately 1.52 volts (versus a saturated calomel electrode), where the potential rises to 1.76 volts over a test period of 128 hours.

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example

A. A titanium mesh sample is coated with a layer of conductive tin oxide according to the method of Example (IA) coated.

B. A titanium mesh sample coated with conductive tin oxide as described in Example 4A is, is further continued with an outer layer of ruthenium dioxide according to that described in example (IB) Process coated.

The mains anodes, which are manufactured as described in A and B above, are installed as anodes in chlorine cells and tested with the electrode gap between the anode and the steel cathode being 0.32 cm (1/8 inch) and the The anode and the cathode are separated by a cationic membrane. The cells are exposed to anolyte concentrations in Range from 250 to 310 g NaCl / l at a pH of 4.5 and temperatures from 80 to 90 ° C. The experiments are carried out at a constant current density of 310 mA / cm (2.0 ASI) performed). The anode of Example 4B exhibits an initial potential of approximately 1.32 volts and is essentially stable for a period of 60 days of operation, whereas the anode of Example 4A has an initial potential of shows about 1.50 V and the potential gradually rises to about 1.90 on the 50th day of operation and then a lot increases rapidly on the 52nd day of operation and is completely passivated on the 55th day.

Example 5

5 "x6" anode plates> manufactured as described in Examples (IA) and (IB), are installed in a chlorate cell and tested using two anode plates which are covered by a mild steel cathode envelope are surrounded. The gap between the anode and

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ORIGINAL INSPECTED

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the cathode is 0.32 cm (1/8 inch). The cell is operated at a current density of 620 mA / cm (4.0 ASI) and is maintained at approximately 70 ^° C. The electrolyte composition is in the range from 400 to 550 g NaClO-, and 120 to 150 g NaCl and 1.0 to 1.5 g sodium dichloromate / l and the pH is approximately 6.7.

The anode of example (IA) with an outer coating of conductive tin oxide shows an initial potential of 4.0 V. The potential gradually rises to 5.4 V during the first 40 hours of operation and the anode fails completely less than 2 days of operation. With identical conditions shows the anode of Example (IB) a lower initial potential (3.50 V) and excellent stability and rises to approximately 4.05 V over 91 days of operation.

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Claims (8)

Claims (?) Electrolytic anode, characterized in that it is a valve metal substrate with a conductive coating on it Contains tin oxide and an outer coating of at least one noble metal or noble metal oxide. 2. Electrolytic anode according to claim 1, characterized in that the substrate is titanium. J5 Electrolytic anode according to Claim 2, characterized in that that the conductive tin oxide contains a mixture of tin dioxide and a small amount of antimony oxide. 4. Electrolytic anode according to claim 2, characterized in that that the outer coating is a noble metal oxide. 5. Electrolytic anode according to claim 4, characterized in that the outer coating is ruthenium oxide. 6. Electrolytic anode according to claim 5, characterized in that the conductive tin oxide is a mixture of tin oxide and between about 0.1 and about 20% by weight of antiraon oxide, based on the total weight of the mixture, calculated as SnO ₂ and Sb ₂ O ,, contains. 7. Process for the electrolysis of an aqueous alkali metal chloride solution, chlorine being released at the anode, characterized in that the anode is a composite structure used a valve metal substrate, a coating of conductive tin oxide on the surface thereof and an outer coating on the surface of the conductive tin oxide of at least one noble metal or noble metal oxide contains. 609808/0712 8. The method according to claim 7, characterized in that the anode is a titanium substrate, a coating thereon conductive tin oxide and an outer coating of ruthenium oxide. Process according to Claim 8, characterized in that the conductive tin oxide contains a mixture of tin oxide and between approximately 0.1 and 20% by weight of antimony oxide, based on the total weight of the mixture, calculated as SnOp and Sb ₂ O ₃ . £ 09808/0712