EP1463847B1 - Electrode for conducting electrolysis in acid media - Google Patents

Abstract

Electrode at least comprising an electroconductive support of a titanium-palladium alloy, titanium, tantalum or compounds or alloys of titanium or of tantalum, an electrochemically active coating and an interlayer between the support and the electrochemically active coating, wherein the interlayer consists of titanium carbide and/or titanium boride and is applied to the support by flame or plasma spraying. Process for producing these electrodes and their use in an electrochemical cell for producing chlorine or chromic acid.

Description

The invention relates to stable electrodes for electrolytic processes, in particular for the electrolysis of hydrochloric acid or aqueous solutions of alkali metal dichromate Process for their preparation and their use.

Aqueous solutions of hydrogen chloride, hereafter called hydrochloric acid, fall as By-product in many processes, especially those where organic Hydrocarbon compounds are oxidized with chlorine oxidizing. Economically interesting is the recovery of chlorine from these hydrochloric acids, which can then be used for example for further chlorinations.

The recovery of chlorine, for example, electrolytically in an electrochemical Cell, consisting essentially of an anode compartment with anode and a cathode compartment with cathode, wherein anode and cathode compartment separated by an ion exchange membrane.

The production of chromic acid by electrolysis of sodium dichromate solutions is also in electrochemical cells of said basic structure possible.

For electrolytic processes, in particular for the electrolysis of hydrochloric acid or aqueous solutions of sodium dichromate, a plurality of electrodes are described.

DE 29 08 269 A1 describes bipolar electrodes based on carbon, however have a limited life under electrolysis conditions. Also from DE 44 17 744 C1 electrodes are known based on carbon, wherein a Activation of the cathode side by applying noble metal compounds takes place. To produce these electrodes, a graphite body with a solution of Precious metal compound soaked and then with open gas flame to 200 bis 450 ° C heated.

US-A-5,411,641 discloses a process for producing dry halogen by electrolysis of anhydrous hydrogen chloride in an electrolytic cell, in Anode and cathode have direct contact with a cation exchange membrane. Anode and cathode are based on carbon and are catalytic active material, for example ruthenium oxide coated.

US Pat. No. 5,770,035 discloses a process for the electrolysis of an aqueous hydrochloric acid solution known, wherein an anode of a corrosion-resistant substrate and a electrochemically active coating is used. In the corrosion resistant Substrate is graphite or titanium, titanium alloys, Niobium or tantalum. As electrochemically active coating becomes a standard activation used from mixtures of oxides of ruthenium, iridium and titanium. The cathode is a carbon-based gas diffusion cathode with a coating from a platinum group metal or a corresponding oxide. The long-term stability of the gas diffusion cathode is low, presumably because there is a loss of contact between the carbon-based gas diffusion electrode and the necessary current distribution electrode resting on the gas diffusion cathode comes. Another reason for a loss of contact is the Formation of electrically poorly conductive oxides on the electrodes during shutdowns the electrolysis. The formation of such oxides can be achieved by a coating the power distribution electrode with a metal mixed oxide, which also for the anode coating can be used. The mixed metal oxide However, it adheres poorly to the electrode, so that the long-term stability of the electrode remains unsatisfactory.

The electrodes described are obtained by direct application of the catalytically active Layer produced on a support and have the disadvantage that the service life the electrodes under the conditions of electrolysis unsatisfactory are.

The use of electrodes with roughened surfaces to improve the Lifespan of these electrodes, especially by rough, plasma-sprayed metallic Coatings is described in EP 493 326 A2. The main point is the generation very much rough surfaces.

US Pat. No. 4,392,927 proposes the use of sodium chloride electrolysis Composite electrodes, consisting of an electrically conductive substrate and a electrochemically active cover layer. The electrochemically active surface layer is applied by thermal spraying of a powder on the carrier, wherein the Powder contains in addition to matrix particles and electrocatalytically active particles. When Matrix particles include, for example, titanium oxide, titanium boride and titanium carbide Question, as electrocatalytically active particles of platinum group metals or Iron group or oxides of these metals.

From US-A 4,140,813 is a method for producing electrodes with improved Long-term stability under the conditions of alkali chloride electrolysis known. On a metallic support, preferably of titanium or a titanium alloy is by means of flame or plasma spraying a first coating Applied titanium suboxide. Subsequently, as electrochemically active substance a platinum group element or a compound of such an element applied. Such electrodes have an improved lifetime among the Conditions of sodium dichromate electrolysis on. You can too be used when the sodium chloride electrolysis under acidic conditions is performed or if hydrochloric acid is to be electrolyzed. Especially under the strongly acidic conditions in hydrochloric acid electrolysis or the Alkali dichromate electrolysis at low pH, however, is also the lifetime here not enough.

The use of titanium carbide or boride interlayers is already known from DE-A-23 44 645 and CH-A-665429. in this connection Although titanium substrates, but electrochemically active layers Lead dioxide used.

When testing anodes with conventional anode coatings It turned out that it even after a relatively short period of use to a chipping the active layer comes from the carrier. The cause is on the one hand one basic poor adhesion between support and active layer, on the other hand a corrosion between the active layer and the metallic carrier in question wherein the corrosion deteriorates the adhesion, which ultimately leads to the destruction of the Anode coating leads.

The object of the invention is therefore to provide electrodes with an improved lifetime under the conditions of electrolysis, especially under the strong acidic conditions in the hydrochloric acid electrolysis or performing the alkali metal dichromate electrolysis develop in acidic medium.

Surprisingly, it has now been found that this problem can be solved if electrodes before application of the catalytically active layer with a special Intermediate layer be provided.

The invention therefore relates to an electrode, at least containing one electrically conductive support made of a titanium-palladium alloy, titanium, tantalum or Compounds or alloys of titanium or tantalum, an electrochemically active Coating and an intermediate layer between support and electrochemical active coating, wherein the intermediate layer of titanium carbide and / or Titanium boride persists and is applied to the carrier by flame or plasma spraying is applied. Here, the electrochemically active layer consists of ruthenium dioxide or iridum dioxide, or a mixed metal oxide which is one contains these oxides.

Compared to those described in US Pat. No. 4,392,927 for the sodium chloride electrolysis Composite electrodes containing only one electrochemically active surface layer containing, in addition to matrix particles, also electrocatalytically active particles, the electrodes of the invention are characterized by increased stability, since by using an intermediate layer both the adhesion to the carrier, as well the adhesion of the catalytically active layer is improved.

The electrodes according to the invention can be used as an anode, as a cathode and also as a cathodic power distributor. They show a very high resistance when used in hydrochloric acid electrolysis or alkali metal dichromate electrolysis in acidic medium. For example, these electrodes are also extremely stable in the electrolysis of hydrochloric acid with a concentration of <20% by weight of HCl at temperatures up to 70 ° C. and high specific current densities of up to 8 kA / m ² . Compared to intermediate layers of titanium oxide or titanium suboxide, the intermediate layers of titanium carbide and titanium boride are characterized by being extremely dense. As a result, an attack by aggressive media, such as hydrochloric acid on the carrier is prevented. In addition, the adhesion of the electrochemically active layer is significantly improved.

The loading of the carrier with the intermediate layer is preferably from 10 to 5000 g / m ² .

In a particular embodiment, the intermediate layer consists of more than one Layer, i. the intermediate layer is multilayered by flame or Plasmaspritzen applied.

Preferably, the intermediate layer is a layer of titanium carbide.

The electrodes of the invention can be, for example, by applying a Intermediate layer by means of flame or plasma spraying on a support and subsequent application of an electrochemically active coating to the Produce intermediate layer, wherein during application of the intermediate layer by Flame or plasma spraying, titanium carbide and / or titanium boride powder of different Grain sizes, i. with a particle size distribution.

The carrier used here is a net, woven fabric, braid, knitted fabric, fleece or foam a titanium-palladium alloy, titanium, tantalum or compounds or alloys of the titan or tantalum.

The used titanium carbide and / or titanium boride powder for applying the Interlayers by flame or plasma spraying preferably have grain sizes from 10 to 200 μm.

For the purposes of this application, particle size is understood to mean the particle diameter, as determined by sieve analysis, for example.

The flame or plasma spraying is done in the usual way. For example, can Titanium carbide or titanium boride powder by means of a commercially available plasma burner be applied to the carrier. Details of the plasma spraying technique can, for example, the brochure "plasma spraying technology, basics and Applications, 1975 "of the company Plasma-Technik AG Plasma gas, for example, a mixture of nitrogen and hydrogen, wherein the volume ratio of nitrogen to hydrogen, for example between 70/30 and 95/5, in an amount of, for example, 5 to 20 1 / min and as Carrier nitrogen can be used. The injection process can, for example, at a current of 200 to 400 amperes and a voltage of 50 to 90 volts be performed. The distance between plasma torch and carrier can for example, be 130 to 200 mm.

The application of the electrochemically active coating can be known per se Done way. For example, it is possible to proceed in such a way that a solution or Dispersion of a compound of an element of the platinum metal group (Ru, Rh, Pd, Os, Ir, Pt) and optionally a compound of titanium on the intermediate layer applied and by subsequent thermal treatment to the corresponding Oxides is implemented. Advantageously, this procedure is repeated several times.

The electrodes according to the invention can be used, for example, as gas-evolving Electrodes are used.

Preference is given to the use of the electrodes in an electrochemical cell for Production of chlorine from aqueous hydrochloric acid solutions or from chromic acid a sodium dichromate / chromic acid solution with evolution of oxygen.

The used electrochemical cell can, for example, an anode compartment with anode and a cathode compartment with gas diffusion electrode and current collector containing anode compartment and cathode compartment through a cation exchange membrane are separated from each other and as the anode, cathode and / or current collector an electrode according to the invention is used.

In the cathode compartment, an oxygen-containing gas, for example, pure oxygen, a mixture of oxygen and inert gases, especially nitrogen, or Air are introduced, preferably oxygen or an oxygen-rich gas.

The oxygen-containing gas is advantageously supplied in an amount such that oxygen is superstoichiometrically based on the amount theoretically required according to equation 1.

When using the electrodes in an electrochemical cell for production of chlorine from aqueous hydrochloric acid solutions becomes the aqueous solution of hydrogen chloride usually introduced into the anode chamber. The temperature of the supplied aqueous solution of hydrogen chloride is preferably 30 to 90 ° C, particularly preferably 50 to 70 ° C.

In particular, aqueous solutions of hydrogen chloride with a hydrogen chloride concentration be used by <20 wt .-%.

The hydrochloric acid electrolysis is preferably at a pressure in the anode compartment greater than 1 bar absolute, more preferably 1.05 to 1.4 bar.

However, the electrodes according to the invention can also be advantageously used in an electrochemical Cell for the production of chromic acid from an aqueous Alkali dichromate solution, in particular from an aqueous sodium dichromate solution deploy. The use is particularly advantageous if the electrolysis of the aqueous sodium dichromate solution is carried out under acidic conditions, because in this Case conventional electrodes rapidly lose activity.

It is also conceivable to manufacture the electrodes in an electrochemical cell of chlorine from aqueous hydrochloric acid solutions as an electrical power distributor of a gas diffusion electrode to use for the reduction of oxygen.

In the following examples, the method according to the invention is explained further, the examples are not intended to limit the general inventive idea to be understood.

example 1

The surface of an expanded metal made of a standard titanium-palladium alloy (Grade 11 titanium) was blasted with steel gravel at a surface roughness of 30 to Roughened 40 μm. Subsequently, the expanded metal was mixed with a 20% by weight Hydrochloric acid pickled for approx. 10 minutes. This also allowed the remains of the blasting agent be removed.

On the pretreated expanded metal was by means of a plasma coating plant applied a layer of titanium carbide of the type plasma technology. This was Plasma powder from H.C. Starck, type AMPERIT 570.3, used. The Grain size distribution was after Microtrac to - 5.6 microns and sieve analysis determined by Rotap to +45.

The plasma gas was helium at a flow rate of 1.3 l / min. and nitrogen at a flow rate of 2.5 l / min. used. The carrier gas used to transport the plasma powder to the burner was nitrogen at 6.5 l / min. used. The burner output was 560 A at 62 V. The plasma torch was moved in the soundproof system by an oscillating mast. The lifting speed was 12 m / min. The horizontal step length was 10 mm per double stroke. The burner distance was about 150 mm at an angle of 90 °. The titanium carbide layer had a basis weight of 50 to 80 g / m ² .

Subsequently, an electrochemically active layer of RuO ₂ and TiO _{2 was} applied to the expanded metal provided with the intermediate layer. For this purpose, a mixture of TiCl ₃ and RuCl ₃ (molar ratio 1: 1) was dissolved in dilute hydrochloric acid (about 2N HCl) and applied to the expanded metal by means of a brush. The coated expanded metal was then heated in air to 500 ° C. This process was repeated several times, preferably 4 to 12 times.

The coated expanded metal was used as the anode and / or cathode mesh, which is known as Power supply of an oxygen-consuming cathode served, i. used as a power distributor.

Example 2 (comparative example)

The surface of an expanded metal made of a standard titanium-palladium alloy (Grade 11 titanium) was blasted with steel gravel at a surface roughness of 30 to Roughened 40 μm. Subsequently, the expanded metal was mixed with a 20% by weight Hydrochloric acid pickled for approx. 10 minutes. This also allowed the remains of the blasting agent be removed.

An electrochemically active layer of RuO ₂ and TiO _{2 was} applied to the pretreated expanded metal. The application was carried out as described in Example 1.

The coated expanded metal was used as the anode and / or cathode mesh, which is known as Power supply of a Sauerstoffverzehrkathode served, used.

Example 3 (electrode test)

In an electrochemical cell containing an anode compartment with anode, a cation exchange membrane and a cathode compartment with oxygen-consuming cathode and current collector were installed and tested with the necessary periphery as an anode and as a current collector described in Examples 1 and 2, respectively, with active surfaces of 100 cm ² ,

From a storage vessel, an aqueous hydrochloric acid solution (15-30 wt .-%) by means of a pump in an anolyte and from there by means of another Pump via a heat exchanger in the anode compartment of an electrochemical cell pumped. Part of the depleted hydrochloric acid solution was used together with the The anode developed chlorine gas via a line in a columnar vessel, in which was a gas / liquid separation, discharged. About a line in the Liquid of the columnar vessel was immersed, became a certain Pressure in the electrochemical cell and set in the anolyte. This was pressed the cation exchange membrane on the oxygen-consuming cathode, the in turn on the power distribution.

Oxygen was introduced via a pipe into a vessel which was filled with water and used for moistening the oxygen, passed. The moistened oxygen was fed to the cathode compartment, was reduced at the oxygen-consuming cathode and reacted with the protons migrated across the cation exchange membrane to water. Residual oxygen was combined with the condensate formed in a condensate removed. The excess oxygen and the condensate were removed from the electrochemical cell.

The test of the anode was carried out as follows:

An aqueous approximately 30% strength by weight hydrochloric acid solution was metered into a hydrochloric acid circuit such that the acid concentration in the anolyte circulation and in the cell was about 12-15% by weight HCl. The temperature of the anolyte solution was adjusted to 60-70 ° C. The electrolysis was operated at a current density of 5 kA / m ² . The cation exchange membrane used was a membrane based on a perfluorosulfonate polymer from DuPont (type Nafion® 324). An oxygen-consuming cathode from E-TEK based on carbon with platinum catalyst was used. The complete cell housing was made of PTFE (polytetrafluoroethylene) or PVDF (polyvinylidene fluoride).

During the course of the electrolysis, the anode and the current distributor were examined at regular intervals and the degree of destruction was determined. The determination was made qualitatively by examining the anode and the current distributor under the light microscope. Quantitatively, the degree of destruction was determined by layer thickness measurements by means of X-ray fluorescence measurement. The results of the investigations are summarized in Table I (anode) and Table II (current distributor). The degree of destruction is given in%, which is to be understood as meaning the proportion of active coating which has been removed in comparison to the layer thickness of active coating originally present. Condition of the anode coatings: Runtime [days] Degree of destruction [%]
Anode according to Example 1 Degree of destruction [%]
Anode according to Example 2 50 0 - 100 <1 - 200 ~ 2 ~ 30 280 5 ~ ~ 50 (new activation) 408 <10 Attempt canceled -: no determination made State of coating of cathode current distributor: Runtime [days] Degree of destruction [%]
Power distributor according to Example 1 Degree of destruction [%]
Power distributor according to Example 2 50 0 ~ 2 100 0 ~ 3 200 0 ~ 10 280 <1 ~ 20 408 <1 Attempt canceled

Surprisingly, the investigations have shown that the manufactured in Example 1 Anode an extremely high stability under the above conditions showed. The anode potential was still unchanged after a period of 408 days. The comparison test with an anode manufactured according to Example 2 had because of Destruction of the anode coating terminated after a period of 280 days become.

The degree of destruction of the power distribution used was in use an electrode according to the invention according to Example 1 significantly lower than when using an electrode according to Example 2.

Claims (8)

1. Electrode containing at least an electrically conductive support made of a titanium-palladium alloy, titanium, tantalum or compounds or alloys of titanium or tantalum, an electrochemically active coating and an intermediate layer between the support and the electrochemically active coating, the intermediate layer consisting of titanium carbide and/or titanium boride and being applied onto the support by flame or plasma spraying, characterised in that the electrochemically active layer consists of ruthenium dioxide or a mixed metal oxide which contains ruthenium dioxide, or of iridium oxide or a mixed metal oxide which contains iridium oxide.
2. Electrode according to Claim 1, characterised in that 10-5000 g/m² of the intermediate layer are applied onto the support.
3. Electrode according to at least one of Claims 1 or 2, characterised in that the intermediate layer is applied in multiple layers.
4. Method for the production of an electrode according to one of Claims 1 to 3 by applying an intermediate layer onto a support and subsequently applying an electrochemically active coating onto the intermediate layer, characterised in that titanium carbide and/or titanium boride powders having different particle sizes are used for applying the intermediate layer by flame or plasma spraying.
5. Method according to Claim 4, characterised in that the powders used have particle sizes of from 10 to 200 µm.
6. Use of an electrode according to at least one of Claims 1 to 3 as a gas-evolving electrode.
7. Use of an electrode according to at least one of Claims 1 to 3 in an electrochemical cell to produce chlorine from aqueous hydrochloric acid solutions or to produce chromic acid from aqueous alkali metal dichromate solutions.
8. Use of an electrode according to at least one of Claims 1 to 3 as an electrical current distributor of a gas diffusion electrode for the reduction of oxygen in an electrochemical cell to produce chlorine from aqueous hydrochloric acid solutions.