WO2003052168A2

WO2003052168A2 - Electrode

Info

Publication number: WO2003052168A2
Application number: PCT/SE2002/002118
Authority: WO
Inventors: Ingemar Björn Arne JOHANSSON; Åsa Carolina OXELMARK; Katrin Denker
Original assignee: Akzo Nobel N.V.; Eka Chemicals Ab
Priority date: 2001-12-19
Filing date: 2002-11-21
Publication date: 2003-06-26
Also published as: WO2003052168A3; AU2002353705A8; AU2002353705A1

Abstract

The invention relates to a method for preparing a substantially lead-free turbulence promoting electrode, wherein an amount of turbulence promoting particles having a particle size from about 30 to about 2000 mm are contacted with at least one electrode and brought to adhere thereto, such that from about 5 to about 50 % of the projected electrode surface area is covered with the particles. The invention also relates to a turbulence promoting electrode obtainable by the method and an electrode comprising turbulence promoting elevated parts arranged on at least one surface of the electrode. The height of the elevated parts is from about 30 to about 2000 mm. The invention further relates to the use of the electrode in an electrolytic cell for the production of alkali metal chlorate and alkali metal hydroxide.

Description

Electrode

The present invention relates to a method for preparing a substantially lead-free turbulence promoting electrode. It also relates a turbulent promoting electrode, and the use thereof in an electrolytic cell.

Background of the invention

In electrolysis processes, electrochemical reactions usually take place on electrodes immersed in the electrolyte. Electrodes are usually made up of a conductive electrode substrate on which an electrocatalytic material capable of accelerating selective electrolytic reactions is deposited. This is described in e.g. US 5,227,030 which discloses a method for providing a metal-surfaced electrode substrate with an electrocatalytic layer.

Anodes for oxygen evolution are disclosed in US 4,425,217 for electrowinning applications, wherein lead-based electrode substrates are used. However, lead-based electrode substrates cannot normaiiy be used in e.g. chiorate eiectroiytes since its softness render them unreliable.

A problem faced in many electrochemical reactions, especially in electrolytic cells operated at high current densities, is the limited mass transport of reactants and reaction products towards and away from the electrode due to a hindering diffusion layer. A diffusion layer is formed at the surface of an electrode at which the concentration of a specific substance is different from the concentration of the same substance in the surrounding bulk solution. A diffusion layer arises when the diffusion rate of the substance towards and away from an electrode surface is not sufficiently high. This can occur when the rates of the reactions taking place at the electrode are faster than the thus rate-limiting diffusion rate of the reactants and the products. The thickness of a formed diffusion layer often range from 50-150 μm. Such diffusion layer is formed in the electrolytic production of e.g. alkali metal chlorate or alkali metal hydroxide out of brine. An electrolyte having a laminar flow profile creates a thicker diffusion layer than does a turbulent flow profile. The thicker the diffusion layer, the slower the mass transport of the reactants and the products towards and away from the electrode surface. This is a problem leading to a higher cell voltage of the electrolytic cell which in turn results in higher energy consumption. Thus, only a cell voltage reduction of a few mV may save a considerable amount of energy, especially as products, such as alkali metal chlorate or alkali metal hydroxide, obtained from electrolysis processes often are produced in large quantities. It is an object of the present invention to solve the above problem. The invention The. present invention relates to a method for preparing a substantially lead-free turbulence promoting electrode, wherein an amount of turbulence promoting particles having a particle size from about 30 to about 2000 μm are contacted with at least one substantially lead-free electrode and brought to adhere to the electrode such that from about 5 to about 50 % of the projected electrode surface area is covered with said particles.

It has been found that a reduced cell voltage can be obtained in an operating electrolytic cell by providing an electrode according to the present invention. This is principally achieved by reducing the thickness of the diffusion layer by providing particles which adhere to the electrode surface and protrude from the electrode surface. A laminar flow profile of an electrolyte in an operated electrolytic cell at the initial electrode or close to the initial electrode can by means of the prepared electrode be at least partly replaced by a turbulent flow profile. The electrode of the invention can thus impart a more turbulent flow profile to a flowing electrolyte which reduces the thickness of the diffusion layer. As a consequence, the mass transport of reactants and products towards and away from the electrode is increased, which in turn can lower the cell voltage. Preferably, the method of preparing the electrode is electroless, i.e. no electrical source is needed to adhere the particles to the electrode. By the term "turbulence promoting" as used herein is meant any agitation caused by the electrode in the operated electrolytic cell.

By the term "electrode" as used herein is meant any electrode comprising at least an electrode substrate, since electrode substrates may also work, at least to some extent, as electrodes. By "substantially lead-free turbulence promoting electrode" is meant herein an electrode or electrode substrate substantially free of lead. However, the electrode may contain such a small amount of lead that the electrode will not become damaged in alkaline electrolytes. Preferably, the electrode contains up to about 10 wt% lead, more preferably up to about 5 wt% lead, and most preferably up to about 1 wt% lead. By the term "initial electrode" is meant the electrode or electrode substrate from which the electrode according to the invention is prepared.

The size of the particles, i.e. the diameter thereof, should suitably be selected so that the adhered particles protrude through the formed diffusion layer where the electrode is arranged or at least protrude through the diffusion layer that would be formed at the electrode surface without the adhered particles. In the operated electrolytic cell, the particles adhered on the electrode are preferably of such size that they at least partly protrude or pass through both the diffusion layer formed at the electrode surface and the region of the laminar flow of electrolyte in the electrolytic cell.

In the production of alkali metal chlorate out of brine for instance, the thickness of the diffusion layer typically ranges from about 50 μm to about 150 μm, but the thickness may vary depending on the flow rate of the electrolyte and further parameters in a specific production plant.

Thus, according to one preferred embodiment, the particle size ranges from about 50 to about 500 μm, and most preferably from about 70 to 250 μm. The size of the particles can be optimised in relation to the electrolyte flow profile passing the electrode so as to achieve a maximal turbulence effect.

The electrode substrate or electrode employed in the method may be made of any metal or conductive element which can retain its physical integrity during the preparation and its subsequent use in an electrolytic cell and which can resist alkali metal chlorate or alkali metal hydroxide electrolytes. The electrode must be sufficiently hard such that particles partly embedded in the electrode still projects at least about 30 μm. Suitable electrode substrate materials include electrically conductive metals such as copper, nickel, valve metals such as titanium, tantalum, zirconium or niobium, and alloys or mixtures thereof, preferably nickel which is particularly resistant to alkaline electrolytes, e.g. in the production of alkali metal hydroxide and alkali metal chlorate. Iron-based materials such as iron, steel, stainless steel or other metal alloys, in which iron is a major component are also suitable electrode substrate materials.

The configuration of the electrode substrate used is not critical. A suitable electrode substrate may, for example, take the form of a flat sheet or plate, a curved surface, a convoluted surface, a punched plate, a woven wire screen, an expanded mesh sheet, a rod, or a tube. However, the electrode substrate preferably is a flat sheet or plate.

Preferably, before contacting the electrodes with the particles, it is advantageous to degrease the electrode substrate with a suitable degreasing solvent prior to roughening its surfaces. Removal of grease deposits from the electrode substrate surfaces is desirable to allow chemical etchants to contact the electrode substrate and uniformly roughen the surface. Removal of grease often allows for good contact between the electrode substrate and any possible coating applied thereon. Suitable degreasing solvents are common organic solvents such as acetone and lower alkanes, as well as halogenated solvents. Removal of grease is also advantageous where a roughened surface is not desired.

The electrode substrate may be roughened in order to increase the adhesion of any possible subsequent contacting step with an electrocatalytic coating solution. Suitable techniques employed in the art to roughen the surface include sand blasting, chemical etching and the like. The use of chemical etchants is well known and such etchants include most strong inorganic acids, such as hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid. Hydrazine hydrosulfate and oxalic acid are also suitable chemical etchants.

According to a preferred embodiment, from about 5 to about 50 %, preferably from about 5 to about 45 %, and most preferably from about 10 to about 35 % of the electrode surface is covered with particles. Suitably, the electrode is contacted with from about 1000 to about 100000 μg particles/cm², preferably from about 2000 to about 20000 μg particles/cm² of the projected electrode surface area. Preferably, if the particles are made of e.g. titanium or oxides thereof, from about 1000 to about 10000 μg of particles are contacted per cm² projected electrode surface area.

Suitably, the material of the particles is selected from valve metals such as titanium, tantalum, zirconium, valve metal oxides, e.g. titanium oxides such as TiO₂, tantalum oxides such as Ta₂O₅, zirconium oxides such as ZrO₂, preferably titanium or oxides thereof. Also noble metals or noble metal oxides from the platinum group such as ruthenium, rhodium, iridium, palladium and platinum can be used. Further particle materials include aluminium, silicon, chromium, manganese, iron, cobolt, nickel, copper and zinc and their oxides. Plastics and polymers having a sufficient adhesion to the electrode may also be used as particle materials as well as carbides such as SiC, nitrides such as SiN, and other ceramics. Any of the above enumerated particle materials may also be used in mixtures in individual particles. Particles of one material may of course be adhered to the electrode together with particles of other materials. Preferably, the particles are conductive, particularly if the electrode has a high coverage. According to one preferred embodiment, the particles are contacted with the electrode by pressing the particles into the electrode, e.g. in a conventional press tool. Prior to pressing, the particles are suitably substantially evenly distributed on the initial electrode surface to obtain a substantially even distribution of adhered particles on the electrode. According to a preferred embodiment, another electrode is placed on top of the electrode with substantially evenly distributed particles. The electrodes can then be applied in a press tool to press in the particles in the electrodes. The particles are thus partly embedded or incorporated into the electrodes. The pressure applied on the electrodes suitably ranges from about 200 to about 5000 kg/m², preferably from about 500 to about 1500 kg/m². According to another preferred embodiment, a coating solution, suitably containing electrocatalytic material, is applied on the initial electrode. A coating layer can be formed subsequent to conventional drying and baking steps. Subsequently, a further coating solution may be applied on the first coating layer. Turbulence-promoting particles can then be substantially evenly distributed on the same coating layer of the electrode. This can be performed e.g. by strewing the particles substantially evenly over the electrode. The particles can in this way adhere to the electrode by reacting chemically with coating material in the coating solution before a coating layer is formed on the electrode surface. However, this is normally only possible when non-inert particles capable of undergoing chemical reactions with coating material in the coating solution is employed.

According to another preferred embodiment, the particles are adhered to the initial electrode by applying on the electrode a particle containing coating solution or by applying a coating solution and the particles separately on the electrode surface area. The resulting particle-containing solution on the electrode can be applied by means of spray coating, painting, rolling, or immersing the electrodes in the solution or other suitable methods, preferably by spray coating. According to one preferred embodiment, subsequent to applying the particles and the solution on the electrode surface area according to any of the embodiments as described herein, the electrode is baked at a temperature ranging from about 400 to about 600°C, preferably from about 450 to about 550°C. The time of baking suitably is from about 10 minutes to about 8 hours, preferably from about 30 minutes to about 4 hours. Good adhesion can in this manner be achieved between the initial electrode and the particles.

According to another preferred embodiment, the turbulence promoting particles are brought to adhere to the electrode by distributing the particles over the electrode in substantially parallel lines to adhere particles in such structure. This structure of particles on the electrode surface is particularly useful in electrolytic cells in which the electrodes are arranged in such a way that the parallel lines are oriented perpendicularly to the flow direction of the electrolyte. Turbulence can in this way be created when the laminar flow profile hits the lines of particles which thus promote a turbulence effect. Electrodes with adhered particles may be subsequently coated by any suitably means with an electrocatalytic metal precursors, e.g. by applying an electrocatalytic solution containing a metal from the platinum group, e.g. ruthenium, rhodium, osmium, iridium, palladium and platinum. Preferred precursor include metal salts of halides or nitrates of ruthenium, palladium, and platinum or mixtures thereof. The invention also relates to an electrode obtainable by the method.

The invention further relates to an electrode having turbulence promoting elevated parts on at least one surface of a substantially lead-free electrode. The elevated parts have a height above the electrode surface from about 30 to about 2000 μm. This means that the elevated parts protrude from about 30 to about 2000 μm from the surface of the initial electrode. The elevated parts may be particles adhered to the electrode but can also be peaks or other elevated formations. The elevated parts may for example be the result of deformation of the initial electrode per se. The further dimensions of the elevated parts in the plane of the electrode surface suitably also are in the range from about 30 to about 2000 μm. The elevated parts preferably have a height, and further dimensions in the plane of the electrode surface, from about 50 to about 500 μm, and most preferably from about 70 to 250 μm. According to one preferred embodiment, at least about 30 μm, preferably at least about 50 μm, and most preferably at least about 60 μm of the parts protrude out from the initial electrode surface so as to cause a turbulence promoting effect on electrolyte flow passing the electrode in the operated electrolytic cell.

The electrode of the invention suitably has from about 0.5 to about 150 million elevated parts/m², preferably from about 1 to about 100 million elevated parts/m², and most preferably from about 5 to about 50 million elevated parts/m² projected electrode surface area.

According to one preferred embodiment, from about 3 to about 95 %, preferably from about 50 to about 90 %, and most preferably from about 70 to about 90 % of the projected electrode surface area is covered with elevated parts.

According to another preferred embodiment, from about 5 to about 50 %, preferably from about 5 to about 45 %, and most preferably from about 10 to about 35 % of the electrode surface is covered with particles.

Preferably, the elevated parts are turbulence promoting particles adhered on at least one surface of the electrode.

These particle densities correspond approximately to weight densities from about 1000 to about 100000 μg particles/cm², and preferably from about 2000 to about 20000 μg particles/cm² of projected electrode surface area.

The part of the particles not protruding from the electrode is embedded or incorporated in the electrode. The embedment of the particles secures the adhesion to the initial electrode. The particles are suitably substantially evenly distributed over the entire electrode or at least over one surface thereof. The particles may also be arranged in substantially parallel lines over the electrode surface to impart efficient turbulence on the passing electrolyte. Further parameters and details of the turbulence promoting electrode are evident from the method of preparing the electrode as described herein.

The invention also relates to the use of the electrode of the invention in an electrolytic cell for the production of alkali metal chlorate or alkali metal hydroxide, preferably alkali metal chlorate. The electrode of the invention is preferably not used in electrolytic cells having membranes.

Suitably, alkali metal chlorate is produced by electrolysis of an electrolyte containing alkali metal chloride according to the overall formula: MeCI + 3H₂O -» MeCIO₃ + 3H₂, where Me represents an alkali metal. The process is cyclic, where in a first step the chloride electrolyte is brought to an electrolyser for the formation of hypochlorite, whereupon the solution is brought further to reaction vessels for further reaction to chlorate. Subsequently, chlorate formed is separated by crystallization or can be used in chlorine dioxide generation. The crystallization of chlorate can be brought about by evaporation or cooling. Evaporation means that the water is evaporated and condensed in a separate step, either indirectly in a heat exchanger or, more frequently, directly in the cooling water. Cooling crystallization means that the temperature is lowered to such an extent, that the chlorate electrolyte becomes saturated with chlorate whereby crystals precipitate.

Brief description of the drawings Fig.1 shows a side view of a conventional electrode surface. Fig.2 shows a side- view of an electrode surface provided with turbulence-promoting particles.

Detailed description of the drawings

Fig.1 shows a side view of a conventional electrode (1 ) at which a diffusion layer (dashed line) (3) is formed. Arrows (4) show the direction of a laminar electrolyte flow profile passing the electrode surface (1 ).

Fig.2 shows a side view of an electrode (1) provided with turbulence-promoting particles (2) according to the invention. A diffusion layer (dashed line) (3) is formed at the electrode (thinner than that of Fig.1 ). The particles (2) promote a turbulence effect on the laminar flow profile so that the flow profile of the electrolyte becomes more turbulent which reduces the thickness of the diffusion layer.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the gist and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the claims. The following examples will further illustrate how the described invention may be performed without limiting the scope of it. Example 1

Two titanium plates (electrodes), having a thickness of 3 mm and the further dimensions 80 x 400 mm, were degreased, blasted, and etched. The degreasing treatment was performed by an alkaline degreasing agent. The blasting was carried out with aluminium oxide. The obtained R_a-value was 5 μm. The etching was performed in 20 wt% boiling hydrochloric acid. The plates were then rinsed and dried.

0.6 g TiO₂ (Amperit™ from Starck) with a particle size ranging from 38-100 μm was substantially evenly spread on one side of one of the titanium plates. The other plate was placed on top of the powder-applied surface of the first plate. The assembled piece of plates were applied in a pressing tool. A pressure of 1000 kg/cm² was then applied during 15 seconds. After termination of the pressing operation, the plates were removed from one another. The particles were substantially evenly distributed and partly embedded in both titanium plates which resulted in good adhesion to the plates. The particle density was approximately 26 million particles/m² covering about 12% of the electrode surface. Subsequently, the plates were spray-coated to deposit several layers of RuO₂-TiO₂ thereon. After each coating solution had been deposited, the electrode (anode) was dried for 10 minutes at 100 °C. The ruthenium and titanium content of each deposited layer were about 0.75 g of each metal/m² electrode surface are. Totally 18 g ruthenium and titanium/m² was deposited on the anode. The anode was subsequently baked for 4 hours at 500 °C.

The prepared anode was tested in a chlorate cell with a titanium cathode. The gap between the electrodes was 3 mm. The electrolyte contained 100 g NaCI/litre, 600 g NaCIOs/ litre, 5 g Na₂Cr₂O₇/litre, and about 3 g NaCIO/litre. The current density was 3 kA/m², the pH 6.5, the temperature was 70 °C, and the linear flow rate was 0.44 m/s. The cell voltage measured was 3.27 V.

Example 2 An anode was prepared as in example 1 , with the only difference that no TiO₂ particles were pressed into the titanium plates. A test of the anode in the same cell was performed under the same conditions as in example 1. The cell voltage was 3.34 V, thus 70 mV higher than for the turbulence promoting anode.

Claims

1. Method for preparing a turbulence promoting electrode, wherein an amount of turbulence promoting particles having a particle size from about 30 to about 2000 μm are contacted with at least one substantially lead-free electrode and brought to adhere to said electrode, such that from about 5 to about 50 % of the projected electrode surface area is covered with said particles.

2. Method according to claim 1 , wherein the particles are pressed into the electrode such that the particles are partly embedded in the electrode.

3. Method according to any of the preceding claims, wherein the particles are contacted with the electrode by applying a particle containing coating solution.

4. Method according to any of the preceding claims, wherein the particles have a particle size from about 50 to about 500 μm.

5. Method according to any of the preceding claims, wherein the particles are made of metal oxides, metals, ceramics, plastics, polymers or mixtures thereof.

6. Method according to any of the preceding claims, wherein the particles are brought to adhere in substantially parallel lines to the electrode.

7. Turbulence promoting electrode obtainable by a method according to any of claims 1-6.

8. Turbulence promoting electrode having turbulence promoting elevated parts on at least one surface of a substantially lead-free electrode, said turbulence promoting elevated parts having a height from about 30 to about 2000 μm above the surface of the electrode.

9. Electrode according to claim 8, wherein the elevated parts have dimensions in the plane of the electrode surface from about 30 to about 2000 μm.

10. Electrode according to claim 8 or 9, wherein the electrode has from about

0.5 to about 150 million elevated parts/m² of the projected electrode surface area.

11. Electrode according to any of claims 8-10, wherein the elevated parts cover from about 5 to about 50 % of the projected electrode surface area.

12. Electrode according to any of claims 8-11 , wherein the elevated parts are turbulence promoting particles.

13. Use of a turbulence promoting electrode as claimed in any of claims 7-12 in an electrolytic cell for production of alkali metal chlorate or alkali metal hydroxide.