EP0135687B1 - Gas evolving metal electrode - Google Patents

Description

The present invention relates to a gas-evolving metal electrode, e.g. can be used in particular as an anode in amalgam cells for chlor-alkali electrolysis.

In the manufacture of e.g. Chlorine by electrolysis of aqueous alkali chloride solutions are generally used today titanium anodes with active layers containing noble metals. These so-called dimensionally stable anodes have the advantage over the previously mainly used graphite electrodes that the external dimensions do not change during operation. The disadvantage of these anodes lies in the relatively high production costs, due to the high price of titanium and its complex processing, as well as the use of noble metal in the active layer.

However, the use of titanium as the electrode base material allows a large number of different geometrical designs in comparison to graphite in order to fulfill the required function as a gas-generating electrode. In particular, the production of very flat electrode surfaces (+ 1 mm difference / m ² electrode surface) has become possible. This in turn allows the distance from the anode to the cathode to be significantly reduced. Since the electrolyte solution, in the case of chloralkali electrolysis the NaCI solution, has an electrical resistance, the aim is to reduce the voltage or energy losses caused thereby to a minimum by keeping the distance between the electrodes as small as possible.

At the same time, however, a certain minimum distance must be ensured for carrying out the electrolysis reaction, which also ensures that, if possible, no short circuits can occur.

In OE-AS 20 41 250 an electrode construction is described in which the working electrode surface, that of the cathode, e.g. in chlorine production using the amalgam process, the mercury cathode, made of expanded metal, perforated sheet metal, wire mesh or the like. is formed. A U-shaped conductor rail is used for uniform current distribution, which must also give the expanded metal the necessary mechanical rigidity in order to achieve the required planarity. It is easy to see that the production of such a conductor rail is very problematic, since titanium sheets have to be pressed into a U-shape and titanium tends to “spring back” after such a pressing process. In addition, notches have to be incorporated into this conductor rail , which reduce the stresses that occur when the expanded metal is welded to the rail, and also allow subsequent correction of the working surface of the anode in order to achieve a better planarity of the electrode surface During the electrolysis reaction, the gas bubbles formed are now discharged upwards through the openings in the expanded metal, perforated plate, etc. For further gas discharge, openings must therefore also be worked into the relatively large conductor rail Conductor rails can also be fixed with stable cross struts.

A good escape of the gas bubbles in the construction described in DE-OS 1814 567 is to be brought about by a complex system of primary and secondary conductor rails, this system also giving the working electrode surface the necessary strength (and thus planarity). This solution has to be bought with high material and manufacturing costs, especially since many welding processes and an exact adjustment of the individual parts are required.

A similar construction is described in DE-OS 20 45 560. Here, the strength of the electrode surface, which consists of a substantially horizontally extending perforated titanium structure, is increased by round rods which are arranged in parallel at a distance on the structure and which also ensure the current distribution. These round bars are connected to rectangular rails arranged transversely thereto, which take on the task of supplying power. The core of these rods and rails is made of aluminum, which in turn is surrounded by titanium. This anode assembly is also based on a complex manufacturing process. In addition, the titanium sheathing of the aluminum must be absolutely tight at all points, in particular also at the weld seams, since with the slightest damage to the titanium sheath, the electrode is rapidly destroyed by the anodic dissolution of the aluminum which takes place in the presence of chloride.

A similar construction is described in DE-DS 27 21 958, in which essential components of the electrode consist of titanium parts, the core of which is filled with rods made of other metals, which are predominantly liquid at operating temperature, in order to improve the power conduction and save expensive titanium , conductive material are embedded.

In order to achieve good gas discharge, in DE-OS 25 25 497 vertically arranged titanium webs with a rectangular cross-section are connected to one another at a certain distance so that the gas bubbles can be drawn off through the resulting gaps. In addition, it is required that these webs are also activated on the sides so that chlorine separation can also take place here. This is said to increase the effective electrode area since the horizontal sections of the webs opposite the mercury cathode make up only a small proportion of the area compared to the geometrical area of the entire electrode. However, as recent studies have shown (Chem. Ing. Techn. 52 (1980), 48-51), it can be considered certain that these side surfaces cannot make any relevant contributions to chlorine separation due to their greater distance from the counterelectrode. In addition, the electrically insulating gas bubbles deposited on the horizontal surface must pass through these spaces, which additionally impedes the flow of current in this area.

US Pat. No. 4,055,847 describes a complicated structure for achieving the necessary strength of the electrode surface and for achieving a good current distribution. It consists of a spider-shaped power distribution system, in which additional support ribs are required. As stated in the patent specification, melting and casting processes are necessary to produce the corresponding structures. However, as is generally known, these are complex and expensive in the processing of metals, in particular titanium or valve metals, since valve metals can only be melted in an argon atmosphere with a strict exclusion of air for metallurgical reasons. Another possible solution is shown in DE-OS 29 49 495. However, this construction also requires a primary and secondary power distribution system, which - analogous to that of DE-OS 1814 567 - require high manufacturing and material costs. Due to the relatively open construction of this system from flat profiles, however, gas bubble tearing and mass transfer on the electrode surface are to be improved.

DE-OS 30 08 116 describes an electrode construction which has only a primary distribution system, but which is also relatively complex. The oval profiles used here are created by flattening the round bars. This should achieve a ratio of working to projected electrode area of> 1. However, no attention is paid here (cf. J. Cramer, Chem. Ing. Techn. 52, 1980, pp. 48-51) that the solution to be electrolyzed opposes the passage of the electric current, so that the electrode surfaces are all the less to the course of the electrolysis reaction, ie Gas evolution contribute the further away they are from the counter electrode. The reason for this is the fact that the streamlines are looking for the shortest possible route as they pass through the solution to be electrolyzed. This fact should be clarified in the following explanations: In this construction, the gas bubble removal from the working electrode surface takes place more quickly due to the strong curvature than with the flat profiles used in DE-OS 29 49 495, but these relatively small degrees of curvature have the disadvantage that that in the areas closest to the cathode, there are very high current densities during electrolysis. This causes higher separation potentials and thus also a higher cell voltage or energy consumption. The areas further away from the cathode are disadvantaged by a thicker electrolyte layer with a correspondingly higher electrical resistance. This also has an unfavorable effect on the cell voltage.

Finally, German utility model 72 07 894 describes a gas-developing electrode which consists of a plate which is interspersed with channels which widen near and to a surface of the electrode. These channels can be continuously conical or venturi-like. This is intended to achieve an improved electrolyte circulation. Apart from the fact that this electrode can only be produced with complex manufacturing technology, this electrode has not found any technical use, since plate-shaped electrodes in particular have disadvantages with regard to gas removal.

• - Möglichst geringer Materialeinsatz zur Herstellung des Elektrodenkörpers.
• - Trotz vereinfachter Konstruktion gute mechanische Festigkeit bei gleichzeitig hoher Reparaturfreundlichkeit und guter Planarität der Elektrodenoberfläche.
• - Die Elektrodenprofile sollten nach Möglichkeit keine Kanten aufweisen, da an diesen Stellen erhöhter Verschleiß der Aktivschicht auftritt.
• - Der Gasblasenabriß sollte durch neue Profilquerschnitte, die nach hydrodynamischen Gesichtspunkten gestaltet sind, verbessert werden. Da Gasblasen für den Stromdurchgang 'Isolatoren" darstellen, dient deren rascher Abtransport einer Erniedrigung des Energiebedarfs für die Elektrolyse.
• - Die Verteilung der Stromlinien auf der arbeitenden Elektrodenfläche sollte möglichst gleichmäßig sein.

The present invention was based on the object of developing a mold for a gas-developing metal electrode which should have the following properties:

• - As little material as possible for the manufacture of the electrode body.
• - Despite the simplified construction, good mechanical strength combined with high ease of repair and good planarity of the electrode surface.
• - If possible, the electrode profiles should have no edges, as increased wear of the active layer occurs at these points.
• - The gas bubble tear should be improved by new profile cross sections, which are designed according to hydrodynamic aspects. Since gas bubbles represent 'insulators' for the passage of current, their rapid removal serves to lower the energy requirement for electrolysis.
• - The distribution of the current lines on the working electrode surface should be as even as possible.

This object is achieved by a gas-developing metal electrode for electrolysis cells, in particular anode for amalgam cells for chloralkali electrolysis, which consists of profiles arranged parallel to one another in a horizontal plane, the effective electrode surface facing the counterelectrode being curved and the profiles having a transverse profile with it a power supply provided power supply are connected to each other, which is characterized in that the curvature of the effective electrode surface in the area of the gaps into one with a smaller radius (r) passes, the radius (R) determining the curvature of the effective electrode surface being from 7 to 180 mm and the smaller radius (r) being 0.5 to 4 mm, and the profiles being closed off at the top by two side surfaces which emerge tangentially from the curvature (22, 25), which form an angle (alpha) of 20 to 120 ° at their intersection.

Solid or hollow profiles can be used as profiles for the electrodes according to the invention. When using full profiles, more titanium is required than with hollow profiles, but this disadvantage is offset by the advantage that the resistance is reduced and the voltage drop is reduced. In addition, full profiles are easier to process.

Furthermore, the effective electrode area, i.e. the surface which can be projected onto the counterelectrode is curved in such a way that the curvature increases from the center towards the edges.

• 1) einerseits sollte die Arbeitsfläche möglichst eben sein, wodurch ein gleichmäßiger Abstand Anodenfläche zu Gegenelektrode gewährleistet ist, was für eine gleichmäßige Stromdichteverteilung günstig, für den erforderlichen Gasblasenabtransport jedoch ungünstig ist und
• 2) andererseits sollte die Arbeitsfläche möglichst gekrümmt sein, wodurch sich die oben beschriebenen Vor- bzw. Nachteile umkehren.

When it comes to the curvature of the middle part, two opposing requirements have to be met, namely

• 1) on the one hand, the working surface should be as flat as possible, which ensures a uniform distance between the anode surface and the counterelectrode, which is favorable for a uniform current density distribution, but is unfavorable for the necessary gas bubble removal and
• 2) on the other hand, the work surface should be as curved as possible, which reverses the advantages and disadvantages described above.

In the following, the gas-developing metal electrode according to the invention will be explained with reference to FIG. 1, which is a perspective view and FIG. 2, in which two profiles are shown enlarged from viewing direction X.

The effective electrode surface consists of profiles arranged parallel to each other. The mechanical and electrical connection of these electrode profiles to one another is carried out by welding with one or more titanium webs (2) which have a shape specially developed for the purpose described. Titanium bodies (5) with an internal thread are attached to these webs. The internal thread serves to receive a power supply (4), e.g. a copper bolt. If necessary, this can be protected from the electrolyte solution (and thus anodic dissolution) by a welded-on titanium tube (5). The power supply to the individual electrode profiles takes place exclusively through a simple primary conductor system made of titanium webs (2). This is easy to manufacture from commercially available titanium sheets of appropriate thickness (matched to the current load) by simply punching them out. Since, with increasing distance from the contacting, less and less current has to be transported to the electrode profiles through this conductor system, since the number of profiles still to be supplied is reduced, the line cross section of this component also decreases. In Figure 1 this can be seen from the fact that the width of the titanium web (2) is reduced. This also helps to minimize the amount of material used.

In FIG. 2, two hollow electrode profiles according to the invention, according to viewing direction X from FIG. 1, are shown schematically, enlarged compared to FIG.

The work surface 21, i.e. the surface which can be projected onto the counterelectrode is curved according to the invention in such a way that the curvature on the sides, i.e. to the neighboring profile or to the cell wall becomes stronger. The curvature is determined by the radius R and the two smaller radii r. The hollow profile is closed at the top by the two meeting side surfaces 22 and 25, which are continued tangentially from the curved working surface.

In this way, the cross section of the gap zone between two profiles is given the profile of a nozzle-shaped rounded inlet zone and a calming zone which widens upward like a diffuser. As a result of the slight curvature, the gas bubbles formed on the work surface move towards the edges of the profiles and, due to the intensifying curvature, experience a desired uniform acceleration in contrast to an abrupt tearing off of the gas bubbles on an edge-shaped profile, which results in a higher pressure loss connected is. As a result, the gas is brought to the speed required to pass the narrowest point of the gap between two profiles with a minimal pressure loss. Due to the lower pressure drop, the gas bubbles reach a higher speed, which entrains a larger amount of liquid. This leads to an improved exchange of the electrolyte solution in front of the work surface. Immediately afterwards to the narrowest point, the gas flows into the expanding calming zone, the opening angle of which is designed in such a way that the gas bubbles reach their normal buoyancy speed largely without loss of pressure.

Due to the above-described effects that promote gas extraction, it is also possible to allow profile constructions with relatively large web widths S, which can be 6 to 50 mm, while the web widths in some of the previously known constructions. lie significantly below 6 mm. The advantage of using a profile with a large web width is obvious because, given the cell dimensions, fewer profiles are needed.

In order for the effects described above to be fully effective, the profiles should have certain geometric dimensions have, which are selected depending on the cell conditions.

As mentioned above, the work surface has less curvature in the middle than at the edges. Thus, the curvature in the central part is determined by a circular line whose radius R is 7-180 mm, preferably 15 to 25 mm, while on both sides the curvature becomes stronger and in a circular line with a radius r of 0.5 to 4 mm passes. The two radii should be chosen so that R / _r i: 5.

The inventive design of the work surface on the one hand ensures that, due to the relatively large radius of the circular line in the central region, an almost optimal, uniform distance of the work surface from the counter-cathode is ensured, the relatively small curvature of which, however, is already sufficient for rapid removal of the gas bubbles formed. Due to the stronger rounding at the transition of the work surface into the tangential side surfaces, edges are avoided at this point, which, as is known, are subject to increased wear.

von 5 bis 1800 beträgt.Depending on the radius R of the central circular line and the web width S, the height of the circular arc results (ie the greatest distance between the web width S and the working surface 21), which, however, should meet the condition that

is from 5 to 1800.

The inclination of the side surfaces can also be varied within wide limits. This inclination is determined by the angle with which they meet and which can expediently be from 20 ° to 120 °.

The technical outlay for producing the metal electrodes according to the invention, which are particularly suitable as anodes for chloralkali electrolysis, is low. To manufacture them, the individual electrode profiles (5) need only be welded continuously to the distributor webs (2). Notches into which the profiles are inserted can be incorporated into these webs for better fixation. The relatively long weld seam ensures good current transfer from the distributor web to the profiles.

Although the electrode construction according to the invention is characterized by an extraordinarily simple structure, its mechanical strength is excellent, essentially also due to the cross-sectional shape of the profiles according to the invention. The electrodes are also very easy to repair. If a profile is damaged, e.g. by short-circuiting, the respective electrode profiles can easily be exchanged individually or brought to the required planarity by corresponding messages.

The extremely low wear of the electrodes due to the absence of edges has already been mentioned above.

The rapid removal of the gas bubbles as described above enables profiles with widths on an effective electrode surface to be realized which were previously unknown. In other words, the ratio of effective electrode area, in which there is a largely uniform current density distribution, to geometric electrode area can be significantly improved.

From the description of the claimed electrode it also follows that only the actually effective surface of the electrode base body has to be provided with an active layer, since here there is a construction in which different partial areas of the profiles are each optimized with regard to the task to be performed. The side opposite the counterelectrode is so designed that the working electrode surface can optimally fulfill its function in the electrolysis process. The other sections of the electrode profile are optimized according to hydrodynamic criteria and simple manufacture. The construction is therefore very suitable for applying the activation solution by dipping, rolling or brushing. Since it is relatively easy to coat only the working electrode surface (which is desirable but not a requirement), the amount of activation solution required is reduced to a minimum. This is particularly advantageous when using activation solutions which contain expensive noble metals or noble metal compounds, for example in the case of the known Ru0 _2- containing active layers for anodic chlorine deposition.

Last but not least, this construction can be coated very well with the aid of spraying processes - in particular thermal spraying processes - since the working electrode surface has no sharp edges and since it is not necessary to coat side surfaces which are difficult to access.

Claims (4)

1. A gas-evolving metal electrode for electrolysis cells, in particular an anode for mercury cells for chlor-alkali electrolysis, which consists of profiles arranged parallel to one another in a horizontal plane, the effective electrode surface facing the counter-electrode being curved and the profiles being connected to one another by means of current distributors which are at right angles to the profiles and provided with a current supply, wherein the curvature of the effective electrode surface changes, in the region of the gaps, to a curvature with a smaller radius (r), the radius (R) which determines the curvature of the effective electrode surface being from 7 to 180 mm and the smaller radius (r) being from 0.5 to 4 mm, and wherein the profiles are closed at the top by means of two lateral surfaces (22, 23) which are tangential to the curvature and enclose an angle (alpha) of from 20 to 120 at the point at which they meet.

2. A gas-evolving metal electrode as claimed in claim 1, wherein R/r

5.

3. A gas-evolving metal electrode as claimed in claims 1 and 2, wherein the difference in height h, between the ribwidth 5 and the point nearest to the counterelectrode, which difference is determined by the curvature of the working surface, satisfies the condition

> Band < 1800.

4. A gas-evolving metal electrode as claimed in claims 1 to 3, wherein the profiles are solid profiles.

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