MXPA96005764A - Improved electrode for demembr electrolyzers - Google Patents

Abstract

The present invention relates to an electrode for electrochemical processes wherein gaseous products are formed, said electrode comprising a metallic sheet formed by means of a suitable tool in order to produce a profile of the type of "Venetian window" consisting of curved bands, characterized in that a mesh is fixed to said sheet, because said mesh, provided with an electrocatalytic coating, has the same profile of the "Venetian window" type as that of the sheet, and because the profiles of the sheet and mesh are coincident.

Description

IMPROVED ELECTRODE FOR MEMBRANE ELECTROLYZERS STATE OF THE ART At present, the ion exchange membrane electrolysis process is the preferred method for the industrial production of chlorine and caustic soda from brine, that is, a concentrated aqueous solution of sodium chloride, although opportunities may be contemplated. promising for other industrial applications, such as the production of hydrogen and oxygen by electrolysis of solutions of alkali metal hydroxides. However, in view of the current great importance of chlor-alkali electrolysis, the following description will be made with reference to this process, without trying to limit the invention to said process. The process of chlor-alkali electrolysis is characterized by a long-term uniform operation, provided that certain technical aspects are adequately approached. Two of these aspects are represented by the reciprocal interaction between the electrodes and the ion exchange membranes and by the operational service life of the electrodes. In relation to the first of these aspects, it must be taken into account that the turbulence of electrolysis can easily cause tremors in the delicate ion exchange membrane. To avoid this problem, which would easily cause the membrane to rupture, normally the two compartments of each elemental cell, which form an industrial electrolyzer, are characterized by a pressure difference that actually keeps the membrane pressed against one of the electrodes, generally the anode in chlor-alkali electrolysis with membrane. The other electrode can also be pressed against the membrane by means of suitable resilient systems, thus increasing the mechanical stability of the membrane itself (technology known as "zero separation"). Alternatively, the other electrode can be separated from the membrane that is pushed against the first electrode by the pressure difference, as already said (technology known as "finite separation" or "narrow separation"). In any case, the membrane is in contact with at least one electrode, whose geometry is extremely important. Various electrode geometries are known in the art, from the so-called expanded metal to the plates cut into parallel bands provided with sharp profiles that act as means for diverting gases (see European Publication No. 0 101 099) to the "Venetian window" electrodes. "(see European Publication No. 0 189 535), obtained by cutting metal sheets with suitable tools. In order to obtain the best behavior of the membrane, it is important that the portions of the electrode made of solid metal have the smallest possible dimensions, since the diffusion of the sodium chloride brine within the interstices between the membrane and the metal decelerates and , as a consequence, the liquid inside the interstices is diluted progressively. The dilution of the brine leads to the formation of blisters in the membrane. Another deterioration mechanism is derived from the stagnation of chlorine bags within the membrane / metal interstices. This stagnation causes the formation of sodium chloride crystals inside the membrane, whose structure becomes permanently altered, thus deteriorating its behavior (see Modern Chlor-Alkali Technnology, Vol. 4, Elsevier Applied Science, 1990, pages 109-123). These membrane damage phenomena are more easily controlled with the expanded metal electrodes where the dimensions of the mesh openings and the solid metal portions can be varied to a large extent by suitably adapting various parameters, such as the arrow between cuts. and its length, as well as the degree of expansion. The situation is much more critical with other geometries, in particular with "Venetian window" electrodes that, on the other hand, offer remarkable advantages with respect to the local fluodynamics of the gas-liquid mixtures of the electrolysis products (see European Publication No 0 189 535). In fact, with the "Venetian window" electrodes, there are large contact areas between the membrane and the solid metal portions of the electrodes and, therefore, there is a high risk of damage, as indicated above, more likely the greater the the current density during the operation in industrial electrolysers. To solve the problem of damage to the membrane, several solutions have been suggested, such as the corrugation of the surface of the membrane that has to come into contact with the electrode. Said corrugation can be obtained through a partial corrosion of the surface, for example by means of a plasma beam or by applying a layer of hydrophilic powder which prevents the adhesion of the gas bubbles. Alternatively, the surface of the electrode can be corrugated by engraving holes and channels therein in accordance with a herringbone pattern, produced by laser equipment (see US Patent No. 5,114,547). As regards the second aspect, that is to say the operational service life of the electrodes, this depends on the structure of the electrodes comprising a metallic substrate having the geometries mentioned above, provided with an electrocatalytic coating. For example, when the electrodes act as anodes (positive polarity), the substrate is titanium and the coating is of oxides be metals of the platinum group with a thickness of some microns. When the electrodes act as cathodes (negative polarity), the substrate is nickel or carbon steel or stainless steel, coated with a thin film (some microns) of Raney nickel, metals of the platinum group or oxides thereof, alone or in combination. The service life of these electrocatalytic coatings depends on the operating conditions, in particular the temperature, current density, electrolyte concentration and presence of poisons capable of preventing electro-catalytic activity ("poisoning"). In any case, after a certain period of operation, the electrodes have to be renewed (in the following description: reactivation). The simplest way is to transport the structures, where the electrodes are secured, to a point in the installation where the electrodes are released from the support structures and replaced by new electrodes. Obviously, this operation is of long duration (transport, mechanical operations) and expensive (total renewal of the electrodes including the metallic substrate). A possible alternative is to fix, usually with spot welding, a new electrode on the surface of the spent one. For this purpose, fine meshes having suitable dimensions in the openings and especially a small thickness are used (see European Publication No. 0 044 035). This method has the substantial disadvantage of altering the local geometry of the membrane-electrode contact, thus greatly modifying the fluodynamics of the electrolyte and gas mixtures produced. This drawback is of particular importance when the fine activated mesh is applied to exhausted electrodes of the "Venetian window" type or of similar geometry. Therefore, it is evident that the solutions proposed by the state of the art (for example, corrugation of the membrane or of the surface of the electrode) have only reduced the impact of the membrane-electrode contact width, notably increasing the costs of production (for example, use of laser equipment) or solve a problem (reactivation of depleted electrodes using fine activated meshes) but give rise to other drawbacks (worse local fluxodynamic gas-electrolyte mixtures). OBJECTS OF THE INVENTION The object of the present invention is to provide a new electrode capable of completely solving the problems affecting the state of the art, in particular with respect to the geometry of the contact area between the membrane and the "window" type electrodes Venetian "or similar geometries, when the electrodes are exhausted after a period of operation. As regards the latter aspect, the electrode of the present invention has a structure such that the reactivation can be carried out in situ without having to transport the spent electrode systems to other points of the installation. Another object of the present invention is to provide a new electrode structure with an electrocatalytic coating that strongly reduces the problems associated with the membrane-electrode contact and also allows an easy reactivation of the coating when it is exhausted. Brief Description of the Figures Figure 1 is a front view of an electrode of the "Venetian window" type. Figure 2 is a cross-section of the electrode structure of Figure 1. The electrode is obtained from a sheet metal formed with a special tool that, at the same time, cuts bands in the sheet and bends them. Figure 3 shows a composite structure comprising the electrode of Figure 1 provided with an activated flat sheet, used to renew the electrocatalytic activity of the electrode according to the teachings of the state of the art. Figure 4 is a front view of the preferred embodiment of the present invention. A flat mesh made of the same metal as the sheet and previously provided with an electrocatalytic coating, is formed using the same tool used for the electrode of Figure 1. Therefore, the shaped mesh has the same profile as the laminar electrode as shown in figure 5.

Figures 6 and 7 show the matching profiles of the shaped mesh of Figures 4 and 5 applied to the sheet of Figures 1 and 2. Description of the Invention A preferred embodiment of the present invention is illustrated in Figures 4, 5, 6 and 7. The mesh provided with an electrocatalytic coating fixed on the electrode of Figure 1, known in the art, ensures several advantages that will be explained in the following description. In the first place, the mesh, characterized by a smaller thickness than the electrode, adheres perfectly to the laminar profile of the electrode and can be effectively secured therein by spot welding. The solution proposed by the state of the art and illustrated in Figure 3 is negatively affected by several problems concerning the welding, probably due to the small contact area between the flat sheet and the curved profiles of the electrode of the " Venetian window ". Therefore, the welding process known in the art is poorly reliable and disjunctions are possible with the consequent unequal distribution of current. In addition to the possibility of resorting to a simpler and more reliable welding procedure, the preferred embodiment of the present invention maintains all the advantageous flux-negative characteristics of the electrode of FIG. 1 belonging to the state of the art.

According to another advantage, the present invention provides an electrode, whose curved profiles have an irregular profile particularly useful for preventing the membrane from adhering to the metal thus preventing the negative phenomena of dilution of the solution of saddic chloride and gas trapping. This result is obtained in an efficient manner, at low cost and with a simple construction method, in particular when the dimensions of the openings in the mesh are smaller than the width of the electrode bands of the "Venetian window" type. Preferably, the mesh is obtained by expanding a sheet having a suitable thickness. As a consequence, the preferred embodiment of the invention adds all the advantages offered by different inventions of the state of the art, that is to say, the reactivation using a flat sheet and the elimination of the problem of dilution in the interstices and the trapping of gas by the fact to engrave the surface of the electrode with channels according to herringbone design. In addition, these advantages are combined in a single element, easy to produce at low cost, able to maintain the fluodináminas characteristics of the structures of the state of the art. For this reason, the preferred embodiment of the present invention is useful not only for the reactivation of depleted electrodes, but also for installation in new electrolysers. In this case, the production process contemplates the following steps: - forming a metal sheet to obtain the structure and profile of figures 1 and 2. Contrary to the teachings of the state of the art, this structure is not provided with an electrocatalytic coating; - expansion of a thin sheet to form the mesh characterized by having adequate dimensions in the openings of the mesh and by a smaller thickness with respect to the shaped sheet. The mesh is provided with a suitable electrocatalytic coating. The mesh is then formed with the same tool used to form the metal sheet. In this way, a shaped mesh is obtained that perfectly adapts to the shaped sheet. In this way, the sheet-mesh assembly can be welded more easily and the welding reliability is improved. As a conclusion, in the composite structure of the present invention, the two components have different and complementary functions. In particular, the shaped mesh, which has a sufficient thickness, ensures the necessary rigidity to the electrode assembly and with its profile provides the best local fluodymine. The mesh has the main function of providing the assembly with the necessary electrocatalytic activity and with the necessary surface corrugation to avoid damage to the membrane caused by dilution in too small interstices and by gas trapping., as indicated above. In another less preferred embodiment of the present invention, a thin sheet is employed instead of the mesh. In this case, the sheet is provided with a suitable electrocatalytic coating and then formed with the same tool used to form the thicker sheet. In this way, the thin sheet, provided with the electrocatalytic coating, adheres perfectly to the profile of the thicker shaped sheet. Obviously, the use of the sheet can be resorted only in the case of reactivation of depleted electrodes. However, the use of the thin sheet involves higher costs than the fine mesh and the profile of the electrode assembly is uniform. Therefore, in the absence of the necessary corrugation, the membrane can be damaged, as is the case with the electrodes of FIG. 1 of the state of the art. On the contrary, similarly to the fine mesh, the welding of the thin sheet, previously shaped as indicated, is easy and reliable. In addition, also with the thin film, the fluodinamine typical of the original electrode is maintained. The above discussion clearly illustrates the distinguishing features of the present invention and certain preferred embodiments thereof. However, other modifications are possible without deviating from the scope of the invention, which is limited only by the individual claims.

Claims (9)

1. CLAIMS 1.- An electrode for electrochemical processes where gaseous products are formed, said electrode comprising a metal sheet formed by means of a suitable tool in order to produce a profile of the type of "Venetian window" consisting of curved bands, characterized in that a mesh is fixed to said sheet; because said mesh, provided with an electrocatalytic coating, has the same profile of the "Venetian window" type as that of the sheet; and because the profiles of the sheet and the mesh are coincident.
2. 2. An electrode according to claim 1, characterized in that said metal sheet has an exhausted electrocatalytic coating.
3. 3. An electrode according to claim 1, characterized in that the profile of the "Venetian window" type mesh is formed by means of the same type of tool used to form said sheet.
4. 4. An electrode according to claim 1, characterized in that said mesh has a thickness less than the thickness of the sheet and the dimensions of the openings are less than the width of the bands of both the mesh and the sheet.
5. 5. An electrode according to claim 1, characterized in that the mesh is fixed to the sheet by welding.
6. 6. An electrode according to claim 5, characterized in that said welding is a spot welding with electrical resistance.
7. 7. An electrode according to claim 1, characterized in that said mesh is expanded metal. 8. An electrode according to claim 1, characterized in that it is suitable for use as a cathode and that the sheet and the mesh are made of nickel and the mesh is provided with an electrocatalytic coating for the evolution of hydrogen in alkaline electrolyte. 9. An electrode according to claim 1, characterized in that it is suitable for use as an anode and that the sheet and the mesh are made of titanium and the mesh is provided with an electrocatalytic coating for the release of chlorine from alkaline solutions of chlorides. 10.- Electrolysis process, characterized in that it comprises using at least the cathode of claim 8. 11.- Electrolysis process, characterized in that it comprises using at least the anode of claim 9. 12.- Method for reactivating an electrode made of a metal sheet having a profile of the "Venetian window" type consisting of curved bands obtained by shaping with a suitable tool, said sheet having an electrocatalytic coating worn out, characterized in that it comprises: - providing an electrocatalytic coating in a mesh made of the same metal than the sheet; - forming said sheet by means of a suitable tool in order to obtain a "Venetian window" type profile; - placing said mesh having a "Venetian window" profile on said sheet, so that the profiles of the mesh and the sheet are coincident; and - fixing said mesh and said sheet by welding. 13. Method according to claim 12, characterized in that said welding is a spot welding with electrical resistance. 14. Method according to claim 12, characterized in that said mesh has a thickness less than the thickness of the sheet and the dimensions of the openings of the mesh are smaller than the width of the bands of both the mesh and the sheet. 15. Method for reactivating an electrode made of a metal sheet having a profile of the "Venetian window" type consisting of curved bands obtained by shaping with a suitable tool, said sheet having an exhausted electrocatalytic coating, characterized in that it comprises: - providing an electrocatalytic coating in a thin flat sheet made of the same metal as the exhausted sheet; - forming said thin flat sheet by means of a suitable tool in order to obtain a type profile of "Venetian window"; - positioning said thin sheet having a "Venetian window" profile on said exhausted sheet, so that the profiles of the thin sheet and the exhausted sheet are coincident; and - fixing said mesh and said sheet by welding. 16. Method according to claim 15, characterized in that said welding is a spot welding with electrical resistance. 17. An electrolysis cell, characterized in that it comprises at least the cathode of claim
8. 8. 18. An electrolysis cell, characterized in that it comprises at least the anode of claim
9. 9.