Electrode assembly, electrode structures and electrolysers

Abstract

The present invention relates to an electrode assembly, electrode structures and an electrolyser using said assemblies/structures, and in particular provides an electrode assembly comprising an anode structure and a cathode structure, each of said anode structure and cathode structure comprising i) a flange which can interact with a flange on an another electrode structure to hold a separator in between the two, ii) an electrolysis compartment which contains an electrode, and which in use contains a liquid to be electrolysed, iii) an inlet for the liquid to be electrolysed and iv) an outlet header for evolved gas and spent liquid, wherein the outlet header on one of the anode structure and the cathode structure is an external outlet header and the outlet header on the other one of the anode structure and the cathode structure is an internal outlet header, as well as to electrolysers comprising a plurality of such electrode assemblies.

Claims

1. An electrode assembly comprising an anode structure and a cathode structure, each of said anode structure and cathode structure comprising i) a flange which can interact with a flange on another electrode structure to hold a separator in between the two, ii) an electrolysis compartment which contains an electrode, and which in use contains a liquid to be electrolysed, iii) an inlet for the liquid to be electrolysed and iv) an outlet header for evolved gas and spent liquid, wherein the outlet header on one of the anode structure and the cathode structure is an external outlet header wherein the external outlet header is an outlet volume by which gases evolved during electrolysis exit the electrode structure and which is provided on the electrode structure outside of the electrolysis compartment, and the outlet header on the other one of the anode structure and the cathode structure is an internal outlet header wherein the internal outlet header is an outlet volume by which gases evolved during electrolysis exit the electrode structure and which is provided on the electrode structure inside of the electrolysis compartment, and further where one or both of the outlet headers comprise one or more internal cross members located along part of or all of the length of and attached internally to the sides of the header.

2. An electrode assembly as claimed in claim 1 wherein each outlet header is an outlet volume which is provided on the individual anode or cathode structure and by which evolved gas exits the anode or cathode structure to an electrolyser collection header which is a volume which collects the gases evolved during electrolysis from the exits of multiple outlet headers and passes them to further processing.

3. An electrode assembly as claimed in claim 2 wherein each outlet header is an extended volume aligned parallel with the long horizontal axis of the electrode structure.

4. An electrode assembly as claimed in claim 1 where V.sub.E/(A.sub.E×L.sub.E) is less than 1, where V.sub.E is the internal volume of the external outlet header in cm.sup.3, A.sub.E is the internal cross sectional area at the exit end of the header L.sub.E is the internal length, more preferably where the external outlet header is tapered, and in particular increases in cross-section area in the direction of gas/liquid flow towards its exit port or ports.

5. An electrode assembly as claimed in claim 1 where the cross members are strips running internally along the length of the outlet header(s), attached to the sides of the header(s), and preferably wherein the cross members are provided with holes through the strips communicating from top to bottom.

6. An electrode assembly as claimed in claim 1 where the external outlet header of the electrode structure which has said external outlet header occupies space which is vertically above the adjacent electrode structure in an electrode module, electrode unit, modular electrolyser or filter press electrolyser.

7. An electrode assembly according to claim 1 wherein the outlet header on the anode structure is an external outlet header and the outlet header on the cathode structure is an internal outlet header.

8. An electrode assembly according to claim 1 wherein in each of said anode structure and cathode structure: the electrolysis compartment comprises a pan with a dished recess, the flange being around the periphery thereof for supporting a gasket capable of sealing the separator between the anode surface of an anode structure and the cathode surface of a cathode structure such that the anode surface is substantially parallel to and faces but is spaced apart from the cathode surface by the separator and is hermetically sealed to the separator, and an electrode spaced from the pan but connected to the pan by electrically conductive pathways between the pan and the electrode with the proviso that where the electrode structure is an anode structure the electrode may be directly electrically connected to the pan, and preferably wherein the dished recess of one of the anode structure and cathode structure is provided with a plurality of outwardly projecting projections and the other of the anode structure and cathode structure is provided with a plurality of inwardly projecting projections the projections being such that the outwardly projecting projections can mate with the inwardly projecting projections in an adjacent electrode structure or electrode module in a modular electrolyser and wherein the electrically conducting pathways comprise one or more conductive posts which are provided with shoulders on which baffles may be mounted to improve circulation of fluids and gas in the electrode structure.

9. A modular or filter press electrolyser comprising a plurality of electrode assemblies as claimed in claim 1, and preferably which comprises 5-300 electrode assemblies.

10. A process for the electrolysis of an alkali metal halide which comprises subjecting an alkali metal halide to electrolysis in a modular or filter press electrolyser comprising a plurality of electrode assemblies, and preferably which comprises 5-300 electrode assemblies, said electrode assemblies each comprising an anode structure and a cathode structure, each of said anode structure and cathode structure comprising i) a flange which can interact with a flange on another electrode structure to hold a separator in between the two, ii) an electrolysis compartment which contains an electrode, and which in use contains a liquid to be electrolysed, iii) an inlet for the liquid to be electrolysed and iv) an outlet header for evolved gas and spent liquid, wherein the outlet header on one of the anode structure and the cathode structure is an external outlet header wherein the external outlet header is an outlet volume by which gases evolved during electrolysis exit the electrode structure and which is provided on the electrode structure outside of the electrolysis compartment, and the outlet header on the other one of the anode structure and the cathode structure is an internal outlet header wherein the internal outlet header is an outlet volume by which gases evolved during electrolysis exit the electrode structure and which is provided on the electrode structure inside of the electrolysis compartment.

11. An electrode structure comprising: i) a pan with a dished recess and a flange which can interact with a flange on a second electrode structure to hold a separator in between the two and the dished recess further having a plurality of inwardly or outwardly projecting projections which can mate with corresponding projections on a third electrode structure in an electrode unit or in a modular electrolyser, and ii) an inlet for liquid to be electrolysed and iii) an outlet header for evolved gas and spent liquid, wherein the outlet header of the claimed electrode structure is an internal outlet header wherein the internal outlet header is an outlet volume by which gases evolved during electrolysis exit the electrode structure and which is provided on the electrode structure inside of the electrolysis compartment and the outlet headers of the second and third electrode structures are external outlet headers wherein the external outlet header is an outlet volume by which gases evolved during electrolysis exit the electrode structure and which is provided on the electrode structure outside of the electrolysis compartment, and further wherein each outlet header is an outlet volume which is provided on the individual electrode structure and by which evolved gas exits the electrode structure to an electrolyser collection header which is a volume which collects the gases evolved during electrolysis from the exits of multiple outlet headers and passes them to further processing.

12. An electrode structure as claimed in claim 11 wherein each outlet header is an extended volume aligned parallel with the long horizontal axis of the electrode structure.

13. An electrode structure according to claim 11 wherein the internal outlet header comprises one or more internal cross members located along part of or all of the horizontal length of and attached internally to the sides of the header.

14. An electrode structure according to claim 11 which is a cathode structure.

15. A method of refurbishing an electrode assembly which method comprises: a) providing an electrode structure as defined in claim 8, b) refurbishing said electrode structure, which refurbishment comprises: i) detaching the electrode and attached current carriers from the pan or from posts to which the current carriers are connected, and ii) subsequently reattaching to the pan the same electrode after refurbishment thereof or a replacement electrode by attachment of the current carriers associated with the refurbished or replacement electrode to the pan or said posts, and c) reassembling the electrode structure.

16. A method of refurbishing an electrode assembly which method comprises: a) providing an electrode structure as defined in claim 8, b) refurbishing the electrode of said electrode structure whilst the electrode is still attached to the pan.

17. A method for refurbishing a modular or filter press electrolyser which comprises refurbishing one or more electrode structures according to the methods of claim 15.

Description

(1) The present invention is further illustrated by reference to, but is in no way limited by, the following drawings, in which:

(2) FIG. 1 is a cross-section of the top part of a bipolar electrode unit according to the present invention;

(3) FIG. 2 is a cross-section of the top part of an electrode module according to the present invention;

(4) FIGS. 3A and 3B show, respectively, examples of “spiders” suitable for use in the anode and cathode structures.

(5) FIGS. 4A and 4B show close-ups of the preferred structures of the cross-members in the external and internal outlet headers.

(6) FIG. 5 is an isometric view looking at an anode structure according to the present invention;

(7) FIG. 6 is a cross-section of the bottom part of a bipolar electrode unit according to the present invention; and

(8) In FIG. 1 there is shown a bipolar electrode unit comprising an anode structure (10) and a cathode structure (30).

(9) The anode structure (10) comprises a flange (11), and a dished recess (12) with an inwardly projecting projection (13), which forms an electrolysis compartment (14) containing an anode (15). The anode structure has an external outlet header (16). The anode (15) is typically in the form of a perforated plate.

(10) The cathode structure (30) comprises a flange (31), and a dished recess (32) with an outwardly projecting projection (33), which forms an electrolysis compartment (34) containing a cathode (35). The cathode structure has an internal outlet header (36). The cathode (35) is typically in the form of a perforated plate.

(11) The anode structure (10) is electrically connected to the cathode structure (30) via a conductivity enhancing device (50) disposed between the inwardly projecting projection (13) on the anode structure (10) and the outwardly projecting projection (33) on the cathode structure (30).

(12) In practise there are multiple inwardly and outwardly projecting projections on each electrode structure, and multiple conductivity enhancing devices such that when the two electrode structures are urged together, the conductivity enhancing devices afford good electrical continuity between the peaks of the cathode structure projections (33) and the anode structure projections (13). The conductivity enhancing device may be in the form of an abrasion device or (more preferably) a bimetallic disc. When the bipolar electrode unit is supplied pre-assembled for use in a filter press bipolar electrolyser, it is possible for the conductivity enhancing device (50) to be omitted completely and instead for the anode and cathode structure to be electrically and mechanically connected together by welding, explosion bonding or a screw connection.

(13) The anode and cathode structures further comprise electrically conductive posts (17, 37), which connect to the respective projections (13, 33), electrically insulating cushions (18, 38) and current carriers which are each in a form having a central portion from which two or more legs radiate (hereinafter referred to as “spiders”)(19, 39). The spiders (19, 39) are mounted between the respective posts (17, 37) and the respective electrodes (15, 35). At the location of the respective posts (17, 37), the electrodes (15, 35) are apertured and the cushions (18, 38) are received within the holes and rest on the central base of the spiders (19, 39).

(14) Flow of liquor from the anode electrolysis compartment (14) to the external outlet header (16) takes place via an exit slot at the upper end of the anode structure (10), the exit slot being located immediately above the anode (15).

(15) Flow of liquor from the cathode electrolysis compartment (34) to the internal outlet header (36) takes place via a slot in the internal outlet header in the upper region of the cathode structure (30).

(16) In FIG. 2 there is shown an electrode module comprising an anode structure (10) and a cathode structure (30). The anode and cathode structures are broadly as defined for FIG. 1 and the same numbering is used as for the corresponding features already described for FIG. 1. However, the respective electrode structures are in this Figure joined with the anode (15) and cathode (35) facing each other with a membrane (51) in between. In particular, the flanges (11, 31) are provided with backing flanges (20, 40) with holes to accept bolts (not shown) for bolting the anode structure (10) and the cathode structure (30) with two gaskets (52) and the membrane (51) to form a module according to the present invention. The membrane (51) passes down through the electrode module between the anode (15) and cathode (35), providing fluid separation between the respective electrolysis compartments (14, 34) of said anode and cathode structures (10, 30).

(17) The spider (19) in the anode electrolysis compartment (14) comprises a disc-shaped central section (21) which can be connected to the end of the post (17), e.g. by welding, screw-fixing or push-fit connectors, and a number of legs (22) which radiate from the central section (21) and are connected at their free ends, e.g. by welding, to the anode (15). Usually the legs (22) are arranged so that the current supply via the post (17) is distributed to a number of equispaced points surrounding the post (17).

(18) The spider (39) in the cathode electrolysis compartment (34) comprises a disc-shaped central section (41) which can be connected to the end of the post (37), e.g. by welding, screw-fixing or push-fit connectors, and a number of legs (42) which radiate from the central section (41) and are connected at their free ends, e.g. by welding, to the cathode (35). Usually the legs (42) are arranged so that the current supply via the post (37) is distributed to a number of equispaced points surrounding the post (37).

(19) In practice, during the production of the electrode structures (10, 30), the spiders (19, 39) may be welded or otherwise connected to the electrodes (15, 35) and the spiders may then be subsequently welded or otherwise secured to the posts (17, 37). This arrangement facilitates replacement or repair of the anode/cathode plates or renewal/replacement of any electrocatalytically-active coating thereon.

(20) Also shown in FIG. 2 are baffles (23, 43) which may serve to partition, respectively, each anode compartment and each cathode compartment into two communicating zones to provide liquor recirculation as discussed further below. The provision of baffles in either compartment is optional, but it is particularly preferred that baffles are provided in the anode compartment. Without wishing to be bound by theory it is believed that recirculation in the anode compartment is useful in providing increased rates of electrolysis, for example by facilitating operation at higher current density.

(21) The baffles (23, 43) may be mounted on the electrically conductive posts (17, 37). Each of the posts may be provided with a shoulder (24, 44) to facilitate installation and accurate location of the baffles.

(22) Also shown in FIG. 2 are a cross-member (25) in the external outlet header (16) of the anode and a cross-member (45) in the internal outlet header (36) of the cathode.

(23) FIGS. 3A and 3B show, respectively, examples of suitable “spiders” for use in the anode and cathode structures.

(24) With respect to FIG. 3A the spider comprises a disc-shaped central section (21) and 4 legs (22) which radiate from the central section (21). The legs (22) radiate symmetrically so that in use the current supply is distributed to a number of equispaced points.

(25) Especially when intended for use in the electrolysis of alkali metal halides, the anode spiders are fabricated from a valve metal or alloy thereof.

(26) With respect to FIG. 3B the spider comprises a disc-shaped central section (41) and 4 legs (42) which radiate from the central section (41). The legs (42) radiate symmetrically so that in use the current supply is distributed to a number of equispaced points.

(27) Especially when intended for use in the electrolysis of alkali metal halides, the cathode spiders may be may be fabricated from materials such as stainless steel, nickel or copper.

(28) As shown, the legs (42) of the cathode spider are longer and configured to be relatively springy, whilst the legs (22) of the anode spider are shorter and more rigid.

(29) FIGS. 4A and 4B show respectively close-ups of the preferred structures of the cross-members (25) and (45). The preferred structure of the cross-member (25) in the external outlet header (16) is in the form of a “ladder” type arrangement, whilst the preferred structure of the cross-members (45) in the internal outlet header (36) is in the form of plates with round holes. As shown in FIG. 4B, there may be more than one cross-member (45) in the outlet header (36). Although only a single cross-member (25) is shown in FIGS. 1 and 2 there may also be more than one cross-member in the outlet header (16)

(30) FIG. 5 shows an anode structure (10) in more detail, showing inwardly projecting frusto-spherical projections (13) and a tapered external outlet header (16). FIG. 5 also exemplifies the locations for the measurements of the A.sub.E and L.sub.E.

(31) FIG. 6 shows a cross-section of the bottom part of a bipolar electrode unit. As with the Figures above the same numbering is used as for the corresponding features already described. In this Figure the anode structure is provided with an anode inlet tube (26) whilst the cathode structure is provided with a cathode inlet tube (46). Ports (not shown) are provided in the respective inlet tubes for discharge of liquor into the respective electrolysis compartments, and are preferably formed such that liquor discharged therefrom is directed towards the back of the pans behind the baffles (23, 43) to aid mixing. The baffles (23, 43) extend vertically within the respective anode and cathode compartments from the lower end of the electrode structure to the upper ends thereof and form two channels within each electrode structure which communicate at least adjacent the top and bottom of the structure.

(32) In a second aspect the present invention provides a modular or filter press electrolyser comprising a plurality of electrode assemblies according to the first embodiment.

(33) For example, the present invention may provide a filter press electrolyser comprising a plurality of connected bipolar electrode units, adjacent bipolar electrode units being connected via a separator and sealing means between flanges on the adjacent units. The separator and sealing means are preferably as described between electrode structures when configured as an electrode module in the first aspect.

(34) Alternatively, the present invention may provide a modular electrolyser comprising a plurality of connected electrode modules. In this case the electrode modules may be connected to each other by providing suitable electrical connections between adjacent modules.

(35) For example, the recessed dish of the anode pan and the recessed dish of the cathode pan in adjacent modules are electrically joined, preferably at the apices of the projections.

(36) Electrical conductivity may be achieved by the use of interconnectors or by close contact between the electrode structures. Electrical conductivity may be enhanced by the provision of conductivity-enhancing materials or conductivity-enhancing devices on the outer surface of the pans. As examples of conductivity-enhancing materials may be mentioned inter alia conductive carbon foams, conductive greases and coatings of a high-conductivity metal, e.g. silver or gold.

(37) When connecting adjacent electrode modules together connections via welding, explosion bonding or a screw connection are not preferred. Instead connections are preferred which are formed by close physical contact between the adjacent electrode structures.

(38) Electroconductivity-enhancing devices which can enhance the contact include electroconductive bimetallic contact strips, discs or plates, electroconductive metal devices, such as washers, or electroconductive metal devices adapted to (a) abrade or pierce the surface of the pans by cutting or biting through any electrically-insulating coating thereon, e.g. an oxide layer, and (b) at least inhibit formation of an insulating layer between the device and the surface of the pan (which may be referred to as an “abrasion device”).

(39) Such devices are described further in U.S. Pat. No. 6,761,808.

(40) The number of modules or bipolar units may be chosen by the skilled man in the light of inter alia the required production volume, available power and voltage and certain constraints known to the skilled man. Typically, however, a modular or filter press electrolyser according to the second aspect of the present invention comprises 5-300 modules.

(41) In a third aspect there is provided a process for the electrolysis of an alkali metal halide which comprises subjecting an alkali metal halide to electrolysis in a modular or filter press electrolyser according to the second aspect.

(42) The modular or filter press electrolyser according to the third aspect of the present invention may be operated according to known methods. For example, it is typically operated at pressures between 50 and 600 kPa (0.5 and 6 bar) absolute pressure, preferably between 50 and 180 kPa (500 and 1800 mbar).

(43) Liquid to be electrolysed is fed to the inlet-tubes in each electrode structure. For example, where the electrolyser is to be used for brine electrolysis inlet-tubes allow caustic to be charged to the cathode structure and brine to be charged to the anode structure. Products, namely chlorine and depleted brine solution from the anode structure and hydrogen and caustic from the cathode structure, are recovered from the respective headers.

(44) The electrolysis may be operated at high current density, i.e. >6 kA/m.sup.2.

(45) In a yet further aspect the present invention provides an electrode structure comprising: i) a pan with a dished recess and a flange which can interact with a flange on a second electrode structure to hold a separator in between the two and the dished recess further having a plurality of inwardly or outwardly projecting projections which can mate with corresponding projections on a third electrode structure in an electrode unit or in a modular electrolyser, ii) an inlet for liquid to be electrolysed and iii) an outlet header for evolved gas and spent liquid,
wherein the outlet header is a tapered external outlet header which increases in cross-section area in the direction of gas/liquid flow towards the exit ports.

(46) Preferably the external outlet header in this aspect comprises one or more internal cross members located along part of or all of the horizontal length of and attached internally to the sides of the header.

(47) In this aspect the depth of the external outlet header may exceed the depth of the claimed electrode structure. In particular, when connected to said second and/or third electrode structure in an electrode module, electrode unit or modular electrolyser, the external outlet header of the claimed electrode structure can occupy space which is vertically above the second and/or third electrode structures.

(48) Other features of the electrode structure may be generally as described in the first aspect. For example, the preferred electrode structure comprises a dished recess is provided with a plurality of inwardly projecting projections.

(49) In a most preferred embodiment of this aspect the flange is around the periphery of the dished recess and being for supporting a gasket capable of sealing the separator between the electrode surface of the claimed electrode structure and the electrode surface of the second electrode structure such that the electrode surfaces are substantially parallel to and face each other, but are spaced apart from each other by the separator and are hermetically sealed to the separator. Further, the electrode structure comprises an electrode spaced from the pan but connected to the pan by electrically conductive pathways between the pan and the electrode with the proviso that where the claimed electrode structure is provided with a plurality of inwardly projecting projections the electrode may be directly electrically connected to the pan.

(50) The electrode structure is preferably an anode structure. In particular, as already described the separator is most prone to damage caused by the formation of a gas space adjacent the separator on the anode side in the upper region of an electrolysis compartment, and also because the separation of formed chlorine from spent brine is the most problematic. The external outlet header located above the electrolysis compartment allows to minimise these problems because its location moves the gas disengagement area away from the separator and also provides increased flexibility to design its shape and size to improve the separation.

(51) In a yet further aspect the present invention provides an electrode structure comprising: i) a pan with a dished recess and a flange which can interact with a flange on a second electrode structure to hold a separator in between the two and the dished recess further having a plurality of inwardly or outwardly projecting projections which can mate with corresponding projections on a third electrode structure in an electrode unit or in a modular electrolyser, and ii) an inlet for liquid to be electrolysed and iii) an outlet header for evolved gas and spent liquid,
wherein the outlet header of the claimed electrode structure is an internal outlet header and the outlet headers of the second and third electrode structures are external outlet headers.

(52) Preferably the internal outlet header in this aspect comprises one or more internal cross members located along part of or all of the horizontal length of and attached internally to the sides of the header.

(53) In this aspect the depth of the external outlet headers of the second and third electrode structures may exceed the depth of said electrode structures such that when the claimed electrode structure is connected to said second and/or third electrode structures in an electrode module, electrode unit or modular electrolyser, the external outlet header of the second or third electrode structures can occupy space which is vertically above the claimed electrode structure.

(54) Again, other features of the electrode structure may be generally as described in the first aspect. For example, the preferred electrode structure comprises a dished recess is provided with a plurality of outwardly projecting projections.

(55) In a most preferred embodiment of this aspect the flange is around the periphery of the dished recess and being for supporting a gasket capable of sealing the separator between the electrode surface of the claimed electrode structure and the electrode surface of the second electrode structure such that the electrode surfaces are substantially parallel to and face each other, but are spaced apart from each other by the separator and are hermetically sealed to the separator. Further, the electrode structure comprises an electrode spaced from the pan but connected to the pan by electrically conductive pathways between the pan and the electrode with the proviso that where the claimed electrode structure is an anode structure the electrode may be directly electrically connected to the pan.

(56) The electrode structure is this aspect is preferably a cathode structure.

(57) Finally, the present invention also provides methods of refurbishing electrode assemblies or modular or filter press electrolysers according to the above aspects.

(58) For example, a method of refurbishing an electrode assembly may comprise: a) providing an electrode structure, b) refurbishing said electrode structure, which refurbishment comprises: i) detaching the electrode and attached current carriers from the pan or from posts to which the current carriers are connected, and ii) subsequently reattaching to the pan the same electrode after refurbishment thereof or a replacement electrode by attachment of the current carriers associated with the refurbished or replacement electrode to the pan or said posts, and c) reassembling the electrode structure.

(59) When an anode or cathode plate of an electrode structure is in need of refurbishment it may be removed from the structure by removing any cushions to expose the central sections of the current carriers and thereby allow their detachment from the posts or, where no posts are present, from the electrode pan. Once the current carriers have been detached, the anode or cathode can then be removed for refurbishment. Refurbishment of an electrode may involve repair of sections of the electrode and/or recoating as required. Alternatively the electrode may be replaced with an entirely new electrode. The new or refurbished electrode is then physically and electrically reattached, for example by spot welding, threaded fasteners or push-fit connectors.

(60) Refurbishment of electrode assemblies used in the chloralkali process is generally required every few years.

(61) In an alternative embodiment, the method of refurbishing an electrode assembly may comprise: a) providing an electrode structure, and b) refurbishing the electrode of said electrode structure whilst the electrode is still attached to the pan. The refurbishment may comprise: i) Removal of electrode coating from electrode contained in the pan in-situ, ii) Decontaminating, cleaning and drying the electrode structure, iii) Repairing any structural damage, e.g. by removal of damaged mesh or electrode blades and rewelding in uncoated replacements, iv) Recoating the repaired electrodes in-situ in the pan.