Electrode assembly for multipolar electrolytic cells

Abstract

An electrode assembly adapted for arrangement between the terminal monopolar electrodes of a multipolar electrolytic cell includes a compartment wall, an anode carried by a lateral surface of the wall and provided with means for movement of the anode in a direction toward an opposed cathode surface to maintain a narrow gap between the electrodes and improve the cell power efficiency; a cathode carried by the opposed lateral surface of the same compartment wall and optionally provided with means for movement in a direction toward an opposed anode surface for maintenance of a narrow spacing of the electrodes. The electrode assembly provides for the construction of electrolytic cells of the membrane or diaphragm or diaphragm-less types which are useful for production of chlorine and caustic and for various other products of electrolytic processes.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrode assembly for construction of multipolar electrolytic cells capable of operating at optimum power efficiencies and requiring a minimum of maintenance over extended periods of time. More particularly this invention relates to bipolar electrode assemblies for construction of multipolar electrolytic cells of commercial size wherein the electrodes are large and must be maintained in closely spaced uniform planar relationship to each other for the cell to operate at optimum power efficiency and at low maintenance requirements. The electrode assembly of this invention is particularly adapted for electrochemical conversion of aqueous sodium chloride to chlorine and caustic in a diaphragm or membrane cell. It should be understood however that the electrode assembly of this invention is not limited to the construction of cells for the production of chlorine and caustic only for it may be used in cells for other electrochemical operations such as, for example, the manufacture of alkali and alkaline earth metal hypochlorites, chlorates, perchlorates, and various organic compounds.

2. Description of the Prior Art

It has been customary in the prior art to use multipolar electrolytic cells for various chemical reactions as this type of cell enables the cell to be constructed in compact form and eliminates the exposed busbar and metallic connections required for the introduction and withdrawal of the electric current to and from the electrodes of monopolar cells. The exposed parts and connections of monopolar cells are subject to attack by gases evolved during electrolysis and other chemicals of the cell environment which cause corrosion and contamination of the electrolyte.

In the general use of prior multipolar cells, bipolar electrodes are positioned intermediate to the terminal monopolar electrodes in any desired manner in closely spaced compact position, sealed at their edges to the cell side and bottom walls to prevent leakage between the adjacent cell compartments which are separated by partitions. Electrical connections are made only to the terminal monopolar electrodes and the electrolyte is circulated in contact with the electrodes in each of the individual compartments. The sum total of the voltage of the intermediate bipolar electrodes is equal to the voltage between the monopolar electrodes. In one type of prior art, multipolar cell the bipolar electrodes are constructed as a single sheet of titanium which serves as the compartment partition. One side of the titanium sheet is uncoated and acts as a cathode and the other side has at least a partially active surface coating and operates as the anode portion of the bipolar electrode. This assembly is positioned so that the anode surface of one titanium sheet is positioned adjacent the cathode position of an opposed assembly and any desired number of assemblies may be arranged between the monopolar terminal electrodes. A similar electrode assembly for multipolar cells which has also been previously used is a graphite plate assembly which functions in the same manner as titanium sheets, the plates serving as compartment partitions with one lateral surface of the plate operating as the anode and the opposed lateral surface acting as a cathode. Such graphite bipolar electrode assemblies are still in use but because of their limited resistance to erosion and chemical attack have been, and are being, replaced by the metallic sheet type bipolar assemblies. Another type of bipolar electrode assembly presently in use comprises a number of metallic solid or foraminous anode and cathode sheets which are positioned in opposed closely spaced manner within an electrolytic cell, each pair being separated by partitions and the total number of assemblies being arranged between terminal monopolar electrodes.

As previously mentioned, the graphite type electrodes are subject to erosion which results in contamination of the electrolyte and reduction of power efficiency. The increased voltage required to overcome the resistance of the larger quantity of the electrolyte present between the constantly increasing spaces between the electrodes caused by the continuous erosion reduces power efficiency. The titanium sheets which serve as the cell partition and provide a cathode on one lateral surface and an anode on the opposed lateral sheet surface have the disadvantage of being subject to corrosion of the cathode surface by the formation of titanium hydride, with resultant deterioration of the cathode surface, disintegration of the entire sheet and failure of the cell. The bipolar assemblies, wherein opposed sheets are positioned within individual compartments intermediate monopolar terminal electrodes, suffer from the disadvantage of the difficulty of maintenance of a closely spaced electrode gap across the entire lateral surface of the electrodes. Consequently, the establishment of a desired predetermined minimum voltage drop between the electrodes by maintaining a minimum distance between the entire cathode and anode surfaces to reduce the internal resistance loss in the electrolyte layer is virtually impossible. Even if the closely spaced electrodes are fabricated to close tolerance flat, uniformly planar sheets and mounted at a predetermined minimum operating distance from each other in a rigid frame, the varying operating parameters of electrolytic processes such as temperature, agitation of the electrolyte and variable stress in different areas of the lateral surface induced by inherent mechanical characteristics cause variations in the spacing of the electrodes. Such variations in spacing cause objectionably high current density at random areas of the working surfaces of the electrodes or electrode segments, resultant inefficient cell operation and, if not corrected, ultimate premature failure of the electrodes.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a bipolar electrode assembly for multipolar electrolytic cells in which the lateral surface spacing between the opposed electrode surfaces may be established and maintained at a predetermined uniform minimum distance.

It is a further object of this invention to provide bipolar electrode assemblies for multipolar electrolytic cells whereby the cells constructed with such bipolar electrodes arranged intermediate terminal monopolar electrodes are capable of operating at optimum power efficiency and minimum maintenance over extended time periods.

It is a still further object of this invention to provide bipolar electrode assemblies for multipolar electrolytic cells which cells, constructed from such bipolar assemblies arranged between monopolar terminal electrodes, are capable of producing, by electrochemical processes, products such as caustic and chlorine, alkali and alkaline earth metal hypochlorites, chlorates, perchlorates and various organic compounds.

It is a further object of this invention to minimize the close tolerance requirements in fabrication of the planar surfaces of large electrodes required for such bipolar electrode assemblies.

Broadly, this invention comprises at least one bipolar electrode assembly adapted for arrangement intermediate to the terminal monopolar electrodes of a multipolar electrolytic cell. The assembly includes an electrically non-conductive partition at least one rigid electrical conductor mounted in and extending beyond each lateral surface of the partition, a dimensionally stable anode and a cathode, respectively connected to the rigid conductor in opposed parallel relation to the partition at opposed surfaces of the conductor, and movable conductive means connecting the non-working lateral surface of at least one electrode to a portion of the rigid conductor facing the non-working lateral surface of said electrode for adjustable movement of the working surface of the electrode in a direction toward the working lateral surface of an opposed electrode of an adjacent bipolar assembly. The assembly is useful for constructing a bipolar electrolytic cell having monopolar electrodes positioned in each of the two terminal compartments of the cell and at least one bipolar electrode assembly of the invention arranged intermediate said terminal monopolar electrodes and means for connecting the terminal electrodes to the respective positive and negative poles of an electrical power source. Such a cell may be used for various electrolytic processes wherein either a diaphragm or a diaphragm-less type cell is required.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical view of one type of bipolar electrode assembly of this invention showing a number of resilient members as the electrically conductive means for movement of the anode, the resilient members being shown connected to several individual rigid electrical connectors.

FIG. 2 is a view similar to FIG. 1 illustrating the anode constructed in segment form, all other parts being similar to FIG. 1.

FIG. 3 is a view similar to FIG. 1 but additionally including a diaphragm positioned over the cathode and a number of electrically conductive resilient members for movement of the cathode, such assembly being adaptable for use in diaphragm type multipolar electrolytic cells.

FIG. 4 is another view of the bipolar electrode assembly of this invention wherein the electrically conductive means for adjustably moving the anode consists of a unitary resilient member connected to the surface of the number of rigid electrically conductive members and a rigid bar connected to one surface of the cathode and a portion of the surface of a number of electrically conductive rigid members.

FIG. 5 is another view of the bipolar electrode assembly of this invention showing a number of resilient members as the electrically conductive means for movement of the cathode only.

FIG. 6 is a view similar to FIG. 5 showing the cathode in segmented form.

FIG. 7 is another view similar to FIG. 3 illustrating the anode and cathode, respectively, constructed of segments.

Referring to the drawings, the electrode assembly is shown generally at numeral 10 and comprises an electrically non-conductive partition 11, which may be constructed of any suitable non-conducting material such as polyvinyl chloride, polyethylene, polyvinylidene chloride and various other plastics and the like. Rigid electrically conductive metal stud members or studs 12 extend through the partition and beyond both opposed lateral surfaces thereof and may be any suitable electrically conductive metal resistant to the corrosive conditions of the cell environment such as titanium, titanium sheathed copper and tantalum. The rigid conductive members 12 may be of various configurations provided they perform the function of conducting the current between the electrodes of the cell assembly and are usually designed of a geometrical configuration to provide maximum current-carrying efficiency. The movable electrically conductive members 15 for movement of the electrodes may be constructed of any suitable electrically conductive metal which is resistant to the cell environment and which is sufficiently resilient to permit movement of the electrodes. Generally, the means 15 for movement of the electrodes are constructed of metal such as titanium, titanium-sheathed brass, steel and nickel but any suitably resilient material resistant to the cell environment may be used. The shape of the electrically conductive members for adjustably moving the electrodes can vary with the design of the cell and may be a plurality of resilient members connected to individual rigid conductors extending through the partitions or may be in the form of a unitary resilient member connected to a number of the electrical conductors. The means for movement of the anodes are a plurality of resilient members connected to a plurality of rigid members 12 in FIGS. 1 to 3, inclusive and FIGS. 6 and 7, and a unitary resilient member connected to a plurality of rigid members 12, in FIG. 4. The means for adjustable movement of the cathode are shown as a plurality of resilient members in FIGS. 3, 5, 6 and 7.

The anodes 13 comprise an electrically conductive substrate with a surface coating thereon of a solid solution of at least one precious metal oxide and at least one valve metal oxide. The electrically conductive substrate may be any metal which is not adversely affected by the cell environment during use and also has the capability, if a breakdown in the surface coating develops of preventing detrimental reaction of the electrolyte with the substrate. The size of the anodes and anode segments may vary provided foraminous or solid anodes of suitable close tolerance flatness for forming the structural bipolar electrode assembly are used. Generally, the substrate is selected from the valve metals including titanium, tantalum, niobium and zirconium. Expanded mesh titanium sheet is preferred at the present time.

In the solid solutions an interstitial atom of a valve metal oxide crystal lattice host structure is replaced with an atom of precious metal. This solid solution structure distinguishes the coating from physical mixtures of the oxides since pure valve metal oxides are, in fact, insulators. Such substitutional solid solutions are electrically conductive, catalytic and electrocatalytic.

In the above-mentioned solid solution host structure the valve metals include titanium, tantalum, niobium and zirconium while the implanted precious metals encompass platinum, ruthenium, palladium, iridium, rhodium and osmium. Titanium dioxide-ruthenium dioxide solid solutions are preferred at this time. The molar ratio of valve metal to precious metal varies between 0.2-5:1, approximately 2:1 being presently preferred.

If desired, the solid solutions may be modified by the addition of other components which may either enter into the solid solution itself or admix with same to attain a desired result. For instance, it is known that a portion of the precious metal oxide, up to 50%, may be replaced with tin dioxide without substantial detrimental effect on the overvoltage. Likewise, the defect solid solution may be modified by the addition of cobalt compounds particularly cobalt titanate. Solid solutions modified by the addition of cobalt titanate, which serves to stabilize and extend the life of the solid solution, are described more completely in co-pending application Ser. No. 104,743 filed Jan. 7, 1971, now abandoned. Other partial substitutions and additions are encompassed. Another type of dimensionally stable anode coating which may be used with good results in the practice of this invention consists of mixtures of chemically and mechanically inert organic polymers and solid solutions of valve metal and precious metal oxides as at least a partial-coating on the electrically conductive substrate. Particularly useful materials in such anode coatings are the above-described solid solutions in admixture with fluorocarbon polymers such as polyvinyl fluoride, polyvinylidene fluoride and the like coated on at least part of the surface of an electrically conductive substrate consisting of the above-described valve metals and other suitable metals. Such anode coatings and preparation thereof are disclosed and more completely described in co-pending application Ser. No. 111,752 filed Feb. 1, 1971.

One other type of dimensionally stable anode capable of satisfactory use in this invention consists of a valve metal substrate bearing a coating of precious metals or precious metal alloys, particularly platinum and alloys thereof on at least part of its surface.

The above-mentioned preferred solid solution coatings are described in more detail in British Pat. No. 1,195,871.

The cathodes 14 may be foraminous as shown and may be any metal capable of sustaining the corrosive cell conditions. A useful metal is generally selected from the group consisting of stainless steel, nickel, titanium, steel, lead and platinum. In some cases the cathodes may be coated with the solid solutions above-described for coating the dimensionally stable anodes. When the assembly is designed for movement of the anode only, the cathodes may be either directly attached to a number of rigid electrically conductive members 12 as shown in FIGS. 1 and 2 or to a unitary rigid member 17, as illustrated in FIG. 4. The working face of the cathode is covered by a diaphragm or membrane 19, when the multipolar cell is to be used as a diaphragm or membrane cell.

When the electrode assembly is utilized for construction of a multipolar electrolytic cell, the cell casing may be any of the usual types of construction materials such as polyvinyl fluoride, polyvinylidene fluoride, various other reinforced plastics and any other materials which are inert to the environment of the particular cell electrolytes and resultant products of the process. To construct a multipolar electrolytic cell, monopolar electrodes are positioned at each of the terminal compartments at the ends of the casing and, for example, may be connected to the end walls of the casing. At least one bipolar electrode assembly is then arranged intermediate the terminal monopolar electrodes with the adjacent anodes and cathodes spaced as close as possible without causing a short circuit. However, any desired number of bipolar electrode assemblies may be arranged in the tank dependent upon the production volume and design features of the particular cell.

It will be noted from the above description that anodes and cathodes of adjacent bipolar electrode assemblies are positioned in closely spaced parallel, substantially face-to-face relation. Because of such closely spaced positioning of the electrodes electrically non-conductive spacers may be and preferably are interwoven through or positioned within the openings of foraminous electrodes or electrode segments to prevent electrical contact of the opposed electrode or segment surfaces. When flat or cylindrical elements are used as spacers they are generally interwoven through alternate openings on the outer edges of the lateral surfaces of the electrodes or electrode segments, but may also be interwoven through other openings in the foraminous electrodes. The electrically non-conductive spacers should be constructed of materials inert to the cell environment and may have any suitable geometric configuration. Generally, the spacers are polyvinylidene chloride, polyvinyl chloride, chlorinated polyvinylfluoride, polyvinylfluoride, tetrafluoroethylene and the like and may be of solid or hollow, cylindrical, flat or other suitable configuration. Other types of spacers capable of satisfactory use are electrically non-conductive strips provided with projections adapted to be tightly engaged within the electrode openings and button-type members such as semi-spherical elements arranged on opposite sides of the electrode openings and joined by an engaging member such as a stem extending through the electrode openings. The spacers are preferably arranged to prevent electrical contact by shorting between the opposed electrode surfaces and, at the same time, provide maximum flow of the electrolyte solution through the openings in the electrodes.

The bipolar electrode assembly of this invention offers many advantages over the prior art electrodes utilized in multipolar electrolytic cells. The electrically conductive means for moving the electrodes assure maximum power efficiency by maintaining the smallest possible space for electrolyte flow between the electrodes plus assuring a low IR of the current passage through the electrolyte. The flexibility of movement of the electrodes resulting from the electrically conductive means for movement of the electrodes maintains even large size electrodes in a flat or planar position thus assuring uniform spacing of electrodes as well as compensating for wear rates of individual sections of said electrodes. The bipolar electrode assembly also affords a great flexibility in construction of multipolar electrolytic cells in that any number of the assemblies may be mounted intermediate the monopolar terminal electrodes at each end of the cell thus providing for variable production rates, ease of construction and disassembly of said cells for cleaning or other maintenance services. The bipolar electrode assembly may be used for any size multipolar cell and enables the use of large size electrodes with excellent power efficiency. The construction of the cells is simple in that the electrode assemblies have been preconstructed and it is merely necessary to position the desired number of assemblies intermediate the terminal monopolar electrodes and connect the terminal electrodes to negative and positive poles of an electrical power source. Also, large size cells can be economically constructed since the precise tolerances in fabricating the planar surfaces of electrodes required by prior art cell construction to obtain maximum power efficiency are not required as the bipolar electrode assemblies of the present invention provide such close tolerance without precise surface fabrication.

Claims

I claim:

1. A generally planar bipolar electrode assembly adapted for closely spaced parallel series arrangement intermediate to the terminal monopolar electrodes of a multipolar electrolytic cell having side, bottom and end walls, comprising:

an electrically non-conducting, liquid tight partition, at least one rigid electrical conductor extending through said partition and beyond each lateral surface thereof;

a dimensionally stable anode and a cathode connected to the rigid conductor on opposite sides of and parallel to the partition;

movable, resilient, electrically conductive means connecting the non-working lateral surface of one electrode to the rigid conductor, the movable, resilient means being capable of causing movement of the lateral working surface of the electrode in a direction toward the working lateral surface of an opposed electrode of opposite charge of an adjacent bipolar assembly while maintaining intraelectrode electrical integrity.

2. The electrode assembly according to claim 1 wherein the anode comprises a number of segments and electrically conductive, resilient, movable means are connected to a non-working lateral surface of each anode segment and to the rigid conductor.

3. The electrode assembly according to claim 1 wherein the movable electrically conductive means is a unitary resilient member connected to the non-working lateral surface of the anode and to a plurality of rigid electrically conductive members.

4. The electrode assembly according to claim 1 wherein movable, resilient, electrically conductive means connect both the anode and cathode to the rigid conductor.

5. The electrode assembly according to claim 4 wherein the anode and cathode respectively are segmented.

6. The electrode assembly according to claim 1 wherein the movable electrically conductive means is a plurality of resilient members connected to the non-working lateral surface of the cathode and to a plurality of rigid electrically conductive members.

7. The electrode assembly according to claim 1 wherein the cathode is constructed of a plurality of segments and a movable, resilient, electrically conductive means is connected to each segment and to a rigid conductor.

8. The electrode assembly according to claim 1 wherein at least one electrically non-conducting spacer is carried by the working lateral surface of the anode or the cathode.