Expandable Electrodes

Abstract

An electrode is provided which, after insertion in an electrolytic cell, may be caused to expand thereby reducing the electrode-electrode gap and, hence, increasing the power efficiency of the electrolysis. Particularly, an anode for use in a diaphragm-type electrolytic cell for the production of chlorine and caustic is described together with a method for assembling the cell.

Description

BACKGROUND OF THE INVENTION

While electrolytic processes are employed for a number of purposes such as the production of chemicals and the plating of conductive surfaces, one of the most commercially significant applications of electrolysis is the production of halogens, particularly chlorine, and alkali metal hydroxides, particularly sodium hydroxide, by the electrolysis of aqueous alkali metal halide solutions, particularly sodium chloride solutions, in diaphragm-type electrolytic cells. The configuration and operation of these diaphragm cells is well known to those skilled in the art and, while the design of the cell may vary considerably from one manufacturer to another, it may be said very broadly that most such designs consist of three basic elements; the anode base, the cathode can and the cover. In some instances, of course, the anodes may depend from the top or sides of the cell rather than extend from the bottom, said top or side thus becoming the "base" for the purposes contemplated herein. Still, the general relationship between component parts remains essentially the same. The anode base may be considered to be the vehicle for both supporting the anodes within the cell compartment and conducting the electrolyzing current to the anode risers. In one of the more prevalent designs, which is suitable for the purpose of illustration, the anodes are disposed in a vertical manner in uniformly spaced rows across the width of the anode base. The cathode can, which generally rests upon the anode base and is insulated therefrom, may be considered as a unit construction, which, in addition to carrying the active cathodic surfaces, serves to divide the cell into a series of anolyte and catholyte compartments. The active cathodic surfaces themselves serve generally as the vehicle, or supporting structure, for the diaphragm, which is often a layer of asbestos fibers serving to separate the anode and cathode compartments of the cell. The function of the cell cover, of course, needs no explanation. A general embodiment of diaphragm cells of the type alluded to in the foregoing is represented by U. S. Pat. No. 2,987,463 which shows such a cell employing graphite anodes.

While a substantial portion of the chlorine and caustic produced in the world today is produced in diaphragm-type electrolytic cells such as the foregoing, a number of problems are inherent in such cells serving to limit their further application and also to impose a handicap upon the degree of efficiency with which the existing cells may be operated. To illustrate this point, most cells in commercial use today are operated with a considerable and discrete gap between the electrodes. Obviously such operation is inefficient since the electrolyte which fills said gap has a considerable resistance of its own to the passage of current and therefore significant quantities of energy are wasted, serving only to raise the temperature of the electrolyte and ultimately limit the current density at which the cell may be operated. Generally the distance between the anode and the cathode is on the order of 0.5 inch. When graphite anodes are employed, this distance becomes, with use, even greater owing to the attrition of the anode, the original thickness of the anode being of the order of 1.25 inches.

While such a gap is inefficient, it has been tolerated heretofore for the reason that assembly of a cell employing the electrodes spaced more closely together is impractically difficult. A number of factors contribute to this difficulty of assembly. To begin with, the cathodes, which are generally steel screens, become misshapen and distorted through use and with age so that a regular surface is no longer presented. Furthermore, the diaphragm material is generally deposited, by vacuum, onto the surface of the cathode from an asbestos slurry and, because of the nature of the slurry and the process for applying same, a diaphragm of non-uniform thickness is often obtained. Perhaps more significantly, the process of imbedding graphite anode blades in the anode base, for instance as described in the aforementioned U. S. Pat. No. 2,987,463, is subject to difficulties such that over the height of the anode, which is usually more than two feet, a displacement from the vertical approaching 0.5 inch in one direction or the other is not uncommon. Therefore it will be seen that when an attempt is made to place the cathode can, carrying the deformable diaphragms thereon, over the vertically-disposed anode blades in such a manner as to present an alternating array of anodes and diaphragm-coated cathodes, difficulties will be presented in the form of destruction of diaphragms, snapping of the relatively brittle anode blades and the like. These difficulties of assembly and the need to provide a space for brine circulation and gas release are the primary reasons that most cells are operated at an average initial anode-cathode gap of 0.5 inch.

Recent years have seen the introduction, in the field of diaphragm cell electrolysis of sodium chloride solutions as in many others, of the so-called dimensionally stable anodes. Generally these dimensionally stable anodes comprise an electrically conductive, electrocatalytically active coating, for example platinum or a precious metal oxide, on an electrically conductive substrate, generally a valve metal such as titanium. These new anodes, owing to the very fact of their dimensional stability, a property not heretofore available to commercial operators of electrolytic cells, have resulted in a profusion of actual and speculated new cell designs. While many of these new designs are of interest and certain of them incorporate a reduction in the anode-cathode gap, it is obvious that the well-established diaphragm cell industry is not going to immediately discard all existing equipment in favor of new, and in some cases yet unproven, cell designs.

For this reason, a large amount of attention has been directed to adapting the existing diaphragms cells, with a minimum of investment, for operation with the dimensionally stable anodes. Usually the changes involved have centered around the area of the anode base, the cumbersome anode bases used with the graphite anodes of the old technology being for the most part discarded. In their place, simple one-piece bases, to which and/or through which the anodes may be fixed and attached to a source of current, have been introduced. Obviously, however, in addition to the savings inherent in the dimensionally stable anodes, their success in these applications resides further in the ability to use the remaining component parts of the existing diaphragm cells, namely the cathode can and the cell cover, without further modification. This being the case, it then becomes readily apparent that the problem of the existence of a considerable anode-cathode gap still remains. Thus, while it may be possible to align somewhat more accurately the vertical disposition of the anodes themselves, the cathode-cathode gap remains fixed, and in some instances, non-uniform. At least two factors have served to prevent the substantial reduction of the anode-cathode gap even in those cells equipped with dimensionally stable anodes. In the first place, the cost of the materials of construction of the anode working faces and the anode risers is so great that a "massive" anode, i.e., one of sufficient dimension to substantially fill the anolyte compartment thereby reducing the anode-cathode gap, is economically unfeasible. Furthermore, the non-uniformity inherent in the diaphragm-coated cathodes remains and the new factor of occasional unevenness of the anode working face is introduced, thereby again presenting considerable assembly problems.

STATEMENT OF THE INVENTION

Therefore, it is a primary object of the present invention to provide a means for reducing the electrode gap in an electrolytic cell thereby resulting in increased efficiency of operation without a sacrifice in ease of assembly of the cell.

It is a further object of the present invention to provide a simple and practical method for assembling a diaphragm-type electrolytic cell for the production of chlorine and caustic, said cell after assembly having a minimal anode-cathode gap.

These and further objects of the present invention will become apparent to those skilled in the art from the description and claims which follow.

These objects are obtained by the provision of an electrode structure which is distinguished by a movable, electrically conductive means connecting the electrode riser with the electrode working face. By this means an electrode may be installed in an electrolytic cell in a contracted state, the movability of the connecting means thereafter allowing the electrode to expand by moving the electrode working face away from the riser thereby reducing the gap between said expandable electrode and the adjacent electrode in the cell.

The advantages of such an electrode and its use in an electrolytic cell are many. For example, when used as an anode in a diaphragm cell, assembly is simple since the anodes, in contracted form, readily accommodate the diaphragm-coated cathodes when the cathode can is placed over the anode array. Expansion of the anode once in place is likewise quite simple and serves to significantly reduce or substantially eliminate the anode-cathode gap. Elimination or reduction of this gap can result in a voltage savings for this factor alone of up to 0.3 volts at 1 ampere per square inch (a.s.i.). Furthermore, it follows that because of the reduction in heat generated in the cell, it may be operated at a high current density without boiling, hence allowing more production per unit area occupied by the cell. Other economies are realized in the savings on materials of construction. For example, it is theoretically possible to supply the necessary electrolyzing current to the working face of the anode using a small diameter riser. As a practical matter, however, such an economy has not generally been realized heretofore since a large diameter riser was needed to maintain the established 0.5 inch anode-cathode gap. According to the present invention, however, the anode-cathode gap is no longer dependent upon the diameter of the anode riser. The number of risers used per cell and the cost of materials of construction of these risers make such savings considerable. Also, in previously proposed anode designs, the anode working face has been directly attached to the riser as be welding. When eventually the anode fails and requires re-coating, it has been difficult to separate the anode face from the riser without materially damaging both components especially where a copper-cored titanium riser is used. In the newly proposed design, however, damage to the riser is avoided since it is only necessary to disconnect the working face from the movable connecting means, a much simpler operation allowing immediate re-use of both riser and connector. While the foregoing are the most significant advantages, a number of other advantages will be pointed out or will appear as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention reference will be made to the drawings, in which:

FIG. 1 illustrates an anode of this invention in contracted form and its relation to the opposed cathodes.

FIG. 2 illustrates the same relationship where the anode has now been expanded.

FIG. 3 is a view, partially cut away, of one embodiment of an expanded anode.

FIG. 4 is a top view of the anode of FIG. 3 in contracted form.

FIG. 5 is a top view of a further embodiment of the present invention.

FIG. 6 is an expanded view of another embodiment of the present invention, unassembled.

FIG. 7 is a top view of the assembled electrode, in expanded form, of FIG. 6.

FIGS. 8 and 9 show top views of a further embodiment of the present invention, in expanded and contracted form, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FOr the purpose of describing the present invention reference will be made hereinafter almost exclusively to the provision of an expandable anode for use in the electrolysis, in a diaphragm cell, of sodium chloride to produce chlorine and caustic. It should be understood, however, that while described as an anode, where conditions of use warrant the electrode may also be used as a cathode or in some instances as both the anode and the cathode. The novel concept of the present invention resides in the expandability of the electrode and while it is at present thought to be most useful as an anode for the electrolysis of sodium chloride, where like considerations as to ease of assembly, reduction of electrode gap and the like apply, it may just as well function as a cathode. Likewise, during the following discussion relating to the electrolysis of sodium chloride it must be appreciated that, for the most part, the details of construction of the surrounding cell components, such as the cover, the cathode can and the anode base itself, are important only to the extent that they allow the incorporation in the cell, with practical advantage, of the expandable anode.

Broadly, the anode construction of the present invention may be considered to consist of three components; the anode riser, the anode working face and the movable connecting means. Before considering the various embodiments which serve to illustrate the inventive concept, it may be desirable to define to some extent the form which the various components may take.

Anode risers of the type generally employed in the present invention are not, with the exception of their often smaller diameter, in and of themselves unknown. Essentially, as with other dimensionally stable anode configurations, the riser serves as a means to conduct the electrolyzing current from the current supply to the anode working face. The primary considerations for the configuration of this riser and the materials of construction used therein are that the riser have sufficient cross-section and an adequate degree of electrical conductivity to supply the total current needed to the anode working face with a minimum of voltage lost to resistance. Furthermore, at least that portion of the riser in contact with the electrolyte must be resistant to this medium under cell conditions. Generally, for the purpose of modification of existing diaphragm cells for use with dimensionally stable anodes, and in many cases for the design of new cells of this type, the riser will merely comprise an electrically conductive material in the shape of a rod. This rod, whether hollow or solid, may be either totally constructed of a valve metal such as titanium, tantalum or an alloy thereof which is resistant to the electrolyte or, more desirably, the rod may have only the outer surface thereof coated with the valve metal, the interior being of a more electrically conductive material such as copper or aluminum.

In most respects the anode working face is itself not unknown to those skilled in the art. Basically the anode working face comprises an electrically conductive, electrolyte-resistant material, especially a valve metal such as titanium, tantalum or alloys thereof, bearing on its surface an electrically conductive, electrocatalytically active coating which may consist of a precious metal, a precious metal oxide or other suitable materials. Only those surfaces of the anode working face at which it is desired to generate chlorine need of course be coated. The physical form of the anode working face may vary from a solid sheet to a foraminate sheet such as expanded metal. In those instances where the anode, after expansion, is to be located in close proximity to the cathode and diaphragm, it will generally be desirable to use a foraminous anode working face in order to provide for release of the chlorine generated and for circulation of electrolyte, both within the anolyte compartment and from the anolyte compartment through the diaphragm into the catholyte. Normally chlorine rises vertically up the face of the anode through the substantial electrolyte gap. However, when this gap is eliminated, it will be obvious that, with at least the same amount of chlorine being generated in reduced space, serious problems of gas release and attrition of the diaphragm, as well as the possibility of forcing chlorine through the diaphragm, will exist. Hence a foraminous anode working face is to be preferred. (Note, however, that if the gap is only reduced and not substantially eliminated, as may be the case in other electrolytic applications, an imperforate working face may readily be employed.) In those instances where the anode is actually held in contact with the diaphragm it will be desirable either to coat only those portions of the anode face not touching the diaphragm or to provide a means for inactivating any coating which is in contact with the diaphragm, these precautions serving to prevent chlorine from being forced through the diaphragm by generation in close contact therewith. Generally the size of the anode surface will correspond to that of the opposing cathode.

The most important characteristics of the movable electrically conductive connecting means relate to the design and configuration thereof. However, certain generalities as to materials of construction and the like may be set forth. Electrical conductivity is of course one of the most important requirements for the material used to fabricate the connecting means. The conductivity and the cross-sectional area of the connecting means are dependent upon the amount of current which must be carried from the riser to the working face. A further consideration is that the material must be resistant to corrosive cell conditions. Also, in a number of the designs disclosed hereinafter it is necessary that the material have a certain flexibility and elasticity, or "memory." This is for the reason that in these designs, in order to allow assembly and disassembly of the cell, it is necessary that the connecting means be forced to move in one direction and, upon removal of the force, return to the original position. Finally, consideration must be given to the ease of welding the connecting means to the working face and the riser since in most instances this will be the manner of connection. Considering all of the foregoing, it is generally found that a valve metal in the form of a sheet or bar is most useful. In some instances a core of a more electrically conductive material may be present. Examples of the foregoing are titanium and copper-cored titanium.

With the above as general background, reference will now be made to the drawings for the purpose of illustrating certain preferred embodiments by which the present invention may be carried into effect.

FIG. 1 illustrates a side view of an anode of the present invention, in contracted form. It will be seen that the anode comprises riser 1 and two anode working faces 3 attached thereto by movable, electrically conductive connecting means 5. The distance between each anode working face and its opposing diaphragm-coated 7 cathode 9 is considerable, typically on the order of one-half inch. The anode is maintained in position in base 11, said base being protected from the cell environment by electrically insulating and corrosion resistant layer 13, typically of rubber, which layer also serves, in combination with flange 15 on anode riser 1, to provide a compressible seal which prevents leakage of electrolyte through the hole in the anode base accommodating the riser.

FIG. 2 shows the same cell but in this instance the anode has been caused or allowed to expand by means of flexible, movable connecting and conducting means 5 so that the anode-cathode gap has been substantially completely eliminated.

FIGS. 3 and 4 show an anode of the present invention in its expanded and contracted forms, respectively, and illustrate one means of obtaining such expansion. In FIG. 3, anode working faces 3 are divorced from anode riser 1 but remain in electrical connection with said riser by means of movable connecting means 5. It can be seen that the connecting means, which take the form of a titanium sheet bent into the proper configuration, are provided with slots 19 to receive corrosion-resistant spacing bars 17. In assembling the cell, bar 17 will not be present (FIG. 4) and, owing to the fact that the "memory" of the connecting means is in a direction towards the riser, the anode will be in a collapsed or contracted form. After the cathode can is inserted over the anode base, an alternating array of anodes and cathodes is presented with a considerable anode-cathode gap between each electrode in the array. Spacing bars 17, which are cut on an angle to provide ease of insertion, are then forced into slots 19 thereby causing the anode working faces to move a distance away from anode riser 1 predetermined by the width of spacing bars 17. The bars remain in place during operation and, when it is desired for any reason to disassemble the cell, spacing bars 17 may be removed causing the anode, again because of the "memory" of connecting means 5, to contract and allow easy disassembly.

FIG. 4 is a top view of the anode construction of FIG. 3 with the spacing bars removed, or not yet inserted, illustrating the close proximity of the riser, the connecting means and the working faces. FIG. 4 further illustrates that the riser need not be constructed of one material but may be, for example, a titanium sheath 21 over a copper core 23. A further characterizing feature of the embodiment of FIG. 4, which may be employed with any of the embodiments falling within the scope of the invention, are projecting spacers 25. These projecting spacers are distributed over the anode working face and are constructed of a electrically non-conductive material. The purpose of the spacers is to maintain a uniform anode-cathode gap over the entire interface and to prevent any possibility of a metallic anode-cathode contact with consequent shorting. While the movable connecting means may provide for the expansion of the anode working face uniformly with respect to a vertical center line, irregularities in the working face itself, the diaphragm, or the cathode screen supporting the diaphragm may cause variations in the anode-cathode gap. Through the use of the projecting spacers this non-uniformity is reduced since the pushing of the anode working face against the diaphragm-coated cathode, whether by positive force as shown in FIG. 4 or spring means (memory) as in FIGS. 8 and 9, causes the anode face to straighten with respect to the anode-cathode gap. The relationship between the spacers fixed on the anode working face and the surface which they contact, generally the diaphragm of the diaphragm-coated cathode, is referred to herein as a "pushing proximity."

FIG. 5 illustrates one of the more simple means for providing an expandable electrode. In this embodiment movable connecting means 5, again of titanium, each consist of merely a single sheet shaped so that, when welded to riser 1 and electrode working face 3, the memory of the connecting means is directed toward the riser. In this embodiment each electrode working face 3 carries on its interior, channels 27 adapted to receiver spacer bars 17. The expansion of the electrode is accomplished as in the embodiment of FIGS. 3 and 4.

FIGS. 6 and 7 are expanded and top views, respectively, of an embodiment of the invention wherein positive adjustment of the distance of electrode working face 3 from electrode riser 1 is possible. In FIG. 6 it may be seen that connecting means 5 consist, for each working face 3, of two strips of material connected to the side of the working face facing the riser. In this embodiment connecting means 5 need not be flexible or resilient and are provided with machined slots 33. In FIG. 6 it is shown that for each slot on connecting means 5, there is a corresponding threaded projection 29 on riser 1. FIG. 7 best illustrates the assembly of such an electrode and shows that connecting means 5 fit over projections 29 by way of slots 33 and, when in the desired position, are held in place by means of nuts 31. While this embodiment may be somewhat more expensive to construct than those preceding, it has the advantage that the electrode working face 3 may be readily removed for re-coating or other operations since the connecting means are not fastened to the riser by relatively permanent means such as welding.

FIGS. 8 and 9 represent a further embodiment of an expandable electrode of the present invention in contracted and expanded form, respectively. In this embodiment, the memory of the connecting means is in a direction away from the riser thus requiring some method of holding the electrode in contracted form. Thus, flexible connecting means 5, attached by welding to electrode working faces 3 and electrode riser 1, are bent at points 35, shown best in FIG. 9, to form lips over which clamping bars 37 (FIG. 8) may be positioned to hold the electrode in a contracted position. In this embodiment, in order to allow proper movement of the anode face, each face is present in two sections with a slight gap 39 being left in the center as shown best in FIG. 8. After installation in the cell, bars 37 may be removed causing the electrode to assume an expanded position as in FIG. 9.

It will be understood by those skilled in the art that many other embodiments are possible and will be suggested by the above. For example, where a positive means of moving the electrode faces is desired, a turnbuckle arrangement may be provided wherein opposing screws tend to move the faces with respect to the riser either directly, thereby acting themselves as the movable connecting means and conducting current, or indirectly by exerting force on movable connecting means which in turn move the faces.

Diaphragm-type electrolytic cells equipped with the expandable anodes of the present invention are assembled and used for the electrolysis of sodium chloride solutions with the result that, in addition to the remarkable ease of assembly and ultimate disassembly of the cells so equipped, significant operating advantages are obtained. Particularly it is found that cells equipped with the expandable anodes, after expansion, operate at an advantage of 0.3 volts at 1 a.s.i. as compared to cells equipped with comparable anodes in which the anode working face is attached directly to the riser, therefore leaving the usual anode-cathode gap of about 0.5 inch. This voltage advantage in turn allows the operation of cells so equipped at current densities as much as 1.35 times as great as is possible with "non-expandable" dimensionally stable anodes.

It is further observed that cells employing the expandable anodes of the present invention wherein the anode working face is constructed of an expanded metal, thereby allowing release of chlorine gas through the anode face, not only make operation at a substantially zero anode-cathode gap possible, a very significant advantage in itself because of the voltage advantage so obtained, but also surprisingly result in a sodium hydroxide solution product having a substantially lower chlorate-caustic ratio than with anodes having imperforate working faces attached directly to the riser. One immediate and practical advantage of such a finding is that according to the practice of this invention it is therefore possible to produce directly sodium hydroxide solutions of a more concentrated nature than ordinary, while still maintaining the same level of chlorates. This of course yields substantial savings in treatment and evaporation steps normally employed to concentrate the caustic solution.

While the invention has been described with reference to many specific and preferred embodiments thereof, it should be understood that these references are not intended to be limiting since alterations and modifications may be made which are within the intended scope of the appended claims.

Claims

I claim:

1. An anode assembly which comprises: a common anode riser disposed from a cell base, at least two opposed working faces and movable, electrically conductive means connecting said faces to opposite sides of the common riser, whereby the distance of the faces from the riser may be varied without disrupting the electrical integrity of the anode assembly.

2. An electrode assembly which comprises: a common electrode riser, at least two opposed electrode working faces and movable, electrically conductive means connecting said faces to opposite sides of the common riser, whereby the distance of the faces from the riser may be varied without disrupting the electrical integrity of the electrode assembly.

3. An electrode assembly as in claim 2 wherein non-conductive projecting spacers are carried on the side of the working face not connected to the riser through the connecting means, said spacers serving to maintain a uniform distance between the electrode faces and any surface in pushing proximity thereto.

4. An electrode assembly which comprises a common electrode riser, at least two opposed electrode working faces and movable, electrically conductive means connecting said faces to opposite sides of said riser wherein the movable, electrically conductive means are flexible connecting means having a memory in a direction from the electrode working face toward the electrode riser and wherein the assembly includes slots on the working faces facing the riser and adapted to receive spacing bars which serve to expand the electrode.

5. An electrode assembly which comprises a common electrode riser, at least two opposed electrode working faces and movable, electrically conductive means connecting said faces to opposite sides of said riser wherein the movable electrically conductive means are flexible connecting means having a memory in a direction away from the electrode riser and wherein said means are provided with lips over which clamping bars may be positioned to maintain said electrode in a contracted form.

6. An electrode assembly which comprises an electrode riser, at least one electrode working face and movable electrically conductive means connecting said face to said riser wherein the movable, electrically conductive means are flexible connecting means having a memory in the direction from the electrode working face toward the electrode riser and wherein the assembly includes a slot on the working face facing the riser and adapted to receive a spacing bar which serves to expand the electrode.

7. An electrode assembly which comprises a common electrode riser, at least two opposed electrode working faces and movable, electrically conductive means connecting said faces to opposite sides of said riser wherein the movable, electrically conductive means are flexible connecting means having a memory in a direction away from the electrode riser and wherein removable clamping means are positioned to maintain said electrode assembly in a contracted form.

8. A method for assembling an electrolytic cell comprising a cathode can with cathodes fixed therein, and a cell base supporting anode risers disposed therefrom, each of said risers serving to conduct current to a pair of anode working faces, which method comprises:

A. placing the cathode can over the anodes disposed from the cell base in such a manner than an alternating array of anodes and cathodes is presented, said anodes and cathodes being separated by a discrete and considerable anode-cathode gap;

B. causing said gap to be reduced by expanding the anode, with respect to the anode face-anode face distance;

C. maintaining electrical integrity between each pair of anode faces and its riser through movable electrically conductive connecting means.

9. A method as in claim 8 wherein the anode face-anode face spacing is expanded, and maintained in an expanded state, by inserting at least one spacing bar between the anode faces associated with each riser.

10. A method as in claim 8 wherein the anode face-anode face spacing is expanded, and maintained in an expanded state, by the spring action of the electrically conductive connecting means which have a memory in a direction away from the riser.