Patent Publication Number: US-3878082-A

Title: Diaphragm cell including means for retaining a preformed sheet diaphragm against the cathode

Description:
[451 Apr. 15, 1975 United States Patent [191 Gokhale DIAPHRAGM CELL INCLUDING MEANS FOR RETAINING A PREFORMED SHEET Primary EXaminerJ0hn H. Mack DIAPHRAGM AGAINST T CATHODE Assistant Examiner-W. l. Solomon Attorney, Agent, or Firm-Bernhard R. Swick; Joseph [75] Inventor. fiiitizlllrshan D. Gokhale, Trenton, D Michaels; Robert E Dunn [73] Assignee: BASF Wyandotte Corporation,  
 Wyandotte, Mich.  
 Feb. 19, 1974 ABSTRACT Filed: The conventional diaphragm electrolytic cell for the [21] A L N 443,614 production of chlorine from an aqueous alkali metal chloride solution as exemplified by the well-known Hooker series of cells and/or Diamond series of cells,  
 is modified to use a preformed sheet material as the diaphragm by the use of elasto-polymeric retainers. Crescent-shaped, expansible elasto-polymeric retain- 6 5 glad 4 M m 21 0 &amp; 5 n 2 /4 I 4 O n 2 no m 2 u 2 u 4 m 0 2 m C G S t U i .4 U 5 :58] Field of Search 204/252, 263, 266, 256,  
 ers are used in combination with U-shaped compressible elasto-polymeric retainers to obtain the desired result.  
 :56] References Cited UNITED STATES PATENTS 10 Claims, 14 Drawing Figures RETHHEEAPR 1 5191s 3 878, 082  
 sum 1 95 3 FIG. I H62 snimaufg P; TE&#39;NTEBAPR 1 51975 DIAPHRAGM CELL INCLUDING MEANS FOR RETAINING A PREFORMED SHEET DIAPHRAGM AGAINST THE CATHODE BACKGROUND 1. Field of the Invention This invention relates to an improvement in the conventional diaphragm electrolytic cell for the production of chlorine from an aqueous alkali metal chloride solution.  
 2. Description of the Prior Art The production of chlorine from an aqueous alkali metal chloride solution by the use of a diaphragm electrolytic cell is well known. This type of cell is described in some detail in the well-known textbook Chlorine, [is Manufacture, Properties and Uses, J. S. Sconce, Editor. American Chemical Society. Monograph No. 154, Reinhold Publishing Corporation, New York, N.Y., (1962). beginning at page 90. Among the cells described therein is the Hooker cell which has a finger type of cathode construction and the Diamond cell which has a flattened tube type of cathode construction. Both of these cells and others similarly described diaphragm cells employed as the diaphragm an asbestos diaphragm made in situ from a waterbased slurry. However, the asbestos slurry type diaphragm has an important advantage in these cells inasmuch as the diaphragm conforms to the convoluted contours of the cathode and presents no attachment problems. It has been suspected for quite some time that the asbestos utilized in the diaphragm is injurious to health and, therefore, desirable to find a more suitable diaphragm material to avoid the use of injurious materials of construction in the building and operation of electrolytic cells. However, as Sconce points out in his text at page 90. the use of asbestos paper wrapped over the finger type cathode and sealed at the top and bottom with cement and putty provided a poor seal and loss of current efficiency, The use of other materials at the diaphragm in electrolytic apparatus appears to be limited to situations where the anode/cathode configuration is such that the diaphragm material can be maintained as a flat sheet simply by stretching between the opposing anode and cathode. An early patent, U.S. Pat. No. 1,464,689. discloses simply stretching a diaphragm material between a pair of cooperating jaws when the anode is flat. A later U.S. patent, U.S. Pat. No. 3,335,079, discloses that in an electrodialysis apparatus having a flat vertical anode and a flat vertical cathode at opposite ends of the unit the diaphragm or membrane between the two electrodes can be installed with one free end, if desired.  
 SUMMARY OF THE INVENTION In accordance with this invention there is provided in an electrolytic cell of the diaphragm type for the production of chlorine from an aqueous alkali metal chloride solution wherein the conventional asbestos-type diaphragm has been replaced by a preformed sheet diaphragm, the improvement comprising the use in combination of:  
 1. an expansible elasto-polymeric crescent-shaped retainer for retaining said diaphragm in place at the junction of the vertical cathode wall and the vertical inner cathode ring wall on each side of said cathode, and  
 2. a compressible elasto-polymeric U-shaped retainer for retaining said diaphragm in place at least at the junction of the curved top or bottom portion of the cathode and the inner cathode ring wall, one said U-shaped retainer being attached to the top of said cathode and another said U-shaped retainer being attached to the bottom of said cathode. said crescent-shaped retainers and said U-shaped retainers in cooperation both maintain said preformed sheet diaphragm in place and prevents a flow of electrolyte around the end of said diaphragm from one electrode chamber to the other electrode chamber of said cell.  
 DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of the cathode in a diaphragm electrolytic cell having the finger-type construction.  
  FIG. 2 is a top view of the cathode in a diaphragm electrolytic cell having the flattened tube type of construction.  
  FIG. 3 is the side view of cathode in a diaphragm electrolytic cell-type cell having the finger-type construction taken along line 3-3 of FIG. 1 and FIG. 3a is the side view of same taken along line 3a-3a of FIG. 1 while FIG. 3b is an end view of the same cell taken along line 3b3b of FIG. 1.  
  FIG. 4 is the top view of cathode having a finger-type construction taken along line 44 of FIG. 30, wherein the cathode has been extended to full length and diaphragm with appropriate retainer added.  
  FIG. 5 is a cross-section of expansible elastopolymeric crescent-shaped retainer used between adjacent cathodes before installation.  
  FIG. 6a and FIG. 6b are cross-sections of modifications of the retainer shown in FIG. 5.  
  FIG. 7a is a cross-section of a compressible elastopolymeric U-shaped retainer used on the curved top and bottom sections of the cathode. Modifications of the U-shaped retainer are shown in FIG. 7b. FIG. and FIG. 7d.  
  FIG. 8 is a partial three-quarter view of the cathode shown in FIG. 3a showing the use of the retainers of FIG. 5 in conjunction with the retainers of FIG. 7.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT In FIGS. 1 and 2 are shown the top plan for two very common types of convoluted cathodes utilized in electrolytic cells. In both FIGS. 1 and 2 the outside of the cathode ring can 10 is typically a solid iron metal or ferrous material so that there is no loss of the catholyte within the cell. Provision is made for the withdrawal of the catholyte and gaseous products by appropriate exits not shown on the drawing as they are immaterial to the understanding of the present invention. The inner wall of the cathode ring 12 and cathode 14 are made out of a ferrous metal composition as a formaminous sheet which can either be a woven wire-mesh or a punched metal material. However, because of cost considerations and convenience of fabrication, it has been found that the woven wire approach is the preferred method. The same ferrous metal is typically used in the preparation of the inner cathode wall retaining ring 12 and mesh cathode 14. As shown in FIG. 3, 3a and 3b, the catholyte chamber is principally defined by the space between the cathode ring can 10, the inside cathode ring 12 and the interior of the cathode 14. The completion of that foregoing definitionis made by the bending of the inside cathode ring 12 along its top and uottom circumferential edges and out of contact with he cathodes 14 so that the extreme edge of the inside :athode ring 12 becomes a flared flange 28 in contact vith the cathode ring can forming the top and botom of the catholyte chamber. The flared flange 28 is ittached to the cathode ring can 10 by suitable me- :hanical joint. e.g.. a crimpling or clamping action or velding and the like. It will be readily appreciated that his joint is beyond the scope of the present invention. he catholyte chamber is continuous between the cathde ring can 10 and inside cathode ring 12.  
  In FIG. 3a the cathodes 14 are shown in crosssection lhile the iron mesh inside cathode ring 12 being 10- ated at a distance from the line 311-311 of FIG. 1 is hown as a side view. At this point it should be noted hat the cathode 14 of FIG. 1 and FIG. 2 present subtantially the same cross-section and appearance. Genrally, the present practice is to make the entire inside athode ring wall 12 completely of open mesh work or ther formaminous material and, therefore, the inside /all above the cathode 14 as well as below the cathode 4 is capable of accepting transfer of ions and electroyte in the same manneras cathode 14. Thus, the presnt practice using an asbestos slurry to prepare an asestos diaphragm requires that the entire cathode ring e immersed into the slurry so that the entire opening my be covered. However, the over-all efficiency of the lectrolytic cell is only very, very slightly changed if the pening on the inside cathode ring 12, i.e., those not on me cathode 14 per se, are closed.  
  On the outside or anode-facing side of cathode 14 is laced the continuous preformed diaphragm 16. In the ase of the finger-type construction as shown in FIG. 1, 1e preformed diaphragm 16 can take on the configuraon of an envelope or bag and slipped over the individal cathode 14 in much the same manner as one would ag a book or similar type object as shown in FIG. 4. l the case of the tube-type cathode 14 of FIG. 2, then re preformed diaphragm material 16 is wrapped round the outside of the cathode l4 and overlapped ightly so as to insure that there is a complete coverage f the cathode. However, in either instance, the diahragm 16 is not self-adhering and will not maintain it- :If on cathode 14 without any further attention. Since )r various reasons the diaphragm 16 does eventually ecome inoperative, provision must also be made for ie removal and replacement of the diaphragm 16 &#39;ithout the destruction of cathode 14 or inner cathode ng 12. Thus, it is necessary to find a means of mainlining the diaphragm 16 in place, blocking the passage fthe electrolyte from one chamber to another without 1e intervention of the diaphragm l6 and yet provide simple and easy means of changing the diaphragm 16 t such time as the diaphragm 16 change is required.  
 This invention provides a series of elastomeric (or .asto-polymeric) retainers 18, 22 shown in FIGS. 5, Ga 1d 6b and 7a, 7b, 7c and 7d which when utilized in )mbination effectively maintain the diaphragm 16 in lace. The use of one type of retainer 18 is shown in IG. 4 and the combination use of retainers 18, 22 is iown in FIG. 8.  
 The expansible elasto-polymeric crescent-shaped re- .iner 18 of FIG. 5 is constructed of chemically inert 1C1 electrically non-conducting thermoplastic material lch as polytetrafluoroethylene, polyvinylidene fluode polymers and polyolefins derived from an olefin )ntaining 2 to 4 carbon atoms including polyethylene,  
 polypropylene. polybutene-l and mixtures thereof. The polytetrafluoroethylene polymer is a preferred polymer. Furthermore, the polymeric material may be a mixture of several different molecular weights of the same type of polymer. It is well known by formulators and compounders skilled in the polymeric arts how to add inert, non-conducting yet reinforcing fillers and other modifiers to enhance the performance of a polymer for a particular application. Since this is really beyond the scope of this invention and for the sake of brevity, further discussions of these matters will not be made at this time.  
  The crescent-shaped retainer 18 is so molded that retainer 18 after fabrication is completed has an elastic quality. When the crescent free ends 19 of the retainer 18 are brought together and then released of the constricting force the ends 19 will again expand or return onto their original position until and unless an intervening force stops the further movement back to the original place. It is important to note that the retainer 18 has a gradually tapering end 19 as is typical of concavoconvex objects such as the moon is in the first or fourth quarter. The tapering permits a gradual release of pressure on diaphram 16 so that at the end of terminal point of crescent retainer 18 there is no sudden change or stress on the diaphragm l6 and as a result no stress cracks are induced into the diaphragm 16.  
  It is an optional but useful modification of retainer 18 as shown in FIG. 6 to add a retaining lug or strip 20 on the convex side of retainer 18 and near the crescent tapered terminal end 19 of retainer 18 so that additional gripping force may be obtained to maintain the diaphragm 16 in place. Advantageously, the cathode 14 will be provided with a receiving groove (not shown on the drawing) corresponding to retaining lug 20 so that the retaining lug 20 can depress the diaphragm 16 into the groove and further apply advantageous mechanical force in maintaining the diaphragm 16 in its proper location.  
  The retainer 18 can be as shown in FIG. 8 additionally modified to have at the longitudinal terminus of the retainer 18 a flap 21 molded onto and as a part of the retainer 18 so that the flap 21 will cover and seal the formaminous nature of the iron mesh inside cathode ring 12 above the cathode area.  
  Finally, as shown in FIG. 8, the retainer 18 can be modified to have a biased cut at the longitudinal ends of the retainer 18 so that retainer 18 when in contact- 22 will have a mitered look or appearance as shown in&#39; FIG. 8. The mitered junction of retainer 18 and retainer 22 will insure a better seal and clamping action on diaphragm 16.  
  The compressible elasto-polymeric U-shaped retainer 22 of FIG. 7a is made from the same class of material as is retainer 18. However, in this case retainer 22 is so molded that when in use the free ends 23 of the U-shape are spread apart in order to slip over cathode l4 and diaphragm l6 and, thereafter, seek to return to its original position thereby exerting pressure on diaphragm 16 and retaining the diaphragm 16 in place. It will be noted that the free ends 23 of the U-shaped retainer 22 are also tapered for the same reasons that the free ends 19 of retainer 18 are tapered.  
  As shown in FIG. 70, the U-shaped retainer 22 can be modified by the addition of retaining lugs 20. However, in this case the retaining lugs 20 are placed on the inside, that is, the concave side or the retainer 22. As pointed out in the discussion of the crescent-shaped retainer 18, the addition of the lugs helps to retain the diaphragm 16 in place. Again. the holding is that with the lugs 20 can be further enhanced by the provision of a corresponding groove in the cathode 14.  
  The longitudinal end of U-shaped retainer 22 which will be adjacent the iron mesh inside cathode ring 12 is also mitered so as to complete the mitered fit and junction with crescent-shaped retainer 18 referred to in previous paragraph and illustrated in FIG. 8. Additionally. the mitered end of the U-shaped retainer 22 can have molded there unto a flap to further cover the iron mesh inside cathode ring 12 when it is a formaminous member above the cathode 14.  
  FIG. 7b shows in cross-section other modification of U-shaped retainer 22 which can be advantageous. This is the insertion within and running along the longitudi nal length of U-shaped retainer 22 a compressible metal spring 24 which is completely enclosed within the U-shaped retainer 22. In a similar manner crescentshaped retainer 18 of FIG. 5 can be modified.  
  In the event that the cathode 14 is of the finger-type construction as illustrated in FIG. 1, then U-shaped retainer 22 need be only a longitudinally short piece or short distance of retainer inasmuch as the diaphragm 16 is a continuous diaphragm having no joint at the top or bottom of cathode l4 and needs to only be secured at the junction of the cathode l4 and inside cathode ring 12. To obtain the tightest fit the longitudinal end of retainer 22 in contact with retainer 18 should have a mitered cut for close fit. The opposite end of retainer 18 can be of any desired configuration. However, when the cathode 14 construction is of the flattened tube type as illustrated in FIG. 2, the diaphragm 16 must be of a sheet nature and wrapped around and overlapped on cathode 14. In this case then the longitudinal length of U-shaped retainer 22 must be such that the retainer 22 extends the entire length of the cathode l4 and, consequently, must have both ends with a mitered effect as discussed above to obtain the most effective seal.  
  Irrespective of which type of cathode 14 is employed or which type of retainer 18, 22 is employed the free end 19, 23 of the retainer 18, 22 is tapered and the amount of taper is controlled by the amount of diaphragm l6 retention power desired, crushability of the diaphragm l6 and the economic compelling reason to avoid covering any more of the cathode 14 surface than is necessary for the satisfactory clamping of diaphragm 16 to cathode 14.  
  In some instances the diaphragm employed in the cell will be particularly delicate with respect to the holding or gripping force of the retainers 18, 22. In such circumstance a compressible cushion or padding 26 can be employed to absorb part of the holding force of the retainer 18, 22. The compressible cushion or padding 26 can be attached to the taper portion of the retainer 18, 22 which is in contact with the diaphragm between the free end 19, 23 and the retaining lug 20, or where the retaining lug 20 would be if present, as is illustrated in FIG. 6b (crescent-shaped retainer 18) and FIG. 7d (U-shaped retainer 22). It will be readily apparent that the padding 26 will be continuous along the length of the retainer 18, 22. The padding 26 is also made from the same class of materials as previously described for the retainer 18, 22 but preferably will be in a sponge or foam form. The padding 26 can be stuck or attached by adhesive means, thus it can be applied and used as a tape. The tape form padding 26 can be varied in width as required with widths of from about one-fourth to one and one-half inches being suitable with thickness from about one-sixteenth to about three-eighths or more being suitable.  
  In the operation of the electrolytic cell the diaphragm 16 will eventually become inoperative due to a variety of conditions associated with the cell and must then be replaced. However, the replacement of the diaphragm 16 in no way requires that new crescent-shaped retainers l8 and U-shaped retainers 22 be installed, the previously used retainers 18, 22 being satisfactory for reuse with the new diaphragm 16 provided that the elasticity of the retainers 18, 22 appears to be substantially the same, that is, they appear to have the requisite amount of retaining or gripping properties.  
  It has previously been noted that in adapting the existing finger-type construction of cathode 14 as shown in FIG. 1 and the flattened tube-type cathode construction of FIG. 2 with a continuous sheet of polymeric diaphragm 16 that the formaminous nature of the iron mesh inside cathode ring 12 presented certain problems with control of flow of electrolyte and/or gases and, therefore, flaps 21, 25 on the crescent-shaped retainer l8 and U-shaped retainer 22 are envisioned. However, this problem can also be rectified by sealing the formaminous area of inside cathode ring 12 with a variety of permanent sealing materials such as melting into it a polymeric material such as polytetrafluoroethylene so that part of the polymer is forced through the formaminous openings while the rest of the polymer sheet remains as a continuous sheet on the inside of the cell adjacent to the cathode l4 and exposed to the electrolytes employed. It is, of course, recognized that in subsequent manufacturing of replacement cathode 14 that the formaminous area above and below the cathodes 14 on the inside cathode ring 12 can be omitted.  
  The foregoing methods and apparatus have been described in the foregoing specification for the purpose of illustration and not limitation. Many other modifications and ramifications will naturally suggest themselves to those skilled in the art based on this disclosure. These are intended to be comprehended as within the scope of the invention.  
  The embodiment of the invention in which an exclusive property or priviledge is claimed are defined as follows:  
  1. In a diaphragm-type electrolytic cell for the production of chlorine from an aqueous alkali metal chloride solution and having therein cathodes wherein the conventional asbestos-type diaphragm has been replaced by a preformed sheet diaphragm the improvement comprising in combination:  
 1. an expansible elasto-polymeric crescent-shaped retainer for retaining said diaphragm in place at the junction of the vertical cathode wall and the vertical inner cathode ring wall on each side of the cathode, and  
 2. compressible elasto-polymeric U-shaped retainers for retaining said diaphragm in place at least at the junction of the curved top and bottom portions of the cathode and the inner cathode ring wall, one said U-shaped retainer being attached to the top of said cathode and another said U-shaped retainer being attached to the bottom of said cathode, said crescent-shaped retainers and said U-shaped retainers in cooperation maintaining said preformed sheet diaphragm in place and preventing the flow of electrolyte around the end of said diaphragm from one electrolyte chamber to the other electrolyte chamber in said cell. 2. The improvement of claim 1 wherein the retainers ave on each side thereof adjacent to said diaphragm locking lug to further engage said cathode and said iaphragm.  
  3. The improvement of claim 1 wherein said retainers -e made of polytetrafluoroethylene polymer. 4. The improvement of claim 1 wherein said retainers ave a flange to cover the formaminous inner cathode ng wall adjacent to the top and bottom of the cathode. 5. The improvement of claim 1 wherein each said U- iaped retainer internally contains therein a metal )ring to enhance the holding action of the free end of said retainer.  
  6. The improvement of claim 1 wherein said U- shaped retainers extend the entire length of said cathode.  
 7. The improvement of claim 1 wherein said U- shaped retainers are adjacent to only the junction of each retainer to protect said diaphragm.