United States Patent [191 OLeary et al.
DIMENSIONALLY ADJUSTABLE ANODE-DIMENSIONALLY STABLE DIAPHRAGM COMBINATION FOR ELECTROLYTIC CELLS Inventors: Kevin J. OLeary, Cleveland Heights; Charles P. Tomba; Robert W. Fenn, III, both of Painesville, all of Ohio Diamond Shamrock Corporation, Cleveland, Ohio Filed: Mar. 1, 1974 Appl. No.: 447,215
Assignee:
US. Cl. 204/282; 204/252; 204/266; 204/283; 204/296 Int. Cl. C25B 11/02; C25B 13/02 Field of Search 204/252, 266, 283, 296, 204/282 References Cited UNITED STATES PATENTS 7/1972 Fogelman 204/252 X Dec. 23, 1975 3,694,281 9/1972 Leduc 204/296 X 3,803,016 4/1974 Conner, Jr. 204/252 X 3,853,721 12/1974 Darlington et a1 204/296 X FOREIGN PATENTS OR, APPLICATIONS 700,296 12/1964 Canada... ..204/296 Primary Examiner-John H. Mack Assistant Examiner-W. 1. Solomon Attorney, Agent, or FirmTimothy E. Tinkler [57] ABSTRACT A diaphragm cell for the electrolytic production of chlorine and alkali metal hydroxide, in other respects conventional, is improved by the provision of a dimen- 3 Claims, No Drawings DIMENSIONALLY ADJUSTABLE ANODE-DIMENSIONALLY STABLE DIAPHRAGM A large portion of the worlds production of chlorine and caustic by the'electrolysis of brine takes place in diaphragm type cells. In such cells, the adjacent anodes and cathodes are separated by hydraulically permeable diaphragms, generally of asbestos fibers.
In the majority of instances, these asbestos diaphragms are formed by direct deposition of the fibers from a slurry onto the cathode, generally a woven steel mesh. This diaphragm-coated cathode is employed spaced a fixed'distance from the adjacent anode, the latter now being in most instances a dimensionally stable anode, e.g., titanium sheet or expanded metal bearing an electrically conductive, electrocatalytically active coating, often based on a platinum group metal.
Owing to the commercial significance of such cells, means by which the performance thereof may be improved, and especially means by which at least a por tion of the existing cell hardware may be adapted to improved-operation, are in great demand.
One proposal for such improved operation is the expandable or dimensionally adjustable electrode of US. Pat. No. 3,674,676. This electrode design allows fabrication and installation of an anode into an electrolytic cell in a contracted form, thus easing assembly problems. On assembly, the dimensional adjustability comes into play by allowing movement of the electrolytically active anode toward the diaphragmcoated cathode without interrupting the electrical integrity of the anode assembly. This reduction in the interelectrode gap results in a lower operating cell voltage owing to the reduced path the current must travel .through the resistant brine solution. It has been found, however, that in many applications insertion and employment of such a dimensionally adjustable anode in an otherwise conventional diaphragm cell, while resulting in the desired voltage reduction, also lowers the current efficiency at which the electrolytic process proceeds. Hence, a means to take advantage of the voltage reduction without loss in current efficiency is sought. 3
STATEMENT OF THE INVENTION Therefore, it is an object of the present invention to provide an improved diaphragm cell for the production of chlorine and caustic.
It is a further object of the present invention to provide a modified diaphragm cell employing many components of existing cells, thus reducing capital outlays.
There has now been found an improvement in a cell for the electrolysis of aqueous alkali metal chloride solutions comprising a container divided into anodecontaining and cathode-containing compartments by a hydraulically permeable diaphragm, which improvement comprises, in combination:
a. as said anode, a dimensionally adjustable electrode, and
b. as said diaphragm, a dimensionally stable, thermoplastic polymer-modified, asbestos diaphragm. Such a cell operates at a low voltage and a high current efficiency for extended periods of time and is easy to assemble and service when necessary. The entire cathode assembly and cover of an existing diaphragm cell may be employed, the only cell modifications necessary being those required to accommodate the metal anodes.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Essentially any diaphgram cell for the electrolysis of alkali metal halide solutions, especially sodium and potassium chlorides, may be improved by incorporation of the diaphragm and anode combination of this invention. Thus, new designs, such as bipolar cells, may be improved by rendering the anode, the cathode, or both, dimensionally adjustable, that is, by providing for movement of one or both electrodes from the partition separating adjacent cell units toward the adjacent electrode, the diaphragm being disposed therebetween.
However, the invention will find most utility in the modification of existing monopolar diaphragm cells, such'as those described in US. Pat. No. 2,987,463. Here the cathode assembly or can and the cell cover remain unchanged. An anode base capable of accommodating and disposing metal anodes within the cell, such as the metal base that is the subject of US. Pat.
No. 3,591,483, will be required to replace the older copper/lead/concrete/asphalt combination bases used with graphite.
The term dimensionally adjustable anode or electrode refers to those electrodes wherein it is possible on assembly of the cell to move the active anode surface closer to the diaphragm-coated cathode on assembly. Generally, such electrodes will comprise an active anode sheet or face connected to a riser or other primary electrical conductor by a means capable of causing the desired movement while maintaining electrical integrity. The anode face is a metal sheet, continuous or foraminous, generally at least partially coated with an electrically conductive, electrocatalytically active material. The variety of such anodes known to the art may be employed. Preferably, the anode face will be expanded titanium metal, to allow for gas release and electrolyte circulation, coated with a platinum group metal oxide or related material. The riser or other conductor serves to convey current from the cell base or a source external the cell to the operating anode face. Conveniently, this may take the form of a metal pillar, such as copper-cored titanium, extending through the cell base. See, for example, US. Pat. No; 3,591,483, incorporated herein by reference. The anode face is then connected to this conductor by a moveable, conductive, connecting means such as a flexible titanium sheet. Dimensionally adjustable electrodes of a type useful as the anodes of this invention are described in more detail in US. Pat. No. 3,674,676, the teachings of which as to electrode configuration, alternatives, and advantages are incorporated herein by reference. Other designs may suggest themselves to one skilled in the art, it only being essential that the requisite capacity to move the active anode sheet toward the opposed diaphragm-coated cathode on cell assembly be present.
Combined with the dimensionally adjustable anode is a dimensionally stable, polymer-modified, asbestos diaphragm. Dimensional stability with respect to the diaphragm refers to resistance to swelling in the aqueous electrolyte as well as resistance to chemical and mechanical attrition. These desirable properties are achieved by treating conventional (usually chrysotile) asbestos fibers with various chemically and mechanically resistant thermoplastic polymers. Included, and particularly to be preferred, are the various polyfluorocarbons such as poly(vinyl fluoride), poly(vinylidene fluoride), polytetrafluoroethylene, polyperfluoroethylene propylene, and the like. Also useful are certain of the chlorinated resins such as poly(vinylidene chloride) and the chloro-fluoro materials such as polychlorotrifluoroethylene and polychlorotrifluoroethylene polyethylene copolymers. In addition, it is possible to impart favorable properties to the diaphragm if the polymer employed also has cation exchange properties. Diaphragms of this type are described in Dutch Published Application 72/ 12225 and exemplified by the use of poly(perfluoroethylene-trifluoroethylene sulfonic acid) with asbestos.
These polymers may be incorporated into the asbestos diaphragm by preforming the asbestos fibers as a diaphragm or sheet and subsequently soaking same in a solution of the appropriate monomers, followed by in situ polymerization. In this fashion, however, a continuous polymer coating on the asbestos fibers is obtained and the advantageous ion exchange properties of the asbestos are thereby lost. Generally, the polymers will be incorporated in the asbestos as such, by either dissolving or suspending same in an appropriate media followed by soaking the preformed asbestos sheet or diaphragm therein or drawing the polymer into the asbestos, for example, by vacuum. Subsequently, the treated asbestos structure is dried and cured, i.e., raised to the temperature at which the thermoplastic polymer melts, thereby locking together adjacent asbestos fibers on cooling and imparting the desired dimensional stability.
A preferred method of obtaining the desired dimensionally'stable diaphragm is by deposition from a slurry of asbestos fibers and thermoplastic polymer particles in the form of polymer granules or fibers. This uniform slurry is then drawn onto the cathode screen or other sheet-forming support, followed by drying and curing at the appropriate elevated temperature to obtain a discontinuous but coherent polymer coat. This method and the results obtained thereby are described more fully in copending US. Ser. No. 324,508, filed Jan. 17, 1973.
In each of the foregoing instances the object is to obtain a dimensionally stable diaphragm comprising an asbestos fiber lattice stabilized with a thermoplastic polymer that has been subjected to fusion to bind adjacent asbestos fibers together and, preferably, provide a discontinuous polymer coating on said fibers.
Preferably, in order to avoid leakage and obtain the maximum advantage, the dimensionally stable diaphragm will have been applied directly onto the cathode surface, usually a wire screen. If this is the case, then the diaphragm-coated cathode assembly may be positioned over the array of dimensionally adjustable anodes to provide the desired alternating anode-cathode arrangement, following which the anodes may be allowed or caused to expand, thus reducing the interelectrode gap. The cell may then be operated conventionally but with the above-described advantages.
We claim:
1. An improvement in a cell for the electrolysis of aqueous alkali metal chloride solutions comprising a container divided into anode-containing and cathodecontaining compartments by a hydraulically permeable diaphragm, which improvement comprises, in combination:
a. as said anode, a dimensionally adjustable electrode comprising an anode-working face, an anode riser, and a moveable means for connecting the anode working face and anoderiser with electrical integrity, and b. as said diaphragm, a dimensionally stable, heat-fused, polyfluorocarbon-asbesto's diaphragm, said diaphragm having a discontinuous polyfluorocarbon coating binding the asbestos together.
2. A cell as in claim 1 wherein said polyfluorocarbon is polytetrafluoroethylene.
3. An improvement in a cell for the electrolysis of aqueous alkali metal chloride solutions comprising a container divided into anode-containing and cathodecontaining compartments by a hydraulically permeable diaphragm, which improvement comprises, in combination:
a. as said anode, a dimensionally adjustable electrode comprising an anode-working face, an anode riser, and a moveable means for connecting the anode working face and anode riser with electrical integrity, and
b. as said diaphragm, a dimensionally stable, heatfused, polymer-asbestos diaphragm, said polymer being selected from the group consisting of poly(- vinylfluoride), poly(vinylidene fluoride), polytetrafluoroethylene, polyperfluoroethylene propylene, and polychlorotrifluoroethylene-polyethylene, said diaphragm having a discontinuous polymer coating binding the asbestos together.