Method and Apparatus for Installing a Membrane to an Electrolytic Cell

The present invention relates to a method and an apparatus of installation of membranes to an electrolytic cell, more particularly, to a finger type electrolytic cell of an alkali metal chloride solution.

Sodium hydroxide has been commercially produced using asbestos or modified asbestos diaphragm cells. However, sodium hydroxide manufactured by the asbestos or modified asbestos method is poor in quality and about 0.9 to about 1.2% by weight of sodium chloride is usually contained in a 50% sodium hydroxide liquor. Sodium chloride contained in sodium hydroxide liquor produced by the asbestos or modified asbestos cell may be removed by an ammonia extraction method, hydrated sodium hydroxide method or the like, but when each of these methods is put into practice on an industrial scale, sodium hydroxide liquor is purified, at best, only to an extent ranging from about 500 to 1,000 ppm, and still worse, a relatively large expenditure is required for purification. Sodium hydroxide used for the rayon industry can permit the presence of only 200 ppm or less of sodium chloride in a 50% sodium hydroxide liquor. Accordingly, it is rather difficult to produce sodium hydroxide usable for the rayon industry at a reasonable and moderate cost by the purification of sodium hydroxide produced by asbestos or modified asbestos diaphragm methods.

When the asbestos or modified asbestos diaphragm cells are converted to the ion exchange membrane cells according to the present invention, not only is the quality of the product improved, but also the operation of an electrolytic plant becomes feasible. That is, due to the fact that there is no falling out of salts in the evaporation system, washing of the slurry lines and the vessels and the like is not required, and the operation may be carried out automatically. Another advantage obtained by the conversion of the asbestos or modified asbestos diaphragm method to the ion exchange membrane method is that a cell liquor hardly containing NaCl is obtained. Further advantage is that sodium hydroxide which is consumed in own factories or within the Kombinat may be supplied directly without being concentrated by evaporation to from 40 to 50%, while asbestos or modified asbestos diaphragm cell liquor containing a large amount of sodium chloride must be concentrated to from 40 to 50%, even though self-consumed in the factories or consumed within the Kombinat, and a low concentrated sodium hydroxide provides satisfaction. By converting to the ion exchange membrane method, sodium hydroxide substantially containing no sodium chloride is obtained, and thus may be supplied for a variety of uses immediately by being cooled to a desired temperature, or may be mixed with a 50% concentrated sodium hydroxide to a desired concentration and then supplied for use.

However, it is very difficult to install an ion exchange membrane to a finger type electrolytic cell comprising winding portions. The installation in a plain way of an ion exchange membrane onto the curved surfaces of a cathode does not fail to provide increased concentration of an alkali metal chloride contained in an alkali metal hydroxide liquor prepared.

On the other hand, hydrodynamically permeable perforated membranes are known as microporous membranes. It is also difficult to locate these membranes to a finger type electrolytic cell which is typical and common as an asbestos diaphragm cell, so that these are not yet in use on an industrial scale.

In order to solve the problems as aforesaid, the present inventors have studied a method and an apparatus which enable simple and easy installation of a substantially flat membrane to a finger type electrolytic cell.

A first object of the present invention is to provide a method and an apparatus for installing a cation exchange membrane and/or a microporous membrane to a finger type electrolytic cell very feasibly.

A second object of the present invention is to provide a method and an apparatus for converting an asbestos or modified asbestos diaphragm cell to a cation exchange membrane cell, producing an alkali metal hydroxide liquor containing little amount of an alkali metal chloride or the like.

A third object of the present inention is to provide a method and an apparatus for producing an alkali metal hydroxide of high quality hardly containing an alkali metal chloride or the like by the use of a cation exchange membrane installation frame.

A fourth object of the present invention is to provide a method and an apparatus which are free from a danger to health or environmental contamination due to asbestos by the conversion of an asbestos or modified asbestos diaphragm cell to an ion exchange membrane cell.

A fifth object of the present invention is to provide a method and an apparatus which enable the production of an alkali metal hydroxide in an economically advantageous way.

These and other objects can be accomplished by the steps of covering the horizontal surfaces of a cathode not opposing an anode with a membrane installation frame having a collar, positioning the membrane substantially parallel to the vertical surfaces of the cathode, and installing and clipping the membrane to the collar, whereby installation of the membrane is facilitated and electrolysis can be carried out at a low cell voltage while reducing the concentration of an alkali metal chloride contained in an alkali metal hydroxide liquor.

As finger type electrolytic cell useful in the present invention there are included not only a finger type construction cell such as that described at page 93, Chlorine--Its Manufacture, Properties and Uses, edited by J. S. Scone, issued Reinhold Publishing Corporation, New York, 1962, incorporated herein by reference, but also a flattened tube type construction. Nowadays, a flattened tube type construction cell is also generally referred to as a finger type electrolytic cell. As alkali metals, there are included sodium, potassium and the like.

An anode in a finger type electrolytic cell is commonly located and embedding to an electro-conductive bottom plate or side plate. A membrane installation frame is positioned to cover the horizontal surfaces of a cathode not opposing the anode. When the membrane installation frame is electrically conductive, it is usually insulated from the cathode by insertion of a packing, lining and any other suitable means.

It is a feature of the present invention that installation of the substantially flat membrane to the curved surfaces of the cathode could be facilitated by providing the collar with the installation frame. The collar is preferably located at about 90 degrees or less, more preferably from about 90 degrees to about 30 degrees to the flat portion of the membrane installation frame. The most preferably is about 90 degrees since neither expansion nor contraction of the membrane is required. It is desired to locate, at least, the collar close to the bottom plate embedding the anode at about 60 degrees to about 30 degrees, thereby facilitating assembly of the anode and the cathode to which the membrane was positioned. In the case of less than 90 degrees, the membrane has to be flared to conform with the collar angle. The smaller the angle, the larger flare of the membrane. While the flared membrane is made from a polymer by heat-moulding without so much difficulty, it is rather difficult to process the flat membrane to be the flared membrane. Accordingly, the collar angle is decided upon according to difficulty or easiness of processing of the membrane and the operation efficiency. Therefore, the lower limit of 30 degrees is a matter of practice, not theory. Inversely, in the case of more than 90 degrees, the membrane must be installed to the collar by being contracted or folded. Care must be taken that the operation efficiency of assemblage of the anode and the cathode may be disturbed, when the collar angle close to the anode enbedded bottom plate is more than 90 degrees.

The membrane is installed and secured to the collar of the membrane installation frame by clipping. Securing by clipping may be executed by the use of clips; press plates and clips; packing and clips; press plates, packings and clips; clips incorporated to the press plates; packings and clips incorporated to the press plates; and the like. Combined use of bolts and clips results in an effective securing. For example, to the curved portion to which the membrane is rather difficult to be secured by clips alone or clips and the press plates, is the securing executed by bolts alone or together with the press plates, if desired, and to the straight portion the securing is achieved by clips singly or in combination of the press plates.

The membrane is secured to the collar even without using the press plate, but the press plate enables easier and more complete securing. In cases where no press plate is employed, it is necessary to provide the secured portion of membrane with hardness by thickening that portion beforehand upon the production of membrane or by folding the membrane twofold or threefold and heat-fusing. More effective securing is also obtained by applying clips at shorter intervals than in the case where the press plate is used. When a microporous membrane is employed, insufficient securing is not necessarily problematical as compared with ion exchange membrane, since it is inherently hydrodynamically permeable.

The membrane installation frame, the press plate and the like have to be made from materials which are resistant to anolyte or the like. As these materials, titanium, zirconium, hafnium, tantalum or an alloy thereof such as titanium-palladium, titanium-tantalum, hastelloy or the like. As the organic materials other than the metallic materials as aforesaid, FRP, heat-resistant polyvinyl chloride, polypropylene, fluorocarbon polymer, fluorocarbon polymer or rubber lined metals or the like. Examples of fluorocarbon polymer are polymers of tetrafluoroethylene, hexafluoropropylene, perfluoroalkylvinylether, copolymers thereof, polytrifluoro-chloroethylene, polyfluorovinylidene and the like.

When the membrane installation frame is made from an electro-conductive material, a packing as an insulator is interposed between the frame and the cathode, otherwise, lining or coating is applied to the surface of the cathode or the surface of the frame facing the cathode. When the membrane installation frame made from plastics is employed, the press plate had better be used to the side of the collar as well. As the materials for clips, press plates or clips-incorporated press plates, metallic materials such as titanium, zirconium, hafnium, tantalum or an alloy thereof, for example, titanium-palladium alloy, titanium-tantalum alloy and the like are preferable. Clips, press plates or clips-incorporated press plates made from organic materials such as polytetrafluoroethylene, polyhexafluoropropylene or the like may also be employed.

In an attempt to accomplish more perfect securing and sealing, it is preferred to interpose a packing between the collar and the membrane, further between the membrane and the press plate, if desired. As a packing, materials resistant to anolyte such as polytetrafluoroethylene are preferable. As polytetrafluoroethylene, "Teflon (Registered Trademark of E. I. Du Pont de Nemours and Company)" thin film or the like may be suitable used which are known as a porous film or tape seal. Thin film of fluorocarbon polymer other than polytetrafluoroethylene such as tetrafluoroethylene-hexafluoropropylene copolymer may also be used. Sealants such as "Teflon" paste, silicone grease and the like may be used with a packing to provide more perfect securing and sealing.

The membrane formed in a cylindrical shape is preferably used. The cylindrical membrane is obtained by extrusion moulding, heat-sealing, sticking by adhesives and the like.

When the cation exchange membrane is employed, cell liquor concentration (sodium hydroxide concentration) can be enhanced over 30 to 40%. As carbon exchange membranes, polyfluorocarbon membrane with sulfonic acid group, sulfon amide group, carboxylic acid group, for example, "Nafion #110," "#215," "#315," "#425" and the like which are produced and sold by E. I. Du Pont de Nemours and Company may be exemplified.

In cases where the present invention is applied to a microporous membrane, there are advantages that back flow of OH.sup.- is reduced due to the amount of flux per a unit area of membrane being increased and the sodium hydroxide concentration in a cell liquor can be raised at the same oxygen concentration in chlorine. The operation costs can be therefore reduced on account of the decrease in cell voltage and the increase in the cell liquor concentration. Although it is difficult to apply an ion exchange membrane method to an electrolysis plant where the subterranean brine is used, if a microporous membrane method is carried out according to the method and apparatus of the present invention, operation costs can be reduced. As microporous membranes, "Nafion #701", "#710" or the like are enumerated which are manufactured and sold by E. I. Du Pont de Nemours and Company.

FIG. 1 or FIG. 5 is a perspective vertical sectional view of a finger type electrolytic cell wherein the membrane installation frame is located so as to cover the horizontal surfaces of the cathode not opposing the anode, to which the membrane is installed.

FIG. 2 or FIG. 6 illustrates an enlarged partially sectional view of FIG. 1 or FIG. 5, respectively.

FIG. 3 or FIG. 7 is a sectional view of the important part illustrating the portion of membrane installation to the installation frame.

FIG. 4 is a perspective view of a clip and FIG. 8 is a perspective view of a clips-incorporated plate.

FIG. 9 is a perspective vertical sectional view of a finger type electrolytic cell wherein the membrane is installed to the membrane installation frame having the one collar located at about 45 degrees to the flat portion of the frame, close to the anode-enbedded bottom plate (hereinafter referred to as lower collar) and the other collar (upper collar) located at about 90 degrees.

FIG. 10 is an enlarged partially sectional view of FIG. 9 and FIG. 11 is a section view of the important part illustrating the portion of membrane installation to the installation frame.

The present invention will be illustrated in more detail by way of examples, which examples are not to be construed in any manner to be limiting of the invention.

As is shown in FIG. 1, membranes 4 formed in a cylindrical shape are positioned to collars 7 located at about 90 degrees to the flat portion 7a of a membrane installation frame. The membrane is positioned substantially parallel to the vertical surfaces 11 of cathodes and secured by means of clips 3 to the collars using press plate 2, interposing a packing 6 between the collar 7 and the membrane 4 (FIG. 3). At the curved portions, securing is conducted by means of bolts and nuts 12. Thus, a cathode compartment 8 and an anode compartment 9 are perfectly isolated from each other (FIG. 2). The membranes installation frames 1 are located in a manner that the horizontal surfaces 10 of the cathode at right angles to a locating direction of the anode 13 to a bottom plate 14 are covered with the flat portion 7a of the installation frame. The bottom plate 14 is insulated from the cathode box 5 by inserting an insulator 15 between the two. A suitable number of clips shown in FIG. 4 are employed at desired intervals. As a membrane, the cation exchange membrane "Nafion #315" produced by E. I. Du Pont de Nemours and Company is employed.

To the anode compartment is a hydrochloric acid-containing sodium chloride solution supplied and deionized water is fed to the cathode compartment, then 2,000 A electric current per unit anode is supplied to the cell. The anode current density is 25 A/dm.sup.2. The brine supplied is 3 N with respect to the NaCl concentration and the HCl concentration is 0.2 N in the brine. After the continuous operation for 7 days, the NaCl concentration is as low as 20 ppm in the catholyte and the NaOH concentration is 16% in the catholyte.

A cylindrically formed membrane 4 is positioned, as is depicted in FIG. 5, substantially parallel to the vertical surfaces of the cathode and secured to the collars 7 which are located at about 90 degrees to the flat portion 7a of the membrane installation frame, using clips-incorporated press plate 3a. At the curved portion securing is executed using a U-form press plate 2a to the collars 7 by way of a packing 6 inserted therebetween with bolts and nuts. Labor and time required for the installation of membrane to the electrolytic cell are markedly reduced by using clips-incorporated press plates.

One degree flared cylindrical membrane 4 is positioned parallel to the surfaces of the cathode, then applied to the collars 7, the upper is located at about 90 degrees and the lower (close to the bottom plate enbedding the anode) is located at about 45 degrees to the flat portion 7a of the membrane installation frame 1. The membrane is secured and sealed by means of clips 3 and the press plate 2 to the collars. At the curved portion, securing is achieved by the use of screw bolts. Between the collar 7 and the membrane 4 is a packing 6 interposed (FIG. 11). The cathode compartment 8 is thus isolated from the anode compartment 9 (FIG. 10). Assemblage of the membrane installed cathode box and the anode enbedded bottom plate is effected by rotating the cathode box at an angle of 180 degrees. By locating the collar 7 at about 45 degrees to the flat portion 7a, operation efficiency of assembling the anode enbedded bottom plate and the membrane installed cathode box is outstandingly improved.