US2326101A - Diaphragm for chlorine cells - Google Patents

Description

1943. w. T. NICHOLS DIAPHRAGM FOR CHLORINE CELLS Filed Oct. 11, 1940 Patented Aug. 3, 1943 UNITED STATES PATENT OFFICE DIAPHRAGM FOR CHLORINE CELLS William T. Nichols, near St. Albans, W. Va., assigner to Wcstvaco Chlorine Products Corporation, New York, N. Y., a corporation of Delaware Application October 11, 1940, Serial No. 360,839

1 Claim. (Cl. 204-295) This invention relates to diaphragms for chlorine cells; and it comprises in a diaphragm type cell for producingchlorine and alkali, a multicludes a set of anodes, usually graphite rods or slabs, and a cathode of perforated 0r slotted sheet steel against which a diaphragm is placed.

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the urge of osmotic and electrical forces, toward the anode. These opposing movements tend to form what is sometimes referred to as a neutral layer, which represents the boundary where the alkali moving toward the anode meets and is neutralized by the acid .anolyte moving toward the cathode. For efficient cell performance, it is desirable that this "neutral layer be established actually "within the diaphragm. .Such positioning prevents excessive entry of hydroxyl ions into the anode compartment with resulting deterioration in cell performance. I

The diaphragm material must possess high chemical resistance to acid, alkali, and strong Sometimes, and advantageously, there is a layer of woven wirev cloth between the perforated sheet steel cathode and the diaphragm, this giving better support to the diaphragm and also having some advantages as regards free liberation of hydrogen bubbles formed on the cathode. The

cathode and diaphragm define an anode chamber containing a body of brine into which the anodes enter. Vorce Patents Nos. 1,286,844 and 2,078,517

illustrate good embodiments of this type. When current passes between the anode and the-cathode, chlorine is produced in the'anode chamber and a caustic liquor containing some salt appears on the face of the cathode.

In all these cells it is desirable to have as little electrical resistance as possible. A change of a fraction of an ohm in cell resistance means a considerable change'in the power input required by the cell. Forthis reason anodes and cathodes are placed as close together as possible and the diaphragm is made reasonably thin.

The efficiency of a diaphragm type chlorine cell depends in considerable measure upon the proper functioning of the diaphragm.. This serves first as a bubble screen to prevent actual intermingling of the two different gases formed in the cell. Second, it functions to regulate the rate of flow of liquid from the anode to the cathode compartment of the cell. Third, it func-' tions to prevent the intermingling of the alkali formed on the cathode surface with the anode liquor, 'which liquor possesses some degree of acidity. In a typical diaphragm cell, the acid anolyte liquor is moving steadily .through'the -diaphragmtoward the cathode under the influence of what are primarily hydraulic forces. The alkali formed on the cathode tends to move, under oxidizing and reducing conditions. Asbestos fiber, usually in the form of commercial asbestos paper, has been found to possess more desirable properties in this respect than other commercially available products ,and is almost universally employed as a diaphragm material. Canadian asbestos, or chrysotile, though not as chemically resistant as other types of asbestos, makes better paper and is generally used. I

In some chlorine cells where cathode design is complex, the diaphragm is sometimes formed on the cathode by inserting the cathode in a suspension of asbestos fiber's and forcing liquid through the cathode screen by applying pressure or vacuum to the cathode structure, thus depositing a layer of fibers of desired thickness and physical characteristics. However, in such a cell as that shown in the Vorce patents earlier men: tioned, it is more economical and more convenient to use pre-formed asbestos sheets than it is to deposit a diaphragm layerby use of a suspension of asbestos fibers.

In anordinary asbestos paper, the felting together of the fibers is not particularly pronounced and as a result, such paper has but very little strength when wetted. The same conditions prevail in the layer of asbestos fibers which may be formed against a cathode structure by use of a.

. suspension of asbestos. In spite of this known low wet strength it is ordinarily expected that the diaphragm will remain in place and retain in gencral its original texture when the cell is filled with.

' liquid and is operating to produce chlorine and caustic, since hydraulic forces tend to holdit in position. In commercial cells there is always a higher liquid head in the anode than in-the oathode compartment. However, it has been found that while this expectation is true asregards a cell before an electrical load is applied, it is not true as regards an electrically loaded cell. When electrical load is applied to a cell, electrical forces phragm material present when electrical load is first applied to the cell, the actual operating diaphragm, that is the disintegrated and re-formed structure, is not uniform from point to point on the cathode and is of an open porous structure which allows of the entry of insoluble impurities (precipitated calcium and magnesium compounds from the brine and anode disintegration products) into the diaphragm structure itself, with resulting adverse efi'ects on the hydraulic and electrical characteristics of the diaphragm.

A restraint on the disintegration of .the initial diaphragm can be made by positioning against the surface of the asbestos paper or formed asbestos layer, a sheet of'some other material having suitable porosity and high strength when wetted; this serving as a clamping member to hold down the underlying asbestos layer and to prevent disintegration -and substantial changes in its texture. Because of chemical necessities, very few materials can be considered for this purpose. Among these, glass cloth and asbestos cloth offer possibilities; but the former does not withstand the chemical conditions with which it is met, over any extended period. The latter has adequate chemical resistance but, as ordinarily used, is so thick and so closely woven that, although it adequately restrains the disintegration of the asbestos paper diaphragm it imposes too much added electrical resistance on the cell.

A wide variety of asbestos papers are on the market, intended for various uses. Such papers are largely used for thermal insulation in buildings. Among these papers which are commercially available are some having high wet strength,

and thus adapted for use in damp situations. Some of these papers, which are commercially known as high wet strength papers, have extraordinarily high resistance to bursting forces and tensile stresses when wetted, and have a much higher ratio of wet strength to dry strength than that found in ordinary asbestos -paper.

For example, a typical asbestos paper or the usual type, 'which has been frequently used heretofore as a diaphragm paper in thicknesses of about 0.06 inch, has a dry Mullen strength of about 16' to 19 (for this thickness) and a wet Mullen strength of about 6. The ratio of dry to wet Mullen strengths for such paper is thus about 3. In contrast, a commercial high wet strengt paper having a thickness of only 0.01 inch has a dry Mullen strength of 9 to 10 (average) and a wet Mullen strength above 6. The wet Mullen strength of this paper is thus equal to that of ordinary paper having six times its thickness, and its ratio of dry to wet Mullen strengths is less than 2, and usually about 1.5.

While these comparative results of Mullen tests are quite outstanding, the differences inthe papers are shown even more clearly by tensile strength measurements. Thus, ordinary asbestos paper of 0.06 inch thickness has a dry tensile strength of about 9.6 pounds per inch (average) in the longitudinal direction (with the machine direction) and about 3.84 pounds per inch (average) in the transverse direction. A high wet strength paper of 0.01 inch thickness has generally similar dry tensile strength values, of

9.5 pounds per inch (average) in the longitudinal direction and 3.4 pounds per inch (average) in the transverse direction. When wetted, however. the tensile strength of the ordinary paper dropped to less than .34 pound per inch in the longitudinal direction, and to 0.00 in the transverse direction. The ratios of dry to wet tensile strengths for this usual type of paper are thus 30.0 in the longitudinal direction, and infinity in the transverse direction. Characteristic ratios for this type of paper are at least 20.

The high wet strength paper showed much less deterioration when wetted. Its wet tensile strength in the longitudinal direction averaged 4.85 pounds per inch; and in the transverse direction 1.66 pounds per inch. Theaverage ratios of dry to wet tensile strength for this type of paper are therefore slightly less than 2 in the 101igitudinal direction, and slightly more than 2 in the transverse direction.

These high wet strength papers have a closer texture than the ordinary asbestos papers, and owe their properties to a more thorough felting, or calendering, or both. These papers, as here- .inbefore noted, are known commercially. Such a high wet strength paper may be produced in accordance with the method disclosed in the Collier Patent No, 2,237,337 although it will be appreciated from the foregoing that the invention may be carried out with high wet strength papers having the foregoing properties, irrespective of their specific method of manufacture. .They are too impervious under cell conditions to serve satisfactorily whenused alone as a diaphragm in electrolytic cells. In order to use such paper as a diaphragm, a very thin layer of it must be employed to realize proper liquid flow; in fact, the layer must be so thin that the neutral layer cannot be positioned within the diaphragm. Inferior cell performance therefore results when this paper is used alone.

It has been found that by a proper combination of this high wet strength paper with the ordinary commercial asbestos paper, the advantages'of a stabilized diaphragm-structure can be realized while at the same time retaining the more desirable cell performance characteristics which are obtained when ordinary asbestos diaphragm paper is used. To this end, the normal low-strength asbestos paper is placed against the cathode structure and is then covered by a very thin layer of high wet strength paper. The normal paper may have a thickness of the order of .05 to .06' inch. The high wet strength paper, on the other hand, need have a thickness of only about .01 inch. When so used, the primary function of the high wet strength-paper is that of a restraining membrane. The conventional asbestos paper .continues to exhibit the desirable electrical and hydraulic characteristics inherent in its structure, but it is prevented from disintegrating and non-uniformly rearranging itself by the mechanical restraining action of the extremely. thin sheet of high wet strength paper imposed on it.

For use in this manner, the high wet strength paper should have a Mullen strength of at least 5 when wet, for a thickness of .01 inch; and it advantageously has a low ratio of dry to wet tensile strengths, in both directions. Such paper is sufliciently strong for the purpose, and does not add materially to the cell resistance.

With a composite diaphragm of the type indicated, the original high efiiciency of the cell remains practically constant for a long period of time and the composition of the catholyte liquor also remains reasonably constant. As an indication of the resistance to disintegration possessed by the new composite diaphragm, it is observed that although graphite dust resulting from disintegration of anodes is found distributed rather evenly throughout an ordinary asbestos diaphragm which has been in service for some months, in the case of the composite diaphragm of the present invention, the graphite dust is confined almost completely to the anode side of the composite sheet. The suspended carbon coming from the anode does not penetrate the restraining sheet or reach the thick main layer of asbestos paper, which retains its original color.

In the accompanying drawing there is shown diagrammatically an-example of a specific emgraphite anode I; one of a set of anodes in an-' nular arrangement. The cathode carries a layer of woven steel wire cloth8, which in turn carries a layer of regular asbestos diaphragm paper 9, usually 0.05 to 0.06 inch thick. On this paper there is applied, in accordance with this invention, a sheet I of the high-wet strength, asbestos paper previously described; this sheet being usually about 0.01 inch thick. The composite diaphragm separates the anode space H nique in operation. As a matter of fact, they are containing the anolyte [2 (which is NaCl brine) easier to place in operation than is a cell containingthe usual low wet strength asbestos paper alone, for they can be filled with brine without the necessity of using any special precautions to prevent rupture of the diaphragm. Furthermore, while cells having diaphragms of ordinary paper frequently require the addition of asbestos slurry to the anolyte compartment shortly after the cell has been placed in operation to repair defacts in the diaphragm structure resulting fromv action of the brine, no such expedient is required when employing the composite diaphragms of the present invention.

The operating characteristics of a cell containing the composite diaphragms are, and remain, superior to those of a cell containing an ordinary diaphragm. The purity of chlorine is high, the chlorate content of cell liquor is low,

graphite losses are somewhat reduced, and the substantially non-conductive diaphragm between said cathode and anode and disposed in close relation to the cathode, said diaphragm consist- .ing of a layer of porous asbestos paper about 0.05 to 0.06 inch in thickness of the type relatively susceptible to disintegration when employed alone in said cell, said layer of paper being sufiiciently thick to substantially restrict flow of cell liquor toward the cathode, and a second layer disposed on that surface of said relatively thick layer which faces the anode, and consisting of a layer of porous, high wet strength asbestos paper, about 0.01 inch in thickness and having a Mullen wet strength of at least 5, and of closer texture than the first layer of asbestos paper, the two layers having suificient combined resistance to hydrostatic head of the cell liquor to regulate the fiow in accordance with the rate of electrolysis.

WILLIAM T. NICHOLS.

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