US3383295A - Process for replacing the diaphragm cathode assembly in an electrochemical cell - Google Patents

Description

R. J. cox 3,383,295

-CATHODE ASSEMBLY May 14, 1968 PROCESS FOR REPLACING THE DIAPHRAGM IN AN ELECTROCHEMICAL CELL 2 Sheets-Sheet 1 Filed April 2, 1964 (PRIOR ART) INVENTOR. ROBERT J. COX

May 14, 1968 R. J. cox 3,383,295

PROCESS FOR REPLACING THE DIAPHRAGM-CATHODE ASSEMBLY IN AN ELECTROCHEMICAL CELL Filed April 2, 1964 2 Sheets-Sheet 2 INVENTOR. ROBERT J, COX W m SK ATTORNEY United States Patent 3,383,295 PROCESS FOR REPLACING THE DIAPHRAGM CATHODE ASSEMBLY IN AN ELECTRO- CHEMICAL CELL Robert J. Cox, Believille, Mich., assignor to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed Apr. 2, 1964, Ser. No. 356,705 2 Claims. (Cl. 204-128) ABSTRACT OF THE DISCLOSURE This invention relates to a replacement diaphragmcathode assembly for chlorine-producing electrolytic cells which require a new diaphragm when the old diaphragm has become clogged. The new cathode to which the new diaphragm is attached compensates for the increased interelectrode gap which results from anode erosion during use. The replacement cathode has a larger dimension than the cathode originally employed but is geometrically similar thereto. The increment in cathode size is substantially equal to the amount of anode material consumed during electrolysis whereby the interelectrode spacing, and hence the IR drop, is kept essentially constant without changing anodes.

This invention relates to cathodes used in electrolytic diaphragm-type cells and more particularly relates to new replacement cathodes and diaphragm-cathode assemblies for improving the efiiciency of operation of such electrolytic cells.

Diaphragm-type electrolytic cells are widely used in industry, particularly in the preparation of caustic soda, chlorine and hydrogen by the electrolysis of sodium chloride brines. Most of such cells consist of a carbon anode and a perforated sheet metal cathode, separated by a diaphragm which is most commonly produced by depositing a layer of asbestos fibers on that surface of the cathode which is nearest the anode.

In the electrolysis of NaCl, sodium hydroxide and hydrogen are formed at the cathode and chlorine is given ofi at the anode. The diaphragm inhibits transmigration of the hydroxyl ions to the anode or of the chlorine to the cathode because of the flow of anolyte through the diaphragm to the catholyte side. This is of major importance in minimizing chlorate and hypochlorite impurities in the sodium hydroxide produced and in minimizing electrical resistance. The construction of diaphragm-type cells is well known and is discussed at length in a number of textbooks including Industrial Chemistry by E. R. Riegel, 5th edition (1949), pages 92-110. US. 2,987,463 to Baker et al. discloses the construction of one such electrolytic cell which is now in commercial operation.

It is a characteristic of diaphragm-type cells in commercial operations involving less than completely pure input solutions (for example, brine used in producing chlorine, hydrogen and caustic soda), that the diaphragms will gradually become plugged with impurities. This plugging reduces permeability and thereby increases electrical resistance and requires that the diaphragm be removed for washing or, more commonly, for complete replacement.

In the many cells currently in commercial operation which have diaphragms integrally attached to foraminous cathodes, the entire diaphragm-cathode assembly is conventionally removed for replacement of the diaphragm. The cell is then fitted with a cathode onto which it attached a new diaphragm and returned to operation.

A second operating characteristic of most modern diaphragm-type electrochemical cells is the slow, but con- Patented May 14, 1968 stant, consumption of their anodes. Substantially all such cells employ graphitized carbon anodes and while various methods have been advanced for minimizing consumption of the anode, it is periodically necessary to remove the cells from service for replacement of the carbon anodes. During such replacement the entire cell is usually refurbished including cleaning of the exterior and interior, and replacement of the anodes and diaphragm. Ideally, the replacement of anodes is scheduled when the diaphragm has become sufiiciently plugged that its replacement is also necessary. Such scheduling reduces the amount of diaphragm rebuilding necessary in the operation of such cells.

The cost of electricity is an important portion of the total cost of most electrochemically produced products, and the electrical loss due to the resistance of the cell (the so-called IR drop) is the subject of many efforts at cost reduction. This IR loss is particularly undesirable in some operations because it generates hea t in the body of liquid within the cell. Since modern high amperage electrochemical cells operate at temperatures which may approach the boiling point of the electrolyte, excessive heating is highly objectionable.

The present invention comprises a specially constructed replacement diaphragm-cathode assembly for periodic substitution in electrolytic cells, which assembly is so designed as to have, when in place, all points on the active surfaces of its cathode closer to the center point of the cell anode than were the corresponding points on the active surfaces of the original cathode. The new assembly of the present invention is preferably substituted at those times when replacement of the diaphragm would conventionally be required due to plugging but in some cases may be substituted when reduction in IR drop through the electrolyte is sufficient to require or justify the substitution but diaphragm replacement would not be required. The assembly is so constructed as to compensate for the decrease in anode dimensions caused by consumption of the anode during operation of the cell.

Use of the new replacement assembly provides savings in electrolyte IR loss, permits lower cell operating temperatures and can reduce the frequency of rebuilding. All these valuable advantages are obtained without any additional operating steps other than those now conventionally used. In most instances, no modification of other parts of the cell is necessary for employment of the present invention. Since spare conventional diaphragm-cathode assemblies must now be inventoried in virtually all commercial diaphragm cell installations, use of the present invention does not require any substantial increase in capital investment.

The following definitions apply to terms used throughout this application. By encircle is meant to substantially surround on all active sides though not necessarily on the top and bottom. Cathodes in some electrolytic cells encircle the anodes. By foraminous is meant porous or perforate. By compensate for the decrease in dimensions of the anode or compensate for the consumption of the anode is meant to offset the increase in distance between points on the cathode and the anode caused by gradual consumption of the anode during electrolysis. This compensation is accomplished by return of the gap between anode and cathode to approximately the value found in a new or completely rebuilt electrolytic cell. By a plugged diaphragm is meant one which by virtue of impurities clogging its apertures causes an objectionably high IR drop across the diaphragm and/ or substantially decreases the diaphragms permeability to fluid flow. By an efiicient diaphragm is meant one through which both the IR drop and the resistance to fluid flow are acceptably low.

FIGURE 1 is a top plan view of a portion of typical approximately 30,000 amp. cell for the electrolysis of 3 alkali metal chloride brines as described in detail in US. 2,987,463.

FIGURE 2 is a detail showing two of the individual anodes and diaphragm-cathode assemblies as they appear when a new cell is placed into service.

FIGURE 3 is a. detail similar to FIGURE 2 showing the anode and diaphragm-cathode assembly at a point at which a diaphragm has become plugged and requires replacement.

FIGURE 4 is a detail similar to FIGURES 2 and 3 showing the anodes and the diaphragm-cathode assembly immediately after replacement of the plugged diaphragm according to conventional practice.

FIGURE 5 shows the anode and cathode immediately after replacement of the diaphragm-cathode assembly according to the practice of the present invention.

FIGURE 6 is a horizontal cross-section of an anode and diaphragm-cathode assembly with their original outlines shown in broken lines.

In the above described drawings, 1 is the anode in a new cell or immediately after replacement of the anode assembly in an older cell as is conventionally done when the cell is completely rebuilt.

Element 2 is the diaphragm-cathode assembly which is composed of the diaphragm 3 coated directly onto a metallic cathode screen 4. A pair of cathode end screens 5 onto which a diaphragm has been deposited is included in the diaphragm-cathode assembly 2 to provide complete encirclement of the anode on all sides, the top and bottom being open.

In operation, the anode is, of course, positively charged and the cathode is negatively charged and current flows between the anode and cathode, passing through the diaphragm, Various mechanical, chemical and eletrical effects cause the anode to be consumed slowly and to be reduced in each of its major dimensions. The diaphragm will simultaneously become gradually plugged with impurities and will require renewal after a time interval which varies widely according to the operating characteristics and conditions of the cell and the purity of the brine being electrolyzed. This interval of time is usually from about one third to about one half the life of conventional graphite anodes.

The plugged diaphragm and its supporting cathode are shown as item 7 in FIGURE 3, which also shows the typical partially consumed anode at this point in the anodes life. Note the substantially increased distance between both the sidewalls and the end screens of the cathode and the partially consumed anode. Since electricity must flow across this gap, the electrical resistance of the electrolyte is related to the width of the gap. At this point the plugged diaphragm which also contributes to electrical resistance may be adding roughly .25 volt per cell to the IR drop, the increased gap between the active surfaces of the anode and the cathode may be adding an additional .15 volt per cell and the increased resistivity due to partial depletion of the anode may have caused an additional .05 volt per cell. The original cell voltage was roughly 3.6 and this has now risen to perhaps 4.05 volts per cell.

FIGURE 4 shows the cathode assembly 8 with new diaphragm which has been substituted for the plugged diaphragm-cathode assembly 7. The assembly 8 is in all respects identical with the original diaphragm-cathode assembly 2 and has been constructed by stripping the plugged diaphragm from an assembly and redepositing a new diaphragm on the bare cathode screen thus exposed. The diaphragm has been redeposited on both the side and the end screens of the cathode and the voltage drop due to the diaphragm is now reduced to its original value. Total voltage of the cell is therefore about 3.8 volts.

The above description and FIGURES l, 2, 3, and 4 have illustrated the conventional method of operating cells similar to those disclosed US. 2,987,463. Operation of the many widely varied types of diaphragm cells is in general similar, and the problems in constantly increasing IR drop are equal or even greater in magnitude in most such cells.

Cell operation according to the present invention begins identically with conventional operation. That is, FIG- URES l and 2 describe the first two stages in the operation with the new diaphragm cathode assemblies.

At the point corresponding to roughly /3 to /2 the life of anodes, the diaphragm becomes sufficiently plugged to require replacement as in conventional operations and the voltage drop through the diaphragm and that due to the increase gap between the active surfaces of the cathode and the anode are identical to those in conventional operations. Accordingly, in the practice of the present invention, the plugged diaphragm-cathode assembly is removed at this point, the anode remains in place, and a new specially constructed replacement diaphragm-cathode assembly 9 is inserted around the anode in the cell, as shown in FIGURE 5. This step may be repeated as required to replace the diaphragm and reduce the gap throughout the life of anode assembly. With good quality brine and other good conditions of operation, it is usually required only once during the life of anodes. However, it is not extremely unusual for three diaphragm to be installed during anode life.

In a typical installation of cells, for example where the diaphragms are replaced at roughly the midlife of the anode, approximately half the cathodes in service would have conventional dimensions while the other half would have modified dimensions. Therefore, in a new installation of cells the capital investment required to obtain advantages of the present invention would not be significantly more than present practice where all cathodes have the same dimensions. In the case of an existing installation of cells, expenditures for revising approximately half the cathodes would, of course, be required.

The replacement cathodes used in the practice of the present invention pose no unusual problems in fabrication. In general, they will be fabricated of materials and by use of methods which are identical to those used in or to fabricate existing cathodes. The preferred cathodes in most electrolytic cells are steel screens and/or perforated steel sheets reinforced as necessary with steel bars and/or strips.

The replacement cathodes will generally be of approximately the same height as the original cathodes except in cases where the anode configuration is such that the height of the anode is substantially decreased during electrolysis. In most instances, the replacement cathode will be geometrically similar to the original cathode with only the internal horizontal cross sectional dimensions reduced in the replacement cathode. The preferred method of determining the proper horizontal cross-sectional dimensions for the replacement cathode is to compare new anodes to anodes removed from a cell at the time that the diaphragm required replacement. These incremental differences can then be proportioned to modify the internal horizontal cross-sectional dimensions for the replacement cathode. That is, if loss of graphite from the active surface of anodes has reduced the cross-sectional dimensions (thickness and width) by /2" at points about halfway up from the anode base, the corresponding cross-sectional dimensions transverse to the replacement cathodes should be /2" less. This procedure establishes the same gap be tween anode and cathode faces as when the anodes were new and installed with the original cathodes.

FIGURE 6 illustrates the measurements of the incremental reduction in the dimensions of anodes and application of these dimensions to modify the horizontal cross-section of the original cathode so as to obtain a replacement cathode which compensates for the reduction of the anode at each point.

In some instances it will be desirable to design the contour of the horizontal cross section of the replacement cathode so as to compensate for the rounding which frequently occurs during consumption of the anode. Also, it will sometimes be useful to slant the surfaces of the cathode from the vertical to compensate for an anode which is more rapidly consumed at one end.

In FIGURE 6, B is the distance (gap) between the anode and the cathode when the cell is new, and B is the distance between the anode and the cathode after partial consumption of the anode and just after the insertion of the replacement mathode. B is equal to B indicating that the original anode-cathode gap has been restored. Also in FIGURE 6, A is the distance which the active surface of the anode has moved back due to gradual consumption of the anode. A is the distance which the nearest point on the cathode has been moved forward by the design of the replacement cathode. Here again, A equal A and both surfaces have moved equal distances thus restoring the original anode-cathode gap.

The new replacement assembly is constructed of materials identical with those now conventionally used in diaphragm cathode assemblies for the various electrolytic cells and the diaphragm applied to the conventional manner. However, the replacement diaphragm cathode assembly of the present invention has been so altered in dimensions from the conventional assembly, that the surface of the cathode is closer to the anode. That is, all points on the active surface of the cathode are closer to the center point of the anode than were the corresponding points on the active surface of the original cathode. The interior distance between the two side screens is less and the interior distance between the two end screens is less than on the conventionally used diaphragm cathode as sembly. These decreased dimensions are so calculated as to compensate for the further decrease in anode dimensions caused by consumption of the anode during the operation of the cell up to the point where the anodes are to be replaced if operational conditions and/or practices call for two diaphragms per anode life or up to the point where the diaphragm is to be replaced and a similarly further modified cathode is to be installed if conditions and/ or practice call for three diaphragms per anode life or if advantages of the present invention justify replacement with a further modified cathode. Several successive replacement cathodes can thus be installed, each modified to a degree suitable to achieve optimun economy in cell operation.

The new diaphragm-cathode assembly isconstructed of materials identical with those used in conventional diaphragm cathode assemblies and the diaphragm is applied in a conventional manner. However, dimensions of the diaphragm-cathode assembly of the present invention are different from the conventional assembly. The internal distance between the side screens of the cathode and between the end screens of the cathode are each lessened in order to compensate for the decrease in anode dimensions caused by consumption of the anode during operation of the cell up to the point where the diaphragm requires replacement or it is otherwise advantageous to replace the cathode with one of the present invention. By so compensating the distance between the new substitute cathode and the partially depleted anode is rendered aproximately equal to that in a new cell or in a cell which has been completely rebuilt with new anode and cathode. The IR drop through the diaphragm has been decreased 'to its original value by substitution of a new diaphragm as is true in the conventional case. The IR drop through the electrolyte is now approximately identical with that for the cell when it was first placed into service rather than being approximately .15 or more volts per cell higher as is true after substitution of a conventional diaphragm cathode assembly.

Most importantly, this lower IR drop of 0.15 volt per cell or more continues throughout the life of the diaphragm which is deposited on the replacement diaphragm-cathode assembly of the present invention, and this valuable decrease in power consumption is achieved with no extra steps and no modification of other parts of the cell.

Because of the reduced electrolyte resistance throughout the life of the replacement diaphragm, such a cell may be operated for a longer time prior to rebuilding with consequent decrease in rebuilding costs and better utilization of anodes by more complete consumption of the anodes and, therefore, reduction of waste due to scrapped carbon. In some cases the greater improvement in efficiency caused by replacements of diaphragm cathode assemblies according to the method of the present invention will permit more frequent economic replacement of such assemblies and further reduce electrical losses and operating costs of the cells.

In some cases it may be advisable to employ larger anodes where this is possible with existing cell design or can be accommodated by design of new cells. Use of thicker :and/ or wider carbon anodes will permit successive changes of diaphragm cathode assemblies employing a series of successivelysmaller internal dimensions. Thus, a thick anode, installed in a cell adapted to the purpose can be used in conjunction with 4 or 5 different diaphragmcathode assemblies having suitably different dimensions to greatly extend the period before complete rebuilding when replacement of the anode is required. This would normally require that the internal distance between the side screens of the cathode and between the end screens of the cathode, when measured in the anolyte section, be increased as much as the new anode thickness and/or width is increased. Correspondingly, the cell size would have to be increased. However, the average IR loss through the anodes would be reduced; and graphite scrap loss would be reduced. Decrease in the amount of downtime of a given cell couples with reduced costs for complete rebuilding of cells will substantially improve the economics of producing chlorine, hydrogen and caustic soda or potassium hydroxide or other products through electrolysis.

When using the assemblies of the present invention with anodes of conventional thickness, it will usually be possible to consume the anode to a smaller thickness before replacing it entirely, and thus the amount of carbon scrap in discarded anodes can be substantially reduced. In this respect, there is an optimum condition between reduced IR drop through the electrolyte (to be reduced with the present invention) and the increasing IR drop through the anode. Therefore, objectives, cost of power, rebuilding costs including labor, graphite, diaphragms and other materials used in rebuilding must be considered in light of the particular installation of cells since they vary to some degree throughout the country and from time to time.

While the above described typical embodiment of the present invention involves the use of a sandwich cell employing finger or tube type cathode-diaphragms extending upward from the bottom of the cell or across the cell, the invention should be understood to be readily adaptable to a wide variety of cells which encompasses most of the currently used diaphragm type cells for the electrolysis of solutions of soluble salts in water.

It should be understood also that the invention is applicable to cells in which the diaphragms are not directly deposited on the cathodes but are composed of separately mounted pieces which are positioned between the anodes and cathodes substantially perpendicular to the cathodes.

The invention is also applicable to cells in which the diaphragms are not built on the cathodes by depositing fibers to form a porous diaphragm through lamination of these fibers but the diaphragms are porous sheet material (usually made of asbestos fibers) which cover the cathodes.

In summary, the present invention comprises a replace ment diaphragm-cathode assembly for use in an electrolytic chlorine producing cell having a partially consumed anode and a diaphragm-cathode assembly comprising a foraminous metallic cathode onto which is deposited a permeable diaphragm, or onto which is placed a sheet of porous diaphragm material, the internal dimensions of the replacement diaphragm-cathode assembly being such as to cause the cathode to more closely encircle the partially consumed anode whereby the reduction in dimensions of said anode is compensated for by the reduced internal dimensions of said replacement diaphragmcathode assembly. (It is to be understood that each reference to an anode means at least one anode and similarly for other cell components.)

This invention also comprises the process of using the above similar diaphragm cathode replacement assembly. In the operation of an electrolytic diaphragm type chlorine producing cell having an anode which is gradually consumed, a first cathode, and a first diaphragm attached to said first cathode, said first diaphragm being gradually rendered inoperative by accumulation of impurities during the electrolysis; the process of replacing said first cathode and said first diaphragm at a time when said first diaphragm has by plugging become substantially reduced in efiiciency with an eificient replacement diaphragm attached to a replacement cathode having dimensions and configurations such that said replacement cathode compensates for the decrease in dimensions of said anode.

In sandwich type electrolytic diaphragm chlorine producing cells which comprise a set of alternatively spaced anode chambers and cathode chambers separated by diaphragms, the invention comprises the process of gradually increasing the thickness of the cathode chambers so as to restore approximately the original spacing between anode and cathode as said anodes are gradually consumed. By thickness is meant the internal dimension of the chamber measured in a direction transverse to the major surface of the anode and cathode.

Stated differently, the invention comprises, in a sandwich type electrolytic diaphragm cell chlorine producing comprising alternately spaced anodes and original cathodes enclosing cathode chambers, said anodes and cathodes being separated by diaphragms; a replacement cathode enclosing a cathode chamber larger in a dimension transverse to said anodes and said cathodes than was said original cathode.

It should be understood that the present invention is capable of a wide variety of adaptions and modifications without departing from the scope and the spirit thereof and that the preceding examples are intended to illustrate the invention and not to limit it in any manner or to any degree.

What is claimed is:

1. In the operation of an electrolytic diaphragm type chlorine-producing cell having an anode which is gradually consumed, a cathode, and a diaphragm attached to said cathode; the process of operating said cell until the anode is partially consumed and the diaphragm is partially lugged, and then replacing said cathode and said diaphragm, comprising the steps of substituting a replacement cathode having a configuration which includes a dimensional substituting substantially equal to the dimensional decrement of the partially consumed anode, attaching a new diaphragm to said replacement cathode, and mounting said replacement cathode in said cell in spaced juxtaposition with the partially consumed anode such that said replacement cathode compensates for the decrease in dimensions of said anode.

2. In the operation of a sandwich type electrolytic diaphragm chlorine-producing cell having a set of alternately spaced anode chambers and cathode chambers separated by diaphragms, the process of operating said cell until the anode is partially consumed and the diaphragm is partially plugged, and then replacing clogged diaphragms comprising the steps of substituting a replacement cathode having dimensions which when mounted in the cell will have an active surface spaced from the anode surface by a distance equal to the original interelectrode spacing, and attaching a new diaphragm to said cathode surface so as to restore approximately the original gap between anode and cathode as said anodes are gradually consumed.

References Cited UNITED STATES PATENTS 1,152,772 9/1915 Wheeler 204-283 2,306,757 12/1942 Schifibauer, et al. 204-260 2,328,665 9/1943 Munson 204-286 2,569,578 10/1951 Rieger 204-206 2,731,412 1/1956 Ferrand 204223 3,275,538 9/1966 l-laupt et al. 204l43 JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

I H. M. FLOURNOY, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,383 ,295 May 14 1968 Robert J. Cox It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 24, "diaphragm" should read diaphragms Column 5, line 9, "mathode" should read cathode Column 6, line 32, "couples" should read coupled Column 7, lil 10, after "above" insert and Column 8, line 12, "substituting" should read change Signed and sealed this 23rd day of September 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. E.

Attesting Officer Commissioner of Patents

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