US 3855104 A
Describes a process and apparatus for the electolysis of HCl containing solutions in unipolar and bipolar cells using graphite electrodes in which the chlorine and hydrogen and the anolyte and catholyte liquor produced in the process are kept separate by means of separate discharge channels in the electrodes with which the active faces of the anodes and cathodes communicate and through which the acid leaves the cells in such a manner as to give longer diaphragm life.
Claims available in
Description (OCR text may contain errors)
d U Z O O o 12-17-74 XR 398559104 BEST AVAILABLE COPY United States Patent 1 1 1111 3,855,104 Messner 1 Dec. 17 1974 [541 PROCESS AND APPARATUS FOR THE 1,247,694 11/1917 Levin 204/256 ELECTROLYSIS OF HCL CONTAINING 9 23 32 1 l iams 1 SOLUTIONS WITH GRAPHITE 3.647.672 3/1972 Mehandjiev 204/278 x ELECTRODES WHICH KEEP THE 1.790.249 1/1931 Roth 204/270 x  Inventor: Georg Messner, Munich, Germany  Assignee: Oronzio De Nora Impianti Elettrochimici S.p.A., Milan. Italy  Filed: Mar. 15, 1973  Appl. N0.: 341,593
l30| Foreign Application Priority Data Mar. 21. 1972 Germany 2213603 [52} US. Cl 204/129, 204/128. 204/256, 204/258, 204/266, 204/270. 204/278, 204/283, 204/294  Int. Cl.. 801k 3/04, BOlk 3/10  Field of Search 204/128, 256, 258, 266, 204/270, 278, 283, 294, 129
 References Cited UNITED STATES PATENTS 1.575.627 3/1926 Heinze 204/278 1.131.859 3/1915 Parks 204/2-70 CHLORINE AND HYDROGEN GASES SEPARATE FOREIGN PATENTS OR APPLICATIONS 566,090 11/1958 Canada 204/256 Primary ExaminerJohn H. Mack Assistant ExaminerW. 1. Solomon Attorney, Agent, or FirmHammond & Littell  ABSTRACT Describes a process and apparatus for the electrolysis of HCl containing solutions in unipolar and bipolar cells using graphite electrodes in which the chlorine and hydrogen and the anolyte and catholyte liquor produced in the process are kept separate by means of separate discharge channels in the electrodes with which the active faces of the anodes and cathodes communicate and through which the acid leaves the cells in such a manneras to give longer diaphragm life.
18 Claims, 29 Drawing Figures BEST AVAILABLE COPY PATENTED 3.855.104
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9E3? AVNLABLE COPY PATEHTED DEC 1 H974 (SHEET 100! 16 BEST AVAlLABLE COPY PATENTEDDECI 71974 3,855,10 sum 12UF 16 4 EEST AVAILABLE COPY PATEHTEQEEE 1 71974 SHEET 13'JF 16 FIG .2l0
BEST AVAILABLE COPY PATENTEU 5551 7 7 SHEET NUF 16 fig? AVAILABLE COPY PXJENTED U53 1 71974 SHEET lSUF 16 BEST AVAILABLE COPY PROCESS AND APPARATUS FOR THE ELECTROLYSIS OF HCL CONTAINING SOLUTIONS WITH GRAPHITE ELECTRODES WHICH KEEP THE CHLORINE AND HYDROGEN GASES SEPARATE Hydrochloric acid has been electrolyzed on graphite electrodes, to produce chlorine and hydrogen, for many years. Apparatus built proponderantly on the same principles and of the same materials as used from the beginning of this industry, with some enlargements and small improvements has been used; see Ullmann, 3rd Edition, Vol. 5, page 303. The standard capacity unit, today, is a bipolar electrolyzer loaded to 10,000 Amp. with 30 5O electrode elements in series, producing in the case of 50 elements approximately 15,000 kg chlorine per day.
The weak point of all prior electrolyzers is the space between the electrodes, especially the diaphragm, which has the duty to keep the two produced gases separate. This diaphragm is a cloth of polyvinyl chloride fibers, which is assembled between the electrodes and held in place by the framesof synthetic material, similar to a drum-skin, See, for example, my prior US. Pat. No. 3,236,760.
The supply of HCl is provided by introduction of hydrochloric acid with a concentration of more than l8% HCl, i.e., more than the concentration of the depleted acid, into the cells between the electrodes. This feed acid has not only to substitute the previously electrolytically decomposed HCl, but it takes off also the Joule heat produced in the electrolyzer. Thus, feed acid in the prior art is introduced in two separate streams of HCl on each side of each diaphragm one flows through the anodic room, the other through the cathodic room, losing a fraction of its HCl content, while passing through the cell. The feed rate of the acid is such as to keep the HCl concentration of the depleted acid at approximately l8% by weight, corresponding to the maximum electrical conductivity of the electrolyte.
The purpose of the splitting of the feed acid into two parts is to eliminate as quickly as possible the chlorine gas bubbles in the anode space and the hydrogen gas bubbles in the cathode space, which increase the elec tric resistance of the electrolyte and the cell voltage. At the usual cell temperature of 70 C about 21 vol.% of water vapor are present in the Cl and H gases.
As long as the diaphragm cloth is not damaged, fairly pure chlorine and hydrogen is produced. The purity is 99.7 99.9 vol.% on the dry basis.
Polyvinyl chloride, as compared with other materials, is the more resistant material for the diaphragms. ln HCl electrolyzers, however, it undergoes a slow modification by chlorination, which makes the fibers brittle, so that finally they do not have sufficient tensile strength to withstand the conditions of use in a HCl electrolysis cell. Cracks and holes are formed in the cloth and its capacity to keep the gases separate is reduced after a few months to the point where not only the purity of the gases becomes unsatisfactory, but the danger of explosion of the formed mixtures of Cl, and H, is increased. In order to eliminate this danger, it is necessary to stop the electrolyzer and to replace the damaged diaphragms by new PVC diaphragms. Fluttering and flexing of the diaphragms caused partially by the introduction of the HCl feed on each side of the diaphragms contributes to their rapid destruction.
The present invention as described in the following examples largely overcomes this defect.
One of the basic principles of the apparatus and process of the present invention is to avoid the selfsupporting diaphragm as used previously, which when submitted to the action of strong mechanical forces in the prior art cells, produced fluttering, a phenomena which not only includes the high power peaks of the fluttering, but also causes the continuous bending of the fibers as they become more and more brittle and leads to rapid destruction of the diaphragms.
One of the principal objects of this invention is the substitution of the two feed acid streams used in the prior art, by a single feed stream which pushes the diaphragms which are pervious to the gases and electrolyte against the solid supports of the anode or cathode faces, thereby reducing bending and fluttering of the diaphragms. l have found that the diaphragms can be protected from the mechanical stresses if they are supported as fully as possible on the more or less solid grids of the anode and cathode faces. This results in a longer life for the diaphragm or diaphragms. In addition, the diaphragms according to the present invention may be supported on rigid grids or networks made of resistant material, such as Teflon, Plexiglass and so on. Such a grid increases the electrode gap and the ohmic resistance of the cell by a slight amount, but prolongs the life of the diaphragms.
Another object of the invention is to support the diaphragm or diaphragms on a rectangular parallelepiped of graphite, provided with blades and slots, used as the electrodes, i.e., anodes and cathodes, provided at regular intervals with bore holes or channels approximaely 20 40 mm in diameter, parallel to the large surface planes of the electrodes and preferably parallel to their shorter rectangular dimension, which bore holes conduct the produced gases and depleted electrolyte away from the electrode surfaces and eventually out of the cell. The inside surfaces of the said bore holes are painted with a suitable liquid, for instance, with a cold curing phenolic resin, in order to avoid erosion of the soft graphite by the moving gases and liquids during the operation of the cell. This simple resin painting is very useful, because it allows the use of a lower graphite quality while still providing adequate discharge channels for the discharge of the formed gases and of the spent electrolyte, a portion of which may be recirculated as later described.
In order to keep the surfaces of the graphite blades electrically active, the resin painting, including hardening of the resin layer, should be done afterdrilling of the bore holes, but before cutting the slots on the surfaces of the graphite electrodes.
After the hardening of the rein, a set of saw discs on a common axle is used to cut a multiple of parallet slots on the surfaces of the graphite plates provided as electrodes, the direction of the parallel slots with reference to the bore holes preferably being an angle of approximately although this angle may vary substantially from 90. The slots are of such a depth that they penetrate a few millimeters into the cavities of the bore holes where they cross the bore holes, creating in this way a connection between the bore holes and the cavities of the slots. The slots should have a width of approximately 1 2 mm and a depth of 5 20 mm, while the graphic blades between the slots should have a thickness of approximately 1 3 mm. During electrolysis, the graphite blades are the electrodes (i.e., anodes and cathodes), while the slots receive the two produced gases and discharge them into the corresponding bore holes. The bore holes and the slots in the graphite electrodes may be produced by other methods.
Referring now to the drawings which show preferred forms of embodiment of the invention and some, but not all, of the possible modifications thereof, and are intended only as illustrative embodiments.
FIG. I is a schematic illustration of a unipolar graphite electrode used as a terminal electrode at either end of a bipolar HCI cell assembly;
FIG. 2 shows a similar unipolar graphite electrode with additional slots at right angles to the slots shown in FIG. 1;
FIGS. 3 and 3a are. respectively, a face view and a 7 FIG. II is a vertical sectional view through a bipolar electrode along the line 11 ll of FIG. 13;
FIG. 12 is a vertical section along the line 12 12 of FIG. 11;
FIG. 13 is a horizontal section in diagrammatical outline, of an assembled cell along the line 13 13 of FIG. 11;
FIG. 14 is a detail of one form of electrolyte feed and FIG. 140 shows anotherform of electrolyte feed, using Venetian blinds instead of sheet diaphragms;
FIGS. 15, 16 and 160 are, respectively, a detail, a face view and a side view of a Plexiglass or Teflon frame for holding the diaphragms against the electrode faces;
FIG. 17 is a sectional view along the line 17 17 of FIG. 13;
FIG. 18 is a schematic end view of the electrolyzer shown in FIG. 13, this view being substantially the same from either end of the electrolyzer;
FIG. 19 is a vertical section along the line 19 19 of FIG. 20, parallel to the direction of the slots and graphite blades, showing the electrodes in horizontal position and showing diaphragms on only one face of the cathodes;
FIG. 20 is a horizontal section along the line 20 20 of FIG. 19;
FIG. 21 shows an enlarged vertical section through a slightly different form of electrode package, showing diaphragms in dash lines along both the anodic and cathodic faces of the bipolar electrodes;
FIG. 21a is a cross sectional detail of FIG. 21',
FIG. 22 shows an enlarged view of the top part of a bipolar electrode in horizontal position with the diaphragm omitted;
FIG. 23 is a horizontal section along the line 23 23 of FIG. 19;
FIG. 24 is a view of a sight glass arrangement along the line 24 24 of FIG. 23, for checking the electrolyte level in single cells; and
FIG. 25 shows an example of a bipolar electrolyzer with the electrode faces in horizontal position.
BEST AVAILABLE COPY The unipolar graphite electrode shown schematically in FIGS. 1 and 2, comprises a graphite slab 1 having bore holes 2 for discharging the gas and depleted electrolyte and horizontal slots 3 separated by blades 4. The slots 3 are preferably about I to 2 mm wide and about 5 to 20 mm deep and intersect the bore holes 2, so that gases produced on the electrode and depleted electrolyte flowing into the slots will go into the resin lined bore holes 2 and be discharged at the top of the electrode. The first bore hole in FIG. I has been cut away to show the communication between the slots 3 and the bore holes 2. Vertical slots 3a shown in FIG. 2, may be provided to increase the surface area of the blades 4. The slots 3 and 3a need not be at one to the other. The slots 3a may be at any desired angle to the slots 3, such as 5 to 90. The blades 4 are preferably about I to 3 mm thick.
The electrodes 1 shown in FIGS. 1 and 2, may constitute the positive and negative electrodes of a single cell, or the terminal positive and the terminal negative electrode of a bipolar electrolyzer which may, for example, have forty or more bipolar intermediate electrodes such as shown in FIGS. 3 and 4.
The slots may be formed in the graphite slab 1 in any suitable manner, as by spaced circular saw blades on a common axis, or by building the electrode from individual plates of graphite having holes 2 and having the slots and blades given above therein. The plates are stacked one on top of the other, with the smaller plates between the larger ones and then cemented together to form electrodes of the construction shown in FIGS. 1 and 2.
In use, the electrodes of FIGS. 1, 2 and 3 may be covered with diaphragms which rest on the crest of the graphite blades 4. In FIGS. 1 and 2, the diaphragms have been omitted to give greater clarity to these figures. The direction of the slots 3 and the bore holes 2 need not be, respectively, horizontal and vertical or at 90 each to the other. The slots 3 can, for example, be vertical and the resin lined bore holes 2 horizontal, preferably with a slight upward tilt toward the discharge end of the electrolyzer as will be described hereafter. The crest of the blades 4 may have shallow slots, about 0.1 to 1 mm, cut into the graphite surface before the deeper slots 3 are cut, to give the crest of the blades additional roughness.
FIGS. 3, 3a and 4 illustrate a bipolar electrode 5 suitable for use as an intermediate electrode in a multiple unit bipolar electrolyzer. The front and back faces of the electrode of FIG. 3 are provided with slots 3 and blades 4, as the FIGS. 1 and 2, and the slots 3, shown in dash lines in FIG. 4, communicate with independent sets of bore holes 2 and 2a in the graphite slabs to discharge gases and electrolyte from the anodic and cathodic faces of the electrode through the bore holes.
Further alternative forms of bipolar electrodes are shown schematically in FIGS. 6, 7, 8, 9 and 10, in which the slots 3, shown in dash lines, are in garland or wave form communicating with the bore holes 2 and 20, one of which discharges Cl, and depleted I-ICl acid through the bore holes 2 from the anodic side of the electrodes, and the other holes 20 discharge H, and depleted acid from the cathodic side of these bipolar electrodes, as shown in FIGS. 8, 9 and 10. The wave or garland form of the slots 3 may be produced by circular saw blades moving in and out of graphite slabs l, or this form of electrode may be produced by alternate layers BEST AVAILABLE COPY of thin plates having the wave shape illustrated and rectangular plates forming the blades 4, stacked one on top of the other to the height desired and cemented together. Other methods of forming the alternate slots 3 and blades 4 may be used. In any form of construction, the bore holes 2 and 2a are coated on the inside with a suitable coating such as a cold curing phenolic resin, to reduce erosion of the bore holes by the gases and liquids moving therethrough during operation of the cell.
The bore holes 2 and may be arranged in a single plane as illustrated in FIG. 7 or in a zigzag pattern as illustrated in FIGS. 4 and 6. When the bore holes 2 or 2a are located beyond the center of the electrode, with reference to the anodic face, the average depth of the anodic slots may be made bigger than the slots on the cathodic side of the electrodes, to provide a larger wear dimension of the anodic graphite blades. This form is shown in FIG. 10. Various combinations of slots and bore holes may be made. The slots may be formed by sawing the graphite slabs 1, by assembling and cementing together the stacked layers of slotted plates and rectangular plates or in other ways and the bore holes 2 may be formed by drilling (before the formation of the slots) or by cementing together graphite sections formed by extrusion, block pressing machining or the like, to provide bore holes 2 and 2a in the graphite sections la of FIG. 6.
In use, the bore holes 2 and 2a when in vertical or nearly vertical position, are more or less flooded with electrolyte and in order to facilitate quick release of the gases and downward recirculation of the electrolyte, the bore holes may be subdivided by guide vanes 6 of Teflon, Plexiglass or the like, preferably into two un.- equal parts as illustrated in FIG. 5, so that the gas bubbles rise, together with the electrolyte in the larger section of the bore holes, while the smaller section allows downward recirculation of the electrolyte. A similar result may be secured by providing larger bore holes in the graphite electrodes and parallel smaller holes which communicate with the larger bore holes only near the top and bottom of the electrodes, so that the gas bubbles and electrolyte will rise in the larger bore holes and when the gas bubbles are released at the top of the bore holes the smaller bore holes will provide passage for the downward recirculation of the electrolyte, freed from the gas bubbles.
FIG. 13 is a horizontal sectional plan view along the line 13 13 of FIG. 11, showing an electrolyzer with a number of bipolar electrodes, the negative and the positive terminal electrodes and the gas collector domes 7 and 7a for C12 and H respectively. For clearness, gaps have been left between the electrode components 8, 9, 10, 11 and 12 of the electrolyzer where spacing gaskets are used and the slotted electrode faces 15 and comb-like feed frames 14 (see FIG. 14) for the feed acid are omitted. The slotted electrode faces 15 are in direct contact with diaphragms 13 which have an appropriate permeability for liquid and current; the diaphragms may be deposited polyvinyl chloride or be prefabricated.
Deposited diaphragms may contain loose PVC fibers with the addition, if desired, of other types of fibers, even chemically less resistant, such as asbestos, rayon, cotton, etc, with minor quantities of binders which may be added as liquids, pastes, emulsions, etc.
Prefabricated diaphragms may have the character of fleeces, containing PVC fibers or other resistant fibers, added asbestos fibers, rayon fibers, cotton and other less resistant fibers as well as colloid binders introduced, for example, by impregnation of the fiber fleece with a diluted alcoholic solution of phenolic resin, a solution of alkali silicate or alkali cellulose Xanthogenate or with mixtures of the two latter solutions or of other materials and subsequent treatment with acids, heat, etc.
Other types of prefabricated diaphragms are cloths, such as PVC cloths, impregnated with chemical agents, as above mentioned, in order to give to the diaphragms the correct permeability for the electrolyte. In order to maintain the optimum distance between the cathodic and anodic diaphragms of about 6 to 10 mm, chemically resistant gaskets 14 of this thickness are provided between the flanges of the Haveg frames 16 through surrounding channels 17 in the frames 16 (see FIG. 14). The space between the diaphragms 13 is filled with electrolyte and is the space where the feed acid is fed in.
Deposited diaphragms as well as prefabricated diaphragms can be formed with spray guns as, forinstance, in the case of glass fiber reinforced polymers.
The introduction of the feed acid into the diaphragm interspace is done from side walls or bottom walls of the periphery of the Haveg frames 16, according to the dimension of the cell. As shown in FIG. 11, the feed acid from the horizontal-feed acid channels 18 18 located in the side walls of the elements frames is distributed among all elements 8, 9, 10, ll, 12, etc., of the electrolyzer and through the channels 18a, 18b and 18c of each element frames 16 (see FIG. 11). The feed acid starting from these channels 18 18a, etc. flows into the inner part of the comb-like Plexiglass frames, situated between the gaskets l4 and the space between the diaphragm of a single electrolytic cell, as illustrated by the arrows in FIG. 14.
The thin comb-like Plexiglass frame 14 shown in FIGS. 16 and 16a and in an enlarged detail in FIG. 15, is held fast between the Haveg frame 16 as shown in FIG. 14 all around the frame flanges 16; it is comb-like only on the sides and bottom, while the upper portion of the Plexiglass frame 14 has rectangular holes for the passage of Cl H and depleted acid flowing to the C1 and H manifolds 7 and 7a. The Plexiglass frames are held in position between gaskets all around the cell frames 16 and the gaskets transmit the pressure also to the rims of the two diaphragms holding them in position. In FIG. 14, the two diaphragms 13 are indicated as dash lines on the crests of the graphite blades 4; they extend to the neighborhood of the final C1 and H, distribution channels at the top of the Haveg frames 16.
FIG. 14 shows as a vertical section the lowest parts of vertical bore holes 20 for H .-ln each bore hole, the lower end of the guide sheet 6 can be seen around which the electrolyte stream, coming downwards in the