FI88410C - ELEKTROLYSCELL - Google Patents

Description

1 88410

This invention relates to an improved structure of an electrode for use in an electrochemical membrane cell.

More particularly, the present invention relates to a multilayer electrode used in the commercial production of concentrated hydrogen sulfite solutions in an electrochemical membrane cell.

Many attempts have been made to develop a process for the electrochemical production of alkali metal hydrogen sulfite, e.g., sodium hydrogen sulfite or potassium hydrogen sulfite, which can compete commercially with conventional zinc reduction processes using either sodium amalgam or metallic iron. Electrochemical processes for the preparation of hydrogen sulfites involve the reduction of bisulfite ions to hydrogen sulfite ions. For this process to be economical, the cell must use current densities sufficient to produce concentrated hydrogen sulfite solutions at high current-to-flux ratios.

In addition, when solutions that are strong reducing agents and effective as bleaching agents must be used in the paper industry, the formation of undesired by-product thiosulfate as an impurity from hydrogen sulfite must be minimized. However, at high hydrogen sulfite concentrations, this by-product reaction becomes difficult. to control.

. . In addition, previous electrochemical pathways to hydrogen sulfite have produced aqueous solutions that are unstable and rapidly decompose. This high rate of decomposition of hydrogen sulfide tends to increase as the pH decreases: / ·· or the reaction temperature increases. One way to control the rate of halide distribution is to reduce the time the solution is in the cell and to maintain the current density as high as possible up to a critical current density at the upper side of which secondary reactions occur as a result of cathode polarization.

A few known processes for making hydrogen sulfite salts electrochemically require the use of water-miscible organic solvents, such as methanol, to reduce the solubility of hydrogen sulfite and prevent its decomposition inside the cell. Expensive recovery of methanol and hydrogen sulfite makes this method uneconomical.

10 The use of zinc as a stabilizer for hydrogen sulphites in electrochemical processes has also been reported, but due to environmental factors, this is no longer economically feasible or desirable.

U.S. Patent No. 4,144,146, issued March 13, 1979 to B. Leuthner et al., Describes an electrochemical process for producing hydrogen sulfite solutions in an electrolytic membrane cell. The process uses high rotational speeds for the catholyte passing through the inlet at the bottom of the cell and removed at the top of the cell, so as to provide advantageous removal of the gases produced during the reaction. The catholyte flow over the surface of the cathodes is maintained at a rate of at least 1 cm / s and the cathode is formed of fibrous mats of compressed sintered fibers with a mesh size of 5 mm or less. The process is described as producing concentrated alkali metal hydrogen sulfite solutions at commercially useful current densities; however, the required cell voltages are high, ranging from 5 to 10 volts. This results in excessive energy consumption. There is no mention in publication 30 of the concentrations of thiosulfate impurity in product solutions.

The availability of electrodes with high mass-to-mass transfer surface area, high surface area to volume ratio, and sufficient porosity has limited the development of a commercially viable electrochemical cell model for the production of aqueous alkali metal sulfites 3,88410 with low alkali metal sulphate sulfates. as impurities.

These and other problems are solved in the model of the present invention using an improved multilayer 5 electrode in an electrolytic film cell for the production of alkali metal hydrogen sulfite.

It is an object of the present invention to provide an improved multilayer electrode for use in an electrochemical cell.

Another object of the present invention is to provide an improved multilayer electrode which is highly porous and which causes a low total pressure drop in the electrolyte fluid flowing through the electrolyte.

It is a further object of the present invention to provide an improved multilayer cathode for use in an electrochemical membrane cell that produces aqueous alkali metal hydrogen sulfite solutions with lower concentrations of alkali metal thiosulfate impurities at high current densities.

It is a feature of the present invention that the improved multilayer electrode has high oxidation and reduction selectivity at high current densities in a non-gas generating reaction.

Another feature of the present invention is that the improved multilayer electrode has at least a ____. the first layer of fibers and the second less tii-. . vis a layer of fibers compressed and joined together.

*; Yet another feature of the present invention is that the flow path of the catholyte forces the catholyte to make multiple passages through a multilayer porous cathode formed of sintered wire fibers disposed in a perforated support layer or sheet. ; and the network.

Yet another feature of the present invention is that a cathode flow barrier is used to direct the catholyte flow through the cathode.

4,88410

An advantage of the present invention is that a high oxidation / reduction selectivity is achieved at high current densities in an electrolytic film cell using a multilayer electrode.

Another advantage of the present invention is that there is a low horizontal drop in the total electrolyte pressure across the multilayer electrode as the electrolyte makes several passes through the electrode.

Yet another advantage of the present invention is that the low total pressure drop of the electrolyte and the multilayer electrode design do not require the use of a high pressure pump to recycle the electrolyte.

These and other objects, features and advantages of the invention are provided in an electrolytic film cell which electrically produces alkali metal hydrogen sulfite by electrochemically reducing the alkali metal bisulfite component of a recycled aqueous catholyte solution in a cell support layer, first fiber layer, second fiber layer, and mesh layer.

The objects, features and advantages of the invention will become apparent upon examination of the following detailed description of the invention, particularly when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic exploded view of the electrolyte cell 10 showing electrolyte flow paths and ion flow paths; Fig. 2 is an elevational view of the anode side of a bipolar cell electrode showing a portion of the anode rods covering the anode backplate with some of these rods shown cut away;

Fig. 3 is an enlarged partial sectional view 35 taken along lines 3-3 of Fig. 2 showing the anode rods when attached to the electrode; 5,88410

Figure 4 is a side elevational view of the roof side of the bipolar electrode?

Fig. 5 is a side sectional view of the bipolar electrode element of the electrolyte cell showing the flow path of the catholyte 5 through the porous cathode in the cathode compartment from the catholyte distribution grooves to the catholyte collection slots or conduits;

Figure 6 is a side elevational view of a separator network interposed between the anode rods and the membrane; and Fig. 7 is a partial side sectional view of the cathode plate showing many layers of the electrode structure.

As seen in an exploded and partially schematic representation in Figure 1, a filter compression type membrane electrolyte cell, generally shown at 10, is shown as consisting of an anode backplate 11, separator means 21, cation selective membrane 25, porous cathode plate 26 and cathode plate 26.

The anode back plate 11 and the cathode back plate 28 form opposite sides of the bipolar electrode, which may be machined from a stainless steel plate or cast from stainless steel. For example, the stainless steel plate may be formed of 304L or 316 type stainless steel, 31.7 mm thick, which is corrosion resistant and is simply made by machining a flat plate to form chambers through which anolyte and catholyte fluids can pass through them. anolyte and catholyte chambers. The thickness of the stainless steel plate provides the rigidity of the structure and extremely precise evenness.

The cathode plate 26 is mounted on the cathode plate 28 with screws (not shown) screwed to the cathode support bases 31, while the anode rods 12 may be welded, for example by TIG welding, to a stainless steel plate. without twisting.

1 35 The anode structure can be seen in more detail in Figures 2-4. As shown in Fig. 2, the anode rear plate 11,88410 has a plurality of parallel, vertically extending anode rods 12 welded in the upper and lower portions of the rods to the anode plate 11. These rods 12 extend across the entire width of the anode plate 11, although for the sake of simplicity of representation. the continuous side-by-side arrangement is not shown in Fig. 2, because the rods in the center of the anode back plate 11 have been completely removed. These rods are, for example, nickel wire rods with a diameter of 3.17 mm 10 and spaced apart so that a gap 20 of about 1.59 mm is formed between the adjacent rods. These anode rods 12 may be formed of nickel 200 or some other corrosion resistant compound that provides low overvoltage properties. Vertical placement of the anode rods 12 with the gap 20 between the anode rods, cf. briefly, Figure 3, provides clear flow channels at the base of the anode back plate 11 where the anolyte fluid enters through the anolyte inlet ports 18 into the top of the anolyte distribution groove 15. The anolyte fluid flows vertically upward in the gaps 20 between the anode rods into the anolyte collection groove 16 before the fluid exits the cell through the anolyte outlet ports 19. The vertical placement of the anode rods 12 provides a uniform current distribution across the anode and prevents gas clogging that can result from the formation of gas bubbles, which can thus reduce the current density in a functioning cell.

Both the anolyte inlet ports 18 and the anolyte outlet ports 19 have transition slots 18 'and 19' respectively, which are machined into a stainless steel plate. The anolyte inlet transition slots 18 'are machined into the anolyte distribution groove 15 to provide a smooth transition surface that is conical and avoids erosion: corrosion, which can interfere with the smooth flow of anolyte to cell 10, and can cause metal contamination when erosion and corrosion occur. The transition notches 19 'of the anolyte outlet 7,88410 are both similarly positioned and machined.

The anode seal groove 14 is machined into the anode back plate 11 around its entire peripheral area. For example, the groove is 9.5 mm wide and 4.8 mm deep to receive a rectangular anode seal (not shown) that is 9.5 mm wide and 9.5 mm deep. This seal may have a material sludge, for example a material sold under the trademark GORE-TEX or TEFLON, placed on the seal 10 to come into contact with the plastic separator means 21 when the cell is compressed and assembled.

The plastic separator means 21 is formed of any material resistant to corrosion of the anolyte, and preferably polypropylene is used. An 8 mesh poly-15 polypropylene product with an open area of about 40% has been used successfully, as has a polyethylene mesh filled with titanium dioxide. The separator means 21 has a separator frame 22 fixedly around the edge area and a separator network 24 on the inside of the separator frame 22. The mesh 24 has been treated with a hydrophilic coating to prevent gas bubbles from adhering to the mesh and the adjacent membrane due to the capillary effect. Titanium dioxide coating attached. network 24, has been successfully used as a hydrophilic coating. By preventing the formation of gas bubbles in the ____: membrane and network, fluctuations in the cell voltage during operation are avoided.

. . The use of the separator means 21 is also successful; ; the formation of locally high acidity regions adjacent to the film when the film contacts the nickel anode rods 12. The fact that the film 25 contacts the nickel anode rods 12 can create pockets with high acidity because the sulfur species are oxidized by sulfuric acid. box as a result of the slow passage of sulfur species 35 back through the membrane during cell operation. The nickel oxide coating on the anode tabs 12 decomposes and 8 88410 nickel corrosion occurs. This corrosion passes through the membrane to the cathode side of the cell 10. There, this nickel corrosion is reduced to the metallic space with a hydrogen sulfite solution. This nickel in the metal space adheres tightly to the film on the cathode side and impairs the transfer of ions and liquid through the film.

The anode is designed so that the anolyte electrolyzed in the cell 10 is any suitable electrolyte capable of feeding alkali metal ions and water molecules into the cathode compartment. Suitable anolyte is, for example, alkali metal halides, alkali metal hydroxides or alkali metal persulphates. The choice of anolyte depends in part on the desired product.

When a halogen gas such as chlorine or bromine is desired, an aqueous solution of an alkali metal chloride or bromide is used as the anolyte. Alkali metal hydroxide solutions are selected when producing oxygen gas or hydrogen peroxide. If sulfuric acid is the desired product, alkali metal persulfate is used. However, alternative materials of construction 20, such as titanium group metals for the moieties moistened with the anolyte with the alkali metal chloride anolyte, would be required for each particular anolyte used.

In each case, light solutions of the selected electrolyte are used as the anolyte. For example, when sodium chloride is selected as the alkali metal chloride, suitable solutions for the anolyte contain about 12 to 25% by weight NaCl. Alkali metal hydroxide solutions, such as sodium hydroxide, contain about 5-40% by weight NaOH.

Cell 10 has been used primarily with alkaline soda. When alkaline soda (NaOH) is used, water and alkaline soda enter through the anolyte inlet 18 and the solution flows along a high velocity flow path between the slits 20 between adjacent anode rods 12 and 35 at the rear of the anolyte compartment toward the top of cell 10. Thus, most of the volume flow of 9,88410 anolyte fluid occurs between the anode tanks 12 and within the hydrophilically treated separator network. Sodium ions migrate across the membrane as a result of the electrolysis reaction 5 to form oxygen, water and sodium ions, 4NaOH -> 02 + 4Na + + 2H2O

The spent base passes out with oxygen and water through the anolyte outlets 19.

The cathode backplate 28 is best seen in Figure 4, while the monolithic structure of an electrode machined from a solid stainless steel plate can be seen in Figure 5. Because the cell is bipolar, the cathode is on one side of the stainless steel plate on the roof plate 28, while the anode backplate 11 and the anodes are on the opposite side. As best seen in Figure 4, the cathode backplate has catholyte inlets 35 on opposite sides 20 of the bottom of the cathode backplate 28 which feed the catholyte into the catholyte distribution groove 32. The catholyte distribution groove 32, catholyte inlet ports 35, and machined cathodes 35 just above the respective anolyte distribution grooves., 15, anolyte openings 18 and anolyte transition notches 18 '25, but on the opposite side of the solid stainless steel electrode plate.

] The lower catholyte chamber 38 is located immediately above the catholyte distribution groove 32. The lower catholyte chamber 38 is separated from the upper catholyte chamber 30 by a substantially horizontal cathode flow barrier 30. The flow barrier 30 extends over the entire width of the catholyte chamber and projects outwardly from the plane of the catholyte backplate 28, as can be seen. also in Figures 1 and 5. The cathode flow barrier 30 interrupts ‘· *; 35 vertical flow of catholyte fluid upward from the lower catholyte chamber 38 to the upper catholyte chamber 39 10 8841 0, thereby causing the catholyte fluid to flow through the path shown by the arrows in Figure 1, which route it twice through the cathode plate 26 on its way to upper catholyte chamber 39. effective surface area, but requires the use of a highly porous cathode plate that allows at least 30% by volume of catholyte fluid to flow through the porous cathode plate 26 while maintaining a minimum residence time of the catholyte in the cell. As will be explained in more detail below, once the catholyte fluid has reached the upper catholyte chamber 39, it enters the catholyte collection groove 34 and exits the cell through machined catholyte outlet slots 36 'and catholyte outlets 36.

The trickle holes 17, which can be seen in Figures 4 and 5, can be used in the cathode flow barrier 30 to allow hydrogen gas to rise from the lower catholyte chamber 38 to the upper catholyte chamber 39. Alternatively or simultaneously, trickle holes 33 shown in Figure 5 can be used to allow hydrogen gas to pass and between the walls of the upper catholyte chamber 38 and 39 and the cathode plate 26 just below the cathode flow barrier 30 and then back through the cathode plate 26 opposite the catholyte collection groove 34.

The cathode plate 26 is held in place on the catholyte caliper plate 28 by a plurality of screws (not shown) located in the plurality of cathode support supports 31 within the lower and upper catholyte chambers 38 and 39.

The cathode plate 26 is a very porous multilayer structure. It comprises a support layer formed of torn stainless steel. This backing layer forms a mounting base and protects the inner metal fiber felt layer, which is formed of, for example, 15% ti-35 fibers, very fine 4-8 micron fibers, and 15% dense 25 micron fibers superimposed on another 88410. For example, an 18 mesh wire mesh with a wire diameter of 0.009 inches is then placed on top of the fibrous felt to form a cathode having a porosity of preferably between 80 and 85%. The cathode-5 plate 26 is thus a 4-layer sintered composite in which all materials are made of stainless steel, preferably 304 or 316 stainless steel, and of a suitable plate size. A very efficient surface area of the cathode plate 26 is achieved by using a low-density metal felt formed of very fine elements.

The groove 29 of the cathode seal is seen in Figure 4 extending around the edge region of the cathode backplate 28. Although not shown, a 9.53 mm round EPDM, 15 ethylene propylene diene monomer sealant has been used to fit in the groove 29 of the cathode seal to provide a liquid seal.

Reduction occurs at the cathode in cell 10 due to electrolysis of a buffered aqueous solution 20 of alkali metal bisulfite. A typical reduction is as follows: 4NaHSO3 + 2e "+ 2Na + - ^ Na2S2C> 4 + 2Na2SO3 + 2H2O.

25

The spent base and sulfur dioxide are mixed to form NaHSO 3, which is fed into the catholyte distribution groove 32 through the catholyte inlets 35 and the catholyte transition slots 35 '. This catholyte fluid rises vertically upwards until it passes through the cathode plate 26, as can best be seen in the figures. . 5 or 1. The cathode flow barrier 30 acts as a barrier to direct vertical flow of catholyte fluid upward from the lower "*" catholyte chamber 38 to the upper catholyte chamber 39.

35 Upper and lower catholyte chambers 38 and 39 and roof ·; ··; there is a cathode gap between the electrodes of about 3.18 mm between the walls of the cathode support 26 placed on the cathode support bases 31, 12 8841 0. The catholyte fluid then passes through the cathode plate 26 and continues to flow upward through the cathode film gap until it reaches the cathode flow barrier 30.

At this point, the catholyte fluid passes through the highly porous cathode plate 26 back to the upper catholyte chamber 39 and then to the catholyte collection groove 34. The cell product solution containing Na 2 S 2 O 4 (dithionate) exits the cell 10 through the catholyte outlet ports 36 'and the outlet port 10'.

A buffer solution containing about 40 to 80 g / l of bisulfite is used with the catholyte due to the formation of sodium thiosulfate resulting from the reduction and decomposition of hydrogen sulfite (dithionate) and the change in pH of the catholyte as the bisulfite is consumed and the sulfite is formed in the next reaction. in accordance with

Na 2 S 2 O 4 + 2e ~ + 2Na + + 2NaHSO 3 -? Na 2 S 2 O 3 + 2Na 2 SO 3 + H 2 O.

This hydrogen sulfite decomposition reaction is electrically controlled in the presence of electrons. As the potential is increased, the current density and, to a certain point, the reaction rate of this undesired thiosulfate-producing reaction are also increased.

The value of the multilayer cathode plate 26 is particularly indicative of its selectivity. Because the multilayer electrode has an increased surface area, it needs a lower voltage or lower potential to control the primary reaction to produce the desired hydrogen sulfite product, thereby reducing the amount of undesired thiosulfate that the hydrogen sulfite decomposes. reaction produces. The increased surface area allows the potential to be kept at a lower level, whereby the primary or desired hydrogen sulfite-producing reaction is 13 8 8 41 0 predominant and substantially below the level at which the hydrogen sulfite decomposition reaction becomes a factor.

The use of a monolithic cell body, i.e., a bipolar cell body or backplate formed of a single 5 stainless steel plate machined into an anode backplate on one side and an anode backplate on the opposite side, provides several significant congenital operating advantages. First, there is no displacement or dimensional instability of any kind due to the joining of two separate pieces of material to form the electrode. The use of a single machined plate also results in a reduction in the number of actual cell components. The most recent, and perhaps most significant, is the elimination of electrical loss from contact between two separate anode and cathode elements that would otherwise have some differences in placement and size. This special structure contributes to the lower electrical energy consumption of the cell.

The hydraulic pressure in the cell 10 is formed by keeping the membrane 25 pressed against the separator means 21 and away from the cathode plate 26. Keeping the membrane 25 in this position also allows a flow path through the cathode plate. The cathode flow barrier 30 further contributes to the hydraulics of the cell 10 by achieving a constant pressure over the entire height of the cathode as a result of the flow inversion characteristics achieved through the plurality of flow paths through the cathode plate 26.

The electrolyte cell 10 operates at current densities, V. ', which are sufficient to produce alkali metal hydrogen sulfite solutions having the desired concentrations. For example, when producing sodium hydrogen sulfite for commercial sale, the solutions contain about 120 to 160 g / l. However, since the alkali metal hydrogen sulfite solutions commercially sold are usually diluted before use, these dilute hydrogen-containing solutions can also be produced directly by the process.

14 8841 0 2 Current densities of at least 0.5 kA / τη are used. Preferably, the current density is in the range of about 1.0 to 4.5, and even more preferably about 2.0 to 3.0 kA / m. At these high current densities, the electrolyte cell 10 operates to produce the required amount of a very pure alkali metal hydrogen sulfite solution that can be used commercially without further concentration or purification.

The electrolytic membrane cell 10 uses a cation exchange membrane between the anode and cathode compartments, which prevents all substantial migration of sulfur-containing ions from the cathode compartment to the anode compartment. A wide variety of cation exchange films containing a variety of polymeric resins and functional groups can be used, assuming that the films have the required sulfur ion selectivity to prevent sulfur ion precipitation within the films. Such precipitation can clog the membranes as a result of diffusion of sulfur species through the membranes and subsequent oxidation to form an acid within the membranes, which acid causes hydrogen sulfide and thiosulfate to decompose to sulfur under acidic conditions. This selectivity can be confirmed by analyzing the anolyte for sulfate ions.

Suitable cation exchange membranes are those that are inert, flexible, and substantially impermeable to the hydrodynamic flow of electrolyte and the passage of gaseous products produced in the cell. Cation exchange membranes are known to contain solid anionic active groups that allow cation penetration and exchange and exclude anions from an external source. In general, the resin film has a matrix or crosslinked polymer associated with charged wastes, such as —SO-, —COO-, —PO-, —hpO 2 '- ^ AsO 3 3a -SeO 3, and mixtures thereof. Resins that can be used to form films include, for example, fluorocarbons, vinyl compounds, polyolefins, and copolymers thereof. Preferred are cation exchange films which are 88410 formed from fluorocarbon polymers having several dependent sulfonic acid groups or carboxylic acid groups or mixtures of sulfonic acid groups and carboxylic acid groups. The terms "sulfonic acid group" and "carboxylic acid-5 groups" are intended to include sulfonic acid salts or carboxylic acid salts in processes such as hydrolysis. Suitable cation exchange films are sold commercially by E.I. DuPont de Nemours & Co., Inc., under the trademark "Nafion", Asahi Glass Company under the trademark "Flemion", Asahi Chemical Company under the trademark "Aciplex". Perfluorinated sulfonic acid films are also available from Dow Chemical Company.

The membrane 25 is interposed between the anode and the cathode and separated from the cathode by a cathode film gap wide enough to allow the catholyte to flow between the cathode plate 26 and the membrane 25 from the lower catholyte chamber 38 to the upper catholyte chamber 39 and prevent gas clogging, but not wide enough. Depending on the shape of the cathode plate 26 used, this cathode film gap 20 is a distance of about 0.05 to 10 mm, and preferably about 1 to 4 mm. The cathode film gap can be maintained by hydraulic pressure or mechanical means. This pattern and the catholyte flow path allow almost all of the catholyte fluid to contact the 25 active areas of the cathode. In addition, in connection with this model, most of the electrolytic reaction occurs in the cathode region closest to the anode.

* '* · Suitable porous baffles 26 used in the cell 10 have at least one layer with a total surface area to volume ratio of greater than 100 cm / cm, 2 3 preferably 250 cm / cm and even more preferably larger. 2 3: than 500 cm / cm. These structures have a porosity of at least 60% and preferably about 70-90%, where the porosity is% of the pore volume. The ratio of the total surface area to the working area of the porous cathode plate 26, where the working area is the facade area of the cathode plate 26, is at least about 16,88410 30: 1 and preferably at least about 50: 1; for example, from about 80: 1 to 100: 1.

The cathode plate 26 consists of four layers, as best seen in Figure 7. The first layer is a support layer 41, which is a perforated stainless steel plate approximately 0.91 mm thick and with 1.59 mm holes 42 3, 18 mm, with 60 ° stepped centers. These holes in the first layer 41 give the perforated stainless steel sheet an open area of approximately 23%. The second layer 44 is preferably formed of fibers 45 of stainless steel 304 having a diameter of 20 to 100 and preferably about 25. The second layer 44 has a density of about 0.00297 N / cm. Alternatively, the second layer 15 may be a woven mesh having a square web of 30 fibers / 25.5 mm x 30 fibers / 25.4 mm and an open area of about 23%. The third layer 48 is much less dense than the second layer 44 and is comprised of stainless steel fibers 304 having a diameter of about 4 to 16 and preferably about 8. The density of the third layer 48 is about 0.00575 N / cm 2. A limiting factor in sizing the diameter of the fibers of the second layer 44 and the third layer 48 is that the fibers 45 of the second layer 44 cannot extend through the fibers of the third layer 48. The fourth layer 50 is a mesh metal fabric, preferably 18 x 18 mesh, with the individual wires of the metal fabric having a diameter of about 0.28 mm. The fourth layers 41, 44, 48 and 50 are pressed together and joined by sintering in a reducing environment such as hydrogen, ammonia or carbon monoxide to form a single sheet having a preferred thickness of about 3.94 mm t about 0. , 20 mm.

The cathode plate 26 of this model provides a high mass transfer capacity because the surface area of the electrode is the same as the pin-35 ta area used for electrolysis. Because the cathode plate 26 is relatively thin, it requires a large surface area to volume ratio, thus forming a large total surface area. This is especially applicable to the third layer, which is the most active layer and critical in determining cathode properties. The large area 26 of the cathode plate 5 is measured as the ratio of the total area of the individual fibers 45, 49 to the total or working area of the entire cathode plate 26. The relationship between these parameters can be expressed so that the thickness of the individual layer of the cathode plate 26 times the surface area to volume ratio is 10 equal to the outer surface area. Selecting the fibers 45 of the second layer 44 with a larger single diameter than the fibers 49 of the third layer 48 and equal porosity provides a structure with a larger surface area near the film 45. The fibers 45 and fibers 15 may just as well be selected from a variety of stainless steel, nickel , steel, copper, carbon-graphite or ferrous and non-ferrous alloys. The cathode plates 26 could have an electrically active coating, such as ruthenium oxide 20 or platinum, added to the surface area to provide a more active electrode surface area.

Current is supplied to cell 10 through anode and cathode current conductor plates (not shown). Discs that are. copper and electrode sizes, are placed against the end cap and the end anode in each cell 10. The electrical connections are made directly to these copper plates. An insulating plate made of, for example, polyvinyl chloride or other suitable plastic and a pressing plate (neither shown) made of, for example, stainless steel or steel are placed against each end of the cell 10 before being assembled. to form a corrugated structure around the desired number of electrodes placed between them.

The cell of the present invention could also be designed. *: 35 The bogie is unipolar, which requires that both sides of each stainless steel plate be treated in the same way and that the half electrodes be used as end electrodes in the assembled cell. The current conductors in the single-pole model would then be conventional copper electrical connectors for each electrode.

In addition, the cell of the present invention could be used in electrochemical reactions other than the production of hydrogen sulfite. Typical is the preparation of organic products by electrochemistry, such as the electrochemical transformations of pyridines through oxidation or reduction reactions in a cation exchange membrane cell of the present design.

Using the new design of cell 10, concentrated alkali metal hydrogen sulfite solutions with low concentrations of early metal thiosulfate as an impurity are produced in electrolytic membrane cells operating at high current densities, substantially reduced cell voltages, and high current efficiencies.

In order to show the results obtained by way of example, the following examples are given without intending to limit the scope of the present invention to that shown therein.

Example 1

A cell of the type shown in Figures 1-5 was assembled from three stainless steel plates mounted on a rack to form two anode / cathode pairs, each with an active electrode area of about: 2, 0.172 m. The plates formed two half-electrodes, one a cathode and the other an anode, deposited around a bipolar electrode with opposite anode-30 and cathode sides. The outer dimensions of the electrode plates were; width about 43 cm, height about 47 cm and thickness about: 2.5 cm.

The anodes were formed of about 47 nickel 200 rods with a diameter of 3.18 mm and 35 welded to the anode back plate, as shown generally in Figure 2, with a spacing between the rods of about 19,841,460 mm. The anolyte collection and distribution grooves were about 31.75 mm wide and about 15.50 mm deep.

The cathode plate was formed of a 4-layer plate cut to the correct size. The first layer 5 was a support layer formed of torn stainless steel with a thickness of 0.914 mm and holes 1.59 mm with 3.16 mm, 60 ° stepped centers 2 and a 23% open area. The second layer was a layer having a density of 0.00297 N / cm, consisting of stainless steel fibers 10,304 with a diameter of about 25 μm.

2

The third layer was a layer having a density of 0.00575 N / cm, consisting of stainless steel fibers 304 with a diameter of about 8 μm. The fourth layer was an 18 x 18 inch mesh formed of a metal fabric having a diameter of about 0.23 mm. These layers were compressed and joined by sintering under a hydrogen environment to form a single sheet about 3.94 mm thick. The cathode plate was cut to form a cathode plate that was approximately 47.0 cm x 43.2 cm.

The cathode plate was mounted on a stainless steel cathode back plate using 20 screws with a diameter of about 3.2 mm, which were placed on the cathode support bases inside the catholyte chambers. A small coating of a suitable electrical connector was applied to the threads of the screws and a silicone adhesive was placed over the end of each screw to prevent the screws from becoming an active part of the cathode assembly.

·: * ·· Six holes with a diameter of 4.2 mm were drilled in the cathode plate to allow inside the cell. Removal of 30 trapped gas bubbles. Three of these holes were drilled near the top of the cell opposite. . catholyte collection groove and three just below the cathode flow barrier.

* - '* Separator means were formed of polypropylene mesh treated with a titanium dioxide coating.

The separators were placed in 1.59 mm thick separator frames, 20 S8410, which had been cut just to fit inside the seal-slaughter in the cell.

Sealing grooves about 9.53 mm wide and about 4.75 mm deep were machined into both anode and cathode-5 backplates. On the anode side of the cell, an approximately 9.53 mm square seal was used in conjunction with an approximately 12.7 mm wide strip formed of an approximately 1.52 mm thick R

GORE-TEX sealing tape placed on top of the seal. A rubber O-ren-10 gas having a diameter of about 9.60 mm was used in the cathode sealing groove. The cell was assembled using the adjustable hydraulic assembly system described in U.S. Patent 4,430,179, which compressed the cell so that a gap of approximately 3.18 mm between the anode and cathode plates was maintained. The cell was then secured with locknuts.

The cell was used continuously for 42 days. Cell p used a NAFION NX 906 perfluorinated membrane soaked in about 2% sodium hydroxide solution for at least four hours before assembly.

The cell operated at a temperature of approximately 25 ° C with a total catholyte flow rate of about 6 g / min and a total anolyte flow rate of about 4 g / min. Excess anolyte containing about 19% sodium hydroxide was continuously purified and added to the catholyte circuit while the anolyte was continuously replenished by the addition of about 35% sodium hydroxide solution at about 69 g per minute. About 230 ml of deionized water per minute was continuously added to the catholyte, as was sulfur dioxide added to the catholyte to maintain a pH between about 5.4 and 5.8 and a sulfite-bisulfite molar ratio between about 1: 3 and 1: 8.

The product catalyst was aspirated from the cell continuously at a rate of about 287 ml per minute and analyzed periodically over each day. The product catalyst, 2i 88410 shown in Table I below, was analyzed from samples taken at the same time each day. This data represents the operation of the cell during 4 days of operation under optimal conditions. The content of sodium hydrogen sulfite, sodium thiosulfate, sodium sulfite and sodium bisulfite was analyzed from the catholyte 5.

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Example 2

A cell similar to the model of Example 1 was assembled using nine bipolar electrode plates and two semi-electrode plates, one as an anode and one as a cathode, each with an active electrode area of about 0.051 m. The same type of cathode plate and anode rods as in Example 1 were used, except that the anode and cathode back plates were about 34.3 cm x 34.3 cm thick and about 3.02 mm thick. Behavior 10 contained a perfluorinated sulfonic acid film having a thickness of about 0.05 mm and an equivalent weight of about 1,000 (g / g molar equivalent exchange capacity) available from Applicant for U.S. Patent 4,470,888.

The separating means were a mesh made of polyethylene used with titanium dioxide with a mesh about 1.78 mm thick and openings of about 9.65 mm and an open area of about 60%. The separator was treated with a mixture of chromium and sulfuric acids available from Fisher Scientific under the name CHROMERGE to provide the necessary hydrophilic surface. The separator grid was mounted on a 3.2 mm separator frame extending approximately 6.4 mm past the edge of the cell.

The cell was sealed using O-rings with a diameter of both anode and ka-todita plate. The 25th place was about 7.37 mm. Approximately 22.23 mm strip G0RE-TEXR

the tape was used between the separator frame and the film.

· * · 'The cell operated at a total catholyte flow rate of 13 g per minute and a total anolyte flow rate of 6 g per minute. 35% sodium hydroxide solution was continuously added to the anolyte at 93 g per minute. Excess anolyte containing about 15% sodium hydroxide was continuously purified and added to the catholyte circuit. : system. In addition, about 320 ml of deionized water per minute was added to the catholyte, while sulfur dioxide 35 was continuously added to the catholyte to maintain the pH.

I have about 5.4 to 5.8 and a sulfite-bisulfite molar ratio of between about 1: 3 and about 1: 8.

The cell operated at a temperature of about 25 ° C, with a total catalyst flow rate of about 13 g mi-5 minutes and a total anolyte flow rate of about 6 g per minute. The cell operated continuously for more than 30 days without a significant change in voltage factor or product formation.

The product catalyst was continuously removed from cell no-10 at a rate of about 350 ml per minute and analyzed periodically during each day. The product catalyst, shown in Table II below, was analyzed from samples taken at the same time each day. These data represent cell performance during 4-day operation under optimized conditions. The catholyte was analyzed for sodium hydrogen sulfite, sodium thiosulfate, sodium sulfite, and sodium bisulfite.

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Although the preferred structure has been shown and described above, incorporating the principles of the foregoing invention, it is to be understood that the invention is not limited to the particular details thus set forth, but in fact very different means may be used to practice the broader ideas of the invention. For example, although the anode backplate has been shown and described to use circular wire rods on its surface, flat rectangular beams or otherwise structures of suitable geometric shape, such as triangular, pentagonal, hexagonal, octagonal, etc., could equally well be used. such additives could be present in the electrolysis. The separator network could also be mounted in a cell between the membrane and the cathode plate, in which case the hydraulic pressure is changed by forcing the membrane away from the anode rods and against the separator network. The scope of the appended claims is intended to cover all obvious changes in details, materials, and arrangement of parts that come to mind to one skilled in the art upon reading this disclosure.

Claims (27)

An electrolytic cell (10) having a top and a bottom and an anolyte and a catholyte flowing therethrough, characterized in that it comprises in combination (a) an anode (11); (b) a cation exchange membrane (25) adjacent to the anode (11); (c) separator means (21) between the anode (11) and the membrane (25) to prevent the membrane from contacting the anode (H); (d) a porous multilayer cathode plate (26) having a first surface (48) adjacent the film (25) and an opposite second surface, the second surface being a support layer (41) of perforated material overlying the second fibrous layer (44); ) and a third fibrous layer (48) on top of this second layer, which is less dense than the second layer (44) and which forms at least a part of the first surface (48) adjacent to the film; and (e) a cathode backplate (28) adjacent the opposite second surface of the cathode plate and having a flow barrier (30) extending substantially horizontally across the cathode backplate defining an upper catholyte chamber (39) 25 and a lower catholyte chamber (38), thereby interrupting the flow barrier (30) a substantially vertical flow of catholyte from the bottom of the cell (10) to the top of the cell, causing substantially all of the catholyte to change direction and pass twice through the porous cathode plate (26) transversely to the first surface and opposite the second surface of the cathode plate (28); and exits the cell (10). A cell according to claim 1, characterized in that the catholyte enters the cell 35 (10) via at least one catholyte inlet (35) which feeds into the lower catholyte chamber (38). A cell according to claim 2, characterized in that the at least one catholyte inlet (35) further feeds the catholyte into the distribution slot (32) through a conical transition slot (35 '). A cell according to claim 3, characterized in that the catholyte exits the cell (10) through at least one catholyte outlet (36). A cell according to claim 4, characterized in that the catholyte flow barrier (30) further has at least one gas trickle hole (17) extending substantially vertically therethrough, connecting directly to the upper catholyte chamber (39) directly to the upper catholyte chamber (39) to allow gas passing through. The cell of claim 5, characterized in that the anode further comprises an anode back plate (11) having at least one anolyte inlet (18) for introducing anolyte into the cell and at least one anolyte outlet (19) for removing anolyte from the cell. A cell according to claim 6, characterized in that the at least one anolyte inlet opening (18) further feeds the anolyte into the distribution groove (15) through at least one conical anolyte transition notch (18 '). A cell according to claim 7, characterized in that the anode further comprises a plurality of anode rods (12) extending between the anolyte distribution groove (15) and the anolyte collection groove (16), which are parallel and vertically aligned. The cell of claim 8, characterized in that the catholyte further comprises a buffered aqueous alkali metal bisulfite solution. A cell according to claim 9, characterized in that the alkali metal bisulfite is sodium bisulfite. A cell according to claim 10, characterized in that the anolyte comprises a mixture of sodium hydroxide and deionized water. An electrode (26) compressed and sintered in a reducing environment for use in an electrochemical cell, characterized in that it comprises in combination at least: (a) a support layer (41) formed of a perforated material; (b) on top of the support layer of the second fiber layer (44)? and (c) a third fiber layer (48), the fibers (49) being smaller in diameter than the fibers (45) of the second layer 15 (44) and disposed on the second layer. Electrode according to Claim 12, characterized in that the support layer (41) of perforated material has an open area of approximately 23%. An electrode according to claim 12, characterized in that the individual fibers (45) in the second fiber layer (44) have a diameter of about 20 to 100 μm. An electrode according to claim 14, characterized in that the individual fibers (45) in the second fiber layer (44) have a diameter of about 25 μιη. Electrode according to Claim 12, characterized in that the fibers 30 (45) of the second layer (44) are a woven mesh. An electrode according to claim 14, characterized in that the second fibrous layer (44) has a density of about 0.00297 N / cm 2. An electrode according to claim 14, characterized in that the individual fibers (49) in the third fiber layer (48) have a diameter of about 4 to 16 μm. An electrode according to claim 18, characterized in that the individual fibers (49) in the third fiber layer (48) have a diameter of about 8 μπι. An electrode according to claim 17, characterized in that the third fiber layer (48) has a density of about 0.00575 N / cm 2. An electrode according to claim 12, characterized in that the fibers of the second layer (44) and the third layer (48) are selected from the group consisting of stainless steel, nickel, steel, copper and carbon graphite. The electrode of claim 12, characterized in that the electrode further comprises a fourth layer (50) on top of the third fibrous layer, which is a reticulated metal fabric. An electrode according to claim 22, characterized in that the electrode is a cathode. An electrode according to claim 23, characterized in that the ratio of the total area of the electrode to the working area is at least about 30: 1. Electrode according to Claim 24, characterized in that the total surface area to volume ratio of the third fiber layer (48) is greater than 100 cm 2 / cm 2. An electrode according to claim 25, characterized in that the porosity of the electrode is at least about 60%. An electrode according to claim 26, characterized in that the porosity of the electrode is about 70 to 90%. 3i 8 8 410 Patentkrav ϊ