JPH0153355B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention is capable of exchanging cations,
A method for the electrolysis of an aqueous alkali metal chloride solution in an electrolytic cell having a gas- and liquid-permeable anode and a gas- and liquid-permeable cathode separated by a membrane substantially impermeable to fluid flow, comprising: an alkali metal chloride aqueous solution is supplied to the anode, and water is supplied to the cathode, while maintaining the space between the surface of the anode and the surface of the cathode substantially at the thickness of the film. This invention relates to an electrolytic method. Recently, electrolytic cells using ion exchange membranes in place of traditional asbestos membranes have been developed, particularly for the electrolysis of brine. Cation exchange membranes are electrically conductive to electrolytes in the operating state, but are impermeable to fluids such as liquids and gases. In operation,
When an alkali metal halide is introduced into the anode, gaseous halogen is generated on the surface of the anode. Alkali metal ions selectively pass through the cation exchange membrane,
Alkali metal ions are combined with hydroxide ions generated at the cathode by electrolysis of water to form alkali metal hydroxide. Electrolysers with cation exchange membranes offer many benefits over conventional diaphragm electrolysers. Electrolysers with cation exchange membranes produce relatively pure solutions of alkali metal hydroxides that, as with porous diaphragms, must be diluted with brine and the hydroxides subsequently separated and purified. There is no problem, and it can be performed more effectively and easily than electrolysis. In order to fully utilize the characteristics of non-porous diaphragms,
It is desirable to minimize the distance between the electrodes (ie, the electrode gap); such a reduction has a significant effect on the operating voltage and, ultimately, the energy efficiency of the electrolytic process. Commercial cation exchange membranes are sensitive to current density, which must be kept within certain desirable limits for their effective operation. The current density should be approximately constant over the entire surface to avoid creating mechanical and electrical stresses that would damage the membrane. In the well-known diaphragm type electrolytic cell, the parameters listed above are largely influenced by structural tolerance limits, and considering the dimensions of the electrode surface of commercially available electrolytic cells, the electrode space (on the order of a few millimeters) )
For very small, unavoidable deviations from the strict parallelism of the anode and cathode surfaces will lead to some fluctuations in the current density across the surface of the diaphragm. As a result, previous attempts to correct current density locally in various regions of the diaphragm have been unsuccessful. According to this invention, the electrolysis of an aqueous solution of an alkali metal halide is carried out using a cation exchange membrane electrolytic cell, and the electrode space in the electrolytic cell is much smaller than that of a known electrolytic cell. However, the electrode space is constant over the entire electrode surface, and the conventional strict mechanical intersection is not considered. It is an object of this invention to maintain a substantially constant current density in an electrolytic cell equipped with an ion-selective cation exchange membrane between an anode and a cathode, and to avoid mechanical and An object of the present invention is to provide a method for effectively electrolyzing alkali metal chlorides using an electrolytic cell that reduces electrical stress. Other objects and benefits of the invention will be further elucidated as follows. Preferred electrolytic cells for carrying out the method of the invention consist of a cathode vessel made of steel or other conductive material that does not corrode in the catholyte environment, the top of the vessel being made of titanium, which is passive under conditions of anodic polarization. occluded by other valve metal plates or covers;
The titanium cover plate is provided with an aperture into which at least one, preferably a series of tubular anodes are welded, extending substantially the entire height of the vessel;
The tube wall of the tubular anode is perforated (except at the top of the tube wall near where it is welded to the titanium plate) to permit liquid and gas permeation. The anode is dimensionally stable, typically made of titanium or other valve metal, with at least a portion of its active surface coated with a conductive electrocatalyst that can withstand the anodic conditions and is non-passive, preferably platinum, palladium, rhodium or ruthenium. and iridium, or its oxide or mixed oxide coating. The lower end of the tubular anode is closed with a plug of inert material, preferably plastic, and is provided with a concentric threaded hole.
The permeable wall of the tubular anode is covered on the outside with a cationic membrane, forming an anode chamber on the inside of the tubular anode. The lower end of the cathode vessel is closed with a plate, preferably an inert plastic plate, and a device for supplying brine or other anolyte to the interior of the numerous tubular anodes, primarily plastic inlet tubes, is provided. , the flange of this tube is provided with a flange for sealing against the bottom plate of the container. The anolyte is delivered through a tubular fitting threaded into a threaded hole in the plug of the tubular anode. A preferred vessel for carrying out the method of the invention has an outlet in its upper part for discharging the catholyte gas and an outlet in its lower part for discharging the catholyte and for recycling the diluted catholyte or water into the cathode chamber. An introduction tube is provided. The anode welded to the cover of the container communicates with the upper chamber of the container through the hole in the cover, and the anode gas is separated from the electrolyte in this upper chamber, escapes from the outlet, is sent to the gas recovery system, and is removed from the electrolyte. is recycled to the resaturation system before being fed back into the electrolyzer. The cathode of an electrolytic cell consists of a cathode container and cathode material contained therein, such as pieces, globules, balls, cylinders, Raschig rings, metal wool, or other particulate, loose conductive material. These particles are packed tightly into the cathode container to at least the height of the permeable wall of the tubular anode coated with a cation exchange membrane. The cathode material is in contact with the inner wall of the vessel and the outer surface of the cation exchange membranes on the numerous tubular anodes, pressing against the membranes by the weight of the packing. The conductive cathode material is graphite, lead, iron, nickel, cobalt, vanadium, molybdenum, or its alloys, intermetallic compounds, metal hydrides, carbides, and nitrides, or has good conductivity and can withstand cathode conditions. It can be any other substance. Materials exhibiting low hydrogen overpotentials such as iron, nickel and their alloys are particularly suitable for brine electrolysis.
On the other hand, for example, Fe
Particulate matter with high hydrogen overvoltage, such as lead and lead alloys, is preferred for the ring reduction of () to Fe().
The cathode material can also include plastic, ceramic, or other non-conductive material coated with a layer of conductive, cathodic resistant material. The titanium plate or cover to which the tubular anode is welded is insulated from the cathode chamber by an insulating gasket. It is connected to the positive terminal of the current distribution network and the cathode chamber is connected to the negative terminal of the current distribution network. The cathode material is polarized to the cathode side and functions as a cathode, and the porosity of the cathode material helps the cathode gas to be discharged rapidly and contributes to cathodically protecting the inner wall of the cathode container. There is. Electrode spacing occurs at the geometrically indeterminate surface of the cathode material adjacent to the surface of the cation exchange membrane and in the geometrically indeterminate surface of the mesh of the permeable wall of the tubular anode to which the cation exchange membrane is attached. The local deflection of the electrolyte flux line narrows it almost as much as the thickness of the cation exchange membrane. The space between the cathode material and the anode is essentially the thickness of the cation exchange membrane during the electrolytic process. The above configuration of the electrolytic cell does not cause sudden local current density differences that could cause mechanical and electrical stresses that could destroy the cation exchange membrane.
Produces a uniform current density over the entire electrode area. Although the electrolytic cell implementing the method of the present invention having a plurality of tubular anodes has a much larger ratio of the electrode surface to the volume occupied by the electrolytic cell than conventional commercially available diaphragm electrolytic cells, First, it has the advantage of being extremely small. In the drawings showing an electrolytic cell for carrying out the method of the present invention, a rectangular container and a circular tube anode are shown. However, the anode tube can be of other shapes, such as oval, hexagonal, or other polygonal shapes, and these shapes fall within the scope of the term "tube" in this specification, and the container may also be rectangular. ,
It can be cylindrical or other shape. Although a single concentric cylindrical anode housed within a cylindrical container is not preferred for practicing the invention, the desired capacity can still be achieved using a large number of cells. As shown in FIG. 1, the electrolytic cell consists of a rectangular cathode vessel 1 made of steel or nickel, or alloys thereof, or other conductive and cathodically durable metals. A cover 2 made of titanium or other anodic passive valve metal bolted to the vessel 1
closes the container at the top. An insulating gasket 3 is provided between the cathode vessel 1 and the titanium cover 2. A titanium tubular anode 4 is welded to a hole in the cover 2 and projects above the cover as shown in the drawing. The tube wall of the tubular anode 4 is provided with holes and other perforations, which are provided in the bottom of the anode 4 from slightly below the cover 2. The perforations 6 of the anode can be formed by welding a reticular or expanded titanium plate to the solid top 5 or can be constructed integrally with the top. The surface of the perforated portion 6 of the tubular anode 4 is suitably coated with an electrocatalytic coating. The coating is one that resists anodic conditions in a non-passive manner and primarily contains noble metals or oxides of noble metals. The tubular anode 4 may be closed at its lower end by welding a titanium stopper or closure 7, or may be made of a chemical-resistant plastic such as PVC with a concentric threaded hole 7a, as shown in FIG. It is preferable that A preferably tubular cation exchange membrane 8 is placed over the anode 4 and fastened to the unperforated top of the anode and to the cylindrical outer surface of the plug 7 with a plastic band 9. This mounting method easily and completely achieves a fluid seal between the cation exchange membrane and the perforated portion of the anode 4, which is troublesome in ordinary filter press electrolyzers. Preferably, the cation exchange membrane 8 is permeable to cations and impermeable to fluids such as liquids and gases. Suitable membrane materials are fluoride polymers or copolymers containing sulfonic acid groups. This type of material is extremely flexible and can be made into tubular shapes by injection or by hot gluing flat sheets. The thickness of this seed film is about one-tenth of a millimeter. The container 1 is rotated 180° to facilitate filling and filled with cathode material 10. The vessel is then closed with a rectangular plate 11 having a perforation at the base of each of the anodes 4. Preferably, this plate is made of inert plastic. A rectangular brine distribution box 12 made of inert plastic is also welded to the plate 11 and closed by a closing plate 13 with an opening 14 for introducing brine. Board 11 and rectangular container 1
A gasket can be provided between the bottoms of the flanges 27. The flange 27 of the plate 11 can be bolted to the bottom flange of the container 1 and the closure plate 1
3 can be bolted to the bottom of the distribution box 12. The brine distribution box has an anode 4 with a tubular connector 15.
is connected inside. One end of this connector has a flange and is screwed into the screw hole 7a of the plug 7. A seal or gasket is provided between the flange of connector 15 and brine distribution box 12. The cathode container is filled with particulate matter up to the top of the permeable portion 6 of the tubular anode 4. The cathode vessel is provided with one or more outlets 17 for discharging hydrogen above the level of the granular layer 10, and below it is provided with an adjustable gooseneck outlet 18 for discharging the catholyte. . Above the top surface of the cathode material 10, a water sprinkler or spray tube 24 extends horizontally the length of the vessel and is provided with a series of holes to control the concentration of alkali metal hydroxide formed within the cathode chamber. Water is added to the cathode container to adjust the dilution. In order to dilute the hydroxide generated at the cathode and maintain the hydroxide concentration in the catholyte flowing out from the electrolytic cell within 25% (by weight) to 43% (by weight), water is introduced into the cathode chamber through the water sprinkler pipe 24. It is advisable to add constantly. The top of each tubular anode 4 is connected to a rectangular tank 19 extending over the entire top of the electrolyzer vessel 1 . The level of the electrolyte in the tank 19 is maintained constant by a gooseneck type discharge pipe 20 for discharging the electrolyte. The electrolyte discharged from tube 20 is sent to a resaturation system before being recycled into the electrolytic cell via electrolyte inlet tube 14. The halogen generated at the anode is discharged through an outlet 21 separated from the electrolyte in the tank 19. The plate or cover 2 to which the tubular anode 4 is welded is directly connected to the positive terminal of the power source by a connecting member 22, and the cathode container 1 is connected to the negative terminal by a connecting member 23. FIG. 2 is a cross-sectional view taken along line 1--1 of FIG. 1, and the elements of the electrolytic cell described with respect to FIG. 1 are designated by the same reference numerals. The location of the water sprinkler tube 24 is indicated by a broken line at a location higher than the level of the particles 10 of cathode material in the cathode vessel 1 . The electrolytic cell shown in the drawing has six tubular anodes in a rectangular casing, but the number of anodes can be varied laterally, multiple rows can be used, and The shape of the electrolytic cell and the anode can also be different from those shown in the drawings. The cylindrical surface of the tubular anode 4 is much wider than the volume of the container 1, and when compared with electrolytic cells used in the general market, although the current density for the electrolytic cells is the same, a small electrolytic cell has a high current density. You can increase the rate of output. In operation, a concentrated brine (120-310 g/) of, for example, NaCl is delivered through the inlet 14 to the distribution box 12 and rises through each of the tubular anodes 4 to deposit chlorine on the electrocatalyst coated surface of that anode. generate. Sodium ions pass through a cation exchange membrane and combine with hydroxide ions produced at the cathode by water electrolysis to form sodium hydroxide. The chlorine rises through the electrolyte in the tubular anode 4 and enters the tank 19, where it is separated from the liquid and discharged through the outlet 21. The rising chlorine bubbles cause the electrolyte within the tubular anode 4 to rapidly flow upwards. The brine from which chlorine has been removed is discharged from the outlet 20 at a constant level.
and is recycled to the resaturation system before being reintroduced into the cell via inlet 14. The hydrogen produced on the surface of the cathode material adjacent to the membrane 8 passes through the granular bed 10 and collects on the upper surface of the cathode vessel, from where it is discharged via the outlet 17. The sodium hydroxide solution is discharged via an adjustable gooseneck outlet 18. Adjustable gooseneck outlet 1
8 maintains the catholyte level at the same level as the top of the cathode bed 10. The catholyte is circulated through a sodium hydroxide recovery device located outside the electrolytic cell, and the diluted sodium hydroxide solution that flows out is reintroduced into the cathode chamber via the sprinkler pipe 24. The operating temperature can vary between 30°C and 100°C, but is preferably kept at about 85°C. The PH value of the anolyte can be varied between 1 and 6, and the current density can be between 1000 and 5000 A/ ^m2 . According to Figures 1 and 2 explained above, an experimental electrolytic cell was constructed, and two tubular anodes made by activating a porous titanium plate with oxides of Ru and Ti were attached to it. 4 was established. The two tubular anodes have a total anode surface area of approximately 19000mm ² , a diameter of 20mm, and a working height of 150mm.
And so. A cathode container with a rectangular cross section was made of stainless steel, and its internal dimensions were 70 mm x 40 mm. The two tubular membranes were made by overlapping the edges of sheets of "Nafion (R) 315" (trade name) manufactured by DuPont, USA and joining them by heating. The cathode material that presses the membrane with its own weight was of two types: a nickel ball with a diameter of 2.5 mm and a compressed nickel wire with an indeterminate length and a diameter of 0.25 mm. In contrast, a comparative electrolytic cell was created in the same manner as the experimental electrolytic cell, but in this comparative electrolytic cell, a concentric tube made of porous nickel plate was used, without using a cathode filling material, and a diaphragm was used. It was mounted around an anode coated with chlorine and bolted to the cathode vessel to act as the cathode of the electrolytic cell. The distance between the anode surface and the cathode surface was kept as constant as possible, and the minimum distance was 3 mm to the maximum distance 3.5 mm. This experimental electrolytic cell and comparative electrolytic cell were tested under the following conditions. Anolyte Sodium chloride 200g / Cathode solution Sodium hydroxide 16wt% Electrolysis temperature 75-85℃ PH (Anolyte) 4-4.5 Current density (on the anode surface) 3000A/m ^{2 The results of experiments over 2} are shown in the table below. It is as follows. 【table】

[Brief explanation of drawings]

FIG. 1 is a sectional view of an electrolytic cell in which the method of the invention is carried out, and FIG. 2 is a sectional view taken along the line - in FIG. 1, with various parts on the cross section indicated by dotted lines. In the drawings, main parts are indicated by the following symbols. DESCRIPTION OF SYMBOLS 1... Cathode container, 2... Cover, 3... Gasket, 4... Tubular anode, 6... Perforation part of anode, 8
... Cation exchange membrane, 10 ... Cathode particulate filling material, 19 ... Tank, 27 ... Flange.

Claims (1)

Claims: 1. A gas- and liquid-permeable anode and a gas- and liquid-permeable cathode separated by a membrane capable of exchanging cations and substantially impermeable to fluid flow. In a method for electrolyzing an aqueous alkali metal chloride solution in an electrolytic cell, the space between the surface of the anode and the surface of the cathode is maintained at substantially the thickness of the membrane during electrolysis, and the space between the surface of the anode and the surface of the cathode is maintained substantially at the thickness of the membrane, and the space between the surface of the anode and the surface of the cathode is An electrolysis method characterized in that a membrane is pressed, an aqueous alkali metal chloride solution is supplied to the anode, and water is supplied to the cathode.