FR2468660A1 - ELECTROLYTIC CELL AND METHOD FOR ELECTROLYSIS OF AQUEOUS SOLUTION OF ALKALI METAL HALIDE - Google Patents

Abstract

The invention relates to an electrolytic cell of the finger-type type and to a process for producing a halogen, hydrogen and an aqueous liquor of an alkali metal hydroxide by electrolysis of an aqueous solution of a halide of alkali metal. The cell comprises a series of dimensionally stable expandable anodes 2 and a cathode compartment comprising a cathode 1 located between the adjacent anodes, as well as a cation exchange membrane 3 arranged between the anodes and the adjacent cathodes, spacers 4 being interposed between opposite sides of the anodes to maintain uniform spacing between the anode and the cathode. Electrolysis at low and constant cell voltage.

Description

The present invention relates generally to cells

  electrolytic processes and electrolysis processes of a solution of an alkali metal halide using a cation exchange membrane. More particularly, the invention relates to an electrolytic cell of the digitiform type comprising a cation exchange membrane between the cathodes and expandable anodes and dimensionally

  stable spacers being arranged between the opposite sides

  anodes so as to maintain a uniform spacing between the anodes and the cathodes; it also relates to a process using such a cell for the electrolysis of an aqueous solution of a

  halogenide of an alkali metal, pressure being exerted on the compartment

cathodic effect.

  Halogens, hydrogen and alkali metal hydroxides have been produced on an industrial scale by a mercury process and a diaphragm process. However, the mercury process poses a risk of environmental contamination due to mercury losses and

  the operational efficiency of such a process therefore decreases year by year.

  On the other hand, a diaphragm process normally uses a liquid transfer disphragm, usually consisting of

  asbestos, this diaphragm dividing an electrolytic cell into a

  anode and a cathode compartment. The anode compartment and the cathode compartment respectively contain several anodes and several cathodeso. According to the diaphragm method, brine is introduced into the anode compartment in which the gaseous halogen is produced, then the brine is passed through the diaphragm for its admission in

  the cathode compartment o is produced an alkali metal hydroxide.

  The hydroxide thus obtained contains a high proportion of

  alkali metal and its use is therefore limited.

  Recently, it was proposed to replace the usual diaphragm with a mem-

  cation exchange brane of the selectively permeable type. Such a membrane allows the passage of ions but leaves only with difficulty

pass a hydraulic current.

  According to the ion exchange membrane process, the

  brine in the anode compartment in which the halogen is produced.

  The alkali metal ions then pass through the membrane and are selectively directed to the cathode compartment. The alkali metal ion reacts with the hydroxyl ion produced by the electrolysis of

  water to thereby form an alkali metal hydroxide.

  The electrolytic processes with ion exchange membranes have the advantage of allowing the production of an alkali metal hydroxide of relatively high purity and high concentration.

  compared to usual diaphragm processes. With regard to

  If the electrolytic cell makes it possible to perform the electrolysis with an ion exchange membrane, an electrolytic cell of the filter press type can be used, such a tank being well known in this industry. Normally, to carry out the electrolysis processes with an ion exchange membrane, a certain distance is maintained

  between the electrodes and a cation exchange membrane. Various

  These have been constructed by focusing on the distance to be provided between the electrodes and the cation exchange membrane or on the uniformity of the spacing between the electrodes, since the absence of such uniform spacing increases the voltage. cell power. Efforts have also been made to improve the surface finish of the electrodes, which obviously results in a high cost of the electrolytic cell. Processes allowing

  eliminate the disadvantages of high cell voltage and

  in the alkali metal halide, which are specific to electrolytic processes with a conventional ion exchange membrane, have been studied and proposed by some of the authors of the present invention, for example in the patent application. Japanese Unexamined No. 47,877 / 1979, in which an electrolytic cell capable of maintaining a low electrical voltage is described by contacting an anode and a

  cathode with a cation exchange membrane. This process is applicable

  to electrolytic cells very varied but especially to the electrolytic cells of the digitiform type. According to this method, it is also possible to use a dimensionally stable expandable anode. This anode is able to expand under the force of a spring and allows moderate contact of the anode and cathode with the cation exchange membrane by adjusting the expansion force

spring.

  An electrolytic cell of the digitive type according to the invention does not only include a construction cell of the finger-like type, such as the cell described on page 93 of Chlorine-Its Manufacture, Properties and Uses, by J.S. Scone, published by Reinhold Publishing.

  Corporation, New York, 1962, but also a flattened tube model.

  At present, a flattened tube type cell is also

  considered an electrolytic cell of the finger type.

  In an electrolysis method using a cation exchange membrane, it is stipulated, for example, in Unexamined Japanese Patent Application No. 109,899/1975, that the pressure is exerted on a cathode compartment in order to contact the membrane. with an anode

  and thus establish conditions for electrolysis.

  However, when an electrolysis is carried out with a cell of the digitiform type comprising a cation exchange membrane with a fixed cathode and a dimensionally stable expandable anode, in which cell the cathode compartment is subjected to an increasing pressure, it is confronted with the disadvantage of a variable spacing between the anode and the cathode according to the pressure exerted, which results in changes in the cell voltage. In Figure 4, the dashed line shows the change in cell voltage according to the pressure

  exercised on the cathode compartment. This is so because the space

  between the anode and the cathode increases as the two end portions of the dimensionally stable expansible anode are brought closer together, thereby changing the average spacing between the anode and the cathode (see FIG. spacing between the anode and the cathode and the position of the cation exchange membrane), since this exchange membrane is pushed against the anode during an increase in

  the pressure exerted on the cathode compartment.

  The present invention has for its main object an electrolytic cell

  and a process for producing halogen, hydrogen and an aqueous alkali metal hydroxide liquor by electrolysis of an aqueous solution of an alkali metal halide under a low cell voltage. The invention also aims to provide an electrolytic cell and an electrolysis process ensuring the maintenance of a uniform spacing so as to place the anode and the cathode in positions immediately.

adjacent to the membrane.

  More specifically, the invention provides a finger type electrolytic cell for the production of a halogen, hydrogen and an aqueous liquor of an alkali metal hydroxide by electrolysis of an aqueous solution of a halide of alkali metal, which cell comprises a series of dimensionally stable expansible anodes, a cathode compartment having a cathode disposed between adjacent anodes, and a cation exchange membrane located between the anodes and adjoining cathodes, spacers being interposed between the opposite sides of the anodes so as to maintain a

  uniform spacing between anodes and cathodes.

  Figure 1 and Figure 2 each show in horizontal section an embodiment of the invention. On the vertical surface of a cathode 1 or along this surface is disposed a cation exchange membrane 3; an expandable anode

  dimensionally stable 2 is disposed inside the membrane 3.

  Between the opposite sides of the dimensionally stable expandable anode 2, spacer elements 4 are interposed to maintain a

  uniform spacing between anode and cathode. The placement of the

  cation exchange brane on an electrolytic cell of the

  This form can be done without difficulty as is described, for example, in Unexamined Japanese Utility Model Applications Nos. 100 952/1979 and 26 015/1980, according to which a cylindrical membrane

  The cation exchanger is fixed with the help of an installation frame of mem-

  branes on the electrolytic cell of the finger type by means of

bolts or staples.

  According to the present invention, space elements can be used.

  of any form, if their only function is to fix and maintain the spacing between the anodes and the cathodes, by suppressing the expansion beyond the desired spacing, that is, without shrinking the distance between opposite sides of the dimensionally expandable anode

  stable. It is also possible to use a spacing element with elastic

  cited, for example a spring. As can be seen in FIG. 1, it is also possible to use an S-shaped spacing member having a certain elasticity to support the two ends of opposite sides of the anode; It would also be possible to have two or more springs of a suitable length, as can be seen in FIG. 2, between the opposite sides of the anode. We can also interpose

  a plate type spacing member between opposite sides of the anode.

  The appropriate material used for the spacers is of the type whose surface or all is halogen-resistant. This may be, for example, an erosion-resistant plastic material, such as a heat-resistant vinyl chloride resin, a fluorinated polymer such as a tetrafluoroethylene polymer, an erosion-resistant metal. such as titanium etc. The cation exchange membrane used according to the invention may in particular be a fluoropolymer membrane containing groups

  cation exchange, an example of which is a perfluoropolymer membrane

  perfluorosulphonic acid hydrocarbon, product sold by Du Pont

  of Nemours under the trademark "Nafio'n". The perfluorohydrocarbon polymer

  The perfluorosulphonic acid moiety used in the following examples has the formula:

* CF 2 CH 2 CF 2 --- *

n 0 F2

CF F

{m, I

CF2-CF2-SO3H

  in which the concentration of the exchange groups is indicated as being about 1100 to 1500 g of dry membrane per equivalent of S03Q exchange groups. In addition, such a cation exchange membrane may also contain weak acidic groups (carboxylic acids, phosphoric acids, etc ...) in isolation or in combination with the group

sulfonic acid.

  The material constituting the cathode, suitable for the implementation

  of the invention is a conductive material of electricity and capac-

  resistant to catholyte, such as iron, steel, nickel or alloys thereof. The cathode can be for example in the form of a

  expanded wire mesh, a metal plate with perforations

  rations or slits, rods or the like.

  2, the material constituting the anode, suitable for the implementation of the invention is a metal of the type "valve metal" resistant to anolytes such as titanium, tantalum, zirconium, tungsten, etc. A metal which can be used for the production of the anode can be a platinum group metal, a mixture of metal oxide of the metal valve type and platinum group metals. The anode can have various forms, for example the shape of an expanded wire mesh, a metal plate having perforations or slots, rods or the like. When an aqueous solution of alkali metal chloride is electrolyzed by means of an electrolytic cell of the digitive type, according to the invention, brine is continuously introduced into

  the anode compartment having expandable anodes

  The spacing elements are interposed between the opposite sides and the chlorine gas and the sodium ion (Na) are generated in this compartment. The chlorine gas thus prepared is evacuated from the cell by suction under a negative pressure of 10 to 10 mm H20 relative to the atmospheric pressure. Brine

  exhausted is evacuated from the cell by overflow.

  The sodium ion passes through the membrane to reach the cathode compartment where it reacts with the hydroxyl ion produced in this compartment to obtain sodium hydroxide. The sodium hydroxide thus obtained is diluted with water introduced into the cathode compartment

  to obtain an aqueous liquor of sodium hydroxide having a concentration

  10 to 40%, after which this liquor is discharged from the cell.

  The hydrogen gas produced in the cathode compartment is discharged from the cell, the pressure exerted from 20 to 200, and preferably from

  at 150 mm H2O with respect to atmospheric pressure, being contr8-

  by the pressure produced by a water seal or by the suction

  developed by a fan compressor.

  In the cathode compartment, the pressure is exerted by the hy-

  gaseous and this pressure is transmitted through the membrane to the expandable anode dimensionally stable. Nevertheless, this dimensionally stable expandable anode is not shrunk due to spacers installed between the opposite sides of the anodes and cathodes,

  so that the uniform spacing between the anode and the cathode is pre-

  Vee. As a result, the electrolysis is carried out under conditions such that the respective surfaces of the anode and the cathode are in contact with each other.

  intimate contact with the cation exchange membrane.

  In Figure 4 the solid line shows the relationship between the pressure

  the cathode compartment and the electrical voltage of the cathode

  lule during the implementation of the electrolysis process according to the invention.

  It goes without saying that a substantially constant cell voltage is maintained even when the cathode compartment is subjected to an increased pressure; According to the invention, the cell voltage is maintained at a low

  stable value because the spacing between the anode and the cathode does not increase

  not more than one wants even when increased pressure is imposed

to the cathode compartment.

  The following example and the comparative example below serve to illustrate

  the invention without in any way limiting its scope.

EXAMPLE

  A monopolar electrolytic cell of the fingertip type, comprising a FRP top cover, a series of dimensionally stable expansible anodes and a cathode compartment having a cathode between each pair of adjacent anodes, anodes between opposite ends of opposite sides of which are installed S-shaped spacer elements as shown in FIG. 1, are attached membranes in "Nafion No. 315" (Du Pont de Nemours) which are cylindrical in shape and which are interposed between the anodes and the

  cathodes, so as to bring the two electrodes into contact with the

  face of the membrane. In the anode compartment, continual introduction

  The solution is an aqueous solution of sodium chloride acidified with hydrochloric acid, while deionized water is continually introduced into the cathode compartment. The cell is activated using a current of 2000 amperes while exerting a pressure on the face of the cathode. The pressure exerted is 130 mm H 20, the current density is 23.5 A / dm and the temperature is set at 900C. The operation of the cell is continued at a constant voltage of 3.2 volts

during 6 months.

COMPARATIVE EXAMPLE

  A comparative example is carried out using the same electrolytic cell of the digitiform type comprising cylindrical membranes in "Nafion No. 315" except that spacing elements are not interposed.

  between opposite sides of the dimensionally stable expandable anode.

  The cell is powered with a current of 2,000 amperes. We impose on

  one side of the cathode a pressure of 130 mm H2O; the density of the neck

  rant is 23.5 A / dm; and the temperature is 80 ° C. The cell voltage is 3.6 volts, that is, a value greater than 0.4

  volt to that obtained when we incorporate spacing elements.

  These two examples make it possible to confirm that the invention provides

  electrolysis under a low and stable cell voltage.

Claims (8)

  1. Electrolytic cell of the digitiform type for the production of a halogen, of hydrogen and of an aqueous liquor of an alkali metal hydroxide by electrolysis of an aqueous solution of an alkali metal halide, characterized in that it comprises a series of dimensionally stable expansible anodes (2), a cathode compartment having a cathode (1) disposed between the adjacent anodes, and a cation exchange membrane (3) located between the anodes and the adjacent catlims, spacers (4) being interposed between opposite sides of the anodes to maintain uniform spacing between the anode and the cathode.   2. Cell according to claim 1, characterized in that the   spacers (4) fulfill an expansion function.   3. Cell according to claim 2, characterized in that each spacer element (4) is S-shaped and has a certain elasticity. 4. Cell according to claim 2, characterized in that each   spacer (4) is of the spring type.   5. Cell according to claim 1, characterized in that each   spacer (4) is of the plate type.   6. A process for producing a halogen, hydrogen and an aqueous liquor of an alkali metal hydroxide, characterized in that it consists in introducing an aqueous solution of alkali metal halide in the anode compartment of an electrolytic cell of the finger-like type,   cell which is as defined in any of the claims   1 to 5, electrolyzing said aqueous alkali metal halide solution while exerting pressure on the cathode compartment so that the respective surfaces of the anode (2) and the cathode (1) are in intimate contact with the cathode compartment. cation exchange membrane; and withdrawing the aqueous liquor of alkali metal hydroxide and the hydrogen of the cathode compartment while withdrawing the solution   aqueous alkali metal halide and the chlorine of the anode compartment   than. 7. Process according to claim 6, characterized in that a pressure of 20 to 200 mm is exerted on the cathode compartment.   H20 relative to atmospheric pressure.   8. Method according to claim 7, characterized in that said   pressure-is between 50 and 150 mm H 20.