CH650031A5 - DEVICE FOR DETERMINING HALOGES BY ELECTROLYSIS OF AN AQUEOUS SOLUTION OF ALKALI METAL HALOGENIDES AND METHOD FOR THEIR OPERATION. - Google Patents

Description

The invention relates to a device for obtaining halogen by electrolysis of an aqueous solution of alkali metal halides between an anode and a cathode, which are separated from one another by a cation exchange membrane, and to a method for operating this device.

The production of halogens, such as chlorine, by electrolysis of sodium chloride solution with lye (NaOH) as the second product is of great industrial importance. The chlor / alkali industry produces millions of tons of chlorine and caustic soda annually. The main electrolytic processes by which chlorine has been produced are the so-called mercury cell process and the diaphragm cell process. The mercury process includes the electrolysis of an alkali metal chloride solution in a cell between a graphite or metal anode, which is also referred to as a dimensionally stable anode.

Chlorine is released at the anode and the alkali metal migrates into the mercury and forms a corresponding amalgam. This amalgam is then reacted with water in a decomposition reaction to form sodium hydroxide solution and hydrogen. For all practical purposes, the mercury cell process for the production of chlorine is out of date. Mercury is such a dangerous substance and regulations for the control of mercury and other types of pollution have become so strict that the days of the mercury cell are over. In addition to this contamination aspect, mercury cells for chlorine production are also expensive and complex. The use of mercury itself leads to problems regarding the size and complexity of the cell, since care was required when handling this material. In addition, mercury is expensive and must be used in large quantities. In addition, the need for the decomposition stage of the amalgam formed, including the associated equipment to produce sodium hydroxide solution and hydrogen, proves to be an additional increase in price.

Mercury is not used in the diaphragm cell, but it contains porous electrodes that are separated by a microporous diaphragm. The. The space between the electrodes is filled with brine and separated by a microporous diaphragm, which can take the form of an overlying porous diaphragm, which the cathode electrolyte (hereinafter referred to as "Katholyt") and the anode electrolyte (hereinafter referred to as " Anolyte »called) chambers separate. One of the serious disadvantages of a diaphragm cell is that the pores in the diaphragm allow mass transport or the hydraulic flow of the sodium chloride solution through the diaphragm. As a result, the catholyte, e.g. the alkali produced at the cathode considerable amounts of sodium chloride. This means the production of a contaminated and diluted lye. On the other hand, the hydroxide generated at the cathode can migrate through the porous diaphragm to the anode, where it is electrolyzed to produce oxygen. The generation of oxygen at the anode is disadvantageous for various reasons. The oxygen not only contaminates the chlorine formed there, but also attacks the anode.

Because mass transfer between the chambers has so many undesirable effects, a number of arrangements have been proposed to overcome or at least partially solve these problems. One of them is Auf2

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maintaining a pressure difference across the diaphragm so as to keep the mass transport of the electrolytes between the anolyte and catholyte chambers as low as possible. At best, however, these solutions have proven to be only partially effective.

In order to eliminate the disadvantages associated with the diaphragm cell and the mass transport through the porous diaphragm, it has been proposed to use selective membranes in cells for chlorine generation with regard to the ionic permeability, in order to separate the anolyte and catholyte chambers. These permeability-selective membranes that are used in these cells are typically cationic membranes that allow the selective passage of positive cations and keep the passage of negatively charged anions to a minimum. Since these membranes are not porous, they hinder the back migration of the alkali from the catholyte chamber into the anolyte chamber and similarly prevent the transport of the brine anolyte in the catholyte chamber and the dilution of the alkali there. However, it has been found that the membrane cells also have certain disadvantages which limit their use to a large extent. One of the main disadvantages of the membrane cell is that it requires high cell voltage. This high cell voltage is only partly attributable to the use of the membrane and is mainly caused by the fact that electrodes are used in the known membrane cells at a physical distance from the membrane. As a result of this spatial distance between the electrodes and the membrane, in addition to the voltage drop across the membrane, the cell has a voltage drop in the electrolyte between the electrodes and the membrane, and these cells are further subject to a voltage drop due to gas bubble formation or mass transfer. Since the catalytic electrodes are at a distance from the membrane, the chlorine is generated at a distance from the membrane. This leads to the formation of a gaseous layer between the electrode and membrane. This gaseous layer interrupts the electrolyte path between the electrode and membrane and thereby partially blocks the ions from the membrane. This interruption of the electrolytic path between the electrode and the membrane naturally leads to a further voltage drop which increases the cell voltage required for the production of chlorine and obviously reduces the voltage effectiveness of the cell.

The device according to the invention is characterized by the combination of the features specified in claim 1. Advantageous embodiments of the device and a method for operating the same result from the dependent claims.

Halogens, e.g. Chlorine, bromine, etc. by electrolysis of an aqueous alkali metal halide solution, e.g. of a NaCl solution on the anode of an electrolytic cell, which contains a solid polymer electrolyte in the form of a cation exchange membrane in order to separate the cell into a catholyte and an anolyte chamber. The catalytic electrodes on which chlorine and lye are produced are very thin, porous, gas-permeable, catalytic electrodes which are connected to and embedded in opposite surfaces of the membrane, so that the chlorine is generated directly at the electrode-membrane interface . This means that the electrodes have very low overvoltages for the chloride ion discharge and the production of lye.

The catalytic electrodes have a catalytic material which comprises at least one reduced platinum group metal oxide which has been thermally stabilized by heating the reduced oxide in the presence of oxygen. In a preferred embodiment, the electrodes consist of such oxide particles which are bound with polytetrafluoroethylene particles. Examples of useful platinum group metals are platinum, palladium, iridium, rodium, ruthenium and osmium. Particles of these metals can be contained in the electrodes.

The preferred reduced metal oxides for chlorine generation are the reduced oxides of ruthenium or iridium. The electrocatalyst can be a single reduced platinum group metal oxide such as ruthenium oxide, iridium oxide, platinum oxide, etc. However, mixtures or alloys of reduced platinum group metal oxides have been found to be more stable. An electrode made of reduced ruthenium oxide with up to 25% reduced iridium oxide and preferably 5-25% by weight reduced iridium oxide has proven to be very stable. Graphite or other conductive extender is added in an amount up to 50% by weight, and preferably 10-30% by weight. The extender should have excellent conductivity and low halogen overvoltage and should be much cheaper than the platinum group metals so that it is possible to create a considerably cheaper yet very effective electrode.

One or more reduced oxides of a valve metal, such as titanium, tantalum, niobium, zirconium, hafnium, vanadium or tungsten, can be added to stabilize the electrode against oxygen, chlorine and the generally harsh electrolysis conditions. Up to 50% by weight of the valve metal is useful, with the preferred amount being in the range of 25-50% by weight.

At least one of the catalytic electrodes is connected to the. liquid-impermeable ion-transporting membrane. By connecting one or both of the electrodes to the membrane, the drop in electrolyte voltage between the electrodes and the membrane is minimized, as is the gas mass transpiration, since no gas layer is formed between the electrode and membrane. This leads to a significant reduction in cell voltage and the significant economic benefits that result.

The invention is explained in more detail below with reference to the drawing, in which

1 is a diagrammatic representation of an electrolytic cell of the present invention and

Figure 2 is a schematic representation of the cell and the reactions taking place in different parts of the cell.

In Fig. 1, 10 denotes a halogen electrolysis cell, which consists of a cathode compartment 11 and an anode compartment 12, which are separated from one another by a solid polymer electrolyte membrane 13, the membrane being preferably a hydrated, permeability-selective cation exchange membrane. Electrodes are connected to the opposite surfaces of the membrane 13 and comprise particles of a fluorocarbon such as Teflon coated with thermally stabilized, reduced oxides of ruthenium (RuOx) or iridium (IrOx) or stabilized, reduced oxides of ruthenium-iridium (RuIr ) Ox, ruthenium-titanium (RuTi) Ox, ruthenium-tantalum-iridium (RuTaIr) Ox or ruthenium-graphite are connected. The cathode 14 is connected to and preferably embedded in one side of the membrane, and a catalytic anode, not shown, is connected to and preferably embedded in the opposite side of the membrane. The Teflon-bound cathode is similar to the anode catalyst. Suitable catalyst materials include finely divided metals such as platinum, palladium, gold, silver, manganese, cobalt or nickel, spinels or reduced platinum group metal oxides such as Pt-Ir ox, Pt-RuOx, further graphite and suitable combinations

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of the aforementioned materials.

Current collectors in the form of metal sieves 15 and 16 are pressed against the electrodes. The entire membrane and electrode assembly is firmly supported by the housing members 11 and 12 by means of the seals 17 and 18, which seals are made of any material that is resistant to the cell environment, namely alkali, chlorine, oxygen, aqueous sodium chloride in the case of electrolysis of brine and HCl, HBr in the case of other hydrogen halides, is stable or inert. One form of such a seal is a filled rubber seal available from the Irving Moore Company, Cambridge, Mass. is marketed under the trade name EPDM.

An aqueous brine is introduced through an electrolyte inlet 19 which communicates with the chamber 20. Used electrolyte and halogens, such as chlorine, are removed through an outlet line 21. A cathode inlet line 22 communicates with the cathode chamber 11 to allow the introduction of cathode chamber electrolyte, water, or aqueous NaOH that is more dilute than that electrochemically formed at the interface electrode / electrolyte.

The water serves two separate functions. Part of the water is electrolyzed to produce hydroxyl anions, and these hydroxyl anions combine with the sodium cations transported through the membrane to form lyes (NaOH). However, the water also rinses the embedded cathode electrode in order to dilute the highly concentrated alkali formed at the membrane / electrode interface and thus minimize diffusion of the alkali through the membrane back into the anode-electrolyte chamber. The cathode outlet conduit 24 communicates with the cathode chamber 11 to remove the dilute liquor, plus any hydrogen and excess water discharging at the cathode. An energized cable 23 is inserted into the cathode chamber and a comparable cable, not shown, is inserted into the anode chamber. These cables connect the current-carrying screens 16 and 15 to the power source.

Figure 2 diagrammatically illustrates the reactions taking place in the cell during the electrolysis of an aqueous brine, and is used to understand the electrolysis process in the way the cell works. An aqueous solution of sodium chloride is introduced into the anode chamber, which is separated from the cathode chamber by the cationic membrane 13. The membrane 13 is a composite membrane with a layer 26 of high water content (20 to 35%, based on the dry weight of the membrane) on the anode side and a layer 27 of low water content (5 to 15%, based on the dry weight of the membrane), both are separated from each other by a Teflon fabric 28.

In order to make the cathode efficiency optimal, the membrane 13 is provided with a cathode-side ion-repellent barrier layer to repel the hydroxyl ions and to block or minimize the migration of the alkali back to the anode. The repellent properties of the barrier layer which repels anions on the cathode side can be further enhanced by chemically modifying the perfluorosulfonic acid membrane on the cathode side to form a thin layer of a polymer having a low water content. This can e.g. done by converting the polymer to form a substituted sulfonamide membrane layer. The cathode side layer 27 has a high milliequivalence or is converted into a weak acid form (sulfonamide) and thereby the water content of this part of the composite membrane is reduced. This increases the film's salt repellency and minimizes the diffusion of sodium hydroxide through the membrane back to the anode. The membrane can also be a homogeneous film of a low water content polymer such as Nafion 150, perfluorocarboxylic acid, etc.

The Teflon-bound catalysts made from reduced noble metal oxides contain at least one thermally stabilized, reduced platinum metal oxide, such as ruthenium, iridium or ruthenium iridium, with or without the addition of reduced oxides of titanium, niobium or tantalum and particles of graphite. These catalysts are pressed into the surface of the membrane 13. The current collectors 15 and 16, which are shown only partially in FIG. 2 for the sake of clarity,

are pressed against the surface of the catalytic electrodes and connected to the positive or negative connection of the energy source in order to create the electrolysis potential between the cell electrodes. The sodium chloride solution introduced into the anode chamber is electrolyzed at the anode 29 to produce chlorine, as illustrated by the bubbles 30 in FIG. 2. The sodium ions Na + are transported through the membrane 13 to the cathode 14. A stream of water or aqueous NaOH, designated 31, is introduced into the cathode chamber and acts as a catholyte. This aqueous stream rinses the surface of the Teflon-bound catalytic cathode 14 to dilute the caustic formed at the membrane to cathode interface and thereby reduce the back diffusion of the caustic through the membrane to the anode.

Part of the water catholyte is electrolyzed at the cathode to form hydroxyl ions and gaseous hydrogen. The hydroxyl ions combine with the sodium ions transported through the membrane to form sodium hydroxide at the interface from the membrane to the electrode. The sodium hydroxide slightly wets the Teflon portion of the bound electrode and migrates to the surface where it is diluted by the aqueous stream rinsing the surface of the electrode. Concentrated sodium hydroxide in the range of 4.5 to 6.5 molar is formed on the cathode using the water rinse of the cathode. However, some of the sodium hydroxide, which is indicated by the arrow 33, migrates back through the membrane 13 to the anode. The sodium hydroxide transported to the anode is oxidized to produce water and oxygen as shown by bubble 34. This is a parasitic response that reduces the effectiveness of the cathode current. Oxygen generation at the anode is undesirable because it can have adverse effects on the electrode and membrane. In addition, the oxygen dilutes the chlorine generated at the anode, so additional treatment is required to remove the oxygen. The reactions in the different parts of the cell are as follows:

Anode reaction

(Principle): 2Cl _-> Cl2 | + 2e_ (1)

Membrane transport: 2Na + + H20 (2)

Cathode reaction: 2H20-v20H- + H2Î-2e- (3a) 2Na ++ 20H -> 2Na0H (3b) Anode reaction: 40H ~ - »- 02f + 2H20 + 4e ~ (4) overall reaction 2NaCl + 2H20 -> - 2NaOH + Cbf (5) (principle): + H2 |

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The new arrangement for the electrolysis of aqueous solutions of brine, which is described in the present application, is characterized by the fact that the catalytic sites of the electrodes are in direct contact with the cation exchange membrane and the ion-exchanging acid residues which are connected to the polymer chain , where these acid-exchanging residues are sulfonic acid residues-SChH • H2O or carboxylic acid residues-COOH • H2O. There is therefore no remarkable voltage drop in either the anolyte or catholyte chamber. Such an electrolyte voltage drop is characteristic of the existing systems and methods in which the electrode and membrane are separated from one another, and it can be of the order of 0.2 to 0.5 volts. Eliminating or significantly reducing this voltage drop is one of the main advantages of the present invention since it has a very significant effect on the overall cell voltage and the economics of the process.

Since chlorine is generated directly at the interface of the anode to the membrane, there is also no voltage drop due to the so-called “bubble effect”, which is a loss of gas mixture and mass transport due to the interruption or blocking of the electrolyte path between the electrode and the membrane.

In systems according to the prior art, the chlorine-releasing catalytic electrode is separated from the membrane. The gas is formed directly on the electrode and results in a gas layer in the space between the membrane and the electrode. This interrupts the electrolyte path between the electrode collector and the membrane and blocks the passage of Na + ions and thus increases the voltage drop.

The Teflon-bound catalytic electrode contains reduced oxides of platinum group metals, such as ruthenium, iridium or ruthenium-iridium, in order to keep the chlorine overvoltage at the anode as low as possible. The reduced ruthenium oxides are stabilized against chlorine and oxygen evolution, so that a stable anode is obtained. The stabilization takes place initially through temperature stabilization, i.e. by heating the reduced ruthenium oxides to a temperature below which the decomposition to the pure metal begins. The reduced oxides are heated to 350-750 ° C. for 30 minutes to 6 hours, and preferably for one hour to temperatures in the range of 550-600 ° C. The Teflon-bonded, reduced ruthenium oxides of the anode are further stabilized by mixing with graphite and / or mixing with the reduced oxides of other platinum metals, such as iridium IrOx in a range from 5 to 25% iridium, with 25% being preferred or with those of Pt , Rh, etc. or with reduced oxides of valve metals such as titanium TiOx, with 25 to 50% of TiOx being preferred, or tantalum (25% or more). It has also been found that a ternary alloy of reduced oxides of titanium, ruthenium and iridium (Ru, Ir, Ti) Ox or of tantalum, ruthenium and iridium (Ru, Ir, Ta) Ox, which is bonded with Teflon, is very effective in the manufacture of a stable, long-life anode. In the case of the ternary alloy, the composition preferably consists of 5 to 25% by weight of reduced oxides of iridium, about 50% by weight of reduced oxides of ruthenium and the rest is a valve metal such as titanium. For a binary alloy of reduced oxides of ruthenium and titanium, the preferred electrode contains 50% by weight of titanium and the rest is ruthenium. Titanium has the additional advantage that it is much cheaper than ruthenium or iridium, making it an effective extender that reduces costs while stabilizing the electrode in an acidic environment and against HCl, chlorine and oxygen evolution . Other valve metals, such as niobium, tantalum, zirconium or hafnium, can be used in the electrode structure instead of titanium.

The alloys of the reduced noble metal oxides are mixed with the reduced oxides of titanium or other valve metals with Teflon to form a homogeneous mixture. The anode content of Teflon can range from 15 to 50% by weight, although 20 to 30% by weight is preferred. The Teflon used is the material sold by Dupont Corporation under the trade name T-30, although other fluorocarbons can also be used equally. Typical amounts of noble metal for the anode are 0.6 mg / cm 2 of the electrode surface, the preferred range being from 1 to 2 mg / cm 2. The current collector for the anode can be a placed niobium mesh with fine mesh, with which a good contact to the electrode surface is obtained. However, an expanded titanium mesh, which is coated with ruthenium oxide, iridium oxide, valve metal oxide or mixtures thereof, can also be used as the collector structure for the anode. Yet another possible anode collector structure can be a noble metal or oxide plated mesh that is attached to a Ti or Ti alloy plate by welding or joining.

The anode current collector, which is in engagement with the bonded anode layer, has a higher chlorine overvoltage than the catalytic surface layer of the anode electrode. This reduces the likelihood of an electrochemical reaction, such as chlorine evolution, on the surface of the current distributor, since these reactions are more likely because of the lower overvoltage on the electrocatalytic anode electrode surface and because the voltage drop to the collector network is greater.

The cathode is preferably a bound mixture of Teflon particles and platinum black with a platinum black quantity of 0.4 to 4 mg / cm 2. However, the other catalytic materials mentioned above can also be used. Similar to the anode, the cathode is preferably connected to the surface of the cation exchange membrane and embedded therein. The cathode is made very thin with a thickness of about 0.05 to 0.075 mm or less and preferably with a thickness of about 0.012 mm, it is porous and has a low teflon content.

The thickness of the cathode can be quite significant. It can be reflected in reduced water or by rinsing and penetrating aqueous NaOH into the cathode, reducing the effectiveness of the cathode current. The cells were made with thin cathodes from about 0.012 to about 0.05 mm thick made of platinum black with 15% Teflon. The current efficiency of cells with thin cathodes was about 80% with 5 molar NaOH, at 88 to 91 ° C and a supply of 290 g NaCl / 1 to the anode. With a 0.075 mm thick cathode made of ruthenium and graphite, the current efficiency was reduced to 54% with 5 molar NaOH. The following Table A shows the relationship of current efficiency to thickness and indicates that thicknesses that do not exceed 0.05 to 0.075 mm have the best performance.

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Table A

cell

cathode

Cathode thickness

Current efficiency

(mm)

in% (with n-NaOH)

1

Platinum black

0.050-0.075

64 (4.0)

2nd

Platinum black

0.050-0.075

73 (4.5)

3rd

Platinum black

0.025-0.05

75 (3.1)

4th

Platinum black

0.025-0.05

82 (5.0)

5

Platinum black

0.012

78 (5.5)

6

5% platinum black

0.075

78 (3.0)

on graphite

7

15% RuOx

0.075

54 (5.0)

on graphite

8th

Platinized

0.250-0.37

57 (5.0)

Graphite fabric

The electrode is made permeable to gas so that the gases developed at the interface between the electrode and the membrane can easily escape. It is made porous in order to allow the rinse water to penetrate to the interface between the electrode and membrane at which the NaOH is formed and to allow the supplied brine to access the membrane and the catalytic electrode sites. This supports the dilution of the highly concentrated NaOH formed before it wets the Teflon and reaches the electrode surface, where it is further diluted by the water flowing over the electrode surface. It is important to dilute the NaOH at the membrane-electrode interface, as the concentration is greatest there. In order to maximize the penetration of water into the cathode, the teflon content should be 15 to 30% by weight.

do not exceed, since Teflon is hydrophobic. With a good porosity, a limited teflon content, a thin cross-section and a rinsing liquid made of water or dilute alkali, the NaOH concentration is controlled in order to reduce the migration of NaOH through the membrane, which is further supported by an anion-repellent barrier layer on the cathode side .

The current collector for the cathode must be carefully selected because the highly corrosive alkali present on the cathode attacks many materials, especially while the cell is switched off. The current collector can be a nickel network because nickel is resistant to alkali. However, the current collector can also be made from a plate made of corrosion-resistant steel, onto which a mesh made of corrosion-resistant steel is welded. Another current collector structure for the cathode that is resistant or inert to alkaline solution is graphite or graphite in combination with a nickel mesh that is pressed onto the plate and against the surface of the electrode.

The cathode current collector which is in engagement with the bound cathode layer is made of a material with a higher hydrogen overvoltage than that of the catalytic cathode surface. The likelihood of an electrochemical reaction, such as the evolution of hydrogen on the current collector, is therefore reduced because of the lower overvoltage at the electrode and because the cathode electrode shields the current collector to a certain extent.

The membrane 13 is preferably a stable hydrated cationic membrane which is distinguished by ion transport selectivity. The cation exchange membrane allows the passage of positively charged sodium cations and minimizes the passage of negatively charged anions. Different types of ion exchange resins can be processed into membranes to enable the selective transport of cations. Two types are the so-called sulfonic acid cation exchange resins and the carboxylic acid cation exchange resins. In the sulfonic acid exchange resins which are preferred, the ion-exchanging groups are hydrated sulfonic acid residues S03H-H2PO which are linked to the polymer chain by sulfonation. The ion-exchanging acid radicals are not mobile within the membrane, but are firmly connected to the polymer chain, which ensures that the electrolyte concentration does not change.

The perfluorocarbon cation exchange membranes containing sulfonic acid groups are preferred because they ensure excellent cation transport, are highly stable and are not influenced by acids and strong oxidizing agents, and also have excellent thermal stability and are essentially unchangeable over time. A specifically preferred group of cation polymer membranes is sold by DuPont Company under the trade name Naflon and this is a membrane in which the polymer is a hydrated copolymer of polytetrafluoroethylene and polysulfonyl fluoride vinyl ether which has pendant sulfonic acid groups. These membranes are used in the hydrogen form, which is usually the form obtained from the manufacturer. The ion exchange capacity of a given sulfonic acid cation exchange membrane is dependent on the milliequivalent weight of the S03-H residue per gram of the dry polymer. The greater the concentration of the sulfonic acid residues, the greater the ion exchange capacity and the greater the ability of the hydrated membrane to transport cations. However, as the membrane's ion exchange capacity increases, the water content increases and the membrane's ability to repel salt decreases.

The rate at which the sodium hydroxide travels from the cathode to the anode side increases with the ion exchange capacity. The migration back reduces the effectiveness of the cathode current and also leads to the generation of oxygen at the anode with all the associated undesirable consequences. The preferred ion exchange membrane for use in brine electrolysis is therefore a laminate consisting of a thin film (about 0.05 mm thick) with a milliequivalent weight of 1500 and a low water content (5 to 15%) with high Salt repellency combined with a film of about 0.1 mm or more in thickness, high ion exchange capacity, a milliequivalent weight of 1100 and a Teflon fabric. One form of such a layer concentration is sold by DuPont under the trade name Naflon 315. Other forms of laminates or constructions in which the cathode side layer consists of a thin resin film of low water content (5 to 15%) in order to optimize salt rejection, while the anode side of the membrane is a film with high water content to promote the ion exchange capacity , are available under the trade names Naflon 355, 376, 390, 227 and 214.

The ion exchange membrane is prepared by soaking in lye (3 to 8 molar) for a period of one hour to determine the water content of the membrane and its ion transport properties. In the case of the layer membrane connected by a Teflon fabric, it may be desirable to clean the membrane or the Teflon fabric by heating at reflux for 3 to 4 hours in 70% HNO3. The cathode side barrier layer should have a low water content based on the water absorption of the persulfonic acid groups. This leads to more effective anion (hydroxyl) rejection. Blocking or rejecting the hydroxyl ions prevents the migration

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the alkali is considerably reduced, thus increasing the current efficiency of the cell and reducing the generation of oxygen at the anode. In an alternative laminate construction, the cathode side layer of the membrane is chemically modified by converting the sulfonic acid group to one with less water absorption. This can e.g. by reacting a surface layer of the polymer to form a layer of sulfonamide groups. There are various implementation options for the formation of the sulfonamide surface layer. One such method involves reacting the surface of the Nafion membrane in the sulfonyl fluoride form with amines such as ethylenediamine to form the substituted sulfonamide membranes. This sulfonamide layer acts as a very effective barrier layer for anions. By rejecting the hydroxyl ions on the cathode side, the back migration of the alkali is obviously considerably reduced.

The reduced platinum group metal oxides of ruthenium, iridium, ruthenium / iridium, etc. with or without the reduced oxides of the valve metals, such as titanium or graphite, which bind with the Teflon particles to form the porous gas-permeable catalytic electrodes, are obtained by thermal decomposition of the mixed ones Metal salts in the absence or presence of excess sodium salts, e.g. nitrate, carbonate, etc. The manufacturing process is a modification of the Adams process of producing platinum by using thermally decomposable halides of iridium, titanium or ruthenium, e.g. of iridium chloride, ruthenium chloride or titanium chloride. For example, to produce the binary oxide (Ru, Ir) Ox, the finely divided salts of ruthenium and iridium are mixed with one another in the same weight ratio of ruthenium and iridium as is desired in the binary oxide. Excess sodium nitrate or equivalent alkali metal salts are added and the mixture is melted for 3 hours at 500 to 600 ° C in a silica bowl. The resulting product is washed thoroughly to remove the nitrates and halides still present. The suspension of the mixed or alloyed oxides is then reduced electrochemically at room temperature or by passing hydrogen bubbles through the mixture. The product is thoroughly dried, ground and sieved through a nylon mesh, the particles typically having a diameter of about 37 p.m after sieving.

The alloy of the reduced oxides of ruthenium and iridium is then thermally stabilized by heating for one hour at a temperature of 500 to 600 ° C. The electrode is then produced by mixing the reduced, thermally stabilized platinum group metal oxides with polytetrafluoroethylene particles. A commercially available product is Teflon T-30 from DuPont.

However, the reduced noble metal oxides, such as RuOx, can also be mixed with a conductive carrier, such as graphite, transition metal carbides and valve metals, in order to improve stability and only require small amounts of noble metal (0.5 mg / cm 2).

If a graphite-ruthenium electrode is used, the powdered graphite (such as Poco-Graphite 1748 from the Union Oil Company) is mixed with 15 to 30% Teflon T-30. This mixture of graphite and Teflon is used to mix the reduced metal oxides.

The mixture of noble metal oxide particles and Teflon particles or of graphite and the reduced oxide particles is arranged in a mold and heated until the mass is sintered into a decal-like shape, which is then bonded to and into the surface of the membrane by applying pressure and heat embeds. Various methods can be used to connect and embed the electrode in the membrane, including the method described in detail in U.S. Patent No. 3,134,697, in which the electrode structure is pressed into the surface of a partially polymerized ion exchange membrane, causing the sintered, porous , gas-absorbing particle mixture integrally connects with the membrane and is embedded in its surface.

To produce chlorine, aqueous alkali metal chloride solution, such as NaCl solution, is led into the anolyte chamber. The feed rate is preferably in the range of 200 to 2000 ml / min / 930 cm2 / 108 mA / cm2. The brine concentration should be kept in the range of 2.5 to 5 moles (150-300 g / l), with a 5 molar solution being preferred since the cathode current efficiency increases directly with the concentration of the solution. At the same time, the increased concentration of brine reduces the evolution of oxygen at the anode, which occurs through water electrolysis. In contrast, with decreasing concentration of the anolyte, the evolution of oxygen increases because of the relative amount of water present at the anode, which competes with the NaCl for the catalytic reaction sites. Since the water concentration increases with decreasing brine concentration, water is increasingly electrolyzed at the anode with the evolution of oxygen. Water electrolysis at the anode also reduces the cathodic effectiveness, since the H + ions generated during the water electrolysis migrate through the membrane and combine with the hydroxyl ions to form water, so that these hydroxyl ions are not available for lye formation.

Maintaining the above flow rate into the anolyte chamber ensures that the anode is continuously supplied with fresh brine.

If the feed rate is reduced, the residence time of the brine and in particular the residence time of the exhausted brine increases. This exhausted brine with its relatively high water content is then present longer at the anode and this leads to increased water electrolysis with the associated generation of oxygen and the transport of the hydrogen ions through the membrane. Both the concentration of the brine and the feed rate thus influence the development of oxygen at the anode and the transport of the hydrogen ions through the membrane.

It may also be desirable to carry out the electrolysis at higher than atmospheric pressure to promote removal of the gaseous electrolysis products. By pressurizing the anolyte and catholyte chamber, the size of the gas bubbles formed on the electrodes is reduced. The smaller gas bubbles separate more easily from the electrode and thus promote the removal of the gaseous electrolysis products from the cell. Another benefit is the elimination or reduction of the formation of gas films on the electrode surface, which can block the access of the anolyte or catholyte solution to the electrode. In a hybrid cell in which only one electrode is connected to the membrane, the reduction in particle size reduces the voltage loss due to the gas barriers and the mass transport in the space between the unbound electrode and the membrane, since the electrolyte path is less interrupted by the smaller bubbles .

Oxygen can also be generated at the anode due to the migration of sodium hydroxide back from the cathode. This NaOH migrates through the membrane due to the steep concentration gradient at the membrane interface and the limited capacity of the cationic

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see membranes, repelling salts is a function of the water content of the membrane. For a 5 molar sodium hydroxide solution, 5 to 30% by weight of the sodium hydroxide formed in the cathode migrate back through the membrane, depending on the type of membrane used. Oxygen is then formed at the anode by the electrochemical oxidation of the OH "anions according to the following equation:

40H - »- 2H2O + Oit + 4e -.

The vol .-% content of oxygen, which is generated by the migration of lye at the anode, is about half the wt .-% of lye. Therefore, when 5 to 30% by weight of the alkali migrates to the anode, 2V2 to 15% by volume of oxygen develop there. By using a layer or other membrane in which the cathode side of the membrane is a film with a high equivalent weight, low water content and increased ability to repel anions (hydroxyl), the migration of the alkali to the anode can be limited.

In addition, oxygen production at the anode can be further reduced by acidifying the brine solution. The H + ions of the acidified brine combine with the hydroxyl ions and this prevents the oxidation of the hydroxyl ions. The evolution of oxygen can be reduced by an order of magnitude or more (from 5 to 10% by volume oxygen to 0.2 to 0.4% by volume) by adding at least 0.25 mol HCl to the brine. If the HCl concentration is less than 0.25 mol, the evolution of oxygen rapidly increases from 0.2-0.4% by volume to the amounts normally observed without acidification, e.g. from 5-10% by volume.

For the process to be carried out optimally, the brine purity must be very high, e.g. The Ca ++ and Mg ++ content may only be low; it should be kept at 0.5 ppm or less in order to avoid deterioration of the membrane due to the absorption of calcium and magnesium ions. A concentration of these ions above 20 ppm leads to an impairment of the performance of the cell within days. The brine must therefore be cleaned in order to keep the total calcium and magnesium ions to less than 2 ppm and preferably less than 0.5 ppm.

At 325 mA / cm2 the operating voltage of the cells with bound electrodes is in the range of 2.9 to 3.6 volts depending on the electrode composition and the brine supplied is preferably kept at a temperature of 80-90 ° C, since cell voltage and overall effectiveness of the Cell can be improved considerably at the higher operating temperatures. E.g. a cell with a Teflon-bound electrode made of the reduced oxide of a ruthenium / iridium mixture was operated at different temperatures with a current density of 325 ° mA / cm2. At 90 ° C the cell voltage was 3.02 volts. At 35 ° C the cell voltage rose to 3.6 volts. For a current density of 215 mA / cm2, the cell required a cell voltage of 2.6 volts at 90 ° C. At the latter current density, the cell voltage rose to 3.15 volts when operating at 35 ° C. A temperature range of 80-90 ° C is therefore preferred in view of the overall effectiveness. Although the cell voltage drops at lower current densities, operation with a current density at 325 mA / cm2 is preferred, since this enables economical operation in relation to the capital invested, e.g. in terms of the size and cost of a plant required to produce a given tonnage of chlorine and / or lye daily.

The cell is made of such materials that are resistant or inert to brine and chlorine in the case of the anolyte chamber and to highly concentrated lye and hydrogen in the catholyte chamber. The end plates of the cell can therefore be made of pure titanium or corrosion-resistant steel and the seals of filled rubber, such as EPDM. The anode current collectors can be made from placed niobium meshes, stretched titanium meshes coated with RuOx, IrOx, valve metal oxides and their mixtures and attached to a titanium plate or a mesh plated with noble metal or noble metal oxide that is attached to a palladium / titanium plate will. The cathode current collector can be a plate made of nickel, mild steel or corrosion-resistant steel, with which a mesh made of corrosion-resistant steel is welded, or a plate with which a nickel mesh is connected. Other materials, such as graphite, which are resistant to lye or inert and are not subject to hydrogen embrittlement, can also be used to manufacture the cathode current collector.

The invention will now be explained in more detail by means of examples.

Examples

Cells were built and tested that had ion exchange membranes and Teflon-bonded electrodes with reduced precious metal oxide embedded in the membrane to demonstrate the effect of the various parameters on the effectiveness of the cell in brine electrolysis, and in particular to To illustrate the operating voltage characteristics of the cell.

Table I illustrates the effect of the various combinations of reduced noble metal oxides on the cell voltage. The cells were equipped with electrodes which had various specific combinations of reduced noble metal oxides, which were bound with Teflon particles and embedded in a 0.15 mm thick cationic ion exchange membrane. The cell was operated with a current density of approximately 325 mA / cm 2 at 90 ° C., feed rates of 200 to 2000 ml / min and a concentration of the feed of 5 mol.

A cell was constructed according to the prior art and contained a dimensionally stabilized anode spaced from the membrane and a cathode mesh made of corrosion-resistant steel similarly spaced.

This control cell was operated under the same conditions.

It can be seen from the data given in Table I that the cell operating potentials in the method according to the invention were in the range from 2.9 to 3.6 volts. Compared to a cell according to the prior art, control cell No. 4, under the same operating conditions, a voltage improvement of 0.6 to 1.5 V was realized. The resulting operational efficiencies and economic benefits are clearly evident.

5

10th

15

20th

25th

30th

35

40

45

50

55

60

9 650 031

Table I

Cell anode cathode supplied current density cell temperature current active membrane

No. brine mA / cm2 voltage (V) ° C samkeit (5 molar

NaOH)

1

6 mg / cm2 (Ru, 25% Ir) Ox

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.2-3.3

90

85%

DuPont Nafion 315 laminate

2nd

6 mg / cm2 (Ru, 25% Ir) Ox

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.3-3.6

90

78%

DuPont 1500 EW Nafion

3rd

6 mg / cm2 (Ru, 25% Ir) Ox

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

2.9

90

66%

DuPont 1500 EW Nafion

4th

Dimensionally stable mesh anode at a distance from the membrane

Mesh made of corrosion-resistant steel at a distance from the membrane

5 molar (290 g / 1)

323

4.2-4.4

90

81%

DuPont 1500 EW Nafion

5

4 mg / cm2 (Ru, 50% Ti) Ox

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.6-3.7

90

85%

DuPont Nafion 315 laminate

6

4 mg / cm2 (Ru 25% Ir-25% Ta) Ox

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.5-3.6

90

86%

DuPont

7

6 mg / cm2 (RuOx graphite)

2 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.0

90

89%

DuPont

8th

6 mg / cm2

4 mg / cm2

5molar

323

3.4

80

83%

DuPont

(RuOx)

Platinum black

(290 g / 1)

1500 EW Nafion

9

6 mg / cm2 (Ru-5Ir) Ox

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.4-3.7

90

73%

DuPont 1500 EW Nafion

10th

2 mg / cm2 (IrOx)

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.1-3.5

90

80%

DuPont Nafion 315 laminate

11

2 mg / cm2 (IrOx)

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.2-3.6

90

65%

DuPont Nafion 315

Laminate

A cell similar to cell No. 7 of Table I was constructed and operated at 90 ° with the addition of saturated brine. The cell potential (V) as a function of the current density is given in Table II below.

Table II

Cell voltage (V) Current density (mA / cm2)

Cell with Teflon-bound catalytic anode and 45 cathode with reduced noble metal oxides, embedded in a cation exchange membrane, was introduced at 90 ° C with the addition of a saturated brine with a current density of 323 mA / cm2 and a feed rate of 2 to 5 ml / min / 6.25 cm2 of the electrode area operated. The 50 percent by volume of oxygen in the chlorine was determined as a function of the cathodic current efficiency.

3.2 2.9 2.7 2.4

430 323 215 108

Table III

55

Cathodic current efficiency (%)

Oxygen development (vol .-%)

89 60 86 84 80

2.2 4.0

5.8

8.9

These results show that the cell operating potential becomes smaller with reduced current density. However, the relationship between current density and cell voltage determines the relationship between operating and capital costs in chlorine electrolysis. However, it becomes clear that even at very high current densities (approximately 325 and 430 mA / cm 2), significant improvements in the cell voltage can be achieved in the process for chlorine production according to the invention.

Table III illustrates the effect of cathodic current efficiency on oxygen evolution. A

Table IV illustrates the controlling effect of the acidification of the brine on the evolution of oxygen. The vol .-% content of oxygen in the chlorine was determined for different HCl concentrations in the brine.

650 031

10th

Table IV

Acid (HCl) concentration (mol) O2 (vol.%)

0.05

2.5

0.075

1.5

0.10

0.9

0.15

0.5

0.25

0.4

From these data it becomes clear that the oxygen development due to the electro-chemical oxidation of the migrated OH - is reduced by preferential reaction of the OH ~ with the H + with the formation of H2O.

A cell similar to cell # 1 of Table I was constructed and operated at 323 mA / cm2 with the addition of saturated NaCl solution which had been acidified with 0.2 N HCl. The cell voltage was measured at various operating temperatures from 35 to 90 ° C.

A cell similar to cell No. 7 of Table I was constructed and operated at 215 mA / cm 2 with the addition of a solution (approximately 5 molar) containing 290 g NaCl / 1, which was not acidified. The cell voltage was measured at various operating temperatures from 35 to 90 ° C. The results were converted to a current density of 323 mA / cm2.

Table V

Cell # 1 voltage (V)

Cell No. 7 voltage (V) converted to 323 mA / cm2 (measured at 215 mA / cm2)

temperature

° C

3.65

3.50 (3.15)

35

3.38

3.30 (2.98)

45

3.20

3.20 (2.90)

55

3.15

3.12 (2.78)

65

3.10

3.05 (2.72)

75

3.05

2.97 (2.65)

85

3.02

2.95 (2.63)

90

These results show that the best operating voltage is obtained in the range of 80 to 90 ° C. However, it should be pointed out that even at 35 ° C., the voltage with the catalyst and electrolyser according to the invention is at least 0.5 volt better than with the chlorine electrolysers according to the prior art, which were operated at 90 ° C.

A series of cells with composite membranes were constructed, each of which had anion-repellent barrier layers on the cathode side in the form of chemically modified sulfonamide layers. The membranes were 0.187 mm thick membranes of the type sold by Dupont under the trade name Nafion. The cactus side of the membrane was modified to a depth of 0.037 mm by reaction with ethylenediamine to form the sulfonamide barrier layer in order to promote the rejection of hydroxyl ions and the migration back of To keep the alkali on the anode side as low as possible. An anode made of (Ru 25 Ir) Ox particles with 20% T-30 Teflon binder and a quantity of noble metal of 6 mg / cm 2 was connected to the membrane. A

Platinum black particle cathode mixed with 15% T-30 Teflon binder and an amount of 4 mg platinum black / cm 2 was connected to the other side of the membrane.

A brine solution with a concentration of 280 5 to 315 g / l of NaCl was fed to the anode chamber and distilled water was filled into the cathode chamber. The cells were operated at a current density of 325 mA / cm 2 and at a temperature of 85-90 ° C. The following results were obtained.

10th

Table VI

cell

Cell

temperature

NaOH concentration

Cathodes

tension

° C

Efficiency in moles

in %

1

2.68

85 °

5.1

89.6

2nd

2.78

89 °

4.8

87.6

3rd

2.76

90 °

4.8

91.6

These results show that the use of a composite membrane with a cathode-side anion-repellent barrier layer made of chemically modified sulfonamide leads to a considerable improvement in the cathode current effectiveness without affecting the voltage effectiveness of the method. This clearly shows that the use of such a membrane with associated electrodes leads to considerable improvements in the current efficiency and thus in the overall economy of the method 30.

If the NaCl electrolysis is carried out in a cell in which both electrodes are connected to the surface of an ion-transporting membrane, a maximum improvement is obtained. However, the improved performance 35 of the method is achieved for all structures in which at least one of the electrodes is connected to the surface of the ion-transporting membrane. Such a cell is called a hybrid cell. The improvement in such a hybrid cell is somewhat less than in a cell in which both electrodes are connected to the membrane. Nevertheless, the improvement in a hybrid cell is clear, namely 0.3 to 0.5 volt better than in the cells according to the prior art.

A series of cells were constructed and salt-45 lye electrolysis carried out therein to compare the results of cells with two electrodes connected to the membrane with the results in hybrid cells in which either only the anode or only the cathode was connected to the membrane and to be compared with a cell according to the state of the art in which none of the electrodes were connected to the membrane.

All of these cells were made with membranes made of Nafion 315, the cells were operated at 90 ° C., and an alkali was introduced at a concentration of 55 about 290 g / l. The amount of catalyst in the bound electrode for platinum black was 2 g / 930 cm2 at the cathode and 4 g / 930 cm2 at the anode for RuOx graphite and RuOx. The current efficiency at 323 mA / cm2 was essentially the same for all cells (84 to 85% for 5 molar NaOH). Table VII shows the cell voltages for the different cells.

11

Table VII

650 031

cell

anode

cathode

Cell voltage (V) at 323 mA / cm2

1

Ru graphite

Platinum black

2.9

(bound)

(bound)

2nd

Platinized niobium net

Platinum black

3.5

(not bound)

(bound)

3rd

Platinized niobium net

Platinum black

3.4

(not bound)

(bound)

4th

Ru graphite

Nickel mesh

3.5

(bound)

(not bound)

5

RuOx

Nickel mesh

3.3

(bound)

(not bound)

6

Platinized niobium net

Nickel mesh

3.8

(not bound)

(not bound)

The cell voltage of cell # 1 equipped with two Teflon-bonded electrodes is almost 1 volt better than the prior art cell in which none of the electrodes was a Teflon-bonded electrode and which is listed under cell # 6. The hybrid cells with bound cathode Nos. 2 and 3 and the hybrid cells with bound anode Nos. 4 and 5 are worse by 0.4 to 0.6 volts than those with two bound electrodes, but still by 0.3 to 0.5 Volts better than cell 6 without a teflon-bound electrode.

By reacting the brine anolyte and the water catholyte on catalytic electrodes that are directly connected to and embedded in the cationic membrane to develop chlorine at the anode and hydrogen and high purity caustic at the cathode is a much better method of manufacture of chlorine from brine has become possible. This arrangement means that the catalytic sites in the electrodes are in direct contact with the membrane and the acid-exchanging residues in the membrane.

20 bran and this leads to a much more voltage efficient method in that the required cell potential is considerably lower (up to 1 volt or more) than with known methods. The use of the highly effective fluorocarbon-bound reduced noble metal oxide catalysts 25 as well as the fluorocarbon graphite / reduced noble metal catalysts with the low overvoltages further improve the effectiveness of the process.

The term "valve metal" used in the present application is defined in U.S. Patent 3,948,451, and it denotes a subset of transition metals, e.g. Includes Ti, Ta, Zr, Mo, Nb and W. These valve metals conduct the current in the anodic direction and resist the passage of current in the cathodic direction. They are exposed to the electrolyte and conditions in an electrolyte cell, e.g. for the production of chlorine and NaOH, sufficiently resistant to be used as an electrode material.

G

1 sheet of drawings

Claims (14)

650 031 PATENT CLAIMS 1. Device for obtaining halogen by electrolysis of an aqueous solution of alkali metal halides between an anode and a cathode, which are separated from one another by a cation exchange membrane, characterized in that at least one of the electrodes is gas-permeable and comprises an electrically conductive, catalytic material, which is in direct contact with the membrane to form an interface at which a gas is deposited as a product of the electrolysis. 2. Device according to claim 1, characterized by an embodiment of the catalytic electrode together with the membrane as a one-piece composite structure. 3. Device according to claim 2, characterized in that the catalytic material is in the form of particles which are embedded on one side of the membrane in order to create a gas- and electrolyte-permeable electrode. 4. Device according to one of claims 1 to 3, characterized by an electron-conducting current distributor which has a surface in contact with an electrode, this surface having a higher overvoltage for the separated gas than the catalytic electrode. 5. Device according to one of claims 3 and 4, characterized in that the electrically conductive, catalytic particles consist of thermally stabilized, reduced oxides of metals of the platinum group. 6. The device according to claim 5, characterized in that the particles are interconnected by means of a fluorocarbon polymer. 7. The device according to claim 5, characterized in that the particles consist of thermally stabilized, reduced oxide of ruthenium and are further stabilized by a content of thermally stabilized, reduced oxide, which consists of that of reduced iridium, tantalum, titanium, Niobium and hafnium oxides formed group is selected. 8. Device according to one of claims 1 to 7, characterized in that the cathodic side of the membrane has a lower water content than the remaining part in order to create an anion barrier layer which repels the hydroxyl ions and the diffusion of alkali through the membrane to the anode keeps a minimum. 9. Device according to one of claims 1 to 8, characterized in that the membrane is formed as a laminate of two layers, the property of repelling anions on the cathode side is greater than on the anode side. 10. The device according to claim 9, characterized in that the membrane consists of a polymeric fluorocarbon and has an anion-repellent sulfonamide barrier layer on the cathode side. 11. The device according to one of claims 1 to 10, characterized in that at least one electrode comprises a mixture of graphite particles and of particles of a metal of the platinum group and / or an oxide of a metal of the platinum group. 12. The method for operating the device according to claim 1, characterized in that an aqueous solution of alkali metal halides is brought into contact with an anode which contains bound, thermally stabilized reduced ruthenium oxides. 13. The method according to claim 12, characterized in that an aqueous solution of alkali metal halides is brought into contact with an anode which contains bound, reduced ruthenium oxides and graphite. 14. The method according to claim 12, characterized in that the aqueous solution of alkali metal halides HCl is added to a concentration of at least 0.25 mol.