SE453602B - PROCEDURE FOR THE PREPARATION OF CHLORINE AND ALKALIMETAL HYDROXIDE - Google Patents

Description

1453602; 27 r 7 7 '7 I A.

U.S. Pat. No. 3,475,302 relates to a process for the electrolytic purification of hydrogen gas in an electrolyser with a porous, hydrogen permeable catalytic anode. U.S. Pat. No. 4,039,409 discloses an oxygen-generating platinum-iridine electrode in a water-electrolytic cell and is acid concentration or purification cell. In oxygen purification - diluted or impure oxygen is supplied to the cathode chamber of the cell and oxygen is released at the anode, while water is supplied. . to the anode in water electrolysis.d HCl; can_be present in electrolysis * but the purpose is the production of essentially rental oxygen and the electrode is effective as oxygen generating and non_ säsamikióru fi vikianae egetektra. The mercury cell process involves electrolysis of alkali metal chloride solution in a cell between a graphite or metal anode and mercury cathode, whereby chlorine is released at the anode and the alkali metal forms alkali metal amalgam which is treated in a decomposition reaction to which water is formed. of sodium hydroxide and hydrogen.

However, the mercury cell process for the production of chlorine has now been abandoned for practical purposes.

The diaphragm cell has porous electrodes separated by a micro-porous diaphragm and is filled with sodium chloride solution.

The diaphragm may be in the form of an overlying porous diaphragm separating the catholyte and anolyte spaces. A serious disadvantage is that the pores in the diaphragm allow a mass transport or hydraulic transport of sodium chloride solution through the diaphragm, so that the sodium hydroxide formed at the cathode contains significant amounts of sodium chloride and gives impure and dilute sodium hydroxide solution. The by-oxide formed at the cathode can migrate back through the porous separator to the anode and be electrolyzed to oxygen. The oxygen not only causes a low degree of purity of the chlorine but also attacks the anode.

P.g.a. the mass transport of anolyte and catholyte between the spaces 453 602 has so many undesirable effects that a number of arrangements have been proposed to minimize or eliminate these problems, one of which is to maintain a pressure difference across the diaphragm to ensure that the mass transport of electrolytes between the anolyte and catholyte compartments is minimized. However, each such solution is at best only partially effective.

To overcome the inconveniences associated with the diaphragm cell and the mass transport of electrolyte through the porous diaphragm, the use of ion permeation selective membranes in chlorine production cells has been proposed to separate the anolyte and catholyte chambers. The pamselective membranes used in these cells are typically cationic membranes which allow selective passage of negatively charged anions.

Since these membranes are not porous, they have a tendency to prevent the return of sodium hydroxide from the catholyte space to the anolyte space and likewise to prevent the sodium chloride anolyte from being transported to the catholyte space and diluting the sodium hydroxide. However, it has been found that membrane cells still have certain disadvantages that limit their use on a large scale. One of the significant disadvantages of membrane type cells of the prior art type is that they are characterized by high cell voltage. This is partly due to the membrane properties themselves. This inconvenience is largely due to the fact that previously known membrane cell constructions use electrodes that are kept physically separated from the membrane. As a result of this physical gap between the electrodes and the membrane, in addition to IR cases through the membrane, the cell also exhibits electrolyte IR cases in the electrolyte between the electrodes and the membrane before the ion transport and also shows voltage drops due to gas bubble formation or mass transport effects. Because the catalytic electrodes are kept separate from the membrane, chlorine is formed at a distance from the membrane. This causes the formation of a gas layer between the electrode and the membrane. This gas layer interrupts the electrolyte path between the electrode and the membrane and partially blocks the ions from the membrane. This av- n; 453 602 breaking of the electrolyte path between the electrode and the membrane naturally results in a further IR drop, which increases the cell voltage required for the generation of chlorine and obviously reduces the voltage efficiency of the cell.

The advantages of the invention appear from the following description: According to the present invention, chlorine and alkali metal hydroxides are prepared by electrolysis of an aqueous solution of alkali metal chloride, such as NaCl solution at the anode of an electrolytic cell comprising a solid polymer electrolyte in the form of a cation exchange membrane catholyte and anolyte compartments. The catalytic electrodes at which chlorine and sodium hydroxide are formed are thin, porous, gas-permeable catalytic electrodes, at least one of which is bonded to the cation exchange membranes over the entire contact surface, so that chlorine is formed immediately at the electrode-membrane interface. This means that the electrodes have a very low overvoltage for chlorine evolution and the formation of sodium hydroxide.

The catalytic electrodes comprise a catalytic material which at least comprises a reduced platinum group metal oxide which is thermally stabilized by heating the reduced oxides in the presence of oxygen. According to a preferred embodiment, the electrodes with polytetrafluoroethylene particles (PTFE particles) contain bonded thermally stabilized, reduced oxides of platinum group metal, i.e. platinum, palladium, iridium, rhodium, ruthenium and osmium. The preferred reduced metal oxides for chlorine production are reduced oxides of ruthenium or iridium. The electrolyte catalyst may be a single, reduced platinum group metal oxide, for example ruthenium oxide, iridium oxide, platinum oxide, etc.

However, it has been found that mixtures or alloys of reduced platinum group metal oxides are more stable. Thus, an electrode of reduced ruthenium oxides containing up to 25% reduced oxides of iridium and preferably 5-25% iridium oxide, by weight, has been found to be very stable. Graphite or other conductive fillers or excipients, for example ruthenized titanium, are added in an amount of up to 50% by weight and preferably 10-30%. The excipient should have good conductivity with low halogen overvoltage and should be significantly cheaper than platinum group metals so that significantly less expensive but highly efficient electrodes are possible.

One or more reduced oxides of anodic oxide film-forming metal, hereinafter also referred to as "valve metal1", eg titanium, tantane, niobium, zirconium, hafnium, vanadium and tungsten, may be added to stabilize the electrode against oxygen, chlorine and generally against Up to 50% by weight of valve metal is useful and the preferred amount is 25-50% by weight. At least one of the catalytic electrodes is bonded to the liquid impermeable ion transporting membrane. By bonding one or both of the electrodes to the membrane, "electrolyte" is minimized. The IR "fall between the electrode and the membrane as well as gas mass transport losses due to the formation of a gas layer between the electrode and the membrane. This entails a significant reduction of the cell voltage and significant economic benefits through this reduction.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the following claims. However, the invention is described in detail below with reference to the accompanying drawing.

Figure 1 shows a schematic of an electrolytic cell constructed according to the invention.

Figure 2 schematically shows the cell and the reactions that take place in different parts of the cell.

Figure 1 shows an electrolytic cell 10 which consists of a cathode space 11 and an anode space 12 separated by a membrane 13 of solid gipolymer electrolyte, which preferably consists of a hydrated, permselective cation membrane. To the anode surface of the membrane 13 are bonded electrodes comprising particles of a Fluorocarbon polymer (PTFE), for example a material sold by Dupont Company, under the trademark "Teflon" ®, bonded to stabilized, reduced oxides of ruthenium f (RuOx), or iridium (IrOx), stabilized reduced oxides ¿of ruthenium-iridium (RuIr) Ox, ruthenium-titanium (RuTi) Ox, ruthenium-titanium-iridium ((RuTiIr) Ox, ruthenium-tantalum-iridium ~ (RuTaIr) Ox cathode 14 is bonded to and embedded in one side of the membrane and a catalytic anode: not shown is bonded to and embedded in the opposite side of the membrane.The PTFE bonded cathode is similar to the anode catalyst. Catalyst materials include finely divided metals of platinum, palladium, gold, silver, spinels, manganese, cobalt, nickel, reduced Pt group metal oxides, Pt-Ir OX, Pt-Ru Ox, graphite and suitable combinations of these materials.

Current collectors in the form of metal meshes (or grids, screens) 15 and 16 are pressed against the electrodes. The entire membrane electrode assembly is fixedly supported between housing elements 11 and 12 by means of gaskets 17 and 18 which are made of any material which is durable or inert to the cell conditions, including chlorine, oxygen, aqueous solution of sodium chloride and sodium hydroxide. such a gasket is a filled rubber gasket marketed by the Irving Moore Company, Cambridge, Massachusetts, AFS, under the trademark EPDM. The aqueous solution of sodium chloride such as anolyte is introduced through an electrolyte compaction 19 connected to the anode chamber 20. Consumed electrolyte and chlorine gas are removed through an outlet line 21 which also passes through the housing. A cathode inlet conduit 22 communicates with the cathode chamber 11 and allows the introduction of the catholyte, water, or aqueous solution of NaOH (more dilute than that formed electrochemically at the electrode / electrolyte interface) into the cathode chamber. Hydrogen has two different functions Some of the water is electrolyzed to form hydroxyl anions (OH-), which combine with the sodium cations transported towards the membrane to form sodium hydroxide (NaOH). The liquid also sweeps over the embedded cathode electrode and dilutes the highly concentrated hydroxide which: is formed at the membrane / electrode interface and reduces the diffusion gave hydroxide back through the membrane into the anolyte chamber.> The cathode outlet conduit 23 communicates with the cathode chamber A current to line 24 is inserted into the cathode chamber and a similar line, not shown, is inserted into the anode chamber.These electrical lines connect the current conducting networks (grids) 15 and 16 to an electric current source.

Figure 2 shows a sketch of the reactions that take place in the cell during the electrolysis of sodium chloride and is useful for understanding the electrolysis process and the way in which the cell functions. An aqueous solution of sodium chloride is introduced into the anode chamber, which is separated from the cathode chamber by the cation membrane 13. To optimize the cathode efficiency, the membrane 13 is provided with a cathode-side ion-repellent barrier layer which rejects hydroxyl ions and blocks or minimizes hydrocide return to the anode. The membrane 13 consists, as explained in more detail below, of a composite membrane made of a layer 26 with a high water content (20 - 35% based on the dry weight of the membrane), on the anode side and a cathode side layer 27 with a low water content (5 - 15% based on the dry weight of the membrane), which are separated by a PTFE fabric 28.

The repellency properties of the anion-repellent barrier layer of the cathode side can be further improved by chemical modification of the membrane on the cathode side to form a thin layer of a low water polymer.

In one embodiment, this is accomplished by modifying the polymer to form a substituted sulfonamide membrane layer. The cathode side layer 27 has a high MEW value or a slightly acidic conversion form (sulfonamide), whereby the water content of this part of the laminate membrane is reduced. Thereby, sodium hydroxide is increased again through the membrane to the anode. 453 602 the salt repellency of the film and the diffusion of the membrane is reduced Ikan also consists of a homogeneous film of a membrane with a low water content (nafionyl, perfluorocarboxylic acid, etc.).

PTFE-bonded reduced noble metal oxide catalysts containing stabilized reduced oxides of ruthenium or pyridium or ruthenium-iridium with or without reduced oxides of titanium, niobium or tantalum and graphite are pressed, such as current collectors 15 and 16, which are shown only partially to make the figure clearer. , is shown, into the surface of the membrane 13.; pressed against the surface of the catalytic electrodes and connected: to positive resp. negative pole of the current source to provide the electrolyzing potential across the cell electrodes.

The sodium chloride solution introduced into the anode chamber is electrolyzed at the anode 29 and gives chlorine formation immediately at the surface as shown sketchily with the bubble formation 30. The sodium ions (Na +) are transported through the membrane 13 to the cathode 14. shown at 31, catholyte. bound catalytic cathode 14 and dilutes the hydroxide as A stream of water or aqueous solution of NaOH, which is introduced into the cathode chamber and acts as the water stream sweeps over the surface of the PTFE- formed at the membrane-cathode interface and reduces the re-diffusion of hydroxide through the membrane to the anode.

A portion of the aqueous catholyte is electrolyzed at the cathode by an alkaline reaction to form hydroxyl ions (OH-) and gaseous hydrogen. The hydroxyl ions combine with the sodium ions transported through the membrane to form sodium hydroxide at the membrane / electrode interface. The sodium hydroxide readily wets the PTFE portions of the bonded electrode and migrates to the surface where the hydroxide is diluted with the stream of water sweeping over the surface of the electrode. With a stream of water sweeping over the cathode, a concentrated sodium hydroxide with a concentration of 4.5 - 6.5 M can easily be obtained at the cathode. Even with dilution, a certain amount of sodium hydroxide, as shown by the arrow 33, will migrate again through the membrane 13 to the 453 602. Sodium hydroxide transported to the anode is oxidized to form water and oxygen as shown by bubbling at 34. This is, of course, a parasitic reaction which: reduces the current efficiency of the cathode. The formation of oxygen itself is undesirable because it can have troublesome effects on the electrode and the membrane. In addition, oxygen dilutes the chlorine formed at the anode so that treatment is required to remove the oxygen. The reaction in different parts of the cell is as follows:: at anøaem 2 c1 '-> cizf + za' (1) (main reaction) _ šMembrane transport: 2Na + + H20 (2) .At the cathode: 2H2O ~ * 20H + H24 -2e (3a ) _ 'ma * + 20x11 »zuaoa (ab) at the anode: 4on" -> 02 + 21420 + 1st' (4) (parasitic) Total: 2Na c1 + 21120 »znao fl + cl2 + fl z (5) (main reaction ) The new arrangement for electrolysis of aqueous solutions of sodium chloride described is characterized in that the catalytic sites in the electrodes are in direct contact with the cation membrane and ion-exchanging acid groups attached to the polymer backbone (both when these groups are sulfonic acid, or carbonic acid groups COOH X H 2 O).

As a result, there is no IR case of significant size in the anolyte or catholyte chambers (this IR case is commonly referred to as an "electrolyte IR case"). The "electrolyte-IR case" characterizes previously known systems and processes in which electrostatic groups SO3 HXH the root and the membrane are separated and may be of the order of 0.2 - 0.5 volts. The elimination or significant reduction of this voltage drop is of course one of the essential advantages of the invention as this has an obvious and very significant effect on the total cell voltage and the economy of the process.Because chlorine is formed directly at the interface between anode and membrane no IR case occurs due to the so called "bubble effect" which is a loss by gas shielding and mass transport due to the breakdown or blocking of the electrolyte path between the electrode rod and the membrane.As pointed out above, in prior art systems the chlorine-discharging catalytic electrode is separated from the membrane. The gas is formed directly at the electrode and this means that a gas layer is obtained in the space between the membrane and the electrode. This causes an interruption of the electrolyte path between the electrode collector and the membrane, which blocks the passage of Na + ions and thereby an increase in the IR case.

§Electrodes: The PTFE-bonded catalytic electrode contains reduced platinum group metal oxides mentioned above, such as ruthenium, iridium or ruthenium-iridium to minimize chlorine overvoltage at the anode. The reduced ruthenium oxides are stabilized against chlorine and oxygen evolution so that a stable anode is obtained. Stabilization is initially achieved by temperature stabilization (thermal), i.e. by heating the reduced oxides of ruthenium to a temperature below the temperature at which the reduced oxides begin to decompose to pure metal. The reduced oxides are therefore preferably heated to 350-750 ° C for a period of from 30 minutes to 6 hours, the preferred thermal stabilization method being carried out by heating the reduced oxides for one hour to a temperature in the range 550-600 ° C. ° C. The PTFE-bonded anode containing reduced oxides of ruthenium is further stabilized by mixing with graphite and / or mixing with reduced oxides of other platinum group metals, for example IIÛX i00m Range 5 - 25% iridium, with 25% being preferred, or platinum rhodium, etci, as well as reduced oxides of valve metals, such as titanium (Ti) OX, with preferably 25-50% TiOx, or reduced oxides of 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 tantalum, ruthenium and iridium (Ru, Ur, Ti) 0x, or tantalum, ruthenium and iridium 453 602 ll (Ru, Ir, Ta) Ox bound with PTFE is very effective in providing a stable, long-lasting anode. For ternary alloys, the composition is preferably 5 to 25% by weight of reduced oxides of iridium, about 50% by weight of reduced oxides of ruthenium and the remainder a valve metal, for example titanium. For a binary alloy of reduced oxides of ruthenium and titanium, the preferred amount is 50% by weight of titanium and the remainder ruthenium.

Of course, titanium has the additional advantage that the substance is much less expensive than both ruthenium and iridium, which is why this substance is an effective excipient that reduces costs and at the same time stabilizes the electrode in an acid environment and against the development of chlorine and oxygen. Other anodic oxide film-forming metals, for example niobium, Nb, tantalum, Ta, zirconium, Zr or hafnium, Hf, can be used with good results instead of Ti in the electrode constructions.

The alloys of the reduced platinum group metal oxides are mixed together with the reduced oxides of titanium or other valve metals with PTFE into a homogeneous mixture.

The PTFE content of the anode may be 15 - 50% by weight, but 20 - 30% by weight is preferred. PTFE is of the type marketed by Dupont Corporation under the name T-30, but other fluorocarbon polymers can also be used with good results.

Typical loads with Pt group metals, etc. for the anode are at least 0.6 mg / cm 2 of the electrode surface and the preferred range is 1-2 mg / cm 2. The current collector for the anode can consist of a platinum niobium network with a fine mesh width that provides good contact with the electrode surface. Alternatively, a tensile titanium mesh coated with ruthenium oxide, iridium oxide, transition metal oxide and mixtures of these materials can also be used as the anode collector structure. Other anode collector structures may be Pt group metal or Pt group metal oxide coated mesh attached to the sheet by welding or bonding.

The anode current collector in contact with the bonded anode layer has a higher chlorine overvoltage than the catalytically active anode layer of the electrode. This reduces the likelihood that electrochemical reactions, such as chlorine evolution, will occur on the current distributor surface because these reactions are more likely to occur on the electrocatalytic anode electrode surface because it has a lower overvoltage and p.g.a. the higher IR case "to the connector network.

The cathode is preferably a bonded mixture of PTFE particles and platinum black with a platinum black loading of 0.4 - 4 mg / cm 2. As previously pointed out, other catalytic materials such as Palladium, gold, silver, gspinels, manganese, cobalt, nickel, graphite as well as the reduced oxides can be used (on the anode, Ru Ir Ox, etc.) with equally good results. Like the anode, the cathode is preferably bonded to and embedded in the surface of the cation membrane. The cathode is made very thin, 0.05 - 0.076 mm or less, and preferably about 0.013 mm and is porous and has a low PTFE content.

The thickness of the cathode can be considerable. This can be reflected in the form of reduced flushing with water or aqueous solution of NaOH and penetration of the cathode, thus reducing the cathode current efficiency. Cells were made with thin (about 0.013 - 0.05 mm) bonded cathodes of Pt black - 15% PTFE.

The current efficiency of cells with thin cathodes was about 80% at SM NaOH when operated at 88-91 ° C with 290 g / L NaCl solution supplied to the anode and with the same current density values (3200 A / dmz). With a Ru-graphite cathode with a thickness of 0.076 mm, the current efficiency was reduced to 54% at SM NaOH. Table A shows the relationships between cathode efficiency and thickness, and it can be seen that a thickness not exceeding 0.05 - 0.67 mm gives the best properties. 453 602 13 i; ššELL A "N 'Current efficiency: Cell Cathode Cathode thickness (mm)% (M NaOH)' 1 Pt-black 0.05 - 0.076 64 (4.om) 2 Pt-black 0.05 - 0.076 73 (4.5 M) 3 Pt-Black 0.025 - 0.05 75 (3.l M) 4 Pt-black 0.025 - 0.05 82 (5 M) 5 Pt-Black 0.013 78 (5.5 M) 6 5% Pt-black 0.076 78 (3.0 M) on graphite 7 15% Ru O 0.076 54 (5.0 M) on graphitex 8 Platinumised 0.25 - 0.38 57 (5 M) graphite fabric The electrode is made gas permeable to allow gases that develop at the electrode / membrane interface to be easily removed. to allow penetration of the sweeping water to the cathode / membrane interface where NaOH is formed and to allow the starting material sodium chloride solution to easily reach the membrane and the catalytic sites of the electrode.

The first-mentioned enhancer dilution of highly concentrated NaOH when it is formed and before NaOH wets PTFE and rises to the electrode surface so that further dilution with water sweeping over the electrode surface is obtained. It is important to dilute at the interface of the membrane where the NaOH concentration is highest. To maximize water penetration at the cathode, the PTFE content should not exceed 15-30% by weight as PTFE is hydrophobic. With good porosity, limited PTFE content, thin cross-section and sweeping water or dilute hydroxide solution, the NaOH concentration is regulated so that migration of NaOH through the membrane is reduced. In addition to regulating the structural properties of the cathode and utilizing water or dilute hydroxide solution that sweeps past to reduce the NaOH concentration, hydroxide return can be further reduced by providing an anion-repellent barrier layer on the cathode side. The current current collector for the cathode must be carefully selected because the highly corrosive hydroxide present at the cathode attacks many materials, especially during shutdown. The current collector may be in the form of a nickel network because nickel is resistant to hydroxide. Alternatively, the current collector may be: made of a stainless steel plate with a stainless steel mesh welded to the plate. A second cathode current structure that is resistant to or inert in the hydroxide solution is graphite or graphite in combination with a nickel mesh pressed to the paste (pate) and to the surface of the electrode. The cathode current collector in contact with the bonded cathode layer is made of materials which have a higher hydrogen overvoltage than the catalytic electrode surface. This also reduces the probability that such an electrochemical reaction as hydrogen evolution takes place on the current distributor, since such reactions are more likely to occur on the electrocatalytic cathode surface because it has a lower overvoltage and because the cathode also shields the collector to some extent.

Membrane The membrane 13 is preferably a stable, hydrated cation membrane which is characterized by ion transport selectivity.

The cation exchange membrane allows the passage of positively charged sodium cations and minimizes the passage of negatively charged anions. There are different types of ion exchange resins that can be formed into membranes that provide selective transport of cation.

Two classes of such resins are called sulfonic acid cation exchange resins and carboxyl cation exchange resins. In the sulfonic acid ion exchange resins, which are the preferred type, the ion exchange groups are hydrated sulfonic acid groups (SO 33 X H 2 O) which are attached to the polymer backbone by sulfonation. The ion-exchanging acid groups are not mobile in the membrane but are firmly bound to the polymer backbone, which ensures that the electrolyte concentration does not vary.

As noted above, perfluorocarbon-sulfonic acid cation exchange membranes are preferred because they provide very good cation transport, are highly stable, unaffected by acids and: strong oxidizing agents, exhibit very good thermal stability, and are substantially unchanged over time.

A preferred class of preferred cation exchange polymer membranes is sold by Dupont Company, AFS, under the trademark "Nafion". These membranes are hydrated copolymers of polytetrafluoroethylene (PTFE) and polysulfonyl fluoride vinyl ether containing sulfonic acid groups attached thereto. These membranes can be used in the hydrogen form, which is usually the condition in which: they are obtained from the manufacturer.

The ion exchange capacity (JBK) for = a certain sulfonic cation exchange membrane depends on the milliequivalent weight (MEV) of SO 3 groups per gram of dry polymer. The higher the content of sulfonic acid groups, the greater the ion exchange capacity and thus the ability of the hydrated membrane to transport cations. However, as the ion exchange capacity of the membrane increases, the water content also increases and the ability of the membrane to reject salts decreases. Thus, the rate at which sodium hydroxide travels from the cathode side to the anode side increases with JBK. As a result, a preferred ion exchange membrane for use in electrolysis of sodium chloride is a laminate consisting of a thin (thickness 0.05 mm) of cation exchange membrane with 1500 MEV and low water content (5 - 15%). , which exhibits high salt repellency, bonded to a film with a thickness of 0.1 mm or more with a high ion exchange capacity, 1100 MEV, with a PTFE fabric. such laminate construction is commercially available from Dupont. This results in a reduction of the cathodic anode with all the undesirable consequences thereof.

One form of a Other forms of laminate or constructions available are Nafion 355, 376, 390, É27, 214, in which the cathode side consists of a thin layer or film of resin with a low water content (5 - 15%) for optimizing the salt repellency, while the anode side of the Company under the trademark Nafion 315. the membrane consists of a film with a high water content which improves the ion exchange capacity.

The ion exchange membrane is prepared by leaching in hydroxide solution (3 - 8 M) for a period of one hour to fix the water content of the membrane and ion transport properties for conversion to sulfonate form. For a laminated membrane bonded to a PTFE fabric, it may be desirable to clean the membrane or PTFE fabric by holding it in 70% HNO 3 under reflux for 3 or 4 hours. As briefly stated above, the barrier layer on the cathode side should be characterized by a low water content on a water-absorbent persulfonic acid group basis. This results in more efficient rejection of anion (hydroxide ion). By blocking or rejecting hydroxide ions, the return of hydroxide is significantly reduced, which increases the current efficiency of the cell and reduces the evolution of oxygen at the anode. In an alternative laminated construction, the cathode side layer of the membrane is chemically modified. The functional groups at the surface layer of the polymer are modified so that they exhibit lower water absorption than the membrane in sulfonic acid form. This can be accomplished by reacting a surface layer of the polymer to form a layer of sulfonamide groups. Various kinds of reactions can be used to produce the surface layer of sulfonamide. One such method involves reacting the surface of the Nafion membrane when present in sulfonyl fluoride form with amines, such as ethylenediamine (EDA), to form substituted sulfonamide membranes.

This sulfonamide layer acts as a very effective barrier layer for anions. By rejecting the hydroxide anions on the cathode side, the return migration of hydroxide (NaOH) is obviously substantially reduced.

Electrode preparation The reduced platinum group metal oxides of ruthenium, iridium, ruthenium-iridium, etc., with or without the reduced oxides of transition metals, such as titanium, or of graphite, which are bonded with the PTFE particles to form the porous, gas-permeable, catalytic the electrodes, are prepared by thermal decomposition of mixed metal salts in the absence or presence of excess sodium salts, such as nitrates, carbonates, etc. "The preparation method used is a modification of Adam's method of platinum production by incorporating thermally decomposable halides of iride. , titanium, or ruthenium, ie salts of these metals, such as iridium chloride, ruthenium chloride or titanium chloride. As an example, in the preparation of (ruthenium, iridium) Ox as a binary alloy, the finely divided salts of ruthenium and iridium are used in admixture in the same weight ratio of ruthenium to iridium as desired in the alloy. An excess of sodium nitrate or equivalent alkali metal salts is incorporated and the mixture is melted in a silica bowl at 500-600 ° C for 3 hours. The residue is washed thoroughly to remove any remaining nitrates and halides. The resulting suspension of mixed and alloyed oxides is reduced at room temperature using an electrocarbon reduction method, or alternatively by bubbling hydrogen through the mixture. The product is carefully dried, ground and sieved through a nylon mesh. Typically, the particles after sieving have a size of 3.7; 4m in diameter.

The alloy of the reduced oxides of ruthenium and iridium is then subjected to thermal stabilization by heating for one hour to 500-600 ° C. The electrode is prepared by mixing the reduced, thermally stabilized platinum group metal oxides with the particles of polytetrafluoroethylene, PTFE. A suitable form of such particles is marketed by Dupont, AFS, under the name TeflonG'I-30.

The reduced Pt metal oxides, such as RuOx, can be mixed with a conductive carrier, for example graphite, transition metal carbides, transition metals to improve stability and allow low platinum group metal loading (0.5 mg / cm 2 in use).

In the case of graphite-ruthenium, powdered graphite (for example "Poco graphite 1748", Union Oil Co.) is mixed with 15-30% Teflon (T-30) based on the weight of the graphite-Teflon mixture. The reduced metal oxides are mixed with the graphite-PTFE mixture. 453 602 18 °: The mixture of Pt metal particles and PTFE particles or * of graphite and the reduced oxide particles is arranged in a mold and heated until the composition is sintered to a decal form, which is then bound to and embedded in the surface of the membrane by the action of pressure and heat. Various methods can be used: to bind and embed the electrode in the membrane, the method described in U.S. Patent No. 3,134,697 being used. In this process, the electrode structure is forced into the surface of a partially polymerized ion exchange membrane so that the sintered, porous, gas-absorbent particle mixture is bonded to the membrane and embedded in the surface of the membrane tube.

PIOCESE ARAMETRAL Chlorine generation is accomplished by introducing an aqueous solution of alkali metal chloride, for example NaCl, into the anolyte chamber.

The feed rate is preferably in the range 200 - zooo cnÉ / fninut / amz / ioo A / amz (cc per minute / per ftz / ioo Asa).

The sodium chloride concentration should be kept within the range 2.5 - SM (150 - 300 g / l), and a 5 molar solution of about 300 g / l is preferred as the cathode current efficiency increases directly with the concentration. At the same time, an increase in the sodium chloride concentration leads to a reduced oxygen evolution at the anode due to water electrolysis. As the concentration of the anolyte decreases, the oxygen evolution increases due to an increase in the relative amount of water present at the anode and which competes with NaCl for catalytic reaction sites. As a result, an additional amount of water will be electrolyzed to form oxygen at the anode. Electrolysis of water at the anode also lowers the cathode efficiency because hydrogen ions H + formed during the electrolysis of water travel through the membrane and combine with hydroxide ions (OH_) to form water instead of using these hydroxyl ions to form hydroxide.

Maintaining the supply to the anolyte chamber within the above range ensures that the anode is continuously supplied with fresh starting material.

: If the feed rate is reduced, the residence time of the starting material and in particular the residence time of the depleted sodium chloride solution will increase. The depleted sodium chloride solution, which has a relatively high water content, is present for a longer time at the anode and this tends to increase the water electrolysis with consequent formation of oxygen and transport of hydrogen ions through the membrane. Thus, the concentration of the sodium chloride solution and the inflow thereof affect the development of oxygen at the anode and the transport of hydrogen through the membrane.

It may also be desirable to carry out the electrolysis at superatmospheric pressure to improve the removal of gaseous electrolysis products. If the anolyte and catholyte chambers are placed under superatmospheric pressure, the size of the gas bubbles formed at the electrodes is reduced.

The smaller gas bubbles can be released much more easily from the electrode and the electrode surface, which improves the removal of gaseous electrolysis products from the cell. An additional advantage is that this tends to eliminate or reduce the formation of gas films at the electrode surface. Such films can block easy access of the anolyte and catholyte solutions to the electrode. In a hybrid cell arrangement in which only one electrode is bonded to the membrane, reduction of the bubble size results in a reduction of the gas shielding and mass transport losses (IR loss due to "bubble effect") in the space between the unbound electrode and the membrane because the interruption of the electrolyte path becomes smaller with smaller bubbles »Oxygen evolution Oxygen evolution at the anode due to electrolysis of water can, as pointed out above, be minimized by keeping the flow within the specified range and by keeping the sodium chloride concentration high. However, oxygen can also be formed in the anode due to sodium hydroxide reflux from the cathode. NaOH travels through the membrane due to the high concentration gradient at the membrane interface and the limited capacity of cation membranes to reject salts, which, as pointed out above, is a function of the water content of the membrane. For a SM NaOH solution, as much as 5 to 30% by weight of the sodium hydroxide formed at the cathode can migrate back through the membrane, depending on the material used. Oxygen is formed at the anode by electrochemical oxidation of OH- according to the following reaction formula: 4oH "-) 21120 + oz + 4e 'Volume percent of oxygen formed at the anode due to migration of hydroxide is about half of the weight percent of hydroxide. As pointed out above, migration of hydroxide to the anode can be limited by the use of a laminated membrane or another membrane in which the cathode side of the membrane is constituted by a layer or a 15% by weight of oxygen to develop if 5 to 30% by weight of hydroxide migrates to the anode. film of cationic resin with high equivalent weight and low water content, which increases the anion (hydroxide) repellency of the membrane.

In addition to minimizing the transport of hydroxide through the membrane by improving the salt repellency of the membrane, oxygen evolution at the anode can be further reduced by acidifying the sodium chloride solution. Hydrogen ions (H +) from the acidified sodium chloride solution are combined with hydroxyl ions (OH +), thereby preventing oxidation of the hydroxyl ions.

Oxygen evolution can be reduced by an order of magnitude or more (from S - 10 volume percent oxygen to 0.2 - 0.4 volume percent by adding at least 0.25 molar HCl. If the HCl solution is less concentrated than 0.25 M HCl, oxygen evolution rises rapidly from 0.2 - 0.4 volume percent to normally observed levels, ie from 5 - 10 volume percent.

For optimal operation of the process and the cell, the degree of purity of the sodium chloride solution must be high, i.e. the content of Ca ++ and Mg ++ must be low. The content of calcium and magnesium ions should be kept at 0.5 ppm (parts per million) or less to avoid destruction of the membrane due to that calcium and magnesium ions in the added sodium chloride solution undergo exchange in the membrane. Concentrations above 20 ppm cause the cell's properties to deteriorate sharply within a few days. As a result, the sodium chloride solution must be purified to maintain the total content at less than 2 ppm and preferably less than 0.5 ppm.

At 32 A / dmz, the operating voltage of cells with bound electrodes is 2.9 - 3.6 volts, depending on the electrode composition, and the supplied starting material is preferably kept at a temperature of from 80 to 90 ° C because the cell voltage and the total efficiency of the cell improves significantly at higher operating temperatures. As an example, a cell operating at 32 A / dmz and utilizing a PTFE-bonded reduced oxide of ruthenium-iridium mixture was operated at different temperatures. At 90 ° C, the cell voltage was 3.02 volts. For the same cell, the cell voltage at 3s ° C rose to 3.6 volts. A cell operating at 22 A / dm 2 and 90 ° C required a cell voltage of 2.6 volts. At the same current density but operating at 35 ° C, the cell voltage rose to 3.15 volts. Thus, a temperature of 80-90 ° C is preferred but consideration of the total efficiency. Although, as shown above, the cell voltage drops at lower current densities, work at 32 A / cm 2 or more is preferred because working at these current density values entails economic benefits in terms of plant costs, i.e. the size of the cost of a plant required for the production of a certain amount of chlorine and / or hydroxide per day.

The materials of which the cell is constructed are those which are resistant or inert to sodium chloride and chlorine in the anolyte chamber and are resistant to highly concentrated hydroxide and hydrogen in the catholyte chamber. The end plates 22 ”of the cell can thus be made of pure titanium or stainless steel, packings of filler-containing rubber, for example EPDM. Anode current collectors, such as those described above, can be made of platinum-plated niobium mesh, sheet metal or expanded titanium meshes coated with RuOx, IrOx, transition metal oxides and mixtures of such substances bonded or adhered to a titanium sheet, or a bonded precious metal or noble metal oxide coated mesh, which is attached to or bonded to a palladium-titanium sheet. The cathode current collector can be a plate of nickel, mild steel or stainless steel with a stainless steel mesh welded to this plate, or a plate with a nickel mesh attached to the plate. Other materials, such as graphite, which are resistant or inert to hydroxide and do not undergo hydrogen embrittlement can be used in the manufacture of cathode current collectors.

As noted above, these current collector materials all have higher hydrogen overvoltage in the cathode, or chlorine overvoltage in the anode, so that the electrochemical reaction, for example evolution of hydrogen and / or chlorine, takes place preferably at the catalytic surfaces of the electrode, and in especially at the interface between these electrocatalytic anodes and the membrane.

The invention is described in more detail with exemplary embodiments.

Cells containing ion exchange membranes with PTFE-bonded reduced noble metal oxide electrodes embedded in the membrane were fabricated and tested to illustrate the effect of various parameters on the activity of the cell in electrolysis of sodium chloride solution and to illustrate in particular the working voltage properties of the cell.

Table I shows the effect on the cell voltage of the various combinations, of reduced Pt metal oxides. Cells are constructed with electrodes containing various specific combinations of reduced Pt metal oxides bound to PTFE particles and deposited in a 0.15 mm thick cation membrane. These M cells were operated with a current density of 32 A / dmz at 90 ° C, a starting material feed of 200 - 2000 ml / minute and with a concentration of SM of the supplied starting material.

A cell was constructed according to the prior art and contained a dimensionally stabilized anode arranged at a certain distance from the membrane and a stainless steel cathode network arranged in a similar manner at certain intervals. This control cell was operated under the same conditions.

The values clearly show that in the method according to the invention, the working potential of the cell is in the range 2.9 - 3.6 volts. When compared with a typically previously known arrangement (comparative cell no. 4) under the same working conditions, a voltage improvement of 0.6 - 1.5 V was obtained. The advantages in terms of working efficiency and economy are clear. 453 602 0 wuæc fl smq m fl m co fl umz ucomun vumø fl ema co fi wmz ucomnn _ = ° fl «~ = zu oom fi U = ° @ == øo fl wmz sm com fl unonan co fi wmz man co fl wøz ucomsn co fl wo man man u Gom Go uo man Go = = ° «« ~ z sm oon fl u = ° @ == = ° «~« z zu øon fl »= ° @ = n æuwøuauq man co fl wøz ucomnn NSOMZ Emm nøunsuz» Hm> m | u ~ ~ ao \ w = q nh fifl xfmlu m ~ eo \ wz Q vHm> m | um ~ au \ w: «HHm> w | um ~ au \ m = q ßum> mxum NE0 \ mZ N uum> m | um NEU \ mS e» ~ m> m | ß @ ~ au \ w = q QÜGNHÉ. HHHU GB-H NMQ OOG On I N- fl MM ._ Now com n. @ - Hn mm = N fi ß com>. ~ 1 <.m mm = Nmæ ooæ en mm: Raw Qom 0. ” mm = “wa nam @ .fl | n.m mm ¶_ = una com ~ .m 1 w.n mm = N fi w 000 .fc I N1 ~ mm _.

MQ »ooa @. ~ Mm = Näß cum @ .n | n.n mm. = ^ Q \ m ° @ ~ v www nam fi. ~ «~ .1 mm nn.

QS ° u .. ° u - »dn ëaxæ fammwmuw Jwwn fl c .mn fl ncwmm um fi pmu | m @ = mm ~ ø | MHw> f flfi wu .fi mnpm -ø fi uo fl x | HHwu _ wss fl ußmz H Admš fi um> Hmm um u» um mau \ m: = uHß> mlum ~ au \ wz Q wovmx ñxø »HV _ eu \ w = NN x ^ Q» NV eu \ m = N x ~ QANH W | = «V ~ au \ m = @ Ax: mv ~ su \ m = Q | xo: mv ~ au \ w = w o fi øa“ m ~ | »H« n ~: mv ~ eu \ w = <K xo fifi a “om : mv ~ eu \ w = Q |: mHHmE .uocmum fi amm H fi nmummuo fl mcma fi o K ° ^ ~ H NWN: mv ~ eu \ @ = QK QAHH Nn ~ == v ~ su \ w = Q xo ^ ~ H NWN: mv _ ~ au \ w = 0 ¶ ° = <HH ou a .MZ flfi wo 453 602 25 ° -One cell similar to cell 7 in Table I was constructed and operated at 90 ° C in a starting material of saturated sodium chloride solution. The cell potential (volts) as a function of current density (ASF) was measured as indicated in Table II.

TABLE II Cell recovery, volts Current density Afdmz 3.2 43 2.9 32 2.7 27 2.4 ll These values show that the working potential of the cell is lowered when the current density is lowered. However, the choice of current density against cell voltage is a balance between operating costs and construction costs in the chlorine electrolysis. However, it is essential that even at very high current density values (32-43 A / dmz) significant improvements (of the order of 1 volt or more) are obtained regarding the cell voltage in the chlorine production process according to the invention.

Table III shows the effect of the cathode current efficiency on the evolution of oxygen. A cell with catalytic anodes and cathodes of PTFE-bonded reduced Pt metal oxide embedded in a cation membrane was operated at 90 ° C with a saturated sodium chloride solution having a current density of 32 A / dmz and a discharge rate of 0.3 - 0.8 ml. / minute / cmz electrode area. The volume percentage of oxygen in the chlorine was determined as a function of the current efficiency of the cathode. 453 602 26 _§ABLE III Cathode current efficiency (%) Oxygen evolution (volume percentage) 89 2.2 86 4.0 84 5.8 80 _ K 8.9 Table IV shows the regulating effect that acidification of the sodium chloride solution has on oxygen evolution. The volume percentage of oxygen in the chlorine was measured at different concentrations of HCl in the sodium chloride solution.

TABLE IV Acid content (HCl) (M) Trit volume% 0.05 2.5 0.075 1.5 0.10 0.9 0.15 0.5 0.25 0.4 It appears from these values that oxygen evolution due to Electrochemical oxidation of returning OH- is reduced by the preferred reaction of OH- chemical with E + to H 2 O.

A cell, similar to cell No. 1 Table 1, was constructed and operated with a saturated NaCl solution acidified with 0.2 M HCl and at 32 A / dmz. The cell voltage was measured at different operating temperatures from 35 to 90 ° C.

A cell similar to cell No. 7 in Table 1 was constructed and operated with 290 g / l (about 5M) / l NaCl as a starting material (non-acidified) at 22 A / dm 2. The operating voltage of the cell was measured at different operating temperatures from 35 to 90 ° C. The values were normalized to 32 A / amz. 1.4 -ï-53 602 27 ° .TABLE V Voltage for cell no. 7 _ in volts normalized to iceil no. 1 32 A / dm2 (values for Voltage volts 22 A / dm2) Temperature OC 3.65 3.50 (3.15 ) 35 3.38 3.30 (2.98) 45 3.2 3.20 (2.9) 55 3.15 3.12 (2.78) 65 3.10 i 3.05 (2.72) 75 3.05 2.97 (2.65) 85 3.02 2.95 (2.63) 90 These values show that the best operating voltage is obtained in the range 80 - 90 ° C. It should be noted, however, that even at 35 ° C, the voltage when using the method and the device according to the invention is at least 0.5 volts better than when using previously known chlorine electrolysers operating at 50 ° C.

A number of cells were constructed with the composite membrane with anion-repellent cathode side barrier layers in the form of chemically modified sulfonamide layers. The membranes consisted of membranes with a thickness of 0.18 mm of a type marketed by E.I. Dupont under the trademark Nafion. The cathode side of the membrane was modified to a depth of 0.04 mm by reaction with ethylenediamine (EDA) to form the sulfonamide barrier layer to improve the rejection of hydroxyl ion and minimize the return of hydroxide to the anode side. An anode consisting of particles of (Ru 25 Ir) 0x with 20% Tef10nQ9¶H30 as binder with a Pt metal loading of 6 mg / cm 2 was bonded to the membrane. A kated of platinum black particles mixed with 15% T-30 as binder with a loading of 4 mg / cm 2 was bound to the other side of the membrane.

A sodium chloride solution with a content of 280 - 315 g / l of NaCl was added to the anode chamber and distilled water was added 455 602 / J 28 F to the cathode chamber. The cells were operated with a current density of 32.7_Å / dmz and a temperature in the range 85-90 ° C, and the following: values of cell voltage, hydroxide concentration and cathode efficiency were achieved with the composite anion-repellent barrier layer.

TABLE VI Cathode ïCell Cell voltage Temperature OC M NaOH efficiency% l 2.68 85 5.1 89.6 2 2.78 89 4.8 87.6 3 2.76 90 4.8 91.6 These values clearly show that the use of a composite membrane with an anion-repellent barrier layer on the cathode side of the chemically modified sulfonamide type entails a significant improvement in the cathode current efficiency without affecting the voltage value during the process. Current efficiencies of around 88 to about 92% are achieved with a process carried out in a cell of this type. This clearly shows that the use of such a membrane with bonded electrodes significantly improves the current efficiency and thus the overall economy of the process.

When electrolysis of NaCl is performed in a cell in which both electrodes are bonded to the surface of an ion transporting membrane, maximum improvement is achieved. However, improved process properties are achieved with all constructions in which at least one of the electrodes is bonded to the surface of the ion transporting member (hybrid cell). The improvement of such a hybrid construction is slightly less than when both electrodes are bonded. Nevertheless, the improvement is very considerable (0.3 - 0.5 volts better than the voltage requirements in previously known processes).

A number of cells were constructed and electrolysis of sodium chloride solution was performed to compare the results in a fully bonded cell (both electrodes) with the results in hybrid cell constructs (anode bound only and cathode bound only) and with the results for previously known non- bonded structures (neither of the electrodes bonded).

All cells were constructed with membranes of Nafion 315, the cell was operated at 90 ° C with a sodium chloride solution containing 290 g / l.

The catalyst load on the bonded electrode was 21 g / m2 for the cathode for Pt black and 43 g / m2 on the anode for Ruox graphite and Rn0x graphite and Ru0x. The current efficiency at 32 A / dmz was essentially the same for all cells (84-85% for SM NaOH). Table VII shows the cell voltage for different cells: TABLE VII Cell voltage at Cell Anode Cathode 32 A / dmz, volts 1 RuOx graphite Pt-black 2.9 (bound) (bound) 2 Platinum-plated Pt-black 3.5 niobium (not ( bound) bound) 3 Platinum-plated Pt-black 3.4 niobium nets (unbound) bound) 4 Ru-graphite Ni-nets 3.5 (bound) (unbound) 5 Ru0 Ni-nets 3.3 (bušdet) (non-bonded) bound) 6 Platinumized Ni network 3.8 niobene network (non (bound) bound) It appears that the cell voltage of the fully bound cell No. 1 is almost 1 volt better than the voltage of the previously known control cell No. 6 which has no binding at all. 'Hybrid cells with bound cathode, 2 and 3, and hybrid cells with bound anode, 4 and 5, are about 0.4 - 0.6 volts worse than the fully bound cell but still 0.3 - 0.5 volts better than before known processes carried out in a cell without any bound electrodes.

It is clear that a very superior process for the production of chlorine from sodium chloride solution is made possible by reacting the sodium chloride anolyte and the aqueous catholyte with catalytic electrodes bound directly to and embedded in the cation membrane to develop chlorine at the anode and hydrogen and high purity hydroxide at the cathode. With this arrangement, the catalytic sites in the electrodes are in direct contact with the membrane and the acid exchange groups in the membrane, which means that the process becomes much more voltage efficient so that the required cell potential is significantly better (up to one volt or more) than previously known processes. . The use of highly efficient fluorocarbon-bonded reduced noble metal oxide catalysts and low overvoltage fluorocarbon-graphite-reduced noble metal catalysts further improves the efficiency of the process. »V

Claims (9)

453 602 PATENT REQUIREMENTS Process for the production of chlorine and alkali metal hydroxide by electrolysis of alkali metal chloride aqueous solution between a porous and gas permeable catalytic anode and porous and gas permeable catalytic cathode, which are separated by and in electrical contact with a cation exchange polymer membrane with an electrically conductive current grille or a perforated plate in contact with the anode resp. the cathode, continuously introducing a stream of water into the cathode chamber in contact with the catalytic cathode as a source of hydroxyl ions at the cathode and so that the stream of water continuously sweeps over the cathode to dilute hydroxide formed at the cathode, conducting electric current between the electrodes of the electrode. rolysis of the alkali metal chloride at the anode to form chlorine and for electrolysis of water at the cathode to form alkali metal hydroxide and hydrogen and continuously removes chlorine from the anode chamber and hydroxide and hydrogen from the cathode chamber, characterized in that at least one of the porous and catalytic anode the porous and gas permeable catalytic cathode is bonded to the cation exchange membranes over the entire contact surface between the catalytic electrode and the membranes, so that the catalytically active sites of the electrode are in contact with the ion exchange radicals in the membranes, so that electrolysis can be performed directly at the membrane. electrode gray nsytan. 2. A method according to claim 1, characterized in that the anode bound to the membranes resp. the cathode contains electrocatalytic particles, which are bonded and bonded to the membranes by means of a binder containing a fluorinated polymer. ' 3. A process according to claim 2, characterized in that the fluorinated polymer is polytetrafluoroethylene. 453 602 O 52 4. A method according to claim 3, characterized in that the content of the polytetrafluoroethylene in the electrode is between 15 and 50% by weight and preferably between 20 and 30% by weight. 5. A process according to any one of claims 2-4, characterized in that the electrocatalytic particles comprise at least one partially reduced, non-stoichiometric, thermally stabilized oxide of platinum group metal. 6. A process according to claim 5, characterized in that the electrocatalytic particles further contain at least one partially reduced, non-stoichiometric, thermally stabilized oxide of anodic oxide film-forming metal. 7. A method according to claim 6, characterized in that the platinum group metal is ruthenium and iridium and that the anodic oxide film-forming metal is titanium, niobium, tantalum and hafnium. 8. A method according to any one of claims 5-7, characterized in that the content of platinum group metal in the electrode bound to the membranes exceeds 0.6 mg / cm 2 and preferably amounts to 1-2 mg / cm 2. 9. A method according to any one of claims 2 and 5-8, characterized in that the electrode bound to the membranes also contains particulate graphite. A method according to any one of the preceding claims, characterized in that the electrode bound to the membranes has a maximum thickness of 75 μm and preferably amounts to about 12 μm.