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

Description

1453602; 27 r 7 7' 7 I A .

U.S. Patent No. 3,475,302 relates to a method for electrolytic purification of hydrogen gas in an electrolysis cell with a porous, hydrogen-permeable catalytic anode. U.S. Patent No. 4,039,409 describes an oxygen-evolving platinum-iridium electrode in a water electrolysis cell and is an oxygen concentration or purification cell. In oxygen purification, dilute 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. HCl may be present in the electrolysis, but the purpose is to produce substantially pure oxygen and the electrode is effective as an oxygen-evolving and non-toxic electrode. -- _ pben s{k§_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 liberated at the anode and the alkali metal forms alkali metal amalgam, which is processed in a decomposition reaction in which the amalgam reacts with water to form sodium hydroxide and hydrogen.

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

The diaphragm cell has porous electrodes separated by a microporous 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 chambers. A serious disadvantage is that the pores in the diaphragm allow mass or hydraulic transport of sodium chloride solution through the diaphragm, so that the sodium hydroxide formed at the cathode contains substantial amounts of sodium chloride and produces an impure and dilute sodium hydroxide solution. The hydrogen peroxide formed at the cathode can migrate back through the porous separator to the anode and be electrolyzed to oxygen. The oxygen not only results in low purity of the chlorine but also attacks the anode.

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

To overcome the disadvantages associated with the diaphragm cell and the mass transport of electrolyte through the porous diaphragm, it has been proposed to use ion perm-selective membranes in chlorine production cells to separate the anolyte and catholyte compartments. The perm-selective membranes used in these cells are typically cation membranes which allow the selective passage of negatively charged anions.

Since these membranes are not porous, they tend to prevent the return of sodium hydroxide from the catholyte compartment to the anolyte compartment and likewise to prevent the sodium chloride anolyte from being transported to the catholyte compartment and diluting the sodium hydroxide. However, it has been found that membrane cells still have certain disadvantages which limit their use on a large scale. One of the essential disadvantages of membrane type cells of the previously known type is that they are characterized by high cell voltage. This is partly due to the membrane properties themselves. This disadvantage is largely due to the fact that previously known membrane cell designs utilize electrodes which are kept physically separate from the membrane. As a result of this physical gap between the electrodes and the membrane, the cell exhibits, in addition to IR drops through the membrane, electrolyte IR drops in the electrolyte between the electrodes and the membrane prior to ion transport and also exhibits voltage drops due to gas bubble formation or mass transport effects. Since the catalytic electrodes are kept separate from the membrane, chlorine is formed at a distance from the membrane. This results in 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 interruption of the electrolyte path between the electrode and the membrane naturally results in a further IR drop, which increases the cell voltage required for chlorine generation and obviously reduces the voltage efficiency of the cell.

The advantages of the invention are apparent from the following description: According to the present invention, chlorine and alkali metal hydroxides are produced 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 which divides the cell into 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 overpotential for chlorine evolution and formation of sodium hydroxide.

The catalytic electrodes comprise a catalytic material comprising at least one reduced platinum group metal oxide which is thermally stabilised by heating the reduced oxides in the presence of oxygen. In a preferred embodiment, the electrodes contain polytetrafluoroethylene (PTFE) particles bonded with thermally stabilised reduced platinum group metal oxides, 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, e.g. 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. 453 602 Graphite or other conductive fillers or extenders, for example ruthenized titanium, are added in an amount of up to 50% by weight and preferably 10-30%. The extender 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 made possible.

One or more reduced oxides of anodic oxide film-forming metal, hereinafter also referred to as "valve metal", e.g. titanium, tantalum, niobium, zirconium, hafnium, vanadium and tungsten, may be added to stabilize the electrode against oxygen, chlorine and generally against demanding electrolysis conditions. 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, the "electrolyte IR" drop between the electrode and the membrane is minimized as well as gas mass transport losses due to the formation of a gas layer between the electrode and the membrane. This results in a substantial reduction in the cell voltage and significant economic benefits through this reduction.

The novel features believed to be characteristic of the invention are set forth with particularity in the following claims. The invention will, however, be described in detail hereinafter with reference to the accompanying drawings.

Figure 1 shows schematically an electrolysis 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 consisting of a cathode compartment 11 and an anode compartment 12 separated by a membrane 13 of solid polymer 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), such as a material sold by the Dupont Company under the trademark "Teflon"®, bonded to stabilized reduced oxides of ruthenium (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 or ruthenium-graphite. The 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. Suitable catalyst materials include finely divided metals of platinum, palladium, gold, silver, spinel, clay, 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 screens) 15 and 16 are pressed against the electrodes. The entire membrane electrode assembly is firmly supported between housing elements 11 and 12 by gaskets 17 and 18 which are made of any material which is resistant or inert to the cell conditions, including chlorine, oxygen, aqueous sodium chloride and sodium hydroxide. One type of such gasket is a filled rubber gasket sold by the Irving Moore Company, Cambridge, Massachusetts, AFS, under the trade name EPDM. The aqueous sodium chloride solution as anolyte is introduced through an electrolyte inlet 19 which is in communication with the anode chamber 20. Spent electrolyte and chlorine gas are removed through an outlet line 21 which also passes through the housing. A cathode inlet line 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. Water network has two different functions. A portion 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 formed at the membrane/electrode interface and reduces the diffusion of hydroxide back through the membrane into the anolyte chamber. The cathode outlet line 23 communicates with the cathode chamber 11 for removal of dilute sodium hydroxide plus any hydrogen discharged at the cathode and any excess water. A current to line 24 is introduced into the cathode chamber and a similar line, not shown, is introduced into the anode chamber. These electrical lines connect the current-carrying grids (grids) 15 and 16 to an electrical power source.

Figure 2 schematically shows 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 operates. 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-repelling barrier layer that repels hydroxyl ions and blocks or minimizes the return migration of hydroxide to the anode. The membrane 13 consists, as explained in more detail below, of a composite membrane made of a high water content layer 26 (20-35% based on the dry weight of the membrane) on the anode side and a low water content layer 27 (5-15% based on the dry weight of the membrane) on the cathode side, which are separated by a PTFE fabric 28.

The repellency properties of the cathode side anion-repelling barrier layer can be further improved by chemically modifying the membrane on the cathode side to form a thin layer of a polymer with low water content.

In one embodiment, this is achieved by modifying the polymer to form a substituted sulfonamide membrane layer. The cathode side layer 27 has a high MEW value or is converted to a weakly acidic form (Sulfonamide), thereby reducing the water content of this portion of the laminate membrane. This increases the salt rejection capacity of the film and reduces the diffusion of sodium hydroxide back through the membrane to the anode. The membrane may also consist of a homogeneous film of a membrane with a low water content (NaF10-100, perfluorocarboxylic acid, etc.).

PTFE-bonded reduced noble metal oxide catalysts containing stabilized reduced oxides of ruthenium or iridium or ruthenium-iridium with or without reduced oxides of titanium, niobium or tantalum and graphite are pressed, as shown, into the surface of the membrane 13. Current collectors 15 and 16, which are shown only partially for clarity, are pressed against the surface of the catalytic electrodes and connected to the positive and negative poles of the power 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 produces chlorine immediately at the surface as shown schematically by the bubble formation 30. The sodium ions (Na+) are transported through the membrane 13 to the cathode 14. A stream of water or aqueous solution of NaOH, which is introduced into the cathode chamber and acts as a catalyst, is shown at 31, the catalytic cathode 14 and dilutes the hydroxide formed at the interface between the membrane and the cathode and reduces the re-diffusion of hydroxide through the membrane to the anode.

A portion of the aqueous catholyte is electrolyzed at the cathode in an alkaline reaction to form hydroxyl ions (OH_) and hydrogen gas. 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 parts of the bonded electrode and migrates to the surface where the hydroxide is diluted by the water stream sweeping over the surface of the electrode. With a water stream sweeping over the cathode, a concentrated sodium hydroxide solution 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 arrow 33 will migrate back through membrane 13 to the anode. Sodium hydroxide transported to the anode is oxidized and forms water and oxygen as shown by bubble formation at 34. This is of course a parasitic reaction which reduces the current efficiency of the cathode. The formation of oxygen in itself is undesirable since it can have adverse effects on the electrode and 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 the anode 2 Cl-> Cl- + Cl- (1) (main reaction) Membrane transport: 2Na+ + H20 (2) At the cathode: 2H2O-20H + H24 -2e (3a) 20x11» zuaoa (ab) at the anode: 4on"-> 02 + 21420 + 1:e' (4) (parasitic) Total: 2Na c1 + 21120» znaofl +cl2+flz (5) (main reaction) The new arrangement for electrolysis of aqueous solutions of sodium chloride described is characterized by the catalytic sites in the electrodes being in direct contact with the cation membrane and ion-exchange acid groups, which are bound to the polymer main chain (both when these groups consist of sulfonic acid 20, or carboxylic acid groups COOH X H20).

As a result, no IR drop of any significant magnitude occurs in the anolyte or catholyte chambers (this IR drop is commonly referred to as the "electrolyte IR drop"). The "electrolyte IR drop" characterizes previously known systems and processes in which the electrode and the membrane are separated and may be of the order of 0.2 - 0.5 volts. The elimination or substantial reduction of this voltage drop is of course one of the _ essential advantages of the invention since this has an obvious and very significant effect on the total cell voltage and the economics of the process. Since chlorine is formed directly at the interface between the anode and the membrane, no IR drop occurs due to the so-called "bubble effect" which is a loss through gas shielding and mass transport due to the interruption or blockage of the electrolyte path between the electrode and the membrane. As previously mentioned, in prior art systems the chloride discharging catalytic electrode is separated from the membrane. The gas is formed directly at the electrode and this results in a gas layer being obtained in the space between the membrane and the electrode. This results in a disruption of the electrolyte path between the electrode collector and the membrane which blocks the passage of Na+ ions and thereby increases the IR drop.

§Electrodes: The PTFE-bonded catalytic electrode contains reduced oxides of platinum group metals as mentioned above, such as ruthenium, iridium or ruthenium-iridium to minimize the chlorine overvoltage at the anode. The reduced ruthenium oxides are stabilized against chlorine and oxygen evolution so as to obtain a stable anode. 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 of 550-600°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, iridium (5-25%, 25% preferred), or platinum, rhodium, etc., and also reduced oxides of valve metals, such as titanium (Ti)Ox, 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)Ox, or tantalum, ruthenium and iridium (Ru, Ir, Ta)Ox bonded with PTFE is very effective in providing a stable, long-lived anode. For ternary alloys, the composition is preferably 5-25% by weight of reduced oxides of iridium, about 50% by weight of reduced oxides of ruthenium, and the balance a valve metal, such as titanium. For a binary alloy of reduced oxides of ruthenium and titanium, the preferred amount is 50% by weight of titanium and the balance ruthenium.

Titanium has the additional advantage of being much less expensive than both ruthenium and iridium, which makes it an effective extender that reduces costs and at the same time stabilizes the electrode in an acidic environment and against the development of chlorine and oxygen. Other anodic oxide film-forming metals, such as niobium, Nb, tantalum, Ta, zirconium, Zr or hafnium, Hf, can be used with good results instead of Ti in electrode designs.

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

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

Typical loadings of Pt group metals, etc. for the anode are at least 0.6 mg/cm2 of the electrode surface and the preferred range is 1 - 2 mg/cm2. The current collector for the anode may be a platinized niobium mesh with a fine mesh size that provides good contact with the electrode surface. Alternatively, an expanded titanium mesh coated with ruthenium oxide, iridium oxide, transition metal oxide, and mixtures of these materials may also be used as the anode collector structure. Other anode collector structures may be Pt group metal or Pt group metal oxide-covered mesh attached to the plate 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 surface layer of the electrode. This reduces the likelihood of electrochemical reactions, such as chlorine evolution, taking place on the current distribution surface since these reactions are more likely to occur on the electrocatalytic anode electrode surface because of its lower overvoltage and due to the higher IR drop to the connector network.

-The cathode preferably consists of a bonded mixture of PTFE particles and platinum black with a platinum black loading of 0.4 - 4 mg/cm2. As previously pointed out, other catalytic materials, such as Palladium, gold, silver, spinels, manganese, cobalt, nickel, graphite as well as the reduced oxides can be used (on the anode, Ru Ir Ox, etc.) with equally good results. The cathode, like the anode, 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 reduced water or aqueous NaOH flushing and penetration of the cathode, thus reducing the cathode current efficiency. Cells were manufactured 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/dm2). 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 relationship between cathode efficiency and thickness, and it is clear that a thickness not exceeding 0.05 - 0.76'mm gives the best properties. _ 'ra 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.o m) 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 Platinized 0.25 - 0.38 57 (5M) graphite fabric The electrode is made gas permeable to allow gases evolved at the electrode/membrane interface to escape easily. It is made porous to allow penetration of the by-passing water to the cathode/membrane interface where NaOH is formed and to allow the starting material, the sodium chloride solution, to easily reach the membrane and the catalytic sites of the electrode.

The former improves the dilution of high concentration NaOH when it is formed and before NaOH wets the 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 wt.% because PTFE is hydrophobic. With good porosity, limited PTFE content, thin cross-section and by-passing water or dilute hydroxide solution, the NaOH concentration is controlled so that migration of NaOH through the membrane is reduced. In addition to controlling the structural properties of the cathode and utilizing water or dilute hydroxide solution sweeping by to reduce the NaOH concentration, re-migration of hydroxide can be further reduced by arranging an anion-repellent barrier layer on the cathode side. 453 602 14 The cathode current collector must be carefully selected because the highly corrosive hydroxide solution present at the cathode attacks many materials, especially during shutdown. The current collector may be in the form of a nickel mesh since nickel is resistant to hydroxide. Alternatively, the current collector may be made of a stainless steel sheet with a stainless steel mesh welded to the sheet. Another cathode current construction that is resistant to or inert in the hydroxide solution is graphite or graphite in combination with a nickel mesh pressed into the paste and onto the surface of the electrode. The cathode current collector in contact with the bonded cathode layer is made of a material that has a higher hydrogen overvoltage than the catalytic electrode surface. This also reduces the likelihood of an electrochemical reaction such as hydrogen evolution taking place on the current distributor since such reactions are more likely to occur on the electrocatalytic cathode surface since this has a lower overvoltage and since the cathode also shields the collector to some extent.

Membrane The membrane 13 preferably consists of 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 various types of ion exchange resins that can be formed into membranes that provide selective transport of cations.

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 (SO33 X H20) which are bonded to the polymer backbone by sulfonation. The ion exchange acid groups are not mobile in the membrane but are firmly bonded to the polymer backbone, ensuring 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, are not affected by acids and strong oxidizing agents, exhibit very good thermal stability, and are substantially unchanged over time.

A particular class of cation exchange polymer membranes which are preferred are sold by the Dupont Company, AFS, under the trade name "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 (IEC) of a given sulfonic cation exchange membrane depends on the milliequivalent weight (MEV) of SO3 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. The rate at which sodium hydroxide migrates from the cathode side to the anode side thus increases with IEC. The current efficiency (KB) and also results in oxygen formation at As a result, a preferred ion exchange membrane for use in the 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 rejection, bonded to a film with a thickness of 0.1 mm or more with high ion exchange capacity, 1100 MEV, with a PTFE fabric. Such a laminate construction is marketed by Dupont. This results in a reduction of the cathodic anode with all the undesirable consequences thereof.

A form of a Other forms of laminates 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 low water content (5 - 15%) to optimize salt rejection, while the anode side of the membrane consists of a film with high water content that improves 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 membrane's water content and ion transport properties for conversion to the sulfonate form. For a laminated membrane bonded to a PTFE fabric, it may be desirable to clean the membrane or PTFE fabric by keeping it in 70% HNO3 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 the basis of water-absorbing persulfonic acid groups. This results in more effective rejection of the anion (hydroxide ion). By blocking or rejecting hydroxide ions, the remigration of hydroxide is significantly reduced, which increases the current efficiency of the cell and reduces oxygen evolution 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 the sulfonic acid form. This can be accomplished by reacting a surface layer of the polymer to form a layer of sulfonamide groups. Various types of reactions can be utilized to produce the sulfonamide surface layer. One such method involves reacting the surface of the Nafion membrane when it is in the 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 repelling the hydroxide anions on the cathode side, the remigration of hydroxide (NaOH) is apparently significantly 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 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 the Adam method for platinum preparation by incorporating thermally decomposable halides of iridium, titanium, or ruthenium, i.e. salts of these metals, such as iridium chloride, ruthenium chloride, or titanium chloride. For example, in the preparation of (ruthenium, iridium)Ox as a binary alloy, the finely divided salts of ruthenium and iridium in a mixture in the same weight ratio of ruthenium to iridium desired in the alloy. An excess of sodium nitrate or equivalent alkali metal salts is incorporated and the mixture is melted in a silica crucible at 500-600°C for 3 hours. The residue is washed carefully to remove nitrates and halides which still remain. 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 dried carefully, 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 thermally stabilized by heating for one hour to 500-600°C. The electrode is prepared by mixing the reduced, thermally stabilized platinum group metal oxides with particles of polytetrafluoroethylene, PTFE. A suitable form of such particles is sold by Dupont, AFS, under the designation TeflonG'I-30.

The reduced Pt metal oxides, such as RuOx, can be mixed with a conductive support, such as graphite, transition metal carbides, transition metals to improve stability and allow low platinum group metal loadings (0.5 mg/cm2) in use.

In the case of graphite-ruthenium, powdered graphite (e.g., "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 into a decal form, :which is then bonded to and embedded in the surface of the membrane §by the action of pressure and heat. Various methods can be used :to bond and embed the electrode in the membrane, whereby the method described in U.S. Patent 3,134,697 can be 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-absorbing particle mixture is bonded to the membrane and embedded in the surface of the membrane.

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

The feed rate is preferably in the range of 200 - 300 cc/min/amz/100 A/amz (cc per minute/per ftz/100 Asa).

The sodium chloride concentration should be kept within the range of 2.5 - SM (150 - 300 g/l), and a 5-molar solution of about 300 g/l is preferred because the cathode current efficiency increases directly with the concentration. At the same time, an increase in the sodium chloride concentration results in a decrease in oxygen evolution at the anode due to water electrolysis. When 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 competing with NaCl for catalytic reaction sites. As a result, additional water will be electrolyzed with the formation of oxygen at the anode. Electrolysis of water at the anode also reduces the cathode efficiency because hydrogen ions H+ formed during the electrolysis of water migrate through the membrane and combine with hydroxide ions (OH_) to form water instead of these hydroxyl ions being utilized to form hydroxide.

Maintaining the flow to the anolyte chamber within the above-mentioned 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 electrolysis of water with the consequent formation of oxygen and transport of hydrogen ions through the membrane. Thus, the concentration of the sodium chloride solution and its inflow affect the evolution of oxygen at the anode and the transport of hydrogen ions through the membrane.

It may also be desirable to conduct the electrolysis at superatmospheric pressure to improve the removal of gaseous electrolysis products. Placing the anolyte and catholyte chambers under superatmospheric pressure reduces the size of the gas bubbles formed at the electrodes.

The smaller gas bubbles can be released much more easily from the electrode and electrode surface, which improves the removal of gaseous electrolysis products from the cell. A further 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, reducing 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 unbonded electrode and the membrane since the interruption of the electrolyte path is less with smaller bubbles. Oxygen evolution Oxygen evolution at the anode due to the electrolysis of water can, ~as pointed out in the foregoing, be minimized by keeping the flow within the specified range and by keeping the sodium chloride concentration high. However, oxygen can also be formed at the anode due to the recirculation of sodium hydroxide from the cathode. NaOH migrates 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 noted above, is a function of the water content of the membrane. For a SM NaOH solution, as much as 5-30% 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' The volume percentage of oxygen formed at the anode due to the migration of hydroxide is about half the weight percentage of hydroxide. Thus, 2.5-15% by volume of oxygen will be evolved if 5-30% by weight of hydroxide migrates to the anode. As noted above, migration of hydroxide to the anode can be limited by using a laminated membrane or other membrane in which the cathode side of the membrane is formed by a layer or film of cation resin with high equivalent weight and low water content, which increases the anion (hydroxide) rejection capacity of the membrane.

In addition to minimizing the transport of hydroxide through the membrane by improving the membrane's salt rejection capability, oxygen evolution at the anode can be further reduced by acidifying the sodium chloride solution. Hydrogen ions (H+) from the acidified sodium chloride solution combine 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 vol.% oxygen to 0.2 - 0.4 vol.%) by adding at least 0.25 molar HCl. If the HCl solution is less concentrated than 0.25M HCl, oxygen evolution rises rapidly from 0.2 - 0.4 vol.% to normally observed levels, i.e. from 5 - 10 vol.%.

For optimal operation of the process and the cell, the 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 maintained at 0.5 ppm (parts per million) or less to avoid membrane damage due to calcium and magnesium ions in the supplied sodium chloride solution undergoing exchange in the membrane. Concentrations above 20 ppm cause the cell properties to deteriorate significantly 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/dm2 the operating voltage for cells with bonded electrodes is 2.9 - 3.6 volts, depending on the electrode composition, and the feedstock is preferably maintained at a temperature of from 80 to 90°C since the cell voltage and overall efficiency of the cell are significantly improved at higher operating temperatures. As an example, a cell operating at 32 A/dm2 and utilizing a PTFE-bonded reduced oxide of ruthenium-iridium mixture was operated at various 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/dm2 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 in view of the overall efficiency. Although, as shown above, the cell voltage drops at lower current densities, operation at 32 A/cm2 or more is preferred because operation at these current density values brings economic advantages in terms of plant costs, i.e. the size of the cost of a plant required to produce a certain amount of chlorine and/or hydroxide per day.

The materials from which the cell is constructed consist of materials that 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, gaskets of filled rubber, for example EPDM. Anode current collectors, such as those described above, can be made of platinized niobium mesh, sheet or expanded mesh of titanium coated with RuOx, IrOx, transition metal oxides and mixtures of such materials bonded or adhered to a titanium sheet, or a mesh coated with a bonded noble metal or noble metal oxide, which is attached to or bonded to a palladium-titanium sheet. The cathode current collector can be a sheet of nickel, mild steel or stainless steel with a stainless steel mesh welded to this sheet, or a sheet with a nickel mesh attached to the sheet. Other materials, for example 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 overvoltages at the cathode, or chlorine overvoltages at the anode, so that the electrochemical reaction, e.g., evolution of hydrogen and/or chlorine, takes place preferentially at the catalytic surfaces of the electrode, and in particular 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 operation of the cell during electrolysis of sodium chloride solution and to illustrate in particular the operating voltage characteristics of the cell.

Table I shows the effect on the cell voltage of the various combinations of reduced Pt metal oxides. Cells were constructed with electrodes containing various specific combinations of reduced Pt metal oxides bound to PTFE particles and embedded in a cationic membrane 0.15 mm thick. These cells were operated with a current density of 32 A/dm2 at 90°C, a feedstock flow rate of 200-2000 ml/minute and a feedstock concentration of SM.

A cell was constructed according to prior art and contained a dimensionally stabilized anode spaced from the membrane and a similarly spaced stainless steel cathode mesh. This comparison 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 of 2.9 - 3.6 volts. When compared with a typical previously known arrangement (comparison 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 clearly evident. 453 602 0 wuæcflsmq mflm coflumz ucomun vumøflema cofiwmz ucomnn _=°fl«~= zu oomfi U=°@== øoflwmz sm comfl unonan cofiwmz man coflwøz ucomsn coflwøz man Goawuz unoann uuwflflann man Mm cøflwaz nunnan Gofiumz zm oomfl ~=°=== =°««~z sm oonfl u=°@== =°«~«z zu øonfl »=°@=n æuwøuauq man coflwøz ucomnn NSOMZ Emm nøunsuz »Hm>m|u~ ~ao\w= q nhfiflxfmlu m ~eo\wz Q vHm>m| u m ~au\w: « HHm>w|u m ~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 O.n I N-fl MM ._ Now com n.@ - H.n mm = Nfiß com >.~ 1 <.m mm = Nmæ ooæ e.n mm : Raw Qom 0." mm = "wa nam @.fl | n.m mm ¶_ = una com ~.m 1 w.n mm = Nfiw 000 .fc I N1~ mm _.

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

QS °u..°u - »dn ëaxæ fammwmuw Jwwnflc .mnflncwmm umfipmu |m@=mm~ø |MHw> fflfiwu .fimnpm -øfiuoflx |HHwu _ wssflußmz H Admšfi lcøflamñ WW: Hmum uufluwßmøu >m vmz »um>m|um mau\m: = uHß>mlum ~au\wz Q wovmx ñxø »HV _ eu\w= N N x ^ Q »NV eu\m= N x ~ QANH W | =«v ~au\m= @ Axe :mv ~su\m= Q |xo :mv ~au\w= w ofiøa "m~|»H «n~ :mv ~eu\w= < K xofifia "om :mv ~eu\w= Q |:mHHmE .uocmumfiamm Hfinmummuoflmcmafio K °^~H NWN :mv ~eu\@= Q K QAHH Nn~ ==v ~su\w= Q xo^~H NWN :mv _ ~au\w= 0 ¶°=< HH ou a .MZ flfiwo 453 602 25 ° -A 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 the current density (ASF) was measured as given in Table II.

TABLE II Cell voltage, volts Current density Afdmz 3.2 43 2.9 32 2.7 27 2.4 11 These values show that the working potential of the cell is lowered when the current density is lowered. The choice of current density versus cell voltage is, however, a trade-off between operating costs and plant costs in the chlorine electrolysis. It is, however, 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 in the cell voltage in the chlorine production process according to the invention.

Table III shows the effect of cathode current efficiency on oxygen evolution. 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 at a current density of 32 A/dm2 and a discharge rate of 0.3 - 0.8 ml/minute/cm2 electrode area. The volume percent oxygen in the chlorine was determined as a function of the cathode current efficiency. 453 602 26 _§TABLE III Cathode current efficiency (%) Oxygen evolution (volume percent) 89 2.2 86 4.0 84 5.8 80 _ K 8.9 Table IV shows the controlling effect that acidification of the sodium chloride solution has on oxygen evolution. The volume percent oxygen in the chlorine was measured at different concentrations of HCl in the sodium chloride solution.

TABLE IV Acidity (HC1) (M) Sgrevolym % 0.05 2.5 0.075 1.5 0.10 0.9 0.15 0.5 0.25 0.4 It is apparent from these values that oxygen evolution due to electrochemical oxidation of migrating OH- is reduced by preferential reaction of OH- chemically with E+ to H20.

A cell, similar to cell no. 1 in Table 1, was constructed and operated with a saturated NaCl solution acidified with 0.2M HCl and with 32 A/dm2. The cell voltage was measured at various 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 (ca. 5M)/l NaCl as starting material (non-acidified) at 22 A/dm2. The working voltage of the cell was measured at various working temperatures from 35 to 90°C. The values were normalized to 32 A/am2. 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 working voltage is obtained within the range 80 - 90°C. It should be noted, however, that even at 35°C the voltage when using the method and device according to the invention is at least 0.5 volts better than when using previously known chlorine electrolysis devices operating at 50°C.

A number of cells were constructed with the composite membrane with anion-repelling 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 sold by E.I.Dupont under the trade name 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 migration of hydroxide to the anode side. An anode consisting of particles of (Ru 25 Ir)0x with 20% Tef10nQ9¶H30 as a binder with a Pt metal loading of 6 mg/cm2 was bonded to the membrane. A cathode of platinum black particles mixed with 15% T-30 as a binder with a loading of 4 mg/cm2 was bonded to the other side of the membrane.

A sodium chloride solution containing 280-315 g/l of NaCl was supplied to the anode chamber and distilled water was supplied to the cathode chamber at 455,602 /J 28 F. The cells were operated at a current density of 32.7_Å/ dm2 and a temperature in the range of 85-90°C, and the following values for cell voltage, hydroxide concentration and cathode efficiency were achieved with the composite anion-repelling 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-repelling barrier layer on the cathode side of chemically modified sulfonamide type results in a substantial improvement in the cathode current efficiency without affecting the voltage value in 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 results in a substantial improvement in the current efficiency and thus in the overall economy of the process.

When electrolysis of NaCl is carried out 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 designs in which at least one of the electrodes is bonded to the surface of the ion-transporting means (hybrid cell). The improvement of such a hybrid design is somewhat less than when both electrodes are bonded. Nevertheless, the improvement is very considerable (0.3 - 0.5 volts better than the voltage requirements of previously known processes).

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

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

The catalyst loading on the bonded electrode was 21 g/m2 for the cathode for Pt black and 43 g/m2 for 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 various cells: TABLE VII Cell Voltage at Cell Anode Cathode 32 A/dmz, volts 1 RuOx-graphite Pt-black 2.9 (bonded) (bonded) 2 Platinized Pt-black 3.5 Niobium mesh (not (bonded) bonded) 3 Platinized Pt-black 3.4 Niobium mesh (not 'bonded) bonded) 4 Ru-graphite Ni-mesh 3.5 (bonded) (not bonded) 5 Ru0 Ni-mesh 3.3 (bonded) (non-bonded) 6 Platinized Ni-mesh 3.8 Niobium mesh (not (bonded) bonded) It is seen that the cell voltage for the fully bonded cell no. 1 is almost 1 volt better than the voltage for the previously known control cell no. 6 which is completely free of bonding. Hybrid cells with bonded cathode, 2 and 3, and hybrid cells with bonded anode, 4 and 5, are about 0.4 - 0.6 volts worse than the fully bonded cell but still 0.3 - 0.5 volts better than previously known processes carried out in a cell without any bonded electrodes.

It is apparent that a far superior process for the production of chlorine from sodium chloride solution is made possible by causing the sodium chloride anolyte and the water catholyte to react at catalytic electrodes bonded directly to and embedded in the cation membrane to evolve 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 makes the process much more voltage efficient so that the required cell potential is substantially better (up to one volt or more) than previously known processes. The use of highly efficient fluorocarbon-bonded reduced noble metal oxide catalysts and fluorocarbon-graphite-reduced noble metal oxide catalysts with low overvoltage 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.