JP2013194323A - Electrolysis method of alkali metal chloride using oxygen-consuming electrode having orifice - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an improved electrolysis unit for chlor-alkali electrolysis using an OCE with a micro-gap arrangement and a silver catalyst as an electrocatalytic substance, in which damage in the course of startup and shutdown and an intervening shutdown period is avoided.SOLUTION: An oxygen-consuming electrode, which is for use in an electrolysis cell with a micro-gap arrangement especially having a distance of 0.01 mm to 2 mm between an ion exchange membrane and the oxygen-consuming electrode, has at least one carrier element in the form of a sheet-like structure and a coating with a gas diffusion layer and a catalytically active component, wherein the coating with the gas diffusion layer is provided with an aperture having a diameter or a height of 0.5 mm to 20 mm, preferably 1 mm to 1 mm, and preferably having a cross-sectional area accounting for 0.05% to 15% of the total OCE area.

Description

  The present invention relates to an oxygen-consuming electrode, particularly for use in chloralkali electrolysis, and to its manufacture, electrolysis apparatus, and a method for electrolysis of aqueous solutions of alkali metal chlorides adapted to specific operating parameters. Furthermore, the present invention relates to the use of the oxygen-consuming electrode in chloralkali electrolysis or fuel cell technology.

  The present invention is known per se for the electrolysis of aqueous alkali metal chlorides in the form of a gas diffusion electrode and using an oxygen consuming electrode typically comprising a gas diffusion layer comprising an electrically conductive support and a catalytically active component. This is derived from the electrolysis method.

  Various proposals for the operation of oxygen-consuming electrodes in industrial scale electrolysis cells are known in principle from the prior art. The basic concept is to replace the hydrogen generating cathode with an oxygen consuming electrode (cathode) in electrolysis (eg in chloralkali electrolysis). A review of possible cell designs and solutions can be found in a publication by Moussallem et al., “Chlor-Alkali Electricity with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”. Appl. Electrochem, 38, (2008), pages 1177-1194.

  An oxygen consuming electrode (hereinafter also referred to as OCE for short) needs to satisfy a series of requirements in order to be usable in an industrial electrolysis tank. For example, the catalyst used and all other materials should be chemically stable to pure oxygen at a concentrated alkali metal hydroxide solution and typically at a temperature of 80-90 ° C. Similarly, since the electrodes are usually installed and operated in an electrolytic cell having an area size exceeding 2 m 2 (industrial scale), a high degree of mechanical stability is required. Further desired properties are as follows: high electrical conductivity, small layer thickness, large internal surface area and high electrochemical activity of the electrolysis catalyst. Appropriate hydrophobic and hydrophilic pores and appropriate pore structure for gas and electrolyte conduction. Long-term stability and low manufacturing costs are further specific requirements for industrially usable oxygen-consuming electrodes.

  In the case of an OCE arrangement in the cathode element where an electrolyte gap exists between the membrane and the OCE, the hydrostatic pressure forms a pressure gradient across the height of the electrode on the catholyte side, whereas it is constant over that height on the gas side. The problem arises from the pressure. Due to this effect, hydrophobic pores can also be submerged and liquid can enter the gas side in the region of the lower electrode. On the other hand, if the gas pressure is too high at the top of the OCE, the liquid is displaced from the hydrophilic pores and oxygen can enter the catholyte. Both of these reduce the performance of the OCE. In practice, this effect limits the build height of the OCE to about 30 cm unless further measures are taken. In industrial electrolysis, the electrodes are typically constructed at a height of 1 m or higher. Various techniques have been described for realizing such a build height.

  WO2001 / 57290A1 is a kind of free-falling liquid thin film along the OCE, called a falling film for short. The cells to be introduced are described (finite gap arrangement). In this arrangement, the liquid column has no effect on the liquid side of the OCE, and the hydrostatic pressure profile is not configured across the cell build height. However, the construct described in WO2001 / 57290A1 is very complex. In order to ensure a homogeneous alkali flow and contact between the homogeneous OCE and the catholyte, the percolator, ion exchange membrane and OCE must be positioned very accurately.

  In another embodiment, the ion exchange membrane divides, in the electrolysis cell, the anode space and the cathode space that directly contacts the OCE without an intervening space for introducing and removing the alkali metal hydroxide solution. This arrangement is referred to as a zero gap arrangement. Also, the zero gap configuration is typically used in fuel cell technology. Here, there is a drawback that the alkali metal hydroxide solution to be formed needs to pass through the OCE to the gas side and then flow downstream in the OCE. In this process, the pores in the OCE must not be blocked by the alkali metal hydroxide solution and crystallization of the alkali metal hydroxide should not be present in the pores. It has been found that very high alkali metal hydroxide concentrations can occur here, but at this high concentration it has been stated that ion exchange membranes lack long-term stability (Lipp et al., J. Appl. Electrochem. 35, 2005, p. 1015—Los Alamos National Laboratory, “Peroxide formation chlor-alkali electrification with carbon-based ODC”).

  A further arrangement, sometimes referred to as “zero gap”, is more precisely defined as “microgap” and is described in JP3553775 and US6117286A1. In this arrangement, due to its absorbency, there is an additional layer of pore hydrophilic material between the ion exchange membrane and the OCE that absorbs the formed aqueous alkali and discharges at least a portion of the alkali downward. . The means of aqueous alkali discharge is determined by the introduction of OCE and cell design. The advantage of this arrangement is that the amount of alkali that flows down to the opposite side of the OCE is less. As a result, the OCE side (reverse side) facing the gas side comes into more contact with oxygen. Furthermore, the OCE pore system is exposed to a small amount of liquid, so that a larger pore volume is utilized for gas transfer. In contrast to the finite gap arrangement described above, the non-aqueous alkali metal hydroxide solution (alkali) is introduced from the gap between the OCE and the ion exchange membrane as a result of application and discharge, and the pores present in the micro gap The material absorbs aqueous alkali and passes through the microgap in a horizontal or vertical direction. The arrangement described in JP3553775 and US6117286A1 is hereinafter referred to as “microgap”. High performance electrolysis cells having a microgap arrangement are easier to construct and operate than electrolysis cells in a finite gap arrangement.

  US 6117286 A1 describes a further embodiment of a “microgap” OCE configuration in which an aqueous alkaline solution is introduced from the pore hydrophilic material to the side of the OCE facing the gas side from the slot in the OCE. The orifice serves to allow the alkali metal hydroxide solution to drip from the pore hydrophilic material to the gas side of the OCE and downstream in the gas space. The advantage of this arrangement is that the alkali metal hydroxide solution is removed on the gas side of the OCE without any liquid film formation on the OCE. However, the construction in the illustrated embodiment is complex.

  US4333262A1 describes an OCE for a zero gap arrangement with a series of orifices through which the formed alkali metal hydroxide solution passes to the gas side of the OCE. The opening area of the orifice is 2 to 80%, preferably 5 to 25% of the electrode area. The orifice is intended to promote the flow of the alkali metal hydroxide solution over the OCE. In this case, the alkali metal hydroxide solution flows downstream on the gas side of the OCE, but this prevents oxygen from approaching the OCE. In a particular embodiment, the placement of additional highly porous separator material between the membrane and the OCE is described. Materials that may be described for further separators are papers made of aluminum oxide or other ceramic fibers, PVC or polypropylene woven fabrics and fine nickel, in addition to paper made of zirconium oxide fibers. This description does not present further separator material functions. The description of the hydrophobic PVC and polypropylene material means that the separator material does not serve to remove the alkali metal hydroxide solution as a microgap arrangement according to JP 3553775 and US6117286A1. Disadvantages of the embodiment described in US Pat. No. 4,332,662 A1 include the relatively high loss of electrode area and the above-mentioned obstruction of oxygen supply to the OCE.

  The oxygen-consuming electrode is typically composed of a support element, such as a porous metal plate or metal wire mesh, and an electrochemically active coating. The electrochemically catalytically active coating is microporous and is composed of hydrophilic and hydrophobic components. The hydrophobic component makes it difficult for the electrolyte to penetrate and thus makes the corresponding pore in the OCE unblocked due to the transfer of oxygen to the catalytically active site. The hydrophilic component allows the electrolyte to penetrate into the catalytically active site and carry hydroxide ions away from the OCE. The hydrophobic component used is generally a fluorinated polymer, such as polytetrafluoroethylene (PTFE), which further serves as a polymer binder for the catalyst particles. In the case of an electrode having a silver catalyst, for example, silver serves as a hydrophilic component. Many compounds have been described as electrochemical catalysts for the reduction of oxygen. However, only platinum and silver have gained practical importance as catalysts for the reduction of oxygen in alkaline solutions.

Platinum has a very high catalytic activity for the reduction of oxygen. Due to the high cost of platinum, it is used exclusively in support form. A preferred support material is carbon. However, the stability for carbon-supported platinum-based electrodes is insufficient in long-term operation, probably because platinum also catalyzes the oxidation of the support material. Carbon further promotes the undesirable formation of H ₂ O ₂ that also causes oxidation. Silver also has a high catalytic activity for the reduction of oxygen. Silver can be used in a carbon-supported form or as fine metallic silver. Carbon-supported silver catalysts are more durable than the corresponding platinum catalysts, but long-term stability under conditions at one of the oxygen-consuming electrodes is also limited, especially when used for chloralkali electrolysis.

  For the production of OCE comprising an unsupported silver catalyst, the silver is preferably introduced at least partially in the form of a silver oxide that is reduced to metallic silver. The reduction is generally performed when the electrolytic cell is first started up. The reduction of the silver compound also causes a change in the crystal arrangement, and more specifically, cross-linking between individual silver particles. This results in an overall connection of the structures.

  It was observed that when the electrolysis current was turned off, the silver catalyst could be oxidized again. Oxidation is clearly promoted in the half cell by oxygen and moisture. Oxidation can lead to rearrangements in the catalyst structure, which has a negative impact on the activity of the catalyst and thus on the performance of the OCE.

  It has also been found that the performance, especially the required electrolysis voltage, depends considerably on the starting conditions in OCE with silver catalyst. This is true for both the first start of OCE and further start after shutdown. One of the problems of the present invention is to find specific conditions for operations that ensure high performance of the OCE, in particular for shutting down and starting an OCE with a silver catalyst.

  A further major element of the electrolysis cell is an ion exchange membrane. The membrane is permeable to cations and water, but is substantially impermeable to anions. Ion exchange membranes are exposed to strong stresses in the electrolysis cell: they must be stable to chlorine on the anode side and stable at a temperature of about 90 ° C. for strong alkaline stresses on the cathode side. is there. Perfluorinated polymers such as PTFE typically do not withstand these stresses. Ions are carried from sulfonate and / or carboxylate groups that are polymerized into these polymers. Carboxylate groups exhibit high selectivity, and polymers containing carboxylate groups have a lower water absorption and higher electrical resistance than polymers containing sulfonate groups. In general, multilayer films are used with a thick layer containing sulfonate groups on the anode side and a thin layer containing carboxylate groups on the cathode side. The membrane is provided with a hydrophilic layer on the cathode side or on both sides. In order to improve the mechanical properties, the membrane is reinforced by embedding fabrics or knits and the reinforcement is preferably incorporated into the layer containing sulfonate groups.

  Due to the composite structure, the ion exchange membrane is sensitive to changes in the media surrounding it. Different molar concentrations can lead to the formation of a significant osmotic pressure gradient between the anode and the cathode. When the electrolyte concentration decreases, the membrane swells with improved water absorption. When the electrolyte concentration is increased, the membrane releases water and as a result contracts, and in extreme cases, the detachment of water can cause solid deposition in the membrane. Thus, concentration changes can cause collapse and damage in the membrane. This can result in delamination of the layer structure (blister formation), which results in a decrease in mass transfer of mass from the membrane.

  Furthermore, pinholes that can result in mixing of the anolyte and catholyte and in extreme cases cracks can occur.

  In a production plant, it is desirable to operate the electrolysis cell for a period of several years without opening in the meantime. However, due to demand volume changes and failures in the production parts upstream and downstream of the electrolysis, the electrolysis cells in the production plant necessarily have to be repeatedly shut down and restarted.

  Stopping and restarting the electrolysis cell results in conditions that can cause damage to the cell elements and significantly reduce their lifetime. More specifically, oxidative damage has damage to the OCE and damage to the membrane, but is found in the cathode space.

  The prior art discloses few modes of operation where the risk of damage to the electrolysis cell can be reduced during startup and shutdown.

  A known measurement from conventional membrane electrolysis is the maintenance of the polarization voltage, which means that when the electrolysis is terminated, the potential difference is not adjusted downward to zero but is maintained at the level of the polarization voltage. means. In practice, a voltage somewhat higher than that required for polarization is set, resulting in a constant low current and consequently a slight electrolysis. However, in the case of the use of OCE, this measurement alone is insufficient to prevent oxidative damage to the OCE during start-up and shutdown.

  Publication JP 2004-300510A describes an electrolysis method in which the corrosion in the cathode space uses a microgap arrangement by stopping the cell with a sodium hydroxide solution in the gas space. Thus, the inflow of the gas space with sodium hydroxide solution protects the cathode space from corrosion, but provides improper protection from damage to the electrodes and membranes by stopping and starting or during the stopping period.

  US4578159A1 purges the cathode space with 35% sodium hydroxide solution before starting the cell, starts the cell at a low current density, and gradually increases the current density for electrolysis using a zero gap configuration Describes that damage to the membrane and electrodes can be prevented. This procedure reduces the risk of damage to the membrane and OCE during start-up, but does not provide protection from damage during and during a stop.

  The document US 4364806 A1 discloses that the exchange of oxygen with nitrogen prevents corrosion in the cathode space after regulating the electrolysis current downward. According to WO2008009661A2, the addition of a small proportion of hydrogen to oxygen results in an improvement in protection from corrosion damage. However, the described method is particularly complex from a safety point of view and requires the introduction of further equipment for supplying nitrogen and oxygen. Upon restart, the OCE pores are partially filled with nitrogen and / or hydrogen, which prevents the supply of oxygen to the reactive sites. The method does not provide protection from damage to the ion exchange membrane and requires high safety with respect to avoiding explosive gas mixtures.

  The final technical report “Advanced Chlor-Alkali Technology” by Jerzy Christunoff (Los Alamos National Laboratory, DOE Award 03EE-2F / Ed190403, 2004) details the temporary outage and the start of zero-gap cell. . In the case of operation stop, after the electrolysis current stops, the oxygen supply is stopped and replaced with nitrogen. The wetness of the gas stream is increased to wash away residual sodium hydroxide solution. On the anode side, the brine is replaced with hot water (90 ° C.). This procedure is repeated until a stability voltage (open circuit voltage) is obtained. The cell is then cooled and then the supply of wet nitrogen and the pumping of water on the anode side are stopped.

  For restarting, the anode side is first filled with brine and water and nitrogen are introduced on the cathode side. The cell is then heated to 80 ° C. The gas supply is then switched to oxygen and a polarization voltage with a low current flow is applied. The current density is then increased, the pressure at the cathode is increased, and the temperature is raised to 90 ° C. The brine and water supply is then adjusted to obtain the desired concentration on the anode and cathode sides.

  The decrease in NaOH concentration due to increased water supply at the cathode side, with the maintenance of NaCl concentration at the anode side, corresponds to an increase in diffusion of chloride ions into the cathode space, corrosiveness in the cathode space and OCE inertness. Exists with a negative impact on chemistry. By restarting, pure water is initially charged on the cathode side and concentrated brine is charged on the anode side, and a higher chloride burden is again expected on the cathode side.

  Replacing the anolyte with water at 90 ° C. during shutdown operations is expected to swell and expand the membrane, increasing the risk of cracks and other damage. The described procedure is extremely complex, and more specifically, a very high level of complexity is required for industrial electrolysis processes.

  It should be noted that the techniques described so far for starting and shutting down the OCE are disadvantageous and only provide improper protection from damage. More specifically, the described method does not provide sufficient protection in the case of an OCE having a microgap configuration.

International Publication No. 2001 / 57290A1 Pamphlet Japanese Patent No. 3553775 US Pat. No. 6,117,286 U.S. Pat. No. 4,332,662 JP 2004-300510 A US Pat. No. 4,578,159 US Pat. No. 4,364,806 International Publication No. 2008/009661 Pamphlet

Moussallem, “Chlor-Alkali Electrolysis with Oxygen Depolarized Methods: History, Present Status and Future Prospects”, J. Am. Appl. Electrochem, 38, 2008, 1177-1194. Lipp, “Peroxide formation chlor-alkali electrolysis with carbon-based ODC” —Los Alamos National Laboratory, J. Mol. Appl. Electrochem. Volume 35, 2005, page 1015 Jerzy Christunoff, “Advanced Chlor-Alkali Technology”, Los Alamos National Laboratory, DOE Award 03EE-2F / Ed190403, 2004

  The object of the present invention is improved for chloralkali electrolysis using OCE with microgap configuration and silver catalyst as electrocatalyst material, which prevents damage during start-up and shutdown processes and intervening shutdown periods. It was to find an electrolyzer. The operating parameters for the operation of such a device, in particular for starting up and shutting down, should be easily implemented, so that the compatibility can be achieved with membranes, electrodes and / or other components of the electrolysis cell. Should be prevented from damage.

  The invention comprises a support element in the form of at least one sheet-like structure and a coating with a gas diffusion layer and a catalytically active component, the coating with the gas diffusion layer being 0.5 mm to 20 mm, preferably Ion exchange in particular in a microgap arrangement, characterized in that it has an opening with a diameter or height of 1 mm to 1 mm and preferably having a cross-sectional area of 0.05% to 15% of the total OCE area An oxygen consuming electrode is provided for use in an electrolysis cell having a distance of between 0.01 mm and 2 mm between the membrane and the oxygen consuming electrode.

1 is a schematic view of an OCE having a horizontal slot 1. Schematic of OCE with slot 2 arranged on a diagonal. FIG. 3 is a schematic diagram of an OCE having a horizontally arranged row of holes 3. Schematic of OCE having holes 4 uniformly arranged on the electrode area. A schematic cross section through an OCE 8 having a horizontal slot 5 and a guide plate 6 and an ion exchange membrane 7.

  The present invention further provides an electrolytic cell comprising chloralkali electrolysis, wherein the oxygen consuming electrode (OCE) is in a microgap configuration, particularly at a distance of 0.01 mm to 2 mm between the ion exchange membrane and the oxygen consuming electrode. Method for performing the cell comprising at least one anode space containing an anolyte, an ion exchange membrane, a cathode space having an oxygen consuming electrode containing a silver-containing catalyst as a cathode, and a flat surface between the OCE and the membrane Having a pore element, the process element has a thickness of 0.01 mm to 2 mm, and in the first step before the application of the electrolysis voltage between the anode and the cathode, the oxygen consuming electrode is 1000 ppm or less on the gas side, Preferably alkali metal hydroxide aqueous solution having a chloride ion content of 700 ppm or less, more preferably 500 ppm or less In wet, after introduction into the subsequent introduction and cathode space of the oxygen gas to the anode space of the anolyte provides a method characterized by applying the electrolysis voltage.

The present invention is also a method for performing chloralkali electrolysis with an electrolytic cell including the oxygen consuming electrode in a microgap configuration, the cell comprising at least one anode space containing an anolyte, an ion exchange membrane, silver A cathode space having an OCE with a catalyst containing and a flat pore element between the OCE and the membrane, the process element having a thickness of 0.01 mm to 2 mm, at the end of the electrolysis operation, For at least the following steps:
The step of flushing the anolyte to remove chloride and cooling the anolyte,
・ Process to cut electrolytic voltage,
A process of discharging the anode space,
Preferably replenishing the anode space with one of the following liquids and then discharging the anode space: 4 mol / L or less of diluted alkali metal chloride solution or deionized water,
Replenish the cathode space with one of the following liquids and then drain the cathode space: dilute alkali metal chloride solution or deionized water of 4 mol / L or less,
Are provided in this order.

  These two variants of electrolysis are, in the preferred embodiment, combined with each other to meet any of the conditions described for starting for electrolysis and for shutting down. This also includes the preferred variants described below.

  At the cathode, strong oxidation states are present due to oxygen, which are no longer replenished by the electrolysis current at shutdown. After cutting off the electrolysis current, further chloride ions diffuse from the membrane into the increased area into the cathode space. Chloride ions facilitate the corrosion process, and further, oxidation of the silver catalyst can form insoluble silver chloride. Damage to the electrode and to the entire cathode space occurs.

  When the electrolysis voltage decreases, the mass transfer through the membrane caused by the current flow also stops. The membrane begins to become deficient in water, and there may be shrinkage and solid precipitation followed by cracks and pinhole formation, facilitating the passage of anions over the membrane. Secondly, due to restart, excessively low water content interferes with mass transfer from the membrane. This results in a pressure increase and delamination at the interface.

  Inhomogeneities in the water and / or ion distribution in the membrane and / or OCE cause local surges in current and mass transfer, which in turn leads to damage to the membrane or OCE.

  There are also problems due to precipitation of alkali metal chloride salts on the anode side. A significant permeability gradient between the anolyte and the catholyte results in the movement of water from the anodic space to the cathodic space. As long as electrolysis is in operation, water migration from the anode space is countered by the loss of chloride and alkali metal ion anodes, and the concentration of alkali metal chloride is reduced under standard electrolysis conditions in the anode space. . When the electrolysis is switched off, the water transfer from the anode space to the cathode space caused by osmotic pressure remains. The concentration of the anolyte is higher than the saturation limit. This results in the precipitation of alkali metal chloride salts that cause damage to the membrane, particularly in the boundary region to the membrane.

  Surprisingly, it has been found that a novel OCE having an opening that is small compared to the total electrode area maintains its performance over a long period of time by a series of simple means in the course of starting and shutting down. The OCE of the present invention is particularly suitable for an industrial electrolysis apparatus having an electrode having a height of 1 m or more, but is not limited thereto. The OCEs of the present invention and methods for operating these OCEs are particularly suitable for the electrolysis of aqueous sodium chloride and aqueous potassium chloride solutions.

  The opening in the OCE can be arranged in any form. Preference is given to circular, oval, rectangular or trapezoidal cross sections. A possible embodiment is shown in FIGS. Particularly preferred are slots that can be arranged horizontally, vertically or diagonally. FIG. 1 shows an OCE having horizontal slots 1 at three different heights. Vertical slots can be similarly arranged. FIG. 2 shows slots 2 arranged diagonally. FIG. 3 shows an opening in the form of a hole, and the hole 3 in FIG. 3 is over-displayed with respect to the size of the OCE. FIG. 4 shows a variant with holes 4 arranged homogeneously on the OCE, here again the size of the holes is over-represented with respect to the OCE area. Similarly, more holes can be placed on top of the OCE (not shown).

  The diameter or height of the opening (shorter side in the case of a rectangular cross-section, shorter diameter in the case of an oval, similarly applies to other shapes) 0.5 mm to 20 mm, preferably 1 mm -10 mm.

  The opening somewhat reduces the area of the catalytically active layer. However, the loss is small and acceptable in the long-term stability advantage for multiple start and stop operations. This is particularly achievable when the total area of the opening is 0.05% to 15% of the electrode area. Therefore, preferred is an oxygen consuming electrode characterized in that the total area of the opening with respect to the electrode side facing the ion exchange membrane is 0.05 to 15%, preferably 1 to 12% of the electrode area. .

  The openings can be uniformly distributed over the electrode area. Preferred is an arrangement where the majority of the openings are in the upper third of the electrodes. This majority means over 60% of the sum of all passage orifices. In a particularly preferred embodiment, one slot is present in each case at a distance of 0.5 cm to 10 cm from the upper and lower ends. Similarly, it can be mentioned that the slots are arranged vertically or diagonally.

  This opening extends horizontally or is given a gradient in the direction of the passage (disposed diagonally, see FIG. 2).

  From the passage orifice, it is also possible for the sodium hydroxide solution to pass from the gap between the membrane and the OCE to the side of the OCE facing the gas side. As shown in principle in FIG. 5, here, during the construction period, the device installed in the cathode half shell takes in the sodium hydroxide solution as the sodium hydroxide solution passes through, and faces the sodium hydroxide solution to the gas side. It is possible to take away from the OCE side. Especially in the case of slots, for example the guide plate 6 below the slot 5 is placed on the structure of the cathode half shell, takes up the alkali metal hydroxide solution, and drip the alkali metal hydroxide solution, for example, It is possible to prevent contact with the OCE anymore.

  Preferably, the total area of the opening with respect to the electrode side facing the ion exchange membrane is 0.05 to 15%, preferably less than 1 to 12% of the OCE area.

  The openings can preferably be arranged to be continuous through the total oxygen consuming electrode, i.e. through both the catalytically active layer and the support element. The opening can affect the catalytically active layer only and can be arranged such that the support element remains in the opening. Since the carrier element is essentially permeable and remains in the region of the opening, it does not compromise the mechanical stability of the OCE. In some embodiments, for example in the case of certain OCE manufacturing methods, it may be advantageous to leave the carrier element in the opening.

  The oxygen electrode having an opening of the present invention can be manufactured by various techniques.

  For example, in an OCE manufactured by a dry or wet process, the openings can be made by stamping, cutting, drilling, machining or other drilling techniques. In general, the opening is made here from the entire electrode, including the carrier element.

  It is also possible to set conditions under which the electrode has a desired opening by measurement during the manufacture of the electrode. This can be done, for example, by application of strips, cylinders or catalyst compositions corresponding to the dimensions of the opening before compression and fitting of similar dimensional displacements. In this embodiment, the carrier element preferably remains in its original form. The replacement body can also be placed in opposite positions on both sides of the carrier element. The replacement body is arranged so that it can be removed again in the course of the manufacturing method. This can be done by removing the substitute and by melting or other physical or chemical techniques.

  The operating parameters for the start and stop of the electrolysis cell are as follows: OCE of the invention with silver catalyst and microgap configuration and standard alkali metal chloride concentration (anolyte) of 2.5-4.0 mol / L and An electrolysis cell having an alkali metal hydroxide concentration (catholyte) of 8-14 mol / L is described, but is not limited to the implementation of the described procedure. According to the invention, for starting and stopping such an electrolytic cell, before starting, the cathode space is wetted with a sodium hydroxide solution having low contamination with chloride ions, It is also possible to use further embodiments in which the anolyte is discharged in the first step, preferably the anodic space is flushed, and the catholyte is discharged and the cathodic space is flushed in subsequent steps after the voltage has been switched off. It is.

Microgap arranged, starting of the electrolysis apparatus having an ion-exchange membrane soaked in alkaline water according OCE and the prior art having a silver catalyst, in a first step, performed by wetting the cathode space in alkaline water. Wetting is performed, for example, by filling the cathode space with an alkali metal hydroxide solution and immediately thereafter discharging the alkali metal hydroxide solution. The concentration of alkaline water used is between 0.01 and 13.9 mol / L, preferably 0.1 to 4 mol of alkali metal hydroxide per liter. Alkaline water should be substantially free of chloride and chlorate ions.

  Preference is given to a method characterized in that the alkali metal hydroxide solution introduced into the catholyte supplied before application of the electrolysis voltage has a chlorate ion content of 20 ppm or less, preferably 10 ppm or less.

  The temperature of the alkali metal hydroxide solution for wetting is 10 to 95 ° C, preferably 15 to 60 ° C.

  The residence time of the alkali metal hydroxide solution in the cathode space is immediately drained, that is, the alkali metal hydroxide solution is immediately released again from the cathode space after filling, and for 200 minutes.

  After releasing the alkali metal hydroxide solution from the cathode space, oxygen is added. Preference is given to releasing the sodium hydroxide solution and adding oxygen to replace the sodium hydroxide solution introduced with oxygen. Usually, the positive pressure at the cathode of 10 to 100 mbar is set according to the arrangement in the cell.

  The concentration is determined by titration or other methods known to those skilled in the art.

  For wetting of the cathode space, preference is given to using alkali metal hydroxide solutions from conventional production. Alkali from shutdown operations is not particularly suitable for wetting prior to cell startup due to contamination with chloride ions.

  After releasing the alkali metal hydroxide solution from the cathode space, oxygen is added. Preference is given to releasing the sodium hydroxide solution and adding oxygen to replace the sodium hydroxide solution introduced with oxygen. Usually, the positive pressure at the cathode of 10 to 100 mbar is set according to the arrangement in the cell.

  After discharging the alkali metal hydroxide solution from the cathode space, the anode space is filled with an alkali metal chloride solution (brine). Brine meets the purity requirements conventionally used for membrane electrolysers. After filling the anode space, brine is introduced by pump circulation from the anode space according to normal equipment conditions. In the process of pump circulation, the anolyte can be heated. The temperature of the supplied brine is selected to establish a temperature of 30-95 ° C. at the output from the anode space. The alkali metal chloride concentration in the fed anolyte is between 150 and 330 g / L.

After filling the anode space and setting up the anode circulation, the electrolysis voltage is applied in the next step. This is done immediately after reaching the anode charge and the temperature of the brine exiting the anode space beyond 60. Subsequent to filling the anode space, it is advantageous if at least part of the polarization voltage or electrolysis voltage is applied. The polarization voltage or electrolysis voltage is adjusted to establish a current density of 0.01 A / m ^{2 to} 40 A / m ² , preferably 10 to 25 A / m ² . At this current density, the time should be 30 minutes or less, preferably 20 minutes or less.

Overall, the total time for start-up should be minimized. The time after filling the anode space with brine and reaching an electrolysis power of> 1 kA / m ² should be less than 240 minutes, preferably less than 150 minutes. The electrolysis current preferably increases at a rate of 3 to 400 A / m ² per minute. The electrolysis cell is then subjected to design parameters such as a concentration of 2.5 to 4.0 moles of alkali metal per liter in the anode space, a current density of ^{3 to} 6 kA / m ^{2 and} a 50% to 100% excess oxygen in the gas supply. Operate with. The oxygen introduced into the cathode compartment is preferably saturated with water vapor at room temperature (ambient temperature). This can be done, for example, by passing oxygen through a water bath before introducing it into the cathode compartment. Likewise, it is conceivable to carry out the wetting at higher temperatures. The concentration of sodium hydroxide solution flowing out of the cathode compartment is essentially established by selection of the alkali metal chloride concentration in the ion exchange membrane and in the anode space between 8 and 14 mol / L. The alkali metal hydroxide solution advantageously flows out of the cathode space spontaneously.

  The described method is suitable for both starting an electrolysis cell containing the inventive OCE after initial start-up and shut-down of the electrolyzer after introduction of silver-containing, in particular silver oxide-containing OCE, which has not yet been operated.

In stopping the electrolysis cell, the following steps:
Reducing the electrolysis voltage and removing chlorine from the anolyte so that less than 10 ppm of active chlorine is present in the anolyte;
Reducing the temperature of the anolyte to below 60 ° C. (20-60 ° C.) and discharging the anode space;
Preferably replenishing the anode space with one of the following liquids: dilute alkali metal chloride solution or deionized water of 4 mol / L or less,
-Discharging the anode space preferably after 0.01 to 200 minutes;
Filling the cathode space with one of the following liquids: 0.01-4 mol / L or less of diluted alkali metal chloride solution or deionized water,
A step of discharging the cathode space preferably after 0.01 to 200 minutes;
In this order.

In the first step, the electrolysis voltage is adjusted downward. In the present invention, the voltage can be adjusted down to zero. Preferably, after reducing the electrolysis current, the voltage is maintained and cut only after a drop to a chlorine content in the anode space of <10 mg / L, preferably less than 1 mg / L. Chlorine content is understood here to mean the total content of dissolved chlorine in zero or more oxidation states. Residual chlorine is preferably removed from the anode space by supplying a chlorine-free anolyte simultaneously with the removal of the chlorine-containing anolyte or by pump circulation of the anode circuit with simultaneous separation and removal of chlorine gas. To do. Voltage during this operation, 0.01~40A / ^{m 2,} preferably adjusted to establish a current density of 10~25A / ^{m 2.} After turning off the electrolysis voltage and the polarization voltage, the anolyte is discharged.

  After turning off the electrolysis voltage, the anolyte is cooled to a temperature below 60 ° C. and then discharged.

  Thereafter, the anode space is flushed. The flush is performed using highly diluted brine, water or preferably deionized water having an alkali metal chloride content of 0.01-4 mol / L. The flush is preferably performed by filling the anode space once and releasing the flush liquid immediately. The flash is charged and discharged in two or more steps, for example, first with dilute brine having an alkali metal chloride content of 1.5-2 mol / L and then with a NaCl content of 0.01 mol / L. It may be done by filling and draining with highly diluted brine or having diluted water. The flash solution may be released again immediately after the anode space is completely filled, or may remain in the anode space for up to 200 minutes and then released. After discharge, a small residual amount of flash solution remains in the anode space. Thereafter, the anode space is encased or shut down without direct contact with the surrounding ambient air.

  The catholyte still present after discharging the anode space is discharged from the cathode space and then the cathode gas space is flushed. The flush is performed with highly diluted alkaline water, water or preferably deionized water having an alkali metal hydroxide content of up to 4 mol / L. The flush is preferably done by filling the gas space once and releasing the flush liquid immediately. The flash is charged and discharged in two or more steps, for example, first with a diluted alkali having an alkali metal hydroxide content of 1.05 to 3 mol / L and then with 0.01 mol / L of alkali metal water. It may be carried out by further filling and discharging with highly diluted alkali having an oxide content or with deionized water. The flash solution may be released again immediately after the cathode space is completely filled, or it may stay in the anode space for up to 200 minutes and then be released. After discharge, a small residual amount of flash solution remains in the cathode space. The cathode space is put into a case or brought into a shutdown state without directly contacting the surrounding outside air.

  The oxygen supply can be stopped when the polarization voltage is turned off. The oxygen supply is preferably stopped before the discharge and flushing of the cathode space, and the orifice for supplying oxygen serves for the discharge or degassing of the cathode space during the filling process.

  After discharging the anode space and the cathode space, the electrolysis cell with the wet membrane can remain ready for sudden start-up without degrading the performance of the electrolysis cell over time in the installed state. In the case of downtime extending over several weeks, it is appropriate to flush or wet the anode space with dilute alkali metal chloride aqueous solution and the cathode space with dilute alkali metal hydroxide aqueous solution at regular intervals for stabilization. It is. Preference is given to flushing at intervals of 1-12 weeks, particularly preferred at intervals of 4-8 weeks. The concentration of the diluted alkali metal chloride solution for flashing or wetting is 1 to 4.8 mol / L. The flash solution may be released again immediately after the anode space is completely filled, or it may stay in the anode space for up to 200 minutes and then be released. The concentration of the alkali metal hydroxide solution used for flashing or wetting is between 0.1 and 10 mol / L, preferably between 1 and 4 mol / L. The temperature of the brine or alkali metal hydroxide solution can be between 10 and 80 ° C, preferably between 15 and 40 ° C. The flash solution may be released again immediately after the cathode space is completely filled, or it may stay in the cathode space for up to 200 minutes and then be released.

  A further embodiment of the method of the invention comprises the step of flushing the electrode spaces, understood to mean the cathode and anode spaces of the electrolysis cell, with a wet gas. For this purpose, for example, water saturated nitrogen is introduced into the anode space. Alternatively, oxygen may be introduced. The gas volume flow rate can be measured and volume exchange of 2 to 10 times can be performed. For a gas volume of 100 liters, the gas volume flow rate may be 1 L / hr to 200 L / hr at a temperature of 5-40 ° C., and the temperature of the gas is preferably ambient temperature, ie 15-25 ° C. . The purge gas is saturated with water at the gas temperature.

  The electrolytic cell removed from the operation by the above method is returned to the operation again by the above method. In the case of compatibility with the described process steps, the electrolysis cell can go through many start-up and shut-down cycles without any hindrance in cell performance.

  The invention will now be described in detail, by way of example, with reference to the drawings.

FIG. 1 is a schematic view of an OCE having a horizontal slot 1.

FIG. 2 is a schematic view of an OCE having slots 2 arranged diagonally.

3 is a schematic view of an OCE having a horizontally arranged row of holes 3. FIG.

FIG. 4 is a schematic view of an OCE having holes 4 uniformly arranged on the electrode area.

FIG. 5 is a schematic cross section through an OCE 8 having a horizontal slot 5 and a guide plate 6 and an ion exchange membrane 7.

Example 1
A powder mixture consisting of 7 wt% PTFE powder, 88 wt% silver (I) oxide and 5 wt% silver powder was applied to a mesh of nickel wire and pressed to form an oxygen consuming electrode (OCE). . The electrode was placed in an electrolyzer having an area of 100 cm ² with a DuPONT N2030 type ion exchange membrane and a PW3MFBP carbon woven fabric from Zoltek having a thickness of 0.3 mm. A carbon woven fabric was placed between the OCE and the membrane. The electrolyzer in assembly has an anolyte supply and discharge, an anode space with an anode made of coated titanium (ruthenium oxide coating), an OCE as a cathode, and a gas space for oxygen and oxygen A cathode space having an inlet and an outlet, and an ion exchange membrane disposed between the anode space and the cathode space. The OCE, carbon woven fabric and ion exchange membrane were compressed onto the anode at a pressure of about 30 mbar with high pressure in the cathode furnace.

  In the first step, the cathode space was filled with 30 wt% sodium hydroxide solution having a content of 20 ppm chloride ions and a content of <10 ppm chlorate ions at 80 ° C. and then discharged again. In the discharge process, oxygen was supplied and the resulting gas space was filled with oxygen. After discharge, a positive pressure of 30 mbar was established on the cathode side.

In the next step, the anode space was filled with chlorine having a concentration of 210 g NaCl / L at 70 ° C. and the anode circulation was put into operation. Immediately after reaching the constant operation of the anode circulation, the electrolysis voltage was applied. Controls electrolysis current, the electrolytic current of 1 kA / ^{m 2} after 5 minutes, and the electrolytic current of 3 kA / ^{m 2} was obtained after 30 minutes. The equipment was operated for 3 days at a power of 3 kA / m 2 and an electrolysis voltage of 1.90 to 1.95 V, a concentration of 32% by weight of removed sodium hydroxide solution and a temperature in the electrolysis cell of 88 ° C.

Example 2
A powder mixture consisting of 7 wt% PTFE powder, 88 wt% silver (I) oxide and 5 wt% silver powder was applied to a mesh of nickel wire and pressed to form an oxygen consuming electrode. The effective OCE area was 10 × 10 cm. Using a cutter knife, one horizontal slot, each 2 mm high from the electrode, was cut 1 cm above the bottom and 1 cm below the top. 4% of the OCE area was removed. The OCE was installed in an electrolysis apparatus having a DuPONT N2030 type ion exchange membrane and a Zoltek PW03 carbon woven fabric having a thickness of 0.3 mm between the OCE and the membrane.

  In the first step, the cathode space was filled with 30 wt% sodium hydroxide solution having a content of 20 ppm chloride ions and a content of <10 ppm chlorate ions at 80 ° C. and then discharged again. In the discharge process, oxygen was supplied and the resulting gas space was filled with oxygen. After discharge, a positive pressure of 30 mbar was established on the cathode side.

In the next step, the anode space was filled with chlorine having a concentration of 210 g NaCl / L at 70 ° C. and the anode circulation was put into operation. Immediately after reaching the constant operation of the anode circulation, the electrolysis voltage was applied. Controls electrolysis current, the electrolytic current of 1 kA / ^{m 2} after 5 minutes, and the electrolytic current of 4 kA / ^{m 2} was obtained after 30 minutes. The equipment was operated for 10 days at a power of 4 kA / m ² and an electrolysis voltage of 2.0 to 2.2 V, a concentration of 32% by weight of removed sodium hydroxide solution and a temperature in an electrolysis cell of 90 ° C. The average electrolysis voltage was 2.14V.

  By making the slots in OCE, no reduction in electrolysis power was seen.

Example 3

  The electrolyzer according to Example 1 was taken out of operation as follows after 3 days of operation:

The electrolyzer was operated down to 90 A / m ² . The anolyte was circulated for over 60 minutes until a chlorine content of 1 mg / L was obtained, and then the electrolysis current was turned off. Within this time, the anolyte was cooled to 70 ° C. The anode space was drained and then filled with deionized water to overflow and immediately drained again.

  Thereafter, the liquid remaining in the cathode space was discharged, the oxygen supply was stopped, the cathode space was filled, overflowed with deionized water, and immediately discharged again.

  After 50 hours of shutdown, the electrolyzer from Example 2 was returned to operation as follows:

  In the first step, the cathode space was filled with a 32 wt% sodium hydroxide solution having a content of 20 ppm chloride ions and a content of <10 ppm chlorate ions at 80 ° C. and then discharged again. In the discharge process, oxygen was supplied and the resulting gas space was filled with oxygen. After discharge, a positive pressure of 30 mbar was established on the cathode side.

In the next step, the anode space was filled with chlorine having a concentration of 210 g NaCl / L at 70 ° C. and the anode circulation was put into operation. Immediately after reaching the constant operation of the anode circulation, the electrolysis voltage was applied. Controls electrolysis current, the electrolytic current of 1 kA / m ² after 5 minutes, 30 minutes after the electrolysis current of 3 kA / m ^2, the temperature at 32% by weight of the removed concentration and 88 ° C. of the electrolytic cell of the sodium hydroxide solution Obtained.

The electrolysis voltage at 3 kA / m ² was 1.8 to 1.9V. The electrolyzer did not show a decrease compared to the period before shutdown, and in fact, an improvement by 100 mV was observed.

Claims (12)

  For use in an electrolysis cell in a microgap arrangement, it has a support element in the form of at least one sheet-like structure and a coating with a gas diffusion layer and a catalytically active component, the coating having said gas diffusion layer comprising An oxygen consuming electrode comprising one or more openings having a diameter or height of 0.5 mm to 20 mm, preferably 1 mm to 10 mm.   2. The oxygen consuming electrode according to claim 1, characterized in that the total area of the opening on the side of the electrode facing the ion exchange membrane is 0.05 to 15%, preferably 1 to 12% of the electrode area. .   3. The oxygen electrode according to claim 1 or 2, characterized in that only the coating on the gas diffusion layer of the support element and the coating of the catalytically active component have openings.   3. The oxygen electrode according to claim 1, wherein the opening is continuous through the coating and the carrier element.   The oxygen consuming electrode according to any one of claims 1 to 4, wherein the opening is formed by punching, cutting, machining, or punching from the oxygen consuming electrode.   The corresponding orifice is maintained in the method for manufacturing an electrode, in the course of the coating of the carrier element, by the insertion of a removable substitution which is subsequently removed after the coating has been completed. The oxygen-consuming electrode according to any one of 1 to 3.   The oxygen consuming electrode according to any one of claims 1 to 3, wherein the oxygen consuming electrode has a distance of 0.01 mm to 2 mm between the ion exchange membrane and the oxygen consuming electrode.   A method for performing chloralkali electrolysis with an electrolysis cell comprising an oxygen-consuming electrode according to claims 1-7 in a microgap arrangement, the cell comprising an anode and an alkali metal chloride. At least one anode space having an anolyte, an ion exchange membrane, a cathode space having an oxygen-consuming electrode comprising a silver-containing catalyst as a cathode, and an OCE having a thickness of 0.01 mm to 2 mm and into which the catholyte flows And a flat pore element between the membranes, and before applying the electrolysis voltage, in the first step, the oxygen-consuming electrode on the gas side is less than 1000 ppm, preferably less than 700 ppm, more preferably less than 500 ppm chloride. Wetting with an aqueous alkali metal hydroxide solution with an ionic content and subsequent introduction of the anolyte into the anode space And applying an electrolysis voltage after introduction of the oxygenous gas into the cathode space. A method for performing chloralkali electrolysis with an electrolysis cell comprising an oxygen-consuming electrode according to any of claims 1 to 7 in a microgap arrangement, the cell comprising at least one anode space containing an anolyte. An ion exchange membrane, a cathode space with OCE with a silver-containing catalyst and a flat pore element between the OCE and the membrane, the pore element having a thickness of 0.01 mm to 2 mm, and electrolysis At the end of the operation, at least the following steps are taken for shutdown:
・ Process to cut electrolytic voltage,
A process of discharging the anode space,
Preferably replenishing the anode space with one of the following liquids and then discharging the anode space: 4 mol / L or less of diluted alkali metal chloride solution or deionized water,
Replenish the cathode space with one of the following liquids and then drain the cathode space: dilute alkali metal chloride solution or deionized water of 4 mol / L or less,
Performing in this order.   The alkali metal hydroxide solution used for wetting the cathode space before application of the electrolysis voltage is characterized in that it has a chlorate ion content of 20 ppm or less, preferably 10 ppm or less, more preferably 4 ppm or less, The method of claim 7.   After the electrolysis cell is shut down and discharged, it is repeated as necessary, and the anode space is changed to 1.7 to 3.4 mol / L, preferably 2 to every 1 to 12 weeks, preferably every 4 to 8 weeks. A dilute alkali metal chloride aqueous solution having a content of 2 to 2.9 mol / L and a dilute alkali metal hydroxide aqueous solution of 2 to 9 mol / L, preferably 4 to 6 mol / L in the cathode space. 10. A method according to claim 8 and 9, characterized in that it is flushed.   12. Process according to any one of claims 7 to 11, characterized in that the alkali metal chloride is sodium chloride or potassium chloride, preferably sodium chloride.

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