JP6549816B2 - Electrolysis of alkali metal chlorides using an oxygen-consuming electrode with an orifice - Google Patents

Description

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

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

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

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

  In the case of an arrangement of OCEs in the cathode element where an electrolyte gap is present between the membrane and the OCE, on the catholyte side, the hydrostatic pressure forms a pressure gradient over the height of the electrode, whereas on the gas side it is constant over that height Problems arise from pressure. Due to this effect, in the area of the lower electrode, hydrophobic pores may also be flooded and liquid may enter the gas side. On the other hand, if the gas pressure is too high at the top of the OCE, the liquid may be displaced from the hydrophilic pores and oxygen may 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 the realization of such build heights.

  WO 2001/57290 A1 is a type of free-falling liquid film along the OCE, abbreviated as falling film, from a flat pore element, called a percolator, in which the liquid is placed from the top to the downstream between the OCE and the ion exchange membrane Describe the cell to be introduced (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 build height of the cell. However, the constructs described in WO 2001/57290 A1 are quite complex. The percolator, the ion exchange membrane and the OCE have to be arranged very precisely in order to ensure a homogeneous alkaline flow and contact of the homogeneous OCE with the catholyte.

  In another embodiment, the ion exchange membrane divides the anode space and the cathode space directly in contact with the OCE in the electrolytic cell without an intervening space for introducing and removing the alkali metal hydroxide solution. This arrangement is referred to as a zero gap arrangement. Also, zero gap arrangements are typically used in fuel cell technology. Here, the alkali metal hydroxide solution that is formed has the disadvantage that it has to pass the OCE to the gas side and then flow downstream at the OCE. In this process, the pores in OCE should not be blocked by the alkali metal hydroxide solution and there should be no crystallization of alkali metal hydroxide in the pores. It has been found that very high alkali metal hydroxide concentrations can occur here, but at this high concentration the ion exchange membrane is stated to lack long-term stability (Lipp et al., J. Appl. Electrochem. 35, 2005, 1015-Los Alamos National Laboratory, "Peroxide formation during chlor-alkali electrolysis with carbon-based ODC".

  A further arrangement, sometimes referred to as "zero gap", is more precisely defined as "micro gap" and is described in JP 3 535 775 and US 6 117 286 A1. In this arrangement, due to its absorbency, a further layer of porous hydrophilic material is present between the ion exchange membrane and the OCE, which absorbs the formed aqueous alkali and discharges at least a part of the alkali downwards. . The means of aqueous alkaline drainage is determined by the introduction of OCE and cell design. The advantage of this arrangement is that the amount of alkali flowing downward to the opposite side of the OCE is less. As a result, the side of the OCE facing the gas side (opposite side) is more in contact with oxygen. Furthermore, the pore system of OCE is exposed to small amounts of liquid, so that a larger pore volume is available for gas transport. In contrast to the above finite gap arrangement, 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 ejection, and the pores present in the micro gap The substance absorbs aqueous alkali and passes through the microgap towards horizontal or vertical direction. The arrangements described in JP 3553 775 and US 6 117 286 A1 are hereinafter referred to as "microgaps". A high performance electrolytic cell with a microgap arrangement is easier to construct and operate than an electrolytic cell in a finite gap arrangement.

  No. 6,117,286 A1 describes a further embodiment of the “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 downstream of the pore hydrophilic substance on the gas side of the OCE and its 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.

  US4332662A1 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 facilitate the flow of the alkali metal hydroxide solution than the OCE. In this case, the alkali metal hydroxide solution flows downstream on the gas side of the OCE, which prevents the access of oxygen to the OCE. In certain embodiments, the arrangement of additional highly porous separator materials between the membrane and the OCE is described. Possible substances which may be mentioned for the further separator are, in addition to the paper made of zirconium oxide fibers, the paper made of aluminum oxide or other ceramic fibers, PVC woven or polypropylene woven and pore nickel. This description does not present the function of further separator materials. The description of 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 3553 775 and US 6117 286 A1. Disadvantages of the embodiments described in US Pat. No. 4,332,662 A1 include relatively high loss of electrode area and impaired oxygen supply to the above OCEs.

  The oxygen consuming electrode is typically comprised of a support element, such as a porous metal plate or metal wire mesh, and an electrochemically catalytically 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, thus making the corresponding pores in the OCE unblocked for transfer of oxygen to the catalytically active site. The hydrophilic component penetrates the electrolyte to the catalytically active site, allowing hydroxide ions to be carried away from the OCE. The hydrophobic component used is generally a fluorinated polymer such as polytetrafluoroethylene (PTFE), which additionally serves as a polymer binder for the particles of the catalyst. In the case of an electrode with a silver catalyst, for example, silver acts 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 carrier form. The preferred carrier material is carbon. However, the stability for carbon-supported platinum-based electrodes is insufficient in long-term operation, presumably because platinum also catalyzes the oxidation of the support material. Carbon further promotes the undesirable formation of H ₂ O _{2 which also} causes oxidation. Silver also has high catalytic activity for the reduction of oxygen. Silver can be used in carbon supported form and as fine metallic silver. Carbon supported silver catalysts are more durable than the corresponding platinum catalysts, but the long term stability under conditions at one of the oxygen consuming electrodes is also limited, especially for use for chloralkali electrolysis.

  In the case of the preparation of OCE comprising an unsupported silver catalyst, the silver is preferably at least partially introduced in the form of silver oxide which is reduced to metallic silver. The reduction is generally carried out when starting the electrolysis cell first. Also, the reduction of the silver compound causes a change in the arrangement of the crystals, and more specifically, cross linking between the individual silver particles. This results in an overall connection of the structure.

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

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

  A further major element of the electrolysis cell is the ion exchange membrane. The membrane is permeable to cations and water but substantially impermeable to anions. Ion exchange membranes are subjected to strong stress in electrolytic cells: they need to be stable on the anode side to chlorine and stable on the cathode side at a temperature of about 90 ° C. against strong alkaline stress is there. Perfluorinated polymers, such as PTFE, typically do not withstand these stresses. The ions are carried from the sulfonate and / or carboxylate groups which are polymerized into these polymers. Carboxylate groups exhibit high selectivity, and polymers containing carboxylate groups have lower water absorption than polymers containing sulfonate groups and have higher electrical resistance. In general, multilayer films are used in thick layers containing sulfonate groups on the anode side and thin layers containing carboxylate groups on the cathode side. The membrane is provided with a hydrophilic layer on the cathode side or both sides. In order to improve the mechanical properties, the membrane is reinforced by embedding of textiles or knits, 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 medium surrounding it. Different molar concentrations can result in 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. If the electrolyte concentration is increased, the membrane will release water, resulting in shrinkage and in extreme cases, water detachment can cause solids to precipitate out of the membrane. Thus, concentration changes can cause collapse and damage in the membrane. This may result in delamination of the layer structure (blister formation), as a result of which the mass is reduced in mass transfer from the membrane.

  In addition, pinholes and, in the extreme case, cracks can occur which can lead to mixing of the anolyte and catholyte.

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

  Shutdown and restart of the electrolytic cell creates conditions that can cause damage to the cell elements and significantly reduce their lifetime. More specifically, oxidative damage is found in the cathode space, with damage to OCE and damage to the membrane.

  The prior art hardly discloses aspects of operation where the risk of damage to the electrolytic cell can be reduced during start up and shut down.

  A known measurement from conventional membrane electrolysis is the maintenance of the polarization voltage, which means that when the electrolysis is finished, 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 the polarization is set, a constant low current and consequently slight electrolysis occur. However, in the case of the use of OCE, this measurement alone is insufficient to prevent oxidative damage to OCE in the process of start up and shutdown.

  The publication specification JP 2004-300510 A describes an electrolysis method in which the corrosion in the cathode space uses a microgap arrangement by stopping the cell with sodium hydroxide solution in the gas space. Thus, the influx of sodium hydroxide solution in the gas space protects the cathode space from corrosion, but gives inadequate protection from damage to the electrodes and the membrane by the stop and start or during the stop period.

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

  The document US Pat. No. 4,364,806 A1 discloses preventing corrosion in the cathode space after the exchange of oxygen with nitrogen regulates the electrolysis current downwards. According to WO 2008009661 A2, the addition of a small proportion of hydrogen to oxygen results in an improvement in the protection from corrosion damage. However, the described method is complicated, especially from the safety point of view, and requires the introduction of further equipment for nitrogen and oxygen supply. At restart, the pores of the OCE are partially filled with nitrogen and / or hydrogen, which prevents the supply of oxygen to the reactive sites. The method does not provide protection against damage to the ion exchange membrane and requires high safety for the avoidance of explosive gas mixtures.

  A final technical report by Jerzy Chlistunoff "Advanced Chlor-Alkali Technology" (Los Alamos National Laboratory, DOE Award 03EE-2F / Ed 190403, 2004) details the conditions for temporary stopping and zero gap cell start-up. . In the case of shutdown, after the electrolysis current is stopped, the oxygen supply is stopped and replaced by nitrogen. The wetting of the gas stream is increased to flush out the remaining sodium hydroxide solution. On the anode side, the brine is replaced with hot water (90 ° C.). This procedure is repeated until a stable voltage (open circuit voltage) is obtained. The cell is then cooled and then the wet nitrogen supply and the water pumping on the anode side is turned off.

  For the restart, first, the anode side is 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 the polarization voltage at 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.degree. The brine and water supplies are then adjusted to obtain the desired concentrations on the anode and cathode sides.

  The decrease in NaOH concentration due to the increased water supply on the cathode side, the maintenance of the NaCl concentration on the anode side, the corresponding increase in the diffusion of chloride ions to the cathode space, the corrosion in the cathode space and the inactivation of OCE Exist with an adverse effect on With restart, pure water is introduced first on the cathode side and concentrated brine is introduced on the anode side, higher chloride loadings are again expected on the cathode side.

  By replacing the anolyte with water during the shutdown operation at 90 ° C., swelling and swelling of the membrane is expected which increases the risk of cracking and other damage. The described procedure is extremely complex and, more specifically, requires a very high level of complexity for industrial electrolysis.

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

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

Moussallem, "Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects", J. Org. Appl. Electrochem, 38, 2008, pages 1177 to 1194 Lipp, "Peroxide formation during chlor-alkali electrolysis with carbon-based ODC" -Los Alamos National Laboratory, J. Org. Appl. Electrochem. 35, 2005, 1015 Jerzy Chlistunoff, "Advanced Chlor-Alkali Technology", Los Alamos National Laboratory, DOE Award 03EE-2F / Ed 190 403, 2004

  The object of the present invention is an improved for chlor-alkali electrolysis with OCE with silver catalyst as a microgap arrangement and electrocatalyst material, preventing damage during start up and shutdown processes and intervening shutdowns. It was to find the electrolyzer. The operating parameters for the operation of such a device, in particular for startup and shutdown, should be easily implemented, whereby the compatibility is determined by the membranes, electrodes and / or other components of the electrolysis cell. Damage to the body should be prevented.

  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 said gas diffusion layer being 0.5 mm to 20 mm, preferably Ion exchange, in particular in the microgap arrangement, characterized in that it has an opening with a diameter or height of 1 mm to 1 mm, preferably with a cross-sectional area of up to 0.05% to 15% of the total OCE area Provided is an oxygen consuming electrode for use in an electrolytic cell having a distance of 0.01 mm to 2 mm between the membrane and the oxygen consuming electrode.

FIG. 2 is a schematic view of an OCE with a horizontal slot 1; FIG. 5 is a schematic view of an OCE with slots 2 diagonally arranged. FIG. 10 is a schematic view of an OCE with a horizontal array of holes 3; Schematic of OCE with holes 4 uniformly arranged on the electrode area. Schematic cross section through OCE 8 with horizontal slot 5 and guide plate 6 and ion exchange membrane 7.

  The invention further relates to an electrolysis cell comprising chloralkali electrolysis, in the microgap arrangement, in particular at a distance of 0.01 mm to 2 mm, between the ion exchange membrane and the oxygen consuming electrode, of said oxygen consuming electrode (OCE) A method for performing, wherein the cell comprises at least one anode space containing anolyte, an ion exchange membrane, a cathode space having an oxygen-consuming electrode containing a silver-containing catalyst as cathode, and a flat between OCE and the membrane With a pore element, the process element has a thickness of 0.01 mm to 2 mm and, in the first step, up to 1000 ppm of oxygen-consuming electrode on the gas side, before the application of the electrolytic voltage between the anode and the cathode Alkali metal hydroxide aqueous solution preferably 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 chlor-alkali electrolysis by means of an electrolysis cell comprising said oxygen-consuming electrode in a microgap arrangement, said cell comprising at least one anode space comprising an anolyte, an ion exchange membrane, silver Cathode space with OCE with contained catalyst and flat pore element between OCE and membrane, the process element having a thickness of 0.01 mm to 2 mm, and at the end of the electrolysis operation, shutdown In order to at least the following steps:
Flushing the anolyte to remove chloride and cooling the anolyte,
・ Step of cutting the electrolysis voltage,
A step of discharging the anode space,
-Preferably, replenishing the anode space with one of the following liquids and then discharging the anode space: diluted alkali metal chloride solution or deionized water of 4 mol / L or less,
Replenishing the cathode space with one of the following liquids and then draining the cathode space: 4 mol / L or less of diluted alkali metal chloride solution or deionized water,
Provide a method characterized in that the steps are performed in this order.

  These two variants of the electrolysis process are, in a preferred embodiment, combined with one another in order to meet the described conditions both for the start up for the electrolysis and for the shutdown. This also includes the preferred variants described below.

  At the cathode, strong oxidation states exist by oxygen, which are no longer replenished by the electrolysis current at shutdown. After the electrolysis current is turned off, chloride ions further diffuse from the membrane into the cathode space. Chloride ions accelerate the corrosion process, and further, oxidation of silver catalysts can form insoluble silver halides. Damage to the electrode and damage to the entire cathode space occur.

  If the electrolysis voltage is reduced, mass transfer through the membrane caused by the current flow also stops. The membrane begins to break down in water and shrinkage and solid precipitation may be present, followed by cracking and pinhole formation, promoting the passage of anions through the membrane. Second, on restart, too low a water content interferes with mass transfer from the membrane. This results in pressure build up and delamination at the interface.

  Heterogeneity in the water and / or ion distribution in the membrane and / or OCE causes a local surge in current and mass transfer, which in turn leads to damage to the membrane or OCE.

  There are also problems due to the deposition of the alkali metal chloride salt on the anode side. The pronounced permeability gradient between the anolyte and the catholyte results in the migration of water from the anode space to the cathode space. As long as the electrolysis is in operation, the migration of water from the anode space is countered by the loss of chloride and alkali metal ion anodes, and the concentration of alkali metal chlorides is reduced under standard electrolysis conditions in the anode space . When the electrolysis is switched off, there is a transfer of water from the anode space to the cathode space caused by the osmotic pressure. The concentration of the anolyte will be higher than the saturation limit. The result is the deposition of alkali metal chloride salts which cause damage to the membrane, especially in the border region to the membrane.

  Surprisingly, it has been found that the novel OCE, which has a small opening compared to the total electrode area, maintains its performance over time by means of a series of simple means in the process of start-up and shutdown. The OCE of the present invention is particularly suitable for industrial electrolytic devices having an electrode with a height of 1 m or more, but is not limited thereto. The OCEs of the invention and the process for operating these OCEs are particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.

  The openings in the OCE can be arranged in any form. Preferred are circular, oval, rectangular or trapezoidal cross sections. Possible embodiments are shown in FIGS. Particularly preferred are slots which may be arranged horizontally, vertically or diagonally. FIG. 1 shows an OCE having horizontal slots 1 at three different heights. Vertical slots may be arranged as well. FIG. 2 shows the slots 2 arranged diagonally. FIG. 3 shows the opening in the form of a hole, the hole 3 in FIG. 3 being over-represented with respect to the size of the OCE. FIG. 4 is a variant with holes 4 arranged homogeneously on the OCE, again the size of the holes being over-represented with respect to the OCE area. Similarly, more holes may 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, the same applies for other shapes as well) 0.5 mm to 20 mm, preferably 1 mm ~ 10 mm.

  The openings reduce the area of the catalytically active layer somewhat. However, the losses are low and acceptable in the long-term stability advantage over multiple start and stop operations. This is particularly achievable if the total area of the openings is between 0.05% and 15% of the electrode area. Therefore, preference is given to an oxygen-consuming electrode characterized in that the total area of the opening with respect to the side of the electrode facing the ion exchange membrane is 0.05 to 15%, preferably 1 to 12% of the electrode area. .

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

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

  From the passage orifice, it is also possible for 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 incorporates sodium hydroxide solution as sodium hydroxide solution passes through, and the sodium hydroxide solution faces the gas side. It is possible to take away from the side of the OCE. In particular in the case of the slot, for example, the guide plate 6 under the slot 5 is placed on the structure of the cathode half shell, taking up the alkali metal hydroxide solution, and dripping the alkali metal hydroxide solution, for example It is possible to no longer contact the OCE.

  Preferably, the total area of the openings with respect to the side of the electrode 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 all the oxygen consuming electrodes, ie through both the catalytically active layer and the support element. The openings affect only the catalytically active layer and may be arranged such that the support elements remain in the openings. The carrier element is essentially permeable and remains in the area of the openings, so that 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 move away from the carrier element in the opening.

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

  For example, in an OCE manufactured by the dry or wet method, the openings can be made by stamping, cutting, drilling, machining or other drilling techniques. In general, the openings here are made of the entire electrode including the carrier element.

  From the measurement during manufacture of the electrode, it is also possible to set conditions under which the electrode has the desired opening. This can be done, for example, by applying a strip, cylinder or catalyst composition corresponding to the size of the opening before compression and fitting in a similar size substitute. In this embodiment, the carrier element preferably remains in its original form. The substitutes can also be placed in opposite positions on both sides of the carrier element. The substitutes are arranged so that they can be removed again in the course of the manufacturing method. This can be done by removing the substitute and also by melting or other physical or chemical methods.

  The operating parameters for starting and stopping the electrolysis cell are, according to the invention, OCE according to the invention with a silver catalyst and a microgap arrangement and a standard alkali metal chloride concentration of 2.5 to 4.0 mol / l (anolyte) and Although described for an electrolytic cell having an alkali metal hydroxide concentration (catholyte) of 8 to 14 moles / L, it is not limited to the practice of the described procedure. According to the invention, in order to start and stop such an electrolysis cell, the cathode space is wetted with sodium hydroxide solution with low contamination with chloride ions prior to start-up, the electrolysis in the process of stopping It is also possible to use a further embodiment in which in the first step the anolyte is discharged, preferably the anode space is flushed and in the subsequent steps the catholyte discharged and the cathode space flushed in a first step after de-energizing. It is.

The starting of the electrolysis apparatus with microgap arrangement, OCE with silver catalyst and ion exchange membrane soaked with alkaline water according to the prior art is carried out in a first step by wetting the cathode space with alkaline water. Wetting takes place, for example, by filling the cathode space with an alkali metal hydroxide solution and immediately thereafter draining 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.

  Preferred is a method characterized in that the alkali metal hydroxide solution introduced into the catholyte supplied prior to application of the electrolytic voltage has a content of chlorate ions 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 between immediate discharge, i.e. immediately releasing the alkali metal hydroxide solution from the cathode space again after filling is complete, and for 200 minutes.

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

  Concentrations are determined by titration or other methods known to those skilled in the art.

  For wetting of the cathode space, it is preferred to use an alkali metal hydroxide solution from conventional production. The alkali from the shutdown operation is not suitable for wetting prior to start up of the cell, in particular due to contamination with chloride ions.

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

  After releasing 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 the normal equipment conditions. The anolyte can be heated in the process of pump circulation. 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 supplied anolyte is between 150 and 330 g / L.

After filling the anode space and raising the anode circulation, the electrolysis voltage is applied in the next step. This is done immediately after the filling of the anode and the temperature of the brine leaving the anode space above 60 has been reached. It is advantageous if the filling of the anode space is followed by at least a part of the polarization voltage or the electrolysis voltage. The polarization 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 startup should be minimized. The time after filling of the anode space with brine and reaching an electrolytic 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 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 ambient temperature (ambient temperature). This can be done, for example, by passing oxygen through a water bath before introducing it into the cathode compartment. It is likewise conceivable to carry out the wetting at higher temperatures. The concentration of sodium hydroxide solution flowing out of the cathode compartment is essentially established from the choice of the alkali metal chloride concentration in the ion exchange membrane and 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 any start up of the electrolytic cell comprising the OCE according to the invention after the initial start-up and shut-down of the electrolyzer after the introduction of the silver-containing, in particular silver oxide-containing OCE which has not yet been operated.

The following steps in stopping the electrolysis cell:
Lowering the electrolytic voltage and removing chlorine from the anolyte to cause less than 10 ppm of active chlorine to be present in the anolyte,
Reducing the temperature of the anolyte to below 60 ° C. (20 to 60 ° C.) and draining the anode space,
-Preferably replenishing the anode space with one of the following liquids: diluted 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: a diluted alkali metal chloride solution of 0.01 to 4 mol / L or less or deionized water,
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 lowering the electrolytic current, the voltage is maintained and only after dropping to a chlorine content in the anode space of <10 mg / L, preferably less than 1 mg / L. The chlorine content is here understood to mean the total content of dissolved chlorine in the oxidation state of zero or more. Residual chlorine is preferably removed from the anode space by providing chlorine-free anolyte simultaneously with removal of the chlorine-containing anolyte, or by pump circulation of the anode circuit with simultaneous chlorine gas separation and removal. Do. The voltage during this operation is adjusted to establish a current density of 0.01 to 40 A / m ² , preferably 10 to 25 A / m ² . After the electrolysis voltage and the polarization voltage are turned off, the anolyte is discharged.

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

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

  The catholyte still present after evacuating the anode space is discharged from the cathode space and then the gas space of the cathode is flushed. The flash is performed with highly diluted alkaline water, water or preferably deionized water with an alkali metal hydroxide content of up to 4 mol / L. The flash is preferably performed by filling the gas space once and releasing the flash liquid immediately. The flash is, for example, first charged and discharged with dilute alkali having an alkali metal hydroxide content of 1.05 to 3 mol / L in two or more steps, and then 0.01 mol / L alkali metal water It may also be done by further filling and draining with highly diluted alkali with oxide content or with deionized water. The flash solution may be released again immediately after the cathode space is completely filled, or may stay in the anode space for up to 200 minutes and then release. After discharge, a small residual amount of flush solution remains in the cathode space. The cathode space is either enclosed in the case or shut down without direct contact with the surrounding ambient air.

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

  After releasing the anode space and the cathode space, the electrolysis cell with the wet membrane can be kept ready for a sudden start-up without degrading the performance of the electrolysis cell for a long time in the installed state. In the case of down time extending over several weeks, for stabilization it is appropriate to flush or wet the anode space with dilute aqueous alkali metal chloride solution and the cathode space with dilute aqueous alkali metal hydroxide solution at regular intervals It is. Preference is given to flushing at intervals of 1 to 12 weeks, particularly preferred at intervals of 4 to 8 weeks. The concentration of the diluted alkali metal chloride solution for flushing or wetting is 1 to 4.8 mol / L. The flash solution may be released again immediately after the anode space has been completely filled, or may remain in the anode space for up to 200 minutes and then released. The concentration of the alkali metal hydroxide solution used to flush or wet 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 may 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 may remain in the cathode space for up to 200 minutes and then released.

  A further embodiment of the method of the invention comprises the step of flushing the electrode space, which is understood to mean the cathode and anode space of the electrolysis cell, with a wetting 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 to perform 2 to 10 volume exchange. For a 100 L gas volume, the gas volume flow rate may be 1 L / hr to 200 L / hr at a temperature of 5-40 ° C., the temperature of the gas is preferably ambient temperature, ie 15-25 ° C. . The purge gas is saturated with water at the temperature of the gas.

  The electrolysis cell removed from the operation according to 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 a number of start-up and turn-down cycles without interruption in the cell's performance.

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

FIG. 1 Schematic of OCE with horizontal slot 1

FIG. 2 shows a schematic of an OCE with slots 2 arranged diagonally.

FIG. 3 shows a schematic view of an OCE with a horizontal arrangement of holes 3;

FIG. 4 shows a schematic of an OCE with holes 4 uniformly distributed over the electrode area.

FIG. 5 a schematic cross section through OCE 8 with horizontal slot 5 and guide plate 6 and 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 electrolytic device having an area of 100 cm ² with a DuPONT N 2030 type ion exchange membrane and a PW3 MFBP 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 anolyte feed 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, 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 by high pressure in a cathode furnace.

  In the first step, the cathode space was filled at 80 ° C. with a 30% by weight sodium hydroxide solution having a content of 20 ppm of chloride ions and a content of chlorate ions of <10 ppm and then drained again. In the course of the evacuation, oxygen was supplied and the gas space obtained 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. The electrolysis voltage was switched on immediately after reaching a constant operation of the anode circulation. 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 installation was operated 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. for 3 days.

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. One horizontal slot, each 2 mm high from the electrodes, was cut 1 cm above the bottom and 1 cm below the top using a cutter knife. 4% OCE area was removed. The OCE was placed on an ionizer membrane of DuPONT type N2030 and an electrolyzer having a Zoltek PW03 carbon woven fabric with a thickness of 0.3 mm between the OCE and the membrane.

  In the first step, the cathode space was filled at 80 ° C. with a 30% by weight sodium hydroxide solution having a content of 20 ppm of chloride ions and a content of chlorate ions of <10 ppm and then drained again. In the course of the evacuation, oxygen was supplied and the gas space obtained 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. The electrolysis voltage was switched on immediately after reaching a constant operation of the anode circulation. 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 installation was operated 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 the electrolysis cell of 90 ° C. for 10 days. The average electrolytic voltage was 2.14V.

  By making the slots at the OCE, there was no reduction in the electrolytic power.

Example 3

  After 3 days of operation, the electrolyzer according to Example 1 was brought out of operation as follows:

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, then the electrolysis current was turned off. Within this time, the anolyte was cooled to 70.degree. The anode space was drained and then filled and flooded with deionized water and drained again immediately.

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

  Fifty hours after shutdown, the electrolyzer from Example 2 was returned to operation as follows:

  In the first step, the cathode space was filled at 80 ° C. with a 32% by weight sodium hydroxide solution having a content of 20 ppm of chloride ions and a content of chlorate ions of <10 ppm and then drained again. In the course of the evacuation, oxygen was supplied and the gas space obtained 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. The electrolysis voltage was switched on immediately after reaching a constant operation of the anode circulation. 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 in

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

Claims (15)

A method for performing chloralkali electrolysis by means of an electrolytic cell comprising an oxygen-consuming electrode in a microgap arrangement, said oxygen-consuming electrode comprising a carrier element in the form of at least one sheet-like structure , and a gas An oxygen-consuming electrode comprising a coating having a diffusion layer and a catalytically active component, said coating having one or more openings having a diameter or height of 0.5 mm to 20 mm, The cell comprises an anode and at least one anode space having an anolyte comprising an alkali metal chloride, an ion exchange membrane, a cathode space having an oxygen-consuming electrode comprising a silver-containing catalyst as cathode, and 0.01 mm to It has a thickness of 2 mm, and have a flat pores elements between the oxygen-consuming electrode and the ion-exchange membrane catholyte flows Before the application of the electrolysis voltage, in the first step, the oxygen-consuming electrode is wetted on the gas side with an aqueous alkali metal hydroxide solution having a content of chloride ions of 1000 ppm or less and subsequently to the anode space of the anolyte Applying an electrolytic voltage after the introduction of oxygen and the introduction of an oxygen gas into the cathode space.   A method according to claim 1, characterized in that, in the first step, the oxygen-consuming electrode is wetted on the gas side with an aqueous alkali metal hydroxide solution having a content of chloride ions of 700 ppm or less before application of the electrolysis voltage. Method described.   A method according to claim 1, characterized in that, in the first step, the oxygen-consuming electrode is wetted on the gas side with an aqueous alkali metal hydroxide solution having a content of chloride ions of 500 ppm or less before application of the electrolysis voltage. Method described. The chloralkali electrolysis, oxygen consuming electrode A method for performing by containing Nde comprising an electrolytic cell in the micro-gap arrangement, the oxygen-consuming electrode, the carrier element in the form of at least one sheet-like structures, as well as gas An oxygen-consuming electrode comprising a coating having a diffusion layer and a catalytically active component, said coating having one or more openings having a diameter or height of 0.5 mm to 20 mm, The cell comprises an anode and at least one anode space having an anolyte comprising an alkali metal chloride , an ion exchange membrane, a cathode space having an oxygen-consuming electrode comprising a silver-containing catalyst as cathode , and 0.01 mm to It has a thickness of 2 mm, and have a flat pores elements between the oxygen-consuming electrode and the ion-exchange membrane catholyte flows At the end of the electrolysis operation, for shutdown, at least the following steps:
・ Step of cutting the electrolysis voltage,
Discharging the anolyte from the anode space,
Filling the cathode space with one of the following liquids and then draining the liquid : 4 mol / L or less of diluted alkali metal hydroxide solution or deionized water,
A method, characterized in that it is performed in this order. Alkali metal hydroxide solution used to wet the cathode space prior to application of the electrolysis voltage, characterized by having a content of less chlorate ions 20 ppm, in any one of claims 1 3 Method described. After shutting down and discharging the electrolysis cell, repeat as necessary, every 1 to 12 weeks, with the anode space with dilute aqueous alkali metal chloride solution having a content of 1.7 to 3.4 mol / L, and 5. The method according to claim 4, characterized in that the cathode space is flushed with 2 to 9 mol / l of diluted alkali metal hydroxide solution.   The method according to any one of claims 4 to 6, wherein the alkali metal chloride is sodium chloride or potassium chloride. 8. The total area of the openings with respect to the side of the oxygen-consuming electrode facing the ion exchange membrane is characterized in that it is 0.05 to 15% of the area of the oxygen-consuming electrode. The method described in. The method according to any one of claims 4 to 8, characterized in that only the covering has an opening. A method according to any one of claims 4 to 8 , characterized in that the openings are continuous through the coating and the carrier element. 11. A method according to any one of claims 4 to 10 , characterized in that the oxygen consuming electrode has a distance of 0.01 mm to 2 mm between the ion exchange membrane and the oxygen consuming electrode. 6. The overall area of the opening relative to the side of the oxygen-consuming electrode facing the ion exchange membrane is characterized in that it is 0.05 to 15% of the area of the oxygen-consuming electrode. The method according to one item. The method according to any one of claims 1 to 3, 5 and 12, characterized in that only the coating has an opening.   14. A method according to any one of claims 1-3, 5 and 12-13, characterized in that the openings are continuous through the coating and the carrier element. The method according to any of claims 1 to 3, 5 and 12 to 14 , characterized in that the oxygen-consuming electrode has a distance of 0.01 mm to 2 mm between the ion exchange membrane and the oxygen-consuming electrode. .

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