Membrane electrolysis process for alkaline chloride solutions using gas diffusion electrodes

Abstract

The present invention relates to a method for the electrolysis of alkali chlorides with oxygen-depolarized electrodes, said method having specific operating parameters for shutdown and restart.

Description

Technical Field

[0001] The present invention relates to a method for the electrolysis of an aqueous solution of an alkali metal chloride by means of a gas diffusion electrode according to specific operating parameters.

[0002] The present invention arises from electrolysis methods known per se, for example from electrolysis methods for the electrolysis of aqueous alkali metal chloride solutions by means of gas diffusion electrodes which usually comprise a gas diffusion layer having a conductive support and a catalytically active component. This arrangement has a narrow gap through which an electrolyte flows between the gas diffusion electrode and the ion exchange membrane.

Background Art

[0003] Various proposals for operating gas diffusion electrodes as oxygen depolarized electrodes in industrial-sized electrolytic cells are generally known from the prior art. The basic idea here is to replace the hydrogen generating cathode of the electrolysis (e.g. chlor-alkali electrolysis) with an oxygen depolarized electrode (cathode). An overview of the 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. Appl. Electrochem. 38 (2008) 1177 - 1194.

[0004] Gas diffusion electrodes (hereinafter also abbreviated as GDEs) must meet many requirements in order to be used in industrial electrolyzers. Thus, the catalysts and all other materials used must be chemically stable up to a temperature of typically 90° C. with respect to the electrolyte and the gases supplied to the electrodes and the compounds formed at the electrodes, such as hydroxide ions or hydrogen. Similarly, the area is usually 2 m 2High mechanical stability is required so that the electrodes can be installed and operated in an electrolysis device having a larger size (industrial size). Further desirable properties are high conductivity, low layer thickness, high internal surface area and high electrochemical activity of the electrolysis catalyst. Appropriate hydrophobic and hydrophilic pores as well as an appropriate pore structure are required for the conduction of gases and electrolytes. Long-term stability and low manufacturing costs are further specific requirements that an industrially applicable oxygen depolarizing electrode must meet.

[0005] WO2001 / 57290A1 describes a cell for chlor-alkali electrolysis, where the liquid is carried from above downwards across a sheet-like porous element known as a percolator, which is installed between a gas diffusion electrode and an ion exchange membrane that is a kind of free-falling film, generally referred to as a falling film, along the gas diffusion electrode (mini-gap arrangement). In this arrangement, only a very small liquid column acts on the liquid side of the gas diffusion electrode, and a high hydrostatic pressure profile is not constructed across the structural height of the cell.

[0006] A further arrangement described in JP3553775 and US6117286A1, which is sometimes also referred to as "zero-gap", but more precisely described as "micro-gap". In this arrangement, a further layer of porous hydrophilic material that takes in an alkali metal hydroxide solution formed by suction and from which at least part of the alkali can flow out downwards is located between the ion exchange membrane and the GDE. The possibility of the alkali metal hydroxide solution flowing out is determined by the installation of the GDE and the cell design. In contrast to the above-described arrangement in the mini-gap design, there is no aqueous alkali metal hydroxide solution (alkali) carried by supply and discharge through the gap between the GDE and the ion exchange membrane; the porous material present in the micro-gap takes in the formed alkali metal hydroxide solution and conducts it further in a horizontal or vertical direction.

[0007] The oxygen depolarizing electrode typically consists of a support element, such as a porous metal plate or a woven fabric made of metal wires, and an electrochemically catalytically active coating. The electrochemically active coating is microporous and consists of hydrophilic and hydrophobic components. The hydrophobic component makes it difficult for the electrolyte to penetrate and thus keeps the appropriate pores in the GDE empty for the transport of oxygen to the catalytically active sites. The hydrophilic component enables the passage of the electrolyte to the catalytically active sites and the outward transport of hydroxide ions from the GDE. Fluorine-containing polymers such as polytetrafluoroethylene (PTFE) are generally used as the hydrophobic component and also serve as a polymer binder for the catalyst particles. In the case of an electrode with a silver catalyst, the silver acts, for example, as a hydrophilic component.

[0008] Many compounds have been described as electrochemically catalysts for the reduction of oxygen. However, only platinum and silver have achieved practical importance as catalysts for the reduction of oxygen in alkaline solutions.

[0009] Platinum has very high catalytic activity towards the reduction of oxygen. Due to the high cost of platinum, it is used exclusively in a supported form. The preferred support material is carbon. However, perhaps because platinum also catalyzes the oxidation of the support material, the stability of platinum-based electrodes supported on carbon during long-term operation is insufficient. In addition, carbon promotes the undesirable formation of H2O2, which also leads to oxidation. Silver, likewise, has high electrocatalytic activity towards the reduction of oxygen.

[0010] Silver can be used in a form supported on carbon or also as finely divided metallic silver. Although silver catalysts supported on carbon are more durable than the corresponding platinum catalysts, their long-term stability under the conditions in the oxygen depolarizing electrode, especially during use for chlor-alkali electrolysis, is also limited.

[0011] In the manufacture of a GDE having an unsupported silver catalyst, silver is preferably introduced at least partially in the form of silver oxide and then reduced to metallic silver. This reduction generally occurs during the initial startup of the electrolytic cell. In the reduction of the silver compound, a change in the arrangement of the microcrystals also occurs, particularly the formation of crosslinks between individual silver particles. This leads to overall structural integration.

[0012] A further central element of the electrolytic cell is the ion exchange membrane. The membrane is permeable to cations and water and almost impermeable to anions. The ion exchange membranes in the electrolytic cell are subject to great stress: they must be resistant to chlorine on the anode side and to strong alkaline conditions at a temperature of about 90 °C on the cathode side. Perfluorinated polymers such as PTFE usually withstand these stresses. Ion transport occurs via acidic sulfonate groups and / or carboxylate groups polymerized in these polymers. Carboxylate groups show greater selectivity, and carboxylate-containing polymers have lower water absorption and higher electrical resistance than polymers containing sulfonate groups. Generally, multilayer membranes having a thicker layer containing sulfonate groups on the anode side and a thinner layer containing carboxylate groups on the cathode side are commonly used. The membrane is provided with a hydrophilic layer on the cathode side or on both sides. To improve the mechanical properties, the membrane is reinforced by the insertion of a woven or non-woven fabric, which is preferably incorporated into the layer containing sulfonate groups.

[0013] Due to their complex structure, ion exchange membranes are sensitive to changes in the medium surrounding them. As a result of different molar concentrations, a high osmotic pressure gradient can be formed between the anode side and the cathode side. When the electrolyte concentration decreases, the membrane swells due to an increase in water absorption. When the electrolyte concentration increases, the membrane releases water and consequently shrinks; in extreme cases, mechanical damage such as the precipitation of solids in the membrane or cracks in the membrane can occur as a result of water drainage.

[0014] Therefore, the concentration change may cause defects and damage on the membrane. Delamination (blister formation) between the layers of the layer structure may occur, resulting in impaired mass transfer or membrane selectivity.

[0015] Furthermore, holes (pinholes) and in extreme cases cracks may occur, through which undesirable mixing of the anolyte and catholyte may occur.

[0016] When the electrolysis voltage switch is turned off, the mass transfer through the membrane caused by the current flow also stops, and furthermore, undesirable concentration changes may occur in the alkali metal chloride-containing electrolyte in the anode space (brine) and the alkali metal hydroxide solution present in the cathode space. The membrane may dehydrate and shrink, and solids may precipitate, resulting in the formation of holes and making it easier for anions to pass through the membrane. When the electrolytic cell is restarted, the too-low water content hinders mass transfer through the membrane, resulting in an increase in osmotic pressure and delamination at the interface between the sulfonic acid group-containing layer and the carboxylic acid group-containing layer typically used in such membranes.

[0017] The non-uniformity of the water and / or ion distribution in the membrane and / or gas diffusion electrode may result in local peaks in electrical transport and mass transfer during a new start-up, which may cause damage to the membrane or gas diffusion electrode.

[0018] Problems are also presented by the precipitation of alkali metal chloride salts on the anode side. The high osmotic pressure gradient between the anode liquor and the cathode liquor results in the transport of water from the anode space to the cathode space. As long as the electrolysis is operating, the transport of water from the anode space is counteracted by the loss of chloride and alkali metal ions, and as a result, the concentration of the alkali metal chloride decreases in the anode space under normal electrolysis conditions. When the electrolysis switch is turned off, the transport of water from the anode space to the cathode space caused by the osmotic pressure persists. The concentration in the anode liquor increases beyond the saturation limit. The precipitation of the alkali metal chloride salt occurs especially in the boundary region to the membrane or even in the membrane, which can lead to damage to the membrane.

[0019] In a production plant, it is desirable to operate the electrolytic cell for periods of years without opening the electrolytic cell during this time. However, due to fluctuations and malfunctions in the emissions in the production area upstream or downstream of the electrolysis, the electrolytic cell in the production plant will inevitably have to be repeatedly stopped and restarted.

[0020] During the stop and restart of the electrolytic cell, conditions occur that can damage cell elements such as the anode, ion exchange membrane, gas diffusion electrode, or further components used in the cell, significantly shortening their lifespan and also impairing the performance of the electrolysis. In particular, oxidative damage in the cathode space, damage to the gas diffusion electrode, and damage to the membrane are known.

[0021] Operating modes that can reduce the risk of damaging the electrolytic cell during start-up and shutdown are hardly known from the prior art.

[0022] The first Japanese publication JP2004-300510A describes an electrolysis process using a micro-gap arrangement, where it is said that corrosion in the cathode space when the cell is stopped can be prevented by flooding the gas space with a sodium hydroxide solution. Flooding the gas space with the sodium hydroxide solution, according to this publication, protects the cathode space from corrosion, but is insufficient in protecting the electrodes and membranes from damage during stop and start or during rest.

[0023] US4578159A1 states that in an electrolysis process using a "zero-gap" arrangement, damage to the membranes and electrodes can be avoided by flushing the cathode space with a 35% sodium hydroxide solution before starting the cell, or by starting the cell at a low current density and gradually increasing the current density. By this procedure, the risk of damage to the membranes and gas diffusion electrodes during start-up is reduced, but protection against damage during stop and rest is not provided.

[0024] It is known from document US4364806A1 that corrosion in the cathode space is said to be reduced by replacing oxygen with nitrogen after adjusting the electrolysis current. According to WO2008009661A2, the addition of a small amount of hydrogen to nitrogen is said to result in an improvement in protection against corrosive damage. However, the methods mentioned are complex, especially with regard to safety, and require the installation of additional equipment for the introduction of nitrogen and hydrogen. At restart, the pores of the gas diffusion electrodes are partially filled with nitrogen and / or hydrogen, which hinders the supply of oxygen to the reaction sites. Furthermore, the method provides no protection against damage to the ion exchange membranes and requires a high level of safety measures to avoid explosive gas mixtures.

[0025] The Final Technical Report "Advanced Chlor-Alkali Technology" by Jerzy Chlistunoff (Los Alamos National Laboratory, DOE Award 03EE-2F / Ed190403, 2004) describes the conditions for temporary shutdown and switch-on of a zero-gap cell. During shutdown, after interrupting the electrolysis current, the oxygen supply is interrupted and replaced with nitrogen. The humidification of the gas flow 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 open-circuit voltage is achieved. Then, after cooling the cell, the supply of wet nitrogen on the anode side and the pump circulation of water are stopped.

[0026] For a new startup, the anode side is first filled with brine, and water and nitrogen are introduced into the cathode side. Next, the cell is heated to 80 °C. Then, the gas supply is switched to oxygen, and a polarization voltage is applied with a low current flow. Thereafter, the current density increases and the pressure inside the cathode rises; its temperature rises to 90 °C. The supplies of brine and water are then adjusted so that the desired concentrations are achieved on the anode and cathode sides.

[0027] This procedure can be carried out with only extremely difficult operation of industrial cells and will result in a diluted electrolyte-containing solution that has to be discarded.

Prior Art Documents

Patent Documents

[0028]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

[0029] [Non-Patent Document 1] Moussallem et al, “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177 - 1194 [Summary of the Invention]

[0030] Startup For startup as described in EP2639337A2, the volumetric flow rate and / or composition of the cathode liquid supplied to the gap is set such that the aqueous solution of alkali metal hydroxide exiting the cathode gap has a chloride ion content of 1000 ppm or less before the electrolysis voltage is applied after the electrolysis voltage is applied between the anode and the cathode and the anolyte and the oxygen-containing gas are introduced into the cathode space.

[0031] According to the prior art of EP2639337A2, humidified oxygen is introduced before startup of the cell having a limited gap arrangement in the cathode liquid circuit, the gauge pressure corresponding to the configuration within the cell is set in the cathode half-cell, and that gauge pressure is generally 10 - 100 mbar relative to the pressure within the anode.

[0032] However, when starting up and shutting down according to the method of EP2639337A2, it has been found that, contrary to expectations, the performance of the electrolysis is impaired when these procedures are repeatedly carried out.

[0033] Techniques for starting and stopping gas diffusion electrodes as described in the prior art are disadvantageous and leave the fact that they provide only inadequate protection against damage.

[0034] The object of the present invention is to find suitable improved operating parameters for the start-up and stop, in particular the stop and the provisional stop, of an electrolysis cell for chlor-alkali electrolysis using a gas diffusion electrode having a mini-gap arrangement and a silver catalyst as the electrocatalytic substance, where the improved parameters are simple to implement and damage to the membrane, electrodes and / or other components of the electrolysis cell is avoided when they are attached.

[0035] For the purposes of the present invention, the mini-gap arrangement means any arrangement of an electrolysis cell having an electrolyte gap through which the cathode liquid flows between the oxygen depolarizing electrode and the membrane, the gap having a gap width of at least 0.01 mm, in particular a gap width of 3 mm or less. In an electrolysis cell according to the principle of the falling film cell preferably used, the cathode liquid flows from top to bottom in the direction of gravity in a vertically arranged electrolysis cell. Other arrangements with alternative flow directions or horizontally arranged electrolysis cells are also intended to be encompassed by the present invention.

[0036] The above-mentioned problems and drawbacks of the previously known methods are overcome by providing the electrolysis process of the present invention.

[0037] Surprisingly, it has been found that an electrolysis device containing a gas diffusion electrode having a silver catalyst can be repeatedly started and stopped without being damaged by an improved sequence of these steps and is also not damaged during rest. This method is particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.

[0038] The above technical object is achieved according to the present invention by a specific sequence of voltage reduction and voltage reduction and exchange of the adhering electrolyte when the electrolysis cell is stopped.

[0039] The present invention provides a method for chlor-alkali electrolysis using an electrolytic cell in a gap arrangement, in particular where the distance between the ion exchange membrane and the gas diffusion electrode is from 0.01 mm to 3 mm, where the cell has at least one anode space having an anode and an anolyte containing an alkali metal chloride, an ion exchange membrane, a cathode space having a gas diffusion electrode as a cathode containing a silver-containing catalyst, and in particular a sheet-like porous element having a thickness of 0.01 mm to 3 mm, through which the catholyte flows between the gas diffusion electrode and the membrane, and at the end of the electrolysis process, in particular for shutdown, at least the following steps: a) Reducing the electrolysis voltage and removing chlorine from the anolyte so that less than 10 mg / l of active chlorine is present in the anolyte by maintaining an electrolysis voltage of 0.1 to 1.4 V per element and a current density greater than zero; b) Adjusting the pH of the anolyte to a value in the range of pH 2 to pH 12; c) Retaining under these conditions as long as an electrolyte is present in the catholyte gap (or the electrolyte flows through the latter); And when emptying the electrolytic cell (for example, in the case of maintenance and repair work on the electrolytic cell where it is necessary to open the electrolytic cell): d) Cooling the anolyte to less than 70 °C while maintaining an electrolysis voltage in the range of 0.1 to 1.4 V; e) Switching off the electrolysis voltage at a temperature of < 55 °C; f) Emptying the cathode gap; g) Emptying the anode space; h) Preferably, refilling the anode space with one of a dilute alkali metal chloride solution having a maximum concentration of 4 mol / l or deionized water, and then emptying the anode space; i) Filling the cathode space with one of a dilute alkali metal hydroxide solution having a maximum concentration of 10 mol / l or deionized water, and then emptying the cathode space are carried out in this order.

[0040] One means known from conventional membrane electrolysis is the maintenance of the polarization voltage. That is, the voltage is not adjusted to zero at the end of the electrolysis. Instead, the residual voltage is maintained such that a residual current flows in the normal electrolysis direction, obtaining a constant small current density, and as a result, electrolysis occurs slightly. When stopping the electrolysis, cooling of the electrolyte is necessary, and as a result, the potential changes. Therefore, this means alone is not sufficient to prevent damage to the electrodes during startup and shutdown when using a gas diffusion electrode.

[0041] Also, it has been observed that the oxidation of the silver catalyst can occur again when the electrolysis current switch is turned off. The oxidation is clearly promoted by oxygen and moisture in the half-cell. In particular, immediately after turning off the electrolysis switch, in addition to the sodium chloride-containing brine, chlorine, hypochlorite, and chlorate are present on the anode side. On the cathode side, there are a sodium hydroxide solution, an electrocatalyst such as silver, and oxygen. By turning off the electrolysis current switch, the system is left to itself, and an electrochemical reaction occurs depending on the potential, concentration, temperature, and pressure. As a result of the oxidation of the cathode catalyst, for example, the oxidation of silver to silver oxide, rearrangement of the catalyst microstructure may occur, which affects the activity of the catalyst and thus the performance of the gas diffusion electrode.

[0042] In the new method, the alkali metal chloride is preferably sodium chloride or potassium chloride, particularly preferably sodium chloride.

[0043] The alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide, particularly preferably sodium hydroxide.

[0044] In a preferred new method, oxygen gas is supplied to the side of the gas diffusion electrode that faces away from the cathode liquid during operation. The oxygen gas flow to the gas diffusion electrode is preferably maintained during the shutdown of the electrolysis by the new method.

[0045] The purity of oxygen corresponds to the customary concentration and purity requirements in electrolysis using a gas diffusion electrode; oxygen having a content exceeding 98.5% by volume is preferably used.

[0046] The temperature of the supplied cathode liquid is adjusted during operation so that a temperature in the range of 70 to 95 °C, preferably 75 to 90 °C, is established at the output from the cathode space. The temperature difference between the anode liquid output and the cathode liquid input is preferably set to less than 20 °C during operation and during shutdown. Such a small temperature difference avoids damage to the ion exchange membrane.

[0047] To remove chlorine from the anode liquid in step a), brine having an NaCl content of 180 g / l (3.07 mol / l) to 330 g / l (5.64 mol / l) is supplied to the anode space in a preferred embodiment. In this way, the chlorine gas present in the anode space is eliminated and the content of dissolved / dispersed chlorine is reduced.

[0048] The determination of the concentrations disclosed in this patent application is carried out, in particular, by titration or by another analytical method generally known to those skilled in the art.

[0049] To reduce the electrolysis voltage in step a) to a range of 0.1 to 1.4 V, a current density of preferably greater than zero to 20 A / m 2 up to, preferably 0.1 A / m 2 ~20 A / m 2 is maintained. Under these conditions, electrolysis operates until the anode liquid no longer contains Cl2, i.e., until the content of chlorine in the oxidized state of 0 is reduced from >0 to less than 10 mg / l. The measurement of the absence of chlorine in the anode liquid is carried out, in particular, by redox titration such as iodine titration or by testing the anode liquid with iodine-starch paper.

[0050] Maintaining the brine pH in the range of 2 to 12, preferably 6 to 9, during step a) is required to avoid chlorine generation at lower pH.

[0051] The temperature of the anolyte in steps a) and b) is preferably at least 65 °C, particularly preferably at least 70 °C.

[0052] The maintenance of a differential pressure of at least 5 mbar between the cathode space and the anode space is particularly preferably ensured during shutdown.

[0053] As preparation for emptying the electrolytic cell, in step d) the anolyte is cooled to a temperature below 70 °C while simultaneously maintaining an electrolysis voltage of 0.1 - 1.4 V. This is a further difference from the prior art - where cooling is carried out without maintaining the electrolysis voltage.

[0054] Switching off the electrolysis voltage in step e) is carried out at an electrolyte temperature of <55 °C, preferably <50 °C.

[0055] The cathode gap (mini - gap) is then emptied in step f) (for example, by switching off the pump for the supply of the catholyte). Here too, there is a difference from the prior art, since in the latter the mini - gap is emptied only after the anode space has been emptied.

[0056] Emptying the anode space in step g) is done by discharging the anolyte and, in particular, subsequently flushing h) the anode space with an alkali metal chloride solution having a maximum concentration of 4 mol / l or deionized water.

[0057] Finally, in step i), the cathode gap (mini - gap) is flushed with a dilute sodium hydroxide solution or deionized water to remove chloride residues and empty the cathode mini - gap. In contrast to the prior art, here the cathode gap is flushed again to remove chloride after the anode space has been emptied. This avoids corrosion on the nickel connection flanges of the cell due to overly high chloride values in the alkali remaining in the cathode space.

[0058] If necessary, it is particularly preferably possible to empty the residue in the anode space.

[0059] The difference from the procedures known from the prior art is that, particularly with respect to EP263337A2, when the electrolysis voltage drops, the current density is not kept constant. Instead, regardless of what current density is established, the electrolysis voltage is set in the range of 0.1 to 1.4 V, which is known as constant potential operation. What is important here is that the current flows from the anode to the cathode, i.e., the flow direction of the original electrolysis current direction is maintained, and that the current is greater than zero in any case. Furthermore, rather than the anode space being emptied first, the cathode gap is emptied immediately after the electrolysis voltage switch is turned off, as described in EP263337A. Emptying the anode space can take up to 150 minutes, depending on the cell structure with industrial structural dimensions, particularly in the case of industrial electrolysis elements. Similarly, the pH of the brine is not considered in the prior art, while according to the present invention, this is optimally between 2 and 12.

[0060] The gas diffusion electrode is efficiently protected by the method of the present invention. The cell can also be cooled to below 70 °C without chlorine being generated on the anode side as a result of the constant potential operation. This is important from a safety perspective when the electrolysis element is later opened for maintenance work or repair.

[0061] Preferred details of the shutdown of membrane electrolysis using a gas diffusion electrode are described below In the first step, the electrolysis voltage is adjusted downward. Here, the voltage is adjusted to a value of 0.1 to 1.4 V. When the temperature of the anode liquid is > 65 °C, the concentration of NaCl exceeds 200 g / l (3.41 mol / l), and the concentration of the alkali metal hydroxide in the cathode liquid is < 28 wt% (9.1 mol / l) at a cathode liquid temperature of > 65 °C, the chlorine content in the anode space is reduced to < 10 mg / l, preferably less than 1 mg / l. Here, the pH of the anode liquid at the output from the electrolysis cell is between 2 and 12, preferably between 6 and 9.

[0062] For this purpose, the chlorine content is the total content of dissolved chlorine in an oxidation state of 0 or higher. The removal of the remaining chlorine from the anode space is preferably carried out by supplying a chlorine-free anolyte and simultaneously discharging the chlorine-containing anolyte, or by pumping the anolyte in the anode circuit and simultaneously removing and discharging chlorine gas.

[0063] According to the prior art, i.e., EP263337A2, the voltage is set so that a current density of 0.01 - 20 A / m 2 , preferably 10 - 18 A / m 2 is established during chlorine-free flushing. Under these conditions, since the generation of chlorine resumes, the electrolysis does not operate at a temperature below 70°C. The cooling of the electrolysis can be carried out according to the method of the present invention when the electrolysis voltage at a temperature below 70°C is 1.4 V or less and the pH of the brine is in the range of 2 - 12. In this state, the electrolysis can be interrupted for a long time without damaging the gas diffusion electrode. Compared with the prior art, the electrolysis voltage continues to be applied.

[0064] When restarting the electrolytic cell, its load can be increased again at any time.

[0065] When emptying the electrolytic cell, it is particularly preferred to carry out the following further steps: · Switch off the voltage supply · First, empty the cathode space within 0.01 - 2 minutes · After emptying the cathode space, empty the anode space within 0.01 - 200 minutes; emptying the cathode space and the anode space can be carried out optionally in parallel after switching off the voltage supply · After emptying the anode space, optionally flush the anode space · Flushing is carried out with water, preferably deionized water, using very dilute brine having an alkali metal chloride content of 0.01 to 4 mol / l. Flushing is preferably carried out by filling the anode space once or by filling only partially the anode space and immediately discharging the flushing liquid. Flushing can also be carried out in two or more stages, for example, first filling the anode space with dilute brine having an alkali metal chloride content of 1.5 to 2 mol / l, discharging it, and then further filling and discharging it with very dilute brine having an NaCl content of 0.01 mol / l or deionized water. The flushing solution can be discharged again immediately after completely filling the anode space or can remain in the anode space for a maximum of 200 minutes and then be discharged. After discharging, a small amount of residual flushing solution remains in the anode space. Then, the anode space remains piped or blocked without direct contact with the ambient atmosphere. The brine complies with the normal purity requirements for membrane electrolysis in chlor-alkali electrolysis. · Flushing of the cathode space is carried out using an alkali metal hydroxide solution having a concentration of 12 mol / l or less, preferably 0.01 to 4 mol / l, which is supplied to the cathode space for 0.01 minutes to 60 minutes and then discharged again. The alkali metal hydroxide solution from normal production is preferably used for flushing the cathode space. The alkali from the shutdown procedure is usually not very suitable for flushing due to contamination by chloride ions. Similarly, flushing can be carried out using deionized water. After the flushing operation, the cathode space is emptied. · Oxygen supply can be interrupted, in particular, by switching off the voltage. Oxygen supply is preferably interrupted after emptying and flushing the cathode space. · Reducing the differential pressure between the cathode chamber and the anode chamber · Lowering the pressure at which the electrolysis element operates to ambient pressure · Closing the electrolysis element to avoid air ingress.

[0066] After emptying / flushing the anode space and the cathode space, an electrolytic cell with a wet membrane can be kept in a built-in state and ready for rapid startup for a relatively long period of time without impairing the performance capacity of the electrolytic cell. In the case of a break of several weeks, in order to achieve stabilization, it is desirable to flush or soak the anode space with a diluted aqueous alkali metal chloride solution and the cathode space with a diluted aqueous alkali metal hydroxide solution at regular intervals. The flushing is preferably carried out at intervals of 1 to 12 weeks, particularly preferably at intervals of 4 to 8 weeks. The concentration of the diluted alkali metal chloride solution used for flushing or wetting is 1 to 4.8 mol / l. The flushing solution can be drained again immediately after completely filling the anode space, or can be present in the anode space for a maximum of 200 minutes and then drained. The concentration of the alkali metal hydroxide solution used for flushing or wetting is 0.1 to 10 mol / l, preferably 1 to 4 mol / l. The temperature of the brine or the alkali metal hydroxide solution can be in the range of 10 to 80°C, preferably 15 to 40°C. The flushing of the mini-gap cathode shell can be carried out for 0.1 to 10 minutes.

[0067] The present invention also provides a method for startup, in particular a method for restarting after a new method for shutdown.

[0068] It is a method for chlor-alkali electrolysis using a membrane electrolytic cell, particularly in a mini-gap arrangement between an ion exchange membrane and a gas diffusion electrode, where the distance between the ion exchange membrane and the gas diffusion electrode is from 0.01 mm to 3 mm, the cell having at least one anode space for containing an anolyte containing an alkali metal chloride, an ion exchange membrane, a cathode space having a gas diffusion electrode as a cathode containing a silver-containing catalyst, and a sheet-like porous element in the gap between the ODE and the membrane, the thickness of the element being particularly from 0.01 mm to 3 mm, through which the catholyte flows during operation, and for the start of the electrolysis process, at least the following steps: j) Filling the anode space with an anode liquid having a temperature of at least 50 °C and passing the anode liquid therethrough; k) Preheating the cathode liquid to a temperature of at least 50 °C; l) Filling the cathode space and the porous element with the preheated cathode liquid having a concentration of 7.5 - 10.5 mol / l and passing the cathode liquid therethrough; m) Setting the electrolysis voltage to a value in the range of 0.1 - 1.4 V; n) Independently setting the temperatures of the cathode liquid and the anode liquid exiting the cell to a temperature in the range of 70 - 100 °C and maintaining them; o) Setting the concentration of the cathode liquid in the supply to the cell so that an alkali metal hydroxide concentration in the range of 7.5 - 12 mol / l is obtained at the output; p) Setting the concentration of the anode liquid in the supply to the cell so that an alkali metal chloride concentration in the range of 2.9 - 4.3 mol / l is obtained at the output; q) Setting the production current density to at least 2 kA / m 2 and preferably at least 4 kA / m 2 ; These steps are carried out in this order.

[0069] The restart of the electrolysis is carried out, in particular, as follows: The anode liquid is introduced into the anode space of the cell as in step j), and in particular, is heated to at least 50 °C in a circuit having a heat exchanger. The cathode liquid is heated to a temperature of at least 50 °C outside the cell, for example, in a circuit having a storage container and a heat exchanger, for step k).

[0070] When the anode chamber is filled and the anode liquid has a temperature of at least 50 °C, the cathode gap (mini-gap) is filled by introducing a preheated alkali metal hydroxide solution having a temperature of at least 50 °C into the gap as in step l). This procedure is different from the prior art where the cathode space is first filled and then the anode space is filled - the procedure according to the present invention avoids unduly high chloride values in the alkali and thus any corrosion problems.

[0071] As soon as the cathode gap is filled with the alkali metal hydroxide solution, an electrolysis voltage of at least 0.4 V is applied, preferably in step m), particularly within 0.01 to 10 minutes, and as a result, a current density of at least 0.2 A / m 2 is established.

[0072] The anode liquid and the cathode liquid are subsequently heated to a temperature of at least 70 °C as in step n), and preferably the current density is then increased.

[0073] The increase in the current density to the production current density in step q) is particularly preferably carried out at a rate of 0.018 kA / (m 2 until the current density in the electrolysis element reaches at least 2 kA / m 2 *min) to 0.4 kA / (m 2 *min).

[0074] The determination of the concentration is carried out by titration or another method generally known to those skilled in the art, unless otherwise specified.

[0075] The electrolytic cell stopped according to the above new method is restarted according to the above new method. When the steps of the described method are observed, the electrolytic cell can undergo many start-up and shut-down cycles without impairing the performance of the cell.

Example

[0076] The gas diffusion electrode used in the examples was manufactured as described in EP1728896B1 as follows: A powder mixture consisting of 7 wt% PTFE powder, 88 wt% silver(I) oxide, and 5 wt% silver powder was applied to a nickel wire-made gauze and pressed to obtain an oxygen depolarizing electrode.

[0077] The electrode had a DuPont type N982 ion exchange membrane (manufactured by Chemours), and was installed in an electrolysis unit with an area of 100 cm 2 and a 3 mm gap between the gas diffusion electrode and the ion exchange membrane.

[0078] The electrolysis unit, in the assembled state, had an anode space with an anode made of titanium expanded metal coated with a commercially available DSA coating for chlorine production from Denora, consisting of an anode liquid inlet and outlet and a mixed oxide of ruthenium oxide / iridium oxide, and a cathode space with a gas diffusion electrode as the cathode, having a gas space for oxygen and an oxygen inlet and outlet, a liquid outlet and an ion exchange membrane, which were arranged between the anode space and the cathode space. A lower pressure prevailed in the anode space than in the cathode space, so that as a result of the higher pressure in the cathode chamber, the ion exchange membrane was pressed onto the anode structure at a pressure of about 30 mbar.

[0079] The electrolytic cell was operated at an electrolyte temperature of about 85 °C, a brine concentration of about 210 g / l (3.58 mol / l) of NaCl and a sodium hydroxide concentration of about 31 wt% (10.4 mol / l). The cell voltage was corrected to 32 wt% (10.79 mol / l) sodium hydroxide and 90 °C by conventional standard methods.

[0080] The electrolyte was introduced into the cell from below and withdrawn again from the top of the cell.

[0081] Oxygen was supplied to the gas space of the cathode. Here, oxygen with a purity exceeding 99.5% by volume was used. After being humidified with water at room temperature, the oxygen was introduced into the gas space of the cathode half shell. The amount of oxygen was adjusted such that a stoichiometric excess of 1.5 times the amount of oxygen required based on the set current intensity was always introduced. The oxygen was supplied to the gas space from the top and discharged at the bottom.

[0082] The electrolysis unit had a gap of approximately 3 mm between the oxygen depolarizing electrode and the ion exchange membrane. This gap was filled with a porous PTFE woven fabric as a percolator and spacer.

[0083] The production current density was 6 kA / m 2 was.

[0084] Example 1 - Startup Before starting the cathode circuit, oxygen saturated with water was supplied to the cathode space at room temperature so that the pressure in the cathode gas space became 59 mbar. The hydrostatic pressure of the sodium hydroxide solution at the lowest point in the cell was 32 mbar.

[0085] Thereafter, an external cathode liquid circuit containing approximately 31 wt% (10.4 mol / l) of sodium hydroxide solution was started, and the sodium hydroxide solution was heated. However, the sodium hydroxide solution had not yet been carried through the cell

[0086] In the next step, according to the present invention, the anode liquid circuit was started, and the anode space was filled with an anode liquid having a concentration of approximately 210 g of NaCl / l (3.58 mol / l). While maintaining the anode circuit and the anode liquid was being conveyed through the cell, the anode liquid was heated to 50°C by a heat exchanger present in the anode circuit.

[0087] After the sodium hydroxide solution reached a temperature of 50°C, the sodium hydroxide solution having a temperature of 50°C was supplied to the cell, filled the cathode gap within 30 seconds, and then an electrolysis voltage of 1.08 V was applied. Thereby, a current density of 10 mA / cm 2 was established.

[0088] The pH of the flowing-out anode liquid was 8.

[0089] The electrolyte was heated from 50 °C to 70 °C within 1 hour. After the temperatures of the flowing-out anode liquid and cathode liquid reached 70 °C, the electrolysis voltage was increased, and here, the electrolysis voltage was increased so that the current density was increased by 50 mA / cm 2 every 2 minutes up to a current density of 600 mA / cm 2

[0090] The concentration was adjusted after the start so that the concentration of the flowing-out brine became about 210 g / l (3.59 mol / l) and the concentration of the sodium hydroxide solution became about 31.5 wt% (10.6 mol / l).

[0091] The cell was operated under these conditions for at least 24 hours.

[0092] Example 2 - Shutdown - According to the present invention The electrolysis unit was operated at a current density of 600 mA / cm 2

[0093] For stopping, the current density was decreased to 1.5 mA / cm 2 . For this purpose, the main rectifier was disconnected and the switch of the polarization rectifier was turned on. Next, the polarization rectifier took over the maintenance of the current density of 1.5 mA / cm 2 . The operation at the low current density was maintained for 1.5 hours. After this, the anode liquid did not contain chlorine. This method is carried out in an industrial electrolysis apparatus for safety reasons. One of the reasons is that chlorine compounds such as chlorine or hypochlorite do not diffuse from the anode liquid through the ion exchange membrane into the cathode liquid, where they cause corrosion of the cell components or gas diffusion electrodes. Based on experience, the chlorine-free flushing stage takes about 1.5 hours in an industrial electrolysis apparatus.

[0094] The electrolyte circuit remained operating at the same volumetric flow rate as during electrolysis operation at 600 mA / cm 2 . The O2 supply was maintained as well.​​

[0095] During the chlorine-free flushing stage, the temperatures of the anode liquid and the cathode liquid were reduced from 85 °C to 70 °C. The cell voltage during this stage was about 1.16 V, and the pH of the anode liquid flowing out of the cell was pH 8.2.

[0096] After 1.5 hours, the temperatures of the anode liquid and the cathode liquid were lowered to 50 °C, and the polarization rectifier was operated at a constant potential. Here, a voltage of 1.16 V was maintained, and the current was appropriately reduced.

[0097] After cooling the anode liquid and the cathode liquid, the polarization rectifier was disconnected, and the cathode liquid was immediately discharged from the cathode space. This was done over a period of about 30 seconds. After emptying the cathode space, the anode space was discharged within 1 hour.

[0098] The anode space was filled with deionized water from the bottom up to a height of up to 50% of the cell height and immediately discharged again.

[0099] The cathode space was similarly flushed by newly turning on the switch of the cathode liquid pump and supplying the cathode liquid to the cathode space. For this purpose, the switch of the cathode liquid pump was turned on for about 10 seconds. The cathode liquid gap was emptied within 15 seconds.

[0100] Then, the cell was left standing for 10 hours.

[0101] Then, startup was carried out as described in Example 1.

[0102] A total of 32 rest periods (stop processes) were carried out.

[0103] At the beginning of the experiment, the cell voltage at a current density of 600 mA / cm 2 was 2.48 V.

[0104] After 32 rest periods, the cell voltage at a current density of 600 mA / cm 2 was 2.48 V.

[0105] The cell voltage did not change, and no damage occurred to the gas diffusion electrode and further components.

[0106] Example 3 - Shutdown - Comparative example The electrolysis unit was started in the same manner as in Example 1. Shutdown was performed as follows according to the prior art: · Reduce the electrolysis current to 1.8 mA / cm 2 · The electrolyte circuit remained operating at the same volumetric flow rate as the electrolysis operation, similar to the O2 supply. · While maintaining the current density at 1.8 mA / cm 2 the temperature of the electrolyte was reduced to 75 °C within 1.5 hours. · Switch off the voltage supply · Immediately after switching off the voltage supply, first the anode space was emptied over a period of about 1 hour. · After emptying the anode space, the cathode space was emptied. · Then the anode space was filled from below with deionized water. Here, the anode space was only half filled and immediately drained again. · The cathode gap was further flushed with the cathode liquid. After discharging the anode liquid, the cathode liquid was also discharged from the cathode gap. · Then the cell was left standing for 10 hours. · Startup was performed as described in Example 1. · Five rests were performed according to the above procedure for shutdown · At the beginning of the experiment, the cell voltage at a current density of 400 mA / cm 2 was 2.11 V. · After five rests, the cell voltage at a current density of 400 mA / cm 2 was 2.14 V.

[0107] The cell voltage increased by 30 mV and damage to the gas diffusion electrode occurred.​

Claims

A method for chlor-alkali electrolysis using an electrolytic cell in a gap configuration, said cell comprising at least one anodic space having an anode and an anolyte containing an alkali metal chloride, an ion exchange membrane, a gas diffusion electrode serving as a cathode containing a silver-containing catalyst, a cathodic space, and a sheet-like porous element, through which the catholyte flows between the gas diffusion electrode and the membrane, and at the end of the electrolysis method, at least the following steps: a) Lowering the electrolysis voltage and removing chlorine from the anolyte so that less than 10 mg / l of active chlorine is present in the anolyte by maintaining an electrolysis voltage of 0.1 to 1.4 V per element and a current density greater than zero; b) Adjusting the pH of the anolyte to a value in the range of pH 2 to pH 12 during step a); c) Retaining under these conditions as long as the electrolyte is present in the catholyte gap; characterized in that these steps are carried out in this order.

2. The method according to claim 1, characterized in that the alkali metal chloride is sodium chloride or potassium chloride.

3. For emptying the electrolytic cell, a further step: d) Cooling the anolyte to a temperature below 70 °C while maintaining the electrolysis voltage in the range of 0.1 to 1.4 V; e) Switching off the electrolysis voltage at an electrolytic solution temperature below 55 °C; f) Emptying the cathodic gap; g) Emptying the anodic space; i) Filling the cathodic space with one of a dilute alkali metal hydroxide solution having a maximum concentration of 10 mol / l or deionized water, and then emptying the cathodic space; characterized in that these steps are carried out in this order, according to the method of claim 1 or 2.

4. The method according to claim 3, characterized in that the alkali metal hydroxide contained in the dilute alkali metal hydroxide solution is sodium hydroxide or potassium hydroxide.

5. Between step g) and step i), a further step: h) Refilling the anodic space with one of a dilute alkali metal chloride solution having a maximum concentration of 4 mol / l or deionized water, and then emptying the anodic space; characterized in that this step is carried out, according to the method of claim 3 or 4.

6. The method according to any one of claims 1 to 5, wherein oxygen gas is supplied to the side of the gas diffusion electrode facing away from the cathode liquid. **Claim 7**: The method according to any one of claims 1 to 6, characterized in that the oxygen gas flow to the gas diffusion electrode is maintained when the electrolysis switch is turned off. **Claim 8**: For the start of the electrolysis method, at least the following steps: j) filling the anode space with an anode liquid having a temperature of at least 50 °C and passing the anode liquid therethrough; k) preheating the cathode liquid to a temperature of at least 50 °C; l) filling the cathode space and the porous element with the preheated cathode liquid having a concentration in the range of 7.5 to 10.5 mol / l and passing the cathode liquid therethrough; m) setting the electrolysis voltage to a value in the range of 0.1 to 1.4 V; n) maintaining the temperatures of the cathode liquid and the anode liquid exiting the cell independently of each other at temperatures in the range of 70 to 100 °C; o) setting the concentration of the cathode liquid in the supply to the cell such that an alkali metal hydroxide concentration in the range of 7.5 to 12 mol / l is obtained at the output; p) setting the concentration of the anode liquid in the supply to the cell such that an alkali metal chloride concentration in the range of 2.9 to 4.3 mol / l is obtained at the output; q) setting the production current density to a value of at least 2 kA / m2 are carried out in this order, the method according to any one of claims 1 to 7. **Claim 9** The increase in current density to the manufacturing current density in step q) is carried out at a rate of 0.018 kA / (m ２ *min) to 0.4 kA / (m ２ *min) until the current density in the electrolysis element reaches at least 2 kA / m ２ The method according to claim 8, characterized in that it is carried out in this way.