Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
  • C25B1/46—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in diaphragm cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
  • C25B11/03—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form perforated or foraminous
  • C25B11/031—Porous electrodes
  • C25B11/032—Gas diffusion electrodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
  • C25B11/081—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound the element being a noble metal
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B13/00—Diaphragms; Spacing elements
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/02—Process control or regulation
  • C25B15/023—Measuring, analysing or testing during electrolytic production
  • C25B15/025—Measuring, analysing or testing during electrolytic production of electrolyte parameters
  • C25B15/027—Temperature
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/02—Process control or regulation
  • C25B15/023—Measuring, analysing or testing during electrolytic production
  • C25B15/025—Measuring, analysing or testing during electrolytic production of electrolyte parameters
  • C25B15/029—Concentration
  • C25B15/031—Concentration pH
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/60—Constructional parts of cells
  • C25B9/65—Means for supplying current; Electrode connections; Electric inter-cell connections

Definitions

• the invention relates to a method for the electrolysis of aqueous solutions of alkali metal chlorides using gas diffusion electrodes while maintaining certain operating parameters.
• the invention is based on electrolysis processes known per se, for example for the electrolysis of aqueous alkali metal chloride solutions using gas diffusion electrodes, which usually comprise an electrically conductive carrier and a gas diffusion layer with a catalytically active component.
• gas diffusion electrodes which usually comprise an electrically conductive carrier and a gas diffusion layer with a catalytically active component.
• the arrangement is such that there is a narrow gap between the gas diffusion electrode and the ion exchange membrane through which an electrolyte flows.
• the gas diffusion electrode - also referred to below as GDE for short - must meet a number of requirements in order to be used in technical electrolyzers.
• the catalyst and all other materials used must be chemically stable against the electrolyte used and the gases supplied to the electrode as well as the compounds formed on the electrode such as hydroxide ions or hydrogen at a temperature of typically up to 90 ° C.
• a high degree of mechanical stability is also required so that the electrodes can be installed and operated in electrolysers with an area of usually more than 2 m 2 (technical size).
• Other desired properties are: high electrical conductivity, a low layer thickness, a high internal surface area and a high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and an appropriate pore structure for conducting gas and electrolyte are necessary. Long-term stability and low manufacturing costs are further special requirements for a technically usable oxygen consumption electrode.
• WO2001/57290 A1 A cell for chlor-alkali electrolysis is described, in which the liquid is guided from top to bottom along the gas diffusion electrode in a kind of freely falling liquid film, known as a falling film, via a flat porous element, a so-called percolator, attached between the gas diffusion electrode and the ion exchange membrane (mini-gap arrangement). With this arrangement only one loads very low liquid column on the liquid side of the gas diffusion electrode, and no high hydrostatic pressure profile builds up over the height of the cell.
• An oxygen consumption electrode typically consists of a support element, for example a plate made of porous metal or a 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 components make it more difficult for electrolyte to penetrate and thus keep the corresponding pores in the GDE free for the transport of oxygen to the catalytically active centers.
• the hydrophilic components enable the electrolyte to penetrate into the catalytically active centers and the removal of the hydroxide ions from the GDE.
• a fluorine-containing polymer such as polytetrafluoroethylene (PTFE) is usually used as the hydrophobic component, which also serves as a polymeric binder for catalyst particles.
• PTFE polytetrafluoroethylene
• the silver serves as a hydrophilic component.
• Platinum has a very high catalytic activity for the reduction of oxygen. Because of the high cost of platinum, it is only used in supported form.
• the preferred carrier material is carbon.
• platinum also catalyzes the oxidation of the carrier material. Carbon also promotes the undesirable formation of H 2 O 2 , which also causes oxidation.
• Silver also has high electrocatalytic activity for reducing oxygen.
• Silver can be used in carbon-supported form and also as finely divided metallic silver. Although the carbon-supported silver catalysts are more durable than the corresponding platinum catalysts, their long-term stability is also limited under the conditions in one of the oxygen consumption electrodes, particularly when used for chlor-alkali electrolysis.
• the silver is preferably introduced at least partially in the form of silver oxides, which are then reduced to metallic silver.
• the reduction usually occurs when the electrolytic cell is put into operation for the first time.
• the silver compounds are reduced, there is also a change in the arrangement of the crystallites, in particular bridge formation between individual silver particles. This leads to an overall solidification of the structure.
• the membrane is permeable to cations and water and largely impermeable to anions.
• the ion exchange membranes in electrolytic cells are exposed to a lot of stress: They have to be resistant to chlorine on the anode side and strong alkaline stress on the cathode side at temperatures around 90°C.
• Perfluorinated polymers such as PTFE usually withstand these stresses. Ion transport occurs via acidic sulfonate groups and/or carboxylate groups polymerized into these polymers. Carboxylate groups show higher selectivity; the polymers containing carboxylate groups have lower water absorption and have a higher electrical resistance than polymers containing sulfonate groups.
• multilayer membranes are used with a thicker layer containing sulfonate groups on the anode side and a thinner layer containing carboxylate groups on the cathode side.
• the membranes are provided with a hydrophilic layer on the cathode side or on both sides.
• the membranes are reinforced by inserting fabrics or nonwovens; the reinforcement is preferably incorporated into the layer containing sulfonate groups.
• the ion exchange membranes are sensitive to changes in the media surrounding them. Strong osmotic pressure gradients can be built up between the anode and cathode sides through different molar concentrations. When electrolyte concentrations decrease, the membrane swells due to increased water absorption. When the electrolyte concentrations increase, the membrane releases water and thereby shrinks. In extreme cases, water removal can lead to the precipitation of solids in the membrane or to mechanical damage such as cracks in the membrane.
• holes pinholes
• cracks can occur, which can lead to undesirable mixing of anolyte and catholyte.
• An inhomogeneity of the water and/or ion distribution in the membrane and/or the gas diffusion electrode can lead to local peaks in current and mass transport when the system is restarted and, as a result, to damage to the membrane or the gas diffusion electrode.
• the precipitation of alkali metal chloride salts on the anode side also causes problems.
• the strong osmotic gradient between anolyte and catholyte results in water transport from the anode to the cathode space.
• the transport of water from the anode space is counteracted by a loss of chloride and alkali ions, so that the concentration of alkali chloride drops in the anode space under normal electrolysis conditions.
• the electrolysis is switched off, the water transport from the anode to the cathode space caused by the osmotic pressure remains.
• the concentration in the anolyte rises above the saturation limit.
• Alkaline chloride salts precipitate, especially in the border area to the membrane or even in the membrane, which can lead to damage to the membrane.
• electrolysis cells are desirably operated for periods of several years without being opened in the meantime.
• electrolysis cells in production plants inevitably have to be shut down and restarted repeatedly.
• JP 2004-300510 A An electrolysis process using a micro gap arrangement is described, in which corrosion in the cathode space is to be prevented by flooding the gas space with caustic soda when the cell is switched off. Flooding the gas space with caustic soda protects the cathode space from corrosion, but offers inadequate protection against damage to the electrode and the membrane during decommissioning and startup or during standstill.
• US 4578159A1 describes an electrolysis process using a "zero gap" arrangement that damage to the membrane and electrode can be avoided by rinsing the cathode compartment with 35% sodium hydroxide solution before starting up the cell, or by starting up the cell with a low current density and gradually increasing the current density. This procedure reduces the risk of damage to the membrane and gas diffusion electrode during commissioning, but does not offer any protection against damage during decommissioning and standstill.
• the anode side is first filled with brine, and water and nitrogen are added to the cathode side.
• the cell is then heated to 80°C.
• the gas supply is switched to oxygen and a polarization voltage with low current flow is applied.
• the current density is then increased and the pressure in the cathode is increased, the temperature rises to 90°C.
• the brine and water supply are then adjusted so that the desired concentrations are achieved on the anode and cathode sides.
• document EP 2 639 337 A2 relates to a method for chlorine-alkali electrolysis with an electrolysis cell with an oxygen consumption electrode, the electrolysis cell having at least one anode space with an anode and an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with an oxygen consumption electrode which has a silver-containing catalyst, and one through which the catholyte flows Electrolyte gap between the oxygen consumption electrode and the membrane, wherein before applying the electrolysis voltage between the anode and cathode, the volume flow and / or the composition of the catholyte supplied to the gap is adjusted so that the aqueous solution of alkali metal hydroxide leaving the electrolyte gap has a chloride ion content of at most 1000 ppm and after introducing the anolyte and an oxygen-containing gas into the cathode space, the electrolysis voltage is applied.
• document EP 2 639 338 A2 relates to a method for chlorine-alkali electrolysis with an electrolysis cell in a micro gap arrangement, the cell having at least one anode compartment with an anode and an anolyte containing alkali chloride, an ion exchange membrane, a cathode compartment which has at least one oxygen consumption electrode as a cathode, which has a silver-containing catalyst , and a 0.01 mm to 2 mm thick flat porous element through which catholyte flows, arranged between the SVE and the membrane and a gas space for oxygen-containing gas, the method being characterized in that before applying the electrolysis voltage between the anode and cathode in one In the first step, the oxygen consumption electrode is wetted with an aqueous alkali hydroxide solution with a maximum chloride ion content of 1000 ppm and after subsequent introduction of the anolyte into the anode space and an oxygen-containing gas into the gas space of the cathode space, the electrolysis
• document EP 2 639 339 A2 relates to a method for chlorine-alkali electrolysis with a special oxygen consumption electrode with a silver-containing catalyst, the cell having at least one anode compartment with an anode and an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode compartment with said oxygen consumption electrode as a cathode and a 0.01 mm to 2 mm thick flat porous element through which catholyte flows between the SVE and the membrane, and the method is characterized in that before applying the electrolysis voltage, in a first step, the oxygen consumption electrode on the gas side is filled with an aqueous alkali hydroxide solution with a chloride ion content of is wetted to a maximum of 1000 ppm and after subsequent introduction of the anolyte into the anode compartment and an oxygen-containing gas into the cathode compartment, the electrolysis voltage is applied.
• the volume flow and / or the composition of the catholyte supplied to the gap is adjusted so that the aqueous solution of alkali metal hydroxide leaving the cathode gap has a chloride ion content of at most 1000 ppm and after introduction of the anolyte and an oxygen-containing gas is applied to the cathode compartment.
• the EP 2639337 A2 before starting up a cell with a finite gap arrangement of the catholyte circuit, humidified oxygen is added and an overpressure is set in the cathode half-cell according to the configuration in the cell, usually at a level of 10 - 100 mbar compared to the pressure in the anode.
• the object of the present invention is to find suitable improved operating parameters for the commissioning and decommissioning, in particular for the decommissioning and interim shutdowns, of an electrolytic cell for chlor-alkali electrolysis using a gas diffusion electrode with a mini-gap arrangement and silver catalyst as an electrocatalytic substance, which are easy to carry out and if they are adhered to, damage to the membrane, electrode and/or other components of the electrolytic cell can be avoided.
• Mini-gap arrangement in the sense of the invention means any arrangement of an electrolytic cell which has an electrolyte gap between the oxygen consumption electrode and the membrane through which the catholyte flows, the gap having a gap width of at least 0.01 mm and in particular having a gap width of at most 3 mm.
• catholyte flows from top to bottom in a vertically arranged electrolysis cell following gravity.
• Other arrangements with alternative flow directions or horizontally arranged electrolytic cells are also intended to be covered by the invention.
• electrolyzers containing a gas diffusion electrode with a silver catalyst can be put into operation and deactivated repeatedly without damage due to the improved sequence of these steps and are not damaged even when stopped.
• the process is particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.
• the technical problem described above is achieved according to the invention in that a specific sequence of voltage reduction and exchange of the electrolytes is adhered to when the electrolytic cell is decommissioned.
• a measure known from conventional membrane electrolysis is the maintenance of a polarization voltage, which means that when the electrolysis ends the voltage is not reduced to zero, but a residual voltage is maintained so that a residual current flows in the usual electrolysis direction, so that a constant low current density results and electrolysis occurs to a small extent. If the electrolysis is to be taken out of operation, the electrolytes must be cooled down, which changes the potentials. Therefore, this measure alone is not sufficient to prevent damage to the electrode when using gas diffusion electrodes when putting them into or out of operation.
• oxidation of the silver catalyst can occur again when the electrolysis current is switched off. Oxidation is apparently promoted by the oxygen and moisture in the half-cell. Especially immediately after the electrolysis has been switched off, chlorine, hypochlorite and chlorate are present on the anode side in addition to the brine containing sodium chloride. On the cathode side, caustic soda, an electrocatalyst such as silver and oxygen. Through When the electrolysis current is switched off, the system is left to its own devices and electrochemical reactions occur that depend on the potential, concentrations, temperatures and pressures. The oxidation of the cathodic catalyst, for example silver to silver oxide, can lead to rearrangements in the catalyst structure, which have a negative effect on the activity of the catalyst and thus the performance of the gas diffusion electrode.
• the alkali metal chloride in the new process is preferably sodium chloride or potassium chloride, particularly preferably sodium chloride.
• the alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide, particularly preferably sodium hydroxide.
• the gas diffusion electrode is exposed to oxygen gas on its side facing away from the catholyte during operation.
• the oxygen gas flow to the gas diffusion electrode is preferably maintained when the electrolysis is switched off in accordance with the new process.
• the purity of the oxygen corresponds to the concentrations and purity requirements usual in electrolysis with a gas diffusion electrode; oxygen with a content of over 98.5% by volume is preferably used.
• the temperature of the catholyte supplied is regulated during operation so that a temperature of 70 - 95 °C, preferably 75 - 90 °C, is achieved in the discharge from the cathode compartment.
• a temperature difference between the anolyte outlet and the catholyte inlet of less than 20 ° C is set during operation and when decommissioning. Such a small temperature difference avoids damage to the ion exchange membrane.
• a brine is supplied to the anode compartment, the NaCl content of which is 180 g/L (3.07 mol/L) to 330 g/L (5.64 mol/L ) amounts. This frees the anode space of any chlorine gas present and reduces the dissolved/dispersed chlorine content.
• concentrations disclosed in this application are determined in particular by titration or another analysis method that is generally known to those skilled in the art.
• a current density of greater than zero up to 20 A/m 2 preferably from 0.1 A/m 2 up to 20 A/ m 2 maintained.
• the electrolysis is carried out until the anolyte is Cl 2 -free, meaning that the content of chlorine with an oxidation state of zero and > 0 is less than 10 mg/L.
• the measurement of the freedom from chlorine in the anolyte is carried out in particular by means of redox titration such as iodometry or by checking the anolyte using iodine-starch paper.
• Maintaining the brine pH in the range from 2 to 12, preferably pH 6 to 9, during step a) is necessary to avoid any chlorine development at lower pH.
• the temperature of the anolyte in steps a) and b) is preferably at least 65 ° C, particularly preferably at least 70 ° C.
• the anolyte is cooled in step d) to a temperature below 70 ° C while maintaining an electrolysis voltage of 0.1 to 1.4 V. This is another difference from the prior art - done here cooling without maintaining the electrolysis voltage.
• the electrolysis voltage is switched off in step e) at an electrolyte temperature of ⁇ 55°C, preferably at a temperature of ⁇ 50°C
• the cathode gap (mini-gap) is then emptied in step f) (e.g. by switching off the pump for the catholyte feed).
• step f e.g. by switching off the pump for the catholyte feed.
• the anode space is emptied in step g) by draining the anolyte and in particular subsequent rinsing h) of the anode space with an alkali metal chloride solution with a maximum of 4 mol/l or with demineralized water (deionized water).
• step i) the cathode gap (mini-gap) is finally rinsed with diluted sodium hydroxide solution or demineralized water to remove chloride residues and empty the cathodic mini-gap.
• the cathode gap is rinsed again to remove chloride. This avoids, for example, corrosion on the cell's nickel connecting flanges due to excessive chloride values in the lye remaining in the cathode compartment.
• the residual emptying of the anode space can then particularly preferably take place.
• the gas diffusion electrode is efficiently protected by the process according to the invention. Thanks to potentiostatic operation, the cell can also be cooled below 70°C without chlorine being developed on the anode side. This is important from a safety perspective if the electrolysis elements are to be opened later for maintenance work or repairs.
• the electrolysis voltage is reduced.
• the voltage is reduced to a value of 0.1 to 1.4 V.
• the chlorine content in the anode space is reduced to ⁇ 10 mg/l, preferably less than 1 mg/l.
• the pH value of the anolyte in the outlet from the electrolysis cell is 2 to 12, preferably 6 to 9.
• Chlorine content is understood to mean the total content of dissolved chlorine in the oxidation state 0 and higher.
• the remaining chlorine is preferably removed from the anode space in such a way that chlorine-free anolyte is supplied while chlorine-containing anolyte is removed, or by pumping the anolyte around in the anode circuit with simultaneous separation and removal of chlorine gas.
• the EP263337A2 when flushing C12-free, the voltage is adjusted so that a current density of 0.01 to 20 A/m 2 is achieved, preferably 10 to 18 A/m 2 . Under these conditions, the electrolysis is not operated below a temperature of 70°C, otherwise the development of chlorine will start again.
• the cooling of the electrolysis can be carried out according to the method according to the invention if the electrolysis voltage is not more than 1.4 V below a temperature of 70 ° C, the pH value of the brine being between 2 and 12. In this state, the electrolysis can remain for many hours without damaging the gas diffusion electrode. Compared to the prior art, the electrolysis voltage remains applied.
• the load can be increased again at any time.
• the electrolytic cell with the moist membrane can be kept installed over a longer period of time for short-term commissioning without affecting the performance of the electrolytic cell.
• Rinsing is preferred at intervals of 1 - 12 weeks, particularly preferably at intervals of 4 - 8 weeks.
• the concentration of the diluted alkali chloride solution used for rinsing or wetting is 1 - 4.8 mol/L.
• the rinsing solution can be drained off again immediately after the anode compartment has been completely filled or it can remain in the anode compartment for up to 200 minutes and then be drained off.
• concentration of the alkali metal hydroxide solution used for rinsing or wetting is 0.1 to 10 mol/l, preferably between 1 and 4 mol/l.
• the temperature of the brine or alkali metal hydroxide solution can be between 10 and 80°C, but preferably 15 to 40°C.
• the mini-gap cathode shells can be rinsed for a period of 0.01 to 10 minutes.
• the cathode gap (mini-gap) is filled according to step l) by filling the preheated alkaline solution with a temperature of at least 50°C into the gap .
• This procedure is different from the prior art; the cathode compartment is filled first and then the anode compartment - the procedure according to the invention avoids excessively high chloride values in the lye and thus possible corrosion problems.
• an electrolysis voltage of at least 0.4 V is preferably applied in step m), in particular within 0.01 to 10 minutes, so that a current density of at least 0.2 A/m 2 is achieved.
• Anolyte and catholyte are then heated according to step n) to a temperature of at least 70 ° C and the current density is then preferably increased.
• the increase in current density to production current density in step q) is particularly preferably carried out with a gradient of 0.018 kA/(m 2 *min) to 0.4 kA/(m 2 *min) until the current density at the electrolysis element is at least 2 kA/m 2 .
• concentrations are determined by titration or other methods generally known to those skilled in the art.
• the electrolytic cell that was taken out of operation according to the above new process is put back into operation according to the new process described above. If the process steps described are adhered to, the electrolytic cell can go through a large number of start-up and shutdown cycles without the cell's performance being impaired.
• the gas diffusion electrode used in the examples was according to EP1728896B1 prepared as follows: A powder mixture consisting of 7% by weight of PTFE powder, 88% by weight of silver-I oxide and 5% by weight of silver powder was applied to a network of nickel wires and pressed into an oxygen consumption electrode.
• the electrode was installed in an electrolysis unit with an area of 100 cm 2 with an ion exchange membrane from DuPONT type N982 (manufacturer Chemours) and a distance between the gas diffusion electrode and the ion exchange membrane of 3 mm.
• the electrolysis unit When assembled, the electrolysis unit has an anode chamber with anolyte inlet and outlet, with an anode consisting of a titanium expanded metal, which is coated with a commercially available DSA coating for chlorine production from Denora, consisting of a mixed oxide of ruthenium/iridium oxide was and a cathode space with the gas diffusion electrode as a cathode and with a gas space for the oxygen and oxygen inlets and outlets, a liquid drain and an ion exchange membrane, which are arranged between the anode and cathode spaces.
• the electrolysis cell was operated with a brine concentration of approximately 210 g/L (3.58 mol/L) NaCl and a sodium hydroxide concentration of approximately 31% by weight (10.4 mol/L) at electrolyte temperatures of approximately 85°C .
• the cell voltage was corrected to 32% by weight (10.79 mol/L) sodium hydroxide solution and 90°C using standard methods.
• the electrolytes were introduced into the cell from below and removed again from the top of the cell.
• Oxygen was supplied to the gas space of the cathode. Oxygen with a quality of more than 99.5% oxygen by volume was used. The oxygen was moistened with water at room temperature before it was introduced into the gas space of the cathode half-shell. The amount of oxygen was regulated so that 1.5 times the stoichiometric excess of the required amount of oxygen was always supplied based on the set current intensity. The oxygen is supplied into the gas space from above and removed below.
• the electrolysis unit had a gap of approximately 3 mm between the oxygen consumption electrode and the ion exchange membrane. This gap was filled with a porous PTFE fabric as a percolator and spacer.
• the production current density was 6 kA/m 2 .
• the anolyte circuit was put into operation and the anode space was filled with an anolyte with a concentration of approximately 210 g NaCl/l (3.58 mol/L). While the anode circuit was maintained and the anolyte was passed through the cell, the anolyte was heated to 50 ° C by a heat exchanger located in the anode circuit.
• the 50°C hot caustic soda was passed into the cell and, after filling the cathode gap, an electrolysis voltage of 1.08 V was applied within 30 seconds. This resulted in a current density of 10 mA/cm 2 .
• the pH value of the anolyte running off was 8.
• the electrolytes were heated from 50°C to 70°C within 1 hour. After the temperature of the draining anolyte and catholyte reached 70° C., the electrolysis voltage was increased, the electrolysis voltage being 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 .
• concentrations were adjusted so that the concentration of the outflowing brine was approx. 210 g/l (3.59 mol/L) and that of the caustic soda was approx. 31.5% by weight (10.6 mol/L). .
• the cell was operated under these conditions for at least 24 hours.
• the electrolysis unit was operated at a current density of 600 mA/cm 2 .
• the current density was reduced to 1.5 mA/cm 2 .
• the main rectifier was switched off and the polarization rectifier was switched on.
• the polarization rectifier then takes over the maintenance of a current density of 1.5 mA/cm 2 . Operation at the low current density was maintained for 1.5 hours.
• the anolyte is then chlorine-free. This process is carried out in technical electrolysers for safety reasons.
• chlorine or chlorine compounds, for example hypochlorite will not diffuse from the anolyte into the catholyte via the ion exchange membrane and lead to corrosion of cell components or the gas diffusion electrode.
• the chlorine-free rinsing phase lasts approx. 1.5 hours.
• Electrolyte circuits remained in operation with the same volume flows as in electrolysis operation at 600 mA/cm 2 .
• the O2 supply was also maintained.
• the temperature of the anolyte and catholyte is reduced from 85°C to 70°C.
• the cell voltage in this phase was approximately 1.16 V and the pH value of the anolyte draining from the cell was pH 8.2.
• the temperature of the anolyte and catholyte is reduced to 50 ° C, with the polarization rectifier being operated potentiostatically.
• the voltage remains at 1.16V and the current is reduced accordingly.
• the polarization rectifier is switched off and the catholyte is immediately drained from the cathode compartment. This takes place in approximately 30 seconds. After emptying the cathode compartment, the anode compartment is drained within 1 hour.
• the anode space is filled with desalinated water from below to a maximum of 50% of the cell height and immediately drained again.
• the cathode gap was also flushed by switching on the catholyte pump again and feeding catholyte into the cathode space.
• the catholyte pump was switched on for approx. 10 seconds.
• the catholyte gap then ran empty within 15 s.
• the cell was then left to stand for 10 h.
• the cell voltage was 2.48 V at a current density of 600 mA/cm 2
• the cell voltage was 2.48 V at a current density of 600 mA/cm 2
• the cell voltage remained unchanged and there was no damage to the gas diffusion electrode or other components.
• the cell voltage increased by 30mV and the gas diffusion electrode was damaged.

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