Membrane Electrolysis Processes For Akaline Chloride Solutions, Using A Gas-diffusion Electrode

Abstract

The invention relates to processes for the electrolysis of alkali chlorides by means of oxygen-depolarized electrodes, said processes having specific operating parameters for shut-down and restarting.

Description

[0001] The invention relates to a process for the electrolysis of aqueous solutions of alkali metal chlorides by means of gas diffusion electrodes with adherence to particular operating parameters.

[0002] The invention proceeds from electrolysis processes known per se, e.g. for the electrolysis of aqueous alkali metal chloride solutions by means of gas diffusion electrodes which usually comprise an electrically conductive support and a gas diffusion layer having a catalytically active component. The arrangement is such that there is a narrow gap through which an electrolyte flows between gas diffusion electrode and ion exchange membrane.

[0003] Various proposals for operating the gas diffusion electrode as oxygen-depolarized electrode in electrolysis cells of industrial size are known in principle from the prior art. The basic idea here is to replace the hydrogen-evolving cathode of the electrolysis (for example in chloralkali electrolysis) by the oxygen-depolarized electrode (cathode). An overview of possible cell designs and solutions may 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] The gas diffusion electrode, hereinafter also referred to as GDE for short, has to meet a number of requirements in order to be able to be used in industrial electrolyzers. Thus, the catalyst and all other materials used have to be chemically stable to the electrolyte used and the gases supplied to the electrode and also the compounds formed at the electrode, e.g. hydroxide ions or hydrogen, at a temperature of typically up to 90.degree. C. A high degree of mechanical stability is likewise required so that the electrodes can be installed and operated in electrolyzers having a size of usually more than 2 m.sup.2 in area (industrial size). Further desirable properties are: a 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 the conduction of gas and electrolyte are necessary. The long-term stability and low production costs are further particular requirements which an industrially usable oxygen-depolarized electrode has to meet.

[0005] WO 2001/57290 A1 describes a cell for chloralkali electrolysis in which the liquid is conveyed from the top downward over a sheet-like porous element, known as a percolator, installed between gas diffusion electrode and ion exchange membrane in a type of free-falling liquid film, referred to as falling film for short, along the gas diffusion electrode (minigap arrangement). In this arrangement, only a very small column of liquid acts on the liquid side of the gas diffusion electrode and no high hydrostatic pressure profile is built up over the construction height of the cell.

[0006] A further arrangement, which is sometimes also referred to as "zero gap" but would be more precisely formulated as "microgap", is described in JP 3553775 and U.S. Pat. No. 6,117,286 A1. In this arrangement a further layer composed of a porous hydrophilic material which takes up the alkali metal hydroxide solution formed due to its suction force and from which at least part of the alkali can flow away in a downward direction is located between the ion exchange membrane and the GDE. The possibility of the alkali metal hydroxide solution flowing away is determined by the installation of the GDE and the cell design. In contrast to the above-described arrangements in the minigap design, no aqueous alkali metal hydroxide solution (alkali) is conveyed by supply and discharge through the gap between the GDE and ion exchange membrane; the porous material present in the microgap takes up the alkali metal hydroxide solution formed and conducts it further in the horizontal or vertical direction.

[0007] An oxygen-depolarized electrode typically consists of a support element, for example a plate of porous metal 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 constituents. The hydrophobic constituents make penetration of electrolyte difficult and thus keep the appropriate pores in the GDE free for transport of oxygen to the catalytically active sites. The hydrophilic constituents allow passage of the electrolyte to the catalytically active sites and outward transport of the hydroxide ions from the GDE. A fluorine-containing polymer such as polytetrafluoroethylene (PTFE) is generally used as hydrophobic component and additionally serves as polymeric binder for particles of the catalyst. In the case of electrodes having a silver catalyst, the silver serves, for example, as hydrophilic component. Many compounds have been described as electrochemical catalyst for the reduction of oxygen. However, only platinum and silver have attained practical importance as catalyst for the reduction of oxygen in alkaline solutions.

[0008] Platinum has a very high catalytic activity for the reduction of oxygen. Owing to the high cost of platinum, this is used exclusively in supported form. A preferred support material is carbon. However, the stability of platinum-based electrodes supported on carbon in long-term operation is unsatisfactory, presumably because platinum also catalyzes the oxidation of the support material. In addition, carbon promotes the undesirable formation of H.sub.2O.sub.2, which likewise brings about oxidation. Silver likewise has a high electrocatalytic activity for the reduction of oxygen.

[0009] Silver can be used in carbon-supported form and also as finely divided metallic silver. Although carbon-supported silver catalysts are more durable than the corresponding platinum catalysts, their long-term stability under the conditions in an oxygen-depolarized electrode, in particular during use for chloralkali electrolysis, is also limited.

[0010] In the production of GDEs having an unsupported silver catalyst, the silver is preferably introduced at least partly in the form of silver oxides which are then reduced to metallic silver. The reduction generally occurs during the first start-up of the electrolysis cell. In the reduction of the silver compounds, a change in the arrangement of the crystallites, in particular bridge formation between individual silver particles, also occurs. This leads overall to consolidation of the structure.

[0011] A further central element of the electrolysis cell is the ion exchange membrane. The membrane is permeable to cations and water and largely impermeable to anions. The ion exchange membranes in electrolysis cells are subject to great stress: they have to be resistant to chlorine on the anode side and strongly alkaline conditions on the cathode side at temperatures of about 90.degree. 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 display greater selectivity and the carboxylate-containing polymers have a smaller water absorption and a higher electrical resistance than polymers containing sulfonate groups. In general, 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 generally used. The membranes are provided with a hydrophilic layer on the cathode side or on both sides. To improve the mechanical properties, the membranes are reinforced by insertion of woven fabrics or nonwovens, and the reinforcement is preferably incorporated in the layer containing sulfonate groups.

[0012] Due to the complex structure, the ion exchange membranes are sensitive to changes in the media surrounding them. High osmotic pressure gradients can be built up between anode side and cathode side as a result of different molar concentrations. When the electrolyte concentrations decrease, the membrane swells due to increased water absorption. When the electrolyte concentrations increase, the membrane releases water and shrinks as a result; in the extreme case, precipitation of solids in the membrane or mechanical damage such as cracks in the membrane can occur as a result of withdrawal of water.

[0013] Concentration changes can thus bring about defects and damage on the membrane. Delamination of the layer structure (blister formation) can occur, as a result of which mass transfer or the selectivity of the membrane is impaired.

[0014] Furthermore, holes (pinholes) and in the extreme case cracks can occur, and undesirable mixing of anolyte and catholyte can occur through these.

[0015] When the electrolysis voltage is switched off, the mass transfer through the membrane brought about by the flow of current also stops, and in addition undesirable concentration changes in the alkali metal chloride-containing electrolyte in the anode space (brine) and the alkali metal hydroxide solution present in the cathode space can occur. The membrane becomes depleted in water, and shrinkage and solid precipitates and consequently hole formation can occur and the passage of anions through the membrane is made easier. When the electrolysis cell is started up again, a water content which is too low hinders mass transfer through the membrane, as a result of which an increase in the osmotic pressure and delamination at the interfaces between the sulfonic acid group-containing and carboxylic acid group-containing layers typically used in such membranes can occur.

[0016] An inhomogeneity of the water and/or ion distribution in the membrane and/or the gas diffusion electrode can lead to local peaks in electricity transport and mass transfer on renewed start-up and consequently to damage to the membrane or the gas diffusion electrode.

[0017] Problems are also presented by the precipitation of alkali metal chloride salts on the anode side. The high osmotic gradient between anolyte and catholyte results in transport of water from the anode space into the cathode space. As long as the electrolysis is in operation, the transport of water from the anode space is countered by a loss of chloride and alkali metal ions, so that the concentration of alkali metal chloride decreases in the anode space under customary electrolysis conditions. When the electrolysis is switched off, the transport of water from the anode space into the cathode space caused by the osmotic pressure persists. The concentration in the anolyte increases to above the saturation limit. Precipitation of alkali metal chloride salts occurs, in particular in the boundary region to the membrane or even in the membrane, which can lead to damage to the membrane.

[0018] In production plants, it is desirable to operate electrolysis cells over periods of a number of years without them being opened during this time. However, due to fluctuations in offtake quantities and malfunctions in production regions upstream or downstream of the electrolysis, electrolysis cells in production plants inevitably have to be repeatedly shut down and started up again.

[0019] In the shutting down and restarting of the electrolysis cells, conditions which lead to damage to the cell elements such as anode, ion exchange membrane, gas diffusion electrode or further components used in the cell and can significantly shorten their life and also impair the performance of the electrolysis occur. In particular, oxidative damage in the cathode space, damage to the gas diffusion electrode and damage to the membrane have been found.

[0020] Few modes of operation by means of which the risk of damage to the electrolysis cells during start-up and shutdown can be reduced are known from the prior art.

[0021] The Japanese first publication JP 2004-300510 A describes an electrolysis process using a microgap arrangement, in which corrosion in the cathode space on shutdown of the cell is said to be prevented by flooding of the gas space with sodium hydroxide solution. The flooding of the gas space with sodium hydroxide solution protects the cathode space against corrosion according to this publication, but it offers insufficient protection against damage to the electrode and the membrane on shutdown and start-up or during the downtime.

[0022] U.S. Pat. No. 4,578,159 A1 states that damage to membrane and electrode is avoided in an electrolysis process using a "zero gap" arrangement by flushing of the cathode space with 35% strength sodium hydroxide solution before start-up of the cell or by starting the cell at a low current density and gradually increasing the current density. This procedures reduces the risk of damage to membrane and gas diffusion electrode during start-up, but offers no protection against damage during shutdown and the downtime.

[0023] It is known from the document U.S. Pat. No. 4,364,806 A1 that corrosion in the cathode space is said to be reduced by replacement of the oxygen by nitrogen after regulating down the electrolysis current. According to WO 2008009661 A2, the addition of a small proportion of hydrogen to the nitrogen is said to give an improvement in protection against corrosive damage. However, the methods mentioned are complicated, in particular in respect of safety aspects, and require installation of additional facilities for introduction of nitrogen and hydrogen. On restarting, the pores of the gas diffusion electrode are partially filled with nitrogen and/or hydrogen, which hinders the supply of oxygen to the reactive sites. In addition, the method does not offer any protection against damage to the ion exchange membrane and demands a high level of safety measures in order to avoid explosive gas mixtures.

[0024] In the Final Technical Report "Advanced Chlor-Alkali Technology" by Jerzy Chlistunoff (Los Alamos National Laboratory, DOE Award 03EE-2F/Ed190403, 2004), conditions for temporary shutdown and switching-on of zero gap cells are described. On shutdown, the oxygen supply is interrupted and replaced by nitrogen after interruption of the electrolysis current. The humidification of the gas stream is increased in order to wash out the remaining sodium hydroxide solution. On the anode side, the brine is replaced by hot water (90.degree. C.). The procedure is repeated until a stable open circuit voltage has been achieved. The cells are then cooled, and the supply of humid nitrogen and the pumped circulation of the water on the anode side are then stopped.

[0025] For renewed start-up, the anode side is firstly filled with brine, and water and nitrogen are introduced on the cathode side. The cell is then heated to 80.degree. C. The gas supply is then changed over to oxygen and a polarization voltage is applied at a low current flow. The current density is subsequently increased and the pressure in the cathode is increased; the temperature rises to 90.degree. C. Brine and water supply are subsequently adapted so that the desired concentrations are achieved on the anode side and cathode side.

[0026] This procedure can be carried out only with extreme difficulty for operation of an industrial cell and leads to dilute electrolyte-containing solutions which have to be disposed of being obtained.

[0027] Start-Up

[0028] For start-up as described in EP 2639337 A2, the volume flow and/or the composition of the catholyte fed to the gap is set so that the aqueous solution of alkali metal hydroxide leaving the cathode gap has a content of chloride ions of not more than 1000 ppm before the electrolysis voltage is applied between anode and cathode and the electrolysis voltage is applied after introduction of the anolyte and an oxygen-containing gas into the cathode space.

[0029] According to the prior art of EP 2639337 A2, humidified oxygen is introduced before start-up of a cell having a finite gap arrangement of the catholyte circuit and a gauge pressure corresponding to the configuration in the cell is set in the cathode half cell, which gauge pressure is generally 10-100 mbar relative to the pressure in the anode.

[0030] However, it has been found when carrying out the start-up and shutdown according to the methods of EP 2639337 A2 that the performance of the electrolysis is impaired contrary to expectations when these procedures are carried out repeatedly.

[0031] The fact remains that the techniques for starting up and shutting down a gas diffusion electrode as described hitherto in the prior art are found to be disadvantageous and offer only insufficient protection against damage.

[0032] It is an object of the present invention to find suitable improved operating parameters for start-up and shutdown, in particular for shutdown and interim downtimes of an electrolysis cell for chloralkali electrolysis using a gas diffusion electrode with minigap arrangement and silver catalyst as electrocatalytic substance, which improved parameters are simple to carry out and damage to membrane, electrode and/or other components of the electrolysis cell is avoided when they are adhered to.

[0033] Minigap arrangement means, for the purposes of the invention, any arrangement of an electrolysis cell which has an electrolyte gap through which catholyte flows between oxygen-depolarized electrode and membrane, where the gap has a gap width of at least 0.01 mm and in particular has a gap width of not more than 3 mm. In the electrolysis cell according to the principle of a falling film cell which is preferably used, catholyte flows from the top downward in the direction of gravity in a vertically arranged electrolysis cell. Other arrangements with an alternative flow direction or a horizontally arranged electrolysis cell are also intended to be encompassed by the invention.

[0034] The abovementioned problems and disadvantages of the processes known hitherto are overcome by the provision of the electrolysis process of the invention.

[0035] It has surprisingly been found that electrolyzers containing a gas diffusion electrode having a silver catalyst can repeatedly be started up and shut down without damage by means of the improved sequence of these steps and also suffer no damage during the downtime. The process is particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.

[0036] The above-described technical object is achieved according to the invention by a specific sequence of voltage reduction and replacement of the electrolytes being adhered to on shutdown of the electrolysis cell.

[0037] The invention provides a process for chloralkali electrolysis using an electrolysis cell in a gap arrangement, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell comprises at least one anode space with anode and an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode which comprises a silver-containing catalyst and an in particular from 0.01 mm to 3 mm thick sheet-like porous element through which catholyte flows between gas diffusion electrode and membrane, characterized in that at the end of the electrolysis process, in particular for shutdown, at least the following steps are carried out in this order: [0038] a) lowering of the electrolysis voltage and removal of chlorine from the anolyte so that less than 10 mg/l of active chlorine is present in the anolyte by maintaining an electrolysis voltage per element of from 0.1 to 1.4 V and a current density which is greater than zero, [0039] b) setting of the pH of the anolyte to a value in the range from pH 2 to pH 12, [0040] c) residence under these conditions as long as electrolyte is present in the catholyte gap (or electrolyte flows through the latter),

[0041] and in the case of emptying of the electrolysis cell (e.g. in the case of maintenance and repair work on the electrolysis cell in which the electrolysis cell has to be opened): [0042] d) cooling of the anolyte to a temperature below 70.degree. C. with maintenance of the electrolysis voltage in the range from 0.1 to 1.4 V, [0043] e) switching off of the electrolysis voltage at a temperature of <55.degree. C., [0044] f) emptying of the cathode gap, [0045] g) emptying of the anode space, [0046] h) preferably renewed filling of the anode space with one of the following liquids: dilute alkali metal chloride solution having a maximum concentration of 4 mol/l or deionized water, and subsequent emptying of the anode space, [0047] i) filling of the cathode space with one of the following liquids: dilute alkali metal hydroxide solution having a maximum concentration of 10 mol/l or deionized water, with subsequent emptying of the cathode space.

[0048] One measure known from conventional membrane electrolysis is maintenance of a polarization voltage, i.e. the voltage is not regulated down to zero on ending of the electrolysis but instead a residual voltage is maintained so that a residual current flows in the usual electrolysis direction, so that a constant small current density results and as a result an electrolysis occurs to a small extent. If the electrolysis is to be shut down, cooling of the electrolyte is necessary, as a result of which the potentials change. This measure alone is therefore not sufficient to prevent damage to the electrode during start-up and shutdown when using gas diffusion electrodes.

[0049] It has also been observed that oxidation of the silver catalyst can occur again on switching off the electrolysis current. The oxidation is obviously promoted by the oxygen and the moisture in the half cell. In particular immediately after switching off the electrolysis, chlorine, hypochlorite and chlorate are present in addition to the sodium chloride-containing brine on the anode side. On the cathode side, sodium hydroxide solution, an electrocatalyst such as silver and oxygen are present. Due to switching off of the electrolysis current, the system is left to itself and electrochemical reactions which depend on the potential, the concentrations, the temperatures and pressures occur. As a result of the oxidation of the cathodic catalyst, e.g. silver to silver oxide, rearrangements in the catalyst microstructure can occur and these have adverse effects on the activity of the catalyst and thus the performance of the gas diffusion electrode.

[0050] In the new process, the alkali metal chloride is preferably sodium chloride or potassium chloride, particularly preferably sodium chloride.

[0051] The alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide, particularly preferably sodium hydroxide.

[0052] In a preferred new process, the gas diffusion electrode is supplied with oxygen gas on its side facing away from the catholyte during operation. The oxygen gas stream to the gas diffusion electrode is preferably maintained during shutdown of the electrolysis according to the new process.

[0053] The purity of the oxygen corresponds to the concentrations and purity requirements customary in electrolysis using a gas diffusion electrode; oxygen having a content of more than 98.5% by volume is preferably used.

[0054] The temperature of the catholyte fed in is regulated during operation so that a temperature of 70-95.degree. C., preferably 75-90.degree. C., is established in the output from the cathode space. A temperature difference between anolyte output and catholyte input is preferably set to less than 20.degree. C. during operation and during shutdown. Such a small temperature difference avoids damage to the ion exchange membrane.

[0055] To remove chlorine from the anolyte in step a), a brine having a content of NaCl of from 180 g/l (3.07 mol/l) to 330 g/l (5.64 mol/l) is fed into the anode space in a preferred embodiment. In this way, the anode space is freed of chlorine gas present and the content of dissolved/dispersed chlorine is reduced.

[0056] Determination of the concentrations disclosed in the present patent application is carried out, in particular, by titration or another analytical method which is known in principle to a person skilled in the art.

[0057] To reduce the electrolysis voltage to a range from 0.1 to 1.4 V in step a), a current density of from greater than zero to 20 A/m.sup.2, preferably from 0.1 A/m.sup.2 to 20 A/m.sup.2, is preferably maintained. Under these conditions, the electrolysis is operated until the anolyte is Cl.sub.2-free, i.e. the content of chlorine having the oxidation state zero is from >0 to less than 10 mg/l. The measurement of the absence of chlorine in the anolyte is, in particular, carried out by means of redox titration such as iodometry or by testing of the anolyte by means of iodine-starch paper.

[0058] The maintenance of the brine pH in the range from 2 to 12, preferably from 6 to 9, during step a) is required in order to avoid any chlorine evolution at a lower pH.

[0059] The temperature of the anolyte in steps a) and b) is preferably at least 65.degree. C., particularly preferably at least 70.degree. C.

[0060] Maintenance of a differential pressure of at least 5 mbar between cathode space and anode space is particularly preferably ensured during shutdown.

[0061] As preparation for emptying of the electrolysis cell, the anolyte is cooled in step d) to a temperature below 70.degree. C. with simultaneous maintenance of an electrolysis voltage of from 0.1 to 1.4 V. This is a further difference from the prior art--cooling is carried out there without maintenance of the electrolysis voltage.

[0062] The switching off of the electrolysis voltage in step e) is carried out at a temperature of the electrolytes of <55.degree. C., preferably at a temperature of <50.degree. C.

[0063] The cathode gap (minigap) is subsequently emptied in step f) (e.g. by switching off the pump for the catholyte feed). Here too, there is a difference from the prior art since in the latter the minigap is emptied only after emptying of the anode space.

[0064] The emptying of the anode space in step g) is carried out by draining the anolyte and, in particular, subsequent flushing h) of the anode space with alkali metal chloride solution having a maximum concentration of 4 mol/1 or with deionized water.

[0065] Finally, in step i), the cathode gap (minigap) is flushed with dilute sodium hydroxide solution or deionized water to remove chloride residues and empty the cathodic minigap. In contrast to the prior art, the cathode gap here is flushed again in order to remove chloride after the anode space has been emptied. This avoids, for example, corrosion on nickel connecting flanges of the cell by excessively high chloride values in the alkali remaining in the cathode space.

[0066] If required, residual emptying of the anode space can particularly preferably then be carried out.

[0067] The difference from the procedure known from the prior art, in particular relative to EP 263337 A2, is that when the electrolysis voltage is lowered, the current density is not kept constant but instead the electrolysis voltage is set in the range from 0.1 to 1.4 V, in what is known as potentiostatic operation, regardless of what current density is established. The important thing here is that a current flows from the anode to the cathode, i.e. the flow direction in the original electrolysis flow direction is retained, and that the current is in any case greater than zero. Furthermore, the cathode gap is emptied immediately after switching off of the electrolysis voltage rather than the anode space being emptied first, as described in EP 263337 A. The emptying of the anode space, particularly in the case of industrial electrolysis elements, takes up to 150 minutes, depending on cell construction at an industrial construction size. Likewise, the pH of the brine is not taken into consideration in the prior art, while according to the invention this is optimally from 2 to 12.

[0068] The gas diffusion electrode is efficiently protected by the process of the invention. The cell can also be cooled to below 70.degree. C. without chlorine being evolved on the anode side as a result of the potentiostatic operation. This is important from safety aspects if the electrolysis elements are to be opened later for maintenance work or repair.

[0069] Preferred Details for Shutdown of Membrane Electrolysis Using a Gas Diffusion Electrode are Described Below

[0070] In a first step, the electrolysis voltage is regulated down. Here, the voltage is regulated down to a value of from 0.1 to 1.4 V. At a temperature of the anolyte of >65 C..degree. and a concentration of greater than 200 g/l (3.41 mol/l) of NaCl and an alkali metal hydroxide concentration in the catholyte of <28% by weight (9.1 mol/l) at a catholyte temperature of >65.degree. C., the chlorine content in the anode space is lowered to <10 mg/l, preferably less than 1 mg/l. Here, the pH of the anolyte in the output from the electrolysis cell is from 2 to 12, preferably from 6 to 9.

[0071] For the present purposes, the chlorine content is the total content of dissolved chlorine in the oxidation state 0 and above. Removal of the remaining chlorine from the anode space is preferably effected by chlorine-free anolyte being fed in with simultaneous discharge of chlorine-containing anolyte, or by pumping of the anolyte in the anode circuit with simultaneous removal and discharge of chlorine gas.

[0072] According to the prior art, namely EP 263337 A2, the voltage is set during flushing free of Cl2 so that a current density of from 0.01 to 20 A/m.sup.2, preferably from 10 to 18 A/m.sup.2, is established. Under these conditions, the electrolysis is not operated below a temperature of 70.degree. C., since otherwise chlorine evolution recommences. The cooling of the electrolysis can be carried out according to the process of the invention when the electrolysis voltage below a temperature of 70.degree. C. is not more than 1.4 V, with the pH of the brine being in the range from 2 to 12. In this state, the electrolysis can be suspended for many hours without the gas diffusion electrode being damaged. Relative to the prior art, the electrolysis voltage continues to be applied.

[0073] If the electrolysis cell is to be started up again, the load can be increased again at any time.

[0074] When the electrolysis cell is to be emptied, the following further steps are particularly preferably carried out: [0075] switching off of the voltage supply [0076] firstly emptying of the cathode space within from 0.01 to 2 minutes [0077] after emptying of the cathode space, emptying of the anode space is carried out within from 0.01 to 200 minutes; the emptying of cathode space and anode space can optionally be carried out in parallel after switching off of the voltage supply [0078] after emptying of the anode space, optionally flushing of the anode space [0079] flushing is carried out using greatly diluted brine having an alkali metal chloride content of from 0.01 to 4 mol/l, with water or, preferably, with deionized water. Flushing is preferably carried out by one-off filling of the anode space or else only partial filling of the anode space and immediate draining of the flushing liquid. Flushing can also be carried out in two or more stages, for example by the anode space firstly being filled with a dilute brine having an alkali metal chloride content of 1.5-2 mol/l and drained and then being further filled with greatly diluted brine having an NaCl content of 0.01 mol/l or with deionized water and drained. The flushing solution can be drained off again immediately after complete filling of the anode space or remain for up to 200 minutes in the anode space and then be drained off. After draining, a small residual amount of flushing solution remains in the anode space. The anode space then remains piped or shut off without direct contact with the surrounding atmosphere. The brine is in accordance with the purity requirements usual for membrane electrolyses in chloralkali electrolysis. [0080] Flushing of the cathode space is carried out using an alkali metal hydroxide solution having a concentration of not more than 12 mol/l, preferably from 0.01 to 4 mol/l, which is fed to the cathode space for from 0.01 minutes to 60 minutes and is subsequently drained off again. Alkali metal hydroxide solution from normal production is preferably used for flushing the cathode space. Alkali from shutdown procedures is less suitable for flushing, mostly because of contamination with chloride ions. Flushing can likewise be carried out using deionized water. After the flushing operation, the cathode space is emptied. [0081] The oxygen supply can be, in particular, shut off with switching off of the voltage. The oxygen supply is preferably shut off after emptying and flushing of the cathode space. [0082] Reduction of the differential pressure between cathode chamber and anode chamber [0083] Lowering of the pressure at which the electrolysis element is operated to ambient pressure [0084] Closure of the electrolysis element in order to avoid entry of air.

[0085] After emptying/flushing of anode space and cathode space, the electrolysis cell with the moist membrane can be kept ready over a relatively long period of time in the built-in state for a quick start-up without the performance capability of the electrolysis cell being impaired. In the case of downtimes of a number of weeks, it is advisable 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 in order to effect stabilization. Flushing is preferably carried out at intervals of 1-12 weeks, particularly preferably at intervals of 4-8 weeks. The concentration of the dilute alkali metal chloride solution used for flushing or wetting is 1-4.8 mol/l. The flushing solution can be drained off again immediately after complete filling of the anode space or reside in the anode space for up to 200 minutes and then be drained off. The concentration of the alkali metal hydroxide solution used for flushing or wetting is from 0.1 to 10 mol/l, preferably from 1 to 4 mol/l. The temperature of the brine or the alkali metal hydroxide solution can be in the range from 10 to 80.degree. C., but is preferably from 15 to 40.degree. C. The flushing of the minigap cathode shells can be carried out for a period of from 0.1 to 10 minutes.

[0086] The invention also provides a process for start-up, in particular for restarting after the new process for shutdown.

[0087] It is a process for chloralkali electrolysis using a membrane electrolysis cell in a minigap arrangement between ion exchange membrane and gas diffusion electrode, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell has at least one anode space with anode for accommodating an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode, which comprises a silver-containing catalyst, and a sheet-like, porous element in the gap between ODE and membrane, which element has a thickness of, in particular, from 0.01 mm to 3 mm and through which catholyte flows during operation, characterized in that, for start-up of the electrolysis process, at least the following steps are carried out in this order: [0088] j) filling of the anode space with anolyte having a temperature of at least 50.degree. C. and passage of the anolyte through it, [0089] k) preheating of catholyte to a temperature of at least 50.degree. C., [0090] l) filling of the cathode space and the porous element with preheated catholyte having a concentration of from 7.5 to 10.5 mol/l and passage of the catholyte through them, [0091] m) setting of the electrolysis voltage to a value in the range from 0.1 to 1.4 V, [0092] n) setting and maintenance of the temperature of the catholyte and anolyte leaving the cell independently of one another to a temperature in the range from 70 to 100.degree. C., [0093] o) setting of the concentration of the catholyte in the feed to the cell so that an alkali metal hydroxide concentration in the range from 7.5 to 12 mol/l is obtained in the output, [0094] p) setting of the concentration of the anolyte in the feed to the cell so that an alkali metal chloride concentration in the range from 2.9 to 4.3 mol/l is obtained in the output, [0095] q) setting of the production current density to a value of at least 2 kA/m.sup.2, preferably at least 4 kA/m.sup.2.

[0096] The restarting of the electrolysis is, in particular, carried out as follows:

[0097] Anolyte is, as per step j), introduced into the anode space of the cell and, in particular, heated to at least 50.degree. C. in a circuit with heat exchanger,

[0098] Catholyte is, for step k), heated to a temperature of at least 50.degree. C. outside the cell, e.g. in a circuit with storage vessel and heat exchanger.

[0099] When the anode chamber has been filled and the anolyte has a temperature of at least 50.degree. C., the cathode gap (minigap) is filled as per step 1) by the preheated alkali metal hydroxide solution having a temperature of at least 50.degree. C. being introduced into the gap. This procedure is different from the prior art in which the cathode space is filled first and the anode space is then filled--the procedure according to the invention avoids excessively high chloride values in the alkali and thus any corrosion problems.

[0100] As soon as the cathode gap has been filled with alkali metal hydroxide solution, an electrolysis voltage of at least 0.4 V is preferably applied in step m), in particular within from 0.01 to 10 minutes, so that a current density of at least 0.2 A/m.sup.2 is established.

[0101] Anolyte and catholyte are subsequently heated to a temperature of at least 70.degree. C. as per step n) and the current density is then preferably increased.

[0102] The increase in the current density to the production current density in step q) is particularly preferably effected at a rate of from 0.018 kA/(m.sup.2*min) to 0.4 kA/(m.sup.2*min) until the current density at the electrolysis element is at least 2 kA/m.sup.2.

[0103] The determination of the concentrations is, unless indicated otherwise, carried out by titration or another method which is known in principle to a person skilled in the art.

[0104] The electrolysis cell which has been shut down according to the above new process is restarted according to the above-described new process. When the process steps described are adhered to, the electrolysis cell can go through many start-up and running-down cycles without the performance of the cell being impaired.

EXAMPLES

[0105] The gas diffusion electrode used in the examples was produced as described in EP 1728896 B1, 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 gauze made of nickel wires and pressed to give an oxygen-depolarized electrode.

[0106] The electrode was installed in an electrolysis unit having an area of 100 cm.sup.2 with a DuPONT type N982 ion exchange membrane (manufactured by Chemours) and a spacing between gas diffusion electrode and ion exchange membrane of 3 mm.

[0107] The electrolysis unit has, in the assembled state, an anode space having an anolyte inlet and outlet and an anode consisting of titanium expanded metal which was coated with a commercial DSA coating for chlorine production from Denora, consisting of a mixed oxide of ruthenium oxide/iridium oxide, and a cathode space having the gas diffusion electrode as cathode and having a gas space for the oxygen and oxygen inlets and outlets, a liquid outlet and an ion exchange membrane, which are arranged between anode space and cathode space. A lower pressure prevailed in the anode space than in the cathode space, so that the ion exchange membrane was pressed onto the anode structure with a pressure of about 30 mbar as a result of the higher pressure in the cathode chamber.

[0108] The electrolysis cell was operated at a brine concentration of about 210 g/l (3.58 mol/l) of NaCl and a sodium hydroxide concentration of about 31% by weight (10.4 mol/l) at electrolyte temperatures of about 85.degree. C. The cell voltage was corrected to 32% by weight (10.79 mol/l) of sodium hydroxide and 90.degree. C. by customary standard methods.

[0109] The electrolytes were each introduced into the cell from below and taken off again from the top of the cell.

[0110] Oxygen was fed to the gas space of the cathode. An oxygen having a purity of more than 99.5% by volume of oxygen was used here. The oxygen was humidified with water at room temperature before being introduced into the gas space of the cathode half shell. The amount of oxygen was regulated so that a 1.5-fold stoichiometric excess over the amount of oxygen required based on the current strength set was always introduced. The oxygen is fed from the top into the gas space and discharged at the bottom.

[0111] The electrolysis unit had a gap of about 3 mm between oxygen-depolarized electrode and ion exchange membrane. This gap was filled with a porous PTFE woven fabric as percolator and spacer.

[0112] The production current density was 6 kA/m.sup.2.

Example 1--Start-Up

[0113] Before start-up of the catholyte circuit, oxygen saturated with water was fed at room temperature into the cathode space so that the pressure in the cathode gas space was 59 mbar. The hydrostatic pressure of the sodium hydroxide solution at the lowest point in the cell was 32 mbar.

[0114] After this, an external catholyte circuit containing an about 31% strength by weight (10.4 mol/l) sodium hydroxide solution was started up and the sodium hydroxide solution was heated, but the sodium hydroxide solution was not yet conveyed through the cell.

[0115] In the next step, the anolyte circuit was, according to the invention, started up and the anode space was filled with an anolyte having a concentration of about 210 g of NaCl/l (3.58 mol/l). While the anode circuit was maintained and the anolyte was conveyed through the cell, the anolyte was heated to 50.degree. C. by means of a heat exchanger present in the anode circuit.

[0116] After the sodium hydroxide solution had attained a temperature of 50.degree. C., the sodium hydroxide solution having a temperature of 50.degree. C. was fed into the cell and, after filling of the cathode gap within 30 seconds, an electrolysis voltage of 1.08 V was applied. This resulted in a current density of 10 mA/cm.sup.2 being established.

[0117] The pH of the outflowing anolyte was 8.

[0118] The electrolyte was heated from 50.degree. C. to 70.degree. C. within 1 hour. After the temperature of the outflowing anolyte and catholyte of 70.degree. C. had been attained, the electrolysis voltage was increased, with the electrolysis voltage being increased such that the current density was raised every 2 minutes by 50 mA/cm.sup.2 up to a current density of 600 mA/cm.sup.2.

[0119] The concentrations were regulated after start-up so that the concentration of the outflowing brine was about 210 g/l (3.59 mol/l) and that of the sodium hydroxide solution was about 31.5% by weight (10.6 mol/l).

[0120] The cell was operated for at least 24 hours under these conditions.

Example 2--Shutdown--According to the Invention

[0121] The electrolysis unit was operated at a current density of 600 mA/cm.sup.2.

[0122] For shutdowns, the current density was reduced to 1.5 mA/cm.sup.2. For this purpose, the main rectifier was disconnected and the polarization rectifier was switched in. The polarization rectifier then takes over maintenance of a current density of 1.5 mA/cm.sup.2. The operation at the low current density was maintained for 1.5 hours. After this, the anolyte is chlorine-free. This process is carried out in industrial electrolyzers for safety reasons. One of the reasons is that chlorine or chlorine compounds such as hypochlorite do not diffuse from the anolyte through the ion exchange membrane into the catholyte and lead there to corrosion of cell components or the gas diffusion electrode. On the basis of experience, the phase of chlorine-free flushing takes about 1.5 hours in industrial electrolyzers.

[0123] Electrolyte circuits remained in operation with the same volume flows as in electrolysis operation at 600 mA/cm.sup.2. The O.sub.2 supply was likewise maintained.

[0124] During the phase of chlorine-free flushing, the temperature of the anolyte and of the catholyte was reduced from 85.degree. C. to 70.degree. C. The cell voltage during this phase was about 1.16 V and the pH of the outflowing anolyte from the cell was pH 8.2.

[0125] After 1.5 hours, the temperature of anolyte and catholyte is reduced to 50.degree. C., with the polarization rectifier being operated potentiostatically. Here, the voltage of 1.16 V is maintained and the current is appropriately decreased.

[0126] After cooling of anolyte and catholyte, the polarization rectifier is disconnected and the catholyte is immediately drained from the cathode space. This occurs over a time of about 30 seconds. After emptying of the cathode space, the anode space is drained within 1 hour.

[0127] The anode space is filled with deionized water from below up to a height of max. 50% of the cell height and immediately drained off again.

[0128] The cathode gap was likewise flushed by renewed switching-on of the catholyte pump and feeding of catholyte into the cathode space. For this purpose, the catholyte pump was switched on for about 10 seconds. The catholyte gap ran empty within 15 seconds.

[0129] The cell was then allowed to stand for 10 hours.

[0130] The start-up was then carried out as described in Example 1.

[0131] A total of 32 downtimes (shutdown processes) were carried out.

[0132] At the beginning of the experiment, the cell voltage at a current density of 600 mA/cm.sup.2 was 2.48 V.

[0133] After 32 downtimes, the cell voltage at a current density of 600 mA/cm.sup.2 was 2.48 V.

[0134] The cell voltage remained unchanged and damage to the gas diffusion electrode and further components did not occur.

Example 3--Shutdown--Comparative Example

[0135] An electrolysis unit was started up as in Example 1. Shutdown was carried out according to the prior art, as follows: [0136] reduction of the electrolysis current to 1.8 mA/cm.sup.2 [0137] electrolyte circuits remained in operation with the same volume flow as in electrolysis operation, likewise the O.sub.2 supply [0138] the temperature of the electrolytes was reduced to 75.degree. C. within 1.5 hours while a current density of 1.8 mA/cm.sup.2 is maintained. [0139] the voltage supply was switched off [0140] immediately after switching off of the voltage supply, the anode space was firstly emptied over a time of about 1 hour. [0141] after emptying of the anode space, the cathode space was emptied. [0142] the anode space was then filled from below with deionized water, with the anode space being filled only halfway and immediately drained again. [0143] the cathode gap was flushed further with catholyte. After draining off of the anolyte, the catholyte was also drained from the cathode gap. [0144] the cell was then allowed to stand for 10 hours. [0145] start-up was carried out as described in Example 1. [0146] 5 downtimes according to the above-described procedure for shutdown were carried out [0147] at the beginning of the experiment, the cell voltage at a current density of 400 mA/cm.sup.2 was 2.11 V [0148] after 5 downtimes, the cell voltage at a current density of 400 mA/cm.sup.2 was 2.14 V.

[0149] The cell voltage increased by 30 mV, and damage to the gas diffusion electrode occurred.

Claims

1.-8. (canceled)

9. A process for chloralkali electrolysis using an electrolysis cell in a gap arrangement, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell comprises at least one anode space with anode and an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode which comprises a silver-containing catalyst and an in particular from 0.01 mm to 3 mm thick sheet-like porous element through which catholyte flows between gas diffusion electrode and membrane, the electrolysis process comprises at least the following steps in this order: a) lowering of the electrolysis voltage and removal of chlorine from the anolyte so that less than 10 mg/l of active chlorine is present in the anolyte by maintaining an electrolysis voltage per element of from 0.1 to 1.4 V and a current density which is greater than zero, b) and setting of the pH of the anolyte to a value in the range from pH 2 to pH 12 during step a), c) residence under these conditions as long as electrolyte is present in the catholyte gap (or electrolyte flows through the latter), and optionally for emptying of the electrolysis cell the further steps: d) cooling of the anolyte to a temperature below 70.degree. C. with maintenance of the electrolysis voltage in the range from 0.1 to 1.4 V, e) switching off of the electrolysis voltage at an electrolyte temperature of <55.degree. C., f) emptying of the cathode gap, g) emptying of the anode space, h) preferably renewed filling of the anode space with one of the following liquids: dilute alkali metal chloride solution having a maximum concentration of 4 mol/l or deionized water, and subsequent emptying of the anode space, i) filling of the cathode space with one of the following liquids: dilute alkali metal hydroxide solution having a maximum concentration of 10 mol/l or deionized water, with subsequent emptying of the cathode space.

10. The process as claimed in claim 9, wherein the alkali metal chloride is sodium chloride or potassium chloride.

11. The process as claimed in claim 9, wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

12. The process as claimed in claim 9, wherein the gas diffusion electrode is supplied with oxygen gas on its side facing away from the catholyte.

13. The process as claimed in claim 9, wherein the oxygen gas flow to the gas diffusion electrode is maintained when the electrolysis is switched off.

14. A process for chloralkali electrolysis using a membrane electrolysis cell in a gap arrangement between ion exchange membrane and gas diffusion electrode, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell has at least one anode space with anode for accommodating an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode, which comprises a silver-containing catalyst, and a sheet-like, porous element in the gap between ODE and membrane, which element has a thickness of, in particular, from 0.01 mm to 3 mm and through which catholyte flows during operation, wherein, for start-up of the electrolysis process, at least the following steps are carried out in this order: j) filling of the anode space with anolyte having a temperature of at least 50.degree. C. and passage of the anolyte through it, k) preheating of catholyte to a temperature of at least 50.degree. C., l) filling of the cathode space and the porous element with preheated catholyte having a concentration of from 7.5 to 10.5 mol/l and passage of the catholyte through them, m) setting of the electrolysis voltage to a value in the range from 0.1 to 1.4 V, n) setting and maintenance of the temperature of the catholyte and anolyte leaving the cell independently of one another to a temperature in the range from 70 to 100.degree. C., o) setting of the concentration of the catholyte in the feed to the cell so that an alkali metal hydroxide concentration in the range from 7.5 to 12 mol/l is obtained in the output, p) setting of the concentration of the anolyte in the feed to the cell so that an alkali metal chloride concentration in the range from 2.9 to 4.3 mol/l is obtained in the output, q) setting of the production current density to a value of at least 2 kA/m.sup.2, preferably at least 4 kA/m.sup.2.

15. The process as claimed in claim 14, wherein the increase in the current density to the production current density in step q) is carried out at a rate of from 0.018 kA/(m.sup.2*min) to 0.4 kA/(m.sup.2*min) until the current density at the electrolysis element is at least 2 kA/m.sup.2.

16. The process as claimed in claim 14, wherein the start-up is a restarting of an electrolysis cell which has been shut down according to a process comprising at least the following steps in this order: a) lowering of the electrolysis voltage and removal of chlorine from the anolyte so that less than 10 mg/l of active chlorine is present in the anolyte by maintaining an electrolysis voltage per element of from 0.1 to 1.4 V and a current density which is greater than zero, b) and setting of the pH of the anolyte to a value in the range from pH 2 to pH 12 during step a), c) residence under these conditions as long as electrolyte is present in the catholyte gap (or electrolyte flows through the latter), and optionally for emptying of the electrolysis cell the further steps: d) cooling of the anolyte to a temperature below 70.degree. C. with maintenance of the electrolysis voltage in the range from 0.1 to 1.4 V, e) switching off of the electrolysis voltage at an electrolyte temperature of <55.degree. C., f) emptying of the cathode gap, g) emptying of the anode space, h) preferably renewed filling of the anode space with one of the following liquids: dilute alkali metal chloride solution having a maximum concentration of 4 mol/l or deionized water, and subsequent emptying of the anode space, i) filling of the cathode space with one of the following liquids: dilute alkali metal hydroxide solution having a maximum concentration of 10 mol/l or deionized water, with subsequent emptying of the cathode space.