DE102012204041A1 - Process for the electrolysis of alkali chlorides with oxygen-consuming electrodes having openings - Google Patents

Abstract

An oxygen-consuming electrode, in particular for use in chloralkali electrolysis, with a novel coating and its production, an electrolysis apparatus, as well as parameters for starting and starting up this electrolysis apparatus are described, in the maintenance of which damage to the cell is avoided.

Description

The invention relates to an oxygen-consuming electrode, in particular for use in the chloralkali electrolysis and its preparation, an electrolysis apparatus, and to a process for the electrolysis of aqueous solutions of alkali metal chlorides while maintaining certain operating parameters. The invention further relates to the use of this oxygen-consuming electrode in the chloralkali electrolysis or fuel cell technology.

The invention is based on electrolytic processes known per se for the electrolysis of aqueous alkali chloride solutions by means of oxygen-consuming electrodes, which are designed as gas diffusion electrodes and usually comprise an electrically conductive support and a gas diffusion layer with a catalytically active component.

Various proposals for operating the oxygen-consuming electrodes in electrolytic cells in technical size are known in principle from the prior art. The basic idea is to replace the hydrogen-evolving cathode of the electrolysis (for example, in the chloralkali electrolysis) by the oxygen-consuming electrode (cathode). An overview of the possible cell designs and solutions may be published by Moussallem et al "Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects", J. Appl. Electrochem. 38 (2008) 1177-1194 , are taken.

The oxygen-consuming electrode - also called SVE for short in the following - has to fulfill a number of requirements in order to be usable in technical electrolysers. Thus, the catalyst and all other materials used must be chemically stable to concentrated alkali hydroxide solutions and to pure oxygen at a temperature of typically 80-90 ° C. Similarly, a high degree of mechanical stability is required that the electrodes in electrolyzers of a size of usually more than 2 m ² surface (technical size) can be installed and operated. Further desired properties are: a high electrical conductivity, a small layer thickness, a high internal surface and a high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and a corresponding pore structure for the conduction of gas and electrolyte. The long-term stability and low production costs are further special requirements for a technically usable oxygen-consuming electrode.

A problem with the arrangement of an SVE in a cathode element, in which there is an electrolyte gap between membrane and SVE, results from the fact that on the side of the catholyte by the hydrostatic pressure a pressure gradient over the height of the electrode forms, on the gas side opposite to the amount of constant pressure. This can lead to flooding of the hydrophobic pores in the lower part of the electrode as well as the liquid getting onto the gas side. On the other hand, if the gas pressure in the upper part of the SVE is too high, fluid can be displaced from the hydrophilic pores and oxygen can reach the side of the catholyte. Both effects reduce the performance of the SVE. In practice, this means that the height of a SVE is limited to about 30 cm, unless further measures are taken. In technical electrolysers usually electrodes with a height of 1 m and higher are installed. For the realization of appropriate heights various technologies are described.

In WO 2001/57290 A1 describes a cell in which the liquid is guided from top to bottom via a flat porous element, a so-called percolator, placed between the SVE and ion exchange membrane in a type of free-falling liquid film, called a falling film for short, along the SVE (finite gap arrangement). With this arrangement, no liquid column is loaded on the liquid side of the SVE, and no hydrostatic pressure profile is built up over the height of the cell. However, the structure described in WO 2001/57290 A1 is very complicated. To ensure a uniform liquor flow and uniform loading of the SVE with catholyte, percolator, ion exchange membrane and SVE must be positioned very accurately.

In another embodiment, the ion exchange membrane, which separates the anode from the cathode space in the electrolysis cell, is placed directly on the SVE without a space into which the alkali hydroxide solution is introduced and removed (catholyte gap). This arrangement is also referred to as a "zero gap" arrangement. The zero gap arrangement is also commonly used in fuel cell technology. The disadvantage here is that the forming alkali metal hydroxide solution must be passed through the SVE to the gas side and then flows down to the SVE. This should not lead to clogging of the pores in the SVE by the alkali hydroxide solution or to crystallization of alkali hydroxide in the pores. It has been found that in this case a very high concentration of alkali metal hydroxide can arise, it being described that the ion exchange membrane is not long-term stable at these high concentrations ( Lipp et al, J. Appl. Electrochem. 35 (2005) 1015 - Los Alamos National Laboratory "Peroxide formation during chlorine-alkali electrolysis with carbon-based ODC" ).

Another arrangement, which is sometimes also referred to as "zero gap" or more precisely formulated as "micro gap" is in JP 3553775 and US 6117286 A1 described. In this arrangement is located between the ion exchange membrane and the SVE another layer of a porous hydrophilic material which receives the resulting alkali due to their suction power and from which at least a portion of the liquor can flow down. The ability to drain the caustic is determined by the installation of the SVE and the cell design. The advantage of this arrangement is that the amount of liquor flowing down the back of the SVE becomes smaller. As a result, the gas side facing side of the SVE (back) remains more accessible to the oxygen. Furthermore, the pore system of the SVE is loaded with less liquid, whereby more pore volume for gas transport is available. In contrast to finite gap arrangements described above, no aqueous alkali hydroxide solution (liquor) is passed through the gap between the SVE and the ion exchange membrane by charging and discharging; the porous material located in the microcolumn absorbs the resulting alkali and conducts it horizontally or vertically Direction further. The following is the in JP 3553775 and US 6117286 A1 described arrangement as "Micro Gap". A powerful electrolytic cell with a micro gap configuration is easier to build and operate than an electrolytic cell in finite gap configuration.

In US 6117286 A1 A further embodiment of the "Micro Gap" SVE arrangement is described in which the alkali hydroxide solution of porous hydrophilic material is guided through the slot in the SVE to the side of the SVE facing the gas side. The openings serve to bring the alkali metal hydroxide solution via the porous hydrophilic material to the gas side of the SVE and drop it into the gas space. The advantage of this arrangement is that the alkali hydroxide solution is discharged on the gas side of the SVE without forming a liquid film on the SVE, which would hinder the access of the oxygen to the SVE. The structure in the embodiments shown, however, is expensive.

In US 4332662 A1 there is described an SVE for a zero gap arrangement having a series of openings through which the resulting alkali hydroxide solution is directed to the gas side of the SVE. The free area of the openings is 2-80% of the electrode area, preferably 5-25%. The openings are intended to facilitate the flow of the alkali hydroxide solution through the SVE. The alkali hydroxide solution then drains off the gas side of the SVE, hindering the access of oxygen to the SVE. In a particular embodiment, the attachment of another highly porous separator material between membrane and SVE is described. As possible materials for the additional separator, a paper made of zirconium oxide fibers, but also paper made of alumina or other ceramic fabrics, PVC or polypropylene fabric and porous nickel. It can not be deduced from the description what function the additional separator material has. The mention of the hydrophobic materials PVC and polypropylene suggests that the separator material does not serve to dissipate the alkali hydroxide solution, as in the micro-gap arrangements accordingly JP 3553775 and US 6117286 A1 , Disadvantages of the embodiments described in US 4332662 A1 include the relatively high loss of electrode area and the mentioned obstruction of the oxygen supply to the SVE.

An oxygen-consuming electrode typically consists of a carrier element, for example a porous metal plate or a metal wire mesh, and an electrochemically catalytically active coating. The electrochemically active coating is microporous and consists of hydrophilic and hydrophobic components. The hydrophobic components make it difficult to pass through electrolyte and thus keep the corresponding pores in the SVE free for the transport of the oxygen to the catalytically active centers. The hydrophilic components allow the penetration of the electrolyte to the catalytically active centers and the removal of the hydroxide ions from the SVE. The hydrophobic component used is generally a fluorine-containing polymer such as polytetrafluoroethylene (PTFE), which also serves as a polymeric binder for particles of the catalyst. For silver-catalyzed electrodes, for example, silver serves as the hydrophilic component. As the electrochemical catalyst for the reduction of oxygen, a variety of compounds are described. Practical importance as a catalyst for the reduction of oxygen in alkaline solutions, however, have only platinum and silver obtained.

Platinum has a very high catalytic activity for the reduction of oxygen. Because of the high cost of platinum, this is used exclusively in supported form. Preferred carrier material is carbon. However, the durability on carbon-supported platinum-based electrodes in continuous operation is inadequate, presumably because platinum also catalyzes the oxidation of the support material. Carbon also favors the unwanted formation of H ₂ O ₂ , which also causes oxidation. Silver also has high electrocatalytic activity for the reduction of 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 also limited their long-term stability under the conditions in one of the oxygen-consuming electrode, especially when used for the chlor-alkali electrolysis.

In the preparation of SVE with unsupported silver catalyst, the silver is preferably introduced at least partially in the form of silver oxides, which are then reduced to metallic silver. The reduction usually takes place when the electrolysis cell is first put into operation. In the reduction of the silver compounds, there is also a change in the arrangement of the crystallites, in particular also to bridge formation between individual silver particles. Overall, this leads to a solidification of the structure.

It has been observed that oxidation of the silver catalyst can occur again when the electrolysis current is switched off. The oxidation is apparently favored by the oxygen and the moisture in the half cell. The oxidation 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 SVE.

It has also been found that the performance, in particular the required electrolysis voltage, of a SVE with silver catalyst is significantly dependent on the conditions at start-up. This applies both to the first commissioning of an SVE and to further commissioning after a standstill. It is one of the objects of the present invention to find specific conditions for the operation, and in particular the start-up and shutdown, of a SVE with silver catalyst which ensure a high performance of the SVE.

Another key 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 exposed to heavy loads: they must 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. The ion transport takes place via acidic sulfonate groups and / or carboxylate groups copolymerized into these polymers. Carboxylate groups show a higher selectivity, the carboxylate group-containing polymers have a lower water absorption and have a higher electrical resistance than sulfonate-group-containing polymers. In general, multilayer membranes are used with a thicker sulfonate group-containing layer on the anode side and a thinner, carboxylate-containing layer on the cathode side. The membranes are provided on the cathode side or on both sides with a hydrophilic layer. To improve the mechanical properties of the membranes are reinforced by the insertion of fabrics or nonwovens, the reinforcement is preferably incorporated in the sulfonate group-containing layer.

Due to the complex structure of the ion exchange membranes are sensitive to changes in the surrounding media. By different molar concentrations strong osmotic pressure gradient can be built up between the anode and cathode side. When the electrolyte concentrations decrease, the membrane swells due to increased water absorption. As the concentration of electrolyte increases, the membrane releases and shrinks water, in extreme cases dehydration may precipitate solids in the membrane.

Changes in concentration can cause disruption and damage to the membrane. It can lead to a delamination of the layer structure (blistering), whereby the mass transport through the membrane deteriorates.

Furthermore, holes (pinholes) and in extreme cases, cracks can occur, which can cause mixing of anolyte and catholyte.

In production plants, electrolysis cells are desirably operated for periods of several years without being opened in the meantime. However, due to fluctuating quantities and disruptions in the electrolysis upstream or downstream production areas, electrolysis cells in production plants must inevitably be repeatedly shut down and restarted.

When the electrolysis cells are switched off and restarted, conditions occur which can damage the cell elements and considerably reduce their service life. In particular, oxidative damage in the cathode space, damage to the SVE and damage to the membrane were found.

Fewer modes of operation are known from the prior art with which the risk of damage to the electrolysis cells during commissioning and decommissioning can be reduced.

A measure known from conventional membrane electrolysis is the maintenance of a polarization voltage, that is to say that the potential difference does not occur when the electrolysis is terminated zero down, but maintained at the level of the polarization voltage upright. Practically, a voltage slightly higher than that required for the polarization is set, so that a constant low current density is applied and to a small extent thereby an electrolysis takes place. However, this measure alone is not sufficient when using SVE in order to prevent oxidative damage of SVE during commissioning or decommissioning.

In the published patent application JP 2004-300510 A an electrolytic process using a micro gap arrangement is described, in which the shutdown of the cell by flooding the gas space with sodium hydroxide to prevent corrosion in the cathode compartment. The flooding of the gas space with caustic soda protects the cathode space from corrosion, but provides insufficient protection against damage to the electrode and the membrane during commissioning and commissioning or during standstill.

US 4578159 A1 describes for an electrolysis process using a "zero gap" arrangement that by flushing the cathode compartment with 35% sodium hydroxide solution prior to commissioning of the cell, or by commissioning the cell with low current density and gradually increasing the current density damage to membrane and electrode are avoided. This procedure reduces the risk of damage to the diaphragm and SVE during commissioning, but offers no protection against damage during decommissioning and shutdown.

From the Scriptures US Pat. No. 4,364,806 A1 It is known that by replacing the oxygen with nitrogen after lowering the electrolysis current, the corrosion in the cathode space is to be reduced. To WO 2008009661 A2 the addition of a small amount of hydrogen to the nitrogen should result in an improvement in protection against corrosive damage. However, the methods mentioned are complicated particularly in terms of safety aspects and require the installation of additional facilities for nitrogen and hydrogen supply. When restarted, the pores of the SVE are partially filled with nitrogen and / or hydrogen, which hinders the supply of oxygen to the reactive centers. Also, the method provides no protection against damage to the ion exchange membrane and makes high safety requirements to avoid explosive gas mixtures.

In the Final Technical Report "Advanced Chlor-Alkali Technology" by Jerzy Chlistunoff (Los Alamos National Laboratory, DOE Award 03EE-2F / Ed190403, 2004 ) lists conditions for temporarily turning off and on zero gap cells. When switching off the oxygen supply is interrupted and replaced by nitrogen after interrupting the electrolysis. The moistening of the gas stream is increased to wash out the remaining soda algae. On the anode side, the brine is replaced by hot water (90 ° C). The process is repeated until a stable voltage (open circuit voltage) is reached. The cells are then cooled, after which the supply of wet nitrogen and the pumping of the water are adjusted on the anode side.

For restarting, the anode side is first filled with brine, on the cathode side, water and nitrogen are added. The cell is then heated to 80 ° C. Then the gas supply is switched to oxygen and a polarization voltage with low current flow is applied. Subsequently, the current density is increased and the pressure in the cathode increased, the temperature rises to 90 ° C. Brine and water feed are subsequently adjusted to achieve the desired concentrations on the anode and cathode sides.

Reducing the NaOH concentration by increasing the supply of water to the cathode side while maintaining the NaCl concentration on the anode side increases the diffusion of chloride ions into the cathode compartment, with consequent negative effects on corrosion in the cathode compartment and deactivation of the SVE. When restarting, pure water is placed on the cathode side and brine concentrated on the anode side, so that an even higher chloride load on the cathode side can also be expected here.

Substitution of the anolyte by 90 ° C hot water during the shutdown process is expected to cause swelling and expansion of the membrane, increasing the risk of cracking and other damage. The procedure described is very complex, insbesondere for technical electrolysis process is a very high cost necessary.

It should be noted that the techniques described so far for commissioning and decommissioning an SVE are unfavorable and offer only insufficient protection against damage. In particular, the described methods do not provide sufficient protection in a SVE with micro gap arrangement.

The object of the present invention is to find improved electrolysis devices for the chloralkali electrolysis using a Mico Gap SVE and silver catalyst as the electrocatalytic substance which damage during commissioning and decommissioning and the interim shutdown is avoided. The operating parameters for the operation of such a device, in particular for the startup and shutdown should be easy to perform, in their compliance damage to membrane, electrode and / or other components of the electrolysis cell should be avoided.

The invention relates to an oxygen-consuming electrode for use in electrolysis cells with micro gap arrangement, in particular with a distance of 0.01 mm to 2 mm between the ion exchange membrane and oxygen-consuming electrode, comprising at least one carrier element in the form of a sheet and a coating with a gas diffusion layer and a catalytic active component, characterized in that the coating with the gas diffusion layer passages having a diameter or a height of 0.5 mm-20 mm, preferably 1 mm-1 mm, preferably with a cross-sectional area of 0.05% up to 15% of the total SVE Has surface.

Another object of the invention is a method for operating a chlor-alkali electrolysis with an electrolytic cell containing such oxygen-consuming electrode (SVE) in a micro gap configuration, in particular with a distance of 0.01 mm to 2 mm between the ion exchange membrane and oxygen-consuming electrode, the cell at least an anode compartment with anolyte, an ion exchange membrane, a cathode compartment with an oxygen-consuming electrode as the cathode, which contains a silver-containing catalyst and a 0.01 mm to 2 mm thick sheet-like porous element between SVE and membrane, characterized in that before application of the electrolysis voltage between Anode and cathode in a first step, the oxygen-consuming electrode on the gas side with an aqueous alkali hydroxide solution containing chloride ions of at most 1000 ppm, preferably at most 700 ppm, more preferably at most 500 ppm wetted and after subsequent insertion tion of the anolyte in the anode compartment and an oxygen-containing gas in the cathode compartment, the electrolysis voltage is applied.

• – Chlorfreispülen des Anolyten und Abkühlung des Anolyten
• – Abschalten der Elektrolysespannung
• – Entleeren des Anodenraums
• – Bevorzugt ein erneutes Befüllen des Anodenraums mit einer der nachstehenden Flüssigkeiten: verdünnte Alkalichlorid-Lösung mit maximal 4 mol/l oder deionisiertes Wasser, mit anschließendem Entleeren des Anodenraums
• – Befüllung des Kathodenraums mit einer der nachstehenden Flüssigkeiten: verdünnte Alkalihydroxid-Lösung mit maximal 4 mol/l oder deionisiertes Wasser, mit anschließendem Entleeren des Kathodenraums

The invention also provides a method for operating a chlor-alkali electrolysis with an electrolytic cell containing such an oxygen-consuming electrode in a micro gap configuration, wherein the cell has at least one anode compartment with anolyte, an ion exchange membrane, a cathode compartment with the SVE with silver-containing catalyst and a 0.01 mm to 2 mm thick porous element between SVE and membrane, characterized in that at the end of the electrolysis process for decommissioning at least the following steps are carried out in this sequence:

• - Chlorine flushing of the anolyte and cooling of the anolyte
• - Switch off the electrolysis voltage
• - emptying the anode compartment
• Preference is given to refilling the anode compartment with one of the following liquids: dilute alkali metal chloride solution with a maximum of 4 mol / l or deionized water, with subsequent emptying of the anode compartment
• - Filling the cathode compartment with one of the following liquids: dilute alkali hydroxide solution with a maximum of 4 mol / l or deionized water, with subsequent emptying of the cathode compartment

These two variants of the electrolysis process are combined in a preferred embodiment with each other, so that both the conditions described for the commissioning of the electrolysis and for decommissioning are met. This also includes the preferred variants described below.

In the cathode prevail by the oxygen strongly oxidative conditions, which are no longer compensated by the electrolysis current when switching off. After switching off the electrolysis, chloride ions also increasingly diffuse through the membrane into the cathode space. Chloride ions promote corrosion processes, furthermore, insoluble silver chloride can form on oxidation of the silver catalyst. Damage to the electrode and also to the entire cathode space occurs.

When the electrolysis voltage ceases, the mass transport through the membrane due to the flow of current also comes to a standstill. The membrane depleted of water, it can shrink and solid precipitation and, as a result, to cracks and hole formation, the passage of anions through the membrane is facilitated. When restarting again, too low a water content prevents mass transport through the membrane. Which leads to an increase in pressure and delamination at the boundary layers.

Inhomogeneities of the water and / or ion distribution in the membrane and / or the SVE lead when restarting to local peaks in the current and mass transport resulting in damage to the membrane or the SVE.

The precipitation of alkali metal chloride salts on the anode side is also problematic. The strong osmotic gradient between anolyte and catholyte results in water transport from the anode to the cathode compartment. As long as the electrolysis is in operation, the water transport is from the anode compartment a loss of chloride and alkali ion anode, so that the concentration of alkali chloride at conventional electrolysis conditions in the anode compartment decreases. When the electrolysis is stopped, the transport of water due to the osmotic pressure remains from the anode into the cathode compartment. The concentration in the anolyte rises above the saturation limit. It comes to the precipitation of alkali chloride salts, especially at the interface to the membrane, which lead to damage to the membrane.

It has surprisingly been found that such a new SVE with its low passages compared to the entire electrode surface by a sequence of simple measures during commissioning and decommissioning their performance over a long time. The SVEs according to the invention are particularly suitable for technical electrolysis devices with electrodes of 1 m height and higher, but are not limited to such. The SVE according to the invention and the method for operating this SVE are particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.

The passages in the SVE can be configured in any desired form. Preferred are round, oval, rectangular or trapezoidal cross-sections. Possible embodiments are in the 1 to 4 shown. Particularly preferred are slots that can be arranged horizontally, vertically or diagonally. 1 shows an SVE with horizontally arranged slots 1 , which are mounted in three different heights. Similarly, vertical slots could also be arranged. 2 shows diagonally arranged slots 2 , 3 shows passages in hole form, with the holes 3 in the 3 are oversized in relation to the SVE size. 4 represents a variant with holes evenly distributed over the SVE surface 4 Here again, the size of the holes is disproportionate to the SVE area. Likewise, the holes can be placed increasingly in the upper part of the SVE (not shown).

The diameter or height of the passages (in the case of a rectangular cross-section the smaller side, in the case of oval the smaller diameter, to be used analogously for other geometry) is 0.5 mm-20 mm, preferably 1 mm-10 mm.

Through the passages, the surface of the catalytically active layer is slightly reduced. However, it turns out that the losses are small and are acceptable in view of the benefits of long-term stability in a variety of take-off and launch operations. This is especially true when the total area of the apertures is 0.05% -15% of the electrode area. Therefore, an oxygen-consuming electrode is also preferred, which is characterized in that with respect to the side of the electrode facing the ion-exchange membrane, the total area of the passages is 0.05 to 15%, preferably 1 to 12% of the electrode surface.

The passages can be evenly distributed over the electrode surface. Arrangements are preferred in which the majority of the passages are in the upper third of the electrode. The majority here means more than 60% of the sum of all passage openings. In a particularly preferred embodiment, each slot is at a distance from the top or bottom of 0.5 cm to 10 cm. It is also conceivable that the slots are arranged vertically or diagonally.

The passages may be horizontal in the direction of passage or be provided with a slope (diagonally arranged, see 2 ).

Through the passage openings and the caustic soda can reach from the gap between the membrane and SVE on the gas side facing the SVE. Constructively, devices can be installed here in the cathode half-shell, which receive the passing caustic soda and lead away from the gas side facing the SVE, as in 5 shown in principle. Especially with slots, for example, below the slots 5 baffles 6 be incorporated on the structure of the cathode half-shell, which absorb the alkali hydroxide solution and drain, for example, so that they no longer come into contact with the SVE.

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

The passages may preferably be formed continuously through the entire oxygen-consuming electrode, that is to say both through the catalytically active layer and the carrier element. The passages may be designed so that they relate only to the catalytically active layer and the carrier element remains in the passages. Since the carrier element is inherently permeable and remains in the region of the passage, the mechanical stability of the SVE is not impaired. Leaving the carrier element in the passages may be advantageous in some embodiments, for example in certain SVE manufacturing processes.

The preparation of the oxygen electrodes with passages according to the invention can be carried out by various techniques.

Thus, in SVE produced by the dry or wet process, the passages can be created by punching, cutting, drilling, milling or other perforation techniques. In this case, passages are generally created through the entire electrode, including the carrier element.

Also can be created by measures during the manufacture of the electrodes requirements so that the electrode has the desired passages. This can be done, for example, by inserting strips, cylindrical bodies or similarly dimensioned displacement bodies corresponding to the dimensions of the passages before application and compaction of the catalyst mass. Preferably, in this embodiment, the carrier element remains in its original form. The displacer body may also be positioned so as to be oppositely disposed on both sides of the support member. The displacement bodies are designed so that they can be removed during the manufacturing process. This can be done by removal, but also by melting or other physical or chemical methods.

The operating parameters for commissioning and decommissioning of an electrolytic cell are described below for an electrolytic cell with an SVE according to the invention with silver catalyst and micro gap configuration and a common alkali metal chloride concentration (anolyte) of 2.5 to 4.0 mol / l and a Alkalihydroxid- Concentration (catholyte) of 8 to 14 mol / L described in more detail, without wishing to limit the execution of the described procedure. According to the invention, further embodiments may be used for the commissioning or decommissioning of such an electrolysis cell, in which prior to startup the cathode compartment is wetted with a sodium hydroxide solution having low contamination with chloride ions, and in which, when decommissioning after switching off the electrolysis voltage in a first step drained of the anolyte and preferably the anode compartment is rinsed and drained in a subsequent step, the catholyte and the cathode compartment is rinsed

The commissioning of an electrolysis unit with micro gap arrangement, an SVE with silver catalyst and an according to the prior art watered ion exchange membrane is carried out in such a way that in a first step, the cathode space is wetted with alkali. The wetting takes place, for example, by filling the cathode space with alkali hydroxide solution and immediately emptying it again. The concentration of the alkali metal hydroxide solution to be used is between 0.01 and 13.9 mol / l, preferably 0.1 to 4 mol of alkali metal hydroxide per liter. The alkali lye must be as free as possible of chloride and chlorine ions.

Preference is given to a method which is characterized in that the alkali hydroxide solution introduced before the application of the electrolysis voltage in the catholyte inlet has a content of chloride ions of at most 1000 ppm, preferably at most 700 ppm, particularly preferably at most 500 ppm.

Preference is also given to a process which is characterized in that the alkali hydroxide solution introduced before the application of the electrolysis voltage in the catholyte inlet has a content of chlorate ions of at most 20 ppm, preferably at most 10 ppm.

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

The residence time of the alkali metal hydroxide solution in the cathode chamber is between immediate emptying, that is, after complete filling, the alkali metal hydroxide solution is discharged from the cathode compartment immediately and 200min.

After draining the alkali hydroxide solution from the cathode compartment, oxygen is added. Preferably, the draining of the sodium hydroxide solution and the addition of oxygen in such a way that the oxygen displaces the charged sodium hydroxide solution. The overpressure in the cathode is set according to the configuration in the cell, usually in the amount of 10 to 100 mbar.

The concentrations are determined by titration or another method known to the person skilled in the art.

Alkali hydroxide solution from regular production is preferably used for wetting the cathode space. Leaching from shutdowns is not suitable for wetting before commissioning the cell, mainly because of the contamination with chloride ions.

After draining the alkali hydroxide solution from the cathode compartment, the oxygen is added. Preferably, the draining of the sodium hydroxide solution and the addition of oxygen in such a way that the oxygen displaces the charged sodium hydroxide solution. The overpressure in the cathode is adjusted according to the configuration in the cell, usually in the amount of 10-100 mbar.

After draining the alkali metal hydroxide solution from the cathode compartment, the anode compartment is filled with alkali metal chloride solution (brine). The brine corresponds to the purity requirements customary for membrane electrolysis. After filling the Anode space, the brine is performed in accordance with the usual equipment conditions by pumping in a circle through the anode compartment. During pumping, the anolyte can be heated up. The temperature of the brine fed in is selected such that a temperature of 30-95 ° C. is established in the discharge from the anode compartment. The concentration of the anolyte supplied to alkali chloride is between 150 and 330 g / L

After filling the anode compartment and starting up the anode circuit, the electrolysis voltage is applied in the next step. This is preferably carried out immediately after filling the anode and reaching a temperature of the sols leaving the anode space of more than 60. It is advantageous if, after filling the anode space, at least the polarization voltage or the electrolysis voltage is switched on. The polarization voltage or electrolysis voltage is adjusted so that a current density of 0.01 A / m ² to 40 A / m ² , preferably 10 to 25 A / m ² sets. The time at this current density should be at most 30 minutes, preferably 20 minutes.

Overall, the entire period for commissioning is to be kept as short as possible. The time after filling the anode space with brine and achieving an electrolysis capacity of> 1 kA / m ² should be less than 240 minutes, preferably less than 150 minutes. Preferably, the electrolysis stream is increased at a rate of 3 to 400 A / m ² per minute. The electrolytic cell is then operated with the design parameters, for example with a concentration of 2.5 to 4.0 moles of alkali metal chloride per liter in the anode compartment, a current density of 3-6 kA / m ² and a 50% to 100% excess of oxygen in the gas supply. The oxygen which is introduced into the cathode compartment is preferably saturated with water vapor at room temperature (ambient temperature). This can be done, for example, by passing the oxygen through a water bath before it is introduced into the cathode compartment. It is also conceivable that the humidification takes place at a higher temperature. The concentration of the effluent from the cathode compartment sodium hydroxide solution is essentially by the choice of ion exchange membrane and the alkali metal chloride concentration in the anode compartment between 8 and 14 mol / L a. The alkali hydroxide solution advantageously flows out of the cathode compartment automatically.

The method described is suitable both for the first commissioning of electrolysis units after installation of a previously not operated silver-containing, in particular containing silver oxide SVE invention, as well as for the commissioning of electrolysis cells containing an inventive SVE after decommissioning.

• – Erniedrigung der Elektrolysespanung und Entfernung von Chlor aus dem Anolyten, so dass weniger als 10ppm aktives Chlor im Anolyten vorhanden ist
• – Erniedrigung der Temperatur des Anolyten auf weniger als 60°C (20–60°C) und Entleeren des Anodenraums
• – Bevorzugt ein erneutes Befüllen des Anodenraums mit einer der nachstehenden Flüssigkeiten: verdünnte Alkalichlorid-Lösung mit maximal 4 mol/l oder deionisiertes Wasser
• – Entleeren des Anodenraums, bevorzugt nach 0,01 bis 200 min
• – Befüllung des Kathodenraums mit einer der nachstehenden Flüssigkeiten: verdünnte Alkalihydroxid-Lösung mit 0,01 bis 4 mol/l oder deionisiertes Wasser
• – Entleeren des Kathodenraums, bevorzugt nach 0,01 bis 200 min

When decommissioning the electrolysis cell, the following steps are carried out in this sequence:

• Lowering the electrolysis voltage and removing chlorine from the anolyte so that less than 10 ppm of active chlorine is present in the anolyte
• - Lowering the temperature of the anolyte to less than 60 ° C (20-60 ° C) and emptying the anode compartment
• Preference is given to refilling the anode compartment with one of the following liquids: dilute alkali metal chloride solution with a maximum of 4 mol / l or deionized water
• - Emptying of the anode compartment, preferably after 0.01 to 200 min
• - Filling the cathode compartment with one of the following liquids: dilute alkali hydroxide solution with 0.01 to 4 mol / l or deionized water
• - Emptying of the cathode space, preferably after 0.01 to 200 min

In a first step, the electrolysis voltage is regulated down. Here, the voltage can be adjusted down to zero. Preferably, after the electrolysis current has been shut down, a voltage is maintained and this is switched off after reduction of the chlorine content in the anode compartment to <10 mg / l, preferably less than 1 mg / l. Under chlorine content here is the total content of dissolved chlorine in the oxidation state 0 and understood higher. The removal of the remaining chlorine from the anode chamber is preferably carried out in such a way that chlorine-free anolyte is supplied with simultaneous removal of chlorine-containing anolyte, or by pumping the anode circuit with simultaneous removal and removal of chlorine gas. The voltage is adjusted during this process so that a current density of 0.01 to 40 A / m ^2, preferably 10 to 25 A / m ² sets. After switching off the electrolysis and polarization voltage of the anolyte is drained.

After switching off the electrolysis, the anolyte is cooled to a temperature below 60 ° C and then drained

Subsequently, the anode compartment is rinsed. The rinsing is carried out with strongly diluted brine with an alkali metal chloride content of 0.01 to 4 mol / l, with water or, preferably, with deionized water. The rinsing is preferably carried out by filling the anode chamber once and immediately draining the rinsing liquid. The rinse can also be done in two or more stages, for example, first with a diluted brine with a Alkali chloride content of 1.5-2 mol / l filled and drained and then further filled with highly diluted brine with a NaCl content of 0.01 mol / l or with deionized water and drained. The rinse solution can be released again immediately after the anode cavity has been completely filled, or it can remain in the anode compartment for up to 200 minutes and then be drained off. After draining, a small residual amount of rinse solution remains in the anode compartment. The anode compartment then remains cased or shut off without direct contact with the surrounding atmosphere.

After emptying the anode compartment still existing catholyte is drained from the cathode compartment, then the gas space of the cathode is rinsed. The rinsing is carried out with strongly diluted alkali metal hydroxide solution with an alkali metal hydroxide content of up to 4 mol / l, with water or, preferably, with deionized water. The rinsing is preferably carried out by a single filling of the gas space and immediate draining of the rinsing liquid. The rinsing may also be carried out in two or more stages, for example filling and draining first with a dilute caustic having an alkali metal hydroxide content of 1.05-3 mol / l and then further with highly dilute caustic having an alkali hydroxide content of 0 , 01 mol / l or filled with deionized water and drained. The rinse solution can be drained off again immediately after the cathode compartment has been completely filled or can remain in the cathode compartment for up to 200 minutes and then be drained off. After draining, a small residual amount of rinse solution remains in the cathode compartment. The cathode compartment remains cased or shut off without direct contact with the surrounding atmosphere.

The oxygen supply can be turned off by switching off the polarization voltage. Preferably, the oxygen supply is switched off before emptying and rinsing of the cathode space, the opening for the oxygen supply is used during filling for venting or degassing of the cathode space.

After emptying the anode and cathode compartments, the electrolytic cell with the moist membrane can be kept ready for a short-term start-up in the installed state for a relatively long period of time without the efficiency of the electrolysis cell being impaired. At standstill over several weeks, it is appropriate for stabilization at regular intervals to rinse the anode compartment with dilute aqueous alkali chloride solution and the cathode compartment with dilute aqueous alkali hydroxide solution or wet. Preference is given to rinsing at intervals of 1-12 weeks, more preferably at intervals of 4-8 weeks. The concentration of dilute alkali chloride solution used for rinsing or wetting is 1-4.8 mol / l. The rinse 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. The concentration of the alkali hydroxide solution used for rinsing or wetting amounts to 0.1 to 10 mol / l. preferably between 1 and 4 mol / l. The temperature of the brine or the alkali metal hydroxide solution may be between 10 and 80 ° C, but preferably 15-40 ° C. The rinse solution can be drained off again immediately after the cathode compartment has been completely filled or can remain in the cathode compartment for up to 200 minutes and then be drained off.

Another embodiment of the method consists of flushing the electrode spaces, including the cathode and anode spaces of the electrolysis cell, with moistened gas. For this purpose, for example, saturated nitrogen is introduced into the anode space with water. Alternatively, oxygen can also be introduced.

The gas volume flow will be measured so that a 2 to 10-fold volume change can take place. At a gas volume of 100 liters, the gas volume flow may be 1 l / h to 200 l / h at a temperature of 5 to 40 ° C, preferably the temperature of the gas is ambient, i. 15-25 ° C amount. The saturation of the purge gas with water takes place at the temperature of the gas.

The electrolysis cell taken out of operation by the above method is put back into operation by the method described above. By adhering to the process steps described, the electrolysis cell can undergo a large number of startup and shutdown cycles without impairing the performance of the cell.

The invention will be explained in more detail by way of example with reference to the figures. Show it:

1 a schematic view of an SVE with horizontal slots 1

2 a schematic plan view of an SVE with diagonally arranged slots 2

3 a schematic plan view of an SVE with horizontally arranged rows of holes 3

4 a schematic plan view of an SVE with evenly distributed on the electrode surface holes 4

5 a schematic cross section through an SVE 8th with horizontal slots 5 and baffles 6 and ion exchange membrane 7

Examples

example 1

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 net of nickel wires and pressed to an oxygen-consuming electrode (SVE). The electrode was installed in an electrolysis unit having an area of 100 cm ² with an ion exchange membrane of DuPONT type N2030 and a carbon fabric PW3MFBP from Zoltek with a thickness of 0.3 mm between SVE and membrane. The electrolysis unit has in assembly an anode compartment with Anolytzu- and effluent, with an anode of coated titanium (coating ruthenium) a cathode compartment with the SVE as the cathode and a gas space for the oxygen and oxygen sour and derivatives, a liquid drain and a carbon fabric and a Ion exchange membrane, which are arranged between the anode and cathode compartment. SVE, carbon cloth and ion exchange membrane were pressed onto the anode by a higher pressure in the cathode chamber at a pressure of about 30 mbar.

In the first step, the cathode compartment was filled with a 30 wt .-%, 80 ° C hot caustic soda solution containing 20 ppm of chloride ions and a content of <10 ppm chlorination and then emptied again. When emptying oxygen was supplied, so that the resulting gas space was filled with oxygen. After emptying, an overpressure of 30 mbar was set on the cathode side.

In the next step, the anode compartment was filled with 70 ° C brine with a concentration of 210 g NaCl / l and put the anode circuit in operation. Immediately after reaching a constant run of the anode circuit, the electrolysis voltage was applied. The electrolysis current was adjusted so that after 5 minutes an electrolysis current of 1 kA / m ² , and after 30 minutes an electrolysis current of 3 kA / m ^{2 was} reached. The plant was operated for 3 days with a capacity of 3 kA / m ² and an electrolysis voltage of 1.90-1.95 V, a concentration of the discharged caustic soda solution of 32 wt .-% and a temperature in the electrolysis cell of 88 ° C. ,

Example 2

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 net of nickel wires and pressed to an oxygen-consuming electrode. The effective SVE area was 10 x 10 cm. A horizontal continuous slit of 2 mm height 1 cm above the lower edge or 1 cm below the upper edge was cut into the electrode by means of a cutter knife. 4% of the SVE area was removed. The SVE was installed in an electrolysis unit with an ion exchange membrane of DuPONT N2030 and a carbon fabric from Zoltek, PW03 with a thickness of 0.3 mm between SVE and membrane.

In the first step, the cathode compartment was filled with a 30% strength, 80 ° C. sodium hydroxide solution containing 20 ppm of chloride ions and a content of <10 ppm chlorination, and then emptied again. When emptying, oxygen was introduced, so that the resulting gas space was filled with oxygen. After emptying, an overpressure of 30 mbar was set on the cathode side.

In the next step, the anode compartment was filled with 70 ° C brine with a concentration of 210 g NaCl / l and the anode circuit was put into operation. Immediately after reaching a constant run of the anode circuit, the electrolysis voltage was applied. The electrolysis current was adjusted so that after 5 minutes an electrolysis current of 1 kA / m ² , and after 30 minutes an electrolysis current of 4 kA / m ^{2 was} reached. The system was operated for 10 days with a capacity of 4 kA / m ² and an electrolysis voltage of 2.0-2.2 V, a concentration of the discharged caustic soda solution of 32 wt .-% and a temperature in the electrolytic cell of 90 ° C. , The mean value of the electrolysis voltage was 2.14 V.

By attaching the slots in the SVE, there was no significant degradation in electrolysis performance.

Example 3

The electrolysis unit according to Example 1 was taken out of service after a running time of 3 days as follows:
The electrolysis current was reduced to 90 A / m ² . The anolyte was circulated for 60 minutes until a chlorine content of <1 mg / L was reached, then the electrolysis current was turned off. The anolyte cooled to 70 ° C during this time. The anode compartment was drained, with deionized water until overflow and immediately emptied again.

Subsequently, remaining liquid was drained in the cathode compartment, turned off the oxygen supply and the cathode compartment filled with deionized water to overflow and immediately emptied again.

The electrolysis unit from Example 2 was put back into operation 50 hours after decommissioning as follows:
In the first step, the cathode compartment was filled with a 32 wt .-%, 80 ° C hot sodium hydroxide solution containing 20 ppm of chloride ions and a content of <10 ppm chlorination and then emptied again. When emptying oxygen was supplied, so that the resulting gas space was filled with oxygen. After emptying, an overpressure of 30 mbar was set on the cathode side.

In the next step, the anode compartment was filled with 70 ° C brine with a concentration of 210 g NaCl / l and the anode circuit was put into operation. Immediately after reaching a constant run of the anode circuit, the electrolysis voltage was applied. The electrolysis current was adjusted so that after 5 minutes an electrolysis current of 1 kA / m ² , and after 30 minutes an electrolysis current of 3 kA / m ^{2 was} achieved at a concentration of sodium hydroxide solution of 32 wt .-% and a temperature in the Electrolysis cell of 88 ° C.

The electrolysis voltage at 3 kA / m ² was 1.8 to 1.9 V. The electrolysis unit showed no deterioration with respect to the time before standstill, even an improvement of 100 mV was observed.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2001/57290 A1 [0006]
• JP 3553775 [0008, 0008, 0010]
• US 6117286 A1 [0008, 0008, 0009, 0010]
• US 4332662 A1 [0010]
• JP 2004-300510 A [0024]
• US 4578159 A1 [0025]
• US 4364806 A1 [0026]
• WO 2008009661 A2 [0026]

Cited non-patent literature

• Moussallem et al "Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects", J. Appl. Electrochem. 38 (2008) 1177-1194 [0003]
• Lipp et al, J. Appl. Electrochem. 35 (2005) 1015 - Los Alamos National Laboratory "Peroxide formation during chlorine-alkali electrolysis with carbon-based ODC" [0007]
• "Advanced Chlor-Alkali Technology" by Jerzy Chlistunoff (Los Alamos National Laboratory, DOE Award 03EE-2F / Ed190403, 2004 [0027]

Claims (12)

Oxygen-consuming electrode for use in electrolysis cells with micro gap configuration), comprising at least one carrier element in the form of a sheet and a coating with a gas diffusion layer and a catalytically active component, characterized in that the coating with the gas diffusion layer one or more passages with a diameter or a Height of 0.5 mm-20 mm, preferably 1 mm-10 mm. An oxygen-consuming electrode according to claim 1, characterized in that, with respect to the side of the electrode facing the ion exchange membrane, the total area of the passages is 0.05 to 15%, preferably 1 to 12% of the electrode area. Oxygen electrode according to one of claims 1 to 2, characterized in that only the coating of the carrier element with the gas diffusion layer and the catalytically active component has passages. Oxygen electrode according to one of claims 1 to 2, characterized in that the passages are formed continuously through the coating and carrier element. Oxygen-consuming electrode according to one of claims 1 to 4, characterized in that the passages are formed by punching, cutting, milling, drilling from an oxygen-consuming electrode. Oxygen-consuming electrode according to one of claims 1 to 3, characterized in that corresponding openings in the manufacturing process of the electrode during the coating of the support element are kept free by inserting removable displacement bodies, which are subsequently removed after production of the coating. Oxygen-consuming electrode according to one of Claims 1 to 3, characterized in that the oxygen-consuming electrode has a spacing of 0.01 mm to 2 mm between the ion-exchange membrane and the oxygen-consuming electrode. A method for operating a chlor-alkali electrolysis with an electrolytic cell containing an oxygen-consuming electrode according to any one of claims 1-7, in a micro gap configuration, the cell having at least one anode compartment with anode and an alkali chloride-containing anolyte, an ion exchange membrane, a cathode compartment with an oxygen-consuming electrode characterized in that prior to application of the electrolysis voltage in a first step, the oxygen-consuming electrode on the gas side with a. As a cathode containing a silver-containing catalyst and a 0.01 mm to 2 mm thick flat, catholyte-flowed porous element between SVE aqueous alkali hydroxide solution having a content of chloride ions of at most 1000 ppm, preferably at most 700 ppm, more preferably at most 500 ppm wetted and after subsequent introduction of the anolyte into the anode compartment and an oxygen-containing gas in the cathode compartment the electrolysis voltage is applied. A method for operating a chlor-alkali electrolysis with an electrolytic cell containing an oxygen-consuming electrode according to any one of claims 1-7, in a micro gap configuration, said cell having at least one anode compartment with anolyte, an ion exchange membrane, a cathode compartment with an SVE with silver-containing catalyst and a 0.01 mm to 2 mm thick sheet-like porous element between SVE and membrane, characterized in that at least the following steps in the sequence are carried out at the end of the electrolysis process for decommissioning: - Switch off the electrolysis voltage - emptying the anode compartment Preference is given to refilling the anode compartment with one of the following liquids: dilute alkali metal chloride solution with a maximum of 4 mol / l or deionized water, with subsequent emptying of the anode compartment - Filling the cathode compartment with one of the following liquids: dilute alkali hydroxide solution with a maximum of 4 mol / l or deionized water, with subsequent emptying of the cathode compartment Method according to one of claims 7, characterized in that the alkali hydroxide solution used before applying the electrolysis voltage for wetting the cathode space has a content of chlorate ions of at most 20 ppm, preferably at most 10 ppm, more preferably at most 4 ppm. A method according to claim 8 and 9, characterized in that after switching off and emptying of the electrolytic cell and then optionally repeated at intervals of 1 to 12 weeks, preferably 4 to 8 weeks, the anode compartment with a dilute alkali chloride solution with a content of 1.7 to 3 , 4 mol / l, preferably 2.2 to 2.9 mol / l and the cathode space with a dilute alkali hydroxide solution with a content of 2 to 9 mol / l, preferably 4 to 6 mol / l is rinsed. Method according to one of claims 7-11, characterized in that it is in the Alkali chloride is sodium chloride or potassium chloride, preferably sodium chloride.

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