KR20130105505A - Process for electrolysis of alkali metal chlorides with oxygen-consuming electrodes having orifices - Google Patents

Abstract

PURPOSE: A method of electrolyzing a chloride alkali metal is provided to use a silver catalyst as an electro catalyst, and to electrolyze a chloride alkali metal by using oxygen depolarized cathodes (OCE) that has a micro gap arrangement. CONSTITUTION: A method of electrolyzing a chloride alkali metal includes the coating of at least one carrier element with a sheetlike structure shape, a catalyst activating component, and a gas diffusion layer. The coating of the gas diffusion layer uses an oxygen consumption electrode that has at least one opening in diameter and height of 0.5 mm to 20 mm, and preferably 1 mm to 10 mm. The ratio of the entire area of an opening to the area of electrode facing an ion exchanging film is 0.05 to 15%, preferably 1 to 12% of the electrode area. The only coatings of a catalyst activating component and at least one carrier element of the gas diffusion layer have openings.

Description

PROCESS FOR ELECTROLYSIS OF ALKALI METAL CHLORIDES WITH OXYGEN® CONSUMING ELECTRODES HAVING ORIFICES}

The present invention relates in particular to an oxygen consuming electrode for use in chlor-alkali electrolysis, to its preparation, to an electrolysis device and to a method for the electrolysis of aqueous alkali metal chloride solution according to certain operating parameters. The present invention further relates to the use of this oxygen consuming electrode in chloralkali electrolysis or fuel electric technology.

The present invention is an electrolysis known per se as electrolysis of an aqueous alkali metal chloride solution using an oxygen consuming electrode in the form of a gas diffusion electrode and generally comprising a gas diffusion layer comprising an electrically conductive carrier and a catalytically active component. It started from the method.

Various attempts at the operation of oxygen consuming electrodes in industrial scale electrolysis cells are known theoretically from the prior art. The basic idea is to replace the hydrogen releasing cathode with an oxygen consuming electrode (cathode) in electrolysis (eg chloralkaline electrolysis). For an overview of possible cell designs and solutions, see Mussallem et al., “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.

Oxygen consuming electrodes (hereinafter also referred to as OCE for short) must meet a set of requirements in order to be available in industrial electrolysers. For example, the catalyst and all other materials used must generally be chemically stable against pure oxygen and concentrated alkali metal hydroxide solutions at temperatures of 80-90 ° C. Similarly, high mechanical stability is required for the electrodes to be installed and operated in electrolyzers of size generally larger than 2 m 2 (industrial scale). Another preferred feature is: high electrical conductivity, thin layer thickness, large internal surface area and high electrochemical activity of the electrode catalyst. Appropriate hydrophobic and hydrophilic pores and suitable pore structures are required for conduction of gas and electrolyte. Long term stability and low production costs are other specific requirements for industrially available oxygen consuming electrodes.

For OCE devices in cathode elements where there is an electrolyte gap between the membrane and the OCE, the hydrostatic pressure on the catholyte side forms a pressure gradient along the height of the electrode, whereas the hydrostatic pressure on the gas side is constant with height. The problem arises. As a result, the hydrophobic hole is also filled at the bottom of the electrode, and liquid can flow to the gas side. On the other hand, if there is an excessively high gas pressure above the OCE, the liquid may be replaced from the hydrophilic pore and oxygen may go to the catholyte side. Both results reduce the performance of the OCE. In practice, the result is that the height of the structure of the OCE is limited to about 30 cm unless further action is taken. In industrial electrolysers, electrodes are generally made at a height of 1 m or more. Various techniques have been described for the implementation of such structure heights.

WO2001 / 57290 A1 is a kind of free-falling liquid film (simply called a drop film) from top to bottom through a flat porous element called a percolator mounted between OCE and an ion exchange membrane, which conducts liquid along the OCE. Cells are described (finite gap arrangement). In this arrangement, the liquid column is not on the liquid side of the OCE, and no hydrostatic pressure profile is established depending on the structural height of the cell. However, the structure described in WO2001 / 57290A1 is very complicated. To ensure uniform alkali flow and uniform contact between the OCE and the catholyte, the percolator and the ion exchange membrane and the OCE must be positioned very accurately.

In another embodiment, an ion exchange membrane in which an anode space is divided from a cathode space in an electrolysis cell is added directly to the OCE without the space (catholyte gap) between which an alkali metal hydroxide solution is introduced into and removed from it. Attached. This arrangement is also called a zero gap arrangement. This zero gap arrangement is also commonly used in fuel cell technology. What is undesirable here is that the alkali metal hydroxide solution formed must pass the OCE to the gas side and then flow down in the OCE. In this process, the pores in the OCE must not be blocked by alkali metal hydroxide solutions and there should be no crystallization of alkali metal hydroxides in the pores. It has been found here that very high concentrations of alkali metal hydroxide may occur, but it has been described that ion exchange membranes at such high concentrations lack long-term stability. (Lipp et al., J. Appl. Electrochem. 35 (2005) 1015-Los Alamos National Laboratory "Peroxide formation during chlor-alkali electrolysis with carbon-based ODC")

Another arrangement, sometimes referred to as "zero gap", but more precisely made of "micro gap", is described in JP3553775 and US6117286A1. In this arrangement, another layer of porous hydrophilic material is present between the ion exchange membrane and the OCE that absorbs the resulting aqueous alkali due to its absorbency and from which at least some alkali can be discharged downward. The method of discharging aqueous alkali is determined by the cell design and the installation of the OCE. The advantage of this arrangement is that less alkali flows down from the back of the OCE. As a result, the face of the OCE facing the gas side (back side) becomes more accessible to oxygen. In addition, the hole system of the OCE is exposed to less amount of liquid so that more hole space can be used for gas transport. Unlike the finite gap arrangement described above, aqueous alkali metal hydroxide solution (alkali) is not conducted through the gap between the OCE and the ion exchange membrane as a result of application and discharge; The porous material present in the micro gaps absorbs the resulting aqueous alkali and allows it to pass through in the horizontal or vertical direction. The arrangement described in JP3553775 and US6117286A1 is hereinafter referred to as "micro gap". High performance electrolysis cells with a microgap structure are easier to build and operate than electrolysis cells with finite gap arrangements.

US6117286A1 describes another embodiment of a "microgap" OCE arrangement in which an aqueous alkaline solution is conducted from the porous hydrophilic material to the side facing the gas side of the OCE through a slot of the OCE. The orifice acts to allow the alkali metal hydroxide solution to pass through the porous hydrophilic material to the gas side of the OCE and to fall down in its gas space. An advantage of this arrangement is that the alkali metal hydroxide solution can be withdrawn from the gas side of the OCE without forming any liquid film on the OCE that prevents oxygen from accessing the OCE. However, the structure shown in this embodiment is complex.

US4332662A1 describes OCE in a zero gap arrangement, which has a series of orifices and the resulting alkali metal hydroxide solution passes through to the gas side of the OCE. The open area of the orifice is 2 to 80%, preferably 5 to 25% of the electrode area. The orifice is intended to promote the flow of the alkali metal hydroxide solution through the OCE. The alkali metal hydroxide solution then flows down from the gas side of the OCE, but this prevents oxygen from accessing the OCE. In certain embodiments, another high porosity separator material arrangement between the membrane and the OCE is described. Possible materials referred to as additional separators include paper made of zirconium oxide fibers, as well as paper made of aluminum oxide or other ceramic fabrics, PVC fabrics or polypropylene fabrics and porous nickel. This description does not mention the function of additional separator materials. The mention of hydrophobic PVC and polypropylene materials indicates that the membrane material does not serve to withdraw the alkali metal hydroxide solution, as in the microgap arrangement according to JP3553775 and US6117286A1. Disadvantages of the embodiment described in US4332662A1 include the relatively high loss of electrode area and the disturbance of oxygen supply to the OCE described above.

Oxygen consuming electrodes generally comprise a support element such as a metal wire mesh or a porous metal plate and an electrochemical catalytically active coating. This electrochemically active coating is microporous and contains hydrophilic and hydrophobic components. The hydrophobic component makes the electrolyte difficult to pass through and therefore keeps the corresponding pores in the OCE unobstructed for transport of oxygen to the catalytically active site. The hydrophilic component allows the electrolyte to pass through the catalytically active site, allowing hydroxide ions to be transported away from the OCE. The hydrophobic component used is typically a fluorinated polymer, such as polytetrafluoroethylene (PTFE), which additionally serves as a polymeric binder for the particles of the catalyst. For example, in the case of electrodes with silver catalysts, silver serves as a hydrophilic component. Many compounds have been described as electrochemical catalysts in the reduction of oxygen. Only platinum and silver, however, had practical significance as a catalyst for the reduction of oxygen in alkaline solutions.

Platinum has a very high catalytic activity in the reduction of oxygen. Because of the high price of platinum, it is only used in supported form. Carbon is preferred as the support material. However, long-term stability of the carbon-supported platinum-based electrodes is inadequate, perhaps because platinum can also catalyze the oxidation of the support material. In addition, carbon promotes the production of unwanted H ₂ O ₂ , which likewise causes oxidation. Silver similarly has high electrocatalytic activity against the reduction of oxygen. Silver can be used as a carbon support and can also be used as fine metal silver. Even if the carbon supported silver catalyst is more durable than the corresponding platinum catalyst, its long-term stability is also limited under conditions of being in one of the oxygen consuming electrodes, especially when used for chloralkaline electrolysis.

For the preparation of OCE comprising an unsupported silver catalyst, silver is preferably at least partly introduced in the form of silver oxide and subsequently reduced to metallic silver. This reduction is usually carried out when the electrolysis cell first starts up. Reduction of the silver compound also leads to a change in the arrangement of the crystals and also in particular to the formation of bridges between the individual silver particles. This causes overall hardening of the structure.

It has been observed that the silver catalyst can be oxidized again if the electrolysis current is interrupted. This oxidation is apparently promoted by oxygen and moisture in the half cell. Oxidation can lead to rearrangement of the catalyst structure, which adversely affects the activity of the catalyst and adversely affects the performance of the OCE.

In OCE with silver catalysts, it has also been found that the performance, in particular the required electrolysis voltage, depends significantly on the starting conditions. This applies to both the first start of the OCE and another start after the stop. It is one of the objectives of the present invention to find out the operation of the OCE with silver catalyst and in particular the specific conditions of stopping and starting, thereby ensuring the high performance of the OCE.

Another key element of electrolysis cells is ion exchange membranes. The membrane is permeable to cations and water and substantially impermeable to anions. In electrolysis cells the ion exchange membranes are in a state of severe stress: the ion exchange membranes must be stable against chlorine on the anode side and alkaline severe stress on the cathode side at temperatures around 90 ° C. Perfluorinated polymers, such as PTFE, generally withstand this stress. Ions are transported through sulfonate groups and / or carboxylate groups polymerized with such polymers. Carboxylate groups show higher selectivity; Polymers containing carboxylate groups have less water absorption and higher electrical resistance than polymers containing sulfonate groups. Usually, a multilayer film is used having a thicker layer comprising sulfonate groups on the anode side and a thinner layer comprising carboxylate groups on the cathode side. Such membranes are provided with a hydrophilic layer on the cathode side or both sides. Reinforce the membrane with inlaying of weaving or knitting to improve the mechanical properties; This reinforcement is preferably combined into a layer comprising sulfonate groups.

Because of the complex structure, ion exchange membranes are sensitive to changes in the media surrounding them. Different molarities can create a significant osmotic pressure gradient between the anode side and the cathode side. As the electrolyte concentration decreases, the water absorption increases and the membrane expands. As the electrolyte concentration increases, the membrane releases water and consequently shrinks; In extreme cases, the water can escape and cause a settling of solids in the membrane. So changing the concentration can interfere with the membrane and cause damage. As a result, delamination of the layer structure may occur (blister formation), which results in worse mass transfer through the membrane.

In addition, very small pinholes and, in extreme cases, cracking may occur, which may result in mixing of the catholyte and the anolyte.

In production plants it is desirable to operate the electrolysis cell over several years without opening it for that period. However, due to changes in demand volume and faults in the upstream and downstream production zones of electrolysis, it is inevitable that the electrolysis cells in the plant must be repeatedly stopped and backed up.

Stopping and restarting an electrolysis cell results in conditions that can damage the cell element and significantly reduce its life. Furthermore, oxidative damage was found in the cathode space, in particular with damage to the OCE and damage to the membrane.

The prior art rarely discloses an operating mode that can reduce the risk of damage to the electrolysis cell in the course of starting and stopping.

A measure of the persistence of the polarization voltage from known membrane electrolysis is known, which means that the potential difference is not adjusted to zero at the end of the electrolysis but is maintained at the degree of polarization voltage. In practical terms, the voltage is set somewhat higher than required for polarization so that the current density is consistently low and the resulting electrolysis is low. However, in the case of the use of OCE, this measure alone is insufficient to prevent oxidative damage of OCE during start-up and shutdown.

Published specification JP 2004-300510 A describes an electrolysis method using a microgap batch, wherein corrosion in the cathode space can be prevented by filling the gas space with sodium hydroxide solution upon shutdown of the cell. Thus, filling the gas space with sodium hydroxide solution protects the cathode space from corrosion, but does not adequately protect against damage to the electrodes and membranes during stop and start-up or during stop times.

US 4578159A1 discloses that in electrolysis using a zero-gap device, purging the cathode space with 35% sodium hydroxide solution or starting the cell at a low current density and gradually increasing the current density prior to the cell start-up. Explain that damage to the electrodes can be prevented. This process reduces the risk of damage to the membrane and the OCE at start-up, but does not provide any protection from damage at stop and during downtime.

The document US4364806A1 discloses that exchanging oxygen with nitrogen after down-regulating the electrolysis current will prevent corrosion in the cathode space. According to WO2008009661A2 the addition of a small proportion of hydrogen to nitrogen will improve protection from erosion damage. However, the method mentioned is particularly complicated from the safety point of view and involves the installation of additional equipment for the supply of nitrogen and hydrogen. Upon restart, the pores of the OCE are partially filled with nitrogen and / or hydrogen, which prevents oxygen from being supplied to the reactive site. The method also orders high safety requirements to avoid explosive gas mixtures without providing any protection from damage to the ion exchange membrane.

Jerzy Chlistunoff's The Final Technical Report "Advanced Chlor-Alkali Technology" (Los Alamos National Laboratory, DOE Award 03EE-2F / Ed190403, 2004) describes the conditions for temporary shutdown and start-up of zero-gap cells. It will be described in detail. In the case of a stop, the oxygen supply stops and the oxygen supply is stopped and replaced with nitrogen. Increase the wetting of the gas stream to flush out the remaining sodium hydroxide solution. On the anode side the brine is replaced with hot water (90 ° C.). This process is repeated until a stable voltage (open circuit voltage) is obtained. The cell is then cooled and the pumped circulation of water and supply of wet nitrogen at the anode side is stopped.

On restart, the anode side initially fills with brine; The cathode side introduces water and nitrogen. The cell is then heated to 80 ° C. The gas supply is then converted to oxygen and a polarization voltage of low current flow is applied. Thereafter, the current density is increased and the pressure of the cathode is increased; The temperature rises to 90 ° C. The brine and water feed is subsequently adjusted to achieve the desired concentration on the anode and cathode sides.

When the NaOH concentration decreases as a result of increased water supply to the cathode side and maintenance of NaCl concentration on the anode side, an increase in chloride ion diffusion into the cathode space has a corresponding adverse effect on corrosion in the cathode space and inactivation of the OCE. Happens with Upon restarting, pure water is initially charged on the cathode side and concentrated brine on the anode side, where much higher chloride loads are also expected to occur on the cathode side.

When the anolyte is replaced by water at 90 ° C. during shutdown, expansion and expansion of the membrane is expected, which increases the risk of cracking and other damage. The process described is very complex; Industrial electrolysis methods in particular require even higher levels of complexity.

The techniques described up to now regarding the starting and stopping of the OCE are undesirable and should only be seen as inadequate protection from damage. The described method furthermore does not provide sufficient protection, especially in the case of OCE in a micro gap arrangement.

One object of the present invention is to find an improved electrolysis unit for chlor-alkali electrolysis using OCE with a microgap batch and using a silver catalyst as the electrocatalyst material, wherein damage during start, stop and stop time processes between Avoid. The operating parameters of the operation of such units, in particular the start and stop, should be easy to carry out, and following this will prevent damaging the membrane, the electrodes and / or other components of the electrolysis cell.

The present invention is an oxygen consuming electrode for use in an electrolysis cell of a microgap batch, and particularly, the distance between the ion exchange membrane and the oxygen consuming electrode is 0.01 mm to 2 mm, and the coating and sheet similarity of the catalytically active component and the gas diffusion layer ( sheetlike) having at least one carrier element in the form of a structure, wherein the coating to the gas diffusion layer has a diameter or height of 0.5 mm to 20 mm, preferably 1 mm to 1 mm, and preferably 0.05% to 15% of the total OCE area. And having an opening of up to% cross-sectional area.

The present invention also proposes a method for electrolyzing chloral alkali with an electrolysis cell comprising the oxygen consuming electrode (OCE) of microgap structure, in particular the distance between the ion exchange membrane and the oxygen consuming electrode is 0.01 mm to 2 mm, the cell comprises a silver containing catalyst and a flat porous element having a thickness between 0.01 mm and 2 mm between the OCE and the membrane, the cathode space having an oxygen consuming electrode as cathode, an ion exchange membrane, an anolyte Alkali metal hydroxides having at least one anode space and having a chloride ion content of at most 1000 ppm, preferably at most 700 ppm and more preferably at most 500 ppm in a first step prior to the application of the electrolysis voltage between the anode and the cathode. Wet the oxygen-consuming electrode on the gas side with an aqueous solution, and subsequently introduce and contain the anolyte solution into the anode space. It characterized by applying an electrolysis voltage after the introduction of the cathode space of the base.

The present invention also provides a method of performing chlor-alkali electrolysis to an electrolysis cell including the oxygen consuming electrode having a microgap structure, the cell comprising a silver-containing catalyst, OCE having a thickness of 0.01 mm to 2 mm and a membrane Having a cathode space with an OCE with a flat porous element in between, an ion exchange membrane, at least one anode space comprising an anolyte, and at the end of the electrolysis operation, carrying out at least the following steps in this order for stopping Features:

-Clean the anolyte solution free of chlorine and cool the anolyte solution

-Blocks electrolysis voltage

-Empty the anode space

Refill the anode space preferably with one of the following liquids: up to 4 mol / l diluted alkali metal chloride solution or deionized water, followed by emptying the anode space.

Filling the cathode space with one of the following liquids: up to 4 mol / l diluted alkali metal hydroxide solution or deionized water, followed by emptying the cathode space.

In a preferred embodiment these two variants of the electrolysis method are combined with each other to follow both conditions described for starting and stopping the electrolysis. This also includes the preferred variant described below.

Oxygen conditions at the cathode create strong oxidative conditions that are no longer compensated for by electrolysis currents at rest. In addition, after the electrolysis current is cut off, the extent to which chloride ions spread through the membrane into the cathode space is increased. Chloride ions promote the corrosion process; In addition, oxidation of the silver catalyst can produce insoluble silver chloride. As a result, there is damage to the electrodes and even to the entire cathode space.

When the electrolysis voltage drops, the material transfer through the membrane due to the current flow also stops. The membrane is deficient in water; There may be shrinkage and precipitation of solids, followed by cracks and formation of tiny holes; The passage of negative ions through the membrane is facilitated. In turn, excessively low water content at restart impedes mass transfer through the membrane. The result is an increase in pressure and delamination at the boundary.

Upon restarting, non-uniformity of water and / or ion distribution in the membrane and / or OCE results in partial spikes in current and mass transfer, which in turn causes damage to the membrane or OCE.

The problem is also manifested by the precipitation of alkali metal chloride salts on the anode side. The significant osmotic pressure gradient between the anolyte and the catholyte causes water transport from the anode space to the cathode space. As long as electrolysis is operating, water transport out of the anode space is countered by the loss of chloride and alkali metal ions of the anode, and the concentration of alkali metal chloride in the anode space falls below standard electrolysis conditions. When electrolysis is interrupted, water transport from the anode space to the cathode space caused by osmotic pressure remains. The concentration in the anolyte rises above the saturation limit. As a result, precipitation of alkali metal chloride salts occurs, particularly at the boundary of the membrane, which causes damage to the membrane.

Surprisingly, it has been found that these new OCEs with small openings relative to the total electrode area maintain their performance for a long time through a series of simple means in the course of starting and stopping. The OCE of the present invention is particularly suitable for, but not limited to, industrial electrolysis units having electrodes of 1 m or higher height. The OCEs of the invention and the method of operating such OCEs are particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.

The openings in the OCE can be configured in any form. Preference is given to round, oval, rectangular or trapezoidal cross sections. Possible embodiments are shown in FIGS. 1 to 4. Particular preference is given to slots which can be arranged horizontally, vertically or diagonally. 1 shows an OCE with horizontal slots 1 at three different heights. Vertical slots can also be arranged in a similar manner. 2 shows the slots 2 arranged diagonally. FIG. 3 shows a hole-shaped opening in which the hole 3 in FIG. 3 is larger than the OCE size. 4 is a variant in which the holes 4 are evenly spread over the OCE region; Here too, the size of the hole is larger than that of the OCE region. Equally more holes can be arranged in the upper part of the OCE (not shown).

The diameter or height of the opening (shorter side for rectangular cross-sectional area, shorter diameter for elliptical cross-sectional area; similarly applies for other geometries) is 0.5 mm to 20 mm, preferably 1 mm to 10 mm.

The opening somewhat reduces the area of the catalytically active layer. However, the losses are small and considered acceptable given the advantages of long term stability during many start and stop operations. This can be achieved especially when the total area of the openings is from 0.05% to 15% of the electrode area. Thus, an oxygen consuming electrode is also preferred, characterized in that the total area of the opening with respect to the surface facing the ion exchange membrane in the electrode is 0.05 to 15%, preferably 1 to 12% of the electrode area.

The opening may be evenly spread over the electrode region. Preferred arrangements are where the majority of the openings are in the upper third of the electrode. The majority here means more than 60% of the total of the total passage orifices. In a particularly preferred embodiment one slot in each case is located at a distance of 0.5 cm to 10 cm from the upper and lower edges. It is thus possible for the slots to be arranged in the vertical or horizontal direction.

The opening may be horizontal in the direction of passage, or may be provided with an inclination (arranged diagonally, see FIG. 2).

The sodium hydroxide solution may pass through the passage orifice from the gap between the membrane and the OCE to face the gas side of the OCE. In terms of construction, it is possible here to absorb the sodium hydroxide solution passing through the device installed in the cathode half-shell as shown in principle in FIG. 5 and away from the facing side of the OCE. In particular in the case of slots, for example, a guide plate 6 under the slot 5 is installed in the structure of the cathode half-shell, which absorbs the alkali metal hydroxide solution and causes it to flow for example further It is possible to avoid contact with the OCE.

Preferably, the total area of the opening with respect to the surface facing the ion exchange membrane of the electrode is 0.05-15%, preferably 1% -12%, of the OCE area.

The opening can preferably be configured to be continuous through the entire oxygen consuming electrode, ie through both the catalytically active layer and the carrier element. The opening can be configured so that it only affects the catalytically active layer and the carrier element remains in the opening. The mechanical stability of the OCE is intact because the carrier element is inherently permeable and it remains in the region of the opening. In some embodiments, it may be advantageous to leave the carrier element in the opening, for example in the case of certain OCE manufacturing methods.

Oxygen electrodes of the present invention with openings can be produced by various techniques.

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

It is also possible to set conditions so that the electrode has the desired opening through measures during electrode manufacture. This can be achieved, for example, by engraving alternative shapes of similar dimensions that correspond to the dimensions of the strip, cylinder or opening before adding and compressing the catalyst composition. In this embodiment the carrier element preferably remains in its original form. Alternative shapes may also be positioned to align in opposite directions in both directions of the carrier element. The alternative shape is configured to be removed again in the course of the manufacturing method. This can be accomplished by dissolving them as well as by melting or other physical or chemical methods.

In an electrolysis cell having a silver catalyst and OCE of the present invention with a microgap structure and a standard alkali metal chloride concentration (anode solution) of 2.5 to 4.0 mol / l, alkali metal hydroxide concentration (catholyte solution) of 8 to 14 mol / l In the following, operating parameters relating to the start and stop of the electrolysis cell will be described, and it is hoped that this is not a limitation on the performance of the described process. According to the present invention, in starting and stopping such an electrolysis cell, the cathode space is wetted with sodium hydroxide solution with low chloride ion contamination prior to start-up, and the anolyte solution is the first step after the electrolysis voltage is cut off during the stop. It is also possible to use another embodiment which releases and preferably washes the anode space, then releases the catholyte and washes the cathode space in a subsequent step.

According to the prior art, the starting of an electrolysis unit of OCE with a microgap batch and an ion exchange membrane moistened with alkaline water and a silver catalyst is carried out in the first step by wetting the cathode space with aqueous alkali. For example, wetting is accomplished by filling the cathode space with an alkali metal hydroxide solution and then emptying it immediately. The concentration of the aqueous alkali used is from 0.01 to 13.9 mol / l, preferably from 0.1 to 4 mol per liter of alkali metal hydroxide. The aqueous alkali should be very substantially free of chloride and chlorate ions.

The method is characterized in that the alkali metal hydroxide solution introduced into the catholyte feed prior to the application of the electrolysis voltage has chloride ions in a content of 1000 ppm or less, preferably 700 ppm or less, more preferably 500 ppm or less. Do.

Also preferred is a method wherein the alkali metal hydroxide solution introduced into the catholyte feed prior to the application of the electrolysis voltage has a chlorate ion in a content of up to 20 ppm, preferably up to 10 ppm.

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

The residence time of the alkali metal hydroxide solution in the cathode space is between 200 minutes and immediately emptied, which means that the alkali metal hydroxide solution is immediately discharged again from the cathode space after being completely filled.

The alkali metal hydroxide solution is released from the cathode space and then oxygen is added. It is desirable to release the sodium hydroxide solution and add oxygen in a manner that replaces the sodium hydroxide solution into which oxygen has been introduced. The positive pressure at the cathode is usually sized from 10 to 100 mbar, depending on the structure in the cell.

Concentration is determined by titration or other method known to those skilled in the art.

Preference is given to using alkali metal hydroxide solutions of regular production for the wetting of the cathode space. Alkali from the stationary operation is inadequate for wet before start-up of the cell, especially due to chloride ion contamination.

Oxygen is added after the alkali metal hydroxide solution is released from the cathode space. It is preferred to release the sodium hydroxide solution and add oxygen in such a way that oxygen replaces the introduced sodium hydroxide solution. The positive pressure at the cathode is usually sized from 10 to 100 mbar, depending on the structure in the cell.

After the alkali metal hydroxide solution is released from the cathode space, the anode space is filled with an alkali metal chloride solution (brine). This brine meets the usual purity requirements in membrane electrolysers. After filling the anode space, the brine is conducted through the anode space by the pump circulation in accordance with normal equipment conditions. The anolyte can be heated in the pump circulation process. The temperature of the brine to be fed is chosen to be formed at a temperature of 30 to 95 ° C. in the output from the anode space. The alkali metal chloride concentration in the supplied anolyte is 150 to 330 g / l.

Fill the anode space and start the anode cycle, then apply the cell decomposition voltage in the next step. This is preferably done immediately after the anode has been filled and the temperature of the brine coming out of the anode space has reached a temperature above 60 ° C. It is advantageous to turn on at least the polarization voltage or the electrolysis voltage after filling the anode space. The polarization voltage or electrolysis voltage is set so that the current density is formed between 0.01 A / m 2 and 40 A / m 2, preferably 10 to 25 A / m 2. The time should not exceed 30 minutes, preferably 20 minutes, at this current density.

Overall, the total startup time should be kept to a minimum. The anode space should be filled with brine and the time after achieving electrolysis power exceeding 1 kA / m 2 should be less than 240 minutes, preferably less than 150 minutes. The electrolysis current is preferably increased at a rate of 3 to 400 A / m 2 every minute. The electrolysis cell is then operated with design parameters, for example 2.5 to 4.0 mol of alkali metal chloride per liter in the anode space, a current density of 3 to 6 kA / m 2 and 50% to 100% excess oxygen in the gas supply. The oxygen introduced into the cathode zone is preferably saturated with water vapor at room temperature (room temperature). This can be done, for example, by passing oxygen through a water bath prior to introduction into the cathode zone. Wetting may also be considered at higher temperatures. The concentration of sodium hydroxide solution flowing out of the cathode zone is essentially formed between 8 and 14 mol / l through the selection of alkali metal chloride concentrations in the ion exchange membrane and anode space. Advantageously, the alkali metal hydroxide solution spontaneously flows out of the cathode space.

The method described in both starting the electrolysis unit for the first time after installation of the silver-containing, especially silver oxide-containing OCE of the present invention which has never been operated before and starting the electrolysis cell containing the OCE of the present invention after stopping, Suitable.

In stopping the electrolysis cell, the following steps are performed in this order:

-Lowering the electrolysis voltage to remove less than 10 ppm of active chlorine from the anolyte and removing chlorine from the anolyte

-Lower the temperature of the anolyte to less than 60 ° C (20 to 60 ° C) and empty the anode space;

Preferably filling the anode space with one of the following liquids: up to 4 mol / l diluted alkali metal chloride solution or deionized water

The anode space is preferably emptied after 0.01 to 200 minutes

Filling the cathode space with one of the following liquids: 0.01-4 mol / l of diluted alkali metal hydroxide solution or deionized water

The cathode space is preferably emptied after 0.01 to 200 minutes.

In the first stage, the electrolysis voltage is down regulated. In this situation the voltage can be regulated down to zero. It is desirable that the voltage is maintained after the electrolysis current is exhausted and only cut off after the chlorine content in the anode space has been reduced to less than 10 mg / l, preferably less than 1 mg / l. Here chlorine content is understood to mean the total content of chlorine dissolved in an oxidation state of zero or more. The remaining chlorine is preferably removed from the anode space in such a way that anolyte free of chlorine is supplied with simultaneous removal of chlorine-containing anolyte or by pump circulation of the anode circuit with simultaneous separation and removal of chlorine gas. The voltage during this operation is adjusted so that the current density is formed between 0.01 and 40 A / m 2, preferably between 10 and 25 A / m 2. The anode liquid is discharged after the electrolysis voltage and the polarization voltage are cut off.

After blocking the electrolysis voltage, the anolyte is cooled down below 60 ° C and then discharged.

Thereafter, the anode space is flushed. The washing is carried out with highly diluted brine with an alkali metal chloride content of 0.01 to 4 mol / l, with water, or preferably with deionized water. Washing is preferably carried out by filling the anode space once and releasing the flush liquid immediately. The washing can also be carried out in two or more stages, for example after the initial filling with diluted brine with an alkali metal chloride content of 1.5 to 2 mol / l and then emptied, and very diluted brine with a NaCl content of 0.01 mol / l. Do this by further filling with furnace or deionized water and then emptying. This washing solution can be discharged again immediately after the anode space has been completely filled or it can be released after staying in the anode space for up to 200 minutes. After discharge, a small residual amount of wash solution remains in the anode space. Thereafter, the anode space remains stationary or surrounded without direct contact with the surrounding atmosphere.

After the anode space is emptied, the catholyte still present is discharged from the cathode space and then washed off the gas space of the cathode. The washing is carried out with highly diluted aqueous alkali having an alkali metal hydroxide content of 4 mol / l or less, with water, or preferably with deionized water. The washing is preferably carried out by filling the gas space once and releasing the washing liquid immediately. The washing can also be carried out in two or more stages, for example, firstly filled and emptied with diluted alkali having an alkali metal hydroxide content of 1.05 to 3 mol / l and then very diluted with an alkali metal hydroxide content of 0.01 mol / l. Fill with more alkaline or deionized water and empty. This washing solution can be discharged again immediately after the cathode space has been completely filled, or it can be discharged after staying in the cathode space for up to 200 minutes. After discharge, a small residual amount of washing solution remains in the cathode space. The cathode space remains stationary or enclosed without direct contact with the surrounding atmosphere.

The oxygen supply can be cut off when the polarization voltage is cut off. It is desirable to shut off the oxygen supply before emptying and cleaning the cathode space; In the filling process, the oxygen supply orifice assists in venting or degassing the cathode space.

After the anode space and the cathode space are emptied, the electrolysis cell with the wet membrane can be maintained to enable immediate start-up without damaging the performance of the electrolysis cell in an installed state over a long period of time. For stabilization periods of several weeks it is appropriate to wet or wash the anode space with diluted aqueous alkali metal chloride solution and the cathode space with diluted aqueous alkali metal hydroxide solution at regular intervals for stabilization. Washing is preferably done at 1-12 week intervals, in particular at 4-8 week intervals. The concentration of the diluted alkali metal chloride solution used for washing or wetting is 1 to 4.8 mol / l. This wash solution can be discharged again immediately after the anode space is completely filled or it can be discharged after staying in the anode space for up to 200 minutes. The concentration of alkali metal hydroxide solution used for washing or wetting is 0.1 to 10 mol / l, preferably 1 to 4 mol / l. The temperature of the alkali metal hydroxide solution or of the brine may be 10 to 80 ° C, but preferably 15 to 40 ° C. The wash solution can be discharged again immediately after the cathode space has been completely filled or can be released after staying in the cathode space for up to 200 minutes.

Another embodiment of the present invention includes washing the electrode space with wet gas, which can be understood as meaning the cathode space and the anode space of the electrolysis cell. For this purpose, for example, water-saturated nitrogen is introduced into the anode space. Alternatively, oxygen can also be introduced. The gas volume flow rate will be such that a volume exchange of 2 to 10 times can be performed. The gas volume flow rate of the 100 liter gas volume may be 1 l / h to 200 l / h at a temperature of 5 to 40 ° C., and the gas temperature is preferably at room temperature, ie 15 to 25 ° C. The purge gas is saturated with water at the temperature of the gas.

Surprisingly, it has been found that these new OCEs with small openings relative to the total electrode area maintain their performance for a long time through a series of simple means in the course of starting and stopping. The OCE of the present invention is particularly suitable for, but not limited to, industrial electrolysis units having electrodes of 1 m or higher height. The OCEs of the invention and the method of operating such OCEs are particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.

An electrolysis cell that is out of operation by the above method can be returned to the operating state by the method described previously. By following the described method steps, the electrolysis cell can go through a number of start and stop cycles of operation without causing any damage to the cell's performance.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. The drawing shows:
Figure 1. Schematic diagram of an OCE with a horizontal slot 1
2. Schematic diagram of the OCE with slots 2 arranged diagonally
3. Schematic diagram of an OCE with a row of holes 3 arranged horizontally
4. Schematic diagram of the OCE with holes 4 evenly spread over the electrode region
5. Schematic cross section through OCE 8 with horizontal slot 5 and guide plate 6 and ion exchange membrane 7

Example  One

A powder mixture consisting of 7% by weight of PTFE powder, 88% by weight of silver oxide (I) and 5% by weight of silver powder was added to a mesh of nickel wires and subjected to pressure to form an oxygen consuming electrode (OCE). Electrodes were installed in a 100 cm 2 area electrolysis unit with Zoltek's PW3MFBP carbon fabric 0.3 mm thick and DuPont N2030 type ion exchange membrane. Carbon fabrics were arranged between the OCE and the membrane. The electrolysis unit has an ion exchange membrane and carbon fabric and liquid outlet, oxygen inlet and outlet, and gas space for oxygen arranged in the assembly between the anode space and the cathode space, the cathode space where OCE is used as the cathode, There is a supply and discharge and an anode space with an anode made of coated titanium (ruthenium oxide coating). OCE, carbon fabric and ion exchange membranes were pressed over the anode at a pressure of about 30 mbar because of the higher pressure in the cathode chamber.

In the first step the cathode space was filled with 30% by weight sodium hydroxide solution with 20 ppm chloride ion content and chlorate ion content less than 10 ppm at 80 ° C. and emptied again. Oxygen was supplied during the emptying process so that the resulting gas space was filled with oxygen. After emptying, a positive pressure of 30 mbar was formed on the cathode side.

In the next step the anode space was filled with brine with a concentration of 210 g NaCl / l at 70 ° C. and the anode cycle was activated. The electrolysis voltage was turned on immediately after achieving the operation of a constant anode cycle. The electrolysis current was controlled so that the electrolysis current was 1 kA / m 2 after 5 minutes and the electrolysis current was 3 kA / m 2 after 30 minutes. The plant operation was operated for 3 days at a power of 3 kA / m 2, an electrolysis voltage of 1.90-1.95 V, a concentration of 32 wt% removed sodium hydroxide solution and a temperature in an electrolysis cell at 88 ° C.

Example  2

A powder mixture consisting of 7% by weight of PTFE powder, 88% by weight of silver oxide (I) and 5% by weight of silver powder was added to nickel death and pressurized to produce an oxygen consuming electrode. The effective OCE area was 10 * 10 cm. A cutter knife was used to cut one horizontal slot every 2 mm height across the electrode, 1 cm above the bottom and 1 cm below the top. 4% of the OCE area was removed. The OCE was installed in an electrolysis unit with a 0.3 mm thick Soltec PW03 carbon fabric and DuPont N2030 type ion exchange membrane between the OCE and the membrane.

In the first step the cathode space was filled with 30% by weight sodium hydroxide solution having a chloride ion content of 20 ppm and a chlorate ion content of less than 10 ppm at 80 ° C. and emptied again. Oxygen was supplied during the emptying process so that the resulting gas space was filled with oxygen. After emptying, a positive pressure of 30 mbar was formed on the cathode side.

In the next step the anode space was filled with brine with a concentration of 210 g NaCl / l at 70 ° C. and the anode cycle was activated. The electrolysis voltage was turned on immediately after achieving a constant operation of the anode circulation. The electrolysis current was controlled so that after 5 minutes the electrolysis current was 1 kA / m 2 and after 30 minutes the electrolysis current was 4 kA / m 2. The plant operation was run over 10 days with a power of 4 kA / m 2, an electrolysis voltage of 2.0-2.2 V, a 32 wt% removed sodium hydroxide solution concentration, and a temperature in an electrolysis cell at 90 ° C. The average electrolysis voltage was 2.14 V.

As a result of slotting in the OCE, no significant deterioration in electrolysis power was found.

Example  3

After 3 days of operation, the electrolysis unit according to Example 1 is out of operation as follows:

Electrolysis current was reduced to 90 A / m 2. The anolyte was circulated for more than 60 minutes until a chlorine content of less than 1 mg / l was obtained, after which the electrolysis current was interrupted. Within this time the anolyte was cooled to 70 ° C. The anode space was emptied, then filled with deionized water and immediately emptied again.

Thereafter, the remaining liquid in the cathode space was released, the oxygen supply was shut off and the cathode space was flooded with deionized water and immediately emptied again.

After 50 hours of stopping, the electrolysis unit from Example 2 was returned to operation as follows:

The first step filled the cathode space with 32% by weight sodium hydroxide solution with 20 ppm chloride ion content and less than 10 ppm chlorate ion content at 80 ° C. and then emptied again. Oxygen was supplied during the emptying process so that the resulting gas space was filled with oxygen. After emptying, a positive pressure of 30 mbar was formed on the cathode side.

In the next step the anode space was filled with brine with a concentration of 210 g NaCl / l at 70 ° C. and the anode cycle was activated. Immediately after constant operation of the anode cycle, the electrolysis voltage was turned on. The electrolysis current was controlled so as to obtain an electrolysis current of 1 kA / m 2 after 5 minutes and an electrolysis current of 3 kA / m 2 after 30 minutes at the temperature of the electrolysis cell at 88 ° C. and a concentration of 32% by weight of the sodium hydroxide solution removed. .

The electrolysis voltage at 3 kA / m 2 was 1.8 to 1.9 V. The electrolysis unit did not show any degradation compared to the period before the shutdown; In fact, as much as 100 mV of improvement was observed.

1 horizontal slot
2 diagonally arranged slots
3 holes
4 holes
5 horizontal slots
6 guide plate
7 ion exchange membrane
8 OCE (Oxygen Consumption Electrode)

Claims (12)

At least one carrier element in the form of a sheet-like structure and at least one opening having a coating of the catalytically active component and the gas diffusion layer, wherein the coating to the gas diffusion layer has a diameter or height of 0.5 mm to 20 mm, preferably 1 mm to 10 mm An oxygen consuming electrode for use in an electrolysis cell having a microgap structure, characterized in that it has a.  The oxygen consuming electrode according to claim 1, wherein the total area of the opening with respect to the surface of the electrode facing the ion exchange membrane is 0.05 to 15%, preferably 1 to 12% of the electrode area. 3. An oxygen consuming electrode according to claim 1 or 2, wherein only the coating of the catalytically active component and of the carrier element to the gas diffusion layer has an opening. 3. Oxygen consuming electrode according to claim 1 or 2, wherein the opening is continuous throughout the coating and carrier element. The oxygen consuming electrode according to any one of claims 1 to 4, wherein the opening is formed by punching, cutting, machining, or drilling from the oxygen consuming electrode. 4. The process according to claim 1, wherein the corresponding orifice is kept clean in the carrier element coating process by insertion of a removable substitute which is subsequently removed after completion of the coating in the method of producing the electrode. Oxygen consumption electrode. The oxygen consuming electrode according to any one of claims 1 to 3, wherein the oxygen consuming electrode has a distance of 0.01 mm to 2 mm between the ion exchange membrane and the oxygen consuming electrode. A method of performing chlor-alkali electrolysis to an electrolysis cell comprising an oxygen consuming electrode according to any one of claims 1 to 7 of a micro-gap structure, the cell comprising an anode liquid containing an alkali metal chloride and an anode The anode space above, the ion exchange membrane and the cathode space having a thickness of 0.01 mm to 2 mm arranged between the OCE and the membrane, through which the catholyte flows a flat porous element and a silver-containing catalyst and with an oxygen consuming electrode as cathode Aqueous alkali metal hydroxide solution having a chloride ion content of at most 1000 ppm, preferably at most 700 ppm, more preferably at most 500 ppm, of the oxygen-consuming electrode on the gas side as a first step prior to the application of the electrolysis voltage. Oxygen, and subsequent introduction of the anolyte into the anode space and the oxygenous gas Performing chlor-alkali electrolysis characterized in that for applying the electrolysis voltage after the subsequent introduction into the cathode space. A method for performing chloralkali electrolysis into an electrolysis cell comprising an oxygen consuming electrode according to any one of claims 1 to 7 of a microgap structure, the cell comprising at least one anode space, an ion exchange membrane comprising an anolyte solution And a cathode space with an OCE with a silver-containing catalyst and a flat porous element between the membrane and the OCE with a thickness of 0.01 mm to 2 mm, at least following steps are carried out in this order for stopping at the end of the electrolysis operation. Method for performing chlor-alkali electrolysis, characterized in that:
-Cuts off the electrolysis voltage
-Free the anode space
Preferably the anode space is refilled with one of the following liquids: dilute alkali metal chloride solution or up to 4 mol / l or deionized water, and subsequently emptying the anode space.
The cathode space is filled with one of the following liquids: up to 4 mol / l diluted alkali metal hydroxide solution or deionized water, and subsequently emptying the cathode space. 8. The chlorine ion content of claim 7, wherein the alkali metal hydroxide solution used to wet the cathode space prior to application of the electrolysis voltage is 20 ppm or less, preferably 10 ppm or less, more preferably 4 ppm or less. Characterized by having a. The anode space according to claim 8 or 9, wherein the anode space is optionally repeated between 1.7 and 3.4 mol / l, preferably every 1 to 12 weeks, preferably every 4 to 8 weeks, after emptying the stationary and electrolytic cells. Washing with a diluted alkali metal chloride solution of 2.2 to 2.9 mol / l, and washing the cathode space with a diluted alkali metal hydroxide solution of 2 to 9 mol / l, preferably 4 to 6 mol / l. How to. Process according to any of claims 7 to 11, characterized in that the alkali metal chloride is sodium chloride or potassium chloride, preferably sodium chloride.

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