EP2639337A2 - Method for the electrolysis of alkali chlorides with oxygen consumption electrodes - Google Patents

Abstract

The process for electrolysis of alkali metal chlorides with an electrolysis cell such as a falling-film cell having an oxygen-consuming electrode, operated according to a finite gap arrangement, is claimed. The electrolysis cell includes an anode space with an anode and an anolyte comprising alkali metal chloride, an ion exchange membrane, a cathode space with an oxygen-consuming electrode as a cathode that comprises a silver-containing catalyst, and an electrolyte gap between oxygen-consuming electrode and membrane through which the catholyte flows. The process for electrolysis of alkali metal chlorides with an electrolysis cell such as a falling-film cell having an oxygen-consuming electrode, operated to a finite gap arrangement, is claimed. The electrolysis cell includes an anode space with an anode and an anolyte comprising alkali metal chloride, an ion exchange membrane, a cathode space with an oxygen-consuming electrode as a cathode that comprises a silver-containing catalyst, and an electrolyte gap between the oxygen-consuming electrode and the membrane through which the catholyte flows, where an application of electrolysis voltage between anode and cathode is preceded by adjustment of the volume flow rate and/or composition of the catholyte supplied to the gap to result in an aqueous solution of alkali metal hydroxide leaving the cathode gap having a content of chloride ions of 500 parts per million (ppm). The electrolysis voltage is applied after introduction of the anolyte and of an oxygen gas into the cathode space. The alkali metal hydroxide solution introduced in the catholyte feed before application of the electrolysis voltage has a content of alkali metal chlorate of 10 ppm. Duration between the introduction of the catholyte and the application of the electrolysis voltage is 150 minutes. A temperature difference between anolyte feed and catholyte drain of less than 20[deg] C is established after the introduction of the catholyte and anolyte. At the end of the electrolysis operation, after the electrolysis voltage is switched off, the alkali metal chloride solution is removed from the anode space, then the anode space is flushed with fresh alkali metal chloride solution until the chlorine content of oxidation state >= 0 in the analyte is less than 10 ppm, then the analyte temperature is lowered and then the analyte is released from the anode space in a first step and in a second step, the introduction of the catholyte is ended and the catholyte is released from the electrolyte gap. The draining analyte has 4-4.6 mol/l of alkali metal chloride. The electrolysis voltage is switched off after attainment of chlorine content in the anolyte of less than 1 mg/l. A positive pressure of greater than 10 mbar relative to the anode space is maintained until the emptying and flushing steps are ended in the cathode space. A discharge of the electrolysis cell is repeated at intervals for 4-8 weeks, the anode space has 2.2-4.8 mol/l of dilute alkali solution, and the cathode space has 4-10 mol/l of alkali metal hydroxide solution.

Description

The invention relates to a method for the electrolysis of aqueous solutions of alkali metal chlorides by means of oxygen-consuming electrodes while maintaining certain operating parameters.

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 ChloralkaliElektrolyse) 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 are just as necessary as a tightness, so that the gas and liquid space remain separated. 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 results from the fact that forms on the side of the catholyte by the hydrostatic pressure, a pressure drop across the height of the electrode, which precludes a constant pressure on the gas side over the height. 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, liquid 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 the In practice, this means that the height of a SVE is limited to about 30 cm, unless further measures are taken.

A preferred solution to this problem is provided by an arrangement in which the catholyte is guided down the SVE from top to bottom via a flat porous element, a so-called percolator, placed between the SVE and the ion exchange membrane, in a type of free-falling liquid film, called a falling film for short , 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. A description of this arrangement can be found in WO 2001/57290 A1 ,

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 for the passage of an alkali, termed the catholyte gap for short. This arrangement is also referred to as a "zero gap" arrangement, as opposed to a "finite gap" arrangement in which the alkali hydroxide solution is passed through a defined narrow gap between SVE and the membrane. 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 or to crystallization of alkali hydroxide in the pores. It has been found that in this case also a very high concentration of alkali metal hydroxide can arise, it being described that the ion exchange membrane is not long-term stable at this high concentration ( Lipp et al, J. Appl. Electrochem. 35 (2005) 1015 - Los Alamos National Laboratory "Peroxide formation during chlorine-alkali electrolysis with carbon-based ODC").

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 and platinum-based electrodes in continuous operation is insufficient, 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, their long-term stability under the conditions in one of the oxygen-consuming electrodes, especially when used for the chlor-alkali electrolysis, is also limited.

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 put into operation for the first time. 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, of a SVC 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 a heavy load: they must be resistant to chlorine on the anode side and strong alkaline stress on the cathode side at a temperature around 90 ° C. Perfluorinated polymers such as PTFE usually withstand these stresses. Ion transport takes place via sulfonate groups or carboxyl groups copolymerized into these polymers. Carboxyl groups show a higher selectivity, have a lower water absorption and have a higher electrical resistance than sulfonate groups. In general, multilayer membranes are used with a thicker sulfonate group-containing layer on the anode side and a thinner, carboxyl-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-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 cause precipitation of solids in the membrane or mechanical destruction of 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 up to several years without being opened in the meantime. However, due to fluctuating purchase quantities and disruptions in the electrolysis upstream or downstream production areas, electrolysis cells in production plants must inevitably be repeatedly switched off and on again.

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

There are known from the prior art few modes of operation, with which the risk of damage to the electrolysis cells when starting and stopping can be reduced.

A measure known from conventional membrane electrolysis is the maintenance of a polarization voltage, that is, when the electrolysis is terminated, the potential difference is not reduced to zero, but maintained at the level of the polarization voltage. Practically, a voltage slightly higher than that required for the polarization is set, so that a constant low current flows and a small amount of electrolysis takes place. However, this measure alone is not sufficient when using SVE in order to prevent oxidative damage of SVE at standstill.

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 4578159A1 describes for an electrolysis process using a "zero gap" arrangement that by flushing the cathode compartment with 35% sodium hydroxide solution prior to commissioning 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 US4364806A1 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 WO2008009661A2 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 and require the installation of additional facilities for the supply of nitrogen and hydrogen. In addition, the addition of hydrogen increases the safety risk in the operation of such electrolyzers by the formation of explosive gas mixtures, since in the cathode compartment residues of oxygen may be present. When restarted, the pores of the SVE are partially filled with nitrogen, which is the supply of oxygen to the reactive centers with special needs. Also, the method provides no protection against damage to the ion exchange membrane.

Jerzy Chlistunoff's Final Technical Report "Advanced Chlor-Alkali Technology" (Los Alamos National Laboratory, DOE Award 03EE-2F / Ed190403, 2004) outlines 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 humidification of the gas stream is increased to wash out the remaining NaOH. On the anode side, the brine is replaced by hot water (90 ° C). The process is repeated until a stable polarization 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.

To restart, 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.

The described known methods are complicated to perform, this is especially true for technical electrolysis systems, where safety aspects have an increased importance. Furthermore, not all processes can be transferred to electrolysis cells with finite gap arrangement.

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.

The object of the present invention is to find an improved electrolytic process for chloralkali electrolysis using a SVE in Finite Gap arrangement with suitable operating parameters for commissioning and decommissioning of the electrolytic cell with SVE with silver catalyst as an electro-catalytic substance, which are easy to perform and damage their compliance be avoided on membrane, electrode and / or other components of the electrolysis cell.

The object is achieved in that when commissioning an electrolysis cell in finite gap arrangement with SVE with silver catalyst on the cathode side, an aqueous alkali metal hydroxide solution with little contamination with chloride - and possibly other anions presented and that the filling of the anode compartment with brine takes place only after commissioning of the catholyte circuit; and that regardless of the decommissioning of an electrolytic cell after switching off the electrolysis in a first step, the anolyte is concentrated, then cooled and then discharged and is discharged in a subsequent step, the catholyte.

The invention relates to a process for chlor-alkali electrolysis with an electrolysis cell oxygen-consuming electrode, preferably operated according to the principle of finite gap arrangement, particularly preferably according to the principle of a falling film cell, wherein the electrolytic cell at least one anode compartment with anode and an alkali metal chloride-containing anolyte, an ion exchange membrane , a cathode compartment having an oxygen-consuming electrode as the cathode, which has a silver-containing catalyst, and an electrolyte gap between the oxygen-consuming electrode and the membrane through which the catholyte flows, characterized in that, before the electrolysis voltage is applied between anode and cathode, the volume flow and / or the composition of the Slit supplied catholyte is adjusted so that the electrolyte solution leaving the aqueous solution of alkali metal hydroxide has a content of chloride ions of at most 1000 ppm, preferably at most 700 ppm, especially bevo Rzugt has at most 500 ppm and after introduction of the anolyte and an oxygen-containing gas in the cathode compartment, the electrolysis voltage is applied.

Finite gap arrangement according to the invention means any arrangement of an electrolytic cell having a flowed through by the catholyte electrolyte gap between oxygen-consuming electrode and membrane, wherein the gap has a gap width of at least 0.1 mm and in particular has a gap width of at most 5 mm. In the preferably used electrolysis cell according to the principle of the falling film cell, catholyte flows in a vertically arranged electrolysis cell following the principle of gravity from top to bottom. Other arrangements with alternative flow direction or horizontally arranged electrolytic cell should also be included in the invention.

Another object of the invention is a method for chlor-alkali electrolysis with an electrolytic cell with oxygen-consuming electrode, preferably operated according to the finite gap principle, for example, a falling film cell, wherein the cell at least one anode compartment with anode and an alkali metal chloride-containing anolyte, an ion exchange membrane, a cathode compartment with a Oxygen-consuming electrode with silver-containing catalyst and having a flowed through by the electrolyte gap between the oxygen-consuming electrode and membrane, characterized in that at the end of the electrolysis process after switching off the electrolysis in a first step, the concentration of the discharged from the anode compartment alkali chloride solution increases, then the anode compartment with fresh alkali chloride solution is rinsed until the content of chlorine of the oxidation state 0 and greater than 0 in the anolyte is in particular less than 10 ppm, then the temperature of the anolyte is lowered and then the anolyte is discharged from the anode compartment and in a subsequent step, the supply of the catholyte terminated and the catholyte is drained from the electrolyte gap.

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, and insoluble silver chloride may form on oxidation of the silver catalyst. There is a risk of damage to the electrode and also the entire cathode space.

When the electrolysis voltage is switched off, the mass transfer through the membrane due to the flow of current also comes to a standstill, and undesired changes in the concentration of the brine and the alkali metal hydroxide solution can occur. The membrane is depleted of water, it can cause shrinkage and Feststoffausfällungen and as a result to a hole, the passage of anions through the membrane is facilitated. On reuse, too low a water content prevents mass transport across the membrane, which can lead to an increase in osmotic pressure and delamination at the interfaces between the sulfonic acid group-containing and carboxylic acid group-containing layers typically used in such membranes.

Inhomogeneity of the water and / or ion distribution in the membrane and / or the SVE can lead to local peaks in the current and mass transport and, as a consequence, to 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 a water transport from the anode to the cathode compartment. As long as the electrolysis is in operation, the transport of water from the anode compartment is precluded by a loss of chloride and alkali ions, so that the concentration of alkali chloride in conventional electrolysis conditions in the anode compartment decreases. When the electrolysis is stopped, the water transport due to the osmotic pressure remains from the anode compartment to the cathode compartment. The concentration in the anolyte rises above the saturation limit. It comes to the precipitation of alkali metal chloride salts, especially in the border region to the membrane or even in the membrane, which can lead to damage to the membrane.

With the provision of the novel electrolysis process according to the invention, the aforementioned problems and disadvantages of the previously known processes are overcome.

Surprisingly, it has been found that electrolysers containing an SVE with silver catalyst can be repeatedly put into operation and put out of operation without damage by the sequence of these comparatively simpler steps and can not be damaged even when at a standstill. The method is particularly suitable for the electrolysis of aqueous sodium chloride and potassium chloride solutions.

The operating parameters for commissioning and decommissioning an electrolytic cell with SVE are described below for an electrolytic cell with SVE with silver catalyst and finite gap arrangement, which can be operated as follows: The concentration of the alkali metal chloride solution (anolyte) 2.9-4 , 3 mol / l and an alkali hydroxide concentration (catholyte) of 8.0 - 12 mol / l is described in detail as a special embodiment, without wishing to limit the execution of the procedure described so. In particular, for the commissioning or for decommissioning of such an electrolysis cell, further embodiments may be used, in which the contamination of chloride and other anions of the aqueous solution of the alkali metal hydroxide solution flowing out of the leaching gap does not exceed certain limit values and the filling of the Anodenraums with aqueous alkali chloride solution takes place only after commissioning of the catholyte circuit; and in which, during decommissioning, the order of concentration changes and draining of the anolyte and thereafter subsequent draining of the catholyte is maintained. The commissioning of an electrolysis unit with finite gap arrangement, an SVE with silver catalyst and an according to the prior art watered ion exchange membrane is carried out, for example as follows:
→ Start-up, catholyte side

Before starting up the catholyte circuit, humidified oxygen is added and in the cathode half cell an overpressure is set according to the configuration in the cell, generally in the amount of 10-100 mbar compared to the pressure in the anode. The purity of the oxygen corresponds to the concentrations and purity requirements customary in electrolysis with SVE; preference is given to oxygen having a residual gas content of <10% by volume. The moistening of the oxygen can take place at room temperature or at the temperature prevailing in the cell. In particular, the moistening can be carried out at a temperature which corresponds to the cell temperature.

The catholyte circuit is put into operation after the start of the oxygen supply. The catholyte (aqueous alkali metal hydroxide solution) can be supplied from the top into the cathode gap, flows through the cathode gap, is discharged again in the lower region and can be partly returned after concentration adjustment by means of a pump back into the upper region of the cathode gap. To minimize the flow rate, a flow brake, e.g. a flat porous element to be incorporated into the cathode gap. The concentration of the supplied alkali metal hydroxide solution in this step preferably has a concentration kept lower by up to 3.5 mol / l than in the later electrolysis; it is preferably 7.5-10.5 mol / l. The concentration of the alkali hydroxide solution in the later electrolysis is usually in the range of 8 to 12 mol / l, preferably 9.5 to 11.5 mol / l.

The concentration of chloride ions in the discharged catholyte is at most 1000 ppm, preferably <700 ppm, more preferably <500 ppm. This is based on the above-mentioned concentration of alkali hydroxide in the catholyte.

The concentration of alkali chlorate, in particular sodium chlorate, in the discharged catholyte is at most 20 ppm, preferably <15 ppm, more preferably <10 ppm. This is based on the above-mentioned concentration of alkali hydroxide in the catholyte.

The determination of the concentrations is carried out by titration or another analysis method basically known to the person skilled in the art.

For the commissioning of the catholyte circuit alkali hydroxide solution from regular production is preferably used. Alkali hydroxide solution from shutdowns is less suitable for commissioning, mainly due to contamination with chloride ions. The temperature of the supplied catholyte is controlled so that a temperature of 50-95 ° C, preferably 75-90 ° C sets in the discharge from the cathode space. The temperature of the exiting catholyte can additionally be influenced by the temperature of the anolyte. Thus, by lowering the Anolytzulauftemperatur the Katholytzulauftemperatur be increased. Preferably, a temperature difference between Anolytzulauf and Katholytablauf of less than 20 ° C is set.

In a particular embodiment, the new method is used so that less than 240 minutes, between the beginning of the introduction of the catholyte and the application of the electrolysis voltage, preferably less than 150 minutes. By continuous, proportionate exchange of the catholyte in the circuit of the electrolyzer, the catholyte circulation can be extended without electricity up to 360 minutes. As a result of the exchange, the chloride ion concentration of the alkali metal hydroxide solution leaving the cathode gap is kept low.
→ Commissioning anode side

After commissioning of the catholyte circuit, the anode compartment is filled with concentrated aqueous alkali metal chloride solution. The concentration of the alkali chloride solution fed in this step is preferably kept higher by 0.5-1.5 mol / l than in the later electrolysis, it is preferably 2.9-5.4 mol / l. The concentration of the supplied alkali metal chloride solution in the later electrolysis is usually in the range of 4.8 to 5.5 mol / l, preferably 5.0 to 5.4 mol / l. The brine corresponds to the purity requirements customary for membrane electrolysis. After filling the anode compartment, the brine is passed in accordance with the usual equipment conditions by pumping in the circulation through the anode compartment. The temperature of the brine in the discharge from the anode compartment should be 50-95.degree. C., preferably 70-90.degree. C., before an electrolysis voltage is applied. If the temperature is lower, the anolyte is heated in the circuit.

After filling the anode compartment and starting up the anode circuit and reaching a temperature of 60-70 ° C, the electrolysis voltage is applied in the next step. Overall, the entire period for commissioning is to be kept as short as possible. It should be less than 240 minutes, preferably less than 150 minutes, between the start of the catholyte and anolyte circulation and the switching on of the electrolysis current. The increase in the current intensity until the desired current intensity is reached is preferably carried out in technical electrolyzers having an area of, for example, 2.7 m ² at a rate of 0.05-1 kA / min. The electrolysis cell is then run with the design parameters, for example, with a concentration of 2.9 to 4.3 moles of alkali chloride per liter in the anode compartment and a concentration of 8 -12 moles of alkali metal hydroxide per liter in the outlet of the cathode, a current density of 3 -. 6 kA / m ² and a 30% to 100% excess of oxygen in the gas supply. The method described is suitable both for the first commissioning of electrolysis units after the installation of a silver-containing, in particular a silver oxide-containing SVE, as well as for the commissioning of electrolysis cells with SVE after decommissioning.

The decommissioning of the electrolysis cell takes place, for example, as follows:
→ Decommissioning - anode side

In the process, which includes special conditions for the shutdown of the electrolytic cell, after reducing the electrolysis current to a current density of 5 - 35A / m ^2, the concentration of the effluent from the anode space brine is increased to 4.0 to 5.3 mol / l ,

In another preferred embodiment of the method, which includes special conditions for the shutdown of the electrolysis cell, the electrolysis voltage is switched off after reaching a chlorine content in the anolyte of <10 mg / l, preferably <1 mg / l. Under chlorine content here is the total content of dissolved in the anolyte chlorine in the oxidation state 0 and higher understood.

It is particularly preferred to maintain an overpressure of the cathode space gas with respect to the anode space gas of> 10 mbar until the end of the emptying and purging of the cathode space. This possibly prevents oscillations of the membrane during operation, which can lead to mechanical stresses and cracks in the membrane.

To achieve the chlorine-free (at most 10 ppm Cl of the oxidation state 0 and higher) of the anolyte, a brine having an alkali metal chloride content of 4.0 to 5.5 mol / l, preferably 4.3 to 5.4 mol / l fed. The temperature of the supplied concentrated anolyte depends on the residual chlorine content in the anode compartment and the electrolysis voltage. At a temperature of less than 70 ° C, the polarization voltage would rise, so that it comes back to a chlorine evolution. The temperature of the supplied anolyte is therefore adjusted so that sets a temperature of above 70 ° C in the outlet. After reaching a chlorine-free state, ie <10 ppm chlorine in the anolyte and exchange with concentrated brine, the temperature of the incoming brine is adjusted so that the temperature of the effluent brine is lowered to 45 - 55 ° C and then emptied of the anode space of brine , There remain small residual amounts of concentrated anolyte in the anode compartment.

The polarization voltage can be maintained until the anolyte is drained. Preferably, the polarization voltage is switched off after reaching a chlorine content in the anode compartment of ≦ 10 ppm, more preferably <1 ppm.
→ Decommissioning cathode compartment

After emptying the anode compartment, the catholyte circulation is interrupted and the remaining catholyte is discharged. The cathode gap can still be rinsed with dilute aqueous alkali metal hydroxide solution. The concentration of the alkali hydroxide solution used for rinsing is 2 to 10 mol / l, preferably 4 to 9 mol / l. In a further embodiment, the lower third of the catholyte space is rinsed. This can be done, for example, that from below alkali hydroxide solution in the cathode compartment guided and then discharged again. There remain small residual amounts of aqueous alkali metal hydroxide solution in the cathode gap.

The oxygen supply can be adjusted by switching off the electrolysis voltage. Preferably, the oxygen supply is adjusted after emptying the cathode space, wherein the adjustment of the oxygen supply before, during or after rinsing of the cathode space can be carried out with alkali hydroxide solution. The overpressure in the cathode compartment of about 10 -100 mbar relative to the anode compartment is maintained during the shutdown process.
→ standstill

After emptying the anode and cathode compartments, the electrolysis cell with the moist membrane can be kept ready for further commissioning over an extended period of time without the capacity 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.

In another embodiment of the method, which includes special conditions for the shutdown of the electrolytic cell, after switching off and emptying the electrolytic cell repeatedly at intervals of 1 to 12 weeks, preferably 4 to 8 weeks, the anode compartment with a dilute alkali chloride solution containing 2 , 2 to 4.8 mol / l and the cathode compartment with an alkali hydroxide solution containing 4 to 10 mol / l rinsed.

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 is calculated so that a 2 to 1 Ofacher volume change can take place. The gas volume flow may be 1 1 / h to 200 1 / h at a temperature of 5 to 40 ° C, preferably the temperature of the gas ambient temperature, i. 15-25 ° C amount. The saturation of the purge gas takes place at the temperature of the gas.

The same procedure is followed with the cathode compartment. Particularly preferably, the gas on the cathode side is oxygen.

Another embodiment of the method is to separate the anode and cathode compartments from the ambient air. The rooms can be closed, for example, after emptying. To compensate for temperature fluctuations of the environment and the so connected volume change, the rooms can also be closed by a liquid immersion.

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 method steps described, the electrolysis cell can run through a large number of startup and shutdown cycles without impairing the performance of the cell.

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 oxygen-consuming electrode was installed in a finite-gap electrolysis unit. Here, the sodium hydroxide solution in the gap between the membrane (ion exchange membrane: type N2030 manufacturer DuPont) and SVE, which contains a porous tissue, fed. The electrolysis unit has in assembly an anode compartment with Anolytzu- and -ablauf, with an anode of coated titanium (coating mixture of iridium and ruthenium) a cathode compartment with the SVE as the cathode and a gas space for the oxygen and Sauerersstoffzu- and- derivatives, a Liquid outlet and a supply and discharge for the sodium hydroxide in the gap and an ion exchange membrane, which are arranged between the anode and cathode compartment. The gap was about 1mm. As the anode, a titanium anode of Fa Uhde was used, which had an above-mentioned coating. The sodium hydroxide volume flow was about 110 1 / h per square meter of the geometric cathode area. Below, the caustic soda is led out of the gap into the gas space and from there via an outlet pipe out of the cathode space.

Before the catholyte cycle was started up, saturated oxygen was added to the cathode compartment at room temperature so that an overpressure to the anode compartment of 40 mbar was established in the cathode compartment.

The amount of oxygen was adjusted so that always 1.5 times the stoichiometric excess is supplied to the required amount of oxygen due to the set current.

Thereafter, the cathode cycle with a 30 wt .-%, about 50 ° C hot caustic soda was put into operation

In the next step, the anode compartment was filled with 50 ° C brine at a concentration of 230 to 300 g NaCl / l and the anode circuit was put into operation. While the anode circulation was maintained, the heating of the anolyte in a heat exchanger connected in the anode circuit was started.

The sodium hydroxide solution leaving the gap between membrane and SVE had a chloride ion content of 320 ppm and a sodium chlorate content of <10 ppm.

Immediately after reaching the temperature of the draining anolyte of 70 ° C and the draining catholyte of 70 ° C, the electrolysis voltage was applied. The electrolysis current was adjusted so that after 6 minutes, an electrolysis current of 1 kA / m ² , and after 30 minutes, an electrolysis current of 4 kA / m ^{2 was} reached. The cell voltage at 4kA / m ² was 2.1V, the temperature of the effluent electrolyte about 88 ° C.

The concentrations were adjusted after commissioning so that the concentration of the effluent brine was about 230g / l and that of caustic soda about 31.5 wt.%.

Example 2

• Der Elektrolysestrom wurde auf 18 A/m² heruntergeregelt.
• Der Anolyt-Kreislauf wurde weiter betrieben, wobei fortlaufend chlorfreie Sole mit der Konzentration von 300g/l zugeführt wurde. Der Anolyt kühlte in dieser Zeit auf 75 °C ab. Nach Erreichen eines Chlorgehaltes von < 1 mg/l im ablaufenden Anolyten wurde der Elektrolysestrom abgeschaltet. Hiernach wurde der Anolyt weiter abgekühlt, dabei auf eine Konzentration von 250 - 270g/l durch Zugabe von Wasser verdünnt und bei einer Temperatur von 50°C abgelassen.

The electrolysis unit according to Example 1 was taken out of service after a period of 10 days as follows:

• The electrolysis current was regulated down to 18 A / m ² .
• The anolyte cycle was continued, continuously feeding chlorine-free brines at the concentration of 300 g / l. The anolyte cooled to 75 ° C during this time. After reaching a chlorine content of <1 mg / l in the draining anolyte, the electrolysis current was switched off. Thereafter, the anolyte was further cooled, while diluted to a concentration of 250 - 270g / l by the addition of water and drained at a temperature of 50 ° C.

After draining the anolyte, the oxygen supply was interrupted and turned off the Katholytzufuhr and drained the catholyte.

• Zuerst wurde bei Raumtemperatur mit Wasser gesättigter Sauerstoff (99,9Vol%) dem Kathodenraum zugeführt, mit dem ein Überdruck zum Anodenraum von 40mbar eingestellt wurde.
• Im ersten Schritt wurde der Kathodenkreislauf mit einer 30%igen, 50 °C heißen Natronlauge mit einem Gehalt an Chloridionen von 20 ppm und einem Gehalt an Natriumchlorat von < 10 ppm gefüllt.

The electrolysis unit was put back into operation after 48 hours after decommissioning as follows:

• First, oxygen saturated with water at room temperature (99.9% by volume) was fed to the cathode compartment, which was used to set an overpressure to the anode compartment of 40 mbar.
• In the first step, the cathode circuit was filled with a 30%, 50 ° C hot caustic soda solution containing 20 ppm of chloride ions and <10 ppm of sodium chlorate.

In the next step, the anode compartment was filled with 50 ° C brine with a concentration of 250 g NaCl / l and the anode circuit was put into operation. Immediately after further heating of the electrolytes and reaching a temperature of the electrolytes (anolyte and catholyte) in the course of about 70 ° C, the electrolysis voltage was applied. The electrolysis current was adjusted so that after 10 minutes an electrolysis current of 1 kA / m ² , and after 90 minutes an electrolysis current of 4 kA / m ^{2 was} applied. The concentration of the sodium hydroxide solution was 31.5 Wt .-%, the brine concentration in the process was 210g / l and the temperature of the effluent electrolyte 88-90 ° C.

The electrolysis voltage at 4 kA / m ² was 2.1 V. The standstill was no deterioration of the performance of the electrolysis unit.

Example 3

The electrolysis unit from example 2 was operated for 150 days. During this period, the electrolysis unit was shut down 11 times according to the conditions in Example 2 and respectively put back into operation accordingly. The shutdown time was 10 stops between 4 and 48 hours and at a standstill 140 hours. During the long standstill, the cathode and anode chambers were sealed tightly to air after emptying, so that no residual moisture could escape.

Some elements of the electrolytic cell were taken out of service after 150 days according to the conditions in Example 2 and then opened. On visual inspection, no solid precipitation, deposits, damage to the membrane, or corrosion damage to the SVK or cathode were seen.

Example 4

In a laboratory cell, the influence of a different chloride content in the sodium hydroxide solution on the performance of the oxygen-consuming cathode (composition as in Example 1) was investigated. The laboratory cell had an SVK, membrane and anode area of 100 cm ² each. The anode (coated titanium anode as in Example 1) was charged with so much brine that the brine running out of the cell had a concentration of 210 g / L and a temperature of 90 ° C. The concentration of the effluent from the cell sodium hydroxide solution was 32 wt.% And the sodium hydroxide solution had a temperature of 90 ° C. The leaching gap between the membrane (type as in Example 1) and SVK was 3 mm. The liquor was pumped from bottom to top through the gap. The experimental conditions were chosen so that the chloride content in the effluent liquor was achieved as shown in the results table. The current density at which the cell voltage was determined was 4 kA / m ² .

Results

Chloride content cell voltage 1000 ppm 2.43V 500 ppm 2,38V 250 ppm 2,26V 10 ppm 2,27V

At 1000 ppm chloride, a noticeable performance loss is observed, below 250ppm no performance loss.

Claims (13)

Method for chlor-alkali electrolysis with an electrolysis cell with oxygen-consuming electrode, preferably operated according to the finite gap arrangement, wherein the electrolysis cell at least one anode compartment with anode and an alkali metal chloride-containing anolyte, an ion exchange membrane, a cathode compartment with an oxygen-consuming electrode as cathode, the catalyst containing a silver and, having an electrolyte gap between the oxygen-consuming electrode and the membrane through which the catholyte flows, characterized in that the volumetric flow and / or the composition of the catholyte fed to the gap is adjusted prior to application of the electrolysis voltage between anode and cathode so that the aqueous solution leaving the electrolyte gap of alkali metal hydroxide has a content of chloride ions of at most 1000 ppm, preferably at most 700 ppm, more preferably at most 500 ppm, and after introduction of the anolyte and an oxygen-containing gas i n the cathode compartment, the electrolysis voltage is applied. A method according to claim 1, characterized in that an electrolytic cell is used according to the principle of the falling film cell as the electrolysis cell. Method according to one of claims 1 and 2, characterized in that prior to application of the electrolysis in the Katholytzulauf initiated alkali hydroxide solution has a content of alkali metal chlorate of at most 20 ppm, preferably at most 15 ppm, more preferably at most 10 ppm. Method according to one of claims 1 to 3, characterized in that between the beginning of the introduction of the catholyte and the application of the electrolysis voltage less than 240 minutes, preferably less than 150 minutes. Method according to one of claims 1 to 4, characterized in that after the initiation of the introduction of the catholyte and the anolyte, a temperature difference between Anolytzulauf and Katholytablauf of less than 20 ° C is set. Process for the chlor-alkali electrolysis with an electrolysis cell with oxygen-consuming electrode, preferably operated according to the principle of a finite gap arrangement, the cell having at least one anode compartment with anode and an alkali metal chloride-containing anolyte, an ion exchange membrane, a cathode compartment with an oxygen-consuming electrode with silver-containing catalyst and a Having flowed through the electrolyte between the electrolyte gap between the oxygen-consuming electrode and membrane, characterized in that at the end of the electrolysis process after Switch off the electrolysis in a first step, the concentration of the discharged from the anode compartment alkali chloride solution increases, then the anode compartment is rinsed with fresh alkali chloride solution until the content of chlorine of the oxidation state 0 and greater than 0 in the anolyte is in particular less than 10 ppm , then the temperature of the anolyte is lowered and then the anolyte is drained from the anode compartment and in a subsequent step, the initiation of the catholyte is stopped and the catholyte is discharged from the electrolyte gap. A method according to claim 6, characterized in that the concentration of the effluent anolyte, an alkali metal chloride content of 2.2 to 4.8 mol / l, preferably 3.5 to 4.6 mol / l, particularly preferably 4.0 to 4 , 6 mol / l. Method according to one of claims 6 and 7, characterized in that an electrolytic cell is used according to the principle of the falling film cell as the electrolytic cell. Method according to one of claims 6 to 8, characterized in that the electrolysis voltage after reaching a chlorine content in the anolyte of <10 mg / l, preferably <1 mg / l is turned off. Method according to one of claims 6 to 9, characterized in that until the end of the emptying and rinsing in the cathode space an overpressure relative to the anode space of> 10 mbar is maintained. Method according to one of claims 6 to 10, characterized in that after switching off and emptying of the electrolytic cell repeatedly at intervals of 1 to 12 weeks, preferably 4 to 8 weeks, the anode compartment with a dilute alkali chloride solution having a content of 2.2 to 4.8 mol / l and the cathode compartment is rinsed with an alkali hydroxide solution containing 4 to 10 mol / l. Method according to one of claims 6 to 11, characterized in that it is combined with a method according to one of claims 1 to 5. Method according to one of claims 1 to 12, characterized in that it is the alkali metal chloride is sodium chloride or potassium chloride, in particular sodium chloride.