EP0717791B1 - Electrolytic cell with multiple partial electrodes and at least one antipolar counter-electrode - Google Patents

Abstract

The invention pertains to an electrolytic cell with an electrolyte, two or more partial electrodes and at least one counter-electrode that is antipolar thereto. A different potential is applied across each of the individual partial electrodes (T1, T2, T3) - relative to the counter-electrode (15).

Description

The invention relates to an electrolytic cell according to Preamble of claim 1.

An electrolytic cell can be referred to as a chemical reactor be in the under supply of electrical Energy on the electrode surfaces chemical reactions take place.

As the current flows through the cell, the charge carriers become transported to the electrode surfaces where after passing through the double, not with the Electrode reactions the actual chemical turnover expires. The after the overall reaction ("cell reaction") converted amount of substance results according to the law of Faraday and is therefore directly from electrolysis time and Electricity dependent. The current density is also a Measure of the speed of the electrochemical reaction.

The maximum current load of a cell is determined by the Speed of the migration of the ions in the electric Field and the diffusion of the ions in the diffusion layer limited at the electrode. If you increase the Current density of an electrochemical cell steadily, so one finally reaches a point at which the reaction is controlled by diffusion, d. H. the speed the desired electrochemical reaction is limited by transport phenomena at the electrode and can by further increasing the electrode potential / the cell voltage is not increased much will. The maximum current density that can be achieved Limit current density called. This limit current density depends among other things from: concentration of the electroactive Species in the electrolyte, diffusion coefficient of electroactive species (starting materials, products), thickness of the Diffusion boundary layer, electrolyte circulation.

In direct technical electrolysis processes, attempts are made to offer the species to be converted in as large a concentration as possible in order to achieve high current densities. Accordingly, technical raw material production processes (e.g. chlorine-alkali electrolysis, chlorate production, hydrogen peroxide production, permanganate production, fluorine production) achieve current densities of 1 kA / m ² (100 mA / cm ² ) and higher.

In numerous electrolysis processes (e.g. indirect electrolytic oxidation / reduction, waste water treatment, various preparative areas of application) the electrodes used can only be operated with a relatively low current density (e.g. 0.5-1 mA / cm ² ). The reason for this lies in the limitation of the conversion rate on the electrode surface due to transport phenomena (diffusion-controlled implementation). Since the species to be converted is often present in a relatively low concentration, it is not possible to increase the concentrations due to the experimental working conditions / framework conditions, and the increase in the rate of turnover due to an increase in the working potential of the working electrodes in diffusion-controlled processes is no longer possible, an increase in space-time -The yield of an electrolytic cell can practically only be achieved in a constructive way. In the case of indirect electrochemical processes, it is often necessary to maintain a relatively narrow potential range, since otherwise undesirable side effects (water decomposition, side reactions) occur, which in the most favorable case only lead to a reduction in the current efficiency.

Because in most electrolysis processes the space-time yield only by reacting to one of the two Electrodes are limited, special measures are required Increase in electrode turnover usually only one of the two electrodes required, basically however possible on both electrodes.

Numerous constructive solutions to increase the space-time yield and to increase the effective electrode area are proposed in the specialist and patent literature been.

Starting from electrolytic cells with massive plate electrodes in various geometric arrangements (parallel, cylindrical, rolled; unmoved, flowed through Cells), were also cells with three-dimensional Electrodes proposed, these electrodes without electrolyte circulation, flowed through or through can be operated. Three-dimensional electrodes can e.g. B. from perforated sheets, sieves, nets, wires, Sintered plates are built up. The use of Bulk electrodes (fixed bed electrodes, fluidized bed electrodes) has been described.

The advantage of using three-dimensional electrodes lies in the significantly increased effective electrode area, because the (porous) material also up to a certain Electrode depth is electrochemically effective. These electrodes show at high electrolysis currents low effective bed depths due to the ohmic resistance in the electrolyte the voltage drop in solution is enlarged, so that only a small part of the used electrode the required potential level is achieved relative to the electrolyte.

If you increase the potential (voltage) of the electrode Increasing the activity of the more distant areas of the Electrode, so is the potential of the front Electrode areas raised too much, so there the undesirable side reactions already mentioned enter.

If a three-dimensional electrode therefore reaches the maximum effective bed depth which is characteristic of an electrode process, it is no longer possible to further increase the effective electrode area by simply increasing the electrode thickness. In the case of the example of the iron (III) triethanolamine complexes described in the specialist literature, the voltage drop in the electrolyte when using a porous cathode within the cathode used at the above-mentioned concentrations (6 g / l (1.18.10 ^-2 mol / l) Fe ₂ (SO ₄ ) ₃ .6H ₂ O, 34 g / l (0.22 mol / l) triethanolamine, 20 g / l (0.5 mol / l) sodium hydroxide solution) should not exceed 150 mV, otherwise either the front one (the anode facing) part of the cathode already enters the area of the electrolyte / mediator decomposition, or the rear (facing away from the anode) part of the porous electrode no longer has optimal electrochemical activity and therefore becomes ineffective or even becomes effective as a bipolar electrode, which leads to undesirable Side reactions can lead.

Breite des zulässigen Arbeitsplateaus der Grenzstromdichte definiert durch das eingesetzte elektrochemische System;

Leitfähigkeit des Elektrolyten;

Elektrolytanteil in der porösen Elektrode (ein hoher Anteil an leitfähigem Elektrolyt in der Elektrode erlaubt die Ausnutzung einer höheren Bettiefe der Elektrode, da durch den verringerten Ohmschen Widerstand des Elektrolyten der Elektrolytspannungsabfall innerhalb der Elektrode verringert wird);

Stromdichte im Elektrolyten (je nach Höhe der Strombelastung im Elektrolyten und dessen Leitfähigkeit ergibt sich ein Spannungsabfall, sodaß die optimale Elektrodenbettiefe auch von der Stromdichte im Elektrolyten abhängt). Im Falle des Eisen(III)-Triethanolamin-Komplexes 6g/l (1.18.10^-2 mol/l) Fe₂(SO₄)₃.6H₂O, 34 g/l (0.22 mol/l) Triethanolamin, 20 g/l (0.5 mol/l) Natronlauge beträgt der wünschenswerte obere Grenzwert des Spannungsabfalls innerhalb der Elektrode 150 mV, bei Verwendung von 3 g/l (5.09.10^-3 mol/l) Fe₂(SO₄)₃.6H₂O, 25 g/l (0.153 mol/l) Bicin (N,N-bis(2-hydroxyethyl)glycin), 20 g/l (0.5 mol/l)Natronlauge kann der Spannungsunterschied innerhalb der porösen Kathode 250 mV betragen;

Strömungsgeschwindigkeit des Elektrolyten durch die Elektrode;

und schließlich

Diffusionskoeffizient und Konzentration der umzusetzenden Spezies.

Basically, the possible depth of a three-dimensional electrode depends on various factors:

Width of the permissible working plateau of the limit current density defined by the electrochemical system used;

Conductivity of the electrolyte;

Proportion of electrolyte in the porous electrode (a high proportion of conductive electrolyte in the electrode allows the use of a greater bed depth of the electrode, since the reduced ohmic resistance of the electrolyte reduces the electrolyte voltage drop within the electrode);

Current density in the electrolyte (depending on the level of the current load in the electrolyte and its conductivity, there is a voltage drop, so that the optimal electrode bed depth also depends on the current density in the electrolyte). In the case of the iron (III) -triethanolamine complex 6g / l (1.18.10 ^-2 mol / l) Fe ₂ (SO ₄ ) ₃ .6H ₂ O, 34 g / l (0.22 mol / l) triethanolamine, 20 g / l (0.5 mol / l) sodium hydroxide solution, the desirable upper limit for the voltage drop within the electrode is 150 mV, when using 3 g / l (5.09.10 ^-3 mol / l) Fe ₂ (SO ₄ ) ₃ .6H ₂ O , 25 g / l (0.153 mol / l) bicine (N, N-bis (2-hydroxyethyl) glycine), 20 g / l (0.5 mol / l) sodium hydroxide solution, the voltage difference within the porous cathode can be 250 mV;

Flow rate of the electrolyte through the electrode;

and finally

Diffusion coefficient and concentration of the species to be converted.

An increase in system performance can accordingly with established procedural conditions after State of the art only by increasing the Electrode face (width, height) can be reached. This has a forced increase depending on the Process required diaphragm area and area the counter electrode. The electrolyte content of the This increases the electrolysis cell and its construction costs disadvantageous measure.

As WO-A-91/14026 shows, one can Increasing the effective electrode bed depth thereby achieve that instead of a three-dimensional Electrode two or more arranged one behind the other Sub-electrodes are provided, which with different Cell voltage (voltage relative to the counter electrode) operate. Raising cell voltage for sub-electrodes which are more distant from the counter electrode is only required to the extent that the voltage drop in the electrolyte to differences in operating conditions of the partial electrodes. So that leaves everyone despite the voltage drop in the electrolyte Partial electrode one - based on the adjacent Electrolytes - essentially the same voltage achieve so that optimal at each sub-electrode Electrolysis conditions prevail. So it is prevented that the voltage drop in the electrolyte is different between the anode-facing side of the cathode and back of the cathode based on the electrolyte outside of a favorable area, for example much higher than 150 mV (triethanolamine complexes) or 250 mV (bicin complexes) and the anode distant Sections of the cathode's effectiveness lose or become effective as bipolar electrodes or the parts near the anode the potential range of the Achieve electrolyte decomposition.

The partial electrodes are constructed so that the electrolyte can flow through them, in order to also provide the partial electrodes remote from the anode to enable electrolytic contact to the anode and to prevent the sub-electrodes near the anode in undesirable Act as bipolar electrodes.

The object of the invention is an electrolytic cell of the type mentioned Generic design with a high space-time yield easy to design.

According to the invention this is solved by the features of claim 1.

The thickness of the partial electrode can therefore depend on the Conductivity of the electrolyte, the existing current density in the electrolyte and the permissible voltage difference between the front and back of the flowable Optimal partial electrode selected.

Further advantages and details of the invention will be explained in more detail in the figure description below.

1 shows the schematic structure of an electrolysis cell according to the prior art, the Fig. 2nd shows the course of the current density depending on between the electrode and the adjacent electrolyte prevailing voltage, Fig. 3 shows schematically the structure of an embodiment of an inventive Electrolysis cell, Fig. 4 shows the Voltage curve in an electrolysis cell according to the invention with two partial electrodes, Fig. 5 shows one partially broken front view of an inventive Partial electrode, Fig. 6 shows a cross section through this sub-electrode, which show FIGS. 7 to 10 Exemplary embodiments for voltage or current supplies of the individual partial electrodes of an inventive Electrolysis cell, Fig. 11 shows the structure an alternative embodiment of a partial electrode from folded sieve fabrics, which Fig. 12 shows in a schematic embodiment for the dimensioning of sub-electrodes relevant sizes that Fig. 13 shows an alternative embodiment, at which the partial electrodes have different thicknesses.

Fig. 1 also shows schematically the voltage drop of the applied cell voltage in an electrolytic cell the prior art, which the various sub-steps assigned to the electrochemical reaction sequence is.

Basically, the total voltage required for an electrolysis process is divided into various ohmic voltage drops and voltage components for carrying out the electrode reaction. The voltage to be applied is the sum of all voltage drops during the operation of the electrochemical cell. (1) U _tot Cell voltage (electrode opposite the counter electrode) (2) U _K Ohmic voltage drop in the leads of the cathode including existing cover layers up to the electrode / electrolyte interface (3) U _A Ohmic voltage drop in the supply lines of the anode including existing cover layers up to the electrode / electrolyte interface (4) U _ZK thermodynamic decomposition voltages on the cathode to carry out the electrochemical reaction (5) U _ZA thermodynamic decomposition voltages at the anode to carry out the electrochemical reaction (6) U _PK Polarization and overvoltage in the diffusion boundary layer of the cathode (7) U _PA Polarization and overvoltage in the diffusion boundary layer of the anode (8th) U _EK Ohmic voltage drop of the electrolyte in the catholyte (9) U _EA Ohmic voltage drop of the electrolyte in the anolyte (10) U _D Ohmic voltage drop caused by the diaphragm

As already explained, exact compliance with the cathode potential (corresponding to U _ZK ) or anode potential (U _ZA ) measured in relation to the adjacent electrolyte is required in various electrochemical processes in order to ensure the selectivity of the reaction.

According to the prior art, iron (III) triethanolamine complexes in aqueous alkaline solution as a reducing agent that can be regenerated by electrochemical reduction use to reduce dyes.

2 shows an example of the potential range which can be used for the indirect electrochemical reduction of iron (III) -triethanolamine complexes in aqueous alkaline solution (the current density is plotted as a function of the potential difference or voltage between the electrode and the adjacent electrolyte). The concentrations in the electrolyte are: 6 g / l (1.18.10 ^-2 mol / l) Fe ₂ (SO ₄ ) ₃ .6H ₂ O, 34 g / l (0.22 mol / l) triethanolamine, 20 g / l (0.5 mol / l) sodium hydroxide solution. When using a flat electrode made of copper sheet, a diffusion limit current of 0.35-0.40 mA / cm ^{2 is} achieved with a liquor circulation of 145 ml / min and a cathode volume of approx. 100 ml (corresponding to 1.45 circulations / min.), Whereby the usable potential difference range is designated A in Fig. 2 and is, for example, 150 mV. If the desired working range A of approx. 150 mV width shown in FIG. 2 is left, the current density either drops below the diffusion limit current density or undesirable side reactions (iron deposition, hydrogen evolution) occur. Depending on the design of the electrolysis cell, the cell voltage in these experiments is approx. 2.2 V. If the concentration of Fe ₂ (SO ₄ ) ₃ .6H ₂ O is reduced as part of recipe optimization, the current density is further reduced to the extent that the concentration of iron (III) triethanolamine complex decreases.

As these comments show, is for the implementation of an electrochemical reaction Cell voltage to the electrochemical cell required for the actual implementation of the starting products but there is potential for a targeted synthesis the working electrode is relevant to the electrolyte.

Because of the current flow in the electrolyte This changes ohmic voltage drop Potential level in the electrolyte with the distance from the Counter electrode. Would you like to increase the space-time yield in Fig. 1 instead of the simple one Copper anode is a three-dimensional one through which the electrolyte can flow Insert the electrode all over the same Potential against the counter electrode (cathode) is located on the counter electrode (anode) facing front of the electrode due to the voltage drop a higher relative voltage in the electrolyte to the electrolyte than this to that of the counter electrode opposite rear side of the three-dimensional Electrode would be the case. Because the voltage drop in the electrolyte and on the other hand the relative voltage of the respective electrode area relative to the currently adjacent electrolyte is decisive for the chemical implementation, can with a shared potential three-dimensional electrode always only make a compromise: When the voltage is on relative to the electrolyte the front of the electrode in the desired work area then lies due to the voltage drop the back of the electrode already in the electrolyte in a low voltage area relative to the Electrolytes, so that the implementation there is no longer optimal or does not occur at all. On the other hand, do you want that the electrode on the back is correct in terms of voltage relative to the electrolyte, it automatically results that the front is too high in terms of voltage is relative to the electrolyte and therefore undesirable Side effects occur. So it can be done with a such three-dimensional potentials Electrode through which the electrolyte can flow in general not achieve that both the rear and also the front of the electrode in a desired one Working range of the voltage relative to the electrolyte (for example of 150 mV as shown in FIG. 2 with "A") lies. The situation is of course all the more critical the smaller this approved work area the voltage is relative to the adjacent electrolyte.

In order to avoid this problem, the invention now essentially proposes, instead of a large three-dimensional electrode which is operated everywhere with the same cell voltage U _ges , to provide a plurality of sub-electrodes which each have a different potential with respect to the counter-electrode and the thickness of the sub-electrodes 1 to vary.

3 shows an exemplary embodiment of an electrolytic cell according to the invention with a counter electrode 15 (cathode) and three mutually spaced equipolar sub-electrodes (anodes) T ₁ , T ₂ , T ₃ , which according to the invention are at different potentials and through which preferably the same currents I1, I2 and I3 flow. The electrolyte is circulated by an external pump 14. The strength of the electrolyte circulation required depends, for example, on the conversion in the electrolyte (catholyte 12, anolyte 13, diaphragm 11) and has the aim of ensuring that the concentration ratios within the electrolysis cell are largely homogeneous.

4 shows the voltage curve in an electrolytic cell _according to the invention with two sub-electrodes T ₁ and T ₂ , which are at different cell voltages U _GES.1 and U _GES.2 compared to the counter electrode (cathode 15). Otherwise, the numerals on the right in FIG. 4 denote the same types of voltage drop as in FIG. 1, with the proviso that the subscript refers to the first partial electrode T ₁ or T ₂ , the second partial electrode T ₂ . The ohmic voltage drop in the electrolyte inside the flowable electrode T _{1 is} identified by 21. The voltage drop in the electrolyte 9 ₂ + 21 is compensated for by a higher potential at the partial electrode T ₂ compared to the partial electrode T ₁ (measured in each case with respect to the cathode 15). This ensures that the electrochemically relevant voltage difference between the electrode surface and the adjacent electrolyte is essentially the same in spite of the different potential position of the electrolyte in the partial electrode T ₁ , T ₂ at these two partial electrodes T ₁ , T ₂ .

The thickness d (cf. FIG. 12) of the sub-electrodes T ₁ , T ₂ , T ₃ etc. will advantageously be chosen so that the potential difference between the sub-electrode and the adjacent electrolyte both on the front of the sub-electrode facing the counter electrode and on that of the Rear side of the partial electrode facing away from the counter electrode lies in the same predeterminable potential difference range. The predeterminable potential difference range with the width A (see FIG. 2) lies between two potential difference values V ₁ , V ₂ . The upper potential difference value V ₂ is determined by the fact that it does not yet have any side reactions, in particular hydrogen evolution. Such side reactions occur for potential difference values above the value V ₂ and operation in this potential range is therefore undesirable. The lower potential difference value V ₁ is determined by the fact that the desired reaction diffusion-controlled (ie essentially independent of the potential) takes place only from this point on. In the area between the two potential difference values V ₁ , V ₂ , the reaction thus runs in the so-called diffusion-controlled area. For each electrolysis system, the values V ₁ , V ₂ and thus their difference V ₂ - V _{1 are} known or can be easily determined.

If you now make the sub-electrodes thin, so their thickness d in relation to the lateral dimensions a, b (cf. FIG. 12) is vanishingly small (so-called two-dimensional electrode), the above Condition that namely the potential difference between the partial electrode and the electrolyte both on the front as well as the back of the electrode is in a given range, easily meet because this potential difference at the front and Practical on the back for two-dimensional electrodes is the same size. However, the invention now provides for the finite To use width A of the potential difference range, around the sub-electrodes with a finite thickness (so-called three-dimensional electrode) can and still adhere to the condition that the Potential difference on the front and on the back of the partial electrode measured to the adjacent one Electrolyte in the specified potential difference range lies. Such a finite thickness allows under a larger effective electrode area (Surface in contact with the electrolyte stands) because the electrolyte also inside, for example porous or made of folded sieve fabrics Partial electrode finds an electrode surface.

However, the partial electrodes must not be made as thick as desired, since one would end up with the original problem that either hydrogen evolution already starts at the front of the partial electrode while the back works normally or the front works normally and the back is essentially ineffective. The invention therefore proposes in a preferred embodiment that the thickness d of the partial electrodes T ₁ , T ₂ , T _{3 is} dimensioned, that the voltage drop in the electrolyte over the thickness of the respective partial electrode is between 50% and 100%, preferably between 60% and 90% of the width A of the predeterminable potential difference range A is.

The dimensioning of the electrode thickness is therefore as follows: First, the values V ₁ , V ₂ (e.g. V ₁ = -1150 mV, V ₂ = -900 mV) are determined (for example from tables or other literature) Potential differences between the electrode and the adjacent electrolyte, between which the desired reaction takes place without side reactions. By forming the difference and the amount A = | V ₂ - V ₁ | the width of the potential difference range is obtained in which the reaction proceeds satisfactorily (e.g. A = 250 mV). On the other hand, the electrolysis cell to be dimensioned knows the current flow (e.g. I = 2A) through the electrolyte in the area of the partial electrode to be dimensioned. If the specific resistance of the electrolyte is also known (e.g. ρ = 8 Ωcm), it is now easy to determine the voltage drop U in the electrolyte per unit length l (measured in the direction of the imaginary connecting line against the electrode partial electrode) (U / 1 = RI / 1 = ρ.I / F = 40 mV / cm), where F (e.g. 400 cm ² ) is the visible surface of the partial electrode. One can now easily calculate the path length l in the electrolyte that is causing a voltage U to drop that corresponds to the width A of the potential difference range (e.g. from U / l = 40 mV = A / l with A = 250 mV it follows that l = 6 , 25 cm). The teaching according to the invention in the preferred exemplary embodiment now consists in taking 50 to 100%, preferably 60 to 90% of this path length (for example l = 6.25 cm) and the thickness d of the partial electrodes with the value obtained in this way dimension (preferably d = 3.75 cm to 5.625 cm in the selected example).

This ensures a high efficiency of the electrodes, on the other hand also that at the front and Back of the electrode for the reaction essentially the same conditions prevail, so that not side reactions already occur during the front the back works normally or at normal working front the back no longer works. In practice, the thickness values of the partial electrodes conveniently often greater than 1 cm, preferably are larger than 5 cm.

Since, in the construction according to the invention with several partial electrodes, the current flow around the area of the individual electrodes is different (increasing current flow, the closer the partial electrode is to the counter electrode), the voltage drop per unit length in the electrolyte is also different in the area of the individual electrodes. If this is taken into account in the dimensioning rule for the thickness of the electrodes mentioned above, a preferred embodiment is obtained in which the thickness of the partial electrodes T ₁ , T ₂ , T ₃ etc. increases with increasing distance from the counter electrode 15. In other words, in one and the same electrolytic cell, partial electrodes through which less current flows (that is, which are further away from the counterelectrode) can be built up thicker without any undesirable side effects. This allows the efficiency, in particular the effectively available electrode surface, to be increased still further.

An exemplary embodiment with partial electrodes, the thickness d ₁ to d ₄ of which increases with the distance from the counter electrode 15, is shown in FIG. 13.

In the case of a normal two-dimensional electrode, for example a plate, the effective partial electrode surface which is in contact with the electrolyte is essentially 2F, where F is the visible surface in the direction of the imaginary connecting line between the partial electrode and the counter electrode (see FIG. 12). In the case of three-dimensional electrodes, there is the possibility that the electrolyte also comes into contact with the electrode surfaces there, for example porous electrodes are possible. However, it is also possible to provide a plurality of layers of electrically conductive material, for example a plurality of folded layers of metallic sieve fabrics are suitable, as are shown schematically in FIG. 11. In reality, the individual folded layers of the screen fabrics will lie closer together than in FIG. 11. One or more layers can be led upwards out of the partial electrode T ₁ in order to form a connecting lug 19. The layers of the folded screen fabric or fabrics can be connected to one another, for example, along the lines X ₁ , X ₂ , preferably by gluing or soldering. Such an electrode can be produced inexpensively on inexpensive starting materials and, in relation to the face F with a relatively small thickness d, has a high effective partial electrode surface which is in contact with the electrolyte (namely the sum of the surface of all the wires from which the screen fabric is formed). . The value of at least five times greater effective surface area than view surface F, which is advantageous according to the invention, can thus be achieved.

A further advantageous dimensioning of the electrodes is that the effective partial electrode surface divided by the volume V is at least 100 dm ^-1 . In other words, an electrode with a volume of 1 l then has at least an effective surface area of 1 m ² .

Especially in reduction experiments with iron (III) triethanolamine complexes 6 g / l (1.18.10 ^-2 mol / l) Fe ₂ (SO ₄ ) ₃ .6H ₂ O, 34 g / l (0.22 mol / l) triethanolamine, 20 g / l (0.5 mol / l) caustic soda, wound partial electrodes made of copper wire have proven to be particularly suitable. 5 and 6 show the structure of such a partial electrode in section and in the front view. These electrodes are produced by fastening (wrapping around) copper wire strands, each consisting of a plurality of individual wires 20, on a rectangular metal frame 18 (15 strands of Cu wire each with a diameter of 0.1 mm and a strand length of 260 m). The effective area (total surface area of the copper wires) of a partial electrode is approx. 1m ² . The thickness of such an electrode is approximately 3-4 mm. A change in the electrode thickness when flowing through is prevented by a plastic wire grid 17 attached to the outside of the electrode. Instead of copper wire, other materials can also be used, restrictions mainly result from the material resistance of the electrode material. By soldering the wire windings on the top of the electrode, there is a uniform current supply 19 to the entire electrode surface.

The electrode construction should be designed so that a uniform potential drop in the electrolyte is achieved is to make optimal use of the electrode surfaces to ensure. The three-dimensional Depending on the application, electrodes can be of various types Materials are manufactured. The structure can from electrically conductive sieve fabrics, wires, sintered plates or other porous constructions. Also the use of fixed bed / fluidized bed electrodes is possible.

The counter electrode can be used as a common electrode for all partial electrodes are executed. In contrast to the usual, known electrode arrangements through this shared power supply the three-dimensional Working electrodes also include an electrode that is behind a similar working electrode is arranged with operated with the same effectiveness. The loss of Effectiveness, which is due to the ohmic resistance in the Electrolytes can be produced, so according to the invention by slightly increasing the amount invested Cell voltage of the more distant electrodes can be avoided.

The power supply can be from a common power supply unit operated from the each electrode with its own control device (e.g. Constant current control) with the nominal current becomes. 7 to 10 show possible structures of a such electrolytic cell including electrical Wiring diagram. The regulation of the cell voltage of the partial electrodes 16 can according to the representations separate power supply units (Fig. 7), by Ballast resistors (Fig. 8), by a common Power supply with adjustable sub-distributors (Fig. 9), by additional voltage supplies (Fig. 10).

Extensive tests showed that 10 electrodes in a row with the same electrochemical turnover (For example, all of those shown in FIG. 2 Work area) can be operated. In principle, of course, is also a higher or lower Number of partial electrodes possible.

It has to equalize the electrode reactions proved to be favorable to circulate the electrolyte, which is not absolutely necessary. The installation of a Diaphragm is essential in those cases the anolyte and catholyte do not mix should. For the basic functionality of the electrochemical cell, however, this is not a necessary one Condition.

With an optimized design, a large electrode area can be realized with a small space requirement (approx. 0.5 l system volume / m ² electrode area).

Basically, a common counter electrode Electrode are used. In appropriate areas of application is also a similar subdivision the counter electrode in partial electrodes with their own power supply possible.

• indirekte elektrochemische Reduktion/Oxidation (z.B. von Indigodispersionen, Küpenfarbstoffen, Azofarbstoffen) mit Hilfe von Eisenkomplexsalzen
• direkte elektrochemische Reaktion von Azofarbstoffen und anderen Textilfarbstoffen zur Abwasserentfärbung
• direkte Reduktion von löslichen Azofarbstoffen
• Cyanidbeseitigung in Galvanikabwässern
• Sulfitoxidation in Prozeß- und Abwässern
• Schwermetallabreicherung in Abwässern
• präparative Aufgabenstellungen in der anorganischen/organischen Chemie
• Wasserzersetzung zur Bildung von Wasserstoff/Sauerstoff

A cell that works according to the features described above can be used advantageously for a wide variety of purposes, in particular

• indirect electrochemical reduction / oxidation (eg of indigo dispersions, vat dyes, azo dyes) with the help of iron complex salts
• direct electrochemical reaction of azo dyes and other textile dyes for wastewater decolorization
• direct reduction of soluble azo dyes
• Cyanide removal in electroplating waste water
• Sulfite oxidation in process and waste water
• Heavy metal depletion in waste water
• preparative tasks in inorganic / organic chemistry
• Water decomposition to form hydrogen / oxygen

Application example 1: Indirect electrochemical reduction of an indigo dispersion

An electrolysis cell for the indirect electrochemical reduction of dispersed indigo with the aid of iron-triethanolamine complexes in an aqueous alkaline medium has two electrode spaces separated by a sound diaphragm with a square face area of 400 cm ² . In the anode compartment (content approx. 2 l) there is a technically common anode, such as that used for B. is used for water electrolysis or chlorine-alkali electrolysis. 10 cathodes with separate power supplies are installed in parallel in the cathode compartment. Design features (electrode fixation, intermediate layer made of non-conductive material) prevent contact between the partial electrodes. Each electrode is constructed as a wire electrode (15 strands of copper wire each with a diameter of 0.1 mm and a strand length of 260 m, the effective area of a partial electrode is approx. 1 m ² ). The partial cathodes are easy to manufacture by means of wound copper wires. By connecting 10 electrodes in series, an effective total area of 10 m ² can be achieved, the view area to the diaphragm / anode space being only 400 cm ² .

A flow occurs with the help of a circulation pump the partial electrodes and a thorough mixing of the cathode space, so that differences in concentration in the cathode compartment be avoided. The performance of the circulation pump is 14-15 l / min, so that the one located in the cathode compartment Pumped around 10 liters in less than 1 min becomes. The cell is powered by a Main supply unit reached, the scheme the supply currents of the sub-electrodes through downstream Partial flow controller takes place (Fig. 9.).

When using a mediator consisting of 2 g / l Fe ₂ (SO) ₃ .6H ₂ O, 8 g / l triethanolamine and 6.5 g / l caustic soda as well as a concentration of 5 g / l indigo powder, current densities are due to investigations with plate electrodes 0.1 mA / cm ² achievable. When using this mediator in the electrolytic cell with divided cathodes and a total area of 10 m ² , total currents of 10 A can be realized (1 A per partial electrode), which confirms the effectiveness of the electrodes arranged one behind the other. The cell voltages with respect to the counter electrode (anode) differ depending on the electrode position and increase with a total current of 9 A from the electrode closest to the anode to the most distant 10th electrode as follows: Electrode no. 1 2nd 3rd 4th 5 6 7 8th 9 10th Cell voltage (V) 14.0 16.1 18.0 19.4 20.8 21.7 22.6 23.1 23.6 23.8

The different necessary for the operation Cell voltages are clearly visible. The tensions on the individual sub-electrodes relative to the adjacent one However, electrolytes are due to the voltage drop essentially the same in the electrolyte so that essentially the same electrochemical on all partial electrodes Conditions prevail. After an electrolysis period of 2.5 hours is the indigo dye in the total batch volume of 15.85 l completely reduced.

Example of use 2: Production of a reduced iron (III) triethanolamine complex

In the electrolysis cell described in application example 1, instead of the 10 cathodes described in example 1, there are only two wire electrodes which are 10 cm apart. The solution used contains the following chemicals: 2 g / l Fe ₂ (SO ₃ ) .6H ₂ O, 8 g / l triethanolamine and 6.5 g / l caustic soda. The cell voltages or voltage drops present in the system at a total current of 2 A (1 A per partial electrode) are shown in FIG. 4, which has already been described. When the more distant cathode has a current load of 1 A between the anode nearer the electrode and the more distant electrode, an ohmic voltage drop of approx. 1.5 V occurs in the electrolyte, which is compensated for by an increase in the cell voltage of the more distant electrode, so that both electrodes have the same electrochemical effectiveness demonstrate.

The formation of the iron (II) complexes by electrochemical Reduction takes place according to Faraday's Law with an electricity yield of over 50%.

Example of use 3: Heavy metal removal from an aqueous solution

Approx. 9 l of a solution of 0.36 g / l CuSO ₄ .5H ₂ O (0.0015 mol / l, 92.8 ppm Cu) and 9.1 g / l (0.028 mol / l) Na ₂ SO ₄ .10H ₂ O as well as 3 ml / l Concentrated acetic acid is electrolyzed in an electrolysis cell with three electrodes of the type described above (area 1 m ² each) at a total current of initially 0.56 A (approx. 0.19 A per electrode), the Cu being deposited on the cathodes. The working potential of the cathodes increases from -31 mV to -680 mV (reference electrode Ag / AgCl, 3M KCl). The electrolyte circulation is 14-15 l / min, the cell voltage is between 4.99 and 5.51 V depending on the electrode. The decrease in the Cu concentration after certain time intervals is described in the following table: Time [min] 0 60 120 180 240 Cu concentration [ppm] 98.3 57.0 24.6 6.7 0.22

Example of use 4: Decolorization of solutions from dyes

A solution of 1 g / l Remazolschwarz B (Hoechst), 2.2 g / l (0.055 mol / l) NaOH and 9.1 g / l (0.028 mol / l) Na ₂ SO ₄ .10H ₂ O are in an electrolytic cell with three electrodes electrolyzed according to the type described above (area 1 m ² each) with a total current of 0.85 A (0.28 A per electrode). The electrolyte circulation is 14-15 l / min, the cell voltage is 4.1 to 4.65 V at the beginning. The working potential of the cathodes increases from -563 mV to -912 mV (reference electrode Ag / AgCl, 3M KCl). The dye solution is diluted with water and the destruction of the dye is determined by measuring the absorbance at 600 nm. The decrease in the color of the solution due to destruction of the dye after certain time intervals is described in the following table: Time [min] 0 15 45 60 90 Absorbance (at 600 nm) 1,938 1,788 1,138 1,015 0.719

The invention can generally be used for the deposition of Materials, especially heavy metals from preferably use aqueous solutions, the partial electrodes, preferably consist of a different material than the material to be separated from the solution. The The partial electrodes can then be cleaned, for example by chemical detachment or simply by changing the polarity Partial and counter electrodes are made.

On the other hand, the one according to the invention is suitable Electrolysis cell also particularly suitable for carrying out Reduction and / or oxidation reactions in which the reactants remain in the dissolved system (so-called mediator technology). So it happens with these Reactions no deposition of material on the Electrodes, which are also used for fine-pored or Do not clog the fine-mesh version. It can then be sub-electrodes with great effectiveness Surface and therefore high efficiency with a long service life be used.

Claims (24)

1. Electrolytic cell with an electrolyte, having two or more electrically conductive partial electrodes through which can flow electrolytes, and at least one counterelectrode antipolar with respect thereto, the individual partial electrodes (T₁, T₂, T₃), in each case with respect to the counterelectrode (15), are at a different potential, characterized in that the thickness (d) of the partial electrodes (T₁, T₂, T₃) is such that the potential difference between the partial electrode and the adjacent electrolyte, both at the front of the partial electrode facing the counterelectrode and at the back of the partial electrode remote from the counterelectrode is in the same predeterminable potential difference range (between V₁ and V₂), the predeterminable potential difference range being between a lower potential difference value (V₁) (measured between the partial electrode and the adjacent electrolyte) and an upper potential difference value (V₂) (measured between the partial electrode and adjacent electrolyte), the upper potential difference value (V₂) being determined in that at the latter no secondary reactions (particularly hydrogen evolution) take place, the lower potential difference value (V₁) being determined in that as from the latter value the desired reaction takes place in diffusion-controlled manner, and that the thickness (d) of the partial electrodes (T₁, T₂, T₃) is such that the voltage drop in the electrolyte is between 50 and 100% of the width of the predeterminable potential difference range (A) over the thickness of the particular partial electrode.
2. Electrolytic cell according to claim 1, characterized in that when using iron (III) triethanolamine complexes 6 g/l (1.18.10^-2 mole/l) Fe₂(SO₄)₃. 6H₂O, 34 g/l (0.22 mole/l) triethanolamine, 20 g/l (0.5 mole/l) caustic soda solution as the electrolyte, the width (A) of the predeterminable potential difference range is 150 mV.
3. Electrolytic cell according to claim 1, characterized in that when using 3 g/l (5.09.10^-3 mole/l) Fe₂(SO₄)₃.6H₂O, 25 g/l (0.153 mole/l) bicine (N,N-bis(2-hydroxyethyl)glycine), 20 g/l (0.5 mole/l) caustic soda solution as the electrolyte, the width of the predeterminable potential difference range is 250 mV.
4. Electrolytic cell according to claim 1, characterized in that when applying a process for the reduction of dispersed indigo dye, the width of the predeterminable potential difference range within a partial electrode when using iron (III) triethanolamine complexes (6 g/l (1.18.10^-2 mole/l) Fe₂(SO₄)₃.6H₂O, 34 g/l (0.22 mole/l) triethanolamine, 20 g/l (0.5 mole/l) caustic soda solution as the electrolyte is 350 mV.
5. Electrolytic cell according to one of the claims 1 to 4, characterized in that the thickness (d) of the partial electrodes (T₁, T₂, T₃) is such that the voltage drop in the electrolyte is between 60 and 90% of the width of the predeterminable potential difference range (A) of the thickness of the particular partial electrode.
6. Electrolytic cell according to one of the claims 1 to 5, characterized in that the electrolyte-permeable partial electrodes (T₁, T₂, T₃) has a thickness of more than 1 mm, preferably more than 5 mm, measured in the direction of the partial electrode-counterelectrode connecting line.
7. Electrolytic cell according to one of the claims 1 to 6, characterized in that the liquid-permeable partial electrodes (T₁, T₂, T₃) are at least partly constructed from conductive gauzes, wires or sintered plates.
8. Electrolytic cell according to claim 7, characterized in that at least one partial electrode (T₁, T₂, T₃) has a metal frame, preferably rectangle-positioned profile frames, around which the wires are wound individually or in prefabricated wire bundles.
9. Electrolytic cell according to one of the claims 1 to 8, characterized in that the partial electrodes (T₁, T₂, T₃) are formed from two or more electroconductive layers, particularly folded out gauzes.
10. Electrolytic cell according to one of the claims 1 to 9, characterized in that the partial electrodes (T , T₂, T₃) are at least partly made from electrically conductive, porous material.
11. Electrolytic cell according to one of the claims 1 to 10, characterized in that the partial electrodes (T₁, T₂, T₃) are at least partly constructed from bulk material electrodes, fixed bed electrodes or fluidized bed electrodes.
12. Electrolytic cell according to one of the claims 1 to 11, characterized in that the surface of the partial electrodes (T₁, T₂, T₃) is provided with a mesh of electrically insulating material, preferably a plastic wire mesh.
13. Electrolytic cell according to one of the claims 1 to 12, characterized in that the partial electrodes (T₁, - T₄) of an electrolytic cell have different thicknesses (d₁ to d₄).
14. Electrolytic cell according to claim 13, characterized in that the thickness (d₁ - d₄) of the partial electrodes (T₁ - T₄) increases with increasing distance from the counterelectrode.
15. Electrolytic cell according to one of the claims 1 to 14, characterized in that the effective partial electrode surface in contact with the electrolyte is at least 5 times and preferably at least 10 times larger than the viewing surface (F) on the partial electrode.
16. Electrolytic cell according to one of the claims 1 to 15, characterized in that the effective partial electrode surface, divided by the volume, is at least 100 dm^-1.
17. Electrolytic cell according to one of the claims 1 to 16, characterized in that separate power supply units (fig. 7) are provided for the split power supply of the individual partial electrodes (T₁, T₂, T₃).
18. Electrolytic cell according to one of the claims 1 to 16, characterized in that a common voltage source is provided for all the partial electrodes and between said source and the partial electrodes are incorporated different additional voltage sources (fig. 10).
19. Use of an electrolytic cell with an electrolyte, with two or more partial electrodes and at least one counterelectrode antipolar thereto, wherein the individual partial electrodes (T₁, T₂, T₃) in each case with respect to the counterelectrode (15), are at different potential, according to one of the claims 1 to 18, for producing iron (II) complexes.
20. Use of an electrolytic cell with an electrolyte, with two or more partial electrodes and at least one counterelectrode antipolar thereto, wherein the individual partial electrodes (T₁, T₂, T₃) in each case with respect to the counterelectrode (15), are at different potential, according to one of the claims 1 to 18, for the indirect, electrochemical reduction of dispersed or dissolved dyes.
21. Use of an electrolytic cell with an electrolyte, with two or more partial electrodes and at least one counterelectrode antipolar thereto, wherein the individual partial electrodes (T₁, T₂, T₃) in each case with respect to the counterelectrode (15), are at different potential, according to one of the claims 1 to 18, for the separation of materials, heavy metals from preferably aqueous solutions, wherein the partial electrodes are preferably made from a different material to that to be separated from the solution.
22. Use according to claim 21, characterized in that the cleaning of the partial electrodes takes place by chemical dissolving or polarity reversal of the counterelectrode and partial electrodes.
23. Use of an electrolytic cell with an electrolyte, with two or more partial electrodes and at least one counterelectrode antipolar thereto, wherein the individual partial electrodes (T₁, T₂, T₃), in each case with respect to the counterelectrode (15), are at different potentials, according to one of the claims 1 to 18, for electrochemical decolourizing of waste waters.
24. Use of an electrolytic cell with an electrolyte, with two or more partial electrodes and at least one counterelectrode antipolar thereto, wherein the individual partial electrodes (T₁, T₂, T₃), in each case with respect to the counterelectrode (15), are at different potentials, according to one of the claims 1 to 18, for performing reduction and/or oxidation reactions, in which the reactants remain in the dissolved system.

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