EP2401422A2

EP2401422A2 - Method and device for regenerating peroxodisulfate pickling solution

Info

Publication number: EP2401422A2
Application number: EP10722522A
Authority: EP
Inventors: Wolfgang Thiele; Hans-Jürgen FÖRSTER; Gerd Heinze
Original assignee: Eilenburger Elektrolyse- und Umwelttechnik GmbH; Eilenburger Elecktrolyse- und Umwelttechnik GmbH
Current assignee: Eilenburger Elektrolyse- und Umwelttechnik GmbH; Eilenburger Elecktrolyse- und Umwelttechnik GmbH
Priority date: 2009-01-09
Filing date: 2010-01-07
Publication date: 2012-01-04
Also published as: WO2010078866A2; DE102009004155A1; WO2010078866A3

Abstract

The aim of the invention is to regenerate peroxodisulfate pickling solutions for copper materials exhausted in a circulatory method in a divided recycling electrolysis cell, in that the dissolved copper is first separated at the cathode and then the caustic peroxodisulfate is reoxidized at the anode. According to the invention, a pickling solution circulating past the pickling bath and the recycling electrolysis cell is used, said solution having excess sulfate and free sulfuric acid having a total sulfate content of 1 to 3 mol/l, in addition to 0.2 to 1.0 mol/l of peroxodisulfate, wherein the dissolved copper is reclaimed from the exhausted pickling solution in the recycling electrolysis cell at planar cathodes having cathode current densities of 4 to 7 A/dm^2.

Description

Process and apparatus for regenerating peroxodisulfate pickling solutions

The invention relates to a method and an electrolysis cell for regenerating pickling solutions containing peroxosulphate for copper and copper materials, on the one hand, the redeemed copper recovered, on the other hand, the mordant Peroxodisul- fat and / or Peroxomonosulfat can be regenerated. The term pickling is to be understood below all other based on similar principles chemically abrasive surface treatment methods, such as firing, etching, glazing, demetallizing, deburring u. a. When demetallizing z. For example, the dissolution of layers consisting of copper and nickel by means of peroxosulfate demetallization solutions is in principle equivalent to the pickling of copper-nickel alloys. The new regeneration process is intended to make it easy to pickle copper and copper alloys in a closed loop process, to completely or partially regenerate the etch chemicals, and to recover the redeemed copper and possibly other alloying constituents. As a result, the recovered metals can be reused, the consumption of pickling chemicals drastically reduced and the accumulation of exhausted pickling solutions that can be disposed of only with great effort, be completely avoided.

The prior art in persulfate recycling pickling processes for copper materials, especially in printed circuit board manufacturing, is determined by the use of platinum anodes with smooth shiny surfaces to regenerate the mordant peroxodisulfate. Only with this it has been possible to achieve a sufficient oxygen overvoltage by means of specially adapted electrolyte compositions and electrolysis conditions, whereby the regeneration of the peroxodisulfate has become possible in the first place.

Only recently has it also been possible to produce peroxodisulfuric acid and its salts using anodes which have been coated with diamond (DE 199 48 184, DE 100 19 393). Anodes from the valve metal niobium have proven particularly suitable, equipped with a 3 to 10 μm thick diamond layer, wherein the diamond was rendered sufficiently conductive by doping, preferably with boron. Both inventions are concerned exclusively with the production of peroxodisulfuric acid and of peroxodisulfates of the alkali metals and of ammonium. The use of diamond-coated anodes has also been reported for regeneration proposed by Peroxodisulfatlösungen z. However, such diamond-coated anodes were provided only as an alternative to the platinum anodes, but without adapting the entire persulfate recycling process as a combination of metal recovery and Persulfatregenerierung the specific features of the use of diamond-coated anodes. The options for process rationalization resulting from the replacement of anodes made of smooth platinum by diamond-coated anodes in the persulfate recycling process have not yet been deliberately applied and, for the most part, not yet recognized at all.

On smooth platinum anodes, the peroxodisulfate regeneration takes place at high anodic current densities of 40 to 70 A / dm ² and a high total sulfate content (sulfuric acid / sulfate concentrations) of preferably> 3 mol / l and with additional use of potential-enhancing additives. Since only maximum current densities of 1, 5 to 2.5 A / dm ^{2 are} possible to recover the copper from the spent pickling solutions in a compact, firmly adhering form, using conventional, simple plate electrolysis cells, there is a required current density ratio for the Anodic Persulfate Formation for Cathodic Copper Deposition from 20 to 30. These extremely different requirements for both electrode reactions are the main reason why it has not been possible to combine both electrode processes in a simply constructed electrolysis cell divided by cation exchange membranes.

So far proposed and partially already in pilot plants technically realized persulfate recycling technologies (DD 211 129, DE 41 37 022, DE 195 06 832) therefore work with separate Vorentkupferungszellen in which deposited the bulk of the copper with the required low current densities in a compact form while regeneration of the peroxodisulfate occurs in a downstream split persulfate recycling cell. In this case, the remaining copper still contained in the largely decuperated pickling solution is deposited in the persulfate recycling cell as a result of the too high cathodic current densities in powder form. Special devices have been developed to discharge the copper powder separating in the cathode compartments of the regeneration cell with the two-phase flow gas-liquid (hydrogen-catholyte) from the cathode compartments while maintaining specific electrolysis and flow conditions and periodically using peroxosulphate. releasing the spent pickling solution (see also Metal Surface 52, 1999, H. 11). The highly active copper powder also reacts with the residual peroxosulphate so that the electrolyte passing into the anode compartment is virtually free of peroxymonosulfate formed by hydrolysis. This is also necessary because in the case of platinum anodes, a residual amount of peroxomonosulfate has a depolarizing effect on the anode potential, as a result of which the current efficiency is markedly reduced.

However, the required high expenditure on equipment for two separate electrolysis cells and the associated increased electric energy consumption had a negative impact on the profitability of this recycling process.

Alternatively, special electrolysis cells have been proposed in which the mass transport to the cathode is increased by special, usually costly measures so strong that copper with higher current density deposited in a compact form and thereby the discrepancy between the cathodic and anodic current density can be reduced.

In DE 101 12 075, such a special split electrolytic cell with rotating cathode and the use of pulsating cathodic currents to increase the mass transfer to the cathode has been proposed. This made it possible for the first time to increase the cathodic current density required for compact copper deposition to such an extent that a combination of cathodic copper deposition and anodic persulfate regeneration could be realized in only one cell while at the same time minimizing the anode area. For this purpose, it was also necessary to arrange the anodes in small-volume, separate anode chambers in order to realize the high current concentrations required for a sufficient current yield. Only in this way was it possible to keep the residence time in the anode chambers until the final peroxodisulfate concentration reached so low that it would not be possible to form peroxomonosulfate by hydrolysis and thus to reduce the current yield by the anode depolarization. Due to the greatly increased mass transport to the cathode by the rotating cathode and the pulsating cathodic currents was also achieved that all residual peroxosulphate is reduced at the cathode. This does not lead to a depolarization of the platinum anode by the peroxymonosulfate formed by hydrolysis during the pickling process. A pilot plant operating according to this process was successfully tested for printed circuit board production (see Galvanotechnik, (2002) pp. 1983-1991). A disadvantage was the relatively high expenditure on equipment for the rotating cathode and the required division of the anode compartment into a larger number of small-volume anode pockets, which in conjunction with the relatively low current capacity of a single cell with a rotating cathode to a high equipment cost for a complete technical Recycling plant led.

The presently achieved technical status of the persulfate recycling process is therefore characterized by the fact that both the variant of a regeneration in separate cells, as well as the variant for regeneration in a special cell with rotating cathode and pulsating cathodic currents require a relatively large amount of equipment, whereby the efficiency of the process was severely impaired. Therefore, these recycling process variants have not been able to prevail extensively in business practice until today.

To make matters worse, that the acceptance of these based on the use of platinum anodes process is greatly limited by the PCB manufacturers in that the composition of the pickling solutions had to be considerably changed in some cases and adapted to the requirements of Persulfatregeneration in order to achieve a sufficiently high current efficiency. In particular, the need to use such potential increasing additives as water pollutants, such as thiocyanate and / or thiourea, which also partially form toxic gaseous decomposition products (eg hydrogen cyanide or cyanogen) in the pickling recycling process, has led to legitimate concerns about possible risks in terms of environmental compatibility and quality assurance in printed circuit board production.

The problem to be solved by the invention therefore consists in avoiding the disadvantages of the known persulfate recycling methods and by the combination of both electrolysis processes in a comparatively simply constructed recycling electrolysis cell with planar cathodes, dispensing with the use of rotation Cathodes significantly reduce the equipment required and thereby significantly improve the efficiency of the recycling process.

This problem has been solved according to the invention by the recycling pickling process shown in claims 1 to 15 using the persulfate recycling electrolytic cell described in claims 16 to 23 in a surprisingly simple manner. This was achieved by the use of diamond-coated anodes with targeted use of their specific properties for the further development of the persulfate recycling pickling process.

The most important basis for this was the surprising finding that, in contrast to the platinum anodes, these diamond-coated anodes do not depolarize the anode in the presence of peroxomonosulphate formed by hydrolysis, thereby reducing the current yield of the peroxodisulphate regeneration by residual amounts Peroxomonosulfate can be avoided. This also eliminated the complete destruction of the peroxosulphates remaining in the exhausted pickling solution which was considered necessary in the prior art before the anodic reoxidation of peroxodisulphate in the pickling solution depleted in copper could be carried out. It is therefore possible to work with cathodic copper depletion under conditions of electrolysis which do not lead to complete cathodic reduction of the remaining peroxosulphates. As a result, some of the peroxosulphates still present in the exhausted pickling solution can be obtained and used further for the pickling process. This results in the principal possibility cathodically perform the copper recovery under such conditions, as they are normally present on planar cathodes in metal recovery cells, so without special measures to increase the mass transfer are necessary.

In the anodic reoxidation of the peroxodisulfate on diamond-coated anodes, another advantage is added. Due to the low sensitivity to the peroxymonosulfate which forms by hydrolysis, it has not been found necessary to comply with the high anodic current concentrations required by platinum anodes by an extreme reduction of the anolyte volume. Rather, the anodes according to the invention broken down into strips or rods could be Common anode bag can be arranged. This also eliminates the additional equipment costs for the previously required division of the anode compartment to a larger number of small-volume anode bags, each of which had to be equipped with its own supply and discharge of anolyte solution.

It has already been known that with the use of diamond-coated electrodes in persulfate electrolysis to achieve high current yields on the use of potential-enhancing additives can be completely dispensed with and that you can stay with the anodic current density and the sulfate / sulfuric acid content of the electrolyte solution below the valid range for platinum anodes to still sufficient current yields of Peroxodisulfat neoplasm to achieve. It has been found that a reduction of the anodic current density on diamond-coated anodes even leads to an increase in the current efficiency, in contrast to the platinum anodes, which are known to be associated with a reduction in current density, a significant current yield reduction.

The targeted use of the possibilities associated therewith in the context of this persulfate-recycling pickling process according to the invention finally led to the surprising finding that current densities at the planar cathodes of 4 to 7 A / dm ² and at the diamond-coated anodes of 25 to 50 A. / dm ² it is possible to recover the copper in a compact form, to reoxidize peroxodisulfate in good yield and to obtain part of the peroxodisulfate still present in the spent pickling solution and to continue to use it in the pickling process. This was particularly surprising because the sum of the measures described has succeeded in considerably reducing the quotients of the anodic and cathodic current densities required from platinum anodes from 20 to 30 to 6 to 10 in the past. This has made it possible, by dividing the anode into such 5 to 12 mm wide strips or bars with respect to the planar cathode reduced total area of the diamond-coated anodes reduced for the persulfate recycling process current density difference by the area ratio cathode anode in a simple built-up recycling cell with planar cathodes to compensate. The insensitivity of the diamond-coated anodes with respect to existing peroxomonosulfates can also be used according to the invention so that a partial stream of the spent pickling solution can be fed directly into the anode chambers, bypassing the catholyte circuit, without reducing the anodic current efficiency of the peroxodisulfate regeneration. This procedure is advantageous in that, in addition, copper passes into the cathode space through the transfer of copper ions through the cation exchange membranes and can be deposited there. A corresponding amount of spent pickling solution then needs to be fed to compensate for the copper balance less in the catholyte cycle. However, even less residual peroxodisulfate is fed into the catholyte, as a result of which an even greater proportion of the remaining peroxodisulfate remains in the regenerate and can be used further for the pickling process.

The sulfates / peroxosulfates used are in particular those of the alkali metals and ammonium, preferably the sodium sulfate / peroxosulfate. However, it is also possible to use sulfates / peroxosulfates of other metals, alone or in admixture with sulfates / peroxosulfates of the alkali metals. It has surprisingly been found that the metal sulfates of alloy constituents such as nickel, zinc or even iron itself, which are enriched in pickling of alloys in the pickling bath, are capable of forming peroxodisulfates which are capable of completely or partially replacing the sodium peroxosulfate in the regenerate. It may also be advantageous to use magnesium sulphate / peroxosulphate as the pickling medium, less as an alloy constituent but as a substitute for sodium sulphate / peroxosulphate. This has particular advantages with regard to a room temperature more than twice as high molar solubility of magnesium sulfate compared to sodium sulfate.

As planar cathodes to be used according to the invention, it is possible to use both sheets made of copper, stainless steel or titanium, and copper expanded metals, in one or more layers or in combination with cathode sheets. Thus, the electrochemically active surface can be increased and also the amount of copper to be deposited per cathode surface can be increased until the cathode is changed.

An inventive recycling electrolysis cell according to claims 16 to 23 and their integration in the entire persulfate recycling process is exemplary and schematically shown in the figure 1. Figure 2 shows a cross-section illustrating the arrangement of the rod-divided anode within the anode pocket to the planar cathode (with stylized streamline profile).

The recycling electrolysis cell consists of the cell container 3, in which a bilaterally acting planar cathode 4 and two anode cassettes 5 are arranged. The cathode-facing sides of the anode cassettes consist of vertical slotted plates in which the cation exchange membranes 6 are clamped. The niobium anodes 7 of 5 to 12 mm width, which are subdivided into vertical strips or rods, are provided with a coating of doped diamond 8. They are arranged at a distance of 50 to 70 mm from the slots in the edge plates of the anode casings, as illustrated in FIG. 2 using the example of an anode which is subdivided into bars and a planar cathode which acts only on one side.

The exhausted pickling solution is fed from the pickling tank 1 by means of the metering pump 2 into the catholyte circuit, which is circulated by means of the catholyte circulation pump 10 out of the catholyte circulation vessel 9 via the cathode chamber. One of the metered amount of pickling solution corresponding proportion of the stationary catholyte enters at 11 in the anode cassettes 5, flows through them and passes through a gas separator as reclaim back into the pickling pan 1. By means of the metering pump 14, an additional amount of a peroxodisulfate solution from the container 13, also to compensate for leaching losses to pickling solution.

Application Examples Example 1 (Comparative Example)

In a laboratory test cell divided by a cation exchange membrane, the anulfate regeneration anodes process was investigated using a pre-decarburized pickling solution still containing peroxosulfate, using the diamond-coated anodes to be used in the invention in comparison to the smooth platinum anodes. The laboratory test cell had an effective anode area of 30 cm ² . The cathode consisted of several stainless steel expanded mesh, the separation of anolyte and catholyte was a NAFION N450 cation exchange membrane. The catholyte used was dil. Sulfuric acid, the anolyte solution had the following basic composition: 250 g / l sodium sulfate 50 g / l sodium peroxodisulfate

100 g / l sulfuric acid

5 g / l copper

Part of the solution was electrolyzed immediately after the sodium peroxodisulfate had been added. Another part was used only after a reaction time of 24 h at 50 ⁰ C after about 8 g / l peroxomonosulfuric had formed by hydrolysis (corresponds to a hydrolyzed portion of the peroxodisulfate used of about 33%). 600 ml of a circulating conveyed via the anode compartment and a cooler anolyte solution over 150 minutes at a temperature of about 30 ⁰ C and a current of 12 A (i = 40 A / dm ²⁾ were electrolyzed. The NaPS total concentration was determined every 30 min and, after 150 min, the formation of new peroxodisulfate and from this the current yield was calculated. The following tests were carried out:

Experiment 1: Niobium anode coated with boron-doped diamond (about 5 μm), anolyte without previous hydrolysis.

Experiment 2: Diamond electrode as in verse 1, but after previous hydrolysis. Experiment 3: Comparative experiment with smooth platinum anode, without previous hydrolysis and without potential-increasing additives.

Experiment 4: Comparative experiment as in verse 3, but after previous hydrolysis. Experiment 5: Comparative experiment with smooth platinum anode, without previous hydrolysis, but with potential-increasing additive (0.2 g / l NaCNS, 0.1 g / l HCl). Experiment 6: Comparative experiment as in verse 5, but after previous hydrolysis). The results are summarized in Table 1: TABLE 1

The results make it clear that under the novel anodic electrolysis conditions (current density, pickling bath composition) and the intended omission of a potential-increasing additive, it is not possible to replace the diamond-coated anodes to be used according to the invention with smooth platinum anodes. This is true regardless of whether a part of the remaining residual persoxosulfate is already hydrolyzed to peroxymonosulfate or not. Only if the remaining peroxosulphate is present exclusively as peroxodisulphate, a comparable current yield of peroxodisulphate new formation is obtained on platinum anodes when using the potential-increasing additive which is undesirable for the printed circuit board production. In the case of peroxomonosulfate, which is in most cases inevitable, even if the potential-enhancing addition of platinum anodes is present, the current efficiency drops sharply.

Example 2:

In the same divided laboratory test cell of Example 1, the following starting solutions with different cations and total sulfate contents were electrolyzed with the diamond-coated anode under the same conditions (current density 40 A / dm ² , electrolysis time 90 min) (in contrast to Example 1 without starting persulfate contents).

Experiment 1: 250 g / l Na ₂ SO ₄ + 100 g / l H ₂ SO ₄

Experiment 2: 400 g / l MgSO ₄ .7H ₂ O + 100 g / l H ₂ SO ₄

Experiment 3: 300 g / l ZnSO ₄ × 7 H ₂ O + 150 g / l Na ₂ SO ₄ + 100 g / l H ₂ SO ₄

Experiment 4: 200 g / l NiSO ₄ × 6 H ₂ O + 150 g / l Na ₂ SO ₄ + 100 g / l H ₂ SO ₄

The results obtained are summarized in the following Table 2:

It can be seen that it also with the enriched in the pickling metal sulfates, z. B. of zinc sulfate in the pickling of brass or nickel sulfate in the pickling of copper-nickel alloys is possible to achieve at least the same current yields in the regeneration as pure sodium sulfate / sulfuric acid solutions. Interestingly, the significantly higher current efficiency obtained with magnesium sulfate / sulfuric acid solutions. Obviously, this is not only due to a higher initial sulfate content due to the better solubility of magnesium sulphate compared to sodium sulfate, but probably also due to the replacement of nations by Mg ²⁺ ions.

Example 3

In a similar to the process scheme Fig. 1 constructed recycling pilot plant, a recycling electrolysis cell was used, which was equipped with two double-acting cathode plates made of stainless steel with an electrochemically effective total area of 200 dm ² . The three anode cassettes each contained 10 vertical, 8 mm wide diamond coated niobium anode strips. The middle anode pocket was bilateral and contained two NAFION cation exchange membranes clamped in two plastic plates each fitted with vertical slots opposite the anode strips. The two edge-anode cassettes were constructed analogously, but they each contained only one cation exchange membrane and were unilateral. The total electrochemically active anode area was 24 dm ² . This results in a cathode / anode area ratio of 8.33.

The stationary catholyte was circulated by circulation pump. By means of a metering pump, 55 l of the pickling solution from the pickling tank were fed into the catholyte cycle every hour. This pickling solution had the following composition:

49 g / l sodium peroxodisulfate

150 g / l sodium sulfate

100 g / l sulfuric acid

15 g / l copper

Stationary concentrations of 4.8 g / l copper and 13 g / l peroxosulphate were calculated in the catholyte cycle, calculated as sodium peroxodisulphate. The copper separated in a compact easily removable form on the Edeistahlkathodenplatten. Electrolyzed with a current of 950 A _1, a cathodic current density of about 4.8 A / dm ² and an anodic current density of 39.6 A / dm ² accordingly. Each hour, 561 g of copper were deposited cathodically and 1,980 g of sodium peroxodisulfate was reduced. The 55 l / h catholyte solution fed with 13 g / l NaPS into the anode cassettes was anodically concentrated to 65.5 g / l NaPS. This corresponds to an amount of sodium peroxodisulfate regenerate of 3,603 g / h fed into the pickling bath. Of these anodically 2.888 g / h were formed, corresponding to a real current yield of 65.0%. Including in the calculation the amount of NaPS residual contained in the catholyte fed in, which would have had to be completely reduced in previous recycling processes, results in an apparent anodic current efficiency of 81.1%.

On average, the NaPS consumption without recycling was about 4,400 g / h, including the carry-over and decomposition losses, which remained approximately constant with and without recycling. Thus, about 81.9% of the amount of NaPS used without recycling could be recovered by the recycling process, in addition to the recovery of 561 g / h of copper. The resulting difference in NaPS consumption from the total consumption without recycling (with approximately the same NaPS loss amount) is: 4,400 - 3,603 = 797 g / h NaPS, this amount must be added to the pickling process in the form of a NaPS solution in addition to the regenerate ,

Claims

claim

1. A recycling pickling process for copper and copper alloys using an aqueous recycle solution containing from 0.2 to 1.0 mol / l of a peroxosulphate in addition to excess sulphate and free sulfuric acid, the exhausted at 0.15 to 0.5 mol / a copper-enriched and peroxosulphate-depleted exhausted pickling solution is electrochemically regenerated in a cation exchange membrane separated recycling electrolysis cell, characterized in that a partial flow of the exhausted pickling solution is fed into a circulating over the cathode compartments of the cell catholyte cycle, wherein the copper at one or more planar Cathodic plates at cathodic current densities of 4 to 7 A / dm ^{2 is} depleted to 0.05 to 0.15 mol / l and the remaining remaining peroxosulfate catholyte is then fed to the anode chambers, in which the peroxodisulfate in the form of peroxodisulfate in the 5 to 12 mm wide vertical strips or rods au aged anode is reoxidized from a diamond-coated valve metal with current densities of 20 to 50 A / dm ² .

2. The method according to claim 1, characterized in that the circulating solution contains a total sulfate content of 1 to 3 mol / l.

3. The method according to claims 1 and 2, characterized in that the anodes have a coating of a made conductive by doping diamond with a thickness of 3 to 10 microns and that is used as a valve metal niobium, titanium, tantalum or zirconium.

4. The method according to claims 1 to 3, characterized in that the anode chambers supplied, largely depleted in copper catholyte still contains peroxosulfates.

5. The method according to claims 1 to 4, characterized in that a partial stream of the peroxosulfate-containing spent pickling solution is fed in addition to the largely decuperated catholyte directly into the anode chambers.

6. Process according to claims 1 to 5, characterized in that the planar cathodes consist of titanium, stainless steel or copper sheets.

7. The method according to claims 1 to 6, characterized in that the planar cathodes consist of one or more contacted with each other copper expanded metal layers.

8. The method according to claim 7, characterized in that the cathodes formed from a plurality of expanded metal layers comprise a center plate with power supply, with which the copper-expanded metal layers applied on both sides are electrically connected.

9. The method according to claim 8, characterized in that the center plate made of copper, titanium or stainless steel.

10. The method according to claims 1 to 9, characterized in that the sulfates / peroxosulfates of the alkali metals and / or ammonium are used alone or in admixture with each other.

11.A method according to claims 1 to 10, characterized in that sulfates / peroxosulfates of magnesium, zinc, nickel, iron alone or in admixture with alkali metal sulfates / Peroxosulfaten be used.

12. The method according to claims 1 to 11, characterized in that the anodically reoxidized peroxodisulfate before being fed into the pickling bath completely o- partially hydrolyzed to Peroxomonosulfat.

13. The method according to claims 1 to 12, characterized in that the circulation solution known additives are added to increase the pickling rate.

14. The method according to claims 1 to 13, characterized in that during the pickling of copper alloys, a part of the decuperated catholyte solution for Recovery of enriching alloying components is removed and worked up by known methods.

15. The method according to claims 1 to 14, characterized in that for the regeneration of several individual cells or a plurality of cathode and / or anode chambers of a cell are successively flowed through by the pickling solution to be regenerated.

16. An electrolytic cell for regenerating exhausted peroxosulphate pickling solutions for copper and copper alloys according to the described in claims 1 to 15 recycling pickling process for copper and copper alloys, consisting of:

• an electrolysis tank

• At least one arranged in the electrolysis tank cathode

• At least one arranged in the electrolysis tank anode bag with einseitg or clamped on both sides cation exchange membranes and a single-sided or double-acting anode with inlets and outlets for the anolyte, characterized in that

Planar cathodes are used in the form of sheets or expanded metals,

The anode pockets on the side facing the cathode are provided with vertical slits in which the cation exchange membranes are clamped,

The anodes consist of a diamond-coated valve metal,

The anodes are subdivided into power supply lines in vertical anode strips or anode bars arranged opposite the slots in the edge plates of the anode pockets,

The anode strips are 5 to 12 mm wide and are arranged at a distance of 40 to 70 mm from one another and from 50 to 70 mm from the planar cathode,

The total electrochemical area of the anode strips is 10 to 25% of the area of the planar cathodes.

17. An electrolytic cell according to claim 16, characterized by the use of boron-doped diamond layers of 3 to 10 microns thickness.

18. electrolytic cell according to claims 16 and 17, characterized by the use of niobium as a valve metal.

19. An electrolytic cell according to claims 16 to 18, characterized by planar cathodes in the form of sheets of copper, stainless steel or titanium.

20. An electrolytic cell according to claims 16 to 19, characterized by planar cathodes, which consist of one or more contacted with each other copper expanded metal layers.

21. An electrolytic cell according to claims 16 to 20, characterized by the arrangement of cooling surfaces in the electrolysis tank.

22. An electrolytic cell according to claims 16 to 21, characterized by a catholyte circuit via an external cooler.

23. An electrolytic cell according to claims 16 to 22, characterized by the hydrodynamic coupling of a plurality of anode pockets.