EP1753894B1 - Method for producing peroxodisulfates in aqueous solution - Google Patents

Abstract

A process for preparing or regenerating peroxodisulfuric acid and its salts by electrolysis of an aqueous solution containing sulfuric acid and/or metal sulfates at diamond-coated electrodes without addition of promoters is described, with bipolar silicon electrodes which are coated with diamond on one side and whose uncoated silicon rear side serves as cathode being used.

Description

The invention relates to a process for the preparation or regeneration of peroxydisulphuric acid and its salts by electrolysis of an aqueous solution containing sulfuric acid and / or metal sulphates. As used herein, the term "metal sulfates" includes sulfates of metals such as zinc, nickel or iron, as well as sulfates of alkali metals, alkaline earth metals or ammonium sulfate. For example, alkali metal sulfates or alkaline earth sulfates can be used as the metal sulfates, alkali metal sulfates or ammonium sulfate are preferably used. It is also possible to use mixtures of different metal sulfates, such as, for example, magnesium sulfate, zinc sulfate or else nickel or iron sulfate, preferably in the regeneration of etching and pickling solutions.

It is known in the prior art that for the production of peroxodisulfates of the alkali metals and of the ammonium, diamond-coated electrodes can be used made of valve metals, preferably of niobium, or of ceramic materials, preferably of silicon [ DE 199 48 184.9 . DE 100 19 683 ]. The diamond layer is rendered conductive by doping with a trivalent or pentavalent element, preferably with boron. In comparison to the exclusively smooth anodized platinum anodes used in the production of peroxodisulfate, there are advantages in that, owing to the high potential achievable at the diamond surface, it is not necessary to add to the electrolyte sufficiently higher additions to achieve sufficiently high current yields, as is indispensable for platinum anodes is. In the preferred use of thiocyanates as polarizers but it comes to cyanide-loaded anode gases, which require complex gas purification measures required. When using diamond-coated anodes can be dispensed with.

Another advantage of the diamond-coated anodes in peroxodisulfate production is that even with lower sulfate content in the anolyte even significantly higher current yields can be achieved than when using platinum anodes.

However, despite the good resistance especially of the diamond-coated silicon electrodes, there were a number of disadvantages in their use. So there was the problem of a suitable power supply. As a result of the relatively low electrical conductivity of the silicon base body, the contact had to be made over the entire surface of the electrode back, so that the current transport needs only to flow from the contacted back over the small thickness of the silicon electrode of about 1 to 2 mm to the diamond coating. Although this problem could be solved in principle by gluing the preferably metallized rear sides of the silicon plates on a good conductive metallic substrate by means of an electrically conductive adhesive, but the effort to be operated is relatively large.

Another disadvantage of the diamond-coated silicon electrodes of the prior art is their limited dimensions of currently maximum 200 × 250 mm. In order to still be able to provide large-area anodes for use in technical electrolysis cells, was in the EP 1 229 149 proposed to stick a larger number of such silicon-diamond electrodes by means of electrically conductive adhesive on a metal base plate, for example of a valve metal, and to seal the edges by a corrosion-resistant resin, such as epoxy resin. However, the effort to be made, for example, to provide the conductive adhesive, for example made of epoxy resin with silver particles, and for the complete removal of the oxide layers on the surfaces to be joined is very high. In addition, such an electrode construction for the production of peroxodisulfate has proven to be insufficient corrosion resistant, so that only a short service life of less than one year could be achieved.

Another possibility of the prior art, to build electrolysis cells with sufficiently large current capacity, is the series connection of a larger number of bipolar silicon diamond electrodes. In FR 2790268 B1 is proposed such a bipolar electrolysis cell, wherein the bipolar electrodes consist of a ceramic substrate which is completely covered with a diamond film. However, this cell is not specifically proposed for the production of peroxodisulfates, but for applications for pollutant degradation or for water disinfection.

Also DE 200 05 681 describes the use of double-sided diamond-coated bipolar electrodes.

In EP 1 254 972 For example, an electrolysis cell construction suitable for different applications is proposed, which can be monopolar or bipolar, as undivided or split cell. In the bipolar design, diamond-coated silicon wafer electrodes are again used exclusively on both sides. For the production of peroxodisulfates, these cells can be used effectively with diamond electrodes coated on both sides and the relatively complex cell construction only for small persulfate throughputs. If you want to increase the throughput to technically relevant areas by a larger number of bipolar single cells, it comes in this construction to yield reductions by the strongly with the total voltage leakage currents in the supply and discharge lines.

The object underlying the present invention was therefore to provide a process for the preparation or regeneration of peroxodisulfuric acid and / or their salts, wherein the disadvantages of the previous methods and electrolysis cells shown at least partially be avoided. It has been found that peroxodisulfates can advantageously be produced in undivided or divided electrolysis cells in a simple manner using bipolar, unilaterally doped diamond-coated silicon electrodes, the uncoated silicon backs acting directly as cathodes.

According to the invention, the coating of the silicon electrode has a thickness of about 1 to about 20 μm, preferably about 5 μm.

It was highly surprising that it only takes the coating of the anode side of the bipolar electrode in order to achieve satisfactory results with the then uncoated silicon back side acting as a cathode. In the case of an undivided bipolar cell, moreover, it has surprisingly been found that with a silicon cathode according to the invention lower persulfate losses result from cathodic reduction, in comparison to the metal cathodes conventionally used in the preparation of persulfate in the prior art.

It has also been found that not only high persulfate formation rates can be achieved with the bipolar electrodes according to the invention, but that this can also be done with the lowest possible cell voltages and thus low specific electrical energy consumption. This is based on the one hand on the finding that the silicon-cathode surfaces are freed by the cathodic load of initially present, poorly conducting oxide layers and in the course of electrolysis these can still be kept completely free. For example, in the endurance test (see Example 1), it could be demonstrated that with increasing operating time the cell voltage is even further reduced, while with the diamond-coated silicon electrodes of the prior art glued onto a metal support, a reverse tendency can be observed due to the increasing corrosion.

Thus, the method of the invention advantageously enables the production of peroxodisulfuric acid and / or its salts on a true bipolar electrode with high current efficiency and low electrical energy consumption, although only the low-conductivity silicon is used as the cathode. In addition, there are no costs for a cathode coating.

Another advantage of the bipolar, single-sided diamond coated silicon electrodes of the present invention is the lower catalytic activity of the silicon back surface as compared to a metallized back electrode, e.g. made of platinum or stainless steel. It has been found that this results in lower reduction losses of peroxodisulfate when electrolyzed in an undivided electrolysis cell. This results in undivided cells that the increase in the peroxodisulfate concentration with the electrolysis time is somewhat steeper and the achievable final concentration is higher than when using a metallized cathode under otherwise identical electrolysis conditions.

In comparison with the bipolar electrodes of the prior art coated on both sides with doped diamond, there are thus advantageously both cost savings for the electrodes themselves and for the electrolysis cells equipped therewith, as well as the achievable lower electrical energy consumption.

The process according to the invention for the preparation of peroxodisulfuric acid and / or its salts can be carried out both in undivided electrolysis cells and in electrolysis cells which are divided, for example by ion exchange membranes or porous diaphragms.

The bipolar, one-sided diamond-coated silicon electrodes according to the invention are particularly suitable for relatively simple constructed undivided electrolysis cells, as they are eg in DE G 200 05 681.6 for the disinfection of waters. It is advantageous for the current input when the monopolar edge anodes from a consist of diamond-coated valve metal. The term "valve metal" refers to a metal that coats with anodic poling with an oxide layer that does not become conductive even at high voltages. When connected as an anode, the metal blocks. Connected as a cathode, the oxide layer dissolves and current flows reasonably uninhibited. Valve metals behave like a rectifier with different polarity. Examples of suitable valve metals are, for example, tantalum, titanium, niobium and zirconium. In the context of the present invention, preference is given to using niobium.

The monopolar edge cathodes are preferably made of a suitable, highly conductive material, such as e.g. Stainless steel, hastelloy, platinum and impregnated graphite. Highly alloyed stainless steels or Hastelloy are preferably used in the context of the present invention. Also, a metallized backside silicon peripheral cathode contacted with a power supply plate made of a highly conductive material, e.g. Copper, because of the good durability in undivided cells can be used. In particular, when using edge electrodes made of metallic materials, the current input due to the good conductivity in a simple manner and without major voltage drops can be optimally realized.

In an electrolytic cell, several electrode stacks consisting of bipolar electrodes and edge electrodes with a current supply can also be electrically connected in parallel. If necessary, the distance of the bipolar electrodes can be adjusted or fixed by spacers. By means of such electrode stacks connected in parallel, it is possible to accommodate larger current capacities in an electrolytic cell without requiring an unacceptably high total voltage. Thus, the voltage can also be optimally adapted to the available rectifier voltage. In addition, by the short-circuit currents in the common supply and discharge lines for the electrolyte solutions can be further minimized, which also in a known manner by the arrangement can be supported by additional resistance paths in these lines.

Undivided bipolar cells constructed according to the invention can be used particularly advantageously if the peroxodisulfate concentration does not have to be too high for the particular application in question, such as e.g. for oxidative pollutant degradation in process solutions and wastewater. As can be seen from Example 2, in an undivided cell with the bipolar electrodes according to the invention in batch mode sodium peroxodisulfate reaction solutions containing 50 to 100 g / l still with current efficiencies between 75 and 50% and specific electrical energy consumption between 1.3 and 1.9 kWh / kg are produced very effectively.

Even better current yields or the same yields at higher final peroxodisulfate concentrations can be achieved by shielding the cathode by means of suitable, material transport to the cathode surface inhibiting materials, as can be seen from Example 3. Suitable materials for these purposes are, for example, PVC gauzes. In undivided cells, sodium peroxodisulfate concentrations of 150 to 200 g / l can still be produced with acceptable current yields of around 50% with the method according to the invention, albeit with higher cell voltages.

If higher final concentrations of peroxodisulfates, for example in the range from 200 to 400 g / l of sodium peroxodisulfate, are desired, the use of divided electrolysis cells with the bipolar silicon electrodes according to the invention is preferred. As can be seen from Example 4, thus current efficiencies between about 75 and 85% can be achieved, but with a more complex cell structure and higher cell voltages of about 5.5 to 6 V. However, it is still comparatively very favorable specific electrical energy consumption of less than 2 , 0 kWh / kg achievable.

Another surprising effect of the process according to the invention is the very low removal rates on the silicon cathodes observed in the continuous test with acid persulfate-containing electrolyte in undivided electrolysis cells. Thus, in an approximately 7-month long-term experiment (see Example 1) in an undivided cell with a steady sodium peroxodisulfate content of about 150 g / l surprisingly low removal rates of only 2 to 3 microns were found. This was particularly surprising because under these very highly corrosive conditions, a 10 to 100 times greater erosion was observed even in platinum cathodes of the prior art. Even cathodes made of graphite or high-alloy stainless steels proved in such peroxodisulfate containing sulfuric acid electrolyte solutions as unsuitable because not sufficiently resistant to corrosion.

Examples Example 1:

An analog DE G 200 05 681.6 constructed undivided bipolar electrolysis cell contained 9 bipolar silicon electrodes, coated on one side with about 3 microns boron-doped diamond (average about 3,000 ppm boron). The edge anode used was a power supply-equipped, one-sided diamond-coated niobium electrode. The peripheral cathode with power supply was Hastelloy. The bipolar electrodes had a dimension of 100 x 33 mm (33 cm ² ). The average distance of the approximately 1 mm thick bipolar electrodes was set by spacers to about 2 mm. The electrolysis current was adjusted to 16.5 A constant, corresponding to an anodic and cathodic current density of 0.5 A / cm ² . The total current capacity of the electrolysis cell resulted from this with 10 × 16.5 = 165 A. The electrolyte used was 2 l of a 300 g / l sodium sulfate and 200 g / l sulfuric acid-containing aqueous solution. It was circulated from a recycle vessel at a rate of about 600 l / h via a heat exchanger and through the cell (batch mode). The electrolysis operation was maintained over 5000 h, with only the evaporated or decomposed water was added. In steady state, a concentration between 170 and 190 g / l sodium peroxodisulfate at a steady state temperature of about 35 ° C was. The total voltage at start-up was 50 V. The mean cell voltage developed in the course of continuous operation as follows: Operating time of 5 h 50 h 500 h 5000 h Mean cell voltage 4.95 V 4,60 V 4.35 V 4.18 v

After 5,000 hours of operation, the electrodes were removed and weight loss determined. From this, the average decrease of the silicon electrode thickness to an average of 3 μm was calculated. The thickness of the silicon cathode thus only decreases by about 10 μm per year.

Example 2:

With the undivided electrolysis cell from Example 1, the dependence of the current yield on the final concentration of sodium peroxodisulfate (NaPS) achieved was determined under the same electrolysis conditions (current density, temperature, batch mode, electrolyte composition). The following results were obtained: Final concentration of NaPS in g / l 25 50 75 100 125 150 Current efficiency of NaPS formation in% 84 77 64 50 40 34

For the final concentration of 50 g / l, a specific electric energy consumption of 1.23 kWh / kg was found for the most favorable cell voltage of approximately 4.2 V after longer operating time, and 50% for the final concentration of 100 g / l NaPS. sinking current output still of 1.89 kWh / kg.

Example 3:

The same undivided electrolysis cell as in Examples 1 and 2 was equipped with a resting on the cathodes of the bipolar electrode plates and the peripheral cathode PVC gauze, which was pressed by a Kunststoffspacer to the surface. Under the same electrolysis conditions as in Example 2 was electrolysed again. The following current yields, based on the final NaPS concentration achieved, were obtained. Final concentration of NaPS in g / l 50 75 100 125 150 175 200 Current efficiency of NaPS formation in% 84 77 73 68 61 54 49

Even in the concentration range between 100 and 200 g / l relatively cheap current yields were achieved, which were on average about 20% higher than without shielding the cathode surfaces. However, the cell voltages were about 0.8 V higher due to the additional resistance of the gauze shield. Nevertheless, e.g. at 150 g / l NaPS final concentration still a very favorable specific electric energy consumption of about 1.85 kWh / kg.

Example 4:

The nine bipolar electrodes and the two monopolar edge electrodes of the undivided electrolysis cell used in Examples 1 to 3 were used in a bipolar split cell. For the separation of anolyte and catholyte cation exchange membranes were used, fixed on both sides by anode and cathode spacers made of plastic. The limited by sealing frame anodes and cathode compartments had a thickness of 2 - 3 mm. Anolyte and catholyte were circulated in separate circuits with the interposition of a heat exchanger. The catholyte used was a 500 g / l sulfuric acid. The anolyte again consisted of a 200 g / l sulfuric acid and 300 g / l of sodium sulfate-containing aqueous solution. In order to avoid an excessive depletion of sodium sulfate both at the desired high final NaPS concentrations by the consumption of peroxodisulfate, as well as by the upper charge of Na ⁺ ions through the cation exchange membrane in the catholyte, 100 g / l were in the anolyte during the electrolysis Dissolved sodium sulfate (in total so 400 g / l sodium sulfate). The anodic and cathodic current density was set to 0.5 A / cm ^2, respectively.

• bei 200 g/ NaPS-Endkonzentration eine Stromausbeute von 86 %
• bei 300 g/l NaPS-Endkonzentration eine Stromausbeute von 82 %
• bei 400 g/l NaPS-Endkonzentration eine Stromausbeute von 74 %

Under otherwise comparable electrolysis conditions, the following current yields for different final NaPS concentrations were obtained:

• at 200 g / NaPS final concentration, a current efficiency of 86%
• at 300 g / l NaPS final concentration, a current efficiency of 82%
• At 400 g / l NaPS final concentration, a current efficiency of 74%

The mean cell voltages were in the range of 5.5 and 6 V. For the final concentration of 400 g / l, a still very low specific electric energy consumption of about 1.8 kWh / kg could be achieved.

Claims (7)

1. Process for producing peroxodisulphuric acid and/or its salts by electrolysis of aqueous solutions of sulphuric acid and/or metal sulphates at diamond-coated electrodes without addition of promoters, characterized in that bipolar silicon electrodes are used which are coated on one side with doped diamond and whose uncoated silicon rear side serves as a cathode.
2. Process according to claim 1, characterized in that the electrolysis is carried out in undivided electrolytic cells.
3. Process according to claim 1, characterized in that the electrolysis is carried out in electrolytic cells which are divided by ion-exchange membranes or porous diaphragms.
4. Process according to one of the claims 1 to 3, characterized in that a diamond-coated anode composed of a valve metal e.g. niobium and provided with a power supply is used as an edge anode.
5. Process according to one of the claims 1 to 4, characterized in that stainless steel, Hastelloy, platinum, impregnated graphite or silicon which has been metallized on one side is used for the edge cathode provided with a power supply.
6. Process according to one of the claims 1 to 5, characterized in that a plurality of electrode stacks comprising bipolar electrodes and edge electrodes provided with a power supply are connected electrically in parallel within an electrolytic cell.
7. A bipolar undivided or divided electrolytic cell equipped with bipolar electrodes coated with diamond on one side for use in a process according to one of the claims 1 to 6.