Method for producing peroxodisulfates in aqueous solution

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.

Claims

1. A process for preparing peroxodisulfuric acid or a salt thereof comprising the step of performing electrolysis of an aqueous solution of at least one of sulfuric acid or a metal sulfate at diamond-coated electrodes without addition of a promoter, wherein each of the diamond-coated electrodes are bipolar silicon electrodes which are coated only on one side with doped diamond and have an uncoated silicon rear side which serves as a cathode, wherein the process is conducted in an acidic, persulfate containing electrolyte, wherein the peroxodisulfuric acid or salt thereof is prepared in an amount of at least 50 g/L.

2. The process of claim 1, wherein the electrolysis is carried out in undivided electrolysis cells.

3. The process of claim 2, wherein a diamond-coated anode composed of a valve metal and provided with a power supply lead is used as a boundary anode.

4. The process of claim 3, wherein the valve metal is niobium.

5. The process of claim 2, wherein stainless steel, Hastelloy, platinum, impregnated graphite or silicon which has been metallized on one side is used for the boundary cathode provided with a power supply lead.

6. The process of claim 1, wherein the electrolysis is carried out in electrolysis cells which are divided by at least one of an ion-exchange membrane or a porous diaphragm.

7. The process of claim 6, wherein a diamond-coated anode composed of a valve metal and provided with a power supply lead is used as a boundary anode.

8. The process of claim 7, wherein the valve metal is niobium.

9. The process of claim 6, wherein stainless steel, Hastelloy, platinum, impregnated graphite or silicon which has been metallized on one side is used for the boundary cathode provided with a power supply lead.

10. The process of claim 1, wherein a diamond-coated anode composed of a valve metal and provided with a power supply lead is used as a boundary anode.

11. The process of claim 10, wherein the valve metal is niobium.

12. The process of claim 1, wherein stainless steel, Hastelloy, platinum, impregnated graphite or silicon which has been metallized on one side is used for the boundary cathode provided with a power supply lead.

13. The process of claim 1, wherein a plurality of electrode stacks comprising bipolar electrodes and boundary electrodes with power supply lead are connected electrically in parallel within an electrolysis cell.

14. The process of claim 1, wherein the wherein the peroxodisulfuric acid or salt thereof is prepared in an amount of from 50 g/L to 100 g/L.

15. The process of claim 1, wherein the wherein the peroxodisulfuric acid or salt thereof is prepared in an amount of from 200 g/L to 400 g/L.

16. The process of claim 1, wherein the wherein the peroxodisulfuric acid or salt thereof is prepared in an amount of from 150 g/L.

Description

EXAMPLES

Example 1

(1) An undivided bipolar electrolysis cell having a construction analogous to that in DE G 200 05 681.6 contained 9 bipolar silicon electrodes coated on one side with about 3 μm of boron-doped diamond (average about 3000 ppm of boron). A niobium electrode coated on one side with diamond and provided with a power supply lead served as boundary anode. The boundary cathode with power supply lead comprised Hastelloy. The bipolar electrodes had a dimension of 100×33 mm (33 cm.sup.2). The mean spacing of the about 1 mm thick bipolar electrodes was set to about 2 mm by means of spacers. The electrolysis current was regulated at a constant 16.5 A, corresponding to an anodic and cathodic current density of 6.5 A/cm.sup.2. The total current capacity of the electrolysis cell was thus 10×16.5=165 A. 2 l of an aqueous solution containing 300 g/l of sodium sulfate and 200 g/l of sulfuric acid served as electrolyte. It was circulated at a rate of about 600 l/h from a circulation reservoir via a heat exchanger and through the cell by pumping (batch operation). Electrolysis operation was maintained for 5000 hours, with only the water which had evaporated or been decomposed being replaced. In steady-state operation, a concentration of 170-190 g/l of sodium peroxodisulfate was established at a steady-state temperature of about 35° C. The total voltage on start-up was 50 V. The mean cell voltage changed as follows over the course of continuous operation:

(2) TABLE-US-00001 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

(3) After 5000 hours of operation, the electrodes were removed and the weight loss was determined. The mean decrease in the silicon electrode thickness was calculated therefrom as an average of 3 μm. The thickness of the silicon cathode thus decreases by only about 10 μm per year.

Example 2

(4) The dependence of the current yield on the final concentration of sodium peroxodisulfate (NaPS) achieved was determined by means of the undivided electrolysis cell from example 1 under the same electrolysis conditions (current density, temperature, batch operation, electrolyte composition). The following results were obtained:

(5) TABLE-US-00002 Final concentration of 25 50 75 100 125 150 NaPS in g/l Current yield of 84 77 64  50  40  34 NaPS formation in %

(6) At the favorable cell voltage of about 4.2 V established after a prolonged period of operation, the specific electric energy consumption was 1.23 kWh/kg for a final concentration of 50 g/l; for a final concentration of 100 g/l of NaPS, it was still 1.89 kWh/kg despite the fact that the current yield had dropped to 50%.

Example 3

(7) The same undivided electrolysis cell as in examples 1 and 2 was equipped with a PVC gauze resting on the cathodes of the bipolar electrode plates and the boundary cathode; this gauze could be pressed onto the surface by means of a plastic spacer. Electrolysis was again carried out under the same electrolysis conditions as in example 2. The following current yields, based on the final NaPS concentration achieved, were obtained.

(8) TABLE-US-00003 Final concentration 50 75 100 125 150 175 200 of NaPS in g/l Current yield of 84 77  73  68  61  54  49 NaPS formation in %

(9) Even in the concentration range from 100 to 200 g/l, relatively favorable current yields were obtained and these were an average of about 20% higher than without shielding of the cathode surfaces. However, the cell voltages were about 0.8 V higher due to the additional resistance of the gauze shielding. Nevertheless, a very favorable specific electric energy consumption of about 1.85 kWh/kg was still obtained at, for example, a final NaPS concentration of 150 g/l.

Example 4

(10) The nine bipolar electrodes and the two monopolar boundary electrodes of the undivided electrolysis cell used in examples 1 to 3 were used in a divided bipolar cell. Cation-exchange membranes which were fixed on both sides by means of anode and cathode spacers made of plastic were used for separating anolyte and catholyte. The anode and cathode spaces bounded by sealing frames had a thickness of 2-3 mm each. Anolyte and catholyte were circulated in separate circuits through a heat exchanger. 500 g/l of sulfuric acid served as catholyte. The anolyte once again consisted of an aqueous solution containing 200 g/l of sulfuric acid and 300 g/l of sodium sulfate. To avoid an excessively large decrease in the sodium sulfate concentration due to both consumption to form peroxodisulfate and the transport of Na.sup.+ ions through the cation-exchange membrane into the catholyte at the desired high final. NaPS concentrations, a further 100 g/l of sodium sulfate were dissolved in the anolyte during the electrolysis (i.e. a total of 400 g/l of sodium sulfate). The anodic and cathodic current densities were each set to 0.5 A/cm.sup.2.

(11) Under otherwise comparable electrolysis conditions, the following current yields were obtained for various final NaPS concentrations:

(12) at a final NaPS concentration of 200 g/l, a current yield of 86%

(13) at a final NaPS concentration of 300 g/l, a current yield of 82%

(14) at a final NaPS concentration of 400 g/l, a current yield of 74%

(15) The mean cell voltages were in the range from 5.5 to 6 V. At the final concentration of 400 g/l, a still very low specific electric energy consumption of about 1.8 kWh/kg could thus be achieved.