Undivided electrolytic cell and use of the same

Abstract

The invention relates to a method for producing an ammonium peroxydisulfate or alkali metal peroxydisulfate, to an undivided electrolytic cell which is composed of individual components, and to an electrolytic device composed of a plurality of said electrolytic cells.

Claims

1. A process for the preparation of an ammonium or alkali metal peroxydisulphate, comprising: anodic oxidation of an aqueous electrolyte comprising a salt chosen from ammonium sulphate, alkali metal sulphate and/or the corresponding hydrogen sulphate in a tubular electrolytic cell comprising at least one anode and one cathode wherein a diamond layer arranged on a conductive carrier and doped with a tri- or pentavalent element is used as the anode, the tubular electrolytic cell comprises an undivided electrolyte chamber between the anode and the cathode, and the aqueous electrolyte comprises no promoter for increasing the decomposition voltage of water to oxygen and has a total solids content of from about 0.5 to about 650 g/l.

2. The process of claim 1, wherein the alkali metal sulphate and/or the corresponding hydrogen sulphate is selected from the group consisting of sodium sulphate, potassium sulphate, sodium hydrogen sulphate, potassium hydrogen sulphate and mixtures thereof.

3. The process of claim 1, wherein the anode carrier material is selected from the group consisting of silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements, aluminium and mixtures thereof.

4. The process of claim 1, wherein a boron-doped and/or phosphorus-doped diamond layer is used.

5. The process of claim 4, wherein the boron-doped and/or phosphorus-doped diamond layer is doped up to an extent of 10,000 ppm in the crystal structure.

6. The process of claim 1, wherein the diamond layer has a film thickness of from about 0.5 m to about 5.0 m.

7. The process of claim 1, wherein a boron-doped diamond layer on a niobium or titanium carrier is used as the anode.

8. The process of claim 1, wherein the cathode is formed from lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and/or acid-resistant high-grade steels.

9. The process of claim 1, wherein multiple electrolytic cells are combined.

10. The process of claim 9, wherein the multiple electrolytic cells are combined in the form of a double tube package or two-dimensionally.

11. The process of claim 1, wherein the electrolyte has an acidic or neutral pH.

12. The process of claim 1, wherein the electrolyte is moved in circulation through the electrolytic cell during the process.

13. The process of claim 12, further comprising a sluicing out of electrolyte solution from the electrolyte circulation.

14. The process of claim 12, further comprising a procedure in which the peroxydisulphates produced are obtained by crystallisation and separating off of the crystals from the electrolyte solution to form an electrolyte mother liquor.

15. The process of claim 14, further comprising a recirculation of the electrolyte mother liquor, to increase the content of acid, sulphate and/or hydrogen sulphate in the electrolytic cell.

16. The process of claim 4, wherein the anodic oxidation is carried out at an anodic current density of from about 50 to about 1,500 mA/cm.sup.2.

17. The process of claim 1, wherein the electrolyte comprises about 100 to about 500 g/1 of persulphate.

18. The process of claim 1, wherein the electrolyte comprises about 0.1 to about 3.5 mol of sulphuric acid per 1 of electrolyte solution.

19. The process of claim 1, wherein the diamond layer has a film thickness of from about 0.8 m to about 2.0 m.

20. The process of claim 1, wherein the diamond layer has a film thickness of about 1.0 m.

21. The process of claim 1, wherein the anodic oxidation is carried out at an anodic current density of from about 250 to about 1,350 mA/cm.sup.2.

22. The process of claim 1, wherein the anodic oxidation is carried out at an anodic current density of from about 400 to about 1200 mA/cm.sup.2.

Description

FIGURES

(1) FIG. 1: Comparison of current efficiencies of different cell types with and without rhodanide (promoter)

(2) FIG. 2a: Current/voltage in Pt/HIP and diamond electrodes

(3) FIG. 2b: Current/yield in Pt/HIP and diamond electrodes

(4) FIG. 3: Electrolytic cell according to the inventionplan view

(5) FIG. 4: Cross-section of an electrolytic cell according to the invention.

(6) FIG. 5: Individual components of the electrolytic cell according to the invention.

(7) FIG. 6: Distributor device

(8) FIG. 3 shows a possible embodiment of an electrolytic cell according to the present invention.

(9) A cross-section of this model is shown in diagram form in FIG. 4. Through the inlet tube (1) the electrolyte enters into the distributor device (2a) and is fed from there in a flow-assisted manner to the electrolyte chamber (3). The electrolyte chamber (3) is formed by the annular gap between the outer surface of the anode (4) and the inner surface of the cathode (5). The electrolytic product is collected by the distributor device (2b) and transferred into the outflow tube (6). Seals (7) close the electrolyte chamber between the inlet and outlet tube and the inner surface of the cathode.

(10) In a preferred embodiment, the distributor device (2) can be configured such that the distributor device simultaneously takes over the sealing of the electrolyte chamber.

(11) FIG. 5 shows the individual components of the electrolytic cell according to the invention. The numbering is analogous to FIG. 4. Further components for sealing the electrolytic cell and for assembly are shown in FIG. 5 but are not numbered. These components are known to the person skilled in the art and can be replaced as desired.

(12) FIG. 6 is an enlarged representation of the distributor device (2). The distributor device has a connection point (21) for an outlet or inlet tube and a connection point (22) for the anode (4). The connection point for the anode forms a hollow cylinder which ends flush with the anode tube or rod (4).

(13) Radial bores (23) are distributed over the periphery of the hollow cylinder of the distributor device. By the radial bores (23) in the distributor device the electrolyte can be fed homogeneously into the electrolyte chamber, and can be removed effectively after passage through the electrolyte chamber. Preferably, the distributor device has 3, more preferably 4 and still more preferably 5 radial bores.

EXAMPLE

(14) The preparation of the various peroxydisulphates takes place according to the following mechanisms:

(15) Sodium Peroxydisulphate:

(16) Anode reaction: 2SO.sub.4.sup.2.fwdarw.S.sub.2O.sub.8.sup.2+2e.sup.

(17) Cathode reaction: H.sup.++2e.sup..fwdarw.H.sub.2

(18) Crystallisation: 2Na.sup.++S.sub.2O.sub.8.sup.2Na.sub.2S.sub.2O.sub.8

(19) Overall: Na.sub.2SO.sub.4+H.sub.2SO.sub.4.fwdarw.Na.sub.2S.sub.2O.sub.8+H.sub.2

(20) Ammonium Peroxydisulphate:

(21) Anode reaction: 2SO.sub.4.sup.2.fwdarw.S.sub.2O.sub.8.sup.2+2e.sup.

(22) Cathode reaction: H.sup.++2e.sup..fwdarw.H.sub.2

(23) Crystallisation: 2NH.sub.4.sup.++S.sub.2O.sub.8.sup.2(NH.sub.4).sub.2S.sub.2O.sub.8

(24) Overall: (NH.sub.4).sub.2SO.sub.4+H.sub.2SO.sub.4.fwdarw.Na.sub.2S.sub.2O.sub.8+H.sub.2

(25) Potassium Peroxydisulphate:

(26) Anode reaction: 2SO.sub.4.sup.2.fwdarw.S.sub.2O.sub.8.sup.2+2e.sup.

(27) Cathode reaction: H.sup.++2e.sup..fwdarw.H.sub.2

(28) Crystallisation: 2K.sup.++S.sub.2O.sub.8.sup.2K.sub.2S.sub.2O.sub.8

(29) Overall: K.sub.2SO.sub.4+H.sub.2SO.sub.4.fwdarw.K.sub.2S.sub.2O.sub.8+H.sub.2

(30) The preparation according to the invention of sodium peroxydisulphate is described by way of example in the following.

(31) On the one hand a two-dimensional and on the other hand a three-dimensional cell comprising a boron-doped niobium anode coated with diamond (diamond anode according to the invention) was used for this.

(32) Electrolyte Starting Composition:

(33) Temperature: 25 C.

(34) Sulphuric acid content: 300 g/l

(35) Sodium sulphate content: 240 g/l

(36) Sodium persulphate content: 0 g/l

(37) Active anode area in the cell types used: Tubular cell with platinum-titanium anode: 1,280 cm.sup.2 Tubular cell with diamond-niobium anode: 1,280 cm.sup.2 Flat cell with diamond-niobium anode: 1,250 cm.sup.2

(38) Cathode material: acid-resistant high-grade steel: 1.4539

(39) Solubility limit (sodium persulphate) of the system approx. 65-80 g/l.

(40) Current Densities:

(41) The electrolyte was concentrated accordingly by being circulated (see FIGS. 1 and 2).

(42) Results:

(43) From the course of the current efficiency as a function of the changed sodium persulphate content (FIG. 1) it can be clearly seen that over the entire operating range appropriate for this cell from approx. 100 g/l to about 350 g/l, even without addition of a promoter, the diamond anode used achieves significantly higher current efficiencies than are known for conventional platinum-coated titanium anodes with added promoter.

(44) From the course of the current efficiency as a function of the current density in the preparation of sodium peroxydisulphate using a platinum anode (comparative examples) with the addition of a corresponding promoter and in a diamond anode which is doped with boron and is to be used according to the invention, in each case incorporated in an undivided electrolytic cell (FIGS. 2a+2b), it follows that at a current density of 100-1,500 mA/cm.sup.2 a current efficiency of more than 75% can be obtained.

(45) In contrast, however, the experiments also showed that conventional titanium anodes coated with Pt foil achieved current efficiencies of only at best 60-65% within this operating range, in spite of the addition of a sodium thiocyanate solution as a promoter. Without the addition of a promoter, on the other hand, current efficiencies of only about 35% are achieved, which the present invention confirms.

(46) In summary, it can be confirmed that even without addition of a potential-increasing agent the current efficiency of a diamond-coated niobium anode is about 10% higher than in a cell with a conventional platinum-titanium anode and addition of a potential-increasing agent, and is about 40% higher than in a cell with a conventional platinum-titanium anode without addition of a potential-increasing agent.

(47) The drop in voltage at a diamond-coated anode is about 0.9 volt higher than in a comparable cell with a platinum-titanium anode. It was furthermore found that the current efficiency with a diamond electrode to be used according to the invention without the addition of a promoter decreases only slowly with increasing total content of sodium peroxydisulphate in the electrolyteunder the experimental conditions, for example, at a current efficiency equal to or above 65% electrolyte solutions having a sodium peroxydisulphate content of about 400-650 g/l can be obtained.

(48) Using a conventional platinum anode and co-using a promoter in the electrolyte, in contrast, only equally high peroxydisulphate concentrations of about 300 g/l can be obtained, and indeed at a current efficiency of about 50%.

(49) Random investigations on a similar system with potassium ions from potassium sulphate produced similarly good results.

(50) It is surprising to the person skilled in the art that the process according to the invention can be carried out at high conversions with current densities which are easy to handle industrially without spatial separation of the anolyte and catholyte and without the use of a promoter, with a simultaneously high current efficiency at simultaneously high persulphate and solids concentrations in undivided electrolytic cells without the addition of a promoter.

(51) In the course of the investigations of this invention, it was found that the preparation of ammonium and essentially alkali metal peroxydisulphates with a high current efficiency is also accordingly possible in an undivided cell when a diamond thin film electrode doped with a tri- or pentavalent element is used as the anode. Surprisingly, the cell can also be employed economically appropriately at a very high solids content, essentially peroxydisulphate content, and at the same time the use of a promoter can be dispensed with completely and the electrolysis can be carried out at a high current density, resulting in further advantages, in particular with respect to installation and capital costs.

(52) Summary:

(53) The use of an undivided cell renders possible electrolyte solutions having very high solids concentrations, as a result of which in turn the expenditure of energy in the obtaining of the salt, essentially the crystallisation and the evaporation of water, is significantly reduced, but at least to 25% of that of a divided cell, directly proportionally to the increase in the solids content.

(54) In spite of dispensing with the use of a promoter and therefore dispensing with the purification measures required for the electrolytic gas, higher conversions and higher persulphate concentrations can be obtained in the electrolyte sluiced out.

(55) The operating current density can be reduced significantly compared with platinum anodes with an equally high production quantity, as a result of which less ohmic losses occur in the system and therefore the outlay on cooling is reduced and the degree of freedom in the design of the electrolytic cells and the cathodes is increased.

(56) At the same time the current efficiency and therefore the production quantity can be increased with increasing current density.

(57) Due to the outstanding abrasion resistance of the diamond-coated anode, very much higher flow rates can be used compared with a Pt anode built up similarly in construction.