Undivided electrolytic cell and use thereof

Abstract

The present invention relates to a method for producing an ammonium peroxodisulphate or an alkali-metal peroxodisulphate, to an undivided electrolytic cell constructed from individual components and to an electrolytic apparatus constructed from a plurality of electrolytic cells of this type.

Claims

1. An electrolysis cell, comprising: (a) at least one tubular cathode; (b) at least one rod-shaped or tubular anode, which comprises a conductive support coated with a conductive diamond layer; (c) at least one inlet tube; (d) at least one outlet tube; and, (e) at least two distributing devices, wherein the distributing devices comprise at least one connector for one outlet or inlet tube and one connector for the anode, wherein the distributing devices connect the anode to a current source and the connector for the anode forms a hollow cylinder having radial holes distributed over the periphery of the hollow cylinder.

2. The electrolysis cell of claim 1, wherein the electrolysis cell comprises a common electrolytic space without a diaphragm.

3. The electrolysis cell of claim 1, wherein the spacing between the anode outer surface and the cathode inner surface is between 1 and 20 mm.

4. The electrolysis cell of claim 1, wherein the internal diameter of the cathode is between 10 and 400 mm.

5. The electrolysis cell of claim 1, wherein the anode and the cathode, each independently of one another, are between 20 and 120 cm long.

6. The electrolysis cell of claim 1, wherein the conductive support is selected from the group consisting of silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements, and/or aluminium or combinations of these elements.

7. The electrolysis cell of claim 1, wherein the diamond layer is doped with at least one trivalent or at least one pentavalent main group or B-group element.

8. The electrolysis cell of claim 1, wherein the cathode is made from lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and/or iron alloys.

9. The electrolysis cell of claim 1, wherein an electrolyte of the electrolysis cell is fed through the inlet tube.

10. The electrolysis cell of claim 1, wherein an electrolysis product is removed via the outlet tube of the electrolysis cell.

11. The electrolysis cell of claim 1, wherein the distributing device distributes the electrolyte into an electrolytic space.

12. The electrolysis cell of claim 1, wherein the components of the electrolysis cell can be individually replaced.

13. The electrolysis cell of claim 1, wherein the distributing device is permanently connected to the anode.

14. An electrolysis apparatus comprising at least two of the electrolysis cells of claim 1, wherein the electrolyte flows through the electrolysis cells one after the other and the electrolysis cells are electrochemically connected in parallel.

15. The electrolysis cell of claim 1, wherein the diamond layer is doped with boron and/or phosphorous.

16. The electrolysis cell of claim 1, wherein the cathode is made from acid-resistant high-grade steel.

17. The electrolysis cell of claim 1, wherein an electrolytic space is present as an annular gap between the anode and the cathode.

Description

FIGURES

(1) FIG. 1 shows current efficiencies in comparison with different cell types with and without rhodanide (promoter).

(2) FIG. 2a shows current/voltage in Pt/HIP and diamond electrodes.

(3) FIG. 2b shows current/efficiency in Pt/HIP and diamond electrodes.

(4) FIG. 3 is a plan view of an electrolytic cell according to the invention.

(5) FIG. 4 is a cross-section of an electrolytic cell according to the invention.

(6) FIG. 5 shows the individual components of the electrolytic cell according to the invention.

(7) FIG. 6 shows the distributing 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 schematically in FIG. 4. The electrolyte enters the distributing device (2a) through the inlet tube (1) and is fed from there to the electrolyte space (3) in a flow-optimised manner. The electrolyte space (3) is formed by the annular gap between the outer surface of the anode (4) and the inner surface of the cathode (5). The electrolysis product is collected by the distributing device (2b) and is transferred into the outlet tube (6). Seals (7) close the electrolyte space between the inlet tube and outlet tube and the inner surface of the cathode.

(10) In a preferred embodiment, the distributing device (2) can be constructed such that the distributing device takes on the function of sealing the electrolyte space at the same time.

(11) FIG. 5 shows the individual components of the electrolytic cell according to the invention. The numbering is identical 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 a person skilled in the art and can be replaced as desired.

(12) FIG. 6 is an enlarged view of the distributing device (2). The distributing devices comprise a connector (21) for an inlet or outlet tube and a connector (22) for the anode (4). The connector for the anode forms a hollow cylinder, which is flush with the anode tube or rod (4).

(13) Radial holes (23) are distributed over the periphery of the hollow cylinder of the distributing device. Through the radial holes (23) in the distributing device, the electrolyte can be fed uniformly into the electrolytic space and, after passing through the electrolytic space, can be effectively conducted away. The distributing device preferably comprises three, more preferably four, and still more preferably five, radial holes.

EXAMPLE

(14) The various peroxodisulphates are produced according to the following mechanisms:

(15) Sodium Peroxodisulphate:

(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 Peroxodisulphate:

(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 Peroxodisulphate:

(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) In the following, the production according to the invention of sodium peroxodisulphate is described by way of example.

(31) Both a two-dimensional and a three-dimensional cell, consisting of a boron-doped, diamond-coated niobium anode (diamond anode according to the invention), was used for this purpose.

(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 surface area in the cell types used: Tubular cell with platinum-titanium anode: 1280 cm.sup.2 Tubular cell with diamond-niobium anode: 1280 cm.sup.2 Flat cell with diamond-niobium anode: 1250 cm.sup.2

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

(39) Solubility boundary (sodium persulphate) of the system: approximately 65 to 80 g/l.

(40) Current Densities:

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

(42) Results:

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

(44) From the progression of the current efficiency as a function of the current density during production of sodium peroxodisulphate using a platinum anode (comparative example) with the addition of corresponding promoter and in a boron-doped diamond anode to be used according to the invention, each installed in an undivided electrolytic cell (FIGS. 2a and 2b), it follows that a current efficiency of over 75% can be obtained at a current density of from 100 to 1500 mA/cm.sup.2.

(45) By contrast, however, the tests showed that conventional Pt-foil-coated titanium anodes only reached current efficiencies of at most 60 to 65% within this operating range, despite the addition of a sodium rhodanide solution as a promoter. However, without the addition of a promoter, current efficiencies of only 35% are achieved, and this substantiates the present invention.

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

(47) The drop in voltage at a diamond-coated anode is approximately 0.9 volts higher than in a comparable cell comprising a platinum-titanium anode. Furthermore, it was shown that the current efficiency in a diamond electrode to be used according to the invention without the addition of a promoter and having an increasing total sodium peroxodisulphate content in the electrolyte only decreases slowlyin some test conditions, for example at a current efficiency of equal to or greater than 65%, electrolyte solutions having a sodium peroxodisulphate content of approximately 400 to 650 g/l can be obtained.

(48) By using a conventional platinum anode and also using a promoter in the electrolyte, by contrast only equally high peroxodisulphate concentrations of approximately 300 g/l can be obtained, and this is at a current efficiency of approximately 50%.

(49) Brief tests on a similar system using potassium ions from potassium sulphate produced similarly good results.

(50) It is surprising to a person skilled in the art that the method according to the invention can be performed at high levels of conversion by technically well manageable current densities without the spatial division of the anolyte and the catholyte and without the use of a promoter, at a high current efficiency and at high persulphate and solids concentrations in undivided cells without the addition of a promoter.

(51) As part of the tests for this invention, it was determined that the production of ammonium peroxodisulphates, but primarily alkali-metal peroxodisulphates having a high current efficiency, is accordingly also possible in an undivided cell by using a diamond thin-film electrode doped with a trivalent or pentavalent element. Surprisingly, the cell can also be used in an economically viable manner with a very high solids content, i.e. peroxodisulphate content, and at the same time the use of a promoter can be completely omitted and electrolysis can be performed at high current densities, from which further advantages result, particularly in respect of installation and purchasing costs.

CONCLUSION

(52) The use of an undivided cell makes possible electrolytic solutions having very high solids concentrations, whereby in turn the energy expenditure for salt extraction, essentially crystallisation and water evaporation, is significantly reduced directly proportionally to the increase in the proportion of solids, but is reduced at least to 25% of that of a divided cell.

(53) Despite a promoter not being required and thus the purification measures required for the electrolysis gases being omitted, higher levels of conversion and higher persulphate concentrations can be achieved in the removed electrolyte.

(54) The operating current density can be significantly reduced with respect to platinum anodes at identical production volumes, whereby less ohmic losses occur in the system and thus the energy required for cooling is reduced, and the degree of freedom in the design of the electrolytic cells and the cathodes is increased.

(55) At the same time, the current efficiency and thus the production volume can be increased in the case of an increased current density.

(56) Owing to the excellent abrasion resistance of the diamond-coated anode, much higher flow speeds can be used compared with a structurally similar Pt anode.