EP0400326B1 - Diaphragm cord - Google Patents

Description

The invention relates to a device for preventing cathodic reductions, in particular for preventing cathodic reductions of anodically produced peroxodisulfate, in electrolytic cells in which the anode space is not separated by a separator, such as e.g. a ceramic or glass frit, separated from the cathode compartment, but only a single electrolyte compartment between the cathode and anode (so-called single-chamber cells). In such a cell type, appropriate measures must be taken to prevent the products produced at the anodes from reaching the counter electrode, since they are reduced there.

It is known to wrap an asbestos cord around cathode surfaces to reduce cathodic reduction processes. In a known industrial process (EWM process) for producing ammonium peroxodisulfate, the cathode rods are wrapped with asbestos cord to prevent cathodic reduction (see Ullmanns Enzyklopadie der Technische Chemie, 3rd edition, volume 13, pages 216-221; DE-PS 257 276). Although this device has proven itself over decades, numerous attempts have been made to substitute asbestos as a diaphragm material because of its health hazard. Asbestos dust has a local irritant effect on the mucous membranes of the eyes and the respiratory tract, and leads to dust lung diseases (asbestosis); the use of asbestos materials is therefore subject to legal restrictions in many countries.

The efforts to replace the asbestos in diaphragm lines with other materials have so far been unsuccessful despite numerous and intensive attempts. Neither inorganic fiber materials, such as alumosilicates, quartz or glass fiber fabrics, nor organic polymers, e.g. based on polyvinyl chloride or polyester (e.g. polyethylene terephthalate), showed a reasonably comparable behavior as the known blue asbestos cord: They were developed by chemically attacked the cathode surface, which led to their mechanical decay and loss of function; a pure PeCe® line was sufficiently stable after a few attempts, but an insulating gas cushion formed between the line and the cathode, causing the cell tension to rise to uneconomically high values. If a technically equivalent asbestos replacement cannot be realized, increasing scarcity and legal restrictions on the use of asbestos would lead to the abandonment of a long-established and inexpensive cell type.

It was therefore an object of the present invention to provide cathodes with which the disadvantages indicated above can be overcome and whose properties are comparable to or even exceed those of cathodes having asbestos diaphragms. For example, the flexibility of the cord in a mechanical winding device should not lead to any problems; the resistance should be guaranteed for at least 2 years with the same or even lower cell voltage compared to the asbestos cord, and with an equally good current yield (ie with a correspondingly low cathodic reduction). In addition, the material and manufacturing price of the diaphragm cord should also be within economically justifiable limits. A health risk to the staff should also be excluded. With some anode constructions, for example in the cells of the above-mentioned "EWM process" for the production of ammonium peroxodisulfate, there are further additional significant problems which have to be taken into account when solving the task: the anodes made of platinum wires, which are attached to titanium rods provided with sprouts are fixed, often come into contact with the cathode diaphragm cord winding in the electrolysis cell due to assembly inaccuracies and the small distance, which makes local electrolyte overheating possible. While the inorganic asbestos cord is only slightly damaged, organic materials, such as polyacrylonitrile or PeCe®, which are inherently stable, ie without "anode contact", become less as a result of this Convection and high current density destroy overheated electrolytes (the very aggressive Caro's acid is formed at high temperatures), which causes the winding to open, fall off and expose the underlying cathode surface. This leads to an increase in the cathodic reduction and thus also to corresponding losses in current efficiency, which can be considerable if the cathodic hydrogen evolution strips the textile cord as it were. The cord parts, which are no longer fixed, can then lead to blockages in the cells and thus to a standstill.

The above-mentioned objects are achieved according to the invention by the use of a cord, which consists of a network of a mixture of a material that is resistant to an electrolyte and the cathode and a fluorine-containing polymer, for covering cathode surfaces to form a diaphragm. Preferred embodiments thereof are the subject of claims 2 to 15.

The fibers resistant to electrolytes and cathodes are preferably those made from polyvinyl chloride (PVC), in particular post-chlorinated polyvinyl chloride (e.g. PeCe®) and / or polyacrylonitrile (PAN); The fluorine-containing polymers used are preferably polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymers (FEP, and / or in particular polytetrafluoroethylene (PTFE).

The fibers made of the electrolytically and cathodically resistant material preferably form the main component of the diaphragm cord; in particular, the quantitative ratio is 95 to 70% by volume of material resistant to the electrolyte and the cathode and 5 to 30% by volume of fluorine-containing polymers.

The braid can consist of two or more yarns, each of which is formed from fibers of the same material group is formed, but it can also be formed from yarns which consist of two or more different fibers of the same or both material groups.

The yarns can be produced in a manner known and customary in textile technology by twisting (from threads) or spinning (from fibers) and processing them into braids; the diaphragm cords used according to the invention are therefore easy and inexpensive to obtain.

The proportion of the fluorine-containing polymers in the diaphragm cord used according to the invention and their structure (structure of the braid) are selected so that the mechanical strength (cohesion) of the diaphragm cords when used in each case by the thermally and chemically stable fluorine-containing polymers, such as e.g. PTFE, is also guaranteed if, e.g. due to contact with the anode wires, high overheating occurs.

The structure of the diaphragm cord used according to the invention (structure of the braid) can be one of the structures customary in textile technology when processing yarns into cords (braids); it depends in particular on the desired mechanical strength and the intended use of the diaphragm cord, but also on the proportion of fluorine-containing polymers in the braid.

The diaphragm cords used according to the invention preferably have a diameter of 2 to 5 mm; in a preferred embodiment they are formed as a round braid, and in another preferred embodiment as a band, in particular with a width of 6 to 10 mm and a thickness of 2 to 3 mm.

The diaphragm cords used according to the invention generally consist of 10 to 25 individual yarns, which in turn contain 20 to 100 elementary fibers with a diameter of preferably 10 to 50 μm, and in particular 10 to 30 μm. The material resistant to the electrolyte and the cathode is preferably spun from individual fibers with a thickness of 10 to 50 μm, in particular 30 μm, into yarns with a thickness of 100 to 900 μm, in particular from 300 to 400 μm; the fluorine-containing polymer is preferably spun from individual fibers with a diameter of 10 to 50 μm, in particular 30 μm, into yarns with a diameter of 50 to 150 μm.

The textile-technical admixture of the proportion of fluorine-containing polymer is expediently such that from e.g. 12 coils with the main component (e.g. PAN, PeCe®) and e.g. 4 coils with the fluorine-containing polymer (e.g. PTFE, PVDF, FEP) is processed into a round braid, resulting in a proportion of approx. 25 to 30% by volume of fluorine-containing polymer, which is evenly distributed in the matrix of the main component. In an analogous manner, different proportions of fluorine-containing polymer can be set by varying the ratio of the number of coils.

The type and mesh structure of the diaphragm cord used according to the invention, as well as their composition from fibers of different hydrophilicity, means that the hydrogen formed on the cathode can easily escape (in fine bubbles) through the winding in the electrolyte. The use of a pure fluoropolymer cord (e.g. PTFE cords), on the other hand, leads to the formation of gas cushions between the cathode and the diaphragm winding, as a result of which the cell voltage rises to unusually high values. The diaphragm cord used according to the invention, on the other hand, has excellent degassing behavior, which has an effect even in a reduced cell tension compared to asbestos.

As tests have shown, e.g. in the electrolysis for the production of ammonium peroxodisulfate in sulfuric acid ammonium sulfate solutions as diaphragm cords, which are made on the coolable cathode tubes from e.g. Graphite or stainless steel can be wound, those made of polyacrylonitrile (PAN) and post-chlorinated polyvinyl chloride (PeCe®) can be used. PAN has proven to be particularly favorable, possibly because of its hydrophilic behavior in the electrolyte. Compared to asbestos, there are reduced cell tensions, even after the build-up of the hydroxide cover layers in and on the fabric winding, which are formed from the inevitable impurities in the electrolyte, namely Fe³⁺ and Mg²⁺ and others, through the alkalization of the cathode surface and the diaphragm winding. Diaphragm windings made from a post-chlorinated polyvinyl chloride cord (PeCe® cord, the cell voltages of which are 0.1 to 0.2 volts (presumably due to the lower hydrophilicity) above those of PAN diaphragm cords, also show a similarly good behavior.

However, if these cathode wraps come into contact with the platinum wires of the anodes - which is inevitable in the long run in technical operation because of the small distances between them - they are locally cut at the contact points and the wrapping falls off. With the diaphragm cords used according to the invention, this can be reliably avoided since the fluoropolymer portion remains undamaged; upon contact with the platinum wires of the anodes, the mechanical cohesion of the diaphragm cord is not released and local damage remains insignificant due to the exposed cathode surface, which is only a few millimeters wide.

The use of the diaphragm cords according to the invention for covering cathode surfaces is particularly suitable for use in the electrolytic production of peroxodisulfates, such as ammonium persulfate. Iron and / or magnesium ions are preferably added to the electrolyte solution in an amount of> 2 mM / l.

Possible cathodes are the cathodes which are customarily known and customary for electrolytic production processes, and in particular the cathodes customary for the electrolytic production of peroxodisulfates, such as e.g. Graphite or stainless steel cathodes. The cathodes can be of any shape appropriate for the intended use and e.g. be designed as cathode rods with a prism or circular cross section, plates, etc. Coolable tube cathodes are primarily used, in particular for the production of ammonium peroxodisulfate.

The surfaces of the cathodes exposed to the electrolyte are wrapped with the diaphragm cord used in accordance with the invention, the wrapping preferably being single-layer, spirally tangential and completely covering the cathode surface.

FIG. 1 shows the use of a diaphragm cord according to the invention using the example of a tubular cathode (1); FIG. 1a shows a wrap in which the diaphragm cord (2) has a circular cross section, and FIG. 1b shows a wrap with a diaphragm cord (2) according to the invention, which is designed as a band.

The present invention also relates to a tubular cathode, which is characterized in that it is wrapped with a diaphragm cord as described above in a single layer, spirally tangent and completely covers the cathode surface.

The examples below are intended to explain the invention in more detail without restricting it thereto.

Examples

The tests for testing the diaphragm cords were carried out in a model electrolysis cell, which consists of a transparent PVC tube with a diameter of 150 mm, which is welded to a PVC floor on one side and a flange on the other and forms the cell container standing upright. Four coolable cathode tubes made of post-compressed graphite are arranged symmetrically in the flange lid, and the anode construction equipped with platinum wire is located in a center line. The cathode tubes, 860 mm in length, are wrapped with the diaphragm cord, ie over a length of about 750 mm, beyond the electrolyte limit; At this height there is the drain of the electrolyte, the inlet by means of a hose nozzle above the bottom of this cylinder cell. It represents the 98th part of an operating cell and, with the help of gas analysis, allows the determination of the current yield and, since the cylinder wall is transparent, the visual observation of the processes inside the cell.

Example 1:

Graphite tubes with a diameter of 30 mm are tightly wrapped with a diaphragm cord of PAN (80% by volume) and PTFE (20% by volume) with a diameter of 3.5 mm (4.3 g / m) according to the invention with 330 turns per meter. After insertion into the model cell, it is filled with an operating electrolyte for the production of ammonium peroxodisulfate, which contains 100 to 220 g / l (NH₄) ₂S₂O ₈, approx. 500 g / l (NH₄) ₂SO₄ and 20 to 150 g / l H₂SO₄. NH₄SCN is metered in according to the amount of peroxomonosulfate formed by hydrolysis. After several weeks of operation, there was no visual or electron-optical change in the PAN fibers of the diaphragm cord.

Comparative example:

A tubular cathode as described in Example 1 is produced, with the exception that instead of the PAN / PTFE diaphragm cord used according to the invention, a diaphragm cord which contains only PAN is used. If this cathode diaphragm winding is brought into intimate contact with pure PAN cords by twisting the anodes with their platinum wires, the PAN fibers are cut at the contact points and finally the Cord winding lifted from the cathode tube; Similarly, the cathodic reduction increases to values up to 100%. The disintegrating fiber pieces hinder the flow of electrolyte and thus significantly interfere with process technology and safety.

On the other hand, if a diaphragm cord made of PAN with 20% by volume of PTFE is used as the diaphragm cord according to the invention, the wrapping of the cathode tubes is retained upon contact with the platinum wires of the anodes. The injured part of the diaphragm is only very small and is irrelevant to the overall yield of the electrolysis. A persulfate reduction current efficiency between 4 and 7% is observed both before and after contact with the anodes.

Example 2:

In an operating cell with 98 anodes, a diaphragm cord made of PAN with 20% by volume PTFE was used to wrap the 210 cathode tubes according to the invention, and with a winding density as specified in Example 1.

After one year of operation, the cell was opened: no changes or significant damage to the cathode windings were discernible. The cell had an operating voltage that was 0.5 volts below the analogue positioned cells with asbestos development, with otherwise the same peroxodisulfate yield. The energy saving corresponds to approx. 7%. The holes in the cord at places with overlying anodes were less than 1 ‰ and played no role in the overall yield. Short circuits between the anode and cathode were avoided due to the intact PTFE yarns.

Claims (16)

1. Use of a cord, which consists of a braided fabric of a mixture of material, which is resistant with respect to an electrolyte and the cathode, and a fluorine-containing polymer, for sheathing cathode surfaces while forming a diaphragm.
2. Use according to Claim 1, characterised in that the material which is resistant with respect to the electrolyte and the cathode forms the major constituent of the braided fabric.
3. Use according to Claim 1, characterised in that the material which is resistant with respect to the electrolyte and the cathode is present in a quantity of 95 to 70 % by volume and the fluorine-containing polymer is present in a quantity of 5 to 30 % by volume.
4. Use according to any one of Claims 1 to 3, characterised in that the material which is resistant with respect to the electrolyte and the cathode is polyacrylonitrile and/or post-chlorinated polyvinyl chloride.
5. Use according to any one of the preceding Claims, characterised in that the fluorine-containing polymer is polytetrafluoroethylene, a copolymer of tetrafluoroethylene/hexafluoropropylene and/or polyvinylidene fluoride.
6. Use according to any one of the preceding Claims, characterised in that the braided fabric consists of a mixture of polyacrylonitrile and polytetrafluoroethylene.
7. Use according to any one of Claims 1 to 5, characterised in that the braided fabric consists of a post-chlorinated polyvinyl chloride and polytetrafluoroethylene.
8. Use according to any one of the preceding Claims, characterised in that the cord consists of 10 to 25 individual threads which, in turn, contain 20 to 100 elementary fibres with a diameter of 10 to 50 µm.
9. Use according to any one of the preceding Claims, characterised in that the material which is resistant with respect to the electrolyte and the cathode is spun from individual fibres with a diameter of 10 to 50 µm to form threads with a diameter of 100 to 900 µm.
10. Use according to any one of the preceding Claims, characterised in that the fluorine-containing polymer is spun from individual fibres with a diameter of 10 to 50 µm to form threads with a diameter of 50 to 150 µm.
11. Use according to any one of the preceding Claims, characterised in that the cord is in the form of a round braided fabric.
12. Use according to any one of the preceding Claims, characterised in that the cord has a diameter of 2 to 5 mm.
13. Use according to any one of Claims 1 to 10, characterised in that the cord is in the form of a tape with a width of 6 to 10 mm and a thickness of 2 to 3 mm.
14. Use according to any one of the preceding Claims for a cathode for the electrolytic preparation of peroxide sulphates, in particular of ammonium peroxide disulphate.
15. Use according to Claim 14, characterised in that the electrolyte contains iron and/or magnesium ions in a quantity of > 2 mM/l.
16. A tubular cathode characterised in that, in accordance with any one of Claims 1 to 13, it is enveloped by the diaphragm cord in a single layer, helically at a tangent and fully covering the cathode surface.

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