EP2872673B1 - Undivided electrolytic cell and use of the same - Google Patents

Description

In one aspect, the present invention relates to an electrolysis cell and its use for producing an ammonium or alkali metal peroxodisulfate.

It is known in the prior art to produce alkali metal and ammonium peroxodisulphate by anodic oxidation of an aqueous solution containing the corresponding sulphate or hydrogen sulphate and to obtain the resulting salt from the anolyte by crystallization. Since in this process the decomposition voltage is above the decomposition voltage of the anodic oxygen formation from water, a so-called promoter, usually thiocyanate as sodium or ammonium thiocyanate, is used to increase the decomposition voltage of water to oxygen (oxygen overvoltage) on a platinum anode that is commonly used.

Rossberger ( US 3915816 (A )) describes a process for the direct production of sodium persulfate. Undivided cells with platinum-coated anodes based on titanium are described as electrolysis cells. The current yields shown are based on the addition of a potential-increasing promoter.

According to DE 27 57 861 sodium peroxodisulfate is produced with a current efficiency of 70 to 80% in an electrolysis cell with a cathode protected by a diaphragm and a platinum anode by adding a neutral aqueous anolyte solution with an initial content of 5 to 9% by weight of sodium ions, 12 to 30% by weight Sulfate ions, 1 to 4% by weight ammonium ions, 6 to 30% by weight peroxodisulfate ions and a potential-increasing promoter, such as, in particular, thiocyanate Using a sulfuric acid solution as a catholyte at a current density is electrolyzed by at least 0.5 to 2 A / cm ² . After the peroxodisulfate has crystallized out and separated off from the anolyte, the mother liquor is mixed with the cathode product, neutralized and fed back to the anode.

1. 1. Um die Sauerstoffentwicklung zu vermindern ist der Einsatz eines Promoters notwendig.
2. 2. Um die beschriebenen hohen Stromausbeuten zu erreichen, ist es erforderlich, dass Anode und Kathode durch den Einsatz einer geeigneten Membran räumlich getrennt sind. Die hierzu erforderlichen Membranen sind sehr stark abbrassionsempfindlich.
3. 3. Das Erfordernis einer hohen Stromdichte und damit eines hohen Anodenpotentials, um eine wirtschaftlich akzeptable Stromausbeute zu erhalten.
4. 4. Die mit der Herstellung der Platinanode verbundenen Probleme insbesondere mit Hinblick auf den Erhalt einer für technische Zwecke akzeptablen Stromausbeute und hohen Lebensdauer der Anode. Beispielhaft zu nennen ist hier der kontinuierliche Platinabtrag, der bis zu 1 g/to Produkt im Persulfat vorhanden sein kann. Dieser Platinabtrag wirkt einerseits produktverunreinigend und führt andererseits zum Verbrauch eines wertvollen Rohstoffs wodurch nicht zuletzt auch die Verfahrenskosten erhöht werden.
5. 5. Die Herstellung von Persulfaten mit niedrigem Löslichkeitsprodukt, i.w. Kalium- und Natriumpersulfat, ist so nur in extrem hoher Verdünnung möglich. Dies macht einen hohen Energieeintrag bei der Kristallbildung erforderlich.
6. 6. Bei Verwendung des sogenannten Konvertierverfahren müssen hergestellte Persulfate aus einer Ammoniumpersulfatlösung umkristallisiert werden. Hieraus resultiert in der Regel eine verringerte oder sogar mangelnde Reinheit des Produktes.

Disadvantages of this procedure are:

1. 1. The use of a promoter is necessary to reduce the development of oxygen.
2. 2. In order to achieve the high current yields described, it is necessary that the anode and cathode are spatially separated by using a suitable membrane. The membranes required for this are very sensitive to abrasion.
3. 3. The requirement of a high current density and thus a high anode potential in order to obtain an economically acceptable current yield.
4. 4. The problems associated with the production of the platinum anode, in particular with regard to obtaining a current yield that is acceptable for technical purposes and a long service life of the anode. One example is the continuous removal of platinum, which can contain up to 1 g / to product in the persulfate. This removal of platinum on the one hand has the effect of contaminating the product and on the other hand leads to the consumption of a valuable raw material, which not least increases the process costs.
5. 5. The production of persulfates with a low solubility product, mainly potassium and sodium persulfate, is only possible in extremely high dilution. This makes a high energy input necessary for the crystal formation.
6. 6. When using the so-called conversion process, produced persulfates must be recrystallized from an ammonium persulfate solution. As a rule, this results in a reduced or even insufficient purity of the product.

EP-B 0 428 171 discloses a filter press type electrolytic cell for the production of peroxo compounds including ammonium peroxodisulfate, sodium peroxodisulfate and potassium peroxodisulfate. Platinum foils applied hot isostatically to a valve metal are used as anodes. A solution of the corresponding sulfate containing a promoter and sulfuric acid is used as the anolyte. This method also has the aforementioned problems.

In the process of DE 199 13 820 Peroxodisulfates are produced by anodic oxidation of an aqueous solution containing neutral ammonium sulfate. For the purpose of producing sodium or potassium peroxodisulfate, the solution obtained from the anodic oxidation, which contains ammonium peroxodisulfate, is reacted with sodium hydroxide solution or potassium hydroxide solution. After crystallization and separation of the corresponding alkali metal peroxodisulfate, the mother liquor is recycled in a mixture with the catholyte produced in the electrolysis. In this process, too, the electrolysis takes place in the presence of a promoter on a platinum electrode as the anode.

Although peroxodisulfates have been obtained on an industrial scale for decades by anodic oxidation on a platinum anode, these processes still have serious disadvantages (see also the list above). It is always necessary to add promoters, also known as polarizers, in order to increase the oxygen overvoltage and improve the current yield. As oxidation products of these promoters, which inevitably arise as by-products during the anodic oxidation, toxic substances get into the anode exhaust gas and must be removed in a gas scrubber. High current yields also require a separation of anolyte and catholyte. The anodes, which are usually completely covered with platinum, always require a high current density. This results in a current load on the anolyte volume, the separator and the cathode, whereby additional measures to reduce the cathodic current density by a three-dimensional structuring of the electrolysis cell and activation are required. In addition, there is a high thermal load on the unstable peroxodisulfate solution. In order to minimize this load, constructive measures have to be taken, and the cooling effort also increases. Because of the limited heat dissipation, the electrode area must be limited, and this increases the installation effort per cell unit. In order to cope with the high current load, additional electrode support materials with high heat transfer properties usually have to be used, which in turn are susceptible to corrosion and expensive.

Michaud, PA, et al. teach in Electro Chemical and Solid-State Letters, 3 (2) 77-79 (2000 ) the production of peroxodisulfuric acid by anodic oxidation of sulfuric acid using a thin-film diamond electrode doped with boron. This document teaches that such electrodes have a higher overvoltage for oxygen than platinum electrodes. However, the publication gives no indication of the industrial production of ammonium and alkali metal peroxodisulfates using boron-doped diamond thin-film electrodes. It is known that sulfuric acid on the one hand and hydrogen sulfates, in particular neutral sulfates, on the other hand, behave very differently in the anodic oxidation. Despite the increased overvoltage of the oxygen at the boron-doped diamond electrode, the main side reaction, in addition to the anodic oxidation of sulfuric acid, is the development of oxygen and also of ozone.

As part of their patent EP 1148155 B1 Stenner and Lehmann recognized as early as 2001 that when using a diamond-coated, split electrolysis cell for the production of persulfates, no additional promoter is required in order to achieve such high current yields. The disadvantage of this process is, primarily because of the sensitive separators, as already described above, that the production of persulfates with a low solubility product, iw Potassium and sodium persulfate, is only possible with extremely high dilution, i.e. below the solubility limit, which requires a high input of energy during crystal formation and the discharge of salt in the course of evaporation and drying.

The international patent application WO 2011/066632 describes, in connection with the purification of water, a cylindrical electrolysis cell which each comprises a tubular cathode and anode, the anode comprising a conductive support coated with a conductive diamond layer. There are also inlet pipes, outlet pipes and, if necessary, a diaphragm to separate the electrolyte space. The electrolysis cell is specially designed for the processing of wastewater and accordingly not designed for high solid concentrations in the electrolyte solution. Distribution facilities are not available.

The object of the present invention is to provide an electrolysis cell which can be used in a technical process for the production of ammonium and alkali metal peroxodisulfates, the disadvantages of the known processes being able to be avoided or at least only occurring to a lesser extent. The use of a diamond-coated, undivided cell for the production of persulfates, in particular those with a low solubility potential in sulfate and sulfuric acid-containing electrolyte solutions or electrolyte suspensions, is to be made possible in order to not only have the electrochemical advantages shown in the course of this invention, but also in particular those already from other applications of a diamond coating The carrier's known, mechanical and abrasive properties can also be used for the electrochemical oxidation of sulfates in suspensions, as listed above.

1. (a) mindestens eine röhrenförmige Kathode,
2. (b) mindestens eine stab- oder röhrenförmige Anode, die einen mit einer leitfähigen Diamantschicht beschichteten leitfähigen Träger umfasst,
3. (c) mindestens ein Zulaufrohr,
4. (d) mindestens ein Ablaufrohr, und
5. (e) mindestens zwei Verteilereinrichtungen, wobei die Verteilereinrichtungen mindestens eine Anschlussstelle für mindestens ein Zu- oder Ablaufrohr und eine Anschlussstelle für die Anode aufweisen und wobei die erste Verteilereinrichtung dazu ausgebildet ist, Elektrolyt aus dem Zulaufrohr in einen Elektrolytraum zu verteilen und die zweite Verteilereinrichtung dazu ausgebildet ist, umgesetzten Elektrolyt zu sammeln und über das Ablaufrohr abzuführen,

und wobei der Elektrolytraum als Ringspalt zwischen innenliegender Anode und außenliegender Kathode gebildet ist.To solve this problem, the present application accordingly provides an electrolysis cell comprising the components:

1. (a) at least one tubular cathode,
2. (b) at least one rod-shaped or tubular anode which comprises a conductive support coated with a conductive diamond layer,
3. (c) at least one inlet pipe,
4. (d) at least one drain pipe, and
5. (e) at least two distributor devices, wherein the distributor devices have at least one connection point for at least one inlet or outlet pipe and one connection point for the anode, and wherein the first distributor device is designed to distribute electrolyte from the inlet pipe into an electrolyte space and the second distributor device for this purpose is designed to collect converted electrolyte and discharge it via the drain pipe,

and wherein the electrolyte space is formed as an annular gap between the inner anode and the outer cathode.

Also described herein is a process for the preparation of an ammonium or alkali metal peroxodisulfate comprising
anodic oxidation of a salt from the series ammonium sulfate, alkali metal sulfate and / or the corresponding aqueous electrolyte containing hydrogen sulfate in an electrolytic cell,
comprising at least one anode and one cathode,
wherein a diamond layer arranged on a conductive support and doped with a trivalent or pentavalent element is used as the anode,
wherein the electrolysis cell comprises an undivided electrolysis space between the anode and the cathode and the aqueous electrolyte does not contain a promoter for increasing the decomposition voltage from water to oxygen.

The salt used for the anodic oxidation from the series ammonium sulfate, alkali metal sulfate and / or the corresponding hydrogen sulfates can be any desired alkali metal sulfate or corresponding hydrogen sulfate. For example, the use of sodium and / or potassium sulfate and / or the corresponding hydrogen sulfate is possible.

“Promoter” or also “polarizer” in the context of the present application is any agent which is known to the person skilled in the art as an additive when carrying out an electrolysis to increase the decomposition voltage of water to oxygen or to improve the current yield. An example of such a promoter used in the prior art is thiocyanate, such as, for example, sodium or ammonium thiocyanate. Such a promoter is not used in the process described. In other words, in the method described herein, the electrolyte has a promoter concentration of 0 g / l. By dispensing with a promoter during the process, there are, for example, no cleaning requirements relating to typical electrolysis gases.

In the method described herein, an anode is used which comprises a diamond layer arranged on a conductive support and doped with a 3- or 5-valent element. One advantage of this feature is the very high wear resistance of the diamond coating. Long-term tests have shown that such electrodes have a minimum life of more than 12 years.

The anode used can be of any shape.

Any anode carrier material known to the person skilled in the art can be used. For example, the carrier material can be selected be out of the group consisting of silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements and / or aluminum or combinations of the elements.

The cathode used in the method described herein is preferably made of lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and / or acid-resistant stainless steels, as they are known to the person skilled in the art. The cathode can have any spatial configuration.

In the electrolysis cell used in the process described, the electrolyte space between the anode and cathode is undivided, i.e. there is no separator between anode and cathode. The use of an undivided cell enables electrolyte solutions with very high solids concentrations, which in turn significantly reduces the energy expenditure in salt production, essentially crystallization and water evaporation, in direct proportion to the increase in the solids content, but at least to 25% of that of a divided cell.

The method described is carried out in a three-dimensional cell. The cell is designed as a tubular cell. The use of a tube geometry, i.e. a tube cell, Consisting of an inner tube as the anode, preferably made of diamond-coated niobium, and an outer tube as the cathode, preferably made of acid-resistant stainless steel, is an advantageous construction with low material costs. The use of an annular gap as a common electrolyte space leads to a uniform and thus low flow loss Flow through and thus a high utilization of the available electrolysis surfaces, which in turn means a high current yield. The production costs of such a cell are low compared to a so-called flat cell.

Several electrolysis cells can be combined, preferably in the form of a double tube package.

The electrolyte used preferably has an acidic, preferably sulfuric acid, or neutral pH.

The electrolyte can be circulated through the electrolytic cell during the process. This prevents an undesirable high electrolyte temperature in the cell, which accelerates the decomposition of the persulfates.

The method can include discharging electrolyte solution from the electrolyte circuit. This can be done in particular to obtain peroxodisulfate produced. A further aspect therefore relates to the recovery of generated peroxodisulfates by crystallization and separation of the crystals from the electrolyte solution with the formation of an electrolyte solution, the electrolyte solution preferably being removed from the electrolyte circuit beforehand was discharged. A recirculation of the electrolyte mother liquor is possible, especially if before generated peroxodisulfates were separated, with increasing the content of acid, sulfate and / or hydrogen sulfate in the electrolysis cell.

The anodic oxidation can be carried out at an anodic current density of 50-1500 mA / cm ^2, in particular about 50-1200 mA / cm ² . A current density used particularly preferably is in the range from 60-975 mA / cm ² .

The electrolyte used preferably has a total solids content of about 0.5 to 650 g / l. The (working) electrolyte preferably contains about 100 to about 500 g / l persulfate, more preferably about 150 to about 450 g / l persulfate and most preferably 250-400 g / l persulfate. The method thus enables high solids concentrations in the electrolyte solution in particular, without the addition of a potential-increasing agent or promoter and the resulting requirements for exhaust gas and wastewater treatment with high current yields in the peroxodisulfate production at the same time.

Furthermore, the electrolyte solution preferably contains from about 0.1 to about 3.5 mol of sulfuric acid per liter (l) of electrolyte solution, more preferably 1-3 mol of sulfuric acid per liter of electrolyte solution, and most preferably 2.2-2.8 mol of sulfuric acid per liter of electrolyte solution.

In summary, an electrolyte with the following composition is particularly preferably used in the process described herein: 150 to 500 g of persulfate and 0.1 to 3.5 mol of sulfuric acid per mol of electrolyte solution per liter of electrolyte. The total solids content is preferably 0.5 g / l to 650 g / l, more preferably 100-500 g / l and most preferably 250-400 g / l, the sulphate content being variable. The promoter portion is 0 g / l.

The invention relates to a built up from individual components, Undivided electrolysis cell, an electrolysis device composed of several such electrolysis cells, and its use for the oxidation of an electrolyte, as defined in the claims.

“Electrolysis” is understood to mean a chemical change caused by the passage of current through an electrolyte, which is expressed in a direct conversion of electrical energy into chemical energy through the mechanism of electrode reactions and ion migration. The technically most important electrochemical conversion is the electrolysis of saline solution, which produces caustic soda and chlorine gas. The production of inorganic peroxides is nowadays carried out on an industrial scale in electrolysis cells.

In large-scale industrial processes, it is particularly desirable that the reactions can be carried out at high concentrations of starting materials and corresponding products. High product concentrations ensure that the end product is easy to work up, since the solvent must be removed if the reaction products are in solution. In the electrolysis of highly concentrated electrolytes, the energy consumption of the downstream processing of the electrolysis products can thus also be reduced.

However, applications with very high solids content place high demands on the components of the electrolysis cell due to the abrasive effect of the electrolyte. In particular, the diaphragm, which in divided electrolysis cells prevents the reaction products of the anode and cathode compartments from mixing, does not permanently withstand electrolysis processes at high concentrations. In the case of high solids contents, electrolysis can therefore only be carried out in undivided cells, in which the anode compartment and the cathode compartment do not have to be spatially separated by the use of a suitable membrane. Such undivided cells are used in particular when neither reactants nor Products made on anode or cathode in are changed in a disturbing way by the other electrode process or react with one another.

Furthermore, the anode and cathode materials must also meet the mechanical requirements at high solids concentrations and therefore be extremely wear-resistant.

In order to make the electrolysis as economical as possible, the electrolysis cells must be designed so that the electrolysis can be carried out at the highest possible current densities. This is only possible if the anode and cathode have good electrical conductivity and are chemically inert to the electrolyte. Usually graphite or platinum is used as anode material. However, these materials have the disadvantage that they do not have sufficient abrasion resistance at high solids concentrations.

The manufacture of mechanically extremely stable and inert electrodes is discussed in DE 199 11 746 disclosed. Electrodes are coated with an electrically conductive diamond layer, the diamond layer being applied using a chemical vapor deposition (CVD) process.

The object of the present invention is to provide an electrolysis cell which enables a continuous and optimized electrolysis process at high solids concentrations (up to about 650 g / l) and at high current density ranges (up to about 1500 mA / cm ² ). The electrolysis cell should be adapted to the electrochemical reactions to be carried out and individual components should be able to be exchanged easily without the actual cell body being destroyed.

1. (a) mindestens eine röhrenförmige Kathode
2. (b) mindestens eine stab- oder röhrenförmige Anode, die einen mit einer leitfähigen Diamantschicht beschichteten leitfähigen Träger umfasst,
3. (c) mindestens ein Zulaufrohr,
4. (d) mindestens ein Ablaufrohr, und

mindestens zwei Verteilereinrichtungen, wobei die Verteilereinrichtungen mindestens eine Anschlussstelle für mindestens ein Zu- oder Ablaufrohr und eine Anschlussstelle für die Anode aufweisen und wobei die erste Verteilereinrichtung dazu ausgebildet ist, Elektrolyt aus dem Zulaufrohr in einen Elektrolytraum zu verteilen und die zweite Verteilereinrichtung dazu ausgebildet ist, umgesetzten Elektrolyt zu sammeln und über das Ablaufrohr abzuführen, und wobei der Elektrolytraum als Ringspalt zwischen innenliegender Anode und außenliegender Kathode gebildet ist.
gelöst werden.Surprisingly, an electrolysis cell made it possible to complete the task:

1. (a) at least one tubular cathode
2. (b) at least one rod-shaped or tubular anode which comprises a conductive support coated with a conductive diamond layer,
3. (c) at least one inlet pipe,
4. (d) at least one drain pipe, and

at least two distributor devices, wherein the distributor devices have at least one connection point for at least one inlet or outlet pipe and one connection point for the anode, and wherein the first distributor device is designed to distribute electrolyte from the inlet tube into an electrolyte compartment and the second distributor device is designed to to collect converted electrolyte and to discharge it via the drain pipe, and wherein the electrolyte space is formed as an annular gap between the inner anode and the outer cathode.
be solved.

- In the electrolytic cell, the anode and cathode are arranged concentrically to one another, so that - the electrolyte space is formed as an annular gap between the inner anode and the outer cathode. - -The diameter of the cathode is therefore larger than that of the anode.

In a preferred embodiment, the electrolyte space does not contain a membrane or a diaphragm. In this case it is an electrolysis cell with a common electrolyte space, i. the electrolysis cell is undivided.

The distance between the anode outer surface and the cathode inner surface is preferably between 1-20 mm, more preferably between 1-15 mm, even more preferably between 2-10 mm and most preferably between 2-6 mm.

The inner diameter of the cathode is preferably between 10-400 mm, more preferably between 20-300 mm, even more preferably between 25-250 mm.

In a preferred embodiment, the anode and cathode are each independently between 20-120 cm, more preferably between 25-75 cm long.

The length of the electrolyte space is preferably at least 20 cm, more preferably at least 25 cm, and at most preferably 120 cm, more preferably 75 cm.

The cathode used according to the invention is preferably made of lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and / or iron alloys, in particular made of stainless steel, in particular acid-resistant stainless steel. In a preferred embodiment, the cathode is made of acid-resistant stainless steel.

The base material of the rod-shaped or tubular, preferably tubular, anode is preferably silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements, and / or aluminum, or combinations of the elements.

The anode support material can be identical to the anode base material or different. In a preferred embodiment, the anode base material functions as a conductive carrier. Any desired conductive material known to the person skilled in the art can be used as the conductive carrier. Particularly preferred carrier materials are silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements, and / or aluminum, or combinations of the elements. Silicon, titanium, niobium, tantalum, tungsten or carbides of these elements, more preferably niobium or titanium, even more preferably niobium, are particularly preferably used as the conductive support.

A conductive diamond layer is applied to this carrier material. The diamond layer can be doped with at least one 3-valent or at least one 5-valent main or sub-group element. The doped diamond layer is thus an n-conductor or a p-conductor. It is preferred here that a boron-doped and / or phosphorus-doped diamond layer is used. The amount of doping is set in such a way that the desired, usually just sufficient, conductivity is achieved. For example, when doped with boron, the crystal structure can contain up to 10,000 ppm, preferably from 10 ppm to 2000 ppm, boron and / or phosphorus.

The diamond layer can be applied over the entire surface or in sections, preferably on the entire outer surface of the rod-shaped or tubular anode. The conductive diamond layer is preferably free of pores.

Processes for applying the diamond layer are known to the person skilled in the art. The diamond electrodes can be produced in two special CVD (Chemical Vapor Deposition) processes. These are the microwave plasma CVD and hot wire CVD processes. In both cases, the gas phase, which is activated to the plasma by microwave irradiation or thermally by hot wires, is made up of methane, hydrogen and possibly other additives, in particular a gaseous compound of the dopant.

By using the boron compound such as trimethyl boron, a p-type semiconductor can be provided. Using a gaseous phosphorus compound as a dopant, an n-type semiconductor is obtained. By depositing the doped diamond layer on crystalline silicon, a particularly dense and pore-free layer is obtained. The diamond layer is preferably applied in a film thickness of about 0.5-5 μm, preferably about 0.8-2.0 μm and particularly preferably about 1.0 μm to the conductive carrier used according to the invention. In another embodiment, the diamond layer is preferably applied in a film thickness of 0.5-35 μm, preferably 5-25 μm, most preferably 10-20 μm, to the conductive support used according to the invention.

As an alternative to the deposition of the diamond layer on a crystalline material, the deposition can also take place on a self-passivating metal such as titanium, tantalum, tungsten, or niobium. To produce a particularly suitable boron-doped diamond layer on a silicon single crystal, PA Michaud (Electrochemical and Solid State Letters, 3 (2) 77-79 (2000 )) referenced.

In the context of the present invention, the use of an anode comprising a niobium or titanium support with a boron-doped diamond layer, in particular with a diamond layer doped with up to 10,000 ppm boron, is particularly preferred.

The diamond-coated electrodes are characterized by a very high mechanical strength and abrasion resistance.

The anode and / or the cathode, more preferably the anode and the cathode, even more preferably the anode, is preferably connected to the power source via the distributor device. In the event that the anode and cathode are connected to the power source via the distributor device, it must be ensured that the distributor device is electrically insulated accordingly. In any case, good electrical contact between the anode and / or cathode and the distributor device must be ensured.

The distributor device also ensures a homogeneous feed of the electrolyte from the feed pipe into the electrolyte space. After the electrolyte has passed the electrolyte compartment, the converted electrolyte (electrolysis product) is effectively collected with the help of at least one upstream distributor and discharged via a drain pipe.

The distributor devices according to the invention preferably consist, independently of one another, of silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements and / or aluminum or combinations of the elements, particularly preferably of titanium.

The distributor devices have at least one connection point for at least one outlet or inlet pipe and one connection point for the anode. Forms the connection point for the anode a possibly closed hollow cylinder that is flush with the anode tube or -staff completes. In the case of tubular anodes, the hollow cylinder in the distributor device can seal the anode tube tightly so that no electrolyte can get into the interior of the anode. Alternatively, the connection point of the distributor device to the anode can have a relief bore in the anode tube. This prevents electrolyte from flowing off into the anode tube if the pressure on the distributor element is too high.

The optionally closed hollow cylinder of the distributor device can be attached to the carrier material of the anode or directly to the diamond-coated carrier. In the latter case, the carrier and the distributor device are separated from one another by the conductive diamond layer. In a particularly preferred embodiment, the distributor device is irreversibly connected to the anode, particularly preferably welded. This is particularly advantageous when working with high currents. For example, the anode and the distributor device can be welded by diffusion welding, electron beam welding or laser welding.

Radial bores are distributed over the circumference of the hollow cylinder of the distributor device. The distributor device preferably has 3, more preferably 4 and even more preferably 5 radial bores. Through the radial bores in the distributor device, the electrolyte can be distributed homogeneously and in a streamlined manner into the electrolyte space and, after passing through the electrolyte space, the electrolysis product can be effectively removed.

The electrolyte is fed to the electrolysis cell and the distributor device via the feed pipe. The electrolysis product is discharged from the electrolysis cell via the drain pipe after the electrolysis product has been collected in the distributor device. In a preferred embodiment, the distributor device is designed in such a way that it also tightly seals the tubular cathode so that no electrolyte or electrolysis product can escape from the cathode.

• Abdichtung der röhrenförmigen Anode, so dass kein Elektrolyt in den Anodeninnenraum eintreten kann oder Druckregulierung durch Entlastungsbohrung in den Anodenraum oder/und
• elektrische Kontaktierung der Anode oder/und Kathode mit der Stromquelle oder/und
• strömungsgünstige und homogene Verteilung des Elektrolyten in den Elektrolytraum (optimale hydraulische Verteilung über die gesamte Austauschfläche) oder/und
• effektive Abführung des Elektrolyseprodukts aus dem Elektrolytraum oder/und
• Abdichtung der röhrenförmigen Kathode oder/und
• Verringerung der Strömungsverluste.

The distribution device fulfills several tasks independently of one another:

• Sealing of the tubular anode so that no electrolyte can enter the anode interior or pressure regulation by means of a relief bore in the anode area and / or
• electrical contacting of the anode and / or cathode with the power source and / or
• Streamlined and homogeneous distribution of the electrolyte in the electrolyte space (optimal hydraulic distribution over the entire exchange surface) and / or
• effective removal of the electrolysis product from the electrolyte space and / or
• Sealing of the tubular cathode and / or
• Reduction of flow losses.

The components anode, cathode, distributor device, inlet and outlet pipe can be assembled to form an electrolysis cell using appropriate assembly devices known to those skilled in the art.

Due to the modular construction of anode, cathode, distributor device, inlet and outlet pipe, the individual components can be made of different materials and can be exchanged or replaced individually if damaged. It has thus succeeded in a simple way to connect the diamond anode according to the invention and the other components, which are made from cheaper materials, to one another to form an electrolysis cell which is very compact in design.

The tubular electrolytic cell is also characterized by high Strength while using little material. Parts that wear out over time due to the abrasive electrolytes, for example, can be replaced individually, so that economical use of materials is guaranteed in this respect as well. In the tubular electrolysis cell, the flow is favorable to the electrolyte space, which prevents flow losses and the surface is optimally used for the electrochemical exchange of substances. A continuous and homogeneous electrolysis process with high solids concentrations and current density ranges is possible due to the electrode materials and electrode arrangement.

Another aspect of the present invention is an electrolysis device which comprises at least two electrolysis cells according to the invention, wherein the electrolyte passes through the electrolysis cells one after the other and the electrolysis cells are operated in an electrochemically connected parallel. Thus, the system services can be implemented flexibly and without limits.

The electrolysis cell according to the invention or the electrolysis device according to the invention is particularly suitable for oxidizing an electrolyte. As stated above, the undivided electrolysis cell is particularly suitable for the oxidation of an electrolyte when neither the electrolyte nor the electrolysis product, which are produced or converted at the anode or cathode, are disruptively altered by the other electrode process or react with one another.

The electrolysis cells according to the invention can be operated with a current density between 50-1500 mA / CM ² , preferably 50-1200 mA / cm ² , more preferably 60-975 mA / cm ² and thus enable large-scale and economical processes.

The electrolysis cells / electrolysis devices according to the invention can also be used at very high solids contents between 0.5-650 g / l, preferably 100-500 g / l, more preferably 150-450 g / l and even more preferably 250-400 g / l can be used.

The electrolysis cells / devices according to the invention are particularly suitable for the anodic oxidation of sulfate to peroxodisulfate.

The electrolysis cells / electrolysis devices according to the invention have proven themselves in particular for the production of peroxodisulfates, for example in a method described herein.

In the electrolytic cell used according to the invention, the electrolyte space between the anode and cathode is undivided, i.e. there is no separator between anode and cathode. The use of an undivided cell enables electrolyte solutions with very high solids concentrations, which in turn significantly reduces the energy expenditure in salt production, essentially crystallization and water evaporation, in direct proportion to the increase in the solids content, but at least to 25% of that of a divided cell. According to the invention, the use of a promoter is also not necessary.

The electrolyte used preferably has an acidic, preferably sulfuric acid, or neutral pH. The electrolyte can be circulated through the electrolytic cell during the process. This prevents an undesirable high electrolyte temperature in the cell, which accelerates the decomposition of the persulphates.

The electrolyte solution is discharged from the electrolyte circuit to recover the peroxodisulfate produced. The peroxodisulfate produced can be obtained by crystallization and separation of the crystals from the electrolyte solution to form an electrolyte liquor.

The electrolyte used preferably has a total solids content of about 0.5 to 650 g / l at the start of the electrolysis. The electrolyte preferably contains from about 100 to about 500 g / l sulfate, more preferably from about 150 to about 450 g / l sulfate, and most preferably from 250-400 g / l sulfate at the start of the reaction. The use of the electrolysis cell / device according to the invention thus enables high solids concentrations in the electrolyte solution, without the addition of a potential-increasing agent or promoter and the resulting requirements for exhaust gas and wastewater treatment with high current yields at the same time Peroxodisulfate production.

Furthermore, the electrolyte solution preferably contains from about 0.1 to about 3.5 mol of sulfuric acid per liter (l) of electrolyte solution, more preferably 1-3 mol of sulfuric acid per liter of electrolyte solution, and most preferably 2.2-2.8 mol of sulfuric acid per liter of electrolyte solution.

In summary, an electrolyte with the following composition is particularly preferably used: 150 to 500 g of sulfate and 0.1 to 3.5 mol of sulfuric acid per liter of starting electrolyte per liter of electrolyte solution. The total solids content is preferably 0.5 g / l to 650 g / l, more preferably 100-500 g / l and most preferably 250-400 g / l. The promoter portion is 0 g / l.

characters

Figure 1:

Current yields in comparison of different cell types with and without rhodanide (promoter).

Figure 2a:

Current / voltage in Pt / HIP and diamond electrodes.

Figure 2b:

Current / yield in Pt / HIP and diamond electrodes.

Figure 3:

Electrolytic cell according to the invention - top view

Figure 4:

Cross section of an electrolytic cell according to the invention

Figure 5:

Individual components of the electrolytic cell according to the invention

Figure 6:

Distribution device

Figure 3 shows a possible embodiment of an electrolytic cell according to the present invention.

A cross section of this model is in Figure 4 shown schematically. The electrolyte reaches the distributor device (2a) through the feed pipe (1) and is fed from there to the electrolyte chamber (3) in a streamlined 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 distributor device (2b) and transferred into the drain pipe (6). Seals (7) close off the electrolyte space between the inlet or outlet pipe and the inner surface of the cathode.

In a preferred embodiment, the distributor device (2) can be designed so that the distributor device simultaneously seals the electrolyte space.

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

Figure 6 is an enlarged view of the distribution device (2). The distributor device has a connection point (21) for an outlet or inlet pipe and a connection point (22) for the anode (4). The connection point for the anode is formed by a hollow cylinder that is flush with the anode tube or rod (4).

Radial bores (23) are distributed over the circumference of the hollow cylinder of the distributor device. The electrolyte can be fed homogeneously into the electrolyte space through the radial bores (23) in the distributor device and can be effectively removed after passing through the electrolyte space. The distributor device preferably has 3, more preferably 4 and even more preferably 5 radial bores.

example

The various peroxodisulphates are produced according to the following mechanisms:

Sodium peroxodisulfate:

• Anode reaction:
  
  2SO ₄ ^2- → S ₂ O ₈ ^2- + 2e ^-
  
  
• Cathodic reaction
  
  2H ⁺ + 2e ^- → H ₂ ↑
  
  
• Crystallization:
  
  2Na ⁺ + S ₂ O ₈ ^2- Na ₂ S ₂ O ₈ ↓
  
  
• total
  
  Na ₂ SO ₄ + H ₂ SO ₄ → Na ₂ S ₂ O ₈ + H ₂ ↑
  
  

Ammonium peroxodisulfate:

• Anode reaction:
  
  2SO ₄ ^2- → S ₂ O ₈ ^2- + 2e ^-
  
  
• Cathodic reaction
  
  2H + + 2e ^- → H ₂ ↑
  
  
• Crystallization:
  
  2NH ₄ + + S ₂ O ₈ ^2- (NH ₄ ) ₂ S ₂ O ₈ ↓
  
  
• total
  
  (NH ₄ ) ₂ SO ₄ + H ₂ SO ₄ → Na ₂ S ₂ O ₈ + H ₂ ↑
  
  

Potassium peroxodisulfate:

• Anode reaction:
  
  2SO ₄ ^2- → S ₂ O ₈ 2 ^- + 2e ^-
  
  
• Cathodic reaction
  
  2H + + 2e ^- → H ₂ ↑
  
  
• Crystallization:
  
  2K ⁺ + S ₂ O ₈ ^2- K ₂ S ₂ O ₈ ↓
  
  
• total
  
  K ₂ SO ₄ + H ₂ SO ₄ → K ₂ S ₂ O ₈ + H ₂ ↑
  
  

The production of sodium peroxodisulfate according to the invention is described below by way of example.

For this purpose, on the one hand a two-dimensional and on the other hand a three-dimensional cell, consisting of a boron-doped diamond-coated niobium anode (diamond anode according to the invention) was used.

Initial electrolyte composition:

Temperature: 25 ° C
Sulfuric acid content: 300 g / l
Sodium sulfate content: 240 g / l
Sodium persulfate content: 0 g / l

• Röhrenzelle mit Platin-Titan-Anode: 1280 cm²
• Röhrenzelle mit Diamant-Niob-Anode: 1280 cm²
• Flachzelle mit Diamant-Niob-Anode: 1250 cm²

Kathodenwerkstoff: säurebeständiger Edelstahl: 1.4539
Löslichkeitsgrenze (Natriumpersulfat) des Systems ca. 65 - 80 g/l.Active anode area for the cell types used:

• Tubular cell with platinum-titanium anode: 1280 cm ²
• Tubular cell with diamond-niobium anode: 1280 cm ²
• Flat cell with diamond-niobium anode: 1250 cm ²

Cathode material: acid-resistant stainless steel: 1.4539
Solubility limit (sodium persulphate) of the system approx. 65 - 80 g / l.

Current densities:

The electrolyte was appropriately concentrated by circulating (see Figures 1 and 2 ).

Results:

From the course of the current yield as a function of the changed

Sodium persulfate content ( Figure 1 ), it can be clearly seen that the diamond anode used achieves significantly higher current yields than with conventional platinum-coated titanium anodes over the entire working range of approx. 100 g / l to approx. 350 g / l, which is useful for this cell, even without the addition of a promoter added promoter are known.

From the course of the current yield as a function of the current density in the production of sodium peroxodisulfate using a platinum anode (comparative examples) with the addition of the appropriate promoter and in a boron-doped diamond anode to be used according to the invention, each installed in an undivided electrolysis cell ( Figure 2a + 2b), it follows that at a current density of 100 - 1500 mA / cm2 a current yield of over 75% is available.

In contrast, the tests also showed that conventional Pt foil-coated titanium anodes only achieved current yields of at best 60-65% within this working range, despite the addition of a sodium rhodanide solution as a promoter. Without the addition of a promoter, on the other hand, current yields of only about 35% are achieved, which confirms the present invention.

In summary, it can be confirmed that the current yield of a diamond-coated niobium anode is around 10% higher than with a cell with a conventional platinum-titanium anode and the addition of a potential-increasing agent and around 40% higher than with a cell with a conventional one, even without the addition of a potential-increasing agent Platinum-titanium anode without the addition of a potential-increasing agent.

The voltage drop across a diamond-coated anode is about 0.9 volts higher than that of a comparable cell with a platinum-titanium anode. Furthermore, it was found that the current yield with a diamond electrode to be used according to the invention without the addition of a promoter only increases with an increasing total content of sodium peroxodisulfate in the electrolyte slowly decreases - under the experimental conditions, for example, electrolyte solutions with a sodium peroxodisulfate content of around 400 - 650 g / l can be obtained with a current yield of 65% or more.

By contrast, when using a conventional platinum anode and also using a promoter in the electrolyte, only equally high peroxodisulfate concentrations of about 300 g / l can be obtained with a current yield of about 50%.

Bullet-point investigations on a similar system with potassium ions from potassium sulfate produced similarly good results.

It is surprising for the person skilled in the art that the method according to the invention at high conversions with technically easily manageable current densities without the spatial separation of anolyte and catholyte and without the use of a promoter with simultaneously high current yield with simultaneously high persulfate and solid concentrations in undivided electrolysis cells without the addition a promoter can be carried out.

In the course of the investigations into this invention, it was found that the production of ammonium and largely alkali metal peroxodisulfates with a high current yield is also possible in an undivided cell by using a diamond thin-film electrode doped with a trivalent or pentavalent element as the anode . Surprisingly, the cell can be used economically even with a very high solids content, i.e. peroxodisulfate content, and at the same time the use of a promoter can be completely dispensed with and the electrolysis can be carried out at a high current density, which results in further advantages, especially with regard to installation and acquisition costs.

Summary:

The use of an undivided cell enables electrolyte solutions with very high solids concentrations, which in turn significantly reduces the energy expenditure in salt production, essentially crystallization and water evaporation, in direct proportion to the increase in the solids content, but at least to 25% of that of a divided cell.

Despite the omission of the use of a promoter and thus the omission of the necessary cleaning measures for the electrolysis gas, higher conversions and higher persulfate concentrations are available in the discharged electrolyte.

The working current density can be significantly reduced compared to platinum anodes with the same high production quantity, which means that fewer ohmic losses occur in the system and thus the cooling effort is reduced and the degree of freedom in the design of the electrolysis cells and the cathodes is increased.

At the same time, as the current density increases, the current yield and thus the production volume can be increased.

Due to the excellent abrasion resistance of the diamond-coated anode, much higher flow velocities can be used compared to a Pt anode with a similar construction.

Claims (13)

1. Electrolytic cell, comprising the components:

   (a) at least one tubular cathode;

   (b) at least one rod-shaped or tubular anode which comprises a conductive carrier coated with a conductive diamond layer;

   (c) at least one inlet tube;

   (d) at least one outlet tube; and

   (e) at least two distributor apparatuses, wherein the distributor apparatuses have at least one connection point for at least one inlet or outlet tube and a connection point for the anode, and wherein the first distributor apparatus is designed to distribute electrolyte from the inlet tube into an electrolyte space and the second distributor apparatus is designed to collect converted electrolyte and to discharge said electrolyte via the outlet tube, and wherein the electrolyte space is formed as an annular gap between the inner anode and the outer cathode.
2. Electrolytic cell according to claim 1, wherein the electrolytic cell has a common electrolyte space without a diaphragm.
3. Electrolytic cell according to either claim 1 or claim 2, wherein the distance between the outer surface of the anode and the inner surface of the cathode is between 1 and 20 mm.
4. Electrolytic cell according to any of claims 1-3, wherein the inner diameter of the cathode is between 10 and 400 mm.
5. Electrolytic cell according to any of claims 1-4, wherein the anode and the cathode are, in each case independently of one another, between 20 and 120 cm long.
6. Electrolytic cell according to any of claims 1-5, wherein the carrier 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. Electrolytic cell according to any of claims 1-6, wherein the diamond layer is doped with at least one trivalent or at least one pentavalent main-group element or transition element, in particular boron and/or phosphorus.
8. Electrolytic cell according to any of claims 1-7, wherein the cathode is made of lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and/or iron alloys, in particular of acid-resistant stainless steel.
9. Electrolytic cell according to any of claims 1-8, wherein the components of the electrolytic cell are individually replaceable.
10. Electrolysis device, comprising at least two electrolytic cells according to any of claims 1-9, wherein the electrolyte passes through the electrolytic cells one after the other and the electrolytic cells are electrochemically connected in parallel.
11. Use of the electrolytic cell according to any of claims 1-9 or of the electrolysis device according to claim 10 for oxidising an electrolyte.
12. Use according to claim 11, wherein the current density is between 50 and 1500 mA/cm².
13. Use according to either claim 11 or claim 12 for producing peroxydisulfate.

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