WO2014009536A1

WO2014009536A1 - Undivided electrolytic cell and use of the same

Info

Publication number: WO2014009536A1
Application number: PCT/EP2013/064809
Authority: WO
Inventors: Michael Müller; Patrick Keller; Markus Schiermeier
Original assignee: United Initiators Gmbh & Co. Kg
Priority date: 2012-07-13
Filing date: 2013-07-12
Publication date: 2014-01-16
Also published as: TW201406998A; EP2872673B1; KR20150034171A; EP2872673A1

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.

Description

Undivided electrolytic cell and its use

description

In one aspect, the present invention relates to a process for the preparation of an ammonium or alkali metal peroxodisulfate.

It is known in the art to prepare alkali metal and ammonium peroxodisulfate by anodic oxidation of an aqueous solution containing the corresponding sulfate or bisulfate, and to recover the resulting salt by crystallization from the anolyte. Since in this method, the decomposition voltage over the decomposition voltage of the anodic oxygenation is water, to increase the decomposition voltage of the water to oxygen (oxygen overvoltage) on a commonly used platinum anode, a so-called promoter, usually thiocyanate as sodium or ammonium thiocyanate used.

Rossberger (US 3915816 (A)) describes a process for the direct production of sodium persulfate. As electrolysis cells undivided cells are described with platinum-coated titanium-based anodes. The illustrated current yields 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 electrolytic cell with a diaphragm-protected cathode and a platinum anode by a neutral aqueous Anolytiösung with an initial content of 5 to 9 wt .-% sodium ions, 12 to 30% by weight of sulfate ions, 1 to 4% by weight of ammonium ions, 6 to 30% by weight of peroxodisulfate ions and a potential-increasing promoter, in particular thiocyanate, using a sulfuric acid solution as the catholyte at a current density of at least 0.5 to 2 A / cnr electrolyzed. After crystallization and separation of peroxodisulfate from the anolyte, the mother liquor is mixed with the cathode product, neutralized and fed back to the anode.

Disadvantages of this method are:

1 . In order to reduce the evolution of oxygen, the use of a promoter is necessary.

2. In order to achieve the described high current efficiencies, it is necessary that the anode and cathode are spatially separated by the use of a suitable membrane. The membranes required for this purpose are very sensitive to abrasion.

3. The requirement of a high current density and thus a high

Anode potential to obtain an economically acceptable current efficiency.

4. The problems associated with the manufacture of the platinum anode, in particular with regard to obtaining a technically acceptable current efficiency and high anode life. To name a few examples is the continuous platinum removal, which can be up to 1 g / to product in persulfate. This platinum removal on the one hand contaminates the product and on the other hand leads to the consumption of a valuable raw material, which not least also increases the process costs.

5. The preparation of low solubility persulfates, i.w. Potassium and sodium persulfate, is possible only in extremely high dilution. This requires a high energy input in crystal formation.

6. When using the so-called conversion process produced persulfates must be recrystallized from an ammonium persulfate solution. This usually results in a reduced or even lack of purity of the product. EP-B 0 428 171 discloses a filter press-type electrolytic cell for producing peroxo compounds, including ammonium peroxodisulfate, sodium peroxodisulfate and potassium peroxodisulfate. As anodes here hot isostatically applied to a valve metal platinum foils are used. The anolyte used is a solution of the corresponding sulfate containing a promoter and sulfuric acid. This method also has the aforementioned problems.

In the process according to DE 199 13 820, peroxodisulfates are prepared by anodic oxidation of an aqueous solution containing neutral ammonium sulfate. For the purpose of preparing sodium or potassium peroxodisulfate, the solution obtained from anodic oxidation containing ammonium peroxodisulfate is reacted with caustic soda or potassium hydroxide solution. After crystallization and separation of the corresponding Al kalimetallperoxodisulfats the mother liquor is recycled in admixture with the catholyte produced during the electrolysis. Also in this method, the electrolysis is carried out 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 methods continue to adhere to serious disadvantages (see also preceding list). There is always an addition of promoters, also referred to as polarizers, required to increase the oxygen overvoltage and improve the current efficiency. As oxidation products of these promoters, which inevitably arise as by-products during anodic oxidation, toxic substances enter the anode exhaust gas and have to be removed in a gas scrubber. High current yields also require separation of anolyte and catholyte. The usually over the entire surface covered with platinum anodes always require a high current density. This results in a current load of the anolyte, 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 labile peroxodisulfate solution. In order to minimize this load, constructive measures must be taken, and the cooling effort increases in addition. Because of the limited heat dissipation, the electrode area must be limited, and this increases the installation cost per unit cell. In order to cope with the high current load, it is generally necessary to additionally use electrode support materials having high heat transfer properties, which in turn are susceptible to corrosion and expensive.

P.A. Michaud et al. in Electro Chemical and Sol id-State Letters, 3 (2) 77-79 (2000) teach the preparation of peroxodisulfuric acid by anodic oxidation of sulfuric acid using a boron doped diamond thin film electrode. This document teaches that such electrodes have a higher overvoltage for oxygen than platinum electrodes. However, the document gives no indication of the industrial production of ammonium and alkali metal peroxodisulfates using boron-doped diamond thin-film electrodes. Namely, 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 major side reaction besides the anodic oxidation of sulfuric acid is the evolution of oxygen and, in addition, ozone.

In the course of their invention described in the patent EP 1 1481 55 B1 Stenner and Lehmann recognized in 2001 that when using a diamond-coated, divided electrolysis cell for the production of persulfates no additional promoter is required to achieve such high current yields. Disadvantage of this method is mainly due to the sensitive separators, as already described above, that the preparation of persulfates with low solubility product, iw Potassium and sodium persulfate, only with extremely high dilution is possible, ie below the solubility limit, which requires a high energy input in the crystal formation and the salt discharge in the course of evaporation and drying.

Accordingly, it is an object of the present invention to provide a technical process for the preparation of ammonium and alkali metal peroxodisulfates, which avoids the disadvantages of the known processes or at least only to a lesser extent and the use of a diamond-coated, undivided cell for the production of persulfates , in particular those with low solubility potential in sulfate and sulfuric acid-containing electrolyte solutions or electrolyte suspensions, in addition to the demonstrated in the present invention electrochemical advantages, in particular also known from other applications of a diamond-coated carrier, mechanical and abrasive properties for the electrochemical oxidation of Sulfates in suspensions, as listed above to harness.

To achieve this object, the present application accordingly provides a process for the preparation of an ammonium or alkali metal peroxodisulfate, comprising

an anodic oxidation of a salt of the ammonium sulfate series,

Alkali metal sulphate and / or aqueous electrolyte containing the corresponding bisulphate in an electrolytic cell,

comprising at least one anode and one cathode,

wherein a diamond layer deposited on a conductive support and doped with a trivalent or pentavalent element is used as the anode,

wherein the electrolytic 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 of water to oxygen. The ammonium sulfate, alkali metal sulfate and / or the corresponding bisulfate salt used for anodizing may be any of the alkali metal sulfate or corresponding bisulfate. In the context of the present application, however, the use of sodium and / or potassium sulfate and / or the corresponding hydrogen sulfate is particularly preferred.

For the purposes of the present invention, "promoter" or else "polarizer" is any means known to those skilled in the art as an additive in carrying out an electrolysis to increase the decomposition voltage of water to oxygen or to improve the current efficiency Thiocyanate such as, for example, sodium or ammonium thiocyanate is not used according to the invention, in other words the electrolyte has a promoter concentration of 0 g / l in the process according to the invention The method eliminates, for example, cleaning requirements regarding the formation of typical electrolysis gases.

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

The anode used can be of any shape.

Any suitable anode material known to a person skilled in the art can be used for this purpose. In a preferred embodiment in the present invention, the support material is selected from the group consisting of silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements and / or aluminum or combinations of the elements.

On this substrate, the doped with a 3- or 5-valent element diamond layer is applied. The doped diamond layer is thus an n-conductor or a p-type conductor. It is preferred that a boron-doped and / or phosphorus-doped diamond layer is used. The amount of doping is adjusted so that the desired, usually just the sufficient, conductivity is achieved. For example, when doped with boron, the crystal structure may contain up to 10000 ppm of boron.

The diamond layer may be applied over the entire surface or in sections, for example, exclusively on the front side or exclusively on the backside of the carrier material.

Methods for applying the diamond layer are known in the art. The production of the diamond electrodes can be carried out in particular in two special CVD methods (chemical vapor deposition technique). These are microwave plasma CVD and hot wire CVD. In both cases, the gas phase, which is activated by microwave irradiation or thermally activated by hot wires to the plasma, from methane, hydrogen and optionally further additives, in particular a gaseous compound of the dopant.

By using a boron compound such as trimethylboron, a p-type semiconductor can be provided. By using a gaseous phosphorus compound as a dopant, an n-type semiconductor is obtained. By deposition of the doped diamond layer on crystal inem a silicon a particularly dense and non-porous layer is obtained - a film thickness of 1 pm is usually sufficient. The diamond layer is preferably in a film thickness of about 0.5 pm to 5 pm, preferably about 0.8 pm to about 2.0 pm and more preferably about 1, 0 pm on the applied Anodenträgermateriai used according to the invention.

As an alternative to depositing 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. For producing a particularly suitable boron-doped diamond layer on a silicon single crystal, reference is made to the above-mentioned article by P.A. Michaud referred.

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 a diamond layer boron-doped in the crystal structure up to 10000 ppm, is particularly preferred.

The cathode used in the process according to the invention is preferably formed from lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and / or acid-resistant stainless steels, as known to the person skilled in the art. Spatially, the cathode can be configured arbitrarily.

In the electrolytic cell used in the invention, the electrolyte space between anode and cathode is undivided, i. there is no separator between anode and cathode. The use of an undivided cell allows electrolyte solutions with very high solids concentrations, which in turn significantly reduces the energy required for salt recovery, essentially crystallization and water evaporation, directly proportional to the solids increase, but at least 25% of that of a divided cell.

The method according to the invention is carried out in preferred embodiments in a two-dimensional or three-dimensional cell. The cell is preferably designed as a flat or tubular cell.

In particular, the use of a tube geometry, ie 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, represents, with low material costs, an advantageous construction. The use of an annular gap as the common electrolyte space is preferred and leads to a uniform and thus low-flow throughflow and thus to a high utilization of the available electrolysis areas, which in turn means a high current efficiency. The manufacturing costs of such a cell are low in relation to a so-called flat cell.

In a preferred embodiment of the method according to the invention several electrolysis cells, preferably in the form of a double tube package or two-dimensional, summarized.

The electrolyte used in the process according to the invention preferably has an acidic, preferably sulfur-acidic, or neutral pH.

In a further preferred embodiment of the invention, the electrolyte is circulated through the electrolytic cell during the process. This prevents a, the decomposition of the persulfate accelerating and thus undesirable high electrolyte temperature in the cell.

In a further preferred embodiment, the method comprises a discharge of electrolyte solution from the electrolyte circuit. This can be done in particular for the production of peroxodisulfate produced. A further preferred embodiment therefore relates to the recovery of peroxodisulfates produced by crystallization and separation of the crystals from the electrolyte solution to form an electrolyte solution, wherein the electrolyte solution has been preferably previously discharged from the electrolyte circuit. A further preferred embodiment comprises a recirculation of the electrolyte mother liquor, in particular if previously produced peroxodisulfates were separated, increasing the content of acid, sulfate and / or hydrogen sulfate in the electrolysis cell.

The anodization is preferably carried out according to the invention at an anodic current density of 50-1 500 mA / cm ² and more preferably about 50-1 200 mA / cm ² . A particularly preferred current density is in the range of 60-975 mA / cm ² .

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

Further, the electrolytic solution preferably contains about 0.1 to about 3.5 moles of sulfuric acid per liter of (I) electrolytic solution, more preferably 1-3 moles of sulfuric acid per liter of electrolyte solution, and most preferably 2.2 to 2.8 moles of sulfuric acid per liter of electrolyte solution.

In summary, in the process according to the invention it is particularly preferred to use an electrolyte having the following composition: per liter of electrolyte 1, 50 to 500 g of persulfate and 0.1 to 3.5 mol of sulfuric acid per mole 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, with the sulfate content being variable. The promoter content is 0 g / l.

The invention further relates to a constructed of individual components, undivided electrolysis cell, an electrolysis device constructed from a plurality of such electrolysis cells, and their use for the oxidation of an electrolyte.

By "electrolysis" is meant a chemical change caused by the passage of electricity through an electrolyte, which is expressed in a direct conversion of electrical energy into chemical energy through the mechanism of the electrode reactions and ion migration Saline solution, in which sodium hydroxide solution and chlorine gas are produced, and the production of inorganic peroxides is nowadays carried out industrially in electrolysis cells.

In large-scale processes, it is particularly desirable that the reactions can be run at high concentrations of reactants and corresponding products. High product concentrations ensure easy work-up of the final product since the solution must be removed from reaction products in solution. In the case of the electrolysis of highly concentrated electrolytes, it is thus also possible to reduce the energy expenditure of the subsequent work-up of the electrolysis products.

However, applications with very high solids content make high demands on the components of the electrolysis cell due to the abrasive effect of the electrolyte. In particular, the diaphragm, which prevents a mixture of the reaction products of the anode and cathode compartments in divided electrolysis cells, does not permanently withstand electrolysis processes at high concentrations. At high solids contents, therefore, electrolysis can be carried out only in undivided cells, in which the anode space and the cathode space need not be spatially separated by the use of a suitable membrane. Such undivided cells are used in particular when neither starting materials nor products which are produced at the anode or cathode, in be disturbed by the other electrode process changed or react with each other.

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

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 possibly high current densities. This is only possible if anode and cathode have good electrical conductivity and are chemically inert to the electrolyte. Usually, the anode material used is graphite or platinum. However, these materials have the disadvantage that they do not have sufficient abrasion resistance at high solids concentrations.

The production of mechanically very stable and inert electrodes is disclosed in DE 1 99 1 1 746. In this case, electrodes are coated with an electrically conductive diamond layer, wherein the diamond layer is applied by a chemical vapor deposition method (CVD).

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 performed and individual components can be easily replaced without the actual cell body is destroyed.

Surprisingly, the object was achieved by an electrolysis cell comprising the components:

(a) at least one tubular cathode,

(B) at least one rod or tubular anode, the one with a comprising conductive diamond layer coated conductive support,

(c) at least one inlet pipe,

(d) at least one drainpipe and

(e) at least one distributor device,

be solved.

Preferably, in the electrolytic cell anode and cathode are arranged concentrically to each other, so that preferably the electrolyte space forms as an annular gap between the inside anode and the outside cathode. In this embodiment, therefore, the diameter of the cathode is larger than that of the anode.

In a preferred embodiment, the electrolyte space contains no membrane or no diaphragm. In this case, it is an electrolytic cell with a common electrolyte space, i. the electrolysis cell is undivided.

Preferably, the distance between the anode outer surface and the cathode inner surface is between 1 -20 mm, more preferably between 1 -1 5 mm, even more preferably between 2-1 0 mm, and most preferably between 2-6 mm.

The inner diameter of the cathode is preferably between 1 0-400 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 most preferably 1 20 cm, more preferably 75 cm. The cathode used in accordance with the invention is preferably made of lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and / or iron alloys, in particular of stainless steel, in particular of 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 may be identical to or different from the anode base material. In a preferred embodiment, the anode base material functions as a conductive carrier. As the conductive support, any conductive material known in the art can be used. Particularly preferred support materials are sil icium, 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, is particularly preferably used as the conductive support.

On this substrate, a conductive diamond layer is applied. The diamond layer may be doped with at least one 3- or at least one 5-valent main or subgroup element. The doped diamond layer is thus an n-conductor or a p-type conductor. It is preferred that a boron-doped and / or phosphorus-doped diamond layer is used. The amount of doping is adjusted so that the desired, usually just the sufficient, conductivity is achieved. For example, in the case of doping with boron, the crystal structure may contain up to 10,000 ppm, preferably from 10 ppm to 2,000 ppm, of boron and / or phosphorus. The diamond layer may 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 non-porous.

Methods for applying the diamond layer are known in the art. The preparation of the diamond electrodes can be carried out in particular in two special CVD processes (Chemical Vapor Deposition). These are the microwave plasma CVD and the hot wire CVD method. In both cases, the gas phase, which is activated by microwave irradiation or thermally activated by hot wires to the plasma, from methane, hydrogen and optionally further additives, in particular a gaseous compound of the dopant.

By using the boron compound such as trimethylboron, a p-type semiconductor can be provided. By using a gaseous phosphorus compound as a dopant, an n-type semiconductor is obtained. By deposition of the doped diamond layer on crystal inem a silicon a particularly dense and non-porous layer is obtained. The diamond layer is preferably applied to the conductive support used according to the invention in a film thickness of about 0.5-5 μm, preferably about 0.8-2.0 μm and particularly preferably about 1 μm. 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 deposition of the diamond layer on a crystalline material, the deposition can also be carried out on a self-passivating metal, such as titanium, tantalum, tungsten, or niobium. For the preparation of a particularly suitable boron-doped diamond layer on a silicon single crystal, reference is made to PA Michaud (Electrochemical and Solid State Letters, 3 (2) 77-79 (2000)). In the context of the present invention, the use of an anode comprising a nitrile or titanium support with a boron-doped diamond layer, in particular with a diamond layer doped with up to 10000 ppm of boron, is particularly preferred.

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

Preferably, the anode and / or the cathode, more preferably the anode and the cathode, even more preferably the anode is connected to the power source via the distributor means. In the event that the anode and cathode are connected via the distributor device to the power source, it must be ensured that the distributor device is electrically insulated in accordance with electrically. In any case, ensure good electrical contact between anode and / or cathode and distribution device.

The distribution device further ensures a homogeneous feed of the electrolyte from the inlet pipe into the electrolyte space. After the electrolyte has passed the electrolyte space, the reacted electrolyte (electrolysis product) is effectively collected by means of at least one upstream distribution device 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 distribution devices preferably have at least one connection point for at least one discharge or supply pipe and a connection point for the anode. The connection point for the anode forms an optionally closed Hohlzyl inder, which is flush with the anode tube or completed. In the case of tubular anodes, the hollow cylinder in the distributor devices can close off the anode tube 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 into the anode tube. This prevents electrolyte from flowing 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 mounted on the support material of the anode or directly on the diamond-coated carrier. In the latter case, therefore, 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 at high currents. For example, the anode and the manifold may be welded by diffusion bonding, electron beam welding or laser welding.

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

The electrolyte is preferably supplied via the inlet pipe of the electrolytic cell and in particular the distributor device. The electrolysis product is preferably removed from the electrolysis cell via the outlet pipe, in particular after the electrolysis product has been collected in the distributor device. In a preferred embodiment, the manifold is designed to seal the tubular cathode so that no electrolyte or electrolysis product can escape from the cathode.

The distributor 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 through relief hole in the anode compartment and / or

• electrical contacting of the anode and / or cathode with the power source and / or

^• aerodynamic 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, distribution device, inlet and outlet pipe can be assembled by an appropriate, known in the art, mounting devices to an electrolytic cell.

Due to the modular design of anode, cathode, distribution device, inlet and outlet pipe, the individual components may be formed in different materials and replaced or replaced individually in case of damage. It has thus been possible to easily connect the diamond anode according to the invention and the other components, which are made of cheaper materials, to one another in a very compact electrolytic cell in construction.

The tubular electrolysis cell is also characterized by high Strength with low material usage. Parts that wear over time, for example due to the abrasive electrolytes, can be replaced individually, so that in this respect an economical use of materials is guaranteed. In the tubular electrolysis cell the Elekrolytraum is flow streamlined, thereby avoiding flow losses and the surface for the electrochemical mass transfer are optimally utilized. A continuous and homogeneous electrolysis process at high solids concentrations and current density ranges is possible due to the electrode material and electrode arrangement.

Another aspect of the present invention is an electrolysis device which comprises at least two electrolytic cells according to the invention, wherein the electrolyte passes through the electrolysis cells in succession and the electrolysis cells are operated electrochemically in parallel. Thus, the plant services are flexible and feasible without limits.

The electrolytic cell according to the invention or the electrolysis device according to the invention is particularly suitable for the oxidation of an electrolyte. As stated above, the undivided electrolysis cell is particularly suitable for the oxidation of an electrolyte when neither electrolyte nor electrolysis product, which are produced or reacted at the anode or cathode, are changed in a disturbing way by the respective other electrode process or react with each other.

The electrolysis cells according to the invention can be operated with a current density between 50-1 500 mA / cm ² , preferably 50-1 200 mA / cm ² , more preferably 60-975 mA / cm ² , thus enabling large-scale and economic processes.

The electrolysis cells / electrolysis devices according to the invention can moreover be used at very high solids contents of between 0.5-650 g / l, preferably 100-500 g / l, more preferably 1-50-450 g / l and even more preferably 250-400 g / l.

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 particularly suitable for the production of peroxodisulfates.

It is known in the prior art to prepare alkali metal peracid and ammonium peroxodisulfate by anodic oxidation of an aqueous solution containing the corresponding sulfate or bisulfate and to recover the resulting salt by crystallization from the anolyte. Since in this method the decomposition voltage above the decomposition voltage of the anodic oxygen formation from water liegt, to increase the decomposition voltage of the water to oxygen (oxygen overvoltage) at a platinum anode usually used a so-called promoter or polarizer, usually ichi thiocyanate as sodium or ammonium thiocyanate used.

Rossberger (US Pat. No. 391 581 6 (A)) describes a process for the direct preparation of sodium persulfate. As electrolysis cells undivided cells are described with platinum-coated titanium-based anodes. The illustrated current yields 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 electrolytic cell with a diaphragm protected by a cathode and a platinum anode by a neutral aqueous Anolytiosung with an initial content of 5 to 9 wt .-% sodium ions, 1 2 to 30% by weight of sulfate ions, 1 to 4% by weight of ammonium ions, 6 to 30% by weight of peroxodisulfate ions and a potential-increasing promoter, in particular thiocyanate, using a Sulfuric acid solution as a catholyte at a current density of at least

0.5 to 2 A / cnr electrolyzed. After the crystallization and separation of peroxodisulfate from the anolyte, the mother liquor is mixed with the cathode product, neutralized and fed back to the anode.

Disadvantages of this method are:

3. The requirement of a high current density and thus a high

Anode potential to obtain an economically acceptable current efficiency.

4. The problems associated with the manufacture of the platinum anode, in particular with regard to obtaining a technically acceptable current efficiency and long anode life. To name a few, here is the continuous platinum removal, which can be up to 1 g / t of product in persulfate. This platinum removal on the one hand contaminates the product and on the other hand leads to the consumption of a valuable raw material, which not least also increases the process costs.

5. The preparation of low solubility persulfates, i.w. Potassium and sodium persulfate, is possible only in extremely high dilution. This makes a high energy input in crystal formation erforderl I required.

6. When using the so-called conversion process produced persulfates must be recrystallized from an ammonium persulfate solution. This usually results in a reduced or even lack of purity of the product. EP-B 0 428 1 71 discloses a filter press-type electrolytic cell for producing peroxo compounds, including ammonium peroxodisulfate, sodium peroxodisulfate and potassium peroxodisulfate. The anodes used here are hot isostatic platinum foils applied to a valve metal. The anolyte used is a solution of the corresponding sulfate containing a promoter and sulfuric acid. This method also has the aforementioned problems.

In the process according to DE 199 13 820, peroxodisulfates are prepared 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 Al kal imetallperoxodisulfats the mother liquor is recycled in admixture with the catholyte produced during the electrolysis. Also in this method, the electrolysis is carried out 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 of a Piatinanode, these methods continue to adhere to serious disadvantages (see also above list). There is always an addition of promoter, also called polarizer, required to increase the oxygen overvoltage and improve the current efficiency. As oxidation products of these promoters, which inevitably arise as by-products during anodic oxidation, toxic substances enter the anode exhaust gas and have to be removed in a gas scrubber. High current yields also require separation of anolyte and catholyte. The usually over the whole area covered with platinum anodes always require a high current density. This results in a current load of the anolyte, the separator and the cathode, creating additional Measures to reduce the cathodic current density by a three-dimensional structuring of the electrolytic cell and activation are required. In addition, there is a high thermal load on the labile peroxodisulfate solution. In order to minimize this load, constructive measures must be taken, and the cooling effort increases in addition. Because of the limited heat dissipation, the electrode area must be limited, and this increases the installation cost per unit cell. In order to cope with the high current load, electrode support materials with high heat transfer properties, which in turn are susceptible to corrosion and expensive, must generally be used as well.

P.A. Michaud et al. in Electro Chemical and Solid-State Letters, 3 (2) 77-79 (2000) teach the preparation of peroxodisulfuric acid by anodic oxidation of sulfuric acid using a boron doped diamond thin film electrode. This document teaches that such electrodes have a higher overvoltage for oxygen than platinum electrodes. However, the document gives no indication of the technical production of ammonium and Al kalimetallperoxodisulfaten using boron-doped diamond thin-film electrodes. Namely, 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 major side reaction besides the anodic oxidation of sulfuric acid is the evolution of oxygen and, in addition, ozone.

In the course of their invention described in the patent EP 1 1481 55 B1 Stenner and Lehmann recognized in 2001 that when using a diamond-coated, divided electrolysis cell for the production of persulfates no additional promoter is required to achieve such high current yields. Disadvantage of this method is mainly due to the sensitive separators, as already described above, that the preparation of low solubility persulfates, i .w. Potassium and sodium persulfate, only with extremely high dilution is possible, ie below the solubility limit, which requires a high energy input in the crystal formation and the salt discharge in the course of evaporation and drying.

The used for the anodic oxidation salt from the series ammonium sulfate, Al kalimetallsulfat and / or the corresponding bisulfate may be any alkali metal sulfate or corresponding bisulfate. In the context of the present application, however, the use of sodium and / or potassium sulfate and / or the corresponding hydrogen sulfate is particularly preferred.

In the electrolysis cell used according to the invention, the electrolyte space between anode and cathode is undivided, ie. there is no separator between anode and cathode. The use of an undivided cell allows electrolyte solutions with very high solids concentrations, which in turn significantly reduces the energy required for salt recovery, essentially crystallization and water evaporation, directly proportional to the solids increase, but at least 25% of that of a divided cell. According to the invention, the use of a promoter is not necessary.

For the purposes of the present invention, "promoter" is any suitable agent which is known to the person skilled in the art as an additive in carrying out an electrolysis for increasing the decomposition voltage of water to oxygen or for improving the current efficiency Promoter is thiocyanate such as sodium or ammonium thiocyanate.

The electrolyte used preferably has an acidic, preferably sulfur-acidic, or neutral pH. The electrolyte may be circulated through the electrolytic cell during the process. This prevents a, the decomposition of the persulfate accelerating and thus undesirable high electrolyte temperature in the cell.

A discharge of electrolyte solution from the electrolyte circuit takes place to obtain generated peroxodisulfate. The produced peroxodisulfate can be obtained by crystallizing and separating the crystals from the electrolytic 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 beginning of the electrolysis. The electrolyte preferably contains from about 100 to about 500 g / L of sulfate, more preferably from about 150 to about 450 g / L of sulfate, and most preferably 250-400 g / L of sulfate at the beginning of the reaction. The use of the electrolysis cell / device according to the invention thus allows high concentrations of solids in the electrolyte solution, without the addition of a potential-enhancing agent or promoter and the resulting requirements for exhaust and wastewater treatment at the same time high current yields in Peroxodisulfatherstellung.

In summary, in the process according to the invention it is particularly preferred to use an electrolyte having the following composition: per liter of starting electrolyte, 150 to 500 g of sulphate and 0.1 to 3.5 mol of sulfuric acid per liter of electrolyte solution. The total solids content is preferably from 0.5 g / l to 650 g / l, more preferably from 1 00 to 500 g / l, and most preferably from 250 to 400 g / l. The promoter content is 0 g / l. characters

FIG. 1: Current yields in comparison of different cell types with and without rhodanide (promoter).

FIG. 2a: current / voltage in Pt / HIP and diamond electrodes.

FIG. 2b: current / yield in Pt / H IP and diamond electrodes.

Fig ur 3: electrolysis cell according to the invention - top view

FIG. 4: Cross section of an electrolysis cell according to the invention

FIG. 5: individual components of the electrokineal cell according to the invention

FIG. 6: Distribution device

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

A cross section of this model is shown schematically in FIG. Through the inlet pipe (1), the electrolyte passes into the distributor device (2a) and is supplied from there aerodynamically to the electrolyte space (3). 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 distributor device (2b) and transferred into the discharge pipe (6). Seals (7) close the electrolyte space between inlet and outlet pipe and inner surface of the cathode.

In a preferred embodiment, the distributor device (2) can be designed such that the distributor device simultaneously undertakes the sealing of the electrolyte space.

FIG. 5 shows the individual components of the electrolysis cell according to the invention. The numbering is analogous to FIG. 4. Further components for sealing the electrolysis cell and for mounting are shown in FIG. 5 but not numbered. These components are known in the art and can be exchanged bel. FIG. 6 is an enlarged view of the distributor device (2). The distribution devices has a connection point (21) for a waste or inlet pipe and a connection point (22) for the anode (4). The connection point for the anode forms a hollow cylinder which terminates flush with the anode tube or rod (4).

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

example

The production of the various peroxodisulfates is carried out according to the following mechanisms:

sodium:

Anode reaction: 2S0 ₄ ^{2 "} S ₂ 0 ₈ ^{2 '} + 2 &

Cathode reaction H ⁺ + 2e ^" - H ₂ †

Crystallization: 2Na ⁺ + S ₂ 0 ₈ ^{2 "} Na ₂ S ₂ O ₈ j

Total Na ₂ S0 ₄ + H ₂ S0 ₄ ^■ -> Na ₂ S ₂ 0 ₈ + H ₂ †

ammonium:

Anode reaction: 2S0 ₄ ^{2 ~} -> S ₂ 0 ₈ ² - + 2 &

Cathode reaction H + + 2e -> H2 †

Crystallization: 2NH ₄ + + S ₂ 0 ₈ ^2- (NH ₄ ) ₂ S ₂ O ₈ j

Total (NH ₄ ) ₂ S0 ₄ + H ₂ S0 ₄ -> Na ₂ S ₂ 0 ₈ + H ₂ | Ka lium peroxo disulfa t:

Anode reaction: 2SO ^"-> _S; 0 _R 2 ^'+ ^2e"

Cathode reaction H + + 2e ^" -> H ₂ †

Crystallization: 2K ⁺ + S ₂ 0 ₈ ^{2 "} K ₂ S ₂ 0 _B j

Total K ₂ S0 ₄ + H ₂ S0 ₄ -> K ₂ S ₂ 0 ₈ + H ₂ |

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

For this purpose, on the one hand a 2-dimensional and on the other a three-dimensional line, consisting of a boron-doped diamond-coated niobium anode (diamond anode according to the invention) was used.

Electrolyte initial composition:

Temperature: 25 ° C

Sulfuric acid content: 300 g / l

Sodium sulfate content: 240 g / l

Sodium persulfate content: 0 g / l

Active anode area in the cell types used:

Tubular cell with platinum-titanium anode: 1 280 cm ²

Tubular cell with diamond-niobium anode: 1280 cm ²

- Flat cell with diamond-niobium anode: 1 250 cm ²

Cathode material: acid-resistant stainless steel: 1 .4539

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

Current densities:

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

Results:

From the course of the current efficiency as a function of the change Sodium persulphate content (FIG. 1), it can be clearly seen that the diamond anode used has a potency of approx. 1000 g / l to about 350 g / l, even without the addition of a promoter achieved significantly higher current efficiency, as they are known from conventional platinum-coated titanium anodes with added promoter.

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

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

In summary, it can be confirmed that the current efficiency of a diamond-coated niobium anode is about 1 0% higher even without the addition of a potential-increasing agent than in a cell with conventional platinum-titanium anode and adding a potential-increasing agent and about 40% higher than in a cell Cell with conventional platinum titanium anode without addition of a potential-enhancing agent.

The voltage drop across a diamond-coated anode is about 0.9 volts higher than for a comparable cell with a platinum-titanium anode. It was further shown that the current yield in a diamond electrode to be used according to the invention without the addition of a promoter only increases with increasing total content of sodium peroxodisulfate in the electrolyte slowly decreases - under the experimental conditions, for example, at a current efficiency of equal to or above 65% electrolyte solutions with a Natriumperoxodisulfatgehalt of about 400 - 650 g / l win.

In contrast, using a conventional platinum anode and concomitant use of a promoter in the electrolyte, only the same high Peroxod isulfatkonzentrationen of about 300 g / l, and obtained at a Stormausbeute of about 50%.

Block-type investigations on a similar system with potassium ions from potassium sulfate gave similarly good results.

It is surprising for the skilled worker that the inventive method at high conversions with technically easy to handle current densities without the spatial separation of anolyte and catholyte and without the use of a promoter at the same time high current efficiency with high persulfate and solid concentrations in undivided electrolysis cells without the addition a promoter can be performed.

In the course of investigations of this invention, it has been found that the production of ammonium and iw but alkali metal peroxodisulfates with high current efficiency, even in an undivided cell, is therefore possible by using as anode an oxide doped with a trivalent or pentavalent element becomes . Surprisingly, the cell, even at very high solids content, i .w. Peroxodisulfatgehalt economically I can be used sensibly and at the same time completely dispensed with the use of a promoter and the electrolysis can be carried out at high current density, resulting in further advantages, especially with regard to installation and acquisition costs result. Summary:

The use of an undivided cell allows electrolyte solutions with very high solids concentrations, which in turn significantly reduces the energy required for salt recovery, essentially crystallization and water evaporation, directly proportional to the increase in solids but at least 25% of that of a divided cell.

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

The working current density can be significantly reduced compared to platinum anodes with the same amount of production, whereby less 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, the current efficiency and thus the production quantity can be increased with increasing current density.

Due to the excellent abrasion resistance of the diamond coated anode, much higher flow rates can be used compared to a structurally similar Pt anode.

Claims

claims

1 . Electrolysis cell comprising the components:

(a) at least one tubular cathode,

(b) at least one rod or tubular anode comprising a conductive support coated with a conductive diamond layer,

(c) at least one inlet pipe,

(d) at least one drainpipe, and

(e) at least two distribution devices.

2. The electrolytic cell according to claim 1, wherein the electrolyte space is formed as an annular gap between inner-lying anode and outer-lying cathode.

3. electrolytic cell according to one of claims 1 or 2, wherein the electrolytic cell has a common electrolyte space without diaphragm.

4. The electrolytic cell according to any one of claims 1 -3, wherein the distance between the anode outer surface and the cathode inner surface is between 1 and 20 mm.

5. An electrolytic cell according to any one of claims 1-4, wherein the inner diameter of the cathode is between 10 and 400 mm.

6. An electrolytic cell according to any one of claims 1-5, wherein the anode and the cathode are each independently between 20 and 1 20 cm long.

7. Elekrolysezelle according to any one of claims 1-6, wherein the carrier is selected from the group consisting of sil icium, germanium, Titanium, zirconium, niobium, tantalum, molybdenum, tungsten, carbides of these elements, and / or aluminum, or combinations of the elements.

8. Elekrolysezelle according to any one of claims 1-7, wherein the diamond layer is doped with at least one three or at least one pentavalent main or sub-group element, in particular boron and / or phosphorus.

9. Elekrolysezelle according to any one of claims 1 -8, wherein the cathode is made of lead, carbon, tin, platinum, nickel, alloys of these elements, zirconium and / or iron alloys, in particular made of acid-resistant stainless steel.

1 0. An electrolytic cell according to any one of claims 1-9, wherein the electrolyte is supplied to the electrolysis cell through the inlet pipe.

1 1. Elekrolysezelle according to any one of claims 1 -1 0, wherein the electrolysis product is removed via the drain pipe of the electrolysis cell.

1 2. Elekrolysezelle according to any one of claims 1 -1 1, wherein the distribution device distributes the electrolyte into the electrolyte space.

1 3. Elekrolysezelle according to any one of claims 1 -1 2, wherein the anode is connected via the distribution device to the power source.

1 4. Elekrolysezelle according to any one of claims 1 -1 3, wherein the components of the electrolysis cell are individually exchangeable.

1 5. Elekrolysezelle according to one of claims 1 -14, wherein the distributor device is irreversibly connected to the anode.

1 6. Electrolysis apparatus comprising at least two electrolysis cells according to one of claims 1 -1 5, wherein the electrolyte passes through the electrolysis cells in succession and the electrolytic cells are connected in parallel electrochemically. s 17. Use of the electrolytic cell according to any one of claims 1-15 or the electrolysis apparatus according to claim 1 6 for the oxidation of an electrolyte.

Use according to claim 17, wherein the current density is between 50 and 1500 mA / cm ² .

1 9. Use according to any one of claims 1 7 or 1 8, wherein the electrolyte has a solids content between 1 50-850 g / l. i 5. 20. Use according to one of claims 1 7-19 for the preparation of

Peroxodisulfate.