EP1148155B2 - Process for producing alkali metal and ammonium peroxide disulphate - Google Patents

Process for producing alkali metal and ammonium peroxide disulphate Download PDF

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Publication number
EP1148155B2
EP1148155B2 EP01109242A EP01109242A EP1148155B2 EP 1148155 B2 EP1148155 B2 EP 1148155B2 EP 01109242 A EP01109242 A EP 01109242A EP 01109242 A EP01109242 A EP 01109242A EP 1148155 B2 EP1148155 B2 EP 1148155B2
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Prior art keywords
anolyte
anode
catholyte
ammonium
peroxodisulfate
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EP1148155A2 (en
EP1148155B1 (en
EP1148155A3 (en
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Thomas Dr. Lehmann
Patrick Stenner
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Evonik Operations GmbH
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Evonik Degussa GmbH
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/01—Products
    • C25B1/28—Per-compounds
    • C25B1/29—Persulfates
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/01—Products
    • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis

Definitions

  • the invention relates to a process for the preparation of sodium and ammonium peroxodisulfate by anodic oxidation of a sodium or ammonium sulfate or hydrogen sulfate containing aqueous solution.
  • Sodium peroxodisulfate is produced at a current efficiency of 70-80% in an electrolytic cell having a diaphragm protected cathode and a platinum anode, by adding a neutral aqueous anolyte solution having an initial content of 5 to 9% by weight of sodium ions, 12 to 30% by weight.
  • a filter press-type electrolytic cell for producing peroxo compounds, including ammonium peroxodisulfate, sodium peroxodisulfate and potassium peroxodisulfate.
  • peroxo compounds including ammonium peroxodisulfate, sodium peroxodisulfate and potassium peroxodisulfate.
  • 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.
  • peroxodisulfates are prepared by anodic oxidation of an aqueous solution containing neutral ammonium sulfate.
  • the solution obtained from the anodic oxidation which contains ammonium peroxodisulfate, is reacted with caustic soda or potassium hydroxide; after crystallization and separation of the corresponding alkali metal peroxodisulfate, the mother liquor is recycled in admixture with the catholyte produced during electrolysis.
  • the electrolysis takes place in the presence of a promoter on a platinum electrode as the anode.
  • PA Michaud 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 boron doped diamond thin film electrode.
  • This document teaches that such electrodes have a higher overvoltage for oxygen than platinum electrodes, but it can not be deduced from this document whether boron-doped diamond thin-film electrodes can also be used for the industrial production of ammonium and alkali metal peroxodisulfates.
  • sulfuric acid on the one hand and hydrogen sulphates, in particular neutral sulphates on the other hand behave very differently in anodic oxidation.
  • the major side reaction besides the anodic oxidation of sulfuric acid is the evolution of oxygen and, in addition, ozone.
  • the object of the present invention is to demonstrate a technical process for the preparation of ammonium and sodium peroxodisulfates, which has the disadvantages of the known processes, at least to a lesser extent.
  • the use of a promoter can be dispensed with completely and the electrolysis can be carried out at low current density, resulting in further advantages.
  • the present invention accordingly provides a process for the preparation of a peroxodisulfate from the series of ammonium and sodium peroxodisulfate, by the anodic oxidation of an ammonium sulfate or sodium hydrogen sulfate containing aqueous electrolyte in an electrolytic cell, comprising at least one anode, a cathode and anolyte space, this by a separator separated from a Katholytraum is characterized in that one uses as an anode disposed on a conductive support and made conductive by doping with a trivalent or pentavalent element diamond layer and added to the anolyte no promoter.
  • the subclaims are directed to preferred embodiments of this method.
  • the effective as an anode conductive diamond layer is doped in their preparation by doping with one or more tri- or pentavalent elements with such an amount that sufficient conductivity results.
  • the doped diamond layer is thus an n-conductor or a p-type conductor.
  • the conductive diamond layer is on a conductive support material, which may be selected from the series silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum and tungsten and carbides of said elements.
  • a conductive diamond layer may also be applied to aluminum.
  • Particularly preferred support materials for the diamond layer are silicon, titanium, niobium, tantalum and tungsten and carbides of these elements.
  • a particularly suitable electrode material for the anode is a boron-doped diamond thin film on silicon.
  • the preparation of the diamond electrodes can be carried out in two special CVD (chemical vapor deposition technique). These are the microwave plasma CVD and the hot wire CVD process.
  • 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.
  • a gaseous compound of the dopant By using a boron compound such as trimethylboron, a p-type semiconductor is formed.
  • a gaseous phosphorus compound as a dopant, an n-type semiconductor is obtained.
  • deposition of the doped diamond layer on crystalline silicon a particularly dense and non-porous layer is obtained - a film thickness of 1 ⁇ m is usually sufficient.
  • the deposition can also take place on a self-passivating metal, such as titanium, tantalum, tungsten or niobium.
  • a self-passivating metal such as titanium, tantalum, tungsten or niobium.
  • ammonium and sodium peroxodisulfate can be carried out in customary electrolysis cells, which may also be combined in the form of a filter pack.
  • Anode space and cathode space are separated by a separator.
  • the separator may, for example, be a conventional porous material made of an oxidic material, but an ion exchange membrane is preferred.
  • Suitable cathodes are those materials which are already known in the art, such as lead, carbon, tin, zirconium, platinum, nickel and their alloys, with lead being preferred.
  • the electrolysis cell comprises a circuit for the liquid anolyte and a further circuit for a liquid catholyte. According to the invention, no promoter is added to the anolyte.
  • the starting anolyte contains per liter 300 to 500 g of ammonium sulfate and 0 to 0.2 mol of sulfuric acid per mole of ammonium sulfate.
  • a substantially neutral starting anolyte is preferred.
  • catholyte is a sulfuric acid ammonium sulfate solution.
  • the anodic oxidation is carried out at an anodic current density in the range of 50 to 1000 mA / cm 2 , preferably 400 to 900 mA / cm 2 .
  • ammonium peroxodisulfate is obtained in a manner known per se, the work-up preferably comprising a vacuum crystallization and separation of the crystals from the mother liquor.
  • the anolyte mother liquor is recirculated into the electrolysis after increasing the content of ammonium sulfate or hydrogen sulfate - this can be done by mixing with the catholyte produced and, if necessary, adding a base.
  • Sodium peroxodisulfate is prepared by adding an anolyte solution containing sodium hydrogen sulfate in an amount of 300 to 700 g / l NaHSO 4 at a current density in the range from 50 to 1000 mA in an electrolysis cell with an anolyte and catholyte circuit separated by a separator, in particular an ion exchange membrane / cm 2 , in particular 400 to 900 mA / cm 2 , oxidized anodically, wherein the catholyte used is a sulfuric acid sodium hydrogen sulfate solution.
  • the example of sodium peroxodisulfate shows at medium current density the dependence of the current efficiency on the concentration of sodium peroxodisulfate with a diamond or platinum electrode and that the current efficiency in a diamond electrode according to the invention to be used with increasing content of sodium peroxodisulfate in the anolyte only slowly decreases - under the experimental conditions can be For example, at a current efficiency of equal to or greater than 75% anolyte solutions with a Natriumperoxodisulfatgehalt of about 400 g / l win.
  • the working current density can be significantly reduced compared to platinum anodes, whereby 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.
  • a further advantage is that the conductive diamond anodes to be used according to the invention can be produced in any desired form and corrosion-prone connection points, such as weld seams and the like, are not present. As a result, a longer electrode life is achieved.
  • the electrolytic cell includes a lead cathode and a boron-doped diamond anode on a Si wafer.
  • the diamond anode was connected to a metal plate (power distributor).
  • the diamond anode was replaced by a mirror-finished platinum sheet ground with diamond powder.
  • the electrolyte chambers were separated by an ion exchange membrane (DuPont, Nafion 430) in the anode compartment and cathode compartment. The distance between the electrodes was 2.2 cm.
  • the round electrode area was 38.48 cm 2 .
  • the following table shows the operating parameters and the specific energy consumption.
  • the table shows the comparison of the electrolysis results with Pt and a diamond anode.
  • NaHSO 4 was oxidized anodically in the cell described above (B1 / VB1).
  • the anolyte consisted of a NaHSO 4 solution containing 610 g NaHSO 4 / l. After setting the current density samples were taken and analyzed after a predetermined time. In calculating the current efficiency, a linear volume decrease was assumed.
  • the curves showed the current efficiency as a function of the sodium peroxodisulfate (NaPS) concentration achieved in the anolyte using a diamond electrode (B2) or a Pt anode (VB2).
  • NaPS sodium peroxodisulfate
  • the anolyte contained no promoter. Only when using an anolyte with a prohibitively high promoter concentration - 0.6 g NH 4 SCN / l - was it possible to achieve current efficiencies close to those of Example B 2.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
  • Electrodes For Compound Or Non-Metal Manufacture (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

Description

Die Erfindung betrifft ein Verfahren zur Herstellung von Natrium- sowie von Ammoniumperoxodisulfat durch anodische Oxidation eines ein Natrium- oder Ammoniumsulfat oder -hydrogensulfat enthaltenden wässrigen Lösung.The invention relates to a process for the preparation of sodium and ammonium peroxodisulfate by anodic oxidation of a sodium or ammonium sulfate or hydrogen sulfate containing aqueous solution.

Es ist bekannt, Alkalimetall- und Ammoniumperoxodisulfat durch anodische Oxidation eines das entsprechende Sulfat oder Hydrogensulfat enthaltenden wässrigen Lösung herzustellen und das Salz durch Kristallisation aus dem Anolyt zu gewinnen.It is known to prepare alkali metal and ammonium peroxodisulfate by anodic oxidation of an aqueous solution containing the corresponding sulfate or bisulfate, and to recover the salt by crystallization from the anolyte.

Gemäß DE-PS 27 57 861 wird Natriumperoxodisulfat mit einer Stromausbeute um 70 bis 80 % in einer Elektrolysezelle mit einer durch ein Diaphragma geschützten Kathode und einer Platinanode hergestellt, indem eine neutrale wäßrige Anolytlösung mit einem Anfangsgehalt von 5 bis 9 Gew.-% Natriumionen, 12 bis 30 Gew.-% Sulfationen, 1 bis 4 Gew.-% Ammoniumionen, 6 bis 30 Gew.-% Peroxodisulfationen und einem potentialerhöhenden Mittel, einem sogenannten Promoter, wie insbesondere Thiocyanat, unter Verwendung einer Schwefelsäure-Lösung als Katholyt bei einer Stromdichte von mindestens 0,5 bis 2 A/cm2 elektrolysiert wird. Nach dem Auskristallisieren und Abtrennen von Peroxodisulfat aus dem Anolyten wird die Mutterlauge mit dem Kathodenprodukt vermischt, neutralisiert und wieder der Anode zugeführt. Nachteile dieses Verfahrens sind 1. das Erfordernis des Einsatzes eines Promoters, um die Sauerstoffentwicklung zu vermindern, 2. das Erfordernis einer hohen Stromdichte und damit eines hohen Anodenpotentials, um eine wirtschaftlich akzeptable Stromausbeute zu erhalten und 3. die mit der Herstellung der Platinanode verbundenen Probleme mit Hinblick auf den Erhalt einer für technische Zwecke akzeptablen Stromausbeute und hohen Lebensdauer der Anode.According to DE-PS 27 57 861 Sodium peroxodisulfate is produced at a current efficiency of 70-80% in an electrolytic cell having a diaphragm protected cathode and a platinum anode, by adding a neutral aqueous anolyte solution having an initial content of 5 to 9% by weight of sodium ions, 12 to 30% by weight. Sulfate ions, 1 to 4 wt .-% ammonium ions, 6 to 30 wt .-% peroxodisulfate ions and a potential-increasing agent, a so-called promoter, in particular thiocyanate, using a sulfuric acid solution as the catholyte at a current density of at least 0.5 to 2 A / cm 2 is 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 process are 1. the requirement of using a promoter to reduce the evolution of oxygen, 2. the requirement of a high current density and thus a high anode potential in order to obtain an economically acceptable current efficiency, and 3. the problems associated with the production of the platinum anode with a view to obtaining a technically acceptable current efficiency and anode life.

Aus der EP-B 0 428 171 ist eine Elektrolysezelle vom Filterpressentyp zur Herstellung von Peroxoverbindungen, darunter Ammoniumperoxodisulfat, Natriumperoxodisulfat und Kaliumperoxodisulfat bekannt. Als Anoden werden hier heißisostatisch auf einem Ventilmetall aufgebrachte Platinfolien verwendet. Als Anolyt wird eine einen Promoter und Schwefelsäure enthaltenden Lösung des entsprechenden Sulfats verwendet. Auch dieses Verfahren weist die zuvor genannten Probleme auf.From the EP-B 0 428 171 For example, a filter press-type electrolytic cell is known 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.

Im Verfahren der DE-OS 199 13 820 werden Peroxodisulfate durch anodische Oxidation einer neutrales Ammoniumsulfat enthaltenden wässrigen Lösung hergestellt. Zum Zwecke der Herstellung von Natrium- oder Kaliumperoxodisulfat wird die aus der anodischen Oxidation erhaltene Lösung, welche Ammoniumperoxodisulfat enthält, mit Natronlauge oder Kaliumlauge umgesetzt; nach der Kristallisation und Abtrennung des entsprechenden Alkalimetallperoxodisulfats wird die Mutterlauge im Gemisch mit dem bei der Elektrolyse erzeugten Katolyt recycliert. Auch in diesem Falle erfolgt die Elektrolyse in Gegenwart eines Promoters an einer Platinelektrode als Anode.In the process of DE-OS 199 13 820 For example, 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 the anodic oxidation, which contains ammonium peroxodisulfate, is reacted with caustic soda or potassium hydroxide; after crystallization and separation of the corresponding alkali metal peroxodisulfate, the mother liquor is recycled in admixture with the catholyte produced during electrolysis. Also in this case, the electrolysis takes place in the presence of a promoter on a platinum electrode as the anode.

Obgleich Peroxodisulfat bereits seit Jahrzehnten in technischem Maßstab durch anodische Oxidation an einer Platinanode gewonnen werden, haften diesen Verfahren weiterhin gravierende Nachteile an:
Es ist immer ein Zusatz von Polarisatoren, auch Promotoren genannt, erforderlich, um die Sauerstoffüberspannung zu erhöhen und die Stromausbeute zu verbessern; Oxidationsprodukte dieser Promotoren gelangen als toxische Substanzen in das Anodenabgas und müssen in einer Gaswäsche entfernt werden.Although peroxodisulfate has been obtained on an industrial scale for decades by anodic oxidation on a platinum anode, these methods continue to have serious disadvantages:
It is always an addition of polarizers, also called promoters, required to increase the oxygen overvoltage and improve the current efficiency; Oxidation products of these promoters arrive as toxic substances in the anode exhaust gas and must be removed in a gas scrubber.

Die üblicherweise ganzflächig mit Platin bedeckten Anoden erfordern stets eine hohe Stromdichte. Dadurch kommt es zu einer hohen Strombelastung des Anolytvolumens, des Separators und der Kathode, wodurch zusätzliche Maßnahmen zur Herabsetzung der kathodischen Stromdichte durch eine dreidimensionale Strukturierung und Aktivierung erforderlich wird. Hinzu kommt eine hohe thermische Belastung der labilen Peroxodisulfatlösung. Um diese Belastung zu minimieren, müssen konstruktive Maßnahmen ergriffen werden, und der Kühlaufwand steigt zusätzlich. Wegen der limitierenden Wärmeabfuhr muss die Elektrodenfläche begrenzt werden, und hiermit steigt der Installationsaufwand pro Zelleneinheit. Um die hohe Strombelastung zu bewältigen, müssen in der Regel zusätzlich Elektroden-Stützmaterialien mit hohen Wärmeübertragungseigenschaften verwendet werden, die ihrerseits korrosionsanfällig und teuer sind.The usually over the entire surface covered with platinum anodes always require a high current density. This results in a high current load of the anolyte volume, the separator and the cathode, whereby additional measures to reduce the cathodic current density by a three-dimensional structuring and activation is 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 in addition.

P.A. Michaud et. al. lehren in Electro Chemical and Solid-State letters, 3(2) 77-79 (2000) die Herstellung von Peroxodischwefelsäure durch anodische Oxidation von Schwefelsäure unter Verwendung einer mit Bor dotierten Diamant-Dünnschichtelektrode. Dieses Dokument lehrt, dass derartige Elektroden eine höhere Überspannung für Sauerstoff aufweisen, als Platinelektroden, jedoch lässt sich diesem Dokument nicht entnehmen, ob mit Bor dotierte Diamant-Dünnschichtelektroden auch zur technischen Herstellung von Ammonium- und Alkalimetallperoxodisulfaten eingesetzt werden können. Es ist nämlich bekannt, dass sich Schwefelsäure einerseits und Hydrogensulfate insbesondere neutrale Sulfate andererseits bei der anodischen Oxidation sehr unterschiedlich verhalten. Trotz der erhöhten Überspannung des Sauerstoffs an der mit Bor dotierten Diamant-Elektrode ist die Hauptnebenreaktion neben der anodischen Oxidation von Schwefelsäure die Entwicklung von Sauerstoff und zusätzlich von Ozon. PA Michaud 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 boron doped diamond thin film electrode. This document teaches that such electrodes have a higher overvoltage for oxygen than platinum electrodes, but it can not be deduced from this document whether boron-doped diamond thin-film electrodes can also be used for the industrial production of ammonium and alkali metal peroxodisulfates. It is known that sulfuric acid on the one hand and hydrogen sulphates, in particular neutral sulphates, on the other hand behave very differently in 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.

Aufgabe der vorliegenden Erfindung ist es, ein technisches Verfahren zur Herstellung von Ammonium und Natriumperoxodisulfaten aufzuzeigen, das die Nachteile der bekannten Verfahren zumindest in geringerem Umfang aufweist. Überraschenderweise wurde gefunden, dass die Herstellung von Ammonium- und Natriumperoxodisulfaten mit hoher Stromausbeute möglich ist, indem als Anode eine mit einem drei- oder fünfwertigen Element dotierte Diamant-Dünnschichtelektrode verwendet wird. Überraschenderweise kann auf den Einsatz eines Promotors vollständig verzichtet werden und die Elektrolyse bei niedriger Stromdichte durchgeführt werden, woraus weitere Vorteile resultieren.The object of the present invention is to demonstrate a technical process for the preparation of ammonium and sodium peroxodisulfates, which has the disadvantages of the known processes, at least to a lesser extent. Surprisingly, it has been found that the production of ammonium and sodium peroxodisulfates with high current efficiency is possible by doping as the anode one with a tri- or pentavalent element Diamond thin-film electrode is used. Surprisingly, the use of a promoter can be dispensed with completely and the electrolysis can be carried out at low current density, resulting in further advantages.

Gegenstand der vorliegenden Erfindung ist demgemäß ein Verfahren zur Herstellung eines Peroxodisulfats aus der Reihe von Ammonium- und Natriumperoxodisulfat, durch die anodische Oxidation eines Ammoniumsulfat oder Natriumhydrogensulfat enthaltenden wässrigen Elektrolyts in einer Elektrolysezelle, umfassend mindestens eine Anode, eine Kathode und einen Anolytraum, wobei dieser durch einen Separator von einem Katholytraum getrennt ist das dadurch gekennzeichnet ist, dass man als Anode eine auf einem leitfähigen Träger angeordnete und durch Dotierung mit einem drei- oder fünfwertigen Element leitfähig gemachte Diamantschicht verwendet und dem Anolyten keinen Promoter zusetzt. Die Unteransprüche richten sich auf bevorzugte Ausführungsformen dieses Verfahrens.The present invention accordingly provides a process for the preparation of a peroxodisulfate from the series of ammonium and sodium peroxodisulfate, by the anodic oxidation of an ammonium sulfate or sodium hydrogen sulfate containing aqueous electrolyte in an electrolytic cell, comprising at least one anode, a cathode and anolyte space, this by a separator separated from a Katholytraum is characterized in that one uses as an anode disposed on a conductive support and made conductive by doping with a trivalent or pentavalent element diamond layer and added to the anolyte no promoter. The subclaims are directed to preferred embodiments of this method.

Die als Anode wirksame leitfähige Diamantschicht wird bei ihrer Herstellung durch Dotierung mit einem oder mehreren drei- oder fünf-wertigen Elementen mit einer solchen Menge dotiert, dass eine ausreichende Leitfähigkeit resultiert. Die dotierte Diamantschicht ist somit ein n-Leiter oder ein p-Leiter. Zweckmäßigerweise befindet sich die leitfähige Diamantschicht auf einem leitfähigen Trägermaterial, wobei dieses ausgewählt sein kann aus der Reihe Silicium, Germanium, Titan, Zirconium, Niob, Tantal, Molybdän und Wolfram sowie Carbiden der genannten Elemente. Alternativ kann eine leitfähige Diamantschicht auch auf Aluminium aufgebracht werden. Besonders bevorzugte Trägermaterialien für die Diamantschicht sind Silicium, Titan, Niob, Tantal und Wolfram sowie Carbide dieser Elemente.The effective as an anode conductive diamond layer is doped in their preparation by doping with one or more tri- or pentavalent elements with such an amount that sufficient conductivity results. The doped diamond layer is thus an n-conductor or a p-type conductor. Conveniently, the conductive diamond layer is on a conductive support material, which may be selected from the series silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum and tungsten and carbides of said elements. Alternatively, a conductive diamond layer may also be applied to aluminum. Particularly preferred support materials for the diamond layer are silicon, titanium, niobium, tantalum and tungsten and carbides of these elements.

Ein besonders geeignetes Elektrodenmaterial für die Anode ist eine bordotierte Diamant-Dünnschicht auf Silicium.A particularly suitable electrode material for the anode is a boron-doped diamond thin film on silicon.

Die Herstellung der Diamant-Elektroden kann in zwei speziellen CVD-Verfahren (chemical vapor deposition technic) erfolgen. Es handelt sich um das Mikrowellen-Plasma-CVD- und das Heißdraht-CVD-Verfahren. In beiden Fällen entsteht die Gasphase, die durch Mikrowellenbestrahlung oder thermisch durch heiße Drähte zum Plasma aktiviert wird, aus Methan, Wasserstoff und ggf. weiteren Zusätzen, insbesondere einer gasförmigen Verbindung des Dotierungsmittels. Durch Verwendung einer Borverbindung, wie Trimethylbor, entsteht ein p-Halbleiter. Unter Einsatz einer gasförmigen Phosphorverbindung als Dotierungsmittel wird ein n-Halbleiter erhalten. Durch Abscheidung der dotierten Diamantschicht auf kristallinem Silicium wird eine besonders dichte und porenfreie Schicht erhalten - eine Filmdicke um 1 µm ist üblicherweise ausreichend. Alternativ zur Abscheidung der Diamantschicht auf einem kristallinen Material kann die Abscheidung auch auf einem selbst passivierenden Metall, wie Titan, Tantal, Wolfram oder Niob erfolgen. Zur Herstellung einer besonders geeigneten bordotierten Diamantschicht auf einem Silicium-Einkristall auf den vorerwähnten Artikel von P.A. Michaud verwiesen.The preparation of the diamond electrodes can be carried out in two special CVD (chemical vapor deposition technique). These are the microwave plasma CVD and the hot wire CVD process. 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 is formed. By using a gaseous phosphorus compound as a dopant, an n-type semiconductor is obtained. By deposition of the doped diamond layer on crystalline silicon, a particularly dense and non-porous layer is obtained - a film thickness of 1 μm is usually sufficient. As an alternative to 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. For the preparation of a particularly suitable boron-doped diamond layer on a silicon single crystal on the aforementioned article by P.A. Michaud referred.

Die Herstellung von Ammonium- und Natriumperoxodisulfat kann in üblichen Elektrolysezellen, die auch in Form eines Filterpakets zusammengefasst sein können, durchgeführt werden. Anodenraum und Kathodenraum sind hierbei durch einen Separator getrennt. Bei dem Separator kann es sich beispielsweise um ein übliches poröses Material aus einem oxidischen Material handeln, bevorzugt wird jedoch eine Ionenaustauschermembran. Als Kathode eignen sich solche Materialien, wie sie im Stand der Technik bereits bekannt sind, wie Blei, Kohlenstoff, Zinn, Zirkon, Platin, Nickel und deren Legierungen, wobei Blei bevorzugt wird.The production of ammonium and sodium peroxodisulfate can be carried out in customary electrolysis cells, which may also be combined in the form of a filter pack. Anode space and cathode space are separated by a separator. The separator may, for example, be a conventional porous material made of an oxidic material, but an ion exchange membrane is preferred. Suitable cathodes are those materials which are already known in the art, such as lead, carbon, tin, zirconium, platinum, nickel and their alloys, with lead being preferred.

Gemäß einer bevorzugten Ausführungsform umfasst die Elektrolysezelle einen Kreislauf für den flüssigen Anolyten und einen weiteren Kreislauf für einen flüssigen Katholyten. Erfindungsgemäß wird dem Anolyt kein Promoter zugesetzt.According to a preferred embodiment, the electrolysis cell comprises a circuit for the liquid anolyte and a further circuit for a liquid catholyte. According to the invention, no promoter is added to the anolyte.

Zur Herstellung von Ammoniumperoxodisulfat enthält der Startanolyt pro Liter 300 bis 500 g Ammoniumsulfat und 0 bis 0,2 Mol Schwefelsäure pro Mol Ammoniumsulfat. Ein im wesentlichen neutraler Startanolyt wird bevorzugt. Katholyt ist in diesem Fall eine schwefelsaure Ammoniumsulfatlösung. Die anodische Oxidation wird bei einer anodischen Stromdichte im Bereich von 50 bis 1000 mA/cm2, vorzugsweise 400 bis 900 mA/cm2, durchgeführt. Aus einem aus dem Anolytkreislauf ausgeschleusten Anolytstrom wird in an sich bekannter Weise Ammoniumperoxodisulfat gewonnen, wobei die Aufarbeitung vorzugsweise eine Vakuumkristallisation und Abtrennung der Kristalle von der Mutterlauge umfasst. Die Anolyt-Mutterlauge wird nach Erhöhung des Gehalts an Ammoniumsulfat oder -hydrogensulfat in die Elektrolyse rezirkuliert- dies kann durch Vermischen mit dem erzeugten Katholyten und bei Bedarf Zugabe einer Base erfolgen.For the preparation of ammonium peroxodisulfate the starting anolyte contains per liter 300 to 500 g of ammonium sulfate and 0 to 0.2 mol of sulfuric acid per mole of ammonium sulfate. A substantially neutral starting anolyte is preferred. In this case, catholyte is a sulfuric acid ammonium sulfate solution. The anodic oxidation is carried out at an anodic current density in the range of 50 to 1000 mA / cm 2 , preferably 400 to 900 mA / cm 2 . From an anolyte stream discharged from the anolyte circuit, ammonium peroxodisulfate is obtained in a manner known per se, the work-up preferably comprising a vacuum crystallization and separation of the crystals from the mother liquor. The anolyte mother liquor is recirculated into the electrolysis after increasing the content of ammonium sulfate or hydrogen sulfate - this can be done by mixing with the catholyte produced and, if necessary, adding a base.

Natriumperoxodisulfat wird hergestellt indem man in einer Elektrolysezelle mit einem mittels eines Separators, insbesondere einer Ionenaustauschermembran, voneinander getrennten Anolyt- und Katholytkreislauf eine Natriumhydrogensulfat in einer Menge von 300 bis 700 g/l NaHSO4 enthaltende Anolytlösung bei einer Stromdichte im Bereich von 50 bis 1000 mA/cm2, insbesondere 400 bis 900 mA/cm2, anodisch oxidiert, wobei als Katholyt eine schwefelsaure Natriumhydrogensulfatlösung verwendet wird.Sodium peroxodisulfate is prepared by adding an anolyte solution containing sodium hydrogen sulfate in an amount of 300 to 700 g / l NaHSO 4 at a current density in the range from 50 to 1000 mA in an electrolysis cell with an anolyte and catholyte circuit separated by a separator, in particular an ion exchange membrane / cm 2 , in particular 400 to 900 mA / cm 2 , oxidized anodically, wherein the catholyte used is a sulfuric acid sodium hydrogen sulfate solution.

Aus dem Verlauf der Stromausbeute in Abhängigkeit der Stromdichte bei der Herstellung von Ammoniumperoxodisulfat unter Verwendung einer Platinelektrode (Vergleichsbeispiele) und einer erfindungsgemäß zu verwendenden mit Bor dotierten Diamantelektrode folgt, dass bei einer Stromdichte von 100 mA/cm2 eine Stromausbeute von über 95 % erhältlich ist. Zwar nimmt die Stromausbeute mit zunehmender Stromdichte ab, jedoch liegt die Stromausbeute bei einer Stromdichte von 1000 mA/cm2 noch deutlich über 80 %. Demgegenüber lässt sich unter Verwendung einer herkömmlichen Platinanode bei niedrigen Stromdichten überhaupt kein Ammoniumperoxodisulfat gewinnen, und bei höheren Stromdichten ist die Stromausbeute um 10 bis 20 % niedriger als unter Verwendung einer erfindungsgemäß eingesetzten Diamantelektrode.From the course of the current efficiency as a function of the current density in the production of ammonium peroxodisulfate using a platinum electrode (comparative examples) and a boron-doped diamond electrode to be used according to the invention, it follows that a current density of 100 mA / cm 2 gives a current efficiency of over 95% , Although the current efficiency decreases with increasing current density, the current efficiency at a current density of 1000 mA / cm 2 is still well above 80%. On the other hand, no ammonium peroxodisulfate can be obtained at all at low current densities using a conventional platinum anode, and at higher current densities, the current efficiency is 10 to 20% lower than when using a diamond electrode used in the present invention.

Das Beispiel von Natriumperoxodisulfat zeigt bei mittlerer Stromdichte die Abhängigkeit der Stromausbeute von der Konzentration an Natriumperoxodisulfat mit einer Diamant- bzw. Platinelektrode und dass die Stromausbeute bei einer erfindungsgemäß zu verwendenden Diamantelektrode mit zunehmendem Gehalt an Natriumperoxodisulfat im Anolyten nur langsam abnimmt - unter den Versuchsbedingungen lassen sich beispielsweise bei einer Stromausbeute von gleich oder über 75 % Anolytlösungen mit einem Natriumperoxodisulfatgehalt von etwa 400 g/l gewinnen. Unter Verwendung einer herkömmlichen Platinanode und Mitverwendung eines Promotors im Anolyten lassen sich demgegenüber nur Peroxodisulfatkonzentrationen von etwa 300 g/l, und zwar bei einer Stromausbeute von etwa 25 % erhalten.The example of sodium peroxodisulfate shows at medium current density the dependence of the current efficiency on the concentration of sodium peroxodisulfate with a diamond or platinum electrode and that the current efficiency in a diamond electrode according to the invention to be used with increasing content of sodium peroxodisulfate in the anolyte only slowly decreases - under the experimental conditions can be For example, at a current efficiency of equal to or greater than 75% anolyte solutions with a Natriumperoxodisulfatgehalt of about 400 g / l win. In contrast, using a conventional platinum anode and the concomitant use of a promoter in the anolyte, it is only possible to obtain peroxodisulfate concentrations of about 300 g / l, with a current yield of about 25%.

Es war nicht vorhersehbar, dass das erfindungsgemäße Verfahren bei hohen Umsätzen mit technisch gut handhabbaren Stromdichten ohne den Einsatz eines Promoters bis zu hohen Umsätzen bei gleichzeitig hoher Stromausbeute durchgeführt werden kann. Da in dem zitierten Artikel von P.A. Michaud einerseits auf die Sauerstoffbildung als Hauptnebenreaktionen hingewiesen wurde und andererseits die anodische Oxidation von Schwefelsäure bei maximal 200 mA/cm2 bei nur sehr geringem Umsatz durchgeführt wurde, war nicht zu erwarten, dass Ammonium- und Natriumperoxodisulfate in einfacher und sehr wirtschaftlicher Weise unter Verwendung einer dotierten Diamant-Anode hergestellt werden können. Ausser dem Wegfall des Einsatzes eines Promotors und damit Wegfall von erforderlichen Reinigungsmaßnahmen des Anodengases sind höhere Umsätze und höhere Persulfat-Konzentrationen im auslaufenden Anolyt erhältlich, wodurch wiederum der Aufwand für die Kristallisation gemindert wird. Die Arbeitsstromdichte kann gegenüber Platinanoden deutlich herabgesetzt werden, wodurch weniger Ohm'sche Verluste im System auftreten und damit der Kühlaufwand vermindert und der Freiheitsgrad in der Gestaltung der Elektrolysezellen und der Kathoden erhöht wird. Ein weiterer Vorteil besteht darin, dass die erfindungsgemäß zu verwendenden leitfähigen Diamant-Anoden in beliebiger Form herstellt werden können und korrosionsanfällige Verbindungsstellen, wie Schweißnähte und dergleichen nicht vorhanden sind. Dadurch wird eine längere Elektrodenlebenszeit erreicht.It was not foreseeable that the process according to the invention can be carried out at high conversions with industrially easy-to-handle current densities without the use of a promoter up to high conversions with simultaneously high current efficiency. Since in the cited article by PA Michaud on the one hand on the oxygen formation was pointed out as a major side reactions and on the other hand, the anodic oxidation of sulfuric acid at a maximum of 200 mA / cm 2 was carried out at very low conversion, it was not expected that ammonium and Natriumperoxodisulfate in simpler and can be made very economically using a doped diamond anode. Except the elimination of the use of a promoter and thus omission of necessary cleaning measures of the anode gas higher sales and higher persulfate concentrations are available in the expiring anolyte, which in turn reduces the cost of crystallization. The working current density can be significantly reduced compared to platinum anodes, whereby 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. A further advantage is that the conductive diamond anodes to be used according to the invention can be produced in any desired form and corrosion-prone connection points, such as weld seams and the like, are not present. As a result, a longer electrode life is achieved.

Die Erfindung wird anhand der nachfolgenden Beispiele und Vergleichsbeispiele weiter erläutert.The invention will be further illustrated by the following examples and comparative examples.

Beispiel 1 (B1) und Vergleichsbeispiel 1 (VB1)Example 1 (B1) and Comparative Example 1 (VB1) Herstellung von AmmoniumperoxodisulfatPreparation of ammonium peroxodisulfate

Die Elektrolysezelle enthält eine Bleikathode und eine mit Bor dotierte Diamantanode auf einem Si-Wafer. Die Diamantanode war mit einer Metallplatte (Stromverteiler) verbunden. Im Vergleichsbeispiel wurde die Diamantanode durch ein mit Diamantpulver geschliffenes, spiegelblankes Platinblech ersetzt. Die Elektrolytkammern waren durch eine Ionenaustauschmembran (Fa. DuPont, Nafion 430) in Anodenraum und Kathodenraum getrennt. Der Abstand der Elektroden betrug 2,2 cm. Die runde Elektrodenfläche betrug 38,48 cm2. Katholyt und Anolyt wurden im Kreis gepumpt, wobei das Katholytvolumen 2 l und das Anolytvolumen V = 0,3 1 betrug.The electrolytic cell includes a lead cathode and a boron-doped diamond anode on a Si wafer. The diamond anode was connected to a metal plate (power distributor). In the comparative example, the diamond anode was replaced by a mirror-finished platinum sheet ground with diamond powder. The electrolyte chambers were separated by an ion exchange membrane (DuPont, Nafion 430) in the anode compartment and cathode compartment. The distance between the electrodes was 2.2 cm. The round electrode area was 38.48 cm 2 . Catholyte and anolyte were pumped in a circle, the catholyte volume being 2 l and the anolyte volume V = 0.3 l.

Die Anfangskonzentrationen betrugen: Katholyt c(Ammoniumsulfat) = 520 g/l c(Schwefelsäure) = 400 g/l Anolyt c(Ammoniumsulfat) = 400 g/l c(Ammoniumperoxodisulfat) = 120 g/l The initial concentrations were: catholyte c (ammonium sulfate) = 520 g / l c (sulfuric acid) = 400 g / l anolyte c (ammonium sulfate) = 400 g / l c (ammonium peroxodisulfate) = 120 g / l

Die Apparatur wurde auf 45 °C temperiert. Anolyt und Katholyt wurden im Kreis geführt. Der Anolyt wurde hierbei aufkonzentriert von c0(APS) = 120 g/l auf cE(APS) = 290 g/l. Anschließend wurde durch Vakuumkristallisation aus dem Anolyt (NH4)2S2O8 auskristallisiert.The apparatus was heated to 45 ° C. Anolyte and catholyte were circulated. The anolyte was concentrated here from c 0 (APS) = 120 g / l to c E (APS) = 290 g / l. Subsequently, crystallization by vacuum crystallization from the anolyte (NH 4 ) 2 S 2 O 8 .

Der nachfolgenden Tabelle sind die Betriebsparameter und der spezifische Energieverbrauch zu entnehmen.The following table shows the operating parameters and the specific energy consumption.

Die Tabelle zeigt den Vergleich der Elektrolyse-Ergebnisse mit Pt- und einer Diamant-Anode. Nr. Elektrodentyp Promoterzusatz Stromdichte Stromausbeute Umsatz spezif. Energieverbrauch (g/l) (A/cm2) (%) (%) (kWh/kg) B 1 dot. Diamant 0,0 0,4 92,5 48 2,31 0,0 0,8 85,9 54 3,15 0,0 0,9 85,9 60 3,2 VB 1 Platin, blank 0,1 0,4 79,7 47 2,74 0,1 0,8 73,6 3,65 0,1 0,9 68,2 49 4,58 0,0 0,5 45,3 4,4 0,0 0,8 33 7,6 The table shows the comparison of the electrolysis results with Pt and a diamond anode. No. electrode type promoter additional current density current yield sales specif. power consumption (G / l) (A / cm 2 ) (%) (%) (KWh / kg) B 1 dot. diamond 0.0 0.4 92.5 48 2.31 0.0 0.8 85.9 54 3.15 0.0 0.9 85.9 60 3.2 VB 1 Platinum, blank 0.1 0.4 79.7 47 2.74 0.1 0.8 73.6 3.65 0.1 0.9 68.2 49 4.58 0.0 0.5 45.3 4.4 0.0 0.8 33 7.6

Bei vergleichbaren Elektrolysebedingungen wurden mit einer Pt-Anode ohne Zusatz eines üblichen Promotors sehr schlechte Ergebnisse erzielt. Mit Zusatz von Ammoniumrhodanid als Promoter liegen die Ergebnisse mit Pt immer noch ca. 10-15 % unter denjenigen, die mit einer Diamant-Anode erzielt wurde. Der spezifische Energieverbrauch ist bei Verwendung einer dotierten Diamantelektrode anstelle einer Pt-Elektrode bei einer Stromdichte von 0,9 A/cm2 um 30 % geringer und zusätzlich ist der Umsatz wesentlich höher.At comparable electrolysis conditions were obtained with a Pt anode without the addition of a conventional promoter very poor results. With the addition of ammonium rhodanide as promoter, the results with Pt are still about 10-15% lower than those achieved with a diamond anode. The specific energy consumption is 30% lower when using a doped diamond electrode instead of a Pt electrode at a current density of 0.9 A / cm 2 and in addition the conversion is much higher.

Beispiel 2 (B2) und Vergleichsbeispiel 2 (VB2)Example 2 (B2) and Comparative Example 2 (VB2)

In der zuvor (B1/VB1) beschriebenen Zelle wurde NaHSO4 anodisch oxidiert. Der Anolyt bestand aus einer NaHSO4-Lösung mit 610 g NaHSO4/l. Nach Einstellung der Stromdichte wurden nach vorgegebener Zeit Proben gezogen und analysiert. Bei der Berechnung der Stromausbeute wurde eine lineare Volumenabnahme angenommen.NaHSO 4 was oxidized anodically in the cell described above (B1 / VB1). The anolyte consisted of a NaHSO 4 solution containing 610 g NaHSO 4 / l. After setting the current density samples were taken and analyzed after a predetermined time. In calculating the current efficiency, a linear volume decrease was assumed.

Die Kurven zeigten die Stromausbeute in Abhängigkeit von der erzielten Natriumperoxodisulfat (NaPS)-Konzentration im Anolyt unter Einsatz einer Diamantelektrode (B2) bzw. einer Pt-Anode (VB2).The curves showed the current efficiency as a function of the sodium peroxodisulfate (NaPS) concentration achieved in the anolyte using a diamond electrode (B2) or a Pt anode (VB2).

In VB 2 enthielt der Anolyt keinen Promoter. Erst unter Einsatz eines Anolyten mit prohibitiv hoher Promoterkonzentration - 0,6 g NH4SCN/l - konnten Stromausbeuten erreicht werden, die denen des Beispiels B 2 nahekamen.In VB 2, the anolyte contained no promoter. Only when using an anolyte with a prohibitively high promoter concentration - 0.6 g NH 4 SCN / l - was it possible to achieve current efficiencies close to those of Example B 2.

Claims (4)

  1. A process for the production of ammonium peroxodisulfate, comprising anodic oxidation of an aqueous electrolyte containing ammonium sulfate in an electrolytic cell comprising at least one anode, one cathode and one anolyte compartment, this being separated from a catholyte compartment by a separator,
    characterised in that
    a diamond layer mounted on a conductive carrier and made conductive by doping with a tri- or pentavalent element is used as the anode and no promoter is added to the anolyte, an aqueous solution of 300 to 500 g/l ammonium sulfate and 0 to 0.2 mol sulfuric acid per mol ammonium sulfate is used as the anolyte, in particular a neutral anolyte, and a sulfuric acid ammonium sulfate solution is used as the catholyte, the anodic oxidation is carried out at a current density in the range of 50 to 1000 mA/cm2, in particular 400 to 900 mA/cm2, and ammonium peroxodisulfate is then crystallised out and separated from the anolyte in a known manner.
  2. A process for the production of sodium peroxodisulfate, comprising anodic oxidation of an aqueous electrolyte containing sodium hydrogen sulfate in an electrolytic cell comprising at least one anode, one cathode and one anolyte compartment, this being separated from a catholyte compartment by a separator,
    characterised in that
    a diamond layer mounted on a conductive carrier and made conductive by doping with a tri- or pentavalent element is used as the anode and no promoter is added to the anolyte, the electrolytic cell has an anolyte and catholyte circulation separated from one another by means of the separator, an anolyte solution containing sodium hydrogen sulfate in an amount of 300 to 700 g/l NaHSO4 is oxidised anodically at a current density in the range of 50 to 1000 mA/cm2, in particular 400 to 900 mA/cm2, and a sulfuric acid sodium hydrogen sulfate solution is used as the catholyte.
  3. The process according to claim 1 or 2,
    characterised in that
    a diamond layer doped with boron on a carrier from the series silicon, germanium, titanium, zirconium, niobium, tantalum, molybdenum and tungsten and carbides of these elements is used as the anode.
  4. The process according to one of claims 1 to 3,
    characterised in that
    an ion exchange membrane is used as the separator.
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DE10019683A1 (en) 2001-10-25
JP5259899B2 (en) 2013-08-07
PL347119A1 (en) 2001-10-22
US20020014418A1 (en) 2002-02-07
CZ20011317A3 (en) 2002-02-13
EP1148155B1 (en) 2005-06-08
IL142638A0 (en) 2002-03-10
AR027804A1 (en) 2003-04-09
AU3710001A (en) 2001-10-25
JP5570627B2 (en) 2014-08-13
EP1148155A3 (en) 2001-11-21
ES2240269T5 (en) 2012-02-03
DE50106427D1 (en) 2005-07-14
JP2013136842A (en) 2013-07-11
BR0101530A (en) 2001-12-04
SK5202001A3 (en) 2002-01-07
ATE297477T1 (en) 2005-06-15
MXPA01003938A (en) 2003-08-20
CA2344499A1 (en) 2001-10-20
CA2344499C (en) 2010-08-03

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