EP0391192A2 - Electrolytic process for manufacturing alkali dichromates and chromic acid - Google Patents

Electrolytic process for manufacturing alkali dichromates and chromic acid Download PDF

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EP0391192A2
EP0391192A2 EP90105661A EP90105661A EP0391192A2 EP 0391192 A2 EP0391192 A2 EP 0391192A2 EP 90105661 A EP90105661 A EP 90105661A EP 90105661 A EP90105661 A EP 90105661A EP 0391192 A2 EP0391192 A2 EP 0391192A2
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sodium
solution
chromic acid
alkali
membranes
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German (de)
French (fr)
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EP0391192B1 (en
EP0391192A3 (en
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Helmut Dr. Klotz
Rainer Dr. Weber
Norbert Dr. Lönhoff
Hans-Dieter Dr. Block
Hans Dieter Pinter
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Bayer AG
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Bayer AG
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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
    • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
    • 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
    • C25B1/22—Inorganic acids
    • 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

Definitions

  • the invention relates to a process for the production of alkali dichromates and chromic acid by electrolysis of alkali monochromate and / or alkali dichromate solutions in electrolysis cells, the anode and cathode spaces of which are separated by cation exchange membranes.
  • membranes based on perfluorinated polymers with exchanger groups are preferably used as cation exchange membranes.
  • These membranes can have both a single-layer and a two-layer structure, the two-layer membranes generally more effectively suppressing the diffusion of hydroxide ions through the membrane, which leads to a higher current efficiency of the electrolysis.
  • this improved current efficiency is mostly associated with a higher cell voltage than is achieved when using single-layer membranes.
  • Such cation exchange membranes are described, for example, in "H Simmrock, E. Griesenbeck, J. Jörissen and R. Rodermund, Chemie-Ing. Techn. 53 (1981), No. 1, pp. 10 to 25” and are described, for example, under the name Nafion® (Manufacturer: EI DuPont De Nemours & Co., Wilmington, Del./USA) commercially available.
  • single-layer membranes have the advantage that, compared to double-layer membranes, they are less sensitive to polyvalent cations, in particular calcium and strontium ions in the alkali metal chromate and / or alkali metal dichromate solutions, which lead to deterioration - and consequently - to deterioration the functionality of the membrane.
  • the object of the invention was to provide a process for the preparation of alkali dichromates and chromic acid which does not have the disadvantages described.
  • the invention thus relates to a process for the production of alkali dichromates and / or chromic acid by electrolysis of alkali monochromate and / or alkali dichromate solutions in electrolysis cells whose anode and cathode spaces are separated by cation exchange membranes, which is characterized in that the cation exchange membranes are single-layer membranes based on are perfluorinated polymers with sulfonic acid groups as cation exchange groups and an aqueous solution with a pH of 4 to 14 is generated in the cathode compartment of the cells.
  • the aqueous solution preferably consists of a solution containing alkali monochromate and / or alkali dichromate, particularly preferably of a solution containing sodium monochromate and / or sodium dichromate.
  • Such solutions are obtained by adding an alkali dichromate-containing solution to the cathode compartment of the cells is, which may also contain alkali monochromate or chromic acid. It is advantageous to supply an alkali chromate-containing solution to the cathode compartment, in which 70 to 95% of the chromate ions are in the form of dichromations and 5 to 30% are in the form of monochromations.
  • Such solutions are obtained, for example, in the production of sodium dichromate solution from sodium monochromate solution by acidification with carbon dioxide under pressure.
  • the aqueous solution can also consist of a sodium carbonate-containing solution, which can also contain portions of sodium hydroxide or sodium bicarbonate.
  • a sodium carbonate-containing solution which can also contain portions of sodium hydroxide or sodium bicarbonate.
  • Such solutions are obtained by adding water or dilute solution containing sodium ions to the cells and adding carbon dioxide to the solution in the cathode compartment inside or outside it.
  • an aqueous sodium dichromate-containing solution with a pH of 6 to 7.5 is generated in the cathode compartment.
  • FIG. 1 The method according to the invention is explained in more detail with reference to FIG. 1.
  • the variant of the method according to the invention described in FIG. 1 represents a particularly advantageous embodiment.
  • Chromium ore is broken down by alkaline oxidative digestion with soda and atmospheric oxygen at 1000 to 1100 ° C in the presence of a lean agent in a rotary kiln (1).
  • the resulting clinker is then leached with water or dilute chromate solution and adjusted to a pH between 7 and 9.5 with a solution containing sodium dichromate (2).
  • Soluble alkali compounds of iron, aluminum and silicon are converted into insoluble and easily filterable hydroxides or oxyhydrates, which are separated off together with the insoluble constituents of the kiln clinker (3).
  • the resulting sodium monochromate solution with a content of 300 to 500 g / l of Na2CrO4 can then, as described in EP-A-47 799, be freed from dissolved vanadate by adding calcium oxide at pH values of 10 to 13 (4).
  • the sodium monochromate solution is then concentrated to a content of 750 to 1000 g / l of Na2CrO4 by single or multi-stage evaporation (5).
  • the sodium monochromate solution can, if appropriate, be evaporated from the main amount before the evaporation (5) by adding or in situ generation of sodium carbonate by precipitation as carbonates on alkaline earth metal ions and other polyvalent cations.
  • the precipitation is preferably carried out at temperatures from 50 to 100 ° C., at pH values between 8 and 12 and with an approximately 2 to 10-fold molar excess of carbonate, based on the amount of alkaline earth metal ions.
  • a pH value below 6.5 is set by one-stage or multi-stage supply of carbon dioxide up to a final pressure of 4 to 15 bar at a final temperature of not more than 50 ° C. and in this way a 70 to with precipitation of sodium hydrogen carbonate 95% conversion of sodium chromate to sodium dichromate achieved (6).
  • the sodium bicarbonate is separated from the resulting suspension under continuing carbon dioxide pressure or, after the pressure has been let down, the sodium bicarbonate is rapidly separated off before it reacts with the sodium dichromate.
  • the resulting sodium monochromate / sodium dichromate solution, separated from the sodium bicarbonate, is now separated into two streams after removal of a partial stream for the pH adjustment of the leached kiln clinker divided.
  • Material stream I is used for the electrolytic production of chromic acid and material stream II is used for the production of sodium dichromate solutions and crystals.
  • stream I is divided into two sub-streams and fed to the anode and cathode compartments of two-chamber electrolysis cells with single-layer membranes as partitions (7).
  • Suitable single-layer membranes are, for example, Nafion®117, Nafion®417, Nafion®423 and Nafion®430, the active exchanger groups of which are sulfonic acid.
  • the single-layer membranes can also have coatings which reduce the gas bubble adhesion or promote the wetting of the membrane with electrolytes.
  • Such membranes are described, for example, in “FY Masuda, J. Appl. Electrochem. 16 (1986), p. 317 ff".
  • Membranes with reduced gas bubble adhesion can also be obtained by physical treatment, such as mechanical roughening or Corana treatment. Corresponding methods are described in US-A-4 610 762 and EP-A-72 485.
  • the electrolysis is preferably carried out in several stages: a substream of stream I is introduced into the anode chamber of the first stage and then after a partial conversion of the monochromations into dichromations and optionally chromic acid or after a partial conversion of the dichromations into chromic acid Ren stages supplied, which cause a further partial conversion to chromic acid until in the last stage a degree of conversion of dichromate in chromic acid from 55 to 70% corresponding to a molar ratio of sodium ions to chromic acid from 0.45: 0.55 to 0.30: 0.70 is reached.
  • the number of stages can be chosen to be of any size, 6 to 15-stage electrolysis being preferred.
  • the other partial stream of stream I if necessary after admixing a partial stream of the sodium chromate solution before evaporation to 750 to 1000 g / l, is fed to all the cathode spaces of the electrolytic cells at such a rate that a pH of 6 in the solution leaving the cells up to 7.5.
  • This sodium dichromate and sodium monochromate-containing solution is optionally fed in after concentration of the carbon dioxide acidification (6), the monochromations formed being converted back into dichromations. It is also possible to return the solution of the cathode compartments to another point in the process, such as, for example, in the pH adjustment (2) or before the alkali cleaning (4).
  • the chromic acid formed during the electrolysis and a solution containing sodium dichromate is brought to a water content of approx. 12 to 22 wt.% Water at temperatures between 55 and 110 ° C by evaporation, the majority of the chromic acid crystallizing out (8).
  • the resulting suspension is then centrifuged at 50 to 110 ° C a solid consisting essentially of crystalline chromic acid and separated into a liquid phase, hereinafter called mother liquor (9).
  • the mother liquor obtained is placed in the electrolysis at a suitable point, i.e. in a level of dichromate conversion degree that is as similar as possible.
  • a suitable point i.e. in a level of dichromate conversion degree that is as similar as possible.
  • part of the mother liquor is removed and used in the residual acidification of stream II or, if stream II has not been removed, in the sodium dichromate process at a point before the sodium chromate solution cleaning, e.g. returned to the pH setting (2).
  • the crystalline chromic acid is freed from adhering mother liquor by washing once or several times with 10 to 50% by weight, based on the weight of the solid, of saturated or almost saturated chromic acid solution and by centrifuging each time after washing.
  • the washed pure chromic acid crystals can now be used immediately or after drying.
  • the solution of stream II is fed to the residual acidification (10).
  • this residual acidification is carried out with mother liquor from the chromic acid filtration (9). However, it can also be done partially or entirely by electrolysis and / or by adding sulfuric acid.
  • the solution obtained after the residual acidification (10) is then concentrated to about 60 to 70% by weight Na2Cr2O7 ⁇ 2 H2O to produce sodium dichromate solution.
  • the solution is concentrated to about 1650 g / l Na2Cr2O7 ⁇ 2 H2O (11) and then cooled to 30 to 40 ° C (12), sodium dichromate precipitating in the form of Na2Cr2O7 ⁇ 2 H2O crystals.
  • the crystals are then centrifugally separated from the mother liquor and dried at temperatures of approx. 70 to 85 ° C.
  • the electrolytic cells used in the examples consisted of anode compartments made of pure titanium and cathode compartments made of stainless steel.
  • Cation exchange membranes from DuPont with the names Nafion® 324 and Nafion® 430 were used as membranes, Nafion® 324 being a two-layer membrane and Nafion® 430 being a single-layer membrane.
  • the cathodes were made of stainless steel and the anodes were made of titanium with the electrocatalytically active coatings mentioned in the individual examples.
  • the distance between the electrodes and the membrane was 1.5 mm in all cases.
  • Sodium dichromate solutions containing 800 g / l Na2Cr2O7 ⁇ 2 H2O were introduced into the anode compartments. The rate of introduction was chosen so that a molar ratio of sodium ions to chromium (VI) of 0.6 was established in the anolytes leaving the cells.
  • the electrolysis temperature was 80 ° C in all cases and the current density was 3 kA / m 2 projected front area of the anodes and cathodes, which area was 11.4 cm x 6.7 cm.
  • the single-layer Nafion® 430 membrane was used to separate the anode and cathode compartments.
  • the anode was a titanium anode with an iridium oxide-containing electrocatalytically active layer, as described, for example, in US Pat. No. 3,878,083.
  • titanium anodes with a platinum layer produced by means of hot-dip galvanization were used, as described in "G. Dick, Galvanotechnik 79 (1988), No. 12, pp. 4066-4071".
  • Table 1 shows, by using a single-layer membrane instead of a two-layer membrane and producing chromate-containing catholytes, a significantly lower cell voltage is achieved with a high current efficiency.
  • Table 1 example membrane Catholyte Medium cell voltage Average current efficiency Trial period 2nd Nafion® 324 20% sodium hydroxide solution 4.9 volts 56% 100 days 3rd Nafion® 324 Chromate solution, pH 6.5 5.2 volts 65% 100 days 4th Nafion® 430 Chromate solution, pH 6.5 4.7 volts 64% 100 days 5 Nafion® 430 chromate solution, pH 13.4 4.5 volts 62% 100 days

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  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
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Abstract

The present invention relates to an electrolytic process for manufacturing alkali dichromates and/or chromic acid from alkali metal monochromates and/or alkali metal dichromate solutions in electrolysis cells whose anode and cathode spaces are separated by cation exchanger membranes. These membranes are single-layer membranes based on perfluorinated polymers having sulphonic acid groups as cation exchanger groups. An aqueous solution having a pH of 4 to 14 is produced in the cathode space of the cells.

Description

Die Erfindung betrifft ein Verfahren zur Herstellung von Alkalidichromaten und Chromsäure durch Elektrolyse von Alkalimonochromat- und/oder Alkalidichromat-Lösungen in Elektrolysezellen, deren Anoden- und Kathodenräume durch Kationenaustauschermembranen getrennt sind.The invention relates to a process for the production of alkali dichromates and chromic acid by electrolysis of alkali monochromate and / or alkali dichromate solutions in electrolysis cells, the anode and cathode spaces of which are separated by cation exchange membranes.

Gemäß US-A-3 305 463 und CA-A-739 447 erfolgt die elektrolytische Herstellung von Alkalidichromaten und Chromsäure in Elektrolysezellen, deren Elektrodenräume durch Kationenaustauschermembranen getrennt sind. Bei der Erzeugung von Natriumdichromat werden Natriummono­chromatlösungen oder -suspensionen in den Anodenraum der Zelle eingeleitet und in eine Natriumdichromatlösung um­gewandelt, indem Natriumionen selektiv durch die Membran in den Kathodenraum überführt werden. Zur Herstellung von Chromsäure werden Natriumdichromat oder Natrium­monochromat oder eine Mischung von Natriumdichromat und -monochromat in den Anodenraum eingeleitet und in eine Chromsäure-haltige Lösung überführt. Im Kathodenraum wird bei beiden Prozessen eine wäßrige Lösung von Natriumhydroxid erhalten.According to US-A-3 305 463 and CA-A-739 447, the electrolytic production of alkali dichromates and chromic acid takes place in electrolysis cells, the electrode spaces of which are separated by cation exchange membranes. In the production of sodium dichromate, sodium monochromate solutions or suspensions are introduced into the anode compartment of the cell and converted into a sodium dichromate solution by selectively transferring sodium ions through the membrane into the cathode compartment. To produce chromic acid, sodium dichromate or sodium monochromate or a mixture of sodium dichromate and monochromate are introduced into the anode compartment and into a Chromic acid-containing solution transferred. In both processes, an aqueous solution of sodium hydroxide is obtained in the cathode compartment.

Als Kationenaustauschermembranen werden bei den ge­nannten Verfahren vorzugsweise chemisch, thermisch und mechanisch ausreichend stabile Membranen auf Basis von perfluorierten Polymeren mit Austauschergruppen einge­setzt. Diese Membranen können sowohl einen einschichti­gen als auch zweischichtigen Aufbau aufweisen, wobei die zweischichtigen Membranen in der Regel die Diffusion von Hydroxydionen durch die Membran effektiver unterdrücken, was zu einer höheren Stromausbeute der Elektrolyse führt. Diese verbesserte Stromausbeute ist jedoch zu­meist mit einer höheren Zellspannung verbunden als sie bei Einsatz von einschichtigen Membranen erreicht wird.In the processes mentioned, chemically, thermally and mechanically sufficiently stable membranes based on perfluorinated polymers with exchanger groups are preferably used as cation exchange membranes. These membranes can have both a single-layer and a two-layer structure, the two-layer membranes generally more effectively suppressing the diffusion of hydroxide ions through the membrane, which leads to a higher current efficiency of the electrolysis. However, this improved current efficiency is mostly associated with a higher cell voltage than is achieved when using single-layer membranes.

Solche Kationenaustauschermembranen sind beispielsweise in "H Simmrock, E. Griesenbeck, J. Jörissen und R. Rodermund, Chemie-Ing. Techn. 53 (1981), Nr 1, S. 10 bis 25" beschrieben und sind beispielsweise unter dem Namen Nafion® (Hersteller: E.I. DuPont De Nemours & Co., Wilmington, Del./USA) im Handel erhältlich.Such cation exchange membranes are described, for example, in "H Simmrock, E. Griesenbeck, J. Jörissen and R. Rodermund, Chemie-Ing. Techn. 53 (1981), No. 1, pp. 10 to 25" and are described, for example, under the name Nafion® (Manufacturer: EI DuPont De Nemours & Co., Wilmington, Del./USA) commercially available.

Einschichtige Membranen haben neben der erzielbaren niedrigeren Zellspannung den Vorteil, daß sie im Ver­gleich zu zweischichtigen Membranen weniger empfindlich gegenüber mehrwertigen Kationen, insbesondere Calcium und Strontiumionen in den Alkalichromat- und/oder Alkalidichromatlösungen sind, die zu Einlagerungen - und in Folge davon - zu einer Verschlechterung der Funk­tionsfähigkeit der Membran führen.In addition to the lower cell voltage that can be achieved, single-layer membranes have the advantage that, compared to double-layer membranes, they are less sensitive to polyvalent cations, in particular calcium and strontium ions in the alkali metal chromate and / or alkali metal dichromate solutions, which lead to deterioration - and consequently - to deterioration the functionality of the membrane.

Die Aufgabe der Erfindung bestand darin, ein Verfahren zur Herstellung van Alkalidichromaten und Chromsäure be­reitzustellen, das die beschriebenen Nachteile nicht aufweist.The object of the invention was to provide a process for the preparation of alkali dichromates and chromic acid which does not have the disadvantages described.

Es wurde nun gefunden, daß die Herstellung von Alkali­dichromaten und Chromsäure durch Elektrolyse besonders vorteilhaft durchgeführt werden kann, wenn als Kation­austauschermembranen einschichtige Membranen mit Sulfon­säuregruppen eingesetzt werden und im Kathodenraum der Elektrolysezellen eine wäßrige Alkaliionen enthaltende Lösung mit einem pH-Wert von 4 bis 14 erzeugt wird.It has now been found that the production of alkali dichromates and chromic acid by electrolysis can be carried out particularly advantageously if single-layer membranes with sulfonic acid groups are used as cation exchange membranes and an aqueous solution containing alkali ions with a pH of 4 to 14 is produced in the cathode compartment of the electrolysis cells .

Gegenstand der Erfindung ist somit ein Verfahren zur Herstellung von Alkalidichromaten und/oder Chromsäure durch Elektrolyse von Alkalimonochromat- und/oder Alkalidichromatlösungen in Elektrolysezellen, deren Anoden- und Kathodenräume durch Kationenaustauscher­membranen getrennt sind, welches dadurch gekennzeichnet ist, daß die Kationenaustauschermembranen einschichtige Membranen auf Basis von perfluorierten Polymeren mit Sulfonsäuregruppen als Kationenaustauschergruppen sind und im Kathodenraum der Zellen eine wäßrige Lösung mit einem pH-Wert von 4 bis 14 erzeugt wird.The invention thus relates to a process for the production of alkali dichromates and / or chromic acid by electrolysis of alkali monochromate and / or alkali dichromate solutions in electrolysis cells whose anode and cathode spaces are separated by cation exchange membranes, which is characterized in that the cation exchange membranes are single-layer membranes based on are perfluorinated polymers with sulfonic acid groups as cation exchange groups and an aqueous solution with a pH of 4 to 14 is generated in the cathode compartment of the cells.

Die wäßrige Lösung besteht vorzugsweise aus einer Alkalimonochromat- und/oder Alkalidichromat-haltigen Lösung, besonders bevorzugt aus einer Natriummono­chromat- und/oder Natriumdichromat-haltigen Lösung. Solche Lösungen werden erhalten, indem dem Kathodenraum der Zellen eine Alkalidichromat-haltige Lösung zugeführt wird, die auch Alkalimonochromat- oder Chromsäureanteile enthalten kann. Es ist vorteilhaft, dem Kathodenraum eine Alkalichromat-haltige Lösung zuzuführen, in der die Chromationen zu 70 bis 95 % als Dichromationen und zu 5 bis 30 % als Monochromationen vorliegen. Solche Lö­sungen werden beispielsweise bei der Herstellung von Natriumdichromatlösung aus Natriummonochromatlösung durch Ansäuerung mit Kohlendioxid unter Druck erhalten.The aqueous solution preferably consists of a solution containing alkali monochromate and / or alkali dichromate, particularly preferably of a solution containing sodium monochromate and / or sodium dichromate. Such solutions are obtained by adding an alkali dichromate-containing solution to the cathode compartment of the cells is, which may also contain alkali monochromate or chromic acid. It is advantageous to supply an alkali chromate-containing solution to the cathode compartment, in which 70 to 95% of the chromate ions are in the form of dichromations and 5 to 30% are in the form of monochromations. Such solutions are obtained, for example, in the production of sodium dichromate solution from sodium monochromate solution by acidification with carbon dioxide under pressure.

Die wäßrige Lösung kann auch aus einer Natriumcarbonat-­haltigen Lösung bestehen, die auch Anteile von Natrium­hydroxid oder Natriumbicarbonat enthalten kann. Zu solchen Lösungen gelangt man, indem den Zellen Wasser oder verdünnte, Natriumionen enthaltende Lösung zuge­führt wird und die Lösung des Kathodenraumes innerhalb oder außerhalb desselben mit Kohlendioxid versetzt wird. In einer besonders bevorzugten Variante des er­findungsgemäßen Verfahrens wird im Kathodenraum eine wäßrige Natriumdichromat-haltige Lösung mit einem pH-­Wert von 6 bis 7,5 erzeugt.The aqueous solution can also consist of a sodium carbonate-containing solution, which can also contain portions of sodium hydroxide or sodium bicarbonate. Such solutions are obtained by adding water or dilute solution containing sodium ions to the cells and adding carbon dioxide to the solution in the cathode compartment inside or outside it. In a particularly preferred variant of the method according to the invention, an aqueous sodium dichromate-containing solution with a pH of 6 to 7.5 is generated in the cathode compartment.

Bei Durchführung des erfindungsgemäßen Verfahrens werden Stromausbeuten erhalten, die denen beim Ein­satz von zweischichtigen Membranen vergleichbar sind und unter den bisher vorgeschlagenen Arbeitsbedingun­gen nicht erreichbar sind. Die Zellspannungen liegen jedoch deutlich niedriger als bei der Elektrolyse in Zellen, deren Elektrodenräume mit einer zweischich­tigen Membran getrennt sind. Einlagerung von Verbin­dungen mehrwertiger Kationen in der Membran werden ver­mieden, wodurch die Lebensdauer der Membran erheblich verlängert wird, was einen kontinuierlichen und dauer­haften Betrieb der Elektrolyse gewährleistet.When the method according to the invention is carried out, current yields are obtained which are comparable to those when using two-layer membranes and cannot be achieved under the working conditions previously proposed. However, the cell voltages are significantly lower than in the case of electrolysis in cells whose electrode spaces are separated by a two-layer membrane. The storage of compounds of multivalent cations in the membrane is avoided, which considerably increases the life of the membrane is extended, which ensures continuous and permanent operation of the electrolysis.

Das erfindungsgemäße Verfahren wird anhand Fig. 1 näher erläutert. Die in Fig. 1 beschriebene Variante des er­findungsgemäßen Verfahrens stellt eine besonders vor­teilhafte Ausführungsform dar.The method according to the invention is explained in more detail with reference to FIG. 1. The variant of the method according to the invention described in FIG. 1 represents a particularly advantageous embodiment.

Chromerz wird durch alkalisch oxidativen Aufschluß mit Soda und Luftsauerstoff bei 1000 bis 1100°C in Gegenwart eines Magerungsmittels in einem Drehrohrofen aufge­schlossen (1). Der dabei entstehende Ofenklinker wird anschließend mit Wasser oder verdünnter Chromatlösung gelaugt und mit Natriumdichromat-haltiger Lösung auf einen pH-Wert zwischen 7 und 9,5 eingestellt (2). Dabei werden lösliche Alkaliverbindungen des Eisens, Aluminiums und Siliciums in unlösliche und gut filtrier­bare Hydroxide bzw. Oxyhydrate umgewandelt, die gemein­sam mit den unlöslichen Bestandteilen des Ofenklinkers abgetrennt werden (3). Die entstandene Natriummono­chromatlösung mit einem Gehalt von 300 bis 500 g/l an Na₂CrO₄ kann dann, wie in der EP-A-47 799 beschrieben, durch Zugabe von Calciumoxid bei pH-Werten von 10 bis 13 von gelöstem Vanadat befreit (4) werden.Chromium ore is broken down by alkaline oxidative digestion with soda and atmospheric oxygen at 1000 to 1100 ° C in the presence of a lean agent in a rotary kiln (1). The resulting clinker is then leached with water or dilute chromate solution and adjusted to a pH between 7 and 9.5 with a solution containing sodium dichromate (2). Soluble alkali compounds of iron, aluminum and silicon are converted into insoluble and easily filterable hydroxides or oxyhydrates, which are separated off together with the insoluble constituents of the kiln clinker (3). The resulting sodium monochromate solution with a content of 300 to 500 g / l of Na₂CrO₄ can then, as described in EP-A-47 799, be freed from dissolved vanadate by adding calcium oxide at pH values of 10 to 13 (4).

Die Natriummonochromatlösung wird anschließend durch ein- oder mehrstufige Verdampfung auf Gehalte von 750 bis 1000 g/l an Na₂CrO₄ eingeengt (5). Die Natriummono­chromatlösung kann gegebenenfalls vor der Eindampfung (5) durch Zusatz bzw. in situ-Erzeugung von Natrium­carbonat durch Fällung als Carbonate von der Hauptmenge an Erdalkaliionen und anderen mehrwertigen Kationen be­freit werden. Die Fällung wird dabei vorzugsweise bei Temperaturen von 50 bis 100°C, bei pH-Werten zwischen 8 und 12 und mit einem ca. 2 bis 10-fachen molaren Carbonatüberschuß, bezogen auf die Menge an Erdalkali­ionen, durchgeführt.The sodium monochromate solution is then concentrated to a content of 750 to 1000 g / l of Na₂CrO₄ by single or multi-stage evaporation (5). The sodium monochromate solution can, if appropriate, be evaporated from the main amount before the evaporation (5) by adding or in situ generation of sodium carbonate by precipitation as carbonates on alkaline earth metal ions and other polyvalent cations. The precipitation is preferably carried out at temperatures from 50 to 100 ° C., at pH values between 8 and 12 and with an approximately 2 to 10-fold molar excess of carbonate, based on the amount of alkaline earth metal ions.

In der nunmehr konzentrierten Lösung wird durch ein­stufige oder mehrstufige Zufuhr von Kohlendioxid bis zu einem Enddruck von 4 bis 15 bar bei einer Endtemperatur nicht über 50°C ein pH-Wert unter 6,5 eingestellt und auf diese Weise unter Ausfällung von Natriumhydrogen­carbonat eine 70 bis 95 %ige Umwandlung des Natrium­chromats in Natriumdichromat erreicht (6).In the now concentrated solution, a pH value below 6.5 is set by one-stage or multi-stage supply of carbon dioxide up to a final pressure of 4 to 15 bar at a final temperature of not more than 50 ° C. and in this way a 70 to with precipitation of sodium hydrogen carbonate 95% conversion of sodium chromate to sodium dichromate achieved (6).

Aus der entstehenden Suspension wird unter weiterbe­stehendem Kohlendioxid-Druck das Natriumhydrogencarbonat abgetrennt oder aber nach dem Entspannen das Natrium­hydrogencarbonat vor seiner Rückreaktion mit dem Natriumdichromat rasch abgetrennt.The sodium bicarbonate is separated from the resulting suspension under continuing carbon dioxide pressure or, after the pressure has been let down, the sodium bicarbonate is rapidly separated off before it reacts with the sodium dichromate.

Das abgetrennte Natriumhydrogencarbonat wird, gegebe­nenfalls nach Zusatz von Natronlauge, durch thermische Behandlung in Soda überführt, welches beim Chromerz­aufschluß (1) eingesetzt wird.The sodium hydrogen carbonate which has been separated off, if appropriate after addition of sodium hydroxide solution, is converted into soda by thermal treatment, which is used in the chromium ore digestion (1).

Die resultierende, vom Natriumhydrogencarbonat abge­trennte Natriummonochromat-/Natriumdichromat-Lösung wird nach Entnahme eines Teilstroms für die pH-Einstellung des gelaugten Ofenklinkers nunmehr in zwei Stoffströme geteilt. Der Stoffstrom I wird der elektrolytischen Her­stellung von Chromsäure und der Stoffstrom II wird der Herstellung von Natriumdichromatlösungen und -kristallen zugeführt.The resulting sodium monochromate / sodium dichromate solution, separated from the sodium bicarbonate, is now separated into two streams after removal of a partial stream for the pH adjustment of the leached kiln clinker divided. Material stream I is used for the electrolytic production of chromic acid and material stream II is used for the production of sodium dichromate solutions and crystals.

Zur elektrolytischen Herstellung von Chromsäure wird der Stoffstrom I in zwei Teilströme aufgeteilt und den Anoden- und Kathodenräumen von Zweikammer-Elektrolyse­zellen mit einschichtigen Membranen als Trennwände zugeleitet (7). Geeignete einschichtige Membranen sind beispielsweise Nafion®117, Nafion®417, Nafion®423 und Nafion®430, deren aktive Austauschergruppen Sulfonsäure sind.For the electrolytic production of chromic acid, stream I is divided into two sub-streams and fed to the anode and cathode compartments of two-chamber electrolysis cells with single-layer membranes as partitions (7). Suitable single-layer membranes are, for example, Nafion®117, Nafion®417, Nafion®423 and Nafion®430, the active exchanger groups of which are sulfonic acid.

Die einschichtigen Membranen können auch Beläge aufweisen, die die Gasblasenhaftung vermindern bzw. die Benetzung der Membran mit Elektrolyten fördern. Solche Membranen sind beispielsweise in "F.Y. Masuda, J. Appl. Electrochem. 16 (1986), S. 317 ff" beschrieben. Mem­branen mit verminderter Gasblasenhaftung sind auch durch eine physikalische Behandlung, wie beispielsweise mecha­nische Aufrauhung oder Corana-Behandlung, erhältlich. Entsprechende Verfahren sind in der US-A-4 610 762 und der EP-A-72 485 beschrieben.The single-layer membranes can also have coatings which reduce the gas bubble adhesion or promote the wetting of the membrane with electrolytes. Such membranes are described, for example, in "FY Masuda, J. Appl. Electrochem. 16 (1986), p. 317 ff". Membranes with reduced gas bubble adhesion can also be obtained by physical treatment, such as mechanical roughening or Corana treatment. Corresponding methods are described in US-A-4 610 762 and EP-A-72 485.

Die Elektrolyse wird vorzugsweise mehrstufig ausgeführt: Ein Teilstrom des Stoffstroms I wird in die Anodenkammer der ersten Stufe eingeführt und nach einer teilweisen Umwandlung der Monochromationen in Dichromationen und gegebenenfalls Chromsäure bzw. nach einer teilweisen Um­wandlung der Dichromationen in Chromsäure sodann weite­ ren Stufen zugeführt, die eine teilweise weitere Umwand­lung in Chromsäure bewirken, bis in der letzten Stufe ein Umwandlungsgrad des Dichromats in Chromsäure von 55 bis 70 % entsprechend einem molaren Verhältnis von Natriumionen zu Chromsäure von 0,45:0,55 bis 0,30:0,70 erreicht ist. Die Zahl der Stufen kann beliebig groß gewählt werden, wobei eine 6 bis 15-stufige Elektrolyse bevorzugt ist.The electrolysis is preferably carried out in several stages: a substream of stream I is introduced into the anode chamber of the first stage and then after a partial conversion of the monochromations into dichromations and optionally chromic acid or after a partial conversion of the dichromations into chromic acid Ren stages supplied, which cause a further partial conversion to chromic acid until in the last stage a degree of conversion of dichromate in chromic acid from 55 to 70% corresponding to a molar ratio of sodium ions to chromic acid from 0.45: 0.55 to 0.30: 0.70 is reached. The number of stages can be chosen to be of any size, 6 to 15-stage electrolysis being preferred.

Allen Kathodenräumen der Elektrolysezellen wird der andere Teilstrom des Stoffstroms I gegebenenfalls nach Zumischung eines Teilstromes der Natriumchromat-Lösung vor Eindampfung auf 750 bis 1000 g/l mit einer solchen Geschwindigkeit zugeleitet, so daß sich in der die Zellen verlassenden Lösung ein pH-Wert von 6 bis 7,5 einstellt. Diese Natriumdichromat- und Natriummono­chromat-haltige Lösung wird gegebenenfalls nach Auf­konzentrieren der Kohlendioxid-Ansäuerung (6) zugeführt, wobei die gebildeten Monochromationen wieder in Di­chromationen umgewandelt werden. Es ist auch möglich, die Lösung der Kathodenräume an eine andere Stelle im Prozeß, wie beispielsweise in die pH-Einstellung (2) oder vor die Laugenreinigung (4) zurückzuführen.The other partial stream of stream I, if necessary after admixing a partial stream of the sodium chromate solution before evaporation to 750 to 1000 g / l, is fed to all the cathode spaces of the electrolytic cells at such a rate that a pH of 6 in the solution leaving the cells up to 7.5. This sodium dichromate and sodium monochromate-containing solution is optionally fed in after concentration of the carbon dioxide acidification (6), the monochromations formed being converted back into dichromations. It is also possible to return the solution of the cathode compartments to another point in the process, such as, for example, in the pH adjustment (2) or before the alkali cleaning (4).

Die bei der Elektrolyse gebildete Chromsäure und einen Rest Natriumdichromat enthaltende Lösung wird durch Ein­dampfen auf einen Wassergehalt von ca. 12 bis 22 Gew.-% Wasser bei Temperaturen zwischen 55 und 110°C gebracht, wobei der überwiegende Teil der Chromsäure auskristal­lisiert (8). Die entstandene Suspension wird an­schließend durch Zentrifugieren bei 50 bis 110°C an einen im wesentlichen aus kristalliner Chromsäure be­stehenden Feststoff und in eine flüssige Phase, im fol­genden Mutterlauge genannt, aufgetrennt (9).The chromic acid formed during the electrolysis and a solution containing sodium dichromate is brought to a water content of approx. 12 to 22 wt.% Water at temperatures between 55 and 110 ° C by evaporation, the majority of the chromic acid crystallizing out (8). The resulting suspension is then centrifuged at 50 to 110 ° C a solid consisting essentially of crystalline chromic acid and separated into a liquid phase, hereinafter called mother liquor (9).

Die erhaltene Mutterlauge wird, gegebenenfalls nach Ver­dünnung mit Wasser, in die Elektrolyse an eine geeignete Stelle, d.h. in eine Stufe möglichst ähnlichen Di­chromat-Umwandlungsgrades, zurückgeführt. Um eine starke Anreicherung von Verunreinigungen im System zu vermei­den, wird ein Teil der Mutterlauge ausgeschleust und bei der Restansäuerung des Stoffstroms II verwendet oder, falls ein Stoffstrom II nicht entnommen worden ist, in den Natriumdichromat-Prozeß an eine Stelle vor der Natriumchromatlösung-Reinigung, z.B. in die pH-Einstel­lung (2) zurückgeführt. Die kristalline Chromsäure wird durch einmaliges oder mehrmaliges Waschen mit 10 bis 50 Gew.-%, bezogen auf das Gewicht des Feststoffs, ge­sättigter oder nahezu gesättigter Chromsäurelösung und durch jeweils an jeden Waschvorgang anschließendes Zentrifugieren von anhaftender Mutterlauge befreit. Die gewaschenen reinen Chromsäurekristalle können nun direkt oder nach einer Trocknung ihrer Verwendung zugeführt werden.The mother liquor obtained, optionally after dilution with water, is placed in the electrolysis at a suitable point, i.e. in a level of dichromate conversion degree that is as similar as possible. In order to avoid a strong accumulation of impurities in the system, part of the mother liquor is removed and used in the residual acidification of stream II or, if stream II has not been removed, in the sodium dichromate process at a point before the sodium chromate solution cleaning, e.g. returned to the pH setting (2). The crystalline chromic acid is freed from adhering mother liquor by washing once or several times with 10 to 50% by weight, based on the weight of the solid, of saturated or almost saturated chromic acid solution and by centrifuging each time after washing. The washed pure chromic acid crystals can now be used immediately or after drying.

Zur Herstellung von Natriumdichromatlösungen und -kristallen wird die Lösung des Stoffstroms II der Rest­ansäuerung (10) zugeführt. Diese Restansäuerung wird, wie oben erwähnt, mit Mutterlauge aus der Chromsäure­filtration (9) durchgeführt. Sie kann aber auch teil­weise oder gänzlich durch Elektrolyse und/oder durch Versetzen mit Schwefelsäure erfolgen.To produce sodium dichromate solutions and crystals, the solution of stream II is fed to the residual acidification (10). As mentioned above, this residual acidification is carried out with mother liquor from the chromic acid filtration (9). However, it can also be done partially or entirely by electrolysis and / or by adding sulfuric acid.

Die nach der Restansäuerung (10) erhaltene Lösung wird anschließend zur Erzeugung von Natriumdichromatlösung auf ca. 60 bis 70 Gew.-% Na₂Cr₂O₇ · 2 H₂O eingeengt. Zur Herstellung von Natriumdichromatkristallen wird die Lö­sung auf ca. 1650 g/l Na₂Cr₂O₇ · 2 H₂O eingeengt (11) und anschließend auf 30 bis 40°C abgekühlt (12), wobei Natriumdichromat in Form von Na₂Cr₂O₇ · 2 H₂O-Kristallen ausfällt. Die Kristalle werden anschließend in Schleudern von der Mutterlauge getrennt und bei Temperaturen von ca. 70 bis 85°C getrocknet.The solution obtained after the residual acidification (10) is then concentrated to about 60 to 70% by weight Na₂Cr₂O₇ · 2 H₂O to produce sodium dichromate solution. To produce sodium dichromate crystals, the solution is concentrated to about 1650 g / l Na₂Cr₂O₇ · 2 H₂O (11) and then cooled to 30 to 40 ° C (12), sodium dichromate precipitating in the form of Na₂Cr₂O₇ · 2 H₂O crystals. The crystals are then centrifugally separated from the mother liquor and dried at temperatures of approx. 70 to 85 ° C.

Das erfindungsgemäße Verfahren soll anhand der folgenden Beispiele erläutert werden.The process according to the invention will be explained with the aid of the following examples.

BeispieleExamples

Die in den Beispielen verwendeten Elektrolysezellen be­standen aus Anodenräumen aus Rein-Titan und Kathodenräumen aus Edelstahl. Als Membranen wurden Kationenaustauschermembranen der Firma DuPont der Be­zeichnung Nafion® 324 und Nafion® 430 eingesetzt, wobei Nafion® 324 eine zweischichtige Membran und Nafion® 430 eine einschichtige Membran ist.The electrolytic cells used in the examples consisted of anode compartments made of pure titanium and cathode compartments made of stainless steel. Cation exchange membranes from DuPont with the names Nafion® 324 and Nafion® 430 were used as membranes, Nafion® 324 being a two-layer membrane and Nafion® 430 being a single-layer membrane.

Die Kathoden bestanden aus Edelstahl und die Anoden aus Titan mit den in den einzelnen Beispielen erwähnten elektrokatalytisch aktiven Beschichtungen. Der Abstand der Elektroden zur Membran betrug in allen Fällen 1,5 mm. In die Anodenräume wurden Natriumdichromat­lösungen mit 800 g/l Na₂Cr₂O₇ · 2 H₂O eingeleitet. Die Geschwindigkeit des Einleitens wurde so gewählt, daß sich in den die Zellen verlassenden Anolyten ein molares Verhältnis von Natriumionen zu Chrom (VI) von 0,6 einstellte.The cathodes were made of stainless steel and the anodes were made of titanium with the electrocatalytically active coatings mentioned in the individual examples. The distance between the electrodes and the membrane was 1.5 mm in all cases. Sodium dichromate solutions containing 800 g / l Na₂Cr₂O₇ · 2 H₂O were introduced into the anode compartments. The rate of introduction was chosen so that a molar ratio of sodium ions to chromium (VI) of 0.6 was established in the anolytes leaving the cells.

In den Kathodenräumen der Zellen wurde entweder Natronlauge oder eine Natriumchromat-haltige Lösung erzeugt.Either sodium hydroxide solution or a solution containing sodium chromate was generated in the cathode spaces of the cells.

Die Elektrolysetemperatur betrug in allen Fällen 80°C und die Stromdichte betrug 3 kA/m² projizierte vordere Fläche der Anoden und Kathoden, wobei diese Fläche 11,4 cm·6,7 cm betrug.The electrolysis temperature was 80 ° C in all cases and the current density was 3 kA / m 2 projected front area of the anodes and cathodes, which area was 11.4 cm x 6.7 cm.

Beispiel 1example 1

In diesem Beispiel wurde die einschichtige Membran Nafion® 430 zur Trennung von Anoden- und Kathodenraum verwendet. Die Anode war eine Titananode mit einer Iridiumoxid-haltigen elektrokatalytisch aktiven Schicht, wie sie beispielsweise in der US-A 3 878 083 beschrieben ist.In this example, the single-layer Nafion® 430 membrane was used to separate the anode and cathode compartments. The anode was a titanium anode with an iridium oxide-containing electrocatalytically active layer, as described, for example, in US Pat. No. 3,878,083.

Dem Kathodenraum wurde Wasser mit einer solchen Ge­schwindigkeit zugeführt, so daß 10 %.ige Natronlauge die Zelle verließ.Water was fed into the cathode compartment at such a rate that 10% sodium hydroxide solution left the cell.

Während einer Elektrolysezeit von 61 Tagen stellte sich eine mittlere Zellspannung von 4,2 Volt ein. Die durch­schnittliche Stromausbeute lag in diesem Zeitraum bei 38 %.An average cell voltage of 4.2 volts was established during an electrolysis time of 61 days. The average electricity yield during this period was 38%.

Nach Beendigung des Versuchs wurde dem Kathodenraum anstelle von Wasser eine Natriumdichromatlösung mit 800 g/l Na₂Cr₂O₇· 2 H₂O zugeführt. Die Geschwindigkeit des Einleitens wurde dabei so eingestellt, daß der die Zelle verlassende Katholyt ein pH-Wert von 6,5 bis 7,0 aufwies. Dabei stellte sich während der Versuchsdauer von 9 Tagen eine unveränderte mittlere Zellspannung von 4,2 Volt ein. Die Stromausbeute stieg auf durchschnitt­lich 63 5 an.After completion of the experiment, a sodium dichromate solution containing 800 g / l Na₂Cr₂O₇ · 2 H₂O was fed to the cathode compartment instead of water. The rate of introduction was adjusted so that the catholyte leaving the cell had a pH of 6.5 to 7.0. An unchanged mean cell voltage of 4.2 volts was established during the test period of 9 days. The electricity yield rose to an average of 63 5.

Durch Erzeugung eines chromathaltigen Katholyten an­stelle von Natronlauge wrde demnach bei gleichbleibender Zellspannung die Stromausbeute erheblich gesteigert.By generating a chromate-containing catholyte instead of sodium hydroxide solution, the current yield would be increased considerably while maintaining the cell voltage.

Beispiele 2, 3, 4, 5:Examples 2, 3, 4, 5:

In diesen Beispielen wurden Titananoden mit einer schmelzgalvanisch erzeugten Platinschicht verwendet, wie sie in "G. Dick, Galvanotechnik 79 (1988), Nr 12, S. 4066 - 4071" beschrieben sind.In these examples, titanium anodes with a platinum layer produced by means of hot-dip galvanization were used, as described in "G. Dick, Galvanotechnik 79 (1988), No. 12, pp. 4066-4071".

Bei den Beispielen 2 und 3 wurden die zweischichtigen Membran Nafion® 324 und bei den Beispielen 3 und 5 die einschichtige Membran Nafion® 430 eingeetzt.In Examples 2 and 3 the two-layer membrane Nafion® 324 and in Examples 3 and 5 the single-layer membrane Nafion® 430 were used.

Als Katholyten wurden erzeugt:

  • Beispiel 2: 20 %ige Natronlauge durch Zufuhr von Wasser zum Kathodenraum
  • Beispiel 3 und 4: Chromathaltige Lösungen mit einem mittleren pH-Wert von 6,5 durch Zufuhr von Natriumdichromatlösung mit 800 g/l Na₂Cr₂O₇·2 H₂O.
  • Beispiel 5: Chromathaltige Lösung mit einem mittleren pH-Wert von 13,4 durch Zufuhr von Natriumdichromatlösung mit 600 g/l Na₂Cr₂O₇·H₂O.
The following were produced as catholytes:
  • Example 2: 20% sodium hydroxide solution by supplying water to the cathode compartment
  • Examples 3 and 4: Chromate-containing solutions with an average pH of 6.5 by supplying sodium dichromate solution with 800 g / l Na₂Cr₂O₇ · 2 H₂O.
  • Example 5: Chromate-containing solution with an average pH of 13.4 by supplying sodium dichromate solution with 600 g / l Na₂Cr₂O₇ · H₂O.

Die Ergebnisse der Versuche sind in Tabelle 1 zusammen­gestellt.The results of the tests are summarized in Table 1.

Wie Tabelle 1 zeigt, wird durch Einsatz einer einschich­tigen Membran anstelle einer zweischichtigen Membran und Erzeugung von chromathaltigen Katholyten eine deutlich geringere Zellspannung bei hoher Stromausbeute erreicht. Tabelle 1 Beispiel Membran Katholyt Mittlere Zellspannung Mittlere Stromausbeute Versuchszeit 2 Nafion® 324 20 %ige Natronlauge 4,9 Volt 56 % 100 Tage 3 Nafion® 324 chromathaltige Lösung, pH 6,5 5,2 Volt 65 % 100 Tage 4 Nafion® 430 chromathaltige Lösung, pH 6,5 4,7 Volt 64 % 100 Tage 5 Nafion® 430 chromathaltige Lösung, pH 13,4 4,5 Volt 62 % 100 Tage As Table 1 shows, by using a single-layer membrane instead of a two-layer membrane and producing chromate-containing catholytes, a significantly lower cell voltage is achieved with a high current efficiency. Table 1 example membrane Catholyte Medium cell voltage Average current efficiency Trial period 2nd Nafion® 324 20% sodium hydroxide solution 4.9 volts 56% 100 days 3rd Nafion® 324 Chromate solution, pH 6.5 5.2 volts 65% 100 days 4th Nafion® 430 Chromate solution, pH 6.5 4.7 volts 64% 100 days 5 Nafion® 430 chromate solution, pH 13.4 4.5 volts 62% 100 days

Claims (3)

1. Verfahren zur Herstellung von Alkalidichromaten und/oder Chromsäure durch Elektrolyse von Alkali­monochromat- und/oder Alkalidichromatlösungen in Elektrolysezellen, deren Anoden- und Kathodenräume durch Kationenaustauschermembranen getrennt sind, dadurch gekennzeichnet, daß die Kationenaus­tauschermembranen einschichtige Membranen auf Basis von perfluorierten Polymeren mit Sulfonsäuregruppen als Kationenaustauschergruppen sind und im Katho­denraum der Zellen eine wäßrige Lösung mit einem pH-Wert von 4 bis 14 erzeugt wird.1. A process for the preparation of alkali dichromates and / or chromic acid by electrolysis of alkali monochromate and / or alkali dichromate solutions in electrolysis cells whose anode and cathode spaces are separated by cation exchange membranes, characterized in that the cation exchange membranes are single-layer membranes based on perfluorinated polymers with sulfonic acid groups as cations are and an aqueous solution with a pH of 4 to 14 is generated in the cathode compartment of the cells. 2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die wäßrige Lösung eine Natriummonochromat- und/oder Natriumdichromat- und/oder Natrium­carbonat-haltige Lösung ist.2. The method according to claim 1, characterized in that the aqueous solution is a sodium monochromate and / or sodium dichromate and / or sodium carbonate-containing solution. 3. Verfahren nach Anspruch 1 oder 2, dadurch gekenn­zeichnet, daß der pH-Wert der wäßrigen Natrium­dichromat-haltigen Lösung 6 bis 7,5 beträgt.3. The method according to claim 1 or 2, characterized in that the pH of the aqueous sodium dichromate-containing solution is 6 to 7.5.
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US3305463A (en) * 1962-03-16 1967-02-21 Pittsburgh Plate Glass Co Electrolytic production of dichromates
GB2051868A (en) * 1979-05-29 1981-01-21 Diamond Shamrock Corp Choice of operation parameters for chromic acid production method using a three compartment cell
GB2051869A (en) * 1979-05-29 1981-01-21 Diamond Shamrock Corp Electrolytic production of chromic acid using two-compartment and three-compartment cells

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DE59009265D1 (en) 1995-07-27
EP0391192B1 (en) 1995-06-21
DE3911065A1 (en) 1990-10-11
KR960016417B1 (en) 1996-12-11
US5127999A (en) 1992-07-07
RO108989B1 (en) 1994-10-31
ES2075083T3 (en) 1995-10-01
DD298004A5 (en) 1992-01-30
JP2904860B2 (en) 1999-06-14
RU1806221C (en) 1993-03-30
PL163448B1 (en) 1994-03-31
KR900016501A (en) 1990-11-13
EP0391192A3 (en) 1991-12-11
CA2013782A1 (en) 1990-10-06
JPH02285084A (en) 1990-11-22
TR26262A (en) 1995-02-15
AR246559A1 (en) 1994-08-31
BR9001593A (en) 1991-05-07
ZA902626B (en) 1991-01-30

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