EP0356806B1 - Process for the production of chromic acid - Google Patents
Process for the production of chromic acid Download PDFInfo
- Publication number
- EP0356806B1 EP0356806B1 EP89115034A EP89115034A EP0356806B1 EP 0356806 B1 EP0356806 B1 EP 0356806B1 EP 89115034 A EP89115034 A EP 89115034A EP 89115034 A EP89115034 A EP 89115034A EP 0356806 B1 EP0356806 B1 EP 0356806B1
- Authority
- EP
- European Patent Office
- Prior art keywords
- chromic acid
- electrolysis
- dichromate
- solution
- membrane
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- 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
Definitions
- the invention relates to a process for the production of chromic acid by electrolysis of dichromate and / or monochromate solutions in electrolysis cells, the anode and cathode spaces of which are separated by cation exchange membranes.
- the electrolytic production of chromic acid takes place in an electrolytic cell, the electrode spaces of which are separated by a cation exchange membrane.
- a solution of alkali dichromate - usually sodium dichromate -, alkali monochromate or a mixture of alkali dichromate with alkali monochromate is introduced into the anode compartment and converted into a solution containing chromic acid by selectively transferring alkali ions through the membrane into the cathode compartment.
- alkali dichromate usually sodium dichromate -, alkali monochromate or a mixture of alkali dichromate with alkali monochromate
- the electrolysis is operated so that constant ratios of chromic acid to alkali ions are established in continuous operation.
- the solutions formed in the anode compartment of the cell are concentrated, it being possible for the crystallization to take place, for example, at 60 to 100.degree.
- the crystallized chromic acid is separated off, washed and dried.
- the object of the invention was to provide a process for the production of chromic acid by electrolysis which does not have the disadvantages described.
- the invention relates to a process for the production of chromic acid by electrolysis of dichromate and / or monochromate solutions in electrolysis cells, the anode and cathode spaces of which are separated by a cation exchange membrane, which is characterized in that the chromic acid content of the solution in the anode space increases periodically compared to a continuous operating state becomes.
- This increase is preferably carried out by lowering the throughput of the dichromate and / or monochromate solution through the anode compartment of the cell.
- the process according to the invention is preferably operated so that the periodic increase in the chromic acid concentration is carried out after an electrolysis time of 1 to 100 days.
- the choice of the time of this measure depends on the content of polyvalent cations in the dichromate and / or monochromate solution. If these cations are very low, the measure can also be carried out after more than 100 days.
- 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 Du Pont with the designation Nafion® 324 were used as membranes.
- the cathodes were made of stainless steel and the anodes were made of expanded titanium with an electrocatalytically active layer of tantalum oxide and iridium oxide. Such anodes are described, for example, in US Pat. No. 3,878,083.
- the distance between the electrodes and the membrane was 1.5 mm in all cases.
- Sodium dichromate solutions with a content of 800 g / l Na2Cr2O7 ⁇ 2 H2O and with the levels of impurities listed in the individual examples were introduced into the anode compartments.
- the electrolysis temperature was 80 ° C. in all cases and the current density was 3 kA / m2 of the projected area of the anode and cathode facing the membrane, this area being 11.4 cm ⁇ 6.7 cm.
- these solutions were converted into solutions containing chromic acid, the rate of introduction of the sodium dichromate solutions being chosen such that molar ratios of sodium ions to chromium (VI) of 0.8 and 0.4 were established in the anolytes.
- the electrolysis cells were operated in such a way that molar ratios of sodium ions to chromium (VI) of 0.8 and in each case 3 days of 0.4 were obtained in the anolyte.
- the membrane had and was like the membrane of Example 1 interspersed with white inclusions.
- the life of the membrane was considerably extended under the selected conditions with high calcium contents in the electrolyte.
Landscapes
- 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)
- Inorganic Compounds Of Heavy Metals (AREA)
Description
Die Erfindung betrifft ein Verfahren zur Herstellung von Chromsäure durch Elektrolyse von Dichromat- und/oder Monochromat-Lösungen in Elektrolysezellen, deren Anoden-und Kathodenräume durch Kationenaustauschermembranen getrennt sind.The invention relates to a process for the production of chromic acid by electrolysis of dichromate and / or monochromate solutions in electrolysis cells, the anode and cathode spaces of which are separated by cation exchange membranes.
Gemäß der CA-A-739 447 erfolgt die elektrolytische Herstellung von Chromsäure (CrO₃) in einer Elektrolysezelle, deren Elektrodenräume durch eine Kationenaustauschermembran getrennt sind. In den Anodenraum wird eine Lösung von Alkalidichromat - in der Regel Natriumdichromat -, Alkalimonochromat oder eine Mischung von Alkalidichromat mit Alkalimonochromat eingeleitet und in eine Chromsäure-haltige Lösung umgewandelt, indem Alkaliionen selektiv durch die Membran in den Kathodenraum überführt werden. Durch Variation der dem Anodenraum der Zelle zugeführten Alkalidichromatmenge und durch Variation der Stromstärke lassen sich verschiedene Chromsäure- und Alkaliionenkonzentrationen in der den Anodenraum verlassenden Lösung einstellen. Üblicherweise wird die Elektrolyse so betrieben, daß sich im kontinuierlichen Betrieb konstante Verhältnisse von Chromsäure zu Alkaliionen einstellen.According to CA-A-739 447, the electrolytic production of chromic acid (CrO₃) takes place in an electrolytic cell, the electrode spaces of which are separated by a cation exchange membrane. A solution of alkali dichromate - usually sodium dichromate -, alkali monochromate or a mixture of alkali dichromate with alkali monochromate is introduced into the anode compartment and converted into a solution containing chromic acid by selectively transferring alkali ions through the membrane into the cathode compartment. By varying the amount of alkali dichromate supplied to the anode compartment of the cell and by varying the current intensity, different chromic acid and alkali ion concentrations can be set in the solution leaving the anode compartment. Usually the electrolysis is operated so that constant ratios of chromic acid to alkali ions are established in continuous operation.
Zur Erzeugung von Chromsäurekristallen werden die in dem Anodenraum der Zelle gebildeten Lösungen eingeengt, wobei die Kristallisation beispielsweise bei 60 bis 100°C erfolgen kann. Die auskristallisierte Chromsäure wird abgetrennt, gewaschen und getrocknet.To generate chromic acid crystals, the solutions formed in the anode compartment of the cell are concentrated, it being possible for the crystallization to take place, for example, at 60 to 100.degree. The crystallized chromic acid is separated off, washed and dried.
Bei der Durchführung dieses Verfahrens kommt es zur Bildung von Einlagerungen von Verbindungen mehrwertiger Ionen, insbesondere von Erdalkaliverbindungen, die die Funktionsfähigkeit der Membran bereits nach kurzer Zeit verschlechtert, bis hin zum völligen Ausfall der Membran. Ursache dieser Einlagerungen sind geringe Gehalte an mehrwertigen Kationen, insbesondere Calcium- und Strontiumionen, in den als Elektrolyt verwendeten Alkalidichromatlösungen, wie sie in technischen Verfahren, beschrieben in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A 7, 1986, S. 67-97, erhältlich sind.When this method is carried out, compounds of polyvalent ions, in particular alkaline earth compounds, are formed, which deteriorates the functionality of the membrane after a short time, up to the complete failure of the membrane. The cause of these deposits are low levels of polyvalent cations, in particular calcium and strontium ions, in the alkali dichromate solutions used as electrolyte, as described in technical processes, described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A 7, 1986, p. 67- 97 are available.
Die Aufgabe der Erfindung bestand darin, ein Verfahren zur Herstellung von Chromsäure durch Elektrolyse bereitzustellen, das die beschriebenen Nachteile nicht aufweist.The object of the invention was to provide a process for the production of chromic acid by electrolysis which does not have the disadvantages described.
Überraschend wurde nun gefunden, daß die genannten Nachteile nicht auftreten, wenn der Chromsäuregehalt der Lösung im Anodenraum der Zelle gegenüber einem kontinuierlichen Betriebszustand periodisch erhöht wird.Surprisingly, it has now been found that the disadvantages mentioned do not occur if the chromic acid content of the solution in the anode compartment of the cell is periodically increased compared to a continuous operating state.
Gegenstand der Erfindung ist ein Verfahren zur Herstellung von Chromsäure durch Elektrolyse von Dichromat-und/oder Monochromatlösungen in Elektrolysezellen, deren Anoden- und Kathodenräume durch Kationenaustauschermembran getrennt sind, welches dadurch gekennzeichnet ist, daß der Chromsäuregehalt der Lösung im Anodenraum gegenüber einem kontinuierlichen Betriebszustand periodisch erhöht wird.The invention relates to a process for the production of chromic acid by electrolysis of dichromate and / or monochromate solutions in electrolysis cells, the anode and cathode spaces of which are separated by a cation exchange membrane, which is characterized in that the chromic acid content of the solution in the anode space increases periodically compared to a continuous operating state becomes.
Diese Erhöhung wird vorzugsweise durch Erniedrigung des Durchsatzes der Dichromat- und/oder Monochromatlösung durch den Anodenraum der Zelle vorgenommen. Es ist aber auch möglich, die periodische Erhöhung des Chromsäuregehalts durch Erhöhung der Stromstärke und/oder durch externe Zufuhr von Chromsäure oder Chromsäurelösung zu realisieren.This increase is preferably carried out by lowering the throughput of the dichromate and / or monochromate solution through the anode compartment of the cell. However, it is also possible to realize the periodic increase in the chromic acid content by increasing the current strength and / or by external supply of chromic acid or chromic acid solution.
Das erfindungsgemäße Verfahren wird vorzugsweise so betrieben, daß die periodische Erhöhung der Chromsäurekonzentration nach einer Elektrolysezeit von 1 bis 100 Tagen durchgeführt wird. Die Wahl des Zeitpunktes dieser Maßnahme ist abhängig vom Gehalt an mehrwertigen Kationen in der Dichromat- und/oder Monochromatlösung. Bei sehr geringen Gehalten dieser Kationen kann die Maßnahme auch nach mehr als 100 Tagen erfolgen. Bei Durchführung des erfindungsgemäßen Verfahrens werden Einlagerungen vermieden bzw. gebildete Einlagerungen werden aufgelöst, wodurch die Lebensdauer der Membran erheblich verlängert wird, was einen kontinuierlichen und dauerhaften Betrieb der Elektrolyse gewährleistet.The process according to the invention is preferably operated so that the periodic increase in the chromic acid concentration is carried out after an electrolysis time of 1 to 100 days. The choice of the time of this measure depends on the content of polyvalent cations in the dichromate and / or monochromate solution. If these cations are very low, the measure can also be carried out after more than 100 days. When carrying out the method according to the invention, deposits are avoided or formed deposits are dissolved, as a result of which the life of the membrane is considerably extended, which ensures continuous and permanent operation of the electrolysis.
Die in den Beispielen verwendeten Elektrolysezellen bestanden aus Anodenräumen aus Rein-Titan und Kathodenräumen aus Edelstahl. Als Membranen wurden Kationenaustauschermembranen der Firma Du Pont mit der Bezeichnung Nafion® 324 verwendet. Die Kathoden bestanden aus Edelstahl und die Anoden aus Titanstreckmetall mit einer elektrokatalytisch aktiven Schicht aus Tantaloxid und Iridiumoxid. Solche Anoden sind beispielsweise in der US-A 3 878 083 beschrieben.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 Du Pont with the designation Nafion® 324 were used as membranes. The cathodes were made of stainless steel and the anodes were made of expanded titanium with an electrocatalytically active layer of tantalum oxide and iridium oxide. Such anodes are described, for example, in US Pat. No. 3,878,083.
Der Abstand der Elektroden zur Membran betrug in allen Fällen 1,5 mm. In die Anodenräume wurden Natriumdichromatlösungen mit einem Gehalt von 800 g/l Na₂Cr₂O₇ · 2 H₂O und mit den in den einzelnen Beispielen aufgeführten Gehalten an Verunreinigungen eingeleitet.The distance between the electrodes and the membrane was 1.5 mm in all cases. Sodium dichromate solutions with a content of 800 g / l Na₂Cr₂O₇ · 2 H₂O and with the levels of impurities listed in the individual examples were introduced into the anode compartments.
Den Kathodenräumen wurde Wasser mit einer solchen Geschwindigkeit zugeführt, so daß 20 %ige Natronlauge die Zellen verließ. Die Elektrolysetemperatur war in allen Fällen 80°C und die Stromdichte betrug 3 kA/m² der der Membran zugewandten projizierten Fläche von Anode und Kathode, wobei diese Fläche 11,4 cm · 6,7 cm betrug.Water was supplied to the cathode compartments at such a rate that 20% sodium hydroxide solution left the cells. The electrolysis temperature was 80 ° C. in all cases and the current density was 3 kA / m² of the projected area of the anode and cathode facing the membrane, this area being 11.4 cm × 6.7 cm.
Die bei diesem Versuch verwendeten Natriumdichromatlösungen wiesen folgende Gehalte an Erdalkaliionen auf:
- Calcium:
- 196 bis 197 ppm
- Strontium:
- kleiner 0,5 ppm
- Magnesium:
- kleiner 0,5 bis 1,1 ppm
- Calcium:
- 196 to 197 ppm
- Strontium:
- less than 0.5 ppm
- Magnesium:
- less than 0.5 to 1.1 ppm
Diese Lösungen wurden in der beschriebenen Elektrolysezelle elektrolytisch in chromsäurehaltige Lösungen umgewandelt. Die Geschwindigkeit des Einleitens der Natriumdichromatlösungen wurde so gewählt, daß sich in dem die Zelle verlassenden Anolyten ein molares Verhältnis von Natriumionen zu Chrom(VI) von ca. 0,8 einstellte. Während des Versuchs stieg die Zellspannung von anfänglich 4,7 V rasch auf 6,2 V an und erreichte nach 18 Tagen 7,0 V. Die durchschnittliche Stromausbeute lag während dieser Zeit bei ca. 68 %. Am 25. Tag fiel die Zellspannung auf 3,8 V und die Stromausbeute auf ca. 46 % ab, was anzeigte, daß die Funktionsfähigkeit der Membran erheblich verschlechtert war. Nach Beendigung des Versuchs nach 29 Tagen war die Membran vollständig mit weißen Einlagerungen durchsetzt, die im wesentlichen aus Calciumhydroxid bestanden. Darüberhinaus wies die Membran an mehreren Stellen ca. 3 bis 5 mm große Blasen auf, von denen einige aufgeplatzt waren. Die Membran war demnach zerstört.These solutions were converted electrolytically into solutions containing chromic acid in the electrolysis cell described. The rate of introduction of the sodium dichromate solutions was chosen so that a molar ratio of sodium ions to chromium (VI) of about 0.8 was established in the anolyte leaving the cell. During the test, the cell voltage rose rapidly from the initial 4.7 V to 6.2 V and reached 7.0 V after 18 days. The average current yield during this time was approximately 68%. On the 25th day, the cell voltage dropped to 3.8 V and the current efficiency to about 46%, which indicated that the functionality of the membrane had deteriorated considerably. After the end of the experiment after 29 days, the membrane was completely covered with white deposits, which consisted essentially of calcium hydroxide. In addition, the membrane had bubbles of approximately 3 to 5 mm in size at several points, some of which had burst. The membrane was therefore destroyed.
Bei diesem Versuch wurden Natriumdichromatlösungen mit folgenden Gehalten an Erdalkaliionen eingesetzt:
- Calcium:
- 196 bis 201 ppm
- Strontium
- kleiner 0,5 ppm
- Magnesium
- kleiner 0,5 ppm
- Calcium:
- 196 to 201 ppm
- strontium
- less than 0.5 ppm
- magnesium
- less than 0.5 ppm
Diese Lösungen wurden in der beschriebenen Elektrolysezelle in chromsäurehaltige Lösungen umgewandelt, wobei die Geschwindigkeit des Einleitens der Natriumdichromatlösungen so gewählt wurden, daß sich abwechselnd in den Anolyten molare Verhältnisse von Natriumionen zu Chrom (VI) von 0,8 und 0,4 einstellten. Die Elektrolysezellen wurden dabei so betrieben, daß sich jeweils 4 Tage molare Verhältnisse von Natriumionen zu Chrom (VI) von 0,8 und jeweils 3 Tage von 0,4 im Anolyten einstellten.In the electrolysis cell described, these solutions were converted into solutions containing chromic acid, the rate of introduction of the sodium dichromate solutions being chosen such that molar ratios of sodium ions to chromium (VI) of 0.8 and 0.4 were established in the anolytes. The electrolysis cells were operated in such a way that molar ratios of sodium ions to chromium (VI) of 0.8 and in each case 3 days of 0.4 were obtained in the anolyte.
Im Lauf des Versuchs stieg die Zellspannung von anfänglich 4,2 V innerhalb von 52 Tagen auf 5,2 V an. Die durchschnittlich Stromausbeute lag während dieser Zeit bei 40 %. Am 54. Tag fiel die Spannung auf 3,9 V und die durchschnittliche Stromausbeute auf 30 % ab, was wie im Beispiel 1 erläutert eine Störung der Funktionsfähigkeit der Membran anzeigte.In the course of the experiment, the cell voltage rose from 4.2 V initially to 5.2 V within 52 days. The average electricity yield during this period was 40%. On the 54th day, the voltage dropped to 3.9 V and the average current yield to 30%, which, as explained in Example 1, indicated a fault in the functionality of the membrane.
Nach Beendigung des Versuchs nach 64 Tagen wies die Membran wie die Membran des Beispiels 1 Blasen auf und war mit weißen Einlagerungen durchsetzt. Durch Anwendung des erfindungsgemäßen Verfahrens wurde jedoch bei den gewählten Bedingungen mit hohen Calciumgehalten im Elektrolyten die Lebensdauer der Membran erheblich verlängert.After the end of the experiment after 64 days, the membrane had and was like the membrane of Example 1 interspersed with white inclusions. By using the method according to the invention, however, the life of the membrane was considerably extended under the selected conditions with high calcium contents in the electrolyte.
Claims (4)
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE3829125A DE3829125A1 (en) | 1988-08-27 | 1988-08-27 | METHOD FOR THE PRODUCTION OF CHROME ACID |
| DE3829125 | 1988-08-27 |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| EP0356806A2 EP0356806A2 (en) | 1990-03-07 |
| EP0356806A3 EP0356806A3 (en) | 1990-04-18 |
| EP0356806B1 true EP0356806B1 (en) | 1992-06-24 |
Family
ID=6361724
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| EP89115034A Expired - Lifetime EP0356806B1 (en) | 1988-08-27 | 1989-08-15 | Process for the production of chromic acid |
Country Status (10)
| Country | Link |
|---|---|
| US (1) | US5071522A (en) |
| EP (1) | EP0356806B1 (en) |
| JP (1) | JP2839154B2 (en) |
| AR (1) | AR244351A1 (en) |
| BR (1) | BR8904279A (en) |
| CA (1) | CA1337808C (en) |
| DE (2) | DE3829125A1 (en) |
| ES (1) | ES2032636T3 (en) |
| MX (1) | MX171486B (en) |
| TR (1) | TR24639A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6063252A (en) * | 1997-08-08 | 2000-05-16 | Raymond; John L. | Method and apparatus for enriching the chromium in a chromium plating bath |
Family Cites Families (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA739447A (en) * | 1966-07-26 | W. Carlin William | Electrolytic production of chromic acid | |
| US2099658A (en) * | 1933-11-09 | 1937-11-16 | Gilbert | Preparation of chromic acid and sparingly soluble chromates |
| US2213249A (en) * | 1934-07-23 | 1940-09-03 | Armstrong Cork Co | Insulation board and method of making the same |
| US3305463A (en) * | 1962-03-16 | 1967-02-21 | Pittsburgh Plate Glass Co | Electrolytic production of dichromates |
| GB1399402A (en) * | 1972-02-10 | 1975-07-02 | Marley Homes Ltd | Reinforced woodwool slab |
| DE2854228C2 (en) * | 1978-12-15 | 1983-11-24 | Ytong AG, 8000 München | Multi-layer sheet made of aerated concrete, as well as process for their manufacture |
| FR2455984A1 (en) * | 1979-05-10 | 1980-12-05 | Dvihally Sandor | Mfr. of resin bonded particle board - with decorative relief pattern |
| CA1141640A (en) * | 1979-06-08 | 1983-02-22 | Thomas A. Pilgrim | Building components |
| WO1983001410A1 (en) * | 1981-10-26 | 1983-04-28 | Teare, John, W. | Method and apparatus for producing concrete panels |
| DE3313641C2 (en) * | 1983-04-15 | 1986-06-12 | M.A.N.- Roland Druckmaschinen AG, 6050 Offenbach | Device for applying a layer of a liquid fine ceramic mass to a carrier |
| GB2236876A (en) * | 1989-10-12 | 1991-04-17 | Bpb Industries Plc | Control of the manufacture of plaster board |
| DE9313351U1 (en) * | 1993-09-04 | 1993-11-04 | Wedi, Helmut, 48282 Emsdetten | Plate-shaped component |
| ES2107599T3 (en) * | 1992-10-29 | 1997-12-01 | Helmut Wedi | PROCEDURE FOR THE MANUFACTURE OF SEMI-FINISHED CORNER PRODUCTS FROM LAMINATED COMPOSITE PLATES. |
-
1988
- 1988-08-27 DE DE3829125A patent/DE3829125A1/en not_active Withdrawn
-
1989
- 1989-08-08 MX MX017106A patent/MX171486B/en unknown
- 1989-08-15 DE DE8989115034T patent/DE58901743D1/en not_active Expired - Lifetime
- 1989-08-15 ES ES198989115034T patent/ES2032636T3/en not_active Expired - Lifetime
- 1989-08-15 EP EP89115034A patent/EP0356806B1/en not_active Expired - Lifetime
- 1989-08-22 TR TR89/0693A patent/TR24639A/en unknown
- 1989-08-24 JP JP1216182A patent/JP2839154B2/en not_active Expired - Lifetime
- 1989-08-25 CA CA000609436A patent/CA1337808C/en not_active Expired - Fee Related
- 1989-08-25 BR BR898904279A patent/BR8904279A/en not_active Application Discontinuation
- 1989-08-25 AR AR89314754A patent/AR244351A1/en active
-
1991
- 1991-02-25 US US07/663,031 patent/US5071522A/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| BR8904279A (en) | 1990-04-17 |
| DE58901743D1 (en) | 1992-07-30 |
| DE3829125A1 (en) | 1990-03-01 |
| CA1337808C (en) | 1995-12-26 |
| TR24639A (en) | 1991-12-25 |
| EP0356806A2 (en) | 1990-03-07 |
| MX171486B (en) | 1993-10-29 |
| US5071522A (en) | 1991-12-10 |
| JP2839154B2 (en) | 1998-12-16 |
| JPH02102127A (en) | 1990-04-13 |
| ES2032636T3 (en) | 1993-02-16 |
| EP0356806A3 (en) | 1990-04-18 |
| AR244351A1 (en) | 1993-10-29 |
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|---|---|---|---|
| PUAI | Public reference made under article 153(3) epc to a published international application that has entered the european phase |
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| PUAL | Search report despatched |
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