EP0261255A1 - Process for working up an aqueous phosphoric-acid solution - Google Patents
Process for working up an aqueous phosphoric-acid solution Download PDFInfo
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- EP0261255A1 EP0261255A1 EP86112992A EP86112992A EP0261255A1 EP 0261255 A1 EP0261255 A1 EP 0261255A1 EP 86112992 A EP86112992 A EP 86112992A EP 86112992 A EP86112992 A EP 86112992A EP 0261255 A1 EP0261255 A1 EP 0261255A1
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- acid solution
- phosphoric acid
- iron
- oxalic acid
- decontamination
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
- G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
- G21F9/04—Treating liquids
- G21F9/06—Processing
Definitions
- the invention relates to a method for preparing a aqueous phosphoric acid solution which is used up in the course of chemical and / or electrochemical decontamination of parts of the system which are radioactively contaminated on its surface and is thereby substantially saturated with iron, with evaporation and the solids formed being conditioned for final storage.
- Phosphoric acid electrolyte baths have been used for electrochemical decontamination for several years. After prolonged use, the iron content and activity in the electrolyte solution increase. With iron concentrations of over 100 g Fe / l, the use of the electrolyte becomes uneconomical because the decontamination does not succeed or the time and personnel expenditure is extremely high. That is why these electrolytes are discarded. Since they are radioactively contaminated, they must be disposed of and conditioned. The following two variants can be considered:
- the electrolyte containing approx. 30 - 40% phosphoric acid is diluted 50 times with water. This is necessary in order to avoid precipitation of Na3PO4 ⁇ 12 H2O during the subsequent neutralization with sodium hydroxide solution.
- a pH of 7 is adjusted with vigorous stirring with NaOH.
- the previously dissolved iron phosphate precipitates out as a precipitate that can be decanted off.
- the iron phosphate precipitate binds the Most of the radioactivity, so that the supernatant sodium phosphate solution is below the official exemption limit for radioactive waste water.
- the water can also be subjected to a precipitation / flocculation process.
- the usual technique today is evaporation of the electrolyte solution.
- the acid must be neutralized (pH 10) in order to protect the evaporator and to maintain a solid product that can be stored for final storage.
- Na3 PO4 ⁇ 12 H2O and iron phosphate form as a mixture.
- approx. 1,000 kg of iron phosphate and 5,200 kg of sodium orthophosphate with 12 H2O are formed, which are conditioned.
- the high volume of waste is to be emphasized in this process, because both the sodium orthophosphate and the iron phosphate must be disposed of as radioactive waste.
- Electrochemical decontamination is a method for decontaminating large parts most economical, while pickling in an acid bath is preferred for smaller parts and those with complicated geometries.
- the suitability of a decontamination process depends on the fulfillment of several prerequisites: on the one hand, the radioactive contamination should be removed as completely as possible, but only as little inactive material as possible removed; Furthermore, a minimal amount of secondary wastes and a minimal release of radioactive substances via water and air are desirable, and finally the operating personnel dealing with the decontamination should be exposed to the lowest possible radiation exposure.
- Processing the contaminated workpieces by pickling or electrochemical decontamination not only leads to a constant increase in activity in the acid baths, but also, due to the increase in the concentration of iron, to a gradual exhaustion of the baths.
- the direct disposal of these iron-contaminated baths would contradict the requirement to minimize secondary waste; on the other hand, regeneration of the baths with the aim of reducing the iron content enables the operation to continue, which corresponds to the above requirement.
- the invention has for its object to carry out the method of the type mentioned in such a way that only relatively small amounts of radioactive waste have to be disposed of.
- the solution to this problem according to the invention is that the used phosphoric acid solution with an aqueous Oxalic acid solution is combined, that the iron oxalate formed is separated off and conditioned after pyrolysis, and that the remaining phosphorus solution is evaporated to a phosphoric acid content of 15 to 65% by weight and then used for decontamination of further system parts.
- the invention is based on the idea already indicated above that a minimization of the radioactive waste is only possible if the used phosphoric acid solution is processed in such a way that it is subsequently reusable and only the iron extracted from the solution is sent to the final storage.
- the reduction treatment is carried out electrochemically in a stainless steel trough connected as a cathode with an immersed graphite anode surrounded by a diaphragm.
- the reduction can also be brought about by using spent phosphoric acid solution as a decontamination solution (pickling) for a few days without electricity; this procedure is preferably used when many small parts have to be treated.
- Optimal precipitation conditions for the iron oxalate result when the used phosphoric acid solution is placed in a template the cold oxalic acid solution is entered.
- an oxalic acid solution in this connection which has an oxalic acid content of 5 to 15% by weight, in particular 10% by weight.
- the iron oxalate formed is expediently separated off by sedimentation and / or filtration.
- the remaining phosphoric acid solution is best evaporated to a phosphoric acid content of about 40% by weight, because this phosphoric acid content enables both purely chemical decontamination (pickling) and electrochemical decontamination. Finally, for cost reasons, it is also advisable to condense the evaporated water and use fresh oxalic acid solution for the preparation.
- the process according to the invention is described in more detail below: One begins with the selection of dismantled components for the subsequent decontamination process; it may be necessary to disassemble larger parts into workable pieces beforehand.
- the chemical decontamination takes place either by pickling in an acid bath (approx. 40% phosphoric acid, approx. 60 ° C) or by electrochemical decontamination, because the workpiece connected as anode is also immersed in approx. 40% phosphoric acid; under the influence of a voltage of approximately 15 V, a current of approx. 1,000 to several 1,000 A flows between a stainless steel trough connected as a cathode and the workpiece, the strength of which increases with increasing temperature and with increasing egg singe content decreases.
- the temperature must be stabilized to approx. 70 ° C with the help of the cooling water.
- the attack of the acid dissolves the accessible metallic surface of the workpiece. This, as well as the effect of the resulting gaseous hydrogen (during pickling) or oxygen (during electrochemical decontamination) mechanically removes corrosion layers adhering to the surface.
- the workpiece is removed from the bath and sprayed with deionized water. The cleaned workpiece is checked for any remaining activity and, if the measurements confirm that it falls below the exemption limit, is subjected to what is known as harmless recycling based on official approval.
- the cycle of phosphoric acid begins with the previously described processes of pickling and electrochemical decontamination.
- the electrolyte used for this absorbs a considerable amount of dissolved iron (up to over 100 g / l), which leads to a reduction in effectiveness.
- the electrolyte is stored in a designated container and, if the proportion of divalent iron in the total iron is less than about 80%, filled into the stainless steel trough for reduction.
- a graphite anode surrounded by a diaphragm dips into this. Under the influence of a DC voltage, the trivalent iron coming into contact with the tub connected as a cathode is reduced, while at the graphite anode O2, CO2 and CO ent stand; this also leads to the gradual consumption of the anode.
- the electrolyte is poured into the reaction vessel, in which the same volume of cold oxalic acid solution has already been placed, and mixed intimately with the same while stirring.
- the precipitation of iron oxalate, FeC2O4 ⁇ 2H2O begins within a minute; this precipitation is prevented by vigorous stirring.
- the resulting suspension is pumped into a cylindrical plastic container, on the bottom of which there is a filter basket. The sedimentation of the precipitation that collects in the filter basket takes place in this container over the course of a few hours.
- the supernatant clear solution is either added immediately to the phosphoric acid evaporator or, to remove residual suspended solid particles, through a further filter basket into the filtrate collecting container, from where the pumping into the evaporator takes place.
- the now low-iron electrolyte is heated in the evaporator until enough water has been distilled off at a boiling temperature of approximately 102 ° C. that the phosphoric acid content is approximately 40 to 65% by weight.
- the electrolyte is then ready for use again and can be used again for pickling or for electrochemical decontamination, which closes the cycle.
- the water also circulates in a closed cycle. All water supplied to the storage tank comes from the condenser of the evaporator system; it is used for Spraying off the workpieces, for preparing oxalic acid solution and for washing the iron oxalate.
- the spray water (contaminated by phosphoric acid and iron phosphate) is also used to produce oxalic acid solution.
- the solid oxalic acid (oxalic acid dihydrate; H2C2O4 ⁇ 2H2O) is mixed with such an amount of water at room temperature and stirred that an approximately 10% oxalic acid solution is formed. This is stored in a storage container and reacted with the reduced electrolyte if necessary.
- the iron oxalate which is almost free of phosphoric acid after repeated washing, is first dried. Thermal decomposition of the iron oxalate at about 250 ° C gives a mixture of iron oxides (FeO, Fe2O3 and others), which are either filled as such in cast barrels or, after mixing with cement and water, are filled into roller-tire barrels and are finally stored after setting.
- FeO, Fe2O3 and others iron oxides
- the gases generated during pyrolysis (CO, CO2, water vapor) are passed over a catalyst, whereby CO is converted into CO2.
- CO gases generated during electrochemical decontamination, pickling and reduction
- H2 water vapor, O2; CO and CO2 from the graphite anode
- electrochemical decontamination plays the most important role, while pickling accounts for less than 10% of the total throughput of the iron.
- 24 t of material can be decontaminated in the course of about 6 to 7 weeks.
- Pickling accounts for around 2 t (corresponding to a total consumption of 250 l phosphoric acid) and electrochemical decontamination approx. 22 t (corresponding to 2,750 l phosphoric acid); for the latter process (assuming a power yield of 50%) a power consumption of 422,000 Ah is required, which corresponds to a work of 6,330 kWh at a voltage of 15 V on average.
- the precipitation of the iron oxalate requires (assumptions: reduction in the amount of iron by 80%; complete conversion of the oxalic acid) 542 kg oxalic acid dihydrate (price per kg currently DM 2.92, totaling approximately DM 1,600.00), which in at least 4,500 l of water (deionized water or spray water) must be dissolved.
- iron oxalate (FeC2O4 ⁇ 2H2O) fall to 773 kg; the sedimentation vessels, which also hold 2 m3 and have to be filled 3 3/4 times, therefore have to hold 206 kg of iron oxalate per filling, which collects in the filter basket.
- the volumes of decantate / filtrate (7.5 m3) and wash water (1 m3), totaling 8.5 m3, are sufficient to fill the phosphoric acid evaporator eight times. Since the 8.5 m3 must be reduced to about 3 m3, i.e. 5.5 m3 have to be distilled off, a time requirement of at least 35 hours for the distillation (the distillate decrease at maximum heating output is approx. 158 l / h).
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Abstract
Bei einem Verfahren zum Aufbereiten einer bei einer chemischen und/oder elektrochemischen Dekontamination von an ihrer Oberfläche radioaktiv verschmutzten Anlageteilen verbrauchten und dabei mit Eisen im wesentlichen gesättigten wässrigen Phosphorsäurelösung wird eingedampft und werden gebildete Feststoffe für eine Endlagerung konditioniert. Im Rahmen eines solchen Verfahrens kommt man zu besonders geringen Mengen endzulagernder Feststoffe, wenn die verbrauchte Phosphorsäurelösung mit einer wässrigen Oxalsäurelösung vereinigt wird, das dabei gebildete Eisenoxalat abgetrennt und nach Pyrolyse konditioniert wird und die zurückbleibende Phosphorsäurelösung auf einen Phosphorsäuregehalt von 15 bis 65 Gew.% eingedampft und danach für eine Dekontamination weiterer Anlageteile eingesetzt wird.In a method for processing a chemical phosphorus and / or electrochemical decontamination of parts of the system which are radioactively contaminated on their surface and which are thereby substantially saturated with iron, aqueous phosphoric acid solution is evaporated and the solids formed are conditioned for final storage. In such a process, particularly small amounts of solids to be disposed of are obtained if the spent phosphoric acid solution is combined with an aqueous oxalic acid solution, the iron oxalate formed is separated off and conditioned after pyrolysis, and the remaining phosphoric acid solution is evaporated to a phosphoric acid content of 15 to 65% by weight and is then used for decontamination of other system parts.
Description
Die Erfindung betrifft ein Verfahren zum Aufbereiten einer bei einer chemischen und/oder elektrochemischen Dekontamination von an ihrer Oberfläche radioaktiv verschmutzten Anlageteilen verbrauchten und dabei mit Eisen im wesentlichen gesättigten wässrigen Phosphorsäurelösung, wobei eingedampft wird und gebildete Feststoffe für eine Endlagerung konditioniert werden. - Chemische Dekontamination meint dabei insbesondere ein Beizen.The invention relates to a method for preparing a aqueous phosphoric acid solution which is used up in the course of chemical and / or electrochemical decontamination of parts of the system which are radioactively contaminated on its surface and is thereby substantially saturated with iron, with evaporation and the solids formed being conditioned for final storage. - Chemical decontamination means pickling in particular.
Phosphorsäureelektrolytbäder werden zur elektrochemischen Dekontamination seit mehreren Jahren eingesetzt. Nach längerem Einsatz steigt der Eisengehalt und die Aktivität in der Elektrolytlösung an. Bei Eisenkonzentrationen von über 100 g Fe/l wird der Einsatz des Elektrolyten unwirtschaftlich, weil der Dekontaminationserfolg ausbleibt oder der Zeit- und Personalaufwand extrem hoch wird. Deswegen werden diese Elektrolyten verworfen. Da sie radioaktiv kontaminiert sind, müssen sie entsorgt und konditioniert werden. Hierfür kommen folgende beiden Varianten in Betracht:Phosphoric acid electrolyte baths have been used for electrochemical decontamination for several years. After prolonged use, the iron content and activity in the electrolyte solution increase. With iron concentrations of over 100 g Fe / l, the use of the electrolyte becomes uneconomical because the decontamination does not succeed or the time and personnel expenditure is extremely high. That is why these electrolytes are discarded. Since they are radioactively contaminated, they must be disposed of and conditioned. The following two variants can be considered:
Als erstes wird der ca. 30 - 40 % Phosphorsäure enthaltende Elektrolyt auf das 50fache mit Wasser verdünnt. Das ist notwendig, um bei der anschließenden Neutralisierung mit Natronlauge eine Ausfällung des Na₃PO₄·12 H₂O zu vermeiden. In einem zweiten Schritt wird unter intensivem Rühren mit NaOH ein pH-Wert von 7 eingestellt. Während der Neutralisation fällt das vorher gelöste Eisenphosphat als gut abdekantierbarer Niederschlag aus. Der Eisenphosphatniederschlag bindet den größten Teil der Radioaktivität, so daß die überstehende Natriumphosphatlösung unterhalb der behördlichen Freigrenze für radioaktives Abwasser liegt. Ergänzend kann das Wasser noch einem Fällungs-/Flockungsverfahren unterzogen werden. Vorteilhaft ist dieses Verfahren, weil pro 3.000 l Elektrolytbad nur ca. 1.000 kg Eisenphosphat zur Konditionierung übrig bleiben. Nachteilig ist, daß das radioaktive Abwasser aus einem 3.000 l Elektrolytbad ca. 1.800 kg Na₃ PO₄ enthält, das sind ca. 1.500 kg PO⁻⁻⁻. Aus Gründen des Umweltschutzes ist dieses Verfahren daher nur in Sonderfällen möglich.First, the electrolyte containing approx. 30 - 40% phosphoric acid is diluted 50 times with water. This is necessary in order to avoid precipitation of Na₃PO₄ · 12 H₂O during the subsequent neutralization with sodium hydroxide solution. In a second step, a pH of 7 is adjusted with vigorous stirring with NaOH. During the neutralization, the previously dissolved iron phosphate precipitates out as a precipitate that can be decanted off. The iron phosphate precipitate binds the Most of the radioactivity, so that the supernatant sodium phosphate solution is below the official exemption limit for radioactive waste water. The water can also be subjected to a precipitation / flocculation process. This process is advantageous because only about 1,000 kg of iron phosphate remain for conditioning per 3,000 l of electrolyte bath. The disadvantage is that the radioactive waste water from a 3,000 l electrolyte bath contains about 1,800 kg Na₃ PO₄, that is about 1,500 kg PO⁻⁻⁻. For reasons of environmental protection, this procedure is therefore only possible in special cases.
Die heute übliche Technik ist das Eindampfen der Elektrolytlösung. Zur Schonung des Verdampfers und zum Erhalt eines festen endlagerungsfähigen Produktes muß die Säure neutralisiert (pH 10) werden. Es bilden sich Na₃ PO₄·12 H₂O und Eisenphosphat als Gemisch. Pro 3.000 1-Charge Elektrolytbad entstehend ca. 1.000 kg Eisenphosphat und 5.200 kg Natriumorthophosphat mit 12 H₂O, welche konditioniert werden. Hervorzuheben ist an diesem Verfahren das hohe Abfallaufkommen, denn sowohl das Natriumorthophosphat und das Eisenphosphat müssen als radioaktiver Abfall entsorgt werden.The usual technique today is evaporation of the electrolyte solution. The acid must be neutralized (pH 10) in order to protect the evaporator and to maintain a solid product that can be stored for final storage. Na₃ PO₄ · 12 H₂O and iron phosphate form as a mixture. For every 3,000 1-batch electrolyte bath, approx. 1,000 kg of iron phosphate and 5,200 kg of sodium orthophosphate with 12 H₂O are formed, which are conditioned. The high volume of waste is to be emphasized in this process, because both the sodium orthophosphate and the iron phosphate must be disposed of as radioactive waste.
Das Ziel des nachfolgend beschriebenen Verfahrens ist die Dekontamination demontierter oberflächig radioaktiv verschmutzter Anlageteile bei gleichzeitiger Minimierung der Menge radioaktiver Abfälle. Als Methode zur Dekontamination großflächiger Teile ist die elektrochemische Dekontamination am wirtschaftlichsten, während für kleinere Teile und solche mit komplizierter Geometrie das Beizen in einem Säurebad zu bevorzugen ist. Die Eignung eines Dekontaminationsverfahrens hängt von der Erfüllung mehrerer Voraussetzungen ab: Zum einen soll die radioaktive Verunreinigung möglichst vollständig entfernt, aber nur möglichst wenig inaktives Material mitabgetragen werden; des weiteren ist eine minimale Menge von Sekundärabfällen sowie eine minimale Freisetzung radioaktiver Stoffe über Wasser und Luft wünschenswert, und schließlich soll das mit der Dekontamination befaßte Bedienungspersonal einer geringstmöglichen Strahlenbelastung ausgesetzt werden. Die Bearbeitung der kontaminierten Werkstücke durch Beizen bzw. elektrochemische Dekontamination führt nicht nur zu einer ständigen Zunahme der Aktivität in den Säurebädern, sondern auch, bedingt durch die Zunahme der Konzentration des Eisens, zu einer allmählichen Erschöpfung der Bäder. Die direkte Beseitigung dieser eisenbelasteten Bäder widerspräche der Forderung nach Minimierung der Sekundärabfälle; hingegen ermöglicht eine Regenerierung der Bäder mit dem Ziel der Herabsetzung des Eisengehaltes eine Fortsetzung des Betriebes, was der obigen Forderung entspricht.The aim of the process described below is to decontaminate dismantled parts of the system that are radioactive contaminated, while at the same time minimizing the amount of radioactive waste. Electrochemical decontamination is a method for decontaminating large parts most economical, while pickling in an acid bath is preferred for smaller parts and those with complicated geometries. The suitability of a decontamination process depends on the fulfillment of several prerequisites: on the one hand, the radioactive contamination should be removed as completely as possible, but only as little inactive material as possible removed; Furthermore, a minimal amount of secondary wastes and a minimal release of radioactive substances via water and air are desirable, and finally the operating personnel dealing with the decontamination should be exposed to the lowest possible radiation exposure. Processing the contaminated workpieces by pickling or electrochemical decontamination not only leads to a constant increase in activity in the acid baths, but also, due to the increase in the concentration of iron, to a gradual exhaustion of the baths. The direct disposal of these iron-contaminated baths would contradict the requirement to minimize secondary waste; on the other hand, regeneration of the baths with the aim of reducing the iron content enables the operation to continue, which corresponds to the above requirement.
Der Erfindung liegt die Aufgabe zugrunde, das Verfahren der eingangs genannten Art so zu führen, daß nur verhältnismäßig kleine Mengen radioaktiver Abfälle entsorgt werden müssen.The invention has for its object to carry out the method of the type mentioned in such a way that only relatively small amounts of radioactive waste have to be disposed of.
Die erfindungsgemäße Lösung dieser Aufgabe besteht darin, daß die verbrauchte Phosphorsäurelösung mit einer wässrigen Oxalsäurelösung vereinigt wird, daß das dabei gebildete Eisenoxalat abgetrennt und nach Pyrolyse konditioniert wird, und daß die zurückbleibende Phosphorlösung auf einen Phosphorsäuregehalt von 15 bis 65 Gew.% eingedampft und danach für eine Dekontamination weiterer Anlageteile eingesetzt wird.The solution to this problem according to the invention is that the used phosphoric acid solution with an aqueous Oxalic acid solution is combined, that the iron oxalate formed is separated off and conditioned after pyrolysis, and that the remaining phosphorus solution is evaporated to a phosphoric acid content of 15 to 65% by weight and then used for decontamination of further system parts.
Die Erfindung geht hierbei von der oben schon angedeuteten Überlegung aus, daß eine Minimierung des radioaktiven Abfalls nur möglich ist, wenn die verbrauchte Phosphorsäurelösung so aufbereitet wird, daß sie anschließend wiederverwendbar ist und nur das der Lösung entzogene Eisen der Endlagerung zugeführt wird.The invention is based on the idea already indicated above that a minimization of the radioactive waste is only possible if the used phosphoric acid solution is processed in such a way that it is subsequently reusable and only the iron extracted from the solution is sent to the final storage.
Für die weitere Ausgestaltung bestehen im Rahmen der Erfindung mehrere Möglichkeiten. So kommt man zu einer besonders hohen Eisenentfernung bzw. Reinheit der Phosphorsäurelösung, wenn die verbrauchte Phosphorsäurelösung vor Vereinigung mit der Oxalsäurelösung einer Reduktionsbehandlung unterworfen wird, bis mindestens 80 % des Eisens in zweiwertiger Form vorliegt. Nach einer bevorzugten Ausführungsform wird die Reduktionsbehandlung auf elektrochemischem Wege in einer als Kathode geschalteten Edelstahlwanne mit einer eintauchenden, von einem Diaphragma umgebenen Graphitanode durchgeführt. Die Reduktion kann aber auch dadurch herbeigeführt werden, daß verbrauchte Phosphorsäurelösung einige Tage lang stromlos als Dekontaminationslösung (Beizung) eingesetzt wird; diese Verfahrensweise wird vorzugsweise dann angewendet, wenn viele Kleinteile zu behandeln sind. Optimale Ausfällungsbedingungen für das Eisenoxalat ergeben sich, wenn die verbrauchte Phosphorsäurelösung in eine Vorlage der kalten Oxalsäurelösung eingegeben ist. Außerdem ist es empfehlenswert, wenn in diesem Zusammenhang mit einer Oxalsäurelösung gearbeitet wird, die einen Oxalsäuregehalt von 5 bis 15 Gew.%, insbes. 10 Gew.%, aufweist. Das gebildete Eisenoxalat wird zweckmäßigerweise durch Sedimentation und/oder Filtration abgetrennt. Ferner empfielt es sich aus Gründen der Energieersparnis, das abgetrennte Eisenoxalat vor der Pyrolyse trocknen zu lassen. Jedenfalls dampft man die zurückbleibende Phosphorsäurelösung am besten auf einen Phosphorsäuregehalt von etwa 40 Gew.% ein, weil dieser Phosphorsäuregehalt sowohl eine rein chemische Dekontamination (Beizung) als auch eine elektrochemische Dekontamination ermöglicht. Schließlich und endlich empfielt es sich aus Kostengründen zusätzlich, daß abgedampfte Wasser zu kondensieren und zur Zubereitung frische Oxalsäurelösung einzusetzen.There are several possibilities for the further configuration within the scope of the invention. This leads to a particularly high iron removal or purity of the phosphoric acid solution if the spent phosphoric acid solution is subjected to a reduction treatment before being combined with the oxalic acid solution until at least 80% of the iron is in divalent form. According to a preferred embodiment, the reduction treatment is carried out electrochemically in a stainless steel trough connected as a cathode with an immersed graphite anode surrounded by a diaphragm. The reduction can also be brought about by using spent phosphoric acid solution as a decontamination solution (pickling) for a few days without electricity; this procedure is preferably used when many small parts have to be treated. Optimal precipitation conditions for the iron oxalate result when the used phosphoric acid solution is placed in a template the cold oxalic acid solution is entered. In addition, it is advisable to work with an oxalic acid solution in this connection which has an oxalic acid content of 5 to 15% by weight, in particular 10% by weight. The iron oxalate formed is expediently separated off by sedimentation and / or filtration. Furthermore, for reasons of energy saving, it is advisable to let the iron oxalate separated dry before pyrolysis. In any case, the remaining phosphoric acid solution is best evaporated to a phosphoric acid content of about 40% by weight, because this phosphoric acid content enables both purely chemical decontamination (pickling) and electrochemical decontamination. Finally, for cost reasons, it is also advisable to condense the evaporated water and use fresh oxalic acid solution for the preparation.
Im folgenden wird das erfindungsgemäße Verfahren näher beschrieben:
Man beginnt mit der Auswahl demontierter Bauteile für den nachfolgenden Dekontaminationsprozeß; gegebenenfalls ist zuvor eine Zerlegung größerer Teile in bearbeitungsgerechte Stücke erforderlich. Die Dekontamination auf chemischem Wege geschieht entweder durch Beizen in einem Säurebad (ca. 40 %ige Phosphorsäure, ca. 60 °C) oder durch elektrochemische Dekontamination, weil das als Anode geschaltete Werkstück gleichfalls in eine ca. 40 %ige Phosphorsäure getaucht wird; unter dem Einfluß einer Spannung von etwa 15 V fließt zwischen einer als Kathode geschalteten Edelstahlwanne und dem Werkstück ein Strom von ca. 1.000 bis einigen 1.000 A, dessen Stärke mit wachsender Temperatur zu- und mit steigendem Ei sengehalt abnimmt. Da sich der Elektrolyt durch den Stromf luß erwärmt, muß mit Hilfe des Kühlwassers die Temperatur auf ca. 70 °C stabilisiert werden. Beim Beizen wie auch beim elektrochemischen Dekontaminieren wird durch den Angriff der Säure die zugängliche metallische Oberfläche des Werkstückes aufgelöst. Dadurch sowie durch die Wirkung des entstehenden gasförmigen Wasserstoffs (beim Beizen) bzw. Sauerstoffs (bei der elektrochemischen Dekontamination) werden auf der Oberfläche haftende Korrosionsschichten mechanisch entfernt. Nach der Abtragung der die Aktivität enthaltenden Schichten wird das Werkstück dem Bad entnommen und mit Deionat abgespritzt. Das gereinigte Werkstück wird auf etwa vorhandene Restaktivität überprüft und, wenn die Messungen das Unterschreiten der Freigrenze bestätigen, aufgrund behördlicher Genehmigung der sog. schadlosen Verwertung zugeführt.The process according to the invention is described in more detail below:
One begins with the selection of dismantled components for the subsequent decontamination process; it may be necessary to disassemble larger parts into workable pieces beforehand. The chemical decontamination takes place either by pickling in an acid bath (approx. 40% phosphoric acid, approx. 60 ° C) or by electrochemical decontamination, because the workpiece connected as anode is also immersed in approx. 40% phosphoric acid; under the influence of a voltage of approximately 15 V, a current of approx. 1,000 to several 1,000 A flows between a stainless steel trough connected as a cathode and the workpiece, the strength of which increases with increasing temperature and with increasing egg singe content decreases. Since the electrolyte heats up due to the current flow, the temperature must be stabilized to approx. 70 ° C with the help of the cooling water. When pickling and electrochemical decontamination, the attack of the acid dissolves the accessible metallic surface of the workpiece. This, as well as the effect of the resulting gaseous hydrogen (during pickling) or oxygen (during electrochemical decontamination) mechanically removes corrosion layers adhering to the surface. After the layers containing the activity have been removed, the workpiece is removed from the bath and sprayed with deionized water. The cleaned workpiece is checked for any remaining activity and, if the measurements confirm that it falls below the exemption limit, is subjected to what is known as harmless recycling based on official approval.
Der Kreislauf der Phosphorsäure beginnt mit den bereits beschriebenen Vorgängen des Beizens und der elektrochemischen Dekontamination. Der dazu verwendete Elektrolyt nimmt dabei erhebliche Menge gelösten Eisens auf (bis über 100 g/l), was zu einer Herabsetzung der Wirksamkeit führt. Nach der Sättigung mit Eisen wird der Elektrolyt in einem dafür vorgesehenen Behälter gelagert und, falls der Anteil des zweiwertigen Eisens am Gesamteisen geringer als etwa 80 % ist, zur Reduktion in die Edelstahlwanne gefüllt. In diese taucht eine von einem Diaphragma umgebene Graphitanode. Unter dem Einfluß einer Gleichspannung wird das mit der als Kathode geschalteten Wanne in Berührung kommende dreiwertige Eisen reduziert, während an der Graphitanode O₂, CO₂ und CO ent stehen; dies führt auch zum allmählichen Verbrauch der Anode.The cycle of phosphoric acid begins with the previously described processes of pickling and electrochemical decontamination. The electrolyte used for this absorbs a considerable amount of dissolved iron (up to over 100 g / l), which leads to a reduction in effectiveness. After saturation with iron, the electrolyte is stored in a designated container and, if the proportion of divalent iron in the total iron is less than about 80%, filled into the stainless steel trough for reduction. A graphite anode surrounded by a diaphragm dips into this. Under the influence of a DC voltage, the trivalent iron coming into contact with the tub connected as a cathode is reduced, while at the graphite anode O₂, CO₂ and CO ent stand; this also leads to the gradual consumption of the anode.
Nach Beendigung der Reduktion, falls der Eisen(II)-Anteil mindestens 80 % beträgt, wird der Elektrolyt in das Reaktionsgefäß gefüllt, in dem bereits das gleiche Volumen kalter Oxalsäurelösung vorgelegt ist, und mit derselben unter Rühren innig vermischt. Die Ausfällung von Eisenoxalat, FeC₂O₄·2H₂O, setzt innerhalb einer Minute ein; durch kräftiges Rühren wird das Absetzen dieses Niederschlages verhindert. Nach der totalen Durchmischung wird die entstandene Suspension in einen zylindrischen Kunststoffbehälter umgepumpt, auf dessen Boden sich ein Filterkorb befindet. In diesem Behälter erfolgt im Verlaufe einiger Stunden die Sedimentation des Niederschlages, der sich im Filterkorb sammelt. Die überstehende klare Lösung wird entweder sofort in den Phosphorsäureverdampfer oder, zur Entfernung restlicher suspendierter Feststoffpartikel, durch einen weiteren Filterkorb in den Filtratauffangbehälter gegeben, von wo aus das Umpumpen in den Verdampfer erfolgt.After the reduction has ended, if the iron (II) content is at least 80%, the electrolyte is poured into the reaction vessel, in which the same volume of cold oxalic acid solution has already been placed, and mixed intimately with the same while stirring. The precipitation of iron oxalate, FeC₂O₄ · 2H₂O, begins within a minute; this precipitation is prevented by vigorous stirring. After total mixing, the resulting suspension is pumped into a cylindrical plastic container, on the bottom of which there is a filter basket. The sedimentation of the precipitation that collects in the filter basket takes place in this container over the course of a few hours. The supernatant clear solution is either added immediately to the phosphoric acid evaporator or, to remove residual suspended solid particles, through a further filter basket into the filtrate collecting container, from where the pumping into the evaporator takes place.
Im Verdampfer wird der nunmehr eisenarme Elektrolyt so lange erhitzt, bis bei einer Siedetemperatur von ca. 102 °C soviel Wasser abdestilliert ist, daß der Phosphorsäuregehalt etwa 40 bis 65 Gew.% beträgt. Der Elektrolyt ist dann wieder gebrauchsfertig und kann erneut zum Beizen oder zur elektrochemischen Dekontamination verwendet werden, womit der Kreislauf geschlossen ist.The now low-iron electrolyte is heated in the evaporator until enough water has been distilled off at a boiling temperature of approximately 102 ° C. that the phosphoric acid content is approximately 40 to 65% by weight. The electrolyte is then ready for use again and can be used again for pickling or for electrochemical decontamination, which closes the cycle.
Auch das Wasser zirkuliert in einem geschlossenen Kreislauf. Alles dem Speicherbehälter zugeführte Wasser entstammt dem Kondensator der Verdampferanlage; verwendet wird es zum Abspritzen der Werstücke, zum Ansetzen von Oxalsäurelösung und zum Waschen des Eisenoxalates. Auch das (durch Phosphorsäure und Eisenphosphat verunreinigte) Abspritzwasser wird zur Herstellung von Oxalsäurelösung verwendet.The water also circulates in a closed cycle. All water supplied to the storage tank comes from the condenser of the evaporator system; it is used for Spraying off the workpieces, for preparing oxalic acid solution and for washing the iron oxalate. The spray water (contaminated by phosphoric acid and iron phosphate) is also used to produce oxalic acid solution.
Die feste Oxalsäure (Oxalsäuredihydrat; H₂C₂O₄·2H₂O) wird bei Raumtemperatur mit einer solchen Wassermenge versetzt und gerührt, daß eine etwa 10 %ige Oxalsäurelösung entsteht. Diese wird in einem Vorratsbehälter gespeichert und bei Bedarf mit dem reduzierten Elektrolyten zur Reaktion gebracht.The solid oxalic acid (oxalic acid dihydrate; H₂C₂O₄ · 2H₂O) is mixed with such an amount of water at room temperature and stirred that an approximately 10% oxalic acid solution is formed. This is stored in a storage container and reacted with the reduced electrolyte if necessary.
Das nach mehrmaligem Waschen nahezu phosphorsäurefreie Eisenoxalat wird zunächst getrocknet. Durch thermische Zersetzung des Eisenoxalats bei etwa 250 °C erhält man ein Gemisch von Eisenoxiden (FeO, Fe₂O₃ u. a.), das entweder als solches in Gußfässer gefü llt oder nach Vermischen mit Zement und Wasser in Rollreifenfässer abgefüllt und nach dem Abbinden endgelagert wird.The iron oxalate, which is almost free of phosphoric acid after repeated washing, is first dried. Thermal decomposition of the iron oxalate at about 250 ° C gives a mixture of iron oxides (FeO, Fe₂O₃ and others), which are either filled as such in cast barrels or, after mixing with cement and water, are filled into roller-tire barrels and are finally stored after setting.
Die bei der Pyrolyse entstehenden Gase (CO, CO₂, Wasserdampf) werden über einen Katalysator geführt, wobei CO in CO₂ umgewandelt wird. Zusammen mit den beim elektrochemischen Dekontaminieren, Beizen und Reduzieren entstehenden Gasen (H₂, Wasserdampf, O₂; CO und CO₂ aus der Graphitanode) werden sie unter ständiger Kontrolle auf Aktivitätsfreiheit in den Abluftkamin geleitet.The gases generated during pyrolysis (CO, CO₂, water vapor) are passed over a catalyst, whereby CO is converted into CO₂. Together with the gases generated during electrochemical decontamination, pickling and reduction (H₂, water vapor, O₂; CO and CO₂ from the graphite anode), they are conducted into the exhaust air stack under constant control to ensure that they are free of activity.
Unter den Dekontaminationsverfahren spielt die elektrochemische Dekontamination die bedeutendste Rolle, während das Beizen mit weniger als 10 % am Gesamtdurchsatz des Eisens beteiligt ist. Mit einer Badfüllung (3.000 l regenerierter Elektrolyt; 40 %ige Phosphorsäure mit einer Eisenkonzentration von 20 g/l) können im Verlaufe von etwa 6 bis 7 Wochen 24 t Material dekontaminiert werden. Dabei entfallen auf das Beizen etwa 2 t (entsprechend einem Gesamtverbrauch von 250 l Phosphorsäure) und auf die elektrochemische Dekontamination ca. 22 t (entsprechend 2.750 l Phosphorsäure); für den letzteren Vorgang ist (unter der Annahme einer Stromausbeute von 50 %) ein Stromverbrauch von 422.000 Ah erforderlich, was bei einer Spannung von durchschnittlich 15 V einer Arbeit von 6.330 kWh entspricht.Among the decontamination processes, electrochemical decontamination plays the most important role, while pickling accounts for less than 10% of the total throughput of the iron. With one bath filling (3,000 l of regenerated electrolyte; 40% phosphoric acid with an iron concentration of 20 g / l), 24 t of material can be decontaminated in the course of about 6 to 7 weeks. Pickling accounts for around 2 t (corresponding to a total consumption of 250 l phosphoric acid) and electrochemical decontamination approx. 22 t (corresponding to 2,750 l phosphoric acid); for the latter process (assuming a power yield of 50%) a power consumption of 422,000 Ah is required, which corresponds to a work of 6,330 kWh at a voltage of 15 V on average.
Zum Reinigen der elektrochemisch dekontaminierten und der gebeizten Teile werden pro Bad und Woche fast 500 l Phosphorsäure verbraucht, in 6 bis 7 Wochen also 3 m³; die Masse der gereinigten Teile beläuft sich auf knapp 24 t. Ein Prozent des Eisens, also 240 kg, gelangt dabei in die zum Beizen und zum elektrochemischen Dekontaminieren verwendete Phosphorsäure sowie in geringerem Umfang in das Abspritzwasser.To clean the electrochemically decontaminated and pickled parts, almost 500 l of phosphoric acid are used per bath and week, ie 3 m³ in 6 to 7 weeks; the mass of the cleaned parts amounts to almost 24 t. One percent of the iron, i.e. 240 kg, gets into the phosphoric acid used for pickling and electrochemical decontamination, and to a lesser extent into the spray water.
Zur Reduktion des Eisens sind (Annahmen; Gesamt-Eisengehalt, ggf. hergestellt durch Eintrag des im Abspritzwasser ausgefallenen Eisenphosphats in das Säurebad, 100 g/l; Anteil des zweiwertigen Eisens am Gesamteisen anfangs 30, bei Beendigung der Reduktion 100 %, Stromausbeute 50 %) 201.500 Ah erforderlich.To reduce the iron, (assumptions; total iron content, possibly produced by entering the iron phosphate precipitated in the spray water into the acid bath, 100 g / l; proportion of divalent iron in the total iron at the beginning 30, Completion of the reduction 100%, current efficiency 50%) 201,500 Ah required.
Die Ausfällung des Eisenoxalates erfordert (Annahmen: Verringerung der Eisenmenge um 80 %; vollständige Umsetzung der Oxalsäure) 542 kg Oxalsäuredihydrat (Preis pro kg z. Z. DM 2,92, insgesamt also ca. DM 1.600,--), das in mindestens 4.500 l Wasser (Deionat bzw. Abspritzwasser) aufzulösen ist.The precipitation of the iron oxalate requires (assumptions: reduction in the amount of iron by 80%; complete conversion of the oxalic acid) 542 kg oxalic acid dihydrate (price per kg currently DM 2.92, totaling approximately DM 1,600.00), which in at least 4,500 l of water (deionized water or spray water) must be dissolved.
Die Summe der Volumina von 3.000 l Elektrolyt und 4.500 l Oxalsäurelösung, also 7,5 m³, entspricht 3 3/4 Füllungen des 2 m³ fassenden Reaktionsgefäßes.The sum of the volumes of 3,000 l of electrolyte and 4,500 l of oxalic acid solution, ie 7.5 m³, corresponds to 3 3/4 fillings of the 2 m³ reaction vessel.
An Eisenoxalat (FeC₂O₄·2H₂O) fallen 773 kg an; die gleichfalls 2 m³ fassenden Sedimentationsgefäße, die 3 3/4 mal gefüllt werden müssen, haben pro Füllung folglich 206 kg Eisenoxalat aufzunehmen, das sich im Filterkorb sammelt.On iron oxalate (FeC₂O₄ · 2H₂O) fall to 773 kg; the sedimentation vessels, which also hold 2 m³ and have to be filled 3 3/4 times, therefore have to hold 206 kg of iron oxalate per filling, which collects in the filter basket.
Wieviel Wasser zum Waschen des Eisenoxalats benötigt wird, richtet sich nach dem gewünschten Grad der Entfernung der anhaftenden Phosphorsäure; man kann von insgesamt etwa 1.000 Litern ausgehen.How much water is needed to wash the iron oxalate depends on the desired degree of removal of the adhering phosphoric acid; one can assume a total of about 1,000 liters.
Die Volumina von Dekantat/Filtrat (7,5 m³) und Waschwasser (1 m³), zusammen also 8,5 m³, reichen für eine achtmalige Befüllung des Phosphorsäureeindampfers aus. Da die 8,5 m³ auf etwa 3 m³ eingeengt werden müssen, also 5,5 m³ abzudestillieren sind, ist von einem Zeitbedarf von mindestens 35 Stunden für das Abdestillieren auszugehen (die Destillat abnahme beträgt bei maximaler Heizleistung ca. 158 l/h).The volumes of decantate / filtrate (7.5 m³) and wash water (1 m³), totaling 8.5 m³, are sufficient to fill the phosphoric acid evaporator eight times. Since the 8.5 m³ must be reduced to about 3 m³, i.e. 5.5 m³ have to be distilled off, a time requirement of at least 35 hours for the distillation (the distillate decrease at maximum heating output is approx. 158 l / h).
Aus dem getrockneten Eisenoxalat (773 kg, ca. 0,6 m³) läßt sich durch thermische Behandlung ein Gemisch von Eisenoxiden herstellen, das etwa 56 % leichter als das Oxalat ist (343 kg, ca. 0,3 m³). From the dried iron oxalate (773 kg, approx. 0.6 m³), a mixture of iron oxides can be produced by thermal treatment, which is about 56% lighter than the oxalate (343 kg, approx. 0.3 m³).
Claims (10)
Priority Applications (5)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
DE8686112992T DE3662476D1 (en) | 1986-09-20 | 1986-09-20 | Process for working up an aqueous phosphoric-acid solution |
EP86112992A EP0261255B1 (en) | 1986-09-20 | 1986-09-20 | Process for working up an aqueous phosphoric-acid solution |
US07/090,958 US4749455A (en) | 1986-09-20 | 1987-08-28 | Method of treating contaminated aqueous phosphoric acid solutions |
JP62228689A JPS63145995A (en) | 1986-09-20 | 1987-09-14 | Method of treating phosphoric acid aqueous solution |
KR1019870010463A KR920000291B1 (en) | 1986-09-20 | 1987-09-19 | Method of treating contaminated aqueous phosphoric acid solutions |
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EP86112992A EP0261255B1 (en) | 1986-09-20 | 1986-09-20 | Process for working up an aqueous phosphoric-acid solution |
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EP0261255A1 true EP0261255A1 (en) | 1988-03-30 |
EP0261255B1 EP0261255B1 (en) | 1989-03-15 |
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EP86112992A Expired EP0261255B1 (en) | 1986-09-20 | 1986-09-20 | Process for working up an aqueous phosphoric-acid solution |
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US (1) | US4749455A (en) |
EP (1) | EP0261255B1 (en) |
JP (1) | JPS63145995A (en) |
KR (1) | KR920000291B1 (en) |
DE (1) | DE3662476D1 (en) |
Cited By (1)
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WO2012175098A1 (en) | 2011-06-23 | 2012-12-27 | Babcock Noell Gmbh | Process and plant for decontaminating phosphoric acid solution |
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KR101370573B1 (en) * | 2012-08-20 | 2014-03-06 | 한국원자력연구원 | A separation technique of restricted nuclide in radioactive waste samples for the determination of Tc-99, Sr-90, Fe-55, Nb-94 and Ni-59(Ni-63) |
JP5997579B2 (en) * | 2012-10-19 | 2016-09-28 | 日本碍子株式会社 | Treatment method of radioactive metal waste |
CN106460137B (en) * | 2014-04-22 | 2019-08-16 | 泰尔生态有限公司 | For going derusting and oxide skin from steel in hot-dip galvanizing technique and for making pickling solution regeneration method and preparation |
AU2015339815B2 (en) * | 2014-10-30 | 2019-05-02 | Jesa, Sa | Method for removing iron in the manufacture of phosphoric acid |
Citations (4)
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US3008904A (en) * | 1959-12-29 | 1961-11-14 | Jr Benjamin M Johnson | Processing of radioactive waste |
US3582402A (en) * | 1968-04-16 | 1971-06-01 | Atcor Inc | Technique for decontaminating metal surfaces in nuclear reactors |
EP0065844A1 (en) * | 1981-05-27 | 1982-12-01 | Prodeco, Inc. | Process for stripping uranium from an alkyl pyrophosphoric acid |
US4549985A (en) * | 1982-06-07 | 1985-10-29 | General Electric Company | Waste disposal process |
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JPS5822381B2 (en) * | 1978-07-31 | 1983-05-09 | 日本車輌製造株式会社 | Luxury diesel locomotive |
US4615776A (en) * | 1983-10-21 | 1986-10-07 | Shinko-Pfaudler Company | Electrolytic decontamination process and process for reproducing decontaminating electrolyte by electrodeposition and apparatuses therefore |
JPS60204900A (en) * | 1984-03-29 | 1985-10-16 | Touden Kankyo Eng Kk | Regenerating method of electrolytic polishing liquid |
US4701246A (en) * | 1985-03-07 | 1987-10-20 | Kabushiki Kaisha Toshiba | Method for production of decontaminating liquid |
-
1986
- 1986-09-20 DE DE8686112992T patent/DE3662476D1/en not_active Expired
- 1986-09-20 EP EP86112992A patent/EP0261255B1/en not_active Expired
-
1987
- 1987-08-28 US US07/090,958 patent/US4749455A/en not_active Expired - Lifetime
- 1987-09-14 JP JP62228689A patent/JPS63145995A/en active Pending
- 1987-09-19 KR KR1019870010463A patent/KR920000291B1/en not_active IP Right Cessation
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3008904A (en) * | 1959-12-29 | 1961-11-14 | Jr Benjamin M Johnson | Processing of radioactive waste |
US3582402A (en) * | 1968-04-16 | 1971-06-01 | Atcor Inc | Technique for decontaminating metal surfaces in nuclear reactors |
EP0065844A1 (en) * | 1981-05-27 | 1982-12-01 | Prodeco, Inc. | Process for stripping uranium from an alkyl pyrophosphoric acid |
US4549985A (en) * | 1982-06-07 | 1985-10-29 | General Electric Company | Waste disposal process |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
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WO2012175098A1 (en) | 2011-06-23 | 2012-12-27 | Babcock Noell Gmbh | Process and plant for decontaminating phosphoric acid solution |
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EP0261255B1 (en) | 1989-03-15 |
DE3662476D1 (en) | 1989-04-27 |
US4749455A (en) | 1988-06-07 |
KR880004500A (en) | 1988-06-04 |
JPS63145995A (en) | 1988-06-18 |
KR920000291B1 (en) | 1992-01-11 |
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