JP2008304280A - Actinoid adsorption material and method for treating radioactive waste liquid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an actinoid adsorption material which makes it possible to separate and recover actinoid selectively over a wide range of sulfate concentration and also has a great adsorption capacity. <P>SOLUTION: The actinoid adsorption material selectively separates actinoid by including trioctyl phosphine oxide (TOPO) in the form of solid powder as an organic compound to selectively produce complexes with the actinoid in a carrier of a calcium alginate gel. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an adsorbent that selectively recovers actinides from a radioactive liquid waste containing actinoids typified by uranium, and a method for treating a radioactive liquid waste using this adsorbent. As the radioactive liquid waste, in particular, a sulfuric acid acidic liquid waste having a relatively high concentration of 0.1 mol / L to 1 mol / L is assumed.

  Research and development of various decontamination methods for uranium waste generated in facilities handling nuclear fuel materials is underway.

  For metal wastes, sulfuric acid decontamination, which is immersed in sulfuric acid decontamination solution and dissolves uranium, is one of the promising methods. When metal waste is decontaminated with sulfuric acid, metals such as iron dissolve in addition to uranium. The decontamination waste liquid is neutralized by recovering sulfuric acid as necessary and then adding sodium hydroxide. At this time, the metal ions dissolved in the decontamination waste liquid are precipitated as sludge. Since this sludge contains uranium, it will be treated as radioactive waste.

  If uranium can be separated and recovered from the decontamination waste liquid, the sludge does not contain uranium, and therefore, there is a possibility that it can be treated as non-radioactive waste. As a method for separating and recovering uranium, a solvent extraction method, an adsorption method, a precipitation method and the like have been known for a long time. Assuming that the concentration of uranium in the decontamination waste liquid is on the order of 100 ppm, it is difficult to separate and recover quantitatively and selectively by the precipitation method. In the solvent extraction method, the number of extractors and tanks increases, and the system becomes complicated. When the concentration of ions to be collected is not so high, such as decontamination waste liquid, the adsorption method is considered most suitable.

  As an adsorbent for selectively separating and recovering uranium, an ion exchange resin has been known for a long time. In recent years, a method for separating uranium with an adsorbent in which an extractant or a chelating agent is encapsulated in microcapsules has been researched and developed (for example, see Patent Document 1). Furthermore, although the separation object is not uranium but an alkali metal, research and development of microcapsules encapsulating a solid inorganic adsorbent is also underway (see, for example, Patent Documents 2 and 3).

Japanese Patent Laying-Open No. 2007-14841 (pages 4-5) Japanese Patent No. 3007967 (page 6, FIGS. 1 and 2) (Japanese Patent No. 3116092 (page 7, FIGS. 1 and 2))

In order to suppress the generation amount of radioactive waste, when separating and recovering uranium from the decontamination waste liquid, a separation method that satisfies the following requirements is required. That is,
a. Less coexisting ions with uranium b. Small amount of reagent required for separation c. The waste liquid treatment speed is realistic d. The amount of secondary waste generated is small.

  When there are many coexisting ions accompanying uranium, the volume of uranium waste after separation cannot be effectively reduced. Therefore, the method needs to selectively separate and recover uranium. Next, when there are many reagents required for separation, there is a concern about an increase in the size of the separation system and an increase in the amount of secondary waste generated. Therefore, it is necessary for the adsorbent to adsorb uranium with a high distribution coefficient and to have a large adsorption capacity. Further, from the viewpoint of the waste liquid treatment speed, it is necessary that the adsorbent has a high adsorption reaction speed. Furthermore, from the viewpoint of ultimately reducing the volume of waste, an adsorbent that can be easily thermally decomposed is desirable. From the above viewpoint, the existing adsorbent is evaluated.

  Examples of ion exchange resins include cation exchange resins and anion exchange resins. From the viewpoint of separating and recovering uranium from the decontamination waste liquid, there are the following problems. Since the cation exchange resin also adsorbs coexisting metal ions such as iron ions to uranium, it is difficult to selectively separate uranium. On the other hand, an anion exchange resin has a lower uranium partition coefficient as the sulfuric acid concentration increases. Usually, the decontamination waste liquid is about 0.1 to 1 mol / L, but uranium cannot be adsorbed efficiently from the sulfuric acid aqueous solution having such a concentration. In addition, both the cation exchange resin and the anion exchange resin are not so easily reduced by thermal decomposition.

  Reagents that selectively bind to uranium, for example, microcapsules in which an organic compound such as tributyl phosphate or di2-ethylhexyl phosphate is encapsulated in a calcium alginate gel can selectively adsorb uranium. However, when a large amount of these liquid organic compounds are encapsulated in microcapsules, they are released from the calcium alginate gel prepared in an aqueous solution system. Therefore, the addition rate cannot usually be increased. As a result, the adsorption capacity of the microcapsule is reduced.

  An object of the present invention is to selectively separate and recover actinides represented by uranium in a wide sulfuric acid concentration range, and to have a large distribution coefficient and adsorption capacity, and to suppress the amount of secondary waste, and the adsorbent It is providing the processing method of the radioactive waste liquid which used NO.

  In order to solve the above problems, the present invention proposes an actinide adsorbent in which an organic compound that selectively forms a complex with an actinide is encapsulated in a solid powder form in a calcium alginate gel carrier.

  In order to solve the above-mentioned problems, the present invention provides a method for treating a radioactive liquid waste containing actinoid ions, wherein the radioactive liquid waste is passed through a column packed with the actinide adsorbent and adsorbs the actinoid ions. A processing method is proposed.

  There is no known solid inorganic adsorbent having selectivity for uranium, such as an inorganic adsorbent having selectivity for an alkali metal. Organic compounds are known as those that selectively complex with uranium.

  In the present invention, a microcapsule containing a solid organic compound is used instead of a known liquid compound among organic compounds. Specifically, trioctylphosphine oxide (TOPO) that selectively adsorbs to uranium in a wide sulfuric acid concentration range is included in a calcium alginate gel.

  Since TOPO is a solid powder, it is not released from the gel even if the amount added to the alginic acid gel is increased. Therefore, the adsorption capacity of the microcapsules can be increased. Also, handling is easier when the particle size is increased by microencapsulation rather than directly handling the TOPO powder. Furthermore, as a characteristic of TOPO, it is expected that uranium is adsorbed with a high distribution coefficient and can be well separated from coexisting elements such as iron.

  On the other hand, the TOPO-encapsulated microcapsules also have problems. First, there was concern that the adsorption reaction of uranium with solid TOPO was slow. Conventionally, TOPO has been used so as to dissolve in an organic solvent such as xylene and extract uranium in the waste liquid into the organic solvent. Compared to this, it is generally expected that uranium in the waste liquid and solid TOPO are directly complexed and adsorbed to be a very slow reaction.

  Therefore, the inventors have found through experiments that the microcapsules can be adsorbed at a practical speed by optimizing the particle size of the microcapsules, and solved the speed problem.

  Unlike the case where an inorganic adsorbent is included, when an organic compound such as TOPO is included, there is a problem with the strength of the microcapsules. When excessive TOPO was added, an event was observed in which the microcapsules weakened and crushed.

  On the other hand, the inventors investigated the relationship between the TOPO addition ratio and the adsorption performance to clarify the minimum required TOPO addition ratio, and at the addition ratio, the TOPO-encapsulated microcapsules had a stable shape. It was confirmed that it could be maintained, and the strength problem was solved.

  Since calcium and sodium alginate gel contains calcium and sodium, there is a concern that even if used microcapsules are treated by pyrolysis, the amount of waste generated is not reduced.

  The inventors have found by analysis that when calcium alginate gel is immersed in a sulfuric acid aqueous solution, calcium and sodium ions and hydrogen ions undergo an exchange reaction, and there are almost no non-volatile components remaining in the microcapsules. It was. That is, when applied to sulfuric acid acidic waste liquid treatment, if the used microcapsules are thermally decomposed, the amount of waste generated can be suppressed, and the problem of secondary waste can be solved.

  In the known technique of microencapsulating an organic compound having selectivity for uranium, the present invention cannot be achieved by simply selecting solid TOPO from various reagents.

  The present invention is an experimental study conducted by the inventors and is based on solving problems specific to solid TOPO, such as speed problems, strength problems, and problems related to the amount of waste generated. Adsorption material with good adsorption capacity and waste volume reduction has been obtained.

  As the organic compound, a phosphine oxide compound or a carbamoylphosphine oxide compound may be employed instead of TOPO.

  According to the present invention, actinides can be selectively separated and recovered from a sulfuric acid-based decontamination waste liquid in a wide concentration range. Since the adsorption method is applied, the separation equipment can be simplified as compared with the solvent extraction method. Moreover, the TOPO microcapsules can be easily thermally decomposed after being brought into contact with the acidic decontamination waste liquid, and no residue derived from the capsule components is produced. Therefore, the generation amount of uranium waste can be suppressed by the separation and recovery method using the TOPO microcapsules.

  Next, with reference to FIGS. 1-5, an Example is described to the processing method of the actinide adsorbent and radioactive waste liquid by this invention.

  First, a method for preparing TOPO microcapsules will be described. Dissolve sodium alginate in water to 1% by weight. A predetermined amount of TOPO powder that has been pulverized well in a mortar in advance is added to this solution and stirred well to disperse the powder. This TOPO suspension is dropped into an aqueous calcium chloride solution. At this time, in order to obtain microcapsules having a desired particle diameter, the diameter of the droplet is controlled. The droplets quickly gel and become microcapsules containing TOPO. The microcapsules are collected by filtration.

  The characteristics of the TOPO microcapsules thus prepared will be described. TOPO has been conventionally used to dissolve uranium by dissolving in an organic solvent such as xylene by a solvent extraction method.

  In the present invention, since TOPO is complexed with uranium in the waste liquid in a solid state, there is a concern that the adsorption reaction is slow. On the other hand, if the particle size of the microcapsules is reduced, it is expected that the adsorption reaction can be accelerated. Therefore, the inventors determined an appropriate particle size range through experiments.

  FIG. 1 is a diagram showing the relationship between the uranium distribution coefficient and the adsorption time using the average particle size of microcapsules as a parameter.

  When the particle size is less than 1 mm, the time to reach adsorption equilibrium is 2 to 3 hours, which is a practical adsorption rate. It has been found that even if the particle size is further reduced, the complex formation reaction between TOPO and uranium is rate-determined, so that it does not become faster than a certain level. If the particle size is too small, it becomes difficult to handle as a powder. From the above examination, it was found that the particle size of the microcapsules should be 0.5 mm to 1 mm as a condition for achieving a practical adsorption rate.

  When TOPO is encapsulated as a solid, there is a concern that the strength of the alginic acid gel decreases. Therefore, we want to minimize the TOPO ratio. Conversely, uranium adsorption performance was thought to increase with the proportion of TOPO. An attempt was made to obtain the minimum TOPO ratio from the dependence of the adsorption performance on the TOPO ratio.

  FIG. 2 is a graph showing the relationship between the uranium distribution coefficient and the TOPO ratio.

  It was found that when the proportion of TOPO was 6% by weight or more, the distribution coefficient of uranium hardly changed. Therefore, various investigations were conducted with the TOPO ratio set to approximately 6% by weight. As a result, it was confirmed that the TOPO-containing microcapsules were stable at a ratio of 6% by weight.

  It is known that calcium alginate gel used as a carrier acts as a cation exchanger. From this, there was a concern that the calcium alginate gel itself adsorbs cations such as iron and could not be separated well from uranium. Therefore, adsorption tests of iron and uranium were conducted from various concentrations of sulfuric acid.

  FIG. 3 is a graph showing the relationship between the uranium partition coefficient and the sulfuric acid concentration.

  The sulfuric acid conditions tested are in the concentration range expected for decontamination waste liquid. Due to the characteristics of the cation exchange reaction, the partition coefficient decreases as the acid concentration increases. In the sulfuric acid concentration range assumed as the decontamination waste liquid, the adsorption of iron ions by the cation exchange reaction of the calcium alginate gel is suppressed, while the adsorption reaction of uranium by the complex formation between TOPO and uranium can be confirmed. It was.

  The calcium alginate gel contains calcium and sodium as an impurity component. When the TOPO microcapsules used for the uranium adsorption treatment were pyrolyzed, they became residues, and there was a concern that the amount of uranium waste could not be suppressed much. On the other hand, as already mentioned, since the alginate gel itself is a cation exchanger, when it is brought into contact with the acidic waste liquid, the calcium ions and sodium ions originally contained cause an exchange reaction with the hydrogen ions in the solution. I could also expect the possibility that none of them would remain.

  Therefore, the inventors conducted a thermal decomposition test of the TOPO-encapsulated microcapsules. When 1 g of microcapsules not brought into contact with the acid solution was subjected to a thermal decomposition treatment at 500 ° C. for 30 minutes, about 4 mg of residue remained. On the other hand, the microcapsule sample in which the used microcapsule was simulated and contacted with the acid solution decreased to 0.4 mg or less due to thermal decomposition. This is because calcium ions and sodium ions originally contained are exchanged with hydrogen ions in the solution. From the above, it was found that TOPO microcapsules can be easily pyrolyzed, and when applied to the treatment of acidic waste liquid, the amount of final waste generated can be suppressed.

  Based on the above experimental studies, the inventors have found that the TOPO-encapsulated microcapsules selectively adsorb uranium at a practical adsorption rate, that the shape during the adsorption process is stable, and that the used microcapsules are It was found that the amount of waste can be reduced by thermal decomposition easily. At the same time, the microcapsule specifications were optimized. Next, a waste liquid treatment process using this microcapsule will be described.

  FIG. 4 is a diagram showing a system configuration of the waste liquid treatment system according to the present invention.

  This waste liquid treatment system includes a decontamination waste liquid supply tank 1, a pump 2, an adsorption tower 3, a waste liquid receiving tank 5, and a used resin receiving tank 6. The adsorption tower 3 is filled with the TOPO-containing microcapsules 4 according to the present invention.

  In order to treat the decontamination waste liquid, the decontamination waste liquid is transferred from the decontamination waste liquid supply tank 2 to the adsorption tower 3 by the pump 2, and an actinide such as uranium is adsorbed. The waste liquid exiting the adsorption tower is sent to the waste liquid receiving tank 5. The waste liquid in the waste liquid receiving tank 5 is neutralized after confirming the uranium concentration and, if necessary, collecting sulfuric acid in the sulfuric acid recovery step. The sludge generated at this time is separated and may be treated as non-radioactive waste containing no uranium. The supernatant liquid from which sludge is separated may be treated as non-radioactive waste liquid.

  The TOPO-containing microcapsules 4 are taken out from the adsorption tower when uranium is saturated. The used TOPO-containing microcapsules 4 are heat-treated at 500 ° C. for about 30 minutes in a pyrolysis furnace. Since the main component is uranium, the residue is treated as radioactive waste.

  The feasibility of waste liquid treatment by the adsorption tower was confirmed by a small-scale test. The adsorption tower had a diameter of 1 cm and a height of 10 cm, and was filled with about 4 g of TOPO microcapsules. The composition of the simulated decontamination waste liquid has a sulfuric acid concentration of 0.1 mol / L, a uranium concentration of 100 ppm, and an iron concentration of 1500 ppm.

  FIG. 5 is a diagram showing breakthrough curves when simulated decontamination waste liquid is passed through the adsorption tower at various feed rates by the column method. At a relatively slow but practical space velocity SV (Space Velocity) = 3, uranium was efficiently adsorbed, and 1 g of adsorbent was able to adsorb uranium in about 50 mL of waste liquid. The saturated adsorption capacity was about 6 mgU / g, which was equivalent to the existing ion exchange resin.

  According to the present embodiment, uranium can be selectively and efficiently separated from a sulfuric acid-based decontamination waste liquid in a wide concentration range. Since the adsorption method is applied, the separation equipment can be simplified as compared with the solvent extraction method. Moreover, the TOPO microcapsules can be easily thermally decomposed after being brought into contact with the acidic decontamination waste liquid, and no residue derived from the capsule components is produced. Thus, according to the separation and recovery method using the TOPO microcapsules of the present invention, the amount of uranium waste generated can be suppressed.

  As the organic compound, a phosphine oxide compound or a carbamoylphosphine oxide compound may be employed instead of TOPO.

It is a figure which shows the relationship between the distribution coefficient of uranium, and adsorption time by using the average particle diameter of microcapsules as a parameter. It is a figure which shows the relationship between the distribution coefficient of uranium, and the ratio of TOPO. It is a figure which shows the relationship between the distribution coefficient of uranium, and a sulfuric acid concentration. It is a figure which shows the system | strain structure of the waste liquid processing system by this invention. It is a figure which shows the breakthrough curve when simulated decontamination waste liquid is passed through the adsorption tower at various supply speeds by the column method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Decontamination waste liquid supply tank 2 Pump 3 Adsorption tower 4 TOPO inclusion microcapsule 5 Waste liquid receiving tank 6 Used resin receiving tank

Claims (6)

  An actinide adsorbent that contains an organic compound that selectively forms a complex with an actinide in the form of a solid powder in a calcium alginate gel carrier. In the actinide adsorbent according to claim 1,
An actinide adsorbent, wherein the organic compound is trioctylphosphine oxide. In the actinide adsorbent according to claim 2,
The actinide adsorbent has a particle size of 0.5 to 1 mm;
An actinide adsorbent characterized in that the encapsulated trioctylphosphine oxide is approximately 6% by weight. In the actinide adsorbent according to claim 1,
The actinide adsorbent, wherein the organic compound is a phosphine oxide compound or a carbamoylphosphine oxide compound. In the method for treating radioactive liquid waste containing actinide ions,
A method for treating a radioactive waste liquid, comprising passing the radioactive waste liquid through a column packed with the actinide adsorbent according to any one of claims 1 to 4 to adsorb actinide ions. In the processing method of the radioactive liquid waste of Claim 5,
A method for treating radioactive liquid waste, wherein a space velocity when the radioactive liquid waste is passed through a column packed with an actinide adsorbent is approximately 3.

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