JPS62176913A - Process for separation and recovery of cesium from treating liquid containing sodium salt - Google Patents

Abstract

PURPOSE:Cs is separated and recovered efficiently and inexpensively from acidic soln. contg. Cs by neutralizing the acidic soln. contg. Cs, removing generated precipitate, then allowing the filtrate to contact with an adsorbent comprising zinc ferrocyanate to adsorb Cs. CONSTITUTION:Acidic soln. contg. Cs such as waste liquid generated from a retreating facility of used nuclear fuel, etc., is neutralized by adding NaOH, Na2CO3, NaHCO3, or their mixture to the acidic soln. Generated insoluble compds. are then separated and removed, and the filtrate is allowed to contact with an adsorbent comprising zinc ferrocyanate at 1-10pH to adsorb Cs. Thus, Cs is separated and recovered. This process is particularly effective when the concn. of Cs is >=10<-3>M. When radioactive Cs having <=10<-3>M concn. is to be treated, the treatment may be more effective if non-radioactive Cs is added as carrier so as to increase the total Cs concn. to >=10<-3>M.

Description

[Detailed Description of the Invention] <Field of Application of the Invention> The present invention relates to a method for separating and recovering cesium from cesium-containing waste liquid, and particularly relates to a method for separating and recovering cesium from cesium-containing waste liquid, and in particular for high-level radioactivity generated from nuclear power-related facilities such as spent nuclear fuel reprocessing facilities. This paper relates to a method for separating and recovering radioactive cesium in waste liquid.

<Background of the Invention> The Burex method is becoming mainstream as a method for reprocessing spent nuclear fuel, but in the case of the final method, high-level radioactive waste liquid containing concentrated nitric acid (2 to 4 M) is generated during processing.

In addition, high-level radioactive waste liquid may also be generated by concentrating medium-level radioactive waste liquid from each process in a reprocessing plant. Safe treatment and disposal methods for high-level radioactive waste liquid have not yet been established, and this is a major obstacle to completing the nuclear fuel cycle. At present, the most promising treatment method is actively developing a treatment method that converts high-level radioactive waste liquid into a vitrified substance, but the final treatment method has the following drawbacks.

First, radioactive cesium such as all, IB7, c, and +u easily evaporates below the vitrification treatment temperature, making it difficult to deal with this problem. Furthermore, it is difficult to completely suppress the elution of radionuclides from the vitrified material during flooding, and radioactive cesium is the nuclide that is most easily eluted. moreover,
Strong radioactivity and a large amount of heat generation are major factors that make it difficult to pre-evaluate the safety of vitrified materials during handling and transportation, long-term storage, and storage stability. This should be considerably reduced if radioactive strontium and cesium, which account for more than 90% of the total radioactivity and heat generated, can be removed in advance. In addition, these radionuclides have great industrial utility value, and radioactive cesium in particular is expected to be in great demand in the future as an irradiation source for sterilizing surplus sludge from activated sludge treatment of urban sewage and other sources, as well as for sterilizing various foods.

Based on the above background, a method for pre-separating radioactive strontium and cesium from high-level radioactive waste liquid has recently attracted attention (for example, "input na alterna").
tive strategy for comm
ercia+high-1 level radio-a
tive management-, IAEA-5
M-261734 (1983)).

As a method for removing cesium from high-level radioactive waste liquid,
Adsorption methods using inorganic ion exchangers or selective ion exchange resins, solvent extraction methods using crown ether, coprecipitation methods using a combination of heavy metal salts and soluble ferrocyanide or ferricyanide salts, chemical treatment methods using cesium precipitation reagents, etc. It is publicly known.

Among these, conventional development has focused on inorganic ion exchangers such as zeolite, ammonium phosphomolybdate, and insoluble ferrocyanide in terms of decontamination coefficient (initial concentration 10 concentration after decontamination), radiation resistance, and fecal resistance. It's been directed at me. However, these inorganic ion exchangers do not have sufficient decontamination coefficients or adsorption amounts in the batch method due to interference from various fission products and other dissolved components of heavy metals dissolved in the concentrated nitric acid waste solution, and the column method There are many problems in practical application, such as the problem of particle size.

On the other hand, recently, a method for separating radioactive cesium from concentrated nitric acid waste solution, rather than directly, but after neutralizing it with sodium hydroxide, is moving toward practical use.
t and eonditioning of waste
es from nuclear fuel p
rocessing plants in
the United States of
America-, IAEA-CM-437187 (1
984)).

The advantage of this method is that the dissolved heavy metal components that interfere with cesium separation are separated as a small amount of high-level radioactive waste consisting of hydroxide precipitates, and the radioactive cesium remaining in the treated solution is removed.
By removing strontium, plutonium, and technetium, dissolved salts can be disposed of as low-level radioactive waste. Furthermore, there is the advantage that the difficulties associated with vitrification treatment can be eliminated or alleviated by effectively utilizing the recovered radioactive cesium for various purposes or by separately disposing of it as a stable solidified material such as a ceramic solidified product.

In the case of this method, radioactive cesium is removed using an ion exchange method using a special selective ion exchange resin.
anees in Ceramics, Volume 8, p.
183, 92, 1984) and the precipitation method using concentrated sodium tetraphenylborate as a precipitant (lteport
DP-MS-82-46, 1982) has been developed. Of these, the ion exchange method has drawbacks in terms of resin stability under high radioactivity and complicated column operations, and is not suitable for removing high concentrations of radioactive cesium. In addition, in the case of concentrated sodium tetraphenylboronate, potassium, which coexists in a relatively large amount, must be quantitatively precipitated, which requires a large amount of extremely expensive reagent. Due to the low power, etc., it is not suitable for removing high concentrations of radioactive cesium. In addition, if a large amount of potassium co-precipitates and the sodium concentration is less than 2M, cesium will re-elute from the precipitate, so it is necessary to include a sodium salt of 2M or more as a washing solution for the precipitate. There were some difficulties in raising the degree. However, on the other hand, each of the above methods has a very high decontamination coefficient of 10" to 106 or more.
There are many advantages.

Purpose of the present invention> Since the concentration of cesium in actual high-level radioactive waste liquids is quite high (maximum 4 x 10 M when the cooling period is short), the present invention uses the above-mentioned ion exchange method and tetraphenylboron. This provides a method to efficiently separate and recover most of the radioactive cesium in a concentrated state before treatment using a known method with a high decontamination coefficient such as the concentrated sodium method.

<Configuration of the Invention> The present invention achieves the above objects by using zinc dicyanide as an adsorbent for removing relatively high concentration of cesium ions contained in solutions containing sodium salts such as sodium nitrate. 4′! Make it moldy.

It is generally known that the heavy metal ferrocyanide easily adsorbs cesium ions, but as the coexisting concentration of hydrogen ions or sodium ions increases, the distribution coefficient Kd (
Adsorbed cesium 1 per gram of adsorbent! to/solution 1rn1
It was thought that if the residual cesium per unit area gradually decreased and the sodium ion concentration increased to IM, it would decrease to 104 or less (Radiochemical and
Radioanilytical Letters.

Volume 42, 9329-340.1980). The present inventors ζ have discovered that zinc ferrocyanide exhibits a Kd value of 104 or higher even in the coexistence of 3M sodium nitrate, and has properties that are not found in ordinary adsorbents, such as a region in which the Kd value increases as the amount of cesium adsorbed increases. Heading, we arrived at the present invention.

The features of the present invention will be explained in detail using curves showing the relationship between the Kd value and the amount of cesium adsorbed in FIGS. 1 to 3.

In addition to zinc ferrocyanide, potassium zinc ferrocyanide was used as an adsorbent for comparison. The adsorption experiment consisted of 2
This was done by shaking at 5°C for 7 days. In the figure, f represents the ratio of the liquid amount (rni) to the weight (g) of the adsorbent. Figure 1 shows 3M using zinc ferrocyanide as an adsorbent.
This is a case where cesium is adsorbed from a sodium nitrate solution. The Kd value is small when the adsorption amount is low, but increases rapidly as it increases, and in the case of f=1000, it showed 104 or more when the adsorption amount was 03 to 4 meq/g. Figure 2 shows the case where cesium is adsorbed from an aqueous solution. When compared with FIG. 1, the Kd value shows a slightly higher value, but the trends are very similar, including differences depending on the f value. On the other hand, FIG. 3 shows the case where cesium was adsorbed from a 3M sodium nitrate solution using potassium zinc ferrocyanide as an adsorbent. The Kd value is significantly lower overall than in FIG. 1 except for the region where the amount of adsorption is small.

However, when the adsorption amount is low, the Kd value is high and approaches a constant value, and no difference in Kd value due to f is observed, indicating the normal behavior of an adsorbent. As mentioned above, zinc ferrocyanide exhibits an abnormal adsorption behavior that is unimaginable according to conventional common sense, and in particular, it exhibits an abnormally large adsorption force in areas where the adsorption amount force 9 is large. Compared to zinc potassium ferrocyanide, which was considered to be a typical cesium adsorbent, the adsorption amount is 3 meq/
It was found that the adsorption force was about 100 times that of ζ at around g, which was found to be very preferable for the purpose of the present invention. The cause of the above abnormality is not clear, but in this case it is not adsorption due to simple ion exchange, but an intermediate compound is generated between cesium and zinc ferrocyanide, which has a strong adsorption force against cesium. It is also possible to show it.

In the present invention, the adsorbent made of zinc ferrocyanide is prepared by zinc ferrocyanide powder or its granules, a suspension of zinc ferrocyanide, or by precipitating zinc ferrocyanide in a porous carrier. Adsorbents and the like are preferably used.

Zinc ferrocyanide 1. T,! It is usually prepared by bringing ferrocyanide ions and zinc ions into contact in an aqueous solution, but it is known that when potassium ions coexist, zinc potassium ferrocyanide tends to be preferentially produced. On the other hand, the present inventors have found that when ferrocyanide ions are reduced in the presence of zinc ions, zinc ferrocyanide is easily produced even in the presence of potassium ions. Therefore, the precipitate produced by the treatment of iron cyano complex-containing wastewater according to the method of the present inventors, such as Japanese Patent Publication No. 57-5598, can be utilized.

In the present invention, when the cesium concentration is 10 M or more, the cesium removal rate and the amount of cesium adsorbed per unit weight of zinc ferrocyanide increase, allowing efficient treatment. In addition, to remove radioactive cesium below 10-3M,
Using non-radioactive cesium as a carrier, the total cesium concentration is 10
It is effective to add 3M or more.

As the coexisting concentration of sodium increases, the Kd value gradually decreases, but remains sufficiently high up to around 5M.

In the case of Burex waste liquid, the sodium concentration can be kept sufficiently low by decomposing a portion of the nitric acid (denitrification) before neutralization.

In order to avoid decomposition of zinc ferrocyanide under alkaline conditions and oxidation by nitric acid under acidic conditions, the pH of the solution is preferably set in the range of 1 to 10 during adsorption treatment.

Solid-liquid separation is relatively easy, and natural sedimentation, filtration, centrifugation, etc. can be applied.

When carrying out batch processing under the above conditions in the method of the present invention, when the cesium concentration is 1 x 10 to 4 x lOM, by appropriately setting the amount of zinc ferrocyanide added in the range of 1 to 10 g/l, The recovery rate, residual concentration, and adsorption amount of cesium from a 3M sodium ion-containing solution were 9 each.
0~99J1xlO~1.5xlOM, 1~4maq/
It becomes. If the residual concentration is high, if the treatment solution is subjected to batch processing again, even if the initial concentration is 4 x 10-''M, the residual concentration can be lowered to 1xlO=M or less by performing the batch processing twice.

Regarding the effects of coexisting salts other than sodium ions, anions have almost no effect, and hydrogen ions, alkali metal ions, and alkaline earth metal ions can be ignored if they are very small compared to sodium ions. The most sensitive heavy metals are advantageously already removed by a neutralization precipitation process. Further, it is clear from known knowledge that zinc ferrocyanide has radiation resistance and that there is almost no difference in adsorption behavior between ordinary cesium ions and its isotope ions. Therefore, the method of the present invention provides Cs Xc in radioactive waste liquid,
It can also be applied to cesium isotopes such as below,
The present invention will be specifically explained based on examples, but the optimum conditions based on the present invention should be determined depending on the individual circumstances due to the nature of the target waste liquid. Of course, the present invention is not limited to this example.

Example 1 Zn2 Fe (CN) 0.01~O,I in terms of 6
g of 7 z O zinc cyanide in an Erlenmeyer flask with a screw cap (
25mt'), and add 1xl of cesium chloride to it.
Q-1~5xlO-''M 3M HaNO3 aqueous solution 1
0- was added and shaken at 25°C for 7 days. Note that zinc ferrocyanide was precipitated from an aqueous solution using sodium ferrocyanide and zinc nitrate in a conventional manner, washed with water, and air-dried. After the contents were quickly filtered with a membrane filter, the residual cesium concentration in the filtrate was analyzed. Table 1 shows the cesium recovery rate and cesium adsorption amount calculated based on the results, and FIG. 1 shows the relationship between the distribution coefficient (Kd) and the cesium adsorption amount.

Note that the adsorption amount and Kd value were calculated according to the following formula.

Adsorption amount = (] Initial cesium concentration (M) I - [Residual cesium concentration (M) I) f Table 1 Reference example 1 Same ferrocyan as in Example 1 with 001 to 01 g in terms of Zn2 Fe (CM) 6 Weigh out zinc chloride into an Erlenmeyer flask (25 mJ) with a screw cap, and add 1 l of cesium chloride to it.
An aqueous solution 10- containing 2M of After the contents were quickly filtered with a membrane filter, the residual cesium concentration in the 'a liquid was analyzed. The distribution coefficient (K
Figure 2 shows the relationship between dl and the amount of cesium adsorbed. As shown in the figure, the Kd value is small where the amount of adsorption is low, and conventional wisdom shows that zinc ferrocyanide is not an excellent adsorbent. On the other hand, in areas where the amount of adsorption is large, there are regions that surprisingly exhibit very large Kd values, and as shown in FIG. 1, it is recognized that a similar tendency is exhibited even in the coexistence of sodium nitrate. As a result, for example, the Kd value when adsorbing 2.5+ aeq/g of cesium from a 3M sodium nitrate solution is larger than that when the amount of cesium adsorbed from an aqueous solution is 0.5 meq/g or less. It can be seen that zinc chloride exhibits a high adsorption capacity that is unimaginable according to conventional wisdom in the region of high adsorption amount.

Comparative Example 1 Zinc potassium ferrocyanide air-dried product 0.01-012g
into an Erlenmeyer flask (25rni) with a screw cap, and add 1 x 10-4 to 6 x 10-" of cesium chloride to it.
Potassium zinc ferrocyanide was precipitated from an aqueous solution by a conventional method using potassium ferrocyanide and zinc sulfate, washed with water, and air-dried. After the contents were quickly filtered with a membrane filter, the residual cesium concentration in the filtrate was analyzed. FIG. 3 shows the relationship between the distribution coefficient (Kd) calculated based on the results and the amount of cesium adsorbed. In addition, when calculating the amount of cesium adsorption and Kd, the weight of potassium zinc ferrocyanide is calculated based on the weight of potassium zinc ferrocyanide, which is equivalent to the contained Fe(CM)- and the weight of potassium zinc ferrocyanide.
(CM), and the same applies to the following comparative examples.

Example 2 Zn2-Fe(CM)6 converted to 00
1 g of the same zinc ferrocyanide as in Example 1 was weighed into an Erlenmeyer flask (25-) with a screw cap, and a 1M NaNO3 aqueous solution 10- containing IX10=M of cesium chloride was added thereto, followed by shaking at 25°C. In the same manner, a case where sodium nitrate was not included was also tested. After shaking for a specified time,
The contents were quickly filtered with a membrane filter, and the residual cesium concentration in the filtrate was analyzed. Figure 4 shows the change in cesium adsorption rate over time. From the figure, it can be seen that almost equilibrium was reached in 2 days, and the adsorption rate was higher in the presence of sodium ions. In addition, the Kd value at equilibrium is NaNO3
5. for the aqueous solution not containing and the 1M NaNO3 aqueous solution, respectively. 08xlO", 4. 02xlO÷-
It was found that the difference was small.

Comparative Example 2 001 g of each of the typical cesium adsorbent, zinc potassium ferrocyanide, commercially available ammonium phosphomolybutate (manufactured by Kanto Kagaku Co., Ltd.) or commercially available mordenite (manufactured by Two-Tone Co., Ltd., 100Na) as in Comparative Example 1 was charged with a screw cap. Weigh into an Erlenmeyer flask (25+nt') and add 1x cesium chloride to it.
Add 10− of 1M NaNO3 aqueous solution containing 10M,
Shake at 25°C until equilibrium is reached.

In the same manner, a case where sodium nitrate was not included was also tested. The contents were filtered with a membrane filter, and the residual cesium concentration in the filtrate was analyzed.

Table 2 shows the Kd values calculated based on the results.

From a comparison with Example 1, it was found that these adsorbents tend to lose their cesium adsorption power in the coexistence of sodium nitrate, and their performance is considerably inferior to that of zinc ferrocyanide.

Table 2 Table 3 Example 3 0.01 g of zinc ferrocyanide in terms of Zn2 Fe (CN) 5 was placed in an Erlenmeyer flask with a screw cap (25 rnl).
The total liquid volume was 10-'M, and the concentrations of cesium chloride and sodium nitrate were each 3 x 10-'M, 0.5
Add a mixed aqueous solution of both salts so that M
Shake for days. As zinc ferrocyanide, those shown in Table 3 with different preparation methods were used. In addition, since ZrO2 is mixed with ferrocyanide of heavy metals other than zinc,
The amount added was determined by analyzing the amount of Fa(CN),'- contained and expressed as the equivalent amount of Zn2Fe(CM)6. Moreover, ZFC2-4 was used in the state of a suspension. After the contents were quickly filtered with a membrane filter, the residual cesium concentration in the filtrate was analyzed. The Kd value was calculated based on the results and is shown in the solid curve in FIG.

Comparative Example 3 Same as Comparative Example 1, air-dried potassium zinc ferrocyanide 0
．． 0118 g of NaNo, 5 was weighed into an Erlenmeyer flask (25 mil) with a screw cap, and a 0-5 MNaNo, 5 aqueous solution 10- containing 3xlOM of cesium chloride was added thereto, and the mixture was shaken at 25°C for 7 days. After the contents were quickly filtered with a membrane filter, the residual cesium concentration in the filtrate was analyzed.

The Kd value was calculated based on the results, and the relationship with the sodium nitrate concentration is shown in the broken line curve in FIG.

Example 4 The same zinc ferrocyanide as in Example 1, weighing 0.01 g in terms of ZnzFe (CM) 6, was seized in an Erlenmeyer flask (25 m7) with a screw cap, and a simulated waste liquid 10- was added and shaken at 25°C for a predetermined time. The contents were quickly filtered with a membrane filter, and the residual cesium concentration in i* was analyzed. Cesium recovery rate after a predetermined period of time calculated based on the results, CE Because the adsorption amount of cesium ions is extremely large even in batch adsorption treatment, for example, the amount of adsorption of cesium ions in high-concentration sodium nitrate solutions generated during the treatment of high-level radioactive waste liquids with a cesium concentration of 10 M or more generated from Burex reprocessing facilities. It is possible to efficiently separate and recover radioactive cesium in a concentrated state using an extremely inexpensive adsorbent, making it easier to treat high-level radioactive waste liquid, and making effective use of radioactive cesium, which is expected to have great effects. You can do it.

[Brief explanation of drawings]

Figures 1 to 3 are drawings for explaining the present invention in detail, and Figures 1 and 2 show the relationship between Kd value and cesium adsorption amount when zinc ferrocyanide is used. Figure 3 shows the results in the presence and absence of sodium nitrate (3M), respectively. For comparison, Figure 3 shows the relationship between Kd value and cesium adsorption amount when zinc potassium ferrocyanide is used. The results are shown in the presence of FIG. 4 is a diagram showing the influence of the coexistence of sodium nitrate on the cesium adsorption rate when using the method of the present invention. FIG. 5 is a diagram comparing the influence of the coexisting sodium nitrate concentration when cesium is adsorbed by the method of the present invention using zinc ferrocyanide prepared in a different manner, compared to when potassium zinc ferrocyanide is used. Pole 3 figure begging umushi” Quantity (meq/g) Certain S1 figure NaNO2 (M) January 8, 1986

Claims (1)

[Claims] (1) After neutralizing the cesium-containing acidic solution by adding sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, or a mixture thereof to remove the heavy metals contained therein as insoluble compounds, the treated solution is In the method of separating and recovering cesium remaining in
Method for separating and recovering cesium from a sodium salt-containing treatment solution, characterized by adsorbing cesium by contacting with an adsorbent made of zinc ferrocyanide in step 10 (2) Cesium concentration in the treatment solution after neutralization treatment is 10＾-
(3) The method according to claim 1 in which the cesium in the cesium-containing acidic solution is C_S^1^3^7, C
Method (4) of claim 1 which is a cesium isotope such as _S^1^3^4. Neutralization by adding sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, or a mixture thereof to an acidic solution containing radioactive cesium. In the method of separating and recovering the radioactive cesium remaining in the treatment solution after treatment and removing the contained heavy metals as insoluble compounds, non-concentration is performed so that the total cesium concentration in the treatment solution is 10^-^3M or more. After adding radioactive cesium as a carrier, pH 1
A method for separating and recovering radioactive cesium from a sodium salt-containing treatment solution, characterized by adsorbing radioactive cesium by bringing it into contact with an adsorbent made of zinc ferrocyanide in steps 10 to 10. (5) Zinc ferrocyanide contains zinc ions The method according to claim 1, which is a precipitate produced by reacting ferricyanide with a reducing agent as described below.