A method for separating and purifying rare earth elements in high-level radioactive liquid waste

Abstract

The present invention relates to the field of nuclear fuel cycle and waste liquid treatment, and specifically to a method for separating and purifying rare earth elements in high-level waste liquid. The present invention mainly comprises the following operating steps: adding ammonium formate salt to high-level waste liquid to generate rare earth element formate precipitate, and separating rare earth elements from high-level waste liquid by filtering the precipitate. The obtained rare earth element formate is suspended in a formic acid aqueous solution, stirred and slurried, so that the actinide formate and other impurities with high solubility are transferred to the liquid phase, filtered to obtain the rare earth element formate solid, and the actinide formate and other impurities are retained in the mother liquor, thereby purifying the rare earth elements. The present invention is simple to operate, has a stable process, and uses cheap and radiation-resistant raw materials, is easy to industrialize, and can be used for the treatment of nuclear waste high-level waste liquid.

Description

Technical Field

[0001] The invention relates to the field of nuclear fuel circulation and waste liquid treatment, and in particular to a method for separating and purifying rare earth elements in high-level waste liquid. Background Art

[0002] The biggest problem with nuclear energy is that it produces a large amount of spent fuel. If it is not properly handled, highly toxic radioactive elements will be released into the environment, which will have an impact on humans for hundreds to hundreds of thousands of years. Therefore, the issue of spent fuel disposal has become a focus of worldwide attention.

[0003] The mature primary treatment process for spent fuel is to dissolve it with nitric acid, then use tributyl phosphate to extract and recover the unfissioned uranium and byproduct plutonium, and re-make nuclear fuel for reuse. This is the PUREX process. The waste liquid produced after the PUREX process contains nitric acid and a large amount of fission elements. It has a complex composition and is difficult to treat. It is called high-level waste liquid and is a hot topic of research around the world. The main components are as follows:

[0004] Table 1 Main elements in spent fuel Burnup depth 33000MWd / t (low enriched uranium)

[0005]

[0006] The current treatment method for high-level radioactive liquid waste is to vitrify the residue obtained after vaporizing nitric acid and water in the waste at about 1000℃ and bury it underground for long-term storage. Since high-level radioactive liquid waste contains long-lived α nuclides, such as actinides such as americium and curium, their half-lives are long, and it takes hundreds of thousands of years for the radioactivity to be reduced to a level that is harmless to the environment and humans. Therefore, the main treatment goals for high-level radioactive liquid waste are as follows: the first goal is to reduce the amount of high-level radioactive waste, and the second goal is to separate the long-lived α nuclides, and then transform the α nuclides with a small proportion but high toxicity, actinides, into stable or short-lived nuclides through the transmutation process. Since high-level radioactive liquid waste contains a high proportion of rare earths, rare earth elements have a large neutron absorption cross-section, which seriously affects the transmutation of actinides. Therefore, the separation of rare earth elements and actinides is the key link to achieve separation and transmutation. The physical and chemical properties of actinides and rare earth elements are very similar, and the separation of the two is a major problem in nuclear waste treatment.

[0007] Among the studies after the PUREX process, the more mature one is the TRPO process of Tsinghua University: using trialkyl (C6-C8) oxyphosphorus extractants, rare earth elements and actinides are extracted simultaneously and the waste liquid is solidified, and then the extracted rare earth and actinide mixture is further separated. Similar to the TRPO process are the CMPO process of the United States and the CMP process of the China Institute of Atomic Energy, but these processes cannot achieve the separation of rare earth and actinide elements, and currently cannot be applied on a large scale. In addition, the current research direction of research institutions is mainly focused on screening new extractants. Patents CN104894372, CN104928477, and CN114107667A disclose related new extractants, but these new extractants have problems such as complex molecular structure, high manufacturing cost, and difficulty in purification. They will also degrade due to the high radiation environment during the separation process, which increases the difficulty of extraction and is difficult to put into practical application; the high-level waste liquid treatment technology is still under continuous exploration. Summary of the invention

[0008] The object of the present invention is to provide a method for separating and purifying rare earth elements in high-level radioactive waste liquid to solve the problems raised in the above-mentioned background technology.

[0009] In order to solve the above technical problems, the present invention provides the following technical solution: a method for separating and purifying rare earth elements in high-level radioactive liquid waste, comprising the following steps:

[0010] Step 1: adding ammonium formate salt to high-level radioactive waste;

[0011] Step 2: heating and stirring to react to generate rare earth element formate;

[0012] Step 3: Filter to obtain rare earth element formate solid, thereby separating rare earth elements from high-level waste liquid;

[0013] Step 4: The rare earth element formate is suspended in a formic acid aqueous solution, stirred and slurried, and the actinide element formate and other impurities are transferred to the mother liquor;

[0014] Step 5: Filter to obtain purified rare earth element formate solid, thereby achieving separation and purification of rare earth elements from high-level radioactive waste liquid.

[0015] Furthermore, in step 1, the rare earth element is any one or more of yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), and dysprosium (Dy).

[0016] Furthermore, in step 1, the ammonium formate salt is any one of tetramethylammonium formate, tetraethylammonium formate, tetrapropylammonium formate, and tetrabutylammonium formate.

[0017] Furthermore, in step 4, the concentration of the formic acid aqueous solution is 1%-99%.

[0018] Furthermore, in step 4, the preferred concentration of the formic acid aqueous solution is 50%.

[0019] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: the purpose of the present invention is to provide a method for separating and purifying rare earth elements in high-level waste liquid, and to separate rare earth elements from high-level waste liquid by utilizing the low solubility of rare earth element formate in water. The separated rare earth elements do not contain actinides and can be treated as short-lived nuclear waste. This method can provide convenience for subsequent treatment processes. Since most of the rare earth elements have been separated, the burden of extracting actinides in the subsequent TRPO process is reduced. If the separated waste liquid is vitrified at high temperature, it can play a role in reducing the amount of high-level waste. The ammonium formate salt can be decomposed into gaseous products before vitrification, and the amount of radioactive waste will not be increased. In addition, formic acid is cheap, easy to recycle, and can withstand radioactive irradiation. Therefore, the process has a high degree of stability, high separation efficiency, and good application prospects. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] The specific preparation method of simulated high-level liquid waste A is as follows:

[0022] Table 2 Ingredients of simulated high-level liquid waste A

[0023] Raw material name Added amount (based on elements) Raw material name Added amount (based on elements) Nitric acid(70%) 36g(0.4mol) Gadolinium Oxide 0.12g Lanthanum Oxide 1.3g Rubidium nitrate 0.35g(4mmol) Cerium carbonate 2.8g Cesium carbonate 2.6 g (19 mmol) Praseodymium carbonate 1.2g Strontium carbonate 0.85g(10mmol) Neodymium Oxide 4.2g Barium Hydroxide 1.5g(11mmol) Samarium Oxide 0.84g Hydroiodic acid (50%) 0.3g Europium Oxide 0.18g water 150g

[0024] According to the ingredient list in Table 2, 36g of nitric acid (70wt.%) was added to the raw materials in the ingredient list, mixed and stirred to dissolve, and then an appropriate amount of water was added to dilute to 200g to obtain simulated high-level radioactive waste liquid A; the waste liquid contained 74mmol of rare earth elements and 15ppm of uranium (the content was relative rare earth elements, from rare earth carbonates and rare earth oxides). The above raw materials were purchased from Aladdin Reagent.

[0025] Embodiment 1:

[0026] Step 1: 19.3 g of tetramethylammonium hydroxide and 9.2 g of formic acid are used to prepare an ammonium formate solution, and the ammonium formate solution is slowly added dropwise to 25 g of simulated high-level waste liquid A to obtain a mixed solution, wherein the dropping time is 30 min;

[0027] Step 2: Heat the mixed solution to 90°C and stir to react for 1 hour;

[0028] Step 3: Cool to room temperature, filter to obtain solid powder, and dry at 150° C. to obtain 2.3 g of mixed rare earth element formate (theoretical output value: 2.56 g), with a yield of 89.8%.

[0029] Embodiment 2:

[0030] Step 1: 34 g of tetraethylammonium hydroxide and 9.2 g of formic acid are used to prepare an ammonium formate solution, and the ammonium formate solution is slowly added dropwise to 25 g of simulated high-level waste liquid A to obtain a mixed solution, wherein the adding time is 30 min;

[0031] Step 2: Heat the mixed solution to 95°C and stir to react for 1 hour;

[0032] Step 3: Cool to room temperature, filter to obtain solid powder, and dry at 150°C to obtain 2.2 g of mixed rare earth element formate (theoretical output value: 2.56 g), with a yield of 86%.

[0033] Table 3

[0034] Sample name Uranium content (relative to total rare earth elements) Simulated high-level liquid waste A 15ppm Example 1 1.2ppm Example 2 1.2ppm

[0035] in conclusion:

[0036] The data of Examples 1-2 show that the use of ammonium formate as a precipitant can achieve the separation of rare earth elements from high-level radioactive liquid waste.

[0037] Embodiment 3:

[0038] Take 2 g of the mixed rare earth element formate separated in Example 1, add 30 ml of formic acid solution (1 wt.%), heat to 90° C., and stir for 2 hours; cool to room temperature, filter and dry at 150° C. to obtain 1.81 g of purified rare earth element formate solid, with a yield of 90.5%.

[0039] Embodiment 4:

[0040] Take 2 g of the mixed rare earth element formate separated in Example 1, add 30 ml of formic acid solution (50 wt.%), heat to 95° C., and stir for 2 hours; cool to room temperature, filter and dry at 150° C. to obtain 1.85 g of purified rare earth element formate solid, with a yield of 92.5%.

[0041] Embodiment 5:

[0042] Take 2 g of the mixed rare earth element formate separated in Example 1, add 30 ml of formic acid solution (80 wt.%), heat to 100° C., and stir for 2 hours; cool to room temperature, filter and dry at 150° C. to obtain 1.9 g of purified rare earth element formate solid, with a yield of 95%.

[0043] The rare earth element formate solid obtained in Examples 3-5 was sampled, dissolved with nitric acid and the uranium content was detected using ICP-MS. The experimental results are shown in Table 4 below.

[0044] Table 4

[0045] Sample name Uranium content (relative to total rare earth elements) Example 3 0.11ppm Example 4 0.10ppm Example 5 0.08ppm

[0046] Conclusion: The data of Examples 3-5 show that the use of aqueous formic acid solution can be used to purify rare earth element formate solids, and the purification effect is best when the concentration of the aqueous formic acid solution is 80wt.%.

[0047] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for purifying rare earth elements in high-level radioactive liquid waste, characterized in that: The following steps are involved: Step 1: adding ammonium formate salt to high-level radioactive waste liquid containing rare earth elements and actinide elements; the ammonium formate salt is any one of tetramethylammonium formate, tetraethylammonium formate, tetrapropylammonium formate, and tetrabutylammonium formate; Step 2: stirring the reaction, filtering, and drying to obtain a rare earth element formate solid; Step 3: Purify the rare earth element formate solid obtained in step 2.

2. The method for purifying rare earth elements in high-level radioactive liquid waste according to claim 1, characterized in that: In step 1, the rare earth element is any one or more of yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, and dysprosium.

3. The method for purifying rare earth elements in high-level liquid waste according to claim 1, characterized in that: In step 2, the reaction temperature is 90-100°C.

4. The method for purifying rare earth elements in high-level radioactive liquid waste according to claim 1, characterized in that: In step 3, the method for purifying the rare earth element formate solid comprises the following steps: S1: suspending a rare earth element formate solid in a formic acid aqueous solution, heating and stirring; the formic acid aqueous solution has a mass concentration of 1-80%; S2: Filter and dry to obtain the purified rare earth element formate solid.

5. The method for purifying rare earth elements in high-level radioactive liquid waste according to claim 4, characterized in that: In S1, the stirring temperature is 90-100°C.

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