Method for fission nuclide separation and LiCl recovery of fused salt electrolysis process post-treatment waste

Abstract

The invention discloses a fission nuclide separation and LiCl recovery method for post-treatment waste of a fused salt electrolysis method, relates to the technical field of nuclear waste treatment, and innovatively provides a fused salt nuclide gradient separation strategy based on the specific formation characteristic of halide perovskite and the solubility difference of chlorine salt in an organic solvent. The method comprises the following steps: separating monovalent Cs < + > and trivalent rare earth fission products Re < 3 + > in a halide perovskite form, separating bivalent Sr < 2 + > element in a strontium carbonate precipitation form, separating LiCl through a fractional dissolution method by taking an organic solvent as a separation medium, and synchronously separating KCl by adopting an evaporative crystallization process, thereby realizing step-by-step separation of the monovalent Cs < + >, bivalent Sr < 2 + > and trivalent Nd < 3 + > fission products in a molten salt system after electrolytic refining. And efficient recovery of eutectic molten salt LiCl and KCl is synchronously completed.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear waste treatment, and particularly relates to a method for separating fission nuclides from waste by molten salt electrolysis and recovering LiCl. Background Art

[0002] As a low-carbon and efficient basic energy source, nuclear energy is an important pillar of the global energy transformation and has important strategic significance in addressing climate change, reducing dependence on fossil energy, and ensuring energy security. According to the rapid development plan of nuclear power, it is optimistically estimated that by 2050, the installed nuclear power capacity in China will reach 4×10⁵ Mwe, accounting for 15% of the total power generation. The annual output of spent fuel is expected to reach 7700 tHM, and the off-reactor storage demand for spent fuel will increase to approximately 20000 tHM, far exceeding the existing storage capacity. Spent fuel has characteristics such as high radioactivity and decay heat release. If no management measures are taken for the discharged spent fuel, it will cause serious harm to the environment and human health. Therefore, the prerequisite for large-scale development and utilization of nuclear energy includes not only a long-term and reliable nuclear fuel supply but also the ability to safely manage and dispose of a large amount of radioactive waste.

[0003] Spent fuel reprocessing is to process the nuclear fuel discharged from the reactor, extract uranium and plutonium materials to make new nuclear fuel, and vitrify the high-level radioactive waste and send it to deep geological disposal. The use of reprocessing can greatly improve the utilization rate of uranium resources, significantly reduce the disposal cost of high-level waste, and shorten the supervision time.

[0004] Among the spent fuel reprocessing methods, the molten salt electrolysis dry reprocessing technology shows significant engineering advantages compared with the traditional PUREX aqueous reprocessing process, including simplified process flow, high operation safety, and less radioactive waste production. It is considered to be the core link and key technology of the future advanced nuclear fuel cycle system.

[0005] The metallurgical process of electrolyzing a molten metal salt electrolyte to extract and purify metals by converting electrical energy into chemical energy is called molten salt electrolysis. The molten salt electrolysis dry reprocessing usually uses lithium chloride-potassium chloride (LiCl-KCl) eutectic salt as the electrolyte salt. With electrolytic refining as the core, it combines electrolytic reduction and extraction to separate actinides from fission products, selectively deposits and recovers uranium (U), plutonium (Pu), and minor actinides at the cathode to achieve a closed-loop fuel cycle. Fission products such as cesium (Cs), strontium (Sr), and rare earth elements (RE) will dissolve in the eutectic molten salt in the form of chlorides.

[0006] The dry reprocessing technology of molten salt electrolysis can not only effectively improve the utilization efficiency of nuclear fuel, but also significantly reduce the production of radioactive waste. However, after processing multiple batches of spent fuel, the enrichment degree of fission products in the molten salt reaches 15-20 wt%, resulting in changes in the properties of the eutectic molten salt. At this time, the recovery efficiency of U and Pu elements decreases, and the recovery quality deteriorates, so it needs to be treated according to the method of radioactive nuclear waste disposal.

[0007] There are mainly two types of nuclear waste in the dry reprocessing of spent fuel, namely metal waste and molten salt waste. Metal waste mainly includes cladding materials and undissolved noble metal fission materials; molten salt waste is mainly chloride molten salt containing alkali metals, alkaline earth metals, rare earth elements and minor actinides. Metal waste is relatively stable and the treatment method is relatively simple. It can be vitrified or a suitable amount of metal Zr can be added to form a eutectic Zr alloy with the metal waste for deep geological disposal. Molten salt waste mainly comes from the waste salt after electrolytic reduction, electrolytic refining and electrolytic extraction. The waste salt contains long-lived radioactive fission products and a large amount of chloride ions, and its structure is unstable. It needs to be transformed into a stable form of substance before geological disposal.

[0008] Table 1 below lists the advantages and disadvantages of different separation and purification technologies for fission nuclides in molten salt waste. The separation methods for rare earth fission nuclides mainly include oxidation precipitation method, carbonate precipitation method, phosphate precipitation method and ion exchange method. Precipitation methods generally have the advantages of simple operation and high efficiency, but there are many limiting conditions in the separation process; Cs + 、Sr 2+ etc. are mainly separated by zone melting crystallization method, ion exchange method and carbonate precipitation method. These methods generally require controlling separation conditions and time; the separation process of halides is mainly to remove chlorine by adding foreign chemical components, and the volatilized chlorine also needs to be further adsorbed and treated with alkali metals, with complex operation and long time consumption.

[0009] Table 1 Separation technology Separation object Advantages Disadvantages Oxidation precipitation <![CDATA[RE 3+ > Does not introduce new chemical components and has high purification efficiency High-temperature environment with high operation risk; unable to separate Cs and Sr Phosphate precipitation <![CDATA[RE 3+ 、Cs + 、Sr 2+ > Fast reaction time and high efficiency Will introduce phosphate impurities; low separation efficiency of Cs and Sr Carbonate precipitation <![CDATA[AE 2+ 、RE 3+ > Does not introduce new chemical components and can precipitate alkaline earth metals <![CDATA[The separation process requires strict control of the carbonate addition amount; generating CO 2 waste gas]]> Vacuum distillation <![CDATA[Cs + , RE 3+ > Simple and efficient operation Can only separate Cs element; needs to be combined with other technologies to achieve rare earth element separation Ion exchange <![CDATA[Cs + 、Sr 2+ 、RE 3+ > Low-temperature molten salt ion exchange technology has been widely studied and the technology is relatively mature Large amount of waste generated after separation; difficult to handle high-temperature molten salt Zone melting crystallization <![CDATA[Cs + 、Sr 3+ > Does not introduce new chemical components and has good separation effect The separation process is complex and time-consuming SAP / U-SAP process Halide High subsequent solidification efficiency of waste after dehalogenation Complex dehalogenation operation; will introduce new chemical components In summary, for different types of fission products, there are significant differences in the separation and purification effects. It is difficult to achieve the efficient and thorough separation of all multivalent fission products through a single separation technology in actual engineering; adopting a multi-technology collaborative strategy can combine economy and technical feasibility, which is the development direction of future advanced molten salt separation and purification technology and is still being explored.

[0010] At this time, the proportion of LiCl-KCl eutectic salt in the molten salt still exceeds 80 wt%, which has significant separation and recovery value. LiCl has high value, and its recovery can greatly reduce the economic cost. Moreover, after separating fission products, the volume of molten salt waste will be greatly reduced, alleviating the subsequent radioactive waste storage pressure. Summary of the Invention

[0011] The present invention aims to provide a method for separating fission nuclides and recovering LiCl from waste by molten salt electrolysis, propose a molten salt nuclide gradient separation strategy, and successfully achieve the separation of monovalent Cs in the molten salt system after electrolytic refining + , divalent Sr 2+ and trivalent Nd 3+ step-by-step separation of fission products, and simultaneously complete the efficient recovery of eutectic molten salts LiCl and KCl.

[0012] A method for separating fission nuclides and recovering LiCl from waste by molten salt electrolysis includes: Step 1, realizing Cs + separation by precipitation method: Prepare a hydrochloric acid solution with a concentration of 30 - 35%, add the molten salt waste to be treated to the hydrochloric acid solution, heat it to 50 - 95 °C and continuously stir vigorously to ensure that the salts are completely dissolved; After the system reaches thermal equilibrium, add InCl 3 in accordance with the stoichiometric ratio to initiate a solid-phase synthesis reaction, stir and heat until the reaction is complete. After the reaction, use a microporous filter membrane to filter and separate the Cs + solid phase and the solution system; Step 2, selectively recovering LiCl with an organic solvent: Add CsCl with the corresponding molar ratio to the solution system to initiate a reaction. After the reaction is complete, perform vacuum distillation to obtain a mixed salt system containing LiCl, KCl, NaCl, SrCl 2 , and Cs 2 , NaNdCl 6 ; Then dissolve the mixed salt in an organic solvent so that LiCl is completely dissolved, and KCl, NaCl, SrCl 2 , and Cs 2 , NaNdCl 6 form a solid-phase precipitate; After the reaction is complete, use a microporous filter membrane to filter and separate the mixed salt solid phase and the organic solution containing LiCl. The organic solution containing LiCl is dried to obtain high-purity LiCl; Step 3, separating Sr 2+ / Nd 3+ and recovering KCl: Dissolve the mixed salt solid phase in deionized water, add K 2 CO 3 in accordance with the stoichiometric ratio to initiate a precipitation reaction. Sr 2+ reacts with CO 3 2- to form a white flocculent SrCO 3 precipitate, and the precipitate is separated by filtration through a microporous filter membrane; Implement temperature gradient regulation on the filtrate. As the temperature decreases, white KCl salt gradually precipitates. Collect the product to complete the recovery of KCl. Evaporate the remaining solution to dryness to obtain Cs that is green as a whole. 2 NaNdCl 6 , a mixed waste of KCl, NaCl.

[0013] Preferably, the organic solvent in step 2 is methanol, ethanol or DMF.

[0014] Based on the specific formation characteristics of halide perovskites and the solubility differences of chlorides in organic solvents, the present invention innovatively proposes a molten salt nuclide gradient separation strategy. Separate monovalent Cs 2 NaInCl 6 and trivalent rare earth fission products Re 2 NaNdCl 6 in the form of halide perovskites (Cs + ), separate divalent Sr 3+ element in the form of strontium carbonate precipitate, separate LiCl by fractional dissolution using an organic solvent as the separation medium, and simultaneously separate KCl by an evaporation crystallization process, so as to achieve the step-by-step separation of monovalent Cs 2+ , divalent Sr + and trivalent Re 2+ fission products in the molten salt system after electrolytic refining, and simultaneously complete the efficient recovery of eutectic molten salts LiCl and KCl. The gradient separation process of the present invention can achieve a Cs 3+ separation rate of 95%, an S + separation rate of 93%, a high-purity LiCl recovery rate of 89.7%, and a KCl recovery rate of 73.3%. Its economic value is far superior to the existing molten salt waste treatment methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a schematic diagram of the Cs 2+ separation process realized by the precipitation method; Figure 2 is the XRD pattern of the product Cs + precipitate under four different process conditions; 2 NaInCl 6 is a schematic diagram of the selective recovery process of LiCl by an organic solvent; Figure 3 Fig. 4(a) is the X-ray diffraction pattern of the recovered LiCl when the organic solvent is methanol; Fig. 4(b) is the X-ray diffraction pattern of the recovered LiCl when the organic solvent is DMF; Fig. 4(c) is the X-ray diffraction pattern of the recovered LiCl when the organic solvent is ethanol; ​Figure 5 For r 2+ / Nd 3+ Schematic diagram of the Cs separation and KCl recovery process; Figure 6(a) shows the X-ray diffraction pattern of the product SrCO 3 ; Figure 6(b) shows the X-ray diffraction pattern of the product KCl; Figure 6(c) shows the X-ray diffraction pattern of the remaining mixture; Figure 6(d) shows the physical picture of the product. Detailed implementation manners

[0016] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners. The embodiments of the present invention are given for the purpose of illustration and description, and are not exhaustive or limit the present invention to the disclosed form. Many modifications and variations are obvious to those of ordinary skill in the art. The embodiments are selected and described to better illustrate the principles and practical applications of the present invention, and enable those of ordinary skill in the art to understand the present invention and design various embodiments with various modifications suitable for specific purposes.

[0017] To verify the effectiveness of the present invention in the separation of fission nuclides and the recovery of LiCl from waste after molten salt electrolysis, the specific composition of the molten salt waste after molten salt electrolysis is simulated as shown in Table 2, where K 2 CO 3 and InCl 3 are the drugs required for the subsequent separation process. Since the contents of trivalent nuclides such as La 3+ , Y 3+ , Ce 3+ are relatively small, their chemical properties are similar to those of Nd 3+ , and they all belong to rare earth elements. Therefore, in the subsequent separation experiments, Nd 3+ is used to represent all trivalent rare earth fission nuclides.

[0018] Table 2

[0019] I. Separation of Cs by precipitation method (the process is shown in + ) Figure 1 ) 1. Prepare a hydrochloric acid solution with a concentration of 30-35%. The hydrochloric acid solution can be prepared by diluting concentrated hydrochloric acid with deionized water and adjusting it to the target concentration.

[0020] 2. Add the simulated salt (simulated molten salt waste) to the hydrochloric acid solution according to the ratio shown in Table 2, place it in a thermostatic magnetic stirrer, heat it to 50-95 °C, and continuously stir vigorously to ensure that the salts are completely dissolved.

[0021] 3. After the system reaches thermal equilibrium, add InCl in stoichiometric ratio 3 to initiate the solid-phase synthesis reaction shown in Equation (1). Under acidic solution conditions, CsCl, NaCl, and InCl 3 spontaneously react to form a milky white flocculent Cs 2 NaInCl 6 halide perovskite precipitate; continue stirring and maintain the temperature until the reaction is complete, so that the radioactive Cs generated by fuel fission + is completely precipitated.

[0022] (1) After the reaction, use a microporous filter membrane to filter and separate the Cs + solid phase and the solution system.

[0023] Table 3 shows the actual yields and separation efficiencies of Cs 2 NaInCl 6 under different hydrochloric acid concentrations and reaction temperatures.

[0024] Table 3

[0025] Verify the phase composition of the product Cs 2 NaInCl 6 precipitate under different process conditions (different hydrochloric acid concentrations and reaction temperatures) by XRD (X-ray diffraction), as Figure 2 shown. As can be seen from Table 2, the diffraction peaks of the products obtained under the first four process conditions can be accurately matched with the standard pdf card of halide perovskite Cs 2 NaInCl 6 (PDF#77-2227), and the product belongs to the Fm3m space group of the cubic crystal system structure. No impurity phases such as cesium chloride are detected, indicating that the product phase is pure, confirming the successful precipitation of radioactive Cs + . Considering the Cs + element separation efficiency and product quality, the hydrochloric acid concentration of 30 wt% and the reaction temperature of 90 °C are determined as the optimal process combination. Under this condition, the Cs+ separation efficiency can reach 95.5%, and at the same time, the halide perovskite product has both a pure phase composition and excellent crystallinity.

[0026] II. Selective recovery of LiCl with organic solvents (the process is shown in Figure 3 ) 1. Add CsCl with the corresponding molar ratio to the solution system to initiate the reaction shown in Equation (2). The trivalent fission product Nd 3+ will react with Cs + , Na + to form a soluble halide perovskite Cs 2 NaNdCl6 Remain in the liquid phase.

[0027] 2. After sufficient reaction, carry out vacuum distillation to obtain a mixed salt system containing LiCl (target product), KCl (eutectic salt component), NaCl (unreacted substance), SrCl 2 (divalent nuclide) and Cs 2 NaNdCl 6 (rare earth complex).

[0028] (2) 3. Dissolve the mixed salt in an organic solvent so that LiCl is completely dissolved, and KCl, NaCl, SrCl 2 and Cs 2 NaNdCl 6 form a solid-phase precipitate. The molecular structures, dielectric constants, and polarities among different organic solvent systems are different, resulting in great differences in the solubility of salts. In this invention, the influence of organic solvents methanol, ethanol, and DMF on the recovery efficiency of LiCl is verified. See Table 4.

[0029] Table 4

[0030] Detect the phase composition of the recovered product by XRD to evaluate the recovery purity and recovery rate of LiCl. Figure 4(a) 、 4(b) 4(c) are the X-ray diffraction patterns of the recovered LiCl when the organic solvents are methanol, DMF, and ethanol respectively. It can be seen from the diffraction data that the products of the three solvent systems are mainly composed of LiCl (PDF#89-3611) as the main crystal phase, but all are accompanied by the formation of the LiCl·H 2 O hydrated phase (PDF#73-1273), which is due to the strong hygroscopicity of LiCl and its coordination with environmental moisture during the vacuum drying process. When methanol is used as the organic solvent, there are more miscellaneous peaks in the recovered product. This is mainly because of the high polarity of methanol, which will dissolve too many impurity salts other than LiCl, resulting in a decrease in the purity of the recovered LiCl and making it difficult to meet the requirements of the electrolysis process for the salt-based regenerative material.

[0031] Therefore, considering the recovery rate and purity of the LiCl product comprehensively, ethanol is determined as the preferred solvent for the LiCl recovery process, and the LiCl recovery rate can reach 89.7%.

[0032] After the reaction is completed, use a microporous filter membrane to filter and separate the mixed salt solid phase and the organic solution containing LiCl. The organic solution containing LiCl is dried to obtain high-purity LiCl.

[0033] The core of the entire LiCl recovery process lies in utilizing the solubility differences of chloride salts in organic solvents.

[0034] III. Sr 2+ / Nd 3+ Separation and KCl Recovery (for the process, see Figure 5 ) 1. Dissolve the mixed salts (containing KCl, NaCl, SrCl 2 , Cs 2 NaNdCl 6 ) in solid phase in deionized water, add K 2 CO 3 according to the stoichiometric ratio to initiate the precipitation reaction shown in formula (3). Sr 2+ reacts with CO 3 2- to form white flocculent SrCO 3 precipitate, and the precipitate is separated by filtration through a microporous membrane.

[0035] (3) Apply temperature gradient regulation to the filtrate. Compared with NaCl and Cs 2 NaNdCl 6 , the solubility of KCl changes greatly with temperature. As the temperature decreases, white KCl salt gradually precipitates. Collect the product to complete the KCl recovery.

[0036] Evaporate the remaining solution to dryness to obtain a greenish Cs 2 NaNdCl 6 , KCl, NaCl mixed waste. The separation and recovery rates of the products are quantitatively analyzed and summarized in Table 5. Among them, the separation rate of SrCO 3 can reach 93%, and the recovery rate of KCl can reach 73.3%.

[0037] Table 5 Product Actual output (g) Theoretical calculated output (g) Separation / recovery efficiency (%) <![CDATA[SrCO 3 > 0.13 0.14 93.0 KCl 1.51 2.06 73.3 Figure 6(a) is the X-ray diffraction pattern of the product SrCO 3 . The diffraction pattern shows that the main phase is orthorhombic SrCO 3 . The KCl phase is also detected in the product, mainly due to the entrainment of the mother liquor during the solid-liquid separation process. The phase semi-quantitative analysis of the SrCO 3 product is carried out by Rietveld refinement, and it is found that its actual composition contains 86.2 wt% SrCO 3 and 13.8 wt% KCl. This basically does not affect the separation efficiency of Sr element, indicating that the separation of the fission element Sr is successfully achieved by the reaction precipitation method.

[0038] Figure 6(b) is the X-ray diffraction pattern of the KCl product. Its diffraction peaks are clear and sharp, and there are no other miscellaneous peaks, indicating that the product is pure without impurities, verifying that the temperature gradient crystallization method can achieve the efficient recovery of KCl.

[0039] Figure 6(c) is the X-ray diffraction pattern of the remaining mixture. The residue is mainly cubic-phase Cs 2 NaNdCl 6 with a small amount of KCl.

[0040] Figure 6(d) is a physical picture of the product. Among them, KCl and SrCO 3 are white powders, and the mixture of Cs 2 NaNdCl 6 , KCl and NaCl is light green.

[0041] IV. The Cs 2 NaInCl 6 halide perovskite precipitate, the white flocculent SrCO 3 precipitate, and the mixed waste of Cs 2 NaNdCl 6 , KCl, and NaCl can be further combined with borosilicate, phosphate, or bismuthate glass and prepared into a form of glass-ceramic solidified body through rapid hot pressing or spark plasma sintering process, and then subjected to waste geological disposal treatment.

[0042] Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art and related fields without creative efforts shall fall within the protection scope of the present invention.

Claims

1. A method for separating fission nuclides and recovering LiCl from waste after molten salt electrolysis, characterized in that: include: Step 1: Precipitation method to achieve Cs + Separation: Prepare a 30-35% hydrochloric acid solution, add the molten salt waste to be treated into the hydrochloric acid solution, heat it to 50-95°C and continue to stir vigorously to ensure that the salt is completely dissolved; After the system reaches thermal equilibrium, InCl3 is added according to the stoichiometric ratio to initiate the solid phase synthesis reaction, and the mixture is stirred and heated until the reaction is fully reacted. After the reaction is completed, Cs is separated by filtering with a microporous membrane. + Solid phase and solution systems; Step 2, organic solvent selectively recovers LiCl: Adding CsCl in a corresponding molar ratio into the solution system to initiate a reaction, and performing reduced pressure distillation after sufficient reaction to obtain a mixed salt system comprising LiCl, KCl, NaCl, SrCl2 and Cs2NaNdCl6; Then, the mixed salt is dissolved in an organic solvent, so that LiCl is completely dissolved and KCl, NaCl, SrCl2 and Cs2NaNdCl6 form a solid phase precipitate; After the reaction is completed, the mixed salt solid phase and the organic solution containing LiCl are separated by filtering with a microporous filter membrane, and the organic solution containing LiCl is dried to obtain high-purity LiCl.

2. The method for separating fission nuclides and recovering LiCl from waste after molten salt electrolysis according to claim 1, characterized in that: Also includes: Step 3, Sr 2+ / Nd 3+ Separation and KCl recovery: The mixed salt solid phase was dissolved in deionized water, and K2CO3 was added in a stoichiometric ratio to initiate the precipitation reaction. 2+ With CO3 2- A white flocculent SrCO3 precipitate is formed, and the precipitate is separated by filtering through a microporous membrane; The filtrate is subjected to temperature gradient control, and as the temperature decreases, white KCl salt is gradually precipitated, and the product is collected to complete KCl recovery; the remaining solution is evaporated to dryness to obtain a mixed waste of Cs2NaNdCl6, KCl, and NaCl that is green overall.

3. The method for separating fission nuclides and recovering LiCl from waste after molten salt electrolysis according to claim 1, characterized in that: The organic solvent in step 2 is methanol, ethanol or DMF.

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