Method for synthesizing sodalite from kaolinite and solidifying radioactive waste salt in situ and application of method

Abstract

The invention discloses a method for synthesizing sodalite from kaolinite and solidifying radioactive waste salt in situ and application of the method, the method comprises the following steps: mixing kaolinite, inorganic alkali, molten salt and chlorine-containing radioactive waste salt, and then carrying out heat treatment, at least one of the inorganic alkali and the molten salt containing Na element; a mixture of kaolinite, inorganic alkali, molten salt and chlorine-containing radioactive waste salt is converted and synthesized into sodalite, and in-situ capture of splitting elements and chlorine elements is achieved. According to the method, the natural clay mineral kaolinite which is rich in natural reserves, stable in chemical property and rich in silicon and aluminum sources is used as a raw material, sodalite is synthesized in a molten salt system, radioactive waste salt is solidified synchronously, and the high-temperature molten salt electrolytic refining and electrolytic reduction processes can be better linked in the process. According to the method, efficient capturing and fixing of various splitting elements and chlorine elements in the radioactive waste salt can be achieved, the stability and safety of a solidified body are improved, the treatment cost and energy consumption are reduced, and the treatment process is simplified.

Description

Technical Field

[0001] The present invention relates to the technical field of radioactive substance treatment, and particularly relates to a method for synthesizing sodalite from kaolinite and in-situ solidifying radioactive waste salt and its application. Background Art

[0002] Nuclear energy is an energy source with high energy density, economy and relatively clean, and is currently widely used globally. However, with the rapid development of the nuclear energy industry, spent fuel treatment has become a key problem restricting the sustainable development of nuclear energy. Generally, spent fuel contains radioactive waste salts with extremely strong water solubility and high radioactivity, which are mainly composed of various alkali metals, alkaline earth metals, actinides, lanthanides and various radioactive products generated by their fission. If not properly treated, it is extremely easy to cause serious threats to the ecological environment and human health.

[0003] At present, the treatment methods of radioactive waste salts include wet and dry treatment technologies. The traditional wet treatment technology has a complex operation process, requires a large amount of chemical reagents, and is prone to secondary pollution. Although common dry post-treatment technologies such as molten salt electrolytic refining and electrolytic reduction can extract valuable materials, they will also generate a large amount of radioactive waste. For example, the electrolytic reduction of waste salts in the LiCl-Li 2 O system contains various fission products such as Cs, Sr, Ba, Ce, Zr, Mo, Ru and a large amount of chlorine element. They have extremely strong migration and transformation abilities in the environment and are extremely difficult to be effectively captured and fixed. Therefore, how to finally treat and dispose of radioactive waste salts will also affect the development of the nuclear energy industry to a certain extent.

[0004] Solidification and stabilization technology is an effective means for the final treatment and disposal of radioactive waste salts. However, some existing solidification materials and methods have problems such as high energy consumption, poor solidification effect on certain specific elements, and high costs. For example, common cement and asphalt solidification technologies have poor thermal stability and radiation stability and can only be used to treat low-level radioactive waste. Although glass and glass-ceramic immobilization technologies can, to a certain extent, achieve the immobilization of radionuclides, the sintering temperature required is usually higher than 1000 °C, which not only has problems of high energy consumption and high cost but also easily causes the volatilization and escape of target radionuclides, resulting in secondary pollution. In addition, the above-mentioned solidification technologies have low solidification ability for fission elements in the form of chlorides and are difficult to effectively treat radioactive waste salts containing a large amount of chlorides. Therefore, there is an urgent need to develop a new method for the solidification treatment of radioactive waste salts that is efficient, energy-saving, and environmentally friendly. The artificial rock solidification technology simulates the formation process of natural minerals and fixes fission elements and other radioactive substances in the mineral lattice or adsorbed inside the mineral skeleton through ion exchange, chemical bonding, etc., achieving effective binding of radioactive substances. This solidified body has good chemical stability, thermal stability, radiation stability, and mechanical strength, etc., and can reduce the migration and diffusion of harmful substances to the surrounding environment, thus providing a reliable guarantee for the safe disposal of harmful substances.

[0005] The composition and structure of sodalite are beneficial to the fixation of radioactive waste salts. Through the process mechanisms such as sodalite synthesis, ion exchange, and adsorption, target radionuclides and chlorine elements can be embedded in its lattice skeleton, thereby achieving the efficient solidification and stabilization of radioactive waste salts. However, the existing sodalite synthesis technology has a complex process and is difficult to achieve the synchronous solidification of radioactive waste salts. For example, the crystallization time of the microwave heating method is relatively long, and the synthesized sodalite products have problems such as low crystallinity and insufficient target radionuclide loading. In order to better connect the high-temperature molten salt electrolytic refining and electrolytic reduction processes in the nuclear waste resource recycling and complete disposal process, directly synthesizing sodalite from the molten waste salt system and synchronously solidifying radionuclides has greater technical advantages. Summary of the Invention

[0006] The present invention aims to solve the problems existing in the solidification technology of radioactive waste salts generated after the treatment of spent fuel by molten salt electrolytic refining and electrolytic reduction, such as complex operation, high cost, serious secondary pollution, and insufficient solidification ability for various radioactive fission elements and chlorine elements.

[0007] To achieve the above object, the present invention provides a method for synthesizing sodalite from kaolinite and in-situ solidifying radioactive waste salts, including, Mixing kaolinite, inorganic alkalis, molten salts, and radioactive waste salts containing chlorine, and then performing heat treatment, wherein at least one of the inorganic alkalis and molten salts contains Na element; Convert a mixture of kaolinite, inorganic alkalis, molten salts, and radioactive waste salts containing chlorine into sodalite to obtain a sodalite solidified body, achieving in-situ capture of fission product elements and chlorine elements.

[0008] Preferably, the radioactive waste salt includes at least one of electrolytic reduction salt LiCl-Li 2 O system or radioactive waste salt in the electrolytic refining salt LiCl-KCl system.

[0009] Further, the molten salt includes at least one of alkali metal chlorides.

[0010] Preferably, the alkali metal chloride is NaCl, or at least one of the eutectic salt systems of KCl-NaCl, KCl-LiCl, and NaCl-LiCl.

[0011] Further, the inorganic alkalis may be at least one of LiOH, NaOH, and KOH.

[0012] Further, the mass ratio of the radioactive waste salt containing chlorine to the total mass of kaolinite, inorganic alkalis, and molten salt is 0.1-1.

[0013] Further, the molar ratio of kaolinite, inorganic alkalis, and molten salt is 2-5:5-10:1-3.

[0014] Preferably, the inorganic alkali is NaOH.

[0015] Preferably, the molar ratio of kaolinite, NaOH, and molten salt is 3:6:2, and the molten salt adopts a eutectic salt system. Therefore, the equation for the conversion of kaolinite into sodalite is as follows, .

[0016] Preferably, the mass ratio of the radioactive waste salt containing chlorine to the total mass of kaolinite, inorganic alkalis, and molten salt is 0.25.

[0017] Further, the mesh number of the mixture of kaolinite, inorganic alkalis, molten salt, and radioactive waste salt containing chlorine is 50-200.

[0018] Preferably, kaolinite, inorganic alkalis, molten salt, and radioactive waste salt containing chlorine are mixed by ball milling. The ball milling is carried out at a rotation speed of 100-800 rpm and a ball-to-material ratio of 5-15 for 0.5-4 h, and then sieved through a 50-200 mesh sieve after ball milling.

[0019] Further, the mixture of kaolinite, inorganic alkalis, molten salt, and radioactive waste salt containing chlorine is transferred to a closed heat-resistant container for heat treatment.

[0020] Further, for the heat treatment, the temperature is raised to 750 - 900°C at a heating rate of 3 - 10°C / min and held for 1 - 8 h, and then cooled at a cooling rate of 3 - 10°C / min.

[0021] It should be noted that the gas atmosphere for the heat treatment does not need to be strictly limited. Even in some specific scenarios, for example, in the presence of oxygen, Ce will generate CeClO and other substances with valence changes, and at the same time compete for Cl in sodalite, which to a certain extent inhibits the best solidification effect of Ce. However, this process can still achieve a good level of Ce solidification treatment. Preferably, the heat treatment is carried out in a protective gas atmosphere, and the protective gas atmosphere is provided by gases such as Ar, He, N 2 etc.

[0022] Further, the radioactive waste salt containing chlorine also undergoes drying treatment.

[0023] Further, the kaolinite also undergoes activation treatment before mixing. The activation treatment includes crushing kaolin to obtain kaolin powder with a particle size of 50 - 200 mesh; holding the 50 - 200 mesh kaolin powder at 600 - 850°C for 1 - 6 h.

[0024] The present invention also provides an application of the above method in treating radioactive waste salt.

[0025] The mechanism involved in the present invention is to use processes such as lattice solidification, adsorption, chemical coordination, and ion exchange to embed fission products and chlorine elements in the waste salt into the artificial rock skeleton, lattice, or adsorbed on the inner and outer parts of the structure, etc., to achieve targeted capture of different elements and lattice solidification of isomorphous substitution, thereby converting the waste salt into a stable and harmless solid form, effectively reducing the risk of migration, diffusion, and leakage of radioactive substances, and being beneficial for long-term geological storage.

[0026] Compared with the prior art, the present invention has the following beneficial effects: Compared with the existing glass or glass-ceramic solidification technology, the present invention can achieve the efficient synthesis of sodalite and synchronously solidify radioactive waste salt at a lower temperature, with a simple operation process, no need to add additional binders or secondary glass coating, no secondary pollution problem, and being environmentally friendly.

[0027] The present invention has good compatibility with fission products and chlorine elements, can effectively encapsulate various metal ions in radioactive waste salt, prevent their release and migration, and the normalized leaching rate of fission products is less than 5×10 -4 g / (cm 2 ·d).

[0028] The present invention uses kaolinite, a natural clay mineral that is abundant in nature, chemically stable, and rich in silicon and aluminum sources, as a raw material to synthesize sodalite in a molten salt system and simultaneously solidify radioactive waste salts, which can better connect with high-temperature molten salt electrolytic refining and electrolytic reduction processes in terms of technology. The present invention can efficiently capture and fix various fission products and chlorine elements in radioactive waste salts, improve the stability and safety of the solidified body, reduce the treatment cost and energy consumption, and simplify the treatment process, thereby providing a new solution for the safe and effective disposal of radioactive waste salts. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0030] Figures 1 - 4 Shows the surface morphologies of the sodalite solidified body synthesized in Example 1 at different magnifications; Figure 5 Shows the element distribution map of the sodalite solidified body synthesized in Example 1; Figure 6a and Figure 6b Respectively show the XRD patterns of the sodalite solidified bodies synthesized in Examples 1-4, and Examples 1 and 5-7; Figure 7 Shows the FTIR test chart of the sodalite solidified body synthesized in Example 1; Figure 8 Shows the XRD patterns of the sodalite solidified body synthesized in Example 1 after irradiation with different doses; Figure 9 Shows the normalized leaching rate chart of the sodalite solidified body in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0032] Next, in combination with specific embodiments of the present invention and the accompanying drawings of the specification, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0033] Example 1 A method for synthesizing sodalite from kaolinite and in-situ solidifying radioactive waste salts is as follows S1. Transfer kaolinite, NaOH, molten salt, and radioactive waste salts containing chlorine to a ball mill and ball mill for 2 h to mix evenly, and then perform drying treatment to obtain a precursor with a mesh size of 50 meshes. S2. Load the precursor into an alumina crucible, cover and seal it, and then transfer it to a high-temperature molten salt furnace under an Ar atmosphere. Heat it to 800 °C at a heating rate of 5 °C / min and hold for 2 h, and then cool it to room temperature at a cooling rate of 5 °C / min to obtain a sodalite solidified body.

[0034] In this embodiment, the molten salt is NaCl-LiCl (the molar ratio of NaCl to LiCl is 0.35), and the molar ratio of kaolinite, NaOH, and molten salt is 3:6:2. The mass ratio of the radioactive waste salts containing chlorine to the total mass of kaolinite, NaOH, and molten salt is 0.25.

[0035] In this embodiment, the radioactive waste salts containing chlorine are waste salts that have not been treated by front-end processes such as precipitation and vacuum distillation, and are simulated by the chlorides in Table 1 below. The simulated radioactive waste salts containing chlorine are dried in a forced-air drying oven at 180 °C for 18 h before treatment to remove the moisture therein.

[0036] Table 1

[0037] In this embodiment, the kaolinite is also subjected to activation treatment before mixing, and the steps are as follows. T1. Crush the kaolinite in a crusher for 20 min and pass through a 50-mesh sieve to obtain kaolinite powder. T2. Transfer the 50-mesh kaolinite powder to a muffle furnace under an Ar atmosphere and sinter at 800 °C for 2 h to remove the crystal water.

[0038] Examples 2-4 Compared with Example 1, the difference is that in step S2, it is held at 750 °C, 850 °C, and 900 °C for 2 h respectively.

[0039] Examples 5-7 Compared with Example 1, the difference lies in that in step S2, it is kept at 800 °C for 1, 4, and 8 h respectively.

[0040] Test Example The scanning electron microscope (SEM, Zeiss SIGMA) was used to observe the microscopic morphology of the sodalite solidified body synthesized in Example 1. From Figures 1 - 4 it can be seen that the sodalite solidified body synthesized in Example 1 presents an aggregated cubic block shape, with a flat surface and no cracks. Further, the energy dispersive spectrometer (EDS, Oxford X-max 55) was used to analyze the element distribution of the sodalite solidified body synthesized in Example 1. From Figure 5 it can be seen that the signal intensity distributions of elements such as Cs, Sr, Ba, Ce, Zr, Mo, and Ru on the surface of the sodalite solidified body are uniform, confirming that the above seven fission products have been effectively solidified in the sodalite solidified body.

[0041] X-ray diffraction (XRD) was used for the phase identification of the sodalite solidified bodies synthesized in Examples 1-7. Figure 6a and Figure 6b respectively show the XRD patterns of the sodalite solidified bodies in Examples 1-4 and Examples 1 and 5-7. It can be seen that at a relatively low sintering temperature of 800 °C, sodalite can be successfully synthesized only after 1-2 h of sintering treatment.

[0042] Fourier transform infrared spectroscopy (FTIR) was used to test the functional groups of the sodalite solidified body synthesized in Example 1. From Figure 7 it can be seen that the characteristic peaks of typical sodalite Al-O-Si, O-Si-O, and O-Al-O appear in the product, further confirming the successful synthesis of sodalite.

[0043] Using 60 a Co radiation source (2 million curies), the samples were irradiated with γ-rays with a total dose of 100 kGy (1.2 kGy / h, 83 h) and 200 kGy (1.2 kGy / h, 166 h) to analyze the radiation resistance performance of the sodalite solidified body synthesized in Example 1. From Figure 8 it can be seen that after irradiation with a total dose of 100 kGy and 200 kGy, the phase of the sodalite solidified body did not change, proving its excellent radiation resistance performance.

[0044] Analyze the normalized leaching rates of the sodalite solidified bodies obtained in Example 1, Example 8, and Example 9 according to the PCT-A method of the American Society for Testing and Materials (ASTM). The specific steps include: grinding the synthesized sodalite solidified body in an agate mortar for 30 minutes and passing it through a 100-mesh sieve. Take 2 g of the solidified body powder and place it in a reaction kettle, add 20 mL of deionized water, and soak it at a constant temperature of 90 °C. Take the leaching solution on the 1st, 3rd, 7th, 14th, and 28th days respectively, and measure the ion concentration in the leaching solution by inductively coupled plasma mass spectrometry (ICP-MS) to calculate the normalized leaching rate.

[0045] From Figure 9 the results of the normalized leaching rates, it can be seen that the normalized leaching rates of the 7 fission product elements of the sodalite solidified body obtained in Example 1 are less than 5×10 -4 g / (cm 2 ·d) after 28 days, indicating that the sodalite solidified body obtained by the treatment method of the present invention effectively solidifies the fission product elements in the waste salt.

[0046] In summary, the present invention can achieve the efficient synthesis of sodalite at a relatively low temperature and simultaneously solidify radioactive waste salt. The operation process is simple, without the need for additional binders or secondary glass coating, and there is no problem of secondary pollution, which is environmentally friendly. The present invention has good compatibility with fission product elements and chlorine elements, can effectively encapsulate various metal ions in radioactive waste salt, prevent their release and migration, and the normalized leaching rate of fission product elements is less than 5×10 -4 g / (cm 2 ·d) after 28 days. The raw materials for synthesizing sodalite in the present invention are widely sourced and inexpensive. The synthesized solidified body has good chemical stability and durability, and can be applied to the immobilization of radioactive waste in molten salt-based and solid waste salts. Generally speaking, the present invention has wide industrial applicability and can be applied to nuclear power plants, nuclear fuel reprocessing plants, and other facilities that generate radioactive waste salt. This method can effectively treat radioactive waste salt and convert it into a stable solidified body, which is beneficial to the safe storage and final disposal of radioactive waste, helps reduce the environmental risk of radioactive waste salt, and promotes the sustainable development of the nuclear energy industry.

[0047] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, 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 synthesizing sodalite from kaolinite and in-situ solidification of radioactive waste salt, characterized in that: include, Mixing kaolinite, inorganic alkali, molten salt, and chlorine-containing radioactive waste salt and then heat treating the mixture, wherein at least one of the inorganic alkali and the molten salt contains the element Na; A mixture of kaolinite, inorganic alkalis, molten salt and chlorine-containing radioactive waste salt is converted into sodalite to obtain a sodalite solid body, thereby achieving in-situ capture of fragmentation elements and chlorine elements.

2. The method according to claim 1, characterized in that The molten salt includes at least one of alkali metal chlorides.

3. The method according to claim 1, characterized in that The molar ratio of the kaolinite, the inorganic base and the molten salt is 2-5:5-10:1-3.

4. The method according to claim 1, characterized in that: The mass ratio of the chlorine-containing radioactive waste salt to the total mass of kaolinite, inorganic alkalis and molten salt is 0.1-1.

5. The method according to claim 1, characterized in that The mesh size of the kaolinite, inorganic alkali, molten salt and chlorine-containing radioactive waste salt after mixing is 50-200 meshes.

6. The method according to claim 1, characterized in that The kaolinite, inorganic alkali, molten salt and chlorine-containing radioactive waste salt are mixed and transferred to a sealed heat-resistant container for heat treatment.

7. The method according to claim 1, characterized in that The heat treatment is performed by heating the temperature to 750-900°C at a heating rate of 3-10°C / min and maintaining the temperature for 1-8h, and then cooling the temperature at a cooling rate of 3-10°C / min.

8. The method according to claim 1, characterized in that The chlorine-containing radioactive waste salt is further dried.

9. The method according to any one of claims 1 to 8, characterized in that: The kaolinite is also subjected to activation treatment before mixing, and the activation treatment comprises: The kaolin is crushed to obtain 50-200 mesh kaolin powder; Keep 50-200 mesh kaolin powder at 600-850℃ for 1-6h.

10. Use of the method according to any one of claims 1 to 9 in treating radioactive waste salt.

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