Method for solidifying Sr, U and Ce in electrolytic refining waste salt by using zirconolite type ceramic and application

Abstract

The invention discloses a method for solidifying Sr, U and Ce in electrolytic refining waste salt through zirconolite type ceramic and application, and belongs to the technical field of solidification of high-radioactivity nuclear waste. According to the method, a K2CO3 precipitant is added into LiCl-KCl molten salt containing SrCl2, UCl4 and CeCl3 after electrolytic refining, so that precipitates, namely SrCO3, UO2 and CeO2, of splitting elements are obtained. The preparation method comprises the following steps: dissolving a precipitate in water, washing away residual molten salt on the surface of the precipitate, mixing the precipitate with perovskite matrix materials CaCO3, ZrO2 and TiO2, and sintering at 1200-1400 DEG C for 3-24 hours to obtain a ceramic solidified body, the chemical formula of a ceramic main body is Ca < 1-x > Sr < x > Zr < 1-y > UyTi2O7 and Ca < 1-x > Sr < x > Zr < 1-y > CeyTi2O7, x is greater than or equal to 0.2, and y is less than or equal to 0.6. The main phase of the ceramic solidified body prepared by the method is zirconolite, the embedded elements are uniformly distributed, the chemical property is stable, the nuclide leaching rate is low, and the ceramic solidified body has good high-level waste inclusiveness and is suitable for long-term deep geological storage.

Description

Technical Field

[0001] The present invention belongs to the technical field of solidification treatment of radioactive nuclear waste, and particularly relates to a method and application for solidifying Sr, U, and Ce in electrolytic refining waste salt with perovskite-type ceramics. Background Art

[0002] As a clean energy source, nuclear energy has developed rapidly globally. High-level radioactive waste is the waste produced in nuclear fuel reprocessing. Reprocessing of spent fuel is an essential link in realizing fuel cycle. This technology focuses on improving the utilization rate of resources, reducing the waste to be geologically disposed, and reducing the harm of nuclear energy to the biosphere. The molten salt electrolytic refining method is currently the most widely studied spent fuel treatment method. Among them, the lithium chloride-potassium chloride eutectic salt is the most commonly used molten salt system. This system can exist stably at high temperatures and has good electrical conductivity and dissolution ability, making it suitable for electrochemical reactions. According to the difference in deposition potential, U, Pu, and a small amount of actinides can be separated from spent fuel. However, fission products such as RE, alkali metals, and alkaline earth metals will accumulate in the molten salt. When these elements accumulate to a certain concentration, it will affect the normal progress of electrolysis. To realize the reuse of the solvent salt and minimize radioactive waste, it is necessary to regularly remove active fission products such as RE, Cs, and Sr from the waste molten salt and purify the solvent salt. Separating the fission products in the molten salt by adding a suitable precipitant has been proven to be feasible.

[0003] The above-obtained precipitate is radioactive, so it is necessary to solidify this high-level radioactive waste. The host material of solid waste must have the characteristics of large packaging capacity, strong chemical, mechanical, thermal, and radiation stability. The host crystal can enable simulated elements to enter its lattice through isomorphic substitution, thereby realizing the chemical durability of stable nuclides. Perovskite belongs to the fluorite derivatives in ceramic-based materials and has been widely studied by scholars due to its good chemical stability and radiation stability for treating the embedding of radioactive nuclides.

[0004] Currently, in the field of nuclide solidification, the perovskite-type ceramic solidified bodies prepared by traditional high-temperature synthesis processes generally face challenges such as a long synthesis cycle and poor chemical stability of the finished products. This phenomenon has a non-negligible impact on the safety of radioactive substances in such materials. So far, there has been no report on the ceramic solidified body of Sr, U(Ce) co-doped perovskite. Therefore, researching and developing a new type of ceramic solidification substrate with excellent stability, high accommodation capacity, and low leaching characteristics has become a key topic that needs to be overcome in this field. Summary of the Invention

[0005] The present invention provides a method for solidifying Sr, U, and Ce in electrolytic refining waste salt with ceramics, and the prepared ceramic substrate has excellent performance in terms of stability, anti-leaching rate, mechanical properties, etc.

[0006] The present invention provides a method for solidifying Sr, U, and Ce in electrolytic refining waste salt with perovskite zirconate ceramics, which comprises the following steps:

[0007] Step 1: Melting the eutectic salt LiCl-KCl containing SrCl2, UCl4 or SrCl2, UCl4, adding a precipitant K2CO3 to the molten salt, separating the liquid after precipitation, and retaining the bottom precipitate; washing the cooled precipitate with deionized water and drying to obtain pure precipitates SrCO3, UO2, and CeO2;

[0008] Step 2: Mixing the precipitate with CaCO3, ZrO2, and TiO2, grinding in a ball mill, and screening to obtain a mixed powder; putting the mixed powder into a crucible for sintering, and cooling to room temperature to obtain a perovskite zirconate ceramic solidified body.

[0009] Further, in step 1, when adding the precipitant, the temperature of the molten salt is 450-550 °C.

[0010] Further, in step 1, the molar ratio of the precipitant added is K2CO3:SrCl2:UCl4:CeCl3 = 1:1:2:2 / 3.

[0011] Further, in step 2, the rotation speed of the ball mill is 200-450 r / min, rotating forward and backward for 2-4 h each; the aperture of the sieve is 100-200 mesh.

[0012] Further, the specific steps of putting the mixed powder into a crucible for sintering in step 2 are as follows:

[0013] Step 2.1: Putting the mixed powder into a corundum crucible and heating it to 800-900 °C in a muffle furnace for insulation for 2 h, and then cooling it to room temperature with the furnace to obtain a pre-sintered ceramic powder;

[0014] Step 2.2: Grinding the pre-sintered ceramic powder to remove air holes and then screening it, and then putting the ceramic powder into a corundum crucible again and heating it to 1300-1400 °C in a muffle furnace for insulation for 3-24 h, and then cooling it to room temperature with the furnace to obtain a perovskite zirconate ceramic solidified body.

[0015] Further, the heating rate of the muffle furnace is 5-10 °C / min.

[0016] Further, in step 2.2, the aperture of the sieve is 100-200 mesh.

[0017] The present invention also provides a perovskite zirconate ceramic solidified body prepared by the method for solidifying Sr, U, and Ce in electrolytic refining waste salt with perovskite zirconate ceramics, and the chemical formula of the perovskite zirconate ceramic solidified body is Ca 1-x Srx Zr 1-y U y Ti2O7 or Ca 1-x Sr x Zr 1-y Ce y Ti2O7, where 0.2 ≤ x, y ≤ 0.6.

[0018] Furthermore, the anti-leaching performance of the perovskite zirconia ceramic solidified body was analyzed. The Sr leaching rate was 6.15×10 - 4 g·m 2 ·d -1 , and the Ce leaching rate was 6.28×10 -7 g·m 2 ·d -1 .

[0019] The present invention also provides an application of a method for solidifying Sr, U, and Ce in electrolytic refining waste salt with perovskite zirconia ceramics in the field of treating high-level radioactive waste.

[0020] The principle of the present invention is as described below:

[0021] (1) In step (1) of the scheme, after adding the precipitant K2CO3, the precipitation reaction formulas are: SrCl2 + K2CO3 = SrCO3↓ + 2KCl, UCl4 + 2K2CO3 = UO2↓ + 4KCl + 2CO2↑, 4CeCl3 + 6K2CO3 + O2 = 4CeO2↓ + 12KCl + 6CO2↑.

[0022] (2) In step (2) of the scheme, the perovskite ore-type ceramic body is ABTi2O7, where A is a divalent element Ca, Sr, with a ratio of (1 - x):x, and B is a tetravalent element Zr, U or Zr, Ce, with a ratio of (1 - y):y. The chemical formula of the ceramic main body is Ca 1-x Sr x Zr 1- y U y Ti2O7, Ca 1-x Sr x Zr 1-y Ce y Ti2O7 (0.2 ≤ x, y ≤ 0.6).

[0023] The beneficial effects of the present invention are as follows:

[0024] 1. The present invention obtains the required solidified elements by directly adding a precipitant to the eutectic LiCl-KCl molten waste salt, which can be applied to the solidification treatment of the waste salt generated in the dry reprocessing of spent fuel by molten salt electrolysis; the present invention mainly aims at the nuclide solidification in the electrolytic refining process, and precipitates the chloride waste salt obtained by anode dissolution and cathode deposition in the previous process by adding a precipitant, so as to facilitate subsequent waste treatment.

[0025] 2. The present invention designs a ceramic solidified body with perovskite zirconolite as the ceramic phase. By using the high compatibility of perovskite zirconolite ceramics with nuclides, various radioactive elements such as fission products and uranium elements are successfully solidified at the same time. For radioactive nuclides, in perovskite zirconolite ceramics, strontium and cerium (uranium) are solidified at the same time. Through charge compensation, the purpose of co-solidification of the two nuclides is achieved. It is prepared by the traditional solid-phase sintering method. The solidified body has an inclusion rate of more than 30%, the internal element distribution is uniform, it has good high-level waste inclusiveness, and its chemical properties are stable. The Sr leaching rate is 6.15×10 -4 g·m 2 ·d -1 , and the Ce leaching rate is only 6.28×10 -7 g·m 2 ·d -1 . The nuclide leaching rate is low, which is suitable for long-term deep geological storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the normalized leaching rate data graph of Ca 0.6 Sr 0.4 Zr 0.6 Ce 0.4 Ti2O7 ceramic under PCT standard;

[0027] Figure 2 is the XRD pattern of Ca 0.6 Sr 0.4 Zr 0.6 U 0.4 Ti2O7 ceramic;

[0028] Figure 3 is the SEM and EDS patterns of Ca 0.6 Sr 0.4 Zr 0.6 U 0.4 Ti2O7 ceramic powder. DETAILED DESCRIPTION OF THE INVENTION

[0029] The following examples describe the present invention in more detail;

[0030] The present invention discloses a method for solidifying Sr, U, and Ce in electrolytic refining waste salt with perovskite zirconolite ceramics, which includes the following steps:

[0031] (1) Removal of target substances from molten salts by precipitation method: First, eutectic salts LiCl-KCl containing SrCl2, UCl4, and CeCl3 were melted at 500 °C respectively. Subsequently, precipitant K2CO3 was added to the molten salts. After precipitation was completed, the upper-layer molten salt solution was poured out at 500 °C, leaving the bottom precipitate. After cooling, the precipitate was washed with deionized water to remove the molten salts attached to the precipitate, and pure precipitates SrCO3, UO2, and CeO2 were obtained after drying.

[0032] (2) Ceramic solidification of precipitates: Further, the above precipitates SrCO3, UO2, and CeO2 were mixed with perovskite zirconolite matrix materials CaCO3, ZrO2, and TiO2 in a certain proportion, placed in a planetary ball mill, and alcohol solvent was added for sufficient ball milling and mixing. After taking out, it was sieved to obtain mixed powder, and the particle size of the ceramic powder was all below 200 μm; the mixed powder was placed in a corundum crucible and heated in a muffle furnace at a heating rate of 5 °C / min to 10 °C / min to 800 - 900 °C, pre-fired at this temperature for 2 h, and then heated to 1300 - 1400 °C for sintering for 3 - 24 h, and cooled to room temperature with the furnace to directly obtain perovskite ceramic blocks.

[0033] Example 1

[0034] Taking LiCl-KCl as the molten salt system with a mass ratio of LiCl:KCl = 38 g:45 g, it was heated to 500 °C for melting, SrCl2, CeCl3, and UCl4 were added. After complete dissolution, precipitant K2CO3 was added, and it was stirred every 4 h. After obtaining the salt-containing precipitate, it was washed with deionized water to remove the molten salts attached to the precipitate, and pure precipitates SrCO3, CeO2, and UO2 were obtained after drying.

[0035] Using Ce 4+ to replace U 4+ for the simulated solidification of U (Note: The ionic radii of Ce 4+ and U 4+ are very close and have similar reaction characteristics): Weigh the molar ratio of CaCO3:SrCO3:ZrO2:CeO2:TiO2 = 0.6:0.4:0.6:0.4:2 to prepare the chemical formula Ca 0.6 Sr 0.4 Zr 0.6 Ce 0.4The ceramic of Ti2O7 is added with an appropriate amount of absolute ethanol, mixed and ground in a ball mill until uniform, transferred to a corundum crucible after passing through a 100-mesh sieve, placed in a muffle furnace, heated from room temperature to 850 °C at a heating rate of 5 - 10 °C / min, held for 2 hours, cooled and taken out for grinding and degassing, passed through a 200-mesh sieve again, then placed in the muffle furnace and heated to 1400 °C, held for 3 h, and cooled to room temperature with the furnace and taken out to obtain a ceramic product as a black-brown dense block.

[0036] Leaching rate performance test of the solidified body: Soak the powder with deionized water according to the PCT (Product conformance testing) standard, soak it statically at 90 °C for 28 days, and the normalized leaching rate of Sr element is 6.15×10 -4 g·m 2 ·d -1 , and the normalized leaching rate of Ce element is 6.28×10 -7 g·m 2 ·d -1 , as shown in the attachment Figure 1 , and the solidification effect is good.

[0037] Example 2

[0038] The difference from Example 1 is that reagents with a molar ratio of CaCO3:SrCO3:ZrO2:CeO2:TiO2 = 0.4:0.6:0.4:0.6:2 are weighed to prepare a ceramic of the chemical formula Ca 0.84 Sr 0.6 Zr 0.4 Ce 0.6 Ti2O7, added with an appropriate amount of absolute ethanol, mixed and ground in a ball mill until uniform, transferred to a corundum crucible after passing through a 100-mesh sieve, placed in a muffle furnace, heated from room temperature to 850 °C at a heating rate of 5 - 10 °C / min, held for 2 hours, cooled and taken out for grinding and degassing, passed through a 200-mesh sieve again, then placed in the muffle furnace and heated to 1400 °C, held for 3 hours, and cooled to room temperature with the furnace and taken out to obtain a ceramic product as a black-brown dense block.

[0039] Example 3

[0040] The difference from Example 1 is that reagents with a molar ratio of CaCO3:SrCO3:ZrO2:CeO2:TiO2 = 0.8:0.2:0.8:0.2:2 are weighed to prepare a ceramic of the chemical formula Ca 0.8 Sr 0.2 Zr 0.8 Ce 0.2For the ceramic of Ti2O7, an appropriate amount of absolute ethanol was added, and they were mixed and ground in a ball mill until uniform. After passing through a 100-mesh sieve, it was transferred to a corundum crucible, placed in a muffle furnace, and heated from room temperature to 900 °C at a heating rate of 5 - 10 °C / min, held for 2 hours, cooled and taken out for grinding and degassing. After passing through a 200-mesh sieve again, it was put into the muffle furnace and heated to 1400 °C, held for 3 hours, and then cooled to room temperature with the furnace and taken out. The obtained ceramic product was a black-brown dense block.

[0041] Example 4

[0042] The difference from Example 1 was that reagents with a molar ratio of CaCO3:SrCO3:ZrO2:UO2:TiO2 = 0.6:0.4:0.6:0.4:2 were weighed to prepare a chemical formula of Ca 0.6 Sr 0.4 Zr 0.6 U 0.4 For the ceramic of Ti2O7, they were mixed and ground in a mortar until uniform. After passing through a 100-mesh sieve, it was transferred to a corundum crucible, placed in a muffle furnace, and heated from room temperature to 800 °C at a heating rate of 5 - 10 °C / min, held for 2 hours, cooled and taken out for grinding and degassing. After passing through a 200-mesh sieve again, it was put into the muffle furnace and heated to 1400 °C, held for 24 hours, and then cooled to room temperature with the furnace and taken out. The obtained ceramic product was a brown dense block. Its XRD is shown in the appendix Figure 2 , SEM and EDS are shown in the appendix Figure 3 .

[0043] Comparative Example 1

[0044] The difference from Example 1 was that for the ceramic of the chemical formula Ca 0.6 Sr 0.4 Zr 0.6 Ce 0.4 Ti2O7, after holding at 1100 °C for 3 hours, was sintered to obtain a ceramic product which was a loose block from light pink to pink.

[0045] In summary, the method of the present invention is applicable to the molten salt system of chlorides of various radioactive elements, and directly synthesizes perovskite zirconia ceramic solidified bodies through precipitation products. Utilizing the high compatibility of perovskite zirconia ceramics with nuclides, various radioactive elements such as fission products and uranium elements are successfully solidified simultaneously, enabling the ceramic solidified body to have a large embedding rate, and the leaching rate of radioactive elements is within the standard range. The leaching rate of Ce is only 6.28×10 -7 g·m 2 ·d -1 .

[0046] To clearly illustrate the embodiments of the present invention or the prior art solutions, the following will briefly introduce the drawings required in the description of the specific embodiments or the prior art. Obviously, the said drawings only represent one of several specific implementation forms of the present invention. For those skilled in the art, without additional innovative efforts, other possible design solutions can also be deduced based on the provided graphic materials.

Claims

1. A method for solidifying Sr, U and Ce in electrolytic refining waste salt with perovskite-zircon ceramics, characterized in that: The following steps are involved: Step 1: melt the eutectic salt LiCl-KCl containing SrCl2, CeCl3 or SrCl2, UCl4, add the precipitant K2CO3 to the molten salt, separate the liquid after precipitation, and retain the bottom precipitate; wash the cooled precipitate with deionized water, and dry it to obtain pure precipitates SrCO3, UO2, CeO2; Step 2: Mix the precipitate with CaCO3, ZrO2 and TiO2, grind them in a ball mill, and sieve to obtain a mixed powder; put the mixed powder into a crucible for sintering, and cool it to room temperature to obtain a perovskite zircon ceramic solid body.

2. The method for solidifying Sr, U and Ce in electrolytic refining waste salt with perovskite-zircon type ceramics according to claim 1, characterized in that: In the step 1, when the precipitant is added, the temperature of the molten salt is 450-550°C.

3. The method for solidifying Sr, U and Ce in electrolytic refining waste salt with perovskite-zircon type ceramics according to claim 1, characterized in that: In step 1, the precipitant is added in a molar ratio of K2CO3:SrCl2:UCl 4: :CeCl3=1:1:2:2 / 3.

4. The method for solidifying Sr, U and Ce in electrolytic refining waste salt with perovskite-zircon type ceramics according to claim 1, characterized in that: In the step 2, the rotation speed of the ball mill is 200-450 r / min, and the forward and reverse rotations are 2-4 hours each; and the sieve aperture is 100-200 mesh.

5. The method for solidifying Sr, U and Ce in electrolytic refining waste salt with perovskite-zircon type ceramics according to claim 1, characterized in that: The mixed powder is placed in a crucible for sintering in step 2, which specifically includes the following steps: Step 2.1: Place the mixed powder in a corundum crucible and heat it to 800-900° C. in a muffle furnace for 2 hours, and then cool it to room temperature to obtain a pre-fired ceramic powder; Step 2.2: Grind the pre-sintered ceramic powder to remove pores and then sieve it. Then put the ceramic powder into the corundum crucible again and place it in a muffle furnace and heat it to 1300-1400°C for 3-24 hours. After cooling to room temperature in the furnace, a solidified peroximate zircon ceramic body is obtained.

6. The method for solidifying Sr, U and Ce in electrolytic refining waste salt with perovskite-zircon type ceramics according to claim 5, characterized in that: The muffle furnace heating rate is 5-10°C / min.

7. The method for solidifying Sr, U and Ce in electrolytic refining waste salt with perovskite-zircon type ceramics according to claim 5, characterized in that: In the step 2.2, the sieve aperture is 100 to 200 meshes.

8. The perovskite zircon ceramic solidified body prepared by the method according to any one of claims 1 to 7, characterized in that: The chemical formula of the perovskite zircon ceramic solidified body is Ca 1-x Sr x Zr 1-y U y Ti2O7 or Ca 1-x Sr x Zr 1-y Ce y Ti2O7, where 0.2≤x, y≤0.

6.

9. The peroximate zircon ceramic solidified body according to claim 8, characterized in that: The anti-leaching performance of the perovskite zircon ceramic solid body was analyzed, and the Sr leaching rate was 6.15×10 -4 g·m 2 ·d -1 , Ce leaching rate is 6.28×10 -7 g·m 2 ·d -1 .

10. Use of the method according to any one of claims 1 to 7 in the field of treating high-level radioactive waste.

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