A method for separating lanthanide and actinide oxides based on a carbonate molten salt medium

Abstract

This invention discloses a method for separating lanthanide and actinide oxides using a carbonate molten salt medium, belonging to the field of nuclear fuel reprocessing. The method uses carbonate as the separation medium. Under a CO2 atmosphere, lanthanide oxides undergo carbonation and dissolution, while actinide oxides such as uranium have extremely low solubility. The actinide oxides are recovered in solid form, achieving lanthanum-actinide separation. Then, the carbonate molten salt containing dissolved lanthanide oxides is heated and the CO2 atmosphere is removed, allowing the lanthanide oxides to precipitate and be recovered as solid powder. The purified carbonate molten salt can be reused. The carbonate molten salt produced by this method can be reused, and the entire separation process is simple, easy to operate, and highly efficient. As a technological reserve for the development of high burnup spent fuel reprocessing technology, it can be used for dry reprocessing of spent fuel independently, or as a pretreatment for water-based reprocessing processes such as PUREX.

Description

Technical Field

[0001] This invention belongs to the field of nuclear fuel reprocessing, specifically relating to a method for separating lanthanide and actinide oxides based on a carbonate molten salt medium. Background Technology

[0002] To reduce greenhouse gas emissions and meet ever-increasing energy demands, nuclear energy has developed rapidly. The expansion of global nuclear power capacity results in a large amount of spent fuel each year, primarily composed of usable uranium and newly generated actinide oxides such as plutonium. Because lanthanides such as neodymium and samarium in spent fuel have high neutron absorption cross-sections, lanthanum-actinium separation must be achieved through reprocessing to ensure that the recovered actinide metal oxides can be returned to the reactor for continued use. However, the physicochemical properties of lanthanides and actinides are very similar, and the high-level radioactive handling involved makes the engineering of spent fuel reprocessing extremely difficult.

[0003] Currently, the primary industrial-scale spent fuel reprocessing process is the PUREX process based on the nitric acid system. This process has significant, even irreplaceable, advantages in processing spent fuels with low burnup, such as those from pressurized water reactors. With the development of fourth-generation nuclear reactors such as fast neutron reactors, the demand for processing spent fuels with high burnup, strong radioactivity, high plutonium content, and high heat release rates is increasing. When processing high-burnup spent fuels, the PUREX process experiences more severe radiolysis of its organic solvents, further exacerbating the formation of the third phase, leading to difficulties in extraction and separation operations and affecting the yields of products such as uranium and plutonium. Dry reprocessing using non-aqueous systems is considered an effective method for processing future high-burnup, short-cooling-period spent fuels.

[0004] Dry post-processing mainly employs metal electrolytic refining, oxide electrodeposition, and fluorination volatilization to separate lanthanum and actinide. These technologies are either energy-intensive or highly corrosive to equipment, requiring further in-depth research and development. Compared to other molten salts, carbonate molten salts offer higher thermal stability, are readily available and inexpensive, and cause less corrosion to equipment. They are commonly used in high-temperature heat transfer and storage, fuel cells, and organic waste pyrolysis. This invention leverages these advantages of carbonate molten salts, using them as a medium to process spent fuel after oxidation and volatilization, achieving the separation of lanthanide oxides and actinide oxides. Summary of the Invention

[0005] To address the problems of high energy consumption and complex operation in the separation of lanthanum and actinides in existing technologies, this invention provides a method for separating lanthanide oxides and actinide oxides from spent fuel that is simple in process, highly efficient in separation, and easy to purify and reuse the dissolving medium. Specifically, the method involves dissolving and separating lanthanide oxides and actinide oxides from spent fuel in a carbonate molten salt system, and recovering actinide oxides in the form of solid insoluble matter.

[0006] This invention provides a method for separating lanthanide and actinide oxides based on a carbonate molten salt medium, comprising the following steps:

[0007] Step 1: After the carbonate system is stirred evenly, it is loaded into the reactor and vacuumed and washed in sequence. After the carbonate system is heated to a molten state, actinide oxide and lanthanide oxide are added and CO2 is introduced. The mixture is kept at this temperature for a period of time.

[0008] Step 2: After the reaction is complete, the mixed molten salt is subjected to solid-liquid separation. The actinide oxides are recovered in solid form, while the lanthanide oxides are dissolved in the molten salt by carbonation.

[0009] Step 3: Remove the CO2 atmosphere from the reactor, keep the molten salt containing lanthanide carbonates at a certain temperature for a period of time, and the carbonation-dissolved lanthanide oxides will precipitate again in solid form. After solid-liquid separation, the lanthanide oxides will be recovered.

[0010] Further, in step 1, the carbonate system is a binary or ternary carbonate system of lithium carbonate, sodium carbonate, and potassium carbonate; the carbonate system is first pre-dried at a temperature of 50~200℃ for 4~12 hours.

[0011] Furthermore, in step 1, the vacuuming and gas washing operations are repeated 3 to 5 times; the gas used for washing is high-purity CO2; and the melting temperature of the carbonate system is 400 to 650°C.

[0012] Further, in step 1, the actinide oxide is UO2 or U3O8; the lanthanide oxide is one or more of La2O3, Nd2O3, Sm2O3, Gd2O3, Dy2O3, Er2O3, Yb2O3, and Lu2O3; the lanthanide oxide accounts for 0.1% to 30% of the mass of the mixture of actinide oxide and lanthanide oxide; the total mass of the mixture of actinide oxide and lanthanide oxide should not exceed 20 wt% of the mass of carbonate, and the mixture is pre-dried at a temperature of 50 to 200°C for 4 to 12 hours.

[0013] Furthermore, there are two methods for introducing CO2. The first method involves directly adding CO2 gas into the reactor using one or more corundum tubes. The CO2 gas flow rate increases accordingly with the mass of the carbonate system; when the mass of the carbonate system is 6g, the gas flow rate is 5~30mL / min. The second method involves compressing a mixture of (NH4)2CO3 with actinide oxides and lanthanide oxides into tablets, and then adding the tablets together with the mixture of actinide oxides and lanthanide oxides into the reactor. During the heat preservation process, (NH4)2CO3 undergoes in-situ thermal decomposition to generate CO2 gas. The mass ratio of (NH4)2CO3 to lanthanide oxides in the tablets is greater than 40; the tableting pressure is 10~700MPa.

[0014] Furthermore, in step 1, the heat preservation temperature is 400~650℃, and the time is 10min~24h.

[0015] Furthermore, in step 2, the solid-liquid separation is carried out by filtration, in which the mixed molten salt is poured into a metal filter screen and the insoluble actinide oxides are retained on the metal filter screen; the solid-liquid separation is carried out in a reactor at 400~650℃ and under a CO2 atmosphere.

[0016] Furthermore, the metal filter screen is made of molybdenum, platinum, or stainless steel, and has a mesh size of 500-1000 mesh.

[0017] Furthermore, in step 3, the method for removing the CO2 atmosphere in the reactor is to replace the atmosphere in the reactor with air or an inert gas; the heat preservation temperature is 650~1000℃, and the time is 0.5~3h.

[0018] The beneficial effects of this invention are as follows:

[0019] (1) This invention is the first to propose using carbonates as a separation medium. Under a CO2 atmosphere, there is a significant difference in the solubility of uranium oxides and lanthanide oxides in molten carbonate, which can be used to separate lanthanide oxides and actinide oxides in high burnout spent fuels, and provides a certain technical reserve for the development of dry reprocessing technology. The use of carbonates as a separation medium in this invention can meet the requirements of spent fuel processing in fast reactors with high burnout and strong radioactivity, and the molten carbonate has high thermal stability, low corrosion to equipment, and is relatively inexpensive.

[0020] (2) The method of the present invention is based on the selective dissolution technology of lanthanide and actinide in carbonate molten salt. Under CO2 atmosphere, lanthanide elements undergo carbonation and dissolution, while the solubility of actinide oxides such as uranium is extremely low, so as to achieve the separation of lanthanide and actinide. The whole process is simple, easy to operate, and efficient in separation. A purification and reuse scheme is proposed for the carbonate molten salt of lanthanide elements that have been separated and dissolved. Only heating and changing the atmosphere are needed to achieve the precipitation of lanthanide elements. After solid-liquid separation, the molten salt can be purified, which facilitates the reuse of carbonate and reduces the volume of solid waste. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the separation principle of the method for separating lanthanide and actinide oxides according to the present invention;

[0022] Figure 2 This is the XRD pattern of the molten salt cooled after the neodymium oxide was dissolved in Example 1 of the present invention;

[0023] Figure 3 This is a diagram illustrating the molten salt purification effect after dissolving lanthanide elements in Example 3 of the present invention.

[0024] Figure 4 This is a diagram illustrating the effect of molten salt recycling in Embodiment 4 of the present invention. Detailed Implementation

[0025] The present invention will now be further described with reference to the accompanying drawings.

[0026] This invention primarily utilizes a CO2 atmosphere to carbonate and dissolve lanthanide oxides, while uranium oxides do not dissolve, thus achieving the separation of lanthanum and actinium from spent fuel. Simultaneously, by removing the CO2 atmosphere from the upper gas phase of the molten salt, the lanthanide oxides are re-precipitated, purifying the molten salt and allowing for reuse. The invention is further described in detail below with reference to specific embodiments. The examples given are merely illustrative and not intended to limit the scope of the invention. Unless otherwise specified, all methods described are conventional. Unless otherwise specified, all raw materials were purchased from chemical reagent companies.

[0027] This invention discloses a method for separating lanthanide and actinide oxides based on a carbonate molten salt medium, comprising the following steps:

[0028] Step 1: Mix the carbonates in the specified proportions and then dry them;

[0029] Step 2: Dry the uranium oxide powder and lanthanide oxides, and mix them evenly in a certain proportion;

[0030] Step 3: Add the carbonate from Step 1 to the corundum crucible, load it into the reactor, and perform vacuuming and gas washing operations to achieve the change of atmosphere inside the container.

[0031] Step 4: Heat and maintain the temperature of the reactor until the carbonate melts. Then add the solid mixture from Step 2 and introduce CO2 gas into it at a certain flow rate to achieve bubbling and stirring.

[0032] Step 5: After the reaction is complete, a solid-liquid separation operation is performed. Uranium oxides are deposited at the bottom of the molten salt and will be recovered in solid form, while lanthanide oxides are dissolved in the molten salt by carbonation.

[0033] Step 6: After solid-liquid separation, the temperature of the molten salt is further increased, and the atmosphere is changed to air or an inert atmosphere. After holding at this temperature for a certain period of time, the dissolved lanthanide elements precipitate out again in solid form. After solid-liquid separation, the purification of carbonate molten salt can be achieved.

[0034] In step 1 above, the carbonate is an alkali metal carbonate, mainly a binary or ternary carbonate system composed of lithium carbonate, sodium carbonate, and potassium carbonate; for example: binary systems: 52%Li₂CO₃-48%Na₂CO₃, 50%Li₂CO₃-50%Na₂CO₃, 43%Li₂CO₃-57%K₂CO₃, 50%Li₂CO₃-50%K₂CO₃, etc.; ternary systems: 33%Li₂CO₃-34%Na₂CO₃-33%K₂CO₃, 44%Li₂CO₃-30%Na₂CO₃-26%K₂CO₃, 61%Li₂CO₃-22%Na₂CO₃-17%K₂CO₃, etc. The melting point of the carbonate system is between 400 and 650℃.

[0035] In step 2 above, the uranium oxide powder is UO2 or U3O8, and the lanthanide oxide includes one or more of La2O3, Nd2O3, Sm2O3, Gd2O3, Dy2O3, Er2O3, Yb2O3, and Lu2O3, with the lanthanide oxide accounting for 0.1% to 30% of the mixture content.

[0036] In steps 1 and 2 above, the drying temperature is 50~200℃ and the drying time is 4~12h. In order to prevent UO2 oxidation, step 2 should be vacuum drying.

[0037] In step 3 above, the vacuuming and gas washing operations are generally repeated 3 to 5 times, with high-purity CO2 used for the gas washing operation.

[0038] In step 4 above, the carbonate melting temperature is 400~650℃ and the time is 15~60min; after adding the solid mixture, the reaction temperature is 400~650℃, and the holding time is 10min~24h depending on the type and mass of lanthanide oxides added; increasing the reaction temperature will cause the lanthanide carbonates to undergo thermal decomposition, decomposing into the corresponding oxides, which is not conducive to the dissolution reaction.

[0039] In step 4 above, CO2 is bubbled into the molten salt using a corundum tube or generated by in-situ thermal decomposition of (NH4)2CO3 added to the carbonate. For in-situ thermal decomposition of (NH4)2CO3, uranium oxide and lanthanide oxides need to be thoroughly mixed with (NH4)2CO3, wherein the mass ratio of (NH4)2CO3 to lanthanide oxides is >40. The mixture is then pressed into tablets at a pressure of 10~700 MPa. Once the molten salt has been heated to the set temperature, the mixture is added to the molten salt.

[0040] In step 5 above, the filtration operation is carried out at the same temperature and atmosphere as in step 4. The solid-liquid separation operation can be carried out by filtration. Molten salt is poured into a metal filter screen. The filter screen material can be molybdenum, platinum or stainless steel, with a mesh size of 500~1000 mesh. Insoluble uranium oxide will be retained on the metal filter screen.

[0041] In step 6 above, the temperature is raised to 650~1000℃, the CO2 atmosphere is removed, and the lanthanides are re-precipitated. Introducing air or other inert gases is a method for removing the CO2 atmosphere, and the solid-liquid separation method is the same as that used in step 5.

[0042] Example 1

[0043] 1.95 g of lithium carbonate, 1.89 g of sodium carbonate, and 2.16 g of potassium carbonate were mixed thoroughly in a molar ratio of 44%-30%-26% and placed in an alumina crucible. Vacuuming and gas purging were performed. After the molten salt melted at 450°C, 0.05 g of Nd₂O₃ was added to the carbonate molten salt. The mixture was then stirred by blowing gas into the molten salt through an alumina tube at a flow rate of 12 mL / min for 120 min at 450°C. Tests showed that Nd₂O₃ dissolved efficiently in the carbonate molten salt, with a solubility of 98.6% after 1 hour. XRD analysis indicated that the lanthanide oxides were converted to Nd₂O₂CO₃.

[0044] Example 2

[0045] 1.95g of lithium carbonate, 1.89g of sodium carbonate, and 2.16g of potassium carbonate were added to an alumina crucible at a molar ratio of 44%-30%-26% and mixed thoroughly. Vacuuming, gas purging, and heating were then performed. After the molten salt melted, 1g of U3O8, 0.05g of La2O3, 0.05g of Nd2O3, 0.05g of Sm2O3, 0.05g of Gd2O3, 0.05g of Dy2O3, 0.05g of Er2O3, 0.05g of Yb2O3, and 0.05g of Lu2O3 were added to the carbonate molten salt. The mixture was stirred by blowing gas into different molten salt environments at a flow rate of 12mL / min using an alumina tube at 450℃ for a time controlled within 540min. The test results showed that the solubility of uranium in the carbonate molten salt was only 0.072%, while other lanthanide elements were completely dissolved.

[0046] Example 3

[0047] 1.95g of lithium carbonate, 1.89g of sodium carbonate, and 2.16g of potassium carbonate were mixed evenly in a molar ratio of 44%-30%-26% and placed in an alumina crucible. Vacuuming, gas purging, and heating were performed. After the molten salt melted, 0.05g of Nd₂O₃ and 0.05g of Sm₂O₃ were simultaneously added to the carbonate molten salt. The mixture was stirred by blowing gas into the molten salt through an alumina tube at a flow rate of 12mL / min at 450℃ for 60 minutes. It was found that under a CO₂ atmosphere, increasing the temperature from 450℃ to 650℃ slightly decreased the solubility of neodymium and samarium. The solubility of neodymium decreased from 91.3% to 85.8%, and the solubility of samarium decreased from 86.4% to 83.2%. Furthermore, the solubility of both neodymium and samarium when dissolved simultaneously was lower than their solubility when dissolved individually. After raising the temperature to 650℃, air was introduced, and the standing time was controlled at 60 minutes. The molten salt clarified and solidified. Further measurements showed that the solubility rates of neodymium and samarium were 2.6% and 2.3%, respectively. Figure 3 As shown. Subsequent filtration and separation steps can separate neodymium and samarium from the carbonate in solid form, thus purifying the carbonate.

[0048] Example 4

[0049] 1.95g lithium carbonate, 1.89g sodium carbonate, and 2.16g potassium carbonate were mixed evenly in a molar ratio of 44%-30%-26% and placed in an alumina crucible. Vacuuming, gas purging, and heating were performed. After the molten salt melted, 0.05g Nd₂O₃ and 0.05g Sm₂O₃ were simultaneously added to the carbonate molten salt. The mixture was stirred by blowing air into the molten salt through an alumina tube at a flow rate of 18mL / min at 450℃ for 120min. After sampling, the temperature was raised to 650℃ and air was introduced. After standing for 60min, solid-liquid separation was performed to obtain purified molten salt. This process was repeated three times. When the carbonate was recycled three times, it was found that the solubility of neodymium and samarium in the carbonate did not decrease significantly after the addition of lanthanide oxides to the carbonate molten salt. The solubility of neodymium remained at 99.53%, while the solubility of samarium was 98.79%. Figure 4 As shown in the figure. The study found that after repeated recycling, the carbonates still maintain a high solubility for lanthanide carbonates formed by the carbonation of lanthanide oxides, allowing for multiple separations and reuses.

[0050] Example 5

[0051] 1.95g of lithium carbonate, 1.89g of sodium carbonate, and 2.16g of potassium carbonate were mixed evenly in a molar ratio of 44%-30%-26% and placed in an alumina crucible. Vacuuming and gas purging were performed. After the molten salt melted at 450℃, 0.05g of Nd₂O₃ and 5g of (NH₄)₂CO₃ were mixed, compressed into tablets, and added to the molten carbonate. At 450℃, (NH₄)₂CO₃ underwent in-situ thermal decomposition, with gas blowing and stirring in the molten salt for 120 minutes. Tests showed that Nd₂O₃ dissolves efficiently in the molten carbonate.

[0052] Comparative Example 1

[0053] 15g of a ternary alkali metal carbonate was mixed thoroughly in a molar ratio of 33%Li-34%Na-33%K and placed in an alumina crucible. The mixture was heated to 600℃ under an air atmosphere. After the molten salt melted, 1g UO2, 0.6g Nd2O3, and 0.6g Sm2O3 were added to the carbonate molten salt, respectively. Air was then introduced through an alumina tube at a flow rate of 10 mL / min for 1 hour. The final solubility of uranium in the molten salt was determined to be less than 0.102%, neodymium oxide 0.660%, and samarium oxide 0.548%. Under these conditions, both lanthanide and actinide oxides exhibited low solubility and remained undissolved in the carbonate, indicating they could not be directly separated by filtration.

[0054] In summary, the method of this invention firstly, at a certain temperature, controls the upper layer of the molten salt to be in a CO2 atmosphere, causing the lanthanide oxides (Ln₂O₃) to carbonate and dissolve in the carbonate molten salt, while the actinide oxides deposit at the bottom of the molten salt, thus achieving lanthanum-actinide separation. Secondly, heating the molten salt containing dissolved lanthanides in an air or inert gas atmosphere causes the lanthanide oxides to precipitate again, achieving purification and reuse of the molten salt. In a carbonate system, the introduction of carbon dioxide causes the lanthanide oxides to convert into carbonates and dissolve in the molten salt. In other molten salts, such as chloride molten salts, the lanthanide carbonates formed will exist as precipitates. Currently, there are no reports on efficient separation of lanthanide and actinide oxides in carbonate molten salts, nor on methods for purifying and reusing the separated molten salt. The above description is a further detailed explanation of the invention in conjunction with specific preferred embodiments, and it should not be considered that the specific implementation of the invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the invention, and all such modifications or substitutions should be considered within the scope of protection of the invention.

Claims

1. A method for separating lanthanide and actinide oxides based on a carbonate molten salt medium, characterized in that: Includes the following steps: Step 1: After the carbonate system is stirred evenly, it is loaded into the reactor and vacuumed and washed in sequence. After the carbonate system is heated to a molten state, actinide oxide and lanthanide oxide are added and CO2 is introduced. The mixture is kept at this temperature for a period of time. Step 2: After the reaction is complete, the mixed molten salt is subjected to solid-liquid separation. The actinide oxides are recovered in solid form, while the lanthanide oxides are dissolved in the molten salt by carbonation. Step 3: Remove the CO2 atmosphere from the reactor, keep the molten salt containing lanthanide carbonates at a certain temperature for a period of time, and the carbonation-dissolved lanthanide oxides will precipitate again in solid form. After solid-liquid separation, the lanthanide oxides will be recovered.

2. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: In step 1, the carbonate system is a binary or ternary carbonate system of lithium carbonate, sodium carbonate, and potassium carbonate; the carbonate system is first pre-dried at a temperature of 50~200℃ for 4~12 hours.

3. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: In step 1, the vacuuming and gas washing operations are repeated 3 to 5 times; the gas used for washing is high-purity CO2; and the melting temperature of the carbonate system is 400 to 650°C.

4. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: In step 1, the actinide oxide is UO2 or U3O8; the lanthanide oxide is one or more of La2O3, Nd2O3, Sm2O3, Gd2O3, Dy2O3, Er2O3, Yb2O3, and Lu2O3; the lanthanide oxide accounts for 0.1% to 30% of the mass of the mixture of actinide oxide and lanthanide oxide; the total mass of the mixture of actinide oxide and lanthanide oxide is not higher than 20 wt% of the mass of the carbonate system; the mixture of actinide oxide and lanthanide oxide is pre-dried at a temperature of 50 to 200°C for 4 to 12 hours.

5. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: The method of introducing CO2 can be to directly bubble CO2 gas into the molten salt through a corundum tube; the CO2 gas flow rate increases accordingly with the increase of the mass of the carbonate system, and when the mass of the carbonate system is 6g, the gas flow rate is 5~30mL / min.

6. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: The method for introducing CO2 involves compressing a mixture of (NH4)2CO3 with actinide oxides and lanthanide oxides into tablets. During the heat preservation process, (NH4)2CO3 undergoes in-situ thermal decomposition to generate CO2. The mass ratio of (NH4)2CO3 to lanthanide oxides in the tablets is greater than 40.

7. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: In step 1, the heat preservation temperature is 400~650℃ and the time is 10min~24h.

8. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: In step 2, the solid-liquid separation is carried out by filtration. The mixed molten salt is poured into a metal filter screen, and the insoluble actinide oxides are retained on the metal filter screen. The solid-liquid separation is carried out at 400~650℃ under a CO2 atmosphere.

9. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 8, characterized in that: The metal filter screen is made of molybdenum, platinum, or stainless steel, with a mesh size of 500-1000 mesh.

10. The method for separating lanthanide and actinide oxides based on carbonate molten salt medium according to claim 1, characterized in that: In step 3, the method for removing the CO2 atmosphere in the reactor is to replace the atmosphere in the reactor with air or an inert gas; the heat preservation temperature is 650~1000℃, and the time is 0.5~3h.

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