Method for separating lanthanides and actinides from chloride aqueous solution

Abstract

The invention relates to the technical field of spent fuel treatment, and discloses a method for separating lanthanide and actinide from a chloride aqueous solution, which comprises the following steps: dissolving a raw material containing lanthanide oxide and actinide oxide into an aqueous solution of aluminum trichloride hexahydrate; after stirring and mixing, standing and separating to obtain a liquid phase containing lanthanide oxides and a solid phase containing actinide oxides, so that lanthanide and actinide are separated, and lanthanide impurities are effectively and selectively dissolved and removed from the actinide oxides. And adding oxalic acid into the liquid phase to precipitate, filtering and calcining to obtain the recovered lanthanide oxide. According to the method, the lanthanide oxide is dissolved and separated from the irradiation nuclear fuel in a high-selectivity mode through the aluminum chloride aqueous solution, the aluminum chloride aqueous solution can efficiently dissolve the lanthanide oxide, the uranium oxide and the thorium oxide are hardly dissolved, the effects of efficient separation and process simplification are achieved, the dissolving efficiency of the lanthanide oxide is high, and the separation effect is good.

Description

Technical Field

[0001] The invention relates to the technical field of spent fuel treatment, and in particular to a method for separating lanthanides and actinides by using a chloride aqueous solution. Background Art

[0002] Nuclear energy is an efficient, green and stable new energy source. As an important component of military, medical and industrial sectors, nuclear energy represents the core competitiveness of energy. The development of nuclear energy has become one of the keys to solving the world's energy shortage and global warming problems. Under the background and goals of "dual carbon", the development of China's nuclear energy will not only be used for power generation, but will also promote the green and low-carbon transformation of nuclear energy. The stable and reliable supply of nuclear fuel and the safe handling of spent fuel are the key ways to achieve the diversified application of nuclear energy, and the latter is a global problem that has not yet been fully solved.

[0003] Spent fuel is mainly composed of unused uranium (96%), nascent plutonium (1%), trace actinides (0.1%) and other fission products. The efficient recovery of uranium and plutonium is not only a prerequisite for ensuring the stable operation of nuclear power development, but also an important force to ensure strategic nuclear deterrence and long-term safety. Therefore, the recovery of uranium and plutonium in spent fuel and the separation of lanthanides from long-lived, highly radioactive minor actinides are important means to reduce energy consumption and environmental threats. The recycling process usually adopts the splitting and transmutation (P&T) strategy to separate radioactive nuclides and convert them into less harmful isotopes through neutron bombardment, but the lanthanides in the waste will also have a strong capture phenomenon of neutrons, thereby preventing the transmutable actinides from capturing neutrons, resulting in a decrease in transmutation efficiency. Therefore, the core of the P&T process is to separate lanthanides and minor actinides with similar properties to improve the transmutation efficiency.

[0004] The current separation process mostly adopts solvent extraction, which will produce a large amount of highly radioactive waste liquid, organic wastewater, high-salt waste liquid and other pollution, causing a large environmental burden and hindering the industrialization of the spent fuel treatment process. In order to achieve the effective separation of lanthanum and actinium and the green recovery of uranium and plutonium, it is necessary to adopt a new green system other than traditional hydrometallurgy. Some researchers have proposed to dissolve the spent fuel by selecting a new solvent with significant solubility differences between actinide and lanthanide oxides, while separating lanthanide and actinide elements, it also reduces environmental pollution from the source. In the literature (Fang-Li et al. Inorg. Chem. 2019, 58), a method for separating lanthanum and actinium in spent fuel using carboxyl-functionalized ionic liquid [Hbet][Tf2N] was reported. Since [Hbet][Tf2N] has an excellent dissolution effect on most lanthanide oxides, and the dissolution rate of actinide oxides such as UO2 in this ionic liquid is almost 0, it can achieve efficient separation of lanthanide and actinide oxides and recycling of lanthanide elements, providing a new green treatment method for spent fuel.

[0005] CN113265551A discloses a method for separating rare earth (lanthanide) element oxides, which utilizes the difference in solubility of rare earth elements and transition metal oxides in high-concentration zinc chloride solution to selectively dissolve and separate rare earth elements from rare earth secondary resources, and efficiently recover rare earth elements. A mixed solid raw material composed of rare earth elements and transition metal oxides is treated with a high-concentration zinc chloride solution, and the rare earth elements can be almost completely dissolved to obtain a primary treatment liquid and a first solid slag, which is composed of transition metal oxides and can be directly recycled; the primary treatment liquid and sodium sulfate are subjected to a first precipitation process to obtain a secondary treatment liquid and a first precipitation, and the secondary treatment liquid is a pure zinc chloride solution, which can be circulated to the first dissolution process to achieve reuse; then a sodium hydroxide solution and the first precipitation are used to perform a second precipitation process to generate a tertiary treatment liquid and a second precipitation; then a hydrochloric acid solution is used to dissolve the second precipitation to obtain a rare earth chloride solution, and the rare earth elements in the chloride aqueous solution are precipitated and calcined by oxalic acid, thereby realizing the recovery of rare earth elements.

[0006] Compared with the traditional acid leaching method, the dissolution method reduces the consumption of chemical substances and environmental pollution, and the operation process is safe and simple. However, in the field of lanthanum / actinium separation, since the properties of the two types of elemental oxides are similar, unlike the chemical properties of rare earth oxides and transition metal oxides, which are quite different, it is difficult to achieve separation with ordinary inorganic salt solutions. Summary of the invention

[0007] Aiming at the problem of difficulty in extracting and separating lanthanum / actinium, the present invention provides a method for separating lanthanide elements and actinide elements by using a chloride aqueous solution. It is found that aluminum trichloride has high selectivity for lanthanide oxides, and can achieve efficient separation of lanthanum and actinium elements and effective recovery of lanthanide oxides, thereby achieving the effect of reducing energy and material consumption and radioactive pollutants.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for separating lanthanide elements and actinide elements by using chloride aqueous solution comprises the steps of: dissolving raw materials containing lanthanide element oxide and actinide element oxide in an aqueous solution of aluminum chloride hexahydrate, stirring and mixing, and standing for separation to obtain a liquid phase containing lanthanide element oxide and a solid phase containing actinide element oxide, thereby separating lanthanum and actinide elements.

[0010] The present invention adopts a specific inorganic salt solution formula and a selective leaching process to achieve selective dissolution and separation of lanthanum and actinium. The principle may be that in a chloride aqueous solution, metal ions combine with hydroxyl groups, and hydrogen ions are ionized in the water, so that the solution itself is acidic to a certain extent. When the ratio of water molecules to metal ions in the high-valent metal ion salt solution is low enough, the external hydration effect is reduced, and the acidity of the solution is enhanced, which is conducive to its reaction with lanthanide elements with stronger alkalinity, thereby selectively dissolving and removing lanthanide impurities in a mixture raw material of actinide oxides.

[0011] The lanthanide oxides are La2O3, Pr6O 11 , Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3 or more.

[0012] Preferably, the lanthanide oxide is one or more of La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. These types of lanthanide oxides have better solubility and better extraction and separation effects.

[0013] The solid-liquid ratio of the raw material and the aqueous solution is 1:20-70, and the solid-liquid ratio of the raw material and the aqueous solution is preferably 1:40-70. The increase in the concentration of the chloride aqueous solution, the decrease in the solid-liquid ratio, and the increase in the reaction temperature and reaction time are all conducive to the dissolution of lanthanide elements by the chloride aqueous solution. However, too high a temperature and too long a reaction time will greatly increase the loss of material energy and reduce the separation efficiency. Therefore, in the condition optimization experiment, when the continued change of the conditions has little effect on the dissolution effect of the lanthanide oxide, it is regarded as one of the best conditions for the dissolution experiment. Under this condition, the chloride aqueous solution has a good dissolution effect on the lanthanide oxide (dissolution rate ≥ 95%), and has almost no dissolution effect on the oxides of uranium and thorium (dissolution rate ≤ 0.1%), and the separation effect between lanthanide elements and uranium and thorium is good.

[0014] The mixing temperature is 45-85° C., preferably 65° C.-85° C., at which the solution has a higher dissolution rate for lanthanide oxides and a better separation effect.

[0015] The mixing time is more than 2 hours, preferably more than 4 hours, and more preferably, the mixing time is 4-7 hours. About 7 hours can basically achieve 99% separation effect.

[0016] The mass ratio of aluminum chloride hexahydrate to water in the aqueous solution of aluminum chloride hexahydrate is 1:5 to 45. Preferably, the mass ratio of aluminum chloride hexahydrate to water is 1:5 to 30.

[0017] The actinide oxides include one or more of UO2 and ThO2.

[0018] Preferably, the process further comprises adding oxalic acid to the liquid phase for precipitation, filtering and calcining to obtain the recovered lanthanide oxide. Oxalic acid is added to the filtrate containing the lanthanide, and the lanthanide is precipitated in the form of lanthanide oxalate, and then the impurities in the oxalate solid are removed by high temperature calcination to form new lanthanide oxide, thereby realizing the recovery of the lanthanide.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The present invention utilizes an aqueous solution of aluminum chloride to selectively dissolve and separate lanthanide elements from irradiated nuclear fuel. The aqueous solution of aluminum chloride can efficiently dissolve lanthanide oxides, but has almost no dissolving effect on uranium and thorium oxides, thereby achieving the effect of efficient separation and simplified process.

[0021] (2) The method for separating lanthanum and actinium elements of the present invention has high dissolution efficiency for lanthanide oxides and good separation effect.

[0022] (3) Compared with the conventional hydrochloric acid complete dissolution method, the chloride aqueous solution of the present invention has the advantages of being safe to use, reducing the generation of acidic waste liquid and not generating volatile gases. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the dissolution rate of 14 lanthanide oxides and UO2 and ThO2 by aluminum chloride solution in Example 6.

[0024] Figure 2 This is the XRD spectrum of La2O3 obtained by oxalic acid precipitation and further calcination in Example 7. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with embodiment. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Those skilled in the art can make modifications or equivalent substitutions based on the understanding of the technical scheme of the present invention without departing from the spirit and scope of the technical scheme of the present invention, and all should be included in the protection scope of the present invention.

[0026] In the following specific embodiments, the lanthanide oxides used are all purchased from the market, and UO2 and ThO2 are prepared in the laboratory. Since there are many types of lanthanide oxides, Examples 1-5 select La2O3, Er2O3 and Lu2O3 as representatives of lanthanide oxides for dissolution and separation experiments. The dissolution rate D is calculated according to the following formula:

[0027]

[0028] Where W M,d is the mass fraction of metal in the extraction solution, m l is the mass of the solution, m M,r is the mass of metal in the oxide.

[0029] Example 1

[0030] Aqueous chloride solutions (KCl, ZnCl2, NiCl2, FeCl3 and AlCl3) of the same molar concentration (0.276mmol / L) were prepared, lutetium oxide and aqueous chloride solutions were mixed at a solid-liquid ratio of 1:50, and the mixture was stirred on a magnetic stirrer at a temperature of 85°C and a speed of 300rpm for 5 hours. After the reaction, the filtrate was filtered using a 0.45μm filter membrane, and the dissolution rates of lanthanide elements in different aqueous chloride solutions were calculated. The dissolution data are summarized in Table 1. It can be seen from Table 1 that in aqueous metal chloride solutions of the same concentration, the dissolution rate of lutetium oxide in aluminum chloride solution is close to 100%, and is much greater than the solubility of lutetium oxide in other chloride salt solutions.

[0031] Table 1 Dissolution data of lutetium oxide in chloride solution

[0032]

[0033]

[0034] Example 2

[0035] Aluminum trichloride (hexahydrate) solid was mixed with deionized water and placed on a magnetic stirrer, stirred at 600 rpm for 10 min to obtain an aluminum chloride solution. The prepared aluminum chloride solution was mixed with five lanthanide oxides (La2O3, Er2O3, Lu2O3, UO2 and ThO2) at a solid-liquid ratio of 1:50, and the mixture was placed on a magnetic stirrer at a temperature of 85°C and a speed of 300 rpm for 5 h. After the reaction was completed, the mixed solution was filtered using a 0.45 μm filter membrane, and the dissolution rate of lanthanide elements in the filtrate was calculated. The data are summarized in Table 2.

[0036] As can be seen from Table 2, the solution with m(H2O):m(AlCl3·6H2O) of 15:1 (H2O-AlCl3-15:1) can dissolve La2O3, Er2O3 and Lu2O3 at a rate of more than 96%, and the dissolution rates of UO2 and ThO2 are 0.03% and 0.21% respectively. The solution has a significantly better dissolution effect on lanthanide oxides than UO2 and ThO2.

[0037] Table 2 Dissolution data of La2O3, Er2O3 and Lu2O3 in aluminum chloride solutions of different concentrations

[0038]

[0039]

[0040] Example 3

[0041] An aluminum chloride solution with a m(H2O):m(AlCl3·6H2O) ratio of 30:1 was prepared by the method in Example 2, and mixed with five oxides (La2O3, Er2O3, Lu2O3, UO2 and ThO2) at a solid-liquid ratio of 1:70, and the mixture was placed on a magnetic stirrer at a temperature of 85°C and a speed of 300 rpm for 5 h. The dissolution rates of lanthanide elements in the obtained filtrate were calculated, and the dissolution data are summarized in Table 3.

[0042] Table 3 Dissolution data of La2O3, Er2O3 and Lu2O3 at different solid-liquid ratios

[0043]

[0044] It can be seen from Table 3 that when the mass ratio of lanthanide oxides to aluminum chloride solution is 1:70, the dissolution rates of aluminum chloride solution for La2O3, Er2O3 and Lu2O3 can reach more than 99%, and the dissolution rates for UO2 and ThO2 are 0.06% and 0.18% respectively. The solubility difference between this solution and lanthanide oxides and UO2 / ThO2 is huge, and it has the potential to achieve the separation of the two.

[0045] Example 4

[0046] The aluminum chloride solution prepared in Example 3 was mixed with five oxides (La2O3, Er2O3, Lu2O3, UO2 and ThO2) at a solid-liquid ratio of 1:50, and the mixture was stirred on a magnetic stirrer at a temperature of 75°C and a speed of 300 rpm for 5 hours. The dissolution rate of lanthanide elements in the obtained filtrate was calculated, and the data are summarized in Table 4.

[0047] It can be seen from Table 4 that under this condition, the dissolution rate of aluminum chloride solution for La2O3, Er2O3 and Lu2O3 can reach more than 93%, and the dissolution rates for UO2 and ThO2 are 0.06% and 0.25% respectively. The large dissolution difference makes it possible to effectively separate lanthanide oxides and UO2 / ThO2 in the solution.

[0048] Table 4 Dissolution data of La2O3, Er2O3 and Lu2O3 at different temperatures

[0049]

[0050] Example 5

[0051] The aluminum chloride solution prepared in Example 3 was mixed with five oxides (La2O3, Er2O3, Lu2O3, UO2 and ThO2) at a solid-liquid ratio of 1:50, and the mixture was placed on a magnetic stirrer at a temperature of 85°C and a speed of 300 rpm for 7 hours. The dissolution rate of lanthanide elements in the obtained filtrate was calculated, and the data are summarized in Table 5.

[0052] Table 5 Effect of dissolution time on the solubility of La2O3, Er2O3 and Lu2O3

[0053]

[0054] It can be seen from Table 5 that under this condition, the dissolution rate of aluminum chloride solution for La2O3, Er2O3 and Lu2O3 can reach more than 96%, and the dissolution rates for UO2 and ThO2 are 0.06% and 0.16% respectively. The solubility of this solution for lanthanide oxides and UO2 / ThO2 is very different, and the separation of the two can be achieved.

[0055] Example 6

[0056] The aluminum chloride solution prepared in Example 3 was mixed with 14 lanthanide oxides and UO2 and ThO2 at a solid-liquid ratio of 1:50, and the mixture was placed on a magnetic stirrer at a temperature of 75°C and a speed of 300 rpm for 7 hours. The dissolution rate of each metal element in the obtained filtrate was calculated. The results are as follows: Figure 1 The data are summarized in Table 6.

[0057] Table 6 Dissolution rate of oxides in aluminum chloride (hexahydrate) solution

[0058] Lanthanide oxides Dissolution rate (%) <![CDATA[La2O3]]> 98.98 <![CDATA[CeO2]]> 0.05 <![CDATA[Pr6O 11 ]]> 69.21 <![CDATA[Nd2O3]]> 99.27 <![CDATA[Sm2O3]]> 98.59 <![CDATA[Eu2O3]]> 99.28 <![CDATA[Gd2O3]]> 99.38 <![CDATA[Tb4O7]]> 37.39 <![CDATA[Dy2O3]]> 96.05 <![CDATA[Ho2O3]]> 96.94 <![CDATA[Er2O3]]> 98.13 <![CDATA[Tm2O3]]> 98.50 <![CDATA[Yb2O3]]> 96.46 <![CDATA[Lu2O3]]> 95.57 <![CDATA[UO2]]> 0.03 <![CDATA[ThO2]]> 0.20

[0059] It can be seen from Table 6 that under this condition, aluminum chloride solution has a good dissolution effect on most lanthanide oxides, while Pr6O 11 The dissolution rates of high-valent metal oxides such as UO2 and Tb4O7 are relatively low. At this time, the dissolution rates of UO2 and ThO2 are 0.03% and 0.20%, respectively, which are very different from the dissolution of lanthanide oxides. Therefore, under the optimal conditions, aluminum chloride solution can effectively separate most lanthanide oxides and UO2 / ThO2.

[0060] Example 7

[0061] Take the filtrate obtained after dissolving La2O3 in Example 4, add oxalic acid solid with a mass of 1.6 times the equivalent of lanthanide elements, and place the mixture on a magnetic stirrer at a temperature of 25°C and a speed of 300rpm for 6 hours. After stirring, the mixture will be allowed to stand for 18 hours at the same temperature for aging, and then filtered to obtain a white solid with the main component of La2(C2O4)3. Then, the obtained solid is placed in a muffle furnace for calcination at a calcination temperature of 900°C. After calcination for 6 hours, La2O3 solid is obtained, and its XRD spectrum is as follows: Figure 2 As shown, the recovery of lanthanide elements was achieved.

Claims

1. A method for separating lanthanides and actinides from aqueous chloride solution, characterized in that: The method comprises the following steps: dissolving raw materials containing lanthanide oxide and actinide oxide in an aqueous solution of aluminum chloride hexahydrate, stirring and mixing, and then standing and separating to obtain a liquid phase containing lanthanide oxide and a solid phase containing actinide oxide.

2. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The lanthanide oxides are La2O3, Pr6O 11 , Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3 or more.

3. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The lanthanide oxide is one or more of La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3.

4. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The solid-to-liquid ratio of the raw material to the aqueous solution is 1:20-70.

5. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The solid-to-liquid ratio of the raw material to the aqueous solution is 1:40-70.

6. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The mixing temperature is 45-85°C.

7. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The mixing time is more than 2 hours.

8. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The mass ratio of aluminum chloride hexahydrate to water in the aqueous solution of aluminum chloride hexahydrate is 1:5-45.

9. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The actinide oxides include one or more of UO2 and ThO2.

10. The method for separating lanthanides and actinides from aqueous chloride solution according to claim 1, characterized in that: The method also includes adding oxalic acid to the liquid phase for precipitation, filtering and calcining to obtain the recovered lanthanide oxide.

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