CN114908381A - Method for removing rare earth ions in waste molten salt - Google Patents

Abstract

The invention provides a method for removing rare earth ions in radioactive waste molten salt, which comprises the step of adding Li into the waste molten salt at the temperature of 300-800 DEG C ₂ O reacts to remove the rare earth ions. By adding Li into the radioactive waste molten salt at 300-800 deg.C ₂ The O is subjected to liquid phase reaction, so that the generation of a large amount of waste gas is avoided, and the method has the advantages of high reaction rate and less environmental pollution. In addition, the method also ensures the rare earth ion removal effect, and does not introduce impurity ions which are difficult to remove or generate a large amount of waste.

Description

A method for removing rare earth ions in waste molten salt

technical field

The invention relates to the field of post-processing of nuclear power spent fuel, in particular to a method for removing rare earth ions in waste molten salt, in particular to a method for removing rare earth ions in waste molten salt produced by an electrolytic refining method.

Background technique

The development of clean and efficient green energy is imminent. Nuclear power represents an important development direction of green energy, is currently booming, and is expected to become one of the mainstream energy sources in the future.

It is difficult to ensure the sustainable development of nuclear power only by relying on natural uranium resources, and efficient utilization of uranium resources, especially nuclear power fuel, is inevitable. At present, the nuclear power industry mostly adopts the water reprocessing process to treat the spent fuel produced by the nuclear power industry.

Compared with the water reprocessing process, the dry reprocessing process has obvious advantages such as short cooling period of spent fuel, few steps, and small waste volume. Dry post-treatment processes mainly include fluorination volatilization method, molten salt liquid-liquid extraction method and high temperature chemical method (such as electrolytic refining method) using molten salt as a medium. Among them, electrolytic refining is the most promising. Electrolytic refining still produces a large amount of waste molten salt with rare earth ions. In order to minimize the amount of nuclear waste, the waste molten salt needs to be purified and reused.

At present, the methods developed by various countries to remove rare earth ions in waste molten salt include phosphate precipitation, zeolite adsorption, and oxygen precipitation. Phosphate radicals are difficult to remove in the phosphate precipitation method, and impurity ions are introduced into the reaction system. The zeolite adsorption method uses 4A-type zeolite with a large structure, which produces a large amount of waste, and it is also difficult to remove the rare earth ions in the waste molten salt. Although the oxygen precipitation method does not introduce impurity ions when removing rare earth ions, and the removal rate of rare earth ions is also good, this method produces a large amount of waste gas and pollutes the environment; at the same time, this method is a gas-liquid reaction, the contact of the reactants is incomplete, and the reaction rate is slow .

Therefore, it is necessary to develop a method for removing rare earth ions in waste molten salt with fast reaction rate and less environmental pollution.

SUMMARY OF THE INVENTION

In view of this, the main purpose of the present invention is to provide a method for removing rare earth ions in radioactive waste molten salt which can solve at least part of the above problems.

According to the present disclosure, a method for removing rare earth ions in radioactive waste molten salt is provided, which includes adding Li ₂ O to the waste molten salt at 300° C.˜800° C. for reaction to remove the rare earth ions.

According to some embodiments, Li ₂ O is added gradually.

According to some embodiments, the method further includes, during the reaction, testing the reaction system by an electrochemical method, obtaining an electrochemical curve, and judging the reaction progress according to the obtained electrochemical curve.

According to some embodiments, the electrochemical method comprises cyclic voltammetry, square wave voltammetry, or open circuit chronopotentiometry.

According to some embodiments, when the oxidation and/or reduction peaks of Li ₂ O appear in the electrochemical curve, it is determined that the reaction progress is that the rare earth ions have been removed, the addition of Li ₂ O is stopped, and the reaction is terminated.

According to some embodiments, a plurality of different regions of the reaction system are tested to obtain a plurality of the electrochemical graphs, wherein,

If no oxidation and/or reduction peaks of Li ₂ O appear in the multiple electrochemical curves, it is judged that the reaction progress is that the rare earth ions are still not removed in the reaction system, and Li ₂ O is continued to be added;

If at least one of the electrochemical curves does not show the oxidation and/or reduction peaks of Li ₂ O, and at least one of the electrochemical curves shows the oxidation and/or reduction peaks of Li ₂ O, then It is judged that the reaction progress is a region in the reaction system where the rare earth ions have not been removed and a region where the rare earth ions have been completely removed and Li ₂ O is excessive, stop adding Li ₂ O, and continue the reaction; or

If the oxidation and/or reduction peaks of Li ₂ O appear in the multiple electrochemical curves, it is judged that the reaction progress is that the rare earth ions in the reaction system have been completely removed, the addition of Li ₂ O is stopped and the reaction is terminated .

The plurality of different zones include upper, middle and lower parts of the reaction system. The plurality of electrochemical graphs are at least three.

According to some embodiments, before performing the electrochemical method test, the spent molten salt is allowed to stand for 2-10 minutes, such as 3, 4, 5, 6, 7 minutes.

According to some embodiments, a three-electrode system is used in the electrochemical method, wherein the working electrode is a tungsten wire or a molybdenum wire, the auxiliary electrode is a graphite rod, a molybdenum rod or a tungsten rod; and the reference electrode is Ag/AgCl.

According to some embodiments, the method further includes, after the reaction is completed, using vacuum distillation or molten salt filtration to remove the precipitate in the reaction system to obtain a molten salt from which rare earth ions are removed. Optionally, when there is excess Li ₂ O in the rare earth ion-removing molten salt, the method further includes electrolyzing the rare earth ion-removing molten salt at 400° C.˜800° C. to remove the excess Li ₂ O.

According to some embodiments, the electrolysis adopts a potentiostatic method, and when the electrolysis current reaches 1 mA, the electrolysis is stopped.

According to some embodiments, the electrolysis uses a three-electrode system, wherein the working electrode is a tungsten wire or a molybdenum wire, the auxiliary electrode is a graphite rod, a molybdenum rod or a tungsten rod; and the reference electrode is Ag/AgCl.

By adding Li ₂ O to the radioactive waste molten salt at 300°C to 800°C to carry out the liquid phase reaction, the generation of a large amount of waste gas is avoided, and the reaction rate is fast and the environmental pollution is less. At the same time, the method also ensures the removal effect of rare earth ions, and does not introduce impurity ions that are difficult to remove or generate a large amount of waste.

Description of drawings

Fig. 1 shows the electrochemical curves obtained by using square wave voltammetry to test the molten salt without adding Li ₂ O and the reaction system after accumulatively adding 0.20g and 0.40g Li ₂ O to the molten salt and reacting respectively in the embodiment. picture;

Figure 2 shows the electrochemical curves obtained by using square wave voltammetry to test the reaction system obtained by accumulatively adding 0.50g, 0.60g, 0.65g, 0.70g Li ₂ O to molten salt and reacting respectively in the embodiment;

3 shows the electrochemical curves obtained by using square wave voltammetry to test the reaction system obtained by accumulatively adding 0.75g, 0.80g, 0.85g, 0.90g Li ₂ O in molten salt and reacting respectively in the embodiment;

Fig. 4 shows the electrochemical curve obtained by using square wave voltammetry to test the reaction system after accumulatively adding 0.70g Li ₂ O in molten salt and reacting in the embodiment;

Fig. 5 shows the electrochemical curve obtained by using square wave voltammetry to test the reaction system after accumulatively adding 0.75g Li ₂ O to molten salt and reacting in the embodiment;

Fig. 6 shows the electrochemical curve obtained by using square wave voltammetry to test the reaction system after accumulatively adding 0.90g Li ₂ O to molten salt and reacting in the embodiment;

Fig. 7 shows the electrochemical graph obtained after electrolysis of the filtered molten salt using cyclic voltammetry in the embodiment;

FIG. 8 shows an electrochemical curve obtained by using square wave voltammetry to test the filtered molten salt after electrolysis.

Detailed ways

The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

Throughout the specification, unless specifically stated otherwise, terms used herein are to be understood as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. In case of conflict, the present specification takes precedence.

The present disclosure is intended to provide a method for removing rare earth ions in waste molten salt with fast reaction rate and less environmental pollution, so as to be applied in the field of nuclear power spent fuel reprocessing, especially in the field of dry reprocessing of nuclear power spent fuel, more particularly The field of purification of waste molten salt produced by electrolytic refining. Among them, the waste molten salt produced during electrolytic refining contains such as LiCl-KCl eutectic salt and a large amount of rare earth elements, such as La, Ce, Pr, Nd, Y and other rare earth elements with a total content of >1000ppm.

In order to achieve the above purpose, the present application provides a method for removing rare earth ions in radioactive waste molten salt, comprising adding Li ₂ O to the waste molten salt at 300°C to 800°C, preferably 450°C to 550°C for reaction to remove rare earth ions .

The inventors found that by adding Li ₂ O to the radioactive waste molten salt containing molten salt and a large amount of rare earth elements (for example, elements such as La, Ce, Pr, Nd, Y, etc., with a total content of >1000 ppm), the displacement reaction is carried out while stirring, The soluble rare earth chloride ions in the waste molten salt are converted into insoluble rare earth oxychloride or rare earth oxide precipitates to remove rare earth ions. The reaction speed is fast, the reaction is thorough, the removal rate of rare earth elements is high, and no other ions are introduced, and no waste gas is generated. And even if there is excess lithium oxide, it can be easily removed by electrolysis.

The present disclosure has no particular limitation on the radioactive waste molten salt, and any waste molten salt produced by the post-processing of the electrolytic refining of nuclear power spent fuel can be applied to the method of the present invention. The molten salt is usually two or more mixed salts, typically, it may be a LiCl-KCl eutectic salt, but not limited thereto.

The specific steps of the method are exemplified below. The following exemplary description is only for the convenience of understanding, and the present method is not limited to the content described below.

Since the method involves the handling of radioactive materials, an unmanned automated device is used to remotely control the reaction process. The method uses a reaction system equipped with an automatic lifting system and a remotely controllable stirring device to carry out the reaction. The material of the stirring paddle can be corundum, stainless steel, nickel-based alloy, tungsten and other materials that are resistant to high temperature and radiation corrosion.

The radioactive waste molten salt is introduced into the above reaction system, the temperature is raised to 300°C to 800°C, and Li ₂ O is added under stirring to carry out the reaction. Preferably, the reaction temperature is 450°C to 550°C, for example 500°C. The reaction time is 0.1～2h. It should be noted that the reaction time can be adjusted according to the amount of the waste molten salt and the reaction temperature, and the reaction time is not particularly limited in the present application.

Since the reaction of the method is a liquid-phase reaction, the generation of waste gas is avoided, and the reaction rate is fast and the environmental pollution is less. In addition, on the premise of ensuring the removal effect of rare earth ions, the method does not introduce impurity ions that are difficult to remove, and does not generate a large amount of waste.

According to some embodiments, Li ₂ O is added gradually. That is, at the beginning of the reaction, the amount of Li ₂ O added is lower than the chemical reaction equivalent, and Li ₂ O is continuously added as the reaction proceeds. For example, the same amount of Li ₂ O may be added in sequence during the reaction; the addition amount of Li ₂ O may also be continuously reduced as the reaction proceeds, for example, Li ₂ O may be added in a manner of decreasing the addition amount in a gradient manner, but not limited to this. In this way, the excess of Li ₂ O can be avoided to a certain extent.

According to a preferred embodiment, the method further includes, during the reaction, using an electrochemical method to test the reaction progress in the reaction system, so that the reaction end point can be judged in real time, and the reaction can be terminated in time. Due to the low redox potential of Li ₂ O, once excess Li ₂ O exists in the reaction system, the oxidation and/or reduction peaks of Li ₂ O can be detected, thereby confirming the completion of the reaction. In this way, the amount of reagent, ie, Li ₂ O, can be reduced, and post-processing steps after an excess of Li ₂ O can be avoided.

Specifically, according to some embodiments, the electrochemical method for monitoring the progress of the reaction may be cyclic voltammetry, square wave voltammetry, or open circuit chronopotentiometry. Cyclic voltammetry is more commonly used due to the small change in the electrode surface before and after monitoring. Monitoring with open-circuit chronopotentiometry can achieve higher accuracy, but for longer periods of time. Square wave voltammetry is more sensitive to electrochemical signals and can quickly obtain electrochemical curves, which is the preferred electrochemical monitoring method. The electrochemical curve is obtained by electrochemical method, and the reaction progress is judged accordingly.

Specifically, according to some embodiments, when judging the progress of the reaction, when the oxidation and/or reduction peaks of Li ₂ O appear in the electrochemical curve, it is judged that the reaction progress is that rare earth ions have been removed, the addition of Li ₂ O is stopped and the reaction is terminated. In the existing method of judging the reaction end point by metal potential, there are often errors in judgment due to the existence of the electrochemical detection limit. It is more accurate to judge the end point of the reaction by the presence or absence of the oxidation and/or reduction peaks of Li ₂ O, and the excess of Li ₂ O can be avoided more accurately.

Further, according to a preferred embodiment, a plurality of different regions of the reaction system are tested to obtain a plurality of electrochemical curves. When judging the progress of the reaction, if no oxidation and/or reduction peaks of Li ₂ O appeared in the multiple electrochemical curves, it was judged that the reaction progress was that there were still rare earth ions in the reaction system that had not been removed, and Li ₂ O was continued to be added; If there is no oxidation and/or reduction peak of Li ₂ O in at least one of the electrochemical curves, and there is an oxidation and/or reduction peak of Li ₂ O in at least one of the electrochemical curves, the reaction process is judged as There are still areas in the reaction system where the rare earth ions have not been removed and areas where the rare earth ions have been completely removed and the Li ₂ O is excessive, stop adding Li ₂ O, and continue to stir for the reaction; or Li ₂ O appears in multiple electrochemical curves. Oxidation and/or reduction peaks, it is judged that the reaction progress is that the rare earth ions in the reaction system have been completely removed, the addition of Li ₂ O is stopped and the reaction is terminated.

For example, three different regions of the reaction system (eg, upper, middle, and lower parts of the reaction system) are tested, and three electrochemical curves are obtained. When judging the progress of the reaction, if the oxidation and/or reduction peaks of Li ₂ O do not appear in the three pictures, it means that there are still rare earth ions in the reaction system that have not been removed. Continue to add Li ₂ O and stir; if at least one of the three pictures appears The oxidation and/or reduction peaks of Li ₂ O are detected, and at least one of the oxidation and/or reduction peaks of Li ₂ O does not appear, indicating that the reaction time is insufficient and the reaction continues; if the oxidation and/or reduction of Li ₂ O appears in all three figures Or the reduction peak, indicating that the rare earth ions have been completely removed, stop adding Li ₂ O and end the reaction. Other numbers of regions of the reaction system can also be tested to obtain a corresponding number of electrochemical plots. The present disclosure does not limit the number of test areas and electrochemical graphs. This judgment method of testing multiple regions of the reaction system to obtain multiple electrochemical graphs can avoid making limited or biased judgments based only on the electrochemical graphs of a certain or part of the regions due to inconsistent reaction processes in different regions. The problem. Therefore, the method of multi-regional testing of the reaction system can not only ensure the complete removal of rare earth ions in the entire reaction system, but also more accurately avoid the excess of Li ₂ O.

According to some embodiments, before the electrochemical method test, the stirring is temporarily stopped, and the reaction system is allowed to stand for a period of time, such as 2-10 min, such as 3, 4, 5, 6, 7 min, preferably 5 min.

According to some embodiments, the method further includes removing the precipitate in the reaction system by a vacuum distillation method or a molten salt filtration method after the reaction ends to obtain a molten salt from which rare earth ions are removed. Specifically, the vacuum distillation method utilizes the saturated vapor pressure difference between chlorides and oxides or oxychlorides to separate them, and the used distillation apparatus is bulky and the distillation time is long, but the purity of the obtained separation product is relatively high. Although the filtration method has higher requirements on the filter material, it can quickly separate the chloride and the precipitate, which is the preferred separation method.

According to some embodiments, the method further comprises electrolyzing the rare earth ion-removing molten salt at 400°C to 800°C, preferably 450°C to 550°C (eg 500°C) to remove excess Li ₂ O, if any . Specifically, the electrolysis adopts a potentiostatic method, and when the electrolysis current reaches 1 mA, the electrolysis is stopped. Taking advantage of the fact that the decomposition voltage of Li ₂ O is lower than that of molten salt, the electrolytic current of 1 mA is used as the judgment end point, and the potentiostatic electrolysis method is used to remove the excess Li ₂ O in the waste molten salt well, making the waste molten salt Not only is it purified, but it can also be recycled, which minimizes the reprocessing waste of nuclear power spent fuel.

According to some embodiments, the electrochemical method for obtaining the electrochemical graph or the electrolysis for removing excess Li ₂ O in the spent molten salt can use a three-electrode system, wherein the working electrode is a tungsten wire or a molybdenum wire (for example, three lengths of Different working electrodes are assembled together and separated by ceramics. The working electrodes are inserted into the molten salt. The three working electrodes are located at the upper, middle and bottom of the molten salt, respectively. The conduction state of the three electrodes is controlled to obtain different areas of the molten salt in turn. The electrochemical curve), the auxiliary electrode is a graphite rod, a molybdenum rod or a tungsten rod; the reference electrode is Ag/AgCl. Preferably, the material of the working electrode is molybdenum, and when the rare earth concentration is relatively high, a relatively accurate electrochemical signal can still be obtained. Remotely use the three-electrode system that cooperates with the electrochemical workstation and can be automatically raised and lowered to obtain the electrochemical curve online. It is easy to operate, and there is no need to interrupt the treatment process of the waste molten salt, and it is not necessary to take out the sample of the waste molten salt, which avoids the operation. risk of exposure to nuclear radiation.

Hereinafter, the present application will be further described through specific examples.

Example

Weigh 38g LiCl, 45g KCl, 2g LaCl ₃ , 2g CeCl ₃ and 2g NdCl ₃ in turn for the preparation of simulated waste molten salt (hereinafter referred to as molten salt); 0.05g, 0.05g, 0.05g, 0.05g, 0.05g, 0.05g of Li ₂ O were used for later use.

Put LiCl and KCl in a corundum crucible, heat to 500 ° C to melt the two chlorides, then add LaCl ₃ , CeCl ₃ and NdCl ₃ , and heat and stir to 500 ° C to melt the three chlorides added later, to prepare molten salt.

Heating to maintain the molten state of the molten salt, let stand for 5 minutes, use the three-electrode system (Mo working electrode, graphite auxiliary electrode, Ag/AgCl reference electrode) connected to the electrochemical workstation to enter the middle of the molten salt, and use square wave voltammetry to enter the middle of the molten salt. The test method is used to obtain the square wave voltammetry curve, as shown in curve 1 in Figure 1.

Add the above Li ₂ O weighed in sequence while stirring, let the system react for 1 min after each addition of Li ₂ O, and then let it stand for 5 min. The square wave voltammetry curve obtained by the above method is shown in Figure 1-6 .

The molten salt and the precipitate were separated by molten salt filtration.

Using platinum as the anode and molybdenum as the cathode, the separated molten salt is electrolyzed at a constant potential until the current is less than 1 mA to remove excess Li ₂ O. Subsequently, the molten salt was electrochemically tested by cyclic voltammetry and square wave voltammetry (both methods used Mo working electrode, graphite counter electrode, Ag/AgCl reference electrode) to obtain cyclic voltammetry curves and square wave voltammetry The results are shown in Figure 7 (cyclic voltammetry curve) and Figure 8 (square wave voltammetry curve).

Among them, in each figure, peak A in each curve is the oxidation and/or reduction peak of Li ion (A1 is the oxidation peak of Li ion, A2 is the reduction peak of Li ion), and peak B is La ion, Ce ion and Nd ion The eutectic reduction peak of ions, peak C is the reduction peak of Li ₂ O, and peak D represents the reduction peak of one of the two-step 3-electron transfer process when the Nd ³⁺ concentration is high.

Curves 1-11 in Fig. 1-3 correspond to the square wave voltammetry curves measured above without adding Li 2 O and gradually adding Li ₂ O respectively. Referring specifically to Figures 1 and 2, the peak B in curves 1-7 decreases continuously until disappearing with the addition of Li ₂ O, indicating that with the addition of Li ₂ O, the concentration of La ion, Ce ion and Nd ion in the molten salt increases. The concentration is continuously decreased until it falls below the detection limit of the electrochemical workstation. The intensity of peak C in curves 8-11 in Fig. 3 increases continuously with the addition of Li ₂ O, indicating that with the addition of Li _{2 O, the concentration of Li 2 O} in the molten salt is higher and higher, but the peak intensity increases with the addition of Li 2 O. The amplitude becomes smaller and smaller, indicating that when Li ₂ O in the molten salt reaches a certain concentration, it starts to promote the conversion of REClO (where RE represents La ion, Ce ion or Nd ion) to RE ₂ O ₃ . It should be noted that, compared with curve 2, the peak B in curve 3 did not decrease significantly with the addition of Li ₂ O, and an abnormality occurred, which was attributed to a part of REClO generated in the molten salt floating on the surface of the molten salt. Li ₂ O first contacts and reacts with it to generate RE ₂ O ₃ , and a large part of it is consumed.

Further reference is made to Figures 4-6. Fig. 4 shows curve 7 in Fig. 2 alone, in which peak B has just disappeared; Fig. 5 shows curve 8 in Fig. 3 alone, in which peak C has just appeared; and Fig. 5 alone shows the graph In curve 11 in 3, the larger peak C appears in curve 11. The reaction systems corresponding to curves 7, 8 and 11 were sampled respectively, and the obtained samples were subjected to ICP (Inductively Coupled Plasma) test. The results show that when peak B disappears, the total residual amount of rare earth ions in the waste salt is 1458 ppm, when peak C just appears in the curve, the total residual amount of rare earth ions in the waste salt is 294 ppm, and when a larger peak C appears, rare earth ions The total residual amount dropped to 47ppm. It can be seen that when the peak C just appeared in the curve, the total residual amount of rare earth ions was only about 300 ppm, that is, the rare earth ions were basically completely removed, and Li ₂ O was only slightly excessive. At this time, the reaction can be more accurately avoided. Li ₂ O excess. If Li ₂ O is continued to be added, even when Li ₂ O is obviously excessive, although the intensity of peak C continues to increase, the total residual amount of rare earth ions only decreases slightly, which undoubtedly increases the burden of subsequent electrolysis.

Further referring to Figures 7-8, the reduction peak C of Li ₂ O disappears, and only the oxidation and/or reduction peak A of Li ions in the two figures indicates that Li ₂ O in the molten salt has been completely removed.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention.

Claims (10)

1. A method for removing rare earth ions from radioactive waste molten salt includes adding Li to the waste molten salt at 300-800 deg.C ₂ O reacts to remove the rare earth ions.

2. The process of claim 1, wherein Li is added stepwise ₂ O。

3. The method of claim 2, further comprising testing the reaction system during the reaction using an electrochemical method to obtain an electrochemical profile, and judging the progress of the reaction based on the obtained electrochemical profile.

4. The method of claim 3, wherein the electrochemical method comprises cyclic voltammetry, square wave voltammetry, or open-circuit chronopotentiometry.

5. The method of claim 3, wherein Li is present in the electrochemical plot ₂ When the oxidation and/or reduction peak of O, judging the reaction process is that the rare earth ions are removed, and stopping adding Li ₂ O and finishing the reaction.

6. The method of claim 3, wherein a plurality of different regions of the reaction system are tested to obtain a plurality of the electrochemical profiles, wherein,

absence of Li in none of the multiple electrochemical plots ₂ Judging the reaction process is that the rare earth ions are not removed in the reaction system if the oxidation and/or reduction peak of O, and continuously adding Li ₂ O；

At least one electrochemical profile of the plurality of electrochemical profiles is free of Li ₂ Oxidation and/or reduction peaks of O, and at least one electrochemical diagram showing Li ₂ The oxidation and/or reduction peak of O, judging that the reaction process is the region in which the rare earth ions are not removed and the region in which the rare earth ions are completely removed and Li still exist in the reaction system ₂ In the region where O is excessive, stopping the addition of Li ₂ O, continuing the reaction; or

Li is present in all of the multiple electrochemical plots ₂ The oxidation and/or reduction peak of O, the reaction process is judged to be that the rare earth ions in the reaction system are completely removed, and Li addition is stopped ₂ O and finishing the reaction.

7. The method of claim 3, wherein the waste molten salt is left to stand for 2-10min before the electrochemical method test is performed.

8. The method of claim 3, wherein the electrochemical process uses a three-electrode system in which the working electrode is a tungsten or molybdenum wire and the auxiliary electrode is a graphite rod, molybdenum rod or tungsten rod; the reference electrode was Ag/AgCl.

9. The method of any one of claims 1 to 8, wherein the method further comprises removing precipitates in the reaction system by using a vacuum distillation method or a molten salt filtration method after the reaction is finished to obtain a molten salt with rare earth ions removed; and optionally, electrolyzing the fused salt for removing the rare earth ions at 400-800 ℃ to remove excessive Li ₂ O, preferably, the electrolysis uses a three-electrode system, wherein the working electrode is a tungsten wire or a molybdenum wire, and the auxiliary electrode is a graphite rod, a molybdenum rod or a tungsten rod; the reference electrode was Ag/AgCl.

10. The method of claim 9, wherein the electrolysis is performed by potentiostatic method, and the electrolysis is stopped when the electrolysis current reaches 1 mA.

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