Electrolytic reduction method for metal cerium in cerium-containing high-temperature fluoride molten salt waste residues

Abstract

The invention belongs to the technical field of nuclear reactor spent fuel circulation and material design, and discloses a method for electrolytically reducing metal cerium from cerium-containing high-temperature fluoride molten salt waste slag, which comprises the following steps: heating the cerium-containing high-temperature fluoride molten salt waste slag until the cerium-containing high-temperature fluoride molten salt waste slag is completely molten to obtain molten state molten salt waste slag; the method comprises the following steps: by taking molten-state molten salt waste residues as an electrolyte, carrying out four-stage pulse electrolysis treatment on the electrolyte for 50-200 times by utilizing a three-electrode system, so as to obtain rare earth metal cerium mixed with molten salt. According to the method, the cerium-containing high-temperature fluoride molten salt waste residues in the molten state are subjected to four-stage pulse electrolysis treatment through a three-electrode system, and electrolytic reduction of metal cerium in the cerium-containing high-temperature fluoride molten salt waste residues can be selectively achieved by regulating and controlling technological parameters and pulse electrolysis times of pulse electrolysis; and the process is simple to operate, short in treatment period and beneficial to popularization and application.

Description

Technical Field

[0001] The present invention relates to the technical field of spent nuclear fuel cycle and material design of nuclear reactors, and particularly relates to an electrolytic reduction method for cerium metal in high-temperature fluoride molten salt waste residue containing cerium. Background Art

[0002] The thorium-based molten salt reactor nuclear energy system, whose full English name is Thorium Molten Salt Reactor Nuclear Energy System, abbreviated as TMSR, is one of the six candidates for the fourth-generation advanced nuclear energy system. It includes three subsystems: thorium-based nuclear fuel, molten salt reactor, and comprehensive utilization of nuclear energy, and has characteristics such as high inherent safety, less nuclear waste, better anti-proliferation performance, and economy.

[0003] TMSR uses fluoride salts as the coolant and fuel salt of the reactor. The nuclear fuel is uniformly dissolved in the carrier salt composed of LiF and BeF2 in the form of fluorides such as ThF4, UF4, or TRUFx, and it is possible to extract or supplement fuel during the reactor operation state to perform on-line plus off-line mode fuel processing and fuel cycle. However, after the uranium is separated by fluorination volatilization and the carrier salt is separated by vacuum distillation from the molten salt waste residue, there are still residues of thorium and fission products in the form of fluorides in the waste salt. Most of the fission products are lanthanide elements with relatively large neutron absorption cross-sections, also known as neutron absorption poisons, which are likely to have a negative impact on the chain reaction. Among them, Ce, Nd, and La are the three elements with the highest content in the spent fuel fission products, and CeF3 is even an analogue of PuF3. Therefore, the separation of cerium metal in the molten salt waste residue can further reduce the volume of α-radioactive waste and improve the nuclear fuel cycle rate.

[0004] In the prior art, currently, the methods for separating cerium metal from cerium-containing mixtures mainly include the air oxidation separation method and the PUREX process based on liquid-liquid extraction technology. Among them, the air oxidation separation method mainly extracts cerium from its compounds by roasting or oxidizing the cerium-containing mixture in the air and controlling the roasting temperature and atmosphere to achieve the valence regulation of cerium. However, due to the high radioactivity and ionic complexity of the molten salt waste residue of the reactor, the above air oxidation separation method is not applicable. The PUREX process based on liquid-liquid extraction technology separates by using the difference in the distribution ratio of different target ions in the organic solvent. However, the process operation of this separation method is complex, and the decontamination factor is not high, and it is not completely applicable to the separation of cerium metal in the molten salt waste residue. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a method for electrolytic reduction of cerium metal in a cerium-containing high-temperature fluoride molten salt waste residue. The present invention heats the cerium-containing high-temperature fluoride molten salt waste residue to form a molten molten salt waste residue as an electrolyte, and performs a four-stage pulsed electrolysis cycle treatment on the electrolyte 50 to 200 times by adopting a three-electrode system, so as to realize the electrolytic reduction treatment of the molten molten salt waste residue by means of pulsed electrolysis, and by adjusting the process parameters and the number of pulsed electrolysis, the formation of oxygen cluster complexes with different compositions by F - ions and metal ions can be effectively avoided, thereby selectively realizing the electrolytic reduction of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue.

[0006] The method for electrolytic reduction of cerium metal in a cerium-containing high-temperature fluoride molten salt waste residue of the present invention is realized through the following technical solutions:

[0007] The present invention takes into account that the molten salt waste residue of TMSR is mainly fluoride molten salt. Compared with the general chloride salt system, the melting point of the molten salt waste residue of TMSR is higher, it is more likely to absorb water, and the electrochemical window is smaller. Especially for metal ions in high-temperature fluoride molten salt, there are huge differences in the electrochemical reaction process. F - ions are easy to form oxygen cluster complexes with different compositions with metal ions, resulting in difficult separation of target ions. The present invention discovers that this problem can be solved by removing the water and oxygen content in the molten salt waste residue. Therefore, the electrolytic reduction operation in the present invention is mainly carried out in a glove box with an extremely low water and oxygen content of ≤1 ppm. Before electrolytic separation, the molten salt waste residue is pretreated at a high temperature of 673K to remove excess water molecules. During this period, the water and oxygen content in the glove box will continue to rise. It should be emphasized that during the actual operation process, pay attention to keeping the cycle open until the water and oxygen content ≤1 ppm before starting electrolytic separation, and ensure that the water and oxygen content ≤1 ppm during the electrolysis process.

[0008] In order to realize the separation and extraction of cerium metal from the molten salt waste residue of TMSR, the present invention provides a method for electrolytic reduction of cerium metal in a cerium-containing high-temperature fluoride molten salt waste residue, and specifically includes the following steps:

[0009] Step 1, heat the cerium-containing high-temperature fluoride molten salt waste residue to completely melt the cerium-containing high-temperature fluoride molten salt waste residue, and obtain a molten molten salt waste residue.

[0010] It should be noted that the present invention does not limit the specific heating temperature and heating time of the heating treatment, as long as it can ensure that the cerium-containing high-temperature fluoride molten salt waste residue is heated to complete melting, so that the cerium-containing high-temperature fluoride molten salt waste residue can be used as an electrolyte to realize the electrolytic reduction of cerium therein through pulsed electrolysis treatment.

[0011] Step 2: Using the molten salt waste residue as the electrolyte, the electrolyte is subjected to a four-stage pulsed electrolysis cycle treatment 50 to 200 times by using a three-electrode system to reduce the cerium ions in the molten salt waste residue to metallic cerium. After the electrolysis is completed, the electrolysis products generated on the working electrode in the three-electrode system are taken out and cooled to obtain rare earth metal cerium mixed with the molten salt.

[0012] It should be noted that in the present invention, by using the molten salt waste residue as the electrolyte and adopting the pulsed potential electrolysis method, the electrolyte is subjected to a four-stage pulsed electrolysis cycle treatment 50 to 200 times by using a three-electrode system to reduce the cerium ions in the molten salt waste residue to metallic cerium, realizing the electrolytic reduction of metallic cerium in the cerium-containing high-temperature fluoride molten salt waste residue.

[0013] To ensure that the electrolytic reduction of metallic cerium in the cerium-containing high-temperature fluoride molten salt waste residue can be achieved through 50 to 200 times of four-stage pulsed electrolysis treatment, preferably, each four-stage pulsed electrolysis treatment in the present invention is carried out through the following steps: First, apply a first potential, and the applied first potential is negative to the reduction potential of Ce and is between the reduction potential of Li + and the reduction potential of Ce 3+ By applying the first potential for 3 to 5 s, so that Ce 3+ is concentrated on the upper surface of the cathode, that is, the working electrode in the three-electrode system in a short time. Subsequently, according to the reduction potential of Ce in the current CV curve, adjust the applied potential to the reduction potential of Ce in the current CV curve as the second potential, and apply the second potential for 55 to 60 s to achieve the selective deposition of Ce. Then, adjust the applied potential to a positive potential of 0.1 V to 0.5 V as the third potential, and the application time is 1 to 3 s, so as to redissolve the loose layer on the surface of the deposited phase on the cathode surface into the molten salt electrolyte by applying this positive potential. Finally, adjust the applied potential to 0 V, and the application time is 2 to 30 s to keep the molten salt system basically stable, so that the concentration of cerium ions in the molten salt is uniform and the current is constant in each cycle, facilitating the subsequent deposition of metallic cerium. By repeating the above four-stage pulsed electrolysis treatment 50 to 200 times, the selective electrolytic reduction of cerium ions in the molten salt waste residue is achieved.

[0014] In some preferred embodiments of the present invention, the second potential is -1.6 V to -1.7 V.

[0015] In some preferred embodiments of the present invention, the first potential is -1.7 V to -1.75 V.

[0016] In some preferred embodiments of the present invention, in the three-electrode system, the working electrode used is an inert metal electrode that is resistant to high temperature and molten salt corrosion.

[0017] In some more preferred embodiments of the present invention, the inert metal electrode resistant to high-temperature molten salt corrosion is one or more of tungsten and molybdenum.

[0018] In some preferred embodiments of the present invention, in the three-electrode system, the reference electrode used is a noble metal electrode resistant to high temperature and molten salt corrosion.

[0019] In some more preferred embodiments of the present invention, the noble metal electrode resistant to high temperature and molten salt corrosion is one or more of gold, platinum, and palladium.

[0020] In some preferred embodiments of the present invention, in the three-electrode system, the counter electrode used is a platinum electrode or a graphite electrode.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention heats the cerium-containing high-temperature fluoride molten salt waste residue to form a molten molten salt waste residue, uses the molten molten salt waste residue as an electrolyte, and performs a four-stage pulsed electrolysis cycle treatment on the electrolyte 50 to 200 times by adopting a three-electrode system, so as to realize the electrolytic reduction treatment of the molten molten salt waste residue by means of pulsed electrolysis. By adjusting the process parameters and the number of pulsed electrolysis times of the pulsed electrolysis, the formation of oxygen cluster complexes with different compositions by F- ions and metal ions can be effectively avoided, thereby selectively realizing the electrolytic reduction of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue. Among them, each four-stage pulsed electrolysis treatment of the present invention is carried out through the following steps: first, apply a first potential, and the applied first potential is negative of the reduction potential of Ce and is between the reduction potential of Li + and the reduction potential of Ce 3+ By applying the first potential for 3 s to 5 s, so that Ce 3+ is concentrated on the upper surface of the working electrode in the cathode, that is, the three-electrode system in a short time. Subsequently, according to the reduction potential of Ce in the current CV curve, adjust the applied potential to the reduction potential of Ce in the current CV curve as the second potential, and apply the second potential for 55 s to 60 s to achieve the selective deposition of Ce. Then, adjust the applied potential to a positive potential of 0.1 V to 0.5 V as the third potential, and the application time is 1 s to 3 s, so as to redissolve the loose layer on the surface of the deposited phase on the cathode surface into the molten salt electrolyte by applying this positive potential. Finally, adjust the applied potential to 0 V, and the application time is 2 s to 30 s to keep the molten salt system basically stable, so that the concentration of cerium ions in the molten salt is uniform and the current is constant in each cycle, so as to facilitate the subsequent deposition of cerium metal. By repeating the above four-stage pulsed electrolysis treatment 50 to 200 times, the selective electrolytic reduction of cerium ions in the molten molten salt waste residue is achieved. The process operation of the present invention is simple and the treatment cycle is short, which is conducive to popularization and use.

[0023] In the three - electrode system adopted in the present invention, the working electrode uses an inert metal electrode resistant to high - temperature molten salt corrosion, and the reference electrode uses a noble metal electrode resistant to high - temperature and molten salt corrosion, enabling the three - electrode system adopted in the present invention to be recyclable. Furthermore, it can effectively reduce the cost of molten salt electrolysis for recovering cerium metal, and is more convenient to operate than self - made molten salt electrolyte electrodes, with a shorter operation process.

[0024] By regulating the process conditions of pulsed electrolysis, the present invention ensures the uniformity of cerium ion concentration and the constancy of current in the molten salt for each cycle. After pulsed electrolysis reduction for 50 to 200 times, the cerium concentration in the molten salt decreases significantly, and the deposition of cerium metal is in - situ monitored at the electrode cross - section. Compared with the electrolysis reduction collection test, the state of the original electrolysis product is more significant, which can effectively reduce the secondary pollution of reactor waste molten salt and achieve the volume reduction and recycling of waste molten salt. Brief Description of the Drawings

[0025] Figure 1 It is the loading voltage cycle for each four - stage pulsed electrolysis treatment in Example 1.

[0026] Figure 2 It is the variation of current with time during pulsed potential electrolysis of the molten - state molten salt waste residue in Example 1 at 873K.

[0027] Figure 3 It is the cyclic voltammetry curve before and after the electrolysis reduction treatment in step 2.2) in Example 1.

[0028] Figure 4 It is the XRD pattern of the cathode electrolysis product in Example 1.

[0029] Figure 5 It is the SEM - EDS spectrum of the cathode electrolysis product in Example 1. Among them, Figure a is the SEM spectrum of the cathode electrolysis product in Example 1, Figure b is the EDS distribution spectrum of Ce element in Figure a, Figure c is the EDS distribution spectrum of K element in Figure a, Figure d is the EDS distribution spectrum of W element in Figure a, Figure e is the EDS distribution spectrum of Na element in Figure a, and Figure f is the EDS distribution spectrum of F element in Figure a. Detailed Embodiments

[0030] Next, the technical solutions in the embodiments of the present invention will be described clearly and completely.

[0031] It should be noted that in the following various embodiments of the present invention, a mixed molten salt of CeF3, LiF, NaF, and KF powders is used to simulate the cerium - containing high - temperature fluoride molten salt waste residue of TMSR, and the mixed molten salt adopted in the following various embodiments of the present invention is specifically prepared through the following steps:

[0032] 1) Inside a glove box filled with an argon atmosphere, LiF powder with a purity of 99.9%, NaF powder with a purity of 99.99%, KF powder with a purity of 99.9%, and CeF3 powder with a purity of 99.99% were dried at 673K for 12h to remove moisture for later use. The water and oxygen content inside the glove box was lower than 1ppm, obtaining dried LiF powder, NaF powder, KF powder, and CeF3 powder.

[0033] 2) The dried LiF powder, NaF powder, and KF powder were mixed according to a molar percentage of 46.5:11.5:42 to obtain a molten salt matrix.

[0034] 3) Dried CeF3 powder was added to the obtained molten salt matrix, and the addition amount of the dried CeF3 powder was 5wt% of the total mass of the molten salt matrix, and mixed evenly to obtain a mixed molten salt.

[0035] Example 1

[0036] This example provides an electrolytic reduction method for cerium metal in a cerium-containing high-temperature fluoride molten salt waste residue, including the following steps:

[0037] Step 1, melting of the cerium-containing high-temperature fluoride molten salt waste residue:

[0038] Using the mixed molten salt prepared above in the present invention to simulate the cerium-containing high-temperature fluoride molten salt waste residue, by weighing 50g of the above mixed molten salt and adding it to a 100mL nickel crucible, and keeping it at 673K for 12h to remove excess moisture. After the water and oxygen content in the glove box ≤ 1ppm, it was heated to 873K at a heating rate of 5°C / min and kept for 1h to fully dissolve and mix the molten salt, making the obtained high-temperature fluoride salt closer to the molten state of the waste molten salt, that is, obtaining a molten-state molten salt waste residue.

[0039] Step 2, four-stage pulse electrolysis treatment:

[0040] 2.1) Using the molten-state molten salt waste residue obtained in the above step 1 as the electrolyte, using platinum wire as the reference electrode, using graphite electrode as the counter electrode, using mm tungsten rod as the working electrode, and inserting the reference electrode, counter electrode, and working electrode into the electrolyte.

[0041] 2.2) Using a three-electrode system to perform four-stage pulse electrolysis treatment on the electrolyte, and the loading voltage cycle of each four-stage pulse electrolysis treatment is as Figure 1 shown, that is, first adjusting the applied potential to -1.7V and applying it for 5s; then, adjusting the applied potential to -1.67V and applying the potential for 55s; then, adjusting the applied potential to 0.3V and applying it for 3s; finally, adjusting the applied potential to 0V and applying it for 2s.

[0042] 2.3) Repeat step 2.3) 100 times.

[0043] 2.4) After electrolysis is completed, take out and cool the electrolysis products generated at the cathode.

[0044] Example 2

[0045] This example provides an electrolytic reduction method for cerium metal in cerium-containing high-temperature fluoride molten salt waste residue, including the following steps:

[0046] Step 1, melting of cerium-containing high-temperature fluoride molten salt waste residue:

[0047] Using the mixed molten salt prepared above in the present invention to simulate the cerium-containing high-temperature fluoride molten salt waste residue, by weighing 50 g of the above-mentioned mixed molten salt and adding it to a 100 mL nickel crucible, keeping it at 673 K for 12 h to remove excess moisture. After the water and oxygen content in the glove box ≤ 1 ppm, heat it at a heating rate of 5 °C / min to 873 K and keep it for 1 h to fully dissolve and mix the molten salt, so that the obtained high-temperature fluoride salt is closer to the molten state of the waste molten salt, that is, the molten state molten salt waste residue is obtained.

[0048] Step 2, four-stage pulse electrolysis treatment:

[0049] 2.1) Using the molten state molten salt waste residue obtained in step 1 above as the electrolyte, using the platinum wire as the reference electrode, using the graphite electrode as the counter electrode, using the tungsten rod with a diameter of

[0050] 2.2) Using the three-electrode system to perform four-stage pulse electrolysis treatment on the electrolyte, the loading voltage cycle of each four-stage pulse electrolysis treatment is as shown in Figure 1 shown, that is, first adjust the applied potential to -1.72 V and apply it for 4 s; then, adjust the applied potential to -1.6 V and apply the potential for 60 s; then, adjust the applied potential to 0.1 V and apply it for 2 s; finally, adjust the applied potential to 0 V and apply it for 10 s.

[0051] 2.3) Repeat step 2.3) 50 times.

[0052] 2.4) After electrolysis is completed, take out and cool the electrolysis products generated at the cathode.

[0053] Example 3

[0054] This example provides an electrolytic reduction method for cerium metal in cerium-containing high-temperature fluoride molten salt waste residue, including the following steps:

[0055] Step 1, melting of cerium-containing high-temperature fluoride molten salt waste residue:

[0056] Use the mixed molten salt prepared above in the present invention to simulate the cerium-containing high-temperature fluoride molten salt waste residue. By weighing 50 g of the above mixed molten salt and adding it to a 100 mL nickel crucible, keep it at 673 K for 12 h to remove excess moisture. After the water and oxygen content in the glove box ≤ 1 ppm, heat it to 873 K at a heating rate of 5 °C / min and keep it for 1 h to fully dissolve and mix the molten salt, making the obtained high-temperature fluoride salt closer to the molten state of the waste molten salt, that is, obtaining the molten state molten salt waste residue.

[0057] Step 2, four-stage pulsed electrolysis treatment:

[0058] 2.1) Use the molten state molten salt waste residue obtained in Step 1 above as the electrolyte, use the platinum wire as the reference electrode, use the graphite electrode as the counter electrode, and use a tungsten rod with a diameter of

[0059] as the working electrode, and insert the reference electrode, counter electrode and working electrode into the electrolyte. Figure 1 2.2) Use the three-electrode system to perform four-stage pulsed electrolysis treatment on the electrolyte. The loading voltage cycle of each four-stage pulsed electrolysis treatment is as

[0060] shown, that is, first adjust the applied potential to -1.75 V and apply it for 35 s; then, adjust the applied potential to -1.7 V and apply it for 57 s; then, adjust the applied potential to 0.5 V and apply it for 1 s; finally, adjust the applied potential to 0 V and apply it for 30 s.

[0061] 2.3) Repeat Step 2.3) 200 times.

[0062] Comparative Example 1

[0063] This comparative example provides an electrolytic reduction method for cerium metal in cerium-containing high-temperature fluoride molten salt waste residue, including the following steps:

[0064] Step 1, melting of cerium-containing high-temperature fluoride molten salt waste residue:

[0065] Use the mixed molten salt prepared above in the present invention to simulate the cerium-containing high-temperature fluoride molten salt waste residue. By weighing 50 g of the above mixed molten salt and adding it to a 100 mL nickel crucible, keep it at 673 K for 12 h to remove excess moisture. After the water and oxygen content in the glove box ≤ 1 ppm, heat it to 873 K at a heating rate of 5 °C / min and keep it for 1 h to fully dissolve and mix the molten salt, making the obtained high-temperature fluoride salt closer to the molten state of the waste molten salt, that is, obtaining the molten state molten salt waste residue.

[0066] Step 2, four-stage pulsed electrolysis treatment:

[0067] 2.1) Using the molten molten salt waste obtained in the above step 1 as the electrolyte, with the platinum wire as the reference electrode, with the graphite electrode as the counter electrode, and with a tungsten rod of mm as the working electrode, insert the reference electrode, counter electrode and working electrode into the electrolyte.

[0068] 2.2) Use a three-electrode system to perform four-stage pulsed electrolysis treatment on the electrolyte: First, apply a potential of -1.7 V for 5 s; then, adjust the applied potential to -1.65 V and apply the potential for 55 s; then, adjust the applied potential to 0.3 V and the application time to 5 s; finally, adjust the applied potential to 0 V and the application time to 20 s.

[0069] 2.3) Repeat step 2.3) 100 times.

[0070] 2.4) After the electrolysis is completed, take out and cool the electrolysis products generated at the cathode.

[0071] Comparative Example 1 compared with Example 1, increased the application time of positive potential and zero potential, and it was expected to improve the electrolytic reduction efficiency of cerium. However, the actual experimental results showed that after the electrolysis in Comparative Example 1 was completed, the present invention found that there were fewer deposits on the working electrode in Comparative Example 1. In addition, agglomerated electrolysis products were found in the electrolyte solution, which may be due to the longer positive potential electrolysis time, resulting in more serious shedding of electrolysis products, hindering the deposition of metallic cerium around the electrode.

[0072] Comparative Example 2

[0073] This comparative example provides a method for electrolytic reduction of metallic cerium in cerium-containing high-temperature fluoride molten salt waste residue, including the following steps:

[0074] Step 1, melting of cerium-containing high-temperature fluoride molten salt waste residue:

[0075] Using the mixed molten salt prepared above in the present invention to simulate the cerium-containing high-temperature fluoride molten salt waste residue, by weighing 50 g of the above mixed molten salt and adding it to a 100 mL nickel crucible, keep it at 673 K for 12 h to remove excess moisture. After the water and oxygen content in the glove box ≤ 1 ppm, heat it at a heating rate of 5 °C / min to 873 K and keep it for 1 h to fully dissolve and mix the molten salt, so that the obtained high-temperature fluoride salt is closer to the molten state of the waste molten salt, that is, the molten molten salt waste residue is obtained.

[0076] Step 2, constant potential electrolytic reduction of metallic cerium:

[0077] 2.1) Using the molten molten salt waste obtained in the above step 1 as the electrolyte, with A platinum wire is used as the reference electrode, and A graphite electrode is used as the counter electrode, and A tungsten rod with a diameter of

[0078] 2.2) Using a three - electrode system, the electrolyte is subjected to potentiostatic electrolysis. The potentiostatic electrolysis uses the reduction potential of cerium, - 1.67 V, in the current cyclic voltammogram curve, and the potentiostatic electrolysis time is 1 h.

[0079] 2.3) After the electrolysis is completed, the electrolysis products generated at the cathode are taken out and cooled.

[0080] The difference between Comparative Example 2 and Example 1 is that in Comparative Example 2, common potentiostatic electrolysis is used instead of cyclic pulse electrolysis. Theoretically, the electrolysis process of Comparative Example 2 is simpler. However, powder diffraction analysis of the electrolysis products after electrolysis in Comparative Example 2 in the present invention shows that the impurity content in the electrolysis products after electrolysis in Comparative Example 2 is relatively high.

[0081] Experimental part

[0082] Since the electrolytic reduction effects of Examples 1 - 3 are similar, in the present invention, taking Example 1 as an example, the current change during the entire electrolysis process in step 2.2) is monitored and recorded as Figure 2 shown below.

[0083] Figure 2 is the current change with time during pulse potential electrolysis of the molten salt waste residue in Example 1 at 873 K. It can be seen that during the entire electrolysis process, the current remains relatively constant, indicating that the change in ion concentration caused by electrolysis has no significant effect on the pulse potential electrolysis process. Since the concentration of Ce 3+ near the electrode and the current density are relatively stable, the electrode potential fluctuation caused by concentration polarization is small, and other side reactions and impurity depositions are not likely to occur.

[0084] The present invention also conducts cyclic voltammetry tests on the system before and after the electrolytic reduction treatment in step 2.2) of Example 1. The test temperature is 873 K, the scanning rate is 200 mV / s, and the test results are as Figure 3 shown below.

[0085] Figure 3 are the cyclic voltammogram curves before and after the electrolytic reduction treatment in step 2.2) of Example 1. It can be seen that after electrolysis, Li +The impurity peak shifted about 62 mV to the left, and the corresponding Ce(III)-Ce(0) redox peak decreased significantly, indicating that part of Ce(III) in the molten salt was electrolytically separated. Since the peak current in the CV curve is related to the concentration, under the same conditions, the ratio of the cathodic peak current (Ipc) corresponding to Ce(III)-Ce(0) measured before and after electrolysis can be used to estimate the effect of electrolytic separation according to Equation 1.

[0086] Degree of decrease in the concentration of Ce(III) in the molten salt after electrolysis = voltage before electrolysis / Ipc Equation 1.

[0087] From Figure 1 The ratio obtained was Ipc. According to Equation 1, the degree of decrease in the concentration of Ce(III) in the molten salt after electrolysis = -0.06854 V / -0.02448 V = 2.80, indicating that the concentration of Ce(III) in the molten salt decreased by about 2.8 times after electrolysis.

[0088] The present invention also cooled and removed the cathode electrode with electrolytic products after the electrolysis in Example 1 was completed, and removed and ground the electrolytic products on its surface into powder, and analyzed its phase composition on an X-ray powder diffractometer, and the test analysis results are as Figure 4 shown

[0089] Figure 4 is the XRD pattern of the cathode electrolytic product in Example 1. It can be seen from it that Ce mainly exists in the form of metal state in the electrolytic product, and there are also a small amount of signals of CeF3 in the spectrum. LiF, KF, and NaF in the spectrum come from the molten salt contaminated during the sample collection and transfer process, and KF is prone to moisture absorption during the transfer process, so there is also a signal of KF·2H2O in the spectrum.

[0090] The present invention also immediately transferred a group of cathode electrodes with electrolytic products after the electrolysis in Example 1 was completed out of the glove box and carried out condensation and solidification in a vacuum transparent condensation mold, and polished them with 800-mesh dry sandpaper on a grinding and polishing machine, and further tested the SEM-EDS of the electrode products, and the test results are as Figure 5 shown.

[0091] Figure 5 is the SEM-EDS spectrum of the cathode electrolytic product in Example 1. Among them, Figure a is the SEM spectrum of the cathode electrolytic product in Example 1, Figure b is the EDS distribution spectrum of Ce element in Figure a, Figure c is the EDS distribution spectrum of K element in Figure a, Figure d is the EDS distribution spectrum of W element in Figure a, Figure e is the EDS distribution spectrum of Na element in Figure a, and Figure f is the EDS distribution spectrum of F element in Figure a.

[0092] From Figure 5It can be seen that the shadow area at the bottom of the SEM image represents the tungsten rod substrate (focused on the deposit), and the deposits protruding on the electrode surface are the electrolysis products, showing obvious metallic luster. Similarly, in the EDS mapping spectrum analysis, a large amount of Na and K metal elements are observed in the products, and this result is in good agreement with the data obtained by XRD, mainly due to the molten salt contaminated during the transfer of the electrolysis products. Due to the extremely high corrosiveness of the molten FLiNaK salt, the diffusion of W ions is observed in the electrolysis products. It is worth noting that a large amount of Ce is observed to be evenly distributed in the electrolysis products, and this result indicates that it is feasible to separate cerium from the fluoride molten salt system by pulse electrolysis.

[0093] Obviously, the above embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

Claims

1. A method for electrolytically reducing cerium metal in a cerium-containing high-temperature fluoride molten salt waste residue, characterized in that, Including the following steps: Heating the cerium-containing high-temperature fluoride molten salt waste residue to melt the cerium-containing high-temperature fluoride molten salt waste residue, obtaining a molten salt waste residue; Using the molten salt waste residue as an electrolyte and performing a four-stage pulsed electrolysis cycle treatment on the electrolyte 50 to 200 times by using a three-electrode system to reduce the cerium ions in the molten salt waste residue to metallic cerium; after electrolysis is completed, taking out and cooling the electrolysis products generated on the working electrode in the three-electrode system to obtain rare earth metal cerium mixed with molten salt; Wherein, each of the four-stage pulsed electrolysis treatments is carried out through the following steps: First, applying a first potential for 3 s to 5 s; subsequently, adjusting the applied potential to a second potential for 55 s to 60 s; then, adjusting the applied potential to a third potential for 1 s to 3 s; finally, adjusting the applied potential to 0 V as the fourth potential for 2 s to 30 s; Wherein, the first potential is negative to the reduction potential of Ce; The second potential is the reduction potential of Ce; The third potential is 0.1 V to 0.5 V.

2. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue as described in claim 1, wherein The second potential is -1.6 V to -1.7 V.

3. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue according to claim 2, characterized in that, The first potential is -1.7 V to -1.75 V.

4. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue according to claim 1, characterized in that, In the three-electrode system, the working electrode used is an inert metal electrode resistant to high-temperature molten salt corrosion.

5. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue as described in claim 4, characterized in that, The inert metal electrode resistant to high-temperature molten salt corrosion is one or more of tungsten and molybdenum.

6. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue as described in claim 1, wherein, In the three-electrode system, the reference electrode used is a noble metal electrode resistant to high-temperature molten salt corrosion.

7. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue according to claim 6, characterized in that, The noble metal electrode resistant to high-temperature molten salt corrosion is one or more of gold, platinum, and palladium.

8. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue according to claim 1, characterized in that, In the three-electrode system, the counter electrode used is a platinum electrode or a graphite electrode.

9. The electrolytic reduction method of cerium metal in the cerium-containing high-temperature fluoride molten salt waste residue as described in claim 1, characterized in that, The water content of the cerium-containing high-temperature fluoride molten salt waste residue ≤ 1 ppm.

Images