CN114908381B - Method for removing rare earth ions in waste molten salt - Google Patents
Method for removing rare earth ions in waste molten salt Download PDFInfo
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- 150000003839 salts Chemical class 0.000 title claims abstract description 83
- 238000000034 method Methods 0.000 title claims abstract description 72
- 229910052761 rare earth metal Inorganic materials 0.000 title claims abstract description 62
- -1 rare earth ions Chemical class 0.000 title claims abstract description 52
- 239000002699 waste material Substances 0.000 title claims abstract description 34
- 238000006243 chemical reaction Methods 0.000 claims abstract description 109
- 229910018068 Li 2 O Inorganic materials 0.000 claims abstract description 88
- 239000002901 radioactive waste Substances 0.000 claims abstract description 10
- 230000009467 reduction Effects 0.000 claims description 28
- 230000003647 oxidation Effects 0.000 claims description 24
- 238000007254 oxidation reaction Methods 0.000 claims description 24
- 238000005868 electrolysis reaction Methods 0.000 claims description 19
- 238000012360 testing method Methods 0.000 claims description 19
- 238000004365 square wave voltammetry Methods 0.000 claims description 17
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 claims description 12
- 238000002848 electrochemical method Methods 0.000 claims description 11
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 claims description 11
- 238000002484 cyclic voltammetry Methods 0.000 claims description 8
- 230000008569 process Effects 0.000 claims description 8
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 7
- 229910021607 Silver chloride Inorganic materials 0.000 claims description 7
- 239000010439 graphite Substances 0.000 claims description 7
- 229910002804 graphite Inorganic materials 0.000 claims description 7
- HKZLPVFGJNLROG-UHFFFAOYSA-M silver monochloride Chemical compound [Cl-].[Ag+] HKZLPVFGJNLROG-UHFFFAOYSA-M 0.000 claims description 7
- 229910052721 tungsten Inorganic materials 0.000 claims description 7
- 239000010937 tungsten Substances 0.000 claims description 7
- 239000002244 precipitate Substances 0.000 claims description 6
- 150000002910 rare earth metals Chemical class 0.000 claims description 6
- 238000001914 filtration Methods 0.000 claims description 5
- 238000004769 chrono-potentiometry Methods 0.000 claims description 3
- 238000005292 vacuum distillation Methods 0.000 claims description 2
- 150000002500 ions Chemical class 0.000 abstract description 13
- 238000003912 environmental pollution Methods 0.000 abstract description 5
- 239000012535 impurity Substances 0.000 abstract description 5
- 239000002912 waste gas Substances 0.000 abstract description 5
- 230000000694 effects Effects 0.000 abstract description 3
- 239000007791 liquid phase Substances 0.000 abstract description 3
- 101000878457 Macrocallista nimbosa FMRFamide Proteins 0.000 description 9
- 238000003756 stirring Methods 0.000 description 8
- 239000002915 spent fuel radioactive waste Substances 0.000 description 7
- 238000007670 refining Methods 0.000 description 5
- 238000004821 distillation Methods 0.000 description 4
- KWGKDLIKAYFUFQ-UHFFFAOYSA-M lithium chloride Chemical compound [Li+].[Cl-] KWGKDLIKAYFUFQ-UHFFFAOYSA-M 0.000 description 4
- 229910001416 lithium ion Inorganic materials 0.000 description 4
- 229910052750 molybdenum Inorganic materials 0.000 description 4
- 239000011733 molybdenum Substances 0.000 description 4
- 238000012544 monitoring process Methods 0.000 description 4
- 238000001556 precipitation Methods 0.000 description 4
- 230000035484 reaction time Effects 0.000 description 4
- 229910019142 PO4 Inorganic materials 0.000 description 3
- 229910021536 Zeolite Inorganic materials 0.000 description 3
- 150000001805 chlorine compounds Chemical class 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 238000011161 development Methods 0.000 description 3
- HNPSIPDUKPIQMN-UHFFFAOYSA-N dioxosilane;oxo(oxoalumanyloxy)alumane Chemical compound O=[Si]=O.O=[Al]O[Al]=O HNPSIPDUKPIQMN-UHFFFAOYSA-N 0.000 description 3
- 230000005496 eutectics Effects 0.000 description 3
- 239000010452 phosphate Substances 0.000 description 3
- NBIIXXVUZAFLBC-UHFFFAOYSA-K phosphate Chemical compound [O-]P([O-])([O-])=O NBIIXXVUZAFLBC-UHFFFAOYSA-K 0.000 description 3
- 239000010457 zeolite Substances 0.000 description 3
- 229910004664 Cerium(III) chloride Inorganic materials 0.000 description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 2
- 229910013618 LiCl—KCl Inorganic materials 0.000 description 2
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 2
- 229910052770 Uranium Inorganic materials 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- VYLVYHXQOHJDJL-UHFFFAOYSA-K cerium trichloride Chemical compound Cl[Ce](Cl)Cl VYLVYHXQOHJDJL-UHFFFAOYSA-K 0.000 description 2
- 229910052593 corundum Inorganic materials 0.000 description 2
- 239000010431 corundum Substances 0.000 description 2
- 230000001186 cumulative effect Effects 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 238000009616 inductively coupled plasma Methods 0.000 description 2
- 229910052746 lanthanum Inorganic materials 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
- 238000000926 separation method Methods 0.000 description 2
- 238000001179 sorption measurement Methods 0.000 description 2
- 238000000141 square-wave voltammogram Methods 0.000 description 2
- JFALSRSLKYAFGM-UHFFFAOYSA-N uranium(0) Chemical compound [U] JFALSRSLKYAFGM-UHFFFAOYSA-N 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 229910052727 yttrium Inorganic materials 0.000 description 2
- 229910052779 Neodymium Inorganic materials 0.000 description 1
- 230000005856 abnormality Effects 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 239000000919 ceramic Substances 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000000835 electrochemical detection Methods 0.000 description 1
- 238000003682 fluorination reaction Methods 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 238000000622 liquid--liquid extraction Methods 0.000 description 1
- FUJCRWPEOMXPAD-UHFFFAOYSA-N lithium oxide Chemical compound [Li+].[Li+].[O-2] FUJCRWPEOMXPAD-UHFFFAOYSA-N 0.000 description 1
- 229910001947 lithium oxide Inorganic materials 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 239000003758 nuclear fuel Substances 0.000 description 1
- 230000033116 oxidation-reduction process Effects 0.000 description 1
- 229910052697 platinum Inorganic materials 0.000 description 1
- 239000012857 radioactive material Substances 0.000 description 1
- 229910001404 rare earth metal oxide Inorganic materials 0.000 description 1
- 239000000376 reactant Substances 0.000 description 1
- 230000036632 reaction speed Effects 0.000 description 1
- 230000027756 respiratory electron transport chain Effects 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 238000000638 solvent extraction Methods 0.000 description 1
- 239000010935 stainless steel Substances 0.000 description 1
- 229910001220 stainless steel Inorganic materials 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 238000005303 weighing Methods 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C5/00—Electrolytic production, recovery or refining of metal powders or porous metal masses
- C25C5/04—Electrolytic production, recovery or refining of metal powders or porous metal masses from melts
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W30/00—Technologies for solid waste management
- Y02W30/50—Reuse, recycling or recovery technologies
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Electrolytic Production Of Metals (AREA)
- Removal Of Specific Substances (AREA)
Abstract
The invention provides a method for removing rare earth ions in radioactive waste molten salt, which comprises the steps of adding Li 2 O into the waste molten salt at 300-800 ℃ for reaction so as to remove the rare earth ions. Li 2 O is added into the radioactive waste molten salt at 300-800 ℃ to carry out liquid phase reaction, so that 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 ensures the effect of removing rare earth ions, and does not introduce impurity ions which are difficult to remove or generate a large amount of waste.
Description
Technical Field
The invention relates to the field of nuclear power spent fuel aftertreatment, in particular to a method for removing rare earth ions in waste molten salt, and especially relates to a method for removing rare earth ions in waste molten salt generated by an electrolytic refining method.
Background
There is an urgent need to develop clean and efficient green energy sources. Nuclear power represents an important development direction of green energy, is currently being developed vigorously, and is expected to become one of mainstream energy in the future.
It is difficult to ensure sustainable development of nuclear power only by means of natural uranium resources, and efficient utilization of uranium resources, particularly nuclear fuel, becomes necessary. Currently, the nuclear power industry mostly adopts a water method post-treatment process to treat spent fuel produced in the nuclear power industry.
Compared with the water method post-treatment process, the dry post-treatment process has the obvious advantages of short spent fuel cooling period, less steps, small waste volume and the like. The dry post-treatment process mainly comprises a fluorination volatilization method, a molten salt liquid-liquid extraction method, a high-temperature chemical method (such as an electrolytic refining method) taking molten salt as a medium, and the like. Among them, the electrolytic refining method has the most development prospect. The electrorefining process still produces a large amount of waste molten salt with rare earth ions, and in order to minimize the amount of nuclear waste, it is necessary to purify and reuse the waste molten salt.
The methods for removing rare earth ions in waste molten salt developed in various countries at present are a phosphate precipitation method, a zeolite adsorption method, an oxygen precipitation method and the like. Phosphate is difficult to remove in the phosphate precipitation method, and impurity ions are introduced into the reaction system. The zeolite adsorption method uses large-structure type 4A zeolite, generates a large amount of waste, and also has difficulty in removing rare earth ions in waste molten salt. Although the oxygen precipitation method does not introduce impurity ions when removing rare earth ions, and the rare earth ion removal rate is good, the method generates a large amount of waste gas and pollutes the environment; meanwhile, the method is a gas-liquid reaction, the contact of reactants is incomplete, and the reaction rate is slow.
Therefore, there is a need to develop a method for removing rare earth ions from waste molten salt with a fast reaction rate and little environmental pollution.
Disclosure of Invention
Accordingly, a primary object of the present invention is to provide a method for removing rare earth ions from radioactive waste molten salt, which solves at least some of the above problems.
According to the present disclosure, there is provided a method for removing rare earth ions from radioactive waste molten salt, comprising adding Li 2 O to the waste molten salt at 300 ℃ to 800 ℃ to react to remove the rare earth ions.
According to some embodiments, li 2 O is added stepwise.
According to some embodiments, the method further comprises testing the reaction system during the reaction using an electrochemical method, obtaining an electrochemical profile, and determining the progress of the reaction based on the obtained electrochemical profile.
According to some embodiments, the electrochemical method comprises cyclic voltammetry, square wave voltammetry, or open circuit chronopotentiometry.
According to some embodiments, when oxidation and/or reduction peaks of Li 2 O appear in the electrochemical plot, the reaction progress is judged as the rare earth ion has been removed, li 2 O addition is stopped and the reaction is ended.
According to some embodiments, a plurality of different regions of the reaction system are tested, a plurality of the electrochemical profiles are obtained, wherein,
If oxidation and/or reduction peaks of Li 2 O do not appear in the electrochemical graphs, judging that the reaction process is that the rare earth ions are still not removed in the reaction system, and continuing to add Li 2 O;
At least one electrochemical curve graph of the plurality of electrochemical curve graphs does not have oxidation and/or reduction peaks of Li 2 O, and at least one electrochemical curve graph has oxidation and/or reduction peaks of Li 2 O, judging that the reaction process is the region where the rare earth ions are not removed and the region where the rare earth ions are completely removed and Li 2 O is excessive in the reaction system, stopping adding Li 2 O, and continuing the reaction; or (b)
And judging the reaction progress as that the rare earth ions in the reaction system are completely removed when oxidation and/or reduction peaks of Li 2 O appear in the electrochemical graphs, stopping adding Li 2 O and ending the reaction.
The plurality of distinct regions includes an upper portion, a middle portion, and a lower portion of the reaction system. The plurality of electrochemical profiles is at least 3.
According to some embodiments, the spent molten salt is allowed to stand for 2-10 minutes, e.g., 3,4, 5,6,7 minutes, before the electrochemical method test is performed.
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 and the auxiliary electrode is a graphite rod, a molybdenum rod or a tungsten rod; the reference electrode is Ag/AgCl.
According to some embodiments, the method further comprises removing the precipitate in the reaction system to obtain molten salt with rare earth ions removed by vacuum distillation or molten salt filtration after the reaction is finished. Optionally, when excess Li 2 O is present in the rare earth ion-removing molten salt, the method further comprises electrolyzing the rare earth ion-removing molten salt at 400 ℃ to 800 ℃ to remove excess Li 2 O.
According to some embodiments, the electrolysis is performed using potentiostatic methods, and the electrolysis is stopped when the electrolysis current reaches 1 mA.
According to some embodiments, 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 is Ag/AgCl.
Li 2 O is added into the radioactive waste molten salt at 300-800 ℃ to carry out liquid phase reaction, so that a large amount of waste gas is avoided, and the method has the advantages of high reaction rate and less environmental pollution. Meanwhile, the method also ensures the effect of removing the rare earth ions, and does not introduce impurity ions which are difficult to remove or generate a large amount of waste.
Drawings
FIG. 1 shows electrochemical graphs obtained in examples by using square wave voltammetry to test molten salt without Li 2 O and a reaction system after adding 0.20g and 0.40g Li 2 O to the molten salt and reacting, respectively;
FIG. 2 shows electrochemical graphs obtained in examples using square wave voltammetry to test reaction systems after cumulative addition of 0.50g, 0.60g, 0.65g, 0.70g Li 2 O to molten salt and reaction, respectively;
FIG. 3 shows electrochemical graphs obtained in examples using square wave voltammetry to test reaction systems after cumulative addition of 0.75g, 0.80g, 0.85g, 0.90g Li 2 O to molten salt and reaction, respectively;
FIG. 4 shows an electrochemical plot obtained in the reaction system after 0.70g of Li 2 O was added cumulatively to molten salt and reacted using square wave voltammetry test in the example;
FIG. 5 shows an electrochemical plot obtained in the reaction system after 0.75g Li 2 O was added cumulatively to molten salt and reacted using square wave voltammetry test in the example;
FIG. 6 shows an electrochemical plot obtained in the reaction system after 0.90g of Li 2 O was added cumulatively to molten salt and reacted using square wave voltammetry test in the example;
FIG. 7 shows an electrochemical plot obtained after electrolysis of the filtered molten salt using cyclic voltammetry testing in the examples;
fig. 8 shows an electrochemical plot obtained after electrolysis of the filtered molten salt tested using square wave voltammetry in the examples.
Detailed Description
The following description of the embodiments of the present application will be made clearly and fully with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some, but not all embodiments of the application. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present application without making any inventive effort, are intended to fall within the scope of the present application.
Throughout the specification, unless specifically indicated otherwise, the terms used herein should be understood as meaning as commonly used in the art. Accordingly, unless defined otherwise, 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 will control.
The disclosure aims to provide a method for removing rare earth ions in waste molten salt with high reaction rate and less environmental pollution, which is applied to the field of nuclear spent fuel post-treatment, particularly the field of nuclear spent fuel dry post-treatment, and more particularly the field of purifying waste molten salt generated by an electrolytic refining method. Wherein the waste molten salt produced during electrolytic refining contains rare earth elements such as LiCl-KCl eutectic salt and a large amount of rare earth elements, for example La, ce, pr, nd, Y with total content of more than 1000 ppm.
In order to achieve the above purpose, the application provides a method for removing rare earth ions in radioactive waste molten salt, which comprises the steps of adding Li 2 O into the waste molten salt at 300-800 ℃, preferably 450-550 ℃ for reaction so as to remove the rare earth ions.
The inventors found that by adding Li 2 O to a radioactive waste molten salt containing the molten salt and a large amount of rare earth elements (for example, la, ce, pr, nd, Y and the like in total content > 1000 ppm), the soluble rare earth chloride ions in the waste molten salt are converted into insoluble rare earth oxychloride or rare earth oxide precipitate to remove the rare earth ions while stirring for substitution reaction. The reaction speed is high, the reaction is thorough, the rare earth element removal rate is high, other ions are not introduced, and waste gas is not generated. But also can be removed by electrolysis conveniently even if there is an excess of lithium oxide.
The present disclosure is not particularly limited to radioactive waste molten salt, and any waste molten salt produced by post-treatment of nuclear power spent fuel electrorefining may be applied to the method of the present invention. The molten salt is usually a mixed salt of two or more, and typically may be LiCl-KCl eutectic salt, but is not limited thereto.
The specific steps of the method are exemplarily described below. The following exemplary description is for ease of understanding only, and the present method is not limited to what is described below.
Because the method involves the treatment of radioactive materials, unmanned automated equipment is used to remotely control the progress of the reaction. The method uses a reaction system with an automatic lifting system and a stirring device capable of being controlled remotely to carry out reaction. The stirring paddle can be made of corundum, stainless steel, nickel-based alloy, tungsten and other high-temperature-resistant and radiation-resistant materials.
Introducing radioactive waste molten salt into the reaction system, heating to 300-800 ℃, and adding Li 2 O under stirring for reaction. Preferably, the reaction temperature is from 450 ℃ to 550 ℃, for example 500 ℃. The reaction time is 0.1-2 h. The reaction time may be adjusted according to the amount of the waste molten salt and the reaction temperature, and the present application is not particularly limited in the reaction time.
The reaction of the method is liquid phase reaction, so that waste gas is avoided, and the method has the advantages of high reaction rate and less environmental pollution. In addition, on the premise of ensuring the removal effect of the rare earth ions, the method does not introduce impurity ions which are difficult to remove and does not generate a large amount of waste.
According to some embodiments, li 2 O is added stepwise. That is, at the beginning of the reaction, the addition amount of Li 2 O was lower than the chemical reaction equivalent, and Li 2 O was added continuously as the reaction proceeded. For example, equal amounts of Li 2 O may be added sequentially to the reaction; the addition amount of Li 2 O may be reduced continuously as the reaction proceeds, for example, li 2 O may be added in a manner of decreasing the addition amount gradient, but is not limited thereto. This way, an excess of Li 2 O can be avoided to some extent.
According to a preferred embodiment, the method further comprises the step of testing the reaction progress in the reaction system by using an electrochemical method in the reaction process, so that the reaction endpoint can be judged in real time, and the reaction can be terminated in time. Since the oxidation-reduction potential of Li 2 O is low, once an excessive amount of Li 2 O is present in the reaction system, the oxidation and/or reduction peak of Li 2 O can be detected, thereby confirming the completion of the reaction. The amount of the reagent, i.e., li 2 O, used can be reduced in this manner, and the post-treatment step after the excess of Li 2 O can be avoided.
In particular, according to some embodiments, the electrochemical method of monitoring the progress of the reaction may be cyclic voltammetry, square wave voltammetry, or open circuit chronopotentiometry. The cyclic voltammetry is more commonly used because of small degree of surface change of the electrodes before and after monitoring. Higher accuracy can be obtained but longer using open circuit chronopotentiometric monitoring. Square wave voltammetry is sensitive to the response of electrochemical signals, and can quickly obtain an electrochemical graph, so that the square wave voltammetry is a preferred electrochemical monitoring method. And obtaining an electrochemical graph by an electrochemical method, and judging the reaction progress according to the electrochemical graph.
Specifically, according to some embodiments, when the oxidation and/or reduction peaks of Li 2 O appear in the electrochemical plot at the time of judging the progress of the reaction, the progress of the reaction is judged as having been removed of the rare earth ion, the addition of Li 2 O is stopped, and the reaction is ended. In the conventional method for judging the reaction end point by using the metal potential, the judgment is often in error due to the existence of an electrochemical detection limit. The mode of judging the reaction end point by whether the oxidation and/or reduction peak of Li 2 O appears or not is more accurate, and the excess of Li 2 O can be avoided more accurately.
Further, according to a preferred embodiment, a plurality of electrochemical profiles are obtained by testing a plurality of different regions of the reaction system. When judging the reaction progress, judging that the reaction progress is that the rare earth ions still remain unremoved in the reaction system and then continuously adding Li 2 O if oxidation and/or reduction peaks of Li 2 O do not appear in the multiple electrochemical graphs; at least one electrochemical curve graph in the plurality of electrochemical curve graphs does not have oxidation and/or reduction peaks of Li 2 O, and at least one electrochemical curve graph has oxidation and/or reduction peaks of Li 2 O, judging that the reaction process is a region in which rare earth ions are not removed and a region in which the rare earth ions are completely removed and Li 2 O is excessive in the reaction system, stopping adding Li 2 O, and continuing stirring to perform the reaction; or oxidation and/or reduction peaks of Li 2 O appear in a plurality of electrochemical graphs, judging the reaction progress to be that the rare earth ions in the reaction system are completely removed, stopping adding Li 2 O and ending the reaction.
For example, three electrochemical plots were obtained for three different regions of the reaction system (e.g., upper, middle, lower portions of the reaction system). When judging the reaction progress, if oxidation and/or reduction peaks of Li 2 O do not appear in all three diagrams, indicating that rare earth ions are still not removed in the reaction system, continuing to add Li 2 O, and stirring; if at least one of the three images shows oxidation and/or reduction peaks of Li 2 O, and at least one oxidation and/or reduction peak of Li 2 O does not show, the reaction time is insufficient, and the reaction is continued; if oxidation and/or reduction peaks of Li 2 O appear in all three figures, the rare earth ions are completely removed, the addition of Li 2 O is stopped and the reaction is ended. Other numbers of zones of the reaction system may also be tested to obtain a corresponding number of electrochemical profiles. The present disclosure does not limit the number of test areas and electrochemical profiles. The judging mode of testing a plurality of areas of the reaction system to obtain a plurality of electrochemical graphs can avoid the problem that the reaction progress of different areas is inconsistent, and the judgment of limitation or deviation is made only based on the electrochemical graph of one area or part of areas. Therefore, the mode of carrying out multi-region test on the reaction system can not only ensure that the rare earth ions of the whole reaction system are completely removed, but also more accurately avoid the excess of Li 2 O.
According to some embodiments, stirring is temporarily stopped and the reaction system is allowed to stand for a period of time, such as 2-10min, e.g. 3,4, 5, 6, 7min, preferably 5min, before performing the electrochemical method test.
According to some embodiments, the method further comprises removing the precipitate in the reaction system to obtain molten salt with rare earth ions removed by a reduced pressure distillation method or a molten salt filtration method after the reaction is finished. Specifically, the reduced pressure distillation method separates chlorides from oxides or oxychlorides by using a saturated vapor pressure difference, and the distillation apparatus used is large in volume and long in distillation time, but the purity of the obtained separated matters is high. Although the filtration method has high requirements on the filter medium, the separation of the chloride from the precipitate can be performed rapidly, and is a preferred separation method.
According to some embodiments, the method further comprises electrolyzing the rare earth ion-removed molten salt at 400-800, preferably 450-550 ℃ (e.g., 500 ℃) to remove excess Li 2 O, if any. Specifically, the electrolysis was performed by a potentiostatic method, and when the electrolysis current reached 1mA, the electrolysis was stopped. By utilizing the characteristic that the decomposition voltage of Li 2 O is smaller than that of molten salt, the electrolysis current of 1mA is used as a judging end point, and the excess Li 2 O in the waste molten salt is well removed by adopting a potentiostatic method electrolysis method, so that the waste molten salt is purified and can be recycled, and the minimization of nuclear power spent fuel post-treatment waste is realized.
According to some embodiments, the electrochemical method for obtaining an electrochemical graph or the electrolysis for removing excessive Li 2 O in the waste molten salt can use a three-electrode system, wherein the working electrodes are tungsten wires or molybdenum wires (for example, three working electrodes with different lengths are assembled together and separated by ceramics, the working electrodes are inserted into the molten salt, the three working electrodes are respectively positioned at the upper part, the middle part and the bottom of the molten salt, and the conducting states of the three electrodes are controlled to obtain the electrochemical graph of different areas of the molten salt in sequence), and the auxiliary electrodes are graphite rods, molybdenum rods or tungsten rods; the reference electrode is Ag/AgCl. Preferably, the working electrode is made of molybdenum, and when the rare earth concentration is high, a relatively accurate electrochemical signal can be obtained. The electrochemical graph is obtained on line by using the three-electrode system which is matched with the electrochemical workstation and can automatically rise and fall, the operation is convenient, the treatment process of the waste molten salt is not required to be interrupted, the sample of the waste molten salt is not required to be taken out, and the danger that an operator suffers from nuclear radiation is well avoided.
The present application will be further illustrated by the following examples.
Examples
38G of LiCl, 45g of KCl, 2g of LaCl 3、2g CeCl3 and 2g of NdCl 3 are sequentially weighed for preparing simulated waste molten salt (hereinafter referred to as molten salt); and sequentially weighing 0.2g, 0.1g, 0.05g and 0.05g of Li 2 O for later use.
LiCl and KCl were placed in a corundum crucible, heated to 500℃to melt both chlorides, then LaCl 3、CeCl3 and NdCl 3 were added, and heated and stirred to 500℃to melt the 3 chlorides added later to prepare molten salts.
Heating to maintain molten salt in molten state, standing for 5min, introducing into the middle of molten salt by using a three-electrode system (Mo working electrode, graphite auxiliary electrode, ag/AgCl reference electrode) connected with an electrochemical workstation, and testing by square wave voltammetry to obtain square wave voltammetry curve, as shown in curve 1 in figure 1.
Sequentially adding the sequentially weighed Li 2 O while stirring, reacting the system for 1min after adding Li 2 O each time, and standing for 5min, wherein the square wave voltammogram obtained by the method is shown in figures 1-6.
And separating molten salt and precipitate by a molten salt filtration method.
And (3) carrying out constant potential electrolysis on the separated molten salt by taking platinum as an anode and molybdenum as a cathode until the current is less than 1mA so as to remove excessive Li 2 O. Subsequently, the molten salt was electrochemically tested by cyclic voltammetry and square wave voltammetry (both methods using Mo working electrode, graphite auxiliary electrode, ag/AgCl reference electrode) to obtain cyclic voltammetry and square wave voltammetry, and the results are shown in fig. 7 (cyclic voltammetry) and fig. 8 (square wave voltammetry).
In each graph, peak a is an oxidation and/or reduction peak of Li ion (A1 is an oxidation peak of Li ion, A2 is a reduction peak of Li ion), peak B is a eutectic reduction peak of La ion, ce ion and Nd ion, peak C is a reduction peak of Li 2 O, and peak D represents a reduction peak of one of the two steps 3 electron transfer processes when the concentration of Nd 3+ is high.
Curves 1-11 in FIGS. 1-3 correspond to the square wave voltammograms measured without and after stepwise addition of Li 2 O, respectively, as described above. Referring specifically to fig. 1 and 2, the peak B in curves 1-7 decreases continuously with the addition of Li 2 O until it disappears, indicating that with the addition of Li 2 O, the concentrations of La, ce and Nd ions in the molten salt decrease continuously until it falls below the detection limit of the electrochemical workstation. The increasing intensity of peak C in curves 8-11 of FIG. 3 with the addition of Li 2 O, shows that with the addition of Li 2 O, the concentration of Li 2 O in the molten salt is higher and higher, but the increasing amplitude of the peak intensity is smaller and smaller, which shows that when the concentration of Li 2 O in the molten salt reaches a certain concentration, conversion of REClO (where RE represents La ion, ce ion or Nd ion) to RE 2O3 is started. It should be noted that, in contrast to curve 2, the peak B in curve 3 did not significantly decrease with the addition of Li 2 O, and an abnormality occurred, and since a part of REClO generated in the molten salt floated on the surface of the molten salt, the added Li 2 O was first contacted and reacted with it, generating RE 2O3, and a large part was consumed.
With further reference to fig. 4-6. Fig. 4 shows curve 7 in fig. 2 alone, in which curve 7 peak B has just disappeared; fig. 5 shows the curve 8 of fig. 3 alone, with the peak C just occurring in the curve 8; and fig. 5 shows the curve 11 of fig. 3 alone, with a larger peak C occurring in the curve 11. The reaction systems corresponding to curves 7, 8 and 11 were sampled, and the resultant samples were subjected to ICP (inductively coupled plasma) test. The result shows that when the peak B disappears, the total residual quantity of the rare earth ions in the waste salt is 1458ppm, when the peak C just appears in the curve, the total residual quantity of the rare earth ions in the waste salt is 294ppm, and when the larger peak C appears, the total residual quantity of the rare earth ions is reduced to 47ppm. It can be seen that when the peak C just appears in the curve, the total residual amount of rare earth ions is only about 300ppm, namely the rare earth ions are basically and thoroughly removed, and meanwhile, li 2 O is only slightly excessive, and the reaction can be finished to more accurately avoid the excessive Li 2 O. If Li 2 O is continuously added, even if Li 2 O is obviously excessive, the intensity of the peak C is continuously increased, but the total residual amount of rare earth ions is only slightly reduced, and the burden of subsequent electrolysis is certainly increased.
With further reference to fig. 7-8, the reduction peak C of Li 2 O has disappeared, and only the oxidation and/or reduction peak a of Li ions in both figures, indicating that Li 2 O has been completely removed from the molten salt.
The foregoing description is only of the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention.
Claims (8)
1. A method for removing rare earth ions in radioactive waste molten salt comprises the steps of gradually adding Li 2 O into the waste molten salt at 300-800 ℃ to react so as to remove the rare earth ions, testing a reaction system in the reaction process by using an electrochemical method, obtaining an electrochemical graph by using a cyclic voltammetry, square wave voltammetry or open-circuit chronopotentiometry, judging the reaction progress according to the obtained electrochemical graph, and judging that the reaction progress is that the rare earth ions are removed when oxidation and/or reduction peaks of Li 2 O appear in the electrochemical graph, stopping adding Li 2 O and ending the reaction.
2. The method of claim 1, wherein a plurality of different regions of the reaction system are tested to obtain a plurality of said electrochemical profiles, wherein,
If oxidation and/or reduction peaks of Li 2 O do not appear in the electrochemical graphs, judging that the rare earth ions are still not removed in the reaction system in the reaction process, and continuing to add Li 2 O;
At least one electrochemical curve graph of the plurality of electrochemical curve graphs does not have oxidation and/or reduction peaks of Li 2 O, and at least one electrochemical curve graph has oxidation and/or reduction peaks of Li 2 O, judging that the reaction process is the region where the rare earth ions are not removed and the region where the rare earth ions are completely removed and Li 2 O is excessive in the reaction system, stopping adding Li 2 O, and continuing the reaction; or (b)
And judging the reaction progress as that the rare earth ions in the reaction system are completely removed when oxidation and/or reduction peaks of Li 2 O appear in the electrochemical graphs, stopping adding Li 2 O and ending the reaction.
3. The method of claim 1, wherein the spent molten salt is allowed to stand for 2-10 minutes before the electrochemical method test is performed.
4. The method of claim 1, wherein the electrochemical process 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 is Ag/AgCl.
5. The method according to any one of claims 1 to 4, further comprising removing precipitates in the reaction system after the reaction is completed by a vacuum distillation method or a molten salt filtration method to obtain a molten salt from which rare earth ions are removed.
6. The method of claim 5, further comprising electrolyzing the rare earth ion-removed molten salt at 400-800 ℃ to remove excess Li 2 O.
7. The method of claim 6, wherein 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 is Ag/AgCl.
8. The method according to claim 7, wherein the electrolysis is performed by a potentiostatic method, and the electrolysis is stopped when the electrolysis current reaches 1 mA.
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