Lithium isotope enrichment device, multistage lithium isotope enrichment device, and lithium isotope enrichment method

Abstract

A lithium isotope concentration device 10 according to the present invention is provided with a process tank 1 which is divided into a supply tank 11 and a recovery tank 12 by means of an electrolyte membrane 2 that has lithium ion conductivity; and a main power supply 51, which is connected between electrodes 31, 32 that are provided on both surface of the electrolyte membrane 2, and a sub power supply 52, which is connected between the electrode 31 and a sub electrode 33 that is provided within the supply tank 11 at a distance from the electrolyte membrane 2, alternately apply a voltage.

Description

[Technical Field]

[0001] The present invention relates to a lithium isotope enrichment apparatus and a multistage lithium isotope enrichment apparatus for separating lithium isotopes, and a lithium isotope enrichment method. [Background technology]

[0002] Lithium (Li) is 7 Li and 6 There are two stable isotopes of Li, with natural abundances of 92.41 mol% and 7.59 mol%. 7 Li and mass number 6 6 Li has very different properties, e.g. 7 Li is used to adjust the pH (hydrogen ion concentration) of the reactor coolant. 6 Lithium is used to produce tritium, a fuel for fusion reactors. 7 Li and 6 Technologies have been developed to enrich and separate Li into a state with fewer isotopes of other isotopes, and methods such as the amalgamation method, molten salt method, distillation method, and the extraction of lithium ions from seawater, etc., Li + The adsorption method and the electrodialysis method (for example, Patent Document 1) are known as methods for selectively recovering methyl methacrylate.

[0003] Compared to the amalgamation method, which uses a large amount of mercury, the molten salt method, which heats lithium compounds at high temperatures, and the distillation method, the adsorption method and electrodialysis method are relatively superior in terms of environmental impact. On the other hand, these methods have the disadvantage of being able to move at high speeds due to their small mass. 6 Li +However, the isotope separation factor is small, and the productivity as an enrichment method is low. Therefore, the inventors have studied lithium recovery technology (e.g., Patent Documents 2 and 3), which selectively recovers Li from seawater, etc., by electrodialysis using an electrolyte membrane with lithium ion conductivity, in order to enrich lithium isotopes. They have found that in such Li recovery, the isotope separation factor is large only for a short period of time immediately after starting operation, and have invented a method to increase the efficiency by applying a voltage intermittently or by applying positive and negative voltages alternately (Patent Document 4, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5429658 [Patent Document 2] Patent No. 6233877 [Patent Document 3] Japanese Patent Application Publication No. 2019-141807 [Patent Document 4] Japanese Patent Application Publication No. 2019-141808 [Non-patent literature]

[0005] [Non-Patent Document 1] Shunsuke Honda, Kiyoto Shin-mura, Kazuya Sasaki, “Lithium isotope enrichment by electrochemical pumping using solid lithium electrolytes”, Journal of the Ceramic Society of Japan, Volume 126, Issue 5, pp 331-335, May 2018 Summary of the Invention [Problem to be solved by the invention]

[0006] In the method described in Patent Document 4, when voltage is applied intermittently, the effect is low unless a certain period of time is set aside for stopping application, and there is a risk of time efficiency decreasing. When applying positive and negative voltages alternately, when the voltage is applied in the opposite direction, the collected 6 Li + Therefore, it is necessary to adjust the voltage and application time to suppress this as much as possible. As such, there is room for further improvement in order to increase productivity.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a lithium isotope enrichment apparatus, a multistage lithium isotope enrichment apparatus, and a lithium isotope enrichment method that are safe, efficient, and highly productive. [Means for solving the problem]

[0008] As a result of extensive research, the inventors have discovered that when applying a voltage intermittently for electrodialysis, a high effect can be obtained even if the time during which voltage application is stopped is shortened by forming a potential difference in the supply side Li aqueous solution (before isotope enrichment) during the period when voltage application is stopped.

[0009] That is, the lithium isotope enrichment apparatus according to the present invention includes a treatment tank partitioned into a first tank and a second tank, and the treatment tank is accommodated in the first tank. 6 Li and 7 Li and Li in the state of lithium ions from an aqueous solution 6and recovering an aqueous solution containing lithium ions with a high Li isotope ratio in the second tank. The lithium isotope enrichment apparatus according to the present invention includes a lithium ion conductive electrolyte membrane that separates the treatment tank, porous electrodes provided in contact with both sides of the lithium ion conductive electrolyte membrane, a sub-electrode provided in the first tank at a distance from the surface of the lithium ion conductive electrolyte membrane facing the first tank and the porous electrode, and a power supply that applies a voltage alternately between the porous electrodes and between the porous electrode on the first tank side and the sub-electrode, with the porous electrode on the first tank side being positive. Furthermore, the multistage lithium isotope enrichment apparatus according to the present invention comprises two or more of the lithium isotope enrichment apparatuses described above, connected together so that the treatment tanks are integrated, and the lithium ion conductive electrolyte membranes of the lithium isotope enrichment apparatuses are arranged spaced apart from each other so as to separate the integrated treatment tank into three or more tanks, and the second tank of one of two adjacent lithium isotope enrichment apparatuses also serves as the first tank of the other.

[0010] The lithium isotope enrichment method according to the present invention is a treatment tank partitioned into a first tank and a second tank by a lithium ion conductive electrolyte membrane, and includes: 6 Li and 7 Li and Li in the state of lithium ions from an aqueous solution 6 The lithium isotope enrichment method according to the present invention comprises alternately performing a first step of applying a positive voltage to the first tank side of porous electrodes provided in contact with both surfaces of the lithium ion conductive electrolyte membrane relative to the second tank side, and a second step of applying a negative voltage to the porous electrode on the first tank side to a sub-electrode provided in the first tank at a distance from the porous electrode and the surface of the lithium ion conductive electrolyte membrane on the first tank side. [Effects of the Invention]

[0011] According to the lithium isotope enrichment device and the lithium isotope enrichment method of the present invention, 6 An aqueous solution with a higher Li isotope ratio can be recovered efficiently, safely, and with high productivity. 6 The isotope ratio of Li can be made even higher. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram illustrating the configuration of a lithium isotope enrichment device according to a first embodiment of the present invention. [Figure 2] 4 is a time chart illustrating the transition of the applied voltage of the power supply device of the lithium isotope enrichment device according to the first embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram illustrating another configuration of the lithium isotope enrichment device according to the first embodiment of the present invention. [Figure 4] FIG. 2 is a schematic diagram of the lithium isotope enrichment apparatus shown in FIG. 1 illustrating the electrodialysis of lithium ions. [Figure 5A] 2 is an enlarged view of a main part illustrating the behavior of lithium ions in an initial state during electrodialysis of lithium ions using the lithium isotope enrichment device shown in FIG. 1. FIG. [Figure 5B] 2 is an enlarged view of a main part illustrating the behavior of lithium ions immediately after the start of movement in electrodialysis of lithium ions using the lithium isotope enrichment device shown in FIG. 1. FIG. [Figure 5C] 2 is an enlarged view of a main part illustrating the behavior of lithium ions during migration in electrodialysis of lithium ions using the lithium isotope enrichment device shown in FIG. 1. FIG. [Figure 5D] 2 is an enlarged view of a main part illustrating the behavior of lithium ions after the movement of lithium ions stops during electrodialysis using the lithium isotope enrichment apparatus shown in FIG. 1. FIG. [Figure 6] This is a model that explains ionic conduction in electrolytes. [Figure 7]10 is a graph illustrating the applied voltage dependence of the amount of movement per unit time and the isotope ratio in electrodialysis of lithium ions, based on a simulation. [Figure 8] FIG. 3 is a schematic diagram illustrating the configuration of a lithium isotope enrichment device according to a modified example of the first embodiment of the present invention. [Figure 9] 6 is a time chart illustrating the transition of the applied voltage of the power supply device of the lithium isotope enrichment device according to the modified example of the first embodiment of the present invention. [Figure 10] 1 is a schematic diagram illustrating the configuration of a multistage lithium isotope enrichment device according to a first embodiment of the present invention. [Figure 11A] FIG. 2 is a schematic diagram illustrating a lithium isotope enrichment method using a multistage lithium isotope enrichment apparatus according to a modified example of the first embodiment of the present invention. [Figure 11B] FIG. 2 is a schematic diagram illustrating a lithium isotope enrichment method using a multistage lithium isotope enrichment apparatus according to a modified example of the first embodiment of the present invention. [Figure 12] FIG. 4 is a schematic diagram illustrating the configuration of a multistage lithium isotope enrichment device according to a second embodiment of the present invention. [Figure 13A] FIG. 13 is a schematic diagram illustrating a lithium isotope enrichment method using the multistage lithium isotope enrichment device shown in FIG. [Figure 13B] FIG. 13 is a schematic diagram illustrating a lithium isotope enrichment method using the multistage lithium isotope enrichment device shown in FIG. [Figure 13C] FIG. 13 is a schematic diagram illustrating a lithium isotope enrichment method using the multistage lithium isotope enrichment device shown in FIG. [Figure 14] FIG. 4 is a schematic diagram illustrating the configuration of a lithium isotope enrichment device according to a second embodiment of the present invention. [Figure 15] FIG. 15 is a schematic diagram of the lithium isotope enrichment device shown in FIG. 14, illustrating electrodialysis of lithium ions in the lithium isotope enrichment method. [Figure 16] FIG. 10 is a schematic diagram illustrating the configuration of a lithium isotope enrichment device according to a first modified example of the second embodiment of the present invention. [Figure 17]FIG. 17 is a schematic diagram of the lithium isotope enrichment device shown in FIG. 16, illustrating electrodialysis of lithium ions in the lithium isotope enrichment method. [Figure 18] FIG. 10 is a schematic diagram illustrating the configuration of a lithium isotope enrichment device according to a second modified example of the second embodiment of the present invention. [Figure 19] FIG. 19 is a schematic diagram of the lithium isotope enrichment device shown in FIG. 18, illustrating electrodialysis of lithium ions in the lithium isotope enrichment method. [Figure 20] 1 is a graph showing the amount of lithium ion migration and the lithium isotope separation coefficient according to an example and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0013] Embodiments of a lithium isotope enrichment apparatus and a lithium isotope enrichment method according to the present invention will be described with reference to the drawings. In the drawings, the size of certain elements may be exaggerated and the shapes may be simplified for clarity. In addition, in the description of each embodiment, elements that are the same as those in the previous embodiment are designated by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0014] [First embodiment] (Lithium isotope enrichment device) 1, a lithium isotope enrichment apparatus 10 according to a first embodiment of the present invention includes a treatment tank 1, an electrolyte membrane (lithium ion conductive electrolyte membrane) 2 that separates the treatment tank 1 into two sections, a first electrode 31 and a second electrode 32 (electrodes with a porous structure) coated on each side of the electrolyte membrane 2, a third electrode (auxiliary electrode) 33, a power supply unit 5, an agitator (circulation means) 6, and a cooling unit 7. The treatment tank 1 is divided by the electrolyte membrane 2 into a supply tank (first tank) 11 that contains a Li-containing aqueous solution ASi, 6and a recovery tank (second tank) 12 that stores an aqueous solution for Li recovery ASo. A third electrode 33 is provided in the supply tank 11, spaced apart from the electrolyte membrane 2. The power supply device 5 includes a main power supply 51 and an auxiliary power supply 52, each of which is a DC power supply, and a switching element 5s1, and the main power supply 51 and the auxiliary power supply 52 apply voltage alternately by switching the switching element 5s1. The main power supply 51 has a positive (+) terminal connected to a first electrode 31 provided on the supply tank 11 side, and a negative (-) terminal connected to a second electrode 32 provided on the recovery tank 12 side. The auxiliary power supply 52 has a positive terminal connected to the first electrode 31, and a negative terminal connected to the third electrode 33. The agitator 6 circulates the Li-containing aqueous solution ASi in the supply tank 11. The cooling device 7 cools the liquid in the recovery tank 12. 6 The electrolyte membrane 2 is cooled via the aqueous solution ASo for recovering Li. Hereinafter, each element constituting the lithium isotope enrichment device according to the first embodiment of the present invention will be described.

[0015] The treatment tank 1 contains a Li-containing aqueous solution ASi and 6 The treatment tank 1 is made of a material that does not corrode or change in quality even when it comes into contact with the aqueous solution ASo for recovering Li. The treatment tank 1 is not particularly limited in shape, etc., as long as it has a volume corresponding to the required treatment capacity.

[0016] The electrolyte membrane 2 is an electrolyte having lithium ion conductivity and has electron e - Furthermore, it is preferable that the Li-containing aqueous solution ASi does not conduct Li. + When the electrolyte membrane 2 contains metal ions other than those mentioned above, it is preferable that the electrolyte membrane 2 does not conduct these metal ions. More preferably, the electrolyte membrane 2 is made of a ceramic material having these properties. Specifically, lithium lanthanum titanium oxide (La 2 / 3-x Li 3x Such an electrolyte membrane 2 has a certain proportion of lattice defects, and the size of the lattice defect sites is small, so Li +For example, in a solid electrolyte with a perovskite (ABO3) structure (A = Li, La, or vacancy, B = Ti) such as LLTO, there are vacancies (A-site defects) in some A-sites. As explained in the lithium isotope enrichment method below, Li is ionized in the A-site defects. + penetrates and Li ions are formed between the neighboring A-site defects. + Hereinafter, a site where Li can exist, such as the A site, will be referred to as a Li site, and a Li site with a vacancy will be referred to as a Li site defect.

[0017] The first electrode 31 and the second electrode 32 are a pair of electrodes for applying a voltage between both surfaces of the electrolyte membrane 2, with the first electrode 31 being in contact with the surface of the electrolyte membrane 2 facing the supply tank 11 (hereinafter referred to as the front surface, as appropriate), and the second electrode 32 being in contact with the surface of the electrolyte membrane 2 facing the recovery tank 12 (hereinafter referred to as the back surface, as appropriate). The first electrode 31 and the second electrode 32 have a porous structure, such as a mesh structure, so that a voltage can be applied to a wide area of ​​the electrolyte membrane 2 while the aqueous solutions ASi and ASo can come into contact with a sufficient area on each surface of the electrolyte membrane 2.

[0018] The first electrode 31 is preferably made of an electrode material that has electronic conductivity and is stable in the Li-containing aqueous solution ASi even when a voltage is applied, and that has catalytic activity for the reactions of the following formulas (1) and (2): The second electrode 32 is preferably made of an electrode material that has electronic conductivity and is stable in the Li-containing aqueous solution ASi even when a voltage is applied, and that has catalytic activity for the reactions of the following formulas (1) and (2): + It began to contain 6 The first electrode 31 and the second electrode 32 are preferably made of an electrode material that is stable when a voltage is applied even in the aqueous solution for Li recovery ASo, and that has catalytic activity for the reactions of the following formulas (3) and (4). The first electrode 31 and the second electrode 32 are preferably made of a material that can be easily processed into the above-mentioned shapes. Platinum (Pt), for example, is a preferred electrode material for the first electrode 31 and the second electrode 32. In each formula, the Li contained in the electrolyte membrane 2 (electrolyte) is + Li + The following formula (2) shows the Li in the aqueous solution (Li-containing aqueous solution ASi). +The following formula (4) shows the reaction in which Li in the electrolyte membrane 2 moves + is an aqueous solution ( 6 The reaction of transferring the Li to the aqueous solution for Li recovery (ASo) is shown below. [ka]

[0019] The third electrode 33 is an electrode that enables a lower potential to be formed in the Li-containing aqueous solution ASi than the surface of the electrolyte membrane 2. Therefore, the third electrode 33 is preferably arranged in the supply tank 11 so as not to contact the electrolyte membrane 2 and the first electrode 31 and to be parallel to the first electrode 31. Furthermore, in order to keep the voltage V2 applied between the third electrode 33 and the first electrode 31 low, as described below, the third electrode 33 is preferably arranged close enough to the first electrode 31 to prevent short-circuiting. Furthermore, the third electrode 33 preferably has a mesh-like shape or the like that allows the aqueous solution to pass through, so that the Li-containing aqueous solution ASi in contact with the surface of the electrolyte membrane 2 (first electrode 31) in the supply tank 11 is continuously replaced. Like the first electrode 31, the third electrode 33 is preferably formed from an electrode material that is stable in the Li-containing aqueous solution ASi even when a voltage is applied and that is easily processed into the above-mentioned shape. The third electrode 33 is preferably made of such an electrode material, for example, platinum (Pt), or carbon (C) can also be used.

[0020] The power supply device 5 includes two DC power supplies 51 and 52, a switching element 5s1, a drive circuit for the switching element 5s1, and other components. The power supply device 5 alternately applies DC voltages from the power supplies 51 and 52. The main power supply 51 has its positive pole connected to the first electrode 31 and its negative pole connected to the second electrode 32, applying a positive voltage V1 (voltage +V1) to the first electrode 31 relative to the second electrode 32. The auxiliary power supply 52 has its positive pole connected to the first electrode 31 and its negative pole connected to the third electrode 33, applying a negative voltage V2 (voltage -V2) to the third electrode 33 relative to the first electrode 31 when the main power supply 51 is not applying the voltage +V1, as shown in FIG. 2 . To this end, the switching element 5s1 is configured to connect the first electrode 31 to either the positive pole of the main power supply 51 or the positive pole of the auxiliary power supply 52, or to neither. In other words, the power supply device 5 includes two DC pulse power supplies synchronized with each other. Ideally, power supplies 51 and 52 should have a built-in capacitor or the like to have high time response so as to output a square wave as shown in Fig. 2. Details of the magnitudes of voltages V1 and V2 and the timing of their application will be described later, but it is preferable that voltage V2 be smaller than voltage V1.

[0021] As described above, the power supply device 5 is only required to apply voltages V1 and V2 of predetermined polarity and magnitude alternately between the first electrode 31 and the second electrode 32 and between the third electrode 33 and the first electrode 31, respectively. The circuit configuration shown in FIG. 1 is merely an example. For example, as shown in FIG. 3, the power supply device 5A may include a variable power supply 51A that can switch between two voltage levels, V1 and V2, and a switching element 5s1 that switches the connection destination of the negative electrode. Alternatively, two DC power supplies may be connected in series via a switching element, with two power supplies applying voltage V1 and one power supply applying voltage V2 (not shown). In the lithium isotope enrichment device 10 shown in FIGS. 1 and 3, when the power supply device 5, 5A applies voltage V1 between the first electrode 31 and the second electrode 32, the third electrode 33 is in an open state (see FIG. 4), and when applying voltage V2 between the third electrode 33 and the first electrode 31, the second electrode 32 is in an open state. Alternatively, the power supply device 5, 5A may be configured so that when a voltage +V1 is applied between the first electrode 31 and the second electrode 32, the third electrode 33 is connected to the same potential as the first electrode 31. Also, the power supply device 5, 5A may be configured so that when a voltage −V2 is applied between the third electrode 33 and the first electrode 31, the second electrode 32 is connected to the same potential as the first electrode 31.

[0022] The agitator 6 is a device for circulating the Li-containing aqueous solution ASi in the supply tank 11 so that the Li-containing aqueous solution ASi in contact with the first electrode 31 is continuously replaced during operation, and is provided as needed. A known device can be used as the agitator 6, and for example, a screw may be rotated, or the Li-containing aqueous solution ASi may be circulated to the outside of the treatment tank 1 by a pump. The lithium isotope enrichment apparatus 10 is also provided with an agitator 6 in the recovery tank 12, 6 The aqueous solution ASo for recovering Li may be circulated.

[0023] The cooling device 7 is provided as needed to maintain the electrolyte membrane 2 at a predetermined temperature. 6The electrolyte membrane 2 is cooled via the aqueous solution ASo for recovering Li. The cooling device 7 may be a known device for cooling a liquid, and preferably has a temperature adjustment function. In this embodiment, the cooling device 7 is an immersion type, and a pipe (refrigerant pipe) through which the refrigerant flows is located inside the recovery tank 12. 6 The cooling device 7 is installed by immersing it in the aqueous solution ASo for recovering Li. The cooling device 7 is only required to be able to keep the electrolyte membrane 2 at a predetermined temperature. 6 The aqueous solution for Li recovery ASo does not need to be kept at a uniform liquid temperature. However, depending on the volume of the treatment tank 1, a stirrer may be provided. The refrigerant pipe of the cooling device 7 is the same as the treatment tank 1, and the aqueous solution for Li recovery ASo and the 6 The cooling device 7 is made of a material that does not corrode or otherwise deteriorate even when it comes into contact with the aqueous solution ASo for recovering Li, and its shape is not particularly limited. For example, in order to efficiently cool the electrolyte membrane 2, the refrigerant pipe is installed in a serpentine shape in accordance with the dimensions of the plate-shaped electrolyte membrane 2, facing closely over a wide area of ​​the electrolyte membrane 2. Depending on the thickness of the electrolyte membrane 2, the refrigerant pipe may be inserted into both the supply tank 11 and the recovery tank 12. The cooling device 7 may also be configured such that the treatment tank 1 has a double structure (jacket tank) and the refrigerant flows inside it (jacket portion). Alternatively, the treatment tank 1 may be configured to cool the Li-containing aqueous solution ASi or 6 The Li recovery aqueous solution ASo may be circulated outside the treatment tank 1 by a pump and cooled by a heat exchanger.

[0024] The temperature of the electrolyte membrane 2 will be described in detail later, but it is set to 30° C. or less so that the aqueous solutions ASi and ASo do not freeze. 6 When the aqueous solution ASo for Li recovery is pure water at the start of operation of the lithium isotope enrichment apparatus 10 (start of electrodialysis), the temperature of the electrolyte membrane 2 is set to 0°C or higher. 6 Alternatively, the temperature of the aqueous solution ASo for Li recovery can be measured.

[0025] The Li-containing aqueous solution ASi is a Li source, 7 Li and 6 Li cations 7 Li + , 6 Li+ The aqueous solution containing the lithium isotope enrichment device 10 is, for example, an aqueous solution of lithium hydroxide (LiOH). At the start of operation of the lithium isotope enrichment device 10, 7 Li + , 6 Li + The Li-containing aqueous solution ASi contains Li + A higher concentration is preferable, and at the start of operation of the lithium isotope enrichment apparatus 10, Li + It is more preferable that the solution is a saturated or supersaturated aqueous solution of the above. 6 The aqueous solution for Li recovery ASo is a solution for recovering lithium ions Li from the aqueous solution containing Li ASi. + ,especially, 6 Li + At the start of operation of the lithium isotope enrichment apparatus 10, the aqueous solution is, for example, pure water. 7 Li and 6 Li( 7 Li + and 6 Li + ) are collectively called Li(Li + ) is called.

[0026] The lithium isotope enrichment apparatus 10 further comprises an operating Li-containing aqueous solution ASi, 6 A liquid level sensor or the like may be provided to detect fluctuations in the amount of the aqueous solution ASo for Li recovery. In addition, in order to prevent carbon dioxide (CO2) in the atmosphere from unintentionally dissolving in the aqueous solutions ASi and ASo and causing lithium carbonate (Li2CO3) to precipitate, the lithium isotope enrichment device 10 6 It is preferable that the aqueous solution ASo for recovering Li is configured so as not to be exposed to the atmosphere. Furthermore, for safety reasons, it is preferable that the lithium isotope enrichment apparatus 10 is provided with an exhaust means for exhausting H2 and O2 generated during operation (by the reactions of formulas (1) and (3)) so as not to fill the inside.

[0027] (Lithium isotope enrichment method) A lithium isotope enrichment method according to an embodiment of the present invention alternately performs a first step of applying a positive voltage V1 to a first electrode 31 provided on the front surface of an electrolyte membrane 2 relative to a second electrode 32 provided on the back surface, and a second step of applying a negative voltage V2 to a third electrode 33 relative to the first electrode 31. First, with reference to Fig. 4, electrodialysis of lithium ions using the lithium isotope enrichment apparatus according to the first embodiment will be described. Note that the agitator 6 and cooling device 7 are omitted from the lithium isotope enrichment apparatus 10 shown in Fig. 4.

[0028] As shown in FIG. 4, in the lithium isotope enrichment apparatus 10, the main power supply 51 of the power supply device 5 applies a positive voltage V1 (voltage +V1) to the first electrode 31 relative to the second electrode 32. Then, hydroxide ions (OH - ) undergoes the reaction shown in equation (1) below, generating electrons e - is released to the first electrode 31, generating water (H2O) and oxygen (O2). In the Li-containing aqueous solution ASi, OH - As the charge in the Li-containing aqueous solution ASi decreases, the charge balance is maintained. + The reaction of the following formula (2), in which migrates into the electrolyte membrane 2, occurs near the electrolyte membrane 2. Combining the reaction of the following formula (1) and the reaction of the following formula (2), the reaction of the following formula (5) occurs near the first electrode 31. On the other hand, near the second electrode 32, 6 The H2O in the aqueous solution ASo for Li recovery is electron e - By supplying hydrogen (H) and OH, the reaction shown in the following formula (3) occurs. - Generates. 6 In the aqueous solution for Li recovery ASo, OH - As the amount of Li increases, the amount of Li in the electrolyte membrane 2 increases to maintain the charge balance. + The reaction of the following formula (4), in which the oxygen ions migrate, occurs near the electrolyte membrane 2. Combining the reaction of the following formula (3) and the reaction of the following formula (4), the reaction of the following formula (6) occurs near the second electrode 32. [ka]

[0029] When these reactions occur, the Li-containing aqueous solution ASi, the electrolyte membrane 2 (electrolyte), and 6 The Li contained in each of the aqueous solutions for Li recovery ASo + Due to the electrochemical potential difference, Li is released from the Li-containing aqueous solution ASi. + permeates the electrolyte membrane 2 6 These reactions are carried out by transferring the Li-containing aqueous solution ASi to the first electrode 31 and the second electrode 32. 6 Each electron e - Therefore, the larger the voltage V1, the faster the rate at which Li moves from the first electrode 31 side to the second electrode 32 side in the electrolyte membrane 2. + However, in reality, when the voltage V1 becomes larger than a certain level, the amount of electrons e - Since Li + The mobility of Li continues to increase, but the voltage dependence decreases. + The behavior of the electrolyte membrane 2 when it permeates the electrolyte membrane 2 will be described in detail with reference to FIGS. 5A to 5C. FIGS. 5A to 5C are enlarged cross-sectional views of the vicinity of the electrolyte membrane 2 of the lithium isotope enrichment device 10, in which the electrodes 31 and 32 are in partial contact with both sides of the electrolyte membrane 2. The aqueous solutions ASi and ASo are 7 Li + , 6 Li + Only the above are indicated by encircling each one.

[0030] When no voltage is applied, as shown in Figure 5A, 7 Li + , 6 Li + The Li-containing aqueous solution ASi floats, and alternately repeats adsorption to and desorption from the surface of the electrolyte membrane 2. From this state, as shown in FIG. 5B, a voltage V1 (voltage +V1) is applied to the first electrode 31 and to the second electrode 32, with a positive voltage and a negative voltage, respectively. In the figure, a positive charge is represented by a + in a circle, and a negative charge is represented by a - in a circle. Then, the Li-containing aqueous solution ASi + (7 Li + , 6 Li + ) dissolves in the electrolyte membrane 2 as a reaction of the above formula (2). At this time, Li adsorbed in the vicinity of the Li site defect on the surface of the electrolyte membrane 2 + Since the electrolyte membrane 2 has a potential gradient where the potential on the back side is lower than that on the front side due to the electrodes 31 and 32, the Li + The Li ions hop to the nearby Li site defects in the deeper part of the electrolyte membrane 2. + The electrons repeatedly move from the Li site defects of the electrolyte membrane 2 to the neighboring Li site defects, and finally, as shown in FIG. 5C, the electrons move from the Li site defects on the back surface to the neighboring Li site defects as shown in the reaction of the formula (4). 6 Transfer to aqueous solution ASo for Li recovery.

[0031] In addition, on the surface of the electrolyte membrane 2, Li adsorbed near the Li site defects + As a result of the migration of Li to the depths of the electrolyte membrane 2, another Li adsorbed in the vicinity of the vacant Li site defect is absorbed. + Li may migrate and penetrate, or new Li may emerge from the Li-containing aqueous solution ASi. + These Li + Similarly, Li moves through the electrolyte membrane 2. + The Li site defect moves to Li + The Li adsorbed on the surface is released through the newly generated Li site defects on the surface of the electrolyte membrane 2. + can start moving to the back side.

[0032] Li + The inter-site migration (hopping) of the ions in the electrolyte membrane 2 will be described in more detail with reference to Fig. 6. Fig. 6 shows a model for explaining ion conduction in the electrolyte, where x is the position in the thickness direction of the electrolyte membrane 2, E p indicates the potential energy. In the electrolyte membrane 2, Li + ( 7 Li+ , 6 Li + ) exists stably at the Li site where the potential energy is minimum, but the nearby Li site is a vacancy (represented by a dashed circle), and the activation energy E a When the site receives more than 1000 keV of energy, the site defect has an energy barrier E m can move by jumping over (hopping) (E a =E D / 2+E m , E D : defect formation energy). Also, we can assume that the ion is thermally vibrating at the position of the potential energy minimum with a frequency of Γ0, and it can hop at a frequency (hopping rate Γ) corresponding to this frequency (frequency factor) Γ0. The frequency Γ0 is inversely proportional to the square root of the ion's mass. 6 Li has a mass of 7 Since it is 6 / 7 times smaller than Li, the frequency Γ0 7 Li, and as will be described in detail later, the average migration speed in the electrolyte membrane 2 is 7 For example, for a Li site defect in the electrolyte membrane 2, two equidistant locations are located nearby. 7 Li + and 6 Li + If exists, 6 Li + It is assumed that the latter will jump preferentially.

[0033] Also, at the Li site of the electrolyte membrane 2, i.e., in the ground state 7 Li + , 6 Li + is the zero-point vibration hω I The potential energy increases by the amount of zero-point vibration hω I is isotope dependent, 7 Li + than 6 Li + Similarly, in the excited state, the zero-point vibration hω S teeth 6 Li+ is larger. Therefore, 7 Li + Mass less than 6 Li + In both the ground and excited states, the zero-point vibration hω I ,hω S The potential energy is high considering 6 Li + The potential energy difference between these excited states and the ground state is the energy barrier E m is small (E m ( 7 Li + )>E m ( 6 Li + )), i.e., the activation energy E a is small. As a result, 6 Li + The energy received is 7 Li + Hopping can occur even when the value is smaller than Li + is the activation energy E of the received energy a Therefore, the larger the applied voltage V1 between both sides of the electrolyte membrane 2, the greater the mobility μ. + The amount of movement per unit time increases.

[0034] Also, 6 Li + is better 7 Li + At a voltage V1 smaller than the activation energy E a The surplus energy for the 6 Li + The mobility μ is higher in the case of the Γ0 frequency ratio. 6 Li + , 7 Li + Movement amount and movement Li + The simulation shows the dependence of the isotope ratio of 1000 to 10 ... a The distribution of is approximated by a normal distribution.6 Li + , 7 Li + For each of these, the activation energy E a ( 6 Li + < 7 Li + Li exceeding the + The ratio of activation energy E a The probability density of the normal distribution with the mean value is calculated, and the cumulative value is multiplied by the frequency Γ0 ratio to obtain the relative value of the mobility μ. 7 Li + and 6 Li + The ratio was calculated as 1:1.

[0035] As shown in Figure 7, 6 Li + , 7 Li + The mobility of each increases from 0 in an S-shaped curve as the applied voltage increases. 7 Li + For activation energy E a Small 6 Li + However, the voltage is shifted to the smaller side and is higher by the frequency Γ0 ratio. 6 Li + Therefore, when the applied voltage is applied to the electrolyte membrane 2, 6 Li + The smaller the range of movement, 6 Li + but 7 Li + As the applied voltage increases, 6 Li + , 7 Li + As the mobilities of these atoms converge, the difference between them becomes smaller, and the isotope ratio converges to (√(7 / 6)) / (1+√(7 / 6)).

[0036] However, in reality, when the voltage V1 is increased,6 Li + , 7 Li + It is believed that the voltage dependence drops significantly before the mobility of ions reaches the limit shown in FIG. 7. In detail, as described above, when the voltage V1 applied between both sides of the electrolyte membrane 2 exceeds a certain value, some of the transition metal ions constituting the electrolyte membrane 2 are reduced (for example, if the electrolyte membrane 2 is LLTO, Ti 4+ +e - →Ti 3+ ), the electrolyte membrane 2 transfers electrons e from the recovery tank 12 side to the supply tank 11 side. - As a result, most of the applied electrical energy is transferred to electrons e - is consumed in the conduction of Li + The voltage dependence of the mobility of Li + Furthermore, when some of the transition metal ions constituting the electrolyte membrane 2 are reduced, the ionic radius of the reduced ions increases (for example, if the electrolyte membrane 2 is LLTO, Ti 4+ <Ti 3+ ), Li + As the bottleneck for movement of Li widens, + of 6 The Li isotope ratio drops sharply. - The temperature of the electrolyte membrane 2 rises due to Joule heat generated in the electrolyte membrane 2, and as will be described later, the moving Li + of 6 The Li isotope ratio will decrease.

[0037] The ion mobility μ is related to the ion diffusion coefficient D by the following equation (7) (T: temperature (K), k: Boltzmann's constant). The diffusion coefficient D is proportional to the hopping rate Γ (a: average distance between sites (jump length), n c : carrier density, f: correlation effect coefficient determined by the ion and its surroundings, d: dimension of the diffusion field). In addition, the frequency factor Γ0 in equation (8) is proportional to the temperature T as expressed in the following equation (9), and (Z s vib / ZI vib ) is inversely proportional to the square root of the mass number m, so the frequency factor Γ0 is inversely proportional to the square root of the mass number m (h: Planck's constant, Z s vib : phonon partition function at the saddle point, Z I vib : phonon partition function in the initial state, C1: constant). From equations (8) and (9), the diffusion coefficient D is expressed by the following equation (10). From equations (7) and (10), the ion mobility μ is expressed by the following equation (11) (C2: constant). As shown in equation (11), the ion mobility μ is 7 Li + For mass number m and activation energy E a Small 6 Li + is higher.

number

[0038] Immediately after the start of voltage application as shown in FIG. 5B, the Li adsorbed on the surface of the electrolyte membrane 2 + in 7 Li + , 6 Li is the Li in the Li-containing aqueous solution ASi. + However, 7 Li + and 6 Due to the difference in migration rate between Li and Li, the Li site defects in the electrolyte membrane 2 6 Li + of Li-containing aqueous solution ASi 6 Li isotope ratio ( 6 Li / ( 7 Li+ 6 Li)) and migration between Li site defects in the electrolyte membrane 2, and 6 Lithium can also be transferred to the aqueous solution ASo. 6 Li + As a result, the Li adsorbed on the surface of the electrolyte membrane 2 + Among them, 6 Li +The surface is filled with new ions from the Li-containing aqueous solution ASi. 7 Li + , 6 Li + By repeating this process, the remaining Li in the Li-containing aqueous solution ASi gradually + of 6 Li isotope ratio ( 6 Li / ( 7 Li+ 6 Li)) decreases, so newly moving Li + of 6 The Li isotope ratio decreases. + of 6 The Li isotope ratio is at a maximum immediately after the start of application of the voltage +V1, and then decreases exponentially as the application time passes (see Patent Document 4).

[0039] Therefore, the lithium isotope enrichment method according to this embodiment involves applying a voltage +V1 for a short period of time (first step). 6 The recovery of a small amount of Li with a high Li isotope ratio is repeated. Just before the voltage application is stopped, when the movement of Li has progressed to a certain extent by the application of voltage +V1, as described above, the Li adsorbed on the surface of the electrolyte membrane 2 + teeth, 6 Li + Since the Li preferentially migrated into the electrolyte membrane 2, the Li remaining in the Li-containing aqueous solution ASi at this point + than 6 The Li isotope ratio is considered to be low (see FIG. 5C). Therefore, before the next application of the voltage +V1, 7 Li + , 6 Li + When the voltage +V1 is stopped and the device is in a non-applied state, Li + The movement of the ions in the electrolyte membrane 2 stops, and in the Li-containing aqueous solution ASi, 6 Li in aqueous solution ASo for Li recovery +The Li atoms float in the aqueous solutions ASi and ASo, respectively, and are repeatedly adsorbed to and released from the front and back surfaces of the electrolyte membrane 2 (see FIG. 5A). However, in the Li-containing aqueous solution ASi, the Li atoms adsorbed on the surface of the electrolyte membrane 2 are repeatedly adsorbed to and released from the back surface of the electrolyte membrane 2 (see FIG. 5A). + However, because the application of voltage +V1 had been causing the particles to be electrically attracted to the deeper side (back side) of the electrolyte membrane 2 until just before, they do not immediately detach from the surface of the electrolyte membrane 2 even when the application of voltage +V1 is stopped.

[0040] Therefore, in the lithium isotope enrichment method according to this embodiment, when the application of the voltage +V1 is stopped, the auxiliary power supply 52 starts to apply a negative voltage V2 (voltage −V2) relative to the first electrode 31 to the third electrode 33 (second step). The application of the voltage −V2 generates a potential gradient in the Li-containing aqueous solution ASi such that the vicinity of the surface of the electrolyte membrane 2 is positive and the vicinity of the third electrode 33 is negative. As a result, as shown in FIG. 5D, the Li adsorbed on the surface of the electrolyte membrane 2 is released. + However, they quickly detach due to electrostatic repulsion.

[0041] By applying voltage -V2, Li is released from the surface of electrolyte membrane 2 in a short time. + Therefore, when the application of voltage −V2 is stopped and the application of voltage +V1 by the main power supply 51 is started again, new Li appears on the surface of the electrolyte membrane 2 as shown in FIG. 5B. + This Li adsorbed on the surface of the electrolyte membrane 2 + is the Li concentration in the Li-containing aqueous solution ASi at this point. + Therefore, the Li adsorbed on the surface of the electrolyte membrane 2 just before the previous application of the voltage +V1 was stopped (see FIG. 5C) + than 6 As a result, even with the short application of voltage +V1, 6 In this way, by alternately applying a voltage +V1 between both sides of the electrolyte membrane 2 (between the first electrode 31 and the second electrode 32) (first step) and a voltage -V2 between the third electrode 33 and the first electrode 31 (second step) in a short time, it is possible to recover Li with a high Li isotope ratio, compared to applying the voltage +V1 continuously. 6It is possible to recover a large amount of Li with a high Li isotope ratio.

[0042] As mentioned above, the larger the voltage V1, the + The mobility of Li per hour is high. + However, when the voltage V1 exceeds a certain value, the electrolyte membrane 2 begins to exhibit electronic conductivity, and as a result, the amount of Li + The energy efficiency of increasing the mobility of Li decreases, and the mobility of Li + of 6 The Li isotope ratio decreases rapidly. On the other hand, the smaller the voltage V1, 7 Li + against 6 Li + Therefore, the voltage V1 is preferably a voltage at which the electrolyte membrane 2 does not exhibit electronic conductivity, and is preferably as low as possible. However, the lower the voltage V1, the greater the mobility of Li + Therefore, it is preferable to set the voltage to a level that does not cause excessive degradation of productivity. Specifically, although it depends on the electronic conductivity of the electrolyte membrane 2 and the electrode performance that determines the electrode reaction overvoltage, when a voltage exceeding 2.0 V is applied, electronic conductivity may be exhibited in the electrolyte membrane 2. On the other hand, Li + As long as the voltage V1 can be moved in the electrolyte membrane 2, there is no particular restriction on the lower limit of the voltage V1. For example, although it depends on the electronic conductivity of the electrolyte membrane 2 and the hydrogen ion concentrations of the aqueous solutions ASi and ASo, it is preferable to set the voltage V1 in the range of 0.5 V or more. The voltage V1 is set to a value which is determined by the time of one continuous application (electrodialysis period) t ED In combination with 6 It is preferable to set it depending on the effect of Li concentration.

[0043] In addition, the larger the voltage V2, the stronger the electric field applied to the Li-containing aqueous solution ASi between the surface of the electrolyte membrane 2 and the third electrode 33, and the greater the amount of Li + is attracted to the third electrode 33 at high speed, and the Li adsorbed on the surface of the electrolyte membrane 2 +is released in a shorter time. On the other hand, when voltage V2, i.e., the potential difference between first electrode 31 and third electrode 33, exceeds a certain value, an electrolysis reaction of HO (reactions of formulas (1) and (3)) occurs in the Li-containing aqueous solution ASi, and energy efficiency decreases. Therefore, voltage V2 is preferably set to a magnitude that does not cause an electrolysis reaction of HO, and is at most smaller than voltage V1. For such a voltage V2, it is preferable that the third electrode 33 be positioned at a short distance from the first electrode 31 so that the electric field between the surface of the electrolyte membrane 2 and the third electrode 33 is large.

[0044] Voltage +V1 applied continuously for one time (electrodialysis period) t ED , and the time for which voltage -V2 is applied continuously (reset period) t RST is not specifically specified, 6 It is preferable that the electrodialysis period t is set so that the Li recovery efficiency is sufficiently high. ED The shorter the time, the more Li is recovered. 6 The Li isotope ratio is high, and specifically, the reset period t is preferably 1 second or less, and more preferably about 0.5 seconds. RST is the Li adsorbed on the surface of the electrolyte membrane 2 by the application of the previous voltage +V1. + It is sufficient that the distance is sufficiently separated, preferably completely separated. 6 The recovery efficiency of Li did not improve, and the electrodialysis period t ED Period t CYC The ratio to the reset period t becomes lower, and the time efficiency (productivity) decreases. RST The larger the voltage V2 is and the shorter the distance between the third electrode 33 and the first electrode 31, the shorter the time required to obtain the effect.

[0045] It is also preferable that the voltage +V1 and the voltage -V2 are not applied simultaneously. + Since the concentration of is relatively low near the surface of the electrolyte membrane 2, when voltage -V2 is applied while voltage +V1 is applied, 6 Li in aqueous solution ASo for Li recovery +The movement of Li is hindered, resulting in a decrease in energy efficiency. + of 6 It is preferable that the voltage −V2 is not applied immediately after the start of application of the voltage +V1, at which the Li isotope ratio is maximized. On the other hand, during the non-application period t int1 ,t int2 However, there may be a no-voltage period t int1 ,t int2 If the time is long, productivity will decrease. Therefore, the application of voltage +V1 (electrodialysis period t ED ) after the application of voltage -V2 (reset period t RST ) to start (t int1 ≧0), and it is more preferable to start applying the voltage −V2 as soon as possible after the application of the voltage +V1 is stopped. int1 =0) is more preferable. RST ) After the stop of the application of voltage +V1 (electrodialysis period t ED ) to start (t int2 ≧0), and the application of voltage +V1 is started after the application of voltage −V2 is stopped (t int2 >0) is more preferable. The timings for starting and stopping the application of the voltages +V1 and -V2 are preferably set in accordance with the timing accuracy of the power supply device 5, etc., so that the voltages +V1 and -V2 are not applied simultaneously.

[0046] In the lithium isotope enrichment method according to this embodiment, Li in the Li-containing aqueous solution ASi on the supply side is + The higher the concentration, the greater the Li + The mobility is higher, 6 Therefore, the Li-containing aqueous solution ASi can be used to concentrate Li. + Higher concentrations are preferred, and Li + It is more preferable that the aqueous solution is a saturated or supersaturated aqueous solution of Li. +In order to suppress the decrease in concentration, for example, every time a predetermined operation application time has elapsed, or 6 Li in aqueous solution ASo for Li recovery + When the concentration drops and reaches a predetermined value, it is preferable to replace the Li-containing aqueous solution ASi in the supply tank 11, and it is also preferable to circulate the Li-containing aqueous solution ASi between the supply tank 11 and the outside of the treatment tank 1 during operation. + The amount of Li remaining in the Li-containing aqueous solution ASi due to the migration of + of 6 Li isotope ratio ( 6 Li / ( 7 Li+ 6 Li)) is suppressed, so newly moving Li + of 6 The decrease in the Li isotope ratio is suppressed, 6 Li can be concentrated more efficiently. In addition, the decrease in the volume of the Li-containing aqueous solution ASi due to the reaction of formula (1) can be compensated for. 6 As for the aqueous solution ASo for recovering Li, the volume of the solution decreases due to the reaction of formula (3), so it is preferable to add water (HO) or the like to the recovery tank 12 as needed. In the lithium isotope enrichment apparatus 10, it is preferable that the liquid levels in the supply tank 11 and the recovery tank 12 are the same during operation.

[0047] Furthermore, as shown in the above formula (11), the ion mobility μ also depends on the temperature T, and the degree of this dependency varies with the activation energy E a is affected by. 7 Li + , 6 Li + The mobility of each increases exponentially with increasing temperature, but the activation energy E a Large 7 Li + As a result, as the temperature increases, the mobility of 7 Li + and expensive 6 Li + The ratio of the moving Li + of 6 The Li isotope ratio is Li+ The temperature range applicable in this embodiment is from the freezing point to the boiling point of the aqueous solution ASi or ASo, 6 When the aqueous solution for Li recovery ASo is pure water at the start of electrodialysis, the temperature is 0 to 100° C. For this reason, the temperature of the electrolyte membrane 2 is preferably 20° C. or lower, more preferably 15° C. or lower, even more preferably 10° C. or lower, and even more preferably 5° C. or lower.

[0048] 6 After Li enrichment is complete 6 For example, after evaporating the water as needed to concentrate the Li from the aqueous solution ASo for Li recovery, lithium carbonate (Li2CO3) is generated and precipitated by bubbling carbon dioxide (CO2) or the like. 6 Alternatively, after producing lithium carbonate, lithium hydroxide (LiOH) can be produced and precipitated by cooling or evaporating the water to create a supersaturated state, and Li can be recovered. 6 Li can also be recovered.

[0049] (Variation) In the lithium isotope enrichment device according to the above embodiment, when the application of voltage between both sides of the electrolyte membrane is stopped, a potential gradient is formed in the Li-containing aqueous solution in the supply tank, with a high potential near the surface of the electrolyte membrane, thereby efficiently desorbing lithium ions adsorbed to the surface of the electrolyte membrane. However, when the application of voltage between both sides of the electrolyte membrane is resumed, the lithium ion concentration in the Li-containing aqueous solution is relatively low near the surface of the electrolyte membrane. This means that it takes time for lithium ions to adsorb to the surface of the electrolyte membrane, and the mobility of lithium ions in the electrolyte membrane is not high immediately after the application of voltage is resumed, resulting in insufficient energy efficiency. Therefore, the following configuration is adopted so that after the lithium ions are temporarily desorbed from the surface of the electrolyte membrane, the low concentration of lithium ions near the surface of the electrolyte membrane is resolved before the application of voltage between both sides of the electrolyte membrane is resumed. A lithium isotope enrichment device and a lithium isotope enrichment method according to a modified example of the first embodiment of the present invention will now be described with reference to FIGS. 8 and 9.

[0050] As shown in Fig. 8, a lithium isotope enrichment apparatus 10B according to a modification of the first embodiment of the present invention includes a treatment tank 1, an electrolyte membrane (lithium ion conductive electrolyte membrane) 2 that partitions the treatment tank 1 into two, a first electrode 31 and a second electrode 32 (electrodes with a porous structure) coated on each side of the electrolyte membrane 2, a third electrode (auxiliary electrode) 33, a power supply 5B, an agitator (circulation means) 6, and a cooling device 7. The power supply 5B includes a main power supply 51 and an auxiliary power supply 52B that applies voltage with reversed polarity, with the main power supply 51 connected to the first electrode 31 and the second electrode 32, and the auxiliary power supply 52B connected to the first electrode 31 and the third electrode 33. Therefore, the lithium isotope enrichment apparatus 10B according to this modification has a configuration in which the auxiliary power supply 52, which is a DC power supply, is replaced with the auxiliary power supply 52B in the lithium isotope enrichment apparatus 10 according to the embodiment shown in Fig. 1.

[0051] The power supply device 5B includes a main power supply 51, which is a DC power supply, and a secondary power supply 52B that can apply voltages with reversed polarity, and further includes a switching element 5s1 and its drive circuit, etc., which switch between the main power supply 51 and the secondary power supply 52B. As in the above embodiment, the main power supply 51 has its positive pole connected to the first electrode 31 and its negative pole connected to the second electrode 32, and intermittently applies a positive voltage V1 (voltage +V1) to the first electrode 31 relative to the second electrode 32. The secondary power supply 52B is connected to the first electrode 31 and the third electrode 33, and applies a DC voltage V2 with its polarity reversed alternately. 9, when the main power supply 51 is not applying the voltage +V1, the auxiliary power supply 52B first applies a negative voltage V2 (voltage -V2) to the third electrode 33 relative to the first electrode 31 during one period in which the voltage +V1 is stopped, and then reverses the polarity and applies a positive voltage V2 (voltage +V2) to the third electrode 33 relative to the first electrode 31. The timing of application of the voltages V1 and V2 will be described in detail later.

[0052] The power supply device 5B may also be configured to include, for example, a variable power supply 51A (see FIG. 3) and a switching element that switches the connection destinations of both electrodes. In the lithium isotope enrichment device 10B shown in FIG. 8, similar to the above embodiment, when the power supply device 5B applies a voltage +V1 between the first electrode 31 and the second electrode 32, the third electrode 33 is in an open state, and when the power supply device 5B applies a voltage −V2 or +V2 between the third electrode 33 and the first electrode 31, the second electrode 32 is in an open state. Alternatively, the power supply device 5B may be configured to connect the third electrode 33 to the same potential as the first electrode 31 when applying a voltage +V1 between the first electrode 31 and the second electrode 32. Furthermore, the second electrode 32 may be configured to connect to the same potential as the first electrode 31 when applying a voltage −V2 or +V2 between the third electrode 33 and the first electrode 31.

[0053] (Lithium isotope enrichment method) The lithium isotope enrichment method according to the modified embodiment of the present invention involves the following steps, repeated in this order: a first step of applying a positive voltage V1 to a first electrode 31 provided on the front surface of an electrolyte membrane 2 relative to a second electrode 32 provided on the back surface; a second step of applying a negative voltage V2 to a third electrode 33 relative to the first electrode 31; and a third step of applying a positive voltage V2 to the third electrode 33 relative to the first electrode 31. ED ) and the second step (reset period t RST ) is as explained in the above embodiment (see FIG. 2). In the lithium isotope enrichment method according to this modification, after the second step and before the next first step, a preparation period t PREP Establish a.

[0054] As described in the above embodiment, after the application of voltage +V1 (first step), the application of voltage −V2 (second step) causes the Li adsorbed on the surface of the electrolyte membrane 2 to be removed, as shown in FIG. 5D. + For more information, see Li + is released from the surface of the electrolyte membrane 2, which is positively charged, by electrostatic repulsion. +is unevenly distributed in the vicinity of the third electrode 33 due to electrostatic attraction, and as a result, Li is relatively concentrated in the vicinity of the surface of the electrolyte membrane 2. + The lithium isotope enrichment method according to this modification involves applying a voltage −V2 from the secondary power supply 52B in the second step, then reversing the polarity of the secondary power supply 52B to apply a positive voltage V2 (voltage +V2) to the third electrode 33 relative to the first electrode 31 (third step). The application of the voltage +V2 generates a potential gradient in the Li-containing aqueous solution ASi such that the vicinity of the surface of the electrolyte membrane 2 is negative and the vicinity of the third electrode 33 is positive. As a result, the Li that was unevenly distributed near the third electrode 33 in the second step is gradually reduced. + is quickly separated from the third electrode 33 by electrostatic repulsion, and Li + The electrostatic repulsion at the surface of the electrolyte membrane 2 is quickly released, and the relative Li + The low concentration state is resolved. Furthermore, the Li ions that were unevenly distributed near the third electrode 33 are + and Li suspended in the Li-containing aqueous solution ASi. + However, they are attracted to the negatively charged surface of the electrolyte membrane 2 by electrostatic attraction, and become highly concentrated in the vicinity, and some of them are adsorbed onto the surface. + is the Li concentration in the Li-containing aqueous solution ASi at this point. + It has the same isotopic ratio as

[0055] When the application of the voltage +V2 is stopped and the application of the voltage +V1 is started again by the main power supply 51, the Li adsorbed on the surface of the electrolyte membrane 2 is removed as shown in FIGS. 5B and 5C. + moves into the electrolyte membrane 2. In this way, by applying a voltage -V2 between the third electrode 33 and the first electrode 31, then reversing the polarity to apply a voltage +V2, and then applying a voltage +V1 between both surfaces of the electrolyte membrane 2 (between the first electrode 31 and the second electrode 32), immediately after the start of application of the voltage +V1, 6 A large amount of Li with a high Li isotope ratio can be recovered.

[0056] The magnitude of the voltage applied between the third electrode 33 and the first electrode 31 in the second step does not have to be the same as that in the third step. In the third step, the larger the voltage V2, the stronger the electric field applied to the Li-containing aqueous solution ASi between the surface of the electrolyte membrane 2 and the third electrode 33, and the greater the amount of Li + As the Li moves away from the third electrode 33 at high speed, it is attracted to the surface of the electrolyte membrane 2. + The low concentration state is resolved in a shorter time, and Li + On the other hand, as explained in the above embodiment, it is preferable to set the size to such an extent that the electrolysis reaction of H2O does not occur in the Li-containing aqueous solution ASi.

[0057] Voltage +V2 is continuously applied for one time (preparation period) t PREP is not particularly specified, and the electrodialysis period t ED and reset period t RST Similarly, 6 It is preferable that the preparation period t be set so that the Li recovery efficiency is sufficiently high. PREP is the Li charge near the surface of the electrolyte membrane 2 generated by the application of the immediately preceding voltage -V2. + It is enough to eliminate the low concentration state, and Li + It is preferable that the concentration of Li becomes high near the surface of the electrolyte membrane 2 and that a large amount of Li is adsorbed on the surface. + Even if the electrode is attracted, the Li + There is a limit to how much the mobility of PREP The longer the electrodialysis period t ED Period t CYC The ratio to the production time decreases, and productivity decreases. PREP is the reset period t RST Similarly, the higher the voltage V2 and the shorter the distance between the third electrode 33 and the first electrode 31, the shorter the effect can be obtained.

[0058] Reset period t RST and preparation period t PREPIf there is a long non-application period during which neither voltage -V2 nor +V2 is applied between the third electrode 33 and the first electrode 31, productivity will decrease. Therefore, it is preferable to start applying the voltage +V2 as soon as possible after the application of the voltage -V2 is stopped, and it is even more preferable to start applying the voltage +V2 at the same time as the application of the voltage -V2 is stopped, as shown in FIG. 9. In addition, the application of the voltage +V2 (preparation period t PREP ) After stopping, the application of voltage +V1 (electrodialysis period t ED ) until the start of the no-voltage period t int3 If the time is long, productivity will decrease, and the effect of applying voltage +V2 will also decrease. Therefore, it is preferable to start applying voltage +V1 as soon as possible after stopping the application of voltage +V2. It is also preferable to start applying voltage +V1 at the same time as stopping the application of voltage +V2 (t int3 = 0) is more preferable. ED ) the voltage +V2 may be applied. Therefore, after the application of the voltage +V1 is started, the application of the voltage +V2 may be stopped. In this case, PREP is set assuming that it is from the start of application of voltage +V2 to the start of application of voltage +V1.

[0059] In the lithium isotope enrichment method according to this embodiment and its modified example, as described above, the lower the temperature of the electrolyte membrane 2, 6 Although it is possible to recover Li with a high isotope ratio, the Li-containing aqueous solution ASi and 6 The aqueous solution ASo for Li recovery must be cooled above its freezing point to prevent it from freezing. 6 The aqueous solution ASo for recovering Li may contain a solute that does not permeate the electrolyte membrane 2 so that the freezing point drops below 0°C. Such a solute is one that the aqueous solution ASi, ASo containing it does not corrode the electrolyte membrane 2 or the electrodes 31, 32. Specific examples include salts such as sodium chloride (NaCl, table salt), magnesium chloride (MgCl2), calcium chloride (CaCl2), potassium chloride (KCl), and organic solvents such as ethylene glycol. As described above, after the completion of electrodialysis, 6 From aqueous solution ASo for Li recovery6 When carbon dioxide gas is bubbled to recover Li, sodium chloride is particularly preferred because it does not produce precipitates (carbonates) other than lithium carbonate and has a large freezing point depression. 6 When the water content of the aqueous solution for Li recovery ASo is evaporated before bubbling carbon dioxide gas to concentrate Li, it is preferable to remove the salt precipitated by the decrease in water content by a general method such as filtration before bubbling. 6 Before bubbling carbon dioxide gas into the aqueous solution ASo for recovering Li, conventional electrodialysis or the like (see, for example, Patent Document 3) may be performed at a temperature of 0°C or higher, for example, room temperature or higher, to selectively recover Li into pure water or the like. According to such a method, the electrolyte membrane 2 is cooled to 0°C or lower, more preferably to below 0°C, 6 The Li isotope ratio can be further increased and efficiently enriched.

[0060] [Multi-stage lithium isotope enrichment device] (First embodiment) The lithium isotope enrichment apparatus 10, 10B (see FIGS. 1 and 8) according to the above-described embodiment and its modified example, 6 Aqueous solution containing high Li isotope ratio ( 6 The aqueous solution for Li recovery ASO is obtained in the recovery tank 12. 6 The aqueous solution for Li recovery ASo is introduced into the emptied supply tank 11 and operated. 6 Therefore, by forming a cascade structure in which the recovery tank 12 of the lithium isotope enrichment device 10 is integrally connected to the supply tank 11 of another lithium isotope enrichment device 10, an aqueous solution containing Li with a high Li isotope ratio can be obtained. 6 It is possible to enrich Li. Hereinafter, a multistage lithium isotope enrichment device according to a first embodiment of the present invention will be described with reference to FIG.

[0061] A multistage lithium isotope enrichment apparatus 20 according to a first embodiment of the present invention includes a treatment tank 1A, four electrolyte membranes (lithium ion conductive electrolyte membranes) 22, 23, 24, and 25 arranged in parallel at intervals so as to partition the treatment tank 1A in one direction into five tanks 11, 12, 13, 14, and 15, a first electrode 31 and a second electrode 32 (electrodes with a porous structure) coated on both sides of the electrolyte membranes 22, 23, 24, and 25, a third electrode (auxiliary electrode) 33 arranged opposite the first electrode 31 in the tanks 11, 12, 13, and 14, four power supply units 5, and an agitator (circulation means) 6 arranged in each of the tanks 11, 12, 13, and 14. The multistage lithium isotope enrichment apparatus 20 has a structure in which four lithium isotope enrichment apparatuses 10 are connected so that each treatment tank 1 is integrated into a treatment tank 1A, and the recovery tank 12 of one of two adjacent lithium isotope enrichment apparatuses 10, 10 is also used as the supply tank 11 of the other. The electrolyte membranes 22, 23, 24, 25 can each have the same configuration as the electrolyte membrane 2 of the lithium isotope enrichment apparatus 10, and will be referred to as the electrolyte membrane 2 as appropriate when not particularly distinguished from one another.

[0062] The tank 11 at the end of the supply tank 11 side of the lithium isotope enrichment apparatus 10 (the left side in FIG. 10 , hereinafter referred to as the supply side) is the supply tank 11, similar to the lithium isotope enrichment apparatus 10, and contains the Li-containing aqueous solution ASi. 6 Contains aqueous solutions AS1, AS2, AS3, and ASo for Li recovery. 6 The aqueous solutions AS1, AS2, AS3, and ASo for Li recovery are used in the lithium isotope enrichment device 10. 6 Lithium ions Li recovered from the Li-containing aqueous solution ASi in the same way as the Li recovery aqueous solution ASo + At the start of operation of the multistage lithium isotope enrichment apparatus 20, the aqueous solution is, for example, pure water. The side of the lithium isotope enrichment apparatus 10 on which the recovery tank 12 is located (the right side in FIG. 10) is referred to as the recovery side. In the multistage lithium isotope enrichment apparatus 20, the tank 15 at the end of the recovery side is the recovery tank.

[0063] In the multistage lithium isotope enrichment apparatus 20, a second electrode 32, a third electrode 33, and a first electrode 31 are arranged in each of the tanks 12, 13, and 14, excluding the tanks 11 and 15 at both ends, in that order from the supply side. The second electrode 32 and the first electrode 31 in each tank are arranged with sufficient spacing between them, i.e., the four electrolyte membranes 2 are arranged with sufficient spacing between them. Furthermore, the second electrode 32 and the third electrode 33 in each tank are preferably arranged with a sufficient distance between them, and further preferably with sufficient spacing between them. Therefore, the tanks 12, 13, and 14 are designed to have a sufficient length in the partition direction of the treatment tank 1A (the connecting direction of the lithium isotope enrichment apparatus 10, the left-right direction in FIG. 10 ). Meanwhile, the third electrode 33 and the first electrode 31 in each tank are preferably arranged close enough to avoid short-circuiting, as described in the configuration of the lithium isotope enrichment apparatus 10.

[0064] The power supply device 5 is as described in the configuration of the lithium isotope enrichment device 10, and includes a main power supply 51 and a secondary power supply 52, and is configured so that these power supplies do not operate simultaneously. Furthermore, in the multistage lithium isotope enrichment device 20, it is preferable that the four power supply devices 5 are configured to operate independently, in other words, so that the main power supply 51 can be connected to the electrodes 31 and 32, and the secondary power supply 52 can be connected to the electrodes 31 and 33, independently for each power supply device 5. To distinguish between the four power supply devices 5 in the multistage lithium isotope enrichment device 20, as shown in FIG. 10 , they are referred to as power supply devices 5(1), 5(2), 5(3), and 5(4) in order from the supply tank 11 side. In this embodiment, the power supply device 5 includes two interlocking switching elements 5s1 and 5s3. As described in the configuration of the lithium isotope enrichment device 10, the switching element 5s1 switches the connection destination of the first electrode 31. The switching element 5s3 connects / disconnects the third electrode 33 and the auxiliary power supply 52, i.e., switches the auxiliary power supply 52 ON / OFF. In the multistage lithium isotope enrichment apparatus 20, the power supplies 51 and 52 are preferably floating power supplies, or only one of the power supplies 51 and 52 may be grounded. For example, the power supply device 5(1) may be configured so that the positive electrode of the main power supply 51 and the negative electrode of the auxiliary power supply 52 are grounded when each is driven so that the Li-containing aqueous solution ASi in the supply tank 11 is grounded to a reference potential. However, if the resistance between the second electrode 32 and the first electrode 31 in each of the tanks 12, 13, and 14 is sufficiently high, two or more power supplies 51 and 52 may each be grounded.

[0065] The agitator 6 circulates the aqueous solutions ASi, AS1, AS2, and AS3 in the tanks 11, 12, 13, and 14, respectively. The multistage lithium isotope enrichment apparatus 20 further includes an agitator 6 in the recovery tank 15, 6The aqueous solution ASo for Li recovery may be circulated. The multistage lithium isotope enrichment apparatus 20 may further include a cooling device 7 (see FIG. 1) for cooling the electrolyte membranes 22, 23, 24, and 25, as necessary (not shown). When the agitator 6 and the immersion-type cooling device 7 are installed in the tanks 12, 13, and 14, they are preferably disposed between the second electrode 32 (electrolyte membrane 2) and the third electrode 33. The other elements are as described in the configuration of the lithium isotope enrichment apparatuses 10 and 10B.

[0066] (Method for enriching lithium isotopes using a multistage lithium isotope enrichment device) The method of enriching lithium isotopes using the multistage lithium isotope enrichment apparatus 20 according to this embodiment is the same as the method using the lithium isotope enrichment apparatus 10. 7 Li, 6 A Li-containing aqueous solution ASi containing Li at a natural abundance ratio is poured into the tank, and pure water is poured into the other tanks 12, 13, 14, and 15. As described above, the Li in the electrolyte membrane 2 + The mobility is calculated by the ratio of the Li + Therefore, immediately after the start of operation, the amount of Li in the aqueous solution AS1, which is pure water in the tank 12, is increased by 100%. + That is, Li only moves through the electrolyte membrane 22. + Li in aqueous solution AS1 + Increasing the concentration is preferable in terms of energy efficiency. For this purpose, only the power supply device 5(1) is driven. Then, the aqueous solution AS1 is charged to a predetermined amount of Li. + When the concentration reaches a desired value, preferably a saturated concentration, the power supply unit 5(2) is driven to transfer Li from the aqueous solution AS1 to the aqueous solution AS2 in the tank 13. + Start moving.

[0067] Li from the Li-containing aqueous solution ASi to the aqueous solution AS1, and from the aqueous solution AS1 to the aqueous solution AS2 + The migration proceeds in parallel, and the aqueous solution AS2 + When the concentration reaches the predetermined value, the power supply unit 5 (3) is driven to supply Li from the aqueous solution AS2 to the aqueous solution AS3 in the tank 14. +Start the movement. By repeating this and finally driving all the power supply devices 5 of the multi-stage lithium isotope concentrator 20, in the figure, Li moves from left to right + and the aqueous solutions AS1, AS2, AS3, and ASo in the respective tanks 12, 13, 14, and 15 change from pure water at the start of operation to 6 aqueous LiOH solutions containing Li with different Li isotope ratios at different concentrations. 6 The Li isotope ratio increases in the order of ASi < AS1 < AS2 < AS3 < ASo. Therefore, even if the isotope separation coefficient due to the movement of Li in one electrolyte membrane 2 is not large, + Li with a high Li isotope ratio can be recovered from the recovery tank 15, so it is not necessary to extremely reduce the Li 6 mobility due to low temperature or low voltage, and the productivity can be increased. + The multi-stage lithium isotope concentrator 20 is designed such that, for example, when driving the main power supply 51 of the power supply device 5(2) and applying a voltage +V1 between the first electrode 31 and the second electrode 32 coated on both sides of the electrolyte membrane 23, even if driving the main power supply 51 or the auxiliary power supply 52 of other power supply devices 5, no substantial electric field is generated between the first electrode 31 and the third electrode 33 or between the second electrode 32 and the third electrode 33 in the tank 12, and also no substantial electric field is generated between the second electrode 32 and the third electrode 33 or between the first electrode 31 and the third electrode 33 in the tank 13. Or even if an electric field is generated, it is designed to be sufficiently weak so as not to inhibit the reactions of formulas (1) and (3) in the vicinity of each of the electrodes 31 and 32 on both sides of the electrolyte membrane 23. For this purpose, as described above, the interval between the electrolyte membranes 2 and the interval between the second electrode 32 and the third electrode 33 are arranged to be sufficiently wide. With such a configuration, two or more adjacent power supply devices 5 can be driven simultaneously. For example, the power supplies 51 and 52 of four power supply devices 5 can be synchronized and the main power supply 51 can be driven simultaneously as shown in FIG. 10, or the main power supply 51 and the auxiliary power supply 52 of different power supply devices 5 can be driven simultaneously.

[0069] The number of electrolyte membranes 2 in the multi-stage lithium isotope concentrator 20 is not particularly specified, and the more there are, that is, the more lithium isotope concentrators 10 are connected,​​​​6 10, the lithium isotope enrichment device 10 is connected in one direction, and all adjacent electrolyte membranes 2, 2 are arranged facing each other. However, for example, the lithium isotope enrichment device 10 may be connected by bending at 90° at one or more locations, and adjacent electrolyte membranes 2, 2 may be arranged perpendicular to each other. In the bent portion of the tank separated by the perpendicularly arranged electrolyte membranes 2, 2, a third electrode 33 is arranged parallel to and adjacent to the first electrode 31 coated on the surface of the electrolyte membrane 2 on the recovery side.

[0070] (Variation) The multistage lithium isotope enrichment apparatus 20 may have a configuration in which a plurality of lithium isotope enrichment apparatuses 10B are connected together, that is, the power supply apparatus 5 is replaced with a power supply apparatus 5B.

[0071] The multistage lithium isotope enrichment device 20 according to the embodiment simultaneously enriches Li in adjacent electrolyte membranes 2. + To enable this movement, the electrolyte membranes 2 are arranged with a wide gap between them, and therefore a treatment tank 1A that is long in the direction in which the lithium isotope enrichment device 10 is connected is required. Here, the lithium isotope enrichment device 10 intermittently applies a voltage between both surfaces of the electrolyte membrane 2. Therefore, the multistage lithium isotope enrichment device 20 staggers the timing of voltage application to adjacent electrolyte membranes 2, thereby making it possible to reduce the influence of reactions near the electrodes 31 and 32 when a voltage is applied between the two surfaces of the electrolyte membrane 2. Hereinafter, a multistage lithium isotope enrichment device according to a modification of the first embodiment of the present invention will be described with reference to FIGS. 11A and 11B.

[0072] A multistage lithium isotope enrichment apparatus 20A according to a modification of the first embodiment of the present invention has a structure in which four lithium isotope enrichment apparatuses 10 are connected so that each treatment vessel 1 is integrated into a treatment vessel 1A, similar to the multistage lithium isotope enrichment apparatus 20 according to the previous embodiment, except that it includes a power supply unit 50 that includes four power supply units 5 (see FIG. 10). Furthermore, similar to the multistage lithium isotope enrichment apparatus 20, the multistage lithium isotope enrichment apparatus 20A is also provided with an agitator 6 and a cooling unit 7 (not shown) as necessary. In this modification, it is preferable that the cooling unit 7 is configured so that a refrigerant circulates through the treatment vessel 1A as a jacket tank.

[0073] The power supply device 50 includes a main power supply 51 connected between the first electrode 31 and the second electrode 32 on both sides of each electrolyte membrane 2, and a secondary power supply 52 connected between the third electrode 33 and the first electrode 31, which are disposed opposite each other within the same tank (11, 12, 13, 14). The main power supply 51 and the secondary power supply 52 connected to the same first electrode 31 alternately apply voltage. That is, the power supply device 50 includes four power supplies 5 (see FIG. 10). To distinguish between these main power supplies 51, as shown in FIGS. 11A and 11B, they are referred to as main power supplies 51(1), 51(2), 51(3), and 51(4) in order from the supply tank 11 side. Similarly, they are referred to as secondary power supplies 52(1), 52(2), 52(3), and 52(4). Furthermore, the power supply device 50 is configured so that the main power supplies 51 connected to the electrodes 31 and 32 on both sides of adjacent electrolyte membranes 2 are not simultaneously driven (i.e., are not connected to the electrodes 31 and 32). For this purpose, the power supply device 50 is configured to supply Li to two or more adjacent power supply devices 5 of the connected lithium isotope enrichment device 10 in a group, with one device in each group alternately supplying Li + In this case, it is preferable to synchronize one unit of each set. In this example, two units are used as a set, so that the main power supply 51(1) and the main power supply 51(3), and the main power supply 51(2) and the main power supply 51(4) are synchronized. For this purpose, all the power supply devices 5 are synchronized with each other in a period t CYC is the same and the electrodialysis period t ED is the period t CYC Less than half of (t ED <t CYC / 2), and the reset period tRST is the period t CYC Less than 1 / 2 of (t RST ≦t CYC / 2). As in the multistage lithium isotope enrichment apparatus 20, the power supplies 51 and 52 are preferably floating power supplies, or in the multistage lithium isotope enrichment apparatus 20A, only one of the power supplies 51 and 52 may be grounded.

[0074] (Method for enriching lithium isotopes using a multistage lithium isotope enrichment device) A lithium isotope enrichment method using a multistage lithium isotope enrichment apparatus 20A according to this modification will be described with reference to Figures 11A and 11B. This modification is similar to the above embodiment except for the application timing of voltages +V1 and -V2 by power supply device 50.

[0075] As shown in FIG. 11A, when the main power supplies 51(1) and 51(3) are connected, the power supply device 50 disconnects the main power supplies 51(2) and 51(4). As a result, a voltage +V1 is applied between both surfaces of each of the electrolyte membranes 22 and 24. Then, Li + The Li-containing aqueous solution ASi in the supply tank 11 passes through the electrolyte membrane 22 to the aqueous solution AS1 in the tank 12, and passes through the electrolyte membrane 24 from the aqueous solution AS2 in the tank 13 to the aqueous solution AS3 in the tank 14. On the other hand, the Li-containing aqueous solution ASi passes through the electrolyte membrane 23 (between the aqueous solution AS1 and the aqueous solution AS2) and the electrolyte membrane 25 (between the aqueous solution AS3 and the aqueous solution ASo). + At this time, the auxiliary power supplies 52(2), 52(4) can be connected to apply a voltage -V2 between the third electrode 33 and the first electrode 31 in the tanks 12, 14. This causes the Li + An electric field E2 is generated from the surface (first electrode 31) of the electrolyte membranes 23 and 25 where the Li is not moving toward the supply side, and the Li adsorbed on the surface of the electrolyte membranes 23 and 25 + is released (see Figure 5D).

[0076] Next, as shown in FIG. 11B, the power supply device 50 disconnects the main power supplies 51(1) and 51(3) and the auxiliary power supplies 52(2) and 52(4), and connects the main power supplies 51(2) and 51(4). This applies a voltage +V1 between both surfaces of each of the electrolyte membranes 23 and 25. Then, Li + moves from the aqueous solution AS1 in the tank 12 through the electrolyte membrane 23 to the aqueous solution AS2 in the tank 13, and from the aqueous solution AS3 in the tank 14 through the electrolyte membrane 25 to the aqueous solution ASo in the tank 15. On the other hand, in the electrolyte membrane 22 (between the aqueous solution ASi and the aqueous solution AS1) and the electrolyte membrane 24 (between the aqueous solution AS2 and the aqueous solution AS3), Li + At this time, the auxiliary power supplies 52(1), 52(3) can be connected to apply a voltage -V2 between the third electrode 33 and the first electrode 31 in the tanks 11, 13. This causes the Li + An electric field E2 is generated from the surface (first electrode 31) of the electrolyte membranes 22 and 24 where the Li is not moving toward the supply side, and the Li adsorbed on the surface of the electrolyte membranes 22 and 24 + is released (see Figure 5D).

[0077] 11A, the power supply device 50 disconnects the main power supplies 51(2) and 51(4) and the auxiliary power supplies 52(1) and 52(3), connects the main power supplies 51(1) and 51(3), and also connects the auxiliary power supplies 52(2) and 52(4). In this way, by alternately connecting the main power supplies 51(1) and 51(3) and the main power supplies 51(2) and 51(4), the Li + The secondary power supplies 52(2), 52(4) and the secondary power supplies 52(1), 52(3) are also alternately connected to move the electrolyte membranes 22, 24 and the electrolyte membranes 23, 25 intermittently and alternately. + Li + As in the previous embodiment, immediately after starting operation, only the power supply device 5(1) is driven to supply Li only to the electrolyte membrane 22. + It is preferable to move the power supply units 5(2), 5(3), and 5(4) one by one in stages and then drive them.

[0078] In this modified example, Li + A secondary power supply 52 is connected between the third electrode 33 and the first electrode 31 in a vessel on the recovery side of the electrolyte membrane 2 through which the electrolyte membrane 2 is moving, and a voltage -V2 is applied. Accordingly, the second electrode 32 and the third electrode 33 are designed to be spaced apart so that an electric field is not substantially generated between the third electrode 33 and the opposing second electrode 32, or, if an electric field is generated, the electric field is weak enough not to inhibit the reaction of formula (3) near the second electrode 32 due to the application of a voltage +V1. Specifically, it is preferable to arrange the third electrode 33 at a sufficiently short distance from the first electrode 31 in the same vessel so that an electric field E2 of the required strength can be obtained with a small voltage V2. This prevents the reaction near the second electrode 32 from being inhibited by the application of the voltage -V2, even if the distance between the second electrode 32 and the third electrode 33 is not large. With this configuration, the multistage lithium isotope enrichment apparatus 20A according to this modified example can be made smaller by shortening the gap between the electrolyte membranes 2 and shrinking the lithium isotope enrichment apparatus 10 in the connecting direction.

[0079] The multistage lithium isotope enrichment apparatus 20A can also be configured by connecting a plurality of lithium isotope enrichment apparatuses 10B, similar to the multistage lithium isotope enrichment apparatus 20. For this purpose, the power supply device 50 is provided with a secondary power supply 52B (see FIG. 8) instead of the secondary power supply 52. ​​In this case, for example, when the main power supplies 51(1) and 51(3) are connected, the secondary power supplies 52(2) and 52(4) are connected with their polarities reversed midway. Therefore, during the reset period t RST and preparation period t PREP The sum of these is the period t CYC Less than 1 / 2 of (t RST +t PREP ≦t CYC / 2).

[0080] (Second embodiment) In a multistage lithium isotope enrichment apparatus in which the lithium isotope enrichment apparatuses 10, 10B (see FIGS. 1 and 8) according to the above-described embodiments and their modifications are connected so that each treatment tank 1 is integrated with the treatment tank 1A as in the first embodiment and its modifications, the recovery tank 12 of one of two adjacent lithium isotope enrichment apparatuses 10, 10 is also used as the supply tank 11 of the other, as described above. Furthermore, the third electrode 33 and the second electrode 32 provided in each of the supply tank 11 and recovery tank 12 thus integrated can be integrated. Below, a multistage lithium isotope enrichment apparatus according to a second embodiment of the present invention will be described with reference to FIG. 12.

[0081] A multistage lithium isotope enrichment apparatus 20B according to a second embodiment of the present invention includes a treatment tank 1A, six electrolyte membranes (lithium ion conductive electrolyte membranes) 22, 23, 24, 25, 26, and 27 arranged in parallel at intervals so as to partition the treatment tank 1A in one direction into seven tanks 11, 12, 13, 14, 15, 16, and 17, a first electrode 31 and a second electrode 32 (electrodes with a porous structure) coated on both sides of the electrolyte membranes 22, 23, 24, 25, 26, and 27, a third electrode (auxiliary electrode) 33 arranged facing the first electrode 31 in the tank 11, and a power supply unit 50C. Similar to the multistage lithium isotope enrichment apparatus 20, the multistage lithium isotope enrichment apparatus 20B may also include a stirrer (circulation means) 6 and a cooling device 7 (not shown) as necessary. The multistage lithium isotope enrichment apparatus 20B has a structure in which six lithium isotope enrichment apparatuses 10B are connected so that each treatment tank 1 is integrated with the treatment tank 1A, and the recovery tank 12 of one of two adjacent lithium isotope enrichment apparatuses 10B, 10B doubles as the supply tank 11 of the other. The second electrode 32 in one recovery tank 12 doubles as the third electrode 33 in the other supply tank 11. Therefore, in the multistage lithium isotope enrichment apparatus 20B, the third electrode 33, spaced apart from the electrolyte membrane 22, is provided only in the supply tank 11 at the supply side. The electrolyte membranes 22, 23, 24, 25, 26, and 27 can each have the same configuration as the electrolyte membrane 2 of the lithium isotope enrichment apparatuses 10, 10B, and will be referred to as the electrolyte membrane 2 as appropriate when not particularly distinguished from one another. The cooling device 7 is preferably configured so that the treatment tank 1A serves as a jacket tank through which a refrigerant flows.

[0082] As described above, in the multistage lithium isotope enrichment apparatus 20B, the second electrode 32 disposed in each of the vessels 12, 13, 14, 15, and 16 also serves as the third electrode 33 of the same vessel. Therefore, it is preferable that the distance between the electrolyte membranes 2 is short so that the second electrode 32 is disposed as the third electrode 33 close enough to the opposing first electrode 31 so as not to short-circuit. In other words, it is preferable that the vessels 12, 13, 14, 15, and 16, excluding the vessels 11 and 17 at both ends, are short in the partition direction of the treatment vessel 1A (the connecting direction of the lithium isotope enrichment apparatus 10B). With this configuration, the multistage lithium isotope enrichment apparatus 20B can reduce the number of parts and can be made smaller by shrinking in the connecting direction.

[0083] The power supply device 50C includes six power supply devices 5C1, 5C2, 5C3, 5C4, 5C5, and 5C6, arranged in order from the supply side, each of which has a main power supply 51 and a secondary power supply 52. ​​The power supply devices 5C1, 5C2, 5C3, 5C4, 5C5, and 5C6 each correspond to the power supply device 5B of the lithium isotope enrichment device 10B, and will be referred to as power supply device 5C as appropriate when not specifically distinguished from one another. In the power supply device 5C, like the power supply device 5B, the main power supply 51 and the secondary power supply 52 alternately apply DC voltage, and the secondary power supply 52 reverses the polarity. Specifically, the power supply device 5C alternately repeats the following steps in this order: first step: connecting the positive and negative electrodes of the main power supply 51 to the first electrode 31 and the second electrode 32, respectively, and applying a voltage +V1; second step: turning off the main power supply 51 and connecting the positive and negative electrodes of the auxiliary power supply 52 to the first electrode 31 and the third electrode 33 or the adjacent second electrode 32 on the supply side, respectively, and applying a voltage -V2; third step: swapping the positive and negative electrodes of the auxiliary power supply 52 and connecting them, and applying a voltage +V2. To achieve this, the power supply device 5C further includes switching elements 5s1, 5s2, and 5s3 that switch the connections of the electrodes 31, 32, and 33. However, because the second electrode 32 of one of the adjacent lithium isotope enrichment devices 10B also serves as the third electrode 33 of the other, the switching element 5s3 is also shared by the switching element 5s2 of the adjacent power supply device 5C on the supply side, except for the power supply device 5C1. In the multistage lithium isotope enrichment apparatus 20B, the power supplies 51 and 52 are preferably floating power supplies, or only one of the power supplies 51 and 52 may be grounded.

[0084] In the multistage lithium isotope enrichment device 20B, Li + It is preferable that the voltage -V2 is not applied between the third electrode 33 or the second electrode 32 and the first electrode 31 in the recovery tank, as well as in the supply tank of the electrolyte membrane 2 where Li+ is moving. If a potential gradient occurs in the aqueous solution in the recovery tank, with the vicinity of the surface of the electrolyte membrane 2 being positive, the movement of Li+ is inhibited, and energy efficiency decreases. For this reason, the power supply device 50C is configured so that three or more adjacent power supply devices 5C of the connected lithium isotope enrichment device 10B are grouped together, and each device in each group is alternately supplied with Li+. + Here, the switching elements 5s1, 5s2, and 5s3 are three-position switching elements that simultaneously perform connection / disconnection so that three units are grouped together.

[0085] The switching element 5s1 is a triple-throw switch that switches the connection destination of the first electrode 31 between three options: the positive electrode of the main power supply 51, the positive electrode of the auxiliary power supply 52, and the negative electrode of the auxiliary power supply 52. ​​The switching element 5s2 connects / disconnects the second electrode 32 to the negative electrode of the main power supply 51. The switching element 5s3 is a double-throw switch that switches the connection destination of the third electrode 33 between three options: the positive electrode of the auxiliary power supply 52, the negative electrode of the auxiliary power supply 52, and no connection. The switching element 5s2, which also serves as the switching element 5s3, is a double-throw switch that switches the connection destination of the second electrode 32 between three options: the negative electrode of the main power supply 51 and the positive electrode of the auxiliary power supply 52 of the adjacent power supply device 5C on the recovery side, the negative electrode of this auxiliary power supply 52, and no connection.

[0086] It is also preferable to synchronize one unit in each set. Here, three units are used in a set, so power supply unit 5C1 and power supply unit 5C4, power supply unit 5C2 and power supply unit 5C5, and power supply unit 5C3 and power supply unit 5C6 are synchronized. For this purpose, all power supply units 5C are synchronized with a period t CYC is the same and the electrodialysis period t ED , reset period t RST , and the preparation period t PREP are period t CYC Less than 1 / 3 of (t ED <tCYC / 3, t RST <t CYC / 3, t PREP <t CYC The power supplies 5C1 and 5C4, the power supplies 5C2 and 5C5, and the power supplies 5C3 and 5C6 are set to the electrodialysis period t ED are driven so as not to overlap.

[0087] (Method for enriching lithium isotopes using a multistage lithium isotope enrichment device) A lithium isotope enrichment method using multistage lithium isotope enrichment apparatus 20B according to this embodiment will be described with reference to Figures 13A, 13B, and 13C. Note that in Figures 13A, 13B, and 13C, the main power supplies 51 and sub-power supplies 52 of power supply devices 5C1, 5C2, 5C3, 5C4, 5C5, and 5C6 are indicated as main power supplies 51(1), 51(2), 51(3), 51(4), 51(5), and 51(6), and sub-power supplies 52(1), 52(2), 52(3), 52(4), 52(5), and 52(6).

[0088] As shown in FIG. 13A, when the power supply devices 5C1 and 5C4 are connected to the main power supplies 51(1) and 51(4), the other power supply devices 5C2, 5C3, 5C5 and 5C6 have the main power supply 51 disconnected. As a result, a voltage +V1 is applied between both surfaces of each of the electrolyte membranes 22 and 25. Then, O2 is generated near the first electrode 31 covering the front surface of the electrolyte membranes 22 and 25, and H2 is generated near the second electrode 32 covering the rear surface of the electrolyte membranes 22 and 25 (omitted in FIG. 13A, see FIG. 4). Then, Li + The Li-containing aqueous solution ASi in the supply tank 11 passes through the electrolyte membrane 22 to the aqueous solution AS1 in the tank 12, and the Li-containing aqueous solution AS3 in the tank 14 passes through the electrolyte membrane 25 to the aqueous solution AS4 in the tank 15. + At this time, the power supply devices 5C2 and 5C5 connect the sub-power supplies 52(2) and 52(5), and Li +A voltage +V2 can be applied between the second electrode 32 and the first electrode 31 in the recovery side tanks 12 and 15 of the electrolyte membranes 22 and 25 through which Li is moving. As a result, an electric field E2 (electric field +E2) is generated in the aqueous solutions AS1 and AS4 toward the recovery side, as shown by the arrows with dotted patterns in the figure. + moves along the electric field, so Li in aqueous solutions AS1 and AS4 + is attracted to the vicinity of the first electrode 31 on the recovery side (near the surfaces of the electrolyte membranes 23 and 26). + is quickly separated from the rear surface (second electrode 32) of the electrolyte membranes 22 and 25 and moves to the recovery side in the aqueous solutions AS1 and AS4. + The concentration of Li in the electrolyte membrane 22, 25 decreases, and the concentration gradient with the surface increases. + The mobility increases.

[0089] Furthermore, at this time, the power supply devices 5C3 and 5C6 connect the auxiliary power supplies 52(3) and 52(6) to apply a voltage -V2 between the second electrode 32 and the first electrode 31 in the tanks 13 and 16. As a result, as shown by the arrows with dotted patterns in the figure, the voltage of Li + An electric field E2 (electric field −E2) is generated from the surface (first electrode 31) of the electrolyte membranes 24 and 27 to which the Li has not moved toward the supply side, and the Li adsorbed on the surface of the electrolyte membranes 24 and 27 + is rapidly withdrawn (see Figure 5D).

[0090] Next, as shown in FIG. 13B, power supply devices 5C1 and 5C4 disconnect main power supplies 51(1) and 51(4) and connect auxiliary power supplies 52(1) and 52(4), and power supply devices 5C2 and 5C5 disconnect auxiliary power supplies 52(2) and 52(5) and connect main power supplies 51(2) and 51(5). Furthermore, power supply devices 5C3 and 5C6 reverse the polarity of auxiliary power supplies 52(3) and 52(6). This applies a voltage +V1 between both surfaces of each of electrolyte membranes 23 and 26. Then, O2 and H2 are generated near electrodes 31 and 32, respectively, which are coated on both surfaces of electrolyte membranes 23 and 26 (omitted in FIG. 13B, see FIG. 4). Then, Li +The aqueous solution AS1 in the tank 12 passes through the electrolyte membrane 23 to the aqueous solution AS2 in the tank 13, and the aqueous solution AS4 in the tank 15 passes through the electrolyte membrane 26 to the aqueous solution AS5 in the tank 16. On the other hand, in the other electrolyte membranes 22, 24, 25, and 27, Li + does not move. Also, at this time, Li + Since a voltage +V2 is applied between the second electrode 32 and the first electrode 31 in the tanks 13 and 16 on the recovery side of the electrolyte membranes 23 and 26 through which Li is moving, an electric field +E2 is generated in the aqueous solutions AS2 and AS5, as indicated by the arrows with dotted patterns in the figure. The electric field +E2 causes the Li concentration in the aqueous solutions AS2 and AS5, which had been unevenly distributed near the second electrode 32 on the supply side due to the voltage -V2 applied to the tanks 13 and 16 until just before (see FIG. 13A), to decrease. + moves to the recovery side and accumulates in the Li + Furthermore, since the voltage +V2 was applied to the tanks 12 and 15 on the supply side of the electrolyte membranes 23 and 26 until just before (see FIG. 13A), the concentration of Li near the surfaces of the electrolyte membranes 23 and 26 in the aqueous solutions AS1 and AS4 decreased. + As a result, the electrolyte membranes 23 and 26 have a high Li concentration. + The concentration gradient of Li becomes larger + The mobility increases.

[0091] Furthermore, at this time, Li + A voltage -V2 is applied between the third electrode 33 and the first electrode 31 in the tank 11 on the supply side of the electrolyte membranes 22 and 25 (see FIG. 13A) where Li was moving, and between the second electrode 32 and the first electrode 31 in the tank 14. As a result, an electric field -E2 is generated in the aqueous solutions ASi and AS3, as shown by the arrows with dotted patterns in the figure. Then, Li adsorbed on the surfaces of the electrolyte membranes 22 and 25 moves. + is rapidly withdrawn (see Figure 5D).

[0092] Next, as shown in FIG. 13C, power supply devices 5C2 and 5C5 disconnect main power supplies 51(2) and 51(5) and connect secondary power supplies 52(2) and 52(5), and power supply devices 5C3 and 5C6 disconnect secondary power supplies 52(3) and 52(6) and connect main power supplies 51(3) and 51(6). Furthermore, power supply devices 5C1 and 5C4 reverse the polarity of secondary power supplies 52(1) and 52(4). This applies a voltage +V1 across both surfaces of each of electrolyte membranes 24 and 27. O2 and H2 are then generated near electrodes 31 and 32, respectively, which are coated on both surfaces of electrolyte membranes 24 and 27 (omitted in FIG. 13C; see FIG. 4). Li + The aqueous solution AS2 in the tank 13 passes through the electrolyte membrane 24 to the aqueous solution AS3 in the tank 14, and the aqueous solution AS5 in the tank 16 passes through the electrolyte membrane 27 to the aqueous solution ASo in the tank 17. On the other hand, in the other electrolyte membranes 22, 23, 25, and 26, Li + At this time, the gap between the third electrode 33 and the first electrode 31 in the tank 11 and the gap between the Li + A voltage +V2 is applied between the second electrode 32 and the first electrode 31 in the tank 14 on the recovery side of the electrolyte membrane 24 through which Li is moving, and an electric field +E2 is generated in the aqueous solutions ASi and AS3, as indicated by the arrows with dotted patterns in the figure. The electric field +E2 causes the Li concentration in the aqueous solutions ASi and AS3 to decrease. This is because the voltage -V2 was applied to the tanks 11 and 14 until just before (see FIG. 13B). + In addition, since the voltage +V2 was applied to the tanks 13 and 16 on the supply side of the electrolyte membranes 24 and 27 until just before (see FIG. 13B), the Li + As a result, the electrolyte membranes 24 and 27 contain Li + The concentration gradient of Li becomes larger + The mobility increases.

[0093] Furthermore, at this time, Li +A voltage -V2 is applied between the second electrode 32 and the first electrode 31 in the recovery tanks 12 and 15 of the electrolyte membranes 23 and 26 (see FIG. 13B) where Li has been transferred. As a result, an electric field -E2 is generated in the aqueous solutions AS1 and AS4, as shown by the arrows with dotted patterns in the figure. Then, the Li adsorbed on the surfaces of the electrolyte membranes 23 and 26 moves. + is rapidly withdrawn (see Figure 5D).

[0094] 13A, power supplies 5C3 and 5C6 disconnect main power supplies 51(3) and 51(6) and connect secondary power supplies 52(3) and 52(6), and power supplies 5C1 and 5C4 disconnect secondary power supplies 52(1) and 52(4) and connect main power supplies 51(1) and 51(4). Furthermore, power supplies 5C2 and 5C5 reverse the polarity of secondary power supplies 52(2) and 52(5). In this manner, multistage lithium isotope enrichment apparatus 20C operates by alternately connecting and disconnecting power supplies 51 and 52 among power supplies 5C1 and 5C4, power supplies 5C2 and 5C5, and power supplies 5C3 and 5C6. This causes voltage +V1 to be applied intermittently across both surfaces of each electrolyte membrane 2. Then, in the tank on the supply side of this electrolyte membrane 2, after the application of voltage +V1 was stopped, voltage -V2 was applied, and Li that had been adsorbed on the surface of the electrolyte membrane 2 was removed. + While Li was released quickly, the voltage +V2 was applied before the next voltage +V1 was applied. + is attracted to the surface of the electrolyte membrane 2 and becomes highly concentrated. Therefore, the multistage lithium isotope enrichment apparatus 20C is a small apparatus that is reduced in size in the partition direction of the treatment vessel 1A, 6 Li with a high Li isotope ratio can be recovered from the recovery tank 17 in a time-efficient manner.

[0095] Similar to the multistage lithium isotope enrichment apparatus 20, the multistage lithium isotope enrichment apparatus 20B may be formed by connecting lithium isotope enrichment apparatuses 10B by bending them at one or more 90° positions, and arranging adjacent electrolyte membranes 2, 2 perpendicular to each other. In the bent portion of the tank separated by the perpendicularly arranged electrolyte membranes 2, 2, the second electrode 32 does not also serve as the third electrode 33; that is, the third electrode 33 is arranged in addition to the electrodes 31, 32, and is arranged parallel to and adjacent to the first electrode 31 coated on the surface of the electrolyte membrane 2 on the recovery side.

[0096] (Variation) The multistage lithium isotope enrichment apparatus 20B may be configured by connecting a plurality of lithium isotope enrichment apparatuses 10. In this case, instead of the auxiliary power supply 52 applying voltage +V2 (third step), a period is provided in which no voltage is applied (application is stopped) between the third electrode 33 or the second electrode 32 and the first electrode 31 in the same vessel.

[0097] Second Embodiment As explained in the first embodiment, during operation, the Li-containing aqueous solution ASi contains Li + Therefore, it is preferable to maintain the Li concentration of the Li-containing aqueous solution ASi at a high level, except for replacing the Li-containing aqueous solution ASi in the supply tank 11 or circulating it between the supply tank 11 and the treatment tank 1. + In order to maintain the concentration, the following configuration is adopted: A lithium isotope enrichment device according to a second embodiment of the present invention will be described below with reference to FIG.

[0098] (Lithium isotope enrichment device) A lithium isotope enrichment apparatus 10C according to a second embodiment of the present invention includes a treatment tank 1B, an electrolyte membrane (lithium ion conductive electrolyte membrane for lithium replenishment) 21, an electrolyte membrane (lithium ion conductive electrolyte membrane) 22, a first electrode (electrode with a porous structure) 31, a second electrode (electrode with a porous structure) 32, a third electrode (auxiliary electrode) 33, a fourth electrode (first electrode for lithium replenishment) 41, a fifth electrode (second electrode for lithium replenishment) 42, a power supply unit 5 incorporating a main power supply 51 and an auxiliary power supply 52, a power supply (lithium replenishment power supply) 53, an agitator (circulation means) 6, and a cooling device 7. The treatment tank 1B is divided into a replenishment tank (lithium replenishment tank) 1z containing a Li-containing aqueous solution AS', a supply tank (first tank) 11 containing a Li-containing aqueous solution ASi, and a supply tank (second tank) 12 containing a Li-containing aqueous solution ASi, by the electrolyte membranes 21 and 22. 6 and a recovery tank (second tank) 12 that stores the aqueous Li recovery solution ASo. The lithium isotope enrichment apparatus 10C is configured such that, compared to the lithium isotope enrichment apparatus 10 (see FIG. 1 ) according to the first embodiment, an electrolyte membrane 21 that separates the treatment tank 1B on the supply tank 11 side of the electrolyte membrane 22, a replenishing tank 1z that is separated from the supply tank 11 by the electrolyte membrane 21, electrodes 41 and 42 provided in each tank of the supply tank 11, and a power supply 53 connected between the electrodes 41 and 42 are added. The remaining configuration is the same as that of the lithium isotope enrichment apparatus 10 according to the first embodiment, and may include a liquid level sensor, exhaust means, etc., as necessary.

[0099] The lithium isotope enrichment apparatus 10C, which is composed of the refill tank 1z, the supply tank 11, the electrolyte membrane 21 separating them, the electrodes 41, 42, and the power supply 53, is a lithium recovery apparatus using a lithium recovery method that uses an electrolyte membrane having lithium ion conductivity, similar to the lithium isotope enrichment apparatus 10 (for example, Patent Documents 2 and 3). This lithium recovery apparatus recovers Li from the Li-containing aqueous solution AS' contained in the refill tank 1z. + is transferred to the Li-containing aqueous solution ASi contained in the supply tank 11. That is, the lithium isotope enrichment apparatus 10C according to this embodiment is an apparatus with a cascade structure in which the lithium recovery apparatus and the lithium isotope enrichment apparatus 10 according to the first embodiment are connected by integrating the treatment tanks of the respective apparatuses with the supply tank 11.

[0100] The electrolyte membrane 22 has the same configuration as the electrolyte membrane 2 of the lithium isotope enrichment device 10 according to the above embodiment. The electrolyte membrane 21 can also have the same configuration as the electrolyte membrane 22.

[0101] The fourth electrode 41 and the fifth electrode 42 are a pair of electrodes for applying a voltage between both sides of the electrolyte membrane 21. The fourth electrode 41 is provided in the refill tank 1z, and the fifth electrode 42 is provided in the supply tank 11, each in contact with or facing the electrolyte membrane 21. One or both of the fourth electrode 41 and the fifth electrode 42 preferably have a porous structure and are in contact with the electrolyte membrane 21. It is more preferable that one of them is in contact with the electrolyte membrane 21. It is even more preferable that the fourth electrode 41 is in contact with the electrolyte membrane 21, as shown in FIG. 14 (see Patent Document 3). By having at least one of the electrodes 41 and 42 in contact with the electrolyte membrane 21, a voltage can be applied to a wide area of ​​the electrolyte membrane 21. Furthermore, by having one of the electrodes 41 and 42 in contact with the electrolyte membrane 21 and the other separated from it, even if the voltage V3 applied by the power supply 53 connected between the electrodes 41 and 42 is relatively high, the potential difference between both sides of the electrolyte membrane 21 is suppressed, and as will be described later, the Li in the electrolyte membrane 21 is reduced. + This can reduce the decline in energy efficiency of travel.

[0102] As described above, the fourth electrode 41 is provided in contact with the surface (surface) of the electrolyte membrane 21 facing the refill tank 1z, and has a porous structure, such as a mesh structure, similar to the electrodes 31 and 32, so that a voltage can be applied across a wide area of ​​the electrolyte membrane 21 while the Li-containing aqueous solution AS' comes into contact with a sufficient area of ​​the surface of the electrolyte membrane 21. The fourth electrode 41 is formed of an electrode material that is electronically conductive and stable in the Li-containing aqueous solution AS' even when a voltage is applied. Furthermore, similar to the first electrode 31, the fourth electrode 41 is preferably made of a material that has catalytic activity for the reactions of the following formulas (1) and (2), and is also preferably a material that can be easily processed into the above-mentioned shape. Platinum (Pt), for example, is a preferred electrode material for the fourth electrode 41. [ka]

[0103] The fifth electrode 42 is disposed in the supply tank 11 so as not to contact the electrolyte membrane 21, preferably at a short distance from the electrolyte membrane 21, and preferably parallel to the electrolyte membrane 21. Like the third electrode 33, the fifth electrode 42 has a large contact area with the Li-containing aqueous solution ASi and preferably has a mesh-like shape through which the aqueous solution can pass, so that the Li-containing aqueous solution ASi in contact with the surface of the electrolyte membrane 21 in the supply tank 11 is constantly replaced. The fifth electrode 42 is preferably formed of an electrode material that is electronically conductive and stable even when a voltage is applied to the Li-containing aqueous solution ASi. Furthermore, the fifth electrode 42 is preferably formed of a material that has catalytic activity for the reaction represented by the following formula (3). Platinum (Pt) is a preferred example of such an electrode material. Alternatively, the fifth electrode 42 can be made of carbon (C), copper (Cu), or stainless steel, which are stable at potentials lower than the potential at which the reaction represented by the following formula (3) occurs. These materials preferably have Pt fine particles, which function as a catalyst, supported on their surfaces. The fifth electrode 42 may have a porous structure similar to the fourth electrode 41 and be provided in contact with the electrolyte membrane 21. [ka]

[0104] Furthermore, in the lithium isotope enrichment apparatus 10C, the fifth electrode 42, the third electrode 33, and the first electrode 31 are disposed in one supply tank 11. The supply tank 11 is designed to have a sufficient length in the partition direction (between the electrolyte membranes 21, 22) so that the fifth electrode 42 is disposed at a sufficient distance from the third electrode 33 or the first electrode 31. More specifically, even if the first electrode 31 is connected to the main power supply 51, or the electrodes 33, 31 are connected to the auxiliary power supply 52, and the fifth electrode 42 is connected to the power supply 53, the design is such that an electric field is not substantially generated between the fifth electrode 42 and the first electrode 31 or the third electrode 33, or even if an electric field is generated, the electric field is sufficiently weak so as not to inhibit the reactions near the fifth electrode 42 and the first electrode 31.

[0105] The power supply 53 is a DC power supply similar to the main power supply 51, and has a positive electrode connected to the fourth electrode 41 and a negative electrode connected to the fifth electrode 42, and applies a positive voltage V3 (voltage +V3) to the fourth electrode 41 relative to the fifth electrode 42. + The power supply 53 may be a variable power supply so that the mobility can be adjusted.

[0106] The agitator 6 circulates the Li-containing aqueous solution ASi in the supply tank 11, as in the first embodiment. The lithium isotope enrichment apparatus 10C further includes an agitator 6 in the replenishment tank 1z to circulate the Li-containing aqueous solution AS', or in the recovery tank 12 to 6 The aqueous solution ASo for recovering Li may be circulated. In addition, when the cooling device 7 is provided in the lithium isotope enrichment apparatus 10C, the Li in the electrolyte membrane 21 may be circulated. + In order not to reduce the mobility, it is preferable not to cool the electrolyte membrane 21. For this purpose, for example, as shown in FIG. 14, the cooling device 7 is of an immersion type, and the refrigerant pipe is connected to the inside of the recovery tank 12. 6 It is immersed in the aqueous solution ASo for recovering Li and placed facing the electrolyte membrane 22.

[0107] The Li-containing aqueous solution AS' is converted into the Li-containing aqueous solution ASi during the operation of the lithium isotope enrichment apparatus 10C. + This is a Li source that supplies Li to maintain a high concentration. The Li-containing aqueous solution AS' is 7 Li and 6 Li cations 7 Li + , 6 Li + The aqueous solution containing Li in its natural abundance is, for example, an aqueous solution of lithium hydroxide (LiOH), similar to the Li-containing aqueous solution ASi. + At low concentrations, Li in the electrolyte membrane + Since the mobility is low, it is preferable that the mobility is higher at the start of operation of the lithium isotope enrichment apparatus 10C. + It is more preferable that the aqueous solution of Li-containing solution ASi is a saturated or supersaturated aqueous solution of Li-containing solution ASi and Li-containing solution ASi. 6As in the first embodiment, the aqueous solution ASo for recovering Li is, for example, a saturated or supersaturated aqueous solution of LiOH and pure water at the start of operation of the lithium isotope enrichment apparatus 10C.

[0108] (Lithium isotope enrichment method) A lithium isotope enrichment method using a lithium isotope enrichment apparatus according to a second embodiment of the present invention will be described with reference to Fig. 15. In Fig. 15, the agitator 6 and the cooling device 7 are omitted. In a lithium isotope enrichment apparatus 10C according to this embodiment, the lithium isotope enrichment is performed by converting the Li-containing aqueous solution ASi in a supply tank 11 into the Li-containing aqueous solution ASi in a recovery tank 12. 6 Li in aqueous solution ASo for Li recovery + The movement is the same as in the first embodiment (see FIGS. 4 and 5A to 5D). Furthermore, the movement of Li from the Li-containing aqueous solution AS' in the replenishing tank 1z to the Li-containing aqueous solution ASi in the supply tank 11 + The migration occurs from the Li-containing aqueous solution ASi by applying a voltage +V1. 6 Li in aqueous solution ASo for Li recovery + It's the same as moving.

[0109] 15, in the lithium isotope enrichment apparatus 10C, the power supply 53 applies a positive voltage V3 (voltage +V3) to the fourth electrode 41 relative to the fifth electrode 42. Then, in the vicinity of the fourth electrode 41, the OH - The reaction shown in the following equation (1) occurs, and electrons e - is released to the fourth electrode 41, generating H2O and O2, and - In the Li-containing aqueous solution AS', OH - As the charge decreases, Li + The reaction of the following formula (2), in which H2O in the Li-containing aqueous solution ASi dissolves in the electrolyte membrane 21, occurs on the surface of the electrolyte membrane 21, i.e., in the vicinity of the fourth electrode 41. Meanwhile, in the vicinity of the fifth electrode 42, H2O in the Li-containing aqueous solution ASi releases electrons e - By supplying H2 and OH, the reaction shown in the following formula (3) occurs. - Then, OH - As the temperature increases, the Li in the electrolyte membrane 21 increases to maintain the charge balance.+ The reaction of the following formula (4) occurs near the back surface of the electrolyte membrane 21. As a result, the Li contained in the Li-containing aqueous solution AS', the electrolyte membrane 21, and the Li-containing aqueous solution ASi near the electrolyte membrane 21 + Due to the electrochemical potential difference, Li is released from the Li-containing aqueous solution AS'. + permeates the electrolyte membrane 21 and moves into the Li-containing aqueous solution ASi. [ka]

[0110] From the reaction of formula (2) to the reaction of formula (4), that is, Li in the Li-containing aqueous solution AS' + penetration of Li from the surface of the electrolyte membrane 21 to the inside, + and Li in the electrolyte membrane 21 + The transfer of Li to the Li-containing aqueous solution ASi is + via the electrolyte membrane 22 (electrolyte membrane 2) 6 This is the same as the transfer to the aqueous solution ASo for recovering Li, as explained in the first embodiment.

[0111] The larger the voltage V3, the greater the amount of Li in the electrolyte membrane 21. + Since the mobility is high (see FIG. 7), the Li + It is preferable to set the voltage V3 in accordance with the mobility. In addition, since the power supply 53 can be continuously energized, the Li-containing aqueous solution AS' can be easily transferred to the Li-containing aqueous solution ASi. + The movement is from the Li-containing aqueous solution ASi by intermittent energization of the main power supply 51. 6 Li in aqueous solution ASo for Li recovery + 15, during operation of the lithium isotope enrichment apparatus 10C (while the main power supply 51 is running), the power supply 53 is constantly driven to continuously supply Li to the Li-containing aqueous solution ASi. + Alternatively, the power supply 53 may be driven for a short time at regular intervals, and a large voltage V3 may be applied to add Li to the Li-containing aqueous solution ASi. +can be moved at high speed.

[0112] Here, the voltage V3 is large, and the potential difference between the two surfaces is large enough to reduce some of the transition metal ions constituting the electrolyte membrane 21 (for example, if the electrolyte membrane 21 is LLTO, Ti 4+ +e - →Ti 3+ ) voltage or more, the electrolyte membrane 21 exhibits electronic conductivity. Then, the electrons e - generates Joule heat, so Li + The energy efficiency of movement drops sharply, and even if the voltage V3 is further increased, the increase in Li + The mobility does not increase. Specifically, although it depends on the electronic conductivity of the electrolyte membrane 21 and the electrode performance that determines the electrode reaction overvoltage, when a voltage exceeding 2.0 V is applied as a potential difference between both sides, electronic conductivity may be exhibited in the electrolyte membrane 21. In the lithium isotope enrichment device 10C, one of the electrodes 41, 42 that sandwich the electrolyte membrane 21 from both sides is disposed away from the electrolyte membrane 21. Therefore, even if the voltage V3 is increased to a certain extent, the potential difference between the two sides of the electrolyte membrane 21 is unlikely to reach the voltage, but if it is further increased, it will reach the voltage, so it is preferable to set the voltage at or below this voltage.

[0113] According to the lithium isotope enrichment apparatus 10C of this embodiment, the Li-containing aqueous solution ASi is constantly or periodically 6 Li isotope ratio is natural ratio + is replenished, so the Li in the Li-containing aqueous solution ASi + Maintain high concentrations of 6 Since the rate of decrease in the Li isotope ratio can be slowed down, long-term continuous operation is possible without circulating the Li-containing aqueous solution ASi with the outside. However, if the operation continues for a certain period of time, the amount of Li remaining in the aqueous solution ASi will increase. 7 Li + By 6 The Li isotope ratio was significantly lower than the natural ratio, and the Li in the Li-containing aqueous solution AS' + The concentration of Li in the electrolyte membrane 21 decreases with voltage V3. +Since the mobility decreases and the energy efficiency decreases, it is preferable to replace the Li-containing aqueous solutions AS' and ASi in the replenishing tank 1z and the supply tank 11.

[0114] (First Modification) The lithium isotope enrichment apparatus 10C according to this embodiment is configured to recover Li from the Li-containing aqueous solution ASi by a known lithium recovery method. + Therefore, the Li replenishment in the Li-containing aqueous solution ASi + However, unlike seawater, Li + In low concentration aqueous solutions, Li + The mobility of Li + Since the rate is limited by the diffusion of Li, it is difficult to increase the voltage V3, and the energy efficiency is low. Furthermore, chloride ions contained in seawater deteriorate the catalytic activity of the fourth electrode 41 and are adsorbed on the surface of the electrolyte membrane 21, resulting in the formation of Li + The mobility decreases. + Li from aqueous solutions containing low concentrations of + In order to efficiently replenish the

[0115] 16, a lithium isotope enrichment apparatus 10D according to a first modification of the second embodiment of the present invention includes a treatment vessel 1B, an electrolyte membrane (lithium ion conductive electrolyte membrane for lithium replenishment) 21, an electrolyte membrane (lithium ion conductive electrolyte membrane) 22, a first electrode (electrode with a porous structure) 31, a second electrode (electrode with a porous structure) 32, a third electrode (auxiliary electrode) 33, a fourth electrode (first electrode for lithium replenishment) 41, a fifth electrode (second electrode for lithium replenishment) 42, a sixth electrode 44, a power supply device 5 incorporating a main power supply 51 and an auxiliary power supply 52, a power supply (power supply for lithium replenishment) 53, a power supply 55, an ion exchange membrane 8, and an agitator (circulation means) 6. The treatment vessel 1B is circulated by the ion exchange membrane 8 and the electrolyte membranes 21 and 22 into a raw material vessel 1y containing a Li-containing aqueous solution SW, a replenishment vessel (lithium replenishment vessel) 1z containing a Li-containing aqueous solution AS′, a supply vessel (first vessel) 11 containing a Li-containing aqueous solution ASi, and a secondary vessel 12 containing a Li-containing aqueous solution ASi. 6and a recovery tank (second tank) 12 for storing an aqueous Li recovery solution ASo. The lithium isotope enrichment apparatus 10D is configured by adding an ion exchange membrane 8 that separates the treatment tank 1B from the replenishment tank 1z side of the electrolyte membrane 21, a sixth electrode 44 provided in the raw material tank 1y separated from the replenishment tank 1z by the ion exchange membrane 8, and a power supply 55 connected between the electrodes 44 and 41 to the lithium isotope enrichment apparatus 10C according to the second embodiment (see FIG. 14). Note that in the lithium isotope enrichment apparatus 10D, the fourth electrode 41 is provided in contact with the surface of the electrolyte membrane 21 facing the replenishment tank 1z. The remaining configuration is the same as that of the lithium isotope enrichment apparatus 10C according to the previous embodiment, and may further include a cooling device 7, a liquid level sensor, an exhaust means, and the like, as necessary.

[0116] The ion exchange membrane 8 is Li + As a result, the Li-containing aqueous solution AS' in the replenishing tank 1z contains cations containing at least Cl. - The ion exchange membrane 8 is a cation exchange membrane that allows cations to pass through and blocks anions, and a Li + ,K + ,Na + Examples of suitable ion exchange membranes include monovalent cation-permselective ion exchange membranes that allow only monovalent cations such as α- and β-blocking agents to pass through, and bipolar monovalent ion-permselective ion exchange membranes that allow only monovalent ions to pass through. These ion exchange membranes may be known, such as SELEMION (registered trademark) CMV (manufactured by AGC Engineering Co., Ltd.) and NEOSEPTA CSE (manufactured by ASTOM Corporation) as cation exchange membranes, SELEMION (registered trademark) CSO (manufactured by AGC Engineering Co., Ltd.) as monovalent cation-permselective ion exchange membranes, and NEOSEPTA CIMS (manufactured by ASTOM Corporation) as bipolar monovalent ion-permselective ion exchange membranes. In the lithium isotope enrichment apparatus 10D, a short distance between the sixth electrode 44 and the fourth electrode 41 is preferred. Therefore, the ion exchange membrane 8 is preferably positioned so as to shorten the distance from the electrolyte membrane 21 (fourth electrode 41). Therefore, the replenishing tank 1z is preferably short in the direction of the partition of the treatment tank 1B.

[0117] The sixth electrode 44 is paired with the fourth electrode 41 to detect the Li in the Li-containing aqueous solution SW. + The sixth electrode 44 is an electrode for transferring cations including cations (including cations (C1) and cations (C2)) to the Li-containing aqueous solution AS' and for making the surface of the electrolyte membrane 21 have a relatively low potential in the Li-containing aqueous solution AS'. The sixth electrode 44 is preferably arranged parallel to the fourth electrode 41 in the raw material tank 1y, and is preferably arranged so as to shorten the distance between the sixth electrode 44 and the fourth electrode 41 across the ion exchange membrane 8. The sixth electrode 44 preferably has a mesh-like shape or the like so as to increase the contact area with the Li-containing aqueous solution SW. The sixth electrode 44 is preferably formed of an electrode material that is electronically conductive and stable even when a voltage is applied in the Li-containing aqueous solution SW, and further preferably has a material that has catalytic activity for the reaction of the following formula (1). When the Li-containing aqueous solution SW contains halide ions, the sixth electrode 44 is further used for oxidation of the halide ions, for example, chloride ions (Cl - ), a material having catalytic activity for the reaction of the following formula (12) is preferred. As such an electrode material for sixth electrode 44, for example, carbon (C), platinum (Pt), or carbon supporting platinum fine particles as a catalyst is preferred. [ka]

[0118] The power supply 55 is a DC power supply similar to the power supply 53, and has a positive electrode connected to the sixth electrode 44 and a negative electrode connected to the fourth electrode 41, i.e., connected in series to the positive electrode of the power supply 53. The power supply 55 applies a voltage V5 to generate an electric field E3 (see FIG. 17) in the Li-containing aqueous solution SW, AS′, thereby detecting the Li in the Li-containing aqueous solution SW. + The cations containing Li are transferred to the Li-containing aqueous solution AS', and the surface of the electrolyte membrane 21 is set to a relatively low potential in the Li-containing aqueous solution AS'. + The power source 53 and the power source 55 are preferably configured to be capable of being turned on and off independently of each other.

[0119] The Li-containing aqueous solution SW is converted into the Li-containing aqueous solution ASi during the operation of the lithium isotope enrichment device 10D. +The Li source supplies Li via the Li-containing aqueous solution AS' to maintain a high concentration. The Li-containing aqueous solution SW contains lithium ions Li + In addition to K + ,Na + ,Ca 2+ Other metal ions such as M n+ Examples of such aqueous solutions include seawater, brine discarded after extracting salt from seawater, groundwater such as hot spring water, and an aqueous solution obtained by crushing used lithium ion secondary batteries and dissolving them in acid and then adjusting the pH. The Li-containing aqueous solution AS' is, as in the above embodiment, 7 Li and 6 Li cations 7 Li + , 6 Li + The aqueous solution containing Li at a natural abundance ratio, for example, a lithium hydroxide (LiOH) aqueous solution. However, in this modification, it may be pure water before the operation (isotope enrichment) of the lithium isotope enrichment device 10D is started. 6 As in the first embodiment, the aqueous solution ASo for recovering Li is, for example, a saturated or supersaturated aqueous solution of LiOH and pure water at the start of operation of the lithium isotope enrichment apparatus 10D.

[0120] A lithium isotope enrichment method using a lithium isotope enrichment apparatus according to a first modified example of the second embodiment of the present invention will be described with reference to Fig. 17. In a lithium isotope enrichment apparatus 10D according to this modified example, pure water is contained in the replenishing tank 1z as the Li-containing aqueous solution AS', and first, the power supply 55 is driven to extract Li from the Li-containing aqueous solution SW contained in the raw material tank 1y. + Li in the Li-containing aqueous solution AS' is transferred by transferring cations including + When the Li concentration reaches a certain level, it is preferable to start driving the power supply 53. + The movement will be explained below. Note that the agitator 6 is omitted in Fig. 17.

[0121] In the lithium isotope enrichment apparatus 10D, the power supplies 55 and 53 connected in series can be regarded as one power supply (referred to as power supply 55-53). The power supplies 55-53 apply a positive voltage (V5+V3) to the sixth electrode 44 relative to the fifth electrode 42. At the same time, the power supply 53 applies a positive voltage V3 to the fourth electrode 41 relative to the fifth electrode 42. Then, the following reactions occur in the raw material tank 1y and the replenishing tank 1z. In the vicinity of the sixth electrode 44 and the fourth electrode 41, the OH in the Li-containing aqueous solutions SW and AS' is converted into OH. - The reaction shown in the following equation (1) occurs, and electrons e - Electrons e are released to the sixth electrode 44 and the fourth electrode 41 to generate H2O and O2. - The Li-containing aqueous solution SW releases Cl - If the electrons are present, the reaction of the following formula (12) further occurs near the sixth electrode 44, and electrons e - In the Li-containing aqueous solution SW and AS', OH - As a result of the decrease in the amount of Li and other anions, the Li in the Li-containing aqueous solution AS' is reduced to maintain the charge balance. + The reaction of the following formula (2), in which the reactant ions migrate into the electrolyte membrane 21, occurs on the surface of the electrolyte membrane 21, that is, in the vicinity of the fourth electrode 41. [ka]

[0122] Furthermore, an electric field E3 is generated between the electrodes 44 and 41 of the Li-containing aqueous solution SW and AS′ by application of a voltage V5 from the power source 55, and a potential gradient is formed in which the sixth electrode 44 has a higher potential than the surface of the electrolyte membrane 21 on which the fourth electrode 41 is provided. - and Cl - is attracted to the sixth electrode 44 by electrostatic attraction. + The cations containing the cations move along the electric field E3, permeate the ion exchange membrane 8, and are attracted to the surface of the electrolyte membrane 21.

[0123] Meanwhile, in the supply tank 11, the following reaction occurs as in the previous embodiment: Near the fifth electrode 42, H2O in the Li-containing aqueous solution ASi converts to electrons e - By supplying H2 and OH, the reaction shown in the following formula (3) occurs. - Then, OH - As the temperature increases, the Li in the electrolyte membrane 21 increases to maintain the charge balance. + The reaction of the following formula (4) occurs in the vicinity of the electrolyte membrane 21. [ka]

[0124] In the lithium isotope enrichment apparatus 10D according to this modification, as described above, a potential gradient is formed in the Li-containing aqueous solution AS′ by applying the voltage V5 from the power supply 55, and thus the cation Li + However, the Li atoms in the Li-containing aqueous solution AS' are attracted to the surface of the electrolyte membrane 21 (fourth electrode 41) by electrostatic attraction, and become relatively concentrated in this vicinity. + Even at low concentrations, Li + can be sufficiently diffused to the surface of the electrolyte membrane 21, and Li + The mobility does not decrease.

[0125] The stronger the electric field E3, the + is attracted to the surface of the electrolyte membrane 21 and the Li + However, when the voltage V5 is increased to strengthen the electric field E3 and reach the voltage at which water electrolysis occurs (+1.229 V vs. SHE, 25°C), the reaction of formula (3) occurs in the Li-containing aqueous solution AS' near the fourth electrode 41, generating H2. This reaction is - Therefore, the reaction of the above formula (1) in the vicinity of the fourth electrode 41 is -When the vicinity of the fourth electrode 41, i.e., the surface of the electrolyte membrane 21 on the side of the refill tank 1z, reaches the H2 generating potential, some of the transition metal ions constituting the electrolyte membrane 21 are reduced (for example, if the electrolyte membrane 21 is LLTO, Ti 4+ +e - →Ti 3+ ) potential, and the electrolyte membrane 21 exhibits electron transfer properties. As a result, the Li + The energy efficiency of the transfer drops sharply. Therefore, the voltage V5 is set to be lower than the voltage at which electrolysis of water occurs.

[0126] Furthermore, even if the voltage V5 is less than the voltage at which electrolysis of water occurs, if it exceeds a certain magnitude relative to the voltage V3, a current flows from the fourth electrode 41 to the negative electrode of the power supply 55, that is, the fourth electrode 41 generates electrons e - and the reaction of formula (3) occurs nearby, generating H2. As a result, the electrolyte membrane 21 exhibits electron transfer properties. Therefore, it is preferable that the voltage V5 is large enough to prevent current from flowing from the fourth electrode 41 toward the negative electrode of the power source 55. To achieve this, for example, an ammeter may be connected in series to the fourth electrode 41 (connecting the ammeter between the fourth electrode 41 and the connection between the power sources 55 and 53) and the voltages V3 and V5 may be applied while measuring the current.

[0127] In the lithium isotope enrichment apparatus 10D according to this modification, the raw material tank 1y may be open to the sea or the like via a filter or the like. Furthermore, the lithium isotope enrichment apparatus 10D may not include an ion exchange membrane 8, i.e., the treatment tank 1B may be divided into three compartments, similar to the lithium isotope enrichment apparatus 10C according to the previous embodiment, and seawater or the like may be stored in the replenishing tank 1z as the Li-containing aqueous solution AS'. Application of a voltage V5 increases the Cl concentration in the Li-containing aqueous solution AS'. - Since anions such as Cl are repelled from the fourth electrode 41 and the electrolyte membrane 21, the ion exchange membrane 8 is not required. - is hardly adsorbed on the surface of the electrolyte membrane 21, and the fourth electrode 41 is - Since the electrolyte membrane 21 is not easily deteriorated by +The mobility does not decrease. + is attracted to the surface of the fourth electrode 41, i.e., the electrolyte membrane 21, and Li + The mobility can be increased. Furthermore, since there is no ion exchange membrane 8 between the sixth electrode 44 and the fourth electrode 41, the electric field E3 can be strengthened relative to the voltage V5. Furthermore, when the ion exchange membrane 8 is not provided, an LiOH aqueous solution may be stored in the refill tank 1z as the Li-containing aqueous solution AS', as in the previous embodiment. As the operation time passes, the LiOH content of the Li-containing aqueous solution AS' decreases. + Even if the concentration decreases, the Li + Since the mobility is unlikely to decrease, the frequency of replacing the Li-containing aqueous solution AS' can be reduced. + When the concentration drops below a certain level, the power supply 55 starts applying the voltage V5.

[0128] According to the lithium isotope enrichment apparatus 10D of this modification, Li is added to the Li-containing aqueous solution ASi from the Li-containing aqueous solution SW such as seawater. + Therefore, for the Li-containing aqueous solution ASi, before the operation (isotope enrichment) of the lithium isotope enrichment apparatus 10D is started, pure water is placed in the supply tank 11, and Li is supplied from the Li-containing aqueous solution SW via the Li-containing aqueous solution AS'. + In other words, the lithium isotope enrichment apparatus 10D can be a lithium recovery / isotope enrichment combined apparatus with a cascade structure in which the lithium recovery apparatus, the lithium isotope enrichment apparatus 10, and the treatment tank are integrated. In this case, first, the power sources 53 and 55 are driven to convert the lithium isotope into the target Li-containing aqueous solution ASi. + After the LiOH aqueous solution is prepared to a certain concentration (for example, a saturated LiOH aqueous solution), the power supply device 5 is driven to start isotope enrichment.

[0129] (Second Modification) The lithium isotope enrichment apparatus 10D according to the above modification can be a combined lithium recovery and isotope enrichment apparatus.+ To make an aqueous solution, a large amount of Li + Therefore, it takes time to start isotope enrichment. + In order to increase the mobility, the following configuration was adopted.

[0130] As shown in FIG. 18, a lithium isotope enrichment apparatus 10E according to a second modified example of the second embodiment of the present invention includes a treatment tank 1B, an electrolyte membrane (lithium ion conductive electrolyte membrane for lithium replenishment) 21, an electrolyte membrane (lithium ion conductive electrolyte membrane) 22, a first electrode (electrode with a porous structure) 31, a second electrode (electrode with a porous structure) 32, a third electrode (auxiliary electrode) 33, a fourth electrode (first electrode for lithium replenishment) 41, a fifth electrode (second electrode for lithium replenishment) 42A, an auxiliary electrode 43, a sixth electrode 44, a power supply device 5 incorporating a main power supply 51 and an auxiliary power supply 52, a power supply (power supply for lithium replenishment) 53, a power supply 54, a power supply 55, an ion exchange membrane 8, and an agitator (circulation means) 6. The treatment tank 1B is divided into a raw material tank 1y containing the Li-containing aqueous solution SW, a replenishment tank (lithium replenishment tank) 1z containing the Li-containing aqueous solution AS′, a supply tank (first tank) 11 containing the Li-containing aqueous solution ASi, and a secondary battery tank 12 containing the Li-containing aqueous solution SW. 6 and a recovery tank (second tank) 12 that stores an aqueous solution ASo for recovering Li. The lithium isotope enrichment apparatus 10E is configured similarly to the lithium isotope enrichment apparatus 10D (see FIG. 16 ) according to the first modified example of the second embodiment, except that a fifth electrode 42A has a porous structure similar to the fourth electrode 41 and is in contact with the electrolyte membrane 21, an auxiliary electrode 43 is provided in the supply tank 11 facing the fifth electrode 42A and spaced apart from the electrolyte membrane 21 and the fifth electrode 42A, and a power source 54 is connected between the electrodes 42A and 43. The rest of the configuration is the same as that of the lithium isotope enrichment apparatus 10D according to the modified example, and may include a cooling device 7, a liquid level sensor, an exhaust means, and the like, as necessary.

[0131] The fifth electrode 42A is an electrode paired with the fourth electrode 41 for applying a voltage between both surfaces of the electrolyte membrane 21, and is also an electrode for relatively increasing the potential of the surface (back surface) of the electrolyte membrane 21 facing the supply tank 11 in the Li-containing aqueous solution ASi. Therefore, the fifth electrode 42A has a porous structure and is provided in contact with the surface of the electrolyte membrane 21 facing the supply tank 11. The fifth electrode 42A is formed of an electrode material that is electronically conductive and stable in the Li-containing aqueous solution ASi even when a voltage is applied. The fifth electrode 42A is preferably made of a material that has catalytic activity for the reactions of the following formulas (1) and (4), and is also preferably a material that can be easily processed into the above-described shape. An example of such an electrode material for the fifth electrode 42A is platinum (Pt). [ka]

[0132] The auxiliary electrode 43 is an electrode for forming a lower potential than the back surface of the electrolyte membrane 21 in the Li-containing aqueous solution ASi and is also an electrode for applying a voltage in pair with the fourth electrode 41. Therefore, the auxiliary electrode 43 is preferably arranged in the supply tank 11 facing the fifth electrode 42A and parallel to the fifth electrode 42A without contacting the electrolyte membrane 21 or the fifth electrode 42A. Furthermore, as described below, the auxiliary electrode 43 is preferably arranged close enough to the fifth electrode 42A to prevent short-circuiting. Furthermore, the auxiliary electrode 43 preferably has a mesh-like shape or the like to increase the contact area with the Li-containing aqueous solution ASi. The auxiliary electrode 43 is preferably formed of an electrode material that is electronically conductive and stable in the Li-containing aqueous solution ASi even when a voltage is applied, and further preferably has catalytic activity for the reaction of the following formula (3). Platinum (Pt), for example, is a preferred electrode material for the auxiliary electrode 43. Alternatively, the auxiliary electrode 43 may be made of carbon (C), copper (Cu), or stainless steel, which are stable at a potential lower than the potential at which the reaction of the following formula (3) occurs. It is more preferable that these materials have Pt fine particles, which function as a catalyst, supported on their surfaces. [ka]

[0133] The power supply 54 is a DC power supply like the power supply 53, and has a positive electrode connected to the fifth electrode 42A and a negative electrode connected to the sub-electrode 43, i.e., connected in series to the negative electrode of the power supply 53. The power supply 54 applies a voltage V4 to create a potential in the Li-containing aqueous solution ASi that is lower than that of the back surface of the electrolyte membrane 21, thereby suppressing the development of electronic conductivity in the electrolyte membrane 21.

[0134] A lithium isotope enrichment method using a lithium isotope enrichment apparatus according to a second modified example of the second embodiment of the present invention will be described with reference to Fig. 19. In a lithium isotope enrichment apparatus 10E according to this modified example, pure water is stored in the replenishing tank 1z and the supply tank 11 as the Li-containing aqueous solutions AS' and ASi. Then, as in the lithium isotope enrichment apparatus 10D according to the first modified example (see Fig. 17), first, the power supply 55 is driven to extract Li from the Li-containing aqueous solution SW stored in the raw material tank 1y. + Li in the Li-containing aqueous solution AS' is transferred by transferring cations including + When the Li concentration reaches a certain level, it is preferable to start driving the power supplies 53 and 54. + The movement will be explained below. Note that the agitator 6 is omitted in Fig. 19.

[0135] In the lithium isotope enrichment apparatus 10E, the power supplies 55, 53, and 54 connected in series can be considered as a single power supply (referred to as power supplies 55-53-54). Similarly, the power supplies 53 and 54 can be considered as a single power supply (referred to as power supplies 53-54). The power supplies 55-53-54 apply a positive voltage (V5+V3+V4) to the sixth electrode 44 with respect to the sub-electrode 43. At the same time, the power supplies 53-54 apply a positive voltage (V3+V4) to the fourth electrode 41 with respect to the sub-electrode 43. Then, in the raw material tank 1y and the replenishing tank 1z, the reaction of the following formula (1) occurs near the sixth electrode 44 and the fourth electrode 41, and the reaction of the following formula (12) occurs near the sixth electrode 44, as in the lithium isotope enrichment apparatus 10D. As a result, the Li in the Li-containing aqueous solution AS' increases. +The reaction of the following formula (2) occurs near the fourth electrode 41, in which the reactant ions migrate into the electrolyte membrane 21. [ka]

[0136] Meanwhile, the following reaction occurs in the supply tank 11. In the vicinity of the sub-electrode 43, the application of a voltage (V3+V4) from the power source 53-54 converts H2O in the Li-containing aqueous solution ASi into electrons e - By supplying H2 and OH, the reaction shown in the following formula (3) occurs. - As a result, H + Since the amount of Li in the electrolyte membrane 21 decreases, the amount of Li in the electrolyte membrane 21 decreases on the back surface of the electrolyte membrane 21, i.e., in the vicinity of the fifth electrode 42A. + The reaction of the following formula (4) occurs, in which OH moves to the Li-containing aqueous solution ASi. At the same time, the power supply 54 applies a positive voltage V4 of a predetermined magnitude based on the voltages V3 and V5 to the fifth electrode 42A relative to the sub-electrode 43. Then, in the vicinity of the fifth electrode 42A, OH in the Li-containing aqueous solution ASi moves. - The reaction shown in the following equation (1) occurs, and electrons e - To the fifth electrode 42A, H2O and O2 are generated. As a result, an imbalance in charge occurs in the vicinity of the fifth electrode 42A, resulting in an excess of cations due to the reactions of the following formulas (1) and (4). However, to compensate for the deficiency of cations occurring near the sub-electrode 43 due to the reaction of the following formula (3), and along the electric field E4 generated by the application of voltage V4, Li + The voltage V3 and the voltage V4 are quickly transferred from the vicinity of the fifth electrode 42A to the vicinity of the sub-electrode 43, and as a result, the imbalance of the charges in the Li-containing aqueous solution ASi is eliminated. The relative relationship between the magnitudes of the voltages V3 and V4 will be described later. [ka]

[0137] In the lithium isotope enrichment apparatus 10E according to this modification, the effect of applying the voltage V5 is the same as that of the lithium isotope enrichment apparatus 10D according to the first modification. In this modification, the application of the voltage V4 also generates an appropriate potential difference in the Li-containing aqueous solution ASi, with the fifth electrode 42A being positive. Then, the electrons e supplied from the sub-electrode 43 to the Li-containing aqueous solution ASi are - moves from the fifth electrode 42A on the back surface of the electrolyte membrane 21 to the positive electrode of the power supply 54, and the potential of the fifth electrode 42A is maintained high at approximately the O2 generating potential. Since the O2 generating potential is higher than the reduction potential of the transition metal ions constituting the electrolyte membrane 21, the electrons e - Therefore, the voltage V3 can be set to a voltage equal to or greater than the applied voltage that causes the electrolyte membrane 21 to reach the reduction potential of at least one transition metal ion constituting the electrolyte membrane 21. In other words, when such a large voltage V3 is applied without applying the voltage V4, the electrons e are released from the negative electrode side (supply tank 11 side) of the electrolyte membrane 21. - However, in this modification, as described above, the application of voltage V4 prevents the electrolyte membrane 21 from reaching the reduction potential of the transition metal ions, and the electrolyte membrane 21 does not absorb electrons e - does not transmit.

[0138] If the voltage V4 is not large enough relative to the voltage V3, a current flows from the fifth electrode 42A to the negative electrode of the power supply 53, i.e., the fifth electrode 42A receives electrons e -The fifth electrode 42A receives the Li-containing solution ASi, and the reaction of formula (3) occurs in the vicinity, generating H2. As a result, electron transfer properties are exhibited in the electrolyte membrane 21. Therefore, the voltage V4 is set to a value that prevents current from flowing from the fifth electrode 42A toward the negative electrode of the power source 53. The lower the resistance between the fifth electrode 42A and the sub-electrode 43 (the resistance of the Li-containing aqueous solution ASi), the smaller the voltage V4 can be. Therefore, it is preferable that the distance between the fifth electrode 42A (electrolyte membrane 21) and the sub-electrode 43 is short. However, as the voltage V4 increases within this range, the current flowing from the power source 54 to the fifth electrode 42A increases, and the generation of O2 (reaction of formula (1)) in the vicinity of the fifth electrode 42A and the generation of H2 (reaction of formula (3)) in the vicinity of the sub-electrode 43 become larger than the Li-containing solution ASi. + Therefore, for example, an ammeter may be connected in series to fifth electrode 42A (connecting the ammeter between the connection between power supplies 53 and 54 and fifth electrode 42A) and voltage V4 may be applied while measuring the current.

[0139] Li-containing aqueous solution ASi is the target + When the concentration reaches, for example, a saturated LiOH aqueous solution, the power supply 53 is driven to start isotope enrichment. + The movement is due to the intermittent energization of the main power supply 51, and the Li + Therefore, after starting the isotope enrichment, the voltage V3 can be reduced, and if necessary, the voltage V5 can be reduced, and the power supply 54 can be stopped (see FIGS. 18 and 17).

[0140] The lithium isotope enrichment apparatuses 10C, 10D, and 10E according to the second embodiment and its modifications may be configured to be connected to the supply tank 11 of the lithium isotope enrichment apparatus 10B (see FIG. 8) according to the modification of the first embodiment. Similarly, the lithium isotope enrichment apparatuses 10C, 10D, and 10E may be configured to be connected to the supply tank 11 of the multistage lithium isotope enrichment apparatuses 20, 20A, and 20B (see FIGS. 10, 11A, 11B, and 12). [Example]

[0141] The lithium isotope enrichment device and the lithium isotope enrichment method according to the present invention have been described above as embodiments for carrying out the present invention, but examples in which the effects of the present invention have been confirmed will be described below. It goes without saying that the present invention is not limited to these examples and the above embodiments, and that various changes and modifications based on these descriptions are also included in the spirit of the present invention.

[0142] The voltage application conditions were changed and the amount of change in the lithium isotope ratio was measured for the lithium isotope enrichment devices according to the embodiment and the modified examples of the present invention shown in FIGS. 1 and 8.

[0143] (Construction of a lithium isotope enrichment device) The lithium isotope enrichment device uses a 50mm x 50mm, 0.5mm thick plate of La as the electrolyte membrane. 0.57 Li 0.29 TiO3 (lithium ion conductive ceramics LLTO, manufactured by Toho Titanium Co., Ltd.) was used. The first and second electrodes (porous electrodes) were formed in the center of each side of the electrolyte membrane. These electrodes were 10 μm thick, 0.5 mm wide, and spaced 0.5 mm apart. Grid-shaped electrodes measuring 19.5 mm × 20.5 mm were then formed. Lead wires were then formed to connect these electrodes to a power source. The first and second electrodes and lead wires were formed by screen-printing Pt paste onto the surface of the electrolyte membrane and firing it at 900 °C for 1 h in air. A 20 mm × 20 mm Pt mesh electrode was used as the third electrode (sub-electrode). The electrolyte membrane with the electrodes formed on it was placed in an acrylic treatment tank, which was divided into a supply tank and a recovery tank. The third electrode was placed in the supply tank so that it directly faced the first electrode on the electrolyte membrane surface (distance between the third electrode and the electrolyte membrane: 50 mm). The treatment tank was then placed in a thermostatic chamber with temperature control. A main power supply was connected between the first electrode and the second electrode with the first electrode serving as a positive electrode, and a secondary power supply was connected between the third electrode and the first electrode, thereby forming a lithium isotope enrichment device.

[0144] A Li-containing aqueous solution is added to the supply tank of the lithium isotope enrichment device. 7 Li: 92.23 mol%, 6 Li: 1 mol / l lithium hydroxide solution containing 7.77 mol% Li was added to the recovery tank. 6 As an aqueous solution for Li recovery, 150 ml of pure water was added to each of the first, second, and third electrodes so that they were completely immersed. The temperatures of the lithium hydroxide aqueous solution and pure water in the treatment tank were then adjusted to 20°C.

[0145] (Lithium isotope enrichment experiment) The applied voltage V1 between the first and second electrodes by the main power supply is + The voltage V2 applied between the third electrode and the first electrode by the secondary power supply was set to 2.0 V, which indicates electrical conductivity but does not exhibit electronic conductivity or is sufficiently small. The voltage V2 applied between the third electrode and the first electrode by the secondary power supply was set to 1.0 V (equivalent to 200 V / m). In Example 1, a voltage +V1 (the first electrode is the positive pole of the main power supply) was applied for 1.0 second, followed by a voltage -V2 (the third electrode is the negative pole of the secondary power supply) for 0.5 seconds, with a 0.5-second application-free period in between (see FIG. 2 and Table 1). In Example 2, a voltage +V1 was applied for 1.0 second, followed by a voltage -V2 for 0.5 seconds, followed by a voltage +V2 (the third electrode is the positive pole of the secondary power supply) for 0.5 seconds, with a 0.5-second application-free period in between (see FIG. 9 and Table 1). In Comparative Example 1, a voltage +V1 was applied for 1.0 second and then stopped for 1.5 seconds, alternately and repeatedly. In Examples 1, 2, and Comparative Example 1, a cumulative voltage +V1 application time was 3600 seconds. In Comparative Example 2, a voltage +V1 was continuously applied for 1 hour (3600 seconds). In both cases, the aqueous solutions in the supply tank and recovery tank were stirred while the voltage was applied. The voltages V1 and V2, the voltage application cycle, and the operating time are shown in Table 1. For the voltage application cycles in Table 1, the application time is shown in parentheses, and the time alone indicates the period during which no voltage was applied.

[0146] After the experiment, the aqueous solution in the recovery tank was collected and 7 Li, 6The amount of Li was measured using an inductively coupled plasma mass spectrometer (ICP-MS) (Elan drc-e, manufactured by PerkinElmer Co., Ltd.). 7 Li, 6 From the amount of Li, the total applied time of voltage +V1 per hour + amount of movement( 7 Li, 6 Li per hour of operation) + the amount of movement, and 6 The Li isotope separation factor was calculated. 6 The Li isotope separation factor is (of the aqueous solution in the recovery tank after voltage application) 6 Li / 7 Li) molar ratio) / (( 6 Li / 7 Li) molar ratio. + Movement amount (Li per voltage + V1 application time (current application time) + Travel amount, Li + mobility), Li per operating hour + the amount of movement, and 6 The Li isotope separation factors are shown in Table 1 and FIG.

[0147] [Table 1]

[0148] As shown in Table 1 and FIG. 20, in comparison with Comparative Example 2 in which a continuous current was applied, Examples 1, 2, and Comparative Example 1 in which a voltage +V1 was applied intermittently between both surfaces of the electrolyte membrane all showed a significant improvement. 6 Furthermore, in Examples 1 and 2, in which a voltage -V2 was applied between the third electrode and the first electrode of the Li-containing aqueous solution in the supply tank while the application of the voltage +V1 was stopped, the Li isotope separation coefficient was higher than in Comparative Example 1, in which no voltage was applied. 6 The Li isotope separation factor was high. This suggests that in a Li-containing aqueous solution, applying a negative voltage to the third electrode spaced from the surface of the electrolyte membrane can separate the Li adsorbed on the surface of the electrolyte membrane. + The intermittent application of voltage +V1 6It can be said that the Li concentration effect can be further improved. In addition, in Example 2, in which the voltage -V2 was applied after the voltage +V2 was applied while the application of the voltage +V1 was stopped, the Li + The mobility is significantly higher, about 3.7 times that of Example 1, and the Li + The amount of movement was also about three times higher. This indicates that the application of voltage +V2 increased the Li + It can be said that the decrease in mobility due to application of voltage -V2 is eliminated and even improved, thereby increasing both time efficiency and energy efficiency. [Explanation of symbols]

[0149] 10, 10B, 10C, 10D, 10E Lithium Isotope Enrichment Device 20, 20A, 20B Multistage Lithium Isotope Enrichment Apparatus 1, 1A, 1B Treatment tank 11 Supply tank (1st tank) 12 Recovery tank (second tank) 1z Refill Tank (Lithium Refill Tank) 2. Electrolyte membrane (lithium ion conductive electrolyte membrane) 21 Electrolyte membrane (lithium ion conductive electrolyte membrane for lithium replenishment) 22,23,24,25,26,27 Electrolyte membrane (lithium ion conductive electrolyte membrane) 31 First electrode (electrode with porous structure) 32 Second electrode (electrode with porous structure) 33 Third electrode (auxiliary electrode) 41 Fourth electrode (first electrode for lithium replenishment) 42, 42A 5th electrode (2nd electrode for lithium replenishment) 50,50C power supply 5,5A,5B power supply 51 Main power supply 51A variable power supply 52,52B Sub power supply 53 Power supply (lithium refill power supply) 6. Agitator (circulation means) 7 Cooling device ASi Li-containing aqueous solution ASo 6 Lithium recovery solution

Claims

1. A treatment tank is provided which is divided into a first tank and a second tank, and the first tank contains 6 Li and 7 Li and Li in the state of lithium ions, 6 A lithium isotope enrichment apparatus that recovers an aqueous solution containing lithium ions having a high Li isotope ratio in the second tank, a lithium ion conductive electrolyte membrane that separates the treatment tank; Porous electrodes provided on both sides of the lithium ion conductive electrolyte membrane in contact with each other; a secondary electrode provided in the first tank at a distance from the surface of the lithium ion conductive electrolyte membrane facing the first tank and the porous electrode; a power supply unit that applies a voltage alternately between the electrodes of the porous structure and between the electrode of the porous structure on the first tank side and the auxiliary electrode, with the electrode of the porous structure on the first tank side being positive.

2. 2. The lithium isotope enrichment apparatus according to claim 1, wherein the power supply device applies a voltage between the porous electrode on the first tank side and the auxiliary electrode, with the porous electrode on the first tank side being positive, then applies a voltage with the auxiliary electrode being positive, and then applies a voltage between the porous electrodes.

3. 3. The lithium isotope enrichment device according to claim 1, wherein the power supply device comprises: a main power supply connected between the electrodes of the porous structure, with the first tank side as a positive electrode; and a secondary power supply connected between the electrode of the porous structure on the first tank side and the secondary electrode.

4. 3. The lithium isotope enrichment apparatus according to claim 1, further comprising a circulation means for circulating the aqueous solution contained in the first tank.

5. the treatment tank is partitioned into a lithium replenishment tank, the first tank, and the second tank in this order; a lithium ion conductive electrolyte membrane for lithium replenishment that separates the lithium replenishment tank from the first tank; a first electrode for replenishing lithium disposed in the lithium replenishing tank; a second electrode for replenishing lithium disposed in the first tank in contact with or facing the lithium ion conductive electrolyte membrane for replenishing lithium; a lithium replenishment power source connected between the first lithium replenishment electrode and the second lithium replenishment electrode, with the first lithium replenishment electrode serving as a positive electrode; The lithium replenishment tank contains 6 Li and 7 3. The lithium isotope enrichment device according to claim 1, wherein lithium ions are transferred from an aqueous solution containing Li in the form of lithium ions to the aqueous solution contained in the first tank.

6. 3. The lithium isotope enrichment device according to claim 1, further comprising a cooling device for cooling the lithium ion conductive electrolyte membrane.

7. A multistage lithium isotope enrichment apparatus comprising two or more lithium isotope enrichment apparatuses according to claim 1 or 2, connected together so that the treatment tanks are integrated, the lithium ion conductive electrolyte membranes of the lithium isotope enrichment device are arranged apart from each other so as to separate the integrated treatment tank into three or more tanks; A multistage lithium isotope enrichment apparatus, characterized in that the second tank of one of two adjacent lithium isotope enrichment apparatuses also serves as the first tank of the other.

8. The multistage lithium isotope enrichment apparatus according to claim 7, characterized in that the power supply devices of the two lithium isotope enrichment apparatuses do not simultaneously apply a voltage between the electrodes of the porous structure.

9. 9. The multistage lithium isotope enrichment apparatus according to claim 8, wherein the porous electrode on the second tank side of one of the two lithium isotope enrichment apparatuses also serves as the auxiliary electrode of the other one.

10. a circulation means for circulating the aqueous solution contained in at least one of the partitioned treatment tanks, 10. The multistage lithium isotope enrichment apparatus according to claim 9, wherein the auxiliary electrode is provided in a tank containing the aqueous solution circulated by the circulation means, the auxiliary electrode being spaced apart from the lithium ion conductive electrolyte membrane.

11. a lithium ion conductive electrolyte membrane for replenishing lithium, which further partitions the partitioned treatment chamber so that a lithium replenishment chamber is provided at the end of the first chamber side; a first electrode for replenishing lithium disposed in the lithium replenishing tank; a second electrode for refilling lithium disposed in the first tank adjacent to the lithium refilling tank so as to be in contact with or face the lithium refilling lithium ion conductive electrolyte membrane; a lithium replenishment power source connected between the first lithium replenishment electrode and the second lithium replenishment electrode, with the first lithium replenishment electrode serving as a positive electrode; The lithium replenishment tank contains 6 Li and 7 8. The multistage lithium isotope enrichment apparatus according to claim 7, wherein lithium ions are transferred from an aqueous solution containing Li in the form of lithium ions to the aqueous solution contained in the first tank adjacent to the lithium replenishment tank.

12. 8. The multistage lithium isotope enrichment apparatus according to claim 7, further comprising a cooling device for cooling the aqueous solution contained in at least one of the partitioned treatment tanks.

13. In a treatment tank partitioned into a first tank and a second tank by a lithium ion conductive electrolyte membrane, 6 Li and 7 Li and Li in the state of lithium ions, 6 A method for enriching lithium isotopes, in which an aqueous solution containing lithium ions having a high Li isotope ratio is recovered in the second tank, a first step of applying a positive voltage to the first tank side of porous electrodes provided in contact with both surfaces of the lithium ion conductive electrolyte membrane, relative to the second tank side; and a second step of applying a negative voltage to the porous electrode on the first tank side, the porous electrode and a sub-electrode provided in the first tank at a distance from the surface of the lithium ion conductive electrolyte membrane on the first tank side.

14. a third step of applying a positive voltage to the auxiliary electrode relative to the porous electrode on the first tank side; 14. The method for enriching lithium isotopes according to claim 13, wherein the first step, the second step, and the third step are repeated in this order.

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