Lithium isotope enrichment device, multi-stage lithium isotope enrichment device, and lithium isotope enrichment method

Abstract

A lithium isotope concentration device 10 is provided with a treatment tank 1 partitioned into a supply tank 11 and a recovery tank 12 by an electrolyte membrane 2 having lithium-ion conductivity, the electrolyte membrane 2 being cooled by a cooling device 6 via a Li-containing aqueous solution ASi in the supply tank 11 to obtain a low temperature at which the 6Li isotope separation coefficient is larger, and a positive voltage being applied, by a power supply 5 connected between electrodes 31, 32 provided on both surfaces of the electrolyte membrane 2, to the supply tank 11-side electrode 31.

Description

Technical Field

[0001] The present invention relates to a lithium isotope enrichment device and a multi-stage lithium isotope enrichment device for separating lithium isotopes, as well as a lithium isotope enrichment method.

Background Art

[0002] Lithium (Li) 7 has two stable isotopes, 6 Li and 7 Li, with natural abundance ratios of 92.41 mol% and 7.59 mol%, respectively. 6 Li with a mass number of 6 and 7 Li with a mass number of 7 have significantly different properties. For example, 6 Li is used to adjust the pH (hydrogen ion concentration) of the coolant in nuclear reactors. On the other hand, 7 Li is used in the production of tritium, the fuel for fusion reactors. Therefore, 6 techniques for enriching and separating + Li and

[0003] 6 6 Li to a state with less of the other isotope have been developed, including the amalgam method, the molten salt method, the distillation method, and the adsorption method and electrodialysis method (for example, Patent Document 1), which is also a method for selectively recovering lithium ions + Li from seawater and the like.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Japanese Patent No. 5429658 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2019 - 141808 [Non - Patent Document]

[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] [Problems to be Solved by the Invention]

[0006] The methods described in Patent Document 2 and the like have room for further improvement in order to increase the isotope separation factor.

[0007] The present invention has been made in view of the above problems, and an object thereof is to provide a safe and more efficient lithium isotope enrichment device, a multi - stage lithium isotope enrichment device, and a lithium isotope enrichment method. [Means for Solving the Problems]

[0008] As a result of intensive research on the enrichment of lithium isotopes by electrodialysis, the present inventors 7 Li + and 6 Li + were found to have different degrees of temperature dependence and voltage dependence of mobility, and came up with the idea of setting the temperature and applied voltage within an appropriate range.

[0009] That is, the lithium isotope enrichment device according to the present invention includes a treatment tank partitioned into a first tank and a second tank, and is housed in the first tank, 6 Li and 7 Li in an aqueous solution containing them in the state of lithium ions, from the aqueous solution, 6 An apparatus for recovering an aqueous solution containing lithium ions having a higher isotope ratio of Li in the second tank, a lithium ion conductive electrolyte membrane partitioning the treatment tank, and porous structures provided in contact with both sides of the lithium ion conductive electrolyte membrane An electrode, a power supply device for applying a voltage between the electrodes, and a cooling device for cooling the lithium ion conductive electrolyte membrane. Further, the multi-stage lithium isotope enrichment device according to the present invention includes two or more of the above-described lithium isotope enrichment devices connected so that the treatment tanks are integrated, and each lithium ion conductive electrolyte membrane of the lithium isotope enrichment device is separated from each other so as to partition the integrated treatment tank into three or more tanks. One of the second tanks of two adjacent lithium isotope enrichment devices also serves as the first tank of the other, and the cooling device is configured to cool an aqueous solution contained in at least one of the partitioned treatment tanks.

[0010] With such a configuration, lithium ions move at a low temperature where the difference in mobility between 6 Li + and 7 Li + is large, and 6 More Li moves and is recovered.

[0011] 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 is housed in the first tank, 6 Li and 7 Li in an aqueous solution containing them in the state of lithium ions, from the aqueous solution, 6A method for recovering an aqueous solution containing lithium ions with a high isotope ratio of Li in the second tank. And the lithium isotope enrichment method according to the present invention is to apply a voltage with the first tank side being positive between the electrodes with a porous structure provided in contact with each of both sides of the lithium ion conductive electrolyte membrane while cooling the lithium ion conductive electrolyte membrane to 20 °C or lower. Alternatively, the lithium isotope enrichment method according to the present invention is to apply a voltage of 1.5 V or lower with the first tank side being positive between the electrodes with a porous structure provided in contact with each of both sides of the lithium ion conductive electrolyte membrane.

[0012] By such a method, 6 Li + and 7 Li + the lithium ions are moved under conditions where the difference in mobility between them is large, 6 Li + and more of it can be moved and recovered.

Advantages of the Invention

[0013] According to the lithium isotope enrichment apparatus and the lithium isotope enrichment method of the present invention, 6 an aqueous solution with a higher isotope ratio of Li can be efficiently and safely recovered. Further, according to the multi-stage lithium isotope enrichment apparatus of the present invention, 6 the isotope ratio of Li can be made even higher.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 3C

Figure 4

Figure 5

Figure 6A

Figure 6B

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0015] Embodiments for carrying out the lithium isotope enrichment device and the lithium isotope enrichment method according to the present invention will be described with reference to the drawings. In the drawings, for the sake of clarity of explanation, the sizes of specific elements etc. may be exaggerated, and the shapes may be simplified.

[0016] 〔Lithium Isotope Enrichment Device〕 As shown in FIG. 1, a lithium isotope enrichment apparatus 10 according to an embodiment of the present invention includes a treatment tank 1, an electrolyte membrane (lithium ion conductive electrolyte membrane) 2, a first electrode 31 and a second electrode 32 (electrodes) coated on each surface of the electrolyte membrane 2, a power supply device 5, and a cooling device 6. The treatment tank 1 is divided by the electrolyte membrane 2 into a supply tank (first tank) 11 that contains an Li-containing aqueous solution ASi and 6 a recovery tank (second tank) 12 that contains an aqueous solution ASo for Li recovery. The power supply device 5 has a DC power supply, and the positive (+) pole of the power supply is connected to the first electrode 31 provided on the supply tank 11 side, and the negative (-) pole is connected to the second electrode 32 provided on the recovery tank 12 side. The cooling device 6 cools the electrolyte membrane 2 through the Li-containing aqueous solution ASi in the supply tank 11. Hereinafter, each element constituting the lithium isotope enrichment apparatus according to the embodiment of the present invention will be described.

[0017] The treatment tank 1 is made of a material that does not deteriorate due to corrosion or the like when it comes into contact with the Li-containing aqueous solution ASi and 6 the aqueous solution ASo for Li recovery. The treatment tank 1 only needs to have a volume corresponding to the required processing capacity, and its shape and the like are not particularly limited.

[0018] The electrolyte membrane 2 is an electrolyte having lithium ion conductivity, and it is preferable that electrons e - do not conduct. Further, when the Li-containing aqueous solution ASi contains metal ions other than Li + , it is preferable that the electrolyte membrane 2 does not conduct these metal ions. More preferably, it is a ceramic electrolyte having these properties. Specifically, lithium lanthanum titanate (La 2 / 3-x Li 3x TiO3, also referred to as LLTO) and the like can be mentioned. Such an electrolyte membrane 2 has lattice defects at a certain ratio, and since the size of the lattice defect sites is small, Li +Metal ions with a larger diameter than [description] do not conduct. For example, in a solid electrolyte having a perovskite (ABO3) structure (A = Li, La, or a vacancy, B = Ti) such as LLTO, some A sites have vacancies (A site defects). And, as will be described in the lithium isotope enrichment method below, Li + enters the A site defects, and Li + moves between neighboring A site defects. Hereinafter, a site where Li can exist such as the A site is called a Li site, and a Li site having a vacancy is called a Li site defect.

[0019] The first electrode 31 and the second electrode 32 are a pair of electrodes provided to contact each surface of the electrolyte membrane 2 and apply a voltage from both sides. The first electrode 31 and the second electrode 32 have a porous structure such as a mesh so that while applying a voltage over a wide range of the electrolyte membrane 2, an aqueous solution ASi, ASo contacts a sufficient area of each surface of the electrolyte membrane 2. The first electrode 31 is provided on the surface of the electrolyte membrane 2 on the supply tank 11 side (hereinafter, appropriately referred to as the surface), has catalytic activity and electronic conductivity for the reaction of the following formula (1), and is an electrode material stable even when a voltage is applied in the Li-containing aqueous solution ASi, for example, formed of platinum (Pt). The second electrode 32 is provided on the surface of the electrolyte membrane 2 on the recovery tank 12 side (hereinafter, appropriately referred to as the back surface), has catalytic activity and electronic conductivity for the reaction of the following formula (2), and when the reaction proceeds to contain Li + and becomes 6 it is an electrode material stable even when a voltage is applied in the Li recovery aqueous solution ASo, for example, formed of Pt.

Chemical formula

[0020] The power supply device 5 is a DC power supply device, the positive electrode is connected to the first electrode 31, and the negative electrode is connected to the second electrode 32 to apply a predetermined voltage V.

[0021] The cooling device 6 is provided to set the electrolyte membrane 2 to a predetermined temperature, and the Li-containing aqueous solution ASi or 6Cool the electrolyte membrane 2 through the aqueous solution ASo for Li recovery. The cooling device 6 can apply a known device for cooling a liquid and preferably has a temperature adjustment function. In the present embodiment, the cooling device 6 is of a plunge type (immersion type), and a pipe (refrigerant pipe) through which the refrigerant flows is immersed in the Li-containing aqueous solution ASi in the supply tank 11. The cooling device 6 only needs to be able to set the electrolyte membrane 2 to a predetermined temperature, and it is not necessary to make the Li-containing aqueous solution ASi and 6 the aqueous solution ASo for Li recovery have a uniform liquid temperature. However, depending on the volume of the treatment tank 1 and the like, a stirring device may be provided. Details of the temperature of the electrolyte membrane 2 will be described later, but it is 30°C or lower, and for example, 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 (electrodialysis start) of the lithium isotope enrichment device 10, it is set to 0°C or higher. The temperature of the electrolyte membrane 2 can be measured by substituting the liquid temperature of the Li-containing aqueous solution ASi or 6 the aqueous solution ASo for Li recovery. The refrigerant pipe of the cooling device 6 is made of a material that does not deteriorate such as corrosion even when it comes into contact with the Li-containing aqueous solution ASi and 6 the aqueous solution ASo for Li recovery, and its shape is not particularly specified. For example, in order to efficiently cool the electrolyte membrane 2, the refrigerant pipe meanders in a planar shape according to the dimensions of the plate-shaped electrolyte membrane 2 and is installed so as to face in the vicinity over a wide area of the electrolyte membrane 2. Also, depending on the thickness of the electrolyte membrane 2 and the like, the refrigerant pipe may be put into both the supply tank 11 and the recovery tank 12. Further, the cooling device 6 may be configured such that the treatment tank 1 has a double structure (jacket tank) and a refrigerant flows through its interior (jacket portion). Alternatively, it may be configured to circulate the Li-containing aqueous solution ASi or 6 the aqueous solution ASo for Li recovery outside the treatment tank 1 with a pump and cool it with a heat exchanger.

[0022] The Li-containing aqueous solution ASi is a Li source, and 7 Li and 6 the cations of Li 7 Li + , 6 Li +An aqueous solution containing, for example, an aqueous solution of lithium hydroxide (LiOH). At the start of operation of the lithium isotope enrichment apparatus 10, 7 Li + , 6 Li + is contained in the natural abundance ratio. 6 The aqueous solution ASo for Li recovery is lithium ions Li recovered from the Li-containing aqueous solution ASi + , in particular, 6 Li + is an aqueous solution for accommodating a large amount of, and at the start of operation of the lithium isotope enrichment apparatus 10, it is, for example, pure water. In this specification, 7 Li and 6 Li( 7 Li + and 6 Li + ) are collectively referred to as Li (Li + ) when not distinguishing each other.

[0023] 〔Lithium Isotope Enrichment Method〕 The lithium isotope enrichment method according to an embodiment of the present invention will be described. First, with reference to FIG. 2, the electrodialysis of lithium ions by the lithium isotope enrichment apparatus according to the embodiment will be described. In the lithium isotope enrichment apparatus 10 shown in FIG. 2, the cooling device 6 is omitted.

[0024] In the lithium isotope enrichment apparatus 10, when the power supply device 5 applies a positive voltage +V to the first electrode 31 with respect to the second electrode 32, in the vicinity of the first electrode 31, hydroxide ions (OH - ) in the Li-containing aqueous solution ASi cause the reaction of the following formula (3), releasing electrons e - to the first electrode 31 and generating water (H2O) and oxygen (O2). In the Li-containing aqueous solution ASi, as OH - decreases, in order to maintain the charge balance, the reaction of the following formula (4) in which Li + in the Li-containing aqueous solution ASi moves into the electrolyte membrane 2 occurs in the vicinity of the electrolyte membrane 2. Combining the reactions of the following formula (3) and the following formula (4), the reaction of the following formula (1) occurs in the vicinity of the first electrode 31. On the other hand, in the vicinity of the second electrode 32,6 In the aqueous solution ASo for Li recovery, H2O of the aqueous solution ASo supplies electrons e - to cause the reaction of the following formula (5), generating hydrogen (H2) and OH - . 6 In the aqueous solution ASo for Li recovery, as OH - increases, in order to maintain the charge balance, the reaction of the following formula (6) occurs near the electrolyte membrane 2 where Li + in the electrolyte membrane 2 moves. Combining the reactions of the following formula (5) and the following formula (6), the reaction of the following formula (2) occurs near the second electrode 32. [Chemical formula]

[0025] When these reactions occur, due to the electrochemical potential differences of Li 6 contained in each of the Li-containing aqueous solution ASi, the electrolyte membrane 2 (electrolyte), and + the aqueous solution ASo for Li recovery (Li + (ASi), Li + (electrolyte), Li + (ASo)), Li in the Li-containing aqueous solution ASi permeates through the electrolyte membrane 2 and + moves to the aqueous solution ASo for Li recovery. These reactions are faster as the amount of movement of each electron e 6 per unit time from the Li-containing aqueous solution ASi to the first electrode 31 and from the second electrode 32 to the aqueous solution ASo for Li recovery is larger. Therefore, the larger the voltage +V, the larger the amount (mobility) of Li 6 per unit time moving from the first electrode 31 side to the second electrode 32 side through the electrolyte membrane 2. However, actually, when the voltage +V becomes larger than a certain level, the electrolyte membrane 2 starts to conduct electrons e - as well. Therefore, although the mobility of Li + continues to increase, its voltage dependence decreases. Here, Li - + + ​​The behavior of the electrolyte membrane 2 when it permeates the electrolyte membrane 2 will be described in detail with reference to Figs. 3A to 3C. Figs. 3A to 3C 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 them.

[0026] When no voltage is applied, as shown in Figure 3A, 7 Li + , 6 Li + The Li-containing aqueous solution ASi floats in the Li-containing aqueous solution ASi, and alternately repeats adsorption to and desorption from the surface of the electrolyte membrane 2. From this state, as shown in FIG. 3B, a voltage of + is applied to the first electrode 31, and a voltage of - is applied to the second electrode 32. 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 + ( 7 Li + , 6 Li + ) is dissolved in the electrolyte membrane 2 as the reaction of formula (4). At this time, the Li adsorbed near the Li site defect on the surface of the electrolyte membrane 2 is dissolved in the electrolyte membrane 2 as the reaction of formula (4). + Since the electrolyte membrane 2 has a potential gradient in which 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 jump (hop) to the Li site defects in the vicinity of the deeper part of the electrolyte membrane 2. + The electrons migrate from the Li site defects of the electrolyte membrane 2 to the neighboring Li site defects repeatedly, and finally, as shown in FIG. 3C, the electrons migrate from the Li site defects on the back surface to the neighboring Li site defects in the electrolyte membrane 2 as shown in the reaction of formula (6). 6 The mixture is transferred to the aqueous solution ASo for Li recovery.

[0027] In addition, on the surface of the electrolyte membrane 2, Li adsorbed near the Li site defect +As a result of the migration of Li to the depths of the electrolyte membrane 2, another Li + The Li-containing aqueous solution ASi may migrate and penetrate, or new Li may be generated from the aqueous solution ASi. + These Li + Similarly, Li moves through the electrolyte membrane 2. + The Li site defect is converted to Li + The Li site defects newly generated on the surface of the electrolyte membrane 2 are filled with Li and vacant again, and the Li adsorbed on the surface is transported through the newly generated Li site defects on the surface of the electrolyte membrane 2. + can start moving to the back side.

[0028] Li + The inter-site migration (hopping) of the ion in the electrolyte membrane 2 will be described in more detail with reference to FIG. 4. FIG. 4 shows a model for explaining ion conduction in an electrolyte, where x is the position in the thickness direction of the electrolyte membrane 2, E p indicates potential energy. In the electrolyte membrane 2, Li + ( 7 Li + , 6 Li + ) is stable at the Li site where the potential energy is minimal, 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 times the energy barrier between the sites, E m It can move by jumping over (hopping) (E a =E D / 2+E m , E D : defect formation energy). Also, it can be assumed that the ion is thermally vibrating at the position of the potential energy minimum with a frequency of Γ0, and 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 is 7times (√(7 / 6)) of Li. Specifically, as will be described later, the average migration speed in the electrolyte membrane 2 is 7 times (√(7 / 6)) higher than that of Li. Also, for example, for a certain Li site defect in the electrolyte membrane 2, at two equidistant locations in the vicinity, 7 Li + and 6 Li + exist, 6 Li + is presumed to jump preferentially.

[0029] Also, at the Li site in the electrolyte membrane 2, that is, in the ground state, 7 Li + , 6 Li + has a potential energy that is higher by the amount of the zero-point vibration hω I . The zero-point vibration hω I depends on the isotope, 7 Li + is larger than 6 Li + . Similarly, in the excited state, the zero-point vibration hω S is 6 Li + is larger. Therefore, 7 Li + with a smaller mass than 6 Li + has a higher potential energy considering the zero-point vibrations hω I , hω S in both the ground state and the excited state. However, 6 Li + has a smaller energy barrier E m which is the potential energy difference between these excited state and ground state (E m ( 7 Li + ) > E m ( 6 Li + ), that is, the activation energy E a is small. As a result, 6 Li + receives less energy than 7 Li +Even if it is smaller, it can hop. Li + Li has more surplus with respect to the activation energy E in the received energy a , the more the mobility μ, that is, the amount of movement per unit time increases. Therefore, the larger the applied voltage +V, the higher the mobility of Li + . Also 6 Li + has 7 Li + at a smaller voltage +V, the surplus energy with respect to the activation energy E becomes equal, and at this time a Li 6 has a higher mobility μ by the ratio of the frequency Γ0. Fig. 5 shows the time per unit time by simulation + of 6 Li + , 7 Li + the amount of movement and the applied voltage dependence of the isotope ratio of the moving Li + . The simulation approximated the distribution of the activation energy E following the Maxwell-Boltzmann distribution by a normal distribution. Specifically a for each of Li 6 Li + , 7 Li + , for each received energy, the ratio of Li a ( 6 Li + < 7 Li + exceeding the activation energy E + was calculated from the probability density of the normal distribution having the activation energy E a as the average value, and the relative value of the mobility μ was obtained by multiplying the cumulative value by the ratio of the frequency Γ0. The isotope ratio was calculated assuming that the abundance ratios of Li 7 before movement + and 6 Li + were 1:1

[0030] As shown in Fig. 5 6 Li + , 7 Li +The mobility increases in an S-shaped curve from 0 as the applied voltage +V increases, 7 Li + for which the activation energy E a is small 6 Li + shifts and changes to the lower voltage +V side and is also higher by the ratio of the frequency Γ0. Note that limμ in Fig. 5 represents the limit of the 6 Li + amount of movement per unit time. Therefore, the smaller the applied voltage +V in the electrolyte membrane 2 within the 6 Li + moving range, the 6 Li + more 7 Li + moves with respect to 6 Li + . 7 Li + When the mobilities of 6 Li + , 7 Li + converge respectively, the difference between them becomes smaller and the isotope ratio converges to (√(7 / 6)) / (1 + √(7 / 6)). However, in reality, when the voltage +V is increased, 6 Li + , 7 Li + before the mobilities reach the limit as shown in Fig. 5, it is considered that the voltage dependence significantly decreases. Specifically, as described above, when the voltage applied between both sides of the electrolyte membrane 2 becomes larger than a certain value, a part of the transition metal ions constituting the electrolyte membrane 2 is reduced (for example, if the electrolyte membrane 2 is LLTO, Ti 4+ + e - → Ti 3+ ), and the electrolyte membrane 2 starts to conduct electrons e - from the recovery tank 12 side to the supply tank 11 side. As a result, most of the given electrical energy is consumed for the conduction of electrons e - , so the voltage dependence of the mobility of Li + decreases, and the energy efficiency in the movement of Li + decreases. Furthermore, when a part of the transition metal ions constituting the electrolyte membrane 2 is reduced, the ionic radius of this reduced ion becomes larger (for example, if the electrolyte membrane 2 is LLTO, Ti 4+ < Ti3+ )), Li + The bottleneck for the movement of Li spreads, so the Li + of 6 isotope ratio will decrease rapidly. Also, since the temperature of the electrolyte membrane 2 increases due to the Joule heat generated by the electrons e - conducted through the electrolyte membrane 2, as will be described later, the Li + of 6 isotope ratio will decrease. Specifically, although it depends on the electronic conductivity of the electrolyte membrane 2, etc., when a voltage above 2.0 V is applied, electronic conductivity can appear in the electrolyte membrane 2. Therefore, the voltage +V is preferably 2.0 V or less. Furthermore, since the isotope separation coefficient increases as the applied voltage becomes smaller, the voltage +V is more preferably 1.5 V or less, and even more preferably 1.0 V or less. On the other hand, if Li + can be moved in the electrolyte membrane 2, the lower limit of the voltage +V is not particularly defined. For example, although it depends on the electronic conductivity of the electrolyte membrane 2 and the hydrogen ion concentration of the aqueous solutions ASi and ASo, etc., it is preferably set in the range of 0.5 V or more. However, since the mobility μ of Li + decreases as the applied voltage +V becomes smaller, it is preferable that the productivity does not decrease too much.

[0031] Also, the ion mobility μ has the relationship with the diffusion coefficient D of the ion as shown in the following formula (7) (T: temperature (K), k: Boltzmann constant). The diffusion coefficient D is proportional to the hopping rate Γ as shown in the following formula (8) (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). Also, the frequency factor Γ0 in formula (8) is proportional to the temperature T as shown in the following formula (9), and also, since (Z s vib / Z I vib ) is inversely proportional to the square root of the mass number m, the frequency factor Γ0 is inversely proportional to the square root of the mass number m (h: Planck constant, Z s vib : phonon distribution function at the saddle point, Z I vib: Phonon distribution function in the initial state, C1: constant). From equations (8) and (9), the diffusion coefficient D is expressed by the following equation (10). And 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, the mass number m and the activation energy E a with small 6 Li + is higher. And the ion mobility μ depends on the temperature T, and the degree of this dependence is affected by the activation energy E a .

Number

[0032] Based on equation (11), regarding the temperature dependence, the relative 7 Li + , 6 Li + amount of movement and the movement Li + isotope ratio of are calculated and shown in Figure 6A. The activation energy E a is 7 Li + is calculated as 0.30 eV, 6 Li + is calculated as 0.25 eV. The isotope ratio is calculated with the abundance ratio of 7 Li + and 6 Li + before movement being 1:1 as in the simulation of the voltage dependence. As shown in Figure 6A, 7 Li + , 6 Li + mobilities each increase exponentially as the temperature increases, but a Li 7 with a large activation energy E + has a greater temperature dependence. Therefore, as the temperature increases, the ratio of 7 Li + with low mobility to 6 Li + with high mobility decreases. Specifically, as shown in Figure 6A,6 The Li isotope ratio sharply decreases from near 1 in the low-temperature range where the mobility is extremely low, and then gradually decreases while becoming gentle as the temperature increases, converging to (√(7 / 6)) / (1 + √(7 / 6)). Thus, the lower the temperature, the higher the proportion of mobile Li + in 6 Li + is high.

[0033] The applicable temperature range in this embodiment is above the freezing point and below the boiling point of the aqueous solutions ASi and ASo, 6 and is 0 to 100°C when the aqueous solution ASo for Li recovery is pure water at the start of electrodialysis. Figure 6B shows an enlarged view at -50 to 50°C in Figure 6A. As shown in Figure 6B, in the temperature range applicable in this embodiment, 6 the temperature dependence of the Li isotope ratio is approximately linear and increases as the temperature decreases. From this, 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 still more preferably 5°C or lower.

[0034] In the lithium isotope enrichment method according to this embodiment, as the voltage application time elapses, the Li + in 6 Li isotope ratio ( 6 Li / ( 7 Li + 6 Li)) of the Li remaining in the Li-containing aqueous solution ASi decreases, so the Li isotope ratio of the newly moving Li + in 6 decreases. Therefore, 6 to more efficiently concentrate Li, for example, 6 when the aqueous solution ASo for Li recovery reaches a predetermined Li + concentration or when a predetermined voltage application time has elapsed, the Li-containing aqueous solution ASi in the supply tank 11 may be replaced. Also, in order to make the isotope separation coefficient larger, the voltage +V may be intermittently applied or alternately applied with the reverse voltage -V′ (V > V′) so as to repeat the state where the isotope separation coefficient is large immediately after the start of voltage application (operation) (see Patent Document 2 and Non-Patent Document 1). After the electrodialysis is completed, 6For the aqueous solution ASo for Li recovery, for example, after evaporating moisture as necessary to concentrate Li, lithium carbonate (Li2CO3) is generated by carbon dioxide (CO2) bubbling or the like and precipitated, 6 Li can be recovered. Alternatively, after generating lithium carbonate, further by cooling or evaporating moisture to form a supersaturated state to generate lithium hydroxide (LiOH) and precipitate it, 6 Li can also be recovered.

[0035] (Modified Example) In the lithium isotope enrichment method according to this embodiment, the lower the temperature of the electrolyte membrane 2, 6 the higher the Li isotope ratio can be increased for recovery (see FIGS. 6A and 6B), but it is necessary to cool the Li-containing aqueous solution ASi and 6 the aqueous solution ASo for Li recovery above its freezing point so that they do not freeze. Therefore, the aqueous solutions ASi, ASo, especially 6 the aqueous solution ASo for Li recovery may contain a solute that does not permeate the electrolyte membrane 2 so that its freezing point drops below 0°C. Such a solute should not corrode the electrolyte membrane 2, electrodes 31, 32, etc. when the aqueous solutions ASi, ASo containing it. Specifically, salts such as sodium chloride (NaCl, table salt), magnesium chloride (MgCl2), calcium chloride (CaCl2), potassium chloride (KCl), or organic solvents such as ethylene glycol can be mentioned. As described above, after the electrodialysis is completed, 6 from the aqueous solution ASo for Li recovery 6 When bubbling carbon dioxide to recover Li, sodium chloride, which does not generate precipitates (carbonates) other than lithium carbonate and has a large freezing point depression, is particularly preferable. Note that, 6 when the aqueous solution ASo for Li recovery is concentrated by evaporating moisture before bubbling carbon dioxide, it is preferable to remove the salts precipitated due to the decrease in moisture by a general method such as filtration before bubbling. Alternatively, the recovered 6Regarding the aqueous solution ASo for Li recovery, before bubbling carbon dioxide gas, ordinary electrodialysis or the like (for example, refer to JP-A-2019-141807) 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 lower than 0 °C, 6 the Li isotope ratio can be further increased and concentrated efficiently.

[0036] 〔Multi-stage lithium isotope enrichment apparatus〕 The lithium isotope enrichment apparatus 10 according to the present invention is for the Li-containing aqueous solution ASi in the supply tank 11 6 an aqueous solution containing Li with a high Li isotope ratio ( 6 the aqueous solution ASo for Li recovery) is obtained in the recovery tank 12. Therefore, after this Li recovery 6 the aqueous solution ASo for Li recovery is put into the emptied supply tank 11, whereby 6 an aqueous solution containing Li with a high Li isotope ratio is obtained. Therefore, as shown in FIG. 7, by adopting a cascade structure in which the recovery tank 12 of the lithium isotope enrichment apparatus 10 is connected so as to be integrated with the supply tank 11 of another lithium isotope enrichment apparatus 10, stepwise 6Li can be concentrated. Such a multi-stage lithium isotope concentrator 20 includes a processing tank 1A, four electrolyte membranes (lithium ion conductive electrolyte membranes) 2 arranged in parallel at intervals so as to partition the processing tank 1A into five tanks 11, 12, 13, 14, 15 in one direction, a first electrode 31 and a second electrode 32 (electrodes) coated on each surface of the electrolyte membrane 2, a power supply device 50, and a cooling device 6A. The power supply device 50 includes the power supply device 5 (see FIG. 1) of the lithium isotope concentrator 10 connected between the electrodes 31 and 32 on both sides of each electrolyte membrane 2, and is configured to connect adjacent power supply devices 5 in series. When applying a voltage intermittently, it is preferable to synchronize all the power supply devices 5. In the multi-stage lithium isotope concentrator 20, the second electrode 32 and the first electrode 31 provided on the respective opposing surfaces of two adjacent electrolyte membranes 2 are connected by a conductor and short-circuited. The cooling device 6A includes two refrigerant pipes introduced into the tanks 12 and 14 to cool the four electrolyte membranes 2. If all the electrolyte membranes 2 can be cooled to a predetermined temperature, the number of refrigerant pipes and the configuration of the cooling device 6A are not limited. Other elements are as described in the configuration of the lithium isotope concentrator 10. That is, the multi-stage lithium isotope concentrator 20 has a structure in which four lithium isotope concentrators 10 are connected so that their respective processing tanks 1 are integrated into the processing tank 1A, and one recovery tank 12 of two adjacent lithium isotope concentrators 10, 10 is also used as the supply tank 11 of the other.

[0037] The method for concentrating lithium isotopes by the multi-stage lithium isotope concentrator 20 is the same as the method by the lithium isotope concentrator 10. In the figure, an aqueous Li-containing solution ASi containing 7 Li, 6 Li in the natural abundance ratio is introduced into the supply tank 11 at the left end, and pure water is introduced into the other tanks 12, 13, 14, 15. By applying a positive voltage +V to each of the plurality of power supplies of the power supply device 50, + Li moves from left to right in the figure, and the pure water in each of the tanks 12, 13, 14, 15 becomes 6 aqueous solutions AS1, AS2, AS3, ASo containing Li with different Li isotope ratios at different concentrations. 6The Li isotope ratio increases in the order of ASi < AS1 < AS2 < AS3 < ASo. Therefore, even if the isotope separation coefficient due to the movement in one electrolyte membrane 2 of Li + is not large, 6 Li with a high Li isotope ratio can be recovered. Thus, it is not necessary to extremely reduce the Li + mobility, and the productivity can be increased.

[0038] In the multi-stage lithium isotope enrichment device 20, the number of the first electrode 31 and the second electrode 32 provided for each electrolyte membrane 2 is not particularly defined. The more there are, that is, the more the lithium isotope enrichment devices 10 are connected, 6 the Li with a high Li isotope ratio can be recovered. Further, in FIG. 7, the lithium isotope enrichment devices 10 are connected in one direction, and all the adjacent electrolyte membranes 2, 2 are arranged facing each other. However, for example, they may be connected by bending at 90° at one or two places so that the adjacent electrolyte membranes 2, 2 are arranged perpendicular to each other.

Example

[0039] As described above, the lithium isotope enrichment device and the lithium isotope enrichment method according to the present invention have been described in terms of the mode for carrying out the present invention. Hereinafter, examples in which the effects of the present invention have been confirmed will be described. It should be noted that the present invention is not limited to this example and the above-described mode, and it goes without saying that various changes and modifications based on these descriptions are also included in the gist of the present invention.

[0040] Regarding the lithium isotope enrichment device according to the embodiment of the present invention shown in FIG. 1, the voltage application conditions were changed, and the change amount of the lithium isotope ratio was measured.

[0041] (Fabrication of lithium isotope enrichment device) The lithium isotope enrichment device uses, as the electrolyte membrane, a plate-shaped La with a size of 50 mm × 50 mm and a thickness of 0.5 mm 0.57 Li 0.29TiO3 (lithium ion conductive ceramic LLTO, manufactured by Toho Titanium Co., Ltd.) was used. Comb-shaped electrodes with a thickness of 10 μm, a width of 0.5 mm, and a spacing of 0.5 mm were formed as the first electrode and the second electrode in a 19.5 mm × 20.5 mm region at the center of each side of this electrolyte membrane. Further, lead wires for connecting to a power supply and connecting to this electrode were formed. The first electrode, the second electrode, and the lead wires were formed by screen-printing Pt paste on the surface of the electrolyte membrane and firing at 900 °C for 1 h in the atmosphere. The electrolyte membrane on which the electrodes and the like were formed was placed in a treatment tank made of an acrylic plate, partitioned into a supply tank and a recovery tank, and the treatment tank was housed in a thermostatic bath having a temperature adjustment function to form a lithium isotope enrichment device.

[0042] As an Li-containing aqueous solution in the supply tank of the lithium isotope enrichment device, 7 Li: 92.23 mol%, 6 a 1 mol / l lithium hydroxide aqueous solution containing Li at Li: 7.77 mol% was placed in the recovery tank 6 Pure water as an aqueous solution for Li recovery was poured in 150 ml portions so that the first electrode and the second electrode were completely immersed. Then, the liquid temperatures of the lithium hydroxide aqueous solution and the pure water in the treatment tank were adjusted to 20 °C.

[0043] A power supply device was connected to the first electrode and the second electrode. After adjusting to a predetermined liquid temperature and allowing 12 hours or more to pass, 2.0 V was applied for 3600 seconds with the first electrode as the positive electrode. While the voltage was being applied, the current value was measured with an ammeter connected in series to the power supply device, and the aqueous solutions in the supply tank and the recovery tank were stirred respectively. After the voltage application, the aqueous solution in the recovery tank was recovered, and the amount of 7 Li, 6 Li in the aqueous solution was measured with an inductively coupled plasma mass spectrometry (ICP-MS) device (Elan drc-e, manufactured by PerkinElmer Co., Ltd.). The supply tank and the recovery tank were newly replaced, the liquid temperature was changed, and 2.0 V was similarly applied for 3600 seconds in each case. The liquid temperature was changed in 5 °C increments from 20 to 50 °C and also adjusted to 0 °C. Further, at a liquid temperature of 20 °C, the voltage was applied in 0.25 V increments from 1.0 to 2.0 V and also at 0.5 V, each for 3600 seconds in the same manner. After each voltage application, similarly, in the aqueous solution recovered from the recovery tank7 Li 6 The amount of Li was measured, 7 Li, 6 from the amount of Li, 6 the Li isotope separation coefficient was calculated. 6 The Li isotope separation coefficient is defined as (the molar ratio of Li / 6 Li in the aqueous solution in the recovery tank after voltage application) / (the molar ratio of Li / 7 Li in the lithium hydroxide aqueous solution in the supply tank before voltage application). Graphs showing the temperature dependence of the Li 6 movement amount (white circles: ○) and the Li isotope separation coefficient (black circles: ●) for an application time of 3600 seconds are shown in Fig. 8 and the graph showing the applied voltage dependence is shown in Fig. 9, respectively. 7 6 Li + movement amount (white circles: ○) and 6 the graph of the temperature dependence of the Li isotope separation coefficient (black circles: ●) is shown in Fig. 8, and the graph of the applied voltage dependence is shown in Fig. 9, respectively.

[0044] As shown in Fig. 8, the lower the temperature, 6 the higher the Li isotope separation coefficient. On the other hand, 6 Li + the movement amount decreased, but even at 0 °C it remained at about 1 / 2 of that at 25 °C, indicating that a sufficient amount could be recovered. Also, as shown in Fig. 9, the lower the applied voltage, 6 the higher the Li isotope separation coefficient, but 6 Li + the movement amount decreased and was extremely small at 0.5 V. At 20 °C and 1.25 V, 6 the Li isotope separation coefficient is equivalent to that at 0 °C and 2.0 V, but 6 Li + the movement amount is about 1 / 2, and the efficiency was lower compared to Li isotope enrichment by cooling. From these facts, it can be said that by combining cooling and voltage control, 6 the efficiency can be improved while increasing the Li isotope separation coefficient. 6

Explanation of symbols

[0045] 10 Lithium isotope enrichment device 20 Multi-stage lithium isotope enrichment device 1,1A Treatment tank 11 Supply tank (first tank) ​​12 Recovery tank (second tank) 2 Electrolyte membrane (lithium ion conductive electrolyte membrane) 31 First electrode (electrode) 32 Second electrode (electrode) 5 Power supply device 50 Power supply device 6,6A Cooling device Aqueous solution containing Si and Li ASo 6 Aqueous solution for Li recovery

Claims

1. A lithium isotope enrichment apparatus including a processing tank partitioned into a first tank and a second tank, recovering, in the second tank, an aqueous solution containing lithium ions having a higher lithium isotope ratio than that of the aqueous solution from an aqueous solution containing Li and Li in a lithium ion state and accommodated in the first tank, wherein: ６ Li and ７ Li in a lithium ion state, and ６ recovering, in the second tank, an aqueous solution containing lithium ions having a higher lithium isotope ratio than that of the aqueous solution. A lithium isotope enrichment device, comprising: a lithium ion conductive electrolyte membrane that partitions the treatment tank; electrodes with a porous structure provided in contact with each of the two surfaces of the lithium ion conductive electrolyte membrane; a power supply device that applies a voltage between the electrodes; and a cooling device that cools the lithium ion conductive electrolyte membrane.

2. The lithium isotope enrichment device according to claim 1, wherein the power supply device intermittently applies a voltage.

3. The lithium isotope enrichment device according to claim 1, wherein the power supply device alternately applies positive and negative voltages.

4. The lithium isotope enrichment device according to any one of claims 1 to 3, wherein the cooling device sets the temperature of the lithium ion conductive electrolyte membrane to 20°C or lower.

5. The lithium isotope enrichment device according to any one of claims 1 to 4, wherein the cooling device cools an aqueous solution accommodated in at least one of the first tank and the second tank.

6. A multi-stage lithium isotope enrichment device comprising two or more lithium isotope enrichment devices according to any one of claims 1 to 5, connected such that the treatment tanks are integrated, wherein the lithium ion conductive electrolyte membranes of the respective lithium isotope enrichment devices are arranged spaced apart from each other so as to partition the integrated treatment tank into three or more tanks, wherein one of the second tanks of two adjacent lithium isotope enrichment devices also serves as the first tank of the other, and the cooling device cools an aqueous solution accommodated in at least one of the partitioned treatment tanks.

7. In a treatment tank partitioned into a first tank and a second tank by a lithium ion conductive electrolyte membrane, an aqueous solution containing Li and Li in a lithium ion state and stored in the first tank, ６ Li and ７ Li is recovered from the aqueous solution in the second tank as an aqueous solution containing lithium ions having a higher isotope ratio of Li than that of the aqueous solution. A method for concentrating lithium isotopes, ６ wherein A lithium isotope enrichment method, characterized in that while cooling the lithium ion conductive electrolyte membrane to 20°C or lower, a voltage is applied between the electrodes with a porous structure provided in contact with each of the two surfaces of the lithium ion conductive electrolyte membrane, with the first tank side being positive.

8. The aqueous solutions accommodated in the first tank and the second tank each have a freezing point of less than 0°C, and the lithium isotope enrichment method according to claim 7, characterized in that the voltage is applied while cooling the lithium ion conductive electrolyte membrane to a temperature above the freezing point and less than 0°C.

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