Method for separating lithium salt by using bipolar membrane

Abstract

The invention relates to a method for separating lithium salt by using a bipolar membrane, which comprises the following steps: putting a lithium salt aqueous solution into a salt chamber between an anion exchange membrane and a cation exchange membrane which are adjacent in a bipolar electroosmosis device, and putting water into an acid chamber between the bipolar membrane and the anion exchange membrane which are adjacent, and a step of feeding water into the alkali chamber between the adjacent bipolar membrane and cation exchange membrane, and a step of applying a current to the bipolar electroosmosis device to obtain an aqueous solution of lithium hydroxide and simultaneously obtain an aqueous solution of acid in the form of a by-product, by controlling the concentration of the aqueous solution of lithium hydroxide discharged from the alkali chamber or the concentration of the aqueous solution of acid discharged from the acid chamber, the current efficiency of the entire process is improved.

Description

Technical Field

[0001] This embodiment relates to a method for separating lithium salts using bipolar membranes. Specifically, it is a method for separating lithium salts using bipolar membranes with improved current efficiency. Background Art

[0002] Lithium sulfate is decomposed into lithium hydroxide and sulfuric acid by electric energy through an ion exchange membrane. However, the higher the concentration, the more likely it is to re-pollute through the membrane. In this way, if re-pollution occurs, the decomposed substances will be lost, resulting in a decrease in current efficiency and thus a decline in economy.

[0003] H + ions in concentrated sulfuric acid diffuse into the salt chamber through an anion exchange membrane (AEM), while OH- ions in concentrated LiOH diffuse into the salt chamber through a cation exchange membrane (CEM). As the concentrations in the acid chamber and the base chamber increase, their diffusion also increases. On the other hand, their diffusion reduces the overall resistance of the bipolar electrodialysis (BPED) system, making it easier to maintain the maximum constant current. However, by reducing the voltage applied to the membrane, it may also reduce the amount of substances decomposed through the membrane.

[0004] In addition, SO42- ions in the acid chamber migrate to the base chamber through bipolar membranes (BPM) under the action of electrostatic force, resulting in an increase in the S impurity content in LiOH. The content of S impurities passing through the BPM shows a trend of increasing with the increase in the voltage applied to the BPM. As described above, according to the concentrations in the acid chamber and the base chamber, the total amount of H + and OH- ions diffusing through the membrane will reduce the current efficiency due to the total amount of decomposed substances being diluted by diffusion, and the resistance change will be affected by the change in the voltage applied in the solution chamber and the change in the impurity content.

[0005] Therefore, it is necessary to develop a method for separating lithium salts that can improve the current efficiency while reducing impurities. Summary of the Invention

[0006] Technical Problem

[0007] The present invention aims to provide a method for separating lithium salts using bipolar membranes with improved current efficiency while reducing the generation of impurities.

[0008] Technical Solution

[0009] A method for separating lithium salts using a bipolar membrane according to an embodiment of the present invention includes: putting an aqueous lithium salt solution into a salt chamber between an adjacent anion exchange membrane and a cation exchange membrane in a bipolar electro-osmosis device, putting water into an acid chamber between an adjacent bipolar membrane and the anion exchange membrane, and putting water into an alkali chamber between the adjacent bipolar membrane and the cation exchange membrane; and applying an electric current to the bipolar electro-osmosis device to obtain an aqueous lithium hydroxide solution and obtain an aqueous acid solution as a by-product.

[0010] By controlling the concentration of the aqueous lithium hydroxide solution discharged from the alkali chamber or the concentration of the aqueous acid solution discharged from the acid chamber, the current efficiency of the entire process is increased.

[0011] The control of the concentration of the aqueous lithium hydroxide solution discharged from the alkali chamber is controlled by measuring the pH value of the aqueous lithium hydroxide solution discharged from the alkali chamber.

[0012] The control of the concentration of the aqueous acid solution discharged from the acid chamber is controlled by measuring the pH value of the aqueous acid solution discharged from the acid chamber.

[0013] The current efficiency and the ionic conductivity of the aqueous lithium hydroxide solution discharged from the alkali chamber or the ionic conductivity of the aqueous acid solution discharged from the acid chamber satisfy the following relationship.

[0014] (Efficiency, %) = -A x ln(S LiOH ) + B

[0015] (Here, efficiency (%) is the current efficiency, S M is the ionic conductivity of the discharged aqueous alkali solution or aqueous acid solution, and -50 < A < -10, 50 < B < 250.).

[0016] The electrodialysis device is composed of a unit pair stack layer formed by arranging a bipolar membrane; an auxiliary gasket; an anion exchange membrane; an auxiliary gasket; a cation exchange membrane; and an auxiliary gasket in sequence, and the last bipolar membrane is in contact with the auxiliary gasket and the metal electrode.

[0017] The step of putting water into the acid chamber between the adjacent bipolar membrane and the anion exchange membrane is to put the aqueous acid solution discharged from the acid chamber after further mixing.

[0018] The step of putting water into the alkali chamber between the adjacent bipolar membrane and the cation exchange membrane is to put the aqueous lithium hydroxide solution discharged from the alkali chamber after further mixing.

[0019] Advantages of the Invention

[0020] According to an embodiment of the present invention, by controlling the concentration of the discharged aqueous solution, the current efficiency of the lithium salt separation process can be improved.

[0021] According to an embodiment of the present invention, by controlling the concentration of the discharged aqueous solution, the generation of impurities can be controlled, which has advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematically shows a bipolar electrodialysis device according to an embodiment of the present invention.

[0023] Figure 2 Schematically shows a method for separating lithium salts using a laboratory-scale electrodialysis device.

[0024] Figure 3 Shows the composition of an experimental device for separating lithium salts on an experimental scale.

[0025] Figure 4 Shows Figure 3 the control system interface of the lithium salt separation experimental device.

[0026] Figure 5 Shows the use of Figure 3 the current-voltage change curve of the experiment using the lithium salt separation experimental device in

[0027] Figure 6 Shows the analysis results of the change in current efficiency caused by the change in ionic conductivity in Example 5.

[0028] Figure 7 Shows the analysis results of the change in the content of S impurities in the alkaline aqueous solution generated according to the change in ionic conductivity in Example 5.

[0029] Figure 8 Shows the analysis results of the change in the content of lithium impurities in the acidic aqueous solution generated as the ionic conductivity changes in Example 5.

[0030] Figure 9 Shows the analysis results of the change in current efficiency with respect to the S concentration of the acidic aqueous solution produced and the Li concentration of the alkaline aqueous solution produced in Example 5.

[0031] Figure 10 Shows the analysis results of the change in the S impurity concentration in the alkaline aqueous solution produced according to the S concentration of the acidic aqueous solution produced and the Li concentration of the alkaline aqueous solution produced in Example 5.

[0032] Figure 11 Shows the analysis results of the change in the Li impurity concentration in the acidic aqueous solution produced according to the S concentration of the acidic aqueous solution produced and the Li concentration of the alkaline aqueous solution produced in Example 5.

[0033] Figure 12 Shows the analysis results of the change in current efficiency according to the change in the concentration of the produced aqueous acid solution and aqueous alkali solution.

[0034] Figure 13 Shows the concentration ratio of the LiOH solution and the analysis results of S impurities in the aqueous alkali solution.

[0035] Figure 14 Shows the correlation results of the lithium ion conductivity and current efficiency analyzed according to Example 5. Detailed Description of the Invention

[0036] The terms first, second, third, etc. are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another part, component, region, layer, and / or segment. Therefore, without departing from the scope of the present invention, the first part, component, region, layer, and / or segment described below may also be described as the second part, component, region, layer, and / or segment.

[0037] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. Unless otherwise clearly indicated to the contrary in the context, the singular forms used are also intended to include the plural forms. It should also be understood that the term "comprising" used in the specification may specifically refer to a certain property, field, integer, step, action, element, and / or component, but does not exclude the existence or addition of other properties, fields, integers, steps, actions, elements, and / or components.

[0038] Although not otherwise defined, the meanings of all terms (including technical terms and scientific terms) used herein are the same as those commonly understood by those of ordinary skill in the art to which the present invention pertains. For terms defined in a dictionary, they should be interpreted as having a meaning consistent with the relevant technical literature and the content disclosed herein, and should not be interpreted in an idealized or overly formal sense.

[0039] The embodiments of the present invention will be described in detail below. However, the content is only presented as an example and does not thereby limit the present invention, which is only defined by the scope of the following claims.

[0040] Figure 1 Schematically shows a bipolar electrodialysis device according to an embodiment of the present invention

[0041] Refer to Figure 1, according to an embodiment of the present invention, the bipolar electrodialysis device may include pairs 10, 20 that are repeatedly stacked between the positive electrode 910 and the negative electrode 920. The first pair 10 may sequentially include a bipolar membrane 210, an auxiliary gasket 311, an anion exchange membrane (AEM) 410, an auxiliary gasket 312, a cation exchange membrane (CEM) 510, and an auxiliary gasket 313. The first pair 10 may be arranged adjacent to the second pair 20 having the same structure. At this time, the auxiliary gasket 313 of the first pair 10 may be arranged adjacent to the bipolar membrane 220 of the second pair 20.

[0042] The bipolar membrane 210, the auxiliary gasket 311, and the anion exchange membrane 410 of the first pair 10 constitute an acid chamber 610. The anion exchange membrane 410, the auxiliary gasket 312, and the cation exchange membrane 510 constitute a salt chamber 710. In addition, the cation exchange membrane 510, the auxiliary gasket 313, and the bipolar membrane 220 may constitute an alkali chamber 810.

[0043] A high-concentration lithium salt aqueous solution is introduced into the salt chamber 710 of the first pair 10, a low-concentration acid or distilled water is introduced into the acid chamber 600, and a low-concentration lithium hydroxide aqueous solution or distilled water is introduced into the alkali chamber 800 to perform bipolar electrodialysis. At the same time, a high-concentration lithium salt aqueous solution is introduced into the second pair 20 to perform the same electrodialysis as the first pair 10.

[0044] In the bipolar electrodialysis device, water-splitting occurs on each bipolar membrane, and the cations and anions in the lithium salt move toward the negative electrode 920 and the positive electrode 910 due to the electrophoresis effect, respectively.

[0045] More specifically, the acid group meets the hydrogen generated by hydrolysis in the first bipolar membrane 210 on the positive electrode side and turns into an acid. The lithium ions that move to the negative electrode through the cation exchange membrane 510 react with the generated hydroxyl groups (OH-) in the second bipolar membrane 220 and turn into lithium hydroxide (LiOH).

[0046] As an example, the overall reaction formula may be as follows.

[0047]

[0048] At this time, the high-concentration lithium salt aqueous solution injected into the salt chamber 710 decomposes into lithium ions and acid groups and is discharged, and a partially remaining low-concentration lithium salt aqueous solution is generated and may be discharged outside the electrodialysis device.

[0049] A high-concentration acid is generated and discharged in the acid chamber 610, and lithium hydroxide is generated and discharged in the alkali chamber 810.

[0050] On the other hand, in the electrodialysis device, the maximum voltage restricted by the membrane supplier can be applied, or the current within the range of current restricted by the membrane supplier per unit membrane operating area can be applied. Specifically, when the internal resistance of the electrodialysis device is low, the maximum constant current is applied; when the internal resistance increases, the maximum constant voltage is applied for electrodialysis.

[0051] In addition, the H + ions of the concentrated sulfuric acid diffuse into the salt chamber 710 through the anion exchange membrane 410, and the OH− ions of the concentrated LiOH can diffuse into the salt chamber 710 through the cation exchange membrane 510. Their diffusion tendency increases with the increase in the concentrations of the acid chamber 610 and the alkali chamber 810. In addition, the SO42− ions in the acid chamber 620 of the second unit pair 20 move to the alkali chamber 810 of the first unit pair 10 under the action of electrostatic force through the second bipolar membrane 220, thereby increasing the S impurity content in LiOH. The S impurity content diffused through the bipolar membrane and generated in the alkali chamber increases with the increase in the voltage applied to the bipolar membrane.

[0052] In one embodiment of the present invention, the method for separating lithium salts using the bipolar electrodialysis device includes putting an aqueous lithium salt solution into the salt chamber between adjacent anion exchange membranes and cation exchange membranes in the bipolar electrodialysis device, putting water into the acid chamber between adjacent bipolar membranes and anion exchange membranes, and putting water into the alkali chamber between adjacent bipolar membranes and cation exchange membranes, and applying a current to the bipolar electrodialysis device to obtain an aqueous lithium hydroxide solution, and simultaneously obtaining an aqueous acid solution as a by-product.

[0053] As described above, the high-concentration aqueous lithium salt solution can be decomposed into lithium ions and acid radical ions in the salt chamber of the electrodialysis device, and the remaining low-concentration aqueous lithium salt solution is generated and then discharged to the outside.

[0054] The water or low-concentration aqueous acid solution can be converted into a high-concentration aqueous acid solution and discharged in the acid chamber of the electrodialysis device. The concentration, pH value, and ionic conductivity of the discharged high-concentration aqueous acid solution can be measured.

[0055] At the same time, the low-concentration aqueous lithium hydroxide solution can be converted into a high-concentration aqueous lithium hydroxide solution and discharged in the alkaline region of the electrodialysis device. The concentration, pH value, and ionic conductivity of the discharged high-concentration aqueous lithium hydroxide solution can be measured.

[0056] If the measured concentration, pH value, or ionic conductivity of the aqueous solution is higher than the target value of the present invention, the measured concentration, pH value, or ionic conductivity of the aqueous solution can be controlled within the target value range of the present invention by methods such as increasing the amount of water input.

[0057] In the present invention, the concentration or pH value or ionic conductivity of the target aqueous solution can be determined by the target value of the current efficiency having the economic efficiency of the lithium salt separation process in the present invention. Specifically, the current efficiency can be defined by considering the total amount of electric charge applied to the lithium salt separation system and the amount of electric charge used in the subsequent concentration of the lithium hydroxide aqueous solution, etc., and can be calculated by the following specific formula.

[0058] η = (LiOH conversion power consumption (conversion charge)) / (Charge consumption) = (△[LiOH] x △Mass) / (F x △Q)

[0059] Here, η is the current efficiency, F is the Faraday constant, △Q is the amount of electric charge supplied within the reference time, △[LiOH] is the change value of the concentration of the lithium hydroxide aqueous solution concentrated within the reference time, and △Mass is the change value of the amount of the lithium hydroxide aqueous solution within the reference time.

[0060] In the present invention, it is confirmed by experimental results that when the ionic conductivity of the discharged aqueous solution is kept low, the current efficiency increases, and the contents of S impurities in the discharged high-concentration lithium hydroxide aqueous solution and Li impurities in the discharged high-concentration acid aqueous solution are reduced.

[0061] However, the lower the concentration of the lithium hydroxide aqueous solution, the greater the concentration multiple during the concentration process for producing the subsequent lithium hydroxide product, resulting in an increase in the process cost of the concentration process. Therefore, in order to ensure the best economic efficiency during the lithium salt separation process, it is necessary to consider the factors of the process cost reduction brought about by the increase in the current efficiency and the cost increase during the subsequent lithium hydroxide concentration process to determine the optimal current efficiency range.

[0062] In an embodiment of the present invention, the current efficiency of the lithium salt separation process and the ionic conductivity can satisfy the following relational expression 1.

[0063] [Relational expression 1]

[0064] (Efficiency, %) = -A x ln(S M ) + B

[0065] Where (efficiency (efficiency), %) is the current efficiency, S Mis the ionic conductivity of the discharged alkaline aqueous solution or acidic aqueous solution, and -50 < A < -10, 50 < b < 250.

[0066] The said relational expression may be specifically as follows.

[0067] [Relational Expression 2]

[0068] (efficiency, %) = -A x ln(S LiOH ) + B

[0069] wherein, (efficiency, %) represents the current efficiency, S LiOH represents the ionic conductivity of the discharged lithium hydroxide aqueous solution, -50 < A < -10, 50 < b < 250.

[0070] The said constants A and B are variables that vary according to process variables including the material system, bipolar membrane type, system design, current value, etc., and these constants can be obtained after determining the lithium salt process system.

[0071] Therefore, in the lithium salt separation process, the target value of the ionic conductivity that satisfies the optimal current efficiency can be determined according to the said Relational Expression 1 or Relational Expression 2.

[0072] It can be seen from the said relational expression that if the value of the ionic conductivity rises above the target value, the current efficiency will be lower than the optimum, and the economy of the lithium salt separation process will be reduced.

[0073] Hereinafter, embodiments of the present invention will be described in detail. However, this is only presented as an example, and the present invention is not limited thereby. The present invention is only defined by the scope of the following claims.

[0074] (Preparation of Lithium Salt Raw Material)

[0075] Lithium salt separation experiments were conducted, and the lithium salt composition was measured by the ICP - AES method and shown in Table 1 below.

[0076]

Table 1

[0077]

[0078] (Lithium Salt Separation Experiment Method)

[0079] Figure 2 Generally shows the method of separating lithium salts using a laboratory - scale electrodialysis device.

[0080] Refer to Figure 2, The experiment of separating lithium salts using the laboratory-scale electrodialysis device of the present invention includes the steps of inputting a high-concentration aqueous lithium salt solution, water (or a low-concentration aqueous acid solution, or a low-concentration aqueous lithium hydroxide solution) into the bipolar electrodialysis device, discharging the concentrated aqueous acid solution, the concentrated aqueous lithium hydroxide solution, and the diluted aqueous lithium salt solution, and recycling the discharged concentrated aqueous acid solution and the concentrated aqueous lithium hydroxide solution to the electrodialysis device.

[0081] Meanwhile, the high-concentration lithium salt is decomposed into lithium ions and acid radical ions in the salt chamber of the electrodialysis device, and a residual low-concentration aqueous lithium salt solution is generated, which can then be discharged to the outside.

[0082] After the water or the low-concentration aqueous acid solution is converted into a high-concentration aqueous acid solution in the acid chamber of the electrodialysis device, it can be supplied to the acid circulation tank in the subsequent stage. Meanwhile, the ionic conductivity of the solution in the acid circulation tank can be measured.

[0083] At the same time, the water or the low-concentration aqueous lithium hydroxide solution can be converted into a high-concentration aqueous lithium hydroxide solution in the alkali chamber of the electrodialysis device, and then supplied to the alkali circulation tank in the subsequent stage. At this time, the ionic conductivity of the solution in the alkali circulation tank can be measured.

[0084] If the ionic conductivity of the solution measured in the acid circulation tank reaches above the target value of the ionic conductivity set in the present invention, the high-concentration acid solution in the acid circulation tank can be discharged to the outside, or a part of the high-concentration acid solution can be discharged to the outside, and then water is added to the acid circulation tank for dilution, and then recycled back to the acid chamber.

[0085] In addition, if the ionic conductivity of the solution measured in the alkali circulation tank reaches above the target value of the ionic conductivity aimed to be achieved in the present invention, the high-concentration aqueous lithium hydroxide solution in the alkali circulation tank can be discharged to the outside, or a part of the high-concentration aqueous lithium hydroxide solution can be discharged to the outside, and water is added to the alkali circulation tank for dilution, and then recycled back to the alkali chamber.

[0086] (Lithium salt separation experimental device)

[0087] Figure 3 Shows the composition of the lithium salt separation experimental equipment, Figure 4 Shows Figure 3 The control system interface of the lithium salt separation experimental device in

[0088] Refer to Figure 3 and Figure 4 , The lithium salt separation experimental device of the present invention mainly includes a lithium salt storage tank and a lithium salt circulation tank, an acid storage tank and an acid circulation tank, an alkali storage tank and an alkali circulation tank, a pH adjustment liquid tank, a distilled water tank and an electrode liquid circulation tank, as well as a bipolar membrane electrodialysis (BPED) stack and a control system.

[0089] On the other hand, the BPED stack is a three-layer BPED stack, which is composed of pairs of basic units 1 of BPM / spacer gasket / AEM / spacer gasket / CEM / spacer gasket and is formed by repeated stacking. The last BPM of each layer is in contact with the end spacer gasket / metal electrodes (+ electrode, - electrode), and the electrolyte for their respective conductivities circulates inside the solution chamber.

[0090] Meanwhile, in the BPED stack, solutions suitable for each chamber are respectively introduced into the solution chambers formed by the gaskets and membranes. That is, distilled water or dilute sulfuric acid aqueous solution is introduced into the acid chamber formed by BPM / gasket / AEM, and the sulfate (SO42-) ions supplied through the AEM are discharged as high-concentration sulfuric acid. Distilled water or dilute LiOH aqueous solution is introduced into the alkali chamber composed of BPM / gasket / CEM and discharged as high-concentration LiOH aqueous solution. Meanwhile, a high-concentration Li2SO4 solution is supplied to the salt chamber composed of AEM / gasket / CEM and discharged as a low-concentration Li2SO4. That is to say, after the solutions supplied from each circulation tank change in concentration within the BPED stack, they flow back into the circulation tank again to change the concentration. Meanwhile, the conductivity of the solution in the circulation tank is being measured. Using these measured values, when the conductivity value of the solution is within the target range, the ionic conductivity of the solution in each circulation tank is controlled by discharging the solution or diluting it with distilled water, thereby controlling the concentrations of the acid chamber, alkali chamber, and salt chamber.

[0091] Meanwhile, the above control system can verify the data measured in the above various device configurations, especially the measured values of the ionic conductivity of the solutions in the acid circulation tank, alkali circulation tank, and salt circulation tank in real time.

[0092] Figure 5 Shows the current-voltage change curve of the experiment using the Figure 3 lithium salt separation experimental device in

[0093] Refer to Figure 5 , in the lithium salt separation experiment, the operation is carried out with a current per unit area of the membrane of about 80 mA / cm2.

[0094] (Examples 1 to 4)

[0095] Using the lithium salts shown in Table 1 as raw materials, lithium salt separation experiments were carried out by changing the ionic conductivity of the solutions entering the acid chamber, salt chamber, and alkali chamber. The ionic conductivity can refer to the ionic conductivity of the solutions circulated and supplied to the acid chamber, salt chamber, and alkali chamber in their respective circulation tanks.

[0096] The components contained in the solutions discharged from the acid chamber, salt chamber, and alkali chamber and the concentrations of each component were analyzed and summarized in Table 2 below.

[0097]

Table 2

[0098]

[0099] It can not only be confirmed that the main elemental composition changes with the change in ionic conductivity of acid / base, but also the relative concentration of impurities changes. This is because the change in ionic conductivity of the solution causes the change in the diffusion behavior of impurities.

[0100] (Example 5)

[0101] In the same manner as in Example 1, an experiment was conducted to confirm the change in current efficiency caused by the change in ionic conductivity of the aqueous acid solution and the aqueous base solution.

[0102] The experimental analysis results are summarized in Table 3.

[0103]

Table 3

[0104]

[0105] Figure 6 It shows the analysis results of the change in current efficiency caused by the change in ionic conductivity in Example 5.

[0106] Figure 6 a of shows the influence of the change in ionic conductivity in the aqueous acid solution on the current efficiency, while Figure 6 b of shows the influence of the change in ionic conductivity in the aqueous base solution on the current efficiency.

[0107] Here, the current efficiency is calculated by the current efficiency measurement formula defined above.

[0108] Referring to Figure 6 , it can be confirmed that as the ionic conductivity values of the aqueous acid solution and the aqueous base solution increase, the current efficiency shows a downward trend.

[0109] Figure 7 It shows the analysis results of the change in the content of S impurities in the aqueous base solution generated according to the change in ionic conductivity in Example 5.

[0110] Figure 7 a of is the analysis result of the change in the content of S impurities in the aqueous base solution generated according to the change in ionic conductivity of the aqueous acid solution, Figure 7 b of represents the analysis result of the change in the content of S impurities in the aqueous base solution generated according to the change in ionic conductivity of the aqueous base solution.

[0111] Referring to Figure 7 , it can be confirmed that as the ionic conductivity values of the aqueous acid solution and the aqueous base solution increase, the content of S impurities in the generated aqueous lithium hydroxide solution shows an upward trend. This is considered to be due to the increase in acid concentration in the acid chamber, resulting in an increase in the migration of S impurities from the acid chamber to the base chamber through the bipolar membrane under the action of the electric field.

[0112] Figure 8 It shows the analysis results of the change in the lithium impurity content in the generated aqueous acid solution with the change in ionic conductivity in Example 5.

[0113] Figure 8 a of [reference] shows the change results of the lithium impurity content in the generated aqueous acid solution analyzed according to the change in the ionic conductivity of the aqueous acid solution, Figure 8 and b of [reference] shows the change results of the lithium impurity content in the generated aqueous acid solution analyzed according to the change in the ionic conductivity of the aqueous alkali solution.

[0114] Reference Figure 8 , it can be confirmed that as the ionic conductivity of the aqueous acid solution and the value of the ionic conductivity of the aqueous alkali solution increase, the Li impurity content in the generated aqueous acid solution shows an increasing trend. This is considered to be due to the increase in the LiOH concentration in the alkaline region, resulting in the migration of lithium ion impurities from the alkaline region to the acidic region under the action of the electric field.

[0115] Figure 9 It shows the analysis results of the change in current efficiency with the S concentration of the aqueous acid solution produced in Example 5 and the Li concentration of the aqueous alkali solution produced.

[0116] Figure 9 a1 of [reference] is the result of analyzing the change in current efficiency with the change in the S mass concentration of the produced aqueous acid solution, Figure 9 and b1 of [reference] is the result of analyzing the change in current efficiency with the Li mass concentration in the produced aqueous alkali solution.

[0117] Figure 9 a2 of [reference] shows the analysis results of the change in current efficiency with the change in the S molar concentration in the produced acid solution, Figure 9 and b2 of [reference] shows the analysis results of the change in current efficiency with the Li molar concentration in the produced alkali solution.

[0118] Reference Figure 9 , it can be confirmed that as the S concentration in the produced aqueous acid solution and the Li concentration in the produced aqueous alkali solution increase, the current efficiency shows a downward trend. This is because the high-concentration acid and alkali aqueous solutions decomposed from the brine solution diffuse through the membrane and move to the salt chamber.

[0119] Figure 10 It shows the analysis results of the change in the S impurity concentration in the produced aqueous alkali solution according to the S concentration of the aqueous acid solution produced in Example 5 and the Li concentration of the aqueous alkali solution produced.

[0120] Figure 10 a1 of [reference] is the result of analyzing the change in the sulfur impurity mass concentration in the produced aqueous alkali solution with the change in the sulfur mass concentration in the produced aqueous acid solution, Figure 10b2 is the result of analyzing the change in the mass concentration of sulfur impurities in the produced aqueous alkali solution with respect to the change in the mass concentration of lithium in the produced aqueous alkali solution.

[0121] Figure 10 a2 analyzed the influence of the change in the molar concentration of S in the produced aqueous acid solution on the molar concentration of S impurities in the produced aqueous alkali solution, while Figure 10 b2 shows the result of the influence of the change in the molar concentration of Li in the produced aqueous alkali solution on the molar concentration of S impurities therein.

[0122] Reference Figure 10 , it can be confirmed that as the concentration of S in the produced aqueous acid solution or the concentration of Li in the produced aqueous alkali solution increases, the concentration of S impurities in the produced aqueous alkali solution shows an upward trend. This is considered to be due to the increase in S ions passing through the bipolar membrane.

[0123] Figure 11 shows the result of analyzing the change in the concentration of Li impurities in the produced aqueous acid solution according to the S concentration of the produced aqueous acid solution and the Li concentration of the produced aqueous alkali solution in Example 5.

[0124] Figure 11 a1 analyzed the influence of the change in the mass concentration of S in the produced aqueous acid solution on the change in the mass concentration of Li impurities in the aqueous acid solution, Figure 11 b1 shows the result of analyzing the change in the mass concentration of Li impurities in the aqueous acid solution with respect to the mass concentration of Li in the produced aqueous alkali solution.

[0125] Figure 11 a2 is the result of analyzing the change in the molar concentration of Li impurities in the produced aqueous acid solution caused by the change in the molar concentration of S in the produced aqueous acid solution, Figure 11 b2 shows the result of analyzing the change in the molar concentration of Li impurities in the aqueous acid solution according to the change in the molar concentration of Li in the produced aqueous alkali solution.

[0126] Reference Figure 11 , it can be confirmed that as the concentration of S in the produced aqueous acid solution or the concentration of Li in the produced aqueous alkali solution increases, the concentration of Li impurities in the produced aqueous acid solution shows an upward trend. This is considered to be due to the increase in the diffusion phenomenon of Li on the membrane in the alkali chamber solution at high concentrations.

[0127] Figure 12 shows the analysis result of the change in current efficiency according to the concentration change of the produced aqueous acid solution and aqueous alkali solution.

[0128] Figure 12 a shows the analysis result of the change in current efficiency according to the mass concentration change of the produced aqueous acid solution and aqueous alkali solution, Figure 12b shows the analysis results of the change in current efficiency according to the change in the equivalent concentration of the aqueous acid solution and the produced aqueous alkali solution.

[0129] Referring to Figure 12 a, it can be confirmed that as the mass concentration of the produced aqueous acid solution or the mass concentration of the produced aqueous alkali solution increases, the current efficiency shows a downward trend. This is considered to be due to the phenomenon of increased ion penetration through the membrane at high concentrations as described above. On the other hand, it can be confirmed that the trend of change in current efficiency caused by the change in the mass concentration of the produced aqueous acid solution is not consistent with that caused by the change in the mass concentration of the produced aqueous alkali solution.

[0130] Referring to Figure 12 b, it can also be confirmed that when the equivalent concentration of the produced aqueous acid solution or the equivalent concentration of the produced aqueous alkali solution increases, the current efficiency shows a downward trend. In addition, it can be confirmed that the trend of change in current efficiency caused by the change in the equivalent concentration of the produced aqueous acid solution and the change in the equivalent concentration of the produced aqueous alkali solution is almost the same.

[0131] Here, the equivalent concentration refers to the number of equivalents of solute contained in each liter of solution, and the unit is (N). The equivalent refers to the number of moles of H + ions dissolved when 1 mole is dissolved.

[0132] As the concentrations of the aqueous acid solution and the aqueous alkali solution decrease, in order to prepare a high-concentration aqueous acid solution and a high-concentration aqueous alkali solution, it is necessary to remove the water in the solution or perform a solution concentration process. At the equivalent concentration of the aqueous acid solution and the equivalent concentration of the aqueous alkali solution, the additional concentration ratio of the LiOH aqueous solution is as Figure 13 shown, and at this time, the LiOH alkali solution is concentrated to a LiOH (molar) concentration of 3.4 M. The results show that as the solution concentration decreases, compared with the 3.4 M LiOH solution, the concentration ratio increases by up to more than 3 times at most.

[0133] As the concentrations of the aqueous acid solution and the aqueous alkali solution decrease, the content of S impurities in the LiOH solution is measured to decrease. After concentrating this solution to a LiOH concentration of 3.4 M, the calculated S concentration results in the cooled solution are as Figure 13 shown. As shown in the figure, the S impurity concentration in the LiOH solution prepared at low concentrations is significantly reduced to 1 / 5 to 1 / 10. This phenomenon is considered to be due to the reduction of the non-uniform voltage phenomenon applied on the BPM during low-concentration operation.

[0134] Figure 14 shows the correlation results of the lithium ion conductivity of lithium hydroxide and the current efficiency analyzed according to Example 5.

[0135] Referring to Figure 14, it can be seen that in the bipolar electroosmosis device, the ionic conductivity value of the discharged lithium hydroxide increases, and the current efficiency gradually rises, showing a certain non-linear growth trend, and the following relational expression can be derived.

[0136] (Efficiency, %) = -20.99 x ln(S LiOH ) + 170.1

[0137] Here, (efficiency %) is the current efficiency, and S LiOH is the ionic conductivity of the discharged aqueous lithium hydroxide solution.

[0138] According to the above relational expression, the ionic conductivity of the aqueous lithium hydroxide solution can be determined to achieve the best current efficiency with excellent economy in the lithium salt separation process.

[0139] On the other hand, in the above relational expression, the constants -20.99 and 170.1 are determined by factors such as the membrane type, substance type, system design, current value, etc. applicable to the bipolar electrodialysis device. When the bipolar device is determined and the types of substances to be separated and produced are determined, these values can be determined.

[0140] That is, the experimental results of Example 5 of the present invention can confirm that the [Relational Expression 2] is satisfied.

[0141] (Efficiency, %) = -A x ln(S LiOH ) + B

[0142] Here, (efficiency %) is the current efficiency, and S LiOH is the ionic conductivity of the discharged aqueous lithium hydroxide solution, -50 < A < -10, 50 < B < 250.

[0143] The present invention is not limited to the above embodiments and can be manufactured in various different forms. Those of ordinary skill in the technical field to which the present invention pertains should be able to understand that without changing the technical idea or essential features of the present invention, it can also be implemented in other specific forms. Therefore, the above-described embodiments are exemplary in all aspects and should not be regarded as having any limitations.

[0144]

Symbol Explanation

[0145] 10, 20: Unit pair

[0146] 110: Positive electrode

[0147] 120: Negative electrode

[0148] 210, 220: Bipolar membrane

[0149] 311, 312, 313, 321, 322, 323: Auxiliary gasket

[0150] 410, 420: Anion exchange membrane

[0151] 510, 520: Cation exchange membrane

[0152] 610, 620: Acid chamber

[0153] 710: Salt chamber

[0154] 810: Base chamber

[0155] 910: Positive electrode

[0156] 920: Negative electrode

Claims

1. A method for separating lithium salts using bipolar membranes, comprising: feeding an aqueous lithium salt solution into a salt chamber between adjacent anion exchange membranes and cation exchange membranes in a bipolar electroosmosis device, feeding water into an acid chamber between adjacent bipolar membranes and anion exchange membranes, and feeding water into an alkali chamber between adjacent bipolar membranes and cation exchange membranes; and applying an electric current to the bipolar electroosmosis device to obtain an aqueous lithium hydroxide solution and, as a by-product, an aqueous acid solution, by controlling the concentration of the aqueous lithium hydroxide solution discharged from the alkali chamber or the concentration of the aqueous acid solution discharged from the acid chamber to improve the current efficiency of the entire process.

2. The method for separating lithium salts using bipolar membranes according to claim 1, wherein the control of the concentration of the aqueous lithium hydroxide solution discharged from the alkali chamber is achieved by measuring the pH value of the aqueous lithium hydroxide solution discharged from the alkali chamber to control the concentration of the aqueous lithium hydroxide solution discharged from the alkali chamber.

3. The method for separating lithium salts using bipolar membranes according to claim 1, wherein the control of the concentration of the aqueous acid solution discharged from the acid chamber is achieved by measuring the pH value of the aqueous acid solution discharged from the acid chamber.

4. The method for separating lithium salts using bipolar membranes according to claim 1, wherein the current efficiency and the ionic conductivity of the aqueous lithium hydroxide solution discharged from the alkali chamber or the ionic conductivity of the aqueous acid solution discharged from the acid chamber satisfy the following relationship, (Efficiency, %) = -A x ln(S LiOH ) + B (Here, the efficiency (%, current efficiency), S M is the ionic conductivity of the discharged alkaline aqueous solution or acidic aqueous solution, and -50 < A < -10, 50 < b < 250).

5. The method for separating lithium salts using bipolar membranes according to claim 1, wherein the electrodialysis device is composed of a unit pair stack layer formed by arranging in sequence a bipolar membrane; an auxiliary gasket; an anion exchange membrane; an auxiliary gasket; a cation exchange membrane; and an auxiliary gasket, and the last bipolar membrane is in contact with the auxiliary gasket and the metal electrode.

6. The method for separating lithium salts using bipolar membranes according to claim 1, wherein feeding water into the acid chamber between adjacent bipolar membranes and anion exchange membranes is achieved by further mixing the aqueous acid solution discharged from the acid chamber and then feeding it in.

7. The method for separating lithium salts using bipolar membranes according to claim 1, wherein feeding water into the alkali chamber between adjacent bipolar membranes and cation exchange membranes is achieved by further mixing the aqueous lithium hydroxide solution discharged from the alkali chamber and then feeding it in.

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