Separation method of lithium salts using bipolar membranes

Abstract

An embodiment of the present invention can provide a method for separating lithium salts using a bipolar membrane, comprising the steps of: introducing an aqueous lithium salt solution into a salt compartment between adjacent anion and cation exchange membranes of a bipolar electrodialysis device; introducing water into an acid compartment between adjacent bipolar and anion exchange membranes; and introducing water into a base compartment between adjacent bipolar and cation exchange membranes; and applying a current to the bipolar electrodialysis device to obtain an aqueous lithium hydroxide solution and an aqueous acid solution as a by-product. The current efficiency of the entire process can be improved by controlling the concentration of the aqueous lithium hydroxide solution discharged from the base compartment or the concentration of the aqueous acid solution discharged from the acid compartment.

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention relate to methods for separating lithium salts using bipolar membranes, and more particularly to methods for separating lithium salts using bipolar membranes with improved current efficiency. [Background technology]

[0002] Lithium sulfate is decomposed by electrical energy through an ion exchange membrane into lithium hydroxide and sulfuric acid, or the higher the concentration, the more it passes through the membrane and becomes recontaminated. If this recontamination occurs, the decomposed substances are lost, the current efficiency decreases, and the economy decreases.

[0003] H of concentrated sulfuric acid + The ions diffuse through the anion exchange membrane (AEM) into the salt compartment, where they are converted to the OH of concentrated LiOH. - Ions diffuse through the cation exchange membrane (CEM) into the salt compartment. This diffusion rate increases as the concentrations in the acid and base compartments increase. This diffusion rate reduces the overall resistance of the bipolar electrodialysis (BPED) system, making it easier to maintain a maximum constant current, but it can also reduce the voltage applied to the membrane, reducing the amount of material decomposed through the membrane.

[0004] Also, the SO4 in the acid chamber 2- The ions pass through the bipolar membranes (BPM) by electrostatic force and move to the base compartment, increasing the S impurity content in LiOH. The S impurity content passing through the BPM tends to increase as the voltage applied to the BPM increases. The H impurity that diffuses through the membrane due to the concentrations in the acid and base compartments as described above + , O.H. -The total amount of ions is the total amount of dissolved substances that have diffused and diluted, reducing the current efficiency, and the change in electrical resistance affects the change in the applied voltage in the solution chamber and the change in impurity content.

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

[0006] An embodiment of the present invention provides a method for separating lithium salts using a bipolar membrane with reduced impurity generation and improved current efficiency. [Means for solving the problem]

[0007] A method for separating lithium salts according to one embodiment of the present invention may include the steps of: introducing an aqueous lithium salt solution into a salt compartment between adjacent anion exchange membranes and cation exchange membranes of a bipolar electrodialysis device; introducing water into an acid compartment between adjacent bipolar membranes and anion exchange membranes; and introducing water into a base compartment 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 an aqueous acid solution as a by-product.

[0008] The current efficiency of the entire process can be improved by controlling the concentration of the aqueous lithium hydroxide solution discharged from the base compartment or the concentration of the aqueous acid solution discharged from the acid compartment.

[0009] Controlling the concentration of the lithium hydroxide aqueous solution discharged from the base compartment may be controlling the pH value of the lithium hydroxide aqueous solution discharged from the base compartment by measuring it.

[0010] Controlling the concentration of the aqueous acid solution discharged from the acid chamber may be achieved by measuring and controlling the pH value of the aqueous acid solution discharged from the acid chamber.

[0011] The current efficiency and the ionic conductivity of the lithium hydroxide aqueous solution discharged from the base compartment or the ionic conductivity of the acid aqueous solution discharged from the acid compartment may satisfy the following relationship:

[0012] (efficiency,%) = -A × ln(S LiOH )+B (Here, (efficiency, %) is the current efficiency, S M is the ionic conductivity of the aqueous base or acid solution being discharged, and -50 <A<-10、50<b<250である。) The electrodialysis device is constructed by stacking unit pairs formed by sequentially arranging a bipolar membrane, a spacer gas cat; an anion exchange membrane, a spacer gas cat; a cation exchange membrane, and a spacer gas cat, and the last bipolar membrane can be in contact with the spacer gas cat and a metal electrode.

[0013] The introduction of the water into the acid compartment between the adjacent bipolar membrane and anion exchange membrane may be performed by further mixing the water with the aqueous acid solution discharged from the acid compartment.

[0014] The introduction of the water into the base chamber between the adjacent bipolar membrane and cation exchange membrane may be performed by further mixing the lithium hydroxide aqueous solution discharged from the base chamber and introducing the water. [Effects of the Invention]

[0015] According to one embodiment of the present invention, there is an advantage that the current efficiency of the lithium salt separation process can be improved by controlling the concentration of the aqueous solution being discharged.

[0016] According to one embodiment of the present invention, there is an advantage that the generation of impurities can be controlled by controlling the concentration of the aqueous solution being discharged. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram of a bipolar electrodialysis device according to one embodiment of the present invention. [Figure 2] FIG. 2 shows a schematic diagram of a method for separating lithium salts using a lab-scale electrodialysis device. [Figure 3] Figure 3 shows the experimental setup for separating lithium salts on a lab scale. [Figure 4] Figure 4 shows the control system screen of the lithium salt separation experimental apparatus shown in Figure 3. [Figure 5] FIG. 5 shows the current-voltage change curves in an experiment using the lithium salt separation experimental device shown in FIG. [Figure 6] FIG. 6 shows the results of analyzing the change in current efficiency due to the change in ionic conductivity in Example 5. [Figure 7] FIG. 7 shows the results of analyzing the change in the content of S impurity in the basic aqueous solution produced in Example 5 according to the change in ionic conductivity. [Figure 8] FIG. 8 shows the results of analyzing the change in the content of Li impurities in the produced acid aqueous solution due to the change in ionic conductivity in Example 5. [Figure 9] FIG. 9 shows the results of analyzing the change in current efficiency depending on the S concentration of the acidic aqueous solution produced in Example 5 and the Li concentration of the basic aqueous solution produced. [Figure 10] FIG. 10 shows the results of analyzing the change in the S impurity concentration in the produced aqueous base solution depending on the S concentration in the aqueous acid solution produced in Example 5 and the Li concentration in the aqueous base solution produced. [Figure 11]FIG. 11 shows the results of analyzing the change in Li impurity concentration in the produced acidic aqueous solution depending on the S concentration of the produced acidic aqueous solution and the Li concentration of the produced base aqueous solution in Example 5. [Figure 12] FIG. 12 shows the results of analyzing the change in current efficiency depending on the concentration of the produced aqueous acid solution and aqueous base solution. [Figure 13] FIG. 13 shows the analytical results for the concentration ratio of the LiOH solution and the S impurity in the aqueous base solution. [Figure 14] FIG. 14 shows the results of analyzing the correlation between lithium hydroxide ion conductivity and current efficiency according to Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0018] In describing the present invention, terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited to these. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Thus, a first part, component, region, layer, or section described below may also be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0019] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. As used in the specification, the term "comprising" refers to the inclusion of certain features, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0020] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless otherwise defined.

[0021] Hereinafter, embodiments of the present invention will be described in detail, but these are presented by way of example only and are not intended to limit the present invention, which is defined only by the scope of the claims that follow.

[0022] FIG. 1 is a schematic diagram of a bipolar electrodialysis device according to one embodiment of the present invention.

[0023] 1, a bipolar electrodialysis device according to one embodiment of the present invention may include unit pairs 10 and 20 repeatedly stacked between a positive electrode 910 and a negative electrode 920. The first unit pair 10 may include a bipolar membrane 210, a spacer gasket 311, an anion exchange membrane (AEM) 410, a spacer gasket 312, a cation exchange membrane (CEM) 510, and a spacer gasket 313, arranged in this order. The first unit pair 10 may be adjacent to a second unit pair 20 having the same configuration, and the spacer gasket 313 of the first unit pair 10 may be adjacent to the bipolar membrane 220 of the second unit pair 20.

[0024] The bipolar membrane 210, spacer gas cat 311, and anion exchange membrane 410 of the first unit pair 10 can form an acid chamber 610, and the anion exchange membrane 410, spacer gas cat 312, and cation exchange membrane 510 can form a salt chamber 710. In addition, the cation exchange membrane 510, spacer gas cat 313, and bipolar membrane 220 can form a base chamber 810.

[0025] Bipolar electrodialysis can be performed by feeding a high-concentration lithium salt solution into the salt compartment 710 of the first unit pair 10, feeding a low-concentration acid or distilled water into the acid compartment 600, and feeding a low-concentration lithium hydroxide solution or distilled water into the base compartment 800. At the same time, a high-concentration lithium salt solution can be fed into the second unit pair 20, and the same electrodialysis as that of the first unit pair 10 can be performed.

[0026] In the bipolar electrodialysis device, water-splitting occurs at each bipolar membrane, and the cations and anions in the lithium salt migrate to the negative electrode 920 and the positive electrode 910, respectively, due to the electrophoretic effect.

[0027] More specifically, the acid group is converted into an acid by contacting with the hydrolyzed hydrogen in the first bipolar membrane 210 on the positive electrode side, and the lithium ions that move to the negative electrode through the cation exchange membrane 510 are converted into hydroxyl groups (OH - ) and converted to lithium hydroxide (LiOH).

[0028] As an example, the overall reaction scheme is as follows:

[0029] Li2SO4(aq) ⇔ 2Li + (base chamber) + SO4 2- (acid chamber) H2O ⇔ H + (acid chamber)+OH - (base chamber) Li2SO4(aq)+2H2O ⇔ 2LiOH(aq, base chamber)+H2SO4(aq, acid chamber) At this time, the high-concentration lithium salt aqueous solution introduced into the salt chamber 710 is decomposed into lithium ions and acid groups, and a low-concentration lithium salt aqueous solution is generated with some of the remaining lithium salts remaining, and can be discharged outside the electrodialysis device.

[0030] A high concentration acid is generated and discharged from the acid chamber 610, and lithium hydroxide is generated and discharged from the base chamber 810.

[0031] Alternatively, the maximum voltage limited by the membrane supplier may be applied to the electrodialysis device, or a current within the current range limited by the membrane supplier per membrane operating area may be applied. Specifically, when the internal resistance of the electrodialysis device is low, the maximum constant current may be applied, and when the internal resistance increases, the maximum constant voltage may be applied to perform electrodialysis.

[0032] In addition, the H of the concentrated sulfuric acid + The ions diffuse through the anion exchange membrane 410 into the salt chamber 710, forming the OH of the concentrated LiOH. - Ions can diffuse through the cation exchange membrane 510 into the salt compartment 710. These diffusions tend to increase as the concentrations in the acid compartment 610 and the base compartment 810 increase. Also, the SO4 in the acid compartment 620 of the second unit pair 20 2- The ions pass through the second bipolar membrane 220 by electrostatic force and move to the base chamber 810 of the first unit pair 10, thereby increasing the content of S impurities in the LiOH. The content of S impurities generated in the base chamber after passing through the bipolar membrane tends to increase as the voltage applied to the bipolar membrane increases.

[0033] A method for separating lithium salts using the bipolar electrodialysis device according to one embodiment of the present invention may include the steps of: introducing an aqueous lithium salt solution into a salt compartment between adjacent anion exchange membranes and cation exchange membranes of the bipolar electrodialysis device; introducing water into an acid compartment between adjacent bipolar membranes and anion exchange membranes; and introducing water into a base compartment 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 an aqueous acid solution as a by-product.

[0034] As described above, the high-concentration lithium salt aqueous solution is decomposed into lithium ions and acid ions in the salt chamber of the electrodialysis device, and the remaining low-concentration lithium salt aqueous solution can be generated and discharged to the outside.

[0035] The water or low-concentration acid solution can be switched to a high-concentration acid solution in the acid chamber of the electrodialysis device and discharged. The concentration or pH value and ionic conductivity of the discharged high-concentration acid solution can be measured.

[0036] Meanwhile, the low-concentration lithium hydroxide aqueous solution can be switched to a high-concentration lithium hydroxide aqueous solution in the base compartment of the electrodialysis device and discharged. The concentration, pH value, and ionic conductivity of the discharged high-concentration lithium hydroxide aqueous solution can be measured.

[0037] If the measured concentration, pH value, or ionic conductivity of the aqueous solution is equal to or higher than the target value of the present invention, the measured concentration, pH value, or ionic conductivity of the aqueous solution can be controlled to fall within the target value range of the present invention by, for example, increasing the amount of water added.

[0038] The target concentration, pH value, or ionic conductivity of the aqueous solution in the present invention can be determined based on a target current efficiency that is economical for the lithium salt separation process in the present invention. Specifically, the current efficiency can be defined by taking into account the amount of charge used to concentrate the lithium hydroxide aqueous solution by subsequent addition relative to the total amount of charge applied to the lithium salt separation system, and can be calculated using the following equation:

[0039] η=(LiOH conversion charge) / (Charge consumption)=(△[LiOH]×△Mass) / (F×△Q) where η is the current efficiency, F is the Faraday constant, ΔQ is the amount of charge supplied over a reference time period, Δ[LiOH] is the change in concentration of the concentrated lithium hydroxide aqueous solution over a reference time period, and ΔMass is the change in mass of the lithium hydroxide aqueous solution over a reference time period.

[0040] In the present invention, it was confirmed through experimental results that when the ionic conductivity of the discharged aqueous solution is maintained low, the current efficiency increases and the content 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.

[0041] However, as the concentration of the lithium hydroxide aqueous solution becomes lower, the concentration rate increases in the subsequent concentration process for producing lithium hydroxide products, resulting in an increase in the process cost of the concentration process. Therefore, in order to ensure optimal economic efficiency in the lithium salt separation process, the optimal current efficiency range must be determined taking into account the process cost savings due to improved current efficiency and the cost increase in the subsequent lithium hydroxide concentration process.

[0042] In one embodiment of the present invention, the current efficiency of the lithium salt separation process and the ionic conductivity may satisfy the following relationship 1:

[0043] [Equation 1] (efficiency,%) = -A × ln(S M)+B where (efficiency, %) is the current efficiency, S M is the ionic conductivity of the discharged aqueous base or acid solution, and -50 <A<-10、50<b<250である。

[0044] Specifically, the above relational expression is as follows:

[0045] [Equation 2] (efficiency,%) = -A × ln(S LiOH )+B where (efficiency, %) is the current efficiency, S LiOH is the ionic conductivity of the discharged lithium hydroxide aqueous solution, and -50 <A<-10、50<b<250である。

[0046] The constants A and B are variables that vary depending on process variables including the material system, the type of bipolar membrane, the system design, the current value, etc., and may be constants that are obtained when the lithium salt process system is determined.

[0047] Therefore, in the lithium salt separation step, a target value of ionic conductivity that satisfies the optimum current efficiency can be determined using the above-mentioned Relational Formula 1 or 2.

[0048] From the above relational expression, it can be seen that when the ionic conductivity value rises above the target value, the current efficiency falls below the optimum value, and the economic efficiency of the lithium salt separation process decreases.

[0049] Hereinafter, embodiments of the present invention will be described in detail, but they are presented by way of example only and are not intended to limit the present invention, which is defined only by the scope of the claims set forth below.

[0050] (Preparing lithium salt raw materials) A lithium salt separation experiment was carried out, and the lithium salt composition was measured by ICP-AES and is shown in Table 1 below.

[0051] [Table 1]

[0052] (Lithium salt separation experiment method) FIG. 2 shows a schematic diagram of a method for separating lithium salts using a lab-scale electrodialysis device.

[0053] Referring to FIG. 2, an experiment for separating lithium salts using the lab-scale electrodialysis device of the present invention includes the steps of feeding a high-concentration lithium salt aqueous solution and water (or a low-concentration acid aqueous solution or a low-concentration lithium hydroxide aqueous solution) into a bipolar electrodialysis device, discharging a concentrated acid aqueous solution, a concentrated lithium hydroxide aqueous solution, and a diluted lithium salt aqueous solution, and circulating the discharged concentrated acid aqueous solution and concentrated lithium hydroxide aqueous solution back to the electrodialysis device.

[0054] Meanwhile, the high-concentration lithium salt is decomposed into lithium ions and acid ions in the salt chamber of the electrodialysis device, and the remaining low-concentration lithium salt aqueous solution is generated and discharged to the outside.

[0055] The water or low-concentration acid solution can be switched to a high-concentration acid solution in the acid chamber of the electrodialysis device and then supplied to the acid circulation tank in the downstream stage. At this time, the ionic conductivity of the solution in the acid circulation tank can be measured.

[0056] Meanwhile, the water or low-concentration lithium hydroxide aqueous solution can be switched to a high-concentration lithium hydroxide aqueous solution in the base chamber of the electrodialysis device and then supplied to a downstream base circulation tank, and the ionic conductivity of the solution in the base circulation tank can be measured.

[0057] If the measured ionic conductivity of the aqueous solution in the acid circulation tank is equal to or greater than the target ionic conductivity value of the present invention, the high-concentration aqueous acid solution in the acid circulation tank can be discharged to the outside, or a portion of the high-concentration aqueous acid solution can be discharged to the outside and water can be added to the acid circulation tank to dilute it, and then the diluted solution can be circulated back into the acid chamber.

[0058] Furthermore, if the measured ionic conductivity of the aqueous solution in the base circulating tank is equal to or greater than the target ionic conductivity value of the present invention, the high-concentration lithium hydroxide aqueous solution in the base circulating tank can be discharged to the outside, or a portion of the high-concentration lithium hydroxide aqueous solution can be discharged to the outside and water can be introduced into the base circulating tank to dilute it, and then the diluted solution can be circulated back to the base chamber.

[0059] (Lithium salt separation experimental device) FIG. 3 shows the configuration of the lithium salt separation experimental equipment, and FIG. 4 shows a control system screen of the lithium salt separation experimental equipment of FIG.

[0060] Referring to Figures 3 and 4, the lithium salt separation experimental apparatus 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, a base storage tank and a base circulation tank, a pH adjusting solution tank, a distilled water tank and an electrode solution circulation tank, as well as a bipolar membrane electrodialysis (BPED) stack and a control system.

[0061] The BPED stack is a three-chamber BPED stack, and is constructed by repeatedly stacking pairs of BPM / spacer gasket / AEM / spacer gasket / CEM / spacer gasket as a basic unit. The last BPM of each layer is in contact with the end spacer gasket / metal electrode (+ pole, - pole), and electrolyte for electrical conduction circulates inside each solution chamber.

[0062] Meanwhile, in the BPED stack, the solution chambers formed by the spacer gasket and membrane were filled with the appropriate solution. In other words, the acid chamber formed by the BPM / spacer gasket / AEM was filled with distilled water or diluted sulfuric acid solution. This was the sulfuric acid (SO4 2- ) ions, resulting in a high-concentration sulfuric acid solution. Distilled water or a dilute LiOH solution flows into the base chamber, composed of a BPM / spacer gasket / CEM, and is discharged as a high-concentration LiOH solution. Meanwhile, a high-concentration Li2SO4 solution is supplied to the salt chamber, composed of an AEM / spacer gasket / CEM, and is discharged as a low-concentration Li2SO4 solution. In other words, the solutions supplied from each circulation tank undergo concentration changes within the BPED stack, then flow back into the circulation tanks, where the concentration is changed again. Meanwhile, the conductivity of the solutions in the circulation tanks is measured. Using this measurement, if the solution's conductivity falls within the target range, the solution is either discharged or diluted with distilled water to control the ionic conductivity of the solution in each circulation tank, thereby controlling the concentration of each acid, base, and salt chamber.

[0063] Meanwhile, the control system can check the data measured in each of the device configurations, and in particular, can check the measured values ​​of the ionic conductivity of the solutions in the acid circulation tank, base circulation tank, and salt circulation tank in real time.

[0064] FIG. 5 shows the current-voltage change curves in an experiment using the lithium salt separation experimental device shown in FIG.

[0065] Referring to Figure 5, in the lithium salt separation experiment, the current per unit area of ​​the membrane was 80 mA / cm 2 I drove at a moderate pace.

[0066] Examples 1 to 4 Using the lithium salts listed in Table 1 as raw materials, lithium salt separation experiments were conducted by varying the ionic conductivity of the solutions flowing into the acid compartment, salt compartment, and base compartment. The ionic conductivity may be the ionic conductivity of the solutions in the circulation tanks that circulate the solutions to the acid compartment, salt compartment, and base compartment.

[0067] The components and concentrations of each component contained in the solutions discharged from the acid, salt and base compartments were analyzed and summarized in Table 2 below.

[0068] [Table 2]

[0069] It can be seen that changing the ionic conductivity of the acid / base not only changes the main elemental components but also the relative concentrations of impurities, because changing the ionic conductivity of the solution changes the impurity diffusion behavior.

[0070] Example 5 An experiment was carried out in the same manner as in Example 1 to confirm the change in current efficiency due to the change in ionic conductivity of the acidic and basic aqueous solutions.

[0071] The experimental analysis results are summarized in Table 3 below.

[0072] [Table 3]

[0073] FIG. 6 shows the results of analyzing the change in current efficiency due to the change in ionic conductivity in Example 5.

[0074] FIG. 6(a) shows the change in current efficiency due to the change in ionic conductivity of an acidic aqueous solution, and FIG. 6(b) shows the change in current efficiency due to the change in ionic conductivity of a basic aqueous solution.

[0075] Here, the current efficiency was calculated by the current efficiency measurement formula defined above.

[0076] Referring to FIG. 6, it can be seen that the current efficiency tends to decrease as the ionic conductivity of the acidic aqueous solution and the ionic conductivity of the basic aqueous solution increase.

[0077] FIG. 7 shows the results of analyzing the change in the content of S impurity in the basic aqueous solution produced in Example 5 according to the change in ionic conductivity.

[0078] Figure 7(a) shows the results of an analysis of the change in the content of S impurities in the generated base aqueous solution due to the change in the ionic conductivity of the acid aqueous solution, and Figure 7(b) shows the results of an analysis of the change in the content of S impurities in the generated base aqueous solution due to the change in the ionic conductivity of the base aqueous solution.

[0079] Referring to Figure 7, it can be seen that the content of S impurities in the produced lithium hydroxide aqueous solution tends to increase as the ionic conductivity of the acid aqueous solution and the ionic conductivity of the base aqueous solution increases. This is thought to be due to an increase in the amount of S impurities passing through the bipolar membrane from the acid compartment to the base compartment in the electric field due to an increase in the acid concentration in the acid compartment.

[0080] FIG. 8 shows the results of analyzing the change in the content of Li impurities in the produced acid aqueous solution due to the change in the ionic conductivity of the aqueous solution added in Example 5.

[0081] FIG. 8(a) shows the results of analyzing the change in the Li impurity content in the generated acidic aqueous solution due to the change in the ionic conductivity of the acidic aqueous solution, and FIG. 8(b) shows the results of analyzing the change in the Li impurity content in the generated acidic aqueous solution due to the change in the ionic conductivity of the base aqueous solution.

[0082] Referring to Figure 8, it can be seen that the content of Li impurities in the acidic aqueous solution increases as the ionic conductivity of the acidic aqueous solution and the ionic conductivity of the basic aqueous solution increases. This is thought to be due to an increase in the lithium ion impurities that move to the acidic compartment in the electric field due to an increase in the LiOH concentration in the basic compartment.

[0083] FIG. 9 shows the results of analyzing the change in current efficiency depending on the S concentration of the acidic aqueous solution produced in Example 5 and the Li concentration of the basic aqueous solution produced.

[0084] Figure 9(a1) shows the results of analyzing the change in current efficiency depending on the mass concentration (S) of the produced acid aqueous solution, and Figure 9(b1) shows the results of analyzing the change in current efficiency depending on the mass concentration (Li) of the produced base aqueous solution.

[0085] Figure 9(a2) shows the results of analyzing the change in current efficiency depending on the S molar concentration of the produced acidic aqueous solution, and Figure 9(b2) shows the results of analyzing the change in current efficiency depending on the Li molar concentration of the produced basic aqueous solution.

[0086] Referring to Figure 9, it can be seen that the current efficiency tends to decrease as the S concentration in the produced acid solution and the Li concentration in the produced base solution increase. This is thought to be because the high-concentration acid and base solutions resulting from the decomposition of the salt solution diffuse through the membrane and move to the salt chamber.

[0087] FIG. 10 shows the results of analyzing the change in the S impurity concentration in the produced aqueous base solution depending on the S concentration in the aqueous acid solution produced in Example 5 and the Li concentration in the aqueous base solution produced.

[0088] FIG. 10(a1) shows the results of analyzing the change in the mass concentration of S impurity in the produced aqueous base solution depending on the change in the mass concentration of S in the produced aqueous acid solution, and FIG. 10(b2) shows the results of analyzing the change in the mass concentration of S impurity in the produced aqueous base solution depending on the mass concentration of Li in the produced aqueous base solution.

[0089] FIG. 10(a2) shows the results of analyzing the change in the molar concentration of S impurities in the produced aqueous base solution depending on the S molar concentration of the produced aqueous acid solution, and FIG. 10(b2) shows the results of analyzing the change in the molar concentration of S impurities in the produced aqueous base solution depending on the Li molar concentration of the produced aqueous base solution.

[0090] Referring to Figure 10, it can be seen that the concentration of S impurities in the produced aqueous base solution tends to increase as the S concentration in the produced aqueous acid solution increases or the Li concentration in the produced aqueous base solution increases. This is thought to be due to an increase in the number of S ions passing through the bipolar membrane.

[0091] FIG. 11 shows the results of analyzing the change in Li impurity concentration in the produced acidic aqueous solution depending on the S concentration of the produced acidic aqueous solution and the Li concentration of the produced base aqueous solution in Example 5.

[0092] FIG. 11(a1) shows the results of analyzing the change in the mass concentration of Li impurity in the produced acidic aqueous solution depending on the change in the mass concentration of S in the produced acidic aqueous solution, and FIG. 11(b1) shows the results of analyzing the change in the mass concentration of Li impurity in the produced acidic aqueous solution depending on the mass concentration of Li in the produced base aqueous solution.

[0093] FIG. 11(a2) shows the results of analyzing the change in the Li impurity molar concentration in the produced acidic aqueous solution depending on the S molar concentration of the produced acidic aqueous solution, and FIG. 11(b2) shows the results of analyzing the change in the Li impurity molar concentration in the produced acidic aqueous solution depending on the Li molar concentration of the produced base aqueous solution.

[0094] Referring to Figure 11, it can be seen that the Li impurity concentration in the produced acid solution tends to increase as the S concentration in the produced acid solution increases or the Li concentration in the produced base solution increases. This is thought to be due to the increase in Li diffusion through the membrane when the solution in the base chamber is highly concentrated.

[0095] FIG. 12 shows the results of analyzing the change in current efficiency depending on the concentration of the produced aqueous acid solution and aqueous base solution.

[0096] FIG. 12(a) shows the results of analyzing the change in current efficiency depending on the mass concentration of the produced acid aqueous solution and the produced base aqueous solution, and FIG. 12(b) shows the results of analyzing the change in current efficiency depending on the normal concentration of the produced acid aqueous solution and the produced base aqueous solution.

[0097] Referring to Figure 12(a), it can be seen that the current efficiency tends to decrease as the mass concentration of the produced acid aqueous solution or the mass concentration of the produced base aqueous solution increases. This is thought to be due to the aforementioned phenomenon of an increase in the amount of ions permeating the membrane at high concentrations. However, it can also be seen that the trends in the change in current efficiency due to changes in the mass concentration of the produced acid aqueous solution and the mass concentration of the produced base aqueous solution do not match.

[0098] 12(b), it can be seen that the current efficiency tends to decrease as the normal concentration of the produced acid solution or the normal concentration of the produced base solution increases. It can also be seen that the change in current efficiency due to the change in the normal concentration of the produced acid solution and the change in the normal concentration of the produced base solution are almost the same.

[0099] Here, normal concentration is the number of equivalents of solute contained in 1 L of solution, and the unit is (N). The equivalent is the number of H dissolved when 1 mol is dissolved. + is the number of moles of ions.

[0100] To produce high-concentration acid and base solutions due to the decrease in the concentration of the aqueous acid or base solution, a process for removing water from the solution or concentrating the solution is required. The concentration ratio of the LiOH solution at normal concentrations of the aqueous acid and base solutions is shown in Figure 13. The LiOH base solution was concentrated at a LiOH (molar) concentration of 3.4M. As can be seen from the results, as the solution concentration decreases, the concentration ratio increases by up to three times compared to the 3.4M LiOH solution.

[0101] It was determined that the S impurity content in the LiOH solution decreased as the concentration of the acidic and basic aqueous solutions decreased. Figure 13 shows the calculated S concentration in a solution when this solution was concentrated to a LiOH concentration of 3.4M. As shown in the figure, the S impurity concentration in the LiOH solution produced at a low concentration was significantly reduced to 1 / 5 to 1 / 10. This phenomenon is believed to be due to a reduction in the occurrence of uneven voltage being applied to the BPM when operating at a low concentration.

[0102] FIG. 14 shows the results of analyzing the correlation between the lithium hydroxide ion conductivity and current efficiency produced in Example 5.

[0103] Referring to Figure 14, it can be seen that as the ionic conductivity of lithium hydroxide discharged from the bipolar electrodialysis device increases, the current efficiency gradually increases, showing a certain nonlinear increasing trend, and the following relational equation can be derived.

[0104]

number

[0105] where (efficiency, %) is the current efficiency, S LiOH is the ionic conductivity of the discharged lithium hydroxide aqueous solution.

[0106] From the above relational expression, it can be confirmed that the ionic conductivity of the lithium hydroxide aqueous solution can be derived to achieve an optimal current efficiency that is economically excellent in the lithium salt separation process.

[0107] Meanwhile, in the above relational expression, the constants -20.99 and 170.1 are determined depending on the type of membrane, type of material, system design, current value, etc. applied to the bipolar electrodialysis device, and are values ​​that can be determined once the bipolar device, the type of material to be separated, and the type of product material are determined.

[0108] That is, it can be confirmed that the experimental results of Example 5 of the present invention satisfy the above-mentioned [Relational Expression 2].

[0109]

number

[0110] where (efficiency, %) is the current efficiency, S LiOH is the ionic conductivity of the discharged lithium hydroxide aqueous solution, and -50 <A<-10、50<b<250である。

[0111] The present invention is not limited to the above-described embodiments, and can be manufactured in various different forms, and a person skilled in the art to which the present invention pertains should understand that the present invention can be embodied in other specific forms without changing the technical concept or essential characteristics of the present invention. Therefore, the above-described embodiments should be understood to be illustrative in all respects and not limiting. [Explanation of symbols]

[0112] 10, 20: unit pair 110: Positive electrode 120: Negative electrode 210, 220: Bipolar membrane 311, 312, 313, 321, 322, 323: Spacer Gas Cat 410, 420: Anion exchange membrane 510, 520: Cation exchange membrane 610, 620: Acid chamber 710: Salt Room 810: Base Room 910: Positive electrode 920: Negative electrode

Claims

1. introducing an aqueous lithium salt solution into a salt compartment between adjacent anion and cation exchange membranes of a bipolar electrodialysis device, introducing water into an acid compartment between adjacent bipolar and anion exchange membranes, and introducing water into a base compartment between adjacent bipolar and cation exchange membranes; and Applying an electric current to the bipolar electrodialysis device to obtain a lithium hydroxide solution and an acid solution as a by-product; The current efficiency of the entire process is improved by controlling the concentration of the lithium hydroxide aqueous solution discharged from the base compartment or the concentration of the acid aqueous solution discharged from the acid compartment. Separation of lithium salts using bipolar membranes.

2. Controlling the concentration of the lithium hydroxide aqueous solution discharged from the base chamber includes: The pH value of the lithium hydroxide aqueous solution discharged from the base chamber is measured and controlled. A method for separating lithium salts using the bipolar membrane of claim 1.

3. Controlling the concentration of the aqueous acid solution discharged from the acid chamber includes: The pH value of the acid solution discharged from the acid chamber is measured and controlled. A method for separating lithium salts using the bipolar membrane of claim 1.

4. The current efficiency and the ionic conductivity of the lithium hydroxide aqueous solution discharged from the base compartment or the ionic conductivity of the acid aqueous solution discharged from the acid compartment satisfy the following relationship: A method for separating lithium salts using the bipolar membrane of claim 1. (efficiency、%)=-A×ln(S ＬｉＯＨ )+B (where (efficiency, %) is the current efficiency, S Ｍ is the ionic conductivity of the discharged aqueous base or acid solution, and is -50<A<-10, 50<b<250.)

5. The electrodialysis device is configured by stacking unit pairs each formed by sequentially arranging a bipolar membrane, a spacer gas cat; an anion exchange membrane, a spacer gas cat; a cation exchange membrane, and a spacer gas cat; The final bipolar membrane is the one in contact with the spacer gas cat and the metal electrode. A method for separating lithium salts using the bipolar membrane of claim 1.

6. Injecting the water into an acid chamber between the adjacent bipolar membrane and an anion exchange membrane includes: The acid aqueous solution discharged from the acid chamber is further mixed and added. A method for separating lithium salts using the bipolar membrane of claim 1.

7. introducing the water into a base chamber between the adjacent bipolar membrane and the cation exchange membrane; The lithium hydroxide aqueous solution discharged from the base chamber is further mixed and added. A method for separating lithium salts using the bipolar membrane of claim 1.

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