Method and apparatus for recovering alkali metal salts

Abstract

The present invention is a method for recovering an alkali metal salt from a solution X containing at least one of a compound HA having a pKa of 3.0 or more and 5.0 or less and an anion A forming the compound HA, and an alkali metal ion, comprising: (1) a pH adjustment step of adjusting the pH of the solution X to 0 or more and 4.0 or less when the pH of the solution X exceeds 4; (2) an aluminum salt addition step of adding an aluminum salt to the solution X so that the molar concentration of aluminum is 0.25 times or more the total molar concentration of anion A, which is the sum of the molar concentration of the compound HA in the solution X and the molar concentration of anion A present as an ion in the solution X; and (3) a nanofiltration step of separating the alkali metal ion from the solution X after steps (1) and (2) to the permeate side using a nanofiltration membrane.

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for recovering alkali metal salts. [Background technology]

[0002] In recent years, the demand for mineral resources has increased significantly with the development of industry and the economy worldwide. Even among mineral resources that are indispensable for a wide range of industries, including the semiconductor industry, there are many resources that are technically difficult to extract as single elements, that are economically unprofitable due to high costs of mining and refining, and that are localized in specific regions, even if there are large reserves in the earth's crust. On the other hand, environmental issues have also been brought into sharp focus, and the construction of a recycling-oriented society is desired. In particular, the development of electric vehicles, motors, and batteries used in them has accelerated, as they have attracted attention for their ability to reduce carbon dioxide emissions. In particular, with regard to batteries, lithium-ion secondary batteries are expected to be the main battery for electric vehicles due to their energy density and light weight.

[0003] Lithium compounds are used, for example, as electrode material for lithium-ion batteries, as an additive for heat-resistant glass, and for surface acoustic wave filters. High-purity lithium carbonate is used as filters and transmitters in mobile phones and car navigation systems. Lithium bromide is used as a refrigerant absorbent in absorption refrigerators for large air conditioning in buildings and factories. Lithium hydroxide is used as a raw material for grease in automobiles and for lithium batteries (primary and secondary). Metallic lithium is used as foil for the negative electrode of primary batteries and as a raw material for butyl lithium used in synthetic rubber catalysts.

[0004] Lithium is extracted from resources such as salt lake brine and ores, but new lithium supply sources are needed to meet the increasing demand for lithium for use in electric vehicles. In particular, there is a demand for technology to efficiently recover lithium from waste materials and waste liquid generated during the manufacturing process of lithium-ion batteries.

[0005] For example, lithium ion battery electrolytes generally use lithium salts (LiPF6, LiBF4, LiAsF6, etc.) that contain fluorine as a constituent element, but they are known to undergo hydrolysis when water is mixed in (see, for example, Non-Patent Document 1). If the electrolyte undergoes hydrolysis, the performance of the battery deteriorates, so electrolytes that have deteriorated during use in the manufacturing process of lithium ion batteries are discarded. Since the electrolyte contains a lot of lithium, recovery is required, but the fluorine contained in the electrolyte becomes an impurity when recovering lithium as lithium carbonate or lithium hydroxide, so separation is required.

[0006] Patent Document 1 describes a method in which an electrolyte is heated, the fluorine-containing solvent is removed, and the residue is leached with water to obtain an aqueous lithium hydroxide solution. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-197149 [Non-patent literature]

[0008] [Non-Patent Document 1] Tetsuya Kawamura and 2 others, "Decomposition reaction of LiPF6 electrolyte for lithium batteries", Bulletin of the Kyushu University Graduate School of Science and Technology, Kyushu University Graduate School of Science and Technology, December 2002, Vol. 24, No. 3, pp. 281-288 Summary of the Invention [Problem to be solved by the invention]

[0009] However, in the method described in Patent Document 1, although the amount of fluorine in the electrolyte is significantly reduced, some fluorine remains as lithium fluoride, which is insufficient for obtaining high-purity lithium hydroxide or lithium carbonate.

[0010] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a method for recovering high purity alkali metal salts from waste materials, waste liquids, ores and the like containing mineral resources. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention has the following configuration. [1] A method for recovering an alkali metal salt from a solution X containing at least one of a compound HA having a pKa of 3.0 or more and 5.0 or less and an anion A forming the compound HA, and an alkali metal ion, the method comprising the following steps (1) to (3): (1) a pH adjustment step of adjusting the pH of the solution X to 0 or more and 4.0 or less when the pH of the solution X exceeds 4.0; (2) an aluminum salt addition step in which an aluminum salt is added to the solution X so that the molar concentration of aluminum (mol / L) is 0.25 times or more the total molar concentration of anion A (mol / L) which is the sum of the molar concentration (mol / L) of the compound HA in the solution X and the molar concentration (mol / L) of anion A present as an ion in the solution X; (3) A nanofiltration step in which alkali metal ions are separated from the solution X after steps (1) and (2) by a nanofiltration membrane to the permeate side. [2] The method for recovering an alkali metal salt according to [1] above, wherein in the step (1), the pH of the solution X is adjusted to 1.0 or more and 3.0 or less. [3] The method for recovering an alkali metal salt according to [1] or [2] above, wherein the pKa of the compound HA is 3.0 or more and 4.0 or less. [4] The method for recovering an alkali metal salt according to any one of [1] to [3] above, wherein in the step (2), an aluminum salt is added to the solution X so as to satisfy any one of the following conditions (a) to (d): Condition (a): When the pH of solution X is 1.0 or more and 1.4 or less, the molar concentration of the aluminum in solution X is 0.5 or more times the molar concentration of the total anion A in solution X. Condition (b): When the pH of solution X is greater than 1.4 and equal to or less than 1.9, the molar concentration of aluminum relative to the molar concentration of the total anion A in solution X is 0.4 or more times. Condition (c): When the pH of solution X is greater than 1.9 and equal to or less than 2.4, the molar concentration of aluminum relative to the molar concentration of the total anion A in solution X is 0.3 or more times. Condition (d): when the pH of solution X is greater than 2.4 and equal to or less than 3.0, the molar concentration of the aluminum relative to the molar concentration of the total anion A in solution X is 0.25 or more times. [5] The method for recovering an alkali metal salt according to any one of [1] to [4] above, wherein the difference between the magnesium sulfate removal rate when a 2000 mg / L aqueous magnesium sulfate solution at 25°C and pH 6.5 is allowed to permeate through the nanofiltration membrane at an operating pressure of 0.5 MPa and a magnesium chloride removal rate when a 2000 mg / L aqueous magnesium chloride solution at 25°C and pH 6.5 is allowed to permeate through the nanofiltration membrane at an operating pressure of 0.5 MPa and is 20% or less. [6] The method for recovering an alkali metal salt according to any one of [1] to [5] above, wherein the difference between the glucose removal rate when a 1000 mg / L aqueous glucose solution at 25°C and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa and the isopropyl alcohol removal rate when a 1000 mg / L aqueous isopropyl alcohol solution at 25°C and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa is 40% or more, and the glucose removal rate is 70% or more and 95% or less. [7] The method for recovering an alkali metal salt according to any one of [1] to [6] above, wherein the compound HA contains fluorine. [8] The method for recovering an alkali metal salt according to any one of [1] to [7] above, wherein the alkali metal salt contains lithium. [9] An apparatus for recovering an alkali metal salt, comprising: a tank for storing a solution containing alkali metal ions; an aluminum salt adding means for adding an aluminum salt to the solution; and a nanofiltration means for separating the alkali metal ions from the solution.

[10] The alkali metal salt recovery device according to [9], further comprising a pH adjusting means for adjusting the pH of the solution. Effect of the Invention

[0012] The method and apparatus for recovering an alkali metal salt of the present invention makes it possible to recover salts of alkali metals such as lithium and cesium at high purity from a solution containing alkali metal ions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The following describes in detail the embodiments of the present invention; however, the present invention is not limited to the following description, and can be modified in any manner without departing from the gist of the present invention. In this specification, the unit of "molar concentration" is mol / L. Furthermore, in this specification, "mass" is synonymous with "weight."

[0014] <1> Method for recovering alkali metal salts The method for recovering an alkali metal salt of the present invention is a method for recovering an alkali metal salt from a solution X containing at least one of a compound HA having a pKa of 3.0 or more and 5.0 or less and an anion A forming the compound HA, and an alkali metal ion, and includes the following steps (1) to (3): (1) A pH adjustment step in which the pH of solution X is adjusted to a value between 0 and 4.0 when the pH of solution X exceeds 4.0. (2) An aluminum salt addition step in which an aluminum salt is added to solution X so that the molar concentration of aluminum (mol / L) is 0.25 times or more the total molar concentration of anion A (mol / L), which is the sum of the molar concentration (mol / L) of compound HA in solution X and the molar concentration (mol / L) of anion A present as an ion in solution X. (3) A nanofiltration step in which alkali metal ions are separated from the solution X after steps (1) and (2) by a nanofiltration membrane to the permeate side.

[0015] <2> solution Solution X contains alkali metal ions. The alkali metal ions contained in solution X are ions that constitute an alkali metal salt, and the alkali metal salt is composed of an alkali metal ion and one or more conjugate bases (e.g., fluoride ions, chloride ions, nitrate ions, sulfate ions, carbonate ions, acetate ions, etc.). Examples of alkali metal salts include salts of lithium, sodium, potassium, rubidium, and cesium, but from the viewpoint of the value of the object to be recovered, it is preferable that solution X contains a salt of lithium. That is, solution X contains lithium ions (hereinafter, "Li + It is preferred that the fluororesin composition includes a fluororesin.

[0016] The solution X containing alkali metal ions is preferably a solution in which a material containing lithium is dissolved in an acid, specifically, waste materials, waste liquids, ores, and slags generated in the manufacturing process of lithium-ion batteries.

[0017] A lithium ion battery is composed of components such as a positive electrode material, a negative electrode material, a separator, and an electrolyte. Any of these components containing lithium can be used as the material for solution X. The acid that dissolves the material containing lithium preferably contains at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid.

[0018] The method of dissolving the lithium-containing material with an acid includes, for example, immersing the material in an acidic aqueous solution, but other methods may be used as long as they can elute the desired alkali metal ions.

[0019] When the lithium-containing material is an electrolyte, for example, LiPF6 is contained in an organic solvent such as ethylene carbonate (EC) or dimethyl carbonate (DMC), but it is preferable to dilute it with water from the viewpoint of the separation property of the nanofiltration membrane described later. By diluting the electrolyte with water, LiPF6 in the electrolyte is hydrolyzed to hydrofluoric acid, phosphoric acid, and lithium hydroxide, and lithium becomes easy to permeate the nanofiltration membrane, and fluorine easily forms a compound with aluminum as described later. In addition, the hydrolysis of LiPF6 is promoted by adding aluminum.

[0020] From the viewpoint of the elution efficiency of the alkali metal ions, the temperature of the acidic aqueous solution to be contacted is preferably 10° C. or more and 100° C. or less. From the viewpoints of cost and safety, the temperature of the acidic aqueous solution is more preferably 20° C. or more and 90° C. or less.

[0021] The solution obtained by dissolving a lithium-containing material in an acid does not always have a constant composition, and the composition may vary depending on the variation in the composition of various ions in the material, the dissolution conditions in the acid, and the like.

[0022] Furthermore, solution X contains at least one of a compound HA having a pKa of 3.0 to 5.0 and an anion A forming said compound HA. Compound HA generates an anion A by ionization. As a result of intensive research, the inventors have found that compound HA having a pKa of 3.0 to 5.0 and / or an anion A forming said compound HA forms a compound with aluminum ions under acidic conditions of pH 4.0 or less, and the permeability of the nanofiltration membrane is significantly reduced. Since the alkali metal salt to be recovered permeates the nanofiltration membrane, the reduction in the permeability of the compound formed from compound HA and / or anion A forming said compound HA contributes to improving the purity of alkali metal salts in the nanofiltration membrane permeate.

[0023] The pKa of the compound HA is more preferably 3.0 or more and 4.0 or less, from the viewpoint of easiness of forming a compound with aluminum.

[0024] pKa is defined by the following equation (1) using the concentration of the compound HA at equilibrium (HA → H + A) at 25°C, the concentration of the anion that constitutes the compound HA (A), and the acid concentration (H). pKa=-log([H]×[A] / [HA]) ···(1)

[0025] The pKa can be determined by any method such as titration. Examples of the compound HA include hydrogen fluoride, acetic acid, and formic acid. Among them, the compound HA is more preferably hydrogen fluoride, since it is easy to form a compound with aluminum ions.

[0026] If the concentration ratio of the total anion A molar concentration (mol / L) (hereinafter simply referred to as the "total anion A molar concentration"), which is the sum of the molar concentration (mol / L) of compound HA and the molar concentration (mol / L) of anion A present as an ion in solution X, to the molar concentration (mol / L) of alkali metal ions to be recovered in solution X, is P, P can be expressed by the following formula (2). Concentration ratio P = {total anion A molar concentration (mol / L)} / {molar concentration of alkali metal ions to be recovered (mol / L)} (2) Specifically, the total anion A molar concentration corresponds to the sum of the concentration of compound HA at equilibrium [HA] contained in solution X and the concentration of anions [A] that compose compound HA. When anion A comes only from compound HA added to solution X, the total anion concentration A corresponds to the anion concentration A when compound HA is completely ionized.

[0027] The concentration ratio P of the solution X is preferably 0.01 or more and 10 or less, and more preferably 0.05 or more and 7 or less.

[0028] Solution X may contain an organic compound. When the solution is an electrolyte for a lithium-ion battery, examples of the organic compounds include those mentioned above. These organic compounds may act as foulants and may cause a decrease in the recovery efficiency of the alkali metal ions, so these foulants may be removed by a pretreatment process described below.

[0029] When the solution X contains lithium ions as alkali metal ions, the lithium ion concentration in the solution is preferably 0.5 mg / L or more and 10,000 mg / L or less. When the lithium ion concentration in the solution is 0.5 mg / L or more, the efficiency of recovery of lithium ions by membrane separation is improved. Furthermore, when the lithium ion concentration in the solution is 10,000 mg / L or less, the osmotic pressure difference is suppressed from becoming large, and the efficiency of membrane separation is improved. The lithium ion concentration in the solution is more preferably 10 mg / L or more and 8,000 mg / L or less, and even more preferably 100 mg / L or more and 6,000 mg / L or less.

[0030] <3> Process (1): pH adjustment process Step (1) is a step of adjusting the pH of solution X to 0 or more and 4.0 or less. The pH adjustment step is performed when the pH of solution X exceeds 4.0. By making the pH of solution X to be treated 4.0 or less, a compound can be efficiently formed between aluminum ions and anions A ionized from compound HA in solution X and / or anions A that form compound HA contained in solution X. In addition, by making the pH of solution X 4.0 or less, precipitation of aluminum hydroxide on the membrane surface can be suppressed in the nanofiltration step described below, and membrane separation using a nanofiltration membrane can be efficiently performed.

[0031] An example of a method for adjusting the pH of solution X to 0 or more and 4.0 or less is to add an acid. The acid to be added is not particularly limited, and examples thereof include sulfuric acid, hydrochloric acid, and nitric acid.

[0032] The pH of solution X is preferably adjusted to 0 or more and 3.0 or less, and more preferably 1.0 or more and 3.0 or less. When the pH is 0 or more, deterioration of the nanofiltration membrane due to acid can be suppressed, and when the pH is 3.0 or less, the permeability of the nanofiltration membrane to alkali metal ions can be maintained at a high level.

[0033] The pH adjustment step is preferably carried out before the aluminum salt addition step described below. By carrying out the pH adjustment step before the aluminum salt addition step, precipitation of aluminum hydroxide can be suppressed, and the nanofiltration step described below can be carried out efficiently.

[0034] <4> Step (2): Adding aluminum salt Step (2) is a step of adding an aluminum salt in an amount equivalent to a molar concentration of 0.25 or more times the total anion A molar concentration (mol / L) (total anion A molar concentration), which is the sum of the molar concentration (mol / L) of compound HA in solution X and the molar concentration (mol / L) of anion A present as anion in solution X as ions. By adding an aluminum salt so that the molar concentration (mol / L) of aluminum is 0.25 or more times the total anion A molar concentration (mol / L), most of the anions A that form compound HA in solution X form compounds with aluminum ions, which are removed by the downstream nanofiltration membrane. For example, if the anion A is a fluoride ion, then a compound AlF4, which is the fluoride of aluminum ion, will be formed if the molar concentration of aluminum ions is relatively low relative to the molar concentration of fluoride ions. - As the molar concentration of aluminum ions relative to fluorine ions increases, the main component of the aluminum ion fluoride becomes AlF4 - →AlF3→AlF2 + →AlF 2+ and transition occurs.

[0035] The aluminum salt added in step (2) is not particularly limited, and examples thereof include aluminum sulfate, aluminum nitrate, and aluminum chloride.

[0036] At higher pH levels, aluminum ions tend to form compounds with anions A. Therefore, it is preferable to adjust the amount of aluminum salt added depending on the pH of solution X. Specifically, the aluminum salt is added in an amount that satisfies any one of the following conditions (a) to (d). Condition (a): When the pH of solution X is between 1.0 and 1.4, the molar concentration of aluminum in solution X is 0.5 or more times the molar concentration of the total anion A in solution X. Condition (b): When the pH of solution X is greater than 1.4 and less than 1.9, the molar concentration of aluminum in solution X is 0.4 times or more the molar concentration of the total anion A in solution X. Condition (c): When the pH of solution X is greater than 1.9 and less than 2.4, the molar concentration of aluminum in solution X is 0.3 times or more the molar concentration of the total anion A in solution X. Condition (d): When the pH of solution X is greater than 2.4 and less than 3.0, the molar concentration of aluminum in solution X is 0.25 or more times the molar concentration of the total anion A in solution X.

[0037] The aluminum salt is preferably added in an amount such that the molar concentration of aluminum relative to the total anion A molar concentration is 10 times or less, more preferably 1 time or less, and even more preferably 0.7 times or less. By adding the aluminum salt in an amount of 10 times or less, it is possible to prevent excessive aluminum from being mixed into the permeate in the subsequent nanofiltration step. In other words, it is preferable to adjust the molar concentration of aluminum relative to the total anion A molar concentration in accordance with the pH of solution X within the range of 0.25 to 10 times.

[0038] The metal ion concentration can be determined, for example, by analyzing the solution to be measured using an ICP optical emission spectrometer (e.g., a Hitachi P-4010 ICP (inductively coupled plasma optical emission spectrometry) instrument) and quantifying the concentration (mol / L) of various ions.

[0039] The means for measuring the total anion A concentration in solution X is not particularly limited as long as it can measure the molar concentration of compound HA in solution X and the molar concentration of anion A in solution X, but for example, an ion chromatograph can be used for analysis in the case of inorganic substances, or a gas chromatograph can be used for analysis in the case of organic substances, and the concentration (mol / L) of each ion can be quantified and determined. The ion chromatograph can measure the concentration of anion A when compound HA is completely ionized.

[0040] When a precipitate containing aluminum hydroxide is produced in the aluminum salt addition step, it is preferable to remove the precipitate by performing solid-liquid separation using a filter press or the like.

[0041] <5> Step (3): Nanofiltration step In the present invention, solution X after the steps (1) and (2) (hereinafter also referred to as "solution X'") is treated with a nanofiltration membrane to separate the alkali metal salt into the permeate side.

[0042] <5-1> Nanofiltration membrane The nanofiltration membrane used in the present embodiment may have a fractionation characteristic between a reverse osmosis membrane and an ultrafiltration membrane. A membrane commonly known as a reverse osmosis membrane can remove most organic substances and ions. On the other hand, an ultrafiltration membrane usually does not remove most ionic species and removes high molecular weight organic substances.

[0043] In order to separate alkali metal ions and polyvalent metal ions, it is preferable that the nanofiltration membrane has a charge on the membrane surface and is capable of both separation by pores (size separation) and electrostatic separation by charge. Such a nanofiltration membrane preferably satisfies either of the following conditions 1 and 2. <Condition 1 for nanofiltration membrane> The difference between the magnesium sulfate removal rate when a 2000 mg / L aqueous magnesium sulfate solution at 25°C and pH 6.5 is passed through the membrane at an operating pressure of 0.5 MPa and the magnesium chloride removal rate when a 2000 mg / L aqueous magnesium chloride solution at 25°C and pH 6.5 is passed through the membrane at an operating pressure of 0.5 MPa is 20% or less. <Condition 2 for nanofiltration membrane> The difference between the glucose removal rate when a 1000 mg / L aqueous glucose solution at pH 6.5 and 25°C is passed through the membrane at an operating pressure of 0.5 MPa and the isopropyl alcohol removal rate when a 1000 mg / L aqueous isopropyl alcohol solution at pH 6.5 and 25°C is passed through the membrane at an operating pressure of 0.5 MPa is 40% or more, and the glucose removal rate is 70% or more and 95% or less. Hereinafter, in this specification, when it is simply stated that "magnesium sulfate removal rate" is used, it means the magnesium sulfate removal rate when a 2000 mg / L aqueous magnesium sulfate solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa; when it is stated that "magnesium chloride removal rate" is used, it means the magnesium chloride removal rate when a 2000 mg / L aqueous magnesium chloride solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa; when it is stated that "glucose removal rate" is used, it means the glucose removal rate when a 1000 mg / L aqueous glucose solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa; and when it is stated that "isopropyl alcohol removal rate" is used, it means the isopropyl alcohol removal rate when a 1000 mg / L aqueous isopropyl alcohol solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa.

[0044] It is more preferable that the difference between the magnesium sulfate removal rate and the magnesium chloride removal rate is 15% or less under condition 1. It is also more preferable that the difference between the glucose removal rate and the isopropyl alcohol removal rate is 50% or more under condition 2, and the glucose removal rate is 80% or more and 90% or less. It is even more preferable that the nanofiltration membrane satisfies both of the above conditions 1 and 2.

[0045] By having the glucose removal rate of the nanofiltration membrane, the difference between the glucose removal rate and the isopropyl alcohol removal rate, and the difference between the magnesium sulfate removal rate and the magnesium chloride removal rate all within the above ranges, selective separation and recovery of alkali metal ions and polyvalent metal ions can be performed with high efficiency and over the long term, making it possible to achieve a highly efficient process. In particular, the inventors have found that when the glucose removal rate and the difference between the glucose removal rate and the isopropyl alcohol removal rate are within the above ranges, the selective separation of compounds of alkali metal ions and aluminum anions A having various valences is high. Furthermore, the inventors have found that by setting the difference between the magnesium sulfate rejection rate and the magnesium chloride rejection rate within the above range, the charge on the membrane surface becomes close to neutral, making fouling due to inorganic scale such as aluminum fluoride less likely to occur, and enabling long-term operation. Inorganic scale is likely to occur during filtration in a relatively high pH range.

[0046] Nanofiltration membranes are made of polymers such as cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides, and vinyl polymers. Nanofiltration membranes may be made of only one material, or may be made of multiple materials. The membrane structure may be an asymmetric membrane having a dense layer on at least one side of the membrane and gradually increasing micropores from the dense layer toward the inside of the membrane or toward the other side, or a composite semipermeable membrane having a very thin separation functional layer made of another material on the dense layer of the asymmetric membrane.

[0047] The composite semipermeable membrane is preferably, for example, a membrane having a porous support membrane containing polysulfone and a separation functional layer containing polyamide provided on the porous support membrane. The composite semipermeable membrane may also have a substrate in addition to the porous support membrane and the separation functional layer, in which case the porous support membrane is provided on the substrate. The polyamide is a thin film formed on the porous support membrane by a polycondensation reaction between a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide.

[0048] The separation functional layer in the composite semipermeable membrane preferably contains 50% by mass or more of a semi-aromatic crosslinked polyamide obtained by interfacial polycondensation of a divalent or higher polyfunctional aliphatic amine compound and a divalent or higher polyfunctional aromatic acid halide, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably consists of only a semi-aromatic crosslinked polyamide. By containing 50% by mass or more of a semi-aromatic crosslinked polyamide, excessive densification due to π-π interactions derived from aromatic rings in the semi-aromatic crosslinked polyamide is suppressed, and excellent alkali metal ion permeability is obtained. Furthermore, the present inventors have found, as a result of intensive research, that when the relative humidity during the interfacial polycondensation is controlled to 80% or more, the resulting composite semipermeable membrane exhibits particularly excellent membrane performance under acidic conditions. The relative humidity can be adjusted by using a precision air conditioning device, for example.

[0049] The polyfunctional aliphatic amine is preferably a cycloaliphatic diamine, more preferably a 4,4'-bipiperidine or piperazine derivative.

[0050] The molecular weight of the alicyclic diamine is preferably 90 or more and 160 or less. When the molecular weight of the alicyclic diamine is 90 or more, the diffusion coefficient of the amine becomes small, and polyamide is gradually formed during the interfacial polycondensation, so that a separation functional layer having a uniform pore size in the film thickness direction is easily formed from the early to middle stages of the polycondensation. In addition, usually, at the end of the initial stage of polycondensation, oligomers are generated excessively on the surface of the support in contact with the organic layer, and the pores on the surface of the support are blocked, which causes the pore size distribution in the film thickness direction to become non-uniform, but by making the molecular weight of the alicyclic diamine 160 or less, the molecular weight of the generated oligomers can be reduced and the interaction with the semi-aromatic crosslinked polyamide can be reduced, so that after the separation functional layer is formed by the polycondensation reaction, the oligomers are easily detached from the separation functional layer, and a separation functional layer with a uniform pore size in the film thickness direction can be easily formed. In addition, by making the molecular weight of the alicyclic diamine 90 or more and 160 or less, it is easy to form a membrane pore size that satisfies the above condition 2.

[0051] Examples of alicyclic diamines having a molecular weight of 90 or more and 160 or less include substituted piperazines in which the piperazine ring is substituted with an alkyl group having 1 to 3 carbon atoms (e.g., 2-methylpiperazine, 2-ethylpiperazine, 2-normal propylpiperazine, 2,2-dimethylpiperazine, 2,2-diethylpiperazine, 2,3-dimethylpiperazine, 2,3-diethylpiperazine, 2,5-dimethylpiperazine, 2,5-diethylpiperazine, 2,6-dimethylpiperazine, 2,6-diethylpiperazine, 2,3,5,6-tetramethylpiperazine, etc.) and homopiperazine.

[0052] The polyfunctional aromatic acid halide is an aromatic acid halide having two or more halogenated carbonyl groups in one molecule, and is not particularly limited as long as it reacts with the polyfunctional aliphatic amine to give a semi-aromatic crosslinked polyamide. As the polyfunctional aromatic acid halide, for example, halides such as 1,3,5-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid, 1,3-benzenedicarboxylic acid, 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid can be used. Among the polyfunctional aromatic acid halides, acid chlorides are preferred, and in particular, from the standpoints of economy, ease of availability, ease of handling, ease of reactivity, and the like, trimesoyl chloride, which is an acid halide of 1,3,5-benzenetricarboxylic acid, isophthaloyl chloride, which is an acid halide of 1,3-benzenedicarboxylic acid, terephthaloyl chloride, which is an acid halide of 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid chloride, which is an acid halide of 1,3,5-benzenetrisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid chloride, which is an acid halide of 1,3,6-naphthalenetrisulfonic acid, are preferred. The polyfunctional aromatic acid halide may be used alone or in a mixture of two or more kinds, but by mixing the trifunctional trimesic acid chloride, 1,3,5-benzenetrisulfonic acid chloride, or 1,3,6-naphthalenetrisulfonic acid chloride with either one of the bifunctional isophthalic acid chloride or terephthalic acid chloride, the molecular gap of the polyamide crosslinked structure is expanded, and a membrane with a uniform pore size distribution can be controlled over a wide range. The mixing molar ratio of the trifunctional acid chloride to the bifunctional acid chloride is preferably 1:20 to 50:1, and more preferably 1:1 to 20:1. In addition, by appropriately adjusting the ratio of the polyfunctional aromatic acid halide to the polyfunctional aliphatic amine added during interfacial polycondensation, it becomes easier to form a membrane pore size that satisfies the above condition 1.

[0053] The above-mentioned composite semipermeable membrane can be obtained, for example, by forming a porous support membrane on a substrate, and then forming a separation functional layer containing a semi-aromatic crosslinked polyamide on the porous support membrane by polycondensing a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide.

[0054] <5-2>Separation by nanofiltration membrane Since alkali metal ions easily permeate nanofiltration membranes and aluminum ions have difficulty permeating nanofiltration membranes, it is possible to separate the alkali metal ions from a compound composed of aluminum and anion A (hereinafter also referred to as "compound B") in solution X (solution X') after steps (1) and (2) have been performed. The nanofiltration membrane is preferably used in a state where it is incorporated into an element such as a spiral type.

[0055] By sending solution X' to the nanofiltration membrane unit, it is possible to obtain a permeate with a smaller concentration ratio P relative to solution X' and a concentrated solution with a larger concentration ratio P. Alkali metal ions that easily pass through the nanofiltration membrane are separated into the permeate side. In order to increase the purity of the alkali metal salt, the permeate may be passed through the nanofiltration membrane unit again to obtain a permeate with a lower concentration ratio P. The permeate may also be nanofiltered multiple times.

[0056] In the nanofiltration step, it is preferable to supply the solution to the nanofiltration membrane at an operating pressure in the range of 0.1 MPa to 8 MPa. If the operating pressure is 0.1 MPa or more, the membrane permeation rate is improved, and if it is 8 MPa or less, damage to the nanofiltration membrane can be suppressed. The operating pressure is more preferably 0.5 MPa to 6 MPa, and even more preferably 1 MPa to 4 MPa.

[0057] <6> Concentration process The alkali metal ion concentration (mg / L) in the permeate of the final-stage nanofiltration membrane unit is preferably 1 mg / L or more, more preferably 10 mg / L or more, and even more preferably 100 mg / L or more. By having an alkali metal ion concentration of 1 mg / L or more, the recovery efficiency of the alkali metal salt in the subsequent stage is improved.

[0058] In order to increase the alkali metal ion concentration (mg / L) in the permeate from the final nanofiltration membrane unit, the permeate from the final nanofiltration membrane unit may be concentrated. The method of concentration is not particularly limited, but may include concentration by evaporation or concentration using a reverse osmosis membrane.

[0059] <7> Organic matter removal process Solution X' may be subjected to an organic matter removal step prior to the nanofiltration step of solution X'. When solution X' is a liquid containing an electrolyte and the organic solvent and water are separated, it is preferable to separate the oil phase and the water phase using any oil-water separation method. Organic matter dissolved in water is preferably removed using an ultrafiltration membrane. Since high molecular weight organic matter can be removed by ultrafiltration, fouling of the nanofiltration membrane can be suppressed by removing high molecular weight organic matter. The water phase in the oil-water separation or the permeate of the ultrafiltration is used in the nanofiltration step of solution X'.

[0060] <8> Recovery process In this step, the alkali metal salt is recovered from the permeate (aqueous alkali metal salt solution) obtained in the nanofiltration step of step (3).

[0061] The alkali metal salt can be recovered by a known method. For example, when the alkali metal salt is a potassium salt, the recovery is carried out by utilizing the temperature dependency of solubility or by adding a poor solvent such as ethanol. Lithium salts have a lower solubility than other alkali metal salts. For example, sodium carbonate and potassium carbonate have a high solubility in water (20 g or more per 100 mL of water), but lithium carbonate dissolves only at 1.33 g per 100 mL of water at 25°C. Therefore, by adding carbonate to the permeate, lithium can be recovered as lithium carbonate. The solubility of lithium carbonate decreases further at high temperatures, so the aqueous solution may be heated.

[0062] <Collection device> The alkali metal salt recovery apparatus of the present invention for carrying out the alkali metal salt recovery method of the present invention comprises a tank for storing a solution containing alkali metal ions (solution X), an aluminum salt adding means for adding an aluminum salt to the solution, and a nanofiltration means for separating the alkali metal ions from the solution. It is more preferable that the recovery apparatus of the present invention further comprises a pH adjustment means for adjusting the pH of the solution.

[0063] The nanofiltration means preferably includes a pressure vessel (hereinafter also referred to as "vessel") filled with a spiral-type element (hereinafter also referred to as "nanofiltration membrane element") incorporating a nanofiltration membrane capable of separating alkali metal ions, and a nanofiltration membrane unit capable of supplying a solution to the vessel by a high-pressure pump. The nanofiltration membrane unit may be configured by connecting multiple vessels in parallel or series, and each vessel may be filled with multiple nanofiltration membrane elements. Nanofiltration membrane elements having any diameter and length can be used. Nanofiltration membrane elements vary in size depending on the membrane area, and for the same membrane type, the larger the membrane area, the more liquid volume can be processed per unit time. The size and number of nanofiltration membrane elements can be determined arbitrarily depending on the scale of the solution to be processed.

[0064] The aluminum salt adding means provided in the recovery apparatus of the present invention is not particularly limited as long as it has a mechanism for adding aluminum salt to a solution. For example, it may be equipment for adding aluminum salt as a solid to a tank storing a solution having a stirring means, or equipment for adding aluminum salt directly to the solution from a tank storing an aqueous solution of high concentration aluminum salt by a chemical feed pump. Among these, from the viewpoint of the size of the equipment and the solubility of aluminum salt in the solution, it is preferable to have a mechanism for adding aluminum salt to the solution from a tank storing high concentration aluminum salt by a chemical feed pump.

[0065] Methods for adding a high concentration aluminum salt from a chemical injection pump include a flow rate proportional method in which a constant amount of a high concentration aqueous aluminum salt solution is added from the chemical injection pump in proportion to the permeation flow rate of the nanofiltration means, and a timer method in which a constant amount of a high concentration aqueous aluminum salt solution is added from the chemical injection pump at regular intervals, and among these, the flow rate proportional method is more preferred.

[0066] There are no particular limitations on the pH adjustment means as long as it has a mechanism for adjusting the pH of the solution. For example, a chemical injection pump for adding an acidic or alkaline aqueous solution to the solution and equipment for measuring the pH of the solution, such as a pH meter, may be installed in a tank for storing the solution. Furthermore, it is preferable to have a mechanism for feeding back the pH value of the solution to the chemical injection pump to adjust the pH of the solution. EXAMPLES

[0067] The present invention will be described below with reference to examples, but the present invention is not limited to these examples in any way. In the examples and comparative examples, measurements were carried out as follows.

[0068] <Nanofiltration membrane performance> (Glucose removal rate and isopropyl alcohol removal rate of nanofiltration membrane) The permeate obtained when a 1000 mg / L glucose aqueous solution at 25 ° C. and pH 6.5 was permeated through a nanofiltration membrane at an operating pressure of 0.5 MPa as the feed water, and the permeate obtained when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 was permeated through a nanofiltration membrane at an operating pressure of 0.5 MPa as the feed water, and the glucose concentration and isopropyl alcohol concentration of each feed water were calculated using the following formula. The isopropyl alcohol removal rate and glucose removal rate were calculated using the following formula. Isopropyl alcohol removal rate (%) = 100 × {1-(isopropyl alcohol concentration in permeate water / isopropyl alcohol concentration in feed water)} Glucose removal rate (%) = 100 × {1-(glucose concentration in permeate water / glucose concentration in feed water)} The isopropyl alcohol concentration was determined using a gas chromatograph (GC-18A, manufactured by Shimadzu Corporation), and the glucose concentration was determined using a refractometer (RID-6A, manufactured by Shimadzu Corporation).

[0069] (Magnesium sulfate removal rate and magnesium chloride removal rate of nanofiltration membrane) The magnesium sulfate (MgSO4) concentration and magnesium chloride (MgCl2) concentration of each feed water were measured. The permeate was measured when a 2000 mg / L aqueous magnesium sulfate solution at 25°C and pH 6.5 was permeated through a nanofiltration membrane at an operating pressure of 0.5 MPa. The permeate was measured when a 2000 mg / L aqueous magnesium chloride solution at 25°C and pH 6.5 was permeated through a nanofiltration membrane at an operating pressure of 0.5 MPa. The practical salinity, i.e., MgSO4 concentration and MgCl2 concentration, of the feed water and permeated water was obtained by measuring the electrical conductivity with an electrical conductivity meter manufactured by Toa Denpa Kogyo Co., Ltd. The MgSO4 removal rate and MgCl2 removal rate were calculated based on the thus obtained MgSO4 concentration and MgCl2 concentration and the following formula. MgSO4 removal rate (%) = 100 x {1 - (MgSO4 concentration in permeate water / MgSO4 concentration in feed water)} MgCl2 removal rate (%) = 100 × {1-(MgCl2 concentration in permeate water / MgCl2 concentration in feed water)}

[0070] <Nanofiltration membrane> (Nanofiltration membrane A) Nonwoven fabric made of polyester fibers (breathability 1cc / cm 2 A 18.0 mass% dimethylformamide (DMF) solution of polysulfone was cast onto a substrate (100 / s) at room temperature (25°C) to a thickness of 180 μm, and then immediately immersed in pure water and left for 5 minutes to produce a porous support membrane (160 μm thick) made of fiber-reinforced polysulfone. Next, the membrane surface temperature of the porous support membrane was adjusted to 25 ° C. while blowing air adjusted to 25 ° C. to remove excess moisture. A 30 ° C. aqueous solution containing 2.0 mass % piperazine, 250 ppm sodium dodecyl diphenyl ether disulfonate, and 1.0 mass % trisodium phosphate was applied to the surface of the porous support membrane and left to stand for 15 seconds, and then nitrogen was blown from an air nozzle to remove excess aqueous solution to form a coating layer of an amine aqueous solution on the porous support membrane. Furthermore, a 38 ° C. n-decane solution containing 0.2 mass % trimesic acid chloride (hereinafter referred to as "TMC") was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand for 1 minute at a relative humidity of 70% and a temperature of 25 ° C. to perform interfacial polycondensation, and two fluids (pure water and air) were blown onto the membrane surface to remove the solution on the surface. After that, it was washed with pure water at 80 ° C. to obtain a nanofiltration membrane A.

[0071] (Nanofiltration membrane B) Nanofiltration membrane B was prepared in the same manner as nanofiltration membrane A, except that the piperazine was changed to 2,5-dimethylpiperazine, and a 38°C n-decane solution containing 0.2 mass% TMC was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand at a relative humidity of 80% and 25°C for 1 minute.

[0072] (Nanofiltration membrane C) Nanofiltration membrane C was obtained by producing a nanofiltration membrane in the same manner as nanofiltration membrane A, except that the TMC concentration was 0.5 mass %.

[0073] The membrane performances of nanofiltration membrane A, nanofiltration membrane B and nanofiltration membrane C are shown in Table 1.

[0074] [Table 1]

[0075] <Liquid to be treated> (Solutions Xa-Xf) On page 282, line 14 of Non-Patent Document 1, a 1.0 mol / L LiPF6 solution is described as an electrolyte. To simulate the molar concentration of lithium (Li) and fluorine (F) when this electrolyte is diluted 10 times with water, Li2SO4 was added to water to give 0.05 mol / L and HF to give 0.60 mol / L, and the pH was adjusted with sodium hydroxide or sulfuric acid to the values ​​shown in Table 2 to prepare solutions Xa to Xf. Note that Li and F in Table 2 are the molar concentrations in a fully ionized state.

[0076] [Table 2]

[0077] (Solution Xa-2) Solution Xa-2 was prepared by using solution Xa but containing acetate ions at the same molar concentration as compound HA instead of fluorine.

[0078] (Solution Xa-3) Solution Xa-3 was prepared by using solution Xa but containing formate ions at the same molar concentration as compound HA instead of fluorine.

[0079] (Solution Xa-4) Solution Xa-4 was prepared by using solution Xa but containing phosphate ions at the same molar concentration as compound HA instead of fluorine.

[0080] (Solution Xa-5) Solution Xa-5 was prepared by using solution Xa but containing cyanide ions instead of fluorine as compound HA at the same molar concentration.

[0081] <Evaluation of the nanofiltration process> (concentration ratio P) The concentration ratio P was calculated based on the molar concentrations of various ions in the solution according to the following formula. Concentration ratio P = {molar concentration of total anion A in solution (mol / L)} / {molar concentration of lithium ions (mol / L)}

[0082] (Pressure ratio) Nanofiltration membrane is used in cross-flow filtration, permeate flow rate: concentrate flow rate = 1:9, permeation flux 0.4m 2 / m 3 Constant flow rate filtration was performed under conditions of 100 / s. The pressure ratio was defined as the ratio of the operating pressure (MPa) after 24 hours to the operating pressure (MPa) after 1 hour from the start of constant flow rate operation. A higher pressure ratio indicates more fouling of the nanofiltration membrane.

[0083] Example 1 Aluminum sulfate was added to solution Xa at 25°C so that the Al concentration was 0.20 mol / L. Nanofiltration membrane A was used for cross-flow filtration with a permeate flow rate: concentrate flow rate = 1:9 and a permeation flux of 0.4 m 2 / m 3 Filtration was performed at a constant flow rate of 1000 ml / s.

[0084] Example 2 The same procedure as in Example 1 was carried out, except that aluminum sulfate was added so that the Al concentration became 0.25 mol / L.

[0085] Example 3 The same procedure as in Example 1 was carried out, except that aluminum sulfate was added so that the Al concentration was 0.30 mol / L.

[0086] Example 4 The same procedure as in Example 1 was repeated, except that solution Xb was used instead of solution Xa.

[0087] Example 5 The same procedure as in Example 4 was carried out, except that aluminum sulfate was added so that the Al concentration became 0.25 mmol / L.

[0088] Example 6 The same procedure as in Example 1 was repeated, except that solution Xc was used instead of solution Xa.

[0089] Example 7 The same procedure as in Example 1 was repeated, except that solution Xd was used instead of solution Xa.

[0090] Example 8 The same procedure as in Example 1 was carried out, except that solution Xe was used instead of solution Xa.

[0091] Example 9 The same procedure as in Example 5 was repeated, except that nanofiltration membrane B was used instead of nanofiltration membrane A.

[0092] Example 10 The same procedure as in Example 1 was repeated, except that solution Xa-2 was used instead of solution Xa.

[0093] Example 11 The same procedure as in Example 1 was repeated, except that solution Xa-3 was used instead of solution Xa.

[0094] Example 12 The same procedure as in Example 8 was repeated, except that nanofiltration membrane B was used instead of nanofiltration membrane A.

[0095] Example 13 The same procedure as in Example 8 was repeated, except that nanofiltration membrane C was used instead of nanofiltration membrane A.

[0096] Comparative Example 1 The same procedure as in Example 1 was repeated except that no aluminum sulfate was added.

[0097] Comparative Example 2 The same procedure as in Example 1 was carried out, except that aluminum sulfate was added so that the Al concentration was 0.05 mmol / L.

[0098] Comparative Example 3 Except for using solution Xf instead of solution Xa, the same procedure as in Example 1 was carried out. As a result, the membrane surface was clogged with precipitates in the nanofiltration step, making it impossible to carry out filtration.

[0099] Comparative Example 4 The same procedure as in Example 1 was repeated, except that solution Xa-4 was used instead of solution Xa.

[0100] Comparative Example 5 The same procedure as in Example 1 was repeated, except that solution Xa-5 was used instead of solution Xa.

[0101] The analysis results of the composition of the initial permeate one hour after the start of constant flow rate operation and the pressure ratios performed for Examples 1 to 13 and Comparative Examples 1, 2, 4 and 5 are shown in Tables 3 and 4. The molar ratios of "Al / total anion A" in Tables 3 and 4 were calculated from the concentration of anion A when compound HA was completely ionized.

[0102] [Table 3]

[0103] [Table 4]

[0104] In the above examples and comparative examples, the lower the concentration ratio of the permeate through the nanofiltration membrane and the lower the pressure ratio, the more advantageous it is. From the above results, it was found that Examples 1 to 13, which are the alkali metal salt recovery methods of the present invention, were able to recover alkali metal salts with higher purity relative to impurities such as fluorine as compared with Comparative Examples 1 to 5.

[0105] Although the present invention has been described in detail using specific embodiments, it is clear to those skilled in the art that various modifications and variations are possible without departing from the intent and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-075075) filed on April 28, 2023, and is incorporated by reference in its entirety. [Industrial Applicability]

[0106] INDUSTRIAL APPLICABILITY The present invention can be suitably used as a method for efficiently separating and recovering alkali metals such as lithium from waste materials, waste liquids, ores, slag, and the like generated in the manufacturing process of lithium ion batteries.

Claims

1. A method for recovering an alkali metal salt from a solution X containing at least one of a compound HA having a pKa of 3.0 or more and 5.0 or less and an anion A forming the compound HA, and an alkali metal ion, the method comprising the steps of: (1) a pH adjustment step of adjusting the pH of the solution X to 0 or more and 4.0 or less when the pH of the solution X exceeds 4.0; (2) An aluminum salt addition step in which an aluminum salt is added to the solution X so that the molar concentration (mol / L) of aluminum is 0.25 times or more the total molar concentration (mol / L) of anion A, which is the sum of the molar concentration (mol / L) of the compound HA in the solution X and the molar concentration (mol / L) of anion A present as an ion in the solution X. (3) A nanofiltration step in which alkali metal ions are separated from the solution X after steps (1) and (2) by a nanofiltration membrane to the permeate side.

2. 2. The method for recovering an alkali metal salt according to claim 1, wherein in the step (1), the pH of the solution X is adjusted to 1.0 or more and 3.0 or less.

3. 3. The method for recovering an alkali metal salt according to claim 1 or 2, wherein the pKa of the compound HA is 3.0 or more and 4.0 or less.

4. 3. The method for recovering an alkali metal salt according to claim 1, wherein in the step (2), an aluminum salt is added to the solution X so as to satisfy any one of the following conditions (a) to (d): Condition (a): When the pH of solution X is 1.0 or more and 1.4 or less, the molar concentration of the aluminum in solution X is 0.5 or more times the molar concentration of the total anion A in solution X. Condition (b): When the pH of solution X is greater than 1.4 and equal to or less than 1.9, the molar concentration of aluminum relative to the molar concentration of the total anion A in solution X is 0.4 times or more. Condition (c): When the pH of solution X is greater than 1.9 and equal to or less than 2.4, the molar concentration of aluminum relative to the molar concentration of the total anion A in solution X is 0.3 or more times. Condition (d): when the pH of solution X is greater than 2.4 and equal to or less than 3.0, the molar concentration of aluminum relative to the molar concentration of the total anion A in solution X is 0.25 or more times.

5. 3. The method for recovering an alkali metal salt according to claim 1 or 2, wherein a difference between a magnesium sulfate removal rate when a 2000 mg / L aqueous magnesium sulfate solution having a pH of 6.5 and a concentration of 25° C. is allowed to permeate through the nanofiltration membrane at an operating pressure of 0.5 MPa and a magnesium chloride removal rate when a 2000 mg / L aqueous magnesium chloride solution having a pH of 6.5 and a concentration of 25° C. is allowed to permeate through the nanofiltration membrane at an operating pressure of 0.5 MPa is 20% or less.

6. 3. The method for recovering an alkali metal salt according to claim 1 or 2, wherein the difference between the glucose removal rate when a 1000 mg / L glucose aqueous solution at 25 ° C. and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa and the isopropyl alcohol removal rate when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa is 40% or more, and the glucose removal rate is 70% or more and 95% or less.

7. 3. The method for recovering an alkali metal salt according to claim 1 or 2, wherein the compound HA contains fluorine.

8. 3. The method for recovering an alkali metal salt according to claim 1 or 2, wherein the alkali metal salt comprises lithium.

9. An apparatus for recovering an alkali metal salt for carrying out the method for recovering an alkali metal salt according to claim 1 or 2, comprising: A tank for storing a solution containing alkali metal ions; an aluminum salt adding means for adding an aluminum salt to the solution; a nanofiltration means for separating alkali metal ions from said solution; An apparatus for recovering alkali metal salt comprising:

10. 10. The apparatus for recovering an alkali metal salt according to claim 9, further comprising a pH adjusting means for adjusting the pH of the solution.