Method for recovering alkali metal salt and device for recovering alkali metal salt

Abstract

The present invention relates to a recovery method of an alkali metal salt comprising the following processes 1 and 2. Process 1: a solution X containing an alkali metal ion is sent as a treated liquid A to a nanofiltration membrane unit A, separated into a permeate A and a concentrate B, and the concentrate B is mixed with the remaining portion of the treated liquid A, and sent again to the nanofiltration membrane unit A, to further obtain a first nanofiltration process of the permeate A. Process 2: the permeate A obtained in the process 1 or a concentrate of the permeate A is sent as a treated liquid B to a nanofiltration membrane unit A, separated into a permeate C and a concentrate D, and the concentrate D is mixed with the remaining portion of the treated liquid B, and sent again to the nanofiltration membrane unit A, to further obtain the permeate C, or the permeate A obtained in the process 1 or a concentrate of the permeate A is sent as a treated liquid B to a nanofiltration membrane unit B, separated into a permeate C and a concentrate D, and the concentrate D is mixed with the remaining portion of the treated liquid B, and sent again to the nanofiltration membrane unit B, to further obtain a second nanofiltration process of the permeate C.

Description

Technical Field

[0001] This invention relates to a method and apparatus for recovering alkali metal salts. Background Technology

[0002] In recent years, with the development of the world economy, the demand for mineral resources has expanded significantly. For example, the demand for lithium as a material for lithium-ion batteries is increasing, and lithium carbonate is also used as an additive in heat-resistant glass and in surface acoustic wave filters. In particular, high-purity lithium is used as filters and transmitters in mobile phones and car navigation systems.

[0003] In addition, cobalt is widely used in various industries as an alloying element for specialty steels and magnetic materials. For example, specialty steels are used in aerospace, generators, and special tools, while magnetic materials are used in small headphones and small motors. Cobalt is also used as a raw material for the positive electrode of lithium-ion batteries, and the demand for cobalt is increasing with the widespread use of mobile information processing terminals such as smartphones, as well as batteries for automobiles and energy storage.

[0004] Nickel, with its high luster and corrosion resistance, is used in stainless steel, and in recent years, its demand as a material for lithium-ion batteries, similar to cobalt, has been increasing. Thus, given the rising demand for various rare metals, measures are being promoted from the perspective of recycling valuable resources to recover rare metals such as lithium, cobalt, and nickel from used lithium-ion batteries or waste generated during their manufacturing processes.

[0005] For example, the recycling of waste lithium-ion batteries is progressing towards practical applications, primarily focusing on rare metals such as cobalt and nickel. However, since solvent extraction using chelating agents is the mainstream method, it not only has a significant environmental impact but also presents cost disadvantages (Non-Patent Document 1). To address this issue, a method for separating and recycling lithium from acid-leached aqueous solutions of waste lithium-ion batteries using separation membranes such as ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes has been disclosed (Patent Document 1). However, in this separation and recycling method, the nanofiltration membrane is a single-stage process, so unless the performance of the nanofiltration membrane is drastically improved, it is difficult to recover lithium with high purity and high efficiency.

[0006] Therefore, a separation and recovery method using a nanofiltration membrane for multi-stage processing has been disclosed (Patent Document 2). That is, this method is a continuous process that improves lithium purity by passing the liquid that has passed through the nanofiltration membrane through it again, and recovers residual lithium by passing the liquid that has not passed through the nanofiltration membrane through it.

[0007] Prior art literature

[0008] Patent Document 1: International Publication No. 2019 / 018333

[0009] Patent Document 2: International Publication No. 2021 / 215484

[0010] Non-Patent Document 1: "Report on the Mineral Resource Basic Preparation Survey Project (Basic Survey Related to the Formulation of Mineral Resource Security Strategy) for Exploration and Other Businesses to Promote Mineral Resource Development in 2018", Environment and Energy Division, Mitsubishi Research Laboratories, Inc., March 2018. Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] However, the method described in Patent Document 2 is complex and continuous. Therefore, if the composition of the liquid being processed changes or the separation performance changes due to the deterioration of the nanofiltration membrane, the nanofiltration membrane processing step may become unstable. There is room for improvement in maintaining the predetermined lithium purity and recovery rate.

[0013] The purpose of this invention is to provide a method for the stable, high-purity, and efficient recovery of alkali metal salts from waste materials, waste liquids, or ores generated in the manufacturing process of lithium-ion batteries or their manufacturing processes with fewer steps.

[0014] Methods for solving problems

[0015] To achieve the above objectives, the present invention adopts the following technical configuration.

[0016] (1) A method for recovering alkali metal salts, comprising the following steps 1 and 2.

[0017] Step 1: A solution X containing alkali metal ions is transported to the nanofiltration membrane unit A as the liquid to be treated, and separated into permeate A and concentrate B. The concentrate B is then mixed with the remaining portion of the liquid to be treated A and transported to the nanofiltration membrane unit A again to further obtain the first nanofiltration process of the permeate A.

[0018] Step 2: The permeate A or its concentrate obtained in Step 1 is fed to the nanofiltration membrane unit A as the treated liquid B, separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed to the nanofiltration membrane unit A again to further obtain the permeate C. Alternatively, the permeate A or its concentrate obtained in Step 1 is fed to the nanofiltration membrane unit B as the treated liquid B, separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed to the nanofiltration membrane unit B again to further obtain the permeate C in this second nanofiltration step.

[0019] (2) Based on the alkali metal salt recovery method described in (1) above,

[0020] The permeate C is obtained by sequentially processing N solutions X (N: an integer greater than 2) by passing the solutions X through step 1 and step 2. Step 2 uses the nanofiltration membrane unit B. During the period after step 1 is performed on the kth solution X(k) (k: an integer greater than 1 and less than (N-1)) of the N solutions X, step 1 is performed in parallel on the (k+1)th solution X(k+1).

[0021] (3) The method for recovering alkali metal salts according to (1) or (2) above,

[0022] The process includes a step of diluting at least one of the liquid to be treated, A, and the liquid to be treated, B.

[0023] (4) The method for recovering alkali metal salts according to (2) or (3) above also includes the following step 3.

[0024] Step 3: A reverse osmosis filtration step in which at least one of the kth permeate A(k) and the kth permeate C(k) is concentrated in at least one of the solutions X(k).

[0025] (5) The method for recovering alkali metal salts as described in (4) above,

[0026] The process 3 is performed only once on the permeate C(k).

[0027] (6) The method for recovering alkali metal salts according to any one of (1) to (5) above,

[0028] The pH value of solution X is below 4.

[0029] (7) The method for recovering alkali metal salts according to any one of (1) to (6) above,

[0030] The alkali metal ions include lithium ions.

[0031] (8) The method for recovering alkali metal salts according to any one of (1) to (7) above includes the following step 4.

[0032] Step 4: There are N solutions X (N: an integer greater than 2). After the completion of Step 2 for the kth solution X(k) (k: an integer greater than 1 and less than (N-1)) among the N solutions X, the remaining portion of the kth processed liquid B(k) mixed with the kth concentrate D(k) is added to the mth solution X(m) (m: an integer greater than (k+1) and less than N) or the mth processed liquid A(m).

[0033] (9) The method for recovering alkali metal salts according to any one of (1) to (8) above,

[0034] At least one of the nanofiltration membrane units A and B has a nanofiltration membrane that has a porous support membrane and a separation functional layer.

[0035] When a positron beam is irradiated onto the surface of the separation functional layer side of the nanofiltration membrane, the average pore size R1 and average pore size R2 of the separation functional layer, derived by positron annihilation lifetime determination, satisfy 0.90≤R1 / R2≤1.10.

[0036] R1: Average aperture under a positron beam intensity of 0.1 keV.

[0037] R2: Average aperture under a positron beam intensity of 0.5 keV.

[0038] (10) The method for recovering alkali metal salts according to any one of (1) to (9) above,

[0039] At least one of the steps 1 and 2 is carried out with a constant permeation flow rate, the change in operating pressure over time is monitored, and at least one of the steps 1 and 2 is terminated when the recovery rate A (%) of alkali metal ions reaches the target value based on the following formula (2).

[0040]

[0041] In formula (2), the recovery rate of alkali metal ions is A (%), the operating pressure is P (Pa), the initial operating pressure is P0 (Pa), and the initial liquid volume of the object being treated is V0 (m³). 3 ), alkali metal ion removal rate R (%) of nanofiltration membrane, liquid recovery rate S (%) of nanofiltration process, supply flow rate Q F (m 3 / s), concentrate flow rate Q c (m 3 / s), filtering end time t = tb.

[0042] (11) The method for recovering alkali metal salts according to any one of (4) to (10) above,

[0043] At least one of the permeate A(k) and the permeate C(k) contains neutral molecules that do not have a charge under conditions with a pH value below 3, and the reverse osmosis filter membrane used in the reverse osmosis filtration process is a low-removal reverse osmosis membrane that achieves an isopropanol removal rate of 70% or more and less than 85% when permeating an isopropanol aqueous solution at 25°C and a pH value of 6.5 under an operating pressure of 0.5 MPa.

[0044] (12) The method for recovering alkali metal salts as described in (11) above,

[0045] The neutral molecule is a boron compound.

[0046] (13) The method for recovering alkali metal salts as described in (12) above,

[0047] In step 3, there is a recycling step in which the concentrate obtained in the reverse osmosis filtration step is mixed with the solution supplied to the reverse osmosis filtration step.

[0048] (14) The method for recovering alkali metal salts according to any one of (11) to (13) above,

[0049] The device includes a step of conveying the permeate obtained in the reverse osmosis filtration process to a high-removal reverse osmosis membrane unit, and adding the obtained permeate as dilution water for at least one of the treated liquid A and the treated liquid B. The high-removal reverse osmosis membrane unit is equipped with a high-removal reverse osmosis membrane that achieves an isopropanol removal rate of 85% or more and 95% or less when permeating an isopropanol aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa.

[0050] (15) A device for recovering alkali metal salts, comprising:

[0051] A solution containing alkali metal ions is used as the liquid to be treated, A, and is separated into permeate A and concentrate B by the first nanofiltration membrane unit.

[0052] A first circulation mechanism that mixes the concentrated liquid B with the remaining portion of the liquid A being treated;

[0053] The permeate A or the concentrate of the permeate A is used as the treated liquid B, and the second separation mechanism separates it into permeate C and concentrate D through the second nanofiltration membrane unit.

[0054] A second circulation mechanism that mixes the concentrated liquid D with the remaining portion of the treated liquid B;

[0055] A dilution mechanism for adding dilution water to at least one of the liquid to be treated, A, and B;

[0056] A flow control mechanism capable of controlling the flow rates of permeate A and concentrate B in the first separation unit and permeate C and concentrate D in the second separation unit; and

[0057] A flow control mechanism that synchronizes the flow rate of the added dilution water in the dilution mechanism with the flow rate of the permeate when the treated liquid with added dilution water is delivered to the nanofiltration membrane unit.

[0058] The effects of the invention

[0059] According to the method for recovering alkali metal salts of the present invention, alkali metal salts such as lithium or cesium can be stably and efficiently recovered from solutions containing alkali metal ions with high purity with fewer steps. Attached Figure Description

[0060] Figure 1 This is a schematic flowchart illustrating a method for recovering alkali metal salts according to one embodiment of the present invention.

[0061] Figure 2 This is a schematic flowchart illustrating a method for recovering alkali metal salts according to another embodiment of the present invention.

[0062] Figure 3 This is a schematic flowchart illustrating a method for recovering alkali metal salts according to another embodiment of the present invention.

[0063] Figure 4 This is a simplified flowchart illustrating the recovery method of alkali metal salts in the comparison approach.

[0064] Figure 5 This is a schematic flowchart illustrating a method for recovering alkali metal salts according to another embodiment of the present invention.

[0065] Figure 6 This is a simplified flowchart illustrating the recovery method of alkali metal salts in the comparison approach.

[0066] Figure 7 This is a simplified flowchart illustrating the recovery method of alkali metal salts in the comparison approach.

[0067] Figure 8 This is a schematic flowchart illustrating a method for recovering alkali metal salts according to another embodiment of the present invention.

[0068] Figure 9 This is a schematic flowchart illustrating a method for recovering alkali metal salts according to another embodiment of the present invention.

[0069] Figure 10 This is a schematic flowchart illustrating a method for recovering alkali metal salts according to another embodiment of the present invention. Detailed Implementation

[0070] The embodiments of the present invention will be described in detail below, but the present invention is not limited to the following description. Any modifications can be made to implement the invention without departing from the spirit of the invention.

[0071] (1) Methods for recovering alkali metal salts

[0072] The method for recovering alkali metal salts of the present invention is a method for recovering alkali metal salts from a solution containing alkali metal ions, comprising the following steps 1 and 2.

[0073] Step 1: A solution X containing alkali metal ions is transported to the nanofiltration membrane unit A as the liquid to be treated, and separated into permeate A and concentrate B. The concentrate B is then mixed with the remaining portion of the liquid to be treated A and transported to the nanofiltration membrane unit A again to further obtain the first nanofiltration process of the permeate A.

[0074] Step 2: The permeate A or its concentrate obtained in Step 1 is fed to the nanofiltration membrane unit A as the treated liquid B, separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed to the nanofiltration membrane unit A again to further obtain the permeate C. Alternatively, the permeate A or its concentrate obtained in Step 1 is fed to the nanofiltration membrane unit B as the treated liquid B, separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed to the nanofiltration membrane unit B again to further obtain the permeate C in this second nanofiltration step.

[0075] The alkali metal salt recovery method of this embodiment involves sequentially processing N solutions X (N: an integer greater than or equal to 2) to obtain the permeate C by passing the solutions X through step 1 and step 2. Preferably, step 2 uses the nanofiltration membrane unit B. During the period between performing step 1 on the k-th solution X(k) (k: an integer greater than or equal to 1 and less than (N-1)) of the N solutions X and performing step 2, step 1 is performed in parallel on the (k+1)-th solution X(k+1). A series of processing steps that semi-continuously perform the batch processing steps in step 1 and step 2 on multiple solutions is called a semi-batch processing step.

[0076] (2) Nanofiltration process

[0077] In the nanofiltration process, a nanofiltration membrane is used to separate a solution containing alkali metal ions into a permeate and a concentrate.

[0078] The ratio of alkali metal ion concentration to polyvalent metal ion concentration in the permeate (hereinafter referred to as the "alkali metal ion ratio") is higher than that of solution X, while the alkali metal ion ratio in the concentrate is lower than that of solution X.

[0079] The concentration of polyvalent metal ions is calculated, for example, by summing the equivalent concentrations of cobalt or nickel ions. Similarly, the concentration of alkali metal ions is calculated, for example, by summing the equivalent concentrations of lithium or cesium ions. Depending on the element, alkali metals sometimes exist in solution as polyatomic ions rather than monatomic ions; the equivalent concentration is the concentration assumed to exist as a monatomic ion. Regarding the concentrations of the aforementioned polyvalent and alkali metal ions, for example, a Hitachi P-4010 ICP (Inductively Coupled Plasma Lucidum) analyzer can be used to analyze the solution being measured and quantify the concentration (mg / L) of each ion.

[0080] (2-1) Solutions containing alkali metal ions

[0081] Solution X containing alkali metal ions only needs to contain at least one alkali metal ion and one or more conjugate bases (e.g., chloride ions, nitrate ions, sulfate ions, carbonate ions, acetate ions, etc.). The alkali metal ions and conjugate bases in solution X can exist in the form of alkali metal salts; examples of alkali metal salts include salts of lithium, sodium, potassium, rubidium, and cesium. From the viewpoint of the value of the recovered material, the presence of lithium salts is preferred. That is, the alkali metal ion in the solution containing alkali metal ions preferably contains lithium ions (hereinafter also referred to as "Li"). + ”).

[0082] In the presence of N solutions X containing alkali metal ions, each solution X only needs to contain at least one alkali metal ion and one or more conjugate bases. The composition of the solution, such as the concentration of alkali metal ions, the concentration of polyvalent metal ions, and the pH value, can vary in each solution X. Preferably, all solutions X contain Li as the alkali metal ion. + .

[0083] The solution X containing alkali metal ions preferably contains, in addition to alkali metal ions, at least one or more polyvalent metal ions. Examples of polyvalent metal ions include alkaline earth metals such as magnesium, calcium, and strontium, typical elements (aluminum, tin, lead, etc.), and transition elements (iron, copper, cobalt, manganese, etc.).

[0084] Furthermore, the solution X containing alkali metal ions may contain neutral molecules that are not charged under conditions where the pH value is below 3, and the molecular weight of these neutral molecules is preferably below 70. Examples of neutral molecules include boron compounds such as formic acid, acetic acid, and boric acid. Boron compounds are sometimes added as additives to the electrolyte of lithium-ion batteries to improve battery characteristics, and therefore are sometimes included in the solution X containing alkali metal ions. For example, in the case of boron compounds, they may become impediments to the refining process during lithium recovery, but can be removed in the reverse osmosis filtration process described later. The boron concentration (mg / L) in solution X is preferably below the concentration of the alkali metal ions to be recovered, more preferably below 0.5 (mg / L) × the concentration of the alkali metal ions to be recovered, and even more preferably below 0.1 (mg / L) × the concentration of the alkali metal ions to be recovered.

[0085] The pH value of the solution X containing alkali metal ions is preferably above 0 and below 4.

[0086] The pH value of the solution X containing alkali metal ions is preferably 4 or less, more preferably 3.5 or less, even more preferably 3 or less, and even more preferably 2.5 or less. By keeping the pH value below 4, the permeability of polyvalent metal ions is maintained at a low level during the nanofiltration process, while the permeability of alkali metal ions is increased.

[0087] Furthermore, the pH value of the solution X containing alkali metal ions is preferably 0 or higher, more preferably 0.5 or higher, and even more preferably 1 or higher. By maintaining a pH value of 0 or higher, the decrease in the selective separation performance of alkali metal ions relative to polyvalent metal ions of the nanofiltration membrane during long-term operation can be suppressed.

[0088] The solution X containing alkali metal ions is preferably a solution in which a lithium-containing material is dissolved in acid. Specifically, lithium-ion batteries and the waste materials, waste liquids, ores, and slag generated during their manufacturing processes can be cited as examples of lithium-ion materials. Among these, lithium-ion batteries are preferred due to their high reusability requirements and the high purity of the rare metals they contain.

[0089] A lithium-ion battery consists of components such as a positive electrode material, a negative electrode material, a separator, and an electrolyte. Any material containing lithium can be used as the material in solution X. The acid used to dissolve the lithium-containing material preferably contains at least one acid selected from hydrochloric acid, sulfuric acid, and nitric acid. The solution obtained by dissolving the lithium-ion battery components in acid may contain, in addition to lithium ions, elements such as nickel, cobalt, and manganese.

[0090] Dissolving substances containing alkali metals with acid can be achieved, for example, by immersing the substance in an acidic aqueous solution. However, other methods can also be used as long as the target alkali metal ions can be dissolved. From the viewpoint of alkali metal ion dissolution efficiency, the temperature of the acidic aqueous solution in contact is preferably 10°C or higher and 100°C or lower. Furthermore, from the viewpoints of cost and safety, 20°C or higher and 80°C or lower is more preferred.

[0091] The solution obtained by dissolving a substance containing an alkali metal in an acid does not always have a fixed composition. Its composition can change due to variations in the composition of the various ions in the substance and the dissolution conditions in the acid. That is, given N solutions X, the compositions of each solution X may differ.

[0092] There is no particular limitation on the volume of solution X containing alkali metal ions to be recovered; if there are N solutions X, the volumes of the N solutions X can be different. From the viewpoint of processing efficiency in each process, the volume of solution X is preferably 10L or more and 10,000L or less.

[0093] Solution X containing alkali metal ions may contain organic compounds. For example, in the case where solution X is the acid solution of a lithium-ion battery, organic compounds such as polyvinylidene fluoride (PVDF), polyolefins, and carbonates can be found in adhesives, separators, electrolytes, etc., which connect active materials to current collectors. These organic compounds may act as impurities and reduce the recovery efficiency of alkali metal ions; therefore, these impurities can be removed by the ultrafiltration process described later.

[0094] When N solutions X containing alkali metal ions each contain 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. By maintaining a lithium ion concentration of 0.5 mg / L or more in the solution, the lithium ion recovery efficiency achieved by membrane separation is improved. Furthermore, by maintaining a lithium ion concentration of 10,000 mg / L or less in the solution, the increase in osmotic pressure difference can be suppressed, thereby improving the efficiency of membrane separation. More preferably, the lithium ion concentration in the solution is 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.

[0095] The alkali metal salt recovery method of this embodiment is applicable even when the alkali metal ion ratio in the solution X containing alkali metal ions is 2.4 or less. Generally, when the alkali metal ion ratio is 2.4 or less, the separation and recovery of alkali metal ions and polyvalent metal ions becomes more difficult. However, in the alkali metal salt recovery method of this embodiment, the selective separation of alkali metal ions and polyvalent metal ions is high, enabling effective recovery of alkali metal ions. Furthermore, the alkali metal salt recovery method of this embodiment is also applicable when the alkali metal ion ratio in the solution X containing alkali metal ions is 1 or less, and further, 0.5 or less.

[0096] (2-2) Nanofiltration membrane

[0097] The nanofiltration membrane used in the alkali metal salt recovery method of this embodiment only needs to have a hierarchical characteristic between the reverse osmosis membrane and the ultrafiltration membrane. Preferably, the difference between the glucose removal rate when passing through a 1000 mg / L glucose aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa and the isopropanol removal rate when passing through a 1000 mg / L isopropanol aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa is 20% or more. The nanofiltration membrane is further preferably characterized by a glucose removal rate of 1000 mg / L glucose aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa, and an isopropanol removal rate of 1000 mg / L isopropanol aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa, with a glucose removal rate of 70% or higher. Furthermore, the magnesium sulfate removal rate is 95% or higher when passing through 2000 mg / L magnesium sulfate aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa. In the following description, when referred to only as "glucose removal rate," it refers to the glucose removal rate when a 1000 mg / L glucose aqueous solution at 25°C and pH 6.5 is passed through the solution at an operating pressure of 0.5 MPa; when referred to only as "isopropanol removal rate," it refers to the isopropanol removal rate when a 1000 mg / L isopropanol aqueous solution at 25°C and pH 6.5 is passed through the solution at an operating pressure of 0.5 MPa; and when referred to only as "magnesium sulfate removal rate," it refers to the magnesium sulfate removal rate when a 2000 mg / L magnesium sulfate aqueous solution at 25°C and pH 6.5 is passed through the solution at an operating pressure of 0.5 MPa.

[0098] Reverse osmosis membranes, as commonly known, remove most organic matter and ions. On the other hand, ultrafiltration membranes typically do not remove most types of ions, but rather high molecular weight organic matter.

[0099] To separate alkali metal ions from polyvalent metal ions, nanofiltration membranes preferably have an electrical charge on their surface, enabling both size-based separation (separation through fine pores) and electrostatic separation (separation through electrical charge). For example, when using a nanofiltration membrane where the difference between glucose removal rate and isopropanol removal rate is 40% or more, and the difference between the magnesium sulfate removal rate and the magnesium chloride removal rate when permeating a 2000 mg / L magnesium chloride aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa is 20% or less, both size-based and electrostatic separation are possible. Hereinafter, when referred to only as "magnesium chloride removal rate" in this specification, it refers to the magnesium chloride removal rate when permeating a 2000 mg / L magnesium chloride aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa.

[0100] Materials used for nanofiltration membranes include cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides, and vinyl polymers. Nanofiltration membranes can be composed of only one material or multiple materials. Furthermore, their membrane structure can be an asymmetric membrane with a dense layer on at least one side, gradually expanding to larger pores towards the interior or the other side; or a composite semi-permeable membrane with a very thin separation functional layer of other materials on the dense layer of an asymmetric membrane.

[0101] As a composite semipermeable membrane, a preferred example is a membrane having a porous support membrane comprising polysulfone and a separation functional layer comprising polyamide disposed on the porous support membrane. Alternatively, in addition to the porous support membrane and the separation functional layer, the composite semipermeable membrane may also include a substrate, in which case the porous support membrane is disposed on the substrate. The polyamide is a thin film formed on the porous support membrane through an interfacial polycondensation reaction between a polyfunctional aliphatic amine and a polyfunctional aromatic acyl halide.

[0102] In the alkali metal salt recovery method of this embodiment, it is preferable that at least one of the nanofiltration membrane units A and B has a porous support membrane and a separation functional layer. A positron beam is irradiated from the surface of the nanofiltration membrane on the separation functional layer side. The average pore size R1 and average pore size R2 of the separation functional layer, derived by positron annihilation lifetime determination, satisfy 0.90 ≤ R1 / R2 ≤ 1.10. Here, R1 and R2 are defined as follows.

[0103] R1: Average aperture under a positron beam intensity of 0.1 keV.

[0104] R2: Average aperture under a positron beam intensity of 0.5 keV.

[0105] The so-called "positron annihilation lifetime determination method" refers to the method of non-destructively evaluating information such as the size, number density, and size distribution of pores ranging from 0.1 to 10 nm based on the time (from hundreds of picoseconds to tens of nanoseconds) from the time of positron incident on the sample to the time of annihilation.

[0106] Furthermore, the measurement region in the depth direction from the sample surface can be adjusted by the energy of the positron beam incident on the sample. Higher energy means a deeper portion from the sample surface is included in the measurement region, but this depth is affected by the sample density. For example, when measuring the separation functional layer of a composite semipermeable membrane, if a positron beam with an energy of approximately 0.1 keV is irradiated from the separation functional layer side of the composite semipermeable membrane, a region with a depth of 1.0–5.0 nm from the sample surface is typically measured; if a positron beam with an energy of approximately 0.5 keV is used, a region with a depth of 10–50 nm from the sample surface is typically measured. Additionally, if other layers, such as a protective layer, are provided on the separation functional layer, the average pore size of the separation functional layer can be measured by removing these other layers beforehand.

[0107] In this embodiment, the thickness of the separation functional layer in the composite semipermeable membrane is preferably 15 nm or more and 50 nm or less. Therefore, under the condition of a positron beam intensity of 0.1 keV, the average pore size reflecting the surface side (the side opposite to the porous support membrane side) of the separation functional layer, and under the condition of 0.5 keV, the average pore size reflecting the porous support membrane side of the separation functional layer, can be said to be that the closer R1 / R2 is to 1, the more uniform the pore size is in the thickness direction. It is speculated that by making the pore size uniform in the thickness direction, the direction of ion diffusion in the separation functional layer becomes uniform, and the permeation resistance of monovalent ions with sizes that can move freely in the separation functional layer is suppressed. As a result, it is believed that excellent selective separation performance of monovalent ions / multivalent ions is achieved. Therefore, R1 / R2 is more preferably 0.92 or more and 1.05 or less, and even more preferably 0.94 or more and 1.03 or less.

[0108] Furthermore, R1 is preferably 0.55 nm or more and 0.70 nm or less, more preferably 0.57 nm or more and 0.68 nm or less, and even more preferably 0.60 nm or more and 0.65 nm or less. By keeping R1 within the above range, the effect of suppressing the permeation resistance of alkali metal ions and hindering the permeation of polyvalent metal ions becomes significant.

[0109] To ensure that R1 and R2 satisfy the above relationship, for example, a method can be used to control the relative humidity to be high, for example, 80% or more, during the interfacial polycondensation of the polyfunctional aliphatic amine compound and the polyfunctional aromatic acyl halide, and to make the molecular weight of the polyfunctional aliphatic amine forming the separation functional layer in the composite semipermeable membrane 90 or more.

[0110] The separation functional layer of the composite semipermeable membrane preferably contains 50% by mass or more of a semi-aromatic crosslinked polyamide obtained by interfacial condensation of a divalent or higher polyfunctional aliphatic amine compound with a divalent or higher polyfunctional aromatic acyl halide, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably composed solely of semi-aromatic crosslinked polyamide. By containing 50% by mass or more of the semi-aromatic crosslinked polyamide, excessive densification caused by π-π interactions of the aromatic rings in the semi-aromatic crosslinked polyamide can be suppressed, resulting in excellent alkali metal ion permeability. Furthermore, through in-depth research, the inventors have found that when the relative humidity during interfacial condensation is controlled to a high level, for example, 80% or more, the resulting composite semipermeable membrane exhibits particularly excellent membrane performance under acidic conditions. The relative humidity can be adjusted using a precision air conditioning device or the like. By maintaining the atmospheric humidity during interfacial condensation at 80% or more, the evaporation of moisture from the formed polyamide can be suppressed, as can the insolubility caused by intermolecular hydrogen bonds in the remaining amino-rich oligomers. Therefore, after forming a separation functional layer through interfacial polycondensation reaction, oligomers can be effectively removed. Thus, the pore size expansion associated with the swelling of semi-aromatic cross-linked polyamides when the membrane is used under acidic conditions can be suppressed, resulting in a composite semi-permeable membrane exhibiting excellent multivalent ion removal properties, namely, predetermined glucose removal rate, isopropanol removal rate, magnesium sulfate removal rate, etc.

[0111] The multifunctional aliphatic amine is preferably an alicyclic diamine, and more preferably a piperidine derivative or a piperazine derivative.

[0112] Furthermore, the molecular weight of the alicyclic diamine is preferably 90 or higher. When the molecular weight of the alicyclic diamine is 90 or higher, the diffusion coefficient of the amine decreases, and polyamide is gradually formed during interfacial polycondensation. Therefore, from the initial to the middle stage of interfacial polycondensation, a separation functional layer with uniform pore size in the film thickness direction is easily formed. On the other hand, the molecular weight of the alicyclic diamine is preferably 160 or lower. Usually, at the beginning and end of the interfacial polycondensation, excessive oligomers are generated on the surface of the support in contact with the organic layer, and the pores on the support surface are blocked, which is the main reason for the uneven pore size distribution in the film thickness direction. However, when the molecular weight of the alicyclic diamine is 160 or lower, the molecular weight of the generated oligomers is smaller, which can reduce the interaction with the semi-aromatic crosslinked polyamide. Therefore, after the separation functional layer is formed by the interfacial polycondensation reaction, the oligomers can easily detach from the separation functional layer, and a separation functional layer with uniform pore size in the film thickness direction can be easily formed.

[0113] Examples of alicyclic diamines with a molecular weight of 90 or more and 160 or less include substituted piperazines (e.g., 2-methylpiperazine, 2-ethylpiperazine, 2-n-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 high-piperazines.

[0114] "Multifunctional aromatic acyl halide" refers to an aromatic acyl halide having two or more halogenated carbonyl groups in one molecule. There are no particular limitations as long as it is obtained by reacting with the aforementioned multifunctional aliphatic amines to obtain a semi-aromatic crosslinked polyamide. Examples of multifunctional aromatic acyl halides that can be used include 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. Among multifunctional aromatic acyl halides, acyl chlorides are preferred, especially from the viewpoints of economy, ease of acquisition, ease of processing, and ease of reactivity. Preferred acyl halides include pyromellitic acid (Pyromellitic chloride), 1,3-benzenedicarboxylic acid (isophthalic acid chloride), 1,4-benzenedicarboxylic acid (terephthalic acid chloride), 1,3,5-benzenedicarboxylic acid (1,3,5-benzenedicarboxylic acid chloride), and 1,3,6-naphthalenetrisulfonic acid (1,3,6-naphthalenetrisulfonic acid chloride). These multifunctional aromatic acyl halides can be used alone or in mixtures of two or more. By mixing any one of the difunctional isophthalic acid chloride or terephthalic acid from trifunctional pyromellitic acid chloride, 1,3,5-benzenedicarboxylic acid chloride, or 1,3,6-naphthalenetrisulfonic acid chloride, the intermolecular gaps in the polyamide crosslinking structure are expanded, allowing for wide-ranging control of membranes with uniform pore size distribution. The preferred molar ratio of trifunctional acyl chloride to difunctional acyl chloride is 1:20 to 50:1, more preferably 1:1 to 20:1.

[0115] The aforementioned composite semipermeable membrane can be obtained, for example, by forming a porous support membrane on a substrate, and then subjecting a multifunctional aliphatic amine and a multifunctional aromatic acyl halide to interfacial polycondensation on the porous support membrane to form a separation functional layer containing a semi-aromatic crosslinked polyamide.

[0116] (2-3) Separation via nanofiltration membrane

[0117] Alkali metal ions can easily pass through nanofiltration membranes, while polyvalent metal ions have difficulty passing through them, thus enabling the separation of alkali metal ions from polyvalent metal ions. Nanofiltration membranes are preferably used in a configuration such as a spiral-shaped element.

[0118] (2-3-1) Process 1: First Nanofiltration Process

[0119] The first nanofiltration step (step 1) involves feeding solution X, as the liquid to be treated (A), into nanofiltration membrane unit A, separating it into permeate A and concentrate B. Concentrate B is then mixed with the remaining portion of the liquid to be treated (A) and fed back into nanofiltration membrane unit A to further obtain permeate A. In step 1, concentrate B can be treated more than twice using nanofiltration membrane unit A. The number of repetitions of this treatment can be arbitrarily set. Furthermore, as described later, the repeated treatment in step 1 can end when the recovery rate reaches a certain value. By mixing concentrate B with the remaining portion of the liquid to be treated (A) while permeating the liquid to be treated (A) through nanofiltration membrane unit A, residual alkali metal ions in concentrate B can be re-permeated through the nanofiltration membrane, thereby improving the recovery rate of alkali metal ions.

[0120] As the first nanofiltration process proceeds, the volume of permeate A increases, the recovery rate (%) of alkali metal ions increases, and the volume of treated liquid A and the ratio of alkali metal ions in treated liquid A decrease.

[0121] In the first nanofiltration step, it is preferable to obtain a permeate with an alkali metal ion ratio of 2 or more and 1000 or less, more preferably a permeate with an alkali metal ion ratio of 10 or more and 700 or less, and even more preferably a permeate with an alkali metal ion ratio of 20 or more and 500 or less. By making the alkali metal ion ratio 2 or more, the processing time of the subsequent second nanofiltration step can be shortened, and by making the alkali metal ion ratio 1000 or less, the processing time of this step can be shortened.

[0122] Furthermore, in the first nanofiltration step, the recovery rate of alkali metal ions is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. If the recovery rate of alkali metal ions in the first nanofiltration step is 80% or more, the recovery cost of alkali metal ions can be reduced. The recovery rate of alkali metal ions in the nanofiltration step is defined in each step by the following formula (1).

[0123] The recovery rate (%) of alkali metal ions in the nanofiltration process = {(permeate volume of the nanofiltration process) × (alkali metal ion concentration in the permeate of the nanofiltration process)} / {(liquid volume of the object being treated) × (initial alkali metal ion concentration in the liquid of the object being treated)} ... Equation (1)

[0124] In the nanofiltration process, it is preferable to supply the solution to the nanofiltration membrane at an operating pressure of 0.1 MPa or higher and 8 MPa or lower. If the operating pressure is 0.1 MPa or higher, the membrane permeation rate increases; if it is 8 MPa or lower, damage to the nanofiltration membrane can be suppressed. More preferably, the operating pressure is 0.5 MPa or higher and 6 MPa or lower, and even more preferably, 1 MPa or higher and 4 MPa or lower.

[0125] As the first nanofiltration step proceeds, the osmotic pressure of the treated liquid A increases, consequently increasing the operating pressure required to obtain the same flow rate of permeate A. Therefore, in order to facilitate continued filtration under conditions of increased osmotic pressure, the alkali metal salt recovery method of this embodiment preferably includes a step of diluting the treated liquid A. By diluting the treated liquid A, its osmotic pressure decreases, allowing the first nanofiltration step to continue and increasing the recovery rate of alkali metal ions; therefore, this is preferable.

[0126] Methods for diluting the liquid to be treated (A) include, for example, adding dilution water directly to the liquid to be treated (A) or adding dilution water to the concentrate (B). From a simplicity perspective, the method of directly adding dilution water to the liquid to be treated (A) is preferred.

[0127] The dilution water can be pure water, acidic solution, etc., without particular limitation. It is preferred to use a permeate with a low concentration of metal ions generated in the reverse osmosis filtration process described later, because it can efficiently separate and recover alkali metal ions and reuse acidic aqueous solutions.

[0128] As for the operation control method in the first nanofiltration process, examples include constant flow filtration and low-pressure filtration. There are no particular limitations. When the liquid to be treated, A, is diluted, constant flow filtration is preferred. If constant flow filtration is used, the flow rate of the added dilution water can also be kept constant, making control easier.

[0129] Under constant flow filtration conditions, from Li + From the viewpoint of recovery efficiency, the liquid flow rate is preferably 1% or more of the liquid volume relative to solution X within 1 minute. From the viewpoint of ease of control, the liquid flow rate is preferably 50% or less of the liquid volume relative to solution X within 1 minute.

[0130] The extent of the first nanofiltration process, i.e., the recovery rate of alkali metal ions, can be determined by appropriately sampling and analyzing the composition of the treated liquid A. However, since the analysis of the liquid composition takes time, it is preferable to be able to continuously monitor the recovery rate of alkali metal ions.

[0131] As a method for monitoring the recovery rate of alkali metal ions during the first nanofiltration process, when the nanofiltration process is carried out at a constant permeation flow rate, the operating pressure is related to the recovery rate of alkali metal ions, as shown in Equation (2) below. Therefore, it is preferable to monitor the change in operating pressure over time using the following formula while determining the recovery rate A (%) of alkali metal ions, and then determine the end time of the first nanofiltration process. By monitoring the operating pressure according to Equation (2) while determining the end time of the first nanofiltration process, the time required for liquid composition analysis can be reduced, and alkali metal ions can be effectively recovered. In addition, the target recovery rate A (%) of alkali metal ions can be appropriately set. For at least one of process 1 and process 2, the process can be terminated when the recovery rate reaches the target value.

[0132]

[0133] In the above formula (2), the recovery rate of alkali metal ions A (%), the operating pressure P (Pa), the initial operating pressure P0 (Pa), and the initial liquid volume V0 (m³) of the object to be treated are specified. 3 ), alkali metal ion removal rate R (%) of nanofiltration membrane, liquid recovery rate S (%) of nanofiltration process, supply flow rate Q F (m 3 / s), concentrate flow rate Q c (m 3 / s), the filtering end time t=tb.

[0134] The liquid recovery rate S in the nanofiltration process is given by S = {(Q} F -Q c ) / Q F}×100 definition.

[0135] Figures 1-3 , Figure 5 and Figures 8-10 This is a schematic flow chart illustrating a method for recovering alkali metal salts according to one embodiment of the present invention. Solution X is conveyed to ultrafiltration membrane unit 1 (described later), and the resulting permeate is conveyed to first tank (container) 5a. The permeate (treated liquid A) stored in first tank 5a is conveyed to nanofiltration membrane unit A (2a), separating into permeate A and concentrate B. Permeate A is conveyed to second tank 5b at a certain flow rate, and concentrate B is conveyed to first tank 5a, where it is mixed with the remaining portion of treated liquid A in first tank 5a. Furthermore, the first nanofiltration step can be performed while dilution water is added to first tank 5a at the same flow rate as permeate A. Additionally, the dilution water may contain the permeate obtained through the reverse osmosis filtration step (described later). Figure 3 and Figure 9In the example shown, the permeate obtained through the reverse osmosis filtration process described later is further transported to the high-removal reverse osmosis membrane unit 4, and the obtained permeate is used as dilution water.

[0136] (2-3-2) Process 2: Second Nanofiltration Process

[0137] In the second nanofiltration step (step 2), the permeate A obtained in the first nanofiltration step (step 1) or a concentrate of the permeate A obtained in step 1 is fed to nanofiltration membrane unit A as the treated liquid B, where it is separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed back to nanofiltration membrane unit A to further obtain the permeate C. Alternatively, the permeate A obtained in step 1 or a concentrate of the permeate A is fed to nanofiltration membrane unit B as the treated liquid B, where it is separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed back to nanofiltration membrane unit B to further obtain the permeate C. In step 2, the concentrate D can be treated more than twice using nanofiltration membrane unit A or nanofiltration membrane unit B. The number of repetitions of this treatment can be arbitrarily set. Furthermore, as mentioned above, the repeated treatment in step 2 can end when the recovery rate reaches a certain value.

[0138] The concentrate of permeate A can be prepared by methods such as concentrating permeate A using a reverse osmosis membrane unit described later, without particular limitation.

[0139] From the viewpoint of shortening processing time, the second nanofiltration process preferably uses permeate A as the liquid to be treated B, and preferably uses nanofiltration membrane unit B.

[0140] In the second nanofiltration process, it is preferable to obtain a permeate C with an alkali metal ion ratio of 10 or more, more preferably a permeate C with an alkali metal ion ratio of 100 or more, and even more preferably a permeate C with an alkali metal ion ratio of 200 or more. If the alkali metal ion ratio is 10 or more, it can be said that the purity of the alkali metal ions is sufficiently high.

[0141] Furthermore, in the second nanofiltration process, the recovery rate of alkali metal ions is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. If the recovery rate of alkali metal ions in the nanofiltration process of the permeate is 80% or more, the recovery cost of alkali metals can be reduced.

[0142] The second nanofiltration step must be performed at least once. If the alkali metal ion ratio of solution X is low and the second nanofiltration step is performed only once, and the alkali metal ion ratio of the permeate C does not reach the target value, the second nanofiltration step can be performed multiple times, using permeate C as the treated solution B, until the alkali metal ion ratio of the obtained permeate reaches the target value. When performing multiple second nanofiltration steps, the nanofiltration membrane unit used after the second step can be either nanofiltration membrane unit A or B, or other nanofiltration membrane units can be used.

[0143] In the second nanofiltration process, similar to the first nanofiltration process, it is preferable to include a process for diluting the liquid to be treated, B. Furthermore, the progress of the process can be controlled based on the above formula (2).

[0144] Examples of operational control methods in the second nanofiltration process include constant flow filtration and low-pressure filtration.

[0145] Under constant flow filtration conditions, from Li + From the perspective of recovery efficiency, the liquid flow rate is preferably 1% or more of the initial liquid volume of the liquid being treated B within 1 minute. From the perspective of ease of control, the liquid flow rate is preferably 50% or less of the initial liquid volume of the liquid being treated B within 1 minute.

[0146] exist Figure 1 , Figure 3 , Figure 8 and Figure 9 In the example shown, a nanofiltration process for solution X is performed in nanofiltration membrane unit A (2a). Permeate A, which has passed through nanofiltration membrane unit A and is stored in tank 2 (5b), is transported as treated solution B to tank 3 (5c). Treated solution B is then transported to nanofiltration membrane unit B (2b), and permeate C is transported at a constant flow rate to tank 4 (5d), thereby performing a second nanofiltration process. At this time, the concentrate D that has not passed through nanofiltration membrane unit B (2b) is mixed with the remaining portion of treated solution B in tank 3 (5c). Alternatively, the second nanofiltration process can be performed while diluting water is added to treated solution B in tank 3 (5c) at the same flow rate as permeate C.

[0147] exist Figure 5 and Figure 10In the example shown, a nanofiltration process for solution X is performed in nanofiltration membrane unit A (2a). Permeate A, which has passed through nanofiltration membrane unit A and is stored in tank 2 (5b), is transported as treated solution B to tank 1 (5a) and then back to nanofiltration membrane unit A (2a). Permeate C is transported to tank 2 (5b) at a constant flow rate, thereby performing a second nanofiltration process. At this time, the concentrate D that has not passed through nanofiltration membrane unit A (2a) is mixed with the remaining portion of treated solution B in tank 1 (5a). Alternatively, the second nanofiltration process can be performed while diluting water is added to treated solution B in tank 1 (5a) at the same flow rate as permeate C.

[0148] exist Figure 5 and Figure 10 Since the same nanofiltration membrane unit A(2a) as the first nanofiltration process is used in the second nanofiltration process, it is preferable to clean the nanofiltration membrane unit A(2a), the first tank 5a, and the second tank 5b before implementing the second nanofiltration process.

[0149] In addition, Figure 8 In the example shown, the remaining portion of the treated liquid B(k) in the third tank 5c after the k-th solution X(k) is treated is added to the first tank 5a containing the p-th treated liquid A(p). Additionally, in Figure 10 In the example shown, the remaining portion of the treated liquid B(k) that was left in tank 1 5a after the k-th solution X(k) was treated is added to tank 1 5a containing the m-th treated liquid A(m). Here, m represents an integer greater than (k+1) and less than N, and p represents an integer greater than (k+2) and less than N.

[0150] On the other hand, Figure 2 In the example shown, to implement the reverse osmosis filtration process described later, the liquid that has passed through the nanofiltration membrane unit A (2a) and is stored in the second tank 5b is transported to the fifth tank 5e. The liquid in the fifth tank 5e is then transported to the first reverse osmosis membrane unit 3a, and the resulting concentrate is transported to the sixth tank 5f. The concentrate stored in the sixth tank 5f is transported to the third tank 5c, and the liquid in the third tank 5c is transported to the nanofiltration membrane unit B (2b) as the treated liquid B. The permeate C is transported to the fourth tank 5d at a constant flow rate. The concentrate D is mixed with the remaining portion of the treated liquid B in the third tank 5c, and dilution water is added to the treated liquid B in the third tank 5c at the same flow rate as the permeate C, while the second nanofiltration process is performed.

[0151] (3) Semi-batch processing procedure

[0152] As mentioned above, since the liquid composition of solution X can sometimes change, it is very difficult, and in some cases impossible, to stabilize the liquid composition obtained in the nanofiltration membrane treatment step and maintain the predetermined lithium purity and recovery rate in a process that continuously transports N (N: an integer greater than or equal to 2) solutions X containing alkali metal ions. Furthermore, if this instability in the nanofiltration membrane treatment process is eliminated to achieve a continuous process capable of high-purity and efficient lithium recovery, the number of nanofiltration membrane stages becomes excessive, leading to increased costs and further process complexity. Additionally, in batch processing, after the treatment of the kth (k: an integer greater than or equal to 1 and less than (N-1)) solution X(k) is completed, the treatment of the next (k+1)th solution X(k+1) begins, resulting in longer processing times.

[0153] Therefore, the alkali metal salt recovery method of this embodiment involves sequentially processing N solutions X (N: an integer greater than or equal to 2) to obtain permeate C from the solutions X via steps 1 and 2. Preferably, step 2 uses a nanofiltration membrane unit B. During the period between performing the first nanofiltration step (step 1) on the kth solution X(k) (k: an integer greater than or equal to 1 and less than (N-1)) among the N solutions X and performing the second nanofiltration step (step 2), the first nanofiltration step (step 1) is performed in parallel on the (k+1)th solution X(k+1) among the N solutions X. In this way, by adopting a semi-batch processing structure that performs the batch processing steps in steps 1 and 2 semi-continuously on multiple solutions, alkali metal ions can be effectively recovered.

[0154] In addition, in order to effectively recover alkali metal ions, it is preferable to add the remaining portion of the treated liquid B(k) after the second nanofiltration process of solution X(k) to solution X (m: an integer above (k+1) and below N) or treated liquid A(m) to perform the first nanofiltration process of solution X(m).

[0155] In this case, the recovery rate of alkali metal in the second nanofiltration process is preferably 50% or more and 95% or less, more preferably 60% or more and 90% or less, and even more preferably 70% or more and 85% or less.

[0156] In the second nanofiltration step, by ensuring that the alkali metal recovery rate is below 95%, the permeation of the permeate with a low alkali metal ratio in the later stages of the nanofiltration step can be suppressed. To suppress the permeation of the permeate with a low alkali metal ratio in the later stages of the nanofiltration step, it is preferable not to add dilution water in the second nanofiltration step.

[0157] The remaining portion of the treated liquid B(k) is the liquid after filtering solution X(k) once using a nanofiltration membrane. If the composition of solution X(m) and solution X(k) remains largely unchanged, the alkali metal ratio is higher in solution X(m). Therefore, by adding the remaining portion of the treated liquid B(k) to solution X(m) or the treated liquid A(m), all the alkali metals in the remaining portion of the treated liquid B(k) can be recovered, and the alkali metal ratio of solution X(m) is also increased. Consequently, the alkali metal ratio of the permeate from the first nanofiltration step of solution X(m) is also increased. That is, the purity and recovery rate of the alkali metals are improved.

[0158] The remaining portion of the treated liquid B(k) is the liquid that has undergone the ultrafiltration process. Therefore, from the viewpoint of reducing the load on the ultrafiltration process, it is more preferable to add the remaining portion of the treated liquid B(k) to the treated liquid A(m) after the ultrafiltration process than to add it to the solution X(m) that has not undergone the ultrafiltration process.

[0159] Therefore, the alkali metal salt recovery method according to this embodiment preferably includes the following step 4.

[0160] Step 4: There are N solutions X (N: an integer greater than 2). After the completion of Step 2 for the kth solution X(k) (k: an integer greater than 1 and less than (N-1)) among the N solutions X, the remaining portion of the kth processed liquid B(k) mixed with the kth concentrate D(k) is added to the mth solution X(m) (m: an integer greater than (k+1) and less than N) or the mth processed liquid A(m).

[0161] Figure 10 An example of implementing step 4 includes adding the remaining portion of the kth processed liquid B(k) mixed with the kth concentrate D(k) to the solution X(k) after step 2 of the solution X(k) is completed.

[0162] Furthermore, if step 4 is combined with the semi-batch processing step and set as step 5 below, alkali metal salts can be recovered more effectively.

[0163] Step 5: After step 2 of the solution X(k) is completed, the remaining portion of the kth treated liquid B(k) mixed with the kth concentrated liquid D(k) is added to the solution X (p: an integer above (k+2) and below N) or the pth treated liquid A(p).

[0164] Figure 8An example of implementing step 5 includes adding the remaining portion of the kth processed liquid B(k) mixed with the kth concentrate D(k) to the solution X(k) after step 2 of the solution X(k) is completed.

[0165] exist Figures 1-3 , Figure 8 and Figure 9 First, for the treated liquid A(1) obtained by conveying solution X(1) to the ultrafiltration membrane unit 1 described later, after the treatment of the nanofiltration membrane unit A(2a) is completed and all the permeate A(1) is conveyed to the second tank 5b, the solution in the first tank 5a is recycled to any tank. Then, solution X(2) is conveyed to the ultrafiltration membrane unit 1 described later, and the treated liquid A(2) obtained is stored in the first tank 5a, and the process is carried out in sequence. The process is also carried out in the same order for solutions X(3) to solutions X(N).

[0166] Furthermore, in the alkali metal salt recovery method according to this embodiment, it is preferable to perform steps 1 and 2 on all N solutions containing alkali metal ions, i.e., from the first solution X(1) to the Nth solution X(N). Alternatively, steps other than steps 1 and 2 may be performed on the N solutions, for example, the reverse osmosis filtration step (step 3) or ultrafiltration step described later may be performed.

[0167] (4) Reverse osmosis filtration process

[0168] The alkali metal salt recovery method of this embodiment preferably includes a reverse osmosis filtration process that concentrates at least one of the permeate A obtained in the first nanofiltration process and the permeate C obtained in the second nanofiltration process.

[0169] In this embodiment, the method for recovering alkali metal salts preferably further includes the following step 3 when there are N solutions X.

[0170] Step 3: A reverse osmosis filtration step in which at least one of the kth permeate A(k) and the kth permeate C(k) is concentrated in at least one solution X(k).

[0171] In the reverse osmosis filtration process, at least one of permeate A and permeate C is fed to the reverse osmosis membrane unit to obtain a concentrate with a higher alkali metal ion concentration than the fed permeate A or permeate C, and a permeate with a lower alkali metal ion concentration than permeate A and permeate C.

[0172] Examples of operational control methods in the reverse osmosis filtration process include constant flow filtration and low-pressure filtration.

[0173] Under constant flow filtration conditions, from Li + From the viewpoint of recovery efficiency, the flow rate of the permeate is preferably 1% or more of the amount of permeate A obtained in the first nanofiltration step or the amount of permeate C obtained in the second nanofiltration step within 1 minute. From the viewpoint of ease of control, it is preferable to have a flow rate of 50% or less of the amount of permeate A obtained in the first nanofiltration step or the amount of permeate C obtained in the second nanofiltration step within 1 minute.

[0174] (4-1) Reverse osmosis membrane

[0175] In the reverse osmosis filtration process, any reverse osmosis membrane that does not allow alkali metal ions to pass through can be used. By using such a reverse osmosis membrane, the loss of lithium ions during the concentration process of alkali metal ions, especially lithium ions, is minimal, and a stable and highly efficient recovery of lithium ions can be achieved. The higher the ion removal rate of the reverse osmosis membrane, the higher the efficiency of the process. However, membranes with high removal rates usually lack water permeability, so a balance should be considered when selecting one.

[0176] Especially when solution X contains neutral molecules, such as boron compounds represented by boric acid, which are uncharged under conditions of pH below 3, these neutral molecules are not removed during the nanofiltration process and are included in the permeate A and permeate C obtained during the nanofiltration process. Therefore, it is preferable to remove neutral molecules while concentrating alkali metal ions through a reverse osmosis filtration process. That is, it is preferable that neutral molecules permeate through the reverse osmosis membrane without alkali metal ions. In particular, a low-removal reverse osmosis membrane that achieves an isopropanol removal rate of 70% or more and less than 85% when permeating with an isopropanol aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa is preferred in that it permeates neutral molecules without alkali metal ions. Here, the aforementioned low-removal reverse osmosis membrane is a membrane that concentrates at least one of the permeate A(k) and the permeate C(k), wherein at least one of the permeate A(k) and the permeate C(k) contains neutral molecules that do not have a charge under conditions where the pH value is below 3, and the aforementioned low-removal reverse osmosis membrane is a reverse osmosis filtration membrane used in the reverse osmosis filtration process.

[0177] Materials used for reverse osmosis membranes include, for example, cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides, and vinyl polymers. Reverse osmosis membranes can be composed of only one material or multiple materials. Furthermore, their membrane structure can be an asymmetric membrane with a dense layer on at least one side, gradually expanding to larger pores towards the interior or the other side; or a composite semi-permeable membrane with a very thin separation functional layer formed of other materials on the dense layer of the asymmetric membrane.

[0178] As a composite semipermeable membrane used as a reverse osmosis membrane, examples include, for instance, a composite semipermeable membrane comprising a substrate, a porous support membrane, and a separation functional layer. Among these, a composite semipermeable membrane containing polyamide in the separation functional layer is preferred. The separation functional layer containing polyamide is obtained by polycondensation of a polyfunctional amine and a polyfunctional acyl halide on a porous support membrane.

[0179] (4-2) Concentration using reverse osmosis membranes

[0180] In the alkali metal salt recovery method of this embodiment, the reverse osmosis filtration step is preferably performed at least once on at least one of the permeate A and permeate C from the nanofiltration step. That is, preferably, in solution X(k), at least one of the kth permeate A(k) obtained in the first nanofiltration step (step 1) and the kth permeate C(k) obtained in the second nanofiltration step (step 2) is performed at least once.

[0181] Regarding the number of reverse osmosis filtration steps, it is preferable to perform the reverse osmosis filtration step only once in solution X(k) for the permeate C(k) obtained in the second nanofiltration step, which can shorten the overall process time. Furthermore, in the case of performing multiple second nanofiltration steps, it is preferable to perform the reverse osmosis filtration step only once for the permeate C(k) obtained in the final second nanofiltration step.

[0182] As described above, when solution X contains neutral molecules, such as boron compounds represented by boric acid, which are uncharged under conditions with a pH value below 3, it is preferable to remove the neutral molecules using a low-removal reverse osmosis membrane while concentrating alkali metal ions. In this case, to effectively remove neutral molecules and concentrate alkali metal ions, it is preferable that the reverse osmosis filtration process includes a circulation step that mixes the concentrate obtained in the reverse osmosis filtration process with the solution supplied to the reverse osmosis filtration process. When a circulation step is included, from the viewpoint of operating pressure, the period for implementing the circulation step is preferably within the range of up to 90% of the pressure resistance value of the reverse osmosis membrane unit.

[0183] Furthermore, in the above-described case, the permeate from the low-removal reverse osmosis membrane contains neutral molecules, and therefore is unsuitable for use as dilution water for the treated liquid A or treated liquid B in the nanofiltration process. The concentration of neutral molecules in the dilution water relative to the concentration of neutral molecules in the treated liquid (mg / L) is preferably 1% or less, more preferably 0.1% or less, and even more preferably 0.01% or less. By ensuring the concentration of neutral molecules (mg / L) relative to the treated liquid is 1% or less, when using the permeate from the low-removal reverse osmosis membrane as dilution water in the semi-batch treatment process of the treated liquid, the accumulation of neutral molecules in the system can be effectively prevented. When using the permeate from the low-removal reverse osmosis membrane as dilution water, it is preferable to supply the permeate from the low-removal reverse osmosis membrane to a high-removal reverse osmosis membrane unit equipped with a high-removal reverse osmosis membrane as defined below, to remove neutral molecules. By passing through the high-removal reverse osmosis membrane, the concentration of neutral molecules in the permeate from the low-removal reverse osmosis membrane can be brought within the range suitable for dilution water as described above.

[0184] "High removal reverse osmosis membrane" refers to a reverse osmosis membrane that removes 85-95% of isopropanol when passing through an isopropanol aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa.

[0185] Figure 1 , Figure 5 , Figure 8 and Figure 10 The example shown is a flowchart of an alkali metal salt recovery process that includes a reverse osmosis filtration step that concentrates the permeate C and a dilution step that uses the permeate obtained in the reverse osmosis filtration step as dilution water for the treated liquid A. Specifically, the permeate C, which has passed through nanofiltration membrane unit B (2b) or nanofiltration membrane unit A (2a) and is stored in tank 4 5d or tank 2 5b, is transported to tank 5e or tank 3 5c. The liquid in tank 5e or tank 3 5c is then transported to the first reverse osmosis membrane unit 3a to perform the reverse osmosis filtration step. The resulting permeate is transported as dilution water for the treated liquid A in tank 1 5a, and the concentrate is transported to tank 6 5f. The liquid in tank 6 5f can be recycled to any tank.

[0186] Figure 9The example shown is a flowchart of an alkali metal salt recovery process that includes a reverse osmosis filtration step to concentrate permeate C and a dilution step to use the permeate obtained in the reverse osmosis filtration step as dilution water for the treated liquid A after treatment with a high-removal reverse osmosis membrane. Specifically, permeate C, which has passed through nanofiltration membrane unit B (2b) and is stored in tank 4 5d, is transported to tank 5e. The liquid in tank 5e is then transported to reverse osmosis membrane unit 1 3a to perform the reverse osmosis filtration step. The concentrate obtained in the reverse osmosis filtration step is transported to tank 6 5f. The liquid in tank 6 5f can be recycled to any tank. Furthermore, the permeate obtained in the reverse osmosis filtration step is transported to high-removal reverse osmosis membrane unit 4. The concentrate obtained in high-removal reverse osmosis membrane unit 4 is discharged, and the permeate is supplied to tank 1 5a, which can be used as dilution water for the treated liquid A.

[0187] Figure 3 The example shown is a reverse osmosis filtration process that concentrates the permeate C, and a flowchart of an alkali metal salt recovery process that uses the permeate obtained in the reverse osmosis filtration process as dilution water for the treated liquid A after treatment with a high-removal reverse osmosis membrane. This reverse osmosis filtration process includes a step of circulating the concentrate. Specifically, the permeate C, which has passed through nanofiltration membrane unit B (2b) and is stored in tank 4 (5d), is transported to tank 5 (5e). The liquid in tank 5 (5e) is then transported to reverse osmosis membrane unit 3a, and the concentrate obtained in reverse osmosis membrane unit 3a is mixed into tank 5 (5e) while the reverse osmosis filtration process is performed. Furthermore, the permeate obtained in the reverse osmosis filtration process is transported to high-removal reverse osmosis membrane unit 4, where the concentrate is discharged, and the permeate is supplied to tank 5a for use as dilution water for the treated liquid A. After the reverse osmosis filtration process is completed, the concentrate of permeate C, i.e., the liquid in tank 5 (5e), ​​can be recycled to any tank.

[0188] Figure 2The example shown is a flowchart of an alkali metal salt recovery process comprising a reverse osmosis filtration step for concentrating permeate A, a reverse osmosis filtration step for concentrating permeate C, and a dilution step in which the permeate obtained in each reverse osmosis filtration step is used as dilution water for the treated liquid A. Specifically, firstly, the liquid passing through tank 5b of nanofiltration membrane unit A (2a) is transferred to tank 5e. The liquid in tank 5e is transferred to reverse osmosis membrane unit 3a, and the resulting concentrate is transferred to tank 6f, where the permeate is added as dilution water to the treated liquid A. Next, the liquid in tank 6f is transferred to tank 3c. The liquid in tank 3c is transferred to nanofiltration membrane unit B (2b) as treated liquid B, and the permeate C stored in tank 4d is transferred to tank 7g. The liquid in tank 7g is transferred to reverse osmosis membrane unit 3b, and the resulting concentrate is transferred to tank 8h, where the permeate is added as dilution water to the treated liquid A. The liquid from tank #8, after 5 hours, can be recycled into any tank.

[0189] (5) Ultrafiltration process

[0190] For solution X(k), ultrafiltration can be performed before the first nanofiltration step. Ultrafiltration removes high molecular weight organic matter, which inhibits scaling of the nanofiltration membrane.

[0191] When multiple solutions are mixed to obtain solution X(k), these solutions can be subjected to ultrafiltration separately. The permeate from the ultrafiltration membrane unit is used as the treated liquid A(k) in the first nanofiltration step.

[0192] exist Figures 1-3 In the process, solution X(k) is delivered to ultrafiltration membrane unit 1, and permeate is delivered to tank 5a.

[0193] (6) Recycling process

[0194] In this process, alkali metal salts are recovered from the permeate C containing alkali metal ions obtained in the second nanofiltration process or from the concentrate of the permeate C obtained in the reverse osmosis filtration process. The recovery process preferably includes the concentration of the aqueous solution of the alkali metal salt.

[0195] The recovery of alkali metal salts can be carried out by known methods. For example, in the case of potassium salts, the temperature dependence of solubility can be utilized, or the recovery can be carried out by adding undesirable solvents such as ethanol.

[0196] Compared to other alkali metal salts, lithium salts have lower solubility. For example, sodium carbonate and potassium carbonate have high solubility in water (over 20 g per 100 mL of water), while lithium carbonate dissolves only 1.33 g per 100 mL of water at 25°C. Therefore, lithium can be recovered as lithium carbonate by adding carbonates to permeate C containing alkali metal ions or a concentrate of permeate C. The solubility of lithium carbonate decreases further at high temperatures, so the aqueous solution can be heated.

[0197] (7) Comparison Method

[0198] Figure 4 , Figure 6 and Figure 7 This is a simplified flowchart illustrating the recovery process of alkali metal salts in the comparison method.

[0199] Figure 4 The process composition relative to Figure 1 The difference in the process configuration lies in the fact that the concentrate B of nanofiltration membrane unit A (2a) and the concentrate D of nanofiltration membrane unit B (2b) are drained without being mixed with the liquids in tank 1 5a and tank 3 5c, respectively. Other than that... Figure 1 The same. Under the above process, Li exists. + The problem of low recovery rate.

[0200] Figure 6 The process configuration is similar to the previous one, except that it lacks the third tank 5c, the nanofiltration membrane unit B (2b), and the fourth tank 5d (i.e., it lacks the second nanofiltration process), and the liquid from the second tank 5b is transferred to the fifth tank 5e. Figure 1 The example shown is the same. In the case of the above process, there is Li in the recovered liquid. + The problem of decreasing purity.

[0201] Figure 7 The process configuration is a continuous processing process. Solution X(1) is fed to ultrafiltration membrane unit 1. The permeate from ultrafiltration membrane unit 1 is mixed with dilution water and then fed to nanofiltration membrane module A(2a). The permeate A from nanofiltration membrane unit A(2a) is fed to nanofiltration membrane unit B(2b), and the concentrate B from nanofiltration membrane unit A(2a) is recycled to tank 5a. The permeate C from nanofiltration membrane unit B(2b) is fed to the first reverse osmosis membrane unit 3a. The concentrate D from nanofiltration membrane unit B(2b) is mixed with the concentrate B from nanofiltration membrane unit A(2a) and fed to tank 5a. The concentrate from the first reverse osmosis membrane unit 3a is fed to tank 5e. The permeate from the first reverse osmosis membrane unit 3a is used as part of the dilution water. After processing the total amount of solution X(1), solutions X(2) and X(3) are processed in the same order. In the above process, Li exists. +The problem of low recovery rate.

[0202] (8) Alkali metal salt recovery device

[0203] The alkali metal salt recovery apparatus of the present invention comprises:

[0204] A solution containing alkali metal ions is used as the liquid to be treated, A, and is separated into permeate A and concentrate B by the first nanofiltration membrane unit.

[0205] A first circulation mechanism that mixes the concentrated liquid B with the remaining portion of the liquid A being treated;

[0206] The permeate A or the concentrate of the permeate A is used as the treated liquid B, and the second separation mechanism separates it into permeate C and concentrate D through the second nanofiltration membrane unit.

[0207] A second circulation mechanism that mixes the concentrated liquid D with the remaining portion of the treated liquid B;

[0208] A dilution mechanism for adding dilution water to at least one of the liquid to be treated, A, and B;

[0209] A flow control mechanism capable of controlling the flow rates of permeate A and concentrate B in the first separation unit and permeate C and concentrate D in the second separation unit; and

[0210] A flow control mechanism that synchronizes the flow rate of the added dilution water in the dilution mechanism with the flow rate of the permeate when the treated liquid with added dilution water is delivered to the nanofiltration membrane unit.

[0211] The alkali metal salt recovery device of the present invention may be a recovery device comprising a first separation device, a first circulation device, a second separation device, a second circulation device, a dilution device, a flow control device a, and a flow control device b.

[0212] The first separation device includes a first nanofiltration membrane unit. In the first separation device, the solution containing alkali metal ions, i.e., the treatment liquid A, is separated into permeate A and concentrate B by passing through the first nanofiltration membrane unit.

[0213] In the first circulation device, the concentrated solution B is mixed with the remaining portion of the treated solution A.

[0214] In the second separation device, the permeate A or the concentrate of the permeate A, as the treated liquid B, is separated into permeate C and concentrate D by the second nanofiltration membrane unit.

[0215] In the second circulation device, the concentrated liquid D is mixed with the remaining portion of the treated liquid B.

[0216] In the dilution device, dilution water is added to at least one of the liquid to be treated, A and B.

[0217] In the flow control device a, the flow rates of permeate A and concentrate B in the first separation device and permeate C and concentrate D in the second separation device are controlled.

[0218] In the flow control device b, the added flow rate of dilution water in the dilution mechanism is synchronized with the permeate flow rate when the treated liquid with added dilution water is delivered to the nanofiltration membrane unit.

[0219] In the first and second separation devices, the nanofiltration membrane unit is preferably a pressure vessel (container) with helical elements filled with nanofiltration membranes, capable of supplying a solution containing alkali metal ions to the vessel via a high-pressure pump. Multiple nanofiltration membrane units can be connected in parallel or series, or each vessel can be filled with multiple nanofiltration membrane elements. The helical elements of the nanofiltration membrane can be elements with arbitrary diameters and lengths. The size of the helical elements varies depending on the membrane area; for the same type of membrane, a larger membrane area allows for processing a greater volume of liquid per unit time. The size and number of helical elements of the nanofiltration membrane can be arbitrarily determined based on the scale of the liquid A being treated.

[0220] As a flow control device a, in order to maintain a constant flow rate of the permeate and concentrate in the nanofiltration membrane unit, it is preferable to have an instrument (flow meter) capable of measuring the flow rates of the permeate and concentrate in the nanofiltration membrane unit. Regarding the flow rate control of the permeate, it is preferable that the high-pressure pump has a mechanism capable of receiving data from the permeate flow meter at any time and controlling the output of the high-pressure pump to achieve a constant permeate flow rate. Regarding the flow rate control of the concentrate, it is preferable to have a solenoid valve near the concentrate flow meter, and this solenoid valve preferably has a mechanism capable of receiving data from the concentrate flow meter at any time and controlling the concentrate flow rate to keep it constant.

[0221] In the first and second circulation devices, it is preferable to have a tank (raw water tank) for filling the liquid to be treated, and piping for circulating the concentrate from the nanofiltration membrane unit back to the raw water tank.

[0222] The dilution equipment preferably includes a tank (dilution water tank) for filling with dilution water. It also preferably includes a pump (dilution water delivery pump) for transferring the dilution water from the dilution water tank to the raw water tank.

[0223] As flow control device b, the dilution water delivery pump preferably has a mechanism that can receive data from the permeate flow meter at any time and deliver dilution water at the same flow rate as the permeate flow rate.

[0224] The aforementioned equipment preferably uses raw materials that are resistant to the liquid properties or operating pressure of the liquid being processed.

[0225] In order to recover alkali metal salts, the recovery device of the present invention can, in addition to the above, also select pumps, piping, valves, tanks, containers, temperature control machines, and measuring instruments (pH meters, conductivity meters, flow meters, pressure gauges, etc.) in any combination.

[0226] Example

[0227] The present invention will now be described with reference to examples, but the invention is not limited to these examples in any way. The measurements in the examples and comparative examples were performed as follows.

[0228] <Performance of Nanofiltration Membranes and Reverse Osmosis Membranes>

[0229] (Glucose removal rate and isopropanol removal rate of nanofiltration membrane)

[0230] Based on the glucose concentration in the permeate and the supply water when a 1000 mg / L glucose aqueous solution at 25°C and pH 6.5 is used as supply water, and the isopropanol concentration in the permeate and the supply water when a 1000 mg / L isopropanol aqueous solution at 25°C and pH 6.5 is used as supply water, and the isopropanol concentration in the permeate and the supply water when a 1000 mg / L isopropanol aqueous solution at 25°C and pH 6.5 is used as supply water, the isopropanol removal rate and glucose removal rate are calculated using the following formula.

[0231] Isopropanol removal rate (%) = 100 × (1 - (isopropanol concentration in permeate / isopropanol concentration in feed water)) Glucose removal rate (%) = 100 × (1 - (glucose concentration in permeate / glucose concentration in feed water))

[0232] In addition, the concentration of isopropanol was determined using a gas chromatograph (GC-18A manufactured by Shimadzu Corporation), and the concentration of glucose was determined using a refractometer (RID-6A manufactured by Shimadzu Corporation).

[0233] (Magnesium sulfate removal rate and magnesium chloride removal rate of nanofiltration membrane)

[0234] The MgSO4 removal rate and MgCl2 removal rate are calculated using the following formulas: (1) The magnesium sulfate (MgSO4) concentration in the permeate and the magnesium sulfate concentration in the supply water is calculated when a 2000 mg / L magnesium sulfate (MgSO4) aqueous solution at 25°C and pH 6.5 is passed through a nanofiltration membrane at an operating pressure of 0.5 MPa. (2) The magnesium chloride (MgCl2) concentration in the permeate and the magnesium chloride concentration in the supply water is calculated when a 2000 mg / L magnesium chloride (MgCl2) aqueous solution at 25°C and pH 6.5 is passed through a nanofiltration membrane at an operating pressure of 0.5 MPa.

[0235] Regarding the concentrations of magnesium sulfate and magnesium chloride, the conductivity of the supply water and permeate was measured using a conductivity meter manufactured by Toa Denpa Kogyo Co., Ltd., to obtain their respective practical salt concentrations, namely MgSO4 concentration and MgCl2 concentration.

[0236] MgSO4 removal rate (%) = 100 × {1 - (MgSO4 concentration in permeate / MgSO4 concentration in feed water)}

[0237] MgCl2 removal rate (%) = 100 × {1 - (MgCl2 concentration in permeate / MgCl2 concentration in feed water)}

[0238] (Positron annihilation lifetime determination method using positron beam method)

[0239] For nanofiltration membranes A and B, which are described later, the average pore size is derived.

[0240] The positron annihilation lifetime of the separated functional layer was determined using the positron beam method. The composite semi-permeable membrane was freeze-dried under reduced pressure at -30°C and cut into 1.5cm × 1.5cm squares as test samples. Using a thin-film corresponding positron annihilation lifetime measuring device equipped with a positron beam generator (described in detail, for example, in Radiation Physics and Chemistry, 58, 603, Pergamon (2000)), the separated functional layer side of the test sample was measured at room temperature vacuum with beam intensities of 0.1keV and 0.5keV using a photomultiplier tube and a barium difluoride scintillation counter, with a total count of 5 million. The results were analyzed using POSITRONFIT. Based on the average lifetime τ of the third component obtained from the analysis, the average pore size at a beam intensity of 0.1keV was taken as R1, and the average pore size at a beam intensity of 0.5keV as R2, using the Tao-Eldrup formula. R1 / R2 was calculated. The resulting value was taken as the "pore size distribution".

[0241] (Solution X)

[0242] The acidic aqueous solution A containing rare metals described in Table 1 of Patent Document 2 contains Li + Ni 2+ Co 2+ Mn 2+ The concentration method involves dissolving lithium sulfate, nickel sulfate, cobalt sulfate, and manganese sulfate in water, and adjusting the pH to 1 with sulfuric acid. Then, this aqueous solution is diluted 1.2 times with a sulfuric acid aqueous solution at pH 1 to prepare solution Xa (number 1).

[0243] Compared to solution Xa (serial number 1), except for Li+ Except for concentrations set at 1 / 2, prepare the solution using the same method, and use it as solution Xa (serial number 2).

[0244] Compared to solution Xa (serial number 1), except for Li + Except for concentrations set at 1 / 4, prepare the solution using the same method, and use it as solution Xa (serial number 3).

[0245] Compared to solutions Xa (numbers 1-3), solutions Xb (numbers 1-3) were prepared using the same method, except that the pH value was adjusted to 3.7.

[0246] Compared with solutions Xa (numbers 1 to 3), except that boric acid was added to make the boron concentration 50 mg / L, solutions were prepared in the same way and were referred to as solutions Xc (numbers 1 to 3).

[0247] The concentrations of various ions in the solutions obtained above were quantified using a Hitachi P-4010 ICP (Inductively Coupled Plasma Lucidum Analyzer) device, and the results are shown in Table 1.

[0248] In addition, the volume of each of the 1 to 3 components constituting solutions Xa, Xb and Xc is 1000L.

[0249] Table 1

[0250]

[0251] <Fabrication of Nanofiltration Membranes and Reverse Osmosis Membranes>

[0252] (Nanofiltration Membrane A)

[0253] In a nonwoven fabric made of polyester fibers (air permeability 1cc / cm) 2 A 18.0% by mass dimethylformamide (DMF) solution of polysulfone was cast onto a substrate at room temperature (25°C) to form a 180 μm thick layer. The substrate was then immediately immersed in pure water and allowed to stand for 5 minutes to form a porous support membrane (160 μm thick) made of fiber-reinforced polysulfone.

[0254] Next, air adjusted to 25°C was blown to remove excess moisture, and the surface temperature of the porous support membrane was simultaneously adjusted to 25°C. A 30°C aqueous solution containing 2.0% by mass piperazine, 250 ppm sodium dodecyl diphenyl ether disulfonate, and 1.0% by mass trisodium phosphate was coated onto the surface of the porous support membrane. After standing for 15 seconds, nitrogen gas was blown from an air nozzle to remove excess aqueous solution, thereby forming an amine aqueous solution coating layer on the porous support membrane. Then, a 38°C n-decane solution containing 0.2% by mass trimellityl chloride (hereinafter referred to as "TMC") was uniformly coated onto the entire surface of the porous support membrane. After standing for 1 minute at 70% relative humidity and 25°C, interfacial condensation occurred. Two fluids (pure water and air) were blown onto the membrane surface to remove the surface solution. Finally, the membrane was washed with 80°C pure water to obtain nanofiltration membrane A.

[0255] (Nanofiltration Membrane B)

[0256] Piperazine was designated as 2,5-dimethylpiperazine. A 38°C n-decane solution containing 0.2% TMC was uniformly coated onto the entire surface of the porous support membrane. The membrane was then left to stand for 1 minute at 80% relative humidity and 25°C. Otherwise, the nanofiltration membrane was prepared using the same method as nanofiltration membrane A to obtain nanofiltration membrane B.

[0257] (Nanofiltration membrane E)

[0258] KOCH's SelRO (registered trademark) MPS-34 was used as the nanofiltration membrane E.

[0259] The membrane properties of nanofiltration membrane A, nanofiltration membrane B and nanofiltration membrane E are shown in Table 2.

[0260] Nanofiltration membranes A, B, and E are wound into a spiral shape by any method and used as membrane elements (hereinafter referred to as "8-inch elements") with a diameter of 20.32 cm and a length of 102 cm.

[0261] Table 2

[0262]

[0263] (Reverse osmosis membrane C)

[0264] A porous support membrane was fabricated using the same method as nanofiltration membrane A. Air, adjusted to 25°C, was blown through the membrane to remove excess moisture, and the membrane surface temperature was simultaneously adjusted to 25°C. After immersion in an aqueous solution containing 5.0% m-phenylenediamine (hereinafter referred to as "m-PDA") for 15 seconds, nitrogen gas was blown through an air nozzle to remove excess aqueous solution. Then, a 30°C n-decane solution containing 0.18% TMC was uniformly coated onto the entire surface of the porous support membrane. The membrane was then allowed to stand at 30°C for 1 minute, and two fluids (pure water and air) were blown through the membrane surface to remove the surface solution. Finally, the membrane was washed with pure water at 80°C to obtain reverse osmosis membrane C.

[0265] (Reverse osmosis membrane D)

[0266] The m-PDA was set to 1.8% by mass, and the TMC was changed to 0.07% by mass. Otherwise, it was manufactured using the same method as reverse osmosis membrane C.

[0267] The membrane properties of the aforementioned reverse osmosis membrane C and reverse osmosis membrane D are shown in Table 3.

[0268] Reverse osmosis membranes C and D are wound into spirals using any method and used as 8-inch elements.

[0269] Table 3

[0270]

[0271] As shown in Table 3, reverse osmosis membrane C is a high-removal reverse osmosis membrane, while reverse osmosis membrane D is a low-removal reverse osmosis membrane.

[0272] <An Evaluation of the Recovery of Alkali Metal Salts>

[0273] (Alkali metal ion ratio)

[0274] The alkali metal ion ratio is calculated using the following formula, based on the concentrations of various ions in the solution.

[0275] Alkali metal ion ratio = Lithium ion concentration / (Cobalt ion concentration + Nickel ion concentration + Manganese ion concentration)

[0276] (Li + Recovery rate

[0277] Calculate Li using the following formula + Recovery rate.

[0278] Li + Recovery rate (%) = {(volume of liquid finally concentrated by the reverse osmosis membrane unit (L)) × (Li in the liquid finally concentrated by the reverse osmosis membrane unit)} + Concentration (mg / L)) / {(Initial volume of solution X (L))×Initial Li in solution X+ Concentration (mg / L)

[0279] Furthermore, in solution X(k), when the remaining portion of the treated liquid B(k) from the second nanofiltration step is recovered, the Li contained in the remaining portion of the treated liquid B(k) + Li in solution X(k) is recovered by adding it to the treated liquid A(m) (m: an integer greater than (k+1) and less than N) or the treated liquid A(p) (p: an integer greater than (k+2) and less than N), and is therefore considered to be recovered. + The recovery rate (k) is calculated by the following formula. The following formula was used in Example 9, but when the number of solutions N = 3, even if k = 2 or 3, it is considered that the Li contained in the remaining portion of the treated liquid B(k) in the second nanofiltration step is... + It was recycled.

[0280] Li + Recovery rate (k)(%) = {(volume of liquid ultimately concentrated by the reverse osmosis membrane unit in the treatment of solution X(k) (L)) × (Li in the liquid ultimately concentrated by the reverse osmosis membrane unit in the treatment of solution X(k)} + Concentration (mg / L) + (Volume (L) of the remaining portion of the treated liquid B(k) in the second nanofiltration step) × (Li in the remaining portion of the treated liquid B(k) in the second nanofiltration step) + Concentration (mg / L)) / {(Initial volume of solution X(k) (L))×(Li in the initial liquid of solution X(k)) + Concentration (mg / L) + (Volume (L) of the remaining portion of the treated liquid B (k-1; k≥2) added to solution X (k) in the second nanofiltration step) × (Li in the remaining portion of the liquid in the treated liquid B (k-1; k≥2) added to solution X (k) in the second nanofiltration step) + Concentration (mg / L)

[0281] (Li + purity)

[0282] Li + Purity is set as the ratio of alkali metal ions in the liquid ultimately concentrated by the reverse osmosis membrane unit.

[0283] (Boron concentration ratio)

[0284] The boron concentration ratio is defined as the ratio of the boron concentration to the lithium ion concentration in the liquid ultimately concentrated by the reverse osmosis membrane unit.

[0285] (Total processing time)

[0286] Total processing time is defined as the total time required to complete the processing of each of the following solutions X, numbered 1 to 3.

[0287] (Example 1)

[0288] exist Figure 1 In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a. A semi-batch processing step is performed according to the sequence 1 to 3 of solution Xa to recover alkali metal salts. Furthermore, the pressure resistance value of the first reverse osmosis membrane unit 3a is 8 MPa. The nanofiltration process is performed by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the nanofiltration process is confirmed by monitoring the operating pressure using equation (2). In the first nanofiltration process, the alkali metal ion recovery rate is 93%. Filtration continues in the second nanofiltration process until the alkali metal ion recovery rate reaches 96%. The reverse osmosis filtration process is performed by constant flow filtration with a permeate flow rate of 60 L / min until the operating pressure reaches 7 MPa. Additionally, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0289] The results of implementing this process are shown in Table 4.

[0290] (Example 2)

[0291] Except for the alkali metal salt recovery process in numbers 1 to 3, which use solution Xb instead of solution Xa, the same method as in Example 1 was used.

[0292] The results of implementing this process are shown in Table 4. At a pH value as high as 3.7, compared with Example 1 at a pH value of 1.0, the permeability of alkali metal ions decreased and the total processing time increased, but lithium ions could be recovered with high purity and high recovery rate.

[0293] (Example 3)

[0294] exist Figure 2In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a and reverse osmosis membrane unit 3b. A semi-batch processing step is performed according to the sequence 1 to 3 of solution Xa to implement the alkali metal salt recovery process. Furthermore, the pressure resistance value of the second reverse osmosis membrane unit 3b is 8 MPa. The nanofiltration step is performed by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the first and second nanofiltration steps is confirmed by monitoring the operating pressure using equation (2). In the first nanofiltration step, the alkali metal ion recovery rate is 93%, and filtration continues in the second nanofiltration step until the alkali metal ion recovery rate reaches 96%. The reverse osmosis filtration step is performed by constant flow filtration with a permeate flow rate of 15 L / min until the operating pressure reaches 7 MPa. In addition, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit for use.

[0295] The results of implementing this process are shown in Table 4. If a reverse osmosis membrane unit is used for concentration after each nanofiltration step, the total processing time increases slightly compared to the case where only one RO concentration is performed after a subsequent nanofiltration step, but lithium ions can be recovered with high purity and high recovery rate.

[0296] (Example 4)

[0297] Except for the use of nanofiltration membrane B, the alkali metal salt recovery process was carried out using the same method as in Example 1.

[0298] The results of implementing this process are shown in Table 4. It can be seen that using nanofiltration membrane B that meets the predetermined performance results in improved purity and recovery rate, and enables processing in a short time.

[0299] (Example 5)

[0300] Instead of monitoring the operating pressure using formula (2), the recovery rate of alkali metal ions in the nanofiltration process was confirmed by appropriate analysis of the permeate. Otherwise, the alkali metal salt recovery process was carried out using the same method as in Example 4.

[0301] The results of implementing this process are shown in Table 4. Due to the time required for proper analysis, the processing time is increased compared to the case of monitoring the operating pressure using Equation (2).

[0302] (Example 6)

[0303] Except for the use of solution Xc, the alkali metal salt recovery process was carried out using the same method as in Example 4.

[0304] The results of implementing this process are shown in Table 4.

[0305] (Example 7)

[0306] exist Figure 9 In the process configuration shown, nanofiltration membrane B is used as the nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), reverse osmosis membrane D is used as the reverse osmosis membrane unit 3a, and reverse osmosis membrane D is used as the high-removal reverse osmosis membrane unit 4. A semi-batch processing step is performed in the order of solutions Xc1 to Xc3 to implement the alkali metal salt recovery process. The first and second nanofiltration steps are performed by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the first and second nanofiltration steps is confirmed by monitoring the operating pressure using equation (2). The alkali metal ion recovery rate is 93% in the first nanofiltration step, and filtration continues in the second nanofiltration step until the alkali metal ion recovery rate reaches 96%. The reverse osmosis filtration step is performed by constant flow filtration with a permeate flow rate of 60 L / min until the operating pressure reaches 7 MPa. Furthermore, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0307] The results of implementing this process are shown in Table 4. It can be seen that boron can be removed by using reverse osmosis membrane D, which is a low-removal membrane, for concentration.

[0308] (Example 8)

[0309] exist Figure 3 In the process configuration shown, nanofiltration membrane B is used as the nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), reverse osmosis membrane D is used as the reverse osmosis membrane unit 3a, and reverse osmosis membrane D is used as the high-removal reverse osmosis membrane unit 4. The semi-batch processing steps are performed in the order of solutions Xc1 to 3 to implement the alkali metal salt recovery process. The first nanofiltration step and the second nanofiltration step are performed with a constant flow rate of 60 L / min. The alkali metal ion recovery rate of the first nanofiltration step and the second nanofiltration step is confirmed by monitoring the operating pressure using formula (2). The alkali metal ion recovery rate is 93% in the first nanofiltration step. Filtration continues in the second nanofiltration step until the alkali metal ion recovery rate becomes 96%. The reverse osmosis filtration step is performed with a constant flow rate of 60 L / min and continues until the operating pressure becomes 7 MPa. In addition, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0310] The results of implementing this process are shown in Table 4. It can be seen that by using reverse osmosis membrane D as a low-removal membrane for concentration and by setting a circulation step in the reverse osmosis filtration process, boron can be further removed compared to Example 7.

[0311] (Example 9)

[0312] exist Figure 8 In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a. A semi-batch processing step is performed in the order of solutions Xa 1 to 3 to recover alkali metal salts. At this time, the treated liquid B (1) from step 2 in solution Xa1 (i.e., solution X (1)) is added to the treated liquid A (3) in solution Xa3 (i.e., solution X (3)). Furthermore, the pressure resistance value of the first reverse osmosis membrane unit 3a is 8 MPa. The nanofiltration process is performed by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using equation (2). In the first nanofiltration process, the alkali metal ion recovery rate was 93%. In the second nanofiltration process, filtration continued until the alkali metal ion recovery rate reached 80%. The reverse osmosis filtration process was implemented by constant flow filtration at a permeate flow rate of 60 L / min until the operating pressure reached 7 MPa. In addition, four 8-inch elements were connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0313] The results of implementing this process are shown in Table 4. It can be seen that by adding the remaining portion of the treated solution B(k) to the treated solution A(p), lithium ions can be recovered with higher purity and higher recovery rate.

[0314] (Comparative Example 1)

[0315] exist Figure 4 In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a. A semi-batch processing step is performed according to the sequence of solutions Xa, from 1 to 3, to achieve alkali metal salt recovery. Furthermore, the permeate flow rate is 60 L / min throughout all processes. Additionally, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0316] The results of implementing this process are shown in Table 5. It can be seen that the lithium recovery rate is low when the nanofiltration process does not have a recycling step.

[0317] (Example 10)

[0318] exist Figure 5 In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a. The alkali metal salt recovery process is carried out in the order of solution Xa number 1 to 3. In addition, the pressure resistance value of the first reverse osmosis membrane unit 3a is 8 MPa. The nanofiltration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the nanofiltration process is confirmed by monitoring the operating pressure using formula (2). The alkali metal ion recovery rate is 93% in the first nanofiltration process. In the second nanofiltration process, filtration continues until the alkali metal ion recovery rate reaches 80%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min until the operating pressure reaches 7 MPa. In addition, the permeate flow rate is 60 L / min in all processes. In addition, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0319] The results of implementing this process are shown in Table 5.

[0320] (Comparative Example 2)

[0321] exist Figure 6 In the system configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a. A semi-batch processing step is performed according to the sequence of solutions Xa, from 1 to 3, to achieve alkali metal salt recovery. Furthermore, the permeate flow rate is 60 L / min throughout all steps. Additionally, four 8-inch elements are connected in series in each nanofiltration membrane unit and each reverse osmosis membrane unit.

[0322] The results of implementing this process are shown in Table 5. It can be seen that the purity of lithium is low when only the first nanofiltration step is performed.

[0323] (Comparative Example 3)

[0324] exist Figure 7 In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a. A continuous processing step is performed according to the sequence of solutions Xa, from 1 to 3, to achieve alkali metal salt recovery. Furthermore, the permeate flow rate is 60 L / min throughout all steps. Additionally, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0325] The results of implementing this process are shown in Table 5. It can be seen that the lithium recovery rate is low in the continuous processing steps.

[0326] (Comparative Example 4)

[0327] Except for the use of nanofiltration membrane E, the alkali metal salt recovery process was carried out using the same method as in Comparative Example 1.

[0328] The results of implementing this process are shown in Table 5.

[0329] (Example 11)

[0330] Except for the use of nanofiltration membrane E, the alkali metal salt recovery process was carried out using the same method as in Example 1.

[0331] The results of implementing this process are shown in Table 5. It can be seen that by applying the process involved in this embodiment, the lithium recovery rate is improved compared with Comparative Example 4.

[0332] (Example 12)

[0333] Except for the use of nanofiltration membrane E, the alkali metal salt recovery process was carried out using the same method as in Example 9.

[0334] The results of implementing this process are shown in Table 5. It can be seen that even when using the nanofiltration membrane E, lithium ions can be recovered with higher purity and higher recovery rate by adding the remaining portion of the treated solution B(k) to the treated solution A(p).

[0335] (Example 13)

[0336] exist Figure 10 In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane unit A (2a), and reverse osmosis membrane C is used as the reverse osmosis membrane unit 3a. The alkali metal salt recovery process is carried out in the order of solution Xa number 1 to 3. In addition, the pressure resistance value of the first reverse osmosis membrane unit 3a is 8 MPa. The nanofiltration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the nanofiltration process is confirmed by monitoring the operating pressure using formula (2). The alkali metal ion recovery rate is 93% in the first nanofiltration process. In the second nanofiltration process, filtration continues until the alkali metal ion recovery rate reaches 80%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min until the operating pressure reaches 7 MPa. In addition, the permeate flow rate is 60 L / min in all processes. In addition, four 8-inch elements are connected in series in each nanofiltration membrane unit and reverse osmosis membrane unit.

[0337] The results of implementing this process are shown in Table 5. It can be seen that by adding the remaining portion of the treated liquid B(k) to the treated liquid A(m), lithium ions can be recovered with higher purity and higher recovery rate.

[0338] Table 4

[0339]

[0340] Table 5

[0341]

[0342] In the above examples and comparative examples, higher lithium recovery rate and lithium purity are more advantageous, while lower boron concentration ratio and total processing time are more advantageous.

[0343] The results above show that the alkali metal salt recovery method of the present invention, namely Examples 1-8, compared with Comparative Examples 1-4, can recover alkali metal salts with high purity and high recovery rate in a short time. Furthermore, the results of Examples 6-8 show that even in the presence of neutral molecules such as boron, they can be removed efficiently, and the total processing time can be shortened.

[0344] The present invention has been described in detail with reference to specific embodiments, but it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. Furthermore, this application is based on Japanese Patent Application No. 2022-192379, filed November 30, 2022, the entirety of which is incorporated herein by reference. All references cited herein are incorporated in their entirety.

[0345] Industry availability

[0346] This invention is suitable as a method for effectively separating and recovering alkali metals such as lithium from waste materials, waste liquids, ores or slags generated in the manufacturing process of lithium-ion batteries or their manufacturing processes.

[0347] Explanation of reference numerals in the attached figures

[0348] 1 Ultrafiltration Membrane Unit

[0349] 2a Nanofiltration Membrane Unit A

[0350] 2b Nanofiltration Membrane Unit B

[0351] 3a First Reverse Osmosis Membrane Unit

[0352] 3b Second Reverse Osmosis Membrane Unit

[0353] 4 high-removal reverse osmosis membrane units

[0354] 5a, first can

[0355] 5b, second can

[0356] 5c, 3rd can

[0357] 5d, 4th can

[0358] 5e, the 5th can

[0359] 5f 6th can

[0360] 5g, 7th can

[0361] 5h, 8th can

Claims

1. A method for recovering alkali metal salts, comprising sequentially performing batch processing steps on N solutions X containing alkali metal ions, wherein the solutions X are subjected to steps 1 and 2 below to obtain permeate, wherein N is an integer greater than or equal to 2. Step 1 is the first nanofiltration step: the solution X, as the treated liquid A, is transported to the nanofiltration membrane unit A, where it is separated into permeate A and concentrate B. The concentrate B is then mixed with the remaining portion of the treated liquid A and transported back to the nanofiltration membrane unit A to further obtain the permeate A. Process 2 is the second nanofiltration process: The permeate A or its concentrate obtained in step 1 is fed to the nanofiltration membrane unit A as the treated liquid B, where it is separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed back to the nanofiltration membrane unit A to further obtain permeate C. Alternatively, The permeate A or its concentrate obtained in step 1 is fed to the nanofiltration membrane unit B as the treated liquid B, where it is separated into permeate C and concentrate D. The concentrate D is then mixed with the remaining portion of the treated liquid B and fed back to the nanofiltration membrane unit B to further obtain the permeate C. The recovery method further includes step 4: after step 2 of the kth solution X(k) in the N solutions X is completed, the remaining portion of the kth treated liquid B(k) mixed with the kth concentrate D(k) is added to the mth solution X(m) or the mth treated liquid A(m), where k is an integer greater than or equal to 1 and less than or equal to N-1, and m is an integer greater than or equal to k+1 and less than or equal to N.

2. The method for recovering alkali metal salts according to claim 1, Step 2 uses the nanofiltration membrane unit B. During the period after performing step 1 on the kth solution X(k) of the N solutions X, step 2 is performed in parallel on the (k+1)th solution X(k+1), where k is an integer greater than or equal to 1 and less than or equal to N-1.

3. The method for recovering alkali metal salts according to claim 1 or 2, The process includes a step of diluting at least one of the liquid to be treated, A, and the liquid to be treated, B.

4. The method for recovering alkali metal salts according to claim 2, It also includes the following process 3. Step 3 is a reverse osmosis filtration step: in at least one of the kth permeate A(k) and the kth permeate C(k), the solution X(k) is concentrated.

5. The method for recovering alkali metal salts according to claim 4, The process 3 is performed only once on the permeate C(k).

6. The method for recovering alkali metal salts according to claim 1 or 2, The pH value of solution X is below 4.

7. The method for recovering alkali metal salts according to claim 1 or 2, The alkali metal ions include lithium ions.

8. The method for recovering alkali metal salts according to claim 1 or 2, At least one of the nanofiltration membrane units A and B has a nanofiltration membrane that has a porous support membrane and a separation functional layer. When a positron beam is irradiated onto the surface of the separation functional layer side of the nanofiltration membrane, the average pore size R1 and average pore size R2 of the separation functional layer, derived by positron annihilation lifetime determination, satisfy 0.90 ≤ R1 / R2 ≤ 1.

10. R1: Average aperture under the condition of positron beam intensity of 0.1 keV. R2: Average aperture under the condition of positron beam intensity of 0.5 keV.

9. The method for recovering alkali metal salts according to claim 1 or 2, At least one of steps 1 and 2 is performed at a constant permeation flow rate, the change in operating pressure over time is monitored, and at least one of steps 1 and 2 is terminated when the recovery rate A of alkali metal ions reaches the target value based on Equation 2 below, where the recovery rate A is in percentage (%). In Equation 2, the recovery rate A of alkali metal ions is expressed in %, the operating pressure P is expressed in Pa, the initial operating pressure P0 is expressed in Pa, and the initial liquid volume V0 of the treated object is expressed in m³. 3 The alkali metal ion removal rate R of the nanofiltration membrane is expressed in %, and the liquid recovery rate S of the nanofiltration process is expressed in %, with a supply flow rate Q. F The unit is m 3 / s, concentrate flow rate Q c The unit is m 3 / s, filter end time t=tb.

10. The method for recovering alkali metal salts according to claim 4 or 5, At least one of the permeate A(k) and the permeate C(k) contains neutral molecules that do not have a charge under conditions with a pH value below 3, and the reverse osmosis filter membrane used in the reverse osmosis filtration process is a low-removal reverse osmosis membrane that achieves an isopropanol removal rate of 70% or more and less than 85% when permeating an isopropanol aqueous solution at 25°C and a pH value of 6.5 under an operating pressure of 0.5 MPa.

11. The method for recovering alkali metal salts according to claim 10, The neutral molecule is a boron compound.

12. The method for recovering alkali metal salts according to claim 11, In step 3, there is a recycling step in which the concentrate obtained in the reverse osmosis filtration step is mixed with the solution supplied to the reverse osmosis filtration step.

13. The method for recovering alkali metal salts according to claim 10, The device includes a step of conveying the permeate obtained in the reverse osmosis filtration process to a high-removal reverse osmosis membrane unit, and adding the obtained permeate as dilution water for at least one of the treated liquid A and the treated liquid B. The high-removal reverse osmosis membrane unit is equipped with a high-removal reverse osmosis membrane that achieves an isopropanol removal rate of 85% or more and 95% or less when permeating an isopropanol aqueous solution at 25°C and pH 6.5 under an operating pressure of 0.5 MPa.

14. An alkali metal salt recovery device, comprising a batch processing step of sequentially processing N solutions X containing alkali metal ions to obtain permeate, wherein N is an integer greater than or equal to 2. The device includes: The solution X is used as the liquid to be treated, and is separated into permeate A and concentrate B by the first separation mechanism through the first nanofiltration membrane unit. A first circulation mechanism that mixes the concentrated liquid B with the remaining portion of the liquid A being treated; The permeate A or the concentrate of the permeate A is used as the treated liquid B, and the second separation mechanism separates it into permeate C and concentrate D through the second nanofiltration membrane unit. A second circulation mechanism that mixes the concentrated liquid D with the remaining portion of the treated liquid B; An apparatus that adds the remaining portion of the kth treated liquid B(k) which is mixed with the kth concentrate D(k) to the mth solution X(m) or the mth treated liquid A(m), wherein k is an integer greater than or equal to 1 and less than or equal to N-1, and m is an integer greater than or equal to k+1 and less than or equal to N; A dilution mechanism for adding dilution water to at least one of the liquid to be treated, A, and B; A flow control mechanism capable of controlling the flow rates of permeate A and concentrate B in the first separation unit and permeate C and concentrate D in the second separation unit; and A flow control mechanism that synchronizes the flow rate of the added dilution water in the dilution mechanism with the flow rate of the permeate when the treated liquid with added dilution water is delivered to the nanofiltration membrane unit.

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