Method and apparatus for recovering alkali metal salts

Abstract

The present invention relates to a method for recovering an alkali metal salt, said method comprising steps 1 and 2 below. Step 1 is a first nanofiltration step in which a solution X containing alkali metal ions is fed, as a to-be-treated liquid A, to a nanofiltration membrane unit A and is thereby separated into a liquid permeate A and a concentrated liquid B, then the concentrated liquid B is mixed with the residue of the to-be-treated liquid A and the result thereof is fed to the nanofiltration membrane unit A again, and more of the liquid permeate A is obtained. Step 2 is a second nanofiltration step in which: the liquid permeate A obtained in step 1 or a concentrated liquid of the liquid permeate A is fed, as a to-be-treated liquid B, to the nanofiltration membrane unit A and is thereby separated into a liquid permeate C and a concentrated liquid D, then the concentrated liquid D is mixed with the residue of the to-be-treated liquid B and the result thereof is fed to the nanofiltration membrane unit A again, and more of the liquid permeate C is obtained; or alternatively, the liquid permeate A obtained in step 1 or a concentrated liquid of the liquid permeate A is fed, as a to-be-treated liquid B, to a nanofiltration membrane unit B and is thereby separated into a liquid permeate C and a concentrated liquid D, then the concentrated liquid D is mixed with the residue of the to-be-treated liquid B and the result thereof is fed to the nanofiltration membrane unit B, and more of the liquid permeate C is obtained.

Description

[Technical field]

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

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

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

[0004] Nickel is used in stainless steel, taking advantage of its luster and high corrosion resistance, and in recent years, like cobalt, its demand as a material for lithium-ion batteries has been increasing. As the demand for various rare metals increases, efforts are being made to recover rare metals such as lithium, cobalt, and nickel from used lithium-ion batteries and waste materials generated during their manufacturing process, from the perspective of recycling valuable resources.

[0005] For example, practical application of resource recovery from waste lithium-ion batteries has been promoted, mainly for rare metals such as cobalt and nickel, but the mainstream method is a solvent extraction method using a chelating agent, which has problems such as a large environmental burden and cost disadvantages (Non-Patent Document 1). To solve this problem, a separation and recovery method using separation membranes such as ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes from an aqueous solution obtained by acid leaching waste lithium-ion batteries has been disclosed (Patent Document 1). However, in this separation and recovery method, since the nanofiltration membrane is a one-stage process, it is difficult to recover lithium at a high purity and in a high amount unless the nanofiltration membrane performance is extremely high.

[0006] Therefore, a separation and recovery method that uses a nanofiltration membrane in multiple stages has been disclosed (Patent Document 2). This method is a continuous process in which the liquid that has passed through the nanofiltration membrane is passed through the nanofiltration membrane again to improve the lithium purity, and the liquid that has not passed through the nanofiltration membrane is passed through the nanofiltration membrane to recover the remaining lithium. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2019 / 018333 [Patent Document 2] International Publication No. 2021 / 215484 [Non-patent literature]

[0008] [Non-Patent Document 1] "Report on the FY2017 Mineral Resource Infrastructure Development Survey for Exploration and Other Businesses to Promote Mineral Resource Development (Basic Survey on the Formulation of a Mineral Resource Securing Strategy)," Mitsubishi Research Institute, Inc., Environment and Energy Business Division, March 2018. Summary of the Invention [Problem to be solved by the invention]

[0009] However, the method described in Patent Document 2 involves a complex and continuous process, and therefore there is a risk that the treatment process using the nanofiltration membrane may become unstable if the composition of the liquid to be treated changes or if the separation performance changes due to deterioration of the nanofiltration membrane, etc., and there is room for improvement in terms of maintaining a specified lithium purity and recovery rate.

[0010] An object of the present invention is to provide a method for stably recovering high-purity and high-volume alkali metal salts with a small number of steps from lithium ion batteries and waste materials, waste liquids, ores, etc. generated in the manufacturing process thereof. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention has the following configuration. (1) A method for recovering an alkali metal salt, comprising the following steps 1 and 2: Step 1: A first nanofiltration step in which a solution X containing alkali metal ions is sent to a nanofiltration membrane unit A as a treated liquid A, and separated into a permeated liquid A and a concentrated liquid B, and further, the concentrated liquid B is mixed with the remainder of the treated liquid A and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid A. Step 2: A second nanofiltration step in which the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit A as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid C, or the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit B as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent again to the nanofiltration membrane unit B to further obtain the permeated liquid C. (2) A method for recovering an alkali metal salt, in which a process for obtaining the permeate C from the solution X through the step 1 and the step 2 is sequentially performed on N solutions X (N: an integer of 2 or more), in which the step 2 uses the nanofiltration membrane unit B, and while the step 2 is being performed after the step 1 on a k-th solution X(k) (k: an integer of 1 to (N-1)) among the N solutions X, the step 1 is performed in parallel on a (k+1)-th solution X(k+1), the method for recovering an alkali metal salt described in (1) above. (3) The method for recovering an alkali metal salt according to (1) or (2) above, further comprising a step of diluting at least one of the liquid A to be treated and the liquid B to be treated. (4) The method for recovering an alkali metal salt according to (2) or (3) above, further comprising the following step 3: Step 3: A reverse osmosis filtration step for concentrating at least one of the kth permeate A(k) and the kth permeate C(k) in at least one of said solutions X(k). (5) The method for recovering an alkali metal salt according to (4) above, wherein the step 3 is carried out only once on the permeated liquid C(k). (6) The method for recovering an alkali metal salt according to any one of the above (1) to (5), wherein the solution X has a pH of 4 or less. (7) The method for recovering an alkali metal salt according to any one of the above (1) to (6), wherein the alkali metal ions include lithium ions. (8) A method for recovering an alkali metal salt according to any one of (1) to (7) above, comprising the following step 4: Step 4: A step of adding, after completion of step 2 for the kth solution X(k) (k: an integer of 1 or more and (N-1) or less) among the N solutions X, the remainder of the kth liquid to be treated B(k) having been mixed with the kth concentrated liquid D(k) to the mth solution X(m) (m: an integer of (k+1) or more and N or less) or the mth liquid to be treated A(m). (9) The nanofiltration membrane of at least one of the nanofiltration membrane unit A and the nanofiltration membrane unit 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 side of the separation functional layer, and the average pore size R1 and the average pore size R2 of the separation functional layer derived from a positron annihilation lifetime measurement method satisfy 0.90≦R1 / R2≦1.10. The method for recovering an alkali metal salt according to any one of the above (1) to (8). R1: Average hole diameter at positron beam intensity of 0.1 keV R2: Average hole diameter at positron beam intensity of 0.5 keV (10) A method for recovering an alkali metal salt according to any one of the above (1) to (9), wherein at least one of the steps 1 and 2 is carried out at a constant permeation flow rate, and at least one of the steps 1 and 2 is terminated when a recovery rate A (%) of the alkali metal ion reaches a target value based on the following formula (2) while monitoring a change over time of the operating pressure:

[0012]

number

[0013] [In formula (2), the recovery rate A of alkali metal ions (%), the operating pressure P (Pa), the initial operating pressure P0 (Pa), and the initial liquid volume to be treated V0 (m 3 ), alkali metal ion removal rate of nanofiltration membrane R (%), liquid recovery rate of nanofiltration process S (%), supply flow rate Q F (m 3 / s), concentrate flow rate Q c (m 3 / s), and the end time of filtration is t=tb. (11) The method for recovering an alkali metal salt according to any one of the above (4) to (10), wherein at least one of the permeated liquid A(k) and the permeated liquid C(k) contains neutral molecules having no charge under conditions of pH 3 or less, and the reverse osmosis filtration membrane used in the reverse osmosis filtration step is a low-rejection reverse osmosis membrane having an isopropyl alcohol removal rate of 70% or more and less than 85% when an aqueous isopropyl alcohol solution having a pH of 6.5 and an operating pressure of 0.5 MPa and a temperature of 25°C is passed through the membrane. (12) The method for recovering an alkali metal salt according to the above (11), wherein the neutral molecule is a boron compound. (13) The method for recovering an alkali metal salt according to (12) above, further comprising a circulation step of mixing the concentrated liquid obtained in the reverse osmosis filtration step with a solution to be supplied to the reverse osmosis filtration step in the step 3. (14) A method for recovering an alkali metal salt according to any one of (11) to (13) above, comprising a step of feeding the permeated liquid obtained in the reverse osmosis filtration step to a high-removal reverse osmosis membrane unit equipped with a high-removal reverse osmosis membrane having an isopropyl alcohol removal rate of 85% or more and 95% or less when an aqueous isopropyl alcohol solution having a pH of 6.5 at 25°C is permeated therethrough under an operating pressure of 0.5 MPa, and adding the permeated liquid obtained as dilution water to at least one of the liquid A to be treated and the liquid B to be treated. (15) a first separation means for separating a solution containing alkali metal ions as a treated liquid A into a permeate liquid A and a concentrate liquid B by a first nanofiltration membrane unit; a first circulation means for mixing the concentrated liquid B with the remainder of the liquid A to be treated; A second separation means for separating the permeated liquid A or a concentrate of the permeated liquid A as a liquid to be treated B into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit; A second circulation means for mixing the concentrated liquid D with the remainder of the liquid to be treated B; A dilution means for adding dilution water to at least one of the liquid A and the liquid B; a flow rate control means capable of controlling the flow rates of the permeated liquid A and the concentrated liquid B in the first separation means, and the permeated liquid C and the concentrated liquid D in the second separation means; An alkali metal salt recovery apparatus comprising: a flow control means for synchronizing the flow rate of dilution water added in the dilution means with the flow rate of the permeated liquid when the treated liquid to which the dilution water is added is sent to a nanofiltration membrane unit. Effect of the Invention

[0014] The method for recovering an alkali metal salt of the present invention makes it possible to stably recover a high-purity, high-volume salt of an alkali metal such as lithium or cesium from a solution containing alkali metal ions with a small number of steps. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic flow diagram showing a method for recovering an alkali metal salt according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. [Diagram 3] FIG. 3 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. [Figure 4] FIG. 4 is a schematic flow diagram showing a method for recovering an alkali metal salt in a comparative example. [Diagram 5] FIG. 5 is a schematic flow diagram showing a method for recovering metal salts according to another embodiment of the present invention. [Figure 6] FIG. 6 is a schematic flow diagram showing a method for recovering an alkali metal salt in a comparative example. [Figure 7] FIG. 7 is a schematic flow diagram showing a method for recovering an alkali metal salt in a comparative example. [Figure 8] FIG. 8 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. [Figure 9] FIG. 9 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. [Figure 10] FIG. 10 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The following describes in detail the embodiments of the present invention; however, the present invention is not limited to the following description, and can be modified in any manner without departing from the gist of the present invention.

[0017] (1) Method for recovering alkali metal salts The method for recovering an alkali metal salt of the present invention is a method for recovering an alkali metal salt from a solution containing alkali metal ions, and comprises the following steps 1 and 2. Step 1: A first nanofiltration step in which a solution X containing alkali metal ions is sent to a nanofiltration membrane unit A as a treated liquid A, and separated into a permeated liquid A and a concentrated liquid B, and further, the concentrated liquid B is mixed with the remainder of the treated liquid A and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid A. Step 2: A second nanofiltration step in which the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit A as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid C, or the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit B as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent again to the nanofiltration membrane unit B to further obtain the permeated liquid C.

[0018] The method for recovering an alkali metal salt according to this embodiment is a method for recovering an alkali metal salt, in which the process of obtaining the permeate C from the solution X through the process 1 and the process 2 is sequentially performed on N solutions X (N: an integer of 2 or more), and it is preferable that the nanofiltration membrane unit B is used in the process 2, and while the process 2 is being performed after the process 1 on the k-th solution X(k) (k: an integer of 1 to (N-1)) among the N solutions X, the process 1 is performed in parallel on the (k+1)-th solution X(k+1). A series of process steps in which the batch process steps in the processes 1 and 2 are performed semi-continuously on a plurality of solutions is called a semi-batch process step.

[0019] (2) Nanofiltration process In the nanofiltration process, a nanofiltration membrane is used to separate a solution containing alkali metal ions into a permeate and a retentate.

[0020] The ratio of the alkali metal ion concentration to the polyvalent metal ion concentration in the permeated liquid (hereinafter referred to as the “alkali metal ion ratio”) is higher than that in solution X, and the alkali metal ion ratio in the concentrated liquid is lower than that in solution X.

[0021] The concentration of polyvalent metal ions is calculated by the sum of the ion-equivalent concentrations of, for example, cobalt ions and nickel ions. The concentration of alkali metal ions is calculated by the sum of the ion-equivalent concentrations of, for example, lithium ions and cesium ions. Some alkali metal elements may exist in a solution as polyatomic ions rather than as monoatomic ions, but the converted concentration is the concentration when it is assumed that they exist as monoatomic ions. The concentrations of the polyvalent metal ions and alkali metal ions can be determined by, for example, analyzing the solution to be measured using a Hitachi P-4010 ICP (inductively coupled plasma emission spectrometry) device to quantify the concentrations (mg / L) of various ions.

[0022] (2-1) Solution containing alkali metal ions Solution X containing alkali metal ions may contain at least alkali metal ions 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 may be present in the form of alkali metal salts, and examples of alkali metal salts include salts of lithium, sodium, potassium, rubidium, and cesium. Among these, it is preferable to contain a salt of lithium from the viewpoint of the value of the object to be recovered. That is, the solution containing alkali metal ions contains lithium ions (hereinafter, "Li + It is preferred that the fluororesin composition includes a fluororesin.

[0023] When there are N solutions X containing alkali metal ions, each solution X only needs to contain at least an alkali metal ion and one or more conjugate bases, and the composition of the solution, such as the alkali metal ion concentration, polyvalent metal ion concentration, and pH, may differ for each solution X. Among them, all solutions X contain Li as the alkali metal ion. + It is preferred that the compound contains

[0024] The solution X containing alkali metal ions preferably contains at least one polyvalent metal ion in addition to the alkali metal ions, such as those of alkaline earth metals (e.g., magnesium, calcium, and strontium), typical elements (e.g., aluminum, tin, and lead), and transition elements (e.g., iron, copper, cobalt, and manganese).

[0025] The solution X containing alkali metal ions may contain neutral molecules that are not charged under conditions of pH 3 or less, and the neutral molecules preferably have a molecular weight of 70 or less. Examples of neutral molecules include boron compounds such as formic acid, acetic acid, and boric acid. Among them, boron compounds may be added as additives to the electrolyte of a lithium ion battery for the purpose of improving the characteristics of the battery, and therefore may be contained in the solution X containing alkali metal ions. For example, when a boron compound is contained, it becomes a purification inhibitor during lithium recovery, but it can be removed by the reverse osmosis filtration process described later. The boron concentration (mg / L) in the solution X is preferably equal to or less than the alkali metal ion concentration of the target to be recovered, more preferably equal to or less than 0.5 (mg / L) times the alkali metal ion concentration of the target to be recovered, and even more preferably equal to or less than 0.1 (mg / L) times the alkali metal ion concentration of the target to be recovered.

[0026] The solution X containing alkali metal ions preferably has a pH of 0 or more and 4 or less. The pH of 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 having a pH of 4 or less, the permeability of alkali metal ions is increased while the permeability of polyvalent metal ions is maintained low in the nanofiltration step. Moreover, the pH of solution X containing alkali metal ions is preferably 0 or more, more preferably 0.5 or more, and even more preferably 1 or more. By having a pH of 0 or more, it is possible to suppress a decrease in the selective separation performance of the nanofiltration membrane for alkali metal ions relative to polyvalent metal ions during long-term operation.

[0027] The solution X containing alkali metal ions is preferably a solution in which a material containing lithium is dissolved in acid. Specific examples of the material containing lithium include lithium ion batteries and waste materials, waste liquids, ores, and slags generated in the manufacturing process thereof. Among these, lithium ion batteries are preferred because of the high demand for reuse and the high purity of the rare metals contained therein. A lithium ion battery is composed of components such as a positive electrode material, a negative electrode material, a separator, and an electrolyte. Of these components, any component containing lithium can be used as the material of solution X. The acid for dissolving the material containing lithium preferably contains at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid. The solution obtained by dissolving the components of a lithium ion battery with an acid contains, in addition to lithium ions, for example, nickel, cobalt, manganese, and the like.

[0028] An example of a method for dissolving an alkali metal-containing substance with an acid is a method of immersing the substance in an acidic aqueous solution. However, other methods may be used as long as the desired alkali metal ions can be eluted. The temperature of the acidic aqueous solution to be contacted is preferably 10°C or higher and 100°C or lower from the viewpoint of the elution efficiency of the alkali metal ions. Moreover, from the viewpoints of cost and safety, it is more preferably 20°C or higher and 80°C or lower. A solution in which an alkali metal-containing substance is dissolved in an acid does not always have a constant composition, but may vary depending on the composition of various ions in the substance and the dissolution conditions in the acid, etc. In other words, if there are N solutions X, the compositions of the N solutions X may differ from each other.

[0029] The amount of solution X containing the alkali metal ions to be recovered is not particularly limited, and when there are N solutions X, the N solutions X may have different amounts. From the viewpoint of the processing efficiency in each step, the amount of solution X is preferably 10 L or more and 10,000 L or less.

[0030] Solution X containing alkali metal ions may contain organic compounds. For example, when solution X is an acid solution for a lithium-ion battery, organic compounds such as polyvinylidene fluoride (PVDF), polyolefin, and carbonate ester derived from the binder, separator, electrolyte, etc. that connects the active material to the current collector may be included. These organic compounds may become foulants and cause a decrease in the recovery efficiency of alkali metal ions, so these foulants may be removed by an ultrafiltration process described later.

[0031] When the solution X containing N alkali metal ions contains lithium ions as alkali metal ions, the lithium ion concentration in the solution is preferably 0.5 mg / L or more and 10,000 mg / L or less. When the lithium ion concentration in the solution is 0.5 mg / L or more, the recovery efficiency of lithium ions by membrane separation is improved. Furthermore, when the lithium ion concentration in the solution is 10,000 mg / L or less, the osmotic pressure difference is suppressed from increasing, and the efficiency of membrane separation is improved. The lithium ion concentration in the solution is more preferably 10 mg / L or more and 8,000 mg / L or less, and even more preferably 100 mg / L or more and 6,000 mg / L or less.

[0032] The method for recovering an alkali metal salt according to the present embodiment can be suitably used when the alkali metal ion ratio of the solution X containing alkali metal ions is 2.4 or less. In general, when the alkali metal ion ratio is 2.4 or less, it becomes difficult to separate and recover the alkali metal ions and the polyvalent metal ions, but the method for recovering an alkali metal salt according to the present embodiment has high selective separation between the alkali metal ions and the polyvalent metal ions, and can effectively recover the alkali metal ions. In addition, the method for recovering an alkali metal salt according to the present embodiment can be suitably used when the alkali metal ion ratio of the solution X containing alkali metal ions is 1 or less, and further 0.5 or less.

[0033] (2-2) Nanofiltration membrane The nanofiltration membrane used in the alkali metal salt recovery method according to this embodiment is only required to have fractionation characteristics that are positioned between a reverse osmosis membrane and an ultrafiltration membrane, and it is preferable that the difference between the glucose removal rate when a 1000 mg / L aqueous glucose solution at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa and the isopropyl alcohol removal rate when a 1000 mg / L aqueous isopropyl alcohol solution at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa is 20% or more. Among these, it is even more preferable that the nanofiltration membrane has a difference of 40% or more between the glucose removal rate when a 1000 mg / L glucose aqueous solution at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa and the isopropyl alcohol removal rate when a 1000 mg / L isopropyl alcohol aqueous solution at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa, the glucose removal rate is 70% or more, and the magnesium sulfate removal rate when a 2000 mg / L magnesium sulfate aqueous solution at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa is 95% or more. Hereinafter, when the term "glucose removal rate" is used simply in this specification, it means the glucose removal rate when a 1000 mg / L aqueous glucose solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa; when the term "isopropyl alcohol removal rate" is used, it means the isopropyl alcohol removal rate when a 1000 mg / L aqueous isopropyl alcohol solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa; and when the term "magnesium sulfate removal rate" is used, it means the magnesium sulfate removal rate when a 2000 mg / L aqueous magnesium sulfate solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa. Membranes commonly known as reverse osmosis are capable of removing most organics and ions, whereas ultrafiltration membranes typically do not remove most ionic species and remove high molecular weight organics.

[0034] In order to separate alkali metal ions and polyvalent metal ions, it is preferable that the nanofiltration membrane has a charge on the membrane surface, and both separation by pores (size separation) and electrostatic separation by charge are possible. For example, in the case of a nanofiltration membrane in which the difference between the glucose removal rate and the isopropyl alcohol removal rate is 40% or more, and the glucose removal rate is 70% or more, and the difference between the magnesium sulfate removal rate and the magnesium chloride removal rate when a 2000 mg / L magnesium chloride aqueous solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa is 20% or less, both size separation and electrostatic separation are possible. Hereinafter, when simply described in this specification as "magnesium chloride removal rate", it means the magnesium chloride removal rate when a 2000 mg / L magnesium chloride aqueous solution at 25°C and pH 6.5 is permeated at an operating pressure of 0.5 MPa.

[0035] Nanofiltration membranes are made of polymers such as cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides, and vinyl polymers. Nanofiltration membranes may be made of only one material, or may be made of multiple materials. The membrane structure may be an asymmetric membrane having a dense layer on at least one side of the membrane and gradually increasing micropores from the dense layer toward the inside of the membrane or the other side, or a composite semipermeable membrane having a very thin separation functional layer made of another material on the dense layer of the asymmetric membrane.

[0036] The composite semipermeable membrane is preferably, for example, a membrane having a porous support membrane containing polysulfone and a separation functional layer containing polyamide provided on the porous support membrane. The composite semipermeable membrane may also have a substrate in addition to the porous support membrane and the separation functional layer, in which case the porous support membrane is provided on the substrate. The polyamide is a thin film formed on the porous support membrane by an interfacial polycondensation reaction between a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide.

[0037] In the method for recovering an alkali metal salt according to this embodiment, the nanofiltration membrane of at least one of the nanofiltration membrane unit A and the nanofiltration membrane unit B has a porous support membrane and a separation functional layer, and the average pore size R1 and the average pore size R2 of the separation functional layer derived by positron annihilation lifetime measurement method by irradiating a positron beam from the surface of the nanofiltration membrane on the separation functional layer side preferably satisfy 0.90≦R1 / R2≦1.10. Here, R1 and R2 are defined as follows. R1: Average hole diameter at positron beam intensity of 0.1 keV R2: Average hole diameter at positron beam intensity of 0.5 keV

[0038] The "positron annihilation lifetime measurement method" is a technique that measures the time (on the order of hundreds of picoseconds to tens of nanoseconds) between when a positron enters a sample and when it annihilates, and non-destructively evaluates information such as the size, number density, and size distribution of 0.1 to 10 nm vacancies based on the annihilation lifetime.

[0039] The measurement area in the depth direction from the sample surface can be adjusted by the energy amount of the positron beam incident on the sample. The higher the energy, the deeper the area from the sample surface will be included in the measurement area, but the depth depends on the density of the sample. For example, when measuring the separation functional layer of a composite semipermeable membrane, if a positron beam with an energy of about 0.1 keV is irradiated from the separation functional layer side of the composite semipermeable membrane, a region at a depth of 1.0 to 5.0 nm from the sample surface is usually measured, and if a positron beam with an energy of about 0.5 keV is irradiated, a region at a depth of 10 to 50 nm from the sample surface is usually measured. 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 the other layers in advance.

[0040] 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 0.1 keV positron beam intensity, the average pore size of the surface side (opposite to the porous support membrane side) of the separation functional layer is reflected, and under the condition of 0.5 keV, the average pore size of the porous support membrane side of the separation functional layer is reflected, and it can be said that the closer R1 / R2 is to 1, the more uniform the pore size is in the thickness direction. It is presumed that the uniform pore size in the thickness direction makes the direction in which ions diffuse in the separation functional layer uniform, and the permeation resistance of monovalent ions, which are sizes that can move freely in the separation functional layer, is suppressed. As a result, it is considered that excellent monovalent ion / multivalent ion selective separation performance is realized. Therefore, it is more preferable that R1 / R2 is 0.92 to 1.05, and even more preferable that it is 0.94 to 1.03.

[0041] 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. When R1 is within the above range, the effect of inhibiting the permeation of polyvalent metal ions while suppressing the permeation resistance of alkali metal ions is remarkable. In order for R1 and R2 to satisfy the above relationship, for example, the relative humidity during the interfacial polycondensation of a polyfunctional aliphatic amine compound and a polyfunctional aromatic acid halide described later is controlled to be high, for example, 80% or more, and the molecular weight of the polyfunctional aliphatic amine that forms the separation functional layer in the composite semipermeable membrane is set to 90 or more.

[0042] The separation functional layer in the composite semipermeable membrane preferably contains 50% by mass or more of a semi-aromatic crosslinked polyamide obtained by interfacial polycondensation of a divalent or higher polyfunctional aliphatic amine compound and a divalent or higher polyfunctional aromatic acid halide, more preferably 80% by mass or more, and even more preferably 90% by mass or more, and is particularly preferably composed of only a semi-aromatic crosslinked polyamide. By containing 50% by mass or more of a semi-aromatic crosslinked polyamide, excessive densification due to π-π interactions derived from aromatic rings in the semi-aromatic crosslinked polyamide is suppressed, and excellent alkali metal ion permeability is obtained. Furthermore, as a result of intensive research, the present inventors have found that when the relative humidity during the interfacial polycondensation is controlled to a high level, for example, 80% or more, the obtained composite semipermeable membrane exhibits particularly excellent membrane performance under acidic conditions. The relative humidity can be adjusted by using a precision air conditioning device, etc. By setting the atmospheric humidity during the interfacial polycondensation to 80% or more, evaporation of water from the formed polyamide can be suppressed, and insolubilization due to intermolecular hydrogen bonds of oligomers with many amino groups generated in excess can be suppressed. This allows oligomers to be efficiently removed after the separation functional layer is formed by the interfacial polycondensation reaction, thereby suppressing the expansion of pore size associated with swelling of the semi-aromatic crosslinked polyamide when the membrane is operated under acidic conditions, and it is believed that a composite semipermeable membrane can be obtained that exhibits excellent multivalent ion removal properties, i.e., a specified glucose removal rate, isopropyl alcohol removal rate, magnesium sulfate removal rate, etc.

[0043] The polyfunctional aliphatic amine is preferably an alicyclic diamine, more preferably a bipiperidine derivative or a piperazine derivative.

[0044] In addition, the molecular weight of the alicyclic diamine is preferably 90 or more. When the molecular weight of the alicyclic diamine is 90 or more, the diffusion coefficient of the amine is small, and polyamide is gradually formed during interfacial polycondensation, so that a separation functional layer with a uniform pore size in the film thickness direction is easily formed from the initial to middle stages of interfacial polycondensation. On the other hand, the molecular weight of the alicyclic diamine is preferably 160 or less. Usually, at the initial and final stages of interfacial polycondensation, oligomers are excessively generated on the surface of the support in contact with the organic layer, and the pores on the surface of the support are blocked, which causes the pore size distribution in the film thickness direction to become non-uniform. However, when the molecular weight of the alicyclic diamine is 160 or less, the molecular weight of the generated oligomer is small, and the interaction with the semi-aromatic crosslinked polyamide can be reduced, so that after the separation functional layer is formed by the interfacial polycondensation reaction, the oligomer is easily detached from the separation functional layer, and a separation functional layer with a uniform pore size in the film thickness direction is easily formed.

[0045] Examples of alicyclic diamines having a molecular weight of 90 or more and 160 or less include substituted piperazines in which the piperazine ring is substituted with an alkyl group having 1 to 3 carbon atoms (e.g., 2-methylpiperazine, 2-ethylpiperazine, 2-normal propylpiperazine, 2,2-dimethylpiperazine, 2,2-diethylpiperazine, 2,3-dimethylpiperazine, 2,3-diethylpiperazine, 2,5-dimethylpiperazine, 2,5-diethylpiperazine, 2,6-dimethylpiperazine, 2,6-diethylpiperazine, 2,3,5,6-tetramethylpiperazine, etc.), and homopiperazine.

[0046] The term "polyfunctional aromatic acid halide" refers to an aromatic acid halide having two or more halogenated carbonyl groups in one molecule, and is not particularly limited as long as it reacts with the polyfunctional aliphatic amine to give a semi-aromatic crosslinked polyamide. Examples of polyfunctional aromatic acid halides that can be used include halides of 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 the polyfunctional aromatic acid halides, acid chlorides are preferred, and in particular, from the standpoints of economy, ease of availability, ease of handling, ease of reactivity, and the like, trimesoyl chloride, which is an acid halide of 1,3,5-benzenetricarboxylic acid, isophthaloyl chloride, which is an acid halide of 1,3-benzenedicarboxylic acid, terephthaloyl chloride, which is an acid halide of 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid chloride, which is an acid halide of 1,3,5-benzenetrisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid chloride, which is an acid halide of 1,3,6-naphthalenetrisulfonic acid, are preferred. The polyfunctional aromatic acid halide may be used alone or in combination of two or more kinds, but by mixing the trifunctional trimesic acid chloride, 1,3,5-benzenetrisulfonic acid chloride, or 1,3,6-naphthalenetrisulfonic acid chloride with either one of the bifunctional isophthalic acid chloride or terephthalic acid chloride, the molecular gap of the polyamide crosslinked structure is enlarged, and the membrane with a uniform pore size distribution can be controlled over a wide range. The molar ratio of the trifunctional acid chloride to the bifunctional acid chloride is preferably 1:20 to 50:1, more preferably 1:1 to 20:1.

[0047] The above-mentioned composite semipermeable membrane can be obtained, for example, by forming a porous support membrane on a substrate, and then forming a separation functional layer containing a semi-aromatic crosslinked polyamide by interfacial polycondensation of a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide on the porous support membrane.

[0048] (2-3) Separation by nanofiltration membrane Since alkali metal ions easily permeate nanofiltration membranes and polyvalent metal ions have a hard time permeating nanofiltration membranes, it is possible to separate alkali metal ions from polyvalent metal ions. The nanofiltration membrane is preferably used in a state where it is incorporated into an element such as a spiral type.

[0049] (2-3-1) Step 1: First nanofiltration step The first nanofiltration step (step 1) is a step in which solution X is sent to the nanofiltration membrane unit A as the treated liquid A, separated into permeate liquid A and concentrated liquid B, and then the concentrated liquid B is mixed with the remainder of the treated liquid A and sent again to the nanofiltration membrane unit A to obtain further permeate liquid A. In step 1, the concentrated liquid B can be treated two or more times with the nanofiltration membrane unit A. The number of times this treatment is repeated can be set as desired. In addition, the repeated treatment in step 1 may be terminated when the recovery rate reaches a certain value, as described later. By permeating the treated liquid A through the nanofiltration membrane unit A while mixing the concentrated liquid B with the remainder of the treated liquid A, the alkali metal ions remaining in the concentrated liquid B can be permeated again through the nanofiltration membrane, making it possible to increase the recovery rate of the alkali metal ions. As the first nanofiltration process progresses, the volume of permeated liquid 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.

[0050] In the first nanofiltration step, it is preferable to obtain a permeate having an alkali metal ion ratio of 2 or more and 1000 or less, more preferably 10 or more and 700 or less, and even more preferably 20 or more and 500 or less. Having an alkali metal ion ratio of 2 or more makes it possible to shorten the processing time of the subsequent second nanofiltration step, and having an alkali metal ion ratio of 1000 or less makes it possible to shorten the processing time of this step.

[0051] In addition, in the first nanofiltration step, the recovery rate of the 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 the alkali metal ions in the first nanofiltration step is 80% or more, the cost of recovering the alkali metal ions can be reduced. The recovery rate of the alkali metal ions in the nanofiltration step is defined by the following formula (1) for each step.

[0052] Recovery rate of alkali metal ions in nanofiltration process (%) = {(volume of permeate in target nanofiltration process) × (concentration of alkali metal ions in permeate in target nanofiltration process)} / {(volume of liquid to be treated) × (initial concentration of alkali metal ions in liquid to be treated)} ··· Equation (1)

[0053] In the nanofiltration step, it is preferable to supply the solution to the nanofiltration membrane at an operating pressure in the range of 0.1 MPa to 8 MPa. If the operating pressure is 0.1 MPa or more, the membrane permeation rate is improved, and if it is 8 MPa or less, damage to the nanofiltration membrane can be suppressed. The operating pressure is more preferably 0.5 MPa to 6 MPa, and even more preferably 1 MPa to 4 MPa.

[0054] In the first nanofiltration step, the osmotic pressure of the treated liquid A increases as the step proceeds, and the operating pressure required to obtain the same flow rate of the permeated liquid A increases accordingly. Therefore, the method for recovering an alkali metal salt according to this embodiment preferably includes a step of diluting the treated liquid A in order to facilitate continuation of the filtration when the osmotic pressure increases. Diluting the treated liquid A is preferable because it reduces the osmotic pressure of the treated liquid A, allowing the first nanofiltration step to be continued and increasing the recovery rate of the alkali metal ions. Examples of methods for diluting the liquid to be treated A include a method of directly adding dilution water to the liquid to be treated A and a method of adding dilution water to the concentrated liquid B. Among these, the method of directly adding dilution water to the liquid to be treated A is preferred because it is simple.

[0055] The dilution water is not particularly limited and may be pure water, an acidic solution, or the like. However, it is preferable to use the permeate having a low metal ion concentration produced in the reverse osmosis filtration process described below, since this allows for highly efficient separation and recovery of alkali metal ions and reuse of the acidic aqueous solution.

[0056] The operation control method in the first nanofiltration step is not particularly limited, and may be, for example, constant flow rate filtration, low pressure filtration, etc., but constant flow rate filtration is preferred when diluting the treated liquid A. With constant flow rate filtration, the flow rate of the dilution water added can also be kept constant, making control easier. In the case of constant flow filtration, Li + From the viewpoint of recovery efficiency, the permeate flow rate is preferably 1% or more of the volume of solution X per minute, and from the viewpoint of ease of control, it is preferably 50% or less of the volume of solution X per minute.

[0057] The degree of progress of the first nanofiltration step, i.e., the recovery rate of alkali metal ions, can be known by appropriately sampling the treated liquid A and analyzing the liquid composition. However, since analyzing the liquid composition takes time, it is preferable to be able to constantly monitor the recovery rate of alkali metal ions. As a method for monitoring the recovery rate of alkali metal ions during the progress of the first nanofiltration step, when the nanofiltration step is performed at a constant permeation flow rate, there is a correlation between the operating pressure (operating pressure) and the recovery rate of alkali metal ions as shown in the following formula (2). Therefore, it is preferable to grasp the recovery rate A (%) of alkali metal ions while monitoring the change over time of the operating pressure using the following formula, and determine the end time of the first nanofiltration step. By determining the end time of the first nanofiltration step while monitoring the operating pressure using formula (2), the time required for analyzing the liquid composition can be reduced, and alkali metal ions can be efficiently recovered. The target recovery rate A (%) of alkali metal ions can be set appropriately. At least one of steps 1 and 2 can be terminated on the condition that the recovery rate reaches a target value.

[0058]

number

[0059] In the above formula (2), the recovery rate A (%) of alkali metal ions, the operating pressure P (Pa), the initial operating pressure P0 (Pa), and the initial liquid volume to be treated V0 (m 3 ), alkali metal ion removal rate of nanofiltration membrane R (%), liquid recovery rate of nanofiltration process S (%), supply flow rate Q F (m 3 / s), concentrate flow rate Q c (m 3 / s), and the filtration end time t=tb.

[0060] The liquid recovery rate S of the nanofiltration process is S = {(Q F -Q c ) / Q F}×100.

[0061] 1 to 3, 5, and 8 to 10 are schematic flow diagrams showing a method for recovering an alkali metal salt according to one embodiment of the present invention. The solution X is sent to an ultrafiltration membrane unit 1 described later, and the resulting permeate is sent to a first tank 5a. The permeate (liquid A to be treated) stored in the first tank 5a is sent to a nanofiltration membrane unit A (2a) and separated into permeate A and concentrated liquid B. The permeate A is sent to a second tank 5b at a constant flow rate, and the concentrated liquid B is sent to the first tank 5a and mixed with the remaining part of the liquid A to be treated in the first tank 5a. The first nanofiltration step can be performed while adding dilution water to the first tank 5a at the same flow rate as the flow rate of the permeate A. The dilution water may contain a permeate obtained by a reverse osmosis filtration step described later. In the example shown in FIG. 3 and FIG. 9, the permeate obtained by the reverse osmosis filtration step described later is further sent to a high-removal reverse osmosis membrane unit 4, and the resulting permeate is used as dilution water.

[0062] (2-3-2) Step 2: Second nanofiltration step In the second nanofiltration step (step 2), the permeate A obtained in the first nanofiltration step (step 1) or the concentrated solution of the permeate A obtained in step 1 is sent to the nanofiltration membrane unit A as the treated liquid B, separated into the permeate C and the concentrated solution D, and further mixed with the remaining part of the treated liquid B and sent to the nanofiltration membrane unit A again to obtain the permeate C. Alternatively, the permeate A or the concentrated solution of the permeate A obtained in step 1 is sent to the nanofiltration membrane unit B as the treated liquid B, separated into the permeate C and the concentrated solution D, and further mixed with the remaining part of the treated liquid B and sent to the nanofiltration membrane unit B again to obtain the permeate C. In step 2, the concentrated solution D can be treated two or more times with the nanofiltration membrane unit A or the nanofiltration membrane unit B. The number of times this treatment is repeated can be set arbitrarily. In addition, the repeated treatment in step 2 may be terminated when the recovery rate reaches a certain value, as described above. The concentrated solution of permeate A can be prepared by concentrating permeate A using a reverse osmosis membrane unit described below, but there is no particular limitation to this. In the second nanofiltration step, from the viewpoint of shortening the treatment time, it is preferable to use the permeated liquid A as the liquid to be treated B, and it is also preferable to use the nanofiltration membrane unit B.

[0063] In the second nanofiltration step, it is preferable to obtain a permeate C having an alkali metal ion ratio of 10 or more, more preferable to obtain a permeate C having an alkali metal ion ratio of 100 or more, and even more preferable to obtain a permeate C having 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. In the second nanofiltration step, the recovery rate of the 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 the alkali metal ions in the nanofiltration step of the permeate is 80% or more, the cost of recovering the alkali metal can be reduced.

[0064] The second nanofiltration step is carried out at least once. If the alkali metal ion ratio of solution X is low and the alkali metal ion ratio of permeate C does not reach the target value when the second nanofiltration step is carried out once, the second nanofiltration step may be carried out multiple times using permeate C as the treated liquid B until the alkali metal ion ratio of the resulting permeate reaches the target value. When the second nanofiltration step is carried out multiple times, the nanofiltration membrane unit used from the second time onwards may be either nanofiltration membrane unit A or B, or another nanofiltration membrane unit may be used.

[0065] Similarly to the first nanofiltration step, the second nanofiltration step preferably includes a step of diluting the to-be-treated liquid B. In addition, the progress of the step can be grasped by the above formula (2).

[0066] Examples of the operation control method in the second nanofiltration step include constant flow rate filtration, low pressure filtration, and the like. In the case of constant flow filtration, Li + From the viewpoint of recovery efficiency, it is preferable that the permeate flow rate is 1% or more of the initial volume of the liquid B to be treated per minute, and from the viewpoint of ease of control, it is preferable that the permeate flow rate is 50% or less of the initial volume of the liquid B to be treated per minute.

[0067] In the examples shown in Figures 1, 3, 8 and 9, the nanofiltration process of solution X is performed in the nanofiltration membrane unit A (2a), and the permeated liquid A that has permeated the nanofiltration membrane unit A and is stored in the second tank 5b is sent to the third tank 5c as the liquid to be treated, the liquid to be treated B is sent to the nanofiltration membrane unit B (2b), and the permeated liquid C is sent to the fourth tank 5d at a constant flow rate, thereby performing the second nanofiltration process. At this time, the concentrated liquid D that did not permeate the nanofiltration membrane unit B (2b) is mixed with the remaining part of the liquid to be treated B in the third tank 5c. In addition, the second nanofiltration process can be performed while adding dilution water to the liquid to be treated B in the third tank 5c at the same flow rate as the flow rate of the permeated liquid C.

[0068] In the example shown in FIG. 5 and FIG. 10, the nanofiltration process of the solution X is performed in the nanofiltration membrane unit A (2a), and the permeated liquid A that has permeated the nanofiltration membrane unit A and been stored in the second tank 5b is sent to the first tank 5a as the liquid to be treated B, and is sent again to the nanofiltration membrane unit A (2a), and the permeated liquid C is sent to the second tank 5b at a constant flow rate, thereby performing the second nanofiltration process. At this time, the concentrated liquid D that did not permeate the nanofiltration membrane unit A (2a) is mixed with the remaining part of the liquid to be treated B in the first tank 5a. In addition, the second nanofiltration process can be performed while adding dilution water to the liquid to be treated B in the first tank 5a at the same flow rate as the flow rate of the permeated liquid C.

[0069] In Figures 5 and 10, since the second nanofiltration step uses the same nanofiltration membrane unit A (2a) as the first nanofiltration step, it is preferable to wash the nanofiltration membrane unit A (2a), the first tank 5a, and the second tank 5b before carrying out the second nanofiltration step.

[0070] Furthermore, in the example shown in Fig. 8, the remainder of the liquid to be processed B(k) remaining in the third tank 5c after the kth solution X(k) has been processed is added to the first tank 5a containing the pth liquid to be processed A(p). Moreover, in the example shown in Fig. 10, the remainder of the liquid to be processed B(k) remaining in the first tank 5a after the kth solution X(k) has been processed is added to the first tank 5a containing the mth liquid to be processed A(m). Here, m is an integer between (k+1) and N, and p is an integer between (k+2) and N.

[0071] On the other hand, in the example shown in Fig. 2, the liquid that has permeated the nanofiltration membrane unit A (2a) and is stored in the second tank 5b is sent to the fifth tank 5e, and then the liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a to perform the reverse osmosis filtration process described later, and the resulting concentrated liquid is sent to the sixth tank 5f. The concentrated liquid stored in the sixth tank 5f is sent to the third tank 5c, and the liquid in the third tank 5c is sent to the nanofiltration membrane unit B (2b) as the liquid to be treated B, the permeated liquid C is sent to the fourth tank 5d at a constant flow rate, the concentrated liquid D is mixed with the remaining part of the liquid to be treated B in the third tank 5c, and dilution water is added to the liquid to be treated B in the third tank 5c at the same flow rate as the flow rate of the permeated liquid C, while performing the second nanofiltration process.

[0072] (3) Semi-batch processing process As described above, the composition of solution X may fluctuate, so in a process in which N (N: an integer of 2 or more) solutions X containing alkali metal ions are continuously pumped, it is extremely difficult, and in some cases impossible, to stabilize the composition of the solution obtained in the nanofiltration membrane treatment process and maintain the specified lithium purity and recovery rate. In addition, if a continuous process is realized to eliminate such instability in the nanofiltration membrane treatment process and enable high-purity and high-recovery lithium, the number of nanofiltration membrane stages will be excessive, resulting in problems of increased costs and further complication of the process. In addition, in a batch treatment process, the treatment of the kth (k: an integer of 1 to (N-1)) solution X(k) is completed before the treatment of the next (k+1)th solution X(k+1) is started, which causes a problem of long treatment time.

[0073] Therefore, the method for recovering an alkali metal salt according to this embodiment is a method for recovering an alkali metal salt, in which the process of obtaining a permeate C from the solution X through steps 1 and 2 is sequentially performed on N solutions X (N: an integer of 2 or more), and in step 2, a nanofiltration membrane unit B is used, and while the first nanofiltration step (step 1) is performed on the k-th solution X(k) (k: an integer of 1 to (N-1)) among the N solutions X and then the second nanofiltration step (step 2) is performed, it is preferable to perform the first nanofiltration step (step 1) in parallel on the (k+1)-th solution X(k+1) among the N solutions X. In this way, by configuring the batch processing steps in steps 1 and 2 as semi-batch processing steps that are performed semi-continuously on a plurality of solutions, the alkali metal ions can be efficiently recovered.

[0074] Furthermore, in order to efficiently recover alkali metal ions, it is preferable to add the remainder of the treated liquid B(k) after completion of the second nanofiltration step of solution X(k) to solution X (m: an integer equal to or greater than (k+1) and equal to or less than N) or the treated liquid A(m) and carry out the first nanofiltration step of solution X(m).

[0075] In this case, in the second nanofiltration step, the recovery rate of the alkali metal 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. In the second nanofiltration step, by setting the recovery rate of the alkali metal to 95% or less, the permeation of the permeate having a low alkali metal ratio in the later stage of the nanofiltration step can be suppressed. In order to suppress the permeation of the permeate having a low alkali metal ratio in the later stage of the nanofiltration step, it is preferable not to add dilution water in the second nanofiltration step.

[0076] The remainder of the treated liquid B(k) is a liquid obtained by filtering solution X(k) once through a nanofiltration membrane, and if the compositions of solutions X(m) and X(k) do not vary significantly, the alkali metal ratio is higher than that of solution X(m). Therefore, by adding the remainder of the treated liquid B(k) to solution X(m) or solution A(m), the entire amount of the alkali metal in the remainder of the treated liquid B(k) can be recovered, and the alkali metal ratio of solution X(m) is improved, so that the alkali metal ratio of the permeate of the first nanofiltration step of solution X(m) is also improved. In other words, the purity and recovery rate of the alkali metal are improved.

[0077] Since the remainder of the treated liquid B(k) is a liquid that has undergone the ultrafiltration process, it is more preferable from the viewpoint of reducing the load of the ultrafiltration process to add the remainder of the treated liquid B(k) to the treated liquid A(m) after the ultrafiltration process rather than to solution X(m) that has not been subjected to the ultrafiltration process.

[0078] Therefore, the method for recovering an alkali metal salt according to this embodiment preferably includes the following step 4. Step 4: A step of adding, after completion of step 2 for the kth solution X(k) (k: an integer of 1 to (N-1)) among the N solutions X, the remainder of the kth liquid to be treated B(k) having been mixed with the kth concentrated liquid D(k) to the mth solution X(m) (m: an integer of (k+1) to N) or the mth liquid to be treated A(m).

[0079] FIG. 10 shows an example of carrying out the step 4, which includes a step of adding the remainder of the kth treated liquid B(k) having been mixed with the kth concentrated liquid D(k) to the solution X (m: an integer equal to or greater than (k+1) and equal to or less than N) after the step 2 for the solution X(k) is completed.

[0080] Furthermore, by combining the semi-batch treatment step with the step 4 described above as the step 5 described below, the alkali metal salt can be recovered more efficiently. Step 5: After completion of step 2 for the solution X(k), the remainder of the kth treated liquid B(k) having been mixed with the kth concentrated liquid D(k) is added to the solution X (p: an integer equal to or greater than (k+2) and equal to or less than N) or the pth treated liquid A(p).

[0081] FIG. 8 shows an example of carrying out the step 5, which includes a step of adding the remainder of the kth treated liquid B(k) having been mixed with the kth concentrated liquid D(k) to the solution X (p: an integer equal to or greater than (k+2) and equal to or less than N) after the step 2 for the solution X(k) is completed.

[0082] In Figures 1 to 3, 8 and 9, first, solution X(1) is sent to the ultrafiltration membrane unit 1 described later to obtain a treated liquid A(1). When the treatment in the nanofiltration membrane unit A(2a) is completed and all of the permeated liquid A(1) is sent to the second tank 5b, the solution in the first tank 5a is collected in an arbitrary tank. Thereafter, solution X(2) is sent to the ultrafiltration membrane unit 1 described later to obtain a treated liquid A(2), which is stored in the first tank 5a, and the process proceeds sequentially. The process proceeds in the same manner for solutions X(3) to X(N).

[0083] In the method for recovering an alkali metal salt according to this embodiment, it is preferable to carry out steps 1 and 2 on all N solutions containing alkali metal ions, i.e., the first solution X(1) to the Nth solution X(N). In addition, steps other than steps 1 and 2 may be carried out on the N solutions, for example, a reverse osmosis filtration step (step 3) or an ultrafiltration step described below may be carried out.

[0084] (4) Reverse osmosis filtration process The method for recovering an alkali metal salt according to this embodiment preferably includes a reverse osmosis filtration step for concentrating at least one of the permeate A obtained in the first nanofiltration step and the permeate C obtained in the second nanofiltration step. In particular, when N solutions X are present, the method for recovering an alkali metal salt according to this embodiment preferably further includes the following step 3. Step 3: A reverse osmosis filtration step for concentrating at least one of the kth permeate A(k) and the kth permeate C(k) in at least one solution X(k).

[0085] In the reverse osmosis filtration process, at least one of permeate A and permeate C is sent to a reverse osmosis membrane unit to obtain a concentrated liquid having a higher alkali metal ion concentration than permeate A or permeate C sent thereto, and a permeate having a lower alkali metal ion concentration than permeate A and permeate C.

[0086] Examples of operation control methods in the reverse osmosis filtration step include constant flow rate filtration and low pressure filtration. In the case of constant flow filtration, Li + From the viewpoint of recovery efficiency, the permeate flow rate is preferably 1% or more of the volume of permeate A obtained in the first nanofiltration step or the volume of permeate C obtained in the second nanofiltration step per minute, and from the viewpoint of ease of control, it is preferably 50% or less of the volume of permeate A obtained in the first nanofiltration step or the volume of permeate C obtained in the second nanofiltration step per minute.

[0087] (4-1) Reverse osmosis membrane In the reverse osmosis filtration step, any reverse osmosis membrane that does not allow alkali metal ions to pass through may be used. By using such a reverse osmosis membrane, the loss of lithium ions during the process of concentrating alkali metal ions, particularly lithium ions, is extremely small, and highly efficient recovery of lithium ions is stably achieved. The higher the ion removal rate of the reverse osmosis membrane, the higher the efficiency of the process. However, since membranes with high removal rates generally have poor water permeability, it is preferable to select a membrane that takes this balance into consideration.

[0088] In particular, when solution X contains neutral molecules that are not charged under conditions of pH 3 or less, such as boron compounds typified by boric acid, the neutral molecules are not removed in the nanofiltration step, and are also contained in the permeate A and permeate C obtained in the nanofiltration step. Therefore, it is preferable to remove neutral molecules while concentrating alkali metal ions by the reverse osmosis filtration step. In other words, it is preferable to prevent alkali metal ions from passing through the reverse osmosis membrane and allow neutral molecules to pass through. In particular, a low-removal reverse osmosis membrane that has an isopropyl alcohol removal rate of 70% or more and less than 85% when an aqueous isopropyl alcohol solution at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa is preferable in that it does not allow alkali metal ions to pass through it and allows neutral molecules to pass through it. Here, the low-rejection reverse osmosis membrane concentrates at least one of the permeated liquid A(k) and the permeated liquid C(k), and at least one of the permeated liquid A(k) and the permeated liquid C(k) contains neutral molecules that are uncharged under conditions of pH 3 or less, and the low-rejection reverse osmosis membrane is a reverse osmosis filtration membrane used in the reverse osmosis filtration process.

[0089] As the material of the reverse osmosis membrane, for example, a polymer such as a cellulose acetate polymer, polyamide, sulfonated polysulfone, polyacrylonitrile, polyester, polyimide, vinyl polymer, etc. is used. The reverse osmosis membrane may be composed of only one material, or may be composed of multiple materials. The membrane structure may be an asymmetric membrane having a dense layer on at least one side of the membrane and having gradually larger micropores from the dense layer toward the inside of the membrane or the other side, or a composite semipermeable membrane having a very thin separation functional layer formed of another material on the dense layer of the asymmetric membrane.

[0090] A specific example of a composite semipermeable membrane used as a reverse osmosis membrane is a composite semipermeable membrane comprising a substrate, a porous support membrane, and a separation functional layer. Among them, a composite semipermeable membrane containing a polyamide in the separation functional layer is preferred. The separation functional layer containing a polyamide is obtained by polycondensing a polyfunctional amine and a polyfunctional acid halide on a porous support membrane.

[0091] (4-2) Concentration using reverse osmosis membrane In the method for recovering an alkali metal salt according to this embodiment, the reverse osmosis filtration step is preferably carried out at least once for at least one of the permeate A and the permeate C of the nanofiltration step. That is, it is preferably carried out at least once for 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) of the solution X(k).

[0092] It is more preferable to carry out the reverse osmosis filtration step only once for the permeate C(k) obtained in the second nanofiltration step for solution X(k), in order to shorten the time for the entire process. When the second nanofiltration step is carried out multiple times, it is preferable to carry out the reverse osmosis filtration step only once for the permeate C(k) obtained in the final second nanofiltration step.

[0093] As described above, when solution X contains neutral molecules that are not charged under conditions of pH 3 or less, such as boron compounds typified by boric acid, it is preferable to concentrate alkali metal ions while removing neutral molecules using a low-rejection reverse osmosis membrane. In this case, it is preferable that the reverse osmosis filtration step includes a circulation step in which the concentrated liquid obtained in the reverse osmosis filtration step is mixed with the solution to be supplied to the reverse osmosis filtration step, in that the alkali metal ions can be concentrated while efficiently removing neutral molecules. When the circulation step is included, the period during which the circulation step is performed is preferably within a range until the pressure reaches 90% of the withstand pressure value of the reverse osmosis membrane unit, from the viewpoint of the operating pressure.

[0094] In the above case, the permeate of the low-removal reverse osmosis membrane contains neutral molecules, and therefore is not suitable for use as dilution water for the treated liquid A or the treated liquid B in the nanofiltration process. The neutral molecule concentration in the dilution water is preferably 1% or less, more preferably 0.1% or less, and even more preferably 0.01% or less, relative to the neutral molecule concentration (mg / L) in the treated liquid. By being 1% or less relative to the neutral molecule concentration (mg / L) in the treated liquid, accumulation of neutral molecules in the system can be significantly prevented when the permeate of the low-removal reverse osmosis membrane is used as dilution water to perform a semi-batch treatment process for the treated liquid. When the permeate of the low-removal reverse osmosis membrane is used as dilution water, it is preferable to send the permeate of the low-removal reverse osmosis membrane to a high-removal reverse osmosis membrane unit equipped with a high-removal reverse osmosis membrane defined below to remove neutral molecules. By passing through the high-removal reverse osmosis membrane, the neutral molecule concentration in the permeate of the low-removal reverse osmosis membrane can be set to a range suitable for the dilution water.

[0095] The term "high-rejection reverse osmosis membrane" refers to a reverse osmosis membrane that has a rejection rate of 85 to 95% of isopropyl alcohol when an aqueous solution of isopropyl alcohol at pH 6.5 at 25°C is passed through it under an operating pressure of 0.5 MPa.

[0096] The examples shown in Figs. 1, 5, 8 and 10 are flow diagrams of an alkali metal salt recovery process including a reverse osmosis filtration step for concentrating permeated liquid C and a dilution step for using the permeated liquid obtained in the reverse osmosis filtration step as dilution water for the liquid A to be treated. Specifically, the permeated liquid C that has permeated through the nanofiltration membrane unit B (2b) or the nanofiltration membrane unit A (2a) and is stored in the fourth tank 5d or the second tank 5b is sent to the fifth tank 5e or the third tank 5c, and the liquid in the fifth tank 5e or the third tank 5c is sent to the first reverse osmosis membrane unit 3a to perform the reverse osmosis filtration step. The permeated liquid obtained is sent as dilution water for the liquid A to be treated in the first tank 5a, and the concentrated liquid is sent to the sixth tank 5f. The liquid in the sixth tank 5f can be collected in any tank.

[0097] The example shown in FIG. 9 is a flow diagram of an alkali metal salt recovery process including a reverse osmosis filtration step for concentrating the permeated liquid C, and a dilution step for treating the permeated liquid obtained in the reverse osmosis filtration step with a high-removal reverse osmosis membrane and then using it as dilution water for the liquid A to be treated. Specifically, the permeated liquid C that has permeated through the nanofiltration membrane unit B (2b) and stored in the fourth tank 5d is sent to the fifth tank 5e, and the liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a, and the reverse osmosis filtration step is performed. The concentrated liquid obtained in the reverse osmosis filtration step is sent to the sixth tank 5f. The liquid in the sixth tank 5f can be collected in any tank. Furthermore, the permeated liquid obtained in the reverse osmosis filtration step is sent to the high-removal reverse osmosis membrane unit 4, and the concentrated liquid obtained in the high-removal reverse osmosis membrane unit 4 is discharged, and the permeated liquid is supplied to the first tank 5a, and can be used as dilution water for the liquid A to be treated.

[0098] The example shown in FIG. 3 is a flow diagram of an alkali metal salt recovery process in which the reverse osmosis filtration step for concentrating the permeated liquid C includes a step of circulating the concentrated liquid, and includes a dilution step in which the permeated liquid obtained in the reverse osmosis filtration step is treated with a high-removal reverse osmosis membrane and then used as dilution water for the liquid A to be treated. Specifically, the permeated liquid C that has permeated through the nanofiltration membrane unit B (2b) and stored in the fourth tank 5d is sent to the fifth tank 5e, the liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a, and the concentrated liquid obtained in the first reverse osmosis membrane unit 3a is mixed with the fifth tank 5e while performing the reverse osmosis filtration step. Furthermore, the permeated liquid obtained in the reverse osmosis filtration step is sent to the high-removal reverse osmosis membrane unit 4, and the concentrated liquid obtained in the high-removal reverse osmosis membrane unit 4 is discharged, and the permeated liquid is supplied to the first tank 5a, and can be used as dilution water for the liquid A to be treated. After the reverse osmosis filtration step is completed, the liquid in the fifth tank 5e, which is the concentrated liquid of the permeated liquid C, may be collected in any tank.

[0099] The example shown in FIG. 2 is a flow diagram of an alkali metal salt recovery process including a reverse osmosis filtration step for concentrating permeated liquid A, a reverse osmosis filtration step for concentrating permeated liquid C, and a dilution step for using the permeated liquid obtained in each reverse osmosis filtration step as dilution water for the liquid A to be treated. Specifically, first, the liquid in the second tank 5b that has permeated through the nanofiltration membrane unit A (2a) is sent to the fifth tank 5e. The liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a, the resulting concentrated liquid is sent to the sixth tank 5f, and the permeated liquid is added to the liquid A to be treated as dilution water. Next, the liquid in the sixth tank 5f is sent to the third tank 5c. The liquid in the third tank 5c is sent to the nanofiltration membrane unit B (2b) as the liquid B to be treated, and the permeated liquid C stored in the fourth tank 5d is sent to the seventh tank 5g. The liquid in the seventh tank 5g is sent to the second reverse osmosis membrane unit 3b, the resulting concentrated liquid is sent to the eighth tank 5h, and the permeated liquid is added as dilution water to the liquid to be treated A. The liquid in the eighth tank 5h can be collected in any tank.

[0100] (5) Ultrafiltration process Solution X(k) may be subjected to ultrafiltration prior to the first nanofiltration step, which can remove high molecular weight organic matter, and by removing the high molecular weight organic matter, it is possible to suppress fouling of the nanofiltration membrane. When multiple solutions are mixed to obtain solution X(k), each of these multiple solutions may be subjected to ultrafiltration. The permeate of the ultrafiltration membrane unit is used as the treated liquid A(k) in the first nanofiltration step. In Figs. 1 to 3, the solution X(k) is sent to the ultrafiltration membrane unit 1, and the permeated liquid is sent to the first tank 5a.

[0101] (6) Recovery process In this step, an alkali metal salt is recovered from the permeate C containing alkali metal ions obtained in the second nanofiltration step or a concentrate of the permeate C obtained through the reverse osmosis filtration step. The recovery step preferably includes concentrating the aqueous alkali metal salt solution.

[0102] The alkali metal salt can be recovered by a known method. For example, when the alkali metal salt is a potassium salt, the recovery is carried out by utilizing the temperature dependency of solubility or by adding a poor solvent such as ethanol. Lithium salts have a lower solubility than other alkali metal salts. For example, sodium carbonate and potassium carbonate have a high solubility in water (20 g or more per 100 mL of water), but lithium carbonate dissolves only 1.33 g per 100 mL of water at 25°C. Therefore, lithium can be recovered as lithium carbonate by adding carbonate to permeate C or a concentrated solution of permeate C, which contains alkali metal ions. The solubility of lithium carbonate decreases further at high temperatures, so the aqueous solution may be heated.

[0103] (7) Comparative form 4, 6 and 7 are schematic flow diagrams showing the alkali metal salt recovery process in comparative form. The difference between the process configuration in FIG. 4 and the process configuration in FIG. 1 is that the concentrated liquid B from the nanofiltration membrane unit A (2a) and the concentrated liquid D from the nanofiltration membrane unit B (2b) are discharged without being mixed with the liquid in the first tank 5a and the third tank 5c, respectively. The rest is the same as in FIG. 1. + The problem is that the recovery rate is low.

[0104] The process configuration of FIG. 6 is the same as the example shown in FIG. 1 except that there is no third tank 5c, nanofiltration membrane unit B (2b), or fourth tank 5d, i.e., there is no second nanofiltration step, and the liquid in the second tank 5b is sent to the fifth tank 5e. In the above process, the Li + The problem is that the purity is low.

[0105] The process configuration of FIG. 7 is a continuous treatment process, in which the solution X (1) is sent to the ultrafiltration membrane unit 1, and the permeate of the ultrafiltration membrane unit 1 is sent to the nanofiltration membrane unit A (2a) while being mixed with dilution water. The permeate A of the nanofiltration membrane unit A (2a) is sent to the nanofiltration membrane unit B (2b), and the concentrated liquid B of the nanofiltration membrane unit A (2a) is recovered in the first tank 5a. The permeate C of the nanofiltration membrane unit B (2b) is sent to the first reverse osmosis membrane unit 3a, and the concentrated liquid D of the nanofiltration membrane unit B (2b) is mixed with the concentrated liquid B of the nanofiltration membrane unit A (2a) and sent to the first tank 5a. The concentrated liquid of the first reverse osmosis membrane unit 3a is sent to the fifth tank 5e. The permeate of the first reverse osmosis membrane unit 3a is used as part of the dilution water. After the entire amount of solution X(1) is treated, solutions X(2) and X(3) are treated in the same manner. + The problem is that the recovery rate is low.

[0106] (8) Alkali metal salt recovery device The alkali metal salt recovery apparatus of the present invention comprises: a first separation means for separating a solution containing alkali metal ions as a treatment liquid A into a permeate liquid A and a concentrate liquid B by a first nanofiltration membrane unit; a first circulation means for mixing the concentrated liquid B with the remainder of the liquid A to be treated; A second separation means for separating the permeated liquid A or a concentrate of the permeated liquid A as a liquid to be treated B into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit; A second circulation means for mixing the concentrated liquid D with the remainder of the liquid to be treated B; A dilution means for adding dilution water to at least one of the liquid A and the liquid B; a flow rate control means capable of controlling the flow rates of the permeated liquid A and the concentrated liquid B in the first separation means, and the permeated liquid C and the concentrated liquid D in the second separation means; The dilution means includes a flow rate control means for synchronizing the flow rate of dilution water added with the flow rate of the permeated liquid when the liquid to be treated to which the dilution water is added is sent to the nanofiltration membrane unit.

[0107] The alkali metal salt recovery apparatus of the present invention comprises: The system includes a first separation device, a first circulation device, a second separation device, a second circulation device, a dilution device, a flow rate control device a, and a flow rate control device b, The first separation equipment includes a first nanofiltration membrane unit, and in the first separation equipment, a liquid to be treated A, which is a solution containing alkali metal ions, is separated into a permeate A and a concentrated liquid B by the first nanofiltration membrane unit, In the first circulation facility, the concentrated liquid B is mixed with the remainder of the treated liquid A, In the second separation equipment, the permeated liquid A or a concentrate of the permeated liquid A is separated into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit as a liquid to be treated B; In the second circulation facility, the concentrated liquid D is mixed with the remainder of the treated liquid B, In the dilution equipment, dilution water is added to at least one of the liquid to be treated A and the liquid to be treated B, In the flow rate control equipment a, the flow rates of the permeate A and the concentrate B in the first separation device, and the flow rates of the permeate C and the concentrate D in the second separation device are controlled; In the flow rate control equipment b, the flow rate of dilution water added in the dilution means is synchronized with the flow rate of the permeate when the treated liquid to which the dilution water is added is sent to the nanofiltration membrane unit. It may be a recovery device.

[0108] In the first and second separation facilities, the nanofiltration membrane unit preferably has a pressure vessel filled with spiral elements of nanofiltration membrane, and is structured so that a solution containing alkali metal ions can be supplied to the vessel by a high-pressure pump. The nanofiltration membrane unit may have a plurality of vessels connected in parallel or series, and each vessel may be filled with a plurality of nanofiltration membrane elements. The spiral elements of the nanofiltration membrane may have any diameter and length. The size of the spiral elements of the nanofiltration membrane varies depending on the membrane area, and for the same membrane type, the larger the membrane area, the more liquid volume can be treated per unit time. The size and number of the spiral elements of the nanofiltration membrane can be determined arbitrarily depending on the scale of the liquid A to be treated.

[0109] The flow rate control equipment a preferably has an instrument (flow meter) capable of measuring the flow rates of the permeate and concentrate of the nanofiltration membrane unit in order to keep the flow rates of the permeate and concentrate of the nanofiltration membrane unit constant. For the control of the flow rate of the permeate, the high-pressure pump preferably has a mechanism that receives data from the permeate flow meter at any time and controls the output of the high-pressure pump so as to maintain a constant permeate flow rate. For the control of the concentrate flow rate, it is preferable to have an electromagnetic valve near the concentrate flow meter, and the electromagnetic valve preferably has a mechanism that receives data from the concentrate flow meter at any time and controls the concentrate flow rate to be constant. It is preferable that the first circulation equipment and the second circulation equipment have a tank (raw water tank) for filling the liquid to be treated, and have piping for circulating the concentrated liquid discharged from the nanofiltration membrane unit to the raw water tank. The dilution equipment preferably has a tank (dilution water tank) for filling with dilution water, and a pump (dilution water supply pump) for supplying dilution water from the dilution water tank to the raw water tank. As the flow rate control equipment b, the dilution water delivery pump preferably has a mechanism for receiving data from the permeate flow meter at all times and delivering dilution water at the same flow rate as the permeate flow rate. The above-mentioned equipment is preferably made of materials that are resistant to the properties of the liquid to be treated and the operating pressure. In order to achieve the recovery of alkali metal salts, the recovery device of the present invention can, in addition to the above, select and combine pumps, piping, valves, tanks, vessels, temperature control devices, and instruments (pH meter, conductivity meter, flow meter, pressure gauge, etc.) in any desired manner. EXAMPLES

[0110] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. Measurements in the examples and comparative examples were carried out as follows.

[0111] <Performance of nanofiltration and reverse osmosis membranes> (Glucose removal rate and isopropyl alcohol removal rate of nanofiltration membrane) The glucose concentration of the permeate and the feed water when a 1000 mg / L glucose aqueous solution at 25 ° C. and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa as the feed water, and the isopropyl alcohol concentration of the permeate and the feed water when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa were calculated using the following formulas. The isopropyl alcohol removal rate and the glucose removal rate were calculated using the following formulas from the isopropyl alcohol concentration of the permeate and the feed water when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa. Isopropyl alcohol removal rate (%) = 100 x (1 - (isopropyl alcohol concentration in permeate / isopropyl alcohol concentration in feed water)) Glucose removal rate (%) = 100 × (1-(glucose concentration in permeate / glucose concentration in feed water)) The isopropyl alcohol concentration was determined using a gas chromatograph (GC-18A, manufactured by Shimadzu Corporation), and the glucose concentration was determined using a refractometer (RID-6A, manufactured by Shimadzu Corporation).

[0112] (Magnesium sulfate removal rate and magnesium chloride removal rate of nanofiltration membrane) The MgSO4 removal rate and MgCl2 removal rate were calculated using the following formulas from the magnesium sulfate concentrations in the permeate and feed water when a 2000 mg / L magnesium sulfate (hereinafter also referred to as "MgSO4") aqueous solution at 25°C and pH 6.5 was permeated through a nanofiltration membrane at an operating pressure of 0.5 MPa, and from the magnesium chloride concentrations in the permeate and feed water when a 2000 mg / L magnesium chloride (hereinafter also referred to as "MgCl2") aqueous solution at 25°C and pH 6.5 was permeated through a nanofiltration membrane at an operating pressure of 0.5 MPa. The magnesium sulfate and magnesium chloride concentrations were determined by measuring the electrical conductivity of the feed water and permeate using an electrical conductivity meter manufactured by Toa Denpa Kogyo Co., Ltd. to obtain the practical salinity, i.e., MgSO4 concentration and MgCl2 concentration, respectively. MgSO4 removal rate (%) = 100 x {1 - (MgSO4 concentration in permeate / MgSO4 concentration in feed water)} MgCl2 removal rate (%) = 100 × {1-(MgCl2 concentration in permeate / MgCl2 concentration in feed water)}

[0113] (Positron annihilation lifetime measurement using positron beam method) The average pore diameters were derived for nanofiltration membrane A and nanofiltration membrane B described below. The positron annihilation lifetime measurement of the separation functional layer was carried out using the positron beam method as follows. The composite semipermeable membrane was freeze-dried under reduced pressure at -30°C and cut into 1.5 cm x 1.5 cm squares to prepare test samples. The separation functional layer side of the test sample was measured with a barium difluoride scintillation counter using a photomultiplier tube at a total count of 5 million at room temperature under vacuum using a thin film positron annihilation lifetime measurement device equipped with a positron beam generator (this device is described in detail in, for example, Radiation Physics and Chemistry, 58,603, Pergamon (2000)). From the average lifetime τ of the third component obtained by the analysis, the average pore size when the beam intensity is 0.1 keV was derived as R1 and the average pore size when the beam intensity is 0.5 keV was derived as R2 using the Tao-Eldrup formula, and R1 / R2 was calculated. The obtained value is called the "pore size distribution".

[0114] (Solution X) Li in rare metal-containing acidic aqueous solution A described in Table 1 of Patent Document 2 + , Ni 2+ , Co 2+ , Mn 2+ Lithium sulfate, nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in water to achieve the desired concentration, and sulfuric acid was used to adjust the pH to 1. The resulting aqueous solution was then diluted 1.2-fold with an aqueous sulfuric acid solution of pH 1 to prepare solution Xa (number 1). For solution Xa (number 1), Li + A solution was prepared in the same manner except that the concentration was halved to prepare solution Xa (number 2). For solution Xa (number 1), Li + A solution was prepared in the same manner except that the concentration was 1 / 4, and this was designated solution Xa (number 3). Solutions Xb (numbers 1 to 3) were prepared in the same manner as solution Xa (numbers 1 to 3), except that the pH was adjusted to 3.7. Solutions Xa (numbers 1 to 3) were each prepared in the same manner as solution Xa (numbers 1 to 3), except that boric acid was added to each of them to set the boron concentration to 50 mg / L, to give solutions Xc (numbers 1 to 3). The concentrations of various ions in the solution obtained above were quantified using a Hitachi P-4010 ICP (inductively coupled plasma atomic emission spectrometry) apparatus. The results are shown in Table 1. The liquid volumes of the numbers 1 to 3 constituting the solutions Xa, Xb and Xc were each 1000 L.

[0115] [Table 1]

[0116] <Preparation of nanofiltration and reverse osmosis membranes> (Nanofiltration membrane A) Nonwoven fabric made of polyester fibers (breathability 1cc / cm 2 A 18.0 mass% dimethylformamide (DMF) solution of polysulfone was cast onto a substrate (100 / s) at room temperature (25°C) to a thickness of 180 μm, and then immediately immersed in pure water and allowed to stand for 5 minutes to produce a porous support membrane (160 μm thick) made of fiber-reinforced polysulfone.

[0117] Next, the membrane surface temperature of the porous support membrane was adjusted to 25 ° C. while blowing air adjusted to 25 ° C. to remove excess moisture. A 30 ° C. aqueous solution containing 2.0 mass % piperazine, 250 ppm sodium dodecyl diphenyl ether disulfonate, and 1.0 mass % trisodium phosphate was applied to the surface of the porous support membrane and left to stand for 15 seconds, and then nitrogen was blown from an air nozzle to remove excess aqueous solution to form a coating layer of an amine aqueous solution on the porous support membrane. Furthermore, a 38 ° C. n-decane solution containing 0.2 mass % trimesic acid chloride (hereinafter, "TMC") was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand for 1 minute at a relative humidity of 70% and a temperature of 25 ° C. to perform interfacial polycondensation, and two fluids (pure water and air) were blown onto the membrane surface to remove the solution on the surface. After that, it was washed with pure water at 80 ° C. to obtain a nanofiltration membrane A.

[0118] (Nanofiltration membrane B) Nanofiltration membrane B was prepared in the same manner as nanofiltration membrane A, except that the piperazine was changed to 2,5-dimethylpiperazine, and a 38°C n-decane solution containing 0.2 mass% TMC was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand at a relative humidity of 80% and 25°C for 1 minute.

[0119] (Nanofiltration Membrane E) SelRO (registered trademark) MPS-34 from KOCH was used as nanofiltration membrane E.

[0120] The membrane performances of nanofiltration membrane A, nanofiltration membrane B and nanofiltration membrane E are shown in Table 2. Nanofiltration membrane A, nanofiltration membrane B and nanofiltration membrane E were each spirally wound by an arbitrary method and used as a membrane element having a diameter of 20.32 cm and a length of 102 cm (hereinafter referred to as "8 inch element").

[0121] [Table 2]

[0122] (Reverse osmosis membrane C) A porous support membrane was prepared in the same manner as nanofiltration membrane A, and the membrane surface temperature of the porous support membrane was adjusted to 25 ° C. while blowing air adjusted to 25 ° C. to remove excess water. After immersing for 15 seconds in an aqueous solution containing 5.0 mass% of m-phenylenediamine (hereinafter, "m-PDA"), nitrogen was blown from an air nozzle to remove excess aqueous solution, and then a 30 ° C. n-decane solution containing 0.18 mass% TMC was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand at 30 ° C. for 1 minute, and two fluids (pure water and air) were blown onto the membrane surface to remove the solution on the surface. After that, it was washed with pure water at 80 ° C. to obtain reverse osmosis membrane C.

[0123] (Reverse osmosis membrane D) This was produced in the same manner as in the case of reverse osmosis membrane C, except that the m-PDA content was changed to 1.8% by mass and the TMC content was changed to 0.07% by mass.

[0124] The membrane performance of the reverse osmosis membrane C and the reverse osmosis membrane D is shown in Table 3. Each of the reverse osmosis membranes C and D was spirally wound by any method and used as an 8-inch element.

[0125] [Table 3]

[0126] From the results in Table 3, it was found that reverse osmosis membrane C was a high-rejection reverse osmosis membrane, and reverse osmosis membrane D was a low-rejection reverse osmosis membrane.

[0127] <Evaluation of alkali metal salt recovery> (alkali metal ion ratio) The alkali metal ion ratio was calculated from the concentrations of various ions in the solution according to the following formula. Alkali metal ion ratio = lithium ion concentration / (cobalt ion concentration + nickel ion concentration + manganese ion concentration)

[0128] (Li + Recovery rate) Li + The recovery rate was calculated using the following formula. Li + Recovery rate (%) = {(amount of liquid finally concentrated in the reverse osmosis membrane unit (L)) × (Li in the liquid finally concentrated in the reverse osmosis membrane unit)} + Concentration (mg / L)} / {(Initial volume of solution X (L)) × Li in the initial solution of solution X + Concentration (mg / L)}

[0129] In addition, in the case of recovering the remainder of the treated liquid B(k) in the second nanofiltration step, the Li contained in the remainder of the treated liquid B(k) is + is recovered by adding it to the treated liquid A(m) (m: an integer between (k+1) and N) or to the treated liquid A(p) (p: an integer between (k+2) and N). Therefore, it is considered to have been recovered, and the Li in solution X(k) is +The recovery rate (k) was calculated using the following formula. In Example 9, the following formula was used. In the case of N=3, even for k=2 and 3, the recovery rate (k) of Li contained in the remaining part of the treated liquid B(k) in the second nanofiltration step was calculated. + was considered to be recoverable. Li + Recovery rate (k) (%) = {(amount of liquid (L) finally concentrated in the reverse osmosis membrane unit in the treatment of solution X(k)) × (amount of Li in the liquid finally concentrated in the reverse osmosis membrane unit in the treatment of solution X(k))} + Concentration (mg / L) + (Liquid volume (L) of remaining liquid B(k) in the second nanofiltration process) × (Li in the remaining liquid B(k) in the second nanofiltration process) + Concentration (mg / L)} / {(Initial volume of solution X(k) (L)) × (Li in the initial solution of solution X(k) + Concentration (mg / L)) + (Liquid volume (L) of the remaining liquid B (k-1: k≧2) in the second nanofiltration process added to solution X(k)) × (Li in the remaining liquid B (k-1: k≧2) in the second nanofiltration process added to solution X(k) + Concentration (mg / L)}

[0130] (Li + purity) Li + The purity was determined as the alkali metal ion ratio in the liquid finally concentrated by the reverse osmosis membrane unit.

[0131] (Boron concentration ratio) The boron concentration ratio was defined as the ratio of the boron concentration to the lithium ion concentration in the liquid finally concentrated in the reverse osmosis membrane unit.

[0132] (Total Processing Time) The total treatment time was defined as the sum of the times required to complete treatments of the various solutions X, numbers 1 to 3.

[0133] Example 1 In the process configuration shown in FIG. 1, nanofiltration membrane A was used as the nanofiltration membrane of nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a. A semi-batch treatment process was carried out in the order of solution Xa numbers 1 to 3, and alkali metal salts were recovered. The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was carried out at a constant flow rate of 60 L / min permeate flow rate. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using formula (2). Filtration was continued until the alkali metal ion recovery rate reached 93% in the first nanofiltration process and 96% in the second nanofiltration process. The reverse osmosis filtration process was carried out at a constant flow rate of 60 L / min permeate flow rate and continued until the operating pressure reached 7 MPa. Four 8-inch elements were connected in series for each nanofiltration membrane unit and reverse osmosis membrane unit. The results of this process are shown in Table 4.

[0134] Example 2 An alkali metal salt recovery process was carried out in the same manner as in Example 1, except that solutions Xb Nos. 1 to 3 were used instead of solution Xa. The results of carrying out this process are shown in Table 4. When the pH was as high as 3.7, the permeability of alkali metal ions decreased and the total processing time increased compared to Example 1 where the pH was 1.0, but lithium ions could be recovered with high purity and high recovery rate.

[0135] Example 3 In the process configuration shown in FIG. 2, a nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), and a reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a and the second reverse osmosis membrane unit 3b. A semi-batch treatment process was carried out in the order of numbers 1 to 3 of the solution Xa, and an alkali metal salt recovery process was carried out. The pressure resistance value of the second reverse osmosis membrane unit 3b was 8 MPa. The nanofiltration process was carried out at a constant flow rate with a permeate flow rate of 60 L / min. The alkali metal ion recovery rates of the first nanofiltration process and the second nanofiltration process were confirmed by monitoring the operating pressure using the formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 96%. The reverse osmosis filtration process was carried out at a constant flow rate with a permeate flow rate of 15 L / min, and continued until the operating pressure reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit was equipped with four 8-inch elements connected in series.

[0136] The results of this process are shown in Table 4. When concentration using a reverse osmosis membrane unit was performed after each nanofiltration step, the total processing time increased slightly compared to when RO concentration was performed only once after the second nanofiltration step, but lithium ions were recovered with high purity and high recovery rate.

[0137] Example 4 The alkali metal salt recovery process was carried out in the same manner as in Example 1, except that nanofiltration membrane B was used. The results of carrying out this process are shown in Table 4. It can be seen that when nanofiltration membrane B satisfying the specified performance was used, the purity and recovery rate improved and processing could be carried out in a short time.

[0138] Example 5 The alkali metal salt recovery process was carried out in the same manner as in Example 4, except that the alkali metal ion recovery rate in the nanofiltration step was confirmed by appropriately analyzing the permeate without monitoring the operating pressure using equation (2). The results of carrying out this process are shown in Table 4. Since appropriate analysis requires time, the processing time was increased compared to when the operating pressure was monitored using equation (2).

[0139] Example 6 The alkali metal salt recovery process was carried out in the same manner as in Example 4, except that solution Xc was used. The results of this process are shown in Table 4.

[0140] Example 7 In the process configuration shown in FIG. 9, the nanofiltration membrane B was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), the reverse osmosis membrane D was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the reverse osmosis membrane D was used as the reverse osmosis membrane of the high-removal reverse osmosis membrane unit 4. A semi-batch treatment process was carried out in the order of solutions Xc1 to 3, and an alkali metal salt recovery process was carried out. The first nanofiltration process and the second nanofiltration process were carried out at a constant flow rate of 60 L / min permeate flow rate. The alkali metal ion recovery rates of the first nanofiltration process and the second nanofiltration process were confirmed by monitoring the operating pressure using the formula (2), and filtration was continued until the alkali metal ion recovery rate was 93% in the first nanofiltration process and 96% in the second nanofiltration process. The reverse osmosis filtration process was carried out at a constant flow rate of 60 L / min permeate flow rate, and continued until the operating pressure reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit was equipped with four 8-inch elements connected in series. The results of carrying out this process are shown in Table 4. It can be seen that boron was removed by concentrating using reverse osmosis membrane D, which has a low removal rate.

[0141] Example 8 In the process configuration shown in FIG. 3, the nanofiltration membrane B was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), the reverse osmosis membrane D was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the reverse osmosis membrane D was used as the reverse osmosis membrane of the high-removal reverse osmosis membrane unit 4. A semi-batch treatment process was carried out in the order of solutions Xc1 to 3, and an alkali metal salt recovery process was carried out. The first nanofiltration process and the second nanofiltration process were carried out at a constant flow rate of 60 L / min permeate flow rate. The alkali metal ion recovery rates of the first nanofiltration process and the second nanofiltration process were confirmed by monitoring the operating pressure using the formula (2). Filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 96%, and the reverse osmosis filtration process was carried out at a constant flow rate of 60 L / min permeate flow rate and the operating pressure was continued until it reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit was equipped with four 8-inch elements connected in series. The results of carrying out this process are shown in Table 4. It can be seen that by concentrating using reverse osmosis membrane D, which is a low-removal membrane, and providing a circulation process in the reverse osmosis filtration process, more boron was removed than in Example 7.

[0142] Example 9 In the process configuration shown in FIG. 8, a nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), and a reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a. Semi-batch treatment steps were carried out in the order of solution Xa numbers 1 to 3, and alkali metal salts were recovered. At this time, the treated liquid B (1) after step 2 in solution Xa1 (i.e., solution X (1)) was added to the treated liquid A (3) in solution Xa3 (i.e., solution X (3)). The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration step was carried out at a constant flow rate of 60 L / min permeate flow rate. The alkali metal ion recovery rate in the nanofiltration process was confirmed by monitoring the operating pressure using equation (2), and filtration was continued until the alkali metal ion recovery rate reached 93% in the first nanofiltration process and 80% in the second nanofiltration process, while the reverse osmosis filtration process was carried out at a constant flow rate of 60 L / min of the permeate and continued until the operating pressure reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series. The results of carrying out this process are shown in Table 4. It can be seen that by adding the remainder of the treated liquid B(k) to the treated liquid A(p), lithium ions can be recovered with higher purity and higher recovery rate.

[0143] Comparative Example 1 In the process configuration shown in FIG. 4, nanofiltration membrane A was used as the nanofiltration membrane of nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and a semi-batch treatment process was carried out in the order of numbers 1 to 3 for solution Xa, to carry out an alkali metal salt recovery process. Note that all steps were carried out so that the permeate flow rate was 60 L / min. Note that four 8-inch elements were connected in series for each nanofiltration membrane unit and reverse osmosis membrane unit. The results of carrying out this process are shown in Table 5. It can be seen that when the nanofiltration step does not have a circulation step, the lithium recovery rate is low.

[0144] Example 10 In the process configuration shown in FIG. 5, the nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a), and the reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the alkali metal salt recovery process was carried out in the order of solution numbers 1 to 3 of the solution Xa. The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was carried out at a constant flow rate of 60 L / min permeate flow rate. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 80%, and the reverse osmosis filtration process was carried out at a constant flow rate of 60 L / min permeate flow rate and continued until the operating pressure reached 7 MPa. In addition, the process was carried out so that the permeate flow rate was 60 L / min in all processes. In addition, four 8-inch elements were connected in series and used in each nanofiltration membrane unit and reverse osmosis membrane unit. The results of this process are shown in Table 5.

[0145] Comparative Example 2 In the system configuration shown in Figure 6, nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a), and reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and a semi-batch treatment process was carried out in the order of numbers 1 to 3 for solution Xa, to carry out the alkali metal salt recovery process. Note that the permeate flow rate was set to 60 L / min in all steps. Note that four 8-inch elements were connected in series for each nanofiltration membrane unit and reverse osmosis membrane unit. The results of this process are shown in Table 5. It can be seen that the lithium purity is low when only the first nanofiltration step is used.

[0146] Comparative Example 3 In the process configuration shown in FIG. 7, nanofiltration membrane A was used as the nanofiltration membrane of nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and continuous treatment steps were carried out in the order of numbers 1 to 3 for solution Xa, to carry out the alkali metal salt recovery process. Note that all steps were carried out so that the permeate flow rate was 60 L / min. Note that each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series. The results of this process are shown in Table 5. It can be seen that the lithium recovery rate is low in the continuous processing process.

[0147] Comparative Example 4 An alkali metal salt recovery process was carried out in the same manner as in Comparative Example 1, except that nanofiltration membrane E was used. The results of this process are shown in Table 5.

[0148] Example 11 The process for recovering an alkali metal salt was carried out in the same manner as in Example 1, except that nanofiltration membrane E was used. The results of carrying out this process are shown in Table 5. It can be seen that by applying the process according to this embodiment, the lithium recovery rate is improved compared to Comparative Example 4.

[0149] Example 12 The alkali metal salt recovery process was carried out in the same manner as in Example 9, except that nanofiltration membrane E was used. The results of carrying out this process are shown in Table 5. It can be seen that even when nanofiltration membrane E was used, lithium ions could be recovered with higher purity and higher recovery rate by adding the remainder of treated liquid B(k) to treated liquid A(p).

[0150] Example 13 In the process configuration shown in FIG. 10, the nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a), and the reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the alkali metal salt recovery process was carried out in the order of solution numbers 1 to 3 of the solution Xa. The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was carried out at a constant flow rate of 60 L / min permeate flow rate. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 80%, and the reverse osmosis filtration process was carried out at a constant flow rate of 60 L / min permeate flow rate and continued until the operating pressure reached 7 MPa. The process was carried out so that the permeate flow rate was 60 L / min in all processes. Each nanofiltration membrane unit and reverse osmosis membrane unit was used with four 8-inch elements connected in series. The results of carrying out this process are shown in Table 5. It can be seen that by adding the remainder of the treated liquid B(k) to the treated liquid A(m), it is possible to recover lithium ions with higher purity and higher recovery rate.

[0151] [Table 4]

[0152] [Table 5]

[0153] In the above examples and comparative examples, the higher the lithium recovery rate and lithium purity, the more advantageous it is, while the lower the boron concentration ratio and total treatment time, the more advantageous it is. From the above results, it was found that Examples 1 to 8, which are the alkali metal salt recovery methods of the present invention, can recover alkali metal salts with high purity, at a high recovery rate, and in a short time, compared to Comparative Examples 1 to 4. Furthermore, from the results of Examples 6 to 8, it was found that even when neutral molecules such as boron are contained, these can be removed with high efficiency and the total treatment time can be shortened.

[0154] Although the present invention has been described in detail with reference to specific embodiments, it is apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2022-192379) filed on November 30, 2022, and is incorporated by reference in its entirety. In addition, all references cited herein are incorporated in their entirety. [Industrial Applicability]

[0155] INDUSTRIAL APPLICABILITY The present invention can be suitably used as a method for efficiently separating and recovering alkali metals such as lithium from lithium ion batteries and waste materials, waste liquids, ores, slag, and the like generated in the manufacturing process thereof. [Explanation of symbols]

[0156] 1 Ultrafiltration membrane unit 2a Nanofiltration Membrane Unit A 2b Nanofiltration membrane unit B 3a First reverse osmosis membrane unit 3b Second reverse osmosis membrane unit 4. High-removal reverse osmosis membrane unit 5a First Tank 5b Second Tank 5c Third Tank 5d 4th Tank 5e 5th Tank 5f 6th Tank 5g 7th Tank 5h 8th Tank

Claims

1. A method for recovering an alkali metal salt, comprising the following steps 1 and 2: Step 1: A first nanofiltration step in which a solution X containing alkali metal ions is sent to a nanofiltration membrane unit A as a treated liquid A, and separated into a permeated liquid A and a concentrated liquid B, and further, the concentrated liquid B is mixed with the remainder of the treated liquid A and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid A. Step 2: The permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit A as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D. The concentrate D is further mixed with the remaining part of the liquid to be treated B and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid C. Or, The permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to a nanofiltration membrane unit B as a liquid to be treated B, and separated into a permeated liquid C and a concentrate D. The concentrate D is further mixed with the remainder of the liquid to be treated B and sent again to the nanofiltration membrane unit B to further obtain the permeated liquid C. A second nanofiltration step.

2. A method for recovering an alkali metal salt, comprising the steps of: obtaining the permeate C from the solution X through the steps 1 and 2, and sequentially carrying out the steps on N solutions X (N: an integer of 2 or more), The step 2 uses the nanofiltration membrane unit B, 2. The method for recovering an alkali metal salt according to claim 1, wherein, while carrying out step 1 and then step 2 on a k-th solution X(k) (k: an integer of 1 or more and (N-1) or less) among the N solutions X, step 1 is carried out in parallel on a (k+1)-th solution X(k+1).

3. 3. The method for recovering an alkali metal salt according to claim 1, further comprising 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 an alkali metal salt according to claim 2, further comprising the following step 3: Step 3: A reverse osmosis filtration step for concentrating at least one of the kth permeate A(k) and the kth permeate C(k) in at least one of said solutions X(k).

5. 5. The method for recovering an alkali metal salt according to claim 4, wherein step 3 is carried out only once on the permeate C(k).

6. 3. The method for recovering an alkali metal salt according to claim 1, wherein the solution X has a pH of 4 or less.

7. 3. The method for recovering an alkali metal salt according to claim 1, wherein the alkali metal ions include lithium ions.

8. The method for recovering an alkali metal salt according to claim 1 or 2, comprising the following step 4: Step 4: a step of adding, after completion of step 2 for a k-th solution X(k) (k: an integer of 1 or more and (N-1) or less) among the N solutions X, a remainder of the k-th liquid to be treated B(k) having been mixed with the k-th concentrated liquid D(k) to an m-th solution X(m) (m: an integer of (k+1) or more and N or less) or the m-th liquid to be treated A(m).

9. The nanofiltration membrane of at least one of the nanofiltration membrane unit A and the nanofiltration membrane unit 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 side of the separation functional layer, and the average pore size R1 and the average pore size R2 of the separation functional layer derived by a positron annihilation lifetime measurement method satisfy 0.90≦R1 / R2≦1.

10.

3. The method for recovering an alkali metal salt according to claim 1 or 2. R1: Average pore diameter at a positron beam intensity of 0.1 keV R2: Average pore diameter at a positron beam intensity of 0.5 keV

10. 3. The method for recovering an alkali metal salt according to claim 1 or 2, wherein at least one of the steps 1 and 2 is carried out at a constant permeation flow rate, and at least one of the steps 1 and 2 is terminated when a recovery rate A (%) of the alkali metal ion reaches a target value based on the following formula (2) while monitoring a change over time of the operating pressure: [0010] [In the formula (2), the recovery rate A of alkali metal ions is (%), the operating pressure P is (Pa), and the initial operating pressure P is ０ (Pa), initial liquid volume V to be processed ０ (m ３ ), alkali metal ion removal rate R (%) of the nanofiltration membrane, liquid recovery rate S (%) of the nanofiltration process, and supply flow rate Q Ｆ (m ３ / s), concentrate flow rate Q ｃ (m ３ / s), and the filtration end time t = tb.

11. At least one of the permeated liquid A(k) and the permeated liquid C(k) contains neutral molecules that are not charged under conditions of pH 3 or less, and the reverse osmosis filtration membrane used in the reverse osmosis filtration step is a low-removal reverse osmosis membrane that has an isopropyl alcohol removal rate of 70% or more and less than 85% when an aqueous isopropyl alcohol solution having a pH of 6.5 and an operating pressure of 0.5 MPa is passed through the membrane. The method for recovering an alkali metal salt according to claim 4 or 5.

12. 12. The method for recovering an alkali metal salt according to claim 11, wherein the neutral molecule is a boron compound.

13. 13. The method for recovering an alkali metal salt according to claim 12, further comprising a circulation step of mixing the concentrated liquid obtained in the reverse osmosis filtration step with the solution to be supplied to the reverse osmosis filtration step in the step 3.

14. 12. The method for recovering an alkali metal salt according to claim 11, further comprising the steps of: feeding the permeated liquid obtained in the reverse osmosis filtration step to a high-removal reverse osmosis membrane unit having a high-removal reverse osmosis membrane having an isopropyl alcohol removal rate of 85% or more and 95% or less when an aqueous isopropyl alcohol solution having a pH of 6.5 and an operating pressure of 0.5 MPa is passed through the high-removal reverse osmosis membrane; and adding the permeated liquid obtained as dilution water to at least one of the treated liquid A and the treated liquid B.

15. a first separation means for separating a solution containing alkali metal ions as a treatment liquid A into a permeate liquid A and a concentrate liquid B by a first nanofiltration membrane unit; a first circulation means for mixing the concentrated liquid B with the remainder of the liquid A to be treated; A second separation means for separating the permeated liquid A or a concentrate of the permeated liquid A as a liquid to be treated B into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit; A second circulation means for mixing the concentrated liquid D with the remainder of the liquid to be treated B; A dilution means for adding dilution water to at least one of the liquid A and the liquid B; a flow rate control means capable of controlling the flow rates of the permeated liquid A and the concentrated liquid B in the first separation means, and the permeated liquid C and the concentrated liquid D in the second separation means; An alkali metal salt recovery apparatus comprising: a flow control means for synchronizing the flow rate of dilution water added in the dilution means with the flow rate of the permeated liquid when the treated liquid to which the dilution water is added is sent to a nanofiltration membrane unit.

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