Alkali metal salt recovery method and alkali metal salt recovery device

Abstract

The present invention pertains to a method for recovering an alkali metal salt, said method comprising the following steps 1 and 2. Step 1: a first nanofiltration step in which a solution X containing alkali metal ions is fed as a liquid to be treated A to a nanofiltration membrane unit A, separated into a permeate A and a concentrate B, mixed with the remaining part of the liquid to be treated A, and fed again to the nanofiltration membrane unit A to further obtain the permeate A; step 2: feeding the permeate A obtained in step 1 or a concentrated solution of the permeate A as a liquid to be treated B to a nanofiltration membrane unit A, separating the permeate A or the concentrated solution into a permeate C and a concentrated solution D, mixing the concentrated solution D with the remainder of the liquid to be treated B, feeding the mixture again to the nanofiltration membrane unit A, and further obtaining the permeate C, or the concentrated solution of the permeate A or the concentrated solution of the permeate A; and a second nanofiltration step in which the permeate A or the concentrate of the permeate A obtained in step 1 is conveyed as a liquid to be treated B to a nanofiltration membrane unit B and separated into a permeate C and a concentrate D, the concentrate D is mixed with the remainder of the liquid to be treated B, the mixture is conveyed again to the nanofiltration membrane unit B, and the permeate C is further obtained.

Description

Technical Field

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

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

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

[0004] Nickel, using its high luster and corrosion resistance, is used as stainless steel, and in recent years, like cobalt, the demand as a material for lithium-ion batteries is increasing. Thus, in the situation where the demand for various rare metals is soaring, from the viewpoint of recycling precious resources, measures are being promoted to recover rare metals such as lithium, cobalt, and nickel from waste materials generated from used lithium-ion batteries or their manufacturing processes.

[0005] For example, the resource recovery from waste lithium-ion batteries is being promoted mainly for rare metals such as cobalt and nickel, but since the solvent extraction method using a chelating agent is the mainstream, there are problems not only of a large environmental load but also of being disadvantageous in terms of cost (Non-Patent Document 1). To solve this problem, a method of separating and recovering from an aqueous solution of acid-leached waste lithium-ion batteries using separation membranes such as ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes has been disclosed (Patent Document 1). However, in this separation and recovery method, since the nanofiltration membrane is a one-stage process, it is difficult to recover lithium with high purity and high efficiency unless the performance of the nanofiltration membrane is extremely improved.

[0006] Therefore, a separation and recovery method in which the nanofiltration membrane is set in multiple stages has been disclosed (Patent Document 2). That is, 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 remaining lithium is recovered by passing the liquid that has not passed through the nanofiltration membrane through the nanofiltration membrane.

[0007] Prior Art Documents

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

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

[0010] Non-Patent Document 1: "Report on the Exploration and Other Projects for Promoting Mineral Resource Development in the 29th Year of Heisei (Basic Survey on Mineral Resources Development Related to the Formulation of Mineral Resource Assurance Strategy)", Mitsubishi Research Institute, Inc., Environmental and Energy Business Unit, March 2018 Summary of the Invention

[0011] Problems to be Solved by the Invention

[0012] However, since the method described in Patent Document 2 is complex and continuous in process, when the composition of the liquid to be treated changes, or when the separation performance changes due to deterioration of the nanofiltration membrane or the like, the treatment process of the nanofiltration membrane may become unstable, and there is still room for improvement in maintaining a predetermined lithium purity and recovery rate.

[0013] An object of the present invention is to provide a method capable of stably recovering an alkali metal salt with high purity and high efficiency from waste materials, waste liquids, ores, etc. generated from lithium ion batteries or their manufacturing processes with a small number of steps.

[0014] Means for Solving the Problems

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

[0016] (1) A method for recovering an alkali metal salt, comprising the following Step 1 and Step 2.

[0017] Step 1: A first nanofiltration step of feeding a solution X containing alkali metal ions as a liquid to be treated A to a nanofiltration membrane unit A, separating it into a permeate A and a concentrate B, mixing the concentrate B with the remaining part of the liquid to be treated A, and feeding it again to the nanofiltration membrane unit A to further obtain the permeate A.

[0018] Step 2: Feeding the permeate A obtained in Step 1 or a concentrate of the permeate A as a liquid to be treated B to the nanofiltration membrane unit A, separating it into a permeate C and a concentrate D, mixing the concentrate D with the remaining part of the liquid to be treated B, and feeding it again to the nanofiltration membrane unit A to further obtain the permeate C, or feeding the permeate A obtained in Step 1 or a concentrate of the permeate A as a liquid to be treated B to a nanofiltration membrane unit B, separating it into a permeate C and a concentrate D, mixing the concentrate D with the remaining part of the liquid to be treated B, and feeding it again to the nanofiltration membrane unit B to further obtain the permeate C, which is a second nanofiltration step.

[0019] (2) According to the method for recovering alkali metal salts described in (1) above,

[0020] The treatment of obtaining the permeate C from the solution X through the above-mentioned step 1 and step 2 is sequentially performed on N (N: an integer of 2 or more) of the solutions X. In the period when step 2 using the nanofiltration membrane unit B is performed after step 1 is performed on the 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 performed in parallel on the (k + 1)-th solution X(k + 1).

[0021] (3) According to the method for recovering alkali metal salts described in (1) or (2) above,

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

[0023] (4) According to the method for recovering alkali metal salts described in (2) or (3) above, it further includes the following step 3.

[0024] Step 3: A reverse osmosis filtration step of concentrating at least one of the k-th permeate A(k) and the k-th permeate C(k) in at least one of the solutions X(k).

[0025] (5) According to the method for recovering alkali metal salts described in (4) above,

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

[0027] (6) According to the method for recovering alkali metal salts described in any one of (1) to (5) above,

[0028] The pH value of the solution X is 4 or less.

[0029] (7) According to the method for recovering alkali metal salts described in any one of (1) to (6) above,

[0030] The alkali metal ions include lithium ions.

[0031] (8) According to the method for recovering alkali metal salts described in any one of (1) to (7) above, it includes the following step 4.

[0032] Step 4: There are N (N: an integer of 2 or more) of the solutions X. After step 2 of the k-th solution X(k) (k: an integer of 1 or more and (N−1) or less) among the N solutions X ends, the remaining part of the k-th liquid to be treated B(k) mixed with the k-th concentrated liquid D(k) is added to the 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).

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

[0034] The nanofiltration membrane included in 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.

[0035] A positron beam is irradiated from the surface on the separation functional layer side of the nanofiltration membrane, and the average pore diameters R1 and R2 of the separation functional layer derived by positron annihilation lifetime measurement method satisfy 0.90 ≤ R1 / R2 ≤ 1.10.

[0036] R1: The average pore diameter under the condition that the positron beam intensity is 0.1 keV

[0037] R2: The average pore diameter under the condition that the positron beam intensity is 0.5 keV

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

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

[0040]

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

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

[0043] At least one of the permeate A(k) and the permeate C(k) contains a neutral molecule that does not carry a charge under the condition that the pH value is 3 or less, and the reverse osmosis filtration membrane used in the reverse osmosis filtration process is a low-removal reverse osmosis membrane that satisfies that the removal rate of isopropanol is 70% or more and less than 85% when an isopropanol aqueous solution at 25°C and a pH value of 6.5 permeates under an operating pressure of 0.5 MPa.

[0044] (12) The recovery method of the alkali metal salt according to the above (11).

[0045] The neutral molecule is a boron compound.

[0046] (13) The recovery method of the alkali metal salt according to the above (12).

[0047] In the process 3, a circulation process is provided in which the concentrated liquid obtained in the reverse osmosis filtration process is mixed with the solution supplied to the reverse osmosis filtration process.

[0048] (14) The recovery method of the alkali metal salt according to any one of the above (11) to (13).

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

[0050] (15) A recovery device for an alkali metal salt, comprising:

[0051] A first separation mechanism that uses a solution containing alkali metal ions as the liquid to be treated A and separates it into a permeate A and a concentrated liquid B through a first nanofiltration membrane unit;

[0052] A first circulation mechanism that mixes the concentrated liquid B with the remaining part of the liquid to be treated A;

[0053] A second separation mechanism that uses the permeate A or the concentrated liquid of the permeate A as the liquid to be treated B and separates it into a permeate C and a concentrated liquid D through a second nanofiltration membrane unit;

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

[0055] A dilution mechanism that adds dilution water to at least one of the liquid to be treated A and the liquid to be treated B;

[0056] A flow control mechanism that can control the respective flows of the permeate A and the concentrated liquid B in the first separation mechanism and the permeate C and the concentrated liquid D in the second separation mechanism; and

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

[0058] Effect of the Invention

[0059] According to the method for recovering alkali metal salts of the present invention, salts of alkali metals such as lithium or cesium can be stably and efficiently recovered from a solution containing alkali metal ions with high purity using a smaller number of steps. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a schematic flowchart showing a method for recovering an alkali metal salt according to an embodiment of the present invention.

[0061] Figure 2 It is a schematic flowchart showing a method for recovering an alkali metal salt according to another embodiment of the present invention.

[0062] Figure 3 It is a schematic flowchart showing a method for recovering an alkali metal salt according to another embodiment of the present invention.

[0063] Figure 4 It is a schematic flowchart showing a method for recovering an alkali metal salt in a comparative method.

[0064] Figure 5 It is a schematic flowchart showing a method for recovering an alkali metal salt according to another embodiment of the present invention.

[0065] Figure 6 It is a schematic flowchart showing a method for recovering an alkali metal salt in a comparative method.

[0066] Figure 7 It is a schematic flowchart showing a method for recovering an alkali metal salt in a comparative method.

[0067] Figure 8 It is a schematic flowchart showing a method for recovering an alkali metal salt according to another embodiment of the present invention.

[0068] Figure 9 It is a schematic flowchart showing a method for recovering an alkali metal salt according to another embodiment of the present invention.

[0069] Figure 10 It is a schematic flowchart showing a method for recovering an alkali metal salt according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0070] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following description, and can be arbitrarily modified and implemented within the scope of the gist of the present invention.

[0071] (1) Method for Recovering Alkali Metal Salt

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

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

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

[0075] The method for recovering alkali metal salts according to the present embodiment is a method for recovering alkali metal salts in which N (N: an integer of 2 or more) solutions X are sequentially subjected to the treatment of obtaining the permeate C from the solution X through Step 1 and Step 2. Preferably, in Step 2, the nanofiltration membrane unit B is used. During the period from performing Step 2 after performing Step 1 on the 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 performed in parallel on the (k + 1)-th solution X(k + 1). A series of treatment steps in which the batch treatment steps in Step 1 and Step 2 are performed semi-continuously on multiple solutions is referred to as a semi-batch treatment step.

[0076] (2) Nanofiltration step

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

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

[0079] The concentration of polyvalent metal ions is calculated as the total of the ion-converted concentrations of, for example, cobalt ions or nickel ions. In addition, the concentration of alkali metal ions is calculated as the total of the ion-converted concentrations of, for example, lithium ions or cesium ions. Depending on the element, alkali metals sometimes exist in solution not as monatomic ions but as polyatomic ions, and the converted concentration is the concentration assuming they exist as monatomic ions. Regarding the above polyvalent metal ion and alkali metal ion concentrations, for example, a P-4010 type ICP (high-frequency inductively coupled plasma optical emission spectrometry) device manufactured by Hitachi, Ltd. can be used to analyze the solution to be measured and quantify the concentrations (mg / L) of various ions.

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

[0081] The solution X containing alkali metal ions only needs to contain at least alkali metal ions and one or more conjugate bases (such as chloride ions, nitrate ions, sulfate ions, carbonate ions, acetate ions, etc.). The alkali metal ions and conjugate bases in solution X can exist in the form of alkali metal salts. Examples of alkali metal salts include salts of lithium, sodium, potassium, rubidium, and cesium. Among them, from the perspective of the value of the object to be recovered, a lithium salt is preferably contained. That is, lithium ions (hereinafter also referred to as "Li" + ) are preferably contained as alkali metal ions in the solution containing alkali metal ions.

[0082] When there are N solutions X containing alkali metal ions, each solution X only needs to contain at least alkali metal ions 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 value can be different in each solution X. Among them, it is preferable that Li + is contained as the alkali metal ion in all solutions X.

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

[0084] In addition, the solution X containing alkali metal ions may contain neutral molecules that do not carry a charge under the condition that the pH value is below 3, and the molecular weight of the neutral molecule is preferably 70 or less. Examples of the neutral molecule include formic acid, acetic acid, and boron compounds such as boric acid. Among them, boron compounds are sometimes added as additives to the electrolyte of lithium-ion batteries for the purpose of improving battery characteristics, and thus may be contained in the solution X containing alkali metal ions. For example, in the case of containing boron compounds, they will become substances that hinder purification during lithium recovery, but can be removed in the reverse osmosis filtration process described later. The boron concentration (mg / L) in the solution X is preferably below the concentration of the alkali metal ions to be recovered, more preferably below the concentration of the alkali metal ions to be recovered × 0.5 (mg / L), and further preferably below the concentration of the alkali metal ions to be recovered × 0.1 (mg / L).

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

[0086] The pH value of the solution X containing alkali metal ions is preferably 4 or less, more preferably 3.5 or less, further preferably 3 or less, and even more preferably 2.5 or less. By setting the pH value to 4 or less, in the nanofiltration process, the permeation rate of polyvalent metal ions is maintained at a low level, and the permeation rate of alkali metal ions becomes high.

[0087] In addition, the pH value of the solution X containing alkali metal ions is preferably 0 or more, more preferably 0.5 or more, and further preferably 1 or more. By setting the pH value to 0 or more, it is possible to suppress the decrease in the selective separation performance of alkali metal ions relative to polyvalent metal ions of the nanofiltration membrane during long-term operation.

[0088] The solution X containing alkali metal ions is preferably a solution obtained by dissolving a lithium-containing material with an acid. Specific examples of the lithium-containing material include waste materials, waste liquids, ores, and slag generated in lithium-ion batteries and their manufacturing processes. Among them, lithium-ion batteries are preferred in terms of high requirements for reuse and high purity of rare metals contained.

[0089] A lithium-ion battery is composed of components such as a positive electrode material, a negative electrode material, a separator, and an electrolyte. Among these components, as long as they are lithium-containing materials, they can be used as materials for the solution X. As the acid for dissolving the lithium-containing material, an acid containing at least one selected from hydrochloric acid, sulfuric acid, and nitric acid is preferred. In the solution obtained by dissolving the components of the lithium-ion battery with an acid, in addition to lithium ions, for example, nickel, cobalt, manganese, etc. are also contained.

[0090] A method of dissolving a substance containing an alkali metal with an acid, for example, a method of immersing the substance in an acidic aqueous solution can be cited. However, as long as the target alkali metal ions can be dissolved out, other methods can also be adopted. From the viewpoint of the dissolution efficiency of alkali metal ions, the temperature of the acidic aqueous solution in contact is preferably 10°C or higher and 100°C or lower. Further, from the viewpoints of cost and safety, it is more preferably 20°C or higher and 80°C or lower.

[0091] The solution obtained by dissolving a substance containing an alkali metal with an acid does not always have a constant composition, but the composition may vary due to variations in the ionic compositions of the substances and the dissolution conditions in the acid. That is, in the case where there are N solutions X, the compositions of the N solutions X may sometimes be different from each other.

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

[0093] The solution X containing alkali metal ions may contain an organic compound. For example, in the case where the solution X is an acid dissolution solution of a lithium-ion battery, organic compounds such as polyvinylidene fluoride (PVDF), polyolefin, and carbonate from adhesives, separators, electrolytes, etc. that connect active materials to current collectors can be cited. These organic compounds may become impurities and cause a decrease in the recovery efficiency of alkali metal ions, and thus these impurities can be removed by the ultrafiltration process described later.

[0094] When lithium ions are contained as alkali metal ions in each of the N solutions X containing alkali metal ions, the lithium ion concentration in the solution is preferably 0.5 mg / L or more and 10,000 mg / L or less. By making the lithium ion concentration in the solution 0.5 mg / L or more, the recovery efficiency of lithium ions achieved by membrane separation is improved. Further, by making the lithium ion concentration in the solution 10,000 mg / L or less, an increase in the osmotic pressure difference can be suppressed, and the efficiency of membrane separation can be improved. The lithium ion concentration in the solution is more preferably 10 mg / L or more and 8,000 mg / L or less, and further preferably 100 mg / L or more and 6,000 mg / L or less.

[0095] The method for recovering alkali metal salts according to this embodiment can also be applied when the alkali metal ion ratio in the solution X containing alkali metal ions is 2.4 or less. Generally, when the alkali metal ion ratio is 2.4 or less, it becomes more difficult to separate and recover alkali metal ions and polyvalent metal ions. However, in the method for recovering alkali metal salts according to this embodiment, the selectivity for separating alkali metal ions from polyvalent metal ions is high, and alkali metal ions can be effectively recovered. In addition, the method for recovering alkali metal salts according to this embodiment can also be applied when the alkali metal ion ratio in the solution X containing alkali metal ions is 1 or less, and further when it is 0.5 or less.

[0096] (2-2) Nanofiltration membrane

[0097] The nanofiltration membrane used in the method for recovering alkali metal salts according to this embodiment only needs to have a fractionation property between the reverse osmosis membrane and the ultrafiltration membrane. Preferably, the difference between the glucose rejection rate when a 1000 mg / L glucose aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa and the isopropanol rejection rate when a 1000 mg / L isopropanol aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa is 20% or more. Among them, the nanofiltration membrane is more preferably such that the difference between the glucose rejection rate when a 1000 mg / L glucose aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa and the isopropanol rejection rate when a 1000 mg / L isopropanol aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa is 40% or more, the above glucose rejection rate is 70% or more, and the magnesium sulfate rejection rate when a 2000 mg / L magnesium sulfate aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa is 95% or more. Hereinafter, in the specification of this application, when only "glucose rejection rate" is mentioned, it refers to the glucose rejection rate when a 1000 mg / L glucose aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa. When "isopropanol rejection rate" is mentioned, it refers to the isopropanol rejection rate when a 1000 mg / L isopropanol aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa. When "magnesium sulfate rejection rate" is mentioned, it refers to the magnesium sulfate rejection rate when a 2000 mg / L magnesium sulfate aqueous solution at 25°C and a pH of 6.5 permeates under an operating pressure of 0.5 MPa.

[0098] Membranes generally known as reverse osmosis membranes can remove most organic substances and ions. On the other hand, ultrafiltration membranes generally do not remove most ion species but remove high-molecular-weight organic substances.

[0099] In order to separate alkali metal ions from polyvalent metal ions, the nanofiltration membrane preferably has a charge on the membrane surface and can perform two types of separations: separation by pores (size separation) and electrostatic separation by charge. For example, when using a nanofiltration membrane with a difference between the glucose rejection rate and the isopropanol rejection rate of 40% or more and a glucose rejection rate of 70% or more, and a difference between the magnesium sulfate rejection rate and the magnesium chloride rejection rate when a 2000 mg / L magnesium chloride aqueous solution at 25 °C and a pH value of 6.5 permeates under an operating pressure of 0.5 MPa of 20% or less, both size separation and electrostatic separation can be performed. Hereinafter, when only "magnesium chloride rejection rate" is recorded in the specification of this application, it refers to the magnesium chloride rejection rate when a 2000 mg / L magnesium chloride aqueous solution at 25 °C and a pH value of 6.5 permeates under an operating pressure of 0.5 MPa.

[0100] As the material of the nanofiltration membrane, polymers such as cellulose acetate-based polymers, polyamides, sulfonated polysulfones, polyacrylonitriles, polyesters, polyimides, and vinyl polymers can be used. The nanofiltration membrane can be composed of only one material or multiple materials. In addition, its membrane structure can also be an asymmetric membrane having a dense layer on at least one side of the membrane and gradually having fine pores with large pore diameters from the dense layer to the inside of the membrane or the other side, or a composite semi-permeable membrane having a very thin separation functional layer formed of other materials on the dense layer of the asymmetric membrane.

[0101] As the composite semi-permeable membrane, for example, a membrane having a porous support membrane containing polysulfone and a separation functional layer containing polyamide provided on the porous support membrane is preferably used. In addition, in addition to the porous support membrane and the separation functional layer, the composite semi-permeable membrane can also have a substrate. In this case, the porous support membrane is provided on the substrate. Polyamide is a thin film formed by an interfacial polycondensation reaction of a polyfunctional aliphatic amine and a polyfunctional aromatic acyl halide on the porous support membrane.

[0102] In the method for recovering alkali metal salts according to this embodiment, it is preferable that the nanofiltration membrane provided in 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 diameters R1 and R2 of the separation functional layer derived by positron annihilation lifetime measurement method satisfy 0.90 ≤ R1 / R2 ≤ 1.10 when irradiating a positron beam from the surface on the separation functional layer side of the nanofiltration membrane. Here, R1 and R2 are defined as follows.

[0103] R1: The average pore diameter under the condition that the positron beam intensity is 0.1 keV

[0104] R2: The average pore diameter under the condition that the positron beam intensity is 0.5 keV

[0105] The so-called "positron annihilation lifetime measurement method" refers to a method for non-destructively evaluating information such as the size, number density, and size distribution of voids with a size of 0.1 to 10 nm based on the annihilation lifetime by measuring the time (in the order of several hundred picoseconds to several tens of nanoseconds) from the incidence of positrons on a sample to their annihilation.

[0106] In addition, the measurement region in the depth direction from the sample surface can be adjusted by the energy of the positron beam incident on the sample. The higher the energy, the deeper the part from the sample surface is included in the measurement region, but the depth is affected by 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 with a depth of 1.0 to 5.0 nm from the sample surface is usually measured. If it is a positron beam with an energy of about 0.5 keV, a region with a depth of 10 to 50 nm from the sample surface is usually measured. In addition, when 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 other layers in advance.

[0107] In the present embodiment, the film 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 that the positron beam intensity is 0.1 keV, it reflects the average pore size on the surface side (the side opposite to the porous support membrane side) of the separation functional layer. Under the condition of 0.5 keV, it reflects the average pore size on the porous support membrane side of the separation functional layer. It can be said that the closer R1 / R2 is to 1, the more uniform the pore size is in the film thickness direction. It is speculated that by making the pore size uniform in the film thickness direction, the direction of ion diffusion in the separation functional layer becomes uniform, and the permeation resistance of monovalent ions with a size capable of freely moving in the separation functional layer is suppressed. As a result, it is considered that excellent monovalent ion / multivalent ion selective separation performance is achieved. Therefore, R1 / R2 is more preferably 0.92 or more and 1.05 or less, and further preferably 0.94 or more and 1.03 or less.

[0108] In addition, 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 further preferably 0.60 nm or more and 0.65 nm or less. By making R1 within the above range, the permeation resistance of alkali metal ions is suppressed, and the effect of hindering the permeation of multivalent metal ions becomes significant.

[0109] In order to make R1 and R2 satisfy the above relationship, for example, a method can be cited in which the relative humidity during the interfacial polycondensation of a polyfunctional aliphatic amine compound and a polyfunctional aromatic acyl halide described later is controlled to be relatively high, for example, 80% or more, and the molecular weight of the polyfunctional aliphatic amine forming the separation functional layer in the composite semipermeable membrane is 90 or more.

[0110] The separation functional layer in the composite semipermeable membrane preferably contains 50% by mass or more of semi-aromatic crosslinked polyamide obtained by interfacial polycondensation of a polyvalent aliphatic amine compound having a valence of 2 or more and a polyvalent aromatic acyl halide having a valence of 2 or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, and particularly preferably consists only of semi-aromatic crosslinked polyamide. By containing 50% by mass or more of semi-aromatic crosslinked polyamide, excessive densification caused by π-π interaction of aromatic rings in the semi-aromatic crosslinked polyamide can be suppressed, and excellent alkali metal ion permeability can be obtained. In addition, as a result of in-depth research by the present inventors, it was found that when the relative humidity during interfacial polycondensation is controlled to be high, for example, 80% or more, the obtained composite semipermeable membrane exhibits particularly excellent membrane properties under acidic conditions. In addition, the relative humidity can be adjusted by using a precision air conditioning device or the like. By making the atmospheric humidity during interfacial polycondensation 80% or more, evaporation of water in the formed polyamide can be suppressed, and insolubilization caused by intermolecular hydrogen bonding of the remaining oligomers with a large amount of amino groups can be suppressed. As a result, oligomers can be effectively removed after the separation functional layer is formed by the interfacial polycondensation reaction, so that an increase in pore size accompanied by swelling of the semi-aromatic crosslinked polyamide when the membrane is used under acidic conditions can be suppressed, and a composite semipermeable membrane showing excellent removal performance for polyvalent ions, that is, a predetermined glucose removal rate, isopropyl alcohol removal rate, magnesium sulfate removal rate, etc. can be obtained.

[0111] The polyvalent aliphatic amine is preferably an alicyclic diamine, more preferably a bipiperidine derivative or a piperazine derivative.

[0112] 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 becomes small, and polyamide is gradually formed during interfacial polycondensation. Therefore, from the initial stage to the middle stage of interfacial polycondensation, it is easy to form a separation functional layer with uniform pore size in the film thickness direction. On the other hand, the molecular weight of the alicyclic diamine is preferably 160 or less. Generally, in 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 is the main reason for the non-uniform pore size distribution in the film thickness direction. However, when the molecular weight of the alicyclic diamine is 160 or less, the molecular weight of the generated oligomers becomes small, and the interaction with the semi-aromatic crosslinked polyamide can be reduced. Therefore, after the separation functional layer is formed by the interfacial polycondensation reaction, the oligomers are easily detached from the separation functional layer, and it is easy to form a separation functional layer with uniform pore size in the film thickness direction.

[0113] As an alicyclic diamine having a molecular weight of 90 or more and 160 or less, for example, a substituted piperazine in which a piperazine ring is substituted with an alkyl group having 1 to 3 carbon atoms (for example, 2-methylpiperazine, 2-ethylpiperazine, 2-n-propylpiperazine, 2,2-dimethylpiperazine, 2,2-diethylpiperazine, 2,3-dimethylpiperazine, 2,3-diethylpiperazine, 2,5-dimethylpiperazine, 2,5-diethylpiperazine, 2,6-dimethylpiperazine, 2,6-diethylpiperazine, 2,3,5,6-tetramethylpiperazine, etc.), homopiperazine can be cited.

[0114] "Polyfunctional aromatic acyl halide" refers to an aromatic acyl halide having two or more halo carbonyl groups in one molecule, and there is no particular limitation as long as a semi-aromatic crosslinked polyamide is obtained by reaction with the above polyfunctional aliphatic amine. As the polyfunctional aromatic acyl halide, for example, halides such as 1,3,5-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid, 1,3-benzenedicarboxylic acid, 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid, 1,3,6-naphthalenetrisulfonic acid can be used. Among the polyfunctional aromatic acyl halides, acyl chlorides are preferred, and in particular, from the viewpoints of economy, ease of acquisition, ease of handling, and ease of reactivity, the acyl halide of 1,3,5-benzenetricarboxylic acid, i.e., trimesoyl chloride, the acyl halide of 1,3-benzenedicarboxylic acid, i.e., isophthaloyl chloride, the acyl halide of 1,4-benzenedicarboxylic acid, i.e., terephthaloyl chloride, the acyl halide of 1,3,5-benzenetrisulfonic acid, i.e., 1,3,5-benzenetrisulfonyl chloride, and the acyl halide of 1,3,6-naphthalenetrisulfonic acid, i.e., 1,3,6-naphthalenetrisulfonyl chloride are preferred. The above polyfunctional aromatic acyl halides can be used alone or in combination of two or more. By mixing any one of the bifunctional isophthaloyl chloride and terephthaloyl chloride in the trifunctional trimesoyl chloride, 1,3,5-benzenetrisulfonyl chloride, and 1,3,6-naphthalenetrisulfonic acid, the molecular gap of the polyamide crosslinked structure is enlarged, and a membrane having a uniform pore size distribution can be controlled over a wide range. The mixing molar ratio of the trifunctional acyl chloride to the bifunctional acyl chloride is preferably 1:20 to 50:1, more preferably 1:1 to 20:1.

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

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

[0117] Alkali metal ions easily permeate through the nanofiltration membrane, and polyvalent metal ions hardly permeate through the nanofiltration membrane. Therefore, alkali metal ions and polyvalent metal ions can be separated. The nanofiltration membrane is preferably used in a state assembled in an element such as a spiral type.

[0118] (2-3-1) Step 1: First nanofiltration step

[0119] The first nanofiltration process (Process 1) is a process in which Solution X is fed as the liquid to be treated A to the nanofiltration membrane unit A, separated into permeate A and concentrate B, and then the concentrate B is mixed with the remaining portion of the liquid to be treated A and fed again to the nanofiltration membrane unit A to further obtain permeate A. In Process 1, the concentrate B can be treated two or more times with the nanofiltration membrane unit A. The number of repetitions of this treatment can be set arbitrarily. In addition, as described later, the repeated treatment in Process 1 can be terminated when the recovery rate reaches a certain value. By mixing the concentrate B with the remaining portion of the liquid to be treated A while allowing the liquid to be treated A to pass through the nanofiltration membrane unit A, the alkali metal ions remaining in the concentrate B can pass through the nanofiltration membrane again, and the recovery rate of the alkali metal ions can be increased.

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

[0121] In the first nanofiltration process, 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 still more preferably 20 or more and 500 or less. By making the alkali metal ion ratio 2 or more, the treatment time of the subsequent second nanofiltration process can be shortened, and by making the alkali metal ion ratio 1000 or less, the treatment time of this process can be shortened.

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

[0123] Recovery rate of alkali metal ions in the nanofiltration process (%) = {(liquid volume of the permeate in the nanofiltration process as the object) × (alkali metal ion concentration in the permeate of the nanofiltration process as the object)} / {(liquid volume to be treated) × (initial alkali metal ion concentration in the liquid to be treated)} ··· Formula (1)

[0124] In the nanofiltration process, it is preferable to supply the solution to the nanofiltration membrane within the range of an operating pressure of 0.1 MPa or more and 8 MPa or less. If the operating pressure is 0.1 MPa or more, the membrane permeation rate increases, 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 or more and 6 MPa or less, and still more preferably 1 MPa or more and 4 MPa or less.

[0125] As the first nanofiltration process proceeds, the osmotic pressure of the liquid to be treated A increases, and accordingly, the operating pressure for obtaining the permeate A of the same flow rate becomes larger. Therefore, in the method for recovering alkali metal salts according to the present embodiment, in order to easily continue filtration in the case where the osmotic pressure increases, it is preferable to include a step of diluting the liquid to be treated A. By diluting the liquid to be treated A, the osmotic pressure of the liquid to be treated A decreases, the first nanofiltration process can be continued, and the recovery rate of alkali metal ions can be increased, which is therefore preferable.

[0126] As a method for diluting the liquid to be treated A, for example, 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 can be cited. Among them, from the viewpoint of simplicity, a method of directly adding dilution water to the liquid to be treated A is preferable.

[0127] The dilution water may be pure water, an acidic solution, etc., and is not particularly limited. It is preferable to use the permeate with a low metal ion concentration generated in the reverse osmosis filtration process described later because alkali metal ions can be separated and recovered efficiently, and the acidic aqueous solution can be reused.

[0128] As an operation control method in the first nanofiltration process, for example, constant flow filtration, low-pressure filtration, etc. can be cited, and there is no particular limitation. In the case of diluting the liquid to be treated A, constant flow filtration is preferable. If it is constant flow filtration, the flow rate of the added dilution water can also be constant, making the control easier.

[0129] In the case of constant flow filtration, from the + viewpoint of the recovery efficiency of Li, the permeate flow rate is preferably a liquid volume of 1% or more with respect to the liquid volume of solution X permeating within 1 minute, and from the viewpoint of ease of control, it is preferably a liquid volume of 50% or less with respect to the liquid volume of solution X permeating within 1 minute.

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

[0131] As a method for monitoring the recovery rate of alkali metal ions during the first nanofiltration process, in the case of performing the nanofiltration process at a constant permeation flow rate, as shown in the following formula (2), the operating pressure (operating pressure) is related to the recovery rate of alkali metal ions. Therefore, it is preferable to monitor the change over time of the operating pressure through the following formula while grasping the recovery rate A (%) of alkali metal ions and determining the end time of the first nanofiltration process. By determining the end time of the first nanofiltration process while monitoring the operating pressure according to formula (2), the time required for analyzing the liquid composition can be reduced, and alkali metal ions can be effectively recovered. In addition, the target recovery rate A (%) of alkali metal ions can be set appropriately. For at least one of Process 1 and Process 2, it can be ended on the condition that the recovery rate reaches the target value.

[0132]

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

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

[0135] Figures 1 to 3 , Figure 5 and Figures 8 to 10 are schematic flowcharts showing the method for recovering alkali metal salts according to an embodiment of the present invention. The solution X is transported to the ultrafiltration membrane unit 1 described later, and the obtained permeate is transported to the first tank (container) 5a. The permeate (liquid to be treated A) stored in the first tank 5a is transported to the nanofiltration membrane unit A (2a) and separated into a permeate A and a concentrate B. The permeate A is transported to the second tank 5b at a constant flow rate, and the concentrate B is transported to the first tank 5a and mixed with the remaining part of the liquid to be treated A in the first tank 5a. In addition, the first nanofiltration process 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. In addition, as the dilution water, it can contain the permeate obtained through the reverse osmosis filtration process described later. In Figure 3 and Figure 9In the example shown, the permeate obtained through the reverse osmosis filtration process described later is further sent to the high-removal reverse osmosis membrane unit 4, and the obtained permeate is used as dilution water.

[0136] (2-3-2) Process 2: Second nanofiltration process

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

[0138] The concentrate of the permeate A can be prepared by a method such as concentrating the permeate A with a reverse osmosis membrane unit described later, and there is no particular limitation.

[0139] From the viewpoint of shortening the treatment time, it is preferable to use the permeate A as the liquid to be treated B in the second nanofiltration process, and it is also preferable to use the nanofiltration membrane unit B.

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

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

[0142] The second nanofiltration step is carried out at least once. When the ratio of alkali metal ions in solution X is small and the ratio of alkali metal ions in the permeate C does not reach the target value in the case where the second nanofiltration step is carried out once, the second nanofiltration step using the permeate C as the liquid to be treated B can be carried out multiple times until the ratio of alkali metal ions in the obtained permeate reaches the target value. In the case of carrying out the second nanofiltration step multiple times, the nanofiltration membrane unit used after the second time can be either the nanofiltration membrane unit A or B, or other nanofiltration membrane units can also be used.

[0143] In the second nanofiltration step, similarly to the first nanofiltration step, it is preferable to have a step of diluting the liquid to be treated B. In addition, the progress of the step can be grasped based on the above formula (2).

[0144] As the operation control method in the second nanofiltration step, for example, constant flow filtration, low-pressure filtration, etc. can be cited.

[0145] In the case of constant flow filtration, from the viewpoint of the recovery efficiency of Li + , the permeate flow rate is preferably such that a liquid volume of 1% or more relative to the initial liquid volume of the liquid to be treated B permeates within 1 minute, and from the viewpoint of ease of control, it is preferably such that a liquid volume of 50% or less relative to the initial liquid volume of the liquid to be treated B permeates within 1 minute.

[0146] In Figure 1 , Figure 3 , Figure 8 and Figure 9 In the examples shown, the nanofiltration step of solution X is carried out in the nanofiltration membrane unit A (2a), the permeate A that permeates through the nanofiltration membrane unit A and is stored in the second tank 5b is transported as the liquid to be treated B to the third tank 5c, the liquid to be treated B is transported to the nanofiltration membrane unit B (2b), and the permeate C is transported to the fourth tank 5d at a constant flow rate, thereby carrying out the second nanofiltration step. At this time, the concentrate D that does not permeate through 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 step can be carried out 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 permeate C.

[0147] In Figure 5 and Figure 10In the example shown, in the nanofiltration membrane unit A (2a), a nanofiltration process of solution X is carried out. The permeate A that permeates through the nanofiltration membrane unit A and is stored in the second tank 5b is transported as the liquid to be treated B to the first tank 5a, and then transported again to the nanofiltration membrane unit A (2a). The permeate C is transported to the second tank 5b at a constant flow rate, thereby implementing the second nanofiltration process. At this time, the concentrate D that does not permeate through the nanofiltration membrane unit A (2a) is mixed with the remaining part of the liquid to be treated B in the first tank 5a. Additionally, the second nanofiltration process can be implemented while adding dilution water to the liquid to be treated B in the first tank 5a at the same flow rate as that of the permeate C.

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

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

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

[0151] (3) Semi-batch treatment process

[0152] As described above, since the liquid composition of Solution X sometimes varies, in the process of continuously transporting N (N: an integer of 2 or more) solutions X containing alkali metal ions, it is very difficult, and in some cases impossible, to stabilize the liquid composition obtained in the treatment process of the nanofiltration membrane and maintain a predetermined lithium purity and recovery rate. In addition, if this instability in the treatment process of the nanofiltration membrane is eliminated and a continuous process capable of recovering lithium with high purity and high efficiency is achieved, the number of stages of the nanofiltration membrane becomes excessive, resulting in problems of increased cost and further complication of the process. In addition, in the batch treatment process, after the treatment of the k-th (k: an integer of 1 or more and (N - 1) or less) solution X(k) is completed, the treatment of the next (k + 1)-th solution X(k + 1) is started, and there is a problem of a longer treatment time.

[0153] Therefore, the method for recovering an alkali metal salt according to the present embodiment is a method for recovering an alkali metal salt in which N (N: an integer of 2 or more) solutions X are sequentially subjected to a treatment of obtaining a permeate C from the solution X through Step 1 and Step 2. Preferably, in Step 2, a nanofiltration membrane unit B is used. During the period when the second nanofiltration step (Step 2) is performed after the first nanofiltration step (Step 1) is performed on the k-th solution X(k) (k: an integer of 1 or more and (N - 1) or less) among the N solutions X, the first nanofiltration step (Step 1) is concurrently performed on the (k + 1)-th solution X(k + 1) among the N solutions X. Thus, by adopting a semi-batch treatment step structure in which the batch treatment steps in Steps 1 and 2 are performed semi-continuously on a plurality of solutions, alkali metal ions can be effectively recovered.

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

[0155] 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 still more preferably 70% or more and 85% or less.

[0156] In the second nanofiltration step, by making the recovery rate of the alkali metal 95% or less, the permeation of the permeate with 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 with 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.

[0157] The remaining portion of the liquid to be treated B(k) is the liquid obtained by filtering the solution X(k) once through a nanofiltration membrane. If the compositions of the solution X(m) and the solution X(k) do not vary significantly, the alkali metal ratio is higher than that of the solution X(m). Therefore, by adding the remaining portion of the liquid to be treated B(k) to the solution X(m) or the liquid to be treated A(m), all the alkali metals in the remaining portion of the liquid to be treated B(k) can be recovered, and the alkali metal ratio of the solution X(m) is also increased. Consequently, the alkali metal ratio of the permeate in the first nanofiltration step of the solution X(m) is also increased. That is, the purity and recovery rate of the alkali metal are improved.

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

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

[0160] Step 4: There are N (N: an integer of 2 or more) of the solution X. After the end of step 2 of the k-th solution X(k) (k: an integer of 1 or more and (N - 1) or less) among the N solution X, a step of adding the remaining portion of the k-th liquid to be treated B(k) mixed with the k-th concentrate D(k) to the 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).

[0161] Figure 10 This is an example of implementing step 4, including a step of adding the remaining portion of the k-th liquid to be treated B(k) mixed with the k-th concentrate D(k) to the solution X (m: an integer of (k + 1) or more and N or less) after the end of step 2 of the solution X(k).

[0162] In addition, if combined with the semi-batch treatment step and step 4 is set as the following step 5, the alkali metal salt can be recovered more effectively.

[0163] Step 5: After the end of step 2 of the solution X(k), a step of adding the remaining portion of the k-th liquid to be treated B(k) mixed with the k-th concentrate D(k) to the solution X(p: an integer of (k + 2) or more and N or less) or the p-th liquid to be treated A(p).

[0164] Figure 8This is an example of implementing the said process 5, including the process of adding the remaining portion of the k-th treated liquid B(k) mixed with the k-th concentrate D(k) to the solution X(p: an integer greater than or equal to (k + 2) and less than or equal to N) after the end of the said process 2 of the solution X(k).

[0165] In Figures 1 to 3 、 Figure 8 and Figure 9 firstly, for the treated liquid A(1) obtained by transporting the solution X(1) to the ultrafiltration membrane unit 1 described later, at the stage where the treatment of the nanofiltration membrane unit A(2a) is completed and all the permeate A(1) is transported to the second tank 5b, the solution in the first tank 5a is recovered into an arbitrary tank. Then, the solution X(2) is transported to the ultrafiltration membrane unit 1 described later, and the obtained treated liquid A(2) is stored in the first tank 5a, and the processes are carried out in sequence. For the solutions X(3) to X(N), the processes are also carried out in the same order.

[0166] In addition, in the method for recovering alkali metal salts according to the present embodiment, it is preferable to perform process 1 and process 2 on all N solutions containing alkali metal ions, that is, from the first solution X(1) to the N-th solution X(N). In addition, processes other than the above-mentioned processes 1 and 2 can also be performed on the N solutions. For example, a reverse osmosis filtration process (process 3) or an ultrafiltration process described later can also be performed.

[0167] (4) Reverse osmosis filtration process

[0168] The method for recovering alkali metal salts according to the present embodiment preferably includes a reverse osmosis filtration process for concentrating at least one of the permeate A obtained in the first nanofiltration process and the permeate C obtained in the second nanofiltration process.

[0169] Among them, in the case where there are N solutions X, the method for recovering alkali metal salts according to the present embodiment preferably further includes the following process 3.

[0170] Process 3: A reverse osmosis filtration process for concentrating at least one of the k-th permeate A(k) and the k-th permeate C(k) in at least one solution X(k).

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

[0172] As an operation control method in the reverse osmosis filtration process, for example, constant flow filtration, low-pressure filtration, etc. can be cited.

[0173] In the case of filtration at a constant flow rate, from the viewpoint of the recovery efficiency of Li + Regarding the permeate flow rate, it is preferably 1% or more of the liquid volume of the permeate A obtained in the first nanofiltration step or the permeate C obtained in the second nanofiltration step per minute. From the viewpoint of ease of control, it is preferably 50% or less of the liquid volume of the permeate A obtained in the first nanofiltration step or the permeate C obtained in the second nanofiltration step per minute.

[0174] (4-1) Reverse osmosis membrane

[0175] In the reverse osmosis filtration step, any reverse osmosis membrane that does not permeate alkali metal ions can be used. By using such a reverse osmosis membrane, the loss of lithium ions during the concentration process of alkali metal ions, especially lithium ions, is extremely small, and high-efficiency recovery of lithium ions can be stably achieved. The higher the ion rejection rate of the reverse osmosis membrane, the higher the efficiency of the process. However, membranes with high rejection rates usually lack water permeability. Therefore, it is preferable to select considering the balance.

[0176] Especially when the solution X contains neutral molecules such as boron compounds represented by boric acid that do not have a charge under conditions where the pH value is 3 or less, the neutral molecules are not removed in the nanofiltration step and are also included in the permeate A and permeate C obtained in the nanofiltration step. Therefore, it is preferable to remove the neutral molecules while concentrating the alkali metal ions through the reverse osmosis filtration step. That is, it is preferable that the reverse osmosis membrane does not permeate alkali metal ions but permeates neutral molecules. In particular, a low rejection reverse osmosis membrane with an isopropanol rejection rate of 70% or more and less than 85% when an isopropanol aqueous solution at 25°C and a pH value of 6.5 is permeated under an operating pressure of 0.5 MPa is preferable in that it does not permeate alkali metal ions but permeates neutral molecules. Here, the low rejection reverse osmosis membrane is a membrane that concentrates at least one of the permeate A(k) and the permeate C(k), at least one of the permeate A(k) and the permeate C(k) contains neutral molecules that do not have a charge under conditions where the pH value is 3 or less, and the low rejection reverse osmosis membrane is a reverse osmosis filtration membrane used in the reverse osmosis filtration step.

[0177] As materials for the reverse osmosis membrane, for example, polymers such as cellulose acetate-based polymers, polyamides, sulfonated polysulfones, polyacrylonitriles, polyesters, polyimides, and vinyl polymers are used. The reverse osmosis membrane can be composed of only one material or multiple materials. In addition, its membrane structure can also be an asymmetric membrane having a dense layer on at least one side of the membrane and gradually having fine pores with large pore diameters from the dense layer to the inside of the membrane or the other side, or a composite semi-permeable membrane having a very thin separation functional layer formed of other materials on the dense layer of the asymmetric membrane.

[0178] As a composite semi-permeable membrane used as a reverse osmosis membrane, specifically, for example, a composite semi-permeable membrane having a substrate, a porous support membrane, and a separation functional layer can be cited. Among them, a composite semi-permeable membrane containing polyamide in the separation functional layer is preferred. The separation functional layer containing polyamide is obtained by polycondensing a polyfunctional amine and a polyfunctional acyl halide on the porous support membrane.

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

[0180] In the method for recovering an alkali metal salt according to the present embodiment, the reverse osmosis filtration step is preferably performed at least once on at least one of the permeates A and C of the nanofiltration step. That is, preferably in the solution X(k), at least one of the k-th permeate A(k) obtained in the first nanofiltration step (step 1) and the k-th permeate C(k) obtained in the second nanofiltration step (step 2) is performed at least once.

[0181] Regarding the number of times of the reverse osmosis filtration step, it is more preferable to perform the reverse osmosis filtration step only once on the permeate C(k) obtained in the second nanofiltration step in the solution X(k) in terms of shortening the time of the entire process. Furthermore, in the case of performing the second nanofiltration step multiple times, it is preferable to perform the reverse osmosis filtration step only once on the permeate C(k) obtained in the last second nanofiltration step.

[0182] As described above, when the solution X contains neutral molecules such as boron compounds represented by boric acid that do not have a charge under the condition that the pH value is 3 or less, it is preferable to concentrate the alkali metal ions while removing the neutral molecules using a low-removal reverse osmosis membrane. At this time, in terms of effectively removing the neutral molecules and concentrating the alkali metal ions, it is preferable that the reverse osmosis filtration step includes a circulation step of mixing the concentrated liquid obtained in the reverse osmosis filtration step with the solution supplied to the reverse osmosis filtration step. In the case of having a circulation step, from the viewpoint of the operating pressure, the period for implementing the circulation step is preferably within the range up to 90% of the pressure resistance value of the reverse osmosis membrane unit.

[0183] In addition, in the above case, the permeate of the low rejection reverse osmosis membrane contains neutral molecules, so it is not suitable for use as dilution water for the liquid to be treated A or the liquid to be treated B in the nanofiltration process. The concentration of neutral molecules in the dilution water is preferably 1% or less, more preferably 0.1% or less, and still more preferably 0.01% or less, relative to the concentration of neutral molecules (mg / L) in the liquid to be treated. By being 1% or less relative to the concentration of neutral molecules (mg / L) in the liquid to be treated, when performing the semi-batch treatment process of the liquid to be treated using the permeate of the low rejection reverse osmosis membrane as dilution water, the accumulation of neutral molecules in the system can be effectively prevented. When using the permeate of the low rejection reverse osmosis membrane as dilution water, it is preferable to transport the permeate of the low rejection reverse osmosis membrane to a high rejection reverse osmosis membrane unit equipped with a high rejection reverse osmosis membrane defined below to remove neutral molecules. By passing through the high rejection reverse osmosis membrane, the concentration of neutral molecules in the permeate of the low rejection reverse osmosis membrane can be brought within the above range suitable for dilution water.

[0184] The "high rejection reverse osmosis membrane" refers to a reverse osmosis membrane with an isopropanol rejection rate of 85-95% when an aqueous isopropanol solution at 25 °C and a pH value of 6.5 permeates under an operating pressure of 0.5 MPa.

[0185] Figure 1 , Figure 5 , Figure 8 and Figure 10 The examples shown in

[0186] Figure 9The example shown is a flow chart of a process for recovering an alkali metal salt, which includes a reverse osmosis filtration step for concentrating the permeate C and a dilution step for using the permeate obtained in the reverse osmosis filtration step as dilution water for the liquid to be treated A after treatment with a high rejection reverse osmosis membrane. Specifically, the permeate C that passes through the nanofiltration membrane unit B (2b) and is stored in the fourth tank 5d is transported to the fifth tank 5e, and the liquid in the fifth tank 5e is transported to the first reverse osmosis membrane unit 3a to perform the reverse osmosis filtration step. The concentrated liquid obtained in the reverse osmosis filtration step is transported to the sixth tank 5f. The liquid in the sixth tank 5f can be recycled to any tank. Furthermore, the permeate obtained in the reverse osmosis filtration step is transported to the high rejection reverse osmosis membrane unit 4. The concentrated liquid obtained in the high rejection reverse osmosis membrane unit 4 is discharged, and the permeate is supplied to the first tank 5a and can be used as dilution water for the liquid to be treated A.

[0187] Figure 3 The example shown is a flow chart of a process for recovering an alkali metal salt, which includes a reverse osmosis filtration step for concentrating the permeate C and a dilution step for using the permeate obtained in the reverse osmosis filtration step as dilution water for the liquid to be treated A after treatment with a high rejection reverse osmosis membrane. The reverse osmosis filtration step includes a step of circulating the concentrated liquid. Specifically, the permeate C that passes through the nanofiltration membrane unit B (2b) and is stored in the fourth tank 5d is transported to the fifth tank 5e, and the liquid in the fifth tank 5e is transported to the first reverse osmosis membrane unit 3a. While mixing the concentrated liquid obtained in the first reverse osmosis membrane unit 3a into the fifth tank 5e, the reverse osmosis filtration step is performed. Furthermore, the permeate obtained in the reverse osmosis filtration step is transported to the high rejection reverse osmosis membrane unit 4. The concentrated liquid obtained in the high rejection reverse osmosis membrane unit 4 is discharged, and the permeate is supplied to the first tank 5a and can be used as dilution water for the liquid to be treated A. After the reverse osmosis filtration step is completed, the concentrated liquid of the permeate C, that is, the liquid in the fifth tank 5e, can be recycled to any tank.

[0188] Figure 2The example shown is a flowchart of a process for recovering alkali metal salts, which includes a reverse osmosis filtration step for concentrating permeate A, a reverse osmosis filtration step for concentrating permeate C, and a dilution step in which the permeate obtained in each reverse osmosis filtration step is used as dilution water for the liquid to be treated A. Specifically, first, the liquid in the second tank 5b that has passed through the nanofiltration membrane unit A (2a) is transported to the fifth tank 5e. The liquid in the fifth tank 5e is transported to the first reverse osmosis membrane unit 3a, the resulting concentrated liquid is transported to the sixth tank 5f, and the permeate is added to the liquid to be treated A as dilution water. Next, the liquid in the sixth tank 5f is transported to the third tank 5c. The liquid in the third tank 5c is transported to the nanofiltration membrane unit B (2b) as the liquid to be treated B, and the permeate C stored in the fourth tank 5d is transported to the seventh tank 5g. The liquid in the seventh tank 5g is transported to the second reverse osmosis membrane unit 3b, the resulting concentrated liquid is transported to the eighth tank 5h, and the permeate is added to the liquid to be treated A as dilution water. The liquid in the eighth tank 5h can be recovered into any tank.

[0189] (5) Ultrafiltration step

[0190] For solution X(k), ultrafiltration can be performed before the first nanofiltration step. By ultrafiltration, high-molecular-weight organic substances can be removed, and by removing high-molecular-weight organic substances, fouling of the nanofiltration membrane can be inhibited.

[0191] When solution X(k) is obtained by mixing multiple solutions, these multiple solutions can be ultrafiltered separately. The permeate of the ultrafiltration membrane unit is used as the liquid to be treated A(k) for the first nanofiltration step.

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

[0193] (6) Recovery step

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

[0195] The recovery of alkali metal salts can be carried out by a known method. For example, when the alkali metal salt is a potassium salt, it can be carried out by utilizing the temperature dependence of solubility or by adding a poor solvent such as ethanol.

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

[0197] (7) Comparison method

[0198] Figure 4 , Figure 6 and Figure 7 are schematic flowcharts showing the recovery processes of alkali metal salts in the comparison method.

[0199] Figure 4 The process composition of Figure 1 differs from that of Figure 1 in that the concentrates B of the nanofiltration membrane unit A (2a) and the concentrate D of the nanofiltration membrane unit B (2b) are drained without mixing with the liquids in the first tank 5a and the third tank 5c respectively. Except for this, it is the same as + In the case of the above process, there is a problem that the recovery rate of Li

[0200] Figure 6 The process composition of Figure 1 is the same as the example shown in + except that there is no third tank 5c, nanofiltration membrane unit B (2b) and fourth tank 5d, that is, there is no second nanofiltration process, and the liquid in the second tank 5b is transported to the fifth tank 5e. In the case of the above process, there is a problem that the purity of Li

[0201] Figure 7 The process composition of +The problem of low recovery rate.

[0202] (8) Recovery device for alkali metal salts

[0203] The recovery device for alkali metal salts of the present invention comprises:

[0204] A first separation mechanism that uses a solution containing alkali metal ions as the liquid to be treated A and separates it into permeate A and concentrate B through a first nanofiltration membrane unit;

[0205] A first circulation mechanism that mixes the concentrate B with the remaining part of the liquid to be treated A;

[0206] A second separation mechanism that uses the permeate A or the concentrate of the permeate A as the liquid to be treated B and separates it into permeate C and concentrate D through a second nanofiltration membrane unit;

[0207] A second circulation mechanism that mixes the concentrate D with the remaining part of the liquid to be treated B;

[0208] A dilution mechanism that adds dilution water to at least one of the liquid to be treated A and the liquid to be treated B;

[0209] A flow control mechanism that can control the respective flows of the permeate A and the concentrate B in the first separation mechanism and the permeate C and the concentrate D in the second separation mechanism; and

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

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

[0212] The first separation device is equipped with a first nanofiltration membrane unit. In the first separation device, a solution containing alkali metal ions, i.e., the liquid to be treated A, is separated into permeate A and concentrate B through the first nanofiltration membrane unit,

[0213] In the first circulation device, the concentrate B is mixed with the remaining part of the liquid to be treated A,

[0214] In the second separation device, the permeate A or the concentrate of the permeate A is used as the liquid to be treated B and is separated into permeate C and concentrate D through a second nanofiltration membrane unit,

[0215] In the second circulation device, the concentrate D is mixed with the remaining part of the liquid to be treated B,

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

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

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

[0219] In the first separation device and the second separation device, the nanofiltration membrane unit is preferably a pressure vessel (vessel) having a spiral element filled with a nanofiltration membrane, and is configured to be able to supply a solution containing alkali metal ions to the vessel by a high-pressure pump. The nanofiltration membrane units can be connected in parallel or in series, and multiple nanofiltration membrane elements can be filled in each vessel. The spiral element of the nanofiltration membrane can use an element having an arbitrary diameter and length. The spiral element of the nanofiltration membrane has different sizes according to the membrane area. Among the same membrane types, the larger the membrane area, the more liquid volume can be processed per unit time. The size and number of the spiral elements of the nanofiltration membrane can be arbitrarily determined according to the scale of the liquid to be treated A.

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

[0221] In the first circulation device and the second circulation device, it is preferably provided with a tank (raw water tank) filled with the liquid to be treated and a pipe for circulating the concentrate discharged from the nanofiltration membrane unit to the raw water tank.

[0222] In the dilution device, it is preferably provided with a tank (dilution water tank) filled with dilution water. It is preferably provided with a pump (dilution water feed pump) for feeding the dilution water from the dilution water tank to the raw water tank.

[0223] As the flow control device b, it is preferred that the dilution water feed pump has a mechanism capable of receiving the data of the permeate flow meter at any time and feeding the dilution water at the same flow rate as the permeate flow rate.

[0224] Preferably, the above-mentioned device has raw materials resistant to the liquid property or operating pressure of the liquid to be processed.

[0225] In addition to the above, for the recovery device of the present invention to achieve the recovery of alkali metal salts, pumps, pipes, valves, tanks, containers, temperature control machines, measuring instruments (pH meters, conductivity meters, flow meters, pressure gauges, etc.) can be arbitrarily combined.

[0226] Examples

[0227] Hereinafter, examples are given to illustrate the present invention, but the present invention is not limited by any of these examples. The measurements in the examples and comparative examples are carried out as follows.

[0228] <Performance of nanofiltration membrane and reverse osmosis membrane>

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

[0230] Based on the glucose concentration of the permeate and feed water when a 1000 mg / L glucose aqueous solution at 25 °C and pH 6.5 as the feed water permeates through the nanofiltration membrane at an operating pressure of 0.5 MPa, and the isopropanol concentration of the permeate and feed water when a 1000 mg / L isopropanol aqueous solution at 25 °C and pH 6.5 permeates through the nanofiltration membrane at an operating pressure of 0.5 MPa, the isopropanol removal rate and glucose removal rate are calculated using the following formula.

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

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

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

[0234] Based on the magnesium sulfate concentration of 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 as the feed water permeates through the nanofiltration membrane at an operating pressure of 0.5 MPa, and the magnesium chloride concentration of 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 permeates through the nanofiltration membrane at an operating pressure of 0.5 MPa, the MgSO4 removal rate and MgCl2 removal rate are calculated using the following formula.

[0235] Regarding the concentrations of magnesium sulfate and magnesium chloride, the conductivities of the feed water and the permeate were measured using a conductivity meter manufactured by Toa Denpa Kogyo Co., Ltd., and the respective practical salt contents, namely the MgSO4 concentration and the MgCl2 concentration, were obtained.

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

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

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

[0239] For the nanofiltration membrane A and the nanofiltration membrane B described below, the average pore diameter was derived.

[0240] The positron annihilation lifetime measurement of the separation functional layer was carried out as follows using the positron beam method. The composite semipermeable membrane was freeze-dried under reduced pressure at -30°C and cut into a square of 1.5 cm × 1.5 cm as the test sample. Using a thin-film corresponding positron annihilation lifetime measurement device equipped with a positron beam generation device (this device is described in detail, for example, in Radiation Physics and Chemistry, 58, 603, Pergamon (2000)), at a beam intensity of 0.1 keV and 0.5 keV and at room temperature in a vacuum, the separation functional layer side of the test sample was measured using a photomultiplier tube through a barium fluoride scintillation counter with a total count of 5 million, and analysis was performed using POSITRONFIT. Based on the average lifetime τ of the third component obtained from the analysis, using the Tao-Eldrup formula, the average pore diameter at a beam intensity of 0.1 keV was taken as R1, and the average pore diameter at a beam intensity of 0.5 keV was taken as R2 and derived, and R1 / R2 was calculated. The value obtained was used as the "pore size distribution".

[0241] (Solution X)

[0242] To make the Li + , Ni 2+ , Co 2+ , Mn 2+ concentrations the same as those of the rare metal-containing acidic aqueous solution A described in Table 1 of Patent Document 2, lithium sulfate, nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in water, and the pH value was adjusted to 1 with sulfuric acid. Further, this aqueous solution was diluted 1.2 times with a sulfuric acid aqueous solution having a pH value of 1 to prepare Solution Xa (Serial No. 1).

[0243] Relative to Solution Xa (Serial No. 1), except for changing the Li+ Except for setting the concentration to 1 / 2, the solution was prepared in the same manner as Solution Xa (No. 2).

[0244] With respect to Solution Xa (No. 1), except for setting the concentration of Li + to 1 / 4, the solution was prepared in the same manner as Solution Xa (No. 3).

[0245] With respect to Solutions Xa (No. 1 to 3), except for adjusting the pH value to 3.7, Solutions Xb (No. 1 to 3) were prepared in the same manner respectively.

[0246] With respect to Solutions Xa (No. 1 to 3), except for adding boric acid respectively to make the boron concentration 50 mg / L, the solutions were prepared in the same manner respectively as Solution Xc (No. 1 to 3).

[0247] For the solutions obtained above, using a P-4010 type ICP (high-frequency inductively coupled plasma optical emission spectrometry) apparatus manufactured by Hitachi, Ltd., the concentrations of various ions were quantified, and the results are shown in Table 1.

[0248] In addition, the liquid volumes of No. 1 to 3 of Solution Xa, Solution Xb, and Solution Xc were each 1000 L.

[0249] Table 1

[0250]

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

[0252] (Nanofiltration Membrane A)

[0253] On a non-woven fabric made of polyester fibers (air permeability: 1 cc / cm 2 / s), a 18.0 mass% solution of polysulfone in dimethylformamide (DMF) was cast at a thickness of 180 μm at room temperature (25 °C), and immediately immersed in pure water and allowed to stand for 5 minutes, thereby fabricating a porous support membrane made of fiber-reinforced polysulfone (thickness: 160 μm).

[0254] Next, air adjusted to 25°C is blown to remove excess moisture, and at the same time, the membrane surface temperature of the porous support membrane is adjusted to 25°C. An aqueous solution at 30°C containing 2.0% by mass of piperazine, 250 ppm of dodecyl diphenyl ether disulfonate, and 1.0% by mass of trisodium phosphate is coated on the surface of the porous support membrane. After standing for 15 seconds, nitrogen is blown from an air nozzle to remove the excess aqueous solution, thereby forming a coating layer of the amine aqueous solution on the porous support membrane. Further, a 38°C n-decane solution containing 0.2% by mass of trimesoyl chloride (hereinafter referred to as "TMC") is uniformly coated on the entire surface of the porous support membrane, and then left standing at 70% relative humidity and 25°C for 1 minute. Thereby, interfacial polycondensation is carried out, and two kinds of fluids (pure water and air) are blown to the membrane surface to remove the solution on the surface. Then, it is washed with pure water at 80°C to obtain the nanofiltration membrane A.

[0255] (Nanofiltration membrane B)

[0256] Using 2,5-dimethylpiperazine as piperazine, after uniformly coating a 38°C n-decane solution containing 0.2% by mass of TMC on the entire surface of the porous support membrane, it is left standing at 80% relative humidity and 25°C for 1 minute. Except for this, the nanofiltration membrane is prepared in the same manner as the nanofiltration membrane A to obtain the nanofiltration membrane B.

[0257] (Nanofiltration membrane E)

[0258] SelRO (registered trademark) MPS-34 of Koch Company is used as the nanofiltration membrane E.

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

[0260] The nanofiltration membrane A, the nanofiltration membrane B, and the nanofiltration membrane E are respectively wound into a spiral shape by an arbitrary method and used as a membrane element (hereinafter referred to as "8-inch element") with a diameter of 20.32 cm and a length of 102 cm.

[0261] Table 2

[0262]

[0263] (Reverse osmosis membrane C)

[0264] A porous support membrane was fabricated in the same manner as the nanofilter membrane A. Air at 25 °C was blown to remove excess moisture, and at the same time, the surface temperature of the porous support membrane was adjusted to 25 °C. After impregnating in an aqueous solution containing 5.0 mass% of m-phenylenediamine (hereinafter referred to as "m-PDA") for 15 seconds, nitrogen was blown from an air nozzle to remove the excess aqueous solution. Furthermore, a 30 °C n-decane solution containing 0.18 mass% of TMC was uniformly coated on the entire surface of the porous support membrane, and then left standing at 30 °C for 1 minute. Two kinds of fluids (pure water and air) were blown onto the membrane surface to remove the surface solution. Then, it was washed with pure water at 80 °C to obtain a reverse osmosis membrane C.

[0265] (Reverse osmosis membrane D)

[0266] m-PDA was set to 1.8 mass%, and furthermore, TMC was changed to 0.07 mass%. Except for this, it was fabricated in the same manner as the reverse osmosis membrane C.

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

[0268] The reverse osmosis membrane C and reverse osmosis membrane D were each wound into a spiral shape by an arbitrary method and used as 8-inch elements.

[0269] Table 3

[0270]

[0271] From the results in Table 3, it can be seen that the reverse osmosis membrane C is a high rejection reverse osmosis membrane, and the reverse osmosis membrane D is a low rejection reverse osmosis membrane.

[0272] <Evaluation of the recovery of alkali metal salts>

[0273] (Alkali metal ion ratio)

[0274] Using the concentrations of various ions in the solution, the alkali metal ion ratio was calculated by the following formula.

[0275] Alkali metal ion ratio = lithium ion concentration / (cobalt ion concentration + nickel ion concentration + manganese ion concentration)

[0276] (Li + recovery rate)

[0277] The Li + recovery rate was calculated by the following formula.

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

[0279] In addition, in solution X(k), when the remaining portion of the liquid to be treated B(k) in the second nanofiltration step is recovered, the Li contained in the remaining portion of the liquid to be treated B(k) + is recovered by being added to the liquid to be treated A(m) (m: an integer of (k + 1) or more and N or less) or the liquid to be treated A(p) (p: an integer of (k + 2) or more and N or less), and thus is regarded as being recovered. The Li in solution X(k) + recovery rate (k) is calculated by the following formula. The following formula was used in Example 9, but when the number of solutions N = 3, even if k = 2, 3, the Li contained in the remaining portion of the liquid to be treated B(k) in the second nanofiltration step is regarded as + being recovered.

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

[0281] (Li + purity)

[0282] Li + The purity is set to the ratio of alkali metal ions in the liquid finally concentrated by the reverse osmosis membrane unit.

[0283] (Boron concentration ratio)

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

[0285] (Total treatment time)

[0286] The total processing time is defined as the total time required to complete the processing of solutions X with serial numbers 1 to 3.

[0287] (Example 1)

[0288] In Figure 1 In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membrane for the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), and reverse osmosis membrane C is used as the reverse osmosis membrane for the first reverse osmosis membrane unit 3a. A semi-batch treatment process is carried out in the order of serial numbers 1 to 3 of solution Xa, and the recovery of alkali metal salts is carried out. In addition, the pressure resistance value of the first reverse osmosis membrane unit 3a is 8 MPa. The nanofiltration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the nanofiltration process is confirmed by monitoring the operating pressure using Equation (2). The alkali metal ion recovery rate in the first nanofiltration process is 93%, and in the second nanofiltration process, filtration is continued until the alkali metal ion recovery rate reaches 96%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min and continues until the operating pressure reaches 7 MPa. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0290] (Example 2)

[0291] Except for not using solution Xa but using serial numbers 1 to 3 of solution Xb, the recovery process of alkali metal salts is carried out in the same manner as in Example 1.

[0292] The results of implementing this process are shown in Table 4. When the pH value is as high as 3.7, compared with Example 1 with a pH value of 1.0, the permeation rate of alkali metal ions decreases and the total processing time increases, but lithium ions can be recovered with high purity and high recovery rate.

[0293] (Example 3)

[0294] In Figure 2In the process configuration shown, nanofiltration membrane A is used as the nanofiltration membranes of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), and reverse osmosis membrane C is used as the reverse osmosis membranes of the first reverse osmosis membrane unit 3a and the second reverse osmosis membrane unit 3b. The semi-batch treatment process is carried out in the order of serial numbers 1 to 3 of the solution Xa, and the recovery process of the alkali metal salt is carried out. In addition, the pressure resistance value of the second reverse osmosis membrane unit 3b is 8 MPa. The nanofiltration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rates of the first nanofiltration process and the second nanofiltration process are confirmed by monitoring the operating pressure using formula (2) respectively. The alkali metal ion recovery rate in the first nanofiltration process is 93%, and in the second nanofiltration process, filtration is continued until the alkali metal ion recovery rate reaches 96%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 15 L / min respectively, and continues until the operating pressure reaches 7 MPa. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

[0295] The results of implementing this process are shown in Table 4. If concentration is carried out using the reverse osmosis membrane unit each time after the nanofiltration process, compared with the case where RO concentration is carried out only once after the nanofiltration process in the subsequent stage, although the total processing time slightly increases, lithium ions can be recovered with high purity and high recovery rate.

[0296] (Example 4)

[0297] The recovery process of the alkali metal salt is carried out in the same manner as in Example 1 except that nanofiltration membrane B is used.

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

[0299] (Example 5)

[0300] The recovery process of the alkali metal salt is carried out in the same manner as in Example 4 except that instead of monitoring the operating pressure using formula (2), the alkali metal ion recovery rate of the nanofiltration process is confirmed by appropriately analyzing the permeate.

[0301] The results of implementing this process are shown in Table 4. Since appropriate analysis takes time, the processing time increases compared with the case of monitoring the operating pressure using formula (2).

[0302] (Example 6)

[0303] The recovery process of the alkali metal salt is carried out in the same manner as in Example 4 except that solution Xc is used.

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

[0305] (Example 7)

[0306] In Figure 9 In the process configuration shown, as the nanofiltration membranes of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), nanofiltration membrane B is used. As the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, reverse osmosis membrane D is used. As the reverse osmosis membrane of the high-removal reverse osmosis membrane unit 4, reverse osmosis membrane D is used. The semi-batch treatment process is carried out in the order of solution Xc1 to 3, and the recovery process of the alkali metal salt is implemented. The first nanofiltration process and the second nanofiltration process are carried out by constant flow filtration 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 are confirmed by monitoring the operating pressure using Equation (2). In the first nanofiltration process, the alkali metal ion recovery rate is 93%, and in the second nanofiltration process, filtration is continued until the alkali metal ion recovery rate reaches 96%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min and continues until the operating pressure reaches 7 MPa. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0308] (Example 8)

[0309] In Figure 3 In the process configuration shown, as the nanofiltration membranes of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), nanofiltration membrane B is used. As the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, reverse osmosis membrane D is used. As the reverse osmosis membrane of the high-removal reverse osmosis membrane unit 4, reverse osmosis membrane D is used. The semi-batch treatment process is carried out in the order of solution Xc1 to 3, and the recovery process of the alkali metal salt is implemented. The first nanofiltration process and the second nanofiltration process are carried out by constant flow filtration 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 are confirmed by monitoring the operating pressure using Equation (2). In the first nanofiltration process, the alkali metal ion recovery rate is 93%, and in the second nanofiltration process, filtration is continued until the alkali metal ion recovery rate reaches 96%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min and continues until the operating pressure reaches 7 MPa. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0311] (Example 9)

[0312] In Figure 8 In the process configuration shown, the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b) uses the nanofiltration membrane A, the reverse osmosis membrane of the first reverse osmosis membrane unit 3a uses the reverse osmosis membrane C, and the semi-batch treatment process is carried out in the order of the serial numbers 1 to 3 of the solution Xa, and the recovery of the alkali metal salt is carried out. At this time, the treated liquid B(1) after the process 2 in the solution Xa1 (that is, the solution X(1)) is added to the treated liquid A(3) in the solution Xa3 (that is, the solution X(3)). In addition, the pressure resistance value of the first reverse osmosis membrane unit 3a is 8 MPa. The nanofiltration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min. The recovery rate of alkali metal ions in the nanofiltration process is confirmed by monitoring the operating pressure using Equation (2). The recovery rate of alkali metal ions in the first nanofiltration process is 93%, and in the second nanofiltration process, filtration is continued until the recovery rate of alkali metal ions reaches 80%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min until the operating pressure reaches 7 MPa. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0314] (Comparative Example 1)

[0315] In Figure 4 In the process configuration shown, the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b) uses the nanofiltration membrane A, the reverse osmosis membrane of the first reverse osmosis membrane unit 3a uses the reverse osmosis membrane C, and the semi-batch treatment process is carried out in the order of the serial numbers 1 to 3 of the solution Xa, and the recovery process of the alkali metal salt is carried out. In addition, in all processes, it is carried out at a permeate flow rate of 60 L / min. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0317] (Example 10)

[0318] In the process configuration shown in Figure 5 , the nanofiltration membrane of the nanofiltration membrane unit A (2a) is the nanofiltration membrane A, and the reverse osmosis membrane of the first reverse osmosis membrane unit 3a is the reverse osmosis membrane C. The recovery process of the alkali metal salt is carried out in the order of serial numbers 1 to 3 of the solution Xa. In addition, the pressure resistance value of the first reverse osmosis membrane unit 3a is 8 MPa. The nanofiltration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min. The recovery rate of alkali metal ions in the nanofiltration process is confirmed by monitoring the operating pressure using formula (2). The recovery rate of alkali metal ions in the first nanofiltration process is 93%, and in the second nanofiltration process, filtration is continued until the recovery rate of alkali metal ions reaches 80%. The reverse osmosis filtration process is carried out by constant flow filtration with a permeate flow rate of 60 L / min and continues until the operating pressure reaches 7 MPa. In addition, in all processes, it is carried out at a permeate flow rate of 60 L / min. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0320] (Comparative Example 2)

[0321] In the Figure 6 system configuration shown, the nanofiltration membrane of the nanofiltration membrane unit A (2a) is the nanofiltration membrane A, and the reverse osmosis membrane of the first reverse osmosis membrane unit 3a is the reverse osmosis membrane C. The semi-batch treatment process is carried out in the order of serial numbers 1 to 3 of the solution Xa, and the recovery process of the alkali metal salt is implemented. In addition, in all processes, it is carried out at a permeate flow rate of 60 L / min. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0323] (Comparative Example 3)

[0324] In the Figure 7 process configuration shown, the nanofiltration membranes of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b) are the nanofiltration membrane A, and the reverse osmosis membrane of the first reverse osmosis membrane unit 3a is the reverse osmosis membrane C. The continuous treatment process is carried out in the order of serial numbers 1 to 3 of the solution Xa, and the recovery process of the alkali metal salt is implemented. In addition, in all processes, it is carried out at a permeate flow rate of 60 L / min. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements are connected in series for use.

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

[0326] (Comparative Example 4)

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

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

[0329] (Example 11)

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

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

[0332] (Example 12)

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

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

[0335] (Example 13)

[0336] In Figure 10 In the process configuration shown, 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. The recovery process of the alkali metal salt was implemented in the order of numbers 1 to 3 of the solution Xa. In addition, the pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was implemented by constant flow filtration with a permeate flow rate of 60 L / min. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using Equation (2). In the first nanofiltration process, the alkali metal ion recovery rate was 93%, and in the second nanofiltration process, filtration was continued until the alkali metal ion recovery rate reached 80%. The reverse osmosis filtration process was implemented by constant flow filtration with a permeate flow rate of 60 L / min and continued until the operating pressure reached 7 MPa. In addition, in all processes, it was implemented in such a way that the permeate flow rate was 60 L / min. In addition, in each nanofiltration membrane unit and reverse osmosis membrane unit, 4 8-inch elements were connected in series and used.

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

[0338] Table 4

[0339]

[0340] Table 5

[0341]

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

[0343] From the above results, it can be seen that the method for recovering alkali metal salts of the present invention, namely Examples 1 to 8, can recover alkali metal salts with high purity and high recovery rate and in a short time compared with Comparative Examples 1 to 4. In addition, from the results of Examples 6 to 8, it can be seen that even in the case of containing neutral molecules such as boron, they can be efficiently removed, and the total treatment time can be shortened.

[0344] The present invention has been described in detail with reference to specific embodiments, but it is obvious 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. In addition, this application is based on the Japanese Patent Application (Japanese Patent Application No. 2022-192379) filed on November 30, 2022, and the whole of it is incorporated herein by reference. In addition, all references cited herein are incorporated in their entirety.

[0345] Industrial Applicability

[0346] The present invention is suitable as a method for effectively separating and recovering alkali metals such as lithium from waste materials, waste liquids, ores or slag generated from lithium ion batteries or their manufacturing processes.

[0347] Explanation of Reference Numerals

[0348] 1 Ultrafiltration Membrane Unit

[0349] 2a Nanofiltration Membrane Unit A

[0350] 2b Nanofiltration Membrane Unit B

[0351] 3a First Reverse Osmosis Membrane Unit

[0352] 3b Second Reverse Osmosis Membrane Unit

[0353] 4 High Removal Reverse Osmosis Membrane Unit

[0354] 5a First Tank

[0355] 5b Second can

[0356] 5c Third can

[0357] 5d Fourth can

[0358] 5e Fifth can

[0359] 5f Sixth can

[0360] 5g Seventh can

[0361] 5h Eighth can

Claims

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

2. The method for recovering an alkali metal salt according to claim 1, The treatment of obtaining the permeate C from the solution X through Step 1 and Step 2 is sequentially performed on N of the solutions X, where N is an integer of 2 or more. In Step 2, the nanofiltration membrane unit B is used. During the period when Step 2 is performed after Step 1 is performed on the k-th solution X(k) among the N solutions X, Step 1 is performed in parallel on the (k + 1)-th solution X(k + 1), where k is an integer of 1 or more and N - 1 or less.

3. The method for recovering an alkali metal salt according to claim 1 or 2, Comprises 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 comprises the following Step 3: Step 3 is a reverse osmosis filtration step: In at least one of the solutions X(k), at least one of the k-th permeate A(k) and the k-th permeate C(k) is concentrated.

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

6. The method for recovering an alkali metal salt according to claim 1 or 2, The pH value of the solution X is 4 or less.

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

8. The method for recovering an alkali metal salt according to claim 1 or 2, Comprises the following Step 4: Step 4: There are N solutions X. After Step 2 of the k-th solution X(k) among the N solutions X is completed, the remaining part of the k-th liquid to be treated B(k) mixed with the k-th concentrate D(k) is added to the m-th solution X(m) or the m-th liquid to be treated A(m), where N is an integer of 2 or more, k is an integer of 1 or more and N - 1 or less, and m is an integer of k + 1 or more and N or less.

9. The method for recovering an alkali metal salt according to claim 1 or 2, wherein the nanofiltration membrane included in 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 onto the surface on the separation functional layer side of the nanofiltration membrane, and the average pore diameters R1 and R2 of the separation functional layer derived by positron annihilation lifetime spectrometry satisfy 0.90 ≤ R1 / R2 ≤ 1.10, R1: the average pore diameter under the condition that the positron beam intensity is 0.1 keV, R2: the average pore diameter under the condition that the positron beam intensity is 0.5 keV.

10. The method for recovering an alkali metal salt according to claim 1 or 2, at least one of the step 1 and the step 2 is carried out at a constant permeation flux, the change of the operating pressure with time is monitored, and based on the following formula 2, when the recovery rate A of alkali metal ions reaches the target value, at least one of the step 1 and the step 2 is ended, and the unit of the recovery rate A is %, In Formula 2, the unit of the recovery rate A of the alkali metal ions is %, the unit of the operating pressure P is Pa, the unit of the initial operating pressure P0 is Pa, and the unit of the initial liquid volume V0 of the object to be treated is m 3 , the unit of the removal rate R of the alkali metal ions by the nanofiltration membrane is %, the unit of the liquid recovery rate S of the nanofiltration process is %, and the supply flow rate Q F is in m 3 / s, the concentrate flow rate Q c is in m 3 / s, and the filtration end time t = tb.

11. The method for recovering an alkali metal salt according to claim 4 or 5, at least one of the permeate A(k) and the permeate C(k) contains a neutral molecule that has no charge under the condition that the pH value is 3 or less, and the reverse osmosis filtration membrane used in the reverse osmosis filtration step is a low rejection reverse osmosis membrane that satisfies that the rejection rate of isopropanol when an aqueous isopropanol solution at 25 °C and a pH value of 6.5 permeates under an operating pressure of 0.5 MPa is 70% or more and less than 85%, 12. The method for recovering an alkali metal salt according to claim 11, wherein the neutral molecule is a boron compound.

13. The method for recovering an alkali metal salt according to claim 12, in the step 3, a circulation step is provided in which the concentrate obtained in the reverse osmosis filtration step is mixed with the solution supplied to the reverse osmosis filtration step.

14. The method for recovering an alkali metal salt according to claim 11, a step is provided in which the permeate obtained in the reverse osmosis filtration step is transported to a high rejection reverse osmosis membrane unit, and the obtained permeate is added as dilution water for at least one of the liquid to be treated A and the liquid to be treated B, and the high rejection reverse osmosis membrane unit is provided with a high rejection reverse osmosis membrane that satisfies that the rejection rate of isopropanol when an aqueous isopropanol solution at 25 °C and a pH value of 6.5 permeates under an operating pressure of 0.5 MPa is 85% or more and 95% or less.

15. An apparatus for recovering an alkali metal salt, comprising: a first separation mechanism that uses a solution containing alkali metal ions as the liquid to be treated A and separates it into a permeate A and a concentrate B through a first nanofiltration membrane unit; a first circulation mechanism that mixes the concentrate B with the remaining part of the liquid to be treated A; a second separation mechanism that uses the permeate A or the concentrate of the permeate A as the liquid to be treated B and separates it into a permeate C and a concentrate D through a second nanofiltration membrane unit; a second circulation mechanism that mixes the concentrate D with the remaining part of the liquid to be treated B; A dilution mechanism for adding dilution water to at least one of the liquid to be treated A and the liquid to be treated B; A flow rate control mechanism capable of controlling the flow rates of the permeate A and the concentrate B in the first separation mechanism and the permeate C and the concentrate D in the second separation mechanism; and A flow rate control mechanism that synchronizes the addition flow rate of dilution water in the dilution mechanism with the permeate flow rate when the liquid to be treated to which dilution water is added is delivered to the nanofiltration membrane unit.

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