Method for separating alkali metal ions, critical ph determination method, and device for separating alkali metal ions

Abstract

The present invention provides a method by which alkali metal ions can be stably separated from a lithium-ion battery, a waste liquid generated in the manufacturing process thereof, or the like. The present invention pertains to a method for separating alkali metal ions, the method comprising: a nanofiltration step for separating a liquid to be treated containing alkali metal ions into a permeated liquid and a concentrated liquid by means of a nanofiltration membrane unit, and feeding the concentrated liquid to the nanofiltration membrane unit again; a dilution step for adding dilution water to at least one among the liquid to be treated and the concentrated liquid; and a pH control step for adjusting the pH of at least one among the liquid to be treated and the concentrated liquid to a value equal to or lower than a critical pH.

Description

Method for separating alkali metal ions, method for determining critical pH, and apparatus for separating alkali metal ions

[0001] The present invention relates to a method for separating alkali metal ions, a method for determining critical pH, and an apparatus for separating alkali metal ions.

[0002] In recent years, the demand for mineral resources has increased significantly due to global economic development. For example, lithium is in high demand as a material for lithium-ion batteries, and lithium carbonate is also used as an additive for heat-resistant glass and in surface acoustic wave filters. High-purity lithium is used in filters and transmitters for mobile phones and car navigation systems. Lithium is mainly produced by refining brine or lithium-containing ore. However, with the recent increase in demand for lithium, efforts are underway to secure lithium resources.

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

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

[0005] For example, while the practical application of lithium recovery from used lithium-ion batteries has progressed, the predominant method is solvent extraction using a chelating agent, which has problems such as a large environmental impact and cost disadvantages (Non-Patent Document 1). To solve this problem, separation and recovery methods using separation membranes such as ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes from an aqueous solution obtained by acid leaching used lithium-ion batteries have been investigated, and a one-stage process using a nanofiltration membrane has been disclosed (Patent Document 1). However, the method described in Patent Document 1 has problems such as low lithium purity and recovery rate.

[0006] As a means for solving the above problems, a separation and recovery method using a nanofiltration membrane in multiple stages has been disclosed (Patent Document 2). That is, it is a continuous process in which the liquid that has passed through the nanofiltration membrane is passed through another nanofiltration membrane to improve the lithium purity, and the liquid that has not passed through the nanofiltration membrane is passed through another nanofiltration membrane to recover the remaining lithium.

[0007] Furthermore, in the method described in Patent Document 2, dilution water is added to the liquid to be treated in order to reduce the increase in osmotic pressure caused by the concentration of the non-permeated liquid, which allows nanofiltration to be carried out continuously and results in a high recovery of lithium.

[0008] International Publication No. 2021 / 215484 International Publication No. 2019 / 018333

[0009] "Report on the FY2017 Mineral Resource Infrastructure Development Survey Project (Basic Survey on the Formulation of a Mineral Resource Securing Strategy) for Exploration and Other Businesses to Promote Mineral Resource Development," Mitsubishi Research Institute, Inc., Environment and Energy Business Division, March 2018.

[0010] However, the method described in Patent Document 2 does not take into account the pH fluctuations of the treated liquid that occur when dilution water is added to the treated liquid. Because nanofiltration membranes are acid-permeable, the pH is prone to fluctuations during the nanofiltration membrane treatment process. Changes in the liquid composition and pH of the treated liquid due to dilution can lead to problems such as membrane fouling due to scale formation and a decrease in permeate quality due to changes in the ion-selective separation properties of the nanofiltration membrane, resulting in a decrease in lithium purity and recovery rate. To stabilize the pH of the treated liquid during the nanofiltration membrane treatment process, acid must be added from outside the system. However, increasing the amount of acid added raises concerns about increased operating costs, increased impurities in the raw water, and accelerated membrane degradation due to the acid. In other words, continuing operation while minimizing the amount of acid added without causing membrane fouling or changes in the ion-selective separation properties of the nanofiltration membrane is important for stabilizing the nanofiltration membrane treatment process and maintaining a predetermined lithium purity and recovery rate. However, the method described in Patent Document 2 lacks the concept of pH control, posing a problem.

[0011] Therefore, an object of the present invention is to provide a method for separating alkali metal ions, a method for determining critical pH, and an apparatus for separating alkali metal ions, which can stably separate alkali metal ions with high purity and high recovery rate in a treatment process in which changes occur in the liquid composition and pH of the liquid to be treated.

[0012] To achieve the above-mentioned object, the present invention encompasses the following configurations [1] to

[12] . [1] A method for separating alkali metal ions, comprising: a nanofiltration step in which a liquid to be treated containing alkali metal ions is separated into a permeate and a concentrate using a nanofiltration membrane unit and the concentrate is sent back to the nanofiltration membrane; a dilution step in which dilution water is added to at least one of the liquid to be treated and the concentrate; and a pH control step in which the pH of at least one of the liquid to be treated and the concentrate is adjusted to a critical pH or lower. [2] The method for separating alkali metal ions according to [1], wherein the pH control step adjusts the pH of at least one of the liquid to be treated and the concentrate to a value equal to or higher than 0 and lower than the critical pH. [3] The method for separating alkali metal ions according to [2], wherein the pH control step adjusts the pH of at least one of the liquid to be treated and the concentrate to a value equal to or higher than (critical pH - 1) and lower than the critical pH. [4] The method for separating alkali metal ions according to any one of [1] to [3], wherein the total amount of dilution water added to the liquid to be treated in the dilution step is less than the total amount of permeate obtained in the nanofiltration step. [5] The method for separating alkali metal ions according to any one of [1] to [4], wherein the dilution step includes a step of adjusting the amount of dilution water added so that the operating pressure in the nanofiltration step is 30% to 100% of the pressure resistance value of the nanofiltration membrane provided in the nanofiltration membrane unit. [6] The method for separating alkali metal ions according to any one of [1] to [5], wherein the dilution step includes a step of adjusting the amount of dilution water added so that the operating pressure in the nanofiltration step is 50% to 100% of the pressure resistance value of the nanofiltration membrane provided in the nanofiltration membrane unit. [7] The method for separating alkali metal ions according to any one of [1] to [5], wherein the total processing time T Ａ The treatment time T when the operating pressure of the nanofiltration step is 30% or more and 100% or less of the pressure resistance value of the nanofiltration membrane. ３０ The ratio T ３０ / T Ａ[8] The method for separating alkali metal ions according to any one of [1] to [7], wherein the alkali metal ions include lithium ions. [9] The nanofiltration membrane provided in the nanofiltration membrane unit has a magnesium sulfate removal rate when a 2000 mg / L aqueous magnesium sulfate solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa. The difference between the magnesium chloride removal rate and the magnesium chloride removal rate when a 2000 mg / L aqueous magnesium chloride solution at 25 ° C. and pH 6.5 is permeated is 20% or less, and the difference between the glucose removal rate when a 1000 mg / L aqueous glucose solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa and the isopropyl alcohol removal rate when a 1000 mg / L aqueous isopropyl alcohol solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa is 40% or more, and the glucose removal rate is 70% or more and 90% or less. The method for separating alkali metal ions according to any one of [1] to [8] above.

[10] The method for separating alkali metal ions according to any one of [1] to [9] above, wherein the nanofiltration membrane provided in the nanofiltration membrane unit is a composite semipermeable membrane having a support membrane and a separation functional layer containing a semi-aromatic crosslinked polyamide on the support membrane, and the N / O ratio, which is the ratio of the number of nitrogen atoms N to the number of oxygen atoms O on the surface of the separation functional layer as measured by X-ray photoelectron spectroscopy, is 0.7 to 1.3.

[11] A method for determining a critical pH in a method for separating alkali metal ions, which comprises a nanofiltration step of separating a liquid to be treated containing alkali metal ions into a permeate and a concentrate using a nanofiltration membrane unit and sending the concentrate back to the nanofiltration membrane unit, and a dilution step of adding dilution water to at least one of the liquid to be treated and the concentrate, the method comprising the following steps (a) to (c):(a) Pre-processing: In each nanofiltration membrane unit, if the nanofiltration process is performed under constant flow rate operation, the flow rates of the permeate and concentrate at the start of the dilution process are set as filtration conditions, and the liquid to be treated is sent to the nanofiltration membrane unit under these conditions. If the nanofiltration process is performed under a flow rate other than constant, the flow rate ratio of the permeate to the concentrate and the operating pressure at the start of the dilution process are set as filtration conditions, and the liquid to be treated is sent to each nanofiltration membrane unit. The alkali metal ion rejection rate R is measured from the alkali metal ion concentrations of the permeate and concentrate obtained after one hour of operation using the following formula (2):

[0013]

[0014] In the above formula (3), A is the treatment time (s) per unit alkali metal ion recovery rate, V ０ is the initial amount of liquid to be treated (L), Q Ｔwhere R is the permeate flow rate (L / s), and R is the alkali metal ion rejection rate (%) of the nanofiltration membrane. (b) Critical pH measurement step: For each nanofiltration membrane unit, the same permeate flow rate and concentrate flow rate as in the previous step (a) are set as filtration conditions. The treated liquid is sent to the nanofiltration membrane unit, separated into a permeate and a concentrate, and water with an electrical conductivity of less than 10 μS / cm is sent to the treated liquid at the same flow rate as the permeate. The concentrate is mixed with the remainder of the treated liquid. While measuring the pH of the treated liquid, nanofiltration is continued until the change in operating pressure P of the nanofiltration membrane unit per treatment time t (ΔP / Δt) becomes ΔP / Δt > 0. At this time, Δt is set to 5 A. If there is no time when ΔP / Δt > 0 within a treatment time t of 50 A (t≦50 A), the average pH of the treated liquid at the treatment time Δt at which ΔP / Δt > 0 is first achieved is calculated, and this value is designated as the critical pH. However, the maximum critical pH is 4, and if the average value is 4 or higher, the critical pH is 4. (c) Re-measurement step: If there is a point at which ΔP / Δt>0 within 50 A of treatment time t (t≦50 A), an aqueous sulfuric acid solution of pH 1, prepared by adding sulfuric acid to water with an electrical conductivity of less than 10 μS / cm, is sent to the nanofiltration membrane unit used in the critical pH measurement step (b) above, and the nanofiltration membrane unit is operated for one hour under the same operating conditions as the previous step (a) above to clean the unit. After cleaning, the pH of the liquid to be treated is lowered by 0.5, and the critical pH measurement step (b) above is performed again. This step is repeated until the point at which ΔP / Δt>0 becomes 50 A<t.

[12] An alkali metal ion separation device comprising: a nanofiltration means for separating a liquid to be treated containing alkali metal ions into a permeate liquid and a concentrate; a dilution means for adding dilution water to at least one of the liquid to be treated and the concentrate; a circulation means for mixing the concentrate with the remainder of the liquid to be treated; a pH measurement means for measuring the pH of at least one of the liquid to be treated and the concentrate; an acid chemical addition means for adding an acid chemical to at least one of the liquid to be treated, the concentrate, and the dilution water; and an addition amount control means for controlling the amount of the acid chemical added so that the pH of at least one of the liquid to be treated and the concentrate is equal to or lower than a critical pH.

[0015] According to the present invention, it is possible to stably separate alkali metal ions, such as lithium and cesium ions, from a liquid to be treated that contains such ions with high purity and high recovery rate.

[0016] Fig. 1 is a schematic flow diagram showing a critical pH measurement step according to an embodiment of the present invention. Fig. 2 is a schematic graph showing the relationship between the treatment time in the critical pH measurement step and the operating pressure of the nanofiltration membrane unit and the pH of the liquid to be treated. Fig. 3 is a schematic flow diagram showing a method for separating alkali metal ions according to one embodiment of the present invention. Fig. 4 is a schematic flow diagram showing a method for separating alkali metal ions according to another embodiment of the present invention. Fig. 5 is a schematic flow diagram showing a method for separating alkali metal ions according to another embodiment of the present invention. Fig. 6 is a schematic flow diagram showing a method for separating alkali metal ions in a comparative example.

[0017] The following describes in detail the embodiments of the present invention, but the present invention is not limited to the following description and can be modified and implemented as desired within the scope that does not deviate from the gist of the present invention.

[0018] (1) Alkali metal ion separation method The alkali metal ion separation method of the present invention comprises a nanofiltration step in which a liquid to be treated containing alkali metal ions (hereinafter also simply referred to as "liquid to be treated") is separated into a permeate and a concentrate using a nanofiltration membrane unit, and the concentrate is sent back to the nanofiltration membrane unit; a dilution step in which dilution water is added to at least one of the liquid to be treated and the concentrate; and a pH control step in which the pH of at least one of the liquid to be treated and the concentrate is adjusted to a critical pH or lower.

[0019] (2) Separation by Nanofiltration Membrane Unit In the nanofiltration process, a nanofiltration membrane unit is used to separate the alkali metal ion-containing liquid into a permeate and a concentrate. A nanofiltration membrane unit refers to a pressure vessel containing one or more nanofiltration membrane elements, each equipped with a nanofiltration membrane (described below), housed in series or parallel. In the present invention, dilution water, chemical solutions, etc. are not added to the nanofiltration membrane unit. Furthermore, nanofiltration membrane elements with any diameter, length, and separation performance may be used in the nanofiltration membrane unit.

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

[0021] The concentration of polyvalent metal ions can be calculated as the sum of the equivalent ion concentrations of, for example, cobalt ions, nickel ions, etc. Furthermore, the concentration of alkali metal ions can be calculated as the sum of the equivalent ion concentrations of, for example, lithium ions, cesium ions, etc. Some alkali metal elements may exist in a solution as polyatomic ions rather than monoatomic ions, but the equivalent concentration is the concentration assuming they exist as monoatomic ions. The concentrations of polyvalent metal ions and alkali metal ions can be calculated as the concentrations (mg / L) of various ions by, for example, analyzing the solution to be measured using a Hitachi P-4010 ICP optical emission analyzer.

[0022] (2-1) Liquid to be treated The liquid to be treated may contain at least alkali metal ions and one or more conjugate bases (for example, chloride ions, nitrate ions, sulfate ions, carbonate ions, acetate ions, etc.). Examples of alkali metal ions include lithium ions, sodium ions, potassium ions, rubidium ions, and cesium ions. From the viewpoint of the value of the object to be recovered, however, it is preferable that the liquid to be treated contains lithium ions (hereinafter referred to as "Li ions") as alkali metal ions. ＋ Preferably, the compound includes a polyimide-containing polymer.

[0023] In addition to alkali metal ions, the liquid to be treated preferably contains at least one polyvalent metal ion, such as ions of alkaline earth metals such as magnesium, calcium, and strontium, ions of typical elements (such as aluminum, tin, and lead), and ions of transition elements (such as iron, copper, cobalt, and manganese).

[0024] The liquid to be treated is preferably a liquid in which a lithium-containing material is dissolved in acid. Specific examples of the lithium-containing material include lithium-ion batteries, as well as waste materials, waste liquids, ores, and slag generated in the manufacturing process of the batteries. Among these, ores and lithium-ion batteries are preferred as the lithium-containing material because they contain a large amount of lithium. Lithium-ion batteries are more preferred because of the high demand for recycling and the high purity of the rare metals they contain.

[0025] Lithium-containing ores include spodumene (Li ２ O.Al ２ O ３ 4SiO ２ ), lepidolite (K(Li,Al) ３ (Si, Al) ４ O １０ (F, OH) ２ Sulfuric acid is widely used as the acid to dissolve these, and in the case of lithium-containing ores that contain aluminum as the main component, the leachate becomes an aqueous solution that contains a large amount of aluminum ions.

[0026] Lithium-ion batteries are composed of components such as a positive electrode material, a negative electrode material, a separator, and an electrolyte. Any of these components containing lithium can be used as the material for the liquid to be treated. The acid used to dissolve the lithium-containing material preferably contains at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid. When lithium-ion battery components are dissolved in acid, the resulting solution contains, in addition to lithium ions, other ions such as nickel ions, cobalt ions, and manganese ions.

[0027] The method for dissolving the lithium-containing material with an acid includes, for example, immersing the material in an acidic aqueous solution, but other methods may also be used as long as they can elute the desired alkali metal ions.

[0028] The initial amount of the liquid to be treated is not particularly limited, but from the viewpoint of the treatment efficiency in each step, it is set to 10 L / m ２ More than 40L / m ２ It is preferable to do the following:

[0029] The liquid to be treated may contain organic compounds. When the liquid to be treated is an acid solution for a lithium-ion battery, the organic compounds include those derived from the binder that connects the active material to the current collector, the separator, the electrolyte, and the like. Examples of organic compounds include polyvinylidene fluoride (PVDF), polyolefin, and carbonate ester. These organic compounds may act as foulants and may reduce the efficiency of alkali metal ion recovery, so it is preferable to remove these foulants by pretreatment.

[0030] When the liquid to be treated contains lithium ions as alkali metal ions, the lithium ion concentration in the liquid to be treated is preferably 0.5 mg / L or more and 10,000 mg / L or less. When the lithium ion concentration in the liquid to be treated is 0.5 mg / L or more, the efficiency of lithium ion recovery by membrane separation is improved. Furthermore, when the lithium ion concentration in the liquid to be treated is 10,000 mg / L or less, the osmotic pressure difference is not too large, and the efficiency of membrane separation is improved. The lithium ion concentration in the liquid to be treated is more preferably 10 mg / L or more and 8,000 mg / L or less, and even more preferably 100 mg / L or more and 6,000 mg / L or less.

[0031] In the alkali metal ion separation method according to this embodiment, the alkali metal ion ratio of the liquid to be treated is preferably 2.4 or less. If the alkali metal ion ratio is 2.4 or less, it becomes more difficult to separate and recover the alkali metal ions, and therefore the alkali metal ion separation method of the present invention can more effectively selectively separate the alkali metal ions from the polyvalent metal ions. The alkali metal ion ratio of the liquid to be treated is more preferably 1.0 or less, and even more preferably 0.5 or less.

[0032] (2-2) Nanofiltration Membrane Unit The nanofiltration membrane included in the nanofiltration membrane unit used in the alkali metal ion separation method according to this embodiment is positioned between a reverse osmosis membrane and an ultrafiltration membrane, and is not particularly limited as long as it has fractionation properties that allow alkali metal ions to permeate and are difficult for polyvalent metal ions to permeate. Membranes commonly known as reverse osmosis membranes can remove most organic substances and ions. On the other hand, ultrafiltration membranes typically do not remove most ionic species, but remove high-molecular-weight organic substances.

[0033] In order to separate alkali metal ions and polyvalent metal ions, it is preferable that the nanofiltration membrane has a charge on the membrane surface, and can perform both separation by pores (size separation) and electrostatic separation by charge.As such nanofiltration membrane, for example, when passing through a 2000mg / L magnesium sulfate aqueous solution at 25 ℃ and pH 6.5 under an operating pressure of 0.5 MPa, the difference between the magnesium sulfate removal rate and the magnesium chloride removal rate when passing through a 2000mg / L magnesium chloride aqueous solution at 25 ℃ and pH 6.5 under an operating pressure of 0.5 MPa is 20% or less; and when passing through a 1000mg / L glucose aqueous solution at 25 ℃ and pH 6.5 under an operating pressure of 0.5 MPa, the difference between the glucose removal rate and the isopropyl alcohol removal rate when passing through a 1000mg / L isopropyl alcohol aqueous solution at 25 ℃ and pH 6.5 under an operating pressure of 0.5 MPa is 40% or more, and it is preferable to use a nanofiltration membrane whose glucose removal rate is 70% or more and 90% or less. The magnesium sulfate removal rate and the magnesium chloride removal rate can be measured by the method described in the Examples below.

[0034] Here, glucose and isopropyl alcohol are neutral molecules with different molecular weights, and the difference between the glucose removal rate and the isopropyl alcohol removal rate indirectly indicates the pore size distribution of the nanofiltration membrane, corresponding to the selective separation and recovery performance of alkali metal ions and polyvalent metal ions. Therefore, when the difference between the glucose removal rate and the isopropyl alcohol removal rate is 40% or more, the pore size distribution of the nanofiltration membrane is sufficiently narrow, and alkali metal ions and polyvalent metal ions can be selectively separated. The difference between the glucose removal rate and the isopropyl alcohol removal rate is more preferably 45% or more, and even more preferably 50% or more. On the other hand, from the viewpoint of the recovery rate of alkali metal ions, the difference between the glucose removal rate and the isopropyl alcohol removal rate is preferably 70% or less, more preferably 65% ​​or less, and even more preferably 60% or less. The glucose removal rate and the isopropyl alcohol removal rate can be measured by the method described in the examples below.

[0035] Furthermore, the glucose removal rate of the nanofiltration membrane is more preferably 80% or more and 90% or less. When the glucose removal rate is within the above range, polyvalent metal ions can be sufficiently removed.

[0036] The use of a nanofiltration membrane that satisfies the above requirements is advantageous in that it enables selective separation and recovery of alkali metal ions and polyvalent metal ions to be carried out with high efficiency over a long period of time, making it a highly efficient process.

[0037] The removal rate of the nanofiltration membrane can be controlled, for example, by controlling the relative humidity to 80% or more during interfacial polycondensation in the production of the composite semipermeable membrane described below, or by selecting the type, molecular weight, etc. of the polyfunctional aliphatic amine used in the interfacial polycondensation.

[0038] Examples of materials for nanofiltration membranes include cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides, and vinyl polymers. Nanofiltration membranes may be composed of one material or multiple materials. Furthermore, the membrane structure may be an asymmetric membrane having a dense layer on at least one side of the membrane and gradually increasing micropores from the dense layer toward the interior of the membrane or toward the other side, or a composite semipermeable membrane having a very thin separation functional layer made of a different material on the dense layer of the asymmetric membrane.

[0039] The composite semipermeable membrane is preferably, for example, a composite semipermeable membrane having a porous support membrane containing polysulfone and a separation functional layer containing polyamide provided on the porous support membrane. The composite semipermeable membrane may also include a substrate in addition to the porous support membrane and the separation functional layer, in which case the porous support membrane is provided on the substrate. The separation functional layer containing polyamide is a thin film formed on the porous support membrane by a polycondensation reaction between a polyfunctional amine and a polyfunctional acid halide.

[0040] The separating functional layer in the composite semipermeable membrane preferably contains 50% by mass or more of semi-aromatic crosslinked polyamide, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably consists of only semi-aromatic crosslinked polyamide. By containing 50% by mass or more of semi-aromatic crosslinked polyamide, excessive densification due to π-π interactions derived from aromatic rings in the semi-aromatic crosslinked polyamide is suppressed, and excellent alkali metal ion permeability is obtained.

[0041] Furthermore, by controlling the relative humidity during interfacial polycondensation to 80% or more, a composite semipermeable membrane exhibiting excellent membrane performance under acidic conditions can be obtained. The relative humidity can be adjusted by using a precision air conditioning device, for example.

[0042] The term "semi-aromatic crosslinked polyamide" refers to a semi-aromatic polyamide obtained by interfacial polycondensation of a polyfunctional aliphatic amine compound and a polyfunctional aromatic acid halide, which forms a crosslinked structure. For example, the aromatic polyamide may form a crosslinked structure via a crosslinking agent, or at least one of the polyfunctional aromatic amine and the polyfunctional aromatic acid halide may be trifunctional or higher, and the aromatic polyamide may form a network-like crosslinked structure.

[0043] The term "polyfunctional aliphatic amine" refers to an aliphatic amine having two or more amino groups in one molecule, and is not particularly limited as long as it can form a semi-aromatic crosslinked polyamide by reaction with a polyfunctional aromatic acid halide.

[0044] As the polyfunctional aliphatic amine, an alicyclic diamine is preferred, and 4,4'-bipiperidine or a piperazine derivative is more preferred.

[0045] Furthermore, when an alicyclic diamine is used, its molecular weight is preferably 90 or more and 160 or less. When the molecular weight of the alicyclic diamine is 90 or more, the diffusion coefficient of the amine is small, and polyamide is gradually formed during interfacial polycondensation, making it easier to form a separation functional layer with uniform pore size in the film thickness direction from the early to middle stages of polycondensation. Furthermore, usually, at the end of polycondensation, excessive oligomers are generated on the surface of the support in contact with the organic layer, blocking the pores on the support surface, causing uneven 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 is small, reducing interaction with the semi-aromatic crosslinked polyamide. Therefore, after forming the separation functional layer through the polycondensation reaction, the oligomers are easily detached from the separation functional layer, making it easier to form a separation functional layer with uniform pore size in the film thickness direction.

[0046] Examples of alicyclic diamines having a molecular weight of 90 or more and 160 or less include substituted piperazines in which the piperazine ring is substituted with an alkyl group having 1 to 3 carbon atoms (e.g., 2-methylpiperazine, 2-ethylpiperazine, 2-normal propylpiperazine, 2,2-dimethylpiperazine, 2,2-diethylpiperazine, 2,3-dimethylpiperazine, 2,3-diethylpiperazine, 2,5-dimethylpiperazine, 2,5-diethylpiperazine, 2,6-dimethylpiperazine, 2,6-diethylpiperazine, and 2,3,5,6-tetramethylpiperazine), and homopiperazine.

[0047] The term "polyfunctional aromatic acid halide" refers to an aromatic acid halide having two or more halogenated carbonyl groups in one molecule, and is not particularly limited as long as it can form a semi-aromatic crosslinked polyamide upon reaction with a polyfunctional aliphatic amine. Examples of polyfunctional aromatic acid halides that can be used include halides of 1,3,5-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid, 1,3-benzenedicarboxylic acid, 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid. Among polyfunctional aromatic acid halides, acid chlorides are preferred, and in particular, from the standpoints of economy, ease of availability, ease of handling, ease of reactivity, and the like, trimesoyl chloride (hereinafter referred to as "TMC"), which is an acid halide of 1,3,5-benzenetricarboxylic acid, isophthaloyl chloride, which is an acid halide of 1,3-benzenedicarboxylic acid, terephthaloyl chloride, which is an acid halide of 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid chloride, which is an acid halide of 1,3,5-benzenetrisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid chloride, which is an acid halide of 1,3,6-naphthalenetrisulfonic acid, are preferred. The polyfunctional aromatic acid halides may be used alone or in combination of two or more, but by mixing the trifunctional TMC, 1,3,5-benzenetrisulfonic acid chloride, or 1,3,6-naphthalenetrisulfonic acid chloride with either the difunctional isophthalic acid chloride or terephthalic acid chloride, the intermolecular gaps in the polyamide crosslinked structure are enlarged, making it possible to control the pore size distribution of a membrane over a wide range. The molar ratio of the trifunctional acid chloride to the difunctional acid chloride is preferably 1:20 to 50:1, more preferably 1:1 to 20:1.

[0048] The composite semipermeable membrane can be obtained, for example, by forming a porous support membrane on a substrate, and then forming a separation functional layer containing a semi-aromatic crosslinked polyamide on the porous support membrane by polycondensation of a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide.

[0049] The nanofiltration membrane used in the alkali metal ion separation method according to this embodiment is preferably a composite semipermeable membrane having a support membrane and a separation functional layer containing a semi-aromatic crosslinked polyamide obtained by interfacial polycondensation of a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide formed on the support membrane.Furthermore, the nanofiltration membrane preferably has an N / O ratio, which is the ratio of nitrogen atoms N to oxygen atoms O on the surface of the separation functional layer measured by X-ray photoelectron spectroscopy (hereinafter referred to as "XPS"), of 0.7 or more and 1.3 or less.That is, the nanofiltration membrane used in the alkali metal ion separation method according to this embodiment is preferably a composite semipermeable membrane having a support membrane and a separation functional layer containing a semi-aromatic crosslinked polyamide on the support membrane, and an N / O ratio, which is the ratio of the number of nitrogen atoms N to the number of oxygen atoms O on the surface of the separation functional layer measured by XPS, of 0.7 or more and 1.3 or less.

[0050] A high N / O ratio on the surface of the separation functional layer indicates that the ratio of polyfunctional aliphatic amines to polyfunctional aromatic acid halides is high in the semi-aromatic crosslinked polyamide bond forming the surface of the separation functional layer.When the N / O ratio on the surface of the separation functional layer is 0.7 or more, the positive charge of the surface of the separation functional layer due to the polyfunctional aliphatic amines present on the surface of the separation functional layer is strengthened, promoting the permeation of negatively charged conjugate bases and achieving electrical neutrality in the permeate, thereby also promoting the permeation of alkali metal ions.In addition, when the N / O ratio on the surface of the separation functional layer is 1.3 or less, the ratio of polyfunctional aromatic acid halides to polyfunctional aliphatic amines present on the surface of the separation functional layer is appropriate, so that the reduction in the removal ability of multivalent ions due to excessive coarsening of pores is unlikely to occur.

[0051] The N / O ratio of the nanofiltration membrane can be controlled, for example, by the concentration ratio of the polyfunctional aromatic acid halide and the polyfunctional aliphatic amine when performing interfacial polycondensation. The N / O ratio on the surface of the separation functional layer can be measured by the method described in the Examples below.

[0052] (2-3) Nanofiltration Step Since alkali metal ions easily permeate the nanofiltration membrane and polyvalent metal ions do not easily permeate the nanofiltration membrane, alkali metal ions and polyvalent metal ions can be separated.

[0053] In the nanofiltration process of the present invention, the liquid to be treated is sent to a nanofiltration membrane unit and separated into a permeate and a concentrate. In the nanofiltration process, a multi-stage permeation process may be carried out in which the obtained permeate is further treated one or more times with a nanofiltration membrane unit to obtain a permeate with a high alkali metal ion ratio. Alternatively, a nanofiltration membrane unit having two or more nanofiltration membrane elements arranged in series may be used. When the permeate is further separated with a nanofiltration membrane, nanofiltration membranes with the same separation performance may be used, or nanofiltration membranes with different separation performance may be used.

[0054] In the nanofiltration step of the present invention, the obtained concentrate is sent again to the nanofiltration membrane unit and separated into a permeate and a concentrate. Since the concentrate contains alkali metal ions that did not permeate the nanofiltration membrane unit, the concentrate is separated again using the nanofiltration membrane to obtain a permeate, which can increase the recovery rate of alkali metals in the nanofiltration step.

[0055] Methods for sending the obtained concentrated liquid back to the nanofiltration membrane unit include a method in which a circulation step is provided in which the obtained concentrated liquid is mixed with the remainder of the liquid to be treated, a method in which a multi-stage concentration step is provided in which the obtained concentrated liquid is sent to another nanofiltration membrane unit as the liquid to be treated, etc. Among these, the method in which a circulation step is provided is preferred from the viewpoints of ease of control and the ability to obtain a high alkali metal ion recovery rate.

[0056] In the method including the circulation step, the concentrated liquid is mixed with the remainder of the liquid to be treated to form a solution, which is then sent back to the nanofiltration membrane unit. The number of times the concentrated liquid is circulated through the liquid to be treated can be set as desired. In this specification, the solution obtained by mixing the concentrated liquid with the remainder of the liquid to be treated is also considered to be the liquid to be treated.

[0057] In the method using a multi-stage concentration process, the concentrate obtained by another nanofiltration membrane unit may be further sent to a nanofiltration membrane unit to separate it into a permeate and a concentrate, and the number of times the concentrate is sent to the nanofiltration membrane unit (the so-called number of stages) can be set as desired. The permeates obtained by each nanofiltration membrane unit may be recovered individually, but it is preferable to recover them mixed together. When using a multi-stage concentration process, the composition of the permeate obtained in the nanofiltration process described below refers to the composition of a solution obtained by mixing all the permeates.

[0058] The alkali metal ion ratio of the permeate obtained in the nanofiltration step is preferably 2 or more and 1000 or less, more preferably 10 or more and 700 or more, and even more preferably 20 or more and 500 or less. When the alkali metal ion ratio of the permeate is 2 or more, the number of treatments using the nanofiltration membrane unit required to obtain a permeate with a desired alkali metal ion ratio can be reduced, and when it is 1000 or less, the treatment time of the nanofiltration step can be shortened. The recovery rate of alkali metal ions in the nanofiltration step is preferably 90% or more, more preferably 95% or more. When the recovery rate of alkali metal ions in the nanofiltration step is 90% or more, the cost of recovering alkali metal ions can be reduced. The recovery rate of alkali metal ions in the nanofiltration step is defined by the following formula (1): Recovery rate of alkali metal ions in the nanofiltration step (%) = {(total volume of permeate in the nanofiltration step) × (alkali metal ion concentration of permeate in the nanofiltration step)} / {(initial volume of the liquid to be treated) × (initial alkali metal ion concentration of the liquid to be treated)} ... formula (1)

[0059] In the nanofiltration step, the liquid to be treated is preferably supplied to the nanofiltration membrane unit at an operating pressure in the range of 0.1 MPa to 8.0 MPa. An operating pressure of 0.1 MPa or higher improves the membrane permeation rate, while an operating pressure of 8.0 MPa or lower can suppress damage to the nanofiltration membrane. The operating pressure is more preferably 0.5 MPa to 6.0 MPa, and even more preferably 1.0 MPa to 4.0 MPa.

[0060] As described above, the nanofiltration step is preferably continued until the recovery rate of alkali metal ions reaches 90% or more, and more preferably 95% or more. As the nanofiltration step progresses, the osmotic pressure of the treated liquid increases, and the operating pressure required to obtain the permeate increases accordingly. Therefore, in order to continue the nanofiltration step, it is necessary to carry out the dilution step described below.

[0061] Examples of operation control methods for the nanofiltration process include constant flow rate operation and constant pressure operation. Among these, constant flow rate operation is preferred because the nanofiltration process of the liquid to be treated is carried out while adding dilution water. Constant flow rate operation allows the flow rate of the added dilution water to be matched with the filtration flow rate (permeate flow rate), making control easier.

[0062] In the case of constant flow rate operation, from the viewpoint of alkali metal ion recovery efficiency, it is preferable that the permeate flow rate is 1% or more of the volume of the liquid to be treated per minute, and from the viewpoint of ease of control, it is preferable that the permeate flow rate is 50% or less of the volume of the liquid to be treated per minute.

[0063] (2-4) Dilution Step The dilution step is a step of adding dilution water to at least one of the liquid to be treated and the concentrated liquid. When the circulation step is provided, adding dilution water to at least one of the liquid to be treated and the concentrated liquid can suppress the increase in osmotic pressure that accompanies the concentration of the liquid to be treated in the circulation step. This makes it possible to continue the nanofiltration step, thereby increasing the recovery rate of alkali metal ions. Furthermore, when the multi-stage concentration step is provided, the concentrated liquid becomes more concentrated, so adding dilution water can suppress the increase in osmotic pressure.

[0064] The total amount of dilution water added to the treated liquid in the dilution step is not particularly limited, but is preferably less than the total amount of permeate obtained in the nanofiltration step. If the total amount of dilution water added to the treated liquid is less than the total amount of permeate, the amount of acid added in the pH control step described below can be reduced. In addition, since the total amount of treated liquid in the nanofiltration step is reduced, the number of nanofiltration membranes required and the treatment time can be reduced, resulting in a more efficient process.

[0065] Furthermore, the dilution step preferably includes a step of adjusting the amount of dilution water added so that the operating pressure in the nanofiltration step is 30% to 100% of the pressure resistance value of the nanofiltration membrane provided in the nanofiltration membrane unit. Here, the pressure resistance value of the nanofiltration membrane means the pressure resistance value of the nanofiltration membrane element. For example, in the case of a commercially available nanofiltration membrane element, it means the maximum operating pressure stated in the specifications. Note that when a nanofiltration membrane element other than a commercially available nanofiltration membrane element is used, it means the pressure resistance value specified by the membrane manufacturer or the pressure resistance value experimentally determined based on the membrane material and structure.

[0066] In the dilution step, if the total amount of dilution water added is less than the total amount of permeate, the osmotic pressure of the treated liquid increases as the treated liquid is concentrated, and the osmotic pressure of the treated liquid decreases as some of the ions in the treated liquid permeate into the permeate. In this case, by controlling the amount of dilution water added and setting the operating pressure in the nanofiltration step to 30% to 100% of the pressure resistance value of the nanofiltration membrane provided in the nanofiltration membrane unit, the amount of dilution water used can be reduced, and the amount of acid added can be reduced. Since the amount of acid added decreases as the operating pressure in the nanofiltration step increases, the operating pressure in the nanofiltration step is more preferably 50% to 100% of the pressure resistance value of the nanofiltration membrane, and even more preferably 70% to 100%.

[0067] In addition, from the viewpoint of being able to further reduce the amount of acid added, in the nanofiltration step, the total processing time T Ａ The treatment time T ３０ The ratio T ３０ / T Ａ is preferably 0.20 or more, more preferably 0.50 or more, and even more preferably 0.80 or more.

[0068] Similarly, from the viewpoint of being able to further reduce the amount of acid added, in the nanofiltration step, the total processing time T ＡThe treatment time T ５０ The ratio T ５０ / T Ａ is preferably 0.20 or more, more preferably 0.50 or more, and even more preferably 0.80 or more.

[0069] When the operating pressure at the start of the nanofiltration step is 30% or less of the pressure resistance value of the nanofiltration membrane used, the nanofiltration step may be temporarily carried out without adding dilution water, the liquid to be treated is concentrated until the operating pressure reaches 30% or more or 50% or more of the pressure resistance value of the nanofiltration membrane provided in the nanofiltration membrane unit, and then dilution water may be added to adjust the operating pressure. Alternatively, the nanofiltration step may be carried out under conditions where the amount of dilution water is less than the amount of permeated liquid, and the operating pressure may be gradually increased to 30% or more or 50% or more of the pressure resistance value of the nanofiltration membrane. ３０ / T Ａ Yahi T ５０ / T Ａ Therefore, it is preferable to temporarily perform the nanofiltration step without adding dilution water after the start of the nanofiltration step, concentrate the liquid to be treated until the operating pressure of the nanofiltration step is 30% or more or 50% or more of the withstand pressure value of the nanofiltration membrane equipped in the nanofiltration membrane unit, and then add dilution water to adjust the operating pressure of the nanofiltration step.

[0070] The dilution water is not particularly limited and may be pure water, an acidic aqueous solution, etc. Among them, it is preferable to use a permeate having a low metal ion concentration obtained in the reverse osmosis filtration step described below, because this allows for highly efficient separation and recovery of alkali metal ions and reuse of the acidic aqueous solution.

[0071] Furthermore, in a nanofiltration process that includes a circulation process, when the obtained permeate is treated multiple times with nanofiltration membrane units, the dilution process only needs to be performed in at least the first nanofiltration membrane unit, and the application of the dilution process in the other nanofiltration membrane units can be set as desired.

[0072] (2-5) pH Control Step The pH control step is a step of adjusting the pH of at least one of the liquid to be treated and the concentrated liquid to a critical pH or lower. The critical pH is determined in the pre-processing and critical pH measurement steps described below. Examples of methods for adjusting the pH include a method of directly adding an acidic solution to the liquid to be treated and the concentrated liquid, and a method of adding the above-mentioned dilution water to which an acidic solution has been added to the liquid to be treated and the concentrated liquid.

[0073] When the above-mentioned circulation step is provided, the pH of the concentrated liquid may be adjusted before mixing with the remainder of the liquid to be treated, or the pH of the solution (liquid to be treated) after mixing the concentrated liquid with the remainder of the liquid to be treated may be adjusted. The pH of the solution (liquid to be treated) after mixing the concentrated liquid with the remainder of the liquid to be treated may be adjusted by adjusting the pH of the entire liquid to be treated, or the pH of the liquid to be treated supplied to the nanofiltration membrane in the piping when sending the liquid to the nanofiltration membrane unit. Furthermore, in order to stabilize the nanofiltration step, it is preferable to control the pH of the liquid to be treated supplied to the nanofiltration membrane unit through the nanofiltration step to a constant value.

[0074] Furthermore, in the nanofiltration process, when separation using nanofiltration membrane units is performed multiple times, it is sufficient to provide a pH control step for at least the liquid to be treated that is supplied to the first nanofiltration membrane unit. For example, when the above-mentioned multi-stage permeation process is provided, the pH control step for the permeate obtained by the first nanofiltration membrane unit that is supplied as the liquid to be treated to the second nanofiltration membrane unit may be set as desired.

[0075] When the above-mentioned multi-stage concentration process is provided, the pH of the concentrated liquid obtained by the nanofiltration membrane unit may be adjusted in the piping when it is sent as the liquid to be treated to another nanofiltration membrane unit, or the concentrated liquid may be temporarily stored in a tank or the like and the pH adjusted before being sent as the liquid to be treated to another nanofiltration membrane unit. Furthermore, when the multi-stage concentration process is provided, since substances that cause scale formation continue to remain on the concentrated liquid side, it is preferable to provide a pH control process for all concentrated liquids sent to each nanofiltration membrane.

[0076] In addition, in order to stabilize the nanofiltration process, it is preferable to control the pH of the concentrate supplied to each nanofiltration membrane unit throughout the nanofiltration process to a constant value.

[0077] The inventors have found that in a method for separating alkali metal ions, it is useful to control the pH of at least one of the liquid to be treated and the concentrated liquid to below the critical pH. By performing the nanofiltration process while controlling the pH of at least one of the liquid to be treated and the concentrated liquid to below the critical pH, it is possible to suppress the generation of scale on the surface of the nanofiltration membrane and to suppress any accompanying changes in the selective separation properties of the nanofiltration membrane. The pH of at least one of the liquid to be treated and the concentrated liquid is preferably controlled to be greater than or equal to 0 and less than the critical pH, more preferably greater than or equal to (critical pH - 1) and less than the critical pH, and even more preferably greater than or equal to (critical pH - 0.3) and less than the critical pH. By controlling the pH of the liquid to be treated or the concentrated liquid within the above range, the amount of acid chemical solution added to control the pH can be minimized. Adding an acid chemical solution to the liquid to be treated or the concentrated liquid increases the osmotic pressure due to the increase in acid concentration, thereby increasing the pressure required in the concentration process using a reverse osmosis membrane, as described below. Furthermore, because it also increases operating costs and impurities in the raw water, it is preferable to add a small amount of acid chemical solution.

[0078] One possible method for reducing the amount of acid chemical solution added is to control the pH of the liquid to be treated or the concentrated liquid at the scale-forming pH. The scale-forming pH can be determined, for example, by gradually adding sodium hydroxide to the liquid to be treated and visually checking for scale formation. However, even when the pH of the liquid to be treated is controlled near the scale-forming pH, fouling associated with scale formation occurs in the nanofiltration process. Therefore, the inventors discovered a method for determining the critical pH, which is the pH at which scale does not form, taking into account the operating conditions of the process, and completed the present invention.

[0079] In this specification, controlling the pH of the liquid to be treated to be greater than or equal to 0 and less than the critical pH means, for example, determining a set value for the pH to be controlled within the range of greater than or equal to 0 and less than the critical pH, and controlling the pH of the liquid to be treated to that set value, and also includes cases where the pH of the liquid to be treated temporarily falls outside the range of greater than or equal to 0 and less than the critical pH during the process of controlling the pH.

[0080] (2-6) Determination of Critical pH In the alkali metal ion separation method of the present invention, the critical pH of the liquid to be treated is determined by the following method. That is, the critical pH can be determined by the critical pH determination method of the present invention. The critical pH determination method of the present invention is a method for determining the critical pH in a method for separating alkali metal ions, which comprises a nanofiltration step of separating the liquid to be treated containing alkali metal ions into a permeate and a concentrate using a nanofiltration membrane unit and sending the concentrate back to the nanofiltration membrane unit, and a dilution step of adding dilution water to at least one of the liquid to be treated and the concentrate, and comprises the following steps (a) to (c):

[0081] In the nanofiltration process, if a multi-stage concentration process using multiple nanofiltration membrane units is provided, the critical pH is measured for each nanofiltration membrane unit. A single nanofiltration membrane unit in the multi-stage concentration process is classified when additives such as dilution water or an acid chemical solution are added to the concentrated solution and the solution is supplied to a new nanofiltration membrane element. Furthermore, in the case of a multi-stage permeation process in which the solution is treated one or more times with a nanofiltration membrane unit, the following steps (a) to (c) are carried out for the first nanofiltration unit. A single nanofiltration membrane unit in the multi-stage permeation process is classified when the permeated solution is supplied to a new nanofiltration membrane element.

[0082] (a) In each nanofiltration membrane unit in the upstream process, if the nanofiltration process is performed in constant flow rate operation, the flow rates of the permeate and concentrate at the start of the dilution process are set as filtration conditions, and the liquid to be treated is sent to the nanofiltration membrane unit under these conditions. If the nanofiltration process is performed in a manner other than constant flow rate operation, the flow rate ratio of the permeate and concentrate at the start of the dilution process and the operating pressure are set as filtration conditions, and the liquid to be treated is sent to each nanofiltration membrane unit. From the alkali metal ion concentrations of the permeate and concentrate obtained after one hour of operation, the alkali metal ion rejection rate R is measured using the following formula (2), and the operation time A per unit alkali metal ion recovery rate of each nanofiltration membrane unit is calculated using the following formula (3).

[0083] Alkali metal ion rejection rate R (%) = {1 - (alkali metal ion concentration in permeated liquid) / (alkali metal ion concentration in treated liquid)} × 100 (Equation 2)

[0084]

[0085] In the above formula (3), A is the treatment time (s) per unit alkali metal ion recovery rate, V ０ is the initial amount of liquid to be treated (L), Q Ｔ is the permeate flow rate (L / s), and R is the alkali metal ion rejection rate (%) of the nanofiltration membrane.

[0086] Here, the permeate of a nanofiltration membrane unit refers to the sum of the permeates of each nanofiltration membrane element that constitutes the nanofiltration membrane unit. On the other hand, the concentrate of a nanofiltration membrane unit is defined according to the connection configuration of the nanofiltration membrane elements that constitute the nanofiltration membrane unit. In the case of a nanofiltration membrane unit in which nanofiltration membrane elements are connected in parallel, the concentrate is the sum of the concentrates of each nanofiltration membrane element. In the case of a nanofiltration membrane unit in which nanofiltration membrane elements are connected in series, the concentrate of the nanofiltration membrane element located at the last stage is the concentrate of the nanofiltration membrane unit. In a configuration in which parallel and series connections are mixed, the concentrate of the nanofiltration membrane unit is the sum of the concentrates obtained according to each connection configuration.

[0087] (b) Critical pH measurement step: For each nanofiltration membrane unit, the same flow rates of the permeate and concentrate as in the previous step are set as filtration conditions. The liquid to be treated is sent to the nanofiltration membrane unit, separated into a permeate and a concentrate, and water with an electrical conductivity of less than 10 μS / cm is sent to the liquid to be treated at the same flow rate as the permeate. The concentrate is mixed with the remainder of the liquid to be treated. While measuring the pH of the liquid to be treated, nanofiltration is continued until the change in operating pressure P of the nanofiltration membrane unit per treatment time t (ΔP / Δt) becomes ΔP / Δt > 0. At this time, Δt is set to 5 A. If there is no time point within 50 A (t≦50 A) at which ΔP / Δt > 0 occurs when the treatment time t exceeds 50 A, the average pH value of the liquid to be treated at the treatment time Δt at which ΔP / Δt > 0 occurs for the first time is calculated, and this value is designated as the critical pH. However, the maximum critical pH is set to 4, and if the average value is 4 or higher, the critical pH is set to 4. FIG. 1 is a schematic flow diagram showing the critical pH measurement process according to an embodiment of the present invention. For example, as shown in FIG. 1, the liquid to be treated is sent to the first nanofiltration membrane unit 5a at the same flow rates of the permeate and concentrate as in the previous process. At this time, water with an electrical conductivity of less than 10 μS / cm is stored in the first tank 1 as dilution water, and this is sent to the liquid to be treated in the second tank 2 at the same flow rate as the permeate. A constant flow rate operation is performed in which the concentrate 7a from the first nanofiltration membrane unit is mixed with the remainder of the liquid to be treated in the second tank 2. While measuring the pH of the liquid to be treated, nanofiltration is continued until the change in operating pressure P of the nanofiltration membrane unit per treatment time t (ΔP / Δt) becomes ΔP / Δt > 0. As nanofiltration continues, the acid in the liquid to be treated permeates together with the permeate, causing the pH of the liquid to increase and leading to the formation of scale at a certain point. As this scale forms, the operating pressure P of the nanofiltration membrane unit increases.

[0088] Figure 2 shows a schematic diagram of a graph used to determine the critical pH. In a graph showing the relationship between treatment time and the operating pressure P of the nanofiltration membrane unit, the range of Δt where ΔP / Δt > 0, i.e., the slope of ΔP / Δt, becomes positive for the first time, is identified. Then, from the graph showing the relationship between treatment time and the pH of the treated liquid, the average pH of the treated liquid within the identified Δt range is calculated and used as the critical pH.

[0089] If there is a point in time t where ΔP / Δt>0 within 50 A (t≦50 A), the nanofiltration membrane is fouled due to the generation of initial scale, and the nanofiltration membrane used is subjected to the (c) re-measurement step described below. Here, initial scale refers to scale that occurs at the processing time t (t≦50 A) immediately after the start of operation of the nanofiltration membrane unit, and occurs when the pH of the treated liquid at the start of operation is higher than the critical pH. Note that if solids are present in the treated liquid after the (c) re-measurement step described below is performed, it is preferable to remove the solids with a filter before the critical pH measurement step.

[0090] (c) Remeasurement Step: If there is a point in time t within 50 A (t≦50 A) where ΔP / Δt>0, a sulfuric acid aqueous solution of pH 1, prepared by adding sulfuric acid to water with an electrical conductivity of less than 10 μS / cm, is sent to the nanofiltration membrane unit used in the critical pH measurement step, and the nanofiltration membrane unit is cleaned for one hour under the same operating conditions as in the previous step (a). After cleaning, the pH of the liquid to be treated is lowered by 0.5, and the critical pH measurement step is performed again. This process is repeated until the point in time where ΔP / Δt>0 occurs becomes 50 A<t.

[0091] (3) Reverse Osmosis Filtration Step The alkali metal ion separation method according to this embodiment may include a reverse osmosis filtration step in which the permeate obtained in the nanofiltration step is sent to a reverse osmosis membrane unit to obtain a concentrated liquid having a higher alkali metal ion concentration than the permeate from the nanofiltration step, and a permeate having a lower alkali metal ion concentration than the permeate from the nanofiltration step.

[0092] Examples of operation control methods for the reverse osmosis filtration step include constant flow rate filtration, low-pressure filtration, etc. In the case of constant flow rate filtration, from the viewpoint of alkali metal ion recovery efficiency, the permeate flow rate is preferably 1% or more of the permeate volume obtained in the nanofiltration step of the treated liquid per minute, and from the viewpoint of ease of control, it is preferably 50% or less of the permeate volume obtained in the nanofiltration step of the treated liquid per minute.

[0093] (3-1) Reverse Osmosis Membrane: By using a reverse osmosis membrane, loss of alkali metal ions, especially lithium ions, during the process of concentrating them is extremely small, and highly efficient recovery can be stably achieved. The higher the ion removal rate of the reverse osmosis membrane, the more efficient the process becomes. However, since membranes with high removal rates generally have poor water permeability, it is preferable to select a membrane that strikes a balance between these two factors.

[0094] Examples of materials for reverse osmosis membranes include cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides, and vinyl polymers. Reverse osmosis membranes may be made of a single material or a plurality of materials. Furthermore, the membrane structure may be an asymmetric membrane having a dense layer on at least one side of the membrane and having gradually increasing micropores with larger pore sizes from the dense layer toward the interior of the membrane or toward the other side, or a composite semipermeable membrane having a very thin separation functional layer made of a different material on the dense layer of the asymmetric membrane.

[0095] Examples of composite semipermeable membranes include composite semipermeable membranes comprising a substrate, a porous support membrane, and a separation functional layer. Among these, composite semipermeable membranes containing a polyamide in the separation functional layer are preferred. The polyamide-containing separation functional layer is obtained by polycondensation of a polyfunctional amine and a polyfunctional acid halide on a porous support membrane.

[0096] As the reverse osmosis membrane, it is preferable to use a reverse osmosis membrane that has an isopropyl alcohol removal rate of 85% or more and 95% or less when an aqueous isopropyl alcohol solution having a pH of 6.5 is passed through it at 25°C under an operating pressure of 0.5 MPa.

[0097] (3-2) Concentration using a reverse osmosis membrane In order to enable more efficient concentration, the reverse osmosis filtration step preferably includes a circulation step in which the concentrated liquid obtained in the reverse osmosis membrane unit is mixed with the permeated liquid obtained in the nanofiltration step, which is the liquid sent to the reverse osmosis membrane unit.

[0098] When the reverse osmosis filtration step includes a circulation step, it is preferable to operate the reverse osmosis filtration step within a range up to 90% of the pressure resistance value of the reverse osmosis membrane unit, from the viewpoint of operating pressure.

[0099] The drawings according to the embodiments of the present invention will be described below.

[0100] 3 is a schematic flow diagram showing an alkali metal ion separation method including a batch-type circulation process according to one embodiment of the present invention. The liquid to be treated stored in the second tank 2 is sent to a first nanofiltration membrane unit 5a equipped with a nanofiltration membrane, and a nanofiltration process is carried out to separate the liquid to be treated into a permeate 7a from the first nanofiltration membrane unit and a concentrate 8a from the first nanofiltration membrane unit. The permeate 7a from the first nanofiltration membrane unit is sent to a third tank 3 at a constant flow rate, and the concentrate 8a from the first nanofiltration membrane unit is sent to a second tank 2 and mixed with the remaining liquid to be treated in the second tank 2. A dilution process is also carried out in which dilution water in the first tank 1 is added to the second tank 2 at the same flow rate as the permeate 7a from the first nanofiltration membrane unit. A pH control process is also carried out in which an acid solution 11 is added to the second tank 2 as needed to adjust the pH. In this case, the dilution water in the first tank 1 may be an acidic solution, and the dilution process and pH control process may be carried out simultaneously. After the nanofiltration step is completed, the permeate 7a from the first nanofiltration membrane unit stored in the third tank 3 is sent to the reverse osmosis membrane unit 6, where the reverse osmosis filtration step is carried out. The concentrated liquid 10 from the reverse osmosis membrane unit is mixed with the third tank 3 and a circulation step is carried out. By sending the permeate 9 from the reverse osmosis membrane unit to the first tank 1, the permeate 9 can be reused as dilution water for the next batch. After the reverse osmosis filtration step is completed, the liquid in the third tank 3 in which the alkali metal ions are concentrated is recovered.

[0101] 4 is a schematic flow diagram showing a method for separating alkali metal ions according to another embodiment of the present invention, comprising a multi-stage concentration process. The liquid to be treated stored in the second tank 2 is sent to a first nanofiltration membrane unit 5a equipped with a nanofiltration membrane, and separated into a permeate 7a of the first nanofiltration membrane unit and a concentrate 8a of the first nanofiltration membrane unit. Subsequently, as a dilution process, dilution water in the first tank 1 is added to the concentrate 8a of the first nanofiltration membrane unit. Furthermore, an acid chemical solution 11 is appropriately added to the concentrate 8a to which dilution water has been added, and a pH control process is performed to adjust the pH. After that, the liquid is sent to the second nanofiltration membrane unit 5b, and separated into a permeate 7b of the second nanofiltration membrane unit and a concentrate 8b of the second nanofiltration membrane unit. Subsequently, as a dilution process, dilution water in the first tank 1 is added to the concentrate 8b of the second nanofiltration membrane unit. Furthermore, an acid solution 11 is added appropriately to the concentrated solution 8b to which dilution water has been added, and a pH control process is performed to adjust the pH, after which the solution is sent to the third nanofiltration membrane unit 5c and separated into a permeate 7c from the third nanofiltration membrane unit and a concentrated solution 8c from the third nanofiltration membrane unit. The concentrated solution 8c from the third nanofiltration membrane unit is recovered in the third tank 3. The permeates 7a to 7c are mixed and sent to the reverse osmosis membrane unit 6, where the reverse osmosis filtration process is performed. The concentrated solution 10 from the reverse osmosis membrane unit 6 is recovered in the fourth tank 4. The permeate 9 from the reverse osmosis membrane unit is sent to the first tank and may be reused as dilution water. After the reverse osmosis filtration process is completed, the liquid from the fourth tank 4 in which alkali metal ions have been concentrated is recovered.

[0102] 5 is a schematic flow diagram showing a batch-type alkali metal ion separation method according to another embodiment of the present invention. The liquid to be treated stored in the second tank 2 is sent to a first nanofiltration membrane unit 5a equipped with a nanofiltration membrane, and separated into a permeate 7a of the first nanofiltration membrane unit and a concentrate 8a of the first nanofiltration membrane unit. The permeate 7a of the first nanofiltration membrane unit is sent to the third tank 3 at a constant flow rate, and the concentrate 8a of the first nanofiltration membrane unit is sent to the second tank 2 and mixed with the remainder of the liquid to be treated in the second tank 2. A circulation process is performed. In addition, a dilution process is performed in which dilution water in the first tank 1 is added to the second tank 2 at the same flow rate as the permeate 7a of the first nanofiltration membrane unit. In addition, a pH control process is performed in which an acid chemical solution 11 is added to the second tank 2 as appropriate to adjust the pH. At this time, the dilution water in the first tank 1 may be an acidic solution, and the dilution process and pH control process may be performed simultaneously. Next, the permeate 7a from the first nanofiltration membrane unit in the third tank 3 is sent to a second nanofiltration membrane unit 5b equipped with a nanofiltration membrane, where it is separated into the permeate 7b from the second nanofiltration membrane unit and the concentrate 8b. The permeate 7b from the second nanofiltration membrane unit is sent to the fourth tank 4 at a constant flow rate, and the concentrate 8b from the second nanofiltration membrane unit is sent to the third tank 3 and mixed with the remainder of the permeate 7a from the first nanofiltration membrane unit in the third tank 3, in a circulation process. Since the permeate 7a from the first nanofiltration membrane unit is sent to the second nanofiltration membrane unit, the risk of scale formation is low, so the dilution process and pH control process are not necessary. The permeate 7b from the second nanofiltration membrane unit in the fourth tank 4 is sent to the reverse osmosis membrane unit 6, where the reverse osmosis filtration process is performed. The concentrate 10 from the reverse osmosis membrane unit is mixed with the fourth tank 4 and the circulation process is performed. The permeate 9 from the reverse osmosis membrane unit is sent to the first tank and can be reused as dilution water. After the reverse osmosis filtration process is completed, the liquid in the fourth tank 4 in which the alkali metal ions are concentrated is recovered.

[0103] (4) Alkali metal ion separation device The alkali metal ion separation device of the present invention comprises a nanofiltration means for separating a liquid to be treated containing alkali metal ions into a permeate liquid and a concentrate, a dilution means for adding dilution water to at least one of the liquid to be treated and the concentrate, a circulation means for mixing the concentrate with the remainder of the liquid to be treated, a pH measurement means for measuring the pH of at least one of the liquid to be treated and the concentrate, an acid solution addition means for adding an acid solution to at least one of the liquid to be treated, the concentrate, and the dilution water, and an addition amount control means for controlling the amount of acid solution added so that the pH of at least one of the liquid to be treated and the concentrate is below the critical pH.

[0104] The nanofiltration means provided in the separation device of the present invention preferably includes a pressure vessel (hereinafter also referred to as a "vessel") filled with spiral-wound elements (hereinafter also referred to as "nanofiltration membrane elements") incorporating nanofiltration membranes capable of separating alkali metal ions, and a nanofiltration membrane unit capable of supplying a solution to the vessel using a high-pressure pump. The nanofiltration membrane unit may be configured by connecting multiple vessels in parallel or series, with each vessel being filled with multiple nanofiltration membrane elements. Nanofiltration membrane elements of any diameter and length can be used. Nanofiltration membrane elements vary in size depending on the membrane area; for the same membrane type, the larger the membrane area, the greater the amount of liquid that can be processed per unit time. The size and number of nanofiltration membrane elements can be determined arbitrarily depending on the scale of the solution to be processed.

[0105] The dilution means is not particularly limited as long as it has a mechanism that can add dilution water to the liquid to be treated or the concentrated liquid. For example, if a reverse osmosis filtration step is provided, the dilution means may be a facility that adds the permeate from the reverse osmosis filtration unit as dilution water.

[0106] The circulation means is not particularly limited as long as it is configured to send the concentrated liquid to the liquid to be treated.

[0107] The pH measuring means is not particularly limited as long as it has a mechanism capable of measuring the pH of the liquid to be treated or the concentrated liquid. For example, a pH meter or the like may be mounted in a storage tank for the liquid to be treated or in a pipe for transporting the concentrated liquid.

[0108] The acid solution adding means and the addition amount control means are not particularly limited as long as they have a mechanism for adding an acid solution to at least one of the liquid to be treated, the concentrated liquid, and the dilution water, and controlling the amount of acid solution added so that the pH of at least one of the liquid to be treated and the concentrated liquid is below the critical pH. For example, the system may be one that adds the acid solution directly to a tank storing the liquid to be treated that has a stirring means, or one that adds the acid solution to a tank storing the dilution water. Among these, it is preferable to have a chemical feed pump for adding the acid solution to the liquid to be treated, and a mechanism that feeds back the pH value of the liquid to be treated to the chemical feed pump and adjusts the amount of acid solution added. The acid to be added is not particularly limited, and examples include sulfuric acid, hydrochloric acid, and nitric acid.

[0109] Furthermore, it is preferable that the acid solution adding means and the adding amount control means can control the pH of at least one of the liquid to be treated and the concentrated liquid to be greater than or equal to 0 and less than the critical pH, and it is more preferable that they can control it to be greater than or equal to (critical pH - 1) and less than the critical pH.

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

[0111] <Performance of nanofiltration membrane and reverse osmosis membrane> (Glucose removal rate, isopropyl alcohol removal rate) When a 1000 mg / L glucose aqueous solution at 25 ° C. and pH 6.5 was passed through a nanofiltration membrane or a reverse osmosis membrane at an operating pressure of 0.5 MPa, the glucose concentrations of the permeate and the feed solution were measured, and when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 was passed through a nanofiltration membrane at an operating pressure of 0.5 MPa, the isopropyl alcohol removal rate and the glucose removal rate were calculated from the isopropyl alcohol concentrations of the permeate and the feed solution using the following formulas (4) and (5). Isopropyl alcohol removal rate (%)=100×(1-(isopropyl alcohol concentration in permeate / isopropyl alcohol concentration in feed))...formula (4) Glucose removal rate (%)=100×(1-(glucose concentration in permeate / glucose concentration in feed))...formula (5) The isopropyl alcohol concentration was determined using a gas chromatograph (GC-18A, manufactured by Shimadzu Corporation), and the glucose concentration was determined using a refractometer (RID-6A, manufactured by Shimadzu Corporation).

[0112] (Magnesium sulfate removal rate, magnesium chloride removal rate) As a feed solution, 2000 mg / L of magnesium sulfate (hereinafter referred to as "MgSO 4 ") at 25°C and pH 6.5 was used. ４ The magnesium sulfate concentrations of the permeate and feed solutions when an aqueous solution of 2000 mg / L magnesium chloride (hereinafter referred to as "MgCl") at pH 6.5 at 25°C were measured. ２ ") aqueous solution was passed through a nanofiltration membrane at an operating pressure of 0.5 MPa. From the magnesium sulfate concentrations of the permeate and feed solution, MgSO was calculated using the following equations (6) and (7). ４ Removal rate and MgCl ２ The removal rate was calculated. The electrical conductivity of the feed solution and the permeate was measured using an electrical conductivity meter manufactured by Toa Denpa Kogyo Co., Ltd., and the practical salt content of each solution, i.e., MgSO ４ concentration, and MgCl ２ The concentration was calculated. ４ Removal rate (%) = 100 × {1 - (MgSO of permeate)} ４ Concentration / Feed MgSO ４Concentration)} ...Formula (6) MgCl ２ Removal rate (%) = 100 x {1-(MgCl of permeate ２ Concentration / Feed MgCl ２ concentration)} ...Formula (7)

[0113] <Analysis of nanofiltration membrane> (N / O ratio of separation functional layer surface) The composite semipermeable membrane was immersed in hydrochloric acid at 25 ° C. and pH 2 for 30 minutes, then immersed in pure water at 90 ° C. for 30 minutes, the membrane surface was washed, and then dried at room temperature under vacuum to obtain a measurement sample. Wide scan analysis by XPS measurement was performed on the separation functional layer surface of the composite semipermeable membrane under the following conditions, and composition analysis of elements detected in the range of 0 eV to 1400 eV was performed. The obtained number of nitrogen atoms (%) was defined as N, and the number of oxygen atoms (%) was defined as O, and the N / O ratio was calculated. Note that, to calculate the N / O ratio, measurements were repeated 10 times at different positions, and the average value was used. Measurement equipment: X-ray photoelectron spectrometer SSX-100 manufactured by SSI, USA Excitation X-ray: Aluminum Kα1 ray, Kα2 ray (1486.6 eV) X-ray diameter: 200 μm X-ray output: 10 kV, 20 mV Photoelectron escape angle: 90° (inclination of the detector relative to the sample surface)

[0114] <Preparation of Solution to be Treated> (Solution X) Lithium sulfate and aluminum sulfate were dissolved in water to give the concentrations shown in Table 1, and sulfuric acid was used to adjust the pH to 1 to prepare Solution X, which had a lithium to aluminum molar ratio of 1:1 and simulated a sulfuric acid leaching solution of ore. It was assumed that the acid leaching rate from the ore was 100%.

[0115] (Solution Y) Lithium sulfate, nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in water to give concentrations shown in Table 1, and the solution was adjusted to pH 1 using sulfuric acid to prepare Solution Y having a lithium, manganese, cobalt, and nickel molar ratio of 1:0.6:0.2:0.2, which simulates the sulfuric acid leaching solution of used lithium-ion batteries (MCN622). It was assumed that the acid leaching rate from used lithium-ion batteries was 100%.

[0116] (Solution Z) Solution Z was prepared in the same manner as solution X, except that sulfuric acid was used to adjust the pH to 2.3.

[0117]

[0118] <Confirmation of pH at which scale occurs> A portion of Solution X was taken out, and sodium hydroxide was added little by little, followed by stirring. When it was confirmed visually that scale had occurred, the addition of sodium hydroxide was stopped. The pH of Solution X at this time was 3.1. The pH at which this scale occurred was defined as the pH at which scale occurred for Solution X.

[0119] <Preparation of nanofiltration membrane, reverse osmosis membrane, and element> (Nanofiltration membrane A) Nonwoven fabric made of polyester fiber (air permeability 1 cc / cm ２ A 18.0 mass % dimethylformamide solution of polysulfone was cast onto the support membrane (160 μm thick) at room temperature (25° C.) to a thickness of 180 μm, and the support membrane was immediately immersed in pure water and left for 5 minutes to produce a porous support membrane (160 μm thick) made of fiber-reinforced polysulfone.

[0120] Next, air adjusted to 25 ° C was blown to remove excess moisture, and the membrane surface temperature of the porous support membrane was adjusted to 25 ° C. 2,5-dimethylpiperazine 2.0 mass%, dodecyl diphenyl ether disulfonate 250 ppm, 30 ° C aqueous solution containing 1.0 mass% trisodium phosphate was applied to the surface of the porous support membrane and left to stand for 15 seconds, and then nitrogen was blown from the air nozzle to remove excess aqueous solution to form a coating layer of amine aqueous solution on the porous support membrane. Furthermore, a 38 ° C n-decane solution containing 0.2 mass% TMC was uniformly applied to the entire surface of the porous support membrane, and then interfacial polycondensation was performed by leaving it to stand for 1 minute at a relative humidity of 80% and a temperature of 25 ° C., and two fluids (pure water and air) were sprayed on the membrane surface to remove the surface solution. Then, it was washed with pure water at 80 ° C to obtain nanofiltration membrane A.

[0121] (Nanofiltration Membrane B) Nanofiltration membrane B was prepared in the same manner as nanofiltration membrane A, except that 2,5-dimethylpiperazine was used as piperazine and the membrane was left standing at a temperature of 40° C. for 1 minute.

[0122] (Nanofiltration Membrane C) Nanofiltration membrane C was prepared in the same manner as nanofiltration membrane A, except that 2,5-dimethylpiperazine was replaced with piperazine and the TMC concentration was changed to 0.05% by mass.

[0123] (Nanofiltration membrane D) A nanofiltration membrane was produced in the same manner as nanofiltration membrane B, except that the TMC concentration was 0.4 mass%, to obtain nanofiltration membrane D. The membrane performance of nanofiltration membranes A to D is shown in Table 2.

[0124]

[0125] (Reverse osmosis membrane E) A porous support membrane was prepared in the same manner as in nanofiltration membrane A, and the membrane surface temperature of the porous support membrane was adjusted to 25 ° C. while removing excess water by blowing air adjusted to 25 ° C. After immersion for 15 seconds in an aqueous solution containing 1.8% by mass of m-phenylenediamine, nitrogen was blown from an air nozzle to remove excess aqueous solution, and then a 30 ° C n-decane solution containing 0.07% by mass of TMC was uniformly applied to the entire surface of the porous support membrane, and then left to stand for 1 minute at 30 ° C., and two fluids (pure water and air) were sprayed 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 E. The isopropyl alcohol removal rate of the reverse osmosis membrane E was 89.0%.

[0126] (Spiral element) Using the above nanofiltration membrane A, nanofiltration membrane B, nanofiltration membrane C, nanofiltration membrane D or reverse osmosis membrane E, an effective membrane area of ​​0.5 m ２ A spiral element (hereinafter also simply referred to as "element") having a diameter of 6.4 cm and a length of 30 cm was prepared.

[0127] <Evaluation of recovery of alkali metal ions in permeate> The concentration of each ion in the permeate sampled after the nanofiltration step was measured by an ICP emission spectrometer for cations and by ion chromatography for anions.

[0128] (Lithium Recovery Rate) The lithium recovery rate was calculated using the following formula (8): Lithium recovery rate (%) = {(volume of concentrated liquid after reverse osmosis process) × (lithium concentration in concentrated liquid after reverse osmosis process)} / {(initial volume of liquid to be treated) / (initial lithium concentration of liquid to be treated)} Formula (8)

[0129] (Lithium Ion Purity) The purity of lithium ions was calculated using the following formula (9): Lithium purity (%) = 100 × {lithium ion concentration in the concentrated solution after the reverse osmosis process / (lithium ion concentration in the concentrated solution after the reverse osmosis process + sum of polyvalent metal ion concentrations in the concentrated solution after the reverse osmosis process)} (9)

[0130] <Determination of critical pH> The above-mentioned "(a) pre-process" was carried out. Specifically, the solution X, solution Y or solution Z was used in an element using the above-mentioned nanofiltration membrane A, nanofiltration membrane B, nanofiltration membrane C or nanofiltration membrane D, and an initial amount of the liquid to be treated V ０ The volume was set to 20 L, and the cross-flow filtration method was used. The permeate flow rate: the concentrate flow rate = 1:9, and the permeate flow rate Q Ｔ Nanofiltration was performed under a constant flow rate of 0.17 L / min (0.003 L / s). The alkali metal ion rejection rate (lithium ion rejection rate) R was calculated from the alkali metal ion concentrations of the obtained permeate and concentrate using the above-mentioned formula (2). The treatment time A per unit alkali metal ion (lithium ion) recovery rate was calculated from the obtained value using the above-mentioned formula (3).

[0131] Next, the above-mentioned "(b) critical pH measurement step" was carried out. Specifically, in the system configuration shown in FIG. 1, solution X or solution Y was filtered through an element using nanofiltration membrane A by crossflow filtration, with a permeate flow rate: concentrate flow rate = 1:9 and a constant flow rate of 0.17 L / min. Diluted water, which is water with an electrical conductivity of less than 10 μS / cm stored in the first tank 1, was sent to solution X or solution Y in the second tank 2 under the same condition of 0.17 L / min as the permeate flow rate. Furthermore, the obtained concentrate 7 of the nanofiltration membrane unit was also mixed with the remainder of solution X or solution Y in the second tank 2. The operating pressure P of the nanofiltration membrane unit against the treatment time t during the above crossflow filtration was plotted, and the change in operating pressure P of the nanofiltration membrane unit per treatment time t (ΔP / Δt) was calculated, and filtration was continued until ΔP / Δt > 0. At this time, Δt was set to 5 A. If there was no point where ΔP / Δt>0 within 50 A of treatment time t (t≦50 A), the average pH value of the treated liquid at the treatment time when ΔP / Δt>0 first occurred was calculated and used as the critical pH.

[0132] If there was a time point where ΔP / Δt>0 within the treatment time t of 50 A (t≦50 A), the above-mentioned "(C) re-measurement step" was carried out, and then the pH of the treated liquid was lowered by 0.5 and the critical pH measurement step was carried out again. The measurement results are shown in Table 3.

[0133]

[0134] <Evaluation of the amount of dilution water> For Examples 15 to 19, the dilution water amount ratio, which is an index of the effect of reducing the amount of dilution water added, was calculated using the following formula (10). The smaller the dilution water amount ratio, the less dilution water was added, which means a smaller amount of sulfuric acid was added, and the more efficient the process is. Dilution water amount ratio = total amount of dilution water added to the liquid to be treated in the dilution step (L) / total amount of permeate obtained in the nanofiltration step (L) ... formula (10)

[0135] <Evaluation of treatment time in constant pressure operation> For Examples 15 to 19, the total treatment time T Ａ The treatment time T when the operating pressure of the nanofiltration process is 30% or more and 100% or less of the pressure resistance value of the nanofiltration membrane ３０ The ratio T ３０ / T Ａ and the treatment time T when the operating pressure of the nanofiltration process is 50% or more and 100% or less of the pressure resistance value of the nanofiltration membrane. ５０ The ratio T ５０ / T Ａ was calculated. ３０ / T Ａ or T ５０ / T Ａ The larger the value, the longer the time for which constant pressure operation was carried out, and the greater the effect of reducing the amount of dilution water added.

[0136] [Example 1] In a system configuration equipped with a circulation process shown in Figure 3, 20 L of solution X was placed in the second tank 2 as the liquid to be treated, a membrane element using a nanofiltration membrane A as the nanofiltration membrane unit 5a, and a membrane element using a reverse osmosis membrane E as the reverse osmosis membrane unit 6 was used, and in a cross-flow filtration system, the permeate flow rate: concentrated liquid flow rate = 1:9, constant flow rate operation was performed under the conditions of a permeate flow rate of 0.17 L / min, and the nanofiltration process of the liquid to be treated was carried out. At the same time as discharging the permeate 7a of the nanofiltration membrane unit 5a into the third tank 3, concentrated sulfuric acid was added to the RO water obtained through the reverse osmosis membrane, and diluted water adjusted to pH -0.2 was added to the second tank 2 from the first tank 1 at a flow rate of 0.17 L / min, and the dilution process and pH control process were carried out simultaneously. In addition, in the pH control process, concentrated sulfuric acid was added as the acid chemical solution 11 as appropriate, and the pH of the liquid to be treated in the second tank 2 during the nanofiltration process was controlled to -0.2, with a control width of ± 0.1 or less. The pressure resistance value of the nanofiltration membrane unit was 5.0 MPa, and the pressure resistance value of the reverse osmosis membrane unit was 8.0 MPa. The nanofiltration process was continued until the lithium recovery rate in the nanofiltration process reached 90% or more. The maximum operating pressure of the nanofiltration process was 1.5 MPa. After completion of the nanofiltration process, the permeate 7a from the first nanofiltration membrane unit discharged into the third tank 3 was sent to an element using a reverse osmosis membrane E as the reverse osmosis membrane unit 6, and the reverse osmosis filtration process was carried out by constant flow operation using a cross-flow filtration method under conditions of a permeate flow rate: concentrate flow rate = 1:9 and a permeate flow rate of 0.17 L / min. In the reverse osmosis filtration process, filtration was continued until the lithium concentration of the solution in the third tank 3 reached 2.0 g / L or more, or until the operating pressure of the reverse osmosis membrane unit reached 8.0 MPa. The lithium ion concentration was measured using a commercially available lithium ion electrode (DX207-Li ISE HALF-CELL manufactured by METTLER TOLEDO). The operation results of this system are shown in Table 3.

[0137] In this example, both the lithium recovery rate and the lithium ion purity were 90% or more, and lithium was recovered with high efficiency. The maximum operating pressure of the reverse osmosis filtration process reached 8.0 MPa.

[0138] Example 2 The liquid to be treated was filtered in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 0.5 and the pH of the dilution water was set to 0.5. The operation results of this system are shown in Table 3. Compared to Example 1, the pH of the liquid to be treated in the second tank 2 was higher, and both the lithium recovery rate and the lithium ion purity were higher, but as in Example 1, the maximum operating pressure of the reverse osmosis filtration step reached 8.0 MPa.

[0139] Example 3 The liquid to be treated was filtered in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 0.8 and the pH of the dilution water was set to 0.8. The operation results of this system are shown in Table 3. As in Example 2, both the lithium recovery rate and the lithium ion purity were high. Furthermore, compared to Example 2, the sulfuric acid concentration of the liquid to be treated in tank 2 was lower, the maximum operating pressure of the reverse osmosis filtration step was 6.8 MPa, and it was possible to concentrate the lithium concentration of the concentrated liquid to a predetermined value.

[0140] Example 4 The liquid to be treated was filtered in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 1.1 and the pH of the dilution water was also set to 1.1. The operation results of this system are shown in Table 3. Compared to Example 3, the lithium recovery rate and lithium ion purity were similar, but the sulfuric acid concentration of the liquid to be treated in the second tank 2 was lower, and the maximum operating pressure of the reverse osmosis filtration step was reduced.

[0141] Example 5 The liquid to be treated was filtered in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 1.3 and the pH of the dilution water was also set to 1.3. The operation results of this system are shown in Table 3. Compared to Example 4, the lithium recovery rate and lithium ion purity were similar, but the sulfuric acid concentration of the liquid to be treated in the second tank 2 was lower, and the maximum operating pressure of the reverse osmosis filtration step was reduced.

[0142] Example 6 The liquid to be treated was filtered in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 1.9 and the pH of the dilution water was also set to 1.9. The operation results of this system are shown in Table 3. Compared to Example 5, the lithium recovery rate and lithium ion purity were similar, but the sulfuric acid concentration of the liquid to be treated in tank 2 was lower, and the maximum operating pressure of the reverse osmosis filtration step was reduced.

[0143] Example 7 Filtration of the liquid to be treated was carried out in the same manner as in Example 6, except that an element using nanofiltration membrane B was used as the nanofiltration membrane unit. The operation results of this system are shown in Table 3. Even when nanofiltration membrane B was used, the lithium recovery rate was 90% or more, and the lithium ion purity was 85% or more, meaning that lithium could be recovered with high efficiency.

[0144] Example 8 Filtration of the liquid to be treated was carried out in the same manner as in Example 5, except that solution Y was used as the liquid to be treated. The operation results of this system are shown in Table 5. Compared to Example 5, the concentration of polyvalent metal ions in the liquid to be treated increased, which increased the maximum operating pressure in the nanofiltration step and slightly reduced the purity of lithium ions, but lithium was still able to be recovered with high efficiency.

[0145] Example 9 The liquid to be treated was filtered in the same manner as in Example 8, except that the pH of the liquid to be treated in the second tank 2 was controlled to 1.7 and the pH of the dilution water was also set to 1.7. The operation results of this system are shown in Table 5. Compared to Example 8, the lithium recovery rate and lithium ion purity were similar, but the sulfuric acid concentration of the liquid to be treated in the second tank 2 was lower, and the maximum operating pressure of the reverse osmosis filtration step was reduced.

[0146] Example 10 The liquid to be treated was filtered in the same manner as in Example 8, except that the pH of the liquid to be treated in the second tank 2 was controlled to 2.3 and the pH of the dilution water was also set to 2.3. The operation results of this system are shown in Table 5. Compared to Example 9, the lithium recovery rate and lithium ion purity were similar, but the sulfuric acid concentration of the liquid to be treated in tank 2 was lower, and the maximum operating pressure of the reverse osmosis filtration step was reduced.

[0147] [Example 11] In a system configuration equipped with a multi-stage concentration process as shown in Figure 4, 20 L of solution Z was placed in the second tank 2 as the liquid to be treated, and elements using nanofiltration membrane A were used as the first nanofiltration membrane unit 5a, the second nanofiltration membrane unit 5b, and the third nanofiltration membrane unit 5c. In the cross-flow filtration method, the permeate flow rate: concentrated liquid flow rate = 1:1, and the permeate flow rate of the first nanofiltration membrane unit was 0.17 L / min. Constant flow rate operation was performed under the conditions. Subsequently, dilution water adjusted to pH 1.9 at a flow rate of 0.17 L / min was added to the concentrated liquid 8a of the first nanofiltration membrane unit, and a dilution process and a pH control process were carried out. In addition, in the pH control process, concentrated sulfuric acid was added as the acid chemical solution 11 as appropriate to control the pH of the concentrated liquid 8a of the first nanofiltration membrane unit to 1.9. The concentrate 8a of the first nanofiltration membrane unit that had undergone the dilution process and the pH control process was supplied to the second nanofiltration membrane unit 5b, and the cross-flow filtration system was operated at a constant flow rate under the conditions of a permeate flow rate: concentrate flow rate = 1:1 and a permeate flow rate of 0.17 L / min. The dilution process and the pH control process were carried out. The concentrate 8b of the second nanofiltration membrane unit was adjusted to pH 1.9 at a flow rate of 0.17 L / min, and dilution water was added to the concentrate 8b of the second nanofiltration membrane unit. The pH of the concentrate 8b of the second nanofiltration membrane unit was adjusted to 1.9 by adding concentrated sulfuric acid as the acid solution 11. The concentrate 8b of the second nanofiltration membrane unit that had undergone the dilution process and the pH control process was supplied to the third nanofiltration membrane unit 5c, and the cross-flow filtration system was operated at a constant flow rate under the conditions of a permeate flow rate: concentrate flow rate = 1:1 and a permeate flow rate of 0.17 L / min. The third nanofiltration membrane unit 5c was operated at a constant flow rate. The permeates 7a to 7c from the first nanofiltration membrane unit 5a, the second nanofiltration membrane unit 5b, and the third nanofiltration membrane unit 5c were mixed and sent to an element using reverse osmosis membrane E as the reverse osmosis membrane unit 6, and the reverse osmosis filtration step was carried out by constant flow operation using a cross-flow filtration method under conditions of a permeate flow rate: concentrate flow rate = 2:1 and a permeate flow rate of 0.34 L / min. The concentrate 10 obtained in the reverse osmosis filtration step and containing lithium ions was recovered in the fourth tank 4. The operating results of this system are shown in Table 5.Even in the case of a continuous system in which the concentrated liquid from each nanofiltration membrane unit is not returned to the second tank 2, scale did not occur by controlling the pH of the liquid to be treated at or below the critical pH when it was sent to the nanofiltration membrane unit.

[0148] Example 12 Filtration of the liquid to be treated was carried out in the same manner as in Example 6, except that elements using nanofiltration membrane C were used as nanofiltration membrane units 5a and 5b. The operation results of this system are shown in Table 5. When nanofiltration membrane C was used, lithium was able to be recovered with high efficiency, with a lithium recovery rate of 90% or more and a lithium ion purity of 83% or more.

[0149] Example 13 Filtration of the liquid to be treated was carried out in the same manner as in Example 6, except that elements using nanofiltration membrane D were used as nanofiltration membrane units 5a and 5b. The operation results of this system are shown in Table 5. When nanofiltration membrane D was used, lithium was able to be recovered with high efficiency, with a lithium recovery rate of 90% or more and a lithium ion purity of 84% or more.

[0150] [Example 14] In the system configuration shown in Figure 5, the nanofiltration process was performed with a multi-stage permeation process in which the permeate 7a of the first nanofiltration membrane unit 5a was nanofiltrated by the second nanofiltration membrane unit 5b. 20 L of solution X was placed in the second tank 2 as the liquid to be treated, and a membrane element using nanofiltration membrane A was used as the first nanofiltration membrane unit 5a. A cross-flow filtration system was used, with a permeate flow rate: concentrated liquid flow rate = 1:9, and a permeate flow rate of 0.17 L / min. Constant flow rate operation was performed, and nanofiltration of the liquid to be treated was performed. At the same time as discharging the permeate 7a of the first nanofiltration membrane unit 5a into the third tank 3, concentrated sulfuric acid was added to the RO water as an acid chemical solution 11, and diluted water adjusted to pH 1.9 was added to the second tank 2 at a flow rate of 0.17 L / min from the first tank 1. A dilution process and a pH control process were simultaneously performed. Further, in the pH control step, concentrated sulfuric acid was added as an acid chemical solution 11 as appropriate, and the pH of the liquid to be treated in the second tank 2 was controlled to 1.9, with a control range of ±0.1. Subsequently, the permeate 7a of the first nanofiltration membrane unit 5a discharged into the third tank 3 was used as the nanofiltration membrane unit 5b using a membrane element using nanofiltration membrane B, and nanofiltration was performed by cross-flow filtration under the conditions of a permeate flow rate: concentrated liquid flow rate = 1:9 and a permeate flow rate of 0.17 L / min. The permeate 7b of the second nanofiltration membrane unit 5b was discharged into the fourth tank 4. The pressure resistance value of the nanofiltration membrane unit was 5.0 MPa, and the pressure resistance value of the reverse osmosis membrane unit was 8.0 MPa. The nanofiltration step was continued until the lithium recovery rate in the permeate 7b of the second nanofiltration membrane unit 5b discharged into the fourth tank 4 was 90% or more, or until the operating pressure of either the first nanofiltration membrane unit or the second nanofiltration membrane unit reached 5.0 MPa. After the nanofiltration step, the permeate 7b from the second nanofiltration membrane unit 5b discharged into the fourth tank 4 was sent to an element using a reverse osmosis membrane E as the reverse osmosis membrane unit 6, and the reverse osmosis filtration step was carried out by constant flow operation using a cross-flow filtration method under conditions of a permeate flow rate: concentrate flow rate = 1:9 and a permeate flow rate of 0.17 L / min. In the reverse osmosis filtration step, filtration was continued until the lithium concentration reached 2.0 g / L or more or until the operating pressure of the reverse osmosis membrane unit reached 8.0 MPa. The operating results of this system are shown in Table 5.By using multiple permeation stages in the nanofiltration process, the purity of lithium ions was significantly increased, enabling highly efficient recovery of lithium.

[0151] Example 15 Filtration of the liquid to be treated was carried out in the same manner as in Example 6, except that the flow rate of dilution water added from the first tank 1 to the second tank 2 was controlled so that the operating pressure in the nanofiltration step was constant at the initial operating pressure of 1.5 MPa. The operating results of this system are shown in Table 6. When the operating pressure in the nanofiltration step was maintained at the maximum operating pressure of 1.5 MPa and operation was carried out at a constant flow rate and pressure, the dilution water volume ratio decreased, and the maximum operating pressure in the reverse osmosis filtration step was reduced compared to Example 6.

[0152] Example 16 Nanofiltration was carried out at a constant flow rate without adding dilution water until the operating pressure of the nanofiltration step reached 2.0 MPa, and then the flow rate of dilution water added from the first tank 1 to the second tank 2 was controlled so that the operating pressure of the nanofiltration step remained constant at 2.0 MPa. The filtration of the treated liquid was carried out in the same manner as in Example 15, except that the flow rate of dilution water added from the first tank 1 to the second tank 2 was controlled so that the operating pressure of the nanofiltration step remained constant at 2.0 MPa. The operating results of this system are shown in Table 6. When the operating pressure of the nanofiltration step was maintained at 2.0 MPa, which was higher than the initial operating pressure, and the system was operated at a constant flow rate and constant pressure, the dilution water volume ratio was reduced, and the maximum operating pressure of the reverse osmosis filtration step was reduced compared to Example 15.

[0153] Example 17 Nanofiltration was performed without adding dilution water until the operating pressure of the nanofiltration step reached 2.5 MPa, and then the flow rate of dilution water added from the first tank 1 to the second tank 2 was controlled so that the operating pressure of the nanofiltration step remained constant at 2.5 MPa. The operation results of this system are shown in Table 6. When the operating pressure of the nanofiltration step was maintained at 2.5 MPa, which was higher than the initial operating pressure, and the system was operated at a constant flow rate and pressure, the dilution water volume ratio decreased, and the maximum operating pressure of the reverse osmosis filtration step decreased compared to Example 16.

[0154] Example 18 Nanofiltration was performed without adding dilution water until the operating pressure of the nanofiltration step reached 3.0 MPa, and then the flow rate of dilution water added from the first tank 1 to the second tank 2 was controlled so that the operating pressure of the nanofiltration step remained constant at 3.0 MPa. The operation results of this system are shown in Table 6. When the operating pressure of the nanofiltration step was maintained at 3.0 MPa, which was higher than the initial operating pressure, and the system was operated at a constant flow rate and pressure, the dilution water volume ratio decreased, and the maximum operating pressure of the reverse osmosis filtration step decreased compared to Example 17.

[0155] Example 19 Nanofiltration was performed without adding dilution water until the operating pressure of the nanofiltration step reached 4.0 MPa, and then the flow rate of dilution water added from the first tank 1 to the second tank 2 was controlled so that the operating pressure of the nanofiltration step remained constant at 4.0 MPa. The operation results of this system are shown in Table 6. When the operating pressure of the nanofiltration step was maintained at 4.0 MPa, which was higher than the initial operating pressure, and the system was operated at a constant flow rate and pressure, the dilution water volume ratio decreased, and the maximum operating pressure of the reverse osmosis filtration step decreased compared to Example 18.

[0156] Example 20 Filtration of the liquid to be treated was carried out in the same manner as in Example 10, except that the flow rate of dilution water added from the first tank 1 to the second tank 2 was controlled so that the operating pressure in the nanofiltration step was constant at the initial operating pressure of 3.5 MPa. The operating results of this system are shown in Table 6. When the operating pressure in the nanofiltration step was maintained at the maximum operating pressure of 3.5 MPa and operation was carried out at a constant flow rate and pressure, the dilution water volume ratio decreased, and the maximum operating pressure in the reverse osmosis filtration step was reduced compared to Example 10.

[0157] [Example 21] Processing time T of nanofiltration step ３０ The liquid to be treated was filtered in the same manner as in Example 16, except that the flow rate of the dilution water added from the first tank 1 to the second tank 2 was controlled so that the time required for the filtration was 2 hours. The operation results of this system are shown in Table 6. ３０ / T ＡWhen operated at 0.1, the dilution water volume ratio increased compared to Example 16, but the maximum operating pressure of the reverse osmosis filtration step decreased compared to Example 6 because the pH of the dilution water was high.

[0158] [Example 22] Processing time T of nanofiltration step ３０ The liquid to be treated was filtered in the same manner as in Example 16, except that the flow rate of the dilution water added from the first tank 1 to the second tank 2 was controlled so that the time required for the treatment was 4 hours. The operation results of this system are shown in Table 6. ３０ / T Ａ When operated at 0.20, the dilution water volume ratio was reduced compared to Example 21, and the maximum operating pressure of the reverse osmosis filtration step was reduced compared to Example 6.

[0159] [Example 23] Processing time T of nanofiltration step ３０ The liquid to be treated was filtered in the same manner as in Example 16, except that the flow rate of the dilution water added from the first tank 1 to the second tank 2 was controlled so that the time required for the filtration was 9 hours. The operation results of this system are shown in Table 6. ３０ / T Ａ When the dilution water ratio was 0.50, the dilution water ratio was reduced compared to Example 22, and the maximum operating pressure of the reverse osmosis filtration step was reduced compared to Example 6.

[0160] Comparative Example 1 The treated liquid was filtered in the same manner as in Example 1, except that the pH of the treated liquid in the second tank 2 was controlled to 2.3 and the pH of the dilution water was also set to 2.3. The operating results of this system are shown in Table 7. Because the pH of the treated liquid in the second tank 2 was controlled at a value higher than the critical pH, fouling due to scale components occurred during filtration, resulting in a decrease in the lithium recovery rate and lithium ion purity. In addition, the maximum operating pressure of the nanofiltration step increased. Although the controlled pH of this comparative example was lower than the scale-generating pH of Solution X, fouling due to scale components occurred.

[0161] Comparative Example 2 The filtration of the liquid to be treated was carried out in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 3.1 and the pH of the dilution water was also set to 3.1. The operation results of this system are shown in Table 7. The pH of the liquid to be treated in the second tank 2 was controlled at the pH at which scale was generated in Solution X, but fouling due to scale components occurred during filtration, resulting in a decrease in the lithium recovery rate and lithium ion purity. In addition, the maximum operating pressure of the nanofiltration step increased.

[0162] Comparative Example 3: Solution Y was used as the liquid to be treated, and filtration of the liquid to be treated was carried out in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 2.8 and the pH of the dilution water was also set to 2.8. The operation results of this system are shown in Table 7. Because the pH of the liquid to be treated in the second tank 2 was controlled at a value higher than the critical pH, fouling due to scale components occurred during filtration, resulting in a decrease in the lithium recovery rate and lithium ion purity. In addition, the maximum operating pressure of the nanofiltration step increased.

[0163] Comparative Example 4: Solution Z was used as the liquid to be treated, and filtration of the liquid to be treated was carried out in the same manner as in Example 11, except that dilution water adjusted to a pH of 2.3 was used. The operating results of this system are shown in Table 7. Because the pH of the liquid to be treated sent to each nanofiltration membrane unit was controlled at a value higher than the critical pH, fouling due to scale components occurred during filtration, resulting in a decrease in the lithium recovery rate and lithium ion purity. In addition, the maximum operating pressure of the nanofiltration process increased.

[0164] Comparative Example 5 The permeate 7a from the first nanofiltration membrane unit 5a was discharged into the third tank 3, and at the same time, RO water with a pH of 7.0 obtained through the reverse osmosis membrane was added as dilution water from the first tank 1 to the second tank 2 at a flow rate of 0.17 L / min. The filtration of the liquid to be treated was carried out in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was not controlled. The operation results of this system are shown in Table 7. Because the pH of the liquid to be treated in the second tank 2 was not controlled, fouling due to scale components occurred during filtration, the maximum operating pressure in the nanofiltration step reached the withstand pressure value of 5.0 MPa, and the lithium recovery rate and lithium ion purity decreased.

[0165] Comparative Example 6 In the system configuration shown in FIG. 6, filtration of the liquid to be treated was carried out in the same manner as in Example 1, except that the pH of the liquid to be treated in the second tank 2 was controlled to 1.8 and no dilution water was added to the second tank 2. FIG. 6 is a schematic flow diagram showing a method for separating alkali metal ions in a comparative example. As shown in FIG. 6, the liquid to be treated stored in the second tank 2 is sent to a first nanofiltration membrane unit 5a equipped with a nanofiltration membrane, and a nanofiltration process is carried out in which the liquid is separated into a permeate 7a of the first nanofiltration membrane unit and a concentrate 8a of the first nanofiltration membrane unit. The permeate 7a of the first nanofiltration membrane unit is sent to the third tank 3 at a constant flow rate, and the concentrate 8a of the first nanofiltration membrane unit is sent to the second tank 2 and mixed with the remainder of the liquid to be treated in the second tank 2. A circulation process is carried out at this time, in which an acid solution 11 is appropriately added to the second tank 2 to adjust the pH. After the nanofiltration step, the permeate 7a from the first nanofiltration membrane unit in the third tank 3 is sent to the reverse osmosis membrane unit 6, where the reverse osmosis filtration step is performed. The concentrated solution 10 from the reverse osmosis membrane unit is mixed with the third tank 3 and the circulation step is performed. After the reverse osmosis filtration step, the liquid in the third tank 3, in which the alkali metal ions are concentrated, is recovered. Filtration in the nanofiltration step was continued until the lithium recovery rate reached 90% or more or the operating pressure of the nanofiltration step reached 5.0 MPa. Filtration in the reverse osmosis filtration step was continued until the lithium concentration reached 2.0 g / L or more or the operating pressure of the reverse osmosis filtration step reached 8.0 MPa. The operating results of this system are shown in Table 7. Because dilution water was not added to the second tank 2, the maximum operating pressure in the nanofiltration step reached 5.0 MPa, which is the pressure resistance value of the nanofiltration membrane, and the lithium recovery rate was significantly reduced.

[0166]

[0167]

[0168]

[0169]

[0170] In the above examples and comparative examples, the higher the lithium recovery rate and lithium purity, the better, and the lower the maximum operating pressure in the nanofiltration process, the maximum operating pressure in the reverse osmosis filtration process, and the dilution water volume ratio, the better. From the above results, it can be seen that Examples 1 to 23, which are the alkali metal separation methods of the present invention, can separate alkali metal ions at a high purity and high recovery rate at a low pressure compared to Comparative Examples 1 to 6.

[0171] The present invention can be suitably used as a method for efficiently separating and recovering alkali metals such as lithium from lithium ion batteries and waste materials, waste liquids, ores, slag, etc. generated in the manufacturing process of the same.

[0172] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2024-133799) filed on August 9, 2024, the contents of which are incorporated herein by reference.

[0173] 1 First tank 2 Second tank 3 Third tank 4 Fourth tank 5a First nanofiltration membrane unit 5b Second nanofiltration membrane unit 5c Third nanofiltration membrane unit 6 Reverse osmosis membrane unit 7a Permeate of first nanofiltration membrane unit 7b Permeate of second nanofiltration membrane unit 7c Permeate of third nanofiltration membrane unit 8a Concentrate of first nanofiltration membrane unit 8b Concentrate of second nanofiltration membrane unit 8c Concentrate of third nanofiltration membrane unit 9 Permeate of reverse osmosis membrane unit 10 Concentrate of reverse osmosis membrane unit 11 Acid chemical solution

Claims

1. A method for separating alkali metal ions, comprising: a nanofiltration process in which a liquid to be treated containing alkali metal ions is separated into a permeate and a concentrate using a nanofiltration membrane unit, and the concentrate is sent back to the nanofiltration membrane unit; a dilution process in which dilution water is added to at least one of the liquid to be treated and the concentrate; and a pH control process in which the pH of at least one of the liquid to be treated and the concentrate is adjusted to a critical pH or lower.

2. The method for separating alkali metal ions according to claim 1, wherein in the pH control step, the pH of at least one of the liquid to be treated and the concentrated liquid is adjusted to a value equal to or higher than 0 and lower than the critical pH.

3. The method for separating alkali metal ions according to claim 2, wherein in the pH control step, the pH of at least one of the liquid to be treated and the concentrated liquid is adjusted to a value equal to or higher than (critical pH - 1) and lower than the critical pH.

4. The method for separating alkali metal ions according to claim 3, wherein the total amount of dilution water added to the liquid to be treated in the dilution step is less than the total amount of permeate obtained in the nanofiltration step.

5. A method for separating alkali metal ions according to claim 3 or 4, wherein the dilution step includes a step of adjusting the amount of dilution water added so that the operating pressure in the nanofiltration step is 30% or more and 100% or less of the pressure resistance value of the nanofiltration membrane provided in the nanofiltration membrane unit.

6. The method for separating alkali metal ions described in claim 5, wherein the dilution step includes a step of adjusting the amount of dilution water added so that the operating pressure in the nanofiltration step is 50% or more and 100% or less of the pressure resistance value of the nanofiltration membrane provided in the nanofiltration membrane unit.

7. In the nanofiltration step, the total processing time T Ａ The treatment time T when the operating pressure of the nanofiltration step is 30% or more and 100% or less of the pressure resistance value of the nanofiltration membrane ３０ The ratio T ３０ / T Ａ 6. The method for separating alkali metal ions according to claim 5, wherein the .gtoreq..times ...

8. The method for separating alkali metal ions according to any one of claims 1 to 4, wherein the alkali metal ions include lithium ions.

9. The method for separating alkali metal ions according to any one of claims 1 to 4, wherein the nanofiltration membrane provided in the nanofiltration membrane unit has a difference of 20% or less between the magnesium sulfate rejection rate when a 2000 mg / L aqueous magnesium sulfate solution at pH 6.5 is passed through it at an operating pressure of 0.5 MPa at 25°C and 2000 mg / L and the magnesium chloride rejection rate when a 2000 mg / L aqueous magnesium chloride solution at pH 6.5 is passed through it at an operating pressure of 0.5 MPa at 25°C and 2000 mg / L, and a difference of 40% or more between the glucose rejection rate when a 1000 mg / L aqueous glucose solution at pH 6.5 is passed through it at an operating pressure of 0.5 MPa at 25°C and 2000 mg / L and the isopropyl alcohol rejection rate when a 1000 mg / L aqueous isopropyl alcohol solution at pH 6.5 is passed through it at an operating pressure of 0.5 MPa at 25°C and 2000 mg / L, and the glucose rejection rate is 70% or more and 90% or less.

10. A method for separating alkali metal ions as described in claim 8, wherein the nanofiltration membrane provided in the nanofiltration membrane unit is a composite semipermeable membrane having a support membrane and a separation functional layer containing a semi-aromatic crosslinked polyamide on the support membrane, and the N / O ratio, which is the ratio of the number of nitrogen atoms N to the number of oxygen atoms O on the surface of the separation functional layer measured by X-ray photoelectron spectroscopy, is 0.7 or more and 1.3 or less.

11. A method for determining a critical pH in a method for separating alkali metal ions, which comprises: a nanofiltration step in which a liquid to be treated containing alkali metal ions is separated into a permeate and a concentrate using a nanofiltration membrane unit, and the concentrate is sent back to the nanofiltration membrane unit; and a dilution step in which dilution water is added to at least one of the liquid to be treated and the concentrate, the method comprising the following steps (a) to (c): (a) Pre-processing: In each nanofiltration membrane unit, if the nanofiltration process is performed under constant flow rate operation, the flow rates of the permeate and concentrate at the start of the dilution process are set as filtration conditions, and the liquid to be treated is sent to the nanofiltration membrane unit under those conditions. If the nanofiltration process is performed under a flow rate other than constant, the flow rate ratio of the permeate and concentrate at the start of the dilution process and the operating pressure are set as filtration conditions, and the liquid to be treated is sent to each nanofiltration membrane unit. The alkali metal ion rejection rate R is measured from the alkali metal ion concentrations of the permeate and concentrate obtained after one hour of operation using the following formula (2), and the operation time A per unit alkali metal ion recovery rate of each nanofiltration membrane unit is calculated using the following formula (3): Alkali metal ion rejection rate R (%) = {1 - (alkali metal ion concentration of permeate) / (alkali metal ion concentration of liquid to be treated)} × 100 ... formula (2). In the above formula (3), A is the treatment time (s) per unit alkali metal ion recovery rate, V ０ is the initial amount of liquid to be treated (L), Q Ｔ where R is the permeate flow rate (L / s), and R is the alkali metal ion rejection rate (%) of the nanofiltration membrane. (b) Critical pH measurement step: For each nanofiltration membrane unit, the same permeate flow rate and concentrate flow rate as in the previous step (a) are set as filtration conditions. The treated liquid is fed to the nanofiltration membrane unit, separated into a permeate and a concentrate, and water with an electrical conductivity of less than 10 μS / cm is fed to the treated liquid at the same flow rate as the permeate, and the concentrate is mixed with the remainder of the treated liquid. While measuring the pH of the treated liquid, nanofiltration is continued until the change in operating pressure P of the nanofiltration membrane unit per treatment time t (ΔP / Δt) becomes ΔP / Δt > 0. At this time, Δt is set to 5 A. If there is no time when ΔP / Δt > 0 within a treatment time t of 50 A (t≦50 A), the average pH of the treated liquid at the treatment time Δt at which ΔP / Δt > 0 is first achieved is calculated, and this value is designated as the critical pH. However, the maximum critical pH is 4, and if the average value is 4 or higher, the critical pH is set to 4. (c) Re-measurement step: If there is a point at which ΔP / Δt>0 within 50 A of treatment time t (t≦50 A), an aqueous sulfuric acid solution of pH 1, prepared by adding sulfuric acid to water with an electrical conductivity of less than 10 μS / cm, is sent to the nanofiltration membrane unit used in the (b) critical pH measurement step, and the nanofiltration membrane unit is operated for one hour under the same operating conditions as the (a) previous step to clean the unit. After cleaning, the pH of the liquid to be treated is lowered by 0.5, and the (b) critical pH measurement step is performed again. This step is repeated until the point at which ΔP / Δt>0 becomes 50 A<t.

12. An alkali metal ion separation device comprising: a nanofiltration means for separating a liquid to be treated containing alkali metal ions into a permeate liquid and a concentrate; a dilution means for adding dilution water to at least one of the liquid to be treated and the concentrate; a circulation means for mixing the concentrate with the remainder of the liquid to be treated; a pH measurement means for measuring the pH of at least one of the liquid to be treated and the concentrate; an acid chemical addition means for adding an acid chemical to at least one of the liquid to be treated, the concentrate, and the dilution water; and an addition amount control means for controlling the amount of acid chemical added so that the pH of at least one of the liquid to be treated and the concentrate is below a critical pH.

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