Composite nanofiltration membrane for separating lithium and magnesium ions and preparation method of composite nanofiltration membrane

Abstract

The invention discloses a composite nanofiltration membrane for separating lithium and magnesium ions and a preparation method of the composite nanofiltration membrane, belongs to the technical field of membrane separation, and aims to solve the problems that in the prior art, the selectivity and permeability of a nanofiltration membrane are balanced, the separation ratio is reduced due to negative charge on the surface of the nanofiltration membrane, and the surface has defects due to uneven pore size distribution of a polyamide layer. The composite nanofiltration membrane comprises a base membrane, a middle layer and a separation layer which are sequentially stacked from bottom to top, wherein the middle layer is formed by oxidation auto-polymerization of dopamine in a weakly alkaline environment. The method can be used for separating lithium and magnesium ions.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane separation, and in particular relates to a composite nanofiltration membrane for lithium and magnesium ion separation and a preparation method thereof. Background Art

[0002] Lithium ions are mostly present in salt lake brine. Traditional processes such as extraction, adsorption, and ion exchange have drawbacks such as large footprints, high energy consumption, and environmental hazards. Membrane technology offers simple processes, low energy consumption, environmental friendliness, and precise separation. Nanofiltration membranes have a molecular weight between reverse osmosis membranes and ultrafiltration membranes. Their unique charge properties also offer unique properties for separating monovalent and polyvalent salt ions.

[0003] Existing research focuses on the high permeability and selectivity of nanofiltration membranes. A common preparation method involves interfacial polymerization, where an amine monomer (such as polyethyleneimine or piperazine) is dissolved in an aqueous phase and an acyl chloride monomer (trimesoyl chloride) is dissolved in an oil phase. A polycondensation reaction occurs at the interface between the aqueous and oil phases, forming a selective layer on the porous support layer. The selective layer primarily contributes to the nanofiltration membrane's retention. However, due to the low amount of residual amine monomer on the surface, the residual acyl chloride groups generated by the amide condensation reaction are hydrolyzed to carboxyl groups, resulting in a negative charge on the membrane surface and a reduced separation ratio. Furthermore, due to the uneven diffusion of the amine monomer at the interface, the polyamide layer often exhibits uneven pore size distribution and surface defects. Summary of the Invention

[0004] In view of the above analysis, the present invention aims to provide a composite nanofiltration membrane for lithium and magnesium ion separation and a preparation method thereof, so as to solve the problems in the prior art of the trade-off between the selectivity and permeability of the nanofiltration membrane, the negative charge on the surface of the nanofiltration membrane resulting in a reduced separation ratio, and the uneven pore size distribution of the polyamide layer resulting in surface defects.

[0005] The purpose of the present invention is mainly achieved through the following technical solutions.

[0006] In a first aspect, the present invention provides a composite nanofiltration membrane for lithium-magnesium ion separation, comprising a base membrane, an intermediate layer, and a separation layer stacked in sequence from bottom to top, wherein the intermediate layer is formed by oxidative self-polymerization of dopamine in a weakly alkaline environment.

[0007] Furthermore, the separation layer is a polyamide separation layer.

[0008] Furthermore, the base membrane is one of polyethersulfone ultrafiltration membrane, polyvinylidene fluoride ultrafiltration membrane, and polysulfone ultrafiltration membrane.

[0009] Furthermore, the pore size of the basement membrane is 1 nm to 100 nm, and the molecular weight cut-off of the basement membrane is 150 kDa.

[0010] In a second aspect, the present invention further provides a method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation, which is used for preparing the above-mentioned composite nanofiltration membrane. The preparation method comprises the following steps:

[0011] Step 1: dissolving dopamine hydrochloride particles in a buffer solution by ultrasonication to obtain a dopamine solution;

[0012] Step 2: Cover the surface of the base film with a dopamine solution, shake it to allow the dopamine to self-polymerize, and then wash it to form an intermediate layer to obtain a composite film;

[0013] Step 3: Dipping the composite membrane into polyethyleneimine, taking it out and air-drying it to obtain an impregnated composite membrane;

[0014] Step 4: placing the impregnated composite membrane in an organic solvent solution of a cross-linking agent to carry out an interfacial polymerization reaction to form a separation layer, thereby obtaining a composite nanofiltration membrane.

[0015] Furthermore, in step 4, after the interfacial polymerization reaction, the following steps are further included:

[0016] The prepared composite film is heat-treated at a temperature of 50° C. to 70° C. and for a time of 5 min to 10 min.

[0017] Furthermore, in step 1, the buffer solution is tris(hydroxymethyl)aminomethane hydrochloride sodium acetate solution.

[0018] Furthermore, in step 4, the organic solvent is a mixture of one or more of n-hexane, n-heptane, benzene, and toluene in any proportion.

[0019] Furthermore, step 1 includes the following steps:

[0020] Step 11: Dissolve the dopamine hydrochloride particles in a buffer solution;

[0021] Step 12: Place the mixed solution of dopamine hydrochloride and the buffer solution under a nitrogen environment and stir to obtain a dopamine solution.

[0022] Furthermore, in step 1, the concentration of the dopamine solution is 0.1 wt% to 0.4 wt%; the concentration of the buffer solution is 45 to 55 mM, and the pH is 8.0 to 9.0.

[0023] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0024] A) The composite nanofiltration membrane for lithium and magnesium ion separation provided by the present invention introduces an intermediate layer between the base membrane and the separation layer. The addition of the intermediate layer can compensate for the defects on the surface of the composite nanofiltration membrane and reduce the diffusion of polyethyleneimine monomer in the base membrane, thereby reducing the thickness of the separation layer, compressing the mass transfer channel, and effectively regulating the "trade-off" effect of the nanofiltration membrane.

[0025] B) The preparation method of the composite nanofiltration membrane for lithium-magnesium ion separation provided by the present invention utilizes dopamine hydrochloride to self-polymerize on the base membrane to form an intermediate layer, which can control the release and diffusion rate of the subsequent polyethyleneimine monomer during the interfacial polymerization process, reduce the thickness of the separation layer, thereby reducing the mass transfer length, accelerating the mass transfer rate, and thus improving the permeation flux of the composite nanofiltration membrane. At the same time, the separation layer is a polyamide separation layer, which can improve the retention of divalent ions, solving the problem of the difficulty in balancing the selectivity and flux of existing lithium-magnesium ion separation nanofiltration membranes.

[0026] C) The preparation method of the composite nanofiltration membrane for lithium-magnesium ion separation provided by the present invention, the prepared intermediate layer can store more aqueous monomers of polyethyleneimine due to electrostatic adsorption, forming a defect-free separation layer, which is beneficial to improving the lithium-magnesium ion separation performance.

[0027] In the present invention, the above-mentioned technical solutions can be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages will become apparent from the description or be understood through practice of the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the embodiments of the description and the contents particularly pointed out in the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like parts throughout the drawings.

[0029] Figure 1 This is a schematic diagram of the structure of each layer in the composite nanofiltration membrane for lithium and magnesium ion separation provided by the present invention;

[0030] Figure 2a This is an infrared spectrum of the composite nanofiltration membrane prepared in Example 1 of the present invention;

[0031] Figure 2b for Figure 2a A partial enlarged view of

[0032] Figure 3 This is a scanning electron microscope image of a composite film containing an intermediate layer after dopamine is generated in the preparation process of Example 1 of the present invention;

[0033] Figure 4 This is a scanning electron microscope image of the composite nanofiltration membrane prepared in Example 1.

[0034] Reference numerals:

[0035] 1-basement membrane; 2-intermediate layer; 3-separation layer. DETAILED DESCRIPTION

[0036] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0037] The present invention provides a composite nanofiltration membrane for separation of lithium and magnesium ions, see Figure 1 , comprising a base membrane 1 (e.g., a polymer ultrafiltration membrane), an intermediate layer 2 (e.g., a polydopamine intermediate layer) and a separation layer 3 (e.g., a polyamide separation layer) stacked in sequence from bottom to top, wherein the separation layer 3 is a glycine-modified polyamide separation layer, which is formed by interfacial copolymerization of polyethyleneimine, glycine and a cross-linking agent, and the surface of the separation layer 3 has a ridge-valley-like convex and concave structure, and the intermediate layer 2 is formed by oxidative self-polymerization of dopamine in a weakly alkaline environment (see Reaction Formula 2).

[0038] Reaction formula 1 is as follows:

[0039]

[0040] Reaction formula 2 is as follows:

[0041]

[0042] Compared with the prior art, the composite nanofiltration membrane for lithium and magnesium ion separation provided by the present invention, on the one hand, introduces an intermediate layer 2 between the base membrane 1 and the separation layer 3. The addition of the intermediate layer 2 can compensate for the defects on the surface of the composite nanofiltration membrane, reduce the diffusion of polyethyleneimine monomer in the base membrane 1, thereby reducing the thickness of the separation layer 3, compressing the mass transfer channel, and effectively regulating the "trade-off" effect of the nanofiltration membrane.

[0043] On the other hand, the raw materials of the separation layer 3 are copolymerized, which can improve the binding stability of glycine and the polyamide matrix and reduce the loss of glycine during the filtration process compared to the existing blending reaction (polyamide and other additives are usually blended).

[0044] In order to improve the performance of the base membrane 1 in the composite nanofiltration membrane, illustratively, the base membrane 1 is one of polyethersulfone ultrafiltration membrane, polyvinylidene fluoride ultrafiltration membrane, and polysulfone ultrafiltration membrane, the pore size of the base membrane 1 is 1 to 100 nm, and the retention molecular weight of the base membrane 1 is 150 kDa.

[0045] Exemplarily, the cross-linking agent is trimesoyl chloride or 1,2,4,5-benzene tetracarboxylic chloride, preferably trimesoyl chloride. A trifunctional cross-linking agent is used to cause a condensation reaction between polyethyleneimine, glycine and the cross-linking agent, which can improve the binding stability of glycine and the polyamide matrix and reduce the loss of glycine during the filtration process.

[0046] In a second aspect, the present invention provides a method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation, comprising the following steps:

[0047] Step 1: dissolving dopamine hydrochloride particles in a buffer solution (e.g., tris-HCl or sodium acetate) by ultrasonication to obtain a dopamine solution;

[0048] Step 2: Cover the surface of the base film 1 with a dopamine solution, shake it at a temperature of 30°C to 40°C (for example, 35°C) for 1 hour to 4 hours to allow the dopamine to undergo a self-polymerization reaction, and then wash it after the reaction to form an intermediate layer 2 to obtain a composite film;

[0049] Step 3: immersing the composite membrane in a mixed solution of polyethyleneimine and glycine, taking it out and air-drying it, removing the excess mixed solution on the membrane surface, and obtaining an immersed composite membrane;

[0050] Step 4: Place the impregnated composite membrane in an organic solvent solution of a cross-linking agent (for example, a mixture of one or more of n-hexane, n-heptane, benzene, and toluene in any proportion) to carry out an interfacial polymerization reaction, then take it out and perform heat treatment (heat treatment temperature is 50°C to 70°C, heat treatment time is 5min to 10min) to form a separation layer 3 and obtain a composite nanofiltration membrane.

[0051] Compared with the prior art, the preparation method of the composite nanofiltration membrane for lithium and magnesium ion separation provided by the present invention utilizes a combination of copolymerization and interfacial polymerization to prepare the composite nanofiltration membrane. The method is convenient, simple to operate, and the prepared composite nanofiltration membrane has good performance.

[0052] Specifically, on the one hand, by self-polymerizing dopamine hydrochloride on the base membrane 1 to form the intermediate layer 2, the release and diffusion rate of the subsequent polyethyleneimine monomer during the interfacial polymerization process can be controlled, and the thickness of the separation layer 3 can be reduced. The thickness of the separation layer 3 prepared by the present invention is in the range of 40 to 60 nm (the thickness range in the preferred embodiment is 40 nm to 49 nm), which is greatly reduced compared to the thickness of the conventional separation layer (50 nm to 200 nm), thereby reducing the mass transfer length, accelerating the mass transfer speed, and thereby increasing the permeation flux of the composite nanofiltration membrane. At the same time, the separation layer 3 is a polyamide separation layer, which can improve the retention of divalent ions, solving the problem that the selectivity and flux of the existing lithium and magnesium ion separation nanofiltration membrane are difficult to balance.

[0053] On the other hand, by introducing glycine into the separation layer 3, glycine has extremely strong hydrophilicity and can enhance the hydrophilicity of the surface of the composite nanofiltration membrane. At the same time, glycine can promote the positive charge of the surface of the composite nanofiltration membrane through surface protonation in the separation layer 3, which has a promoting effect on the balance between permeability and selectivity. In addition, small molecule amino acids are natural materials that are non-toxic, economical and environmentally friendly.

[0054] On the other hand, the intermediate layer 2 prepared by the present invention can store more aqueous monomers of polyethyleneimine due to electrostatic adsorption, forming a defect-free separation layer 3, which is beneficial to improving the lithium-magnesium ion separation performance.

[0055] It should be noted that in the above step 4, the role of the heat treatment is to optimize the microstructure and interface properties of the membrane through temperature control, and ultimately improve the membrane's separation efficiency for lithium and magnesium ions and its practical application stability.

[0056] Exemplarily, in order to improve the mixing uniformity of the dopamine solution, the above step 1 includes the following steps:

[0057] Step 11: Dissolve the dopamine hydrochloride particles in a buffer solution;

[0058] Step 12: Stir the mixed solution of dopamine hydrochloride and the buffer solution under a nitrogen environment (eg, magnetic stirring) at a stirring speed of 400 rpm to 800 rpm for 1 to 5 min to obtain a dopamine solution.

[0059] In order to ensure the preparation quality of the intermediate layer 2, in the above step 1, the concentration of the dopamine solution is 0.1wt% to 0.4wt%; the concentration of the buffer solution is 45 to 55mM (for example, 50mM), and the pH is 8.0 to 9.0 (for example, 8.5).

[0060] Considering that the molecular weight of polyethyleneimine will affect the quality of the subsequent separation layer 3, illustratively, in the above step 3, the molecular weight of polyethyleneimine is 1,000 to 70,000.

[0061] Similarly, polyethyleneimine and glycine will also affect the quality of the subsequent separation layer 3. For example, in the above step 3, the concentration of polyethyleneimine in the mixed solution of polyethyleneimine and glycine is 0.5wt% to 1wt%; the concentration of glycine in the mixed solution of polyethyleneimine and glycine is 0.1wt% to 0.7wt%.

[0062] Accordingly, the concentration of the cross-linking agent is 0.05% to 0.1 wt%.

[0063] In order to ensure that the mixed solution of polyethyleneimine and glycine fully impregnates the composite membrane, in the above step 3, the immersion time is 8 minutes to 15 minutes (for example, 10 minutes), and the immersion temperature is 15° C. to 30° C.

[0064] In order to further promote the interfacial polymerization reaction, in the above step 4, the temperature of the interfacial polymerization reaction is 20°C to 28°C (for example, 25°C), and the time of the interfacial polymerization reaction is 0.5min to 3min (for example, 0.5min, 1min, 2min or 3min).

[0065] Example 1

[0066] The preparation method of the composite nanofiltration membrane for lithium and magnesium ion separation provided in this embodiment includes the following steps:

[0067] Step a: cleaning the polyethersulfone ultrafiltration membrane ultrafiltration base membrane;

[0068] Dopamine hydrochloride was dispersed in a Tris-HCl buffer solution with a concentration of 50 mM and a pH of 8.5;

[0069] Step b: placing a mixed solution of dopamine hydrochloride and a buffer solution under a nitrogen environment and magnetically stirring (the stirring speed is 800 rpm) for 1 minute to obtain a dopamine solution, wherein the concentration of the dopamine solution is 0.2 wt%;

[0070] Step c: pouring the dopamine solution onto the surface of the polyethersulfone ultrafiltration membrane, placing it in a shaker, shaking it at 35° C. for 2 hours, and washing it to form an intermediate layer to obtain a composite membrane;

[0071] Step d: the composite membrane is immersed in an aqueous solution of polyethyleneimine having a concentration of 1 wt% and a molecular weight of 70,000 and 0.7 wt% of glycine at 25° C. for 10 minutes, and then the excess aqueous solution on the surface is removed to obtain an immersed composite membrane;

[0072] Step e: placing the impregnated composite membrane in a 0.1 wt% hexane solution of trimesoyl chloride, performing interfacial polymerization at 25° C. for 1 minute, removing the residual organic phase solution, and heat treating the membrane at 60° C. for 10 minutes to obtain a composite nanofiltration membrane for lithium and magnesium ion separation.

[0073] Example 2

[0074] The preparation method of the composite nanofiltration membrane for lithium and magnesium ion separation provided in this embodiment includes the following steps:

[0075] Step a: cleaning the polyethersulfone ultrafiltration membrane ultrafiltration base membrane;

[0076] Dopamine hydrochloride was dispersed in a Tris buffer solution with a concentration of 50 mM and a pH of 8.5;

[0077] Step b: placing a mixed solution of dopamine hydrochloride and a buffer solution under a nitrogen environment and magnetically stirring (the stirring speed is 800 rpm) for 1 minute to obtain a dopamine solution, wherein the concentration of the dopamine solution is 0.2 wt%;

[0078] Step c: pouring the dopamine solution onto the surface of the polyethersulfone ultrafiltration membrane, placing it in a shaker, shaking it at 35° C. for 2 hours, and washing it to form an intermediate layer to obtain a composite membrane;

[0079] Step d: the composite membrane is immersed in polyethyleneimine with a concentration of 1 wt% and a molecular weight of 70,000 at 25° C. for 10 minutes, and then excess aqueous solution on the surface is removed to obtain an immersed composite membrane;

[0080] Step e: placing the impregnated composite membrane in a 0.1 wt% hexane solution of trimesoyl chloride, performing interfacial polymerization at 25° C. for 1 minute, removing the residual organic phase solution, and heat treating the membrane at 60° C. for 10 minutes to obtain a composite nanofiltration membrane for lithium and magnesium ion separation.

[0081] It should be noted that the steps of Example 1 and Example 2 are basically the same, the difference being that glycine is not added during the preparation of the separation layer in Example 2.

[0082] Example 3

[0083] The preparation method of the composite nanofiltration membrane for lithium and magnesium ion separation provided in this embodiment includes the following steps:

[0084] Step a: cleaning the polyethersulfone ultrafiltration membrane ultrafiltration base membrane;

[0085] Dopamine hydrochloride was dispersed in a sodium acetate buffer solution with a concentration of 55 mM and a pH of 8.0;

[0086] Step b: placing a mixed solution of dopamine hydrochloride and a buffer solution under a nitrogen environment and magnetically stirring (the stirring speed is 400 rpm) for 5 min to obtain a dopamine solution, wherein the concentration of the dopamine solution is 0.4 wt %;

[0087] Step c: pouring the dopamine solution onto the surface of the polyethersulfone ultrafiltration membrane, placing it in a shaker, shaking it at 40° C. for 1 hour, and washing it to form an intermediate layer to obtain a composite membrane;

[0088] Step d: the composite membrane is immersed in an aqueous solution of polyethyleneimine with a molecular weight of 30,000 and 0.3 wt % glycine at 20° C. for 15 minutes, and then the excess aqueous solution on the surface is removed to obtain an immersed composite membrane;

[0089] Step e: The impregnated composite membrane is placed in a 0.06 wt% 1,2,4,5-benzene tetrakis(carbonyl chloride) toluene solution, and an interfacial polymerization reaction is carried out at 28° C. for 2 minutes. The residual organic phase solution is removed and the membrane is heat-treated at 70° C. for 5 minutes to obtain a composite nanofiltration membrane for lithium and magnesium ion separation.

[0090] Comparative Example 1

[0091] The preparation method of the composite nanofiltration membrane for lithium and magnesium ion separation provided in this comparative example comprises the following steps:

[0092] After cleaning the polyethersulfone ultrafiltration membrane, the ultrafiltration base membrane is immersed in polyethyleneimine with a concentration of 1wt% and a molecular weight of 70,000 for 10 minutes, and the excess aqueous phase solution on the surface is removed to obtain the immersed polyethersulfone ultrafiltration membrane ultrafiltration base membrane; the immersed polyethersulfone ultrafiltration membrane is placed in a 0.1wt% hexane solution of trimesoyl chloride, and an interfacial polymerization reaction is carried out for 1 minute. The residual organic phase solution is removed and the membrane is placed at 60°C for heat treatment for 10 minutes to obtain a composite nanofiltration membrane for lithium and magnesium ion separation.

[0093] It should be noted that the difference between Comparative Example 1 and Example 1 is that no intermediate layer is formed in Comparative Example 1, and glycine is not added during the preparation of the separation layer.

[0094] Infrared spectrum measurement was performed on Example 1, see Figure 2a to Figure 2b , 3500cm -1 ~3300cm -1 The characteristic peak at 1600 cm is attributed to the stretching vibration of OH and NH; -1 The characteristic peak at 900 cm is attributed to the stretching vibration of C=O; -1 The characteristic peak at is attributed to the stretching vibration of -COOH. The above shows that polydopamine is polymerized on the basement membrane to form a polyamide separation layer, and glycine is evenly dispersed.

[0095] Figure 3 This is a scanning electron microscope image of the composite membrane containing the intermediate layer obtained during the preparation process of Example 1. It can be seen from the figure that multiple rod-shaped protrusions (i.e., polydopamine) are formed on the base membrane, and most of them are distributed with large pores. There is no improvement in separation performance, only the pure water flux is increased. Figure 4 This is a scanning electron microscope image of the composite nanofiltration membrane prepared in Example 1. It can be seen that a dense separation layer with a ridge-valley-like protrusion and depression structure is formed on the surface of the polydopamine intermediate layer, which effectively increases the roughness of the separation layer and thereby increases the contact area with the solute.

[0096] The nanofiltration performance of Example 1, Example 2 and Comparative Example 1 was measured. The specific test methods for pure water flux (F) and retention rate (R) are as follows:

[0097] The method for testing the pure water flux using a cross-flow membrane performance evaluation instrument includes the following steps:

[0098] First, the nanofiltration membranes prepared in Example 1, Example 2, and Comparative Example 1 were pre-pressed at a pressure of 0.6 MPa for 30 min using deionized water as feed, and then the pressure was adjusted to 0.5 MPa. After stabilization for 30 min, a certain volume of permeate was taken and the permeation time was recorded. The pure water flux (Lm) was calculated by the following formula: -2 h-1 bar -1 ).

[0099] The salt retention rate test method is as follows: first, the nanofiltration membranes prepared in Example 1, Example 2 and Comparative Example 1 are pre-pressed at a pressure of 0.6 MPa for 30 minutes using deionized water as the feed, and then the pressure is adjusted to 0.5 MPa. After stabilization for 30 minutes, the deionized water is replaced with a 1 g / L MgCl2 solution or LiCl solution. After stable operation for 15 minutes, the permeate and the raw liquid are sampled and the concentration is measured. The retention rate R (%) of the MgCl2 or LiCl solution is calculated according to the following formula, and the retention rate is tested by a conductivity meter.

[0100]

[0101] Where F is the pure water flux, unit is L m -2 h -1 bar -1 , V is the volume of the permeate, unit L, S is the filtration area of ​​the membrane, unit m 2 ; t is the time required to measure the permeate, unit is h; R is the retention rate, unit is %; C p is the permeate concentration, in g / L; C f is the concentration of the raw material solution, in g / L.

[0102] The results showed that the thickness of the separation layer of the composite nanofiltration membrane prepared in Example 1 was 49 nm, and the pure water flux was 17.5 Lm -2 h -1 bar -1 The retention rates of MgCl2 solution and LiCl solution were 93.2% and 30.2%, respectively.

[0103] The thickness of the separation layer of the composite nanofiltration membrane prepared in Example 2 is 58 nm, and the pure water flux is 10.5 L m -2 h -1 bar -1 The rejection rates of MgCl2 solution and LiCl solution were 95.15% and 47.8%, respectively.

[0104] The thickness of the separation layer of the composite nanofiltration membrane prepared in Comparative Example 1 was 89 nm, and the pure water flux was 4.55 L m -2 h -1 bar -1 The rejection rates of MgCl2 solution and LiCl solution were 96.77% and 41.2%, respectively.

[0105] Cross-flow and inductively coupled plasma mass spectrometry were used to perform magnesium and lithium separation tests on Example 1, Example 2, and Comparative Example 1 at a test temperature of 25° C. The specific method is as follows:

[0106] First, the nanofiltration membranes prepared in the embodiment and the comparative example were pre-pressed at a pressure of 0.6 MPa for 30 min using deionized water as feed, and then the pressure was adjusted to 0.5 MPa. After stabilization for 30 min, the feed solution was replaced with a total salt concentration of 2 g / L, Mg 2+ :Li + =20:1 mixed salt solution of lithium chloride and magnesium chloride. After running for 30 minutes, a certain volume of permeate was collected and the Mg in the feed solution and permeate was analyzed by inductively coupled plasma mass spectrometry. 2+ 、Li + The concentration was tested and the magnesium-lithium separation factor was calculated using the following formula.

[0107]

[0108] Where:

[0109] represents the magnesium-lithium separation factor, Indicates Mg in raw material solution 2+ The concentration of , in g / L; Indicates Li in the raw material solution + The concentration of , in g / L; Indicates Mg in the permeate 2+ The concentration of , in g / L; Indicates Li in the permeate + The concentration is in g / L.

[0110] The results showed that for a concentration of 2 g / L, Mg 2+ :Li + =20:1 mixed solution of MgCl2 and LiCl, the separation factor of the composite nanofiltration membrane prepared in Example 1 is 30.5; the separation factor of the composite nanofiltration membrane prepared in Example 2 is 6.33; the separation factor of the composite nanofiltration membrane prepared in Comparative Example 1 is 5.35.

[0111] It can be seen that the embodiments of the present invention (especially Example 1) have higher pure water flux and better separation factor when performing magnesium-lithium separation, and can better achieve magnesium-lithium separation, which shows that the composite nanofiltration membrane has excellent performance and can be applied to lithium extraction from salt lakes.

[0112] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A composite nanofiltration membrane for lithium and magnesium ion separation, characterized in that: The invention comprises a base film, an intermediate layer and a separation layer which are sequentially stacked from bottom to top. The intermediate layer is formed by oxidative self-polymerization of dopamine in a weakly alkaline environment.

2. The composite nanofiltration membrane for lithium and magnesium ion separation according to claim 1, characterized in that: The separation layer is a polyamide separation layer.

3. The composite nanofiltration membrane for lithium and magnesium ion separation according to claim 1, characterized in that: The base membrane is one of a polyethersulfone ultrafiltration membrane, a polyvinylidene fluoride ultrafiltration membrane, and a polysulfone ultrafiltration membrane.

4. The composite nanofiltration membrane for lithium and magnesium ion separation according to claim 1, characterized in that The pore size of the basement membrane is 1 nm to 100 nm, and the molecular weight cut-off of the basement membrane is 150 kDa.

5. A method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation, characterized in that: For preparing the composite nanofiltration membrane according to any one of claims 1 to 4, the preparation method comprises the following steps: Step 1: dissolving dopamine hydrochloride particles in a buffer solution by ultrasonication to obtain a dopamine solution; Step 2: Cover the surface of the base film with a dopamine solution, shake it to allow the dopamine to self-polymerize, and then wash it to form an intermediate layer to obtain a composite film; Step 3: Dipping the composite membrane into polyethyleneimine, taking it out and air-drying it to obtain an impregnated composite membrane; Step 4: placing the impregnated composite membrane in an organic solvent solution of a cross-linking agent to carry out an interfacial polymerization reaction to form a separation layer, thereby obtaining a composite nanofiltration membrane.

6. The method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation according to claim 5, characterized in that: In the step 4, the following steps are further included after the interfacial polymerization reaction: The prepared composite membrane is subjected to heat treatment, wherein the heat treatment temperature is 50° C. to 70° C., and the heat treatment time is 5 min to 10 min.

7. The method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation according to claim 5, characterized in that: In step 1, the buffer solution is tris(hydroxymethyl)aminomethane hydrochloride or sodium acetate solution.

8. The method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation according to claim 5, characterized in that: In step 4, the organic solvent is a mixture of one or more of n-hexane, n-heptane, benzene, and toluene in any proportion.

9. The method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation according to claim 5, characterized in that: The step 1 comprises the following steps: Step 11: Dissolve the dopamine hydrochloride particles in a buffer solution; Step 12: Place the mixed solution of dopamine hydrochloride and the buffer solution under a nitrogen environment and stir to obtain a dopamine solution.

10. The method for preparing a composite nanofiltration membrane for lithium and magnesium ion separation according to claim 5, characterized in that: In step 1, the concentration of the dopamine solution is 0.1 wt% to 0.4 wt%; the concentration of the buffer solution is 45 to 55 mM, and the pH is 8.0 to 9.0.

Images