CN113332860A

CN113332860A - Preparation and application of high-permselectivity magnesium-lithium separation nanofiltration membrane

Info

Publication number: CN113332860A
Application number: CN202110651771.7A
Authority: CN
Inventors: 何涛; 何荣荣; 董晨俊; 徐姗姗
Original assignee: Shanghai Advanced Research Institute of CAS
Current assignee: Shanghai Advanced Research Institute of CAS
Priority date: 2021-06-11
Filing date: 2021-06-11
Publication date: 2021-09-03

Abstract

The invention relates to a preparation method of a nanofiltration membrane for magnesium-lithium separation. Firstly, alternately coating polyelectrolytes with opposite charges, namely polycation and polyanion electrolyte solutions, on the surface of a porous support membrane; coating to the required number of layers and then carrying out crosslinking treatment; the porous support membrane, namely the base membrane, is made of the following high polymer materials, namely polyether sulfone, polysulfone and sulfonated polyether sulfone; coating polyelectrolyte, namely polycation and polyanion electrolyte solution for 1-10 periodic layers; after the polyelectrolyte solution is coated on the surface of the basement membrane, the pore diameter of the membrane is reduced in a cross-linking mode; alternately coating polyelectrolyte solution with opposite charges on the surfaces of the ultra-filtration membrane and the micro-filtration membrane to ensure that the polyelectrolyte solution is deposited on the surface of the base membrane layer by layer in a self-assembly manner, thereby obtaining the high-permeability-selectivity magnesium-lithium separation nanofiltration membrane material; and then crosslinking to obtain the nanofiltration membrane for magnesium-lithium separation. Any polyelectrolyte that is charged and that is soluble and dispersible in water can be used in the layer-by-layer self-assembly process.

Description

Preparation and application of high-permselectivity magnesium-lithium separation nanofiltration membrane

Technical Field

The invention belongs to the technical field of ion separation, and particularly relates to a nanofiltration membrane, which is particularly used for separating magnesium, lithium and salts (solution) thereof.

Background

Lithium is a metal element with the smallest atomic radius and the lightest mass in nature, and lithium and salts thereof are widely used in industries such as batteries, alloy manufacturing, ceramics, new energy and the like due to unique properties, and are regarded as a novel energy and strategic resources. Chinese lithium resources are abundant and account for 16% of the total reserves in the world, while salt lake lithium resources account for 86.5% of the lithium resources proven in China, and are mainly distributed in Qinghai and Tibet regions and respectively account for 49.6% and 28.4% of the total reserves in China. The lithium ions in the Qinghai salt lake coexist with a plurality of multivalent ions and are difficult to separate. Typical ions are magnesium ions; the chemical properties of magnesium and lithium are similar, the hydration radius of magnesium ions is 0.43nm, the hydration radius of lithium ions is 0.38nm, the hydration radius of the magnesium ions and the hydration radius of the lithium ions are different by less than 0.1nm, and the separation of magnesium and lithium is a world problem which troubles scientists for decades. In addition, some brines are rich in calcium ions, and the calcium ions have a hydration radius of 0.41nm and are difficult to separate from lithium ions.

At present, the main technical route for extracting lithium from Qinghai salt lake brine in China is divided into two steps of lithium ion separation and purification and concentration. Lithium ion purification can be achieved by using monovalent/multivalent ion separation nanofiltration membrane materials. Patent CN 212356643U discloses a special membrane integrated device for separating magnesium and lithium from lithium-rich brine. The device comprises a control system, a nanofiltration system I, a nanofiltration system II and a reverse osmosis system; the nanofiltration membrane component is arranged above the reverse osmosis membrane component in the frame and is used for enriching magnesium and lithium to separate as an integral device. Patent CN 109824065 a discloses a method for separating and enriching lithium from magnesium and lithium. Diluting and filtering the old brine in the salt pan to obtain nanofiltration raw water; and (3) treating the nanofiltration raw water by a nanofiltration and reverse osmosis device separation device to complete the separation and enrichment of lithium in the salt lake brine with high magnesium-lithium ratio. The function of the nanofiltration membrane mainly comprises that monovalent ions are ensured to permeate and multivalent ions are intercepted at the same timeThe ion separation technology with low energy consumption has obvious advantages in the field of magnesium and lithium separation of salt lake brine. The preparation of the nanofiltration membrane material with high separation efficiency and high permselectivity is very important. The separation mechanism of the nanofiltration membrane mainly comprises size sieving and electrostatic repulsion effect. The separation of ions depends primarily on the size of the ions and the pore size of the nanofiltration membrane. And the other separation path utilizes the south-of-the-road effect to realize the separation of multivalent and monovalent ions. Divalent Mg²⁺Has a charge intensity higher than that of monovalent Li⁺So that the positively charged nanofiltration membrane is used for separating Mg²⁺And Li⁺In the course of (1), the film is paired with divalent cation Mg² ⁺Has a rejection effect obviously higher than that of univalent cation Li⁺. Thus monovalent cation Li⁺More readily permeate the membrane into the permeate, and divalent cations Mg²⁺Is trapped, and then Mg²⁺Has a higher rejection rate than Li⁺The method is beneficial to magnesium-lithium separation and improves the magnesium-lithium separation coefficient S. However, the commercial nanofiltration membrane is mainly synthesized by an interfacial polymerization method, and the synthesized monomers are trimesoyl chloride and piperazine, so that the surface charge of the obtained membrane material is negative, and the selectivity is greatly reduced. There are reports in the literature that adjusting the pH of brine to below the isoelectric point of commercial membrane surfaces can effect a change from negative to positive charge on the membrane surface, but exhibit separation coefficients generally below 10, probably due to a broad pore distribution of the membrane. Therefore, designing a separation material to realize positive charge of a separation skin layer is the main research direction of magnesium-lithium ion separation at present. The literature reports that a positively charged polyamide composite nanofiltration hollow fiber membrane is prepared for lithium magnesium separation by interfacial polymerization on a polyacrylonitrile ultrafiltration hollow fiber membrane using 1, 4-bis (3-aminopropyl) piperazine and trimesoyl chloride, with a magnesium lithium separation coefficient of 2.6(Desalination 369(2015) 26-36). It has also been reported that cross-linked polyetherimide is used as a carrier, and a composite nanofiltration membrane with a positive charge skin layer is synthesized through interfacial polymerization of branched polyethyleneimine and trimesoyl chloride, and the Separation and Purification Technology 186(2017) 233-242 has a magnesium-lithium Separation coefficient of 9.2. Reports that-NH-rich membranes were designed and prepared by interfacial polymerization of polyethyleneimine with trimesoyl chloride on polyethersulfone ultrafiltration membranes₃ ⁺and-NH₂The positive charge nanofiltration membrane of (1), the magnesium-lithium separation coefficient is 20 (desalinization 449(2019) 57-68). The novel nanofiltration membrane with the magnesium-lithium separation coefficient of 16.1 (desalinization 488(2020)114522) is prepared by doping a graphene oxide additive in an ultrafiltration basal membrane and then carrying out interfacial polymerization reaction of polyethyleneimine and trimesoyl chloride. Polyether sulfone is used as a base film, and three layers of ultrathin nanofiltration membranes are prepared. A carboxylated cellulose nanocrystal is used as an intermediate layer and an ultra-thin polyamide layer prepared by interfacial polymerization of polyethyleneimine and trimesoyl chloride, with a magnesium-lithium Separation coefficient of 12.15(Separation and Purification Technology 230(2020) 115567).

Research results in recent years show that the positively charged nanofiltration membrane shows excellent magnesium-lithium selective separation performance in the magnesium-lithium separation process. According to the electrostatic repulsion effect, the method for improving the separation of magnesium and lithium in the experiment at present comprises the following steps: the literature reports that polyethyleneimine serving as a water-phase monomer is subjected to interfacial polymerization reaction with trimesoyl chloride on the surface of a polyether sulfone ultrafiltration membrane, and a positively charged nanofiltration membrane rich in amino is designed. The literature reports that a layer of single-walled carbon nanotube is paved on the surface of polyether sulfone, then a polyamide layer is prepared through an interfacial polymerization method, so that a nanofiltration membrane has sub-nano holes, a PEI layer is grafted on the surface of the nanofiltration membrane through an amidation reaction, so that the membrane has enhanced positive charges, and a double-layer nanofiltration membrane is formed. The magnesium-lithium separation coefficient is 33.4, and Flux is 12L/m²hbar (Journal of Membrane Science 620(2021) 118862). Amine functionalized ionic liquids 1-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide ([ MimAP ] have also been reported][Tf2N]) Grafted to the surface of a polyamide membrane, [ MimAP][Tf2N]The amine group (b) reacts with the acid chloride group remaining on the surface of the nascent PA membrane. Due to [ MimAP][Tf2N]All modified PA film surfaces are positively charged at pH 6.4. The Mg-Li separation coefficient was 8.15(Journal of Membrane Science 603(2020) 117997). In addition, a great number of documents report that the membrane material is positively charged by adopting doping of nano materials, so that the separation performance is improved. However, the nano-material is adopted to prepare the nano-filtration membrane, how to spread the carbon nano-tube on the surface of the membrane, and how to add the graphene into the reaction monomer, and the like, which only have scientific research value, from the engineering practice point of view,it is difficult to realize large-scale preparation in a short time. In addition, most of the existing preparation processes of the magnesium-lithium separation nanofiltration membrane material adopt organic solvents, which are potentially harmful to the environment.

Aiming at the problem of the magnesium-lithium separation nanofiltration membrane, the invention discloses a novel nanofiltration membrane material system, and the obtained magnesium-lithium ion separation nanofiltration membrane has the permselectivity far higher than that reported in the current patents and documents. Experimental process shows that the retention rate of magnesium chloride can be greatly improved by alternately immersing the base membrane into polyelectrolyte solution with opposite charges. The materials for assembly are alternately deposited on the base film, and the multilayer nano film can be prepared by repeating the process, wherein the magnesium-lithium separation coefficient is over 50, and the optimal film separation coefficient is higher than 100. Compared with a commercial nanofiltration membrane technology and a current mainstream magnesium-lithium separation membrane material preparation technology, the layer-by-layer self-assembly technology is a universal and simple method for manufacturing the ultrathin polyelectrolyte multilayer membrane, the membrane structure and the properties are easy to regulate and control, and the layer-by-layer self-assembly technology is more flexible compared with the conventional nanofiltration membrane and reverse osmosis membrane preparation technologies such as an interface polymerization method and the like; secondly, the water-based technology is green and environment-friendly. The layer-by-layer self-assembly technology is basically non-toxic and harmless water used as a solvent, and compared with film preparation processes such as an interface polymerization method and a phase conversion method which need organic solvents, the layer-by-layer self-assembly technology is non-toxic and harmless, green and environment-friendly, and low in cost.

Disclosure of Invention

The invention aims to provide a novel nanofiltration membrane material with high permeability selectivity prepared by adopting a green layer-layer self-assembly technology, and solves the problems that the prior nanofiltration membrane has low separation coefficient in the separation process of magnesium and lithium ions and the membrane preparation adopts an organic solvent.

The technical scheme adopted by the invention for solving the technical problems is as follows: a preparation method of a high-permeability magnesium-lithium separation membrane material comprises the following steps: firstly, alternately coating polyelectrolytes with opposite charges, namely polycation and polyanion electrolyte solutions, on the surface of a porous support membrane; coating to the required number of layers and then carrying out crosslinking treatment; the used porous support membrane is made of high polymer materials such as polyether sulfone, polysulfone, sulfonated polyether sulfone, polyacrylonitrile, polyolefin materials, polyimide, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polyether imide, sulfonated polyether sulfone, sulfonated polysulfone, aminated polyether sulfone and aminated polypropylene cyanide; coating polyelectrolyte (polycation and polyanion electrolyte solution) for 1-10 periods (double layers), or more periods (layers);

the porous support membrane, i.e., the base membrane, is of the type of a hollow fiber membrane, a flat sheet membrane, and a tubular membrane.

The polycation electrolyte material in the polycation electrolyte solution is selected from one or more of polyacrylamide hydrochloride (PAH), polydimethyldiallylammonium chloride (PDADMAC), Polyethyleneimine (PEI), methacryloyloxyethyl trimethyl ammonium chloride, chitosan, polyvinyl alcohol (PVA), polyacrylamide (SC498) and the like.

The polyanionic electrolyte material in the polyanionic electrolyte solution is selected from one or more of sodium polystyrene sulfonate (PSS), dextran sodium sulfate, sodium carboxymethyl cellulose, sodium Polyacrylate (PAA), sodium polyvinyl sulfonate (PVS), sulfonated polyether ether ketone, Hyaluronic Acid (HA) and the like.

The cross-linking treatment method comprises chemical cross-linking, thermal cross-linking and photocatalytic cross-linking; the chemical crosslinking agent used for chemical crosslinking is one or more of Glutaraldehyde (GA), glyoxal, polyvinyl alcohol (PVA), propionaldehyde, fluorophenyl isocyanate, ethylene glycol and the like.

Further, the polymer coating method is a dip coating method, a spin coating method, a spray coating method, a coating method and a filter pressing method.

Further, the molecular weight of the polyelectrolyte has a direct influence on the thickness and roughness of the nanofiltration membrane; the molecular weight of the polyelectrolyte is between 10k and 1000 kDa. And selecting proper polyelectrolyte according to the pore diameter of the base membrane for coating.

Further, increasing the polyelectrolyte concentration has less effect on pore size. When the polycation and polyanion are once bound, the resulting film can prevent subsequent, more polyelectrolytes from binding to each other, and the resulting steric hindrance limits the addition of more polyelectrolytes to the polyelectrolyte multilayer film. In the examples, the polyelectrolyte concentration is 0.1-20g/L, the polyanion is preferably 2-10g/L, and the polycation is preferably 4-15 g/L.

Further, the electrolyte solution is dispersed in a salt solution, and the concentration of the added salt also has a crucial influence on the growth of the polyelectrolyte multilayer film. There is a competitive relationship between the salt ions and the polyelectrolyte. When the salt concentration is low, the polyelectrolytes are bound together by electrostatic attraction, and this state is called internal compensation (Intrinsic compensation); when the salt concentration is high, the charge of the polyelectrolyte is mainly neutralized by salt ions, and an electrostatic shielding effect (electrostatic shielding) occurs, and this state is called external compensation (external compensation). It is generally believed that the internally compensated membrane is thinner and dense, while the externally compensated membrane is relatively thicker and more open-pored, porous. The internal and external compensation affects not only the growth type of the polyelectrolyte multilayer film, but also the mechanical properties, the polymer chain mobility and the swelling behavior of the polyelectrolyte multilayer film in water. The salt concentration (e.g., NaCl molar concentration range 0.001-3M), more preferably 0.05-1.0 mol/L. The concentration of the salt is relative to the concentration of the polyelectrolyte, and the interaction between the polyelectrolyte and the salt can be regulated and controlled by regulating the concentration ratio of the polyelectrolyte to the salt, so that the pore diameter of the polyelectrolyte multilayer film is regulated. Increasing the polyelectrolyte concentration or salt concentration in the deposition solution increases the thickness of the polyelectrolyte multilayer film, but the effect on pore size can be divided into two distinct regions. In the first growth phase, the polyelectrolyte concentration in the deposition solution increases, resulting in an increase in the polymer deposition rate and a decrease in pore size. In the second growth phase, the pore size increases with increasing polyelectrolyte concentration. The second case is due to less adsorbed polyelectrolyte on the membrane and less agglomerated polyelectrolyte due to the change in the interaction between the polyelectrolyte and the salt.

Further, the use of different salt ions can have an effect on the thickness, rigidity, and swelling properties of the polyelectrolyte membrane. The salt in the electrolyte solution is NaCl or NaNO₃、KBr、NaF、NaBr、MgCl₂And the like. The specific effect of the different salt ion species is due to the different positions of the ions in the Hofmeister Series. Hofmeister Series (as shown in FIG. 1)) Initially, the ability of an ion to denature a protein is increased from left to right.

The influence of different salt ions as background solution on the polyelectrolyte multilayer film is researched by applying Hofmeister Series to a polyelectrolyte system. In Hofmeister Series, ions positioned on the right side of Cl & lt- & gt are called as anotropic ions, the structures of water molecules (water structures breakers) are damaged, the polarization capability is strong, the interaction force with the water molecules is weaker than that between the water molecules, and the ions are not easily combined with the water molecules; at position Cl^-The left ions are called cosmotropic ions, constitute water molecule structures (water structure makers), and the polarization of the water molecules is relatively weak, and the acting force between the water molecules is strong, so that the water molecules are easily combined. When chaotropic ions are used as background electrolyte, salt ions are easier to combine with the polyelectrolyte, so that a stronger charge shielding effect is caused, namely, an external compensation (external-compensation) effect is stronger, polyelectrolyte molecules are caused to be in an aggregated form, and the prepared polyelectrolyte multilayer film is thicker; while polyelectrolyte multilayer films prepared using cosmotropic ions are thinner. According to Hofmeister Series, the thickness and roughness of the multilayer film increase in the following order: anion F^-<Cl^-<Br^-(ii) a Cation K⁺<Na⁺<Li⁺. Furthermore, with NaSCN as the background electrolyte, there is more external compensation. In addition, the polyelectrolyte multilayer film may be reinforced in rigidity due to the increase in external compensation. The entropy of hydration of different anions affects the degree of swelling of the polyelectrolyte multilayer film, with the degree of swelling increasing with more negative entropy. The swelling degree of the polyelectrolyte multilayer film prepared using NaBr as a background electrolyte was much greater than that of the film using NaF.

In the present invention, the polyelectrolyte material is dissolved in a salt solution of a certain concentration. And alternately coating the polyelectrolyte solution with opposite charges on the surface of the base film, and finishing the coating after the required number of layers is reached. Crosslinking may optionally be carried out using a crosslinking agent.

Further, the polyelectrolyte multilayer film formation process is classified into exothermic reaction and endothermic reaction according to different growth modes. In a linear growth mode, delta H is-1000J/mol, and belongs to a strong exothermic reaction; in exponential growth mode,. DELTA.H >0, is an endothermic reaction. According to the formula Δ G ═ Δ H-T Δ S, in linear growth mode, both enthalpy changes (Δ H <0) and entropy changes (Δ S >0) favor the formation of polyelectrolyte complexes (PEC); and in exponential growth mode, enthalpy change (Δ H >0), entropy change (Δ S >0), where temperature (T) has a key effect on film formation. The experimental temperature was 20-30 ℃.

If the nanofiltration membrane is prepared by crosslinking, the carbonyl of the crosslinking agent is protonated firstly, and the amino and the carbonyl undergo nucleophilic addition reaction. After attack by amines, the nitrogen atom is deprotonated, yielding an unstable intermediate called methanolamine; under the weak acid pH, the methanol amine is protonated; further dehydrating to generate two intermediate products; the intermediate product is deprotonated to give the imine (FIG. 2). The crosslinking agent comprises a material containing hydroxyl (such as PVA) and a material containing carbonyl (such as GA) and is used for the carbonyl protonation of GA when the material is crosslinked; then carrying out alcohol addition with PVA; oxygen on hydroxyl in PVA has lone pair electrons and stronger nucleophilicity, and oxygen attacks carbonyl carbon by the lone pair electrons to form hemiacetal; the aldehyde group of GA is a strong polar group, and carbon shows strong electropositivity, so that it is easy to react with nucleophilic reagent to generate acetal reaction (FIG. 3). By covalent cross-linking, the pore size of the LBL membrane can be effectively reduced.

In the process of preparing the nanofiltration membrane, the highest retention rate of magnesium chloride can reach 98% or more.

The rejection (R) in the present invention is defined as: under certain conditions, the concentration of the raw material liquid (C)_f) With the concentration of the permeate (C)_p) The difference is divided by the concentration of the starting solution (C)_f)，R＝(C_f-C_p)/C_f. The nanofiltration membrane containing the polymer coating prepared by the invention has the following desalting performance test conditions: the concentration of the salt solution is 500mg/L, the testing pressure is 3.0MPa, and the system control temperature is 25 ℃.

The magnesium-lithium separation coefficient (S) in the present invention is defined as: under certain conditions, permeateMg in (p) side²⁺And Li⁺Is divided by Mg on the feed side (f)²⁺And Li⁺The ratio of the concentrations of (a) to (b).

The nanofiltration membrane containing the polymer coating prepared by the invention has the test conditions of mixed salt solution concentration of 2000-6000mg/L, magnesium-lithium ratio of 20-60, test pressure of 2.0-4.0bar and system control temperature of 25 ℃.

The permeability coefficient J in the present invention is defined as: mass per unit area per unit time under certain conditions. J ═ Δ m/(ρ a Δ t P), where Δ m (kg) is Δ t (h) the mass gain of the filtrate in the separation time, ρ is the density of the filtrate (1kg L)^-1)，A(m²) For effective filtration area, p (bar) is the transmembrane pressure. The temperature of the system is controlled to room temperature such as 25 deg.C (the same applies hereinafter).

The technical route and the method disclosed by the invention are suitable for preparing hollow fiber membranes, flat plates and tubular membranes, self-supporting or externally-supported membrane materials. Any person having the general knowledge of membrane preparation will be able to prepare any form and membrane material according to the methods provided by the present invention. The method disclosed by the invention is suitable for preparing the nanofiltration material, and the specific implementation method can effectively regulate and control the formula of the membrane preparation liquid and the membrane preparation parameters.

Compared with the prior art, the invention has the following beneficial effects:

1. the membrane material obtained by the invention has excellent osmotic separation performance, the magnesium-lithium separation coefficient is stable between 50 and 120, and the Mg-lithium separation coefficient is stable to Mg²⁺The retention rate of the catalyst is not lower than 98.5 percent; the permselectivity is much higher than that of the existing commercial nanofiltration membranes.

2. The separation performance of the membrane material obtained by the invention is suitable for magnesium-lithium separation. Can separate lithium from natural brine with high efficiency, and provides a good choice for stabilizing lithium supply. The polyelectrolyte solution alternating coating technology has flexibility; the polycation electrolyte and the polyanion electrolyte are combined on the surfaces of the ultrafiltration membrane and the microfiltration membrane, namely the base membrane through layer-to-layer self-assembly, and are crosslinked with the polyelectrolyte through a crosslinking agent to obtain a membrane material with high flux and high separation coefficient for separating magnesium and lithium ions;

3. the adopted water as the solvent is green and environment-friendly, and has no harm to human body.

In a word, the process preparation process is simple and efficient, the structure and the performance of the film can be adjusted by changing the film-making parameters, the method and the assembly materials, and the flexibility is high. Compared with the existing commercial nanofiltration membrane, the membrane material has more excellent permselectivity, and the preparation method takes water as a solvent, is environment-friendly and green, has mild conditions, and provides a high-efficiency and low-cost separation material for extracting lithium from high-magnesium-lithium-ratio brine.

Drawings

FIG. 1: hourmester sequence.

FIG. 2: schiff base reaction mechanism diagram.

FIG. 3: the mechanism diagram of the acetal reaction.

FIG. 4: scanning electron micrographs of sections of the coated polyethersulfone membrane described in example 1.

Detailed Description

The technical solution of the present invention is illustrated by the following specific examples, but the scope of the present invention is not limited thereto:

the materials used in the examples are all commercially available products.

The polyelectrolyte for the preparation of nanofiltration membranes is indicated in the attached table description:

table 1: polyelectrolyte for preparing nanofiltration membrane by layer-by-layer self-assembly

One or more of polyacrylamide hydrochloride (PAH), polydimethyldiallylammonium chloride (PDADMAC), Polyethyleneimine (PEI), methacryloyloxyethyl trimethylammonium chloride (MAM), chitosan, polyvinyl alcohol (PVA), polyacrylamide (SC498), and the like.

Example 1

A polysulfone hollow fiber membrane is selected as a basal membrane to carry out polyelectrolyte coating (MWCO is 7.5kDa, the inner diameter is 0.8nm, and the outer diameter is 1.3 mm). Firstly, polyanionic electrolyte sodium polystyrene sulfonate (Mw is 1,000kDa) is dissolved in 0.5mol/L sodium chloride solution to prepare 6g/L sodium polystyrene sulfonate (PSS) polyelectrolyte solution. Then, the polycation electrolyte material poly dimethyl diallyl ammonium chloride (Mw 200-350kDa) is dissolved in 1.5mol/L sodium chloride solution to prepare 12g/L poly dimethyl diallyl ammonium chloride (PDADMA) polyelectrolyte solution. Glutaraldehyde and polyvinyl alcohol are selected as cross-linking agents, glutaraldehyde is prepared into a solution with the concentration of 0.5 wt%, and polyvinyl alcohol is dissolved in deionized water to prepare a solution with the concentration of 0.5 wt% (without pH adjustment). Inputting the sodium polystyrene sulfonate solution into the hollow fiber membrane, and standing for 6 min; injecting deionized water into the membrane filaments from the bottom to ensure that each membrane filament is immersed by the solution, standing for 8min, and after standing, allowing the solution in the membrane filaments to flow out; the solution on the surface was purged with 0.1bar of nitrogen until a monolayer was finished. In the same way, the polydimethyldiallylammonium chloride solution is fed into the membrane filaments, washed with deionized water and purged with nitrogen until 1 cycle layer is formed, i.e. the double layer is finished. The inner surface of the hollow fiber membrane in the embodiment 1 is coated with 3.5 double layers, and then two cross-linking agents of glutaraldehyde and polyvinyl alcohol are sequentially input into the membrane filaments, so that aldehyde groups of the glutaraldehyde and hydroxyl groups of the polyvinyl alcohol are cross-linked, and the cross-linking time is 12 min. After coating the polyelectrolyte, it is apparent from the cross-sectional view of the membrane that the coating thickness is about 80 nm, see FIG. 4.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The hollow fiber membranes of example 1 were tested at a test pressure of 4 bar. The test results are: permeability coefficient of 11.2L m^-2 h^-1 bar^-1，Mg²⁺Has a rejection of 99.2%, Li⁺The retention rate of (D) was 8.5%. The magnesium-lithium separation coefficient S was 117.7.

Example 2

The polyelectrolyte coating was performed using the same base film as in example 1. First, sodium polystyrene sulfonate PSS (Mw ═ 1,000kDa) was dissolved in 0.5mol/L sodium chloride solution to prepare 4g/L PSS polyelectrolyte solution. Then, a polycationic electrolyte material PAH (Mw ═ 10 to 20kDa) was dissolved in a 2.5mol/L sodium chloride solution to prepare a 10g/L PAH polyelectrolyte solution. GA was used as a crosslinking agent, and a 50% aqueous solution of GA was prepared as a 0.5 wt% solution (pH 5.0). Coating the polyanion electrolyte solution PSS on the inner cavity of the hollow fiber membrane wire in a dead-end filtration mode, and controlling the volume of the extruded coating solution to be 10mL so as to fix the thickness of each layer of coating. Inputting deionized water into the membrane wire after each polyelectrolyte coating, washing the inside of the membrane wire, and then using N₂Purging the inner cavity of the membrane wire until the step is 1 single layer; in the same way, the polyion electrolyte PAH is coated on the inner surface of the hollow fiber membrane. Rinsing the lumen of the membrane wire with deionized water after each polyelectrolyte coating, followed by N₂The lumen of the membrane filaments was purged to 1 bilayer by this step. The above steps are circulated to obtain the required 2.0 membrane modules with double layers. And then glutaraldehyde is adopted to coat the inner cavity of the hollow fiber membrane wire in a dead-end filtration mode.

Preparing a simulated salt lake brine solution: simulating a salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test pressure was 4 bar. The test results are: permeability coefficient of 10.8L m^-2 h^-1bar^-1，Mg²⁺Has a rejection of 99.1%, Li⁺The rejection of (a) was 27.2%. The magnesium-lithium separation coefficient S was 81.1. This example shows that with or without a pressure coating, the magnesium lithium separation coefficients do not differ much when testing mixed salt solutions.

Comparative example 1

Preparing a simulated salt lake brine solution: 4000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺Quality ofThe ratio is 20. The test pressure was 4 bar. The test results are: permeability coefficient of 6.0L m^-2 h^-1 bar^-1，Mg²⁺Has a rejection of 98.5%, Li⁺The rejection of (D) was-25.0%. The magnesium-lithium separation coefficient S was 71.3.

Comparative example 2

Preparing a simulated salt lake brine solution: 6000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test pressure was 4.0 bar. The test results are: permeability coefficient of 4.2L m^-2 h^-1 bar^-1，Mg²⁺Has a rejection of 98.9%, Li⁺The rejection of (D) was-0.8%. The magnesium-lithium separation coefficient S was 64.1.

Example 2, comparative example 1 and comparative example 2 show that: as the concentration of the test simulated brine increases, the electrostatic repulsion of the membrane is weakened by the charge shielding effect caused by the high concentration of the salt, thereby reducing the magnesium-lithium separation coefficient S.

Comparative example 3

In example 2 the final glutaraldehyde cross-linking step of the membrane was eliminated and the material of the membrane obtained was tested against simulated salt lake brine solution: simulating a salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test pressure was 3 bar. The test results are: permeability coefficient of 11.3L m^-2 h^-1 bar^-1，Mg²⁺Has a rejection of 99.0%, Li⁺The rejection of (a) was 21.6%. The magnesium-lithium separation coefficient S was 82.5. This example shows that without chemical crosslinking, the magnesium lithium separation coefficients do not differ much.

Example 3

The polyelectrolyte coating was carried out as in example 1, using polyethersulfone (thickness about 150 μm, MWCO ═ 25 ten thousand Da) as the base film. First, sodium polyacrylate (Mw ═ 4,000k) was dissolved in 0.5mol/L sodium chloride solution to prepare 2g/L sodium polyacrylate polyelectrolyte solution. Then, the polycation electrolyte material poly dimethyl diallyl ammonium chloride (Mw 200-. The cross-linking agent is glutaraldehyde and polyvinyl alcohol, 50% water solution of glutaraldehyde is prepared into 0.5 wt% concentration solution, and polyvinyl alcohol is dissolved in deionized water to prepare 0.5 wt% concentration solution (without pH regulation). Pouring 25mL of polyanionic electrolyte solution sodium polyacrylate on the flat membrane component, completely immersing the surface of the membrane with the solution, standing for 10min, then pouring out the sodium polyacrylate, and cleaning the surface of the membrane with deionized water for 10 min; nitrogen gas sweeps the redundant solution on the surface of the membrane to be clean until the step is 1 monolayer; pouring the poly cationic electrolyte poly dimethyl diallyl ammonium chloride on the flat membrane component by the same method, standing for 5min, then pouring out the sodium polyacrylate, and cleaning the surface of the membrane for 10min by using deionized water; the nitrogen purged the excess solution from the membrane surface to 1 bilayer. The above steps are circulated to obtain the membrane module with 3 double layers. And then pouring the prepared cross-linking agents of glycol and glutaraldehyde onto the surface of the flat membrane in sequence, standing for 15min, pouring the solution, and placing the solution into deionized water for storage until testing and use.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test pressure was 4 bar. The test results are: permeability coefficient of 8.5L m^-2h^-1 bar^-1，Mg²⁺Has a rejection of 98.9%, Li⁺The retention rate of (a) was 30.6%. The magnesium-lithium separation coefficient S was 51.2. This example shows that the hollow fiber membrane is slightly more effective in magnesium-lithium separation than the flat sheet membrane.

Example 4

The film of the present invention is prepared as follows. The polyelectrolyte coating method was changed to spray coating. The supporting layer is a polyether-ether-ketone flat membrane (MWCO is 15 ten thousand Da), 5g/L of sodium polystyrene sulfonate (PSS) solution (Mw is 200 k) is sprayed on the surface of the supporting layer, and the supporting layer is kept stand for 15 min; then 5g/L of polyethyleneimine is sprayed and kept stand for 15 min. 5 bilayers were sprayed alternately.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test pressure was 3 bar. The test results are: permeability coefficient of 6.5L m^-2 h^-1 bar^-1The magnesium-lithium separation coefficient S was 55.4.

Example 5

The support material is a polypropylene cyanide coating composite tubular membrane component (MWCO ═ 90 ten thousand Da) with the inner diameter of 1.25cm and the thickness of 3 mm. First, a 15g/L polyelectrolyte solution of sodium polyvinyl sulfonate was prepared by dissolving sodium polyvinyl sulfonate (Mw ═ 500k) in a 2.0mol/L sodium chloride solution. Then, the polycation electrolyte material, namely methacryloyloxyethyl trimethyl ammonium chloride (79% -81% in water), is dissolved in 2.5mol/L sodium chloride solution to prepare 10g/L of methacryloyloxyethyl trimethyl ammonium chloride polyelectrolyte solution. Glutaraldehyde is used as the crosslinking agent, and glutaraldehyde in 50% water solution is prepared into 2.0 wt% concentration solution (pH is adjusted to 4.1). Injecting a sodium polyvinylsulfonate solution into the inner surface of the tubular membrane; then, deionized water is pumped into the film from the bottom of the film to ensure that the inner surface of the film is immersed by the solution, standing is carried out for 5min, and the solution in the film flows out after the standing is finished; the solution on the surface was purged with 0.1bar of nitrogen until a monolayer was finished. In the same way, methacryloyloxyethyl trimethyl ammonium chloride is injected into the inner surface of the membrane, washed by deionized water and purged by nitrogen, and the process is finished when the membrane is a double layer. Coating 2 double layers, then pumping glutaraldehyde into the inner surface of the membrane from the bottom of the membrane to crosslink aldehyde groups of the glutaraldehyde and amino groups of methacryloyloxyethyl trimethyl ammonium chloride, and cleaning unreacted polyelectrolyte with deionized water for the crosslinked membrane module to be tested.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test pressure was 4 bar. The test results are: permeability coefficient of 15.2L m^-2h^-1 bar^-1，Mg²⁺Has a rejection of 99.1%, Li⁺The retention rate of (a) was 26.4%. The magnesium-lithium separation coefficient S was 78.5.

Example 6

The same base film as in example 1 was used as in example 1. Firstly, sodium polystyrene sulfonate (Mw ═ 650k) is dissolved in 0.5mol/L sodium chloride solution to prepare 5g/L of sodium polystyrene sulfonate polyelectrolyte solution. Then, a polycationic electrolyte material, polyethyleneimine (Mw ═ 10k), was dissolved in a 2.5mol/L sodium chloride solution to prepare a 12g/L polyethyleneimine polyelectrolyte solution. With the preparation process of example 1, 3.0 bilayers were coated. And (4) washing the unreacted polyelectrolyte by using deionized water for the prepared membrane component to be tested.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 40. The test pressure was 4 bar. The test results are: permeability coefficient of 12.1L m^-2 h^-1 bar^-1，Mg²⁺Has a rejection of 99.2%, Li⁺The retention rate of (a) was 30.6%. The magnesium-lithium separation coefficient S was 83.2.

Example 7

The basement membrane in example 2 was used. First, sodium polystyrene sulfonate (Mw. about.650 k) was dissolved in 0.5mol/L sodium chloride solution to prepare 6g/L PSS polyelectrolyte solution. Then, polyacrylamide hydrochloride (Mw ═ 10k) as a polycationic electrolyte material was dissolved in 2.5mol/L sodium chloride solution to prepare 12g/L of PAH polyelectrolyte solution. Glutaraldehyde is used as the crosslinking agent, and glutaraldehyde in a 50% aqueous solution is prepared into a solution with the concentration of 1.0 wt% (the pH is adjusted to 4.5). By adopting the preparation process in the embodiment 1, 2.5 double layers are coated, and then glutaraldehyde is adopted for crosslinking, wherein the crosslinking time is 8 min. And washing the unreacted polyelectrolyte by using deionized water for the cross-linked membrane to be tested.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 60. The test pressure was 4 bar. The test results are: the calculated permeability coefficient is 10.0L m^-2 h^-1 bar^-1， Mg²⁺The rejection of (2) was 99.2%, and the rejection of Li + was 27.9%. The magnesium-lithium separation coefficient S was 87.2.

Example 8

The film of the present invention is prepared as follows. The salt solution NaCl was changed to KBr as in example 6, and the polyelectrolyte solution was dissolved in KBr. 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺In a mass ratio of20. The test results are: permeability coefficient of 12.8L m^-2 h^-1 bar^-1The magnesium-lithium separation coefficient S was calculated to be 79.8.

Example 9

The film of the present invention is prepared as follows. The polyelectrolyte coating method is changed into a dip coating method. Alternately immersing the support material in two solutions with different components, taking out after a certain time, washing with water or other solutions, and repeating the steps for multiple times until the prepared membrane material is obtained. The basement membrane adopts polysulfone flat sheet membrane (MWCO is 56 ten thousand Da), the polyanion electrolyte adopts sodium carboxymethyl cellulose, the polycation electrolyte solution base is poly dimethyl diallyl ammonium chloride, and the concentration of the polyelectrolyte solution is 0.1 g/L. The sodium carboxymethylcellulose is firstly dip-coated, and then the poly dimethyl diallyl ammonium chloride is dip-coated, wherein the number of the dip-coating layers is 10 double layers.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test pressure was 4 bar. The test results are: permeability coefficient of 8.6L m^-2 h^-1 bar^-1The magnesium-lithium separation coefficient S was 63.6.

Example 10

The film of the present invention is prepared as follows. After the polyelectrolyte coating was completed as in example 1, there was no crosslinking of glutaraldehyde and polyvinyl alcohol. 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 20. The test results are: permeability coefficient of 15.5L m^-2 h^-1 bar^-1The magnesium-lithium separation coefficient S was 99.5.

Example 11

Adopting a polyethylene diaphragm flat membrane (with the aperture of 30nm) subjected to plasma hydrophilic modification, wherein the coating solution adopts the following components: dissolving sodium polyvinyl sulfonate (Mw ═ 2,000k) in 1.5mol/L sodium chloride solution to prepare 5g/L sodium polyvinyl sulfonate polyelectrolyte solution; the polycation electrolyte material poly dimethyl diallyl ammonium chloride (Mw ═ 500k) is dissolved in 1.5mol/L sodium chloride solution to prepare 10g/L poly dimethyl diallyl ammonium chloride polyelectrolyte solution. The cross-linking agent is glutaraldehyde and polyvinyl alcohol, glutaraldehyde of 50% water solution is prepared into 1.0 wt% concentration solution (pH is adjusted to 4.1), and polyvinyl alcohol is dissolved in deionized water to prepare 1 wt% concentration solution (pH is not adjusted). After the film was prepared by the process of example 3, it was stored in deionized water until use for testing.

Preparing a simulated salt lake brine solution: 2000ppm MgCl₂The LiCl mixed solution is used as a raw material solution, wherein Mg²⁺/Li⁺The mass ratio was 50. The test pressure was 4 bar. The test results are: permeability coefficient of 8.9L m^-2 h^-1 bar^-1The Mg/Li separation coefficient S was 71.8.

The magnesium lithium separation coefficient data obtained from the tests were compared by examples 1-11. The highest separation coefficient of magnesium and lithium in the current literature is reported to be 33.4. The separation coefficient S of magnesium and lithium obtained by the process method is between 51.2 and 120, wherein the rejection rate of Mg2+ is between 98.0 and 99.5 percent. Further proves that the separation coefficient of magnesium and lithium is obviously improved by alternately coating polyelectrolyte solution on the surface of the basement membrane and then crosslinking through a crosslinking agent, and a good choice is provided for efficiently separating lithium from natural saline water and searching for stable lithium resource supply. The above examples do not indicate a limited scope of application of the patent. Any person skilled in the art of membrane preparation can easily apply the method described in the patent to any other possible system to obtain high performance lithium-magnesium separation nanofiltration membranes.

Claims

1. A preparation method of a magnesium-lithium separation membrane with high permselectivity is characterized by comprising the following steps: firstly, alternately coating polyelectrolytes with opposite charges, namely polycation and polyanion electrolyte solutions, on the surface of a porous support membrane; coating to the required number of layers and then carrying out crosslinking treatment; the used porous support membrane, namely the basement membrane is made of the following high polymer materials, namely polyether sulfone, polysulfone, sulfonated polyether sulfone, sulfonated polysulfone, polyacrylonitrile, polyolefin materials, polyimide, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polyether imide, polyether ether ketone, aminated polysulfone, aminated polyether sulfone and aminated polypropylene cyanide;

coating polyelectrolyte, namely polycation and polyanion electrolyte solution for 1-10 periodic layers; after the polyelectrolyte solution is coated on the surface of the basement membrane, the pore diameter of the membrane is reduced in a cross-linking mode;

the polycation electrolyte material in the polycation electrolyte solution is selected from one or more of polyacrylamide hydrochloride (PAH), polydimethyldiallylammonium chloride (PDADMAC), Polyethyleneimine (PEI), methacryloyloxyethyl trimethyl ammonium chloride, chitosan, polyvinyl alcohol (PVA), polyacrylamide (SC498) and the like, and the concentration of the polycation electrolyte is 0.1-20 g/L;

the polyanion electrolyte material in the polyanion electrolyte solution is selected from one or more of sodium polystyrene sulfonate (PSS), dextran sodium sulfate, sodium carboxymethyl cellulose, sodium Polyacrylate (PAA), sodium polyvinyl sulfonate (PVS), sulfonated polyether ether ketone, Hyaluronic Acid (HA) and the like; the concentration of the polyanionic electrolyte is 0.1-20 g/L; the polyanion is preferably 2-10g/L, and the polycation is preferably 4-15 g/L.

2. The method for preparing a magnesium-lithium separation membrane with high permselectivity according to claim 1, wherein the porous support membrane, i.e., the type of the base membrane, is a hollow fiber membrane, a flat sheet membrane or a tubular membrane.

3. The method for preparing a magnesium-lithium separation membrane having high permselectivity according to claim 1, wherein the presence or absence of cross-linking is provided. If the crosslinking is carried out, the crosslinking method is chemical crosslinking, and the adopted chemical crosslinking agent comprises one or more of glutaraldehyde, glyoxal, polyvinyl alcohol, propionaldehyde, fluorophenyl isocyanate, ethylene glycol and the like; concentration of 0.01-5 wt%, most preferably concentration of 0.1-2 wt%; the pH range is 4-6, preferably 4-4.5.

4. The method for preparing a magnesium-lithium separation membrane with high permselectivity according to claim 1, wherein the polymer coating method is dip coating, spin coating, spray coating, or filter pressing.

5. The method of preparing a magnesium-lithium separation membrane with high permselectivity according to claim 1, wherein the molecular weight cut-off range of the base membrane material is preferably between 5k and 1000kDa, most preferably in the range of 30k-300 kDa.

6. The method for preparing a magnesium-lithium separation membrane with high permselectivity according to claim 1, wherein the polyanionic electrolyte and the polycationic electrolyte are dissolved in the salt solution at a concentration ranging from 0.001mol/L to 3.0mol/L, preferably from 0.05 mol/L to 1.0mol/L, and the salt is NaCl or NaNO₃、KBr、NaF、NaBr、MgCl₂。

7. The method for preparing the magnesium-lithium separation membrane with high permselectivity according to claim 1, wherein the molecular weight of the polyelectrolyte has a direct influence on the thickness and roughness of the nanofiltration membrane; the molecular weight of the polyelectrolyte is between 10k and 1000 kDa.

8. The method of preparing a magnesium-lithium separation membrane with high permselectivity according to claim 1, wherein the coating is performed by selecting a suitable polyelectrolyte according to the pore size of the base membrane.

9. The method for preparing a magnesium-lithium separation membrane with high permselectivity according to claim 1, wherein the separation membrane is a hollow fiber membrane, a flat sheet membrane or a tubular membrane.

10. The application of the magnesium-lithium separation membrane obtained by the preparation method of the magnesium-lithium separation membrane with high osmotic selectivity according to one of claims 1 to 9, wherein the nanofiltration membrane obtained by the polyelectrolyte coating is used for magnesium-lithium separation of salt lake brine, the magnesium-lithium separation coefficient can reach more than 50, and the optimal membrane separation coefficient is higher than 100.