CN115845640B - Positively charged composite nanofiltration membrane and preparation method and application thereof - Google Patents

Abstract

The present invention belongs to the technical field of membrane material preparation and membrane separation, and specifically relates to a positively charged composite nanofiltration membrane and a preparation method and application thereof. The present invention provides a preparation method of a positively charged composite nanofiltration membrane, firstly, an interfacial polymerization reaction is carried out between a polyamine aqueous phase and a polyacyl chloride organic phase to generate a polyamide functional layer, and then a chitosan quaternary ammonium salt is used to adjust the charge of the polyamide layer, and a positively charged composite nanofiltration membrane is prepared by thermal compounding. The present invention improves the positive charge strength of the nanofiltration membrane, strengthens the Donnan effect of the composite nanofiltration membrane, and improves the retention rate of the nanofiltration membrane for multivalent cations. The positively charged composite nanofiltration membrane prepared by the present invention can effectively improve the magnesium-lithium separation performance of a high magnesium-lithium ratio system in brine, has the advantages of simple operation and green environmental protection, and has good industrial application prospects in lithium extraction from salt lakes.

Description

A positively charged composite nanofiltration membrane and its preparation method and application

Technical Field

The invention belongs to the technical field of membrane material preparation and membrane separation, and specifically relates to a positively charged composite nanofiltration membrane and a preparation method and application thereof.

Background technique

Lithium and its compounds are important strategic resources for national economy and national defense construction. The development of lithium extraction technology from salt lakes is of great significance to the national economy and national development.

The technologies for extracting lithium from salt lakes are mainly based on solvent extraction, adsorption, electrochemical deintercalation, selective electrodialysis and membrane separation. Compared with other methods, membrane separation has low cost, simple process, easy operation and green environmental protection, and has received extensive attention. Nanofiltration membrane, as one of the most important methods in membrane separation technology, is a separation membrane with nanoscale pores and a charge on the surface. Based on the synergistic effect of multiple mechanisms such as the Donnan effect, pore size screening, and dielectric effect, it has a high retention rate for divalent and higher high-valent ions and a high permeability for monovalent ions, thereby achieving the separation of ions with different valence states. However, since magnesium and lithium in salt lake brine have very similar chemical properties and hydration radii, the higher the magnesium-to-lithium ratio in the brine, the more difficult it is to extract lithium.

At present, common nanofiltration membranes are synthesized by interfacial polymerization of amine monomers and acyl chloride monomers. The surface charge of the obtained membrane material is generally negative, and the magnesium-lithium separation performance is low, and it cannot have both high permeability and high selectivity. Based on the principle of nanofiltration membrane separation, it is believed that the surface charge of the nanofiltration membrane seriously affects the Donnan effect and dielectric effect of the nanofiltration membrane. Improving the positive charge of the membrane surface can improve the magnesium-lithium separation effect, which is also one of the effective ways to achieve magnesium-lithium separation. In recent years, domestic and foreign researchers have adjusted the surface charge of the nanofiltration membrane by introducing cationic surfactants into traditional negatively charged nanofiltration membranes, and used the Donnan effect to improve the retention performance of the nanofiltration membrane for multivalent cations. The cationic surfactants are mainly nitrogen-containing organic amine derivatives. For example, Chinese patent CN113230888A discloses a method for preparing a positively charged nanofiltration membrane, using polyethyleneimine as a cationic surfactant. The amine groups in polyethyleneimine have weak charge in a neutral solution, and the separation effect for multivalent cations and cationic small molecules is poor. Therefore, it is necessary to develop a positively charged composite nanofiltration membrane with good separation effect for multivalent cations and monovalent cations.

Summary of the invention

In view of this, the purpose of the present invention is to provide a positively charged composite nanofiltration membrane and its preparation method and application. The present invention uses chitosan quaternary ammonium salt to adjust the charge of the polyamide functional layer. The chitosan quaternary ammonium salt can increase the positive charge intensity, strengthen the Donnan effect of the composite nanofiltration membrane, and improve the retention performance of the positively charged composite nanofiltration membrane for cations, thereby improving the separation efficiency of multivalent cations and monovalent cations.

In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

The present invention provides a method for preparing a positively charged composite nanofiltration membrane, comprising the following steps:

The base film is sequentially immersed in a polyamine aqueous solution and a polyacyl chloride organic solution, and then an interfacial polymerization reaction is performed to obtain a base film containing a polyamide functional layer;

The base film containing the polyamide functional layer is immersed in a chitosan quaternary ammonium salt aqueous solution for charge modification to obtain a base film containing the polyamide functional layer and the cationic modified layer;

The base membrane containing the polyamide functional layer and the cationic modified layer is thermally composited to obtain the positively charged composite nanofiltration membrane.

Preferably, the polyamine aqueous solution comprises polyethyleneimine and piperazine, and the mass ratio of polyethyleneimine to piperazine is 1 to 5:1.

Preferably, the polyacyl chloride in the polyacyl chloride organic solution includes one or more of trimesoyl chloride, terephthaloyl chloride and isophthaloyl chloride.

Preferably, the organic solvent in the polyacyl chloride organic solution includes one or more of n-hexane, dodecane and cyclohexane.

Preferably, the material of the base film includes one or more of polyacrylonitrile, polyethersulfone and polysulfone.

Preferably, the concentration of the chitosan quaternary ammonium salt in the chitosan quaternary ammonium salt aqueous solution is 0.1-0.7 wt %.

Preferably, the temperature of the thermal compounding is 50-100° C., and the time is 300-1500 s.

Preferably, the base film is immersed in the polyamine aqueous solution for 5 to 25 minutes.

The present invention also provides a positively charged composite nanofiltration membrane obtained by the preparation method described in the above technical scheme, comprising a base membrane, a polyamide functional layer and a cationic modified layer stacked in sequence, wherein the polyamide functional layer is prepared by interfacial polymerization of a polyamine aqueous solution and a polyacyl chloride organic solution, and the cationic modified layer is provided with positively charged groups by chitosan quaternary ammonium salt.

The present invention also provides the use of the positively charged composite nanofiltration membrane in magnesium-lithium separation.

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

The invention provides a method for preparing a positively charged composite nanofiltration membrane, comprising the following steps: immersing a base membrane in a polyamine aqueous solution and a polyacyl chloride organic solution in sequence, and performing an interfacial polymerization reaction to obtain a base membrane containing a polyamide functional layer; immersing the base membrane containing the polyamide functional layer in a chitosan quaternary ammonium salt aqueous solution to perform charge modification to obtain a base membrane containing a polyamide functional layer and a cationic modified layer; and thermally compounding the base membrane containing the polyamide functional layer and the cationic modified layer to obtain the positively charged composite nanofiltration membrane.

The present invention adopts chitosan quaternary ammonium salt to adjust the charge of the polyamide functional layer. The chitosan quaternary ammonium salt has a high charge density and good film-forming property, can improve the positive charge strength of the composite nanofiltration membrane, strengthen the Donnan effect of the composite nanofiltration membrane, and enhance the retention performance of the positively charged composite nanofiltration membrane for multivalent cations, thereby improving the separation efficiency of multivalent cations and monovalent cations.

Furthermore, the polyamide functional layer of the present invention is prepared by interfacial polymerization reaction between a polyamine aqueous solution and a polyacyl chloride organic solution. The polyamine aqueous solution contains polyethyleneimine and piperazine. The mixture of polyethyleneimine and piperazine is used as an interfacial polymerization aqueous phase reactant, which can reduce the retention capacity of the positively charged composite nanofiltration membrane for monovalent cations, further improve the separation efficiency of multivalent cations and monovalent cations, and improve the separation rate of magnesium and lithium.

The present invention also provides the positively charged composite nanofiltration membrane described in the above technical solution, which can effectively improve the magnesium-lithium separation performance of the high magnesium-lithium ratio system in brine, has the advantages of simple operation and green environmental protection, and has good industrial application prospects in lithium extraction from salt lakes.

The present invention also provides the application of the positively charged composite nanofiltration membrane. The positively charged composite nanofiltration membrane of the present invention is used for magnesium and lithium separation in salt lake brine. The water flux is 6.41Lm ^- 2h ^- 1bar ^-1 , the ^Mg2+ retention rate is as high as 90%, and the Li ⁺ retention rate is as low as 10%, that is, it has moderate water flux, excellent Mg2 ⁺ retention rate and low Li ⁺ retention rate. The data of the embodiments and comparative examples show that the positively charged composite nanofiltration membrane of the present invention has a higher multivalent cation retention rate and has a better magnesium-lithium separation effect in the process of extracting lithium from salt lakes.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

FIG1 is a graph showing the magnesium and lithium retention rate results of Example 1;

FIG2 is a graph showing the magnesium and lithium retention rate results of Example 2;

FIG3 is a graph showing the magnesium-lithium retention rate results of Example 3;

FIG4 is a graph showing the magnesium and lithium retention rate results of Comparative Example 1;

FIG5 is a graph showing the magnesium-lithium retention rate results of Comparative Example 2.

Detailed ways

The present invention provides a method for preparing a positively charged composite nanofiltration membrane, comprising the following steps:

The base film is sequentially immersed in a polyamine aqueous solution and a polyacyl chloride organic solution, and then an interfacial polymerization reaction is performed to obtain a base film containing a polyamide functional layer;

The base film containing the polyamide functional layer is immersed in a chitosan quaternary ammonium salt aqueous solution for charge modification to obtain a base film containing the polyamide functional layer and the cationic modified layer;

The base membrane containing the polyamide functional layer and the cationic modified layer is thermally composited to obtain the positively charged composite nanofiltration membrane.

In the present invention, unless otherwise specified, the materials and equipment used are commercially available products in the art.

The invention immerses the base film in polyamine aqueous solution and polyacyl chloride organic solution in sequence, and then performs interfacial polymerization reaction to obtain the base film containing the polyamide functional layer.

In the present invention, the material of the base film preferably includes one or more of polyacrylonitrile (PAN), polyethersulfone (PES) and polysulfone (PSF).

In the present invention, the polyamine aqueous solution preferably includes polyethyleneimine (PEI) and piperazine (PIP), and the mass ratio of polyethyleneimine to piperazine is preferably 1 to 5:1, more preferably 1:1, 2:1, 3:1, 4:1 or 5:1.

In the present invention, the mass concentration of polyethyleneimine (PEI) in the polyamine aqueous solution is preferably 0.2-1.0 wt%, more preferably 0.6-0.8 wt%; the mass concentration of piperazine (PIP) in the polyamine aqueous solution is preferably 0.1-0.5 wt%, more preferably 0.2 wt%.

In the present invention, the weight average molecular weight of the polyethyleneimine (PEI) is preferably 600-10,000.

In the present invention, the polyamine aqueous solution preferably also includes a water phase additive, and the water phase additive preferably includes one or more of triethylamine, sodium dodecylbenzene sulfonate, sodium carbonate and sodium dodecyl sulfate. The concentration of the water phase additive in the polyamine aqueous solution is preferably 0.1-1.5wt%, more preferably 0.3wt%. The water phase additive can promote the wetting of membrane pores by the water phase solvent and shorten the water phase immersion time.

In the present invention, the time for immersing the base film in the polyamine aqueous solution is preferably 300 to 1500 seconds (5 to 25 minutes), more preferably 5, 10, 15, 20 or 25 minutes, and the purpose of the immersion is to make the polyamine aqueous solution wet the surface of the base film. The present invention has no special requirements for the volume of the polyamine aqueous solution during the immersion, as long as the base film is immersed.

In the present invention, after the base film is immersed in the polyamine aqueous solution, it is preferred to remove excess polyamine aqueous solution. The present invention has no special requirements for the removal method, such as pouring out or rolling the base film with a rubber roller.

In the present invention, the polyacyl chloride in the polyacyl chloride organic solution preferably includes one or more of trimesoyl chloride, terephthaloyl chloride and isophthaloyl chloride.

In the present invention, the mass concentration of the polyacyl chloride in the polyacyl chloride organic solution is preferably 0.05-0.25 wt %, more preferably 0.15 wt %.

In the present invention, the organic solvent in the polyacyl chloride organic solution preferably includes one or more of n-hexane, dodecane and cyclohexane, and the organic solvent is preferably an organic solvent that is immiscible with water.

In the present invention, the immersion time in the polyacyl chloride organic solution is preferably 10 to 300 seconds, more preferably 120 seconds, and the volume of the polyacyl chloride organic solution during the immersion is preferably the same as the volume of the polyamine aqueous solution.

In the present invention, during the interfacial polymerization reaction, the polyacyl chlorides in the polyacyl chloride organic solution and the polyamines in the polyamine aqueous solution undergo interfacial polymerization to form a polyamide layer on the surface of the base film; after the interfacial polymerization reaction, the excess polyacyl chloride organic solution is preferably removed.

After obtaining the base film containing the polyamide functional layer, the present invention immerses the base film containing the polyamide functional layer in a chitosan quaternary ammonium salt aqueous solution for charge modification to obtain a base film containing the polyamide functional layer and a cationic modified layer.

In the present invention, the concentration of the chitosan quaternary ammonium salt in the chitosan quaternary ammonium salt aqueous solution is preferably 0.1-0.7 wt %, more preferably 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 or 0.7 wt %.

In the present invention, the time for immersing the base film containing the polyamide functional layer in the chitosan quaternary ammonium salt aqueous solution is preferably 10 to 600 seconds, more preferably 120 to 300 seconds; after the immersion, it is preferably also included to remove excess chitosan quaternary ammonium salt aqueous solution; during the charge modification process, the amine group of the chitosan quaternary ammonium salt reacts with the unreacted acyl chloride group of the interfacial polymerization.

After obtaining the base membrane containing the polyamide functional layer and the cationic modified layer, the present invention thermally composites the base membrane containing the polyamide functional layer and the cationic modified layer to obtain the positively charged composite nanofiltration membrane.

In the present invention, the temperature of the thermal composite is preferably 50-100°C, more preferably 50, 60, 70, 80 or 90°C; the time is preferably 300-1500s, more preferably 1200s. During the thermal composite process, the base membrane support layer and the polyamide functional layer shrink to different degrees, which can increase the roughness of the membrane surface and improve the membrane flux; the shrinkage of the support layer and the polyamide functional layer makes the membrane pores smaller and the retention rate higher.

The present invention adopts mixed water phase interfacial polymerization and chitosan quaternary ammonium salt surface charge modification to prepare a positively charged composite nanofiltration membrane, which can effectively improve the magnesium-lithium separation performance of the positively charged composite nanofiltration membrane for a high magnesium-lithium ratio system in brine.

The present invention also provides a positively charged composite nanofiltration membrane obtained by the preparation method described in the above technical scheme, comprising a base membrane, a polyamide functional layer and a cationic modified layer stacked in sequence, wherein the polyamide functional layer is prepared by interfacial polymerization of a polyamine aqueous solution and a polyacyl chloride organic solution, and the cationic modified layer is provided with positively charged groups by chitosan quaternary ammonium salt.

In the present invention, the positively charged groups of the cationic modified layer are provided by chitosan quaternary ammonium salt, and the chitosan quaternary ammonium salt can exhibit good positive charging performance in solutions within the entire pH range.

The present invention also provides the use of the positively charged composite nanofiltration membrane in magnesium-lithium separation.

In the present invention, the solution for separation of magnesium and lithium is preferably salt lake brine with a high magnesium to lithium ratio (10 to 70:1), the salt lake brine with a high magnesium to lithium ratio preferably comprises multivalent (including divalent) cations, and the salt lake brine with a high magnesium to lithium ratio preferably contains magnesium chloride and magnesium sulfate.

In the present invention, the parameters for separation of magnesium and lithium preferably include: operating pressure 5 bar, nanofiltration time 60 min, and flow rate 2.5 L/min.

In order to further illustrate the present invention, the positively charged composite nanofiltration membrane provided by the present invention and its preparation method and application are described in detail below in conjunction with the accompanying drawings and examples, but they should not be understood as limiting the scope of protection of the present invention.

Example 1

This example is the preparation of a positively charged composite nanofiltration membrane sample 1, wherein the positively charged nanofiltration selection layer (cation modified layer) in the positively charged composite nanofiltration membrane sample 1 is provided with positively charged groups by chitosan quaternary ammonium salt.

A PEI/PIP mixed aqueous phase solution was prepared, wherein the PEI mass concentration was 0.8wt%, the PIP mass concentration was 0.2wt%, and the PEI to PIP mass concentration ratio was 4:1, 0.3wt% sodium dodecyl sulfate was added, and stirred until completely dissolved as the first phase;

A 0.15 wt% organic phase solution of trimesoyl chloride is prepared, wherein the organic solvent is n-hexane, as the second phase;

0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 wt% chitosan quaternary ammonium salt aqueous solutions were prepared respectively, and stirred until completely dissolved as the third phase;

Immerse the polyacrylonitrile base membrane in the first phase to submerge the base membrane surface so that it contacts the surface for 600 seconds, take it out, and then roll the polyacrylonitrile base membrane with a rubber roller to remove excess solution, pour an equal volume of the second phase and soak for 120 seconds, after the interfacial polymerization reaction is completed, pour an equal volume of the third phase on the base membrane surface and soak for 120 seconds, after the reaction is completed, put it in a 50°C forced air oven for thermal compounding for 1200 seconds and then take it out to obtain a positively charged composite nanofiltration membrane sample 1, which is stored in pure water for later use.

The simulated brine lithium extraction experiment was carried out under the operating conditions of operating pressure 5 bar, nanofiltration time 60 min, and flow rate 2.5 L/min. Table 1 is a table of simulated brine composition. The magnesium and lithium retention rate results of Example 1 are shown in Figure 1.

Table 1 Simulated brine composition

Element Li ⁺ Na ⁺ Mg ²⁺ Ca ^{2+ Cl-} SO ₄ ^2- K ⁺ Content g/L 0.21 102.3 13.5 0.3 188.1 twenty four 8.45

Example 2

This example is the preparation of a positively charged composite nanofiltration membrane sample 2, wherein the positively charged nanofiltration selection layer (cation modified layer) in the positively charged composite nanofiltration membrane sample 2 is provided with positively charged groups by chitosan quaternary ammonium salt.

A PEI/PIP mixed aqueous phase solution was prepared, wherein the PEI mass concentration was 0.8wt%, the PIP mass concentration was 0.2wt%, and the PEI to PIP mass concentration ratio was 4:1, 0.3wt% sodium dodecyl sulfate was added, and stirred until completely dissolved as the first phase;

A 0.15 wt% organic phase solution of trimesoyl chloride is prepared, wherein the organic solvent is n-hexane, as the second phase;

Prepare a 0.3 wt % chitosan quaternary ammonium salt aqueous solution and stir until completely dissolved as the third phase;

Immerse the polyacrylonitrile base membrane in the first phase to submerge the base membrane surface so that it contacts the surface for 600 seconds, take it out, and then roll the polyacrylonitrile support membrane with a rubber roller to remove excess solution, pour an equal volume of the second phase and soak for 120 seconds, and after the reaction is completed, pour an equal volume of the third phase on the surface of the base membrane and soak for 120 seconds. After the reaction is completed, put it in a 50, 60, 70, 80, and 90°C forced air oven for thermal compounding for 1200 seconds, then take it out to obtain the positively charged composite nanofiltration membrane sample 2, which is stored in pure water for later use.

The simulated brine lithium extraction experiment was carried out under the operating conditions of operating pressure 5 bar, nanofiltration time 60 min, and flow rate 2.5 L/min. The simulated brine composition table is shown in Table 1, and the magnesium and lithium retention rate of Example 2 is shown in Figure 2.

Example 3

This example is the preparation of a positively charged composite nanofiltration membrane sample 3, wherein the positively charged nanofiltration selection layer (cation modified layer) in the positively charged composite nanofiltration membrane sample 3 is provided with positively charged groups by chitosan quaternary ammonium salt.

A PEI/PIP mixed aqueous phase solution was prepared, wherein the PEI mass concentration was 0.8wt%, the PIP mass concentration was 0.2wt%, and the PEI to PIP mass concentration ratio was 4:1, 0.3wt% sodium dodecyl sulfate was added, and stirred until completely dissolved as the first phase;

A 0.15 wt% organic phase solution of trimesoyl chloride is prepared, wherein the organic solvent is n-hexane, as the second phase;

Prepare a 0.3 wt % chitosan quaternary ammonium salt aqueous solution and stir until completely dissolved as the third phase;

The polyacrylonitrile base membrane was immersed in the first phase to submerge the base membrane surface, and was kept in contact with the surface for 5, 10, 15, 20, and 25 minutes (300 to 1500 seconds) respectively. Then, the polyacrylonitrile base membrane was taken out and rolled with a rubber roller to remove excess solution. An equal volume of the second phase was poured in and soaked for 120 seconds. After the reaction was completed, an equal volume of the third phase solution was poured on the surface of the base membrane and soaked for 600 seconds. After the reaction was completed, it was placed in an 80°C forced air oven for thermal compounding for 1200 seconds and then taken out to obtain the positively charged composite nanofiltration membrane sample 3, which was stored in pure water for later use.

The simulated brine lithium extraction experiment was carried out under the operating conditions of operating pressure 5 bar, nanofiltration time 60 min, and flow rate 2.5 L/min. The simulated brine composition table is shown in Table 1, and the magnesium and lithium retention rate of Example 3 is shown in Figure 3.

Comparative Example 1

This comparative example is the preparation of negatively charged composite nanofiltration membrane comparative sample 1, wherein the negatively charged nanofiltration selection layer in the negatively charged composite nanofiltration membrane comparative sample 1 uses piperazine (PIP) as a raw material to provide negatively charged groups.

0.1, 0.2, 0.3, 0.4, and 0.5 wt% PIP aqueous solutions were prepared respectively, 0.3 wt% sodium dodecyl sulfate was added, and stirred until completely dissolved as the first phase;

A 0.15 wt% organic phase solution of trimesoyl chloride is prepared, wherein the organic solvent is n-hexane, as the second phase;

Immerse the polyacrylonitrile base membrane in the first phase to submerge the base membrane surface so that it contacts the surface for 600 seconds, take it out, and then roll the polyacrylonitrile base membrane with a rubber roller to remove excess solution, pour in an equal volume of the second phase and soak for 120 seconds. After the reaction is completed, put it in an 80°C forced air oven for thermal compounding for 10 minutes and then take it out to obtain a negatively charged composite nanofiltration membrane sample 1, which is stored in pure water for later use.

A simulated brine lithium extraction experiment was carried out under the operating conditions of an operating pressure of 5 bar, a nanofiltration time of 60 min, and a flow rate of 2.5 L/min. The simulated brine composition table is shown in Table 1, and the magnesium and lithium retention rate of Comparative Example 1 is shown in Figure 4.

Comparative Example 2

This comparative example is the preparation of positively charged composite nanofiltration membrane comparative sample 2, wherein the positively charged nanofiltration selection layer in the positively charged composite nanofiltration membrane comparative sample 2 is provided with positively charged groups by a PEI/PIP mixed aqueous phase.

Prepare a PEI/PIP mixed aqueous phase solution, wherein the PEI mass concentration is 0.2, 0.4, 0.6, 0.8, and 1.0 wt%, the PIP mass concentration is 0.2 wt%, and the PEI and PIP mass concentration ratio is 1:1, 2:1, 3:1, 4:1, and 5:1, respectively, add 0.3 wt% sodium dodecyl sulfate, and stir until completely dissolved as the first phase;

A 0.15 wt% organic phase solution of trimesoyl chloride is prepared, wherein the organic solvent is n-hexane, as the second phase;

Immerse the polyacrylonitrile base membrane in the first phase to submerge the base membrane surface, allow it to contact the surface for 600 seconds, take it out, and then roll the polyacrylonitrile base membrane with a rubber roller to remove excess solution, pour in an equal volume of the second phase and soak for 120 seconds. After the reaction is completed, put it in an 80°C forced air oven for thermal compounding for 10 minutes and then take it out to obtain the positively charged composite nanofiltration membrane comparison sample 2, which is stored in pure water for later use.

A simulated brine lithium extraction experiment was carried out under the operating conditions of an operating pressure of 5 bar, a nanofiltration time of 60 min, and a flow rate of 2.5 L/min. The simulated brine composition table is shown in Table 1, and the magnesium-lithium retention rate results of Comparative Example 2 are shown in Figure 5.

Figure 4 shows the relationship between the negatively charged nanofiltration membrane and the magnesium and lithium retention rates of Comparative Example 1. It can be seen from Figure 4 that the magnesium and lithium retention rates have the same change trend, and the highest magnesium retention rate is only 40%. The lithium retention rate is relatively high, and it does not have a good magnesium-lithium separation effect; Figure 5 shows the relationship between the positively charged composite nanofiltration membrane and the magnesium and lithium retention rates of Comparative Example 2. It can be seen from Figure 5 that the retention rate of magnesium ions increases with the increase of PEI concentration, and the retention rate of lithium ions shows a downward trend, indicating that the positively charged nanofiltration membrane exhibits excellent magnesium-lithium separation performance.

Figure 1 shows the effect of chitosan quaternary ammonium salt mass concentration on magnesium-lithium retention rate in Example 1. With the increase of chitosan quaternary ammonium salt, the positive charge performance of the membrane surface is improved, the retention rate of magnesium ions is further improved compared with Comparative Example 2, and the retention rate of lithium ions remains basically unchanged, indicating that chitosan quaternary ammonium salt can improve the retention rate of magnesium ions by improving the positive charge performance of the membrane surface; Figure 2 shows the effect of heat treatment temperature on magnesium-lithium retention rate in Example 2. Temperatures above 80°C will cause membrane pore collapse, which is not conducive to magnesium-lithium separation; Figure 3 shows the effect of aqueous phase (first phase) immersion time on magnesium-lithium retention rate in Example 3. The longer the immersion time, the better the wettability of the aqueous phase solution to the membrane. The wetting of the membrane is conducive to the interfacial polymerization reaction, generating a polyamide layer with surface defects.

It can be seen from the above examples and comparative example data that the positively charged composite nanofiltration membrane of the present invention has the characteristics of high selectivity and high ^Mg2+ retention rate, and the preparation method is simple to operate, and can be applied to the field of lithium extraction from salt lake brine.

Although the above-mentioned embodiments have made a detailed description of the present invention, they are only some embodiments of the present invention, rather than all embodiments. People can also obtain other embodiments based on the embodiments of the present invention without creative work, and these embodiments all fall within the scope of protection of the present invention.

Claims (8)

1. The preparation method of the positively charged composite nanofiltration membrane is characterized by comprising the following steps of:

Sequentially dipping a base film into a polyamine aqueous solution and a polybasic acyl chloride organic solution, and performing interfacial polymerization reaction to obtain a base film containing a polyamide functional layer; the polyamine aqueous solution comprises polyethyleneimine and piperazine, wherein the mass ratio of the polyethyleneimine to the piperazine is 4:1; the mass concentration of the polyethyleneimine in the polyamine aqueous solution is 0.8wt percent, and the mass concentration of the piperazine is 0.2wt percent;

immersing the base film containing the polyamide functional layer in chitosan quaternary ammonium salt aqueous solution for charge modification to obtain the base film containing the polyamide functional layer and the cation modified layer; the concentration of the chitosan quaternary ammonium salt in the chitosan quaternary ammonium salt aqueous solution is 0.3wt%;

and thermally compounding the base film containing the polyamide functional layer and the cation modified layer to obtain the positively charged composite nanofiltration membrane.

2. The method of claim 1, wherein the polyacyl chloride in the polyacyl chloride organic solution comprises one or more of trimesoyl chloride, terephthaloyl chloride, and isophthaloyl chloride.

3. The method of claim 1 or 2, wherein the organic solvent in the organic solution of polyacyl chloride comprises one or more of n-hexane, dodecane, and cyclohexane.

4. The method of claim 1, wherein the base membrane comprises one or more of polyacrylonitrile, polyethersulfone, and polysulfone.

5. The method according to claim 1, wherein the temperature of the thermal compounding is 50 to 100 ℃ and the time is 300 to 1500s.

6. The method according to claim 1 or 2, wherein the base film is immersed in the aqueous polyamine solution for a period of 5 to 25 minutes.

7. The positively charged composite nanofiltration membrane obtained by the preparation method of any one of claims 1 to 6, which is characterized by comprising a base membrane, a polyamide functional layer and a cation modification layer which are sequentially stacked, wherein the polyamide functional layer is prepared by interfacial polymerization of a polyamine aqueous solution and a polybasic acyl chloride organic solution, and the cation modification layer is prepared by providing a positive group by chitosan quaternary ammonium salt.

8. The use of the positively charged composite nanofiltration membrane of claim 7 in magnesium-lithium separation.