Highly Selective Nanofiltration Membrane for Magnesium-Lithium Separation Based on Polyolefin-Based Membrane, Preparation Method Thereof and Application

Abstract

The present invention discloses a highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane, its preparation method and application. The preparation method is as follows: The polyolefin substrate membrane is brought into single-sided contact with an organic phase monomer solution containing isophthaloyl chloride or terephthaloyl chloride monomers. The excess solution is poured out. The side of the obtained membrane that has been in contact with the organic phase monomer solution is then brought into contact with an aqueous phase monomer solution containing polyethyleneimine, and an interfacial polymerization reaction occurs. The obtained membrane is placed in an oven for heat treatment, and then the membrane is soaked in an organic solution for activation, thus obtaining a polyvinyl composite nanofiltration membrane. The nanofiltration membrane of the present invention has high magnesium ion retention and high magnesium-lithium separation selectivity in a mixed solution with a high magnesium-lithium ratio. The preparation process is simple, the conditions are mild, it is easy to scale up and realize industrial production, and it has good long-term operation stability.

Description

Technical Field

[0001] The present invention relates to a nanofiltration membrane with high selectivity for magnesium-lithium separation, and particularly to a nanofiltration membrane with high selectivity for magnesium-lithium separation based on a polyolefin-based membrane, its preparation method and application, belonging to the field of nanofiltration membranes. Background Art

[0002] Since natural lithium-containing brines contain high concentrations of magnesium ions, there are two key indicators for the separation and extraction of lithium from salt lake brines: lithium content and magnesium-lithium ratio. The current precipitation method requires a large amount of soda ash to precipitate magnesium before lithium extraction, resulting in high production costs and low economic feasibility. Therefore, it is necessary to separate magnesium ions and lithium ions in advance. Due to the very similar chemical properties of lithium ions and magnesium ions, the separation and extraction of lithium in brines have become a technical problem currently faced by the chemical industry.

[0003] In early work, researchers applied commercial nanofiltration (NF) membranes to magnesium-lithium separation and preliminarily verified the possibility of applying nanofiltration membrane separation technology to lithium extraction from salt lakes. According to the Donnan effect, negatively charged nanofiltration membranes have good retention performance for anions, but insufficient retention ability for positively charged ions. By analyzing the separation mechanism of nanofiltration membranes, namely pore size sieving effect and Donnan exclusion effect, it was found that due to the similar sizes of magnesium and lithium ions (the hydrated radius of magnesium ions is 0.43 nm, and the hydrated radius of lithium ions is 0.38 nm), the pore size sieving effect plays a limited role in the magnesium-lithium separation process. And the charge amount of magnesium ions is twice that of lithium ions. Based on this analysis, the electrostatic interaction between the surface charge of the nanofiltration membrane and magnesium ions and lithium ions is the key to improving the magnesium-lithium separation ability of the membrane. A nanofiltration membrane with a positively charged surface may have stronger magnesium-lithium separation performance.

[0004] Predecessors used traditional diamine monomers such as piperazine (PIP) and trimesoyl chloride (TMC) for interfacial polymerization on a porous polyethylene (PE) substrate to obtain a multilayer composite polyamide (TFC-PE / PA) nanofiltration membrane with a negative charge, and its magnesium-lithium ion separation performance is poor. Using TMC and polyamine-based polyethyleneimine (PEI) monomers for inverse interfacial polymerization, the obtained nanofiltration membrane has a relatively high surface positive charge, but its magnesium ion and lithium ion separation performance decreases as the concentration ratio of magnesium and lithium ions in the solution increases. Therefore, how to obtain a nanofiltration membrane that maintains high magnesium-lithium separation selectivity in a solution with a high magnesium-lithium ion concentration ratio is one of the difficulties in current magnesium-lithium separation technology. Summary of the Invention

[0005] In order to solve the deficiencies of the above technologies, the present invention provides a highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane, its preparation method and application. The present invention uses a polyolefin porous membrane as the support substrate membrane, which can greatly reduce the thickness and preparation cost of the membrane. By carrying out interfacial polymerization of polyvinylamine (PEI) and isophthaloyl chloride or terephthaloyl chloride on the polyolefin porous substrate membrane, a composite nanofiltration membrane with high magnesium ion and lithium ion separation performance is prepared, which has a high magnesium-lithium separation efficiency with high magnesium ion retention in a mixed solution containing a high magnesium-lithium ratio.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a preparation method of a highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane, and the preparation method is as follows:

[0007] First, soak the support substrate membrane with an organic phase solution containing an acyl chloride monomer to obtain a support substrate membrane infiltrated with the organic phase monomer, and then soak it in an aqueous phase solution containing an amine monomer for interfacial polymerization. Subsequently, activate the obtained nanofiltration membrane to obtain a magnesium-lithium separation nanofiltration membrane.

[0008] Preferably, it specifically includes the following steps:

[0009] Step S1: Dissolve isophthaloyl chloride or terephthaloyl chloride in an organic solvent to obtain an organic phase solution;

[0010] Step S2: Dissolve the polyvinylimine monomer in water to obtain an aqueous phase solution;

[0011] Step S3: Soak the support substrate membrane with the organic phase solution prepared in Step S1 to obtain a support substrate membrane infiltrated with the organic phase monomer;

[0012] Step S4: Immerse the support substrate membrane infiltrated with the organic phase monomer obtained in Step S3 in the aqueous phase solution prepared in Step S2 for interfacial polymerization reaction to obtain a polyamide thin film composite nanofiltration membrane;

[0013] Step S5: Heat-treat and alcohol-activate the polyamide thin film composite nanofiltration membrane obtained in Step S3 to obtain a nanofiltration membrane with magnesium-lithium separation performance.

[0014] Preferably, in the organic phase solution prepared in Step S1, the concentration of isophthaloyl chloride or terephthaloyl chloride is 0.05 - 0.2 w / v%, and the organic solvent is n-hexane.

[0015] Preferably, in the aqueous phase solution prepared in Step 2, the molecular weight of the polyvinylimine monomer is 700 - 70000 daltons, and the concentration of the polyvinylimine monomer is 0.1 - 2.0 wt%.

[0016] Preferably, in Step S3, the support substrate membrane is a polyolefin porous membrane, and the soaking time is 1 - 10 min.

[0017] Preferably, in step S4, the reaction time of the interfacial polymerization reaction is 1 - 10 min.

[0018] Preferably, in step S5, the heat treatment temperature is 30 - 95 °C, the time is 15 - 60 min, the activation alcohol is isopropanol, and the activation time is 15 - 60 min.

[0019] The highly selective magnesium-lithium separation nanofiltration membrane prepared by the above preparation method.

[0020] Application of the highly selective magnesium-lithium separation nanofiltration membrane in cation separation.

[0021] During the preparation of the nanofiltration membrane, since the PEI molecule contains more amino groups, during the reaction, the formed PA layer contains residual amino groups, which show a relatively high positive charge after protonation. The positively charged nanofiltration membrane generally has a high rejection rate for divalent cations such as Mg 2+ and Ca 2+ and Cu 2+ Therefore, this nanofiltration membrane can be applied to the separation of other multivalent and monovalent cations.

[0022] Traditional commercial nanofiltration membranes are negatively charged and have poor magnesium-lithium separation performance, and cannot effectively separate magnesium and lithium in salt lake brines. During the interfacial polymerization reaction, as the polyamide layer grows, the amine monomers that can diffuse into the reaction zone gradually decrease, while the acyl chloride in the organic phase always maintains a high concentration, resulting in the inevitable formation of a large number of carboxyl-terminated structures, which come from the hydrolysis of residual acyl chloride groups. Therefore, the nanofiltration membranes prepared by traditional interfacial polymerization are usually negatively charged. These membranes are prone to attracting multivalent cations through electrostatic interactions, which is not conducive to the efficient separation of magnesium and lithium cations.

[0023] The present invention discloses a highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane, its preparation method and application. By using the method of reverse interfacial polymerization, using polyamine-based PEI as the aqueous phase monomer and using meta- and para-phthaloyl chloride carrying two acyl chloride groups as the organic phase monomer, a nanofiltration (NF) membrane is prepared by reverse interfacial polymerization, so that the aqueous solution of the polyethyleneimine monomer containing abundant amino groups is always above the phthaloyl chloride monomer with fewer acyl chloride groups. After the polymerization reaction, the obtained PA (polyamide) layer has a relatively large membrane pore size and a relatively high positive charge density, and has high magnesium-lithium separation performance in a solution with a high magnesium ion and lithium ion concentration ratio. The nanofiltration membrane prepared by the present invention has good magnesium-lithium separation performance, can be applied to the fields of water treatment and chemical separation, and the preparation method is simple to operate and is applicable to large-scale continuous production. Description of the Drawings

[0024] Figure 1Schematic diagram of the preparation process of the present invention.

[0025] Figure 2 Scanning electron micrograph of Example 1 of the present invention.

[0026] Figure 3 Scanning electron micrograph of Example 2 of the present invention.

[0027] Figure 4 Magnesium-lithium separation performance diagram of Comparative Example 1 and Comparative Example 2 of the present invention and Example 1 and Example 2 in solutions with different magnesium-lithium ratios.

[0028] Figure 5 Stability performance test diagram of Example 1 and Example 2 of the present invention for 24 consecutive hours. Detailed description of the specific implementation

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and specific examples.

[0030] Materials used in the present invention: There are no special restrictions on the sources of all raw materials in the present invention and the following examples, and they can be commercially available.

[0031] Membrane flux detection method for highly selective lithium-magnesium separation membrane: The membrane permeation selectivity test system is used to test the water permeation flux and salt rejection rate of the membrane. The test system includes a pump, a membrane cell, pipelines, a regulating valve, a pressure and flow detector. The effective membrane area for testing is 19.64 cm 2 , the test pressure is 6 bar, and the test temperature is 25 ± 0.5 °C.

[0032] Salt concentration for testing the single-salt rejection rate: The concentrations of MgCl2 and LiCl are both 1000 ppm.

[0033] Mixed-salt concentration and magnesium-lithium ratio for testing the lithium-magnesium separation performance: The total concentration of MgCl2 and LiCl is 1000 ppm, where Mg 2+ / Li + = 1:1, 5:1, 10:1, 20:1, 50:1, 100:1. Mg 2+ and Li + concentrations are detected using inductively coupled plasma emission spectrometry (ICP-OES, VISTA-MPX, Varian).

[0034] The calculation formula for magnesium-lithium selectivity is as follows: where C Li，p and C Mg，p are the concentrations (g / L) of Li + and Mg 2+ in the permeate, and C Li，f and C Mg，f are the concentrations of Li in the permeate respectively,+ and Mg 2+ Concentration (g / L).

[0035]

Example 1

[0036] Prepare an aqueous phase mixture containing 0.5% polyethyleneimine (molecular weight 70000 Da). Prepare a n-hexane solution containing 0.1% isophthaloyl chloride as the organic phase mixture.

[0037] First, place the organic phase mixture on the surface of the polyolefin support bottom membrane, adsorb for 4 min, remove the excess solution. Then, take the aqueous phase mixture and place it on the surface of the support membrane, react for 4 min, remove the excess solution. Then, place the membrane in a blast drying oven at 60 °C and dry for 15 min. Place the prepared lithium-magnesium separation membrane in an isopropanol solution for activation for 15 min, and then store the membrane in deionized water for further testing of its separation performance.

[0038] After Mg 2+ / Li + = 1:1, 5:1, 10:1, 20:1, 50:1, and 100:1 mixed solutions were tested, and the lithium-magnesium selectivities of this lithium-magnesium separation membrane were 16.82, 23.15, 23.41, 24.58, 25.28, and 27.26 respectively.

[0039]

Example 2

[0040] Prepare an aqueous phase mixture containing 0.5% polyethyleneimine (molecular weight 70000 Da). Prepare a n-hexane solution containing 0.1% terephthaloyl chloride as the organic phase mixture.

[0041] First, place the organic phase mixture on the surface of the polyolefin support bottom membrane, adsorb for 4 min, remove the excess solution. Then, take the aqueous phase mixture and place it on the surface of the membrane, react for 4 min, remove the excess solution. Then, place the membrane in a blast drying oven at 60 °C and dry for 15 min. Place the prepared lithium-magnesium separation membrane in an isopropanol solution for activation for 15 min, and then store the membrane in deionized water for further testing of its separation performance.

[0042] After Mg 2+ / Li + = 1:1, 5:1, 10:1, 20:1, 50:1, and 100:1 mixed solutions were tested, and the lithium-magnesium selectivities of this lithium-magnesium separation membrane were 14.78, 24.93, 28.01, 30.09, 31.46, and 45.64 respectively.

[0043]

Comparative Example 1

[0044] In this comparative example, the monomer of the organic phase mixture was 0.1% phthaloyl chloride, and the others were the same as in Example 1.

[0045] After Mg 2+ / Li + mixed solution tests with ratios of 20:1, 50:1, and 100:1, the lithium / magnesium selectivity of this lithium-magnesium separation membrane was 12.76, 13.20, and 12.06, respectively.

[0046]

Comparative Example 2

[0047] The monomer of the organic phase mixture in this comparative example was 0.087% trimesoyl chloride (TMC), and the others were the same as in Example 2.

[0048] After Mg 2+ / Li + mixed solution tests with ratios of 20:1, 50:1, and 100:1, the lithium / magnesium selectivity of this lithium-magnesium separation membrane was 13.22, 12.82, and 10.82, respectively.

[0049] Figure 1 is a schematic flow chart for preparing the nanofiltration membrane of the present invention. The organic phase monomer is isophthaloyl chloride or terephthaloyl chloride, and the aqueous phase monomer is polyethyleneimine.

[0050] Figure 2 and Figure 3 are the surfaces of the dense polyamide layers obtained in Examples 1 and 2; the surface morphology and cross-sectional morphology of the highly selective lithium-magnesium separation membranes obtained in Examples 1 and 2 of this example were characterized. It can be seen from the analysis that the surface of the highly selective lithium-magnesium separation membrane is smooth, dense, and defect-free.

[0051] Observation Figure 2 and Figure 3 the surface morphology on the left, it can be clearly seen that the porous structure of the polyolefin substrate membrane is covered by newly grown protrusion structures, indicating the successful formation of a thin polyamide layer. There are many wrinkles on the surface of this highly selective lithium-magnesium separation membrane, and it is relatively dense, with no obvious change in the surface morphology. The reaction rate between the acyl chloride group and the amino group is relatively fast, forming a PA layer with a certain thickness. As Figure 2 、 Figure 3 shown in the right figure: Figure 2 、 Figure 3 The surface and cross-sectional morphology diagrams of the highly selective lithium-magnesium separation membrane indicate that PEI and isophthaloyl chloride, terephthaloyl chloride were successfully crosslinked on the top of the PE membrane to form a relatively dense PA separation layer.

[0052] Figure 4 The separation selectivity changes of the examples and comparative examples in mixed solutions with different magnesium / lithium ratios were compared. As Figure 4As shown, the abscissa is the different magnesium-lithium ratios of the mixed solution, and the ordinate is the magnesium-lithium separation coefficient of the nanofiltration membrane. By comparing the test results of the examples and the comparative examples, it can be seen that in the comparative examples, traditional trimesoyl chloride or phthaloyl chloride is used as the organic phase monomer, and its lithium-magnesium selectivity decreases as the magnesium-lithium ratio increases. While for Examples 1 and 2, the magnesium-lithium separation coefficient increases as the magnesium-lithium ratio of the solution increases. The upward trend of the magnesium-lithium separation coefficient in Example 1 is relatively gentle, rising from 16.82 to 27.26. The magnesium-lithium separation coefficient in Example 2 rises faster, from 14.78 to 45.65. Therefore, in Example 2, terephthaloyl chloride is used, which has higher magnesium-lithium separation performance for high magnesium-lithium ratio solutions, and as the magnesium-lithium ratio continues to increase, the prepared nanofiltration membrane has better magnesium-lithium separation performance.

[0053] Figure 5 The long-term stability test was carried out on the lithium-magnesium separation membranes obtained in Examples 1 and 2. After 24 hours of continuous performance testing, the rejection rate of the NF membrane for MgCl2 was relatively stable. However, due to the long-term operation of the testing instrument, the internal temperature of the instrument rose, resulting in a certain degree of fluctuation in the permeability coefficient. The permeability coefficients of Example 1 and Example 2 were basically stable at 3.5 and 3.2, indicating that the prepared lithium-magnesium separation membranes have good long-term stability.

[0054] The nanofiltration membrane prepared by the present invention has good magnesium-lithium separation performance in high magnesium-lithium ratio mixed solutions. At the same time, the preparation method is simple, the conditions are mild, the application range is wide, and it has good long-term operation stability, and can be applied to water treatment, seawater desalination, sewage treatment, dye purification, etc.

[0055] The above embodiments are not limitations on the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the scope of the technical solution of the present invention also fall within the protection scope of the present invention.

Claims

1. Preparation method of a highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane, characterized in that: The preparation method is as follows: First, immerse the support substrate membrane with an organic phase solution containing an acyl chloride monomer to obtain a support substrate membrane infiltrated with the organic phase monomer. Then, immerse it in an aqueous phase solution containing an amine monomer for interfacial polymerization. Subsequently, subject the obtained nanofiltration membrane to activation treatment to obtain a magnesium-lithium separation nanofiltration membrane; Specifically, it includes the following steps: Step S1: Dissolve isophthaloyl chloride or terephthaloyl chloride in an organic solvent to obtain an organic phase solution; Step S2: Dissolve the polyethyleneimine monomer in water to obtain an aqueous phase solution; Step S3: Immerse the support substrate membrane with the organic phase solution prepared in Step S1 to obtain a support substrate membrane infiltrated with the organic phase monomer; Step S4: Immerse the support substrate membrane infiltrated with the organic phase monomer obtained in Step S3 in the aqueous phase solution prepared in Step S2 for interfacial polymerization reaction to obtain a polyamide thin film composite nanofiltration membrane; Step S5: Subject the polyamide thin film composite nanofiltration membrane obtained in Step S4 to heat treatment and alcohol activation to obtain a nanofiltration membrane with magnesium-lithium separation performance.

2. The preparation method of the highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane according to claim 1, wherein: In the organic phase solution prepared in Step S1, the concentration of isophthaloyl chloride or terephthaloyl chloride is 0.05 - 0.2 w / v%, and the organic solvent is n-hexane.

3. The preparation method of the highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane according to claim 1, wherein: In the aqueous phase solution prepared in Step S2, the molecular weight of the polyethyleneimine monomer is 700 - 70000 Daltons, and the concentration of the polyethyleneimine monomer is 0.1 - 2.0 wt%.

4. The preparation method of the highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane according to claim 1, characterized in that: In Step S3, the support substrate membrane is a polyolefin porous membrane, and the immersion time is 1 - 10 min.

5. The preparation method of the highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane according to claim 1, wherein: In Step S4, the reaction time of the interfacial polymerization reaction is 1 - 10 min.

6. The preparation method of the highly selective magnesium-lithium separation nanofiltration membrane based on a polyolefin-based membrane according to claim 1, characterized in that: In Step S5, the temperature of the heat treatment is 30 - 95 °C, the time is 15 - 60 min, the activation alcohol is isopropyl alcohol, and the activation time is 15 - 60 min.

7. A highly selective magnesium-lithium separation nanofiltration membrane prepared by the preparation method according to any one of claims 1 - 6.

8. Application of the highly selective magnesium-lithium separation nanofiltration membrane according to claim 7 in cation separation.

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