Membrane separation and concentration method for extracting lithium from alkaline salt lake brine and application thereof

Abstract

The application relates to the field of membrane separation concentration technology and discloses a membrane separation concentration method for extracting lithium from alkaline salt lake brine and application. The method specifically comprises the following steps: step S1, pretreating alkaline brine to obtain membrane inlet liquid; step S2, sequentially performing nanofiltration treatment on the membrane inlet liquid by using a first-stage nanofiltration system and a second-stage nanofiltration system to obtain second-stage nanofiltration water; and step S3, performing reverse osmosis concentration treatment on the nanofiltration water by using a reverse osmosis system to obtain reverse osmosis concentrated liquid. The application also provides application of the membrane separation concentration method for extracting lithium from alkaline salt lake brine in the removal of impurities and the extraction of lithium from high-altitude low-temperature alkaline brine. Compared with a traditional process, the membrane separation concentration method can greatly reduce magnesium carbonate and calcium sulfate precipitation, avoids complicated processes such as precipitation impurity removal, and can efficiently, with high recovery rate and low cost, extract lithium ions in alkaline salt lake.

Description

Technical Field

[0001] This invention relates to the field of membrane separation and concentration technology, and in particular to a membrane separation and concentration method for lithium extraction from alkaline salt lake brine and its application. Background Technology

[0002] Lithium is a core strategic metal in the new energy field, supporting the development of industries such as energy storage and new energy vehicles. Currently, lithium resources are mainly found in salt lake brines. Lithium extraction from salt lakes is a key technology for achieving self-sufficiency in lithium resources, breaking through the bottleneck of upstream raw material supply, and ensuring the security of the new energy industry chain. The mainstream processes for lithium extraction from salt lakes currently include membrane methods, adsorption methods, and adsorption-membrane coupling processes. Among them, the adsorption-membrane coupling process has been applied on a large scale in neutral and weakly alkaline salt lakes in Qinghai. However, in the highly alkaline salt lakes of Tibet, traditional polyamide nanofiltration and conventional electrodialysis membrane processes are still the main production methods.

[0003] Existing conventional lithium extraction processes and improved patented technologies are poorly adapted to the complex conditions of the highly alkaline salt lakes in Tibet, making it difficult to meet the requirements of stable, low-cost, and high-recovery industrial production. Specific technical shortcomings are as follows: 1. Traditional membrane processes are prone to scaling, have short lifespans, and high maintenance costs. The high pH and high carbonate-bicarbonate content of Tibetan salt lake brine easily cause calcium and magnesium carbonate scale to precipitate on the surface of conventional nanofiltration and electrodialysis membranes, resulting in decreased membrane flux, alkaline aging of membrane elements, and shortened lifespan. Furthermore, the low temperatures in high-altitude winters, unstable photovoltaic power supply, and large seasonal fluctuations in brine concentration mean that pretreatment cannot completely eliminate scaling risks. In addition, strict ecological controls in Tibet, the difficulty of treating concentrated brine, and poor conditions for maintenance and spare parts supply in high-altitude areas prevent the entire membrane system from operating stably at full capacity year-round. High costs for membrane replacement, chemicals, and maintenance severely restrict stable production.

[0004] 2. Existing adsorption and coupling processes have significant technical shortcomings and are unsuitable for alkaline salt lake conditions. The mature adsorption-membrane coupling process used in Qinghai is only applicable to neutral and weakly alkaline brine and cannot be directly used in the highly alkaline salt lakes of Tibet. Existing improved patents also have deficiencies: Chinese patent application CN120001080A provides a special adsorbent for lithium extraction from salt lakes. This adsorbent has a short adsorption life in alkaline brine, requiring frequent replacement, resulting in poor production continuity and high costs; Chinese patent application CN121044605A discloses a lithium extraction process suitable for alkaline salt lakes, but this alkaline lithium extraction process is lengthy, complex, and has high energy and time costs, resulting in poor industrial applicability.

[0005] 3. Existing technologies generally suffer from incompatibility between membrane materials and alkaline brine operating conditions, leading to frequent problems such as system scaling, membrane failure, and operational fluctuations. This directly results in low lithium recovery rates and low raw material utilization. Furthermore, rapid equipment wear and tear, high reagent consumption, and high maintenance costs significantly increase the overall production cost of lithium extraction from alkaline salt lakes, limiting the efficient and economical development of lithium resources in Tibetan alkaline salt lakes.

[0006] Therefore, it is necessary to provide a membrane separation and concentration method for lithium extraction from alkaline salt lake brine and its application to solve the above-mentioned technical problems. Summary of the Invention

[0007] The main objective of this invention is to provide a membrane separation and concentration method and its application for lithium extraction from alkaline salt lake brine, aiming to solve the problems of low lithium recovery rate and high cost of membrane material recycling in existing alkaline salt lake lithium extraction methods.

[0008] To achieve the above objectives, this invention proposes a membrane separation and concentration method for lithium extraction from alkaline salt lake brine, comprising the following steps: Step S1: Pre-treat the alkaline brine to obtain the membrane inlet solution; Step S2: The feed solution is subjected to two-stage nanofiltration treatment to obtain secondary nanofiltration permeate; the process is divided into primary and secondary stages. The first stage uses a large-pore negative charge nanofiltration membrane, which has a sodium sulfate rejection rate of 90-97% in the feed solution and is used to separate divalent anions; the surface structure pore size of the large-pore negative charge nanofiltration membrane is 0.6-1.2 nm. The secondary stage employs a small-pore positive-charge nanofiltration membrane, which has a magnesium chloride rejection rate of greater than 98% and is used to separate divalent cations; the surface structure pore size of the small-pore positive-charge nanofiltration membrane is 0.5-1.0 nm. Step S3: The secondary nanofiltration permeate obtained in step S2 is concentrated by reverse osmosis using a reverse osmosis system to obtain reverse osmosis concentrate, i.e. lithium concentrate. The reverse osmosis system includes a reverse osmosis concentration membrane, which has a sodium chloride rejection rate of more than 99.7%.

[0009] Optionally, step S2 specifically involves pumping the membrane feed solution into the first stage to obtain first-stage nanofiltration permeate and first-stage nanofiltration concentrate, and then pumping the first-stage nanofiltration permeate into the second stage to obtain second-stage nanofiltration permeate and second-stage nanofiltration concentrate.

[0010] Optionally, the primary nanofiltration permeate contains monovalent anions and cations and divalent cations.

[0011] Optionally, the secondary nanofiltration permeate contains monovalent anions and cations.

[0012] Optionally, both the large-pore negative charge nanofiltration membrane and the small-pore positive charge nanofiltration membrane are composed of a three-layer structure: the outer layer is a polyamide selective separation layer, the middle layer is a porous ultrafiltration base membrane, and the bottom layer is a polyester nonwoven fabric.

[0013] Optionally, the specific manufacturing methods of the large-pore negative charge nanofiltration membrane and the small-pore positive charge nanofiltration membrane are as follows: using a non-solvent phase separation method, the homogeneous casting solution after vacuum degassing is coated onto the surface of a polyester nonwoven fabric, the wet membrane thickness is controlled, and it is immersed in a deionized water coagulation bath to solidify and obtain a porous ultrafiltration base membrane; the porous ultrafiltration base membrane is immersed in an aqueous solution containing polyamine monomers, and then contacted with an organic phase solution containing polyacrylamide chlorides, and a polyamide selective separation layer is generated in situ through a liquid-liquid interface polycondensation reaction, and the nanofiltration membrane is obtained after heat treatment and water washing; the homogeneous casting solution is specifically prepared by dissolving polysulfone or polyethersulfone in an organic solvent and adding a pore-forming agent.

[0014] Optionally, the thickness of the polyamide selective separation layer is 50-200 nm.

[0015] Optionally, the first stage includes one or more first-stage nanofiltration membrane elements connected in series, with an operating pressure of 1-6 MPa; the second stage includes one or more second-stage nanofiltration membrane elements connected in series, with an operating pressure of 1-3 MPa; and the reverse osmosis system includes one or more reverse osmosis membrane elements connected in series, with an operating pressure of 4-10 MPa.

[0016] Optionally, the alkaline brine includes lithium ions, sodium ions, potassium ions, calcium ions, magnesium ions, chloride ions, sulfate ions, and carbonate ions, and the pH value of the brine is 7-11.

[0017] In addition, the present invention also provides an application of a membrane separation and concentration method for lithium extraction from alkaline salt lake brine in the removal of impurities and extraction of lithium from high-altitude, low-temperature alkaline brine.

[0018] The application of the technical solution of the present invention has at least the following beneficial effects: (1) The present invention adopts a high-pressure resistant spiral wound nanofiltration structure with a large filling area, which can withstand an operating pressure of 8 MPa and has lower overall operating energy consumption. The main function of the primary nanofiltration membrane is to separate divalent anions, specifically to remove sulfate and carbonate ions from the brine. The secondary nanofiltration membrane is mainly used to separate divalent cations, specifically to remove calcium and magnesium ions from the brine. The two types of nanofiltration membranes used in the present invention can avoid the formation of magnesium carbonate and calcium sulfate precipitates, while ensuring a high enrichment and recovery rate of lithium ions, improving the utilization rate of membrane materials, and reducing production costs.

[0019] (2) Both the large-pore negative-charge nanofiltration membrane and the small-pore positive-charge nanofiltration membrane used in this invention are composed of a three-layer structure. The surface structure is specifically prepared by interfacial polymerization, in which amine monomers and acyl chloride monomers undergo a condensation reaction at the oil-water interface to generate a polyamide selective separation layer with a thickness of 50-200 nm in situ. The surface structure pore size of the large-pore negative-charge nanofiltration membrane is 0.6-1.2 nm, and the surface structure pore size of the small-pore positive-charge nanofiltration membrane is 0.5-1.0 nm. The nanofiltration membrane proposed in this invention achieves separation by relying on sieving effect and surface electrostatic repulsion. The large-pore negative-charge nanofiltration membrane can retain medium and large molecular organic matter and divalent anions, while allowing monovalent anions, cations and divalent cations to pass through. The small-pore positive-charge nanofiltration membrane can retain medium and small molecular organic matter and divalent cations, while allowing monovalent anions and cations to pass through. Using the membrane of this invention, lithium ions can be extracted from alkaline salt lakes with high efficiency, high recovery rate and low cost.

[0020] (3) The membrane separation and concentration method for lithium extraction from alkaline salt lake brine proposed in this invention involves pretreatment, nanofiltration and reverse osmosis of the feed liquid in sequence. Compared with traditional processes, this method can significantly reduce the precipitation of magnesium carbonate and calcium sulfate, increase the lithium ion concentration, and avoid cumbersome processes such as precipitation and impurity removal.

[0021] (4) This invention takes low-cost and high-efficiency lithium extraction membrane separation process as the core, and selects a method that combines with reverse osmosis system to achieve ion sieving through smaller and more precise controllable membrane pore size, thereby completing the concentration and enrichment of lithium element, which has higher treatment efficiency for salt lake brine and stronger application adaptability. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of a membrane separation and concentration method for lithium extraction from alkaline salt lake brine in an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of the large-pore negatively charged nanofiltration membrane and the small-pore positively charged nanofiltration membrane in the embodiments of the present invention; Figure 3 The image shown is a SEM image of the polyamide selective separation layer in this embodiment of the invention, where Mag represents the magnification, WD represents the working distance, EHT represents the accelerating voltage, and Signal A represents the imaging detector (signal channel A).

[0024] Explanation of icon numbers: 1-Polyamide selective separation layer; 2-Porous ultrafiltration base membrane; 3-Polyester nonwoven fabric.

[0025] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] Example 1: Alkaline brine contains a large number of monovalent and divalent cations and anions, as well as poorly ionized elements. The concentrations of lithium ions are 240 mg / L, sodium ions 14.4 g / L, potassium ions 2.2 g / L, calcium ions 57 mg / L, magnesium ions 810 mg / L, chloride ions 11.9 g / L, sulfate ions 19 g / L, and carbonate ions 2.5 g / L. Poorly ionized elements include silicon (30 mg / L) and boron (740 mg / L). The mass concentration of the alkaline brine is 51.73 g / L, its specific gravity is 1.035 g / mL, and its pH value is 9.11.

[0028] Combination Figure 1 The present invention provides a membrane separation and concentration method for lithium extraction from alkaline salt lake brine, comprising the following steps: (Flowchart of the method) Step S1: Pre-treat the alkaline brine by passing it through one or more filtration systems, such as fiber bundle filter, self-cleaning filter, and ultrafiltration system, to remove suspended solids and other impurities, and obtain the inlet liquid. Step S2: The feed solution is first pumped into the first stage for first-stage nanofiltration treatment to obtain first-stage nanofiltration permeate and first-stage nanofiltration concentrate. The first stage uses a large-pore negative charge nanofiltration membrane, preferably the SLNF8 product produced by Hunan Aowei Technology Co., Ltd., which has a sodium sulfate rejection rate of 90-97% in the feed solution. The surface structure pore size of the large-pore negative charge nanofiltration membrane is 0.6-1.2 nm, which is used to separate divalent anions, specifically sulfate and carbonate ions in the brine. The primary nanofiltration permeate is then pumped into the secondary nanofiltration stage for secondary nanofiltration treatment, yielding secondary nanofiltration permeate and secondary nanofiltration concentrate. The secondary stage uses a small-pore positive-charge nanofiltration membrane, preferably SLNF3 produced by Hunan Aowei Technology Co., Ltd., which has a magnesium chloride rejection rate of greater than 98%. The surface structure pore size of the small-pore positive-charge nanofiltration membrane is 0.5-1.0 nm, used to separate divalent cations, specifically calcium and magnesium ions in the brine. Step S3: Pump the secondary nanofiltration permeate obtained in step S2 into the reverse osmosis system for reverse osmosis concentration treatment to obtain reverse osmosis permeate and reverse osmosis concentrate (i.e. lithium concentrate). The reverse osmosis system uses a reverse osmosis concentration membrane, preferably the SLRO3 product produced by Hunan Aowei Technology Co., Ltd., which has a sodium chloride rejection rate of more than 99.7%.

[0029] Both the large-pore negative charge nanofiltration membrane and the small-pore positive charge nanofiltration membrane are composed of a three-layer structure: the outer layer is a polyamide selective separation layer, the middle layer is a porous ultrafiltration base membrane, and the bottom layer is a polyester nonwoven fabric used to provide mechanical support.

[0030] The specific fabrication methods for large-pore negatively charged nanofiltration membranes and small-pore positively charged nanofiltration membranes are as follows: Step ①: Dissolve polysulfone or polyethersulfone in an organic solvent such as N,N-dimethylformamide, and add an appropriate amount of pore-forming agent to prepare a homogeneous casting solution; Step 2: The homogeneous casting solution after vacuum degassing is coated onto the surface of polyester nonwoven fabric using the non-solvent phase separation (NIPS) method. The wet film thickness is controlled, and then the film is immersed in a deionized water coagulation bath. Liquid-liquid phase separation is initiated by the double diffusion exchange of solvent and non-solvent, and the film is solidified into a porous ultrafiltration base membrane with an asymmetric pore structure. After thorough washing with water to remove residual solvent, the film is ready for use. Step 3: Perform interfacial polymerization on the surface of the porous ultrafiltration membrane: First, immerse the porous ultrafiltration membrane in an aqueous solution containing polyamine monomers (such as m-phenylenediamine or piperazine) to allow the surface pores to fully adsorb the aqueous phase. Then, contact it with an organic solution containing polyacrylamide chlorides (such as trimesoyl chloride) to undergo a condensation reaction at the liquid-liquid interface, generating a polyamide selective separation layer with a thickness of 50-200 nm in situ. After heat treatment and water washing, a nanofiltration membrane with both high flux and high selectivity is obtained.

[0031] The above-mentioned nanofiltration membrane is prepared using parameters and reagents from the prior art, with reference to Chinese Patent Publication No. CN114191992B.

[0032] After primary nanofiltration treatment, primary nanofiltration permeate and primary nanofiltration concentrate are obtained. The primary nanofiltration permeate is mainly composed of monovalent anions, cations, and divalent cations, while the primary nanofiltration concentrate achieves enrichment of divalent anions. Secondary nanofiltration treatment yields secondary nanofiltration permeate and secondary nanofiltration concentrate. The secondary nanofiltration permeate is mainly composed of monovalent anions and cations, while the secondary nanofiltration concentrate achieves enrichment of divalent cations. After some ions are removed by different nanofiltration and reverse osmosis systems, compared to the feed solution, the concentration of divalent anions in the primary nanofiltration concentrate increases by 2-5 times; compared to the primary nanofiltration permeate, the concentration of divalent cations in the secondary nanofiltration concentrate increases by 3-10 times; and compared to the secondary nanofiltration permeate, the concentration of monovalent cations in the reverse osmosis concentrate increases by 1.2-3 times.

[0033] The first stage includes one or more first-stage nanofiltration membrane elements connected in series, with an operating pressure of 1-6 MPa, preferably the 2540 product manufactured by Hunan Aowei Technology Co., Ltd.; the second stage includes one or more second-stage nanofiltration membrane elements connected in series, with an operating pressure of 1-3 MPa, preferably the 2540 product manufactured by Hunan Aowei Technology Co., Ltd.; the reverse osmosis system includes one or more reverse osmosis membrane elements connected in series, with an operating pressure of 4-10 MPa, preferably the 2540 product manufactured by Hunan Aowei Technology Co., Ltd.

[0034] During the membrane separation and concentration process, inductively coupled plasma optical emission spectrometry (ICP-OES) and ion chromatography (IC) were used to sample and test the nanofiltration permeate after primary and secondary nanofiltration treatments, respectively. The concentrations of divalent anions, divalent cations, and monovalent cations in the feed solution were measured, and the divalent anion rejection rate (%), divalent cation rejection rate (%), monovalent cation permeability (%), and monovalent cation purity were calculated. The specific calculation method is shown below: Divalent anion rejection rate = (1 - concentration of divalent anions in product water / concentration of divalent anions in feed water) * 100%; Divalent cation rejection rate = (1 - concentration of divalent cations in product water / concentration of divalent cations in feed water) * 100%; Monovalent cation permeability = (monovalent cation concentration in product water / monovalent cation concentration in feed water) * 100%; Monovalent cation purity = (monovalent cation concentration ÷ total cation concentration) * 100%; Total cation concentration = Monovalent cation concentration + Divalent cation concentration.

[0035] Comparative Example 1: In step S2, only secondary nanofiltration is performed; the remaining experimental steps and conditions are the same as in Example 1.

[0036] Comparative Example 2: In step S2, only the first-stage nanofiltration process is performed; the remaining experimental steps and conditions are the same as in Example 1.

[0037] Comparative Example 3: The large-pore, strongly negatively charged nanofiltration membrane used in step S2 has a surface structure with a pore size of 1.3-1.5 nm and a sodium sulfate rejection rate of approximately 60%. The remaining experimental steps and conditions are the same as in Example 1.

[0038] Comparative Example 4: The large-pore, strongly negatively charged nanofiltration membrane used in step S2 has a surface pore size of 0.3-0.5 nm and a sodium sulfate rejection rate of more than 99.5%. The remaining experimental steps and conditions are the same as in Example 1.

[0039] Comparative Example 5: The small-pore positive-charge nanofiltration membrane used in step S2 has a surface pore size of 1.1-1.3 nm and a magnesium chloride rejection rate of approximately 90%. The remaining experimental steps and conditions are the same as in Example 1.

[0040] Comparative Example 6: The small-pore positive-charge nanofiltration membrane used in step S2 has a surface pore size of 0.2-0.4 nm and a magnesium chloride rejection rate of more than 99.5%. The remaining experimental steps and conditions are the same as in Example 1.

[0041] Comparative Example 7: The alkaline brine contained lithium ions at a concentration of 920 mg / L, sodium ions at 95 g / L, potassium ions at 28 g / L, calcium ions at 1 mg / L, magnesium ions at 3 mg / L, chloride ions at 14.6 g / L, sulfate ions at 3.8 g / L, and carbonate ions at 5.2 g / L, among others. The brine had a mass concentration of 290 g / L, a specific gravity of 1.041 g / mL, and a pH of 9.16. The brine also contained poorly ionized elements, including silicon at a concentration of 20 mg / L and boron at 330 mg / L. The remaining experimental procedures and conditions were the same as in Example 1.

[0042] In the membrane separation and concentration processes of Example 1 and Comparative Examples 1-7, samples of the nanofiltration permeate after primary and secondary nanofiltration treatments were taken and tested. The test results are shown in Table 1: Table 1. Test water samples and corresponding results

[0043] Based on the test data obtained from Example 1 and Comparative Examples 1-7, it can be seen that: (1) Compared with Example 1, Comparative Example 1 only performed two-stage nanofiltration, which resulted in the inability to remove divalent anions. Both divalent anions and divalent cations were simultaneously retained by the small-pore positive-charged nanofiltration membrane in the secondary nanofiltration membrane element, leading to severe fouling of the membrane element. At the same time, the operating pressure of the secondary nanofiltration membrane element increased significantly from 3.5 MPa in Example 1 to 6.5 MPa. The secondary nanofiltration membrane element was compressed by the high operating pressure, which reduced the membrane pore size, resulting in a significant decrease in the permeability of monovalent cations, and consequently a significant decrease in the permeability of lithium ions.

[0044] (2) Compared with Example 1, Comparative Example 2 only underwent a first-stage nanofiltration process. Although it did not affect the permeability of monovalent cations, the divalent cations and monovalent cations could not be effectively separated during the test, resulting in low purity of the monovalent cations. Therefore, the lack of a second-stage nanofiltration process affected the purity of lithium ions in Comparative Example 2.

[0045] (3) Compared with Example 1, the surface structure pore size of the large-pore negative charge nanofiltration membrane in Comparative Example 3 is larger, which reduces the retention rate of divalent anions. A large number of divalent anions pass through the ultra-large-pore negative charge nanofiltration membrane with divalent cations and enter the primary nanofiltration product water. Since the main purpose of the secondary nanofiltration process is to retain divalent cations, the content of divalent anions in the product water is ultimately high, which further reduces the purity of monovalent cations.

[0046] (4) Compared with Example 1, the surface structure pore size of the small-pore positive charge nanofiltration membrane in Comparative Example 4 is smaller, the divalent cation rejection rate is increased, and the monovalent cation rejection rate is significantly increased, thereby reducing the monovalent cation permeability. Similarly, the lithium ion permeability is also significantly reduced.

[0047] (5) Compared with Example 1, the surface structure pore size of the small-pore positive charge nanofiltration membrane in Comparative Example 5 is larger, which leads to a large reduction in the retention rate of divalent cations. After secondary nanofiltration treatment, a large number of divalent cations pass through into the feed solution containing monovalent cations, making it impossible to effectively separate divalent cations and monovalent cations.

[0048] (6) Compared with Example 1, the surface structure pore size of the small-pore positive charge nanofiltration membrane in Comparative Example 6 is smaller, the divalent cation rejection rate is increased, the monovalent cation rejection rate is also significantly increased, the permeability is reduced, and the lithium ion permeability is significantly reduced.

[0049] In summary, based on Example 1 and Comparative Examples 1-6, it can be seen that the present invention, in Example 1, combines a large-pore negative charge nanofiltration membrane with a suitable rejection rate and a small-pore positive charge nanofiltration membrane, which can effectively separate divalent anions, divalent cations and monovalent cations, and achieve high-efficiency, high-recovery and low-cost extraction of lithium ions from alkaline salt lakes.

[0050] (7) Compared with Example 1, Comparative Example 7 showed an increase in the concentration of divalent anions, a decrease in the concentration of divalent cations, and a significant increase in the concentration of monovalent cations in the feed solution, leading to an increase in the operating pressure of the first-stage nanofiltration treatment. Experimental data also showed a slight increase in the rejection rate of monovalent cations. High-purity monovalent cations could still be obtained after the second-stage nanofiltration treatment, indicating that the combination of the nanofiltration system and the reverse osmosis system is adaptable to feed solutions with different water quality ranges and can achieve efficient treatment.

[0051] The above description is only a preferred embodiment of the present invention and does not limit the scope of the present invention. All equivalent structural transformations made under the inventive concept of the present invention using the contents of the present invention specification and drawings, or direct / indirect applications in other related technical fields, are included within the protection scope of the present invention.

Claims

1. A membrane separation and concentration method for lithium extraction from alkaline salt lake brine, characterized in that, Includes the following steps: Step S1: Pre-treat the alkaline brine to obtain the membrane inlet solution; Step S2: The feed solution is subjected to two-stage nanofiltration using a nanofiltration system to obtain secondary nanofiltration permeate; the nanofiltration system consists of a primary nanofiltration system and a secondary nanofiltration system. The primary nanofiltration system employs a large-pore negatively charged nanofiltration membrane, which has a sodium sulfate rejection rate of 90-97% in the feed solution and is used to separate divalent anions; the surface structure pore size of the large-pore negatively charged nanofiltration membrane is 0.6-1.2 nm. The secondary nanofiltration system employs a small-pore positive-charge nanofiltration membrane with a magnesium chloride rejection rate greater than 98%, used for separating divalent cations; the surface structure pore size of the small-pore positive-charge nanofiltration membrane is 0.5-1.0 nm. Step S3: The secondary nanofiltration permeate obtained in step S2 is concentrated by reverse osmosis using a reverse osmosis system to obtain reverse osmosis concentrate, i.e. lithium concentrate. The reverse osmosis system includes a reverse osmosis concentration membrane, which has a sodium chloride rejection rate of more than 99.7%.

2. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 1, characterized in that, Specifically, step S2 involves first pumping the membrane feed solution into a primary nanofiltration system to obtain primary nanofiltration permeate and primary nanofiltration concentrate, and then pumping the primary nanofiltration permeate into a secondary nanofiltration system to obtain secondary nanofiltration permeate and secondary nanofiltration concentrate.

3. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 2, characterized in that, The primary nanofiltration permeate contains monovalent anions and cations as well as divalent cations.

4. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 2, characterized in that, The secondary nanofiltration permeate contains monovalent anions and cations.

5. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 1, characterized in that, Both the large-pore negative charge nanofiltration membrane and the small-pore positive charge nanofiltration membrane are composed of a three-layer structure: the outer layer is a polyamide selective separation layer, the middle layer is a porous ultrafiltration base membrane, and the bottom layer is a polyester nonwoven fabric.

6. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 5, characterized in that, The specific fabrication methods for the large-pore negative charge nanofiltration membrane and the small-pore positive charge nanofiltration membrane are as follows: using a non-solvent phase separation method, the homogeneous casting solution after vacuum degassing is coated onto the surface of a polyester nonwoven fabric, the wet membrane thickness is controlled, and it is immersed in a deionized water coagulation bath to solidify and obtain a porous ultrafiltration base membrane; the porous ultrafiltration base membrane is immersed in an aqueous solution containing polyamine monomers, and then contacted with an organic solution containing polyacrylamide chlorides, and a polyamide selective separation layer is generated in situ through a liquid-liquid interface polycondensation reaction, and the nanofiltration membrane is obtained after heat treatment and water washing; the preparation method of the homogeneous casting solution is specifically to dissolve polysulfone or polyethersulfone in an organic solvent and add a pore-forming agent to prepare a homogeneous casting solution.

7. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 6, characterized in that, The thickness of the polyamide selective separation layer is 50-200 nm.

8. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 1, characterized in that, The primary nanofiltration system includes one or more primary nanofiltration membrane elements connected in series, with an operating pressure of 1-6 MPa; the secondary nanofiltration system includes one or more secondary nanofiltration membrane elements connected in series, with an operating pressure of 1-3 MPa; the reverse osmosis system includes one or more reverse osmosis membrane elements connected in series, with an operating pressure of 4-10 MPa.

9. The membrane separation and concentration method for lithium extraction from alkaline salt lake brine according to claim 1, characterized in that, The alkaline brine contains lithium ions, sodium ions, potassium ions, calcium ions, magnesium ions, chloride ions, sulfate ions, and carbonate ions, and the pH value of the brine is 7-11.

10. The application of a membrane separation and concentration method for lithium extraction from alkaline salt lake brine as described in any one of claims 1 to 9 in the removal of impurities and extraction of lithium from high-altitude, low-temperature alkaline brine.

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