Method for extracting lithium and recycling magnesium from salt lake brine and application

Abstract

The invention belongs to the technical field of lithium resource extraction and salt lake resource comprehensive utilization, and relates to a method for extracting lithium and recycling magnesium in salt lake brine. The method comprises the following steps: 1, adding sodium tripolyphosphate into the lithium-containing salt lake brine, adding NH4Cl and ammonia gas, and adjusting the pH value of the brine to 9-10; 2, pretreating the salt lake lithium extraction nanofiltration membrane; step 3, performing nanofiltration to obtain penetrating fluid and residual fluid; step 4, introducing the obtained penetrating fluid into an evaporator for evaporation and concentration, and filtering and separating to obtain a lithium concentrated solution; adding sodium carbonate into the lithium concentrated solution to react to obtain Li2CO3 precipitate; 5, adding sodium polyacrylate, and performing nanofiltration to obtain a residual liquid concentrated solution; and 6, carrying out vacuum freeze drying on the residual concentrated solution to obtain the sodium magnesium-phosphate complex. The method can be used for selectively and efficiently extracting lithium from salt lake brine and synchronously realizing resource utilization of magnesium, not only has efficient separation performance, but also has high salinity adaptability and long-term stability, and the recovered magnesium resource can be used as an additive for geopolymer preparation and has an excellent effect.

Description

Technical Field

[0001] This invention belongs to the technical field of lithium resource extraction and comprehensive utilization of salt lake resources, and relates to a method for extracting lithium and simultaneously utilizing magnesium resources from salt lake brine. Background Technology

[0002] With the continued surge in lithium demand (lithium production is projected to increase 18-20 times by 2050), traditional lithium extraction methods have significant limitations: while hard rock mining offers fast extraction speeds, it consumes large amounts of chemical reagents and causes severe environmental pollution; although salt lake brines, which account for 62.6% of global lithium resources, are more sustainable, they face challenges such as high salinity (100-300 g / L), resulting in traditional evaporation and precipitation processes taking up to 2 years, and high magnesium (Mg) content in the brine. 2+ This interferes with purification and generates a large amount of solid waste; producing 1 ton of lithium carbonate (Li2CO3) requires consuming over 500m³ of solid waste. 3 Freshwater exacerbates challenges such as water scarcity. Existing direct lithium extraction (DLE) technology suffers from limitations due to Li... + With Mg 2+ Similar physicochemical properties make efficient separation difficult, and problems such as difficulty in scaling up materials and complex processes hinder industrial applications.

[0003] The patented technology, "Apparatus and Method for Preparing Lithium Carbonate from Brine Using Solar Energy" (CN101928023A), uses solar energy as a heat source. It concentrates brine through membrane distillation, then uses triple-effect evaporation for forced evaporation and crystallization, precipitating various inorganic salts containing sodium, potassium, and magnesium. Finally, lithium carbonate is obtained through chemical precipitation. This method can also extract various inorganic salts in stages while obtaining lithium carbonate. However, due to the presence of magnesium... 2+ With Li + Difficulty in separation results in lithium recovery rates of only 50%-70% in some deposits.

[0004] The patented technology, "A Method and Apparatus for Extracting Lithium from Salt Lakes" (CN119351778A), utilizes adsorption to selectively adsorb lithium. Simultaneously, it employs ultrafiltration for impurity removal, nanofiltration for magnesium removal, reverse osmosis for lithium concentration concentration, and resin for magnesium and boron removal to improve lithium quality. Finally, battery-grade lithium carbonate is obtained through lithium precipitation, washing, and drying. However, this process requires multiple impurity removal steps, resulting in a cumbersome production flow. Furthermore, membrane fouling and ion concentration polarization are difficult to avoid, leading to reduced membrane selectivity.

[0005] The patented technology, "A Lithium Extraction Process from Salt Lakes Using Redox Coupled Membrane Separation Technology" (CN117144150A), involves preparing a redox coupled membrane, adding sodium thiosulfate as a reducing agent to the salt lake brine, and treating the brine with a polysulfone microporous membrane after oxidation. The lithium-ion solution is then fed into a reverse osmosis membrane concentration system, and a sodium carbonate aqueous solution is added for precipitation treatment, ultimately yielding lithium. This method incorporates lithium iron phosphate into the membrane and utilizes redox adsorption for lithium extraction. However, the lithium iron phosphate gradually degrades during this process, resulting in significant lithium loss. Furthermore, due to the extremely high magnesium-to-lithium ratio in salt lakes, the production process generates a large amount of magnesium, and this technology does not consider the simultaneous and comprehensive utilization of magnesium resources.

[0006] Yuan Hong et al. (Influence of retarder on the properties of slag-red mud-based geopolymers [J]. Bulletin of the Chinese Ceramic Society, 2025, 44(7):2589-2596,2607.) studied the effect of sodium tripolyphosphate on the properties of slag-red mud-based geopolymers. The results showed that when sodium tripolyphosphate was added, the setting and hardening rate of the geopolymer could be reduced by adsorption and complexation of calcium ions in the high-calcium geopolymer. However, due to the large amount added, the compressive strength of the geopolymer decreased by 19.2%. Summary of the Invention

[0007] The technical problem this invention aims to solve is to provide a method and application for lithium extraction and simultaneous magnesium resource utilization from salt lake brine, addressing the shortcomings of existing technologies. This method can be used to selectively and efficiently extract lithium from salt lake brine while simultaneously realizing the resource utilization of magnesium. It not only possesses high separation efficiency but also exhibits high salinity adaptability and long-term stability. Furthermore, the recovered magnesium resources can be used as an additive in geopolymer preparation, demonstrating excellent results.

[0008] To achieve the above objectives, the technical solution adopted by the present invention comprises the following steps: The first aspect of this invention provides a method for extracting lithium and simultaneously utilizing magnesium resources from salt lake brine, comprising the following steps: Step 1: Add sodium tripolyphosphate to the lithium-containing salt lake brine, controlling the molar ratio of tripolyphosphate ions in the sodium tripolyphosphate to Mg in the brine to be 1.1-1.2:1. Add NH4Cl and ammonia to adjust the pH of the brine to 9-10 to obtain the treated salt lake brine. Step 2: Pretreatment of the lithium extraction nanofiltration membrane from the salt lake; Step 3: Using a nanofiltration device, the treated brine from the salt lake is filtered through a two-stage process using a salt lake lithium extraction nanofiltration membrane at a pressure of 2-4 MPa and room temperature to obtain a permeate (lithium-rich solution) and a residual liquid (lithium-rich [Mg(P3O4]2O4]2O4). 10 )] 3- The solution; Step 4: Pass the obtained permeate into an evaporator for evaporation and concentration, filter and separate to obtain lithium concentrate; add sodium carbonate to the lithium concentrate to react, separate the solid and liquid, wash the precipitate, and obtain Li2CO3 precipitate; Step 5: Add sodium polyacrylate to the residual liquid and use a nanofiltration device, using a lithium extraction nanofiltration membrane from a salt lake, under a pressure of 4-5 MPa and room temperature, employ a two-stage filtration process (the first-stage residual liquid enters the second stage for re-filtration) to obtain a concentrated residual liquid. Step 6: Vacuum freeze-dry the concentrated residual liquid to obtain sodium magnesium phosphate complex.

[0009] According to the above scheme, the composition of the salt lake brine meets the following requirements: total salt concentration (TSC) of 120~280 g / L, of which Li concentration is 0.85~2.0 g / L, Mg concentration is 25~50 g / L, Ca concentration is 0-0.06 g / L, Na concentration is 30~90 g / L, and K concentration is 5~15 g / L; if the brine salinity exceeds the target range, it shall be diluted with water.

[0010] According to the above scheme, the salt lake lithium extraction nanofiltration membrane includes one of XN45 nanofiltration membrane, NFG nanofiltration membrane, UA60 nanofiltration membrane or NF-8 nanofiltration membrane.

[0011] According to the above scheme, the preferred nanofiltration membrane for lithium extraction from salt lakes is an XN45 membrane, which has a Mg removal rate ≥99.5% under high salinity. [Mg(P3O) 10 )] 3- The adhesion to the XN45 membrane surface is weak (only 1.1 nN), and this weak interaction can reduce membrane fouling. [Mg(P3O4)] adsorbed on the membrane surface... 10 )] 3- The complex layer can be removed by rinsing with fresh water.

[0012] According to the above scheme, the addition ratio of NH4Cl and ammonia is: the molar ratio of NH4Cl to NH3 is 1:6.5~7.5.

[0013] According to the above scheme, the pretreatment method of the sodium lithium extraction filter membrane from the salt lake is as follows: soaking in deionized water to remove impurities, and then activating with water until the permeation flux is stable. Specifically, the water activation can be the initial filtration of pure water at 10 bar pressure for 2 hours until the permeation flux is stable.

[0014] According to the above scheme, the amount of sodium carbonate used in step four is as follows: sodium carbonate is measured as sodium and added at a sodium-lithium atomic ratio of 1.1-1.3:1.

[0015] According to the above scheme, the reaction described in step four is: stirring the reaction at 70-90℃.

[0016] According to the above scheme, the reaction time in step four is 1-2 hours.

[0017] According to the above scheme, the lithium concentration in the lithium concentrate is 11-14 g / L.

[0018] According to the above scheme, the amount of sodium polyacrylate added in step five is 5-10 mg / L.

[0019] According to the above scheme, step six, vacuum freeze-drying, involves freeze-drying at -20 to -40°C and a vacuum degree of 0.05 to 0.1 mbar for 10 to 12 hours. A second aspect of the present invention provides the application of the sodium magnesium phosphate complex prepared by the above method as an additive in the preparation of geopolymers.

[0020] According to the above scheme, the amount of sodium magnesium-phosphate complex added is 0.1-0.5 wt%. When the amount added is 0.1-0.5 wt%, the setting time is reduced by more than 30%, and the strength is increased by 5-7 MPa.

[0021] By employing the above method, the present invention has the following advantages compared with the prior art: 1. This invention utilizes sodium tripolyphosphate, which has a strong effect on magnesium ions (Mg²⁺). 2+ It exhibits significant selective complexation ability, and when used in conjunction with NH4Cl and ammonia to adjust the pH of the system, it can achieve selective and efficient extraction of lithium from salt lake brine, and has high salinity adaptability and long-term stability.

[0022] The specific explanation is as follows: Sodium tripolyphosphate affects magnesium ions (Mg) 2+ ) exhibits significant selective complexation ability, the core mechanism of which stems from the difference in charge density and polarization: Mg 2+ Due to its high charge density (+2 valence) and strong polarization ability, it can form a stable coordination structure with the five coordinated oxygen atoms of sodium tripolyphosphate ([Mg(P3O4)2]). 10 )] 3- The binding energy of Li is as high as -5.80 eV; while Li + Due to its low charge density (+1 valence) and weak polarization, sodium tripolyphosphate can only partially occupy coordination sites, and its binding energy (1.25 eV) is far from sufficient to form a stable complex. Ultimately, this results in sodium tripolyphosphate's dependence on Mg... 2+ The retention rate exceeds 99.5%, achieving highly efficient rejection of lithium ions.

[0023] In addition Mg 2+ With tripolyphosphate (P3O) 10 5- The complex formed by ([Mg(P3O)) 10 )] 3-It has an extremely high stability constant (logK≈7.5), far exceeding that of Li. + (logK≈3.1), making it difficult for competitive coordination to disrupt the binding at high salinity; furthermore, although increased salinity reduces Mg through the Debye-Hückel effect... 2+ The activity coefficient of Mg2+ decreases, but the activity of tripolyphosphate ligands decreases simultaneously, and the two effects partially cancel each other out. Experiments show that when the salt concentration increases from 10 g / L to 300 g / L, the activity of Mg2+ decreases. 2+ - The conditional stability constant of sodium tripolyphosphate decreases by only about 0.2 orders of magnitude, a negligible effect; and Mg 2+ The high polarizability enhances the overlap of electron clouds with the coordinated oxygen atom, while the tripolyphosphate ion provides multiple coordination sites (P=O, POP, oxygen on the hydroxyl group, etc.), which interact with Mg. 2+ The formation of multidentate coordinated cyclic intracyclic complexes (chelates) results in stable five-membered ring structures, while Li + Due to its weak polarization ability and large ionic radius, sodium tripolyphosphate is more easily solubilized by water molecules at high salinity, further reducing the probability of complexation. Based on the above multiple synergistic effects, in practical applications, sodium tripolyphosphate is effective against Mg. 2+ The retention rate can still be maintained at over 99.5% in high-salinity salt lake brine, which fully demonstrates its highly efficient selective complexation ability for magnesium ions in a high-salinity environment.

[0024] Simultaneously, NH3, by occupying the positions of water molecules in hydrated magnesium ions, can increase the pH required for Mg(OH)2 precipitation, preventing the formation of white flocculent precipitates and significantly extending membrane stability. Furthermore, the formation of [Mg(NH3)]... n ] 2+ With a low stability constant, it readily complexes with sodium tripolyphosphate, effectively reducing the entropy change of the complexation between Mg and sodium tripolyphosphate, thus accelerating the reaction. Ultimately, this enables the selective and efficient extraction of lithium from salt lake brine, while also exhibiting high salinity adaptability and long-term stability.

[0025] In addition, the long chains of sodium tripolyphosphate are large anions with multiple negative charges, which can also play a role in electrostatic repulsion, steric hindrance, and colloidal action to prevent the aggregation and deposition of small precipitates such as magnesium hydroxide and magnesium carbonate, thereby significantly reducing contaminants on the nanofiltration membrane surface and increasing the membrane's service life.

[0026] 2. Simultaneous resource utilization of magnesium, with the prepared magnesium resources serving as excellent additives in geopolymer preparation: This method yields over 95% of the magnesium as sodium magnesium-phosphate complexes, achieving simultaneous resource utilization of magnesium. The recovered magnesium resources can be used as additives in the geopolymer preparation process with excellent results, as detailed below: (1) The negatively charged chain anions in the prepared sodium magnesium phosphate complex can be rapidly and specifically adsorbed onto the surface of metakaolin particles. Utilizing their long molecular chains, they form a strong steric hindrance layer, thereby overcoming the van der Waals forces between particles, disrupting the original flocculation structure of metakaolin in alkaline solution, releasing free water, and promoting better contact and reaction between metakaolin particles and the alkaline activator. Meanwhile, Mg... 2+ The presence of Mg can effectively bridge the negatively charged large silicate clusters of sodium silicate (base activator), to some extent "breaking" their aggregated state, making them smaller and more reactive. 2+ The synergistic effect of tripolyphosphate creates a uniform and efficient reaction environment, which greatly optimizes the kinetic conditions of the geopolymerization reaction, resulting in rapid reaction and solidification of the raw materials, reducing the solidification time by 30%.

[0027] (2) Additives to enhance the compressive strength of geopolymers: Mg 2+ Relative to Na + With a stronger charge, when acting as a charge-balancing ion within the geopolymer gel structure, it can form stronger ionic bonds, significantly enhancing the degree of polymerization of the geopolymer network. Mg 2+ It can also directly participate in the reaction to form tiny, high-strength crystalline phases (such as magnesium silicate hydrate, MSH). Due to the dispersion effect of tripolyphosphate, these crystalline phases can be better distributed in the geopolymer gel, playing a role in micro-reinforcement and further strengthening the matrix. The compressive strength of the geopolymer is significantly improved. Attached Figure Description

[0028] Figure 1 The figure shows the XRD pattern of the prepared lithium carbonate. As shown, the Li₂CO₃ product has sharp diffraction peaks and no other obvious impurity phases were found, indicating that Li₂CO₃ has good crystallinity, complete unit cell structure, and good structural order.

[0029] Figure 2 Morphological analysis of the prepared lithium carbonate. As shown in the figure, the prepared Li₂CO₃ product is in the form of rod-shaped crystals with uniform crystal size and a smooth and intact surface. Detailed Implementation

[0030] Example 1 Step 1: Add sodium tripolyphosphate to the lithium-containing brine, controlling the molar ratio of tripolyphosphate in the sodium tripolyphosphate to Mg in the lithium-containing brine to be 1.2:1. Add appropriate amounts of NH4Cl and ammonia, with a molar ratio of NH4Cl to ammonia of 1:6.5. Adjust the pH of the brine to 9 to obtain the treated brine. The total salt concentration (TSC) of the lithium-containing brine is 122.1 g / L, of which the concentration of Li is 0.9 g / L, the concentration of Mg is 26.1 g / L, the concentration of Ca is 0.03 g / L, the concentration of Na is 51 g / L, and the concentration of K is 6.2 g / L. Step 2: Pretreatment of NFG nanofiltration membrane (MWCO548Da): Soak in deionized water for 12 hours to remove preservatives, and initially filter pure water at 10 bar pressure for 2 hours until the permeation flux stabilizes; Step 3: Using a nanofiltration device, the treated brine from the salt lake is filtered under 2 MPa pressure and room temperature using a two-stage filtration process (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a permeate, i.e., a lithium-rich solution, and a residual liquid, i.e., a [Mg(P3O4]2]2O4 solution. 10 )] 3- The solution; Step 4: Pass the obtained permeate into an evaporator for evaporation and concentration, filter and separate to obtain lithium concentrate with a concentration of 12.2 g / L; add sodium carbonate (Na2CO3) to the lithium concentrate at a sodium-to-lithium atomic ratio of 1.2:1, and stir the reaction at 70°C for 1 hour; separate the solid and liquid, wash the precipitate to obtain Li2CO3 precipitate; Step 5: Add sodium polyacrylate to the residual liquid at a concentration of 10 mg / L, and use a nanofiltration device with an NFG nanofiltration membrane. Under the conditions of 5 MPa pressure and room temperature, a two-stage filtration process is adopted (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a concentrated residual liquid. Step 6: Vacuum freeze-dry the concentrated residual liquid to obtain sodium magnesium phosphate complex.

[0031] Figure 1 The figure shows the XRD pattern of the prepared Li₂CO₃. As shown in the figure, the Li₂CO₃ product has sharp diffraction peaks and no other obvious impurity phases were found, indicating that the Li₂CO₃ has good crystallinity, complete unit cell structure, and good structural order.

[0032] Figure 2 Morphological analysis of the prepared Li₂CO₃. As shown in the figure, the prepared Li₂CO₃ product is in the form of rod-shaped crystals with uniform crystal size and a smooth and intact surface.

[0033] Results analysis: After treatment, 99.33% of the Mg in the salt lake brine was removed. 2+ Intercepted, and Li +The recovery rate reached 89.23%, and the purity of the Li₂CO₃ product reached 99.65% according to the test standard "Determination of Lithium Carbonate by Acid-Base Titration" (GB / T 11064.1-2024). After 360 hours of operation, the lithium flux remained at 3.26 mol / m³. -2 h -1 Compared to the highest level of 3.35 mol m - 2 h -1 The decrease was only 3%. Therefore, sodium tripolyphosphate-assisted nanofiltration can achieve precise separation of magnesium and lithium while maintaining long-term stable membrane operation.

[0034] The obtained sodium magnesium phosphate complex can be used in the preparation of geopolymers. It is prepared according to the following formula: 2220g metakaolin, 3200g sodium silicate solution (1220g sodium silicate, 1980g water), and 17.2g sodium magnesium phosphate complex (0.5wt% of solid material). Sodium silicate is dissolved in water to prepare an alkali activator solution; the alkali activator is added to the metakaolin, and the mixture is mechanically stirred for 5 minutes in a sealed container to obtain a geopolymer slurry; the sodium magnesium phosphate complex is added to the prepared geopolymer slurry, and mechanical stirring continues for 10 minutes. The mixture is then poured into a mold, and the mold is placed on a vibration table and vibrated for 3-10 minutes; the mold is cured at room temperature for 14 days, and then demolded to obtain the prepared geopolymer sample. Compared to the blank sample without the additive, the initial setting time of the final prepared geopolymer with added sodium magnesium phosphate complex was reduced from 74 min to 58 min, and the final setting time was reduced from 351 min to 245 min, a reduction of more than 30% (measurement method: "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement", GB / T 1346-2024). The 14-day strength reached 43.1 MPa, which is 23% higher than the 35 MPa of the blank sample.

[0035] Example 2 Step 1: Add sodium tripolyphosphate to the lithium-containing salt lake brine, controlling the molar ratio of tripolyphosphate in the sodium tripolyphosphate to Mg in the brine to be 1.1:1. Add appropriate amounts of NH4Cl and ammonia, with a molar ratio of NH4Cl to ammonia of 1:7.5. Adjust the pH of the brine to 9.5 to obtain the treated salt lake brine. The total salt concentration (TSC) of the lithium-containing salt lake brine is 202.6 g / L, of which the concentration of Li is 1.9 g / L, the concentration of Mg is 48.1 g / L, the concentration of Ca is 0.02 g / L, the concentration of Na is 71 g / L, and the concentration of K is 5.2 g / L. Step 2: Pretreatment of XN45 nanofiltration membrane (MWCO309Da): Soak in deionized water for 12 hours to remove preservatives, and initially filter pure water at 10 bar pressure for 2 hours until the permeation flux stabilizes; Step 3: Using a nanofiltration device, the treated brine from the salt lake is filtered under 4 MPa pressure and room temperature using a two-stage filtration process (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a permeate, i.e., a lithium-rich solution, and a residual liquid, i.e., a solution rich in [Mg(P3O4)2]. 10 )] 3- The solution; Step 4: Pass the obtained permeate into an evaporator for evaporation and concentration, filter and separate to obtain lithium concentrate with a concentration of 13.2 g / L; add sodium carbonate (Na2CO3) to the lithium concentrate at a sodium-lithium atomic ratio of 1.3:1, and stir the reaction at 90°C for 2 hours; separate the solid and liquid, wash the precipitate, and obtain Li2CO3 precipitate.

[0036] Step 5: Add sodium polyacrylate at a concentration of 8 mg / L to the residual liquid, and use a nanofiltration device with an XN45 nanofiltration membrane. Under the conditions of 4 MPa pressure and room temperature, a two-stage filtration process is adopted (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a concentrated residual liquid.

[0037] Step 6: Vacuum freeze-dry the concentrated residual liquid to obtain sodium magnesium phosphate complex.

[0038] Results analysis: After treatment, 99.61% of the Mg in the salt lake brine was removed. 2+ Intercepted, and Li + The recovery rate reached 89.71%, and the purity of the Li₂CO₃ product reached 99.81% according to the standard "Determination of Lithium Carbonate by Acid-Base Titration" (GB / T 11064.1-2024). After 360 hours of operation, the lithium flux remained at 3.28 mol m⁻¹. -2 h -1 Compared to the highest level of 3.35 mol m - 2 h -1 The decrease was only 2.1%. Therefore, sodium tripolyphosphate-assisted nanofiltration can achieve precise separation of magnesium and lithium while maintaining long-term stable membrane operation.

[0039] The obtained sodium magnesium phosphate complex can be used in the preparation of geopolymers. It is prepared according to the following formula: 2220g metakaolin, 3200g sodium silicate solution (1220g sodium silicate and 1980g water), and 10.32g sodium magnesium phosphate complex (0.3wt% of solid material). Sodium silicate is dissolved in water to prepare an alkali activator solution; the alkali activator is added to the metakaolin, and the mixture is mechanically stirred for 5 minutes in a sealed container to obtain a geopolymer slurry; the sodium magnesium phosphate complex is added to the prepared geopolymer slurry, and mechanical stirring continues for 10 minutes. The mixture is then poured into a mold, and the mold is placed on a vibration table and vibrated for 3-10 minutes; the mold is cured at room temperature for 14 days, and then demolded to obtain the prepared geopolymer sample. Compared to the blank sample without the additive, the initial setting time of the final prepared geopolymer with sodium magnesium phosphate complex was reduced from 74 min to 63 min, and the final setting time was reduced from 351 min to 280 min, a reduction of more than 21% (measurement method: "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement", GB / T 1346-2024). The 14-day strength reached 40.5 MPa, which is 15.7% higher than the 35 MPa of the blank sample.

[0040] Example 3 Step 1: Add sodium tripolyphosphate to the lithium-containing salt lake brine, controlling the molar ratio of tripolyphosphate in the sodium tripolyphosphate to Mg in the brine to be 1.15:1. Add appropriate amounts of NH4Cl and ammonia, with a molar ratio of NH4Cl to ammonia of 1:7.0. Adjust the pH of the brine to 10 to obtain the treated salt lake brine. The total salt concentration (TSC) of the lithium-containing salt lake brine is 149.6 g / L, of which the concentration of Li is 1.3 g / L, the concentration of Mg is 36.2 g / L, the concentration of Ca is 0.02 g / L, the concentration of Na is 32 g / L, and the concentration of K is 5.2 g / L. Step 2: Pretreatment of UA60 nanofiltration membrane (MWCO962Da): Soak in deionized water for 12 hours to remove preservatives, and initially filter pure water at 10 bar pressure for 2 hours until the permeation flux stabilizes; Step 3: Using a nanofiltration device, the treated brine from the salt lake is filtered under 3 MPa pressure and room temperature using a two-stage filtration process (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a permeate, i.e., a lithium-rich solution, and a residual liquid, i.e., a [Mg(P3O4]2]2O4 solution. 10 )] 3- The solution; Step 4: Pass the obtained permeate into an evaporator for evaporation and concentration, filter and separate to obtain lithium concentrate with a concentration of 12.8 g / L; add sodium carbonate (Na2CO3) to the lithium concentrate at a sodium-to-lithium atomic ratio of 1.3:1, and stir the reaction at 80°C for 1.5 hours; separate the solid and liquid, wash the precipitate, and obtain Li2CO3 precipitate; Step 5: Add sodium polyacrylate at a concentration of 6 mg / L to the residual liquid and use a nanofiltration device with a UA60 nanofiltration membrane. Under the conditions of 4.5 MPa pressure and room temperature, a two-stage filtration process is adopted (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a concentrated residual liquid. Step 6: Vacuum freeze-dry the concentrated residual liquid to obtain sodium magnesium phosphate complex.

[0041] Results analysis: After treatment, 99.21% of the Mg in the salt lake brine was removed. 2+ Intercepted, and Li + The recovery rate reached 89.52%, and the purity of the Li₂CO₃ product reached 99.61% according to the standard "Determination of Lithium Carbonate by Acid-Base Titration" (GB / T 11064.1-2024). After 360 hours of operation, the lithium flux remained at 3.21 mol m -2 h -1 Compared to the highest level of 3.35 mol m - 2 h -1 The decrease was only 4.1%. Therefore, sodium tripolyphosphate-assisted nanofiltration can achieve precise separation of magnesium and lithium while maintaining long-term stable membrane operation.

[0042] The obtained sodium magnesium phosphate complex can be used in the preparation of geopolymers. It is prepared according to the following formula: 2220g metakaolin, 3200g sodium silicate solution (1220g sodium silicate, 1980g water), and 3.44g sodium magnesium phosphate complex (0.1wt% of solid material). Sodium silicate is dissolved in water to prepare an alkali activator solution; the alkali activator is added to the metakaolin, and the mixture is mechanically stirred for 5 minutes in a sealed container to obtain a geopolymer slurry; the sodium magnesium phosphate complex is added to the prepared geopolymer slurry, and mechanical stirring continues for 10 minutes. The mixture is then poured into a mold, and the mold is placed on a vibration table and vibrated for 3-10 minutes; the mold is cured at room temperature for 14 days, and then demolded to obtain the prepared geopolymer sample. Compared to the blank sample without the additive, the initial setting time of the final prepared geopolymer with added sodium magnesium phosphate complex was reduced from 74 min to 66 min, and the final setting time was reduced from 351 min to 298 min, a reduction of more than 15% (measurement method: "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement", GB / T 1346-2024). The 14-day strength reached 39.5 MPa, which is 11.2% higher than the 35 MPa of the blank sample.

[0043] Example 4 Step 1: Add sodium tripolyphosphate to the lithium-containing salt lake brine, controlling the molar ratio of tripolyphosphate in the sodium tripolyphosphate to Mg in the brine to be 1.18:1. Add appropriate amounts of NH4Cl and ammonia, with a molar ratio of NH4Cl to ammonia of 1:7.2. Adjust the pH of the brine to 9.8 to obtain the treated salt lake brine. The total salt concentration (TSC) of the lithium-containing salt lake brine is 277.6 g / L, of which the concentration of Li is 1.5 g / L, the concentration of Mg is 43.2 g / L, the concentration of Ca is 0.1 g / L, the concentration of Na is 84 g / L, and the concentration of K is 5.8 g / L. Step 2: Pretreatment of NF-8 nanofiltration membrane (MWCO1913Da): Soak in deionized water for 12 hours to remove preservatives, and initially filter pure water at 10 bar pressure for 2 hours until the permeation flux stabilizes. Step 3: Using a nanofiltration device, the treated brine from the salt lake is filtered under a two-stage filtration process at 3.5 MPa pressure and room temperature (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a permeate, i.e., a lithium-rich solution, and a residual liquid, i.e., a [Mg(P3O4]2]2O4 solution. 10 )] 3- The solution; Step 4: Pass the obtained permeate into an evaporator for evaporation and concentration, filter and separate to obtain lithium concentrate with a concentration of 12.1 g / L; add sodium carbonate (Na2CO3) to the lithium concentrate at a sodium-to-lithium atomic ratio of 1.3:1, and stir the reaction at 85°C for 1.8 hours; separate the solid and liquid, wash the precipitate to obtain Li2CO3 precipitate; Step 5: Add sodium polyacrylate at a concentration of 5 mg / L to the residual liquid and use a nanofiltration device with an NF-8 nanofiltration membrane. Under the conditions of 4 MPa pressure and room temperature, a two-stage filtration process is adopted (the residual liquid from the first stage enters the second stage for re-filtration) to obtain a concentrated residual liquid.

[0044] Step 6: Vacuum freeze-dry the concentrated residual liquid to obtain sodium magnesium phosphate complex.

[0045] Results analysis: After treatment, 99.13% of the Mg in the salt lake brine was removed. 2+ Intercepted, and Li + The recovery rate reached 89.27%, and the purity of the Li₂CO₃ product reached 99.58% according to the test standard "Determination of Lithium Carbonate by Acid-Base Titration" (GB / T 11064.1-2024). After 360 hours of operation, the lithium flux remained at 3.31 mol m -2 h -1 Compared to the highest level of 3.35 mol m - 2 h -1The decrease was only 1.2%. Therefore, sodium tripolyphosphate-assisted nanofiltration can achieve precise separation of magnesium and lithium while maintaining long-term stable membrane operation.

[0046] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for extracting lithium and simultaneously utilizing magnesium resources from salt lake brine, characterized in that: Includes the following steps: Step 1: Add sodium tripolyphosphate to the lithium-containing salt lake brine, controlling the molar ratio of tripolyphosphate ions in the sodium tripolyphosphate to Mg in the brine to be 1.1-1.2:

1. Add NH4Cl and ammonia to adjust the pH of the brine to 9-10 to obtain the treated salt lake brine. Step 2: Pretreatment of the lithium extraction nanofiltration membrane from the salt lake; Step 3: Using a nanofiltration device, the treated brine from the salt lake is filtered through a two-stage process using a salt lake lithium extraction nanofiltration membrane at a pressure of 2-4 MPa and room temperature to obtain a permeate (lithium-rich solution) and a residual liquid (lithium-rich [Mg(P3O4]2O4]2O4). 10 )] 3- The solution; Step 4: Pass the obtained permeate into an evaporator for evaporation and concentration, filter and separate to obtain lithium concentrate; Sodium carbonate was added to the lithium concentrate to react, and the solid-liquid separation and washing of the precipitate were performed to obtain Li2CO3 precipitate. Step 5: Add sodium polyacrylate to the residual liquid and use a nanofiltration device, using a lithium extraction nanofiltration membrane from a salt lake, to obtain a concentrated residual liquid under a two-stage filtration process at a pressure of 4-5 MPa and room temperature. Step 6: Vacuum freeze-dry the concentrated residual liquid to obtain sodium magnesium phosphate complex.

2. The method according to claim 1, characterized in that: The composition of the salt lake brine meets the following requirements: total salt concentration of 120~280 g / L, of which Li concentration is 0.85~2.0 g / L, Mg concentration is 25~50 g / L, Ca concentration is 0-0.06 g / L, Na concentration is 30~90 g / L, and K concentration is 5~15 g / L; if the brine salinity exceeds the target range, it shall be diluted with water.

3. The method according to claim 1, characterized in that: The addition ratio of NH4Cl and ammonia is: NH4Cl to NH3 molar ratio 1:6.5~7.

5.

4. The method according to claim 1, characterized in that: The lithium extraction nanofiltration membrane from the salt lake includes one of the following: XN45 nanofiltration membrane, NFG nanofiltration membrane, UA60 nanofiltration membrane, or NF-8 nanofiltration membrane.

5. The method according to claim 1, characterized in that: The lithium extraction nanofiltration membrane from the salt lake is an XN45 membrane.

6. The method according to claim 1, characterized in that: The pretreatment method for the lithium extraction nanofiltration membrane from the salt lake is as follows: soaking in deionized water to remove impurities, and then activating it with water until the permeation flux is stable.

7. The method according to claim 1, characterized in that: The amount of sodium carbonate used in step four is as follows: sodium carbonate is measured as sodium and added at a sodium-lithium atomic ratio of 1.1-1.3:1; the reaction described in step four is carried out by stirring at 70-90℃; the reaction time is 1-2 hours.

8. The method according to claim 1, characterized in that: The lithium concentration in the lithium concentrate is 11-14 g / L; the amount of sodium polyacrylate added in step five is 5-10 mg / L; the vacuum freeze-drying in step six is ​​freeze-drying for 10-12 hours at -20~-40°C and a vacuum degree of 0.05~0.1 mbar.

9. The application of the sodium magnesium phosphate complex prepared by the method of claim 1 as an additive in the preparation of geopolymers.

10. The application according to claim 9, characterized in that: The amount of sodium magnesium phosphate complex added is 0.1-0.5 wt%.

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