A method and system for efficiently extracting lithium from a mixed salt solution

Abstract

This invention relates to a method and system for efficient lithium extraction from a mixed salt solution, comprising: adding a chelating agent to the mixed salt solution to be treated, adjusting the pH to promote the formation of stable chelates between the chelating agent and divalent or polyvalent cations in the mixed salt solution to obtain feed water; separating the feed water through a membrane, utilizing the membrane pore size sieving mechanism and electrostatic repulsion to achieve lithium-magnesium separation, producing membrane filtration concentrate and membrane filtration permeate; adjusting the pH of the membrane filtration concentrate and performing solid-liquid separation to obtain highly dispersed high-value hydroxide and chelating agent recovery solution; concentrating and enriching the membrane filtration permeate to obtain a lithium-rich concentrate, which is used for lithium precipitation. The solution of this invention solves the technical problems of current membrane-based lithium extraction technologies, such as high requirements for feed water, limited adaptability, high freshwater consumption, low lithium-magnesium separation coefficient, low permeate concentration, and low lithium recovery rate.

Description

Technical Field

[0001] This invention relates to the field of lithium resource extraction technology, specifically to a method and system for efficiently extracting lithium from a mixed salt solution. Background Technology

[0002] Lithium and its compounds are widely used in the national economy, military industry, and aerospace sector. In recent years, with the rollout of new electric vehicles and a significant increase in demand, lithium, as a strategic resource, has increasingly demonstrated its enormous market potential and application prospects. Lithium is mainly distributed in lithium ores, brines, and seawater, with salt lake lithium resources accounting for 58% of the world's proven lithium resources, gradually becoming the cornerstone of global lithium supply. However, the magnesium ion content in salt lake brines is much higher than the lithium ion content. Due to the similar chemical properties of magnesium and lithium, the presence of large amounts of magnesium increases the difficulty of lithium separation and extraction.

[0003] Brine generally refers to liquid minerals with a salt content greater than 5%. Brine that accumulates on the earth's surface is called surface brine or lake brine. Brine that accumulates below the surface is called underground brine. Brine often contains potassium (K). + Na + Ca 2+ Mg 2+ Cl - SO4 2- CO3 2- HCO3 - B, Li + ,Br - I - Lithium can be extracted from brine using ions such as Sr, Rb, and Cs. To date, there are numerous brine lithium extraction processes, primarily including evaporation precipitation, calcination leaching, adsorption, solvent extraction, and membrane separation. Membrane separation, a physical separation method, offers advantages such as environmental friendliness and relatively low equipment and operating costs. Therefore, it is widely used in water purification, brine softening, and the desalination and concentration of organic matter and bioactive substances. Its application in lithium extraction from salt lakes is also becoming increasingly mature.

[0004] For example, Chinese patent CN03108088.X first disclosed a method for separating magnesium and enriching lithium from salt lake brine using nanofiltration membranes. This method utilizes nanofiltration technology to separate magnesium and lithium from salt lake brine, and then uses the lithium-rich permeate to produce high-purity lithium salts. However, the single-stage membrane process in this patent has a low separation coefficient, resulting in a low lithium ion content / concentration in the lithium-rich permeate. Further concentration is required to increase the lithium ion concentration, which also increases energy consumption costs. Furthermore, to improve lithium recovery, the nanofiltration concentrate is recycled to the nanofiltration feed water, leading to an increase in the magnesium-to-lithium ratio, necessitating increased membrane operating pressure and the number of nanofiltration stages (increasing energy consumption and equipment costs). Chinese patents CN201310035015.7 and CN106241841A respectively disclose methods for separating lithium from salt lake brine with a high magnesium-to-lithium ratio. The lithium-rich permeate obtained from nanofiltration is sent to a reverse osmosis or forward osmosis unit for concentration, and then the lithium-rich concentrate after deep magnesium removal is sent to a lithium precipitation unit. However, these solutions have limited magnesium removal efficiency of nanofiltration membrane units and limited lithium ion concentration in nanofiltration permeate, requiring the addition of pretreatment devices such as adsorption. Furthermore, the magnesium and lithium content in the permeate remains high, necessitating the addition of advanced magnesium removal and concentration units, increasing equipment investment costs. US Patent 11219864B2 discloses a coupled process using ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis to achieve efficient separation and enrichment of lithium from brine in salt lakes. However, the nanofiltration membrane in this patent has low separation efficiency, and the need for a large amount of freshwater to dilute the membrane feed concentration results in a low lithium ion concentration in the permeate, requiring a concentration unit composed of reverse osmosis and electrodialysis to increase the lithium ion concentration.

[0005] In summary, current membrane-based lithium extraction technologies mainly suffer from several problems: (1) They require a high magnesium-to-lithium ratio in the nanofiltration feed water. An excessively high magnesium-to-lithium ratio leads to poor economic efficiency in the nanofiltration process, often requiring the addition of an adsorption unit before filtration to remove some magnesium (reduce the magnesium-to-lithium ratio) for nanofiltration separation; (2) The lithium-magnesium selectivity of membrane technology is not high, and the separation coefficient is low, resulting in low lithium recovery rate and still high magnesium-to-lithium ratio in the product water. The former leads to serious lithium resource loss and low resource utilization, while the latter requires the addition of multiple nanofiltration units or even deep magnesium removal units to improve the purity of lithium products, increasing the complexity of the process and equipment costs; (3) The total salt concentration that nanofiltration membranes used for lithium-magnesium separation can withstand is low, and concentration polarization is prone to occur on the membrane surface, leading to reduced membrane flux and decreased separation efficiency, increasing the complexity of the process; to alleviate this phenomenon, a large amount of fresh water is required to dilute the feed water (side) by a high ratio, resulting in a large consumption of fresh water, which limits the use and promotion of nanofiltration technology in freshwater-scarce saline lake areas.

[0006] Therefore, a highly efficient brine utilization scheme that can separate lithium and magnesium and concentrate lithium is of great significance for improving the lithium-ion yield of brine and reducing process costs and energy consumption. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a method and system for efficient lithium extraction from mixed salt solutions, which solves the technical problems of current membrane process lithium extraction technology, such as high requirements for feed water, limited adaptability, large fresh water consumption, low lithium-magnesium separation coefficient, low product water concentration, and low lithium recovery rate.

[0009] (II) Technical Solution

[0010] In a first aspect, the present invention provides a method for efficiently extracting lithium from a mixed salt solution, the method comprising:

[0011] S1. Chelation pretreatment: Add a chelating agent to the mixed salt solution to be treated and adjust the pH to promote the formation of stable chelates between the chelating agent and the divalent or polyvalent cations in the mixed salt solution to obtain the influent.

[0012] S2, Membrane separation: The influent produced in step S1 is separated by membrane separation. Lithium and magnesium are separated by membrane pore size screening mechanism and electrostatic repulsion effect, producing membrane filtration concentrate and membrane filtration permeate.

[0013] S3, Dechelation reaction: Adjust the pH of the membrane filtration concentrate produced by S2 to cause the chelates in the membrane filtration concentrate to undergo a dechelation reaction. At the same time, divalent or polyvalent cations combine with hydroxide ions to form precipitates. After solid-liquid separation, high-value hydroxide precipitates and chelating agent recovery liquid are obtained.

[0014] S4. Concentration and desalination of chelating agent: The chelating agent recovery solution generated in S3 is concentrated by membrane filtration to remove salt from the chelating agent recovery solution and increase the concentration of chelating agent in the chelating agent recovery solution to obtain concentrated chelating agent recovery solution for reuse in step S1.

[0015] S5. Preparation of lithium-rich concentrate: The membrane filtration permeate produced in S2 is concentrated and enriched to increase the lithium concentration, resulting in a lithium-rich concentrate, which is used to prepare lithium salts.

[0016] In this application, the mixed salt solution includes, but is not limited to, surface brine (also known as salt lake brine), underground brine, petroleum brine, or industrially produced brine containing Mg. 2+ Li + (may also contain K) + Na + Ca 2+ The cations are equal to the anions, and the anions are usually Cl. - and / or SO4 2- The waste brine containing plasma. The high-value hydroxide obtained in S3 mainly refers to nano-magnesium hydroxide. Therefore, the solution of this invention can realize lithium extraction from high-salinity brine and produce high-value-added nano-magnesium hydroxide products as a byproduct.

[0017] According to a preferred embodiment of the present invention, in S1, the chelating agent is one or more of organophosphonic acids, aminocarboxylic acids, amino acids, hydroxycarboxylic acids, water-soluble polymers, and polycarboxylic acid chelating agents, or a combination of one or more of the aforementioned chelating agents and dispersants.

[0018] Specifically: organophosphonic acid chelating agents are selected from one or more of ethylenediaminetetramethylenephosphonic acid (EDTMPA), diethylenetriaminepentamethylenephosphonic acid (DETPMP), polyol phosphate (PAPE), or phosphonohydroxyacetic acid (HPAA); aminocarboxylic acid chelating agents are one or more of N-carboxyethyl ethylenediaminetetraacetic acid (HEDTA), ethylene glycol diaminoethyl ether tetraacetic acid (EGTA), or 1,2-cyclohexanediaminetetraacetic acid (DCTA) and their corresponding salts (such as sodium salts); amino acid chelating agents are methionine, lysine, or glycine and their... One or more of the corresponding salts (such as sodium salts); water-soluble polymer chelating agents are one or more of polyacrylic acid (PAA), polyethyleneimine (PEI), hydroxyethyl cellulose (HEC) or polyacrylamide (PAM); hydroxycarboxylic acid chelating agents are one or more of linolenic acid (LA) and malic acid (MA) and their corresponding salts (such as sodium salts); polycarboxylic acid chelating agents are one or more of polymaltate (PGA), poly(methacrylic acid) (PMMA) and poly(co-maleic acid acrylate) (PAMA) and their corresponding salts (such as sodium salts).

[0019] When an ionic chelating agent is selected, it not only increases the steric hindrance of magnesium but also alters its charge, thereby synergistically promoting lithium-magnesium separation and improving the magnesium-lithium separation coefficient through the membrane pore size sieving mechanism and electrostatic repulsion effect (the membrane is negatively charged). The combined use of ionic chelating agents and water-soluble polymeric chelating agents can further increase the size of the magnesium complex, leading to its retention and improving the magnesium-lithium separation efficiency.

[0020] According to a preferred embodiment of the present invention, in S1, the magnesium-to-lithium ratio (by mass) of the mixed salt solution is 0.1-1000:1, the lithium ion mass concentration is 0.01-5 g / L, and the pH value is 6-8.

[0021] According to a preferred embodiment of the present invention, in S1, a pretreatment unit needs to be added depending on whether the mixed salt solution contains sulfate ions. The pretreatment unit is mainly used to remove some sulfate ions and reduce the magnesium-lithium ratio to a certain extent. The pretreatment unit can be a microfiltration membrane and / or a loose nanofiltration membrane.

[0022] Then, before adding the chelating agent to the mixed salt solution to be treated, the concentration of divalent and polyvalent cations in the mixed salt solution is detected, and the amount of chelating agent to be added is determined based on the detection results. The molar ratio of the chelating agent to the divalent and polyvalent cations in the mixed salt solution varies depending on the type of chelating agent. If the chelating agent is a small molecule chelating agent, the molar ratio of the chelating agent to the divalent and polyvalent cations in the mixed salt solution is 0.7-1.3:1. If the chelating agent is a large molecule chelating agent (such as polycarboxylic acid chelating agents - water-soluble copolymers such as PMMA), the molar ratio of the chelating agent to the divalent and polyvalent cations in the mixed salt solution is 0.01-0.1:1. In step S1, the pH range should be adjusted to a range that promotes the chelating agent's ability to chelate with divalent and polyvalent cations and improves the stability of the chelate. For example, the pH of the mixed salt solution is adjusted to 5-10.5, preferably 8-10. The specific pH range depends on the type and dosage of the chelating agent used. Different types and dosages of chelating agents result in different optimal pH values ​​for promoting chelate formation and stability. During the chelation pretreatment in step S1, the mixed salt solution can be kept in a flowing or agitated state to increase the uniformity of mixing between the chelating agent and the mixed salt solution, ensuring that magnesium ions form chelates as much as possible.

[0023] According to a preferred embodiment of the present invention, in step S2, the membrane separation employs a single-stage membrane separation process or a multi-stage membrane separation process. The single-stage membrane separation process includes one or more parallel membrane separation devices. The multi-stage membrane separation process includes membrane separation devices connected in series. The feed water of each stage membrane separation device is treated with chelating agents and pH adjusted as needed. The permeate from each stage membrane separation device serves as the feed water for the next stage membrane separation device. The concentrate from each stage membrane separation device is treated according to step S3 to achieve the recycling of the chelating agent and reduce the cost of reagent input. "As needed" means that if the ratio of chelating agent to divalent and polyvalent ions in S1 already meets the requirements for complete chelation, this step does not require the addition of chelating agent again; otherwise, if the ratio is insufficient, some chelating agent needs to be added.

[0024] The type of chelating agent added to the feed water between the two-stage membrane separation unit can be the same as or different from the type of chelating agent added to the initial mixed salt solution. When adding the chelating agent to the feed water between the two-stage membrane separation unit, the concentration of the chelating agent and divalent or polyvalent cations in the feed water can be detected in time, and a certain amount of chelating agent can be added. The molar ratio of the added chelating agent to the magnesium ion concentration is 0.01-0.4:1.

[0025] The multi-stage membrane separation process achieves the goal of progressively reducing the magnesium-to-lithium ratio in the permeate from each stage of the membrane separation unit and increasing the purity of lithium ions in the permeate. After the multi-stage membrane separation process, the total lithium-magnesium separation coefficient can reach over 2000, and even more preferably over 7000.

[0026] According to a preferred embodiment of the present invention, in S2, the membrane separation pressure difference is 0.1-5 MPa, the inlet water temperature is controlled at 10-50°C, and the volume ratio of membrane filtration permeate to inlet water is controlled at 0.1-5:1; under the aforementioned conditions, efficient separation of magnesium and lithium can be achieved.

[0027] According to a preferred embodiment of the present invention, in S2, cross-flow filtration is used for membrane separation, and the flow rate is 5.0-100 L / h, preferably 20-40 L / h. Under the aforementioned conditions, the influence of concentration polarization on the separation effect can be reduced. The flow rate of 5.0-100 L / h balances membrane treatment efficiency and concentration polarization; too low a flow rate leads to low separation efficiency, while too high a flow rate can easily result in a low magnesium-lithium separation coefficient and may still cause concentration polarization. Cross-flow filtration involves horizontal flow across the membrane surface driven by a pump. The shear force generated when the feed water flows across the membrane surface carries away the particles retained on the membrane surface, thereby keeping the fouling layer at a relatively thin level and reducing the boundary layer thickness caused by concentration polarization.

[0028] According to a preferred embodiment of the present invention, in S2, based on the differences in the type of chelating agent, the magnesium-to-lithium ratio of the mixed salt solution, and the lithium concentration, the membrane combination used in the membrane separation process includes membrane module type and membrane material. The membrane module type is one or more combinations of flat sheet membranes, spiral wound membranes, hollow fiber membranes, and tubular membranes; the membrane material is one or more combinations of microfiltration, ultrafiltration, and nanofiltration membranes. Preferably, the molecular weight cutoff of the membrane material is 100-50000, more preferably 300-10000.

[0029] Specifically, the membrane combination used in the membrane separation process can be set according to the magnesium-to-lithium ratio, lithium concentration, type of chelating agent, and characteristics of the resulting chelate in the mixed salt solution. Three typical cases are as follows:

[0030] (1) When the lithium ion concentration of the brine to be treated is 0-1.0 g / L and the magnesium-lithium ratio is 0-40, if the brine does not contain sulfate, one or more aminocarboxylic acid chelating agents are selected and added alone or in combination, and further treated with a single-stage nanofiltration device. At this time, the total magnesium-lithium separation coefficient is 500-1000 and the magnesium-lithium ratio of the membrane permeate is 0.01-0.1.

[0031] If the brine contains sulfate, it should be pretreated with a loose nanofiltration membrane before adding the chelating agent to remove some sulfate ions and reduce the magnesium-to-lithium ratio. Then, one or more organophosphonic acid chelating agents are added alone or in combination, and further treated with a single-stage nanofiltration unit. At this time, the total magnesium-to-lithium separation coefficient is 800-3000, and the magnesium-to-lithium ratio of the membrane unit permeate is 0.01-0.05.

[0032] (2) When the lithium ion concentration in the sulfate-containing brine to be treated is 0-1.0 g / L and the magnesium-lithium ratio is greater than 40, or when the lithium ion concentration is greater than 1.0 g / L and the magnesium-lithium ratio is 0-40: First, the brine is pretreated with a loose nanofiltration membrane to remove some of the sulfate ions. Then, one or more amino acid chelating agents are added to the brine alone or in combination and the brine is subjected to primary nanofiltration through a loose nanofiltration membrane. After that, one or more hydroxycarboxylic acid chelating agents are added to the primary nanofiltration product water alone or in combination and the brine is subjected to secondary nanofiltration through a compact nanofiltration membrane. At this time, the total magnesium-lithium separation coefficient is 1800-3000 and the magnesium-lithium ratio of the membrane product water is 0.005-0.01.

[0033] (3) When the lithium ion concentration in the sulfate-containing brine to be treated is greater than 1.0 g / L and the magnesium-to-lithium ratio is greater than 40, the brine is first pretreated with a loose nanofiltration membrane to remove some of the sulfate ions. Then, polycarboxylic acid and / or water-soluble polymer chelating agents are added to the brine and subjected to primary ultrafiltration through an ultrafiltration membrane. The permeate from the primary ultrafiltration is then subjected to secondary ultrafiltration or nanofiltration, and the permeate from the secondary ultrafiltration or nanofiltration is subjected to tertiary nanofiltration. Polycarboxylic acid and / or water-soluble polymer chelating agents are added to the influent at each stage as needed. At this time, the total lithium-magnesium separation coefficient of the three-stage membrane filtration device is 5000-10000, and the magnesium-to-lithium ratio of the membrane filtration permeate is 0.01-0.06.

[0034] In practical applications, those skilled in the art can refer to the above three process forms and flexibly select appropriate chelating agents and chelate complexes, as well as increase or decrease the number of membrane filtration stages and membrane combination forms, based on the magnesium-lithium ratio in the mixed salt solution, the initial concentration of magnesium and lithium, the water production conditions of each stage, and the magnesium-lithium concentration requirements in the final water produced by the membrane process.

[0035] According to a preferred embodiment of the present invention, in S3, the pH of the dechelation reaction is 10.5-14, preferably in the range of 11-13, so that there are more hydroxide ions in the membrane filtration concentrate, which in turn promotes the combination of divalent or polyvalent metal cations with hydroxide ions to form precipitates, resulting in highly dispersed, high-value hydroxides (mainly highly dispersed nano-sized magnesium hydroxide). Highly dispersed nano-magnesium hydroxide has extremely high application value and can be used as a flame retardant / smoke suppressant in polymer materials, as well as as an additive in flame-retardant and anti-corrosion coatings.

[0036] According to a preferred embodiment of the present invention, in S3, the pH is adjusted by adding an alkaline reagent, and the amount of alkaline reagent added is such that it can dissociate OH groups. - The calculated molar ratio of magnesium ions in the concentrate from membrane filtration ranges from 1.8 to 2.5. - A slight excess of magnesium promotes the formation of highly dispersed, high-value magnesium hydroxide precipitate. Preferably, the dechelation reaction occurs at a temperature range of 10-70°C, more preferably 15-50°C.

[0037] According to a preferred embodiment of the present invention, in S3, during the dechelation reaction, the membrane process is made to be in a flowing, agitated, or rotating state to increase the dechelation rate.

[0038] According to a preferred embodiment of the present invention, in S3, after the dechelation reaction is completed, a solid-liquid separation unit is used for solid-liquid separation. The solid-liquid separation unit includes, but is not limited to, one or a combination of a sedimentation device, a plate and frame filter press, and a centrifuge. The sedimentation device may include a sedimentation tank and an inclined tube assembly disposed within the sedimentation tank.

[0039] According to a preferred embodiment of the present invention, in step S3, a certain amount of flocculant (such as PAM) or morphology aid (such as PEG) can be added during the dechelation reaction. The flocculant can promote the precipitation of magnesium hydroxide, and the morphology aid can obtain magnesium hydroxide products with predetermined microstructures or sizes, and obtain chelating agent recovery liquid with higher purity, thereby improving the chelating agent reuse rate. The amount of flocculant / morphology aid added can be 0.1-0.5 g / L. After drying, the separated magnesium hydroxide precipitate yields a highly dispersed nano-sized magnesium hydroxide product, solving the magnesium hazard problem caused by existing salt lake lithium extraction processes.

[0040] According to a preferred embodiment of the present invention, in step S4, the chelating agent recovery solution produced in step S3 is further passed through a nanofiltration membrane with a loose separation functional layer to separate salts, thereby increasing the concentration of chelating agent in the concentrated water and reducing the salt content, thus obtaining a concentrated chelating agent recovery solution. This process achieves a chelating agent recovery rate of 95-100%, and the concentrated chelating agent recovery solution can be directly reused in step S1 or used to treat the feed water of each stage of the multi-stage membrane separation process in step S2.

[0041] According to a preferred embodiment of the present invention, in S5, the method for concentrating and enriching the membrane filtration permeate to prepare a lithium-rich concentrate is one or more combinations of reverse osmosis, forward osmosis, membrane distillation, and MVR systems.

[0042] According to a preferred embodiment of the present invention, in step S5, the lithium-rich concentrate has a concentration factor of 5-60 compared to the membrane filtration permeate; the lithium ion content in the lithium-rich concentrate is 25-40 g / L, and the magnesium-to-lithium mass ratio is 0.005-0.1. The lithium-rich concentrate meeting these conditions has optimal economic benefits and can be sent to a subsequent lithium precipitation unit for lithium precipitation. Lithium precipitation uses carbonates, such as sodium carbonate, to generate lithium carbonate, which is then separated into solid and liquid phases, washed, and dried to obtain the lithium carbonate product.

[0043] According to a preferred embodiment of the present invention, in S5, when concentrating the membrane filtration permeate, in order to extract sodium and potassium (sodium and potassium are both monovalent metal cations, which easily enter the membrane filtration permeate along with lithium ions) from the membrane filtration permeate and to improve the purity of the lithium salt product, phase separation technology can be used for stepwise removal during the concentration process. The phase separation technology can be implemented by an MVR system.

[0044] According to phase diagram theory, as temperature increases, the sodium chloride crystalline phase region expands, and the solubility of sodium chloride decreases. The MVR system can lower the boiling point of the membrane filtration product water under vacuum conditions, promoting the transformation of the liquid phase to the gas phase and improving evaporation efficiency. Therefore, the salting-out effect of lithium salts (lithium chloride) on sodium and potassium salts (sodium chloride and potassium chloride) can be utilized to separate sodium and potassium ions from the solution in the form of sodium and potassium salt crystals. Simultaneously, by controlling the evaporation temperature and vacuum level, the crystallization loss of lithium ions can be minimized during the evaporation process. Specifically, the temperature of the MVR system is 0-90℃, preferably 30-90℃, and the vacuum level is 0-0.1MPa, preferably 0.05-0.1MPa.

[0045] Based on the above preferred embodiments, not only can the purity of lithium salt products be improved and the recovery rate of lithium ions in the mixed salt solution be increased, but also the comprehensive utilization of other resources in the mixed salt solution is achieved, yielding sodium and potassium salt (sodium chloride and potassium chloride) products. Furthermore, the purified water produced by concentrating the membrane filtration permeate can be recycled to reduce freshwater resource consumption, which is environmentally friendly.

[0046] In a second aspect, the present invention provides a system for efficient lithium extraction from a mixed salt solution, comprising: a pretreatment unit, a mixing unit, a first acid-base adjustment unit, a membrane separation unit, and a concentration and enrichment unit connected in sequence.

[0047] The pretreatment unit is used to remove insoluble substances and sulfate ions from the water to be treated. The pretreatment unit includes a microfiltration membrane, an ultrafiltration membrane, and / or a loose nanofiltration membrane.

[0048] One end of the mixing unit is connected to the effluent from the pretreatment unit, and the other end is connected to the first acid-base adjustment unit; the mixing unit is also equipped with a chelating agent dosing device;

[0049] The membrane separation unit includes a membrane filtration permeate side and a membrane filtration concentrate side. The membrane filtration permeate side is connected to the concentration and enrichment unit to concentrate and enrich the membrane filtration permeate to obtain a lithium-rich concentrate. The membrane filtration concentrate side is connected to the chelating agent recovery unit.

[0050] The chelating agent recovery unit includes a second acid-base adjustment unit and a solid-liquid separation device. The membrane filtration concentrate is connected to the second acid-base adjustment unit, which is connected to the solid-liquid separation device. The second acid-base adjustment unit is used to adjust the pH of the membrane filtration concentrate to generate a dechelation reaction, and the concentrate is separated by the solid-liquid separation device to obtain the chelating agent recovery solution.

[0051] The solid-liquid separation device is connected to the concentration and desalination unit to perform membrane concentration and desalination of the chelating agent recovery liquid to obtain concentrated chelating agent recovery liquid; the concentrate side of the concentration and desalination unit is connected to the mixing unit for recycling the concentrated chelating agent recovery liquid into the mixing unit.

[0052] Both the first and second acid-base adjustment units are equipped with pH adjustment reagent dosing devices.

[0053] Preferably, the purified water or low-concentration brine produced on the freshwater side of the concentration and enrichment unit and the concentration and desalination unit can be used to prepare pH adjustment reagents, dilute membrane feed water, or prepare chelating agent solutions.

[0054] Preferably, the concentration and enrichment unit is further connected to a lithium precipitation unit for converting the lithium-rich concentrate into a lithium salt product. For example, the lithium salt product is lithium carbonate or lithium phosphate.

[0055] Preferably, the mixing unit is equipped with an online ion concentration detection device to adjust the dosage of the chelating agent according to the concentration of divalent and polyvalent cations in the mixed salt solution entering the mixing unit.

[0056] Preferably, the second acid-base adjustment unit is further provided with a magnesium ion concentration detection device for detecting the magnesium ion concentration and, based on the dissociable OH groups, determining the concentration of magnesium ions. - Add an alkaline reagent within a molar ratio of 1.8 to 2.5 of magnesium ions in the membrane filtration concentrate.

[0057] Preferably, the mixing unit is a pipeline mixer, and the flow rate of the mixed salt solution in the pipeline mixer is in the range of 0.1-2 m / s, preferably in the range of 0.5-1.5 m / s; too fast a flow rate is not conducive to the uniform mixing of the chelating agent and the full reaction with divalent or polyvalent cations.

[0058] Preferably, the membrane separation unit adopts a single-stage membrane separation device or a multi-stage membrane separation device; the single-stage membrane separation device includes one or more parallel membrane separation devices; the multi-stage membrane separation device includes membrane separation devices in series, wherein the permeate of each stage membrane separation device is used as the feed water of the next stage membrane separation device, and the concentrate of each stage membrane separation device enters the second acid-base adjustment unit for dechelation reaction.

[0059] Preferably, the concentration and desalination unit is also connected to the membrane separation unit so that the concentrated chelating agent recovery solution produced by the concentration and desalination unit is added to the interstage feed water of the multi-stage membrane separation device, and the interstage feed water is also connected to the pH adjustment reagent dosing device.

[0060] In the multi-stage membrane separation device, the permeate from each stage of the membrane separation device serves as the feed water for the next stage. A chelating agent is added and the pH is adjusted in each stage of the feed water. The concentrate from each stage of the membrane separation device is sent to the chelating agent recovery unit. Through the multi-stage membrane separation device, the magnesium-to-lithium ratio in the permeate from each stage of the membrane separation device is gradually reduced, and the purity of lithium ions in the permeate is improved. After the multi-stage membrane separation process, the total lithium-magnesium separation coefficient can reach over 2000, and even more preferably over 8000.

[0061] (III) Beneficial Effects

[0062] The technical advantages of this invention are as follows: By adding a chelating agent to a mixed salt solution and adjusting the pH to form a stable chelate, and then coupling it with a membrane separation process, the difference between magnesium and lithium ions (and other divalent ions and lithium ions) is significantly expanded. The size sieving effect and electrostatic repulsion effect (the membrane carries a negative charge) of the membrane are used to achieve efficient separation of magnesium and lithium from salt lakes. This broadens the range of magnesium-to-lithium ratios, total salt concentrations, and membrane materials that can be selected in existing membrane lithium extraction processes, significantly reducing energy consumption, saving freshwater resources, and reducing the cost of supporting facilities such as the initial magnesium removal unit and subsequent deep magnesium removal units. Simultaneously, the dechelation reaction can be achieved by adjusting the pH of the membrane filtration concentrate. Through dechelation, the chelating agent bound to magnesium is released, enabling efficient recovery and utilization of the chelating agent, saving reagent costs and reducing pollution emissions.

[0063] Due to the electrostatic repulsion between magnesium chelates, they exhibit high dispersion in solution, and the chelates also possess good stability. During the dechelation and recovery of the chelating agent, the addition of hydroxide ions causes the magnesium chelates to slowly release monodisperse magnesium ions. Therefore, after dechelation, highly dispersed nano-sized magnesium hydroxide products can be obtained, effectively solving the problems of magnesium hazard (drastic increase in the magnesium-to-lithium ratio and ecological damage in salt lake resources) and the waste of large amounts of magnesium resources caused by the return of magnesium-rich concentrated solution after lithium extraction to salt lakes. Nano-sized magnesium hydroxide products have high application value and are added as flame retardant functional materials in many cable materials and flame retardant coatings.

[0064] The solution of this invention has the advantages of low requirement for magnesium-lithium ratio in mixed salt solutions, wide applicability, high lithium-magnesium separation efficiency, wide range of membrane material selection, low freshwater demand, small fixed investment, and low operating cost. It specifically solves the problems of existing membrane processes, such as stringent requirements for magnesium-lithium ratio and initial ion concentration in feed water, low magnesium-lithium separation efficiency and lithium recovery rate, high energy consumption, and large freshwater consumption. Attached Figure Description

[0065] Figure 1 This is a schematic diagram of a system embodiment 1 for efficiently extracting lithium from a mixed salt solution according to the present invention.

[0066] Figure 2 This is a schematic diagram of a system embodiment 2 for efficiently extracting lithium from a mixed salt solution according to the present invention. Detailed Implementation

[0067] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0068] System Implementation Example 1

[0069] like Figure 1 The diagram shows a schematic representation of an embodiment 1 of the system for efficient lithium extraction from a mixed salt solution according to the present invention. The system includes, in sequence, a mixing unit 1, a first acid-base adjustment unit 2, a membrane separation unit 3, a concentration and enrichment unit 4, and a lithium precipitation unit 5. One end of the mixing unit 1 is the inlet of the mixed salt solution, and the other end is connected to the first acid-base adjustment unit 2. The mixing unit 1 is also equipped with a chelating agent dosing device 11. The membrane separation unit 3 employs a single-stage membrane separation device and includes a membrane filtration permeate side and a membrane filtration concentrate side. The permeate side is connected to the concentration and enrichment unit 4 to concentrate and enrich the membrane filtration permeate to obtain a lithium-rich concentrate. The lithium-rich concentrate enters the lithium precipitation unit 5 to generate lithium salts such as lithium carbonate or lithium phosphate. The membrane filtration concentrate side of the membrane separation unit 3 is connected to a chelating agent recovery unit 6. The chelating agent recovery unit 6 includes a second acid-base adjustment unit 61 and a solid-liquid separation device 62. The membrane filtration concentrate produced by the membrane separation unit 3 enters the second acid-base adjustment unit 61. An alkaline reagent is added to the second acid-base adjustment unit 61 to adjust the pH, triggering a dechelation reaction. After the dechelation reaction is complete, the concentrate is separated by the solid-liquid separation device 62 to obtain the chelating agent recovery solution. The solid-liquid separation device 62 is connected to a concentration and desalination unit 7. The concentration and desalination unit 7 uses membrane concentration and desalination to obtain a concentrated chelating agent recovery solution. The concentration and desalination unit 7 is a nanofiltration membrane with a loose separation functional layer. This nanofiltration membrane is used to increase the chelating agent concentration, thus producing the concentrated chelating agent recovery solution. The concentrate side of the concentration and desalination unit 7 is connected to a mixing unit 1 for recycling the concentrated chelating agent recovery solution back into the mixing unit 1. Both the first acid-base adjustment unit 2 and the second acid-base adjustment unit 61 are equipped with a pH adjustment reagent dosing device 8. The mixing unit 1 is a pipeline mixer equipped with a flow rate control device to control the feed rate of the mixed salt solution, ensuring a flow rate range of 0.1-2 m / s, preferably 0.5-1.5 m / s. Excessive flow rate hinders the uniform mixing of the chelating agent and its sufficient reaction with divalent or polyvalent cations. The purified water or low-concentration brine produced on the freshwater side of the concentration and enrichment unit 4 and the concentration and desalination unit 7 can be used to prepare pH adjustment reagents, dilute membrane feed water, or prepare chelating agent solutions.

[0070] The mixing unit 1 is equipped with an online ion concentration detection device to adjust the dosage of the chelating agent based on the concentration of divalent and polyvalent cations in the mixed salt solution entering the mixing unit. The first acid-base adjustment unit 2 and the second acid-base adjustment unit 61 can also be equipped with pH detection devices to monitor pH in real time and prevent overdosing.

[0071] In addition, the second acid-base adjustment unit 61 is also equipped with a magnesium ion concentration detection device for real-time detection of magnesium ion concentration and for adjusting the concentration based on the dissociable OH groups. - Add an alkaline reagent at a molar ratio of 1.8 to 3.8 of magnesium ions in the membrane filtration concentrate to promote the complete conversion of magnesium ions into magnesium hydroxide precipitate.

[0072] System Implementation Example 2

[0073] See Figure 2 The diagram shows a schematic representation of Embodiment 2 of the system for efficient lithium extraction from a mixed salt solution according to the present invention. Unlike Embodiment 1, this embodiment employs a multi-stage membrane separation unit 3. The multi-stage membrane separation unit comprises membrane separation devices connected in series, with the permeate from each stage serving as the feed water for the next stage. This progressively reduces the magnesium-to-lithium ratio in the permeate and increases the purity of lithium ions in the permeate. The concentrate from each stage enters the chelating agent recovery unit 6 for dechelation and chelating agent recovery, thereby achieving efficient chelating agent recovery, with a recovery rate exceeding 95%.

[0074] In addition, the brine undergoes pretreatment before entering mixing unit 1 to remove organic matter and particulate matter (insoluble solids), ensuring a smoother subsequent treatment process and reducing membrane fouling and clogging. Pretreatment also removes some sulfate ions from the water. Pretreatment can employ microfiltration membranes, loose nanofiltration membranes, or a combination thereof. Microfiltration membranes primarily remove large organic molecules and insoluble matter, while loose nanofiltration membranes primarily remove sulfate ions.

[0075] To ensure a high magnesium-lithium separation coefficient for each stage of the membrane separation unit, a pre-chelation reaction is required for the feed water of each stage. Therefore, in this embodiment, the concentration and desalination unit 7 can be connected to the membrane separation unit 3 to add chelating agents to the interstage feed water of the multi-stage membrane separation unit. In addition, the interstage feed water is also connected to a pH adjustment reagent dosing device 8.

[0076] The following examples further illustrate the concept and technical effects of the present invention; in the following examples, the magnesium-lithium ratio refers to the mass ratio.

[0077] Process Example 1

[0078] This embodiment treats sulfate-containing salt lake brine that is low in magnesium-to-lithium ratio (0-40) and low in lithium (0-1 g / L). The chelating agent used is the small-molecule organophosphonic acid chelating agent DETPMP-5Na. The membrane module used is a single-stage nanofiltration unit (MWCO-500). Before entering the membrane module, the brine is pretreated with a macroporous, loosely porous nanofiltration membrane (MWCO-1000) to partially reduce the sulfate ion concentration and magnesium-to-lithium ratio in the brine. The main parameters of this embodiment are as follows:

[0079] (1) In this embodiment, the mass concentration of lithium ions in the salt lake brine is 0.4 g / L, the magnesium-lithium ratio is 15:1, and the pH value of the brine is 6.9.

[0080] (2) The brine was first pretreated using a macroporous loose nanofiltration membrane (MWCO-1000) to achieve a lithium ion concentration of 0.416 g / L. Then, a chelating agent, DETPMP-5Na (MW: 711.2 Da), was added to the pretreated brine using a pipeline mixer. The brine flow rate in the pipeline mixer was 0.8 m / s, and the molar ratio of DETPMP-5Na dosage to magnesium ions in the pipeline mixer per unit time was 0.7-0.8:1. At this point, the pH of the brine was 7-9. The thoroughly mixed chelated brine was pumped into the first acid-base adjustment unit, where sodium hydroxide was added to promote the chelation stability of DETPMP with divalent and polyvalent cations in the brine until the pH of the brine reached 8.5-9.3. At this point, the lithium ion concentration was 0.4092 g / L, and the magnesium-lithium ratio was 10.264:1.

[0081] (3) The membrane separation unit is a single-stage nanofiltration membrane. The nanofiltration membrane is a polyamide (PA) with a molecular weight cutoff of 500. The effective filtration area is 7 square meters, the operating temperature is 18-24℃, the membrane filtration pressure is 1-1.5 MPa, and the membrane type is a spiral wound membrane. The membrane separation process adopts cross-flow filtration, the brine flow rate is 34 L / h, and the product water to feed water volume ratio of the nanofiltration membrane is controlled at (7-12):25. After 15 days of continuous operation, the membrane flux of the nanofiltration unit stabilizes at 7.5 L·m -2 ·h -1 ·bar -1 .

[0082] Sampling analysis, the composition and separation coefficient of each liquid are shown in Table 1:

[0083] Table 1:

[0084] Liquid flow <![CDATA[Li + g / L]]> <![CDATA[Mg 2+ g / L]]> <![CDATA[Ca 2+ g / L]]> <![CDATA[SO4 2- g / L]]> brine 0.40 6.00 3.50 8.05 Pretreated product water 0.41 4.20 2.28 2.01 Membrane filtration concentrate 0.34 8.39 4.55 3.92 Membrane filtration water production 0.48 0.009 0.00 0.11 Membrane separation coefficient / 546.34 / 21.39

[0085] As shown in Table 1, the content of magnesium and calcium ions in the membrane filtration permeate is significantly reduced. The magnesium-to-lithium ratio in the brine decreases from 15 to 0.01875 in the membrane filtration permeate, and the total lithium-magnesium separation coefficient (total separation coefficient = brine magnesium-to-lithium ratio / membrane unit effluent magnesium-to-lithium ratio) is 800. Therefore, this embodiment can efficiently separate magnesium and lithium from the brine, enriching the lithium concentration in the membrane filtration permeate, which is beneficial for the extraction of lithium ions from salt lake brine. Furthermore, the chelated brine does not require dilution, which is advantageous for its application and promotion in salt lake areas where freshwater is scarce.

[0086] (4) The concentrate from the nanofiltration membrane was pumped into the second acid-base adjustment unit, and sodium hydroxide was added to initiate a dechelation reaction. The molar ratio of hydroxide ions added to magnesium ions in the concentrate was 2:1, and the pH was 12.15-12.40. To promote the dechelation reaction, the temperature was controlled at 27-30℃, and the stirring speed was 1500 rpm. The chelating agent recovery solution and precipitate were further separated using a bag filter. The recovery rate of DETPMP was 97.45%, and the recovery rate of nano-sized magnesium hydroxide was 98.01%. The salt concentration of the chelating agent recovery solution was then reduced by loosening the nanofiltration membrane, and the DETPMP was concentrated and returned to the pipeline mixer.

[0087] (5) The membrane filtration permeate obtained in step (3) is sent to the forward osmosis and MVR system for concentration at a temperature of 50°C and a vacuum of 0.05 MPa. The concentration is 55 times, and sodium and potassium salts are precipitated in steps. The lithium ion concentration in the lithium-rich concentrate is 26.4 g / L and the magnesium-lithium ratio is 0.015 (magnesium is partially crystallized during the concentration process).

[0088] Comparative Example 1

[0089] The brine treated in this comparative example and Example 1 was a low-lithium brine (0-1 g / L) with a low magnesium-to-lithium ratio (0-40). The membrane module used was a single-stage nanofiltration unit (MWCO-500). Similar to Example 1, a macroporous loose nanofiltration membrane (MWCO-1000) was first used to pretreat the brine to partially reduce the sulfate ion concentration and magnesium-to-lithium ratio. The main parameters are as follows:

[0090] (1) In this embodiment, the mass concentration of lithium ions in the salt lake brine is 0.4 g / L, the magnesium-lithium ratio is 15:1, and the pH value of the brine is 6.9.

[0091] (2) The brine was first pretreated using a macroporous loose nanofiltration membrane (MWCO-1000) to achieve a lithium ion concentration of 0.4092 g / L. The pretreated brine was then diluted and pumped into the first acid-base adjustment unit to adjust the pH to 8.5-9.3. After a 5.115-fold dilution, the lithium ion concentration in the brine was 0.08 g / L, and the magnesium-to-lithium ratio was 10.264:1. The brine from the first acid-base adjustment unit was then pumped into the membrane separation unit for lithium and magnesium ion separation. The membrane separation unit used a polyamide (PA) nanofiltration membrane with a molecular weight cutoff of 500. The effective filtration area of ​​the membrane filtration unit was 7 m². 2 The temperature was 18-24℃, the filtration pressure was 1-1.5MPa, and the membrane type was spiral wound. Cross-flow filtration was used, the brine flow rate was 34L / h, and the permeate to feed volume ratio was controlled at (7-12):25. After 15 days of continuous operation, the membrane flux of the nanofiltration unit stabilized at 11.3L·m³. -2 ·h -1 ·bar -1 .

[0092] Sampling analysis, the composition and separation coefficient of each liquid are shown in Table 2:

[0093] Table 2:

[0094]

[0095] As can be seen from Table 2, the magnesium-to-lithium ratio in the membrane filtration permeate is reduced to some extent compared to the brine, but it is still as high as 1.5833. The total lithium-magnesium separation coefficient (total separation coefficient = brine magnesium-to-lithium ratio / membrane unit effluent magnesium-to-lithium ratio) is 9.47, which is difficult to meet the magnesium-to-lithium ratio requirements for preparing industrial-grade or battery-grade lithium carbonate products. It is necessary to add multi-stage membrane filtration or subsequent deep magnesium removal units to reduce the magnesium-to-lithium ratio, which will lead to a sharp increase in the overall process cost.

[0096] As shown in Example 1 and Comparative Example 1, the solution of the present invention does not require dilution with fresh water throughout the entire process and can effectively treat high-concentration brine. Compared with the high magnesium-to-lithium mass ratio in the brine produced in Comparative Example 1 (1.5833) and the overall separation coefficient (9.47), the magnesium-to-lithium ratio in the brine produced in Example 1 decreased to 0.01875 after the addition of the chelating agent, and the separation coefficient reached as high as 800. This demonstrates that the solution of the present invention can effectively treat high-salinity brine and significantly reduce the magnesium-to-lithium ratio in the brine, exhibiting an extremely high separation coefficient for magnesium and lithium in the brine.

[0097] Process Example 2

[0098] The sulfate-free brine treated in this embodiment is a low-lithium brine (0-1 g / L) with a low magnesium-to-lithium ratio (0-40). The chelating agent used is the small-molecule aminocarboxylic acid chelating agent EGTA-4Na, and the membrane module used is a single-stage nanofiltration unit (MWCO-300). The main parameters of this embodiment are as follows:

[0099] (1) In this embodiment, the mass concentration of lithium ions in the salt lake brine is 0.17 g / L, the magnesium-to-lithium ratio is 27:1, and the pH value of the brine is 6.9. Brine without sulfate ions does not require pretreatment.

[0100] (2) Add chelating agent EGTA-4Na (MW: 472.35 Da) to the brine using a pipe mixer. The brine flow rate in the pipe mixer is 0.5 m / s. The molar ratio of EGTA-4Na dosage to magnesium ions in the pipe mixer per unit time is (0.8-1.2):1. At this time, the pH of the brine is 8.6-9.5. Pump the fully mixed chelated brine into the first acid-base adjustment unit and add sodium hydroxide to promote the chelation stability of EGTA-4Na with divalent and polyvalent cations in the brine until the pH of the brine is 10. At this time, the lithium ion concentration is 0.15 g / L and the magnesium-lithium ratio is 27:1.

[0101] (3) The membrane separation unit is a single-stage nanofiltration membrane. The nanofiltration membrane is a polyamide (PA) with a molecular weight cutoff of 300. The effective filtration area is 7 square meters, the operating temperature is 18-24℃, the membrane filtration pressure is 1-1.5 MPa, and the membrane type is a flat sheet membrane. The membrane separation process adopts cross-flow filtration, the brine flow rate is 40 L / h, and the product water to feed water volume ratio of the nanofiltration membrane is controlled at 3.5:1. After 25 days of continuous operation, the membrane flux of the nanofiltration unit stabilizes at 3.7 L·m -2 ·h -1 ·bar -1 .

[0102] Sampling analysis, the composition and separation coefficient of each liquid are shown in Table 1:

[0103] Table 3:

[0104]

[0105]

[0106] As shown in Table 1, the content of magnesium and calcium ions in the membrane filtration permeate was significantly reduced. The magnesium-to-lithium ratio in the chelated brine decreased from 27 to 0.03529 in the membrane filtration permeate, and the total lithium-magnesium separation coefficient (total separation coefficient = brine magnesium-to-lithium ratio / membrane unit effluent magnesium-to-lithium ratio) was 765.00. Therefore, this embodiment can efficiently separate magnesium and lithium from brine, enriching the lithium concentration in the membrane filtration permeate, which is beneficial for the extraction of lithium ions from salt lake brine. Furthermore, the brine does not require dilution with fresh water, which is advantageous for its application and promotion in salt lake areas where fresh water is scarce.

[0107] (4) The concentrate from the nanofiltration membrane was pumped into the second acid-base adjustment unit, and sodium hydroxide was added to initiate a dechelation reaction. The molar ratio of hydroxide ions added to magnesium ions in the concentrate was 2.07:1, and the pH was 12.7-13.1. To promote the dechelation reaction, the temperature was controlled at 30-34℃, and the stirring speed was 2000 rpm. The chelating agent recovery solution and precipitate were further separated using a plate and frame filter. The recovery rate of EGTA was 99.15%, and the recovery rate of nano-sized magnesium hydroxide was 99.41%. Subsequently, the salt concentration of the chelating agent recovery solution was reduced by loosening the nanofiltration membrane, the EGTA was concentrated, and then returned to the pipeline mixer.

[0108] (5) The membrane filtration permeate obtained in step (3) is sent to the forward osmosis and MVR system for concentration at a temperature of 65°C and a vacuum of 0.08 MPa. The concentration is 60 times, and sodium and potassium salts are precipitated in steps. The lithium ion concentration in the lithium-rich concentrate is 10.26 g / L and the magnesium-lithium ratio is 0.030 (magnesium is partially crystallized during the concentration process).

[0109] Process Example 3

[0110] The sulfate-containing brine treated in this embodiment is a low-lithium brine (0-1 g / L) with a high magnesium-to-lithium ratio (>40). The chelating agents used are amino acid-based and hydroxycarboxylic acid-based chelating agents, and the membrane module used is a nanofiltration membrane (MWCO: 800 Da) + nanofiltration membrane separation (MWCO: 500 Da). The main parameters of this embodiment are as follows:

[0111] (1) In this embodiment, the mass concentration of lithium ions in the salt lake brine is 0.48 g / L, the magnesium-to-lithium ratio is 100:1, and the pH value of the brine is 6.5.

[0112] (2) First, the brine is pretreated using a macroporous loose nanofiltration membrane (MWCO-1000). The pretreatment process can reduce the concentration of sulfate and magnesium ions and appropriately increase the concentration of lithium ions.

[0113] Sodium methionine (MW: 398.44) was added to the pretreated brine at a flow rate of 1.2 m / s. The molar ratio of sodium methionine dosage to magnesium ion content in the pipeline mixer per unit time was (0.7-0.8):1, and the pH of the brine was 7.5-8.0. The thoroughly mixed chelated pretreated brine was pumped into the first acid-base adjustment unit, and acid was added to promote the chelation effect of sodium methionine with divalent and polyvalent cations in the brine until the pH of the brine reached 7.0-7.5. Due to the small amounts of sodium methionine and acid added, the lithium ion concentration in the chelated pretreated brine (membrane unit inlet) became 0.5 g / L, and the magnesium-lithium ratio was 80:1.

[0114] (3) The membrane separation unit consists of nanofiltration units connected in series. The first-stage nanofiltration unit consists of a polyamide (PA) nanofiltration membrane with a molecular weight cutoff of 800, and the second-stage nanofiltration unit consists of a polyamide (PA) nanofiltration membrane with a molecular weight cutoff of 500. The effective filtration areas of the two membranes are 7m² and 7m², respectively. 2 The operating temperature is 16-22℃. The first-stage nanofiltration unit has a filtration pressure of 3.4-4.6 MPa, and the nanofiltration unit pressure is 2.5-3.0 MPa. All membranes are spiral wound membranes. The membrane separation process uses cross-flow filtration with a brine flow rate of 22 L / h. To increase lithium ion recovery and separation efficiency, the ratio of permeate to feed volume is controlled at 4:1.

[0115] During the separation process, the chelated pretreated brine is directly pumped into the first-stage nanofiltration separation unit, and the nanofiltration permeate is used as feed water into the second-stage nanofiltration membrane. The concentrations of chelating agents and divalent or polyvalent cations in the nanofiltration unit feed water are monitored regularly, and sodium linolenic acid salt (MW: 278.43), a hydroxycarboxylic acid chelating agent, is added to the feed water (effluent from the first-stage membrane unit) in real time. The optimal molar ratio of chelating agent to magnesium ion concentration is (0.1-0.3):1. The membrane filtration concentrate produced by the two-stage membrane units is pumped into the second acid-base adjustment unit for dechelation reaction. After 60 days of continuous operation, the membrane permeate flux of the two-stage membrane units stabilized at 12.4 and 5.5 L·m⁻¹, respectively. -2 ·h -1 ·bar -1 Sampling analysis revealed the composition and separation coefficients of each liquid sample, as shown in Table 4.

[0116] Table 4:

[0117] Liquid flow <![CDATA[Li + g / L]]> <![CDATA[Mg 2+ g / L]]> <![CDATA[Ca 2+ g / L]]> <![CDATA[SO4 2- g / L]]> Salt lake brine 0.48 48.00 0.11 158.57 Pretreated product water 0.50 40.00 0.06 47.57 First-stage nanofiltration membrane filters the concentrated water. 0.34 170.28 0.26 209.81 First-stage nanofiltration membrane filtration produces water. 0.54 7.43 0.01 7.01 First-stage nanofiltration separation coefficient / 5.81 6.48 7.33 Second-stage nanofiltration membrane filters the concentrate. 0.38 37.03 0.05 33.65 Second nanofiltration membrane filtration produces water 0.58 0.03 0.00 0.35 Second-stage nanofiltration separation coefficient / 266.01 / 21.51

[0118] As shown in Table 4, after the brine is treated by two stages of nanofiltration membrane filtration units, the magnesium ion content in the membrane filtration permeate is significantly reduced. The magnesium-to-lithium ratio decreases from 100 in the brine to 0.0517 in the second-stage membrane filtration permeate, and the total lithium-magnesium separation coefficient (total separation coefficient = brine magnesium-to-lithium ratio / membrane unit effluent magnesium-to-lithium ratio) is 1933.33. This demonstrates that the present invention achieves a very high separation coefficient for magnesium and lithium. In this embodiment, no freshwater or recycled purified water is used to dilute the brine and the nanofiltration unit feedwater, and no freshwater resources are consumed during the filtration process, which is beneficial for the application and promotion of this technology in salt lake areas where freshwater is scarce.

[0119] (4) The concentrated water from the two-stage membrane filtration was pumped into the second acid-base adjustment unit, and sodium hydroxide was added to induce a dechelation reaction. The molar ratio of hydroxide ions added to magnesium ions in the concentrated water was 2:1. At this point, the pH values ​​of the solutions were 12.58-13.01 and 11.51-12.03, respectively. The temperature of the second acid-base adjustment unit was controlled at 27°C, and the stirring speed was 1100 rpm. The solution after the reaction was separated using a bag filter to obtain the chelating agent recovery solution and the precipitate. The recovery rates of sodium methionine and sodium linoleate were 98.47% and 99.17%, respectively, and petal-shaped magnesium hydroxide with a particle size of about 300 nm was obtained. The yields of nano-sized magnesium hydroxide produced from the concentrated water from the two-stage membrane filtration were 96.17% and 99.60%, respectively. Then, the salt concentration of the chelating agent recovery solution was reduced by loosening the nanofiltration membrane, and the sodium methionine and sodium linoleate solutions were concentrated and returned to the pipeline mixer or the feed water of the second-stage membrane of the membrane separation unit.

[0120] (5) The membrane filtration product obtained from the second-stage nanofiltration membrane in step (3) is sent to the reverse osmosis and MVR system for concentration. The temperature is 75℃, the vacuum degree is 0.01Mpa, the concentration factor is 54.827, the mass concentration of lithium ions in the lithium-rich concentrate is 31.8g / L, and the magnesium-lithium ratio is 0.0510 (after concentration, some magnesium crystals further reduce the magnesium-lithium ratio).

[0121] Process Example 4

[0122] The sulfate-containing brine treated in this embodiment is a high-lithium brine (>1 g / L) with a high magnesium-to-lithium ratio (>40). It utilizes a combination of a water-soluble polymeric chelating agent—polyacrylamide (PAM, MW: 100000)—and a macromolecular polycarboxylic acid chelating agent—poly(methacrylic acid) (PMMA, MW: 100000). The membrane module used is a two-stage ultrafiltration (MWCO 50000) + a single-stage nanofiltration (MWCO: 1000). The main parameters of this embodiment are as follows:

[0123] (1) In this embodiment, the mass concentration of lithium ions in the salt lake brine is 1.90 g / L, the magnesium-lithium ratio (by mass) is 71.789:1, and the pH value of the brine is 6.43.

[0124] (2) First, the brine is pretreated using a macroporous loose nanofiltration membrane (MWCO-1500). The pretreatment process can reduce the concentration of sulfate and magnesium ions and appropriately increase the concentration of lithium ions.

[0125] Then, a water-soluble polymeric chelating agent, polyacrylamide (PAM, MW: 100000), was added to the pretreated brine. The brine flow rate was 1.5 m / s, and the molar ratio of magnesium ions in the pipeline mixer per unit time was (0.01-0.02):1. At this point, the pH of the brine was 7.5-8.1. The thoroughly mixed chelated pretreated brine was pumped into the first acid-base adjustment unit, where sodium hydroxide was added to promote the chelation effect of PAM with divalent and polyvalent cations in the brine until the pH of the brine reached 9.3-9.7. The lithium ion concentration in the chelated pretreated brine was 1.996 g / L, and the magnesium-to-lithium ratio was 64:1.

[0126] (3) The membrane separation unit consists of two-stage ultrafiltration and one-stage nanofiltration units connected in series. The ultrafiltration unit is composed of polysulfone (PsF) ultrafiltration membranes with a molecular weight cutoff of 50,000, while the nanofiltration unit is composed of polyamide (PA) nanofiltration membranes with a molecular weight cutoff of 1,000. The effective filtration area of ​​each membrane filtration unit is 7m². 2 The operating temperature is 19-25℃, and the operating pressures for each stage of membrane filtration are 0.4-1MPa, 0.4-0.7MPa, and 1-1.5MPa, respectively. Hollow fiber membranes are used for all stages. Cross-flow filtration is used in the membrane separation process, with a brine flow rate of 45L / h. The volume ratio of permeate to feed liquid in each stage of membrane filtration is controlled at 4:1.

[0127] During the separation process, the chelated pretreated brine is directly pumped into the first-stage ultrafiltration separation, and the ultrafiltration permeate is used as feed water into the second-stage ultrafiltration membrane. The concentrations of chelating agent and divalent or polyvalent cations in the feed water of the second-stage ultrafiltration unit are monitored regularly. A large-molecule polycarboxylic acid chelating agent—poly(methacrylic acid) (PMMA, MW: 100000)—is added to the feed water in real time. The optimal molar ratio of chelating agent to magnesium ion concentration is (0.026-0.03):1, and the pH is adjusted to 10.1-10.5 to ensure the separation effect of the second-stage ultrafiltration membrane. The second-stage ultrafiltration permeate enters the nanofiltration unit to obtain nanofiltration permeate and nanofiltration concentrate. The concentrate from each membrane filtration stage is pumped into the second acid-base adjustment unit for dechelation reaction. After 25 days of continuous operation, the membrane permeate flux of the ultrafiltration and nanofiltration units stabilized at 27.52, 26.37, and 14.37 L·m⁻¹, respectively. -2 ·h -1 ·bar -1 Sampling analysis revealed the composition and separation coefficients of each liquid sample, as shown in Table 5.

[0128] Table 5:

[0129] Liquid flow <![CDATA[Li + g / L]]> <![CDATA[Mg 2+ g / L]]> <![CDATA[Ca 2+ g / L]]> <![CDATA[SO4 2- g / L]]> brine 1.90 136.4 0.10 40.60 Pretreated product water 2.00 127.74 0.04 11.60 First-stage ultrafiltration membrane filters the concentrated water. 1.29 485.40 0.20 13.48 First-stage ultrafiltration membrane filtration produces water. 2.17 38.33 0.001 11.13 First-stage ultrafiltration separation coefficient / 3.63 43.40 1.13 Second-stage ultrafiltration membrane filters the concentrated water. 1.42 168.63 0.005 12.41 Second-stage ultrafiltration membrane filtration produces water. 2.36 5.75 0.00 10.81 Second-stage ultrafiltration separation coefficient / 7.25 / 1.12 Third-stage nanofiltration membrane filters the concentrate. 1.92 28.65 / 52.41 Third-stage nanofiltration membrane filtration produces water. 2.47 0.025 / 0.41 Third-stage nanofiltration separation coefficient / 240.72 / 27.59

[0130] As shown in Table 5, after separation by a three-stage membrane filtration system, the magnesium ion content in the membrane filtration product water is significantly reduced. The magnesium-to-lithium ratio decreases from 71.789 in the brine to 0.0101 in the product water from the third membrane filtration stage. The total lithium-magnesium separation coefficient of the membrane separation unit (total separation coefficient = brine magnesium-to-lithium ratio / membrane unit product water magnesium-to-lithium ratio) is 7092.8. Therefore, the method of this invention can efficiently separate magnesium and lithium from brine. Furthermore, calculations show that the lithium ion yields of the three-stage membrane filtration system are as high as 87.09%, 86.88%, and 83.72%, respectively.

[0131] (4) The concentrated water from the two-stage ultrafiltration unit is pumped into the second acid-base adjustment unit, and sodium hydroxide is added to induce a dechelation reaction. The molar ratio of hydroxide ions added to magnesium ions in the filtered concentrated water is 2.15:1. At this time, the pH of the solution is 11.87-12.04. The temperature of the second acid-base adjustment unit is controlled at 30-35℃, and the stirring speed is 4000 rpm. The solution after the reaction is pumped into an inclined plate filter for separation to obtain the chelating agent recovery solution and precipitate. At this time, the recovery rates of PAM-100000 and PMMA-100000 are 97.18% and 96.92%, respectively. At the same time, nano-sized magnesium hydroxide products are obtained with yields of 98.43% and 97.12%, respectively. Afterward, the salt concentration of the chelating agent recovery solution is reduced by loosening the nanofiltration membrane, and the PAM-100000 and PMMA-100000 solutions are concentrated and returned to the feed water of each stage of the pipeline mixer or membrane separation unit.

[0132] (5) The water filtered by the third-stage nanofiltration membrane in step (3) is sent to the forward osmosis and MVR system for concentration. The concentration is 13.1 times, the temperature is 45℃, the vacuum degree is 0.03Mpa, the mass concentration of lithium ions in the lithium-rich concentrate is 29.8g / L, and the magnesium-lithium ratio is 0.010.

[0133] Process Example 5

[0134] The salt lake brine processed in this embodiment is Longmu Lake brine, which is a low-lithium brine (0-1 g / L) with a low magnesium-to-lithium ratio (0-40). However, Longmu Lake brine contains potassium ions in addition to magnesium and lithium ions. + Na + Cl - SO4 2- In this embodiment, the chelating agent used is a small molecule aminocarboxylic acid chelating agent - 1,2-cyclohexanediamine tetraethylenetetrasodium (DCTA-4Na, MW: 481.13), and the membrane module is a single-stage nanofiltration membrane (MWCO: 1000). The main parameters of this embodiment are as follows:

[0135] (1) In this embodiment, the lithium ion concentration of the treated Longmu Cuo brine is 0.79 g / L, the magnesium-lithium ratio is 14.22:1, and the pH value of the brine is 6.9.

[0136] (2) The brine was first pretreated using a macroporous loose nanofiltration membrane (MWCO-1500). This pretreatment process reduced the concentrations of sulfate and magnesium ions while appropriately increasing the concentration of lithium ions. Then, 1,2-cyclohexanediamine tetraethylenetetrasodium (DCTA-4Na, MW: 481.13) was added to the pretreated brine using a pipe mixer. To increase the mixing degree between DCTA-4Na and the pretreated brine, the brine flow rate was 3.1 / s. The molar ratio of DCTA-4Na dosage to the divalent cation content (magnesium and calcium ions) in the pipe mixer per unit time was (0.75-1.3):1. At this point, the pH of the brine was 9.5-10.4. The concentration of lithium ions in the pretreated brine was chelated to 0.84 g / L, and the magnesium-to-lithium ratio was 5.87:1.

[0137] (3) The membrane separation unit consists of a single-stage polyamide (PA) loose nanofiltration membrane with a molecular weight cutoff of 600. The effective filtration area of ​​the filtration unit is 7m². 2 The operating temperature is 15-30℃, the pressure is 2.0-4.0 MPa, and a flat sheet membrane is used. The membrane separation process adopts cross-flow filtration, the brine flow rate is 6.0 L / h, and the volume ratio of permeate to feed liquid in the membrane unit is controlled at 1:1.

[0138] During the separation process, the chelated pretreated brine is directly pumped into the nanofiltration unit for separation, while the membrane filtration concentrate is pumped into the second acid-base adjustment unit for dechelation reaction. After 90 days of continuous operation, the membrane permeate flux of the nanofiltration unit stabilized at 3.5 L·m⁻¹. -2 ·h -1 ·bar -1 .

[0139] Sampling analysis, the composition and separation coefficient of each liquid are shown in Table 6:

[0140] Table 6:

[0141]

[0142] As shown in Table 6, using the method of this invention, although only a single-stage nanofiltration separation is performed, the magnesium ion content in the membrane filtration permeate is significantly reduced. The magnesium-to-lithium ratio decreases from 14.22 in the brine to 0.005128 in the nanofiltration permeate, and the overall magnesium-to-lithium separation coefficient is 2771.962. This demonstrates that the method of this invention can achieve very high magnesium-to-lithium separation efficiency even in the presence of multiple divalent ions and monovalent metal cations.

[0143] In this embodiment, no fresh water or recycled purified water is used to dilute the brine and the nanofiltration unit feed water, and no fresh water resources are consumed during the filtration process. Furthermore, calculations show that the single-stage lithium-ion yield is as high as 69.64%, which is beneficial for the application and promotion of this technology in salt lake areas where fresh water is scarce.

[0144] (4) The concentrated water from the membrane unit filtration was pumped into the second acid-base adjustment unit, and sodium hydroxide was added to initiate a dechelation reaction. The molar ratio of hydroxide ions added to magnesium ions in the membrane filtration concentrate was 2.05:1, and the pH was 13-14. To promote the dechelation effect, the temperature of the second acid-base adjustment unit was controlled at 15-30℃, and the stirring speed was 5000 rpm. The solution after the reaction was separated using an inclined plate filter to obtain the chelating agent recovery solution and the precipitate. At this point, the recovery rate of DCTA-4Na was 98.47%. Then, the salt concentration of the chelating agent recovery solution was reduced by loosening the nanofiltration membrane, the DCTA-4Na solution was concentrated, and then returned to the pipeline mixer.

[0145] (5) The membrane filtration permeate from step (3) is sent to the reverse osmosis and MVR system for concentration. The concentration factor is 23.60 times, the mass concentration of lithium ions in the lithium-rich concentrate is 27.61 g / L, and the magnesium-lithium ratio is 0.005.

[0146] As shown in Table 6, the concentration of divalent calcium, magnesium, and sulfate ions in the nanofiltration membrane concentrate increases the purity of sodium and potassium ions in the nanofiltration permeate. The nanofiltration permeate can be treated using phase separation technology via an MVR system to precipitate sodium and potassium salts in the concentrate as crystals. During the evaporation process in the MVR system, the temperature is controlled at 30-90℃, and the vacuum degree is 0-0.5 MPa, preferably 0.05-0.1 MPa, to reduce the amount of lithium salt crystals precipitated, resulting in higher purity sodium or magnesium salts and improving lithium yield.

[0147] The above embodiments are all preferred embodiments designed based on the concept of the present invention and specifically according to the characteristics of the brine, including the initial lithium concentration and magnesium-lithium ratio of the brine.

[0148] The chelation-assisted membrane filtration process for high-efficiency lithium extraction from salt lake brine of the present invention can effectively reduce the magnesium-to-lithium ratio in the brine, thereby greatly reducing the magnesium-to-lithium ratio and simplifying the subsequent deep magnesium removal process. The present invention can flexibly combine chelating agents and membrane filtration units according to the lithium ion and magnesium-to-lithium ratio of different salt lakes, and has great flexibility.

[0149] The effects of the technical solution of this invention include:

[0150] (1) The addition of chelating agents can effectively increase the difference between divalent and polyvalent cations and monovalent cations (lithium, sodium, potassium) in salt lake brine, and achieve efficient separation of lithium and magnesium by using membrane size sieving and electrostatic repulsion effect, thus saving equipment investment costs.

[0151] (2) The membrane material selection is wide. Depending on the different influent water quality and the selected chelating agent, both nanofiltration and ultrafiltration membranes can be used in this invention, making the process highly adaptable. Therefore, the membrane separation unit can treat high-concentration brine with a high magnesium-to-lithium ratio, increase lithium flux and lithium purity in the product water, reduce energy consumption and freshwater resource usage, and simplify the lithium-magnesium separation process, which is of great significance to salt lake areas with scarce freshwater.

[0152] (3) The selected chelating agent is environmentally friendly, the recycling process is simple and efficient, and the recycling effect of the chelating agent is stable, which can effectively reduce the amount of reagent added and reduce costs.

[0153] (4) The present invention can be used to treat water with high salt concentration, brine initial concentration and magnesium-lithium ratio range, which can effectively reduce the amount of fresh water used, reduce the load of the later concentration process, thereby reducing the energy consumption of the process and the cost of equipment investment.

[0154] (5) This invention has a mature technical route that is easy to promote. It can be used to directly extract lithium from salt lake brine, or it can be used as a nested separation unit combined with the existing salt field lithium extraction process. It can also be used to treat waste salt solutions containing magnesium, lithium, calcium, potassium and sodium generated in various industrial production processes.

[0155] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for efficiently extracting lithium from a mixed salt solution, characterized by, include: S1. Chelation pretreatment: A chelating agent is added to the mixed salt solution to be treated, and the pH is adjusted to promote the formation of stable chelates between the chelating agent and the divalent or polyvalent cations in the mixed salt solution to obtain influent; the chelating agent is one or more of the following: organophosphonic acids, aminocarboxylic acids, amino acids, hydroxycarboxylic acids, water-soluble polymers, and polycarboxylic acid chelating agents; the magnesium-to-lithium mass ratio of the mixed salt solution is 0.1-1000:1, and the lithium ion mass concentration is 0.01-5 g / L; S2, Membrane Separation: The influent from step S1 is separated by membrane separation. Lithium and magnesium are separated using the membrane pore size sieving mechanism and electrostatic repulsion effect, producing membrane filtration concentrate and membrane filtration permeate. The membrane combination used for membrane separation includes the following three modes: Mode 1: When the lithium ion concentration of the brine to be treated is 0-1.0 g / L and the magnesium-lithium ratio is 0-40, if the brine does not contain sulfate, one or more aminocarboxylic acid chelating agents are added alone or in combination, and further treated with a single-stage nanofiltration device; if the brine contains sulfate, the brine is pretreated with a loose nanofiltration membrane before adding the chelating agent to remove some sulfate ions and reduce the magnesium-lithium ratio, and then one or more organophosphonic acid chelating agents are added alone or in combination, and further treated with a single-stage nanofiltration device. Mode 2: When the lithium ion concentration in the sulfate-containing brine to be treated is 0-1.0 g / L and the magnesium-lithium ratio is greater than 40, or when the lithium ion concentration is greater than 1.0 g / L and the magnesium-lithium ratio is 0-40: First, the brine is pretreated with a loose nanofiltration membrane to remove some of the sulfate ions. Then, one or more amino acid chelating agents are added to the brine alone or in combination and the brine is subjected to primary nanofiltration through a loose nanofiltration membrane. After that, one or more hydroxycarboxylic acid chelating agents are added to the primary nanofiltration product water alone or in combination and the water is subjected to secondary nanofiltration through a compact nanofiltration membrane. Mode 3: When the lithium ion concentration in the sulfate-containing brine to be treated is greater than 1.0 g / L and the magnesium-to-lithium ratio is greater than 40, the brine is first pretreated with a loose nanofiltration membrane to remove some sulfate ions. Then, polycarboxylic acid and / or water-soluble polymer chelating agents are added to the brine and subjected to primary ultrafiltration through an ultrafiltration membrane. The permeate from the primary ultrafiltration is subjected to secondary ultrafiltration or nanofiltration, and the permeate from the secondary ultrafiltration or nanofiltration is subjected to tertiary nanofiltration. Polycarboxylic acid and / or water-soluble polymer chelating agents are added to the influent at each stage as needed. S3, Dechelation reaction: The pH of the membrane filtration concentrate produced in S2 is adjusted to cause the chelates in the membrane filtration concentrate to undergo a dechelation reaction. At the same time, divalent or polyvalent cations combine with hydroxide ions to form precipitates. After solid-liquid separation, high-value hydroxide precipitates and chelating agent recovery liquid are obtained. S4. Chelating agent reuse: The chelating agent recovery solution is reused in step S1; S5. Preparation of lithium-rich concentrate: The membrane filtration permeate produced in S2 is concentrated and enriched to increase its lithium concentration, resulting in a lithium-rich concentrate, which is used to prepare lithium salts.

2. The method of claim 1, wherein, In S1, the chelating agent further includes a dispersant; the pH value of the mixed salt solution is 6-8; the mixed salt solution is surface brine, underground brine, petroleum brine, or industrially produced brine containing Mg. 2+ and Li + Waste brine.

3. The method of claim 1, wherein, In S2, the membrane separation pressure difference is 0.1-5MPa, the inlet water temperature is controlled at 10-50℃, and the volume ratio of membrane filtration permeate to inlet water is controlled at 0.1-5:1; the membrane separation adopts cross-flow filtration, and the flow rate is 5.0-100L / h.

4. The method of claim 1, wherein, In S2, the membrane combination used in the membrane separation process includes membrane module type and membrane material. The membrane module type is one or more of the following: flat sheet membrane, spiral wound membrane, hollow fiber membrane and tubular membrane. The membrane material is one or more of the following: microfiltration membrane, ultrafiltration membrane and nanofiltration membrane.

5. The method of claim 1, wherein, In S3, pH is adjusted by adding an alkaline reagent. The amount of alkaline reagent added is such that it can dissociate OH-. - The molar ratio of magnesium ions in the concentrate from membrane filtration ranges from 1.8 to 2.5; the temperature range for the dechelation reaction is 10-70°C.

6. The method of claim 1, wherein, In S3, after the dechelation reaction is completed, a solid-liquid separation unit is used for solid-liquid separation. The solid-liquid separation unit is one or a combination of a sedimentation device, a plate and frame filter press, and a centrifuge. A certain amount of flocculant is added during the dechelation reaction.

7. The method of claim 1, wherein, S4 further includes: concentrating the chelating agent recovery solution generated in S3 through a membrane, filtering out the salt in the chelating agent recovery solution, increasing the concentration of the chelating agent in the chelating agent recovery solution, and obtaining a concentrated chelating agent recovery solution for reuse in step S1.

8. The method of claim 1, wherein, In S5, during the concentration of membrane filtration permeate, phase separation technology is used to separate sodium and potassium ions from the membrane filtration permeate in a stepwise crystallization manner to obtain sodium and potassium salt products.

9. A system for efficiently extracting lithium from a mixed salt solution, comprising: It includes: The pretreatment unit, mixing unit, first acid-base adjustment unit, membrane separation unit, concentration and enrichment unit, and concentration and desalination unit are connected in sequence. The pretreatment unit is used to remove insoluble substances and sulfate ions from the water to be treated. The pretreatment unit includes a microfiltration membrane and / or a loose nanofiltration membrane. One end of the mixing unit is connected to the effluent from the pretreatment unit, and the other end is connected to the first acid-base adjustment unit; the mixing unit is also equipped with a chelating agent dosing device; The membrane separation unit includes a membrane filtration permeate side and a membrane filtration concentrate side. The membrane filtration permeate side is connected to the concentration and enrichment unit to concentrate and enrich the membrane filtration permeate to obtain a lithium-rich concentrate. The membrane filtration concentrate side is connected to a chelating agent recovery unit. The membrane separation unit employs a single-stage membrane separation device or a multi-stage membrane separation device. The single-stage membrane separation device includes one or more parallel membrane separation devices. The multi-stage membrane separation device includes membrane separation devices connected in series, wherein the permeate from each stage of the membrane separation device serves as the feed water for the next stage of the membrane separation device, and the concentrate from each stage of the membrane separation device enters the chelating agent recovery unit for chelating agent recovery. The chelating agent recovery unit includes a second acid-base adjustment unit and a solid-liquid separation device. The membrane filtration concentrate side is connected to the second acid-base adjustment unit, and the second acid-base adjustment unit is connected to the solid-liquid separation device. The pH of the membrane filtration concentrate is adjusted using the second acid-base adjustment unit to generate a dechelation reaction, and then separated by a solid-liquid separation device to obtain a chelating agent recovery solution. The chelating agent recovery solution is transported to the concentration and desalination unit, where it is concentrated by membrane treatment and desalinated to obtain concentrated chelating agent recovery solution. The concentrated desalination unit is connected to the mixing unit on the concentrate side, which is used to reuse the concentrated chelating agent recovery liquid in the mixing unit; Both the first and second acid-base adjustment units are equipped with pH adjustment reagent dosing devices.

10. The system of claim 9, wherein, The lithium enrichment unit is also connected with a lithium precipitation unit for converting the lithium-rich concentrated solution into lithium salt products.

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