Method and system for efficiently extracting lithium from mixed salt solution

Abstract

The present invention relates to a method and system for efficiently extracting lithium from a mixed salt solution. The method comprises: adding a chelating agent into a mixed salt solution to be treated, and regulating the pH to promote the chelating agent and divalent or multivalent cations in the mixed salt solution to form a stable chelate, so as to obtain influent water; subjecting the influent water to membrane separation, and using a membrane pore size sieving mechanism and electrostatic repulsion to achieve lithium-magnesium separation, so as to generate membrane-filtered concentrated water and membrane-filtered produced water; regulating the pH of the membrane-filtered concentrated water, and subjecting same to solid-liquid separation so as to obtain a high-value hydroxide having a high degree of dispersion and a chelating agent recovery liquid; and performing concentration and enrichment on the membrane-filtered produced water to obtain a lithium-rich concentrated liquid, the lithium-rich concentrated liquid being used for lithium precipitation. The solution of the present invention solves the technical problems of existing membrane process lithium extraction technologies of high requirements for influent water, limited adaptability, large consumption of fresh water, low lithium-magnesium separation factor, low concentration of produced water, low lithium recovery rate, etc.

Description

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

[0001] The present invention relates to the technical field of lithium resource extraction, and in particular to a method and system for efficiently extracting lithium from a mixed salt solution. Background Art

[0002] Lithium and its compounds are widely used in various sectors of the national economy, military industry, and aerospace. In recent years, with the rollout of new electric vehicle products and a significant increase in demand, lithium, as a strategic resource, has increasingly demonstrated significant market potential and application prospects. Lithium is primarily found in lithium ore, brine, and seawater. Salt lake lithium resources account for 58% of the world's proven lithium resources and have gradually become the cornerstone of the global lithium supply. However, the magnesium ion content in salt lake brine is much higher than that of lithium ions. 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 usually refers to liquid minerals with a salt content greater than 5%. Brine that accumulates on the surface is called surface brine or lake brine. Brine that accumulates below the ground is called underground brine. Brine often contains K + 、Na + , Ca 2+ Mg 2+ 、Cl - 、SO4 2- 、CO3 2- 、HCO3 - , B, Li + Br - , I - , Sr, Rb, Cs, and other ions. To date, numerous processes for lithium extraction from brine have been developed, primarily including evaporation and precipitation, calcination and leaching, adsorption, solvent extraction, and membrane separation. Membrane separation is a physical separation method characterized by its 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 a nanofiltration membrane. This method uses nanofiltration technology to separate magnesium and lithium from salt lake brine, and then produces high-purity lithium salts from lithium-rich produced water. However, the separation coefficient of the single-stage membrane process in this patent is low, and the lithium ion content / concentration of the lithium-rich produced water is low. It needs to be further concentrated to increase the lithium ion concentration, which also increases energy consumption costs. In order to improve the recovery rate of lithium, the nanofiltration concentrate is circulated to the nanofiltration influent, resulting in an increase in the magnesium-lithium ratio, and it is necessary to increase the operating pressure of the membrane and the number of nanofiltration stages (increase 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-lithium ratio. The lithium-rich produced water obtained by nanofiltration is sent to a reverse osmosis or forward osmosis unit for concentration, and then the lithium-rich concentrated water after deep magnesium removal is sent to a lithium precipitation unit. However, the magnesium removal effect of the nanofiltration membrane unit in these solutions and the lithium ion concentration in the nanofiltration water are limited, requiring the addition of pre-treatment devices such as adsorption. In addition, the magnesium and lithium content in the water is still high, so it is necessary to add a deep magnesium removal unit and a concentration unit, which increases the equipment investment cost. U.S. Patent US11219864B2 discloses a coupled process using ultrafiltration, nanofiltration, reverse osmosis and electrodialysis to achieve the purpose of efficiently separating and enriching lithium from salt lake brine. However, the separation efficiency of the nanofiltration membrane in this patent is low. Since a large amount of fresh water is required to dilute the membrane feed concentration, the lithium ion concentration in the water is low, and the lithium ion concentration needs to be increased through a concentration unit consisting of reverse osmosis and electrodialysis.

[0005] In summary, the current membrane process lithium extraction technology has several major problems: (1) The magnesium-lithium ratio of the nanofiltration influent is required to be high. An excessively high magnesium-lithium ratio leads to poor economic efficiency of the nanofiltration process, and it is often necessary to add an adsorption unit before filtration to remove part of the magnesium (reduce the magnesium-lithium ratio) in preparation for nanofiltration separation; (2) The lithium-magnesium selectivity of the membrane technology is not high, and the separation coefficient is low, resulting in a low lithium recovery rate and a high magnesium-lithium content in the produced water. The former leads to serious loss of lithium resources and low resource utilization, while the latter requires the addition of multi-stage nanofiltration units or even deep magnesium removal units to improve the purity of the lithium product, which increases the complexity of the process and equipment cost; (3) The total salt concentration that the nanofiltration membrane used for lithium-magnesium separation can withstand is low, and concentration polarization is prone to occur on the membrane surface, resulting in reduced membrane flux and separation efficiency, which increases the complexity of the process; in order to alleviate this phenomenon, a large amount of fresh water is required to dilute the influent (side) by a high multiple, resulting in a large amount of fresh water consumption, which limits the use and promotion of nanofiltration technology in salt lake areas with a shortage of fresh water.

[0006] Therefore, an efficient brine comprehensive utilization scheme that can achieve lithium and magnesium separation and efficient lithium concentration in brine is of great significance for increasing the lithium ion yield of brine and reducing process costs and energy consumption.

[0007] (1) Technical issues to be resolved

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

[0009] (2) 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: adding a chelating agent to the mixed salt solution to be treated, and adjusting the pH to promote the formation of a stable chelate between the chelating agent and the divalent or multivalent cations in the mixed salt solution to obtain influent;

[0012] S2, membrane separation: the influent generated in step S1 is subjected to membrane separation, and lithium and magnesium are separated by utilizing the membrane pore size screening mechanism and electrostatic repulsion effect to generate membrane filtered concentrated water and membrane filtered product water;

[0013] S3, dechelation reaction: adjusting the pH of the membrane filtered concentrated water produced in S2 to cause the chelate in the membrane filtered concentrated water to undergo a dechelation reaction, and at the same time, divalent or multivalent cations combine with hydroxide to form a precipitate, and solid-liquid separation is performed to obtain a high-value hydroxide precipitate and a chelating agent recovery liquid;

[0014] S4, chelating agent concentration and salt reduction: the chelating agent recovery liquid produced in S3 is subjected to membrane concentration to filter out the salt in the chelating agent recovery liquid, thereby increasing the chelating agent concentration in the chelating agent recovery liquid to obtain a concentrated chelating agent recovery liquid for reuse in step S1;

[0015] S5. Preparation of lithium-rich concentrated solution: The membrane filtered water produced in S2 is concentrated and enriched to increase the lithium concentration contained therein to obtain a lithium-rich concentrated solution, which is used to prepare lithium salts.

[0016] In this application, the mixed salt solution includes but is not limited to surface brine (surface brine is also called salt lake brine), underground brine, petroleum brine, or industrial brine containing Mg 2+ 、Li + (May also contain K + 、Na + , Ca 2+ The anion is usually Cl - and / or SO4 2- ) plasma waste brine. The high-value hydroxide obtained in S3 mainly refers to nano-magnesium hydroxide. Therefore, the solution of the present invention can achieve lithium extraction from high-salinity brine and produce high-value-added nano-magnesium hydroxide products as a by-product.

[0017] According to a preferred embodiment of the present invention, in S1, the chelating agent is one or more of organic phosphonic acid, aminocarboxylic acid, amino acid, hydroxycarboxylic acid, water-soluble polymer, polycarboxylic acid chelating agents, or a combination of one or more of the foregoing chelating agents and a dispersant.

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

[0019] When an ionic chelating agent is used, it not only increases the steric hindrance of magnesium but also changes its charge. This synergistic effect of membrane pore size screening and electrostatic repulsion (the membrane is negatively charged) promotes lithium-magnesium separation and improves the magnesium-lithium separation coefficient. The combination of an ionic chelating agent and a water-soluble polymer chelating agent 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-lithium ratio (by mass ratio) 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 according to whether the mixed salt solution contains sulfate ions. The pretreatment unit is mainly used to remove part of the 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 multivalent cations in the mixed salt solution is first detected, and the amount of the chelating agent added is determined according to the test result. Wherein, the molar ratio of the amount of the chelating agent added to the divalent and multivalent cations in the mixed salt solution varies according to the type of the chelating agent. If the chelating agent is a small molecule chelating agent, the molar ratio of the chelating agent to the divalent and multivalent cations in the mixed salt solution is 0.7-1.3:1. If the chelating agent is a macromolecular chelating agent (such as a water-soluble copolymer such as a polycarboxylic acid chelating agent-PMMA), the molar ratio of the chelating agent to the divalent and multivalent cations in the mixed salt solution is 0.01-0.1:1. In S1, the pH range is adjusted to promote the chelating agent to chelate with the divalent and multivalent cations and improve the pH range of the chelate stability. For example, the pH of the mixed salt solution is adjusted to 5-10.5, preferably 8-10. The specific pH range is related to the type and dosage of the selected chelating agent. Different types of chelating agents and different dosages will result in different pH values ​​that are most suitable for promoting chelate formation and chelate stability. During the chelation pretreatment process of step S1, the mixed salt solution can be made into a flowing or agitated state to increase the uniformity of mixing of the chelating agent and the mixed salt solution so that the magnesium ions form chelates as much as possible.

[0023] According to a preferred embodiment of the present invention, in S2, the membrane separation adopts a one-stage membrane separation process or a multi-stage membrane separation process; the one-stage membrane separation process includes one or more parallel membrane separation devices; the multi-stage membrane separation process includes membrane separation devices in a series relationship, wherein the inlet water of each membrane separation device is added with a chelating agent and the pH is adjusted as needed, and the produced water of each membrane separation device is used as the inlet water of the next membrane separation device, and the concentrated water of each membrane separation device is processed according to step S3 to realize the recycling of the chelating agent and reduce the investment cost of the agent. Among them, as needed means that if the ratio of the chelating agent to the divalent and multivalent ions in S1 has met the requirement of complete chelation, then there is no need to add the chelating agent again in this step; otherwise, if it is insufficient, some chelating agent needs to be added.

[0024] The type of chelating agent added to the influent between the two-stage membrane separation device 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 influent between the two-stage membrane separation device, the concentrations of the chelating agent and divalent or multivalent cations in the influent can be detected in time, and a certain amount of chelating agent can be added, and 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 can achieve the purpose of gradually reducing the magnesium-lithium ratio in the produced water of each stage of the membrane separation device and improving the purity of lithium ions in the produced water; after treatment by the multi-stage membrane separation process, the total separation coefficient of lithium and magnesium can reach more than 2000, and preferably more than 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 water production and inlet water is controlled at 0.1-5:1; under the above conditions, efficient separation of magnesium and lithium can be achieved.

[0027] According to a preferred embodiment of the present invention, in S2, membrane separation adopts cross-flow filtration, and the flow rate is 5.0-100L / h, preferably 20-40L / h. Under the above conditions, the influence of concentration polarization on the separation effect can be reduced. The flow rate is determined to be 5.0-100L / h to take into account both the membrane treatment efficiency and the concentration polarization problem; too low a flow rate will lead to low separation efficiency, while too high a flow rate will easily lead to a low magnesium-lithium separation coefficient and may still produce concentration polarization. Cross-flow filtration is to flow in parallel with the membrane surface under the push of a pump. The shear force generated when the influent flows through the membrane surface carries away the particles retained on the membrane surface, thereby keeping the contamination layer at a thin level and reducing the thickness of the boundary layer caused by concentration polarization.

[0028] According to a preferred embodiment of the present invention, in S2, the membrane separation process employs a membrane combination comprising a membrane module type and a membrane material, depending on the type of chelating agent, the magnesium-to-lithium ratio of the mixed salt solution, and the lithium concentration. The membrane module type is a combination of one or more of a flat membrane, a spiral wound membrane, a hollow fiber membrane, and a tubular membrane; and the membrane material is a combination of one or more of a microfiltration membrane, an ultrafiltration membrane, and a nanofiltration membrane. Preferably, the membrane material has a molecular weight cutoff of 100-50,000, more preferably 300-10,000.

[0029] Specifically, the membrane combination used in the membrane separation process can be set according to the magnesium-lithium ratio in the mixed salt solution, the lithium concentration, the type of chelating agent and the properties of the resulting chelate. The more typical situations 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 membrane water production is: 0.01-0.1;

[0031] 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. Then, one or more organic phosphonic acid chelating agents are added alone or in combination, and further treated with a single-stage nanofiltration device. At this time, the total magnesium-lithium separation coefficient is 800-3000, and the magnesium-lithium ratio of the membrane unit water production 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 part of the sulfate, and then one or more amino acid chelating agents are selected to be added to the brine alone or in combination and subjected to primary nanofiltration through a loose nanofiltration membrane, and then one or more hydroxycarboxylic acid chelating agents are selected to be added to the primary nanofiltration water alone or in combination, and subjected to secondary nanofiltration through a tight nanofiltration membrane; at this time, the total magnesium-lithium separation coefficient is 1800-3000, and the membrane water magnesium-lithium ratio 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-lithium ratio is greater than 40, the brine is first pretreated with a loose nanofiltration membrane to remove part of the sulfate, and then a polycarboxylic acid and / or water-soluble polymer chelating agent is selected and added to the brine and subjected to a first-stage ultrafiltration through an ultrafiltration membrane. The first-stage ultrafiltration water is subjected to a second-stage ultrafiltration or nanofiltration, and the second-stage ultrafiltration or nanofiltration water is subjected to a third-stage nanofiltration; polycarboxylic acid and / or water-soluble polymer chelating agent is added to the influent of 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-lithium ratio of the membrane filtration water 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 compounds, as well as increase or decrease the number of membrane filtration stages and membrane combination forms according to the magnesium-lithium ratio in the mixed salt solution, the initial magnesium-lithium concentration, 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 dechelating reaction is 10.5-14, preferably in the range of 11-13, so that the membrane filtered concentrated water contains more hydroxide ions, thereby promoting the combination of divalent or polyvalent metal cations with hydroxide to form a precipitate, thereby obtaining a highly dispersed, high-value hydroxide (mainly highly dispersed nano-magnesium hydroxide). Highly dispersed nano-magnesium hydroxide has extremely high application value and can be used as a flame retardant / smoke suppressant for polymer materials and as an additive for 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 agent, and the amount of the alkaline agent added (in terms of dissociable OH - The molar ratio of magnesium ions in the concentrated water to the membrane filtration is 1.8-2.5, OH - A slight excess of magnesium promotes the formation of high-dispersion, high-value magnesium hydroxide precipitates. Preferably, the dechelating reaction temperature ranges from 10 to 70°C, preferably from 15 to 50°C.

[0037] According to a preferred embodiment of the present invention, in S3, during the dechelating reaction, the membrane process is put into a flowing, stirring or rotating state to increase the dechelating speed.

[0038] According to a preferred embodiment of the present invention, in S3, after the dechelating reaction is completed, solid-liquid separation is performed using a solid-liquid separation unit, wherein 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 S3, a certain amount of flocculant (such as PAM, etc.) or morphology aid (such as PEG) can be added during the dechelating reaction process. The flocculant can promote the precipitation of magnesium hydroxide. The morphology aid can obtain a magnesium hydroxide product with a predetermined microscopic morphology or size, and obtain a higher purity chelating agent recovery liquid, thereby improving the chelating agent reuse rate. The amount of flocculant / morphology aid added can be 0.1-0.5g / L. After the separated magnesium hydroxide precipitate is dried, a highly dispersed nano-scale magnesium hydroxide product is obtained, which solves the magnesium damage problem caused by the existing salt lake lithium extraction process.

[0040] According to a preferred embodiment of the present invention, in step S4, the chelating agent recovery liquid produced in step S3 is further passed through a nanofiltration membrane having a loose separation functional layer to separate salts, thereby increasing the chelating agent concentration in the concentrated water and reducing the salt content, thereby producing a concentrated chelating agent recovery liquid. This process achieves a chelating agent recovery rate of 95-100%. The concentrated chelating agent recovery liquid can be directly reused in step S1 or used to treat the influent of each membrane separation device in 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 filtered water to prepare the lithium-rich concentrate is a combination of one or more of reverse osmosis, forward osmosis, membrane distillation and MVR system.

[0042] According to a preferred embodiment of the present invention, in S5, the lithium-rich concentrate is concentrated 5-60 times compared to the membrane filtration water; 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. Lithium-rich concentrate meeting these conditions has the best economic benefits and can be fed into the subsequent lithium precipitation unit for lithium precipitation. Lithium precipitation uses carbonates, such as sodium carbonate, to produce lithium carbonate, which is then separated into solid and liquid form, washed, and dried to produce the lithium carbonate product.

[0043] According to a preferred embodiment of the present invention, in S5, when the membrane filtration water is concentrated, in order to extract sodium and potassium (sodium and potassium are both monovalent metal cations and easily enter the membrane filtration water together with lithium ions) in the membrane filtration water and improve the purity of the lithium salt product, during the concentration process, phase separation technology can be used for step-by-step removal, and the phase separation technology can be achieved by the MVR system.

[0044] According to phase diagram theory, as the temperature increases, the sodium chloride crystal phase area becomes larger and the solubility of sodium chloride becomes lower. The MVR system can reduce the boiling point of membrane filtration water under vacuum environment, promote the transformation of liquid phase to gas phase in the system, and improve evaporation efficiency. Therefore, the salting-out effect of lithium salt (lithium chloride) on sodium salt and potassium salt (sodium chloride and potassium chloride) can be used to separate sodium ions and potassium ions from the solution in the form of sodium salt and potassium salt crystals. At the same time, by controlling the evaporation temperature and vacuum degree, the crystallization loss of lithium ions can be minimized during the evaporation process. Among them, the temperature of the MVR system is 0-90°C, preferably 30-90°C, and the vacuum degree is 0-0.1MPa, preferably 0.05-0.1MPa.

[0045] Based on the above preferred embodiment, not only can the purity of the lithium salt product be improved, and the recovery rate of lithium ions in the mixed salt solution be increased throughout the entire process, but it also achieves the comprehensive utilization of other resources in the mixed salt solution, obtaining sodium and potassium salt (sodium chloride and potassium chloride) products. Furthermore, the purified water produced by concentrating the membrane filtered water can be recycled, reducing freshwater resource consumption and having environmental benefits.

[0046] In a second aspect, the present invention provides a system for efficiently extracting lithium 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 in the water to be treated, and 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 water outlet of the pretreatment unit, and the other end is connected to the first acid-base adjustment unit; the mixing unit is also provided with a chelating agent dosing device;

[0049] The membrane separation unit includes a membrane filtration water production side and a membrane filtration concentrated water side. The membrane filtration water production side is connected to the concentration and enrichment unit to concentrate and enrich the membrane filtration water production to obtain a lithium-rich concentrated solution; the membrane filtration concentrated water side is connected to a 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 filtered concentrated water 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 membrane filtered concentrated water is pH-adjusted by the second acid-base adjustment unit to produce a dechelating reaction, and is separated by the solid-liquid separation device to obtain a chelating agent recovery liquid;

[0051] The solid-liquid separation device is connected to the concentration and salt reduction unit to perform membrane concentration and desalination on the chelating agent recovery liquid to obtain concentrated chelating agent recovery liquid; the concentrated water side of the concentration and salt reduction unit is connected to the mixing unit to reuse the concentrated chelating agent recovery liquid in the mixing unit;

[0052] The first acid-base regulating unit and the second acid-base regulating unit are both provided with a pH regulating reagent dosing device.

[0053] Preferably, the purified water or brine with lower concentration produced on the fresh water side of the concentration and enrichment unit and the concentration and salt reduction unit can be used to prepare acid-base pH regulating 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, lithium carbonate or lithium phosphate.

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

[0056] Preferably, the second acid-base regulating unit is further provided with a magnesium ion concentration detection device for detecting the magnesium ion concentration and - The alkaline reagent is added in a molar ratio of 1.8-2.5 to the magnesium ions in the concentrated water filtered by the membrane.

[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 uniform mixing of the chelating agent and sufficient reaction with divalent or multivalent 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 a series relationship, wherein the produced water of each stage of the membrane separation device serves as the inlet water of the next stage of the membrane separation device, and the concentrated water of each stage of the membrane separation device enters the second acid-base adjustment unit for dechelating reaction.

[0059] Preferably, the concentration and salt reduction unit is further connected to the membrane separation unit so that the concentrated chelating agent recovery liquid produced by the concentration and salt reduction unit can be added to the interstage influent of the multistage membrane separation device, and the interstage influent is also connected to the pH regulating agent dosing device.

[0060] In the multi-stage membrane separation device, the produced water of each membrane separation device serves as the inlet water of the next membrane separation device, a chelating agent is added to the inlet water of each stage and the pH is adjusted, and the concentrated water of each membrane separation device is transported to the chelating agent recovery unit. Through the multi-stage membrane separation device, the purpose of gradually reducing the magnesium-lithium ratio in the produced water of each membrane separation device and improving the purity of lithium ions in the produced water is achieved; after treatment by the multi-stage membrane separation process, the total lithium-magnesium separation coefficient can reach more than 2000, and more preferably can reach more than 8000. (3) Beneficial effects

[0061] The technical effect of the present invention is that: by adding a chelating agent to the mixed salt solution and adjusting the pH to form a stable chelate, and then coupling it with the membrane separation process, the difference between magnesium and lithium ions (and other divalent ions and lithium ions) is significantly enlarged, and the size screening effect and electrostatic repulsion effect (membrane negative charge) of the membrane are used to separate and realize the efficient separation of magnesium and lithium in salt lakes, broaden the magnesium-lithium ratio, total salt concentration and membrane material selection range of the existing membrane lithium extraction process, significantly reduce energy consumption, save fresh water resources, and reduce the cost and energy consumption of supporting facilities such as the early magnesium removal unit and the subsequent deep magnesium removal unit. At the same time, the dechelation reaction can be achieved by adjusting the pH of the membrane filtered concentrated water, and the chelating agent bound to magnesium is released by dechelation, so that the chelating agent can be efficiently recycled, the reagent cost is saved, and the pollution emission is reduced.

[0062] Due to the electrostatic repulsion between magnesium chelates, the magnesium chelates are highly dispersed in the solution and have good stability. During the dechelation and recovery of the chelating agent, the magnesium chelates slowly release monodisperse magnesium ions with the addition of hydroxide. Therefore, after dechelation, a highly dispersed nano-magnesium hydroxide product can be obtained. This effectively solves the magnesium damage caused by the return of magnesium-rich concentrated solution to the salt lake after lithium extraction (a sharp increase in the magnesium-lithium ratio of salt lake resources and ecological damage) and the waste of a large amount of magnesium resources. Nano-magnesium hydroxide products have high application value and are added as flame retardant functional materials in many cable materials and flame retardant coatings.

[0063] The solution of the present invention has the advantages of low magnesium-lithium ratio requirements for mixed salt solutions, wide applicability, high lithium-magnesium separation efficiency, a wide range of membrane material selection, low fresh water demand, small fixed investment, and low operating costs. It specifically solves the problems of existing membrane processes such as stringent requirements on the influent magnesium-lithium ratio and initial ion concentration, low magnesium-lithium separation efficiency and lithium recovery rate, high energy consumption, and large fresh water consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] FIG1 is a schematic diagram of a system embodiment 1 for efficiently extracting lithium from a mixed salt solution according to the present invention.

[0065] FIG2 is a schematic diagram of a system embodiment 2 for efficiently extracting lithium from a mixed salt solution according to the present invention. DETAILED DESCRIPTION

[0066] In order to better explain the present invention and facilitate understanding, the present invention is described in detail below through specific implementation methods in conjunction with the accompanying drawings.

[0067] System Example 1

[0068] As shown in Figure 1, it is a schematic diagram of a system embodiment 1 for efficiently extracting lithium from a mixed salt solution according to the present invention. The system comprises: 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 connected in sequence. One end of the mixing unit 1 is the water inlet end 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 provided with a chelating agent dosing device 11. The membrane separation unit 3 adopts a primary membrane separation device, and includes a membrane filtration water production side and a membrane filtration concentrated water side. The filtration water production side is connected to the concentration and enrichment unit 4 to concentrate and enrich the membrane filtration water production to obtain a lithium-rich concentrated solution. The lithium-rich concentrated solution enters the lithium precipitation unit 5 to generate lithium salts such as lithium carbonate or lithium phosphate. The membrane filtration concentrated water side of the membrane separation unit 3 is connected to the 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 filtered concentrated water produced by the membrane separation unit 3 enters the second acid-base adjustment unit 61. By adding an alkaline reagent to the second acid-base adjustment unit 61 to adjust the pH, a dechelating reaction is generated. After the dechelating reaction is completed, the chelating agent recovery liquid is separated by the solid-liquid separation device 62. The solid-liquid separation device 62 is connected to the concentration and salt reduction unit 7. The concentration and salt reduction unit 7 is used to perform membrane concentration and desalination on the chelating agent recovery liquid to obtain a concentrated chelating agent recovery liquid. The concentration and salt reduction unit 7 is a nanofiltration membrane with a loose separation functional layer. The nanofiltration membrane is used to increase the chelating agent concentration to obtain a concentrated chelating agent recovery liquid. The concentrated water side of the concentration and salt reduction unit 7 is connected to the mixing unit 1 for recycling the concentrated chelating agent recovery liquid to the mixing unit 1. The first acid-base adjustment unit 2 and the second acid-base adjustment unit 61 are both provided 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, maintaining a flow rate range of 0.1-2 m / s, preferably 0.5-1.5 m / s. Excessive flow rates are detrimental to uniform mixing of the chelating agent and sufficient reaction with divalent or polyvalent cations. The purified water or lower-concentration brine produced by the freshwater side of the concentration and enrichment unit 4 and the concentration and desalination unit 7 can be used to prepare acid-base pH adjustment reagents, dilute membrane feed water, or prepare chelating agent solutions.

[0069] The mixing unit 1 is equipped with an online ion concentration detection device to adjust the dosage of the chelating agent according to the concentration of divalent and multivalent 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 a pH detection device for real-time pH monitoring to avoid overdosing.

[0070] In addition, the second acid-base regulating unit 61 is also provided with a magnesium ion concentration detection device for real-time detection of magnesium ion concentration and the dissociation of OH groups. - An alkaline reagent is added at a molar ratio of 1.8-3.8 to the magnesium ions in the membrane filtered concentrated water to promote the conversion of all magnesium ions into magnesium hydroxide precipitation as much as possible.

[0071] System Example 2

[0072] Referring to FIG2 , there is shown a schematic diagram of a second embodiment of a system for efficiently extracting lithium from a mixed salt solution according to the present invention. Unlike Embodiment 1, the membrane separation unit 3 in this embodiment utilizes a multi-stage membrane separation device. The multi-stage membrane separation device comprises membrane separation devices connected in series, wherein the produced water from each membrane separation device serves as the inlet water for the next membrane separation device, thereby gradually reducing the magnesium-to-lithium ratio in the produced water and improving the lithium ion purity in the produced water. The concentrated water from each membrane separation device enters the chelating agent recovery unit 6 for dechelation and chelating agent recovery, thereby achieving efficient chelating agent recovery, with a chelating agent recovery rate exceeding 95%.

[0073] In addition, the brine is pretreated before entering the mixing unit 1 to remove organic matter and solid particulate matter (solid insoluble matter). This ensures a smoother subsequent treatment process and reduces membrane fouling and clogging. Pretreatment also removes some sulfate ions from the water to be treated. Pretreatment can be performed using a microfiltration membrane, a loose nanofiltration membrane, or a combination thereof. Microfiltration membranes primarily remove large organic molecules and insoluble matter, while loose nanofiltration membranes primarily remove sulfate ions.

[0074] In order to achieve a high magnesium-lithium separation coefficient for each stage of the membrane separation device, it is necessary to pre-chelate the influent of each stage. Therefore, in this embodiment, the concentration and salt reduction unit 7 can be connected to the membrane separation unit 3 to allow the addition of a chelating agent to the interstage influent of the multi-stage membrane separation device. In addition, the interstage influent is also connected to a pH adjustment reagent dosing device 8.

[0075] The following is a further explanation of the concept and technical effects of the present invention in conjunction with process examples; the magnesium-lithium ratio in the following embodiments refers to the mass ratio.

[0076] Process Example 1

[0077] The sulfate-containing salt lake brine treated in this example is low-lithium brine (0-1 g / L) with a low magnesium-to-lithium ratio (0-40). The chelating agent used is the small molecule organic phosphonic acid chelating agent DETPMP-5Na. The membrane assembly used is a single-stage nanofiltration unit (MWCO-500). Before entering the membrane assembly, the brine is pretreated with a macroporous loose nanofiltration membrane (MWCO-1000) to partially reduce the sulfate ion concentration and magnesium-to-lithium ratio in the brine. The main parameters of this example are as follows:

[0078] (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.

[0079] (2) The brine was first pretreated using a macroporous loose nanofiltration membrane (MWCO-1000). The lithium ion concentration in the brine was 0.416 g / L. Then, the 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. The molar ratio of DETPMP-5Na addition to magnesium ions in the pipeline mixer per unit time was 0.7-0.8:1. At this time, the pH of the brine was 7-9. The fully mixed chelated brine was pumped into the first acid-base adjustment unit, and sodium hydroxide was added to promote the chelation stability of DETPMP with divalent and multivalent cations in the brine until the brine pH was 8.5-9.3. At this time, the lithium ion concentration was 0.4092 g / L and the magnesium-lithium ratio was 10.264:1.

[0080] (3) The membrane separation unit is a single-stage nanofiltration membrane. The nanofiltration membrane is polyamide (PA) with a molecular weight cutoff of 500. The effective filtration area is 7 square meters, the operating temperature is 18-24°C, the membrane filtration pressure is 1-1.5 MPa, and the membrane type is a roll membrane. The membrane separation process adopts cross-flow filtration, the brine flow rate is 34 L / h, and the volume ratio of the nanofiltration membrane's water output and water inlet is controlled at (7-12):25. After 15 days of continuous operation, the membrane flux of the nanofiltration unit stabilized at 7.5 L·m -2 ·h -1 bar -1 .

[0081] Sampling analysis, the composition of each feed liquid and the separation coefficient are shown in Table 1:

[0082] Table 1:

[0083] As shown in Table 1, the magnesium and calcium ion contents in the membrane-filtered water are significantly reduced, and the magnesium-to-lithium ratio in the brine is reduced from 15 to 0.01875 in the membrane-filtered water. 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-filtered water, which is beneficial for the extraction of lithium ions from salt lake brine. Furthermore, the chelated brine does not require dilution, which is beneficial for its application and promotion in salt lake areas where fresh water is scarce.

[0084] (4) The concentrated water from the nanofiltration membrane is pumped into the second acid-base adjustment unit, and sodium hydroxide is added to cause a dechelating reaction. The molar ratio of the amount of hydroxide added to the magnesium ion content in the filtered concentrated water is 2:1, and the pH is 12.15-12.40. To promote the dechelating reaction, the temperature is controlled at 27-30°C and the stirring speed is 1500 rpm. A bag filter is further used to separate the chelating agent recovery liquid and the precipitate. The recovery rate of DETPMP is 97.45%, and the recovery rate of nano-magnesium hydroxide is 98.01%. The salt concentration of the chelating agent recovery liquid is then reduced by a loose nanofiltration membrane, and the DETPMP is concentrated and returned to the pipeline mixer.

[0085] (5) The membrane filtered water obtained in step (3) is sent to the forward osmosis and MVR system for concentration at a temperature of 50° C. and concentrated 55 times under a vacuum degree of 0.05 MPa, while sodium salt and potassium salt are precipitated step by step. The mass concentration of lithium ions in the lithium-rich concentrate is 26.4 g / L, and the magnesium-lithium ratio is 0.015 (partial crystallization of magnesium occurs during the concentration process).

[0086] Comparative Example 1

[0087] The salt lake brine treated in this comparative example and Example 1 was low-lithium brine (0-1 g / L) with a low magnesium-to-lithium ratio (0-40). The membrane assembly used was a single-stage nanofiltration unit (MWCO-500). As in Example 1, the brine was pretreated using a macroporous loose nanofiltration membrane (MWCO-1000) to partially reduce the sulfate ion concentration and magnesium-to-lithium ratio in the brine. The main parameters are as follows:

[0088] (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.

[0089] (2) The brine was first pretreated using a macroporous loose nanofiltration membrane (MWCO-1000). The lithium ion concentration in the brine was 0.4092 g / L. The pretreated brine was then diluted and pumped into the first acid-base adjustment unit to adjust the pH of the brine to 8.5-9.3. The lithium ion concentration of the salt lake brine after dilution by 5.115 times was 0.08 g / L, and the magnesium-lithium ratio was 10.264:1. The brine in the first acid-base adjustment unit was pumped into the membrane separation unit for separation of lithium and magnesium ions. The membrane separation unit was a polyamide (PA) nanofiltration membrane with a molecular weight cutoff of 500. The effective filtration area of ​​the membrane filtration unit was 7 m 2 The temperature was 18-24°C, the filtration pressure was 1-1.5 MPa, and the membrane type was a spiral membrane. Cross-flow filtration was used, the brine flow rate was 34 L / h, and the membrane filtration water output and water inlet volume ratio was controlled at (7-12):25. After 15 days of continuous operation, the membrane flux of the nanofiltration unit stabilized at 11.3 L·m -2 ·h -1 bar -1 .

[0090] Sampling analysis, the composition of each feed liquid and the separation coefficient are shown in Table 2:

[0091] Table 2:

[0092] As can be seen from Table 2, the magnesium-lithium ratio in the membrane filtration water product is lower than that in the brine to a certain extent, but it is still as high as 1.5833. The total lithium-magnesium separation coefficient (total separation coefficient = brine magnesium-lithium ratio / membrane unit effluent magnesium-lithium ratio) is 9.47, which is difficult to meet the magnesium-lithium ratio requirements for the preparation of 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-lithium ratio, which will lead to a sharp increase in the cost of the entire process.

[0093] As can be seen from Example 1 and Comparative Example 1, the solution of the present invention does not require the use of fresh water for dilution throughout the entire process and can effectively treat high-concentration brine. Compared with the high magnesium-to-lithium mass ratio of 1.5833 and the total separation coefficient of 9.47 in Comparative Example 1, the magnesium-to-lithium ratio of the produced water in Example 1 is reduced to 0.01875 after the addition of the chelating agent, and the separation coefficient is as high as 800. This shows that the solution of the present invention can effectively treat high-salinity brine and can significantly reduce the magnesium-to-lithium ratio of the brine, and has an extremely high separation coefficient for magnesium and lithium in the brine.

[0094] Process Example 2

[0095] The sulfate-free salt lake brine treated in this example is 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 example are as follows:

[0096] (1) In this embodiment, the mass concentration of lithium ions in the salt lake brine is 0.17 g / L, the magnesium-lithium ratio is 27:1, and the pH value of the brine is 6.9. Brine that does not contain sulfate does not require pretreatment.

[0097] (2) Adding chelating agent EGTA-4Na (MW: 472.35Da) to the brine using a pipeline mixer, the brine flow rate in the pipeline mixer is 0.5m / s, the molar ratio of EGTA-4Na dosage to magnesium ions in the pipeline mixer per unit time is (0.8-1.2):1, and the pH of the brine is 8.6-9.5. The fully mixed chelated brine is pumped into the first acid-base adjustment unit, and sodium hydroxide is added thereto to promote the chelation stability of EGTA-4Na with divalent and multivalent cations in the brine until the pH of the brine reaches 10, at which time the lithium ion concentration is 0.15g / L and the magnesium-lithium ratio is 27:1.

[0098] (3) The membrane separation unit is a single-stage nanofiltration membrane. The nanofiltration membrane is polyamide (PA) with a molecular weight cutoff of 300. The effective filtration area is 7 square meters, the operating temperature is 18-24°C, the membrane filtration pressure is 1-1.5 MPa, and the membrane type is a flat membrane. The membrane separation process adopts cross-flow filtration, the brine flow rate is 40 L / h, and the volume ratio of the nanofiltration membrane's water output and water inlet is controlled at 3.5:1. After 25 days of continuous operation, the membrane flux of the nanofiltration unit stabilized at 3.7 L·m -2 ·h -1 bar -1 .

[0099] Sampling analysis, the composition of each feed liquid and the separation coefficient are shown in Table 1:

[0100] Table 3:

[0101] As shown in Table 1, the magnesium and calcium ion contents in the membrane-filtered water are significantly reduced. The magnesium-to-lithium ratio in the chelated brine is reduced from 27 to 0.03529 in the membrane-filtered water, and the total lithium-magnesium separation coefficient (total separation coefficient = brine magnesium-to-lithium ratio / membrane unit effluent magnesium-to-lithium ratio) is 765.00. Therefore, this embodiment can efficiently separate magnesium and lithium from the brine, enriching the lithium concentration in the membrane-filtered water, which is beneficial for the extraction of lithium ions from salt lake brine. Furthermore, the brine does not need to be diluted with fresh water, which is beneficial for its application and promotion in salt lake areas where fresh water is scarce.

[0102] (4) The concentrated water from the nanofiltration membrane is pumped into the second acid-base adjustment unit, and sodium hydroxide is added to cause a dechelating reaction. The molar ratio of the amount of hydroxide added to the magnesium ion content in the filtered concentrated water is 2.07:1, and the pH is 12.7-13.1. In order to promote the dechelating reaction, the temperature is controlled at 30-34°C, the stirring speed is 2000 rpm, and the plate and frame filter is further used to separate the chelating agent recovery liquid and the precipitate. The recovery rate of EGTA is 99.15%, and the recovery rate of nano-magnesium hydroxide is 99.41%. The salt concentration of the chelating agent recovery liquid is then reduced by a loose nanofiltration membrane, and the EGTA is concentrated and returned to the pipeline mixer.

[0103] (5) The membrane filtered water obtained in step (3) is sent to the forward osmosis and MVR system for concentration at a temperature of 65° C. and concentrated 60 times under a vacuum of 0.08 MPa, while sodium salt and potassium salt are precipitated step by step. The mass concentration of lithium ions in the lithium-rich concentrate is 10.26 g / L, and the magnesium-lithium ratio is 0.030 (partial crystallization of magnesium occurs during the concentration process).

[0104] Process Example 3

[0105] The sulfate-containing salt lake brine treated in this example is low-lithium brine (0-1 g / L) with a high magnesium-to-lithium ratio (>40). The chelating agents used are amino acid and hydroxycarboxylic acid chelating agents. The membrane assembly used is a nanofiltration membrane (MWCO: 800 Da) + nanofiltration membrane separation (MWCO: 500 Da). The main parameters of this example are as follows:

[0106] (1) In this embodiment, the mass concentration of lithium ions in the salt lake brine is 0.48 g / L, the magnesium-lithium ratio is 100:1, and the pH value of the brine is 6.5.

[0107] (2) The brine is first pretreated using a macroporous loose nanofiltration membrane (MWCO-1000). The pretreatment process can reduce the concentrations of sulfate and magnesium ions and appropriately increase the concentration of lithium ions.

[0108] Sodium methionine (MW: 398.44) was added to the pretreated brine at a brine flow rate of 1.2 m / s. The molar ratio of sodium methionine addition to the magnesium ion content in the pipeline mixer per unit time was (0.7-0.8):1, resulting in a brine pH of 7.5-8.0. The thoroughly mixed chelated pretreated brine was pumped into the first acid-base adjustment unit, where acid was added to promote the chelation of sodium methionine with divalent and multivalent cations in the brine until the brine pH 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 feed) reached 0.5 g / L, with a magnesium-to-lithium ratio of 80:1.

[0109] (3) The membrane separation unit is composed of nanofiltration + nanofiltration devices in series. The first stage nanofiltration device is composed of a polyamide (PA) nanofiltration membrane with a molecular weight cutoff of 800, and the second stage nanofiltration device is composed of a polyamide (PA) nanofiltration membrane with a molecular weight cutoff of 500. The effective filtration areas of the two membranes are 7m 2 The operating temperature is 16-22°C, with a first-stage nanofiltration unit pressure of 3.4-4.6 MPa and a nanofiltration unit pressure of 2.5-3.0 MPa. Both membranes use spiral wound membranes. The membrane separation process utilizes cross-flow filtration, with a brine flow rate of 22 L / h. To increase lithium ion recovery and separation efficiency, the membrane filtration water-to-liquid volume ratio is controlled at 4:1.

[0110] During the separation process, the chelated pretreated brine is directly pumped into the first-stage nanofiltration separation, and the nanofiltration product water enters the second-stage nanofiltration membrane as the inlet water. The concentrations of the chelating agent and divalent or multivalent cations in the inlet water of the nanofiltration device are timely detected, and the hydroxycarboxylic acid chelating agent sodium linolenic acid (Linolenic acid sodium salt, MW: 278.43) is added to the inlet water (the outlet water of the first-stage membrane unit) in real time. The optimal molar ratio of the chelating agent addition ratio to the magnesium ion concentration is (0.1-0.3):1. The membrane filtration concentrate produced by the two-stage membrane device is pumped into the second acid-base adjustment unit for dechelation reaction. After 60 days of continuous operation, the membrane permeation flux of the two-stage membrane device stabilized at 12.4 and 5.5 L·m -2 ·h -1 bar -1 Sampling analysis, the composition of each feed liquid and the separation coefficient are shown in Table 4:

[0111] Table 4:

[0112] As can be seen from Table 4, after the brine is treated by two-stage nanofiltration in series with membrane filtration units, the magnesium ion content of the membrane filtration water is greatly reduced, and the magnesium-lithium ratio is reduced from 100 in the salt lake brine to 0.0517 in the second-stage membrane filtration water. The total lithium-magnesium separation coefficient (total separation coefficient = brine magnesium-lithium ratio / membrane unit effluent magnesium-lithium ratio) is 1933.33. It can be seen that the present invention has a very high separation coefficient for magnesium and lithium. In this embodiment, no fresh water or recycled clean water is used to dilute the brine and the nanofiltration unit inlet water. No fresh water resources are consumed during the filtration process, which is conducive to the application and promotion of this technology in salt lake areas where fresh water is scarce.

[0113] (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 cause a dechelating reaction. The molar ratio of the amount of hydroxide added to the magnesium ion content in the concentrated water from the membrane filtration was 2:1. At this time, the pH of the solution was 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 by a bag filter to obtain a chelating agent recovery liquid and a precipitate. The recovery rates of sodium methionine and sodium linolenate were 98.47% and 99.17%, respectively. Petal-shaped magnesium hydroxide with a particle size of about 300 nm was obtained. The yields of nano-magnesium hydroxide produced by the concentrated water from the two-stage membrane filtration were 96.17% and 99.60%, respectively. Afterwards, the salt concentration of the chelating agent recovery liquid was reduced by a loose nanofiltration membrane, and the sodium methionine and sodium linolenate solutions were concentrated and returned to the pipeline mixer or the second-stage membrane inlet water of the membrane separation unit.

[0114] (5) The membrane filtered water obtained from the second-stage nanofiltration membrane in step (3) was sent to the reverse osmosis and MVR system for concentration at a temperature of 75°C, a vacuum degree of 0.01 MPa, a concentration multiple of 54.827, a lithium ion mass concentration of 31.8 g / L in the lithium-rich concentrate, and a magnesium-lithium ratio of 0.0510 (partial magnesium crystallization after concentration further reduced the magnesium-lithium ratio).

[0115] Process Example 4

[0116] The sulfate-containing salt lake brine treated in this example is high-lithium brine (>1g / L) with a high magnesium-to-lithium ratio (>40). A water-soluble polymer chelating agent, polyacrylamide (PAM, MW: 100,000), and a macromolecular polycarboxylic acid chelating agent, poly(methacrylic acid) (PMMA, MW: 100,000), are used in combination. The membrane assembly used is a two-stage ultrafiltration (MWCO 50,000) + a first-stage nanofiltration (MWCO: 1000). The main parameters of this example are as follows:

[0117] (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 ratio) is 71.789:1, and the pH value of the brine is 6.43.

[0118] (2) The brine is first pretreated using a macroporous loose nanofiltration membrane (MWCO-1500). The pretreatment process can reduce the concentrations of sulfate and magnesium ions and appropriately increase the concentration of lithium ions.

[0119] A water-soluble polymer chelating agent, polyacrylamide (PAM, MW: 100,000), was then added to the pretreated brine at a brine flow rate of 1.5 m / s. The PAM dosage was such that the molar ratio of magnesium ions in the pipeline mixer per unit time was (0.01-0.02):1, resulting in a brine pH of 7.5-8.1. The thoroughly mixed chelated pretreated raw brine was pumped into the first acid-base adjustment unit, where sodium hydroxide was added to promote the chelation of PAM with divalent and multivalent cations in the brine until the brine pH reached 9.3-9.7. The lithium ion concentration in the chelated pretreated brine was 1.996 g / L, with a magnesium-to-lithium ratio of 64:1.

[0120] (3) The membrane separation unit consists of two ultrafiltration devices and one nanofiltration device in series. The ultrafiltration unit consists of a polysulfone (Psf) ultrafiltration membrane with a molecular weight cutoff of 50,000, while the nanofiltration unit consists of a polyamide (PA) nanofiltration membrane with a molecular weight cutoff of 1,000. The effective filtration area of ​​each membrane filtration device is 7 m 2 The operating temperature is 19-25°C, and the operating pressures for each membrane filtration stage are 0.4-1MPa, 0.4-0.7MPa, and 1-1.5MPa, respectively. All membranes use hollow fiber membranes. The membrane separation process utilizes cross-flow filtration, with a brine flow rate of 45L / h. The volume ratio of product water to feed liquid for each membrane filtration stage is controlled at 4:1.

[0121] During the separation process, the chelated pretreated brine is directly pumped into the first-stage ultrafiltration separation, and the ultrafiltration product water enters the second-stage ultrafiltration membrane as the influent. The concentrations of the chelating agent and divalent or multivalent cations in the influent of the second-stage ultrafiltration device are promptly detected, and a macromolecular polycarboxylic acid chelating agent - poly (methacrylic acid) (PMMA, MW: 100000) is added to the influent in real time. The optimal molar ratio of the chelating agent addition to the 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 product water enters the nanofiltration unit to obtain nanofiltration product water and nanofiltration concentrate. The concentrate filtered by each membrane is pumped into the second acid-base adjustment unit for dechelation reaction. After 25 days of continuous operation, the membrane permeation fluxes of the ultrafiltration and nanofiltration units stabilized at 27.52, 26.37 and 14.37 L·m, respectively. -2 ·h -1 bar -1 Sampling analysis, the composition of each feed liquid and the separation coefficient are shown in Table 5:

[0122] Table 5:

[0123] As can be seen from Table 5, after separation using three membrane filtration devices in series, the magnesium ion content of the membrane filtration water is greatly reduced, with the magnesium-to-lithium ratio dropping from 71.789 in the salt lake brine to 0.0101 in the third membrane filtration water. The total lithium-magnesium separation coefficient of the membrane separation unit (total separation coefficient = brine magnesium-to-lithium ratio / membrane unit water magnesium-to-lithium ratio) is 7092.8. Therefore, the method of the present invention can efficiently separate magnesium and lithium from brine. Furthermore, calculations show that the lithium ion yields of the three membrane filtration devices are as high as 87.09%, 86.88%, and 83.72%, respectively.

[0124] (4) The concentrated water from the two-stage ultrafiltration device is pumped into the second acid-base adjustment unit, and sodium hydroxide is added to cause a dechelating reaction. The molar ratio of the amount of hydroxide added to the magnesium ion 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°C, and the stirring speed is 4000 rpm. The reacted solution is pumped into an inclined plate filter for separation to obtain a chelating agent recovery liquid and a 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-magnesium hydroxide products are obtained with yields of 98.43% and 97.12%, respectively. Afterwards, the salt concentration of the chelating agent recovery liquid is reduced by a loose nanofiltration membrane, and the PAM-100000 and PMMA-100000 solutions are concentrated and returned to the inlet water of each level of the pipeline mixer or membrane separation unit.

[0125] (5) The water produced by the third-stage nanofiltration membrane filtration of the membrane separation unit in step (3) is sent to the forward osmosis and MVR system for concentration, and is concentrated 13.1 times at a temperature of 45°C and a vacuum degree of 0.03 MPa. The mass concentration of lithium ions in the lithium-rich concentrate is 29.8 g / L, and the magnesium-lithium ratio is 0.010.

[0126] Process Example 5

[0127] The salt lake brine treated in this embodiment is Longmucuo brine, which is a low-lithium brine (0-1g / L) with a low magnesium-lithium ratio (0-40). However, Longmucuo brine contains K in addition to magnesium and lithium ions. + 、Na + 、Cl - 、SO4 2- The chelating agent used in this example is a small molecule aminocarboxylic acid chelating agent - 1,2-cyclohexanediaminetetraethylenetetrasodium (DCTA-4Na, MW: 481.13), and the membrane module used is a single-stage nanofiltration (MWCO: 1000). The main parameters of this example are as follows:

[0128] (1) In this embodiment, the mass concentration of lithium ions in the treated Longmucuo brine is 0.79 g / L, the magnesium-lithium ratio is 14.22:1, and the pH value of the brine is 6.9.

[0129] (2) The brine was first 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. Then, 1,2-cyclohexanediamine tetrasodium (DCTA-4Na, MW: 481.13) was added to the pretreated brine using a pipeline mixer. In order to increase the degree of mixing between DCTA-4Na and the pretreated brine, the flow rate of the brine was 3.1 / s, and the molar ratio of the amount of DCTA-4Na added to the divalent cation content (magnesium and calcium ions) in the pipeline mixer per unit time was (0.75-1.3):1. At this time, the pH of the brine was 9.5-10.4. The concentration of lithium ions in the chelated pretreated brine was 0.84 g / L, and the magnesium-lithium ratio was 5.87:1.

[0130] (3) The membrane separation unit is composed 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 7m 2 The operating temperature is 15-30°C, the pressure is 2.0-4.0 MPa, and the membrane type is a flat membrane. The membrane separation process adopts cross-flow filtration, the brine flow rate is 6.0 L / h, and the volume ratio of the membrane unit's water output to the feed liquid is controlled at 1:1.

[0131] During the separation process, the chelated pre-treated brine is directly pumped into the nanofiltration unit for separation, and the membrane filtered concentrated water is pumped into the second acid-base adjustment unit for dechelation reaction. After 90 days of continuous operation, the membrane permeation flux of the nanofiltration unit was stabilized at 3.5L·m -2 ·h -1 bar -1 .

[0132] Sampling analysis, the composition of each feed liquid and the separation coefficient are shown in Table 6:

[0133] Table 6:

[0134] As can be seen from Table 6, using the method of the present invention, despite only a single-stage nanofiltration separation, the magnesium ion content of the membrane filtration water product is greatly reduced, with the magnesium-to-lithium ratio reduced from 14.22 in the salt lake brine to 0.005128 in the nanofiltration water product, and the overall magnesium-to-lithium separation coefficient is 2771.962. This demonstrates that the method of the present invention can achieve very high magnesium-to-lithium separation efficiency even in the presence of multiple divalent ions and monovalent metal cations.

[0135] In this example, no fresh water or recycled purified water is used to dilute the brine or the nanofiltration unit feedwater, and no fresh water resources are consumed during the filtration process. Furthermore, calculations show a single-stage lithium ion yield of up to 69.64%, facilitating the application and promotion of this technology in salt lake regions where fresh water is scarce.

[0136] (4) The concentrated water from the membrane unit is pumped into the second acid-base adjustment unit, and sodium hydroxide is added to cause a dechelating reaction. The molar ratio of the amount of hydroxide added to the magnesium ion content in the membrane filtered concentrated water is 2.05:1, and the pH is 13-14. In order to promote the dechelating effect, the temperature of the second acid-base adjustment unit is controlled at 15-30°C, and the stirring speed is 5000 rpm. The solution after the reaction is separated by an inclined plate filter to obtain a chelating agent recovery liquid and a precipitate. At this time, the recovery rate of DCTA-4Na is 98.47%. Then, the salt concentration of the chelating agent recovery liquid is reduced by a loose nanofiltration membrane, and the DCTA-4Na solution is concentrated and returned to the pipeline mixer.

[0137] (5) The membrane filtered water produced in step (3) is sent to the reverse osmosis and MVR system for concentration. The concentration ratio 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.

[0138] As shown in Table 6, the concentration of large amounts of divalent calcium, magnesium, and sulfate ions in the nanofiltration concentrate improves the purity of sodium and potassium ions in the nanofiltration product water. Nanofiltration water can utilize phase separation technology to crystallize sodium and potassium salts from the membrane concentrate through an MVR system. During the evaporation process in the MVR system, the temperature is controlled at 30-90°C and the vacuum is maintained at 0-0.5 MPa, preferably 0.05-0.1 MPa, to reduce the amount of lithium salt crystals, resulting in higher-purity sodium or magnesium salts and improving lithium yield.

[0139] 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 of the brine and the magnesium-lithium ratio.

[0140] The chelating-assisted membrane filtration process for efficient lithium extraction from salt lake brine of the present invention can effectively reduce the magnesium-to-lithium ratio in the brine, greatly reduce the magnesium-to-lithium ratio, and thus simplify the subsequent deep magnesium removal process; the present invention can flexibly combine chelating agents and membrane filtration units according to the lithium ions and magnesium-to-lithium ratios of different salt lakes, and has greater flexibility.

[0141] The effects of the technical solution of the present invention include:

[0142] (1) The addition of chelating agents can effectively expand the difference between divalent and multivalent cations and monovalent cations (lithium, sodium, and potassium) in salt lake brine, and achieve efficient separation of lithium and magnesium by utilizing the size screening and electrostatic repulsion effect of the membrane, thus saving equipment investment costs.

[0143] (2) The membrane material selection range is wide. Depending on the different influent water qualities and the selected chelating agent, both nanofiltration membranes and ultrafiltration membranes can be applied to the present invention, and the process applicability is strong. Therefore, the membrane separation unit can treat high-concentration and high-magnesium-to-lithium ratio brines, improve lithium flux and lithium purity in the produced water, reduce energy consumption and freshwater resource usage, and simplify the lithium-magnesium separation process, which is of significant significance to salt lake areas with a shortage of fresh water.

[0144] (3) The selected chelating agent is environmentally friendly, the recovery process is simple and efficient, and the recycling effect of the chelating agent is stable, which can effectively reduce the dosage of the agent and reduce costs.

[0145] (4) The present invention can be used to treat water with high salt concentration, a wide range of initial brine concentration and magnesium-lithium ratio, and can effectively reduce the amount of fresh water used and slow down the load of the subsequent concentration process, thereby reducing process operation energy consumption and equipment investment costs.

[0146] (5) The technical route of this invention is mature and easy to promote. It can be used to directly extract lithium from salt lake brine, or it can be combined with the existing salt field lithium extraction process as a nested separation unit. It can also be used to treat waste salt solutions containing magnesium, lithium, calcium, potassium, and sodium generated in various industrial production.

[0147] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 in that: include: S1. Chelating pretreatment: adding a chelating agent to the mixed salt solution to be treated, and adjusting the pH to promote the chelating agent to form a stable chelate with the divalent or multivalent cations in the mixed salt solution to obtain influent; S2, membrane separation: the influent water produced in step S1 is subjected to membrane separation, and the lithium and magnesium are separated by utilizing the membrane pore size screening mechanism and electrostatic repulsion effect to produce membrane filtration concentrated water and membrane filtration produced water; S3, dechelation reaction: adjusting the pH of the membrane filtration concentrated water produced in S2 to cause the chelate in the membrane filtration concentrated water to undergo a dechelation reaction, and at the same time, divalent or multivalent cations combine with hydroxide to form a precipitate, and after solid-liquid separation, a high-value hydroxide precipitate and a chelating agent recovery liquid are obtained; S4, chelating agent recycling: reusing the chelating agent recovery liquid to step S1; S5, preparing lithium-rich concentrated solution: concentrating and enriching the membrane filtered water produced in S2 to increase its lithium concentration, thereby obtaining lithium-rich concentrated solution, which is used to prepare lithium salts.

2. The method according to claim 1, characterized in that In S1, the chelating agent is one or more of organic phosphonic acid, aminocarboxylic acid, amino acid, hydroxycarboxylic acid, water-soluble polymer, polycarboxylic acid chelating agent, or a combination of one or more of the above chelating agents and a dispersant; the magnesium-lithium mass ratio of the mixed salt solution is 0.1-1000:1, the lithium ion mass concentration is 0.01-5g / L, and the pH value is 6-8; the mixed salt solution is surface brine, underground brine, petroleum brine, or industrial brine containing Mg 2+ , Li + of waste brine.

3. The method according to claim 1, characterized in that In S2, the membrane separation adopts a primary membrane separation process or a multi-stage membrane separation process; the primary membrane separation process includes one or more parallel membrane separation devices; the multi-stage membrane separation process includes membrane separation devices in a series relationship, wherein the influent of each membrane separation device is added with a chelating agent as needed to meet the chelating requirements of the chelating agent and magnesium ions and adjust the pH, the produced water of each membrane separation device is used as the influent of the next membrane separation device, and the concentrated water of each membrane separation device is processed according to step S3.

4. The method according to claim 1, characterized in that In S2, the membrane separation pressure difference is 0.1-5MPa, the inlet water temperature is controlled at 10-50°C, and the volume ratio of membrane filtration water production and 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.

5. The method according to claim 1, characterized in that In S2, the membrane combination used in the membrane separation process includes a membrane component type and a membrane material, wherein the membrane component type is a combination of one or more of a flat membrane, a rolled membrane, a hollow fiber membrane and a tubular membrane; and the membrane material is a combination of one or more of a microfiltration membrane, an ultrafiltration membrane and a nanofiltration membrane.

6. The method according to claim 1, characterized in that In S2, the membrane separation process uses the following three membrane combinations: (1) When the lithium ion concentration of the brine to be treated is 0-1.0 g / L and the magnesium-to-lithium ratio is 0-40, if the brine does not contain sulfate, one or more aminocarboxylic acid chelating agents are selected to be 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 part of the sulfate ions and reduce the magnesium-to-lithium ratio, and then one or more organic phosphonic acid chelating agents are selected to be added alone or in combination, and further treated with a single-stage nanofiltration device; (2) When the lithium ion concentration in the sulfate-containing brine to be treated is 0-1.0 g / L and the magnesium-to-lithium ratio is greater than 40, or when the lithium ion concentration is greater than 1.0 g / L and the magnesium-to-lithium ratio is 0-40: firstly, the brine is pretreated with a loose nanofiltration membrane to remove part of the sulfate, and then one or more amino acid chelating agents are selected to be added to the brine alone or in combination and subjected to primary nanofiltration through a loose nanofiltration membrane, and then one or more hydroxycarboxylic acid chelating agents are selected to be added to the primary nanofiltration product water alone or in combination, and secondary nanofiltration is performed through a tight nanofiltration membrane; (3) When the lithium ion concentration in the sulfate-containing brine to be treated is greater than 1.0 g / L and the magnesium-lithium ratio is greater than 40, the brine is first pretreated with a loose nanofiltration membrane to remove part of the sulfate, and then polycarboxylic acid and / or water-soluble polymer chelating agents are selected and added to the brine and subjected to first-stage ultrafiltration through an ultrafiltration membrane. The first-stage ultrafiltration water is subjected to second-stage ultrafiltration or nanofiltration, and the second-stage ultrafiltration or nanofiltration water is subjected to third-stage nanofiltration; polycarboxylic acid and / or water-soluble polymer chelating agents are added to the influent of each stage as needed.

7. The method according to claim 1, characterized in that In S3, the pH is adjusted by adding an alkaline agent, and the amount of the alkaline agent added is: - The molar ratio of the magnesium ions to the concentrated water filtered by the membrane is in the range of 1.8-2.5; the temperature range of the dechelating reaction is 10-70°C.

8. The method according to claim 1, characterized in that In S3, after the dechelating reaction is completed, a solid-liquid separation unit is used to perform solid-liquid separation, wherein the solid-liquid separation unit is one or a combination of a sedimentation device, a plate and frame filter press, and a centrifuge; and a certain amount of flocculant is added during the dechelating reaction.

9. The method according to claim 1, characterized in that: S4 also includes: performing membrane concentration on the chelating agent recovery liquid produced in S3, filtering out the salt in the chelating agent recovery liquid, increasing the chelating agent concentration in the chelating agent recovery liquid, and obtaining concentrated chelating agent recovery liquid for reuse in step S1.

10. The method according to claim 1, characterized in that In S5, when the membrane filtration water is concentrated, the sodium ions and lithium ions in the membrane filtration water are separated from the membrane filtration water in the form of step-by-step crystallization using phase separation technology to obtain sodium salt and potassium salt products.

11. A system for efficiently extracting lithium from a mixed salt solution, characterized in that: It includes: 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; The pretreatment unit is used to remove insoluble substances and sulfate ions in the water to be treated, and the pretreatment unit includes a microfiltration membrane and / or a loose nanofiltration membrane; One end of the mixing unit is connected to the water outlet of the pretreatment unit, and the other end is connected to the first acid-base adjustment unit; the mixing unit is also provided with a chelating agent dosing device; The membrane separation unit comprises a membrane filtration water production side and a membrane filtration concentrated water side, wherein the membrane filtration water production side is connected to the concentration and enrichment unit to concentrate and enrich the membrane filtration water production to obtain a lithium-rich concentrated solution; the membrane filtration concentrated water side is connected to a chelating agent recovery unit; The chelating agent recovery unit comprises a second acid-base regulating unit and a solid-liquid separation device, the concentrated water side of the membrane filtration is connected to the second acid-base regulating unit, and the second acid-base regulating unit is connected to the solid-liquid separation device; The second acid-base adjustment unit is used to adjust the pH of the membrane filtered concentrated water to produce a dechelating reaction, and the concentrated water is separated by a solid-liquid separation device to obtain a chelating agent recovery liquid; The chelating agent recovery liquid can be reused in the mixing unit; The first acid-base regulating unit and the second acid-base regulating unit are both provided with a pH regulating agent dosing device.

12. The system according to claim 11, characterized in that The concentration and enrichment unit is also connected to a lithium precipitation unit for converting the lithium-rich concentrated solution into a lithium salt product.

13. The system according to claim 11, characterized in that 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 a series relationship, wherein the produced water of each stage of the membrane separation device is used as the inlet water of the next stage of the membrane separation device, and the concentrated water of each stage of the membrane separation device enters the chelating agent recovery unit for chelating agent recovery.

14. The system according to claim 11, characterized in that The system also includes a concentration and salt reduction unit; the chelating agent recovery liquid generated by the chelating agent recovery unit is transported to the concentration and salt reduction unit, and the chelating agent recovery liquid is subjected to membrane concentration and desalination to obtain a concentrated chelating agent recovery liquid; The concentrated water side of the concentration and salt reduction unit is connected to the mixing unit for recycling the concentrated chelating agent recovery liquid to the mixing unit.

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