Lithium resource efficient recovery method based on replacement dialysis process

Abstract

The invention provides a high-efficiency lithium resource recovery method based on a replacement dialysis process, and belongs to the technical field of industrial wastewater treatment and resource recovery. The method depends on a four-compartment replacement dialysis device as a core, and combines a defluorination chelating purification unit, a bipolar membrane electrodialysis in-situ alkali production unit, a reverse osmosis / MVR coupling crystallization unit and the like; directional separation and recycling of the lithium fluoride in the lithium fluoride wastewater are realized; the method comprises the following specific steps: firstly, generating a lithium chloride and sodium fluoride solution through four-compartment replacement dialysis; after lithium chloride is purified and concentrated, part of lithium chloride is subjected to bipolar membrane electrodialysis to prepare lithium hydroxide which is recycled to adjust the pH value, and the rest of lithium chloride reacts with sodium carbonate to generate high-purity lithium carbonate; the sodium fluoride is subjected to pH regulation, reverse osmosis concentration and MVR crystallization to obtain a high-purity product. According to the method, replacement dialysis, bipolar membrane electrolysis and evaporative crystallization technologies are creatively combined, so that the recovery rates of lithium and fluorine are respectively greater than or equal to 98% and greater than or equal to 95%, the water reuse rate is greater than or equal to 90%, no exogenous alkali liquor is added in the whole process, and the method is suitable for efficient low-carbon treatment of high-fluorine wastewater in the lithium battery industry.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment and resource recovery technology, and particularly relates to a method for efficient recovery of lithium resources based on displacement dialysis process. Background Technology

[0002] With the rapid development of the global lithium battery industry, lithium fluoride, as a key raw material for lithium battery electrolytes and cathode materials, has seen continuous growth in production. Consequently, the amount of wastewater discharged from lithium fluoride production has also increased year by year. This type of wastewater contains high concentrations of... (Usually 300-500 mg / L) and (Usually 1000-2000 mg / L), accompanied by , , If heavy metal ions are not handled properly, they will not only waste valuable resources such as lithium and fluorine, but also cause serious soil and water pollution problems.

[0003] Among existing lithium fluoride wastewater treatment processes, the most widely used is the "stepwise precipitation method": calcium chloride is first added to the wastewater to... With calcium fluoride ( The process involves removing Li⁺ as a precipitate, followed by the addition of sodium carbonate to the defluorinated wastewater to precipitate and recover Li⁺ as lithium carbonate. However, this process has the following problems:

[0004] (1) Resource waste and secondary pollution: It precipitates as calcium fluoride, which is classified as HW32 hazardous waste, with treatment costs as high as 2,000-3,000 yuan / ton. This not only fails to recover fluorine resources but also creates pressure for hazardous waste disposal.

[0005] (2) High consumption and high cost of medicines: To ensure , Complete precipitation requires excessive addition of calcium chloride and sodium carbonate, causing reagent costs to account for over 50% of the total treatment cost, while also introducing a large amount of... , Impurity ions can affect the purity of subsequent resource recovery.

[0006] (3) Low water reuse rate: The high-salt wastewater (salt content usually >5%) generated after sedimentation needs to be directly evaporated, which consumes a lot of energy and the water reuse rate is generally less than 50%;

[0007] (4) Low product purity: The purity of lithium carbonate obtained by precipitation method is usually only 95%-98%, which is difficult to meet the demand of the lithium battery industry for high-purity lithium carbonate (purity ≥99.5%).

[0008] To address the aforementioned problems, several improved processes have been proposed in related fields. For example, patent CN202111450265.8 discloses "a method for extracting lithium from high-sodium lithium-containing brine," which uses resin adsorption to selectively adsorb Li⁺. However, this method has two major drawbacks:

[0009] (1) Resin regeneration requires 5% to 10% hydrochloric acid and sodium hydroxide, resulting in secondary pollution;

[0010] (2) Unable to Recycling still requires additional treatment of fluoride ions, which increases the complexity and cost of the process. Summary of the Invention

[0011] To address the issue of simultaneous recovery of lithium, fluorine, and water resources from lithium fluoride wastewater, reduce reagent consumption, and achieve cleaner production, this invention proposes a highly efficient lithium resource recovery method based on displacement dialysis.

[0012] To achieve the above objectives, the present invention provides the following technical solution:

[0013] One of the technical solutions of the present invention:

[0014] A method for efficient recovery of lithium resources based on displacement dialysis process. The method uses a lithium fluoride wastewater treatment system to treat wastewater. The lithium fluoride wastewater treatment system includes a four-compartment displacement dialysis device, a fluoride removal resin tower, a chelation resin tower, an MVR evaporator crystallizer, a lithium carbonate reactor, a bipolar membrane electrodialysis unit, and a reverse osmosis membrane module.

[0015] The four-compartment displacement dialysis device is arranged sequentially along the water flow direction, with compartment 1, compartment 2, compartment 3, and compartment 4. Adjacent compartments are separated by a cation exchange membrane C or anion exchange membrane A. The outlet of compartment 2 is connected sequentially to the inlet of the defluorination resin tower, the inlet of the chelation resin tower, and the inlet of the MVR evaporator crystallizer. The outlet of the MVR evaporator crystallizer is connected to the inlet of the lithium carbonate reactor and the inlet of the bipolar membrane electrodialysis unit. The alkali outlet of the bipolar membrane electrodialysis unit is connected to the pH adjustment liquid inlet of compartment 1. The by-product outlet of the lithium carbonate reactor is connected to the sodium chloride supply outlet of compartment 3. The outlet of compartment 4 is connected to the inlet of the reverse osmosis membrane module, and the concentrate outlet of the reverse osmosis membrane module is connected to another inlet of the MVR evaporator crystallizer.

[0016] The method also includes the following steps:

[0017] Step 1: Pump the lithium fluoride wastewater with pH adjusted to 3-4 into compartment one of the four-compartment displacement dialysis unit; pump the sodium chloride solution into compartment three; and introduce clean water into compartments two and four respectively; start the four-compartment displacement dialysis unit and apply a 10-30V DC electric field. Under the drive of the electric field, the solution in compartment one... They migrate through the cation exchange membrane to compartment two, and from compartment three they migrate to compartment two. A lithium chloride solution is formed; in compartment one It migrates through the anion exchange membrane to compartment four, where it reacts with the Na⁺ that migrates from compartment three to compartment four to form a sodium fluoride solution;

[0018] Step 2: Pump the lithium chloride solution generated in compartment two into the defluorination resin tower to remove residual fluoride from the solution. Then, it is pumped into a chelating resin tower to remove heavy metal ions; the purified lithium chloride solution enters the MVR evaporator crystallizer, and is concentrated by evaporation to obtain a saturated lithium chloride solution with a concentration of 1.3 wt.%. The distilled water generated during the evaporation process is recycled as reverse osmosis permeate.

[0019] Step 3: The saturated lithium chloride solution obtained in Step 2 is divided into two parts. One part is pumped into a bipolar membrane electrodialysis unit to prepare lithium hydroxide through bipolar membrane hydrolysis and ion migration. The lithium hydroxide is recycled to adjust the pH of the lithium fluoride wastewater in compartment 1 in Step 1. The other part is pumped into a lithium carbonate reactor. Sodium carbonate solution is added to the reactor, and the reaction is carried out under stirring to generate lithium carbonate precipitate. The lithium carbonate product is obtained by solid-liquid separation. The sodium chloride solution, a byproduct of the reaction, is returned to compartment 3 of Step 1 for recycling.

[0020] Step 4: Pump the sodium fluoride solution generated in compartment 4 into the pH adjustment tank, add NaOH solution to adjust the pH to 7-8; the adjusted sodium fluoride solution enters the reverse osmosis membrane module and is concentrated 5-10 times by membrane separation; the concentrated sodium fluoride solution enters the MVR evaporator crystallizer and is evaporated and crystallized at a temperature of 60-80℃, and the sodium fluoride product is obtained by solid-liquid separation.

[0021] Preferably, in step 2, the resin filled in the fluoride removal resin tower is... Modified resin, the The modified resin has a particle size of 0.3-0.8 mm and a specific surface area of ​​80-120. .

[0022] Preferably, in step 2, the resin filled in the chelating resin tower is D418 type chelating resin, and the working exchange capacity of the D418 type chelating resin is ≥1.5mmol / g, and the operating temperature is ≤60℃.

[0023] Preferably, in step 1, the initial concentration of the sodium chloride solution is 15-20. The initial volume ratio of compartment one, compartment two, compartment three, and compartment four is 4:1:1.6:1.6.

[0024] Preferably, in step 3, the operating parameters of the bipolar membrane electrodialysis unit are:

[0025] Voltage 2-5V, current density 100-200 Membrane stack temperature 25-40 Feed flow rate 10-20 .

[0026] Preferably, in step 4, the reverse osmosis membrane module uses an anti-fouling aromatic polyamide composite membrane, with an operating pressure of 1.5-2.5. Temperature 20-30 Inlet flow rate: 1.0-1.5 .

[0027] Preferably, in step 3, the reaction temperature of the lithium carbonate reactor is 60-80°C. Stirring speed 200-300 The amount of sodium carbonate solution added is different from that in the lithium chloride solution. The molar ratio is .

[0028] Compared with the prior art, the present invention has the following advantages and technical effects:

[0029] (1) High resource recovery rate: Li (lithium carbonate) recovery rate ≥98%, F (sodium fluoride) recovery rate ≥95%, water reuse rate ≥90%;

[0030] (2) Cost advantage: This invention reduces LiOH consumption by more than 80% through in-situ alkali production via bipolar membrane electrodialysis and reduces the introduction of impurities into lithium chloride. If NaOH is added in this step, sodium chloride impurities will be introduced into lithium chloride, affecting the purity of lithium chloride, and thus affecting the purity of the final product lithium carbonate and the lithium recovery rate. The recycling of NaCl reduces reagent costs by more than 60%.

[0031] (3) Environmentally friendly: No hazardous waste is generated, and carbon emissions are reduced by 60% compared with traditional processes. Attached Figure Description

[0032] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0033] Figure 1 This is a process flow diagram of the present invention for the simultaneous recovery and preparation of lithium carbonate and sodium fluoride using a lithium fluoride wastewater treatment system;

[0034] Figure 2 This is a schematic diagram of the four-compartment displacement dialysis device of the present invention, wherein C is a cation exchange membrane and A is an anion exchange membrane;

[0035] Figure 3This is a schematic diagram of the apparatus for preparing lithium hydroxide using the bipolar membrane electrodialysis unit of the present invention, where BP is the bipolar membrane, C is the cation exchange membrane, and A is the anion exchange membrane. Detailed Implementation

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0038] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0039] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0040] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0041] This invention uses a four-compartment displacement dialysis device as its core, and utilizes an electric field to achieve the directional separation of Li⁺ and F⁻. It combines fluoride removal and chelation purification to remove impurities, bipolar membrane electrodialysis to produce alkali in situ to reduce the addition of exogenous reagents, and reverse osmosis / MVR coupled crystallization to achieve product purification and water resource recovery, ultimately forming an integrated process of separation, purification, conversion and refining.

[0042] A method for efficient recovery of lithium resources based on displacement dialysis process. This method uses a lithium fluoride wastewater treatment system to treat wastewater. The lithium fluoride wastewater treatment system includes a four-compartment displacement dialysis device, a fluoride removal resin tower, a chelation resin tower, an MVR evaporator crystallizer, a lithium carbonate reactor, a bipolar membrane electrodialysis unit, and a reverse osmosis membrane module.

[0043] The four-compartment displacement dialysis unit is arranged sequentially along the water flow direction, with compartments one, two, three, and four. Adjacent compartments are separated by cation exchange membranes C or anion exchange membranes A. The outlet of compartment two is connected sequentially to the inlet of the defluorination resin tower, the inlet of the chelation resin tower, and the inlet of the MVR evaporator crystallizer. The outlet of the MVR evaporator crystallizer is connected to the inlet of the lithium carbonate reactor and the inlet of the bipolar membrane electrodialysis unit. The alkali outlet of the bipolar membrane electrodialysis unit is connected to the pH adjustment liquid inlet of compartment one. The by-product outlet of the lithium carbonate reactor is connected to the sodium chloride supply outlet of compartment three. The outlet of compartment four is connected to the inlet of the reverse osmosis membrane module, and the concentrate outlet of the reverse osmosis membrane module is connected to another inlet of the MVR evaporator crystallizer.

[0044] The four-compartment displacement dialysis device is used to achieve the directional separation of Li⁺ and F⁻. The device is arranged sequentially along the water flow direction: compartment 1 (wastewater compartment), compartment 2 (lithium chloride generation compartment), compartment 3 (sodium chloride circulation compartment), and compartment 4 (sodium fluoride generation compartment). A cation exchange membrane (C, allowing cations such as Li⁺ and Na⁺ to pass through) and anion exchange membrane (A, allowing anions such as F⁻ and Cl⁻ to pass through) are respectively installed between adjacent compartments. The specific membrane arrangement is as follows: cation exchange membrane between compartment 1 and compartment 2, anion exchange membrane between compartment 2 and compartment 3, and cation exchange membrane between compartment 3 and compartment 4.

[0045] The purification unit includes a fluoride removal resin tower and a chelating resin tower, used to remove residual F⁻ and heavy metal ions from the lithium chloride solution;

[0046] The concentration and conversion unit includes an MVR evaporator crystallizer, a lithium carbonate reactor, and a bipolar membrane electrodialysis unit. The MVR evaporator crystallizer is used for the concentration of lithium chloride and sodium fluoride solutions. The bipolar membrane electrodialysis unit is used for in-situ preparation of lithium hydroxide, which is then reused for pH adjustment. The lithium carbonate reactor is used to react lithium chloride with sodium carbonate to produce lithium carbonate.

[0047] The purification and recovery unit includes a reverse osmosis membrane module for pre-concentrating sodium fluoride solution, reducing energy consumption for subsequent MVR evaporation, and simultaneously recovering distilled water.

[0048] The efficient lithium resource recovery method based on displacement dialysis process also includes the following steps:

[0049] Step 1: Four-compartment replacement dialysis separation:

[0050] Pretreatment: The pH of the lithium fluoride wastewater is adjusted to 3-4 using hydrochloric acid (this pH range can prevent Li⁺ from forming lithium hydroxide precipitate and at the same time improve the migration efficiency of F⁻).

[0051] Feeding: Pump the pH-adjusted lithium fluoride wastewater into compartment one, pump a 15-20 wt.% sodium chloride solution into compartment three, and introduce clean water (as the receiving phase for ion migration) into compartments two and four respectively.

[0052] Electric field drive: Start the four-compartment displacement dialysis device and apply a DC electric field of 10-30V (too low a voltage will result in slow ion migration and low processing efficiency; too high a voltage will easily cause concentration polarization and increase energy consumption). Under the action of the electric field, Li⁺ in compartment one migrates to compartment two through the cation exchange membrane, and Cl⁻ in compartment three migrates to compartment two through the anion exchange membrane, forming a lithium chloride solution in compartment two. At the same time, F⁻ in compartment one migrates to compartment four through the anion exchange membrane, and Na⁺ in compartment three migrates to compartment four through the cation exchange membrane, forming a sodium fluoride solution in compartment four.

[0053] Discharge: When the concentration of lithium chloride solution in compartment two reaches 0.8-1.2 wt.% and the concentration of sodium fluoride solution in compartment four reaches 0.5-0.8 wt.%, the electric field is stopped, and the lithium chloride solution in compartment two and the sodium fluoride solution in compartment four are collected respectively; the remaining sodium chloride solution in compartment three (concentration reduced to 10-15 wt.%) is reserved for the next recycling.

[0054] Step 2: Purification and concentration of lithium chloride solution:

[0055] Defluoridation treatment: The lithium chloride solution collected in compartment two is pumped into the defluoridation resin tower, which is filled with La(OH)3 modified resin (La(OH)3 has a strong selective adsorption capacity for F⁻, with an adsorption capacity of up to 10-15 mg / g). The influent flow rate is controlled at 8-12 BV / h (BV is the resin bed volume, and this flow rate can ensure that F⁻ is in full contact with the resin). The F⁻ concentration in the effluent can be reduced to below 1 mg / L.

[0056] Heavy metal removal: The defluorinated lithium chloride solution is pumped into a chelating resin tower, which is filled with D418 type chelating resin (this resin has strong selectivity for heavy metal ions such as Ca²⁺, Cd³⁺, and Cr²⁺, and has a working exchange capacity ≥1.5mmol / g). The influent flow rate is controlled at 5-8 BV / h, and the total concentration of heavy metal ions in the effluent can be reduced to below 0.1mg / L.

[0057] Evaporation and concentration: The purified lithium chloride solution is pumped into the MVR evaporator crystallizer, and the evaporation temperature is controlled at 70-90℃ and the vacuum degree is -0.08~-0.09MPa. After evaporation and concentration, a saturated lithium chloride solution with a concentration of 1.3wt.% is obtained (this concentration is the saturation concentration of lithium chloride at room temperature, which is convenient for subsequent bipolar membrane electrodialysis and lithium precipitation reaction). The distilled water generated during the evaporation process is recycled as reverse osmosis permeate and reused in process makeup water (such as the clean water supply for compartments two and four).

[0058] Step 3: Lithium chloride conversion and alkali solution reuse:

[0059] Alkali production via bipolar membrane electrodialysis: The saturated lithium chloride solution obtained in step 2 is divided into two parts at a volume ratio of 1:4-1:6. The smaller portion (approximately 15%-20%) is pumped into a bipolar membrane electrodialysis unit. This unit consists of alternating bipolar membranes (BP, which can hydrolyze to produce H⁺ and OH⁻), cation exchange membranes (C), and anion exchange membranes (A), forming acid, alkali, and salt chambers. The saturated lithium chloride solution enters the salt chamber, and under the drive of the electric field, Li⁺ migrates through the cation exchange membrane to the alkali chamber, where it interacts with the bipolar membrane. The OH⁻ produced by hydrolysis combines to form lithium hydroxide; Cl⁻ migrates through the anion exchange membrane to the acid chamber and combines with H⁺ produced by the bipolar membrane hydrolysis to form hydrochloric acid (which can be used as a backup acid for adjusting the pH of wastewater); the voltage of the bipolar membrane electrodialysis unit is controlled at 2-5V and the current density is 100-200mA / cm², and finally a lithium hydroxide solution with a concentration of 3-5wt.% is obtained. This solution will be used to adjust the pH of the lithium fluoride wastewater in the first compartment in step 1 to avoid the introduction of impurities by external alkaline solutions (such as NaOH);

[0060] Lithium carbonate preparation: Pump the remaining saturated lithium chloride solution (approximately 80%-85%) from step 2 into the lithium carbonate reactor. Slowly add a sodium carbonate solution with a concentration of 10-15 wt.% to the reactor, controlling the molar ratio of sodium carbonate added to Li⁺ in the lithium chloride solution to 1.05-1.1:1 (excess sodium carbonate ensures complete precipitation of Li⁺). Adjust the reaction temperature to 60-80℃ and the stirring speed to 200-300 r / min. After reacting for 1-2 hours, lithium carbonate precipitate is generated. Centrifuge the reaction solution to obtain wet lithium carbonate, which is then dried at 80-100℃ to obtain a lithium carbonate product with a purity ≥99.8%. The mother liquor generated from centrifugation (mainly composed of sodium chloride) is returned to compartment three of step 1 as a supplement to the sodium chloride circulating liquid.

[0061] Step 4: Sodium fluoride purification:

[0062] pH adjustment: Pump the sodium fluoride solution collected in compartment 4 in step 1 into the pH adjustment tank, add a 5-10 wt.% NaOH solution, and adjust the pH to 7-8 (neutral conditions can prevent F⁻ from forming HF and prevent NaF from hydrolyzing).

[0063] Reverse osmosis concentration: The pH-adjusted sodium fluoride solution is pumped into the reverse osmosis membrane module. An anti-fouling aromatic polyamide composite membrane is used. The operating pressure is controlled at 1.5-2.5 MPa, the temperature at 20-30℃, and the feed water flow rate at 1.0-1.5 m / s to concentrate the sodium fluoride solution to a concentration factor of 5-10 times (the concentration of sodium fluoride after concentration is 2.5-8 wt.%), which can significantly reduce the energy consumption of subsequent MVR evaporation. The permeate (conductivity ≤50 μS / cm) generated during the reverse osmosis process is recycled as water resources and reused in the process makeup water.

[0064] MVR Evaporation Crystallization: The sodium fluoride solution concentrated by reverse osmosis is pumped into an MVR evaporation crystallizer. The evaporation temperature is controlled at 60-80℃ and the vacuum degree is -0.07~-0.08MPa for evaporation crystallization. When the solution reaches a supersaturated state, sodium fluoride crystals precipitate. The crystal slurry is centrifuged to obtain wet sodium fluoride material, which is then dried at 60-80℃ to obtain sodium fluoride product with a purity ≥99.2% (which can be reused as a raw material for fluorochemicals).

[0065] The schematic diagram of the four-compartment displacement dialysis device of the present invention is shown below. Figure 2 Where C is a cation exchange membrane and A is an anion exchange membrane; the schematic diagram of the bipolar membrane electrodialysis unit for preparing lithium hydroxide is shown below. Figure 3 BP is a bipolar membrane, C is a cation exchange membrane, and A is an anion exchange membrane.

[0066] To further verify the feasibility and effectiveness of this invention, lithium fluoride wastewater from a lithium battery company was used as the treatment target for a specific implementation. Detailed parameters are as follows:

[0067] The treatment capacity for lithium fluoride wastewater is 5 m³ / d, and the water quality parameters are shown in the table below:

[0068] Table 1. Raw water quality parameters

[0069]

[0070] The process parameters are set as follows:

[0071] Step 1: Four-compartment replacement dialysis separation:

[0072] Wastewater pH adjustment: Adjust the wastewater pH to 3.5 using 3wt.% hydrochloric acid;

[0073] Feeding parameters: 200L of wastewater is injected into compartment 1, 80L of 18wt.% sodium chloride solution is injected into compartment 3, 50L of clean water is injected into compartment 2, and 80L of clean water is injected into compartment 4.

[0074] Electric field parameters: applied DC voltage 30V, initial current 10A, running time 2h;

[0075] Results: 55L of lithium chloride solution (Li⁺ concentration 1230mg / L, F⁻ concentration 12mg / L) was obtained from compartment two; 75L of sodium fluoride solution (F⁻ concentration 1850mg / L, Na⁺ concentration 1120mg / L) was obtained from compartment four; and 85L of sodium chloride solution (concentration 16.2wt.%) remained in compartment three.

[0076] Step 2: Purification and concentration of lithium chloride solution:

[0077] Defluoridation treatment: The defluoridation resin tower is filled with La(OH)3 modified resin (particle size 0.5mm, specific surface area 100m² / g), the influent flow rate is 10BV / h, and the effluent F⁻ concentration is 0.5mg / L;

[0078] Heavy metal removal: The chelating resin tower is filled with D418 type chelating resin, the influent flow rate is 6 BV / h, and the effluent Ca²⁺ concentration is 0.8 mg / L, Cd³⁺ concentration is <0.01 mg / L, and Cr²⁺ concentration is <0.01 mg / L;

[0079] Evaporation and concentration: The MVR evaporator crystallizer was set at 80℃ and a vacuum of -0.085MPa to obtain 18L of 1.3wt.% saturated lithium chloride solution and recover 32L of reverse osmosis permeate (conductivity 35μS / cm).

[0080] Step 3: Lithium chloride conversion and alkali solution reuse:

[0081] Alkali production by bipolar membrane electrodialysis: 5L of saturated lithium chloride solution was introduced into the bipolar membrane electrodialysis unit. The voltage was 2V, the current density was 150mA / cm², and the temperature was 30℃. 3L of 3wt.% lithium hydroxide solution was obtained (recycled to step 1 to adjust pH), and 3L of 3wt.% hydrochloric acid was also obtained.

[0082] Lithium carbonate preparation: The remaining 13L of saturated lithium chloride solution was introduced into the lithium carbonate reactor, and 12wt.% sodium carbonate solution (molar ratio 1.08:1) was added. The temperature was 70℃, the stirring speed was 250r / min, and the reaction was carried out for 1.5h. After centrifugation, wet lithium carbonate was obtained. After drying, 0.82kg of lithium carbonate product with a purity of 99.85% was obtained. The mother liquor (containing 15wt.% sodium chloride) was returned to compartment three.

[0083] Step 4: Sodium fluoride purification:

[0084] pH adjustment: Adjust the pH of the sodium fluoride solution to 7.5 using 8 wt.% NaOH solution;

[0085] Reverse osmosis concentration: The reverse osmosis membrane module operates at a pressure of 2.0 MPa, a temperature of 25℃, a feed water flow rate of 1.2 m / s, and a concentration factor of 8 times, yielding 9.4 L of concentrate (sodium fluoride concentration of 6.4 wt.%) and recovering 65.6 L of permeate (conductivity of 42 μS / cm).

[0086] MVR evaporation crystallization: The MVR evaporation crystallizer was set at 70℃ and a vacuum of -0.075MPa. After centrifugation and drying, 1.15kg of sodium fluoride product with a purity of 99.3% was obtained.

[0087] Implementation results:

[0088] In this implementation, the lithium recovery rate was 98.2%, the fluorine recovery rate was 95.5%, and the water reuse rate was 91.2%. The reagent cost (only a small amount of sodium carbonate and sodium chloride were added) was reduced by 62% compared with the traditional process. No hazardous waste was generated, and carbon emissions were reduced by 61% compared with the traditional process. The purity of the products met the requirements for industrial applications, which verified the feasibility and superiority of the invention.

[0089] The technical solution of the present invention will be further illustrated by the following embodiments.

[0090] Example 1:

[0091] The efficient lithium resource recovery method based on displacement dialysis includes the following steps:

[0092] Step 1: Pump lithium fluoride wastewater into compartment 1 of the four-compartment displacement dialysis unit. Pump sodium chloride (NaCl) solution into compartment 3 of the four-compartment displacement dialysis unit. Pass clean water into compartments 2 and 4. Turn on the four-compartment displacement dialysis unit and apply a DC electric field. The operating voltage of the four-compartment displacement dialysis unit is 30V, and the initial current is 10A. Add 200L of lithium fluoride wastewater to compartment 1, 50L of clean water to compartment 2, 80L of NaCl solution to compartment 3, and 80L of clean water to compartment 4. Add 50L of 3% sodium sulfate solution to the electrode water tank of the bipolar membrane electrodialysis unit. A LiCl solution (LiCl) is generated in compartment 2. + With a concentration of 1230 mg / L and a lithium conversion efficiency of 97.8%, a NaF solution (conductivity 9.89 mS / cm) was generated in compartment 4.

[0093] The specific operating parameters for displacement dialysis are shown in Table 2;

[0094] Table 2 Displacement dialysis operating parameters

[0095]

[0096] Step 2: The LiCl solution generated in compartment 2 is sequentially passed through a defluorination resin tower, a chelation resin tower, and an MVR evaporator crystallizer to obtain a saturated LiCl solution and reverse osmosis permeate; the influent flow rate is 200 L / h, and the defluorination filtration rate is 10 BV / h; the resin operating parameters are shown in Table 3.

[0097] Table 3. Resin Operating Parameters

[0098]

[0099] Step 3: The 5L LiCl solution obtained in Step 2 is processed into lithium hydroxide (LiOH) using a bipolar membrane electrodialysis unit. The LiOH is recycled to compartment 1 to adjust the pH of the lithium fluoride wastewater and reduce the amount of impurities introduced by other alkalis. The remaining LiCl solution is centrifuged with sodium carbonate (Na2CO3) in a lithium carbonate reactor through a lithium precipitation reaction to obtain high-purity electronic-grade Li2CO3 (purity 99.8%). The byproduct sodium chloride (NaCl) is returned to compartment 3 for recycling. The operating parameters of the bipolar membrane electrodialysis unit are shown in Table 4.

[0100] Table 4. Operating parameters of the bipolar membrane electrodialysis unit

[0101]

[0102] Step 4: Adjust the pH of the NaF solution generated in compartment 4, and then pass it through the reverse osmosis membrane module (RO) and MVR evaporator crystallizer to obtain sodium fluoride product (NaF crystals, purity 99.2%). The RO operating parameters are shown in Table 5.

[0103] Table 5. RO Operating Parameters

[0104]

[0105] As can be seen from Example 1, the present invention achieves a Li (lithium carbonate) recovery rate of ≥98%, a F (sodium fluoride) recovery rate of ≥95%, and a water reuse rate of ≥90%. In-situ alkali production via bipolar membrane electrodialysis reduces LiOH consumption by over 80% and reduces the introduction of impurities into lithium chloride. If NaOH is added in this step, sodium chloride impurities are introduced into the lithium chloride, affecting its purity and consequently the purity of the final product, lithium carbonate, and the lithium recovery rate. The recycling of NaCl reduces reagent costs by over 60%. The production process generates no hazardous waste and is environmentally friendly.

[0106] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for efficient lithium resource recovery based on displacement dialysis process, characterized in that, This method uses a lithium fluoride wastewater treatment system to treat wastewater. The lithium fluoride wastewater treatment system includes a four-compartment displacement dialysis device, a fluoride removal resin tower, a chelation resin tower, an MVR evaporator crystallizer, a lithium carbonate reactor, a bipolar membrane electrodialysis unit, and a reverse osmosis membrane module. The four-compartment displacement dialysis device is arranged sequentially along the water flow direction, with compartment 1, compartment 2, compartment 3, and compartment 4. Adjacent compartments are separated by a cation exchange membrane C or anion exchange membrane A. The outlet of compartment 2 is connected sequentially to the inlet of the defluorination resin tower, the inlet of the chelation resin tower, and the inlet of the MVR evaporator crystallizer. The outlet of the MVR evaporator crystallizer is connected to the inlet of the lithium carbonate reactor and the inlet of the bipolar membrane electrodialysis unit. The alkali outlet of the bipolar membrane electrodialysis unit is connected to the pH adjustment liquid inlet of compartment 1. The by-product outlet of the lithium carbonate reactor is connected to the sodium chloride supply outlet of compartment 3. The outlet of compartment 4 is connected to the inlet of the reverse osmosis membrane module, and the concentrate outlet of the reverse osmosis membrane module is connected to another inlet of the MVR evaporator crystallizer. The method also includes the following steps: Step 1: Pump the lithium fluoride wastewater with pH adjusted to 3-4 into compartment one of the four-compartment displacement dialysis unit; pump the sodium chloride solution into compartment three; and introduce clean water into compartments two and four respectively; start the four-compartment displacement dialysis unit and apply a 10-30V DC electric field. Under the drive of the electric field, the solution in compartment one... They migrate through the cation exchange membrane to compartment two, and from compartment three they migrate to compartment two. A lithium chloride solution is formed; in compartment one It migrates through the anion exchange membrane to compartment four, where it reacts with the Na⁺ that migrates from compartment three to compartment four to form a sodium fluoride solution; Step 2: Pump the lithium chloride solution generated in compartment two into the defluorination resin tower to remove residual fluoride from the solution. Then, it is pumped into a chelating resin tower to remove heavy metal ions; the purified lithium chloride solution enters the MVR evaporator crystallizer, and is concentrated by evaporation to obtain a saturated lithium chloride solution with a concentration of 1.3 wt.%. The distilled water generated during the evaporation process is recycled as reverse osmosis permeate. Step 3: The saturated lithium chloride solution obtained in Step 2 is divided into two parts. One part is pumped into a bipolar membrane electrodialysis unit to prepare lithium hydroxide through bipolar membrane hydrolysis and ion migration. The lithium hydroxide is recycled to adjust the pH of the lithium fluoride wastewater in compartment 1 in Step 1. The other part is pumped into a lithium carbonate reactor. Sodium carbonate solution is added to the reactor, and the reaction is carried out under stirring to generate lithium carbonate precipitate. The lithium carbonate product is obtained by solid-liquid separation. The sodium chloride solution, a byproduct of the reaction, is returned to compartment 3 of Step 1 for recycling. Step 4: Pump the sodium fluoride solution generated in compartment 4 into the pH adjustment tank, add NaOH solution to adjust the pH to 7-8; the adjusted sodium fluoride solution enters the reverse osmosis membrane module and is concentrated 5-10 times by membrane separation; the concentrated sodium fluoride solution enters the MVR evaporator crystallizer and is evaporated and crystallized at a temperature of 60-80℃, and the sodium fluoride product is obtained by solid-liquid separation.

2. The method for efficient lithium resource recovery based on displacement dialysis process according to claim 1, characterized in that, In step 2, the resin filled in the fluoride removal resin tower is... Modified resin, the The modified resin has a particle size of 0.3-0.8 mm and a specific surface area of ​​80-120. .

3. The method for efficient lithium resource recovery based on displacement dialysis process according to claim 1, characterized in that, In step 2, the resin filled in the chelating resin tower is D418 type chelating resin, and the working exchange capacity of the D418 type chelating resin is ≥1.5mmol / g, and the operating temperature is ≤60℃.

4. The method for efficient lithium resource recovery based on displacement dialysis process according to claim 1, characterized in that, In step 1, the initial concentration of the sodium chloride solution is 15-20%. The initial volume ratio of compartment one, compartment two, compartment three, and compartment four is 4:1:1.6:1.

6.

5. The method for efficient lithium resource recovery based on displacement dialysis process according to claim 1, characterized in that, In step 3, the operating parameters of the bipolar membrane electrodialysis unit are as follows: Voltage 2-5V, current density 100-200 Membrane stack temperature 25-40 Feed flow rate 10-20 .

6. The method for efficient lithium resource recovery based on displacement dialysis process according to claim 1, characterized in that, In step 4, the reverse osmosis membrane module uses an anti-fouling aromatic polyamide composite membrane, with an operating pressure of 1.5-2.

5. Temperature 20-30 Inlet flow rate: 1.0-1.5 .

7. The method for efficient lithium resource recovery based on displacement dialysis process according to claim 1, characterized in that, In step 3, the reaction temperature of the lithium carbonate reactor is 60-80°C. Stirring speed 200-300 The amount of sodium carbonate solution added is different from that in the lithium chloride solution. The molar ratio is .

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