Method for concentrating rare earth leaching solution by complexation reinforced membrane

Abstract

This invention provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The method includes the following steps: (1) mixing the rare earth extract and a complexing agent, and adjusting the pH to obtain a pretreated solution; (2) performing membrane filtration on the pretreated solution to obtain a concentrated solution and a permeate; (3) adjusting the pH of the concentrated solution to obtain a concentrated rare earth solution; wherein the aqueous phase obtained from the extraction in step (3) is returned to step (1) for recycling. The method for concentrating rare earth extract using a complexation-enhanced membrane addresses the flux decay during nanofiltration concentration of rare earth-containing extracts. By adding a complexing agent to the extract, rare earth ions are complexed with calcium and magnesium ions. The complexing agent increases the effective size of rare earth ions, improving the retention rate of rare earth ions, reducing the concentration of free calcium and magnesium ions, reducing fouling on the membrane surface, and improving the membrane's operating efficiency.

Description

Technical Field

[0001] This invention relates to the fields of membrane separation and hydrometallurgical technology, and in particular to a method for concentrating rare earth leachate using a complexation-enhanced membrane. Background Technology

[0002] In the rare earth hydrometallurgical process, rare earth ores are leached using a sulfuric acid system to obtain rare earth ions (such as La). 3+ Ce 3+ 、Nd 3+ (etc.) and a large number of impurity ions (Ca 2+ Mg 2+ SO4 2- The extract (etc.) has a relatively low ion concentration and a large water volume. Using nanofiltration membrane technology to concentrate and reduce the volume of this extract is a key technical approach to reduce the processing load of subsequent extraction processes.

[0003] However, this membrane concentration process faces three major technical bottlenecks: some commercial nanofiltration membranes struggle to achieve ideal retention of rare earth ions with small hydration radii, such as lanthanum ions, leading to the loss of rare earth ions through the membrane. Secondly, the presence of Ca in the extract... 2+ Mg 2+ SO4 2- Small molecular ions and concentration polarization exacerbate the accumulation of solutes on the membrane surface, generating enormous osmotic pressure during concentration, leading to a sharp increase in the required driving pressure and a dramatic increase in energy consumption. Simultaneously, as the concentration factor increases, Ca... 2+ Mg 2+ With SO4 2- When the salt concentration exceeds the solubility of calcium sulfate and magnesium sulfate, a dense scale layer crystallizes on the membrane surface, leading to irreversible flux decline. The synergistic effect of these factors ultimately results in decreased membrane flux, increased energy consumption, shortened membrane life, and degraded separation performance, while also increasing cleaning frequency and operating costs.

[0004] In the prior art, the addition of complexing agents to treat rare earth solutions has been reported. For example, CN121023264B discloses a method for separating heavy rare earth ions, which uses a vinyl complexing agent including at least one of vinylphosphonic acid, vinylsulfonic acid, vinylacetic acid, or 4-vinylbenzyl chloride to selectively complex heavy rare earth ions in an aqueous phase. The aqueous phase containing the heavy rare earth ion complex is then extracted with an organic phase containing an extractant to separate different heavy rare earth ions. CN113737030B discloses a method for separating rare earths using a water-soluble polymeric complexing agent. This method uses phosphorylated chitosan (PCS) as a complexing agent and utilizes the difference in shear stability of PCS-RE complexes formed by PCS complexing with different rare earth ions. Shear-discomplexing coupled ultrafiltration is then used to separate mixed rare earth ion solutions. CN121023263A discloses a method for separating heavy rare earth ions based on phosphate-based complexing agents. The phosphate-based complexing agents can form significantly different and reversible heavy rare earth-phosphate complexes with various heavy rare earth elements. The aqueous phase containing these complexes is then extracted and purified from the organic phase to achieve efficient separation of the heavy rare earth ions. However, these technologies primarily focus on the application of complexing agents in the extraction stage, rather than developing their multifunctionality in the membrane concentration stage. The direct use of complexing agents in pretreatment before membrane processing, their complexation behavior with rare earth elements, calcium, and magnesium, and their impact on the membrane process have not been systematically studied and applied. Furthermore, traditional polyacrylic acid or phosphine-based scale inhibitors are mostly high-molecular-weight organic polymers. While they can delay scaling, they do not contribute to reducing osmotic pressure. Moreover, as organic substances, they are easily adsorbed onto the membrane surface or utilized as a nutrient source by microorganisms in the water, thus exacerbating organic and biological fouling of the membrane system.

[0005] To address the shortcomings of existing technologies, this invention provides a complexation pretreatment method that can significantly improve the overall efficiency of the membrane concentration process. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention provides a method for concentrating rare earth extracts using a complexation-enhanced membrane. This invention is a method that simultaneously improves the rare earth retention rate, reduces osmotic pressure, and inhibits calcium and magnesium scaling by using a complexing agent.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] This invention provides a method for concentrating rare earth extracts using a complexation-enhanced membrane, the method comprising the following steps:

[0009] (1) Mix rare earth extract and complexing agent, and adjust pH first to obtain pretreated solution;

[0010] (2) The pretreatment liquid is subjected to membrane filtration to obtain a concentrate and a permeate;

[0011] (3) The concentrated solution is subjected to a second pH adjustment and extraction separation to obtain a concentrated rare earth solution;

[0012] The aqueous phase obtained from the extraction and separation in step (3) is returned to step (1) for recycling.

[0013] This invention adds a complexing agent to the extraction solution to complex rare earth ions with calcium and magnesium ions. The complexing agent increases the effective size of rare earth ions, thereby improving the retention rate of rare earth ions, reducing the concentration of free calcium and magnesium ions, reducing fouling on the membrane surface, and improving the membrane's operating efficiency.

[0014] This invention adds a complexing agent to the feed liquid to bind the originally freely moving small ions (REs). 3+ Ca 2+ Mg 2+ The rare earth element (REE) is transformed into a larger complex molecule, improving the rejection rate of the nanofiltration membrane. Rare earth extracts are complex multi-ion systems; complexation alters the interactions between ions. A single complexing molecule can bind one or more rare earth / calcium / magnesium ions, forming larger, more complex soluble complexes. The number of freely moving small ions in the system is greatly reduced, replaced by fewer, larger complex molecules. According to the van der Hoff equation π = iCRT, osmotic pressure is directly proportional to the molar concentration of solute particles in the solution. Therefore, the particle concentration difference across the membrane decreases significantly, and the osmotic pressure also decreases. The number of free calcium and magnesium ions decreases, and sulfate ions exist in the solution as free ions, increasing the calcium and magnesium content of the ions. 2+ Mg 2+ With SO4 2- The salt concentration is much lower than the solubility of calcium sulfate and magnesium sulfate, which fundamentally blocks their combination with sulfate ions, effectively reducing the deposition of calcium and magnesium ions on the membrane surface. This achieves the purpose of inhibiting inorganic salt scaling, extending the membrane cleaning cycle, and improving the operational stability and economy of membrane concentration.

[0015] As a preferred technical solution of the present invention, the pH of the rare earth leachate is 4 to 5, for example, it can be 4, 4.5 or 5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] As a preferred embodiment of the present invention, the metal ions in the rare earth leachate include RE. 3+ Ca 2+ or Mg 2+ Any one or at least two of them.

[0017] Preferably, the rare earth extract contains RE 3+The salt concentration is 0.1~0.3 g / L, for example, it can be 0.1 g / L, 0.2 g / L or 0.3 g / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] Preferably, the RE 3+ Including La 3+ Ce 3+ Pr 3+ 、Nd 3+ Pm 3+ 、Sm 3+ Eu 3+ Gd 3+ 、Tb 3+ Dy 3+ Ho 3+ Er 3 + Tm 3+ Yb 3+ Lu 3+ ,Sc 3+ Or Y 3+ Any one or at least two of them.

[0019] As a preferred embodiment of the present invention, the complexing agent includes any one or a combination of at least two of diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, sodium poly(4-styrene sulfonate), tetrasodium N,N-bis(carboxymethyl)-L-glutamate, citric acid, or polyethyleneimine. Typical but non-limiting combinations include combinations of diethylenetriaminepentaacetic acid and sodium poly(4-styrene sulfonate), combinations of diethylenetriaminepentaacetic acid and tetrasodium N,N-bis(carboxymethyl)-L-glutamate, combinations of tetrasodium N,N-bis(carboxymethyl)-L-glutamate and citric acid, combinations of tetrasodium N,N-bis(carboxymethyl)-L-glutamate and citric acid, and combinations of tetrasodium N,N-bis(carboxymethyl)-L-glutamate, citric acid, and polyethyleneimine, etc., preferably diethylenetriaminepentaacetic acid.

[0020] In this invention, the complexing agent exhibits an extremely high complexing constant for rare earth ions (e.g., logK RE-DTPA ≈ 18~21). After complexation pretreatment, the effective size of rare earth ions increases, and the retention rate of rare earth ions by the nanofiltration membrane is improved. The complexing agent also has a high complexing constant for Ca... 2+ Mg 2+ Although the complexation constant of the calcium and magnesium ions is lower than that of rare earth elements (e.g., logK Ca-DTPA ≈ 8~10, logK Mg-DTPA ≈ 6~8), it can still occur effectively under given pH conditions. The reduced concentration of free calcium and magnesium ions fundamentally blocks their binding with sulfate ions, effectively inhibiting the nucleation and deposition of inorganic salt scale such as calcium sulfate on the membrane surface, resulting in a significant reduction in osmotic pressure and remarkable energy-saving effect.

[0021] As a preferred technical solution of the present invention, the complexing agent and the rare earth extract containing RE 3+ The molar ratio is (0.5~2):1, for example, it can be 0.5:1, 0.75:1, 1:1, 1.25:1, 1.5:1, 1.75:1 or 2:1, etc., but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0022] This invention combines the complexing agent with RE in the rare earth extract. 3+ When the molar ratio is controlled within the above range, if the complexing agent is lower than the optimal ratio, free ions will still exist in the solution, forming deposits that clog the membrane pores. Furthermore, the positively charged ions neutralize the negatively charged membrane surface, reducing the electrostatic repulsion barrier between the membrane and the pollutants. This leads to the dense deposition of pollutants on the membrane surface, forming a high-resistance filter cake layer, thus reducing the flux. If the complexing agent is higher than the optimal ratio, excessive complexing agent molecules accumulate near the membrane surface, forming a concentration polarization layer or directly adhering to the membrane surface, increasing the permeation resistance of the boundary layer. At the same time, high concentrations of complexing agent may change the viscosity of the solution, increasing the filtration resistance and reducing the flux to a low level.

[0023] As a preferred technical solution of the present invention, the first adjustment of pH to 4.5~8.5 can be, for example, 4.5, 5.5, 6.5, 7, 7.5 or 8.5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] This invention controls the pH within the aforementioned range, thereby optimizing the coordination environment of the complexing agent: under appropriate neutral or weakly alkaline conditions, it can effectively inhibit H+. + The competition with the active groups of the complexing agent, while preventing the hydrolysis and precipitation of metal ions, promotes the formation of stable, soluble complexes between the complexing agent and rare earth, magnesium, and calcium ions. If the pH is too low (too acidic), a large amount of H₂... + Protonation of the carboxyl or amino groups of the complexing agent significantly reduces its effective coordination sites, leading to incomplete or even no complexation. If the pH is too high (too alkaline), rare earth ions, magnesium, and calcium ions easily form hydroxides or basic salt precipitates, which can reduce complexation efficiency or completely destroy the complexation process. Therefore, only by precisely controlling the pH within a suitable range can efficient and stable complexation effects be achieved.

[0025] As a preferred technical solution of the present invention, the membrane material for the membrane filtration treatment includes any one or a combination of at least two of nanofiltration membranes, loose nanofiltration membranes, or ultrafiltration membranes.

[0026] Preferably, the molecular weight cutoff of the membrane material is 200~8000 Da, for example, it can be 200 Da, 500 Da, 800 Da, 2000 Da, 3000 Da, 5000 Da or 8000 Da, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 300~2000 Da.

[0027] As a preferred technical solution of the present invention, the second adjustment of pH to 3~6 can be, for example, 3, 3.5, 4, 4.5, 5, 5.5 or 6, etc., but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 3.

[0028] As a preferred technical solution of the present invention, the extraction separation includes extraction and back extraction.

[0029] Preferably, the extractant used in the extraction includes P204 and / or P507.

[0030] Preferably, the back-extraction agent includes hydrochloric acid or nitric acid, with hydrochloric acid being the preferred choice.

[0031] Preferably, the concentration of the hydrochloric acid is 1~5 mol / L, for example, it can be 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L or 5 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 2 mol / L.

[0032] As a preferred technical solution of the present invention, the volume concentration factor of the concentrate in step (2) reaches 4 to 10 times, for example, it can be 4, 5, 6, 7, 8, 9 or 10, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] Compared with existing technical solutions, the present invention has at least the following beneficial effects:

[0034] (1) The method for concentrating rare earth extract using a complexation-enhanced membrane provided by this invention has the following advantages: the concentration of free rare earth, calcium, and magnesium ions is reduced, with a preferred lanthanum ion rejection rate of 97.67%, a preferred calcium ion rejection rate of 95.09%, and a preferred magnesium ion rejection rate of 97.18%, fundamentally blocking their combination with sulfate ions, effectively inhibiting the nucleation and deposition process of inorganic salt scale such as calcium sulfate on the membrane surface, significantly reducing osmotic pressure, resulting in significant energy savings, and no significant decrease in pure water flux after continuous concentration operation. This invention simultaneously achieves three major functions—concentration and separation, increased flux, and reduced pollution—through a single pretreatment step. The process is simple and efficient, avoiding the complexity and high cost of multi-step processing.

[0035] (2) The method for concentrating rare earth extracts using a complexation-enhanced membrane provided by this invention has the following advantages: Through the precise addition of the complexing agent and the synergistic effect of multiple mechanisms of action, efficient scale inhibition is achieved with lower reagent consumption, reducing the need for frequent chemical cleaning due to membrane scaling, thereby significantly reducing the consumption of acid and alkali cleaning agents and the generation of cleaning wastewater. Simultaneously, the complexing agent is extracted and reused after dissociation, reducing the amount of reagents and alkali consumed in the later stages, thus lowering production costs. The reduced replacement frequency of membrane elements due to scaling damage further extends the service life of the membrane assembly and reduces the overall operating cost of the membrane concentration process. Detailed Implementation

[0036] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0037] In one specific embodiment, the present invention provides a method for concentrating rare earth extracts using a complexation-enhanced membrane, the method comprising the following steps:

[0038] It should be clarified that any use of the process provided in the embodiments of the present invention or any substitution or change of conventional data falls within the protection and disclosure scope of the present invention.

[0039] Example 1

[0040] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane, the method comprising the following steps:

[0041] Prepare a simulated rare earth leaching solution: La2(SO4)3 0.3 g / L, CaSO4 0.1 g / L, MgSO4 0.45 g / L, pH = 4.5.

[0042] (1) Complexation pretreatment: to the La-containing 3+ Ca 2+ Mg 2+ SO4 2- Diethylenetriaminepentaacetic acid (DTPA:La) was added to the rare earth extract. 3+ (Molar ratio = 1:1) and adjust the pH to 7 to allow the complexing agent to react with La. 3+ Ca 2+ Mg 2+ A complexation reaction occurs, forming a La, Ca, Mg-complex, resulting in a pretreated solution;

[0043] (2) Membrane concentration and separation: The pretreated liquid obtained in step (1) is passed into a loose nanofiltration membrane unit with a molecular weight cutoff of 2000 Da for concentration to obtain a high-concentration concentrate and a permeate;

[0044] (3) Complex disruption and recovery: Adjust the pH of the concentrate obtained in step (2) to 3 to disrupt the stability of the complex and release La 3+ Ca 2 + Mg 2+ And complexing agent; then extraction was performed using extractant P507, La 3+ Preferentially entering the organic phase, while complexing agents and Ca... 2+ Mg 2+ Remains in the aqueous phase;

[0045] (4) Complexing agent recycling: The aqueous phase containing the complexing agent obtained in step (3) is returned to step (1) for reuse;

[0046] (5) Rare earth back-extraction: the La-loaded rare earth obtained in step (3) 3+ The organic phase was subjected to hydrochloric acid back-extraction to obtain a concentrated rare earth solution.

[0047] Example 2

[0048] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane, the method comprising the following steps:

[0049] Prepare a simulated rare earth leaching solution: La2(SO4)3 0.3 g / L, CaSO4 0.1 g / L, MgSO4 0.45 g / L, pH = 4.

[0050] (1) Complexation pretreatment: to the La-containing 3+ Ca 2+ Mg 2+ SO4 2- Diethylenetriaminepentaacetic acid (DTPA:La) was added to the rare earth extract. 3+ (Molar ratio = 0.5:1) and adjust the pH to 7 to allow the complexing agent to react with La. 3+ Ca 2+ Mg 2+ A complexation reaction occurs, forming a La, Ca, Mg-complex, resulting in a pretreated solution;

[0051] (2) Membrane concentration and separation: The pretreated liquid obtained in step (1) is passed into an ultrafiltration membrane unit with a molecular weight cutoff of 2000 Da for concentration to obtain a high-concentration liquid and a permeate;

[0052] (3) Complex disruption and recovery: Adjust the pH of the concentrate obtained in step (2) to 3 to disrupt the stability of the complex and release La 3+ Ca 2 + Mg 2+ And complexing agent; then extraction was performed using extractant P507, La 3+Preferentially entering the organic phase, while complexing agents and Ca... 2+ Mg 2+ Remains in the aqueous phase;

[0053] (4) Complexing agent recycling: The aqueous phase containing the complexing agent obtained in step (3) is returned to step (1) for reuse;

[0054] (5) Rare earth back-extraction: the La-loaded rare earth obtained in step (3) 3+ The organic phase was subjected to hydrochloric acid back-extraction to obtain a concentrated rare earth solution.

[0055] Example 3

[0056] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane, the method comprising the following steps:

[0057] Prepare a simulated rare earth leaching solution: La2(SO4)3 0.3 g / L, CaSO4 0.1 g / L, MgSO4 0.45 g / L, pH = 5.

[0058] (1) Complexation pretreatment: to the La-containing 3+ Ca 2+ Mg 2+ SO4 2- Diethylenetriaminepentaacetic acid (DTPA:La) was added to the rare earth extract. 3+ (Molar ratio = 2:1) and adjust the pH to 7 to allow the complexing agent to react with La. 3+ Ca 2+ Mg 2+ A complexation reaction occurs, forming a La, Ca, Mg-complex, resulting in a pretreated solution;

[0059] (2) Membrane concentration and separation: The pretreated liquid obtained in step (1) is passed into a nanofiltration membrane unit with a molecular weight cutoff of 2000 Da for concentration to obtain a high-concentration concentrate and a permeate;

[0060] (3) Complex disruption and recovery: Adjust the pH of the concentrate obtained in step (2) to 6 to disrupt the stability of the complex and release La 3+ Ca 2 + Mg 2+ And complexing agent; then extraction was performed using extractant P507, La 3+ Preferentially entering the organic phase, while complexing agents and Ca... 2+ Mg 2+ Remains in the aqueous phase;

[0061] (4) Complexing agent recycling: The aqueous phase containing the complexing agent obtained in step (3) is returned to step (1) for reuse;

[0062] (5) Rare earth back-extraction: the La-loaded rare earth obtained in step (3) 3+ The organic phase was subjected to hydrochloric acid back-extraction to obtain a concentrated rare earth solution.

[0063] Example 4

[0064] This embodiment provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The only difference between this method and Example 1 is that diethylenetriaminepentaacetic acid is added and the pH value is adjusted to 4.5. All other aspects are the same as in Example 1.

[0065] Example 5

[0066] This embodiment provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The only difference between this method and Example 1 is that diethylenetriaminepentaacetic acid is added and the pH value is adjusted to 8.5. All other aspects are the same as in Example 1.

[0067] Example 6

[0068] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane. The only difference between this method and Example 1 is that the complexing agent is changed to citric acid (CA:La). 3+ The molar ratio was 1:1, and the pH was adjusted to 7. Everything else was the same as in Example 1.

[0069] Example 7

[0070] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane. The only difference between this method and Example 1 is that the complexing agent is changed to sodium poly(4-styrene sulfonate) (PSS:La). 3+ The molar ratio was 1:1, and the pH was adjusted to 7. Everything else was the same as in Example 1.

[0071] Example 8

[0072] This embodiment provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The only difference between this method and Example 1 is that a nanofiltration membrane unit with a molecular weight cutoff of 300 Da is used instead. All other aspects are the same as in Example 1.

[0073] Example 9

[0074] This embodiment provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The only difference between this method and Example 1 is that: in step (1), diethylenetriaminepentaacetic acid is not added separately; only the aqueous phase containing the complexing agent reused in step (3) is used as the complexing agent, and the pH is adjusted to 7. The rest is the same as in Example 1.

[0075] Example 10

[0076] This embodiment provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The only difference between this method and Example 8 is that: in step (1), diethylenetriaminepentaacetic acid is not added separately; only the aqueous phase containing the complexing agent reused in step (3) is used as the complexing agent, and the pH is adjusted to 7. The rest is the same as in Example 8.

[0077] Example 11

[0078] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane. The only difference between this method and Example 1 is the change in the amount of diethylenetriaminepentaacetic acid (DTPA:La). 3+ (Molar ratio = 0.1:1), all other parameters are the same as in Example 1.

[0079] Example 12

[0080] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane. The only difference between this method and Example 1 is the change in the amount of diethylenetriaminepentaacetic acid (DTPA:La). 3+ (Molar ratio = 2.5:1), all other parameters are the same as in Example 1.

[0081] Example 13

[0082] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane. The only difference between this method and Example 1 is the addition of diethylenetriaminepentaacetic acid (DTPA:La). 3+ The molar ratio was 1:1, and the pH was adjusted to 4. Everything else was the same as in Example 1.

[0083] Example 14

[0084] This embodiment provides a method for concentrating rare earth extracts using a complexation-enhanced membrane. The only difference between this method and Example 1 is the addition of diethylenetriaminepentaacetic acid (DTPA:La). 3+ The molar ratio was 1:1, and the pH was adjusted to 10. Everything else was the same as in Example 1.

[0085] Comparative Example 1

[0086] This comparative example provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The only difference between this method and Example 1 is that the complexing agent diethylenetriaminepentaacetic acid is not added, and the aqueous phase containing the complexing agent in step (3) is not returned to step (1) for reuse. All other aspects are the same as in Example 1.

[0087] Comparative Example 2

[0088] This comparative example provides a method for concentrating rare earth extract using a complexation-enhanced membrane. The only difference between this method and Example 8 is that the complexing agent diethylenetriaminepentaacetic acid is not added, and the aqueous phase containing the complexing agent in step (3) is not returned to step (1) for reuse. All other aspects are the same as in Example 8.

[0089] Performance testing

[0090] The membrane performance was evaluated using a cross-flow filtration device (SF-SB, Hangzhou Saifei Membrane Separation Technology Co., Ltd., China). The effective filtration area of ​​this system was 7 cm². 2 Before testing membrane performance, deionized water was used as the pretreatment feed solution. This pretreatment was performed at a constant pressure of 5 bar for 30 min to achieve stable permeate flux, ensure membrane stability, and eliminate the influence of initial compaction on subsequent tests. Nanofiltration concentration tests were then conducted at 25°C, a constant pressure of 10 bar, and a cross-flow velocity of 50 L / h. The experiment was terminated after the volumetric concentration factor reached 8 times. Pure water flux tests were performed at 5 bar before membrane operation, after operation, and after hydraulic cleaning (high cross-flow velocity flushing).

[0091]

[0092] (1) The degree of complexation between the target substance and the complexing agent was evaluated by measuring the membrane’s rejection rate of the target substance. The rejection rate of the membrane for the target substance after the addition of the complexing agent was tested, and the rejection rate of the membrane for the target substance without the addition of the complexing agent was compared. The results are shown in Table 1.

[0093]

[0094] (2) The pure water flux before and after the membrane filtration concentration of 4 to 8 times and the flux change during the filtration process were tested. The results are shown in Table 2.

[0095]

[0096] A comprehensive comparison of Examples 1 and 6-7 shows that, in Example 6, when CA was used as the complexing agent, the 2000 Da loose nanofiltration membrane exhibited rejection rates of 17.71%, 14.64%, and 24.91% for lanthanum, calcium, and magnesium ions, respectively. CA struggled to achieve sufficient complexation of metal ions, thus its improvement effect on the membrane deposition process was relatively limited and failed to reach the desired level. In Example 7, when PSS was used as the complexing agent, the 2000 Da loose nanofiltration membrane showed rejection rates of 36.15%, 19.66%, and 5.94% for lanthanum, calcium, and magnesium ions, respectively. Furthermore, the polymer easily caused physical blockage and accumulation, forming a gel layer on the membrane surface. After complexing with cations, the negative charge on the polymer chains was neutralized, the electrostatic repulsion between polymer chains disappeared, and the change in molecular weight structure led to the formation of a dense, irreversible fouling layer, resulting in a significant decrease in flux and preventing concentration to 8 times. This indicates that DTPA was the optimal complexing agent for cations.

[0097] A comprehensive comparison of Examples 1, 8-10 shows that after adding the recycled complexing agent, the salt rejection rate of the membrane is basically the same as that when the fresh complexing agent is added, indicating that DTPA does not undergo irreversible structural damage during the decomplexing extraction process and has good chemical stability and reusability.

[0098] A comparative analysis of Examples 1-5 and Examples 11-14 shows that increasing the DTPA dosage initially increases the salt rejection rate of the 2000 Da membrane, followed by a decrease. Increasing the pH value causes the rejection rate to initially increase and then level off. This indicates that increasing the DTPA concentration increases the number of coordination sites and improves the efficiency of the coordination reaction, resulting in an increased rejection rate. However, excessively high concentrations lead to severe organic pollution and oversaturation of complexation sites, causing a decrease in the rejection rate. Neutral or weakly alkaline environments promote the dissociation of the carboxyl groups in the DTPA molecule. The dissociated carboxyl oxygen atoms are more likely to provide lone pairs of electrons to form coordinate bonds with the empty orbitals of trivalent ions, significantly enhancing the complexation ability of DTPA for metal ions. However, in strongly alkaline environments, such as in Example 14, metal ions easily form hydroxides or basic salt precipitates, disrupting the complexation process and making it impossible to measure the effective rejection rate.

[0099] A comprehensive comparison of Examples 1-5 and Examples 11-12 shows that, under optimal formulation conditions, the complexing agent forms stable complexes with metal ions. Complexes larger than the membrane pores reduce pore blockage. Simultaneously, the carboxyl and hydroxyl functional groups in the complexing agent molecule coordinate with metal ions, exposing hydrophilic groups and enhancing the hydrophilicity of the complex, maintaining high flux and reducing irreversible fouling. When the complexing agent ratio is lower than optimal, free ions still exist in the solution, forming deposits that cause membrane pore blockage. Furthermore, positively charged ions neutralize the negative charge on the membrane surface, lowering the electrostatic repulsion barrier between the membrane and pollutants, leading to dense pollutant deposition on the membrane surface, forming a high-resistance filter cake layer, thus reducing flux. When the complexing agent ratio is higher than optimal, excessive complexing agent molecules accumulate near the membrane surface, forming a concentration polarization layer or directly adhering to the membrane surface, increasing the boundary layer permeation resistance. Simultaneously, high concentrations of complexing agent may alter the solution viscosity, forming organic fouling, increasing filtration resistance, and reducing flux to a low level.

[0100] A comprehensive comparison of Examples 1 and 8 and Comparative Examples 1 and 2 shows that, after adding the complexing agent, the metal ions combine with the complexing agent to form a larger complex, reducing the positive charge density of the metal ions. Under the synergistic effect of size sieving and the Donnan effect, the salt rejection rate of the membrane is significantly improved. Furthermore, compared to the 2000 Da large-pore membrane in Example 1, the 300 Da small-pore membrane in Example 8 has a more compact pore structure, reducing the probability of complexes entering and clogging the pores, thus resulting in weaker pore clogging. After washing, the permeate flux did not decrease. In Comparative Example 2, the 300 Da small-pore membrane, without the addition of the complexing agent, although having a high rejection rate, showed a significant flux decline. The initial feed flux and the flux after concentration in Comparative Example 2 were both lower than those in Example 8, and the flux recovery rate after washing was also significantly lower. This indicates that relying solely on small-pore physical rejection is insufficient to prevent membrane fouling, while adding a complexing agent can effectively slow down flux decline and improve membrane washability while maintaining high rejection.

[0101] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for concentrating rare earth extract using a complexation-enhanced membrane, characterized in that, The method includes the following steps: (1) Mix rare earth extract and complexing agent, and adjust pH first to obtain pretreated solution; (2) The pretreatment liquid is subjected to membrane filtration to obtain a concentrate and a permeate; (3) The concentrated solution is subjected to a second pH adjustment and extraction separation to obtain a concentrated rare earth solution; The aqueous phase obtained from the extraction and separation in step (3) is returned to step (1) for recycling.

2. The method according to claim 1, characterized in that, The pH of the rare earth leachate is 4-5.

3. The method according to claim 1 or 2, characterized in that, The rare earth leachate contains metal ions including RE. 3 + Ca 2+ or Mg 2+ Any one or at least two of them; Preferably, the rare earth extract contains RE 3+ The salt concentration is 0.1~0.3 g / L; Preferably, the RE 3+ Including La 3+ Ce 3+ Pr 3+ 、Nd 3+ Pm 3+ 、Sm 3+ Eu 3+ Gd 3+ 、Tb 3+ Dy 3+ Ho 3+ Er 3+ Tm 3 + Yb 3+ Lu 3+ ,Sc 3+ Or Y 3+ Any one or at least two of them.

4. The method according to any one of claims 1 to 3, characterized in that, The complexing agent includes any one or a combination of at least two of diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, sodium poly(4-styrenesulfonate), tetrasodium N,N-bis(carboxymethyl)-L-glutamic acid, citric acid, or polyethyleneimine, preferably diethylenetriaminepentaacetic acid.

5. The method according to any one of claims 1 to 4, characterized in that, The complexing agent and rare earth extract containing RE 3+ The molar ratio is (0.5~2):

1.

6. The method according to any one of claims 1 to 5, characterized in that, The first step is to adjust the pH to 4.5-8.

5.

7. The method according to any one of claims 1 to 6, characterized in that, The membrane material used in the membrane filtration process includes any one or a combination of at least two of nanofiltration membranes, loose nanofiltration membranes, or ultrafiltration membranes. Preferably, the membrane material has a molecular weight cutoff of 200-8000 Da, more preferably 300-2000 Da.

8. The method according to any one of claims 1 to 7, characterized in that, The second adjustment is to pH 3-6, preferably 3.

9. The method according to any one of claims 1 to 8, characterized in that, The extraction separation includes extraction and back extraction; Preferably, the extractant used in the extraction includes P204 and / or P507; Preferably, the back-extraction agent includes hydrochloric acid or nitric acid, with hydrochloric acid being preferred; Preferably, the concentration of the hydrochloric acid is 1~5 mol / L, more preferably 2 mol / L.

10. The method according to any one of claims 1 to 9, characterized in that, The volume concentration factor of the concentrate in step (2) reaches 4 to 10 times.

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