A method for continuous separation of metal ions based on ligand selective complexation coupled with preparative vertical free-flow electrophoresis

Abstract

The present application belongs to the field of resource and environmental engineering and electrochemical separation technology, and discloses a metal ion continuous separation method based on ligand selective complexation and preparation type vertical free flow electrophoresis coupling. According to the difference between the complex stability constant and the coordination characteristics of different metal ions and ligands, one or more ligands are added to the solution containing multiple metal ions, so that the metal forms stable complexes with different apparent charges, spatial configurations and electrophoretic mobilities. Then the system is continuously introduced into the PVFE separation device, under the action of an external direct current electric field, different metal species undergo differential directional migration, and continuous separation and enrichment are realized through multiple collection outlets. By adjusting the type of ligand, the concentration of ligand, the pH of the solution, the electric field intensity and the residence time, the high selectivity separation and recovery of metal ions are realized. The present application does not need ion exchange membrane and does not rely on organic extractant, and has the advantages of continuous operation, simple equipment structure, easy modularization and amplification.

Description

Technical Field

[0001] This invention belongs to the field of resource and environmental engineering and electrochemical separation technology, and relates to a continuous separation method for metal ions based on ligand selective complexation and preparative vertical free-flow electrophoresis coupling. Background Technology

[0002] Metal resources are a crucial foundation for the development of new energy, electronic information, advanced manufacturing, and strategic emerging industries. With the rapid development of electric vehicles, energy storage batteries, and wind power, the demand for key metals such as lithium, rare earth elements, nickel, cobalt, and copper continues to grow. However, a large amount of metal resources are widely distributed in complex liquid systems such as metallurgical wastewater, mining wastewater, electroplating wastewater, salt lake brine, and industrial solid waste leachate. These systems have complex compositions, numerous coexisting ions, and many metal ions share similar valence states, ionic radii, and chemical properties, making separation challenging. Currently, metal separation mainly relies on technologies such as solvent extraction, chemical precipitation, adsorption, and membrane separation. While these methods can achieve a certain degree of metal enrichment and recovery, they generally suffer from complex processes, high reagent consumption, high operating costs, potential for secondary pollution, and difficulty in achieving continuous operation, severely restricting the efficient resource utilization of complex multi-metal systems.

[0003] To achieve green and efficient metal separation and recovery, electrochemical separation technology has attracted widespread attention due to its advantages such as low energy consumption and ease of control. Among them, preparative vertical free-flow electrophoresis (PVFE) is a novel membrane-free electrochemical separation technology. It constructs a porous network structure by filling the separation chamber with inert particles, using an electric field to drive the directional migration of charged species, achieving continuous separation and multi-outlet collection. Compared with membrane technologies such as electrodialysis and bipolar membrane electrodialysis, PVFE does not require ion exchange membranes, effectively avoiding problems such as membrane fouling, scaling, and concentration polarization, and exhibits good operational stability and engineering scale-up potential. Furthermore, by introducing complexing ligands for selective coordination control of metal ions, different metal ions can form metal complexes with different stabilities, apparent charges, and electrophoretic mobilities. For example, ligands such as ethylenediaminetetraacetic acid (EDTA) and citric acid can form complexes with significantly different properties with different metal ions. These complexes not only inhibit the hydrolysis and precipitation of metal ions and improve their migration stability in an electric field, but also utilize the differences in complexation abilities between different metal ions and ligands to construct differences in migration rates among metal species. Based on a coupling strategy of ligand-selective complexation and continuous PVFE electrophoretic separation, continuous and highly selective separation and recovery of target metal ions in complex multi-metal systems can be achieved, providing a new technical approach for the efficient utilization of metal resources. Summary of the Invention

[0004] The purpose of this invention is to provide a continuous metal ion separation method based on ligand-selective complexation coupled with preparative vertical free-flow electrophoresis. By utilizing the differences in complexation stability and charge characteristics between different metal ions and ligands, the apparent charge and electrophoretic migration behavior of metal species are controlled. Combined with preparative vertical free-flow electrophoresis technology, continuous and highly selective separation and recovery of target metal ions in complex multi-metal systems can be achieved, thereby solving the problems of low metal separation selectivity, high reagent consumption and difficulty in continuous operation in the prior art.

[0005] The technical solution of the present invention: A method for continuous separation of metal ions based on ligand-selective complexation coupled with preparative vertical free-flow electrophoresis includes the following steps: (1) Perform component analysis on the solution to be separated to determine the types and contents of metal ions; (2) Based on the differences in complexation stability constant and coordination mode between different metal ions and complexing ligands, one or more complexing ligands are added to the solution to be separated so that different metal ions form stable metal complexes with different apparent charge numbers and electrophoretic mobility. (3) The solution to be separated after being regulated by complexed ligands is continuously introduced into a preparative vertical free-flow electrophoresis separation device. Under the action of an external electric field that is intersecting the direction of liquid flow, different metal ions or metal complexes undergo lateral differential migration based on their differences in electrophoretic mobility, thereby achieving spatial separation. (4) Based on the differences in the positional distribution of different metal ions or metal complexes in the separation space, the corresponding metal enrichment liquid is collected through multiple collection outlets, thereby achieving continuous selective separation and recovery of target metal ions; (5) Perform single-stage or multi-stage cyclic separation on the metal enrichment solution obtained in step (4) to improve the enrichment factor of the target metal and the purity of the product.

[0006] The complexing ligand is one or more of citric acid, oxalic acid, tartaric acid, lactic acid, gluconic acid, ethylenediaminetetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA). EDTA and DTPA, due to their strong chelating ability and high negative charge, can react with Fe... 3+ Al 3+ Cu 2+ Ni 2+ High-valence metal cations can form stable anionic metal complexes; citric acid, under acidic conditions, undergoes partial protonation of its carboxyl groups, allowing it to react with Fe. 3+ Al 3+High-valence metal ions form positively charged cationic metal complexes; ligands such as oxalic acid, tartaric acid, lactic acid, and gluconic acid form coordinate bonds with metal ions by providing lone pairs of electrons from the coordinating atoms. Therefore, the metal complexes formed generally maintain the original valence state of the metal ions.

[0007] The molar ratio of the complexed ligand to the metal ion is 1:1 to 20:1.

[0008] In step (2), the pH of the solution to be separated is 1 to 12.

[0009] The difference in apparent charge between the metal complexes formed by the different metal ions is not less than one unit charge.

[0010] The electric field strength in step (3) is 1–1000 V·m -1 .

[0011] The metal ions include Fe. 3+ Al 3+ Cu 2+ Ni 2+ Co 2+ Li + Mg 2+ Ca 2+ And one or more of rare earth metal ions.

[0012] The solution to be separated is red mud leachate, metallurgical waste liquid, electroplating wastewater, mining wastewater, salt lake brine, electronic waste leachate, or waste lithium-ion battery leachate.

[0013] A preparative vertical free-flow electrophoresis separation device includes a power supply system, an electrophoresis separation system, a sample input system, a sample collection system, and a gas collection system; wherein: The power supply section includes a regulated DC power supply 1, the positive terminal of which is connected to the anode plate 2 and the negative terminal is connected to the cathode plate 3, which is used to form an electric field perpendicular to the laminar flow direction of the solution to be separated in the separation chamber 4. The electrophoretic separation system includes a separation chamber 4 for containing the solution to be separated and providing an electromigration separation space for metal ions or metal complexes under the action of an external electric field; the solution to be separated flows continuously in a laminar flow state in the separation chamber 4 from top to bottom; The sample input system is used to continuously input the solution containing metal ions and complexing ligands into the separation chamber 4 through the sample inlet 6; The sample collection system includes two or more outlets 5 located at the lower end of the separation chamber 4, for collecting metal enrichment solutions at different migration positions respectively; The gas collection system includes a gas collection port 7 and a gas guiding structure 8, which are used to collect the gas generated by the electrode reaction.

[0014] Furthermore, the separation chamber 4 is filled with inert particles 10 with a particle size of 0.01 to 3 mm to form a porous medium; Furthermore, the inert particles 10 are one or more of the following: quartz sand, glass microspheres, or polymer particles; Furthermore, the bottom of the separation chamber 4 is provided with two or more water outlets 5, and a baffle 9 is provided between adjacent water outlets 5 to suppress back mixing between water outlets; Furthermore, condensation chambers 11 are provided on both sides of the separation chamber 4, and the condensation chamber 11 is provided with a circulating cooling medium inlet 12 and a circulating cooling medium outlet 13; Furthermore, the gas collection port 7 is located at the top of the separation chamber 4, and is situated on the side of the anode plate 2 and the side of the cathode plate 3, respectively; Furthermore, the gas guiding structure 8 is disposed between the two gas collection ports 7 and is an integrally formed inverted trapezoidal module with a cylindrical channel in the center that communicates with the sample inlet 6; the inclined sidewalls on both sides of the inverted trapezoidal module form a gas guiding surface to guide the gas generated by the electrode reaction to be transported along the guiding surface to the corresponding gas collection port 7, thereby reducing the retention of bubbles in the separation chamber 4. Furthermore, the preparative vertical free-flow electrophoresis separation device adopts a single-stage operation mode or a series operation mode formed by modular stacking of multiple separation units.

[0015] The beneficial effects of this invention are as follows: This invention utilizes ligand-selective complexation to regulate the differences in the existing form, apparent charge, and electrophoretic mobility of metal ions, and combines this with preparative vertical free-flow electrophoresis technology to achieve continuous and highly selective separation and recovery of target metal ions in complex multi-metal systems. This method eliminates the need for ion exchange membranes and organic extractants, effectively avoiding membrane fouling and secondary pollution problems. It boasts advantages such as high separation efficiency, stable operation, and ease of modular scale-up, and can be widely applied to various metal-containing waste liquids and resource recovery systems. Attached Figure Description

[0016] Figure 1 A schematic diagram of the preparative vertical free-flow electrophoresis described in this invention.

[0017] Figure 2 This is a schematic diagram showing the overall structure and partial disassembly of the vertical electrophoresis separation chamber in this invention.

[0018] Figure 3 This is a schematic diagram of the operation mode and cascade configuration of the multi-stage vertical electrophoresis system in this invention.

[0019] In the diagram: 1-Regulated DC power supply; 2-Anode plate; 3-Cathode plate; 4-Separation chamber; 5-Water outlet; 6-Sample inlet; 7-Gas collection port; 8-Gas guiding device; 9-Baffle; 10-Inert particles; 11-Condensation chamber; 12-Circulating cooling medium inlet; 13-Circulating cooling medium outlet; 14-Support; 15-Sample bottle; 16-Single-channel peristaltic pump; 17-Water collection container; 18-Multi-channel peristaltic pump. Detailed Implementation

[0020] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0021] Example 1: Investigation of the migration path of metal complexes in preparative vertical electrophoresis This embodiment uses a mixed system of iron and aluminum ions as the research object to investigate the migration behavior and separation effect of the metal complexes formed after selective ligand complexation in a preparative vertical free-flow electrophoresis apparatus. A mixed solution with iron and aluminum ion concentrations of 15 mg / L was prepared using ferric sulfate and aluminum sulfate. Based on the differences in the complexation stability constants between iron ions and ethylenediaminetetraacetic acid (EDTA) and between aluminum ions and citric acid, solutions were prepared according to the Fe... 3+ EDTA molar ratio 1:1 and Al 3+ Ligands were added to the mixed solution at a Cit molar ratio of 1:2 to form stable Fe–EDTA and Al–Cit complexes, respectively, for iron and aluminum ions. After ligand addition, the pH of the solution was adjusted to 1.5, and stirring was continued until the complexation reaction reached equilibrium. Subsequently, the resulting mixed solution of metal complexes was continuously introduced into a preparative vertical free-flow electrophoresis apparatus for separation. During the separation process, the electric field strength in the separation chamber was controlled at 150 V / m, and the hydraulic residence time was 3 h. Due to the different apparent charge characteristics and electrophoretic mobility of the Fe–EDTA and Al–Cit complexes, they underwent different degrees of lateral migration under the action of the applied electric field, thereby achieving spatial separation of iron and aluminum components. After the separation was stably operated, the effluent from each outlet channel (from the anode side to the cathode side, outlet numbers were 1, 2, 3, 4, 5, and 6), and the concentrations of iron and aluminum were measured. The results are shown in Table 1.

[0022] Table 1 Metal concentrations in the effluent from each outlet

[0023] As shown in Table 1, the concentrations of iron and aluminum in each outlet channel exhibit a clear gradient distribution, with opposite trends. Iron complexes gradually migrate and accumulate towards the anode side, while aluminum complexes gradually migrate and accumulate towards the cathode side, indicating that the two metal complexes exhibit significantly different migration behaviors under the influence of an electric field.

[0024] The above results demonstrate that ligand-selective complexation can effectively alter the form, apparent charge characteristics, and electrophoretic mobility of metal ions, thereby significantly amplifying the migration differences between different metal ions and achieving their directional separation in an electric field. This method provides a technical basis for the continuous and efficient separation and recovery of chemically similar metal ions such as iron and aluminum.

[0025] Example 2: Performance evaluation of the method of the present invention on different metal ion systems. To verify the broad applicability of the method of this invention, based on Example 1, the same separation principle and operating procedure were used to further verify it on various metal systems. First, taking the Sc / Y mixed system as the research object, the charge characteristics and electrophoretic mobility of the two rare earth metal ions were controlled by ligand selective complexation, and its separation effect in a preparative vertical free-flow electrophoresis apparatus was investigated. Second, for the Li / Mg mixed system, the preferential complexation ability of EDTA on high-valence metal ions was utilized to enable Mg... 2+ It forms a stable complex with EDTA, while Li + They remain essentially in a free state, thus establishing differences in apparent charge characteristics and migration behavior between the two, achieving selective migration and removal of Mg ions. Finally, for those containing Cu... 2+ or Ni 2+ In the wastewater system, EDTA was added to form stable metal complexes. The directional migration characteristics of these complexes in an electric field were then utilized to selectively enrich and recover copper and nickel ions. Considering that the separation objective of these systems was primarily the enrichment and recovery of the target metals, rather than migration path characterization, a dual-outlet preparative vertical free-flow electrophoresis apparatus was used for separation. This structure allows for direct collection of enriched products based on the migration direction of the target metals, avoiding the generation of non-target intermediate product streams in multi-outlet apparatuses, improving product recovery rates, and reducing subsequent processing costs. The separation and enrichment results for each system are shown in Table 2.

[0026] Table 2 Separation and enrichment results of different metal systems

[0027] The above results show that the present invention is not only applicable to the separation of metal ions with similar chemical properties such as iron and aluminum, but can also be extended to various systems such as rare earth metals, alkali metals and alkaline earth metals, and transition metals, and has good versatility and application prospects.

[0028] Example 3: Purification performance evaluation of multi-stage vertical electrophoresis Based on the results of Example 2, although the preparative vertical free-flow electrophoresis apparatus exhibited significant selective separation effects on both the ligand-regulated Fe / Al and Sc / Y systems, a small amount of non-target metal ions undergoing antagonistic migration remained in the target metal enrichment solution after single-stage separation, thus limiting the purity of the final product. To further improve the purity of the target metal in the enrichment solution, the target metal enrichment solution obtained during the single-stage separation was used as the feed liquid for the next stage, introduced into the next stage of the preparative vertical free-flow electrophoresis apparatus for further separation and concentration. Each stage of the electrophoresis apparatus operated under the same conditions and in series, performing a total of three stages of concentration. The concentrations of the target metal and impurity metals in the metal enrichment solutions obtained after each stage of concentration were measured, and the product purity was calculated. The results are shown in Table 3.

[0029] Table 3. Metal concentration and product purity in the anolyte after each stage of electrophoresis treatment.

[0030] The results showed that with the increase of the separation stage, both the target metal concentration and product purity continuously improved, while the impurity metal content gradually decreased. This indicates that multi-stage tandem preparative vertical free-flow electrophoresis separation can further achieve deep purification and efficient enrichment of the target metal, providing an effective approach for obtaining high-purity metal products.

Claims

1. A method for continuous separation of metal ions based on ligand-selective complexation coupled with preparative vertical free-flow electrophoresis, characterized in that, Includes the following steps: (1) Perform component analysis on the solution to be separated to determine the types and contents of metal ions; (2) Based on the differences in complexation stability constant and coordination mode between different metal ions and complexing ligands, one or more complexing ligands are added to the solution to be separated so that different metal ions form stable metal complexes with different apparent charge numbers and electrophoretic mobility. (3) The solution to be separated after being regulated by complexed ligands is continuously introduced into a preparative vertical free-flow electrophoresis separation device. Under the action of an external electric field that is intersecting the direction of liquid flow, different metal ions or metal complexes undergo lateral differential migration based on their differences in electrophoretic mobility, thereby achieving spatial separation. (4) Based on the differences in the positional distribution of different metal ions or metal complexes in the separation space, the corresponding metal enrichment liquid is collected through multiple collection outlets, thereby achieving continuous selective separation and recovery of target metal ions; (5) Perform single-stage or multi-stage cyclic separation on the metal enrichment solution obtained in step (4) to improve the enrichment factor of the target metal and the purity of the product.

2. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, The complexing ligand is one or more of citric acid, oxalic acid, tartaric acid, lactic acid, gluconic acid, ethylenediaminetetraacetic acid, and diethylenetriaminepentaacetic acid.

3. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, The molar ratio of the complexed ligand to the metal ion is 1:1 to 20:

1.

4. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, In step (2), the pH of the solution to be separated is 1 to 12.

5. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, The difference in apparent charge between the metal complexes formed by the different metal ions is not less than one unit charge.

6. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, The electric field strength in step (3) is 1–1000 V·m -1 .

7. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, The metal ions include Fe. 3+ Al 3+ Cu 2+ Ni 2+ Co 2+ Li + Mg 2+ Ca 2+ And one or more of rare earth metal ions.

8. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, The solution to be separated is red mud leachate, metallurgical waste liquid, electroplating wastewater, mining wastewater, salt lake brine, electronic waste leachate, or waste lithium-ion battery leachate.

9. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 1, characterized in that, The preparative vertical free-flow electrophoresis separation device includes a power supply system, an electrophoresis separation system, a sample input system, a sample collection system, and a gas collection system; wherein: The power supply section includes a regulated DC power supply (1), the positive terminal of which is connected to the anode plate (2) and the negative terminal is connected to the cathode plate (3), which is used to form an electric field perpendicular to the laminar flow direction of the solution to be separated in the separation chamber (4). The electrophoretic separation system includes a separation chamber (4) for containing the solution to be separated and providing an electromigration separation space for metal ions or metal complexes under the action of an external electric field; the solution to be separated flows continuously in a laminar flow state in the separation chamber (4) from top to bottom; The sample input system is used to continuously input the solution containing metal ions and complexing ligands into the separation chamber (4) through the sample inlet (6); The sample collection system includes two or more outlets (5) located at the lower end of the separation chamber (4) for collecting metal enrichment solutions at different migration positions respectively; The gas collection system includes a gas collection port (7) and a gas guiding structure (8) for collecting the gas generated by the electrode reaction.

10. The method for continuous separation of metal ions based on ligand-selective complexation and preparative vertical free-flow electrophoresis coupling according to claim 9, characterized in that, The separation chamber (4) is filled with inert particles (10) with a particle size of 0.01 to 3 mm to form a porous medium; The inert particles (10) are one or more of the following: quartz sand, glass microspheres or polymer particles; The bottom of the separation chamber (4) is provided with two or more water outlets (5), and baffles (9) are provided between adjacent water outlets (5) to suppress back mixing between water outlets; The separation chamber (4) is provided with condensation chambers (11) on both sides. The condensation chamber (11) is provided with a circulating cooling medium inlet (12) and a circulating cooling medium outlet (13). The gas collection port (7) is located at the top of the separation chamber (4) and is located on the side of the anode plate (2) and the side of the cathode plate (3), respectively. The gas guiding structure (8) is located between two gas collection ports (7) and is an integrally formed inverted trapezoidal module. Its center has a cylindrical channel that communicates with the sample inlet (6). The inclined sidewalls on both sides of the inverted trapezoidal module form a gas guiding surface to guide the gas generated by the electrode reaction to be transported along the guiding surface to the corresponding gas collection port (7), thereby reducing the retention of bubbles in the separation chamber (4). The preparative vertical free-flow electrophoresis separation device can operate in a single-stage mode or in a series mode formed by modular stacking of multiple separation units.

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