A method and device for separating magnesium and lithium by low-voltage electrodialysis assisted by a salt bridge based on lithium super ionic conductor

Abstract

The application provides a salt bridge assisted low-voltage electrodialysis magnesium-lithium separation method and device based on lithium superionic conductor. The method is as follows: lithium superionic conductor is used to divide an electrodialysis cell into an anode chamber and a cathode chamber; during operation, the anode chamber is injected with a raw material solution, and the cathode chamber is injected with an extraction solution; one end of a salt bridge is in contact with the raw material solution, and the other end of the salt bridge is in contact with the extraction solution; the other ends of the two salt bridges are immersed in an auxiliary chamber containing a lithium-free solution; a voltage lower than the solution decomposition voltage is applied to the electrodialysis cell, and the extraction solution in the cathode chamber is transferred and output by a pump and a pipe after electrodialysis. The method solves the problem of charge imbalance between the anode solution and the cathode solution in the process of magnesium-lithium separation by electrodialysis based on lithium superionic conductor by using a salt bridge, instead of realizing charge balance by electrolyzing the solution at a high voltage, thereby effectively reducing the energy consumption of magnesium-lithium separation, and simultaneously having high magnesium-lithium separation efficiency and good application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of material science and chemical engineering, and particularly relates to a low-voltage electrodialysis magnesium-lithium separation method and device based on lithium super ionic conductor and assisted by salt bridge. BACKGROUND

[0002] The application of lithium super ionic conductor in magnesium-lithium separation has the advantages of large lithium flux and remarkable separation effect, and has obvious technical advantages compared with traditional magnesium-lithium separation methods such as precipitation method, extraction method, adsorption method and the like. However, the application of lithium super ionic conductor in magnesium-lithium separation under low voltage cannot realize the charge balance of the anode solution and the cathode solution, and it is necessary to electrolyze the brine under high voltage (>2.0 V) to generate chlorine and hydrogen and the like to ensure the charge balance, which not only needs to handle toxic and harmful gases, but also has high power consumption cost, thereby affecting the popularization of related technologies. If the charge balance in the separation process can be ensured, the application of lithium super ionic conductor in the separation of magnesium ions and lithium ions in salt lake brine and seawater under less than 2.0 V can be realized, the device can be simplified, the energy consumption can be reduced, and the application potential of lithium super ionic conductor in the field of magnesium-lithium separation can be better exerted. SUMMARY

[0003] The purpose of the application is to provide a low-voltage electrodialysis magnesium-lithium separation method based on lithium super ionic conductor and assisted by salt bridge, which aims to balance the charges of anions and cations in the solution by using salt bridge, better exert the magnesium-lithium separation effect of lithium super ionic conductor, realize the application of lithium super ionic conductor in the separation of magnesium ions and lithium ions in salt lake brine and seawater under low voltage, and reduce the magnesium-lithium separation cost.

[0004] To achieve the above purpose, the application provides a low-voltage electrodialysis magnesium-lithium separation method based on lithium super ionic conductor and assisted by salt bridge, which comprises the following steps:

[0005] 1) The electrodialysis cell is separated into an anode chamber and a cathode chamber by lithium super ionic conductor, and when working, the anode chamber is injected with raw material liquid, and the cathode chamber is injected with extraction liquid;

[0006] 2) One end of one salt bridge is in contact with the raw material liquid, one end of the other salt bridge is in contact with the extraction liquid, and the other ends of the two salt bridges are immersed in an auxiliary chamber containing lithium-free solution together;

[0007] 3) A voltage lower than the decomposition voltage of the solution is applied to the electrodialysis cell, and the extraction liquid in the cathode chamber is transferred and output by a pump and a pipe after electrodialysis.

[0008] Preferably, a plurality of electrodialysis cells are used in series, the extraction liquid output by the previous-stage electrodialysis cell is transported into the anode chamber of the next-stage electrodialysis cell to continue electrodialysis, and the electrodialysis liquid of the next-stage electrodialysis cell is returned to the cathode chamber of the previous-stage electrodialysis cell to continue use.

[0009] Preferably, the raw material solution in step 1) is recycled salt lake brine, seawater or the extraction solution delivered from the previous electrodialysis cell, and the extraction solution is a lithium-free solution or the post-electrodialysis solution returned from the next electrodialysis cell.

[0010] Preferably, the lithium-free solution used is one or more of pure water, NaCl, KCl, NH4Cl, Na2SO4, K2SO4, (NH4)2SO4, NaNO3, KNO3 and NH4NO3 aqueous solution.

[0011] Preferably, the lithium superionic conductor used is one or more of oxide conductor, polymer conductor and oxide-polymer composite conductor, and the thickness of the lithium superionic conductor is 1 μm to 10 mm.

[0012] Preferably, the oxide conductor is one or more of Li 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, garnet-type Li7La3Zr2O 12 , perovskite-type Li 3y La 2 / 3–y TiO3 and electrolyte obtained by doping the above substances with other elements, wherein 0 < x < 1 and 0.04 < y < 0.17.

[0013] Preferably, the polymer conductor is composed of a polymer and a conductive lithium salt, the polymer is one or more of polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA), the conductive lithium salt is one or more of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4) and lithium bis-trifluoromethanesulfonimide (LiTFSI), and the molar ratio of the polymer to the conductive lithium salt is (5-20):1.

[0014] Preferably, the salt bridge in step 2) is composed of agar-based gel, inorganic salt and its shell, wherein the inorganic salt is one or more of NaCl, KCl, NH4Cl, Na2SO4, K2SO4, (NH4)2SO4, NaNO3, KNO3 and NH4NO3.

[0015] Preferably, the voltage applied to the electrodialysis cell in step 3) is lower than 2.0 V.

[0016] The application also provides a salt bridge assisted low-voltage electrodialysis device for separating magnesium and lithium based on lithium super ionic conductor, which is used in the method and comprises a power supply, an electric pump, a conduit, at least one electrodialysis cell and a corresponding auxiliary chamber; the electric pump and the conduit are used for leading in and out raw material liquid and extraction liquid; the electrodialysis cell is composed of a shell, a lining, an electrode, lithium super ionic conductor and two salt bridges; the electrodialysis cell is separated into an anode chamber and a cathode chamber by the lithium super ionic conductor, the anode chamber contains the raw material liquid, and the electrode is connected with the positive pole of the power supply; the cathode chamber contains the extraction liquid, and the electrode is connected with the negative pole of the power supply; the auxiliary chamber contains a lithium-free solution, and the anode chamber and the cathode chamber are connected with the auxiliary chamber through the salt bridges.

[0017] Compared with the prior art, the technical scheme of the application has the following beneficial effects:

[0018] (1) The agar salt bridge used in the application has low preparation cost and can be reused;

[0019] (2) The voltage applied to the device in the application is low, the power consumption cost is low, and the Li + No harmful gases such as chlorine are generated during the extraction process, which is environmentally friendly;

[0020] (3) The current efficiency of the application is high, the current is provided by lithium ion migration, and the high magnesium-lithium separation effect of lithium super ionic conductor can be fully utilized. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 Figure 1 is a diagram of a single-stage electrodialysis cell device for extracting lithium according to the application;

[0022] Figure 2 Figure 2 is a schematic diagram of the connection of a single-stage electrodialysis cell for extracting lithium according to the application;

[0023] Figure 3 Figure 3 is a schematic diagram of the connection of a multi-stage electrodialysis cell for extracting lithium according to the application;

[0024] Figure 4 Figure 4 is a graph showing the relationship between the separation coefficient and lithium recovery rate over time according to Example 1 of the application, in which the arrows indicate the ordinate of the lines. DETAILED DESCRIPTION

[0025] The application will be further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only used to explain the application, and not to limit the application.

[0026] Example 1:

[0027] Please refer to Figure 1 and Figure 2 Li 1.5 Al 0.5 Ge1.5 (PO4)3 lithium super ionic conductor separates brine (Mg 2+ concentration of 10 g / L, Li + concentration of 0.2 g / L), pure water, and agar-saturated KCl salt bridge; a voltage of 1.0 V was applied to the electrodialysis cell for 10 h; the extract was taken for Li + , Mg 2+ concentration at the initial, 1 h, 3 h, 6 h, and 10 h, respectively. Figure 4 The separation coefficient and lithium recovery rate obtained in Example 1 are shown in the graph of the change over time. Table 2 shows the separation coefficient (the ratio of the Li / Mg in the extract to the Li / Mg in the original brine) of the device, which is used to judge the selectivity of the device to Li + . The higher the separation coefficient, the better. Table 3 shows the change of the lithium recovery rate over time (the ratio of the Li + concentration in the extract to the Li + concentration in the original brine), which is used to judge the passability of the device to Li + . The higher the recovery rate, the better.

[0028] Table 1 Ion concentration in the extract

[0029]

[0030] Table 2 Separation coefficient

[0031] 1h 3h 6h 10h Separation factor 89.15 96.78 135.36 148.32

[0032] Table 3 Recovery rate of the device to Li +

[0033] 1h 3h 6h 10h % Recovery 1.68 4.55 10.67 19.56

[0034] Example 2:

[0035] Please refer to Figure 1 Li7La3Zr2O 12 ion conductor with a thickness of 1 mm separates brine (Mg 2+ concentration of 15 g / L, Li + concentration of 0.2 g / L), and NaCl solution, and agar-saturated NaCl salt bridge; a voltage of 1.8 V was applied to the electrodialysis cell for 15 h, the Li / Mg in the extract was 2.84, the separation coefficient was higher than 200, and the lithium recovery rate was greater than 13.6%.

[0036] Example 3:

[0037] Please refer to Figure 1 polymer ion conductor (the molar ratio of PEO to LiTFSI is 15:1) with a thickness of 3 mm separates brine (Mg 2+ ​concentration of 10 g / L, Li + concentration of 0.1 g / L, KCl solution, and agar-KCl salt bridge; a voltage of 0.8 V was applied to the electrodialysis cell for 8 h, the Li / Mg in the extract was 2.59, the separation coefficient was higher than 250, and the lithium recovery rate was greater than 10.2%.

[0038] Example 4:

[0039] Please refer to Figure 1 The brine (Mg 1.5 Al 0.5 Ti 1.5 The mass ratio of (PO4)3was 1:4) separated the brine (Mg 2+ concentration of 10 g / L, Li + concentration of 0.15 g / L, Na2SO4solution, and agar-Na2SO4salt bridge; a voltage of 1.1 V was applied to the electrodialysis cell for 8 h, the Li / Mg in the extract was 2.14, the separation coefficient was higher than 140, and the lithium recovery rate was greater than 12.3%.

[0040] Example 5:

[0041] Please refer to Figure 3 The brine (Mg 0.33 La 0.557 TiO3ion conductor separated the brine (Mg 2+ concentration of 10 g / L, Li + concentration of 0.2 g / L, KCl solution, and agar-saturated KCl salt bridge, and three-stage electrodialysis cells were connected in series; a voltage of 1.5 V was applied to each electrodialysis cell for 10 h, the Li / Mg in the extract of the third-stage electrodialysis cell was 192.4, the overall device separation coefficient was higher than 9600, and the lithium recovery rate was greater than 18%.

Claims

1. A salt-bridge-assisted low-voltage electrodialysis method for separating magnesium and lithium based on lithium superionic conductors, characterized in that, The method includes the following steps: 1) The electrodialysis cell is divided into an anode chamber and a cathode chamber by a lithium superion conductor. During operation, the feed solution is injected into the anode chamber and the extract solution is injected into the cathode chamber. 2) One end of one salt bridge is in contact with the raw material solution, and one end of the other salt bridge is in contact with the extract solution. The other ends of the two salt bridges are immersed together in the auxiliary chamber containing lithium-free solution. 3) Apply a voltage lower than the solution decomposition voltage to the electrodialysis cell, and after electrodialysis, transfer the extract from the cathode chamber to the output via pump and pipe; In step 2), the salt bridge is composed of agar-like gel, inorganic salt and its shell, wherein the inorganic salt is one or more of NaCl, KCl, NH4Cl, Na2SO4, K2SO4, (NH4)2SO4, NaNO3, KNO3 and NH4NO3.

2. The method as described in claim 1, characterized in that, Multiple electrodialysis cells are connected in series. The extract from the previous electrodialysis cell is transported to the anode chamber of the next electrodialysis cell for further electrodialysis. The electrodialysis solution from the next electrodialysis cell is returned to the cathode chamber of the previous electrodialysis cell for continued use.

3. The method as described in claim 2, characterized in that, The raw material liquid in step 1) is the circulating brine of a salt lake, seawater, or the extract transported from the previous electrodialysis tank. The extract is a lithium-free solution or the electrodialysis liquid returned from the next electrodialysis tank.

4. The method as described in claim 3, characterized in that, The lithium-free solution used is one or more of the following: pure water, NaCl, KCl, NH4Cl, Na2SO4, K2SO4, (NH4)2SO4, NaNO3, KNO3, and NH4NO3 aqueous solution.

5. The method as described in claim 1, characterized in that, The lithium superion conductor used is one or more of oxide conductors, polymer conductors, and oxide-polymer composite conductors, and the thickness of the lithium superion conductor is 1 μm to 10 mm.

6. The method as described in claim 5, characterized in that, The oxide conductor is Li 1+x Al x Ti 2-x (PO4)3, Li 1+ x Al x Ge 2-x (PO4)3, Garnet-type Li7La3Zr2O 12 Perovskite-type Li 3y La 2 / 3–y One or more of the following electrolytes are obtained by doping TiO3 and higher substances with other elements, wherein 0 < x <1, 0.04< y <0.

17.

7. The method as described in claim 5, characterized in that, The polymer conductor is composed of a polymer and a conductive lithium salt. The polymer is one or more of polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polymethyl methacrylate. The conductive lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate, and lithium bis(trifluoromethanesulfonyl)imide. The molar ratio of polymer to conductive lithium salt is (5-20):

1.

8. The method as described in claim 1, characterized in that, In step 3), the voltage applied to the electrodialysis cell is less than 2.0 V.

9. A salt-bridge-assisted low-voltage electrodialysis device for separating magnesium and lithium based on a lithium superionic conductor, characterized in that, The apparatus is used in the method described in any one of claims 1-8, the apparatus comprising a power supply, an electric pump, a conduit, at least one electrodialysis tank and a corresponding auxiliary chamber; the electric pump and the conduit are used for introducing and exporting the feed solution and the extract; the electrodialysis tank consists of a shell, a liner, electrodes, a lithium superion conductor and two salt bridges; the electrodialysis tank is separated into an anode chamber and a cathode chamber by the lithium superion conductor, the anode chamber contains the feed solution and the electrode is connected to the positive terminal of the power supply; the cathode chamber contains the extract and the electrode is connected to the negative terminal of the power supply; the auxiliary chamber contains a lithium-free solution, and the anode chamber and the cathode chamber are connected by salt bridges and the auxiliary chamber, respectively.

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