Method for removing sodium and sulfur impurities from lithium-phosphorus coexistence system

Abstract

The application provides a method for removing sodium and sulfur impurities from a lithium-phosphorus coexisting system, comprising the following steps: mixing a lithium-phosphorus containing solution with a sulfur removal agent and performing solid-liquid separation to obtain a sulfur removal solution; precipitating and removing phosphorus from the sulfur removal solution to obtain a phosphorus removal solution; adjusting the pH of the phosphorus removal solution, then performing cooling crystallization to remove sulfur and sodium, and performing solid-liquid separation to obtain a preliminary impurity removal solution; passing the preliminary impurity removal solution through two-stage ion adsorption columns to perform deep impurity removal, and obtaining a lithium salt solution; performing directional crystallization and lithium precipitation on the lithium salt solution, then performing solid-liquid separation to obtain lithium salt solid. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexisting system solves the problem that sodium and sulfur impurities in the lithium-phosphorus coexisting system are difficult to be deeply removed by using a multi-process synergistic strategy of precipitating and removing sulfur and phosphorus, cooperatively removing sodium and sulfur by cooling, deeply separating lithium and sodium by two-stage ion sieves, and controlling sodium inclusions by directional crystallization.

Description

Technical Field

[0001] This invention belongs to the field of resource recycling technology and relates to a method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system. Background Technology

[0002] Lithium-ion batteries, especially the highly safe lithium iron phosphate batteries, are widely used in mobile communications, laptop batteries, portable tools, and electric vehicles due to their advantages such as high operating voltage, high specific energy, long cycle life, and low self-discharge. With the increasing use of lithium-ion batteries, the consumption of non-renewable metal resources is also growing. Therefore, recycling high-value, abundant metals from lithium-ion batteries is of great significance.

[0003] Currently, the common method for recycling lithium iron phosphate batteries is wet recycling, such as leaching waste lithium iron phosphate materials. However, impurities inevitably enter the leachate during the leaching process, requiring impurity removal. Existing impurity removal methods are complex and consume large amounts of chemical reagents, leading to secondary pollution. Furthermore, in lithium-phosphorus coexistence systems, due to the presence of sodium... + With Li + Their chemical properties are similar, making efficient separation difficult. Summary of the Invention

[0004] The purpose of this invention is to provide a method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system. The method solves the problem of the difficulty in deeply removing sodium and sulfur impurities in a lithium-phosphorus coexistence system by employing a multi-step synergistic strategy of precipitation to remove sulfur and phosphorus, cooling to remove sodium and sulfur, two-stage ion sieve for deep separation of lithium and sodium, and directional crystallization to control sodium inclusions.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] This invention provides a method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system, the method comprising the following steps:

[0007] (1) The lithium-phosphorus solution is mixed with the desulfurizing agent and then separated into solid and liquid components to obtain the desulfurization solution;

[0008] (2) The desulfurization solution obtained in step (1) is subjected to precipitation to remove phosphorus, thereby obtaining a phosphorus removal solution;

[0009] (3) Adjust the pH of the phosphorus removal solution in step (2), then cool and crystallize to remove sulfur and sodium, and obtain a preliminary impurity removal solution after solid-liquid separation;

[0010] (4) The preliminary impurity removal solution described in step (3) is passed through a two-stage ion adsorption column for deep impurity removal to obtain a lithium salt solution;

[0011] (5) The lithium salt solution described in step (4) is subjected to directional crystallization to precipitate lithium, and then solid-liquid separation is performed to obtain solid lithium salt.

[0012] This invention first removes sulfur through precipitation, then removes phosphorus through precipitation, and finally uses a cooling crystallization method to remove Na. + With residual SO4 2- The sodium and sulfur crystallize out as Na₂SO₄·10H₂O. After solid-liquid separation to remove the crystals, a preliminary purified solution is obtained. This solution is then passed through a two-stage ion adsorption column for selective lithium extraction, removing as much Ca as possible. 2+ Mg 2+ and Fe 3 + The lithium salt is precipitated by directional crystallization, while sodium impurities remain stably in the mother liquor. This process achieves deep separation of lithium and sodium, resulting in a high-purity lithium product with low sodium and low sulfur content.

[0013] Preferably, the desulfurizing agent in step (1) includes a calcium chloride solution.

[0014] This invention removes sulfur by adding calcium chloride solution to generate CaSO4·2H2O precipitate.

[0015] Preferably, the amount of calcium chloride solution added is 1.1 to 1.3 times the theoretical amount of sulfate that can be combined, for example, it can be 1.1 times, 1.15 times, 1.2 times, 1.25 times or 1.3 times, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] Preferably, in the lithium-phosphorus solution of step (1), Li + The content of ions is 2g / L-15g / L, for example, 2g / L, 5g / L, 10g / L or 15g / L; the content of phosphate ions is 0.2g / L-3g / L, for example, 0.2g / L, 0.5g / L, 1g / L, 2g / L or 3g / L; the content of sodium ions is 80g / L-90g / L, for example, 80g / L, 85g / L or 90g / L; the content of sulfate ions is 70g / L-80g / L, for example, 70g / L, 75g / L or 80g / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] Preferably, before the desulfurization solution in step (1) is subjected to step (2), the temperature is raised first, and then carbon dioxide gas is introduced into the system to control the pH of the system, calcium carbonate precipitate is generated, and after solid-liquid separation, the obtained filtrate is then subjected to step (2).

[0018] This invention removes calcium after desulfurization and before dephosphorization by first heating the system and then introducing carbon dioxide gas to control the pH of the system, thereby generating CaCO3 precipitate.

[0019] Preferably, the temperature is first raised to 50℃-60℃, for example, 50℃, 52℃, 54℃, 56℃, 58℃ or 60℃, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] Preferably, the pH of the control system is 8.0-8.5, for example, it can be 8.0, 8.1, 8.2, 8.3, 8.4 or 8.5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] Preferably, the precipitation and phosphorus removal in step (2) includes first adjusting the pH of the system with hydrochloric acid, then adding ferric chloride solution, and obtaining a phosphorus removal solution after solid-liquid separation.

[0022] The present invention first adjusts the pH with hydrochloric acid, then adds FeCl3 solution to generate FePO4·2H2O precipitate for phosphorus removal.

[0023] Preferably, the pH of the system is first adjusted to 4-4.5 using hydrochloric acid, for example, 4, 4.1, 4.2, 4.3, 4.4 or 4.5, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] Preferably, the amount of ferric chloride solution added is 1.5 to 3 times the theoretical amount required for the reaction with phosphorus, for example, it can be 1.5 times, 2 times, 2.5 times or 3 times, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] Preferably, the pH of the phosphorus removal solution in step (2) is adjusted to 6.0-6.5, for example, it can be 6.0, 6.1, 6.2, 6.3, 6.4 or 6.5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] Preferably, the pH of the phosphorus removal solution in step (2) is adjusted using LiOH.

[0027] Preferably, the cooling crystallization temperature in step (3) is 5℃-10℃, for example, it can be 5℃, 6℃, 7℃, 8℃, 9℃ or 10℃; the time is 30min-60min, for example, it can be 30min, 40min, 50min or 60min, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0028] Preferably, after adjusting the pH of the preliminary impurity removal solution in step (3), it is first passed through a first adsorption column, and then eluted with hydrochloric acid to obtain a lithium chloride solution. The pH of the lithium chloride solution is adjusted with LiOH, and then passed through a second adsorption column. The effluent from the second adsorption column is nanofiltered to obtain the lithium salt solution in step (4).

[0029] In this invention, after the initial impurity removal solution passes through two stages of ion adsorption columns, nanofiltration is also performed to remove as many impurity ions as possible, thereby further improving the purity of the product.

[0030] Preferably, the pH of the preliminary impurity removal solution in step (3) is adjusted to 7.2-7.8, for example, it can be 7.2, 7.3, 7.4, 7.5, 7.6, 7.7 or 7.8, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Preferably, the flow rate through the first adsorption column is 1 BV / h to 2 BV / h, for example, it can be 1 BV / h, 1.2 BV / h, 1.4 BV / h, 1.6 BV / h, 1.8 BV / h or 2 BV / h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] Preferably, the pH of the lithium chloride solution is adjusted to 7.3-7.7 using LiOH, for example, 7.3, 7.4, 7.5, 7.6 or 7.7, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] Preferably, the flow rate through the second adsorption column is 1 BV / h to 2 BV / h, for example, it can be 1 BV / h, 1.2 BV / h, 1.4 BV / h, 1.6 BV / h, 1.8 BV / h or 2 BV / h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, both the first adsorption column and the second adsorption column are titanium-based ion sieve adsorption columns.

[0035] The titanium-based ion sieve adsorption columns described in this invention are all H2TiO3 type titanium-based ion sieve adsorption columns, which can efficiently and selectively extract lithium.

[0036] Preferably, the nanofiltration has a molecular weight cutoff of 200-300 Daltons, such as 200 Daltons, 250 Daltons or 300 Daltons, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] Preferably, the directional crystallization lithium precipitation in step (5) includes the following steps: heating the lithium salt solution in step (4) to 90℃-95℃, for example, 90℃, 91℃, 92℃, 93℃, 94℃ or 95℃, and then adding saturated Na2CO3 solution, controlling the cooling rate to carry out directional crystallization lithium precipitation.

[0038] Preferably, the controlled cooling rate is 5℃ / h-10℃ / h, for example, it can be 5℃ / h, 6℃ / h, 7℃ / h, 8℃ / h, 9℃ / h or 10℃ / h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0039] Preferably, after the solid-liquid separation in step (5), the lithium salt solid is further washed and dried to obtain the lithium salt solid.

[0040] Preferably, pure water is used for the washing process.

[0041] Preferably, the drying temperature is 100℃-120℃, for example, it can be 100℃, 105℃, 110℃, 115℃ or 120℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] The method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system described in this invention solves the technical problem of the difficulty in deeply removing sodium and sulfur impurities from lithium-phosphorus coexistence systems through a multi-step synergistic strategy. Specifically, this invention first removes sulfur through precipitation, then removes phosphorus through precipitation, and finally uses a cooling crystallization method to remove sodium impurities. + With residual SO4 2- The sodium and sulfur crystallize out as Na₂SO₄·10H₂O. After solid-liquid separation to remove the crystals, a preliminary purified solution is obtained. This solution is then passed through a two-stage ion adsorption column for selective lithium extraction, removing as much Ca as possible. 2+ Mg 2+ and Fe 3+ The lithium salt is precipitated by directional crystallization, while sodium impurities remain stably in the mother liquor. This process achieves deep separation of lithium and sodium, resulting in a high-purity lithium product with low sodium and low sulfur content. Detailed Implementation

[0044] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0045] Example 1

[0046] This embodiment provides a method for removing sodium and sulfur impurities from a lithium-phosphorus coexisting system. The method includes the following steps:

[0047] (1) Precipitation method to remove sulfur impurities: Add CaCl2 solution to lithium phosphorus solution to generate CaSO4·2H2O precipitate, filter once, heat the above filtrate to 55°C, pass CO2 gas through, control pH to 8.3 to generate CaCO3 precipitate, filter twice to obtain desulfurization solution;

[0048] In the lithium-phosphorus-containing solution, Li + The content of ions is 8 g / L, the content of phosphate ions is 1 g / L, the content of sodium ions is 85 g / L, and the content of sulfate ions is 80 g / L.

[0049] The amount of CaCl2 solution added is 1.1 times the theoretical amount;

[0050] (2) Phosphorus removal: The pH of the desulfurization solution in step (1) is adjusted to 4.2 using dilute HCl, and then FeCl3 solution is added to generate FePO4·2H2O precipitate. After filtration and separation, the phosphorus removal solution is obtained.

[0051] The amount of FeCl3 solution added was twice the theoretical amount;

[0052] (3) Crystallization to remove sodium sulfate: The dephosphorization solution in step (2) is adjusted to pH 6.3 with LiOH, cooled to 8°C, kept warm for 40 minutes, filtered, and a preliminary impurity removal solution is obtained;

[0053] (4) Selective lithium extraction using titanium-based ion sieve: The pH of the preliminary impurity removal solution described in step (3) is adjusted to 7.5, and then passed through an H2TiO3 type titanium-based ion sieve adsorption column at a flow rate of 1.5 BV / h.

[0054] (5) Deep sodium removal: The adsorption column of step (4) was eluted with hydrochloric acid with a concentration of 0.5 mol / L to obtain LiCl solution. The pH of the LiCl solution was adjusted to 7.5 with LiOH and then passed through the second-stage H2TiO3 titanium-based ion sieve adsorption column at a flow rate of 1 BV / h. The effluent was filtered through a nanofiltration membrane.

[0055] (6) Heat the filtrate obtained in step (5) to 95°C, add saturated Na2CO3 solution, control the cooling rate to 8°C / h, so that sodium carbonate crystallizes out in a directional and orderly manner, and sodium impurities remain stably in the mother liquor. Then, separate the crystallized product from the sodium-containing mother liquor by filtration. The crystallized product is washed with pure water 3 times and then dried at 110°C to obtain the lithium salt product.

[0056] Example 2

[0057] This embodiment provides a method for removing sodium and sulfur impurities from a lithium-phosphorus coexisting system. The method includes the following steps:

[0058] (1) Precipitation method to remove sulfur impurities: Add CaCl2 solution to lithium phosphorus solution to generate CaSO4·2H2O precipitate, filter once, heat the above filtrate to 60°C, pass CO2 gas, control pH to 8.0 to generate CaCO3 precipitate, filter twice to obtain desulfurization solution;

[0059] In the lithium-phosphorus-containing solution, Li + The content of ions is 15 g / L, the content of phosphate ions is 3 g / L, the content of sodium ions is 80 g / L, and the content of sulfate ions is 70 g / L.

[0060] The amount of CaCl2 solution added is 1.2 times the theoretical amount;

[0061] (2) Phosphorus removal: The pH of the desulfurization solution in step (1) is adjusted to 4 using dilute HCl, and then FeCl3 solution is added to generate FePO4·2H2O precipitate. After filtration and separation, the phosphorus removal solution is obtained.

[0062] The amount of FeCl3 solution added was 2.5 times the theoretical amount;

[0063] (3) Crystallization to remove sodium sulfate: The pH of the phosphorus removal solution in step (2) is adjusted to 6.0 with LiOH, the temperature is lowered to 5°C, and the solution is kept warm for 30 minutes. After filtration, a preliminary impurity removal solution is obtained.

[0064] (4) Selective lithium extraction using titanium-based ion sieve: The pH of the preliminary impurity removal solution described in step (3) is adjusted to 7.8, and then passed through an H2TiO3 type titanium-based ion sieve adsorption column at a flow rate of 1 BV / h.

[0065] (5) Deep sodium removal: The adsorption column of step (4) was eluted with hydrochloric acid with a concentration of 0.5 mol / L to obtain LiCl solution. The pH of the LiCl solution was adjusted to 7.7 with LiOH and then passed through the second-stage H2TiO3 titanium-based ion sieve adsorption column at a flow rate of 2 BV / h. The effluent was filtered through a nanofiltration membrane.

[0066] (6) Heat the filtrate obtained in step (5) to 90°C, add saturated Na2CO3 solution, control the cooling rate to 5°C / h, so that sodium carbonate crystallizes out in a directional and orderly manner, and sodium impurities remain stably in the mother liquor. Then, separate the crystallized product from the sodium-containing mother liquor by filtration. The crystallized product is washed twice with pure water and then dried at 100°C to obtain the lithium salt product.

[0067] Example 3

[0068] This embodiment provides a method for removing sodium and sulfur impurities from a lithium-phosphorus coexisting system. The method includes the following steps:

[0069] (1) Precipitation method to remove sulfur impurities: Add CaCl2 solution to lithium phosphorus solution to generate CaSO4·2H2O precipitate, filter once, heat the above filtrate to 50°C, pass CO2 gas through, control pH to 8.5 to generate CaCO3 precipitate, filter twice to obtain desulfurization solution;

[0070] In the lithium-phosphorus-containing solution, Li + The content of ions is 2 g / L, the content of phosphate ions is 0.2 g / L, the content of sodium ions is 90 g / L, and the content of sulfate ions is 70 g / L.

[0071] The amount of CaCl2 solution added was 1.3 times the theoretical amount;

[0072] (2) Phosphorus removal: The pH of the desulfurization solution in step (1) is adjusted to 4.5 using dilute HCl, and then FeCl3 solution is added to generate FePO4·2H2O precipitate. After filtration and separation, the phosphorus removal solution is obtained.

[0073] The amount of FeCl3 solution added was three times the theoretical amount;

[0074] (3) Crystallization to remove sodium sulfate: The dephosphorization solution in step (2) is adjusted to pH 6.5 with LiOH, cooled to 10°C, kept warm for 30 minutes, filtered, and a preliminary impurity removal solution is obtained;

[0075] (4) Selective lithium extraction using titanium-based ion sieve: The pH of the preliminary impurity removal solution described in step (3) is adjusted to 7.2, and then passed through an H2TiO3 type titanium-based ion sieve adsorption column at a flow rate of 2 BV / h.

[0076] (5) Deep sodium removal: The adsorption column of step (4) was eluted with hydrochloric acid with a concentration of 0.5 mol / L to obtain LiCl solution. The pH of the LiCl solution was adjusted to 7.3 with LiOH and then passed through the second-stage H2TiO3 titanium-based ion sieve adsorption column at a flow rate of 1 BV / h. The effluent was filtered through a nanofiltration membrane.

[0077] (6) Heat the filtrate obtained in step (5) to 95°C, add saturated Na2CO3 solution, control the cooling rate to 10°C / h, so that sodium carbonate crystallizes out in a directional and orderly manner, and sodium impurities remain stably in the mother liquor. Then, separate the crystallized product from the sodium-containing mother liquor by filtration. The crystallized product is washed with pure water 3 times and then dried at 120°C to obtain the lithium salt product.

[0078] Example 4

[0079] This embodiment provides a method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system. Except for step (1), which involves filtration without heating and introducing CO2 gas to remove calcium, the method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system is the same as in embodiment 1.

[0080] Example 5

[0081] This embodiment provides a method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system. Except for step (5), in which the effluent is not filtered by a nanofiltration membrane, the method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system is the same as that in Example 1.

[0082] Comparative Example 1

[0083] This comparative example provides a method for removing impurities from a lithium-phosphorus coexistence system. The method for removing impurities from the lithium-phosphorus coexistence system is the same as that in Example 1, except that step (1) is not performed.

[0084] Comparative Example 2

[0085] This comparative example provides a method for removing impurities from a lithium-phosphorus coexistence system. The method for removing impurities from the lithium-phosphorus coexistence system is the same as that in Example 1, except that step (2) is not performed and step (3) uses hydrochloric acid to adjust the pH.

[0086] Comparative Example 3

[0087] This comparative example provides a method for removing impurities from a lithium-phosphorus coexistence system. The method for removing impurities from the lithium-phosphorus coexistence system is the same as that in Example 1, except that step (3) is not performed.

[0088] Comparative Example 4

[0089] This comparative example provides a method for removing impurities from a lithium-phosphorus coexistence system. The method for removing impurities from the lithium-phosphorus coexistence system is the same as that in Example 1, except that step (5) is not performed.

[0090] The purity of the lithium salt products obtained in the above examples and comparative examples, as well as the sodium and sulfur content in the lithium salt products, are shown in Table 1:

[0091] Table 1

[0092]

[0093] As can be seen from Table 1 above:

[0094] As shown in Example 1 and Comparative Example 1, the present invention removes sulfur through precipitation, which not only significantly removes sulfur but also avoids introducing other impurities; the introduced calcium ions can be removed through precipitation. As shown in Example 1 and Comparative Example 2, the present invention can further improve product purity through phosphorus removal. As shown in Example 1 and Comparative Example 3, the present invention removes sodium sulfate by cooling crystallization, which can simultaneously remove sodium and sulfur, thus improving product purity. As shown in Example 1 and Comparative Example 4, the present invention achieves deep impurity removal through a two-stage ion adsorption column. As shown in Examples 1 and 4, the present invention does not remove introduced calcium after sulfur removal, which will affect product purity. As shown in Examples 1 and 5, the present invention further removes impurities by performing nanofiltration after passing through a second-stage titanium-based ion sieve adsorption column.

[0095] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for removing sodium and sulfur impurities from a lithium-phosphorus coexistence system, characterized in that, The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system includes the following steps: (1) The lithium-phosphorus solution is mixed with the desulfurizing agent and then separated into solid and liquid components to obtain the desulfurization solution; (2) The desulfurization solution obtained in step (1) is subjected to precipitation to remove phosphorus, thereby obtaining a phosphorus removal solution; (3) Adjust the pH of the phosphorus removal solution in step (2), then cool and crystallize to remove sulfur and sodium, and obtain a preliminary impurity removal solution after solid-liquid separation; (4) The preliminary impurity removal solution described in step (3) is passed through a two-stage ion adsorption column for deep impurity removal to obtain a lithium salt solution; (5) The lithium salt solution described in step (4) is subjected to directional crystallization to precipitate lithium, and then solid-liquid separation is performed to obtain solid lithium salt.

2. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 1, characterized in that, The desulfurizing agent in step (1) includes a calcium chloride solution; Preferably, the amount of calcium chloride solution added is 1.1 to 1.3 times the theoretical amount that binds to sulfate ions; Preferably, in the lithium-phosphorus solution of step (1), Li + The content of ions is 2g / L-15g / L, the content of phosphate ions is 0.2g / L-3g / L, the content of sodium ions is 80g / L-90g / L, and the content of sulfate ions is 70g / L-80g / L.

3. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 2, characterized in that, Before the desulfurization solution described in step (1) is subjected to step (2), the temperature is raised first, and then carbon dioxide gas is introduced into the system to control the pH of the system, calcium carbonate precipitate is generated, and after solid-liquid separation, the obtained filtrate is then subjected to step (2). Preferably, the temperature is first raised to 50℃-60℃; Preferably, the pH of the control system is 8.0-8.

5.

4. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 1 or 2, characterized in that, The precipitation and phosphorus removal in step (2) involves first adjusting the pH of the system with hydrochloric acid, then adding ferric chloride solution, and obtaining a phosphorus removal solution after solid-liquid separation. Preferably, the pH of the system is first adjusted to 4-4.5 using hydrochloric acid; Preferably, the amount of ferric chloride solution added is 1.5 to 3 times the theoretical amount required for the reaction with phosphorus.

5. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 1 or 2, characterized in that, Adjust the pH of the phosphorus removal solution in step (2) to 6.0-6.5; Preferably, LiOH is used to adjust the pH of the phosphorus removal solution in step (2); Preferably, the cooling crystallization temperature in step (3) is 5℃-10℃ and the time is 30min-60min.

6. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 1 or 2, characterized in that, After adjusting the pH of the preliminary impurity removal solution in step (3), it is first passed through the first adsorption column and then eluted with hydrochloric acid to obtain a lithium chloride solution. The pH of the lithium chloride solution is adjusted with LiOH and then passed through the second adsorption column. The effluent from the second adsorption column is then nanofiltered to obtain the lithium salt solution in step (4).

7. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 6, characterized in that, Adjust the pH of the preliminary impurity removal solution described in step (3) to 7.2-7.8; Preferably, the flow rate through the first adsorption column is 1 BV / h to 2 BV / h; Preferably, the pH of the lithium chloride solution is adjusted to 7.3-7.7 using LiOH.

8. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 6, characterized in that, The flow rate through the second adsorption column is 1 BV / h to 2 BV / h; Preferably, both the first adsorption column and the second adsorption column are titanium-based ion sieve adsorption columns.

9. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 1 or 2, characterized in that, Step (5) involves directional crystallization of lithium, which includes the following steps: heating the lithium salt solution in step (4) to 90℃-95℃, then adding a saturated Na2CO3 solution and controlling the cooling rate to carry out directional crystallization of lithium. Preferably, the controlled cooling rate is 5℃ / h-10℃ / h.

10. The method for removing sodium and sulfur impurities from the lithium-phosphorus coexistence system according to claim 1 or 2, characterized in that, After solid-liquid separation in step (5), the mixture was washed and dried to obtain the lithium salt solid. Preferably, pure water is used for the washing process; Preferably, the drying temperature is 100℃-120℃.

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