A method for deep dephosphorization of cobalt sulfate solution by multi-stage synergistic treatment

Abstract

This invention belongs to the field of phosphorus removal technology, and specifically relates to a deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment. This invention relates to a deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment, comprising the following steps: S11. Pretreatment and primary phosphorus removal; S12. Synergistic extraction; S13. Deep purification with resin; S14. Evaporation and crystallization: the exchange liquid obtained in step S13 is concentrated, crystallized, filtered, and washed to obtain cobalt sulfate heptahydrate, and the mother liquor is returned to step S11 for recycling; S15. Phosphorus recovery from phosphorus-containing liquid. The beneficial effects of this invention are: (1) Impurity phosphorus can be deeply purified, and the phosphorus content of the prepared cobalt sulfate heptahydrate is ≤0.00003wt% using the deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment; (2) The process is mild and easy to promote and apply; (3) The resource utilization of solid waste iron slag transforms solid waste into intermediate products, increasing economic value.

Description

Technical Field

[0001] This invention belongs to the field of phosphorus removal technology, and specifically relates to a deep phosphorus removal method for multi-stage synergistic treatment of cobalt sulfate solution. Background Technology

[0002] Cobalt sulfate is a key precursor material for the preparation of cathode materials in ternary lithium batteries, and its purity directly affects battery performance. However, during the hydrometallurgical refining of cobalt raw materials, the cobalt sulfate solution often contains phosphorus impurities, such as phosphates, which can easily lead to structural defects and decreased electrochemical performance in battery materials. Existing phosphorus removal methods include chemical precipitation and extraction. Chemical precipitation uses aluminum or iron salts to precipitate phosphorus, but it easily introduces new metallic impurities, requiring complex subsequent purification steps. Extraction, however, has low removal efficiency.

[0003] CN106564966B discloses a purification process for cobalt sulfate crystallization mother liquor used in battery cathode materials, specifically including the following steps: Step 1, concentrating and crystallizing a cobalt sulfate solution to obtain mother liquor and cobalt sulfate crystals; Step 2, adding water to the mother liquor obtained in Step 1, followed by concentrated sulfuric acid, to obtain P507 back-extracting agent; Step 3, using the P507 back-extracting agent from Step 2 to back-extract cobalt in an extraction tank, while controlling the pH value of the back-extracted cobalt sulfate solution, allowing impurities carried in the mother liquor from the P507 back-extracting agent to enter the P507 organic phase, and the sulfuric acid in the back-extracting agent back-extracts cobalt loaded on the organic phase of P507, with the cobalt entering the P507 back-extracting agent, and the impurities in the mother liquor entering P507, thus achieving the purpose of purifying the crystallization mother liquor. This production process is simple, and the cobalt sulfate solution produced has a cobalt content of not less than 100 g / L and the content of each impurity is less than 0.0005 g / L, meeting the requirements for battery-grade cobalt sulfate solution. This production process is low-cost and high-yield, and the resulting mother liquor can be reused repeatedly, avoiding the situation where the mother liquor is wasted due to excessive impurities. However, the removal depth is insufficient, making it difficult to meet high-end demands. Summary of the Invention

[0004] The purpose of this invention is to provide a deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment, comprising the following steps:

[0005] S11. Pretreatment and primary phosphorus removal: Adjust the pH of the cobalt sulfate solution to 2.0~3.0, stir evenly, add the compound phosphorus removal agent at a dosage ratio of 0.5~2.0 g / L, and react for 0.5~2 h;

[0006] S12. Co-extraction: After the reaction in step S11, the precipitate is separated by membrane filtration of the mixed liquid. The filtrate is co-extracted with a mixture of extractants P507 and Cyanex272 under countercurrent extraction at pH 4.0~5.0 to separate the solvent back-extraction solution and the aqueous phase. The aqueous phase is the cobalt sulfate solution after extraction. The separated precipitate is washed with 0.5mol / L~2.0mol / L sulfuric acid to obtain phosphorus-containing enrichment solution A.

[0007] S13. Deep purification of resin: The aqueous phase obtained in step S12 is fed into an ion exchange column, and after exchange, an exchanged liquid is obtained. After the resin is regenerated, a phosphorus-enriched solution B is obtained.

[0008] S14. Evaporation and crystallization: The exchange liquid obtained in step S13 is concentrated, crystallized, filtered, and washed to obtain cobalt sulfate heptahydrate. The mother liquor is returned to step S11 for recycling.

[0009] S15. Phosphorus recovery from phosphorus-containing solution: Combine the phosphorus-containing enriched solution A from step S12, the phosphorus-containing enriched solution B from step S13, and the solvent back-extraction solution from step S13, and add iron-containing slag. Adjust the pH value to 2.5~4.0, the reaction temperature to 50~80℃, and the reaction time to 0.5~2.0h. Filter to obtain phosphorus-iron slag and phosphorus-precipitated liquid. Separate the organic phase from the phosphorus-precipitated liquid, wash and purify it with water, and it can be reused.

[0010] In the multi-stage processing of this invention, after membrane filtration and synergistic extraction in step S12, most of the impurity ions are removed before entering the resin purification step in step S13. The resin purification pressure is low, the resin can be used continuously, the resin regeneration cycle is long, and the process is stable and reliable.

[0011] Furthermore, the composite dephosphorizing agent is ZIF-67(Co) loaded with nano-FeOOH or lanthanum-modified hydroxyapatite.

[0012] Furthermore, in step S12, the membrane used for membrane filtration is a polyamide nanofiltration membrane, preferably a zirconium phosphate modified nanofiltration membrane.

[0013] Furthermore, in step S12, the volume ratio of P507 to Cyanex272 is 1:1.

[0014] Furthermore, in step S13, the ion exchange column is filled with a strongly basic anion exchange resin and TS-1 titanium-silicon molecular sieve. The aqueous phase first flows through the titanium-silicon molecular sieve and then through the strongly basic anion exchange resin. The mass ratio of the strongly basic anion exchange resin to the TS-1 titanium-silicon molecular sieve is 1:1. The titanium-silicon molecular sieve (TS-1) has a framework of titanium forming Ti-OP bonds with PO4³⁻, which significantly enhances its phosphate adsorption effect.

[0015] Furthermore, the iron slag in step S15 is ferric hydroxide slag.

[0016] Furthermore, the preparation method of the ZIF-67(Co) loaded with nano FeOOH is as follows: ZIF-67(Co) is immersed in a 1 mol / L FeCl3 solution, and 1 mol / L NaOH is added dropwise. FeOOH nanostructures are grown on the surface of ZIF-67(Co) through a co-precipitation reaction. After the reaction is completed, the ZIF-67(Co) loaded with nano FeOOH is obtained by filtration, washing and drying.

[0017] Furthermore, the preparation method of the lanthanum-modified hydroxyapatite is as follows:

[0018] S21. Dissolve hydroxyapatite in 100 times its weight of deionized water to obtain mixture A;

[0019] S22. Then dissolve lanthanum chloride in 100 times its mass of deionized water, and add HCl dropwise until the solution becomes transparent to obtain mixture B. The mass of lanthanum chloride added is 0.5 times the mass of hydroxyapatite in step S21.

[0020] S23. Add mixture A to mixture B, stir for 30 min, adjust the pH to 10-11 with ammonia, stir for another 3 h, let stand for 24 h, filter, wash, dry, crush, and calcine in a muffle furnace at 300℃ for 5 h to obtain lanthanum-modified hydroxyapatite. Rare earth-modified hydroxyapatite shows a significant increase in phosphorus adsorption capacity and adsorption effect because hydroxyapatite provides phosphorus exchange lattice sites, lanthanum strongly adsorbs H2PO4⁻ through Lewis acid interaction, and La³⁺ substitutes for Ca²⁺, inducing lattice distortion, increasing the specific surface area, and enhancing the adsorption effect.

[0021] Furthermore, the method for preparing the zirconium phosphate modified nanofiltration membrane is as follows:

[0022] S31. Prepare 0.2 mol / L ZrOCl2·8H2O solution and 0.2 mol / L Na2HPO4 solution respectively;

[0023] S32. Immerse the polyamide nanofiltration membrane in 0.2 mol / L ZrOCl2·8H2O for 120 min, allowing Zr... 4 Zr ions are adsorbed onto the functional groups on the membrane surface. Afterward, the membrane is removed and washed to remove unadsorbed ions. The membrane is then immersed in a 0.2 mol / L Na₂HPO₄ solution for 60 min to allow the adsorbed Zr ions to be absorbed. 4 ⁺ reacts with PO4³⁻ in solution to form crystals in situ on the membrane surface;

[0024] S33. After the reaction is complete, the membrane is thoroughly washed with deionized water to remove residual ions, and then dried to obtain the zirconium phosphate modified nanofiltration membrane. The zirconium phosphate nanofiltration membrane additionally forms Zr-OP covalent bonds, resulting in a phosphorus rejection rate greater than 99.9%.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] (1) Impurity phosphorus can be deeply purified. A deep phosphorus removal method using multi-stage synergistic treatment of cobalt sulfate solution was adopted, and the content of cobalt sulfate heptahydrate phosphorus in the prepared product was ≤0.00003wt%;

[0027] (2) The process is mild and easy to promote and apply;

[0028] (3) The resource utilization of solid waste iron slag transforms solid waste into intermediate products, thereby increasing its economic value. Detailed Implementation

[0029] The present invention will be further described below with reference to the embodiments. The present invention is not limited to the embodiments. Those skilled in the art should understand that any improvement to the present invention, equivalent substitution of raw materials for the products of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0030] The preparation method of ZIF-67(Co) loaded with nano FeOOH used in the embodiments and comparative examples of the present invention is as follows: ZIF-67(Co) is immersed in 1 mol / L FeCl3 solution, and 1 mol / L NaOH is added dropwise. FeOOH nanostructures are grown on the surface of ZIF-67(Co) through coprecipitation reaction. After the reaction is completed, the ZIF-67(Co) loaded with nano FeOOH is obtained by filtration, washing and drying.

[0031] The preparation method of lanthanum-modified hydroxyapatite used in the embodiments and comparative examples of this invention is as follows:

[0032] S21. Dissolve hydroxyapatite in 100 times its weight of deionized water to obtain mixture A;

[0033] S22. Then dissolve lanthanum chloride in 100 times its mass of deionized water, and add HCl dropwise until the solution becomes transparent to obtain mixture B. The mass of lanthanum chloride added is 0.5 times the mass of hydroxyapatite in step S21.

[0034] S23. Add mixture A to mixture B, stir for 30 min, adjust the pH to 10-11 with ammonia, stir for another 3 h, let stand for 24 h, filter, wash, dry, crush and calcine at 300℃ for 5 h in a muffle furnace to obtain lanthanum modified hydroxyapatite.

[0035] The preparation method of the zirconium phosphate modified nanofiltration membrane used in the embodiments and comparative examples of this invention is as follows:

[0036] S31. Prepare 0.2 mol / L ZrOCl2·8H2O solution and 0.2 mol / L Na2HPO4 solution respectively;

[0037] S32. Immerse the polyamide nanofiltration membrane in 0.2 mol / L ZrOCl2·8H2O for 120 min, allowing Zr... 4 Zr ions are adsorbed onto the functional groups on the membrane surface. Afterward, the membrane is removed and washed to remove unadsorbed ions. The membrane is then immersed in a 0.2 mol / L Na₂HPO₄ solution for 60 min to allow the adsorbed Zr ions to be absorbed. 4 ⁺ reacts with PO4³⁻ in solution to form crystals in situ on the membrane surface;

[0038] S33. After the reaction is complete, the membrane is thoroughly washed with deionized water to remove residual ions, and then dried to obtain a zirconium phosphate modified nanofiltration membrane.

[0039] The elemental contents of the cobalt sulfate solution used in the embodiments and comparative examples of this invention are shown in Table 1.

[0040] Table 1. Element content in cobalt sulfate solution

[0041] Element Ni mg / L Co g / L Mn mg / L Fe mg / L Cu mg / L Al mg / L Zn mg / L P g / L pH Content ＜1.0 119.94 ＜1.0 ＜1.0 ＜1.0 ＜1.0 ＜1.0 2.5 3.42

[0042] Example 1

[0043] The purpose of this invention is to provide a deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment, comprising the following steps:

[0044] S11. Pretreatment and primary phosphorus removal: Take 1L of cobalt sulfate solution from Table 1, adjust the pH to 2.0, stir evenly, add the composite phosphorus removal agent, and react for 1 hour;

[0045] S12. Co-extraction: After the reaction in step S11, the precipitate is separated by membrane filtration of the mixed liquid. The filtrate is co-extracted with a mixture of extractants P507 and Cyanex272 under countercurrent extraction at pH 4.0 to separate the solvent back-extraction solution and the aqueous phase. The aqueous phase is the cobalt sulfate solution after extraction. The separated precipitate is washed with 1 mol / L sulfuric acid to obtain phosphorus-containing enrichment solution A. The volume ratio of P507 to Cyanex272 is 1:1.

[0046] S13. Deep purification of resin: The aqueous phase obtained in step S12 is fed into an ion exchange column, and after exchange, an exchanged liquid is obtained. After the resin is regenerated, a phosphorus-enriched solution B is obtained.

[0047] S14. Evaporation and crystallization: The exchange liquid obtained in step S13 is concentrated, crystallized, filtered, and washed to obtain cobalt sulfate heptahydrate. The mother liquor is returned to step S11 for recycling.

[0048] S15. Phosphorus recovery from phosphorus-containing solution: Combine the phosphorus-containing enriched solution A from step S12, the phosphorus-containing enriched solution B from step S13, and the solvent back-extraction solution from step S13, add ferric hydroxide slag, adjust the pH value to 2.5, the reaction temperature to 50-60℃, the reaction time to 1 hour, filter, and obtain phosphorus-iron slag and phosphorus-precipitated liquid. Separate the organic phase from the phosphorus-precipitated liquid, wash and purify with water, and it can be reused.

[0049] The composite dephosphorizing agent is ZIF-67(Co) loaded with nano FeOOH, and the amount of dephosphorizing agent added is 1g.

[0050] The membrane used for membrane filtration in step S12 is a polyamide nanofiltration membrane.

[0051] In step S13, the ion exchange column is filled with a strong base anion exchange resin and TS-1 titanium-silicon molecular sieve. The aqueous phase first flows through the titanium-silicon molecular sieve and then through the strong base anion exchange resin. The mass ratio of the strong base anion exchange resin to the TS-1 titanium-silicon molecular sieve is 1:1.

[0052] The elemental contents of cobalt sulfate heptahydrate obtained in step S14 are shown in Table 2.

[0053] Table 2

[0054] Element Ni wt% Co wt% Mn wt% Fe wt% Cu wt% Al wt% Zn wt% P wt% Content 0.0001 20.62 0.0001 0.0002 0.0001 0.0001 0.0001 0.00003

[0055] The main material composition of the phosphorus-iron slag is: Co 0.001wt%, P 15.3wt%.

[0056] Example 2

[0057] The purpose of this invention is to provide a deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment, comprising the following steps:

[0058] S11. Pretreatment and primary phosphorus removal: Take 1L of cobalt sulfate solution from Table 1, adjust the pH to 2.5, stir evenly, add the composite phosphorus removal agent, and react for 1 hour;

[0059] S12. Co-extraction: After the reaction in step S11, the precipitate is separated by membrane filtration of the mixed liquid. The filtrate is co-extracted with a mixture of extractants P507 and Cyanex272 under countercurrent extraction at pH 4.5 to separate the solvent back-extraction solution and the aqueous phase. The aqueous phase is the cobalt sulfate solution after extraction. The separated precipitate is washed with 1 mol / L sulfuric acid to obtain phosphorus-containing enrichment solution A. The volume ratio of P507 to Cyanex272 is 1:1.

[0060] S13. Deep purification of resin: The aqueous phase obtained in step S12 is fed into an ion exchange column, and after exchange, an exchanged liquid is obtained. After the resin is regenerated, a phosphorus-enriched solution B is obtained.

[0061] S14. Evaporation and crystallization: The exchange liquid obtained in step S13 is concentrated, crystallized, filtered, and washed to obtain cobalt sulfate heptahydrate. The mother liquor is returned to step S11 for recycling.

[0062] S15. Phosphorus recovery from phosphorus-containing solution: Combine the phosphorus-containing enriched solution A from step S12, the phosphorus-containing enriched solution B from step S13, and the solvent back-extraction solution from step S13, add ferric hydroxide slag, adjust the pH value to 3.0, the reaction temperature to 50-60℃, the reaction time to 1 hour, filter, and obtain phosphorus-iron slag and phosphorus-precipitated liquid. Separate the organic phase from the phosphorus-precipitated liquid, wash and purify with water, and it can be reused.

[0063] The composite dephosphorizing agent is ZIF-67(Co) loaded with nano FeOOH, and the amount of dephosphorizing agent added is 1g.

[0064] In step S12, the membrane used for membrane filtration is a zirconium phosphate modified nanofiltration membrane.

[0065] In step S13, the ion exchange column is filled with a strong base anion exchange resin and TS-1 titanium-silicon molecular sieve. The aqueous phase first flows through the titanium-silicon molecular sieve and then through the strong base anion exchange resin. The mass ratio of the strong base anion exchange resin to the TS-1 titanium-silicon molecular sieve is 1:1.

[0066] The elemental contents of cobalt sulfate heptahydrate obtained in step S14 are shown in Table 3.

[0067] Table 3

[0068] Element Ni wt% Co wt% Mn wt% Fe wt% Cu wt% Al wt% Zn wt% P wt% Content 0.0001 20.66 0.0001 0.0001 0.0001 0.0001 0.0001 0.00002

[0069] The main material composition of the phosphorus iron slag is: Co 0.0005wt%, P 15.6wt%.

[0070] Example 3

[0071] The purpose of this invention is to provide a deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment, comprising the following steps:

[0072] S11. Pretreatment and primary phosphorus removal: Take 1L of cobalt sulfate solution from Table 1, adjust the pH to 3.0, stir evenly, add the composite phosphorus removal agent, and react for 1 hour;

[0073] S12. Co-extraction: After the reaction in step S11, the precipitate is separated by membrane filtration of the mixed liquid. The filtrate is co-extracted with a mixture of extractants P507 and Cyanex272 under countercurrent extraction at pH 5.0 to separate the solvent back-extraction solution and the aqueous phase. The aqueous phase is the cobalt sulfate solution after extraction. The separated precipitate is washed with 1 mol / L sulfuric acid to obtain phosphorus-containing enrichment solution A. The volume ratio of P507 to Cyanex272 is 1:1.

[0074] S13. Deep purification of resin: The aqueous phase obtained in step S12 is fed into an ion exchange column, and after exchange, an exchanged liquid is obtained. After the resin is regenerated, a phosphorus-enriched solution B is obtained.

[0075] S14. Evaporation and crystallization: The exchange liquid obtained in step S13 is concentrated, crystallized, filtered, and washed to obtain cobalt sulfate heptahydrate. The mother liquor is returned to step S11 for recycling.

[0076] S15. Phosphorus recovery from phosphorus-containing solution: Combine the phosphorus-containing enriched solution A from step S12, the phosphorus-containing enriched solution B from step S13, and the solvent back-extraction solution from step S13, add ferric hydroxide slag, adjust the pH value to 3.5, the reaction temperature to 50-60℃, the reaction time to 1 hour, filter, and obtain phosphorus-iron slag and phosphorus-precipitated liquid. Separate the organic phase from the phosphorus-precipitated liquid, wash and purify with water, and it can be reused.

[0077] The composite dephosphorizing agent is lanthanum-modified hydroxyapatite, and the amount of dephosphorizing agent added is 1g.

[0078] In step S12, the membrane used for membrane filtration is a zirconium phosphate modified nanofiltration membrane.

[0079] In step S13, the ion exchange column is filled with a strong base anion exchange resin and TS-1 titanium-silicon molecular sieve. The aqueous phase first flows through the titanium-silicon molecular sieve and then through the strong base anion exchange resin. The mass ratio of the strong base anion exchange resin to the TS-1 titanium-silicon molecular sieve is 1:1.

[0080] The elemental contents of cobalt sulfate heptahydrate obtained in step S14 are shown in Table 4.

[0081] Table 4

[0082] Element Ni wt% Co wt% Mn wt% Fe wt% Cu wt% Al wt% Zn wt% P wt% Content 0.0001 20.68 0.0001 0.0001 0.0001 0.0001 0.0001 0.00001

[0083] The main material composition of the phosphorus iron slag is: Co 0.0004wt%, P 15.8wt%.

[0084] Comparative Example 1

[0085] The TS-1 titanium-silicon molecular sieve in step S13 of Example 1 is removed, and the rest is the same as in Example 1, so it will not be repeated. The content of each element in cobalt sulfate heptahydrate obtained in step S14 is shown in Table 5.

[0086] Table 5

[0087] Element Ni wt% Co wt% Mn wt% Fe wt% Cu wt% Al wt% Zn wt% P wt% Content 0.0001 20.55 0.0002 0.0002 0.0002 0.0001 0.0001 0.00012

[0088] The main material composition of the phosphorus iron slag is: Co 0.001wt%, P 5.45wt%.

[0089] Comparative Example 2

[0090] In Example 1, step S13 was modified so that the aqueous phase first flows through a strongly basic anion exchange resin and then through a titanium-silicon molecular sieve. The rest is the same as in Example 1 and will not be repeated here. The content of each element in the cobalt sulfate heptahydrate obtained in step S14 is shown in Table 5.

[0091] Table 5

[0092] Element Ni wt% Co wt% Mn wt% Fe wt% Cu wt% Al wt% Zn wt% P wt% Content 0.0002 20.50 0.0003 0.0002 0.0002 0.0001 0.0001 0.00020

[0093] The main material composition of the phosphorus-iron slag is: Co 0.001wt%, P 5.22wt%.

[0094] Data from Examples 1-3 show that the method of the present invention has high phosphorus removal efficiency, produces high-purity cobalt sulfate heptahydrate with low impurity content, particularly phosphorus content ≤0.00003wt%, while exhibiting minimal Co loss, demonstrating significant technical effectiveness. A comparison of data from Example 2 and Example 1 indicates that the zirconium phosphate-modified nanofiltration membrane technology is more effective. A comparison of data from Example 2 and Example 3 shows that the lanthanum-modified hydroxyapatite has higher removal efficiency. Data from Comparative Example 1 indicates that TS-1 titanium-silicon molecular sieve is an essential key component. Data from Comparative Example 2 indicates that the order of packing the strongly basic anion exchange resin and the titanium-silicon molecular sieve must not be changed.

[0095] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A deep phosphorus removal method for cobalt sulfate solution through multi-stage synergistic treatment, characterized in that, Includes the following steps: S11. Pretreatment and primary phosphorus removal: Adjust the pH of the cobalt sulfate solution to 2.0~3.0, stir evenly, add the composite phosphorus removal agent, and react for 0.5~2 hours; S12. Co-extraction: After the reaction in step S11, the precipitate is separated by membrane filtration of the mixed liquid. The filtrate is co-extracted with a mixture of extractants P507 and Cyanex272 under countercurrent extraction at pH 4.0~5.0 to separate the solvent back-extraction solution and the aqueous phase. The aqueous phase is the cobalt sulfate solution after extraction. The separated precipitate is washed with 0.5mol / L~2.0mol / L sulfuric acid to obtain phosphorus-containing enrichment solution A. S13. Deep purification of resin: The aqueous phase obtained in step S12 is fed into an ion exchange column, and after exchange, an exchanged liquid is obtained. After the resin is regenerated, a phosphorus-enriched solution B is obtained. S14. Evaporation and crystallization: The exchange liquid obtained in step S13 is concentrated, crystallized, filtered, and washed to obtain cobalt sulfate heptahydrate. The mother liquor is returned to step S11 for recycling. S15. Phosphorus recovery from phosphorus-containing solution: Combine the phosphorus-containing enriched solution A from step S12, the phosphorus-containing enriched solution B from step S13, and the solvent back-extraction solution from step S12, then add iron-containing slag. Adjust the pH value to 2.5~4.0, the reaction temperature to 50~80℃, and the reaction time to 0.5~2.0h. Filter to obtain phosphorus-iron slag and phosphorus-precipitated liquid. Separate the organic phase from the phosphorus-precipitated liquid, wash and purify it with water, and it can be reused. The composite dephosphorizing agent is ZIF-67(Co) loaded with nano FeOOH or lanthanum-modified hydroxyapatite, and the amount of dephosphorizing agent added is 0.5-2.0 g / L; The membrane used for membrane filtration in step S12 is a polyamide nanofiltration membrane; In step S13, the ion exchange column is filled with a strong base anion exchange resin and TS-1 titanium-silicon molecular sieve. The aqueous phase first flows through the titanium-silicon molecular sieve and then through the strong base anion exchange resin. The mass ratio of the strong base anion exchange resin to the TS-1 titanium-silicon molecular sieve is 1:

1.

2. The deep phosphorus removal method according to claim 1, characterized in that, In step S12, the membrane used for membrane filtration is a zirconium phosphate modified polyamide nanofiltration membrane.

3. The deep phosphorus removal method according to claim 1, characterized in that, In step S12, the volume ratio of P507 to Cyanex272 is 1:

1.

4. The deep phosphorus removal method according to claim 1, characterized in that, The iron-containing slag in step S15 is ferric hydroxide slag.

5. The deep phosphorus removal method according to claim 1, characterized in that, The preparation method of ZIF-67(Co) loaded with nano FeOOH is as follows: ZIF-67(Co) is immersed in 1 mol / L FeCl3 solution, and 1 mol / L NaOH is added dropwise. FeOOH nanostructures are grown on the surface of ZIF-67(Co) through coprecipitation reaction. After the reaction is completed, the surface is filtered, washed and dried to obtain ZIF-67(Co) loaded with nano FeOOH.

6. The deep phosphorus removal method according to claim 1, characterized in that, The preparation method of the lanthanum-modified hydroxyapatite is as follows: S21. Dissolve hydroxyapatite in 100 times its weight of deionized water to obtain mixture A; S22. Then dissolve lanthanum chloride in 100 times its mass of deionized water, and add HCl dropwise until the solution becomes transparent to obtain mixture B. The mass of lanthanum chloride added is 0.5 times the mass of hydroxyapatite in step S21. S23. Add mixture A to mixture B, stir for 30 min, adjust the pH to 10-11 with ammonia, stir for another 3 h, let stand for 24 h, filter, wash, dry, crush and calcine at 300℃ for 5 h in a muffle furnace to obtain lanthanum modified hydroxyapatite.

7. The deep phosphorus removal method according to claim 2, characterized in that, The method for preparing the zirconium phosphate modified polyamide nanofiltration membrane is as follows: S31. Prepare 0.2 mol / L ZrOCl2·8H2O solution and 0.2 mol / L Na2HPO4 solution respectively; S32. Immerse the polyamide nanofiltration membrane in 0.2 mol / L ZrOCl2·8H2O for 120 min, allowing the Zr... 4+ Ions are adsorbed onto the functional groups on the membrane surface. Afterward, the membrane is removed and washed to remove unadsorbed ions. Then, the membrane is immersed in a 0.2 mol / L Na₂HPO₄ solution for 60 min to allow the adsorbed Zr to be absorbed. 4+ With PO4 in solution 3- The reaction generates crystals in situ on the membrane surface; S33. After the reaction is complete, the membrane is thoroughly washed with deionized water to remove residual ions, and then dried to obtain a zirconium phosphate modified polyamide nanofiltration membrane.