Method for preparing iron phosphate from phosphorus iron slag

Abstract

The present disclosure provides a method for preparing iron phosphate from phosphorus iron slag, the method comprising the following steps: (1) pretreating the phosphorus iron slag to obtain a mixture containing aluminum phosphate; (2) mixing the mixture with a chloride, obtaining a calcined slag by calcination, and then washing and filtering the calcined slag, mixing the calcined slag with an acid solution, and obtaining solution A after reaction and solid-liquid separation; (3) mixing solution A with elemental iron, obtaining solution B after reaction and solid-liquid separation, adding an iron source and / or a phosphorus source to adjust the phosphorus-iron molar ratio to obtain a raw material solution; (4) mixing the raw material solution with hydrogen peroxide, adjusting the pH for reaction, and obtaining iron phosphate dihydrate through solid-liquid separation and sintering treatment. The method can achieve deep removal of Al and Cu impurities, thereby preparing iron phosphate with high purity. The method is simple and low in cost.

Description

TECHNICAL FIELD

[0001] The present disclosure belongs to the field of resource recycling, and relates to a method for preparing iron phosphate from phosphorus-iron slag. BACKGROUND

[0002] At present, lithium iron phosphate with a regular olivine structure is praised as an ideal positive electrode material of energy storage lithium batteries, and has been mass-produced. However, the service life of lithium ion batteries is limited, which will inevitably lead to a large number of retired power lithium ion batteries. In the waste lithium iron phosphate battery, the mass fraction of lithium iron phosphate is 30%-35%, and the mass fraction of copper foil and aluminum foil is about 10%. The content of valuable metal elements Li, Fe, Cu and Al far exceeds their content in natural minerals, and the arbitrary piling up also causes great waste of valuable resources, which is not in line with the national green and sustainable development strategy.

[0003] Al, Cu and Fe are difficult to separate because of their similar properties, and the presence of these impurities will affect the recovery and preparation of battery-grade iron phosphate from phosphorus-iron slag and the electrochemical performance of lithium iron phosphate battery positive electrode materials. At present, the methods for removing aluminum and copper impurities include using alkaline or acidic solution to leach impurities. However, the impurity content in the recovered and prepared iron phosphate is still high, or resin adsorption is used to remove aluminum, but the cost is high, the process flow is complex, the efficiency is low, and the impurity removal effect is poor. SUMMARY

[0004] The following is a summary of the subject matter described in detail in this document. This summary is not intended to limit the scope of protection of the claims.

[0005] The purpose of the present disclosure is to provide a method for preparing iron phosphate from phosphorus-iron slag. The present disclosure can achieve deep removal of Al and Cu impurities by mixing and sintering phosphorus-iron slag with chlorides and adding elemental iron for replacement during the recovery process, thereby preparing high-purity iron phosphate. The method is simple and low in cost.

[0006] To achieve the purpose of the present disclosure, the present disclosure adopts the following technical solutions:

[0007] In a first aspect, the present disclosure provides a method for preparing iron phosphate from phosphorus-iron slag, which comprises the following steps:

[0008] (1) pretreating the phosphorus-iron slag to obtain a mixture containing aluminum phosphate;

[0009] (2) mixing the mixture containing aluminum phosphate with chlorides, obtaining a calcined slag by calcination, washing and filtering, then mixing the calcined slag with an acid solution, and obtaining a solution A after one-step reaction and solid-liquid separation;

[0010] (3) mixing solution A with elemental iron, after two-step reaction, obtaining solution B through solid-liquid separation, adding iron source and / or phosphorus source to adjust the molar ratio of phosphorus and iron to obtain a raw solution;

[0011] (4) mixing the raw solution and hydrogen peroxide, adjusting pH for three-step reaction, obtaining iron phosphate dihydrate through solid-liquid separation, and obtaining the iron phosphate through sintering treatment.

[0012] The phosphorus-iron slag disclosed in the embodiments of the present disclosure includes, but is not limited to, the phosphorus-iron slag after lithium extraction from lithium iron phosphate batteries or other aluminum-containing iron phosphate precursors.

[0013] The embodiments of the present disclosure can realize phase conversion of aluminum impurities in the phosphorus-iron slag by high-temperature calcination of the phosphorus-iron slag and chlorides, so that the aluminum impurities are converted into aluminum chloride and form a gas that volatilizes. After washing and filtering the obtained calcined slag to remove excess chlorides, the slag obtained after washing is reacted with acid, and after solid-liquid separation, a leaching solution containing iron and phosphorus ions is obtained. After adding iron to replace copper, deep removal of Al and Cu impurities can be realized. By adding iron source or phosphorus source to adjust the phosphorus-iron ratio, a synthetic raw solution with a certain phosphorus-iron ratio is obtained. Hydrogen peroxide is added to adjust the pH to obtain an iron phosphate precursor precipitate. The process is simple and the production cost is low.

[0014] In one embodiment, the pretreatment of step (1) includes:

[0015] Mixing the phosphorus-iron slag with a solvent, adding a phosphoric acid solution, controlling the pH for reaction, and drying to obtain a mixture containing aluminum phosphate.

[0016] In one embodiment, the solvent includes water.

[0017] In one embodiment, the mass concentration of the phosphoric acid solution is 50% to 85%, for example, 50%, 60%, 70%, or 80%, etc.

[0018] In one embodiment, the pH is 1.8 to 3.5, for example, 1.8, 2, 2.5, 3, or 3.5, etc.

[0019] In one embodiment, the temperature of the reaction is 50 to 90°C, for example, 50°C, 60°C, 70°C, 80°C, or 90°C, etc.

[0020] In one embodiment, the reaction time is 1 to 2 hours, for example, 1 hour, 1.2 hours, 1.5 hours, 1.8 hours, or 2 hours, etc.

[0021] In one embodiment, the chlorides of step (2) include sodium chloride, potassium chloride, and iron chloride.

[0022] In one embodiment, the mass fraction of sodium chloride is 15-30%, for example, 15%, 18%, 20%, 25%, or 30%, etc., based on 100% of the mass of the chlorides.

[0023] In one embodiment, the mass fraction of potassium chloride is 15-25%, for example, 15%, 18%, 20%, 22%, or 25%, etc.

[0024] In one embodiment, the mass fraction of iron chloride is 55-75%, for example, 55%, 60%, 65%, 70%, or 75%, etc.

[0025] The embodiments of the present disclosure can reduce the melting point of the eutectic salt to below 200°C by controlling the composition and proportion of the chlorides, and the molten chlorides can better react with aluminum phosphate. After calcination and washing, the generated aluminum oxide can be avoided to remain in the molten chlorides, and the aluminum content in the target product can be further reduced by re-forming aluminum phosphate in acid dissolution. The method is simple and efficient.

[0026] In one embodiment, the mass ratio of the mixture to the chlorides in step (2) is 1:(0.2-0.6), for example, 1:0.2, 1:0.3, 1:0.4, 1:0.5, or 1:0.6, etc.

[0027] The calcination reaction equation of the mixture of the mixture and the chlorides in the embodiments of the present disclosure is as follows (taking sodium chloride as an example):

[0028] AlPO4+3NaCl=AlCl3↑+Na3PO4.

[0029] In one embodiment, the temperature of the calcination treatment in step (2) is 185-270°C, for example, 185°C, 200°C, 220°C, 250°C, or 270°C, etc.

[0030] In one embodiment, the atmosphere of the calcination treatment in step (2) includes air.

[0031] In one embodiment, the time of the calcination treatment in step (2) is 1-2h, for example, 1h, 1.2h, 1.5h, 1.8h, or 2h, etc.

[0032] In one embodiment, the acid solution in step (2) includes sulfuric acid and / or phosphoric acid.

[0033] The ionic equation of the mixture of the calcination residue and the sulfuric acid solution in the embodiments of the present disclosure is as follows:

[0034] 2FePO4+6H + +3SO4 2- =2Fe 3++3SO4 2- +2H2PO4 - +2HPO4 2- .

[0035] The ionic equation for the mixture of roasted slag and phosphoric acid solution described in this embodiment is as follows:

[0036] FePO4+3H + +PO4 3- =Fe 3+ +H2PO4 - +HPO4 2- .

[0037] In one embodiment, the concentration of the acid solution in step (2) is 0.8 to 1.2 mol / L, for example: 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L or 1.2 mol / L, etc.

[0038] In one embodiment, the mass-to-volume ratio of the roasted residue and acid solution in step (2) is 1g:(5-10)mL, for example: 1g:5mL, 1g:6mL, 1g:7mL, 1g:8mL or 1g:10mL, etc.

[0039] In one embodiment, the reaction time in step (2) is 0.5 to 2.5 hours, for example: 0.5 hours, 1 hour, 1.5 hours, 2 hours or 2.5 hours.

[0040] In one embodiment, the elemental iron in step (3) comprises iron powder.

[0041] In one embodiment, solution A in step (3) contains the impurity Cu. 2+ .

[0042] In one embodiment, the elemental iron and the impurity Cu in solution A in step (3) 2+ The molar ratio is (1-2):1, for example: 1:1, 1.2:1, 1.5:1, 1.8:1 or 2:1, etc.

[0043] The embodiments of this disclosure describe elemental iron and impurities Cu in solution. 2+ The ionic equation for the reaction is as follows:

[0044] Fe+Cu 2+ =Fe 2+ +Cu.

[0045] In one embodiment, the reaction time of the two steps in step (3) is 0.5 to 2.5 hours, for example: 0.5 hours, 1 hour, 1.5 hours, 2 hours or 2.5 hours.

[0046] In one embodiment, the iron source in step (3) includes any one or a combination of at least two of ferric sulfate, ferric chloride, or ferric nitrate.

[0047] In one embodiment, the phosphorus source in step (3) includes any one or a combination of at least two of ammonium dihydrogen phosphate, sodium phosphate, ammonium phosphate, or monohydrogen phosphate.

[0048] In one embodiment, the phosphorus-iron molar ratio in step (3) is (1 to 1.1):1, for example: 1:1, 1.02:1, 1.05:1, 1.08:1 or 1.1:1, etc.

[0049] In one embodiment, the molar ratio of hydrogen peroxide and iron in the raw material solution in step (4) is (1 to 1.5):1, for example: 1:1, 1.1:1, 1.2:1, 1.4:1 or 1.5:1, etc.

[0050] In one embodiment, the pH adjuster in step (4) includes ammonia.

[0051] In one embodiment, the pH in step (4) is 1.6 to 2.2, for example: 1.6, 1.8, 2, 2.1 or 2.2, etc.

[0052] In one embodiment, the temperature of the three-step reaction in step (4) is 50–95°C.

[0053] In one embodiment, the reaction time of the three steps in step (4) is 1 to 2 hours, for example: 1 hour, 1.2 hours, 1.5 hours, 1.8 hours or 2 hours.

[0054] In one embodiment, the sintering temperature in step (4) is 350 to 650°C, for example: 350°C, 400°C, 450°C, 500°C or 650°C.

[0055] In one embodiment, the sintering time in step (4) is 3 to 6 hours, for example: 3 hours, 3.5 hours, 4 hours, 5 hours or 6 hours.

[0056] As an optional embodiment of this disclosure, the method includes the following steps:

[0057] (1) Mix the phosphorus iron slag with water, add a phosphoric acid solution with a mass concentration of 50% to 85%, control the pH to 1.8 to 3.5, react at 50 to 90°C for 1 to 2 hours, and filter to obtain a mixture containing aluminum phosphate;

[0058] (2) A mixture containing aluminum phosphate is mixed with chloride at a mass ratio of 1:(0.2-0.6) and calcined at 185-270℃ for 1-2 hours to obtain calcined residue. After washing and filtering the calcined residue, the calcined residue is mixed with acid solution at a mass-volume ratio of 1g:(5-10)mL and reacted in one step for 0.5-2.5 hours. After solid-liquid separation, solution A is obtained.

[0059] (3) Mix solution A with iron powder, and after two-step reaction for 0.5 to 2.5 hours, obtain solution B by solid-liquid separation. Add iron source and / or phosphorus source to adjust the phosphorus-iron molar ratio to (1 to 1.1):1 to obtain raw material solution;

[0060] (4) Mix the raw material solution and hydrogen peroxide, add ammonia to adjust the pH to 1.6-2.2 and carry out a three-step reaction for 1-2 hours. Separate the solid and liquid to obtain ferric phosphate dihydrate, and sinter at 350-650℃ for 3-6 hours to obtain the ferric phosphate.

[0061] This disclosure has the following beneficial effects:

[0062] (1) This disclosure achieves deep removal of Al and Cu impurities by mixing and sintering ferrophosphate slag with chloride and adding elemental iron during the recycling process, thereby producing high-purity ferrophosphate. The method is simple and low-cost.

[0063] (2) The phosphorus iron raw material liquid obtained by the method described in this disclosure has an Al content of less than 0.0039%, a Cu content of less than 0.003%, a P content of more than 12.04%, and an Fe content of more than 25.33%. The recovered ferric phosphate has an iron content of more than 29.35%, a phosphorus content of more than 16.32%, an impurity Al ion content of less than 0.004%, and an impurity copper ion content of less than 0.003%. Detailed Implementation

[0064] The technical solutions of this disclosure will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of this disclosure and should not be construed as specific limitations thereof.

[0065] The main elemental composition of the ferrophosphorus slag used in the embodiments and comparative examples of this disclosure is as follows:

[0066] Al: 0.50%, Cu: 0.32%, P: 8.11%, Fe: 15.34%. Unless otherwise specified, all proportions or percentages in the embodiments and comparative examples disclosed herein are mass ratios.

[0067] The chloride compositions used in the embodiments and comparative examples of this disclosure are as follows:

[0068] Sodium chloride: 20%, potassium chloride: 20%, ferric chloride: 60%.

[0069] Example 1

[0070] This embodiment provides a method for preparing ferric phosphate from ferric phosphate slag, the method comprising the following steps:

[0071] (1) The phosphorus iron slag after lithium extraction from lithium iron phosphate batteries was dispersed in an aqueous solution at a liquid-solid mass ratio of 2:1, and a 70% phosphoric acid solution was added to control the pH at 2.5. The mixture was reacted at 70°C for 1.5 hours. After the reaction was completed, the mixture was filtered and dried at 100°C to obtain a mixture.

[0072] (2) Weigh 100g of the mixture and 40g of chloride, mix thoroughly, place in a muffle furnace, and calcine at 250°C for 1.5h in air atmosphere to obtain calcined residue. After washing and filtering, react the calcined residue in a 1mol / L sulfuric acid solution with a solid-liquid ratio of 1g:7.5mL for 1.5h, and then perform solid-liquid separation to obtain solution A.

[0073] (3) Add iron powder to solution A. The amount of iron powder used is 1.5 times the molar amount of copper in the solution. The reaction time is 1.5h. After the reaction, the solid and liquid are separated to obtain solution B. In solution B, add an iron source or phosphorus source to adjust the phosphorus-iron molar ratio in the system to 1:1.05 according to Fe:P to obtain the raw material solution. The iron supplementing reagent is ferric sulfate, and the phosphorus supplementing reagent is ammonium dihydrogen phosphate.

[0074] (4) The raw material solution and hydrogen peroxide were mixed at a molar ratio of hydrogen peroxide to iron of 1.2:1. Ammonia was added to adjust the pH to 1.9 for co-precipitation reaction. The reaction temperature was 70°C and the reaction time was 1.5 h. After the reaction was completed, solid and liquid were separated to obtain ferric phosphate dihydrate precipitate. The precipitate was calcined in a muffle furnace at 500°C for 4 h to obtain ferric phosphate.

[0075] Example 2

[0076] This embodiment provides a method for preparing ferric phosphate from ferric phosphate slag, the method comprising the following steps:

[0077] (1) The phosphorus iron slag after lithium extraction from lithium iron phosphate batteries was dispersed in an aqueous solution at a liquid-solid mass ratio of 2:1, and an 85% phosphoric acid solution was added to control the pH at 1.8. The mixture was reacted at 50°C for 1 hour. After the reaction was completed, the mixture was filtered and dried at 100°C to obtain a mixture.

[0078] (2) Weigh 100g of the mixture and 60g of chloride, mix them thoroughly, place them in a muffle furnace, and calcine them at 270℃ for 2h in an air atmosphere to obtain calcined residue. After washing and filtering, react the calcined residue in a sulfuric acid solution with a solid-liquid ratio of 1g:10mL and a concentration of 0.8mol / L for 2h, and then perform solid-liquid separation to obtain solution A.

[0079] (3) Add iron powder to solution A. The amount of iron powder used is twice the molar amount of copper in the solution. The reaction time is 2 hours. After the reaction, the solid and liquid are separated to obtain solution B. In solution B, add an iron source or phosphorus source to adjust the phosphorus-iron molar ratio in the system to 1:1.1 according to Fe:P to supplement phosphorus and iron, and obtain raw material solution. The iron supplementing reagent is ferric sulfate, and the phosphorus supplementing reagent is ammonium dihydrogen phosphate.

[0080] (4) The raw material solution and hydrogen peroxide are mixed at a molar ratio of hydrogen peroxide to iron of 1.5:1. Ammonia water is added to adjust the pH to 2.2 for co-precipitation reaction. The reaction temperature is 95℃ and the reaction time is 1h. After the reaction is complete, solid and liquid are separated to obtain ferric phosphate dihydrate precipitate. The precipitate is calcined in a muffle furnace at 650℃ for 3h to obtain ferric phosphate.

[0081] Example 3

[0082] This embodiment provides a method for preparing ferric phosphate from ferric phosphate slag, the method comprising the following steps:

[0083] (1) The phosphorus iron slag after lithium extraction from lithium iron phosphate batteries was dispersed in an aqueous solution at a liquid-solid mass ratio of 2:1, and a 70% phosphoric acid solution was added to control the pH at 3.5. The mixture was reacted at 90°C for 2 hours. After the reaction was completed, the mixture was filtered and dried at 100°C to obtain a mixture.

[0084] (2) Weigh 100g of the mixture and 20g of chloride, mix thoroughly, place in a muffle furnace, and calcine at 185℃ for 1h in air atmosphere to obtain calcined residue. After washing and filtering, react the calcined residue in a 1.2mol / L sulfuric acid solution with a solid-liquid ratio of 1g:5mL for 0.5h, and then perform solid-liquid separation to obtain solution A.

[0085] (3) Add iron powder to solution A. The amount of iron powder used is 1 times the molar amount of copper in the solution. The reaction time is 0.5h. After the reaction, the solid and liquid are separated to obtain solution B. In solution B, add an iron source or phosphorus source to adjust the phosphorus-iron molar ratio in the system to 1:1 according to Fe:P to supplement phosphorus and iron, and obtain raw material solution. The iron supplementing reagent is ferric sulfate, and the phosphorus supplementing reagent is ammonium dihydrogen phosphate.

[0086] (4) The raw material solution and hydrogen peroxide are mixed at a molar ratio of hydrogen peroxide to iron of 1:1. Ammonia water is added to adjust the pH to 1.6 for co-precipitation reaction. The reaction temperature is 50℃ and the reaction time is 2h. After the reaction is complete, solid and liquid are separated to obtain ferric phosphate dihydrate precipitate. The precipitate is calcined in a muffle furnace at 350℃ for 6h to obtain ferric phosphate.

[0087] Example 4

[0088] The only difference between this embodiment and Embodiment 1 is that the mass of sodium chloride is 10g, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0089] Example 5

[0090] The only difference between this embodiment and Embodiment 1 is that the mass of sodium chloride is 70g; all other conditions and parameters are exactly the same as in Embodiment 1.

[0091] Example 6

[0092] The only difference between this embodiment and embodiment 1 is that the roasting temperature in step (2) is 170°C, while the other conditions and parameters are exactly the same as in embodiment 1.

[0093] Example 7

[0094] The only difference between this embodiment and embodiment 1 is that the roasting temperature in step (2) is 300°C, while the other conditions and parameters are exactly the same as in embodiment 1.

[0095] Comparative Example 1

[0096] This comparative example provides a method for preparing ferric phosphate from ferric phosphate slag, the method comprising the following steps:

[0097] (1) The phosphorus iron slag after lithium extraction from lithium iron phosphate batteries was reacted in a sulfuric acid solution with a concentration of 1 mol / L for 1.5 h at a solid-liquid ratio of 1 g: 7.5 mL, and then solid-liquid separation was performed to obtain solution A;

[0098] (2) Add iron powder to solution A. The amount of iron powder used is 1.5 times the molar amount of iron in the solution. The reaction time is 1.5 h. After the reaction, the solid and liquid are separated to obtain solution B.

[0099] (3) In solution B, add an iron source or phosphorus source to adjust the phosphorus-iron molar ratio in the system to 1:1.05 according to Fe:P. The iron supplementing reagent is ferric sulfate, and the phosphorus supplementing reagent is ammonium dihydrogen phosphate.

[0100] (4) Add hydrogen peroxide to solution B. The molar amount of hydrogen peroxide used is 1.2 times the molar amount of iron in the solution. Add ammonia to adjust the pH to 1.9 to carry out co-precipitation reaction. The reaction temperature is 70℃ and the reaction time is 1.5h. After the reaction is complete, solid and liquid are separated to obtain ferric phosphate dihydrate precipitate. It is then calcined in a muffle furnace at 500℃ for 4h to obtain ferric phosphate.

[0101] Comparative Example 2

[0102] The only difference between this comparative example and Example 1 is that iron powder is not added in step (3), while the other conditions and parameters are exactly the same as in Example 1.

[0103] Performance testing:

[0104] (1) The mass concentrations of the main elements in solution B were obtained using ICP testing in the examples and comparative examples. The test results are shown in Table 1.

[0105] Table 1

[0106]

[0107]

[0108] (2) The mass fractions of each component in the iron phosphate prepared by the test examples and comparative examples are shown in Table 2:

[0109] Table 2

[0110] Fe / % P / ％ Al / ％ Cu / % Example 1 29.76 16.66 0.001 0.001 Example 2 29.48 16.39 0.001 0.003 Example 3 29.35 16.32 0.004 0.003 Example 4 29.59 16.56 0.008 0.002 Example 5 29.63 16.24 0.003 0.002 Example 6 29.78 16.93 0.10 0.003 Example 7 28.49 16.77 0.004 0.003 Comparative Example 1 28.78 15.88 0.27 0.05 Comparative Example 2 29.45 16.71 0.002 0.22

[0111] As can be seen from Tables 1-2, and as obtained from Examples 1-3, the phosphorus-iron raw material solution obtained by the method described in this disclosure has an Al content of less than 0.0039%, a Cu content of less than 0.003%, a P content of more than 12.04%, and an Fe content of more than 25.33%. The recovered ferric phosphate has an iron content of more than 29.35%, a phosphorus content of more than 16.32%, an impurity Al ion content of less than 0.004%, and an impurity copper ion content of less than 0.003%.

[0112] A comparison of Examples 1 and 4-5 shows that the amount of chloride added in the method disclosed herein affects the recovery effect. Controlling the mass ratio of the mixture to chloride at 1:(0.2-0.6) results in a better recovery effect. If the amount of chloride added is too large, it leads to the waste of chloride salts and easily introduces new impurities. If the amount of chloride added is too small, the reaction cannot proceed completely, resulting in a high content of aluminum impurities in the calcined phosphorus iron slag.

[0113] Comparing Examples 1 and 6-7, it can be seen that in the method described in this disclosure, the roasting temperature in step (2) affects the recovery effect. The recovery effect is better when the roasting temperature is controlled at 185-270℃. If the roasting temperature is too high, ferric chloride will be generated and volatilized, resulting in the loss of iron. If the roasting temperature is too low, the boiling point of aluminum chloride will not be reached, affecting the removal of aluminum impurities in the ferrophosphate slag.

[0114] As can be seen from the comparison between Example 1 and Comparative Examples 1-2, this disclosure achieves the phase transformation of aluminum impurities in ferrophosphate slag by high-temperature calcination of ferrophosphate slag and chloride salt, so that the aluminum impurities are converted into aluminum chloride and volatilized as gas. The slag obtained after calcination is then reacted with acid, and after solid-liquid separation, a leachate containing iron and phosphorus ions is obtained. After removing copper by adding iron, the deep removal of Al and Cu impurities can be achieved. By adding iron or phosphorus sources to adjust the phosphorus-iron ratio, a synthesis stock solution with a certain phosphorus-iron ratio is obtained. Hydrogen peroxide and ammonia are added to adjust the pH to obtain the precipitate of ferric phosphate precursor. The process is simple and the production cost is low.

Claims

1. A method for preparing ferric phosphate from ferric phosphate slag, the method comprising the following steps: (1) Pretreatment of phosphate iron slag yields a mixture containing aluminum phosphate; (2) A mixture containing aluminum phosphate is mixed with chloride and calcined to obtain calcined residue. After washing and filtration, the calcined residue is mixed with an acid solution. After a one-step reaction, solution A is obtained by solid-liquid separation. (3) Mix solution A with elemental iron. After two-step reaction, obtain solution B by solid-liquid separation. Add iron source and / or phosphorus source to adjust the phosphorus-iron molar ratio to obtain raw material solution. (4) The raw material solution and hydrogen peroxide are mixed, the pH is adjusted and a three-step reaction is carried out. The solid and liquid are separated to obtain ferric phosphate dihydrate, which is then sintered to obtain the ferric phosphate. The chloride in step (2) includes sodium chloride, potassium chloride and ferric chloride. Based on the mass of the chloride as 100%, the mass fraction of sodium chloride is 15% to 30%, the mass fraction of potassium chloride is 15% to 25%, and the mass fraction of ferric chloride is 55% to 75%. The calcination temperature in step (2) is 185 to 270°C.

2. The method as described in claim 1, wherein, The preprocessing in step (1) includes: The phosphorus slag is mixed with a solvent, a phosphoric acid solution is added, the pH is controlled to carry out the reaction, and the mixture is dried to obtain a mixture containing aluminum phosphate.

3. The method as described in claim 2, wherein, The solvent includes water.

4. The method of claim 2, wherein, The phosphoric acid solution has a mass concentration of 50% to 85%.

5. The method of claim 2, wherein, The pH is 1.8 to 3.

5.

6. The method of claim 2, wherein, The reaction temperature is 50–90°C.

7. The method of claim 2, wherein, The reaction time is 1 to 2 hours.

8. The method of claim 1, wherein, The mass ratio of the mixture to the chloride in step (2) is 1:(0.2-0.6).

9. The method of claim 1, wherein, The atmosphere for the roasting process in step (2) includes air.

10. The method of claim 1, wherein, The roasting process in step (2) takes 1 to 2 hours.

11. The method of claim 1, wherein, The acid solution in step (2) includes sulfuric acid and / or phosphoric acid.

12. The method of claim 1, wherein, The concentration of the acid solution in step (2) is 0.8 to 1.2 mol / L.

13. The method of claim 1, wherein, The mass-to-volume ratio of the roasted residue and acid solution in step (2) is 1 g: (5-10) mL.

14. The method of claim 1, wherein, The reaction time in step (2) is 0.5 to 2.5 hours.

15. The method of claim 1, wherein, The elemental iron in step (3) includes iron powder.

16. The method of claim 1, wherein, The solution A described in step (3) contains the impurity Cu. 2+ .

17. The method of claim 1, wherein, The elemental iron and the impurity Cu in solution A mentioned in step (3) 2+ The molar ratio is (1~2):

1.

18. The method of claim 1, wherein, The reaction time for the two steps in step (3) is 0.5 to 2.5 hours.

19. The method of claim 1, wherein, The iron source in step (3) includes any one or a combination of at least two of ferric sulfate, ferric chloride, or ferric nitrate.

20. The method of claim 1, wherein, The phosphorus source in step (3) includes any one or a combination of at least two of the following: ammonium dihydrogen phosphate, sodium phosphate, ammonium phosphate, or monohydrogen phosphate.

21. The method of claim 1, wherein, The phosphorus-iron molar ratio in step (3) is (1-1.1):

1.

22. The method of claim 1, wherein, In step (4), the molar ratio of hydrogen peroxide to iron in the raw material solution is (1-1.5):

1.

23. The method of claim 1, wherein, The pH adjuster mentioned in step (4) includes ammonia.

24. The method of claim 1, wherein, The pH value in step (4) is 1.6 to 2.

2.

25. The method of claim 1, wherein, The temperature of the three-step reaction in step (4) is 50-95℃.

26. The method of claim 1, wherein, The reaction time for the three steps described in step (4) is 1 to 2 hours.

27. The method of claim 1, wherein, The sintering temperature in step (4) is 350–650°C.

28. The method of claim 1, wherein, The sintering process in step (4) takes 3 to 6 hours.

29. The method of claim 1, wherein, The method includes the following steps: (1) Mix the phosphorus iron slag with water, add a phosphoric acid solution with a mass concentration of 50% to 85%, control the pH to 1.8 to 3.5, react at 50 to 90°C for 1 to 2 hours, and filter to obtain a mixture containing aluminum phosphate; (2) A mixture containing aluminum phosphate is mixed with chloride at a mass ratio of 1:(0.2-0.6) and calcined at 185-270°C for 1-2 hours to obtain calcined residue. After washing and filtration, the calcined residue is mixed with acid solution at a mass-volume ratio of 1g:(5-10)mL and reacted in one step for 0.5-2.5 hours. After solid-liquid separation, solution A is obtained. (3) Mix solution A with iron powder, and after two-step reaction for 0.5 to 2.5 hours, obtain solution B by solid-liquid separation. Add iron source and / or phosphorus source to adjust the phosphorus-iron molar ratio to (1 to 1.1):1 to obtain raw material solution; (4) Mix the raw material solution and hydrogen peroxide, add ammonia to adjust the pH to 1.6-2.2 and carry out a three-step reaction for 1-2 hours. Separate the solid and liquid to obtain ferric phosphate dihydrate, and sinter at 350-650℃ for 3-6 hours to obtain the ferric phosphate.

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