A method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag

Abstract

The present invention belongs to the field of metallurgy technology, and specifically relates to a method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate extraction slag, which solves the problem that the waste iron phosphate slag after lithium extraction from waste lithium iron phosphate batteries in the prior art cannot be utilized. Waste lithium iron phosphate extraction slag is used as raw material, potassium hydroxide is used as an additive for calcination, the lithium iron phosphate is decomposed, and then deionized water is leached to dissolve phosphorus into the solution in the form of potassium phosphate. The leachate is purified and evaporated and concentrated to crystallize potassium phosphate, or the leachate is purified and phosphoric acid is added to adjust the pH value and then evaporated and concentrated to crystallize dipotassium hydrogen phosphate or potassium dihydrogen phosphate. The leachate is dried and converted into an ironmaking raw material whose main component is ferroferric oxide. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate extraction slag of the present invention can realize the full utilization of the lithium extraction waste slag, and the entire process is simple and the product has high added value.

Description

Technical field:

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a method for preparing potassium phosphate and potassium dihydrogen phosphate from lithium-extracted slag from waste lithium iron phosphate. Background technology:

[0002] Lithium iron phosphate batteries offer advantages such as high operating voltage, high energy density, long cycle life, excellent safety, low self-discharge, and no memory effect. They are widely used in various new energy vehicles, 3C portable electronic devices, drones, and other fields. Currently, lithium iron phosphate batteries account for approximately half of total lithium-ion battery shipments. The design life of lithium-ion power batteries is generally 6-7 years, with some reaching 10 years. The retirement of a large number of lithium iron phosphate batteries generates a large amount of waste battery materials. The positive electrode material is the most expensive component of lithium iron phosphate battery production and is also the most valuable component of retired lithium iron phosphate batteries.

[0003] After years of research and development, the technology for recycling and reusing lithium from spent lithium iron phosphate battery cathode materials has matured and achieved industrialization. Lithium extraction from spent lithium iron phosphate battery cathode materials produces a large amount of iron phosphate waste residue, primarily composed of iron phosphate and carbon, with small amounts of other substances such as copper, aluminum oxide, and fluoride. Because this waste residue has a low added value and contains some hazardous substances, it is not suitable for direct disposal as ordinary solid waste and has yet to be effectively utilized. Phosphorus, on the other hand, is a crucial resource, crucial for food security, human health, and the safety and stability of key supply chains, including new energy and new energy vehicles. Phosphate rock has become a strategic non-metallic mineral resource, but global reserves are relatively scarce, and its mining has a significant impact on the environment. Excessive phosphate mining can lead to land degradation, water pollution, and ecosystem damage. Therefore, there is an urgent need to increase the recovery and utilization rate of phosphorus from various resources. Summary of the invention:

[0004] The purpose of the present invention is to provide a method for preparing potassium phosphate and potassium dihydrogen phosphate from lithium slag after lithium extraction from waste lithium iron phosphate batteries, so as to solve the problem that the waste iron phosphate slag after lithium extraction from waste lithium iron phosphate batteries cannot be utilized in the prior art.

[0005] The technical solution of the present invention is:

[0006] A method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag comprises the following steps:

[0007] (1) Calcination and conversion of lithium extraction waste residue

[0008] The lithium extraction waste residue is ground to a particle size of less than 0.15 mm and dried, and then evenly mixed with potassium salt and petroleum coke having a particle size of less than 0.15 mm, and the mixture is formed into agglomerates, and the obtained agglomerates are placed in a calcining furnace for calcination to obtain calcined lithium extraction slag;

[0009] The potassium salt is potassium hydroxide or potassium carbonate. The proportion of the potassium salt to the lithium extraction waste residue is determined by the phosphorus content in the lithium extraction waste residue. The potassium hydroxide or potassium carbonate is added according to 100-150% of the theoretical required mass in equation (1) or equation (2).

[0010] The proportion of petroleum coke added is determined by the iron content in the lithium extraction waste slag. The petroleum coke is added in an amount of 100 to 300% of the theoretically required carbon mass in equation (1) or equation (2).

[0011] 6FePO4+18KOH+C=6K3PO4+2Fe3O4+9H2O+CO (1)

[0012] 6FePO4+9K2CO3+C=6K3PO4+2Fe3O4+9CO2+CO (2)

[0013] (2) Leaching and separation of phosphorus and iron from calcined material

[0014] Using deionized water as a leaching agent, the calcined lithium extraction slag is leached, so that the water-soluble potassium phosphate enters the liquid phase, and the remaining insoluble iron slag and impurities remain in the slag phase, thereby achieving the separation of P and Fe; the leaching is carried out at a temperature of 20 to 200° C., the liquid-to-solid mass ratio of the leaching process is 15:1 to 5:1, the leaching time is 0.5 to 10 hours, and after leaching, the leached slag and potassium phosphate leachate are obtained by filtration;

[0015] (3) Potassium phosphate evaporation crystallization

[0016] The potassium phosphate leachate is evaporated and concentrated at a temperature of 90 to 150° C. to a relative density of 1.38 to 1.50 g / ml, and then cooled to below 60° C. in a cooling crystallizer to precipitate potassium phosphate trihydrate crystals, which are then filtered or centrifuged to obtain potassium phosphate trihydrate and concentrated mother liquor.

[0017] (4) Evaporation crystallization of dipotassium hydrogen phosphate and potassium dihydrogen phosphate

[0018] Phosphoric acid is added to the potassium phosphate leachate to adjust the solution pH to 6-9, and the solution is evaporated and concentrated at a temperature of 90-150° C. until the concentration of dipotassium hydrogen phosphate is above 400 g / L, and then cooled to below 40° C. in a cooling crystallizer to precipitate dipotassium hydrogen phosphate crystals, which are filtered or centrifuged to obtain dipotassium hydrogen phosphate and concentrated mother liquor;

[0019] Phosphoric acid is added to the potassium phosphate leachate to adjust the solution pH to 4-6, and the solution is evaporated and concentrated at a temperature of 90-150° C. to a potassium dihydrogen phosphate concentration of 600 g / L or more, and then cooled to below 40° C. in a cooling crystallizer to precipitate potassium dihydrogen phosphate crystals, which are filtered or centrifuged to obtain potassium dihydrogen phosphate and concentrated mother liquor;

[0020] (5) Treatment of leaching residue

[0021] The main components of the leached slag obtained after leaching the calcined lithium extraction slag are Fe3O4 and C, wherein the Fe3O4 content is 60-90wt% and the C content is 2-10wt%. The leached slag is dried at 100-250°C for 2-5h and used as ironmaking raw material.

[0022] In the method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag, in step (1), if the carbon content in the lithium extraction waste slag exceeds the carbon amount required for the reaction, no additional petroleum coke is added.

[0023] The method for preparing potassium phosphate and potassium dihydrogen phosphate from lithium-ion phosphate waste slag comprises the following steps: in the conversion process of step (1), the pelletizing pressure is 50-200 MPa, the calcination temperature is 300° C.-1100° C., and the calcination time is 1-10 hours; after the calcination is completed, the pellets are cooled to below 200° C. and then taken out of the calcination furnace.

[0024] In the method for preparing potassium phosphate and potassium dihydrogen phosphate from lithium ion slag extracted from waste lithium iron phosphate, during the leaching process in step (2), mechanical stirring is adopted or the process is carried out under an ultrasonic environment.

[0025] In the method for preparing potassium phosphate and potassium dihydrogen phosphate from lithium phosphate extraction residue from waste lithium iron phosphate, in the leaching process of step (2), preferably, the leaching is carried out at a temperature of 50 to 100°C.

[0026] The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag comprises the following steps: in step (3), the concentrated mother liquor is returned to the potassium phosphate leachate to adjust the pH to be greater than 12, and then the potassium phosphate or dipotassium hydrogen phosphate or potassium dihydrogen phosphate is evaporated and crystallized.

[0027] In the method for preparing potassium phosphate and potassium dihydrogen phosphate from lithium-extracted slag from waste lithium iron phosphate, in step (3), the concentrated mother liquor is purified and impurities removed when the concentrated mother liquor is circulated 5 to 20 times.

[0028] The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag comprises the following steps: when purifying and removing impurities from the concentrated mother liquor, adding phosphoric acid to the concentrated mother liquor from which potassium phosphate crystals, dipotassium hydrogen phosphate crystals, or potassium dihydrogen phosphate crystals are separated by distillation and crystallization; adjusting the pH of the concentrated mother liquor to below 8 to precipitate aluminum in the form of aluminum hydroxide crystals; and simultaneously adding calcium dihydrogen phosphate to the concentrated mother liquor.

[0029] In the method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag, the amount of calcium dihydrogen phosphate added is 90-120% of the theoretical mass required by equation (3);

[0030] Ca(H2PO4)2+2KF=2KH2PO4+CaF2 (3).

[0031] The method for preparing potassium phosphate and potassium dihydrogen phosphate from lithium iron phosphate residue is followed by filtration or centrifugation to obtain filter residue and filtered mother liquor, and the filtered mother liquor is returned to the potassium phosphate leachate for concentration and crystallization.

[0032] The design idea of ​​the present invention is:

[0033] The invention uses waste lithium iron phosphate lithium extraction waste residue as raw material, calcines with potassium hydroxide as an additive, decomposes the lithium iron phosphate, and then leaches with deionized water to dissolve phosphorus into the solution in the form of potassium phosphate. The leachate is purified and evaporated to concentrate and crystallize potassium phosphate, or the leachate is purified and adjusted in pH value to concentrate and crystallize dipotassium hydrogen phosphate or potassium dihydrogen phosphate. The leachate is dried and converted into an ironmaking raw material whose main component is ferroferric oxide.

[0034] Because lithium extraction waste slag contains impurity elements such as copper, aluminum, and fluorine, most of these impurity elements will enter the slag during the leaching process of the calcined lithium extraction slag. However, a small amount of aluminum and fluorine will enter the potassium phosphate leachate. During the evaporation and crystallization process of the potassium phosphate leachate, these impurities will remain in the concentrated mother liquor, where aluminum mainly exists in the form of potassium aluminate, and fluorine mainly exists in the form of potassium fluoride. As the number of concentrated mother liquor cycles increases, the concentrations of potassium aluminate and potassium fluoride in the mother liquor increase. When the number of mother liquor cycles reaches a certain level, potassium aluminate and potassium fluoride will precipitate together with potassium phosphate, resulting in an increase in the impurity content of potassium phosphate. To prevent aluminum and fluorine from entering the potassium phosphate, the concentrated mother liquor is purified during the concentrated mother liquor circulation process. At this time, phosphoric acid is added to the concentrated mother liquor from which potassium phosphate crystals are separated by distillation and crystallization. The pH of the concentrated mother liquor is adjusted to below 8, causing aluminum to precipitate in the form of aluminum hydroxide crystals. At the same time, calcium dihydrogen phosphate is added to the concentrated mother liquor to remove fluorine.

[0035] The advantages and beneficial effects of the present invention are:

[0036] The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate lithium extraction slag of the present invention can realize the utilization of all components of the lithium extraction waste slag, and the entire process is simple, the product added value is high, and there is no secondary pollution. It is an energy-saving and environmentally friendly comprehensive utilization method of lithium extraction waste slag and has good industrial application prospects. Description of the drawings:

[0037] Figure 1The present invention provides a process flow chart for the comprehensive utilization of waste lithium iron phosphate extraction residue to prepare potassium phosphate trihydrate and iron-rich slag.

[0038] Figure 2 The present invention provides a process flow chart for the comprehensive utilization of waste lithium iron phosphate residue to prepare potassium dihydrogen phosphate and iron-rich slag. Specific implementation method:

[0039] like Figure 1 As shown, the process flow of the comprehensive utilization of waste lithium iron phosphate waste residue of the present invention is as follows:

[0040] The dried waste lithium iron phosphate lithium extraction waste residue (lithium residue) is evenly mixed with potassium salt and petroleum coke, and the mixture is agglomerated. The agglomerates are placed in a calcining furnace for calcination, and calcined lithium extraction residue (calcined residue) is obtained after calcination.

[0041] Deionized water is used as a leaching agent to leach the calcined lithium extraction slag, so that the water-soluble potassium phosphate enters the liquid phase, and the remaining insoluble iron slag and impurities remain in the slag phase, thereby achieving the separation of P and Fe. After leaching, the leached slag and potassium phosphate leachate are obtained by filtration. The leached slag is dried and converted into an ironmaking raw material whose main component is ferroferric oxide.

[0042] The potassium phosphate leachate is distilled and concentrated, and then cooled and crystallized to precipitate potassium phosphate crystals, which are then filtered or centrifuged to obtain potassium phosphate (tripotassium phosphate trihydrate) and concentrated mother liquor; or Figure 2 As shown, the potassium phosphate leachate is distilled and concentrated after adding phosphoric acid to adjust the pH, and then cooled and crystallized to precipitate potassium dihydrogen phosphate crystals, which are then filtered or centrifuged to produce potassium dihydrogen phosphate and concentrated mother liquor. The concentrated mother liquor is returned to the potassium phosphate leachate, and the potassium phosphate or potassium dihydrogen phosphate is then distilled, concentrated, and cooled and crystallized.

[0043] Below, the present invention is further described in detail with reference to the embodiments.

[0044] Example 1

[0045] In this embodiment, the main components of the waste lithium iron phosphate lithium extraction waste residue are TFe 32.79%, P16.66%, C 5.23%, Al2O3 0.72%, Cu≤0.01%, Cl 0.03%, Na0.159%, F 0.072%, and the balance is mainly O.

[0046] The lithium extraction waste slag is ground to a particle size of less than 0.15 mm and dried. The mixture is then uniformly mixed with potassium hydroxide (also less than 0.15 mm in particle size) and petroleum coke, and then agglomerated under a pressure of 100 MPa. The resulting agglomerates are then placed in a calcining furnace for calcination to obtain calcined lithium extraction slag. The potassium hydroxide is added in an amount equal to 120% of the theoretical mass required in equation (1), and no additional petroleum coke is added. The calcination temperature is 600°C for 2 hours. After calcination, the agglomerates are cooled to below 50°C and removed from the calcining furnace.

[0047] Using deionized water as a leaching agent, the calcined lithium extraction slag is ground to a particle size of less than 0.15 mm, added to deionized water, and leached at 80°C for 3 hours. The liquid-to-solid mass ratio during the leaching process is 5:1. After leaching, the residue is filtered to obtain a filter residue (leached residue) and a potassium phosphate leachate. The potassium phosphate leachate has a relative density of ≤1.05 g / ml and a potassium phosphate concentration of 80 g / L. The potassium phosphate leachate is evaporated and concentrated at 100°C to a relative density of 1.40 g / ml, cooled to 30°C, potassium phosphate crystals are precipitated, and centrifuged to obtain potassium phosphate trihydrate and a concentrated mother liquor. The concentrated mother liquor is returned to the next leaching potassium phosphate solution. The separated potassium phosphate trihydrate has a purity of ≥99 wt%.

[0048] The leached residue obtained after leaching the calcined lithium-extraction slag is washed and dried at 150°C for 5 hours before being used as raw material for ironmaking. The leached residue primarily consists of Fe₃O₄ and C, with the Fe₃O₄ content being approximately 80% by weight and the C content being 8%. By mass percentage, the main components are TFe (60%), P (0.03%), C (8%), Al (0.70%), and F (0.18%), with the remainder primarily being O.

[0049] After the concentrated mother liquor has been circulated 10 times, the concentrated mother liquor is purified and impurities removed. During the purification and impurity removal process, phosphoric acid is added to the concentrated mother liquor from which potassium phosphate crystals are separated by distillation and crystallization, and the pH of the concentrated mother liquor is adjusted to below 8, causing aluminum to precipitate as aluminum hydroxide crystals. Simultaneously, calcium dihydrogen phosphate is added to the concentrated mother liquor. The amount of calcium dihydrogen phosphate added is 100% of the theoretical mass required by equation (3). Filtration or centrifugation is then performed to obtain a filter residue and a filtered mother liquor. The filtered mother liquor is then returned to the potassium phosphate leachate for concentrated crystallization.

[0050] Example 2

[0051] In this embodiment, the main components of the waste lithium iron phosphate lithium extraction waste residue are TFe 32.79%, P16.66%, C 5.23%, Al2O3 0.72%, Cu≤0.01%, Cl 0.03%, Na0.159%, F 0.072%, and the balance is mainly O.

[0052] The lithium extraction waste residue is ground to a particle size of less than 0.15 mm and dried. The mixture is then uniformly mixed with potassium hydroxide (also less than 0.15 mm in particle size) and then agglomerated under a pressure of 100 MPa. The resulting agglomerates are then placed in a calcining furnace for calcination to obtain calcined lithium extraction slag. The potassium hydroxide is added in an amount equal to 130% of the theoretical mass required in equation (1), and no additional petroleum coke is added. The calcination temperature is 300°C for 2 hours. After calcination, the agglomerates are cooled to below 50°C and removed from the calcining furnace.

[0053] Using deionized water as a leaching agent, the calcined lithium extraction slag is ground to a particle size of less than 0.075 mm, added to deionized water, and leached at a temperature of 80°C for 1 hour. The liquid-to-solid mass ratio during the leaching process is 2:1. After leaching, the residue is filtered to obtain a filter residue (leached residue) and a potassium phosphate leachate, the concentration of which is about 80 g / L. Phosphoric acid is added to the potassium phosphate leachate to adjust the solution pH to 5.0, so that the potassium phosphate solution is converted into a potassium dihydrogen phosphate solution. The solution is evaporated and concentrated at a temperature of 100°C to a potassium dihydrogen phosphate concentration of 400 g / L. The solution is cooled to 30°C, potassium dihydrogen phosphate crystals are precipitated, and centrifugation is performed to obtain potassium dihydrogen phosphate and a concentrated mother liquor. The concentrated mother liquor is returned to the next leached potassium dihydrogen phosphate solution, and the potassium dihydrogen phosphate obtained by separation has a purity of ≥99wt%.

[0054] The leached residue obtained after leaching the calcined lithium-extraction slag is washed and dried at 150°C for 5 hours before being used as raw material for ironmaking. The leached residue primarily consists of Fe₃O₄ and C, with the Fe₃O₄ content being approximately 76% by weight and the C content being 8%. By mass percentage, the main components are TFe 56%, P 0.20%, C 9.5%, Al 0.80%, and F 0.16%, with the remainder primarily being O.

[0055] After the concentrated mother liquor has been circulated five times, the concentrated mother liquor is purified and impurities removed. While the concentrated mother liquor is being purified and impurities removed, calcium dihydrogen phosphate is added to the concentrated mother liquor from which potassium dihydrogen phosphate crystals are separated by distillation and crystallization. The amount of calcium dihydrogen phosphate added is 90% of the theoretical mass required by equation (3). Filtration or centrifugation is then performed to obtain a filter residue and a filtered mother liquor. The filtered mother liquor is then returned to the potassium phosphate leachate for concentrated crystallization.

[0056] Implementation results indicate that the iron phosphate residue left after lithium extraction from spent lithium iron phosphate batteries contains significant amounts of phosphorus and iron, making it a valuable resource. To increase the economic value of lithium iron phosphate battery recycling and reduce environmental pollution, this invention has developed a clean, environmentally friendly, and streamlined recycling process for iron phosphate residue, truly achieving high-value utilization of all components of spent lithium-ion power batteries.

Claims

1. A method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate residue, characterized in that: The following steps are involved: (1) Calcination and conversion of lithium extraction waste residue The lithium extraction waste residue is ground to a particle size of less than 0.15 mm and dried, and then evenly mixed with potassium salt and petroleum coke with a particle size of less than 0.15 mm, and the mixture is agglomerated. The agglomerates are placed in a calcining furnace for calcination at a calcination temperature of 300° C. to 600° C. to obtain calcined lithium extraction slag; The potassium salt is potassium hydroxide or potassium carbonate. The proportion of the potassium salt to the lithium extraction waste residue is determined by the phosphorus content in the lithium extraction waste residue. Potassium hydroxide or potassium carbonate is added according to 100-150% of the theoretical required mass in equation (1) or equation (2). The proportion of petroleum coke is determined by the iron content in the lithium extraction waste slag. The petroleum coke is mixed according to 100-300% of the theoretical carbon mass required in equation (1) or equation (2); 6FePO4+18KOH+C=6K3PO4+2Fe3O4+9H2O+CO (1) 6FePO4+9K2CO3+C=6K3PO4+2Fe3O4+9CO2+CO (2) (2) Leaching and separation of phosphorus and iron from calcined materials Deionized water is used as a leaching agent to leach the calcined lithium extraction slag, causing the water-soluble potassium phosphate in the slag to enter the liquid phase, while the remaining insoluble iron slag and impurities remain in the slag phase, thereby achieving the separation of P and Fe. The leaching is carried out at a temperature of 20-200°C, with a liquid-to-solid mass ratio of 15:1-5:1 during the leaching process, and the leaching time is 0.5-10 hours. After leaching, the leached slag and potassium phosphate leachate are obtained by filtration. (3) Evaporation crystallization of potassium phosphate, dipotassium hydrogen phosphate, or potassium dihydrogen phosphate The potassium phosphate leachate is evaporated and concentrated at a temperature of 90-150° C. to a relative density of 1.38-1.50 g / ml, and then cooled to below 60° C. in a cooling crystallizer to precipitate potassium phosphate trihydrate crystals, which are then filtered or centrifuged to obtain potassium phosphate trihydrate and concentrated mother liquor. Phosphoric acid is added to the potassium phosphate leachate to adjust the pH of the solution to 6-9, and the solution is evaporated and concentrated at a temperature of 90-150° C. to a concentration of dipotassium hydrogen phosphate of 400 g / L or higher, and then cooled to below 40° C. in a cooling crystallizer to precipitate dipotassium hydrogen phosphate crystals, which are then filtered or centrifuged to obtain dipotassium hydrogen phosphate and a concentrated mother liquor. adding phosphoric acid to the potassium phosphate leachate to adjust the solution pH to 4-6, evaporating and concentrating the solution at a temperature of 90-150° C. to a potassium dihydrogen phosphate concentration of 600 g / L or higher, cooling the solution to below 40° C. in a cooling crystallizer to precipitate potassium dihydrogen phosphate crystals, and filtering or centrifuging the solution to obtain potassium dihydrogen phosphate and concentrated mother liquor; (4) Treatment of leaching residue The main components of the leached slag obtained after leaching of the calcined lithium extraction slag are Fe3O4 and C, of ​​which the Fe3O4 content is 60~90wt% and the C content is 2~10wt%. The leached slag is dried at 100~250℃ for 2~5h and used as ironmaking raw material.

2. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 1, characterized in that: In step (1), if the carbon content in the lithium extraction waste residue exceeds the carbon amount required for the reaction, no additional petroleum coke is added.

3. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 1, characterized in that: During the conversion process of step (1), the pelletizing pressure is 50-200 MPa and the calcination time is 1-10 h. After the calcination is completed, the pellets are cooled to below 200° C. and then taken out of the calcination furnace.

4. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 1, characterized in that: During the leaching process of step (2), mechanical stirring is adopted or the process is carried out under an ultrasonic environment.

5. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 1, characterized in that: During the leaching process of step (2), the leaching is carried out at a temperature of 50-100°C.

6. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 1, characterized in that: In step (3), the concentrated mother liquor is returned to the potassium phosphate leachate to adjust the pH to be greater than 12, and then the potassium phosphate or dipotassium hydrogen phosphate or potassium dihydrogen phosphate is evaporated and crystallized.

7. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 1, characterized in that: In step (3), when the concentrated mother liquor is circulated 5 to 20 times, the concentrated mother liquor is purified and impurities are removed.

8. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 7, characterized in that: When purifying and removing impurities from the concentrated mother liquor, phosphoric acid is added to the concentrated mother liquor from which potassium phosphate crystals are separated by distillation and crystallization, and the pH of the concentrated mother liquor is adjusted to below 8 to precipitate aluminum in the form of aluminum hydroxide crystals, and simultaneously calcium dihydrogen phosphate is added to the concentrated mother liquor; alternatively, calcium dihydrogen phosphate is added to the concentrated mother liquor from which dipotassium hydrogen phosphate crystals or potassium dihydrogen phosphate crystals are separated by distillation and crystallization.

9. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 8, characterized in that: The amount of calcium dihydrogen phosphate added is 90-120% of the theoretical mass required by equation (3); Ca(H2PO4)2+2KF=2KH2PO4+CaF2 (3).

10. The method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate slag according to claim 8, characterized in that: Subsequently, filtration or centrifugation is performed to obtain filter residue and filtered mother liquor, and the filtered mother liquor is returned to the potassium phosphate leachate for concentration and crystallization.

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