A method for preparing battery-grade iron phosphate from a low-quality phosphorus source

Abstract

This invention discloses a method for preparing battery-grade iron phosphate from a low-quality phosphorus source. The method involves preparing a bifunctional impurity remover to remove impurities from the low-quality phosphorus source, obtaining a phosphorus source solution. Simultaneously, iron powder and a flocculant are used to purify ferrous sulfate, a byproduct of titanium dioxide production, resulting in a purified ferrous sulfate solution. The purified ferrous sulfate solution and the phosphorus source solution are then mixed and reacted. An oxidant and a precipitant are added to prepare iron phosphate dihydrate. Finally, the solution is dried and calcined to obtain anhydrous iron phosphate. This invention uses low-cost, low-quality phosphoric acid to replace high-cost refined phosphoric acid, effectively reducing raw material costs while ensuring the quality of the iron phosphate. Furthermore, the iron phosphate prepared using the above process not only meets the impurity content standards but also exhibits excellent electrochemical performance and high tap density, showing broad prospects for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material iron phosphate preparation technology, specifically relating to a method for preparing battery-grade iron phosphate from a low-quality phosphorus source. Background Technology

[0002] With the increasing penetration rate of new energy vehicles, the power and energy storage battery industry has also experienced rapid development. Common battery types include ternary lithium, lithium iron phosphate, lithium cobalt oxide, and lithium manganese oxide batteries. Among them, lithium iron phosphate batteries have surpassed ternary lithium batteries in market share due to their advantages such as high safety performance, long cycle life, low pollution, and low cost, becoming the preferred choice for power and energy storage batteries. As an important component of power and energy storage batteries, lithium iron phosphate cathode materials are exhibiting a trend of parallel development of multiple technical routes under the influence of multiple factors such as technological iteration and raw material price fluctuations. Currently, the industrial preparation of lithium iron phosphate mainly includes four process routes: ferrous oxalate process, iron oxide red process, all-wet process, and solid-phase iron phosphate process. Among these methods, the solid-phase iron phosphate process has gradually become one of the most mainstream lithium iron phosphate production processes due to its advantages such as simple process, more relaxed reaction conditions, stronger controllability, high raw material utilization rate, and good repeatability.

[0003] Currently, the main industrial processes for preparing ferric phosphate include the ammonium method, sodium method, and iron method. The raw materials for these processes mainly include: iron sources (ferrous sulfate, ferric chloride, ferrous oxalate, iron powder); phosphorus sources (industrial phosphoric acid, food-grade phosphoric acid, monoammonium phosphate, diammonium phosphate); oxidants (hydrogen peroxide, oxygen); and precipitants (ammonia, sodium hydroxide). Phosphorus sources constitute the largest component of the raw material cost in ferric phosphate, accounting for approximately 62% of the raw material cost and approximately 36% of the total cost. Therefore, the main method to reduce the production cost of ferric phosphate is to utilize low-quality phosphorus and iron sources, processing them before preparing ferric phosphate. For example, CN 108455547 A discloses a method for preparing low-impurity, high-iron-phosphorus ratio, battery-grade ferric phosphate, which involves removing manganese and magnesium ion impurities from ferrous sulfate at low temperature before reacting it with phosphate and hydrogen peroxide to prepare ferric phosphate. Yuan Wenlong et al. used ferrous sulfate, a byproduct of titanium dioxide production, as an iron source after removing manganese ions from it to synthesize ferric phosphate precursors (Yuan Wenlong, Wang Bixia, Zhao Ying et al. Synthesis of ferric phosphate precursors from ferrous sulfate, a byproduct of titanium dioxide production. Nonferrous Metals Engineering. 2023, 13(7):61-68). CN 114906829 A discloses a method for preparing battery-grade ferric phosphate using agricultural-grade wet-process phosphoric acid. The method involves purifying the agricultural-grade wet-process phosphoric acid by removing fluoride and aluminum, adjusting the pH once, sedimentation, and adjusting the pH twice to obtain a phosphate reaction solution, which is then reacted with ferric sulfate solution to obtain ferric phosphate. CN115818605 A discloses a method for preparing ferric phosphate dihydrate, which uses a fluorine-free impurity removal reagent to remove metal ions from high-impurity phosphoric acid, then extracts and back-extracts to obtain purified phosphoric acid, which is finally reacted with ferric hydroxide to generate ferric phosphate dihydrate.

[0004] Current technologies for purifying phosphoric acid typically involve adding alkaline purifying agents to generate phosphate precipitates, thereby separating phosphorus from impurities. However, this method results in significant phosphorus loss, and because the precipitation rate of metal impurity ions is extremely slow, precipitates continue to form over time after filtration, which is detrimental to long-term industrial storage and also affects the purity of ferric phosphate. Therefore, how to quickly and efficiently purify low-quality phosphorus sources has become one of the main factors restricting the reduction of ferric phosphate production costs. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing battery-grade iron phosphate from a low-quality phosphorus source. The method involves purifying and removing impurities from the low-quality phosphorus source, and then reacting it with ferrous sulfate to obtain high-quality iron phosphate, thereby reducing production costs.

[0006] To achieve the above objectives, the present invention provides a method for preparing battery-grade iron phosphate from a low-quality phosphorus source, comprising the following steps:

[0007] (1) Preparation of purified ferrous sulfate: ferrous sulfate by-product of titanium dioxide is dissolved in water at high temperature to obtain ferrous sulfate solution, then iron powder is added, flocculant is added after the reaction is completed, and after mixing evenly, the purified ferrous sulfate solution is obtained by filtration.

[0008] (2) Phosphorus source pretreatment: Filter the low-quality phosphorus source, add pH adjuster and bifunctional impurity remover to the filtrate; after reaction, filter to obtain phosphorus source solution;

[0009] (3) Preparation of ferric phosphate dihydrate: Add the phosphorus source solution obtained in step (2) to the purified ferrous sulfate solution prepared in step (1), and then add oxidant and precipitant in sequence according to the feeding time of 20~60 min. Adjust the pH value of the solution to 1-3, continue the reaction for 60-70 min, and then filter the filter cake by pressure filtration.

[0010] (4) The filter cake is washed once until the conductivity of the wash water is 1.0~3.0 mS / cm to obtain an amorphous ferric phosphate filter cake. The amorphous ferric phosphate filter cake is slurried with deionized water to obtain a slurry with a solid content of 5-15%. After adjusting the pH value to 1.4-1.9, the temperature is raised to 85-95℃ and kept at the temperature for 50-120 min for crystallization. Finally, the filter cake is filtered and washed a second time until the conductivity of the filter cake wash water is 0.1~0.5 mS / cm to obtain ferric phosphate dihydrate.

[0011] (4) Dry and calcine ferric phosphate dihydrate to obtain battery-grade ferric phosphate.

[0012] Preferably, the Fe in the ferrous sulfate solution in step (1) 2+ The ion concentration is 60~80 g / L; the amount of iron powder used is 1%-3% of the mass of ferrous sulfate; the amount of flocculant used is 0.1‰-0.4‰ of the mass of ferrous sulfate.

[0013] Preferably, the pH of the solution at the end of the reaction in step (1) is 3.0-4.0.

[0014] More preferably, the flocculant is any one of polyacrylamide, sodium polyacrylate, polydimethyldiallylammonium chloride, polyethyleneimine, polyvinylpyridine salt, and sodium carboxymethyl cellulose.

[0015] Preferably, the low-quality phosphorus source in step (2) has a phosphorus mass fraction of 5-9% and an Al content of 3000-10000ppm; the pH adjuster is any one of 0.3mol / L NaOH solution, 25wt% ammonia water, 30wt% urea solution, or 0.3mol / L KOH solution.

[0016] Preferably, the preparation method of the dual-function impurity remover is to mix and dissolve the flocculant and complexing agent with water, add a crosslinking agent, and mix evenly.

[0017] More preferably, the mass ratio of flocculant, complexing agent, water and crosslinking agent is 0.8-1:0.8-1:2:0.05.

[0018] More preferably, the flocculant is polyferric chloride or polyacrylamide; the complexing agent is any one of EDTA, EDTA-2Na, sodium pyrophosphate, sodium tripolyphosphate, and sodium citrate; and the crosslinking agent is styrene.

[0019] Preferably, in step (3), the molar ratio of ferrous ions in the purified ferrous sulfate solution to phosphorus in the phosphorus source solution is 1:1.05~0.95; the molar ratio of oxidant to ferrous ions is 0.5~0.75:1; and the molar ratio of precipitant to phosphorus in the phosphorus source solution is 1.5~2.0:1.

[0020] More preferably, the oxidant is any one of 25wt% hydrogen peroxide, oxygen, 0.1mol / L nitric acid, and 0.1mol / L sodium persulfate solution; the precipitant is any one of 0.1mol / L NaOH solution, 25wt% ammonia, 30wt% urea solution, and 0.1mol / L KOH solution.

[0021] Preferably, the reaction temperature in step (3) is 20-50℃, the reaction pH is 1-3, and the reaction stirring speed is 300-600rpm.

[0022] Preferably, the drying temperature in step (4) is 80-130℃ and the drying time is 10-15h; the calcination temperature is 550-750℃ and the calcination time is 3-5h.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. A bifunctional impurity remover was prepared using flocculant, complexing agent, water, and crosslinking agent. It has a three-dimensional network structure, which can provide a large number of complexed ions and strongly adsorb colloidal particles. Through adhesion, bridging, and crosslinking, it promotes colloidal coagulation and removes most of the impurity metal ions in low-quality phosphorus sources. At the same time, heating the low-quality source can not only accelerate the precipitation rate of metal impurity ions and improve the precipitation rate, but also effectively prevent the low-quality phosphorus source from continuously precipitating after filtration, which would affect industrial storage.

[0025] 2. Ferrous sulfate, a byproduct of the titanium dioxide industry, is further purified by adding iron powder for replacement and flocculant flocculation before being used as an iron source, thereby reducing the impurity content in anhydrous ferric phosphate.

[0026] 3. Using low-quality phosphorus sources and industrial byproduct ferrous sulfate to prepare battery-grade iron phosphate has the advantages of high purity, high yield, high tap density, and low impurity content, while greatly reducing reaction costs and the difficulty of industrial waste treatment.

[0027] 4. The process for removing impurities from low-quality phosphorus sources is simple, has good compatibility with conventional equipment, can be quickly adapted to current process equipment, and has the potential for large-scale application. Attached Figure Description

[0028] Figure 1 The image shows the SEM image of the anhydrous ferric phosphate prepared in Example 1.

[0029] Figure 2 The image shows the XRD pattern of the anhydrous ferric phosphate prepared in Example 1. Detailed Implementation

[0030] The technical solution of the present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention.

[0031] Example 1

[0032] (1) Preparation of purified ferrous sulfate: Take ferrous sulfate, a byproduct of titanium dioxide industry, add water and stir to dissolve it at 80°C, so that the Fe in the solution is purified. 2+ With an ion concentration of 70 g / L, iron powder was added, and the mixture was stirred until the pH of the solution reached 4.0. Heating was then stopped, and polyacrylamide was added. After the precipitate and flocculation were complete, the solution was vacuum filtered to obtain a purified ferrous sulfate solution. The amount of iron powder used was 1.5% of the mass of ferrous sulfate, and the amount of polyacrylamide used was 0.15‰ of the mass of ferrous sulfate.

[0033] (2) Preparation of bifunctional impurity remover: Polyacrylamide and EDTA are added to deionized water, heated to 60°C and slowly stirred to dissolve, and then styrene is added to obtain a bifunctional impurity remover with a network structure; wherein the ratio of polyacrylamide, EDTA, deionized water and styrene is 1:1:2:0.05.

[0034] (3) Phosphorus source pretreatment: Take low-quality phosphorus source and vacuum filter to obtain filtrate. Place the filtrate in a 70℃ constant temperature water bath and add 25wt% ammonia water while stirring at 300rpm to adjust the pH to 2.5. Then add bifunctional impurity remover. After stirring and reacting for 40min, a large amount of impurities precipitate out. Vacuum filter to obtain phosphorus source solution. The phosphorus mass fraction in the low-quality phosphorus source is 5%, and the Al content is 7000ppm. The amount of the bifunctional impurity remover is 2‰ of the mass of the low-quality phosphorus source.

[0035] (4) Preparation of ferric phosphate dihydrate: Take the purified ferrous sulfate solution prepared in step (1), keep the stirring speed at 400 rpm, add the phosphorus source solution prepared in step (3) while stirring, and then add hydrogen peroxide with a content of 25 wt% over a dropping time of 30 min. After the reaction continues for 10 min, the hydrogen peroxide will completely oxidize the ferrous phosphate. Then add ammonia solution with a content of 25 wt% over a dropping time of 30 min to adjust the pH to 2.0. After the reaction continues for 60 min, filter the filter cake by pressure. The molar ratio of ferrous ions to phosphorus is 1:1, and the molar ratio of hydrogen peroxide to ferrous ions is 0.75:1.

[0036] (5) Washing: Use a filter press to wash the filter cake obtained in step (4) until the conductivity of the wash water is less than 2.5 mS / cm, to obtain an amorphous iron phosphate filter cake;

[0037] (6) Crystallization: Add water to the amorphous ferric phosphate filter cake in step (5) until the solid content is 8%, stir thoroughly until the slurry is uniform, add 85wt% refined phosphoric acid to make the pH of the slurry reach 1.8, heat the slurry to keep the temperature at 90℃, filter after 100min, and use a filter press to wash the conductivity of the filter cake washing water to below 0.3 mS / cm to obtain a powdery white ferric phosphate dihydrate filter cake;

[0038] (7) Second washing: Place the ferric phosphate dihydrate filter cake in an oven and dry it at 90°C for 12 hours. Then, crush the dried ferric phosphate dihydrate with a pulverizer and place the material in a sagger in a muffle furnace and calcine it at 550°C for 5 hours to obtain anhydrous ferric phosphate.

[0039] Example 2

[0040] (1) Preparation of purified ferrous sulfate: Take ferrous sulfate, a byproduct of titanium dioxide industry, add water at 80℃ and stir to dissolve it so that the Fe in the solution is reduced. 2+ With an ion concentration of 65 g / L, iron powder was added, and the mixture was stirred until the pH of the solution reached 4.0. Heating was then stopped, and sodium polyacrylate was added. After the precipitation and flocculation were complete, the solution was vacuum filtered to obtain a purified ferrous sulfate solution. The amount of iron powder used was 3% of the mass of ferrous sulfate, and the amount of sodium polyacrylate used was 0.10‰ of the mass of ferrous sulfate.

[0041] (2) Preparation of bifunctional impurity remover: Polyacrylamide and EDTA-2Na were added to deionized water, heated to 60°C and slowly stirred to dissolve, and then styrene was added to obtain a bifunctional impurity remover with a network structure; wherein the mass ratio of polyacrylamide, EDTA-2Na, deionized water and styrene was 1:0.9:2:0.05;

[0042] (3) Phosphorus source pretreatment: Take low-quality phosphorus source and vacuum filter to obtain filtrate. Place the filtrate in a 60℃ constant temperature water bath and add 0.3mol / L NaOH solution while stirring at 300rpm to adjust the pH to 2.8. Then add bifunctional impurity remover. After stirring and reacting for 40min, a large amount of impurities precipitate out. Vacuum filter to obtain phosphorus source solution. The phosphorus mass fraction in the low-quality phosphorus source is 6%, and the Al content is 6000ppm. The amount of the bifunctional impurity remover is 2.5‰ of the mass of the low-quality phosphorus source.

[0043] (4) Preparation of ferric phosphate dihydrate: Take the purified ferrous sulfate solution prepared in step (1), keep the stirring speed at 400 rpm, and add the phosphorus source solution prepared in step (3) while stirring. Then, with a feeding time of 60 min, pass 90 wt% pure oxygen below the surface of the phosphorus source solution and continue the reaction for 10 min. The oxygen will completely oxidize the ferrous ions. Then, with a feeding time of 40 min, add 0.1 mol / L NaOH solution dropwise to adjust the pH to 2.2. Continue the reaction for 60 min and then filter by pressure to retain the filter cake. The molar ratio of ferrous ions to phosphorus is 1:1.05, and the molar ratio of oxygen to ferrous ions is 0.5:1.

[0044] (5) Washing: The filter cake obtained in step (4) is washed using a filter press until the conductivity of the wash water is less than 2.0 mS / cm, to obtain an amorphous iron phosphate filter cake;

[0045] (6) Crystallization: Add water to the amorphous ferric phosphate filter cake from step (5) until the solid content is 12%, stir thoroughly until the slurry is uniform, add 85wt% refined phosphoric acid to make the pH of the slurry reach 1.7, heat the slurry to keep the temperature at 90℃, filter after 70min, and use a filter press to wash the conductivity of the filter cake washing water to below 0.3 mS / cm to obtain a powdery white ferric phosphate filter cake dihydrate;

[0046] (7) Second washing: Place the ferric phosphate dihydrate filter cake in an oven and dry it at 90°C for 12 hours. Then, crush the dried ferric phosphate dihydrate with a pulverizer and place the material in a muffle furnace in a sagger and calcine it at 600°C for 4 hours to obtain anhydrous ferric phosphate.

[0047] Example 3

[0048] (1) Preparation of purified ferrous sulfate: Take ferrous sulfate, a byproduct of titanium dioxide industry, add water at 80℃ and stir to dissolve it so that the Fe in the solution is reduced.2+ With an ion concentration of 75 g / L, iron powder was added, and the mixture was stirred until the pH of the solution reached 3.3. Heating was then stopped, and polydimethyldiallyl ammonium chloride was added. After the precipitate flocculated completely, the solution was vacuum filtered to obtain a purified ferrous sulfate solution. The amount of iron powder used was 1% of the mass of ferrous sulfate, and the amount of polydimethyldiallyl ammonium chloride used was 0.2‰ of the mass of ferrous sulfate.

[0049] (2) Preparation of bifunctional impurity remover: Add polyferric chloride and sodium tripolyphosphate to deionized water, heat to 60°C and stir slowly to dissolve, then add styrene to obtain a bifunctional impurity remover with a network structure; wherein the mass ratio of polyferric chloride, sodium tripolyphosphate, deionized water and styrene is 0.9:1:2:0.05;

[0050] (3) Phosphorus source pretreatment: Take low-quality phosphorus source and vacuum filter to obtain filtrate. Place the filtrate in a 70℃ constant temperature water bath and add 30wt% urea solution while stirring at 300rpm to adjust the pH to 2.0. Then add bifunctional impurity remover. After stirring and reacting for 35min, a large amount of impurities precipitate out. Vacuum filter to obtain phosphorus source solution. The phosphorus mass fraction in the low-quality phosphorus source is 7%, and the Al content is 5000ppm. The amount of the bifunctional impurity remover is 1.5‰ of the mass of the low-quality phosphorus source.

[0051] (4) Preparation of ferric phosphate dihydrate: Take the purified ferrous sulfate solution prepared in step (1), keep the stirring speed at 400 rpm, add the phosphorus source solution prepared in step (3) while stirring, and then add 0.1 mol / L nitric acid at a time of 20 min. Continue the reaction for 10 min until the ferrous phosphate is completely oxidized. Then add 30 wt% urea solution at a time of 20 min to adjust the pH to 2.5. Continue the reaction for 60 min and then filter the filter cake. The molar ratio of ferrous ions to phosphorus is 1:0.95 and the molar ratio of nitric acid to ferrous ions is 0.6:1.

[0052] (5) Washing: Use a filter press to wash the filter cake obtained in step (4) until the conductivity of the wash water is less than 3.0 mS / cm, to obtain an amorphous iron phosphate filter cake;

[0053] (6) Crystallization: Add water to the amorphous ferric phosphate filter cake from step (5) until the solid content is 13%, stir thoroughly until the slurry is uniform, add 85wt% refined phosphoric acid to make the pH of the slurry reach 1.7, heat the slurry to keep the temperature at 90℃, filter after 80min, and use a filter press to wash the conductivity of the filter cake washing water to below 0.3 mS / cm to obtain a powdery white ferric phosphate filter cake dihydrate;

[0054] (7) Second washing: Place the ferric phosphate dihydrate filter cake in an oven and dry it at 90°C for 12 hours. Then, crush the dried ferric phosphate dihydrate with a pulverizer and place the material in a muffle furnace in a sagger and calcine it at 650°C for 5 hours to obtain anhydrous ferric phosphate.

[0055] Example 4

[0056] (1) Preparation of purified ferrous sulfate: Take ferrous sulfate, a byproduct of titanium dioxide industry, add water at 80℃ and stir to dissolve it so that the Fe in the solution is reduced. 2+ With an ion concentration of 75 g / L, iron powder was added, and the reaction was stirred until the pH of the solution reached 3.5. Heating was then stopped, and polyvinylpyridine salt was added. After the precipitation and flocculation were complete, the solution was vacuum filtered to obtain a purified ferrous sulfate solution. The amount of iron powder used was 2.5% of the mass of ferrous sulfate, and the amount of polyvinylpyridine salt used was 0.2‰ of the mass of ferrous sulfate.

[0057] (2) Preparation of bifunctional impurity remover: Add polyferric chloride and EDTA to deionized water, heat to 60°C and stir slowly to dissolve, then add styrene to obtain a bifunctional impurity remover with a network structure; wherein the mass ratio of polyferric chloride, EDTA, deionized water and styrene is 0.8:1:2:0.05;

[0058] (3) Phosphorus source pretreatment: Take low-quality phosphorus source and vacuum filter to obtain filtrate. Place the filtrate in a 70℃ constant temperature water bath and add 0.3mol / L KOH solution while stirring at 300rpm to adjust the pH to 3.0. Then add bifunctional impurity remover. After stirring and reacting for 40min, a large amount of impurities precipitate out. Vacuum filter to obtain phosphorus source solution. The phosphorus mass fraction in the low-quality phosphorus source is 7%, and the Al content is 8000ppm. The amount of the bifunctional impurity remover is 2.0‰ of the mass of the low-quality phosphorus source.

[0059] (4) Preparation of ferric phosphate dihydrate: Take the purified ferrous sulfate solution prepared in step (1), keep the stirring speed at 400 rpm, add the phosphorus source solution prepared in step (3) while stirring, and then add sodium persulfate at a concentration of 0.1 mol / L at a time of 50 min. Continue the reaction for 10 min, and the sodium persulfate will completely oxidize the ferrous ions. Then add KOH solution at a concentration of 0.1 mol / L at a time of 50 min to adjust the pH to 2.5. Continue the reaction for 60 min and then filter the filter cake. The molar ratio of ferrous ions to phosphorus is 1:1.1, and the molar ratio of sodium persulfate to ferrous ions is 0.7:1.

[0060] (5) Washing: Use a filter press to wash the filter cake obtained in step (4) until the conductivity of the wash water is less than 3.0 mS / cm, to obtain an amorphous iron phosphate filter cake;

[0061] (6) Crystallization: Add water to the amorphous ferric phosphate filter cake from step (5) until the solid content is 10%, stir thoroughly until the slurry is uniform, add 85wt% refined phosphoric acid to make the pH of the slurry reach 1.5, heat the slurry to keep the temperature at 90℃, filter after 100min, and use a filter press to wash the conductivity of the filter cake washing water to below 0.3 mS / cm to obtain a powdery white ferric phosphate dihydrate filter cake;

[0062] (7) Second washing: Place the ferric phosphate dihydrate filter cake in an oven and dry it at 90°C for 12 hours. Then, crush the dried ferric phosphate dihydrate with a pulverizer and place the material in a muffle furnace in a sagger and calcine it at 650°C for 3 hours to obtain anhydrous ferric phosphate.

[0063] Example 5

[0064] (1) Preparation of purified ferrous sulfate: Take ferrous sulfate, a byproduct of titanium dioxide industry, add water at 80℃ and stir to dissolve it so that the Fe in the solution is reduced. 2+ With an ion concentration of 60 g / L, iron powder was added, and the mixture was stirred until the pH of the solution reached 3.7. Heating was then stopped, and sodium carboxymethyl cellulose was added. After the precipitate flocculated completely, the solution was vacuum filtered to obtain a purified ferrous sulfate solution. The amount of iron powder used was 2.0% of the mass of ferrous sulfate, and the amount of sodium carboxymethyl cellulose used was 0.4‰ of the mass of ferrous sulfate.

[0065] (2) Preparation of bifunctional impurity remover: Add polyferric chloride and EDTA-2Na to deionized water, heat to 60°C and stir slowly to dissolve, then add styrene to obtain a bifunctional impurity remover with a network structure; wherein the mass ratio of polyferric chloride, EDTA-2Na, deionized water and styrene is 1:0.8:2:0.05;

[0066] (3) Phosphorus source pretreatment: Take low-quality phosphorus source and vacuum filter to obtain filtrate. Place the filtrate in a 70℃ constant temperature water bath and add 25wt% ammonia water while stirring at 500rpm to adjust the pH to 2.2. Then add a bifunctional impurity remover. After stirring and reacting for 60min, a large amount of impurities precipitate out. Vacuum filter to obtain phosphorus source solution. The phosphorus mass fraction in the low-quality phosphorus source is 7%, and the Al content is 9000ppm. The amount of the bifunctional impurity remover is 2.6‰ of the mass of the low-quality phosphorus source.

[0067] (4) Preparation of ferric phosphate dihydrate: Take the purified ferrous sulfate solution prepared in step (1), keep the stirring speed at 400 rpm, add the phosphorus source solution prepared in step (3) while stirring, and then add hydrogen peroxide with a content of 25 wt% over a dropping time of 60 min. Continue the reaction for 10 min, and the hydrogen peroxide will completely oxidize the ferrous phosphate. Then add ammonia solution with a content of 25 wt% over a dropping time of 60 min to adjust the pH to 2.5. Continue the reaction for 60 min and then filter the filter cake by pressure. The molar ratio of ferrous ions to phosphorus is 1:1.2, and the molar ratio of hydrogen peroxide to ferrous ions is 0.75:1.

[0068] (5) Washing: Use a filter press to wash the filter cake obtained in step (4) until the conductivity of the wash water is less than 3.0 mS / cm, to obtain an amorphous iron phosphate filter cake;

[0069] (6) Crystallization: Add water to the amorphous ferric phosphate filter cake from step (5) until the solid content is 13%, stir thoroughly until the slurry is uniform, add 85wt% refined phosphoric acid to make the pH of the slurry reach 1.6, heat the slurry to keep the temperature at 90℃, filter after 120min, and use a filter press to wash the conductivity of the filter cake washing water to below 0.3 mS / cm to obtain a powdery white ferric phosphate dihydrate filter cake;

[0070] (7) Second washing: Place the ferric phosphate dihydrate filter cake in an oven and dry it at 90°C for 12 hours. Then, crush the dried ferric phosphate dihydrate with a pulverizer and place the material in a muffle furnace in a sagger and calcine it at 650°C for 3 hours to obtain anhydrous ferric phosphate.

[0071] Comparative Example 1

[0072] The method and steps are the same as in Example 1, except that step (2) is omitted. The solution obtained after vacuum filtration of a low-quality phosphorus source is directly used as the phosphorus source solution to prepare anhydrous iron phosphate.

[0073] Comparative Example 2

[0074] The method and steps are the same as in Example 1, except that step (2) is omitted. The solution obtained after vacuum filtration of the low-quality phosphorus source is directly used as the phosphorus source solution to prepare anhydrous iron phosphate. The phosphorus mass fraction in the low-quality phosphorus source is 5%, and the content of Al impurities is 4000 ppm.

[0075] Results Detection: The anhydrous ferric phosphate prepared in the above examples and comparative examples was analyzed using manual titration, a tap density meter, and inductively coupled plasma atomic emission spectrometry to determine the content of various substances and the tap density. The results are shown in Table 1.

[0076] Table 1 Physicochemical properties of anhydrous ferric phosphate

[0077]

[0078] The physicochemical properties of anhydrous ferric phosphate in this embodiment are shown in Table 1. Its iron content (36.12%), phosphorus content (20.83%), and iron-to-phosphorus ratio (0.9639) are all within the standard range. Its tap density is 1.15 g / m³, which is significantly higher than the 0.60 g / m³ required by the HG / T4701-2004 standard for ferric phosphate used in batteries. 3 The requirements are met; moreover, in Example 1, when the Al impurity content of the low-quality phosphorus source is 7000 ppm, the Al impurity content of the anhydrous ferric phosphate prepared by the method of this invention is 30.2 ppm < 50 ppm, and the contents of other impurity elements meet the requirements of the mainstream market for the impurity content of anhydrous ferric phosphate.

[0079] The anhydrous ferric phosphate prepared in Example 1 was photographed using a field emission scanning electron microscope and analyzed by X-ray diffraction. The results were obtained from the SEM images (…). Figure 1 As can be seen from the XRD pattern, the anhydrous iron phosphate prepared in Example 1 has uniform primary particle size and relatively dense secondary agglomerates. This morphology of anhydrous iron phosphate has a high tap density and is suitable for high-compact lithium iron phosphate. Figure 2 As can be seen, the intensity and position of the characteristic peaks of the anhydrous iron phosphate prepared in Example 1 correspond one-to-one with the characteristic peaks of the standard anhydrous iron phosphate, with no obvious impurity peaks and no impurity peaks appearing, indicating that its purity is high; the standard anhydrous iron phosphate is derived from Figure 3-24 in Jiang Yan's research on the preparation of battery-grade iron phosphate from ferrous sulfate by-product of Taibao (Jiang Yang. Homogeneous precipitation preparation of battery-grade iron phosphate from ferrous sulfate by-product of titanium dioxide [D]. Central South University, 2022.).

Claims

1. A method for preparing battery-grade iron phosphate from a low-quality phosphorus source, characterized in that: Includes the following steps: (1) Preparation of purified ferrous sulfate: ferrous sulfate by-product of titanium dioxide is dissolved in water at high temperature to obtain ferrous sulfate solution, then iron powder is added, flocculant is added after the reaction is completed, and after mixing evenly, the purified ferrous sulfate solution is obtained by filtration. (2) Phosphorus source pretreatment: Filter the low-quality phosphorus source, add pH adjuster and bifunctional impurity remover to the filtrate; after reaction, filter to obtain phosphorus source solution; (3) Preparation of ferric phosphate dihydrate: Add the phosphorus source solution, oxidant and precipitant obtained in step (2) to the purified ferrous sulfate solution obtained in step (1). After the reaction, the solution is washed once, crystallized and washed twice to prepare ferric phosphate dihydrate. (4) Batteries-grade iron phosphate is obtained by drying and calcining iron phosphate dihydrate; The preparation method of the bifunctional impurity remover is to mix and dissolve the flocculant and complexing agent with water, add the crosslinking agent, and mix evenly. The flocculant is polyferric chloride or polyacrylamide; the complexing agent is any one of EDTA, EDTA-2Na, sodium pyrophosphate, sodium tripolyphosphate, and sodium citrate; and the crosslinking agent is styrene.

2. The method for preparing battery-grade iron phosphate from a low-quality phosphorus source according to claim 1, characterized in that: In step (1), the Fe in the ferrous sulfate solution 2+ The ion concentration is 60~80 g / L; the amount of iron powder used is 1%-3% of the mass of ferrous sulfate; the amount of flocculant used is 0.1‰-0.4‰ of the mass of ferrous sulfate.

3. The method for preparing battery-grade iron phosphate from a low-quality phosphorus source according to claim 2, characterized in that: The flocculant is any one of polyacrylamide, sodium polyacrylate, polydimethyldiallylammonium chloride, polyethyleneimine, polyvinylpyridine salt, and sodium carboxymethyl cellulose.

4. The method for preparing battery-grade iron phosphate from a low-quality phosphorus source according to claim 1, characterized in that: In step (2), the low-quality phosphorus source has a phosphorus mass fraction of 5-9% and an Al content of 3000-10000ppm; the pH adjuster is any one of NaOH solution, ammonia water, urea, and KOH solution.

5. The method for preparing battery-grade iron phosphate from a low-quality phosphorus source according to claim 1, characterized in that: The mass ratio of flocculant, complexing agent, water and crosslinking agent is 0.8-1:0.8-1:2:0.

05.

6. The method for preparing battery-grade iron phosphate from a low-quality phosphorus source according to claim 1, characterized in that: In step (3), the molar ratio of ferrous ions in the purified ferrous sulfate solution to phosphorus in the phosphorus source solution is 1:1.05~0.95; the molar ratio of oxidant to ferrous ions is 0.5~0.75:1; and the molar ratio of precipitant to phosphorus in the phosphorus source solution is 1.5~2.0:

1.

7. The method for preparing battery-grade iron phosphate from a low-quality phosphorus source according to claim 6, characterized in that: The oxidant is any one of hydrogen peroxide, oxygen, nitric acid, and sodium persulfate; the precipitant is any one of NaOH solution, ammonia, urea, and KOH solution.

8. The method for preparing battery-grade iron phosphate from a low-quality phosphorus source according to claim 1, characterized in that: The drying temperature in step (4) is 80-130℃ and the drying time is 10-15h; the calcination temperature is 550-750℃ and the calcination time is 3-5h.

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