Preparation method and application of agricultural iron phosphate

Abstract

The invention discloses a preparation method and application of agricultural iron phosphate. The preparation method comprises the following steps: S1, mixing a phosphorus source, an iron source, an oxidizing agent and a pH regulator to obtain first reaction slurry; the phosphorus source comprises phosphorus salt, and the phosphorus salt comprises agricultural monoammonium phosphate; s2, washing the first reaction slurry to obtain an amorphous iron phosphate filter cake; s3, pulping the amorphous iron phosphate filter cake, adding an acid agent for adjustment, and carrying out heating reaction to obtain iron phosphate slurry; and S4, filtering and washing the iron phosphate slurry, and calcining to obtain the iron phosphate. According to the method, the iron phosphate preparation process is divided into two steps of synthesis and washing, impurities brought in from agricultural grade phosphorus source components can be effectively removed by washing the amorphous iron phosphate slurry once or multiple times, and the method has the characteristics of small primary particles, large secondary particles, compact accumulation and regular polyhedron shape, and is suitable for preparing the lithium iron phosphate material with high electrical property.

Description

Technical Field

[0001] The invention relates to the technical field of battery material preparation, and in particular to a preparation method of agricultural iron phosphate and application thereof. Background Art

[0002] At present, the production process of iron phosphate mainly uses industrial-grade phosphate or phosphoric acid as the phosphorus source, and then combines iron salt or iron powder as the iron source to prepare iron phosphate. There are few processes that directly use agricultural-grade phosphate as the phosphorus source to prepare battery-grade iron phosphate. Industrial-grade phosphate or phosphoric acid has lower impurities, but higher costs. With the increase in the market share of lithium iron phosphate batteries, industrial-grade phosphorus sources are gradually experiencing market shortages. Therefore, the selection of phosphorus sources with lower grade and smaller price fluctuations has a very positive effect on the production of iron phosphate, but low grade means that more impurities are brought in, and more steps are required to remove impurities.

[0003] A method for preparing battery-grade iron orthophosphate has been proposed in existing research, which specifically discloses the use of agricultural-grade phosphorus sources to prepare battery-grade iron orthophosphate, using agricultural monoammonium phosphate or wet-process ammonium phosphate neutralization slurry as a phosphorus source, and sequentially preparing battery-grade iron orthophosphate through dissolution, impurity removal and synthesis steps, especially optimizing the impurity removal process, and removing a large amount of insoluble matter and water-soluble impurities in the phosphorus source by adjusting the pH with an alkaline neutralizer, filtering, defluorination and other steps. Although the method can obtain a purified phosphorus source that can be used to prepare iron phosphate, the impurity removal steps are cumbersome and its electrical properties are average. In another study, phosphoric acid and nitrite were added as the reaction base liquid after adjusting the pH with titanium dioxide ferrous sulfate solution; and phosphorus-containing compound fertilizers were dissolved and filtered to obtain a phosphate solution as a phosphorus source and added to the reaction base liquid, and then hydrogen peroxide was added to react to obtain iron phosphate slurry, washed, and calcined to obtain the iron phosphate. Although the impurity content of the iron phosphate synthesized by this method is low, the use of nitrite as the reaction base liquid has a greater impact on environmental protection and equipment.

[0004] Therefore, there is an urgent need to develop a method for preparing battery-grade iron phosphate with simple impurity removal steps and excellent electrical properties. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a method for preparing agricultural iron phosphate to obtain battery-grade anhydrous iron phosphate with low impurity content, which solves the problem of high impurity content in the direct use of agricultural-grade monoammonium phosphate to prepare iron phosphate materials, simplifies the impurity removal process of the phosphorus source, and further reduces the production cost.

[0006] According to a method for preparing iron phosphate according to a first aspect of the present invention, the method comprises the following steps:

[0007] S1: mixing a phosphorus source, an iron source, an oxidant and a pH adjuster to obtain a first reaction slurry;

[0008] wherein the phosphorus source comprises a phosphorus salt, and the phosphorus salt comprises agricultural monoammonium phosphate;

[0009] S2: washing the first reaction slurry to obtain an amorphous iron phosphate filter cake;

[0010] S3: slurrying the amorphous ferric phosphate filter cake, adding an acid agent for adjustment, and then heating the mixture to obtain ferric phosphate slurry;

[0011] S4: removing impurities from the iron phosphate slurry and then calcining it to obtain the iron phosphate.

[0012] According to some embodiments of the present invention, in step S1, the phosphorus source includes agricultural grade phosphorus salt.

[0013] The present invention obtains the iron phosphate material, which solves the impurity problem caused by the direct use of agricultural-grade phosphorus sources as raw materials and the application problem of battery-grade iron phosphate materials. After the phosphorus source is an agricultural-grade phosphorus source, the present invention does not need to perform additional treatment on the agricultural-grade phosphorus source except for filtering and removing slag once without adding related equipment. The amorphous iron phosphate reaction precursor is generated by controlling the reaction temperature and reaction pH of the first reaction slurry. After washing it in this process, the amount of overall impurities introduced can be significantly reduced. Then, by controlling the reaction pH, reaction temperature and reaction concentration of the second reaction, the performance of the iron phosphate can be significantly controlled; through the preparation method of the present application, a densely stacked, small primary particles, large secondary particles, regular polyhedral micromorphology and low impurity content dihydrate iron phosphate material can be finally prepared by an agricultural-grade phosphorus source. The iron phosphate material of the present application is suitable for preparing lithium iron phosphate materials with high electrical performance.

[0014] According to some embodiments of the present invention, in step S1, the phosphorus source is in solution state, and in the solution state, the solvent of the phosphorus source includes water; the mass ratio of the water to the phosphorus salt is 1 to 4:1, and the concentration of the phosphorus salt is 1.2 mol / L to 2.0 mol / L.

[0015] The mass ratio and concentration within the above range ensure that the agricultural-grade phosphorus source is completely dissolved and forms a uniform solution, preventing crystallization and precipitation.

[0016] According to some embodiments of the present invention, in step S1, in the solution state, the solvent of the phosphorus source includes water; the mass ratio of the water to the phosphorus salt is 1-4:1, and the concentration of the configured phosphorus salt is 1.6 mol / L.

[0017] According to some embodiments of the present invention, in step S1, the method for preparing the phosphorus source comprises: mixing water and the phosphorus source at 20° C. to 50° C. for 10 min to 60 min.

[0018] According to some embodiments of the present invention, in step S1, the method for preparing the phosphorus source comprises: mixing water and the phosphorus source at 40° C. for 45 minutes.

[0019] According to some embodiments of the present invention, the iron source comprises a ferrous iron salt;

[0020] The divalent iron salt includes at least one of ferrous sulfate, ferrous chloride and ferrous acetate.

[0021] According to some embodiments of the present invention, in step S1, the iron source is obtained by dissolving and filtering ferrous sulfate.

[0022] According to some embodiments of the present invention, in a solution state, the concentration of the iron salt is 1.2 mol / L to 2.0 mol / L.

[0023] According to some embodiments of the present invention, in a solution state, the concentration of the iron salt is 1.6 mol / L.

[0024] According to some embodiments of the present invention, the oxidant comprises hydrogen peroxide.

[0025] According to some embodiments of the present invention, the pH adjuster includes at least one of concentrated ammonia solution, sodium hydroxide and ammonium bicarbonate.

[0026] According to some embodiments of the present invention, the phosphorus source, the iron source and the oxidant are in a molar ratio of 0.9-1.1:1:0.5-1.

[0027] By adjusting the molar ratio, the reaction rate and selectivity can be controlled to avoid side reactions or excessive oxidation, which may affect the quality of the final product; at the same time, it is possible to avoid incomplete reactions due to too low a molar ratio, which may affect the purity and performance of the product.

[0028] According to some embodiments of the present invention, in step S1, the pH after mixing is 2.0-3.0.

[0029] The above pH range helps to control the precipitation process of iron phosphate, avoiding the generation of other products and the introduction of more alkaline metal precipitation due to too high pH, ​​while avoiding the inability to completely precipitate the iron phosphate salt due to too low pH.

[0030] According to some embodiments of the present invention, in step S1, the pH after mixing is 2.2.

[0031] According to some embodiments of the present invention, in step S2, the washing method includes at least one of membrane washing and filter press washing.

[0032] According to some embodiments of the present invention, in step S2, the washing conductivity during the washing process is ≤3000 μS / cm.

[0033] According to some embodiments of the present invention, step S3 further includes removing impurities.

[0034] According to some embodiments of the present invention, in step S3, the impurity removal step includes mixing the ferric phosphate slurry and the pH adjuster to obtain a second reaction slurry and then repeating the operation of step S2.

[0035] According to some embodiments of the present invention, in step S3, the pH value of the adjusted system is 0.6-1.5.

[0036] The above pH conditions ensure that amorphous iron phosphate is provided with a crystallization environment.

[0037] According to some embodiments of the present invention, the acid agent includes at least one of sulfuric acid, phosphoric acid, acetic acid, monoammonium phosphate and ammonium sulfate.

[0038] According to some embodiments of the present invention, in step S3, the temperature of the temperature-raising reaction is 85° C. to 95° C.; and / or the concentration of metal ions in the slurry is 0.5 mol / L to 1.5 mol / L.

[0039] Within this concentration range, the quality and properties of the iron phosphate precipitation can be effectively controlled to avoid agglomeration or uneven precipitation due to excessive concentration, which would affect subsequent processing steps; moderate ion concentration helps optimize the reaction rate and prevents excessively fast reaction rates due to excessive concentrations, which may lead to incomplete reactions or the formation of by-products; it also helps control the purity of the generated iron phosphate, reduce the mixing of impurities, and ensure that the final product meets battery-grade standards.

[0040] According to some embodiments of the present invention, in step S3, the temperature of the temperature-raising reaction is 85° C. to 95° C.; and / or the concentration of metal ions in the slurry is 0.8 mol / L.

[0041] According to some embodiments of the present invention, in step S3, the beating time is 30 min to 60 min.

[0042] According to some embodiments of the present invention, in step S3, the insulation time is 30 min to 180 min.

[0043] According to some embodiments of the present invention, in step S4, the impurity removal step includes filtering and washing.

[0044] According to some embodiments of the present invention, in step S4, the washing conductivity during the washing process is ≤1000 us / cm.

[0045] According to some embodiments of the present invention, in step S4, the calcination temperature is 500°C to 600°C.

[0046] According to some embodiments of the present invention, in step S4, the calcination time is 2 to 6 hours.

[0047] According to some embodiments of the present invention, in step S4, the calcination time is 4 hours.

[0048] The present invention divides the iron phosphate preparation process into two steps of synthesis and washing. By washing the amorphous iron phosphate slurry once or multiple times, the impurities introduced from the agricultural-grade phosphorus source component can be effectively removed, and then an acid agent is added for synthesis reaction. It can be seen from the physical and chemical indicators that the obtained iron phosphate has a low impurity content and meets the battery-grade iron phosphate standard. It can be seen from the electron microscope morphology that the obtained iron phosphate has a primary particle size of only between 40nm and 60nm. Its primary particles are small, which is conducive to more uniform carbon coating of lithium iron phosphate obtained when it is used as a precursor to prepare lithium iron phosphate, and higher conductivity, thereby improving its rate performance and discharge capacity; its secondary particles are large, densely stacked, and present regular polyhedrons. Its agglomerate D50 is between 6 and 8um, which is conducive to obtaining a particle size grading effect when preparing lithium iron phosphate and improving the compaction of lithium iron phosphate. It is suitable for preparing lithium iron phosphate materials with high electrical performance.

[0049] The embodiment of the second aspect of the present invention proposes the application of the above preparation method in preparing battery positive electrode materials.

[0050] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0052] Figure 1 A schematic flow chart of a method for preparing battery-grade iron phosphate from agricultural-grade monoammonium phosphate provided in an embodiment of the present invention;

[0053] Figure 2 This is an XRD diagram of iron phosphate provided in Example 1 of the present invention.

[0054] Figure 3 This is a SEM image of a ferric phosphate provided in Example 1 of the present invention under a 50,000x electron microscope;

[0055] Figure 4 This is a SEM image of a ferric phosphate provided in Example 2 of the present invention under a 50,000x electron microscope;

[0056] Figure 5 This is a SEM image of a ferric phosphate provided in Example 3 of the present invention under a 50,000x electron microscope;

[0057] Figure 6 This is a SEM image of a ferric phosphate provided in Example 4 of the present invention under a 50,000x electron microscope;

[0058] Figure 7 This is a SEM image of a ferric phosphate provided in Comparative Example 1 of the present invention under a 50,000x electron microscope;

[0059] Figure 8 This is a SEM image of the iron phosphate provided in Comparative Example 2 of the present invention under a 50,000-fold electron microscope. DETAILED DESCRIPTION

[0060] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0061] Example 1

[0062] This embodiment discloses a method for preparing iron phosphate, which comprises the following steps:

[0063] S1: Weigh agricultural grade monoammonium phosphate (nutrient ≥ 55) (10% N-45% P2O5-0% K2O) and deionized water in a mass ratio of 1:3, heat to 40°C, keep warm and stir for 45 minutes, and then filter. After measuring the phosphorus content of the obtained phosphate filtrate, add water to obtain a phosphate solution (phosphate concentration is 1.6 mol / L), and then weigh the phosphate solution, ferrous sulfate solution (iron salt concentration is 1.6 mol / L), hydrogen peroxide (wt% = 27.5%), and ammonia water (wt% = 21%) in a molar ratio of 1:1:0.6:1. After adding the above solutions into the reactor under co-current conditions, adjust the pH to 2.2, and age for 30 minutes to obtain the first reaction slurry;

[0064] S2: The first reaction slurry is washed by a filter press, and the washing conductivity during the washing process is ≤5000us / cm; the obtained iron phosphate filter cake is added with an appropriate amount of deionized water for pulping, and sulfuric acid (wt%=98%) is added at a molar ratio of 1:0.45 for pulping to obtain an iron-phosphorus-sulfur mixed slurry with a pH of about 0.6; ammonia water (wt%=21%) is added to adjust the pH to 2.3, and the second reaction slurry is obtained by aging for 30 minutes, and the second reaction slurry is washed by a filter press, and the washing conductivity during the washing process is ≤3000us / cm to obtain the amorphous iron phosphate filter cake;

[0065] S3: Weigh amorphous iron phosphate and phosphoric acid (wt%=85%) in a molar ratio of 1:0.2 respectively, add appropriate amounts of deionized water and phosphoric acid to the amorphous iron phosphate filter cake for slurrying to adjust the slurry pH to between 1.2 and 1.4, heat to 90° C. and keep warm for 120 min to obtain the iron phosphate slurry, wherein the concentration of metal ions is 0.8 mol / L;

[0066] S4: After washing the iron phosphate slurry with deionized water, the slurry is placed in a muffle furnace and calcined at 550° C. for 4 hours to obtain the iron phosphate powder.

[0067] The schematic flow diagram of the method for obtaining iron phosphate in Example 1 is as follows Figure 1 As shown, the XRD pattern and electron microscope picture are as follows Figure 2 , Figure 3 As shown, Figure 2 This is an XRD diagram of iron phosphate provided in Example 1. Figure 3 This is a SEM image of the iron phosphate provided in Example 1 of this specification under a 50,000x electron microscope.

[0068] from Figure 2 It can be seen that compared with the characteristic diffraction peak in the standard iron phosphate card (77-0094), the diffraction peak shape and peak position of the iron phosphate prepared in Example 1 of the present invention are completely consistent with it and there is no impurity peak. At the same time, the diffraction peak of the iron phosphate prepared in the embodiment of the present invention is sharp and has a narrow half-height width, which shows that the iron phosphate prepared in Example 1 of the present invention is a pure phase and anhydrous iron phosphate phase with good crystallinity. Figure 2 It can be seen that in Example 1 of the present invention, the anhydrous iron phosphate primary particles obtained by respectively controlling its oxidation process and adding an acid agent to control its crystallization environment are spherical, and the spherical primary particles are orderly stacked to form secondary particles with a particle size distribution, which is suitable for preparing lithium iron phosphate with high electrical performance.

[0069] Example 2

[0070] This embodiment discloses a method for preparing iron phosphate, which comprises the following steps:

[0071] S1: Weigh agricultural grade monoammonium phosphate with nutrient ≥60 (10% N-50% P2O5--0% K2O) and deionized water in a mass ratio of 1:3, heat to 40°C and keep warm and stir for 45 minutes, filter the obtained phosphate filtrate, measure phosphorus and add water to obtain a phosphate solution (phosphate concentration is 1.6 mol / L), then weigh the phosphate solution, ferrous sulfate solution (iron salt concentration is 1.6 mol / L), hydrogen peroxide (wt%=27.5%), and ammonia water (wt%=21%) in a molar ratio of 1:1:0.6:1, add the above solutions into the reactor under co-current conditions, adjust the pH to 2.2, and age for 30 minutes to obtain a reaction slurry;

[0072] S2: washing the reaction slurry with a filter press, wherein the washing conductivity during the washing process is ≤3000 us / cm, to obtain the amorphous iron phosphate filter cake;

[0073] S3: Weigh amorphous iron phosphate and phosphoric acid (wt%=85%) in a molar ratio of 1:0.2 respectively, add appropriate amount of deionized water and sulfuric acid to the amorphous iron phosphate filter cake for slurrying to adjust the pH of the slurry to between 1.2 and 1.4, heat to 90° C. and keep warm for 120 min to obtain the iron phosphate slurry, wherein the concentration of metal ions is 0.8 mol / L;

[0074] S4: After washing the iron phosphate slurry with deionized water, the slurry is placed in a muffle furnace and calcined at 550° C. for 4 hours to obtain the iron phosphate powder.

[0075] Figure 4 This is a SEM image of the iron phosphate provided in Example 2 under a 50,000x electron microscope.

[0076] Example 3

[0077] This embodiment discloses a method for preparing iron phosphate, which comprises the following steps:

[0078] S1: Weigh agricultural grade monoammonium phosphate with nutrient ≥64 (11% N-53% P2O5--0% K2O) and deionized water in a mass ratio of 1:3, heat to 40°C, keep warm and stir for 45 minutes, and then filter to obtain a phosphate solution, wherein the phosphate concentration is 1.6 mol / L, and then weigh the phosphate solution, ferrous sulfate solution (iron salt concentration is 1.6 mol / L), hydrogen peroxide (wt%=27.5%), and ammonia water (wt%=21%) in a molar ratio of 1:1:0.6:1, add the above solutions into the reactor under co-current conditions, adjust the pH to 2.2, and age for 30 minutes to obtain a reaction slurry;

[0079] S2: washing the reaction slurry with a filter press, wherein the washing conductivity during the washing process is ≤3000 us / cm, to obtain the amorphous iron phosphate filter cake;

[0080] S3: Weigh amorphous ferric phosphate, sulfuric acid (wt%=98%) and monoammonium phosphate (wt%=85%) in a molar ratio of 1:0.16:0.16, respectively, add an appropriate amount of deionized water to prepare an acid agent, and then add the phosphoric acid and sulfuric acid to the amorphous ferric phosphate filter cake for slurrying to adjust the slurry pH to between 0.9 and 1.1, heat to 90° C. and keep warm for 120 min, thereby obtaining the ferric phosphate slurry, wherein the concentration of metal ions is 0.8 mol / L;

[0081] S4: After washing the iron phosphate slurry with deionized water, the slurry is placed in a muffle furnace and calcined at 550° C. for 4 hours to obtain the iron phosphate powder.

[0082] Figure 5 This is a SEM image of the iron phosphate provided in Example 3 under a 50,000x electron microscope.

[0083] Example 4

[0084] This embodiment discloses a method for preparing iron phosphate, which comprises the following steps:

[0085] S1: Weigh agricultural grade monoammonium phosphate with nutrient ≥67 (12% N-55% P2O5--0% K2O) and deionized water in a mass ratio of 1:3, heat to 40°C, keep warm and stir for 45 minutes, and then filter to obtain a phosphate solution, wherein the phosphate concentration is 1.6 mol / L, and then weigh the phosphate solution, ferrous sulfate solution (iron salt concentration is 1.6 mol / L), hydrogen peroxide (wt%=27.5%), and ammonia water (wt%=21%) in a molar ratio of 1:1:0.6:1, add the above solutions into the reactor under co-current conditions, adjust the pH to 2.2, and age for 30 minutes to obtain a reaction slurry;

[0086] S2: washing the reaction slurry with a filter press, wherein the washing conductivity during the washing process is ≤3000 us / cm, to obtain the amorphous iron phosphate filter cake;

[0087] S3: Weigh amorphous iron phosphate and phosphoric acid at a molar ratio of 1:0.2 respectively, add appropriate amounts of deionized water and phosphoric acid to the amorphous iron phosphate filter cake for slurrying to adjust the pH of the slurry to between 1.2 and 1.4, heat to 90° C. and keep warm for 120 min to obtain the iron phosphate slurry, wherein the concentration of metal ions is 1.0 mol / L;

[0088] S4: After washing the iron phosphate slurry with deionized water, the slurry is placed in a muffle furnace and calcined at 550° C. for 4 hours to obtain the iron phosphate powder.

[0089] Figure 6 This is a SEM image of the iron phosphate provided in Example 4 under a 50,000x electron microscope.

[0090] Comparative Example 1

[0091] Comparative Example 1: The phosphorus source is industrial grade monoammonium phosphate to prepare iron phosphate, specifically:

[0092] S1: weigh industrial-grade monoammonium phosphate and add an appropriate amount of deionized water to obtain a phosphate solution, weigh the phosphate solution, ferrous sulfate, hydrogen peroxide, and ammonia water in a molar ratio of 1:1:0.6:1, add the above solutions into the reactor under co-current conditions, adjust the pH to 2.2, and age for a certain period of time to obtain the first reaction slurry;

[0093] S2: washing the reaction slurry to obtain the amorphous iron phosphate filter cake;

[0094] S3: Weigh the above-mentioned amorphous iron phosphate and phosphoric acid in a molar ratio of 1:0.2 respectively, add appropriate amounts of deionized water and phosphoric acid to the above-mentioned amorphous iron phosphate filter cake for slurrying to adjust the slurry pH to between 1.2 and 1.4, heat it to 90° C. and keep it warm for 120 minutes to obtain the iron phosphate slurry, and the concentration of metal ions is 0.8 mol / L;

[0095] S4: After washing the iron phosphate slurry with deionized water, the slurry is placed in a muffle furnace and calcined at 550° C. for 4 hours to obtain the iron phosphate powder.

[0096] Figure 7 This is a SEM image of the iron phosphate provided in Example 5 under a 50,000x electron microscope.

[0097] Comparative Example 2

[0098] Comparative Example 2 does not wash the amorphous ferric phosphate to prepare ferric phosphate, specifically:

[0099] S1: Weigh nutrient ≥64 (11% N-53% P2O5--0% K2O) agricultural grade monoammonium phosphate and deionized water in a mass ratio of 1:3, heat to 40℃, keep warm and stir for a certain period of time, and filter to obtain a phosphate solution. Weigh phosphate solution, ferrous sulfate, hydrogen peroxide, and phosphoric acid in a molar ratio of 1:1:0.6:0.15, feed the above solutions into the reactor for a certain period of time, mix, heat to 90℃, and continue the reaction for 3 hours to obtain iron phosphate slurry;

[0100] S2: The iron phosphate slurry is filtered and washed with pure water, and calcined at 550° C. for 4 h to obtain the iron phosphate powder.

[0101] Figure 8 This is a SEM image of the iron phosphate provided in Comparative Example 1 under a 50,000x electron microscope.

[0102] Test Example 1

[0103] In this test example, the iron phosphate prepared in the above-mentioned embodiment and comparative example was tested for performance, and the corresponding properties were obtained as shown in Table 1 below.

[0104] Table 1 Index detection of each embodiment and comparative example

[0105]

[0106] It can be seen from Table 1 that Examples 1 to 4 use industrial-grade monoammonium phosphate to prepare iron phosphate compared to Comparative Example 1, wherein the metal impurity content is substantially the same, proving that the scheme can be used to prepare iron phosphate with a metal impurity content similar to that of industrial-grade monoammonium phosphate using agricultural-grade monoammonium phosphate. In Examples 1 and 2, relative to Comparative Example 2, the amorphous iron phosphate is washed to prepare iron phosphate, and its impurity content is significantly lower, proving that the problem of high impurity content in the raw material can be solved by washing the amorphous iron phosphate once or multiple times. This shows that based on the scheme of the present application, agricultural-grade monoammonium phosphate can be used to produce battery-grade anhydrous iron phosphate with low impurity content, and the battery-grade iron phosphate meets the industry standard for battery-grade iron phosphate, solves the problem of high impurity content in the preparation of iron phosphate materials using agricultural-grade monoammonium phosphate directly, simplifies the impurity removal process of the phosphorus source, and further reduces production costs.

[0107] Furthermore, in order to verify the performance of lithium iron phosphate prepared from the iron phosphate of the present application, lithium source, carbon source, deionized water and additives in a mass ratio of 1:0.25:0.08:2.5:0.008 were added to the iron phosphate obtained in Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2, respectively, and sand milled, wherein the lithium source was lithium carbonate, the carbon source was glucose, and the additive was titanium dioxide; after grinding to a certain particle size, spray drying was performed, and finally calcined at 750°C in a protective gas to obtain lithium iron phosphate.

[0108] The electrical properties of the iron phosphate prepared by Example 1 and Comparative Example 1 were tested, and the electrical performance data obtained are shown in Table 2 below.

[0109] Table 2. Electrical performance test of lithium iron phosphate

[0110]

[0111] As can be seen from the above table, based on the present application scheme, the battery-grade anhydrous iron phosphate obtained by agricultural-grade monoammonium phosphate is used as a precursor to prepare lithium iron phosphate. Compared with the iron phosphate obtained by the conventional method, its compaction density and discharge performance are also improved. When phosphoric acid is used as the acid agent, the compaction density and electrical properties are better when the reaction concentration is 0.8 mol / L. This shows that the preparation method provided by the present application scheme can effectively prepare battery-grade iron phosphate with good discharge performance.

[0112] The above is a description of a specific embodiment of the specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recorded in the claims can be performed in an order different from that in the embodiments and still achieve the desired results. In addition, the processes depicted in the drawings do not necessarily require the specific order or continuous order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

Claims

1. A method for preparing iron phosphate from an agricultural phosphorus source, characterized in that: The preparation method comprises the following steps: S1: mixing a phosphorus source, an iron source, an oxidant and a pH adjuster to obtain a first reaction slurry; wherein the phosphorus source comprises a phosphorus salt, and the phosphorus salt comprises agricultural monoammonium phosphate; S2: washing the first reaction slurry to obtain an amorphous iron phosphate filter cake; S3: slurrying the amorphous ferric phosphate filter cake, adding an acid agent for adjustment, and then heating the mixture to obtain ferric phosphate slurry; S4: removing impurities from the iron phosphate slurry and then calcining it to obtain the iron phosphate.

2. The method according to claim 1, characterized in that In step S1, the phosphorus source is in a solution state, and in the solution state, the solvent of the phosphorus source includes water; the mass ratio of the water to the phosphorus salt is 1 to 4:1, and the concentration of the phosphorus salt is 1.2 mol / L to 2.0 mol / L.

3. The method according to claim 1, characterized in that The iron source includes a ferrous iron salt; The divalent iron salt includes at least one of ferrous sulfate, ferrous chloride and ferrous acetate.

4. The preparation method according to claim 1, characterized in that: The oxidant includes hydrogen peroxide.

5. The method according to claim 1, 3 or 4, characterized in that: In terms of molar ratio, the phosphorus source, the iron source and the oxidant are 0.9-1.1:1:0.5-1.

6. The method according to claim 1, characterized in that In step S1, the pH of the first reaction slurry is 2.0-3.

0.

7. The method according to claim 1, characterized in that In step S3, the pH value of the system after adding the acid agent is adjusted to 0.6-1.

5.

8. The method according to claim 1, characterized in that In step S3, the pH of the second reaction slurry is 2.0-3.

0.

9. The method according to claim 1, characterized in that: The acid agent includes at least one of sulfuric acid, phosphoric acid and acetic acid.

10. The method according to claim 1, characterized in that In step S3, the temperature of the temperature-raising reaction is 85° C. to 95° C.; and / or the concentration of metal ions in the slurry is 0.5 mol / L to 1.5 mol / L.

11. Use of the method according to any one of claims 1 to 9 in preparing a positive electrode material for a battery.

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