Preparation method and application of battery-grade iron phosphate

Abstract

The invention provides a preparation method and application of battery-grade iron phosphate, and relates to the field of lithium ion batteries. Specifically, agricultural-grade ammonium, white carbon black and sodium sulfate are fully mixed in a liquid phase, a first reaction is carried out, and then separation is carried out to obtain a first liquid phase; adding oxalic acid, carrying out a second reaction, and separating to obtain a second liquid phase; adjusting the pH of the second liquid phase through an alkali source, and separating to obtain a third liquid phase; introducing an iron source and an oxidizing agent into the third liquid phase, carrying out a third reaction, and separating to obtain a first solid phase; pulping the first solid phase, adjusting the pH value of the pulping liquid through phosphoric acid, heating and aging, and carrying out solid-liquid separation to obtain a second solid phase; and calcining the second solid phase to obtain the battery-grade iron phosphate. According to the method, agricultural-grade ammonium is used as a phosphorus source, purification is performed through a specific route, the iron phosphate is prepared, the preparation route of the iron phosphate serving as a lithium iron phosphate synthesis raw material is effectively expanded, and the prepared iron phosphate is low in impurity content and high in iron-phosphorus ratio and has good cost performance and application prospects.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and in particular to a preparation method and application of battery-grade iron phosphate. Background Art

[0002] As the core and foundation of new energy batteries, new energy battery materials have ushered in unprecedented development opportunities. Lithium iron phosphate has the advantages of high safety and long life, and has broad market application prospects. Its application fields include but are not limited to power batteries, energy storage batteries, small appliances, etc. In the preparation process of lithium iron phosphate, phosphate railway line is expected to become the mainstream process for preparing lithium iron phosphate with lower cost and better cost performance.

[0003] Furthermore, the production routes of iron phosphate generally include the ammonium method, the sodium method and the iron powder method, etc.; among them, the ammonium method mainly generates iron phosphate by reacting ferrous sulfate and monoammonium phosphate, the sodium method generates dihydrogen phosphate by reacting ferrous sulfate and hydrogen peroxide after phosphoric acidification, and then adjusts the pH with sodium hydroxide to obtain iron phosphate, and the iron powder method prepares iron phosphate by phosphoric acid acidification of iron powder raw materials and then filtering, washing, flash evaporation and other processes. In the above routes, the phosphorus source is one of the direct production raw materials of iron phosphate, and its supply problem is of paramount importance. Agricultural-grade ammonium (also known as agricultural-grade diammonium phosphate) is cheap and has low solid transportation costs. If agricultural-grade ammonium is used as a raw material to prepare battery-grade iron phosphate, it will be beneficial to the further promotion of lithium iron phosphate batteries.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] The first purpose of the present invention is to provide a method for preparing battery-grade iron phosphate. By using agricultural-grade ammonium as the initial raw material for impurity removal and purification and as a phosphorus source, the preparation process route of iron phosphate for lithium iron phosphate synthesis raw materials is effectively expanded, and battery-grade cost-effective iron phosphate is effectively prepared at a lower cost.

[0006] The second object of the present invention is to provide an application of the iron phosphate prepared by the method for preparing battery-grade iron phosphate in lithium-ion batteries.

[0007] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0008] A method for preparing battery-grade iron phosphate comprises the following steps:

[0009] (1) fully mixing agricultural grade ammonium, white carbon black and sodium sulfate in a liquid phase, performing a first reaction, and performing solid-liquid separation to obtain a first liquid phase; adding oxalic acid to the first liquid phase, performing a second reaction, and performing solid-liquid separation to obtain a second liquid phase; adjusting the pH of the second liquid phase to 7.0±0.5 by using an alkali source, and performing solid-liquid separation to obtain a third liquid phase;

[0010] (2) introducing an iron source and an oxidant into the third liquid phase, performing a third reaction, and then performing solid-liquid separation to obtain a first solid phase; slurrying the first solid phase, adjusting the pH of the slurry to 1.65±0.5 with phosphoric acid, and then heating and aging, performing solid-liquid separation to obtain a second solid phase; and calcining the second solid phase to obtain battery-grade iron phosphate.

[0011] The iron phosphate prepared by the method for preparing battery-grade iron phosphate is used in lithium-ion batteries.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] The present invention provides a preparation method and application of battery-grade iron phosphate, which uses agricultural-grade ammonium (monoammonium phosphate) as an initial raw material. It is not only inexpensive and has low solid transportation costs, but also can remove impurities through a simple chemical reaction, thereby synthesizing battery-grade iron phosphate with low impurities and a high iron-phosphorus ratio.

[0014] In the purification and impurity removal method designed in the present invention, phosphate salts with low impurity content can be obtained. Compared with the traditional iron phosphate synthesis process, the consumption of high-cost phosphoric acid and a large amount of alkali products can be avoided, and the cost of raw materials is effectively reduced. At the same time, the prepared iron phosphate has high content and low impurities, which is conducive to the subsequent preparation of the lithium iron phosphate to exert the cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0016] Figure 1 A process flow chart of Example 1 of the present invention is provided. DETAILED DESCRIPTION

[0017] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but those skilled in the art will appreciate that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer is not specified in the reagents or instruments used, they are all conventional products that can be purchased commercially. In addition, the terms "first", "second" and "third" are only used for descriptive purposes and cannot be interpreted as indicating or implying relative importance.

[0018] A first aspect of the present invention is to provide a method for preparing battery-grade iron phosphate.

[0019] The preparation method of the present invention mainly comprises the following two steps; wherein step (1) separates and purifies agricultural-grade ammonium to obtain a phosphorus source for synthesizing iron phosphate, and step (2) generates iron phosphate by using a special phosphorus source and an iron source; thereby, a battery-grade iron phosphate with a high iron-to-phosphorus ratio is prepared based on low raw material costs, which has the advantages of lower cost performance and low transportation cost, and the preparation process is simple and controllable, which is conducive to the further promotion of lithium iron phosphate batteries.

[0020] (1) Agricultural-grade ammonium, white carbon black and sodium sulfate are fully mixed in a liquid phase, and after a first reaction, a solid-liquid separation is performed to obtain a first liquid phase; oxalic acid is added to the first liquid phase, and after a second reaction, a solid-liquid separation is performed to obtain a second liquid phase; the pH of the second liquid phase is adjusted to 7.0±0.5 by an alkali source, and a solid-liquid separation is performed to obtain a third liquid phase.

[0021] As a preferred embodiment, the active ingredient of the agricultural-grade ammonium includes monoammonium phosphate (i.e., diammonium phosphate), and the content of the monoammonium phosphate is ≤65%. However, the preparation process of agricultural-grade monoammonium phosphate is usually to acid-lyze the phosphate ore after crushing it, or to react the phosphate ore with a reducing agent such as coke or natural gas at high temperature, and then neutralize it with ammonia to obtain a crude monoammonium phosphate. The present invention can effectively reduce the cost price by using agricultural-grade ammonium as a raw material, but it is also obvious that agricultural-grade ammonium will introduce a large amount of impurities.

[0022] As a more preferred embodiment, the agricultural-grade ammonium used in the present invention includes at least fluorine, silicon, aluminum, calcium and manganese impurity elements; as a reference value, the agricultural-grade ammonium products obtained by direct sale usually include the following impurity elements: fluorine 5000pm~8000ppm, silicon 800ppm~1800ppm, aluminum 10000ppm~16000ppm, calcium 1000ppm~5000ppm, manganese 100ppm~500ppm. However, it is worth noting that the types of the above impurity elements only list the elements with typical high content, and the content values ​​of the above impurity elements are only for reference, and have strong floating properties according to the phosphate rock source and treatment method used by the agricultural-grade ammonium.

[0023] As a preferred embodiment, the solvent of the liquid phase includes water, that is, the initial raw material is dissolved in water; in some optional embodiments, tap water can be used as the solvent; common soluble ions in tap water include calcium ions, magnesium ions, sodium ions, chloride ions, etc., and the above soluble ion components are either filtered out in step (1) or will not be mixed in the precipitation of the iron phosphate precursor in step (2); at the same time, when tap water is used as the solvent, the cost and feasibility of the preparation method can be further reduced.

[0024] As a preferred embodiment, the agricultural grade ammonium is first dissolved in water, and then the white carbon black and the sodium sulfate are added to prepare the liquid phase, followed by thorough mixing; during the mixing process, auxiliary methods such as oscillation, stirring, centrifugation, ultrasound, and heat treatment may be optionally used, which has a good homogenous and promoting effect on the mixing and the first reaction.

[0025] As a more preferred embodiment, the mass ratio of the agricultural-grade ammonium to the solvent in the liquid phase is (2-3):1, including but not limited to 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, and 3:1, a numerical range consisting of any one or any two of them.

[0026] As a preferred embodiment, the mass ratio of the white carbon black to the fluorine in the agricultural grade ammonium is 1 to 1.5, and the mass ratio of the sodium sulfate to the fluorine in the agricultural grade ammonium is 1 to 1.5.

[0027] As a preferred embodiment, the temperature of the first reaction is 40°C to 60°C; in some optional embodiments, the first reaction is carried out under stirring; the present invention does not limit the time of the first reaction, and those skilled in the art can judge and decide whether to terminate the reaction based on the reaction state, such as no longer continuing to generate precipitates.

[0028] As a preferred embodiment, any of the solid-liquid separation methods involved in the present invention includes but is not limited to decantation, filtration, centrifugation, filter or membrane separation, etc.; in a more preferred embodiment, the solid-liquid separation in this step is carried out by suction filtration in a low-yield production mode or a laboratory scenario, and in a large-scale batch production mode, solid-liquid separation can be carried out by a filter.

[0029] In the present invention, white carbon black and sodium sulfate are used to complex with fluoride ions in the liquid phase to generate sodium fluorosilicate complex precipitate and filter out. In the present invention, white carbon black can be added in a slightly excessive amount because white carbon black is insoluble in water, and unreacted white carbon black can also be filtered out without affecting the purity of the subsequent battery-grade iron phosphate product. Similarly, when sodium sulfate is in excess, although it will not be removed in step (1), it will not affect the purity of the first solid phase obtained by solid-liquid separation in step (2).

[0030] As a preferred embodiment, the temperature of the second reaction is 40°C to 60°C; in some optional embodiments, the second reaction is carried out under stirring; the present invention does not limit the time of the second reaction, and those skilled in the art can judge and decide whether to terminate the reaction based on the reaction state, such as no longer continuing to generate precipitates.

[0031] As a preferred embodiment, the mass ratio of the oxalic acid to the calcium in the agricultural-grade ammonium is 0.7 to 1.2, including but not limited to a numerical range consisting of any one or any two of 0.7, 0.8, 0.9, 1.0, 1.1, and 1.2.

[0032] As a more preferred embodiment, the property characteristics of the second liquid phase can be used to determine whether the amount of oxalic acid added is excessive or insufficient; the pH of the second liquid phase is 3.8-4.2. When the pH of the second liquid phase is higher than the above-mentioned limit value, it is necessary to supplement oxalic acid in an appropriate amount; and when the pH of the second liquid phase is lower than the above-mentioned limit value, if there is an excess of oxalic acid, more of the first liquid phase should be adaptively introduced to avoid the formation of iron oxalate precipitate in the subsequent step (2) that affects the purity of iron phosphate.

[0033] As a preferred embodiment, the phosphorus content in the second liquid phase is 12 wt.% to 15 wt.%.

[0034] In the present invention, calcium oxalate precipitate is formed by the reaction of oxalic acid (H2C2O4) and calcium ions; at the same time, a small amount of impurity ions such as magnesium ions present in the solution will also form precipitates and be filtered out after the second reaction.

[0035] As a preferred embodiment, the alkali source includes at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium carbonate, potassium bicarbonate, potassium hydroxide or ammonia water.

[0036] As a preferred embodiment, the pH of the second liquid phase is first adjusted to 5.8-6.4, solid-liquid separation is performed, and then the pH of the second liquid phase is adjusted to 7.0±0.5, solid-liquid separation is performed to obtain a third liquid phase; thereby, the aluminum ion impurities with a low content in the liquid phase can be further effectively filtered out.

[0037] In the present invention, a phosphorus source raw material is obtained by separation and purification; it is understandable that soluble impurities such as sodium ions, sulfate ions, chloride ions, etc. are still present in the obtained third liquid phase, but these impurity ions do not affect the subsequent synthesis and separation of iron phosphate.

[0038] (2) introducing an iron source and an oxidant into the third liquid phase, performing a third reaction, and then performing solid-liquid separation to obtain a first solid phase; slurrying the first solid phase, adjusting the pH of the slurry to 1.65±0.5 with phosphoric acid, and then heating and aging, performing solid-liquid separation to obtain a second solid phase; and calcining the second solid phase to obtain battery-grade iron phosphate.

[0039] As a preferred embodiment, the iron source includes ferrous sulfate; in the present invention, ferrous sulfate obtained by extracting and purifying titanium dioxide by-products is preferably used; specifically, in the sulfuric acid method for preparing titanium dioxide, titanium concentrate is crushed and mixed with concentrated sulfuric acid, and after acid hydrolysis, a by-product solution containing ferrous sulfate is generated; the solution is filtered, the filtrate is alkali-adjusted, and filtered to obtain a relatively pure ferrous sulfate solution, which is used as the iron source in the present invention.

[0040] In the process of producing titanium dioxide by the sulfuric acid method, a large amount of ferrous sulfate is generated as a by-product. For every ton of titanium dioxide produced, about 2.5 to 3.5 tons of ferrous sulfate crystals are discharged. Therefore, by introducing titanium dioxide by-products, the production cost can be further effectively reduced, and a greener resource comprehensive recycling application can be achieved.

[0041] As a more preferred embodiment, before step (2), the pH of the solution of ferrous sulfate produced as a titanium dioxide byproduct is adjusted to 2.5±0.2 through the third liquid phase, and the concentration of ferrous iron in the solution of ferrous sulfate produced as a titanium dioxide byproduct is 60±2 g / L.

[0042] As a preferred embodiment, when ferrous sulfate produced as a byproduct of titanium dioxide is not used as the iron source, an iron solution containing the iron source is first prepared, and the pH of the iron solution is adjusted to 2.5±0.2 with a small amount of the third liquid phase.

[0043] In this preferred case, in step (2), the pH of the iron source solution is first adjusted to 2.5±0.2 by the third liquid phase. After the third reaction, the pH of the slurry containing ferrous iron is further adjusted to 1.65±0.5 by phosphoric acid before aging. That is, a two-stage method is adopted to adjust the pH of the iron salt, thereby making fuller use of the iron element and improving the iron yield.

[0044] First, when the pH is adjusted for the first time, the iron source is converted into a mixed valence state including divalent iron and trivalent iron. The trivalent iron can first react with the small amount of phosphate in the third liquid phase used to adjust the pH to form amorphous iron phosphate crystal nuclei; then, the iron source is mixed with the oxidant and the third liquid phase and the third reaction is formally carried out. The divalent iron is completely oxidized to trivalent iron, and further precipitated to form a large amount of amorphous iron phosphate; according to the common ion effect, the crystal structure of the common ion Fe 3+ The presence of reduces the solubility of iron phosphate in water, which is more conducive to the precipitation. Furthermore, reducing the amount of phosphoric acid added during the aging stage not only avoids phosphoric acid from dissolving iron phosphate, but also effectively ensures the stability of the iron phosphate lattice.

[0045] As a preferred embodiment, the oxidant includes hydrogen peroxide or oxygen, and hydrogen peroxide is more preferred. Based on the premise of avoiding the introduction of ionic impurities, hydrogen peroxide is the preferred oxidant component, while when oxygen is used, a blower, oxygen supply equipment, etc. need to be introduced, and the operation is more cumbersome, so it is not the first choice.

[0046] As a preferred embodiment, the molar ratio of the third liquid phase to the iron source is 1:(1-1.1) based on the phosphorus atoms in the third liquid phase and the iron atoms in the iron source; at the same time, when the oxidant is hydrogen peroxide, the molar ratio of the third liquid phase to the oxidant is 1:(0.5-1).

[0047] As a preferred embodiment, the temperature of the third reaction is 50°C to 80°C, and the time of the third reaction is 1.5h to 3h; in some optional embodiments, the temperature of the third reaction includes but is not limited to any point value among 50, 55, 60, 65, 70, 75, 80 (°C) or a real number interval formed by any two values, and the time of the third reaction is preferably 2h.

[0048] As a preferred embodiment, after obtaining the first solid phase, it is countercurrent washed several times with deionized water; thereby preventing the impurity ions in the third liquid phase from adhering to the surface of the first solid phase, resulting in impurities in the subsequent finished product.

[0049] As a preferred embodiment, in the slurry obtained after the slurrying, the solid phase particle size D50 is 5.5 μm to 6.5 μm, and D99 is 50 μm to 60 μm.

[0050] As a preferred embodiment, the solvent used in the pulping is deionized water, and the specific operation includes the following steps: fully dissolving the first solid phase in deionized water and stirring until it is evenly dispersed.

[0051] As a more preferred embodiment, in the pulping liquid obtained by the pulping, the solid-liquid ratio of the solvent to the first solid phase is 1:(1-1.5) (g / mL).

[0052] As a preferred embodiment, the temperature of the solvent used for the pulping is 50° C. to 80° C.; meanwhile, the pulping and pH adjustment are both carried out within the same temperature threshold.

[0053] As a preferred embodiment, the temperature is raised to 85°C to 95°C; in some more preferred embodiments, the temperature is raised at a rate of 3 to 8 (°C / 20min).

[0054] In the present invention, by adopting a gradient temperature increase program, part of the energy consumption can be reduced, and it is beneficial to the crystal transformation of iron phosphate, and the iron-phosphorus ratio is effectively improved.

[0055] As a preferred embodiment, the aging includes: starting from the time when the pulping liquid turns white, keeping it warm and standing for 1.5 hours to 3 hours.

[0056] As a preferred embodiment, the calcination temperature is 450°C to 650°C, and the calcination time is 1.5h to 3h; in some optional embodiments, the calcination temperature includes but is not limited to any point value among 450, 460, 480, 500, 520, 550, 580, 600, 650 (°C) or a real number interval composed of any two values, and the calcination time includes but is not limited to any point value among 1.5, 1.6, 1.8, 2, 2.2, 2.5, 2.8, 3 (h) or a real number interval composed of any two values.

[0057] The second aspect of the present invention is to provide use of the iron phosphate prepared by the method for preparing battery-grade iron phosphate as described in the first aspect in lithium-ion batteries.

[0058] It is understood that the use includes but is not limited to: a lithium-ion battery positive electrode prepared from a specific iron phosphate, a method for preparing a lithium-ion battery positive electrode based on a specific iron phosphate, and the lithium-ion battery products or electrical equipment derived from the above. The use can be a product related to the lithium battery industry, or a method for preparing or using a lithium battery product, etc., which is not limited in the present invention.

[0059] Example 1

[0060] Depend on Figure 1The process flow chart of this embodiment is provided as shown.

[0061] S1: Add tap water to commercially available agricultural-grade ammonium to dissolve it, and then add white carbon black and sodium sulfate. After stirring at 50°C for 30 minutes, separate the solid and liquid to obtain filtrate 1; wherein the mass ratio of agricultural-grade ammonium to tap water is 2.5, and the mass ratio of the amount of white carbon black and sodium sulfate used to the fluorine in the agricultural-grade ammonium is 1.25.

[0062] S2: Add oxalic acid to filtrate 1, wherein the mass ratio of oxalic acid to calcium in agricultural grade ammonium is 0.9; after stirring at 50°C for 30 minutes, filter to obtain filtrate 2; after testing, the pH of filtrate 2 is 3.9 and the phosphorus content is 12.75wt.%.

[0063] S3: Sodium carbonate was added to the filtrate 2 to adjust the pH to 7.0±0.2, and the mixture was stirred at room temperature for 1 h. After solid-liquid separation, the filtrate 3 was obtained.

[0064] S4: using the purified ferrous sulfate solution obtained from the titanium dioxide by-product as the iron raw material, adjusting the pH of the ferrous sulfate solution as the iron raw material to 2.5±0.2 through step S3 to obtain solution A.

[0065] S5: Mix the filtrate 3, hydrogen peroxide and solution A, wherein the molar ratio of the three is 1:0.8:1.05, calculated based on the phosphorus in the filtrate 3 and the iron in the solution A; react at 60°C for 2 hours, perform filter pressing, and wash the obtained filter cake three times in countercurrent with deionized water, and collect the washed filter cake.

[0066] S6: Beat the first-wash filter cake with water at 60°C according to the slurry solid content of 15%, and perform aging and beating. After the slurry particles are distinct, add phosphoric acid to the beating liquid to adjust the pH to 1.65±0.2; increase the temperature to 90°C according to the heating program of increasing 5°C every 20 minutes, and keep the temperature for 2 hours after the slurry turns white; after the insulation is completed, perform solid-liquid separation, and the obtained filter cake is countercurrent washed three times with deionized water, and then collected to obtain the second-wash filter cake.

[0067] S7: drying the second-wash filter cake to remove free water on the surface; and calcining the filter cake at 550° C. for 2 h to obtain the anhydrous iron phosphate of this embodiment.

[0068] Table 1 provides a composition test table of the agricultural-grade ammonium raw material and the filtrate 3 used in this embodiment; wherein the P2O5 content is tested by titration, and the unit is wt. %; the ICP test results of the phosphorus source dry material after the filtrate 3 is dried are also taken, and the result units are all in ppm.

[0069] Table 1

[0070] <![CDATA[P2O5]]> F Si Al Ca Mn Agricultural Grade Ammonium 43.36 7093 1248 13297 2976 252 Filtrate 3 12.75 8 31 7 86 52

[0071] Example 2

[0072] It is basically the same as Example 1, except that:

[0073] S1: The mass ratio of agricultural grade ammonium to tap water is 2, and the mass ratio of white carbon black and sodium sulfate to fluorine in agricultural grade ammonium is 1;

[0074] S2: The mass ratio of oxalic acid to calcium in agricultural grade ammonium is 1.2.

[0075] Example 3

[0076] It is basically the same as Example 1, except that:

[0077] S1: The mass ratio of agricultural grade ammonium to tap water is 3, and the mass ratio of white carbon black and sodium sulfate to fluorine in agricultural grade ammonium is 1.5;

[0078] S2: The mass ratio of oxalic acid to calcium in agricultural grade ammonium is 0.7.

[0079] Example 4

[0080] It is basically the same as Example 1, except that:

[0081] The reaction conditions of steps S1 and S2 were adjusted to: stirring at 60°C for 25 min;

[0082] The reaction conditions of step S5 were adjusted to: 80°C for 1.5h;

[0083] The reaction conditions of step S6 were adjusted to: increase the temperature to 85° C. by 3° C. every 20 min.

[0084] Example 5

[0085] It is basically the same as Example 1, except that:

[0086] The reaction conditions of steps S1 and S2 were adjusted to: stirring at 40°C for 40 min;

[0087] The reaction conditions of step S5 were adjusted to: 50°C for 2.5h;

[0088] The reaction conditions of step S6 were adjusted to: increase the temperature to 95° C. by 8° C. every 20 min.

[0089] Example 6

[0090] It is basically the same as Example 1, except that:

[0091] S3: Sodium carbonate was added to the filtrate 2 to adjust the pH to 6.0±0.2, stirred at room temperature for 0.5 h, and after solid-liquid separation, sodium carbonate was further added to adjust the pH to 7.0±0.2, stirred at room temperature for 0.5 h, and filtrate 3 was obtained after solid-liquid separation.

[0092] Table 2 provides a dry material component detection table of the filtrate 3 obtained in each embodiment of the present invention after drying.

[0093] Table 2

[0094] <![CDATA[P2O5]]> F Si Al Ca Mn Example 1 12.75 8 31 7 86 52 Example 2 12.26 11 28 6 32 46 Example 3 13.52 25 38 10 90 42 Example 4 13.86 40 50 18 69 51 Example 5 14.26 38 30 16 100 50 Example 6 14.26 29 41 11 150 53

[0095] Table 3 shows the purity and iron-phosphorus ratio of the anhydrous iron phosphate prepared in each embodiment of the present invention. Table 3 shows that the preparation route of the present invention can obtain an iron phosphate product with high purity and high iron-phosphorus ratio, which can be further used as a raw material for preparing battery-grade lithium iron phosphate materials and has good economic advantages.

[0096] Table 3

[0097] purity(%) Iron-phosphorus ratio Example 1 99.9 0.985 Example 2 99.9 0.980 Example 3 99.9 0.982 Example 4 99.9 0.979 Example 5 99.9 0.983 Example 6 99.9 0.986

[0098] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A method for preparing battery-grade iron phosphate, characterized in that: The steps include: (1) fully mixing agricultural grade ammonium, white carbon black and sodium sulfate in a liquid phase, performing a first reaction, and performing solid-liquid separation to obtain a first liquid phase; adding oxalic acid to the first liquid phase, performing a second reaction, and performing solid-liquid separation to obtain a second liquid phase; adjusting the pH of the second liquid phase to 7.0±0.5 by using an alkali source, and performing solid-liquid separation to obtain a third liquid phase; (2) introducing an iron source and an oxidant into the third liquid phase, performing a third reaction, and then performing solid-liquid separation to obtain a first solid phase; slurrying the first solid phase, adjusting the pH of the slurry to 1.65±0.5 with phosphoric acid, and then heating and aging, performing solid-liquid separation to obtain a second solid phase; and calcining the second solid phase to obtain battery-grade iron phosphate.

2. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: The active ingredients of the agricultural grade ammonium include monoammonium phosphate; Preferably, the content of the monoammonium phosphate in the agricultural grade ammonium is ≤ 65%; Preferably, the impurity elements of the agricultural grade ammonium include fluorine, silicon, aluminum, calcium and manganese.

3. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: In step (1), the mass ratio of the agricultural grade ammonium to the solvent in the liquid phase is (2-3):1; and / or, the mass ratio of the white carbon black to the fluorine in the agricultural grade ammonium is 1 to 1.5; And / or, the mass ratio of the sodium sulfate to the fluorine in the agricultural grade ammonium is 1 to 1.

5.

4. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: The temperature of the first reaction is 40°C to 60°C; And / or, the temperature of the second reaction is 40°C to 60°C.

5. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: The mass ratio of the oxalic acid to the calcium in the agricultural grade ammonium is 0.7 to 1.2; Preferably, after adding oxalic acid, the pH of the second liquid phase is 3.8-4.

2.

6. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: The iron source includes ferrous sulfate, and the oxidant includes hydrogen peroxide or oxygen; Preferably, the ferrous sulfate is prepared using a by-product of the sulfuric acid method titanium dioxide production process as a raw material.

7. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: The molar ratio of the third liquid phase to the iron source is 1:(1-1.1) based on the phosphorus atoms in the third liquid phase and the iron atoms in the iron source. And / or, when the oxidant is hydrogen peroxide, the molar ratio of phosphorus atoms in the third liquid phase to the oxidant is 1:(0.5-1).

8. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: The heating is performed to 85°C to 95°C, preferably, the heating is performed at a rate of 3°C to 8°C per 20 minutes; And / or, the aging duration is 1.5h to 3h.

9. The method for preparing battery-grade iron phosphate according to claim 1, characterized in that: The calcination temperature is 450° C. to 650° C., and the calcination time is 1.5 h to 3 h.

10. Use of the iron phosphate obtained by the method for preparing battery-grade iron phosphate according to any one of claims 1 to 9 in lithium-ion batteries.

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