Low-impurity ferric phosphate, and preparation method therefor and use thereof

Abstract

The present application belongs to the field of the preparation of high-purity ferric phosphate, and provides a low-impurity ferric phosphate, and a preparation method therefor and the use thereof. The preparation method comprises: mixing wet-process phosphoric acid with water to obtain a phosphoric acid solution, adjusting the pH of the phosphoric acid solution, and heating and aging same to obtain an aged slurry; filtering same to obtain a phosphate solution, and adding water and a first oxidant to obtain an aqueous phosphate solution containing the first oxidant; mixing a ferrous sulfate solution with the aqueous phosphate solution containing the first oxidant to obtain a first slurry; heating and aging same, and after the color of the first slurry changes, adding a second oxidant to the first slurry to obtain a second slurry; and performing solid-liquid separation on the second slurry, and sequentially washing, drying and sintering the obtained solid material to obtain a low-impurity ferric phosphate. In the present application, wet-process phosphoric acid is used as a raw material, and therefore the utilization rate of phosphorus in the wet-process phosphoric acid is greatly increased; moreover, prepared anhydrous ferric phosphate has a relatively low impurity content.

Description

Low-impurity iron phosphate and preparation method and application thereof

[0001] This application claims priority to Chinese patent application No. 202410432820.1, filed on April 11, 2024. This application incorporates the entire contents of the aforementioned Chinese patent application. TECHNICAL FIELD

[0002] The present application relates to the technical field of new energy, in particular to a low-impurity iron phosphate and a preparation method and application thereof. BACKGROUND

[0003] Currently, there are three types of mainstream process routes for preparing iron phosphate:

[0004] (1) Sodium method: a mixed solution composed of sodium hydroxide, phosphoric acid and hydrogen peroxide is added dropwise into a divalent iron compound solution to prepare iron phosphate;

[0005] (2) Ammonium method: a mixed solution composed of ammonium dihydrogen phosphate or diammonium hydrogen phosphate and hydrogen peroxide is added dropwise into a divalent iron compound solution to prepare iron phosphate;

[0006] (3) Iron method: a ferrous dihydrogen phosphate solution is prepared from high-purity phosphoric acid and elemental iron, and then hydrogen peroxide is used as an oxidizing agent and a precipitating agent to prepare iron phosphate.

[0007] Most of the aforementioned mainstream synthesis methods of iron phosphate in the industry use wet-process phosphoric acid as raw material, which contains a large amount of impurity elements such as Al, Mg, Mn and Zn. Before being used to prepare iron phosphate, the wet-process phosphoric acid needs to be decontaminated. However, during the decontamination process, a large amount of phosphorus elements and impurity cations are easily generated into phosphate precipitates and filtered out, resulting in a low utilization rate of phosphorus and causing waste of phosphorus resources.

[0008] Therefore, there is an urgent need to develop a low-cost and low-phosphorus-loss preparation method that can significantly improve the utilization rate of phosphorus while obtaining high-purity iron phosphate.

[0009] In view of the above, the present application is proposed.

[0010] SUMMARY

[0011] In view of the technical problems in the background art, the present application provides a low-impurity iron phosphate and a preparation method and application thereof, aiming to reduce phosphorus loss and improve the utilization rate of phosphorus while ensuring the purity of the iron phosphate product.

[0012] In a first aspect, the embodiments of the present application provide a preparation method of a low-impurity iron phosphate, comprising:

[0013] mixing wet-process phosphoric acid and water to obtain a phosphoric acid solution, adjusting the pH of the phosphoric acid solution to 2.0-3.0, and then heating and aging to obtain an aged slurry;

[0014] The aged slurry is filtered to obtain a phosphate solution, after which water and a first oxidizing agent are added to obtain an aqueous phosphate solution containing the first oxidizing agent;

[0015] The ferrous sulfate solution is mixed with the aqueous phosphate solution containing the first oxidizing agent to obtain a first slurry;

[0016] The first slurry is aged by heating, and after the first slurry changes color, a second oxidizing agent is added to the first slurry to obtain a second slurry;

[0017] The second slurry is subjected to solid-liquid separation, and the obtained solid material is sequentially washed, dried, and sintered to obtain low-impurity iron phosphate.

[0018] In the technical scheme of the embodiments of the present application, wet-process phosphoric acid is used as a raw material, and after the wet-process phosphoric acid and water are mixed to obtain a phosphoric acid solution, the pH value of the phosphoric acid solution is adjusted to 2.0-3.0, and then the phosphoric acid solution is aged by heating. The solubility product difference between iron phosphate and aluminum phosphate is used to make aluminum precipitate first, so as to remove aluminum. However, the prepared phosphate solution still contains magnesium and manganese impurities. Thereafter, the divalent iron ions are oxidized by adding an oxidizing agent in two steps. The scheme of step-by-step oxidation makes a small amount of trivalent iron ions exist in the system, which inhibits the precipitation of Mg and Mn, greatly improves the utilization rate of phosphorus in the wet-process phosphoric acid, and the prepared anhydrous iron phosphate has a low impurity content. In addition, compared with the prior art of using high-purity monoammonium phosphate / ammonium phosphate or phosphoric acid as a phosphorus source, the wet-process phosphoric acid raw material used in the embodiments of the present application has a lower cost, which can significantly reduce the process cost.

[0019] In some embodiments, the first oxidizing agent and the second oxidizing agent are each independently selected from one of hydrogen peroxide, ammonium persulfate, and sodium persulfate.

[0020] In the technical scheme of the embodiments of the present application, the first oxidizing agent and the second oxidizing agent are easy to obtain, and are both suitable for oxidizing divalent iron ions.

[0021] In some embodiments, the molar ratio of the first oxidizing agent to ferrous ions in the ferrous sulfate solution is (0.7-0.9):2; and / or

[0022] The molar ratio of the second oxidizing agent added dropwise to the first slurry to ferrous ions in the ferrous sulfate solution is (0.2-0.5):2;

[0023] The total amount of the first oxidizing agent and the second oxidizing agent and the molar ratio of ferrous ions in the ferrous sulfate solution are (1.1-1.4):2.

[0024] In this embodiment, by regulating the amount of the first oxidizing agent and the second oxidizing agent in the two-step oxidation, the amount of the first oxidizing agent is 0.7-0.9 times the total amount of completely oxidizing ferrous iron, and the amount of the second oxidizing agent is 0.2-0.5 times the total amount of completely oxidizing ferrous iron, while the total amount of the two oxidizing agents is slightly greater than the theoretical amount, so that the reaction process is more accurately controlled, the ferrous iron is fully converted into ferric iron, and the introduction of manganese and magnesium elements in the product is reduced.

[0025] In some embodiments, the step of obtaining the aging slurry from the wet-process phosphoric acid comprises:

[0026] The wet-process phosphoric acid and water are mixed to obtain a phosphoric acid solution with a phosphorus mass fraction of 5%-10%;

[0027] The pH value of the phosphoric acid solution is adjusted to 2.0-3.0, and then the temperature is increased for aging to obtain the aging slurry.

[0028] In this embodiment, by regulating the concentration of the phosphoric acid solution, the aluminum impurities can be more fully removed after the temperature is increased for aging, the aluminum ions can be prevented from entering the subsequent ferrous iron oxidation stage, the formation of ferric iron precipitate is not interfered, and the introduction of impurity elements in the product ferric phosphate is reduced.

[0029] In some embodiments, the temperature for the temperature-increasing aging is 85°C-95°C, and the time for the temperature-increasing aging is 1h-4h.

[0030] In this embodiment, by accurately controlling the operating temperature and the aging time for the temperature-increasing aging, the aluminum can be fully deposited, and a better aluminum removal effect can be achieved.

[0031] In some embodiments, in the aqueous phosphate solution containing the first oxidizing agent, the molar concentration of the phosphate is 1.0mol / L-2.5mol / L.

[0032] In this embodiment, by regulating the concentration of the aqueous phosphate solution, the reaction rate during the synthesis of ferric phosphate can be controlled, so that the reaction rate is not too fast, and the product uniformity is improved.

[0033] In some embodiments, the aqueous phosphate solution containing the first oxidizing agent is added dropwise into the ferrous sulfate solution for 10min-60min, and the reaction is continued for 10min-60min after the dropwise addition is completed.

[0034] In this embodiment, the ferrous sulfate solution is used as the bottom solution, and the aqueous phosphate solution is added dropwise to carry out the reaction, so that the first oxidizing agent is slowly added, which is beneficial to controlling the reaction rate and making the ferric phosphate uniformly precipitate.

[0035] In some embodiments, the step of obtaining the second slurry from the first slurry comprises: warming and aging the first slurry, the aging temperature being 80-95℃, after the first slurry changes color, adding a second oxidizing agent to the first slurry, and then, keeping the first slurry for 30-90 minutes to obtain the second slurry.

[0036] In this embodiment, the warming and aging of the first slurry causes a crystal transformation, i.e., the transformation from an amorphous phase to a crystalline state of iron phosphate dihydrate, which in turn causes the first slurry to change color, and then, the remaining ferrous ions are oxidized and a second oxidizing agent is added to continue the oxidation reaction.

[0037] In some embodiments, the washing is countercurrent washing; and / or

[0038] The drying temperature is 90-110℃, and the drying time is 2-12 hours; and / or

[0039] The sintering temperature is 550-650℃, and the sintering time is 1-8 hours.

[0040] In this embodiment, after the solid-liquid separation of the second slurry, an iron phosphate solid material is obtained, the surface residual phosphate ions are removed by washing, the washing water is removed by drying, and then, after sintering, an anhydrous iron phosphate product is obtained.

[0041] In some embodiments, the raw material of the ferrous sulfate solution is a by-product of titanium dioxide; and / or

[0042] The molar concentration of the ferrous sulfate solution is 0.5-1.5 mol / L.

[0043] In this embodiment, by controlling the source of the ferrous sulfate solution, the raw material is easy to obtain, and the process cost is reduced; by controlling the concentration of the ferrous ions, the reaction rate of the generation of iron phosphate is not too fast, which is beneficial to obtain an iron phosphate product with more uniform particle size.

[0044] In a second aspect, the embodiments of the present application provide a low-impurity iron phosphate, which is prepared by the preparation method in any of the above embodiments, wherein the content of the impurity element Mg in the prepared low-impurity iron phosphate is less than or equal to 40 ppm, and the content of the impurity element Mn is less than or equal to 60 ppm.

[0045] In the technical solution of the embodiments of the present application, by optimizing the preparation process of iron phosphate, the deposition of impurities such as Mg and Mn is inhibited through a two-step oxidation process, the utilization rate of phosphorus in wet-process phosphoric acid is greatly improved, and the prepared anhydrous iron phosphate has a low impurity content.

[0046] In a third aspect, the embodiments of the present application provide a lithium iron phosphate positive electrode material, which is prepared by using the low-impurity iron phosphate as a precursor. Since the low-impurity iron phosphate has a low impurity content, the purity of the lithium iron phosphate positive electrode material product is improved, and thus better electrochemical performance is obtained.

[0047] In a fourth aspect, the embodiments of the present application provide a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector. The positive electrode active layer comprises the lithium iron phosphate positive electrode material in the above-mentioned embodiments. Since the electrochemical performance of the lithium iron phosphate positive electrode material is improved, the positive electrode sheet has excellent electrochemical performance.

[0048] In a fifth aspect, the embodiments of the present application provide a secondary battery, which comprises the positive electrode sheet. Based on the excellent electrochemical performance of the positive electrode sheet, the electrochemical performance of the secondary battery product can be further improved.

[0049] The above description is only a summary of the technical solutions of the present application. In order to more clearly understand the technical means of the present application, the embodiments of the present application can be implemented according to the content of the description, and in order to make the above and other purposes, characteristics and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0050] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creating any creative labor.

[0051] FIG. 1 is a flowchart of a preparation method of low-impurity iron phosphate provided by the embodiments of the present application;

[0052] FIG. 2 is a process flowchart of the preparation method of low-impurity iron phosphate provided by the embodiments of the present application;

[0053] FIG. 3 is a scanning electron microscope test result graph of the iron phosphate of embodiment 1 of the present application;

[0054] FIG. 4 is a scanning electron microscope test result graph of the iron phosphate of embodiment 2 of the present application;

[0055] FIG. 5 is a scanning electron microscope test result graph of the iron phosphate of embodiment 3 of the present application;

[0056] FIG. 6 is a scanning electron microscope test result graph of the iron phosphate of embodiment 4 of the present application;

[0057] FIG. 7 is a scanning electron microscope test result graph of the iron phosphate of embodiment 5 of the present application;

[0058] Figure 8 is a scanning electron microscope test result graph of the iron phosphate of Example 6 of the present application;

[0059] Figure 9 is a scanning electron microscope test result graph of the iron phosphate of Example 7 of the present application;

[0060] Figure 10 is a scanning electron microscope test result graph of the iron phosphate of Example 8 of the present application;

[0061] Figure 11 is a scanning electron microscope test result graph of the iron phosphate of Comparative Example 4 of the present application. DETAILED DESCRIPTION

[0062] The embodiments of the technical solutions of the present application will be described in detail below with reference to the accompanying drawings. The following examples are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0063] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "include" and "have" and any variations thereof in the specification and claims of the present application and the above description of drawings are intended to cover non-exclusive inclusion.

[0064] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0065] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily independent or alternative embodiments to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0066] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " herein generally represents an "or" relationship between the associated objects before and after it.

[0067] In the description of the embodiments of the present application, the term "a plurality of" refers to two or more (including two), and similarly, "a plurality of groups" refers to two or more groups (including two groups), and "a plurality of pieces" refers to two or more pieces (including two pieces).

[0068] In the description of the embodiments of the present application, if the directions or positional relationships indicated by the technical terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" are based on the directions or positional relationships shown in the drawings, they are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0069] In the description of the embodiments of the present application, unless otherwise explicitly specified and limited, if the technical terms "mounting", "connection", "connection", "fixing" and the like appear, they should be understood in a broad sense, for example, they can be fixedly connected, or can be detachably connected, or can be integrated; they can be mechanically connected, or can be electrically connected; they can be directly connected, or can be indirectly connected through an intermediate medium; they can be the communication or interaction relationship between two elements. For those skilled in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0070] The current mainstream synthesis method of iron phosphate needs to remove impurities in wet-process phosphoric acid to prepare industrial-grade phosphate or phosphoric acid with low impurities, and then prepare iron phosphate with high-purity refined phosphate or phosphate as the phosphorus source. It is known that wet-process phosphoric acid contains a large amount of impurity elements such as Al, Mg, Mn and Zn. In the process of removing the above impurities, a large amount of phosphate precipitates will be filtered out, which directly leads to a large amount of waste of phosphorus resources.

[0071] In order to improve the comprehensive utilization rate of phosphorus in raw materials, the embodiments of the present application provide a preparation method of low-impurity iron phosphate, which avoids the process route of Mg and Mn precipitation through a step-by-step oxidation scheme, greatly improves the utilization rate of phosphorus in wet-process phosphoric acid, reduces the amount of alkali, reduces the production cost of iron phosphate, and the prepared anhydrous iron phosphate has low impurity content.

[0072] Referring to FIG. 1, in a first aspect, the embodiments of the present application provide a preparation method of low-impurity iron phosphate, comprising the following steps:

[0073] S1, mixing wet-process phosphoric acid and water to obtain a phosphoric acid solution, and adjusting the pH of the phosphoric acid solution to 2.0-3.0, and then heating and aging to obtain an aged slurry;

[0074] S2, filtering the aging slurry to obtain a phosphate solution, and then adding water and a first oxidizing agent to obtain a phosphate aqueous solution containing the first oxidizing agent;

[0075] S3, mixing the ferrous sulfate solution with the phosphate aqueous solution containing the first oxidizing agent to obtain a first slurry;

[0076] S4, aging the first slurry, and adding a second oxidizing agent to the first slurry after the first slurry changes color to obtain a second slurry;

[0077] S5, performing solid-liquid separation on the second slurry, and sequentially performing washing, drying and sintering on the obtained solid material to obtain low-impurity iron phosphate.

[0078] Further, in step S1 of the present application, a wet-process phosphoric acid sample is used as a raw material, and an aluminum removal process is performed first to obtain an aging slurry. Specifically, the preparation method of the phosphate aqueous solution containing the first oxidizing agent includes: mixing the wet-process phosphoric acid and water to obtain a phosphoric acid solution with a phosphorus mass fraction of 5% to 10%; adjusting the pH value of the phosphoric acid solution to 2.0 to 3.0, and then aging at an elevated temperature to obtain the aging slurry.

[0079] Specifically, the mass fraction of phosphorus in the phosphoric acid solution can be 5%, 8%, 10% and the like. When adjusting the pH value of the phosphoric acid solution, the alkali solution used can be ammonia water, sodium hydroxide solution and the like, but is not limited thereto. By adjusting the amount of the alkali solution added, the pH value of the system after the pH value adjustment can be 2.0, 2.5, 3.0 and the like.

[0080] In some embodiments, the temperature of the aging at an elevated temperature is controlled to be 85℃ to 95℃, and the time of the aging at an elevated temperature is controlled to be 1h to 4h. After the pH value adjustment meets the requirements, the temperature of the aging is controlled to be 85℃, 90℃, 95℃ and the like, and the aging time can be 1h, 2h, 3h, 4h and the like.

[0081] In step S2, the aging slurry is filtered to obtain a phosphate solution; the first oxidizing agent and water are added to the phosphate solution to control the concentration of phosphorus, and a phosphate aqueous solution containing the first oxidizing agent is obtained.

[0082] It should be noted that the pH value of the phosphoric acid solution is adjusted to 2.0 to 3.0, and after the temperature is increased, the solubility product difference between iron phosphate and aluminum phosphate is used to make aluminum precipitate first, so as to achieve the purpose of removing aluminum. The reaction slurry after the aging at an elevated temperature is subjected to solid-liquid separation by means of filtration and the like to obtain aluminum-containing filter residue and a phosphate solution, the aluminum-containing filter residue enters other processes for aluminum recovery, and the phosphate solution is mixed with the first oxidizing agent and water, so that the concentration of the phosphate and the amount of the first oxidizing agent meet the process requirements, and a phosphate aqueous solution containing the first oxidizing agent is obtained.

[0083] The application only separates the aluminum impurities in the wet-process phosphoric acid, so that a large amount of Mg and Mn impurities in the wet-process phosphoric acid exist in the form of ions, avoiding the consumption of phosphorus elements caused by the formation of phosphate of Mg and Mn impurities, and improving the utilization rate of phosphorus. Compared with the process route of preparing ammonium phosphate from wet-process phosphoric acid (pH value 4.0-5.0), the pH value (2.0-3.0) of the phosphoric acid solution is adjusted to be lower in the application, thereby reducing the amount of ammonia water.

[0084] In some embodiments, the molar concentration of the phosphate salt in the aqueous phosphate salt solution containing the first oxidizing agent is 1.0 mol / L-2.5 mol / L, such as 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, or 2.5 mol / L. By adjusting the concentration of the phosphoric acid solution and the aqueous phosphate salt solution, the reaction rate during the synthesis of iron phosphate can be controlled, so that the reaction rate is not too fast, and the uniformity of the product is improved. The amount of the first oxidizing agent added is determined according to the content of ferrous ions in the ferrous sulfate solution, and the molar ratio of the first oxidizing agent in the aqueous phosphate salt solution to the ferrous ions in the ferrous sulfate solution is (0.7-0.9):2, such as 0.7:2, 0.8:2, or 0.9:2. That is, when preparing the aqueous phosphate salt solution containing the first oxidizing agent, the amount of the first oxidizing agent added is controlled to be 0.7-0.9 times the total amount of divalent iron.

[0085] In some embodiments, the first oxidizing agent is selected from one of hydrogen peroxide, ammonium persulfate, and sodium persulfate, and can be any of the above, all of which are suitable for oxidizing divalent iron ions.

[0086] Specifically, before step S3, a ferrous sulfate solution also needs to be provided. In the application, the source of ferrous sulfate is not limited, for example, it can be a ferrous sulfate solution obtained by refining and removing impurities from a by-product of titanium dioxide. The ferrous sulfate solution can be diluted with pure water. By adjusting the source of the ferrous sulfate solution, the raw materials are easy to obtain, and the process cost is reduced. The molar concentration of divalent iron in the prepared ferrous sulfate solution can be 0.5 mol / L-1.5 mol / L, such as 0.5 mol / L, 1.0 mol / L, or 1.5 mol / L. By adjusting the concentration of divalent iron ions, the reaction rate of generating iron phosphate can be controlled, thereby facilitating the preparation of iron phosphate products with more uniform particle sizes.

[0087] Specifically, in step S3, the aqueous phosphate solution containing the first oxidizing agent can be added dropwise into the ferrous sulfate solution, the dropwise adding time is 10 min to 60 min, and after the dropwise adding is completed, the reaction is continued for 10 min to 60 min to make the first oxidizing agent fully react. Preferably, the reaction is carried out by using the ferrous sulfate solution as a bottom solution and adding the aqueous phosphate solution containing the first oxidizing agent dropwise, so that the first oxidizing agent is slowly added, which is beneficial to control the reaction rate and make the iron phosphate uniformly precipitate. The dropwise adding time of the aqueous phosphate solution containing the first oxidizing agent can be 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, etc., and the reaction time after the dropwise adding is completed can be 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, etc.

[0088] It should be noted that, in the first-stage oxidation reaction, the amount of the first oxidizing agent is 0.7 to 0.9 times the total amount of the completely oxidized divalent iron, and the amount of the first oxidizing agent is insufficient to completely oxidize the divalent iron ions, and the divalent iron is fully oxidized after the second-stage oxidation reaction.

[0089] Specifically, in step S4, the first slurry is aged by being heated, and after the first slurry changes color, the second oxidizing agent is added dropwise into the first slurry to carry out the second-stage oxidation reaction, and the second slurry is obtained after the reaction. The reason why the first slurry changes color is mainly that the iron phosphate is converted from an amorphous phase to a crystalline state of iron phosphate dihydrate. The present application adds the oxidizing agents in two steps, so that a small amount of trivalent iron ions exists in the system during the color changing process to inhibit the precipitation of impurity ions, so that the overall process does not need a complex impurity removal process, and the utilization rate of phosphorus in the wet-process phosphoric acid is greatly improved.

[0090] In some embodiments, the first oxidizing agent and the second oxidizing agent are each independently selected from one of hydrogen peroxide, ammonium persulfate, and sodium persulfate, and the first oxidizing agent and the second oxidizing agent can be the same or different. When the above several oxidizing agents completely oxidize the divalent iron ions, the molar ratio of the oxidizing agent to the divalent iron ions is 1:2.

[0091] In some embodiments, the molar ratio of the second oxidizing agent added dropwise into the first slurry to the ferrous ions in the ferrous sulfate solution is (0.2-0.5):2, and the molar ratio of the total amount of the first oxidizing agent and the second oxidizing agent to the ferrous ions in the ferrous sulfate solution is (1.1-1.4):2. That is, the total amount of the oxidizing agents in the two-stage oxidation reaction is slightly excessive, the amount of the second oxidizing agent added in the second-stage oxidation reaction is 0.2 to 0.5 times the total amount of the completely oxidized divalent iron, the reaction process is more accurately controlled, the divalent iron is fully converted into trivalent iron, and the introduction of manganese and magnesium elements in the iron phosphate product is reduced.

[0092] Specifically, the amount of the second oxidizing agent added to the first slurry is calculated according to the amount of ferrous ions in the prepared ferrous sulfate solution, and the molar ratio of the second oxidizing agent added to the first slurry to the ferrous ions in the oxidized ferrous sulfate solution can be 0.1:2, 0.2:2, 0.3:2, 0.4:2, 0.5:2, etc. That is, the amount of the second oxidizing agent required for the second-stage oxidation reaction process can be calculated according to the theoretical amount of the second oxidizing agent required for completely oxidizing the ferrous ions in the ferrous sulfate solution, so that the amount of the second oxidizing agent added to the first slurry is 0.2-0.5 times the theoretical amount.

[0093] Specifically, the total amount of the first and second oxidizing agents can also be calculated according to the amount of ferrous ions in the prepared ferrous sulfate solution, and the molar ratio of the total amount of the two-step added oxidizing agents to the ferrous ions in the ferrous sulfate solution can be 1.1:2, 1.2:2, 1.3:2, 1.4:2, etc. When the molar ratio of the oxidizing agent to the divalent iron ions is 1:2, the oxidizing agent can completely oxidize the divalent iron ions, and the total amount of the oxidizing agents controlled in the two-step reaction is 1.1-1.4 times the theoretical amount.

[0094] In some embodiments, the first slurry is aged by heating, and the aging temperature is controlled to be 80-95°C. After the first slurry changes from light white to light pink, a peristaltic pump can be used to add the second oxidizing agent to the slurry after color change, and the second oxidizing agent is controlled to be added for 30-90 min after completion of the addition. The first slurry is aged by heating to cause a crystal transformation from an amorphous phase to a crystalline state of iron phosphate dihydrate, thereby changing the color of the first slurry, and then the second oxidizing agent is added to continue the oxidation reaction.

[0095] Specifically, when the first slurry is aged by heating, the aging temperature can be controlled to be 80°C, 85°C, 90°C, 95°C, etc., and the second oxidizing agent is controlled to be added for 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, etc., after completion of the addition.

[0096] Specifically, in step S5, the iron phosphate dihydrate in the second slurry is separated out, and low-impurity iron phosphate can be prepared after impurity removal, sintering, etc.

[0097] In some embodiments, the second slurry is subjected to solid-liquid separation, and the obtained solid material is sequentially washed, dried, and sintered. The surface residual phosphate ions are removed by washing, the washing water is removed by drying, and anhydrous iron phosphate product is obtained after sintering.

[0098] In some embodiments, the washing can be a reverse flow washing, the solid material can be fully contacted with the washing water, so that the surface residual phosphate ions are fully removed, the washing water is removed by drying, and the anhydrous iron phosphate product is obtained after sintering.

[0099] In some embodiments, the drying temperature is 90-110 DEG C, and the drying time is 2-12 h. Within the drying temperature and time range, the washing water on the surface of the solid material can be more fully removed, and the crystal morphology of the iron phosphate is not affected. Specifically, the drying temperature can be 90 DEG C, 95 DEG C, 100 DEG C, 105 DEG C, 110 DEG C, and the drying time can be 2 h, 5 h, 8 h, 10 h, 12 h, etc.

[0100] In some embodiments, the sintering temperature is 550-650 DEG C, and the sintering time is 1-8 h. Within the sintering temperature and time range, the anhydrous iron phosphate product can be obtained by losing the crystal water of the iron phosphate dihydrate. Specifically, the sintering temperature can be 550 DEG C, 600 DEG C, 650 DEG C, etc., and the sintering time can be 1 h, 3 h, 5 h, 8 h, etc.

[0101] According to the above analysis, referring to FIG. 2, the main steps of the preparation method provided in the application are as follows: dissolving the wet-process phosphoric acid with water to obtain a phosphoric acid solution, adding ammonia water dropwise to the phosphoric acid solution to adjust the pH value to meet the requirements, then heating and aging to obtain an aged aluminum slurry; solid-liquid separation of the aged aluminum slurry to obtain an aluminum-containing filter residue and a refined phosphate salt solution, mixing the refined phosphate salt solution with a first oxidizing agent to obtain a phosphate salt aqueous solution containing hydrogen peroxide, adding the phosphate salt aqueous solution containing hydrogen peroxide to the prepared ferrous sulfate solution to perform a first-stage oxidation reaction, to obtain a first slurry; heating and aging the first slurry, then adding a second oxidizing agent dropwise after color change to perform a second-stage oxidation reaction, to obtain a second slurry; solid-liquid separation and washing of the second slurry to obtain iron phosphate dihydrate; drying and sintering of the iron phosphate dihydrate to obtain a low-impurity anhydrous iron phosphate product.

[0102] In a second aspect, the embodiments of the application provide a low-impurity iron phosphate prepared by the above preparation method, wherein the content of the impurity element Mg in the prepared low-impurity iron phosphate is less than or equal to 40 ppm, and the content of the impurity element Mn is less than or equal to 60 ppm. By optimizing the preparation process of the iron phosphate, the deposition of the impurities such as Mg and Mn is inhibited by a two-step oxidation process, the utilization rate of phosphorus in the wet-process phosphoric acid is greatly improved, and the prepared anhydrous iron phosphate has a low impurity content.

[0103] It should be noted that "low impurities" refers to the low impurity content of the iron phosphate product, and the main impurities are Mg and Mn, wherein the Mg content is less than or equal to 40 ppm, and the Mn content is less than or equal to 60 ppm. In addition to Mg and Mn, the iron phosphate product also contains other impurities such as Na, Ni, Pb, etc., but the content is small, generally less than 12 ppm.

[0104] In a third aspect, the embodiments of the present application provide a lithium iron phosphate positive electrode material prepared from the above low-impurity iron phosphate as a precursor. Exemplarily, the lithium iron phosphate positive electrode material is prepared from the above low-impurity iron phosphate, lithium salt and carbon source as raw materials, through ball milling, drying and calcination.

[0105] Due to the low impurity content in the iron phosphate, the purity of the lithium iron phosphate positive electrode material product is improved, thereby facilitating the obtaining of better electrochemical performance.

[0106] In a fourth aspect, the embodiments of the present application provide a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector, and the positive electrode active layer comprises the above lithium iron phosphate positive electrode material. Due to the improvement of the electrochemical performance of the lithium iron phosphate positive electrode material, the positive electrode sheet is endowed with excellent electrochemical performance.

[0107] In a fifth aspect, the embodiments of the present application provide a secondary battery, which comprises the above positive electrode sheet, and can further comprise a negative electrode sheet, an electrolyte, a separator and the like to form a complete battery structure. Based on the excellent electrochemical performance of the positive electrode sheet, the electrochemical performance of the secondary battery product can be further improved.

[0108] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not specified in the embodiments, it is carried out according to the technology or condition described in the literature in the art or according to the product instruction. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained from the market.

[0109] Embodiment 1

[0110] The embodiment provides a preparation method of low-impurity iron phosphate, which comprises the following steps:

[0111] (1) Preparation of ferrous sulfate solution: a ferrous sulfate solution obtained by refining and removing impurities from a by-product of titanium white powder is taken, and pure water is added to prepare a ferrous sulfate solution with a molar concentration of 1.0 mol / L for standby.

[0112] (2) Preparation of the refined phosphate solution: 484.49 g of the wet-process phosphoric acid sample (mass fraction 78.25%, see Table 1 for specific composition) was taken and diluted with pure water to prepare a phosphoric acid solution with a phosphorus mass fraction of 8.0%. While stirring, ammonia water was added dropwise to the phosphoric acid solution to adjust the pH value to 2.5. After the pH value adjustment was completed, the reaction solution was warmed to 90°C and aged for 2.0 h. After the aging was completed, the slurry was filtered to obtain the refined phosphate solution.

[0113] The filter residue was dried and weighed to have a mass of 31.48 g. The phosphorus content was detected to be 20.88%. The total phosphorus mass carried away by the filter residue was calculated, and the phosphorus loss rate was 5.48%.

[0114] (3) Preparation of the aqueous phosphate solution containing the first oxidizing agent: The refined phosphate solution in step (2) was taken and diluted with pure water and hydrogen peroxide. The molar ratio of the hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution in step (1) was 0.84:2 (i.e., the hydrogen peroxide was added at a proportion of 0.84 times the total amount of divalent iron to be oxidized), to prepare an aqueous phosphate solution with a phosphorus molar concentration of 2.0 mol / L for standby use.

[0115] (4) First-stage oxidation reaction: The reaction kettle was started to stir. 200 mL of the ferrous sulfate solution in step (1) was used as the base solution. 100 mL of the aqueous phosphate solution containing the first oxidizing agent in step (3) was added dropwise into the reaction kettle using a peristaltic pump within 30 min. After the dropwise addition was completed, the reaction was continued for 30 min to obtain a first slurry.

[0116] (5) Second-stage oxidation reaction: The first slurry in step (4) was warmed and aged. The aging temperature was set to 90°C. After the reaction slurry changed from light white to light pink, hydrogen peroxide was continuously added dropwise into the slurry using a peristaltic pump. The molar ratio of the hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution was 0.36:2 (i.e., the hydrogen peroxide was added at a proportion of 0.36 times the total amount of divalent iron to be oxidized). After the dropwise addition was completed, the temperature was maintained for 60 min to obtain a second slurry.

[0117] (6) Preparation of anhydrous iron phosphate: The second slurry in step (5) was filtered and countercurrently washed. The filter cake was dried at 95°C for 6 h and sintered at 600°C for 4 h to obtain anhydrous iron phosphate.

[0118] Example 2

[0119] The difference from Example 1 is only in the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3) of this example, the molar ratio of the hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution in step (1) was 0.7:2. In step (5), the molar ratio of the hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution was 0.5:2.

[0120] Example 3

[0121] The difference from Example 1 is only the amount of hydrogen peroxide added in step (3) and step (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.9:2; in step (5), the molar ratio of the hydrogen peroxide added dropwise to the ferrous ions in the ferrous sulfate solution is 0.3:2.

[0122] Example 4

[0123] The difference from Example 1 is only the amount of hydrogen peroxide added in step (3) and step (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.8:2; in step (5), the molar ratio of the hydrogen peroxide added dropwise to the ferrous ions in the ferrous sulfate solution is 0.4:2.

[0124] Example 5

[0125] This example provides a method for preparing low-impurity iron phosphate, comprising the following steps:

[0126] (1) Preparation of ferrous sulfate solution: Take the ferrous sulfate solution obtained by refining and removing impurities from the by-product of titanium white powder, and prepare a ferrous sulfate solution with a molar concentration of 0.5 mol / L with pure water for standby.

[0127] (2) Preparation of refined phosphate solution: Take 484.49 g of wet-process phosphoric acid sample (mass fraction 78.25%, specific composition see Table 1), and prepare a phosphoric acid solution with a phosphorus mass fraction of 5.0% with pure water. Under stirring, add ammonia water to the phosphoric acid solution to adjust the pH value to 2.0. After the pH value adjustment is completed, the reaction solution is warmed to 85°C and aged for 4.0 h. After aging is completed, the slurry is filtered to obtain a refined phosphate solution.

[0128] (3) Preparation of phosphate aqueous solution containing first oxidant: Take the refined phosphate solution prepared in step (2), add pure water and hydrogen peroxide, and the molar ratio of the amount of hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.7:2 (i.e. the hydrogen peroxide added is 0.7 times the total amount of divalent iron to be oxidized), to prepare a phosphate solution with a phosphorus molar concentration of 1.0 mol / L for standby.

[0129] (4) First-stage oxidation reaction: Start the reaction kettle stirring, take 200 mL of the ferrous sulfate solution in step (1) as the bottom liquid, use a peristaltic pump to add 100 mL of the phosphate aqueous solution containing first oxidant in step (3) into the reaction kettle dropwise within 10 min, continue to react for 60 min after the dropwise addition is completed, to obtain a first slurry.

[0130] (5) Second-stage oxidation reaction: The reaction slurry of step (4) was aged at an aging temperature of 80°C. When the reaction slurry changed from light white to light pink, hydrogen peroxide was continuously added to the slurry using a peristaltic pump. The molar ratio of hydrogen peroxide to ferrous ions in the ferrous sulfate solution was 0.5:2 (i.e., the hydrogen peroxide was added at a proportion of 0.5 times the total amount of divalent iron to be oxidized). After the addition was completed, the reaction was continued for 30 min, and a second slurry was obtained.

[0131] (6) Preparation of anhydrous iron phosphate: The second slurry of step (5) was filtered, washed countercurrently, dried at 90°C for 12 h, and sintered at 550°C for 8 h to obtain anhydrous iron phosphate.

[0132] Example 6

[0133] The present example provides a method for preparing low-impurity iron phosphate, which comprises the following steps:

[0134] (1) Preparation of a ferrous sulfate solution: A byproduct of titanium white powder was refined to remove impurities to obtain a ferrous sulfate solution, which was prepared into a ferrous sulfate solution with a molar concentration of 1.5 mol / L using pure water.

[0135] (2) Preparation of a refined phosphate solution: 484.49 g of a wet-process phosphoric acid sample (mass fraction 78.25%, specific composition shown in Table 1) was prepared into a phosphoric acid solution with a phosphorus mass fraction of 10.0% using pure water. While stirring, sodium hydroxide solution was added dropwise to the phosphoric acid solution to adjust the pH value of the system to 3.0. After the pH value adjustment was completed, the reaction solution was heated to 95°C and aged for 1.0 h. After the aging was completed, the slurry was filtered to obtain a refined phosphate solution.

[0136] (3) Preparation of a phosphate aqueous solution containing a first oxidizing agent: The refined phosphate solution of step (2) was prepared into a phosphate solution with a phosphorus molar concentration of 2.5 mol / L using pure water and hydrogen peroxide ①. The molar ratio of the amount of hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution of step (1) was 0.9:2 (i.e., the hydrogen peroxide was added at a proportion of 0.9 times the total amount of divalent iron to be oxidized).

[0137] (4) First-stage oxidation reaction: The reaction kettle was started to stir. 200 mL of the ferrous sulfate solution of step (1) was used as the base solution. 100 mL of the phosphate aqueous solution containing a first oxidizing agent of step (3) was added dropwise into the reaction kettle using a peristaltic pump within 60 min. After the addition was completed, the reaction was continued for 10 min to obtain a first slurry.

[0138] (5) Second-stage oxidation reaction: The reaction slurry of step (4) was aged at an aging temperature of 95°C. When the reaction slurry changed from light white to light pink, hydrogen peroxide was continuously added to the slurry by using a peristaltic pump. The molar ratio of hydrogen peroxide to ferrous ions in the ferrous sulfate solution was 0.3:2 (i.e., the hydrogen peroxide was added at a ratio of 0.3 times the total amount of divalent iron to be oxidized). After the addition was completed, the reaction was continued for 90 min, and a second-stage slurry was obtained.

[0139] (6) Preparation of anhydrous iron phosphate: The second-stage slurry of step (5) was filtered, washed countercurrently, and dried at 110°C for 2 h. Anhydrous iron phosphate was obtained by sintering at 650°C for 1 h.

[0140] Example 7

[0141] The difference from Example 1 is only in the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3), the molar ratio of hydrogen peroxide ① added to ferrous ions in the ferrous sulfate solution of step (1) is 0.9:2; and in step (5), the molar ratio of hydrogen peroxide ② added to ferrous ions in the ferrous sulfate solution is 0.2:2.

[0142] Example 8

[0143] The difference from Example 1 is only in the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3), the molar ratio of hydrogen peroxide ① added to ferrous ions in the ferrous sulfate solution of step (1) is 0.9:2; and in step (5), the molar ratio of hydrogen peroxide ② added to ferrous ions in the ferrous sulfate solution is 0.5:2.

[0144] Comparative Example 1

[0145] The difference from Example 1 is only in the way of adding hydrogen peroxide to the phosphate. The hydrogen peroxide is added according to the traditional one-step method, at a ratio of 1.2 times the total amount of divalent iron to be oxidized, to ensure complete oxidation of divalent iron in the system. The other steps are the same as in Example 1. The specific steps are as follows:

[0146] (1) Preparation of a ferrous sulfate solution: The same as in Example 1.

[0147] (2) Preparation of a refined phosphate solution: The same as in Example 1.

[0148] (3) Preparation of a phosphate solution: The refined phosphate solution of step (2) was prepared by adding pure water and hydrogen peroxide ① (the hydrogen peroxide was added at a ratio of 1.20 times the total amount of divalent iron to be oxidized) to obtain a phosphate solution with a phosphate molar concentration of 2.0 mol / L.

[0149] (4) Iron phosphate synthesis: open the reactor stirring, using 200 mL of step (1) ferrous sulfate solution as the bottom liquid, using a peristaltic pump to add 100 mL of prepared phosphate solution into the reactor dropwise for 30 min. After the dropwise addition is completed, continue to react for 30 min.

[0150] (5) Crystal transformation: the reaction completed slurry of step (4) is aged by increasing the temperature, and the aging temperature is set to 90°C. After the reaction slurry changes from light white to light pink, continue to keep warm for 60 min.

[0151] (6) Preparation of anhydrous iron phosphate: filter the reaction completed slurry of step (5), countercurrent washing, drying at 95°C for 6h, and sintering at 600°C for 4h to obtain anhydrous iron phosphate.

[0152] Comparative Example 2

[0153] In the comparative example, the conventional phosphoric acid impurity removal scheme is used for the wet-process phosphoric acid, that is, first adjust the wet-process phosphoric acid to a pH value of 7.0, filter, and then use refined phosphoric acid to adjust the pH value of the phosphate to 2.0-3.0. The hydrogen peroxide addition method in the phosphate is according to the traditional one-step scheme, and the hydrogen peroxide addition ratio is 1.2 times the total amount of divalent iron to be completely oxidized. The other steps are the same as in the example, and the specific steps are as follows:

[0154] (1) Preparation of ferrous sulfate solution: take the ferrous sulfate solution obtained by refining and impurity removal from the by-product of titanium white powder, and prepare a ferrous sulfate solution with a divalent iron molar concentration of 1.0 mol / L for use.

[0155] (2) Preparation of refined phosphate solution: take 484.49 g of wet-process phosphoric acid sample (mass fraction 78.25%) and add pure water to prepare a phosphoric acid solution with a phosphorus mass fraction of 8.0%. Under stirring, add ammonia water to the phosphoric acid solution to adjust the pH value of the system to 7.0. After the pH value adjustment is completed, heat the reaction solution to 90°C and age for 2.0h. After aging is completed, filter the slurry, and then use refined phosphoric acid to adjust the pH value to 2.5 to obtain a refined phosphate solution.

[0156] Dry the filter residue and weigh the mass to be 73.01 g. The phosphorus content is 22.90%. Calculate the phosphorus loss according to the total phosphorus mass carried away by the filter residue, and the phosphorus loss rate is 13.95%.

[0157] (3) Preparation of phosphate: take the refined phosphate solution of step (2), add pure water and hydrogen peroxide (the hydrogen peroxide addition ratio is 1.20 times the total amount of divalent iron to be completely oxidized) to prepare a phosphate solution with a phosphorus molar concentration of 2.0 mol / L for use.

[0158] (4) Iron phosphate synthesis: open the reactor stirring, using 200 mL of step (1) ferrous sulfate solution as the bottom liquid, using a peristaltic pump to add 100 mL of prepared phosphate into the reactor in 30 min, after the dropwise addition is completed, continue to react for 30 min.

[0159] (5) Crystal transformation: the reaction completed slurry of step (4) is aged by increasing the temperature, the aging temperature is set to 90℃, after the reaction slurry changes from light white to light pink, continue to keep warm for 60 min.

[0160] (6) Preparation of anhydrous iron phosphate: filter the reaction completed slurry of step (5), countercurrent washing, drying at 95℃ for 6h, sintering at 600℃ for 4h to obtain anhydrous iron phosphate.

[0161] Comparative Example 3

[0162] The difference with Example 1 is only the amount of hydrogen peroxide added in step (3) and step (5). Specifically, in step (3) of this example, the molar ratio of hydrogen peroxide ① added to ferrous ions in step (1) ferrous sulfate solution is 0.95:2; in step (5), the molar ratio of hydrogen peroxide ② added to ferrous ions in step (1) ferrous sulfate solution is 0.1:2.

[0163] Comparative Example 4

[0164] The difference with Example 1 is only the amount of hydrogen peroxide added in step (3) and step (5). Specifically, in step (3) of this example, the molar ratio of hydrogen peroxide ① added to ferrous ions in step (1) ferrous sulfate solution is 0.6:2; in step (5), the molar ratio of hydrogen peroxide ② added to ferrous ions in step (1) ferrous sulfate solution is 0.85:2.

[0165] In this application, the composition of the phosphate obtained after step (2) of the test examples and comparative examples is tested, and the element removal rate and phosphorus loss rate are calculated, and the results are shown in Table 2. The composition and content of impurity elements of anhydrous iron phosphate prepared by the test examples and comparative examples are shown in Table 3.

[0166] Table 1 Wet-process phosphoric acid main component test table

[0167] From the raw material wet-process phosphoric acid test results in Table 1, the main impurity components are Al, Mg, and Mn.

[0168] Table 2 Phosphorus loss and element detection data table of phosphate prepared by the test examples and comparative examples

[0169] Table 3 Impurity element detection data table of anhydrous iron phosphate prepared by the test examples and comparative examples

[0170] The test results of the scanning electron microscope of the iron phosphate of Example 1 are shown in FIG. 3; the test results of the scanning electron microscope of the iron phosphate of Example 2 are shown in FIG. 4; the test results of the scanning electron microscope of the iron phosphate of Example 3 are shown in FIG. 5; the test results of the scanning electron microscope of the iron phosphate of Example 4 are shown in FIG. 6; the test results of the scanning electron microscope of the iron phosphate of Example 5 are shown in FIG. 7; the test results of the scanning electron microscope of the iron phosphate of Example 6 are shown in FIG. 8; the test results of the scanning electron microscope of the iron phosphate of Example 7 are shown in FIG. 9; the test results of the scanning electron microscope of the iron phosphate of Example 8 are shown in FIG. 10; and the test results of the scanning electron microscope of the iron phosphate of Comparative Example 4 are shown in FIG. 11.

[0171] As compared with the test results of the scanning electron microscope of Comparative Example 4, it can be seen from the test results of the scanning electron microscope of Examples 1 to 8 that the low-impurity iron phosphate obtained in Examples 1 to 8 is in a sheet shape, and the excessive amount of the oxidizing agent added in Comparative Example 4 can cause a small increase in the content of impurity ions, and can cause the microstructure of the iron phosphate obtained in Comparative Example 4 to change obviously and be in a block shape, and the performance is correspondingly reduced.

[0172] As can be seen from Table 2, the method provided in the embodiments of the present application can better remove Al impurities, and the removal efficiency can reach 97.78%. At the same time, the Mg and Mn impurities are retained in the liquid phase in the form of ions, thereby reducing the phosphorus loss rate from 13.95% to 5.48%, saving 8.47%, which can significantly reduce the production cost of iron phosphate and has good economic benefits.

[0173] As can be seen from Table 3, compared with the traditional one-step method for preparing iron phosphate, the segmented oxidation scheme provided in the embodiments of the present application can greatly reduce the content of Mg and Mn impurities in anhydrous iron phosphate. The content of Mg is reduced from 91.4 ppm to 23.2 ppm, and the content of Mn is reduced from 233.29 ppm to 56.47 ppm, which indicates that the segmented oxidation scheme provided in the embodiments of the present application can greatly reduce the content of Mg and Mn in anhydrous iron phosphate.

[0174] As can be seen from the corresponding results of Comparative Example 3 in Table 3, the excessive low amount of the oxidizing agent can cause the content of impurity ions Mg and Mn in the anhydrous iron phosphate to increase obviously.

[0175] In summary, the directional impurity removal-segmented oxidation scheme provided in the present application can greatly reduce the loss of phosphorus resources and reduce the production cost of iron phosphate. At the same time, the segmented oxidation scheme can better solve the problem of high content of Mg and Mn impurities in the traditional one-step method for preparing iron phosphate, expand the application scenarios of the one-step method for synthesizing iron phosphate, and make the prepared iron phosphate more suitable for preparing high-performance lithium iron phosphate.

[0176] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art, without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A method for preparing low-impurity ferric phosphate, characterized in that: include: Mixing wet-process phosphoric acid and water to obtain a phosphoric acid solution, adjusting the pH of the phosphoric acid solution to 2.0-3.0, and then heating and aging to obtain an aged slurry; filtering the aged slurry to obtain a phosphate solution, and then adding water and a first oxidant to obtain a phosphate aqueous solution containing the first oxidant; mixing the ferrous sulfate solution with the phosphate aqueous solution containing the first oxidant to obtain a first slurry; heating and aging the first slurry, and adding a second oxidant to the first slurry after the first slurry changes color to obtain a second slurry; The second slurry is subjected to solid-liquid separation, and the obtained solid material is washed, dried and sintered in sequence to obtain the low-impurity ferric phosphate.

2. The preparation method according to claim 1, characterized in that The first oxidant and the second oxidant are independently selected from one of hydrogen peroxide, ammonium persulfate and sodium persulfate.

3. The preparation method according to claim 2, characterized in that The molar ratio of the first oxidant to the ferrous ions in the ferrous sulfate solution is (0.7-0.9):2; and / or The molar ratio of the second oxidant added dropwise to the first slurry to the ferrous ions in the ferrous sulfate solution is (0.2-0.5):2; The molar ratio of the total amount of the first oxidant and the second oxidant to the ferrous ions in the ferrous sulfate solution is (1.1-1.4):

2.

4. The preparation method according to claim 1, characterized in that The step of obtaining the aged slurry from the wet-process phosphoric acid comprises: Mixing wet-process phosphoric acid and water to obtain a phosphoric acid solution containing 5% to 10% by mass of phosphorus; The pH value of the phosphoric acid solution is adjusted to 2.0-3.0, and then the solution is heated and aged to obtain an aged slurry.

5. The preparation method according to claim 4, characterized in that The temperature of the heating aging is 85° C. to 95° C., and the time of the heating aging is 1 hour to 4 hours.

6. The preparation method according to claim 4, characterized in that In the phosphate aqueous solution containing the first oxidant, the molar concentration of phosphate is 1.0 mol / L to 2.5 mol / L.

7. The preparation method according to claim 1, characterized in that The phosphate aqueous solution containing the first oxidant is added dropwise to the ferrous sulfate solution for 10 to 60 minutes, and the reaction is continued for 10 to 60 minutes after the addition is completed.

8. The preparation method according to claim 1, characterized in that The step of obtaining the second slurry from the first slurry includes: heating and aging the first slurry at a temperature of 80°C to 95°C, adding the second oxidant to the first slurry after the first slurry changes color, and then keeping the first slurry warm for 30 minutes to 90 minutes to obtain the second slurry.

9. The preparation method according to claim 1, characterized in that The washing is countercurrent washing; and / or The drying temperature is 90°C to 110°C and the drying time is 2h to 12h; and / or The sintering temperature is 550°C to 650°C, and the sintering time is 1h to 8h.

10. The preparation method according to claim 1, characterized in that The raw material of the ferrous sulfate solution is titanium dioxide by-product; and / or The molar concentration of the ferrous sulfate solution is 0.5 mol / L to 1.5 mol / L.

11. A low-impurity ferric phosphate, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 10, wherein the content of impurity element Mg in the prepared low-impurity ferric phosphate is less than or equal to 40 ppm, and the content of impurity element Mn is less than or equal to 60 ppm.

12. A lithium iron phosphate positive electrode material, characterized in that: It is prepared using the low-impurity ferric phosphate described in claim 11 as a precursor.

13. A positive electrode plate, characterized in that: It comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector, wherein the positive electrode active layer comprises the lithium iron phosphate positive electrode material according to claim 12.

14. A secondary battery, characterized in that: Including the positive electrode sheet as claimed in claim 13.

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