Iron phosphate with low impurity content, its manufacturing method, and use

Abstract

This invention provides iron phosphate with a low impurity content, a method for producing the same, and its use, and belongs to the field of producing high-purity iron phosphate. The production method includes the steps of: mixing wet phosphoric acid and water to obtain a phosphoric acid solution, adjusting the pH of the phosphoric acid solution, raising the temperature and aging it to obtain an aged slurry, filtering it to obtain a phosphate solution, adding water and a primary oxidizing agent to obtain an aqueous phosphate solution containing a primary oxidizing agent; mixing a ferrous sulfate solution with the aqueous phosphate solution containing a primary oxidizing agent to obtain a first slurry; raising the temperature and aging it until the color of the first slurry changes, then adding a secondary oxidizing agent to the first slurry to obtain a second slurry; and separating the second slurry into solid and liquid components, and subjecting the obtained solid material to washing, drying, and sintering in that order to obtain iron phosphate with a low impurity content. In this invention, by using wet phosphoric acid as a raw material, the utilization rate of phosphorus in wet phosphoric acid is greatly improved, and moreover, the impurity content of the produced anhydrous iron phosphate is low.

Description

Technical Field

[0001] This application claims the priority of Chinese Patent Application No. 202410432820.1 with a filing date of April 11, 2024. This application incorporates the full text of the above Chinese patent application by reference.

[0002] This application relates to the field of new energy technologies, and specifically to iron phosphate with low impurity content, its manufacturing method, and uses.

Background Art

[0003] Currently, as the processes for manufacturing iron phosphate, the following three methods are mainstream. (1) Sodium method: A mixed solution composed of sodium hydroxide, phosphoric acid, and hydrogen peroxide is dropped into a divalent iron compound solution to produce iron phosphate. (2) Ammonium method: A mixed solution composed of ammonium dihydrogen phosphate or diammonium hydrogen phosphate and hydrogen peroxide is dropped into a divalent iron compound solution to produce iron phosphate. (3) Iron method: Ferrous dihydrogen phosphate solution is produced using high-purity phosphoric acid and elemental iron, and then hydrogen peroxide is used as an oxidizing agent and precipitating agent to produce iron phosphate.

[0004] Many of the mainstream iron phosphate synthesis methods in the industry described above use wet phosphoric acid as a raw material. Wet phosphoric acid contains a large amount of impurity elements such as Al, Mg, Mn, and Zn, so it is necessary to remove impurities before using it in the production of iron phosphate. In the process of removing impurities, a large amount of phosphorus elements and impurity cations easily form phosphate precipitates and are filtered off, resulting in a low overall utilization rate of phosphorus and waste of phosphorus resources. Therefore, there is an urgent need to develop a low-cost manufacturing method with less phosphorus loss that can significantly improve the utilization rate of phosphorus while obtaining high-purity iron phosphate. From such a perspective, this application has been proposed.

Summary of the Invention

Problems to be Solved by the Invention

[0005] In light of the technical challenges described in the background art, this application provides iron phosphate with a low impurity content, a method for producing the same, and a method for using the same, which reduce phosphorus loss and improve phosphorus utilization while ensuring the purity of the iron phosphate product. In the first aspect, the embodiments of the present application are as follows: The process involves mixing wet phosphoric acid with water to obtain a phosphoric acid solution, adjusting the pH of the phosphoric acid solution to 2.0-3.0, then raising the temperature to mature it and obtain a matured slurry. The process involves filtering the matured slurry to obtain a phosphate solution, then adding water and a primary oxidizing agent to obtain an aqueous phosphate solution containing the primary oxidizing agent, The first step is to mix a ferrous sulfate solution with an aqueous phosphate solution containing a primary oxidizing agent to obtain a first slurry. The first slurry is heated and matured, and when the color of the first slurry changes, the second oxidizing agent is added to the first slurry to obtain the second slurry. The present invention provides a method for producing iron phosphate with a low impurity content, comprising the steps of: separating the second slurry into solid and liquid components; washing, drying, and sintering the obtained solid material in that order to obtain iron phosphate with a low impurity content.

[0006] In the technical solution of the embodiment of this application, wet phosphoric acid is used as a raw material. A phosphoric acid solution is obtained by mixing wet phosphoric acid with water, adjusting the pH of the phosphoric acid solution to 2.0-3.0, and then heating and aging. By utilizing the difference in the solubility product of iron phosphate and aluminum phosphate, the objective of removing aluminum is achieved by precipitating aluminum first. However, the produced phosphate solution still contains magnesium and manganese impurities. Subsequently, an oxidizing agent is added in two stages to oxidize the divalent iron ions. Using a stepwise oxidation scheme, a small amount of trivalent iron ions are present in the system, suppressing the precipitation of Mg and Mn, thereby significantly improving the utilization rate of phosphorus in the wet phosphoric acid, and the produced anhydrous iron phosphate has a low impurity content. Furthermore, compared to existing technologies that use high-purity monoammonium phosphate / diammonium phosphate or phosphoric acid as a phosphorus source, the wet phosphoric acid used in the embodiment of this application has lower raw material costs and can significantly reduce process costs.

[0007] In some embodiments, the first and second oxidizing agents are each independently selected from hydrogen peroxide, ammonium persulfate, and sodium persulfate.

[0008] In the technical solutions of the embodiments of the present application, the first and second oxidizing agents provided are readily available raw materials and both are suitable for use with divalent iron oxide ions.

[0009] In some examples, the molar ratio of the primary oxidizing agent to ferrous ions in the ferrous sulfate solution is (0.7~0.9):2, and / or The molar ratio of the oxidizing agent 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 primary and secondary oxidizing agents to the ferrous ions in the ferrous sulfate solution is (1.1-1.4):2.

[0010] In this example, by adjusting the amounts of the first and second oxidizing agents added in the two-stage oxidation, the amount of the first oxidizing agent is 0.7 to 0.9 times the total amount required to completely oxidize the divalent iron, and the amount of the second oxidizing agent is 0.2 to 0.5 times the total amount required to completely oxidize the divalent iron. Furthermore, by controlling the total amount of oxidizing agents added in two stages to be slightly more than the theoretical amount, the reaction process can be controlled more precisely, allowing for the complete conversion of divalent iron to trivalent iron, and reducing the introduction of manganese and magnesium into the product.

[0011] In some embodiments, the step of obtaining a matured slurry from wet phosphoric acid is, The steps include: mixing wet phosphoric acid with water to obtain a phosphoric acid solution in which the mass fraction of phosphorus is 5% to 10%; The process includes the steps of adjusting the pH of the phosphoric acid solution to 2.0-3.0, then raising the temperature to mature it and obtain a matured slurry.

[0012] In this example, by adjusting the concentration of the phosphoric acid solution, aluminum impurities can be removed more completely after heating and aging. This prevents aluminum ions from entering the subsequent divalent iron oxidation step and inhibits the formation of trivalent iron precipitates, thereby reducing the introduction of impurity elements into the iron phosphate product.

[0013] In some examples, the temperature-controlled aging process involves a temperature of 85°C to 95°C and a duration of 1 to 4 hours.

[0014] In this embodiment, by precisely controlling the operating temperature and maturation time during the temperature-raising maturation process, aluminum can be sufficiently precipitated, thereby improving the aluminum removal effect.

[0015] In some examples, the phosphate aqueous solution containing the first oxidizing agent has a molar concentration of phosphate of 1.0 mol / L to 2.5 mol / L.

[0016] In this example, adjusting the concentration of the phosphate aqueous solution is advantageous for controlling the reaction rate during the synthesis of iron phosphate, resulting in a reaction rate that is not too fast and improving the uniformity of the product.

[0017] In some embodiments, the time for dropping the aqueous phosphate solution containing the first oxidant into the ferrous sulfate solution is 10 min to 60 min, After the dropping is completed, the reaction is continued for 10 min to 60 min.

[0018] In this example, the reaction is carried out by dropping the aqueous phosphate solution with the ferrous sulfate solution as the bottom liquid. Slowly adding the first oxidant is advantageous for controlling the reaction rate and uniformly depositing iron phosphate.

[0019] In some embodiments, the step of obtaining the second slurry from the first slurry includes heating the first slurry to age at an aging temperature of 80°C to 95°C, and when the color of the first slurry changes, dropping the second oxidant into the first slurry, and then keeping the first slurry at a temperature for 30 min to 90 min to obtain the second slurry.

[0020] In this example, the first slurry is heated and aged to cause a crystal transformation from the amorphous phase to the crystalline state of iron phosphate dihydrate, thereby changing the color of the first slurry and oxidizing the remaining divalent iron ions, and then the second oxidant is dropped to continue the oxidation reaction.

[0021] In some embodiments, the washing is countercurrent washing, and / or the drying temperature is 90°C to 110°C, the drying time is 2 h to 12 h, and / or the sintering temperature is 550°C to 650°C, and the sintering time is 1 h to 8 h.

[0022] In this example, the second slurry is subjected to solid-liquid separation to obtain an iron phosphate solid material, ions such as phosphate groups remaining on the surface are removed by washing, the washing water is removed by drying, and sintering is performed to obtain an anhydrous iron phosphate product.

[0023] In some embodiments, the raw material of the ferrous sulfate solution is a by-product from the production of titanium dioxide, and / or The molar concentration of the ferrous sulfate solution is 0.5 mol / L to 1.5 mol / L.

[0024] In this embodiment, by adjusting the source of the ferrous sulfate solution, the raw materials can be easily obtained and the process cost can be reduced. By adjusting the concentration of divalent iron ions, the production reaction rate of iron phosphate is prevented from being too fast, which contributes to obtaining an iron phosphate product having a more uniform particle size.

[0025] In a second aspect, an embodiment of the present application provides iron phosphate with a low impurity content, which is produced by the production method described in any of the above embodiments, has a content of impurity element Mg of 40 ppm or less, and a content of impurity element Mn of 60 ppm or less.

[0026] In the technical solution of the embodiment of the present application, the production process of iron phosphate is optimized, and the precipitation of impurities such as Mg and Mn is suppressed through a two-stage oxidation process, so that the utilization rate of phosphorus in wet phosphoric acid is greatly improved, and the produced anhydrous iron phosphate has a low impurity content.

[0027] In a third aspect, an embodiment of the present application provides a cathode material for lithium iron phosphate, which is produced using the iron phosphate with a low impurity content as a precursor. Since the impurity content in the iron phosphate is low, it helps to improve the purity of the cathode material product of lithium iron phosphate and helps to obtain better electrochemical properties.

[0028] [[ID=B]] In a fourth aspect, an embodiment of the present application provides a positive electrode plate including a positive electrode current collector and a positive electrode active layer provided on at least one side of the positive electrode current collector, and the positive electrode active layer includes the cathode material of lithium iron phosphate in the above embodiment. By improving the electrochemical properties of the cathode material of lithium iron phosphate, the positive electrode plate is provided with excellent electrochemical properties.

[0029] In a fifth aspect, an embodiment of the present application provides a secondary battery including the above positive electrode plate. Based on the excellent electrochemical properties of the positive electrode plate, the electrochemical properties of the secondary battery product can be further improved.

[0030] The above description is merely an outline of the technical solution of the present application. In order to provide a clearer understanding of the technical means of the present application, to enable implementation in accordance with the contents of the specification, and to make the above and other objectives, features, and advantages of the present application clearer and easier to understand, specific embodiments of the present application are given below. [Brief explanation of the drawing]

[0031] To more clearly explain the technical solution of this application, the drawings used in this application are briefly described below. As is clear, the drawings described below represent only some embodiments of this application, and those skilled in the art can derive other drawings from these without any creative work.

[0032] [Figure 1] This is a schematic flowchart illustrating the method for producing iron phosphate with low impurity content according to the embodiments of the present invention. [Figure 2] This is a schematic diagram of the process flow for a method of producing iron phosphate with a low impurity content according to the embodiments of the present invention. [Figure 3] This figure shows the results of a scanning electron microscope test of iron phosphate in Example 1 of the present invention. [Figure 4] This figure shows the results of a scanning electron microscope test of iron phosphate in Example 2 of the present invention. [Figure 5] This figure shows the results of a scanning electron microscope test of iron phosphate in Example 3 of the present invention. [Figure 6] This figure shows the results of a scanning electron microscope test of iron phosphate in Example 4 of the present invention. [Figure 7] This figure shows the results of a scanning electron microscope test of iron phosphate in Example 5 of the present application. [Figure 8] This figure shows the results of a scanning electron microscope test of iron phosphate in Example 6 of the present application. [Figure 9] This figure shows the results of a scanning electron microscope test of iron phosphate in Example 7 of this application. [Figure 10]This figure shows the results of a scanning electron microscope test of iron phosphate in Example 8 of the present invention. [Figure 11] This figure shows the results of a scanning electron microscope test of iron phosphate in Comparative Example 4 of the present invention. [Modes for carrying out the invention]

[0033] The following descriptions will detail embodiments of the technical solution of the present application with reference to the drawings. The following embodiments are provided solely as examples to more clearly illustrate the technical solution of the present application and will not limit the scope of protection of the present application.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art. Terms used herein are used solely to describe specific embodiments and are not intended to limit this application. The terms “including” and “having,” and their variations, in the description of the specification, claims, and drawings above, are intended to cover non-exclusive inclusion.

[0035] In the description of the embodiments of this application, technical terms such as “first,” “second,” etc., are used solely to distinguish different subjects and are not to be understood as indicating or implying relative importance, or implicitly indicating the number, specific order, or primary / secondary relationship of the technical features shown. In the description of the embodiments of this application, “plural” means two or more unless otherwise clearly and specifically limited.

[0036] The “Examples” described herein mean that certain features, structures, or properties described with reference to the Examples may be included in at least one Example of the Application. The appearance of the same term in different parts of the Specification does not necessarily refer to the same Example, nor does it refer to mutually exclusive, independent, or alternative Examples. As those skilled in the art will understand both explicitly and implicitly, the Examples described herein may be combined with other Examples.

[0037] In the description of the embodiments of this application, the term "and / or" merely describes the relationship between related objects, indicating that three relationships are possible. For example, in the case of A and / or B, it can mean that A exists alone, that A and B exist simultaneously, or that B exists alone. In addition, the symbol " / " in this specification generally indicates that the preceding and succeeding related objects are in an "or" relationship.

[0038] In the description of the embodiments of this application, the term "multiple" means two or more (including two), similarly, "multiple sets" means two or more sets (including two sets), and "multiple sheets" means two or more sheets (including two).

[0039] In the description of the embodiments of this application, the orientations or positional relationships indicated by technical terms such as "center," "vertical direction," "horizontal direction," "length," "width," "thickness," "top," "bottom," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inside," "outside," "clockwise," "counterclockwise," "axial direction," "radial direction," and "circumferential direction" are based on the orientations or positional relationships indicated on the surface and are merely intended to facilitate and simplify the description of the embodiments of this application. They do not indicate or imply that the devices or elements shown need to have a specific orientation, or be configured and operated in a specific orientation, and therefore should not be interpreted as limiting the embodiments of this application.

[0040] In the description of the embodiments of this application, unless otherwise specifically defined and limited, technical terms such as “attach,” “connect,” “join,” and “fix” should be understood in a broad sense, and may include, for example, fixed connections, removable connections, or integral connections; mechanical connections, electrical connections; direct connections, indirect connections via an intermediate medium, internal communication between two components, or interaction relationships between two components. A person skilled in the art will understand the specific meaning of the above terms in the embodiments of this application depending on the specific circumstances.

[0041] The current mainstream method for synthesizing iron phosphate requires removing impurities from wet phosphoric acid to produce industrial phosphate or phosphoric acid with low impurity content, and then using the purified, high-purity phosphate or phosphoric acid as the phosphorus source to produce iron phosphate. Wet phosphoric acid is known to contain large amounts of impurity elements such as Al, Mg, Mn, and Zn, and in the process of removing these impurities, a precipitate of phosphate is formed from a large amount of phosphorus elements and impurity cations and filtered out, resulting in a waste of large amounts of phosphorus resources.

[0042] To improve the overall utilization rate of phosphorus in the raw materials, the embodiments of this application provide a method for producing iron phosphate with a low impurity content, which significantly improves the utilization rate of phosphorus in wet phosphoric acid, reduces the amount of alkali used, reduces the production cost of iron phosphate, and results in an anhydrous iron phosphate with a low impurity content.

[0043] Referring to Figure 1, in the first embodiment, the embodiment of the present application is: Step S1 involves mixing wet phosphoric acid and water to obtain a phosphoric acid solution, adjusting the pH of the phosphoric acid solution to 2.0-3.0, then raising the temperature to mature it and obtain a matured slurry. Step S2 involves filtering the matured slurry to obtain a phosphate solution, then adding water and a primary oxidizing agent to obtain an aqueous phosphate solution containing the primary oxidizing agent. Step S3 involves mixing a ferrous sulfate solution with an aqueous phosphate solution containing a primary oxidizing agent to obtain a first slurry. Step S4 involves raising the temperature of the first slurry and allowing it to mature until the color of the first slurry changes, at which point the second oxidizing agent is added to the first slurry to obtain the second slurry. The present invention provides a method for producing iron phosphate with a low impurity content, comprising step S5, which involves separating the second slurry into solid and liquid components, and then subjecting the resulting solid material to washing, drying, and sintering in that order to obtain iron phosphate with a low impurity content.

[0044] Furthermore, in step S1 of this invention, a wet phosphoric acid sample is used as a raw material, and first an aluminum removal step is performed to obtain a matured slurry. Specifically, the method for producing an aqueous phosphate solution containing a primary oxidizing agent includes the steps of: mixing wet phosphoric acid and water to obtain a phosphoric acid solution in which the mass fraction of phosphorus is 5% to 10%; and adjusting the pH value of the phosphoric acid solution to 2.0 to 3.0, then raising the temperature to mature it and obtain a matured slurry.

[0045] Specifically, the mass fraction of phosphorus in the phosphoric acid solution may be 5%, 8%, 10%, etc. Examples of alkaline solutions used to adjust the pH of the phosphoric acid solution include, but are not limited to, aqueous ammonia and sodium hydroxide solution. By adjusting the amount of alkaline solution added, the pH of the system can be adjusted to 2.0, 2.5, 3.0, etc.

[0046] In some embodiments, the temperature for temperature-controlled aging is controlled to 85°C to 95°C, and the aging time is controlled to 1 to 4 hours. After adjusting the pH value to meet the requirements, the temperature is increased for aging, and the aging temperature can be controlled to 85°C, 90°C, 95°C, etc., and the aging time can be controlled to 1 hour, 2 hours, 3 hours, 4 hours, etc.

[0047] In step S2, the aged slurry is filtered to obtain a phosphate solution, and the primary oxidizing agent and water are added to the phosphate solution to adjust the phosphorus concentration and obtain an aqueous phosphate solution containing the primary oxidizing agent.

[0048] Furthermore, the pH of the phosphoric acid solution is adjusted to 2.0-3.0, and after heating, the difference in the solubility product of iron phosphate and aluminum phosphate is used to precipitate the aluminum first, thereby achieving the objective of removing the aluminum. After heating and maturation, the reaction slurry is separated into solid and liquid by means of filtration or other means to obtain an aluminum-containing cake and a phosphate solution. The aluminum-containing cake is subjected to other processes to recover the aluminum, and the phosphate solution, the first oxidizing agent, and water are mixed so that the phosphate concentration and the amount of first oxidizing agent used reliably meet the process requirements, thereby obtaining an aqueous phosphate solution containing the first oxidizing agent.

[0049] In the embodiments of this application, only aluminum impurities are separated from the wet phosphoric acid, and a large amount of Mg and Mn impurities are present in the wet phosphoric acid in ionic form. This avoids the consumption of phosphorus due to the formation of phosphates from the Mg and Mn impurities, thereby improving the utilization rate of phosphorus. Compared to the process scheme for producing monoammonium phosphate using wet phosphoric acid (pH value 4.0~5.0), in the embodiments of this application, the pH value of the phosphoric acid solution is adjusted to a lower value (2.0~3.0), thereby reducing the amount of aqueous ammonia used.

[0050] In some embodiments, the molar concentration of the phosphate in the aqueous phosphate solution containing the primary oxidizing agent may be 1.0 mol / L to 2.5 mol / L, for example, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, or 2.5 mol / L. By adjusting the concentrations of the phosphoric acid solution and the aqueous phosphate solution, the reaction rate during the synthesis of iron phosphate can be controlled, avoiding a reaction rate that is too fast and improving the uniformity of the product. The amount of primary oxidizing agent added is determined based on the content of ferrous ions in the ferrous sulfate solution, and the molar ratio of primary oxidizing agent in the aqueous phosphate solution to ferrous ions in the ferrous sulfate solution may be (0.7 to 0.9):2, for example, 0.7:2, 0.8:2, 0.9:2, etc. In other words, when producing an aqueous phosphate solution containing the primary oxidizing agent, the amount of primary oxidizing agent added is controlled to be 0.7 to 0.9 times the total amount required to completely oxidize divalent iron.

[0051] In some embodiments, the first oxidizing agent is selected from hydrogen peroxide, ammonium persulfate, and sodium persulfate, or any one of these, and all are applicable to the oxidation of ferrous ions.

[0052] Specifically, it is also necessary to provide a ferrous sulfate solution before step S3. In this application, the source of ferrous sulfate is not limited, but it may be a ferrous sulfate solution obtained by purifying a by-product of titanium dioxide production to remove impurities. This ferrous sulfate solution can be diluted by adding pure water. By adjusting the source of the ferrous sulfate solution, raw materials can be easily obtained and process costs can be reduced. Here, the molar concentration of divalent iron in the produced ferrous sulfate solution may be 0.5 mol / L to 1.5 mol / L, for example, 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, etc. By adjusting the concentration of divalent iron ions, the reaction rate of iron phosphate production can be controlled, which is advantageous in obtaining iron phosphate products with a more uniform particle size.

[0053] Specifically, in step S3, an aqueous phosphate solution containing the primary oxidizing agent may be added dropwise to the ferrous sulfate solution over 10 to 60 minutes, and after the addition is complete, the reaction may be continued for 10 to 60 minutes to allow the primary oxidizing agent to react sufficiently. Preferably, the ferrous sulfate solution is used as the bottom solution, and the aqueous phosphate solution containing the primary oxidizing agent is added dropwise to carry out the reaction. Adding the primary oxidizing agent slowly is advantageous for controlling the reaction rate and uniformly precipitating iron phosphate. Here, the dropwise addition time for the aqueous phosphate solution containing the primary oxidizing agent may be 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, etc., and the reaction duration after the addition is complete may be 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, etc.

[0054] In the first oxidation reaction, the amount of primary oxidizing agent used is 0.7 to 0.9 times the total amount needed to completely oxidize the ferrous iron. Therefore, the amount of primary oxidizing agent used is not sufficient to completely oxidize the ferrous iron ions, and the ferrous iron is completely oxidized in the subsequent second oxidation reaction.

[0055] Specifically, in step S4, the first slurry is heated and matured, and when the color of the first slurry changes, the second oxidizing agent is added dropwise to the first slurry to carry out the second oxidation reaction and obtain the second slurry. The reason why the color of the first slurry changes is mainly because iron phosphate changes from an amorphous phase to a crystalline state of iron phosphate dihydrate. In the embodiment of the present invention, since the oxidizing agent is added in two stages, a small amount of trivalent iron ions are present in the system during the color change, which suppresses the precipitation of impurity ions, eliminating the need for a complex impurity removal process throughout the entire process and significantly improving the utilization rate of phosphorus in wet phosphoric acid.

[0056] In some embodiments, the first and second oxidizing agents are each independently selected from hydrogen peroxide, ammonium persulfate, and sodium persulfate, and the first and second oxidizing agents may be the same or different, and in any case in which the above multiple oxidizing agents completely dissolve the divalent iron ions, the molar ratio of the oxidizing agents to the divalent iron ions is 1:2.

[0057] In some embodiments, the molar ratio of the oxidizing agent added dropwise to 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 primary and secondary oxidizing agents to the ferrous ions in the ferrous sulfate solution is controlled to (1.1-1.4):2. That is, the total amount of oxidizing agent used in the second oxidation reaction is slightly excessive, and the amount of secondary oxidizing agent added in the second oxidation reaction is 0.2-0.5 times the total amount required to completely oxidize the divalent iron. By more precisely controlling the reaction process, the introduction of manganese and magnesium elements into the iron phosphate product is reduced while completely converting the divalent iron to trivalent iron.

[0058] Specifically, the amount of oxidizing agent to be added dropwise to the first slurry can be calculated from the amount of ferrous ions in the ferrous sulfate solution produced, and the molar ratio of the oxidizing agent added dropwise to the first slurry to the ferrous ions in the oxidized ferrous sulfate solution can be controlled to 0.1:2, 0.2:2, 0.3:2, 0.4:2, 0.5:2, etc. In other words, the amount of oxidizing agent required for the second oxidation reaction can be calculated based on the theoretical amount of oxidizing agent needed to completely oxidize the ferrous ions in the ferrous sulfate solution. The amount of oxidizing agent added dropwise to the first slurry will be 0.2 to 0.5 times the theoretical amount.

[0059] Specifically, the total amount of the first and second oxidizing agents can be calculated from the amount of ferrous ions in the ferrous sulfate solution produced, and the molar ratio of the total amount of oxidizing agents added in two stages to the ferrous ions in the ferrous sulfate solution may be controlled to 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. In the embodiments of this application, the total amount of oxidizing agent used in the two-step reaction is controlled to 1.1 to 1.4 times the theoretical amount.

[0060] In some embodiments, the first slurry is heated and aged while controlling the aging temperature to 80°C to 95°C. When the first slurry changes from pale white to pale pink, the second oxidizing agent is added dropwise to the slurry using a peristaltic pump, and after the addition of the second oxidizing agent is complete, the slurry is kept warm for 30 to 90 minutes. The first slurry is heated and aged to cause a crystalline transformation from the amorphous phase to the crystalline state of iron dihydrate phosphate, thereby changing the color of the first slurry, and then the oxidation reaction is continued by adding the second oxidizing agent dropwise.

[0061] Specifically, when the first slurry is heated and matured, the maturation temperature may be controlled to 80°C, 85°C, 90°C, 95°C, etc., and the duration of heat retention after the addition of the second oxidizing agent may be 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, etc.

[0062] Specifically, in step S5, iron phosphate dihydrate may be separated from the second slurry, and iron phosphate with a low impurity content may be produced through processes such as impurity removal and sintering.

[0063] In some examples, the second slurry is subjected to solid-liquid separation, and the resulting solid material is subjected to washing, drying, and sintering in that order. Ions such as phosphate groups remaining on the surface are removed by washing, the washing water is removed by drying, and then sintered to obtain an anhydrous iron phosphate product.

[0064] In some embodiments, the washing can be countercurrent washing, which allows the solid material to come into complete contact with the washing water. As a result, ions such as phosphate groups remaining on the surface are completely removed by drying, the washing water is removed by drying, and the material is sintered to obtain an anhydrous iron phosphate product.

[0065] In some embodiments, the drying temperature is 90°C to 110°C, and the drying time is 2 hours to 12 hours. Within this range of drying temperature and drying time, the cleaning water on the surface of the solid material can be removed more completely without affecting the crystalline form of iron phosphate. Specifically, the drying temperature may be 90°C, 95°C, 100°C, 105°C, or 110°C, and the drying time may be 2 hours, 5 hours, 8 hours, 10 hours, or 12 hours.

[0066] In some embodiments, the sintering temperature is 550°C to 650°C, and the sintering time is 1 to 8 hours. Within this range of sintering temperature and time, crystal water is lost from iron phosphate dihydrate, and anhydrous iron phosphate products can be obtained. Specifically, the sintering temperature may be 550°C, 600°C, 650°C, etc., and the sintering time may be 1 hour, 3 hours, 5 hours, 8 hours, etc.

[0067] According to the above analysis, as shown in Figure 2, the main steps of the manufacturing method according to this application are as follows: Add water to wet phosphoric acid and dissolve to obtain a phosphoric acid solution. Add ammonia water dropwise to the phosphoric acid solution to adjust the pH value until the requirements are met, then raise the temperature and age to obtain an aluminum precipitate slurry. Separate the aluminum precipitate slurry into solid and liquid to obtain an aluminum-containing cake and a purified phosphate solution. Mix the purified phosphate solution with a first oxidizing agent to obtain an aqueous phosphate solution containing hydrogen peroxide. Add the aqueous phosphate solution containing hydrogen peroxide dropwise to the prepared ferrous sulfate solution to carry out the first oxidation reaction and obtain a first slurry. Raise the temperature of the first slurry and age until it changes color, then add a second oxidizing agent dropwise to carry out the second oxidation reaction and obtain a second slurry. Separate the second slurry into solid and liquid and wash to obtain iron phosphate dihydrate. Dry and sinter the iron phosphate dihydrate to obtain an anhydrous iron phosphate product with low impurity content.

[0068] In a second embodiment, the embodiment of the present application provides iron phosphate with a low impurity content, manufactured by the above manufacturing method, having an impurity element Mg content of 40 ppm or less and an impurity element Mn content of 60 ppm or less. By optimizing the iron phosphate manufacturing process and suppressing the precipitation of impurities such as Mg and Mn through a two-stage oxidation process, the utilization rate of phosphorus in wet phosphoric acid is greatly improved, and the anhydrous iron phosphate produced has a low impurity content.

[0069] "Low impurity content" refers to a low level of impurities in iron phosphate products. The main impurities are Mg and Mn, with Mg content being 40 ppm or less and Mn content being 60 ppm or less. In addition to Mg and Mn, iron phosphate products also contain other impurities such as Na, Ni, and Pb, but their content is small, usually less than 12 ppm.

[0070] In a third aspect, the embodiments of the present application provide lithium iron phosphate cathode materials produced using the above-mentioned low-impurity iron phosphate as a precursor. As an example, lithium iron phosphate cathode materials are produced by ball milling, drying, and roasting using the above-mentioned low-impurity iron phosphate, lithium salt, and carbon source as raw materials.

[0071] Due to its low impurity content in iron phosphate, it helps improve the purity of lithium iron phosphate cathode material products, which contributes to obtaining better electrochemical properties.

[0072] In a fourth aspect, an embodiment of the present application includes a positive electrode current collector and a positive electrode active layer provided on at least one side of the positive electrode current collector, wherein the positive electrode active layer provides a positive electrode plate containing the lithium iron phosphate positive electrode material described above. By improving the electrochemical properties of the lithium iron phosphate positive electrode material, the positive electrode plate is given excellent electrochemical properties.

[0073] In a fifth aspect, the embodiment of the present application provides a secondary battery that includes the above-mentioned positive electrode plate and further includes a negative electrode plate, electrolyte, separator, etc., which constitute a complete battery structure. Based on the excellent electrochemical properties of the positive electrode plate, the electrochemical properties of the secondary battery product can be further improved.

[0074] The following are some specific examples, but these examples are illustrative and are for interpretation purposes only and should not be understood as limiting the present application. Where specific techniques or conditions are not specified in the examples, they should be carried out in accordance with the techniques or conditions described in the relevant art literature or in accordance with the product description. Unless otherwise specified, the reagents or equipment used are all common products available commercially.

[0075] Example 1 This embodiment provides a method for producing iron phosphate with a low impurity content, comprising the following steps.

[0076] (1) Preparation of ferrous sulfate solution: A ferrous sulfate solution obtained by purifying a by-product of titanium dioxide production to remove impurities was collected, and pure water was added to it to prepare a ferrous sulfate solution with a molar concentration of 1.0 mol / L for use.

[0077] (2) Preparation of purified phosphate solution: 484.49 g of wet phosphoric acid sample (mass fraction 78.25%, specific composition shown in Table 1) was mixed with pure water to prepare a phosphoric acid solution with a mass fraction of 8.0% phosphorus. Ammonia water was added dropwise to the phosphoric acid solution while stirring to adjust the pH of the system to 2.5. After the pH adjustment was complete, the reaction solution was heated to 90°C and aged for 2.0 hours. After aging was complete, the slurry was filtered to obtain a purified phosphate solution.

[0078] The cake was baked and weighed, and its mass was 31.48g. Detection revealed a phosphorus content of 20.88%. The phosphorus loss, calculated as the total amount of phosphorus removed by the cake, was found to be 5.48%.

[0079] (3) Preparation of a phosphate aqueous solution containing the primary oxidizing agent: To the purified phosphate solution from step (2), pure water and hydrogen peroxide were added so that the molar ratio of the amount of hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution from step (1) was 0.84:2 (i.e., the ratio of hydrogen peroxide added was 0.84 times the total amount required to completely oxidize the divalent iron), to prepare a phosphate solution with a molar concentration of phosphorus of 2.0 mol / L for use.

[0080] (4) First oxidation reaction: Stirring in the reaction vessel was started, 200 mL of the ferrous sulfate solution from step (1) was used as the bottom liquid, and 100 mL of the aqueous phosphate solution containing the first oxidizing agent from step (3) was added dropwise to the reaction vessel over 30 minutes using a peristaltic pump. After the addition was complete, the reaction was continued for 30 minutes to obtain the first slurry.

[0081] (5) Second oxidation reaction: The first slurry from step (4) was heated and aged at a set temperature of 90°C. When the reaction slurry changed from pale white to pale pink, hydrogen peroxide was continued to be added dropwise to the slurry using a peristaltic pump until the molar ratio of hydrogen peroxide to ferrous ions in the ferrous sulfate solution was 0.36:2 (i.e., the ratio of hydrogen peroxide added was 0.36 times the total amount required to completely oxidize the divalent iron). After the addition was complete, the temperature was maintained for 60 minutes to obtain the second slurry.

[0082] (6) Production of anhydrous iron phosphate: The second slurry from step (5) was filtered, washed in a countercurrent, dried at 95°C for 6 hours, and sintered at 600°C for 4 hours to obtain anhydrous iron phosphate.

[0083] Example 2 Compared to Example 1, the only difference is the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide (i) added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.7:2, and in step (5), the molar ratio of the dropwise added hydrogen peroxide (ii) to the ferrous ions in the ferrous sulfate solution is 0.5:2.

[0084] Example 3 Compared to Example 1, the only difference is the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide (i) added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.9:2, and in step (5), the molar ratio of the hydrogen peroxide (ii) added dropwise to the ferrous ions in the ferrous sulfate solution is 0.3:2.

[0085] Example 4 Compared to Example 1, the only difference is the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide (i) added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.8:2, and in step (5), the molar ratio of the hydrogen peroxide (ii) added dropwise to the ferrous ions in the ferrous sulfate solution is 0.4:2.

[0086] Example 5 This embodiment provides a method for producing iron phosphate with a low impurity content, comprising the following steps.

[0087] (1) Preparation of ferrous sulfate solution: A ferrous sulfate solution obtained by purifying a by-product of titanium dioxide production to remove impurities was collected, and pure water was added to it to prepare a ferrous sulfate solution with a molar concentration of 0.5 mol / L for use.

[0088] (2) Preparation of purified phosphate solution: 484.49 g of wet phosphoric acid sample (mass fraction 78.25%, specific composition shown in Table 1) was mixed with pure water to prepare a phosphoric acid solution with a mass fraction of 5.0% phosphorus. Ammonia water was added dropwise to the phosphoric acid solution while stirring to adjust the pH of the system to 2.0. After the pH adjustment was complete, the reaction solution was heated to 85°C and aged for 4.0 hours. After aging was complete, the slurry was filtered to obtain a purified phosphate solution.

[0089] (3) Preparation of a phosphate aqueous solution containing the primary oxidizing agent: To the phosphate solution purified in step (2), pure water and hydrogen peroxide were added so that the molar ratio of hydrogen peroxide to ferrous ions in the ferrous sulfate solution from step (1) was 0.7:2 (i.e., the amount of hydrogen peroxide added was 0.7 times the total amount required to completely oxidize the divalent iron), to prepare a phosphate solution with a molar phosphorus concentration of 1.0 mol / L for use.

[0090] (4) First oxidation reaction: Stirring in the reaction vessel was started, and 200 mL of the ferrous sulfate solution from step (1) was used as the bottom liquid. 100 mL of the phosphate aqueous solution containing the first oxidizing agent from step (3) was added dropwise to the reaction vessel over 10 minutes using a peristaltic pump. After the addition was complete, the reaction was continued for 60 minutes to obtain the first slurry.

[0091] (5) Second oxidation reaction: The slurry in which the reaction was completed in step (4) was heated and aged at a set temperature of 80°C. When the reaction slurry changed from pale white to pale pink, hydrogen peroxide was continued to be added dropwise to the slurry using a peristaltic pump so that the molar ratio of hydrogen peroxide added dropwise to ferrous ions in the ferrous sulfate solution was 0.5:2 (i.e., the ratio of hydrogen peroxide added was 0.5 times the total amount required to completely oxidize the divalent iron). After the addition was complete, the temperature was maintained for 30 minutes to obtain the second slurry.

[0092] (6) Production of anhydrous iron phosphate: The second slurry from step (5) was filtered, washed in a countercurrent, dried at 90°C for 12 hours, and sintered at 550°C for 8 hours to obtain anhydrous iron phosphate.

[0093] Example 6 This embodiment provides a method for producing iron phosphate with a low impurity content, comprising the following steps.

[0094] (1) Preparation of ferrous sulfate solution: A ferrous sulfate solution obtained by purifying a by-product of titanium dioxide production to remove impurities was collected, and pure water was added to it to prepare a ferrous sulfate solution with a molar concentration of 1.5 mol / L for use.

[0095] (2) Preparation of purified phosphate solution: 484.49 g of wet phosphoric acid sample (mass fraction 78.25%, specific composition shown in Table 1) was mixed with pure water to prepare a phosphoric acid solution with a mass fraction of 10.0% phosphorus. Sodium hydroxide solution was added dropwise to the phosphoric acid solution while stirring to adjust the pH of the system to 3.0. After the pH adjustment was complete, the reaction solution was heated to 95°C and aged for 1.0 hour. After aging was complete, the slurry was filtered to obtain a purified phosphate solution.

[0096] (3) Preparation of a phosphate aqueous solution containing the primary oxidizing agent: To the phosphate solution purified in step (2), pure water and hydrogen peroxide (i) were added so that the molar ratio of the amount of hydrogen peroxide added to the ferrous ions in the ferrous sulfate solution from step (1) was 0.9:2 (i.e., the amount of hydrogen peroxide added was 0.9 times the total amount required to completely oxidize the divalent iron), to prepare a phosphate solution with a molar concentration of phosphorus of 2.5 mol / L for use.

[0097] (4) First oxidation reaction: Stirring in the reaction vessel was started, and 200 mL of the ferrous sulfate solution from step (1) was used as the bottom liquid. 100 mL of the phosphate aqueous solution containing the first oxidizing agent from step (3) was added dropwise to the reaction vessel over 60 minutes using a peristaltic pump. After the addition was complete, the reaction was continued for 10 minutes to obtain the first slurry.

[0098] (5) Second oxidation reaction: The slurry in which the reaction was completed in step (4) was heated and aged at a set temperature of 95°C. When the reaction slurry changed from pale white to pale pink, hydrogen peroxide was continued to be added dropwise to the slurry using a peristaltic pump so that the molar ratio of hydrogen peroxide added dropwise to ferrous ions in the ferrous sulfate solution was 0.3:2 (i.e., the ratio of hydrogen peroxide added was 0.3 times the total amount required to completely oxidize the divalent iron). After the addition was complete, the temperature was maintained for 90 minutes to obtain the second slurry.

[0099] (6) Production of anhydrous iron phosphate: The second slurry from step (5) was filtered, washed in a countercurrent, dried at 110°C for 2 hours, and sintered at 650°C for 1 hour to obtain anhydrous iron phosphate.

[0100] Example 7 Compared to Example 1, the only difference is the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide (i) 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 dropwise added hydrogen peroxide (ii) to the ferrous ions in the ferrous sulfate solution is 0.2:2.

[0101] Example 8 Compared to Example 1, only the amount of hydrogen peroxide added in steps (3) and (5) is different. Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide (i) added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.9:2, and in step (5), the molar ratio of the hydrogen peroxide (ii) added dropwise to the ferrous ions in the ferrous sulfate solution is 0.5:2.

[0102] Comparative Example 1 In comparison to Example 1, the addition of hydrogen peroxide to the phosphate was carried out by a conventional one-step method, and the addition ratio of hydrogen peroxide was 1.2 times the total amount required to completely oxidize the divalent iron, thus ensuring the complete oxidation of the divalent iron in the system. The remaining steps were the same as in the example. The specific steps are as follows:

[0103] (1) Preparation of ferrous sulfate solution: The same procedure as in Example 1.

[0104] (2) Preparation of purified phosphate solution: The same procedure as in Example 1.

[0105] (3) Preparation of phosphate: To the phosphate solution purified in step (2), pure water and hydrogen peroxide (i) were added (the ratio of hydrogen peroxide added was 1.20 times the total amount required to completely oxidize the divalent iron) to prepare a phosphate solution with a phosphorus molar concentration of 2.0 mol / L for use.

[0106] (4) Synthesis of iron phosphate: Stirring in the reaction vessel was started, and 100 mL of the prepared phosphate was added dropwise to the reaction vessel using a peristaltic pump over 30 minutes, with 200 mL of the ferrous sulfate solution from step (1) as the bottom liquid. After the dropwise addition was complete, the reaction was continued for 30 minutes.

[0107] (5) Crystallization: The slurry in which the reaction was completed in step (4) was heated, and the aging temperature was set to 90°C for aging. When the reaction slurry changed from pale white to pale pink, the temperature was maintained for 60 minutes.

[0108] (6) Preparation of anhydrous iron phosphate: The slurry from step (5) in which the reaction was completed was filtered, washed in a countercurrent, dried at 95°C for 6 hours, and sintered at 600°C for 4 hours to obtain anhydrous iron phosphate.

[0109] Comparative Example 2 In the comparative example, conventional methods for removing impurities from wet phosphoric acid were used; that is, first, the wet phosphoric acid was adjusted to pH 7.0 and filtered, and then the pH of the phosphate was adjusted to 2.0-3.0 using purified phosphoric acid. The addition of hydrogen peroxide to the phosphate was carried out by a conventional one-step method, and the addition ratio of hydrogen peroxide was 1.2 times the total amount required to completely oxidize the divalent iron, thereby ensuring the complete oxidation of the divalent iron in the system. The remaining steps were the same as in the example. The specific steps are as follows.

[0110] (1) Preparation of ferrous sulfate solution: A ferrous sulfate solution obtained by purifying a by-product of titanium dioxide production to remove impurities was collected, and pure water was added to it to prepare a ferrous sulfate solution with a molar concentration of 1.0 mol / L of divalent iron for use.

[0111] (2) Preparation of purified phosphate solution: 484.49 g of wet phosphoric acid sample (mass fraction 78.25%) was mixed with pure water to prepare a phosphoric acid solution with a mass fraction of 8.0% phosphorus. Ammonia water was added dropwise to the phosphoric acid solution while stirring to adjust the pH of the system to 7.0. After the pH adjustment was complete, the reaction solution was heated to 90°C and aged for 2.0 hours. After aging was complete, the slurry was filtered, and the pH of the filtrate was further adjusted to 2.5 with purified phosphoric acid to obtain a purified phosphate solution.

[0112] The cake was baked and weighed, and its mass was 73.01 g. Detection revealed a phosphorus content of 22.90%. The phosphorus loss, calculated as the total amount of phosphorus removed by the cake, was found to be 13.95%.

[0113] (3) Preparation of phosphate: To the phosphate solution purified in step (2), pure water and hydrogen peroxide were added (the ratio of hydrogen peroxide added was 1.20 times the total amount to completely oxidize the divalent iron) to prepare a phosphate solution with a phosphorus molar concentration of 2.0 mol / L for use.

[0114] (4) Iron phosphate synthesis: Stirring in the reaction vessel was started, and 200 mL of the ferrous sulfate solution from step (1) was used as the bottom liquid. 100 mL of the prepared phosphate was added dropwise to the reaction vessel over 30 minutes using a peristaltic pump, and the reaction was continued for 30 minutes after the addition was complete.

[0115] (5) Crystallization: The slurry in which the reaction was completed in step (4) was heated, and the aging temperature was set to 90°C for aging. When the reaction slurry changed from pale white to pale pink, the temperature was maintained for 60 minutes.

[0116] (6) Preparation of anhydrous iron phosphate: The slurry from step (5) in which the reaction was completed was filtered, washed in a countercurrent, dried at 95°C for 6 hours, and sintered at 600°C for 4 hours to obtain anhydrous iron phosphate.

[0117] Comparative Example 3 Compared to Example 1, the only difference is the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide (i) added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.95:2, and in step (5), the molar ratio of the hydrogen peroxide (ii) added dropwise to the ferrous ions in the ferrous sulfate solution is 0.1:2.

[0118] Comparative Example 4 Compared to Example 1, the only difference is the amount of hydrogen peroxide added in steps (3) and (5). Specifically, in step (3) of this example, the molar ratio of the amount of hydrogen peroxide (i) added to the ferrous ions in the ferrous sulfate solution in step (1) is 0.6:2, and in step (5), the molar ratio of the hydrogen peroxide (ii) added dropwise to the ferrous ions in the ferrous sulfate solution is 0.85:2.

[0119] In this application, the composition of the phosphate purified in step (2) of the examples and comparative examples was tested, and the results of calculating the elemental removal rate and phosphorus loss rate are shown in Table 2. The composition and content of impurity elements of the anhydrous iron phosphate prepared in the examples and comparative examples were tested, and the results are shown in Table 3.

[0120] Table 1: Table for detecting the main components of wet phosphoric acid [Table 1] From the detection results of wet phosphoric acid in the raw materials shown in Table 1, the main impurity components are Al, Mg, and Mn.

[0121] Table 2: Table of data for detecting phosphorus loss and elements of phosphates produced in the examples and comparative examples. [Table 2] TIFF2026515322000004.tif12170

[0122] Table 3: Table of data for detecting impurity elements in anhydrous iron phosphate produced in the examples and comparative examples. [Table 3] The results of the scanning electron microscope (SEM) tests of iron phosphate in Example 1 are shown in Figure 3, the results of the SEM in Example 2 are shown in Figure 4, the results of the SEM in Example 3 are shown in Figure 5, the results of the SEM in Example 4 are shown in Figure 6, the results of the SEM in Example 5 are shown in Figure 7, the results of the SEM in Example 6 are shown in Figure 8, the results of the SEM in Example 7 are shown in Figure 9, the results of the SEM in Example 8 are shown in Figure 10, and the results of the SEM in Comparative Example 4 are shown in Figure 11.

[0123] Comparing the scanning electron microscope test results for Examples 1-8 with those for Comparative Example 4, the iron phosphate obtained in Examples 1-8, which had a low impurity content, was in a flake-like form. In Comparative Example 4, too much oxidizing agent was added, slightly increasing the impurity ion content. This significantly altered the microstructure of the iron phosphate obtained in Comparative Example 4, causing it to become lumpy and consequently degrading its performance.

[0124] Table 2 shows that the method of the present invention can better remove Al impurities, achieving a removal efficiency of 97.78%. Furthermore, because Mg and Mn impurities are retained in the liquid phase in ionic form, the phosphorus loss rate decreases from 13.95% to 5.48%, resulting in a savings of 8.47%. This significantly reduces the production cost of iron phosphate, making it economically superior.

[0125] Table 3 shows that using the stepwise oxidation scheme according to the examples of this application significantly reduces the content of Mg and Mn impurities in anhydrous iron phosphate compared to the conventional one-step method for producing iron phosphate. The Mg content decreased from 91.4 ppm to 23.2 ppm, and the Mn content decreased from 233.29 ppm to 56.47 ppm. This indicates that using the stepwise oxidation scheme according to the examples of this application can significantly reduce the content of Mg and Mn in anhydrous iron phosphate.

[0126] From the corresponding results in Comparative Example 3 of Table 3, it was found that if the amount of oxidizing agent added is too small, the content of anhydrous iron phosphate impurity ions Mg and Mn increases significantly.

[0127] As described above, using the directional impurity removal-stepwise oxidation scheme of this invention significantly reduces the loss of phosphorus resources and lowers the production cost of iron phosphate. Furthermore, by adopting a stepwise oxidation scheme, the problem of high Mg and Mn impurity content in the conventional one-step iron phosphate production process is more effectively resolved, expanding the application scenarios of the one-step iron phosphate synthesis process, and thereby making the produced iron phosphate more suitable for the production of high-performance lithium iron phosphate.

[0128] Furthermore, this application is not limited to the embodiments described above. The embodiments described above are merely illustrative, and any embodiment that has substantially the same configuration as the technical concept and exhibits the same effects within the scope of the technical solutions of this application is included within the scope of this application. In addition, other forms constructed by adding various modifications to the embodiments that a person skilled in the art could conceive, or by combining some of the components of the embodiments, are also included within the scope of this application, as long as they do not depart from the spirit of this application.

Claims

1. The process involves mixing wet phosphoric acid with water to obtain a phosphoric acid solution, adjusting the pH of the phosphoric acid solution to 2.0-3.0, then raising the temperature to mature it and obtain a matured slurry. The aging slurry is filtered to obtain a phosphate solution, and then water and a first oxidizing agent are added to obtain an aqueous phosphate solution containing the first oxidizing agent. The steps include: mixing a ferrous sulfate solution with an aqueous phosphate solution containing the first oxidizing agent to obtain a first slurry; The first slurry is heated and matured until the color of the first slurry changes, then a second oxidizing agent is added to the first slurry to obtain a second slurry. A method for producing iron phosphate with a low impurity content, comprising the steps of: separating the second slurry from the solid-liquid material, and subjecting the obtained solid material to washing, drying, and sintering in that order to obtain iron phosphate with a low impurity content.

2. The manufacturing method according to claim 1, characterized in that the first oxidizing agent and the second oxidizing agent are both independently selected from hydrogen peroxide, ammonium persulfate, and sodium persulfate.

3. The molar ratio of the first oxidizing agent to the ferrous ions in the ferrous sulfate solution is (0.7 to 0.9):2, and / or The molar ratio of the second oxidizing agent dropped into the first slurry to the ferrous ions in the ferrous sulfate solution is (0.2 to 0.5):

2. The manufacturing method according to claim 2, characterized in that 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 to 1.4):

2.

4. The step of obtaining the aged slurry from the wet phosphoric acid is: The steps include: mixing wet phosphoric acid with water to obtain a phosphoric acid solution in which the mass fraction of phosphorus is 5% to 10%; The manufacturing method according to claim 1, characterized by comprising the step of adjusting the pH value of the phosphoric acid solution to 2.0 to 3.0, then raising the temperature to mature it and obtain a matured slurry.

5. The manufacturing method according to claim 4, characterized in that the temperature rise and maturation is at a temperature of 85°C to 95°C and for a time of 1 hour to 4 hours.

6. The manufacturing method according to claim 4, characterized in that the phosphate aqueous solution containing the first oxidizing agent has a molar concentration of phosphate of 1.0 mol / L to 2.5 mol / L.

7. The time for dropping the aqueous phosphate solution containing the first oxidizing agent into the ferrous sulfate solution is 10 min to 60 min. The manufacturing method according to claim 1, characterized in that the reaction is sustained for 10 min to 60 min after the dropwise addition is complete.

8. The manufacturing method according to claim 1, characterized in that the step of obtaining the second slurry from the first slurry includes raising the temperature of the first slurry and aging it at a maturation temperature of 80°C to 95°C, and when the color of the first slurry changes, dropping the second oxidizing agent onto the first slurry, and then keeping the first slurry warm for 30 min to 90 min to obtain the second slurry.

9. The cleaning is countercurrent cleaning, and / or The drying temperature is 90°C to 110°C, the drying time is 2 hours to 12 hours, and / or The manufacturing method according to claim 1, characterized in that the sintering temperature is 550°C to 650°C and the sintering time is 1 hour to 8 hours.

10. The raw materials for the ferrous sulfate solution are by-products from the production of titanium dioxide, and / or The manufacturing method according to claim 1, characterized in that the molar concentration of the ferrous sulfate solution is 0.5 mol / L to 1.5 mol / L.

11. Iron phosphate with low impurity content, manufactured by the manufacturing method described in any one of claims 1 to 10, characterized in that the content of the impurity element Mg is 40 ppm or less and the content of the impurity element Mn is 60 ppm or less.

12. A lithium iron phosphate cathode material characterized by being manufactured using the low impurity content of iron phosphate described in claim 11 as a precursor.

13. A positive electrode plate comprising a positive electrode current collector and a positive electrode active layer provided 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 described in claim 12.

14. A secondary battery characterized by including the positive electrode plate described in claim 13.

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