Preparation method of low-impurity battery-grade iron phosphate

Abstract

The invention belongs to the field of lithium ion battery material preparation, and provides a preparation method of low-impurity battery grade iron phosphate, which comprises the following steps: dissolving titanium dioxide ferrous sulfate in water to obtain a solution A, adjusting the pH value of the solution A to 3.0-4.0, reacting, and carrying out solid-liquid separation to obtain a clear liquid; adjusting the pH value of the clear liquid, and then mixing the clear liquid with sodium nitrite to obtain a solution B; adding the solution B into a reaction kettle, introducing compressed oxygen until the pressure in the kettle body is 5-15 bar, reacting until the pressure in the kettle body is reduced to a constant value, and carrying out solid-liquid separation to obtain a purified iron source solution; preparing iron phosphate dihydrate by taking the purified iron source solution and a phosphorus source as raw materials through a precipitation method; and calcining the iron phosphate dihydrate to obtain the low-impurity battery-grade iron phosphate. The preparation method is simple and feasible in process, low in cost, wide in source of required raw materials, good in safety, low in cost, easy to realize large-scale production, good in impurity removal effect and low in iron loss, and the Mn content of the battery-grade iron phosphate prepared from the impurity-removed ferrous sulfate can be as low as 10 ppm or below.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery material preparation, and relates to a method for preparing low-impurity battery-grade iron phosphate, and specifically to a method for preparing low-impurity battery-grade iron phosphate by using titanium dioxide by-product ferrous sulfate as a raw material. Background Art

[0002] As the most commonly used precursor material for lithium iron phosphate in power batteries, iron phosphate is usually synthesized from high-purity divalent or trivalent iron salts, which are relatively expensive. Currently, purifying ferrous sulfate produced as a byproduct of titanium dioxide and using it as an iron source for the preparation of iron phosphate has become one of the mainstream production methods for iron phosphate to reduce production costs.

[0003] The preparation of iron phosphate using ferrous sulfate, a by-product of titanium dioxide, as raw material includes the purification, impurity removal and oxidation process of the iron source. Although the current existing process can produce battery-grade iron phosphate, as the quality and performance requirements of the positive electrode material lithium iron phosphate continue to increase, the impurity content of battery-grade iron phosphate prepared by the existing process is still relatively high, usually in the range of tens to hundreds of ppm, which is difficult to meet high quality requirements.

[0004] At present, most of the impurities are removed by chemical precipitation. Generally, in the initial stage of the process, a pH regulator (mostly alkaline substances such as iron powder) is added to increase the system pH of the iron salt solution prepared from the by-product ferrous sulfate of titanium dioxide, so that the impurity ions form corresponding hydroxides and precipitate. Finally, the precipitate is separated from the iron salt solution by physical methods to achieve the purpose of impurity removal. However, when pH = 6.5 (initial concentration is 1 mol / L), Fe(OH)2 begins to precipitate. Therefore, there is an upper limit to the increase in the pH of the raw material system, while the pH of the hydroxide corresponding to the impurities Mn and Mg is higher than 6.5. If the pH is further increased by ammonia water or liquid alkali for impurity removal, the iron loss caused will also lead to a further increase in production costs. In addition, excessive content of metal impurities manganese and magnesium will not only cause a decrease in the specific capacity and energy density of the positive electrode material, but also damage the life and safety performance of the manufactured battery.

[0005] The oxidation process usually uses a reagent with strong oxidizing properties. Hydrogen peroxide is widely used in the iron source oxidation process because it has strong oxidizing properties and does not produce polluting media after the reaction, which can effectively protect the environment. However, although hydrogen peroxide itself is non-flammable, it can undergo redox reactions with combustibles and reducing agents to release a large amount of heat and oxygen, thereby causing combustion and explosion; in addition, once the human body inhales excessive hydrogen peroxide vapor or mist, it will have a strong irritating effect on the respiratory tract, and long-term contact may also cause contact dermatitis and other diseases. In recent years, with the increasingly stringent national security requirements and the increase in the price of hydrogen peroxide, the large-scale use of hydrogen peroxide as a raw material will bring greater economic risks and safety hazards to enterprises. Summary of the invention

[0006] In view of the above technical problems, the purpose of the present invention is to provide a method for preparing battery-grade iron phosphate with low impurities.

[0007] To achieve the above purpose, the present invention proposes the following solutions: Provide a method for preparing battery-grade iron phosphate with low impurities, including: S1. Dissolve titanium white ferrous sulfate in water to obtain solution A, adjust the pH value of solution A to 3.0 - 4.0, and after reaction, perform solid-liquid separation to obtain a clear liquid; S2. Adjust the pH of the clear liquid to 0.2 - 2, then mix the clear liquid with sodium nitrite to obtain solution B; add solution B to a reaction kettle, and introduce compressed oxygen into the reaction kettle until the pressure in the kettle body is 5 - 15 bar, and carry out the reaction. Wait until the pressure in the kettle drops to a constant value, and perform solid-liquid separation to obtain a purified iron source solution; S3. Use the purified iron source solution and a phosphorus source as raw materials to prepare iron phosphate dihydrate by a precipitation method; specifically including: (1) Mix the phosphorus source solution with a pH regulator to adjust the pH value of the phosphorus source solution to 4.0 - 6.5; dropwise add the iron source solution to the phosphorus source solution and carry out the reaction to obtain a synthesis slurry; (2) Filter and wash the synthesis slurry, then add water to make a slurry to obtain a cleaning slurry; mix the cleaning slurry with phosphoric acid and perform crystallization to obtain an iron phosphate dihydrate slurry; (3) Wash the iron phosphate dihydrate slurry, perform solid-liquid separation and drying to obtain iron phosphate dihydrate; S4. Calcinate the iron phosphate dihydrate to obtain battery-grade iron phosphate with low impurities.

[0008] Further, the sodium nitrite is mixed with the clear liquid in the form of a sodium nitrite solution.

[0009] Further, the mass concentration of sodium nitrite in solution B is 0.06% - 0.13%.

[0010] Further, in step S1, the temperature of the reaction is 40 - 50 °C.

[0011] Further, in step S1, the pH value of solution A is adjusted to 3.0 - 4.0 by adding iron powder or FeO powder, preferably 3.0 - 3.5.

[0012] Further, in step S1, the concentration of ferrous ions in solution A is 70 - 120 g / L.

[0013] Further, in step S2, before mixing the clear liquid with the sodium nitrite solution, adjust the pH value of the clear liquid to 0.8 - 1.2.

[0014] Further, in step S2, the temperature of the reaction is 40-50 °C.

[0015] Further, in step S2, compressed oxygen is introduced into the reaction kettle until the pressure in the kettle body is 10-15 bar.

[0016] Further, in step (1), the temperature of the reaction is 50-60 °C; after the iron source solution is added dropwise, it is kept warm for 30-60 min.

[0017] Further, in step (2), the temperature of the crystallization is 90-95 °C; the duration of the crystallization is 180-300 min.

[0018] Further, in step (3), the phosphoric acid is determined according to the molar ratio of P in the phosphoric acid to Fe in the cleaning slurry being 0.05-0.20.

[0019] Further, in the low-impurity battery-grade iron phosphate, the Mn content is not higher than 15 ppm, the Al content is not higher than 40 ppm, the Mg content is not higher than 15 ppm, and the Ti content is not higher than 20 ppm.

[0020] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects: The provided preparation method of low-impurity battery-grade iron phosphate is simple and easy to implement, with low cost. Using titanium white ferrous sulfate as the iron source, the required raw materials have wide sources, good safety, and low cost, and are easy to realize large-scale production. The impurity removal effect is good and the iron loss is small. The battery-grade iron phosphate with extremely low impurity content is prepared from the ferrous sulfate after impurity removal to meet the requirements of high-quality cathode materials for iron phosphate.

[0021] In the provided preparation method of low-impurity battery-grade iron phosphate, the Mn content in the obtained low-impurity battery-grade iron phosphate product can be as low as below 10 ppm, which can meet the requirements of ultra-high-quality cathode materials for iron phosphate, and the iron source utilization rate can be as high as over 98%. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0023] Figure 1 It is a process flow chart for the preparation of battery-grade iron phosphate. Detailed Embodiments

[0024] In view of the problems that the battery-grade iron phosphate prepared from titanium white ferrous sulfate has relatively high impurity content and is difficult to meet the composition requirements of high-quality cathode materials, or the iron loss content is too high during the impurity removal process of titanium white ferrous sulfate, or the impurity removal process is complex and the production cost is high, etc., through research, the applicant has developed a method for preparing battery-grade iron phosphate with low impurities from titanium white ferrous sulfate. This method can deeply remove impurities such as Mn, Al, and Ti while controlling the reduction of iron loss, and then use the purified ferrous sulfate as a raw material to prepare battery-grade iron phosphate with extremely low impurity content.

[0025] Some embodiments of the present invention provide a method for preparing battery-grade iron phosphate with low impurities, including: S1. Dissolve titanium white ferrous sulfate in water to obtain solution A, adjust the pH value of solution A to 3.0 - 4.0, and after reaction, perform solid-liquid separation to obtain a clear liquid; S2. Adjust the pH of the clear liquid to 0.2 - 2, then mix the clear liquid with sodium nitrite to obtain solution B; add solution B to a reaction kettle, and introduce compressed oxygen into the reaction kettle until the pressure in the kettle body is 5 - 15 bar, and carry out the reaction. Wait until the pressure in the kettle drops to a constant value, and perform solid-liquid separation to obtain a purified iron source solution; S3. Use the purified iron source solution and a phosphorus source as raw materials to prepare iron phosphate dihydrate by a precipitation method; specifically including: (1) Mix the phosphorus source solution with a pH regulator to adjust the pH value of the phosphorus source solution to 4.0 - 6.5; dropwise add the iron source solution to the phosphorus source solution and carry out the reaction to obtain a synthetic slurry; (2) Filter and wash the synthetic slurry, then add water to make a slurry to obtain a washed slurry; mix the washed slurry with phosphoric acid and perform crystallization to obtain an iron phosphate dihydrate slurry; (3) Perform solid-liquid separation and drying on the iron phosphate dihydrate slurry to obtain iron phosphate dihydrate; S4. Calcinate the iron phosphate dihydrate to obtain battery-grade iron phosphate with low impurities.

[0026] The main metal ion impurities in titanium white ferrous sulfate include Ti, Mg, Mn, and Al, and they all mainly exist in the form of sulfates in titanium white ferrous sulfate. In this application, first, iron powder and H +The reaction indirectly regulates the pH value to 3.0 - 4.0. Titanium and aluminum are removed by the hydrolysis of Ti ions and aluminum ions. At the same time, iron powder can reduce ferric iron in the raw materials, reducing iron loss. In titanium white ferrous sulfate, Mg and Mn exist in the form of divalent sulfates, and their initial precipitation pH values are much higher than the pH value range for the synthesis of conventional iron phosphate. In theory, both of them can be completely eluted during the washing process of iron phosphate. The applicant's research found that in actual operation, manganese ions are often more difficult to be eluted than magnesium ions. This is because the Mn ion has a larger radius and a relatively lower charge density, so the formed hydrated ion radius is smaller. During the reaction, the ion with a smaller hydrated radius (Mn ion) binds more tightly to the precipitation product, while the ion with a larger hydrated radius (such as Mg ion) binds weakly due to steric hindrance effect and is thus more easily eluted. Therefore, in the actual production process, Mg ions can often be removed by washing, while Mn ions cannot be washed to a lower level by washing and need to be removed in the iron source solution. Through research, the applicant reacts the iron source solution with sodium nitrite solution under certain pressure and by introducing compressed oxygen, which not only oxidizes ferrous ions to obtain ferric ion solution, but also oxidizes divalent manganese ions so that manganese elements are removed in the form of manganese oxide precipitation, thereby realizing the preparation of low - impurity battery - grade iron phosphate.

[0027] In some preferred embodiments, the sodium nitrite is mixed with the supernatant in the form of a sodium nitrite solution; preferably, the mass concentration of sodium nitrite in solution B is 0.06% - 0.13%, such as 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, 0.13%, etc. For example, the mass concentration of the sodium nitrite solution is 2% - 5%, such as 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc., and the weight ratio of the sodium nitrite solution to the supernatant is 2.5% - 3.5%, such as 2.5%, 2.8%, 3%, 3.2%, 3.5%, etc.

[0028] In some preferred embodiments, in step S1, the temperature of the reaction is 40 - 50°C, such as 40°C, 42°C, 45°C, 48°C, 50°C, etc.

[0029] In some preferred embodiments, in step S1, the pH value of solution A is adjusted to 3.0 - 4.0, more preferably 3.0 - 3.5, such as 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, etc., by adding iron powder or FeO powder. Adjusting the pH value with iron powder or FeO powder can, on the one hand, stably adjust the pH value of solution A, and on the other hand, will not introduce new impurities.

[0030] In some embodiments, in step S1, the concentration of ferrous ions in solution A is 70-120 g / L, more preferably 80-120 g / L, such as 80 g / L, 85 g / L, 90 g / L, 95 g / L, 100 g / L, 105 g / L, 110 g / L, 115 g / L, 120 g / L, etc.

[0031] In some preferred embodiments, in step S2, before mixing the clarified liquid with the sodium nitrite solution, the pH value of the clarified liquid is adjusted to 0.8-1.2, such as 0.8, 0.9, 1.0, 1.1, 1.2, etc.

[0032] In some preferred embodiments, in step S2, compressed oxygen is introduced into the reaction kettle until the pressure in the kettle body is 10-15 bar, such as 10 bar, 11 bar, 12 bar, 13 bar, 14 bar, 15 bar, etc. It has been found through research that by regulating the introduction of compressed oxygen to regulate the pressure in the reaction kettle to 10-15 bar, it is beneficial to reduce the content of impurity Mn in iron phosphate.

[0033] In some preferred embodiments, in step S2, the temperature of the reaction is 40-50 °C, such as 40 °C, 42 °C, 45 °C, 48 °C, 50 °C, etc.

[0034] In some preferred embodiments, in step (1), the temperature of the reaction is 50-60 °C, such as 50 °C, 52 °C, 55 °C, 58 °C, 60 °C, etc.; after the addition of the iron source solution is completed, keep warm for 30-60 min, such as 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, etc.

[0035] In some preferred embodiments, in step (2), the temperature of the crystallization is 90-95 °C, such as 90 °C, 91 °C, 92 °C, 93 °C, 94 °C, 95 °C, etc.; the duration of the crystallization is 180-300 min, such as 180 min, 195 min, 210 min, 225 min, 240 min, 255 min, 270 min, 285 min, 300 min, etc.

[0036] In some preferred embodiments, in step (3), the phosphoric acid is determined according to the molar ratio of P in the phosphoric acid to Fe in the cleaning slurry being 0.05-0.20, such as 0.05, 0.08, 0.10, 0.12, 0.15, 0.18, 0.2, etc.

[0037] In some preferred embodiments, in the low-impurity battery-grade iron phosphate, the Mn content is not higher than 15 ppm, more preferably not higher than 12 ppm, and even more preferably lower than 10 ppm; the Al content is not higher than 40 ppm, more preferably not higher than 37 ppm, and even more preferably lower than 32 ppm; the Mg content is not higher than 15 ppm, more preferably not higher than 12 ppm, and even more preferably lower than 10 ppm; the Ti content is not higher than 20 ppm, more preferably 18 ppm, and even more preferably lower than 15 ppm.

[0038] In some embodiments, the pH regulator can be a conventional reagent, such as ammonia water, liquid caustic soda, etc.

[0039] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and in detail below in conjunction with the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0040] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0041] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchases or can be prepared by existing methods.

[0042] The iron source used in the following examples is ferrous titanyl sulfate, and its main content and main impurity levels are shown in Table 1. It should be noted that in the subsequent examples and comparative examples, the iron element content is determined by the conventional potassium dichromate method for solution, and the detection of other impurity contents is determined by the ICP method.

[0043] Table 1 Example 1 (1) Dissolve the ferrous titanyl sulfate raw material in water to form a solution of about 700 g / L, and heat it to 45 °C for full dissolution. Slowly add iron powder to the solution to adjust the pH value of the solution to 3.0 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0044] (2) Prepare a sodium nitrite solution with a mass concentration of 2.0%, adjust the pH value of the clear solution obtained in step 1 to 1.1, and add the sodium nitrite solution to the clear solution, and the sodium nitrite solution is 3.0% of the weight of the clear solution; (3) Feed the supernatant obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the supernatant until the pressure in the closed pressure-resistant reaction kettle reaches 12 bar. Stop feeding compressed oxygen, control the temperature of the solution in the kettle at 50 °C, and react fully in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. By detecting the oxidation degree of the iron source solution with o-phenanthroline reagent, it is found that the complete oxidation of the iron source is achieved.

[0045] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value at 6.0, slowly drop the mixed phosphorus source solution into the iron source solution, and react and precipitate at 55 °C. After the dropping is completed, keep warm for 40 min to obtain a synthetic slurry.

[0046] (5) Filter and wash the synthetic slurry, then re-pulp and disperse it. Add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), heat up to 95 °C and keep warm for 240 min to obtain an iron phosphate dihydrate slurry.

[0047] (6) After filtering, washing, drying, and roasting the iron phosphate dihydrate slurry, obtain an anhydrous iron phosphate product.

[0048] Example 2 (1) Dissolve the titanium white ferrous sulfate raw material in water to form a solution of about 700 g / L, and heat up to 40 °C for full dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.5 to precipitate titanium and aluminum ions, and filter to obtain the supernatant.

[0049] (2) Prepare a sodium nitrite solution with a mass concentration of 2.5%. Adjust the pH value of the supernatant obtained in step 1 to 1.0, and add the sodium nitrite solution to the supernatant. The sodium nitrite solution is 3.5% of the mass of the supernatant. (3) Feed the supernatant obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the supernatant until the pressure in the closed pressure-resistant reaction kettle reaches 15 bar. Stop feeding compressed oxygen, control the temperature of the solution in the kettle at 45 °C, and react fully in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. By detecting the oxidation degree of the iron source solution with o-phenanthroline reagent, it is found that the complete oxidation of the iron source is achieved.

[0050] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value at 6.5, slowly drop the mixed phosphorus source solution into the iron source solution, and react and precipitate at 50 °C. After the dropping is completed, keep warm for 60 min to obtain a synthetic slurry.

[0051] (5) Filter and wash the synthesized slurry, then re-beat and disperse it. Add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.1), heat up to 95 °C and hold for 180 min to obtain the iron phosphate dihydrate slurry.

[0052] (6) After filtering, washing, drying, and calcining the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0053] Example 3 (1) Dissolve the titanium white ferrous sulfate raw material in water to form a solution of about 700 g / L, and heat up to 45 °C for complete dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.2 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0054] (2) Prepare a sodium nitrite solution with a mass concentration of 3.0%. Adjust the pH value of the clear solution obtained in step 1 to 1.2, and add the sodium nitrite solution to the clear solution. The sodium nitrite solution is 3.5% of the mass of the clear solution. (3) Feed the clear solution obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the clear solution in the closed pressure-resistant reaction kettle until the pressure in the kettle reaches 10 bar. Stop feeding compressed oxygen, control the temperature of the solution in the kettle at 50 °C, and react fully in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. Detect the oxidation degree of the iron source solution through o-phenanthroline reagent, and it is found that complete oxidation of the iron source is achieved.

[0055] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value at 5.5, slowly drip the mixed phosphorus source solution into the iron source solution, and react and precipitate at 50 °C. After the dripping is completed, hold for 45 min to obtain the synthesized slurry.

[0056] (5) Filter and wash the synthesized slurry, then re-beat and disperse it. Add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.2), heat up to 90 °C and hold for 180 min to obtain the iron phosphate dihydrate slurry.

[0057] (6) After filtering, washing, drying, and calcining the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0058] Example 4 (1) Dissolve the titanium white ferrous sulfate raw material in water to form a solution of about 700 g / L, and heat up to 45 °C for complete dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.0 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0059] (2) Prepare a sodium nitrite solution with a mass concentration of 2.0%. Adjust the pH value of the clear liquid obtained in step 1 to 0.2, and add the sodium nitrite solution to the clear liquid. The sodium nitrite solution is 3.0% of the weight of the clear liquid. (3) Feed the clear liquid obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the clear liquid in the pressure-resistant reaction kettle until the pressure in the kettle reaches 12 bar. Stop feeding compressed oxygen, control the temperature of the solution in the kettle at 50 °C, and fully react in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. Detect the oxidation degree of the iron source solution through o-phenanthroline reagent, and it is found that complete oxidation of the iron source is achieved.

[0060] (4) Mix the phosphorus source solution with a pH regulator (ammonia water) and control the pH value at 6.0. Slowly drip the mixed phosphorus source solution into the iron source solution and react to precipitate at 55 °C. After the dripping is completed, keep warm for 40 min to obtain a synthetic slurry.

[0061] (5) Filter, wash, and then re-pulp and disperse the synthetic slurry. Add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), heat up to 95 °C and keep warm for reaction for 240 min to obtain a dihydrate iron phosphate slurry.

[0062] (6) After filtering, washing, drying, and roasting the dihydrate iron phosphate slurry, an anhydrous iron phosphate product is obtained.

[0063] Example 5 (1) Dissolve the titanium white ferrous sulfate raw material in water to form a solution of about 700 g / L, and heat up to 45 °C for full dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.0, precipitate titanium and aluminum ions, and filter to obtain a clear liquid.

[0064] (2) Prepare a sodium nitrite solution with a mass concentration of 2.0%. Adjust the pH value of the clear liquid obtained in step 1 to 1.8, and add the sodium nitrite solution to the clear liquid. The sodium nitrite solution is 3.0% of the weight of the clear liquid. (3) Feed the clear liquid obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the clear liquid in the pressure-resistant reaction kettle until the pressure in the kettle reaches 12 bar. Stop feeding compressed oxygen, control the temperature of the solution in the kettle at 50 °C, and fully react in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. Detect the oxidation degree of the iron source solution through o-phenanthroline reagent, and it is found that complete oxidation of the iron source is achieved.

[0065] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value to 6.0, slowly drop the mixed phosphorus source solution into the iron source solution, react and precipitate at 55 °C, and keep warm for 40 min after the dropping is completed to obtain a synthetic slurry.

[0066] (5) Filter and wash the synthetic slurry, then re-pulp and disperse it, add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), raise the temperature to 95 °C and keep warm for 240 min to obtain an iron phosphate dihydrate slurry.

[0067] (6) After filtering, washing, drying, and roasting the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0068] Example 6 (1) Dissolve the titanium white ferrous sulfate raw material in water to form a solution of about 700 g / L, and raise the temperature to 45 °C for full dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.0 to precipitate titanium and aluminum ions, and filter to obtain a clear solution.

[0069] (2) Prepare a sodium nitrite solution with a mass concentration of 2.0%, adjust the pH value of the clear solution obtained in step 1 to 1.1, add the sodium nitrite solution to the clear solution, and the sodium nitrite solution is 3.0% of the weight of the clear solution; (3) Feed the clear solution obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the clear solution in the closed pressure-resistant reaction kettle until the pressure in the kettle reaches 5 bar, stop feeding compressed oxygen, control the temperature of the solution in the kettle to 50 °C, fully react in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable, filter to remove the precipitate, and obtain an iron source solution after oxidation and impurity removal; Detect the oxidation degree of the iron source solution through o-phenanthroline reagent, and find that complete oxidation of the iron source is achieved.

[0070] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value to 6.0, slowly drop the mixed phosphorus source solution into the iron source solution, react and precipitate at 55 °C, and keep warm for 40 min after the dropping is completed to obtain a synthetic slurry.

[0071] (5) Filter and wash the synthetic slurry, then re-pulp and disperse it, add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), raise the temperature to 95 °C and keep warm for 240 min to obtain an iron phosphate dihydrate slurry.

[0072] (6) After filtering, washing, drying, and roasting the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0073] Comparative Example 1 (1) Dissolve the titanium white ferrous sulfate raw material in water to prepare a solution with a concentration of about 700 g / L, and heat it to 45 °C for complete dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 2.5 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0074] (2) Prepare a sodium nitrite solution with a mass concentration of 2.0%. Adjust the pH value of the clear solution obtained in step 1 to 1.1, and add the sodium nitrite solution to the clear solution. The sodium nitrite solution is 3.0% of the weight of the clear solution. (3) Feed the clear solution obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the clear solution in the closed pressure-resistant reaction kettle until the pressure in the kettle reaches 12 bar. Stop feeding compressed oxygen, control the temperature of the solution in the kettle at 50 °C, and react fully in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. Detect the oxidation degree of the iron source solution through the o-phenanthroline reagent, and it is found that the complete oxidation of the iron source is achieved.

[0075] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value at 6.0, and slowly add the mixed phosphorus source solution dropwise to the iron source solution. React and precipitate at 55 °C, and keep warm for 40 min after the addition is completed to obtain a synthetic slurry.

[0076] (5) Filter and wash the synthetic slurry, then re-pulp and disperse it. Add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), heat it to 95 °C and keep warm for reaction for 240 min to obtain a dihydrate iron phosphate slurry.

[0077] (6) After filtering, washing, drying, and roasting the dihydrate iron phosphate slurry, an anhydrous iron phosphate product is obtained.

[0078] Comparative Example 2 (1) Dissolve the titanium white ferrous sulfate raw material in water to prepare a solution with a concentration of about 700 g / L, and heat it to 45 °C for complete dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.0 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0079] (2) Feed the clear solution into a pressure-resistant reaction kettle, adjust the pH value to 1.1, and then feed compressed oxygen into the clear solution in the closed pressure-resistant reaction kettle until the pressure in the kettle reaches 12 bar. Stop feeding compressed oxygen, control the temperature of the solution in the kettle at 50 °C, and react fully in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. Detect the oxidation degree of the iron oxide source solution through the o-phenanthroline reagent, and it is found that there is an incomplete oxidation situation. Add hydrogen peroxide until the ferrous ions are completely oxidized.

[0080] (3) The phosphorus source solution is mixed with a pH adjuster (ammonia water) to control the pH value to 6.0, and the mixed phosphorus source solution is slowly added dropwise to the iron source solution to react and precipitate at 55°C. After the addition is completed, the solution is kept warm for 40 minutes to obtain a synthetic slurry.

[0081] (4) After filtering and washing the synthetic slurry, the slurry was re-dispersed, phosphoric acid was added (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), the temperature was raised to 95° C. and the reaction was maintained at this temperature for 240 min to obtain dihydrate iron phosphate slurry.

[0082] (5) The dihydrate ferric phosphate slurry is filtered, washed, dried and calcined to obtain an anhydrous ferric phosphate product.

[0083] Comparative Example 3 (1) Dissolve the titanium dioxide ferrous sulfide raw material in water to prepare a solution of about 700 g / L, and heat it to 45°C to fully dissolve it. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.0, precipitate titanium and aluminum ions, and filter to obtain a clear solution.

[0084] (2) preparing a sodium nitrite solution, wherein the mass concentration of the sodium nitrite solution is 2.0%, adjusting the pH value of the clear solution obtained in step 1 to 1.1, and adding the sodium nitrite solution to the clear solution, wherein the mass of the sodium nitrite solution is 3.0% of the mass of the clear solution; (3) The clear liquid obtained in step 2 is introduced into a reactor, and oxygen is continuously introduced into the clear liquid (air is continuously introduced and discharged during the reaction process). The pressure in the reactor is controlled to be normal pressure and the temperature is 50° C. The reaction is fully carried out in the reactor until the pH value of the solution stops increasing and remains stable. The precipitate is filtered out to obtain an iron source solution after oxidation and impurity removal. The degree of oxidation of the iron source solution after impurity removal is detected by a phenanthroline reagent. If incomplete oxidation is found, hydrogen peroxide is added until the ferrous ions are completely oxidized.

[0085] (4) The phosphorus source solution is mixed with a pH adjuster (ammonia water) to control the pH value to 6.0, and the mixed phosphorus source solution is slowly added dropwise to the iron source solution to react and precipitate at 55° C. After the addition is completed, the solution is kept warm for 40 minutes to obtain a synthetic slurry.

[0086] (5) After filtering and washing the synthetic slurry, the slurry was re-dispersed, phosphoric acid was added (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), the temperature was raised to 95° C. and the mixture was kept warm for 240 min to obtain dihydrate iron phosphate slurry.

[0087] (6) The dihydrate ferric phosphate slurry is filtered, washed, dried and calcined to obtain an anhydrous ferric phosphate product.

[0088] Comparative Example 4 (1) Dissolve the titanium white ferrous sulfate raw material in water to form a solution of about 700 g / L, and heat it to 45 °C for complete dissolution. Slowly add iron powder to the solution, adjust the pH value of the solution to 3.0 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0089] (2) Prepare a sodium nitrite solution with a mass concentration of 2.0%, and add the sodium nitrite solution to the clear solution obtained in step 1. The sodium nitrite solution is 3.0% of the weight of the clear solution. (3) Introduce the clear solution obtained in step 2 into a pressure-resistant reaction kettle, and then introduce compressed oxygen into the clear solution until the pressure in the closed pressure-resistant reaction kettle reaches 12 bar. Stop introducing compressed oxygen, control the temperature of the solution in the kettle at 50 °C, and fully react in the closed pressure-resistant reaction kettle until the pressure in the kettle drops to a constant value or the pH value of the solution stops rising and remains stable. Filter to remove the precipitate to obtain an iron source solution after oxidation and impurity removal. Detect the oxidation degree of the iron source solution through o-phenanthroline reagent, and it is found that complete oxidation of the iron source is achieved.

[0090] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value at 6.0, slowly drip the mixed phosphorus source solution into the iron source solution, and react and precipitate at 55 °C. After the dripping is completed, keep warm for 40 min to obtain a synthetic slurry.

[0091] (5) Filter, wash, and then redisperse the synthetic slurry by beating, add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), heat it to 95 °C and keep warm for reaction for 240 min to obtain a dihydrate ferric phosphate slurry.

[0092] (6) After filtering, washing, drying, and roasting the dihydrate ferric phosphate slurry, obtain an anhydrous ferric phosphate product.

[0093] The reaction duration of oxidation and impurity removal in step (3), the recovery rate of iron element (the recovery rate of iron element refers to the percentage of the total amount of iron element contained in the iron source solution to the total amount of iron element in the titanium white ferrous sulfate raw material), the impurity level of the obtained iron source solution, and the dosage of additional hydrogen peroxide in each of the above examples and comparative examples are shown in Table 2.

[0094] Table 2 It can be seen from Table 2 that the content of impurity Al in the ferrous source obtained in step (3) of Comparative Example 1 increases significantly, which indicates that when the pH of the reaction system adjusted with the iron source in step (1) is relatively low, it is not conducive to the precipitation and removal of Al.

[0095] As can be seen from Table 2, the reaction duration in Step (3) of Comparative Example 2 increased significantly, the recovery rate of iron element decreased, and there were still a large amount of ferrous ions and manganese ions not oxidized at the reaction end point. To completely oxidize ferrous ions, a large amount of hydrogen peroxide needed to be additionally added. This indicates that sodium nitrite significantly promoted the oxidative precipitation removal of manganese ions and increased the recovery rate of iron element while increasing the removal rate of manganese ions; in Comparative Example 3, the reaction duration in Step (3) increased significantly, the recovery rate of iron element decreased to some extent, and there were still a large amount of ferrous ions and manganese ions not oxidized at the reaction end point. To completely oxidize ferrous ions, a large amount of hydrogen peroxide needed to be additionally added, indicating that the pressure environment in Step (3) also significantly promoted the oxidative precipitation removal of manganese ions and increased the recovery rate of iron element while increasing the removal rate of manganese ions.

[0096] As can also be seen from Table 2, the recovery rate of iron element in the iron source solution obtained in Comparative Example 4 decreased significantly. After analysis, this was because the pH was not adjusted before adding the sodium nitrite solution in Step (2), resulting in a high pH, which caused ferrous ions to be converted into iron hydroxide precipitates and enter the precipitate during oxidation in Step (3), resulting in a large amount of oxidized ferric precipitates not being recovered and utilized, causing great waste.

[0097] As can be seen from Table 2, by comparing Example 1, Example 4, and Example 5, it was found that when the pH adjustment value of the supernatant was too low before mixing the sodium nitrite solution with the supernatant obtained in Step (1), the removal of Mn was incomplete (see Example 4), and when the pH of the supernatant after adjustment was relatively high, the recovery rate of iron decreased (see Example 5).

[0098] As can be seen from Table 2, by comparing Example 1 and Example 6, it was found that in Step (3), when the pressure in the autoclave decreased at the initial stage of the reaction due to the compressed air introduced, the precipitation removal of Mn was incomplete.

[0099] The main impurity levels of the iron phosphates prepared in the above-mentioned examples and comparative examples are shown in Table 3.

[0100] Table 3 As can be seen from Table 3, the impurity levels of the iron phosphates prepared in each of the examples and comparative examples can all reach the battery grade level. Among them, the impurity Mn content of the battery grade iron phosphate prepared in each example is lower than 60 ppm, far lower than the impurity Mn content of the iron phosphates prepared in Comparative Example 2 and Comparative Example 3; and the impurity Al content of the battery grade iron phosphate prepared in each example is lower than 40 ppm, significantly lower than the impurity Al content of the iron phosphate prepared in Comparative Example 1; combined with Table 2, it can be seen that the recovery rate of iron element content in each example is higher than 85%, significantly higher than the iron element recovery rate in Comparative Example 4. Moreover, the iron phosphates prepared in Examples 1 to 3 not only have an iron element recovery rate as high as over 98%, but also have a low Mn content of 10 ppm or less, a low Mg content of 10 ppm or less, a low Al content of 30 ppm or less, and a low Ti content of 15 ppm or less, with extremely low impurity content, which can meet the requirements of ultra-high-quality cathode materials for the quality of the precursor.

[0101] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing battery-grade iron phosphate with low impurities, characterized in that, Including: S1. Dissolve titanium white ferrous sulfate in water to obtain solution A, adjust the pH value of solution A to 3.0 - 4.0, and after reaction, perform solid-liquid separation to obtain a clear liquid; S2. Adjust the pH of the clear liquid to 0.2 - 2, and then mix the clear liquid with sodium nitrite to obtain solution B; Add solution B into a reaction kettle, and introduce compressed oxygen into the reaction kettle until the pressure in the kettle body is 5 - 15 bar for reaction. Wait until the pressure in the kettle drops to a constant value, and perform solid-liquid separation to obtain a purified iron source solution; S3. Use the purified iron source solution and a phosphorus source as raw materials to prepare iron phosphate dihydrate by a precipitation method; specifically including: (1) Mix the phosphorus source solution with a pH regulator to adjust the pH value of the phosphorus source solution to 4.0 - 6.5; dropwise add the iron source solution into the phosphorus source solution for reaction to obtain a synthetic slurry; (2) Filter and wash the synthetic slurry, then add water to make a slurry to obtain a washed slurry; mix the washed slurry with phosphoric acid, and after crystallization, obtain an iron phosphate dihydrate slurry; (3) Wash, perform solid-liquid separation and dry the iron phosphate dihydrate slurry to obtain iron phosphate dihydrate; S4. Calcinate the iron phosphate dihydrate to obtain low-impurity battery-grade iron phosphate.

2. The preparation method of low-impurity battery-grade iron phosphate as claimed in claim 1, wherein The sodium nitrite is mixed with the clear liquid in the form of a sodium nitrite solution; The mass concentration of sodium nitrite in solution B is 0.06% - 0.13%.

3. The preparation method of low-impurity battery-grade iron phosphate as claimed in claim 1, wherein In step S1, the temperature of the reaction is 40 - 50 °C; In step S1, adjust the pH value of solution A to 3.0 - 4.0 by adding iron powder or FeO powder; In step S1, the concentration of ferrous ions in solution A is 70 - 120 g / L.

4. The preparation method of low-impurity battery-grade iron phosphate as claimed in claim 1, wherein, In step S2, before mixing the clear liquid with the sodium nitrite solution, adjust the pH value of the clear liquid to 0.8 - 1.

2.

5. The preparation method of battery-grade iron phosphate according to claim 1, characterized in that, In step S2, the temperature of the reaction is 40 - 50 °C.

6. The preparation method of battery-grade iron phosphate according to claim 1, characterized in that, In step S2, introduce compressed oxygen into the reaction kettle until the pressure in the kettle body is 10 - 15 bar.

7. The preparation method of low-impurity battery-grade iron phosphate as described in claim 1, characterized in that, In step (1), the temperature of the reaction is 50 - 60 °C; after the addition of the iron source solution is completed, keep warm for 30 - 60 min.

8. The preparation method of low-impurity battery-grade iron phosphate according to claim 1, characterized in that, In step (2), the temperature of the crystallization is 90 - 95 °C; the duration of the crystallization is 180 - 300 min.

9. The preparation method of low-impurity battery-grade iron phosphate as claimed in claim 1, characterized in that, In step (3), the phosphoric acid is determined according to the molar ratio of P in the phosphoric acid to Fe in the washed slurry being 0.05 - 0.

20.

10. The preparation method of low-impurity battery-grade iron phosphate as described in claim 1, characterized in that, In the low-impurity battery-grade iron phosphate, the Mn content is not higher than 15 ppm, the Al content is not higher than 40 ppm, the Mg content is not higher than 15 ppm, and the Ti content is not higher than 20 ppm.

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