Preparation Method of Low-Impurity Battery-Grade Iron Phosphate

Abstract

The present invention belongs to the field of preparation of lithium-ion battery materials, and provides a method for preparing battery-grade iron phosphate with low impurities, which includes: dissolving titanium white ferrous sulfate in water to obtain solution A, adjusting the pH value of solution A to 3.0 - 4.0, and after reaction, performing solid-liquid separation to obtain a supernatant; after adjusting the pH of the supernatant, mixing the supernatant with sodium nitrite to obtain solution B; adding solution B into a reaction kettle, and introducing compressed oxygen until the pressure in the kettle body is 5 - 15 bar, reacting until the pressure in the kettle drops to a constant value, and performing solid-liquid separation to obtain a purified iron source solution; using the purified iron source solution and a phosphorus source as raw materials, preparing iron phosphate dihydrate by a precipitation method; and calcining the iron phosphate dihydrate to obtain battery-grade iron phosphate with low impurities. This preparation method has a simple and feasible process, low cost, wide sources of required raw materials, good safety, low cost, is easy to realize large-scale production, has good impurity removal effect and less iron loss, and the Mn content of the battery-grade iron phosphate prepared from the ferrous sulfate after impurity removal can be as low as below 10 ppm.

Description

Technical Field

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

[0002] As the most commonly used precursor material for lithium iron phosphate in power batteries, the synthesis raw materials of iron phosphate are usually high-purity divalent or trivalent iron salts, and the price is relatively expensive. At present, the purification and refinement of by-product ferrous sulfate of titanium white and its use as an iron source for preparing iron phosphate to reduce production costs have become one of the mainstream production methods of iron phosphate.

[0003] The preparation of iron phosphate by using by-product ferrous sulfate of titanium white as a raw material includes the purification and impurity removal of the iron source and the oxidation process. Although the existing processes can prepare battery-grade iron phosphate at present, with the continuous improvement of the quality and performance requirements of the cathode material lithium iron phosphate, the impurity content of the battery-grade iron phosphate prepared by the existing processes is still relatively high, usually in the range of dozens to hundreds of ppm, and it is difficult to meet the high-quality requirements.

[0004] Currently, chemical precipitation method is mostly used for impurity removal. Generally, at the initial stage of the process, a pH regulator (mostly alkaline substances such as iron powder) is added to increase the pH of the iron salt stock solution prepared from by-product ferrous sulfate of titanium white, so that impurity ions form corresponding hydroxides and precipitate, and finally the precipitate is separated from the iron salt stock solution by physical methods to achieve the purpose of impurity removal. However, Fe(OH)2 starts to precipitate at pH = 6.5 (initial concentration is 1mol / L), so there is an upper limit to the increase range of the pH of the raw material system. The pH values of the corresponding hydroxides of impurities Mn and Mg for precipitation are higher than 6.5. Continuing to increase the pH by ammonia water or liquid caustic soda for impurity removal will also cause further increase in production costs due to iron loss. Moreover, too high contents of metal impurities manganese and magnesium will not only cause a decrease in the specific capacity and energy density of the cathode material, but also damage the life and safety performance of the prepared battery.

[0005] The oxidation process usually uses reagents with strong oxidizing properties for oxidation. In the oxidation process of the iron source, hydrogen peroxide is widely used because of its strong oxidizing property and the fact that it does not produce polluting media after reaction, which can effectively protect the environment. However, although hydrogen peroxide itself is not flammable, it can react with combustibles and reducing agents through redox reactions to release a large amount of heat and oxygen, thus causing combustion and explosion. In addition, once the human body inhales excessive vapor or mist of hydrogen peroxide, it will cause a strong stimulating effect on the respiratory tract, and long-term exposure may also lead to diseases such as contact dermatitis. In recent years, with the increasing strictness of national safety requirements and the increase in the price of hydrogen peroxide year by year, 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 object of the present invention is to provide a method for preparing battery-grade iron phosphate with low impurities.

[0007] To achieve the above object, the present invention proposes the following solutions:

[0008] Provide a method for preparing battery-grade iron phosphate with low impurities, including:

[0009] 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 supernatant;

[0010] S2. Adjust the pH of the supernatant to 0.2 - 2, then mix the supernatant 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 is 5 - 15 bar, perform the reaction, and wait until the pressure in the kettle drops to a constant value, then perform solid-liquid separation to obtain a purified iron source solution;

[0011] S3. Use the purified iron source solution and a phosphorus source as raw materials to prepare iron phosphate dihydrate by precipitation method; specifically including:

[0012] (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 perform the reaction to obtain a synthetic slurry;

[0013] (2) Filter and wash the synthetic 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;

[0014] (3) Wash, perform solid-liquid separation and dry the iron phosphate dihydrate slurry to obtain iron phosphate dihydrate;

[0015] S4. Calcinate the iron phosphate dihydrate to obtain battery-grade iron phosphate with low impurities.

[0016] Further, the sodium nitrite is mixed with the supernatant in the form of a sodium nitrite solution.

[0017] Further, the mass concentration of sodium nitrite in solution B is 0.06% - 0.13%.

[0018] Further, in step S1, the temperature of the reaction is 40 - 50 °C.

[0019] 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.

[0020] Further, in step S1, the concentration of ferrous ions in solution A is 70 - 120 g / L.

[0021] Further, 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.

[0022] Further, in step S2, the temperature of the reaction is 40 - 50 °C.

[0023] Further, in step S2, compressed oxygen is introduced into the reaction kettle until the pressure in the kettle body reaches 10 - 15 bar.

[0024] 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.

[0025] Further, in step (2), the temperature of the crystallization is 90 - 95 °C; the duration of the crystallization is 180 - 300 min.

[0026] 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.

[0027] 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.

[0028] Compared with the prior art, one or more of the above - mentioned technical solutions can achieve at least one of the following beneficial effects:

[0029] 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 achieve large - scale production. The impurity removal effect is good and the iron loss is small. The ferrous sulfate after impurity removal is used to prepare battery - grade iron phosphate with extremely low impurity content to meet the requirements of high - quality cathode materials for iron phosphate.

[0030] In the provided preparation method of low - impurity battery - grade iron phosphate, the Mn content in the obtained low - impurity battery - grade iron phosphate finished 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%. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] 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 following drawings 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.

[0032] Figure 1 It is a process flow diagram for the preparation of battery-grade iron phosphate. Specific embodiments

[0033] Aiming at the problems that the impurity content of the battery-grade iron phosphate prepared from titanium white ferrous sulfate is relatively high and difficult to meet the requirements of the composition of high-quality cathode materials, or the iron loss content in the impurity removal process of titanium white ferrous sulfate is too large, or the impurity removal process is complex and the production cost is high, etc., the applicant has developed a method for preparing low-impurity battery-grade iron phosphate 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.

[0034] Some embodiments of the present invention provide a method for preparing low-impurity battery-grade iron phosphate, including:

[0035] 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;

[0036] 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, perform 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;

[0037] S3. Use the purified iron source solution and a phosphorus source as raw materials to prepare iron phosphate dihydrate by precipitation method; specifically including:

[0038] (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; drop the iron source solution into the phosphorus source solution and perform the reaction to obtain a synthetic slurry;

[0039] (2) Filter and wash the synthetic slurry, then add water to make a slurry to obtain a cleaning slurry; mix the cleaning slurry with phosphoric acid, and after crystallization, obtain an iron phosphate dihydrate slurry;

[0040] (3) Perform solid-liquid separation and drying on the iron phosphate dihydrate slurry to obtain iron phosphate dihydrate;

[0041] S4. Calcinate the iron phosphate dihydrate to obtain low-impurity battery-grade iron phosphate.

[0042] 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, through iron powder and H +The reaction indirectly regulates the pH value to 3.0 - 4.0, removes titanium and aluminum by using the hydrolysis of titanium ions and aluminum ions. At the same time, iron powder can reduce ferric iron in the raw materials and reduce iron loss; in titanium white ferrous sulfate, both 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 radius of Mn ions is larger and the charge density is relatively lower, so the formed hydrated ion radius is actually smaller. In 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 a ferric ion solution, but also oxidizes divalent manganese ions to remove manganese elements in the form of manganese oxide precipitation, thereby realizing the preparation of low - impurity battery - grade iron phosphate.

[0043] 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.

[0044] 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.

[0045] In some preferred embodiments, in step S1, the pH value of solution A is adjusted to 3.0 - 4.0 by adding iron powder or FeO powder, and further preferably to 3.0 - 3.5, such as 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, etc. Adjusting the pH value by using iron powder or FeO powder can, on the one hand, smoothly adjust the pH value of solution A, and on the other hand, will not introduce new impurities.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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 iron source solution is completely added dropwise, it is kept warm for 30-60 min, such as 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, etc.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] In some embodiments, the pH regulator can be a conventional reagent, such as ammonia water, liquid alkali, etc.

[0055] To facilitate the understanding of the present invention, the following will describe the present invention more comprehensively and in detail with reference to 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.

[0056] Unless otherwise defined, all the technical terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The technical 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.

[0057] Unless otherwise specifically stated, all kinds of raw materials, reagents, instruments and equipment used in the present invention can be obtained through market purchase or can be prepared by existing methods.

[0058] The iron source used in the following embodiments is ferrous titanium white, and its main content and main impurity levels are shown in Table 1. It should be noted that in the subsequent embodiments and comparative examples, the iron element content is determined by the conventional potassium dichromate method for measuring solutions, and the detection of other impurity contents is determined by the ICP method.

[0059] Table 1

[0060]

[0061] Example 1

[0062] (1) Dissolve the ferrous titanium white sulfur 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, precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0063] (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;

[0064] (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 sealed 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 sealed 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 phenanthroline reagent, 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 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 dropping is completed to obtain a synthetic slurry.

[0066] (5) Filter, 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 a dihydrate iron phosphate slurry.

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

[0068] Example 2

[0069] (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.

[0070] (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.

[0071] (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 sealed 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 sealed 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 phenanthroline reagent, it is found that complete oxidation of the iron source is achieved.

[0072] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value at 6.5, and slowly add the mixed phosphorus source solution dropwise to the iron source solution. React and precipitate at 50 °C, and keep warm for 60 min after the dropping is completed to obtain a synthetic slurry.

[0073] (5) Filter and wash the synthesized slurry, then beat it into pulp again for dispersion. Add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.1), heat up to 95 °C and keep it for reaction for 180 min to obtain iron phosphate dihydrate slurry.

[0074] (6) After filtering, washing, drying and roasting the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0075] Example 3

[0076] (1) Dissolve the titanium white ferrous sulfate raw material in water to prepare 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 3.2 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0077] (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.

[0078] (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 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 the phenanthroline reagent, and it is found that the complete oxidation of the iron source is achieved.

[0079] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value at 5.5, 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 it for 45 min to obtain a synthesized slurry.

[0080] (5) Filter and wash the synthesized slurry, then beat it into pulp again for dispersion. Add phosphoric acid (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.2), heat up to 90 °C and keep it for reaction for 180 min to obtain iron phosphate dihydrate slurry.

[0081] (6) After filtering, washing, drying and roasting the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0082] Example 4

[0083] (1) Dissolve the titanium white ferrous sulfate raw material in water to prepare 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 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0084] (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.

[0085] (3) Feed the clear liquid obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the clear liquid 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 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 the phenanthroline reagent, and it is found that the complete oxidation of the iron source is achieved.

[0086] (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.

[0087] (5) Filter, wash, and then redisperse the synthetic slurry by repulping. 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.

[0088] (6) After filtering, washing, drying, and roasting the dihydrate iron phosphate slurry, an anhydrous iron phosphate product is obtained.

[0089] Example 5

[0090] (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.

[0091] (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.

[0092] (3) Feed the clear liquid obtained in step 2 into a pressure-resistant reaction kettle, and then feed compressed oxygen into the clear liquid 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 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 the phenanthroline reagent, and it is found that the complete oxidation of the iron source is achieved.

[0093] (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 synthesis slurry.

[0094] (5) Filter and wash the synthesis 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.

[0095] (6) After filtering, washing, drying, and roasting the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0096] Example 6

[0097] (1) Dissolve the titanium white ferrous sulfate raw material in water to form a solution of about 700 g / L, heat up to 45 °C and dissolve it fully. 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.

[0098] (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;

[0099] (3) Pass the clear solution obtained in step 2 into a pressure-resistant reaction kettle, then pass compressed oxygen into the clear solution in the closed pressure-resistant reaction kettle until the pressure in the kettle reaches 5 bar, stop passing 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 the phenanthroline reagent, and it is found that the complete oxidation of the iron source is achieved.

[0100] (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 synthesis slurry.

[0101] (5) Filter and wash the synthesis 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.

[0102] (6) After filtering, washing, drying, and roasting the iron phosphate dihydrate slurry, an anhydrous iron phosphate product is obtained.

[0103] Comparative Example 1

[0104] (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.

[0105] (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.

[0106] (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 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 the phenanthroline reagent, and it is found that the complete oxidation of the iron source is achieved.

[0107] (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.

[0108] (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.

[0109] (6) After filtering, washing, drying, and roasting the dihydrate iron phosphate slurry, an anhydrous iron phosphate product is obtained.

[0110] Comparative Example 2

[0111] (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.

[0112] (2) The supernatant is introduced into a pressure-resistant reaction kettle, the pH value is adjusted to 1.1, and then compressed oxygen is introduced into the supernatant into the closed pressure-resistant reaction kettle until the pressure in the kettle reaches 12 bar. The introduction of compressed oxygen is stopped, the temperature of the solution in the kettle is controlled at 50 °C, and the reaction is fully carried out 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. The precipitate is removed by filtration to obtain an iron source solution after oxidation and impurity removal. By detecting the oxidation degree of the iron oxide source solution with phenanthroline reagent, it is found that there is an incomplete oxidation situation, and hydrogen peroxide is added until the ferrous ions are completely oxidized.

[0113] (3) The phosphorus source solution is mixed with a pH regulator (ammonia water), the pH value is controlled at 6.0, and the mixed phosphorus source solution is slowly dropped into the iron source solution, and the reaction precipitation is carried out at 55 °C. After the dropping is completed, it is kept warm for 40 min to obtain a synthetic slurry.

[0114] (4) The synthetic slurry is filtered, washed, re-slurried and dispersed, phosphoric acid is added (the molar ratio of P in phosphoric acid to Fe in the slurry = 0.15), and the temperature is raised to 95 °C and kept warm for reaction for 240 min to obtain a dihydrate iron phosphate slurry.

[0115] (5) After the dihydrate iron phosphate slurry is filtered, washed, dried and calcined, an anhydrous iron phosphate product is obtained.

[0116] Comparative Example 3

[0117] (1) The titanium white ferrous sulfate raw material is dissolved in water to form a solution of about 700 g / L, and the temperature is raised to 45 °C for full dissolution. Iron powder is slowly added to the solution, the pH value of the solution is adjusted to 3.0, the titanium and aluminum ions are precipitated, and the supernatant is obtained after filtration.

[0118] (2) Prepare a sodium nitrite solution with a mass concentration of 2.0%. Adjust the pH value of the supernatant obtained in step 1 to 1.1, and add the sodium nitrite solution to the supernatant. The mass of the sodium nitrite solution is 3.0% of the mass of the supernatant.

[0119] (3) The supernatant obtained in step 2 is introduced into the reaction kettle, and then oxygen is continuously introduced into the supernatant (continuous intake and continuous exhaust during the reaction process). The pressure in the kettle is controlled at atmospheric pressure and the temperature is 50 °C. The reaction is fully carried out in the reaction kettle until the pH value of the solution stops rising and remains stable. The precipitate is removed by filtration to obtain an iron source solution after oxidation and impurity removal. By detecting the oxidation degree of the obtained iron source solution after impurity removal with phenanthroline reagent, it is found that there is an incomplete oxidation situation, and hydrogen peroxide is added until the ferrous ions are completely oxidized.

[0120] (4) The phosphorus source solution is mixed with a pH regulator (ammonia water), the pH value is controlled at 6.0, and the mixed phosphorus source solution is slowly dropped into the iron source solution, and the reaction precipitation is carried out at 55 °C. After the dropping is completed, it is kept warm for 40 min to obtain a synthetic slurry.

[0121] (5) Filter and wash the synthesized slurry, then beat it into pulp again for dispersion. 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 the temperature for reaction for 240 min to obtain ferric phosphate dihydrate slurry.

[0122] (6) After filtering, washing, drying and roasting the ferric phosphate dihydrate slurry, an anhydrous ferric phosphate product is obtained.

[0123] Comparative Example 4

[0124] (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 to precipitate titanium and aluminum ions, and obtain a clear solution after filtration.

[0125] (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;

[0126] (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 to 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 the phenanthroline reagent, and it is found that the complete oxidation of the iron source is achieved.

[0127] (4) Mix the phosphorus source solution with a pH regulator (ammonia water), control the pH value to 6.0, slowly drip the mixed phosphorus source solution into the iron source solution, and react and precipitate at 55 °C. Keep the temperature for 40 min after the dripping is completed to obtain a synthesized slurry.

[0128] (5) Filter and wash the synthesized slurry, then beat it into pulp again for dispersion. 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 the temperature for reaction for 240 min to obtain ferric phosphate dihydrate slurry.

[0129] (6) After filtering, washing, drying and roasting the ferric phosphate dihydrate slurry, an anhydrous ferric phosphate product is obtained.

[0130] The reaction duration of oxidation and impurity removal in step (3) of each of the above examples and comparative examples, 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 are shown in Table 2.

[0131] Table 2

[0132]

[0133] As can be seen from Table 2, the content of impurity Al in the ferrous source obtained in Step (3) of Comparative Example 1 increased significantly, indicating that when the pH of the reaction system adjusted with the iron source in Step (1) was relatively low, it was not conducive to the precipitation and removal of Al.

[0134] As can be seen from Table 2, the reaction duration of Step (3) in 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 end point of the reaction. To completely oxidize the ferrous ions, a large amount of hydrogen peroxide needed to be added additionally. This indicates that sodium nitrite significantly promoted the oxidation precipitation and removal of manganese ions, and while increasing the removal rate of manganese ions, it also increased the recovery rate of iron elements; the reaction duration of Step (3) in Comparative Example 3 increased significantly, the recovery rate of iron element decreased somewhat, and there were still a large amount of ferrous ions and manganese ions not oxidized at the end point of the reaction. To completely oxidize the ferrous ions, a large amount of hydrogen peroxide needed to be added additionally, indicating that the pressure environment in Step (3) also significantly promoted the oxidation precipitation and removal of manganese ions, and while increasing the removal rate of manganese ions, it also increased the recovery rate of iron elements.

[0135] 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 relatively 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 that could not be recycled, causing great waste.

[0136] 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), it would lead to incomplete removal of Mn (see Example 4), and when the pH of the supernatant after adjustment was relatively high, it would reduce the recovery rate of iron (see Example 5).

[0137] 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 reactor decreased at the initial stage of the reaction due to the compressed air introduced, it would lead to incomplete precipitation and removal of Mn.

[0138] The main impurity levels of the iron phosphates prepared by the steps of the above-mentioned examples and comparative examples are shown in Table 3.

[0139] Table 3

[0140]

[0141] As can be seen from Table 3, the impurity levels of the iron phosphates prepared in each of the examples and comparative examples can reach the battery-grade level. Among them, the impurity Mn content of the battery-grade iron phosphates prepared in each of the examples 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 phosphates prepared in each of the examples 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 rates of iron element content in each of the examples are 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 for the quality of the precursor for ultra-high-quality cathode materials.

[0142] 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 low-impurity battery-grade iron phosphate, characterized in that: Includes: S1, dissolving titanium dioxide ferrous sulfate in water to obtain solution A, adjusting the pH value of solution A to 3.0-4.0, and performing solid-liquid separation after the reaction to obtain a clear solution; S2. Adjust the pH of the clear solution to 0.2-2, and then mix the clear solution with sodium nitrite to obtain solution B; Solution B is added to the reactor, and compressed oxygen is introduced into the reactor until the pressure in the reactor reaches 5-15 bar. The reaction is carried out, and after the pressure in the reactor drops to a constant value, solid-liquid separation is performed to obtain a purified iron source solution; S3. Using the purified iron source solution and phosphorus source as raw materials, preparing ferric phosphate dihydrate by a precipitation method; specifically comprising: (1) mixing a phosphorus source solution and a pH adjuster, and adjusting the pH value of the phosphorus source solution to 4.0-6.5; adding an iron source solution dropwise to the phosphorus source solution, reacting, and obtaining a synthetic slurry; (2) filtering and washing the synthetic slurry, and then adding water to slurry to obtain a clean slurry; mixing the clean slurry with phosphoric acid, and crystallizing the mixture to obtain a dihydrate ferric phosphate slurry; (3) The ferric phosphate dihydrate slurry is washed, solid-liquid separated and dried to obtain ferric phosphate dihydrate; S4. Calcinate ferric phosphate dihydrate to obtain low-impurity battery-grade ferric phosphate.

2. The method for preparing low-impurity battery-grade iron phosphate according to 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 the solution B is 0.06% to 0.13%.

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

4. The method for preparing low-impurity battery-grade iron phosphate according to claim 1, wherein: In step S2, before mixing the clear liquid with the sodium nitrite solution, the pH value of the clear liquid is adjusted to 0.8-1.

2.

5. The method for preparing battery-grade iron phosphate according to claim 1, wherein: In step S2, the reaction temperature is 40-50°C.

6. The method for preparing battery-grade iron phosphate according to claim 1, wherein: In step S2, compressed oxygen is introduced into the reactor until the pressure inside the reactor reaches 10-15 bar.

7. The method for preparing low-impurity battery-grade iron phosphate according to claim 1, wherein: In step (1), the reaction temperature is 50-60° C.; after the iron source solution is added dropwise, the reaction is kept warm for 30-60 minutes.

8. The method for preparing low-impurity battery-grade iron phosphate according to claim 1, wherein: In step (2), the crystallization temperature is 90-95° C.; the crystallization time is 180-300 min.

9. The method for preparing low-impurity battery-grade iron phosphate according to claim 1, wherein: In step (3), the phosphoric acid is determined according to a molar ratio of P in the phosphoric acid to Fe in the cleaning slurry of 0.05 to 0.

20.

10. The method for preparing low-impurity battery-grade iron phosphate according to claim 1, wherein: 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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