A method for producing iron phosphate

Abstract

The application discloses a preparation method of iron phosphate, and relates to the technical field of battery positive electrode materials. The preparation method adds a part of supplementary phosphoric acid in the preparation process of the iron phosphate, and then continuously adds a part of a supplementary divalent iron source to react with the remaining supplementary phosphoric acid, so that the product yield is improved, and the energy consumption is reduced. Meanwhile, the application selects the specific time range of 10-30 minutes before the color of the material changes to white to add the supplementary divalent iron source, the addition amount of the supplementary divalent iron source is adjusted, the iron-phosphorus ratio can be controlled, the high and low of the iron-phosphorus ratio in the iron phosphate is effectively controlled, the yield of the iron phosphate is effectively improved, cost reduction and benefit increase are realized. The control of the iron-phosphorus ratio is beneficial to improving the sand grinding granularity when the iron phosphate is used as a precursor to prepare lithium iron phosphate, reducing the temperature in the sintering process, and realizing energy consumption reduction.

Description

Technical Field

[0001] This invention relates to the field of battery cathode material technology, and more specifically, to a method for preparing iron phosphate. Background Technology

[0002] Compared to ternary lithium batteries, lithium iron phosphate (LFP) has a lower energy density, limiting its application. Therefore, improving the energy density of LFP batteries has become a hot topic in the power battery field. For LFP material manufacturers, increasing the compaction density of the material is currently the most mainstream method to improve energy density. However, increasing the compaction density exacerbates the problems of low electronic conductivity and ion migration rate in LFP materials. Moreover, the physical doping of elements such as titanium, manganese, and vanadium during the LFP preparation process can lead to uneven distribution phenomena such as segregation and agglomeration in the product. Therefore, doping elements are introduced earlier in the preparation process of the LFP precursor—iron phosphate—using chemical methods for iron phosphate doping. This makes it easier for active elements to be incorporated into the crystal lattice rather than the bulk phase, and more easily forms lattice defects that promote lithium-ion diffusion.

[0003] However, precursor doping modification will cause changes in the core physicochemical index, iron-phosphorus ratio. Since the doping element will substitute Fe or P, the iron-phosphorus ratio of the precursor will be unbalanced. The Fe / P ratio is generally between 0.94 and 0.95. The iron-phosphorus ratio in the precursor iron phosphate will have a huge impact on the compaction density and electrical properties of the final product, lithium iron phosphate. If the iron-phosphorus ratio is too low, the electrical properties will be too low, while if the iron-phosphorus ratio is too high, the compaction density will be too low. Therefore, it is necessary to obtain a suitable iron-phosphorus ratio that can balance compaction density and electrical properties.

[0004] Secondly, fourth-generation and above lithium iron phosphate products adopt a two-sintering preparation process. One of the raw materials is a one-sintering product with low compaction density and high electrical performance. To prepare such products, the raw materials and auxiliary materials need to be ground to a very small particle size, which consumes a lot of energy. By controlling the iron-phosphorus ratio of the precursor, the grinding particle size can be increased, and the sintering temperature can be reduced, thus achieving a "double reduction" in energy consumption.

[0005] Therefore, how to improve the current problem of low iron-to-phosphorus ratio and uncontrollable level in the preparation of iron phosphate is a technical problem that needs to be solved.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing iron phosphate.

[0008] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing ferric phosphate, comprising: Using an iron source solution as the base solution, a phosphorus-oxygen solution was added, and after stirring and reacting, the mixture was washed until the conductivity of the washed water was ≤3500 μS / cm, and then filtered to obtain a water-containing filter cake. The aqueous filter cake is mixed with water, pulped, and heated and kept at a constant temperature to obtain a slurry. The slurry is transferred to an aging reactor, and a doping solution is added to the bottom liquid or the aging reactor. The aging reactor is heated to 85-100°C within 20-40 minutes, supplemented with phosphoric acid, and mixed and reacted until crystallization begins. The time for crystallization to begin is 10-30 minutes before the material turns white. The material that begins crystallization is used as preheated slurry. The preheated slurry and the supplemented ferrous iron source are added to the controlled reaction vessel in parallel. After the reaction continues until the color of the material turns white, an oxidant is added and the reaction continues for 60-180 minutes. After the reaction is completed, the material is washed until the conductivity of the water after washing is ≤500 μS / cm, and the precursor is obtained by filtration. The precursor is dried and calcined to obtain the iron phosphate product.

[0009] In an optional embodiment, the P / Fe molar ratio of phosphorus in the supplemented phosphoric acid to iron in the iron source solution is 0.15-1.2; And / or, the time point at which the crystallization begins is characterized by X-ray diffraction patterns, and the crystallization begins when a single prominent characteristic peak is detected in the X-ray diffraction pattern. And / or, the amount of the supplementary ferrous iron source added is 1%-15% of the mass of the iron source solution; And / or, the time period is 20-40 minutes; And / or, the amount of oxidant added is 1-1.3 times the molar amount of Fe in the system.

[0010] In an optional embodiment, the preheated slurry is fed by dripping through a feed coil, and the supplementary ferrous iron source is fed by spraying through a nozzle.

[0011] In an optional embodiment, the feeding time for the preheated slurry dripping is 10-40 minutes, and the feeding time for the supplementary divalent iron source spraying is 4-6 minutes.

[0012] In an optional embodiment, the molar ratio of iron to phosphorus in the iron source solution and the phosphorus-oxygen solution is 1:1-1.1; Preferably, the phosphorus-oxygen solution is introduced into the iron source solution at a flow rate of 7 L / min to 20 L / min; Preferably, the iron source solution and the phosphorus-oxygen solution are stirred and mixed at a speed of 100-350 r / min for 30-120 min.

[0013] In an optional embodiment, the pulping speed is 150-400 r / min, the heat preservation temperature is 35-75℃, the heat preservation reaction time is 30-60 min, and the solid content in the pulp is 5%-20%.

[0014] In an optional embodiment, the concentration of the iron source solution is 20-100 g / L, and the pH of the iron source solution is 0.5-3.5; And / or, the iron source in the iron source solution includes at least one of ferrous sulfate, ferrous oxalate, ferrous nitrate, ferrous chloride, iron powder, ferric oxide, ferric oxide, ferric phosphate slag, and lithium iron phosphate recycled black powder; And / or, the solution in the iron source solution includes at least one of pure water, phosphoric acid, sulfuric acid and hydrochloric acid.

[0015] In an optional embodiment, the doping solution includes at least one of the following: titanium sulfate, titanium oxysulfate, liquid titanium, manganese carbonate, manganese sulfate, manganese acetate, manganese nitrate, manganese acetate, manganese oxalate, manganese chloride, and manganese phosphate aqueous solution. And / or, the amount of the doping solution added is 0.3%-0.7% of the mass of the iron source solution used as the base solution.

[0016] In an optional embodiment, the phosphorus-oxygen solution is a mixture of a phosphorus source solution and an oxidant, wherein the amount of oxidant added to the phosphorus-oxygen solution is 1.0-1.3 times the molar amount of iron in the iron source solution; Preferably, the concentration of the phosphorus source solution is 20-80 g / L, and the pH of the phosphorus source solution is 3.0-8.5; Preferably, the amount of oxidant added accounts for 2.5%-5.5% of the mass of the phosphorus-oxygen solution; Preferably, the phosphorus source solution includes at least one of the following: ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, and an aqueous solution of sodium dihydrogen phosphate. Preferably, the oxidant includes at least one selected from hydrogen peroxide, potassium permanganate, nitric acid, and ammonium persulfate.

[0017] Secondly, the present invention provides an iron phosphate, which is prepared by the preparation method described in any of the foregoing embodiments.

[0018] The present invention has the following beneficial effects: The method for preparing ferric phosphate provided by this invention involves adding a portion of supplementary phosphoric acid during the ferric phosphate preparation process, followed by the addition of a portion of supplementary ferrous iron source. This allows the supplementary iron source to react with the remaining supplementary phosphoric acid, thereby increasing product yield and reducing energy consumption. Furthermore, this invention selects the specific time range of 10-30 minutes before the material turns white as the timeframe for adding the supplementary ferrous iron source. This is because this is the initial and critical period of the transformation from amorphous ferric phosphate to dihydrate ferric phosphate, during which the crystal growth has the most active sites, representing a rapid nucleation stage. The added ferrous iron source reacts with excess phosphoric acid to generate ferrous phosphate, which can be uniformly mixed with the material, avoiding segregation and agglomeration that would lead to uneven iron distribution within the crystals and adversely affect downstream products. By adjusting the amount of supplementary ferrous iron source added, this invention allows for controllable iron-to-phosphorus ratio, effectively controlling the iron-to-phosphorus ratio in ferric phosphate and significantly improving the yield, thus achieving cost reduction and efficiency improvement. Because of the addition of supplemental phosphoric acid, phosphorus was in excess in the aging reactor during the initial crystallization process, resulting in a large amount of free PO4. 3- When a divalent iron source is added, it is oxidized and reacts with phosphate to form iron phosphate, improving product yield and achieving cost reduction and efficiency improvement. The preparation method and process of this invention are simple, requiring no new equipment or factory buildings, ensuring the stability of the original product and avoiding product quality fluctuations caused by process adjustments. In particular, controlling the iron-phosphorus ratio is beneficial for improving the grinding particle size and reducing the temperature during the sintering process when using iron phosphate as a precursor to prepare lithium iron phosphate, achieving a "double reduction" in energy consumption. The lithium iron phosphate product prepared from this iron phosphate can balance compaction density and fully release electrical performance, achieving a balance between the two. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The image shows the XRD pattern of polyhydrate ferric phosphate in the amorphous stage of the initial slurry (before heating) in the aging reactor during step S4 of Example 1. Figure 2 This is the initial XRD pattern of amorphous iron phosphate beginning to transform into crystals 20 minutes before the material turns white in step S4 of Example 1; Figure 3 The image shows the XRD pattern of ferric phosphate dihydrate after adding ferrous iron source and oxidant in step S4 of Example 1. Figure 4This is a SEM image of ferric phosphate dihydrate during the drying stage in step S5 of Example 1. Figure 5 This is a SEM-EDS image of ferric phosphate dihydrate during the drying stage in step S5 of Example 1. Figure 6 This is a SEM image of anhydrous ferric phosphate during the calcination stage in step S5 of Example 1. Figure 7 The image shows the XRD pattern of anhydrous ferric phosphate during the calcination stage in step S5 of Example 1. Figure 8 The XRD pattern of anhydrous ferric phosphate in the calcination stage of step S5 in Comparative Example 5. Figure 9 The image shows the XRD pattern of anhydrous ferric phosphate during the calcination stage in step S5 of Comparative Example 6. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] This invention provides a method for preparing ferric phosphate, which includes the following steps: S1. Using the iron source solution as the base liquid, add water and phosphorus-oxygen solution, stir and react, then wash until the conductivity of the washed water is ≤3500us / cm, and filter to obtain a water-containing filter cake.

[0023] In this invention, the molar ratio of iron to phosphorus in the iron source solution and the phosphorus-oxygen solution is Fe:P = 1:1-1.1 (the amount of water added can be adjusted to control the solid content of the solution to 8%-12%); the concentration of the iron source solution is 20-100 g / L, and the pH of the iron source solution is 0.5-3.5; the phosphorus-oxygen solution is a mixture of phosphorus source solution and oxidant, wherein the amount of oxidant added in the phosphorus-oxygen solution is 1.0-1.3 times the molar amount of iron in the iron source solution, the concentration of the phosphorus source solution is 20-80 g / L, and the pH of the phosphorus source solution is 3.0-8.5; the amount of oxidant added accounts for 2.5%-5.5% of the mass of the phosphorus-oxygen solution.

[0024] In this invention, by controlling the concentration and mass ratio of the iron source solution and the phosphorus-oxygen solution, it is possible to ensure that the water-containing filter cake forms preliminary iron phosphate.

[0025] To ensure the uniformity of mixing between the iron source solution and the phosphorus-oxygen solution, in this invention, the phosphorus-oxygen solution is introduced into the iron source solution at a flow rate of 7 L / min to 20 L / min; the iron source solution and the phosphorus-oxygen solution are stirred and mixed at a speed of 100-350 r / min for 30-120 min.

[0026] In some typical but non-limiting examples, the iron source in the iron source solution includes, but is not limited to, at least one of ferrous sulfate, ferrous oxalate, ferrous nitrate, ferrous chloride, iron powder, ferric oxide, ferric oxide, ferric phosphate slag, and lithium iron phosphate recovery black powder; the solution in the iron source solution includes, but is not limited to, at least one of pure water, phosphoric acid, sulfuric acid, and hydrochloric acid. The phosphorus source solution includes, but is not limited to, at least one of aqueous solutions of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate; the oxidant includes, but is not limited to, at least one of hydrogen peroxide, potassium permanganate, nitric acid, and ammonium persulfate.

[0027] S2. Add water to the water-containing filter cake, beat it into a pulp, and heat it to keep it warm to obtain a slurry.

[0028] Transfer the water-containing filter cake to a pulping reactor, add an appropriate amount of hot pure water, control the solid content of the reactants within the range of 5%-20%, pulp at a speed of 150-400 r / min, heat to 35-75℃, and keep warm for 30-60 min. The solid content in the final pulp is also maintained at 5%-20%.

[0029] S3. Transfer the slurry to the aging reactor. The bottom liquid or aging reactor contains a doping solution.

[0030] In this invention, there are two methods for adding the doping solution. One method is to add it directly to the base solution to achieve synthetic doping. When adding the doping solution to the base solution, it is necessary to control the flow rate and stirring speed, and to continuously stir to ensure that the doping solution and the iron source solution are mixed evenly. Specifically, the flow rate of the doping solution is 0.1-0.3 L / min, the stirring speed is 200-300 r / min, and the stirring time is 30-90 min.

[0031] The second method of addition is to add the doping solution into the aging reactor. After the slurry is transferred to the aging reactor, the doping solution is added to the slurry. The flow rate of the doping solution is controlled at 0.1-0.3 L / min, the stirring speed is 200-300 r / min, and stirring is continued for 30-90 min to ensure that the doping solution and the slurry are mixed evenly.

[0032] Both of the above methods can achieve uniform incorporation of the doping solution into the iron phosphate precursor. The amount of doping solution added is 0.3%-0.7% of the mass of the iron source solution used as the base solution.

[0033] The doping solution includes, but is not limited to, at least one of the following aqueous solutions: titanium sulfate, titanium oxysulfate, titanium solution, manganese carbonate, manganese sulfate, manganese acetate, manganese nitrate, manganese acetate, manganese oxalate, manganese chloride, and manganese phosphate.

[0034] S4. Heat the aging reactor to 85-100℃ within 20-40 minutes, add supplementary phosphoric acid, and mix and react until crystallization begins. The time for crystallization to begin is 10-30 minutes before the material turns white. Use the material that has begun crystallization as preheated slurry.

[0035] In this invention, the P / Fe molar ratio of phosphorus in the supplemented phosphoric acid and iron in the iron source solution is 0.15-1.2. After adding supplemented phosphoric acid, samples are continuously taken and X-ray diffraction (XRD) is used to detect whether the crystallization initiation point has been reached. The crystallization initiation point is characterized by XRD patterns. When a prominent single characteristic peak is detected in the XRD pattern, crystallization is confirmed to have begun. This is because the atomic arrangement of amorphous materials is short-range ordered and long-range disordered. When X-rays irradiate such materials, only the short-range ordered atomic arrangement produces weak coherent scattering. The scattered waves cannot form stable interference enhancement over a large area, ultimately appearing as broadened, flat diffuse peaks (commonly known as "large blobs") in the XRD pattern, without sharp characteristic peaks. When amorphous materials undergo crystallization transformation under external conditions (such as heating), the atoms gradually adjust from a disordered arrangement to a long-range ordered crystal structure, forming a periodic crystal lattice. When X-rays irradiate the crystal, the crystal planes selectively and coherently enhance the scattering of X-rays. This type of scattering signal has high intensity and strong directivity, which is reflected in the XRD pattern as sharp, prominent characteristic diffraction peaks. Therefore, in this invention, the detection of a single prominent characteristic peak in the X-ray diffraction pattern is used to confirm the start of crystal transformation, and the material at this stage is used as the preheating slurry.

[0036] S5. Add the preheated slurry and the supplemented ferrous iron source in parallel to the controlled reaction vessel. Continue the reaction until the material turns white. After a period of time, add the oxidant and continue the reaction for 60-180 minutes. After the reaction is completed, wash the material until the conductivity of the water after washing is ≤500us / cm. Filter to obtain the precursor.

[0037] In this invention, a supplementary ferrous iron source is added 10-30 minutes before the material turns white (i.e., after the crystal transformation begins). The amount of supplementary ferrous iron source added is 1%-15% of the mass of the iron source solution used as the base liquid. This effectively controls the iron-phosphorus ratio of the precursor. By adjusting the amount of ferrous iron added, the iron-phosphorus ratio can be controlled. In this way, the lithium iron phosphate prepared from iron phosphate can achieve both compaction density and full release of electrical performance, achieving a balance between the two. The reason for choosing to add ferrous iron 10-30 minutes before the material turns white is that this is the initial and critical period of the transformation of polyhydrate amorphous iron phosphate into dihydrate crystal iron phosphate. At this time, the active sites for crystal growth are the most numerous, belonging to the explosive nucleation stage. The added ferrous iron source reacts with excess phosphoric acid to generate ferrous phosphate, which can be uniformly mixed with the material to avoid segregation and agglomeration, which would lead to uneven iron distribution in the crystal and adversely affect downstream products.

[0038] The timing of adding ferrous iron is crucial. It must be added 10-30 minutes before the material turns white. If ferrous iron is added after the material has turned white, the amorphous ferric phosphate has already completed its crystalline transformation to crystalline ferric phosphate. The added ferrous iron can only adhere to the surface of the crystals. After oxidation, it combines with phosphate ions and can only grow on the crystal surface, resulting in uneven iron-phosphorus distribution and decreased purity. This has a significant impact on downstream lithium blending products, leading to abnormal electrical performance and compaction density. If ferrous iron is added too early during the slurry stage, a mixture of ferrous and ferric iron will appear in the material. This will cause impurities to form during the amorphous-to-crystalline transformation of ferric phosphate, resulting in decreased product purity and affecting subsequent use.

[0039] Therefore, this invention requires strict control over the timing of adding the supplementary ferrous iron source. Furthermore, the invention also requires strict control over the methods of adding the preheated slurry and the supplementary ferrous iron source. Specifically, the preheated slurry is fed by dripping through a feed coil, while the supplementary ferrous iron source is sprayed through a nozzle. The feeding time for dripping the preheated slurry is 10-40 minutes, and the feeding time for spraying the supplementary ferrous iron source is 4-6 minutes.

[0040] Subsequently, after the material turns white for a period of time (e.g., 20-40 minutes), an oxidant is added. The oxidant can safely oxidize ferrous iron (Fe2+) to ferric iron (Fe3+). The ferric iron then rapidly reacts with free phosphate ions in the slurry to form ferric phosphate. The amount of oxidant added is 1-1.3 times the molar amount of Fe in the system. The oxidant includes, but is not limited to, at least one of hydrogen peroxide, potassium permanganate, nitric acid, and ammonium persulfate.

[0041] S6. The precursor is dried and calcined to obtain the iron phosphate product.

[0042] In this invention, the drying temperature is 80-120℃, the calcination temperature is 650-750℃, and finally, iron phosphate product is obtained.

[0043] The lithium iron phosphate prepared by the above preparation method can balance compaction density and fully release electrical performance.

[0044] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0045] Example 1 This embodiment provides a method for preparing ferric phosphate, which includes the following steps: S1. Take 8 kg of an iron source solution (ferrous sulfate) with a concentration of 65 g / L and a pH of 2, add it to the reactor as the base liquid, start stirring, then add 1.8 kg of 75℃ hot pure water to make up the volume and continue stirring for 10 min at a stirring speed of 200 r / min; take 5.5 kg of a phosphorus-oxygen solution (ammonium dihydrogen phosphate + hydrogen peroxide) with a concentration of 50 g / L and a pH of 7.35, where the amount of hydrogen peroxide added accounts for 3.5% of the mass of the phosphorus-oxygen solution, add it to the reactor containing the iron source base liquid at a flow rate of 10 L / min, stirring the reactor at a stirring speed of 200 r / min during the addition process. After the phosphorus-oxygen solution is added, continue the synthesis reaction for 60 min; after the synthesis reaction is completed, wash the material until the conductivity of the water after washing is 3000 μS / cm, and filter to obtain a solid water-containing filter cake; S2. Transfer the water-containing filter cake to a pulping reactor, add hot pure water with a weight of 3 times that of the water-containing filter cake, control the solid content of the reactants to be 15%, stir at 300 r / min, heat to 55℃, keep warm for 60 min, and obtain slurry. S3. Transfer the slurry to the aging reactor, add 0.5L of titanium source (titanium sulfate solution) at a flow rate of 0.1L / min to ensure that the titanium doping content of the product is maintained at 5000ppm, stir at a stirring speed of 300r / min, and continue stirring for 60min.

[0046] S4. Heat the aging reactor to 95°C within 30 minutes, then add phosphoric acid. The amount of phosphoric acid added is such that the molar ratio of P in the phosphoric acid to Fe in the iron source solution used as the bottom liquid is 0.8. Mix and react until crystallization begins. The time for crystallization to begin is 20 minutes before the material turns white. Use the material that has begun crystallization as the preheated slurry. The S5, 95℃ preheated slurry was fed by drip feeding through a feed coil. 0.4 kg (5% of the iron source solution mass) of ferrous iron source (65 g / L, pH 2 ferrous sulfate solution) was added to the controlled reactor in parallel flow with the 95℃ slurry by spraying through a nozzle. The addition time of the preheated slurry was controlled at 25 min, and the addition time of the ferrous iron source was controlled at 5 min. After the reaction continued until the material turned white, 30 min later, 71 g of oxidant (hydrogen peroxide) was added, ensuring that its molar amount was 1 times the molar amount of ferrous iron in the system. The reaction continued for 180 min. After the reaction, the material was washed until the conductivity of the water after washing was 300 μS / cm, and the precursor was obtained by filtration.

[0047] S6. The precursor is dried at 105°C, and then ferric phosphate dihydrate is calcined at 700°C to obtain the target product, ferric phosphate.

[0048] Example 2 This embodiment provides a method for preparing ferric phosphate, which includes the following steps: S1. Take 8 kg of ferrous sulfate solution (65 g / L, pH 2) as the base liquid in the reactor. Simultaneously, add 0.5 L of titanium source (titanium sulfate solution) at a flow rate of 0.1 L / min to ensure that the titanium doping content of the product is maintained at 5000 ppm. Then, add 1.8 kg of 75℃ hot pure water to make up the volume and stir continuously for 10 min at a stirring speed of 200 r / min. Take 5.5 kg of phosphorus-oxygen solution (ammonium dihydrogen phosphate + hydrogen peroxide) with a concentration of 50 g / L and pH 7.35, where the amount of hydrogen peroxide added accounts for 3.5% of the mass of the phosphorus-oxygen solution. Add it to the reactor containing the iron-titanium mixed base liquid at a flow rate of 10 L / min. During the addition, the stirring speed of the reactor is 200 r / min. After the phosphorus-oxygen solution is added, continue the synthesis reaction for 60 min. After the synthesis reaction is completed, wash the material until the conductivity of the water after washing is 3000 μS / cm, and filter to obtain a solid water-containing filter cake. S2. Transfer the water-containing filter cake to a pulping reactor, add hot pure water with a weight of 3 times the water-containing filter cake, control the solid content of the reactants to be 15%, stir at 300 r / min, keep the reaction temperature at 55℃, and react for 60 min to obtain a slurry. S3. Transfer the slurry to the aging reactor; S4. Control the temperature to 95℃ within 30 minutes, then add phosphoric acid. The amount of phosphoric acid added is such that the molar ratio of P in the phosphoric acid to Fe in the iron source solution used as the base liquid is 0.8. Mix and react until crystallization begins. The time when crystallization begins is 10 minutes before the material turns white. Use the material that begins crystallization as the preheated slurry. The S5, 95℃ preheated slurry was fed by drip feeding through a feed coil. 0.24 kg (3% of the iron source solution mass) of ferrous iron source (65 g / L, pH 2 ferrous sulfate solution) was added to the controlled reactor in parallel flow with the 95℃ preheated slurry by spraying through a nozzle. The addition time of the preheated slurry was controlled at 25 min, and the addition time of the ferrous iron source was controlled at 5 min. After the material turned white, 30 min later, 43 g of oxidant (hydrogen peroxide) was added, ensuring that the molar amount of oxidant was 1 times the molar amount of ferrous iron in the system. The reaction was continued for 180 min. After the reaction, the material was washed until the conductivity of the water after washing was 300 μS / cm, and the precursor was obtained by filtration. S6. The precursor is dried at 105°C, and then ferric phosphate dihydrate is calcined at 700°C to obtain the target product, ferric phosphate.

[0049] Example 3 This embodiment provides a method for preparing ferric phosphate, which includes the following steps: S1. Take 8 kg of ferrous sulfate solution (65 g / L, pH 2) as the base liquid in the reactor. Simultaneously, add 0.5 L of titanium source (titanium sulfate solution) at a flow rate of 0.1 L / min to ensure that the titanium doping content of the product is maintained at 5000 ppm. Then, add 1.8 kg of 75℃ hot pure water to make up the volume and stir continuously for 10 min at a stirring speed of 200 r / min. Take 5.5 kg of phosphorus-oxygen solution (ammonium dihydrogen phosphate + hydrogen peroxide) with a concentration of 50 g / L and pH 7.35, where the amount of hydrogen peroxide added accounts for 3.5% of the mass of the phosphorus-oxygen solution. Add it to the reactor containing the iron-titanium mixed base liquid at a flow rate of 10 L / min. During the addition, the stirring speed of the reactor is 200 r / min. After the phosphorus-oxygen solution is added, continue the synthesis reaction for 60 min. After the synthesis reaction is completed, wash the material until the conductivity of the water after washing is 3000 μS / cm, and filter to obtain a solid water-containing filter cake. S2. Transfer the water-containing filter cake to a pulping reactor, add hot pure water with a weight of 3 times the water-containing filter cake, control the solid content of the reactants to be 15%, stir at 300 r / min, keep the reaction temperature at 55℃, and react for 60 min to obtain a slurry. S3. Transfer the slurry to the aging reactor; S4. Then, control the temperature to rise to 95°C within 30 minutes, and then add phosphoric acid. The amount of phosphoric acid added is such that the molar ratio of P in the phosphoric acid to Fe in the iron source solution used as the base liquid is 0.8. Mix and react until crystallization begins. The time when crystallization begins is 30 minutes before the material turns white. Use the material that begins crystallization as the preheated slurry. The S5, 95℃ preheated slurry was fed by drip feeding through a feed coil. 0.56 kg (7% of the iron source solution mass) of ferrous iron source (65 g / L, pH 2 ferrous sulfate solution) was added to the controlled reactor in parallel flow with the 95℃ preheated slurry by spraying through a nozzle. The addition time of the preheated slurry was controlled at 25 min, and the addition time of the ferrous iron source was controlled at 5 min. After the material turned white, 30 min later, 100.3 g of oxidant (hydrogen peroxide) was added, ensuring that its molar amount was 1 times the molar amount of ferrous iron in the system. The reaction was continued for 180 min. After the reaction, the material was washed until the conductivity of the water after washing was 300 μS / cm, and the precursor was obtained by filtration. S6. The precursor is dried at 105°C, and then ferric phosphate dihydrate is calcined at 700°C to obtain the target product, ferric phosphate.

[0050] Example 4 This embodiment provides a method for preparing ferric phosphate, which includes the following steps: S1. Take 26 kg of ferrous sulfate (20 g / L, pH 1) as the base solution in the reactor and start stirring for 10 min at a speed of 200 r / min. Take 13.75 kg of ammonium dihydrogen phosphate (ammonium dihydrogen phosphate + hydrogen peroxide) (20 g / L, pH 4, with hydrogen peroxide accounting for 2.5% of the mass of the phosphorus-oxygen solution) and add it to the reactor containing the base solution at a flow rate of 7 L / min. During the addition, the reactor is stirred at a speed of 100 r / min. After the phosphorus-oxygen solution is added, continue the synthesis reaction for 30 min. After the synthesis reaction is completed, wash the material until the conductivity of the water after washing is 3000 μS / cm, and filter to obtain a solid water-containing filter cake. S2. Transfer the water-containing filter cake to a pulping reactor, add hot pure water, control the solid content of the reactants to 5%, stir at 150 r / min, keep the reaction temperature at 40℃, and react for 60 min to obtain a slurry. S3. Transfer the slurry material to the aging reactor, add 0.5L of titanium source (titanium liquid) at a flow rate of 0.2L / min to ensure that the titanium doping content of the product is maintained at 5000ppm, stir at a stirring speed of 200r / min, and continue stirring for 90min.

[0051] S4. Heat the aging reactor to 95°C within 30 minutes, then add phosphoric acid. The amount of phosphoric acid added is such that the molar ratio of P in the phosphoric acid to Fe in the iron source solution used as the bottom liquid is 0.8. Mix and react until crystallization begins. The time for crystallization to begin is 20 minutes before the material turns white. Use the material that has begun crystallization as the preheated slurry. The S5, 95℃ preheated slurry was fed by drip feeding through a feed coil. 0.4 kg (1.5% of the iron source solution mass) of ferrous iron source (20 g / L, pH 1 ferrous sulfate solution) was added to the controlled reactor in parallel flow with the 95℃ slurry by spraying through a nozzle. The addition time of the preheated slurry was controlled at 10 min, and the addition time of the ferrous iron source was controlled at 4 min. After the reaction continued until the material turned white, 30 min later, 71 g of oxidant (hydrogen peroxide) was added, ensuring that its molar amount was 1 times the molar amount of ferrous iron in the system. The reaction continued for 180 min. After the reaction, the material was washed until the conductivity of the water after washing was 300 μS / cm, and the precursor was obtained by filtration.

[0052] S6. The precursor is dried at 105°C, and then ferric phosphate dihydrate is calcined at 700°C to obtain the target product, ferric phosphate.

[0053] Example 5 This embodiment provides a method for preparing ferric phosphate, which includes the following steps: S1. Take 5.2 kg of ferrous sulfate solution (100 g / L, pH 3) as the base solution in the reactor and start stirring. Then add 5.2 kg of 75°C hot pure water to make up the volume and continue stirring for 10 min at a stirring speed of 200 r / min. Take 3.44 kg of ammonium dihydrogen phosphate solution (ammonium dihydrogen phosphate + hydrogen peroxide) with a concentration of 80 g / L and pH 8, where the amount of hydrogen peroxide added accounts for 5.5% of the mass of the phosphorus-oxygen solution. Add it to the reactor containing the base solution at a flow rate of 20 L / min. During the addition process, the stirring speed of the reactor is 300 r / min. After the phosphorus-oxygen solution is added, continue the synthesis reaction for 120 min. After the synthesis reaction is completed, wash the material until the conductivity of the washing water is 3000 μS / cm. Filter to obtain a solid water-containing filter cake with a wash water conductivity of 3000 μS / cm. S2. Transfer the water-containing filter cake to a pulping reactor, add hot pure water, control the solid content of the reactants to be 20%, stir at 400 r / min, keep the reaction temperature at 70℃, and react for 30 min to obtain a slurry. S3. Transfer the slurry material to the aging reactor, add 0.5L of titanium source (titanium sulfate solution) at a flow rate of 0.3L / min to ensure that the titanium doping content of the product is maintained at 5000ppm, stir at a stirring speed of 400r / min, and continue stirring for 30min.

[0054] S4. Heat the aging reactor to 95°C within 30 minutes, then add phosphoric acid. The amount of phosphoric acid added is such that the molar ratio of P in the phosphoric acid to Fe in the iron source solution used as the bottom liquid is 0.8. Mix and react until crystallization begins. The time for crystallization to begin is 20 minutes before the material turns white. Use the material that has begun crystallization as the preheated slurry. The S5, 95℃ preheated slurry was fed by drip feeding through a feed coil. 0.4 kg (7.7% of the iron source solution mass) of ferrous iron source (100 g / L, pH 3 ferrous sulfate solution) was added to the controlled reactor in parallel flow with the 95℃ slurry via a spray nozzle. The addition time of the preheated slurry was controlled at 40 min, and the addition time of the ferrous iron source was controlled at 6 min. After the reaction continued until the material turned white, 30 min later, 71 g of oxidant (hydrogen peroxide) was added, ensuring that its molar amount was 1 times the molar amount of ferrous iron in the system. The reaction continued for 180 min. After the reaction, the material was washed until the conductivity of the water after washing was 300 μS / cm, and the precursor was obtained by filtration.

[0055] S6. The precursor is dried at 105°C, and then ferric phosphate dihydrate is calcined at 700°C to obtain the target product, ferric phosphate.

[0056] Comparative Example 1 This comparative example is basically the same as Example 2, except that in this comparative example, step S5 did not add supplementary ferrous iron source and oxidant. Specifically, the temperature was raised to 95°C within 30 minutes, and then phosphoric acid was added. The amount of phosphoric acid added was such that the molar ratio of P in the phosphoric acid to Fe in the iron source solution used as the base liquid was 0.8. After the slurry turned white, the reaction continued for 180 minutes. After the reaction was completed, the material was washed until the conductivity of the water after washing was 300 μS / cm, and the precursor was obtained by filtration.

[0057] Comparative Example 2 This comparative example is basically the same as Example 1, except that in steps S4-S5 of this comparative example, a supplementary ferrous iron source is added within 40 minutes before the material turns white, and the iron-phosphorus ratio is lower than expected.

[0058] Comparative Example 3 This comparative example is basically the same as Example 1, except that in steps S4-S5 of this comparative example, a supplementary ferrous iron source is added within 5 minutes before the material turns white, and the iron-phosphorus ratio is lower than expected.

[0059] Comparative Example 4 This comparative example is basically the same as Example 1, except that in step S5 of this comparative example, the preheated slurry at 95°C is used as the base liquid, and the ferrous iron source is added dropwise directly, resulting in a lower iron-to-phosphorus ratio than expected.

[0060] Comparative Example 5 This comparative example is basically the same as Example 2, except that the ferrous iron source is added during the pulping stage, and the anhydrous ferric phosphate XRD shows impurities at low angles.

[0061] Comparative Example 6 This comparative example is basically the same as Example 3, except that in this comparative example, after the color of the material turns white, the anhydrous iron phosphate XRD shows impurities at low angles.

[0062] Experimental Example 1 This experimental example demonstrates X-ray diffraction testing of ferric phosphate at each stage of step S4 in Example 1.

[0063] in, Figure 1 This is the XRD pattern of the initial slurry (before heating) in the aging reactor. At this stage, the polyhydrated ferric phosphate in the initial slurry is in an amorphous stage, belonging to the non-crystalline state. After adding phosphoric acid, 20 minutes before the material turns white, the amorphous ferric phosphate begins to transform into crystals. Please refer to [link to relevant documentation]. Figure 2 The initial XRD pattern shows that crystalline peaks begin to appear within the amorphous diffraction peaks, indicating a transition state. After adding a supplementary ferrous iron source and oxidant, please refer to [the original text]. Figure 3 The XRD pattern shows that ferric phosphate dihydrate is in a crystalline state. In this embodiment, the addition of ferrous iron was chosen 20 minutes before the material turned white. This is because this is the initial and critical period of the transformation from polyhydrate amorphous ferric phosphate to crystalline ferric phosphate. At this time, the active sites for crystal growth are at their highest, representing an explosive nucleation stage. The added ferrous iron reacts with the excess phosphoric acid to generate ferrous phosphate, which can be uniformly mixed with the material, avoiding segregation and agglomeration. Uneven iron distribution within the crystals would negatively impact downstream products.

[0064] Please refer to the SEM image of ferric phosphate dihydrate during the final drying stage. Figure 4 The SEM images show that ferric phosphate dihydrate is composed of a lamellar structure. Figure 5 This is a SEM-EDS image of ferric phosphate dihydrate prepared in the drying stage in Example 1. The elemental composition of ferric phosphate dihydrate can be seen from the SEM-EDS image. Figure 6 This is a SEM image of anhydrous ferric phosphate during the calcination stage of Example 1. The SEM image shows that anhydrous ferric phosphate is composed of a blocky structure. Figure 7 This is the XRD pattern of anhydrous ferric phosphate during the calcination stage of Example 1. The XRD pattern shows that the anhydrous ferric phosphate has high crystallinity, and the corresponding standard card is 77-0094.

[0065] Experimental Example 2 The iron-to-phosphorus ratio and doping amount of the iron phosphate prepared in the above examples and comparative examples were tested. The testing methods included Fe potassium dichromate titration, P gravimetric method, and ICP method for doping elements. The test results are shown in Table 1.

[0066] Table 1. Statistical table of detection results for different examples

[0067] As can be seen from the table above, comparing the iron-phosphorus ratios of Examples 1-5 and Comparative Example 1, the addition of ferrous iron (Fe2+) does indeed help improve the Fe2+ ratio. Data from Comparative Examples 2-3 and 5-6 show that the timing of the addition of the ferrous iron source is crucial. Adding it outside the time frame specified in this application will significantly reduce the Fe2+ ratio. Furthermore, adding the ferrous iron source during the pulping stage or after the material has turned white will also significantly reduce the Fe2+ ratio. If ferrous iron is added after the material has turned white, the amorphous iron phosphate has already completed its crystalline transformation to crystalline iron phosphate. At this point, the added ferrous iron can only adhere to the crystal surface. After oxidation and combination with phosphate ions, it can only grow on the crystal surface, resulting in uneven distribution of iron and phosphorus in the material, decreased purity, and a significant impact on downstream lithium compounding, leading to abnormal electrical performance and compaction density. If ferrous iron is added too early during the pulping stage, a mixture of ferrous and ferric iron will appear in the material, causing impurities to form during the amorphous-to-crystalline transformation of iron phosphate, resulting in decreased product purity and affecting subsequent use. As can be seen from Comparative Example 4, the method of adding the 95℃ slurry and the ferrous iron source is also crucial. Using the conventional dripping method will result in poor mixing effect of the 95℃ slurry and the ferrous iron source, which will lead to a significant reduction in the iron-phosphorus ratio.

[0068] In addition, X-ray diffraction tests were performed on the calcined anhydrous ferric phosphate of Comparative Examples 5-6 mentioned above, from... Figure 8 and Figure 9 It can be seen that the XRD patterns of the calcined anhydrous ferric phosphate in Comparative Examples 5-6 all show impurity phases near the low angle.

[0069] In summary, the method for preparing ferric phosphate provided by this invention improves product yield and reduces energy consumption by adding a portion of supplementary phosphoric acid and then a portion of supplementary ferrous iron source during the ferric phosphate preparation process, allowing it to react with the remaining phosphoric acid. Furthermore, this invention selects the specific time range of 10-30 minutes before the material turns white to add the supplementary ferrous iron source. This is because this is the initial and critical period of the transformation from amorphous ferric phosphate to dihydrate ferric phosphate, during which the active sites for crystal growth are at their highest, representing a burst nucleation stage. The added supplementary ferrous iron source reacts with excess phosphoric acid to generate ferrous phosphate, which can be uniformly mixed with the material, avoiding segregation and agglomeration that would lead to uneven iron distribution within the crystals and adversely affect downstream products. By adjusting the amount of supplementary ferrous iron source added, this invention allows for controllable iron-to-phosphorus ratio, effectively controlling the iron-to-phosphorus ratio in ferric phosphate, and also effectively improving the yield of ferric phosphate, achieving cost reduction and efficiency improvement. Because of the addition of supplemental phosphoric acid, phosphorus was in excess in the aging reactor during the initial crystallization process, resulting in a large amount of free PO4. 3- When a divalent iron source is added, it is oxidized and reacts with phosphate to form iron phosphate, increasing product yield and achieving cost reduction and efficiency improvement. The preparation method and process of this invention are simple, requiring no new equipment or factory buildings, ensuring the stability of the original product and avoiding product quality fluctuations caused by process adjustments. In particular, controlling the iron-to-phosphorus ratio in iron phosphate is beneficial for improving the grinding particle size and reducing the temperature during the sintering process when using iron phosphate as a precursor for lithium iron phosphate preparation, achieving a "double reduction" in energy consumption. The lithium iron phosphate product prepared from this iron phosphate can balance compaction density and fully release electrical performance, achieving a balance between the two.

[0070] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing ferric phosphate, characterized in that, It includes: Using an iron source solution as the base solution, a phosphorus-oxygen solution was added, and after stirring and reacting, the mixture was washed until the conductivity of the washed water was ≤3500μS / cm, and then filtered to obtain a water-containing filter cake. The aqueous filter cake is mixed with water, pulped, and heated and kept at a constant temperature to obtain a slurry. The slurry is transferred to an aging reactor, and a doping solution is added to the bottom liquid or the aging reactor. The aging reactor is heated to 85-100°C within 20-40 minutes, supplemented with phosphoric acid, and mixed and reacted until crystallization begins. The time for crystallization to begin is 10-30 minutes before the material turns white. The material that begins crystallization is used as preheated slurry. The preheated slurry and the supplementary ferrous iron source are added to the controlled reaction vessel in parallel. The preheated slurry is fed by dripping through the feed coil, and the supplementary ferrous iron source is sprayed through the nozzle. After the reaction continues until the color of the material turns white, an oxidant is added and the reaction continues for 60-180 minutes. After the reaction is completed, the material is washed until the conductivity of the water after washing is ≤500μS / cm, and the precursor is obtained by filtration. The precursor is dried and calcined to obtain the iron phosphate product.

2. The method for preparing ferric phosphate according to claim 1, characterized in that, The P / Fe molar ratio of phosphorus in the supplemented phosphoric acid to iron in the iron source solution is 0.15-1.2; And / or, the time point at which the crystallization begins is characterized by X-ray diffraction patterns, and the crystallization begins when a single prominent characteristic peak is detected in the X-ray diffraction pattern. And / or, the amount of the supplementary ferrous iron source added is 1%-15% of the mass of the iron source solution; And / or, the time period is 20-40 minutes; And / or, the amount of oxidant added is 1-1.3 times the molar amount of Fe in the system.

3. The method for preparing ferric phosphate according to claim 1, characterized in that, The feeding time for the preheated slurry dripping is 10-40 minutes, and the feeding time for the supplementary divalent iron source spraying is 4-6 minutes.

4. The method for preparing ferric phosphate according to claim 1, characterized in that, The molar ratio of iron to phosphorus in the iron source solution and the phosphorus-oxygen solution is 1:1-1.

1.

5. The method for preparing ferric phosphate according to claim 4, characterized in that, The phosphorus-oxygen solution is introduced into the iron source solution at a flow rate of 7 L / min to 20 L / min.

6. The method for preparing ferric phosphate according to claim 4, characterized in that, The iron source solution and the phosphorus-oxygen solution are stirred and mixed at a speed of 100-350 r / min for 30-120 min.

7. The method for preparing ferric phosphate according to claim 1, characterized in that, The pulping speed is 150-400 r / min, the heat preservation temperature is 35-75℃, the heat preservation reaction time is 30-60 min, and the solid content in the pulp is 5%-20%.

8. The method for preparing ferric phosphate according to any one of claims 1-7, characterized in that, The concentration of the iron source solution is 20-100 g / L, and the pH of the iron source solution is 0.5-3.5; And / or, the iron source in the iron source solution includes at least one of ferrous sulfate, ferrous oxalate, ferrous nitrate, ferrous chloride, iron powder, ferric oxide, ferric oxide, ferric phosphate slag, and lithium iron phosphate recycled black powder; And / or, the solution in the iron source solution includes at least one of pure water, phosphoric acid, sulfuric acid and hydrochloric acid.

9. The method for preparing ferric phosphate according to any one of claims 1-7, characterized in that, The doping solution includes at least one of the following aqueous solutions: titanium sulfate, titanium oxysulfate, manganese carbonate, manganese sulfate, manganese nitrate, manganese acetate, manganese oxalate, manganese chloride, and manganese phosphate. And / or, the amount of the doping solution added is 0.3%-0.7% of the mass of the iron source solution used as the base solution.

10. The method for preparing ferric phosphate according to any one of claims 1-7, characterized in that, The phosphorus-oxygen solution is a mixture of a phosphorus source solution and an oxidant, wherein the amount of oxidant added to the phosphorus-oxygen solution is 1.0-1.3 times the molar amount of iron in the iron source solution.

11. The method for preparing ferric phosphate according to claim 10, characterized in that, The concentration of the phosphorus source solution is 20-80 g / L, and the pH of the phosphorus source solution is 3.0-8.

5.

12. The method for preparing ferric phosphate according to claim 10, characterized in that, The amount of oxidant added accounts for 2.5%-5.5% of the mass of the phosphorus-oxygen solution.

13. The method for preparing ferric phosphate according to claim 10, characterized in that, The phosphorus source solution includes at least one of the following: ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, and an aqueous solution of sodium dihydrogen phosphate.

14. The method for preparing ferric phosphate according to claim 10, characterized in that, The oxidant includes at least one of hydrogen peroxide, potassium permanganate, nitric acid, and ammonium persulfate.

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