A method for preparing high-iron phosphorus ratio iron phosphate in low cost and in batches

Abstract

The application provides a method for preparing high-iron-phosphorus-ratio iron phosphate in batches at low cost, and the steps are as follows: S1. reacting a ferrous sulfate solution, a phosphorus salt solution and hydrogen peroxide to obtain amorphous iron phosphate; S2. filtering and washing the amorphous iron phosphate; S3. adding pure water to the filtered cake of the washed amorphous iron phosphate, stirring, heating, adding a crystal transformation promoter after keeping warm, and reacting to obtain iron phosphate dihydrate slurry, and reserving part of the iron phosphate dihydrate slurry; S4. squeezing, drying and calcining the remaining iron phosphate dihydrate slurry; S5. repeating steps S1-S4 in batches; if it is the first batch reaction, the crystal transformation promoter is phosphoric acid, and if it is not the first batch reaction, the crystal transformation promoter is a mixture of hydrogen peroxide and crystal nuclei prepared from the iron phosphate dihydrate slurry reserved in the last batch. The method can greatly save the preparation cost of iron phosphate, and the iron-phosphorus ratio of the prepared iron phosphate is above 0.99.

Description

Technical Field

[0001] This invention relates to the field of ferric phosphate preparation technology, and specifically to a low-cost, multi-batch method for preparing ferric phosphate with a high iron-to-phosphorus ratio. Background Technology

[0002] Iron phosphate (FePO4) is a white or off-white monoclinic crystalline powder with wide industrial applications, primarily in new energy batteries, catalysts, and water treatment. Iron phosphate is a key raw material for the production of lithium iron phosphate batteries, which are widely used in new energy vehicles, electric bicycles, and energy storage systems due to their high safety, long lifespan, and low cost. Iron phosphate can also be used as a component of catalysts to accelerate certain chemical reactions, improving reaction efficiency and selectivity. Furthermore, iron phosphate has the ability to adsorb heavy metal ions and can be used in wastewater treatment to remove harmful substances from water.

[0003] Currently, chemical precipitation is the primary method for preparing iron phosphate in industrial production, with the ammonia and sodium processes being the mainstream technologies. However, existing methods for preparing iron phosphate have several drawbacks, such as high cost, low iron-to-phosphorus ratio, and crystal structure defects. Specifically, traditional methods often require high-purity raw materials and involve complex processes, resulting in high production costs. This not only increases battery manufacturing costs but also limits the large-scale application of iron phosphate. The iron-to-phosphorus ratio is a crucial parameter for iron phosphate materials, directly affecting their electrochemical performance. Existing methods often struggle to precisely control this ratio, leading to unstable performance of the resulting iron phosphate materials and impacting battery performance and lifespan. Furthermore, some methods may result in defects in the iron phosphate crystal structure, such as lattice distortion and uneven grain size, which can affect the conductivity and ion diffusion rate of iron phosphate, thereby impacting battery charge-discharge performance.

[0004] These drawbacks can lead to a decline in the performance of iron phosphate materials in battery applications, such as rapid capacity decay, increased internal resistance, and shortened cycle life. Therefore, researching and developing low-cost, high-performance iron phosphate preparation methods is of great significance for improving the performance and reducing the cost of lithium iron phosphate batteries. Chinese patent application number CN115571865A discloses a method for preparing high-quality iron phosphate. This invention uses a two-step ammonia method to prepare iron phosphate, using divalent iron salts and ammonium phosphate salts as raw materials through steps such as oxidation synthesis, primary washing, high-temperature aging, secondary washing, drying, and calcination. The prepared iron phosphate has high quality and good consistency. However, the process involves two washing steps, resulting in a large amount of rinsing wastewater and increased treatment costs. Furthermore, the addition of phosphoric acid during the aging process increases raw material costs and can easily lead to the formation of phosphorus-containing impurities in the product, resulting in a low iron-to-phosphorus ratio. Chinese patent application number CN117023537A discloses a method for preparing low-temperature, highly dispersible spherical nano-ferric phosphate. This invention uses a sodium process to prepare ferric phosphate, using iron salts and phosphate salts as raw materials. The process involves synthesis, high-temperature aging, one-time washing, drying, and calcination. Although this route uses a one-time washing process, reducing the amount of rinsing wastewater, it is prone to causing the product to exceed the standard for impurity element content. Furthermore, excessive phosphorus source is still required during aging, which increases the cost of raw materials and results in a low iron-to-phosphorus ratio in the product. Summary of the Invention

[0005] To address the problems existing in the background technology, this invention provides a low-cost, multi-batch method for preparing high iron-to-phosphorus ratio ferric phosphate. The method uses a crystal transformation promoter composed of ammonia and ferric phosphate dihydrate to promote the crystal transformation during aging. This method has low production cost, low waste discharge, and a high iron-to-phosphorus ratio in the product.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0007] A low-cost, multi-batch method for preparing high-iron-phosphorus ratio iron phosphate includes the following steps:

[0008] S1. Oxidative synthesis: Ferrous sulfate solution, phosphate salt solution and hydrogen peroxide are reacted to obtain amorphous ferric phosphate;

[0009] S2. Filtration and rinsing: The amorphous ferric phosphate obtained in step S1 is filtered and washed;

[0010] S3. High-temperature aging: Add pure water to the washed amorphous ferric phosphate filter cake, stir evenly and heat up. After keeping it at the temperature for a period of time, add crystal transformation promoter. After reacting for a period of time, obtain ferric phosphate dihydrate slurry. Reserve some of the ferric phosphate dihydrate slurry for the preparation of the next batch of crystal transformation promoter.

[0011] S4. Pressing, drying and calcining: The remaining ferric phosphate dihydrate slurry is pressed, dried and calcined to obtain the high ferric phosphate with phosphorus ratio;

[0012] S5. Repeat steps S1 to S4 in consecutive batches;

[0013] If it is the first batch of reaction, the crystal transformation promoter is phosphoric acid. After the reaction in step S3, the ferric phosphate dihydrate is washed and slurried with water to obtain the washed ferric phosphate dihydrate slurry. Part of it is reserved for the preparation of the next batch of crystal transformation promoter, and the remainder is used in step S4. If it is not the first batch of reaction, the crystal transformation promoter is a mixture of hydrogen peroxide and crystal nuclei. The crystal nuclei are obtained from the ferric phosphate dihydrate slurry reserved in step S3 of the previous batch.

[0014] According to the above scheme, if it is the first batch of reaction, in step S3, the ferric phosphate dihydrate is washed until the conductivity of the effluent is ≤200μs / cm. The residual phosphoric acid can be removed during the washing process. If it is the second and subsequent batches of reaction, since the ferric phosphate dihydrate slurry obtained from the reaction does not contain phosphoric acid, washing is not required.

[0015] According to the above scheme, the molar ratio of ferrous sulfate to phosphate salt is Fe:P = 1:1.02-1.05, and the molar ratio of oxidant to ferrous ions is 0.5-0.6:1.

[0016] Preferably, the concentration of the ferrous sulfate solution is 0.5–1.5 mol / L, and the mass concentration of the phosphate salt solution is 15%–25%.

[0017] According to the above scheme, the phosphate salt is one or more of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, monoammonium hydrogen phosphate, and sodium monohydrogen phosphate, and the oxidant is one or more of hydrogen peroxide, sodium thiosulfate, and ammonium thiosulfate.

[0018] According to the above scheme, in step S1, the pH value is adjusted to 1-2 with ammonia or sodium hydroxide before the reaction. If the phosphate salt is ammonium dihydrogen phosphate or ammonium monohydrogen phosphate, ammonia is preferred for adjusting the pH value; if the phosphate salt is sodium dihydrogen phosphate or sodium monohydrogen phosphate, sodium hydroxide is preferred for adjusting the pH value.

[0019] According to the above scheme, in step S2, the water is washed until the conductivity of the effluent is ≤1ms / cm.

[0020] Based on a hydrogen peroxide content of 27%, the mass ratio of hydrogen peroxide in the crystal transformation promoter to ferric phosphate dihydrate in the seed crystal is 20–50:1.

[0021] According to the above scheme, the solid content of the slurry obtained after adding pure water to amorphous ferric phosphate in step S3 is 8% to 15%. If it is the first batch reaction, the molar ratio of added phosphoric acid to iron ions is 0.05 to 0.2:1. If it is not the first batch reaction, the amount of crystal nucleation promoting conversion agent added is 10% to 40% of the weight of dihydrate ferric phosphate slurry.

[0022] According to the above scheme, the temperature of step S3 is 80-95℃, and the holding time is 0.5-1h.

[0023] According to the above scheme, the calcination temperature in step S4 is 500-600℃ and the time is 1-3h.

[0024] According to the above scheme, in step S4, a filter press is used for pressing, the pressing pressure is ≥1.2MPa, the drying method is flash drying, the drying temperature is 200~300℃, and the calcination is carried out by rotary kiln sintering.

[0025] The beneficial effects of this invention are:

[0026] 1. In this invention, hydrogen peroxide solution is used instead of inorganic acids such as phosphoric acid or sulfuric acid to provide acidity during aging, promoting the transformation of amorphous ferric phosphate crystals into ferric phosphate dihydrate crystal nuclei. Seed crystals are used to provide phosphorus source to promote the growth of ferric phosphate dihydrate crystal nuclei, saving the amount of high-cost inorganic acids and reducing raw material costs. Since non-degradable inorganic acids such as phosphoric acid or sulfuric acid are not added during aging, but easily degradable hydrogen peroxide solution is introduced to provide acidity, the ferric phosphate dihydrate slurry after aging does not contain other anions and cations, so there is no need for secondary washing, which greatly reduces the amount of rinsing wastewater and reduces wastewater treatment costs, resulting in a cost saving of 5% to 10% for ferric phosphate.

[0027] 2. Because no non-decomposable inorganic acids such as phosphoric acid or sulfuric acid are added during the aging process, the present invention is less likely to produce phosphorus-containing impurities during the aging process. Furthermore, the hydrogen peroxide used in the aging process will oxidize the ferrous ions in the amorphous ferric phosphate solution again, resulting in a higher iron-to-phosphorus ratio in the product. The iron-to-phosphorus ratio of the prepared ferric phosphate is above 0.99. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of Embodiment 1 of the present invention;

[0029] Figure 2 This is a scanning electron microscope image of the iron phosphate prepared in Example 1 of the present invention. Detailed Implementation

[0030] The principles and features of the present invention are described below with reference to the accompanying drawings and specific embodiments. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0031] To address the problems of low iron-to-phosphorus ratio and high preparation cost of existing iron phosphate, the inventors provide a low-cost, multi-batch method for preparing high iron-to-phosphorus ratio iron phosphate, comprising the following steps:

[0032] S1. Oxidative synthesis: Ferrous sulfate solution, phosphate salt solution and hydrogen peroxide are reacted to obtain amorphous ferric phosphate;

[0033] S2. Filtration and rinsing: The amorphous ferric phosphate obtained in step S1 is filtered and washed;

[0034] S3. High-temperature aging: Add pure water to the washed amorphous ferric phosphate filter cake, stir evenly and heat up. After keeping it at the temperature for a period of time, add crystal transformation promoter. After reacting for a period of time, obtain ferric phosphate dihydrate slurry. Reserve some of the ferric phosphate dihydrate slurry for the preparation of the next batch of crystal transformation promoter.

[0035] S4. Pressing, drying and calcining: The remaining ferric phosphate dihydrate slurry is pressed, dried and calcined to obtain the high ferric phosphate with phosphorus ratio;

[0036] S5. Repeat steps S1 to S4 in consecutive batches;

[0037] If it is the first batch of reaction, the crystal transformation promoter is phosphoric acid. After the reaction in step S3, the ferric phosphate dihydrate is washed and slurried with water to obtain the washed ferric phosphate dihydrate slurry. Part of it is reserved for the preparation of the next batch of crystal transformation promoter, and the remainder is used in step S4. If it is not the first batch of reaction, the crystal transformation promoter is a mixture of hydrogen peroxide and crystal nuclei. The crystal nuclei are obtained from the ferric phosphate dihydrate slurry reserved in step S3 of the previous batch.

[0038] In some specific embodiments, if it is the first batch of reaction, in step S3, ferric phosphate dihydrate is washed until the conductivity of the effluent is ≤200μs / cm. The washing process can remove residual phosphoric acid. If it is the second and subsequent batches of reaction, since the ferric phosphate dihydrate slurry obtained from the reaction does not contain phosphoric acid, washing is not required.

[0039] In some specific embodiments, the molar ratio of ferrous sulfate to phosphate salt is Fe:P = 1:1.02 to 1.05, and the molar ratio of oxidant to ferrous ions is 0.5 to 0.6:1.

[0040] Preferably, the concentration of the ferrous sulfate solution is 0.5–1.5 mol / L, and the mass concentration of the phosphate salt solution is 15%–25%.

[0041] In some specific embodiments, the phosphate salt is one or more of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, monoammonium hydrogen phosphate, and sodium monohydrogen phosphate, and the oxidant is one or more of hydrogen peroxide, sodium thiosulfate, and ammonium thiosulfate.

[0042] In some specific embodiments, ammonia or sodium hydroxide is added before the reaction in step S1 to adjust the pH to 1-2. If the phosphate salt is ammonium dihydrogen phosphate or ammonium monohydrogen phosphate, ammonia is preferably used to adjust the pH; if the phosphate salt is sodium dihydrogen phosphate or sodium monohydrogen phosphate, sodium hydroxide is preferably used to adjust the pH.

[0043] In some specific embodiments, in step S2, the water is washed until the conductivity of the effluent is ≤1 ms / cm.

[0044] In some specific embodiments, based on a hydrogen peroxide content of 27% by mass, the mass ratio of hydrogen peroxide in the crystal transformation promoter to ferric phosphate dihydrate in the seed crystal is 20-50:1.

[0045] In some specific embodiments, the solid content of the slurry obtained after adding pure water to amorphous ferric phosphate in step S3 is 8% to 15%. If it is the first batch reaction, the molar ratio of added phosphate to iron ions is 0.05 to 0.2:1. If it is not the first batch reaction, the amount of crystal nucleation promoting conversion agent added is 10% to 40% of the weight of dihydrate ferric phosphate slurry.

[0046] In some specific embodiments, the temperature of step S3 is 80-95°C, and the holding time is 0.5-1h.

[0047] In some specific embodiments, the calcination temperature in step S4 is 500-600°C, and the time is 1-3 hours.

[0048] Based on the above embodiments, the present invention provides the following specific examples to further illustrate the invention. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following examples, unless otherwise specified, are generally performed according to the manufacturer's recommended conditions. Unless otherwise stated, percentages and parts are calculated by mass.

[0049] Example 1

[0050] like Figure 1 As shown in the figure, the low-cost, multi-batch method for preparing high-iron-phosphorus ratio iron phosphate provided in this embodiment includes the following specific steps:

[0051] Step 1: Weigh 115g of ammonium dihydrogen phosphate solid, 66g of 22% ammonia solution, and 380g of pure water, mix them thoroughly, then add 76g of 27% hydrogen peroxide solution and stir well to form a phosphate salt solution; measure 1L of 1mol / L ferrous sulfate solution as the base solution, and slowly add the above phosphate salt solution to it. After reacting for a period of time, amorphous ferric phosphate is formed.

[0052] Step 2: Filter the above amorphous ferric phosphate using a filter press and rinse it with pure water. The rinsing is complete when the conductivity of the effluent is ≤1 mS / cm.

[0053] Step 3: Add 1000g of water to the rinsed filter cake for slurrying, heat to 90℃ and keep warm for 30min, then add 11.5g of 85% phosphoric acid and react for 10min to obtain ferric phosphate dihydrate slurry. Rinse the ferric phosphate dihydrate until the conductivity of the effluent is ≤200μs / cm, then add 1000g of water and slurry again to obtain rinsed ferric sulfate dihydrate slurry. Reserve 10% of the rinsed ferric phosphate dihydrate slurry for the preparation of the next batch of crystal transformation promoter.

[0054] Step 4: Press the remaining 90% of the rinsed ferric phosphate dihydrate slurry at a pressure of 1.2 MPa.

[0055] Step 5: Dry the pressed slurry by flash evaporation at a temperature of 200-300°C, and then calcine it in a rotary kiln at a temperature of 550°C for 2 hours to obtain anhydrous ferric phosphate.

[0056] Step 6: Repeat steps 1 to 5 in consecutive batches. The difference is that in step 3, the rinsed filter cake is added to 1000g of water for slurrying, heated to 90℃ and kept at that temperature for 30 minutes. Then, 300g of crystal transformation promoter is added. The crystal transformation promoter is composed of 200g of 27% hydrogen peroxide and 100g of ferric phosphate dihydrate slurry reserved from the previous batch. After reacting for 10 minutes, ferric phosphate dihydrate slurry is obtained. The obtained ferric phosphate dihydrate slurry does not need to be rinsed. 10% is directly reserved for the next batch of seed crystals, and 90% is directly used for step 4.

[0057] The scanning electron microscope image of the anhydrous ferric phosphate prepared in this embodiment is shown below. Figure 2 As shown in the figure, the primary particle morphology is a short rod-shaped structure with small particle size, ranging from 20 to 80 nm. At this size, the material has high activity, which is beneficial to improving the electrical properties of the material.

[0058] Example 2

[0059] The difference between this embodiment and Embodiment 1 is that the amount of crystal transformation promoter added in step 6 is 400g. The crystal transformation promoter is composed of 266.7g of hydrogen peroxide with a mass fraction of 27% and 133.3g of ferric phosphate dihydrate slurry. The remaining steps are the same as in Embodiment 1.

[0060] Example 3

[0061] The difference between this embodiment and Embodiment 1 is that the amount of crystal transformation promoter added in step 6 is 300g. The crystal transformation promoter is composed of 250g of 27% hydrogen peroxide and 50g of ferric phosphate dihydrate slurry. The remaining steps are the same as in Embodiment 1.

[0062] Example 4

[0063] The difference between this embodiment and Embodiment 1 is that the amount of ammonia added in step 1 is 38g, while the rest of the steps are the same as in Embodiment 1.

[0064] Example 5

[0065] The difference between this embodiment and embodiment 1 is that the amount of water added in step 3 is 700g, and the heat preservation time is 1h.

[0066] The properties of the anhydrous ferric phosphate prepared in Examples 1-5 are shown in Table 1 below.

[0067] Table 1. Specific properties of anhydrous ferric phosphate obtained in Examples 1-5

[0068]

[0069] The above results show that the iron-to-phosphorus ratio of the iron phosphate prepared by the preparation method of the present invention is higher than 0.99, indicating an extremely high iron-to-phosphorus ratio.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for low cost batchwise production of high-Fe / P iron phosphate, characterized in that, Comprising the following steps: S1. Oxidation synthesis: reacting a ferrous sulfate solution, a phosphorus salt solution and hydrogen peroxide to obtain amorphous iron phosphate; S2. Filtration and rinsing: filtering and washing the amorphous iron phosphate obtained in step S1; S3. High-temperature aging: adding pure water to the washed amorphous iron phosphate filter cake, stirring uniformly, heating, adding a crystal transformation promoter after a period of time, and obtaining iron phosphate dihydrate slurry after a period of time, and reserving part of the iron phosphate dihydrate slurry for preparing the crystal transformation promoter of the next batch; S4. Squeezing, drying and calcining: squeezing, drying and calcining the remaining iron phosphate dihydrate slurry to obtain the high-iron phosphorus ratio iron phosphate; S5. Repeating steps S1-S4 in continuous batches; If it is the first batch reaction, the crystal transformation promoter is phosphoric acid, and after the reaction in step S3, the iron phosphate dihydrate is rinsed and slurried with water to obtain rinsed iron phosphate dihydrate slurry, part of which is reserved for the preparation of the crystal transformation promoter of the next batch, and the remaining part is used in step S4; if it is not the first batch reaction, the crystal transformation promoter is a mixture of hydrogen peroxide and crystal nucleus, and the crystal nucleus is prepared from the iron phosphate dihydrate slurry reserved in step S3 of the previous batch.

2. The process for low cost multi-batch production of high ferric to phosphorus ratio iron phosphate of claim 1, wherein, The molar ratio of the ferrous sulfate to the phosphorus salt is Fe:P=1:1.02-1.

05.

3. The process for low cost multi-batch production of high ferric to phosphorus ratio iron phosphate of claim 1, wherein, The phosphorus salt is one or more of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium monohydrogen phosphate and sodium monohydrogen phosphate.

4. The method of claim 3, wherein the low cost, multi-batch production of high-Fe, low-P iron phosphate is characterized by, The pH value before the reaction in step S1 is adjusted to 1-2 with ammonia water or sodium hydroxide.

5. The process for low cost multi-batch production of high ferric to phosphorus ratio iron phosphate of claim 1, wherein, The washing in step S2 is performed until the water conductivity is ≤1 ms / cm.

6. The process for low cost multi-batch production of high ferric to phosphorus ratio iron phosphate of claim 1, wherein, If it is the second batch reaction, the crystal seeds in the crystal transformation promoter are prepared from the iron phosphate dihydrate slurry obtained in the previous batch, which is reslurried in water after pressure filtration and washing, and if it is the third or subsequent batch reaction, the crystal seeds in the crystal transformation promoter are the iron phosphate dihydrate slurry reserved in the previous batch, and the mass ratio of hydrogen peroxide to iron phosphate dihydrate in the crystal seeds is 20-50:1, calculated based on 27% hydrogen peroxide.

7. The method of claim 6, wherein the low cost, multi-batch production of high-Fe, low-P iron phosphate is characterized by, The solid content of the slurry obtained after adding pure water to the amorphous iron phosphate in step S3 is 8%-15%, and if it is the first batch reaction, the molar ratio of added phosphoric acid to iron ions is 0.05-0.2:1, and if it is not the first batch reaction, the amount of crystal nucleus transformation promoter added is 10%-40% of the weight of the iron phosphate dihydrate slurry.

8. The method of claim 6 or 7, wherein the method is characterized by, The temperature in step S3 is 80-95℃, and the holding time is 0.5-1h.

9. The process for low cost multi-batch production of high-iron phosphorus ratio iron phosphate of claim 1, wherein, The calcination temperature in step S4 is 500-600℃, and the time is 1-3h.

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