Method for preparing high-tap-density battery-grade iron phosphate from crude phosphoric acid

Abstract

The invention discloses a method for preparing high-tap-density battery-grade iron phosphate from crude phosphoric acid. The method is low in production cost, simple in process, high in tap density of iron phosphate and capable of enabling lithium iron phosphate to be good in compaction performance. Ferric phosphate dihydrate seed crystals are introduced during first-time adding, a small amount of ferric ions can inhibit precipitation of impurity ions, meanwhile, the ferric ions and phosphate ions are uniformly nucleated on the surfaces of the seed crystals, the particle morphology of ferric phosphate is controlled, and the tap density of the ferric phosphate is improved; adding again, oxidizing most ferrous ions into ferric ions, and continuing to nucleate and grow on the surface of the seed crystal together with the phosphorus source, so that the primary particle size of the iron phosphate is relatively large, the secondary particle aggregation is uniform, the unoxidized ferrous ions can form a complex with phosphate radicals to occupy excessive phosphate radicals, and the precipitation of impurities such as Mn and Mg is further inhibited; and finally, the particle size of the iron phosphate generated by combining a small amount of the remaining iron source with the phosphorus source is small, so that the finally formed iron phosphate is wide in particle size distribution and high in tap density.

Description

Technical Field

[0001] The invention relates to a method for preparing a compound, and in particular to a method for preparing high-tap battery-grade iron phosphate by using crude phosphoric acid. Background Art

[0002] With the rapid development of new energy vehicles and energy storage industries, lithium iron phosphate batteries are widely used in electric vehicles and energy storage systems due to their advantages such as good safety performance, long cycle life, high operating voltage, high energy density, low self-discharge rate, and no memory effect, and are gradually occupying a large market share. As one of the raw materials for preparing lithium iron phosphate positive electrode materials, iron phosphate plays a decisive role in improving the safety and cycle stability of lithium iron phosphate batteries. The performance of lithium iron phosphate batteries is mainly determined by iron phosphate, so the optimization of its preparation process and cost control are of great significance to improving the performance of lithium iron phosphate batteries and reducing production costs. At present, iron phosphate is generally prepared by oxidation synthesis using high-purity iron sources and phosphorus sources. The lithium iron phosphate products prepared using it have good stability, relatively balanced compaction and capacity, and good capacity and compaction performance even in winter when the temperature is low. In this process, ferrous sulfate, a byproduct of titanium dioxide, is the iron source of the current mainstream process. Cheap iron sources can effectively reduce the production cost of batteries, but their impurity content is high, especially impurities such as Mn and Mg are difficult to remove, resulting in a high impurity content in iron phosphate, which in turn affects the performance of lithium iron phosphate batteries; the main sources of phosphorus are monoammonium phosphate and phosphoric acid with high purity, which are expensive and account for more than half of the total production cost of iron phosphate, which is the fundamental reason for the high cost of lithium iron phosphate batteries. Crude phosphoric acid has a low price, but its impurity content is high and cannot be directly used in the preparation process. It needs further impurity removal before it can be put into production. Different impurity removal processes and equipment have different requirements for labor costs and energy consumption costs. How to use low-cost raw materials and produce raw materials that meet production needs without increasing a large amount of impurity removal costs is also a problem that needs to be weighed. The document with application number 2024103489920 discloses a method for preparing battery-grade iron phosphate with high specific surface area, using byproducts of titanium dioxide products and crude phosphoric acid as raw materials, and removing impurities by adjusting pH to obtain iron source and phosphorus source for preparing iron phosphate, but the impurity removal process introduces a large amount of sodium ions, and a large amount of washing water is required later; in addition, in order to control the content of impurity elements in iron phosphate, the preparation process is complicated, time-consuming and energy-intensive. At the same time, the iron phosphate prepared by it has a large specific surface area, which may lead to a low tap density, and is easy to block materials during the production process of iron phosphate, affecting the production of iron phosphate. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to reduce production costs, improve the purity of iron phosphate, and improve the tap density of iron phosphate. A method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid is provided.

[0004] Technical solution: The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid of the present invention comprises the following steps:

[0005] (1) removing ferrous sulfate, a byproduct of titanium dioxide, with phosphoric acid to obtain a ferrous sulfate clear solution;

[0006] (2) removing impurities from crude phosphoric acid to obtain a clear phosphate solution;

[0007] (3) firstly adding hydrogen peroxide to the ferrous sulfate solution obtained in step (1) to oxidize part of the ferrous ions, and simultaneously adding ferric phosphate dihydrate seed crystals, and then heating and mixing with the phosphate solution prepared in step (2), adding hydrogen peroxide again to oxidize part of the ferrous ions, heating to an aging temperature and keeping the temperature, and then adding hydrogen peroxide again until the ferrous ions are completely oxidized;

[0008] (4) The solution obtained in step (3) is kept warm, aged, filtered, washed, dried, and calcined to obtain battery-grade iron phosphate.

[0009] Further, in the step (1), the iron content in the ferrous sulfate clear solution is 1-1.5 mol / L, and the step (1) uses phosphoric acid to remove impurities from the ferrous sulfate, adjusts the pH to 1.4-1.8 with phosphoric acid, the heating temperature is 40-60°C, and the stirring time is 1-3h, and the impurity ions Cr, Ti, Al, Zn and part of Mn and Mg are removed in the form of precipitation by filtering. In the process of impurity removal, phosphoric acid is added to make the impurity ions and phosphate ions form precipitation and remove them. The pH value of the precipitation formed by Mn and Mg and phosphate ions is quite different from that of other impurity ions. It is necessary to greatly increase the cost of impurity removal if Mn and Mg are to be removed as cleanly as possible. In order to balance the problem of impurity removal efficiency and cost, part of Mn and Mg are retained in the ferrous sulfate clear solution, but too much Mn content will refine the iron phosphate grains, resulting in an increase in specific surface area, which makes the processing performance of iron lithium worse, and too much Mn ions in iron phosphate will also affect the electrical properties of iron lithium, so it is necessary to suppress the precipitation of Mn and Mg ions in the subsequent preparation process of iron phosphate.

[0010] Further, in the step (2), the phosphorus content in the phosphate clear solution is 2.2-3 mol / L, and the pH value of step (2) is adjusted to 3.0-5.0 with ammonia water, and the heating temperature is 60-80°C and the stirring time is 1-3h for filtering and removing impurities. The pH value is adjusted with ammonia water to remove impurities, which can avoid the introduction of other impurity ions to increase the difficulty and cost of subsequent removal. The concentration of the iron source and the phosphorus source will affect the grain growth process during the synthesis process. Too high a concentration will cause the grains to grow too fast, and too low a concentration will be unfavorable for the growth of the grains, and the solid content of the system is too low, resulting in an increase in the amount of wastewater. Therefore, the iron phosphate synthesis process must be carried out under an appropriate concentration of iron source and phosphorus source to make its reaction more uniform.

[0011] Furthermore, in the step (3), the molar ratio of the iron source to the phosphorus source is 1:1.05 to 1.2. The appropriately proportioned mixture of the iron source and the phosphorus source can control the generation rate, particle size, particle morphology and specific surface area of ​​the iron phosphate, thereby controlling the tap density of the iron phosphate and controlling the iron-phosphorus ratio to be between 0.965 and 0.975, thereby ensuring that the subsequently prepared battery has a stable structure, high safety, and high cycle stability and specific capacity.

[0012] Furthermore, in step (3), the molar ratio of hydrogen peroxide to ferrous sulfate is 0.55-0.65:1, and a slight excess of hydrogen peroxide is used to ensure complete oxidation of the divalent iron. Before mixing, the first addition of hydrogen peroxide is 3%-7% of the total amount; after mixing and heating, the second addition is 80%-90%; after heating to the aging temperature and keeping warm, the remaining 7%-13% is added again. Before mixing, a small amount of hydrogen peroxide oxidizes the divalent iron ions in ferrous sulfate to produce a small amount of trivalent iron ions, which can prevent Mn and Mg ions from precipitating with phosphate when the phosphorus source and the iron source are mixed, thereby reducing the content of impurities in the iron phosphate; after mixing and heating, hydrogen peroxide is added for the second time to oxidize most of the divalent iron ions into trivalent iron ions and form an intermediate with the phosphorus source - basic ammonium ferric phosphate precipitation, and a small part of the unoxidized divalent iron can form a complex with phosphate to occupy excess phosphate, thereby inhibiting the precipitation of impurities such as Mn and Mg; hydrogen peroxide is added for the third time to oxidize the remaining divalent iron, ensure that the phosphorus source and the iron source react completely, and avoid waste of resources.

[0013] Furthermore, in step (3), when hydrogen peroxide is added for the first time, dihydrate iron phosphate seeds with a theoretical yield of 5% to 15% of iron phosphate are added at the same time. The addition of dihydrate iron phosphate seeds allows the iron source to combine with the phosphorus source to uniformly nucleate on the surface of the seeds when hydrogen peroxide is added for the first and second times, which is beneficial to promoting the growth of iron phosphate particles, making the primary iron phosphate particles larger, more tightly agglomerated, and the secondary particles uniform in size. When hydrogen peroxide is added for the third time, the remaining small amount of ferrous ions are oxidized to form precipitation with phosphate ions to form iron phosphate particles with smaller particle sizes, thereby making the final particle size distribution of the iron phosphate wider, increasing the tap density, and further improving the specific capacity and cycle performance of the lithium iron phosphate prepared therefrom.

[0014] Furthermore, in step (3), before the second addition of hydrogen peroxide, concentrated sulfuric acid is used to adjust the pH to 1.8-2.0. In the early stage, due to the influence of the impurity removal effect, the pH of the iron source is 1.4-1.8, and the pH of the phosphorus source is 3.0-5.0. The pH after the two are mixed is greater than 2. The high pH value will affect the morphology, size, uniformity and compactness of the iron phosphate particles, and will produce iron hydroxide impurities during the precipitation process, resulting in the loss of the iron source, affecting the performance and yield of the iron phosphate.

[0015] Furthermore, in step (3), before the last addition of hydrogen peroxide, the temperature is raised again to an aging temperature of 80 to 95° C. for 55 to 75 minutes, and the remaining hydrogen peroxide is added after being kept warm for 1 to 2 hours. The hydrogen peroxide added the second time is added again after the reaction is complete, thereby avoiding the waste of phosphorus source and iron source.

[0016] Furthermore, the aging process time of step (4) is 1 to 3 hours, the washing process is to wash until the conductivity of the filtrate is less than 500 μS / cm, the calcination temperature is 550 to 700° C., and the calcination time is 2 to 4 hours.

[0017] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. The raw materials are ferrous sulfate and crude phosphoric acid, which are byproducts of titanium dioxide, which greatly reduce the production cost of iron phosphate; 2. Hydrogen peroxide is added in batches during the mixing process of the iron source and the phosphorus source, and a small amount of trivalent iron ions inhibit the precipitation of Mn and Mg ions and phosphate ions. At the same time, dihydrate iron phosphate seeds are introduced, so that the iron source and the phosphorus source are uniformly nucleated on the surface of the seeds, which is conducive to promoting the growth of iron phosphate particles; when hydrogen peroxide is added for the second time, most of the ferrous ions are oxidized into trivalent iron ions and continue to nucleate and grow with the phosphorus source on the dihydrate iron phosphate seeds, forming iron phosphate with larger primary particle size, tight agglomeration and uniform secondary particle size, and a small part of the unoxidized divalent iron It can form a complex with phosphate to occupy excess phosphate, further inhibiting the precipitation of impurities such as Mn and Mg; adding hydrogen peroxide for the third time quickly oxidizes the remaining small amount of divalent iron ions to ensure that the phosphorus source and the iron source react more fully and avoid waste of resources. The remaining small amount of iron source combines with the phosphorus source to generate iron phosphate with a smaller particle size, so that the final iron phosphate has a wider particle size distribution and a higher tap density; 3. The impurity removal process is simple and easy to implement, the impurity removal effect is good and no other impurity ions are introduced. The process for preparing iron phosphate is simple and the generation efficiency is high; 4. The method has a simple process and a good impurity removal effect. The iron-phosphorus ratio is controlled at about 97%, the specific surface area is moderate, and the tap density is good, which can make the preparation of lithium iron phosphate have good compaction performance and high cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a SEM image of the iron phosphate prepared in Example 1-3 of the present invention;

[0019] Figure 2 The SEM images of the iron phosphate prepared in Comparative Examples 1-4 of the present invention;

[0020] Figure 3 This is the XRD pattern of the iron phosphate prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The technical solution of the present invention is further described below in conjunction with the examples and drawings. The materials used in the examples are all commercially available.

[0022] Example 1

[0023] (1) Weigh 1800 g of ferrous sulfate, a byproduct of titanium dioxide, and add 5.5 L of pure water. Stir for 30 min until the raw material is completely dissolved. Heat the solution to 50° C. and add phosphoric acid to adjust the pH to 1.6. Continue stirring for 120 min and then filter to obtain a ferrous sulfate clear solution, in which the iron concentration is 1 mol / L.

[0024] (2) Weigh 1000 g of 46% crude phosphoric acid and dilute it with 2 L of pure water, add an appropriate amount of ammonia water to adjust the pH of the solution to 4.0, then heat the solution to 70° C., keep it warm for 2 h, and then perform solid-liquid separation to obtain a phosphate clear solution, in which the phosphorus concentration is 2.2 mol / L;

[0025] (3) According to the molar ratio of hydrogen peroxide to iron ions of 0.55:1, 1664.3 g of hydrogen peroxide (concentration 6%) was weighed, 104 g of dihydrate iron phosphate seed (theoretical iron phosphate yield 10%) was added to the ferrous sulfate solution obtained in step (1), and then 83.2 g (5% of the total amount of hydrogen peroxide) of hydrogen peroxide was added to oxidize part of the ferrous ions, and then 2.79 L of the phosphate solution prepared in step (2) (the molar ratio of phosphorus to iron was 1.15:1) was added and the pH of the mixed solution was adjusted to 1.9 with 98% concentrated sulfuric acid, the mixed solution was heated to 45°C, and 1414.6 g of hydrogen peroxide (85% of the total amount of hydrogen peroxide) was added again to oxidize the divalent iron ions, and then the temperature was continued to be raised to 85°C for 85 min. After the heating was completed, the temperature was kept for 1.5 h, and the remaining 166.5 g of hydrogen peroxide was added again to completely oxidize the divalent iron ions;

[0026] (4) The solution obtained in step (3) is kept warm until the iron phosphate slurry turns white, aged, filtered, washed until the conductivity of the filtrate is less than 500 μS / cm, dried, and calcined at 600° C. for 3 h to obtain anhydrous iron phosphate.

[0027] Example 2

[0028] (1) Weigh 2250 g of ferrous sulfate, a byproduct of titanium dioxide, and add 5.5 L of pure water. Stir for 30 min until the raw material is completely dissolved. Heat the solution to 40° C. and add phosphoric acid to adjust the pH to 1.8. Continue stirring for 180 min and then filter to obtain a ferrous sulfate clear solution, wherein the iron concentration is 1.25 mol / L;

[0029] (2) Weigh 1231 g of 46% crude phosphoric acid and dilute it with 2 L of pure water, add an appropriate amount of ammonia water to adjust the pH of the solution to 3.0, then heat the solution to 80°C, keep it warm for 1 hour, and then perform solid-liquid separation to obtain a phosphate clear solution, in which the phosphorus concentration is 2.76 mol / L;

[0030] (3) According to the molar ratio of hydrogen peroxide to iron ions of 0.6:1, 2269.5 g of hydrogen peroxide (concentration 6%) was weighed, and 187 g of dihydrate iron phosphate seed crystals (theoretical yield of iron phosphate 15%) was added to the ferrous sulfate solution obtained in step (1), and then 158.8 g (7% of the total amount of hydrogen peroxide) of hydrogen peroxide was added to oxidize part of the ferrous ions, and then 2.66 L of the phosphate salt solution prepared in step (2) (the molar ratio of phosphorus to iron was 1.1:1) was added and the pH of the mixed solution was adjusted to 1.8 with 98% concentrated sulfuric acid, and the mixed solution was heated to 45° C. and 1815.6 g of hydrogen peroxide (80% of the total amount of hydrogen peroxide) was added again to oxidize the divalent iron ions, and then the temperature was continued to be raised to 90° C. for 95 min. After the heating was completed and kept warm for 1 h, the remaining 295.1 g of hydrogen peroxide was added again to completely oxidize the divalent iron ions;

[0031] (4) The solution obtained in step (3) is kept warm until the iron phosphate slurry turns white, aged, filtered, washed until the conductivity of the filtrate is less than 500 μS / cm, dried, and calcined at 550° C. for 4 h to obtain anhydrous iron phosphate.

[0032] Example 3

[0033] (1) Weigh 2700 g of ferrous sulfate, a byproduct of titanium dioxide, and add 5.5 L of pure water. Stir for 30 min until the raw material is completely dissolved. Heat the solution to 60° C. and add phosphoric acid to adjust the pH to 1.4. Continue stirring for 60 min and then filter to obtain a ferrous sulfate clear solution, wherein the iron concentration is 1.5 mol / L.

[0034] (2) Weigh 1339 g of 46% crude phosphoric acid and dilute it with 2 L of pure water, add an appropriate amount of ammonia water to adjust the pH of the solution to 5.0, then heat the solution to 60° C., keep it warm for 3 h, and then perform solid-liquid separation to obtain a phosphate clear solution, in which the phosphorus concentration is 3 mol / L;

[0035] (3) According to the molar ratio of hydrogen peroxide to iron ions of 0.65:1, 2950.4 g of hydrogen peroxide (concentration 6%) was weighed, and 75 g of dihydrate iron phosphate seed crystals (theoretical yield of iron phosphate 5%) were added to the ferrous sulfate solution obtained in step (1), and then 88.5 g (3% of the total amount of hydrogen peroxide) of hydrogen peroxide was added to oxidize part of the ferrous ions, and then 3.2 L of the phosphate solution prepared in step (2) (the molar ratio of phosphorus to iron was 1.2:1) was added and the pH of the mixed solution was adjusted to 2.0 with 98% concentrated sulfuric acid, and the mixed solution was heated to 45° C. and 2655.3 g of hydrogen peroxide (90% of the total amount of hydrogen peroxide) was added again to oxidize the divalent iron ions, and then the temperature was continued to be raised to 80° C. for 75 min. After the heating was completed and kept warm for 2 h, the remaining 206.5 g of hydrogen peroxide was added again until the divalent iron ions were completely oxidized;

[0036] (4) The solution obtained in step (3) is kept warm until the iron phosphate slurry turns white, aged, filtered, washed until the conductivity of the filtrate is less than 500 μS / cm, dried, and calcined at 650° C. for 2 h to obtain anhydrous iron phosphate.

[0037] Comparative Example 1

[0038] Compared with Example 1, in step (3) of Comparative Example 1, the hydrogen peroxide was added all at once after the mixed solution was heated to 45° C., and other conditions were the same.

[0039] Comparative Example 2

[0040] Compared with Example 1, in step (3) of Comparative Example 2, hydrogen peroxide was added twice: 85% was added after the mixed solution was heated to 45° C., the temperature was raised to 85° C. again, and the remaining 15% was added after the mixture was kept warm for 1.5 hours, while other conditions were the same.

[0041] Comparative Example 3

[0042] Compared with Example 1, in step (3) of Comparative Example 2, hydrogen peroxide was added twice: the first addition amount was 5%, and the second addition amount was 95% after the mixed solution was heated to 45° C., and other conditions were the same.

[0043] Comparative Example 4

[0044] Compared with Example 1, in step (3) of Comparative Example 4, no iron phosphate dihydrate seed crystals were added before the first addition of hydrogen peroxide, and other conditions were the same.

[0045] The contents of impurity ions in the ferrous sulfate and crude phosphoric acid solutions before and after impurity removal in the above examples and comparative examples were detected, and the data are shown in Tables 1 and 2 for details.

[0046] Table 1 Detection results of impurity content of ferrous sulfate before and after impurity removal

[0047]

[0048] As shown in Table 1, the use of phosphoric acid to remove impurities from ferrous sulfate has a very good effect on removing Ca, Na, Zn, Si, and Al, and the removal rate of Ti ions is more than 99.9%. There are still a large amount of Mn and Mg in the ferrous sulfate clear solution, which are removed by the subsequent mixing process.

[0049] Table 2 Detection results of impurity content of crude phosphate before and after impurity removal

[0050]

[0051] As can be seen from Table 2, the use of ammonia water to remove impurities from crude phosphoric acid has a good effect on removing the impurity ions Fe, Cr, Ti, Al, Zn, Mn, and Mg. The Na ions that cannot be removed and a small amount of Mn and Mg ions are removed by the subsequent mixing process.

[0052] The prepared iron phosphate was analyzed by SEM images and compared Figure 1 and 2 Compared with the comparative example, the primary particles of iron phosphate prepared in Examples 1-3 have more uniform particle size, more complete structure and better compactness. Figure 2 In Comparative Example 4, the primary particle size is small because no iron phosphate dihydrate seed crystals are added. Figure 3 It can be seen from the XRD diagram that the iron phosphate synthesized by this method has no impurity peaks, small peak width and sharp peak value, indicating that the prepared iron phosphate has very good crystallinity. This is mainly due to the addition of hydrogen peroxide in batches during the mixing of the iron source and the phosphorus source: a small amount of hydrogen peroxide first oxidizes part of the ferrous ions to increase the content of trivalent ferrous ions, inhibiting the formation of precipitation of Mn and Mg ions with phosphate ions and mixing into the finished iron phosphate product, thereby reducing the content of impurities in the iron phosphate; the second addition of hydrogen peroxide, most of the divalent iron ions are oxidized into trivalent iron ions and uniformly nucleate with the phosphorus source on the surface of the dihydrate iron phosphate seed crystal, and a small part of the unoxidized divalent iron can form a complex with phosphate to occupy the excess phosphate, further inhibiting the precipitation of impurities such as Mn and Mg; after aging and heat preservation for a certain period of time, hydrogen peroxide is added for the third time to quickly oxidize the remaining small amount of divalent iron ions, and the oxidation efficiency of divalent iron ions is promoted by batch oxidation while shortening the precipitation time of impurity ions such as Mn and Mg, further inhibiting the content of impurities such as Mn and Mg in the finished iron phosphate product.

[0053] The performance of the prepared iron phosphate was tested, see Table 3 for details.

[0054] Table 3 Performance indicators and impurity content of ferric phosphate

[0055]

[0056] It can be verified from Table 3 that, compared with Comparative Examples 1-3, there is no difference in the impurity removal effect between the phosphorus source and the iron source mixed in Example 1-3, and the method of adding hydrogen peroxide in step (3) has a great influence on the impurity content of iron phosphate. The addition of hydrogen peroxide in three batches can effectively inhibit the precipitation of impurity Mn and Mg ions, and the content of Mg and Mn impurities in iron phosphate is reduced by 3 to 4 times; at the same time, compared with comparative examples 1-3, the tap density of iron phosphate in Examples 1-3 is increased by about 10%, which also verifies that a small amount of trivalent iron ions are preferentially oxidized to inhibit Mn and Mg ions from forming precipitation with phosphate ions and mixing into the finished iron phosphate product, thereby reducing the content of impurities in iron phosphate, making the particle morphology and particle size of iron phosphate more uniform and the structure more complete; compared with comparative example 4, the tap density of iron phosphate in Examples 1-3 is significantly improved, which also verifies that the addition of iron phosphate dihydrate seeds makes the iron source and the phosphorus source combine to form uniform nucleation on the surface of the seeds when hydrogen peroxide is added, which is beneficial to promote the growth of iron phosphate particles, making the primary particles of iron phosphate larger, tightly agglomerated, and the secondary particles uniform in particle size, and the remaining small amount of ferrous ions are oxidized and precipitated with phosphate ions when hydrogen peroxide is added for the third time to form iron phosphate particles with smaller particle size, so that the final particle size distribution of iron phosphate is wider, and the tap density is significantly improved.

[0057] In the above preparation method, step (1) and step (2) perform a simple impurity removal process on the raw materials, and preferentially remove Ca, Na, Zn, Si, Al and Ti in ferrous sulfate and Fe, Cr, Ti, Al and Zn in crude phosphoric acid, and the impurity removal effect is very good. Part of the Mn and Mg impurity ions are retained in the clear solution of ferrous sulfate and crude phosphoric acid, and are removed in the subsequent preparation process. The batch addition of hydrogen peroxide in step (3) inhibits the impurities Mn and Mg from entering the iron phosphate grains, greatly reducing the impurity content. The introduction of dihydrate iron phosphate seeds also makes the primary particles of iron phosphate more uniform, the particles are closer to each other, and the secondary particles have a wider particle size distribution, which improves its tap density. The precise control of the molar ratio of phosphorus source to iron source allows the iron-phosphorus ratio of iron phosphate to be controlled at about 97%, the specific surface area is moderate, and the tap density is greatly improved. The lithium iron phosphate battery prepared using the same has good compaction performance and high cycle stability.

Claims

1. A method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid, characterized in that: The steps include: (1) removing ferrous sulfate, a byproduct of titanium dioxide, with phosphoric acid to obtain a ferrous sulfate clear solution; (2) removing impurities from crude phosphoric acid to obtain a clear phosphate solution; (3) firstly adding hydrogen peroxide to the ferrous sulfate solution obtained in step (1) to oxidize part of the ferrous ions, and simultaneously adding ferric phosphate dihydrate seed crystals, and then heating and mixing with the phosphate solution prepared in step (2), adding hydrogen peroxide again to oxidize part of the ferrous ions, heating to an aging temperature and keeping the temperature, and then adding hydrogen peroxide again until the ferrous ions are completely oxidized; (4) The solution obtained in step (3) is kept warm, aged, filtered, washed, dried, and calcined to obtain battery-grade iron phosphate.

2. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 1, characterized in that: The iron content in the ferrous sulfate clear solution prepared in step (1) is 1 to 1.5 mol / L.

3. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 1, characterized in that: In the step (1), when removing impurities, the pH is adjusted to 1.4-1.8 with phosphoric acid, heated to 40° C.-60° C., stirred for 1-3 hours, and then filtered to obtain a ferrous sulfate clear solution.

4. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 1, characterized in that: The phosphorus content in the phosphate clear solution in step (2) is 2.2-3 mol / L.

5. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 1, characterized in that: In the step (2) of removing impurities, the pH is adjusted to 3.0-5.0 by using ammonia water, heated to 60-80° C., stirred for 1-3 hours, and then filtered to obtain a phosphate clear solution.

6. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 1, characterized in that: In the step (3), the molar ratio of the iron source to the phosphorus source is 1:1.05-1.

2.

7. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 1, characterized in that: In the step (3), the molar ratio of hydrogen peroxide to ferrous sulfate is 0.55-0.65:

1.

8. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 7, characterized in that: The amount of ferric phosphate dihydrate seed crystals added in step (3) is 5% to 15% of the theoretical output of ferric phosphate.

9. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 7, characterized in that: In the step (3), the first addition amount of hydrogen peroxide before mixing is 3% to 7% of the total amount; the second addition amount after mixing and heating is 80% to 90% of the total amount; after heating to the aging temperature and keeping warm, the remaining 7% to 13% is added again.

10. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 9, characterized in that: In step (3), before adding hydrogen peroxide for the second time, the pH is adjusted to 1.8-2.0 with concentrated sulfuric acid.

11. The method for preparing high-tap battery-grade iron phosphate using crude phosphoric acid according to claim 9, characterized in that: The aging temperature when adding hydrogen peroxide at the end of step (3) is 80-95° C., the heating time is 55 min-75 min, and the remaining hydrogen peroxide is added after keeping warm for 1-2 hours.

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