Preparation method of compact-structure and high-compaction type iron phosphate material

Abstract

The invention discloses a preparation method of a compact-structure and high-compaction type iron phosphate material, and relates to the technical field of lithium ion, and the preparation method comprises the following steps: S1, mixing iron phosphate slag obtained after lithium extraction of a battery with iron phosphate waste, and dissolving in a sulfuric acid solution to obtain a solution A; s2, adding an alkaline solution to adjust the pH to generate a precipitate B; s3, dissolving in a sulfuric acid solution to obtain a solution C, and filtering to obtain ferrophosphorus liquid; s4, adding a phosphoric acid solution to obtain a mixed solution; s5, adding deionized water; s6, raising the temperature, reacting until the solution is converted into white suspension, and keeping the temperature to obtain an iron phosphate dihydrate precursor; s7, drying the iron phosphate dihydrate precursor at the temperature of 100-200 DEG C, and then calcining at the temperature of 400-600 DEG C for 2-5 hours to obtain anhydrous iron phosphate; according to the method, ferrophosphorus liquid is adopted, iron ions and phosphate ions are provided, the pH and the concentration of a system are adjusted, and the iron phosphate material with a compact structure is obtained through programmed heating.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ions, and particularly to a preparation method of a dense and high-compaction iron phosphate material. Background Art

[0002] As an important precursor of lithium iron phosphate, iron phosphate material not only improves the energy density of the battery, but also significantly reduces the risk of thermal runaway, enabling the battery to operate stably under extreme conditions. In addition, the recyclability of iron phosphate material also conforms to the concept of sustainable development and helps to build a circular economy system. However, the traditional process for preparing iron phosphate usually uses conventional raw materials (such as ferrous sulfate and monoammonium phosphate) and involves relatively complex steps, resulting in high preparation costs, including raw material preparation, reaction, precipitation, filtration, washing, drying and calcination. These steps often require long reaction times and high energy consumption. Although the performance is good and the compaction is high, it is not conducive to large-scale industrial production. The present invention uses low-cost recycled phosphorus-iron liquid to produce high-compaction iron phosphate through short processes. The following is the relevant technical content that has been reported: Patent "A Preparation Method of Iron Phosphate, the Prepared Iron Phosphate and Its Application, CN110451471 A" includes the following steps: S1, prepare a ferrous salt solution, add phosphoric acid, and then add an oxidant to oxidize ferrous ions into ferric ions; S2, take phosphate and / or hydrogen phosphate and mix them with phosphoric acid to prepare a mixed solution; S3, heat the mixed solution prepared in step S2 to (90-100)°C, and then add the ferric ion solution prepared in step S1; S4, calcine the product obtained after the reaction in step S3 to obtain the iron phosphate.

[0003] Patent "Iron Phosphate for High-Compaction Lithium Iron Phosphate and Its Preparation Method, CN117163929A", the preparation method is as follows: S1, prepare a phosphate solution, and then prepare a ferrous solution, and mix the phosphate solution and the ferrous solution to obtain a ferrous phosphate solution; S2, heat up the ferrous phosphate solution, add hydrogen peroxide to obtain a slurry A composed of ferrous phosphate octahydrate and ammonium iron hydroxyphosphate; S3, add a phosphoric acid solution to slurry A, heat up and keep warm, and then add hydrogen peroxide to obtain a white slurry B of iron phosphate dihydrate; S4, after slurry B is filtered, washed and dried, iron phosphate dihydrate with a complex structure of large and small particles is obtained, and after calcination of iron phosphate dihydrate, anhydrous iron phosphate with a complex structure of large and small particles is obtained.

[0004] Patent "A High-Compaction Iron Phosphate Material and Its Preparation Method, CN 115215313 A". The preparation method includes: preparing a first mixture system containing a phosphate source, a ferrous ion source, and a first pH regulator. The first mixture system undergoes a first reaction to obtain a first slurry containing an amorphous ferrous phosphate octahydrate reaction precursor; adding a second pH regulator to the first slurry to obtain a second slurry, and the second slurry is acidic; adding an oxidizing substance to the second slurry to conduct a second reaction to obtain a dihydrate iron phosphate material.

[0005] However, in the patent "A Preparation Method of Iron Phosphate, the Prepared Iron Phosphate and Its Application, CN110451471 A", ferrous salts and phosphates are used. By oxidizing ferrous ions in the ferrous salt solution into ferric ions and then adding the oxidized solution into a phosphate and / or hydrogen phosphate solution, iron phosphate is prepared. Among them, the oxidized ferric ion solution is uneven, and the ferric ions have also undergone hydrolysis reactions in this case, resulting in difficulties in ensuring the internal structure and physical and chemical indexes of the subsequently prepared iron phosphate material.

[0006] Patent "Iron Phosphate for High-Compaction Lithium Iron Phosphate and Its Preparation Method, CN117163929A". By mixing a phosphate solution and a ferrous solution, adding hydrogen peroxide after heating, and then adding phosphoric acid to heat to prepare iron phosphate, the steps are cumbersome and the preparation cost is relatively high.

[0007] In the patent "A High-Compaction Iron Phosphate Material and Its Preparation Method, CN 115215313 A", phosphates and iron salts are used to prepare iron phosphate by a two-step method, including batching, synthesis, primary washing, pulping, conversion, and secondary washing. The process is complex, the preparation process is long, and the cost is relatively high, which does not conform to the current mainstream direction of preparing iron phosphate materials. Summary of the Invention

[0008] To solve the above technical problems, the present invention provides a preparation method of an iron phosphate material with simple process and controllable crystal form. Using a phosphorus-iron liquid prepared from low-cost iron phosphate slag or iron phosphate waste after lithium extraction from batteries, this liquid provides ferric ions and phosphate ions. Uniformly mixing phosphoric acid solution in the phosphorus-iron liquid, adjusting the pH and concentration of the system with deionized water, further optimizing the cumbersome reaction steps, while reducing the generation of intermediates, and directly generating dense-structured dihydrate iron phosphate after heating. At the same time, the iron and phosphorus in the raw materials of the present invention are premixed, making it easier to achieve uniform precipitation of iron and phosphorus elements at the molecular level during the feeding process. Coupled with appropriate process parameters, the uniformity of the structure and chemical composition of iron phosphate is improved, which helps to obtain iron phosphate with uniform pores, ultimately improving the grinding efficiency and reaction activity of lithium iron phosphate, and being beneficial to the preparation of high-compaction lithium iron phosphate. Specifically as follows: A preparation method of a dense-structured high-compaction iron phosphate material, including the following steps: S1. Mix the iron phosphate slag after lithium extraction from the battery and the iron phosphate waste at an iron element molar ratio of 1:3 - 5, and dissolve them in sulfuric acid solution to obtain solution A; S2. Add an alkaline solution to solution A to adjust the pH to 2.0 - 4.0, and precipitate B is formed; S3. Precipitate B is dissolved in sulfuric acid solution again to obtain solution C, and after filtration, phospho - iron liquid is obtained; S4. Add phosphoric acid solution to the phospho - iron liquid to obtain a mixed solution; S5. Add deionized water to the mixed solution obtained in step S4 to adjust the iron ion concentration in the system to 1.0 - 1.5 mol / L and the pH to 0.8 - 1.2; S6. Heat up to 80 - 100 °C, react until the solution turns into a white suspension, and keep warm for 1 - 4 hours to obtain a dihydrate iron phosphate precursor; S7. Dry the dihydrate iron phosphate precursor at 100 - 200 °C, and then calcine it at 400 - 600 °C for 2 - 5 hours to obtain anhydrous iron phosphate.

[0009] Preferably, in step S1, in the iron phosphate slag and the iron phosphate waste, the molar ratio of iron element to phosphorus element is 0.996 - 1.004:1.

[0010] Preferably, in step S2, the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, ammonia water, and magnesium oxide, and the pH is adjusted to 2.5 - 3.5.

[0011] Preferably, in step S4, the molar ratio of the phosphorus element in the phosphoric acid solution to the phosphorus element in the phospho - iron liquid is 0.05 - 0.15:1.

[0012] Preferably, in step S5, the addition amount of deionized water accounts for 1 / 40 - 1 / 5 of the volume of the phospho - iron liquid.

[0013] Preferably, in step S6, the heat - preservation time is 2 - 3 hours, and the reaction temperature is 85 - 95 °C.

[0014] Preferably, the tapped density of the anhydrous iron phosphate is 1.2 - 1.5 g / cm 3 .

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Aiming at the problems in the prior art such as long reaction process, complex reaction process, and sparse structure, the present invention provides a preparation method of a high - tap - density iron phosphate material with a dense structure. By using phospho - iron liquid to provide iron ions and phosphate ions, adjusting the pH and concentration of the system, and heating up in a programmed manner, an iron phosphate material with a dense structure is obtained.

[0016] 2. The present invention prepares iron phosphate materials by directly adding phosphoric acid to a raw material solution in which iron and phosphorus elements are evenly distributed and in appropriate proportions to adjust the pH and concentration of the system. The principle is as follows: Deionized water is used to adjust the pH required for the reaction in the system. Then, at a certain temperature and acidity, the iron ions and phosphate ions in the solution are combined. After turning white, it is kept warm for a period of time to obtain an iron phosphate material with a dense structure.

[0017] 3. The present invention uses iron phosphate slag and iron phosphate waste after lithium extraction from batteries as raw materials. The ratios of iron and phosphorus elements in the iron phosphate slag and waste are close to the theoretical values (Fe:P = 1:1), eliminating the need for additional addition of high-valent iron sources or phosphorus sources and significantly reducing the raw material cost.

[0018] 4. The present invention only requires five steps of "dissolution - precipitation - re-dissolution - reaction - calcination" to complete the preparation, without complex batching and multiple washings. The process flow is greatly simplified, reducing the investment in raw materials and equipment, thus reducing the production cost and shortening the production cycle. This method can achieve continuous and stable production, which is conducive to increasing production capacity and further reducing the cost per unit product. Based on the current market raw material prices, compared with the traditional process using pure chemical reagents such as ferrous sulfate and monoammonium phosphate, the raw material cost of the present invention is reduced by about 60% - 70%.

[0019] 5. The present invention provides a method for preparing an iron phosphate material with a simple process and controllable crystal form. An iron phosphate solution prepared from inexpensive iron phosphate slag or iron phosphate waste after lithium extraction from batteries is used. This solution provides ferric ions and phosphate ions. The phosphoric acid solution is uniformly mixed in the iron phosphate solution. By adjusting the pH and concentration of the system with deionized water, the cumbersome reaction steps are further optimized, and the generation of intermediates is reduced. After heating, dense dihydrate iron phosphate is directly formed. At the same time, the iron and phosphorus in the raw materials of the present invention are premixed, making it easier to achieve uniform precipitation of iron and phosphorus elements at the molecular level during the feeding process. Coupled with appropriate process parameters, the uniformity of the structure and chemical composition of iron phosphate is improved, which helps to obtain iron phosphate with uniform pores, ultimately improving the grinding efficiency and reaction activity of lithium iron phosphate and being conducive to the preparation of high-compact lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Electron microscopy scan of anhydrous iron phosphate prepared in Example 4; Figure 2 Electron microscopy scan of anhydrous iron phosphate prepared in Example 5; Figure 3 Electron microscopy scan of anhydrous iron phosphate prepared in Comparative Example 1; Figure 4 Electron microscopy scan of anhydrous iron phosphate prepared in Comparative Example 2; Figure 5It is the ferrophosphorus liquid of Example 4. Detailed implementation mode

[0021] Example 1 A preparation method of a high-compactness high-tap-density iron phosphate material, comprising the following steps: S1. Mix the iron phosphate slag after lithium extraction from the battery with iron phosphate waste at an iron element molar ratio of 1:3, and dissolve it in a sulfuric acid solution to obtain solution A; S2. Add an alkaline solution to solution A to adjust the pH to 2.0 to form precipitate B; S3. Dissolve precipitate B in a sulfuric acid solution again to obtain solution C, and obtain ferrophosphorus liquid after filtration; S4. Add a phosphoric acid solution to the ferrophosphorus liquid to obtain a mixed solution; S5. Add deionized water to the mixed solution obtained in step S4 to adjust the iron ion concentration in the system to 1.0 mol / L and the pH to 0.8; S6. Heat up to 80 °C, react until the solution turns into a white suspension, and keep it warm for 1 hour to obtain a dihydrate iron phosphate precursor; S7. Dry the dihydrate iron phosphate precursor at 100 °C, and then calcine it at 400 °C for 2 hours to obtain anhydrous iron phosphate.

[0022] Moreover, in step S1, in the iron phosphate slag: the molar ratio of iron element to phosphorus element is 0.996:1; in the iron phosphate waste: the molar ratio of iron element to phosphorus element is 0.996:1.

[0023] Moreover, in step S2, the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, ammonia water, and magnesium oxide, and the pH is adjusted to 2.5.

[0024] Moreover, in step S4, the molar ratio of the phosphorus element in the phosphoric acid solution to the phosphorus element in the ferrophosphorus liquid is 0.05:1.

[0025] Moreover, in step S5, the addition amount of deionized water accounts for 1 / 40 of the volume of the ferrophosphorus liquid.

[0026] Moreover, in step S6, the heat preservation time is 2 hours and the reaction temperature is 85 °C.

[0027] Moreover, the tap density of the anhydrous iron phosphate is 1.2 g / cm 3 .

[0028] Example 2 A preparation method of a high-compactness high-tap-density iron phosphate material, comprising the following steps: S1. Mix the iron phosphate slag after lithium extraction from the battery with iron phosphate waste at an iron element molar ratio of 1:5, and dissolve it in a sulfuric acid solution to obtain solution A; S2. Add an alkaline solution to solution A to adjust the pH to 4.0, forming precipitate B; S3. Redissolve precipitate B in a sulfuric acid solution to obtain solution C, and obtain a phosphoric iron solution after filtration; S4. Add a phosphoric acid solution to the phosphoric iron solution to obtain a mixture; S5. Add deionized water to the mixture obtained in step S4 to adjust the iron ion concentration in the system to 1.5 mol / L and the pH to 1.2; S6. Heat up to 80 - 100 °C, react until the solution turns into a white suspension, and keep warm for 4 hours to obtain a ferric phosphate dihydrate precursor; S7. Dry the ferric phosphate dihydrate precursor at 200 °C and then calcine it at 600 °C for 5 hours to obtain anhydrous ferric phosphate.

[0029] Moreover, in step S1, in the phosphoric iron slag, the molar ratio of iron element to phosphorus element is 1.004:1; in the phosphoric iron waste, the molar ratio of iron element to phosphorus element is 1.004:1.

[0030] Moreover, in step S2, the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, ammonia water, and magnesium oxide, and the pH is adjusted to 3.5.

[0031] Moreover, in step S4, the molar ratio of the phosphorus element in the phosphoric acid solution to the phosphorus element in the phosphoric iron solution is 0.15:1.

[0032] Moreover, in step S5, the added amount of deionized water accounts for 1 / 5 of the volume of the phosphoric iron solution.

[0033] Moreover, in step S6, the heat preservation time is 3 hours and the reaction temperature is 85 - 95 °C.

[0034] Moreover, the tap density of the anhydrous ferric phosphate is 1.5 g / cm 3 .

[0035] Example 3 A preparation method of a structurally dense and high - tap - density ferric phosphate material, comprising the following steps: S1. Mix the phosphoric iron slag after lithium extraction from the battery and the phosphoric iron waste in a molar ratio of iron element 1:4, and dissolve them in a sulfuric acid solution to obtain solution A; S2. Add an alkaline solution to solution A to adjust the pH to 3, forming precipitate B; S3. Redissolve precipitate B in a sulfuric acid solution to obtain solution C, and obtain a phosphoric iron solution after filtration; S4. Add a phosphoric acid solution to the phosphoric iron solution to obtain a mixture; S5. Add deionized water to the mixture obtained in step S4 to adjust the iron ion concentration in the system to 1.2 mol / L and the pH to 1.0; S6. Heat to 90 °C and react until the solution turns into a white suspension. Keep warm for 2.5 hours to obtain the iron phosphate dihydrate precursor. S7. Dry the iron phosphate dihydrate precursor at 150 °C and then calcine it at 500 °C for 3.5 hours to obtain anhydrous iron phosphate.

[0036] Moreover, in the step S1, in the iron phosphate slag, the molar ratio of iron element to phosphorus element is 1:1; in the iron phosphate waste, the molar ratio of iron element to phosphorus element is 1:1.

[0037] Moreover, in the step S2, the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, ammonia water, and magnesium oxide, and the pH is adjusted to 3.

[0038] Moreover, in the step S4, the molar ratio of the phosphorus element in the phosphoric acid solution to the phosphorus element in the phosphorus-iron solution is 1:1.

[0039] Moreover, in the step S5, the addition amount of deionized water accounts for 1 / 20 of the volume of the phosphorus-iron solution.

[0040] Moreover, in the step S6, the heat preservation time is 2.5 hours and the reaction temperature is 90 °C.

[0041] Moreover, the tapped density of the anhydrous iron phosphate is 1.3 g / cm 3 .

[0042] Example 4 A preparation method of a phosphoric acid iron material with a dense structure and high tap density specifically includes the following key steps: (1) Accurately weigh 48.8 kg of phosphorus-iron solution ( Figure 5 ) as the starting material.

[0043] (2) Weigh out 650 g of phosphoric acid solution.

[0044] (3) Measure 3 L of deionized water for standby.

[0045] (4) During the process of slowly adding the weighed 650 g of phosphoric acid solution to the phosphorus-iron solution, continuous stirring is required to ensure uniform mixing of the two; the key to this step is slow addition and sufficient stirring to avoid side reactions caused by local high concentration; after mixing evenly, adjust the pH value of the reaction system to 0.8 with 3 L of deionized water. After adjustment, heat the reaction system to 85 °C.

[0046] (5) When the color of the reaction system gradually changes from wine red to white, it indicates the formation of iron phosphate dihydrate; at this time, keep the system at a constant temperature of 85 °C for 100 minutes.

[0047] (6) Wash the iron phosphate dihydrate after heat preservation treatment to remove impurities and unreacted raw materials adhering to the surface; after washing, perform drying treatment to remove moisture, and then calcine at high temperature to obtain anhydrous iron phosphate.

[0048] Example 5 A preparation method of a dense and high-compact iron phosphate material, which is optimized and adjusted on the basis of Example 1. The main difference is that the concentration and pH value of the reaction system are further finely regulated by increasing the amount of deionized water used.

[0049] The following are the specific steps of this method: (1) Accurately weigh 48.8 kg of phosphorus-iron liquid as the starting reactant.

[0050] (2) Weigh out 650 g of phosphoric acid solution.

[0051] (3) In this example, the amount of deionized water measured is increased to 5 L.

[0052] (4) Slowly and evenly add the weighed 650 g of phosphoric acid solution to the phosphorus-iron liquid, and continuously stir during the addition to ensure that the two can be fully mixed; after mixing, gradually adjust the pH value of the reaction system to 1.2 with 5 L of deionized water; after adjustment, heat the reaction system to 85 °C.

[0053] (5) As the reaction proceeds, when it is observed that the color of the system gradually changes from wine red to white, it indicates that iron phosphate dihydrate has been formed; at this time, keep the system at 85 °C for constant temperature heat preservation for 100 min to obtain iron phosphate dihydrate.

[0054] (6) Wash the generated iron phosphate dihydrate. After washing, perform drying treatment to remove moisture; then, perform calcination treatment at high temperature to convert iron phosphate dihydrate into anhydrous iron phosphate.

[0055] Comparative Example 1 A preparation method of iron phosphate, which is similar to Example 4 and Example 5 in steps, but the difference is that the amount of deionized water added is different. The following are the detailed steps of this Comparative Example 1: (1) Accurately weigh 48.8 kg of phosphorus-iron liquid.

[0056] (2) Weigh out 650 g of phosphoric acid solution.

[0057] (3) Measure 1 L of deionized water for standby.

[0058] Deionized water plays two key roles here: one is to act as a solvent to help the phosphoric acid solution mix better with the ferrophosphorus solution; the other is to be used to adjust the pH value of the system, which is an important factor affecting the growth and structure of iron phosphate crystals.

[0059] (4) Slowly add the weighed 650 g of phosphoric acid solution to the ferrophosphorus solution, and conduct thorough stirring during this process to ensure that the phosphoric acid and the ferrophosphorus solution can be evenly mixed, and avoid side reactions caused by too high local concentration.

[0060] (5) After stirring is completed, use the previously measured 1 L of deionized water to adjust the pH value of the entire system to 0.6.

[0061] (6) After adjusting the pH value, heat the system to 85 °C. During the heating process, it is necessary to closely monitor the temperature change of the system to ensure that the heating rate is appropriate and avoid adverse effects on the reaction caused by too high or too low temperature.

[0062] (7) When the color of the system gradually turns white, it indicates that the formation of iron phosphate crystals has been completed. At this time, keep the system at a constant temperature of 85 °C for 100 min.

[0063] (8) Wash, dry, and calcine the iron phosphate dihydrate after it turns white to obtain an anhydrous iron phosphate product.

[0064] Comparative Example 2 A method for preparing iron phosphate, which is similar to Example 4 and Example 5 in steps, but the difference lies in the different amounts of deionized water added. The following are the detailed steps of this Comparative Example 2: (1) Accurately weigh 48.8 kg of ferrophosphorus solution.

[0065] (2) Weigh out 650 g of phosphoric acid solution.

[0066] (3) Measure 8 L of deionized water for standby.

[0067] (4) Slowly add the weighed 650 g of phosphoric acid solution to the ferrophosphorus solution, and conduct thorough stirring during this process.

[0068] (5) After stirring is completed, use the previously measured 8 L of deionized water to adjust the pH value of the entire system to 1.4.

[0069] (6) After adjusting the pH value, heat the system to 85 °C. During the heating process, it is necessary to closely monitor the temperature change of the system to ensure that the heating rate is appropriate and avoid adverse effects on the reaction caused by too high or too low temperature.

[0070] (7) When the color of the system gradually turns white, it indicates that the formation of iron phosphate crystals has been completed. At this time, keep the system at a constant temperature of 85 °C for 100 min.

[0071] (8) Wash, dry, and calcine the iron phosphate dihydrate after it turns white to obtain anhydrous iron phosphate.

[0072] Experimental Section

[0073] Experiment 1 Perform electron microscopy scanning on the anhydrous iron phosphates prepared in Examples 4 and 5 and Comparative Examples 1 and 2. See specifically Figures 1-4 . It can be seen from the figures that the anhydrous iron phosphates prepared in Examples 4 - 5 exhibit a uniform particle morphology, with a smooth particle surface and no obvious pores, a narrow particle size distribution, and the particles are tightly bonded to form a dense packing structure with no obvious gaps or cracks.

[0074] The anhydrous iron phosphate particles prepared in Comparative Examples 1 and 2 have irregular shapes, a wide particle size distribution, serious agglomeration of some particles, partial melting, lack of uniformity, a rough particle surface, obvious pores and cracks, indicating a loose internal structure of the material, poor compactness, weak bonding between particles, obvious gaps, resulting in a low overall packing density of the material.

[0075] Experiment 2 Perform physical and chemical index tests on the anhydrous iron phosphates prepared in Examples 4 and 5 and Comparative Examples 1 and 2, specifically including Fe content, P content, Fe / P, D50, BET, and tapped density. The results are shown in Table 1.

[0076] Table 1 Physical and Chemical Index Test Results

[0077] It can be seen from Table 1 that the iron and phosphorus contents in Examples 4 and 5 are both relatively large; the iron - phosphorus ratio is stable and both are greater than 96.2%, which is one of the key factors maintaining the stability of the dense packing structure of iron phosphate.

[0078] The D50 particle size reflects the distribution of particle sizes. A smaller particle size usually means more uniform particles, which is beneficial for forming a dense packing structure and thus improving the compaction density of the material. The D50 values of Examples 4 and 5 are both at a relatively small level; the specific surface area is an important parameter for measuring the surface characteristics of the material. A larger specific surface area usually means more active sites, which is beneficial for the insertion and extraction of lithium ions. The specific surface areas of Examples 4 and 5 are both greater than 10 m 2 / g; the tapped density is an important indicator for measuring the compaction performance of the material. For the cathode material of lithium - ion batteries, a higher tapped density is beneficial for improving the energy density and cycle stability of the battery. The tapped densities of Examples 4 and 5 are both far greater than 1.0 m 3 / g.

[0079] The iron-to-phosphorus ratios of Comparative Examples 1 and 2 are both low, the particle size of the finished product is too large, the morphology and sintering resistance of anhydrous iron phosphate are extremely low, and the specific surface area is too low. Due to the loose binding between particles and the large number of internal pores, the tap density is less than 1.0 m 3 / g.

[0080] Experiment 3 The iron phosphates prepared in Examples 4 and 5 and Comparative Examples 1 and 2 were made into lithium iron phosphate materials under the same conditions, and their tap densities and electrochemical properties were measured. After the iron phosphate materials prepared in Example 1 and Example 2 were made into lithium iron phosphate, the tap densities of their powders were both greater than 2.60 g / cm 3 . In terms of electrochemical performance, the discharge capacity at 0.5C is as high as 146 mAh / g. While the tap densities of the powders in the comparative examples are only 2.32 g / cm3 and 2.37 g / cm 3 .

Claims

1. A method for preparing a dense and high-density ferric phosphate material, characterized in that: The following steps are involved: S1, mixing the iron phosphate slag after lithium extraction from the battery with the iron phosphate waste in a molar ratio of 1:3-5 for the iron element, and dissolving them in a sulfuric acid solution to obtain a solution A; S2, adding alkaline solution to solution A to adjust the pH to 2.0-4.0 to generate precipitate B; S3, dissolving the precipitate B in a sulfuric acid solution again to obtain a solution C, and filtering to obtain a ferrophosphorus liquid; S4, adding phosphoric acid solution to the ferrophosphorus liquid to obtain a mixed solution; S5, adding deionized water to the mixed solution obtained in step S4, adjusting the iron ion concentration of the system to 1.0-1.5 mol / L and the pH to 0.8-1.2; S6, heating to 80-100°C, reacting until the solution turns into a white suspension, and keeping the temperature for 1-4 hours to obtain a dihydrate iron phosphate precursor; S7. Dry the iron phosphate dihydrate precursor at 100-200° C., and then calcine at 400-600° C. for 2-5 hours to obtain anhydrous iron phosphate.

2. The method for preparing a dense and high-density ferric phosphate material according to claim 1, characterized in that: In the step S1, in the ferrophosphate slag, the molar ratio of iron to phosphorus is 0.996-1.004:1; in the ferrophosphate waste, the molar ratio of iron to phosphorus is 0.996-1.004:

1.

3. The method for preparing a dense and high-density ferric phosphate material according to claim 1, characterized in that: In step S2, the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, ammonia water, and magnesium oxide, and the pH is adjusted to 2.5-3.

5.

4. The method for preparing a dense and high-density ferric phosphate material according to claim 1, characterized in that: In the step S4, the molar ratio of phosphorus in the phosphoric acid solution to phosphorus in the ferrophosphorus liquid is 0.05-0.15:

1.

5. The method for preparing a dense and high-density ferric phosphate material according to claim 1, characterized in that: In step S5, the amount of deionized water added is 1 / 40-1 / 5 of the volume of the ferrophosphorus liquid.

6. The method for preparing a dense and high-density ferric phosphate material according to claim 1, characterized in that: In step S6, the insulation time is 2-3 hours and the reaction temperature is 85-95°C.

7. A method for preparing a dense and high-density ferric phosphate material according to any one of claims 1 to 6, characterized in that: The tap density of the anhydrous ferric phosphate is 1.2-1.5 g / cm 3 .

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