Large single crystal iron phosphate adaptive to high compaction type lithium iron phosphate and preparation method of large single crystal iron phosphate

Abstract

The invention relates to the technical field of lithium battery positive electrode material preparation, in particular to large single crystal iron phosphate adaptive to high compaction type lithium iron phosphate and a preparation method thereof.The preparation method comprises the following steps that S1, after chemical fertilizer acid is diluted, an impurity removing agent and a flocculating agent are added, and filtering is conducted to obtain a purified chemical fertilizer acid solution; compounding the purified chemical fertilizer acid solution with phosphoric acid to obtain an iron dissolving solution, wherein the mass concentration of phosphoric acid in the iron dissolving solution is 10-45%; s2, adding an iron source into the iron dissolving solution, heating, dissolving and filtering to obtain a ferrophosphorus solution; s3, adding an oxidizing agent I into the ferrophosphorus solution at a certain temperature, and carrying out an oxidation reaction to obtain a reaction solution; s4, adding an oxidizing agent II into the reaction solution, and carrying out heat preservation and aging at a certain temperature to obtain an aged solution; and S5, filtering, washing, drying and calcining the aging solution to obtain the large single crystal iron phosphate. The problems of low production efficiency, high energy consumption, large equipment investment, large environmental pollution and high post-treatment cost of the existing iron phosphate adaptive to high-compaction lithium iron phosphate can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery positive electrode material preparation, and in particular to a large single crystal iron phosphate adapted to high-pressure compaction lithium iron phosphate and a preparation method thereof. Background Art

[0002] The installed capacity of lithium iron phosphate batteries is much higher than that of ternary lithium batteries due to their comprehensive advantages such as excellent high-temperature performance, long cycle life, low price, and environmental friendliness. At present, the installed capacity accounts for as much as 74.0% of the total installed capacity. Lithium iron phosphate batteries are more excellent in terms of energy storage, with a market share of over 94% in domestic electrochemical energy storage. Foreign demand is also growing rapidly, and the export volume of domestic energy storage lithium iron phosphate batteries has skyrocketed. However, with the further development of the new energy industry, the market has higher and higher requirements for the energy density of lithium iron phosphate. Given that its capacity and voltage platform are close to the theoretical value, there is little room for improvement and it is extremely difficult. Therefore, increasing the compaction density of lithium iron phosphate is an effective way to increase its energy density.

[0003] At present, the methods for improving the compaction density of lithium iron phosphate are mostly achieved through particle grading, calcination system regulation and doping. For example, the patent with application publication number CN118373398A discloses a method for preparing ultra-high compaction lithium iron phosphate positive electrode material, which obtains an iron-containing precursor based on iron phosphate and a dispersant; firstly, the iron-containing precursor, lithium source, phosphorus source and first / second carbon source are mixed for first / second sintering to obtain a first / second precursor mixture and ground to a first / second preset particle size to obtain a first / second precursor; secondly, the first / second precursor and the third carbon source are mixed for third sintering to obtain a first lithium iron phosphate positive electrode material; finally, the large-particle first precursor, the small-particle second precursor and the third carbon source form an ultra-high compaction density lithium iron phosphate positive electrode material with titanium-nitrogen dual doping and a second carbon coating. This method also uses particle grading and doping methods to increase the compaction density of lithium iron phosphate. However, compared with conventional processes, this method is more complicated and has an additional mixing process and a sintering process, which results in reduced production efficiency and increased energy consumption.

[0004] The patent application publication number CN118515254A discloses a method for preparing lithium iron phosphate, using Fe2O3 and H3PO4 prepared by calcining iron-rich Fenton sludge as raw materials to prepare iron phosphate; first, the iron phosphate is mixed with a lithium source, a carbon source and an additive A, sand-milled, spray-dried, and then placed in a tubular furnace for sintering at a first temperature to obtain sintering material I; then the graphite mold loaded with sintering material I is placed in a rapid hot pressing sintering furnace chamber and rapidly heated to a second temperature for sintering to obtain sintering material II; finally, sintering material II is crushed to a certain particle size to obtain a high-density lithium iron phosphate material. This method uses a two-step sintering method to prepare high-density lithium iron phosphate, and one of the sintering steps is hot pressing sintering. Compared with the conventional process, a set of hot pressing sintering equipment is added, resulting in an increase in floor space, equipment investment and energy consumption while reducing production efficiency.

[0005] The patent application publication number CN118702083A discloses a method for preparing an iron phosphate precursor suitable for high-pressure compaction type lithium iron phosphate. An amorphous iron phosphate slurry is prepared using a titanium dioxide by-product containing Fe3O4 as a raw material; first, it is stirred and crystallized at high temperature to obtain iron phosphate dihydrate I; secondly, it is stirred and ultrasonically crystallized at low frequency and high temperature to obtain iron phosphate dihydrate II; then, it is stirred and ultrasonically crystallized at high frequency and high temperature to obtain iron phosphate dihydrate III; finally, a bifunctional dispersant is added to iron phosphate dihydrate I, iron phosphate dihydrate II and iron phosphate dihydrate III, and then mixed and graded, and then washed twice, filtered, dried, crushed and sintered to obtain an iron phosphate precursor suitable for high-pressure compaction type lithium iron phosphate. This method starts with the precursor material of lithium iron phosphate and improves the compaction density of lithium iron phosphate by increasing the grading of iron phosphate. However, its bifunctional dispersant is selected from one or more of cyclohexanol, phenol, polyvinyl pyrrolidone, hexadecyltrimethylammonium bromide, and polyethylene glycol. Toxic gases will be produced during the sintering preparation process of lithium iron phosphate, which will increase the cost of three waste treatment and environmental pressure.

[0006] In summary, the current methods for increasing the compaction density of lithium iron phosphate have low production efficiency, high energy consumption, large footprint, high equipment investment, severe environmental pollution during the production process, and high post-processing costs, making it difficult to adapt to large-scale production needs. Summary of the invention

[0007] The present invention provides a large single crystal iron phosphate adapted for high-pressure compaction lithium iron phosphate and a preparation method thereof, which can solve the problems of low production efficiency, high energy consumption, large equipment investment, large environmental pollution and high post-processing cost.

[0008] This application provides the following technical solutions:

[0009] A large single crystal iron phosphate adapted to high-density lithium iron phosphate and a preparation method thereof, comprising the following steps:

[0010] S1. After the fertilizer acid is diluted to a certain concentration, an impurity remover and a flocculant are added, and the mixture is filtered to obtain a purified fertilizer acid solution; the purified fertilizer acid solution is compounded with phosphoric acid to obtain an iron-dissolving solution, wherein the mass concentration of phosphoric acid in the iron-dissolving solution is 10-45%;

[0011] S2, adding an iron source to the iron-dissolving liquid, heating, dissolving, filtering, and obtaining a ferrophosphorus solution;

[0012] S3, adding oxidant I to the ferrophosphorus solution at a certain temperature to carry out oxidation reaction to obtain a reaction solution;

[0013] S4, adding oxidant II to the reaction solution, and performing thermal aging at a certain temperature to obtain an aged solution;

[0014] S5. The aged solution is filtered, washed, dried and calcined to obtain large single crystal iron phosphate.

[0015] The large single crystal iron phosphate prepared by the above preparation method and its application in the preparation of lithium iron phosphate.

[0016] Beneficial effects:

[0017] 1. This technical solution does not use additives and templates. Through the coordinated regulation of the oxidant addition ratio before and after the reaction process, the stirring system and the heating system during the aging process, the size and distribution range of the iron phosphate grains are adjusted, and high-quality large single crystal iron phosphate with high crystallinity and different particle size ranges suitable for high-pressure compaction lithium iron phosphate is prepared, which increases its electrochemical properties, meets the needs of high-performance lithium-ion batteries, and can effectively enhance the market competitiveness of the product.

[0018] 2. Fertilizer acid is a process acid produced during the preparation of phosphoric acid. The concentration of phosphoric acid is 30%-60%. It has not undergone the final impurity removal process and concentration process. The impurity concentration of Mg, Al, etc. exceeds 4000ppm. The cost is lower than that of phosphoric acid, but it cannot be directly used as a phosphorus source to prepare iron phosphate. This technical solution uses fertilizer acid to remove impurities and then compound with phosphoric acid as a phosphorus source to replace pure 85% phosphoric acid to prepare iron phosphate. Compared with using pure 85% phosphoric acid as a phosphorus source to dissolve the iron source, the cost of raw materials is greatly reduced. Moreover, the reagents used in this technical solution, such as impurity removers and oxidants, are all easy-to-obtain low-cost reagents, which further reduce production costs. The manufacturing cost of iron phosphate is reduced by at least 800 yuan / ton. At the same time, by purifying the fertilizer acid solution, the content of harmful substances in the waste liquid is reduced, the difficulty and cost of waste liquid treatment are reduced, the effective utilization of resources is achieved, the waste of chemicals is reduced, and the economic benefits are improved.

[0019] 3. The preparation process of the present invention is simple, convenient and environmentally friendly, with high production efficiency and low energy consumption. For conventional iron phosphate production lines using pure iron as the iron source, there are no extra cumbersome steps, strong compatibility, and only the addition of a fertilizer acid impurity removal reactor is required, with low transformation cost and suitable for large-scale production.

[0020] Preferably, in step S1, the fertilizer acid is diluted with water to prepare a fertilizer acid solution, the mass concentration of phosphoric acid in the fertilizer acid solution is 10-30%, an impurity remover is added for impurity removal, the reaction temperature for impurity removal is 15-60° C., the reaction time for impurity removal is 0.5-3h, and the reaction is carried out to an end point pH of 2.8-8.5; a flocculant is added, and the solution is filtered to obtain a purified fertilizer acid solution.

[0021] Preferably, in step S1, the impurity remover is one or a combination of ammonium carbonate, sodium hydroxide, ammonia water, and ammonia gas, and the molar mass ratio of the impurity remover to the phosphoric acid in the fertilizer acid solution is 0.1 to 5:1; the flocculant is polyacrylamide, and the molar mass ratio of the flocculant to the phosphoric acid in the fertilizer acid solution is 0.0001 to 0.1:1.

[0022] By adding impurity removers and flocculants, impurity ions are effectively removed, ensuring the purity of the fertilizer acid solution, achieving effective utilization of resources, and improving economic benefits.

[0023] Preferably, in step S2, the iron source is one or a combination of iron powder, iron block, and iron rod; the molar mass ratio of the iron source to phosphorus in the molten iron solution is 100-0.35:1; the reaction temperature is 40-90°C, the reaction time is 2-12h, and the Fe content in the ferrophosphorus solution is 0.1%. 2+ The mass concentration is 2.5 to 5wt%.

[0024] Ensure the complete dissolution of iron ions and provide stable raw materials for subsequent reactions.

[0025] Preferably, in step S3, the reaction temperature is 30-80° C., the reaction time is 0.5-3 h, and the molar mass ratio of the oxidant I to the Fe ions in the ferrophosphide solution is 0.8-3:1.

[0026] Preferably, in step S4, the insulation aging temperature is 85-97° C., the insulation aging time is 1-3 h; and the molar mass ratio of the oxidant II to the Fe ions in the ferrophosphide solution is 0.5-30:1.

[0027] This technical solution adopts a staged oxidation reaction. In step S3, oxidant I is added, and preliminary oxidation is performed at 30-80°C to generate iron phosphate crystal nuclei and grow appropriately. In step S4, heating is continued and oxidant II is added, and heat preservation aging is performed at 85-97°C to further grow the iron phosphate crystals. This staged oxidation method avoids the generation of too many crystal nuclei due to the addition of too much oxidant at one time, ensures the controllability of crystal nucleus formation and growth, promotes the gradual growth of iron phosphate crystals, and forms a uniform and stable large single crystal structure.

[0028] If the oxidation reaction temperature of step S3 is lower than 30°C, the reaction is incomplete; if the temperature is higher than 80°C, the iron-phosphorus ratio of iron phosphate will be low; if the molar mass ratio of oxidant I to Fe ions in ferrophosphide solution is less than 0.8:1, too few iron phosphate nuclei are formed during the reaction, resulting in abnormal growth of grains and a serious decrease in specific surface area; if the molar mass ratio of oxidant I to Fe ions in ferrophosphide solution is greater than 3:1, too many iron phosphate nuclei are formed during the reaction, resulting in small growth of iron phosphate grains.

[0029] If the aging reaction temperature of step S4 is lower than 85°C, the iron-phosphorus ratio of iron phosphate will be low; if the temperature is higher than 97°C, it will cause increased energy consumption; if oxidant II is not added, the residual iron in the mother liquor will increase, causing losses, and will cause various indicators of iron phosphate to fail to meet the standards; if the molar mass ratio of oxidant II to Fe ions in ferrophosphorus solution is less than 0.5:1, the iron phosphate grains will be reduced, the residual iron in the mother liquor will increase, and losses will be caused. If the molar mass ratio of oxidant II to Fe ions in ferrophosphorus solution is greater than 30:1, it will cause an increase in raw material costs.

[0030] Preferably, the oxidant I and the oxidant II are one or a combination of oxygen, hydrogen peroxide, air, and ozone; the components of the oxidant I and the oxidant II are the same or different.

[0031] The oxidants used are oxygen, hydrogen peroxide, air, and ozone, which are easily available and of low cost, thus effectively reducing production costs.

[0032] Preferably, in step S5, washing is performed with water for 1 to 3 times; the drying temperature is 80 to 200° C., and the drying time is 6 to 13 hours; the calcination temperature is 550 to 590° C., and the calcination time is 1 to 4 hours.

[0033] The aged solution is filtered and washed to ensure the removal of residual impurities. It is dried at 80-200°C to ensure the dryness of the product. It is calcined at 550-590°C for 1-4 hours to further enhance the stability and crystallinity of the crystals, and improve the physical properties and chemical stability of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a SEM image of the iron phosphate obtained in Example 1 of the present invention.

[0035] Figure 2 This is a SEM image of the iron phosphate obtained in Example 2 of the present invention.

[0036] Figure 3 This is a SEM image of the iron phosphate obtained in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0037] The following is further described in detail through specific implementation methods:

[0038] A large single crystal iron phosphate suitable for high-density lithium iron phosphate and a preparation method thereof, the specific steps are as follows:

[0039] S1. Fertilizer acid is diluted with water to prepare a fertilizer acid solution, and the mass concentration of phosphoric acid in the fertilizer acid solution is 10-30%; an impurity remover is added to remove impurities, and the reaction temperature of the impurity removal is 15-60°C, the reaction time of the impurity removal is 0.5-3h, and the pH value of the reaction to the end point is 2.8-8.5; a flocculant is added, and the reaction is filtered to obtain a purified fertilizer acid solution. The purified fertilizer acid solution is compounded with 85% pure phosphoric acid to obtain an iron-dissolving solution, and the mass ratio of the fertilizer acid solution to 85% pure phosphoric acid is 0.1-0.7:1, and the mass concentration of phosphoric acid in the iron-dissolving solution is 10-45%.

[0040] The preparation water is one or a combination of purified water, washing water, and distilled water. The impurity remover is one or a combination of ammonium carbonate, sodium hydroxide, ammonia water, and ammonia gas, and the molar mass ratio of the impurity remover to the phosphoric acid in the fertilizer acid solution is 0.1 to 5:1. The flocculant is polyacrylamide, and the molar mass ratio of the flocculant to the phosphoric acid in the fertilizer acid solution is 0.0001 to 0.1:1.

[0041] S2, adding an iron source to the iron solution obtained in step S1, wherein the molar mass ratio of the iron source to phosphorus in the iron solution is 100-0.35:1, heating the solution for reaction and dissolution, the reaction temperature is 40-90°C, the reaction time is 2-12h, the dissolution end point is the appearance of black iron slag on the surface of the solution, the solution is green, and the precipitate is removed by suction to obtain a ferrophosphorus solution. At this time, Fe 2+ The concentration is 2.5-5wt%. The iron source is one or a combination of iron powder, iron block, and iron rod.

[0042] S3, heating the ferrophosphorus solution obtained in step S2 to 30-80°C, adding oxidant I for oxidation reaction, and stirring, the stirring speed is 150-400 rpm, the reaction time is 0.5-3h, and a reaction solution is obtained. The molar mass ratio of oxidant I to Fe ions in the ferrophosphorus solution is 0.8-3:1.

[0043] S4, adding oxidant II to the reaction solution obtained in step S3, and performing heat preservation aging at 85-97° C. for 1-3 hours at a stirring speed of 200-350 rpm to obtain an aged solution. The molar mass ratio of oxidant II to Fe ions in the ferrophosphorus solution is 0.5-30:1.

[0044] Oxidant I and oxidant II are one or a combination of oxygen, hydrogen peroxide, air and ozone; the components of oxidant I and oxidant II are the same or different.

[0045] S5. Filter the aged solution obtained in step S4, remove the filtrate, and obtain a filter cake. Wash the filter cake with water, the mass ratio of washing water to filter cake is 2 to 5:1, and the number of washings is 1 to 3 times. Dry the filter cake at a temperature of 80 to 200°C and a drying time of 6 to 13 hours. Calcine the dried filter cake at a temperature of 550 to 590°C and a calcination time of 1 to 4 hours, and cool it naturally to room temperature to obtain a large single crystal iron phosphate. After crushing, batch mixing, and demagnetization, the iron phosphate product is packaged.

[0046] Example 1

[0047] A large single crystal iron phosphate adapted to high-density lithium iron phosphate and a preparation method thereof, comprising the following steps:

[0048] S1. Add fertilizer acid and pure water to prepare a fertilizer acid solution with a phosphoric acid concentration of 25%, pass ammonia gas with a molar mass of 5 times that of phosphoric acid to remove impurities, heat to 50°C and stir to react for 1h, add polyacrylamide with a molar mass of 1 / 1000 of the phosphoric acid in the fertilizer acid solution to flocculate and filter to obtain a purified fertilizer acid solution, and compound with 85% pure phosphoric acid to obtain a 45% phosphoric acid iron-dissolving solution. The impurity content of the fertilizer acid solution and the purified fertilizer acid solution is shown in Table 1.

[0049] Table 1 Comparison of impurity content between fertilizer acid and purified fertilizer acid solution

[0050] Impurity Type Ca Mg Cu Zn Ni Mn Ti Fertilizer Acid 4326 6532 Not detected 87.41 11.49 835.2 68.37 Purification of fertilizer acid solutions 172.8 28.3 Not detected 5.26 Not detected 13.17 Not detected Impurity Type K Na Al Cr Co Sn Pb Fertilizer Acid 2094 563.8 8771 27.24 13.63 Not detected Not detected Purification of fertilizer acid solutions 122.3 134.8 28.37 Not detected Not detected Not detected Not detected

[0051] S2. Put the iron block into the iron solution obtained in step S1, the molar mass ratio of the iron source to the iron solution is 20:1, heat to 80℃, stir to react and dissolve, after 12h, the solution turns green and then filter to remove the precipitate to obtain ferrophosphorus solution. 2+ The ion concentration is 3.5wt%-4wt%.

[0052] S3. Using the ferrophosphorus solution obtained in step S2 as the base solution, the temperature was raised to 60° C., the stirring speed was 200 rpm, and hydrogen peroxide with a molar mass of 1.2 times that of Fe ions in the ferrophosphorus solution was added to carry out oxidation reaction for 1 hour to obtain a reaction solution.

[0053] S4. The reaction solution obtained in step S3 is directly heated, and air is introduced while the temperature is raised to 97°C for heat preservation and aging. The molar mass ratio of air to Fe ions in the ferrophosphorus solution is 30:1, the stirring speed is 300 rpm, and the aging solution is obtained by heat preservation for 2 hours.

[0054] S5. The aged solution obtained in step S4 is filtered, washed twice with 4 times of water, dried at 180° C. for 8 h, calcined at 570° C. for 3 h, crushed, batch mixed, demagnetized, and packaged to obtain the iron phosphate product.

[0055] Figure 1The SEM image of the iron phosphate prepared in this example shows that the anhydrous iron phosphate obtained by the present invention has good crystallization and a large grain size distribution. The tap density of the iron phosphate obtained in this example is 1.02 g / cm 3 The compaction density of the prepared lithium iron phosphate is 2.62g / cm 3 , other indicators are shown in Table 2.

[0056] Example 2

[0057] A large single crystal iron phosphate adapted to high-density lithium iron phosphate and a preparation method thereof, comprising the following steps:

[0058] S1. Add fertilizer acid and washing water to prepare a fertilizer acid solution with a phosphoric acid concentration of 30%, introduce ammonia water with a molar mass of 0.5 times that of the phosphoric acid to remove impurities, heat to 30°C and stir to react for 1.5 hours, add polyacrylamide with a molar mass of 1 / 3000 of the phosphoric acid in the fertilizer acid solution to flocculate, and then filter to obtain a purified fertilizer acid solution, and compound with 85% pure phosphoric acid to obtain a 30% phosphoric acid iron-dissolving solution.

[0059] S2. Put the iron rod into the iron solution obtained in step S1, the molar mass ratio of the iron source to the iron solution is 80:1, heat to 90℃, stir to react and dissolve, after 5 hours, the solution turns green and then filter to remove the precipitate to obtain ferrophosphorus solution. 2+ The ion concentration is 3.5wt%-5wt%.

[0060] S3. The ferrophosphorus solution obtained in step S2 is used as the base liquid, the temperature is raised to 60° C., the stirring speed is 200 rpm, and ozone twice the molar mass of Fe ions in the ferrophosphorus liquid is added to carry out oxidation reaction for 2 hours to obtain a reaction solution.

[0061] S4. The reaction solution obtained in step S3 is directly heated to 85°C for aging for 1 hour, oxygen is introduced and stirring is continued. The molar mass ratio of oxygen to Fe ions in the ferrophosphorus solution is 9:1. The aging solution is obtained by heating for 2 hours.

[0062] S5. The aged solution obtained in step S4 is filtered, washed twice with 3 times of water, dried at 200° C. for 6 h, calcined at 570° C. for 3.5 h, and packaged to obtain the iron phosphate product after crushing, batch mixing, and demagnetization.

[0063] Figure 2 The SEM image of the iron phosphate prepared in this example shows that the anhydrous iron phosphate obtained by the present invention has good crystallization and a large grain size distribution. The tap density of the iron phosphate obtained in this example is 1.13 g / cm 3 The compaction density of the prepared lithium iron phosphate is 2.65g / cm 3 , other indicators are shown in Table 2.

[0064] Example 3

[0065] A large single crystal iron phosphate adapted to high-density lithium iron phosphate and a preparation method thereof, comprising the following steps:

[0066] S1. Add fertilizer acid and distilled recycled water to prepare a solution with a phosphoric acid concentration of 20%, pass sodium hydroxide with a molar mass 1.5 times that of the phosphoric acid to remove impurities, heat to 60°C and stir to react for 0.5h, add polyacrylamide with a molar mass of 1 / 5000 of the phosphoric acid in the fertilizer acid solution to flocculate, and then filter to obtain a purified fertilizer acid solution, which is compounded with 85% pure phosphoric acid to obtain a 30% phosphoric acid iron-dissolving solution.

[0067] S2. Put the iron block into the iron solution obtained in step S1, the molar mass ratio of the iron source to the iron solution is 10:1, heat to 70°C and stir to react and dissolve. After 12 hours, the solution turns green and is filtered to remove the precipitate to obtain ferrophosphorus solution. The Fe ion concentration in the solution is 4wt%-4.5wt%.

[0068] S3. The ferrophosphorus liquid obtained in step S2 is used as the base liquid, the temperature is raised to 40° C., the stirring speed is 180 rpm, and oxygen 3 times the molar mass of Fe ions in the ferrophosphorus liquid is added to carry out an oxidation reaction to obtain a reaction solution.

[0069] S4. The reaction solution obtained in step S3 is directly heated, and ozone is introduced while the temperature is raised to 85°C for heat preservation and aging. The molar mass ratio of ozone to Fe ions in the ferrophosphorus solution is 5:1, the stirring speed is 350 rpm, and the aging solution is obtained by heat preservation for 3 hours.

[0070] S5. The aged solution obtained in step S4 is filtered, washed once with 4 times of water, dried at 150° C. for 13 h, calcined at 580° C. for 2 h, crushed, batch mixed, demagnetized, and packaged to obtain the iron phosphate product.

[0071] The tap density of the iron phosphate obtained in this example is 1.05 g / cm 3 The compaction density of the prepared lithium iron phosphate is 2.61g / cm 3 , other indicators are shown in Table 2.

[0072] Example 4

[0073] A large single crystal iron phosphate adapted to high-density lithium iron phosphate and a preparation method thereof, comprising the following steps:

[0074] S1. Add fertilizer acid and pure water to prepare a fertilizer acid solution with a phosphoric acid concentration of 10%, introduce ammonium carbonate with a molar mass of 0.8 times that of the phosphoric acid to remove impurities, heat to 55°C and stir to react for 0.5h, add polyacrylamide with a molar mass of 1 / 3500 of the phosphoric acid in the fertilizer acid solution to flocculate, and then filter to obtain a purified fertilizer acid solution, and compound with 85% pure phosphoric acid to obtain a 25% phosphoric acid iron-dissolving solution.

[0075] S2. Put iron powder into the iron solution obtained in step S1, the molar mass ratio of iron source to iron solution is 70:1, heat to 50°C and stir to react and dissolve, after 12 hours, the solution turns green and is filtered to remove the precipitate to obtain ferrophosphorus solution, the Fe ion concentration in the ferrophosphorus solution is 2.5wt%-5wt%.

[0076] S3. The ferrophosphorus solution obtained in step S2 was used as the base liquid, the temperature was raised to 70°C, the stirring speed was 300 rpm, and hydrogen peroxide with a molar mass of 1.0 times that of Fe ions in the ferrophosphorus liquid was added to carry out oxidation reaction for 3 hours to obtain a reaction solution.

[0077] S4. The reaction solution obtained in step S3 is directly heated, and hydrogen peroxide is added while the temperature is increased to 85°C for heat preservation and aging. The molar mass ratio of hydrogen peroxide to Fe ions in the ferrophosphorus solution is 0.5:1, the stirring speed is 200 rpm, and the aging solution is obtained by heat preservation for 2 hours.

[0078] S5. The aged solution obtained in step S4 is filtered, washed twice with 2 times of water, dried at 150° C. for 8 h, calcined at 560° C. for 4 h, crushed, batch mixed, demagnetized, and packaged to obtain the iron phosphate product.

[0079] The tap density of the iron phosphate obtained in this example is 1.09 g / cm 3 The compacted density of the prepared lithium iron phosphate is 2.64g / cm 3 , other indicators are shown in Table 2.

[0080] Comparative Example 1

[0081] S1. Add fertilizer acid and washing water to prepare a fertilizer acid solution with a phosphoric acid concentration of 20%, pass sodium hydroxide with a molar mass 1.5 times that of phosphoric acid to remove impurities, heat to 60°C and stir to react for 0.5h, add appropriate amount of polyacrylamide for flocculation and filter to obtain a purified fertilizer acid solution, and compound with 85% pure phosphoric acid to obtain a 20% phosphoric acid iron-dissolving solution.

[0082] S2. Place an iron rod into the iron solution obtained in step S1, wherein the molar mass ratio of the iron source to the iron solution is 50:1, and heat to 90°C and stir to react and dissolve. After 4 hours, the solution turns green and is filtered to remove the precipitate to obtain a ferrophosphorus solution. The Fe ion concentration in the ferrophosphorus solution is 2.5wt%-3wt%.

[0083] S3. The ferrophosphorus solution obtained in step S2 was used as the base liquid, the temperature was raised to 75° C., the stirring speed was 500 rpm, and hydrogen peroxide with a molar mass of 1.5 times that of Fe in the ferrophosphorus liquid was added to react for 3 h to obtain a reaction solution.

[0084] S4. The reaction solution obtained in step S3 is filtered, washed once with 3 times of water, dried at 200° C. for 5 h, calcined at 570° C. for 2.5 h, and packaged to obtain the iron phosphate product after crushing, batch mixing, and demagnetization.

[0085] Figure 3 The SEM image of iron phosphate prepared in this comparative example is shown in the figure. Figure 1 Figure 2 The iron phosphate prepared in the comparative example is flaky and has no large grains. The tap density of the iron phosphate obtained in this example is 0.82 g / cm 3 The compaction density of the prepared lithium iron phosphate is only 2.35g / cm 3 , other indicators are shown in Table 2.

[0086] The impurity contents of the finished ferric phosphates prepared in Examples 1-4 and Comparative Example 1 are shown in Table 2.

[0087] Table 2 Indices of ferric phosphate products prepared in Examples 1 to 4 and Comparative Example 1

[0088]

[0089]

[0090] According to the steps and results of the above embodiments, it can be seen that the method for regulating and preparing large single crystal iron phosphate suitable for high-pressure compaction type lithium iron phosphate of the present invention is simple, the steps are simple, and no additional additives are required. The prepared iron phosphate has the advantages of adjustable morphology and particle size distribution and low impurity content. The tap density of iron phosphate prepared in Comparative Example 1 and the compacted density of lithium iron phosphate prepared therefrom are significantly lower than those in Examples 1-4, indicating that the present invention can achieve the regulation of the size and distribution range of iron phosphate grains only by coordinated regulation of the drop ratio of the oxidant before and after the reaction process, the stirring system, and the heating system during the aging process, and prepare high-quality large single crystal iron phosphate suitable for high-pressure compaction type lithium iron phosphate with high crystallinity and different particle size ranges.

[0091] The above are only embodiments of the present invention. The invention is not limited to the field involved in this implementation case. The common knowledge such as the known specific structure and characteristics in the scheme is not described in detail here. It should be pointed out that for those skilled in the art, several deformations and improvements can be made without departing from the structure of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing large single crystal iron phosphate suitable for high-density lithium iron phosphate, characterized in that: The steps include: S1. After the fertilizer acid is diluted to a certain concentration, an impurity remover and a flocculant are added, and the mixture is filtered to obtain a purified fertilizer acid solution; the purified fertilizer acid solution is compounded with phosphoric acid to obtain an iron-dissolving solution, wherein the mass concentration of phosphoric acid in the iron-dissolving solution is 10-45%; S2, adding an iron source to the iron-dissolving liquid, heating, dissolving, filtering, and obtaining a ferrophosphorus solution; S3, adding oxidant I to the ferrophosphorus solution at a certain temperature to carry out oxidation reaction to obtain a reaction solution; S4, adding oxidant II to the reaction solution, and performing thermal aging at a certain temperature to obtain an aged solution; S5. The aged solution is filtered, washed, dried and calcined to obtain large single crystal iron phosphate.

2. The preparation method according to claim 1, characterized in that: In step S1, fertilizer acid is diluted with water to prepare a fertilizer acid solution, the mass concentration of phosphoric acid in the fertilizer acid solution is 10-30%, an impurity remover is added to remove impurities, the reaction temperature of the impurity removal is 15-60° C., the reaction time of the impurity removal is 0.5-3h, and the reaction is performed until the end point pH is 2.8-8.5; a flocculant is added, and the solution is filtered to obtain a purified fertilizer acid solution.

3. The preparation method according to claim 1, characterized in that: In step S1, the impurity remover is one or a combination of ammonium carbonate, sodium hydroxide, ammonia water, and ammonia gas, and the molar mass ratio of the impurity remover to the phosphoric acid in the fertilizer acid solution is 0.1-5:1; the flocculant is polyacrylamide, and the molar mass ratio of the flocculant to the phosphoric acid in the fertilizer acid solution is 0.0001-0.1:

1.

4. The preparation method according to claim 1, characterized in that: In step S2, the iron source is one or a combination of iron powder, iron block, and iron rod; the molar mass ratio of the iron source to phosphorus in the molten iron solution is 100 to 0.35:1; the reaction temperature is 40 to 90°C, and the Fe in the ferrophosphorus solution is 2+ The mass concentration is 2.5 to 5wt%.

5. The preparation method according to claim 1, characterized in that: In step S3, the reaction temperature is 30-80° C., the reaction time is 0.5-3 h, and the molar mass ratio of the oxidant I to the Fe ions in the ferrophosphide solution is 0.8-3:

1.

6. The preparation method according to claim 1, characterized in that: In step S4, the heat preservation aging temperature is 85-97° C., the heat preservation aging time is 1-3 hours, and the molar mass ratio of the oxidant II to the Fe ions in the ferrophosphorus solution is 0.5-30:

1.

7. The preparation method according to claim 1, characterized in that: The oxidant I and oxidant II are one or a combination of oxygen, hydrogen peroxide, air and ozone; the components of oxidant I and oxidant II are the same or different.

8. The preparation method according to claim 1, characterized in that: In step S5, washing is performed with water for 1 to 3 times; the drying temperature is 80 to 200° C., and the drying time is 6 to 13 hours; the calcination temperature is 550 to 590° C., and the calcination time is 1 to 4 hours.

9. Large single crystal iron phosphate prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the large single crystal iron phosphate according to claim 9 in the preparation of high-density lithium iron phosphate.

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