Preparation method of lithium iron phosphate precursor iron phosphate
By controlling the reaction temperature and the use of surfactant, combining calcium carbonate to adjust pH, iron phosphate dihydrate slurry with different particle sizes is prepared, which solves the problem of filtration difficulties in hydrolysis, and achieves efficient preparation of high-pressure density lithium iron phosphate, which improves the battery energy density and reduces costs.
Patent Information
- Application Number
- CN202510818935.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-18
- Publication Date
- 2025-08-15
AI Technical Summary
The existing hydrolysis method is difficult to filtration when removing impurities ferrous sulfate, which leads to a long solid-liquid separation cycle, affecting production capacity and cost.
The method of controlling the reaction temperature and surfactant was used to prepare iron phosphate slurry of different particle sizes, and through reasonable grading, combined with the use of low-priced calcium carbonate as a pH regulator, the powder accumulation efficiency was optimized and the interparticle porosity was reduced.
The compaction density of lithium iron phosphate is improved, the energy density of lithium iron phosphate batteries is improved, the preparation process is simplified, and the production cost is reduced.
Smart Images
Figure BDA0005456028160000111 
Figure BDA0005456028160000121 
Figure BDA0005456028160000122
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of lithium battery raw material preparation, and in particular to a method for preparing iron phosphate, a precursor of lithium iron phosphate. Background Art
[0002] With the rapid development of the new energy industry, lithium-ion batteries, as core energy storage devices, have become a key technical research focus in the fields of electric vehicles, smart grids, etc. The performance optimization of lithium-ion batteries has become a key point of technical research in the fields of electric vehicles and smart grids. Among them, lithium iron phosphate (LiFePO4), a positive electrode material, has become one of the mainstream choices for power batteries due to its high safety, long cycle life and environmental friendliness. Iron phosphate (FePO4) is a key precursor for the preparation of lithium iron phosphate. Its physical and chemical properties (such as purity, morphology, particle size distribution, etc.) directly determine the electrochemical properties of the final positive electrode material. In order to meet the needs of high-energy-density batteries, improving the compaction density of lithium iron phosphate has become an important technical direction. Iron phosphate, as a raw material for the preparation of lithium iron phosphate, has a direct impact on the compaction density of lithium iron phosphate. The purpose of the present invention is to provide a method for preparing iron phosphate, a precursor of lithium iron phosphate. The prepared iron phosphate can be used as a raw material for lithium iron phosphate to improve the compaction density of lithium iron phosphate, and ultimately improve the energy density of lithium iron phosphate battery packs.
[0003] In the industrial production of iron phosphate, ferrous sulfate, a by-product of the titanium dioxide industry, has become the mainstream raw material source for the preparation of iron phosphate due to its low-cost advantage. However, as a by-product of titanium dioxide, ferrous sulfate has a high content of solid insoluble matter and metal impurities, and needs to be removed before it can be used as a raw material for iron phosphate for synthetic batteries. Currently, there are many reported methods for removing impurities from ferrous sulfate, a by-product of titanium dioxide, including hydrolysis, colloidal precipitation adsorption, chemical precipitation, flocculation and co-precipitation. Among them, the hydrolysis method is low in cost and simple to operate, and is suitable for industrial mass production. The hydrolysis method uses a hydrolysis reaction to remove impurities such as aluminum and titanium in the ferrous sulfate solution. By adding a pH regulator (commonly used are phosphoric acid and ammonia water) to adjust the pH value of the system, the titanyl sulfate in the ferrous sulfate solution is hydrolyzed to form a metatitanic acid precipitate, and the aluminum sulfate is hydrolyzed to form an aluminum hydroxide precipitate. The impurities are then filtered out and the clear liquid is used. This method has a good removal effect on the impurities titanium and aluminum, but because the generated metatitanic acid precipitate is fine, the filtration is very slow, which can easily lead to the Fe in the solution. 2+ Oxidation forms Fe(OH)3 colloid, which is prone to clogging during filtration, making separation difficult. 2+The oxidation problem of ferrous sulfate is currently solved by adding a sedimentation agent and letting the mixture settle, or adding a filter aid before filtration to increase the filtration speed. This not only introduces new impurities, but also increases production costs and prolongs the solid-liquid separation cycle. The low filtration efficiency of the solid-liquid separation link in the hydrolysis method is a key issue that restricts production capacity and cost. Another object of the present invention is to solve the problems of difficult filtration and easy clogging, and long solid-liquid separation cycle in the existing ferrous sulfate hydrolysis method for impurity removal, thereby improving production capacity and saving costs. Summary of the Invention
[0004] In view of this, the present invention provides a method for preparing ferric phosphate, a precursor of lithium iron phosphate, using ferrous sulfate, a by-product of titanium dioxide, as a raw material. Lithium iron phosphate is prepared using the ferric phosphate prepared by the preparation method of the present invention as a raw material, thereby improving the compaction density of the lithium iron phosphate.
[0005] In order to achieve the above object, the present invention provides a method for preparing lithium iron phosphate precursor iron phosphate, which comprises the following steps:
[0006] Step 1, prepare ferrous sulfate solution: take ferrous sulfate heptahydrate, a by-product of titanium dioxide, and dissolve it in water to obtain a ferrous sulfate solution. After removing aluminum and titanium impurities in the ferrous sulfate solution by hydrolysis, the ferrous sulfate solution is obtained. A phosphoric acid solution with a concentration of 70-85wt% is used to adjust the pH of the ferrous sulfate solution to 1-1.5 to prevent Fe 2+ oxidation.
[0007] Step 2, preparing amorphous iron phosphate: adding a mixed solution of hydrogen peroxide and phosphoric acid or phosphate to the ferrous sulfate solution of step 1 at a uniform rate under stirring, controlling the addition time of the mixed solution to be 15 to 20 minutes, and after the addition is completed, controlling the reaction temperature to be 45 to 50° C. and reacting for 30 to 60 minutes to obtain an amorphous iron phosphate slurry, and filtering the amorphous iron phosphate slurry and washing with deionized water to obtain the desired amorphous iron phosphate filter cake; preferably, the hydrogen peroxide is industrial hydrogen peroxide with a mass concentration of 27.5%, and the phosphate is at least one of monoammonium phosphate and diammonium hydrogen phosphate; preferably, the molar ratio of iron to phosphorus in the amorphous iron phosphate slurry is (1.1 to 1.2): 1; preferably, the amount of hydrogen peroxide added is 1.2 times the theoretical calculated value, and the P content of the phosphate solution is 4.5 to 5.5%, so that the reaction end point can be controlled to a lower pH value, so that most of the metal impurities in the amorphous iron phosphate slurry enter the liquid phase and are filtered and separated, effectively improving the product purity.
[0008] Step 3, prepare small particle slurry and large particle slurry of ferric phosphate dihydrate: add water to the amorphous ferric phosphate filter cake prepared in step 2 to make a slurry and divide it into two equal parts: slurry A and slurry B, add phosphoric acid solution, nitric acid solution and surfactant to slurry A, start stirring and heat it to 85°C, and crystallize and age for 150 to 210 minutes to obtain small particle slurry of ferric phosphate dihydrate; add phosphoric acid solution and nitric acid solution to slurry B, start stirring and heat it to 95°C, and crystallize and age for 150 to 210 minutes to obtain large particle slurry of ferric phosphate dihydrate; preferably, control slurry A and slurry B after crystallization and aging. The solid content is 7-10wt%; preferably, the amount of the phosphoric acid solution added is such that the solute mass of the added phosphoric acid solution accounts for 4-6% of the mass of the added amorphous ferric phosphate filter cake, the amount of the nitric acid solution added is such that the solute mass of the added nitric acid solution accounts for 4-6% of the mass of the added amorphous ferric phosphate filter cake, and the amount of the surfactant added is such that the solute mass of the added surfactant accounts for 0.3-0.5% of the mass of the added amorphous ferric phosphate filter cake; preferably, the surfactant in step 3 is a cationic surfactant with a cyano group, and the cyano group (CN-) on the surfactant reacts with Fe 3+ The strong coordination property can enhance the adsorption force and adsorption amount of the surfactant on the surface of the iron phosphate particles, further reducing the agglomeration of the particles. Specifically, the cationic surfactant with a cyano group is at least one of 2-cyanoethyl (dimethyl) ammonium bromide and (cyanomethyl) trimethyl ammonium iodide.
[0009] Step 4: Mixing: The small-particle slurry of ferric phosphate dihydrate and the large-particle slurry are mixed in proportion, filtered, washed, and dried to obtain anhydrous ferric phosphate, which is used as a raw material for preparing lithium iron phosphate. Preferably, the mixing weight ratio of the small-particle slurry of ferric phosphate dihydrate to the large-particle slurry in step 4 is (1.3-1.7):1.
[0010] The present invention removes impurities from ferrous sulfate, a byproduct of titanium dioxide, based on the principle of hydrolysis (titanyl sulfate hydrolyzes to form a metatitanic acid precipitate, and aluminum sulfate hydrolyzes to form an aluminum hydroxide precipitate). The ferrous ions in the removed ferrous sulfate are reduced to ferric ions by an oxidant, and the ferrous ions react with phosphate to form ferric phosphate. The present invention prepares ferric phosphate into a slurry containing ferric phosphate dihydrate of varying particle sizes through crystal transformation and aging. The slurries of varying particle sizes are then mixed in proportion, thereby optimizing powder stacking efficiency and reducing interparticle porosity, resulting in a more compact ferric phosphate for use in preparing lithium iron phosphate with a high compaction density.
[0011] A large number of studies have shown that the particle size of ferric phosphate dihydrate particles and the reaction temperature show a U-shaped correlation. The smallest particle size of ferric phosphate grains can be obtained around 80°C. As the temperature rises, the movement of the particles intensifies, and the collision between the particles is more likely to produce agglomeration, causing the grains to gradually increase. Therefore, the present invention controls the reaction temperature to make the particle size of ferric phosphate dihydrate in slurry A smaller than that in slurry B; on the other hand, the surface of ferric phosphate has a positive charge under a low pH (pH < 2.75) environment. The addition of a cationic surfactant to slurry A is more conducive to the dispersion of ferric phosphate particles, reduces particle agglomeration, and thus makes the particle size of ferric phosphate dihydrate in slurry A further smaller than that in slurry B. Slurry B, therefore, the present invention controls the reaction temperature and the addition of surfactants so that the particle size D50 of the dihydrated iron phosphate obtained in slurry A is 2.1-2.5 μm, and the particle size D50 of the dihydrated iron phosphate obtained in slurry B is 3.3-3.9 μm. By mixing these two slurries of different particle sizes in proportion, the particles of different particle sizes are reasonably graded, the powder stacking efficiency is optimized, and the inter-particle void ratio is reduced, thereby obtaining a more compacted iron phosphate. The iron phosphate obtained by the preparation method of the present invention is used as a raw material to prepare lithium iron phosphate with a high compaction density. After testing, its compaction density is 2.59-2.63 g / cm 3 .
[0012] Furthermore, in some embodiments of the present invention, the pH regulator used in the hydrolysis method in step 1 is calcium carbonate. Since the acidity of the ferrous sulfate solution obtained by dissolving the titanium dioxide byproduct ferrous sulfate in water is relatively strong (the solution contains more sulfuric acid), and the low pH environment will inhibit the hydrolysis reaction of titanyl sulfate, affecting the impurity removal effect, the present invention uses calcium carbonate slurry as a pH regulator to adjust the pH value of the ferrous sulfate solution. On the one hand, CaCO3 can undergo a double decomposition reaction with the free H2SO4 in the ferrous sulfate heptahydrate, thereby consuming the H in the solution. + , causing the pH value of the system to rise, which is conducive to the hydrolysis of titanyl sulfate to form insoluble metatitanic acid. On the other hand, by slowly adding calcium carbonate slurry to the dilute sulfuric acid environment, the nucleation rate of calcium sulfate can be effectively controlled, resulting in large-sized calcium sulfate crystals and improving the filtration efficiency of solid-liquid separation. Due to the low solubility of CaCO3, the CaSO4 precipitate nuclei generated by the reaction of CaCO3 and H2SO4 mainly nucleate in situ on the surface of calcium carbonate. By slowly adding calcium carbonate slurry, calcium sulfate is conducive to forming a coarse flaky crystal structure. This structure can make the filter cake structure more uniform and loose, reduce the formation of dense filter cake, and produce a "bridging" effect during the separation process, reducing filtration resistance and improving filtration rate.
[0013] Specifically, the hydrolysis and impurity removal method of ferrous sulfate, a by-product of titanium dioxide in step 1, comprises the following steps:
[0014] S1. Dissolving ferrous sulfate heptahydrate, a by-product of titanium dioxide, in water to obtain a ferrous sulfate solution; preferably, the mass ratio of ferrous sulfate heptahydrate to water is 1:2;
[0015] S2. Mixing calcium carbonate powder and water in a certain proportion to obtain a pH adjustment slurry; preferably, the mass ratio of calcium carbonate powder to water is 1:5; preferably, the calcium carbonate powder is heavy calcium powder;
[0016] S3. Slowly add the pH adjusting slurry to the ferrous sulfate solution to adjust the pH of the solution to 4-4.5, then react at a temperature of 50-70°C for 90-120 minutes, filter the reactant precipitate to obtain a ferrous sulfate solution. Preferably, the pH adjusting slurry is added to the ferrous sulfate solution under stirring, and stirring ensures that the calcium carbonate powder is evenly dispersed in the pH adjusting slurry to enhance the effect of the pH adjusting slurry. Preferably, the amount of pH adjusting slurry added is such that the mass of the calcium carbonate powder in the added pH adjusting slurry accounts for 0.5-1.1% of the mass of the ferrous sulfate heptahydrate dissolved in the added ferrous sulfate solution. Preferably, the speed at which the pH adjusting slurry is added to the ferrous sulfate solution is such that the pH adjusting slurry is added at a uniform speed within 20-30 minutes.
[0017] Furthermore, the water used in the above technical solution is all desalted water.
[0018] In summary, compared with the prior art, the preparation method of the lithium iron phosphate precursor iron phosphate provided by the present invention has the following advantages:
[0019] (1) The present invention controls the reaction temperature and the addition of surfactants so that the particle size D50 of the dihydrated iron phosphate obtained in slurry A is 2.1-2.5 μm, and the particle size D50 of the dihydrated iron phosphate obtained in slurry B is 3.3-3.9 μm. By mixing the two slurries of different particle sizes in proportion, the particles of different particle sizes are reasonably graded, the powder stacking efficiency is optimized, and the void ratio between particles is reduced, thereby obtaining a more compacted iron phosphate. The iron phosphate obtained by the preparation method of the present invention is used as a raw material to prepare lithium iron phosphate with a high compaction density. The compaction density thereof is 2.59-2.63 g / cm 3 .
[0020] (2) The present invention improves the filtration rate of ferrous sulfate hydrolysis impurity removal by using low-cost calcium carbonate as a pH regulator, solving the problem of Fe 2+ It will be oxidized into iron hydroxide colloid when in contact with air. In addition, the precipitated titanic acid is an extremely fine amorphous colloid, which makes filtration difficult.
[0021] (3) The iron phosphate prepared by the present invention has high purity and high density, which can effectively improve the energy density of the final product, the lithium iron phosphate battery.
[0022] (4) The preparation process of the present invention is simple, low in cost, and can be used for industrial-scale production. DETAILED DESCRIPTION
[0023] In order to make those skilled in the art better understand the technical solution of the present invention, the present invention is further described in detail below with reference to specific embodiments. Unless otherwise specified, the materials and equipment involved in the following embodiments are all commercially available products and can be purchased through commercial channels.
[0024] The impurity content of ferrous sulfate heptahydrate, a by-product of titanium dioxide, used in the following examples is determined by dissolving 500 g of ferrous sulfate heptahydrate, a by-product of titanium dioxide, in 1000 g of desalted water to prepare a ferrous sulfate solution, which is then detected using an inductively coupled plasma emission spectrometer (ICP). The main impurity contents (average values) are shown in Table 1 below.
[0025] Example 1:
[0026] A method for preparing lithium iron phosphate precursor iron phosphate comprises the following steps:
[0027] Step 1, preparing a ferrous sulfate solution: dissolving ferrous sulfate heptahydrate, a by-product of titanium dioxide, in 2 times the mass of desalted water to obtain a ferrous sulfate solution, mixing heavy calcium powder and desalted water in a weight ratio of 1:5 to obtain a pH adjustment slurry, slowly adding the pH adjustment slurry to the ferrous sulfate solution to adjust the pH of the solution to 4, and then reacting at 50°C for 90 minutes, filtering the reactant precipitate to obtain a ferrous sulfate filtrate, sampling for ICP detection, and the results are shown in Table 1 below; 85% phosphoric acid is used to adjust the pH of the ferrous sulfate filtrate to 1 to obtain a ferrous sulfate solution.
[0028] Step 2: Prepare amorphous ferric phosphate: A mixed solution of hydrogen peroxide and phosphate was uniformly added to the ferrous sulfate solution from step 1 while stirring for 15 minutes. The reaction temperature was controlled at 45°C and the reaction was continued for 30 minutes to obtain an amorphous ferric phosphate slurry. The amorphous ferric phosphate slurry was filtered and washed with 5 times deionized water to obtain an amorphous ferric phosphate filter cake. The mass concentration of the hydrogen peroxide was 27.5%, and the amount added was 1.2 times the theoretical calculated value. The phosphate was a monoammonium phosphate solution with a phosphorus content of 4.5%. The molar ratio of iron to phosphorus in the amorphous ferric phosphate slurry was 1.11:1.
[0029] Step 3, prepare small particle slurry and large particle slurry of ferric phosphate dihydrate: the amorphous ferric phosphate filter cake prepared in step 2 is mixed with desalted water to obtain a slurry, and the slurry is divided into slurry A and slurry B, phosphoric acid solution, nitric acid solution and surfactant are added to slurry A, stirring is started and the temperature is raised to 85°C, and crystallization and aging are carried out for 150 minutes to obtain small particle slurry of ferric phosphate dihydrate, and the particle size (D50) in the slurry is detected using a Malvern 3000 particle size analyzer. The results are shown in Table 2 below; phosphoric acid solution and nitric acid solution are added to slurry B, stirring is started and the temperature is raised to 95°C, and crystallization and aging are carried out for 150 minutes to obtain large particle slurry of ferric phosphate dihydrate, and Malvern 3000 particle size analyzer is used to detect the particle size (D50). ... 0 particle size analyzer was used to detect the particle size (D50) in the slurry, and the results are shown in Table 2 below; the solid content of slurry A and slurry B after crystallization and aging was controlled to be 7wt%; the amount of phosphoric acid solution added to slurry A or slurry B was such that the solute mass of the added phosphoric acid solution accounted for 4% of the mass of the added amorphous ferric phosphate filter cake, the amount of nitric acid solution added to slurry A or slurry B was such that the solute mass of the added nitric acid solution accounted for 4% of the mass of the added amorphous ferric phosphate filter cake, and the amount of surfactant added to slurry A was such that the solute mass of the added surfactant accounted for 0.3% of the mass of the added amorphous ferric phosphate filter cake; the surfactant was 2-cyanoethyl(dimethyl)ammonium bromide.
[0030] Step 4, mixing: the small particle slurry of ferric phosphate dihydrate and the large particle slurry were mixed at a weight ratio of 1.3:1, filtered, washed and dried to obtain anhydrous ferric phosphate, and samples were taken for ICP testing. The results are shown in Table 1 below.
[0031] Step 5, calcination: the anhydrous iron phosphate obtained in step 4 was calcined with lithium carbonate and glucose at 750° C. to obtain lithium iron phosphate. The compacted density of the obtained lithium iron phosphate was tested, and the results are shown in Table 3 below.
[0032] Example 2:
[0033] A method for preparing lithium iron phosphate precursor iron phosphate comprises the following steps:
[0034] Step 1, preparing a ferrous sulfate solution: dissolving ferrous sulfate heptahydrate, a by-product of titanium dioxide, in 2 times the mass of desalted water to obtain a ferrous sulfate solution, mixing heavy calcium powder and desalted water in a weight ratio of 1:5 to obtain a pH adjustment slurry, slowly adding the pH adjustment slurry to the ferrous sulfate solution to adjust the pH of the solution to 4.5, and then reacting at 70°C for 120 minutes, filtering the reactant precipitate to obtain a ferrous sulfate filtrate; and adjusting the pH of the ferrous sulfate filtrate to 1.5 with 85% phosphoric acid to obtain a ferrous sulfate solution.
[0035] Step 2: Prepare amorphous ferric phosphate: Slowly add a mixed solution of hydrogen peroxide and phosphate to the ferrous sulfate solution in step 1 while stirring. The addition time of the mixed solution of hydrogen peroxide and phosphate is controlled to be 20 minutes. The reaction temperature is controlled to 50°C, and the reaction is carried out for 60 minutes to obtain an amorphous ferric phosphate slurry. The amorphous ferric phosphate slurry is filtered and washed with 10 times deionized water to obtain an amorphous ferric phosphate filter cake. The mass concentration of the hydrogen peroxide is 27.5%, and the amount added is 1.2 times the theoretical calculated value. The phosphate is a diammonium hydrogen phosphate solution with a phosphorus content of 5.5%. The molar ratio of iron to phosphorus in the amorphous ferric phosphate slurry is 1.19:1.
[0036] Step 3, prepare small particle slurry and large particle slurry of ferric phosphate dihydrate: the amorphous ferric phosphate filter cake prepared in step 2 is mixed with desalted water to obtain a slurry, and the slurry is divided into slurry A and slurry B, phosphoric acid solution, nitric acid solution and surfactant are added to slurry A, stirring is started and the temperature is raised to 85°C, and crystallization and aging are carried out for 210 minutes to obtain small particle slurry of ferric phosphate dihydrate, and the particle size (D50) in the slurry is detected using a Malvern 3000 particle size analyzer. The results are shown in Table 2 below; phosphoric acid solution and nitric acid solution are added to slurry B, stirring is started and the temperature is raised to 95°C, and crystallization and aging are carried out for 210 minutes to obtain large particle slurry of ferric phosphate dihydrate, and Malvern 3000 particle size analyzer is used to detect the particle size (D50). ... 0 particle size analyzer is used to detect the particle size (D50) of the particles in the slurry, and the results are shown in Table 2 below; the solid content of slurry A and slurry B after crystallization and aging is controlled to be 10wt%; the amount of phosphoric acid solution added to slurry A or slurry B is such that the solute mass of the added phosphoric acid solution accounts for 6% of the mass of the added amorphous ferric phosphate filter cake, the amount of nitric acid solution added to slurry A or slurry B is such that the solute mass of the added nitric acid solution accounts for 6% of the mass of the added amorphous ferric phosphate filter cake, and the amount of surfactant added to slurry A is such that the solute mass of the added surfactant accounts for 0.5% of the mass of the added amorphous ferric phosphate filter cake; the surfactant is 2-cyanoethyl (dimethyl) ammonium bromide.
[0037] Step 4, mixing: mixing the small particle slurry of ferric phosphate dihydrate and the large particle slurry at a weight ratio of 1.7:1, filtering, washing and drying to obtain anhydrous ferric phosphate.
[0038] Step 5, calcination: the anhydrous iron phosphate obtained in step 4 was calcined with lithium carbonate and glucose at 750° C. to obtain lithium iron phosphate. The compacted density of the obtained lithium iron phosphate was tested, and the results are shown in Table 3 below.
[0039] Example 3:
[0040] Same as Example 1, except that:
[0041] In step 2, a mixed solution of hydrogen peroxide and phosphoric acid is added to the ferrous sulfate solution; and the surfactant used in step 3 is (cyanomethyl)trimethylammonium bromide.
[0042] Comparative Example 1:
[0043] Same as Example 1, except that:
[0044] The addition time of the hydrogen peroxide and monoammonium phosphate mixture in step 2 is controlled to be 1 min.
[0045] Comparative Example 2:
[0046] Same as Example 1, except that:
[0047] In step 2, the amorphous ferric phosphate slurry is not subjected to filtering, washing, adding water and then slurrying operations. In step 3, the amorphous ferric phosphate slurry is directly divided into slurry A and slurry B and subsequent operations are performed.
[0048] Comparative Example 3:
[0049] Same as Example 1, except that:
[0050] The surfactant used in step 3 is a nonionic surfactant polyethylene glycol.
[0051] Comparative Example 4:
[0052] Same as Example 1, except that:
[0053] Without step 3 and step 4, the preparation method of anhydrous ferric phosphate used in step 5 is as follows:
[0054] The amorphous ferric phosphate filter cake prepared in step 2 is mixed with desalted water to obtain a slurry, phosphoric acid solution and nitric acid solution are added to the slurry, stirring is started and the temperature is raised to 85°C, and the slurry is crystallized and aged for 160 minutes. The particle size (D50) of the slurry after crystallization and aging is detected using a Malvern 3000 particle size analyzer. The results are shown in Table 2 below. The slurry after crystallization and aging is filtered, washed, and dried to obtain anhydrous ferric phosphate. Wherein, the solid content of the slurry after crystallization and aging is controlled to be 8wt%; the amount of phosphoric acid solution added to the slurry is such that the solute mass of the added phosphoric acid solution accounts for 4% of the mass of the amorphous ferric phosphate filter cake added, and the amount of nitric acid solution added to the slurry is such that the solute mass of the added nitric acid solution accounts for 4% of the mass of the amorphous ferric phosphate filter cake added.
[0055] Comparative Example 5:
[0056] Same as Example 1, except that:
[0057] In step 4, the mixing weight ratio of the small particle slurry and the large particle slurry of ferric phosphate dihydrate is 1:1.
[0058] Comparative Example 6:
[0059] Same as Example 1, except that:
[0060] In step 4, the mixing weight ratio of the small particle slurry and the large particle slurry of ferric phosphate dihydrate is 2:1.
[0061] The following are the test results of each embodiment and comparative example.
[0062] Table 1. ICP detection of impurity content
[0063]
[0064] As can be seen from Table 1, the preparation method provided by the present invention significantly reduces the impurity content of anhydrous ferric phosphate.
[0065] Table 2. Particle size of the slurry after crystallization and aging in each embodiment and comparative example
[0066]
[0067] Table 3. Compacted density of lithium iron phosphate prepared in various examples and comparative examples
[0068]
[0069] It can be seen from Tables 2 and 3 that the iron phosphate obtained by the preparation method provided by the present invention is used to prepare lithium iron phosphate, which improves the compaction density of lithium iron phosphate. This is because the present invention prepares iron phosphate into a slurry containing dihydrate iron phosphate with different particle sizes through crystal transformation and aging, and then mixes the slurries with different particle sizes in a specific proportion to obtain a denser iron phosphate, thereby improving the compaction density of lithium iron phosphate.
[0070] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be construed as limiting the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. Persons skilled in the art will appreciate that improvements and modifications may be made without departing from the spirit and scope of the present invention, and such improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. A method for preparing iron phosphate, a precursor of lithium iron phosphate, characterized in that: The following steps are involved: Step 1, preparing a ferrous sulfate solution: dissolving ferrous sulfate heptahydrate, a by-product of titanium dioxide, in water to obtain a ferrous sulfate solution, removing aluminum and titanium impurities in the ferrous sulfate solution by hydrolysis to obtain a ferrous sulfate solution, and adjusting the pH of the ferrous sulfate solution to 1-1.5; Step 2, preparing amorphous ferric phosphate: adding a mixed solution of hydrogen peroxide and phosphoric acid or phosphate to the ferrous sulfate solution of step 1 at a uniform speed under stirring, controlling the addition time of the mixed solution to be 15 to 20 minutes, and after the addition is completed, controlling the reaction temperature to 45 to 50° C. and reacting for 30 to 60 minutes to obtain an amorphous ferric phosphate slurry, and filtering and washing the amorphous ferric phosphate slurry to obtain an amorphous ferric phosphate filter cake; Step 3, preparing a small particle slurry and a large particle slurry of ferric phosphate dihydrate: mixing the amorphous ferric phosphate filter cake prepared in step 2 with water to obtain a slurry, dividing the slurry into slurry A and slurry B, adding a phosphoric acid solution, a nitric acid solution and a surfactant to slurry A, starting stirring and heating to 85°C, and crystallizing and aging for 150 to 210 minutes to obtain a small particle slurry of ferric phosphate dihydrate; adding a phosphoric acid solution and a nitric acid solution to slurry B, starting stirring and heating to 95°C, and crystallizing and aging for 150 to 210 minutes to obtain a large particle slurry of ferric phosphate dihydrate; Step 4, mixing: mixing the small particle slurry of ferric phosphate dihydrate and the large particle slurry in proportion, filtering, washing and drying to obtain anhydrous ferric phosphate, which is used for the preparation of lithium iron phosphate.
2. The preparation method according to claim 1, wherein: The pH regulator used in the hydrolysis method in step 1 is calcium carbonate.
3. The preparation method according to claim 1, wherein: The mass concentration of the hydrogen peroxide in step 2 is 27.5%, and the phosphate is selected from at least one of monoammonium phosphate and diammonium hydrogen phosphate.
4. The preparation method according to claim 1, wherein: The amount of hydrogen peroxide added in step 2 is 1.2 times the theoretical calculated value, and the P content of the phosphate solution is 4.5-5.5%.
5. The preparation method according to claim 1, wherein: The molar ratio of the iron element to the phosphorus element in the amorphous ferric phosphate slurry in step 2 is (1.1-1.2):
1.
6. The preparation method according to claim 1, wherein: In step 3, the solid content of slurry A and slurry B after crystallization and aging is controlled to be 7-10 wt%.
7. The preparation method according to claim 1, wherein: The surfactant in step 3 is a cationic surfactant with a cyano group.
8. The preparation method according to claim 7, characterized in that: The cationic surfactant with a cyano group is at least one of 2-cyanoethyl (dimethyl) ammonium bromide and (cyanomethyl) trimethyl ammonium iodide.
9. The preparation method according to claim 1, wherein: In step 3, the amount of phosphoric acid solution added is such that the solute mass of the added phosphoric acid solution accounts for 4-6% of the mass of the added amorphous ferric phosphate filter cake, the amount of nitric acid solution added is such that the solute mass of the added nitric acid solution accounts for 4-6% of the mass of the added amorphous ferric phosphate filter cake, and the amount of surfactant added is such that the solute mass of the added surfactant accounts for 0.3-0.5% of the mass of the added amorphous ferric phosphate filter cake.
10. The preparation method according to claim 1, characterized in that: In the step 4, the mixing weight ratio of the small particle slurry to the large particle slurry of ferric phosphate dihydrate is (1.3-1.7):1.