Process for the preparation of iron phosphate dihydrate in orthorhombic form

Abstract

The application relates to the technical field of iron phosphate preparation, in particular to a preparation method of iron phosphate dihydrate containing an orthorhombic crystal form, which comprises the following steps: adding a mixed solution of a phosphate and an oxidizing agent into a ferrous solution, controlling the amount of the oxidizing agent, so that there is no ferrous ion in the reaction solution after the mixed solution is added; adding an alkali liquor to the reaction solution to adjust the pH value, and keeping the temperature at 50-60 DEG C for 1-2 hours; filtering and washing the prepared slurry to obtain a solid; after the solid is pulped, adding phosphoric acid to adjust the pH value, and reacting at 80-100 DEG C for 1-2 hours; and filtering, washing and drying the reaction product to obtain the iron phosphate dihydrate containing the orthorhombic crystal form. The preparation method of the iron phosphate dihydrate containing the orthorhombic crystal form is simple in process, short in technological process, low in reaction temperature, and does not need to add a surfactant in the reaction, so that the iron phosphate dihydrate containing the orthorhombic crystal form can be prepared.

Description

Technical Field

[0001] This invention relates to the field of ferric phosphate preparation technology, and specifically to a method for preparing ferric phosphate dihydrate containing orthorhombic crystals. Background Technology

[0002] Lithium iron phosphate (LFP) is a second-generation cathode material for lithium-ion batteries. Batteries made with LFP as the cathode have advantages such as low cost, long cycle life, good thermal stability, and environmental friendliness, and are widely used in power tools, electric vehicles, and energy storage batteries. In existing LFP production processes, the solid-state process, using iron phosphate as the iron-phosphorus source precursor, accounts for the largest proportion. The elemental composition, particle morphology, crystal structure, and other physicochemical properties of iron phosphate play a decisive role in the final LFP's compaction density and electrical properties.

[0003] The chemical structure of ferric phosphate is highly dependent on the preparation method; different methods yield iron-phosphorus compounds with varying crystal structures, sizes, and morphologies. Based on crystal structure, they can be classified into isophosphorus manganese oxide type, amorphous, orthorhombic, monoclinic, and α-quartz crystal systems. Common preparation methods for ferric phosphate include the ammonium method, sodium method, and iron method. The ammonium method uses ferrous salts, phosphoric acid or its salts, oxidizing agents, and ammonia as main raw materials. First, ferrous sulfate is mixed with phosphoric acid and the oxidizing agent, then ammonia is added to adjust the pH value, resulting in a series of reactions that precipitate ferric phosphate. The sodium method uses ferric salts and sodium phosphate as main raw materials, undergoing a metathesis reaction under certain conditions to precipitate ferric phosphate. The iron method involves adding iron powder or iron filings to a reaction vessel containing phosphoric acid, and then slowly adding an oxidizing agent such as hydrogen peroxide at an appropriate temperature, causing the iron to oxidize with the phosphoric acid to form ferric phosphate. The ferric phosphate products prepared by these three methods are primarily monoclinic crystals. However, lithium iron phosphate prepared from monoclinic iron phosphate has a relatively low lithium-ion diffusion rate, which may affect the charge-discharge performance and rate performance of the battery when used as a cathode material for lithium-ion batteries, thus limiting its application in the field of high-performance batteries.

[0004] Studies have found that lithium iron phosphate materials prepared from orthorhombic iron phosphate exhibit significantly better electrochemical performance than those prepared from monoclinic iron phosphate. Therefore, researchers began investigating how to prepare orthorhombic iron phosphate materials. Chinese invention patent application CN 102491302 A discloses an oxidation precipitation method using air as a catalyst. First, divalent iron salt, phosphoric acid, or phosphate is mixed into an aqueous solution in a specific ratio. Then, the prepared aqueous solution of divalent iron salt and phosphoric acid or phosphate, along with a pH adjuster solution, is continuously pumped into a stirred reactor. Air is introduced into the reactor at a certain flow rate using an air compressor. The temperature of the reaction solution in the reactor is controlled and maintained constant within the range of 40-98°C using a constant-temperature water bath. The flow rates of the divalent iron salt and phosphoric acid or phosphate solution, as well as the air flow rate, are kept constant, and the pH value of the reaction solution in the reactor is controlled and maintained constant between 0.5-7.5. After the addition is complete, stirring and aging continue, with continuous air circulation for oxidation. The oxidation precipitation reaction generates ammonium ions (NH4+). + ), hydroxide ions (OH) - The process involves obtaining a crystalline complex of NH4Fe2(OH)(PO4)2·2H2O with water of crystallization. After solid-liquid separation, washing with deionized water, and drying, NH4Fe2(OH)(PO4)2·2H2O powder is obtained. This powder is then calcined in air at 500-700℃ for 2-24 hours to decompose and remove ammonium, hydroxide, and water of crystallization, yielding high-quality battery-grade anhydrous iron phosphate (FePO4) with an orthorhombic crystal structure. The drawback of this technique is the difficulty in controlling the reaction conditions, hindering industrial-scale production.

[0005] As an improvement, Chinese invention patent application CN 115215313 A discloses a method for preparing an orthorhombic high-pressure solid iron phosphate material, comprising the following steps: (1) preparing a first mixture system containing phosphate source, ferrous ion source and a first pH adjuster, the first mixture system undergoes a first reaction to obtain a first slurry containing an amorphous ferrous phosphate octahydrate reaction precursor; (2) adding a second pH adjuster to the first slurry to obtain a second slurry, the second slurry being acidic; (3) adding an oxidizing substance to the second slurry to carry out a second reaction to obtain iron phosphate dihydrate material. The principle of this technical solution is to utilize the characteristic that amorphous ferrous phosphate octahydrate (Fe3(PO)2·8H2O) has very low solubility in its formation environment. Using amorphous ferrous phosphate octahydrate as an intermediate, an acidic second pH adjuster is added to the first slurry for generating the amorphous ferrous phosphate octahydrate precursor. The pH value of the second slurry is controlled to be -0.5 to 1.5, allowing a portion of the generated amorphous ferrous phosphate octahydrate to dissolve rapidly under acidic conditions, thereby increasing the Fe content in the reaction system. 2+ and PO4 3-The concentration is beneficial for subsequent steps (3) after adding oxidizing substances, so that more iron phosphate dihydrate crystal nuclei can be generated at the beginning stage of the reaction. With the addition of oxidizing substances and the generation of iron phosphate dihydrate crystal nuclei, the Fe in the reaction system... 2+ and PO4 3- The concentration decreased compared to the initial stage of the reaction, and with the slow dissolution of the amorphous ferrous phosphate octahydrate precursor and the slow change in pH of the reaction system, a relatively low concentration of Fe could be maintained in the reaction system. 2+ and PO4 3- This process inhibits excessive nucleation of ferric phosphate dihydrate, controls its slow growth, and ultimately forms a densely packed ferric phosphate dihydrate material with large primary particles and a regular polyhedral morphology. The drawback of this technique is that it requires testing the pH and redox potential of the first mixture system beforehand, and determining whether to add a reducing agent based on these parameters, which is a relatively cumbersome operation. Summary of the Invention

[0006] To address the technical problems of difficult-to-control reaction conditions and cumbersome operation in existing methods for preparing orthorhombic ferric phosphate dihydrate, this invention provides a method for preparing ferric phosphate dihydrate containing orthorhombic crystals.

[0007] The technical solution of this invention is as follows: A method for preparing iron phosphate dihydrate containing orthorhombic crystal form includes the following steps: S1. Add a mixed solution of phosphate and oxidant to the ferrous solution, and control the amount of oxidant so that there are no ferrous ions in the reaction solution after the mixed solution is added. S2. Add alkali solution to the reaction solution to adjust the pH value to 1.6~2.0, and keep it at 50~60℃ for 1~2 hours; S3. Filter and wash the obtained slurry to obtain a solid; S4. After the solid is pulped, add phosphoric acid to adjust the pH to 1.5~1.6, react at 80~100℃ for 1~2 h, and the reaction product is obtained after filtration, washing and drying.

[0008] Furthermore, in step S1, the iron source of the ferrous solution is selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous oxalate.

[0009] Furthermore, in step S1, the phosphate is selected from at least one of monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and calcium dihydrogen phosphate.

[0010] Furthermore, in step S1, the oxidant is hydrogen peroxide.

[0011] Furthermore, in step S1, the molar ratio of phosphate to ferrous salt is 1~1.2:1, and the molar ratio of oxidant to ferrous salt is 0.8~1.4:1.

[0012] Furthermore, in step S1, the concentration of the ferrous solution is 0.5~1 mol / L.

[0013] Furthermore, in step S1, the pH value of the mixed solution of phosphate and oxidant is 4 to 7.

[0014] Furthermore, in step S1, a mixed solution of phosphate and oxidant is added to the ferrous solution while stirring at a speed of 150-300 rpm for 0.5-2 h.

[0015] Furthermore, in step S3, after the obtained slurry is filtered for solid-liquid separation, the obtained solid is washed with water until the conductivity is <3000 μs / cm.

[0016] Furthermore, in step S4, the filter cake is pulped to obtain a slurry with a solid content of 8wt%~13wt%.

[0017] Furthermore, in step S4, the reaction product is filtered and washed until the conductivity of the wash water is <500 μs / cm, and then dried at 120℃ for 2~5 h.

[0018] Furthermore, in the orthorhombic ferric phosphate dihydrate obtained in step S4, the content of orthorhombic ferric phosphate dihydrate is 20%~40%.

[0019] The beneficial effects of this invention are as follows: The present invention provides a method for preparing ferric phosphate dihydrate with orthorhombic crystal form. In the process of synthesizing ferric phosphate, a two-step method is adopted through liquid-phase precipitation reaction. The preparation method is simple, the process flow is short, the reaction temperature is low, and no surfactant is required in the reaction to obtain ferric phosphate dihydrate with orthorhombic crystal form. When the temperature is higher, the supersaturation of the system is relatively low. At this time, thermodynamics dominates the preferential nucleation and crystallization of the thermally stable crystal form, thus obtaining ferric phosphate dihydrate with orthorhombic crystal form. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a SEM image of the iron phosphate prepared in Example 1.

[0022] Figure 2 This is a SEM image of the iron phosphate prepared in Example 2.

[0023] Figure 3 This is a SEM image of the iron phosphate prepared in Comparative Example 1.

[0024] Figure 4 These are comparison images of the XRD patterns and standard cards of the iron phosphate prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0025] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0026] In this invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0027] The weight of the relevant components mentioned in this specification can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to this specification is within the scope disclosed in this specification.

[0028] To obtain orthorhombic iron phosphate and thereby prepare lithium iron phosphate with high compaction density and electrical properties, embodiments of the present invention provide a method for preparing orthorhombic iron phosphate dihydrate, comprising the following steps: S1. Add a mixed solution of phosphate and oxidant to the ferrous solution, and control the amount of oxidant so that there are no ferrous ions in the reaction solution after the mixed solution is added. S2. Add alkali solution to the reaction solution to adjust the pH value to 1.6~2.0, and keep it at 50~60℃ for 1~2 hours; S3. Filter and wash the obtained slurry to obtain a solid; S4. After the solid is pulped, add phosphoric acid to adjust the pH to 1.5~1.6, react at 80~100℃ for 1~2 h, and the reaction product is obtained after filtration, washing and drying.

[0029] This invention first involves mixing a divalent iron source, a phosphorus source, and an oxidant for reaction. An alkaline solution is added to the reaction solution to generate a precipitate. The product is then filtered, washed, and slurried. Phosphoric acid is used to adjust the pH of the slurry, and the reaction continues. This process ultimately yields orthorhombic iron phosphate dihydrate, with an orthorhombic iron phosphate dihydrate content of 20%–40%. This results in higher compaction density during the preparation of lithium iron phosphate.

[0030] The iron source of the ferrous solution described in S1 of this invention is selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous oxalate. The Fe concentration in the ferrous solution is controlled at 0.5~1 mol / L. Too high a concentration will lead to rapid subsequent reactions that are difficult to control; too low a concentration will lead to slow reactions that are not conducive to production.

[0031] The phosphate described in S1 of this invention can be at least one of monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and calcium dihydrogen phosphate, wherein calcium dihydrogen phosphate is preferably low-quality calcium dihydrogen phosphate.

[0032] The mixed solution of phosphate and oxidant described in S1 of the present invention is prepared by using a phosphate solution and an oxidant solution. The concentration of phosphate in the phosphate solution used during preparation is controlled at 0.5~1 mol / L. If the concentration is too high, the subsequent reaction will be rapid and difficult to control; if the concentration is too low, the reaction will be slow and not conducive to production.

[0033] The oxidant described in S1 of this invention can be any oxidant capable of oxidizing ferrous ions in the art, preferably hydrogen peroxide. In some embodiments of this invention, the oxidant in S1 is hydrogen peroxide. The concentration of the oxidant used in preparing the mixed solution of phosphate and oxidant described in S1 of this invention is controlled according to the molar number of ferrous ions, for example, set in the range of 0.5~1 mol / L. Exemplarily, the concentration of the oxidant is selected from any one of 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, or a value between any two.

[0034] In the mixed solution of phosphate and oxidant described in S1 of this invention, the pH value of the mixed solution is controlled between 4 and 7. If the pH is too low, the yield will be too low, and if the pH is too high, ferric hydroxide will be generated.

[0035] In S1 of the present invention, the P / Fe ratio of phosphate to ferrous salt is 1.0 to 1.2 in terms of molar ratio. For example, the P / Fe ratio of phosphate to ferrous salt can be any one of 1.0, 1.05, 1.1, 1.15, or 1.2 in terms of molar ratio, or any value between two of them, so as to better control the finished product indicators.

[0036] In S1 of the present invention, in order to achieve complete oxidation, the ratio of oxidant to ferrous salt is 0.8 to 1.4 in molar ratio. For example, the ratio of oxidant to ferrous salt in molar ratio can be any one of 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or any value between any two.

[0037] In S1 of this invention, a mixed solution of phosphate and oxidant is added to a reaction vessel containing ferrous solution while stirring. The stirring speed is 150-300 rpm. Exemplarily, the stirring speed can be any one of 150 rpm, 200 rpm, 250 rpm, and 300 rpm, or any value between two of them. The addition time of the mixed solution is 0.5-2 h. Exemplarily, the addition time of the mixed solution can be any one of 0.5 h, 1 h, 1.5 h, and 2 h, or any value between two of them.

[0038] The alkaline solution added to the reaction solution in S2 of this invention is ammonia. By adding ammonia, the pH value of the reaction solution changes accordingly. The pH value of the reaction solution is preferably adjusted to any one of 1.6, 1.7, 1.8, 1.9, and 2.0, or a value between any two of them.

[0039] In S2 of the present invention, the heat preservation temperature is 50~60℃. For example, the heat preservation temperature can be any one of 50℃, 55℃, and 60℃, or any value between two of them; the heat preservation time can be any one of 1 h, 1.5 h, and 2.0 h, or any value between two of them.

[0040] The specific step S3 of the present invention involves filtering the prepared slurry to separate the solid and liquid, and then washing the obtained solid with water until the conductivity is <3000 μs / cm.

[0041] In S4 of the present invention, the solid content of the slurry formed by solid pulping is controlled at 8wt% to 13wt%. For example, the solid content can be selected from any one of 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, or any value between two of them.

[0042] In step S4 of this invention, phosphoric acid is added to adjust the pH value to 1.5-1.6. The pH adjustment must be strictly controlled. When the pH value of the slurry is below the lower limit of 1.5, the formation of orthorhombic crystals will be inhibited; when the pH value of the slurry is above the upper limit of the 1.6 range, the pH is too high and the impurity content is too high. For example, in step S3 of this invention, the addition of phosphoric acid to adjust the pH value can be selected to adjust the pH value of the slurry to any one of 1.50, 1.51, 1.52, 1.53, 1.54, 1.55, 1.56, 1.57, 1.58, 1.59, or 1.60, or a value between any two.

[0043] In step S4 of this invention, the pH-adjusted slurry is reacted at 80-100°C for 1-2 hours. The main purpose of this step is to form an orthorhombic crystal form. Therefore, the control of reaction temperature and reaction time has a significant impact on the crystal form of the final product. When the reaction temperature is too high, the nucleation rate of ferric phosphate is faster and the number of nuclei is greater, thus inhibiting the formation of the orthorhombic crystal form. Therefore, the upper limit of the reaction temperature should generally not exceed 100°C, preferably not exceeding 95°C, and more preferably not exceeding 90°C. When the reaction temperature is too low, the growth rate is too slow, requiring a longer reaction time to obtain the orthorhombic crystal form, which is not conducive to production. Therefore, the lower limit of the reaction temperature should generally not be lower than 80°C, preferably not lower than 85°C, and more preferably not lower than 90°C. The reaction time can be any one of 1 hour, 1.5 hours, or 2 hours, or any value between two of these.

[0044] In S4 of the present invention, the slurry after pH adjustment is reacted by stirring while reacting, and the stirring speed is 150~300 rpm. For example, the stirring speed can be any one of 150 rpm, 200 rpm, 250 rpm, 300 rpm, or any value between two of them.

[0045] In step S4 of this invention, the reaction solution obtained after the reaction is filtered, the solid is washed with water until the conductivity of the wash water is <500 μs / cm, and finally the washed solid is dried to remove free water.

[0046] Example 1 A method for preparing iron phosphate dihydrate containing orthorhombic crystal form includes the following steps: S1. Prepare 2 L of 0.5 mol / L ferrous sulfate solution and 4 L of a mixed solution of hydrogen peroxide and monoammonium phosphate. The mixed solution is prepared by mixing 2 L of 0.5 mol / L hydrogen peroxide solution and 2 L of 0.5 mol / L monoammonium phosphate solution evenly. The pH value of the mixed solution is measured to be 4. 2 L of ferrous sulfate solution was added to the reaction vessel. The stirring speed of the reaction vessel was set to 200 rpm. The mixed solution was added to the reaction vessel while stirring at room temperature for a total of 1 h. The molar ratio of phosphate to total added ferrous salt was calculated to be 1. After the mixed solution was added, the ferrous ions in the reaction solution were detected. The result was that no ferrous ions were detected in the reaction solution. S2. Continue to maintain the stirring speed of the reactor at 200 rpm, slowly add ammonia water dropwise into the reactor, adjust the pH value of the solution to 1.8, and then keep it at 50℃ for 2 h; S3. After the obtained slurry is filtered and separated into solid and liquid components, the solid obtained is washed with water until the conductivity of the wash water is <3000 μs / cm, and the solid is collected. S4. After washing, the solid is pulped, and the solid content in the resulting pulp is controlled to be 8 wt%. The pulp is transferred to a reaction vessel, phosphoric acid is added to the reaction vessel, the pH of the solution is adjusted to 1.51, the stirring speed of the reaction vessel is set to 200 rpm, and the reaction is carried out at 90℃ for 1.5 h. The reaction product is then filtered, and the solid after solid-liquid separation is washed until the conductivity of the wash water is <500 μs / cm. Then it is dried at 120℃ for 3 h to obtain iron phosphate dihydrate containing orthorhombic crystals.

[0047] The iron phosphate product obtained from S4 was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 1 As shown, the ferric phosphate product prepared in Example 1 contains orthorhombic ferric phosphate dihydrate. The crystal structure of the ferric phosphate product obtained in S4 was characterized using X-ray diffraction (XRD), and the results are as follows. Figure 4 As shown, a comparison with the standard cards of orthorhombic ferric phosphate and monoclinic ferric phosphate reveals that the conclusions are consistent with those of SEM.

[0048] Example 2 A method for preparing iron phosphate dihydrate containing orthorhombic crystal form includes the following steps: S1. Prepare 4 L of 0.5 mol / L ferrous sulfate solution and 8 L of a mixed solution of hydrogen peroxide and monoammonium phosphate. The mixed solution is prepared by mixing 4 L of 0.6 mol / L hydrogen peroxide solution and 4 L of 0.6 mol / L monoammonium phosphate solution evenly. The pH value of the mixed solution is measured to be 4.22. 4 L of ferrous sulfate solution was added to the reaction vessel. The stirring speed of the reaction vessel was set to 150 rpm. The mixed solution was added to the reaction vessel while stirring at room temperature for a total of 1 h. The molar ratio of phosphate to total added ferrous salt was calculated to be 1. After the mixed solution was added, the ferrous ions in the reaction solution were detected. The result was that no ferrous ions were detected in the reaction solution. S2. Increase the stirring speed of the reactor to 200 rpm, slowly add ammonia water dropwise into the reactor, adjust the pH of the solution to 1.8, and then keep it at 50℃ for 2 h. S3. After the obtained slurry is filtered and separated into solid and liquid components, the solid obtained is washed with water until the conductivity of the wash water is <3000 μs / cm, and the solid is collected. S4. After washing, the solid is pulped, and the solid content in the resulting pulp is controlled to be 8 wt%. The pulp is transferred to a reaction vessel, phosphoric acid is added to the reaction vessel, the pH of the solution is adjusted to 1.53, the stirring speed of the reaction vessel is set to 200 rpm, and the reaction is carried out at 95℃ for 1.5 h. The reaction product is then filtered, and the solid after solid-liquid separation is washed until the conductivity of the wash water is <500 μs / cm. Then it is dried at 120℃ for 3 h to obtain iron phosphate dihydrate containing orthorhombic crystals.

[0049] The iron phosphate product obtained from S4 was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 2 As shown, the iron phosphate product prepared in Example 2 contains orthorhombic iron phosphate dihydrate, but the content of orthorhombic iron phosphate dihydrate is lower than that of the product in Example 1. The reason for this may be that after adding phosphoric acid in step S4, the reaction temperature of the slurry is too high, which leads to a faster crystal growth rate and affects the further formation of the orthorhombic crystal.

[0050] Example 3 A method for preparing iron phosphate dihydrate containing orthorhombic crystal form includes the following steps: S1. Prepare 4 L of 0.5 mol / L ferrous sulfate solution and 8 L of a mixed solution of hydrogen peroxide and monoammonium phosphate. The mixed solution is prepared by mixing 4 L of 0.5 mol / L hydrogen peroxide solution and 4 L of 0.5 mol / L monoammonium phosphate solution evenly. The pH value of the mixed solution is measured to be 4. 4 L of ferrous sulfate solution was added to the reaction vessel. The stirring speed of the reaction vessel was set to 300 rpm. The mixed solution was added to the reaction vessel while stirring at room temperature for a total of 0.5 h. The molar ratio of phosphate to total added ferrous salt was calculated to be 1. After the mixed solution was added, the ferrous ions in the reaction solution were detected. The result was that no ferrous ions were detected in the reaction solution. S2. Continue to maintain the stirring speed of the reactor at 300 rpm, slowly add ammonia water dropwise into the reactor, adjust the pH value of the solution to 1.8, and then keep it at 50℃ for 2 h; S3. After the obtained slurry is filtered and separated into solid and liquid components, the solid obtained is washed with water until the conductivity of the wash water is <3000 μs / cm, and the solid is collected. S4. After washing, the solid is pulped, and the solid content in the resulting pulp is controlled to be 8 wt%. The pulp is transferred to a reaction vessel, phosphoric acid is added to the reaction vessel, the pH of the solution is adjusted to 1.58, the stirring speed of the reaction vessel is set to 200 rpm, and the reaction is carried out at 90℃ for 1.5 h. The reaction product is then filtered, and the solid after solid-liquid separation is washed until the conductivity of the wash water is <500 μs / cm. Then it is dried at 120℃ for 3 h to obtain iron phosphate dihydrate containing orthorhombic crystals.

[0051] Example 4 A method for preparing iron phosphate dihydrate containing orthorhombic crystal form includes the following steps: S1. Prepare 4 L of ferrous oxalate solution with a concentration of 0.7 mol / L and prepare 8 L of mixed solution of hydrogen peroxide and low-quality calcium dihydrogen phosphate, wherein the mixed solution is prepared by uniformly mixing 4 L of hydrogen peroxide with a concentration of 0.56 mol / L and 4 L of low-quality calcium dihydrogen phosphate solution with a concentration of 0.84 mol / L. 4 L of ferrous oxalate solution was added to the reactor. The stirring speed of the reactor was set to 300 rpm. The mixed solution was added to the reactor while stirring at room temperature for a total of 2 hours. The molar ratio of phosphate to total added ferrous salt was calculated to be 1.2. After the mixed solution was added, the ferrous ions in the reaction solution were detected. The result was that no ferrous ions were detected in the reaction solution. S2. Continue to maintain the stirring speed of the reactor at 300 rpm, slowly add ammonia water dropwise into the reactor, adjust the pH value of the solution to 2.0, and then keep it at 60℃ for 1 h; S3. After the obtained slurry is filtered and separated into solid and liquid components, the solid obtained is washed with water until the conductivity of the wash water is <3000 μs / cm, and the solid is collected. S4. After washing, the solid is pulped, and the solid content in the resulting pulp is controlled to be 13wt%. The pulp is transferred to a reaction vessel, phosphoric acid is added to the reaction vessel, the pH of the solution is adjusted to 1.52, the stirring speed of the reaction vessel is set to 300 rpm, and the reaction is carried out at 80℃ for 2 h. The reaction product is filtered, and the solid after solid-liquid separation is washed until the conductivity of the wash water is <500 μs / cm. Then it is dried at 120℃ for 3 h to obtain iron phosphate dihydrate containing orthorhombic crystals.

[0052] Example 5 A method for preparing iron phosphate dihydrate containing orthorhombic crystal form includes the following steps: S1. Prepare 2 L of 0.8 mol / L ferrous chloride solution and 4 L of a mixed solution of hydrogen peroxide and potassium dihydrogen phosphate, wherein the mixed solution is prepared by mixing 2 L of 1 mol / L hydrogen peroxide and 2 L of 0.88 mol / L potassium dihydrogen phosphate solution evenly. 2 L of ferrous chloride solution was added to the reaction vessel. The stirring speed of the reaction vessel was set to 150 rpm. The mixed solution was added to the reaction vessel while stirring at room temperature for a total of 1.5 h. The molar ratio of phosphate to total added ferrous salt was calculated to be 1.1. After the mixed solution was added, the ferrous ions in the reaction solution were detected. The result was that no ferrous ions were detected in the reaction solution. S2. Continue to maintain the stirring speed of the reactor at 150 rpm, slowly add ammonia water dropwise into the reactor, adjust the pH value of the solution to 1.6, and then keep it at 55℃ for 1.5 h; S3. After the obtained slurry is filtered and separated into solid and liquid components, the solid obtained is washed with water until the conductivity of the wash water is <3000 μs / cm, and the solid is collected. S4. After washing, the solid is pulped, and the solid content in the resulting pulp is controlled to be 10 wt%. The pulp is transferred to a reaction vessel, phosphoric acid is added to the reaction vessel, the pH of the solution is adjusted to 1.55, the stirring speed of the reaction vessel is set to 150 rpm, and the reaction is carried out at 100℃ for 1 h. The reaction product is then filtered, and the solid after solid-liquid separation is washed until the conductivity of the wash water is <500 μs / cm. Then it is dried at 120℃ for 3 h to obtain iron phosphate dihydrate containing orthorhombic crystals.

[0053] The iron phosphate dihydrate products prepared in Examples 1 to 5 were tested, and it was found that the content of orthorhombic iron phosphate dihydrate in the products was in the range of 20% to 40%.

[0054] Comparative Example 1 The preparation method of ferric phosphate in Comparative Example 1 is basically the same as the preparation method of ferric phosphate dihydrate containing orthorhombic crystal form in Example 1. The main difference lies in the pH value of the slurry adjusted by adding phosphoric acid in step S4. Specifically, the steps are as follows: S1. Prepare 2 L of 0.5 mol / L ferrous sulfate solution and 4 L of a mixed solution of hydrogen peroxide and monoammonium phosphate. The mixed solution is prepared by mixing 2 L of 0.5 mol / L hydrogen peroxide solution and 2 L of 0.5 mol / L monoammonium phosphate solution evenly. The pH value of the mixed solution is measured to be 4. 2 L of ferrous sulfate solution was added to the reaction vessel. The stirring speed of the reaction vessel was set to 200 rpm. The mixed solution was added to the reaction vessel while stirring at room temperature for a total of 1 h. The molar ratio of phosphate to total added ferrous salt was calculated to be 1. After the mixed solution was added, the ferrous ions in the reaction solution were detected. The result was that no ferrous ions were detected in the reaction solution. S2. Continue stirring the reactor at 200 rpm, slowly add ammonia water dropwise to the reactor, adjust the pH of the solution to 1.8, filter the reactants directly without heat preservation, wash the obtained solid with water until the conductivity of the wash water is <3000 μs / cm, and collect the solid. S3. After washing, the solid is pulped, and the solid content in the resulting pulp is controlled to be 8 wt%. The pulp is transferred to a reaction vessel, phosphoric acid is added to the reaction vessel, the pH of the solution is adjusted to 1.11, the stirring speed of the reaction vessel is set to 200 rpm, and the reaction is carried out at 90℃ for 1.5 h. The reaction product is then filtered, and the solid after solid-liquid separation is washed until the conductivity of the wash water is <500 μs / cm. Then it is dried at 120℃ for 3 h to obtain iron phosphate dihydrate containing orthorhombic crystal form, and the orthorhombic crystal form content is less than 20%.

[0055] The iron phosphate product obtained from S3 was observed using a scanning electron microscope (SEM), and the results are as follows: Figure 3 As shown in the figure, the ferric phosphate product prepared in Comparative Example 1 is a monoclinic dihydrate ferric phosphate, indicating that the lower pH during the final step of adding phosphoric acid inhibited the formation of the orthorhombic crystal form. The crystal structure of the ferric phosphate product obtained in S3 was characterized using X-ray diffraction (XRD), and the results are as follows. Figure 4 As shown, a comparison with the standard cards of orthorhombic ferric phosphate and monoclinic ferric phosphate reveals that the conclusions are consistent with those of SEM.

[0056] The iron phosphate dihydrate prepared in Examples 1-3 and Comparative Example 1 was used to prepare lithium iron phosphate under the same conditions. The lithium iron phosphate was then used to prepare coin cells under the same conditions using conventional methods. The coin cell prepared using the lithium iron phosphate product from Example 1 was named #1 coin cell, and so on. The coin cell using the iron phosphate product from Example 2 was named #2 coin cell, the coin cell using the iron phosphate product from Example 3 was named #3 coin cell, and the coin cell using the iron phosphate product from Comparative Example 1 was named #4 coin cell. The electrical properties and compaction density were then tested under the same conditions using conventional methods in the art. The results are shown in Table 1 below.

[0057] Table 1. Results of electrical properties and compaction density tests

[0058] As can be seen from Table 1, compared with Comparative Example 1, the compaction density of lithium iron phosphate prepared using iron phosphate prepared in the embodiments of the present invention as raw material was significantly improved.

[0059] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing ferric phosphate dihydrate containing orthorhombic crystals, characterized in that, Includes the following steps: S1. Add a mixed solution of phosphate and hydrogen peroxide to the ferrous solution. The pH of the mixed solution is 4-7. Control the amount of hydrogen peroxide so that there are no ferrous ions in the reaction solution after the mixed solution is added. S2. Add alkali solution to the reaction solution to adjust the pH value to 1.6~2.0, and keep it at 50~60℃ for 1~2 h; S3. Filter and wash the obtained slurry to obtain a solid. S4. After the solid is pulped, add phosphoric acid to adjust the pH to 1.5~1.6, react at 80~100℃ for 1~2 h, and the reaction product is obtained after filtration, washing and drying.

2. The method for preparing ferric phosphate dihydrate containing orthorhombic crystals as described in claim 1, characterized in that, In step S1, the iron source of the ferrous solution is selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate and ferrous oxalate; The phosphate is selected from at least one of monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and calcium dihydrogen phosphate.

3. The method for preparing ferric phosphate dihydrate containing orthorhombic crystals as described in claim 1, characterized in that, In step S1, the molar ratio of phosphate to ferrous salt is 1~1.2:1, and the molar ratio of oxidant to ferrous salt is 0.8~1.4:

1.

4. The method for preparing ferric phosphate dihydrate containing orthorhombic crystals as described in claim 1, characterized in that, In step S1, the concentration of the ferrous solution is 0.5~1 mol / L.

5. The method for preparing ferric phosphate dihydrate with orthorhombic crystal structure as described in claim 1, characterized in that, In step S1, a mixed solution of phosphate and oxidant is added to the ferrous solution while stirring at a speed of 150-300 rpm for 0.5-2 h.

6. The method for preparing ferric phosphate dihydrate containing orthorhombic crystals as described in claim 1, characterized in that, In step S3, after the obtained slurry is filtered and separated into solid and liquid components, the obtained solid is washed with water until the conductivity is <3000 μs / cm.

7. The method for preparing ferric phosphate dihydrate containing orthorhombic crystals as described in claim 1, characterized in that, In step S4, the filter cake is pulped to obtain a slurry with a solid content of 8wt%~13wt%.

8. The method for preparing ferric phosphate dihydrate containing orthorhombic crystals as described in claim 1, characterized in that, In step S4, the reaction product is filtered and washed until the conductivity of the wash water is <500 μs / cm, and then dried at 120℃ for 2~5 h.

9. The method for preparing ferric phosphate dihydrate containing orthorhombic crystals as described in claim 1, characterized in that, In the orthorhombic ferric phosphate dihydrate obtained in step S4, the content of orthorhombic ferric phosphate dihydrate is 20%~40%.

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