Preparation method of iron phosphate dihydrate containing orthorhombic crystal form

Abstract

The invention relates to the technical field of preparation of iron phosphate, in particular to a preparation method of iron phosphate dihydrate containing orthorhombic crystal form, which comprises the following steps: adding a mixed solution of phosphate and an oxidizing agent into a ferrous solution, and controlling the dosage of the oxidizing agent, so that no ferrous ion exists in the reaction solution after the mixed solution is added; alkali liquor is added into the reaction liquid to adjust the pH value, and heat preservation is conducted for 1-2 h at the temperature of 50-60 DEG C; filtering and washing the prepared slurry to obtain a solid; and pulping the solid, adding phosphoric acid to adjust the pH value, reacting at 80-100 DEG C for 1-2 hours, and filtering, washing and drying the reaction product to obtain the product. According to the preparation method of the iron phosphate dihydrate containing the orthorhombic crystal form, in the process of synthesizing iron phosphate, the two-step method is adopted through the liquid phase precipitation reaction, the preparation method is simple in process, short in technological process and low in reaction process temperature, and the iron phosphate dihydrate containing the orthorhombic crystal form can be prepared without adding a surfactant in the reaction.

Description

Technical Field

[0001] The present invention relates to the technical field of iron phosphate preparation, and particularly relates to a method for preparing dihydrate iron phosphate with an orthorhombic crystal form. Background Art

[0002] Lithium iron phosphate is the cathode material of the second-generation lithium-ion battery. The battery prepared with lithium iron phosphate as the cathode has the advantages of low cost, long cycle life, good thermal stability and environmental friendliness, and has a very wide application in the fields of power tools, electric vehicles and energy storage batteries. In the existing lithium iron phosphate production process, the solid-phase method process using iron phosphate as the iron and phosphorus source precursor accounts for the largest proportion, and the physical and chemical properties such as the elemental composition, particle morphology, and crystal structure of iron phosphate play a decisive role in the tap density and electrical properties of the finally prepared lithium iron phosphate.

[0003] The chemical structure of iron phosphate has a strong dependence on the preparation method. Different preparation methods can obtain iron and phosphorus compounds with different crystal structures, sizes and morphologies. According to its crystal structure, it can be divided into isophosphorite-manganeseite type, amorphous, orthorhombic, monoclinic and α-quartz crystal systems, etc. The commonly used preparation methods of iron phosphate include the ammonium method, the sodium method and the iron method. The ammonium method uses ferrous salt, phosphoric acid or its salt, oxidant and ammonia water as the main raw materials. First, ferrous sulfate is mixed with phosphoric acid and oxidant, and then ammonia water is added to adjust the pH value, and a series of reactions are carried out to generate iron phosphate precipitate; the sodium method uses iron salt and sodium phosphate as the main raw materials, and a double decomposition reaction occurs under certain conditions to generate iron phosphate precipitate; the iron method is to add iron powder or iron filings into a reaction vessel containing phosphoric acid, and at an appropriate temperature, an oxidant such as hydrogen peroxide is slowly added dropwise to cause an oxidation reaction between iron and phosphoric acid to generate iron phosphate. The structure of the iron phosphate products prepared by the above three methods is mainly monoclinic crystal form. However, the lithium ion diffusion rate of the lithium iron phosphate prepared from monoclinic iron phosphate is relatively low. When used as the cathode material of a lithium-ion battery, it may affect the charge and discharge performance and rate performance of the battery, limiting its application in the field of high-performance batteries.

[0004] Research findings show that the electrochemical performance of lithium iron phosphate materials prepared from orthorhombic iron phosphate is significantly superior to that of lithium iron phosphate materials prepared from monoclinic iron phosphate. Therefore, technicians began to study how to prepare orthorhombic iron phosphate materials. Chinese Patent Application CN 102491302 A discloses an oxidative precipitation method using air as a catalyst. First, a divalent iron salt, phosphoric acid or phosphate are mixed to form an aqueous mixture solution. Then, the prepared mixed aqueous solution of divalent iron salt and phosphoric acid or phosphate, and the pH regulator solution are continuously pumped into a stirred reactor respectively. Through an air compressor, air is input into the reactor at a certain flow rate. Through a constant temperature water bath, the temperature of the reaction solution in the reactor is controlled and maintained within the range of 40 - 98 °C. The flow rates of the mixed aqueous solution of divalent iron salt and phosphoric acid or phosphate, and air are kept constant, and the pH value of the reaction solution in the reactor is controlled and adjusted to be 0.5 - 7.5 and kept constant. After the feeding is completed, stirring and aging continue, and air oxidation is continuously carried out. Through the oxidative precipitation reaction, a crystalline complex NH4Fe2(OH)(PO4)2·2H2O containing ammonium root (NH4 + ), hydroxide (OH - ), and crystal water is formed; then, after solid-liquid separation, washing with deionized water, and drying, NH4Fe2(OH)(PO4)2·2H2O powder is obtained; then, the NH4Fe2(OH)(PO4)2·2H2O powder is calcined in an air atmosphere at 500 - 700 °C for 2 - 24 hours to decompose and remove ammonium root, hydroxide, and crystal water, and high-quality battery-grade anhydrous iron phosphate (FePO4) with orthorhombic crystal form is obtained. The disadvantage of this technical solution is that the reaction conditions are difficult to control, and it is difficult to achieve industrial production.

[0005] As an improvement, Chinese Patent Application CN 115215313 A discloses a preparation method of orthorhombic high tap density iron phosphate material, which includes the following steps: (1) preparing a first mixture system containing a phosphate source, a ferrous ion source, and a first pH regulator, and 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 regulator to the first slurry to obtain a second slurry, and the second slurry is acidic; (3) adding an oxidizing substance to the second slurry to carry out a second reaction to obtain a dihydrate iron phosphate 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. Taking amorphous ferrous phosphate octahydrate as an intermediate, then adding an acidic second pH regulator to the first slurry for generating the amorphous ferrous phosphate octahydrate precursor, and controlling the pH value of the second slurry to be -0.5 ~ 1.5, so that a part of the generated amorphous ferrous phosphate octahydrate dissolves rapidly under acidic conditions, thereby increasing the Fe 2+ and PO4 3-Concentration, which is beneficial for the subsequent step (3) of adding an oxidizing substance. At the initial stage of the reaction, more ferrous phosphate dihydrate crystal nuclei can be generated. With the addition of the oxidizing substance and the formation of ferrous phosphate dihydrate crystal nuclei, the Fe 2+ and PO4 3- concentrations in the reaction system decrease compared to the initial stage of the reaction. And with the slow dissolution of the amorphous iron(II) phosphate octahydrate precursor and the slow change of the pH value of the reaction system, a relatively low concentration of Fe 2+ and PO4 3- can be maintained in the reaction system, thereby inhibiting the excessive nucleation of ferrous phosphate dihydrate, controlling its slow growth, and finally forming a ferrous phosphate dihydrate material with dense packing, relatively large primary particles, and a regular polyhedral morphology. The disadvantage of this technical solution is that it is necessary to first test the pH value and redox potential of the first mixture system, and judge whether it is necessary to add a reducing substance to the first mixture system according to the pH value and redox potential, and the operation is relatively cumbersome. SUMMARY OF THE INVENTION

[0006] Aiming at the technical problems of difficult control of reaction conditions and cumbersome operation in the existing preparation method of orthorhombic ferrous phosphate dihydrate, the present invention provides a preparation method of ferrous phosphate dihydrate containing orthorhombic crystal form.

[0007] The technical solution of the present invention is as follows: A preparation method of ferrous phosphate dihydrate containing orthorhombic crystal form, comprising the following steps: S1. Add a mixed solution of phosphate and an oxidant to the ferrous solution, and control the dosage of the oxidant so that there is no ferrous ion in the reaction solution after the mixed solution is added; S2. Add an alkali solution to the reaction solution to adjust the pH value to 1.6 - 2.0, and keep it warm at 50 - 60 °C for 1 - 2 h; S3. Filter and wash the prepared slurry to obtain a solid; S4. After the solid is slurried, add phosphoric acid to adjust the pH value to 1.5 - 1.6, and react at 80 - 100 °C for 1 - 2 h. After the reaction product is filtered, washed, and dried, the product is obtained.

[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 ammonium dihydrogen 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] Further, in step S1, the molar ratio of phosphate to ferrous salt is 1 to 1.2:1, and the molar ratio of oxidant to ferrous salt is 0.8 to 1.4:1.

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

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

[0014] Further, in step S1, while stirring, the mixed solution of phosphate and oxidant is added to the ferrous solution, the stirring speed is 150 to 300 rpm, and the addition time of the mixed solution is 0.5 to 2 h.

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

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

[0017] Further, in step S4, the reaction product is filtered and washed until the conductivity of the washing water < 500 μs / cm, and then dried at 120°C for 2 to 5 h.

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

[0019] The beneficial effects of the present invention are as follows: The preparation method of orthorhombic dihydrate ferric phosphate provided by the present invention, in the process of synthesizing ferric phosphate, through liquid-phase precipitation reaction, adopting a two-step method, the preparation method has a simple process, a short process flow, a low reaction temperature, and no surfactant needs to be added during the reaction, and orthorhombic dihydrate ferric phosphate can be prepared; if the temperature is too high, the supersaturation of the system will be relatively low, and at this time, thermodynamics dominates the preferential nucleation and crystallization of the thermally stable crystal form, and orthorhombic dihydrate ferric phosphate can be prepared. Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 It is the SEM diagram of the ferric phosphate prepared in Example 1.

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

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

[0024] Figure 4 It is the comparison chart of the XRD of the iron phosphate prepared in Example 1 and Comparative Example 1 and the standard card. Detailed implementation manners

[0025] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0026] In the present invention, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or its similar expression means any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b, or c", or, "at least one (item) of a, b, and c" can both represent: a, b, c, a-b (that is, a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple respectively.

[0027] The weights of the relevant components mentioned in the specification of the present invention can not only refer to the specific contents of each component, but also represent the proportional relationship of the weights between each component. Therefore, as long as the contents of the relevant components in the specification of the present invention are enlarged or reduced in proportion, they are within the scope disclosed in the specification of the present invention.

[0028] In order to obtain orthorhombic iron phosphate and thereby prepare lithium iron phosphate with a relatively high tap density and electrical properties, the embodiments of the present invention provide a preparation method of dihydrate iron phosphate containing orthorhombic crystal form, including the following steps: S1. Add a mixed solution of phosphate and oxidant to the ferrous solution, and control the dosage of the oxidant so that there is no ferrous ion in the reaction solution after the mixed solution is added. S2. Add an alkali solution to the reaction solution to adjust the pH value to 1.6 - 2.0, and keep it warm at 50 - 60 °C for 1 - 2 h. S3. Filter and wash the prepared slurry to obtain a solid. S4. After solid beating, add phosphoric acid to adjust the pH value to 1.5 - 1.6, react at 80 - 100 °C for 1 - 2 h, and the reaction product is obtained after filtration, washing, and drying.

[0029] In the present invention, a divalent iron source, a phosphorus source, and an oxidant are first mixed for reaction, an alkali solution is added to the reaction solution to form a precipitate, and the product is filtered, washed, and pulped. Then, phosphoric acid is used to adjust the pH value of the pulp for reaction. Through this process, orthorhombic dihydrate iron phosphate is finally obtained, and the content of orthorhombic dihydrate iron phosphate is 20% - 40%. Thus, a higher tap density can be obtained when preparing lithium iron phosphate.

[0030] The iron source of the ferrous solution described in S1 of the present 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 cause the subsequent reaction to be rapid and difficult to control; too low a concentration will cause the reaction to be slow and unfavorable for production.

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

[0032] The mixed solution of the phosphate and the oxidant described in S1 of the present invention is prepared using a phosphate solution and an oxidant solution. The concentration of the phosphate in the phosphate solution used for preparation is controlled at 0.5 - 1 mol / L. Too high a concentration will cause the subsequent reaction to be rapid and difficult to control; too low a concentration will cause the reaction to be slow and unfavorable for production.

[0033] The oxidant described in S1 of the present invention can be any oxidant in the art that can oxidize ferrous ions, and hydrogen peroxide is preferably used. In some embodiments of the present invention, the oxidant in S1 is hydrogen peroxide. The concentration of the oxidant used for preparing the mixed solution of the phosphate and the oxidant described in S1 of the present invention is controlled according to the molar number of ferrous ions, for example, set within 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 any value between any two of them.

[0034] In the mixed solution of the phosphate and the oxidant described in S1 of the present invention, the pH value of the mixed solution is controlled at 4 - 7. If the pH is too low, the yield is too low; if the pH is too high, iron hydroxide will be formed.

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

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

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

[0038] The lye added to the reaction solution in S2 of the present invention is ammonia water. By adding ammonia water, 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, 2.0, or a value between any two of them.

[0039] In S2 of the present invention, the temperature for heat preservation is 50 to 60 °C. Exemplarily, the temperature for heat preservation can be any one of 50 °C, 55 °C, 60 °C, or a value between any two of them; the heat preservation time can be any one of 1 h, 1.5 h, 2.0 h, or a value between any two of them.

[0040] The specific step in S3 of the present invention is that after the obtained slurry is filtered for solid-liquid separation, the obtained solid is washed with water until the conductivity is < 3000 μs / cm.

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

[0042] In S4 of the present invention, phosphoric acid is added to adjust the pH value to 1.5 - 1.6. The adjustment of the pH value must be strictly controlled. When the pH value of the slurry is lower than the lower limit value of 1.5, the formation of the orthorhombic crystal form will be inhibited; when the pH value of the slurry is higher than the upper limit value of the range of 1.6, the impurity content is relatively high due to too high pH. Exemplarily, when adding phosphoric acid to adjust the pH value as described in S3 of the present invention, the pH value of the slurry can be adjusted to any one of 1.50, 1.51, 1.52, 1.53, 1.54, 1.55, 1.56, 1.57, 1.58, 1.59, 1.60, or any value between any two of them.

[0043] In S4 of the present invention, the slurry after adjusting the pH value is reacted at 80 - 100 °C for 1 - 2 h. The main purpose of this step is to form the orthorhombic crystal form. Therefore, the control of the reaction temperature and reaction time has a great influence on the crystal form of the final product. When the reaction temperature is too high, the nucleation rate of iron phosphate is relatively fast and the number of nuclei is relatively large. Therefore, the formation of the orthorhombic crystal form will be inhibited. Therefore, the upper limit value of the reaction temperature generally should not exceed 100 °C, preferably not exceed 95 °C, and more preferably not exceed 90 °C; when the reaction temperature is too low, the growth rate is too slow and a longer reaction time is required to obtain the orthorhombic crystal form, which is not conducive to production. Therefore, the lower limit value of the reaction temperature generally should 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 h, 1.5 h, 2 h, or any value between any two of them.

[0044] In S4 of the present invention, the slurry after adjusting the pH value is reacted in a way of stirring while reacting, and the stirring speed is 150 - 300 rpm. Exemplarily, the stirring speed can be any one of 150 rpm, 200 rpm, 250 rpm, 300 rpm, or any value between any two of them.

[0045] In S4 of the present invention, after filtering the reaction solution obtained after the reaction, the solid is taken for washing with water until the conductivity of the washing water < 500 μs / cm, and finally the washed solid is dried to remove free water.

[0046] Example 1 A preparation method of dihydrate iron phosphate containing orthorhombic crystal form includes the following steps: S1. Prepare 2 L of ferrous sulfate solution with a concentration of 0.5 mol / L, and prepare a mixed solution of hydrogen peroxide and monoammonium phosphate of 4 L. The mixed solution is prepared by mixing 2 L of hydrogen peroxide solution with a concentration of 0.5 mol / L and 2 L of monoammonium phosphate solution with a concentration of 0.5 mol / L evenly. After measurement, the pH value of the mixed solution is 4; Add 2 L of ferrous sulfate solution to the reaction kettle, set the stirring rate of the reaction kettle to 200 rpm, and while stirring at room temperature, add the mixed solution to the reaction kettle. The total addition time is 1 h. After calculation, in terms of molar ratio, the P / Fe of phosphate to the total added ferrous salts is 1. After the addition of the mixed solution is completed, detect the ferrous ions in the reaction solution. The result is that no ferrous ions are detected in the reaction solution; S2. Keep the stirring rate of the reaction kettle at 200 rpm, slowly drip ammonia water into the reaction kettle, adjust the pH value of the solution to 1.8, and then keep it warm at 50 °C for 2 h; S3. After the prepared slurry is filtered and separated by solid-liquid separation, wash the obtained solid until the conductivity of the washing water is < 3000 μs / cm, and collect the solid; S4. Pulp the solid after washing, control the solid content in the formed slurry to be 8 wt%, transfer the slurry to the reaction kettle, add phosphoric acid to the reaction kettle, adjust the pH value of the solution to 1.51, set the stirring rate of the reaction kettle to 200 rpm, react at 90 °C for 1.5 h, then filter the reaction product, wash the solid after solid-liquid separation until the conductivity of the washing water is < 500 μs / cm, and then dry it at 120 °C for 3 h to obtain iron phosphate dihydrate containing orthorhombic crystal form.

[0047] Use a scanning electron microscope (SEM) to observe the iron phosphate product obtained in S4. The results are as Figure 1 shown. It can be seen that the iron phosphate product prepared in Example 1 contains iron phosphate dihydrate with orthorhombic crystal form. Use an X-ray diffractometer (XRD) to characterize the crystal structure of the iron phosphate product obtained in S4. The results are as Figure 4 shown. By comparing with the standard card of orthorhombic iron phosphate and the standard card of monoclinic iron phosphate, it can be seen that the conclusion is consistent with that of SEM.

[0048] Example 2 A preparation method of iron phosphate dihydrate containing orthorhombic crystal form, comprising the following steps: S1. Prepare 4 L of ferrous sulfate solution with a concentration of 0.5 mol / L, and prepare a mixed solution of 8 L of hydrogen peroxide and ammonium dihydrogen phosphate. The mixed solution is prepared by mixing 4 L of hydrogen peroxide with a concentration of 0.6 mol / L and 4 L of ammonium dihydrogen phosphate solution with a concentration of 0.6 mol / L evenly. After measurement, the pH value of the mixed solution is 4.22; ​ S2. Increase the stirring rate of the reaction kettle to 200 rpm, slowly drop ammonia water into the reaction kettle, adjust the pH value of the solution to 1.8, and then keep it warm at 50 °C for 2 h; S3. After the obtained slurry is filtered for solid-liquid separation, wash the obtained solid with water until the conductivity of the washing water is < 3000 μs / cm, and collect the solid; S4. Pulp the solid after washing, control the solid content in the formed slurry to be 8 wt%, transfer the slurry to the reaction kettle, add phosphoric acid to the reaction kettle, adjust the pH value of the solution to 1.53, set the stirring rate of the reaction kettle to 200 rpm, react at 95 °C for 1.5 h, then filter the reaction product, wash the solid after solid-liquid separation until the conductivity of the washing water is < 500 μs / cm, and then dry it at 120 °C for 3 h to obtain iron dihydrogen phosphate with orthorhombic crystal form.

[0049] Use a scanning electron microscope (SEM) to observe the iron phosphate product obtained in S4, and the results are as Figure 2 shown. It can be seen that the iron phosphate product prepared in Example 2 contains iron dihydrogen phosphate with orthorhombic crystal form, but the content of iron dihydrogen phosphate with orthorhombic crystal form is lower than that of the product in Example 1. Analyzing the reason, it may be that after adding phosphoric acid in step S4, the reaction temperature of the slurry is too high, resulting in an accelerated crystal growth rate and affecting the further formation of the orthorhombic crystal form.

[0050] Example 3 A preparation method of iron dihydrogen phosphate with orthorhombic crystal form, comprising the following steps: S1. Prepare a 4 L ferrous sulfate solution with a concentration of 0.5 mol / L, and prepare an 8 L mixed solution of hydrogen peroxide and monoammonium phosphate. The mixed solution is prepared by mixing 4 L of hydrogen peroxide with a concentration of 0.5 mol / L and 4 L of monoammonium phosphate solution with a concentration of 0.5 mol / L evenly. After measurement, the pH value of the mixed solution is 4; Add 4 L of ferrous sulfate solution to the reaction kettle, set the stirring rate of the reaction kettle to 300 rpm, and while stirring at room temperature, add the mixed solution to the reaction kettle. The total addition time is 0.5 h. After calculation, in terms of molar ratio, the P / Fe of the phosphate salt to the total added ferrous salt is 1. After the addition of the mixed solution is completed, detect the ferrous ions in the reaction solution, and the result is that no ferrous ions are detected in the reaction solution; S2. Keep the stirring rate of the reaction kettle at 300 rpm, slowly drop ammonia water into the reaction kettle, adjust the pH value of the solution to 1.8, and then keep it warm at 50 °C for 2 h; S3. After the obtained slurry is filtered for solid-liquid separation, wash the obtained solid with water until the conductivity of the washing water is < 3000 μs / cm, and collect the solid; S4. Pulp the solid after washing is completed, control the solid content in the formed slurry to be 8 wt%, transfer the slurry to a reaction kettle, add phosphoric acid to the reaction kettle, adjust the pH value of the solution to 1.58, set the stirring rate of the reaction kettle to 200 rpm, react at 90 °C for 1.5 h, filter the reaction product, wash the solid after solid-liquid separation until the conductivity of the washing water is <500 μs / cm, and then dry at 120 °C for 3 h to obtain orthorhombic iron phosphate dihydrate.

[0051] Example 4 A preparation method of orthorhombic iron phosphate dihydrate, comprising the following steps: S1. Prepare a 4 L ferrous oxalate solution with a concentration of 0.7 mol / L, and prepare a mixed solution of 8 L hydrogen peroxide and low-quality calcium dihydrogen phosphate. The mixed solution is prepared by 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 evenly; Add 4 L of ferrous oxalate solution to the reaction kettle, set the stirring rate of the reaction kettle to 300 rpm, and add the mixed solution to the reaction kettle while stirring at room temperature. The total addition time is 2 h. After calculation, in terms of molar ratio, the P / Fe of phosphate to the total added ferrous salt is 1.2. After the addition of the mixed solution is completed, detect the ferrous ions in the reaction solution, and the result is that no ferrous ions are detected in the reaction solution; S2. Keep the stirring rate of the reaction kettle at 300 rpm, slowly drop ammonia water into the reaction kettle, adjust the pH value of the solution to 2.0, and then keep it warm at 60 °C for 1 h; S3. After the prepared slurry is filtered for solid-liquid separation, wash the obtained solid with water until the conductivity of the washing water is <3000 μs / cm, and collect the solid; S4. Pulp the solid after washing is completed, control the solid content in the formed slurry to be 13 wt%, transfer the slurry to a reaction kettle, add phosphoric acid to the reaction kettle, adjust the pH value of the solution to 1.52, set the stirring rate of the reaction kettle to 300 rpm, react at 80 °C for 2 h, filter the reaction product, wash the solid after solid-liquid separation until the conductivity of the washing water is <500 μs / cm, and then dry at 120 °C for 3 h to obtain orthorhombic iron phosphate dihydrate.

[0052] Example 5 A preparation method of orthorhombic iron phosphate dihydrate, comprising the following steps: S1. Prepare 2 L of ferrous chloride solution with a concentration of 0.8 mol / L, and prepare 4 L of a mixed solution of hydrogen peroxide and potassium dihydrogen phosphate. The mixed solution is prepared by mixing 2 L of hydrogen peroxide solution with a concentration of 1 mol / L and 2 L of potassium dihydrogen phosphate solution with a concentration of 0.88 mol / L evenly; Add 2 L of ferrous chloride solution to the reaction kettle, set the stirring rate of the reaction kettle to 150 rpm, and add the mixed solution to the reaction kettle while stirring at room temperature. The total addition time is 1.5 h. After calculation, in terms of molar ratio, the P / Fe of phosphate to the total added ferrous salts is 1.1. After the addition of the mixed solution is completed, detect the ferrous ions in the reaction solution. The result is that no ferrous ions are detected in the reaction solution; S2. Keep the stirring rate of the reaction kettle at 150 rpm, slowly drip ammonia water into the reaction kettle, adjust the pH value of the solution to 1.6, and then keep it warm at 55 °C for 1.5 h; After the prepared slurry is filtered for solid-liquid separation, wash the obtained solid until the conductivity of the washing water < 3000 μs / cm, and collect the solid; S4. Pulp the washed solid, control the solid content in the formed slurry to be 10 wt%, transfer the slurry to the reaction kettle, add phosphoric acid to the reaction kettle, adjust the pH value of the solution to 1.55, set the stirring rate of the reaction kettle to 150 rpm, react at 100 °C for 1 h, filter the reaction product, wash the solid after solid-liquid separation until the conductivity of the washing water < 500 μs / cm, and then dry it at 120 °C for 3 h to obtain orthorhombic iron phosphate dihydrate.

[0053] Detect the iron phosphate dihydrate products prepared in Examples 1 to 5, and it is found that the content of orthorhombic iron phosphate dihydrate in the obtained products is within the range of 20% - 40%.

[0054] Comparative Example 1 The preparation method of iron phosphate in Comparative Example 1 is basically the same as the preparation method of orthorhombic iron phosphate dihydrate in Example 1. The main difference is the pH value of the slurry after adding phosphoric acid in Step S4. The specific steps are as follows: S1. Prepare 2 L of ferrous sulfate solution with a concentration of 0.5 mol / L, and prepare 4 L of a mixed solution of hydrogen peroxide and ammonium dihydrogen phosphate. The mixed solution is prepared by mixing 2 L of hydrogen peroxide solution with a concentration of 0.5 mol / L and 2 L of ammonium dihydrogen phosphate solution with a concentration of 0.5 mol / L evenly. After measurement, the pH value of the mixed solution is 4; Add 2 L of ferrous sulfate solution to the reaction kettle, set the stirring rate of the reaction kettle to 200 rpm, and while stirring at room temperature, add the mixed solution to the reaction kettle. The total addition time is 1 h. After calculation, in terms of molar ratio, the P / Fe of phosphate to the total added ferrous salts is 1. After the addition of the mixed solution is completed, detect the ferrous ions in the reaction solution. The result is that no ferrous ions are detected in the reaction solution; S2. Keep the stirring rate of the reaction kettle at 200 rpm, slowly drip ammonia water into the reaction kettle, adjust the pH value of the solution to 1.8, filter the reactants directly without heat preservation, wash the obtained solid with water until the conductivity of the washing water is < 3000 μs / cm, and collect the solid; S3. Pulp the solid after washing, control the solid content in the formed slurry to be 8 wt%, transfer the slurry to the reaction kettle, add phosphoric acid to the reaction kettle, adjust the pH value of the solution to 1.11, set the stirring rate of the reaction kettle to 200 rpm, react at 90 °C for 1.5 h, then filter the reaction product, wash the solid after solid-liquid separation until the conductivity of the washing water is < 500 μs / cm, and then dry at 120 °C for 3 h to obtain iron phosphate dihydrate with orthorhombic crystal form, and the content of orthorhombic crystal form is less than 20%.

[0055] Use a scanning electron microscope (SEM) to observe the iron phosphate product obtained in S3. The result is as Figure 3 shown. It can be seen that the iron phosphate product prepared in Comparative Example 1 is monoclinic iron phosphate dihydrate, indicating that when phosphoric acid is added in the last step of the reaction, the lower pH inhibits the formation of orthorhombic crystal form. Use an X-ray diffractometer (XRD) to characterize the crystal structure of the iron phosphate product obtained in S3. The result is as Figure 4 shown. By comparing with the standard cards of orthorhombic iron phosphate and monoclinic iron phosphate, it can be seen that the conclusion is consistent with that of SEM.

[0056] Prepare lithium iron phosphate from the iron phosphate dihydrate prepared in Examples 1 to 3 and Comparative Example 1 under the same conditions. Prepare button cells from the obtained lithium iron phosphate under the same conditions according to the conventional method. Name the button cell prepared from the iron phosphate product of Example 1 as 1# button cell, and so on. The button cell with the iron phosphate product of Example 2 as the raw material is 2# button cell, the button cell with the iron phosphate product of Example 3 as the raw material is 3# button cell, and the button cell with the iron phosphate product of Comparative Example 1 as the raw material is 4# button cell. Then test the electrical performance and tap density under the same conditions according to the conventional method in the art. The results are shown in Table 1 below.

[0057] Table 1 Detection results of electrical performance and tap density

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

[0059] Although the present invention has been described in detail by referring to the accompanying drawings and in conjunction with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, those of ordinary skill in the art can make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions should all be within the scope of the present invention. / Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered by the protection scope of the present invention.

Claims

1. A preparation method of orthorhombic iron dihydrogen phosphate, characterized in that, It includes the following steps: S1. Add a mixed solution of phosphate and oxidant to the ferrous solution, and control the dosage of the oxidant so that there is no ferrous ion in the reaction solution after the mixed solution is added completely; S2. Add an alkali solution to the reaction solution to adjust the pH value to 1.6 - 2.0, and keep it warm at 50 - 60 °C for 1 - 2 h; S3. Filter and wash the prepared slurry to obtain a solid; S4. After the solid is slurried, add phosphoric acid to adjust the pH value to 1.5 - 1.6, react at 80 - 100 °C 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 crystal form according to claim 1, wherein: 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, diammonium hydrogen phosphate and calcium dihydrogen phosphate; The oxidant is hydrogen peroxide.

3. The preparation method of a ferric phosphate dihydrate containing an orthorhombic crystal form as claimed in claim 1, wherein, In step S1, the molar ratio of the phosphate to the ferrous salt is 0.8 - 1.4:1, and the molar ratio of the oxidant to the ferrous salt is 0.8 - 1.4:

1.

4. The preparation method of an iron(II) phosphate dihydrate containing an orthorhombic crystal form according to 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 containing orthorhombic crystal form according to claim 1, wherein: In step S1, the pH value of the mixed solution of the phosphate and the oxidant is 4 - 7.

6. The preparation method of an iron(II) phosphate dihydrate containing an orthorhombic crystal form as claimed in claim 1, characterized in that, In step S1, add the mixed solution of the phosphate and the oxidant to the ferrous solution while stirring, the stirring speed is 150 - 300 rpm, and the addition time of the mixed solution is 0.5 - 2 h.

7. The preparation method of a ferric phosphate dihydrate containing an orthorhombic crystal form as claimed in claim 1, characterized in that, In step S3, after the prepared slurry is filtered for solid-liquid separation, the obtained solid is washed with water until the conductivity < 3000 μs / cm.

8. The method for preparing ferric phosphate dihydrate containing orthorhombic crystal form according to claim 1, wherein: In step S4, the filter cake is slurried to obtain a slurry with a solid content of 8wt% - 13wt%.

9. The method for preparing ferric phosphate dihydrate containing orthorhombic crystal form according to claim 1, wherein: In step S4, the reaction product is filtered and washed until the conductivity of the washing water < 500 μs / cm, and then dried at 120 °C for 2 - 5 h.

10. The preparation method of a ferric phosphate dihydrate containing an orthorhombic crystal form as described in claim 1, wherein, In the obtained iron phosphate dihydrate with orthorhombic crystal form in step S4, the content of the orthorhombic iron phosphate dihydrate is 20% - 40%.

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