Preparation method of octahydrate ferrous phosphate

Abstract

The application discloses a preparation method of ferrous phosphate octahydrate, which comprises the following steps: 1) preparing ferrous sulfate solution and phosphorus source solution, protecting by inert gas, adding the ferrous sulfate solution and the phosphorus source solution into a surfactant bottom solution, stirring, adding alkali solution until the pH of the reaction system is 2.5-4.0, stirring for the first-stage reaction, and performing suction filtration to obtain a filter cake; 2) slurring the filter cake according to the original solid content to obtain a reaction solution, protecting by inert gas, heating to 40-50 DEG C, adding alkali solution to adjust the pH of the solution to 4.5-6.0, stirring for the second-stage reaction, performing suction filtration, washing, and drying to obtain the ferrous phosphate octahydrate. The application controls the coprecipitation reaction by using stage temperature and segmented pH value, and performs solid-liquid separation by suction filtration under low pH value, so that the particle size, morphology and purity of the crystal can be controlled. After the first-stage reaction is completed, the slurry is subjected to the first suction filtration, and impurity elements such as Mg and Mn which are easy to precipitate in the system under high pH value are removed.

Description

Technical Field

[0001] This invention relates to the field of ferrous phosphate technology, and more particularly to a method for preparing ferrous phosphate octahydrate. Background Technology

[0002] In the past two years, the installed capacity of lithium iron phosphate (LFP) power batteries has gradually surpassed that of ternary lithium batteries. Currently, the main production process for LFP is the carbothermal reduction method, whose precursor is primarily trivalent iron phosphate, driving the rapid development of LFP precursor production. Mainstream LFP processes include the sodium method, ammonium method, iron method, and iron oxide red method.

[0003] Since 2023, the market prices of lithium iron phosphate and ternary materials have continued to decline, constantly squeezing the profit margins of raw materials. In order to reduce production costs, the production process of lithium iron phosphate has gradually shifted from the higher-cost sodium process to the lower-cost ammonium process.

[0004] To further reduce production costs and improve product quality, the iron-based process and the iron oxide red process have attracted more attention as new precursor processes for lithium iron phosphate. Ferrous phosphate, as a new precursor, has inexpensive raw materials and a simple synthesis process, making it a potential candidate material for next-generation lithium iron phosphate precursors.

[0005] Currently, ferrous phosphate is mainly synthesized using a co-precipitation method. For example, patent CN202010720310.6 discloses a method for preparing ferrous phosphate using ferrous sulfate. This method first uses a solution of ferrous sulfate and water as a raw material, then adds iron powder and liquid alkali to adjust the pH, keeps it at a certain temperature, and then precipitates it. The precipitated product is then added to a sodium hydroxide solution and liquid alkali to synthesize the ferrous phosphate product. This method produces ferrous phosphate with reduced by-product salt formation; the use of sodium hydroxide for ferrous phosphate synthesis is beneficial for the growth of ferrous phosphate crystals and shortens the washing time; the obtained ferrous phosphate product has uniform concentration, good color, low impurity content, and high yield. However, it does not provide product purity test results or product yield data, and it cannot produce high-purity ferrous phosphate octahydrate.

[0006] CN201911188406.6 discloses a method for preparing high-performance ferrous phosphate. This method involves preparing a mixed solution of an iron source and a metal ion additive: ferrous sulfate at a concentration of 0.5–2 mol / L is mixed with the metal ion additive to form an aqueous solution of the iron source, thereby improving the basic electrical performance of the LiFePO4 cathode material. The resulting lithium iron phosphate synthesized from ferrous phosphate exhibits a half-cell 0.2C initial discharge capacity of 162 mAh / g, a full-cell 1C discharge capacity of 142 mAh / g, a capacity retention of 99.5% after 100 cycles at 1C, and an electrode compaction density of 2.54 g / cm³. 3 However, it is impossible to obtain high-purity ferrous phosphate octahydrate.

[0007] Due to the significant temperature and pH sensitivity of ferrous phosphate, residual Mn and Mg impurities in the product are difficult to remove. Ferrous ions in the product are easily oxidized, making commonly used impurity removal processes difficult to apply. This results in high impurity content in ferrous phosphate synthesized by existing methods, and the presence of impurities leads to low crystallinity, poor quality stability, and makes it difficult to synthesize octahydrate ferrous phosphate products with high crystallinity, performance stability, and low impurity content.

[0008] For example, the high-pressure compaction iron phosphate material and its preparation method disclosed in CN202210664327.3 can only produce ferrous phosphate octahydrate in the first slurry during the reaction process, and dihydrate iron phosphate material can be produced from ferrous phosphate octahydrate by controlling the pH value of the first slurry. Although the 0.1C discharge capacity of the obtained iron phosphate can reach up to 162.3mAh / g, it is still impossible to produce ferrous phosphate octahydrate with purity that meets the requirements of subsequent production. Moreover, the content and purity of ferrous phosphate octahydrate in the first slurry are not detected. Summary of the Invention

[0009] This invention provides a method for preparing ferrous phosphate octahydrate.

[0010] The solution of the present invention is:

[0011] A method for preparing ferrous phosphate octahydrate includes the following steps:

[0012] 1) Prepare ferrous sulfate solution and phosphorus source solution. Under the protection of inert gas, add ferrous sulfate solution and phosphorus source solution to surfactant base solution at the same feeding time. Stir and add alkali solution dropwise until the pH of the reaction system is between 2.5 and 4.0. Stir to carry out the first stage of reaction, filter and obtain filter cake.

[0013] 2) After the filter cake is slurried again according to the original solid content, a reaction solution is obtained. Under the protection of inert gas, the temperature is raised and alkali solution is added dropwise to adjust the pH value of the solution to between 4.5 and 6.5. The second stage of reaction is carried out by stirring, followed by filtration, washing, and drying to obtain ferrous phosphate octahydrate.

[0014] The reaction process is divided into two stages using the above method. The first stage allows for slow nucleation and uniform crystal growth at low temperature and pH. The first-stage slurry, with its lower pH, is filtered to remove some refractory impurities. The filter cake is then re-slurried, and the solution is heated to a temperature significantly higher than the first stage for further reaction. Simultaneously, the pH is controlled at a higher level to promote rapid crystal growth, thereby controlling the particle size, morphology, and purity of the resulting ferrous phosphate octahydrate. The inert gas used can be argon or nitrogen.

[0015] The ferrous sulfate solution used in this method can be prepared from industrial-grade ferrous sulfate, or it can be prepared from ferrous sulfate byproducts of titanium dioxide after purification with ammonia, or it can be prepared from the substance obtained by reacting sulfuric acid and iron powder. This method has a wide range of raw material sources, and the resulting product has high purity ferrous phosphate octahydrate with few impurities, which helps reduce production costs for enterprises.

[0016] As a preferred technical solution, the pH of the reaction system during the first stage of reaction in 1) is one of the following: between 3.5 and 4.0, between 3.0 and 3.5, or between 2.5 and 3.0.

[0017] As a preferred technical solution, the pH of the reaction system during the second stage of the reaction in step 2) is one of the following: between 6.0 and 6.5, between 5.5 and 6.0, between 5.0 and 5.5, or between 4.5 and 5.0.

[0018] As a preferred technical solution, the ferrous sulfate solution prepared in step 1) contains Fe 2+ The ion mass concentration is 4.5%–7.0%. Further preferred, the Fe content in the ferrous sulfate solution is... 2+ The ion mass concentration is one of 4.5%, 4.8%, 5.0%, 5.5%, or 6.0%.

[0019] As a preferred technical solution, the solute in the surfactant substrate is at least one of polyethylene glycol, hexadecyltrimethylammonium bromide, and polyvinylpyrrolidone.

[0020] As a preferred technical solution, the solute mass concentration of the surfactant base solution is 0.5‰ to 2‰. More preferably, the solute mass concentration of the surfactant base solution is 1.0‰.

[0021] As a preferred technical solution, in step 1), the temperature at which the ferrous sulfate solution and the phosphorus source solution are added to the surfactant base solution is 10–30°C, and the mixing time is 10–30 min. More preferably, the temperature at which the ferrous sulfate solution and the phosphorus source solution are added to the surfactant base solution is one of 10°C, 20°C, and 30°C, and the mixing time is one of 30 min and 20 min.

[0022] As a preferred technical solution, the temperature of the first stage reaction in step 1) is 10-30°C, and the stirring reaction is carried out for 10-30 minutes.

[0023] As a preferred technical solution, the temperature of the second stage reaction in step 2) is 40–55°C, and the reaction is stirred for 20–30 minutes. More preferably, the temperature of the second stage reaction is one of 40°C, 45°C, 50°C, and 55°C.

[0024] As a preferred technical solution, the ferrous sulfate solution is prepared using industrial-grade ferrous sulfate. This method can be used to prepare the solution with industrial-grade raw materials, which can effectively reduce production costs, and the resulting product has good purity, effectively meeting various subsequent requirements.

[0025] As a preferred technical solution, the phosphorus source solution in 1) is prepared with industrial-grade monoammonium phosphate; the molar ratio of iron to phosphorus is 1 to 1.5:1. More preferably, the molar ratio of iron to phosphorus is one of 1.25:1, 1.4:1, and 1.5:1.

[0026] A method for preparing ferrous phosphate octahydrate using the above-mentioned technical solution includes the following steps:

[0027] 1) Prepare ferrous sulfate solution and phosphorus source solution. Under the protection of inert gas, add ferrous sulfate solution and phosphorus source solution to surfactant base solution at the same feeding time. Stir and add alkali solution dropwise until the pH of the reaction system is between 2.5 and 4.0. Stir to carry out the first stage of reaction, filter and obtain filter cake.

[0028] 2) After the filter cake is slurried again according to the original solid content, a reaction solution is obtained. Under the protection of inert gas, the temperature is raised and alkali solution is added dropwise to adjust the pH value of the solution to between 4.5 and 6.5. The second stage of reaction is carried out by stirring, followed by filtration, washing, and drying to obtain ferrous phosphate octahydrate.

[0029] Advantages of this invention:

[0030] 1) The method for preparing ferrous phosphate octahydrate provided in this application employs a staged temperature-controlled reaction and staged filtration at low pH. First, crystals are nucleated and stabilized at low temperature, and then crystal growth is promoted at high temperature, which shortens the precipitation reaction time and makes the crystal growth more uniform. At the same time, precipitate is precipitated at a lower pH. After the reaction is complete, the slurry is filtered for the first time to remove impurities such as Mg and Mn that are easily precipitated in the system at high pH. Then, the filter cake is slurried again and the pH is adjusted to a higher point to allow the crystals to grow further, thereby obtaining ferrous phosphate octahydrate with low impurity content and stable morphology.

[0031] 2) The method for preparing ferrous phosphate octahydrate provided in this application first adds a surfactant to the reaction substrate to promote uniform mixing of raw materials, and then nucleates and grows in the solvent system of the surfactant, thereby further controlling the uniformity of morphology and obtaining a morphology-controllable ferrous phosphate octahydrate product.

[0032] 3) The method for preparing ferrous phosphate octahydrate provided in this application involves introducing an inert gas into the co-precipitation reaction vessel during the reaction process for protection, ensuring that the vessel maintains a slightly positive pressure environment, reducing the oxygen content in the reaction environment, reducing the oxidation of ferrous iron during the reaction process, and helping to improve the purity of the product. Attached Figure Description

[0033] Figure 1 This is a 10 μm SEM image of ferrous phosphate octahydrate obtained in Example 1 of the present invention;

[0034] Figure 2 This is a 5μm SEM image of ferrous phosphate octahydrate obtained in Example 1 of the present invention;

[0035] Figure 3 The image shows the XRD pattern of ferrous phosphate octahydrate obtained in Example 1 of this invention. Detailed Implementation

[0036] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0037] Example 1:

[0038] 1) Prepare Fe using industrial-grade (purity above 90%) ferrous sulfate. 2+ A ferrous sulfate solution with an ion concentration of 4.5% was prepared using industrial-grade (purity above 98%) monoammonium phosphate to form a phosphorus source solution. The molar ratio of Fe to P in the iron source and phosphorus source was 1:1. The purity of ferrous sulfate and monoammonium phosphate in the following examples is the same, and will not be repeated here.

[0039] 2) Prepare a surfactant base solution with a content of 1‰ using polyethylene glycol;

[0040] 3) At 10℃, ferrous sulfate solution and phosphorus source solution are added to the reaction base solution at the same feeding time (i.e., 1 kg of ferrous sulfate solution and 0.5 kg of phosphorus source solution are pumped into the reaction base solution within 10 min, with the corresponding pumping speed of ferrous sulfate solution being 6 kg / h and the pumping speed of phosphorus source solution being 3 kg / h). The feeding time is controlled at 10 min, and ammonia water is added dropwise at the same time to control the pH of the reaction solution between 2.5 and 3.0. After the feeding is completed, the mixture is stirred and reacted for 20 min in this state before being filtered.

[0041] 4) After re-slurrying the filter cake with deionized water according to the original solid content, heat the solution system. After the temperature rises to 40℃, continue to add alkali solution to adjust the pH value of the solution to between 4.5 and 5.0. After the pH value stabilizes, continue to stir the reaction for 20 minutes.

[0042] 5) The reaction slurry is filtered, washed, and then dried to obtain ferrous phosphate octahydrate;

[0043] 6) The co-precipitation reaction vessel must be protected with nitrogen gas during the reaction process.

[0044] The same feeding time in 3) means that the pump starts and stops feeding at the same time.

[0045] The solid content in 4) refers to the solid content of the slurry before filtration.

[0046] The alkaline solution in 4) is ammonia.

[0047] Example 2:

[0048] 1) Prepare Fe using industrial-grade ferrous sulfate 2+ The solution is a 5.0% ferrous sulfate solution. The phosphorus source solution is prepared with industrial-grade monoammonium phosphate. The molar ratio of Fe to P in the iron source and phosphorus source is 1.25:1.

[0049] 2) Prepare a surfactant base solution with a concentration of 1‰ using hexadecyltrimethylammonium bromide;

[0050] 3) At 20℃, ferrous sulfate solution and phosphorus source solution are added to the reaction base solution at the same time, with the addition time controlled at 20 min. At the same time, ammonia water is added dropwise to control the pH of the solution between 3.0 and 3.5. After the addition is completed, the mixture is stirred and reacted for 30 min in this state before being filtered.

[0051] 4) After re-slurrying the filter cake with deionized water according to the original solid content, heat the solution system. After the temperature rises to 45℃, continue to add alkali solution to adjust the pH value of the solution to 5.0-5.5. After the pH value stabilizes, continue to stir the reaction for 30 minutes.

[0052] 5) The reaction slurry is filtered, washed, and then dried to obtain ferrous phosphate octahydrate;

[0053] 6) The co-precipitation reaction vessel must be protected with nitrogen gas during the reaction process.

[0054] Example 3:

[0055] 1) Prepare Fe using industrial-grade ferrous sulfate 2+ The 5.5% ferrous sulfate solution was prepared with industrial-grade monoammonium phosphate to form a phosphorus source solution, with the Fe and P molar ratio of the iron source to phosphoric acid being 1.5:1.

[0056] 2) Prepare a surfactant base solution with a content of 1‰ using polyvinylpyrrolidone;

[0057] 3) At 20℃, ferrous sulfate solution and phosphorus source solution are added to the reaction base solution at the same time, with the addition time controlled at 20 min. At the same time, ammonia water is added dropwise to control the pH of the solution between 3.0 and 3.5. After the addition is completed, the mixture is stirred and reacted for 10 min in this state before being filtered.

[0058] 4) After re-slurrying the filter cake with deionized water according to the original solid content, heat the solution system. After the temperature rises to 50℃, continue to add alkali solution to adjust the pH value of the solution to between 5.0 and 5.5. After the pH value stabilizes, continue to stir the reaction for 20 minutes.

[0059] 5) The reaction slurry is filtered, washed, and then dried to obtain ferrous phosphate octahydrate;

[0060] 6) The coprecipitation reaction vessel must be protected with an inert gas (argon) during the reaction process.

[0061] Example 4:

[0062] 1) Prepare Fe using industrial-grade ferrous sulfate 2+ The ferrous sulfate solution is 6.0%. The phosphorus source solution is prepared with industrial grade monoammonium phosphate. The molar ratio of Fe to P in the iron source and phosphorus source is 1.5:1.

[0063] 2) Prepare a surfactant base solution with a concentration of 1‰ using polyacrylamide;

[0064] 3) At 30℃, ferrous sulfate solution and phosphorus source solution are added to the reaction base solution at the same time, with the addition time controlled at 30 min. At the same time, ammonia water is added dropwise to control the pH of the solution between 3.5 and 4.0. After the addition is completed, the mixture is stirred and reacted for 10 min in this state before being filtered.

[0065] 4) After re-slurrying the filter cake with deionized water according to the original solid content, heat the solution system. After the temperature rises to 55℃, continue to add alkali solution to adjust the pH value of the solution to between 5.5 and 6.0. After the pH value stabilizes, continue to stir the reaction for 20 minutes.

[0066] 5) The reaction slurry is filtered, washed, and then dried to obtain ferrous phosphate octahydrate;

[0067] 6) The coprecipitation reaction vessel must be protected with an inert gas (argon) during the reaction process.

[0068] Example 5:

[0069] The difference from Example 1 is that a surfactant base solution with a content of 2‰ was prepared using polyethylene glycol, while the other steps are the same.

[0070] Example 6:

[0071] The difference from Example 1 is that a surfactant base solution with a content of 0.5‰ was prepared using polyethylene glycol, while the other steps are the same.

[0072] Comparative example:

[0073] 1) Prepare Fe using industrial-grade ferrous sulfate 2+The 5.0% ferrous sulfate solution was prepared with industrial-grade monoammonium phosphate to form a phosphorus source solution, with the molar ratio of Fe to P in the iron source and phosphorus source being 1.5:1.

[0074] 2) Prepare a surfactant base solution with a concentration of 1‰ using polyacrylamide;

[0075] 3) At 40℃, ferrous sulfate solution and phosphorus source solution are added to the reaction base solution at the same time, with the addition time controlled at 10 min. At the same time, ammonia water is added dropwise to control the pH of the solution between 5.0 and 5.5. After the addition is completed, the mixture is stirred and reacted for 20 min in this state. Then, it is filtered, washed, and dried to obtain ferrous phosphate octahydrate.

[0076] 4) The co-precipitation reaction vessel needs to be protected with nitrogen gas during the reaction process.

[0077] The content of impurity elements in the products obtained in each embodiment and comparative example was detected using the analytical methods for the elements to be tested in the industry standard HG / T 4701-2021 "Iron Phosphate for Batteries". The results are shown in Table 1 below:

[0078] Table 1

[0079]

[0080] As can be seen from the table above, this method can effectively remove various impurities contained in the product by adopting a two-stage reaction. The comparative example without the two-stage reaction cannot effectively remove Al, Ca, and Na impurities. The content of Ni, Co, Mn, and Za impurities is more than twice that of the product obtained by the method provided in this application. The content of Mg impurities in the product obtained by the comparative example is 100 times that of the method provided in this application.

[0081] The physical properties of the products obtained in each embodiment and comparative example were tested. The testing methods were the corresponding methods in the industry standard HG / T 4701-2021 "Iron Phosphate for Batteries" to analyze Fe, P and particle size. The crystallinity value was measured by X-ray diffraction, and the specific testing procedure was the same as the existing method. The results are shown in Table 2 below:

[0082] Table 2

[0083]

[0084] As can be seen from the table above, the contents of iron and phosphorus are similar, with Fe... 2+ / Fe 3+ The ratio is better than that obtained in the comparative example, and the particle size is finer and the crystallinity is higher. A partial SEM image of the product obtained in Example 1 is shown below. Figures 1-2 As shown, the product exhibits a regular crystal structure, and the surface of the crystal structure is regular after magnification, indicating that the obtained product is ferrous phosphate octahydrate. From... Figure 3The XRD pattern of the synthesized product is completely consistent with the standard card of ferrous phosphate octahydrate, and the diffraction peaks of the pattern are sharp, indicating that the product has good crystal form and high crystallinity.

[0085] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing ferrous phosphate octahydrate, characterized in that, Includes the following steps: 1) Prepare ferrous sulfate solution and phosphorus source solution. Under inert gas protection, simultaneously add the ferrous sulfate solution and phosphorus source solution to the surfactant base solution at the same feeding time. Stir and add alkali solution dropwise until the pH of the reaction system is between 2.5 and 4.

0. Then stir to carry out the first stage reaction and filter to obtain filter cake. The solute in the surfactant base solution is at least one of polyethylene glycol, hexadecyltrimethylammonium bromide and polyvinylpyrrolidone. The mass concentration of the solute in the surfactant base solution is 0.5‰ to 2‰. The temperature at which the ferrous sulfate solution and phosphorus source solution are added to the surfactant base solution is 10 to 30°C, and the mixing time is 10 to 30 min. The temperature of the first stage reaction is 10 to 30°C, and the reaction is stirred for 10 to 30 min. 2) After the filter cake is slurried again according to the original solid content, a reaction solution is obtained. Under the protection of inert gas, the temperature is raised and alkali solution is added dropwise to adjust the pH value of the solution to between 4.5 and 6.

5. The second stage reaction is carried out by stirring, filtration, washing and drying to obtain ferrous phosphate octahydrate. The temperature of the second stage reaction is 40-55℃ and the reaction is stirred for 20-30 minutes.

2. The method for preparing ferrous phosphate octahydrate as described in claim 1, characterized in that: The ferrous sulfate solution prepared in step 1) contains Fe 2+ The ion mass concentration is 4.5%–7.0%.

3. The method for preparing ferrous phosphate octahydrate as described in claim 1, characterized in that: The phosphorus source solution in 1) is prepared with industrial-grade monoammonium phosphate; the molar ratio of iron to phosphorus is 1 to 1.5:1.

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