CN118702079A - A method for preparing ferrous phosphate octahydrate - Google Patents

Abstract

The invention discloses a method for preparing ferrous phosphate octahydrate, which comprises the following steps: 1) preparing a ferrous sulfate solution and a phosphorus source solution, and adding the ferrous sulfate solution and the phosphorus source solution to a surfactant base solution under inert gas protection, stirring and dripping alkali solution until the pH value of the reaction system is between 2.5 and 4.0, stirring the first stage of reaction, and filtering to obtain a filter cake; 2) re-slurrying the filter cake according to the original solid content to obtain a reaction solution, heating the filter cake to 40 to 50 DEG C under inert gas protection, and then dripping alkali solution to adjust the pH value of the solution to between 4.5 and 6.0, stirring the second stage of reaction, filtering, washing, and drying to obtain ferrous phosphate octahydrate; the invention adopts stage temperature and segmented pH value to control the coprecipitation reaction, and performs solid-liquid separation in combination with filtering at a low pH value to ensure that the particle size, morphology and purity of the crystals are controllable, and the slurry is filtered for the first time after the first stage of reaction is completed to remove impurity elements such as Mg and Mn that are easy to precipitate in the system under high pH value.

Description

A method for preparing ferrous phosphate octahydrate

Technical Field

The invention relates to the technical field of ferrous phosphate, and in particular to a method for preparing ferrous phosphate octahydrate.

Background Art

In the past two years, the installed capacity of lithium iron phosphate power batteries has gradually exceeded that of ternary power batteries. The main production process of lithium iron phosphate is currently the carbon thermal reduction method. The precursor of the carbon thermal reduction method is mainly trivalent iron source iron phosphate, which has driven the vigorous development of iron phosphate precursors. The mainstream processes of iron phosphate include: sodium method, ammonium method, iron method and iron oxide red method.

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

In order to further reduce production costs and improve product quality, the iron process and iron oxide red process as new iron phosphate precursor processes have received more attention. As a new precursor, ferrous phosphate has low-cost synthetic raw materials and simple synthesis process, and can be called a candidate material for the new generation of lithium iron phosphate precursors.

Existing ferrous phosphate is mainly synthesized by coprecipitation. 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 and keep warm for precipitation, and then adds a sodium solution and liquid alkali to synthesize the obtained ferrous phosphate product. This method produces ferrous phosphate and reduces the formation of by-product salts; the use of sodium to synthesize ferrous phosphate is conducive to 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, but does not provide product purity test results and product yield data, and cannot produce high-purity ferrous phosphate octahydrate.

CN201911188406.6 discloses a method for preparing high-performance ferrous phosphate, which comprises preparing a mixed solution of an iron source and a metal ion additive: mixing ferrous sulfate with a concentration of 0.5 to 2 mol/L with a metal ion additive to form an iron source aqueous solution to improve the basic electrical properties of the LiFePO ₄ positive electrode material. The lithium iron phosphate synthesized from the obtained ferrous phosphate has a half-cell 0.2C first discharge capacity of 162 mAh/g, a full-cell 1C discharge capacity of 142 mAh/g, a 1C cycle 100-week capacity retention rate of 99.5%, and a pole piece compaction density of 2.54 g/cm ^3. However, high-purity ferrous phosphate octahydrate cannot be obtained.

Since ferrous phosphate has outstanding temperature and pH sensitivity, it is difficult to remove the Mn and Mg impurities remaining in the product. The ferrous ions in the product are easily oxidized, making it difficult to use various commonly used impurity removal processes, resulting in a high impurity content in the ferrous phosphate synthesized by the existing method, and due to the presence of impurities, the product has low crystallinity and poor quality stability, and it is difficult to synthesize an octahydrate ferrous phosphate product with high crystallinity, performance stability and low impurity content.

For example, the high-density 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 produce ferrous phosphate dihydrate material 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 162.3 mAh/g, it is still impossible to produce ferrous phosphate octahydrate with a purity that meets the subsequent production requirements, and the content and purity of ferrous phosphate octahydrate in the first slurry are not tested.

Summary of the invention

The invention provides a method for preparing ferrous phosphate octahydrate.

The scheme of the present invention is:

A method for preparing ferrous phosphate octahydrate comprises the following steps:

1) preparing ferrous sulfate solution and phosphorus source solution, adding the ferrous sulfate solution and phosphorus source solution to the surfactant base solution at the same time under the protection of inert gas, adding alkali solution dropwise with stirring until the pH of the reaction system is between 2.5 and 4.0, stirring for the first stage reaction, filtering with suction, and obtaining a filter cake;

2) The filter cake is re-slurried according to the original solid content to obtain a reaction solution, and the solution is heated under the protection of an inert gas, and then an alkali solution is added dropwise to adjust the pH value of the solution to between 4.5 and 6.5, and the second stage reaction is carried out by stirring, suction filtering, washing, and drying to obtain ferrous phosphate octahydrate.

The reaction process is divided into two stages by the above method. In the first stage, the crystals can be slowly nucleated and uniformly grown at low temperature and low pH. The first stage slurry with low pH value is filtered to remove some impurity ions that are difficult to remove. After the filter cake is re-slurried, the solution is heated to a temperature significantly higher than that of the first stage for reaction. At the same time, the pH value of the solution is regulated at a higher level to promote rapid growth of crystals, thereby achieving control of the particle size, morphology and purity of the obtained ferrous phosphate octahydrate. The inert gas used can be argon or nitrogen.

The ferrous sulfate solution used in the method can be prepared from industrial-grade ferrous sulfate, or from the titanium dioxide byproduct ferrous sulfate added with ammonia water to remove impurities, or from the material obtained by reacting sulfuric acid and iron powder. The method has a wide range of raw materials, and the obtained product has high purity and few impurities in ferrous phosphate octahydrate, which is conducive to reducing the production cost of enterprises.

As a preferred technical solution, the pH of the reaction system during the first stage reaction in 1) is one of 3.5 to 4.0, 3.0 to 3.5, and 2.5 to 3.0.

As a preferred technical solution, the pH of the reaction system during the concentrated second stage reaction in 2) is one of 6.0 to 6.5, 5.5 to 6.0, 5.0 to 5.5, and 4.5 to 5.0.

As a preferred technical solution, the Fe ²⁺ ion mass concentration in the ferrous sulfate solution prepared in 1) is 4.5-7.0%. Further preferably, the Fe ²⁺ ion mass concentration in the ferrous sulfate solution is one of 4.5%, 4.8%, 5.0%, 5.5%, and 6.0%.

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

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‰.

As a preferred technical solution, the temperature of the ferrous sulfate solution and the phosphorus source solution added to the surfactant base solution in 1) is 10-30°C, and the mixing time is 10-30 minutes. More preferably, the temperature of the ferrous sulfate solution and the phosphorus source solution added to the surfactant base solution is one of 10°C, 20°C, and 30°C. The mixing time is one of 30 minutes and 20 minutes.

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

As a preferred technical solution, the temperature of the second stage reaction in 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.

As a preferred technical solution, the ferrous sulfate solution is prepared with industrial-grade ferrous sulfate. This method can be prepared with industrial-grade raw materials, which can effectively reduce production costs, and the obtained product has good purity and can effectively meet various subsequent needs.

As a preferred technical solution, the phosphorus source solution in 1) is prepared from 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.

Due to the adoption of the above technical solution, a method for preparing ferrous phosphate octahydrate comprises the following steps:

1) preparing ferrous sulfate solution and phosphorus source solution, adding the ferrous sulfate solution and phosphorus source solution to the surfactant base solution at the same time under the protection of inert gas, adding alkali solution dropwise with stirring until the pH of the reaction system is between 2.5 and 4.0, stirring for the first stage reaction, filtering with suction, and obtaining a filter cake;

2) The filter cake is re-slurried according to the original solid content to obtain a reaction solution, and the solution is heated under the protection of an inert gas, and then an alkali solution is added dropwise to adjust the pH value of the solution to between 4.5 and 6.5, and the second stage reaction is carried out by stirring, suction filtering, washing, and drying to obtain ferrous phosphate octahydrate.

Advantages of the present invention:

1) The method for preparing ferrous phosphate octahydrate provided in the present application adopts a staged temperature control reaction, and cooperates with staged filtration at a low pH value, first nucleates and stabilizes the crystals at a low temperature, and then promotes crystal growth at a high temperature, shortens the precipitation reaction time, and makes the crystal growth more uniform; at the same time, precipitates at a lower pH, and after the reaction is complete, the slurry is filtered for the first time to remove impurity elements such as Mg and Mn that are easily precipitated in the system at a high pH value, and then the filter cake is slurried again, and the pH value is adjusted to a higher point to allow the crystal to grow further, thereby obtaining ferrous phosphate octahydrate with low impurity content and stable morphology.

2) The method for preparing ferrous phosphate octahydrate provided in the present application comprises first adding a surfactant to the reaction base liquid to promote uniform mixing of the raw materials, and performing nucleation and growth in the solvent system of the surfactant, thereby further controlling the uniformity of the morphology and obtaining a ferrous phosphate octahydrate product with controllable morphology.

3) In the method for preparing ferrous phosphate octahydrate provided in the present application, an inert gas is introduced into the coprecipitation reaction vessel for protection during the reaction process to ensure that a slightly positive pressure environment is maintained in the container, thereby reducing the oxygen content in the reaction environment and reducing the oxidation of divalent iron during the reaction, which helps to improve the purity of the product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a 10 μm SEM image of ferrous phosphate octahydrate obtained in Example 1 of the present invention;

FIG2 is a 5 μm SEM image of ferrous phosphate octahydrate obtained in Example 1 of the present invention;

FIG3 is an XRD diagram of ferrous phosphate octahydrate obtained in Example 1 of the present invention.

DETAILED DESCRIPTION

In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further described below in conjunction with specific embodiments.

Embodiment 1:

1) An industrial grade (purity of 90% or more) ferrous sulfate solution with an Fe2 ⁺ ion concentration of 4.5% was prepared, and an industrial grade (purity of 98% or more) monoammonium phosphate solution was prepared. The molar ratio of Fe to P in the iron source and the phosphorus source was 1:1. The purity of ferrous sulfate and monoammonium phosphate in the following examples is the same as this, and will not be repeated here.

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

3) At 10° C., the ferrous sulfate solution and the phosphorus source solution are added to the reaction base liquid at the same time according to 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 liquid within 10 minutes, and the corresponding ferrous sulfate solution pumping speed is 6 kg/h, and the phosphorus source solution pumping speed is 3 kg/h), the feeding time is controlled to be 10 minutes, 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 reaction is stirred in this state for 20 minutes and then filtered.

4) The filter cake was re-slurried with deionized water according to the original solid content, and the solution system was heated. After the temperature rose to 40° C., alkali solution was continued to be added dropwise to adjust the pH value of the solution to between 4.5 and 5.0. After the pH value stabilized, the reaction was continued with stirring for 20 minutes.

5) filtering the reaction slurry, washing it, and then drying it to obtain ferrous phosphate octahydrate;

6) During the reaction, the coprecipitation reaction vessel needs to be protected by nitrogen.

The same feeding time in 3) is to start pumping material at the same time and end feeding at the same time.

The original solid content in 4) is the solid content of the slurry before filtration.

The alkali solution in 4) is aqueous ammonia.

Embodiment 2:

1) preparing a ferrous sulfate solution with an Fe2 ⁺ content of 5.0% with industrial grade ferrous sulfate, and preparing a phosphorus source solution with industrial grade monoammonium phosphate, wherein the molar ratio of Fe to P in the iron source and the phosphorus source is 1.25:1;

2) preparing a surfactant base solution with a content of 1‰ using hexadecyltrimethylammonium bromide;

3) Add the ferrous sulfate solution and the phosphorus source solution to the reaction base solution at the same time at 20°C, and control the addition time to be 20 minutes. At the same time, add ammonia water dropwise to control the pH value of the solution to be between 3.0 and 3.5. After the addition is completed, stir the reaction in this state for 30 minutes and then filter.

4) The filter cake was re-slurried with deionized water according to the original solid content, and the solution system was heated. After the temperature rose to 45° C., alkali solution was continued to be added dropwise to adjust the pH value of the solution to 5.0-5.5. After the pH value stabilized, the stirring reaction was continued for 30 minutes.

5) filtering the reaction slurry, washing it, and then drying it to obtain ferrous phosphate octahydrate;

6) During the reaction, the coprecipitation reaction vessel needs to be protected by nitrogen.

Embodiment 3:

1) using industrial grade ferrous sulfate to prepare a ferrous sulfate solution with an Fe2 ⁺ content of 5.5%, and using industrial grade monoammonium phosphate to prepare a phosphorus source solution, the molar ratio of Fe to phosphoric acid being 1.5:1;

2) Prepare a surfactant base solution with a content of 1‰ using polyvinyl pyrrolidone;

3) Add the ferrous sulfate solution and the phosphorus source solution to the reaction base solution at the same time at 20°C, and control the addition time to be 20 minutes. At the same time, add ammonia water to control the pH value of the solution to be between 3.0 and 3.5. After the addition is completed, stir the reaction in this state for 10 minutes and then filter.

4) The filter cake was re-slurried with deionized water according to the original solid content, and the solution system was heated. After the temperature rose to 50° C., alkali solution was continued to be added dropwise to adjust the pH value of the solution to between 5.0 and 5.5. After the pH value stabilized, the stirring reaction was continued for 20 minutes.

5) filtering the reaction slurry, washing it, and then drying it to obtain ferrous phosphate octahydrate;

6) During the reaction process, the coprecipitation reaction vessel needs to be protected by inert gas (argon).

Embodiment 4:

1) preparing a ferrous sulfate solution with an Fe2 ⁺ content of 6.0% with industrial grade ferrous sulfate, and preparing a phosphorus source solution with industrial grade monoammonium phosphate, wherein the molar ratio of Fe and P in the iron source to the phosphorus source is 1.5:1;

2) Prepare a surfactant base solution with a content of 1‰ using polyacrylamide;

3) Add the ferrous sulfate solution and the phosphorus source solution to the reaction base solution at the same time at 30°C, and control the addition time to 30 minutes. At the same time, add ammonia water to control the solution pH to be between 3.5 and 4.0. After the addition is completed, stir the reaction in this state for 10 minutes and then filter.

4) The filter cake was re-slurried with deionized water according to the original solid content and the solution system was heated. After the temperature rose to 55° C., alkali solution was continued to be added dropwise to adjust the pH value of the solution to between 5.5 and 6.0. After the pH value stabilized, the stirring reaction was continued for 20 minutes.

5) filtering the reaction slurry, washing it, and then drying it to obtain ferrous phosphate octahydrate;

6) During the reaction process, the coprecipitation reaction vessel needs to be protected by inert gas (argon).

Embodiment 5:

The difference from Example 1 is that polyethylene glycol is used to prepare a surfactant base liquid with a content of 2‰, and the other steps are the same.

Embodiment 6:

The difference from Example 1 is that polyethylene glycol is used to prepare a surfactant base liquid with a content of 0.5‰, and the other steps are the same.

Comparative Example:

1) preparing a ferrous sulfate solution with an Fe2 ⁺ content of 5.0% with industrial grade ferrous sulfate, and preparing a phosphorus source solution with industrial grade monoammonium phosphate, wherein the molar ratio of Fe and P in the iron source to the phosphorus source is 1.5:1;

2) Prepare a surfactant base solution with a content of 1‰ using polyacrylamide;

3) Add the ferrous sulfate solution and the phosphorus source solution to the reaction base solution at the same time at 40° C., control the addition time to be 10 min, and simultaneously add ammonia water to control the solution pH to be between 5.0 and 5.5. After the addition is completed, stir the reaction in this state for 20 min, then filter, wash, and dry to obtain ferrous phosphate octahydrate.

4) The coprecipitation reaction vessel needs to be protected by nitrogen during the reaction.

The content of impurity elements in the products obtained in each embodiment and comparative example was detected by the analysis method of 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:

Table 1

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

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

Table 2

As can be seen from the above table, the content of iron and phosphorus elements is close, the ratio of Fe2 ⁺ /Fe3 ⁺ is better than the result obtained in the comparative example, and the particle size is finer and the crystallinity is higher. The local SEM images of the product obtained in Example 1 are shown in Figures 1 and 2. It can be seen that the product has a regular crystal structure, and the surface of the crystal structure is regular after the crystal structure is magnified, indicating that the obtained product is ferrous phosphate octahydrate. It can also be seen from Figure 3 that the XRD spectrum of the synthesized product is completely consistent with the standard card of ferrous phosphate octahydrate spectrum, and the diffraction peak of the spectrum is sharp, indicating that the product has a good crystal form and a high crystallinity.

The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims (8)

1. The preparation method of the ferrous phosphate octahydrate is characterized by comprising the following steps:

1) Preparing ferrous sulfate solution and phosphorus source solution, adding the ferrous sulfate solution and the phosphorus source solution into the surfactant base solution at the same time according to the same charging time under the protection of inert gas, stirring and dripping alkali solution until the pH value of a reaction system is between 2.5 and 4.0, stirring and carrying out first-stage reaction, and carrying out suction filtration to obtain a filter cake;

2) Re-pulping the filter cake according to the original solid content to obtain a reaction solution, heating the reaction solution under the protection of inert gas, then dripping alkali solution to adjust the pH value of the solution to be between 4.5 and 6.5, stirring the solution, and carrying out second-stage reaction, suction filtration, washing and drying to obtain ferrous phosphate octahydrate.

2. The method for preparing ferrous phosphate octahydrate according to claim 1, wherein the method comprises the following steps: the mass concentration of Fe ²⁺ ions in the ferrous sulfate solution prepared in the step 1) is 4.5-7.0%.

3. The method for preparing ferrous phosphate octahydrate according to claim 1, wherein the method comprises the following steps: the solute in the surfactant base solution is at least one of polyethylene glycol, cetyl trimethyl ammonium bromide and polyvinylpyrrolidone.

4. A method for preparing ferrous phosphate octahydrate according to claim 1 or claim 3, wherein: the solute mass concentration of the surfactant base solution is 0.5-2 per mill.

5. The method for preparing ferrous phosphate octahydrate according to claim 1, wherein the method comprises the following steps: the temperature of the ferrous sulfate solution and the phosphorus source solution added into the surfactant base solution in the step 1) is 10-30 ℃, and the mixing time is 10-30 min.

6. The method for preparing ferrous phosphate octahydrate according to claim 1, wherein the method comprises the following steps: the temperature of the first-stage reaction in the step 1) is 10-30 ℃, and the reaction is stirred for 10-30 min.

7. The method for preparing ferrous phosphate octahydrate according to claim 1, wherein the method comprises the following steps: the temperature of the second stage reaction in the step 2) is 40-55 ℃, and the stirring reaction is carried out for 20-30 min.

8. The method for preparing ferrous phosphate octahydrate according to claim 1, wherein the method comprises the following steps: the phosphorus source solution in the step 1) is prepared from industrial monoammonium phosphate; the molar ratio of the iron element to the phosphorus element is 1-1.5:1.