Preparation method of iron phosphate and application thereof

Abstract

This invention discloses a method for preparing iron phosphate and its application, belonging to the field of new energy battery materials technology. The method for preparing iron phosphate provided by this invention includes the following steps: S1. Acid leaching of waste lithium iron phosphate to obtain a leachate; S2. Mixing the leachate with citric acid and then carrying out a hydrothermal reaction; the heating mechanism of the hydrothermal reaction includes a first holding platform at a temperature of 60-70℃ for a holding time > 0.5h, a second holding platform at a temperature of 90-100℃ for a holding time > 0.5h, and a third holding platform at a temperature of 105-160℃, carried out sequentially. The preparation method provided by this invention can produce near-spherical iron phosphate with high uniformity, good dispersibility, and low impurity content, and can utilize waste lithium iron phosphate as raw material. This invention also provides applications of the above preparation method.

Description

Technical Field

[0001] This invention relates to the field of new energy battery materials technology, and in particular to a method for preparing iron phosphate and its application. Background Technology

[0002] In recent years, the new energy industry has developed rapidly. Lithium-ion batteries, due to their high specific energy, high power density, and long lifespan, have been actively and widely developed globally and are mainly used in transportation vehicles such as electric vehicles and electric bicycles, and can also be used as energy storage materials. Based on the cathode material, lithium-ion batteries can be classified into lithium iron phosphate batteries, lithium cobalt oxide batteries, and nickel-cobalt-manganese oxide batteries. Considering cost-effectiveness, lithium iron phosphate batteries have significant advantages and competitiveness. Furthermore, lithium iron phosphate cathode materials also have advantages such as long cycle life, high safety performance, stable structure, abundant raw material sources, and low cost. Most importantly, it has a theoretical capacity of 170 mAh / g and a stable discharge platform of 3.5V. Its capacity and energy density are not significantly inferior to other cathode materials. These advantages make lithium iron phosphate one of the most ideal cathode materials for power batteries; that is, lithium iron phosphate batteries are currently considered a relatively ideal power battery.

[0003] The carbothermal reduction method is currently a commercially available method for preparing lithium iron phosphate (LFP). Specifically, it involves mixing iron phosphate, a reducing carbon source, and a lithium source, followed by heat treatment. This method utilizes iron phosphate as a precursor, which offers advantages such as low cost and environmental safety. However, it also places high demands on the quality of the iron phosphate precursor. Specifically, because iron phosphate and LFP are structurally similar, the morphology and structure of LFP prepared using iron phosphate as a precursor typically remain largely unchanged. The purity, morphology, particle size, and structure of the iron phosphate significantly influence the electrochemical performance of LFP materials. Therefore, researching and preparing iron phosphate precursors with good morphology and uniform particle size is crucial.

[0004] The preparation of iron phosphate for batteries requires precise control of factors such as the iron-to-phosphorus ratio, particle size, and water of crystallization content. There are numerous methods for preparing iron phosphate, including co-precipitation, hydrothermal synthesis, solid-state synthesis, and sol-gel methods. Currently, the mainstream method for preparing iron phosphate is co-precipitation. In co-precipitation, iron and phosphorus sources are prepared into solutions, where the phosphorus source is usually phosphoric acid or phosphate, and the iron source is usually ferrous iron (Fe2+). Oxidizing agents such as hydrogen peroxide are typically added to oxidize the ferrous iron to ferric iron (Fe3+). This method has advantages such as a short process and low energy consumption. However, this method usually requires the addition of alkali to adjust the pH to precipitate the iron phosphate in the system. This process can easily cause localized uneven mixing, leading to inconsistent product size and potentially residual SO4. 2- and NH4 +Impurity ions, such as those present in lithium iron phosphate, are difficult to remove through washing during subsequent processing and are inherited by the lithium iron phosphate. The presence of impurities in lithium iron phosphate severely affects its electrochemical performance. Solid-state and sol-gel methods also suffer from the problem of high impurity levels. While the hydrothermal method can produce lithium iron phosphate with fewer impurities, it requires a multi-stage impurity removal process before the reaction, meaning the raw materials used must be free of impurities.

[0005] Therefore, the iron phosphate prepared using related technologies has a high impurity content or involves very complex processes. Neither is suitable for processes that use waste lithium iron phosphate powder as raw material to prepare iron phosphate. Summary of the Invention

[0006] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a method for preparing ferric phosphate, which can effectively reduce the impurity content in the obtained ferric phosphate and improve the morphological uniformity, crystallinity and tap density of the obtained ferric phosphate.

[0007] The present invention also provides applications of the above preparation method.

[0008] According to an embodiment of a first aspect of the present invention, a method for preparing ferric phosphate is provided, the method comprising the following steps:

[0009] S1. Acid leaching of waste lithium iron phosphate yields leachate;

[0010] S2. The leachate and citric acid are mixed and then subjected to a hydrothermal reaction;

[0011] The heating mechanism of the hydrothermal reaction includes a first heat preservation platform with a temperature of 60-70°C and a heat preservation time of >0.5h, a second heat preservation platform with a temperature of 90-100°C and a heat preservation time of >0.5h, and a third heat preservation platform with a temperature of 105-160°C, which are carried out sequentially.

[0012] The preparation method according to embodiments of the present invention has at least the following beneficial effects:

[0013] Waste lithium iron phosphate contains a variety of impurities, such as iron contamination from the production line, aluminum and copper contamination from the application environment, and calcium and magnesium contamination from industrial water. In traditional technologies, if waste lithium iron phosphate is used as a raw material to produce iron phosphate, it usually requires a strict impurity removal process.

[0014] The preparation method provided by this invention is an optimized hydrothermal method. Specifically, in a closed environment, citric acid is added as a dispersant and morphology control agent, while the heating mechanism of the hydrothermal reaction is controlled. This promotes the transformation of the reaction precipitate from amorphous to highly crystalline particles, allowing the use of waste lithium iron phosphate as a raw material. The resulting iron phosphate exhibits high sphericity (quasi-spherical morphology), high particle size uniformity, good dispersibility, and low impurity content. Throughout the process, the amorphous precipitate, due to its loose structure and large specific surface area, easily adsorbs impurities. During the transformation into a highly crystalline precipitate, the precipitate particles become more compact, the specific surface area decreases, and the adsorbed impurities are expelled.

[0015] In the preparation method provided by this invention, citric acid is selected as a morphology control agent. Compared with citrate, it does not disrupt the pH of the hydrothermal reaction system, and the hydrothermal reaction needs to be carried out at a lower pH to further reduce impurities in the obtained ferric phosphate. In other words, by selecting the type of morphology control agent, this invention avoids the introduction of other reagents and reduces impurities in the obtained product.

[0016] The preparation method provided by this invention can produce ferric phosphate under low acidity conditions without the need to add additional acids or bases, thus saving reagent costs.

[0017] The preparation method provided by this invention offers a green recycling approach for the reuse of waste lithium iron phosphate powder; it also reduces the cost of solid waste treatment, turning waste into treasure and promoting resource recycling and reuse.

[0018] According to some embodiments of the present invention, in step S1, the liquid phase used for acid leaching is a mixture of sulfuric acid and hydrogen peroxide. The concentration of the sulfuric acid is 5.5–6.5 mol / L, specifically about 6 mol / L; the concentration of the hydrogen peroxide is 28–32 wt%, specifically about 30 wt%.

[0019] More specifically, in the liquid phase used for acid leaching, the volume ratio of sulfuric acid to hydrogen peroxide is 15–20:1. For example, it can be approximately 16:1, 17:1, 18:1, or approximately 19:1.

[0020] According to some embodiments of the present invention, in step S1, the solid-liquid mass ratio of the acid leaching is 1:10 to 20. For example, it can be about 1:12, 1:14, 1:15, 1:16 or about 1:18.

[0021] According to some embodiments of the present invention, in step S1, the acid leaching temperature is 65–75°C. For example, it can be approximately 70°C.

[0022] According to some embodiments of the present invention, in step S1, the acid leaching time is 1.5 to 2.5 hours. For example, it can be about 2 hours.

[0023] According to some embodiments of the present invention, in step S1, the heating method of acid leaching includes water bath heating.

[0024] According to some embodiments of the present invention, in step S1, the acid leaching method includes reflux leaching.

[0025] According to some embodiments of the present invention, in step S1, the leachate includes soluble components at the following concentrations:

[0026] Li 3-5 g / L; for example, it could be approximately 4 g / L.

[0027] Fe 65~70g / L; for example, it could be approximately 68g / L;

[0028] P 36~41g / L; for example, it could be about 38g / L or about 40g / L;

[0029] Al 0.2~2g / L; for example, it can be about 0.5g / L, 1.0g / L or about 1.5g / L;

[0030] S 47~50g / L; for example, it could be about 48g / L or about 49g / L;

[0031] Ca 0.15~0.18g / L; for example, it could be about 0.16g / L or about 0.17g / L;

[0032] Na 0.01~0.03g / L; for example, it can be about 0.01g / L.

[0033] According to some embodiments of the present invention, in step S2, the mass of the citric acid is 1 to 10% of the theoretically obtainable mass of ferric phosphate by the preparation method. Specifically, it can be about 7%, 8%, or about 9%.

[0034] According to some embodiments of the present invention, in step S2, the mass of citric acid is 1-5% of the theoretically obtainable mass of ferric phosphate by the preparation method. Specifically, it can be about 1.5%, 2%, 2.5%, 3%, or about 4%.

[0035] According to some embodiments of the present invention, in step S2, the pH of the mixture of the leachate and citric acid is 0.8 to 1.1. Specifically, it can be about 0.9 or about 1.0. Under these pH conditions, the probability of impurity ions, including lithium ions, precipitating with ferric phosphate is very small, meaning the obtained product is relatively pure ferric phosphate.

[0036] According to some embodiments of the present invention, in step S2, the constant temperature of the first insulation platform is 62-68°C. For example, it can be approximately 65°C.

[0037] According to some embodiments of the present invention, in step S2, the temperature holding time of the first insulation platform is 1 to 2 hours. For example, it can be about 1.5 hours.

[0038] According to some embodiments of the present invention, the constant temperature of the second insulation platform is 92-98°C. For example, it can be approximately 95°C.

[0039] According to some embodiments of the present invention, in step S2, the temperature holding time of the second heat preservation platform is 1 to 2 hours. For example, it can be about 1.5 hours.

[0040] According to some embodiments of the present invention, in step S2, the constant temperature of the third insulation platform is 110-160°C. For example, it can be approximately 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, or approximately 155°C.

[0041] According to some embodiments of the present invention, in step S2, the temperature-maintaining time of the third insulation platform is 1 to 5 hours. For example, it can be about 2 hours or about 3 hours.

[0042] According to some embodiments of the present invention, in step S2, the heating rate of the hydrothermal reaction is 10–20 °C / h. Specifically, it can be approximately 12 °C / h, 15 °C / h, or approximately 18 °C / h. In actual production, this heating rate is limited to the heating rate set by the instrument, and there will be a slight difference between this rate and the actual heating rate of the hydrothermal reaction system. However, since the heating rate is relatively slow, this difference is negligible.

[0043] According to some embodiments of the present invention, in step S2, the hydrothermal reaction is carried out under the action of a mass transfer assisting means. Depending on the choice of instrument, the mass transfer assisting means includes stirring or rotation of the reaction vessel.

[0044] The stirring speed and / or the rotation speed of the reaction vessel are 10–30 rpm; specifically, it can be about 15 rpm, 20 rpm, or about 25 rpm. This can increase the mass transfer rate, thereby improving the uniformity of the hydrothermal reaction and ultimately improving the particle size uniformity of the obtained ferric phosphate.

[0045] According to some embodiments of the present invention, step S2 further includes solid-liquid separation, washing, and drying of the obtained solid product after the hydrothermal reaction. The solid-liquid separation method includes at least one of filtration, natural sedimentation, and centrifugation. The washing method is water washing, and the endpoint of the water washing is that the conductivity of the washing liquid is ≤500 μS / cm. The drying temperature is 90–120°C; specifically, it can be about 100°C or about 110°C.

[0046] This yields hydrated ferric phosphate, primarily ferric phosphate dihydrate.

[0047] According to some embodiments of the present invention, the preparation method further includes the following steps performed after step S2:

[0048] S3. The solid product obtained from calcination step S2.

[0049] Therefore, the hydrated ferric phosphate obtained in step S2 can be converted into anhydrous ferric phosphate.

[0050] According to some embodiments of the present invention, in step S3, the calcination temperature is 600°C to 750°C. For example, it can be about 650°C or about 700°C.

[0051] According to some embodiments of the present invention, in step S3, the heating rate of the calcination is 5 to 10 °C / min. For example, it can be about 8 °C / min.

[0052] According to some embodiments of the present invention, in step S3, the calcination atmosphere is an air atmosphere.

[0053] According to some embodiments of the present invention, in step S3, the calcination time is 2 to 4 hours. For example, it can be about 3 hours.

[0054] According to an embodiment of the second aspect of the present invention, an application of the preparation method described herein is provided in the preparation of lithium iron phosphate.

[0055] Since the application employs all the technical solutions of the preparation methods described in the above embodiments, it possesses at least all the beneficial effects brought about by the technical solutions of the above embodiments. Furthermore, since lithium iron phosphate inherits some of the properties of the precursor iron phosphate, the resulting lithium iron phosphate exhibits high sphericity, high uniformity of particle size distribution, good dispersibility, and low impurity content.

[0056] According to some embodiments of the present invention, the preparation of lithium iron phosphate includes mixing the iron phosphate obtained by the preparation method with a lithium source and a reducing carbon source, and then calcining it under conditions that isolate water and oxygen.

[0057] Unless otherwise specified, the term "about" in this invention actually means that the error is allowed to be within ±2%, for example, about 100 is actually 100 ± 2% × 100.

[0058] Unless otherwise specified, "between" in this invention includes the number itself, for example, "between 2 and 3" includes the endpoint values ​​2 and 3.

[0059] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0060] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0061] Figure 1 This is a SEM image of hydrated iron phosphate obtained in step S2 of Example 1 of the present invention;

[0062] Figure 2 This is the XRD pattern of iron phosphate obtained in Example 1 of the present invention.

[0063] Figure 3 This is a SEM image of the hydrated iron phosphate obtained in step S2 of Comparative Example 2 of this invention. Detailed Implementation

[0064] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0065] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0066] Example 1

[0067] This example demonstrates the preparation of iron phosphate, with the following specific steps:

[0068] S1. A 6 mol / L sulfuric acid aqueous solution and a 30 wt% hydrogen peroxide solution were mixed at a volume ratio of 18:1. The resulting mixture was then mixed with waste lithium iron phosphate at a solid-liquid mass ratio of 1:15 for acid leaching. The acid leaching temperature was 70℃, and the heat preservation method was water bath heating. The entire mixture was refluxed and acid-leached for 2 hours. The resulting leachate [Li 4 g / L, Fe 68 g / L, P 39 g / L, Al 1.2 g / L, S 49 g / L, Ca 0.16 g / L, Na 0.02 g / L, pH approximately 1] was determined. The iron and phosphorus contents were measured, and the theoretically obtainable mass of iron phosphate was calculated.

[0069] S2. After adding the acid leaching solution to the reactor, add citric acid at 2% of the theoretically produced iron phosphate mass and mix thoroughly (until the citric acid dissolves). Then, conduct a hydrothermal reaction in a rotary reactor at a rotation speed of 20 rpm. The specific heating mechanism for the hydrothermal reaction is as follows: first, raise the temperature to 60°C and hold for 1 hour; then raise the temperature to 90°C and hold for 1 hour; finally, raise the temperature to 120°C and hold for 2 hours. The heating rate of the hydrothermal reaction is approximately 15°C / hour.

[0070] It should be noted that the total amount of the mixture should be ≤ 2 / 3 of the reactor volume to avoid the mixture boiling violently and causing injury.

[0071] After the hydrothermal reaction is completed, the process is carried out in sequence by filtration, washing with water and drying; the endpoint of water washing is when the conductivity of the washing liquid is ≤500μS / cm; the drying temperature is 100℃.

[0072] This step produces hydrated ferric phosphate, mainly ferric phosphate dihydrate.

[0073] S3. Calcination of the hydrated iron phosphate obtained in step S2. The specific calcination process is as follows: in an air atmosphere, the temperature is increased to 700℃ at a heating rate of 8℃ / min, and the holding time is 3h.

[0074] Example 2

[0075] This example prepares an iron phosphate, which differs from Example 1 in that:

[0076] In step S2, the amount of citric acid added is 1%.

[0077] Example 3

[0078] This example prepares an iron phosphate, which differs from Example 1 in that:

[0079] In step S2, the amount of citric acid added is 5%.

[0080] Example 4

[0081] This example prepares an iron phosphate, which differs from Example 1 in that:

[0082] In step S2, the heating process of the hydrothermal reaction is as follows: first, the temperature is raised to 60℃ and held for 1 hour, then the temperature is raised to 90℃ and held for 1 hour, and finally the temperature is raised to 140℃ and held for 2 hours.

[0083] Example 5

[0084] This example prepares an iron phosphate, which differs from Example 1 in that:

[0085] In step S2, the heating process of the hydrothermal reaction is as follows: first, the temperature is raised to 60℃ and held for 1 hour, then the temperature is raised to 90℃ and held for 1 hour, and finally the temperature is raised to 160℃ and held for 2 hours.

[0086] Example 6

[0087] This example prepares an iron phosphate, which differs from Example 1 in that:

[0088] In step S2, the amount of citric acid added is 8%.

[0089] Example 7

[0090] This example prepares an iron phosphate, which differs from Example 1 in that:

[0091] In step S2, the heating process of the hydrothermal reaction is as follows: first, the temperature is raised to 60℃ and held for 1 hour, then the temperature is raised to 90℃ and held for 2 hours, and finally the temperature is raised to 160℃ and held for 2 hours.

[0092] Comparative Example 1

[0093] This example prepares an iron phosphate, which differs from Example 1 in that:

[0094] In step S2, the heating process of the hydrothermal reaction is to raise the temperature to 120°C and keep it at that temperature for 4 hours.

[0095] Comparative Example 2

[0096] This example prepares an iron phosphate, which differs from Example 1 in that:

[0097] In step S2, citric acid is not added.

[0098] Comparative Example 3

[0099] This example prepares an iron phosphate, which differs from Example 1 in that:

[0100] (1) In step S2, citric acid is not added.

[0101] (2) In step S2, the heating process of the hydrothermal reaction is to raise the temperature to 120°C and keep it at that temperature for 4 hours.

[0102] Comparative Example 4

[0103] This example prepares an iron phosphate, which differs from Example 1 in that:

[0104] In step S2, the heating process of the hydrothermal reaction is as follows: heat up to 60℃ and hold for 1 hour, then heat up to 90℃ and hold for 1 hour, and finally heat up to 100℃ and hold for 2 hours.

[0105] Comparative Example 5

[0106] This example prepares an iron phosphate, which differs from Example 1 in that:

[0107] In step S2, the heating process of the hydrothermal reaction is as follows: first, the temperature is raised to 60℃ and held for 0.5h, then the temperature is raised to 90℃ and held for 0.5h, and finally the temperature is raised to 120℃ and held for 2h.

[0108] Test case

[0109] In this example, the morphology of the ferric phosphate dihydrate obtained in step S2 of the examples and comparative examples was tested using scanning electron microscopy (SEM). The results showed that the ferric phosphate dihydrate obtained in the examples had a near-spherical morphology, and within the SEM field of view, the particle size uniformity was high, the particle dispersion was good, and there was no obvious agglomeration; the morphology of the ferric phosphate dihydrate obtained in all examples was similar. In contrast, no obvious particulate matter was observed in the comparative examples, and the ferric phosphate dihydrate tended to agglomerate into large particles, i.e., the particle size uniformity and dispersion were extremely poor; the morphology of all comparative examples was similar, with the only difference being the degree of agglomeration. The SEM morphology of the ferric phosphate dihydrate obtained in Example 1 is shown below. Figure 1 As shown in the figure, the SEM morphology of the iron phosphate dihydrate obtained in Comparative Example 2 is as follows. Figure 3 As shown.

[0110] The second aspect of this example tested the XRD pattern of the ferric phosphate obtained in the embodiment. The results showed that the characteristic peaks of the XRD pattern of the ferric phosphate obtained in the embodiment matched the characteristic peak positions of the standard ferric phosphate pattern. This indicates that the embodiments of the present invention have indeed produced ferric phosphate with good morphology, dispersibility, and crystallinity. The XRD pattern of the ferric phosphate obtained in Example 1 and its correspondence with the standard pattern are shown below. Figure 2 As shown.

[0111] In the third aspect of this example, ICP-OES was used to test the impurity content in the ferric phosphate obtained in the examples and comparative examples, specifically testing the contents of Al, Ca, Na, and S. The test results are shown in Table 1.

[0112] The fourth aspect of this example tested the tap density of the iron phosphate obtained in the examples and comparative examples. The test results are shown in Table 1.

[0113] Table 1 shows the impurity content and tap density of anhydrous ferric phosphate obtained in the examples and comparative examples.

[0114]

[0115] As shown in Table 1, the addition of citric acid and the staged heating reaction are beneficial to reducing impurities in the ferric phosphate product. The mechanism is that the addition of citric acid can complex with iron, and after the precipitation is formed, it can prevent impurities from accumulating in the precipitate. At the same time, during the staged heating process, the product generated at low temperature is mainly amorphous particles without complete crystal form. As the temperature rises, the amorphous particles transform into particles with higher crystallinity, which can remove some impurities and thus reduce the impurity content of the product. Furthermore, the addition of citric acid is beneficial to increasing the tap density of the product, which is related to the morphology and dispersibility of the product.

[0116] Specifically:

[0117] Comparing Examples 1-3 and Example 6, it can be seen that as the amount of citric acid added increases, the tap density of the obtained ferric phosphate shows a trend of first increasing and then stabilizing, with the inflection point appearing around 2%. However, excessive citric acid will increase reagent costs and subsequent water treatment costs. Therefore, in actual production, the amount of citric acid added can be considered to be ≤5%, and further, it can be between 1% and 2%.

[0118] Comparing Examples 1, 4-5, 7 and Comparative Example 5, it can be seen that within the scope provided by the present invention, changing the heating mechanism of the hydrothermal reaction has no significant impact on the performance of the obtained iron phosphate. However, if it is outside the scope required by the present invention, it will significantly reduce the tap density and increase the impurity content.

[0119] Comparing Example 1 and Comparative Examples 1-3, it can be seen that the addition of citric acid and the specific hydrothermal reaction heating mechanism both have the effect of reducing the impurity content in the obtained ferric phosphate and increasing the tap density; and there is a significant synergy between the two, neither of which can be omitted.

[0120] Comparing Example 1 and Comparative Example 4, it can be seen that the recrystallization process of ferric phosphate dihydrate requires a certain temperature drive. If the temperature is set below the range required by the present invention, ferric phosphate with good crystallinity cannot be formed, which will lead to an increase in impurity content and a decrease in tap density.

[0121] In summary, the iron phosphate prepared by this invention has many advantages, such as high sphericity, high particle size uniformity, good dispersibility, good crystallinity, and low impurity content. Since the lithium iron phosphate prepared from iron phosphate will inherit the morphology and impurities of iron phosphate to a certain extent, it is reasonable to expect that the resulting lithium iron phosphate will also have the above-mentioned advantages in morphology and purity. Furthermore, it is expected that the lithium iron phosphate prepared from the iron phosphate prepared by this invention will have good electrochemical performance and is expected to be widely used in power batteries, energy storage batteries, and 3C small household appliance batteries.

[0122] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A method for preparing ferric phosphate, characterized in that, The preparation method includes the following steps: S1. Acid leaching of waste lithium iron phosphate yields leachate; S2. The leachate and citric acid are mixed and then subjected to a hydrothermal reaction; in step S2, the mass of the citric acid is 1-10% of the theoretically obtainable mass of ferric phosphate by the preparation method. The heating mechanism of the hydrothermal reaction includes a first heat preservation platform with a temperature of 60~70℃ and a heat preservation time of 1~2h, a second heat preservation platform with a temperature of 90~100℃ and a heat preservation time of 1~2h, and a third heat preservation platform with a temperature of 105~160℃ and a heat preservation time of 1~5h, which are carried out sequentially.

2. The preparation method according to claim 1, characterized in that, The preparation method further includes the following steps performed after step S2: S3. The solid product obtained from calcination step S2.

3. The preparation method according to claim 1, characterized in that, In step S2, the mass of citric acid is 1-5% of the mass of iron phosphate theoretically obtainable by the preparation method.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step S1, the solid-liquid mass ratio of the acid leaching is 1:10~20; and / or, the acid leaching temperature is 65~75℃.

5. The preparation method according to any one of claims 1 to 3, characterized in that, In step S1, the liquid phase used for acid leaching is a mixture of sulfuric acid and hydrogen peroxide.

6. The preparation method according to claim 5, characterized in that, The concentration of the sulfuric acid is 5.5~6.5 mol / L, and the concentration of the hydrogen peroxide is 28~32 wt%.

7. The preparation method according to claim 6, characterized in that, The volume ratio of sulfuric acid to hydrogen peroxide is 15~20:

1.

8. The preparation method according to claim 2, characterized in that, In step S3, the calcination temperature is 600℃~750℃.

9. The application of the preparation method according to any one of claims 1 to 8 in the preparation of lithium iron phosphate.

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