Al removal method of crude phosphoric acid, preparation method and application of dihydrate iron phosphate

Abstract

The application discloses a method for removing Al from crude phosphoric acid, a preparation method and application of iron phosphate dihydrate, and relates to the technical field of positive electrode materials. The method for removing Al from crude phosphoric acid comprises the following steps: diluting wet-process phosphoric acid with water to a P concentration of 30-60 g / L, heating, and adding iron hydroxide to adjust the pH to 1-1.5 to obtain a phosphoric acid adjusted solution; adding dehydrated insoluble phosphate as a precipitation promoter to the phosphoric acid adjusted solution to perform a precipitation promotion reaction, and then performing solid-liquid separation to obtain waste residue and a phosphoric acid impurity removal solution. The application can induce and promote the precipitation and separation of the remaining impurity ions from the solution. Meanwhile, the precipitation promoter used in the method does not introduce impurity elements into the system, and the loss of iron caused by the low iron concentration in the system is low, so that the method can remove aluminum while not causing new troubles. Through the method, the aluminum content in the phosphoric acid can be efficiently, simply and low-cost controlled, and the separation of phosphorus and aluminum is realized.

Description

Technical Field

[0001] This invention relates to the field of cathode material technology, and more specifically, to a method for removing Al from crude phosphoric acid, a method for preparing iron phosphate dihydrate, and their applications. Background Technology

[0002] my country's new energy industry is booming, and the demand for power batteries is increasing daily. Since its inception, lithium-ion batteries have received widespread attention due to their advantages such as high specific energy, long lifespan, and low pollution. Among them, lithium iron phosphate batteries have advantages such as high safety, low manufacturing cost, and good electrochemical performance, making them an important branch of lithium batteries. As a precursor material for lithium iron phosphate, the low-cost, high-quality synthesis and production process of iron phosphate is receiving increasing attention. One important method for preparing iron phosphate is the synthesis of iron phosphate dihydrate from phosphoric acid and iron salts, followed by further calcination to remove crystallization water.

[0003] Currently, there are two main methods for producing phosphoric acid: pyrometallurgical and wet processes. Pyrometallurgical phosphoric acid has high purity, but its production cost remains high; wet process phosphoric acid has lower cost, but it contains more impurities. As one of the more abundant impurities in wet process phosphoric acid, aluminum ions, like iron ions, can form insoluble precipitates with phosphate ions. Furthermore, aluminum phosphate and iron phosphate have similar solubility products, making them easily mixed into the product during synthesis and difficult to separate. Industrially, extraction and ammoniation methods are commonly used to remove impurities from wet process phosphoric acid, but these methods suffer from significant phosphorus loss and low utilization value of byproducts.

[0004] To date, most methods for precipitating impurities in phosphoric acid introduce other impurity ions, leading to reduced product quality and increased wastewater treatment pressure.

[0005] Ferric phosphate dihydrate is a precursor to ferric phosphate, and its impurity content significantly affects the quality of the ferric phosphate obtained from calcination, thereby impacting the performance of lithium iron phosphate. Therefore, controlling its impurity content is of paramount importance.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a method for removing Al from crude phosphoric acid, a method for preparing ferric phosphate dihydrate, and its application.

[0008] This invention is implemented as follows: In a first aspect, the present invention provides a method for removing Al from crude phosphoric acid, comprising: Wet-process phosphoric acid is diluted with water to a P concentration between 30-60 g / L to obtain a diluted phosphoric acid solution; The diluted phosphoric acid solution was heated and ferric hydroxide was added to adjust the pH to 1-1.5 to obtain the phosphoric acid-adjusted solution. Dehydrated, insoluble phosphate was used as a precipitant and added to the phosphoric acid-conditioned solution to promote precipitation. Subsequently, solid-liquid separation was performed to obtain waste residue and phosphoric acid impurity removal solution.

[0009] In an optional embodiment, the amount of the precipitant added is 5%-20% of the molar amount of Al in the phosphoric acid-adjusted solution.

[0010] In an optional embodiment, the precipitation-promoting reaction is carried out at a temperature of 50-95°C for 0.5-2 hours, and the precipitation-promoting reaction is carried out under stirring.

[0011] In an optional embodiment, the method for preparing the dehydrated insoluble phosphate includes: dehydrating the insoluble phosphate by sintering at 150-350°C for 4-12 hours.

[0012] In an optional embodiment, the insoluble phosphate includes at least one of hydrated aluminum phosphate, hydrated iron phosphate, hydrated magnesium phosphate, hydrated zinc phosphate, and hydrated calcium phosphate.

[0013] In an optional embodiment, when the sparingly soluble phosphate is hydrated ferric phosphate, after obtaining the waste residue, the waste residue is further subjected to sulfuric acid aging to remove impurities, and ferric phosphate dihydrate is obtained as the product. The aging mother liquor is then recycled for the next waste residue aging, and the number of cycles is 4-5 times. When the sparingly soluble phosphate is at least one of hydrated aluminum phosphate, hydrated magnesium phosphate, hydrated zinc phosphate, and hydrated calcium phosphate, after obtaining the waste residue, the waste residue is washed and dried, and then sintered at 150-350°C for 4-12 hours to dehydrate it, thereby obtaining a regenerated precipitant. The regenerated precipitant is then returned to the precipitation reaction for reuse.

[0014] In an optional embodiment, the heating temperature for heating the phosphoric acid dilution is 50-95°C.

[0015] Secondly, the present invention provides a method for preparing ferric phosphate dihydrate, comprising using a phosphoric acid impurity-removing solution obtained by the Al removal method of crude phosphoric acid as described in any of the above embodiments as raw material, reacting the phosphoric acid impurity-removing solution with ferrous sulfate and hydrogen peroxide to obtain ferric phosphate dihydrate.

[0016] In an optional embodiment, the molar ratio of phosphoric acid to ferrous sulfate in the phosphoric acid purification solution is 1:0.7-1.1; And / or, the molar ratio of ferrous ions in the ferrous sulfate to the hydrogen peroxide is 1:1.1-1.2.

[0017] In an optional embodiment, the step of reacting the phosphoric acid impurity removal solution with ferrous sulfate and hydrogen peroxide includes: first stirring the phosphoric acid impurity removal solution at 70~98°C, adding the ferrous sulfate during the stirring process, and after the ferrous sulfate dissolves, adding the hydrogen peroxide, continuing the reaction at 70~98°C for 4-8 hours, filtering, and washing the precipitate with deionized water.

[0018] In an optional embodiment, the stirring speed is 300~600 rpm.

[0019] Thirdly, the present invention also provides the application of the iron phosphate dihydrate prepared by the preparation method of the second aspect above in the preparation of lithium iron phosphate batteries.

[0020] The present invention has the following beneficial effects: The method for removing Al from crude phosphoric acid provided by this invention involves diluting the wet-process phosphoric acid and adjusting its pH to an acidic range of 1-1.5. Then, a dehydrated, sparingly soluble phosphate is added to the system as a precipitant. The precipitation of the sparingly soluble phosphate induces and promotes the precipitation of impurities (Al, Ca, Mg, etc.) from the solution after phosphoric acid adjustment, which are then separated by precipitation. The reason why sparingly soluble phosphate can act as a precipitant is that the metal ions in sparingly soluble phosphate are usually iron, aluminum, magnesium, etc. These metal ions readily combine with phosphate to form precipitates with similar properties. After high-temperature pretreatment, their structure is destroyed, and dissolution-reprecipitation occurs in the acidic system. This process can cause co-precipitation of other phosphates with similar properties. Simultaneously, the concentrations of phosphate and aluminum ions in wet-process phosphoric acid are relatively high. Under this high-concentration environment, only a small amount of sparingly soluble phosphate needs to be added to promote the precipitation of other impurity ions (especially aluminum ions) from the solution through co-precipitation and induction, thereby achieving Al removal through precipitation separation. Meanwhile, the precipitant used in this invention does not introduce impurity elements into the phosphoric acid dilution solution. Since impurities such as Al, Ca, and Mg in the phosphoric acid dilution solution compete with Fe for precipitation, and the concentrations of Al and Mg in the phosphoric acid dilution solution are higher than those of Fe, Al and Mg will preferentially co-precipitate before Fe, thus avoiding the loss of Fe element caused by co-precipitation. This method can reduce Fe element loss while removing aluminum, laying the foundation for the subsequent preparation of ferric phosphate dihydrate. This method allows for efficient, simple, and low-cost control of the aluminum content in phosphoric acid, achieving phosphorus-aluminum separation. This enables the efficient application of crude phosphoric acid in ferric phosphate synthesis, facilitating the preparation of ferric phosphate dihydrate products with acceptable aluminum content. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a process flow diagram of the preparation method of ferric phosphate dihydrate provided in Embodiment 1 of the present invention; Figure 2 SEM image of ferric phosphate dihydrate prepared by the preparation method of ferric phosphate dihydrate provided in Example 1 of this invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0024] This invention provides a method for removing Al from crude phosphoric acid, comprising the following steps: S1. Dilute wet-process phosphoric acid with water to a P concentration between 30-60 g / L to obtain a diluted phosphoric acid solution.

[0025] The wet-process phosphoric acid selected in this invention can be phosphoric acid obtained by any wet process. The main active component of wet-process phosphoric acid is phosphoric acid (H3PO4), which contains a small amount of sulfate (SO4). 2- ), iron ions (Fe 3+ ), aluminum ions (Al) 3+ ) and magnesium ions (Mg 2+ Impurities such as phosphoric acid are removed. By diluting wet-process phosphoric acid with water to a P concentration between 30-60 g / L, the pH of the system is approximately 1.0-2.0, which is a strongly acidic environment.

[0026] S2. Heat the diluted phosphoric acid solution and add ferric hydroxide to adjust the pH to 1-1.5 to obtain the phosphoric acid-adjusted solution.

[0027] By heating the system to temperatures ranging from 50 to 95°C, the increased temperature promotes the ionization of more H3PO4 molecules into H+. + and H2PO4 - This leads to H in the solution + The concentration increased slightly, and the pH decreased slightly.

[0028] The pH was then adjusted to 1-1.5 by adding ferric hydroxide. The added ferric hydroxide (Fe(OH)3) can undergo an acid-base neutralization reaction with phosphoric acid (H3PO4), and the H+ in the phosphoric acid... + The phosphate combines with the OH⁻ of ferric hydroxide to form stable H₂O, disrupting the dissolution equilibrium of ferric hydroxide and promoting its gradual dissolution to form iron phosphate. In this invention, by heating and adding ferric hydroxide, the pH value of the diluted phosphate solution can be adjusted to ensure that the pH of the adjusted solution is within the acidic range of 1-1.5.

[0029] S3. Preparation of dehydrated, insoluble phosphate.

[0030] Dehydration of sparingly soluble phosphates involves sintering at 150-350℃ for 4-12 hours. The main purpose of this dehydration step is to disrupt the original structure and crystal form of the hydrated phosphate. If the phosphate has high crystallinity and a relatively intact structure, it is difficult for precipitates to form on its surface, making it difficult to induce impurity precipitation and resulting in a relatively poor impurity removal effect. However, after sintering and dehydration disrupts the structure, in addition to its own inducing effect, the sparingly soluble phosphate will partially dissolve and redeprecipitate in high-concentration phosphoric acid due to its structural instability. This redeprecipitation causes impurities to co-precipitate, which enhances the impurity removal effect.

[0031] Among them, the insoluble phosphates include at least one of the following: hydrated aluminum phosphate (e.g., aluminum phosphate dihydrate, aluminum phosphate pentahydrate), hydrated ferric phosphate (e.g., ferric phosphate monohydrate, ferric phosphate dihydrate, ferric phosphate trihydrate, or ferric phosphate tetrahydrate), hydrated magnesium phosphate (e.g., magnesium phosphate tetrahydrate or magnesium phosphate octahydrate), hydrated zinc phosphate (e.g., zinc phosphate monohydrate or zinc phosphate dihydrate), and hydrated calcium phosphate (e.g., calcium phosphate monohydrate or calcium phosphate dihydrate). Since aluminum, iron, magnesium, calcium, and zinc are impurity elements contained in wet-process phosphoric acid, by selecting aluminum phosphate, ferric phosphate, and magnesium phosphate as insoluble phosphates, no other elements are introduced, thus ensuring the purity of the phosphoric acid solution.

[0032] S4. The dehydrated insoluble phosphate is added to the phosphoric acid-conditioned solution as a precipitation promoter to promote precipitation reaction. Then, solid-liquid separation is performed to obtain waste residue and phosphoric acid impurity removal solution.

[0033] In this invention, a sparingly soluble phosphate is added to the system as a precipitant. The precipitation of the sparingly soluble phosphate induces and promotes the precipitation of impurities (Al, Ca, Mg, etc.) from the solution after phosphoric acid conditioning, and these impurities are then separated through precipitation. The reason why sparingly soluble phosphate can act as a precipitant is that the metal ions in sparingly soluble phosphate are usually iron ions, aluminum ions, magnesium ions, etc., which readily combine with phosphate ions to form precipitates. When sparingly soluble phosphate is added to the phosphoric acid conditioning solution, co-precipitation easily occurs during the reaction. At the same time, the concentration of phosphate and aluminum ions in wet-process phosphoric acid is relatively high. Under this high-concentration environment, only a small amount of sparingly soluble phosphate needs to be added to promote the precipitation of other impurity ions (especially aluminum ions) from the solution through precipitation induction, thereby achieving Al removal through precipitation separation. The amount of precipitant added accounts for 5%-20% of the molar amount of Al in the phosphoric acid solution after adjustment. Too little precipitant leads to poor precipitation, while too much, although reducing the Al content, may increase other impurities, hindering the preparation of high-purity phosphoric acid. Furthermore, the precipitant used in this invention does not introduce additional impurity elements into the phosphoric acid dilution solution. Since impurities such as Al, Ca, and Mg in the phosphoric acid dilution solution compete with Fe during precipitation, and the concentrations of Al and Mg are higher than Fe, Al and Mg will preferentially co-precipitate before Fe, thus avoiding Fe loss. This achieves aluminum removal while reducing Fe loss, laying the foundation for subsequent preparation of ferric phosphate dihydrate. After adding the precipitant, the precipitation reaction temperature is 50-95℃, and the reaction time is 0.5-2 hours. Maintaining high-temperature stirring for a period ensures uniform dispersion of the precipitant and promotes the precipitation and separation of other impurity ions from the solution.

[0034] S5. Further process the waste residue obtained from the solid-liquid separation in the impurity removal step to make full use of its effective components.

[0035] When the selected insoluble phosphate is hydrated ferric phosphate, since hydrated ferric phosphate itself has a high value, the impurities can be removed by sulfuric acid aging to obtain ferric phosphate dihydrate with low impurities that can be used as a product. The aging mother liquor can be recycled for the next waste residue aging, and the recycling number can be 4-5 times.

[0036] If other insoluble phosphates are selected, the waste residue can be simply washed and dried, and then sintered at 150-350℃ for 4-12 hours to dehydrate it, thus obtaining a regenerated precipitant.

[0037] The sulfuric acid aging process for waste residue involves using a sulfuric acid solution concentration of 0.3-1.5 mol / L, a reaction solid content of 20%-30%, an aging temperature of 70-95℃, and a time of 1-4 hours.

[0038] By processing the precipitate after impurity removal, the precipitating agent can be recycled, which reduces the overall cost of impurity removal and the cost burden caused by the direct loss of the precipitating agent.

[0039] This invention employs a precipitation-promoting method, a highly efficient, simple, and low-cost approach, to control the aluminum content in phosphoric acid, thereby reducing the Al concentration in the crude phosphoric acid solution and achieving phosphorus-aluminum separation.

[0040] Furthermore, the present invention also provides a method for preparing ferric phosphate dihydrate, which includes using the phosphoric acid impurity-removing solution obtained by the above-mentioned method for removing Al from crude phosphoric acid as raw material, reacting the phosphoric acid impurity-removing solution with ferrous sulfate and hydrogen peroxide to obtain ferric phosphate dihydrate.

[0041] The molar ratio of phosphoric acid to ferrous sulfate in the phosphoric acid impurity removal solution is 1:0.7-1.1; the molar ratio of ferrous ions to hydrogen peroxide in the ferrous sulfate solution is 1:1.1-1.2.

[0042] In this invention, phosphoric acid is a moderately strong acid that partially ionizes in solution to release H+. + This provides an acidic environment for the oxidation of hydrogen peroxide; acidic conditions significantly enhance the oxidizing power of hydrogen peroxide. + Under catalysis, hydrogen peroxide will reduce Fe 2+ Oxidized to Fe 3+ It is reduced to H2O, and the phosphate ions ionized in the solution react with Fe. 3+ A metathesis reaction occurs, producing a sparingly soluble ferric phosphate (FePO4) precipitate that precipitates out of the solution; at the same time, the ferric phosphate (FePO4) molecules combine with surrounding water molecules to form a stable water of crystallization structure, ultimately forming ferric phosphate dihydrate.

[0043] Specifically, the reaction steps of the phosphoric acid purification solution with ferrous sulfate and hydrogen peroxide include: first, stirring the phosphoric acid purification solution at 70~98℃; adding ferrous sulfate during stirring; and after the ferrous sulfate dissolves, adding hydrogen peroxide; continuing the reaction at 70~98℃ for 4-8 hours; filtering; and washing the precipitate with deionized water. The stirring speed is 300~600 rpm.

[0044] The precipitant used in this invention does not introduce additional impurity elements into the phosphoric acid diluent. Since impurities such as Al, Ca, and Mg in the phosphoric acid diluent compete with Fe during precipitation, and the concentrations of impurities Al and Mg in the phosphoric acid diluent are higher than the concentration of Fe, impurities Al and Mg will preferentially co-precipitate before Fe, thereby avoiding the loss of Fe element caused by co-precipitation. This can reduce the loss of Fe element while removing aluminum, laying the foundation for the subsequent preparation of ferric phosphate dihydrate.

[0045] This invention uses a precipitation-promoting method to control the aluminum content in phosphoric acid, thereby reducing the Al concentration in the crude phosphoric acid solution and achieving phosphorus-aluminum separation. This allows crude phosphoric acid to be efficiently used in the synthesis of ferric phosphate, which is beneficial for preparing ferric phosphate dihydrate products with qualified aluminum content.

[0046] In addition, lithium iron phosphate dihydrate with qualified aluminum content can also be selected as a precursor material to prepare lithium iron phosphate batteries.

[0047] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0048] Example 1 Please see Figure 1 This embodiment provides a method for preparing ferric phosphate dihydrate, which includes the following steps: S1. Preparation of phosphoric acid dilution: Take a certain volume of wet-process phosphoric acid, add water and stir to dilute until the P concentration is 60 g / L, to obtain phosphoric acid dilution 1. At this time, the composition of phosphoric acid dilution 1 is P 60 g / L, Fe 0.42 g / L, Al 2.06 g / L, Ca 0.01 g / L, Mg 4.29 g / L; pH is 1.06.

[0049] S2. Adjusting pH: Heat and stir the diluted phosphoric acid solution in a water bath at 50°C. Add ferric hydroxide and finely adjust the pH through the reaction. + The concentration was adjusted to pH=1.5 to obtain the phosphoric acid-adjusted solution.

[0050] S3. Pretreatment of precipitant: Take a certain amount of ferric phosphate dihydrate, sinter at 250℃ for 8 hours, and use the dehydrated ferric phosphate as a precipitant.

[0051] S4. Precipitation reaction: Add powdered precipitant to the phosphoric acid-adjusted solution and stir at 50°C for 2 hours to promote precipitation. The amount of precipitant added is 10% of the molar amount of Al in the system. After filtration, waste residue and phosphoric acid impurity removal solution are obtained.

[0052] S5. Recycling and Aging of Impurity-Removed Waste Residue: After washing, the separated waste residue is transferred to a beaker, and a 0.5 mol / L sulfuric acid solution is added at a solid content of 30%. The mixture is stirred in a 95°C water bath for 4 hours. During the process, the material turns pink. The discharged material is filtered, washed, and dried to obtain low-impurity ferric phosphate dihydrate. The mother liquor is recycled for the next waste residue aging.

[0053] S6. Preparation of ferric phosphate dihydrate: Transfer the beaker containing the phosphoric acid purification solution to a water bath. Set the water bath temperature to 90℃ and the stirring speed to 500 rpm, stirring continuously. Simultaneously, add pre-weighed ferrous sulfate to the system, with the molar amount of ferrous sulfate being 1.0 times the molar amount of phosphoric acid. After the ferrous sulfate dissolves, raise the temperature to 95℃ and add hydrogen peroxide to the system according to a molar ratio of ferrous ions in ferrous sulfate to hydrogen peroxide of 1:1.1 to oxidize the ferrous sulfate. React under water bath heating and stirring for 8 hours, filter, and wash the precipitate with deionized water to obtain the ferric phosphate dihydrate product. See the SEM image for details. Figure 2 ,from Figure 2 It can be seen that the iron phosphate dihydrate product provided in this embodiment has a denser lamination and a more regular overall morphology.

[0054] Example 2 This embodiment is basically the same as Embodiment 1, except that the heat preservation temperature in step S2 is 90°C in this embodiment.

[0055] Example 3 This embodiment is basically the same as Embodiment 1, except that the precipitation reaction temperature in step S4 is 95°C.

[0056] Example 4 This embodiment is basically the same as Embodiment 1, except that in this embodiment, the precipitation reaction time in step S4 is 0.5h.

[0057] Example 5 This embodiment is basically the same as Embodiment 1, except that in this embodiment, the concentration of P in step S1 is 40 g / L, and at this time, the composition of phosphoric acid dilution solution 2 is P 40 g / L, Fe 0.28 g / L, Al 1.53 g / L, Ca 0.01 g / L, and Mg 2.97 g / L.

[0058] Example 6 This embodiment is basically the same as Embodiment 1, except that in this embodiment, magnesium phosphate octahydrate is used as a precipitant in step S3 for sintering and dehydration. In step S5, after obtaining the waste residue, the waste residue is washed and dried, and then sintered at 250°C for 8 hours to dehydrate, obtaining a regenerated precipitant. The regenerated precipitant is returned to the precipitation reaction for reuse.

[0059] Example 7 This embodiment is basically the same as embodiment 1, except that in this embodiment, aluminum phosphate dihydrate, iron phosphate dihydrate and magnesium phosphate octahydrate in a mass ratio of 4:3:3 are selected as sintering dehydration promoters in step S3.

[0060] Example 8 This embodiment is basically the same as embodiment 1, except that in this embodiment, the amount of precipitant added in step S4 is 20% of the molar amount of Al in the system.

[0061] Example 9 This embodiment is basically the same as that of embodiment 1, except that in this embodiment, the amount of precipitant added in step S4 is 5% of the molar amount of Al in the system.

[0062] Example 10 This embodiment is basically the same as embodiment 1, except that the impurity removal waste residue aging and recycling step S5 is omitted in this embodiment.

[0063] Comparative Example 1 This comparative example is basically the same as Example 1, except that in this comparative example, the concentration of P in step S1 is 80 g / L. At this time, the composition of the phosphoric acid dilution 3 is P 80 g / L, Fe 0.55 g / L, Al 3.11 g / L, Ca 0.01 g / L, and Mg 6.03 g / L.

[0064] Comparative Example 2 This comparative example is basically the same as Example 1, except that in this comparative example, ferric hydroxide is added in step S2 to bring the pH to 2.0.

[0065] Comparative Example 3 This comparative example is essentially the same as Example 1, except that in this comparative example, only the pH was adjusted, and no precipitating agent was added. The specific steps include: S1-S2 are the same as in Example 1.

[0066] S3. Preparation of ferric phosphate dihydrate: Transfer the beaker containing the phosphoric acid-adjusted solution to a water bath. Set the water bath temperature to 90℃ and the stirrer speed to 500 rpm, stirring continuously. Simultaneously, add pre-weighed ferrous sulfate to the system, with the molar amount of ferrous sulfate being 1.0 times the molar amount of phosphoric acid. After the ferrous sulfate dissolves, raise the temperature to 95℃ and add hydrogen peroxide to the system according to a molar ratio of ferrous ions in ferrous sulfate to hydrogen peroxide of 1:1.1 to oxidize the ferrous ions. React under water bath heating and stirring for 8 hours, filter, and wash the precipitate with deionized water to obtain the ferric phosphate dihydrate product.

[0067] Comparative Example 4 This comparative example is basically the same as Example 1, except that the pH adjustment step is omitted in this comparative example. The specific steps are as follows: S1 is the same as in Example 1.

[0068] S2. Pretreatment of precipitant: Take a certain amount of ferric phosphate dihydrate, sinter at 250℃ for 8 hours, and use the dehydrated ferric phosphate as a precipitant.

[0069] S3, Precipitation reaction: Add powdered precipitant to phosphoric acid dilution solution and continue stirring at 50°C for 2 hours to promote precipitation. The amount of precipitant added is 10% of the molar amount of Al in the system. After filtration, waste residue and phosphoric acid impurity removal solution are obtained.

[0070] S4 is the same as step S6 in Example 1.

[0071] Comparative Example 5 This comparative example is basically the same as Example 1, except that the pretreatment step of the precipitant in step S3 is omitted in this comparative example. The specific steps are as follows: S1-S2 are the same as in Example 1.

[0072] S3. Precipitation reaction: Add a small amount of ferric phosphate dihydrate powder to the phosphoric acid-adjusted solution and continue stirring at 50°C for 2 hours to promote precipitation. The amount of ferric phosphate dihydrate added is 10% of the molar amount of Al in the system. After filtration, waste residue and phosphoric acid impurity removal solution are obtained.

[0073] S4 is the same as step S6 in Example 1.

[0074] Comparative Example 6 This comparative example is basically the same as Example 1, except that in this comparative example, the dehydration method in step S3 is vacuum dehydration at 0.1 MPa and 50°C.

[0075] Comparative Example 7 This comparative example is basically the same as Example 1, except that in this comparative example, pH adjustment and precipitation reaction are carried out simultaneously. Specifically, it includes the following steps: S1. Preparation of phosphoric acid dilution: Take a certain volume of wet-process phosphoric acid, add water and stir to dilute until the P concentration is 60 g / L, to obtain phosphoric acid dilution 1. At this time, the composition of phosphoric acid dilution 1 is P 60 g / L, Fe 0.42 g / L, Al 2.06 g / L, Ca 0.01 g / L, Mg 4.29 g / L; pH is 1.06.

[0076] S2. Pretreatment of precipitant: Take a certain amount of ferric phosphate dihydrate, sinter at 250℃ for 8 hours, and use the dehydrated ferric phosphate as a precipitant.

[0077] S3. The diluted phosphoric acid solution 1 is heated and stirred in a water bath at 50°C. The same amount of ferric hydroxide and precipitant as in Example 1 are added to it. The precipitation reaction is continued at 50°C for 2 hours. The mixture is then filtered to separate the waste residue and the purified phosphoric acid solution.

[0078] S4 is the same as S6 in Example 1.

[0079] Comparative Example 8 The phosphoric acid dilution solution from Example 1 was adjusted to a ferrous phosphate solution, with a ferrous phosphate concentration of P 40 g / L and Fe 40 g / L. 2+ 72 g / L (Fe / P molar ratio approximately = 1). The ferrous phosphate solution was then purified according to the purification method described in Example 1.

[0080] Experimental Example The impurity element content of the phosphoric acid purification solution and ferric phosphate dihydrate product provided in Examples 1-10 and Comparative Examples 1-8 was tested, and the specific data were obtained by ICP-AES testing. The test results are shown in Tables 1 and 2. Note that Comparative Example 3 omitted the step of using a precipitant for impurity removal; therefore, the component content of the phosphoric acid-adjusted solution was tested in Comparative Example 3. The component content of the phosphoric acid purification solution in Examples 1-10 and Comparative Examples 1-2 and 4-8 is recorded.

[0081] Table 1. Statistical table of composition of phosphoric acid impurity removal solution or phosphoric acid conditioning solution

[0082] Table 2. Impurity content (%) in ferric phosphate products

[0083] As can be seen from Tables 1 and 2, the method for removing Al from crude phosphoric acid provided by this invention can significantly reduce the Al content in the phosphoric acid dilution solution. The effect of Example 10 is basically the same as that of Example 1, but because Example 10 did not perform subsequent treatment on the waste residue after impurity removal, the precipitant was directly lost, significantly increasing the cost. In Comparative Example 1, the P concentration in step S1 was 80 g / L, exceeding the range of this application. At this point, the high P concentration resulted in high solution viscosity, which would have a certain impact on the precipitation of impurities, hindering the precipitation of impurities. The pH required for the precipitation reaction in this application is relatively low. When the pH in Comparative Example 2 was 2, the Al content of impurities was significantly reduced, but the P loss rate in Comparative Example 2 was as high as 9.93%. In Comparative Example 3, only the pH was adjusted without performing the precipitation reaction. It can be seen that the Al content of impurities was significantly higher than in Example 1, and the P loss was also significantly greater than in Example 1. In Comparative Example 4, the pH adjustment step was omitted, and the precipitation reaction was performed directly. In this case, the phosphoric acid dilution solution was not heated and the pH was adjusted with ferric hydroxide, resulting in a significantly higher Al content of impurities than in Example 1. Comparative Example 5 omitted the pretreatment step of the precipitant in step S3 and directly used ferric phosphate dihydrate powder as the precipitant. Since ferric phosphate dihydrate is uncalcined, its crystallinity is high and its structure is relatively intact, making it difficult to induce impurity precipitation, resulting in a relatively poor impurity removal effect. Comparative Example 6, using a low-temperature vacuum treatment method, could not remove the water of crystallization from ferric phosphate dihydrate, and its impurity removal effect was similar to Comparative Example 5. Comparative Example 7 simultaneously carried out pH adjustment and precipitation reaction. In this case, the impurity removal effect of Al was poor, and the phosphorus loss rate was as high as 9.06%. Comparative Example 8 targeted the ferrous phosphate solution for impurity removal, showing a significant increase in phosphorus loss rate, reaching 15.3%.

[0084] In summary, the method for removing Al from crude phosphoric acid provided by this invention involves diluting the wet-process phosphoric acid and adjusting its pH to an acidic range of 1-1.5. Subsequently, a dehydrated, sparingly soluble phosphate is added to the system as a precipitant. The precipitation of the sparingly soluble phosphate induces and promotes the precipitation of impurities (Al, Ca, Mg, etc.) from the solution after phosphoric acid conditioning, which are then separated by precipitation. The reason why sparingly soluble phosphate can act as a precipitant is that the metal ions in sparingly soluble phosphate are usually iron, aluminum, magnesium, etc., which readily combine with phosphate to form precipitates. When sparingly soluble phosphate is added to the phosphoric acid conditioning solution, co-precipitation easily occurs during the reaction. Simultaneously, the concentrations of phosphate and aluminum ions in wet-process phosphoric acid are relatively high. Under this high-concentration environment, only a small amount of sparingly soluble phosphate is needed to promote the precipitation of other impurity ions (especially aluminum ions) from the solution through precipitation induction, thereby achieving Al removal through precipitation separation. Meanwhile, the precipitant used in this invention does not introduce additional impurity elements into the phosphoric acid dilution solution. Since impurities such as Al, Ca, and Mg in the phosphoric acid dilution solution compete with Fe for precipitation, and the concentrations of Al and Mg in the phosphoric acid dilution solution are higher than those of Fe, Al and Mg will preferentially co-precipitate before Fe, thus avoiding the loss of Fe element caused by co-precipitation. This method can reduce Fe element loss while removing aluminum, laying the foundation for the subsequent preparation of ferric phosphate dihydrate. This method allows for efficient, simple, and low-cost control of the aluminum content in phosphoric acid, achieving phosphorus-aluminum separation. This enables the efficient application of crude phosphoric acid in ferric phosphate synthesis, facilitating the preparation of ferric phosphate dihydrate products with acceptable aluminum content.

[0085] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for removing Al from crude phosphoric acid, characterized in that, It includes: Wet-process phosphoric acid is diluted with water to a P concentration between 30-60 g / L to obtain a diluted phosphoric acid solution; The diluted phosphoric acid solution was heated and ferric hydroxide was added to adjust the pH to 1-1.5 to obtain the phosphoric acid-adjusted solution. Dehydrated insoluble phosphate was used as a precipitation promoter and added to the phosphoric acid-conditioned solution to promote precipitation. Subsequently, solid-liquid separation was performed to obtain waste residue and phosphoric acid impurity removal solution. The amount of the precipitant added is 5%-20% of the molar amount of Al in the phosphoric acid-adjusted solution; The precipitation-promoting reaction is carried out at a temperature of 50-95℃ and a reaction time of 0.5-2h, and the precipitation-promoting reaction is carried out under stirring. The method for preparing the dehydrated insoluble phosphate includes: sintering the insoluble phosphate at 150-350℃ for 4-12 hours to dehydrate it; The insoluble phosphates include at least one of hydrated aluminum phosphate, hydrated iron phosphate, hydrated magnesium phosphate, hydrated zinc phosphate, and hydrated calcium phosphate.

2. The method for removing Al from crude phosphoric acid according to claim 1, characterized in that, When the sparingly soluble phosphate is hydrated ferric phosphate, after obtaining the waste residue, the process further includes aging the waste residue with sulfuric acid to remove impurities, thereby obtaining ferric phosphate dihydrate as the product. The aging mother liquor is then recycled for the next waste residue aging, and the number of cycles is 4-5 times. When the sparingly soluble phosphate is at least one of hydrated aluminum phosphate, hydrated magnesium phosphate, hydrated zinc phosphate, and hydrated calcium phosphate, after obtaining the waste residue, the waste residue is washed and dried, and then sintered at 150-350°C for 4-12 hours to dehydrate it, thereby obtaining a regenerated precipitant. The regenerated precipitant is then returned to the precipitation reaction for reuse.

3. The method for removing Al from crude phosphoric acid according to claim 1, characterized in that, The heating temperature for heating the diluted phosphoric acid solution is 50-95℃.

4. A method for preparing ferric phosphate dihydrate, characterized in that, It includes a method for removing Al from crude phosphoric acid as described in any one of claims 1-3, wherein the phosphoric acid impurity-removed solution obtained by the method for removing Al from crude phosphoric acid is used as raw material, and the phosphoric acid impurity-removed solution is reacted with ferrous sulfate and hydrogen peroxide to obtain ferric phosphate dihydrate.

5. The method for preparing ferric phosphate dihydrate according to claim 4, characterized in that, The molar ratio of phosphoric acid to ferrous sulfate in the phosphoric acid purification solution is 1:0.7-1.1; And / or, the molar ratio of ferrous ions in the ferrous sulfate to the hydrogen peroxide is 1:1.1-1.

2.

6. The method for preparing ferric phosphate dihydrate according to claim 4, characterized in that, The steps for reacting the phosphoric acid impurity removal solution with ferrous sulfate and hydrogen peroxide include: first, stirring the phosphoric acid impurity removal solution at 70~98℃ and 300~600rpm; adding the ferrous sulfate during stirring; and after the ferrous sulfate dissolves, adding the hydrogen peroxide; continuing the reaction at 70~98℃ for 4-8 hours; filtering; and washing the precipitate with deionized water.

Images