Process for preparing iron phosphate using coarse phosphoric acid as phosphorus source

Abstract

This invention relates to the field of ferric phosphate preparation technology, specifically a process for preparing ferric phosphate using purified crude phosphoric acid as a phosphorus source, comprising the following steps: S1: Preliminary purification of wet-process crude phosphoric acid, concentration to obtain a phosphate salt solution, and preliminary purification of ferrous sulfate (a byproduct of titanium dioxide production), concentration to obtain an iron salt solution; S2: Mixing the phosphate salt solution and iron salt solution, adding calcium carbonate and stirring, then adding excess oxalic acid, controlling the pH to 1.8-2.0, mixing and stirring, and filtering to obtain an oxidation solution; S3: Adding excess oxidant to the oxidation solution, adjusting the pH to 2.0-3.0, filtering, and obtaining a precipitate of amorphous ferric phosphate; S4: Adding water to the amorphous ferric phosphate and slurrying, controlling the pH to 1.5-2.0, allowing it to stand and age, taking the bottom precipitate and sequentially washing, drying, and calcining to obtain anhydrous ferric phosphate. This invention uses wet-process crude phosphoric acid and ferrous sulfate (a byproduct of titanium dioxide production) as raw materials to prepare high-purity fine-grained ferric phosphate, using low-cost chemical reagents, and the process is simple to operate.

Description

Technical Field

[0001] This invention relates to the field of iron phosphate preparation technology, specifically a process for preparing iron phosphate by using purified crude phosphoric acid as a phosphorus source. Background Technology

[0002] Ferric phosphate, with the chemical formula FePO4, is a monoclinic crystalline powder, primarily existing as the hydrate FePO4·2H2O, and is generally white or light pinkish-white. It possesses excellent thermal properties and a long lifespan, making it a suitable precursor for lithium-ion battery cathodes. Various methods exist for preparing ferric phosphate, mainly including solid-phase, liquid-phase, and gas-phase methods. Currently, co-precipitation is the mainstream commercial production method. This method, while controlling product purity, optimizes reaction conditions to control the morphology and size of the product to meet the needs of different fields.

[0003] Currently, the main phosphorus source for ferric phosphate production is industrial ammonium phosphate. From a process source perspective, using crude phosphoric acid as a substitute for industrial ammonium phosphate could increase its phosphorus value. However, crude phosphoric acid contains high levels of impurities such as Mg, Ca, Al, K, and F, making it difficult to use directly as a synthesis raw material. Current processes for preparing ferric phosphate using crude phosphoric acid as a phosphorus source employ methods such as ion exchange, solvent extraction, and chemical precipitation to completely purify the crude phosphoric acid, removing most impurities, and then using the clarified liquid as the phosphorus source. However, this process essentially still uses pure phosphoric acid as a synthesis raw material to avoid excessive impurities in the product, and the cost of deep purification of crude phosphoric acid is not low, contradicting the initial intention of using crude phosphoric acid to reduce costs.

[0004] Ferrous sulfate heptahydrate is the most important solid byproduct in the production of titanium dioxide using the sulfuric acid process. It is a high-quality iron source. However, unpurified ferrous sulfate heptahydrate contains high levels of impurities such as Ti, Mn, Ca, and Mg, which limits its application as an iron source for the preparation of ferric phosphate.

[0005] In summary, there is an urgent need for a simple and low-cost method to produce ferric phosphate using crude phosphoric acid and ferrous sulfate heptahydrate of titanium dioxide as raw materials to meet industrial demands. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a process for preparing iron phosphate using crude phosphoric acid as a phosphorus source, so as to achieve at least low preparation cost, simple process and high purity of iron phosphate product.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A process for preparing iron phosphate using crude phosphoric acid as a phosphorus source includes the following steps: S1: The wet-process crude phosphoric acid is initially purified to obtain a phosphate salt solution, and the titanium dioxide by-product ferrous iron is initially purified to obtain an iron salt solution. S2: Mix the phosphate salt solution and iron salt solution, add calcium carbonate and stir, then add excess oxalic acid, control the pH value to 1.8~2.0, mix and stir, and then filter to obtain the solution to be oxidized; S3: Add excess oxidant to the solution to be oxidized, adjust the pH to 2.0~3.0, filter, and obtain mother liquor and precipitated amorphous ferric phosphate; S4: Add water to the amorphous ferric phosphate and slurry it. Control the pH value to 1.5~2.0, let it stand and age, and take the bottom precipitate for washing, drying and calcination in sequence to obtain anhydrous ferric phosphate.

[0009] The wet-process crude phosphoric acid is a crude phosphoric acid product produced by the dihydrate sulfuric acid leaching method. Its main component is phosphoric acid, and its main impurities include Mg. 2+ Ca 2+ Al 3+ K + Na + F - wait.

[0010] The ferrous oxide byproduct of the titanium dioxide production process is a byproduct generated during the sulfuric acid process for titanium dioxide production. Its main component is FeSO4, and its main impurities include Fe. 3+ Ti 4+ Mn 2+ Ca 2+ Mg 2+ wait.

[0011] In some embodiments, in step S1, the preliminary purification method of the wet-process crude phosphoric acid is: adding ammonia water dropwise to the wet-process crude phosphoric acid to adjust the pH to 3-4.

[0012] In some embodiments, in step S1, the preliminary purification method of the ferrous byproduct of titanium dioxide is as follows: after dissolving the ferrous byproduct of titanium dioxide, ammonia water is added dropwise to adjust the pH to 4-5.

[0013] In some embodiments, in step S1, the concentration of phosphoric acid in the phosphate salt solution is preferably 0.5~1.5 mol / L; and the concentration of ferrous sulfate in the iron salt solution is preferably 2~2.5 mol / L.

[0014] In some embodiments, in step S2, the oxidant is hydrogen peroxide.

[0015] In some embodiments, in step S2, the amount of phosphate salt solution and iron salt solution added is calculated based on the amount of P in the raw material wet-process crude phosphoric acid and the total Fe in the titanium dioxide by-product ferrous iron, and added at a molar ratio of Fe:P = 0.8~2:1.

[0016] In some embodiments, in step S4, the amount of water added is 1.3 to 1.6 times the mass of the amorphous iron phosphate.

[0017] In some embodiments, in step S4, the pH-regulating agent is phosphoric acid, preferably 80 wt%.

[0018] In some embodiments, in step S4, the washing liquid used is dilute ammonia water, and the concentration is preferably 5~15wt%.

[0019] In some embodiments, in step S4, the calcination temperature is 700~800℃, and the calcination time is preferably 2~4h.

[0020] It is worth noting that the main impurity component in crude phosphoric acid is Mg. 2+ Ca 2+ Al 3+ K + and F - The main impurity component in ferrous sulfate heptahydrate is Ti. 4+ Mn 2+ Ca 2+ Mg 2+ Step S1 of the present invention removes Al by separately adjusting the acidity of crude phosphoric acid and ferrous sulfate heptahydrate. 3+ and Ti 4+ In step S2 of this invention, calcium carbonate is added under acidic conditions to make F - It precipitates as CaF2, while consuming SO4. 2- The ions form a CaSO4 precipitate, then oxalic acid is added, reacting with Ca... 2+ Mn 2+ Mg 2+ Multiple alkali metal ions form complexes, which are then removed by solid-liquid separation. At this point, the solution contains Fe. 2+ Fe 3 + PO4 3- K + and excess C2O4 2- In step S3 of the present invention, Fe is made by adding excess hydrogen peroxide. 2+ All oxidized to Fe 3 + At the same time, C2O4 2- The impurities are oxidized to CO2 and discharged. Furthermore, under acidic conditions, some impurities are slightly soluble (such as CaSO4, MgHPO4, AlPO4·2H2O, etc.). Through precise pH control, the content of these impurities can be further reduced during the first precipitation process via solid-liquid separation. At this point, the main impurity in amorphous ferric phosphate is K.+ In step S4 of this invention, ammonia water is used for washing, so that ammonium ions react with K on the surface of the precipitate. + Ion exchange is performed to remove K. + The objective is to produce an iron phosphate product with extremely high purity, where the content of impurity elements is strictly limited to below 30 ppm, while simultaneously achieving a stable particle size within the range of approximately 30-50 nanometers.

[0021] The beneficial effects of this invention are: 1. This invention uses wet-process crude phosphoric acid and ferrous sulfate (a byproduct of titanium dioxide) as raw materials to prepare high-purity fine-grained ferric phosphate. The overall process does not involve deep purification of the raw materials, and the reagents used are all low-cost chemical reagents. The preparation process does not involve complex equipment.

[0022] 2. Most of the reagents used in this invention can be added in excess and can be removed in subsequent steps. The amount added does not need to be precisely calculated or the content needs to be detected in advance. The operation is simple and convenient.

[0023] 3. This invention ensures that the content of impurity elements in the final product is strictly limited to below 30 ppm by controlling the order of addition of the impurity removal reagents and precise pH control, while simultaneously achieving a stable particle size within the range of approximately 30-50 nanometers. The ferric phosphate product obtained using this preparation process not only exhibits extremely low impurity content but also possesses excellent crystallization properties. Furthermore, the raw materials are widely available and easily obtained in batches, significantly improving the overall quality and market competitiveness of the product. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the process flow of the present invention.

[0025] Figure 2 This is a SEM image of the nanoscale high-purity anhydrous iron phosphate prepared in Example 1 of this invention. Detailed Implementation

[0026] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited to the following description.

[0027] All chemical reagents used in the following examples and comparative examples were AR grade. The crude phosphoric acid raw material was purchased from Shandong Xinhe New Materials. The ferrous sulfate byproduct used was a byproduct generated in the sulfuric acid process for titanium dioxide production, sourced from the production line of Sichuan Longmang Phosphate Chemical Co., Ltd. ICP-OES was used to detect the crude phosphoric acid and ferrous sulfate; the results are shown in Tables 1 and 2.

[0028] Table 1 Content of various components in crude phosphoric acid

[0029] Table 2. Content of various components of ferrous sulfate

[0030] Example 1 This embodiment provides a process for preparing iron phosphate by purifying crude phosphoric acid and using it as a phosphorus source, including the following steps: 1) Phosphate preparation: Transfer 1000g of crude phosphoric acid to a 5L flask, then start the stirring device. Gradually add 15wt% ammonia water to finely adjust the pH of the reaction system until it reaches 5.0±0.1. After adjustment, let it stand for 30 minutes, then separate the mixture using a vacuum filtration device to obtain a clear filtrate. This filtrate is the desired phosphate solution, with a phosphorus content of 5.8%.

[0031] 2) Iron Salt Preparation: Accurately weigh 800g of ferrous sulfate, a byproduct of titanium dioxide production, and transfer it to a 3L flask. Mix it with 1600g of demineralized water and stir until fully dissolved. Then, add 15wt% ammonia solution dropwise to finely adjust the pH to 4.0-5.0. Finally, effectively separate solid impurities by vacuum filtration to obtain a purified iron salt solution.

[0032] 3) Mixing and purification: Transfer the iron salt solution to a 5L flask and start stirring. Then, while stirring, add the phosphate salt solution (at a Fe / P molar ratio of 1:1). After mixing, add 15g of excess calcium carbonate and stir. Stir for 1 hour to produce calcium fluoride precipitate. Then, while stirring, slowly add 25g of excess anhydrous oxalic acid. Control the pH to 1.8~2.0 by controlling the amount of oxalic acid added. A mixed precipitate of calcium oxalate, manganese oxalate, and magnesium oxalate will be produced. Filter and take the filtrate as the solution to be oxidized. 4) Primary precipitation: Add 120 mL of hydrogen peroxide (30 wt% oxidant) to the solution to be oxidized, and adjust the pH to between 2.0 and 3.0 by adding 15 wt% ammonia water to obtain the amorphous iron phosphate that has precipitated for the first time.

[0033] 5) Reprecipitation: Dissolve amorphous ferric phosphate in deionized water at a mass ratio of 1:1.5, then start the stirring device. After the filter cake is completely broken up and evenly dispersed in the water, add 80wt% phosphoric acid to adjust the pH to between 1.5 and 2.0. Stirring should be continued to ensure uniform mixing. After completing the above steps, transfer the resulting slurry to a pre-prepared flask and heat to 95°C for 75 minutes. During the reprecipitation process, closely observe the color change of the slurry. Once it changes from the initial light yellow to pinkish-white, it indicates that the reaction has reached a specific stage. At this point, heat and maintain the temperature to promote further reaction. After the temperature maintenance is complete, perform solid-liquid separation by vacuum filtration. The obtained solid is the desired filter cake, which should be collected and properly stored.

[0034] 6) Washing: Wash the filter cake obtained by vacuum filtration. First, wash it with 10 times the volume of demineralized water, then add 5 times the volume of 2wt% dilute ammonia water, keep it warm and stir for 30 minutes at 50℃, and finally rinse it with demineralized water until the washing solution is neutral.

[0035] 7) Drying and calcination: The washed filter cake was dried in an oven at 120℃. After drying, it was calcined at 700℃ (heating rate of 5℃ / min) and held at this temperature for 4 hours to obtain anhydrous ferric phosphate (yield 97.17%, iron-phosphorus ratio 0.9706). The SEM image is shown below. Figure 2 As shown.

[0036] Example 2 This embodiment provides a process for preparing iron phosphate by using purified crude phosphoric acid as a phosphorus source. The method differs from that in Embodiment 1 in that some process parameters are adjusted, as follows: 1) Phosphate preparation: Transfer 1000g of crude phosphoric acid to a 5L flask, then start the stirrer. Gradually add 15wt% ammonia water to finely adjust the pH of the reaction system until it reaches 5±0.1. After adjustment, let it stand for 30 minutes, then separate the mixture using a vacuum filtration device to obtain a clear filtrate. This filtrate is the desired phosphate solution, with a phosphorus content of 5.8%.

[0037] 2) Iron Salt Preparation: Accurately weigh 800g of ferrous sulfate, a byproduct of titanium dioxide production, and transfer it to a 3L flask. Mix it with 1600g of demineralized water and stir until fully dissolved. Then, add 15wt% ammonia solution dropwise to finely adjust the pH to 4.0-5.0. Finally, effectively separate solid impurities by vacuum filtration to obtain a purified iron salt solution.

[0038] 3) Mixing and purification: Transfer the iron salt solution to a 5L flask and start stirring. Then, while stirring, add the phosphate salt solution (at a Fe / P molar ratio of 1:1). After mixing, add 20g of excess calcium carbonate and stir to produce calcium fluoride precipitate. Then, while stirring, slowly add 30g of excess anhydrous oxalic acid and stir. Control the pH to 1.8~2.0 by controlling the amount of oxalic acid added to produce a mixed precipitate of calcium oxalate, manganese oxalate, and magnesium oxalate. Filter and take the filtrate as the solution to be oxidized. 4) Primary precipitation: Add 150 mL of hydrogen peroxide (30 wt% oxidant) to the solution to be oxidized, and adjust the pH to between 2.0 and 3.0 by adding 15 wt% ammonia water to obtain the amorphous iron phosphate that has been precipitated for the first time.

[0039] 5) Reprecipitation: Dissolve amorphous ferric phosphate in deionized water at a mass ratio of 1:1.3, then start the stirring device. After the filter cake is completely broken up and evenly dispersed in the water, add 80wt% phosphoric acid to adjust the pH to between 1.5 and 2.0. Stirring should be continued to ensure uniform mixing. After completing the above steps, transfer the resulting slurry to a pre-prepared flask and heat to 95℃ for 50 minutes. During the reprecipitation process, closely observe the color change of the slurry. Once it changes from the initial light yellow to pinkish-white, it indicates that the reaction has reached a specific stage. At this point, heat and maintain the temperature to promote further reaction. After the temperature maintenance is complete, perform solid-liquid separation by vacuum filtration. The obtained solid is the desired filter cake, which should be collected and properly stored.

[0040] 6) Washing: Wash the filter cake obtained by vacuum filtration. First, wash it with 10 times the volume of demineralized water, then add 5 times the volume of 2wt% dilute ammonia water, keep it warm and stir for 30 minutes at 50℃, and finally rinse it with demineralized water until the washing solution is neutral.

[0041] 7) Drying and calcining: The washed filter cake was dried in an oven at 120℃. After drying, it was calcined at 800℃ (heating at 5℃ / min) and kept at that temperature for 4 hours to obtain anhydrous ferric phosphate (yield 97.16%).

[0042] Comparative Example 1 This comparative example provides a process for preparing iron phosphate using purified crude phosphoric acid as a phosphorus source. The method is the same as in Example 1, except that the purified phosphate salt and iron salt are directly mixed for preparation without further mixing and impurity removal, as follows: 1) Phosphate preparation: Same as in Example 1.

[0043] 2) Iron salt preparation: Same as in Example 1.

[0044] 3) Mixing and precipitating: Pour the iron salt solution into a 5L flask, then add the phosphate salt solution (at a Fe / P molar ratio of 1:1) while stirring. Then add excess hydrogen peroxide (30wt%, 120mL), and add 15wt% ammonia to adjust the pH to between 2.0 and 3.0. After mixing, the amorphous iron phosphate precipitate is obtained in the first stage.

[0045] 4) Re-precipitation: Same as in Example 1.

[0046] 5) Washing: Same as in Example 1.

[0047] 6) Drying and calcining: Same as in Example 1, to obtain anhydrous ferric phosphate (yield 72.73%).

[0048] Comparative Example 2 This comparative example provides a process for preparing ferric phosphate by purifying crude phosphoric acid and using it as a phosphorus source. The method is the same as in Example 1, except that no further precipitation is performed. Instead, the amorphous ferric phosphate is directly washed and calcined to obtain anhydrous ferric phosphate (yield 78.56%).

[0049] Experimental Example 1 The iron phosphate samples prepared in Examples 1-2 and Comparative Examples 1-2 were detected by ICP-OES (F could not be detected), and the results are shown in Table 3.

[0050] Table 3

[0051] Experiment Example 2 Electron microscopy was performed on the iron phosphate samples prepared in Examples 1-2 and Comparative Examples 1-2. The iron-to-phosphorus ratio, primary particle size, and particle size are shown in Table 4. Table 4

[0052] The average yield of anhydrous ferric phosphate prepared by the co-precipitation method in the industry is 98.15%, with an average iron-to-phosphorus ratio of 0.975, an average primary particle size of 102.3 nm, and an average Dx. 50 The particle size is 3.14 μm. The anhydrous ferric phosphate prepared by the method of this invention has higher purity, higher yield, and smaller atomic size. Comparing Example 1 and Comparative Examples 1-2, it can be seen that the mixed purification step can effectively remove Mg, Ca, and Mn, while the secondary precipitation can significantly reduce the content of Na and K. The process of this invention combines the two and simultaneously achieves precise control of particle size, resulting in ferric phosphate with extremely high yield and purity, and a particle size of less than 50 nanometers.

[0053] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A process for preparing iron phosphate using crude phosphoric acid as a phosphorus source, characterized in that, Includes the following steps: S1: The wet-process crude phosphoric acid is initially purified to obtain a phosphate salt solution, and the titanium dioxide by-product ferrous iron is initially purified to obtain an iron salt solution. S2: Mix the phosphate salt solution and iron salt solution, add calcium carbonate and stir, then add excess oxalic acid, control the pH to 1.8~2.0, mix and stir, then filter to obtain the solution to be oxidized; S3: Add excess oxidant to the solution to be oxidized, adjust the pH to 2.0~3.0, filter, and obtain mother liquor and precipitated amorphous ferric phosphate; S4: Add water to the amorphous ferric phosphate and slurry it. Control the pH value to 1.5~2.0, let it stand and age, and take the bottom precipitate for washing, drying and calcination in sequence to obtain anhydrous ferric phosphate.

2. The process for preparing ferric phosphate according to claim 1, characterized in that, In step S1, the preliminary purification method for the wet-process crude phosphoric acid is as follows: ammonia water is added dropwise to the wet-process crude phosphoric acid to adjust the pH value to 3-4.

3. The process for preparing ferric phosphate according to claim 1, characterized in that, In step S1, the preliminary purification method for the ferrous byproduct of titanium dioxide is as follows: after dissolving the ferrous byproduct of titanium dioxide, ammonia water is added dropwise to adjust the pH to 4-5.

4. The process for preparing ferric phosphate according to claim 1, characterized in that: In step S1, the concentration of phosphoric acid in the phosphate salt solution is 0.5~1.5 mol / L; the concentration of ferrous sulfate in the iron salt solution is 2~2.5 mol / L.

5. The process for preparing ferric phosphate according to claim 1, characterized in that: In step S2, the oxidant is hydrogen peroxide.

6. The process for preparing ferric phosphate according to claim 1, characterized in that: In step S4, the amount of water added is 1.3 to 1.6 times the mass of the amorphous iron phosphate.

7. The process for preparing ferric phosphate according to claim 1, characterized in that: In step S4, the pH-regulating agent is phosphoric acid.

8. The process for preparing ferric phosphate according to claim 1, characterized in that: In step S4, the washing solution used is dilute ammonia.

9. The process for preparing ferric phosphate according to claim 1, characterized in that: In step S4, the calcination temperature is 700~800℃.

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