Process for the preparation of iron phosphate from raffinate acid

Abstract

The application discloses a method for preparing iron phosphate from raffinate acid, and belongs to the technical field of wet-process phosphoric acid resource utilization and iron phosphate preparation. The method comprises the following steps: S1, mixing and extracting phosphoric acid-containing acid liquor and organic phase containing mixed extractant to obtain organic phase loaded with phosphoric acid and raffinate; S2, carrying out liquid-liquid-solid reaction on the organic phase loaded with phosphoric acid and an iron salt solution to generate iron phosphate precipitation, and then carrying out solid-liquid separation to obtain iron phosphate; S3, mixing the organic phase after reaction with ammonia or ammonia water to regenerate, so that residual inorganic acid forms ammonium salt and precipitates metal impurities, and then carrying out solid-liquid separation to obtain regenerated organic phase; and S4, returning the regenerated organic phase to S1 for recycling. The method directly prepares battery-grade iron phosphate from low-value phosphoric acid-containing acid liquor such as raffinate acid through selective extraction and liquid-liquid-solid reaction, omits the stripping and concentration steps in the traditional extraction process, has low impurity content of the product, the organic phase can be recycled, the by-product ammonium salt can be used as nitrogen fertilizer, and high-value utilization of phosphorus resources is realized.

Description

Technical Field

[0001] This invention belongs to the field of wet-process phosphoric acid resource utilization technology, and relates to a method for preparing iron phosphate from residual raffinate. Specifically, it relates to a method for preparing battery-grade iron phosphate by using low-value phosphoric acid-containing solutions such as residual raffinate as raw materials and coupling solvent extraction with liquid-liquid-solid reaction. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as a cathode material for lithium-ion batteries, has become the mainstream choice in new energy vehicles and energy storage due to its advantages such as high safety, long cycle life, and relatively low cost. Iron phosphate (FePO4) is a key precursor in the preparation of lithium iron phosphate, and its purity, crystal morphology, and batch stability directly affect the electrochemical performance of the final battery material. With the rapid development of the lithium iron phosphate industry, the market demand for iron phosphate continues to grow. Developing low-cost, high-stability phosphorus and iron sources has become the core approach to reducing the production cost of iron phosphate.

[0003] Wet-process phosphoric acid is one of the main industrial methods for producing phosphoric acid. It is obtained by decomposing phosphate rock with inorganic acids such as sulfuric acid. Its cost is much lower than that of thermal-process phosphoric acid, but it contains various impurities such as iron, aluminum, magnesium, calcium, sodium, potassium, fluorine, silicon, and sulfate. During the production, purification, and washing processes of wet-process phosphoric acid, phosphoric acid-containing acid solutions such as dilute acid, concentrated acid, washing acid, and raffinate (a byproduct of purification) are generated. Raffinate, the acid remaining after phosphoric acid extraction in the solvent extraction and purification process of wet-process phosphoric acid, still contains a relatively high concentration of phosphoric acid (typically 15 wt%–45 wt% as P2O5). However, due to its high acidity, complex impurities, and large compositional fluctuations, enterprises often find it difficult to directly utilize it for high-value purposes, often facing problems such as large stockpiles, high processing costs, and insufficient release of resource value. If the phosphorus resources in low-value phosphoric acid-containing acid solutions such as raffinate can be converted into battery-grade iron phosphate, it can not only significantly reduce the production cost of iron phosphate but also improve the comprehensive utilization level of byproduct acids from wet-process phosphoric acid, which has significant economic and environmental value.

[0004] In the existing technology, the technical routes for preparing iron phosphate using wet phosphoric acid or residual raffinate mainly include two categories.

[0005] The first type is the direct precipitation method, which involves treating the phosphoric acid solution to remove impurities and then directly reacting it with iron salts to produce ferric phosphate. For example, existing technology discloses a method for preparing high-taper spherical battery-grade ferric phosphate, which uses wet-process phosphoric acid as the phosphorus source and ferric sulfate as the iron source. By adding a precipitant to adjust the pH and controlling the reaction temperature, ferric phosphate is precipitated from the aqueous phase. Although this type of method has a relatively simple process, it has obvious shortcomings: First, metallic impurities, fluorine, silicon, sulfate, etc., in the residual acid can easily co-precipitate with ferric phosphate or be carried into the product, affecting the purity and crystal morphology of the product; second, to reduce the influence of impurities, multiple steps such as neutralization and impurity removal, oxidation, filtration, pH adjustment, precipitation, and aging are usually required, resulting in a long process and many control points; third, fluctuations in the acid composition of the raw materials can directly lead to insufficient batch stability of ferric phosphate.

[0006] The second type is solvent extraction purification, which uses solvent extraction to selectively transfer phosphoric acid from wet-process phosphoric acid into the organic phase, followed by back-extraction to obtain purified phosphoric acid. This purified phosphoric acid is then used as a raw material to prepare ferric phosphate or phosphates. For example, existing technology discloses a wet-process phosphoric acid purification process and its extract regeneration method. This method uses an acidic extractant to extract impurity cations from crude phosphoric acid, or uses an organic solvent to extract phosphoric acid into the organic phase, followed by regeneration of the extractant through reduction back-extraction, acid back-extraction, and other steps to obtain purified phosphoric acid. The Prayon process, widely used in industry, uses a mixed extractant of diisopropyl ether and tributyl phosphate to extract phosphoric acid, followed by back-extraction to obtain purified phosphoric acid. While this type of method can obtain high-purity phosphoric acid, its technical route follows the inherent pattern of "first extraction and purification to obtain purified phosphoric acid, then separate preparation of phosphates." This means that multiple intermediate steps such as extraction, back-extraction, and concentration must be involved, resulting in a long process chain. The concentration and reprecipitation processes introduce additional energy consumption and costs, making it economically unsustainable.

[0007] In summary, existing technologies, whether direct precipitation or solvent extraction purification, face significant technical bottlenecks when dealing with low-value phosphoric acid-containing solutions such as residual raffinate. Direct precipitation struggles to balance product purity and process simplicity; while solvent extraction can purify phosphoric acid, it fails to eliminate the intermediate step of "first obtaining liquid-phase purified phosphoric acid, then solid-phase precipitation," resulting in higher overall process costs. More importantly, existing technologies do not disclose a route for selectively extracting phosphoric acid into the organic phase, bypassing the intermediate step of back-extraction to prepare purified phosphoric acid, and instead directly reacting it with an iron salt solution to simultaneously generate ferric phosphate. Therefore, developing a method that can directly target low-value phosphoric acid-containing solutions such as residual raffinate, selectively transfer phosphoric acid, and simultaneously generate ferric phosphate through a liquid-liquid-solid reaction while simultaneously recycling the organic phase, is of great significance for the high-value utilization of residual raffinate and the low-cost preparation of ferric phosphate. Summary of the Invention

[0008] Therefore, the purpose of this invention is to provide a method for preparing ferric phosphate from residual raffinate, in order to solve the technical problems of impurities easily entering the product, cumbersome process flow, and low resource utilization value when low-value phosphoric acid solutions such as residual raffinate are directly used to prepare ferric phosphate.

[0009] To solve the above technical problems, the present invention provides a method for preparing ferric phosphate from residual raffinate, comprising the following steps: S1: The phosphoric acid solution is mixed with the organic phase containing the extractant for phosphoric acid extraction to obtain an organic phase loaded with phosphoric acid and a raffinate containing metal impurities. S2: The organic phase loaded with phosphoric acid obtained in S1 is mixed with an iron salt solution to carry out a liquid-liquid-solid reaction, so that the phosphoric acid in the organic phase is transferred into the aqueous phase and reacts with iron ions to form iron phosphate precipitate. Solid-liquid separation is performed to obtain iron phosphate and the organic phase after the liquid-liquid-solid reaction. S3: The organic phase obtained from the liquid-liquid-solid reaction in S2 is mixed with ammonia or ammonia water for regeneration, so that the residual inorganic acid in the organic phase is transferred into the aqueous phase and forms ammonium salt, while the entrained or co-extracted metal impurities are precipitated, and the regenerated organic phase is obtained by solid-liquid separation. S4: Return the regenerated organic phase obtained in S3 to S1 for recycling.

[0010] According to a preferred embodiment of the method of the present invention, the phosphoric acid solution in S1 is one or more of the following: wet process dilute phosphoric acid, wet process concentrated phosphoric acid, residual acid produced during the purification of wet process phosphoric acid, phosphate rock acid hydrolysis solution, and washing acid; the phosphorus concentration in the phosphoric acid solution, calculated as P2O5, is 15-45 wt%.

[0011] According to a preferred embodiment of the method of the present invention, the extractant in S1 is a mixed extractant composed of an amine extractant, tributyl phosphate and an alcohol extractant. The amine extractant is selected from one or more of primary amines, secondary amines, tertiary amines and quaternary ammonium salts. The alcohol extractant is selected from one or more of n-octanol, isooctanol, sec-octanol, decanol and isodecanol.

[0012] Preferably, the amine extractant is selected from one or more of trioctylamine, triisooctylamine, trialkylamine, N235, methyltrioctylammonium chloride, and trioctylmethylammonium chloride.

[0013] According to a preferred embodiment of the method of the present invention, the organic phase in S1 is composed of an extractant and a diluent; the diluent is selected from one or more of kerosene, sulfonated kerosene, n-alkanes, isoalkanes, and aromatic solvents; the volume fraction of the extractant in the organic phase is 20-80%, and the volume ratio of amine extractant, tributyl phosphate and alcohol extractant in the extractant is (10-60):(10-60):(5-30).

[0014] According to a preferred embodiment of the method of the present invention, the phosphoric acid extraction in S1 adopts a single-stage or multi-stage extraction, the extraction temperature is 30-60 °C, the volume ratio of organic phase to aqueous phase is 5:1-1:1, and the single-stage extraction time is 5-30 min.

[0015] According to a preferred embodiment of the method of the present invention, the iron salt solution in S2 is one of ferric nitrate solution, ferric chloride solution, and ferric sulfate solution; the inorganic acid radicals formed in S2 and S3 are the same.

[0016] According to a preferred embodiment of the method of the present invention, the molar ratio of iron in the iron salt solution to phosphorus in the loaded phosphoric acid in S2 is 1:(1.01-1.15), the liquid-liquid-solid reaction temperature is 30-60 °C, and the reaction time is 5-30 min.

[0017] According to a preferred embodiment of the method of the present invention, the pH of the liquid-liquid-solid reaction system in S2 is 1.0-2.5; the pH is controlled by adjusting the amount of circulating mother liquor, the concentration of iron salt solution, or the volume ratio of organic phase to aqueous phase.

[0018] According to a preferred embodiment of the method of the present invention, the ferric phosphate obtained in S2 is washed with water, dried and calcined to obtain anhydrous ferric phosphate; wherein the calcination temperature is 450-650 °C and the calcination time is 1-4 h.

[0019] According to a preferred embodiment of the method of the present invention, the ammonia or ammonia water mentioned in S3 is gaseous ammonia, liquid ammonia or ammonia water; the regeneration temperature is 30-60 ℃, the regeneration time is 15-45 min, and the regeneration endpoint pH is 7.0-10.5; the aqueous phase obtained in S3 contains one of ammonium nitrate, ammonium chloride or ammonium sulfate, and is recovered as a nitrogen fertilizer raw material.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses phosphoric acid-containing solutions, such as raffinate, as raw materials. Phosphoric acid is selectively extracted using a mixed extractant composed of amine extractants, tributyl phosphate, and alcohol extractants. This process leaves most metal impurities in the raffinate, significantly reducing the risk of impurities entering the ferric phosphate product. Results show that the content of impurities such as calcium, magnesium, aluminum, sodium, and potassium in the obtained ferric phosphate product is no higher than 60 ppm, the phosphorus-to-iron molar ratio is close to the stoichiometric ratio (0.998-1.005), and the XRD pattern perfectly matches the standard card, meeting the purity requirements for battery-grade ferric phosphate.

[0021] 2. This invention employs a liquid-liquid-solid reaction between an iron salt solution and a phosphoric acid-loaded organic phase, allowing the transfer of phosphoric acid from the organic phase to the aqueous phase and the precipitation of iron phosphate to be completed in the same process. This eliminates the intermediate step of "first back-extracting to prepare purified phosphoric acid and then separately precipitating iron phosphate" in the traditional solvent extraction process, thus shortening the process flow and reducing energy consumption and production costs.

[0022] 3. This invention helps to solve the problems of large stock of residual acid, low utilization value and high treatment cost of by-product raffinate from wet phosphoric acid purification, and realizes the high-value utilization of phosphorus resources in residual acid; at the same time, the ammonium salt phase obtained from the S3 regeneration process can be recovered as nitrogen fertilizer raw material, and the regenerated organic phase can be returned to S1 for recycling, realizing the coupling of extractant recycling and by-product resource recovery. Detailed Implementation

[0023] The following description is based on specific embodiments.

[0024] 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 in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the present invention.

[0025] In this invention, unless otherwise specified, all equipment and raw materials are available from the market or commonly used in the industry. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art.

[0026] The residual acid, wet-process dilute phosphoric acid, and washing acid used in the examples were all taken from the wet-process phosphoric acid production and purification unit; the reagents used, such as trioctylamine, triisooctylamine, N235, methyltrioctylammonium chloride, tributyl phosphate, n-octanol, isooctanol, isodecanol, sulfonated kerosene, kerosene, and isoalkanes, were all industrial grade or chemically pure grade; the ferric chloride solution and ferric sulfate solution used were prepared from the corresponding ferric salts, with concentrations expressed as Fe. 3+ The ammonia used is industrial grade, with a mass fraction of approximately 25%.

[0027] Example 1

[0028] The method for preparing ferric phosphate from residual raffinate described in this embodiment follows these steps: S1. Phosphoric acid extraction: The residual acid from the wet phosphoric acid purification process was selected as the phosphoric acid solution. The phosphoric acid concentration in the residual acid, calculated as P2O5, was 15 wt%, and the contents of the main metal impurities were: Fe 1.2 wt%, Al 0.8 wt%, Mg 0.5 wt%, and Ca 0.3 wt%.

[0029] The extractant was prepared by mixing trioctylamine, tributyl phosphate and isooctanol in a volume ratio of 40:40:20; the extractant was then mixed with sulfonated kerosene in a volume ratio of 30:70 to prepare an organic phase, in which the volume fraction of the extractant in the organic phase was 30%.

[0030] The raffinate and organic phase were subjected to a two-stage countercurrent extraction at 30 °C, with a total volume ratio of organic phase to aqueous phase of 5:1. Each extraction stage lasted 5 min, followed by a 10-min settling period after each stage. After extraction, a phosphoric acid-loaded organic phase and a raffinate containing metal impurities were obtained. The residual P₂O₅ concentration in the raffinate was determined to be approximately 1.2 wt%, and the phosphoric acid extraction rate was approximately 92%.

[0031] S2, Liquid-Liquid-Solid Reaction: The organic phase loaded with phosphoric acid obtained in S1 is mixed with a ferric chloride solution to carry out a liquid-liquid-solid reaction. 3+ The concentration of iron was 2.0 mol / L, and the molar ratio of iron to phosphorus in the loaded phosphate was controlled at 1:1.01. The reaction temperature was 30 ℃, the reaction time was 5 min, and the stirring speed was 300 rpm. During the reaction, the pH of the reaction system was adjusted to 1.5 by adding a portion of the circulating mother liquor collected after the solid-liquid separation in step S2. After the reaction was completed, the mixture was allowed to stand for 10 min to allow the solid phase to settle completely, followed by solid-liquid separation to obtain the iron phosphate filter cake and the organic phase after the liquid-liquid-solid reaction.

[0032] The obtained ferric phosphate filter cake was washed three times with deionized water at a solid-liquid mass ratio of 1:5 each time, until the conductivity of the wash water was less than 200 μS / cm. Then it was dried in a drying oven at 105 ℃ for 4 h, and then calcined in a muffle furnace at 550 ℃ for 3 h to obtain anhydrous ferric phosphate product.

[0033] S3. Ammonia Regeneration: The organic phase obtained from the liquid-liquid-solid reaction in S2 is mixed with 25% ammonia solution at 30 °C for regeneration. The stirring speed is 200 rpm, and the regeneration time is 15 min. The pH at the regeneration endpoint is controlled by adjusting the amount of ammonia added. After regeneration, the mixture is allowed to stand for 15 min to separate the phases. Solid-liquid separation removes the precipitated metallic impurities (hydroxides), yielding a regenerated organic phase and an ammonium chloride solution. The resulting ammonium chloride solution can be concentrated and recovered as a nitrogen fertilizer feedstock.

[0034] S4, Organic Phase Recycling: The regenerated organic phase obtained in S3 is returned to S1 and subjected to the next batch of phosphoric acid extraction under the same conditions as the fresh residual acid, thus recycling the process.

[0035] The anhydrous ferric phosphate product obtained in Example 1 was tested and found to have a P / Fe molar ratio of 1.002; the content of impurities such as calcium, magnesium, aluminum, sodium, and potassium was not higher than 50 ppm; and the D content was determined by laser particle size analysis. 50 Its thickness is 3.2 μm; its BET specific surface area is 7.5 m². 2 / g; XRD analysis showed that the obtained product had characteristic diffraction peaks of iron phosphate, and the product phase was iron phosphate.

[0036] Example 2

[0037] The method for preparing ferric phosphate from residual raffinate described in this embodiment follows these steps: S1. Phosphoric acid extraction: Wet-process dilute phosphoric acid was used as the phosphoric acid solution. The concentration of phosphorus in the dilute acid, calculated as P2O5, was determined to be 20 wt%.

[0038] The extractant was prepared by mixing trioctylamine, tributyl phosphate and n-octanol in a volume ratio of 45:40:15; the extractant was then mixed with kerosene in a volume ratio of 70:30 to prepare an organic phase, in which the volume fraction of the extractant in the organic phase was 70%.

[0039] The above-mentioned wet-process phosphoric acid dilute acid and organic phase were subjected to a two-stage extraction at 45 °C, with a total volume ratio of organic phase to aqueous phase of 3:1. The extraction time for each stage was 15 min, and the phases were allowed to separate after each extraction stage for 15 min. After extraction, a phosphoric acid-loaded organic phase and a raffinate containing metal impurities were obtained. The residual P2O5 concentration in the raffinate was determined to be approximately 2.0 wt%, and the phosphoric acid extraction rate was approximately 90%.

[0040] S2, Liquid-Liquid-Solid Reaction: The organic phase loaded with phosphoric acid obtained in S1 is mixed with a ferric chloride solution to carry out a liquid-liquid-solid reaction. 3+ The concentration of iron was 2.5 mol / L, and the molar ratio of iron to phosphorus in the loaded phosphate was controlled at 1:1.10. The reaction temperature was 45 ℃, the reaction time was 15 min, and the stirring speed was 350 rpm. During the reaction, the pH of the reaction system was adjusted to 1.8 by adding a portion of the circulating mother liquor collected after solid-liquid separation in step S2. After the reaction was completed, the mixture was allowed to stand for 15 min to allow the solid phase to settle completely, followed by solid-liquid separation to obtain iron phosphate filter cake and the organic phase after the liquid-liquid-solid reaction.

[0041] The obtained ferric phosphate filter cake was washed three times with deionized water at a solid-liquid mass ratio of 1:5 each time, until the conductivity of the wash water was less than 200 μS / cm. Then it was dried in a drying oven at 110 ℃ for 3 h, and then calcined in a muffle furnace at 580 ℃ for 2.5 h to obtain anhydrous ferric phosphate product.

[0042] S3. Ammonia Regeneration: The organic phase obtained from the liquid-liquid-solid reaction in S2 is mixed with 25% ammonia solution at 45 °C for regeneration. The stirring speed is 250 rpm, and the regeneration time is 30 min. The pH at the regeneration endpoint is controlled by adjusting the amount of ammonia added. After regeneration, the mixture is allowed to stand for 15 min to separate the phases. Solid-liquid separation removes the precipitated metallic impurities (hydroxides), yielding the regenerated organic phase and an ammonium chloride solution. The resulting ammonium chloride solution can be concentrated and recovered as a nitrogen fertilizer feedstock.

[0043] S4. Organic phase recycling: The regenerated organic phase obtained in S3 is returned to S1 and subjected to the next batch of phosphoric acid extraction with fresh wet phosphoric acid under the same conditions, thus recycling the process.

[0044] The anhydrous ferric phosphate product obtained in Example 2 was tested: the P / Fe molar ratio was 1.005; the content of impurities such as calcium, magnesium, aluminum, sodium, and potassium was not higher than 40 ppm; and the D content was measured by laser particle size analysis. 50 Its thickness is 2.8 μm; its BET specific surface area is 8.1 m². 2 / g; XRD analysis showed that the obtained product had characteristic diffraction peaks of iron phosphate, and the product phase was iron phosphate.

[0045] Example 3

[0046] The method for preparing ferric phosphate from residual raffinate described in this embodiment follows these steps: S1. Phosphoric acid extraction: The washing acid was used as a phosphoric acid solution. The concentration of phosphorus in the washing acid, calculated as P2O5, was determined to be 35 wt%.

[0047] The extractant was prepared by mixing triisooctylamine, tributyl phosphate and isodecanol in a volume ratio of 50:30:20; the extractant was then mixed with isoalkanes in a volume ratio of 70:30 to prepare an organic phase, in which the volume fraction of the extractant in the organic phase was 70%.

[0048] The washing acid and organic phase were subjected to a single-stage extraction at 60 °C with a volume ratio of organic phase to aqueous phase of 1:1 for 30 min. After extraction, the mixture was allowed to stand for 20 min to separate the phases. After extraction, a phosphoric acid-loaded organic phase and a raffinate containing metal impurities were obtained. The residual P₂O₅ concentration in the raffinate was determined to be approximately 5.0 wt%, and the phosphoric acid extraction rate was approximately 85.7%.

[0049] S2, Liquid-Liquid-Solid Reaction: A liquid-liquid-solid reaction is carried out by mixing a ferric sulfate solution with the organic phase loaded with phosphoric acid obtained in S1. 3+ The concentration of iron was 3.0 mol / L, and the molar ratio of iron to phosphorus in the loaded phosphate was controlled at 1:1.15. The reaction temperature was 60 ℃, the reaction time was 30 min, and the stirring speed was 400 rpm. During the reaction, the pH of the reaction system was adjusted to 2.2 by adding a portion of the circulating mother liquor collected after the solid-liquid separation in step S2. After the reaction was completed, the mixture was allowed to stand for 20 min to allow the solid phase to settle completely, followed by solid-liquid separation to obtain the iron phosphate filter cake and the organic phase after the liquid-liquid-solid reaction.

[0050] The obtained ferric phosphate filter cake was washed three times with deionized water at a solid-liquid mass ratio of 1:5 each time, until the conductivity of the wash water was less than 200 μS / cm. Then it was dried in a drying oven at 120 ℃ for 2 h, and then calcined in a muffle furnace at 620 ℃ for 2 h to obtain anhydrous ferric phosphate product.

[0051] S3. Ammonia Regeneration: The organic phase obtained from the liquid-liquid-solid reaction in S2 is mixed with 25% ammonia solution at 60 °C for regeneration. The stirring speed is 300 rpm, and the regeneration time is 45 min. The pH at the end of the regeneration is controlled by adjusting the amount of ammonia added. After regeneration, the mixture is allowed to stand for 20 min to separate the phases. Solid-liquid separation removes the precipitated metallic impurities (hydroxides), yielding the regenerated organic phase and an ammonium sulfate solution. The resulting ammonium sulfate solution can be concentrated and recovered as a nitrogen fertilizer feedstock.

[0052] S4, Organic Phase Recycling: The regenerated organic phase obtained in S3 is returned to S1 and subjected to the next batch of phosphoric acid extraction under the same conditions as fresh washing acid, thus recycling the process.

[0053] The anhydrous ferric phosphate product obtained in Example 3 was tested: the P / Fe molar ratio was 0.998; the contents of impurities such as calcium, magnesium, aluminum, sodium, and potassium were all no higher than 60 ppm; and the D content was measured by laser particle size analysis. 50 Its thickness is 4.1 μm; its BET specific surface area is 6.8 m². 2 / g; XRD analysis showed that the obtained product had characteristic diffraction peaks of iron phosphate, and the product phase was iron phosphate.

[0054] Comparative Example 1 To verify the technical advantages of this invention in reducing impurity content, Comparative Example 1 was set up. The same raffinate as in Example 1 was used as the raw material, with a phosphorus concentration of 15 wt% (based on P2O5). Ferric chloride solution was directly added to this raffinate, controlling the molar ratio of iron to phosphorus to be 1:1.01. The mixture was stirred at 30 °C for 5 min, and then the pH of the system was adjusted to 1.5 with ammonia. After standing, solid-liquid separation was performed to obtain a precipitate. The precipitate was washed with water, dried at 105 °C for 4 h, and calcined at 550 °C for 3 h to obtain the comparative product.

[0055] The test results showed that the P / Fe molar ratio of the product obtained in Comparative Example 1 was 0.985; the contents of impurities such as calcium, magnesium, aluminum, sodium, and potassium were: Ca 320 ppm, Mg 280 ppm, Al 450 ppm, Na 180 ppm, and K 120 ppm, respectively; the D content measured by laser particle size analysis was... 50 Its thickness is 5.6 μm; its BET specific surface area is 12.3 m². 2 / g. The results show that when iron phosphate is precipitated directly with residual acid without extraction and separation, a large amount of metal impurities are co-precipitated with the iron phosphate, resulting in a product purity that is much lower than that of Example 1. Furthermore, the product has a wider particle size distribution and a higher specific surface area, which is not conducive to the subsequent preparation of lithium iron phosphate cathode materials.

[0056] Comparative Example 2 To verify the technical advantages of this invention in simplifying the process and reducing energy consumption, Comparative Example 2 was set up. Using the same residual acid as in Example 1 as the raw material, S1 phosphoric acid extraction was performed under the same extractant composition and extraction conditions as in Example 1 to obtain an organic phase loaded with phosphoric acid. Subsequently, a conventional back-extraction process was used: the organic phase loaded with phosphoric acid was back-extracted with deionized water at a volume ratio of 1:1 at 30 °C for 30 min. After standing and phase separation, a purified phosphoric acid aqueous phase and an organic phase to be regenerated were obtained. The purified phosphoric acid aqueous phase was mixed with a ferric chloride solution, controlling the molar ratio of iron to phosphorus to be 1:1.01, and reacted at 30 °C for 5 min. The pH was adjusted to 1.5, and the solid-liquid separation, washing, drying, and calcination conditions were the same as in Example 1 to obtain the comparative product.

[0057] The test results showed that the P / Fe molar ratio of the product obtained in Comparative Example 2 was 1.001; the contents of impurities such as calcium, magnesium, aluminum, sodium, and potassium were all no higher than 55 ppm, and the product purity was basically equivalent to that of Example 1. However, the total process time of Comparative Example 2 was about 40 minutes longer than that of Example 1, and the back-extraction process caused dilution of the phosphoric acid aqueous phase, requiring additional evaporation and concentration to meet the subsequent precipitation concentration requirements, resulting in a significant increase in energy consumption; at the same time, the back-extraction process generated dilute phosphoric acid wastewater containing trace amounts of organic matter, increasing wastewater treatment costs. In contrast, Example 1 directly generated iron phosphate from the organic phase through a liquid-liquid-solid reaction, omitting the back-extraction and concentration steps, resulting in a shorter process flow and lower overall cost.

[0058] Example 1 The regenerated organic phase obtained in S3 of Example 1 was continuously returned to S1 for recycling to examine its extraction performance stability. Each cycle used the same residual acid feedstock as in Example 1, operated under the same extraction conditions, and then sequentially performed the S2 liquid-liquid-solid reaction and S3 ammonia regeneration, for a total of 20 cycles.

[0059] The P / Fe molar ratio and the content of major impurities of the anhydrous ferric phosphate products obtained after the 1st, 5th, 10th, 15th and 20th cycles were determined respectively, and the results are shown in Table 1.

[0060] Table 1 Results of Organic Phase Cyclic Stability Study

[0061] As can be seen from the results in Table 1, after 20 cycles, the regenerated organic phase still maintains good extraction selectivity, the impurity content of the obtained iron phosphate product is still controlled at a low level, and the P / Fe molar ratio is stable between 1.000 and 1.003, indicating that the organic phase recycling process of the present invention is stable and reliable.

[0062] Example 2: Effect of different extractant compositions on phosphoric acid extraction rate and impurity co-extraction To investigate the effect of the mixed extractant composition on the technical effect, Example 2 was set up. The same raffinate feedstock as in Example 1 was used, and S1 extraction was performed using the following three extractant compositions. The diluent for all three was sulfonated kerosene, and the extraction conditions were the same as in Example 1: Group A: Trioctylamine alone (30% by volume); Group B: A binary mixture of trioctylamine and tributyl phosphate (volume ratio 50:50, total integral 30%). Group C: The ternary mixture of trioctylamine, tributyl phosphate and isooctyl alcohol from Example 1 (volume ratio 40:40:20, total integral 30%).

[0063] After extraction, the concentration of P2O5 and the content of major metal impurities (Fe, Al, Mg) in the loaded organic phase of each group were measured, and the phosphoric acid extraction rate and the co-extraction rate of metal impurities were calculated. The results are shown in Table 2.

[0064] Table 2. Effects of different extractant compositions on phosphoric acid extraction rate and impurity co-extraction rate

[0065] As shown in Table 2, compared with single amine extractants (Group A) and binary mixtures (Group B), the ternary mixed extractant (Group C) significantly reduced the co-extraction rate of metal impurities while maintaining a high phosphoric acid extraction rate, thus effectively reducing the impurity risk of subsequent iron phosphate products, demonstrating the synergistic effect of the ternary mixed extractant.

[0066] In the description of this invention, it should be understood that "-" and "~" represent a range between two values, and this range includes the endpoints. For example, "AB" represents a range greater than or equal to A and less than or equal to B. "A~B" represents a range greater than or equal to A and less than or equal to B.

[0067] In the description of this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can represent three cases: A exists alone, A and B exist simultaneously, and B exists alone.

[0068] In the description of the invention, the numerical values ​​of time, temperature, ratio, and mass involved can be based on actual measurements, standard equipment parameters, simplified rounding results, or within an acceptable error range, ensuring the practicality and repeatability of the invention.

[0069] In the description of this invention, the terms “about” or “approximately” are used to express approximate values ​​or ranges, allowing for a certain degree of error to ensure the flexibility and practicality of the description, while remaining within an acceptable range of error, with the maximum error not exceeding 10% of the corresponding value or range.

[0070] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing ferric phosphate from residual raffinate, characterized in that, The steps include the following: S1: The phosphoric acid solution is mixed with the organic phase containing the extractant for phosphoric acid extraction to obtain an organic phase loaded with phosphoric acid and a raffinate containing metal impurities. S2: The organic phase loaded with phosphoric acid obtained in S1 is mixed with an iron salt solution to carry out a liquid-liquid-solid reaction, so that the phosphoric acid in the organic phase is transferred into the aqueous phase and reacts with iron ions to form iron phosphate precipitate. Solid-liquid separation is performed to obtain iron phosphate and the organic phase after the liquid-liquid-solid reaction. S3: The organic phase obtained from the liquid-liquid-solid reaction in S2 is mixed with ammonia or ammonia water for regeneration, so that the residual inorganic acid in the organic phase is transferred into the aqueous phase and forms ammonium salt, while the entrained or co-extracted metal impurities are precipitated, and the regenerated organic phase is obtained by solid-liquid separation. S4: Return the regenerated organic phase obtained in S3 to S1 for recycling.

2. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, The phosphoric acid solution mentioned in S1 is one or more of the following: wet-process dilute phosphoric acid, wet-process concentrated phosphoric acid, residual acid produced during the purification of wet-process phosphoric acid, phosphate rock acid hydrolysis solution, and washing acid; the phosphorus concentration in the phosphoric acid solution, calculated as P2O5, is 15-45 wt%.

3. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, The extractant in S1 is a mixed extractant composed of amine extractants, tributyl phosphate and alcohol extractants. The amine extractant is selected from one or more of primary amines, secondary amines, tertiary amines and quaternary ammonium salts. The alcohol extractant is selected from one or more of n-octanol, isooctanol, sec-octanol, decanol and isodecanol.

4. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, The organic phase described in S1 is composed of an extractant and a diluent; the diluent is selected from one or more of kerosene, sulfonated kerosene, n-alkanes, isoalkanes, and aromatic solvents; the volume fraction of the extractant in the organic phase is 20-80%, and the volume ratio of amine extractant, tributyl phosphate, and alcohol extractant in the extractant is (10-60):(10-60):(5-30).

5. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, Phosphoric acid extraction in S1 employs single-stage or multi-stage extraction, with an extraction temperature of 30-60 ℃, an organic phase to aqueous phase volume ratio of 5:1-1:1, and a single-stage extraction time of 5-30 min.

6. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, The iron salt solution mentioned in S2 is one of ferric nitrate solution, ferric chloride solution, or ferric sulfate solution; the inorganic acid anions formed in S2 and S3 are the same.

7. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, In S2, the molar ratio of iron to phosphorus in the iron salt solution is 1:(1.01-1.15), the liquid-liquid-solid reaction temperature is 30-60 ℃, and the reaction time is 5-30 min.

8. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, The pH of the liquid-liquid-solid reaction system in S2 is 1.0-2.5; the pH is controlled by adjusting the amount of circulating mother liquor, the concentration of iron salt solution, or the volume ratio of organic phase to aqueous phase.

9. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, The ferric phosphate obtained from S2 was washed with water, dried and calcined to obtain anhydrous ferric phosphate; the calcination temperature was 450-650 ℃ and the calcination time was 1-4 h.

10. The method for preparing ferric phosphate from residual raffinate according to claim 1, characterized in that, The ammonia or ammonia water mentioned in S3 is gaseous ammonia, liquid ammonia, or ammonia water; the regeneration temperature is 30-60 ℃, the regeneration time is 15-45 min, and the regeneration endpoint pH is 7.0-10.5; the aqueous phase obtained in S3 contains one of ammonium nitrate, ammonium chloride, or ammonium sulfate, and is recovered as a nitrogen fertilizer raw material.

Images