A method for fractional purification of residual acid to prepare battery-grade iron phosphate

Abstract

This invention relates to the field of ammonium phosphate solution and ferric phosphate production technology, and discloses a method for preparing battery-grade ferric phosphate through fractional purification of raffinate. The method includes: S1. Pretreatment and organic solvent removal, mixing and preheating the raffinate with ferrous sulfate solution, adding hydrogen peroxide in a pH < 0.5 system, and reacting Fe... 2+ The addition amount is 10%–20% of the molar amount of H2O2; S2. Precipitation and ammoniation: NH4F / NaF precipitant is added and heated, the pH is adjusted to 5.0–5.5 with ammonia water, and the solution is filtered with a filter aid to obtain the first filtrate; S3. Complexation and impurity removal: Gallic acid / tartaric acid complexing agent is added and reacted, and the solution is filtered again to obtain ammonium phosphate solution; S4. Iron phosphate oxidation synthesis: using ammonium phosphate solution and phosphoric acid as phosphorus source, ferrous sulfate as iron source, and hydrogen peroxide as oxidant, the solution is subjected to re-slurry aging, washing, drying, and calcination to obtain battery-grade iron phosphate. This invention effectively degrades residual organic extractants such as TBP / P2O4 in the raffinate acid, with a phosphorus recovery rate of greater than 70%, and the obtained iron phosphate meets battery-grade requirements.

Description

Technical Field

[0001] This invention relates to the field of ammonium phosphate solution and ferric phosphate production technology, and in particular to a method for preparing battery-grade ferric phosphate by fractional purification of residual acid. Background Technology

[0002] With the rapid development of the new energy vehicle and energy storage markets, iron phosphate, as an important battery material, has seen continuous innovation and improvement in its production technology. Currently, the main iron phosphate production processes in the industry include the ammonium process, the sodium process, and the iron process, with the ammonium process being the mainstream. The production process consumes a large amount of phosphoric acid, and the purity of the phosphoric acid directly affects the impurity content in the product. To reduce the impurity content in the phosphoric acid, it needs to be treated before it can be used in iron phosphate production.

[0003] In current phosphoric acid production processes, the extraction rate of phosphoric acid for impurity removal can only reach 70% (P2O5). The remaining portion forms raffinate, a mixture of high-concentration P2O5 and high-impurity iron, magnesium, aluminum, and other substances. Currently, in the production of iron phosphate, the untreated raffinate is directly recycled into the preparation of battery-grade monoammonium phosphate (MAP) and industrial MAP to reuse it. However, due to the high impurity content in the raffinate, the impurity content in the product is too high, or the phosphorus recovery rate is too low.

[0004] Currently, when reusing residual raffinate, it is often used to remove impurities before being applied to ferric phosphate. However, since the residual raffinate is obtained through extraction, there are residual organic extractants (such as TBP, P2O4, etc.) in the raw materials. Directly using chemical precipitation to remove impurities will result in impurity encapsulation and complexation, leading to a high impurity content in the final product. This cannot be effectively removed, ultimately resulting in poor performance and high impurity content in the ferric phosphate product. Furthermore, when the residual raffinate is used directly as a raw material in the preparation of ferric phosphate, the phosphorus yield is low and the loss is significant. The proportion of phosphoric acid that needs to be added to the phosphorus source is too high, which cannot effectively reduce production costs.

[0005] In the prior art, several methods for treating and utilizing residual raffinate have been disclosed. For example, Chinese patent application CN202210227393 discloses a method for preparing battery-grade iron phosphate using ferrous sulfate and phosphoric acid, but its raw material is pure phosphoric acid, which cannot solve the problem of the large amount of metallic impurities and residual organic extractants in the residual raffinate. Another example is Chinese patent application CN202410289777, which discloses a method for preparing disodium hydrogen phosphate or sodium dihydrogen phosphate from residual raffinate, but its product is not battery-grade iron phosphate, and it does not effectively remove the residual organic extractants in the residual raffinate, resulting in difficulty in further improving product purity and a low phosphorus recovery rate.

[0006] Therefore, there is an urgent need for a process and method that can efficiently purify residual acid, effectively remove organic extractants and metal impurities, achieve high phosphorus recovery rate, and ultimately prepare battery-grade iron phosphate. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for preparing battery-grade iron phosphate through fractional purification of raffinate. This method solves the problems in existing technologies where raffinate, as a byproduct of wet phosphoric acid extraction, contains a large amount of metallic impurities (iron, magnesium, aluminum, etc.) and residual organic extractants (such as TBP, P2O4). Direct reuse of this raffinate leads to product purity failing to meet battery-grade requirements and low phosphorus recovery rates. Furthermore, existing technologies lack a comprehensive technical solution for effectively degrading the organic extractants in raffinate and then purifying it through fractional purification to prepare battery-grade iron phosphate.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for preparing battery-grade iron phosphate by fractional purification of residual acid, characterized by comprising the following steps: S1. Pretreatment and organic solvent removal: The residual acid is mixed with a ferrous sulfate solution and preheated to 25–30°C. Then, hydrogen peroxide is added, and the mixture is reacted for 60 min in a system with pH < 0.5 to obtain the treated acid; the Fe in the ferrous sulfate solution... 2+ The concentration is 40–50 g / L, and the amount added is 10%–20% of the molar amount of hydrogen peroxide added; S2. Precipitation and ammoniation: A precipitant is added to the treated acid and heated to 50-55°C. The pH is then adjusted to 5.0-5.5 with an alkaline solution. The reaction is carried out for 60 minutes. A filter aid is then added and the reaction is carried out for 10 minutes. The mixture is then filtered to obtain a primary filtrate. S3. Complexation and impurity removal: Preheat the first filtrate to 65-70°C, add a complexing agent and react for 30 minutes, then filter again to obtain the filtrate, which is the ammonium phosphate solution. S4. Iron phosphate oxidation synthesis: using ferrous sulfate as the iron source, the ammonium phosphate solution and phosphoric acid as the phosphorus source, and hydrogen peroxide as the oxidant, a synthesis reaction is carried out to obtain a primary filter cake. The primary filter cake is then aged to obtain iron phosphate dihydrate, which is then washed, dried, and calcined to obtain battery-grade iron phosphate.

[0009] As a preferred technical solution, in step S1, the residual acid comes from the phosphoric acid extraction process, and its phosphorus pentoxide concentration is 25-30%, and it is diluted with pure water to a phosphorus pentoxide mass concentration of 15-20% before purification.

[0010] As a preferred technical solution, in step S1, the concentration of hydrogen peroxide is 25-30%, and the amount added is 1.0-1.5% of the mass of the residual raffinate.

[0011] As a preferred technical solution, in step S1, the residual organic extractant in the raffinate is tributyl phosphate (TBP) and / or di(2-ethylhexyl) phosphate (P204); in the pretreatment and organic solvent removal step, the chemical oxygen demand (COD) in the raffinate is degraded from the initial 700-1400 mg / L to 100-150 mg / L.

[0012] As a preferred technical solution, in step S2, the precipitant is one or more combinations of NH4F and NaF, and the amount added is 0.5 to 1.0% of the mass of the residual acid.

[0013] As a preferred technical solution, in step S2, the alkaline solution is ammonia water with a concentration of 20%; the filter aid is ethanol or isopropanol, and the amount added is 0.5‰ to 1.0‰ of the mass of the residual acid.

[0014] As a preferred technical solution, in step S3, the complexing agent is gallic acid (C... 10 H 12 O5) and / or tartaric acid (C4H6O6), the amount added is 0.8 to 1.2% of the mass of the residual acid.

[0015] As a preferred technical solution, the complexing agent is a combination of gallic acid and tartaric acid, with a volume ratio of 3:2.

[0016] As a preferred technical solution, in step S4, the conditions for re-aging of the pulp are a temperature of 90±2℃ and a solid content of 12%. The pulp is heated to 90±2℃ and kept at that temperature for 40 to 45 minutes until the material turns white. After turning white, the temperature is kept at that temperature for another 60 minutes.

[0017] As a preferred technical solution, in step S2, adding the filter aid increases the filtration speed by 20-33%. As a preferred technical solution, the phosphorus recovery rate is greater than 70% in the iron phosphate oxidation synthesis step.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) A complete process route for fractional purification was proposed for the first time, realizing the high-value utilization of residual acid. This invention organically combines four steps: Fenton degradation of organic extractant → precipitation and ammoniation to remove metals → complexation to remove impurities → synthesis of iron phosphate, creating a highly efficient conversion path from low-cost residual acid to high-value-added battery-grade iron phosphate.

[0019] (2) The Fenton degradation technology under a superacid system with pH < 0.5 was revealed for the first time, which creatively solved the problem of organic extractant residue. This invention targets the residual TBP / P2O4 organic extractant in raffinate acid, employing ferrous sulfate-catalyzed hydrogen peroxide to generate hydroxyl radicals (·OH) for oxidative degradation. Unlike the conventional Fenton reaction (pH > 2), this invention, under a superacidic system with pH < 0.5, has shown through extensive experiments that if Fe... 2+ Adding more than 20% of the molar amount of H2O2 will cause the system to become turbid, resulting in poor subsequent impurity removal and significant phosphorus loss. Therefore, this invention uses Fe 2+ The addition amount is strictly controlled within 10%–20% of the molar amount of H2O2, which is the optimal range for this specific system. After this step, the COD of the residual acid is reduced from the initial 700–1400 mg / L to 100–150 mg / L, with a removal rate exceeding 85%, eliminating the interference of the organic extractant on the subsequent precipitation and impurity removal processes from the source.

[0020] (3) The fractional purification strategy of precipitation ammoniation + complexation removal significantly improved the removal efficiency and phosphorus recovery rate. In the precipitation-ammoniation step, NH4F / NaF acts as a precipitant, reacting with metal impurity ions to form fluoride precipitates, effectively preventing phosphorus loss caused by the combination of phosphate ions and metal ions. In the complexation and impurity removal step, gallic acid / tartaric acid acts as a complexing agent to further remove residual metal impurity ions from the filtrate, preventing some of the precipitated ions from re-dissolving and forming soluble double salts. Through the synergistic effect of these two steps, the phosphorus recovery rate of this invention reaches over 70%.

[0021] (4) Introducing filter aids significantly improves process efficiency. Adding ethanol or isopropanol as a filter aid after the precipitation and ammoniation step can increase the filtration speed by 20-33%, effectively shortening the production cycle and reducing energy consumption.

[0022] (5) The obtained iron phosphate product meets the battery grade requirements. This invention uses ammonium phosphate solution prepared from residual raffinate to replace battery-grade or industrial-grade monoammonium phosphate as the phosphorus source in the traditional ammonium process. The synthesized iron phosphate product has a low content of various metal impurity elements (see Table 1 for specific data), which can meet the requirements of battery-grade iron phosphate (such as YS / T 1027-2022 standard).

[0023] (6) High phosphorus recovery rate reduces production costs This invention achieves a final phosphorus recovery rate of over 70% through stepwise impurity removal and precise control of reaction conditions, which is significantly higher than the phosphorus recovery rate of existing technologies. This greatly reduces the amount of phosphoric acid to be added, and further reduces the production cost of iron phosphate. Attached Figure Description

[0024] Figure 1A process flow diagram for the preparation of battery-grade iron phosphate based on fractional purification of residual acid provided by the present invention; Figure 2 This is a scanning electron microscope (SEM) image of the iron phosphate prepared in Example 1 of the present invention; Figure 3 This is a scanning electron microscope (SEM) image of the iron phosphate prepared in Example 2 of the present invention; Figure 4 This is a scanning electron microscope (SEM) image of the iron phosphate prepared in Example 3 of the present invention; Figure 5 This is a scanning electron microscope (SEM) image of the iron phosphate prepared in Example 4 of the present invention; Figure 6 This is a scanning electron microscope (SEM) image of the iron phosphate prepared in Example 5 of the present invention; Figure 7 This is a scanning electron microscope (SEM) image of the iron phosphate prepared in Example 6 of the present invention; Figure 8 This is a scanning electron microscope (SEM) image of the iron phosphate prepared in Example 7 of the present invention. Detailed Implementation

[0025] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0026] Unless otherwise specified, all materials and instruments used in the following embodiments were obtained through commercial channels; and all detection methods used are existing methods unless otherwise specified.

[0027] Example 1

[0028] Take 20 kg of residual raffinate and dilute it with water to a phosphorus pentoxide concentration of 18%. Add hydrogen peroxide (30% concentration) at 1.0% of the mass of the residual raffinate, and add ferrous sulfate solution (Fe2+) at 10% of the molar amount of the added hydrogen peroxide. 2+ (Concentration of 40-50 g / L).

[0029] The order of addition is as follows: first, add the ferrous sulfate solution to the diluted residual acid and preheat it to 25°C, then slowly add hydrogen peroxide and continue the reaction at 25°C for 60 minutes.

[0030] After the reaction was completed, NH4F was added to the solution at 0.5% of the mass of the residual acid and the mixture was heated to 50°C. The pH was adjusted to 5.0 with ammonia. Ethanol was added at 0.5‰ of the mass of the residual acid and the mixture was reacted for 10 minutes. The mixture was then filtered once using a plate and frame filter to obtain the first filtrate.

[0031] The filtrate was heated to 65°C, and gallic acid was added at 0.8% of the mass of the residual acid. The reaction was continued for 30 minutes, followed by a second plate and frame filtration to obtain an ammonium phosphate solution. The phosphorus recovery rate was found to be 70–75%.

[0032] Using ferrous sulfate as the iron source, the aforementioned ammonium phosphate solution and phosphoric acid as the phosphorus source, and hydrogen peroxide as the oxidant, a synthesis reaction was carried out to obtain a primary filter cake. The primary filter cake was then subjected to aging at 90±2℃ and a solid content of 12%: the slurry was heated to 90±2℃ and held for 40–45 minutes until the material turned white, and then held for another 60 minutes to obtain ferric phosphate dihydrate. After washing, drying, and calcination, battery-grade ferric phosphate was obtained. The SEM image of the obtained ferric phosphate is shown below. Figure 2 As shown.

[0033] Example 2

[0034] Take 20 kg of residual raffinate and dilute it with water to a phosphorus pentoxide concentration of 18%. Add hydrogen peroxide (30% concentration) at 1.0% of the residual raffinate mass, and add ferrous sulfate solution (Fe2+) at 20% of the added hydrogen peroxide molar amount. 2+ (Concentration of 40-50 g / L).

[0035] The order of addition is as follows: first, add the ferrous sulfate solution to the diluted residual acid and preheat it to 25°C, then slowly add hydrogen peroxide and continue the reaction at 25°C for 60 minutes.

[0036] After the reaction was completed, NH4F was added to the solution at 1.0% of the mass of the residual acid and the mixture was heated to 50°C. The pH was adjusted to 5.5 with ammonia. Ethanol was added at 1.0‰ of the mass of the residual acid and the mixture was reacted for 10 minutes. The mixture was then filtered once using a plate and frame filter press to obtain the first filtrate.

[0037] The filtrate was heated to 70°C, and gallic acid was added at 1.2% of the mass of the residual acid. The reaction was continued for 30 minutes, followed by a second plate and frame filtration to obtain an ammonium phosphate solution. The phosphorus recovery rate was found to be 70–75%.

[0038] Using ferrous sulfate as the iron source, the aforementioned ammonium phosphate solution and phosphoric acid as the phosphorus source, and hydrogen peroxide as the oxidant, a synthesis reaction was carried out to obtain a primary filter cake. The primary filter cake was then subjected to slurry aging, washing, drying, and calcination under the same conditions as in Example 1 to obtain battery-grade iron phosphate. The SEM image of the obtained iron phosphate is shown below. Figure 3 As shown.

[0039] Example 3

[0040] Take 20 kg of residual raffinate and dilute it with water to a phosphorus pentoxide concentration of 18%. Add hydrogen peroxide (30% concentration) at 1.0% of the mass of the residual raffinate, and add ferrous sulfate solution (Fe2+) at 10% of the molar amount of the added hydrogen peroxide. 2+ (Concentration of 40-50 g / L).

[0041] The order of addition is as follows: first, add the ferrous sulfate solution to the diluted residual acid and preheat it to 25°C, then slowly add hydrogen peroxide and continue the reaction at 25°C for 60 minutes.

[0042] After the reaction was completed, NaF was added to the solution at 0.5% of the mass of the residual acid and the mixture was heated to 50°C. Isopropanol was then added at 0.5‰ of the mass of the residual acid and reacted for 10 minutes. The mixture was then filtered once using a plate and frame filter to obtain the first filtrate.

[0043] The filtrate was heated to 70°C, and tartaric acid was added at 0.8% of the mass of the residual acid. The reaction was continued for 30 minutes, followed by a second plate and frame filtration to obtain an ammonium phosphate solution. The phosphorus recovery rate was found to be 70–75%.

[0044] Using ferrous sulfate as the iron source, the aforementioned ammonium phosphate solution and phosphoric acid as the phosphorus source, and hydrogen peroxide as the oxidant, a synthesis reaction was carried out to obtain a primary filter cake. The primary filter cake was then subjected to slurry aging, washing, drying, and calcination under the same conditions as in Example 1 to obtain battery-grade iron phosphate. The SEM image of the obtained iron phosphate is shown below. Figure 4 As shown.

[0045] Example 4

[0046] Take 20 kg of residual raffinate and dilute it with water to a phosphorus pentoxide concentration of 18%. Add hydrogen peroxide (30% concentration) at 1.0% of the residual raffinate mass, and add ferrous sulfate solution (Fe2+) at 20% of the added hydrogen peroxide molar amount. 2+ (Concentration of 40-50 g / L).

[0047] The order of addition is as follows: first, add the ferrous sulfate solution to the diluted residual acid and preheat it to 25°C, then slowly add hydrogen peroxide and continue the reaction at 25°C for 60 minutes.

[0048] After the reaction was completed, NaF was added to the solution at 1.0% of the mass of the residual acid and the mixture was heated to 50°C. The pH was adjusted to 5.0 with ammonia and the reaction was continued for 10 min. Isopropanol was added at 1.0‰ of the mass of the residual acid and the reaction was continued for 10 min. The mixture was then filtered once using a plate and frame filter press to obtain the first filtrate.

[0049] The filtrate was heated to 65°C, and tartaric acid was added at 1.2% of the mass of the residual acid. The reaction was continued for 30 minutes, followed by a second plate and frame filtration to obtain an ammonium phosphate solution. The phosphorus recovery rate was found to be 70–75%.

[0050] Using ferrous sulfate as the iron source, the aforementioned ammonium phosphate solution and phosphoric acid as the phosphorus source, and hydrogen peroxide as the oxidant, a synthesis reaction was carried out to obtain a primary filter cake. The primary filter cake was then subjected to slurry aging, washing, drying, and calcination under the same conditions as in Example 1 to obtain battery-grade iron phosphate. The SEM image of the obtained iron phosphate is shown below. Figure 5 As shown.

[0051] Example 5

[0052] The difference from Example 1 is as follows: Ferrous sulfate solution was mixed with residual raffinate and preheated to 25°C; the reaction time was 50 min; the phosphorus pentoxide concentration in the residual raffinate was 20%; the residual raffinate was diluted with pure water to a phosphorus pentoxide mass concentration of 15% before use; the concentration of hydrogen peroxide used was 25%, and the amount of hydrogen peroxide added was 1.0% of the mass of the residual raffinate; the Fe content in the ferrous sulfate solution was... 2+ The concentration was 40 g / L, and the amount added was 10% of the molar amount of hydrogen peroxide added.

[0053] The precipitant was NH4F, and the complexing agent was gallic acid.

[0054] A precipitant was added to the treated acid and heated to 50°C. The pH was then adjusted to 5.5 with alkali solution, and the reaction continued. A filter aid was added, and the reaction was carried out for another 10 minutes before pressure filtration to obtain the first filtrate. The first filtrate was then heated to 65°C, and gallic acid was added at 0.8% of the mass of the residual acid. The reaction was continued for another 30 minutes, followed by a second plate and frame filtration to obtain an ammonium phosphate solution.

[0055] The subsequent iron phosphate synthesis steps were the same as in Example 1. The SEM image of the obtained iron phosphate is shown below. Figure 6 As shown.

[0056] Example 6

[0057] The difference from Example 1 is as follows: Ferrous sulfate solution was mixed with residual raffinate and preheated to 30°C; the phosphorus pentoxide concentration in the residual raffinate was 30%; the residual raffinate was diluted with pure water to a phosphorus pentoxide mass concentration of 20% before use; the concentration of hydrogen peroxide used was 30%, and the amount of hydrogen peroxide added was 1.5% of the mass of the residual raffinate; the Fe content in the ferrous sulfate solution used was... 2+ The concentration is 50 g / L, and the amount added is 20% of the molar amount of hydrogen peroxide added.

[0058] A precipitant was added to the treated acid and heated to 55°C. The pH was then adjusted to 5.5 with alkali solution, and the reaction continued. A filter aid was added, and the reaction was carried out for another 10 minutes before pressure filtration to obtain the first filtrate. The first filtrate was then heated to 65°C, and gallic acid was added at 1.2% of the mass of the residual acid. The reaction was continued for another 30 minutes, followed by a second plate and frame filtration to obtain an ammonium phosphate solution.

[0059] The subsequent iron phosphate synthesis steps were the same as in Example 1. The SEM image of the obtained iron phosphate is shown below. Figure 7 As shown.

[0060] Example 7

[0061] The difference from Example 1 is as follows: The precipitant is a mixture of NH4F and NaF in a volume ratio of 1:1; The complexing agent is a mixture of gallic acid and tartaric acid in a volume ratio of 3:2; The filter aid is isopropanol.

[0062] The remaining operating steps are the same as in Example 1. The SEM image of the obtained ferric phosphate is shown below. Figure 8 As shown.

[0063] Comparative Experiment Example 1: Verification of Fenton Degradation Effect The same residual acid sample as in Example 1 was taken and tested. The residual organic extractant was found to be tributyl phosphate (TBP) and / or di(2-ethylhexyl) phosphate (P204), and the initial chemical oxygen demand (COD) was 1100 mg / L.

[0064] Pretreatment and organic solvent removal were performed according to step S1 of this application: Ferrous sulfate solution (Fe...) was added to a system with pH < 0.5. 2+ The concentration was 45 g / L, and the amount added was 15% of the molar amount of hydrogen peroxide added. The mixture was preheated to 28°C, and then hydrogen peroxide (concentration 30%, amount added was 1.2% of the mass of the residual acid) was added. The reaction was carried out for 60 min.

[0065] After the reaction, the COD value of the treated acid was measured to be 120 mg / L, and the COD removal rate was 89.1%. Gas chromatography-mass spectrometry (GC-MS) analysis confirmed that the characteristic peak intensities of TBP and / or P2O4 decreased by more than 90%, proving that the organic extractant was effectively degraded.

[0066] Comparative Experiment Example 2: Fe 2+ Increased dosage optimization experiment Take the same residual acid sample as in Example 1, and add ferrous sulfate solution (Fe) at 5%, 10%, 20%, 30%, 50%, and 100% of the molar amount of hydrogen peroxide added, respectively, in a system with pH < 0.5. 2+ (Concentration 45 g / L). The experimental phenomena and results are shown in Table 1 below: Experimental results show that in a superacidic system with pH < 0.5, when Fe 2+When the amount added exceeds 20% of the molar amount of H2O2, the system becomes turbid, the subsequent impurity removal effect decreases, and phosphorus loss increases. When Fe 2+ When the amount of H2O2 added is controlled at 10% to 20% of the molar amount, the system remains clear, the impurity removal efficiency is high, and the phosphorus recovery rate can reach over 70%. Therefore, the 10% to 20% range specified in this application is the optimal range for this specific system.

[0067] Comparative Experiment Example 3: Comparison of Filter Aid Effects Take the same residual acid sample as in Example 1 and process it according to steps S1 and S2 of this application. After the precipitation and amination step is completed, take two portions of the same volume of the mixture.

[0068] Control group: Direct plate and frame filtration.

[0069] Experimental group: Add 0.8‰ of ethanol to the residual acid, stir for 10 min and then filter by plate and frame.

[0070] The comparison revealed that the filtration speed of the experimental group was 26% higher than that of the control group, and the moisture content of the filter cake was lower. Similar results were observed when isopropanol was added.

[0071] Comparative Experiment Example 4: Comparison of Synergistic Effects of Complexing Agents Take the same residual acid sample as in Example 1 and process it according to steps S1 and S2 of this application to obtain the same primary filtrate. In the complexation and impurity removal step, the primary filtrate is treated using the following three methods: Option A: Add only gallic acid (1.0%); Option B: Add only tartaric acid (1.0%). Option C: Add gallic acid and tartaric acid simultaneously (volume ratio of 3:2, total addition 1.0%).

[0072] The obtained ammonium phosphate solution was used to prepare battery-grade iron phosphate under the same conditions. The content of key metal impurities in the final product was detected, and the results are shown in Table 2 below. The results showed that the impurity content of the product from Scheme B was lower than that from Scheme A, while the impurity content of the product from Scheme C (used in combination) was the lowest, proving that gallic acid and tartaric acid have a synergistic effect.

[0073] Performance testing The various indicators of the ammonium phosphate solution and ferric phosphate obtained in Examples 1 to 7 were tested according to existing methods, and the results are listed in Table 3.

[0074] Table 3. Performance indicators of ammonium phosphate solutions and ferric phosphate obtained in Examples 1-7 From Table 3 and Figures 2-8 It is evident that the method provided by this invention can effectively remove metal impurities and various other impurities from the residual acid, while avoiding the removal of phosphorus, increasing the phosphorus recovery rate (>70%), reducing the amount of phosphoric acid added during the preparation of ferric phosphate, and effectively reducing the content of metal impurities in ferric phosphate made from residual acid while reducing production costs. The resulting ferric phosphate product meets battery-grade requirements.

[0075] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing battery-grade iron phosphate by fractional purification of residual acid, characterized in that, Includes the following steps: S1. Pretreatment and organic solvent removal: The residual acid is mixed with a ferrous sulfate solution and preheated to 25–30°C. Then, hydrogen peroxide is added, and the mixture is reacted for 60 min in a system with pH < 0.5 to obtain the treated acid; the Fe in the ferrous sulfate solution... 2+ The concentration is 40–50 g / L, and the amount added is 10%–20% of the molar amount of hydrogen peroxide added; S2. Precipitation and ammoniation: A precipitant is added to the treated acid and heated to 50-55°C. The pH is then adjusted to 5.0-5.5 with an alkaline solution. The reaction is carried out for 60 minutes. A filter aid is then added and the reaction is carried out for 10 minutes. The mixture is then filtered to obtain a primary filtrate. S3. Complexation and impurity removal: Preheat the first filtrate to 65-70°C, add a complexing agent and react for 30 minutes, then filter again to obtain the filtrate, which is the ammonium phosphate solution. S4. Iron phosphate oxidation synthesis: using ferrous sulfate as the iron source, the ammonium phosphate solution and phosphoric acid as the phosphorus source, and hydrogen peroxide as the oxidant, a synthesis reaction is carried out to obtain a primary filter cake. The primary filter cake is then aged to obtain iron phosphate dihydrate, which is then washed, dried, and calcined to obtain battery-grade iron phosphate.

2. The method according to claim 1, characterized in that: In step S1, the residual acid comes from the phosphoric acid extraction process, and its phosphorus pentoxide concentration is 25-30%. Before purification, it is diluted with pure water to a phosphorus pentoxide mass concentration of 15-20%.

3. The method according to claim 1, characterized in that: In step S1, the concentration of hydrogen peroxide is 25-30%, and the amount added is 1.0-1.5% of the mass of the residual raffinate.

4. The method according to claim 1, characterized in that: In step S1, the residual organic extractant in the raffinate is tributyl phosphate and / or di(2-ethylhexyl) phosphate; in the pretreatment and organic solvent removal step, the chemical oxygen demand in the raffinate is reduced from the initial 700-1400 mg / L to 100-150 mg / L.

5. The method according to claim 1, characterized in that: In step S2, the precipitant is one or more combinations of NH4F and NaF, and the amount added is 0.5 to 1.0% of the mass of the residual acid.

6. The method according to claim 1, characterized in that: In step S2, the alkaline solution is ammonia water with a concentration of 20%; the filter aid is ethanol or isopropanol, and the amount added is 0.5‰ to 1.0‰ of the mass of the residual acid.

7. The method according to claim 1, characterized in that: In step S3, the complexing agent is gallic acid and / or tartaric acid, and the amount added is 0.8 to 1.2% of the mass of the residual acid.

8. The method according to claim 7, characterized in that: The complexing agent is a combination of gallic acid and tartaric acid in a volume ratio of 3:

2.

9. The method according to claim 1, characterized in that: In step S4, the conditions for re-aging the pulp are a temperature of 90±2℃ and a solid content of 12%. The pulp is heated to 90±2℃ and kept at that temperature for 40-45 minutes until the material turns white. After turning white, the temperature is kept at that temperature for another 60 minutes.

10. The method according to claim 1, characterized in that: In step S2, the filtration speed is increased by 20-33% after adding the filter aid.

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