A method for recovering phosphorus and rare earths from a rare earth leach solution to produce battery grade iron phosphate

Abstract

The application discloses a method for recovering phosphorus and rare earth from a rare earth leaching solution and preparing battery-grade iron phosphate, and belongs to the field of rare earth hydrometallurgy. In view of the problem that local over-alkalization in the existing alkaline neutralization process leads to co-precipitation of rare earth and generation of magnesium sulfate wastewater, the application uses an organic extractant to extract free acid and moderately controls the acidity, and selectively precipitates Fe-P under the condition of no additional metal ions; the precipitate is restructured into primary iron phosphate through temperature swing crystallization, so that rare earth carried in the primary iron phosphate returns to the solution; battery-grade iron phosphate is obtained through dissolution-reprecipitation; the extractant is back-extracted for regeneration, and rare earth in the raffinate is recovered through P507 extraction. The overall process has a phosphorus recovery rate of >89%, an iron recovery rate of >89%, a rare earth loss rate of <0.008%, no magnesium sulfate wastewater, and the product meets the HG / T 4701-2021 standard. The method realizes the collaborative recovery of phosphorus and rare earth and the high-value utilization of phosphorus and iron resources.

Description

Technical Field

[0001] This invention belongs to the field of rare earth hydrometallurgy, and specifically relates to a method for recovering phosphorus and rare earths from rare earth leachate to prepare battery-grade iron phosphate. Background Technology

[0002] Rare earth elements are crucial strategic resources. Due to their unique physicochemical properties, they have wide applications in electronics, new energy, national defense, and many other fields, earning them the reputation of being the "vitamins" of modern industry and a treasure trove of new materials. Currently, rare earth elements are mainly obtained from minerals such as bastnaesite, monazite (REPO4), ion-adsorption rare earth minerals, and mixed rare earth minerals. For phosphorus-containing rare earth minerals like monazite and mixed rare earth minerals, the industrial process primarily uses concentrated sulfuric acid decomposition to extract rare earth elements. However, during the mineral decomposition process, the associated phosphorus inevitably enters the leachate, forming a solution containing rare earth ions (REPs). 3+ ), iron ions (Fe 3+ ), phosphate (PO4) 3- ) and sulfate (SO4) 2- A complex acidic system.

[0003] In the aforementioned system, there is a strong interaction between phosphate and rare earth elements, leading to the easy formation of rare earth phosphate precipitates during subsequent purification and extraction processes, resulting in significant rare earth losses. Traditional processes typically remove phosphorus from the leachate by using iron salts for phosphorus fixation followed by alkali neutralization and precipitation. For example, alkaline neutralizing agents such as magnesium oxide, calcium oxide, or rare earth carbonate are added to the rare earth sulfate solution to adjust the pH and promote the precipitation of ferric phosphate or ferric hydroxide, thereby removing phosphate. However, this method has several drawbacks: firstly, the resulting phosphorus-containing precipitate often contains impurities such as rare earth elements, iron, and thorium, resulting in a complex composition that is difficult to utilize effectively. It is usually treated as dephosphorization slag or leaching residue, creating solid waste disposal pressure; secondly, the alkali neutralization process easily introduces sodium (Na₂O₃). + Ca 2+ Mg 2+ or NH4 + The presence of exogenous ions not only increases the ion load of the system but may also cause localized over-alkaliness due to uneven mass transfer at the solid-liquid interface, leading to the co-precipitation loss of rare earth phosphates and hydroxides. Furthermore, in strongly acidic rare earth leachates, processes such as phosphate deprotonation, iron hydrolysis, iron phosphate precipitation, rare earth phosphate precipitation, and sulfate complexation are coupled, resulting in a narrow selective separation window between phosphorus, iron, and rare earths, making precise control difficult.

[0004] Several front-end stepwise extraction processes have been proposed for the separation of phosphorus and rare earth elements in phosphorus-containing rare earth ores. For example, patent CN201810545487.X proposes a stepwise leaching process after low-temperature roasting with concentrated sulfuric acid. This involves roasting the phosphorus-containing rare earth concentrate with concentrated sulfuric acid, followed by a primary water leaching to preferentially obtain a phosphoric acid extract, and then a secondary dilute sulfuric acid leaching of the residue to obtain a rare earth extract. Patent CN202111382247.0 further employs multi-stage countercurrent leaching to increase the phosphorus concentration in the phosphoric acid extract, and utilizes a mixed extractant to extract, wash, and back-extract the phosphoric acid extract to obtain the finished phosphoric acid product. However, these front-end processes primarily target phosphorus-containing rare earth concentrates or roasted ores, and their target products are typically phosphoric acid extract and sulfuric acid rare earth extract. For rare earth leaching solutions already formed in actual production, especially systems where phosphorus, iron, rare earth elements, and sulfate coexist, these processes cannot directly solve the problem of selective phosphorus recovery and high-value utilization of phosphorus-containing iron components.

[0005] Regarding the removal of phosphorus from rare earth leaching solutions, some patents have proposed improved solutions. For example, patent CN 114737049 A proposes adding iron ions to the leaching section of rare earth roasted ore, causing the iron ions to react with phosphate ions to form iron phosphate precipitate, which is then removed by solid-liquid separation. Patent CN 116622989 A proposes adding ferrous salt to a rare earth sulfate solution, adjusting the iron-phosphorus molar ratio, and then using an oxidant to remove the Fe... 2+ Oxidized to Fe 3+ Then, the pH of the system is adjusted to a suitable range using a pH adjuster, so that Fe... 3+ Phosphate precipitates with phosphate ions. This type of method can improve phosphate removal efficiency and control rare earth loss to some extent. However, existing phosphorus removal technologies in rare earth leaching solutions still have the following shortcomings: First, ferric salt fixation or ferrous oxidation for phosphorus removal mainly aims to reduce the phosphorus content in the rare earth solution. The resulting ferric phosphate or phosphorus-containing slag is usually treated as phosphorus removal slag or leaching slag, failing to achieve high-value utilization of phosphorus-containing iron precipitates; Second, this type of method usually requires the addition of magnesium oxide, calcium oxide, rare earth carbonate, or other pH adjusters, which can easily increase the ion load of the system and may cause local pH over-extension, leading to co-precipitation of rare earth phosphates or rare earth hydroxides; Third, in acidic rare earth leaching solutions, multiple precipitation and complexation processes are coupled, resulting in a narrow selective separation window between phosphorus, iron, and rare earths, making precise control difficult.

[0006] Therefore, developing a method that can efficiently recover phosphorus and rare earth elements from rare earth sulfate leachate, avoid the introduction of exogenous ions and local over-alkalization, and convert the recovered phosphorus iron components into battery-grade iron phosphate at high value, has significant industrial and environmental value for reducing rare earth losses, reducing solid waste generation, and achieving comprehensive resource utilization. Summary of the Invention

[0007] Therefore, the present invention aims to provide a method for recovering phosphorus and rare earths from rare earth leachate to prepare battery-grade iron phosphate, so as to significantly reduce rare earth loss without introducing exogenous metal ions and avoiding local over-alkalization, thereby realizing high-value utilization of phosphorus and iron resources and closed-loop recycling of rare earths.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for recovering phosphorus and rare earth elements from rare earth leachate to prepare battery-grade iron phosphate includes adding an iron source to a rare earth sulfate leachate to adjust the iron-phosphorus ratio, causing iron to react with phosphate ions to form a precipitate, and treating the precipitate to prepare battery-grade iron phosphate; the method includes the following steps: S1: Add ferric or ferrous salts to rare earth sulfate leaching solution and control the iron-phosphorus molar ratio to 1:1~3:1; S2: The solution obtained in S1 is mixed with the organic phase at a volume ratio of 1:5 to 5:1 for extraction and precipitation reaction. The reaction temperature is 15 to 75°C, and the reaction time is 5 min to 120 min. After the reaction, the raffinate, acid-containing extractant, and Fe-P precipitate are separated. The organic phase consists of an extractant, a phase modifier, and a diluent, kerosene. The volume fraction of the extractant is 10% to 50%, the volume fraction of the phase modifier is 5% to 20%, and the volume fraction of kerosene is 30% to 85%. The extractant is selected from one or more of tributyl phosphate, trialkylphosphine oxide, trioctylamine, tripropylamine, tributylamine, tripentylamine, or trioctyldecyl tertiary amine. The phase modifier is selected from n-butanol, isoamyl alcohol, n-hexanol, n-octanol, or isooctanol. S3: Dissolve the Fe-P precipitate obtained in S2 in dilute sulfuric acid at a temperature of 20~50℃ and a pH of 0~1, add phosphoric acid to control the iron-phosphorus molar ratio to 0.7~1.3, and then raise the temperature to 70~130℃ for thermal crystallization reaction for 2h~10h to obtain primary iron phosphate. The coprecipitated rare earth elements are retained in the solution. S4: Dissolve the primary iron phosphate obtained in S3 in 0.5~4 mol / L sulfuric acid, then adjust the pH to 1.0~4.0 with alkaline solution to carry out precipitation reaction. The reaction temperature is 25~95℃ and the reaction time is 0.5h~10h. Solid-liquid separation yields battery-grade iron phosphate product and sulfate solution.

[0009] According to a preferred embodiment of the method of the present invention, the organic phase in S2 consists of 20% to 40% by volume of trioctylamine or tributyl phosphate, 5% to 15% by volume of isooctanol or n-octanol, and the balance being kerosene.

[0010] According to a preferred embodiment of the method of the present invention, the volume ratio of the organic phase to the solution in S2 is 1:3 to 3:1, the reaction temperature is 25 to 50°C, and the reaction time is 10 min to 90 min.

[0011] According to a preferred embodiment of the method of the present invention, the temperature of the dilute sulfuric acid in S3 is 25~50℃, the pH is 0.2~0.8, the iron-phosphorus molar ratio after adding phosphoric acid is 0.9~1.1, the temperature of the thermal crystallization reaction is 80~120℃, and the reaction time is 2h~6h.

[0012] According to a preferred embodiment of the method of the present invention, the concentration of sulfuric acid in S4 is 1~2.5 mol / L, the alkaline solution is ammonia water with a concentration of 0.5~3 mol / L, the pH is adjusted to 1.0~3.0, the reaction temperature is 60~90℃, and the reaction time is 0.5h~4h.

[0013] According to a further embodiment of the method of the present invention, the method further includes step S5: back-extracting the acid-containing extractant obtained in S2 with an alkaline solution of concentration of 0.5~5 mol / L at an organic phase to aqueous phase volume ratio of 1:1~15:1 to obtain a regenerated extractant and a back-extracting solution; combining the back-extracting solution with the sulfate solution obtained in S4, and obtaining sulfate crystals and mother liquor by concentration, crystallization and solid-liquid separation, wherein the mother liquor is used to prepare the alkaline solution.

[0014] According to a further embodiment of the method of the present invention, step S6 is further included: mixing the raffinate obtained in S2 with saponified P507 extractant at a volume ratio of 1:5 to 5:1 to carry out an extraction reaction to recover rare earth elements in the raffinate; wherein the volume fraction of P507 in the extractant is 10% to 50%, the volume fraction of kerosene is 50% to 90%, the reaction temperature is 25 to 75°C, the reaction time is 5 min to 60 min, and the degree of saponification of P507 is 10% to 40%.

[0015] According to a preferred embodiment of the method of the present invention, in S6, the volume fraction of P507 is 20%~30%, the volume fraction of kerosene is 70%~80%, the reaction temperature is 25~35℃, the reaction time is 10~60min, the degree of saponification of P507 is 10%~30%, and the volume ratio of the raffinate to the extractant is 1:2~4:1.

[0016] According to a preferred embodiment of the method of the present invention, the iron salt in S1 is polyferric sulfate, and the ferrous salt is ferrous sulfate. When the ferrous salt is added, an oxidizing agent is also added to remove the Fe. 2+ Oxidized to Fe 3+ The iron-phosphorus molar ratio is 1.5:1 to 2:1.

[0017] According to a preferred embodiment of the method of the present invention, the solution containing the rare earth element in S3 is recycled back to S3 for dissolving the Fe-P precipitate; the sulfate solution obtained in S4 is concentrated and crystallized, and the mother liquor is recycled for preparing the alkaline solution in S4 and / or S5.

[0018] The present invention achieves the following beneficial effects through the above technical solution: (1) In the extraction, precipitation and recovery of phosphorus iron stage, this invention does not directly add an inorganic alkaline neutralizing agent to the rare earth sulfate leaching solution to adjust the pH. Instead, it uses an organic extractant to extract free acid from the aqueous phase, achieving a mild control of the system's acidity. This process uniformly reduces acidity through liquid-liquid interface mass transfer, rather than through solid-liquid acid-base neutralization reaction. This avoids problems such as local over-alkaliening, large-scale introduction of exogenous ions, and co-precipitation of rare earth phosphates or rare earth hydroxides in traditional alkali neutralization processes, significantly reducing rare earth loss and the burden of subsequent wastewater treatment. At the same time, the acidic organic phase can achieve extractant regeneration and recycling after back-extraction, and the back-extraction solution can be further concentrated and crystallized to obtain sulfate by-products, which helps reduce reagent consumption and improve process economy.

[0019] (2) This invention achieves the conversion of Fe-P precipitate into primary ferric phosphate through temperature-controlled oscillating crystallization, and allows co-precipitated or entrained rare earth elements to re-enter the solution. This process utilizes low-temperature dissolution to break down the amorphous precipitate structure and release entrained rare earth elements, followed by high-temperature thermal crystallization to promote the preferential growth of crystalline ferric phosphate, thereby further reducing rare earth loss. Experiments show that the overall phosphorus recovery rate can reach 89.83%~99.33%, the iron recovery rate can reach 89.87%~98.54%, the rare earth loss rate is as low as 0.004%~0.008%, and the rare earth impurity content in the product is significantly lower than that of traditional processes.

[0020] (3) This invention proposes a complete process route to convert phosphorus in the leachate into battery-grade iron phosphate product that meets the HG / T 4701-2021 standard through a three-stage progressive treatment of extraction precipitation, temperature swing crystallization, and dissolution and reprecipitation. This avoids the generation of phosphorus iron slag in traditional phosphorus removal processes and helps reduce the pressure of solid waste disposal and environmental risks. At the same time, the sulfate solution and raffinate generated in the process can be recycled and utilized, forming a closed-loop recycling system for phosphorus-rare earth synergistic recovery. Detailed Implementation

[0021] The following description is based on specific embodiments.

[0022] 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.

[0023] 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.

[0024] Example 1

[0025] This embodiment provides a method for recovering phosphorus and rare earth elements from rare earth leachate to prepare battery-grade iron phosphate. The specific steps are as follows: S1: Iron-to-phosphorus ratio adjustment. Take 400 mL of rare earth sulfate leaching solution into a 1000 mL beaker. The rare earth concentration in this leaching solution is 29.1 g / L (based on rare earth oxide (REO) content), and the P2O5 concentration is 7.2 g / L. Add solid polyferric sulfate, stir to dissolve and mix thoroughly, controlling the iron-to-phosphorus molar ratio in the solution to be 2:1.

[0026] S2: Extraction and precipitation reaction. Preparation of the organic phase: Measure trioctylamine, isooctyl alcohol, and kerosene, and mix them thoroughly at volume fractions of 30%, 10%, and 60%, respectively. Place the solution obtained in S1 (aqueous phase) and the above organic phase at a volume ratio of 1:3 (aqueous phase:organic phase) in a separatory funnel, and extract by shaking in a 25°C water bath for 30 min. After standing and separating the layers, collect the organic phase (containing acid extractant), the aqueous phase (raffinate), and the solid phase (Fe-P precipitate) separately. Wash the solid phase 2-3 times with a small amount of deionized water and label it "Fe-P precipitate". The precipitation rate of phosphorus was determined to be 99.98%, the precipitation rate of iron was 99.93%, and the rare earth co-precipitation rate was 0.89%.

[0027] S3: Temperature-controlled crystallization. The above Fe-P precipitate was placed in a 250 mL beaker, and 100 mL of dilute sulfuric acid (25℃, pH 0.2) was added. The mixture was stirred until completely dissolved. Phosphoric acid was then added to the solution to maintain an iron-to-phosphorus molar ratio of 1.0. The solution was transferred to a hydrothermal reactor and heated to 120℃ for thermal crystallization for 4 h. After the reaction, the mixture was cooled to room temperature, filtered, and the primary ferric phosphate and filtrate were collected separately. The filtrate was designated "Filtrate I" (containing rare earth elements and can be recycled back to S3). The primary ferric phosphate was washed with deionized water until neutral and designated "Primary Ferric Phosphate". During this process, the phosphorus yield was 98.82%, the iron yield was 98.61%, and the rare earth element retention rate was 99.51%.

[0028] S4: Dissolution-Reprecipitation. Place the above primary ferric phosphate in a 250 mL beaker, add 100 mL of 2 mol / L sulfuric acid, and stir to dissolve. Add 1 mol / L ammonia dropwise to the solution to adjust the pH to 2.0. Heat the solution to 90℃ and stir for 0.5 h. After the reaction is complete, filter, wash with deionized water until neutral, and dry at 60℃ to obtain the battery-grade ferric phosphate product. The filtrate is designated as "Filtrate II" (ammonium sulfate solution).

[0029] S5: Extractant regeneration and sulfate recovery. The acid-containing extractant obtained from S2 is mixed with 1 mol / L ammonia water at a volume ratio of 9:1 (organic phase: aqueous phase) for back-extraction to obtain regenerated extractant and back-extraction solution. The back-extraction solution is combined with filtrate II obtained from S4, and after concentration, crystallization, and solid-liquid separation, ammonium sulfate crystals and mother liquor are obtained. The mother liquor is used to prepare ammonia solution.

[0030] S6: Rare earth recovery. The raffinate obtained from S2 was mixed with P507-kerosene extractant (P507 volume fraction 30%, kerosene volume fraction 70%, saponification degree 30%) at a volume ratio of 1:2 (aqueous phase: organic phase), and extracted at 25℃ for 10 min. The organic phase and aqueous phase were separated. The organic phase was back-extracted to obtain a rare earth enriched solution, and the aqueous phase was the tail liquid after rare earth removal.

[0031] Calculations show that the phosphorus recovery rate in this embodiment is 98.81%, the iron recovery rate is 98.54%, and the rare earth loss rate is 0.004%. The quality indicators of the obtained battery-grade iron phosphate product are shown in Table 1.

[0032] Example 2

[0033] This embodiment provides a method for recovering phosphorus and rare earth elements from rare earth leachate to prepare battery-grade iron phosphate. The specific steps are as follows: S1: Iron-to-phosphorus ratio adjustment. Take a rare earth sulfate leaching solution with a rare earth concentration of 24.8 g / L (based on REO) and a P2O5 concentration of 5.4 g / L. Add ferrous sulfate and hydrogen peroxide to it, and stir to adjust the Fe... 2+Oxidized to Fe 3+ The iron-phosphorus molar ratio in the solution is controlled to be 1.5:1.

[0034] S2: Extraction and precipitation reaction. The organic phase was prepared as follows: 50% (v / v) tributyl phosphate (TBP), 5% n-octanol, and 45% kerosene. The solution obtained in S1 (aqueous phase) and the organic phase were mixed at a volume ratio of 1:1 (aqueous phase:organic phase) in a separatory funnel and extracted by shaking in a 35°C water bath for 10 min. After standing and separating the layers, the organic phase, aqueous phase, and solid phase were collected separately. The solid phase was washed with a small amount of deionized water and labeled as "Fe-P precipitation". During this process, the precipitation rate of phosphorus was 99.81%, the precipitation rate of iron was 99.76%, and the rare earth co-precipitation rate was 1.38%.

[0035] S3: Temperature-controlled crystallization. The Fe-P precipitate was placed in a beaker, and dilute sulfuric acid (50℃, pH 0.8) was added and stirred to dissolve. Phosphoric acid was added to maintain the iron-phosphorus molar ratio at 1.1. The solution was heated to 80℃, and the thermal crystallization reaction was carried out for 6 hours. Filtering yielded primary ferric phosphate and the filtrate (containing rare earth elements, recyclable). The phosphorus yield was 99.52%, the iron yield was 90.09%, and the rare earth element retention rate was 99.44%.

[0036] S4: Dissolution-Reprecipitation. Primary ferric phosphate was dissolved in 1 mol / L sulfuric acid, and the pH was adjusted to 1.0 with 0.5 mol / L ammonia. The reaction was carried out at 60℃ for 4 h. After filtration, washing, and drying, battery-grade ferric phosphate product was obtained.

[0037] S5: Extractant regeneration. The organic phase obtained in S2 is back-extracted with 0.5 mol / L ammonia water at a volume ratio of 5:1 (organic phase: aqueous phase), and the regenerated extractant is recycled. The back-extraction solution is combined with the sulfate solution obtained in S4, concentrated, and crystallized to obtain the ammonium sulfate product.

[0038] S6: Rare earth recovery. The raffinate obtained from S2 was mixed with P507-kerosene extractant (P507 20%, kerosene 80%, saponification degree 10%) at a volume ratio of 3:1 (aqueous phase: organic phase), and extracted at 45℃ for 20 min to recover rare earth.

[0039] Calculations show that the phosphorus recovery rate in this embodiment is 99.33%, the iron recovery rate is 89.87%, and the rare earth loss rate is 0.008%. The quality indicators of the obtained battery-grade iron phosphate product are shown in Table 1.

[0040] Example 3

[0041] This embodiment provides a method for recovering phosphorus and rare earth elements from rare earth leachate to prepare battery-grade iron phosphate. The specific steps are as follows: S1: Iron-to-phosphorus ratio adjustment. Take a rare earth sulfate leaching solution with a rare earth concentration of 31.2 g / L (calculated as REO) and a P2O5 concentration of 9.8 g / L. Add polyferric sulfate to control the iron-to-phosphorus molar ratio in the solution to 3:1.

[0042] S2: Extraction and precipitation reaction. Prepare the organic phase: 40% tributylamine (volume fraction), 15% isoamyl alcohol, and 45% kerosene. Place the solution obtained in S1 (aqueous phase) and the organic phase at a volume ratio of 1:2 (aqueous phase:organic phase) in a separatory funnel and extract by shaking in a 50℃ constant temperature water bath for 90 min. After standing and separating the layers, collect the organic phase, aqueous phase, and solid phase separately. Wash the solid phase with a small amount of deionized water and label it "Fe-P precipitation". In this process, the precipitation rate of phosphorus was 99.91%, the precipitation rate of iron was 98.68%, and the rare earth co-precipitation rate was 0.60%.

[0043] S3: Temperature-controlled crystallization. The Fe-P precipitate was placed in a beaker, and dilute sulfuric acid (35℃, pH 0.6) was added and stirred to dissolve. Phosphoric acid was added to maintain the iron-to-phosphorus molar ratio at 0.9. The solution was heated to 95℃, and the thermal crystallization reaction was carried out for 4 hours. Filtering yielded primary ferric phosphate and the filtrate (containing rare earth elements, recyclable). The phosphorus yield was 89.92%, the iron yield was 98.01%, and the rare earth element retention rate was 99.29%.

[0044] S4: Dissolution-Reprecipitation. Primary ferric phosphate was dissolved in 2.5 mol / L sulfuric acid, and the pH was adjusted to 3.0 with 3 mol / L ammonia. The reaction was carried out at 30°C for 8 h. After filtration, washing, and drying, battery-grade ferric phosphate product was obtained.

[0045] S5: Extractant regeneration. The organic phase obtained in S2 is back-extracted with 3 mol / L ammonia water at a volume ratio of 12:1 (organic phase: aqueous phase), and the regenerated extractant is recycled. The back-extraction solution is combined with the sulfate solution obtained in S4 for further treatment.

[0046] S6: Rare earth recovery. The raffinate obtained from S2 was mixed with P507-kerosene extractant (P507 25%, kerosene 75%, saponification degree 20%) at a volume ratio of 4:1 (aqueous phase: organic phase), and extracted at 35℃ for 60 min to recover rare earth.

[0047] Calculations show that the overall phosphorus recovery rate in this embodiment is 89.83%, the iron recovery rate is 96.71%, and the rare earth loss rate is 0.004%. The quality indicators of the obtained battery-grade iron phosphate product are shown in Table 1.

[0048] Table 1. Sample Indicators and Standard Technical Indicators of Ferric Phosphate

[0049] As shown in Table 1, all indicators of the iron phosphate products obtained in Examples 1-3 meet the standard requirements of HG / T 4701-2021 for battery-grade iron phosphate. Specifically, the iron-to-phosphorus ratio and impurity content (Ca, Mg, Na, heavy metals, etc.) are better than the standard limits, and the tap density and particle size distribution meet the application requirements for battery materials. This indicates that the method described in this invention can stably prepare high-quality battery-grade iron phosphate products.

[0050] Comparative Example 1 To illustrate the technical effects of this invention, a traditional magnesium oxide neutralization method was used for treatment, with the following steps: (1) Take 400 mL of rare earth sulfate leaching solution from the same batch as in Example 1 into a 1000 mL beaker, place it in a 25°C constant temperature water bath, and stir magnetically (300 rpm).

[0051] (2) Add Fe2(SO4)3 solid to control the Fe / P molar ratio in the solution to 2.0.

[0052] (3) While stirring continuously, slowly add MgO powder to adjust and control the final pH of the solution to 3.5. Monitor the pH of the solution during the alkali addition process to avoid local over-alkaliness.

[0053] (4) Maintain the reaction at 25°C for 30 min.

[0054] (5) After the reaction is complete, the mixture is filtered under vacuum, and the filtrate and precipitate are collected separately. The filtrate is recorded as "filtrate A", and the precipitate is recorded as "phosphorus iron slag A". The precipitate is washed three times with deionized water and dried at 60°C for 12 h.

[0055] (6) Place the phosphorus iron slag A in a 500 mL three-necked flask, add 200 mL of dilute sulfuric acid (control the pH to about 0.2~0.6), stir the reaction at 90℃ for 4 h, so that the amorphous FePO4 recrystallizes into FePO4·2H2O, and at the same time, the excess iron and the entrained rare earth dissolve into the solution.

[0056] (7) After the reaction is complete, filter the product and collect the recrystallized product and filtrate separately. Wash the product with deionized water until neutral, dry at 60°C for 12 h, and record it as "Product A1". Record the filtrate as "Filtrate A2" (containing excess Fe). 3+ Rare earth elements can be returned to the leaching process.

[0057] (8) Add NH4HCO3 to filtrate A, control the NH4HCO3 / TREE molar ratio to 4, stir the reaction for 80 min, and recover the rare earth carbonate precipitate.

[0058] The results showed that in Comparative Example 1, the phosphorus removal rate was 99.972%, the residual phosphorus concentration in the solution was 0.87 mg / L, the iron removal rate was 99.866%, the residual iron concentration in the solution was 14.92 mg / L, the rare earth loss rate was 0.016%, the rare earth content of iron phosphate in the product was 529.94 ppm, and magnesium sulfate wastewater was generated.

[0059] Comparative Example 2 After obtaining Fe-P precipitate according to steps S1 and S2 in Example 1, skip the temperature swing crystallization step S3 and directly proceed to the dissolution and reprecipitation treatment in S4, with the remaining conditions being the same as in Example 1.

[0060] The results showed that in Comparative Example 2, the phosphorus removal rate was 99.981%, the residual phosphorus concentration in the solution was 0.58 mg / L, the iron removal rate was 99.932%, the residual iron concentration in the solution was 7.54 mg / L, the rare earth loss rate was 0.046%, the rare earth content of iron phosphate in the product was 1585.56 ppm, and no magnesium sulfate wastewater was generated.

[0061] Comparative Example 3 After obtaining phosphorus iron slag A according to steps (1) to (5) of Comparative Example 1, the acid medium recrystallization in steps (6) to (7) is not carried out. Instead, the temperature swing crystallization, dissolution and reprecipitation, extractant regeneration and rare earth recovery are carried out directly using steps S3 to S6 in Example 1. The other conditions are the same as in Example 1.

[0062] The results showed that in Comparative Example 3, the phosphorus removal rate was 99.969%, the residual phosphorus concentration in the solution was 0.96 mg / L, the iron removal rate was 99.864%, the residual iron concentration in the solution was 15.18 mg / L, the rare earth loss rate was 0.007%, the rare earth content of iron phosphate in the product was 257.02 ppm, and magnesium sulfate wastewater was generated.

[0063] Results Comparison and Mechanism Analysis The main process parameters of Example 1 were compared with those of Comparative Examples 1 to 3, and the results are shown in Table 2.

[0064] Table 2 Comparison of process parameters between each embodiment and the comparative example

[0065] As shown in Table 2, the rare earth loss rate (0.004%) of Example 1 (complete process of the present invention) is significantly lower than that of Comparative Example 1 (0.016%), Comparative Example 2 (0.046%) and Comparative Example 3 (0.007%), and the rare earth impurity content in the product (135.87 ppm) is much lower than that of each comparative example. This proves that the present invention achieves the minimization of rare earth loss through the synergistic combination of "extraction precipitation" and "temperature swing crystallization".

[0066] Comparative Example 2 (extraction of precipitate only, without temperature-controlled crystallization) avoided magnesium sulfate wastewater and localized over-alkaliness through extraction to remove acid. However, because the Fe-P precipitate did not undergo crystal reconstruction, the entrained and adsorbed rare earth elements were not effectively released, resulting in a rare earth content as high as 1585.56 ppm in the final product, with a rare earth loss rate of 0.046%. This indicates that temperature-controlled crystallization plays a crucial role in releasing entrained rare earth elements and controlling product purity.

[0067] Comparative Example 3 (temperature-controlled crystallization only, without extraction and precipitation) used conventional magnesium oxide neutralization to obtain ferrophosphate slag. Although it underwent temperature-controlled crystallization, the initial precipitate already contained a large amount of rare earth elements (comparative Example 1 showed a rare earth loss of 0.016%). Even after recrystallization, the rare earth impurities in the final product were still significantly higher than in Example 1 (257.02 ppm vs 135.87 ppm). This indicates that extraction and precipitation are prerequisites for reducing initial rare earth entrainment and achieving low rare earth loss.

[0068] The differences in the mechanisms of magnesium oxide neutralization and extraction for acid removal are analyzed as follows: The main reaction for the neutralization of magnesium oxide is: , , ; In the extraction-precipitation reaction, the extractant is replaced by R. The main reaction in the extraction-precipitation reaction is as follows: , , ; Extractant regeneration: , Where aq represents the liquid phase, o represents the organic phase, and s represents the solid phase.

[0069] As can be seen from the above reactions, magnesium oxide neutralization is achieved by directly adding alkaline oxides to the aqueous phase to neutralize free sulfuric acid, raising the pH of the system and promoting FePO4 precipitation. This process is an acid-base neutralization reaction, producing the corresponding magnesium sulfate salt and water. Since MgO is a solid phase, its dissolution rate is limited by mass transfer at the solid-liquid interface. The local pH on the particle surface is much higher than that of the bulk solution, leading to preferential nucleation of rare earth phosphates and hydroxides on the particle surface, resulting in rare earth co-precipitation. Simultaneously, the reaction introduces Mg... 2+ The increased ion load in the system and the resulting magnesium sulfate wastewater increased the burden on subsequent treatment.

[0070] The extraction-precipitation method of this invention involves extracting free acids (H+) from the aqueous phase using organic extractants (such as trioctylamine, TBP, etc.). + / HSO4- This process gradually and uniformly reduces the acidity of the aqueous phase during the liquid-liquid interface mass transfer. Instead of directly neutralizing the acid, it transfers it to the organic phase, inducing selective Fe-P precipitation within the preferential supersaturation window of FePO4 regulated by Fe / P. Due to the uniform liquid-liquid mass transfer, there are no localized over-alkali zones in the aqueous phase, effectively suppressing the co-precipitation of rare earth phosphates and hydroxides. Simultaneously, no Mg is introduced during the extraction stage. 2+ The use of exogenous metal ions avoids the generation of magnesium sulfate wastewater. The organic phase loaded with acid can be regenerated and recycled after back-extraction with alkaline solution. The back-extraction solution is concentrated and crystallized to obtain sulfate byproducts, thus realizing the recycling of reagents.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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 recovering phosphorus and rare earth elements from rare earth leachate to prepare battery-grade iron phosphate, comprising adding an iron source to a rare earth sulfate leachate to adjust the iron-phosphorus ratio, causing iron to react with phosphate ions to form a precipitate, and treating the precipitate to prepare battery-grade iron phosphate; characterized in that, The method includes the following steps: S1: Add ferric salt or ferrous salt to rare earth sulfate leaching solution and control the iron-phosphorus molar ratio to 1:1~3:1; S2: The solution obtained in S1 is mixed with the organic phase at a volume ratio of 1:5 to 5:1 for extraction and precipitation reaction. The reaction temperature is 15 to 75°C, and the reaction time is 5 min to 120 min. After the reaction, the raffinate, acid-containing extractant, and Fe-P precipitate are separated. The organic phase consists of an extractant, a phase modifier, and a diluent, kerosene. The volume fraction of the extractant is 10% to 50%, the volume fraction of the phase modifier is 5% to 20%, and the volume fraction of kerosene is 30% to 85%. The extractant is selected from one or more of tributyl phosphate, trialkylphosphine oxide, trioctylamine, tripropylamine, tributylamine, tripentylamine, or trioctyldecyl tertiary amine. The phase modifier is selected from n-butanol, isoamyl alcohol, n-hexanol, n-octanol, or isooctanol. S3: Dissolve the Fe-P precipitate obtained in S2 in dilute sulfuric acid at a temperature of 20~50℃ and a pH of 0~1, add phosphoric acid to control the iron-phosphorus molar ratio to 0.7~1.3, and then raise the temperature to 70~130℃ for thermal crystallization reaction for 2h~10h to obtain primary iron phosphate. The coprecipitated rare earth elements are retained in the solution. S4: Dissolve the primary iron phosphate obtained in S3 in 0.5~4 mol / L sulfuric acid, then adjust the pH to 1.0~4.0 with alkaline solution to carry out precipitation reaction. The reaction temperature is 25~95℃ and the reaction time is 0.5h~10h. Solid-liquid separation yields battery-grade iron phosphate product and sulfate solution.

2. The method according to claim 1, characterized in that, The organic phase described in S2 consists of 20% to 40% by volume of trioctylamine or tributyl phosphate, 5% to 15% by volume of isooctanol or n-octanol, and the balance being kerosene.

3. The method according to claim 1, characterized in that, The volume ratio of the organic phase to the solution in S2 is 1:3 to 3:1, the reaction temperature is 25 to 50°C, and the reaction time is 10 min to 90 min.

4. The method according to claim 1, characterized in that, The dilute sulfuric acid in S3 is at a temperature of 25~50℃ and a pH of 0.2~0.

8. After adding phosphoric acid, the iron-phosphorus molar ratio is 0.9~1.

1. The temperature of the thermal crystallization reaction is 80~120℃ and the reaction time is 2h~6h.

5. The method according to claim 1, characterized in that, The sulfuric acid in S4 has a concentration of 1~2.5 mol / L, the alkaline solution is ammonia water with a concentration of 0.5~3 mol / L, the pH is adjusted to 1.0~3.0, the reaction temperature is 60~90℃, and the reaction time is 0.5h~4h.

6. The method according to claim 1, characterized in that, The method also includes step S5: back-extracting the acid-containing extractant obtained in S2 with an alkaline solution of 0.5~5 mol / L at a volume ratio of organic phase to aqueous phase of 1:1~15:1 to obtain a regenerated extractant and a back-extraction solution; combining the back-extraction solution with the sulfate solution obtained in S4, and obtaining sulfate crystals and mother liquor by concentration, crystallization and solid-liquid separation, wherein the mother liquor is used to prepare the alkaline solution.

7. The method according to claim 1, characterized in that, The process also includes step S6: mixing the raffinate obtained in S2 with saponified P507 extractant at a volume ratio of 1:5 to 5:1 to carry out an extraction reaction to recover rare earth elements from the raffinate; wherein the volume fraction of P507 in the extractant is 10% to 50%, the volume fraction of kerosene is 50% to 90%, the reaction temperature is 25 to 75°C, the reaction time is 5 min to 60 min, and the degree of saponification of P507 is 10% to 40%.

8. The method according to claim 7, characterized in that, The volume fraction of P507 in S6 is 20%~30%, the volume fraction of kerosene is 70%~80%, the reaction temperature is 25~35℃, the reaction time is 10~60min, the saponification degree of P507 is 10%~30%, and the volume ratio of the raffinate to the extractant is 1:2~4:

1.

9. The method according to claim 1, characterized in that, The ferric salt mentioned in S1 is polyferric sulfate, and the ferrous salt is ferrous sulfate. When adding the ferrous salt, an oxidizing agent also needs to be added to remove the Fe. 2+ Oxidized to Fe 3+ The iron-phosphorus molar ratio is 1.5:1 to 2:

1.

10. The method according to claim 1, characterized in that, The solution containing the rare earth element in S3 is recycled back to S3 to dissolve the Fe-P precipitate; the sulfate solution obtained in S4 is concentrated and crystallized, and the mother liquor is recycled to prepare the alkaline solution described in S4 and / or S5.

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