A method for preparing lithium salt and iron phosphate by using smut and phosphogypsum in cooperation

Abstract

The present application relates to a kind of method for preparing lithium salt and iron phosphate by using smut and phosphogypsum synergistically, comprising the following steps: phosphogypsum is washed with water, neutralized and treated, and calcined pretreatment, to obtain activated phosphogypsum;The activated phosphogypsum is mixed with pretreated smut to obtain mixed powder, then water is added to make slurry;Buffer system is added to the slurry to adjust the pH value of the slurry to 2.5-5.5, and hydrogen peroxide is added, heated to obtain reaction slurry;The reaction slurry is subjected to solid-liquid separation to obtain lithium-containing leaching solution and mixed solid phase residue;The lithium-containing leaching solution is subjected to precipitation reaction to obtain lithium salt;The mixed solid phase residue is subjected to acid washing, water washing and calcination to obtain iron phosphate.The present application mixes activated phosphogypsum with smut for synergistic treatment, adjusts the pH value, and carries out calcium-mediated synergistic reaction under the action of hydrogen peroxide, to realize selective and mild leaching of lithium in smut, and prepare lithium salt and iron phosphate.The present application realizes waste treatment and solid waste coupling closed loop cycle.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization technology, specifically relating to a method for preparing lithium salts and iron phosphate by synergistic use of black powder and phosphogypsum. Background Technology

[0002] With the rapid development of the new energy vehicle industry, lithium iron phosphate batteries have been widely used due to their high safety and long cycle life. However, the lifespan of lithium iron phosphate batteries is generally 5-8 years. As early installed power batteries are gradually retired, the disposal of waste lithium iron phosphate batteries has become an urgent environmental and resource recycling problem. The black powder obtained after dismantling, crushing, and sorting waste lithium iron phosphate batteries contains various valuable elements such as lithium, iron, and phosphorus, with a lithium content of 2-5 wt%, possessing extremely high recycling value. Currently, the mainstream technical routes for black powder recycling include pyrometallurgy and hydrometallurgy. Pyrometallurgy has high energy consumption and low lithium recovery rate; hydrometallurgy usually uses inorganic acids (sulfuric acid, hydrochloric acid, nitric acid) for leaching, which has problems such as high reagent consumption, high cost of waste acid and wastewater treatment, and poor selectivity. In recent years, selective lithium extraction technology has become a research hotspot. For example, the acid-free leaching strategy of sodium persulfate can achieve selective lithium recovery under room temperature conditions, but this type of technology has limited adaptability to raw materials, and the iron- and phosphorus-rich slag after lithium extraction still needs further treatment.

[0003] On the other hand, phosphogypsum is an industrial byproduct emitted during the wet-process phosphoric acid production. Approximately 4.5 to 5 tons of phosphogypsum are generated for every ton of phosphoric acid produced. Large-scale stockpiling of this product not only occupies valuable land resources but also poses a serious environmental pollution risk. The presence of soluble phosphorus, fluorine, and heavy metals in phosphogypsum makes direct resource utilization difficult. Existing resource utilization pathways mainly include the building materials sector (gypsum board, cement retarders, etc.), agricultural soil improvement, and chemical raw material recovery. However, due to factors such as large fluctuations in phosphogypsum quality, high impurity content, and limited downstream processing capacity, the comprehensive utilization rate remains low.

[0004] Currently, there are no reported processes for co-treating waste lithium iron phosphate black powder and phosphogypsum to simultaneously produce lithium salts and iron phosphate. Existing black powder recycling technologies require large amounts of added acid and alkali reagents, while the resource utilization of phosphogypsum faces bottlenecks in disposal. These two processes operate independently, lacking systematic coupling. How to co-treat these two widely distributed solid wastes to achieve a closed-loop cycle of waste-to-waste treatment and solid waste coupling is a pressing technical challenge that needs to be addressed in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical deficiencies and provide a method for preparing lithium salts and iron phosphate by synergistically using black powder and phosphogypsum, thereby solving the technical problem that waste lithium iron phosphate black powder and phosphogypsum are difficult to process synergistically in the prior art.

[0006] To achieve the above-mentioned technical objectives, the technical solution provided by this invention is as follows: In a first aspect, the present invention provides a method for synergistically preparing lithium salts and iron phosphate using black powder and phosphogypsum, comprising the following steps: S1, phosphogypsum is subjected to water washing, neutralization treatment and calcination pretreatment to obtain activated phosphogypsum; S2, activated phosphogypsum is mixed with pretreated black powder to obtain mixed powder, and then water is added to form a slurry; S3, a buffer system is added to the slurry to adjust the pH value of the slurry to 2.5-5.5, and hydrogen peroxide is added to carry out a heating reaction to obtain a reaction slurry; S4, the reaction slurry is subjected to solid-liquid separation to obtain a lithium-containing leachate and a mixed solid residue; the lithium-containing leachate is subjected to a precipitation reaction to obtain lithium salt; the mixed solid residue is subjected to acid washing, water washing and calcination to obtain iron phosphate.

[0007] Compared with the prior art, the beneficial effects of the present invention include: This invention utilizes activated phosphogypsum as a calcium and phosphorus source, providing sulfate ions, and mixes it with waste lithium iron phosphate black powder for synergistic treatment. After pH adjustment, a calcium-mediated synergistic reaction is conducted under the action of hydrogen peroxide to achieve selective and mild leaching of lithium from the black powder. Simultaneously, the calcium sulfate in the phosphogypsum reacts with the iron phosphate component in the black powder to generate calcium phosphate precipitate. After solid-liquid separation, lithium-containing leachate and iron phosphate slag are obtained. The lithium-containing leachate can be used to prepare lithium salts through precipitation, while the iron phosphate slag can be used to prepare iron phosphate through acid washing and calcination. This invention synergistically transforms two widely used solid wastes into high-value-added products in a single process, offering advantages such as low reagent consumption, high process coupling, and high comprehensive utilization rate of solid waste, achieving a closed-loop cycle of waste-to-waste treatment and solid-waste coupling. Detailed Implementation

[0008] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0009] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this invention are intended to cover non-exclusive inclusion.

[0010] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0011] To address the current shortcomings of co-processing waste lithium iron phosphate black powder and phosphogypsum, this invention provides a method for co-preparing lithium salts and iron phosphate using black powder and phosphogypsum. This invention breaks away from the conventional approach of separately treating black powder recycling and phosphogypsum resource recovery. It co-reacts waste lithium iron phosphate black powder and phosphogypsum under mild conditions, achieving selective leaching of lithium and pre-enrichment of iron phosphate components through a calcium ion-mediated mechanism. The leachate is then precipitated to obtain lithium salt products, and the solid products are acid-washed and calcined to obtain battery-grade iron phosphate. This achieves a "one-pot" co-conversion of the two solid wastes into high-value-added products.

[0012] In a first aspect, the present invention provides a method for preparing lithium salts and iron phosphate by synergistic use of black powder and phosphogypsum, comprising the following steps: S1, phosphogypsum is obtained by washing, neutralizing and calcining pretreatment to obtain activated phosphogypsum; S2, mix activated phosphogypsum with pretreated black powder to obtain mixed powder, then add water to make slurry; S3, add a buffer system to the slurry to adjust the pH value of the slurry to 2.5-5.5, add hydrogen peroxide, and heat the reaction to obtain the reaction slurry; S4. The reaction slurry is subjected to solid-liquid separation to obtain a lithium-containing leachate and a mixed solid residue. The lithium-containing leachate is subjected to precipitation reaction to obtain lithium salt. The mixed solid residue is subjected to acid washing, water washing and calcination to obtain iron phosphate.

[0013] This invention uses activated phosphogypsum as a calcium and phosphorus source, and provides sulfate ions. It is then mixed with waste lithium iron phosphate black powder for synergistic treatment. After pH adjustment, a calcium-mediated synergistic reaction is carried out under the action of hydrogen peroxide to obtain a lithium-containing leachate (lithium sulfate solution) and iron phosphate slag. The lithium sulfate solution can be used to prepare lithium salts through precipitation reaction, and the iron phosphate slag can be used to prepare iron phosphate through acid washing and calcination.

[0014] In some embodiments, in step S1, the water washing adopts a three-stage countercurrent water washing, with a liquid-to-solid ratio of (2-5):1 for each stage, and a washing time of 30-60 min. The liquid-to-solid ratio refers to the mass ratio.

[0015] Furthermore, the neutralization process is carried out in the final stage of water washing, specifically by adding lime milk to the phosphogypsum slurry obtained from the final stage of water washing to adjust the pH value to 6.5-8.0, followed by washing and filtration.

[0016] In some embodiments, the conditions for calcination pretreatment in step S1 include: a temperature of 160–180°C and a calcination time of 1.5–2.5 h.

[0017] Since phosphogypsum's main component is calcium sulfate dihydrate, it also contains impurities such as free phosphoric acid, fluoride, phosphorus pentoxide, phosphate, and small amounts of heavy metals. In this invention, adjusting the pH serves to neutralize free acid and precipitate fluoride. Additionally, excessively high pH can cause the generated calcium hydroxide to coat the phosphogypsum particles, reducing reactivity. Calcination primarily converts calcium sulfate dihydrate (CaSO4·2H2O) into calcium sulfate hemihydrate (CaSO4·0.5H2O). The dehydrated hemihydrate gypsum has a lamellar, porous crystal structure with many slits, significantly increasing its specific surface area and greatly enhancing its surface activity and reaction rate. Therefore, this invention removes harmful phosphoric acid by performing activation pretreatment on the phosphogypsum. - Soluble P2O5 further transforms inert dihydrate gypsum into porous hemihydrate gypsum, increasing the release rate of calcium ions and enhancing the contact efficiency between the solid and liquid phases, thereby promoting its reaction kinetics with LiFePO4 in black powder.

[0018] In some embodiments, in step S2, the pretreated black powder is a mixture of positive and negative electrode black powder obtained by dismantling, crushing and sorting retired lithium iron phosphate batteries, wherein the lithium content is 2-5 wt%, the iron content is 25-40 wt%, and the phosphorus content is 15-25 wt%; the crushing is carried out by ball milling or impact crushing to a particle size ≤150μm; the sorting is carried out by dry weak magnetic separation to remove iron impurities.

[0019] In some embodiments, in step S2, the mass ratio of activated phosphogypsum to pretreated black powder is (0.6-1.2):1; the solid-liquid ratio in the slurry is 1:(3-8).

[0020] In some embodiments, in step S3, the buffer system includes an acetate-sodium acetate buffer system, a phosphate-phosphate buffer system, a citric acid-sodium citrate buffer system, or a formic acid-sodium formate buffer system. The purpose of adding a buffer system in this invention is not only to adjust the initial pH, but also to control the stability of the solution pH throughout the synergistic reaction process, avoiding excessive dissolution of iron and aluminum or premature precipitation of ferric phosphate due to local acidity fluctuations.

[0021] Furthermore, the buffer system is an acetate-sodium acetate buffer system, and the amount of acetate-sodium acetate buffer system added is 2-10% of the slurry volume. Compared with inorganic buffer systems (such as phosphate-phosphate), the acetate-sodium acetate buffer system used in this invention is easier to decompose and remove during calcination and crystallization post-treatment, without introducing new inorganic impurities; compared with citric acid buffer system, acetate-sodium acetate buffer system is inexpensive and does not over-stabilize iron ions, thus avoiding oxidation difficulties.

[0022] In some embodiments, in step S3, the mass fraction of hydrogen peroxide is 25-35%, and the amount added is 1-10% of the mass of the pretreated black powder.

[0023] In some embodiments, in step S3, the heating reaction is carried out by stirring at 50–95°C for 1–4 h.

[0024] In some embodiments, step S4, obtaining lithium salt by precipitation reaction of lithium-containing leachate specifically includes: adding sodium carbonate solution or phosphoric acid at a theoretical equivalent of 1 to 1.1 times to the lithium-containing leachate, controlling the reaction temperature at 70 to 95°C, the pH at 8 to 11, the precipitation reaction time at 30 to 90 min, filtering, and drying at 105 to 120°C for 4 to 8 h to obtain lithium carbonate or lithium phosphate product.

[0025] Furthermore, phosphoric acid of 1.1 times the theoretical equivalent is added to the lithium-containing leachate, and then sodium hydroxide solution or ammonia is added to adjust the pH to 9-10.

[0026] In some embodiments, in step S4, the pickling is carried out using dilute sulfuric acid with a concentration of 5-15 wt%, the liquid-to-solid ratio is (3-5):1, the pickling temperature is 40-70°C, and the pickling time is 30-90 min. This invention, by controlling the pickling conditions, especially by using dilute sulfuric acid, minimizes the solubility of ferric phosphate, thus avoiding the loss of ferric phosphate (<1%) while effectively removing excess calcium phosphate and other impurities from the solid phase.

[0027] It is understandable that calcium sulfate byproducts can be separated from the mixed solid residue during the acid washing process (calcium sulfate is mainly in the acid washing filtrate, with a small amount entering the water washing filtrate). After washing, it can be used in building materials or soil conditioners, and the specific choice can be made according to the actual situation.

[0028] In some embodiments, in step S4, the water is washed until neutral; further, the liquid-to-solid ratio of the water is (3-5):1, and the washing time is 30-90 minutes, which can be selected according to the situation, as long as the water is washed until neutral.

[0029] In some embodiments, in step S4, the calcination temperature is 500–700°C and the calcination time is 2–5 h.

[0030] The main mechanism of action and advantages of this invention are as follows: (1) Solid waste co-processing and waste treatment: black powder and phosphogypsum are co-processed. Lithium is extracted from calcium sulfate in phosphogypsum, and calcium ions are precipitated from iron phosphate in black powder. This complementary transformation avoids the addition of external acids and alkalis and reduces reagent costs.

[0031] (2) Mild conditions and selective lithium extraction: Calcium ions mediate mild reaction, selectively leaching lithium under strong acid without external strong acid, temperature ≤95℃, reducing the dissolution of iron and aluminum impurities and reducing the impurity removal load.

[0032] (3) Full element recycling and high-value utilization: Lithium is recycled as lithium carbonate or lithium phosphate, iron phosphate is recycled as battery-grade iron phosphate, and calcium is used as calcium sulfate by-product in building materials, achieving full element high-value recycling.

[0033] (4) Short process flow and low overall cost: The lithium extraction from black powder and the resource utilization of phosphogypsum are integrated in the same reactor, the process is compact, the equipment investment is small, the process water is recycled, and the cost is reduced by 20-30%.

[0034] The present invention will be further described in detail below through specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments used that do not specify the manufacturer are all conventional products that can be obtained commercially.

[0035] Example 1 A method for synergistically preparing lithium salts and iron phosphate using black powder and phosphogypsum includes the following steps: S1, Raw material pretreatment: S101, Pretreatment of Phosphogypsum: Phosphogypsum discharged from a phosphate chemical plant was collected. Its main component was CaSO4·2H2O, with a content of 87%, of which the water-soluble P2O5 content was 0.85%, and the pH was 3.2. A three-stage countercurrent washing process was adopted, with a liquid-to-solid ratio of 3:1 and a washing time of 45 min, carried out at room temperature. Lime milk (containing 20wt% CaO) was added to the phosphogypsum slurry in the third stage of washing to adjust the pH to 7.2. After stirring for 30 min, the mixture was filtered. After washing, the water-soluble P2O5 content in the phosphogypsum (filter cake) decreased to 0.08%. The filter cake was calcined at 170℃ for 2 h to obtain activated phosphogypsum with a whiteness of 72%, and its main component was calcium sulfate hemihydrate.

[0036] S102, Black Powder Pretreatment: Take 500g of black powder obtained from the dismantling, crushing, and sorting of retired lithium iron phosphate batteries, with a Li content of 3.2 wt%, an Fe content of 24.3 wt%, and a P content of 13.9 wt%. Use a ball mill to crush the black powder to a particle size ≤150 μm, and then remove iron impurities by dry weak magnetic separation to obtain pretreated black powder.

[0037] S2, Material mixing: The activated phosphogypsum obtained in step S101 and the pretreated black powder obtained in step S102 are mixed at a mass ratio of 1:1, and deionized water is added to prepare a slurry with a solid-liquid ratio of 1:5.

[0038] S3, Material reaction: Add an acetate-sodium acetate buffer system to the slurry at a volume of 5% of the slurry, adjust the pH of the system to 4.5, and add 30% hydrogen peroxide to the reaction system at a volume of 5% of the pretreated black powder. Stir the reaction at 65°C for 2.5 h.

[0039] S4, Solid-Liquid Separation and Product Preparation: S401, the reaction slurry was vacuum filtered to obtain a lithium-containing leachate and a mixed solid residue. The lithium concentration in the lithium-containing leachate was 6.03 g / L. ICP analysis showed that the lithium leaching rate in the leachate reached 94.2%, and the iron and aluminum impurity contents were 12 mg / L and 8 mg / L, respectively.

[0040] S402, after evaporating and concentrating the lithium-containing leachate to 1 / 5 of its original volume, added sodium carbonate solution (20 wt%), controlled the reaction temperature at 85℃ and pH=10, and carried out the precipitation reaction for 60 min. After filtration, it was dried at 110℃ for 6 h to obtain 78.4 g of lithium carbonate product. The purity of the product was analyzed to be 99.3%, which meets the battery-grade lithium carbonate standard.

[0041] S403, the mixed solid residue was acid-washed with 10 wt% sulfuric acid solution at a liquid-solid ratio of 4:1 at 60℃ for 60 min. After filtration, the solid phase was washed with water until neutral, and then calcined at 600℃ for 3 h to obtain 278.9 g of anhydrous iron phosphate product. The Fe / P molar ratio was analyzed to be 0.975 (meeting the requirements for qualified products), and the product purity was 98.5%.

[0042] Example 2 A method for synergistically preparing lithium salts and iron phosphate using black powder and phosphogypsum includes the following steps: S1, Raw material pretreatment: S101, Pretreatment of phosphogypsum: Phosphogypsum was subjected to a three-stage countercurrent water washing process at a liquid-to-solid ratio of 4:1 and a washing time of 50 min at room temperature. Lime milk was added to the phosphogypsum slurry in the third stage of washing to adjust the pH to 6.8. After stirring for 40 min, the mixture was filtered. The water-soluble P2O5 content in the phosphogypsum (filter cake) decreased from 0.92% to 0.09%. The filter cake was calcined at 160℃ for 2.5 h to obtain activated phosphogypsum.

[0043] S102, Black Powder Pretreatment: Take 500 g of black powder (same as in Example 1), crush it with impact to a particle size ≤100 μm, and obtain pretreated black powder after magnetic separation to remove impurities.

[0044] S2, Material mixing: Activated phosphogypsum and pretreated black powder are mixed at a mass ratio of 0.8:1, and deionized water is added to prepare a slurry with a solid-liquid ratio of 1:6.

[0045] S3, Material reaction: Add a phosphate-disodium hydrogen phosphate buffer system to the slurry at a volume of 8% of the slurry, adjust the pH of the system to 4.0, and add hydrogen peroxide with a mass fraction of 27.5% to the reaction system at a mass fraction of 4% of the pretreated black powder. Stir the reaction at 60℃ for 2 hours.

[0046] S4, Solid-Liquid Separation and Product Preparation: S401, after filtration of the reaction slurry, yielded a lithium-containing leachate with a lithium concentration of 4.94 g / L and a lithium leaching rate of 92.7%. The iron and aluminum impurity contents were 15 mg / L and 10 mg / L, respectively.

[0047] S402, after evaporating and concentrating the lithium-containing leachate, added sodium carbonate solution, controlled the reaction temperature at 75℃, pH=9.5, and the precipitation reaction for 80 min. After filtration and drying, 76.7 g of lithium carbonate product with a purity of 99.1% was obtained.

[0048] S403, the mixed solid residue was acid washed with 8 wt% sulfuric acid solution at a liquid-solid ratio of 4:1 at 55℃ for 80 min. After filtration, the solid phase was washed with water until neutral and then calcined at 580℃ for 4 h to obtain 275.4 g of anhydrous iron phosphate product with a Fe / P molar ratio of 0.98 and a purity of 98.2%.

[0049] Example 3 A method for synergistically preparing lithium salts and iron phosphate using black powder and phosphogypsum includes the following steps: S1, Raw material pretreatment: S101, Pretreatment of phosphogypsum: Phosphogypsum was subjected to a three-stage countercurrent water washing process at a liquid-to-solid ratio of 5:1 and a washing time of 60 min at room temperature. Lime milk was added to the phosphogypsum slurry in the third stage of washing to adjust the pH to 7.5. After stirring for 35 min, the mixture was filtered. The water-soluble P2O5 content in the phosphogypsum (filter cake) decreased from 0.78% to 0.07%. The filter cake was calcined at 180℃ for 1.5 h to obtain activated phosphogypsum.

[0050] S102, Black Powder Pretreatment: Take 500 g of black powder (same as in Example 1), crush it into particles with a particle size ≤150 μm by ball milling, and obtain pretreated black powder after magnetic separation to remove impurities.

[0051] S2, Material mixing: Activated phosphogypsum and pretreated black powder are mixed at a mass ratio of 1.2:1, and deionized water is added to prepare a slurry with a solid-liquid ratio of 1:4.

[0052] S3, Material reaction: Add a citric acid-sodium citrate buffer system to the slurry at a volume of 6% of the slurry, adjust the pH of the system to 5.0, and add 30% hydrogen peroxide to the reaction system at a volume of 8% of the pretreated black powder. Stir and react at 60°C for 2.5 hours.

[0053] S4, Solid-Liquid Separation and Product Preparation: S401, after filtration of the reaction slurry, yielded a lithium-containing leachate with a lithium concentration of 6.15 g / L and a lithium leaching rate of 93.5%. The iron and aluminum impurity contents were 18 mg / L and 12 mg / L, respectively.

[0054] S402, after evaporating and concentrating the lithium-containing leachate, added sodium carbonate solution, controlled the reaction temperature at 90℃, pH=10.5, and the precipitation reaction for 45 min. After filtration and drying, 81.8 g of lithium carbonate product with a purity of 98.8% was obtained.

[0055] S403, the mixed solid residue was acid washed with 12 wt% sulfuric acid solution at a liquid-solid ratio of 4:1 at 65℃ for 45 min, filtered, the solid phase was washed with water until neutral, and then calcined at 650℃ for 2.5 h to obtain 288.5 g of anhydrous iron phosphate product with a Fe / P molar ratio of 0.99 and a purity of 98.7%.

[0056] Example 4 A method for synergistically preparing lithium salts and iron phosphate using black powder and phosphogypsum includes the following steps: S1, Raw material pretreatment: S101, Pretreatment of phosphogypsum: Phosphogypsum was subjected to a three-stage countercurrent water washing process at a liquid-to-solid ratio of 2.5:1 and a washing time of 40 min at room temperature. Lime milk was added to the phosphogypsum slurry in the third stage of washing to adjust the pH to 7.0. After stirring for 30 min, the mixture was filtered. The water-soluble P2O5 content in the phosphogypsum (filter cake) decreased from 1.05% to 0.10%. The filter cake was calcined at 175℃ for 2 h to obtain activated phosphogypsum.

[0057] S102, Black Powder Pretreatment: Take 500 g of black powder (same as in Example 1), crush it with impact to a particle size ≤120 μm, and obtain pretreated black powder after magnetic separation to remove impurities.

[0058] S2, Material mixing: Activated phosphogypsum and pretreated black powder are mixed at a mass ratio of 0.6:1, and deionized water is added to prepare a slurry with a solid-liquid ratio of 1:7.

[0059] S3, Material reaction: Add an acetate-sodium acetate buffer system to the slurry at a volume of 4% of the slurry, adjust the pH of the system to 4.8, and add hydrogen peroxide with a mass fraction of 27.5% at a mass fraction of 3% of the pretreated black powder to the reaction system. Stir the reaction at 70℃ for 4 h.

[0060] S4, Solid-Liquid Separation and Product Preparation: S401, after filtration of the reaction slurry, yielded a lithium-containing leachate with a lithium concentration of 4.02 g / L and a lithium leaching rate of 91.8%. The iron and aluminum impurities were 14 mg / L and 7 mg / L, respectively.

[0061] S402, after evaporating and concentrating the lithium-containing leachate, added sodium carbonate solution, controlled the reaction temperature at 90℃, pH=10.5, and the precipitation reaction for 45 min. After filtration and drying, 73.7g of lithium carbonate product with a purity of 99.0% was obtained.

[0062] S403, the mixed solid residue was acid washed with 9 wt% sulfuric acid solution at 50℃ for 75 min, filtered, the solid phase was washed with water until neutral, and then calcined at 550℃ for 4.5 h to obtain 271.3 g of anhydrous iron phosphate product with Fe / P molar ratio of 0.98 and purity of 98.0%.

[0063] The main effects of the above embodiments 1-4 are summarized in Table 1 below.

[0064] Table 1 Main effects of Examples 1-4

[0065] The above embodiments all achieved efficient selective leaching of lithium from black powder (lithium leaching rate ≥ 91.8%), and the resulting lithium salt products and iron phosphate products both met the technical requirements of battery-grade materials, verifying the feasibility and stability of the technical solution of the present invention.

[0066] Comparative Example 1 Compared with Example 1, the only difference is that the activated phosphogypsum is directly replaced with pure calcium sulfate (analytical grade CaSO4·2H2O, purity ≥99%), and no pretreatment activation step is performed; the other steps and conditions are the same as in Example 1.

[0067] The resulting lithium-containing leachate had a lithium concentration of 4.54 g / L and a leaching rate of 71.2%, a decrease of 23% compared to Example 1. The iron and aluminum impurity contents were 35 mg / L and 11 mg / L, respectively. The obtained iron phosphate product weighed 269.1 g, with an Fe / P molar ratio of 1.12 and a purity of 96.8%.

[0068] This is mainly because pure calcium sulfate lacks the crystal nuclei such as calcium phosphate inherent in phosphogypsum, resulting in uneven release of calcium ions and low reactivity. Furthermore, an additional phosphorus source is necessary to maintain the Fe / P stoichiometric balance; otherwise, the iron content of the product will be too high. Additionally, pure calcium sulfate lacks the porous structure of hemihydrate calcium sulfate, leading to uneven release of calcium ions during the reaction. This may cause excessive precipitation in certain areas, clogging the reaction interface and inhibiting the reaction process.

[0069] Comparative Example 2 Compared with Example 1, the only difference is that the phosphogypsum used was not calcined and activated, but only washed and neutralized (pH adjusted to 7.2); the other steps and conditions are the same as in Example 1.

[0070] The resulting lithium-containing leachate had a lithium concentration of 5.67 g / L and a leaching rate of 88.7%. Iron and aluminum impurities were present at concentrations of 22 mg / L and 10 mg / L, respectively. The obtained iron phosphate product weighed 273.6 g, with an Fe / P molar ratio of 1.02 and a purity of 97.2%. XRD analysis showed that the uncalcined phosphogypsum was still predominantly composed of dihydrate gypsum (CaSO4·2H2O), with a specific surface area of ​​only 5.8 m² / g (approximately 11.2 m² / g after activation in Example 1), resulting in a slow calcium ion release rate and limited reaction kinetics.

[0071] Comparative Example 3 Compared with Example 1, the only difference is that the amount of activated phosphogypsum is adjusted to 0.2:1 (i.e., 100g of activated phosphogypsum and 500g of pretreated black powder); the other steps and conditions are the same as in Example 1.

[0072] The resulting lithium-containing leachate had a lithium concentration of 5.28 g / L and a leaching rate of 82.5%. Iron and aluminum impurities were present at concentrations of 28 mg / L and 12 mg / L, respectively. The resulting iron phosphate product weighed 269.8 g, with an Fe / P molar ratio of 0.85 and a purity of 95.6%.

[0073] This is because the amount of phosphogypsum used is too low, resulting in insufficient calcium ions and sulfate ions, which cannot fully decompose lithium iron phosphate in the black powder, causing some lithium to remain in the solid phase and fail to leach out; at the same time, there is an excess of phosphate ions in the system, resulting in a low iron-to-phosphorus ratio in the final iron phosphate product.

[0074] Comparative Example 4 Compared with Example 1, the only difference is that the amount of activated phosphogypsum is adjusted to 1.5:1; the other steps and conditions are the same as in Example 1.

[0075] The resulting lithium-containing leachate had a lithium concentration of 5.77 g / L and a leaching rate of 90.2%, slightly lower than the 94.2% of Example 1. The iron and aluminum impurities were 19 mg / L and 11 mg / L, respectively. The resulting iron phosphate product weighed 267.3 g, had an Fe / P molar ratio of 1.04, a purity of 97.1%, and a residual calcium content of 0.35 wt%.

[0076] This is because the amount of phosphogypsum used is too large, resulting in a serious excess of calcium ions in the reaction system and an excessive amount of Ca3(PO4)2 precipitate. This precipitate cannot be completely dissolved and removed by dilute sulfuric acid in the subsequent acid washing step. Some calcium is mixed into the ferric phosphate product in the form of CaSO4 or Ca3(PO4)2, leading to increased impurities and decreased purity.

[0077] In summary, this invention co-processes waste lithium iron phosphate black powder with phosphogypsum, a byproduct of phosphate chemical industry. First, the phosphogypsum undergoes pretreatment including washing, neutralization, and calcination to obtain activated phosphogypsum. Then, the activated phosphogypsum is mixed with the pretreated black powder, and selective and gentle leaching of lithium from the black powder is achieved through calcium ion mediation in an acetate-sodium acetate buffer system. After solid-liquid separation, lithium salts and anhydrous lithium iron phosphate are prepared separately. This invention breaks with the conventional approach of separately processing black powder and phosphogypsum, co-processing two large-volume solid wastes into high-value-added products in a single process. It has the advantages of low reagent consumption, high process coupling, and high comprehensive utilization rate of solid waste, realizing a closed-loop cycle of waste-to-waste treatment and solid-waste coupling.

[0078] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for synergistically preparing lithium salts and iron phosphate using black powder and phosphogypsum, characterized in that, Includes the following steps: S1, phosphogypsum is obtained by washing, neutralizing and calcining pretreatment to obtain activated phosphogypsum; S2, the activated phosphogypsum is mixed with pretreated black powder to obtain a mixed powder, and then water is added to make a slurry; S3, add a buffer system to the slurry to adjust the pH value of the slurry to 2.5-5.5, add hydrogen peroxide, and heat the reaction to obtain the reaction slurry; S4, the reaction slurry is subjected to solid-liquid separation to obtain a lithium-containing leachate and a mixed solid residue; the lithium-containing leachate is subjected to a precipitation reaction to obtain lithium salt; the mixed solid residue is subjected to acid washing, water washing and calcination to obtain iron phosphate.

2. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S1, the water washing adopts a three-stage countercurrent water washing method, with a liquid-to-solid ratio of (2-5):1 and a washing time of 30-60 min; The neutralization treatment involves adding lime milk to the phosphogypsum slurry obtained from the final water washing stage to adjust the pH value to 6.5–8.0, followed by washing and filtration. The conditions for the calcination pretreatment include: a temperature of 160–180°C and a calcination time of 1.5–2.5 h.

3. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S2, the pretreated black powder is a mixture of positive and negative electrode black powder obtained from the dismantling, crushing, and sorting of retired lithium iron phosphate batteries, wherein the lithium content is 2-5 wt%, the iron content is 25-40 wt%, and the phosphorus content is 15-25 wt%. The crushing process employs ball milling or impact crushing to achieve a particle size ≤150μm. The sorting is performed using magnetic separation.

4. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S2, the mass ratio of the activated phosphogypsum to the pretreated black powder is (0.6-1.2):1; The solid-liquid ratio in the slurry is 1:(3-8).

5. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S3, the buffer system includes an acetate-sodium acetate buffer system, a phosphate-phosphate buffer system, a citric acid-sodium citrate buffer system, or a formic acid-sodium formate buffer system.

6. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 5, characterized in that, The buffer system is an acetate-sodium acetate buffer system, and the amount of acetate-sodium acetate buffer system added is 2 to 10% of the slurry volume.

7. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S3, the hydrogen peroxide has a mass fraction of 25-35% and is added at a rate of 1-10% of the mass of the pretreated black powder.

8. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S3, the heating reaction is carried out by stirring at 50–95°C for 1–4 hours.

9. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S4, the lithium-containing leachate undergoes a precipitation reaction to obtain lithium salt, specifically by: adding sodium carbonate solution or phosphoric acid to the lithium-containing leachate, controlling the reaction temperature at 70–95°C, the pH at 8–11, the precipitation reaction time at 30–90 min, filtering, and drying at 105–120°C for 4–8 h to obtain lithium carbonate or lithium phosphate products.

10. The method for synergistic preparation of lithium salt and iron phosphate using black powder and phosphogypsum according to claim 1, characterized in that, In step S4, the pickling is carried out using dilute sulfuric acid with a concentration of 5-15 wt%, the liquid-to-solid ratio is (3-5):1, the pickling temperature is 40-70℃, and the pickling time is 30-90 min. The water is washed until neutral; The calcination temperature is 500–700℃, and the calcination time is 2–5 h.

Images