Method for recycling lithium iron phosphate black powder

Abstract

The application discloses a recycling method of lithium iron phosphate black powder, which comprises the following steps: mixing sintered lithium iron phosphate black powder with a first acidic reagent and an oxidizing agent, and filtering to obtain a first solution and a first solid; performing impurity removal on the first solution to obtain lithium dihydrogen phosphate; mixing the sintered first solid with a first phosphoric acid solution to obtain a second solution; and performing impurity removal on the second solution to obtain ferrous oxalate.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate technology, and specifically to a method for recycling lithium iron phosphate black powder. Background Technology

[0002] The current market for lithium iron phosphate (manganese) black powder recycling mainly focuses on lithium recovery. However, the phosphate slag after lithium extraction is either discarded or sold as fertilizer raw material, resulting in its economic value not being fully realized. A few companies process the phosphate slag into iron phosphate, but because impurities such as Al, Ti, Ni, Co, and Mn cannot be effectively removed, the batch stability of iron phosphate products is poor and even the pass rate is low. Summary of the Invention

[0003] The purpose of this invention is to provide a method for recycling lithium iron phosphate black powder, which solves the problem of inadequate impurity removal during the recycling process.

[0004] To achieve the objectives of this invention, the following technical solution is provided: This invention provides a method for recycling lithium iron phosphate black powder, comprising: The sintered lithium iron phosphate black powder is mixed with a first acidic reagent and an oxidant for selective lithium extraction and then filtered to obtain a first solution and a first solid. The first solution is then subjected to a purification operation to obtain lithium dihydrogen phosphate. The sintered first solid was mixed with a first phosphoric acid solution and subjected to a second acid leaching to obtain a second solution. The second solution was then subjected to a purification process to obtain ferrous oxalate.

[0005] In one embodiment, the liquid-to-solid ratio of the first acidic reagent and the sintered lithium iron phosphate black powder is (3-6) mL:1g; and / or, the reaction temperature for mixing the sintered lithium iron phosphate black powder with the first acidic reagent is 50℃-90℃; and / or, the addition time of the oxidant is 0.5h-1.5h; and / or, the leaching time after mixing with the first acidic reagent is 1h-5h; and / or, the pH of the first solution is 0.3-1.8.

[0006] In one embodiment, the concentration of the first phosphoric acid solution is 20%-35%; and / or, the liquid-solid ratio of the sintered first solid to the first phosphoric acid solution is (3-7) mL:1g; and / or, the reaction temperature for mixing the sintered first solid with the first phosphoric acid solution is 60℃-90℃; and / or, the leaching time for the secondary acid leaching is 1h-5h.

[0007] In one embodiment, the first solution is subjected to a purification operation to obtain lithium dihydrogen phosphate, comprising: The alkaline reagent and the first solution are mixed to obtain the first pre-solution; A second phosphoric acid solution and a precipitant are added to the first presolution to obtain a second solid; The second solid is subjected to impurity removal treatment to obtain the lithium dihydrogen phosphate.

[0008] In one embodiment, the molar ratio of lithium ions to phosphorus ions in the first presolution and the second phosphoric acid solution is (2.9-3.1):1; and / or, the pH of the first presolution is 9-12; and / or, the reaction time for adding the second phosphoric acid solution to the first presolution is 0.5h-3h; and / or, the reaction temperature for adding the second phosphoric acid solution to the first presolution is 60℃-90℃.

[0009] In one embodiment, the alkaline reagent includes one or more of ammonia water, ammonia gas, sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium bicarbonate.

[0010] In one embodiment, the second solid is subjected to a purification process to obtain the lithium dihydrogen phosphate, comprising: The second solid was washed and dissolved using a third phosphoric acid solution, and a second pre-solution was obtained by acidification. Lithium hydroxide solution is added to the second presol to remove impurities, resulting in a third presol, the pH of which is 3.5-5. Acidification is achieved by adding a fourth phosphoric acid solution to the third presol, resulting in a fourth presol, the pH of which is 1.8-3.5. The fourth presolution was subjected to evaporation and crystallization to obtain the lithium dihydrogen phosphate.

[0011] In one embodiment, the second solution is subjected to a purification operation to obtain ferrous oxalate, comprising: Iron powder was added to the first solution to perform a reduction operation, resulting in a fifth pre-solution; Add the first oxalic acid solution to the fifth presol, mix and perform iron precipitation, then filter to obtain the third solid; The third solid is mixed with the second oxalic acid solution and then aged.

[0012] In one embodiment, the concentration of the first oxalic acid solution is 1 mol / L-4 mol / L; and / or, the addition time of the first oxalic acid solution is 0.5 h-1.5 h; and / or, the reaction time after adding the first oxalic acid solution is 0.5 h-3 h; and / or, the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution is 30 °C-75 °C.

[0013] In one embodiment, the concentration of the second oxalic acid solution is 1.5 mol / L-3 mol / L; and / or, the liquid-to-solid ratio of the third solid to the second oxalic acid solution is (0.8-2) mL:1 g; and / or, the reaction time of the aging operation is 0.5 h-4 h; and / or, the reaction temperature of the aging operation is 60 °C-95 °C.

[0014] The present invention relates to a method for recycling lithium iron phosphate (LFP) black powder. This method involves preparing battery-grade ferrous oxalate and lithium dihydrogen phosphate from LFP black powder, employing a secondary sintering and acid leaching process to achieve selective lithium extraction. By controlling the leaching rate of Al, Ti, Ni, Co, and Mn elements in the black powder to >99% during the lithium extraction process, a lithium-rich solution containing impurities is obtained, which is easy to remove impurities from. For the phosphate-iron slag obtained from selective lithium extraction, phosphoric acid leaching is used, and the leachate is used to prepare ferrous oxalate dihydrate. This invention enables the complete recovery of all elements in lithium iron phosphate black powder. The products, ferrous oxalate dihydrate and lithium dihydrogen phosphate, can be used as raw materials for lithium iron phosphate preparation, achieving recycling. Furthermore, the ferrous oxalate dihydrate prepared by this invention contains less than 0.1% trivalent iron, and the contents of impurity elements such as Ni, Co, Mn, Ti, K, and Na are less than 50 ppm, and the content of Al is less than 100 ppm. The lithium dihydrogen phosphate contains less than 50 ppm of impurity elements such as Ni, Co, Mn, Ti, K, Na, and Al. Moreover, the molar ratio of Li to P in the lithium dihydrogen phosphate is 0.99-1.01, and the contents of elements such as Cu and Zn meet the requirements for the subsequent preparation of lithium iron phosphate cathode. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] Figure 1 This is a flowchart of a method for recycling lithium iron phosphate black powder according to one embodiment; Figure 2 This is a flowchart of one step in a method for recycling lithium iron phosphate black powder according to one embodiment; Figure 3 This is a flowchart of another step in a method for recycling lithium iron phosphate black powder according to one embodiment; Figure 4 This is a flowchart of another step in a method for recycling lithium iron phosphate black powder according to one embodiment; Figure 5 This is a flowchart of a method for recycling lithium iron phosphate black powder according to another embodiment; Figure 6 This is a scanning electron microscope (SEM) image of ferrous oxalate from one embodiment. Figure 7 This is a scanning electron microscope (SEM) image of lithium dihydrogen phosphate according to one embodiment. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and 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.

[0018] It should be noted that when a component is said to be "fixed" to another component, it can be directly on the other component or it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

[0019] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used in this invention includes any and all combinations of one or more of the associated listed items.

[0020] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0021] Please refer to Figure 1 and Figure 5 This invention provides a method for recycling lithium iron phosphate black powder, comprising: Step S10: The sintered lithium iron phosphate black powder is mixed with the first acidic reagent and oxidant for selective lithium extraction and filtration to obtain a first solution and a first solid. The first solution is then subjected to impurity removal to obtain lithium dihydrogen phosphate.

[0022] Step S20: The sintered first solid is mixed with the first phosphoric acid solution for a second acid leaching to obtain a second solution. The second solution is then subjected to a purification operation to obtain ferrous oxalate.

[0023] Optionally, in step S10, the sintering conditions for lithium iron phosphate black powder are as follows: sintering is carried out in a reducing atmosphere of air, N2, or N2+H2 / CO, wherein the volume percentage of hydrogen and carbon monoxide does not exceed 5%, the sintering temperature is 350℃-600℃, and the sintering time is 0.5h-2h. The sintering temperature can be 350℃, 400℃, 450℃, 500℃, 550℃, 600℃, etc., without limitation. The sintering time can be 0.5h, 1h, 1.5h, 2h, etc., without limitation. Lithium iron phosphate black powder is rich in metallic elements such as lithium, iron, and phosphorus. These elements need to exist in specific valence states during the recovery process to ensure the purity of subsequent extraction. In an oxidizing atmosphere, iron (Fe²⁺)... + It is easily oxidized to Fe³ + This leads to the formation of impurity phases (such as LiFeO2, Li3Fe2(PO4)3, etc.), reducing the electrochemical performance of the recycled materials. Sintering in a reducing atmosphere provides protection, preventing iron oxidation and ensuring that iron in the product is in the form of Fe²⁺. + The sintering temperature is too high, which can cause the lithium source particles in the lithium iron phosphate black powder to agglomerate, destroying the material structure. Trace impurities introduced from the raw materials or environment (such as Na, K, Al, Si, etc.) may form a eutectic mixture with lithium iron phosphate at high temperatures. If the sintering temperature is too low, the solid-phase reaction will be incomplete, resulting in impurity residues.

[0024] Optionally, the first acidic reagent can be hydrochloric acid or sulfuric acid added to pure water, and then mixed with the sintered lithium iron phosphate black powder in a certain proportion.

[0025] Optionally, the oxidant may include one or more of potassium permanganate, hydrogen peroxide, ozone, and oxygen. The oxidant is added over a period of 0.5-1.5 hours, and the first solution is a lithium-rich solution.

[0026] Optionally, in step S20, a washing operation is required before sintering the first solid. The first solid is mainly ferrophosphorus slag. The first solid is washed until the conductivity of the washing liquid is less than 1.5 mS / cm, and the washed first solid is then flash-dried. The sintering conditions for the first solid are: sintering in a low-oxygen (i.e., oxygen content less than 5%) or N2 or N2+H2 / CO (≤5%) reducing atmosphere, wherein the volume percentage of hydrogen and carbon monoxide does not exceed 5%, the sintering temperature is 450℃-650℃, and the sintering time is 0.5h-4h. The sintering temperature can be 450℃, 500℃, 550℃, 600℃, 650℃, etc., without limitation. The sintering time can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, etc., without limitation. Sintering in a reducing atmosphere helps reduce the oxidation of ferrous ions. The recycled black powder may contain impurities such as aluminum, copper, and manganese, which are difficult to completely remove during the first sintering. The second sintering can more effectively reduce the impurity content through high-temperature volatilization or chemical reactions (such as oxidative decomposition). After the impurities are reduced, when using the black powder as a raw material to prepare ferrous oxalate, the impurities can be avoided from interfering with the reaction process, reducing the risk of side reactions and improving the purity of ferrous oxalate.

[0027] The present invention relates to a method for recycling lithium iron phosphate (LFP) black powder. This method involves preparing battery-grade ferrous oxalate and lithium dihydrogen phosphate from LFP black powder, employing a secondary sintering and acid leaching process to achieve selective lithium extraction. By controlling the leaching rate of Al, Ti, Ni, Co, and Mn elements in the black powder to >99% during the lithium extraction process, a lithium-rich solution containing impurities is obtained, which is easy to remove impurities from. For the phosphate-iron slag obtained from selective lithium extraction, phosphoric acid leaching is used, and the leachate is used to prepare ferrous oxalate dihydrate. This invention enables the complete recovery of all elements in lithium iron phosphate black powder. The products, ferrous oxalate dihydrate and lithium dihydrogen phosphate, can be used as raw materials for lithium iron phosphate preparation, achieving recycling. Furthermore, the ferrous oxalate dihydrate prepared by this invention contains less than 0.1% trivalent iron, and the contents of impurity elements such as Ni, Co, Mn, Ti, K, and Na are less than 50 ppm, and the content of Al is less than 100 ppm. The lithium dihydrogen phosphate contains less than 50 ppm of impurity elements such as Ni, Co, Mn, Ti, K, Na, and Al. Moreover, the molar ratio of Li to P in the lithium dihydrogen phosphate is 0.99-1.01, and the contents of elements such as Cu and Zn meet the requirements for the subsequent preparation of lithium iron phosphate cathode.

[0028] Furthermore, the present invention has a high recovery rate, with the comprehensive recovery rate of elements such as lithium, iron, and phosphorus exceeding 95%. Specifically, the lithium recovery rate is calculated as follows: lithium recovery rate = (lithium element in lithium dihydrogen phosphate - lithium element in added lithium hydroxide) / lithium in the raw material; iron recovery rate = (iron element in ferrous oxalate - reduced iron) / iron in the raw material; and phosphorus recovery rate = (phosphorus element in lithium dihydrogen phosphate - added phosphorus) / phosphorus in the raw material.

[0029] Electron micrograph of ferrous oxalate prepared using the lithium iron phosphate black powder recycling method of the present invention is shown below. Figure 6 As shown, the electron microscope image of the prepared lithium dihydrogen phosphate is as follows. Figure 7 As shown in the figure, ferrous oxalate has different morphological structures and particle sizes. At the same time, the prepared ferrous oxalate particles and lithium dihydrogen phosphate have regular morphologies and no dust on the particle surface, indicating that the ferrous oxalate crystals and lithium dihydrogen phosphate crystals have good crystallinity.

[0030] In one embodiment, the liquid-to-solid ratio of the first acidic reagent to the sintered lithium iron phosphate black powder is (3-6) mL:1g. Optionally, the liquid-to-solid ratio can be 3 mL:1g, 3.5 mL:1g, 4 mL:1g, 4.5 mL:1g, 5 mL:1g, 5.5 mL:1g, 6 mL:1g, etc., without limitation. The liquid-to-solid ratio directly affects the leaching efficiency and acid consumption. If the liquid-to-solid ratio is too small, the first acidic reagent decomposes faster, the effective concentration decreases, and the lithium-ion leaching rate decreases. While a large liquid-to-solid ratio can reduce the viscosity of the slurry, it increases the equipment load and acid consumption, reducing economic efficiency. A moderate liquid-to-solid ratio can balance lithium-ion leaching efficiency and cost.

[0031] And / or, the reaction temperature for mixing the sintered lithium iron phosphate black powder with the first acidic reagent is 50℃-90℃. Optionally, the reaction temperature can be 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc., without limitation. If the reaction temperature is too low, the reaction will be incomplete, and the lithium leaching rate will be low; if the reaction temperature is too high, the reaction time can be shortened, but excessive evaporation or side reactions may occur. A moderate reaction temperature can ensure the lithium leaching rate while reducing the dissolution loss of iron.

[0032] And / or, the oxidant is added for 0.5-1.5 hours. Optionally, the addition time can be 0.5 hours, 0.6 hours, 0.7 hours, 0.8 hours, 0.9 hours, 1 hour, 1.1 hours, 1.2 hours, 1.3 hours, 1.4 hours, 1.5 hours, etc., without limitation. Rapid addition of the oxidant may result in insufficient oxidation reaction, leading to iron phosphate dissolution or lithium loss. However, excessively long addition time will prolong the production cycle. An appropriate addition time can maintain a stable reaction environment.

[0033] And / or, the leaching time after mixing with the first acidic reagent is 1-5 hours. Optionally, the leaching time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, etc., without limitation. A leaching time that is too short may result in incomplete lithium leaching; a leaching time that is too long may trigger side reactions (such as the formation of LiFe(PO4)(OH) impurities). A moderate leaching time corresponds to a higher lithium leaching rate and fewer impurities.

[0034] And / or, the pH of the first solution is 0.3-1.8. Optionally, the pH of the first solution can be 0.3, 0.5, 0.8, 1, 1.2, 1.4, 1.6, 1.8, etc., without limitation. If the pH is too high, lithium may precipitate in the form of Li3PO4, leading to a decrease in leaching rate; if the pH is too low, iron phosphate dissolves, increasing lithium loss. When the pH is controlled between 0.3 and 1.8, the lithium leaching rate is high, the phosphate content is low, and the product purity is high.

[0035] In one embodiment, the concentration of the primary phosphoric acid solution is 20%-35%. Optionally, the concentration of the primary phosphoric acid solution can be 20%, 25%, 30%, 35%, etc., without limitation. If the concentration of the primary phosphoric acid solution is too low, the decomposition of the primary phosphoric acid solution is accelerated, the effective concentration is reduced, and the iron ion leaching rate decreases. If the concentration of the primary phosphoric acid solution is too high, it will promote the formation of iron-phosphate complexes, reducing the iron ion leaching efficiency. When the concentration of the primary phosphoric acid solution is moderate, the iron ion leaching rate is stable and efficient.

[0036] And / or, the liquid-to-solid ratio of the first solid and the first phosphoric acid solution after sintering is (3-7) mL:1g. Optionally, the liquid-to-solid ratio can be 3mL:1g, 4mL:1g, 5mL:1g, 6mL:1g, 7mL:1g, etc., without limitation. When the liquid-to-solid ratio is too low, the viscosity of the slurry increases, the fluidity deteriorates, and the contact area and diffusion rate between phosphoric acid and black powder particles are limited. When the liquid-to-solid ratio is too high, larger reaction equipment and more phosphoric acid are required, leading to increased production costs. When the liquid-to-solid ratio is moderate, the leaching efficiency of iron ions is high and the production cost is moderate.

[0037] And / or, the reaction temperature for mixing the sintered first solid with the first phosphoric acid solution is 60℃-90℃. Optionally, the reaction temperature can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc., without limitation. At lower reaction temperatures, the dissolution rate of lithium iron phosphate and the migration rate of iron ions are slower, resulting in a lower leaching rate. Excessively high reaction temperatures may cause phosphoric acid decomposition or volatilization, reducing leaching efficiency. Controlling the reaction temperature at 60℃-90℃ ensures both the normal use of the first phosphoric acid solution and leaching efficiency.

[0038] And / or, the leaching time for secondary acid leaching is 1-5 hours. Optionally, the leaching time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, etc., without restriction. A leaching time that is too short may result in incomplete iron leaching; a leaching time that is too long does not significantly improve the iron leaching rate, but only prolongs the experimental period. A moderate leaching time can balance leaching rate and experimental period.

[0039] Please refer to Figure 2 In one embodiment, step S10 involves removing impurities from the first solution to obtain lithium dihydrogen phosphate, including: Step S11: Mix the alkaline reagent and the first solution to remove impurities and obtain the first pre-solution.

[0040] In step S12, a second phosphoric acid solution and a precipitant are added to the first pre-solution to carry out a lithium precipitation reaction, thereby obtaining a second solid.

[0041] Step S13: The second solid is subjected to impurity removal treatment to obtain lithium dihydrogen phosphate.

[0042] Optionally, in step S11, the reaction time of the alkaline reagent is greater than 5 minutes, and solid-liquid separation is performed to obtain the first pre-solution.

[0043] Optionally, in step S12, after adding the second phosphoric acid solution, a precipitant can be added to adjust the pH. The precipitant can be one or more of sodium hydroxide, potassium hydroxide, ammonia water, and ammonia gas, without limitation.

[0044] After adding a second phosphoric acid solution to the first pre-solution, solid-liquid separation is performed. The corresponding solution contains byproducts such as ammonium salt and sodium salt. The byproducts can be obtained by evaporation, crystallization, and drying of the solution.

[0045] In one embodiment, the molar ratio of lithium ions to phosphorus ions in the first pre-solution and the second phosphoric acid solution is (2.9-3.1):1. Optionally, the Li / P molar ratio can be 2.9:1, 2.95:1, 3:1, 3.05:1, 3.1:1, etc., without limitation. Theoretically, the lithium ions in the first pre-solution exist as lithium phosphate, with the chemical formula Li3PO4, and the theoretical Li / P molar ratio is 3:1. Controlling the actual ratio at (2.9-3.1):1 ensures complete reaction of phosphate ions and avoids excessive phosphate residue leading to difficulties in subsequent processing.

[0046] And / or, the pH of the first pre-solution is 9-12. The pH can be 9, 10, 11, 12, etc., without restriction. If the pH is too low, magnesium in the first pre-solution may exist in the form of MgHPO4, making complete precipitation difficult; if the pH is too high, lithium may be lost due to local supersaturation. A pH of 9-12 balances magnesium removal and lithium retention.

[0047] And / or, the reaction time for adding the second phosphoric acid solution to the first pre-solution is 0.5h-3h. Optionally, the reaction time can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, etc., without limitation. If the time is insufficient, lithium phosphate may not be completely converted, resulting in unreacted lithium phosphate or intermediate products remaining in the product; if the time is too long, it will affect the production cycle; controlling the reaction time is sufficient to ensure the complete conversion of lithium phosphate.

[0048] And / or, the reaction temperature for adding the second phosphoric acid solution to the first pre-solution is 60℃-90℃. Optionally, the reaction temperature can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc., without limitation. At lower temperatures, the reaction rate is too slow, which may lead to incomplete dissolution of lithium phosphate and reduced product purity. At higher temperatures, the phosphoric acid may decompose or volatilize, reducing the reaction rate. Controlling the reaction temperature at 60℃-90℃ ensures the normal use of the second phosphoric acid solution while maintaining the reaction rate.

[0049] In one embodiment, the alkaline reagent includes one or more of ammonia water, ammonia gas, sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium bicarbonate. The alkaline reagent is used to precipitate impurity ions such as aluminum ions, magnesium ions, and calcium ions in the first solution.

[0050] Please refer to Figure 3 In one embodiment, step S13 includes: Step S131: Wash the second solid and dissolve it with a third phosphoric acid solution to obtain a second pre-solution through acidification.

[0051] In step S132, lithium hydroxide solution is added to the second presol to remove impurities, resulting in a third presol, the pH of which is 3.5-5.

[0052] In step S133, a fourth phosphoric acid solution is added to the third presol to acidify it, resulting in a fourth presol, the pH of which is 1.8-3.5.

[0053] Step S134: Evaporate and crystallize the fourth presol to obtain lithium dihydrogen phosphate.

[0054] Optionally, in step S131, the washing operation involves multi-stage countercurrent washing using hot water at 60℃-90℃ until the conductivity of the washing solution is less than 1.5mS / cm, followed by solid-liquid separation and dissolution using a second phosphoric acid solution.

[0055] Optionally, in step S132, the lithium hydroxide solution can be a saturated or near-saturated lithium hydroxide solution, the reaction time is greater than 10 min, and a third pre-solution is obtained after solid-liquid separation. This step can be used to remove impurity elements such as iron, aluminum, and titanium. The pH of the third pre-solution is not limited to 3.5, 4, 4.5, or 5.

[0056] Optionally, the fourth phosphoric acid solution added in step S133 may be industrial-grade or battery-grade phosphoric acid. The pH of the fourth pre-solution may be 1.8, 2, 2.5, 3, 3.5, etc., without restriction.

[0057] Please refer to Figure 5Optionally, in step S134, the evaporation crystallization operation includes: evaporating at a temperature of 110℃-130℃ until obvious solid precipitates in the fourth pre-solution; after no obvious solid precipitates, stopping heating and cooling to 30℃-60℃, controlling the cooling time to 0.5h-3h; centrifuging, controlling the water content of the solid to be less than 5%; drying the centrifuged solid at 100℃-120℃ until the water content is <0.35%, thus obtaining the lithium dihydrogen phosphate product. The first phosphoric acid mother liquor obtained after centrifugation can be recycled for the next batch of lithium dihydrogen phosphate impurity removal process.

[0058] Please refer to Figure 4 In one embodiment, step S20 involves removing impurities from the second solution to obtain ferrous oxalate, including: Step S21: Add iron powder to the first solution to perform a reduction operation to obtain the fifth pre-solution.

[0059] Step S22: Add the first oxalic acid solution to the fifth presolution, mix and filter to obtain the third solid.

[0060] Step S23: Mix the third solid with the second oxalic acid solution and perform an aging process.

[0061] Optionally, in step S21, the reduction principle is: n(Fe) = n(copper) + 1 / 2·n(Fe3+). The actual amount of iron powder added is 1-1.5 times the theoretical molar value to ensure the full removal of copper and the full reduction of iron, so that the fifth pre-solution is an iron-phosphorus solution.

[0062] Please refer to Figure 5 Optionally, in step S22, filtration yields a third solid and a second phosphoric acid mother liquor, with the second phosphoric acid mother liquor containing 15%-30% phosphoric acid. Approximately two-thirds of the second phosphoric acid mother liquor is recycled for secondary acid leaching in the next batch in step S20, and the remaining approximately one-third is used for lithium precipitation in step S12 and impurity removal in step S13 (specifically, the acid leaching in step S131). Optionally, the third solid is ferric oxalate.

[0063] Optionally, step S23 also includes a centrifugal drying operation, specifically centrifuging and drying the solid-liquid mixture after aging, with a drying temperature of 80℃-110℃ and a drying time of 1h-6h, and the centrifuged solution can be used for iron precipitation in step S22.

[0064] Optionally, steps S21 and S23 are both carried out in a nitrogen atmosphere to ensure that the content of ferric iron in ferrous oxalate dihydrate is less than 0.1%.

[0065] For the ferrophosphate slag obtained after selective lithium extraction, this invention uses a first phosphoric acid solution for leaching to obtain a first solution and prepare ferrous oxalate dihydrate. The mother liquor from subsequent preparation processes is a phosphoric acid solution. A portion of the phosphoric acid solution is returned to the phosphoric acid leaching process, and the remainder is used to prepare lithium dihydrogen phosphate, enabling the recycling of the phosphoric acid solution with only a small amount of additional replenishment required. Compared to traditional wet recycling processes, this invention generates less wastewater, achieves phosphoric acid recycling, and has lower production costs.

[0066] In one embodiment, the concentration of the first oxalic acid solution is 1 mol / L to 4 mol / L. Optionally, the concentration of the first oxalic acid solution can be 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, etc., without limitation. If the concentration of the first oxalic acid solution is too low, it will prolong the reaction time required to form ferrous oxalate. If the concentration of the first oxalic acid solution is too high, nucleation may occur, resulting in excessively small particle size of the generated ferrous oxalate. A moderate concentration of the first oxalic acid solution ensures that the particle size of the generated ferrous oxalate is not too fine, while also improving the reaction cycle.

[0067] And / or, the addition time of the first oxalic acid solution is 0.5h-1.5h. Optionally, the addition time can be 0.5h, 0.8h, 1h, 1.2h, 1.5h, etc., without limitation. Too short an addition time will lead to excessively high local concentrations, resulting in uneven reaction and affecting product purity. Too long an addition time will prolong the reaction time and reduce the formation efficiency. An appropriate addition time can balance uniform reaction and formation efficiency.

[0068] And / or, the reaction time after adding the first oxalic acid solution is 0.5h-3h. Optionally, the reaction time can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, etc., without limitation. Excessive reaction time will affect the reaction cycle. Insufficient reaction time is not conducive to the complete reaction of oxalic acid and ferrous ions. An appropriate reaction time ensures the complete reaction of oxalic acid and ferrous ions without excessively prolonging the experimental cycle.

[0069] And / or, the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution is 30℃-75℃. Optionally, the reaction temperature can be 30℃, 40℃, 50℃, 60℃, 70℃, 75℃, etc., without limitation. When the reaction temperature is too low, molecular motion weakens, the collision frequency of reactants decreases, resulting in a significant decrease in the reaction rate. When the reaction temperature is too high, oxalic acid easily decomposes into carbon dioxide and water at high temperatures, leading to a decrease in reactant concentration, a decrease in reaction rate, or even failure to fully react with impurity particles. When the reaction temperature is moderate, the concentration of oxalic acid as a reactant is maintained without reducing the reaction rate, thereby improving the precipitation efficiency of impurities.

[0070] In one embodiment, the concentration of the second oxalic acid solution is 1.5 mol / L to 3 mol / L. The concentration of the second oxalic acid solution can be 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, etc., and is not limited. If the concentration of the second oxalic acid solution is too low, incomplete reaction may result in residual ferrous ions in the product, which also affects purity. If the concentration of the second oxalic acid solution is too high, impurities (such as unreacted oxalate ions or byproducts) may be introduced, reducing product purity. A moderate concentration of the second oxalic acid solution ensures product purity.

[0071] And / or, the liquid-to-solid ratio of the third solid and the second oxalic acid solution is (0.8-2) mL:1g. Optionally, the liquid-to-solid ratio can be 0.8 mL:1g, 1 mL:1g, 1.2 mL:1g, 1.4 mL:1g, 1.6 mL:1g, 1.8 mL:1g, 2 mL:1g, etc., without limitation. Due to hindered crystal growth and secondary nucleation, a low liquid-to-solid ratio can lead to irregular morphology, rough surface, and easy agglomeration of aged ferrous oxalate crystals. A high liquid-to-solid ratio will affect equipment and production costs. An appropriate liquid-to-solid ratio helps to save costs while producing stable crystal forms.

[0072] And / or, the aging process can last from 0.5 h to 4 h. Optionally, the aging time can be 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, etc., without limitation. Excessive aging time may lead to crystal aggregation or secondary nucleation, resulting in increased particle size and crystal form transformation. Insufficient reaction time leads to unstable crystal form of the aged product. An appropriate reaction time can form ferrous oxalate with a stable crystal structure.

[0073] And / or, the reaction temperature for aging is 60℃-95℃. Optionally, the reaction temperature for aging can be 60℃, 70℃, 80℃, 90℃, 95℃, etc., without limitation. When the reaction temperature is too low, the system has a high degree of supersaturation, high resistance to particle movement, slow crystal growth, and is prone to forming β-type ferrous oxalate. When the reaction temperature is too high, particle movement intensifies, the crystal growth rate accelerates, and it tends to form the more stable α-type ferrous oxalate. When the reaction temperature is moderate, it helps to form mixed-crystal ferrous oxalate, thereby improving the stress concentration problem of single-crystal ferrous oxalate during sintering.

[0074] The technical solution of the present invention will be described in detail below through specific embodiments.

[0075] Example 1 Please refer to Figure 5 The method for recycling lithium iron phosphate black powder in this embodiment is as follows: In step S10, the lithium iron phosphate black powder sintered in nitrogen is mixed with a first acidic reagent and an oxidant for selective lithium extraction and then filtered to obtain a first solution and a first solid. The sintering temperature is 500℃, and the sintering time is 1.5h; the first acidic reagent is sulfuric acid, the liquid-to-solid ratio of the first acidic reagent and the sintered lithium iron phosphate black powder is 5mL:1g, the reaction temperature for mixing the first acidic reagent is 80℃; the oxidant is hydrogen peroxide, the oxidant is added for 1.0h, and the pH of the first solution is 1.2.

[0076] The purification process for the first solution includes: mixing ammonia with the first solution for 10 minutes to obtain a first pre-solution; adding a second phosphoric acid solution and a precipitant to the first pre-solution to obtain a second solid. The molar ratio of lithium ions to phosphorus ions in the first pre-solution and the second phosphoric acid solution is 3:1; the pH of the first pre-solution is 10; the reaction time for adding the second phosphoric acid solution to the first pre-solution is 2 hours; and the reaction temperature for adding the second phosphoric acid solution to the first pre-solution is 85°C. The second solid is subjected to impurity removal treatment, including: multi-stage countercurrent washing with hot water at 60℃-90℃ until the conductivity of the washing solution is less than 1.5 mS / cm; after solid-liquid separation, it is dissolved using a second phosphoric acid solution; a saturated lithium hydroxide solution is added to the second presolution to obtain a third presolution with a pH of 4.5. A fourth phosphoric acid solution is added to the third presolution to obtain a fourth presolution with a pH of 2.5.

[0077] The fourth pre-solution was subjected to evaporation and crystallization to obtain lithium dihydrogen phosphate. Specifically, the evaporation temperature was 120℃ until obvious solid precipitated from the fourth pre-solution; after no obvious solid precipitated, heating was stopped and the temperature was lowered to 60℃, with the cooling time controlled at 3 hours; centrifugation was performed, controlling the water content of the solid to be less than 5%; the centrifuged solid was dried at 120℃ until the water content was <0.35%, thus obtaining the finished lithium dihydrogen phosphate product.

[0078] Step S20 involves mixing the first solid, sintered in nitrogen, with a first phosphoric acid solution for a second acid leaching to obtain a second solution. The sintering temperature of the first solid is 550℃, and the sintering time is 3 hours; the concentration of the first phosphoric acid solution is 25%; the liquid-to-solid ratio of the sintered first solid to the first phosphoric acid solution is 6 mL:1 g; the reaction temperature for adding the first phosphoric acid solution is 90℃; and the leaching time after adding the first phosphoric acid solution is 3 hours. The second solution was purified to obtain ferrous oxalate. Specifically, iron powder was added to the first solution under a nitrogen atmosphere for reduction to obtain a fifth pre-solution, with the iron powder content being 1.5 times the theoretical molar value. A first oxalic acid solution was added to the fifth pre-solution, mixed, and filtered to obtain a third solid. The concentration of the first oxalic acid solution was 3 mol / L, the addition time was 1.0 h, the reaction time after adding the first oxalic acid solution was 2 h, and the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution was 55 °C. The third solid was mixed with the second oxalic acid solution, with a concentration of 2.0 mol / L, and the liquid-to-solid ratio of the third solid to the second oxalic acid solution was 1.5 mL:1 g. The aging reaction time was 2 h, and the reaction temperature was 90 °C. The solid-liquid mixture after aging was centrifuged and dried at 100 °C for 3 h to obtain ferrous oxalate.

[0079] Example 2 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature for mixing the first acidic reagent is 30°C.

[0080] Example 3 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature for mixing the first acidic reagent is 120°C.

[0081] Example 4 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the oxidant addition time is 0.1 h and the leaching time after mixing with the first acidic reagent is 0.5 h.

[0082] Example 5 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the oxidant addition time is 2 hours and the leaching time after mixing with the first acidic reagent is 3 hours.

[0083] Example 6 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature for adding the second phosphoric acid solution is 40°C, and the reaction time after adding the second phosphoric acid solution is 0.1 h.

[0084] Example 7 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature for adding the second phosphoric acid solution is 120°C, and the reaction time after adding the second phosphoric acid solution is 4 hours.

[0085] Example 8 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the first oxalic acid solution is 0.5 mol / L, and the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution is 20°C.

[0086] Example 9 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the first oxalic acid solution is 5 mol / L, and the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution is 85°C.

[0087] Example 10 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the addition time of the first oxalic acid solution is 0.1 h; and the reaction time after adding the first oxalic acid solution is 0.1 h.

[0088] Example 11 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the addition time of the first oxalic acid solution is 2 hours; and the reaction time after adding the first oxalic acid solution is 4 hours.

[0089] Example 12 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature for adding the first phosphoric acid solution is 40°C, and the leaching time after adding the first phosphoric acid solution is 0.5 h.

[0090] Example 13 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature for adding the first phosphoric acid solution is 120°C, and the leaching time after adding the first phosphoric acid solution is 6 hours.

[0091] Example 14 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature of the aging operation is 40°C and the reaction time of the aging operation is 0.1h.

[0092] Example 15 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature of the aging operation is 120°C and the reaction time of the aging operation is 6 hours.

[0093] Comparative Example 1 The difference between the recycling method of lithium iron phosphate black powder in this comparative example and that in Example 1 is that the lithium iron phosphate black powder is not sintered.

[0094] Comparative Example 2 The difference between the recycling method of lithium iron phosphate black powder in this comparative example and that in Example 1 is that no oxidant is added.

[0095] Comparative Example 3 The difference between the recycling method of lithium iron phosphate black powder in this comparative example and that in Example 1 is that the first solid is not sintered.

[0096] The purity of the prepared lithium dihydrogen phosphate and ferrous oxalate were tested, and the test results are as follows.

[0097] Table 1-1

[0098] Table 1-2

[0099] Table 1-3

[0100] Table 1-4

[0101] Table 1-5

[0102] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0103] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for recycling lithium iron phosphate black powder, characterized in that, include: The sintered lithium iron phosphate black powder is mixed with a first acidic reagent and an oxidant for selective lithium extraction and then filtered to obtain a first solution and a first solid. The first solution is then subjected to a purification operation to obtain lithium dihydrogen phosphate. The sintered first solid was mixed with a first phosphoric acid solution and subjected to a second acid leaching to obtain a second solution. The second solution was then subjected to a purification process to obtain ferrous oxalate.

2. The method for recycling lithium iron phosphate black powder according to claim 1, characterized in that, The liquid-to-solid ratio of the first acidic reagent and the sintered lithium iron phosphate black powder is (3-6) mL:1g; and / or, the reaction temperature for mixing the sintered lithium iron phosphate black powder with the first acidic reagent is 50℃-90℃; and / or, the addition time of the oxidant is 0.5h-1.5h; and / or, the leaching time after mixing with the first acidic reagent is 1h-5h; and / or, the pH of the first solution is 0.3-1.

8.

3. The method for recycling lithium iron phosphate black powder according to claim 1, characterized in that, The concentration of the first phosphoric acid solution is 20%-35%; and / or, the liquid-solid ratio of the sintered first solid to the first phosphoric acid solution is (3-7) mL:1g; and / or, the reaction temperature for mixing the sintered first solid with the first phosphoric acid solution is 60℃-90℃; and / or, the leaching time for the secondary acid leaching is 1h-5h.

4. The method for recycling lithium iron phosphate black powder according to any one of claims 1 to 3, characterized in that, The first solution is subjected to a purification process to obtain lithium dihydrogen phosphate, comprising: The alkaline reagent and the first solution are mixed to remove impurities, resulting in the first pre-solution; A second phosphoric acid solution and a precipitant are added to the first pre-solution to carry out a lithium precipitation reaction, yielding a second solid. The second solid is subjected to impurity removal treatment to obtain the lithium dihydrogen phosphate.

5. The method for recycling lithium iron phosphate black powder according to claim 4, characterized in that, The molar ratio of lithium ions to phosphorus ions in the first presolution and the second phosphoric acid solution is (2.9-3.1):1; and / or, the pH of the first presolution is 9-12; and / or, the reaction time for adding the second phosphoric acid solution to the first presolution is 0.5h-3h; and / or, the reaction temperature for adding the second phosphoric acid solution to the first presolution is 60℃-90℃.

6. The method for recycling lithium iron phosphate black powder according to claim 4, characterized in that, The alkaline reagent includes one or more of ammonia water, ammonia gas, sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium bicarbonate.

7. The method for recycling lithium iron phosphate black powder according to claim 4, characterized in that, The second solid is subjected to a purification process to obtain the lithium dihydrogen phosphate, comprising: The second solid was washed and dissolved using a third phosphoric acid solution, and a second pre-solution was obtained by acidification. Lithium hydroxide solution is added to the second presol to remove impurities, resulting in a third presol, the pH of which is 3.5-5. Acidification is achieved by adding a fourth phosphoric acid solution to the third presol, resulting in a fourth presol, the pH of which is 1.8-3.

5. The fourth presolution was subjected to evaporation and crystallization to obtain the lithium dihydrogen phosphate.

8. The method for recycling lithium iron phosphate black powder according to any one of claims 1 to 3, characterized in that, The second solution is subjected to a purification process to obtain ferrous oxalate, comprising: Iron powder was added to the first solution to perform a reduction operation, resulting in a fifth pre-solution; Add the first oxalic acid solution to the fifth presol, mix and perform iron precipitation, then filter to obtain the third solid; The third solid is mixed with the second oxalic acid solution and then aged.

9. The method for recycling lithium iron phosphate black powder according to claim 8, characterized in that, The concentration of the first oxalic acid solution is 1 mol / L-4 mol / L; and / or, the addition time of the first oxalic acid solution is 0.5 h-1.5 h; and / or, the reaction time after adding the first oxalic acid solution is 0.5 h-3 h; and / or, the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution is 30 °C-75 °C.

10. The method for recycling lithium iron phosphate black powder according to claim 8, characterized in that, The concentration of the second oxalic acid solution is 1.5 mol / L-3 mol / L; and / or, the liquid-solid ratio of the third solid to the second oxalic acid solution is (0.8-2) mL:1 g; and / or, the reaction time of the aging operation is 0.5 h-4 h; and / or, the reaction temperature of the aging operation is 60 °C-95 °C.

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