Method for recycling lithium from waste lithium iron phosphate battery by acid circulation to prepare lithium phosphate

Abstract

The application provides a method for preparing lithium phosphate by recycling lithium from waste lithium iron phosphate batteries through acid circulation, comprising the following steps: pretreating the waste lithium iron phosphate batteries to obtain black powder of lithium iron phosphate batteries; oxidizing and roasting the black powder to fully oxidize and decompose the lithium iron phosphate, thereby obtaining roasted black powder; acid leaching the roasted black powder to obtain a lithium-containing acid leaching solution; mixing hydrogen peroxide solution, alkaline solution and the lithium-containing acid leaching solution, stirring and reacting to obtain a mixed solution; removing impurities from the mixed solution through adsorption to obtain a refined lithium salt solution; performing bipolar membrane electrodialysis on the refined lithium salt solution to obtain lithium hydroxide solution and mixed acid solution; mixing phosphate and the lithium hydroxide solution, performing solid-liquid separation, and then washing and drying to obtain lithium phosphate. The application solves the problems of high sulfuric acid consumption, high acid wastewater discharge, low resource comprehensive utilization rate and low product added value in the existing wet recycling method, realizes efficient recycling of lithium from waste lithium iron phosphate batteries, and significantly improves the economic and environmental benefits of the process.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery recycling technology, and relates to a method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate. Background Technology

[0002] Lithium iron phosphate (LFP) batteries offer advantages such as high safety, long cycle life, and low cost. However, their lifespan is typically only 5-8 years. Used LFP batteries contain large amounts of valuable metals such as lithium, phosphorus, and iron; improper disposal not only causes serious environmental pollution but also wastes precious resources. Therefore, developing efficient, environmentally friendly, and low-cost recycling technologies for used LFP batteries is crucial for achieving resource recycling and promoting the sustainable development of the new energy industry.

[0003] Currently, recycling processes for spent lithium iron phosphate batteries include pyrometallurgical recycling and hydrometallurgical recycling. Pyrometallurgical recycling decomposes battery materials through high-temperature roasting, followed by subsequent processing to recover valuable metals. However, it suffers from drawbacks such as high energy consumption, significant pollution, and low lithium recovery rates. Hydrometallurgical recycling typically uses strong acids such as sulfuric acid as leaching agents to leach lithium from the lithium iron phosphate cathode material. Then, impurities such as iron, aluminum, calcium, and magnesium are removed through chemical precipitation to finally produce lithium carbonate. However, traditional hydrometallurgical recycling consumes large amounts of sulfuric acid, generating substantial amounts of acidic wastewater and difficult-to-treat chemically precipitated residue, resulting in high processing costs and significant environmental pressure. Furthermore, the final product, lithium carbonate, requires further conversion into lithium phosphate for use in the regeneration of lithium iron phosphate cathode materials, increasing the process steps and production costs.

[0004] To reduce acid consumption and costs in the recovery process, some studies have attempted to apply bipolar membrane electrodialysis technology to lithium iron phosphate recovery. Bipolar membrane electrodialysis can decompose salt solutions into acids and bases under a DC electric field, offering advantages such as low energy consumption, no pollution, and high separation efficiency. However, existing technologies mostly only achieve lithium enrichment and lithium hydroxide preparation, failing to effectively separate and reuse the phosphoric acid and sulfuric acid generated during the leaching process, resulting in still high acid consumption. Furthermore, these technologies suffer from cumbersome impurity removal steps, incomplete impurity removal, and low added value of the final product, making large-scale, stable industrial application difficult. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method for recovering lithium from waste lithium iron phosphate batteries and preparing lithium phosphate. This method can solve the problems of high sulfuric acid consumption, large amounts of acidic wastewater discharge, low resource utilization rate, and low product added value in existing wet recycling processes, achieving efficient recovery of lithium from waste lithium iron phosphate batteries and significantly improving the economic and environmental benefits of the process.

[0006] To achieve this objective, the present invention employs the following technical solution:

[0007] This invention provides a method for recovering lithium from spent lithium iron phosphate batteries by recycling the acid, the method comprising the following steps:

[0008] (1) Waste lithium iron phosphate batteries are pretreated to obtain lithium iron phosphate battery black powder;

[0009] (2) The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder;

[0010] (3) The roasted black powder obtained by acid leaching is separated into solid and liquid components to obtain a lithium-containing acidic leachate;

[0011] (4) Mix hydrogen peroxide solution with the lithium-containing acidic leachate to obtain an oxidized lithium-containing acidic leachate; then mix alkaline solution with the oxidized lithium-containing acidic leachate, stir to react, and separate solid and liquid to obtain a purified solution;

[0012] The solutes in the impurity removal solution include LiH2PO4 and Li2SO4;

[0013] (5) The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0014] (6) The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution;

[0015] The mixed phosphate and the lithium hydroxide solution were subjected to solid-liquid separation, followed by washing and drying to obtain lithium phosphate.

[0016] This invention first thoroughly disrupts the stable crystal structure of lithium iron phosphate through oxidative roasting, followed by acid leaching. Then, bipolar membrane electrodialysis is used to separate the refined lithium salt solution into a lithium hydroxide solution and a mixed acid solution. The mixed acid can be directly returned to the acid leaching process, reducing acid consumption and significantly decreasing the generation and discharge of acidic wastewater, thus significantly reducing the cost of acid leaching and wastewater treatment. Subsequently, impurities such as iron and aluminum are removed by hydrogen peroxide combined with pH adjustment using an alkaline solution. Heavy metals such as calcium and magnesium are then removed by adsorption, resulting in targeted and thorough impurity removal and effectively improving the lithium recovery rate. Finally, the lithium hydroxide solution obtained from electrodialysis is directly reacted with phosphate to prepare a high-purity lithium phosphate product. Compared with lithium carbonate prepared by traditional processes, this product can be directly used for the regeneration of lithium iron phosphate cathode materials, eliminating subsequent conversion steps and significantly increasing the added value of the product. In addition, this invention does not require the addition of large amounts of chemical precipitants, avoiding the problem of difficult-to-treat waste residue. The entire process has no emissions of toxic or harmful substances, operates under mild conditions, has strong equipment versatility, and is easy to implement for continuous and large-scale industrial production.

[0017] In some embodiments, the leaching solution used in step (3) includes a mixture of sulfuric acid and phosphoric acid.

[0018] In some embodiments, the mixed acid solution in step (6) is reused for the acid leaching in step (3).

[0019] In some embodiments, the total hydrogen ion concentration in the mixture is 0.7 mol / L to 1.0 mol / L.

[0020] In some embodiments, the molar ratio of sulfuric acid to phosphoric acid in the mixture is 2:1 to 4:1.

[0021] In some embodiments, the acid leaching temperature in step (3) is 80°C to 95°C.

[0022] In some embodiments, the liquid-to-solid ratio of the acid leaching in step (3) is 3:1 to 5:1.

[0023] In some embodiments, the stirring rate of the acid leaching in step (3) is 300 r / min to 500 r / min.

[0024] In some embodiments, the acid immersion time in step (3) is 3h to 5h.

[0025] In some embodiments, the pretreatment in step (1) includes: crushing and gravity sorting the waste lithium iron phosphate batteries after they are fully discharged to obtain coarse black powder; the coarse black powder is decarbonized at 500℃~600℃ for 1h~2h to obtain the lithium iron phosphate battery black powder.

[0026] In some embodiments, the heating rate of the oxidative calcination in step (2) is 5°C / min to 10°C / min.

[0027] In some embodiments, the oxidative calcination temperature in step (2) is 600°C to 700°C.

[0028] In some embodiments, the oxidative calcination time in step (2) is 1.5h to 3h.

[0029] In some embodiments, the concentration of the hydrogen peroxide solution in step (4) is 25wt% to 30wt%.

[0030] In some embodiments, the amount of hydrogen peroxide solution used in step (4) is such that the amount of hydrogen peroxide is 2 to 6 times the molar amount of divalent iron ions in the lithium-containing acidic leachate.

[0031] In some embodiments, the alkaline solution in step (4) is a sodium hydroxide solution with a concentration of 2 mol / L to 5 mol / L.

[0032] In some embodiments, the amount of alkaline solution used in step (4) is to adjust the pH of the system to 4-6.

[0033] In some embodiments, the stirring reaction time in step (4) is 0.5h to 2h.

[0034] In some embodiments, the adsorption and impurity removal in step (5) is performed using an aminophosphonic acid chelating resin or a strongly acidic styrene-based cation exchange resin.

[0035] In some embodiments, during the adsorption and purification process in step (5), the flow rate of the purification solution is 1 BV / h to 3 BV / h.

[0036] In some embodiments, the current density of the bipolar membrane electrodialysis in step (6) is 10 mA / cm². 2 ~30mA / cm 2 .

[0037] In some embodiments, the flow rate of the purified lithium salt solution during bipolar membrane electrodialysis in step (6) is 5 L / h to 15 L / h.

[0038] In some embodiments, the temperature of the bipolar membrane electrodialysis in step (6) is 25°C to 40°C.

[0039] In some embodiments, the phosphate in step (6) includes trisodium phosphate.

[0040] In some embodiments, the mixing temperature in step (6) is 40°C to 80°C.

[0041] In some embodiments, the mixing in step (6) is carried out under stirring conditions at a speed of 200 r / min to 400 r / min.

[0042] As a preferred embodiment of the method provided by the present invention, the method includes the following steps:

[0043] (1) After the waste lithium iron phosphate battery is fully discharged, it is crushed and gravity separated to obtain coarse black powder; the coarse black powder is decarbonized at 500℃~600℃ for 1h~2h to obtain the lithium iron phosphate battery black powder.

[0044] (2) The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder;

[0045] The oxidation calcination is carried out in an air atmosphere, with a heating rate of 5℃ / min~10℃ / min, a temperature of 600℃~700℃, and a time of 1.5h~3h.

[0046] (3) The roasted black powder obtained by acid leaching is separated into solid and liquid components to obtain a lithium-containing acidic leachate;

[0047] The acid leaching solution comprises a mixture of sulfuric acid and phosphoric acid, wherein the total hydrogen ion concentration in the mixture is 0.7 mol / L to 1.0 mol / L, and the molar ratio of sulfuric acid to phosphoric acid is 2:1 to 4:1.

[0048] The acid leaching temperature is 80℃~95℃, the stirring rate is 300r / min~500r / min, and the time is 3h~5h;

[0049] The liquid-to-solid ratio of the acid leaching is 3:1 to 5:1, and the unit of the liquid-to-solid ratio is mL / g;

[0050] (4) Mix hydrogen peroxide solution with the lithium-containing acidic leachate to obtain oxidized lithium-containing acidic leachate; then mix alkaline solution with the oxidized lithium-containing acidic leachate, stir and react for 0.5h~2h, separate solid and liquid to obtain impurity-removed solution;

[0051] The concentration of the hydrogen peroxide solution is 25wt%~30wt%; the amount of hydrogen peroxide solution used is such that the amount of hydrogen peroxide is 2 to 6 times the molar amount of divalent iron ions in the lithium-containing acidic leachate.

[0052] The alkaline solution is a sodium hydroxide solution with a concentration of 2 mol / L to 5 mol / L, and the amount of alkaline solution used is to adjust the pH of the system to 4 to 6.

[0053] The solutes in the impurity removal solution include LiH2PO4 and Li2SO4;

[0054] (5) The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0055] The adsorption and impurity removal is carried out using an aminophosphonic acid chelating resin or a strongly acidic styrene-based cation exchange resin; during the adsorption and impurity removal, the flow rate of the impurity removal solution is 1 BV / h to 3 BV / h.

[0056] (6) The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution;

[0057] Trisodium phosphate and the lithium hydroxide solution were mixed under stirring conditions of 40℃~80℃ and 200r / min~400r / min, and the solid and liquid were separated. After washing and drying, lithium phosphate was obtained.

[0058] The current density of the bipolar membrane electrodialysis is 10 mA / cm². 2 ~30mA / cm 2 The flow rate of the purified lithium salt solution during the bipolar membrane electrodialysis is 5 L / h to 15 L / h, and the temperature of the bipolar membrane electrodialysis is 25℃ to 40℃.

[0059] The drying process involves vacuum drying at 80℃~100℃ and absolute pressure ≤0.02MPa for 6h~12h.

[0060] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] This invention first thoroughly disrupts the stable crystal structure of lithium iron phosphate through oxidative roasting, followed by acid leaching. Then, bipolar membrane electrodialysis is used to separate the refined lithium salt solution into a lithium hydroxide solution and a mixed acid solution. The mixed acid can be directly returned to the acid leaching process, reducing acid consumption and significantly decreasing the generation and discharge of acidic wastewater, thus significantly reducing the cost of acid leaching and wastewater treatment. Subsequently, impurities such as iron and aluminum are removed by hydrogen peroxide combined with pH adjustment using an alkaline solution. Heavy metals such as calcium and magnesium are then removed by adsorption, resulting in targeted and thorough impurity removal and effectively improving the lithium recovery rate. Finally, the lithium hydroxide solution obtained from electrodialysis is directly reacted with phosphate to prepare a high-purity lithium phosphate product. Compared with lithium carbonate prepared by traditional processes, this product can be directly used for the regeneration of lithium iron phosphate cathode materials, eliminating subsequent conversion steps and significantly increasing the added value of the product. In addition, this invention does not require the addition of large amounts of chemical precipitants, avoiding the problem of difficult-to-treat waste residue. The entire process has no emissions of toxic or harmful substances, operates under mild conditions, has strong equipment versatility, and is easy to implement for continuous and large-scale industrial production. Attached Figure Description

[0063] Figure 1 A process flow diagram is provided for the method of this invention. Detailed Implementation

[0064] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0065] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0066] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0067] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0068] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation 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 mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0069] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0070] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0071] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0072] This invention provides a method for recovering lithium from spent lithium iron phosphate batteries by recycling the acid, the method comprising the following steps:

[0073] (1) Waste lithium iron phosphate batteries are pretreated to obtain lithium iron phosphate battery black powder;

[0074] (2) The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder;

[0075] (3) The roasted black powder obtained by acid leaching is separated into solid and liquid components to obtain a lithium-containing acidic leachate;

[0076] (4) Mix hydrogen peroxide solution with the lithium-containing acidic leachate to obtain an oxidized lithium-containing acidic leachate; then mix alkaline solution with the oxidized lithium-containing acidic leachate, stir to react, and separate solid and liquid to obtain a purified solution;

[0077] The solutes in the impurity removal solution include LiH2PO4 and Li2SO4;

[0078] (5) The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0079] (6) The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution;

[0080] The mixed phosphate and the lithium hydroxide solution were subjected to solid-liquid separation, followed by washing and drying to obtain lithium phosphate.

[0081] This invention first thoroughly disrupts the stable crystal structure of lithium iron phosphate through oxidative roasting, followed by acid leaching. Then, bipolar membrane electrodialysis is used to separate the refined lithium salt solution into a lithium hydroxide solution and a mixed acid solution. The mixed acid can be directly returned to the acid leaching process, reducing acid consumption and significantly decreasing the generation and discharge of acidic wastewater, thus significantly reducing the cost of acid leaching and wastewater treatment. Subsequently, impurities such as iron and aluminum are removed by hydrogen peroxide combined with pH adjustment using an alkaline solution. Heavy metals such as calcium and magnesium are then removed by adsorption, resulting in targeted and thorough impurity removal and effectively improving the lithium recovery rate. Finally, the lithium hydroxide solution obtained from electrodialysis is directly reacted with phosphate to prepare a high-purity lithium phosphate product. Compared with lithium carbonate prepared by traditional processes, this product can be directly used for the regeneration of lithium iron phosphate cathode materials, eliminating subsequent conversion steps and significantly increasing the added value of the product. In addition, this invention does not require the addition of large amounts of chemical precipitants, avoiding the problem of difficult-to-treat waste residue. The entire process has no emissions of toxic or harmful substances, operates under mild conditions, has strong equipment versatility, and is easy to implement for continuous and large-scale industrial production.

[0082] In some embodiments, the leaching solution used in step (3) includes a mixture of sulfuric acid and phosphoric acid.

[0083] In some embodiments, the mixed acid solution in step (6) is reused for the acid leaching in step (3).

[0084] In some embodiments, the total hydrogen ion concentration in the mixture is 0.7 mol / L to 1.0 mol / L, for example, it can be 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1.0 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0085] In some embodiments, the molar ratio of sulfuric acid to phosphoric acid in the mixture is 2:1 to 4:1, for example, it can be 2:1, 2.5:1, 3:1, 3.5:1 or 4:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0086] In some embodiments, the acid leaching temperature in step (3) is 80°C to 95°C, for example, it can be 80°C, 83°C, 85°C, 88°C, 90°C, 93°C or 95°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0087] In some embodiments, the liquid-to-solid ratio of the acid leaching in step (3) is 3:1 to 5:1, for example, it can be 3:1, 3.5:1, 4:1, 4.5:1 or 5:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0088] In some embodiments, the stirring rate of the acid leaching in step (3) is 300 r / min to 500 r / min, for example, it can be 300 r / min, 320 r / min, 350 r / min, 380 r / min, 400 r / min, 420 r / min, 450 r / min, 480 r / min or 500 r / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0089] In some embodiments, the pickling time in step (3) is 3h to 5h, for example, it can be 3h, 3.5h, 4h, 4.5h or 5h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0090] In some embodiments, the pretreatment in step (1) includes: crushing and gravity sorting the waste lithium iron phosphate batteries after they are fully discharged to obtain coarse black powder; the coarse black powder is decarbonized at 500℃~600℃ for 1h~2h to obtain the lithium iron phosphate battery black powder.

[0091] The decarbonization treatment temperature is 500℃~600℃, for example, it can be 500℃, 520℃, 540℃, 550℃, 560℃, 580℃ or 600℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0092] The decarbonization treatment time is 1h to 2h, for example, it can be 1h, 1.2h, 1.5h, 1.6h, 1.8h or 2h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0093] The oxidative roasting method in this invention can completely destroy the stable olivine-type crystal structure of waste lithium iron phosphate, completely oxidizing the divalent iron to trivalent iron, while ensuring that lithium exists in the form of soluble lithium phosphate. This not only significantly improves the lithium leaching rate in the subsequent acid leaching process but also avoids the problem of continuously adding large amounts of oxidant during oxidative acid leaching. Oxidative roasting can also simultaneously remove residual organic binders, conductive agents, and other volatile impurities from lithium iron phosphate battery black powder, reducing the content of organic impurities and suspended particulate matter in the lithium-containing acidic leachate, effectively preventing these impurities from contaminating, clogging, and degrading the bipolar membrane electrodialysis stack. Furthermore, the composition of the lithium-containing acidic leachate after oxidative roasting is simpler and more controllable, reducing the interference of other complex impurity ions on the separation effect of bipolar membrane electrodialysis, improving the purity of lithium hydroxide and mixed acid, and ensuring that the mixed acid can be directly reused for acid leaching without additional purification steps.

[0094] In some embodiments, the heating rate of the oxidative calcination in step (2) is 5℃ / min to 10℃ / min, for example, it can be 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0095] In some embodiments, the oxidative calcination temperature in step (2) is 600℃~700℃, for example, it can be 600℃, 620℃, 640℃, 650℃, 660℃, 680℃ or 700℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0096] In some embodiments, the oxidative calcination time in step (2) is 1.5h to 3h, for example, it can be 1.5h, 2h, 2.5h or 3h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0097] In this invention, hydrogen peroxide can completely oxidize the small amount of ferrous ions remaining in the lithium-containing acidic leachate to ferric ions, so that during the stirring reaction, the ferric ions and aluminum ions can form a stable hydroxide precipitate and be efficiently removed. This invention performs separate oxidation to remove iron and aluminum after acid leaching, which avoids lithium co-precipitation loss caused by excessively high local pH, while ensuring thorough removal of iron and aluminum impurities.

[0098] In some embodiments, the concentration of the hydrogen peroxide solution in step (4) is 25wt% to 30wt%, for example, it can be 25wt%, 26wt%, 27wt%, 28wt%, 29wt% or 30wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0099] In some embodiments, the amount of hydrogen peroxide solution used in step (4) is 2 to 6 times the molar amount of divalent iron ions in the lithium-containing acidic leachate. For example, it can be 2 times, 2.1 times, 3 times, 4 times, 5 times, 5.2 times or 6 times, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0100] In some embodiments, the alkaline solution in step (4) is a sodium hydroxide solution with a concentration of 2 mol / L to 5 mol / L, for example, it can be 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L or 5 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0101] In some embodiments, the amount of alkaline solution used in step (4) is to adjust the pH of the system to 4 to 6, for example, 4, 4.2, 4.5, 4.6, 5, 5.2, 5.5 or 6, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0102] In some embodiments, the stirring reaction time in step (4) is 0.5h to 2h, for example, it can be 0.5h, 0.8h, 1h, 1.2h, 1.5h, 1.6h, 1.8h or 2h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0103] In some embodiments, the adsorption and impurity removal in step (5) is carried out using aminophosphonic acid chelating resin or strong acid styrene-based cation exchange resin to remove heavy metal impurities such as calcium and magnesium.

[0104] In some embodiments, during the adsorption and purification process in step (5), the flow rate of the purification solution is 1 BV / h to 3 BV / h, for example, it can be 1 BV / h, 1.5 BV / h, 2 BV / h, 2.5 BV / h or 3 BV / h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0105] In some embodiments, the current density of the bipolar membrane electrodialysis in step (6) is 10 mA / cm². 2 ~30mA / cm 2 For example, it could be 10mA / cm 2 15mA / cm 2 20mA / cm 2 25mA / cm 2 Or 30mA / cm 2 However, this does not limit the listed values; any other unlisted values ​​within the range are also applicable.

[0106] In some embodiments, the flow rate of the purified lithium salt solution during bipolar membrane electrodialysis in step (6) is 5 L / h to 15 L / h, for example, it can be 5 L / h, 6 L / h, 8 L / h, 10 L / h, 12 L / h or 15 L / h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0107] In some embodiments, the temperature of bipolar membrane electrodialysis in step (6) is 25°C to 40°C, for example, it can be 25°C, 26°C, 28°C, 30°C, 32°C, 34°C or 35°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0108] In this invention, the alkaline chamber of the bipolar membrane electrodialysis yields a lithium hydroxide solution, and the acid chamber yields a mixed acid solution.

[0109] In some embodiments, the phosphate in step (6) includes trisodium phosphate.

[0110] In some embodiments, the mixing temperature in step (6) is 40°C to 80°C, for example, it can be 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0111] In some embodiments, the mixing in step (6) is carried out under stirring conditions of 200 r / min to 400 r / min, for example, 200 r / min, 250 r / min, 300 r / min, 350 r / min or 400 r / min, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0112] As a preferred embodiment of the method provided by the present invention, the method includes the following steps:

[0113] (1) After the waste lithium iron phosphate battery is fully discharged, it is crushed and gravity separated to obtain coarse black powder; the coarse black powder is subjected to decarbonization treatment at 500℃~600℃ for 1h~2h in a nitrogen atmosphere to obtain the lithium iron phosphate battery black powder.

[0114] (2) The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder;

[0115] The oxidation calcination is carried out in an air atmosphere, with a heating rate of 5℃ / min~10℃ / min, a temperature of 600℃~700℃, and a time of 1.5h~3h.

[0116] (3) The roasted black powder obtained by acid leaching is separated into solid and liquid components to obtain a lithium-containing acidic leachate;

[0117] The acid leaching solution comprises a mixture of sulfuric acid and phosphoric acid, wherein the total hydrogen ion concentration in the mixture is 0.7 mol / L to 1.0 mol / L, and the molar ratio of sulfuric acid to phosphoric acid is 2:1 to 4:1.

[0118] The acid leaching temperature is 80℃~95℃, the stirring rate is 300r / min~500r / min, and the time is 3h~5h;

[0119] The liquid-to-solid ratio of the acid leaching is 3:1 to 5:1, and the unit of the liquid-to-solid ratio is mL / g;

[0120] (4) Mix hydrogen peroxide solution with the lithium-containing acidic leachate to obtain oxidized lithium-containing acidic leachate; then mix alkaline solution with the oxidized lithium-containing acidic leachate, stir and react for 0.5h~2h, separate solid and liquid to obtain impurity-removed solution;

[0121] The concentration of the hydrogen peroxide solution is 25wt%~30wt%; the amount of hydrogen peroxide solution used is such that the amount of hydrogen peroxide is 2 to 6 times the molar amount of divalent iron ions in the lithium-containing acidic leachate.

[0122] The alkaline solution is a sodium hydroxide solution with a concentration of 2 mol / L to 5 mol / L, and the amount of alkaline solution used is to adjust the pH of the system to 4 to 6.

[0123] The solutes in the impurity removal solution include LiH2PO4 and Li2SO4;

[0124] (5) The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0125] The adsorption and impurity removal is carried out using an aminophosphonic acid chelating resin or a strongly acidic styrene-based cation exchange resin; during the adsorption and impurity removal, the flow rate of the impurity removal solution is 1 BV / h to 3 BV / h.

[0126] (6) The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution;

[0127] Trisodium phosphate and the lithium hydroxide solution were mixed under stirring conditions of 40℃~80℃ and 200r / min~400r / min, and the solid and liquid were separated. After washing and drying, lithium phosphate was obtained.

[0128] The current density of the bipolar membrane electrodialysis is 10 mA / cm². 2 ~30mA / cm 2 The flow rate of the purified lithium salt solution during the bipolar membrane electrodialysis is 5 L / h to 15 L / h, and the temperature of the bipolar membrane electrodialysis is 25℃ to 40℃.

[0129] The drying process involves vacuum drying at 80℃~100℃ and absolute pressure ≤0.02MPa for 6h~12h.

[0130] In this invention, waste lithium iron phosphate batteries are fully discharged and then crushed and gravity-separated to obtain coarse black powder. The coarse black powder is then subjected to decarbonization treatment at 500℃~600℃ for 1h~2h under a nitrogen atmosphere to obtain the lithium iron phosphate battery black powder. The main components of the lithium iron phosphate battery black powder, by mass percentage, are: 92wt%~96wt% LiFePO4, 1wt%~3wt% residual carbon, 0.1wt%~0.5wt% Al, 0.2wt%~0.8wt% Fe, 0.05wt%~0.2wt% Ca, 0.03wt%~0.15wt% Mg, and the balance being impurities.

[0131] To clearly illustrate the technical solution of this invention, the source of the lithium iron phosphate battery black powder in the following embodiments and comparative examples is as follows: Waste lithium iron phosphate batteries were completely discharged, then crushed and gravity-separated to obtain coarse black powder; the coarse black powder was subjected to decarbonization treatment at 550°C for 1.5 hours under a nitrogen atmosphere to obtain lithium iron phosphate battery black powder. X-ray fluorescence spectroscopy (XRF) analysis revealed that the main components of the lithium iron phosphate battery black powder were: 94.2 wt% LiFePO4, 2.1 wt% residual carbon, 0.32 wt% Al, 0.56 wt% Fe, 0.11 wt% Ca, 0.07 wt% Mg, and the balance being impurities.

[0132] Example 1

[0133] This embodiment provides a method for recycling lithium from spent lithium iron phosphate batteries to prepare lithium phosphate, and the process flow diagram is as follows: Figure 1 As shown, it includes:

[0134] S1. The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder.

[0135] The oxidation calcination was carried out in an air atmosphere, with a heating rate of 8°C / min, a temperature of 650°C, and a time of 2.5 h.

[0136] S2. The roasted black powder obtained by acid leaching is subjected to solid-liquid separation to obtain a lithium-containing acidic leachate;

[0137] The acid leaching solution comprises a mixture of sulfuric acid and phosphoric acid, wherein the total hydrogen ion concentration in the mixture is 0.9 mol / L and the molar ratio of sulfuric acid to phosphoric acid is 3:1.

[0138] The acid leaching temperature was 85℃, the stirring rate was 400r / min, and the time was 4h;

[0139] The liquid-to-solid ratio of the acid leaching is 4:1, and the unit of the liquid-to-solid ratio is mL / g;

[0140] S3. Mix hydrogen peroxide solution with the lithium-containing acidic leachate to obtain an oxidized lithium-containing acidic leachate; then mix alkaline solution with the oxidized lithium-containing acidic leachate, stir and react for 1 hour, separate solid and liquid to obtain a purified solution;

[0141] The concentration of the hydrogen peroxide solution is 28 wt%; the amount of hydrogen peroxide solution used is such that the amount of hydrogen peroxide is 4 times the molar amount of divalent iron ions in the lithium-containing acidic leachate.

[0142] The alkaline solution is a 3 mol / L sodium hydroxide solution, and the amount of alkaline solution used is to adjust the pH of the system to 5.

[0143] The solutes in the impurity removal solution include LiH2PO4 and Li2SO4;

[0144] S4. The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0145] The adsorption and impurity removal is carried out using an aminophosphonic acid chelating resin (D402-II); during the adsorption and impurity removal, the flow rate of the impurity removal solution is 2 BV / h.

[0146] S5. The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution.

[0147] Trisodium phosphate (the amount added was determined according to the chemical formula of lithium phosphate) was mixed with the lithium hydroxide solution at a temperature of 60℃ and a stirring speed of 300 r / min. After solid-liquid separation, lithium phosphate was obtained by washing and drying.

[0148] The current density of the bipolar membrane electrodialysis is 20 mA / cm². 2 The flow rate of the purified lithium salt solution during the bipolar membrane electrodialysis is 10 L / h, and the temperature of the bipolar membrane electrodialysis is 30 °C.

[0149] The drying process involved vacuum drying at 90°C and an absolute pressure of 0.02 MPa for 9 hours.

[0150] Example 2

[0151] This embodiment provides a method for recycling lithium from spent lithium iron phosphate batteries to prepare lithium phosphate, including:

[0152] S1. The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder.

[0153] The oxidation calcination was carried out in an air atmosphere, with a heating rate of 5℃ / min, a temperature of 600℃, and a time of 3h.

[0154] S2. The roasted black powder obtained by acid leaching is subjected to solid-liquid separation to obtain a lithium-containing acidic leachate;

[0155] The acid leaching solution comprises a mixture of sulfuric acid and phosphoric acid, wherein the total hydrogen ion concentration in the mixture is 1.0 mol / L and the molar ratio of sulfuric acid to phosphoric acid is 2:1.

[0156] The acid leaching temperature was 95℃, the stirring rate was 300r / min, and the time was 3h;

[0157] The liquid-to-solid ratio of the acid leaching is 5:1, and the unit of the liquid-to-solid ratio is mL / g;

[0158] S3. Mix hydrogen peroxide solution with the lithium-containing acidic leachate to obtain an oxidized lithium-containing acidic leachate; then mix alkaline solution with the oxidized lithium-containing acidic leachate, stir and react for 0.5 h, separate solid and liquid to obtain a purified solution;

[0159] The concentration of the hydrogen peroxide solution is 25 wt%; the amount of hydrogen peroxide solution used is such that the amount of hydrogen peroxide is twice the molar amount of divalent iron ions in the lithium-containing acidic leachate.

[0160] The alkaline solution is a 2 mol / L sodium hydroxide solution, and the amount of alkaline solution used is to adjust the pH of the system to 4.

[0161] The solutes in the impurity removal solution include LiH2PO4 and Li2SO4;

[0162] S4. The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0163] The adsorption and impurity removal is carried out using a strong acid styrene-based cation exchange resin (001*7); during the adsorption and impurity removal, the flow rate of the impurity removal solution is 1 BV / h;

[0164] S5. The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution.

[0165] Trisodium phosphate (the amount added was determined according to the chemical formula of lithium phosphate) was mixed with the lithium hydroxide solution at a temperature of 40℃ and a stirring speed of 200r / min. After solid-liquid separation, lithium phosphate was obtained by washing and drying.

[0166] The current density of the bipolar membrane electrodialysis is 10 mA / cm². 2 The flow rate of the purified lithium salt solution during the bipolar membrane electrodialysis is 5 L / h, and the temperature of the bipolar membrane electrodialysis is 25 °C.

[0167] The drying process involved vacuum drying at 80°C and an absolute pressure of 0.02 MPa for 12 hours.

[0168] Example 3

[0169] This embodiment provides a method for recycling lithium from spent lithium iron phosphate batteries to prepare lithium phosphate, including:

[0170] S1. The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder.

[0171] The oxidation calcination was carried out in an air atmosphere, with a heating rate of 10℃ / min, a temperature of 700℃, and a time of 1.5h.

[0172] S2. The roasted black powder obtained by acid leaching is subjected to solid-liquid separation to obtain a lithium-containing acidic leachate;

[0173] The acid leaching solution comprises a mixture of sulfuric acid and phosphoric acid, wherein the total hydrogen ion concentration in the mixture is 0.7 mol / L and the molar ratio of sulfuric acid to phosphoric acid is 4:1.

[0174] The acid leaching temperature is 80℃, the stirring rate is 500r / min, and the time is 5h;

[0175] The liquid-to-solid ratio of the acid leaching is 3:1, and the unit of the liquid-to-solid ratio is mL / g;

[0176] S3. Mix hydrogen peroxide solution with the lithium-containing acidic leachate to obtain an oxidized lithium-containing acidic leachate; then mix alkaline solution with the oxidized lithium-containing acidic leachate, stir and react for 2 hours, separate solid and liquid to obtain a purified solution;

[0177] The concentration of the hydrogen peroxide solution is 30 wt%; the amount of hydrogen peroxide solution used is such that the amount of hydrogen peroxide is 6 times the molar amount of divalent iron ions in the lithium-containing acidic leachate.

[0178] The alkaline solution is a 5 mol / L sodium hydroxide solution, and the amount of alkaline solution used is to adjust the pH of the system to 6.

[0179] The solutes in the impurity removal solution include LiH2PO4 and Li2SO4;

[0180] S4. The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0181] The adsorption and impurity removal is carried out using an aminophosphonic acid chelating resin (D402-II); during the adsorption and impurity removal, the flow rate of the impurity removal solution is 3 BV / h.

[0182] S5. The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution.

[0183] Trisodium phosphate (the amount added was determined according to the chemical formula of lithium phosphate) was mixed with the lithium hydroxide solution at a temperature of 80℃ and a stirring speed of 400 r / min. After solid-liquid separation, lithium phosphate was obtained by washing and drying.

[0184] The current density of the bipolar membrane electrodialysis is 30 mA / cm². 2 The flow rate of the purified lithium salt solution during the bipolar membrane electrodialysis is 15 L / h, and the temperature of the bipolar membrane electrodialysis is 40 °C.

[0185] The drying process involved vacuum drying at 100°C and an absolute pressure of 0.02 MPa for 6 hours.

[0186] Example 4

[0187] This embodiment provides a method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate. Except for the oxidation roasting temperature of 450°C, the method is the same as that in Example 1.

[0188] Example 5

[0189] This embodiment provides a method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate. Except for the oxidation roasting temperature of 750°C, the method is the same as that in Example 1.

[0190] Example 6

[0191] This embodiment provides a method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate. Except for the amount of hydrogen peroxide solution used, which is twice the molar amount of divalent iron ions in the lithium-containing acidic leachate, the rest is the same as in Example 1.

[0192] Example 7

[0193] This embodiment provides a method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate. Except for the amount of hydrogen peroxide solution used, which is 6 times the molar amount of divalent iron ions in the lithium-containing acidic leachate, the rest is the same as in Example 1.

[0194] Example 8

[0195] This embodiment provides a method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate. Except for the fact that the molar ratio of sulfuric acid to phosphoric acid in the acid leaching solution is 5:1, everything else is the same as in Example 1.

[0196] Example 9

[0197] This embodiment provides a method for recycling lithium from spent lithium iron phosphate batteries to prepare lithium phosphate, except that the current density of bipolar membrane electrodialysis is 5 mA / cm². 2 Except for the above, everything else is the same as in Example 1.

[0198] Example 10

[0199] This embodiment provides a method for recycling lithium from spent lithium iron phosphate batteries to prepare lithium phosphate, except that the current density of bipolar membrane electrodialysis is 40 mA / cm². 2 Except for the above, everything else is the same as in Example 1.

[0200] Comparative Example 1

[0201] This comparative example provides a method for recycling lithium from spent lithium iron phosphate batteries to prepare lithium phosphate. Except for the absence of hydrogen peroxide solution, the method is identical to Example 1, including:

[0202] S1. The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder.

[0203] The oxidation calcination was carried out in an air atmosphere, with a heating rate of 8°C / min, a temperature of 650°C, and a time of 2.5 h.

[0204] S2. The roasted black powder obtained by acid leaching is subjected to solid-liquid separation to obtain a lithium-containing acidic leachate;

[0205] The acid leaching solution comprises a mixture of sulfuric acid and phosphoric acid, wherein the total hydrogen ion concentration in the mixture is 0.9 mol / L and the molar ratio of sulfuric acid to phosphoric acid is 3:1.

[0206] The acid leaching temperature was 85℃, the stirring rate was 400r / min, and the time was 4h;

[0207] The liquid-to-solid ratio of the acid leaching is 4:1, and the unit of the liquid-to-solid ratio is mL / g;

[0208] S3. Mix the alkaline solution with the lithium-containing acidic leachate, stir and react for 1 hour, then separate the solid and liquid to obtain the purified solution;

[0209] The alkaline solution is a 3 mol / L sodium hydroxide solution, and the amount of alkaline solution used is to adjust the pH of the system to 5.

[0210] S4. The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution;

[0211] The adsorption and impurity removal is carried out using an aminophosphonic acid chelating resin (D402-II); during the adsorption and impurity removal, the flow rate of the impurity removal solution is 2 BV / h.

[0212] S5. The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution.

[0213] Trisodium phosphate (the amount added was determined according to the chemical formula of lithium phosphate) was mixed with the lithium hydroxide solution at a temperature of 60℃ and a stirring speed of 300 r / min. After solid-liquid separation, lithium phosphate was obtained by washing and drying.

[0214] The current density of the bipolar membrane electrodialysis is 20 mA / cm². 2 The flow rate of the purified lithium salt solution during the bipolar membrane electrodialysis is 10 L / h, and the temperature of the bipolar membrane electrodialysis is 30 °C.

[0215] The drying process involved vacuum drying at 90°C and an absolute pressure of 0.02 MPa for 9 hours.

[0216] Performance Characterization

[0217] The total recovery rate of lithium and the purity of lithium phosphate in the methods provided in the above embodiments and comparative examples were measured, and the results are shown in Table 1.

[0218] The total lithium recovery rate is the percentage of lithium mass in lithium phosphate relative to the lithium mass in lithium iron phosphate battery black powder; the purity of lithium phosphate is determined according to the standard GB / T 30835-2014 "Battery Grade Lithium Phosphate".

[0219] Table 1

[0220]

[0221] As can be seen from Examples 1 to 3 in Table 1, the method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate provided by the present invention can achieve efficient lithium recovery and preparation of high-purity lithium phosphate.

[0222] A comparison of Examples 4 and 5 with Example 1 shows that when the oxidative roasting temperature is too low, the stable olivine-type crystal structure of lithium iron phosphate cannot be fully destroyed, and the oxidation of ferrous iron is incomplete, resulting in a decrease in the leaching rate of lithium during the subsequent acid leaching process. When the oxidative roasting temperature is too high, lithium iron oxide phase that is difficult to dissolve in acid will be generated, causing irreversible loss of lithium. At the same time, high temperature will cause the material to sinter and agglomerate, further reducing the leaching efficiency.

[0223] A comparison of Examples 6 and 7 with Example 1 shows that when the amount of hydrogen peroxide is too small, the residual ferrous iron in the leachate cannot be completely oxidized to ferric iron, resulting in the inability of iron ions to form a stable ferric hydroxide precipitate for removal during subsequent pH adjustment. This leads to an increase in the iron impurity content and a decrease in purity in the product. When the amount of hydrogen peroxide is too large, the excess hydrogen peroxide will decompose and generate a large number of microbubbles, making the hydroxide precipitate particles too small, which makes filtration and separation difficult. At the same time, it will increase the co-precipitation loss of lithium, resulting in a slight decrease in the total lithium recovery rate.

[0224] A comparison between Example 8 and Example 1 shows that when the overall hydrogen ion molar concentration is constant, excessive sulfuric acid content will lead to an increase in the content of impurity sulfate ions in the lithium phosphate product, which will slightly reduce the purity of lithium phosphate.

[0225] A comparison of Examples 9 and 10 with Example 1 shows that when the current density is too low, the bipolar membrane water dissociation rate is slow, the salt solution is not fully decomposed, and the lithium recovery is incomplete, resulting in a decrease in the total lithium recovery rate. When the current density is too high, membrane polarization will occur, which will not only increase energy consumption but also reduce the purity of lithium phosphate products.

[0226] As can be seen from the comparison between Comparative Example 1 and Example 1, when hydrogen peroxide solution is not used, the ferrous iron in the leachate cannot be oxidized to ferric iron, and a stable hydroxide precipitate cannot be formed, resulting in residual iron impurities and a decrease in product purity.

[0227] In summary, this invention first thoroughly disrupts the stable crystal structure of lithium iron phosphate through oxidative roasting, followed by acid leaching. Then, bipolar membrane electrodialysis is used to separate the refined lithium salt solution into a lithium hydroxide solution and a mixed acid solution. The mixed acid can be directly returned to the acid leaching process, reducing acid consumption and significantly decreasing the generation and discharge of acidic wastewater, thus significantly reducing the cost of acid leaching and wastewater treatment. Subsequently, impurities such as iron and aluminum are removed by hydrogen peroxide combined with pH adjustment using an alkaline solution. Heavy metals such as calcium and magnesium are then removed through adsorption, resulting in targeted and thorough impurity removal and effectively improving the lithium recovery rate. Finally, the lithium hydroxide solution obtained from electrodialysis is directly reacted with phosphate to prepare a high-purity lithium phosphate product. Compared to lithium carbonate prepared by traditional processes, this product can be directly used for the regeneration of lithium iron phosphate cathode materials, eliminating subsequent conversion steps and significantly increasing the added value of the product. In addition, this invention does not require the addition of large amounts of chemical precipitants, avoiding the problem of difficult-to-treat waste residue. The entire process has no emissions of toxic or harmful substances, operates under mild conditions, has strong equipment versatility, and is easy to implement for continuous and large-scale industrial production.

[0228] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for recycling lithium from waste lithium iron phosphate batteries to prepare lithium phosphate, characterized in that, The method includes the following steps: (1) Waste lithium iron phosphate batteries are pretreated to obtain lithium iron phosphate battery black powder; (2) The lithium iron phosphate battery black powder is oxidized and roasted to fully oxidize and decompose the lithium iron phosphate, and then naturally cooled to obtain roasted black powder; (3) The roasted black powder obtained by acid leaching is separated into solid and liquid components to obtain a lithium-containing acidic leachate; (4) Mix the hydrogen peroxide solution with the lithium-containing acidic leachate to obtain an oxidized lithium-containing acidic leachate; Then, the alkaline solution is mixed with the oxidized lithium-containing acidic leachate, stirred and reacted, and the solid and liquid are separated to obtain a purified solution; The solutes in the impurity removal solution include LiH2PO4 and Li2SO4; (5) The impurity removal solution is subjected to adsorption to remove impurities, resulting in a refined lithium salt solution; (6) The refined lithium salt solution is subjected to bipolar membrane electrodialysis to obtain a lithium hydroxide solution and a mixed acid solution; The mixed phosphate and the lithium hydroxide solution were subjected to solid-liquid separation, followed by washing and drying to obtain lithium phosphate.

2. The method according to claim 1, characterized in that, The acid leaching solution used in step (3) includes a mixture of sulfuric acid and phosphoric acid; And / or, the mixed acid solution described in step (6) is reused for the acid leaching described in step (3).

3. The method according to claim 2, characterized in that, The total hydrogen ion concentration in the mixture is 0.7 mol / L to 1.0 mol / L; And / or, in the mixture, the molar ratio of sulfuric acid to phosphoric acid is 2:1 to 4:

1.

4. The method according to claim 2 or 3, characterized in that, The acid leaching temperature in step (3) is 80℃~95℃; And / or, the liquid-to-solid ratio of the acid leaching in step (3) is 3:1 to 5:1, and the unit of the liquid-to-solid ratio is mL / g; And / or, the stirring rate of the acid leaching in step (3) is 300 r / min to 500 r / min; And / or, the acid leaching time in step (3) is 3h~5h.

5. The method according to any one of claims 1 to 4, characterized in that, The pretreatment in step (1) includes: after the waste lithium iron phosphate battery is fully discharged, it is crushed and gravity separated to obtain coarse black powder; the coarse black powder is decarbonized at 500℃~600℃ for 1h~2h to obtain the lithium iron phosphate battery black powder.

6. The method according to any one of claims 1 to 5, characterized in that, The heating rate of the oxidative calcination in step (2) is 5℃ / min~10℃ / min; And / or, the oxidation calcination temperature in step (2) is 600℃~700℃; And / or, the oxidative calcination time in step (2) is 1.5h to 3h.

7. The method according to any one of claims 1 to 6, characterized in that, The concentration of the hydrogen peroxide solution in step (4) is 25wt%~30wt%; And / or, the amount of hydrogen peroxide solution used in step (4) is such that the amount of hydrogen peroxide is 2 to 6 times the molar amount of divalent iron ions in the lithium-containing acidic leachate; And / or, the alkaline solution in step (4) is a sodium hydroxide solution with a concentration of 2 mol / L to 5 mol / L; And / or, the amount of alkaline solution used in step (4) is to adjust the pH of the system to 4~6; And / or, the stirring reaction time in step (4) is 0.5h to 2h.

8. The method according to any one of claims 1 to 7, characterized in that, The adsorption and impurity removal in step (5) is carried out using aminophosphonic acid chelating resin or strong acid styrene-based cation exchange resin. And / or, during the adsorption and purification process described in step (5), the flow rate of the purification solution is 1 BV / h to 3 BV / h.

9. The method according to any one of claims 1 to 8, characterized in that, The current density of the bipolar membrane electrodialysis in step (6) is 10 mA / cm². 2 ~30mA / cm 2 ; And / or, the flow rate of the purified lithium salt solution during bipolar membrane electrodialysis in step (6) is 5 L / h to 15 L / h; And / or, the temperature of the bipolar membrane electrodialysis in step (6) is 25℃~40℃.

10. The method according to any one of claims 1 to 9, characterized in that, The phosphate in step (6) includes trisodium phosphate; And / or, the mixing temperature in step (6) is 40°C to 80°C; And / or, the mixing in step (6) is carried out under stirring conditions at a speed of 200 r / min to 400 r / min.

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