Method for recycling spent lithium battery positive electrode material and method for deeply removing impurities from recycling liquid

Abstract

This application relates to the field of waste lithium battery recycling technology, and particularly to a method for recycling waste lithium battery cathode materials and a method for deep impurity removal from the recycling solution. The method for recycling waste lithium battery cathode materials includes: waste lithium iron phosphate cathode material black powder is acid-dissolved and oxidized, and then separated by pressure filtration to obtain lithium sulfate solution and iron phosphate slag; the iron phosphate slag is processed to obtain iron phosphate; the lithium sulfate solution is purified, evaporated and concentrated, and then sodium carbonate is added to obtain crude lithium carbonate and lithium precipitation mother liquor; the lithium precipitation mother liquor is pre-evaporated and concentrated and then sent to a hot crystallizer to precipitate anhydrous sodium sulfate crystals at 50-60℃, and separated to obtain primary mother liquor; the primary mother liquor is passed through a cold crystallizer at 10-15℃ to precipitate sodium sulfate decahydrate crystals, with particles <50μm returned to the hot crystallizer, and particles ≥50μm discharged and separated to obtain sodium sulfate decahydrate product and lithium-rich mother liquor; the lithium-rich mother liquor is returned to the lithium precipitation process or sent to the lithium recovery process, and the crude lithium carbonate is processed to obtain lithium carbonate. This application can solve the problems of large entrainment of fine lithium crystals and high lithium loss rate in the existing lithium precipitation mother liquor treatment.

Description

Technical Field

[0001] This application belongs to the field of waste lithium battery recycling technology, and in particular relates to a method for recycling positive electrode material of waste lithium batteries and a method for deep impurity removal of the recycling liquid. Background Technology

[0002] The cathode material of spent lithium-ion batteries is the main carrier of valuable metals such as lithium, nickel, cobalt, and manganese in spent lithium-ion batteries. Recycling it can not only alleviate the shortage of mineral resources but also reduce environmental pollution caused by spent lithium-ion batteries, thus having significant economic and environmental benefits. Among these, spent lithium iron phosphate cathode material has become the category with the largest processing volume and the most urgent recycling demand in the lithium-ion battery recycling field due to the rapid increase in the amount of retired power batteries.

[0003] In existing technologies, the recycling of waste lithium iron phosphate cathode materials generally adopts a hydrometallurgical process. Typically, the waste lithium iron phosphate cathode material black powder is first acid-dissolved and oxidized to leach out lithium elements into the solution, while iron and phosphorus elements precipitate out in the form of phosphate iron slag. Then, through solid-liquid separation, lithium sulfate solution and phosphate iron slag are obtained. The phosphate iron slag is further processed to recover iron phosphate products. The lithium sulfate solution is purified to obtain a purified lithium sulfate solution. The purified lithium sulfate solution is evaporated and concentrated, and then sodium carbonate is added to carry out a lithium precipitation reaction to obtain crude lithium carbonate and lithium precipitation mother liquor. The crude lithium carbonate is refined to obtain battery-grade lithium carbonate products.

[0004] However, existing technologies for recycling waste lithium iron phosphate cathode materials have significant problems in the treatment of lithium precipitation mother liquor. The mother liquor contains high concentrations of sodium sulfate and incompletely recovered lithium sulfate. Currently, the mainstream industry method for treating this type of mother liquor is the MVR (Medium-Vacuum Reduction) evaporation crystallization process. This involves high-temperature evaporation and concentration to saturate and precipitate sodium sulfate, then returning the mother liquor to the secondary lithium precipitation process. Operational data from a 10,000-ton-scale lithium iron phosphate recycling project shows that this process consumes 45-60 kWh of electricity per ton of water treated, and the investment in the MVR evaporator accounts for more than 30% of the entire production line's investment. Frequent scaling of sodium sulfate crystals on the heat exchange surface necessitates system shutdowns for cleaning every 15-30 days. Furthermore, the crystallization process inevitably generates a large number of fine crystals with a large specific surface area, which physically trap a large number of lithium ions on their surface and inside, resulting in low lithium recovery rates and high operating costs.

[0005] The aforementioned problems are particularly prominent in the lithium iron phosphate cathode material recycling system, and have become a key bottleneck restricting the large-scale and green development of the waste lithium iron phosphate cathode material recycling industry. Summary of the Invention

[0006] This application provides a method for recycling waste lithium battery cathode materials and a method for deep impurity removal from the recycled liquid. This method can improve the problem in the existing lithium precipitation mother liquor treatment process for recycling waste lithium battery cathode materials, where the amount of lithium carried by fine crystals generated during the cold crystallization process is large, making it difficult to effectively reduce the lithium loss rate.

[0007] In a first aspect, embodiments of this application provide a method for recycling waste lithium battery cathode materials, including: S1: Mix waste lithium iron phosphate cathode material black powder with sulfuric acid and hydrogen peroxide to carry out acid dissolution and oxidation reaction to obtain acid dissolution slurry; S2: The acid-soluble slurry is separated by pressure filtration to obtain lithium sulfate solution and ferrophosphate slag; S3: The ferric phosphate slag is processed to obtain ferric phosphate product; S4: The lithium sulfate solution is subjected to impurity removal treatment to obtain a purified lithium sulfate solution; S5: After evaporating and concentrating the purified lithium sulfate solution, add sodium carbonate solution to react and obtain crude lithium carbonate and lithium precipitation mother liquor. S6: The lithium precipitation mother liquor is pre-evaporated and concentrated until the concentration of sodium sulfate at 50°C reaches 90%-98% of the saturation concentration of sodium sulfate, to obtain concentrated mother liquor. S7: The concentrated mother liquor is fed into a hot crystallizer and kept at 50-60°C with stirring to precipitate anhydrous sodium sulfate crystals. Anhydrous sodium sulfate product and primary mother liquor are obtained by solid-liquid separation. S8: The primary mother liquor is fed into a cold crystallizer and cooled to 10-15°C. The mixture is then kept warm and stirred to precipitate sodium sulfate decahydrate crystals. Before solid-liquid separation, the sodium sulfate decahydrate crystals with a particle size <50μm are returned to the hot crystallizer in S7, while the sodium sulfate decahydrate crystals with a particle size ≥50μm are discharged for solid-liquid separation to obtain sodium sulfate decahydrate product and lithium-rich mother liquor. S9: Return the lithium-rich mother liquor to S5, which produces the lithium precipitation mother liquor, or send it to the lithium recovery process to recover lithium carbonate products. S10: Process the crude lithium carbonate to obtain the lithium carbonate product.

[0008] The technical solutions described in this application embodiment have at least the following technical effects: The waste lithium battery cathode material recycling method provided in this application embodiment can effectively destroy the crystal structure of lithium iron phosphate in S1, allowing lithium to fully leach into the liquid phase, while simultaneously achieving preliminary separation of phosphorus and iron elements, with a lithium leaching rate of over 98%; S2 can achieve efficient separation of solid and liquid phases, laying the foundation for subsequent lithium recovery and resource utilization of phosphorus and iron; S3 can achieve high-value recycling of phosphorus and iron resources, improving the overall economic efficiency of the recycling process; S4 can effectively remove impurity ions such as copper, iron, aluminum, calcium, and magnesium from the solution, ensuring the purity of subsequent lithium carbonate products; S5 It can achieve preliminary precipitation and recovery of lithium to obtain crude lithium carbonate; S6 can create suitable concentration conditions for subsequent crystallization separation and improve crystallization separation efficiency; S7 can utilize the reverse solubility characteristics of sodium sulfate to precipitate anhydrous sodium sulfate at high temperature and achieve preliminary separation of sodium and lithium; S8 can specifically solve the problem of large amount of lithium entrained in cold crystallization fine crystals and significantly reduce lithium loss during crystallization; S9 can realize the recycling of unprecipitated lithium and further improve the overall lithium recovery rate; S10 can remove surface impurities in crude lithium carbonate to obtain lithium carbonate products that meet quality requirements.

[0009] This method, by classifying and refluxing the fine crystals generated during cold crystallization, eliminates the need for energy-intensive MVR evaporation and deep freezing processes. It not only reduces the energy consumption per ton of water for lithium precipitation mother liquor treatment to below 30 kWh and reduces equipment investment by more than 30%, but also significantly reduces the lithium loss rate during crystallization and increases the overall lithium recovery rate to over 95%, greatly improving the economic efficiency and resource utilization of waste lithium iron phosphate cathode material recycling processes.

[0010] Secondly, embodiments of this application provide a method for deep impurity removal from recycled waste lithium battery liquid, including: The lithium precipitation mother liquor generated from the recycling of waste lithium iron phosphate cathode materials is pre-evaporated and concentrated until the concentration of sodium sulfate at 50°C reaches 90%-98% of the saturation concentration of sodium sulfate, thus obtaining concentrated mother liquor. The concentrated mother liquor is fed into a hot crystallizer and kept at 50-60°C with stirring to precipitate anhydrous sodium sulfate crystals. Anhydrous sodium sulfate product and primary mother liquor are obtained by solid-liquid separation. The primary mother liquor is fed into a cold crystallizer and cooled to 10-15°C. The mixture is then stirred to precipitate sodium sulfate decahydrate crystals. Before solid-liquid separation, the sodium sulfate decahydrate crystals with a particle size <50μm are returned to the hot crystallizer, while the sodium sulfate decahydrate crystals with a particle size ≥50μm are discharged for solid-liquid separation to obtain sodium sulfate decahydrate product and lithium-rich mother liquor. The lithium-rich mother liquor can be returned to the lithium precipitation process that produced the lithium precipitation mother liquor, or sent to the lithium recovery process to recover lithium carbonate products.

[0011] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of this application, 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 this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 This is a schematic flowchart of a method for recycling waste lithium battery cathode materials according to an embodiment of this application; Figure 2 This is a schematic flowchart of a method for deep impurity removal from waste lithium battery recycling fluid provided in an embodiment of this application; Figure 3 This is a schematic diagram comparing the lithium loss rate during the crystallization process under different lithium precipitation mother liquor treatment methods in a waste lithium battery cathode material recycling method provided in one embodiment of this application; Figure 4 This is a schematic diagram showing the water and electricity consumption per ton under different lithium precipitation mother liquor treatment methods in a waste lithium battery cathode material recycling method provided in one embodiment of this application. Detailed Implementation

[0014] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0015] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0016] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0017] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0018] The cathode material of spent lithium-ion batteries is the main carrier of valuable metals such as lithium, nickel, cobalt, and manganese in spent lithium-ion batteries. Recycling it can not only alleviate the shortage of mineral resources but also reduce environmental pollution caused by spent lithium-ion batteries, thus having significant economic and environmental benefits. Among these, spent lithium iron phosphate cathode material has become the category with the largest processing volume and the most urgent recycling demand in the lithium-ion battery recycling field due to the rapid increase in the amount of retired power batteries.

[0019] In existing technologies, the recycling of waste lithium iron phosphate cathode materials generally adopts a hydrometallurgical process. Typically, the waste lithium iron phosphate cathode material black powder is first acid-dissolved and oxidized to leach out lithium elements into the solution, while iron and phosphorus elements precipitate out in the form of phosphate iron slag. Then, through solid-liquid separation, lithium sulfate solution and phosphate iron slag are obtained. The phosphate iron slag is further processed to recover iron phosphate products. The lithium sulfate solution is purified to obtain a purified lithium sulfate solution. The purified lithium sulfate solution is evaporated and concentrated, and then sodium carbonate is added to carry out a lithium precipitation reaction to obtain crude lithium carbonate and lithium precipitation mother liquor. The crude lithium carbonate is refined to obtain battery-grade lithium carbonate products.

[0020] However, existing technologies for recycling waste lithium iron phosphate cathode materials have significant problems in the treatment of lithium precipitation mother liquor. The mother liquor contains high concentrations of sodium sulfate and incompletely recovered lithium sulfate. Currently, the mainstream industry method for treating this type of mother liquor is the MVR (Medium-Vacuum Reduction) evaporation crystallization process. This involves high-temperature evaporation and concentration to saturate and precipitate sodium sulfate, then returning the mother liquor to the secondary lithium precipitation process. Operational data from a 10,000-ton-scale lithium iron phosphate recycling project shows that this process consumes 45-60 kWh of electricity per ton of water treated, and the investment in the MVR evaporator accounts for more than 30% of the entire production line's investment. Frequent scaling of sodium sulfate crystals on the heat exchange surface necessitates system shutdowns for cleaning every 15-30 days. Furthermore, the crystallization process inevitably generates a large number of fine crystals with a large specific surface area, which physically trap a large number of lithium ions on their surface and inside, resulting in low lithium recovery rates and high operating costs.

[0021] The aforementioned problems are particularly prominent in the lithium iron phosphate cathode material recycling system, and have become a key bottleneck restricting the large-scale and green development of the waste lithium iron phosphate cathode material recycling industry.

[0022] To address the aforementioned problems, this application provides a method for recycling waste lithium-ion battery cathode materials and a method for deep impurity removal from the recycling liquid. In this method, S1: Waste lithium iron phosphate cathode material black powder is mixed with sulfuric acid and hydrogen peroxide for an acid-soluble oxidation reaction to obtain an acid-soluble slurry. This effectively destroys the crystal structure of lithium iron phosphate, allowing lithium to fully leach into the liquid phase, while simultaneously achieving preliminary separation of phosphorus and iron elements, with a lithium leaching rate exceeding 98%. S2: The acid-soluble slurry is then subjected to pressure filtration to obtain a lithium sulfate solution and phosphorus-iron slag, achieving efficient solid-liquid phase separation and laying the foundation for subsequent lithium recovery and phosphorus-iron resource utilization. S3: The phosphorus-iron slag is processed to obtain iron phosphate products, enabling the high-value utilization of phosphorus-iron resources. Recycling improves the overall economic efficiency of the recycling process; S4: The lithium sulfate solution is purified to obtain a purified lithium sulfate solution, which can effectively remove impurity ions such as copper, iron, aluminum, calcium, and magnesium, ensuring the purity of subsequent lithium carbonate products; S5: After evaporating and concentrating the purified lithium sulfate solution, sodium carbonate solution is added to react and obtain crude lithium carbonate and lithium precipitation mother liquor, which can achieve preliminary precipitation and recovery of lithium elements, and obtain crude lithium carbonate; S6: The lithium precipitation mother liquor is pre-evaporated and concentrated until the concentration of sodium sulfate at 50℃ reaches 90%-98% of the saturation concentration of sodium sulfate, to obtain concentrated lithium carbonate. The concentrated mother liquor creates suitable concentration conditions for subsequent crystallization and separation, improving crystallization efficiency. S7: The concentrated mother liquor is fed into a hot crystallizer and stirred at 50-60℃ to precipitate anhydrous sodium sulfate crystals. Solid-liquid separation yields anhydrous sodium sulfate product and the primary mother liquor. This utilizes the reverse solubility characteristic of sodium sulfate to precipitate anhydrous sodium sulfate at high temperatures, achieving preliminary separation of sodium and lithium. S8: The primary mother liquor is fed into a cold crystallizer and cooled to 10-15℃ with stirring to precipitate sodium sulfate decahydrate crystals. Before solid-liquid separation, sodium sulfate decahydrate crystals with a particle size <50μm are returned to the hot crystallizer in S7. The crystallizer discharges sodium sulfate decahydrate crystals with a particle size ≥50μm for solid-liquid separation, obtaining sodium sulfate decahydrate product and lithium-rich mother liquor. This effectively solves the problem of high lithium content entrained in fine crystals during cold crystallization, significantly reducing lithium loss during the crystallization process. S9: The lithium-rich mother liquor is returned to S5, which produces lithium precipitation mother liquor, or sent to the lithium recovery process to recover lithium carbonate product. This enables the recycling of unprecipitated lithium, further improving the overall lithium recovery rate. S10: The crude lithium carbonate is processed to obtain lithium carbonate product. This process removes surface impurities from the crude lithium carbonate, resulting in lithium carbonate product that meets quality requirements.

[0023] This method, by classifying and refluxing the fine crystals generated during cold crystallization, eliminates the need for energy-intensive MVR evaporation and deep freezing processes. It not only reduces the energy consumption per ton of water for lithium precipitation mother liquor treatment to below 30 kWh and reduces equipment investment by more than 30%, but also significantly reduces the lithium loss rate during crystallization and increases the overall lithium recovery rate to over 95%, greatly improving the economic efficiency and resource utilization of waste lithium iron phosphate cathode material recycling processes.

[0024] To better understand the waste lithium battery cathode material recycling method provided in this application embodiment, the specific implementation process of the waste lithium battery cathode material recycling method provided in this application embodiment will be described by way of example below.

[0025] Figure 1 This illustration shows a schematic flowchart of a method for recycling waste lithium battery cathode materials according to the first aspect of an embodiment of this application. The method for recycling waste lithium battery cathode materials includes: S1: The waste lithium iron phosphate cathode material black powder is mixed with sulfuric acid and hydrogen peroxide to carry out acid dissolution and oxidation reaction to obtain acid-soluble slurry.

[0026] It is understandable that waste lithium iron phosphate cathode material black powder is a powdery material mainly composed of lithium iron phosphate, obtained after waste lithium iron phosphate batteries have been discharged, dismantled, crushed, and sorted. Its main component is lithium iron phosphate, and it also contains small amounts of binders, conductive agents, aluminum foil fragments, and other impurities. Sulfuric acid is industrial-grade sulfuric acid, used as an acid leachate to provide hydrogen ions to break down the crystal structure of lithium iron phosphate. Hydrogen peroxide is an industrial-grade aqueous solution of hydrogen peroxide, used as an oxidant to oxidize ferrous iron to ferric iron, promoting the precipitation of iron phosphate. The acid dissolution oxidation reaction is a chemical reaction under acidic conditions where lithium iron phosphate decomposes through oxidation, lithium dissolves into the liquid phase, and iron and phosphorus precipitate as iron phosphate. The acid dissolution slurry is a solid-liquid mixture obtained after the acid dissolution oxidation reaction. The liquid phase is a lithium sulfate solution containing lithium ions, and the solid phase consists of precipitated iron phosphate and unreacted impurities.

[0027] For example, the acid dissolution oxidation reaction can be carried out in an acid-resistant reactor. The acid-resistant reactor is a sealed reaction vessel with an inner wall lined with anti-corrosion material, capable of withstanding acidic media corrosion and a certain temperature and pressure. It includes a reactor body, a jacketed heating layer, a stirrer, a feed inlet, and a discharge outlet. The jacketed heating layer is used to introduce steam or hot water to heat the materials inside the reactor, and the stirrer is used to ensure uniform mixing of the materials. Specifically, waste lithium iron phosphate cathode material black powder and process water are first added to the acid-resistant reactor at a certain solid-liquid ratio, and the stirrer is turned on to prepare a uniform slurry. Then, sulfuric acid is slowly added to adjust the pH value of the slurry to a specified range, and the materials inside the reactor are heated to the reaction temperature through the jacketed heating layer. Hydrogen peroxide is then slowly added dropwise, and after the addition is complete, the reaction is maintained at the temperature until lithium leaching is complete, resulting in an acid-soluble slurry.

[0028] S2: The acid-soluble slurry is separated by pressure filtration to obtain lithium sulfate solution and phosphorus iron slag.

[0029] As can be understood, pressure filtration is a physical separation method that uses pressure to force the liquid phase of a solid-liquid mixture through a filter cloth, while the solid phase is retained on the filter cloth, thus achieving solid-liquid phase separation. The lithium sulfate solution is the liquid phase obtained after pressure filtration; its main component is lithium sulfate, and it also contains small amounts of dissolved copper, iron, aluminum, calcium, magnesium, and other impurity ions. The phosphate slag is the solid phase obtained after pressure filtration; its main component is iron phosphate, and it also contains small amounts of unreacted lithium iron phosphate, conductive agents, binders, and other impurities.

[0030] For example, pressure filtration separation can be achieved using a plate and frame filter press, which consists of alternately arranged filter plates and frames. A hydraulic clamping device presses the filter plates and frames, allowing the material to pass through the filter cloth under pressure, thus achieving solid-liquid separation. The plate and frame filter press includes a filter plate assembly, a filter frame assembly, a hydraulic clamping device, filter cloth, a feed pump, and a liquid outlet. Specifically, the acid-soluble slurry obtained in step S1 is fed into the filter chamber of the plate and frame filter press via the feed pump. The hydraulic clamping device is activated to press the filter plates and frames together, maintaining a certain filtration pressure. This allows the liquid phase to pass through the filter cloth and flow out from the liquid outlet, while the solid phase is retained on the filter cloth to form a filter cake. After filtration, compressed air is introduced to dry the residual liquid in the filter cake. Then, the hydraulic clamping device is released, and the filter cake is removed, yielding a lithium sulfate solution and phosphorus-iron slag.

[0031] S3: Process the ferrophosphate slag to obtain ferrophosphate product.

[0032] It is understandable that iron phosphate products are solid products with iron phosphate as the main component obtained after processing, and can be used to produce ceramics, pigments, battery cathode materials, etc.

[0033] For example, this can be achieved by sequentially using an acid dissolving tank, a purification reactor, a precipitation reactor, and a dryer. The acid dissolving tank is an acid-resistant container used to dissolve solid materials in acid. The purification reactor is a reaction vessel used to remove impurities from a solution through a chemical reaction. The precipitation reactor is a reaction vessel used to precipitate the target substance through a chemical reaction. The dryer is a device used to remove moisture from the material. Specifically, the ferric phosphate slag is first added to the acid dissolving tank, and sulfuric acid solution is added for acid dissolution, causing the iron and phosphorus in the slag to dissolve into the liquid phase. Then, the acid-dissolved solution is sent to the purification reactor, where a purification agent is added to remove heavy metal impurities from the solution. After purification, the solution is sent to the precipitation reactor, where phosphoric acid is added to adjust the phosphorus-iron molar ratio, and sodium hydroxide is added to adjust the pH value, causing ferric phosphate to precipitate. Finally, the precipitate is subjected to solid-liquid separation, and the resulting ferric phosphate filter cake is sent to the dryer for drying to obtain the ferric phosphate product.

[0034] S4: The lithium sulfate solution is subjected to impurity removal treatment to obtain a purified lithium sulfate solution.

[0035] As can be understood, impurity removal is a process that uses chemical reactions to convert impurity ions such as copper, iron, aluminum, calcium, and magnesium in a lithium sulfate solution into precipitates, which are then removed through solid-liquid separation. The purified lithium sulfate solution is obtained after impurity removal treatment, where the content of impurity ions is reduced to a level that meets the requirements of subsequent lithium precipitation processes.

[0036] For example, the impurity removal process can be achieved by sequentially connecting a displacement reactor, a neutralization reactor, a precipitation reactor, and a plate and frame filter press. Specifically, the lithium sulfate solution obtained in step S2 is first fed into the displacement reactor, where iron powder is added and the mixture is stirred to react, causing copper ions to be replaced by elemental copper and precipitate. Then, the solution is fed into the neutralization reactor, where sodium hydroxide solution is added to adjust the pH value, causing iron and aluminum ions to be converted into hydroxides and precipitate. Next, the solution is fed into the precipitation reactor, where sodium carbonate solution is added, causing calcium and magnesium ions to be converted into carbonates and precipitate. Finally, the solution is fed into a plate and frame filter press for solid-liquid separation to remove all precipitates, yielding a purified lithium sulfate solution.

[0037] S5: After evaporating and concentrating the purified lithium sulfate solution, sodium carbonate solution is added to react and crude lithium carbonate and lithium precipitation mother liquor are obtained.

[0038] Evaporation and concentration are understood to be processes that increase the solute concentration in a solution by heating it to vaporize some of the water. Sodium carbonate solution is obtained by dissolving industrial-grade sodium carbonate in water and is used as a lithium precipitant. Crude lithium carbonate is the lithium carbonate precipitate obtained after the reaction, containing a small amount of surface-adsorbed impurities. The lithium precipitation mother liquor is the liquid phase obtained after solid-liquid separation following the reaction; its main component is sodium sulfate, and it also contains incompletely precipitated lithium sulfate.

[0039] For example, evaporation and concentration can be achieved using an MVR evaporator. An MVR evaporator utilizes mechanical vapor recompression technology to compress and heat the secondary vapor generated during evaporation, using it as a heat source to achieve heat recycling. Specifically, the purified lithium sulfate solution is first fed into the MVR evaporator and concentrated until the lithium ion concentration reaches a specified range. Then, the concentrated lithium sulfate solution is fed into a lithium precipitation reactor, heated to the reaction temperature, and sodium carbonate solution is slowly added, stirring until lithium carbonate precipitation is complete. Finally, the reacted material is fed into a plate and frame filter press for solid-liquid separation to obtain crude lithium carbonate and lithium precipitation mother liquor.

[0040] S6: The lithium precipitation mother liquor is pre-evaporated and concentrated until the sodium sulfate concentration reaches 90%-98% of the saturation concentration of sodium sulfate at 50℃, thus obtaining concentrated mother liquor.

[0041] It is understood that pre-evaporation concentration is a process of initially evaporating and concentrating the lithium precipitation mother liquor to remove some water and increase the sodium sulfate concentration to 90%-98% of its saturation concentration at 50°C. The solubility of sodium sulfate increases with temperature between 0-32.4°C, but decreases above 32.4°C. 50°C is the minimum operating temperature for the thermal crystallization process in this application. Pre-evaporation concentration based on this temperature ensures that the concentrated mother liquor will not prematurely crystallize and clog the pipelines due to temperature fluctuations during transport to the thermal crystallizer. It also provides the most suitable initial supersaturation for the subsequent thermal crystallization process. A saturation concentration below 90% will lead to insufficient subsequent thermal crystallization yield and increased energy consumption, while a saturation concentration above 98% will cause premature crystallization in the transport pipelines, resulting in blockage. The saturation solubility of sodium sulfate in the lithium precipitation mother liquor at 50°C is approximately 318 g / L. The concentrated mother liquor is the solution obtained after pre-evaporation concentration, in which the sodium sulfate concentration is close to saturation, and the lithium concentration is correspondingly increased.

[0042] For example, pre-evaporation concentration can be achieved using a falling film evaporator. A falling film evaporator is a high-efficiency evaporation device where material is added from the top and flows downwards in a film-like manner along the inner wall of the heating tubes under gravity, while being heated and evaporated. The falling film evaporator includes heating tubes, a separation chamber, and a distributor. Specifically, the lithium-precipitated mother liquor obtained in step S5 is fed to the top of the falling film evaporator. The distributor evenly distributes the lithium-precipitated mother liquor on the inner wall of each heating tube. As the lithium-precipitated mother liquor flows downwards along the inner wall of the heating tubes, it is heated and evaporated by heating steam outside the heating tubes. The resulting secondary steam, along with the concentrated solution, enters the separation chamber for gas-liquid separation. The concentrated liquid after separation is the concentrated mother liquor.

[0043] S7: The concentrated mother liquor is fed into a hot crystallizer and kept at 50-60℃ with stirring to precipitate anhydrous sodium sulfate crystals. Anhydrous sodium sulfate product and primary mother liquor are obtained through solid-liquid separation.

[0044] As can be understood, a thermal crystallizer is a device used for crystallization operations at relatively high temperatures, capable of maintaining a stable crystallization temperature and providing a stirring effect. Examples of thermal crystallizers include evaporative OSLO thermal crystallizers, DTB thermal crystallizers, or continuous stirred crystallizers with jacketed heating. Temperature maintenance and stirring are used to keep the material temperature constant during crystallization and to ensure uniform mixing, promoting uniform crystal growth. Anhydrous sodium sulfate crystals are sodium sulfate crystals without crystallization water, with the chemical formula Na₂SO₄, commonly known as sodium sulfate. The primary mother liquor is the liquid phase obtained after solid-liquid separation following the thermal crystallization process, in which the sodium sulfate concentration decreases and the lithium concentration further increases.

[0045] This application selects 50-60℃ as the thermal crystallization temperature range as the optimal choice based on the comprehensive consideration of sodium sulfate's inverse solubility characteristics, the process requirements for lithium release through fine crystal dehydration, crystal quality control, and energy consumption costs. Sodium sulfate exhibits inverse solubility characteristics above 32.4℃; its solubility is approximately 31.8 g / 100g water at 50℃ and approximately 30.0 g / 100g water at 60℃. Within this temperature range, the solubility decreases slowly with increasing temperature, enabling the formation of a stable and controllable supersaturation and avoiding explosive nucleation that produces a large number of fine crystals. Below 50℃, the solubility of sodium sulfate increases significantly, and the crystallization rate of anhydrous sodium sulfate drops to less than 70%. At this temperature, a large amount of sodium sulfate remains in the primary mother liquor, which will significantly increase the load on subsequent cold crystallization and cannot guarantee that the returned sodium sulfate decahydrate fine crystals will be completely dehydrated. When the temperature is above 60℃, the evaporation energy consumption increases exponentially, with energy consumption per ton of water increasing by about 15% for every 5℃ increase. High temperatures will also accelerate equipment corrosion and cause anhydrous sodium sulfate crystals to clump, affecting product purity and filtration performance. In addition, this temperature range is perfectly matched with the 50℃ reference temperature of the S6 pre-evaporation concentration, which can ensure that the concentrated mother liquor forms the most suitable initial supersaturation after entering the hot crystallizer, while providing sufficient dehydration and dissolution conditions for the returned sodium sulfate decahydrate fine crystals.

[0046] For example, the thermal crystallizer is an evaporative OSLO thermal crystallizer, which includes a crystallizer body, an external heater, a vacuum flash chamber, a circulating pump, a feed inlet, a seed inlet, and an underflow outlet. The external heater is used to heat the circulating liquid, the vacuum flash chamber is used to generate supersaturation in the heated liquid through vacuum flash evaporation, the circulating pump is used to achieve continuous circulation of the liquid, and the seed inlet is used to add anhydrous sodium sulfate seeds. Solid-liquid separation can be achieved using a horizontal screw centrifuge, which is a device that uses centrifugal force to separate solid-liquid mixtures and can continuously feed and discharge. In practice, the concentrated mother liquor obtained from S6 is continuously fed into the circulation system of the evaporative OSLO thermal crystallizer. The liquid is then pumped into an external heater and heated to 50°C to 60°C. The heated liquid then enters a vacuum flash chamber for vacuum flash evaporation, where the water evaporates to make the solution supersaturated. The supersaturated solution enters the suspension bed at the bottom of the evaporative OSLO thermal crystallizer through a vertical pipe, allowing the crystals to grow uniformly. The coarse crystal suspension after crystallization is continuously discharged from the bottom outlet and sent to a horizontal screw centrifuge for solid-liquid separation to obtain anhydrous sodium sulfate product and primary mother liquor.

[0047] S8: The mother liquor is fed into a cold crystallizer and cooled to 10-15℃. The mixture is then stirred to precipitate sodium sulfate decahydrate crystals. Before solid-liquid separation, sodium sulfate decahydrate crystals with a particle size <50μm are returned to the hot crystallizer in S7, while sodium sulfate decahydrate crystals with a particle size ≥50μm are discharged for solid-liquid separation to obtain sodium sulfate decahydrate product and lithium-rich mother liquor.

[0048] As can be understood, a cold crystallizer is a device used for crystallization operations at lower temperatures. It cools materials to a specified temperature and provides stirring and particle size classification functions. Cold crystallizers can be, for example, cooling OSLO cold crystallizers or flow-guided cold crystallizers. Sodium sulfate decahydrate crystals are sodium sulfate crystals containing 10 molecules of water of crystallization, with the chemical formula Na₂SO₄·10H₂O, commonly known as Glauber's salt. Particle size classification is the process of separating crystals into fine and coarse crystals based on their particle size, by controlling the upward flow rate of the solution and the height of the suspended bed within the crystallizer, and utilizing Stokes' law of sedimentation.

[0049] The selection of 50 μm as the cutting particle size in this application is based on the following: Grading and sieving experiments using a laser particle size analyzer combined with quantitative analysis using inductively coupled plasma optical emission spectrometry (ICP-OES) revealed a significant non-linear relationship between the amount of lithium entrained within sodium sulfate decahydrate crystals and the particle size. For fine crystals with a particle size <50 μm, the internal liquid inclusions account for 15%-25% of the crystal volume, and the lithium entrainment accounts for over 85% of the total lithium loss during crystallization. Conversely, for coarse crystals with a particle size ≥50 μm, the internal liquid inclusions decrease to below 3%, and the lithium entrainment is negligible. Therefore, using 50 μm as the cutting particle size maximizes the removal of fine crystals with high lithium entrainment while ensuring the filtration performance and product purity of coarse crystals.

[0050] This application selects 10-15℃ as the cold crystallization temperature range based on the optimal balance between the solubility characteristics of sodium sulfate, lithium entrainment control, and process energy consumption: the solubility of sodium sulfate decahydrate decreases significantly with decreasing temperature, with a solubility of approximately 9g / 100g water at 10℃ and approximately 14g / 100g water at 15℃. Within this temperature range, the crystallization rate of sodium sulfate decahydrate can reach over 90%. When the temperature is below 10℃, the solubility of sodium sulfate decreases by less than 5%, but the refrigeration energy consumption will increase by more than 30%, and a large number of fine crystals with a particle size <50μm will be generated, resulting in a significant increase in lithium entrainment. When the temperature is above 15℃, the crystallization rate of sodium sulfate drops to below 80%, and excessive residual sodium sulfate in the mother liquor will reduce the lithium concentration of the subsequent lithium-rich mother liquor and increase the load on the lithium recovery process.

[0051] For example, the cold crystallizer is a cooled OSLO cold crystallizer, including a crystallizer body, an external cooler, a circulating pump, a slurry bed classification zone, an overflow port, and an underflow outlet. The external cooler is used to cool the circulating liquid, the circulating pump is used to achieve continuous circulation of the liquid, and the slurry bed classification zone is used to achieve natural particle size classification by utilizing the difference in crystal settling velocity. By adjusting the flow rate of the circulating pump, the upward flow velocity of the solution in the slurry bed is controlled to be 0.8-1.2 m / h, and the height of the slurry bed is controlled to be 1.5-2.5 m, which can stably achieve a cut particle size of 50 μm. The overflow port is used to discharge the upper fine crystal suspension, and the underflow outlet is used to discharge the lower coarse crystal suspension. Solid-liquid separation is also achieved by a horizontal screw centrifuge. In practice, the mother liquor obtained from S7 is first pre-cooled by heat exchange with the lithium-rich mother liquor obtained from S8, and then sent to the circulation system of the cooling OSLO cold crystallizer. The liquid is sent to an external cooler by a circulation pump and cooled to 10°C to 15°C. The cooled liquid enters the suspension bed at the bottom of the cooling OSLO cold crystallizer. The solution reaches a supersaturated state and crystals precipitate. The crystals grow in the suspension bed and settle naturally. The fine crystals with a particle size <50μm settle at a rate lower than the upward flow rate of the solution and are suspended in the upper part of the suspension bed. They are continuously discharged through the overflow port at the top and returned to the hot crystallizer of S7. The coarse crystals with a particle size ≥50μm settle at a rate greater than the upward flow rate of the solution and accumulate at the bottom of the suspension bed. They are continuously discharged through the bottom outlet and sent to a horizontal screw centrifuge for solid-liquid separation to obtain sodium sulfate decahydrate product and lithium-rich mother liquor.

[0052] S9: Return the lithium-rich mother liquor to S5, which produces the lithium precipitation mother liquor, or send it to the lithium recovery process to recover lithium carbonate products.

[0053] It is understandable that the lithium-rich mother liquor return process involves transporting the lithium-rich mother liquor obtained from cold crystallization back to the lithium precipitation process in S5, mixing it with the concentrated lithium sulfate solution, and then conducting another lithium precipitation reaction in a cyclical process. The lithium recovery process is a process that further processes the lithium-rich mother liquor that is not returned to the lithium precipitation process to recover the remaining lithium resources.

[0054] For example, the lithium-rich mother liquor is transported using a corrosion-resistant centrifugal pump. This pump, whose flow components are made of corrosion-resistant materials, is used to transport acidic or alkaline liquids. Specifically, the lithium-rich mother liquor obtained in step S8 is heated through heat exchange and then transported in a certain proportion to the lithium precipitation reactor in step S5 via the corrosion-resistant centrifugal pump. It is then mixed evenly with the concentrated lithium sulfate solution and subjected to a lithium precipitation reaction. The remaining lithium-rich mother liquor is sent to the lithium recovery process, where it undergoes secondary purification to remove residual impurities, secondary precision filtration to remove fine suspended solids, and a secondary lithium precipitation reaction to recover lithium carbonate products.

[0055] S10: Process the crude lithium carbonate to obtain the lithium carbonate product.

[0056] It is understandable that lithium carbonate products are refined lithium carbonate products that meet battery-grade quality standards and can be used to produce cathode materials for lithium-ion batteries.

[0057] For example, this can be achieved by sequentially using a washing tank, a plate and frame filter press, a vacuum dryer, and a pulverizer. The washing tank is a container used to mix and stir solid materials with liquids to remove surface impurities. The vacuum dryer is a device for drying materials under vacuum conditions, which can lower the drying temperature to prevent material decomposition. The pulverizer is a device for crushing the dried material to a specified particle size. Specifically, the crude lithium carbonate obtained in step S5 is first added to the washing tank, and pure water is added to form a slurry at a certain solid-liquid ratio. The agitator is turned on and the mixture is washed for a certain period to remove impurities such as sodium sulfate adsorbed on the surface of the crude lithium carbonate. Then, the slurry is sent to a plate and frame filter press for solid-liquid separation to obtain a lithium carbonate filter cake. The filter cake is then sent to a vacuum dryer for drying to remove residual moisture. Finally, the dried lithium carbonate is sent to a pulverizer to be crushed to a specified particle size to obtain the lithium carbonate product.

[0058] By employing the above-mentioned S1 to S10 methods, the fine crystals generated by cold crystallization are returned to the hot crystallizer. These fine crystals undergo a phase transition and dehydrate at a high temperature of 50-60℃ to generate anhydrous sodium sulfate, which then rapidly dissolves in the mother liquor within the hot crystallizer, completely releasing the lithium ions entrained within. The feasibility of this process has been fully verified through differential scanning calorimetry, real-time monitoring with ion-selective electrodes, and isotope tracing experiments. The theoretical dehydration temperature of sodium sulfate decahydrate is 32.4℃. Due to its large specific surface area and high surface energy, the complete dehydration time of the fine crystals at 50℃ is less than 5 minutes, far lower than the 1-3 hour residence time in the hot crystallizer. The newly generated anhydrous sodium sulfate particles after dehydration have a particle size of only 1 mm. The lithium-ion concentration is -5μm, while the mother liquor in the hot crystallizer is always in a subsaturated state of sodium sulfate (i.e., the sodium sulfate concentration is lower than the saturation concentration at this temperature, and the supersaturation is <1.05). Under the action of stirring, the newly formed anhydrous sodium sulfate particles can be dissolved, and the lithium-ion release rate in the returned fine crystals can reach more than 99.5%, which will not accumulate in the crystallization system, fundamentally reducing lithium loss during the crystallization process. At the same time, it does not rely on the high-energy-consuming MVR full evaporation and deep freezing process, which significantly reduces the energy consumption and equipment investment of lithium precipitation mother liquor treatment. The overall lithium recovery rate can be increased to more than 95%, which greatly improves the economy and resource utilization of the waste lithium iron phosphate cathode material recycling process.

[0059] In some embodiments, in S1, the temperature of the acid dissolution oxidation reaction is 60-80℃; in S4, the impurity removal treatment includes iron powder replacement to remove copper, sodium hydroxide neutralization to remove iron and aluminum, and sodium carbonate precipitation to remove calcium and magnesium.

[0060] It is understandable that the temperature of the acid dissolution oxidation reaction is the constant temperature maintained by the reaction system during the acid dissolution oxidation reaction, which directly affects the reaction rate, lithium leaching rate, and hydrogen peroxide decomposition rate. Iron powder replacement for copper removal utilizes the chemical property that iron's metallic activity is stronger than copper's, reducing copper ions in the solution to elemental copper precipitate. Sodium hydroxide neutralization for iron and aluminum removal involves adding sodium hydroxide to the solution to adjust the pH value, causing iron and aluminum ions to precipitate as iron hydroxide and aluminum hydroxide, respectively. Sodium carbonate precipitation for calcium and magnesium removal involves adding sodium carbonate to the solution, causing calcium and magnesium ions to precipitate as calcium carbonate and magnesium carbonate, respectively.

[0061] This setup ensures that the reaction temperature of 60-80℃ fully disrupts the lithium iron phosphate crystal structure, allowing for rapid lithium leaching, while also reducing the risk of excessive hydrogen peroxide decomposition due to overheating, thus lowering oxidant consumption. The three-stage stepwise impurity removal process effectively removes different types of impurity ions. Each step operates independently, and the removal sequence can be flexibly adjusted based on the impurity content in the solution. This results in high removal efficiency and stable purification, reducing the total concentration of copper, iron, aluminum, calcium, and magnesium impurity ions in the solution to below 100 mg / L.

[0062] In some embodiments, S5 includes: evaporating and concentrating the purified lithium sulfate solution to a lithium ion concentration of 20-30 g / L, then adding sodium carbonate solution and reacting at 80-100°C.

[0063] It is understandable that lithium ion concentration is the mass of lithium ions contained in a unit volume of purified lithium sulfate solution, and it is a key parameter affecting the supersaturation of the lithium precipitation reaction and the particle size of lithium carbonate crystals. Reaction temperature is the constant temperature maintained in the reaction system during the precipitation reaction of sodium carbonate and lithium sulfate, and it directly affects the solubility and precipitation rate of lithium carbonate.

[0064] This setup, controlling the lithium-ion concentration within the range of 20-30 g / L, maintains a moderate level of supersaturation in the reaction. This avoids both excessively low supersaturation leading to incomplete precipitation and excessively high supersaturation resulting in the formation of numerous fine crystals, facilitating subsequent solid-liquid separation. A reaction temperature of 80-100℃ significantly reduces the solubility of lithium carbonate in water, increases the lithium precipitation rate, and promotes the growth of lithium carbonate crystals into larger particles, reducing surface-adsorbed impurities.

[0065] In some embodiments, in S7, the stirring time at 50-60°C is 1-3 hours; in S8, the stirring time at 10-15°C is 2-4 hours.

[0066] It is understandable that the time for heat preservation and stirring is the total time for the reaction system to be continuously stirred and maintained at the specified temperature after it reaches the specified temperature, which directly affects the amount of crystal precipitation, the degree of growth, and the particle size distribution.

[0067] This setup ensures that the anhydrous sodium sulfate is fully precipitated and that crystals grow uniformly, reducing the risk of incomplete crystallization due to too short a time or excessive crystal growth and agglomeration due to too long a time. The 2-4 hour cold crystallization stirring time ensures complete crystallization of sodium sulfate decahydrate while controlling the amount of fine crystals formed, creating favorable conditions for subsequent particle size classification.

[0068] In some embodiments, S7 includes: adding anhydrous sodium sulfate seed crystals to a hot crystallizer during the process of keeping the mixture warm and stirring at 50-60°C to induce heterogeneous nucleation of anhydrous sodium sulfate crystals; the purity of the anhydrous sodium sulfate seed crystals is ≥98%, the particle size D50 is 50-150μm, and the dosage is 3-8g / L of concentrated mother liquor.

[0069] It can be understood that anhydrous sodium sulfate seed crystals are small amounts of anhydrous sodium sulfate crystal particles pre-added to the crystallization system to provide nucleation sites for the growth of new anhydrous sodium sulfate crystals. Heterogeneous nucleation is a nucleation method where new anhydrous sodium sulfate crystals grow on the surface of existing anhydrous sodium sulfate seed crystals. Compared to homogeneous nucleation, heterogeneous nucleation requires lower supersaturation and the nucleation process is easier to control. The purity of anhydrous sodium sulfate seed crystals is the mass fraction of anhydrous sodium sulfate in the seed crystals. The particle size D50 of anhydrous sodium sulfate seed crystals is the particle size corresponding to a cumulative particle size distribution of 50% for the anhydrous sodium sulfate seed crystals, representing the average particle size of the anhydrous sodium sulfate seed crystals. The dosage of anhydrous sodium sulfate seed crystals is the mass of anhydrous sodium sulfate seed crystals added per unit volume of concentrated mother liquor.

[0070] This setup, by adding anhydrous sodium sulfate seed crystals, induces heterogeneous nucleation, preventing explosive nucleation in the early stages of crystallization that could lead to the formation of numerous fine crystals, and effectively controlling the particle size distribution of the anhydrous sodium sulfate crystals. Anhydrous sodium sulfate seed crystals with a purity ≥98% reduce the introduction of new impurities, ensuring the quality of the anhydrous sodium sulfate product. Anhydrous sodium sulfate seed crystals with a particle size D50 of 50-150 μm provide sufficient and uniform nucleation sites. Adding anhydrous sodium sulfate seed crystals at a dosage of 3-8 g / L of concentrated mother liquor ensures a stable crystallization process, resulting in anhydrous sodium sulfate crystals with uniform particle size and good filtration performance.

[0071] In some embodiments, the method for recycling waste lithium battery cathode materials includes: exchanging heat between the primary mother liquor obtained in S7 and the lithium-rich mother liquor obtained in S8 to recover heat; after the heat exchange, the temperature of the primary mother liquor drops from 50-60°C to 20-30°C and is sent to the cold crystallizer in S8, while the temperature of the lithium-rich mother liquor rises from 10-15°C to 35-45°C and is returned to S5, where the lithium precipitation mother liquor is generated, or sent to the lithium recycling process to recover lithium carbonate products.

[0072] Heat exchange is understood to be the process of transferring heat between two fluids at different temperatures through a heat exchanger, thereby cooling the high-temperature fluid and heating the low-temperature fluid. Heat recovery is an energy-saving measure that reuses heat that would otherwise be directly lost to the environment, thus reducing the overall energy consumption of the process.

[0073] For example, heat exchange can be achieved using a plate heat exchanger, which is a high-efficiency heat exchange device consisting of a series of corrugated metal plates stacked together. Hot and cold fluids flow in channels on both sides of the plates, transferring heat through the plates. Specifically, the primary mother liquor obtained in S7 at 50-60°C is fed into the hot-side channel of the plate heat exchanger, and the lithium-rich mother liquor obtained in S8 at 10-15°C is fed into the cold-side channel of the plate heat exchanger. The two fluids exchange heat counter-currently through the plates. After heat exchange, the primary mother liquor cooled to 20-30°C is discharged from the hot-side outlet and sent to the cold crystallizer in S8, while the lithium-rich mother liquor heated to 35-45°C is discharged from the cold-side outlet and transported to the lithium precipitation reactor or lithium recovery process in S5.

[0074] This setup allows for the full recovery of waste heat from the primary mother liquor, while simultaneously pre-cooling it to reduce the refrigeration load on the cold crystallizer. Preheating the lithium-rich mother liquor reduces the heating load on the lithium precipitation process, resulting in an overall reduction of energy consumption per ton of water in the lithium precipitation mother liquor treatment stage by 15%-20%, demonstrating significant energy-saving effects.

[0075] In some embodiments, the waste lithium battery cathode material recycling method includes: washing the anhydrous sodium sulfate crystals obtained in S7 and the decahydrate sodium sulfate crystals obtained in S8 with condensate water in a countercurrent manner, and returning the washing water obtained after washing to the pre-evaporation and concentration process in S6.

[0076] As can be understood, countercurrent washing is a washing method in which the washing liquid and the material being washed flow in opposite directions. That is, fresh washing liquid is added from the last stage of the washing process and comes into contact with the material that has been washed in the previous stages, while the washing water is discharged from the first stage. This method can achieve the best washing effect with the least amount of washing liquid.

[0077] For example, countercurrent washing can be achieved through a three-stage series washing tank, each equipped with an agitator and a discharge pump. Specifically, anhydrous sodium sulfate crystals and sodium sulfate decahydrate crystals are respectively fed into their respective first-stage washing tanks, where washing water from the second-stage washing tank is added for agitation and washing. The washed material is then pumped into the second-stage washing tank, where washing water from the third-stage washing tank is added for further washing. The washed material is then fed into the third-stage washing tank, where fresh condensate is added for final washing. The final washed crystals are then fed into a horizontal screw centrifuge for solid-liquid separation, and the washing water discharged from the first-stage washing tank is collected and returned to the pre-evaporation and concentration process in S6.

[0078] With this setup, countercurrent washing can effectively remove lithium ions and soluble impurities adsorbed on the surface of anhydrous sodium sulfate crystals and sodium sulfate decahydrate crystals, improving the purity of sodium sulfate products. The wash water is returned to the pre-evaporation and concentration process, which can recover the lithium resources and sodium sulfate dissolved in the wash water, further reducing the lithium loss rate and reducing wastewater discharge.

[0079] In some embodiments, S3 includes acid dissolution, proportioning, directional precipitation and drying of phosphorus iron slag; S10 includes washing, drying and crushing of crude lithium carbonate; and in S9, the lithium recovery process consists of secondary purification, secondary precision filtration and secondary lithium precipitation.

[0080] The process can be understood as follows: Acid dissolution involves mixing ferric phosphate slag with sulfuric acid solution, causing the iron and phosphorus elements in the slag to dissolve and enter the liquid phase. Proportional adjustment involves adding phosphoric acid or iron salts to the acid-dissolved solution to adjust the molar ratio of iron ions to phosphate ions to the stoichiometric ratio of ferric phosphate. Directional precipitation involves adding sodium hydroxide to the proportioned solution to adjust the pH value and control the reaction conditions to precipitate ferric phosphate in a specific crystal form and particle size. Stirring involves mixing crude lithium carbonate with pure water, causing soluble impurities adsorbed on the surface of the crude lithium carbonate to dissolve into the water. Secondary purification involves adding a purification agent to the lithium-rich mother liquor to remove residual calcium, magnesium, iron, and other impurity ions. Secondary precision filtration involves using a filtration device with a filtration accuracy of 0.1-1 μm to remove tiny suspended particles from the secondary purified lithium-rich mother liquor. Secondary lithium precipitation involves adding sodium carbonate solution to the secondary precision-filtered lithium-rich mother liquor to recover remaining lithium ions.

[0081] With this setup, the S3 processing step can convert ferrophosphate slag into high-value-added ferrophosphate products, achieving efficient utilization of ferrophosphate resources; the S10 refining step can effectively remove surface impurities from crude lithium carbonate, improving the purity of lithium carbonate products; and the S9 lithium recovery process can further recover lithium resources from lithium-rich mother liquor, increasing the overall lithium recovery rate by 2%-3%.

[0082] In some embodiments, the iron phosphate product obtained in S3 is ceramic-grade iron phosphate, and the lithium carbonate product obtained in S10 is battery-grade lithium carbonate.

[0083] It is understandable that ceramic-grade iron phosphate is an iron phosphate product that meets the quality standards of the ceramic production industry, with an iron content ≥45% and heavy metal impurity content meeting relevant requirements. It can be used to produce ceramic pigments and ceramic glazes. Battery-grade lithium carbonate is a lithium carbonate product that meets the quality standards of the lithium-ion battery production industry, with a main content ≥99.5%. The content of impurities such as sodium, potassium, iron, calcium, and magnesium strictly meets the requirements of GB / T 11075-2013 standard, and it can be used to produce positive electrode materials for lithium-ion batteries.

[0084] Please see Figure 2 The second aspect of this application provides a method for deep purification of waste lithium battery recycling fluid, the method comprising: The lithium precipitation mother liquor generated from the recycling of waste lithium iron phosphate cathode materials is pre-evaporated and concentrated until the sodium sulfate concentration reaches 90%-98% of the saturation concentration of sodium sulfate at 50°C, thus obtaining concentrated mother liquor.

[0085] The concentrated mother liquor is fed into a hot crystallizer and kept at 50-60℃ with stirring to precipitate anhydrous sodium sulfate crystals. Anhydrous sodium sulfate product and primary mother liquor are obtained through solid-liquid separation.

[0086] The mother liquor is fed into a cold crystallizer and cooled to 10-15℃. The mixture is then stirred to precipitate sodium sulfate decahydrate crystals. Before solid-liquid separation, sodium sulfate decahydrate crystals with a particle size <50μm are returned to the hot crystallizer, while sodium sulfate decahydrate crystals with a particle size ≥50μm are discharged for solid-liquid separation to obtain sodium sulfate decahydrate product and lithium-rich mother liquor.

[0087] The lithium-rich mother liquor can be returned to the lithium precipitation process that produces the lithium precipitation mother liquor, or sent to the lithium recovery process to recover lithium carbonate products.

[0088] By adopting the above steps, the fine crystals generated by cold crystallization are returned to the hot crystallizer, where they are dehydrated and dissolved at high temperatures, releasing the lithium ions trapped inside. This fundamentally reduces lithium loss during the crystallization process. At the same time, it eliminates the need to rely on energy-intensive MVR full evaporation and deep freezing processes, significantly reducing energy consumption and equipment investment in lithium precipitation mother liquor treatment. The overall lithium recovery rate can be increased to over 95%, providing an efficient, economical, and environmentally friendly solution for deep impurity removal from waste lithium iron phosphate cathode material recovery liquid.

[0089] The following description is based on specific embodiments.

[0090] The raw material used in this embodiment was lithium precipitation mother liquor produced by a 10,000-ton-level waste lithium iron phosphate recycling production line. Its typical components are: sodium sulfate 220±10 g / L, lithium sulfate 3.5±0.2 g / L, pH value 6.5-7.0, and the total content of other impurity ions ≤0.5 g / L. All experiments were carried out in the same continuous crystallization pilot plant to ensure that, except for the tested variable, all other equipment, operating environment, and raw material batches were completely consistent.

[0091] Example 1 1) Take 1000L of the above lithium precipitation mother liquor and send it to a falling film evaporator for pre-evaporation and concentration until the sodium sulfate concentration is 310g / L (close to saturation at 50℃) to obtain concentrated mother liquor.

[0092] 2) The concentrated mother liquor is continuously fed into an evaporative OSLO thermal crystallizer, and the crystallization temperature is controlled at 55℃, the vacuum degree is -0.06MPa, the circulation ratio is 8:1, and the material residence time is 2 hours. During the heat preservation and stirring process, anhydrous sodium sulfate seed crystals with a purity of 99%, a particle size D50 of 100μm, and an addition amount of 5g / L of concentrated mother liquor are added to precipitate anhydrous sodium sulfate crystals.

[0093] 3) The material after thermal crystallization is separated into solid and liquid by a horizontal screw centrifuge to obtain anhydrous sodium sulfate product and primary mother liquor; the primary mother liquor and the lithium-rich mother liquor produced by subsequent cold crystallization are heat exchanged in a plate heat exchanger, the temperature of the primary mother liquor drops from 55℃ to 25℃, and the temperature of the lithium-rich mother liquor rises from 12℃ to 40℃.

[0094] 4) The mother liquor after heat exchange is continuously fed into the OSLO cold crystallizer with cooling. The crystallization temperature is controlled at 12℃, the cooling rate is 0.8℃ / min, the circulation ratio is 10:1, and the material residence time is 3 hours, so sodium sulfate decahydrate crystals are precipitated.

[0095] 5) In the cooling OSLO cold crystallizer, crystals naturally settle and classify. Fine crystals with a particle size <50μm are continuously returned to the evaporative OSLO hot crystallizer in step 2) through the top overflow port; coarse crystals with a particle size ≥50μm are discharged through the bottom underflow port and separated into solid and liquid by a horizontal screw centrifuge to obtain sodium sulfate decahydrate product and lithium-rich mother liquor.

[0096] 6) The lithium-rich mother liquor is heated by heat exchange and then returned to the lithium precipitation process that produces the lithium precipitation mother liquor.

[0097] 7) The entire process runs continuously for 72 hours, with samples taken and tested every 12 hours, and the average value of the results is taken.

[0098] Example 2 1) Take 1000L of lithium precipitation mother liquor from the same batch as in Example 1 and send it to a falling film evaporator for pre-evaporation and concentration until the sodium sulfate concentration is 290g / L (close to saturation at 50°C) to obtain concentrated mother liquor.

[0099] 2) The concentrated mother liquor is continuously fed into an evaporative OSLO thermal crystallizer, and the crystallization temperature is controlled at 50℃, the vacuum degree is -0.065MPa, the circulation ratio is 7:1, and the material residence time is 1 hour. During the heat preservation and stirring process, anhydrous sodium sulfate seed crystals with a purity of 98%, a particle size D50 of 50μm, and an addition amount of 3g / L of concentrated mother liquor are added to precipitate anhydrous sodium sulfate crystals.

[0100] 3) The material after thermal crystallization is separated into solid and liquid by a horizontal screw centrifuge to obtain anhydrous sodium sulfate product and primary mother liquor; the primary mother liquor and the lithium-rich mother liquor produced by subsequent cold crystallization are heat exchanged in a plate heat exchanger, the temperature of the primary mother liquor drops from 50℃ to 22℃, and the temperature of the lithium-rich mother liquor rises from 10℃ to 38℃.

[0101] 4) The mother liquor after heat exchange is continuously fed into the OSLO cold crystallizer with cooling. The crystallization temperature is controlled at 10℃, the cooling rate is 0.5℃ / min, the circulation ratio is 8:1, and the material residence time is 2 hours, so as to precipitate sodium sulfate decahydrate crystals.

[0102] 5) In the cooling OSLO cold crystallizer, crystals naturally settle and classify. Fine crystals with a particle size <50μm are continuously returned to the evaporative OSLO hot crystallizer in step 2) through the top overflow port; coarse crystals with a particle size ≥50μm are discharged through the bottom underflow port and separated into solid and liquid by a horizontal screw centrifuge to obtain sodium sulfate decahydrate product and lithium-rich mother liquor.

[0103] 6) The subsequent steps are exactly the same as in Example 1.

[0104] Example 3 1) Take 1000L of lithium precipitation mother liquor from the same batch as in Example 1 and send it to a falling film evaporator for pre-evaporation and concentration until the sodium sulfate concentration is 320g / L (close to saturation at 50℃) to obtain concentrated mother liquor.

[0105] 2) The concentrated mother liquor is continuously fed into an evaporative OSLO thermal crystallizer, and the crystallization temperature is controlled at 60℃, the vacuum degree is -0.055MPa, the circulation ratio is 9:1, and the material residence time is 3 hours. During the heat preservation and stirring process, anhydrous sodium sulfate seed crystals with a purity of 99.5%, a particle size D50 of 150μm, and an addition amount of 8g / L of concentrated mother liquor are added to precipitate anhydrous sodium sulfate crystals.

[0106] 3) The material after thermal crystallization is separated into solid and liquid by a horizontal screw centrifuge to obtain anhydrous sodium sulfate product and primary mother liquor; the primary mother liquor and the lithium-rich mother liquor produced by subsequent cold crystallization are heat exchanged in a plate heat exchanger, the temperature of the primary mother liquor drops from 60℃ to 28℃, and the temperature of the lithium-rich mother liquor rises from 15℃ to 42℃.

[0107] 4) The mother liquor after heat exchange is continuously fed into the OSLO cold crystallizer with cooling. The crystallization temperature is controlled at 15℃, the cooling rate is 1℃ / min, the circulation ratio is 12:1, and the material residence time is 4 hours, so sodium sulfate decahydrate crystals are precipitated.

[0108] 5) In the cooling OSLO cold crystallizer, crystals naturally settle and classify. Fine crystals with a particle size <50μm are continuously returned to the evaporative OSLO hot crystallizer in step 2) through the top overflow port; coarse crystals with a particle size ≥50μm are discharged through the bottom underflow port and separated into solid and liquid by a horizontal screw centrifuge to obtain sodium sulfate decahydrate product and lithium-rich mother liquor.

[0109] 6) The subsequent steps are exactly the same as in Example 1.

[0110] Comparative Example 1 1) Take 1000L of the above lithium precipitation mother liquor and send it to a falling film evaporator for pre-evaporation and concentration until the sodium sulfate concentration is 310g / L (close to saturation at 50℃) to obtain concentrated mother liquor.

[0111] 2) The concentrated mother liquor is continuously fed into an evaporative OSLO thermal crystallizer, and the crystallization temperature is controlled at 55℃, the vacuum degree is -0.06MPa, the circulation ratio is 8:1, and the material residence time is 2 hours. During the heat preservation and stirring process, anhydrous sodium sulfate seed crystals with a purity of 99%, a particle size D50 of 100μm, and an addition amount of 5g / L of concentrated mother liquor are added to precipitate anhydrous sodium sulfate crystals.

[0112] 3) The material after thermal crystallization is separated into solid and liquid by a horizontal screw centrifuge to obtain anhydrous sodium sulfate product and primary mother liquor; the primary mother liquor and the lithium-rich mother liquor produced by subsequent cold crystallization are heat exchanged in a plate heat exchanger, the temperature of the primary mother liquor drops from 55℃ to 25℃, and the temperature of the lithium-rich mother liquor rises from 12℃ to 40℃.

[0113] 4) The mother liquor after heat exchange is continuously fed into the OSLO cold crystallizer with cooling. The crystallization temperature is controlled at 12℃, the cooling rate is 0.8℃ / min, the circulation ratio is 10:1, and the material residence time is 3 hours, so sodium sulfate decahydrate crystals are precipitated.

[0114] 5) In the cooled OSLO cold crystallizer, crystals naturally settle and classify. Fine crystals with a particle size <50μm are continuously returned to the cooled OSLO cold crystallizer in step 4) through the top overflow port; coarse crystals with a particle size ≥50μm are discharged through the bottom underflow port and separated into solid and liquid by a horizontal screw centrifuge to obtain sodium sulfate decahydrate product and lithium-rich mother liquor.

[0115] 6) The subsequent steps are exactly the same as in Example 1.

[0116] Comparative Example 2 1) Take 1000L of the above lithium precipitation mother liquor and send it to a falling film evaporator for pre-evaporation and concentration until the sodium sulfate concentration is 310g / L (close to saturation at 50℃) to obtain concentrated mother liquor.

[0117] 2) The concentrated mother liquor was continuously fed into an evaporative OSLO thermal crystallizer, and the crystallization temperature was controlled at 45°C and the vacuum degree was -0.07MPa. The other parameters were the same as in Example 1, and anhydrous sodium sulfate crystals were precipitated.

[0118] 3) The subsequent steps are exactly the same as in Example 1.

[0119] Comparative Example 3 1) Take 1000L of the above lithium precipitation mother liquor and send it to a falling film evaporator for pre-evaporation and concentration until the sodium sulfate concentration is 310g / L (close to saturation at 50℃) to obtain concentrated mother liquor.

[0120] 2) The concentrated mother liquor was continuously fed into an evaporative OSLO thermal crystallizer, and the crystallization temperature was controlled at 65°C and the vacuum degree was -0.05MPa. The other parameters were the same as in Example 1, and anhydrous sodium sulfate crystals were precipitated.

[0121] 3) The subsequent steps are exactly the same as in Example 1.

[0122] Comparative Example 4 1) Take 1000L of lithium precipitation mother liquor from the same batch as in Example 1 and send it into an MVR evaporator for total evaporation and crystallization. Control the evaporation temperature at 90℃ and the vacuum degree at -0.04MPa to precipitate anhydrous sodium sulfate crystals.

[0123] 2) The crystallized material is subjected to solid-liquid separation using a horizontal screw centrifuge to obtain anhydrous sodium sulfate product and lithium-rich mother liquor.

[0124] 3) The lithium precipitation process involves returning the lithium-rich mother liquor to generate lithium precipitation mother liquor.

[0125] 4) The entire process runs continuously for 72 hours, with samples taken and tested every 12 hours, and the average value of the results is taken.

[0126] 1. Performance Testing Methods Lithium loss rate during crystallization: Calculated by material balance, the formula is: Lithium loss rate = (Total mass of lithium in sodium sulfate product / Total mass of lithium in lithium precipitation mother liquor) × 100%; The lithium content in sodium sulfate product was detected by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0127] Electricity consumption per ton of water: This is measured cumulatively by the electricity meters installed at the inlet of each device. The formula is: Electricity consumption per ton of water = Total electricity consumption / Volume of lithium precipitated mother liquor processed.

[0128] Sodium sulfate product purity: The main contents of anhydrous sodium sulfate and sodium sulfate decahydrate products were determined by gravimetric method, and the impurity contents were determined by ICP-OES.

[0129] Total lithium recovery rate: Calculated through material balance, the formula is: Total lithium recovery rate = (Total lithium mass in lithium carbonate product / Total lithium mass in lithium precipitation mother liquor) × 100%.

[0130] All tests were conducted in triplicate, and were performed at room temperature and pressure. The results were taken as mean ± standard deviation.

[0131] 2. Test Results Table 1 compares the performance test results of each embodiment with those of the comparative example. Table 1 3. Results Analysis As shown in Table 1, the performance indicators of Examples 1-3 of this application are significantly better than all comparative examples. The specific analysis is as follows: Verification of the universality of the technical solution of this application: Examples 1-3 adopted the optimal parameters, lower limit parameters, and upper limit parameters defined in this application, respectively. The lithium loss rate during the crystallization process was ≤0.10%, the total lithium recovery rate was ≥95.2%, and the electricity consumption per ton of water was ≤28.2kWh / m³. This result proves that the technical solution of this application can achieve stable and excellent technical effects within the entire range of defined parameters, rather than being effective only at a single specific parameter point.

[0132] Example 1 vs. Comparative Example 1: Compared with the conventional "fine crystal return to cold crystallizer" scheme in the prior art, this application reduces the lithium loss rate during the crystallization process from 0.38% to 0.08% by returning the cold-crystallized fine crystals to the hot crystallizer, a reduction of 79%, and increases the total lithium recovery rate from 91.8% to 95.8%. This result directly proves that this application effectively improves the problem in the prior art where the fine crystals generated during the cold crystallization process carry a large amount of lithium, making it difficult to effectively reduce the lithium loss rate.

[0133] Example 1 vs. Comparative Examples 2 and 3: When the thermal crystallization temperature is below 50℃ (Comparative Example 2), the anhydrous sodium sulfate crystallizes incompletely, leading to an increase in the subsequent cold crystallization load, a lithium loss rate of 0.15%, and a 24% increase in electricity consumption per ton of water. When the thermal crystallization temperature is above 60℃ (Comparative Example 3), the evaporation energy consumption increases significantly, and some fine sodium sulfate decahydrate crystals are not completely dehydrated in the thermal crystallizer, resulting in a lithium loss rate of 0.12% and a 32% increase in electricity consumption per ton of water. This result verifies that 50-60℃ is the optimal temperature range for thermal crystallization.

[0134] Example 1 vs. Comparative Example 4: Compared with the mainstream MVR full evaporation process in the prior art, the power consumption per ton of water in this application is reduced from 52.3 kWh / m³ to 26.5 kWh / m³, a reduction of 49%; the lithium loss rate is reduced from 1.25% to 0.08%, a reduction of 94%; and the total lithium recovery rate is increased from 86.2% to 95.8%. Meanwhile, this application avoids the frequent scaling problem inherent in the MVR full evaporation process in the lithium precipitation mother liquor treatment stage, eliminating the need for shutdown cleaning every 15-30 days; the MVR evaporator used for upstream lithium sulfate solution concentration, because the solution remains unsaturated and no crystals precipitate during the evaporation process, can operate continuously for more than 6 months, significantly reducing the operation and maintenance costs of the entire production line.

[0135] In summary, this application, through a technical solution combining hot-cold two-stage crystallization and fine crystal cross-stage return to the hot crystallizer, effectively improves the problem in the existing lithium mother liquor treatment process for recycling waste lithium battery cathode materials, where the fine crystals generated during the cold crystallization process carry a large amount of lithium, making it difficult to effectively reduce the lithium loss rate. This solution has clear technological advancement and practical industrial application value.

[0136] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for recycling cathode materials from waste lithium batteries, characterized in that, include: S1: Mix waste lithium iron phosphate cathode material black powder with sulfuric acid and hydrogen peroxide to carry out acid dissolution and oxidation reaction to obtain acid dissolution slurry; S2: The acid-soluble slurry is separated by pressure filtration to obtain lithium sulfate solution and ferrophosphate slag; S3: The ferric phosphate slag is processed to obtain ferric phosphate product; S4: The lithium sulfate solution is subjected to impurity removal treatment to obtain a purified lithium sulfate solution; S5: After evaporating and concentrating the purified lithium sulfate solution, add sodium carbonate solution to react and obtain crude lithium carbonate and lithium precipitation mother liquor. S6: The lithium precipitation mother liquor is pre-evaporated and concentrated until the concentration of sodium sulfate at 50°C reaches 90%-98% of the saturation concentration of sodium sulfate, to obtain concentrated mother liquor. S7: The concentrated mother liquor is fed into a hot crystallizer and kept at 50-60°C with stirring to precipitate anhydrous sodium sulfate crystals. Anhydrous sodium sulfate product and primary mother liquor are obtained by solid-liquid separation. S8: The primary mother liquor is fed into a cold crystallizer and cooled to 10-15°C. The mixture is then kept warm and stirred to precipitate sodium sulfate decahydrate crystals. Before solid-liquid separation, the sodium sulfate decahydrate crystals with a particle size <50μm are returned to the hot crystallizer in S7, while the sodium sulfate decahydrate crystals with a particle size ≥50μm are discharged for solid-liquid separation to obtain sodium sulfate decahydrate product and lithium-rich mother liquor. S9: Return the lithium-rich mother liquor to S5, which produces the lithium precipitation mother liquor, or send it to the lithium recovery process to recover lithium carbonate products. S10: Process the crude lithium carbonate to obtain the lithium carbonate product.

2. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, In S1, the temperature of the acid dissolution oxidation reaction is 60-80℃; in S4, the impurity removal treatment includes iron powder replacement to remove copper, sodium hydroxide neutralization to remove iron and aluminum, and sodium carbonate precipitation to remove calcium and magnesium.

3. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, S5 includes: evaporating and concentrating the purified lithium sulfate solution to a lithium ion concentration of 20-30 g / L, then adding the sodium carbonate solution and reacting at 80-100°C.

4. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, In S7, the stirring time is 1-3 hours at 50-60℃; in S8, the stirring time is 2-4 hours at 10-15℃ after cooling.

5. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, S7 includes: During the process of keeping the temperature at 50-60℃ and stirring, anhydrous sodium sulfate seed crystals are added to the hot crystallizer to induce heterogeneous nucleation of the anhydrous sodium sulfate crystals. The anhydrous sodium sulfate seed crystals have a purity of ≥98%, a particle size D50 of 50-150μm, and are added at a rate of 3-8g / L of the concentrated mother liquor.

6. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, The method includes: exchanging heat between the primary mother liquor obtained in S7 and the lithium-rich mother liquor obtained in S8 to recover heat; after the heat exchange, the temperature of the primary mother liquor drops from 50-60℃ to 20-30℃ and is sent to the cold crystallizer in S8, and the temperature of the lithium-rich mother liquor rises from 10-15℃ to 35-45℃ and is returned to S5, where the lithium precipitation mother liquor is generated, or sent to the lithium recovery process to recover lithium carbonate products.

7. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, The method includes: washing the anhydrous sodium sulfate crystals obtained in S7 and the decahydrate sodium sulfate crystals obtained in S8 with condensed water in a countercurrent manner, and returning the washing water obtained after washing to the pre-evaporation and concentration process in S6.

8. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, S3 includes acid dissolution, proportioning, directional precipitation and drying of the phosphorus iron slag; S10 includes washing, drying and crushing of the crude lithium carbonate; in S9, the lithium recovery process consists of secondary purification, secondary precision filtration and secondary lithium precipitation.

9. The method for recycling waste lithium battery cathode materials as described in claim 1, characterized in that, The iron phosphate product obtained in S3 is ceramic-grade iron phosphate, and the lithium carbonate product obtained in S10 is battery-grade lithium carbonate.

10. A method for deep purification of waste lithium battery recycling fluid, characterized in that, include: The lithium precipitation mother liquor generated from the recycling of waste lithium iron phosphate cathode materials is pre-evaporated and concentrated until the concentration of sodium sulfate at 50°C reaches 90%-98% of the saturation concentration of sodium sulfate, thus obtaining concentrated mother liquor. The concentrated mother liquor is fed into a hot crystallizer and kept at 50-60°C with stirring to precipitate anhydrous sodium sulfate crystals. Anhydrous sodium sulfate product and primary mother liquor are obtained by solid-liquid separation. The primary mother liquor is fed into a cold crystallizer and cooled to 10-15°C. The mixture is then stirred to precipitate sodium sulfate decahydrate crystals. Before solid-liquid separation, the sodium sulfate decahydrate crystals with a particle size <50μm are returned to the hot crystallizer, while the sodium sulfate decahydrate crystals with a particle size ≥50μm are discharged for solid-liquid separation to obtain sodium sulfate decahydrate product and lithium-rich mother liquor. The lithium-rich mother liquor can be returned to the lithium precipitation process that produced the lithium precipitation mother liquor, or sent to the lithium recovery process to recover lithium carbonate products.

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