Method for preparing battery-grade lithium sulfate from lithium iron phosphate waste old pole piece powder

Abstract

The application provides a method for preparing battery-grade lithium sulfate from lithium iron phosphate waste old pole piece powder. In the method, the lithium iron phosphate waste old pole piece powder is slurried, lithium therein is leached out, iron and aluminum are removed through two-step pH adjustment, calcium and magnesium are removed, and finally, the pH is adjusted by using an acid solution to meet the feeding requirements of subsequent lithium sulfate concentration and crystallization. Process water and waste acid generated in the recovery process are all recycled in a closed circuit, and acid consumption and water consumption are greatly reduced.

Description

Technical Field

[0001] This application belongs to the field of resource recycling technology and relates to a method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder. Background Technology

[0002] With the rapid development of the new energy power battery industry, lithium iron phosphate batteries, due to their excellent safety performance, long cycle life, and low cost, have been widely used in new energy vehicles, energy storage, and other fields. As power batteries are being decommissioned in large quantities, the resource recycling of spent lithium iron phosphate electrodes has become a crucial link in the green and sustainable development of the lithium battery industry chain. Lithium sulfate, as a core intermediate raw material for the preparation of battery-grade lithium carbonate and lithium hydroxide, can be used to prepare high-purity battery-grade lithium sulfate from spent lithium iron phosphate electrodes, achieving the recycling of lithium resources. This approach possesses both high economic and environmental value and is currently the mainstream technology direction for the resource recycling of spent lithium iron phosphate.

[0003] Currently, most existing lithium extraction processes from waste lithium iron phosphate electrodes employ a production model of direct acid leaching, single-stage impurity removal, and open material handling. Conventional processes, after acid leaching, typically use a single-stage pH adjustment method to remove impurities such as iron, aluminum, calcium, and magnesium from the solution. However, the impurity precipitation process is prone to causing colloidal encapsulation of liquid-phase lithium salts, resulting in lithium resource loss. Furthermore, to avoid the risk of impurity introduction, traditional processes often use high-cost lithium salt alkaline agents such as lithium carbonate and lithium hydroxide to adjust the pH for impurity removal, significantly increasing the reagent costs for industrial production and resulting in poor economic efficiency.

[0004] Furthermore, existing two-step pH purification and impurity removal processes have significant technical flaws. Most processes maintain a highly alkaline solution environment for extended periods after removing calcium and magnesium impurities, which easily leads to the combination of lithium ions and carbonate ions in the solution to form lithium carbonate solid precipitate. This not only drastically reduces the overall lithium recovery rate but also causes a decrease in the lithium concentration of the refined solution and a reduction in product yield. Simultaneously, traditional recovery processes are mostly open production systems, with process media such as phosphorus iron slag washing water, lithium precipitation mother liquor, and waste acid being directly discharged. This not only results in a significant waste of water and acid resources, leading to high water and acid consumption, but also generates a large amount of process wastewater, increasing environmental protection pressure and failing to meet the energy-saving, water-saving, and environmentally friendly requirements of continuous industrial production.

[0005] Existing closed-loop recycling processes mostly employ an unrestricted full-cycle mother liquor mode. Impurities introduced during production continuously accumulate within the system, making it difficult to achieve dynamic impurity balance. Ultimately, this leads to excessive impurities in lithium sulfate products, failing to meet battery-grade standards and severely restricting product quality and process stability. Therefore, existing lithium extraction processes from waste lithium iron phosphate electrodes generally suffer from numerous problems, including high lithium loss, high reagent and energy costs, low material recycling rates, and significant environmental pressure. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this application is to provide a method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder. In this application, the waste lithium iron phosphate electrode powder is slurried, and the lithium is first leached out. Then, the pH is adjusted in two steps to remove iron and aluminum first, and then to remove calcium and magnesium. Finally, an acid solution is used to adjust the pH to meet the feeding requirements for subsequent lithium sulfate concentration and crystallization. The process water and waste acid generated during the recycling process are all recycled in a closed loop, which greatly reduces acid and water consumption.

[0007] To achieve this objective, the present application adopts the following technical solution: This application provides a method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder, the method comprising the following steps: The waste lithium iron phosphate electrode powder is slurried to obtain slurry material. The slurry material, concentrated sulfuric acid and oxidant are mixed and oxidized and leached. After the first solid-liquid separation, phosphate slag and lithium sulfate leachate are obtained. The lithium sulfate leaching solution is adjusted to a first pH, and then subjected to a second solid-liquid separation to obtain a first filtrate and a first filter residue. The first filtrate is adjusted to a second pH, and then subjected to a third solid-liquid separation to obtain a second filtrate and a second filter residue. The second filtrate is adjusted back to a third pH using an acid solution to obtain a refined lithium sulfate solution. The refined lithium sulfate solution is then concentrated to obtain lithium sulfate crystals and lithium precipitation mother liquor. The lithium precipitation mother liquor is reused in the oxidative leaching step. The phosphorus iron slag is subjected to a first-stage countercurrent washing and a second-stage countercurrent washing in sequence. The first-stage wash water generated from the first-stage countercurrent washing is recycled as a slurrying agent for slurrying waste lithium iron phosphate electrode powder, and the second-stage wash water generated from the second-stage countercurrent washing is recycled as makeup water for the first-stage countercurrent washing. The solid slag obtained from the second-stage countercurrent washing is discharged.

[0008] In some possible implementations, the solid content of the slurry is 25% to 40%.

[0009] In some possible implementations, the oxidant includes hydrogen peroxide.

[0010] In some possible implementations, the temperatures for adjusting the lithium sulfate leachate to a first pH, adjusting the first filtrate to a second pH, and adjusting the second filtrate to a third pH using an acid solution are independently 55°C to 75°C.

[0011] In some possible implementations, the regulator for adjusting the lithium sulfate leachate to a first pH includes lime milk.

[0012] In some possible implementations, the first pH is 3.5 to 5.

[0013] In some possible implementations, the adjuster for adjusting the first filtrate to the second pH includes sodium carbonate.

[0014] In some possible implementations, the second pH is 9 to 11.

[0015] In some possible implementations, the acid solution includes sulfuric acid.

[0016] In some possible implementations, the third pH is 5.5 to 7.5.

[0017] In some possible implementations, the periodically open-circuit discharged lithium mother liquor accounts for 0.5% to 5% of the total volume of lithium mother liquor.

[0018] The numerical range described in this application includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, this application will not exhaustively list the specific point values ​​included in the range.

[0019] Compared with the prior art, this application has the following advantages: (1) In this application, after the waste lithium iron phosphate electrode powder is slurried, the lithium is first leached out. Then, the pH is adjusted in two steps to remove iron and aluminum first, and then calcium and magnesium are removed. Finally, the pH is adjusted using an acid solution to avoid the precipitation of lithium carbonate under a highly alkaline environment and prevent lithium loss. The process water and waste acid generated during the recycling process are all recycled in a closed loop, which greatly reduces acid consumption and water consumption.

[0020] (2) The lithium sulfate obtained from the waste lithium iron phosphate electrode powder described in this application has a purity of over 99.92%, which meets the battery grade standard, and the lithium recovery rate can reach over 96.52%. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder as described in the embodiments of this application. Detailed Implementation

[0022] The technical solution of this application will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely to help understand this application and should not be regarded as specific limitations on this application.

[0023] The scope of this application can be defined by lower and upper limits. The selected lower and upper limits define the boundaries of a specific range. The range defined in this way can be defined by the inclusion or exclusion of endpoints. Any endpoint can be independently selected for inclusion or exclusion, and all lower and upper limits can be arbitrarily combined to form new ranges. That is, any lower limit can be combined with any upper limit to form a valid range. For example, if the ranges of 60~120 and 80~110 are listed for specific parameters, it should be understood that the ranges of 60~110 and 80~120 also fall within the scope of this application. In addition, if the minimum range values ​​1 and 2 are listed, and the maximum range values ​​3, 4 and 5 are also listed, then all ranges of 1~3, 1~4, 1~5, 2~3, 2~4 and 2~5 fall within the scope of this application. In this application, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0~5" means that all real numbers between 0 and 5 have been fully listed in this document, and "0~5" is only a shortened representation of this set of numerical combinations. When a parameter is expressed as an integer ≥2, it is equivalent to listing positive integers that meet the requirements, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. When a parameter is expressed as an integer selected from "2~10", it is equivalent to listing any integer among 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0024] In this application, "at least two combinations" refers to a quantity greater than or equal to 2 unless otherwise specified. For example, "any one or at least two combinations" means any one of the listed items, or a combination of at least two of the listed items formed in a manner that does not conflict and enables the implementation of this application. In this application, unless otherwise specified, the features or solutions corresponding to "and / or" cover any one of two or more related listed items, as well as any and all combinations of the related listed items. The arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a set consisting of A, B, and combinations of A and B, where "containing A and / or B" can be understood, depending on the context of the statement, as containing A, containing B, or simultaneously containing both A and B. In this application, "optional" means that the corresponding feature, component, step or solution is not necessary, that is, it is selected from either "with" or "without". If there are multiple "optional" limitations in a technical solution, unless otherwise specified and there is no technical conflict or mutual constraint, each "optional" limitation is independent and does not affect the others.

[0025] In this application, technical features or solutions described using open-ended terms such as "comprising" or "including" do not exclude additional non-conflicting elements beyond those listed unless otherwise specified. They are considered to disclose both closed-ended features or solutions consisting solely of the listed elements and open-ended features or solutions that may include additional non-conflicting elements beyond the listed elements. For example, if A includes a1, a2, and a3, unless otherwise specified, this means that A can consist only of a1, a2, and a3, or it can include other non-conflicting elements based on a1, a2, and a3. This corresponds to the disclosure of technical solutions such as "A consists of a1, a2, and a3," "A is selected from a1, a2, and a3," and "A not only includes a1, a2, and a3, but may also include other non-conflicting elements." All embodiments and optional embodiments of this application, unless otherwise specified and without technical conflict, can be combined to form new technical solutions, and such combinations fall within the scope of this application. The term "embodiment" as used in this application means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. Those skilled in the art can understand, explicitly and implicitly, that the embodiments described in this application can be combined with other embodiments that do not conflict with the technology.

[0026] In this application, the order in which the steps are written in the methods described in the various embodiments does not imply a strict order of execution. The actual order of execution of each step should be determined according to its function and possible internal logic. Unless otherwise specified, all steps in this application can be executed in the order in which they are written or in any order without technical conflict.

[0027] This application provides a method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder, the method comprising the following steps: The waste lithium iron phosphate electrode powder is slurried to obtain slurry material. The slurry material, concentrated sulfuric acid and oxidant are mixed and oxidized and leached. After the first solid-liquid separation, phosphate slag and lithium sulfate leachate are obtained. The lithium sulfate leaching solution is adjusted to a first pH, and then subjected to a second solid-liquid separation to obtain a first filtrate and a first filter residue. The first filtrate is adjusted to a second pH, and then subjected to a third solid-liquid separation to obtain a second filtrate and a second filter residue. The second filtrate is adjusted back to a third pH using an acid solution to obtain a refined lithium sulfate solution. The refined lithium sulfate solution is then concentrated to obtain lithium sulfate crystals and lithium precipitation mother liquor. The lithium precipitation mother liquor is reused in the oxidative leaching step. The phosphorus iron slag is subjected to a first-stage countercurrent washing and a second-stage countercurrent washing in sequence. The first-stage wash water generated from the first-stage countercurrent washing is recycled as a slurrying agent for slurrying waste lithium iron phosphate electrode powder, and the second-stage wash water generated from the second-stage countercurrent washing is recycled as makeup water for the first-stage countercurrent washing. The solid slag obtained from the second-stage countercurrent washing is discharged.

[0028] The first solid-liquid separation, the second solid-liquid separation, and the third solid-liquid separation in this application independently include filtration and / or pressure filtration.

[0029] The method described in this application does not limit the processing order of lithium sulfate leachate and ferrophosphate slag. The lithium sulfate leachate can be treated first, or the ferrophosphate slag can be subjected to primary countercurrent washing and secondary countercurrent washing first.

[0030] In the method described in this application, the waste lithium iron phosphate electrode powder is first subjected to pulping and oxidative acid leaching to leach out the lithium. Then, the lithium sulfate leaching solution is adjusted to the first pH to selectively precipitate and remove Fe and Al impurities. The first filtrate is adjusted to the second pH to remove Ca and Mg impurities. The pH is then adjusted back to the first pH by an acid solution to obtain a high-lithium, low-sodium refined lithium sulfate solution, thus avoiding the loss of impurities through colloidal encapsulation and lithium co-precipitation. Refined lithium sulfate solution is concentrated and crystallized under controlled temperature to directly produce lithium sulfate that meets battery-grade standards. All lithium precipitation mother liquor is returned to the oxidation acid leaching process for recycling. During long-term operation, a small amount of mother liquor is periodically discharged to control the accumulation of sodium salts in the system, thus stabilizing product purity. After two-stage countercurrent washing, the iron phosphate slag is enriched in the final solid slag for discharge or recycling. All the first wash water generated from the first-stage countercurrent washing is returned to the waste electrode powder slurry preparation section. The residual free acid in the first wash water partially replaces the addition of concentrated sulfuric acid and achieves closed-loop interception of trace lithium within the system, significantly reducing acid and water consumption. The washing water is utilized in stages without the discharge of acidic lithium-containing wastewater. Ultimately, this achieves a comprehensive resource recovery effect, including increased total lithium recovery rate, reduced auxiliary material consumption, stable production of battery-grade lithium sulfate, and a closed-loop three-phase cycle of process water, acid, and lithium.

[0031] In some embodiments, the solid content of the slurry is 25% to 40%, for example: 25%, 28%, 30%, 35% or 40%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] In some embodiments, the oxidant includes hydrogen peroxide.

[0033] In some embodiments, the temperatures for adjusting the lithium sulfate leachate to the first pH, adjusting the first filtrate to the second pH, and using an acid solution to adjust the second filtrate to the third pH are independently 55°C to 75°C, for example: 55°C, 60°C, 65°C, 70°C, or 75°C, etc., and are not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] In some embodiments, the adjusting agent for adjusting the lithium sulfate leachate to a first pH includes lime milk.

[0035] In some embodiments, the first pH is 3.5 to 5, for example: 3.5, 3.8, 4, 4.5 or 5, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0036] This application uses inexpensive lime milk instead of lithium salt alkaline agent to remove iron and aluminum impurities. Iron and aluminum impurities preferentially hydrolyze and precipitate at a pH of 3.5 to 5.0, during which the total amount of impurities is highest. Using low-cost lime milk avoids the large consumption of expensive lithium salts.

[0037] In some embodiments, the adjuster for adjusting the first filtrate to the second pH includes sodium carbonate.

[0038] In some embodiments, the second pH is 9 to 11, such as 9, 9.5, 10, 10.5 or 11, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] This application uses sodium carbonate to directionally precipitate Ca. 2+ Mg 2+ The calcium introduced by lime milk in the previous step is removed. The two-stage pH adjustment is used to remove impurities and avoid the simultaneous formation of a large amount of hydroxide colloids that encapsulate liquid-phase lithium salts, thereby reducing the loss of lithium entrainment in the filter residue.

[0040] In some embodiments, the acid solution includes sulfuric acid.

[0041] In some embodiments, the third pH is 5.5 to 7.5, for example: 5.5, 6, 6.5, 7 or 7.5, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0042] This application uses sulfuric acid to adjust the pH of the solution to near neutral to meet the feed requirements for subsequent lithium sulfate concentration and crystallization.

[0043] In some embodiments, the periodically open-circuit discharged lithium precipitation mother liquor accounts for 0.5% to 5% of the total volume of lithium precipitation mother liquor, for example: 0.5%, 1%, 2%, 4% or 5%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] In this application, during long-term circulation, a small amount of mother liquor, accounting for 0.5% to 5% of the volume of the lithium precipitation mother liquor, is periodically discharged through an open circuit to control the accumulation of sodium salts within the system. The timing can be 1 hour or 3 hours, and can be adjusted according to the composition of the system.

[0045] Example 1 This embodiment provides a method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder. A schematic diagram of the process flow is shown below. Figure 1 As shown, the method includes the following steps: Waste lithium iron phosphate electrode powder was slurried to obtain a slurry with a solid content of 35% (the slurrying agent used in the initial slurrying was water, and the slurrying agent used in subsequent slurries was the first-stage wash water generated from the subsequent first-stage countercurrent washing). The slurry, concentrated sulfuric acid, and hydrogen peroxide were mixed (the amount of concentrated sulfuric acid was 1.1 times the theoretical leaching amount of lithium, and the amount of hydrogen peroxide added was 1.2 times the amount required for complete oxidation). Oxidation leaching was carried out at 25°C, and the filtrate was filtered to obtain phosphate slag and lithium sulfate leachate. The pH of the lithium sulfate leaching solution was adjusted to 4 using lime milk at 65℃, and the solution was filtered to obtain a first filtrate and a first filter residue. The pH of the first filtrate was adjusted to 10.2 using sodium carbonate, and the solution was filtered to obtain a second filtrate and a second filter residue. The pH of the second filtrate was adjusted back to 6.5 using sulfuric acid solution to obtain a refined lithium sulfate solution. The refined lithium sulfate solution was concentrated by MVR evaporation, and lithium sulfate solid was precipitated by temperature-controlled crystallization. The solution was filtered to obtain lithium sulfate crystals and lithium precipitation mother liquor. The lithium precipitation mother liquor was reused in the oxidative leaching step. During the long-term circulation process, a small amount of mother liquor accounting for 2% of the volume of the lithium precipitation mother liquor was periodically discharged through an open circuit to control the accumulation of sodium salts in the system. The phosphorus iron slag is subjected to a first-stage countercurrent washing and a second-stage countercurrent washing in sequence. The first-stage wash water generated from the first-stage countercurrent washing is recycled as a slurrying agent for slurrying waste lithium iron phosphate electrode powder, and the second-stage wash water generated from the second-stage countercurrent washing is recycled as makeup water for the first-stage countercurrent washing. The solid slag obtained from the second-stage countercurrent washing is discharged.

[0046] Example 2 This embodiment provides a method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder. A schematic diagram of the process flow is shown below. Figure 1 As shown, the method includes the following steps: Waste lithium iron phosphate electrode powder was slurried to obtain a slurry with a solid content of 25% (the slurrying agent used in the initial slurrying was water, and the slurrying agent used in subsequent slurries was the first-stage wash water generated from the subsequent first-stage countercurrent washing). The slurry, concentrated sulfuric acid, and hydrogen peroxide were mixed (the amount of concentrated sulfuric acid was 1.1 times the theoretical leaching amount of lithium, and the amount of hydrogen peroxide added was 1.2 times the amount required for complete oxidation). Oxidation leaching was carried out at 25°C, and the filtrate was filtered to obtain phosphate slag and lithium sulfate leachate. The pH of the lithium sulfate leaching solution was adjusted to 5 using lime milk at 55℃, and the solution was filtered to obtain a first filtrate and a first filter residue. The pH of the first filtrate was adjusted to 11 using sodium carbonate, and the solution was filtered to obtain a second filtrate and a second filter residue. The pH of the second filtrate was adjusted back to 7.5 using sulfuric acid solution to obtain a refined lithium sulfate solution. The refined lithium sulfate solution was concentrated by MVR evaporation, and lithium sulfate solid was precipitated by temperature-controlled crystallization. The solution was filtered to obtain lithium sulfate crystals and lithium precipitation mother liquor. The lithium precipitation mother liquor was reused in the oxidative leaching step. During the long-term circulation process, a small amount of mother liquor accounting for 0.5% of the volume of the lithium precipitation mother liquor was periodically discharged to control the accumulation of sodium salts in the system. The phosphorus iron slag is subjected to a first-stage countercurrent washing and a second-stage countercurrent washing in sequence. The first-stage wash water generated from the first-stage countercurrent washing is recycled as a slurrying agent for slurrying waste lithium iron phosphate electrode powder, and the second-stage wash water generated from the second-stage countercurrent washing is recycled as makeup water for the first-stage countercurrent washing. The solid slag obtained from the second-stage countercurrent washing is discharged.

[0047] Example 3 This embodiment provides a method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder. A schematic diagram of the process flow is shown below. Figure 1 As shown, the method includes the following steps: Waste lithium iron phosphate electrode powder was slurried to obtain a slurry with a solid content of 40% (the slurrying agent used in the initial slurrying was water, and the slurrying agent used in subsequent slurries was the first-stage wash water generated from the subsequent first-stage countercurrent washing). The slurry, concentrated sulfuric acid, and hydrogen peroxide were mixed (the amount of concentrated sulfuric acid was 1.1 times the theoretical leaching amount of lithium, and the amount of hydrogen peroxide added was 1.2 times the amount required for complete oxidation). Oxidation leaching was carried out at 75°C, and the filtrate was filtered to obtain phosphorus iron slag and lithium sulfate leachate. The pH of the lithium sulfate leaching solution was adjusted to 3.5 using lime milk at 75℃, and the solution was filtered to obtain a first filtrate and a first filter residue. The pH of the first filtrate was adjusted to 9 using sodium carbonate, and the solution was filtered to obtain a second filtrate and a second filter residue. The pH of the second filtrate was adjusted back to 5.5 using sulfuric acid solution to obtain a refined lithium sulfate solution. The refined lithium sulfate solution was concentrated by MVR evaporation, and lithium sulfate solid was precipitated by temperature-controlled crystallization. The solution was filtered to obtain lithium sulfate crystals and lithium precipitation mother liquor. The lithium precipitation mother liquor was reused in the oxidative leaching step. During the long-term circulation process, a small amount of mother liquor accounting for 5% of the volume of the lithium precipitation mother liquor was periodically discharged through an open circuit to control the accumulation of sodium salts in the system. The phosphorus iron slag is subjected to a first-stage countercurrent washing and a second-stage countercurrent washing in sequence. The first-stage wash water generated from the first-stage countercurrent washing is recycled as a slurrying agent for slurrying waste lithium iron phosphate electrode powder, and the second-stage wash water generated from the second-stage countercurrent washing is recycled as makeup water for the first-stage countercurrent washing. The solid slag obtained from the second-stage countercurrent washing is discharged.

[0048] Example 4 The only difference between this embodiment and Embodiment 1 is that lime milk was used to adjust the pH of the lithium sulfate leachate to 3; all other conditions and parameters are exactly the same as in Embodiment 1.

[0049] Example 5 The only difference between this embodiment and Embodiment 1 is that lime milk was used to adjust the pH of the lithium sulfate leachate to 5.5; all other conditions and parameters are exactly the same as in Embodiment 1.

[0050] Example 6 The only difference between this embodiment and Embodiment 1 is that sodium carbonate is used to adjust the pH of the first filtrate to 8.5; all other conditions and parameters are exactly the same as in Embodiment 1.

[0051] Example 7 The only difference between this embodiment and Embodiment 1 is that sodium carbonate is used to adjust the pH of the first filtrate to 11.5; all other conditions and parameters are exactly the same as in Embodiment 1.

[0052] Example 8 The only difference between this embodiment and Embodiment 1 is that sulfuric acid solution is used to adjust the pH of the second filtrate to 5; all other conditions and parameters are exactly the same as in Embodiment 1.

[0053] Example 9 The only difference between this embodiment and Embodiment 1 is that sulfuric acid solution is used to adjust the pH of the second filtrate to 8; all other conditions and parameters are exactly the same as in Embodiment 1.

[0054] Comparative Example 1 The only difference between this comparative example and Example 1 is that sodium carbonate was directly used to adjust the pH of the lithium sulfate leachate to 10; all other conditions and parameters are exactly the same as in Example 1.

[0055] Performance testing: The purity of lithium sulfate was obtained and the lithium recovery rate was calculated for the test examples and comparative examples. The test results are shown in Table 1: Table 1 As can be seen from Table 1, from Examples 1 to 9, the purity of lithium sulfate obtained from the waste lithium iron phosphate electrode powder described in this application can reach over 97.2%, and the lithium recovery rate can reach over 93.23%. By adjusting the preparation conditions, the purity of the obtained lithium sulfate can reach over 99.92%, meeting the battery-grade standard, and the lithium recovery rate can reach over 96.52%.

[0056] A comparison of Examples 1 and 4-5 shows that in the method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder described in this application, adjusting the pH of the lithium sulfate leachate affects the recovery effect. Controlling the pH of the lithium sulfate leachate at 3.5-5 results in a better recovery effect. If the pH of the lithium sulfate leachate is too low, the removal of iron and aluminum is unsafe, leading to excessive iron and aluminum impurities in the subsequent product. Furthermore, these residual ions will precipitate again during subsequent concentration or pH adjustment, forming fine particles that are difficult to filter and contaminating the final product. If the pH of the lithium sulfate leachate is too high, some Li... + Initially, lithium is adsorbed and co-precipitated by newly generated Fe(OH)3 and Al(OH)3 colloids or entrained into the slag phase, leading to a significant increase in primary lithium loss.

[0057] A comparison of Examples 1 and 6-7 shows that in the method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder described in this application, adjusting the pH of the first filtrate affects the recovery effect. Controlling the pH of the first filtrate at 9-11 results in better recovery. If the pH of the first filtrate is too low, the calcium and magnesium precipitation reaction is incomplete, and residual Ca... 2+ Mg 2+ The content is too high, which cannot meet the stringent requirements for calcium and magnesium in battery-grade lithium sulfate. If the pH of the first filtrate is adjusted too high, it will lead to lithium precipitation and loss.

[0058] A comparison of Examples 1 and 8-9 shows that in the method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder described in this application, the pH of the second filtrate affects the recovery effect. Controlling the pH of the second filtrate between 5.5 and 7.5 results in better recovery. If the pH of the second filtrate is too low, excessive acid is used in the recovery process, introducing excess SO4. 2- Furthermore, corrosion of the evaporation equipment accelerates under acidic conditions. If the pH of the second filtrate is too high, trace amounts of residual Ca will be released during subsequent evaporation and concentration. 2+ Mg 2+ As the concentration increases, secondary precipitation will occur, leading to scaling and clogging of the evaporation equipment. At the same time, CO2 is easily absorbed under alkaline conditions, generating trace amounts of Li2CO3 precipitate, which affects the yield.

[0059] As can be seen from the comparison between Example 1 and Comparative Example 1, this application removes iron and aluminum first and then calcium and magnesium by adjusting the pH in two steps. The two-stage separation avoids the simultaneous generation of a large amount of hydroxide colloids that encapsulate liquid-phase lithium salts, thereby reducing the loss of lithium entrainment in the filter residue.

[0060] The applicant declares that the above description is only a specific implementation of this application, but the protection scope of this application is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application fall within the protection and disclosure scope of this application.

Claims

1. A method for preparing battery-grade lithium sulfate from waste lithium iron phosphate electrode powder, characterized in that, The method includes the following steps: The waste lithium iron phosphate electrode powder is slurried to obtain slurry material. The slurry material, concentrated sulfuric acid and oxidant are mixed and oxidized and leached. After the first solid-liquid separation, phosphate slag and lithium sulfate leachate are obtained. The lithium sulfate leaching solution is adjusted to a first pH, and then subjected to a second solid-liquid separation to obtain a first filtrate and a first filter residue. The first filtrate is adjusted to a second pH, and then subjected to a third solid-liquid separation to obtain a second filtrate and a second filter residue. The second filtrate is adjusted back to a third pH using an acid solution to obtain a refined lithium sulfate solution. The refined lithium sulfate solution is then concentrated to obtain lithium sulfate crystals and lithium precipitation mother liquor. The lithium precipitation mother liquor is reused in the oxidative leaching step and periodically discharged through an open circuit. The phosphorus iron slag is subjected to a first-stage countercurrent washing and a second-stage countercurrent washing in sequence. The first-stage wash water generated from the first-stage countercurrent washing is recycled as a slurrying agent for slurrying waste lithium iron phosphate electrode powder, and the second-stage wash water generated from the second-stage countercurrent washing is recycled as makeup water for the first-stage countercurrent washing. The solid slag obtained from the second-stage countercurrent washing is discharged.

2. The method as described in claim 1, characterized in that, The solid content of the slurry is 25%~40%; And / or, the oxidant includes hydrogen peroxide.

3. The method as described in claim 1 or 2, characterized in that, The temperatures for adjusting the lithium sulfate leachate to the first pH, adjusting the first filtrate to the second pH, and adjusting the second filtrate to the third pH using an acid solution are independently 55°C to 75°C.

4. The method according to any one of claims 1-3, characterized in that, The adjusting agent for adjusting the lithium sulfate leachate to the first pH includes lime milk.

5. The method according to any one of claims 1-4, characterized in that, The first pH is 3.5~5.

6. The method according to any one of claims 1-5, characterized in that, The pH adjuster for adjusting the first filtrate to the second pH includes sodium carbonate.

7. The method according to any one of claims 1-6, characterized in that, The second pH is 9-11.

8. The method according to any one of claims 1-7, characterized in that, The acid solution includes sulfuric acid.

9. The method according to any one of claims 1-8, characterized in that, The third pH is 5.5~7.

5.

10. The method according to any one of claims 1-9, characterized in that, The periodically discharged lithium precipitate mother liquor accounts for 0.5% to 5% of the total volume of lithium precipitate mother liquor.

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