Recycling method for positive electrode of spent lithium iron phosphate battery

Abstract

Provided in the present application is a recycling method for a positive electrode of a spent lithium iron phosphate battery, comprising the following steps: (1) obtaining spent lithium iron phosphate, and roasting the spent lithium iron phosphate under an oxygen-containing atmosphere to obtain a roasted product; (2) mixing the roasted product with an acid solution for leaching treatment, and then performing filtering to obtain a lithium-containing acid solution and a phosphorus-iron-containing filter residue, wherein the acid solution comprises at least a sulfuric acid; (3) mixing the phosphorus-iron-containing filter residue with a phosphoric acid solution for dissolution, and then performing filtering to obtain a first filtrate and a residue; and (4) mixing the first filtrate with water to obtain a mixed solution, subsequently performing a crystallization treatment, then performing filtering to obtain a filter residue and a second filtrate, and calcining the filter residue to obtain a product iron phosphate. The recycling method provided in the present application can improve the leaching rate of lithium, the removal rate of impurities, and the recycling rate of phosphorus and iron, and at the same time, obtain the product iron phosphate that can be used in the field of secondary batteries.

Description

A method for recycling the positive electrode of waste lithium iron phosphate batteries

[0001] This application claims priority to Chinese Patent Application No. 2024117421247, filed on November 29, 2024, entitled "A Method for Recycling the Positive Electrode of Waste Lithium Iron Phosphate Batteries", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of recycling technology, and in particular to a method for recycling the positive electrode of a waste lithium iron phosphate battery. Background Technology

[0003] Lithium iron phosphate (LiFePO4) batteries are rapidly gaining popularity in renewable energy storage for electric vehicles, hybrid vehicles, and smart grids due to their superior safety, relatively high energy density, long cycle life, and low cost. The recycling of LiFePO4 batteries is of great significance for environmental protection and resource recycling.

[0004] Currently, the main methods for recycling lithium iron phosphate (LFP) include direct regeneration, hydrometallurgical processes, and selective lithium extraction. Direct regeneration is a viable option due to its low cost, but it's only suitable for recycling low-impurity LFP products or substandard LFP materials; it cannot be used for LFP materials with high impurity content. Hydrometallurgical methods typically involve acid leaching and leachate purification to extract different elements. For example, inorganic acid salts such as sulfuric acid and phosphoric acid, or organic acids like acetic acid and oxalic acid, are used as leachate to recover LiFePO4 batteries. However, due to the carbon layer and binder covering the waste batteries, wet lithium leaching efficiency is low, and the recycling selectivity is limited. Selective lithium extraction technology has been well-developed. Its advantage lies in first extracting lithium and then separating it to obtain iron phosphate. While this improves lithium recovery, problems remain regarding impurity removal.

[0005] Therefore, there is a need to develop a simple and efficient method for recycling lithium iron phosphate, while converting the iron-phosphate product into high-performance battery-grade iron phosphate. Summary of the Invention

[0006] The purpose of this application is to provide a method for recycling the positive electrode of spent lithium iron phosphate batteries, so as to improve the lithium leaching rate, impurity removal rate, and iron phosphate recovery rate, while obtaining iron phosphate, a product that can be used in the field of secondary batteries. The specific technical solution is as follows:

[0007] This application provides a method for recycling the positive electrode of waste lithium iron phosphate batteries, which includes the following steps:

[0008] (1) Obtain waste lithium iron phosphate, and roast the waste lithium iron phosphate in an oxygen-containing atmosphere to obtain roasted product, wherein the roasting temperature T1 is 400°C to 800°C.

[0009] (2) The roasted material is mixed with an acid solution and leached at high temperature, and then filtered to obtain a lithium acid solution and a phosphorus-containing iron filter residue; wherein the acid solution includes at least sulfuric acid;

[0010] (3) The phosphorus-containing iron filter residue is mixed with phosphoric acid solution and dissolved, and then filtered to obtain the first filtrate and residue;

[0011] (4) The first filtrate is mixed with water to obtain a mixed solution, then crystallized, and then filtered to obtain a filter residue and a second filtrate. The filter residue is calcined to obtain the product iron phosphate.

[0012] In some embodiments of this application, in step (2), the acid solution contains H + The concentration of CO is 1.0 mol / L to 3.5 mol / L; and the leaching treatment temperature T2 is ≥ 80℃.

[0013] In some embodiments of this application, in step (2), the leaching temperature T2 is 80°C to 120°C.

[0014] In some embodiments of this application, in step (2), the acid solution comprises sulfuric acid, or the acid solution comprises sulfuric acid and hydrochloric acid.

[0015] In some embodiments of this application, in step (2), the acid solution includes sulfuric acid and hydrochloric acid, wherein the concentration of hydrochloric acid in the acid solution is C1 and the concentration of sulfuric acid is C2, satisfying: 0.2mol / L≤C1≤2.5mol / L, C2=(C0-C1) / 2.

[0016] In some embodiments of this application, 0.6 mol / L ≤ Cl ≤ 2 mol / L.

[0017] In some embodiments of this application, in step (2), the mass of the roasted material is m1 g, the volume of the acid solution is V1 L, and m1:V1 = (50 to 250):1.

[0018] In some embodiments of this application, the recycling method further includes the following steps:

[0019] (2') Add acid to the lithium-containing acid solution obtained in step (2), and then continue the leaching treatment using the lithium-containing acid solution; wherein the added acid is the same as the acid in the acid solution in step (2).

[0020] In some embodiments of this application, the number of moles of lithium in the roasted material is N1, the number of moles of added sulfuric acid is N2, and 0.95 × 0.5 × N1 ≤ N2 ≤ 1.05 × 0.5 × N1, wherein the number of moles of lithium in the roasted material is based on the number of moles of lithium in waste lithium iron phosphate.

[0021] In some embodiments of this application, the acid solution further includes hydrochloric acid, and the added hydrochloric acid is such that the concentration C1' of hydrochloric acid in the lithium-containing acid solution is maintained at 0.2 mol / L ≤ C1' ≤ 2.5 mol / L.

[0022] In some embodiments of this application, the added hydrochloric acid maintains the concentration C1' of hydrochloric acid in the lithium-containing acid solution at 0.6 mol / L ≤ C1' ≤ 2 mol / L.

[0023] In some embodiments of this application, in step (1), at least one of conditions 1 to 2 is satisfied:

[0024] Condition 1: The oxygen-containing atmosphere contains air or oxygen;

[0025] Condition 2: The calcination temperature T1 is 600℃ to 750℃ and the time t1 is 0.5h to 4h.

[0026] In some embodiments of this application, in step (3), at least one of conditions 3 to 4 is satisfied:

[0027] Condition 3: The concentration of the phosphoric acid solution C3 is 2 mol / L to 10 mol / L;

[0028] The mass of the phosphorus-containing iron filter residue is m2 g, and the volume of the phosphoric acid solution is V2 L, where m2:V2 = (50 to 250):1;

[0029] Condition 4: The dissolution temperature T3 is 20°C to 50°C.

[0030] In some embodiments of this application, in step (4), at least one of conditions 5 to 6 is satisfied:

[0031] Condition 5: The pH of the mixed solution is between 1 and 2;

[0032] Condition 6: The temperature T4 of the crystallization treatment is 80℃ to 110℃ and the time t4 is 2h to 6h.

[0033] In some embodiments of this application, the following steps are also included:

[0034] (5) Heat the second filtrate to make its concentration the same as that of the phosphoric acid solution in step (3), and then return it to the phosphoric acid solution to continue dissolving the phosphoric iron filter residue.

[0035] The beneficial effects of this application are:

[0036] 1. The recycling method provided in this application combines pyrometallurgical and hydrometallurgical processes. Air roasting decomposes carbon and organic matter in waste lithium iron phosphate, facilitating subsequent lithium extraction. Compared to traditional processes using inert gas roasting, this method is simpler, more energy-efficient, and more effective. Furthermore, roasting in air uses oxygen as an oxidant to burn Fe... 2+ Oxidized to Fe 3+ The calcination products Li3Fe2(PO4)3 and Fe2O3 were obtained. Then, lithium was leached into the liquid phase by combining dilute acid solution and high temperature conditions, while Fe and P remained in the solid phase, achieving solid-liquid separation of elements. Compared with the traditional use of oxidants such as hydrogen peroxide, ozone, and persulfate, this method is more cost-effective, reduces reagent consumption, and simplifies the process. In addition, the combination of pyrometallurgical and hydrometallurgical processes reduces the amount of acid and alkali used while increasing the lithium leaching rate, which is beneficial for Li recovery.

[0037] In addition, due to the total H+ in the acid solution + At a concentration of C0 and a leaching temperature of T2, the roasted product Li3Fe2(PO4)3 will generate the intermediate product H3PO4. H3PO4 reacts with Fe2(SO4)3 to form iron phosphate precipitate. H3PO4 also reacts with a small amount of Fe2O3 to form iron phosphate precipitate. Fe2(SO4)3 is mainly obtained from the reaction of Li3Fe2(PO4)3 with sulfuric acid, and a small amount is obtained from the reaction of Fe2O3 with sulfuric acid (the degree of reaction is low). Ultimately, this allows the elements Fe and P to be enriched as much as possible in the phosphorus-containing iron filter residue, so that the phosphorus-containing iron filter residue obtained from the leaching treatment can enter the subsequent process, thereby improving the recovery rate of Fe and P.

[0038] In this application, impurities such as Al and Ca in waste lithium iron phosphate are converted into oxides by air roasting. During the subsequent acid leaching lithium extraction, the impurities are more easily removed by immersing them in the liquid phase, which helps to improve the purity of the iron phosphate product obtained after subsequent iron phosphate crystallization. The impurity content in the final iron phosphate product meets at least the industrial grade requirements.

[0039] 2. In this application, since impurity elements are converted into oxides during the oxidative roasting process, when the acid solution is controlled as a mixed system (sulfuric acid + hydrochloric acid), by further controlling the concentration of hydrochloric acid, the effect of removing impurities with hydrochloric acid can be achieved in the system of this process, effectively removing impurities in the solid phase, and the impurity content in the iron phosphate product further meets the battery-grade requirements.

[0040] 3. In this process, phosphoric acid is used to dissolve the iron phosphate slag and then crystallize it. This process uses only water to adjust the pH of the mixed solution for crystallization, so that battery-grade iron phosphate with an iron-to-phosphorus ratio that meets the requirements of the battery industry can be obtained. Moreover, this process does not introduce other alkaline solutions and impurities, and the phosphoric acid can be recycled.

[0041] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0043] Figure 1 is a flowchart of Example 1;

[0044] Figure 2 is the XRD pattern of the calcined product in Example 1;

[0045] Figure 3 shows photographs of the roasted products at different roasting temperatures in Example 1 and Comparative Example b. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0047] In this application, the phosphorus-containing iron filter residue is a solid phase obtained by leaching and filtration, which mainly contains iron phosphate residue (FePO4) and a small amount of ferric oxide (Fe2O3).

[0048] This application provides a method for recycling the positive electrode of waste lithium iron phosphate batteries, which includes the following steps:

[0049] (1) Obtain waste lithium iron phosphate, and roast the waste lithium iron phosphate in an oxygen-containing atmosphere to obtain roasted product;

[0050] (2) The roasted material is mixed with an acid solution and leached at high temperature, and then filtered to obtain a lithium acid solution and a phosphorus iron filter residue; wherein the acid solution includes at least sulfuric acid;

[0051] (3) The phosphorus-containing iron filter residue is mixed with phosphoric acid solution and dissolved, and then filtered to obtain the first filtrate and residue;

[0052] (4) The first filtrate is mixed with water to obtain a mixed solution, then crystallized, and then filtered to obtain a filter residue and a second filtrate. The filter residue is calcined to obtain the product iron phosphate.

[0053] This application does not particularly limit the method of obtaining waste lithium iron phosphate. Exemplarily, waste lithium iron phosphate batteries are disassembled to obtain the positive electrode. The positive electrode current collector and the positive electrode material layer of the disassembled positive electrode are separated. The obtained positive electrode material layer is also the waste lithium iron phosphate. The waste lithium iron phosphate may contain, but is not limited to, lithium iron phosphate, organic matter, carbon, etc. This application does not limit the aforementioned organic matter and carbon. Exemplarily, the organic matter can be a binder in the positive electrode material layer, and the carbon can be a conductive agent in the positive electrode material layer.

[0054] The recycling method provided in this application combines pyrometallurgical and hydrometallurgical processes. Air roasting decomposes carbon and organic matter in waste lithium iron phosphate, facilitating subsequent lithium extraction and reducing the impact on the purity of the phosphorus-containing iron filter residue. Compared to traditional processes using inert gas roasting, this method is simpler, more energy-efficient, and more effective. Furthermore, roasting in air uses oxygen as an oxidant to burn Fe... 2+ Oxidized to Fe 3+ The lithium is then leached using a dilute acid solution, retaining as much Fe and P as possible in the solid phase. This method is more cost-effective and reduces reagent consumption compared to traditional methods using oxidants such as hydrogen peroxide, ozone, and persulfate, and the process is simpler. Furthermore, the combination of pyrometallurgical and hydrometallurgical processes reduces acid and alkali usage while increasing lithium leaching rates. Air roasting converts impurities such as Al and Ca in the waste lithium iron phosphate into oxides, which are then more easily removed during acid leaching, improving the purity of the iron phosphate crystals and meeting at least the impurity requirements for industrial-grade lithium iron phosphate. Afterward, the iron phosphate slag is dissolved in phosphoric acid and then crystallized. This process uses only water to adjust the pH of the mixed solution for crystallization, resulting in battery-grade iron phosphate with an iron-to-phosphorus ratio that meets battery industry requirements. Moreover, this process does not introduce other alkaline solutions or impurities, and the phosphoric acid is recyclable.

[0055] In this application, the molar ratio of iron to phosphorus in the product iron phosphate is (0.96 to 1):1, which meets the requirements for battery-grade products.

[0056] In some embodiments of this application, in step (1), the roasting of waste lithium iron phosphate in air may include the following reaction: 12LiFePO4+3O2→2Fe2O3+4Li3Fe2(PO4)3 Formula 1.

[0057] In some embodiments of this application, in step (1), condition 1 is satisfied: the oxygen-containing atmosphere contains air or oxygen. Introducing oxygen during the roasting process can decompose the carbon and organic matter in the waste lithium iron phosphate and convert the impurity elements into corresponding oxides so that they can be removed in step (2), which is beneficial to improving the purity of the final product, iron phosphate.

[0058] In some embodiments of this application, in step (1), condition 2 is satisfied: the calcination temperature T1 is 400℃ to 800℃, and the time t1 is 0.5h to 4h; for example, the calcination temperature T1 can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, or 800℃, or between any two of the above numbers. For example, the calcination time t1 can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, or 4h, or between any two of the above numbers. By adjusting the calcination temperature T1 and time t1 within the range of this application, it is beneficial to improve the purity of the final product, iron phosphate. The scheme of this application, when calcined within this temperature range, can obtain a red calcined product, indicating that the calcined product contains Fe2O3. Preferably, the calcination temperature is 600℃~750℃, and the calcination time is 2h~3h. In this process, by adjusting the temperature of oxidative roasting, a roasted product containing Li3Fe2(PO4)3 and Fe2O3 can be obtained, which provides the possibility for subsequent high-temperature leaching treatment and for achieving liquid-solid phase separation of Li, Fe, and P elements.

[0059] In some possible implementations, waste lithium iron phosphate is oxidized and roasted in air. This process needs to be carried out in an oxygen-containing atmosphere, using oxygen as the oxidant. The oxygen content in the air is sufficient to meet the requirements. The roasting temperature is 400℃-800℃, and the roasting time is 0.5h-4h. The waste lithium iron phosphate is placed in a high-temperature furnace, and the oxygen in the air is used to oxidize the Fe... 2+ Oxidized to Fe 3+ The reaction equation is shown in Equation 1. 1 mol of LiFePO4 is decomposed into 1 / 3 mol of Fe2O3 and 2 / 3 mol of Li3Fe2(PO4)3.

[0060] In some embodiments of this application, in step (2), H in the acid solution + The concentration C0 is 1.0 mol / L to 3.5 mol / L; and the leaching temperature T2 is ≥ 80℃, preferably 80℃ to 120℃. For example, H in the acid solution +The concentration C0 can be 1.0 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, or 3.5 mol / L, or any two of the above numbers. For example, the leaching temperature T2 can be 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, or 120℃, or any two of the above numbers. In this application, lithium is leached onto the calcined product at high temperature using an acid solution, by controlling the total H2 in the acid solution... + When the concentration C0 and the leaching temperature T2 are within the above range, Li is recovered in liquid phase, while Fe and P are first dissolved in the liquid phase by acid and then reacted to form solid iron phosphate slag for recovery. This method yields high Li content. + Leaching rate of lithium-containing acid solution (Li + The leaching rate reaches over 99%, and the high-yield phosphorus-containing iron filter residue (the phosphorus-containing iron filter residue yield reaches over 98%) improves the recovery and utilization rate of Li, Fe and P elements, as well as the purity of the solid and liquid phases.

[0061] In some embodiments of this application, in step (2), the leaching treatment time t2 is 2h to 6h, which is conducive to the full leaching of lithium.

[0062] In some embodiments of this application, in step (2), the acid solution includes sulfuric acid, which can react with the calcined product Li3Fe2(PO4)3 to generate water-soluble lithium sulfate and water-soluble intermediate products Fe(H2PO4)3 and Fe2(SO4)3 (see Equation 2). The intermediate product Fe(H2PO4)3 is further decomposed at high temperature to obtain iron phosphate slag and phosphoric acid (see Equation 3). Phosphoric acid can react with Fe2(SO4)3 to generate iron phosphate slag (see Equation 4). At the same time, phosphoric acid can further react with a small amount of calcined product Fe2O3 to obtain iron phosphate slag (see Equation 5). Iron phosphate is a solid phase. Then, through filtration, lithium sulfate exists in the lithium-containing acid solution, and iron phosphate slag exists in the solid phase of phosphorus-containing iron filter residue, thereby separating lithium elements from iron and phosphorus elements. A small amount of unreacted Fe2O3 is also present in the phosphorus-containing iron filter residue. That is, the phosphorus-containing iron filter residue mainly contains iron phosphate slag and a small amount of Fe2O3. The following is a detailed description:

[0063] In some embodiments of this application, in step (2), the acid solution includes sulfuric acid, the calcined product includes Li3Fe2(PO4)3 and Fe2O3, and the reaction of the calcined product with the acid solution includes the following reaction formula: 2Li3Fe2(PO4)3+6H2SO4→3Li2SO4+2Fe(H2PO4)3+Fe2(SO4)3 Formula 2.

[0064] In some embodiments of this application, the leaching temperature T2 is ≥ 80°C, and the intermediate product generated can undergo a decomposition reaction at this temperature. The decomposition reaction includes the following reaction formula: 2Fe(H2PO4)3→2FePO4+4H3PO4 (Formula 3).

[0065] Furthermore, another intermediate product of the acid leaching solution, Fe2(SO4)3, reacts directly with product H3PO4 under the same conditions where Fe(H2PO4)3 decomposes, to generate solid iron phosphate slag. The reaction is shown in Equation 4: Fe2(SO4)3 + 2H3PO4 → 2FePO4 + 3H2SO4 Equation 4.

[0066] In some embodiments of this application, in step (2), a portion of the Fe2O3 in the calcined product continues to react with phosphoric acid (obtained by decomposition of intermediate product) to obtain solid iron phosphate slag, including reaction formula 5: Fe2O3+2H3PO4→2FePO4+3H2O Formula 5.

[0067] In some embodiments of this application, the overall reaction in step (2) includes the following equation: Fe2O3 + 2Li3Fe2(PO4)3 + 3H2SO4 → 3Li2SO4 + 6FePO4 4+ Formula 6 for 3H2O.

[0068] As can be seen from Equation 6, the roasted product (including Li3Fe2(PO4)3 and Fe2O3) obtained after roasting can be acid-leached at high temperature to allow Li element to enter the liquid phase in the form of lithium sulfate, while iron phosphate slag (FePO4) exists in the solid phase.

[0069] In this process, the calcined product selectively leaches Li as lithium sulfate in a dilute acid system (see Equation 2), while Fe and P exist in solid form as much as possible (i.e., they will be present in the filter residue). The dilute acid system can be sulfuric acid or a mixture of sulfuric acid and hydrochloric acid, or other suitable acid systems. The sulfuric acid acts as a solvent for lithium leaching, and the total H₂ in the solution is... + The CO concentration is maintained between 1.0 mol / L and 3.5 mol / L, and the leaching temperature is ≥80℃ (preferably 80℃~120℃). This leaching process is achieved by controlling the total H₂. +The concentration and leaching temperature are within this range, at which the lithium leaching rate reaches 99%, and the yield of phosphorus-containing iron filter residue reaches over 98%. In some embodiments of this application, in step (2), the acid solution includes sulfuric acid and hydrochloric acid. Specifically, the acid solution contains sulfuric acid and hydrochloric acid. Sulfuric acid can react with the calcined material to generate water-soluble lithium sulfate and water-insoluble phosphorus-containing iron filter residue. Hydrochloric acid can react with the oxides formed after the impurity elements are calcined to generate water-soluble chlorides, such as aluminum chloride and calcium chloride. Then, through filtration, lithium sulfate, aluminum chloride, and calcium chloride are present in the lithium-containing acid solution, and the phosphorus-containing iron filter residue is a solid phase. This allows the lithium element to be separated from the iron and phosphorus elements, and further removes the impurity elements in the solid phase, improving the purity of the phosphorus-containing iron filter residue and providing the possibility of obtaining battery-grade product iron phosphate in the future.

[0070] In some embodiments of this application, in step (2), the acid solution includes sulfuric acid and hydrochloric acid, and the calcined product (i.e., the calcined material) includes Li3Fe2(PO4)3, Fe2O3, Al2O3, and CaO. The reaction between the calcined product and the acid solution includes the following reaction formula:

[0071] The reactions of Li3Fe2(PO4)3 and Fe2O3 with acid solution are shown in Equation 6.

[0072] The impurity phase Al2O3 reacts with CaO as shown in Equations 7 and 8: Al2O3 + 6HCl = 2AlCl3 + 3H2O (Equation 7); CaO + 2HCl = CaCl2 + H2O (Equation 8).

[0073] Hydrochloric acid is used to convert solid Al2O3 and CaO into liquid AlCl3 and CaCl2, thereby removing impurities.

[0074] In some embodiments of this application, in step (2), the acid solution includes sulfuric acid and hydrochloric acid, with the concentration of hydrochloric acid in the acid solution being C1 and the concentration of sulfuric acid being C2, satisfying: 0.2 mol / L ≤ C1 ≤ 2.5 mol / L, C2 = (C0 - C1) / 2; preferably, 0.6 mol / L ≤ C1 ≤ 2 mol / L. For example, the concentration of hydrochloric acid, C1, can be 0.2 mol / L, 0.5 mol / L, 0.7 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.7 mol / L, 2 mol / L, 2.2 mol / L, or 2.5 mol / L, or any two of the above values. When the concentrations of sulfuric acid and hydrochloric acid in the acid solution are within the above ranges, sulfuric acid is mainly used for lithium leaching, and hydrochloric acid is mainly used for impurity removal, which can reduce the content of impurity elements in the iron phosphate filter residue, thereby facilitating the obtaining of high-purity iron phosphate.

[0075] In some embodiments of this application, in step (2), the mass of the calcined material is m1 g, the volume of the acid solution is V1 L, and m1:V1 = (50 to 250):1. For example, m1:V1 can be 50:1, 75:1, 100:1, 125:1, 150:1, 175:1, 200:1, 225:1, or 250:1, or any two of the above ratios. By adjusting the value of m1:V1 within the above range, high Li can be obtained. + The high leaching rate of lithium-containing acid solution and the high yield of phosphorus-containing iron filter residue improve the recovery and utilization rate of different elements.

[0076] In some embodiments of this application, the recovery method further includes the following steps: (2') adding acid to the lithium-containing acid solution obtained in step (2), and then continuing the leaching treatment using the lithium-containing acid solution; wherein the added acid is the same as the acid in the acid solution in step (2). This achieves full utilization of the acid solution and full recovery of lithium.

[0077] In some embodiments of this application, the molar number of lithium elements in the calcined product is N1, and the molar number of sulfuric acid added is N2, where 0.95 × 0.5 × N1 ≤ N2 ≤ 1.05 × 0.5 × N1. For example, 2N2 can be 0.95N1, 0.96N1, 0.97N1, 0.98N1, 0.99N1, N1, 1.01N1, 1.02N1, 1.03N1, 1.04N1, or 1.05N1, or any two of the above numbers. That is, the amount of sulfuric acid added is substantially the same as the amount of sulfuric acid consumed during the reaction.

[0078] In some embodiments of this application, the acid solution further includes hydrochloric acid, which is added to maintain the concentration C1' of hydrochloric acid in the lithium-containing acid solution at 0.2 mol / L ≤ C1' ≤ 2.5 mol / L; preferably, 0.6 mol / L ≤ C1' ≤ 2 mol / L. For example, the concentration C1' of hydrochloric acid in the lithium-containing acid solution can be 0.2 mol / L, 0.5 mol / L, 0.7 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.7 mol / L, 2 mol / L, 2.2 mol / L, or 2.5 mol / L, or any two of the above values. That is, the concentration of hydrochloric acid in the lithium-containing acid solution is substantially the same as the concentration of hydrochloric acid in the original acid solution, and the amount of sulfuric acid added is also substantially the same as the amount of sulfuric acid consumed, resulting in the concentration of acid in the lithium-containing acid solution being substantially the same as the concentration of acid in the acid solution, thereby allowing it to be used for lithium leaching treatment to achieve full recovery of lithium.

[0079] In this process, lithium is selectively leached with Li2SO4, while iron and phosphorus mainly exist in the forms of Fe(H2PO4)3, Fe2(SO4)3, and Fe2O3. Based on the characteristics of Fe(H2PO4)3, it decomposes into FePO4 and H3PO4 at high temperatures. The generated H3PO4 can continue to react with Fe2(SO4)3 and some Fe2O3 to generate FePO4 (iron phosphate slag). After filtration, phosphorus-containing iron filter residue and lithium-containing acid solution are obtained. The phosphorus-containing iron filter residue enters the subsequent process to obtain the product iron phosphate. To reuse the product after acid leaching and roasting of lithium-containing acid solution, since some sulfuric acid is consumed during the lithium extraction process (see general equation 6), it is necessary to add the acid consumed by lithium to the obtained lithium-containing acid solution (approximately 0.5 mol of sulfuric acid is added to 1 mol of waste lithium iron phosphate LiFePO4). At the same time, for the mixed acid system (i.e., acid solution containing sulfuric acid and hydrochloric acid), it is necessary to ensure that the concentration of hydrochloric acid in the system is appropriate in each cycle to remove impurities. Then, the lithium-containing acid solution is recycled to continue leaching lithium. This is also a process of lithium enrichment, achieving full recovery of lithium.

[0080] In some embodiments of this application, in step (3), the phosphorus-containing iron filter residue is dissolved in phosphoric acid.

[0081] In some embodiments of this application, in step (3), condition 3 is satisfied: the concentration C3 of the phosphoric acid solution is 2 mol / L to 10 mol / L; for example, the concentration C3 of the phosphoric acid solution can be 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, or 10 mol / L, or between any two of the above numbers. The mass of the phosphorus-containing iron filter residue is m2 g, and the volume of the phosphoric acid solution is V2 L, m2:V2 = (50 to 250):1; for example, m2:V2 can be 50:1, 75:1, 100:1, 125:1, 150:1, 175:1, 200:1, 225:1, or 250:1, or between any two of the above ratios. Phosphoric acid dissolves the ferric phosphate residue in the ferric phosphate filter residue. A small amount of ferric oxide in the ferric phosphate filter residue also dissolves into the phosphoric acid, thus increasing the recovery rate. A portion remains in the residue, which is then filtered to obtain Fe-containing iron. 3+ PO4 3- The first filtrate is then mixed with water to adjust the pH, and then subjected to high-temperature crystallization in step (4) to obtain the product ferric phosphate. By adjusting the concentration of the phosphoric acid solution C3 and the value of m2:V2 within the above range, phosphoric acid and phosphorus-containing iron filter residue can be dissolved to improve the element recovery rate.

[0082] In some embodiments of this application, in step (3), condition 4 is satisfied: the dissolution temperature T3 is between 20°C and 50°C. For example, the dissolution temperature T3 can be 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, or 50°C, or any two of the above numbers. By adjusting the dissolution temperature within the above range, the phosphorus-containing iron filter residue can be fully dissolved, thereby improving the element recovery rate.

[0083] This application does not impose any particular limitation on the dissolution time, as long as the purpose of this application can be achieved. For example, the dissolution time can be from 1 hour to 8 hours.

[0084] In some embodiments of this application, in step (3), satisfying conditions 3 and 4 above enables the phosphorus-containing iron filter residue to be fully dissolved, thereby improving the element recovery rate.

[0085] In some embodiments of this application, in step (4), condition 5 is satisfied: the pH of the mixed solution is between 1 and 2; for example, the pH of the mixed solution can be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2, or between any two of the above numbers. By adjusting the pH of the mixed solution within the above range, it is more conducive to the crystallization of ferric phosphate to obtain the product ferric phosphate.

[0086] In some embodiments of this application, in step (4), condition 6 is met: the crystallization treatment temperature T4 is 80°C to 110°C, and the time t4 is 2h to 6h. For example, the crystallization treatment temperature T4 can be 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, or 110°C, or any two of the above numbers. For example, the crystallization treatment time t4 can be 2h, 3h, 4h, 5h, or 6h, or any two of the above numbers. By adjusting the crystallization treatment temperature T4 and time t4 within the above ranges, it is more conducive to the crystallization of ferric phosphate to obtain the product ferric phosphate.

[0087] In some embodiments of this application, in step (4), satisfying conditions 5 and 6 above is more conducive to the crystallization of ferric phosphate to obtain the product ferric phosphate.

[0088] In some embodiments of this application, in step (4), the ferric phosphate obtained by crystallization is usually ferric phosphate dihydrate. The ferric phosphate dihydrate is calcined to remove the water, thereby obtaining ferric phosphate for use in the battery field. This application does not have any particular restrictions on the calcination temperature and time, as long as the purpose of this application can be achieved. For example, the calcination temperature T5 is 500°C to 800°C and the time t5 is 2h to 6h.

[0089] In this process, the phosphorus-containing iron filter residue is treated through steps (3) and (4) to obtain a ferric phosphate product that meets the requirements. For example, specifically: the obtained phosphorus-containing iron filter residue is dissolved in phosphoric acid, with a phosphoric acid concentration (C3) of 2 mol / L-10 mol / L, a solid-liquid ratio of 50 g / L-250 g / L, and a leaching and dissolution temperature of 20℃-50℃. The purpose is to dissolve the phosphorus-containing iron filter residue as much as possible, especially the ferric phosphate residue, and then filter it to obtain a product containing Fe. 3+ PO4 3- The acid solution is subjected to high-temperature crystallization. The pH of the acid solution during crystallization needs to be controlled. Since phosphoric acid is a moderately strong acid, the pH of the solution can be adjusted to 1.0-2.0 using water. At the same time, no other alkaline solution is introduced in this process, and the water can be removed by evaporation and leaching can be achieved. Then, ferric phosphate is crystallized at high temperature. The reaction temperature T4 is 80℃-110℃ and the time t4 is 2h-6h. After filtration, ferric phosphate dihydrate is obtained. Ferric phosphate dihydrate can be calcined at high temperature to obtain industrial-grade or battery-grade ferric phosphate. The calcination temperature is 500℃-800℃ and the time is 2h-6h.

[0090] In some embodiments of this application, the method further includes the following steps: (5) heating the second filtrate to make its concentration the same as that of the phosphoric acid solution, and then adding it to the phosphoric acid solution to continue dissolving the phosphorus-containing iron filter residue. Specifically, the second filtrate contains phosphoric acid, and the water introduced in step (4) is evaporated and then used to dissolve the phosphorus-containing iron filter residue, thereby achieving full utilization of the phosphoric acid solution and reducing recycling costs.

[0091] Example

[0092] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0093] Test methods and equipment:

[0094] X-ray diffraction (XRD) test:

[0095] The calcined material was analyzed using an X-ray diffractometer to obtain XRD patterns. The XRD tests were performed using a Rigaku Uitima IV instrument with a copper target and Kα rays.

[0096] Example 1

[0097] Figure 1 is a flowchart of Example 1, and the specific steps are as follows:

[0098] 10g of waste lithium iron phosphate was placed in a muffle furnace and roasted at a high temperature (T1 = 600℃) for t1 = 2h in air atmosphere to obtain the roasted product. Carbon and organic matter were removed, while impurities such as Al and Ca existed in the form of oxides, which were removed in the subsequent acid leaching process. The lithium content of the waste lithium iron phosphate was 3.9% by mass. The roasted product contained Fe2O3 and Li3Fe2(PO4)3. The XRD characterization of the roasted product is shown in Figure 2, which shows the characteristic peaks of Fe2O3 and Li3Fe2(PO4)3.

[0099] Place 10g of the roasted material m1 in H + In an acid solution with a total concentration of C0 = 2 mol / L, where the concentration of H2SO4 is C2 = 0.7 mol / L and the concentration of HCl is C1 = 0.6 mol / L, and the volume of the acid solution is 0.1 L, the solid-liquid ratio of the feed is m1:V1 = 100:1. The mixture is then stirred in a water bath at T2 = 85℃ for t2 = 4 h. After the reaction is complete, the mixture is filtered to obtain a filter residue and a filtrate. The filter residue is a phosphorus-containing iron filter residue, and the filtrate is a lithium-containing filtrate, which includes LiCl and Li2SO4. Here, the purpose of sulfuric acid is to leach Li, and hydrochloric acid is used to remove impurities. This single step achieves both lithium leaching and impurity removal. Lithium was selectively leached in the filtrate as lithium sulfate, with a lithium content of 3880 mg / L. The calculated lithium leaching rate was 99.5% (3880 / 3900 = 99.5%). Iron and phosphorus existed in solid form in the filter residue (i.e., phosphorus-containing iron filter residue), with a residue mass of 9.5 g. The yield of the phosphorus-containing iron filter residue was 98.8% (yield calculation: 10 g of lithium iron phosphate, excluding 0.39 g of lithium, leaves 9.61 g, 9.5 / 9.61 = 98.8%). The filter residue was tested for impurity content to ensure that impurities were removed before iron phosphate crystallization. The test results showed that the impurity content in the filter residue was 216 mg / kg Al, 67 mg / kg Ca, and 55 mg / kg Na, achieving the dual effect of Li leaching and impurity removal.

[0100] Add 0.028 mol of sulfuric acid (the concentration of sulfuric acid in the lithium acid solution, C2', is 0.7 mol / L) and 0.02 mol of hydrochloric acid (the concentration of hydrochloric acid in the lithium acid solution, C1', is 0.6 mol / L) to the obtained lithium acid solution, and continue the lithium leaching process. Repeat this cycle five times until the lithium content in the lithium acid solution is enriched to 19300 mg / L.

[0101] The obtained filter residue was placed in phosphoric acid with a concentration of C3 = 6 mol / L. The mass of the phosphoric acid-containing iron filter residue was m2 = 9.5 g, and the volume of the phosphoric acid solution was V2 = 0.095 L, with a m2:V2 ratio of 100:1. The solution was dissolved and filtered at room temperature (T3 = 25℃) to obtain a first filtrate and residue. The first filtrate contained FePO4. The pH of the first filtrate was adjusted to 1.5 using deionized water, and the solution was placed in a water bath at T4 = 95℃ for t4 = 2 h for high-temperature crystallization. The solution was then filtered to obtain a dihydrate ferric phosphate filter residue and a second filtrate, which was also the dilute phosphoric acid filtrate. The dihydrate ferric phosphate was calcined at T5 = 700℃ for t5 = 2 h to obtain anhydrous ferric phosphate. The Fe:P ratio of the anhydrous ferric phosphate was 0.99:1 (meeting the requirements for the iron-to-phosphorus ratio of battery grade). The impurity content met the chemical industry standard (ferric phosphate for batteries, HG / T 4701-2021). The elemental content of the product ferric phosphate is shown in Table 1. The second filtrate is evaporated to remove water and reaches an initial phosphoric acid concentration of 6 mol / L. The process of dissolving the phosphorus-containing iron filter residue continues, and this process is repeated to achieve the recycling of crystalline iron phosphate.

[0102] Table 1

[0103] Example 2

[0104] 10g of waste lithium iron phosphate was placed in a muffle furnace and roasted at a high temperature (T1 = 700℃) for t1 = 2h in air atmosphere to obtain the roasted product. Carbon and organic matter were removed, while impurities such as Al and Ca existed in the form of oxides, which were removed in the subsequent acid leaching process. The lithium content of the waste lithium iron phosphate was 3.9% by mass. The XRD characterization of the roasted product was similar to that shown in Figure 2.

[0105] Place 10g of the roasted material m1 in H + In an acid solution with a total concentration of C0 = 1.2 mol / L, where the concentration of H2SO4 is C2 = 0.4 mol / L and the concentration of HCl is C1 = 0.4 mol / L, and the volume of the acid solution is 0.1 L, the solid-liquid ratio of the feed is m1:V1 = 100:1, and then the mixture is stirred in a water bath at T2 = 90℃ for t2 = 4 h. After the reaction is completed, the mixture is filtered to obtain filter residue and filtrate. The filter residue is also known as phosphorus-iron filter residue, and the filtrate is also known as lithium-containing filtrate. Selective acid leaching of lithium resulted in lithium sulfate in the filtrate, with a lithium content of 3873 mg / L, yielding a lithium leaching rate of 99.3%. Iron and phosphorus existed as solids in the filter residue, with a residue mass of 9.42 g. The phosphorus-iron filter residue yield was 98.0% (yield calculation: 10 g of lithium iron phosphate contains 0.39 g of lithium, leaving 9.61 g, 9.42 / 9.61 = 98.0%). Impurity content testing of the filter residue revealed that the impurities were Al content of 1573 mg / kg, Ca content of 257 mg / kg, and Na content of 198 mg / kg. While efficient leaching of Li was achieved, not all impurities could be removed.

[0106] Add 0.028 mol of sulfuric acid (the concentration of sulfuric acid in the lithium acid solution, C2', is 0.4 mol / L) and 0.021 mol of hydrochloric acid (the concentration of hydrochloric acid in the lithium acid solution, C1', is 0.4 mol / L) to the obtained lithium acid solution, and continue the lithium leaching process. Repeat this cycle five times until the lithium content in the lithium acid solution is enriched to 19289 mg / L.

[0107] The obtained filter residue was placed in phosphoric acid with a concentration of C3 = 5 mol / L. The mass of the phosphoric iron filter residue was m2 = 9.42 g, and the volume of the phosphoric acid solution was V2 = 0.045 L, with a m2:V2 ratio of 209:1. The solution was dissolved and filtered at room temperature (T3 = 25°C) to obtain the first filtrate and residue. The pH of the first filtrate was adjusted to 1.0 using deionized water, and the solution was placed in a water bath at T4 = 95°C for t4 = 2 h for high-temperature crystallization. The solution was then filtered to obtain a dihydrate ferric phosphate filter residue and a second filtrate, which was also the dilute phosphoric acid filtrate. The dihydrate ferric phosphate was calcined at T5 = 600°C for t5 = 2 h to obtain anhydrous ferric phosphate. The Fe:P ratio of the anhydrous ferric phosphate was 0.99:1, meeting the battery-grade standard; however, the impurity content was higher than in Example 1, meeting the industrial-grade standard. The second filtrate was evaporated to remove water to achieve an initial phosphoric acid concentration of 5 mol / L. The process of dissolving the phosphoric iron filter residue was then repeated to achieve a crystalline ferric phosphate cycle.

[0108] Example 3

[0109] Example 3 differs from Example 2 in that it uses H + An acid solution with a total concentration of C0 = 2.2 mol / L was prepared, wherein the concentration of sulfuric acid was C2 = 0.9 mol / L and the concentration of HCl was C1 = 0.4 mol / L; the rest was the same as in Example 2. In this example, the leaching rate of Li reached 99.4%, and the yield of phosphorus-containing iron filter residue was 98.6%. However, the impurities in the filter residue were not completely removed, with Al content of 1378 mg / Kg, Ca content of 203 mg / Kg, and Na content of 178 mg / Kg. The Al content exceeded the standard for battery-grade iron phosphate. The Fe:P ratio in the anhydrous iron phosphate product was 0.98:1, which meets the requirements for the iron-phosphorus ratio of battery-grade products.

[0110] Through Examples 3 and 2, it can be explained that the total H + A CO concentration between 1.0 mol / L and 3.5 mol / L can ensure a Li leaching rate of over 99%, but the removal of impurities Al, Ca, and Na, as well as the total H₂, are affected. + The concentration C0 is not directly related.

[0111] Example 4

[0112] Example 4 differs from Example 2 in that it uses H+ An acid solution with a total concentration of C0 = 3 mol / L was prepared, wherein the concentration of sulfuric acid was C2 = 0.9 mol / L and the concentration of hydrochloric acid was C1 = 1.2 mol / L; the rest was the same as in Example 2. In this example, the leaching rate of Li reached 99.3%, the yield of phosphorus-containing iron filter residue was 98.4%, and the impurities in the filter residue were Al content of 251 mg / Kg, Ca content of 75 mg / Kg, and Na content of 47 mg / Kg, which met the battery-grade standard (HG / T 4701-2021); the Fe:P ratio in the anhydrous iron phosphate product was 0.97:1, which met the battery-grade iron-phosphorus ratio requirements.

[0113] By comparing Example 4 with Example 3, the concentration of hydrochloric acid was increased, which shows that hydrochloric acid has a purification effect. The purification effect is better when the concentration of hydrochloric acid is between 0.6 mol / L and 2.0 mol / L.

[0114] Example 5

[0115] The difference between Example 5 and Example 1 is that H is used. + An acid solution with a total concentration of C0 = 3.4 mol / L was used, wherein the concentration of sulfuric acid was C2 = 0.8 mol / L and the concentration of hydrochloric acid was C1 = 1.8 mol / L; the reaction temperature in the water bath was T2 = 120℃, and the rest was the same as in Example 1. In this example, the leaching rate of Li reached 99.4%, the yield of phosphorus-containing iron filter residue was 98.1%, and the impurities in the filter residue were Al content of 241 mg / Kg, Ca content of 68 mg / Kg, and Na content of 42 mg / Kg. The anhydrous iron phosphate obtained after acid leaching and recrystallization had a Fe:P ratio of 0.99:1, which meets the battery grade standard, and the impurity content also meets the battery grade standard HG / T4701-2021 (see Table 3).

[0116] Example 6

[0117] 10g of waste lithium iron phosphate was placed in a muffle furnace and roasted at a high temperature of 650℃ (T1 = 2.5h) in air atmosphere to obtain the roasted product, in which carbon and organic matter were removed. The lithium content of the waste lithium iron phosphate was 3.9% by mass.

[0118] Place 10g of the roasted material m1 in H +In an acid solution with a total concentration of C0 = 2.8 mol / L, where the concentration of H2SO4 is C2 = 1.4 mol / L and the concentration of HCl is C1 = 0 mol / L, the volume of the acid solution is 0.1 L, and the solid-liquid ratio is m1:V1 = 100:1. The mixture is then stirred in a water bath at T2 = 90℃ for t2 = 4 h. After the reaction is complete, the mixture is filtered to obtain a filter residue and a filtrate. The filter residue contains phosphorus and iron, and the filtrate contains lithium. The purpose of the sulfuric acid here is to leach out Li. Lithium was selectively leached in the form of lithium sulfate in the filtrate, with a lithium content of 3865 mg / L, resulting in a lithium leaching rate of 99.1%. Iron and phosphorus existed in solid form in the filter residue, with a residue mass of 9.43 g. The solid yield of the phosphorus-iron filter residue was 98.1% (yield exceeding 98%). The filter residue was tested for impurities, and the results showed that the impurities in the filter residue were Al content of 2013 mg / Kg, Ca content of 327 mg / Kg, and Na content of 262 mg / Kg. Li leaching was achieved, but the filter residue contained a relatively high amount of impurities.

[0119] Add 0.028 mol of sulfuric acid (the concentration of sulfuric acid C2' in the lithium acid solution is 1.4 mol / L) to the obtained lithium acid solution and continue the lithium leaching process. Repeat this cycle five times until the lithium content in the lithium acid solution is enriched to 17800 mg / L.

[0120] The obtained filter residue was placed in phosphoric acid with a concentration of C3 = 8 mol / L. The mass of the phosphoric acid-containing iron filter residue was m2 = 9.43 g, and the volume of the phosphoric acid solution was V2 = 0.15 L, with a m2:V2 ratio of 63. The solution was dissolved and filtered at room temperature (T3 = 25℃) to obtain the first filtrate and residue. The pH of the first filtrate was adjusted to 2.0 using deionized water, and the solution was placed in a water bath at T4 = 95℃ for t4 = 4 h for high-temperature crystallization. The solution was then filtered to obtain iron phosphate dihydrate filter residue and the second filtrate, which was also the dilute phosphoric acid filtrate. The iron phosphate dihydrate was calcined at T5 = 750℃ for t5 = 3 h to obtain anhydrous iron phosphate. The Fe:P ratio of the anhydrous iron phosphate was 0.99:1, meeting battery-grade standards, but with a relatively high impurity content. The elemental content of the iron phosphate product is shown in Table 2. The dilute acid filtrate was evaporated to remove water, achieving an initial phosphoric acid concentration of 8 mol / L. The process of dissolving the phosphoric acid-containing iron filter residue was then repeated to achieve the cyclic crystallization of iron phosphate.

[0121] Table 2

[0122] Example 7

[0123] Similar to Example 1, the difference lies in the solid-liquid ratio of the feed, m1:V1 = 250:1, where the volume of the acid solution, V1, remains constant, while the mass of the calcined product, m1, changes accordingly. The lithium leaching rate is 98.4%, and the phosphorus-containing iron filter residue yield is 98.9%. The impurities in the filter residue are Al content of 244.8 mg / Kg, Ca content of 76.3 mg / Kg, and Na content of 63.5 mg / Kg, achieving both Li leaching and impurity removal, meeting battery-grade standards (HG / T 4701-2021). The anhydrous iron phosphate product has an Fe:P ratio of 0.99:1, meeting the battery-grade iron-phosphorus ratio requirements.

[0124] Example 8

[0125] Similar to Example 1, the difference lies in the solid-liquid ratio of the feed, m1:V1 = 50:1, where the volume of the acid solution, V1, remains constant, while the mass of the calcined product, m1, changes accordingly. The lithium leaching rate is 98.6%, the phosphorus-containing iron filter residue yield is 99.2%, and the impurities in the filter residue are Al content of 238.4 mg / Kg, Ca content of 80.6 mg / Kg, and Na content of 73.5 mg / Kg. This achieves the dual effect of Li leaching and impurity removal, meeting battery-grade standards (HG / T 4701-2021). The anhydrous iron phosphate product has a Fe:P ratio of 0.98:1, meeting the battery-grade iron-phosphorus ratio requirements.

[0126] Through Examples 1, 7 and 8, it can be seen that when the solid-liquid ratio m1:V1 of the calcined material and the acid solution is satisfied (50 to 250):1 during the mixed leaching process, good results can be achieved.

[0127] Example 9-1

[0128] 10g of waste lithium iron phosphate was placed in a muffle furnace and roasted at a high temperature of 700℃ (T1) for 2 hours in an air atmosphere to obtain the roasted product. This process removed carbon and organic matter, while impurities such as Al and Ca existed in the form of oxides, which were removed in the subsequent acid leaching process. The lithium content in the waste lithium iron phosphate was 3.9% by mass.

[0129] Place 10g of the roasted material m1 in H +In an acid solution with a total concentration of C0 = 3.8 mol / L, where the concentration of H2SO4 is C2 = 1.3 mol / L and the concentration of HCl is C1 = 1.2 mol / L, and the volume of the acid solution is 0.1 L, the solid-liquid ratio of the feed is m1:V1 = 100:1, and then the mixture is stirred in a water bath at T2 = 90℃ for t2 = 4 h. After the reaction is completed, the mixture is filtered to obtain filter residue and filtrate. The filter residue is also known as phosphorus-iron filter residue, and the filtrate is also known as lithium-containing filtrate. The lithium content in the filtrate was 3869 mg / L, and the calculated lithium leaching rate was 99.2%. The mass of the filter residue was 3.4 g, meaning that iron and phosphorus were in solid form in the filter residue. The phosphorus-iron filter residue yield was 35.4% (10 g of lithium iron phosphate, excluding 0.39 g of lithium, left 9.61 g, 3.4 / 9.61 = 35.4%). The loss of iron and phosphorus in the filtrate was about 65%. This example only achieved effective leaching of Li, but the loss of iron and phosphorus was relatively large.

[0130] Example 9-2

[0131] 10g of waste lithium iron phosphate was placed in a muffle furnace and roasted at a high temperature of 700℃ (T1) for 2 hours in an air atmosphere to obtain the roasted product. This process removed carbon and organic matter, while impurities such as Al and Ca existed in the form of oxides, which were removed in the subsequent acid leaching process. The lithium content in the waste lithium iron phosphate was 3.9% by mass.

[0132] Place 10g of the roasted material m1 in H + In an acid solution with a total concentration of C0 = 0.6 mol / L, where the concentrations of H2SO4 are C2 = 0.2 mol / L and HCl are C1 = 0.2 mol / L, and the volume of the acid solution is 0.1 L, the solid-liquid ratio of the feed is m1:V1 = 100:1. The mixture is then stirred in a water bath at T2 = 90℃ for t2 = 4 h. After the reaction is complete, the mixture is filtered to obtain a filter residue and a filtrate. The filter residue contains phosphorus and iron, and the filtrate contains lithium. The lithium content in the filtrate is 2373 mg / L, and the calculated lithium leaching rate is only 60.8%. This indicates that H2SO4... + When the ion concentration is below 1.0 mol / L, lithium leaching is incomplete. The filter residue mass is 8.85 g, meaning that iron and phosphorus are in solid form in the filter residue, with a yield of 92.1%. The loss of iron and phosphorus is relatively small. This filter residue may contain a large amount of Li3Fe2(PO4)3 that has not undergone acid leaching reaction.

[0133] Example 9-3

[0134] 10g of waste lithium iron phosphate was placed in a muffle furnace and roasted at a high temperature of 700℃ (T1) for 2 hours in an air atmosphere to obtain the roasted product. This process removed carbon and organic matter, while impurities such as Al and Ca existed in the form of oxides, which were removed in the subsequent acid leaching process. The lithium content in the waste lithium iron phosphate was 3.9% by mass.

[0135] Place 10g of the roasted material m1 in H + In an acid solution with a total concentration of C0 = 2.8 mol / L, where the concentration of H2SO4 is C2 = 0.8 mol / L and the concentration of HCl is C1 = 1.2 mol / L, and the volume of the acid solution is 0.1 L, the solid-liquid ratio of the feed is m1:V1 = 100 g / L. The mixture is then stirred in a water bath at T2 = 60℃ for t2 = 4 h. After the reaction is complete, the mixture is filtered to obtain filter residue and filtrate. The filter residue contains phosphorus and iron, and the filtrate contains lithium. The lithium content in the filtrate is 3736.2 mg / L, and the calculated lithium leaching rate is 95.8%. The mass of the obtained filter residue is 2.6 g, meaning that iron and phosphorus are in solid form in the filter residue, with a recovery rate of 27.0%, and 73% of iron and phosphorus are lost in the filtrate. This indicates that if the reaction temperature T2 ≥ 80℃ cannot be reached, iron and phosphorus loss will occur (i.e., iron and phosphorus elements will be present in greater quantities in the liquid phase).

[0136] Comparative Example a

[0137] Waste lithium iron phosphate was roasted in a muffle furnace at a high temperature of 700℃ (T1) for 2 hours under air atmosphere to obtain roasted material. This process removed carbon and organic matter, while impurities such as Al and Ca existed in oxide form, which were removed in a subsequent acid leaching process. The lithium content in the waste lithium iron phosphate was 3.9% by mass.

[0138] Place 10g of the roasted material m1 in H + In an acid solution with a total concentration of C0 = 1.4 mol / L, where the HCl concentration is C1 = 1.4 mol / L and the volume of the acid solution is 0.1 L, the solid-liquid ratio of the feed is m1:V1 = 100 g / L. The mixture is then stirred in a water bath at T2 = 90℃ for t2 = 4 h. After the reaction is complete, the mixture is filtered to obtain filter residue and filtrate. The filter residue contains phosphorus and iron, and the filtrate contains lithium. The lithium content in the filtrate is 2652 mg / L, and the calculated lithium leaching rate is only 68%. Furthermore, the mass of the obtained filter residue is 9.13 g, meaning that iron and phosphorus are present in solid form in the filter residue, with a recovery rate of 95.00%.

[0139] This indicates that using only hydrochloric acid to react with the roasted material will prevent Li from being leached out. This is because hydrochloric acid is extremely unstable and easily volatilizes at high temperatures, resulting in incomplete lithium leaching.

[0140] Comparative Example b

[0141] The difference from Example 1 is that the calcination temperature is 900°C.

[0142] Waste lithium iron phosphate was roasted in a muffle furnace at a high temperature (T1 = 900℃) for t1 = 2 hours in an air atmosphere to obtain the roasted product. This process removed carbon and organic matter, while impurities such as Al and Ca existed in the form of oxides. The lithium content in the waste lithium iron phosphate was 3.9% by mass.

[0143] During the experiment, it was found that if the temperature was too high during the oxidative roasting process, such as 900℃, the roasted product turned blackish-purple, unlike the red roasted product produced in the examples, as shown in Figure 3. The left side shows the roasted product from Example 1, which is red, while the right side shows the roasted product from Comparative Example b, which is blackish-purple. This may be because at excessively high temperatures, Li3Fe2(PO4)3 decomposes into lithium phosphate and phosphorus pentoxide. Some of the phosphorus pentoxide further reacts with ferric oxide to form FePO4, and the remaining phosphorus pentoxide volatilizes, leading to a loss of phosphorus (P), affecting the yield of iron phosphate, and reducing the mass of the roasted material.

[0144] The acid leaching reaction was then carried out under the same conditions as in Example 1. It was found that the lithium leaching rate was only 90%, the filter residue mass was 8.9g, and the yield was 92.6%. Although the yield was high, it may still contain a lot of solid-phase roasted products.

[0145] After phosphate dissolution and high-temperature crystallization (experimental conditions were the same as in Example 1), the product obtained had a relatively high impurity content, with Al content at 2500 ppm, Ca content at 417 ppm, and Na content at 318 ppm. This may be because Al, Ca, and other elements generated more stable insoluble oxides during the oxidative roasting process, making it difficult for Al, Ca, and other elements to dissolve into the ferric phosphate product during acid leaching.

[0146] The preparation parameters and performance tests for each embodiment and comparative example are shown in Table 3.

[0147] Table 3 Note: " / " in Table 3 indicates that the corresponding parameter does not exist.

[0148] As can be seen from the above examples and comparative examples:

[0149] 1) Comparing Examples 1 to 6 with Examples 9-1 to 9-3, it can be seen that when the total H+ concentration in the acid solution is 1.0 to 3.5 mol / L and the reaction temperature in the water bath reaches above 80°C, the system can effectively leach Li+, and the yield of the filter residue, i.e., the phosphorus-containing iron filter residue, is also relatively high. This is because under the reaction conditions, the acid reacts more thoroughly with the material after roasting lithium iron phosphate, resulting in a better lithium leaching effect, and the high-temperature decomposition reaction of Fe2(HPO4)3 is also carried out more thoroughly.

[0150] Meanwhile, a comparison with Comparative Example a also shows that the acid in the acid system of this process can be a mixed acid or sulfuric acid; if hydrochloric acid is used alone, its high-temperature volatility makes it impossible to guarantee the total H+ content in the system during the reaction. + The concentration remained within a suitable range, leading to Li+ Incomplete leaching reaction, Li + As the leaching rate decreases, the yield of iron and phosphorus solids in the filter residue also decreases slightly.

[0151] 2) By comparing Examples 1, 4, and 5 with Examples 2 and 3, it can be seen that when a mixed acid system is used, the total H in the system... + With both concentration and reaction temperature met, adjusting the concentration of hydrochloric acid can significantly improve the impurity removal effect. This is because, compared to sulfuric acid, hydrochloric acid reacts more readily with impurity oxides to form soluble salts in the environment of this process.

[0152] Additionally, it should be noted that Examples 9-1 to 9-3 and Comparative Example a, due to their Li + The leaching rate and / or the yield of phosphorus-containing iron filter residue were low, so no impurity content test was performed.

[0153] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0154] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for recycling the positive electrode of a waste lithium iron phosphate battery, comprising the following steps: (1) Obtain waste lithium iron phosphate, and roast the waste lithium iron phosphate in an oxygen-containing atmosphere to obtain roasted product, wherein the roasting temperature T1 is 400°C to 800°C. (2) The roasted material is mixed with an acid solution and leached at high temperature, and then filtered to obtain a lithium acid solution and a phosphorus-containing iron filter residue; wherein the acid solution includes at least sulfuric acid; (3) The phosphorus-containing iron filter residue is mixed with phosphoric acid solution and dissolved, and then filtered to obtain the first filtrate and residue; (4) The first filtrate is mixed with water to obtain a mixed solution, then crystallized, and then filtered to obtain a filter residue and a second filtrate. The filter residue is calcined to obtain the product iron phosphate.

2. The recycling method according to claim 1, wherein, In step (2), the acid solution contains H + The concentration C0 ranges from 1.0 mol / L to 3.5 mol / L; and, The leaching treatment temperature T2 is ≥ 80℃.

3. The recycling method according to claim 2, wherein, In step (2), the leaching temperature T2 is 80°C to 120°C.

4. The recycling method according to any one of claims 1 to 3, wherein, In step (2), the acid solution includes sulfuric acid; or, The acid solution includes sulfuric acid and hydrochloric acid.

5. The recycling method according to claim 4, wherein, In step (2), the acid solution includes sulfuric acid and hydrochloric acid, and the concentration of hydrochloric acid in the acid solution is C1 and the concentration of sulfuric acid is C2, satisfying: 0.2mol / L≤C1≤2.5mol / L, C2=(C0-C1) / 2; Preferably, 0.6 mol / L ≤ Cl ≤ 2 mol / L.

6. The recycling method according to claim 1, wherein, In step (2), the mass of the calcined material is m1g, and the volume of the acid solution is V1L, where m1:V1 = (50 to 250):

1.

7. The recycling method according to claim 1, further comprising the following steps: (2') Add acid to the lithium acid solution obtained in step (2), and then continue the leaching treatment using the lithium acid solution; The acid added is the same as the acid in the acid solution in step (2).

8. The recycling method according to claim 7, wherein, The number of moles of lithium in the calcined product is N1, and the number of moles of sulfuric acid added is N2, with the formula 0.95 × 0.5 × N1 ≤ N2 ≤ 1.05 × 0.5 × N1.

9. The recycling method according to claim 8, wherein, The acid solution also includes hydrochloric acid, and the added hydrochloric acid maintains the concentration C1' of hydrochloric acid in the lithium-containing acid solution at 0.2mol / L≤C1'≤2.5mol / L; Preferably, the added hydrochloric acid maintains the concentration C1' of hydrochloric acid in the lithium-containing acid solution at 0.6 mol / L ≤ C1' ≤ 2 mol / L.

10. The recycling method according to claim 1, wherein, In step (1), at least one of conditions 1 to 2 is satisfied: Condition 1: The oxygen-containing atmosphere contains air or oxygen; Condition 2: The calcination temperature T1 is 600℃ to 750℃ and the time t1 is 0.5h to 4h.

11. The recycling method according to claim 1, wherein, In step (3), at least one of conditions 3 to 4 is satisfied: Condition 3: The concentration of the phosphoric acid solution C3 is 2 mol / L to 10 mol / L; The mass of the phosphorus-containing iron filter residue is m2g, and the volume of the phosphoric acid solution is V2L, where m2:V2 = (50 to 250):1; Condition 4: The dissolution temperature T3 is 20°C to 50°C.

12. The recycling method according to claim 1, wherein, In step (4), at least one of conditions 5 to 6 is satisfied: Condition 5: The pH of the mixed solution is between 1 and 2; Condition 6: The temperature T4 of the crystallization treatment is 80℃ to 110℃ and the time t4 is 2h to 6h.

13. The recycling method according to claim 1, wherein, It also includes the following steps: (5) Heat the second filtrate to make its concentration the same as that of the phosphoric acid solution in step (3), and then return it to the phosphoric acid solution to continue dissolving the phosphoric iron filter residue.

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