A method for regenerating battery-grade iron phosphate from lithium extraction residue of waste lithium iron phosphate

Abstract

The application discloses a method for regenerating battery-grade iron phosphate from lithium-extracted residue of waste lithium iron phosphate, and belongs to the field of lithium battery material recycling and utilization. The method is innovative in that the lithium-extracted residue of waste lithium iron phosphate with high aluminum content is regenerated into battery-grade iron phosphate through a three-step continuous separation process of selective leaching, selective precipitation and selective dissolution recrystallization, so as to realize low-cost deep separation of Fe and Al in the lithium-extracted residue of waste lithium iron phosphate with high aluminum content. According to the concentrations of soluble iron, aluminum and phosphorus elements and in combination with relevant thermodynamic data, the optimal pH condition required for regulating and controlling the selective precipitation process can be accurately calculated, a large amount of experimental cost and time in the early stage can be saved, and the accuracy and efficiency of aluminum removal are greatly improved, so the method has wide adaptability.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery recycling, specifically relating to a method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue. Background Technology

[0002] With the rapid development of the electric vehicle and energy storage industries, lithium iron phosphate (LFP) batteries have been widely used due to their advantages of high safety, long cycle life and low cost. However, the lifespan of LFP batteries is usually only 5 to 10 years, resulting in a large number of waste batteries. These batteries pose environmental risks of heavy metal pollution and contain high-value resources such as lithium, iron and phosphorus. Therefore, the green and efficient recycling of waste LFP batteries has become an important issue for resource sustainability and environmental protection.

[0003] Among the various recycling processes for waste LFP batteries, selective lithium extraction (LES) hydrometallurgical technology is highly favored due to its efficient and economical lithium recovery pathway. This process preferentially separates lithium through controlled leaching while retaining iron and phosphorus in the solid phase, forming "lithium extraction slag" (LES), which is mainly composed of iron phosphate and mixed with impurities such as carbon, aluminum, and copper. Currently, the process of regenerating high-value-added lithium salts from lithium-containing leachates is relatively mature. However, the resource utilization of LES, especially for the regeneration of battery-grade iron phosphate that can be used as a cathode precursor, faces a significant challenge: the deep removal of aluminum impurities. During battery dismantling, the aluminum foil of the cathode is difficult to completely separate, resulting in LES often containing 1%-6% aluminum. In subsequent hydrometallurgical processing, aluminum is further removed in the form of Al. 3+ Form enters the solution, Al 3+ with Fe 3+ The chemical properties of aluminum phosphate and iron phosphate are highly similar. In particular, the precipitation equilibrium constants of aluminum phosphate and iron phosphate are extremely close, which severely limits the separation efficiency of traditional precipitation processes. Although trace aluminum doping (mass fraction <0.05%) can improve the performance of LiFePO4 / C materials, excessive aluminum residue (>500 ppm) will seriously damage the electrochemical performance of regenerated iron phosphate and the finally synthesized lithium iron phosphate materials.

[0004] Existing technologies target AI 3+ / Fe 3+ All separation routes have significant drawbacks: solvent extraction utilizes Fe 3+ It easily forms FeCl4 - Due to the properties of complexes, Fe is preferentially extracted. 3+However, its core objective is to remove iron rather than separate aluminum, and it also causes phosphorus loss. This method has high extractant costs and is difficult to back-extract, making it unsuitable for deep aluminum removal. The strong alkali treatment method utilizes the dual nature of aluminum; while selectively dissolving aluminum with a strong alkali, it also destroys the olivine structure of iron phosphate, leading to the loss of iron and phosphorus resources and the introduction of new impurities, significantly increasing recovery costs. The fluorine complexation method utilizes the reaction of fluoride ions with Al... 3+ A stable complex is formed, enabling selective precipitation of ferric phosphate dihydrate. However, the subsequent complex defluorination steps increase process costs and environmental risks. The iron powder reduction-precipitation method first uses iron powder to reduce Fe... 3+ Restored to Al 3+ Fe with significantly different properties 2+ Separation is then achieved through precipitation. However, this method consumes a large amount of iron powder and subsequent oxidant, resulting in excessively high costs and hindering industrial application. The high-temperature precise pH control method broadens the reaction range by increasing the reaction temperature. 3+ / Fe 3+ The method utilizes the difference in pH window for precipitation to achieve selective precipitation, but the conditions are harsh, the operating window is narrow, and the industrial stability is poor.

[0005] Therefore, existing technologies still lack a new process that is mild, easy to operate, environmentally friendly, cost-controllable, and highly efficient in the deep removal of aluminum from lithium extraction slag and the regeneration of battery-grade iron phosphate. Summary of the Invention

[0006] The purpose of this invention is to provide a method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue, thereby solving the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for regenerating battery-grade lithium iron phosphate from waste lithium iron phosphate residue includes the following steps: 1) Mix waste lithium iron phosphate residue with acid solution at a liquid-to-solid mass ratio of (2-10):1, and react thoroughly at 15-70℃ for a period of time to obtain a mixed slurry. The acid solution contains H... + The molar ratio of aluminum in the lithium extraction slag is (100-3):1; 2) Determine the concentrations of soluble iron, soluble aluminum, and soluble phosphorus in the mixed slurry obtained in step 1). If the concentration of soluble phosphorus in the mixed slurry is lower than the sum of the contents of soluble iron and soluble aluminum, add phosphate to adjust the concentration of soluble phosphorus to be equal to the sum. 3) Based on the precipitation equilibrium thermodynamic data of iron, aluminum and phosphorus, a theoretical relationship model between aluminum precipitation efficiency and initial pH value is constructed. According to the concentration of each element measured in step 2) and the theoretical relationship model, the pH value when the aluminum precipitation efficiency is zero is obtained. 4) Continuously add alkaline solution to the mixed slurry obtained in step 2) to adjust the pH value of the system to the pH value obtained in step 3), and then carry out a selective precipitation reaction at a reaction temperature of 15-70℃. After the reaction is completed, filter out the solid product and wash it repeatedly to obtain the first leaching residue. 5) Place the first leaching residue obtained in step 4) in an acid solution and heat it in a high-temperature water bath to carry out a selective dissolution-recrystallization reaction. After the reaction is completed, filter out the solid product, and repeatedly wash and dry it to obtain the second leaching residue. The liquid-solid mass ratio of the acid solution to the first leaching residue is (2-10):1, and the molar ratio of H+ in the acid solution to iron in the leaching residue is (0-2):1. 6) The second leaching residue obtained in step 5) is placed in a high-temperature oxygen-containing environment for calcination to obtain iron phosphate material.

[0008] Preferably, the steps for constructing the theoretical relationship model include: 31) Based on the hydrolysis constants of Al3+ at the current reaction temperature and the ionization constants of phosphoric acid at the current reaction temperature, a distribution coefficient model of phosphate ions and aluminum ions under different pH conditions was constructed. 32) Based on the concentration of soluble aluminum obtained in step 2), the adjusted concentration of soluble phosphorus, the solubility product constant of AlPO4·2H2O, and the distribution coefficient model, construct a theoretical relationship model between aluminum precipitation efficiency and the initial pH value of the reaction.

[0009] The calculation of the distribution coefficient model is shown in the following formulas (1) and (2): In the above formula, The distribution coefficient of phosphate ions. K is the distribution coefficient of aluminum ions. an (n=1, 2, 3) are the ionization constants of phosphoric acid at each stage, K bn (n=1, 2, 3, 4) are the hydrolysis constants of aluminum ions at each stage; The theoretical relationship between the aluminum precipitation efficiency and the initial pH value of the reaction is calculated according to the following formula (3): In the formula, This refers to the concentration of soluble aluminum ions. The concentration of soluble phosphate ions after adjustment in step 2) is given, in mol / L and K. sp-Al It is the solubility product constant of AlPO4·2H2O.

[0010] Preferably, the pH value at which the aluminum precipitation efficiency is zero in step 3) is obtained by calculating the aluminum precipitation efficiency at different pH values ​​using a theoretical relationship model between the aluminum precipitation efficiency and the initial pH value of the reaction, plotting a curve with pH value on the horizontal axis and aluminum precipitation efficiency on the vertical axis, and reading the intersection of the curve with the horizontal axis.

[0011] Preferably, the acid solution in step 1) is one, two or three of H2SO4, H3PO4 and HCl, the reaction time is 30-180 min and the stirring rate is 0-1000 r / min.

[0012] Preferably, the phosphate in step 2) is one or more of sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, ammonium phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

[0013] Preferably, the alkaline solution in step 4) is one or more of ammonia water, sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution, the selective precipitation reaction time is 30-180 min, and the stirring rate is 0-1000 r / min.

[0014] Preferably, the acid solution in step 5) is one or both of H2SO4 and H3PO4, the selective dissolution-recrystallization reaction temperature is 70-95℃, the reaction time is 30-240 min, the stirring rate is 0-1000 r / min, the drying temperature is 60-80℃, and the drying time is 12-24 h.

[0015] Preferably, in step 6), the high-temperature calcination temperature of the second leaching residue is 500-900℃, the calcination atmosphere is air or pure oxygen, and the calcination time is 2-6 h.

[0016] Preferably, the washing method for the solid product in steps 4) and 5) is as follows: repeatedly pulping and filtering with an acidic solution with a pH of 0.5-3, and then repeatedly pulping and filtering with a large amount of deionized water until the conductivity of the washing solution is less than 500 μS / cm.

[0017] The beneficial effects of this invention are as follows: 1. The present invention provides a method for regenerating battery-grade iron phosphate from waste lithium iron phosphate slag. It innovatively uses a three-step continuous separation process of "selective leaching - selective precipitation - selective dissolution and recrystallization" to regenerate waste lithium iron phosphate slag with high aluminum content into battery-grade iron phosphate with an aluminum content of less than 300 ppm. This achieves deep removal of aluminum and efficient regeneration of battery-grade iron phosphate, solving the core problem of high aluminum residue restricting the resource utilization of lithium phosphate slag. 2. In the selective leaching reaction stage of this invention, based on the thermodynamic property that aluminum residues are more soluble than ferric phosphate, highly selective leaching of aluminum is achieved under precisely controlled acidic conditions, with only a very small amount of iron leaching along with it, minimizing iron loss; in the selective precipitation reaction stage, Fe... 3+ Compared to Al 3+ Thermodynamic properties that make it easier to preferentially form amorphous iron phosphate precipitate: By controlling the pH and reaction temperature of the system, most of the soluble iron and phosphorus in the upstream process slurry precipitate in the form of amorphous iron phosphate, while only a very small amount of aluminum and phosphorus co-precipitate in the form of amorphous aluminum phosphate, significantly improving the iron yield and reducing the amount of aluminum entrained; In the selective dissolution and recrystallization reaction, relying on the stronger recrystallization tendency of iron dihydrate than aluminum dihydrate, by controlling the high temperature water bath conditions and the acidity of the system, most of the iron in the first leaching residue preferentially forms FePO4·2H2O precipitate through the dissolution-recrystallization process, while the crystallization of aluminum is inhibited, thereby achieving the deep removal of aluminum and the directional enrichment of iron step by step; 3. By measuring the concentrations of soluble iron, aluminum, and phosphorus elements in the mixed slurry and combining relevant thermodynamic data, this invention can accurately calculate the optimal pH conditions required for the selective precipitation process, which greatly improves the accuracy of aluminum removal and precipitation efficiency, ensuring that iron and phosphorus elements are maximized in precipitation and enrichment while aluminum elements are maximized in retention, thereby solving the problem of iron and phosphorus element loss during selective leaching. 4. The method of this invention is applicable to complex lithium extraction slag with different carbon, iron, aluminum and phosphorus contents. When the raw material composition changes, it is only necessary to calculate and adjust according to the iron, aluminum and phosphorus element concentrations in the actual filtrate to quickly determine the optimal pH conditions required for selective precipitation process. It provides a precise aluminum removal solution for lithium extraction slag with different sources and compositions, without the need to conduct a large number of exploratory experiments, saving experimental resources, manpower and time costs, and improving process flexibility and industrial adaptability. 5. This invention addresses the differences in the physicochemical properties of iron and aluminum species in waste lithium iron phosphate residue. Without introducing additional strong complexing agents such as fluorides, aluminum removers, or reducing agents, it achieves deep removal of aluminum through mild operating conditions, appropriate acidity control, conventional water bath temperature, rapid reaction rate, low reagent consumption, and precise reaction kinetic control. This also avoids complex subsequent impurity removal processes and potential secondary pollution, resulting in good economic benefits and environmental friendliness, and has broad prospects for industrial application. Attached Figure Description

[0018] Figure 1 This is a process flow diagram of a method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to the present invention. Figure 2 The graph shows the relationship between the theoretical precipitation efficiency of iron and aluminum elements and pH value in Example 1. Figure 3 The graph shows the relationship between the theoretical precipitation efficiency of iron and aluminum and pH value in Example 2. Figure 4 The graph shows the relationship between the theoretical precipitation efficiency of iron and aluminum elements and pH value in Example 3. Figure 5 The XRD patterns of the first leaching residue in Examples 1, 2, and 3 are shown. Figure 6 The XRD pattern of the product obtained from the Al-P simulation system in Experimental Example 4; Figure 7 The XRD pattern of the product obtained from the Al-P simulation system in Experiment Example 4 after high-temperature calcination; Figure 8 The XRD patterns of the second leaching residues in Examples 1, 2, 3, Comparative Examples 1, 2, and 3 are shown. Figure 9 The image shows the XRD pattern of the product obtained from the Al-P system in Experimental Example 7. Detailed Implementation

[0019] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description. In the following embodiments, reagents and instruments not specifically mentioned are commercially available, and experimental operations not specifically mentioned are performed according to the manufacturer's instructions or conventional techniques in the art. Unless otherwise defined, all professional and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein can be applied to the present invention; the endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered as specifically disclosed herein.

[0020] This invention provides a method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue, comprising the following steps: 1) The waste lithium iron phosphate lithium extraction residue and acid solution are mixed at a liquid-solid mass ratio of (2-10):1, and after being fully reacted at a temperature of 15-70℃ for a period of time, a mixed slurry is obtained, wherein the molar ratio of H+ in the acid solution to aluminum in the lithium extraction residue is (100-3):1. 2) Determine the concentrations of soluble iron, soluble aluminum, and soluble phosphorus in the mixed slurry obtained in step 1). If the concentration of soluble phosphorus in the mixed slurry is lower than the sum of the contents of soluble iron and soluble aluminum, add phosphate to adjust the concentration of soluble phosphorus to be equal to the sum. 3) Based on the precipitation equilibrium thermodynamic data of iron, aluminum and phosphorus, a theoretical relationship model between aluminum precipitation efficiency and initial pH value is constructed. According to the concentration of each element measured in step 2) and the theoretical relationship model, the pH value when the aluminum precipitation efficiency is zero is obtained. 4) Continuously add alkaline solution to the mixed slurry obtained in step 2) to adjust the pH value of the system to the pH value obtained in step 3), and then carry out a selective precipitation reaction at a reaction temperature of 15-70℃. After the reaction is completed, filter out the solid product and wash it repeatedly to obtain the first leaching residue. 5) Place the first leaching residue obtained in step 4) in an acid solution and heat it in a high-temperature water bath to carry out a selective dissolution-recrystallization reaction. After the reaction is completed, filter out the solid product, and repeatedly wash and dry it to obtain the second leaching residue. The liquid-solid mass ratio of the acid solution to the first leaching residue is (2-10):1, and the molar ratio of H+ in the acid solution to iron in the leaching residue is (0-2):1. 6) The second leaching residue obtained in step 5) is placed in a high-temperature oxygen-containing environment for calcination to obtain iron phosphate material.

[0021] It should be noted that: in step 1), the aluminum content in the waste lithium iron phosphate residue is usually less than 10,000 ppm and the carbon content is less than 10%. The carbon species include PVDF and its decomposition products, membrane decomposition products, conductive carbon, and graphite materials; in the selective precipitation reaction in step 4), most of the leached iron and phosphorus elements precipitate in the form of amorphous iron phosphate (FePO4·xH2O), while only a small portion of the leached aluminum and phosphorus elements co-precipitate in the form of AlPO4·xH2O.

[0022] In some preferred embodiments of the present invention, the steps for constructing the theoretical relationship model include: 31) Based on Al at the current reaction temperature 3+ Based on the hydrolysis constants of phosphoric acid at various levels and the ionization constants of phosphoric acid at various levels, a distribution coefficient model of phosphate ions and aluminum ions under different pH conditions was constructed. 32) Based on the concentration of soluble aluminum obtained in step 2), the adjusted concentration of soluble phosphorus, the solubility product constant of AlPO4·2H2O, and the distribution coefficient model, construct a theoretical relationship model between aluminum precipitation efficiency and the initial pH value of the reaction.

[0023] Furthermore, the corresponding calculation of the distribution coefficient model is shown in the following formulas (1) and (2): In the formula, The distribution coefficient of phosphate ions. K is the distribution coefficient of aluminum ions. an (n=1, 2, 3) are the ionization constants of phosphoric acid at each stage, K bn (n=1, 2, 3, 4) are the hydrolysis constants of aluminum ions at each stage; The theoretical relationship between the aluminum precipitation efficiency and the initial pH value of the reaction is calculated according to the following formula (3): In the formula, This refers to the concentration of soluble aluminum ions. The concentration of soluble phosphate ions after adjustment in step 2) is given, in mol / L and K. sp-Al It is the solubility product constant of AlPO4·2H2O.

[0024] In some preferred embodiments of the present invention, the pH value at which the aluminum precipitation efficiency is zero in step 3) is obtained by calculating the aluminum precipitation efficiency at different pH values ​​using a theoretical relationship model between the aluminum precipitation efficiency and the initial pH value of the reaction, plotting a curve with pH value on the horizontal axis and aluminum precipitation efficiency on the vertical axis, and reading the intersection of the curve with the horizontal axis.

[0025] In some preferred embodiments of the present invention, the aluminum content in the waste lithium iron phosphate extraction slag in step 1) is 0-10000 ppm, the carbon content is 0-10%, and the carbon species include PVDF and its decomposition products, membrane decomposition products, conductive carbon, and graphite materials. The aluminum content in the waste lithium iron phosphate extraction slag can also be 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, or 9000 ppm, and the carbon content can also be 1, 2, 3, 4, 5, 6, 7, 8, or 9%.

[0026] In some preferred embodiments of the present invention, the acid solution in step 1) is one, two, or three of H2SO4, H3PO4, and HCl, and the liquid-solid mass ratio of the acid solution to the waste lithium iron phosphate lithium extraction residue can also be 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, or 9:1; the acid solution contains H... + The molar ratio of aluminum in the lithium iron phosphate residue is 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, and 90:1.

[0027] In some preferred embodiments of the present invention, the selective leaching reaction time in step 1) is 30-180 min, and the stirring rate is 0-1000 r / min; wherein, the selective leaching reaction temperature can also be 25℃, 35℃, 45℃, 55℃, or 65℃; the reaction time can also be 60 min, 120 min, or 150 min, and the reaction stirring rate can also be 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min, 600 r / min, 700 r / min, 800 r / min, or 900 r / min.

[0028] In some preferred embodiments of the present invention, the phosphate in step 2) is one or more of sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, ammonium phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

[0029] In some preferred embodiments of the present invention, the alkaline solution in step 4) is one or more of ammonia water, sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution, the reaction time is 30-180 min, and the stirring rate is 0-1000 r / min; the selective precipitation reaction temperature can also be 25℃, 35℃, 45℃, 55℃, or 65℃, the reaction time can also be 60 min, 120 min, or 150 min, and the stirring rate can also be 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min, 600 r / min, 700 r / min, 800 r / min, or 900 r / min.

[0030] In some preferred embodiments of the present invention, the acid solution in step 5) is one or both of H2SO4 and H3PO4, and the liquid-solid mass ratio of the acid solution to the first leaching residue can also be selected as 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, or 9:1. The acid solution contains H... + The molar ratio of iron to iron in the leaching residue can also be selected as 0.25:1, 0.5:1, 0.75:1, 1:1, 1.25:1, 1.5:1, or 1.75:1.

[0031] In some preferred embodiments of the present invention, the selective dissolution-recrystallization reaction in step 5) is carried out at a temperature of 70-95°C, for a time of 30-240 min, and with a stirring rate of 0-1000 r / min; wherein, the reaction temperature may also be 75°C, 80°C, 85°C, or 90°C, the time may also be 60 min, 90 min, 120 min, 150 min, 180 min, or 210 min, and the stirring rate may also be 100 r / min, 200 r / min, 300 r / min, 400 r / min, 500 r / min, 600 r / min, 700 r / min, 800 r / min, or 900 r / min.

[0032] In some preferred embodiments of the present invention, the washing method for the solid product in steps 4) and 5) is as follows: first, use an acid solution with a pH of 0.5-3 to repeatedly pulp and filter, and then use a large amount of deionized water to repeatedly pulp and filter until the conductivity of the washing solution is less than 500 μS / cm.

[0033] In some preferred embodiments of the present invention, the drying temperature in step 5) is 60-80℃ and the drying time is 12-24 h; wherein, the drying temperature can also be selected as 65℃, 70℃, or 75℃, and the drying time can also be selected as 14 h, 16 h, 18 h, 20 h, or 22 h.

[0034] In some preferred embodiments of the present invention, the high-temperature calcination temperature of the second leaching residue in step 6) is 500-900℃, the calcination atmosphere is air or pure oxygen, and the calcination time is 2-6 h; wherein, the calcination temperature can also be selected as 600℃, 700℃, or 800℃, and the calcination time can also be selected as 1h, 2h, 3h, 4h, or 5h.

[0035] It should be noted that during the selective precipitation reaction described in step 4), the actual precipitates formed are mainly amorphous iron phosphate (FePO4·xH2O) and a very small amount of amorphous aluminum phosphate (AlPO4·xH2O). Although thermodynamic data for amorphous substances are generally lacking in the published literature, experimental studies have found that the precipitation patterns of these amorphous phases (such as the earlier onset pH of iron precipitation and the continuous increase in precipitation efficiency with increasing pH) show excellent consistency with the precipitation behavior predicted based on the thermodynamic data of the corresponding crystalline phases. The literature (DOI: 10.1021 / ic300346t) further confirms through a large amount of experimental data that the standard enthalpy of formation (Δ) of the amorphous phase... f H 0 ) and Gibbs free energy (Δ f G 0 There is a very strong linear correlation (R0) between the phase and the corresponding crystalline phase. 2 Although the thermodynamic stability of the amorphous phase is always lower than that of the crystalline phase with the same composition (e.g., 0.9999), the order of their relative stability remains unchanged. Therefore, this invention uses widely accepted precipitation equilibrium thermodynamic data of crystalline AlPO4·2H2O and FePO4·2H2O to construct a theoretical model of precipitation efficiency to simulate and predict the precipitation behavior of amorphous iron phosphate / aluminum. Practice has shown that the pH conditions determined by this model can effectively guide experiments, achieving efficient precipitation of iron and highly selective separation of aluminum.

[0036] Similar to the model for aluminum precipitation efficiency, this invention also constructs a theoretical model for the relationship between iron precipitation efficiency and initial pH value, in order to verify that iron can achieve near-complete precipitation under optimal pH conditions. The specific construction steps are as follows: (a) Based on the hydrolysis constant of iron ions and the ionization constants of phosphoric acid at various levels under the current reaction temperature, a distribution coefficient model of phosphate ions and iron ions under different pH conditions is constructed; (b) Based on the soluble iron concentration, the adjusted soluble phosphorus concentration, and the solubility product constant Ksp of FePO4·2H2O measured in step 2), -Fe In addition, a distribution coefficient model was used to construct a theoretical relationship model between iron precipitation efficiency and pH.

[0037] The Fe 3+ The distribution coefficient model is calculated as shown in the following formula (4): In the formula, K is the distribution coefficient of iron ions. cn (n=1, 2, 3, 4) are the hydrolysis constants of iron ions at each stage; The theoretical relationship between the iron precipitation efficiency and the initial pH value of the reaction is calculated according to the following formula (5): In the formula, This refers to the concentration of soluble iron ions. The concentration of soluble phosphate ions after adjustment in step 2) is given, in mol / L and K. sp-Fe It is the solubility product constant of FePO4·2H2O.

[0038] It should be noted that all thermodynamic data involved in this invention, including the hydrolysis constants of aluminum ions, iron ions, ionization constants of phosphoric acid, solubility product constants of FePO4·2H2O, and solubility product constants of AlPO4·2H2O, can be obtained from the "Ran's Handbook of Chemistry" and related materials. Example 1

[0039] 50 g of waste lithium iron phosphate lithium extraction residue #1 (aluminum content 9300 ppm, carbon content 5% (including graphite), iron content 30.5 wt%, phosphorus content 17.5 wt%) was mixed with sulfuric acid solution according to the nH ratio. + The mixture with nAl = 10:1 and a liquid-to-solid mass ratio of 5:1 was mixed and reacted for 120 min at a reaction temperature of 25℃ and a stirring rate of 500 r / min to obtain a mixed slurry by selective leaching. Take a small amount of the mixed slurry obtained in step 1), filter it, wash the solid repeatedly, combine the filtrate and washing liquid, and make up to a certain volume. Use potassium dichromate titration and ICP-OES technology to calculate the concentration of iron, aluminum and phosphorus in the filtrate. The calculation results are shown in Table 1. Add ammonium dihydrogen phosphate to the remaining mixed slurry according to the measurement results so that the concentration of soluble phosphorus in the mixed slurry is equal to the sum of the contents of soluble iron and soluble aluminum. Calculate the precipitation efficiency of aluminum at different pH values ​​under the current reaction temperature using formulas (1)-(3) and the data in Table 1. Calculate the precipitation efficiency of iron and aluminum at different pH values ​​under the current reaction temperature using formulas (1), (4), and (5) and the data in Table 1. Plot the results as shown below. Figure 2 The curve showing the relationship between theoretical precipitation efficiency and pH value is based on... Figure 2 It can be seen that the pH value is 1.2 when the precipitation efficiency of aluminum is zero at the current reaction temperature; Saturated sodium hydroxide solution was continuously added to the mixed slurry obtained in step 2) to adjust the pH of the system to 1.2. After the pH value stabilized, the mixture was reacted at 35℃ and 500 r / min for 120 min to carry out a selective precipitation reaction. After the reaction was completed, the filtrate and solid product were separated by filtration. The obtained solid product was repeatedly washed with acid solution of pH=1.5 and deionized water until the conductivity of the washing solution was lower than 500 μS / cm. The washing solution after each washing was collected and combined with the aforementioned filtrate as the first leachate. The remaining solid after washing was the first leach residue. 5) Mix the first leaching residue obtained in step 4) with a sulfuric acid solution, controlling the molar ratio of hydrogen ions to iron in the system to be nH. + With nFe = 0.75:1 and a liquid-to-solid mass ratio of 5:1, a selective dissolution-recrystallization reaction was carried out at 90℃ and a stirring rate of 500 r / min for 120 min. After the reaction, the filtrate and solid product were separated by filtration. The obtained solid product was repeatedly washed with acid solution of pH=1.5 and deionized water until the conductivity of the washing solution was lower than 500 μS / cm. The washing solution after each washing was collected and combined with the aforementioned filtrate as the second leaching solution. The remaining solid after washing was the second leaching residue. 6) The second leaching residue obtained in step 5) is calcined at 800℃ in a pure oxygen atmosphere for 2 h to finally obtain the iron phosphate material. Example 2

[0040] This embodiment prepares iron phosphate material according to the method of Example 1, the only difference being: in step 1), waste lithium iron phosphate extraction residue #2 (aluminum content 7000 ppm, carbon content 3.5% (including graphite), iron content 31.5 wt%, phosphorus content 18.02 wt%) is used, and sulfuric acid solution is reacted with lithium extraction residue #2 according to nH + :nAl = 25:1 mixing; in step 3), draw as shown Figure 3 The curve showing the relationship between theoretical precipitation efficiency and pH value is based on... Figure 3 It can be seen that the pH value is 1.3 when the aluminum precipitation efficiency is zero at the current reaction temperature; in step 4), the pH of the system is adjusted to 1.3. Example 3

[0041] This embodiment prepares iron phosphate material according to the method of Example 1, the only difference being: in step 1), waste lithium iron phosphate extraction residue #3 (aluminum content 4800 ppm, carbon content 4.2% (including graphite), iron content 29 wt%, phosphorus content 16.78 wt%) is used, and sulfuric acid solution is reacted with lithium extraction residue #3 according to nH + : nAl = 35:1 mixing; in step 3), draw as shown Figure 4The curve showing the relationship between theoretical precipitation efficiency and pH value is based on... Figure 4 It can be seen that the pH value is 1.25 when the aluminum precipitation efficiency is zero at the current reaction temperature; in step 4), the pH of the system is adjusted to 1.25.

[0042] Table 1. Concentrations of soluble iron and aluminum elements and adjusted concentrations of soluble phosphorus ions after selective leaching reactions in Examples 1-3. It should be noted that: by Figures 2-4 The curves showing the relationship between theoretical precipitation efficiency and pH value indicate that the initial precipitation pH of AlPO4·2H2O was 1.2 (Example 1), 1.3 (Example 2), and 1.25 (Example 3), respectively, and increased with increasing pH. The precipitation efficiency of FePO4·2H2O approached 100% starting at pH = 0 and remained relatively stable with increasing pH. Therefore, to achieve selective precipitation of FePO4·2H2O (i.e., a large amount of iron precipitates while aluminum hardly precipitates), the pH should be controlled within the ranges of 0~1.2, 0~1.3, and 0~1.25, respectively. Considering that the actual precipitation forms of iron and aluminum phosphates under the conditions of this invention are FePO4·xH2O and AlPO4·xH2O, their initial precipitation pH is usually higher than that of the corresponding dihydrates. However, thermodynamic data are lacking. Therefore, the initial precipitation pH of AlPO4·2H2O in each system was taken as the actual control target, i.e., Example 1: pH = 1.2, Example 2: pH = 1.3, Example 3: pH = 1.25.

[0043] Comparative Example 1 50 g of waste lithium iron phosphate lithium extraction residue #1 (aluminum content 9300 ppm, carbon content 5% (including graphite), iron content 30.5 wt%, phosphorus content 17.5 wt%) was mixed with sulfuric acid solution according to nH + The mixture of nFe = 0.75:1 and liquid to solid mass ratio of 5:1 was reacted at 90℃ and 500 r / min for 120 min to carry out a selective dissolution-recrystallization reaction. After the reaction, the filtrate and solid product were separated by filtration. The solid product was repeatedly washed with acid solution of pH=1.5 and deionized water until the conductivity of the washing solution was lower than 500 μS / cm. The washing solution after each washing was collected and combined with the aforementioned filtrate as the second leachate. The remaining solid after washing was the second leaching residue. 2) The second leaching residue obtained in step 1) was calcined at 800℃ in a pure oxygen atmosphere for 2 h to finally obtain the iron phosphate material.

[0044] Comparative Example 2 This comparative example prepared iron phosphate material according to the method of Comparative Example 1, the only difference being that: in step 1), waste lithium iron phosphate lithium extraction slag #2 (aluminum content 7000 ppm, carbon content 3.5% (including graphite), iron content 31.5 wt%, phosphorus content 18.02 wt%) was used.

[0045] Comparative Example 3 This comparative example prepared iron phosphate material according to the method of Comparative Example 1, the only difference being that: in step 1), waste lithium iron phosphate lithium extraction slag #3 (aluminum content 4800 ppm, carbon content 4.2% (including graphite), iron content 29 wt%, phosphorus content 16.78 wt%) was used, the rest was the same as Comparative Example 1.

[0046] Experimental Example 1 The mass fractions of iron and aluminum in the filter residue obtained from filtration separation in step 2) of Examples 1-3 were determined by potassium dichromate titration and ICP-OES, respectively. The mass of residual iron and aluminum in the filter residue was calculated based on the total mass of the filter residue. Based on the mass fractions of iron, aluminum and phosphorus elements measured in step 2) of Examples 1-3, the mass of iron and aluminum elements in the filtrate is calculated in combination with the mass of the filtrate. The leaching rates L of iron and aluminum elements after selective leaching reaction in Examples 1-3 were calculated using the following formula (6), and the results are shown in Table 2: L (6) In the above formula, w represents the leaching rate of the target element. S1 Mass fraction of target element in filter residue, m S1 For filter cake quality, w L1 m represents the mass fraction of the target element in the filtrate. L1 For the quality of the filtrate; As shown in Table 2, under precisely controlled acidity and temperature conditions, the selective leaching process can leach approximately 95% of Al from complex lithium extraction slag with varying carbon, iron, aluminum, and phosphorus contents, while only a small portion, approximately 20% of Fe, is leached. This demonstrates that the selective leaching reaction in the method of this invention achieves highly selective leaching of aluminum and minimizes the loss of iron.

[0047] Table 2. Leaching rates of Fe and Al elements after selective leaching reactions in Examples 1-3 Experiment Example 2 The mass fractions of Fe and Al in the first leachate and the first leach residue prepared in Examples 1-3 were determined by potassium dichromate titration and ICP-OES, respectively. The first leachate (m) prepared in Examples 1-3 was measured. L2) and the mass of the first leaching residue (m S2 ); The precipitation rate P of iron and aluminum elements after selective precipitation reaction in Examples 1-3 is calculated according to the following formula (7). 1, The calculation results are shown in Table 3. (7) The total recovery rate R of iron and aluminum elements after selective precipitation reaction in Examples 1-3 was calculated using the following formula (8), and the calculation results are shown in Table 4: R (8) In equations (7) and (8) above, w L2 m represents the mass fraction of the target element in the first leachate. L2 First leachate mass, w G1 To increase the mass fraction of the target element in lithium slag raw material, m G1 To improve the quality of lithium slag raw materials, L is the leaching rate of the target element calculated by formula (6) in Experimental Example 1; Table 3. Precipitation rates of Fe and Al elements after selective precipitation reaction in Examples 1-3 Table 4. Total recovery rate of Fe and Al elements in the first leaching residue of Examples 1-3. As shown in Tables 3 and 4, even with significant fluctuations in the elemental content of the lithium extraction slag, after selective leaching, by adding alkaline solution to the mixed slurry and precisely adjusting the pH to the target value, the iron leached in the first stage of the process can preferentially precipitate with a precipitation rate exceeding 95%, while only a small amount of leached aluminum co-precipitates. Ultimately, the total recovery rate of iron can reach approximately 99%, while the removal rate of aluminum is approximately 92%. This indicates that the method of the present invention can ensure the maximum precipitation and enrichment of iron and phosphorus while maximizing the retention of aluminum under different raw material conditions, demonstrating good selectivity and stability.

[0048] Experimental Example 3 The first leaching residues prepared in Examples 1, 2, and 3 were subjected to XRD characterization analysis, and the results are as follows: Figure 5 As shown; Depend on Figure 5Analysis shows that the XRD patterns of the first leaching residues obtained in Examples 1-3 only show diffraction peaks corresponding to graphite (PDF#00-026-1080) and olivine-type iron phosphate (PDF# 97-009-2199), and no peaks corresponding to FePO4·2H2O were detected. Since olivine-type iron phosphate cannot be directly prepared by liquid-phase synthesis, it is inferred that the Fe element in the first leaching residue precipitates in the form of FePO4·xH2O, and the aluminum element is difficult to detect due to its low content.

[0049] Experiment Example 4 This experimental example further determines the precipitation form of aluminum under selective precipitation reaction conditions by constructing an Al-P simulation system; The specific method is as follows: Weigh a certain amount of aluminum sulfate octadechydrate and dissolve it in a phosphoric acid solution of a certain concentration, so that the mass fraction of aluminum in the solution is 10 times that of aluminum in Example 1 as measured in Experimental Example 1, and the mass fraction of phosphorus is equal to that of phosphorus in Experimental Example 1. Under the conditions of 35℃ and a stirring rate of 500 r / min, gradually add sodium hydroxide solution until a precipitate is formed, so that the mass fraction of aluminum in the solution is equal to that of aluminum in Example 1 as measured in Experimental Example 1. L1-Al 10 times that of the target, the mass fraction of phosphorus is equal to the value measured in Experiment Example 1. L1-P The precipitate was washed, dried, and then characterized by XRD. The results are as follows: Figure 6 As shown in the figure; the precipitate was dehydrated by calcination at 700℃ for 2 h in air atmosphere, and then the dehydrated product was characterized and analyzed by XRD. The results are shown in the figure. Figure 7 As shown; Depend on Figure 6 Analysis shows that the XRD pattern of the precipitate product generally shows a dome-shaped peak, without any obvious high diffraction peaks at a certain angle, proving that the product is disordered over a long range, which does not conform to the long-range ordered characteristics of crystals, thus proving that it is an amorphous substance.

[0050] Depend on Figure 7 Analysis shows that the XRD pattern of the product after high-temperature dehydration corresponds well with the characteristic peaks of AlPO4 (PDF# 97-009-8381, PDF# 00-048-0652). Therefore, it is determined that in the selective precipitation reaction, the co-precipitation form of aluminum in the liquid phase is AlPO4·xH2O.

[0051] Experimental Example 5 The mass fractions of Fe and Al in the second leachates prepared in Examples 1-3 and Comparative Examples 1-3 were determined by potassium dichromate titration and ICP-OES, respectively. The mass of the first leaching residue prepared in Examples 1-3 and the mass of the second leachate prepared in Examples 1-3 and Comparative Examples 1-3 were determined. Calculate the precipitation rate P2 of iron and aluminum elements after the selective dissolution-recrystallization reaction in Examples 1-3 and Comparative Examples 1-3 according to the following formula (9): (9) In the formula, w L3 The mass fraction of the target element in the second leachate, m L3 Second leachate mass, w G2 m represents the mass fraction of the target element in the solid before the selective dissolution-recrystallization reaction. G2 The mass of the solid before the selective dissolution-recrystallization reaction; wherein, the solid before the selective dissolution-recrystallization reaction in Examples 1-3 is the first leaching residue.

[0052] The measurement and calculation results are shown in Table 5: Table 5. Mass fractions of Fe and Al in the raw materials before the selective dissolution-recrystallization reaction and precipitation rates of Fe and Al after the reaction in Examples 1-3 and Comparative Examples 1-3. As shown in Table 5, since the iron content of the three lithium extraction slags (#1, #2, and #3) is similar while the aluminum content varies greatly, the precipitation rates of iron in Examples 1-3 after selective dissolution-recrystallization are similar and relatively high (approximately 95%). However, the precipitation rates of aluminum vary significantly depending on the aluminum content of the raw materials. Comparing Example 1 with Comparative Example 1, Example 2 with Comparative Example 2, and Example 3 with Comparative Example 3, it can be seen that although a high iron precipitation rate can be achieved by selective dissolution-recrystallization alone, the co-precipitation rate of aluminum is significantly higher than in the examples. This results in a higher aluminum residue in the product, which cannot meet the purity requirements of battery-grade iron phosphate. This indicates that without the aforementioned selective leaching and selective precipitation steps, it is difficult to achieve deep removal of aluminum by relying solely on the selective dissolution-recrystallization process, thus highlighting the necessity of the first stage of selective leaching and the second stage of selective precipitation in this invention.

[0053] Experimental Example 6 The second leaching residues prepared in Examples 1-3 and Comparative Examples 1-3 were characterized by XRD analysis, and the results are as follows: Figure 8 As shown; Depend on Figure 8 It can be seen that the XRD patterns of all products correspond well with the characteristic peaks of FePO4·2H2O (PDF# 97-001-5842), and no other impurity peaks were detected. This is because the aluminum content of all products is low, resulting in the content of the corresponding species being below the detection limit and difficult to monitor.

[0054] Experimental Example 7 This experimental example further determines the precipitation form of aluminum under selective dissolution-recrystallization reaction conditions by constructing an Al-P simulation system; The specific method is as follows: A certain amount of aluminum sulfate octahydrate is weighed and dissolved in a phosphoric acid solution of a certain concentration, so that the mass fractions of aluminum and phosphorus in the solution are 0.2 times that of Comparative Example 1. The pH is adjusted to approximately 0.8 with sulfuric acid, and the reaction is carried out at 90℃ and a stirring rate of 500 r / min for 120 min to induce precipitation. The resulting precipitate is filtered, washed, and then characterized by XRD. The results are... Figure 9 As shown; Depend on Figure 9 Analysis shows that the XRD pattern of the obtained product corresponds well with the characteristic peak of AlPO4·2H2O (PDF# 04-011-1602), and there are no other impurity peaks, proving that in the selective dissolution-recrystallization process of this invention, the co-precipitated form of aluminum is AlPO4·2H2O.

[0055] Experimental Example 8 The contents of Fe and Al elements in the iron phosphate products prepared in Examples 1-3 and Comparative Examples 1-3 were determined by potassium dichromate titration and ICP-OES, respectively. The results are shown in Table 6. Table 6. Content of Fe and Al elements in the iron phosphate products prepared in Examples 1-3 and Comparative Examples 1-3 As shown in Table 6 above, the three-step continuous separation process of "selective leaching-selective precipitation-selective dissolution and recrystallization" proposed in this invention (Examples 1-3) can regenerate various high-aluminum-content waste lithium iron phosphate residues (Al content greater than 4500 ppm) into battery-grade iron phosphate with an aluminum content of less than 300 ppm. In contrast, Comparative Examples 1-3 only use the selective dissolution-recrystallization process, which can remove most of the aluminum, but the residual aluminum content in their products is still significantly high, failing to meet the purity requirements of battery-grade iron phosphate. This fully demonstrates the significant advantages of the three-step process of this invention in deep dealuminization.

[0056] In summary, it should be noted that the above description is only a preferred embodiment of the present invention and should not be used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still make some simple deductions, substitutions, or equivalent substitutions of some technical features for the technical solutions described in the foregoing embodiments without departing from the concept of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue, characterized in that, Includes the following steps: 1) Mix waste lithium iron phosphate residue with acid solution at a liquid-to-solid mass ratio of (2-10):1, and react thoroughly at 15-70℃ for a period of time to obtain a mixed slurry. The acid solution contains H... + The molar ratio of aluminum in the lithium extraction slag is (100-3):1; 2) Determine the concentrations of soluble iron, soluble aluminum, and soluble phosphorus in the mixed slurry obtained in step 1). If the concentration of soluble phosphorus in the mixed slurry is lower than the sum of the contents of soluble iron and soluble aluminum, add phosphate to adjust the concentration of soluble phosphorus to be equal to the sum. 3) Based on the precipitation equilibrium thermodynamic data of iron, aluminum and phosphorus, a theoretical relationship model between aluminum precipitation efficiency and initial pH value is constructed. According to the concentration of each element measured in step 2) and the theoretical relationship model, the pH value when the aluminum precipitation efficiency is zero is obtained. 4) Continuously add alkaline solution to the mixed slurry obtained in step 2) to adjust the pH value of the system to the pH value obtained in step 3), and then carry out a selective precipitation reaction at a reaction temperature of 15-70℃. After the reaction is completed, filter out the solid product and wash it repeatedly to obtain the first leaching residue. 5) Place the first leaching residue obtained in step 4) in an acid solution and heat it in a high-temperature water bath to carry out a selective dissolution-recrystallization reaction. After the reaction is completed, filter out the solid product, and repeatedly wash and dry it to obtain the second leaching residue; wherein, the liquid-solid mass ratio of the acid solution to the first leaching residue is (2-10):1, and the acid solution contains H + The molar ratio of iron in the leaching residue to iron is (0-2):1; 6) The second leaching residue obtained in step 5) is placed in a high-temperature oxygen-containing environment for calcination to obtain iron phosphate material.

2. The method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 1, characterized in that: The steps for constructing the theoretical relationship model include: 31) Based on Al at the current reaction temperature 3+ Based on the hydrolysis constants of phosphoric acid at various levels and the ionization constants of phosphoric acid at various levels, a distribution coefficient model of phosphate ions and aluminum ions under different pH conditions was constructed. 32) Based on the concentration of soluble aluminum obtained in step 2), the adjusted concentration of soluble phosphorus, the solubility product constant of AlPO4·2H2O, and the distribution coefficient model, construct a theoretical relationship model between aluminum precipitation efficiency and the initial pH value of the reaction.

3. The method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 2, characterized in that: The calculation of the distribution coefficient model is shown in the following formulas (1) and (2): In the formula, The distribution coefficient of phosphate ions. K is the distribution coefficient of aluminum ions. an (n=1, 2, 3) are the ionization constants of phosphoric acid at each stage, K bn (n=1, 2, 3, 4) are the hydrolysis constants of aluminum ions at each stage; The theoretical relationship between the aluminum precipitation efficiency and the initial pH value of the reaction is calculated according to the following formula (3): In the formula, This refers to the concentration of soluble aluminum ions. The concentration of soluble phosphate ions after adjustment in step 2) is given, in mol / L and K. sp-Al It is the solubility product constant of AlPO4·2H2O.

4. The method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 3, characterized in that: The pH value at which the aluminum precipitation efficiency is zero, as mentioned in step 3), is obtained by calculating the aluminum precipitation efficiency at different pH values ​​using a theoretical relationship model between the aluminum precipitation efficiency and the initial pH value of the reaction. The curve relationship is plotted with pH value on the horizontal axis and aluminum precipitation efficiency on the vertical axis, and the intersection of the curve and the horizontal axis is read.

5. The method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 1, characterized in that: The acid solution in step 1) is one, two or three of H2SO4, H3PO4 and HCl, the selective precipitation reaction time is 30-180 min, and the stirring rate is 0-1000 r / min.

6. The method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 1, characterized in that: In step 2), the phosphate is one or more of sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, ammonium phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

7. The method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 1, characterized in that: The alkaline solution in step 4) is one or more of ammonia water, sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution. The reaction time is 30-180 min and the stirring rate is 0-1000 r / min.

8. The method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 1, characterized in that: In step 5), the acid solution is one or both of H2SO4 and H3PO4. The selective dissolution-recrystallization reaction temperature is 70-95℃, the reaction time is 30-240 min, the stirring rate is 0-1000 r / min, the drying temperature is 60-80℃, and the drying time is 12-24 h.

9. A method for regenerating battery-grade iron phosphate from waste lithium iron phosphate residue according to claim 1, characterized in that: In step 6), the high-temperature calcination temperature of the second leaching residue is 500-900℃, the calcination atmosphere is air or pure oxygen, and the calcination time is 2-6 h.

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