Method for recycling phosphorus-iron slag at low cost and high efficiency and application thereof

Abstract

The application discloses a method for recycling phosphorus-iron slag at low cost and high efficiency and application thereof. The method comprises the following steps: S1, mixing waste phosphorus-iron slag and concentrated sulfuric acid to obtain a mixture, wherein the molar ratio of sulfuric acid to iron in the phosphorus-iron slag is 1.3-1.5:1; S2, stirring the mixture, diluting, removing the solid part, and obtaining a leaching solution; S3, adjusting the pH of the leaching solution to 1.6-2.5, removing the liquid phase part, and obtaining amorphous iron phosphate; S4, adding water to the amorphous iron phosphate, adding phosphoric acid, aging, and obtaining iron phosphate dihydrate; and S5, calcining to obtain battery-grade iron phosphate. The application provides a process for preparing battery-grade iron phosphate by using waste phosphorus-iron slag (by-product after lithium extraction from retired lithium iron phosphate batteries) as raw material, concentrated sulfuric acid leaching, directional impurity removal, controllable conversion and high-temperature sintering.

Description

Technical Field

[0001] This invention relates to the field of industrial solid waste resource utilization and new energy materials technology, specifically to a low-cost and efficient method for recycling phosphorus iron slag and its application. Background Technology

[0002] In recent years, the global new energy vehicle industry has experienced explosive growth. Lithium iron phosphate (LFP) batteries, with their advantages of good thermal stability, long cycle life, and relatively low cost, have captured a major share of the power battery market. The large amount of phosphorus-iron slag generated after lithium extraction from retired batteries has become an industrial solid waste that urgently needs proper disposal. This phosphorus-iron slag mainly consists of iron phosphate, graphite, and a small amount of lithium salt impurities, and is rich in iron and phosphorus. Improper disposal not only occupies land and pollutes the environment but also results in a serious waste of valuable resources.

[0003] Meanwhile, the demand for battery-grade lithium iron phosphate (LFP) continues to grow in the power battery industry chain. As a key precursor for LFP cathode materials, battery-grade LFP must meet stringent requirements such as a precise iron-to-phosphorus ratio, extremely low impurity content (such as aluminum, sodium, potassium, and calcium ions, which are typically required to be below 50 ppm), and uniform crystal morphology. Currently, its production mainly relies on primary mineral resources, facing the dual pressures of resource constraints and rising costs.

[0004] Therefore, the efficient recovery of phosphorus and iron from phosphate slag and their use in the preparation of battery-grade iron phosphate can not only alleviate the pressure of solid waste disposal and reduce environmental pollution, but also replace some of the original resources, realizing the circular economy value of "turning waste into treasure", which has important industrial significance and environmental benefits.

[0005] Current technologies for the resource utilization of phosphate iron slag are mainly divided into three categories: hydrometallurgy, pyrometallurgy, and integrated recycling. However, all of them have significant drawbacks and are difficult to meet the demands of industrialization and high quality. 1) Limitations of pyrometallurgical technology: When decomposing ferric phosphate using the high-temperature roasting method, an inert atmosphere and high-temperature environment must be maintained, which not only consumes more than 3000 kWh / ton of energy, but also produces SO2 and NO. x Harmful gases are present, and environmental treatment costs are high. Patent application CN115385314A discloses a method for recovering iron and phosphorus from phosphate slag. Although this method achieves iron and phosphorus separation through potassium-assisted reduction, the equipment has stringent sealing requirements, significantly increasing initial investment and maintenance costs, and the purity of the product is difficult to reach battery-grade standards.

[0006] 2) Defects of Traditional Hydrometallurgical Processes: Existing acid leaching processes mostly use dilute sulfuric acid or composite acid systems, which have two major problems: First, the leaching efficiency is low. In ferrophosphate slag, iron phosphate is tightly bound to iron oxides, and the recovery rate of iron and phosphorus under dilute acid systems is generally less than 85%, with a reaction cycle of 4-6 hours. Second, the removal of impurities is difficult. Impurities such as aluminum, calcium, and lithium contained in the leachate co-precipitate with iron and phosphorus, requiring multiple extractions and separations, resulting in high reagent consumption and high wastewater treatment costs. For example, patent application CN118026126A discloses a method for recovering iron phosphate from waste ferrophosphate slag. This method uses ammonia to control precipitation, but it is sensitive to fluctuations in the iron-to-phosphorus ratio of the raw material, and the purity and stability of the product are poor.

[0007] 3) Insufficient integrated recycling technology: The process of directly converting phosphorus iron slag into lithium iron phosphate is still in the laboratory stage. It has problems such as uneven reaction system and difficulty in accurately controlling doping elements. The product's cycle performance decays quickly and cannot meet the requirements for reuse in power batteries. Moreover, this technology has extremely high requirements for the initial purity of phosphorus iron slag, making it unsuitable for industrial-grade solid waste with complex composition.

[0008] Patent application CN117566709A discloses a method for selectively recovering lithium from spent lithium iron phosphate batteries and preparing battery-grade iron phosphate. This method involves leaching battery materials at 20-65°C using a specific leaching agent to obtain a lithium-containing leachate and iron phosphate slag mainly composed of iron phosphate and graphite. After deep impurity removal, the leachate can be used to prepare battery-grade iron phosphate. The key to this process is selectively dissolving lithium and impurity elements by controlling the leaching conditions, while simultaneously preventing the iron phosphate slag from undergoing crystal transformation, thus reducing the difficulty of subsequent processing. However, in actual recycling scenarios, iron phosphate slag is usually a byproduct directly generated after lithium extraction from batteries, and its crystal form is not specifically controlled. Therefore, this method is out of touch with industrial realities and has limited applicability. Patent application CN115448279A proposes a method for recovering and preparing battery-grade iron phosphate from commonly available iron phosphate slag. This process uses phosphoric acid to leach the iron phosphate slag and adds iron powder as a reducing agent to improve leaching efficiency. During the process, ferric iron is reduced to ferrous iron. However, this method has significant drawbacks: firstly, phosphoric acid is expensive, and to re-oxidize ferrous iron and synthesize ferric phosphate, additional oxidant and approximately one-third of the total phosphoric acid are required for coordination, resulting in high reagent consumption and complex steps; secondly, the introduction of iron powder further increases raw material costs. Overall, this process is not economically viable and is difficult to apply to large-scale industrial production.

[0009] Therefore, it is of great significance to develop a low-cost and efficient method for recycling waste phosphorus slag, a byproduct of lithium extraction from retired lithium iron phosphate batteries. Summary of the Invention

[0010] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a method for recycling ferrophosphate slag, which is low in cost and has good recycling effect, and the resulting ferrophosphate has a low aluminum content, which can meet the control requirements of battery manufacturers for aluminum content (100ppm) in ferrophosphate.

[0011] The present invention also proposes applications of the above method.

[0012] According to one aspect of the present invention, a method for recovering ferrophosphate slag is provided, comprising the following steps: S1. Mix waste ferrophosphate slag with concentrated sulfuric acid to obtain a mixture, wherein the molar ratio of sulfuric acid to iron in the ferrophosphate slag is 1.3~1.5:1; S2. After stirring the mixture, dilute it, remove the solid portion, and obtain an extract; S3. Adjust the pH of the leachate to 1.6~2.5, remove the liquid phase, and obtain amorphous ferric phosphate; S4. After adding water to the amorphous ferric phosphate and pulping it, phosphoric acid is added and the mixture is aged to obtain ferric phosphate dihydrate. S5. Calcination yields battery-grade iron phosphate.

[0013] The method according to embodiments of the present invention has at least the following beneficial effects: The present invention provides a process for preparing battery-grade iron phosphate using waste iron phosphate slag (a byproduct of lithium extraction from decommissioned lithium iron phosphate batteries) as raw material, through concentrated sulfuric acid leaching, directional impurity removal, controlled conversion, and high-temperature sintering. This process has the following advantages: 1) High resource utilization rate: The recovery rate of iron and phosphorus elements is no less than 95%, realizing the high-value utilization of waste phosphorus and iron slag and effectively alleviating the dependence on primary mineral resources.

[0014] 2) High product purity: Through targeted impurity removal and crystallization process control, the obtained product has low impurity content and accurate iron-phosphorus ratio, and all indicators meet the standard requirements of high-quality battery-grade iron phosphate.

[0015] 3) Good process economy: The overall process is simple, the total reaction cycle can be controlled within 10 hours, the reagent consumption is low, and the amount of production wastewater discharged is reduced by more than 30% compared with traditional methods.

[0016] 4) Significant environmental benefits: It not only avoids the environmental risks caused by the stockpiling of phosphorus iron slag from the source, but also has no harmful gas emissions during the critical high-temperature sintering stage, which is in line with the green manufacturing and related strategic policy orientation.

[0017] According to some embodiments of the present invention, the concentrated sulfuric acid has a mass fraction of 70% or more.

[0018] According to some embodiments of the present invention, the concentrated sulfuric acid has a mass fraction of 70-80%.

[0019] According to some embodiments of the present invention, step S1 further includes the step of diluting commercially available concentrated sulfuric acid.

[0020] According to some embodiments of the present invention, the stirring time in step S2 is 3 to 6 hours.

[0021] According to some embodiments of the present invention, the stirring speed in step S2 is 100~300 rpm.

[0022] According to some embodiments of the present invention, the dilution step S2 specifically includes diluting with water to a phosphorus concentration of 1~1.5 mol / L.

[0023] According to some embodiments of the present invention, the pH is adjusted by adding ammonia in step S3.

[0024] According to some embodiments of the present invention, the mass concentration of the ammonia water is 5-15%.

[0025] According to some embodiments of the present invention, the mass concentration of the ammonia water is 5-10%.

[0026] According to some embodiments of the present invention, the ammonia water is added at a rate of 15~35 mL / min.

[0027] According to some embodiments of the present invention, in step S3, the pH of the leachate is adjusted to 1.8 to 2.2.

[0028] According to some embodiments of the present invention, the aging temperature is 90~95°C.

[0029] According to some embodiments of the present invention, in step S4, amorphous ferric phosphate and water are mixed and pulped at a solid-liquid ratio of 1:3.5~4.5 g / mL. For example, a ratio of 1:4, etc.

[0030] According to some embodiments of the present invention, the amount of phosphoric acid added in step S4 is 0.03~0.05 eq.

[0031] According to some embodiments of the present invention, the calcination temperature is 500~600℃ and the time is 3~5h.

[0032] The present invention also proposes the application of the above method in the recycling of lithium iron phosphate batteries.

[0033] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the recycling process of waste phosphorus iron slag in an embodiment of the present invention. Detailed Implementation

[0035] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available. Unless otherwise specified, the same parameter value is the same in all embodiments. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0036] In the description of this invention, references to terms such as "some embodiments" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0037] In some embodiments of the present invention, a low-cost and efficient method for recovering ferrophosphate slag is provided, comprising the following steps: 1. First, mix concentrated sulfuric acid and water to obtain concentrated sulfuric acid with a mass fraction of 70%~80%. Then, slowly add waste phosphate slag to the obtained concentrated sulfuric acid to obtain a paste-like mixture. The molar ratio of sulfuric acid to iron in the phosphate slag is 1.3-1.5:1. 2. After stirring the paste mixture for 3-6 hours, slowly add water to dilute it, and obtain a mixture of graphite, sulfuric acid and ferric phosphate; 3. Filter the mixture to obtain ferric phosphate leachate; 4. Slowly add ammonia (5-10% by mass) to the ferric phosphate leachate to adjust the pH so that the pH value in the leachate is controlled at 1.6-2.5. After filtration, amorphous ferric phosphate is obtained, and metallic impurities such as aluminum and copper remain in the filtrate. 5. After adding water to the amorphous ferric phosphate and pulping it, a small amount of phosphoric acid is added, and the mixture is aged at a high temperature of 90-95℃ to obtain ferric phosphate dihydrate. 6. Calcine the ferric phosphate dihydrate at a temperature of 500-600℃ for 3-5 hours to obtain battery-grade ferric phosphate.

[0038] Example 1 This example provides a method for recovering phosphorus-iron slag, such as... Figure 1 As shown. The specific process is as follows: ① Add 98% concentrated sulfuric acid to pure water to make the sulfuric acid concentration 10 mol / L, stir at 200 rpm, and add ferric phosphate slag according to the molar ratio of H2SO4:Fe=1.5. After reacting at 65℃ (60~70℃ is also acceptable) for 4 hours, dilute with water to the phosphorus concentration 1.2 mol / L, filter to separate the insoluble residue, and obtain ferric phosphate leachate with a leaching rate of 99%.

[0039] ② Add 1L of leachate to the reactor, slowly add 10% ammonia solution at a rate of 20mL / min, and stir at 300rpm. Adjust the pH of the leachate to 2.0 and age for 1 hour. Filter and wash the slurry to separate impurities such as aluminum and copper, obtaining 220g of pure amorphous ferric phosphate.

[0040] ③ The above 220g of amorphous ferric phosphate was slurried with water until the solid content reached 20%, and 0.05 eq of phosphoric acid was added. The mixture was then heated to 92℃ and aged to convert the amorphous ferric phosphate into α-quartz dihydrate ferric phosphate. After the reaction was completed, the slurry was filtered and washed to obtain 220g of high-purity ferric phosphate dihydrate filter cake.

[0041] ④ Place the filter cake in a porcelain crucible, put it in a muffle furnace, and sinter it at 560℃ for 3 hours to obtain battery-grade iron phosphate.

[0042] The battery-grade iron phosphate was subjected to compositional analysis, as follows: Main content: FePO4 ≥ 99.5 wt%; Iron-phosphorus molar ratio: 1.03; Impurity content: Na≤25ppm, K≤18ppm, Ca≤32ppm, Al≤20ppm, Cu≤5ppm; Crystallization morphology: orthorhombic crystal system, average grain size 2-5μm, uniform morphology.

[0043] Example 2 This example provides a method for recovering phosphorus-iron slag, such as... Figure 1 As shown. The specific process is as follows: ① Add 98% concentrated sulfuric acid to pure water to make the sulfuric acid concentration 10 mol / L, stir at 200 rpm, and add ferric phosphate slag according to the molar ratio of H2SO4:Fe=1.8. After reacting at 65℃ (60~70℃ is also acceptable) for 4 hours, dilute with water to the phosphorus concentration 1.5 mol / L, filter to separate the insoluble residue, and obtain ferric phosphate leachate with a leaching rate of 99%.

[0044] ② Add 1L of leachate to the reactor, slowly add 5% ammonia solution at a rate of 40mL / min, and stir at 300rpm. Adjust the pH of the leachate to 2.0 and age for 1 hour. Filter and wash the slurry to separate impurities such as aluminum and copper, obtaining 260g of pure amorphous ferric phosphate.

[0045] ③ The above 260g of amorphous ferric phosphate was slurried with water until the solid content reached 20%, and 0.05 eq of phosphoric acid was added. The mixture was then heated to 92℃ and aged to convert the amorphous ferric phosphate into α-quartz dihydrate ferric phosphate. After the reaction was completed, the slurry was filtered and washed to obtain 260g of high-purity ferric phosphate dihydrate filter cake.

[0046] ④ Place the filter cake in a porcelain crucible, put it in a muffle furnace, and sinter it at 580℃ for 3 hours to obtain battery-grade iron phosphate.

[0047] The battery-grade iron phosphate was subjected to compositional analysis, as follows: Main content: FePO4 ≥ 99.5 wt%; Iron-phosphorus molar ratio: 1.03; Impurity content: Na≤25ppm, K≤18ppm, Ca≤32ppm, Al≤20ppm, Cu≤5ppm; Crystallization morphology: orthorhombic crystal system, average grain size 2-5μm, uniform morphology.

[0048] Comparative Example 1 This example provides a method for recovering ferrophosphate slag. The specific process is as follows: ① Add 98% concentrated sulfuric acid to pure water to make the sulfuric acid concentration 6 mol / L, stir at 200 rpm, and add ferric phosphate slag according to the molar ratio of H2SO4:Fe=1.5. After reacting at 65℃ (60~70℃ is also acceptable) for 4 hours, dilute with water to the iron concentration 1.2 mol / L, filter to separate the insoluble residue, and obtain ferric phosphate leachate with a leaching rate of 70%.

[0049] ② Add 1L of leachate to the reactor, slowly add 20% ammonia solution at a rate of 10mL / min, and stir at 300rpm. Adjust the pH of the leachate to 2.0 and age for 1 hour. Filter and wash the slurry to separate impurities such as aluminum and copper, obtaining 220g of pure amorphous ferric phosphate.

[0050] ③ The above 220g of amorphous ferric phosphate was slurried with water until the solid content reached 20%, and 0.05 eq of phosphoric acid was added. The mixture was then heated to 92℃ and aged to convert the amorphous ferric phosphate into α-quartz dihydrate ferric phosphate. After the reaction was completed, the slurry was filtered and washed to obtain 220g of high-purity ferric phosphate dihydrate filter cake.

[0051] ④ Place the filter cake in a porcelain crucible, put it in a muffle furnace, and sinter it at 560℃ for 3 hours to obtain battery-grade iron phosphate.

[0052] The battery-grade iron phosphate was subjected to compositional analysis, as follows: Main content: FePO4 ≥ 99.5 wt%; Iron-phosphorus molar ratio: 1.03; Impurity content: Na≤25ppm, K≤18ppm, Ca≤32ppm, Al≤200ppm, Cu≤5ppm; Crystal morphology: orthorhombic crystal system, average grain size 2-5 μm, uniform morphology; The aluminum iron phosphate content obtained by the Comparative Example 1 scheme is close to 200 ppm, which cannot meet the requirements of high-quality battery applications.

[0053] In this invention, concentrated sulfuric acid exhibits unique advantages in the field of mineral leaching due to its strong acidity and strong oxidizing properties. ① It can rapidly disrupt the iron-hydrogen bond structure of ferric phosphate dihydrate in ferric phosphate slag, significantly increasing the dissolution rate of iron and phosphorus, and eliminating the need for reducing agents to reduce ferric iron. Therefore, subsequent production of ferric phosphate does not require the use of oxidants, nor does it require large-scale replenishment of phosphoric acid, significantly reducing production costs. The cost comparison for producing 1 ton of ferric phosphate is shown in Table 1 below: Table 1

[0054] ② Its high concentration characteristic can reduce the water content of the reaction system and reduce the energy consumption of subsequent concentration. In the sulfuric acid-ammonia water system, when n(H2SO4)∶n(Fe)=1.5, the iron and phosphorus recovery rate can be increased to more than 95%, and the countercurrent leaching mode can further enhance the mass transfer efficiency and shorten the reaction time to less than 2 hours. Traditional concentrated sulfuric acid leaching technology still faces key bottlenecks: First, leaching conditions are not optimized; excessively high concentrations easily lead to sulfuric acid decomposition and volatilization, while excessively low concentrations fail to achieve efficient dissolution. Second, impurity separation is incomplete; concentrated sulfuric acid dissolves impurity elements simultaneously, lacking targeted impurity removal processes. Third, product crystallization control is difficult; direct concentration easily forms amorphous iron phosphate, requiring precise control of crystallization parameters to meet battery-grade morphology requirements. Therefore, developing an integrated process of "high-efficiency concentrated sulfuric acid leaching - directional impurity removal - controllable crystallization" to solve the efficiency, purity, and cost issues in the high-value utilization of iron phosphate slag has become a crucial technological direction for breakthroughs in this field. This invention cleverly overcomes the shortcomings of existing technologies, providing a simple, highly efficient, high-purity, and cost-controllable process for preparing battery-grade iron phosphate from waste iron phosphate slag, achieving an efficient combination of solid waste resource utilization and new energy material preparation.

[0055] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for recovering phosphorus-iron slag, characterized in that: Includes the following steps: S1. Mix waste ferrophosphate slag with concentrated sulfuric acid to obtain a mixture, wherein the molar ratio of sulfuric acid to iron in the ferrophosphate slag is 1.3~1.5:1; S2. After stirring the mixture, dilute it, remove the solid portion, and obtain an extract; S3. Adjust the pH of the leachate to 1.6~2.5, remove the liquid phase, and obtain amorphous iron phosphate; S4. After adding water to the amorphous ferric phosphate and slurrying, phosphoric acid is added and the mixture is aged to obtain ferric phosphate dihydrate. S5. Calcination yields battery-grade iron phosphate.

2. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: The concentrated sulfuric acid has a mass fraction of 68% to 80%.

3. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: The stirring time in step S2 is 3-6 hours; and / or the stirring speed in step S2 is 100-300 rpm.

4. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: The dilution step S2 specifically includes diluting with water to a phosphorus concentration of 1~1.5 mol / L.

5. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: In step S3, the pH is adjusted by adding ammonia water, and the mass concentration of the ammonia water is 5-15%.

6. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: The ammonia water is added at a rate of 15-35 mL / min; and / or, in step S3, the pH of the leachate is adjusted to 1.8-2.

2.

7. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: The aging temperature is 90~95℃.

8. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: The amount of phosphoric acid added in step S4 is 0.03~0.05 eq.

9. The method for recovering phosphorus-iron slag according to claim 1, characterized in that: The calcination temperature is 500~600℃, and the time is 3~5h.

10. The application of the method as described in any one of claims 1 to 9 in the recycling of lithium iron phosphate batteries.

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