Comprehensive utilization method of waste lithium iron phosphate lithium extraction slag

Abstract

The invention belongs to the technical field of metallurgy, and particularly relates to a comprehensive utilization method of waste lithium iron phosphate lithium extraction slag. According to the method, the waste lithium iron phosphate lithium extraction slag is used as a raw material, potassium hydroxide is used as an additive for aerobic calcination, carbon in the lithium extraction slag is combusted, lithium iron phosphate is decomposed, then aqueous solution leaching is carried out to enable phosphorus to be dissolved into a solution in a potassium phosphate form, a leaching solution is purified and then evaporated, concentrated and crystallized to form potassium phosphate, and the lithium iron phosphate is obtained. Potassium phosphate is dissolved in water to react with ferric chloride, calcium chloride, magnesium chloride and the like to prepare ferric phosphate, tricalcium phosphate or trimagnesium phosphate, and leaching residues are dried and converted into ironmaking raw materials with iron oxide as a main component. Through the steps of water leaching pretreatment, calcination oxidation, potassium phosphate conversion, leaching residue recovery and the like, the problems that in the prior art, the waste lithium iron phosphate lithium extraction residues are low in additional value and cannot be effectively utilized are solved, the waste lithium iron phosphate lithium extraction residues are effectively utilized, and efficient separation and high-value utilization of phosphorus and iron resources are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a comprehensive utilization method of waste lithium iron phosphate slag for lithium extraction. Background Art

[0002] Lithium iron phosphate batteries, with their core advantages of safety, environmental friendliness, long lifespan, and low cost, have become a mainstream choice for applications across multiple sectors, dominating the power battery and energy storage sectors. Despite challenges with energy density and low-temperature performance, technological advancements will continue to break through these bottlenecks. In the future, the application prospects of lithium iron phosphate batteries in the global power battery and energy storage markets will remain broad. Lithium iron phosphate batteries have a limited service life. When retired, they generate a large amount of waste battery materials, which contain a large number of valuable components such as lithium, phosphorus, and iron. Promoting the recycling of waste lithium iron phosphate battery materials can not only alleviate resource shortages, especially the demand for lithium resources, reduce external dependence, and ensure supply chain security, but also reduce pollution from waste batteries and carbon emissions from the battery industry.

[0003] The positive electrode material is the most expensive component of lithium iron phosphate battery production and is also the most valuable component of retired lithium iron phosphate. Currently, the most valuable lithium in the positive electrode material of used lithium iron phosphate batteries is recovered through a wet leaching process. After leaching, the lithium recovery rate can reach over 98%. The residue remaining after leaching and lithium extraction is mainly composed of FePO4, and may also contain impurities such as graphite, aluminum, copper, and fluoride. Due to the low added value of the residue after lithium extraction and the high cost of comprehensive utilization, it has not been reasonably utilized. In addition, the lithium extraction residue contains some harmful substances and is not suitable for direct treatment as ordinary solid waste. The comprehensive utilization of lithium extraction residue has become a key link in the resource recycling and environmental protection management of used lithium iron phosphate batteries.

[0004] Patent publication number CN118929597A proposes a method for recovering iron phosphate from lithium iron phosphate extraction residue and its application. The step-by-step acid leaching (high and low potential) requires precise control of the redox potential, making the operation complex. Deep aluminum removal requires sodium fluoride and a protective atmosphere, posing a high safety risk. Furthermore, the method fails to fully consider the diversified recovery of phosphorus resources and cannot recover carbon and iron resources from the leached residue.

[0005] Patent publication number CN118206092A proposes a method for recycling and treating waste lithium iron phosphate (LIFP) slag and its application. While the method primarily recovers valuable metals such as lithium, iron, and phosphorus, it fails to fully consider the diverse recovery of phosphorus resources, and the purity of the ferrous phosphate needs to be improved. Furthermore, the method relies on a multi-step acid leaching process (phosphoric acid + hydrochloric acid) and a complex impurity removal process (sodium fluoride for aluminum removal, sulfonated kerosene for extraction, and sulfide for titanium removal), resulting in high reagent consumption, high costs, and the generation of hazardous fluorine and sulfur-containing waste.

[0006] Patent publication number CN118993001A proposes a method for preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate extraction residue. This method focuses on preparing potassium phosphate and potassium dihydrogen phosphate from waste lithium iron phosphate extraction residue and on phosphorus recovery, but does not fully recycle iron. The lack of an effective iron recovery solution prevents efficient utilization of the iron in waste lithium iron phosphate extraction residue. Furthermore, the calcination process requires the addition of a petroleum coke reducing agent (100-300%), which consumes a lot of energy and introduces an additional carbon source. The resulting product is limited to potassium phosphate salts, and does not extend to high-value-added phosphate products such as iron phosphate. Summary of the Invention

[0007] The purpose of the present invention is to provide a comprehensive utilization method for waste lithium iron phosphate lithium extraction slag, to solve the problems of low added value and ineffective utilization of waste lithium iron phosphate lithium extraction slag in the prior art, to effectively utilize the waste lithium iron phosphate lithium extraction slag, to increase its added value, to achieve the recycling of valuable components such as phosphorus and iron, to reduce environmental pollution, and at the same time to optimize the process flow to reduce costs and promote the recycling of resources.

[0008] The technical solution of the present invention is: A comprehensive utilization method for waste lithium iron phosphate slag, comprising the following steps: (1) Water leaching pretreatment of lithium iron phosphate slag The waste lithium iron phosphate lithium extraction residue is ground to a particle size of less than 0.15 mm, added to deionized water, and leached in aqueous solution at a temperature of 20-100° C. for 10-60 minutes, with a liquid-to-solid mass ratio of 15:1-5:1 during the leaching process. After leaching, the aqueous residue and the primary leachate are obtained by filtration; calcium chloride is added to the primary leachate, and fluorine in the primary leachate is separated by sedimentation and filtration. The separated leachate is returned to the aqueous solution leaching step for recycling; (2) Calcination of water-leached slag The water-leached residue is dried at a temperature of 100-300° C. for 1-5 hours, ground to a particle size of less than 0.15 mm, and uniformly mixed with potassium hydroxide powder having a particle size of less than 0.15 mm to form agglomerates at a pressure of 50-200 MPa. The prepared agglomerates are placed in a calcining furnace for calcination. During the calcination process, air is introduced into the calcining furnace at a rate of 1-100 L / min to fully oxidize the carbon in the agglomerates, and calcined lithium-extracting slag is obtained after calcination. (3) Leaching and separation of phosphorus and iron from calcined materials The calcined lithium extraction slag is ground to a particle size of less than 0.15 mm, and then placed in a potassium hydroxide aqueous solution with a concentration of 10-50 g / L for alkali leaching. The alkali leaching is performed at a temperature of 20-80° C., a liquid-to-solid mass ratio of 5:1-2:1, and a leaching time of 0.5-10 h. After the alkali leaching, the leached slag and potassium phosphate leachate are obtained by filtering. (4) Potassium phosphate evaporation crystallization The potassium phosphate leachate is evaporated to 60-80% at a temperature of 90-150°C for concentration, and distilled water and a concentrated solution are obtained after evaporation; the concentrated solution is cooled to below 40°C in a cooling crystallizer to precipitate potassium phosphate trihydrate crystals, which are filtered or centrifuged to obtain potassium phosphate trihydrate and a concentrated mother liquor; the concentrated mother liquor is returned to the potassium phosphate leachate of step (3), and the potassium phosphate is evaporated and crystallized; (5) Conversion of potassium phosphate trihydrate The potassium phosphate trihydrate obtained by evaporation, concentration, cooling and crystallization is dissolved in distilled water, and the pH is adjusted to 6-8 to obtain a potassium phosphate aqueous solution, wherein the liquid-solid mass ratio of the potassium phosphate trihydrate to the distilled water is 2:1-5:1; magnesium chloride, ferric chloride or calcium chloride is dissolved in distilled water in another container, wherein the liquid-solid mass ratio of the magnesium chloride, ferric chloride or calcium chloride to the distilled water is 2:1-5:1 to obtain a corresponding magnesium chloride aqueous solution, ferric chloride aqueous solution or calcium chloride aqueous solution; the potassium phosphate aqueous solution is mixed with one of the magnesium chloride aqueous solution, ferric chloride aqueous solution or calcium chloride aqueous solution, and stirred for conversion, so that a precipitate appears in the solution, and the potassium chloride aqueous solution and trimagnesium phosphate, ferric phosphate or tricalcium phosphate precipitate are obtained after filtration and separation, respectively; (6) Treatment of leaching residue The calcined lithium extraction slag is leached to obtain iron-containing leaching slag, which is calcined at 500-800° C. for 2-5 hours and used as raw material for ironmaking.

[0009] The method for comprehensive utilization of waste lithium iron phosphate slag comprises the following steps: in step (1), during the aqueous solution leaching process, mechanical stirring is adopted, so that all sodium ions and chloride ions enter the primary leachate, and part of the fluoride ions enter the primary leachate.

[0010] The method for comprehensive utilization of waste lithium iron phosphate slag, in step (1), the amount of calcium chloride added is 95-100% of the required mass according to the reaction theory of reaction formula (1) based on the fluoride ion content in the primary leachate; CaCl2+2F - = CaF2+2Cl - (1).

[0011] The method for comprehensively utilizing waste lithium iron phosphate slag for lithium extraction comprises the following steps: in step (2), the proportion of potassium hydroxide to the leached slag is determined by the phosphorus content in the leached slag; the potassium hydroxide is added in an amount of 120-200% of the theoretically required mass in reaction formula (2); the carbon in the leached slag undergoes an oxidation reaction according to reaction formula (3); in addition to reacting with potassium hydroxide according to reaction formula (2), the iron phosphate also generates potassium ferrate K2FeO4 according to reaction formula (4) and potassium ferrite KFeO2 according to reaction formula (5); 2FePO4+6KOH = 2K3PO4+Fe2O3+3H2O (2) C + O2 = CO2 (3) 4FePO4+20KOH+3O2= 4K3PO4+4K2FeO4+10H2O (4) 2FePO4+8KOH = 2K3PO4+2KFeO2+4H2O (5).

[0012] The method for comprehensive utilization of waste lithium iron phosphate slag comprises the following steps: during the calcination process in step (2), the calcination temperature is 200°C to 600°C, and the calcination time is 1 to 10 hours; after the calcination is completed, the agglomerate is taken out of the calcination furnace and cooled in air.

[0013] The method for comprehensive utilization of waste lithium iron phosphate slag for lithium extraction comprises the following steps: in step (3), during the alkali solution leaching process, mechanical stirring is used, potassium phosphate is directly dissolved in the solution, potassium ferrite KFeO2 undergoes a decomposition reaction according to reaction formula (6), and potassium ferrate K2FeO4 undergoes a decomposition reaction according to reaction formula (7); after leaching, phosphorus enters the liquid phase as potassium phosphate, and iron and other impurities remain in the slag phase, thereby achieving separation of P and Fe; 2KFeO2+4H2O = 2Fe(OH)3+2KOH (6) 4K2FeO4+10H2O = 4Fe(OH)3+8KOH+3O2 (7).

[0014] The method for comprehensive utilization of waste lithium iron phosphate slag for lithium extraction, in step (5), trimagnesium phosphate, iron phosphate or tricalcium phosphate precipitate is dried at a temperature of 100-300°C for 1-5 hours to obtain trimagnesium phosphate, iron phosphate or tricalcium phosphate products.

[0015] In the method for comprehensive utilization of waste lithium iron phosphate slag, in step (5), a potassium chloride aqueous solution is evaporated and crystallized at a temperature of 90-150° C. to obtain potassium chloride crystals and distilled water.

[0016] The design concept of the present invention is: In the prior art, the waste residue from lithium extraction from used lithium iron phosphate batteries is primarily composed of iron phosphate (FePO4), also containing impurities such as graphite, aluminum, copper, and fluoride. Due to its low added value and high comprehensive utilization costs, these waste residues have not been effectively utilized. The method of the present invention uses used lithium iron phosphate extraction residue as raw material and aerobic calcination with potassium hydroxide as an additive. This burns the carbon in the residue and simultaneously decomposes the lithium iron phosphate. Then, aqueous leaching is performed to dissolve phosphorus into the solution as potassium phosphate. The leachate is purified and evaporated to concentrate the crystallized potassium phosphate. The potassium phosphate is then dissolved in water and reacted with ferric chloride, calcium chloride, magnesium chloride, and other substances to produce iron phosphate, tricalcium phosphate, or trimagnesium phosphate. The leached residue is then dried and converted into an ironmaking raw material primarily composed of iron oxide. Thus, through the steps of aqueous leaching pretreatment, calcination and oxidation, potassium phosphate conversion, and leach residue recovery, efficient separation and high-value utilization of phosphorus and iron resources are achieved.

[0017] The core innovations and technical advantages of this invention are as follows: (1) Aerobic calcination: The waste lithium iron phosphate extraction slag is mixed with potassium hydroxide and then aerobic calcined to fully oxidize and remove the carbon and decompose the iron phosphate into a water-soluble potassium phosphate form.

[0018] (2) Aqueous solution leaching separation: Aqueous solution leaching is used to make phosphorus enter the liquid phase in the form of potassium phosphate, thereby separating phosphorus from iron and other impurities.

[0019] (3) Conversion and utilization of potassium phosphate: Potassium phosphate is converted into high value-added products such as trimagnesium phosphate, iron phosphate and tricalcium phosphate through steps such as evaporation, concentration and crystallization.

[0020] (4) Recycling of iron slag: The iron element in the leached slag is converted into ironmaking raw materials whose main component is iron oxide, thus realizing the recycling of iron resources.

[0021] Therefore, the present invention adopts a method of aerobic calcination combined with aqueous solution leaching to effectively remove carbon impurities and achieve phosphorus and iron separation. The process flow is reasonably designed. Through the synergistic effect of each step, efficient treatment and resource recovery of waste lithium iron phosphate extraction waste residue are achieved. Not only phosphorus and iron elements are recovered, but also a variety of high value-added products such as trimagnesium phosphate, iron phosphate and tricalcium phosphate are prepared through further conversion steps, which broadens the utilization path of waste lithium iron phosphate extraction waste residue.

[0022] The advantages and beneficial effects of the present invention are: 1. High value-added utilization of lithium extraction slag. The comprehensive utilization method of waste lithium iron phosphate lithium extraction slag of the present invention can achieve the separation of phosphorus and iron. After treatment, products such as trimagnesium phosphate, iron phosphate, or tricalcium phosphate can be obtained. The products have high added value. The separated iron is used as a raw material for ironmaking. The entire process is free of secondary pollution. This is an energy-saving and environmentally friendly method for comprehensive utilization of lithium extraction slag, and has good industrial application prospects.

[0023] 2. Effective separation and recovery of phosphorus and iron. Lithium extraction slag contains a large amount of phosphorus and iron, but existing technologies have difficulty in achieving effective separation and recovery of phosphorus and iron. The method of the present invention achieves effective separation of phosphorus and iron through steps such as water leaching pretreatment, aerobic calcination, and leaching separation. Phosphorus enters the solution in the form of potassium phosphate and is further converted into products such as trimagnesium phosphate, iron phosphate, or tricalcium phosphate; iron exists in the leaching slag in the form of iron oxide and can be used as a raw material for ironmaking, achieving efficient recovery and utilization of phosphorus and iron resources.

[0024] 3. Reduce environmental pollution. Lithium extraction slag contains some harmful substances and should not be directly treated as ordinary solid waste. The method of the present invention comprehensively treats the lithium extraction slag, avoiding environmental pollution caused by harmful substances and achieving harmless treatment of waste lithium iron phosphate extraction slag.

[0025] 4. Optimization and simplification of process flow. The existing lithium slag treatment process is complex and costly. The present invention provides a relatively simple and efficient treatment method. Through reasonable process design and step optimization, it reduces treatment costs and improves treatment efficiency, making the comprehensive utilization of lithium slag more economically feasible and promising for industrial application.

[0026] 5. Compared with patent publication number CN118929597A, the present invention fully utilizes the phosphorus and iron components in waste lithium iron phosphate extraction waste residue, not only recovering valuable metals but also converting other components into useful products in subsequent steps, thus avoiding resource waste. During the water leaching pretreatment step, calcium chloride is added to the primary leachate to effectively remove fluoride ions. At the same time, sodium and chloride enter the primary leachate and are separated and removed, ensuring the purity of the solution in subsequent steps.

[0027] 6. Compared to patent publication number CN118206092A, this invention fully utilizes the phosphorus and iron components in waste lithium iron phosphate extraction residue. Not only are phosphorus and iron recovered, but subsequent steps also convert them into a variety of high-value-added products (such as trimagnesium phosphate, iron phosphate, and tricalcium phosphate), avoiding resource waste. The combined aerobic calcination and water leaching method eliminates the need for large amounts of acid and alkali, reduces the use of chemical reagents, lowers processing costs, and avoids strong corrosion to equipment, minimizing environmental pollution risks.

[0028] 7. Compared with the patent publication number CN118993001A, the present invention realizes the full utilization of phosphorus and iron in waste lithium iron phosphate extraction waste slag. The phosphorus element is converted into products such as potassium phosphate, and the iron element is converted into iron oxide and other products and used as ironmaking raw materials, thereby improving the comprehensive utilization rate of resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The present invention is a process flow chart for the comprehensive utilization of waste lithium iron phosphate slag for lithium extraction. DETAILED DESCRIPTION

[0030] like Figure 1 As shown, the process flow of the comprehensive utilization of waste lithium iron phosphate slag for lithium extraction of the present invention is as follows: (1) Using aqueous solution (deionized water) as a leaching agent, the waste lithium iron phosphate lithium extraction residue is leached with aqueous solution, and after leaching, the aqueous leaching residue and the primary leachate are obtained by filtration.

[0031] (2) The water-leached residue is dried and ground into a particle size of less than 0.15 mm, and mixed with potassium hydroxide powder with a particle size of less than 0.15 mm to form a mass. The mass is placed in a calcining furnace for calcination to obtain calcined lithium-extracting slag.

[0032] (3) The calcined lithium extraction slag is ground to a particle size of less than 0.15 mm and placed in a potassium hydroxide aqueous solution for alkaline leaching. During the leaching process, potassium phosphate is directly dissolved in the solution. After leaching, the leached slag and potassium phosphate leachate are obtained by filtration.

[0033] (4) The potassium phosphate leachate is evaporated, concentrated, and crystallized to precipitate potassium phosphate trihydrate crystals, which are then filtered or centrifuged to obtain potassium phosphate trihydrate and concentrated mother liquor. The concentrated mother liquor is returned to the potassium phosphate leachate, and the potassium phosphate leachate is evaporated, concentrated, and crystallized again.

[0034] (5) Dissolve the potassium phosphate trihydrate obtained by evaporation, concentration, cooling and crystallization in distilled water, and adjust the pH to 6-8 to obtain a potassium phosphate aqueous solution. In another container, dissolve magnesium chloride, ferric chloride or calcium chloride in distilled water to obtain the corresponding magnesium chloride aqueous solution, ferric chloride aqueous solution or calcium chloride aqueous solution. Mix the potassium phosphate aqueous solution with one of the magnesium chloride aqueous solution, ferric chloride aqueous solution or calcium chloride aqueous solution, stir and convert, and a precipitate will appear in the solution. After filtering and separation, potassium chloride aqueous solution and trimagnesium phosphate, ferric phosphate or tricalcium phosphate precipitate are obtained respectively. After drying the trimagnesium phosphate, ferric phosphate or tricalcium phosphate precipitate, trimagnesium phosphate, ferric phosphate or tricalcium phosphate product can be obtained; the potassium chloride aqueous solution is evaporated and crystallized at a temperature of 90-150°C to obtain potassium chloride crystals and distilled water.

[0035] (6) The main components of the leached slag obtained after calcining the lithium extraction slag and leaching it with alkaline solution are Fe2O3 and Fe(OH)3. The leached slag is obtained after calcining and is used as a raw material for ironmaking.

[0036] Below, the present invention is further described in detail with reference to the embodiments.

[0037] Example 1 (trimagnesium phosphate): In this embodiment, the main components of the waste lithium iron phosphate lithium extraction slag are, by mass percentage, TFe (total iron): 32.79%, P: 16.66%, C: 5.23%, Al2O3: 0.72%, Cu<0.01%, Cl: 0.03%, Na: 0.159%, F: 0.072%, and the remainder is mainly O.

[0038] The comprehensive utilization method of waste lithium iron phosphate slag for lithium extraction comprises the following steps: (1) Grind the waste lithium iron phosphate lithium extraction slag to a particle size of less than 0.15 mm, add it to deionized water, and carry out aqueous solution leaching at a temperature of 60°C for 30 minutes. The liquid-solid mass ratio of the leaching process is 10:1. After leaching, filter to obtain aqueous leaching residue and primary leachate; add calcium chloride to the primary leachate, and separate the fluorine in the primary leachate by sedimentation filtration. The separated leachate is returned to the aqueous solution leaching step for recycling; the amount of calcium chloride added is based on the fluoride ion content in the primary leachate, and is added according to 98% of the required mass according to the reaction theory of reaction formula (1).

[0039] (2) The water-leached slag is dried at 200°C for 3 h, ground to a particle size of less than 0.15 mm, and mixed evenly with potassium hydroxide powder with a particle size of less than 0.15 mm to form a pellet. The pelletizing pressure is 100 MPa. The prepared pellets are placed in a calcining furnace for calcination. During the calcination process, air is introduced into the calcining furnace at a rate of 50 L / min to fully oxidize the carbon in the pellets. After calcination, calcined lithium-extracting slag is obtained. The proportion of potassium hydroxide to the water-leached slag is determined by the phosphorus content in the water-leached slag. Potassium hydroxide is added according to 150% of the theoretical required mass in reaction formula (2). During the calcination process, the calcination temperature is 400°C and the calcination time is 5 h. After the calcination is completed, the pellets are taken out of the calcining furnace and cooled in air.

[0040] (3) The calcined lithium extraction slag was ground to a particle size of less than 0.15 mm and placed in a potassium hydroxide aqueous solution with a concentration of 30 g / L for alkaline leaching. The alkaline leaching was carried out at a temperature of 50°C, the liquid-to-solid mass ratio of the leaching process was 3:1, and the leaching time was 5 h. After the alkaline leaching, the leached slag and potassium phosphate leachate were obtained by filtration.

[0041] (4) The potassium phosphate leachate is evaporated to 70% at a temperature of 120° C. for concentration, and distilled water and a concentrated solution are obtained after evaporation; the concentrated solution is cooled to 30° C. in a cooling crystallizer to precipitate potassium phosphate trihydrate crystals, which are filtered or centrifuged to obtain potassium phosphate trihydrate and a concentrated mother liquor; the concentrated mother liquor is returned to the potassium phosphate leachate of step (3), and the potassium phosphate is evaporated and crystallized.

[0042] (5) The potassium phosphate trihydrate obtained by evaporation, concentration, cooling and crystallization is dissolved in distilled water, and the pH is adjusted to 7 to obtain a potassium phosphate aqueous solution, and the liquid-solid mass ratio of potassium phosphate trihydrate to distilled water is 3:1; in another container, magnesium chloride is dissolved in distilled water, and the liquid-solid mass ratio of magnesium chloride to distilled water is 3:1 to obtain a magnesium chloride aqueous solution; the potassium phosphate aqueous solution and the magnesium chloride aqueous solution are mixed and stirred for conversion, and a precipitate will appear in the solution. After filtration and separation, the potassium chloride aqueous solution and the trimagnesium phosphate precipitate are obtained respectively; the trimagnesium phosphate precipitate is dried at a temperature of 200°C for 3 hours to obtain the trimagnesium phosphate product. The potassium chloride aqueous solution is evaporated and crystallized at a temperature of 120°C to obtain potassium chloride crystals and distilled water.

[0043] (6) The calcined lithium slag is leached to obtain iron-containing leaching slag, which is calcined at 650°C for 3 hours and used as raw material for ironmaking.

[0044] After testing, the purity of the finally obtained trimagnesium phosphate was 98.5wt%, the purity of potassium chloride was 99.2wt%, and the iron content in the ironmaking raw materials was 65wt%.

[0045] Example 2 (ferric phosphate): In this embodiment, the main components of the waste lithium iron phosphate lithium extraction slag are TFe: 32.79%, P: 16.66%, C: 5.23%, Al2O3: 0.72%, Cu<0.01%, Cl: 0.03%, Na: 0.159%, F: 0.072%, and the remainder is mainly O.

[0046] The comprehensive utilization method of waste lithium iron phosphate slag for lithium extraction comprises the following steps: (1) Grind the waste lithium iron phosphate lithium extraction slag to a particle size of less than 0.15 mm, add it to deionized water, and carry out aqueous solution leaching at a temperature of 80°C for 20 minutes. The liquid-solid mass ratio of the leaching process is 8:1. After leaching, filter to obtain aqueous leaching residue and primary leachate; add calcium chloride to the primary leachate, separate the fluorine in the primary leachate by sedimentation filtration, and return the separated leachate to the aqueous solution leaching step for recycling; the amount of calcium chloride added is based on the fluoride ion content in the primary leachate, and is 96% of the required mass according to the reaction theory of reaction formula (1).

[0047] (2) The water-leached slag is dried at 150°C for 4 hours, ground to a particle size of less than 0.15 mm, and mixed evenly with potassium hydroxide powder with a particle size of less than 0.15 mm to form a pellet. The pelletizing pressure is 150 MPa. The prepared pellets are placed in a calcining furnace for calcination. During the calcination process, air is introduced into the calcining furnace at a rate of 80 L / min to fully oxidize the carbon in the pellets. After calcination, calcined lithium-extracting slag is obtained. The proportion of potassium hydroxide to the water-leached slag is determined by the phosphorus content in the water-leached slag. Potassium hydroxide is added according to 180% of the theoretical required mass in reaction formula (2). During the calcination process, the calcination temperature is 500°C and the calcination time is 8 hours. After the calcination is completed, the pellets are taken out of the calcining furnace and cooled in air.

[0048] (3) The calcined lithium extraction slag was ground to a particle size of less than 0.15 mm and placed in a potassium hydroxide aqueous solution with a concentration of 40 g / L for alkaline leaching. The alkaline leaching was carried out at a temperature of 60°C, the liquid-to-solid mass ratio of the leaching process was 4:1, and the leaching time was 8 h. After the alkaline leaching, the leached slag and potassium phosphate leachate were obtained by filtration.

[0049] (4) The potassium phosphate leachate is evaporated to 75% at a temperature of 130° C. for concentration, and distilled water and a concentrated solution are obtained after evaporation; the concentrated solution is cooled to 35° C. in a cooling crystallizer to precipitate potassium phosphate trihydrate crystals, which are filtered or centrifuged to obtain potassium phosphate trihydrate and a concentrated mother liquor; the concentrated mother liquor is returned to the potassium phosphate leachate of step (3), and the potassium phosphate is evaporated and crystallized.

[0050] (5) The potassium phosphate trihydrate obtained by evaporation, concentration, cooling and crystallization is dissolved in distilled water, and the pH is adjusted to 6.5 to obtain a potassium phosphate aqueous solution, and the liquid-solid mass ratio of potassium phosphate trihydrate to distilled water is 2:1; in another container, ferric chloride is dissolved in distilled water, and the liquid-solid mass ratio of ferric chloride to distilled water is 2:1 to obtain a ferric chloride aqueous solution; the potassium phosphate aqueous solution and the ferric chloride aqueous solution are mixed and stirred for conversion, and a precipitate will appear in the solution. After filtration and separation, the potassium chloride aqueous solution and the ferric phosphate precipitate are respectively obtained; the ferric phosphate precipitate is dried at a temperature of 150°C for 4 hours to obtain the ferric phosphate product. The potassium chloride aqueous solution is evaporated and crystallized at a temperature of 100°C to obtain potassium chloride crystals and distilled water.

[0051] (6) The calcined lithium slag is leached to obtain iron-containing leaching slag, which is calcined at 700°C for 4 hours and used as raw material for ironmaking.

[0052] After testing, the purity of the final iron phosphate obtained was 99wt%, the purity of potassium chloride was 99.5wt%, and the iron content in the ironmaking raw materials was 68wt%.

[0053] Example 3 (tricalcium phosphate): In this embodiment, the main components of the waste lithium iron phosphate lithium extraction slag are TFe: 32.79%, P: 16.66%, C: 5.23%, Al2O3: 0.72%, Cu<0.01%, Cl: 0.03%, Na: 0.159%, F: 0.072%, and the remainder is mainly O.

[0054] The comprehensive utilization method of waste lithium iron phosphate slag for lithium extraction comprises the following steps: (1) Grind the waste lithium iron phosphate lithium slag to a particle size of less than 0.15 mm, add it to deionized water, and carry out aqueous solution leaching at a temperature of 100°C for 10 minutes. The liquid-solid mass ratio of the leaching process is 6:1. After leaching, filter to obtain aqueous leaching residue and primary leachate; add calcium chloride to the primary leachate, and separate the fluorine in the primary leachate by sedimentation filtration. The separated leachate is returned to the aqueous solution leaching step for recycling; the amount of calcium chloride added is based on the fluoride ion content in the primary leachate, and 97% of the mass required according to the reaction theory of reaction formula (1) is added.

[0055] (2) The water-leached slag is dried at 250°C for 2 h, ground to a particle size of less than 0.15 mm, and mixed evenly with potassium hydroxide powder with a particle size of less than 0.15 mm to form a pellet. The pelletizing pressure is 80 MPa. The prepared pellets are placed in a calcining furnace for calcination. During the calcination process, air is introduced into the calcining furnace at a rate of 30 L / min to fully oxidize the carbon in the pellets. After calcination, calcined lithium-extracting slag is obtained. The proportion of potassium hydroxide to the water-leached slag is determined by the phosphorus content in the water-leached slag. Potassium hydroxide is added according to 130% of the theoretical required mass in reaction formula (2). During the calcination process, the calcination temperature is 300°C and the calcination time is 6 h. After the calcination is completed, the pellets are taken out of the calcining furnace and cooled in air.

[0056] (3) The calcined lithium extraction slag was ground to a particle size of less than 0.15 mm and placed in a potassium hydroxide aqueous solution with a concentration of 20 g / L for alkaline leaching. The alkaline leaching was carried out at a temperature of 40°C, the liquid-to-solid mass ratio of the leaching process was 3:1, and the leaching time was 6 h. After the alkaline leaching, the leached slag and potassium phosphate leachate were obtained by filtration.

[0057] (4) The potassium phosphate leachate is evaporated at a temperature of 110° C. by 65% ​​for concentration, and distilled water and a concentrated solution are obtained after evaporation; the concentrated solution is cooled to 32° C. in a cooling crystallizer to precipitate potassium phosphate trihydrate crystals, which are filtered or centrifuged to obtain potassium phosphate trihydrate and a concentrated mother liquor; the concentrated mother liquor is returned to the potassium phosphate leachate of step (3), and the potassium phosphate is evaporated and crystallized.

[0058] (5) The potassium phosphate trihydrate obtained by evaporation, concentration, cooling and crystallization is dissolved in distilled water, and the pH is adjusted to 7.5 to obtain a potassium phosphate aqueous solution, and the liquid-solid mass ratio of potassium phosphate trihydrate to distilled water is 4:1; in another container, calcium chloride is dissolved in distilled water, and the liquid-solid mass ratio of calcium chloride to distilled water is 3:1 to obtain a calcium chloride aqueous solution; the potassium phosphate aqueous solution and the calcium chloride aqueous solution are mixed and stirred for conversion, and a precipitate will appear in the solution. After filtration and separation, the potassium chloride aqueous solution and tricalcium phosphate precipitate are obtained respectively; the tricalcium phosphate precipitate is dried at 250°C for 2h to obtain the tricalcium phosphate product. The potassium chloride aqueous solution is evaporated and crystallized at 140°C to obtain potassium chloride crystals and distilled water.

[0059] (6) The calcined lithium slag is leached to obtain iron-containing leaching slag, which is calcined at 550°C for 5 hours and used as raw material for ironmaking.

[0060] After testing, the purity of the final tricalcium phosphate obtained was 98.8wt%, the purity of potassium chloride was 99.3wt%, and the iron content in the ironmaking raw materials was 62wt%.

[0061] Implementation results demonstrate that, through a series of processing steps, the present invention converts lithium-extraction slag into a variety of high-value-added products, such as trimagnesium phosphate, iron phosphate, or tricalcium phosphate (with a purity exceeding 98% by weight). The purity of the potassium chloride in these products reaches over 99% by weight, and the iron content in the ironmaking raw material (calcined slag) reaches over 60% by weight, significantly increasing the utilization value of the lithium-extraction slag. This invention is suitable for large-scale recycling of spent lithium iron phosphate batteries, enabling the simultaneous production of phosphate products and ironmaking raw materials. It offers significant economic benefits, a green and low-carbon process, and broad potential for industrialization and application.

Claims

1. A comprehensive utilization method for waste lithium iron phosphate slag, characterized in that: The following steps are involved: (1) Water leaching pretreatment of lithium iron phosphate slag The waste lithium iron phosphate lithium extraction residue is ground to a particle size of less than 0.15 mm, added to deionized water, and leached in aqueous solution at a temperature of 20-100° C. for 10-60 minutes, with a liquid-to-solid mass ratio of 15:1-5:1 during the leaching process. After leaching, the aqueous residue and the primary leachate are obtained by filtration; calcium chloride is added to the primary leachate, and fluorine in the primary leachate is separated by sedimentation and filtration. The separated leachate is returned to the aqueous solution leaching step for recycling; (2) Calcination of water-leached slag The water-leached residue is dried at a temperature of 100-300° C. for 1-5 hours, ground to a particle size of less than 0.15 mm, and uniformly mixed with potassium hydroxide powder having a particle size of less than 0.15 mm to form agglomerates at a pressure of 50-200 MPa. The prepared agglomerates are placed in a calcining furnace for calcination. During the calcination process, air is introduced into the calcining furnace at a rate of 1-100 L / min to fully oxidize the carbon in the agglomerates, and calcined lithium-extracting slag is obtained after calcination. (3) Leaching and separation of phosphorus and iron from calcined materials The calcined lithium extraction slag is ground to a particle size of less than 0.15 mm, and then placed in a potassium hydroxide aqueous solution with a concentration of 10-50 g / L for alkali leaching. The alkali leaching is performed at a temperature of 20-80° C., a liquid-to-solid mass ratio of 5:1-2:1, and a leaching time of 0.5-10 h. After the alkali leaching, the leached slag and potassium phosphate leachate are obtained by filtering. (4) Potassium phosphate evaporation crystallization The potassium phosphate leachate is evaporated to 60-80% at a temperature of 90-150°C for concentration, and distilled water and a concentrated solution are obtained after evaporation; the concentrated solution is cooled to below 40°C in a cooling crystallizer to precipitate potassium phosphate trihydrate crystals, which are filtered or centrifuged to obtain potassium phosphate trihydrate and a concentrated mother liquor; the concentrated mother liquor is returned to the potassium phosphate leachate of step (3), and the potassium phosphate is evaporated and crystallized; (5) Conversion of potassium phosphate trihydrate The potassium phosphate trihydrate obtained by evaporation, concentration, cooling and crystallization is dissolved in distilled water, and the pH is adjusted to 6-8 to obtain a potassium phosphate aqueous solution, wherein the liquid-solid mass ratio of the potassium phosphate trihydrate to the distilled water is 2:1-5:1; magnesium chloride, ferric chloride or calcium chloride is dissolved in distilled water in another container, wherein the liquid-solid mass ratio of the magnesium chloride, ferric chloride or calcium chloride to the distilled water is 2:1-5:1 to obtain a corresponding magnesium chloride aqueous solution, ferric chloride aqueous solution or calcium chloride aqueous solution; the potassium phosphate aqueous solution is mixed with one of the magnesium chloride aqueous solution, ferric chloride aqueous solution or calcium chloride aqueous solution, and stirred for conversion, so that a precipitate appears in the solution, and the potassium chloride aqueous solution and trimagnesium phosphate, ferric phosphate or tricalcium phosphate precipitate are obtained after filtration and separation, respectively; (6) Treatment of leaching residue The calcined lithium extraction slag is leached to obtain iron-containing leaching slag, which is calcined at 500-800° C. for 2-5 hours and used as raw material for ironmaking.

2. The comprehensive utilization method of waste lithium iron phosphate slag according to claim 1, characterized in that: During the aqueous solution leaching process in step (1), mechanical stirring is adopted, so that all the sodium ions and chloride ions enter into the primary leachate, and part of the fluoride ions enter into the primary leachate.

3. The comprehensive utilization method of waste lithium iron phosphate slag according to claim 1 or 2, characterized in that: In step (1), the amount of calcium chloride added is 95-100% of the required mass according to the reaction theory of reaction formula (1) based on the fluoride ion content in the primary leachate; CaCl2+2F - = CaF2+2Cl - (1)。 4. The comprehensive utilization method of waste lithium iron phosphate slag according to claim 1, characterized in that: In step (2), the proportion of potassium hydroxide to the leached residue is determined by the phosphorus content in the leached residue. Potassium hydroxide is added according to 120-200% of the theoretically required mass in reaction formula (2), and the carbon in the leached residue undergoes an oxidation reaction according to reaction formula (3). In addition to reacting with potassium hydroxide according to reaction formula (2), ferric phosphate also generates potassium ferrate K2FeO4 according to reaction formula (4) and potassium ferrite KFeO2 according to reaction formula (5); 2FePO4+6KOH = 2K3PO4+Fe2O3+3H2O (2) C + O2 = CO2 (3) 4FePO4+20KOH+3O2 = 4K3PO4+4K2FeO4+10H2O (4) 2FePO4+8KOH = 2K3PO4+2KFeO2+4H2O (5).

5. The comprehensive utilization method of waste lithium iron phosphate slag according to claim 1 or 4, characterized in that: During the calcination process in step (2), the calcination temperature is 200° C. to 600° C., and the calcination time is 1 to 10 hours. After the calcination is completed, the agglomerate is taken out of the calcination furnace and cooled in air.

6. The comprehensive utilization method of waste lithium iron phosphate slag according to claim 1 or 4, characterized in that: During the alkaline solution leaching process in step (3), mechanical stirring is used, potassium phosphate is directly dissolved in the solution, potassium ferrite KFeO2 undergoes a decomposition reaction according to reaction formula (6), and potassium ferrate K2FeO4 undergoes a decomposition reaction according to reaction formula (7); after leaching, phosphorus enters the liquid phase as potassium phosphate, and iron and other impurities remain in the slag phase, thereby achieving separation of P and Fe; 2KFeO2+4H2O=2Fe(OH)3+2KOH (6) 4K2FeO4+10H2O=4Fe(OH)3+8KOH+3O2 (7).

7. The comprehensive utilization method of waste lithium iron phosphate slag according to claim 1, characterized in that: In step (5), the trimagnesium phosphate, iron phosphate or tricalcium phosphate precipitate is dried at a temperature of 100-300° C. for 1-5 hours to obtain the trimagnesium phosphate, iron phosphate or tricalcium phosphate product.

8. The comprehensive utilization method of waste lithium iron phosphate slag according to claim 1, characterized in that: In step (5), the potassium chloride aqueous solution is evaporated and crystallized at a temperature of 90-150° C. to obtain potassium chloride crystals and distilled water.

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