Method for efficiently recovering lithium and ferrous sulfate heptahydrate in retired lithium iron phosphate through total leaching and green impurity removal

Abstract

The invention relates to the technical field of battery material recovery, and particularly discloses a method for efficiently recovering lithium and ferrous sulfate heptahydrate in retired lithium iron phosphate through total leaching and green impurity removal. Comprising the following steps: 1, adding strong base and a carbon source into black powder, and calcining at high temperature in an inert atmosphere; step 2, dissolving in pure water after calcining, and carrying out suction filtration to obtain a lithium salt solution and ferrous hydroxide; 3, sulfuric acid is added into ferrous hydroxide for suction filtration, and graphite and a ferric salt solution are obtained; drying the ferric salt solution to obtain ferrous sulfate heptahydrate; step 4, adding alkali into the lithium salt solution to obtain a purified lithium salt solution; 5, introducing carbon dioxide into the purified lithium salt solution, and adding alkali to generate lithium carbonate precipitate; and 6, finally carrying out heat preservation, suction filtration, washing and drying to obtain a lithium carbonate finished product. The technical problems that in the prior art, a traditional decommissioned lithium iron phosphate battery grading recycling technology is complex in production process, more waste liquid pollution is generated, and the production cost is relatively high are solved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of battery material recycling, and particularly discloses a method for efficiently recycling lithium and ferrous sulfate heptahydrate in retired lithium iron phosphate by full leaching and green impurity removal. BACKGROUND

[0002] Lithium iron phosphate batteries have been widely used in electronic products, electric vehicles, aerospace and energy storage due to their good cycle performance and high safety. However, the service life of lithium iron phosphate batteries is about 5-7 years, and a large number of lithium iron phosphate batteries are retired at the end of their service life. Due to the high consumption of lithium iron phosphate ion power batteries, a large number of waste lithium iron phosphate batteries will have to be disposed of soon. It is predicted that the global production of waste lithium ion batteries will reach 1.36 million tons by 2025, but only 5% of the waste lithium batteries are recycled, and resources such as iron and phosphorus are often discarded, causing serious environmental pollution and resource waste.

[0003] At present, the recycling process of retired LiFePO4 batteries gradually shows a trend of integration from two independent processes of pyrometallurgy and hydrometallurgy, forming a new process of pyrometallurgy + hydrometallurgy. First, the black powder is calcined by pyrometallurgy, and then the Li, Fe and P elements in the battery material are leached by hydrometallurgy. Finally, lithium is recovered in the form of lithium carbonate in the leaching solution by using sodium carbonate as a precipitant. For example, the patent document with the publication number CN115369249B discloses a recycling method for lithium iron phosphate battery black powder, which is to first calcine the black powder and then use water for leaching and precipitation.

[0004] Although the above process can achieve the recycling of lithium, iron and other resources in the battery, this method has some significant defects: the recycling process does not fully consider the recycling of iron, graphite and other materials, and the iron phosphate slag mixture produced in the leaching process is only simply treated by magnetic separation. For example, the patent document with the publication number CN117509743A also does so for the leaching residue, which is simply used to magnetically separate the iron oxide product for sale. This method not only cannot fully recycle the iron in the filter residue, but also causes the recycled iron and graphite materials to have low purity and low value for sale, and even can only be treated as solid waste, causing serious resource waste.

[0005] In order to improve the value of various materials in the recycling of retired LiFePO4 battery and fully recycle, a grading recycling process appears in the industry. For example, the patent document CN119725837A discloses a grading recycling method of lithium, iron, phosphorus and graphite in waste lithium iron phosphate battery. The process is to calcine the recovered battery black powder, then acid leaching, using sodium carbonate as a precipitant to precipitate lithium in the leaching solution in the form of lithium carbonate, and ball milling the filter residue. After ball milling, sulfuric acid, sodium hydroxide and hydrogen peroxide are added to leach the iron salt solution and graphite filter residue again, and the iron and graphite materials are fully recovered by re-filtering.

[0006] Although the above grading recycling process can grade recycle lithium, iron, phosphorus and graphite materials, when recycling iron and graphite, not only ball milling of the filter residue is needed, but also chemical reagents such as sodium hydroxide and hydrogen peroxide are needed. The production process is complicated, more waste liquid pollution is generated, the production cost is relatively high, and the value of practical application in production is low. SUMMARY

[0007] The present application aims to provide a treatment method for reducing the moisture content of sludge to solve the technical problems of the traditional grading recycling process of retired lithium iron phosphate battery in the prior art, which is not only complicated in production process, generates more waste liquid pollution, but also leads to relatively high production cost, and the value of practical application in production is low.

[0008] To solve the above problems, the technical scheme adopted by the present application is as follows: a treatment method for reducing the moisture content of sludge, comprising the following steps: Step 1, adding strong alkali and carbon source to the black powder after crushing the retired lithium iron phosphate battery to form a black powder mixture, and placing the black powder mixture in an inert atmosphere for high temperature calcination; the high temperature calcination temperature is 550-800℃; the calcination time is 2-5 h; Step 2, dissolving the black powder mixture after high temperature calcination and cooling in pure water, stirring and mixing, then separating by filtration to obtain a lithium salt solution containing metal impurities and a carbon-containing ferrous hydroxide filter residue; the mass of pure water is 2-6 times that of the black powder mixture, and the dissolution reaction time is 1.5-5 h; Step 3, adding sulfuric acid to the carbon-containing ferrous hydroxide filter residue after reaction, then filtering to obtain a graphite filter residue and an iron salt solution; vacuum low-temperature drying the iron salt solution and then filtering to obtain a ferrous sulfate heptahydrate product; Step 4, adding alkali to the lithium salt solution containing metal impurities to precipitate the metal impurities, purifying the lithium salt solution, and then filtering to obtain a purified lithium salt solution after precipitation; Step 5, adding a precipitant to the purified lithium salt solution to generate lithium carbonate precipitate; Step 6: After the lithium carbonate precipitation is completed, the product is kept warm, filtered, washed, and dried to obtain the finished lithium carbonate product.

[0009] The beneficial effects of this implementation plan are as follows: 1. Existing technologies for the graded recycling of retired lithium iron phosphate batteries require ball milling of the filter residue and the addition of chemical reagents such as sodium hydroxide and hydrogen peroxide. This invention, however, utilizes a combination of pyrometallurgical and hydrometallurgical processes. It involves high-temperature melting of a carbon source, strong alkali, and lithium iron phosphate black powder in an inert atmosphere. The absence of oxygen in this inert atmosphere prevents the oxidation of ferrous ions. The carbon source further reduces some ferric ions to ferrous ions while simultaneously protecting them from oxidation. The carbon reacts with the molten strong alkali to form solid ferrous hydroxide, which is insoluble in water and can be easily separated from the system during subsequent water dissolution. Since ferrous hydroxide is more acid-soluble than ferric oxide, the amount of sulfuric acid added can be significantly reduced during subsequent iron recovery, thereby reducing pollutant emissions, simplifying the treatment process, lowering chemical reagent consumption, and effectively reducing overall treatment costs. Therefore, this method not only offers good economic benefits but also significant environmental benefits, providing a green and efficient solution for the selective extraction of lithium from waste lithium iron phosphate batteries.

[0010] 2. Compared with the traditional process of selective leaching using strong acid and hydrogen peroxide, the acid leaching method of the present invention using ferrous sulfate avoids the use of costly hydrogen peroxide and the generation of solid waste iron phosphate slag mixture. For large-scale industrial production, it not only reduces costs and avoids the transportation risks of hydrogen peroxide, but also greatly improves the recycling rate of resources and reduces waste treatment and disposal costs.

[0011] 3. This invention provides a process for preparing ferrous sulfate heptahydrate, which can produce high-purity ferrous sulfate heptahydrate. The raw materials are readily available, the process is simple, low-cost, short-process, and easy to scale up, and it has excellent application prospects.

[0012] Further, the precipitant in step 5 is carbon dioxide and a strong base, and the specific process is: continuously passing carbon dioxide into the purified lithium salt solution, and adding a strong base to increase the pH of the system, so that lithium carbonate precipitates, and the gas flow of the carbon dioxide passed in step 5 is 0.1-3 L / h; after adding the strong base, the pH of the system is 10-12. The process of precipitating lithium elements in the leaching solution by using sodium carbonate in the prior art is that, from the principle point of view, sodium carbonate first undergoes a double decomposition reaction to generate carbon dioxide, carbon dioxide reacts with other forms of lithium ions in the leaching solution to generate soluble LiHCO3 through carbonization reaction, and then lithium carbonate is obtained through pyrolysis reaction at 80-90℃. This way has two defects, one is that, because the carbonization reaction is reversible, when the concentration of LiHCO3 in the solution is too high, the carbonization reaction will reverse, LiHCO3 will decompose into other forms, and only when LiHCO3 is consumed by the pyrolysis reaction to generate lithium carbonate, the carbonization reaction will continue to proceed in the forward direction to generate LiHCO3, therefore, the carbonization reaction always maintains a stable reaction speed, which is difficult to improve. And also leads to the concentration of LiHCO3 to maintain a stable value, the concentration of LiHCO3 as a reactant of the pyrolysis reaction affects the reaction speed of the pyrolysis reaction, and when the concentration of LiHCO3 is stable, the reaction rate of the pyrolysis reaction is also difficult to improve; ultimately, the overall precipitation rate of lithium carbonate is low, and the time is long. Two, because the reaction is reversible, the lithium ions in the leaching solution cannot be completely carbonized into soluble LiHCO3, and the lithium ions in the leaching solution always have a residue, which leads to a precipitation rate of usually only 80-85%, which limits the recovery efficiency of lithium. And the present application uses continuous carbon dioxide, which can make the carbonization reaction proceed in the forward direction by controlling the pressure of carbon dioxide and the reaction temperature, thereby improving the carbonization reaction speed and the concentration of LiHCO3, and further improving the reaction speed of the pyrolysis reaction, and ultimately achieving the purpose of improving the precipitation efficiency of lithium carbonate. And because the carbon dioxide in the carbonization reaction is pressurized in the present application, the concentration of the reactant of the carbonization reaction is increased, and the lithium ions can be almost completely converted into soluble LiHCO3, which maximizes the lithium in the solution, and the precipitation rate of lithium carbonate is more than 90%.

[0013] Further, the strong base added in step 1 is any one of NaOH, KOH, sodium ethoxide and potassium ethoxide; and the carbon source is any one of sucrose, glucose, pitch and petroleum coke.

[0014] Further, the mass ratio of the amount of the strong base added in step 1 to the amount of the lithium iron phosphate black powder is 0.5-3:1; and the amount of the carbon source added is 2%-50% of the total mass of the lithium iron phosphate black powder.

[0015] Further, the temperature of the pure water for dissolving the black powder mixture in step 2 is 50-90℃.

[0016] Further, the concentration of sulfuric acid in step 3 is 0.5-3 mol / L; the reaction time is 0.5 h-3 h, and the addition amount of sulfuric acid is 0.8-4 times the molar amount of iron.

[0017] Further, the graphite filter residue in step 3 can be dried after water washing to make graphite finished product, the water added in water washing is 2-6 times the mass of graphite, and water washing is 2-4 times.

[0018] Further, the vacuum concentration in step 3 is: evaporation concentration at a temperature of 40-80℃, and hot filtration and drying to obtain ferrous sulfate heptahydrate finished product Further, the strong base in step 4 is one of NaOH, KOH, sodium ethoxide and potassium ethoxide; the pH for impurity removal is 7-8.5.

[0019] BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The process flowchart of the present application is, Figure 2 The X-ray diffraction pattern of lithium carbonate of the present application is, Figure 3 The scanning electron microscope pattern of lithium carbonate of the present application is, Figure 4 is a partial enlarged view of Figure 3 .

[0021] DETAILED DESCRIPTION

[0022] The following will be further described in detail through specific embodiments: The embodiments are shown as follows: Figures 1-4 METHOD OVERVIEW Step 1, adding strong base and carbon source into the broken black powder of retired lithium iron phosphate to form black powder mixture, and placing the black powder mixture in inert atmosphere for high temperature calcination; the high temperature calcination temperature is 550-800℃; the calcination time is 2-5 h; Step 2, dissolving the black powder mixture after high temperature calcination and cooling in pure water, and separating by suction filtration to obtain lithium salt solution containing metal impurities and carbon-containing ferrous hydroxide filter residue after stirring and mixing; the pure water is 2-6 times the mass of the black powder mixture, and the dissolution reaction time is 1.5-5 h; Step 3, adding sulfuric acid to the carbon-containing ferrous hydroxide filter residue to react, and then suction filtering to obtain graphite filter residue and iron salt solution; vacuum low-temperature drying the iron salt solution and then suction filtering to obtain ferrous sulfate heptahydrate finished product; Step 4, adding alkali to the lithium salt solution containing metal impurities to precipitate metal impurities, purifying the lithium salt solution, and suction filtering to obtain purified lithium salt solution after precipitation is completed; ​Step 5, add a precipitant to the purified lithium salt solution to generate lithium carbonate precipitate; Step 6, after the lithium carbonate precipitate is completed, heat preservation, filtration, washing, drying to get lithium carbonate finished product. Example 1 As Figure 1 shown, a method for efficiently recovering lithium and ferrous hydroxide from retired lithium iron phosphate by full leaching and green impurity removal, comprising the following steps: (1) Take 157.81 g of LiFePO4 black powder, add 40.03 g of NaOH and 12.80 g of sucrose to it and mix evenly; (2) Place the black powder mixture in an inert atmosphere of nitrogen, calcine at 550°C for 5 h, and take it out after cooling; (3) Dissolve the calcined black powder mixture in 6 times the mass of pure water and stir for 1.5 h, then separate the lithium salt solution containing metal impurities and carbon-containing ferrous hydroxide by filtration after the reaction is completed; the temperature of the pure water is 50°C.

[0023] (4) Add 1 mol / L sulfuric acid to the carbon-containing ferrous hydroxide, and after 2 h of reaction, filter to obtain graphite and ferrous salt solution; the amount of sulfuric acid added is 4 times the molar amount of iron.

[0024] (5) Wash the graphite water 2 times using a plate and frame filter press and dry at 100°C to obtain graphite finished product; the total washing water level is 2 times the mass of graphite.

[0025] (6) Evaporate and concentrate the ferrous salt solution under vacuum at 50°C, then filter while hot, and vacuum dry to obtain ferrous sulfate heptahydrate finished product; (7) Add 0.5 mol / L NaOH to the crude lithium salt solution containing metal impurities to adjust the pH of the solution to 7 for further impurity removal, and after the pH value is adjusted to the target value, the impurity removal is completed, and the purified lithium salt solution is obtained by filtration; (8) After adjusting the pH of the purified lithium salt solution to 10 by adding NaOH, continuously pass in 0.5 L / h of carbon dioxide to generate lithium carbonate precipitate; (9) After heat preservation at 60°C for 2 h, filter and wash twice with 2 times the water, and dry at 80°C to obtain lithium carbonate finished product.

[0026] After washing and drying, the recovery rate and main content of the ferrous sulfate heptahydrate product are 95.23% and 98.71%, respectively, and the recovery rate and main content of the lithium carbonate product are 90.91% and 98.56%, respectively. The X-ray diffraction pattern of the lithium carbonate is shown in Figure 2 , and the scanning electron microscope image of the lithium carbonate is shown in Figure 3 , Figure 4as shown.

[0027] Example 2 As Figure 1 shown, a method for efficiently recycling lithium and ferrous hydroxide in retired lithium iron phosphate by total leaching and green impurity removal includes the following steps: (1) Take 157.83 g of LiFePO4 black powder, add 50.02 g of NaOH and 15.83 g of glucose to it and mix evenly; (2) Place the black powder mixture in an inert atmosphere of nitrogen and calcine at 650°C for 5 h, then take it out after cooling; (3) Dissolve the calcined black powder mixture in 6 times the mass of pure water and stir for 2 h, then separate the lithium salt solution containing metal impurities and carbon-containing ferrous hydroxide by filtration; the temperature of the pure water is 75°C.

[0028] (4) Add 1 mol / L sulfuric acid to the carbon-containing ferrous hydroxide, and after 2 h of reaction, filter to obtain graphite and ferrous salt solution; the amount of sulfuric acid added is 3 times the molar amount of iron.

[0029] (5) Wash the graphite water 3 times with a plate and frame filter press and dry at 100°C to obtain graphite products; the total washing water level is 3 times the mass of graphite; (6) Evaporate and concentrate the ferrous salt solution at 40°C under vacuum, then filter while hot, and vacuum dry to obtain ferrous sulfate heptahydrate products; (7) Add 0.5 mol / L NaOH to the crude lithium salt solution containing metal impurities to adjust the pH to 7 for further impurity removal; after the pH value reaches the target value, the impurity removal is complete, and the purified lithium salt solution is obtained by filtration; (8) After adjusting the pH of the purified lithium salt solution to 11 by adding KOH, continuously pass in 1.5 L / h of carbon dioxide to generate lithium carbonate precipitate; (9) After 2 h of incubation at 85°C, filter and wash twice with 2 times the water, and dry at 80°C to obtain lithium carbonate products.

[0030] After washing and drying, the final recovery rate and main content of ferrous sulfate heptahydrate products are 96.41% and 98.01%, respectively, and the recovery rate and main content of lithium carbonate products are 90.83% and 98.75%, respectively.

[0031] Example 3 As Figure 1 shown, a method for efficiently recycling lithium and ferrous hydroxide in retired lithium iron phosphate by total leaching and green impurity removal includes the following steps: (1) Take 157.80 g of LiFePO4 black powder, add 70.02 g of NaOH and 23.68 g of pitch to it and mix uniformly; (2) Place the black powder mixture in an inert atmosphere of argon, calcine at a high temperature of 700°C for 3 h, and take it out after cooling; (3) Dissolve the calcined black powder mixture in 5 times the mass of pure water, stir for 2 h, and then separate the lithium salt solution containing metal impurities and the ferrous hydroxide containing carbon by suction filtration; the temperature of the pure water is 65°C.

[0032] (4) Add 1.5 mol / L sulfuric acid to the ferrous hydroxide containing carbon, and after 2 h of reaction, suction filtration is performed to obtain graphite and ferrous salt solution; the addition amount of sulfuric acid is 2.5 times the molar amount of iron.

[0033] (5) Wash the graphite water 4 times using a plate and frame filter press, and dry at 100°C to obtain the finished graphite product; the total washing water level is 6 times the mass of graphite; (6) Evaporate and concentrate the ferrous salt solution under vacuum at 60°C, and then suction filter while hot, and vacuum dry to obtain the finished ferrous sulfate heptahydrate product; (7) Add 1 mol / L KOH to the crude lithium salt solution containing metal impurities to adjust the pH of the solution to 8 for further impurity removal; after the pH value is adjusted to the target value, the impurity removal is completed, and suction filtration is performed to obtain the purified lithium salt solution; (8) Continuously pass 1 L / h of carbon dioxide into the purified lithium salt solution and add 1 mol / L sodium ethoxide until the system reaches a final pH of 12, to produce lithium carbonate precipitate; (9) After 2 h of incubation at 90°C, suction filtration is performed, and the product is washed with 3 times the mass of water twice at 80°C, and then dried to obtain the finished lithium carbonate product.

[0034] After washing and drying, the final recovery rate and main content of the ferrous sulfate heptahydrate product are 97.08% and 98.11%, respectively, and the final recovery rate and main content of the lithium carbonate product are 91.03% and 98.36%, respectively.

[0035] Example 4 As shown in Figure 1 , a full leaching green impurity removal and efficient recovery method for lithium and ferrous hydroxide in retired lithium iron phosphate, comprising the following steps: (1) Take 157.80 g of LiFePO4 black powder, add 80.00 g of NaOH and 23.67 g of petroleum coke to it and mix uniformly; (2) Place the black powder mixture in an inert atmosphere of argon, calcine at a high temperature of 750°C for 3 h, and take it out after cooling; (3) The calcined black powder mixture is dissolved in 4 times the mass of pure water and stirred for 3 h, then filtered to separate the lithium salt solution containing metal impurities and the carbon-containing ferrous hydroxide; the temperature of the pure water is 90°C.

[0036] (4) 1.5 mol / L sulfuric acid is added to the carbon-containing ferrous hydroxide, and after reacting for 2 h, graphite and ferrous salt solution are obtained by filtration; the amount of sulfuric acid added is 2 times the molar amount of iron (5) The graphite water is washed 4 times using a plate and frame filter press, and is dried at 100°C to obtain the graphite product; the total washing water level is 5 times the mass of the graphite; (6) The ferrous salt solution is evaporated and concentrated under vacuum at 75°C, then filtered while hot, and vacuum dried to obtain the ferrous sulfate heptahydrate product; (7) 0.5 mol / L sodium ethoxide is added to the crude lithium salt solution containing metal impurities to adjust the pH of the solution to 8.5 for further impurity removal; after the pH value is adjusted to the target value, the impurity removal is completed, and the purified lithium salt solution is obtained by filtration; (8) After adjusting the pH of the purified lithium salt solution to 11.5 by adding NaOH, 2 L / h of carbon dioxide is continuously introduced to generate lithium carbonate precipitate; (9) After 2 h of incubation at 90°C, filtration is performed, and the product is washed twice with 4 times the water, and dried at 80°C to obtain the lithium carbonate product.

[0037] After washing and drying, the recovery rate and main content of the ferrous sulfate heptahydrate product are 96.89% and 97.93%, respectively, and the recovery rate and main content of the lithium carbonate product are 91.93% and 98.27%, respectively.

[0038] Example 5 As shown in Figure 1 , a full leaching green impurity removal and efficient recovery of lithium and ferrous hydroxide from retired lithium iron phosphate includes the following steps: (1) Take 157.80 g of LiFePO4 black powder, add 75.01 g of NaOH and 23.69 g of glucose to it and mix evenly; (2) The black powder mixture is placed in an inert atmosphere and calcined at 800°C for 2 h, and then taken out after cooling; (3) The calcined black powder mixture is dissolved in 3 times the mass of pure water and stirred for 4 h, then filtered to separate the lithium salt solution containing metal impurities and the carbon-containing ferrous hydroxide; the temperature of the pure water is 80°C.

[0039] (4) 2 mol / L sulfuric acid is added to the carbon-containing ferrous hydroxide, and after reacting for 1.5 h, graphite and ferrous salt solution are obtained by filtration; the amount of sulfuric acid added is 1.5 times the molar amount of iron (5) The graphite water is washed 2 times using a plate and frame filter press, and is dried at 100°C to obtain a graphite product; the total washing water level used is 4 times the mass of the graphite; (6) The iron salt solution is evaporated, concentrated and crystallized under vacuum at 60°C, and then filtered while hot, and dried to obtain a ferrous sulfate heptahydrate product; (7) 1.5 mol / L potassium ethoxide is added to the crude lithium salt solution containing metal impurities to adjust the pH of the solution to 8.5 to further remove impurities, and after the pH value is adjusted to the target value, the impurity removal is completed, and the purified lithium salt solution is filtered; (8) After continuously passing 1.5 L / h of carbon dioxide into the purified lithium salt solution and adding 1 mol / L NaOH until the system reaches a final pH of 11, lithium carbonate precipitate is generated; (9) After incubation at 70°C for 2 h, filtration, and washing with 4 times water for 3 times, and drying at 80°C, a lithium carbonate product is obtained.

[0040] After washing and drying, the recovery rate and main content of the ferrous sulfate heptahydrate product are 98.19% and 98.23%, respectively, and the recovery rate and main content of the lithium carbonate product are 91.78% and 98.37%, respectively.

[0041] Example 6 As shown in Figure 1 A method for efficiently recovering lithium and ferrous hydroxide from retired lithium iron phosphate by full leaching and green impurity removal, comprising the following steps: (1) Take 157.82 g of LiFePO4 black powder, add 65.01 g of NaOH and 23.67 g of sucrose to it and mix uniformly; (2) Place the black powder mixture in an inert atmosphere of nitrogen, and calcine at 650°C for 4 h, and then take it out after cooling.

[0042] (3) Dissolve the calcined black powder mixture in 2 times the mass of pure water, stir for 5 h, and then separate the lithium salt solution containing metal impurities and carbon-containing ferrous hydroxide by filtration; the temperature of the pure water is 65°C.

[0043] (4) Add 2 mol / L sulfuric acid to the carbon-containing ferrous hydroxide, react for 1.5 h, and then filter to obtain graphite and ferrous salt solution; the amount of sulfuric acid added is 0.8 times the molar amount of iron.

[0044] (5) The graphite water is washed 2 times using a plate and frame filter press, and is dried at 100°C to obtain a graphite product; the total washing water level used is 4 times the mass of the graphite; (6) The iron salt solution is evaporated, concentrated and crystallized under vacuum at 60°C, and then filtered while hot, and dried to obtain a ferrous sulfate heptahydrate product; (7) adding 1 mol / L sodium hydroxide to the crude lithium salt solution containing metal impurities to adjust the pH of the solution to 8.5 to further remove impurities, and after the pH value is adjusted to the target value, the impurity removal is ended, and the purified lithium salt solution is obtained by filtration; (8) continuously passing 3 L / h of carbon dioxide into the purified lithium salt solution and adding 1 mol / L NaOH until the terminal pH of the system is 11.5, to produce lithium carbonate precipitate; (9) after incubation at 85°C for 2 h, filtration is performed, and the product is washed with 4 times water for 3 times, and dried at 80°C to obtain the lithium carbonate product.

[0045] After washing and drying, the final recovery rate and main content of the ferrous sulfate heptahydrate product are 97.86% and 98.03%, respectively, and the recovery rate and main content of the lithium carbonate product are 92.01% and 98.53%, respectively.

[0046] The above is only an embodiment of the present application, and the specific structure and characteristics of the scheme are not described in detail. It should be noted that for those skilled in the art, without departing from the structure of the present application, a number of modifications and improvements can be made, which should be considered as the protection scope of the present application, and these will not affect the effect and practicality of the patent. The protection scope of the present application should be subject to the content of its claims, and the specific implementation mode and the like recorded in the specification can be used to explain the content of the claims.

Claims

1. A method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate through a green, full-leaching process, characterized in that: Includes the following steps: Step 1: Add a strong alkali and a carbon source to the black powder from the crushed retired lithium iron phosphate and mix evenly to form a black powder mixture. Place the black powder mixture in an inert atmosphere and calcine at a high temperature; the high temperature calcination temperature is 550-800℃; the calcination time is 2-5 hours. Step 2: Dissolve the black powder mixture that has been calcined at high temperature and then cooled in pure water, stir and mix, and then filter to obtain a lithium salt solution containing metal impurities and a carbon-containing ferrous hydroxide filter residue; the mass of the pure water is 2-6 times that of the black powder mixture, and the dissolution reaction time is 1.5-5 h; Step 3: After adding sulfuric acid to the carbon-containing ferrous hydroxide filter residue and reacting, filter to obtain graphite filter residue and iron salt solution; concentrate and crystallize the iron salt solution under vacuum, and then filter to obtain ferrous sulfate heptahydrate product. Step 4: Add alkali to the lithium salt solution containing metal impurities to precipitate the metal impurities and purify the lithium salt solution. After precipitation, filter to obtain the purified lithium salt solution. Step 5: Add a precipitant to the purified lithium salt solution to form lithium carbonate precipitate; Step 6: After the lithium carbonate precipitation is completed, the product is kept warm, filtered, washed, and dried to obtain the finished lithium carbonate product.

2. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 1, characterized in that: In step 5, the precipitant is carbon dioxide and a strong alkali. The specific process is as follows: carbon dioxide is continuously introduced into the purified lithium salt solution, and a strong alkali is added to increase the pH of the system, so that lithium carbonate precipitates are formed. The flow rate of the introduced carbon dioxide is 0.5-3 L / h, and the pH of the system after adding the strong alkali is 10-12.

3. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 1, is characterized in that: The strong alkali added in step 1 is any one of NaOH, KOH, sodium ethoxide, and potassium ethoxide; the carbon source is any one of sucrose, glucose, pitch, and petroleum coke.

4. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 3, is characterized in that: In step 1, the mass ratio of the added strong alkali to the added lithium iron phosphate black powder is 0.25-0.5:1; the added carbon source is 0.8%-10% of the total mass of lithium iron phosphate black powder.

5. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 1, characterized in that: The temperature of the pure water used to dissolve the black powder mixture in step 2 is 50-90℃.

6. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 1, characterized in that: In step 3, the concentration of sulfuric acid is 0.5-3 mol / L; the reaction time is 0.5 h-3 h; and the amount of sulfuric acid added is 0.8 to 4 times the molar amount of iron.

7. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 1, is characterized in that: In step 3, the graphite filter residue can be washed with water and then dried to produce finished graphite products. The mass of water added during washing is 2-6 times the mass of the graphite, and the washing is performed 2-4 times.

8. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 1, characterized in that: In step 3, vacuum concentration is achieved by evaporating and concentrating the product at a temperature of 40-80℃, followed by hot filtration and drying to obtain the ferrous sulfate heptahydrate product.

9. The method for efficient recovery of lithium and ferrous sulfate heptahydrate from decommissioned lithium iron phosphate by full leaching, green impurity removal, and high efficiency according to claim 1, characterized in that: In step 4, the strong alkali is one of NaOH, KOH, sodium ethoxide, or potassium ethoxide; the pH for impurity removal is 7-8.5.

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