Method for recycling lithium carbonate from waste lithium iron phosphate battery black powder

Abstract

The application discloses a method for recycling lithium carbonate from black powder of waste lithium iron phosphate battery, and relates to the field of battery recycling. 3‑ and F ‑ ; the filtrate is adjusted in pH in two stages by using LiOH produced by the method, TiO(OH)2, Fe(OH)3, Al(OH)3 and Cu(OH)2 are precipitated in sequence; after the cation-removing solution is concentrated by a membrane and Ca 2+ is removed by chelating resin, the solution is sent into a bipolar membrane electrodialysis device to convert Li2SO4 into H2SO4 and LiOH; the sulfuric acid is returned to leaching, part of the LiOH is used for adjusting pH, the rest is used for carbonization-pyrolysis with CO2 to generate lithium carbonate, and the crystallization mother liquor is returned to impurity removal. + The application does not introduce Na , no sodium sulfate salt is opened, and cations and anions are deeply removed, so that the comprehensive lithium recovery rate and product purity are finally improved.

Description

Technical Field

[0001] This invention relates to the field of battery recycling, specifically a method for recovering lithium carbonate from waste lithium iron phosphate battery black powder. Background Technology

[0002] The recycling of spent lithium iron phosphate battery black powder is a crucial link in the sustainable development of the new energy industry. Currently, the mainstream recycling process is as follows: lithium is preferentially extracted from the battery black powder through sulfuric acid leaching; the pH of the leachate is adjusted with sodium hydroxide; after removing impurity ions, it is concentrated through mechanical vapor recompression (MVR) or extraction; and sodium carbonate is added to precipitate industrial-grade lithium carbonate.

[0003] However, existing processes have the following significant drawbacks: First, the mainstream process uses NaOH to adjust the pH, making it difficult to control the Na content in the product, which seriously affects the quality of lithium carbonate products. Simultaneously, Na... + First, lithium accumulates within the system, requiring periodic opening of sodium-containing wastewater, causing lithium loss with the wastewater. Second, during the extraction and concentration process, the oil content in the raffinate is difficult to control, not only causing organic pollution of the product but also increasing the difficulty and cost of subsequent wastewater treatment. Third, energy consumption is too high. MVR concentration has high energy consumption, significantly increasing production costs and reducing the economic efficiency of the process. Fourth, the extraction process requires a large amount of NaOH to raise the pH to above 13, requiring the opening of a large amount of sodium sulfate mixed salts in the system, causing lithium loss with mixed salts and increasing solid waste disposal costs. Fifth, anionic impurities are difficult to control effectively. Existing processes have limited control over F in the leachate. - and PO4 3- The lack of effective deep removal methods for anionic impurities affects the quality of the final product. Sixth, in the existing process, the feed liquid temperature of the bipolar membrane electrodialysis unit is greatly affected by environmental fluctuations, leading to changes in membrane impedance and poor operational stability; at the same time, the exothermic process of lithium precipitation is not effectively utilized, resulting in serious energy waste. Summary of the Invention

[0004] The purpose of this invention is to provide a method for recovering lithium carbonate from waste lithium iron phosphate battery black powder, which aims to overcome the technical problems existing in the prior art, such as Na ion pollution, severe open-circuit lithium loss due to sodium sulfate impurities, and low efficiency in removing anionic impurities.

[0005] To achieve the objective, the present invention provides the following technical solution: like Figures 1 to 4 As shown, a method for recovering lithium carbonate from waste lithium iron phosphate battery black powder includes the following steps: Step (1): Mix waste lithium iron phosphate battery black powder with H2SO4 and H2O2 for lithium extraction leaching; after solid-liquid separation, leachate and leachate residue are obtained; Step (2): The leachate from step (1) and the crystallization mother liquor from step (9) are fed into the bottom neutralization zone of the impurity removal reactor; the Ca(OH)2 clear liquid is fed into the middle impurity removal zone of the impurity removal reactor, and the pH of the middle impurity removal zone is controlled at 2.5-4; Li2CO3 and LiOH in the crystallization mother liquor are preferentially neutralized with H2SO4 in the above leachate; the neutralized solution rises to the middle impurity removal zone to react with the Ca(OH)2 clear liquid, and PO4 is precipitated and removed. 3- and F - After filtering the clear liquid in the upper clarification zone of the impurity removal reactor, a deionized solution is obtained. Step (3): Add some of the LiOH produced in step (6) to the above deionized solution, and adjust the pH in two stages; in the first stage, control the pH to 4-6 to precipitate and remove TiO(OH)2 and Fe(OH)3; in the second stage, control the pH to 6-8 to precipitate and remove Al(OH)3 and Cu(OH)2; after filtration, a deionized solution is obtained. Step (4): The above decation solution is concentrated to a Li+ concentration of 2.0-2.3 mol / L by membrane concentration to obtain concentrated solution and deionized water; Step (5): Remove Ca from the above concentrated solution by passing it through an H-type chelating resin. 2+ The residual polyvalent cations yield a lithium sulfate solution; Step (6): The above lithium sulfate solution is fed into a bipolar membrane electrodialysis unit, and Li2... S O4 is converted into H2SO4 and LiOH, with the concentrations of H2SO4 and LiOH controlled at 2–3.5 mol / L respectively; Step (7): Return the H2SO4 produced in step (6) to step (1) as leaching acid to realize the closed-loop circulation of sulfate in the system; Step (8): Part of the LiOH produced in step (6) is returned to step (3) as a pH adjuster; the other part of LiOH is reacted with CO2 in a two-step carbonization-pyrolysis reaction: first, the pH of the carbonization endpoint is controlled to be 8-11 to generate a lithium bicarbonate solution; then the temperature is raised to 70-95℃ for thermal decomposition reaction; after filtration, Li2CO3 and crystallization mother liquor are obtained. Step (9): Return the crystallization mother liquor from step (8) to the impurity removal reactor in step (2) to form a closed loop of lithium and alkalinity in the system.

[0006] Furthermore, in step (2), the above-mentioned impurity removal reactor is equipped with a tubular distributor, through which the crystallization mother liquor is introduced into the bottom neutralization zone.

[0007] Furthermore, the aforementioned impurity removal reactor is vertically fixed with a circulation guide tube; the aforementioned bottom neutralization zone is provided with an external circulation pipe, an external circulation pump, and an internal injection pipe connected in sequence, and the nozzle of the internal injection pipe faces the bottom inlet of the circulation guide tube. In step (2), the solution neutralized in the bottom neutralization zone is extracted and pressurized by the external circulation pump and then injected into the bottom inlet of the circulation guide tube through the internal injection pipe, generating a jet entrainment effect, which forces the neutralized liquid inside and outside the circulation guide tube to rise to below the Ca(OH)2 injection pipe in the middle impurity removal zone.

[0008] Furthermore, the inner wall of the aforementioned impurity removal reactor is equipped with a pH gradient monitoring probe array to monitor the pH values ​​of the bottom neutralization zone, the middle impurity removal zone, and the upper clarification zone, respectively. The flow rate of the crystallization mother liquor is automatically controlled by the PLC based on the pH value of the bottom neutralization zone, maintaining the pH value between 1.5 and 3.0. The amount of Ca(OH)2 clear liquid added is automatically controlled by the PLC based on the pH value of the middle impurity removal zone, maintaining the pH of this zone between 2.5 and 4.

[0009] Furthermore, the above step (8) is completed in a coupled circulation tank, which is divided into an alkaline buffer zone and a carbonization reaction zone by a partition. The LiOH produced in step (6) first enters the alkaline buffer zone, a part of which is pumped to step (3), and the other part overflows to the carbonization reaction zone to carry out a two-step carbonization-pyrolysis reaction with CO2.

[0010] Furthermore, the aforementioned partition is a heat-conducting partition, through which the heat from the carbonization reaction zone is conducted to the alkali buffer zone, maintaining the temperature of the alkali buffer zone at 35–50°C. The aforementioned alkali buffer zone is equipped with a heat exchange coil, and the heat transfer medium in the heat exchange coil enters the plate heat exchanger through the circulation pipeline to preheat the lithium sulfate solution to be entered into the bipolar membrane electrodialysis unit, raising its temperature to 30–45°C.

[0011] Furthermore, in step (4), the above membrane concentration is reverse osmosis membrane concentration, and an ultrafiltration process is set up before membrane concentration to filter suspended solids; the demineralized water is returned to steps (1) and (6) for use.

[0012] Compared with the prior art, the present invention has the following advantages: Firstly, in this invention, the leachate and crystallization mother liquor are preferentially neutralized at the bottom of the impurity removal reactor, and then the mixture rises to the middle to remove PO4 using Ca(OH)2. 3- and F - It utilizes LiOH for staged cation removal, bipolar membrane electrodialysis to generate acids and bases for internal reuse, and a closed-loop mother liquor system. No Na is introduced throughout the entire process. + It eliminates open circuits caused by sodium sulfate and other salts; it achieves a triple closed-loop cycle of sulfate, lithium, and alkalinity, with a comprehensive lithium recovery rate of ≥96%, and the product meets battery-grade standards.

[0013] Secondly, in this invention, the impurity removal reactor is equipped with a tubular distributor. The crystallization mother liquor is introduced into the bottom neutralization zone through the distributor in a fine jet, so that the crystallization mother liquor and the leachate are evenly mixed, accelerating the neutralization reaction, avoiding excessive local acidity, and providing a stable pH environment for subsequent impurity removal.

[0014] Thirdly, in this invention, the impurity removal reactor has a built-in circulating guide tube. The neutralized liquid at the bottom is pressurized by an external circulation pump and injected into the bottom of the guide tube through an internal injection pipe, generating a jet entrainment effect that lifts the neutralized liquid to the central Ca(OH)2 injection zone. Forced circulation and uniform lifting effectively prevent localized over-concentration of Ca(OH)2, significantly inhibiting the formation of fine CaSO4 crystals and preventing them from encapsulating CaF2 and Ca3(PO4)2 precipitation, thus preventing F... - and PO4 3- The removal rate has been increased to over 95%.

[0015] Fourth, in this invention, a pH gradient monitoring probe array is installed on the inner wall of the impurity removal reactor. The PLC automatically controls the flow rate of the crystallization mother liquor based on the bottom pH and automatically controls the amount of Ca(OH)2 added based on the middle pH. This enables precise zoned pH control, ensuring optimal conditions in both the neutralization and impurity removal zones, improving the stability and efficiency of impurity removal, and reducing human intervention.

[0016] Fifth, in this invention, the lithium precipitation step is completed in a coupled circulation tank, which is divided into an alkali buffer zone and a carbonization reaction zone by a partition. LiOH first enters the buffer zone, a portion of which is pumped to adjust the pH, and the other portion overflows to the carbonization zone. This enables precise metering and continuous operation of the alkali solution, avoids additional investment in storage tanks and equipment, simplifies the process, and improves operational reliability.

[0017] Sixth, in this invention, the partition is a thermally conductive partition, and the heat released by the carbonization reaction is conducted to the alkaline buffer zone; the medium in the heat exchange coil in the buffer zone is preheated by the plate heat exchanger to the bipolar membrane feed lithium sulfate solution. This can recover the heat released by lithium precipitation, preheat the electrodialysis feed, and reduce membrane impedance and power consumption; at the same time, the insulation of the alkaline buffer zone is beneficial to the subsequent carbonization reaction, realizing energy cascade utilization. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the process of the present invention.

[0019] Figure 2 This is a partial cross-sectional view of the impurity removal reactor in this invention.

[0020] Figure 3 This is a top view of the tubular distributor in this invention.

[0021] Figure 4 This is a partial cross-sectional view of the coupling circulation tank in this invention. Detailed Implementation

[0022] Specific embodiments of the present invention will now be described with reference to the accompanying drawings. Many details are described below to provide a comprehensive understanding of the invention; however, those skilled in the art will be able to implement the invention without these details.

[0023] like Figures 1 to 3 As shown, a method for recovering lithium carbonate from waste lithium iron phosphate battery black powder includes the following steps: Step (1): The waste lithium iron phosphate battery black powder is mixed with H2SO4 and H2O2 for lithium extraction leaching. After solid-liquid separation, leachate and leaching residue are obtained.

[0024] Among them, the black powder from waste lithium iron phosphate batteries contains LiFePO4 cathode, C carbon anode and trace impurities Al 3+ Cu 2+ Ti 4+ F - Lithium is preferentially extracted by H2SO4 and H2O2, and C powder, ferric phosphate slag, and some unreacted Cu, Al, and F are filtered out of the system as leaching residue.

[0025] Reaction formula:

[0026] Preferably, the reaction temperature is controlled at 60–90℃, the reaction time at 0.5–2h, the pH at 1.0–2.0, and the liquid-to-solid ratio at 3:1–10:1.

[0027] The H2SO4 required for leaching is mainly generated in step (6), and is supplemented as appropriate according to the valence state of LiFePO4 / FePO4 in the black powder and the reaction pH.

[0028] like Figure 1 As shown, the lithium extraction in step (1) is completed in leaching unit 1, which can be an existing plate leaching unit, but is not limited to it; the number of leaching cycles can be 1 or 2 to 3.

[0029] Step (2): The leachate from step (1) and the crystallization mother liquor from step (9) are fed into the bottom neutralization zone 201 of the impurity removal reactor 2; the Ca(OH)2 clear liquid is fed into the middle impurity removal zone 202 of the impurity removal reactor, and the pH of the middle impurity removal zone 202 is controlled at 2.5-4. The crystallization mother liquor is preferentially neutralized with the leachate; the neutralized solution then rises to the middle impurity removal zone 202 to react with the Ca(OH)2 clear liquid; the clear liquid in the upper clarification zone 203 of the impurity removal reactor 2 is filtered to obtain a deionized solution.

[0030] like Figure 1 , Figure 2 and Figure 3As shown, specifically, the impurity removal reactor 2 is a vertical tank structure, internally divided from bottom to top into a bottom neutralization zone 201, a middle impurity removal zone 202, and an upper clarification zone 203. A tubular distributor 21 is fixedly installed in the impurity removal reactor. This tubular distributor 21 consists of a main pipe 211 and multiple branch pipes 212 connected to the main pipe 211. The inlet end of the main pipe 211 is connected to a flange with bolts, and the closed ends of the branch pipes 212 are fixedly connected to corresponding connecting seats on the inner wall of the impurity removal reactor 2 with connecting lugs and bolts. Multiple distribution holes 210 are provided upwards on the main pipe 211 and the branch pipes 212, with a preferred hole diameter of 0.5–2 mm. The crystallization mother liquor flows into the bottom neutralization zone 201 in a fine jet through the tubular distributor 21, preferentially neutralizing with the leachate.

[0031] like Figure 2 As shown, the impurity removal reactor 2 is also vertically fixedly provided with a circulation guide cylinder 25, the outer wall of which is fixedly connected to the inner wall of the impurity removal reactor 2 by multiple connecting rods. The bottom inlet of the circulation guide cylinder 25 is located above the tubular distributor 21, preferably 30-50 cm away; the top outlet of the circulation guide cylinder 25 extends to below the Ca(OH)2 injection pipe 26 in the middle impurity removal zone 203.

[0032] like Figure 2 As shown, the bottom neutralization zone 201 is provided with an external circulation pipe 24, an external circulation pump 23, and an internal injection pipe 22 connected in sequence. The nozzle of the internal injection pipe 22 faces the bottom inlet of the circulation guide cylinder 25. After the solution neutralized in the bottom neutralization zone is drawn out and pressurized by the external circulation pump 23, it is injected into the bottom inlet of the circulation guide cylinder 25 through the internal injection pipe 22, generating a jet entrainment effect, which forces the liquid inside and outside the circulation guide cylinder 25 to flow upward, thereby raising the neutralized liquid in the bottom neutralization zone 201 to below the Ca(OH)2 injection pipe 26 in the middle impurity removal zone 202, promoting the uniform mixing of the neutralized liquid and Ca(OH)2, and avoiding the formation of a large number of CaSO4 fine crystals due to excessively high local Ca(OH)2 concentration, which would then encapsulate CaF2 and Ca3(PO4)2 precipitation.

[0033] like Figure 2 As shown, the inner wall of the impurity removal reactor is equipped with a pH gradient monitoring probe array 27 to monitor the pH values ​​of the bottom neutralization zone 201, the middle impurity removal zone 202, and the upper clarification zone 203, respectively. The flow rate of the crystallization mother liquor is automatically controlled by a PLC based on the pH value of the bottom neutralization zone 201, preferably maintaining the pH value of this zone between 1.5 and 3.0. The amount of Ca(OH)2 clarified solution added is automatically controlled by a PLC based on the pH value of the middle impurity removal zone 202, preferably maintaining the pH value of this zone between 2.5 and 4.

[0034] like Figures 1 to 3 As shown, step (2) specifically includes the following sub-steps: Step (2.1): The mother liquor of crystallization is introduced into the bottom neutralization zone 201 in a fine jet through the tubular distributor 21. The Li2CO3 and LiOH in the mother liquor of crystallization react with the H2SO4 in the leachate. H2SO4+ Li2CO3→ Li2SO4+ H2O + CO2↑ H₂SO₄ + 2LiOH → Li₂SO₄ + 2H₂O This process removes free H2SO4 from the leachate. The process takes place in the bottom neutralization zone, where the pH is maintained between 1.5 and 3.0.

[0035] Step (2.2): After neutralization, part of the solution rises through the circulation guide tube 25 to below the Ca(OH)2 injection pipe 26 in the central impurity removal zone 202, where the F in the solution... - PO4 3- Reaction with Ca(OH)2 clear solution: 3Ca 2+ + 2PO4 3- → Ca3(PO4)2↓(Ksp=2.1×10 -33 ) Ca 2+ + 2F - → CaF 2 ↓ (Ksp=3.9×10) -11 ) Thus removing F - PO4 3- The reaction that produces CaSO4 is significantly suppressed (only trace amounts of Ca are produced). 2+ With SO4 2- reaction).

[0036] This localized isolation design, which involves neutralization followed by impurity removal, effectively avoids the problem in traditional processes where Ca(OH)₂ reacts violently with H₂SO₄ to generate a large amount of fine CaSO₄ crystals that then encapsulate CaF₂ and Ca₃(PO₄)₂ precipitates. This allows F… - and PO4 3- The removal rate has been increased to over 95%.

[0037] Step (2.3): Collect the clear liquid after the reaction in the upper clarification zone, filter it to obtain a solution free of anions.

[0038] Step (3): Add LiOH from step (9) to the anion-removing solution from step (3). After filtration, a cation-removing solution is obtained.

[0039] Specifically, except for anionic liquids containing Al 3+ Cu 2+ Ti 4+Impurities such as these are removed by adding a portion of the LiOH produced in step (6) in two stages to adjust the pH.

[0040] The first stage involves controlling the pH to 4–6 to precipitate and remove TiO(OH)₂ and Fe(OH)₃. Fe 3+ + 3OH - → Fe(OH)3↓ (Ksp=1.0×10 -39 ) Ti 4+ + 2OH - + H2O → TiO(OH)2↓ + 2H + (Ksp=1.0×10) -29 ) The second stage controls the pH to 6-8 for precipitation removal of Al(OH)3 and Cu(OH)2. Al 3+ + 3OH - → Al(OH)3↓(Ksp=3×10 -34 ) Cu 2+ + 2OH - → Cu(OH)2↓(Ksp=6×10 -20 ) like Figure 1 As shown, specifically, the two-stage pH adjustment removal in step (3) is completed in impurity removal reactor one and impurity removal reactor two, respectively. Impurity removal reactor one and impurity removal reactor two are existing impurity removal reactor one, and their specific structures will not be described in detail.

[0041] As a preferred option, in order to control Li + The precipitate is washed away and the wash water is returned to step (1) for leaching pulping.

[0042] Step (4): The above decation solution is concentrated through a membrane to remove Li + The concentration was concentrated to 2.0–2.3 mol / L to obtain concentrated solution and demineralized water.

[0043] like Figure 1 As shown, specifically, the membrane concentration in step (4) is completed in membrane concentration unit 4, which can be an existing reverse osmosis membrane concentration unit, but is not limited thereto. Preferably, an ultrafiltration process is set up before membrane concentration to filter suspended solids.

[0044] Preferably, the desalinated water concentrated by the membrane is returned to steps (1) and (6) for use.

[0045] Step (5): Remove Ca from the above concentrated solution by passing it through an H-type chelating resin. 2+The remaining polyvalent cations are then used to obtain a lithium sulfate solution. The polyvalent cations include +2, +3, and +4 valent cations.

[0046] like Figure 1 As shown, specifically, step (5) is completed within the existing H-type chelating resin column 5. Preferably, the H-type chelating resin column is backwashed and regenerated using sulfuric acid; the backwash solution is returned to step (1) for leaching acid replenishment.

[0047] Step (6): The above lithium sulfate solution is fed into the bipolar membrane electrodialysis unit 6, where Li2SO4 is converted into H2SO4 and LiOH under the action of a DC electric field. Li2SO4+ 2H2O → 2LiOH + H2SO4 Preferably, the concentrations of H2SO4 and LiOH are each controlled to be 2–3.5 mol / L.

[0048] Among them, the bipolar membrane electrodialysis unit 6 is an existing bipolar membrane electrodialysis unit, and its specific structure will not be described in detail.

[0049] Step (7): Return the H2SO4 produced in step (6) to step (1) as leaching acid to achieve a closed-loop circulation of sulfate ions in the system.

[0050] Step (8): Part of the LiOH produced in step (6) is returned to step (3) as a pH adjuster; the other part of LiOH is reacted with CO2 in a two-step carbonization-pyrolysis reaction; after filtration, Li2CO3 and crystallization mother liquor are obtained.

[0051] like Figure 1 and Figure 4 As shown, specifically, the above step (8) is completed in the coupled circulation tank 7, which is divided into an alkali buffer zone 701 and a carbonization reaction zone 702 by a partition 73. The partition 73 is fixed to the stepped surface inside the tank by a sealing weld.

[0052] like Figure 1 and Figure 4 As shown, preferably, the partition 73 is a heat-conducting partition, and the heat of the carbonization reaction zone 702 is conducted to the alkali buffer zone 701 through the partition 73, so that the temperature of the alkali buffer zone is maintained at 35-50°C.

[0053] like Figure 1 and Figure 4 As shown, preferably, the alkaline buffer zone 701 is also equipped with a heat exchange coil 74. The heat transfer medium in the heat exchange coil 74 enters the plate heat exchanger 75 through the circulation pipeline to preheat the lithium sulfate solution to be entered into the bipolar membrane electrodialysis unit, so that its temperature rises to 30-45°C.

[0054] like Figure 1 and Figure 4 As shown, the carbonization reaction zone 702 is equipped with a heating element 72. This heating element 72 is used to raise the temperature of the solution during the pyrolysis stage. It can be an existing immersion heating tube with an internally flowing heating medium or an existing external heating jacket, without much restriction here.

[0055] The LiOH produced in step (6) first enters the alkaline buffer zone 701, a portion of which is pumped to step (3), and the other portion overflows to the carbonization reaction zone 702, where it undergoes a two-step carbonization-pyrolysis reaction with CO2: first, the pH of the carbonization endpoint is controlled to be 8-11 to generate a lithium bicarbonate solution, and then the temperature is raised to 70-95℃ for thermal decomposition reaction.

[0056] like Figure 1 and Figure 4 As shown, preferably, the product is metered and distributed by metering pump 76 and then pumped to step (3).

[0057] like Figure 1 and Figure 4 As shown, preferably, the carbonization reaction zone 702 is provided with a tubular distributor 21, the specific structure of which is the same as that of the tubular distributor 21, and will not be described in detail here.

[0058] Step (9): Return the crystallization mother liquor from step (8) to the impurity removal reactor 2 in step (2) to form a closed loop of lithium and alkalinity in the system.

[0059] The above are merely specific embodiments of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial modifications made to the present invention using this concept shall be considered as infringing upon the protection scope of the present invention.

Claims

1. A method for recovering lithium carbonate from waste lithium iron phosphate battery black powder, characterized in that, Includes the following steps: Step (1): Mix waste lithium iron phosphate battery black powder with H2SO4 and H2O2 for lithium extraction leaching; after solid-liquid separation, leachate and leachate residue are obtained; Step (2): The leachate from step (1) and the crystallization mother liquor from step (9) are fed into the bottom neutralization zone of the impurity removal reactor; the Ca(OH)2 clear liquid is fed into the middle impurity removal zone of the impurity removal reactor, and the pH of the middle impurity removal zone is controlled at 2.5-4; Li2CO3 and LiOH in the crystallization mother liquor are preferentially neutralized with H2SO4 in the leachate; the neutralized solution rises to the middle impurity removal zone to react with the Ca(OH)2 clear liquid, and PO4 is precipitated and removed. 3- and F - After filtering the clear liquid in the upper clarification zone of the impurity removal reactor, a deionized solution is obtained. Step (3): Add some of the LiOH produced in step (6) to the anion-removing solution, and adjust the pH in two stages; in the first stage, control the pH to 4-6 to precipitate and remove TiO(OH)2 and Fe(OH)3; in the second stage, control the pH to 6-8 to precipitate and remove Al(OH)3 and Cu(OH)2; after filtration, a cation-removing solution is obtained. Step (4): The cation-removing solution is concentrated using a membrane to remove Li + The concentration was concentrated to 2.0–2.3 mol / L to obtain concentrated solution and demineralized water; Step (5): Remove Ca from the concentrated solution by passing it through an H-type chelating resin. 2+ The residual polyvalent cations yield a lithium sulfate solution; Step (6): The lithium sulfate solution is fed into a bipolar membrane electrodialysis unit to convert Li2SO4 into H2SO4 and LiOH, and the concentrations of H2SO4 and LiOH are controlled to be 2-3.5 mol / L respectively; Step (7): Return the H2SO4 produced in step (6) to step (1) as leaching acid to realize the closed-loop circulation of sulfate in the system; Step (8): Part of the LiOH produced in step (6) is returned to step (3) as a pH adjuster; the other part of LiOH is reacted with CO2 in a two-step carbonization-pyrolysis reaction: first, the pH of the carbonization endpoint is controlled to be 8-11 to generate a lithium bicarbonate solution; then the temperature is raised to 70-95℃ for thermal decomposition reaction; after filtration, Li2CO3 and crystallization mother liquor are obtained. Step (9): Return the crystallization mother liquor from step (8) to the impurity removal reactor in step (2) to form a closed loop of lithium and alkalinity in the system.

2. The method for recovering lithium carbonate from waste lithium iron phosphate battery black powder according to claim 1, characterized in that, In step (2), the impurity removal reactor is equipped with a tubular distributor, through which the crystallization mother liquor is introduced into the bottom neutralization zone.

3. The method for recovering lithium carbonate from waste lithium iron phosphate battery black powder according to claim 2, characterized in that, The impurity removal reactor is vertically fixed with a circulation guide tube; the bottom neutralization zone is provided with an external circulation pipe, an external circulation pump and an internal injection pipe connected in sequence, and the nozzle of the internal injection pipe faces the bottom inlet of the circulation guide tube. In step (2), the solution neutralized in the bottom neutralization zone is extracted and pressurized by the external circulation pump and then injected into the bottom inlet of the circulation guide tube through the internal injection pipe, generating a jet entrainment effect, which forces the neutralized liquid inside and outside the circulation guide tube to rise to below the Ca(OH)2 injection pipe in the middle impurity removal zone.

4. The method for recovering lithium carbonate from waste lithium iron phosphate battery black powder according to claim 1, characterized in that, The inner wall of the impurity removal reactor is equipped with a pH gradient monitoring probe array to monitor the pH values ​​of the bottom neutralization zone, the middle impurity removal zone, and the upper clarification zone, respectively. The flow rate of the crystallization mother liquor is automatically controlled by the PLC according to the pH value of the bottom neutralization zone to maintain the pH value between 1.5 and 3.

0. The amount of Ca(OH)2 clear liquid added is automatically controlled by the PLC according to the pH value of the middle impurity removal zone to maintain the pH of this zone between 2.5 and 4.

5. The method for recovering lithium carbonate from waste lithium iron phosphate battery black powder according to claim 1, characterized in that, Step (8) is completed in a coupled circulation tank, which is divided into an alkaline buffer zone and a carbonization reaction zone by a partition. The LiOH produced in step (6) first enters the alkaline buffer zone, a part of which is pumped to step (3), and the other part overflows to the carbonization reaction zone to carry out a two-step carbonization-pyrolysis reaction with CO2.

6. The method for recovering lithium carbonate from waste lithium iron phosphate battery black powder according to claim 5, characterized in that, The partition is a heat-conducting partition, through which the heat from the carbonization reaction zone is conducted to the alkali buffer zone, maintaining the temperature of the alkali buffer zone at 35-50°C. The alkali buffer zone is equipped with a heat exchange coil, and the heat transfer medium in the heat exchange coil enters the plate heat exchanger through the circulation pipeline to preheat the lithium sulfate solution to be entered into the bipolar membrane electrodialysis unit, raising its temperature to 30-45°C.

7. The method for recovering lithium carbonate from waste lithium iron phosphate battery black powder according to claim 1, characterized in that, The membrane concentration in step (4) is reverse osmosis membrane concentration. Before membrane concentration, an ultrafiltration process is set up to filter suspended solids. The demineralized water is returned to steps (1) and (6) for use.

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