Method and system for selectively extracting lithium from waste lithium iron phosphate

Abstract

The application discloses a method and system for selectively extracting lithium from waste lithium iron phosphate, which comprises the following steps: using sodium sulfate as a grinding aid to mechanically activate and pretreat waste lithium iron phosphate black powder; performing a first-stage low-acid limited oxygen leaching on the activated material to form a hydrated iron oxide interface layer on the surface of the particles, thereby obtaining a first-stage leaching solution and a first-stage leaching residue; performing a second-stage high-acid strong oxidation leaching on the first-stage leaching residue to realize efficient selective leaching of lithium and inhibition of iron and phosphorus, thereby obtaining a second-stage lithium-containing leaching solution and a second-stage iron and phosphorus leaching residue. 2+ The application uses air as the only oxidant, discards strong oxidants such as hydrogen peroxide in the whole process, and creatively utilizes the iron dissolved by itself to construct a catalytic layer on the surface of the particles, thereby realizing efficient, inexpensive and endogenous circulation of Fe 3+ / Fe.

Description

Technical Field

[0001] This invention relates to the field of lithium recycling technology, specifically to a method and system for selectively extracting lithium from waste lithium iron phosphate. Background Technology

[0002] Lithium iron phosphate (LFP) is an important lithium-ion battery material widely used in electric vehicles and energy storage systems. With the explosive growth of the lithium-ion battery market, the disposal of spent LFP batteries has become increasingly challenging, making their recycling a key research focus. Lithium, being the most expensive raw material and the most economically viable recycling resource in LFP materials, has attracted widespread attention for its recovery, making efficient lithium recovery crucial. Currently, the common process for extracting lithium from spent LFP involves using oxidants (such as hydrogen peroxide, persulfate, and ozone) in an acidic environment to disrupt the olivine structure, causing lithium, iron, and phosphorus to leach out co-extract. Then, multi-stage precipitation or extraction is used to remove impurities, achieving the separation of iron, phosphorus, and lithium. The process has the following limitations: (1) Post-precipitation: Iron and phosphorus are first completely leached into the solution and then separated by precipitation, which leads to equipment corrosion, large solution volume, and subsequent loss of lithium inclusions; (2) Oxidant dependence: Strong oxidants such as hydrogen peroxide are commonly used. In industrial production, hydrogen peroxide is too expensive and is prone to local over-acidity, which can cause excessive heat and gas production, posing a safety risk. Summary of the Invention

[0003] One objective of this invention is to provide a method for selectively extracting lithium from waste lithium iron phosphate. This method achieves selective lithium leaching during the leaching process, enriching iron and phosphorus in the leaching residue, thus solving the separation problem at its source. Furthermore, it eliminates the use of hydrogen peroxide, reducing costs.

[0004] The second objective of this invention is to provide a system for selectively extracting lithium from waste lithium iron phosphate.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, the present invention provides a method for selectively extracting lithium from waste lithium iron phosphate, comprising the following steps:

[0007] (1) Using sodium sulfate as a grinding aid, waste lithium iron phosphate black powder is mechanically activated and pretreated;

[0008] (2) The activated material is subjected to a first stage of low acid and limited oxygen leaching to form a hydrated iron oxide interface layer on the surface of lithium iron phosphate particles, resulting in a first stage of leachate and a first stage of leachate residue.

[0009] (3) The first stage of leaching residue is subjected to a second stage of high acid and strong oxidation leaching to achieve efficient selective leaching of lithium and inhibition of iron and phosphorus, resulting in a second stage of lithium-containing leaching solution and a second stage of iron and phosphorus leaching residue.

[0010] Preferably, in step (1), the mass ratio of the waste lithium iron phosphate black powder to sodium sulfate is 3~5:1.

[0011] Preferably, in step (1), the mechanical activation is performed by ball milling, the ball milling speed is 250~350 r / min, and the ball milling time is 30~60 min.

[0012] Preferably, in step (2), the specific steps of the first stage of low acid and limited oxygen leaching are as follows: the activated material is mixed with 0.3-0.8 mol / L low concentration sulfuric acid at a liquid-to-solid ratio of 5-8:1 mL / g, the pH of the system is adjusted to be between 2.8 and 3.5, and the air flow rate is controlled at 0.1-0.5 VVM (0.1-0.5 cubic meters of air per cubic meter of liquid per minute) at 65-75℃, the oxidation-reduction potential is controlled at +300mV to +400mV, and the reaction is carried out for 30-60 minutes to obtain a first stage of leachate and a first stage of leachate residue.

[0013] Furthermore, the leachate is recycled back to step (2) to achieve lithium enrichment.

[0014] Preferably, in step (3), the specific steps of the second stage of high acid strong oxidation leaching are as follows: at a liquid-solid ratio of 4~6:1mL / g, the first stage leaching residue from step (2) is mixed with 1.0~2.0mol / L high concentration sulfuric acid, the pH of the system is adjusted to between 1.8 and 2.3, the air flow rate is controlled at 1.0~2.0VVM at 80~90℃, the oxidation-reduction potential is controlled at +480mV ~ +580mV, and the reaction is carried out for 90~150min to obtain the second stage lithium-containing leaching solution and the second stage iron-phosphorus leaching residue.

[0015] Furthermore, the lithium-containing leachate described in step (3) is used to prepare lithium carbonate.

[0016] Furthermore, the lithium-containing leachate is used to prepare lithium carbonate, comprising the following steps: adding sodium hydroxide to the lithium-containing leachate for neutralization and impurity removal; and then adding sodium carbonate to the purified lithium-containing solution for alkaline precipitation to obtain the lithium carbonate product.

[0017] Furthermore, the iron-phosphorus leaching residue described in step (3) is used to prepare iron phosphate.

[0018] Furthermore, the iron-phosphorus leaching residue is used to prepare iron phosphate, which includes the following steps: removing impurities from the leaching solution after acid leaching of the iron-phosphorus leaching residue, then adding an oxidant to the iron-phosphorus solution for full oxidation, and finally adding ammonia water to precipitate and obtain iron phosphate.

[0019] Secondly, the present invention also provides a system for selectively extracting lithium from waste lithium iron phosphate, comprising a mechanical activation unit, a leaching reaction unit, a gas supply unit, and a PLC control unit.

[0020] The leaching reaction unit includes a first leaching reactor and a second leaching reactor with identical structures. The leaching reactor includes a heatable vessel body, and a pH sensor and an ORP sensor are installed inside the vessel body. A composite gas plate is horizontally installed at the bottom of the vessel body. The composite gas plate consists of an annular main pipe, a tangential nozzle, and a multi-layer sintered plate. The annular main pipe is connected to the gas supply unit through an air inlet pipe. The tangential nozzle is connected to the annular main pipe and is set along the tangential direction of the annular main pipe. The central axis of the tangential nozzle is inclined upward at an angle of 5° to 15° with the horizontal plane in the vertical direction. The multi-layer sintered plate is a multi-layer sintered metal fiber plate with a gradient pore size made of acid-resistant metal or ceramic, and is evenly distributed in blocks on the gas plate. Air inlets are provided on both sides of the bottom of the multi-layer sintered plate, and the top of the multi-layer sintered plate is an air outlet facing the inside of the reactor.

[0021] The gas supply unit is connected to the air inlet of the multilayer sintered plate and is used to supply gas to the leaching reaction unit.

[0022] The PLC control unit is connected to the pH sensor, ORP sensor and gas flow meter respectively, and is used to monitor and control the pH value, redox potential and air flow rate in the first leaching reactor and the second leaching reactor in real time.

[0023] Preferably, the tangential nozzle is converging, with a large inlet diameter and a small outlet diameter.

[0024] Preferably, the apertures of the multilayer sintered plate from bottom to top are 30~50 micrometers, 20~30 micrometers, and 5~20 micrometers, respectively.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. This invention uses a specific ratio of soluble salt (sodium sulfate) as the grinding medium to efficiently destroy the crystal structure of LiFePO4 through mechanochemical action, exposing Li + The diffusion channels lay the foundation for subsequent selective leaching.

[0027] 2. This invention employs a two-stage leaching functional separation design: Stage 1 (catalytic layer construction): Under low acid and limited oxygen (low ORP) conditions, partial leaching of lithium and controlled dissolution and conversion of iron are achieved. The core objective is to generate a catalytically active hydrated iron oxide interface layer on the particle surface in situ; Stage 2 (selective deep leaching): Under high acid, strong oxidation (high ORP) conditions and with precise pH control, the formed hydrated iron oxide catalyst layer is used to efficiently catalyze the reaction of oxygen in the air with Fe. 2+ The oxidation process enables deep leaching of lithium and in-situ fixation of iron and phosphorus.

[0028] 3. This invention uses air as the sole oxidant: the entire process abandons strong oxidants such as hydrogen peroxide, and creatively utilizes the iron dissolved from the particles to construct a catalytic layer on the particle surface, achieving Fe 2+ / Fe 3+ It is efficient, inexpensive, and has an endogenous cycle.

[0029] 4. This invention designs an air disc to generate a swirling flow through a tangential nozzle, replacing the stirring and leaching of a stirring paddle.

[0030] 5. PLC-based precise control of oxidation-reduction potential and pH: Upgrading the key process parameters (ORP, pH) from manual experience-based judgment to automated closed-loop control is the core technological guarantee for achieving process stability, repeatable results, and industrial reliability. Attached Figure Description

[0031] Figure 1 This is a flowchart of a method for selectively extracting lithium from waste lithium iron phosphate.

[0032] Figure 2 This is the XRD pattern of the waste lithium iron phosphate raw material used in the example;

[0033] Figure 3 This is a schematic diagram of the structure of the reaction vessel in an embodiment of the present invention;

[0034] Figure 4 yes Figure 3 Schematic diagram of the structure of the composite gas disk;

[0035] Figure 5 yes Figure 4 Schematic diagram of the structure of a multilayer sintered plate;

[0036] Figure 6 This is a schematic diagram of the working principle of the composite air-supported disc;

[0037] In the diagram, 1. Reactor body, 2. Feed inlet, 3. Composite gas plate, 3-1. Tangential nozzle, 3-2. Annular header, 3-3. Multi-layer sintered plate, 3-3-1. Multi-layer sintered plate air inlet, 3-3-2. Sintered plate with 30~50 micrometer pore size, 3-3-3. Sintered plate with 20~30 micrometer pore size, 3-3-4. Sintered plate with 5~20 micrometer pore size, 4. Gas pipe, 5. Gas flow meter, 6. Air pump, 7. Discharge port, 8. Discharge control valve, 9. PLC control unit, 10. Numerical display screen, 11. ORP sensor, 12. pH sensor. Detailed Implementation

[0038] Embodiments of this application are described in detail below. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. Rather, embodiments of this application include all variations, modifications, and equivalents falling within the spirit and scope of the appended claims.

[0039] To achieve the above objectives, a method for selectively extracting lithium from waste lithium iron phosphate is proposed according to the first aspect of this application, comprising the following steps:

[0040] Waste lithium iron phosphate black powder was mechanically activated and pretreated using sodium sulfate as a grinding aid.

[0041] Objective: To efficiently disrupt the LiFePO4 crystal structure using mechanical force, thereby exposing Li⁺ diffusion channels and laying the foundation for subsequent selective leaching.

[0042] The activated material is subjected to a first-stage low-acid, limited-oxygen leaching to form a hydrated iron oxide interface layer on the particle surface, resulting in a first-stage leachate and a first-stage leachate residue.

[0043] Objective: To react activated pretreated materials with low-concentration sulfuric acid under limited oxidation conditions, causing some lithium and iron to dissolve; and to remove a small amount of Fe from lithium iron phosphate particles. 2+ Upon oxidation by air, a hydrated iron oxide interface layer is formed in situ on the particle surface. This interface layer, acting as a highly efficient heterogeneous catalyst, can significantly accelerate the oxidation of Fe in solution by oxygen in the air. 2+ The oxidation kinetics process is described. The catalytic mechanism lies in the fact that the hydrated iron oxide interface layer provides a solid-liquid interface, greatly promoting the transfer of electrons from the adsorbed Fe. 2+ The efficiency of transfer to O2 is improved, thereby achieving efficient and stable cyclic oxidation using oxygen in the air as the sole oxidant.

[0044] (3) The first stage of leaching residue is subjected to a second stage of high acid and strong oxidation leaching to achieve efficient selective leaching of lithium and inhibition of iron and phosphorus, resulting in a second stage of lithium-containing leaching solution and a second stage of iron and phosphorus leaching residue.

[0045] Objective: To react a leaching residue with high-concentration sulfuric acid under strong oxidizing conditions, utilizing a pre-formed hydrated iron oxide catalyst layer under high ORP and precisely controlled pH conditions, to efficiently catalyze the oxidation of Fe by oxygen in the air. 2+ The oxidation process enables deep leaching of lithium and in-situ fixation of iron and phosphorus.

[0046] Optionally, in step (1), the mass ratio of the waste lithium iron phosphate black powder to sodium sulfate is 3~5:1.

[0047] Optionally, in step (1), the mechanical activation is performed by ball milling, with a ball milling speed of 250~350 r / min and a ball milling time of 30~60 min.

[0048] Optionally, in step (2), the specific steps of the first stage of low acid and limited oxygen leaching are as follows: the activated material is mixed with 0.3-0.8 mol / L of low concentration sulfuric acid at a liquid-to-solid ratio of 5-8:1 mL / g, the pH of the system is adjusted to be between 2.8 and 3.5, and the air flow rate is controlled at 0.1-0.5 VVM (0.1-0.5 cubic meters of air per cubic meter of liquid per minute) at 65-75℃. The oxidation-reduction potential is controlled at +300mV to +400mV, and the reaction is carried out for 30-60 minutes to obtain a first stage of leachate and a first stage of leachate residue.

[0049] Furthermore, the leachate is recycled back to step (3) to achieve lithium enrichment.

[0050] Optionally, in step (3), the specific steps of the second stage of high acid strong oxidation leaching are as follows: at a liquid-solid ratio of 4~6:1mL / g, the first stage leaching residue in step (2) is mixed with 1.0~2.0mol / L of high-concentration sulfuric acid, the pH of the system is adjusted to between 1.8 and 2.3, the air flow rate is controlled at 1.0~2.0VVM at 80~90℃, the oxidation-reduction potential is controlled at +480mV ~ +580mV, and the reaction is carried out for 90~150min to obtain the second stage lithium-containing leaching solution and the second stage iron-phosphorus leaching residue.

[0051] Furthermore, the lithium-containing leachate described in step (3) is used to prepare lithium carbonate.

[0052] Furthermore, the lithium-containing leachate is used to prepare lithium carbonate, comprising the following steps: adding sodium hydroxide to the lithium-containing leachate for neutralization and impurity removal; and then adding sodium carbonate to the purified lithium-containing solution for alkaline precipitation to obtain the lithium carbonate product.

[0053] Furthermore, the iron-phosphorus leaching residue described in step (3) is used to prepare iron phosphate.

[0054] Furthermore, the iron-phosphorus leaching residue is used to prepare iron phosphate, which includes the following steps: removing impurities from the leaching solution after acid leaching of the iron-phosphorus leaching residue, then adding an oxidant to the iron-phosphorus solution for full oxidation, and finally adding ammonia water to precipitate and obtain iron phosphate.

[0055] According to a second aspect of this application, a system for selectively extracting lithium from waste lithium iron phosphate is provided, comprising a mechanical activation unit, a leaching reaction unit, a gas supply unit, and a PLC control unit.

[0056] The leaching reaction unit includes a first leaching reactor and a second leaching reactor with identical structures. The leaching reactor includes a heatable vessel body, and a pH sensor and an ORP sensor are installed inside the vessel body. A composite gas plate is horizontally installed at the bottom of the vessel body. The composite gas plate consists of a tangential nozzle, an annular header, and a multi-layer sintered plate. The annular header is connected to the gas supply unit through an air inlet pipe. The tangential nozzle is connected to the annular header and is arranged along the tangential direction of the annular header. The central axis of the tangential nozzle is inclined upward at an angle of 5° to 15° with the horizontal plane in the vertical direction. The multi-layer sintered plate is a multi-layer sintered metal fiber plate with gradient pore size made of acid-resistant metal or ceramic, and is evenly distributed in blocks on the gas plate. Air inlets are provided on both sides of the bottom of the multi-layer sintered plate. The air inlets are connected to the gas supply unit through an air inlet pipe. The top of the multi-layer sintered plate is the air outlet, which faces the inside of the reactor.

[0057] The working principle of the composite air circuit is as follows: Figure 6 As shown, when compressed air is ejected at high speed from various tangential nozzles, the airflow moves "along" the vessel wall. The high-speed fluid (air) tends to adhere to the adjacent solid surface (vessel wall). The combined effect of multiple tangential airflows creates a powerful three-dimensional spiral upward flow field rotating around the central axis inside the vessel. The swirling flow creates a low-pressure zone in the central region of the vessel bottom, while the pressure is higher near the vessel wall. This pressure difference drives the slurry to circulate from the high-pressure zone to the low-pressure zone, forming a strong secondary flow. This continuous, bottom-initiated circulation flow effectively re-entrains and suspends settled solid particles, ensuring no sediment at the bottom of the vessel and solving the "dead zone" problem that may exist in traditional stirring. Initial bubble breakage: The initial bubbles ejected from the nozzles are relatively large in diameter. These large bubbles are immediately engulfed in the high-shear swirling flow, subjected to intense turbulent shearing and gas-liquid friction, thus being torn and broken into smaller bubbles.

[0058] The PLC control unit is connected to the pH sensor, ORP sensor and gas flow meter respectively, and is used to monitor and control the pH value, redox potential and air flow rate in the first leaching reactor and the second leaching reactor in real time.

[0059] The tangential nozzle is converging, with a large inlet diameter and a small outlet diameter. This structure efficiently converts the pressure energy of the gas into kinetic energy, causing a sharp increase in the outlet airflow velocity, thereby generating stronger shearing and driving forces.

[0060] The pore sizes of the multi-layer sintered plate, from bottom to top, are 30-50 micrometers, 20-30 micrometers, and 5-20 micrometers, respectively. Air passes through the micropores of the multi-layer sintered plate with specific pore sizes, generating uniform and dense ultrafine microbubbles. The specific surface area of ​​ultrafine microbubbles is much larger than that of ordinary bubbles, resulting in an exponential increase in the mass transfer efficiency of oxygen from the gas phase to the liquid phase. Microbubbles rise slowly and have a longer residence time in the liquid, increasing oxygen utilization from 10-20% in traditional bubbling to over 60%. Oxidation is more uniform, avoiding localized insufficient oxidation.

[0061] To illustrate the advanced nature and innovative aspects of this invention and to make its technical features easier to understand, further explanation is provided in conjunction with specific examples.

[0062] The XRD patterns of the waste lithium iron phosphate battery powder used in the following examples are shown below. Figure 2 As shown in Table 1, the elemental composition and content of its raw materials are as follows.

[0063] Table 1 Elemental Composition and Content

[0064] element Fe P Li C F Al Na Pb Ca K content / % 33.3 18.63 4.12 4.11 1.18 0.17 0.11 0.04 0.04 0.01 Example 1

[0065] A method for selectively extracting lithium from spent lithium iron phosphate, the system used in this method comprising the following components:

[0066] The mechanical activation unit includes a ball mill jar;

[0067] The leaching reaction unit includes a first leaching reactor and a second leaching reactor with identical structures, the structures of which are as follows: Figure 3 As shown, the vessel includes a heatable vessel body 1, which has a reaction chamber for reacting materials inside. The top side of the vessel body 1 has a feed inlet 2 communicating with the reaction chamber, and the bottom of the vessel body 1 has a discharge outlet 7 communicating with the reaction chamber. The discharge outlet 7 is also equipped with a discharge control valve 8.

[0068] The vessel body is equipped with a pH sensor 12 and an ORP sensor 11, and a composite gas disk 3 is horizontally mounted at the bottom of the vessel body. The structure of the composite gas disk is as follows: Figure 4 As shown, the composite gas plate 3 consists of a tangential nozzle 3-1, an annular main pipe 3-2, and a multi-layer sintered plate 3-3. The annular main pipe 3-2 is connected to the gas supply unit via an inlet pipe 4. The tangential nozzle 3-1 is connected to the annular main pipe 3-2 and is arranged along the tangential direction of the annular main pipe 3-2. The central axis of the tangential nozzle 3-1 forms a 10° upward tilt angle with the horizontal plane in the vertical direction. The structure of the multi-layer sintered plate is as follows: Figure 5 As shown, the multi-layer sintered plate 3-3 is a multi-layer sintered metal fiber plate made of ceramic. From bottom to top, it is provided with: sintered plate 3-3-2 with a pore size of 30~50 micrometers, sintered plate 3-3-3 with a pore size of 20~30 micrometers, and sintered plate 3-3-4 with a pore size of 5~20 micrometers. The multi-layer sintered plates are evenly distributed in blocks on the composite gas plate 3. The bottom two sides of the multi-layer sintered plate 3-3 are provided with air inlets 3-3-1, and the top of the multi-layer sintered plate 3-3 is an air outlet facing the inside of the reactor.

[0069] The gas supply unit includes a gas pretreatment unit (not shown in the figure), a gas flow meter 5, and a gas pump 6; the gas pump 6 is connected to the gas flow meter 5, and the gas flow meter 5 is connected to the air inlet 3-3-1 of the multilayer sintered plate 3-3 through the air inlet pipe 4.

[0070] The PLC control unit 9 is connected to the pH sensor 12, the ORP sensor 11 and the gas flow meter 5 respectively, and is used to monitor and control the pH value, oxidation-reduction potential and air flow rate in the first leaching reactor and the second leaching reactor in real time, and display them in real time on the numerical display screen 10.

[0071] The basic process of this method is as follows: Figure 1 As shown, it includes the following steps:

[0072] (1) Pretreatment by mechanical activation of sodium sulfate: 50g of waste lithium iron phosphate black powder was mixed with sodium sulfate at a mass ratio of 3:1 and pretreated by ball milling at 300r / min for 30min in a ball mill jar;

[0073] (2) Leaching with a low-acid catalyst layer: The activated material is mixed with 0.3 mol / L dilute sulfuric acid solution at a liquid-solid ratio of 5:1 mL / g in the first leaching reactor. The pH value is controlled at 3.5, the temperature is raised to 65℃, and the air flow rate is 0.1 VVM. The PLC dynamically adjusts the air valve according to the feedback of the ORP sensor to stabilize the actual ORP between +300 mV and +320 mV (relative to the Ag / AgCl reference electrode) for leaching. The reaction time is 30 min, resulting in a first-stage leaching solution and a first-stage leaching residue. The first-stage leaching solution can be returned to the system for continued use to achieve lithium enrichment.

[0074] (3) Two-stage high-acid selective leaching: The leaching residue from step (2) is mixed with 1.0 mol / L sulfuric acid solution at a liquid-solid ratio of 4:1 mL / g in the second leaching reactor, and the pH value is controlled at 2.3. The reaction temperature is 80℃, the air flow rate is 1.0 VVM, and the PLC controls the oxidation-reduction potential to +480mV ~ +500mV through a dual-loop control for leaching. After reacting for 100 min, the mixture is filtered to obtain the two-stage leaching solution and the two-stage leaching residue.

[0075] The PLC control unit is set with dual core parameters: the target ORP value range is +480mV ~ +500mV, and the target pH value is 2.3.

[0076] ORP control loop: When the measured ORP is below 480mV, the PLC increases the air valve; when it is above 500mV, the air valve decreases, so that the ORP is precisely controlled within the +480mV ~ +500mV window.

[0077] pH control loop: When the measured pH is higher than 2.3, the PLC starts the concentrated sulfuric acid drip pump; when the pH is lower than 2.3, the dripping stops and the pH value is stabilized at 2.3.

[0078] (4) Add sodium hydroxide to the lithium-containing filtrate for neutralization and impurity removal, and then add sodium carbonate for alkaline precipitation to obtain lithium carbonate product;

[0079] (5) Remove impurities from the leachate after acid leaching of iron-phosphorus slag, then oxidize it and add ammonia water to neutralize and precipitate FePO4∙H2O.

[0080] result:

[0081] The Li content in the second-stage leaching residue was 0.27%, the Fe content was 36.35%, and the P content was 20.26%; corresponding to a total leaching rate of 94.11% for lithium, 0.55% for iron, and 0.93% for phosphorus. Example 2

[0082] The reactor system used in this embodiment is basically the same as that in Embodiment 1, except that the tangential nozzle of the composite gas disk has an inclination angle of 8° and the distribution of the sintering plate blocks is further optimized.

[0083] The specific steps for lithium extraction include:

[0084] (1) Mechanical activation pretreatment: Black powder and sodium sulfate were mixed at a mass ratio of 3.5:1 and ball milled in a ball mill jar at 350 r / min for 40 min for pretreatment;

[0085] (2) Construction of a low-acid catalyst layer for leaching: The activated material was mixed with 0.5 mol / L dilute sulfuric acid solution at a liquid-solid ratio of 6:1 mL / g in the first leaching reactor. The pH value was controlled at 3.2, the reaction temperature at 70℃, the air flow rate at 0.3 VVM, and the ORP controlled at +350 mV to +370 mV. The reaction time was 40 min.

[0086] (3) Two-stage high acid selective leaching: The leaching residue from step (2) is mixed with 1.5 mol / L sulfuric acid solution at a liquid-solid ratio of 5:1 mL / g in the second leaching reactor. The reaction temperature is 85℃, the air flow rate is 1.5 VVM, the pH is controlled at 2.0, the ORP is between +520 mV and +540 mV, and the reaction time is 90 min.

[0087] (4) Add sodium hydroxide to the lithium-containing filtrate for neutralization and impurity removal, and then add sodium carbonate for alkaline precipitation to obtain lithium carbonate product;

[0088] (5) Remove impurities from the leachate after acid leaching of iron-phosphorus slag, then oxidize it and add ammonia water to neutralize and precipitate FePO4∙H2O.

[0089] result:

[0090] The Li content in the second-stage leaching residue was 0.16%, the Fe content was 36.42%, and the P content was 20.28%; corresponding to a total leaching rate of 96.48% for lithium, 1.45% for iron, and 1.91% for phosphorus. Example 3

[0091] The reactor system used in this embodiment is basically the same as that in Embodiment 1, except that the tilt angle of the gas disc tangential nozzle is adjusted to 15°.

[0092] The specific steps for lithium extraction include:

[0093] (1) Mechanical activation pretreatment: LFP black powder and sodium sulfate were mixed at a mass ratio of 4:1 and pretreated by ball milling at 250 r / min for 50 min in a ball mill jar;

[0094] (2) Leaching with a low-acid catalyst layer: The activated material was mixed with 0.6 mol / L dilute sulfuric acid solution in the first leaching reactor at a liquid-solid ratio of 7:1 mL / g. The pH value was controlled at 2.8, the reaction temperature at 72℃, the air flow rate at 0.4 VVM, and the ORP was controlled at +370 mV to +390 mV. The reaction time was 50 min.

[0095] (3) Two-stage high acid selective leaching: The leaching residue from step (2) is mixed with 1.8 mol / L sulfuric acid solution at a liquid-solid ratio of 5:1 mL / g in the second leaching reactor. The reaction temperature is 88℃, the air flow rate is 1.8 VVM, the pH is controlled at 1.8, the ORP is between +560 mV and +580 mV, and the reaction time is 120 min.

[0096] (4) Add sodium hydroxide to the lithium-containing filtrate for neutralization and impurity removal, and then add sodium carbonate for alkaline precipitation to obtain lithium carbonate product;

[0097] (5) Remove impurities from the leachate after acid leaching of iron-phosphorus slag, then oxidize it and add ammonia water to neutralize and precipitate FePO4∙H2O.

[0098] result:

[0099] The Li content in the second-stage leaching residue was 0.07%, the Fe content was 36.18%, and the P content was 20.28%; corresponding to a total leaching rate of 98.54% for lithium, 2.25% for iron, and 2.07% for phosphorus. Example 4

[0100] The reaction vessel system used in this embodiment is basically the same as that in Embodiment 1, except that the tilt angle of the gas disc tangential nozzle is adjusted to 12° to enhance swirling and solid suspension.

[0101] The specific steps for lithium extraction include:

[0102] (1) Mechanical activation pretreatment: LFP black powder and sodium sulfate were mixed at a mass ratio of 5:1 and pretreated by ball milling at 300 r / min for 60 min in a ball mill jar;

[0103] (2) Construction of a low-acid catalyst layer for leaching: The activated material was mixed with 0.8 mol / L dilute sulfuric acid solution at a liquid-solid ratio of 8:1 mL / g in the first leaching reactor. The pH value was controlled at 3.0, the reaction temperature at 75℃, the air flow rate at 0.5 VVM, and the ORP was controlled at around +380 mV to +400 mV. The reaction time was 60 min.

[0104] (3) Two-stage high acid selective leaching: The leaching residue from step (2) is mixed with 2.0 mol / L sulfuric acid solution at a liquid-solid ratio of 6:1 mL / g in the second leaching reactor. The reaction temperature is 90℃, the air flow rate is 2.0 VVM, the pH is controlled at 1.8, the ORP is around +540 mV to +560 mV, and the reaction time is 150 min.

[0105] (4) Add sodium hydroxide to the lithium-containing filtrate for neutralization and impurity removal, and then add sodium carbonate for alkaline precipitation to obtain lithium carbonate product;

[0106] (5) Remove impurities from the leachate after acid leaching of iron-phosphorus slag, then oxidize it and add ammonia water to neutralize and precipitate FePO4∙H2O.

[0107] result:

[0108] The Li content in the second-stage leaching residue was 0.06%, the Fe content was 36.24%, and the P content was 20.29%; corresponding to a total leaching rate of 98.71% for lithium, 2.26% for iron, and 2.18% for phosphorus.

[0109] Comparative Example 1: Lithium leaching using traditional process of sulfuric acid + hydrogen peroxide

[0110] Take 50g of LFP black powder, leach it with sulfuric acid (Li, Fe, and P consume 1.2 times the theoretical amount of analytical grade sulfuric acid) and hydrogen peroxide (4 times the theoretical amount), at a temperature of 90℃, react for 4 hours, and then filter.

[0111] The results are shown in Table 2: the iron-phosphorus leaching rate is high, but it requires a large amount of hydrogen peroxide, resulting in low economic benefits.

[0112] Table 2

[0113] Test results Li Fe P Leachate / % 0.14 18.29 5.37 Leaching rate / % Li Fe P Slag 96.96 51.01 74.27

[0114] Comparative Example 2: Sulfuric acid (low acid) + conventional air supply system

[0115] Take 50g of LFP black powder, with a liquid-to-solid ratio of 5:1mL / g, add sulfuric acid (Li, Fe, and P consume 1 times the theoretical amount of analytical grade sulfuric acid), oxidize in air at 90℃ for 1~6h, and then filter.

[0116] The results are shown in Table 3: the iron-phosphorus leaching rate is high, the air oxidation efficiency is low, and the reaction time is long.

[0117] Table 3

[0118]

[0119] Comparative Example 3: Sulfuric acid (high acid) + conventional ordinary air device

[0120] Take 50g of LFP black powder, with a liquid-to-solid ratio of 5:1mL / g, add sulfuric acid (Li, Fe, and P consume 1.5 times the theoretical amount of analytical grade sulfuric acid), oxidize in air at 90℃ for 6 hours, and then filter.

[0121] The results are shown in Table 4: iron and phosphorus were almost completely leached out, making subsequent separation from lithium difficult, and the process resulted in significant lithium entrainment losses.

[0122] Table 4

[0123] Test results Li Fe P Leachate / % 0.03 1.63 3.81 Leaching rate / % Li Fe P Slag 99.83 98.77 94.85

[0124] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for selectively extracting lithium from waste lithium iron phosphate, characterized in that, Includes the following steps: (1) Using sodium sulfate as a grinding aid, waste lithium iron phosphate black powder is mechanically activated and pretreated; (2) The activated material is subjected to a first stage of low acid and limited oxygen leaching to form a hydrated iron oxide interface layer on the surface of lithium iron phosphate particles, resulting in a first stage of leachate and a first stage of leachate residue. The specific steps of the first stage of low acid and limited oxygen leaching are as follows: the activated material is mixed with 0.3-0.8 mol / L low concentration sulfuric acid at a liquid-to-solid ratio of 5-8:1 mL / g, the pH of the system is adjusted to be between 2.8 and 3.5, the air flow rate is controlled at 0.1-0.5 VVM at 65-75℃, the oxidation-reduction potential is controlled at +300mV to +400mV, and the reaction is carried out for 30-60 min to obtain a first stage of leachate and a first stage of leachate residue. (3) The first stage of leaching residue is subjected to a second stage of high acid strong oxidation leaching to achieve efficient selective leaching of lithium and inhibition of iron and phosphorus, resulting in a second stage of lithium-containing leaching solution and a second stage of iron and phosphorus leaching residue. The specific steps of the second stage of high acid strong oxidation leaching are as follows: the first stage of leaching residue in step (2) is mixed with 1.0~2.0mol / L high concentration sulfuric acid at a liquid-to-solid ratio of 4~6:1mL / g, the pH of the system is adjusted to between 1.8 and 2.3, the air flow rate is controlled at 1.0~2.0VVM at 80~90℃, the oxidation-reduction potential is controlled at +480mV ~ +580mV, and the reaction is carried out for 90~150min to obtain a second stage of lithium-containing leaching solution and a second stage of iron and phosphorus leaching residue.

2. The method for selectively extracting lithium from waste lithium iron phosphate according to claim 1, characterized in that, In step (1), the mass ratio of the waste lithium iron phosphate black powder to sodium sulfate is 3~5:

1.

3. The method for selectively extracting lithium from waste lithium iron phosphate according to claim 1, characterized in that, In step (1), the mechanical activation is performed by ball milling, with a rotation speed of 250~350 r / min and a milling time of 30~60 min.

4. The method for selectively extracting lithium from waste lithium iron phosphate according to claim 1, characterized in that, The leachate is recycled back to step (3) to achieve lithium enrichment.

5. A method for selectively extracting lithium from waste lithium iron phosphate according to claim 1, characterized in that, The lithium-containing leachate described in step (3) is used to prepare lithium carbonate.

6. A method for selectively extracting lithium from waste lithium iron phosphate according to claim 5, characterized in that, The lithium-containing leachate is used to prepare lithium carbonate, comprising the following steps: adding sodium hydroxide to the lithium-containing leachate for neutralization and impurity removal; then adding sodium carbonate to the purified lithium-containing solution for alkaline precipitation to obtain the lithium carbonate product.

7. A method for selectively extracting lithium from waste lithium iron phosphate according to claim 1, characterized in that, The iron-phosphorus leaching residue described in step (3) is used to prepare iron phosphate.

8. A method for selectively extracting lithium from waste lithium iron phosphate according to claim 7, characterized in that, The iron-phosphorus leaching residue is used to prepare iron phosphate, which includes the following steps: removing impurities from the leaching solution after acid leaching of the iron-phosphorus leaching residue, then adding an oxidant to the iron-phosphorus solution for full oxidation, and finally adding ammonia water to precipitate and obtain iron phosphate.

9. A selective lithium extraction system applied to any one of claims 1 to 8, characterized in that, It includes a mechanical activation unit, a leaching reaction unit, a gas supply unit, and a PLC control unit. The leaching reaction unit includes a first leaching reactor and a second leaching reactor with identical structures. The leaching reactor includes a heatable vessel body, and a pH sensor and an ORP sensor are installed inside the vessel body. A composite gas plate is horizontally installed at the bottom of the vessel body. The composite gas plate consists of an annular main pipe, a tangential nozzle, and a multi-layer sintered plate. The annular main pipe is connected to the gas supply unit through an air inlet pipe. The tangential nozzle is connected to the annular main pipe and is set along the tangential direction of the annular main pipe. The central axis of the tangential nozzle is inclined upward at an angle of 5° to 15° with the horizontal plane in the vertical direction. The multi-layer sintered plate is a multi-layer sintered metal fiber plate with a gradient pore size made of acid-resistant metal or ceramic, and is uniformly distributed in blocks on the gas plate. The bottom two sides of the multi-layer sintered plate are provided with air inlets, and the top of the multi-layer sintered plate is an air outlet facing the inside of the reactor. The gas supply unit is connected to the air inlet of the multilayer sintered plate and is used to supply gas to the leaching reaction unit. The PLC control unit is connected to the pH sensor, ORP sensor and gas flow meter respectively, and is used to monitor and control the pH value, redox potential and air flow rate in the first leaching reactor and the second leaching reactor in real time.

10. The selective lithium extraction system according to claim 9, characterized in that, The tangential nozzle is converging, with a large inlet diameter and a small outlet diameter.

11. The selective lithium extraction system according to claim 9, characterized in that, The apertures of the multilayer sintered plate from bottom to top are 30~50 micrometers, 20~30 micrometers, and 5~20 micrometers, respectively.

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