A method for pre-extracting lithium by microwave reduction and ultrasonic leaching in cooperation with carbon dioxide

Abstract

The present application belongs to the technical field of waste lithium ion battery recycling and solid waste resource utilization, and discloses a method for pre-extracting lithium by microwave reduction and carbon dioxide ultrasonic leaching. First, biomass waste is used as raw material, impregnated and mixed with pore-forming agent and transition metal salt, then high-temperature pyrolysis, acid pickling and drying are carried out to prepare porous wave-absorbing biochar; then the wave-absorbing biochar is mixed with waste lithium ion battery positive material black powder, and microwave radiation roasting is carried out under a protective gas to obtain a microwave reconstruction product; then the reconstruction product is slurried by adding water, and leaching is carried out under the synergistic action of pressurized carbon dioxide and ultrasonic field, and solid-liquid separation is carried out to obtain a lithium-rich solution and a nickel-cobalt-manganese enriched residue; finally, the lithium-rich solution is heated and analyzed to obtain a lithium carbonate product. The present application realizes waste treatment by waste, has short roasting time, low energy consumption, high lithium selective leaching rate, no strong acid and strong base consumption in the whole process, and the carbon dioxide can be recycled, and has good industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the technical field of waste lithium-ion battery recycling and solid waste resource utilization. Specifically, it relates to a method for selectively extracting lithium from waste cathode materials by using modified biochar as a microwave-absorbing reducing agent, combined with microwave carbothermal reduction and ultrasonic-enhanced leaching processes. Background Technology

[0002] With the increasing penetration rate of new energy vehicles, the number of retired ternary lithium batteries has risen sharply. Recovering high-value metals from waste ternary cathode materials is of great significance for alleviating resource shortages and protecting the ecological environment. Traditional recycling processes mainly include hydrometallurgy and pyrometallurgy. Hydrometallurgy suffers from problems such as high consumption of strong acids and alkalis, lengthy process flows, and high lithium loss rates due to recovery at the end of the process. Pyrometallurgy suffers from problems such as high energy consumption, large equipment investment, long process flows, poor metal separation or low lithium recovery rates, and serious secondary pollution.

[0003] In recent years, the technology of preferentially extracting lithium using pressurized carbon dioxide aqueous solution systems has become a research hotspot due to its advantages such as environmental friendliness and the absence of high-salt wastewater. However, the highly stable layered lattice of ternary materials exhibits strong resistance to leaching. Existing improved methods typically employ conventional tube furnaces for prolonged oxidation or reduction calcination to disrupt the lattice, followed by leaching. Such methods are limited by the low efficiency of conventional heat conduction, resulting in extremely high energy consumption. Furthermore, during the subsequent gas-liquid-solid three-phase carbon dioxide leaching process, the mass transfer resistance is extremely high due to the product layer coating effect, causing the leaching reaction to typically take several hours to complete.

[0004] Microwave metallurgy features bulk heating and rapid temperature rise. Existing research and patent literature have confirmed the feasibility of microwave carbothermal reduction combined with CO2 leaching or ultrasonic leaching in the recycling of waste lithium batteries (such as CN117778753A, CN118006927A, etc.). However, the following problems are common in the existing technology: (1) Using ordinary coal powder or unmodified biomass powder as reducing agent results in limited microwave absorption capacity, long roasting time, and high energy consumption; (2) Product layer coating during leaching leads to slow mass transfer and limited lithium leaching rate; (3) Lack of targeted design for the microwave absorption performance of reducing agent; (4) The synergistic effect of CO2 carbonation leaching and ultrasonic cavitation applied simultaneously in the same reaction system has not been revealed and utilized.

[0005] Current technology has consistently failed to recognize that by regulating the hierarchical porous structure of biochar through pore-forming agents, structural inheritance can be achieved through microwave roasting, enabling the reconstructed cathode products to acquire built-in mass transfer channels, thereby fundamentally amplifying the efficiency of subsequent CO2 ultrasonic leaching. The lack of understanding of the cross-process synergistic effects of "pore-forming—microwave roasting—lithium leaching" has also prevented current processes from overcoming the dual bottlenecks of long roasting times and slow leaching mass transfer.

[0006] Therefore, developing a process for preparing high-performance microwave-absorbing biochar through modification and using it for rapid microwave carbothermal reduction, combined with pressurized CO2 and ultrasonic cavitation synergistic leaching, has significant industrial application potential and ecological value. Summary of the Invention

[0007] To address at least one problem existing in the prior art, the present invention aims to provide a method for pre-extraction of lithium by microwave reduction combined with ultrasonic leaching of carbon dioxide.

[0008] To achieve the objectives of this invention, the specific technical solution is as follows: A method for pre-extraction of lithium using microwave reduction combined with ultrasonic carbon dioxide leaching includes the following steps: (1) The biomass waste powder is impregnated and mixed with a pore-forming agent and a transition metal salt in an aqueous solution, evaporated to dryness, pulverized and then subjected to high-temperature pyrolysis under a protective gas. After washing and drying, porous microwave-absorbing biochar is obtained; wherein the pore-forming agent is one or a combination of urea, ammonium bicarbonate and ammonium oxalate, and the transition metal salt is one or a combination of cobalt chloride, cobalt nitrate, nickel chloride, nickel nitrate, ferric chloride, ferric nitrate or their hydrates. (2) The waste lithium-ion battery cathode material black powder and the microwave absorbing biochar prepared in step (1) are mixed in proportion and microwave irradiated under the protection of protective gas to obtain microwave reconstruction product. (3) The microwave reconstruction product from step (2) is mixed with water and slurried, pressurized carbon dioxide gas is introduced, and an ultrasonic field is applied at the same time to carry out the leaching reaction; after the leaching reaction is completed, solid and liquid are separated to obtain lithium-rich solution and nickel-cobalt-manganese enriched slag.

[0009] Optionally, the process also includes step (4): heating and desorbing the lithium-rich solution from step (3) to precipitate lithium carbonate, which is then filtered, washed and dried to obtain the lithium carbonate product.

[0010] The technical principle of this invention is as follows: (I) Preparation of modified biochar and microwave absorption mechanism This invention modifies the pyrolysis of biomass by using pore-forming agents and transition metal salts, introducing multi-level channels and high dielectric loss components into carbon materials, thereby obtaining a reducing agent with excellent microwave absorption capabilities.

[0011] Pore-forming agents (such as urea) decompose and release ammonia and carbon dioxide gases during pyrolysis, forming a rich hierarchical porous structure within the carbon matrix. This significantly increases the specific surface area, providing a structural basis for multiple microwave reflections within the material and simultaneously increasing the contact area with the cathode material. Transition metal salts (such as iron, cobalt, and nickel salts) are reduced in situ during pyrolysis to generate metal or metal carbide nanoparticles. These particles possess high dielectric loss characteristics and can generate a strong dielectric heating effect in a microwave field. Working synergistically with the carbon matrix, they significantly improve the overall microwave absorption and conversion efficiency of biochar.

[0012] Furthermore, the role of the aforementioned hierarchical porous structure is not limited to enhancing the microwave absorption performance of biochar itself. It can generate a structural inheritance effect during the subsequent microwave carbothermal reduction process: the hierarchical porous biochar no longer forms a traditional point contact mode with the cathode material particles, but rather a surface contact and embedded interface contact. Combined with in-situ loaded high-dielectric-loss nanoparticles, microwave micro-hot spots can be uniformly distributed on the surface and shallow interface of the cathode particles, allowing the reduction reaction to proceed simultaneously from multiple sites into the particle interior. During the reaction, lattice collapse and gas release accompany the process, ultimately causing the obtained microwave reconstruction product to inherit the porous structure characteristics of biochar, forming a loose morphology with penetrating pores and numerous microcracks. This effect extends the role of pore-forming modification from the biochar preparation stage to the calcination reconstruction stage, providing a built-in mass transfer channel for the subsequent leaching process, which is a key structural basis for achieving efficient and selective lithium leaching.

[0013] (II) Microwave Carbothermic Reduction Mechanism Under microwave radiation, the microwave-absorbing biochar rapidly heats up and forms numerous microscopic "hot spots" within the material. The local temperature is much higher than the macroscopic temperature, causing the layered lattice of the cathode material to collapse and reconstruct in a very short time. During this process, lithium is released from the stable layered structure and reacts with carbothermal reduction products to form easily soluble lithium-containing phases (such as Li₂O, Li₂CO₃, or LiOH), while transition metals (Ni, Co, Mn) are reduced to low-valence oxides or metallic states, which are insoluble in water, providing a basis for subsequent selective leaching.

[0014] It should be noted that the cathode material black powder obtained after dismantling, crushing and sorting of waste lithium-ion batteries inevitably contains some residual anode carbonaceous material (mainly graphite). This residual carbon can participate in the carbothermic reduction reaction as an auxiliary reducing agent during microwave roasting, and it also has a certain microwave absorption capacity, which helps the material as a whole to heat up in the microwave field. Therefore, the proportion of modified biochar added in step (2) of this invention has taken into account the contribution of residual carbon in the black powder.

[0015] The microwave carbothermal reduction process can be represented by the following reaction equation (taking ternary NCM materials as an example): 12LiNi1 / 3Co1 / 3Mn1 / 3O2+7C→6Li2CO3+4CoNi+4MnO+CO2↑ The carbon sources include modified biochar and residual negative electrode carbon in black powder. XRD results show that the main microwave carbothermic reduction products are Li2CO3, CoNi alloy, and MnO (such as...). Figure 3 As shown in the figure, CoNi alloy is a solid solution formed by Ni and Co under reducing conditions.

[0016] (III) Mechanism of synergistic leaching by pressurized CO2 and ultrasound One of the core innovations of this invention is that pressurized CO2 carbonation leaching and ultrasonic cavitation effect are applied simultaneously in the same reaction system, forming a coupling of chemical driving force and physical driving force to achieve synergistic effect.

[0017] From a chemical driving force perspective: In a pressurized CO2 aqueous solution system, the lithium-containing phase (Li2O or LiOH) in the microwave reconstruction product reacts with dissolved CO2 to generate soluble lithium bicarbonate. Li₂CO₃ + CO₂ + H₂O → 2LiHCO₃ Lithium bicarbonate dissolves in water and enters the liquid phase. However, transition metal elements and low-valence oxides (Ni, Co, MnO, etc.) do not react significantly with CO2 under the same conditions, or the resulting carbonates have extremely low solubility and remain in the solid phase, thus achieving selective leaching of lithium.

[0018] In terms of physical driving force: the cavitation effect generated by the synchronously applied ultrasonic field forms a strong microjets at the solid-liquid interface, which continuously peels off the dissolved product layer and unreacted solid surface coverings on the particle surface, exposing fresh unreacted interfaces and greatly enhancing the mass transfer process at the gas-liquid-solid three-phase interface.

[0019] Synergistic effect: Pressurized CO2 provides the chemothermodynamic driving force for carbonation leaching, while ultrasonic cavitation provides the physical kinetic driving force for enhanced mass transfer. The two work synchronously in the same reaction system. The chemical reaction continuously consumes the lithium-containing phase at the interface, while ultrasonic exfoliation continuously exposes fresh surfaces, forming a positive feedback loop of "reaction-exfoliation-re-reaction". This reduces the leaching time from several hours to 10-30 minutes, and the single-stage leaching rate of lithium can reach over 96%.

[0020] (iv) Lithium carbonate product recycling As an optional post-treatment step, lithium bicarbonate in lithium-rich solutions can be decomposed into lithium carbonate precipitate by heating: 2LiHCO3→Li2CO3↓+CO2↑+H2O The desorption temperature is controlled at 90~95℃, at which lithium bicarbonate can be rapidly and completely decomposed. The CO2 gas released during the desorption process is collected and returned to step (3) for recycling, realizing a closed-loop circulation of the leaching agent.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention utilizes inexpensive renewable biomass waste and modifies it by impregnation and pyrolysis with pore-forming agents and transition metal salts to prepare porous biochar with excellent microwave absorption performance. It also uses it as a reducing agent, which not only achieves waste treatment but also greatly improves the microwave carbothermal reduction efficiency, shortens the roasting time to 5-20 minutes, and has lower energy consumption than conventional resistance furnaces and simple biomass mixing microwave roasting processes.

[0022] (2) This invention innovatively combines ultrasonic field with pressurized CO2 leaching system, and effectively solves the bottleneck problem of product layer coating and slow mass transfer in gas-liquid-solid three-phase reaction by utilizing ultrasonic cavitation effect, compressing leaching time to 10~30 minutes, and the single-stage leaching rate of lithium can reach more than 96%, and the selectivity for impurity metals such as nickel, cobalt, and manganese is extremely high.

[0023] (3) The process of this invention is short, energy-efficient and efficient. It does not require the use of strong acids or alkalis and the leaching agent CO2 can be recycled. It is a green, low-carbon and sustainable waste lithium battery recycling method with significant industrial application prospects. Attached Figure Description

[0024] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a process flow diagram of an embodiment of the present invention.

[0025] Figure 2 The image is a scanning electron microscope image of the microwave reconstruction product obtained in step (2) of Example 1.

[0026] Figure 3 The XRD patterns of the waste ternary black powder in Example 1 and the microwave reconstruction product obtained in step (2) are shown.

[0027] Figure 4 This is a scanning electron microscope image of the porous absorbing biochar prepared in Example 1 of the present invention. Detailed Implementation

[0028] A method for pre-extraction of lithium using microwave reduction combined with ultrasonic carbon dioxide leaching includes the following steps: (1) The biomass waste powder is impregnated and mixed with pore-forming agent and transition metal salt in an aqueous solution, evaporated to dryness, and then subjected to high-temperature pyrolysis under an inert atmosphere. After washing and drying, porous microwave absorbing biochar is obtained. (2) The waste lithium-ion battery cathode material black powder and the microwave-absorbing biochar prepared in step (1) are mixed in proportion and microwave radiation calcined under the protection of protective gas to reconstruct the lattice of the cathode material and convert lithium into a soluble lithium-containing phase to obtain the microwave reconstruction product. (3) The microwave reconstruction product from step (2) is mixed with water and slurryed, pressurized carbon dioxide gas is introduced, and an ultrasonic field is applied at the same time. The rapid selective dissolution of lithium is achieved by utilizing the synergistic effect of pressurized CO2 carbonation leaching and ultrasonic cavitation stripping. After the leaching reaction is completed, micro-pressure solid-liquid separation is performed to obtain lithium-rich solution and nickel-cobalt-manganese enriched slag.

[0029] Optionally, the process also includes step (4): heating and desorbing the lithium-rich solution from step (3) to precipitate lithium carbonate, which is then filtered, washed and dried to obtain the lithium carbonate product.

[0030] In a specific embodiment, in step (1), the biomass waste is one or a combination of walnut shells, corn stalks, coconut shells, bamboo waste, and pomelo peels.

[0031] In a specific embodiment, in step (1), the transition metal salt is one or a combination of cobalt chloride, cobalt nitrate, nickel chloride, nickel nitrate, ferric chloride, ferric nitrate or their hydrates.

[0032] In a specific embodiment, in step (1), the mass ratio of the biomass waste powder to the transition metal salt is 10:(0.5~1.5).

[0033] Biomass waste is widely available and inexpensive, enabling waste-to-waste treatment. During pyrolysis, transition metal salts can catalyze the formation of more graphitized carbon structures and generate in-situ metal or metal carbide nanoparticles with high dielectric loss, significantly improving the microwave absorption performance of biochar. Urea, as a pore-forming agent and nitrogen source, helps to form a high specific surface area and hierarchical porous structure, enhancing microwave absorption and reaction contact area.

[0034] In a specific embodiment, in step (1), the high-temperature pyrolysis temperature is 700~850℃, the heating rate is 5~10℃ / min, and the holding time is 1~3h. This temperature range is beneficial for obtaining biochar with both good electrical conductivity and abundant surface functional groups.

[0035] In a specific embodiment, in step (1), the impregnation and mixing time is 1 to 3 hours.

[0036] In a specific embodiment, the evaporation temperature is 80~100℃.

[0037] In a specific embodiment, in step (1), the washing is performed sequentially using acid and water. Acid washing can remove unstable metal salts and impurities from the pyrolysis products, further clear the pores, and leave behind metal species with catalytic activity and microwave absorption properties.

[0038] In a specific embodiment, in step (1), the acid washing uses 1~2 mol / L hydrochloric acid, and after acid washing, it is washed with deionized water until neutral.

[0039] In a specific embodiment, in step (2), the waste lithium-ion battery cathode material black powder is one or more of waste ternary black powder, lithium cobalt oxide, lithium manganese oxide, and lithium nickel cobalt aluminum oxide.

[0040] In a specific embodiment, in step (2), the waste lithium-ion battery cathode material black powder contains residual anode carbonaceous material (mainly graphite), and the residual carbon content is generally 5% to 15% of the black powder mass, the specific content depending on the efficiency of the preceding sorting process. The mass ratio of the black powder to the porous microwave-absorbing biochar is 100:(5~15). Since the residual carbon in the black powder can participate in the carbothermic reduction reaction as an auxiliary reducing agent and contribute a certain microwave absorption capacity, the addition ratio of modified biochar in step (2) has taken into account the contribution of the residual carbon.

[0041] In a specific embodiment, in step (2), the microwave frequency of the microwave radiation roasting is 915MHz or 2.45GHz, the microwave power is 600~1200W, and the roasting time is 5~20min.

[0042] The modified microwave-absorbing biochar of this invention can rapidly generate hot spots in a microwave field, enabling the material to reach the reaction temperature in a short time, significantly shortening the calcination time and reducing energy consumption.

[0043] In a specific embodiment, in step (3), the partial pressure of the carbon dioxide gas is 1.5~3.5 MPa.

[0044] In a specific embodiment, in step (3), the frequency of the ultrasonic field is 20~40kHz, the ultrasonic power density is 0.5~2.0W / mL, the leaching temperature is 15~35℃, and the leaching time is 10~30min.

[0045] Pressurized CO2 dissolves in water to form carbonic acid, providing the chemothermodynamic driving force for lithium carbonation leaching. Simultaneously applied ultrasonic fields generate cavitation effects at the solid-liquid interface, forming microjets that continuously strip the product layer from the particle surface, providing the physicokinetic driving force for enhanced mass transfer. Both processes are spatiotemporally coupled within the same reaction system, forming a positive feedback loop of "reaction-stripping-re-reaction," reducing leaching time from several hours to 10-30 minutes, with a single-stage lithium leaching rate exceeding 96%. Room temperature operation reduces energy consumption.

[0046] In a specific embodiment, in step (3), the solid-liquid ratio of the slurry is 10~50g / L.

[0047] In a specific embodiment, in step (3), the solid-liquid separation is carried out under a micro-positive pressure of 0.1~0.3MPa to prevent the dissolved CO2 from escaping and causing lithium carbonate to precipitate prematurely.

[0048] In a specific embodiment, in step (4), the temperature of the heating and desorption process is 90~95℃. At this temperature, lithium bicarbonate (LiHCO3) can be rapidly decomposed into lithium carbonate (Li2CO3) precipitate.

[0049] In a specific embodiment, in step (4), the carbon dioxide gas generated during the analysis process is collected and returned to step (3) for recycling, thus realizing a closed-loop circulation of the leaching agent, which is environmentally friendly.

[0050] In a specific embodiment, the protective gas mentioned in steps (1) and (2) is nitrogen or an inert gas, preferably argon.

[0051] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0052] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0053] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0054] Example 1 This embodiment provides a pre-lithiation method using microwave reduction combined with ultrasonic carbon dioxide leaching, employing methods such as... Figure 1 The process flow diagram shown includes the following steps: (1) Collect agricultural waste walnut shells, crush them to 100 mesh, wash and dry them. Weigh 100g of walnut shell powder, 20g of urea and 10g of ferric chloride hexahydrate, add them to deionized water and stir to soak for 2h, then stir and evaporate to dryness at 80℃, and then crush them. Place the resulting mixed precursor in a tube furnace, heat it to 800℃ at a rate of 5℃ / min under a nitrogen atmosphere, and hold it at that temperature for 2 hours for co-pyrolysis. Wash the pyrolysis product with 1mol / L hydrochloric acid to remove unreacted metal salts, then wash it with deionized water until neutral, and dry it at 110℃ to obtain magnetic porous microwave absorbing biochar with high dielectric loss characteristics.

[0055] (2) Take 100g of waste NCM523 ternary black powder and mix it evenly with 8g of microwave-absorbing biochar prepared in step (1). Place the mixture in a microwave tube furnace, introduce high-purity nitrogen as a protective gas, turn on the microwave (frequency 2.45GHz), set the power to 800W, and radiate for 10min. In the microwave field, the microwave-absorbing biochar rapidly heats up and forms countless microscopic "hot spots" inside the material, causing the ternary material lattice to collapse instantaneously, the transition metal to be reduced, and the lithium element to be converted into an easily soluble phase in situ, thus obtaining the microwave reconstruction product.

[0056] (3) The microwave reconstruction product obtained in step (2) was transferred to a high-pressure reactor equipped with an ultrasonic transducer, and deionized water was added at a solid-liquid ratio of 25 g / L to prepare a slurry. CO2 gas was introduced into the reactor to maintain the total pressure of the system at 2.5 MPa. The ultrasonic generator was turned on, the frequency was set to 20 kHz, and the ultrasonic power density was 1.2 W / mL. The mixture was stirred and leached at room temperature (25 °C) for 15 min, and the microjets generated by ultrasonic cavitation were used to continuously peel off the coating layer on the surface of the particles. After leaching, the mixture was filtered under a micro-positive pressure of 0.2 MPa to achieve solid-liquid separation and obtain a lithium-rich solution and a nickel-cobalt-manganese-enriched slag.

[0057] (4) The lithium-rich solution obtained in step (3) is transferred into a crystallization vessel and heated to 95°C with stirring. The solution is then kept at a constant temperature for 45 minutes to decompose the LiHCO3 in the solution and precipitate a white Li2CO3 precipitate. The CO2 gas released during the precipitation process is collected after condensation and drying for recycling in step (3). The precipitate is filtered, washed with deionized water, and dried under vacuum at 110°C to obtain the lithium carbonate product.

[0058] According to inductively coupled plasma optical emission spectrometry (ICP-OES), the single-stage leaching rate of lithium in this embodiment was 97.6%, and the co-leaching rates of nickel, cobalt, and manganese were all less than 0.05%.

[0059] Figure 2This is a scanning electron microscope (SEM) image of the microwave reconstruction product obtained in Example 1 of this invention. As can be seen from the image, after microwave calcination, the dense layered structure of the original ternary material underwent significant collapse and reconstruction, with numerous obvious cracks and disintegration pores appearing on the particle surface. This indicates that under the action of the micro-hot spots of the microwave-absorbing biochar, lithium elements have directionally dissociated from the stable lattice and migrated to the surface, providing an excellent physical channel and kinetic conditions for subsequent ultrasonic cavitation leaching.

[0060] Figure 3 The image shows the XRD patterns of the waste ternary black powder and the obtained microwave reconstruction product in Example 1 of this invention. As can be seen from the image, the upper figure is the XRD pattern of the waste ternary black powder, showing LiNi... 0.5 Co 0.2 Mn 0.3 Characteristic diffraction peaks of O2 (NCM523). The figure below shows the XRD pattern of the microwave reconstruction product obtained after microwave calcination of the modified microwave-absorbing biochar of this invention. Compared with the figure above, there is a significant change: the characteristic diffraction peaks of the original NCM523 disappear, and are replaced by diffraction peaks of four phases: Li2CO3, CoNi alloy, MnO, and C. Under the microwave dielectric heating effect of the modified microwave-absorbing biochar of this invention, the layered lattice of the NCM523 cathode material collapses and reconstructs in a short time. Lithium migrates directionally to the surface and is transformed into the Li2CO3 phase, while the transition metal remains in the solid phase in the form of CoNi alloy and MnO. A large amount of carbon matrix is ​​retained, laying a solid phase foundation for the selective extraction of lithium by subsequent pressurized CO2 carbonation leaching.

[0061] Figure 4 This is a scanning electron microscope (SEM) image of the magnetic porous microwave-absorbing biochar with high dielectric loss characteristics prepared in Example 1 of this invention. The image shows that after urea pore formation and iron salt catalytic pyrolysis, a rich and uniformly distributed hierarchical network of pores is formed inside the biochar matrix. This well-developed physical structure not only significantly increases the specific surface area but also effectively increases the multiple reflection paths of microwaves within the material, which is the key microscopic basis for improving the overall electromagnetic wave loss and microwave absorption heat generation capability.

[0062] Example 2 This embodiment provides a pre-lithiation method using microwave reduction combined with ultrasonic carbon dioxide leaching, employing methods such as... Figure 1 The process flow diagram shown includes the following steps: (1) Collect corn stalks, crush them to 100 mesh, wash and dry them. Weigh 100g of corn stalk powder, 30g of urea and 8g of ferric nitrate nonahydrate, add them to deionized water and stir to soak for 2h, then stir and evaporate to dryness at 80℃, and then crush them. Place the resulting mixed precursor in a tube furnace, heat it to 750℃ at a rate of 5℃ / min under a nitrogen atmosphere, and hold it at that temperature for 2.5h for co-pyrolysis. Wash the pyrolysis product with 1mol / L hydrochloric acid to remove unreacted metal salts, then wash it with deionized water until neutral, and dry it at 110℃ to obtain straw-based porous microwave absorbing biochar.

[0063] (2) Take 150g of waste NCM811 ternary black powder and mix it evenly with 15g of microwave-absorbing biochar prepared in step (1). Place the mixture in a microwave tube furnace, introduce high-purity argon as a protective gas, turn on the microwave (frequency 2.45GHz), set the power to 900W, and radiate for 8 minutes. In the microwave field, the microwave-absorbing biochar rapidly heats up and forms countless microscopic "hot spots" inside the material, causing the ternary material lattice to collapse instantaneously, the transition metal to be reduced, and the lithium element to be converted into an easily soluble phase in situ, thus obtaining the microwave reconstruction product.

[0064] (3) The microwave reconstruction product obtained in step (2) was transferred to a high-pressure reactor equipped with an ultrasonic transducer, and deionized water was added at a solid-liquid ratio of 30 g / L to prepare a slurry. CO2 gas was introduced into the reactor to maintain the total system pressure at 3.0 MPa. The ultrasonic generator was turned on, the frequency was set to 20 kHz, and the ultrasonic power density was 1.5 W / mL. The mixture was stirred and leached at 20 °C for 20 min, and the microjets generated by ultrasonic cavitation were used to continuously peel off the coating layer on the particle surface. After leaching, the mixture was filtered under a micro-positive pressure of 0.2 MPa to achieve solid-liquid separation and obtain a lithium-rich solution and a nickel-cobalt-manganese-enriched slag.

[0065] (4) The lithium-rich solution obtained in step (3) is transferred into a crystallization vessel and heated to 90°C with stirring. The solution is then kept at a constant temperature for 60 min to decompose the LiHCO3 in the solution and precipitate a white Li2CO3 precipitate. The CO2 gas released during the precipitation process is collected after condensation and drying for recycling in step (3). The precipitate is filtered, washed with deionized water, and dried under vacuum at 120°C to obtain the lithium carbonate product.

[0066] According to ICP-OES testing, the single-stage leaching rate of lithium in this example was 96.8%, and the purity of the resulting lithium carbonate product reached 99.6%.

[0067] Example 3 This embodiment provides a pre-lithiation method using microwave reduction combined with ultrasonic carbon dioxide leaching, employing methods such as... Figure 1 The process flow diagram shown includes the following steps: (1) Collect waste pomelo peels, crush them to 100 mesh, wash and dry them. Weigh 100g of waste pomelo peel powder, 15g of urea and 12g of ferric nitrate nonahydrate, add them to deionized water and stir to soak for 2h, then stir and evaporate to dryness at 80℃, and then crush them. Place the resulting mixed precursor in a tube furnace, heat it to 850℃ at a rate of 8℃ / min under an argon atmosphere, and hold it at that temperature for 1.5h for co-pyrolysis. Wash the pyrolysis product with 1mol / L hydrochloric acid to remove unreacted metal salts, then wash it with deionized water until neutral, and dry it at 110℃ to obtain pomelo peel-based porous microwave absorbing biochar.

[0068] (2) Take 100g of waste NCM111 ternary black powder and mix it evenly with 12g of microwave-absorbing biochar prepared in step (1). Place the mixture in a microwave tube furnace, introduce high-purity argon as a protective gas, turn on the microwave (frequency 2.45GHz), set the power to 1100W, and radiate for 15min. In the microwave field, the microwave-absorbing biochar rapidly heats up and forms countless microscopic "hot spots" inside the material, causing the ternary material lattice to collapse instantaneously, the transition metal to be reduced, and the lithium element to be converted into an easily soluble phase in situ, thus obtaining the microwave reconstruction product.

[0069] (3) The microwave reconstruction product obtained in step (2) was transferred to a high-pressure reactor equipped with an ultrasonic transducer, and deionized water was added at a solid-liquid ratio of 40 g / L to prepare a slurry. CO2 gas was introduced into the reactor to maintain the total pressure of the system at 2.5 MPa. The ultrasonic generator was turned on, the frequency was set to 20 kHz, and the ultrasonic power density was 0.8 W / mL. The mixture was stirred and leached at 35 °C for 30 min, and the microjets generated by ultrasonic cavitation were used to continuously peel off the coating layer on the surface of the particles. After leaching, the mixture was filtered under a micro-positive pressure of 0.2 MPa to achieve solid-liquid separation and obtain a lithium-rich solution and a nickel-cobalt-manganese-enriched slag.

[0070] (4) The lithium-rich solution obtained in step (3) is transferred into a crystallization vessel and heated to 95°C with stirring. The solution is then kept at a constant temperature for 60 minutes to decompose the LiHCO3 in the solution and precipitate a white Li2CO3 precipitate. The CO2 gas released during the precipitation process is collected after condensation and drying for recycling in step (3). The precipitate is filtered, washed with deionized water, and dried under vacuum at 120°C to obtain the lithium carbonate product.

[0071] According to ICP-OES testing, the single-stage leaching rate of lithium in this example was 97.2%, and the purity of the resulting lithium carbonate product reached 99.6%.

[0072] Example 4 This embodiment provides a method for pre-extraction of lithium using microwave reduction combined with ultrasonic carbon dioxide leaching, including the following steps: (1) Collect waste coconut shells, crush them to 100 mesh, wash and dry them. Weigh 100g of waste coconut shell powder, 20g of urea and 15g of nickel chloride hexahydrate, add them to deionized water and stir to soak for 2h, then stir and evaporate to dryness at 80℃, and then crush them. Place the resulting mixed precursor in a tube furnace, heat it to 750℃ at a rate of 5℃ / min under an argon atmosphere, and hold it at that temperature for 1.5h for co-pyrolysis. Wash the pyrolysis product with 1mol / L hydrochloric acid to remove unreacted metal salts, then wash it with deionized water until neutral, and dry it at 110℃ to obtain coconut shell-based porous microwave absorbing biochar.

[0073] (2) Take 500g of waste lithium cobalt oxide black powder and mix it evenly with 40g of microwave-absorbing biochar prepared in (1). Place the mixture in a 915MHz industrial microwave oven, introduce high-purity nitrogen as a protective gas, set the power to 1200W, and radiate heat for 15min. In the microwave field of the longer wavelength band of 915MHz, the microwave-absorbing biochar also generates a strong dielectric heating effect, which causes the overall collapse of the lithium cobalt oxide material lattice in the large volume material system.

[0074] (3) The reconstructed product was transferred to a large-capacity ultrasonic reactor, and deionized water was added at a solid-liquid ratio of 30 g / L. CO2 gas was introduced to maintain the total pressure of the system at 3.0 MPa, and the ultrasonic generator (frequency 30 kHz, power density 1.5 W / mL) was turned on. The mixture was stirred and leached at 30 °C for 20 min. Subsequently, the mixture was separated by vacuum filtration under a slight positive pressure of 0.2 MPa.

[0075] (4) The lithium-rich solution was heated to 95°C and kept at a constant temperature for 60 min to precipitate Li2CO3 and then dried.

[0076] ICP-OES analysis showed that the single-stage lithium leaching rate in this embodiment was 96.5%, and the purity of the resulting lithium carbonate product reached 99.5%. This indicates that the process still possesses excellent lithium extraction efficiency at industrial-grade microwave frequencies.

[0077] Comparative Example 1 The steps of this comparative example are basically the same as those of Example 1, except that in the CO2 leaching process in step (3), the ultrasonic generator is not turned on, and mechanical stirring is used instead. Specifically: (3) The microwave reconstruction product obtained in step (2) is transferred to a high-pressure reactor equipped with an ultrasonic transducer, and deionized water is added at a solid-liquid ratio of 25 g / L to prepare a slurry. CO2 gas is introduced into the reactor to maintain the total pressure of the system at 3.0 MPa. The leaching is carried out at 20°C with mechanical stirring for 15 min. After the leaching is completed, the solid and liquid are separated by vacuum filtration under a slight positive pressure of 0.2 MPa to obtain the solution and residue.

[0078] Testing revealed that, without the application of ultrasound, the single-stage leaching rate of lithium was only 42.5%. This indicates that the cavitation effect generated by the ultrasonic field plays a crucial role in overcoming the mass transfer bottleneck and achieving efficient lithium leaching.

[0079] Comparative Example 2 The steps of this comparative example are basically the same as those in Example 1, except that: (2) Take 100g of waste NCM523 ternary black powder and mix it evenly with 8g of microwave-absorbing biochar prepared in step (1). Place the mixture in a conventional resistance heating tube furnace, introduce high-purity nitrogen as a protective gas, and heat it to 800℃ at a rate of 10℃ / min. Since it relies on external heat conduction, it needs to be kept at the temperature for up to 2 hours to ensure that the reaction is complete.

[0080] Testing revealed that although the final lithium leaching rate in this comparative example reached 95.1%, the total time spent on the calcination process exceeded 2 hours, significantly longer than the 10 minutes in Example 1. This demonstrates the substantial advantages of the modified microwave-absorbing carbon combined with microwave heating in significantly reducing energy consumption and shortening the calcination time.

[0081] Comparative Example 3 The steps of this comparative example are basically the same as those in Example 1, except that: (1) Prepare conventional commercial flake graphite powder; (2) Take 100g of waste NCM523 ternary black powder and mix it evenly with 8g of conventional commercial flake graphite powder. Place the mixture in a microwave tube furnace, introduce high-purity nitrogen as a protective gas, turn on the microwave (frequency 2.45GHz), set the power to 800W, and radiate heat for 10min to obtain the microwave reconstruction product.

[0082] Testing revealed that conventional commercial graphite lacks a well-developed porous structure, resulting in extremely low microwave absorption and conversion efficiency. It is unable to generate a sufficiently high-temperature "hot spot" within a very short 10 minutes, leading to incomplete dissociation of the ternary lattice. Ultimately, the single-step lithium leaching rate was only 68.3%. This indicates that the porous microwave-absorbing biochar specific to this invention is an indispensable core element for achieving rapid microwave reconfiguration.

[0083] Comparative Example 4 The steps of this comparative example are basically the same as those in Example 1, except that: (3) Transfer 100g of microwave reconstruction product to an open ultrasonic reactor, add deionized water to slurry, and control the solid-liquid ratio to 20g / L. Continuously introduce CO2 gas (flow rate 2.0L / min) into the bottom of the reactor to keep the solution CO2 saturated. Turn on the mechanical stirrer (400r / min) and simultaneously turn on the ultrasonic generator, set the frequency to 25kHz and the power density to 1.5W / mL, and react at room temperature (25℃) for 60min.

[0084] (4) Separation and analysis: After the reaction is completed, the mixture is filtered under normal pressure. The filtrate is transferred to a crystallization vessel and heated to 95°C for 60 min to precipitate lithium carbonate. The precipitate is washed and dried at 110°C.

[0085] Test results: Under normal pressure conditions, combined with strong ultrasonic cavitation, the single-stage leaching rate of lithium was 81.2%, indicating that pressurized leaching can significantly improve the lithium ion leaching rate.

[0086] Comparative Example 5 The steps in this comparative example are basically the same as those in Example 1, except that no pore-forming agent or transition metal salt is added in step (1), and the biomass is directly pyrolyzed: (1) Collect walnut shells, crush them to 100 mesh, wash them with water and dry them. Weigh 100g of walnut shell powder, add it to deionized water and stir for 2 hours, then stir and evaporate it to dryness at 80℃, and then crush it. Place the obtained precursor in a tube furnace, heat it to 800℃ at a rate of 5℃ / min under a nitrogen atmosphere, and hold it at that temperature for 2 hours for pyrolysis. Wash the pyrolysis product with deionized water until neutral, and dry it at 110℃ to obtain ordinary biochar.

[0087] (2) Take 100g of waste NCM523 ternary black powder and mix it evenly with 8g of ordinary biochar prepared in step (1). The subsequent microwave roasting, leaching and desorption steps are exactly the same as in Example 1.

[0088] The test results showed that the single-stage leaching rate of lithium in this comparative example was 79.2%.

[0089] Comparative Example 6 The steps in this comparative example are basically the same as those in Example 1, except that in step (1), ordinary coal powder is used instead of modified microwave-absorbing biochar: (1) Take commercial smokeless coal powder and pass it through a 100-mesh sieve.

[0090] (2) Take 100g of waste NCM523 ternary black powder and mix it evenly with 8g of coal powder obtained in step (1). The subsequent microwave roasting, leaching and desorption steps are exactly the same as in Example 1.

[0091] The test results showed that the single-stage leaching rate of lithium in this comparative example was 80.4%.

[0092] Comparative Example 7 The steps of this comparative example are basically the same as those of Example 1, except that CO2 gas is not introduced into the reactor in step (3), and only ultrasonic water leaching is used: (3) The microwave reconstruction product obtained in step (2) is transferred to a high-pressure reactor equipped with an ultrasonic transducer, and deionized water is added at a solid-liquid ratio of 25 g / L to prepare a slurry. No CO2 gas is introduced, and a nitrogen atmosphere is maintained inside the reactor. The ultrasonic generator is turned on, the frequency is set to 20 kHz, and the ultrasonic power density is 1.2 W / mL. The mixture is stirred and leached at room temperature (25 °C) for 15 min. After leaching, the mixture is filtered to achieve solid-liquid separation, resulting in a lithium-rich solution and a nickel-cobalt-manganese-enriched residue.

[0093] (4) The subsequent analysis steps are the same as in Example 1.

[0094] The test results showed that the single-stage leaching rate of lithium in this comparative example was 37.6%.

[0095] Comparative Example 8 The steps of this comparative example are basically the same as those of Example 1, except that in step (3), neither CO2 gas is introduced nor ultrasound is turned on, and only pure water is used for leaching: (3) The microwave reconstruction product obtained in step (2) is transferred to a reactor, and deionized water is added at a solid-liquid ratio of 25 g / L to form a slurry. CO2 gas is not introduced, and ultrasonic waves are not turned on. The leaching is carried out by mechanical stirring at room temperature (25°C) for 15 min. After leaching, the mixture is filtered to achieve solid-liquid separation and obtain a lithium-rich solution and a nickel-cobalt-manganese-enriched residue.

[0096] (4) The subsequent analysis steps are the same as in Example 1.

[0097] The test results showed that the single-stage leaching rate of lithium in this comparative example was 24.1%.

[0098] The key data of the above embodiments and comparative examples are summarized in the table below: Table 1 It can be seen that: Examples 1-4, using the technical solution of the present invention, all achieved a single-stage lithium leaching rate of over 96%; Comparative Example 1 eliminated the ultrasonic field, Comparative Example 2 used conventional resistance furnace roasting instead of microwave radiation roasting, Comparative Example 3 used conventional graphite powder instead of porous microwave-absorbing biochar, Comparative Example 4 used atmospheric pressure CO2 leaching instead of pressurized CO2 leaching, Comparative Example 5 used unmodified ordinary biochar instead of modified microwave-absorbing biochar, Comparative Example 6 used ordinary coal powder instead of modified microwave-absorbing biochar, Comparative Example 7 eliminated CO2 carbonation leaching, and Comparative Example 8 eliminated both CO2 and ultrasound. The lithium leaching rates of all comparative examples showed a significant decrease. The above results indicate that: (1) The preparation process of modified microwave absorbing biochar (pore-forming agent + transition metal salt impregnation pyrolysis) is the core element for realizing microwave rapid calcination; (2) The synergistic effect of pressurized CO2 carbonation leaching and ultrasonic cavitation stripping applied simultaneously in the same reaction system is a key technical means to achieve rapid and selective lithium leaching. (3) The preparation of porous microwave absorbing biochar, microwave radiation calcination, pressurized CO2 and ultrasonic field synergistic leaching and other technical means do not play their roles independently, but under the conditions defined by this invention, they jointly realize the efficient reconstruction of the cathode material lattice and the selective dissolution of lithium, thereby achieving the technical effect of high lithium leaching rate.

[0099] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of the present invention.

Claims

1. A method for pre-extraction of lithium using microwave reduction combined with ultrasonic leaching of carbon dioxide, characterized in that, Includes the following steps: (1) The biomass waste powder is impregnated and mixed with a pore-forming agent and a transition metal salt in an aqueous solution, evaporated to dryness, pulverized, and then subjected to high-temperature pyrolysis under a protective gas. After washing and drying, porous microwave-absorbing biochar is obtained; wherein the pore-forming agent is one or a combination of urea, ammonium bicarbonate, and ammonium oxalate, and the transition metal salt is one or a combination of cobalt chloride, cobalt nitrate, nickel chloride, nickel nitrate, ferric chloride, ferric nitrate, or their hydrates. (2) The waste lithium-ion battery cathode material black powder and the microwave absorbing biochar prepared in step (1) are mixed in proportion and microwave irradiated under the protection of protective gas to obtain microwave reconstruction product. (3) The microwave reconstruction product from step (2) is mixed with water and slurried, pressurized carbon dioxide gas is introduced, and an ultrasonic field is applied at the same time to carry out the leaching reaction; after the leaching reaction is completed, solid and liquid are separated to obtain lithium-rich solution and nickel-cobalt-manganese enriched slag.

2. The lithium extraction method according to claim 1, characterized in that, In step (1), the biomass waste is one or a combination of walnut shells, corn stalks, coconut shells, bamboo waste, and pomelo peels; The mass ratio of the biomass waste to the transition metal salt is 10:(0.5~1.5).

3. The lithium extraction method according to claim 1, characterized in that, In step (1), the temperature of the high-temperature pyrolysis is 700~850℃, the heating rate is 5~10℃ / min, and the holding time is 1~3h.

4. The lithium extraction method according to claim 1, characterized in that, In step (1): The impregnation and mixing time is 1 to 3 hours; The evaporation temperature is 80~100℃; The washing process involves sequentially washing with acid and water; the acid washing uses 1-2 mol / L hydrochloric acid, followed by washing with deionized water until neutral; The drying temperature is 80~105℃.

5. The lithium extraction method according to claim 1, characterized in that, In step (2): The waste lithium-ion battery cathode material black powder is one or a combination of several of the following: waste ternary black powder, lithium cobalt oxide, lithium manganese oxide, and lithium nickel cobalt aluminum oxide. The mass ratio of the waste lithium-ion battery cathode material black powder to the microwave-absorbing biochar is 100:(5~15).

6. The lithium extraction method according to claim 1, characterized in that, In step (2), the frequency of the microwave radiation is 2.45 GHz or 915 MHz, the microwave radiation power is 600~1200 W, and the microwave radiation time is 5~20 min.

7. The lithium extraction method according to claim 1, characterized in that, In step (3): The partial pressure of the pressurized carbon dioxide gas is 1.5~3.5 MPa; The frequency of the ultrasonic field is 20~40kHz, and the ultrasonic power density is 0.5~2.0W / mL; The leaching reaction is carried out at a temperature of 15-35°C for 10-30 minutes.

8. The lithium extraction method according to claim 1, characterized in that, In step (3): The solid-liquid ratio of the microwave reconstruction product mixed with water in the slurry is 10~50g / L; The solid-liquid separation method is micro-pressure filtration, with an operating pressure of 0.1~0.3MPa.

9. The lithium extraction method according to claim 1, characterized in that, The protective gas mentioned in steps (1) and (2) is nitrogen or an inert gas.

10. The method for pre-extracting lithium according to any one of claims 1 to 9, characterized in that, The process also includes step (4): heating and precipitating the lithium-rich solution obtained in step (3) to obtain lithium carbonate, which is then filtered, washed, and dried to obtain the lithium carbonate product; wherein: The temperature for the heating and desorption process is 90~95℃; The carbon dioxide gas released during the analysis process is collected after condensation and drying, and returned to step (3) for recycling.

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