A method and system for lithium sulfide production and solvent recovery

Abstract

The application discloses a lithium sulfide preparation and solvent recovery method and system, and belongs to the technical fields of lithium sulfide preparation and organic solvent recovery. The method comprises the following steps: adding anhydrous lithium hydroxide powder into N-methyl pyrrolidone solvent to form a suspension, introducing hydrogen sulfide gas into a sealed reaction kettle to perform a neutralization reaction, and generating lithium hydrosulfide-N-methyl pyrrolidone slurry; stopping the introduction of the gas, heating the system to 180 DEG C, and pyrolyzing at constant temperature to make lithium hydrosulfide decompose into lithium sulfide solid, while the solvent volatilizes to form steam and tail gas with incondensable gas; introducing the tail gas into a series condenser, controlling the temperature of the condensing zone in the condenser to be 30 DEG C to 45 DEG C, liquefying the steam, and collecting and recovering the steam; and recycling the recovered solvent after impurity removal. The application can recover N-methyl pyrrolidone solvent under high-temperature pyrolysis conditions and reduce solvent loss.

Description

Technical Field

[0001] This invention relates to the field of lithium sulfide preparation and organic solvent recovery technology, and particularly to a method and system for lithium sulfide preparation and solvent recovery. Background Technology

[0002] In existing technologies, lithium sulfide, as a key raw material for all-solid-state lithium battery electrolytes, is prepared through a mainstream method where lithium hydroxide and hydrogen sulfide gas undergo a gas-liquid neutralization reaction in an organic solvent system to generate lithium hydrosulfide intermediates. These intermediates are then subjected to high-temperature pyrolysis to decompose the lithium hydrosulfide into solid lithium sulfide. N-methylpyrrolidone is widely used as a synthesis solvent due to its excellent solubility. However, during the high-temperature pyrolysis stage at approximately 180°C, the temperature approaches the boiling point of N-methylpyrrolidone solvent, causing the solvent to evaporate and form vapor, which is then released along with the reaction tail gas.

[0003] Existing lithium sulfide production processes typically lack dedicated solvent recovery devices, resulting in the direct emission of volatilized vapors as part of the exhaust gas. This leads to organic solvent loss, increased production costs, and potential environmental pollution issues. Current processes focus on moisture control during the synthesis stage, but lack effective specific recovery methods for solvent volatilization losses under specific conditions during the high-temperature pyrolysis stage.

[0004] Therefore, it is necessary to recover N-methylpyrrolidone solvent and reduce solvent loss under the high-temperature pyrolysis conditions of lithium sulfide preparation. Summary of the Invention

[0005] The main objective of this invention is to provide a method and system for lithium sulfide preparation and solvent recovery, which aims to achieve N-methylpyrrolidone solvent recovery and reduce solvent loss under the high-temperature pyrolysis conditions of lithium sulfide preparation.

[0006] To achieve the above objectives, this invention proposes a method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent, comprising the following steps: S1. Anhydrous lithium hydroxide powder is added to N-methylpyrrolidone solvent to form a suspension. In a closed reactor under nitrogen protection and at a temperature of 80℃~120℃, hydrogen sulfide gas is introduced to carry out a neutralization reaction to generate lithium hydrosulfide-N-methylpyrrolidone slurry. S2. Stop the flow of hydrogen sulfide gas, heat the system in the sealed reactor to 180°C and perform isothermal pyrolysis to decompose the lithium hydrosulfide into lithium sulfide solid. At the same time, the N-methylpyrrolidone solvent in the system evaporates upon heating to form N-methylpyrrolidone vapor, which mixes with the non-condensable gas in the system to form tail gas containing N-methylpyrrolidone. S3. The tail gas containing N-methylpyrrolidone is discharged from the closed reactor and introduced into a condenser connected in series in the tail gas delivery pipeline. The temperature of the condensation zone inside the condenser is controlled to be 30℃~45℃, so that the N-methylpyrrolidone vapor in the tail gas containing N-methylpyrrolidone is liquefied and collected into the recovery storage tank, thus completing the separation and recovery of N-methylpyrrolidone solvent. The uncondensed non-condensable gas is transported to the tail gas treatment unit for treatment. S4. The recovered liquid N-methylpyrrolidone solvent is filtered to remove impurities and then recycled back to step S1 as a dispersion solvent for lithium hydroxide.

[0007] Preferably, in step S1, the mass ratio of lithium hydroxide to N-methylpyrrolidone solvent is 1:8 to 1:15, the flow rate of hydrogen sulfide gas is 0.5 L / min to 1.2 L / min, and the stirring speed during the reaction is 300 r / min to 500 r / min.

[0008] Preferably, in step S1, the hydrogen sulfide gas is introduced from the bottom of the suspension, and the constant temperature stirring reaction time is 2 h to 4 h.

[0009] Preferably, in step S2, the pyrolysis heating rate of the system to 180°C is 5°C / min to 8°C / min, and the system is kept at 180°C for 3 to 5 hours.

[0010] Preferably, in step S2, the mixture is continuously stirred during the isothermal pyrolysis process at 180°C.

[0011] Preferably, the tail gas containing N-methylpyrrolidone is discharged from the sealed reactor and then transported to the condenser through a tail gas delivery pipeline. The tail gas delivery pipeline is insulated and heated throughout the process, and the heating temperature is maintained at 100°C to 120°C.

[0012] Preferably, in step S3, the condenser is a shell-and-tube condenser, and the condenser has multiple layers of baffle plates inside.

[0013] Preferably, the condenser uses circulating cooling water as the cooling medium, and the inlet temperature of the circulating cooling water is controlled to be 25℃~30℃ and the outlet temperature is 35℃~42℃.

[0014] Preferably, in step S3, the residence time of the N-methylpyrrolidone-containing tail gas in the condenser is controlled to be greater than or equal to 8 s.

[0015] This application also discloses a system for recovering N-methylpyrrolidone solvent during the preparation of lithium sulfide, including a reaction vessel, a tail gas delivery pipeline, a condensation and recovery unit, a solvent circulation unit, and a tail gas treatment unit. The reactor is a closed pressure-resistant reactor. The reactor is equipped with a heating wire and a low-speed stirring component. The top of the reactor is equipped with a tail gas outlet, which is connected to the tail gas delivery pipeline. The outer side of the exhaust gas delivery pipeline is provided with a heat tracing and insulation component, which is used to maintain the heat tracing temperature of the exhaust gas delivery pipeline at 100℃~120℃. The condensation recovery unit includes a condenser, a recovery storage tank, and a liquid level monitoring component. The condenser is installed in series on the exhaust gas delivery pipeline. The condenser is a shell-and-tube condenser with multiple layers of baffle condenser plates inside. The bottom liquid phase outlet of the condenser is connected to the recovery storage tank through a pipeline. The liquid level monitoring component is installed in the recovery storage tank and is used to monitor the liquid level in the recovery storage tank. The solvent circulation unit includes a delivery pump and a precision filter. The outlet of the recovery storage tank is connected in sequence to the delivery pump and the precision filter. The outlet of the precision filter is connected to the feed inlet of the reactor through a reflux pipe. The exhaust gas treatment unit includes a spray absorption structure, which is connected to the top gas phase outlet of the condenser via a pipe.

[0016] The above technical solution has the following effects: This application addresses the process of reacting hydrogen sulfide with lithium hydroxide and pyrolyzing it at 180°C. By performing isothermal pyrolysis after the neutralization reaction and setting the temperature of the condensation zone inside the condenser for condensation recovery, it can match the volatility characteristics of the solvent during pyrolysis. Compared with processes without a dedicated solvent recovery device, this method can condense and separate the volatilized N-methylpyrrolidone vapor, reducing organic solvent loss and lithium sulfide production costs. Simultaneously, by filtering and recycling the recovered liquid solvent, a closed-loop recycling process is constructed, reducing organic waste gas emissions and simplifying the subsequent harmless treatment of tail gas. Attached Figure Description

[0017] The present invention will now be described with reference to specific embodiments and accompanying drawings, wherein: Figure 1 A flowchart illustrating the method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent provided in the embodiments of this application.

[0018] Figure 2 This is a schematic diagram of the structure of the N-methylpyrrolidone solvent recovery system provided in the lithium sulfide preparation process according to an embodiment of this application. Detailed Implementation

[0019] To make the objectives and technical solutions of this application clearer, the following description, in conjunction with the accompanying drawings and embodiments, further illustrates this application. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit the scope of protection of this application.

[0020] Example 1 like Figure 1 and Figure 2 As shown, this embodiment provides a method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent, specifically applied in an industrial-scale lithium sulfide production line. First, the material mixing and gas-liquid synthesis reaction in step one are carried out. 10 kg of anhydrous lithium hydroxide powder is weighed and added to 100 kg of N-methylpyrrolidone solvent, controlling the mass ratio of lithium hydroxide to N-methylpyrrolidone solvent to be 1:10. A stirring device is started, and the stirring speed is controlled to be stable at 400 r / min, allowing the lithium hydroxide powder to be stirred and dispersed evenly in the solvent, thereby forming a lithium hydroxide and N-methylpyrrolidone suspension. Next, this suspension is introduced into a sealed, pressure-resistant reactor 100, and high-purity nitrogen gas is continuously introduced to replace the air inside the reactor 100, maintaining a slight positive pressure protection of nitrogen throughout the synthesis reaction. The heating device is turned on to heat the system inside the reactor 100 and maintain the temperature at 100°C. Subsequently, hydrogen sulfide gas was continuously bubbled into the bottom of the suspension at a constant flow rate of 0.8 L / min through a gas inlet pipe located at the bottom of the reactor 100. Under these conditions, the temperature was maintained and the reaction was continuously stirred for 3 hours, allowing the lithium hydroxide dispersed in the liquid to undergo a gas-liquid neutralization reaction with the introduced hydrogen sulfide gas, generating a mixed slurry of lithium hydrosulfide and N-methylpyrrolidone, thus completing the gas-liquid synthesis of the reaction intermediate.

[0021] After the gas-liquid synthesis reaction of the aforementioned intermediate is completed, the high-temperature pyrolysis vaporization process of step two is executed. At this time, the introduction of hydrogen sulfide gas into the reactor 100 is stopped, and the setting parameters of the heating controller are adjusted to control the reactor 100 to heat up at a pyrolysis heating rate of 6℃ / min. When the system temperature inside the reactor 100 reaches 180℃, the isothermal pyrolysis stage begins, and isothermal pyrolysis is performed at 180℃ for 4 hours, causing the lithium hydrosulfide intermediate generated in the previous step to decompose and transform into lithium sulfide solid. Throughout the isothermal pyrolysis process at 180℃, the stirring device is continuously stirred at a low speed to prevent the newly generated lithium sulfide solid in the system from sticking together or agglomerating, while ensuring uniform heating of the reactants and promoting solvent vaporization. At the same time, as the system temperature reaches 180℃, which is close to the boiling point of N-methylpyrrolidone solvent, the N-methylpyrrolidone solvent in the system evaporates upon heating and forms N-methylpyrrolidone vapor. The N-methylpyrrolidone vapor mixes with the nitrogen carrier gas and trace amounts of unreacted hydrogen sulfide gas present in the reactor 100 in the top space of the reactor 100, thereby forming an N-methylpyrrolidone-containing tail gas.

[0022] The tail gas condensation and recovery process in step three is then performed. The tail gas containing N-methylpyrrolidone formed by the above mixture is discharged from the tail gas outlet at the top of the reactor 100 and introduced into the tail gas delivery pipeline 200, which is connected in series. To prevent the high-boiling-point N-methylpyrrolidone vapor from prematurely liquefying, condensing, and adhering to the pipe wall due to local cooling during pipeline transportation, thereby causing scaling, blockage, or solvent loss, the tail gas delivery pipeline 200 is equipped with a heat tracing component throughout, controlling the heat tracing temperature to be maintained at 110°C. The tail gas containing N-methylpyrrolidone is transported under the heat tracing protection of 110°C and introduced into the condenser 310 connected in series in the middle section of the pipeline. In this embodiment, the condenser 310 is specifically a shell-and-tube condenser, and the shell side of the condenser 310 is staggered with multiple layers of baffle condensing plates to increase the effective contact heat exchange area between the tail gas and the condensing tube wall, and to extend the heat exchange flow path of the tail gas inside the condenser 310. Condenser 310 uses circulating cooling water as the cooling medium, controlling the inlet temperature of the circulating cooling water at 28℃ and the outlet temperature at 38℃, thereby controlling the temperature of the condensation zone inside condenser 310 at 35℃. The residence time of the N-methylpyrrolidone-containing tail gas in condenser 310 is controlled to be 9 s. Under this condition, the N-methylpyrrolidone vapor in the tail gas undergoes a phase change and liquefies upon cooling. The liquid N-methylpyrrolidone solvent collects downwards along the multi-layer baffle condenser plates under gravity and flows into and is stored in the recovery storage tank 320 through the liquid phase outlet at the bottom of condenser 310. In this process, the single recovery amount of the volatilized N-methylpyrrolidone solvent reaches 8.2 kg, with a recovery rate of 95.3%. The uncondensed trace amounts of hydrogen sulfide gas and non-condensable gases such as nitrogen are discharged from the gas phase outlet at the top of condenser 310 and continue to be transported to the tail gas treatment unit 500 for neutralization and desulfurization treatment.

[0023] Finally, the solvent recycling process in step four is executed. The purity of the liquid N-methylpyrrolidone solvent collected in the recovery storage tank 320 is greater than or equal to 99.5%, making it suitable for use as an industrial-grade solvent for material dispersion. The transfer pump 410 in the solvent circulation unit is started to extract the liquid N-methylpyrrolidone solvent from the recovery storage tank 320 and pass it through a precision filter 420 installed on the pipeline. The precision filter 420 has a filtration accuracy of 0.22 μm to trap and remove trace amounts of lithium sulfide dust, mechanical impurities, or small particles entrained in the liquid phase. After precision filtration and impurity removal, the N-methylpyrrolidone solvent is directly returned to the upstream material mixing process via a return pipeline, where it is again used as a dispersion solvent for lithium hydroxide in the next round of lithium sulfide gas-liquid synthesis reaction, thus achieving a closed-loop recycling of organic solvents in the production process. The final lithium sulfide product prepared in this embodiment has a purity greater than or equal to 99.9%. The purity of the N-methylpyrrolidone solvent was determined by gas chromatography, and the purity of the lithium sulfide product was determined by conventional lithium sulfide purity testing methods in the field; the recovery rate of the N-methylpyrrolidone solvent was calculated using the formula... Calculate, where, For recovery rate, To recover the mass of N-methylpyrrolidone collected in storage tank 320, This represents the mass of N-methylpyrrolidone volatilized during the pyrolysis stage.

[0024] Example 2 like Figure 1 and Figure 2As shown, this embodiment provides another method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent. First, the material mixing and gas-liquid synthesis reaction in step one are performed. 10 kg of anhydrous lithium hydroxide powder is weighed and added to 120 kg of N-methylpyrrolidone solvent, controlling the mass ratio of lithium hydroxide to N-methylpyrrolidone solvent to be 1:12. The stirring mechanism of the reactor 100 is started, and the stirring speed is controlled at 350 r / min to ensure uniform dispersion and form a suspension of lithium hydroxide and N-methylpyrrolidone. Nitrogen gas with a slight positive pressure is introduced into the sealed reactor 100 for gas phase replacement and sealing protection. The heating device is used to heat the system temperature of the suspension and maintain it at a constant 90°C. Subsequently, high-purity hydrogen sulfide gas is continuously introduced to the bottom of the suspension at a gas flow rate of 1.0 L / min, and the reaction is carried out at a constant temperature with stirring for 3.5 h, converting into a slurry of intermediate lithium hydrosulfide and N-methylpyrrolidone. After the synthesis reaction is completed, the high-temperature pyrolysis vaporization process of step two is performed. The hydrogen sulfide gas is stopped, and the heating device is controlled to raise the system temperature in reactor 100 to 180°C at a heating rate of 7°C / min. The system is then kept at a constant temperature of 180°C for 3.5 hours. During this pyrolysis stage, the stirring device is continuously operated to prevent solid agglomeration. Lithium hydrosulfide is pyrolyzed into solid lithium sulfide at 180°C. Simultaneously, the N-methylpyrrolidone solvent in the system evaporates upon heating, generating vapor, which mixes with non-condensable gases to form N-methylpyrrolidone-containing tail gas. Immediately following, the tail gas condensation and recovery process of step three is performed. The N-methylpyrrolidone-containing tail gas is introduced into the tail gas delivery pipeline 200 through the tail gas outlet. The insulation and heating temperature of the tail gas delivery pipeline 200 is maintained at 105°C throughout its operation to prevent solvent condensation and wall formation. The exhaust gas is then introduced into a shell-and-tube condenser 310, with the temperature of the condensation zone inside condenser 310 controlled at 40°C. The inlet temperature of the circulating cooling water is controlled at 26°C, and the outlet temperature is controlled at 36°C. The residence time of the N-methylpyrrolidone-containing exhaust gas in condenser 310 is controlled at 8.5 s. The N-methylpyrrolidone vapor in the exhaust gas undergoes heat exchange and liquefaction on the surface of the multi-layer baffle condenser plate, and the condensed and recovered liquid solvent flows into the recovery storage tank 320. Test results show that the recovery rate of N-methylpyrrolidone solvent in this embodiment is 96.1%. Finally, in step four, the recovered liquid N-methylpyrrolidone solvent is extracted by a transfer pump 410 and filtered through a precision filter 420 with a filtration accuracy of 0.22 μm to remove trace dust impurities. After removal, it is recycled back to the reactor 100 for the preparation of the next batch of materials, thus preparing lithium sulfide product.

[0025] In the above embodiments, the mass ratio of lithium hydroxide to N-methylpyrrolidone solvent, reaction temperature, hydrogen sulfide gas flow rate, stirring speed, pyrolysis heating rate, isothermal pyrolysis time, heating temperature, condenser internal condensation zone temperature, circulating cooling water inlet temperature, circulating cooling water outlet temperature, and tail gas residence time can all be selected at the end or middle of the aforementioned range, and implemented according to the above steps.

[0026] Example 3 like Figure 2As shown, this embodiment provides a system for recovering N-methylpyrrolidone solvent in the above-mentioned lithium sulfide preparation process. The recovery system specifically includes a reactor 100, a tail gas delivery pipeline 200, a condensation recovery unit, a solvent circulation unit, and a tail gas treatment unit 500. The reactor 100 is a sealed, pressure-resistant reactor, internally equipped with heating wires and a low-speed stirring component, used to complete the neutralization synthesis reaction of lithium hydroxide and hydrogen sulfide in the N-methylpyrrolidone solvent system and the subsequent 180°C high-temperature pyrolysis reaction. A tail gas outlet is located at the top of the reactor 100, connected to the inlet of the tail gas delivery pipeline 200 via a flange. The entire outer wall of the tail gas delivery pipeline 200 is wrapped with an electric heating tape as an insulation and heat tracing component, and the electric heating tape is connected to a temperature controller to maintain the heat tracing temperature of the pipeline within the range of 100°C to 120°C throughout. A condensation recovery unit is installed in series in the middle section of the exhaust gas delivery pipeline 200. This unit specifically includes a shell-and-tube condenser 310, a recovery storage tank 320, and a level monitoring component 330. Multiple layers of staggered baffle condenser plates are fixedly installed in the shell side of the condenser 310. The liquid outlet at the bottom of the condenser 310 is connected to the inlet of the recovery storage tank 320 below via a stainless steel pipe. A level sensor is installed on the recovery storage tank 320 as the level monitoring component 330 to monitor the volume and level changes of the recovered N-methylpyrrolidone solvent inside the tank. Simultaneously, the recovery storage tank 320 is a sealed, pressure-resistant tank with a nitrogen pressurization port at the top. This port allows for the replenishment of slightly positive nitrogen to maintain nitrogen protection, isolating the recovered N-methylpyrrolidone solvent from moisture in the outside air and preventing it from absorbing water, deteriorating, or undergoing oxidative decomposition. The solvent circulation unit includes a transfer pump 410 and a precision filter 420. The discharge outlet at the bottom of the recovery storage tank 320 is connected in series via pipes to the suction end of the transfer pump 410 and the inlet end of the precision filter 420. The precision filter 420 is equipped with a polytetrafluoroethylene filter element with a filtration accuracy of 0.22 μm. The discharge outlet of the precision filter 420 is connected to the feed inlet at the top of the reactor 100 via a return pipe, thus forming a closed-loop solvent circulation circuit. The exhaust gas treatment unit 500 includes a spray absorption structure and is connected to the gas phase outlet at the top of the condenser 310 via a gas phase pipe. It is used to receive the non-condensable gas containing trace amounts of hydrogen sulfide and nitrogen remaining after condensation and cooling by the condenser 310.

[0027] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent, characterized in that, Includes the following steps: S1. Anhydrous lithium hydroxide powder is added to N-methylpyrrolidone solvent to form a suspension. In a closed reactor (100) under nitrogen protection and at a temperature of 80℃~120℃, hydrogen sulfide gas is introduced to carry out a neutralization reaction to generate lithium hydrosulfide-N-methylpyrrolidone slurry. S2. Stop the flow of hydrogen sulfide gas, raise the temperature of the system in the sealed reactor (100) to 180°C and perform isothermal pyrolysis to decompose the lithium hydrosulfide into lithium sulfide solid. At the same time, the N-methylpyrrolidone solvent in the system is heated and volatilizes to form N-methylpyrrolidone vapor, which mixes with the non-condensable gas in the system to form tail gas containing N-methylpyrrolidone. S3. The tail gas containing N-methylpyrrolidone is discharged from the closed reactor (100) and introduced into the condenser (310) connected in series in the tail gas delivery pipeline (200). The temperature of the condensation zone inside the condenser (310) is controlled to be 30℃~45℃, so that the N-methylpyrrolidone vapor in the tail gas containing N-methylpyrrolidone is liquefied and collected into the recovery storage tank (320), thus completing the separation and recovery of N-methylpyrrolidone solvent. The uncondensed non-condensable gas is transported to the tail gas treatment unit (500) for treatment. S4. The recovered liquid N-methylpyrrolidone solvent is filtered to remove impurities and then recycled back to step S1 as a dispersion solvent for lithium hydroxide.

2. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 1, characterized in that, In step S1, the mass ratio of lithium hydroxide to N-methylpyrrolidone solvent is 1:8 to 1:15, the flow rate of hydrogen sulfide gas is 0.5 L / min to 1.2 L / min, and the stirring speed during the reaction is 300 r / min to 500 r / min.

3. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 1, characterized in that, In step S1, the hydrogen sulfide gas is introduced from the bottom of the suspension, and the reaction is carried out under constant temperature and stirring for 2 to 4 hours.

4. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 1, characterized in that, In step S2, the system is heated to 180°C at a heating rate of 5°C / min to 8°C / min, and is kept at 180°C for 3 to 5 hours.

5. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 1, characterized in that, In step S2, the mixture is continuously stirred during the isothermal pyrolysis process at 180°C.

6. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 1, characterized in that, The tail gas containing N-methylpyrrolidone is discharged from the closed reactor (100) and then transported to the condenser (310) through the tail gas delivery pipeline (200). The tail gas delivery pipeline (200) is insulated and heated throughout the process, and the heating temperature is maintained at 100℃~120℃.

7. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 1, characterized in that, In step S3, the condenser (310) is a shell-and-tube condenser, and the condenser (310) is provided with multiple layers of baffle condenser plates inside.

8. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 7, characterized in that, The condenser (310) uses circulating cooling water as the cooling medium, and controls the inlet temperature of the circulating cooling water to be 25℃~30℃ and the outlet temperature to be 35℃~42℃.

9. The method for preparing lithium sulfide and recovering N-methylpyrrolidone solvent according to claim 1, characterized in that, In step S3, the residence time of the N-methylpyrrolidone-containing tail gas in the condenser (310) is controlled to be greater than or equal to 8 seconds.

10. A system for recovering N-methylpyrrolidone solvent during the preparation of lithium sulfide, characterized in that, It includes a reaction vessel (100), a tail gas delivery pipeline (200), a condensation recovery unit, a solvent circulation unit, and a tail gas treatment unit (500). The reactor (100) is a closed pressure-resistant reactor. The reactor (100) is equipped with a heating wire and a low-speed stirring component. The top of the reactor (100) is equipped with a tail gas outlet, which is connected to the tail gas delivery pipeline (200). The exhaust gas delivery pipeline (200) is provided with a heat tracing and insulation component on the outside. The heat tracing and insulation component is used to maintain the heat tracing temperature of the exhaust gas delivery pipeline (200) at 100℃~120℃. The condensation recovery unit includes a condenser (310), a recovery storage tank (320), and a liquid level monitoring component (330). The condenser (310) is installed in series on the exhaust gas delivery pipeline (200). The condenser (310) is a shell-and-tube condenser with multiple layers of baffle condenser plates inside. The bottom liquid phase outlet of the condenser (310) is connected to the recovery storage tank (320) through a pipeline. The liquid level monitoring component (330) is installed in the recovery storage tank (320) and is used to monitor the liquid level in the recovery storage tank (320). The solvent circulation unit includes a delivery pump (410) and a precision filter (420). The outlet of the recovery storage tank (320) is connected in sequence to the delivery pump (410) and the precision filter (420). The outlet of the precision filter (420) is connected to the feed inlet of the reactor (100) through a reflux pipe. The exhaust gas treatment unit (500) includes a spray absorption structure, which is connected to the top gas phase outlet of the condenser (310) via a pipe.

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