A process for the recovery of carbon fiber composites by molten salt assisted steam pyrolysis

Abstract

This invention belongs to the field of carbon fiber composite material recycling, and provides a recycling process for carbon fiber composite materials using molten salt-assisted steam pyrolysis, comprising three stages: molten salt treatment, steam pyrolysis, and post-treatment. Through a synergistic mechanism of "efficient resin decomposition by molten salt and surface residue cleaning by steam," the process achieves deep resin pyrolysis while simultaneously recovering pyrolysis products and efficiently recycling high-performance carbon fibers for reuse. This significantly improves the purity and interfacial properties of recycled carbon fibers, reduces energy consumption and secondary pollution, and is particularly suitable for the green recycling of composite materials with high cross-linking density or special coatings, providing an efficient and clean solution for the circular economy of composite materials.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber composite material recycling, and particularly relates to a recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Carbon fiber composites have become an irreplaceable key strategic material in high-end equipment fields such as aerospace, defense, new energy vehicles, and wind power generation. In recent years, with the rapid expansion of the carbon fiber composite market, carbon fiber composite waste has also accumulated rapidly, posing new environmental challenges. Therefore, developing economically feasible carbon fiber composite recycling technologies is of paramount importance.

[0004] Molten salt recycling of carbon fiber is a thermochemical process that uses high-temperature molten salt media to treat waste composite materials. However, this process has significant drawbacks: First, the high-temperature, highly corrosive molten salt places extremely high demands on the reactor material, resulting in huge equipment investment and maintenance costs; second, the recovered fibers require complex washing after the process to remove residual salt, otherwise the fiber performance and subsequent utilization will be severely affected; third, the preparation, recovery, and recycling of molten salt are complex processes that can easily cause secondary pollution. The overall system complexity and operational difficulty of the process limit its large-scale industrial application.

[0005] Steam pyrolysis is a technique for recovering carbon fibers by introducing steam as a reaction medium in an inert or low-oxygen atmosphere. This method requires no additional catalyst and yields high-value gaseous products. However, it also has significant drawbacks: the pyrolysis temperature needs precise control, as excessively high temperatures can directly damage fiber strength; the steam concentration needs to be optimized and balanced, otherwise the yield of liquid products will decrease; even under optimized conditions, pyrolysis carbon may still remain on the fiber surface, requiring additional oxidation steps for removal, increasing energy consumption and process complexity; furthermore, the surface chemical properties of the recovered fibers are altered, typically requiring surface treatment to restore good interfacial bonding properties.

[0006] Therefore, there is an urgent need to develop simpler and more efficient methods for recycling carbon fiber composite materials. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a recycling process for carbon fiber composites using molten salt-assisted steam pyrolysis. Through the synergistic effect of "molten salt impregnation + steam pyrolysis," it solves the issues of low resin decomposition rate, severe fiber damage, and low cleanliness in traditional recycling methods, thereby maximizing the preservation of the mechanical properties of the recycled carbon fibers. The process comprises three stages: molten salt impregnation, steam pyrolysis, and post-treatment.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis, comprising: The carbon fiber composite material was immersed in molten salt and then heated to pyrolyze, yielding the pyrolysis products. The pyrolysis product was immersed in an acid solution, washed, and dried to obtain a sample after molten salt impregnation treatment; The sample impregnated with molten salt was subjected to secondary pyrolysis in a steam environment to collect carbon fibers and pyrolysis products. The carbon fiber surface is coated with TiO2 powder, heated in air, washed, and dried to obtain recycled carbon fiber.

[0009] In a second aspect, the present invention provides recycled carbon fibers recovered by the method described above.

[0010] A third aspect of the present invention provides the application of the above-mentioned recycled carbon fiber in the fields of aerospace, defense, new energy vehicles, and wind power generation.

[0011] A fourth aspect of the present invention provides a recycling system for molten salt-assisted steam pyrolysis of carbon fiber composite materials, comprising: a muffle furnace (1), a fixed-bed reactor (5), a steam boiler (3), and a gas cylinder (2); The exhaust port of the muffle furnace (1) is connected to the first air inlet of the fixed bed reactor (5). The steam inlet of the fixed bed reactor (5) is connected to the exhaust port of the steam boiler (3). The second air inlet of the fixed bed reactor (5) is connected to the exhaust port of the gas cylinder. The bottom of the fixed bed reactor (5) is provided with a discharge port, which is connected to the feed port of the ice water condensation recovery system (6). The fixed bed reactor (5) is also provided with a cracked gas outlet, which is connected to the gas exchange cylinder and the gas bag (7) in sequence.

[0012] Beneficial effects of the present invention (1) Through the synergistic effect of deep decomposition of molten salt and surface activation of water vapor, the pyrolysis temperature is significantly reduced while the resin is completely decomposed and the fiber surface is functionalized in situ, so that the mechanical properties and interfacial bonding strength of recycled carbon fiber are improved simultaneously. (2) The resin decomposes completely, the surface residue rate is extremely low, and the fiber cleanliness is high, making it especially suitable for recycling complex composite materials with high cross-linking density or coatings. (3) The process is highly adaptable, energy consumption and secondary pollution are significantly reduced, the system integration is high, and it has the dual advantages of efficient separation and high-quality recovery. Attached Figure Description

[0013] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. Exemplary embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0014] Figure 1 This is an integrated device diagram of the recycling process of carbon fiber composite material by molten salt-assisted steam pyrolysis according to the present invention. In the diagram, 1 is a muffle furnace, 2 is an N2 bottle, 3 is a steam boiler, 4 is a temperature and time controller, 5 is a fixed bed reactor, 6 is an ice water condensation and recovery system, and 7 is a gas bag. Detailed Implementation

[0015] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. The reagents and raw materials used in this invention are readily available through conventional means, and unless otherwise specified, they are used in accordance with conventional methods in the art or product instructions. Similarly, unless otherwise specified, the test methods of this invention are performed in accordance with conventional methods in the art or industry-standard methods or practices. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described herein are for illustrative purposes only.

[0017] This invention mainly proposes a recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis, including: The carbon fiber composite material was immersed in molten salt and then heated to pyrolyze, yielding the pyrolysis products. The pyrolysis product was immersed in an acid solution, washed, and dried to obtain a sample after molten salt impregnation treatment; The sample impregnated with molten salt was subjected to secondary pyrolysis in a steam environment to collect carbon fibers and pyrolysis products. The carbon fiber surface is coated with TiO2 powder, heated in air, washed, and dried to obtain recycled carbon fiber.

[0018] The carbon fiber composite material uses 3-24K T300-T1000 carbon fiber bundles, and the resin is epoxy resin, phenolic resin, bismaleimide or polyether ether ketone, etc., and is not limited to the above types. The carbon fiber volume fraction is 45%-65%, which can be flexibly adjusted according to design requirements. The side length of the cubic sample is 20-50mm.

[0019] The composition of the molten salt affects the recycling efficiency of carbon fiber composites. Therefore, this invention studies the composition of the molten salt. Preferably, the molten salt is composed of SnCl2 and KCl in a ratio of 4:1-1.5. More preferably, the SnCl2 to KCl ratio is 4:1. Compared with traditional alkali salts, chloride salts, nitrate salts, carbonate salts, and sulfate salts and their combinations, the carbon fibers prepared using the composite molten salt of SnCl2 and KCl exhibit a higher retention rate of mechanical properties.

[0020] The pyrolysis temperature affects the recycling efficiency of carbon fibers. Therefore, this invention studies the pyrolysis temperature. Preferably, the pyrolysis temperature is 200~300℃ and the time is 10~60min, so as to better improve the retention rate of the mechanical properties of carbon fibers.

[0021] This invention involves immersing the pyrolysis products in an acid solution for washing to further remove residual organic matter. Therefore, this invention studies the concentration of the hydrochloric acid solution and the immersion time. Preferably, the acid solution is a hydrochloric acid solution with a concentration of 5%-10% and an immersion time of 30-40 minutes to better remove residual organic matter from the carbon fibers while minimizing damage to the carbon fibers. Ultrasonic cleaning generates powerful shock waves when cavitation bubbles collapse, causing residual impurities to be stripped off and dispersed under the action of the shock waves. Therefore, this invention studies the ultrasonic treatment time, and preferably, the ultrasonic treatment time is 30-40 minutes to obtain better cleaning effect.

[0022] Steam pyrolysis is a technique that uses steam as a reaction medium in an inert or low-oxygen atmosphere to recover carbon fibers. Therefore, based on the previous deep decomposition with molten salt, this invention studies the conditions for steam treatment. Preferably, the steam content in the steam environment is 10%~30%, with the remainder being inert gas. More preferably, the inert gas is N2, and the N2 flow rate is 200 ml / min, to obtain better carbon fiber recovery results.

[0023] The temperature and time of pyrolysis affect the treatment effect of steam pyrolysis. Therefore, this invention studies the temperature and time of secondary pyrolysis. Preferably, the temperature of the secondary pyrolysis is 400~600℃ and the time is 20~40min to obtain a better recovery effect.

[0024] Temperature affects the removal effect of residual pyrolytic carbon on the carbon fiber surface. Therefore, this invention studies the heating temperature. Preferably, the heating temperature in the air is 500~550℃ and the heating time is 5~20 minutes to better remove residual pyrolytic carbon on the carbon fiber surface.

[0025] More specifically, including: The molten salt impregnation stage first involves cutting the carbon fiber composite material into cubic samples using a cutting machine. Then, SnCl2 and KCl are prepared in a specific ratio and placed in a crucible, heated to a molten state in a muffle furnace. The composite material is then immersed in the molten salt in the muffle furnace, with the temperature and time set accordingly. After the pyrolysis is complete, the pyrolysis products are removed from the muffle furnace. After cooling to room temperature, the sample is immersed in hydrochloric acid to remove excess residue from the carbon fiber surface. It is then repeatedly washed with deionized water, rinsed thoroughly, and ultrasonically treated before finally drying in an oven. The degradation rate of the composite material is then analyzed.

[0026] In the steam pyrolysis stage, the sample, after molten salt impregnation, is placed in a fixed-bed reactor. The N2 flow rate is set using a rotor flowmeter, and N2 is introduced to purge air. The target temperature is set via the control panel, and the reactor is heated to the target temperature. Simultaneously, a steam boiler is turned on to generate steam, and water condensed in the pipeline is discharged through dual valves. After the steam generation stabilizes, the steam content in the N2 is adjusted to the target level and introduced into the reactor. The heating time is then set via the control panel, and the pyrolysis reaction begins in the furnace, accompanied by the generation of pyrolysis oil, pyrolysis gas, and pyrolysis carbon. The pyrolysis oil is collected using an ice-water condensation system, and the pyrolysis gas is subsequently collected using a gas bag. The pyrolysis carbon adheres to the surface of the carbon fibers. After the reaction is complete, the reactor is cooled to room temperature, and the pyrolysis furnace is opened to remove the sample and pyrolysis carbon. This completes the recovery of carbon fibers and the collection of pyrolysis products. The pyrolysis products are then subjected to yield, qualitative, and quantitative analysis.

[0027] The post-treatment stage involves coating the surface of the recovered carbon fibers with TiO2 powder after the reaction is complete, followed by heating in air to remove residual pyrolytic carbon. Afterwards, the fibers are washed with deionized water and dried in an oven to obtain regenerated carbon fibers. Finally, the carbon fibers are tested for mechanical properties and other parameters.

[0028] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the specific embodiments are explanations of the present invention and not limitations thereof.

[0029] In the following examples, all carbon fiber composite materials are commercially available products.

[0030] In the following embodiments, the recycling system for carbon fiber composite material by molten salt-assisted steam pyrolysis includes: a muffle furnace 1, a fixed bed reactor 5, a steam boiler 3, and a gas cylinder 2; The exhaust port of the muffle furnace 1 is connected to the first air inlet of the fixed bed reactor 5. The steam inlet of the fixed bed reactor 5 is connected to the exhaust port of the steam boiler 3. The second air inlet of the fixed bed reactor 5 is connected to the exhaust port of the gas cylinder. A discharge port is provided at the bottom of the fixed bed reactor 5. The discharge port is connected to the inlet of the ice water condensation recovery system 6. The fixed bed reactor 5 is also provided with a cracked gas outlet. The cracked gas outlet is connected to the gas exchange cylinder and gas bag 7 in sequence. A temperature and time controller 4 is also provided on the fixed bed reactor 5.

[0031] In the following embodiments, the specific steps for coating the surface of the recycled carbon fiber with TiO2 powder include: (1) Using an aqueous solution containing 5% ethanol by volume as a solvent, nano-TiO2 was prepared into a TiO2 solution with a mass fraction of 5%. The prepared solution was ultrasonically oscillated for 3 hours using an ultrasonic disperser to disperse the nano-TiO2 particles evenly, thus obtaining a TiO2 dispersion.

[0032] (2) The recovered carbon fiber is immersed in TiO2 dispersion for 1 hour and then taken out and hung to dry.

[0033] Example 1 The selected material is a carbon fiber composite material made of T300 3K carbon fiber bundles and epoxy resin, with a carbon fiber volume fraction of 45%. First, the material is cut into 20mm pieces. 20mm A 5mm sample was prepared. Next, 20g of molten salt with a SnCl2:KCl mass ratio of 4:1 was prepared and placed in a crucible, heated to a molten state in a muffle furnace. The composite material was then immersed in the molten salt and placed in the muffle furnace. The reaction temperature was set to 200℃, and the reaction time was 20min. After the reaction, the pyrolysis product was removed and cooled to room temperature. It was then soaked in hydrochloric acid for 30min, repeatedly washed with deionized water, rinsed thoroughly, and ultrasonically treated for 30min. Finally, it was dried in an oven at 60℃ for 6h. After drying, the sample was placed in a fixed-bed reactor. The N2 flow rate was set to 200ml / min, and N2 was introduced to purge air. The reactor temperature was set to 400℃, and after heating to the target temperature, 10% water vapor was introduced into the reactor. The heating time was set to 30min to start the reaction. Simultaneously, the pyrolysis oil was collected through an ice-water condensation system, and the pyrolysis gas was collected subsequently using a gas bag. After the reaction, it was cooled to room temperature, and the recovered carbon fibers and pyrolysis carbon were removed. Finally, TiO2 powder was coated onto the surface of the recycled carbon fibers, and the mixture was heated in air at 500°C for 5 minutes. Afterwards, it was washed with deionized water and dried in an oven to obtain the regenerated carbon fibers.

[0034] Example 2 The selected material is a carbon fiber composite material made of T700 12K carbon fiber bundles and epoxy resin, with the carbon fiber volume fraction being 55%. First, the material is cut into 30mm pieces. 20mm A 5mm sample was prepared. Next, 20g of molten salt with a SnCl2:KCl mass ratio of 4:1 was prepared and placed in a crucible, heated to a molten state using a muffle furnace. The composite material was then immersed in the molten salt and placed in the muffle furnace. The reaction temperature was set to 200℃, and the reaction time was 30min. After the reaction, the pyrolysis product was removed and cooled to room temperature. It was then soaked in hydrochloric acid for 30min, repeatedly washed with deionized water, rinsed thoroughly, and ultrasonically treated for 30min. Finally, it was dried in an oven at 60℃ for 6h. After drying, the sample was placed in a fixed-bed reactor. The N2 flow rate was set to 200ml / min, and N2 was introduced to purge air. The reactor temperature was set to 450℃, and after heating to the target temperature, 20% water vapor was introduced into the reactor. The heating time was set to 30min to start the reaction. Simultaneously, the pyrolysis oil was collected through an ice-water condensation system, and the pyrolysis gas was collected subsequently using a gas bag. After the reaction, it was cooled to room temperature, and the recovered carbon fibers and pyrolysis carbon were removed. Finally, TiO2 powder was coated onto the surface of the recycled carbon fibers, and the mixture was heated in air at 500°C for 10 minutes. Afterwards, it was washed with deionized water and dried in an oven to obtain the regenerated carbon fibers.

[0035] Example 3 The selected material is a carbon fiber composite material made of T800 18K carbon fiber bundles and epoxy resin, with a carbon fiber volume fraction of 60%. First, the material is cut into 40mm pieces. 20mm A 5mm sample was prepared. Next, 20g of molten salt with a SnCl2:KCl mass ratio of 4:1 was prepared and placed in a crucible, heated to a molten state in a muffle furnace. The composite material was then immersed in the molten salt and placed in the muffle furnace. The reaction temperature was set to 200℃, and the reaction time was 20min. After the reaction, the pyrolysis product was removed and cooled to room temperature. It was then soaked in hydrochloric acid for 30min, repeatedly washed with deionized water, rinsed thoroughly, and ultrasonically treated for 30min. Finally, it was dried in an oven at 60℃ for 6h. After drying, the sample was placed in a fixed-bed reactor. The N2 flow rate was set to 200ml / min, and N2 was introduced to purge air. The reactor temperature was set to 500℃, and after heating to the target temperature, 30% water vapor was introduced into the reactor. The heating time was set to 30min to start the reaction. Simultaneously, the pyrolysis oil was collected through an ice-water condensation system, and the pyrolysis gas was collected subsequently using a gas bag. After the reaction, it was cooled to room temperature, and the recovered carbon fibers and pyrolysis carbon were removed. Finally, TiO2 powder was coated onto the surface of the recycled carbon fibers, and the mixture was heated in air at 500°C for 15 minutes. Afterwards, it was washed with deionized water and dried in an oven to obtain the regenerated carbon fibers.

[0036] Example 4 The selected material is a carbon fiber composite material made of T1000 24K carbon fiber bundles and epoxy resin, with a carbon fiber volume fraction of 65%. First, the material is cut into 50mm pieces. 20mm A 5mm sample was prepared. Next, 20g of molten salt with a SnCl2:KCl mass ratio of 4:1 was prepared and placed in a crucible, heated to a molten state in a muffle furnace. The composite material was then immersed in the molten salt and placed in the muffle furnace. The reaction temperature was set to 200℃, and the reaction time was 30min. After the reaction, the pyrolysis product was removed and cooled to room temperature. It was then soaked in hydrochloric acid for 30min, repeatedly washed with deionized water, rinsed thoroughly, and ultrasonically treated for 30min. Finally, it was dried in an oven at 60℃ for 6h. After drying, the sample was placed in a fixed-bed reactor. The N2 flow rate was set to 200ml / min, and N2 was introduced to purge air. The reactor temperature was set to 400℃, and after heating to the target temperature, 25% water vapor was introduced into the reactor. The heating time was set to 30min to start the reaction. Simultaneously, the pyrolysis oil was collected through an ice-water condensation system, and the pyrolysis gas was collected subsequently using a gas bag. After the reaction, it was cooled to room temperature, and the recovered carbon fibers and pyrolysis carbon were removed. Finally, TiO2 powder was coated onto the surface of the recycled carbon fibers, and the mixture was heated in air at 500°C for 15 minutes. Afterwards, it was washed with deionized water and dried in an oven to obtain the regenerated carbon fibers.

[0037] Comparative Example 1 The recycled carbon fiber prepared in Example 1 of patent CN111196879A.

[0038] Comparative Example 2 The recycled carbon fiber prepared in paragraph

[0127] of the specification of patent CN116144072A.

[0039] Experimental Example 1 The tensile strength of the recycled carbon fiber monofilaments prepared in the examples and comparative examples was tested according to ISO 11566:1996. Based on the test results, the tensile strength retention rate of the carbon fiber monofilaments was calculated. The specific results are shown in Table 1.

[0040] Table 1 Tensile strength of recycled carbon fiber monofilaments serial number Carbon fiber monofilament tensile strength retention rate Appearance Example 1 95% The surface is smooth and intact, with no resin residue. Example 2 93% The surface is smooth and intact, with no resin residue. Example 3 93% The surface is smooth and intact, with no resin residue. Example 4 91% The surface is smooth and intact, with no resin residue. Comparative Example 1 90% The surface is smooth and intact, with no resin residue. Comparative Example 2 95% The surface is smooth and intact, with no resin residue. The above description is merely a preferred embodiment of the present invention and is 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 protection of the present invention.

Claims

1. A recycling process for carbon fiber composite materials via molten salt-assisted steam pyrolysis, characterized in that, include: The carbon fiber composite material was immersed in molten salt and then heated to pyrolyze, yielding the pyrolysis products. The pyrolysis product was immersed in an acid solution, washed, and dried to obtain a sample after molten salt impregnation treatment; The sample impregnated with molten salt was subjected to secondary pyrolysis in a steam environment to collect carbon fibers and pyrolysis products. The carbon fiber surface is coated with TiO2 powder, heated in air, washed, and dried to obtain recycled carbon fiber.

2. The recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis as described in claim 1, characterized in that, The molten salt is composed of SnCl2 and KCl in a ratio of 4:1-1.

5.

3. The recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis as described in claim 1, characterized in that, The pyrolysis temperature is 200~300℃ and the time is 10~60min.

4. The recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis as described in claim 1, characterized in that, The acid solution is a hydrochloric acid solution with a concentration of 5%-10%, and the soaking time is 30-40 minutes; Alternatively, the ultrasonic treatment time is 30-40 minutes.

5. The recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis as described in claim 1, characterized in that, In the water vapor environment, the water vapor content is 10%~30%, and the remainder is inert gas.

6. The recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis as described in claim 1, characterized in that, The secondary pyrolysis temperature is 400~600℃, and the time is 20~40min.

7. The recycling process for carbon fiber composite materials by molten salt-assisted steam pyrolysis as described in claim 1, characterized in that, The air is heated to a temperature of 500-550°C for 5-20 minutes.

8. The recycled carbon fiber recovered by the method of any one of claims 1-7.

9. The application of the recycled carbon fiber as described in claim 8 in the fields of aerospace, defense and military industry, new energy vehicles, and wind power generation.

10. A recycling system for carbon fiber composite materials by molten salt-assisted steam pyrolysis, characterized in that, include: Muffle furnace (1), fixed bed reactor (5), steam boiler (3), gas cylinder (2); The exhaust port of the muffle furnace (1) is connected to the first air inlet of the fixed bed reactor (5). The steam inlet of the fixed bed reactor (5) is connected to the exhaust port of the steam boiler (3). The second air inlet of the fixed bed reactor (5) is connected to the exhaust port of the gas cylinder. The bottom of the fixed bed reactor (5) is provided with a discharge port, which is connected to the feed port of the ice water condensation recovery system (6). The fixed bed reactor (5) is also provided with a cracked gas outlet, which is connected to the gas exchange cylinder and the gas bag (7) in sequence.

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