A method for recycling carbon fiber by stepped molten salt

Abstract

The present invention relates to the technical field of carbon fiber recycling, and particularly to a method for recycling carbon fiber by stepwise molten salt. The method includes: placing a carbon fiber reinforced resin matrix composite material in a first composite molten salt for primary pyrolysis; placing the carbon fiber reinforced resin matrix composite material after primary pyrolysis in a second composite molten salt for secondary pyrolysis; wherein, the catalytic property of the first composite molten salt is greater than that of the second composite molten salt, and the melting point of the first composite molten salt is less than that of the second composite molten salt, and ultrasonic waves are applied to the reaction system during both primary pyrolysis and secondary pyrolysis; cleaning and drying the carbon fiber obtained by secondary pyrolysis in sequence to achieve the recycling of carbon fiber. The above solution can solve the problems in the related art such as long heat treatment time, low resin degradation rate, and poor retention strength of the recycled carbon fiber.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon fiber recycling, and particularly relates to a method for recycling carbon fiber by stepped molten salt. Background Art

[0002] As an emerging material, carbon fiber is widely used in cutting-edge fields such as aerospace and wind power generation due to its excellent low density and high strength characteristics. With the increasing demand for carbon fiber application scenarios such as aircraft, the input ratio of carbon fiber in application components also shows a significant growth trend. When in application, carbon fiber is often well bonded and positioned with a thermosetting resin as the matrix to form a carbon fiber reinforced resin matrix composite (CFRP). After a large amount of CFRP reaches the service life and is discarded, there is an urgent need to recycle expensive carbon fiber from it for reuse. The cross-linkable thermosetting resin is difficult to efficiently remove from CFRP, making the directional recycling of carbon fiber a difficult problem.

[0003] In related technologies, pyrolysis, as the only technology for preliminary commercial application, has initially achieved large-scale recycling of carbon fiber. However, the above method has problems such as long heat treatment time, low resin degradation rate, and poor retained strength of the recycled carbon fiber.

[0004] Therefore, there is an urgent need for a method for recycling carbon fiber by stepped molten salt to solve the above problems. Summary of the Invention

[0005] One or more embodiments of this specification describe a method for recycling carbon fiber by stepped molten salt, which can solve the problems in related technologies such as long heat treatment time, low resin degradation rate, and poor retained strength of the recycled carbon fiber.

[0006] One embodiment of this specification provides a method for recycling carbon fiber by stepped molten salt, including:

[0007] Placing the carbon fiber reinforced resin matrix composite in a first composite molten salt for primary pyrolysis;

[0008] Placing the carbon fiber reinforced resin matrix composite after primary pyrolysis in a second composite molten salt for secondary pyrolysis; wherein, the catalytic property of the first composite molten salt is greater than that of the second composite molten salt, and the melting point of the first composite molten salt is less than that of the second composite molten salt. Ultrasonic waves are applied to the reaction system during both primary pyrolysis and secondary pyrolysis;

[0009] Cleaning and drying the carbon fiber obtained by secondary pyrolysis in sequence to achieve the recycling of carbon fiber.

[0010] According to the above embodiment, the step of placing the carbon fiber reinforced resin matrix composite in a first composite molten salt for primary pyrolysis includes:

[0011] The carbon fiber reinforced resin matrix composite is pyrolyzed once in the first composite molten salt and the target organic solvent; wherein, the dielectric constant of the target organic solvent is greater than 3.6 C2 / (N·m 2 ).

[0012] According to the above embodiment, before the carbon fiber reinforced resin matrix composite is pyrolyzed once in the first composite molten salt, it further includes:

[0013] Sort the carbon fiber reinforced resin matrix composite from the raw materials.

[0014] According to the above embodiment, the melting point of the first composite molten salt is 120 - 160 °C, the melting point of the second composite molten salt is 350 - 400 °C, and the boiling point of the target organic solvent is 160 - 200 °C.

[0015] According to the above embodiment, the first composite molten salt includes at least one of alkali salts, chloride salts, and nitrate salts; and / or,

[0016] The target organic solvent includes at least one of alcohols, amines, and alkanolamines; and / or,

[0017] The second composite molten salt includes at least one of carbonate salts and sulfate salts.

[0018] According to the above embodiment, the carbon fiber reinforced resin matrix composite is subjected to primary pyrolysis, secondary pyrolysis, cleaning, and drying by being clamped by two clamps that can move horizontally and vertically.

[0019] According to the above embodiment, the clamping portion of the clamp for clamping the end of the carbon fiber reinforced resin matrix composite can rotate around the axis of the carbon fiber reinforced resin matrix composite.

[0020] According to the above embodiment, the carbon fiber obtained by secondary pyrolysis is sequentially cleaned and dried to realize the recovery of carbon fiber, including:

[0021] The carbon fiber obtained by secondary pyrolysis is sequentially cleaned and dried;

[0022] The carbon fiber after drying and located between the two clamps is sheared to realize the recovery of carbon fiber;

[0023] and / or,

[0024] The carbon fiber obtained by secondary pyrolysis is sequentially cleaned and dried, including:

[0025] The carbon fiber obtained by secondary pyrolysis is washed with water, washed with an organic solvent, and dried.

[0026] According to the above embodiments, after placing the carbon fiber reinforced resin matrix composite after the first pyrolysis in the second composite molten salt for secondary pyrolysis, the following steps are further included:

[0027] Separate the volatile components released from the first pyrolysis and the second pyrolysis to obtain pyrolysis oil and pyrolysis gas;

[0028] Fractionate and purify the obtained pyrolysis oil to realize the recovery of pyrolysis products.

[0029] According to the above embodiments, after separating the volatile components released from the first pyrolysis and the second pyrolysis to obtain pyrolysis oil and pyrolysis gas, the following steps are further included:

[0030] Burn the obtained pyrolysis gas and output the heat generated by the combustion to the first composite molten salt and the second composite molten salt;

[0031] Treat the gas generated by the combustion for tail gas treatment.

[0032] According to the method for recycling carbon fiber by cascade molten salt provided in the embodiments of the present specification, by sequentially performing the first pyrolysis and the second pyrolysis on the carbon fiber reinforced resin matrix composite, the cascade molten salt heat treatment for recycling carbon fiber is realized. Moreover, ultrasonic waves are applied to the reaction system during both the first pyrolysis and the second pyrolysis, so that the heat treatment time can be shortened and the resin degradation rate can be increased; meanwhile, the carbon fiber obtained from the second pyrolysis is sequentially washed and dried to realize the recycling of carbon fiber. Moreover, since the catalytic property of the first composite molten salt is greater than that of the second composite molten salt and the melting point of the first composite molten salt is less than that of the second composite molten salt, the retention strength of the recycled carbon fiber can be improved. Description of the Drawings

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to these drawings.

[0034] Figure 1 Shows a schematic flow chart of a method for recycling carbon fiber by cascade molten salt according to an embodiment;

[0035] Figure 2 Shows a schematic flow chart of a method for recycling carbon fiber by cascade molten salt according to another embodiment;

[0036] Figure 3 Shows a schematic structural diagram of a system for recycling carbon fiber by cascade molten salt according to an embodiment;

[0037] Figure 4 The schematic diagrams before and after a fixture clamping a carbon fiber reinforced resin matrix composite according to an embodiment are shown.

[0038] Reference numerals:

[0039] 1 - Heat treatment unit;

[0040] 11 - Primary reactor;

[0041] 12 - Secondary reactor;

[0042] 13 - Ultrasonic generator;

[0043] 2 - Carbon fiber recovery unit;

[0044] 21 - Water cleaning area;

[0045] 22 - Organic solvent cleaning area;

[0046] 23 - Drying area;

[0047] 24 - Shearing area;

[0048] 3 - Pretreatment unit;

[0049] 4 - Supply unit;

[0050] 41 - Primary molten salt tank;

[0051] 42 - Organic solvent tank;

[0052] 43 - Secondary molten salt tank;

[0053] 5 - Fixture;

[0054] 51 - Clamping part;

[0055] 6 - Pyrolysis product recovery unit;

[0056] 61 - Gas-liquid condensation separator;

[0057] 62 - Fractionator;

[0058] 7 - Waste heat utilization unit;

[0059] 71 - Gas burner;

[0060] 72 - Tail gas processor. Detailed implementation manners

[0061] The solution provided in this specification will be described below with reference to the accompanying drawings.

[0062] Figure 1 The flow schematic diagram of a method for recovering carbon fiber by cascade molten salt according to an embodiment is shown. As Figure 1As shown, the method for recycling carbon fiber by stepped molten salt includes:

[0063] Step 100: Place the carbon fiber reinforced resin matrix composite material in the first composite molten salt for primary pyrolysis;

[0064] Step 102: Place the carbon fiber reinforced resin matrix composite material after primary pyrolysis in the second composite molten salt for secondary pyrolysis; wherein, the catalytic property of the first composite molten salt is greater than that of the second composite molten salt, and the melting point of the first composite molten salt is lower than that of the second composite molten salt. Ultrasonic waves are applied to the reaction system during both primary pyrolysis and secondary pyrolysis;

[0065] Step 104: Clean and dry the carbon fiber obtained by secondary pyrolysis in sequence to achieve the recycling of carbon fiber.

[0066] In this embodiment, by performing primary pyrolysis and secondary pyrolysis on the carbon fiber reinforced resin matrix composite material in sequence, stepped molten salt heat treatment for recycling carbon fiber is achieved. Moreover, ultrasonic waves are applied to the reaction system during both primary pyrolysis and secondary pyrolysis, so that the heat treatment time can be shortened and the resin degradation rate can be increased; meanwhile, the carbon fiber obtained by secondary pyrolysis is cleaned and dried in sequence to achieve the recycling of carbon fiber. Moreover, since the catalytic property of the first composite molten salt is greater than that of the second composite molten salt, and the melting point of the first composite molten salt is lower than that of the second composite molten salt, the retention strength of the recycled carbon fiber can be improved.

[0067] It can be understood that in the embodiments of this specification, the catalytic property of the composite molten salt is based on the resin decomposition rate and the degree of damage to the carbon fiber structure. The former is referenced by the removal rate of the resin, and the latter is referenced by the tensile strength of a single carbon fiber filament, which will not be elaborated here.

[0068] In some embodiments, an inert atmosphere needs to be maintained during both primary pyrolysis and secondary pyrolysis to avoid the oxidation of carbon fiber under high temperature conditions (such as 100 - 400 °C). Among them, the inert atmosphere can be nitrogen, argon, etc., which are not limited in the embodiments of this specification.

[0069] In an embodiment of this specification, step 100 may specifically include:

[0070] Place the carbon fiber reinforced resin matrix composite material in the first composite molten salt and a target organic solvent for primary pyrolysis; wherein, the dielectric constant of the target organic solvent is greater than 3.6 C² / (N·M 2 )

[0071] In this embodiment, in order to further remove the resin during primary pyrolysis, an organic solvent with a dielectric constant greater than 3.6 C² / N·M 2 can be added to the first composite molten salt.

[0072] In some embodiments, the target organic solvent includes at least one of alcohols, amines, and alkanolamines. For example, it can be butanediol, ethylene glycol, ethanolamine, aniline, etc., and the embodiments of this specification do not specifically limit them here.

[0073] In an embodiment of this specification, the melting point of the first composite molten salt is 120 - 160 °C, the melting point of the second composite molten salt is 350 - 400 °C, and the boiling point of the target organic solvent is 160 - 200 °C.

[0074] In this embodiment, in order to ensure that the resin can be removed more thoroughly and the retention strength of the recycled carbon fiber can be ensured, it is necessary to make the first composite molten salt have strong catalytic activity and a low melting point, the target organic solvent have strong polarity and good solubility, and the second composite molten salt have weak catalytic activity and a high melting point. Therefore, it is necessary to ensure that the target organic solvent can maintain a liquid form after being mixed with the first composite molten salt, that is, the boiling point of the target organic solvent is greater than the melting point of the first composite molten salt.

[0075] In an embodiment of this specification, the first composite molten salt includes at least one of alkali salts, chlorides, and nitrates.

[0076] In some embodiments, the alkali salt can be sodium hydroxide, potassium hydroxide, etc., the chloride can be sodium chloride, potassium chloride, etc., the nitrate can be sodium nitrate, potassium nitrate, etc., and the embodiments of this specification do not specifically limit their types here. At the same time, the specific ratio of each molten salt is not specifically limited either, as long as the above characteristics (i.e., melting point, boiling point, catalytic activity, polarity, solubility, etc.) are satisfied.

[0077] In an embodiment of this specification, the second composite molten salt includes at least one of carbonates and sulfates.

[0078] In some embodiments, the carbonate can be sodium carbonate, potassium carbonate, etc., the sulfate can be sodium sulfate, potassium sulfate, etc., and the embodiments of this specification do not specifically limit their types here. At the same time, the specific ratio of each molten salt is not specifically limited either, as long as the above characteristics (i.e., melting point, boiling point, catalytic activity, polarity, solubility, etc.) are satisfied.

[0079] In an embodiment of this specification, before step 100, the above method may specifically further include:

[0080] Sort the carbon fiber reinforced resin matrix composite material from the raw materials.

[0081] In this embodiment, generally speaking, carbon fiber reinforced resin matrix composites are mostly specific components (such as wind turbine blades). Therefore, the carbon fiber reinforced resin matrix composites can be sorted from the raw materials by means of machine vision to remove the distinguishable hybrid components that do not belong to CFRP, and the sorted carbon fiber reinforced resin matrix composites can be sent into the first composite molten salt through a conveyor belt (not shown in the figure). Here, the specific machine vision algorithm is not elaborated in the embodiments of this specification.

[0082] In one embodiment of this specification, step 104 may specifically include:

[0083] Perform water cleaning, organic solvent cleaning and drying on the carbon fibers obtained by secondary pyrolysis.

[0084] In this embodiment, first perform water cleaning on the carbon fibers obtained by secondary pyrolysis, which is beneficial to washing away the residual molten salt and large-particle-size resin on the surface of the carbon fibers; then perform organic solvent cleaning on the carbon fibers after water cleaning, which can dissolve the liquid-phase macromolecular organic matter adhering to the surface of the carbon fibers, so as to completely dissolve and wash away the resin on the surface of the carbon fibers to obtain the desired carbon fibers. Among them, the organic solvent can be acetone, which is not limited in the embodiments of this specification.

[0085] In one embodiment of this specification, after step 102, the above method may specifically further include:

[0086] Separate the volatile components (specifically generated above the molten salt liquid surface, i.e., gas-liquid components) pyrolyzed in the first pyrolysis and the second pyrolysis to obtain pyrolysis oil and pyrolysis gas;

[0087] Perform fractional distillation and purification on the obtained pyrolysis oil to realize the recovery of pyrolysis products.

[0088] In one embodiment of this specification, after the step of "separating the volatile components pyrolyzed in the first pyrolysis and the second pyrolysis to obtain pyrolysis oil and pyrolysis gas", the above method may specifically further include:

[0089] Burn the obtained pyrolysis gas and output the heat generated by the combustion to the first composite molten salt and the second composite molten salt;

[0090] Perform tail gas treatment on the gas generated by the combustion.

[0091] Figure 4 Shows a schematic diagram before and after a fixture clamping a carbon fiber reinforced resin matrix composite according to an embodiment. Please refer to Figure 4 , in one embodiment of this specification, the carbon fiber reinforced resin matrix composite is subjected to the first pyrolysis, the second pyrolysis, cleaning and drying by being clamped by two fixtures 5 that can move horizontally and vertically.

[0092] In this embodiment, by providing the fixture 5, it is beneficial to improve the rate of carbon fiber recovery, and compared with the method of using tweezers in the related art, it is more convenient to obtain the final desired carbon fiber product.

[0093] It can be understood that the fixture 5 can be connected to a motor (not shown in the figure), and the motor can drive the fixture 5 to lift and move horizontally. Details are not described in this embodiment of the specification. At the same time, the fixture 5 can be any structure capable of clamping the carbon fiber reinforced resin matrix composite material, and details and limitations are not described in this embodiment of the specification.

[0094] In some embodiments, the fixture 5 can be made of Hastelloy. Hastelloy is a type of nickel-based alloy, which is currently mainly divided into three series: B, C, and G. It is mainly used in strong corrosive medium occasions where ferritic Cr-Ni or Cr-Ni-Mo stainless steels, non-metallic materials, etc. cannot be used. Of course, the fixture 5 can also be made of other corrosion-resistant alloys, and details are not limited in this embodiment of the specification.

[0095] In an embodiment of this specification, the clamping portion of the fixture for clamping the end of the carbon fiber reinforced resin matrix composite material can rotate around the axis of the carbon fiber reinforced resin matrix composite material.

[0096] In this embodiment, by providing the clamping portion 51 that can rotate around the axis of the carbon fiber reinforced resin matrix composite material, not only can the height of the carbon fiber reinforced resin matrix composite material be adjusted to better immerse it below the molten salt liquid surface, but also during the subsequent cleaning and drying processes, the cleaning and drying processes can be accelerated by rotating the carbon fiber, improving the efficiency of obtaining the final desired carbon fiber.

[0097] It can be understood that the clamping portion 51 is also provided with a rotating motor (not shown in the figure), and details are not described in this embodiment of the specification.

[0098] In an embodiment of this specification, step 104 may specifically include:

[0099] Clean and dry the carbon fiber obtained by secondary pyrolysis in sequence;

[0100] Cut the carbon fiber located between the two fixtures after drying to achieve the recovery of the carbon fiber.

[0101] In this embodiment, after drying, the carbon fiber located between the two fixtures 5 can be cut using scissors to achieve the recovery of the carbon fiber.

[0102] Figure 2 Shows a schematic flow diagram of a method for recovering carbon fiber by stepped molten salt according to another embodiment. Please refer to Figure 2, the method for recycling carbon fiber by cascade molten salt specifically includes the following steps:

[0103] Step 200: Sort the carbon fiber reinforced resin matrix composite material from the raw materials;

[0104] Step 202: Place the carbon fiber reinforced resin matrix composite material in the first composite molten salt and the target organic solvent for primary pyrolysis;

[0105] Step 204: Place the carbon fiber reinforced resin matrix composite material after primary pyrolysis in the second composite molten salt for secondary pyrolysis;

[0106] Step 206: Wash the carbon fiber obtained by secondary pyrolysis with water, wash it with an organic solvent, and dry it;

[0107] Step 208: Shear the dried carbon fiber located between the two clamps to achieve the recycling of carbon fiber;

[0108] Step 210: Separate the volatiles released by primary pyrolysis and secondary pyrolysis to obtain pyrolysis oil and pyrolysis gas;

[0109] Step 212: Fractionally purify the obtained pyrolysis oil to achieve the recycling of pyrolysis products;

[0110] Step 214: Burn the obtained pyrolysis gas and output the heat generated by the combustion to the first composite molten salt and the second composite molten salt;

[0111] Step 216: Treat the gas generated by combustion for tail gas;

[0112] Figure 3 shows a schematic structural diagram of a system for recycling carbon fiber by cascade molten salt. Please refer to Figure 3 , a system for recycling carbon fiber by cascade molten salt, includes a heat treatment unit 1 and a carbon fiber recycling unit 2, wherein:

[0113] The heat treatment unit 1 includes a primary reactor 11 and a secondary reactor 12 connected in sequence. A plurality of ultrasonic generators 13 are provided at the bottoms of the primary reactor 11 and the secondary reactor 12. The primary reactor 11 contains the first composite molten salt, and the secondary reactor 12 contains the second composite molten salt. The primary reactor 11 is used for primary pyrolysis of the carbon fiber reinforced resin matrix composite material, and the secondary reactor 12 is used for secondary pyrolysis of the carbon fiber reinforced resin matrix composite material after primary pyrolysis. The catalytic property of the first composite molten salt is greater than that of the second composite molten salt, and the melting point of the first composite molten salt is less than that of the second composite molten salt;

[0114] The carbon fiber recovery unit 2 includes a cleaning area and a drying area 23 connected in sequence. The cleaning area is used to clean the carbon fibers obtained by secondary pyrolysis, and the drying area 23 is used to dry the cleaned carbon fibers to achieve the recovery of carbon fibers.

[0115] In an embodiment of this specification, the above system further includes a pretreatment unit 3. The pretreatment unit 3 is connected to the first-stage reactor 11. The pretreatment unit 3 is used to separate the carbon fiber reinforced resin matrix composite material from the raw materials and send the separated carbon fiber reinforced resin matrix composite material into the first-stage reactor 11.

[0116] In an embodiment of this specification, the above system further includes a supply unit 4. The supply unit 4 includes a first-stage molten salt tank 41, an organic solvent tank 42, and a second-stage molten salt tank 43. The first-stage molten salt tank 41 is used to supply the first composite molten salt to the first-stage reactor 11, the organic solvent tank 42 is used to supply the target organic solvent to the first-stage reactor 11, and the second-stage molten salt tank 43 is used to supply the first composite molten salt to the second-stage reactor 12.

[0117] In this embodiment, in order to facilitate the supply of the first composite molten salt to the first-stage reactor 11, the supply of the target organic solvent to the first-stage reactor 11, and the supply of the first composite molten salt to the second-stage reactor 12, the first composite molten salt with preset components and a preset ratio can be stored in the first-stage molten salt tank 41 in advance, the target organic solvent with preset components and a preset ratio can be stored in the organic solvent tank 42, and the second composite molten salt with preset components and a preset ratio can be stored in the second-stage molten salt tank 43.

[0118] In an embodiment of this specification, the cleaning area includes a water cleaning area 21 and an organic solvent cleaning area 22 connected in sequence. The organic solvent cleaning area 22 is connected to the drying area 23. The water cleaning area 21 is used to perform water cleaning on the carbon fibers obtained by secondary pyrolysis, and the organic solvent cleaning area 22 is used to perform organic solvent cleaning on the carbon fibers after water cleaning.

[0119] In an embodiment of this specification, the above system further includes a pyrolysis product recovery unit 6. The pyrolysis product recovery unit 6 includes a gas-liquid condensation separator 61 and a fractionator 62 connected in sequence. The gas-liquid condensation separator 61 is used to separate the volatile components (specifically generated above the molten salt liquid level, that is, gas-liquid components) precipitated from the first-stage reactor 11 and the second-stage reactor 12 to obtain pyrolysis oil and pyrolysis gas. The fractionator 62 is used to fractionate and purify the pyrolysis oil obtained by the gas-liquid condensation separator 61 to achieve the recovery of pyrolysis products.

[0120] In this embodiment, by setting the gas-liquid condensation separator 61 and the fractionator 62, the collection of pyrolysis products such as pyrolysis gas and pyrolysis oil can be completed.

[0121] In one embodiment of this specification, the above system further includes a waste heat utilization unit 7. The waste heat utilization unit 7 includes a gas burner 71 and a tail gas processor 72 connected in sequence. The gas burner 71 is connected to the gas-liquid condensation separator 61. The gas burner 71 is used to burn the pyrolysis gas obtained by the gas-liquid condensation separator 61 and output the heat generated by the combustion to the outer walls of the first reactor 11 and the second reactor 12. The tail gas processor 72 is used to perform tail gas treatment on the gas generated by the gas burner 71.

[0122] In this embodiment, by providing the gas burner 71, it is beneficial to burn the pyrolysis gas obtained by the gas-liquid condensation separator 61 and output the heat generated by the combustion to the outer walls of the first reactor 11 and the second reactor 12, thereby realizing the utilization of the waste heat of the volatile matter.

[0123] Please refer to Figure 3 and Figure 4 , in one embodiment of this specification, the above system further includes two clamps 5 that can move horizontally and vertically. The two clamps 5 are used to clamp both ends of the carbon fiber reinforced resin matrix composite material;

[0124] By using the clamps 5 to clamp the carbon fiber reinforced resin matrix composite material, the carbon fiber reinforced resin matrix composite material is successively driven by the clamps 5 to be immersed in the molten first composite molten salt and the second composite molten salt, and the carbon fiber obtained by secondary pyrolysis is successively driven to the cleaning area and the drying area 23.

[0125] In one embodiment of this specification, the carbon fiber recovery unit 2 further includes a shearing area 24. The drying area 23 is connected to the shearing area 24. The shearing area 24 is used to shear the carbon fiber after drying and located between the two clamps 5 to realize the recovery of the carbon fiber.

[0126] The above solution will be introduced below in combination with a specific embodiment (recovering carbon fiber from waste fan blades).

[0127] After the full-process recovery of 20 kg of waste fan blades by the above system, about 8 kg of carbon fiber is obtained. The retention rate of the single-filament tensile strength of the carbon fiber is about 95%, and the total time-consuming for the whole cycle is about 1 h. The specific process can be seen in the following steps:

[0128] 1) Heat and raise the temperature of a total of 200 kg of pre-mixed NaNO3-KNO3-NaNO2 (7-53-40 wt%) to 150 °C in the first reactor 11, and heat and raise the temperature of a total of 200 kg of pre-mixed K2CO3-Li2CO3-Na2CO3 (34.5-32.1-33.4 wt%) to 400 °C in the second reactor 12. Both the first reactor 11 and the second reactor 12 are kept warm for 1 h to fully melt to prepare the eutectic salt, and then 20 kg of ethanolamine is added to the first reactor 11;

[0129] 2) After simply disassembling the discarded fan, the pretreatment unit 3 removes the remaining components other than the blades.

[0130] 3) Fix the discarded fan blades with the fixture 5 and immerse them below the molten salt level in the first-stage reactor 11. Turn on multiple ultrasonic generators 13 on the side and bottom of the first-stage reactor 11 and keep warm for 10 minutes.

[0131] 4) The fixture 5 takes out the discarded fan blades and immerses them below the molten salt level in the second-stage reactor 12. Turn on multiple ultrasonic generators 13 on the side and bottom of the second-stage reactor 12 and keep warm for 10 minutes.

[0132] 5) The fixture 5 takes out the discarded fan blades and immerses them in the water cleaning area 21 for 5 minutes.

[0133] 6) The fixture 5 takes out the discarded fan blades and immerses them in the organic solvent cleaning area 22 for 5 minutes.

[0134] 7) The fixture 5 takes out the carbon fiber and dries it in the drying area 23 to remove moisture.

[0135] 8) The fixture 5 takes out the carbon fiber and removes both ends of the material in the shearing area 24, thus obtaining clean carbon fiber, thereby realizing the recovery of carbon fiber.

[0136] Note: The volatile components released during the pyrolysis process are separated into pyrolysis oil and pyrolysis gas by the gas-liquid condensation separator 61. Among them, the pyrolysis oil is fractionated and collected by the multi-stage fractionator 62, and the pyrolysis gas generates heat through the gas burner 71 to supply heat to the first-stage reactor 11 and the second-stage reactor 12 for molten salt insulation. The next batch of waste can be recycled, and the tail gas is treated by the tail gas processor 72.

[0137] The above describes specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims can be performed in a different order than in the embodiments and still achieve the desired results. Additionally, the processes depicted in the drawings do not necessarily require the specific order or sequential order shown to achieve the desired results. In certain embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0138] Each embodiment in this specification is described in a progressive manner. The same or similar parts between each embodiment can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the device embodiments, since they are basically similar to the method embodiments, the description is relatively simple, and the relevant parts can refer to the partial description of the method embodiments.

[0139] It should be noted that, in this document, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variation thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements but also includes other elements not expressly listed, or elements inherent to such process, method, article or device.

[0140] Finally, it should be noted that: The above embodiments are only used to illustrate the technical solutions of this specification, and are not intended to limit them; Although the technical solutions of this specification have been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; And these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of this specification.

Claims

1. A method for recycling carbon fiber by stepwise molten salt, characterized in that, Including: Placing the carbon fiber reinforced resin matrix composite material in a first composite molten salt for primary pyrolysis; Placing the carbon fiber reinforced resin matrix composite material after primary pyrolysis in a second composite molten salt for secondary pyrolysis; wherein, the catalytic property of the first composite molten salt is greater than that of the second composite molten salt, and the melting point of the first composite molten salt is less than that of the second composite molten salt. Ultrasonic waves are applied to the reaction system during both primary pyrolysis and secondary pyrolysis. The catalytic property of the composite molten salt is evaluated by the resin decomposition rate and the degree of damage to the carbon fiber structure. The resin decomposition rate is referenced by the removal rate of the resin, and the degree of damage to the carbon fiber structure is referenced by the tensile strength of the carbon fiber monofilament; Successively cleaning and drying the carbon fibers obtained by secondary pyrolysis to achieve the recovery of carbon fibers; The carbon fiber reinforced resin matrix composite material is subjected to primary pyrolysis, secondary pyrolysis, cleaning and drying by being clamped by two clamps that can move horizontally and vertically; The clamping part of the clamp for clamping the end of the carbon fiber reinforced resin matrix composite material can rotate around the axis of the carbon fiber reinforced resin matrix composite material. It can not only adjust the height of the carbon fiber reinforced resin matrix composite material to better immerse it below the molten salt liquid level, but also accelerate the cleaning and drying processes by rotating the carbon fiber during subsequent cleaning and drying, improving the efficiency of obtaining the desired carbon fibers in the end.

2. The method for recycling carbon fiber by stepwise molten salt according to claim 1, characterized in that, The step of placing the carbon fiber reinforced resin matrix composite material in the first composite molten salt for primary pyrolysis includes: The carbon fiber reinforced resin matrix composite material is pyrolyzed once in a first composite molten salt and a target organic solvent; wherein, the dielectric constant of the target organic solvent is greater than 3.6 C 2 / (N·M 2 )。 3. The method for recovering carbon fiber from stepped molten salt according to claim 2, characterized in that, Before placing the carbon fiber reinforced resin matrix composite material in the first composite molten salt for primary pyrolysis, it further includes: Sorting the carbon fiber reinforced resin matrix composite material from the raw materials.

4. The method for recycling carbon fiber by stepwise molten salt according to claim 2, wherein The melting point of the first composite molten salt is 120 - 160 °C, the melting point of the second composite molten salt is 350 - 400 °C, and the boiling point of the target organic solvent is 160 - 200 °C.

5. The method for recovering carbon fiber by stepwise molten salt according to claim 4, characterized in that, The first composite molten salt includes at least one of alkali salts, chloride salts and nitrate salts; and / or, The target organic solvent includes at least one of alcohols, amines and alkanolamines; and / or, The second composite molten salt includes at least one of carbonate salts and sulfate salts.

6. The method for recovering carbon fiber by stepwise molten salt according to claim 1, wherein, The step of successively cleaning and drying the carbon fibers obtained by secondary pyrolysis to achieve the recovery of carbon fibers includes: Successively cleaning and drying the carbon fibers obtained by secondary pyrolysis; Cutting the carbon fibers after drying and located between the two clamps to achieve the recovery of carbon fibers; and / or, The step of successively cleaning and drying the carbon fibers obtained by secondary pyrolysis includes: Performing water cleaning, organic solvent cleaning and drying on the carbon fibers obtained by secondary pyrolysis.

7. The method for recycling carbon fiber by stepwise molten salt according to any one of claims 1-6, characterized in that, After placing the carbon fiber reinforced resin matrix composite material after primary pyrolysis in the second composite molten salt for secondary pyrolysis, it further includes: Separating the volatile components evolved from primary pyrolysis and secondary pyrolysis to obtain pyrolysis oil and pyrolysis gas; Performing fractional distillation and purification on the obtained pyrolysis oil to achieve the recovery of pyrolysis products.

8. The method for recycling carbon fiber by stepwise molten salt according to claim 7, characterized in that, After separating the volatile components evolved from primary pyrolysis and secondary pyrolysis to obtain pyrolysis oil and pyrolysis gas, it further includes: The obtained pyrolysis gas is combusted, and the heat generated by the combustion is output to the first composite molten salt and the second composite molten salt; The gas generated by the combustion is subjected to tail gas treatment.

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