A method for extracting caprolactam from waste containing heteropolyamide 6

Abstract

The present invention relates to the technical field of polyamide recycling, and particularly relates to a method for extracting caprolactam from waste polyamide 6 containing impurities. In this method, the waste polyamide 6 containing impurities is mixed with a tin-based metal catalyst and then depolymerized by passing superheated steam. The caprolactam generated by the depolymerization is discharged with the superheated steam and then condensed to obtain an aqueous solution of caprolactam. By using a tin-based metal catalyst in the steam depolymerization, this method can avoid the problem of catalyst deactivation caused by foreign components in the waste polyamide 6, and can also effectively inhibit the formation of coke during depolymerization.

Description

Technical Field

[0001] The present invention relates to the technical field of polyamide recycling, and particularly relates to a method for extracting caprolactam from waste polyamide 6 containing impurities. Background Art

[0002] Polyamide 6 (polycaprolactam) is the most common polyamide polymer, with excellent thermal and mechanical properties, and is widely used in fields such as fibers, films, and engineering plastics. With the increasing annual consumption of polyamide 6, the recycling of waste polyamide 6 products has gradually become a hot and difficult issue. At present, the utilization rate of waste polyamide 6 products is relatively low, and most of them are landfilled or incinerated, resulting in serious waste of resources and environmental pollution.

[0003] The current recycling methods mainly rely on physical methods. For example, the Chinese invention patent with publication number CN112339157A discloses that after cleaning, separating, removing impurities, pulverizing, and drying the polyamide material, it is extruded through a screw at high temperature and melted, and then water-cooled granulated to be used as plastics in low-end industrial fields. This recycling method has low added value, high requirements for raw material purity, and poor product performance of recycled polyamide 6.

[0004] In addition to physical methods, chemical methods can also be used to depolymerize polyamide materials. The quality of the purified caprolactam monomer obtained by depolymerization can reach the excellent grade and can be used to prepare polyamide 6 fibers, etc., with high added value. Generally speaking, the chemical method has lower requirements for raw material purity than the physical method and can perform high-quality regeneration on waste materials that cannot be recycled by the physical method. The chemical depolymerization of polyamide 6 commonly uses steam depolymerization: superheated steam is used as the depolymerizing agent and continuously introduced into the reactor from the bottom. The superheated steam will pass through the molten polyamide 6 and be discharged from the upper part of the reactor. In this process, the superheated steam acts as a depolymerizing agent, a stripping gas, and a heat source. The caprolactam produced by hydrolysis is discharged from the upper part of the reactor together with the superheated steam and can be collected after condensation to obtain an aqueous solution of caprolactam (Research progress on the recycling of waste polyamide fibers [J]. Synthetic Fiber Industry, 2014, 37, 51-55).

[0005] During the steam depolymerization process, a catalyst needs to be added to accelerate the depolymerization reaction so that the steam depolymerization process can be applied to industrial-scale production. Commonly used catalysts include acids and alkalis: acids are generally phosphoric acid, boric acid, etc. For example, phosphoric acid is used as a catalyst during the depolymerization process in the Chinese invention patent with publication number CN103467378A; alkalis are generally alkalis of alkali metals, such as alkali metal hydroxides, alkali metal carbonates, etc., as in the Chinese invention patent with publication number CN101157646B.

[0006] The polyamide 6 waste recycled in society often contains a large amount of non-polyamide 6 components. These components are prone to reacting with traditional catalysts during the depolymerization process, resulting in a decrease or even inactivation of the catalyst activity. For example, waste carpets are one of the important sources of polyamide 6 waste. In polyamide 6 carpets, polyamide 6 fibers are usually fixed between two backing layers by adhesives. The adhesives and backing often contain a large amount of calcium carbonate (CaCO 3 ) fillers. During the acid-catalyzed steam depolymerization process, the calcium carbonate in the polyamide 6 waste carpet will react with phosphoric acid, deactivating the catalyst and reducing the depolymerization rate. In addition, polyamide 6 waste is often mixed with a certain amount of PET polyester. During the base-catalyzed steam depolymerization process, due to the higher hydrolysis activity of the ester bond than the amide bond, the PET molecular chain will depolymerize preferentially. The acidic substances produced by the depolymerization, such as terephthalic acid, will react with the basic catalyst in the system, resulting in catalyst deactivation.

[0007] To solve the above problems, the traditional technology is to perform impurity removal pretreatment on the polyamide 6 waste before steam depolymerization: after the polyamide 6 waste carpet is mechanically disassembled, the polyamide 6 fibers on the surface layer will be separated from the backing material. By taking advantage of the difference in specific gravity between the two and using means such as cyclone separation, polyamide 6 fibers with higher purity (purity ≥ 90%) can be separated; for PET / polyamide 6 blended fabrics, polyamide 6 fibers with higher purity can be separated by selective dissolution methods. For example, both the Chinese invention patent with publication number CN104837903A and the US invention patent with publication number US5430068B use solvents to dissolve and separate the polyester.

[0008] The above pretreatment impurity removal process requires high costs and is difficult to completely remove the foreign components that can cause catalyst deactivation, which results in the fact that in actual production, the polyamide 6 waste containing impurities usually cannot be recycled.

[0009] In addition, during the current steam depolymerization process of waste polyamide 6, there is also the problem of reactor / pipeline coking. Due to the high depolymerization temperature, side reactions inevitably occur during the depolymerization process, and finally coking products are generated. Foreign components such as dyes and additives in the polyamide 6 waste will further exacerbate coking formation. In industrial production, these coking products are easily attached to the reactor wall surface, resulting in a decrease in the heat transfer performance of the reactor and an increase in energy consumption; or they accumulate in the pipeline, causing blockage, production interruption, and even production accidents. Summary of the Invention

[0010] In order to overcome the defects of the above-mentioned prior art, the technical problem to be solved by the present invention is to provide a method for extracting caprolactam from polyamide 6 waste containing impurities, which has high reaction efficiency and is not interfered by impurities.

[0011] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for extracting caprolactam from waste polyamide 6 containing impurities, comprising the following steps: Mixing the waste polyamide 6 containing impurities with a tin-based metal catalyst, and then introducing superheated steam for depolymerization. The caprolactam generated by the depolymerization is discharged with the superheated steam and then condensed to obtain an aqueous solution of caprolactam.

[0012] The beneficial effects of the present invention are as follows: In the extraction method of the present invention, a tin-based metal catalyst is used in the steam depolymerization, which avoids the problem of catalyst deactivation caused by foreign components in the polyamide 6 waste during the depolymerization process, eliminates the pretreatment and impurity removal process of the waste polyamide 6 containing impurities, and can effectively extract caprolactam from the waste polyamide 6 containing impurities; in addition, it can effectively inhibit the formation of coke deposits during the depolymerization, and can ensure the stable and long-term continuous operation of the reaction device. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The figure shows a schematic diagram of the flow direction of steam and materials in Example 14 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] To describe in detail the technical content, the achieved objectives and the effects of the present invention, the following is described in conjunction with the embodiments and the accompanying drawings.

[0015] The most crucial concept of the present invention lies in: Using a tin-based metal catalyst in the steam depolymerization can avoid the problem of catalyst deactivation caused by foreign components in the polyamide 6 waste, and can effectively inhibit the formation of coke deposits during the depolymerization.

[0016] A method for extracting caprolactam from waste polyamide 6 containing impurities according to the present invention comprises the following steps: Mixing the waste polyamide 6 containing impurities with a tin-based metal catalyst, and then introducing superheated steam for depolymerization. The caprolactam generated by the depolymerization is discharged with the superheated steam and then condensed to obtain an aqueous solution of caprolactam.

[0017] From the above description, it can be seen that the beneficial effects of the present invention are as follows: The inventor found through experiments that the tin (II) atoms in the tin-based catalyst have appropriate interactions with the amide groups on the polyamide main chain at the depolymerization temperature. Coupled with the variable valence of the tin atoms, the amide groups can be well activated to complete the catalytic cycle, which has the advantages of high catalytic efficiency and few side reactions, and the active intermediate is not easily interfered by foreign components / impurities and deactivated. Thus, it avoids the problem of catalyst deactivation caused by impurities / foreign components (such as alkaline inorganic fillers like calcium carbonate and PET polyester, etc.) in the polyamide 6 waste during the steam depolymerization process, eliminates the pretreatment and impurity removal process of the waste polyamide 6 containing impurities, and saves costs.

[0018] During the steam depolymerization process, due to the existence of side reactions, coke deposits are likely to form in the reactor. Coke deposition will seriously affect the heat transfer characteristics of the reactor and easily block the pipeline, resulting in production interruption. Therefore, during the production process, it is necessary to stop production regularly to clean the reactor, causing huge cost waste. The catalyst used in the present invention can not only accelerate the depolymerization reaction, avoid the problem of catalyst deactivation caused by non-polyamide 6 components, but also has the advantages of high selectivity, few side reactions, and can reduce the generation of coke deposits, and is suitable for industrial production.

[0019] Further, the tin-based metal catalyst is at least one of stannous oxide, stannous hydroxide, stannous chloride, and tin oxide.

[0020] Further, the addition amount of the tin-based metal catalyst is 0.3-5.0 wt% of the mass of polyamide 6.

[0021] Further, the addition amount of the tin-based metal catalyst is 1.0-3.0 wt% of the mass of polyamide 6.

[0022] From the above description, it can be seen that if the amount of catalyst used is too small, the catalytic effect is not obvious; if the amount is too large, side reactions may be triggered and it is also unfavorable in terms of cost.

[0023] Further, in the waste polyamide 6 containing impurities, the mass fraction of polyamide 6 > 50%. Preferably, the mass fraction of polyamide 6 > 70%.

[0024] From the above description, it can be seen that the higher the mass fraction of polyamide 6, the fewer the impurities and the higher the caprolactam yield obtained.

[0025] Further, the depolymerization temperature during depolymerization is 250-350 °C. Preferably, 270-340 °C.

[0026] From the above description, it can be seen that the higher the depolymerization temperature, the faster the depolymerization rate. However, when the temperature is too high, side reactions are easily triggered, resulting in a decrease in monomer yield.

[0027] Further, an intermittent production process is adopted for production: after the polyamide 6 is completely depolymerized or when the caprolactam concentration in the condensate is lower than 1%, the residue is discharged, and the next batch of waste polyamide 6 containing impurities is added to continue the steam depolymerization.

[0028] Please refer to Figure 1 As shown, further, a continuous production process is adopted for production: the residence time of all materials in the reaction system is 1-4 h, and the flow direction of the steam is opposite to the flow direction of the materials.

[0029] As can be seen from the above description, the residence time of the material in the reaction system is 1 - 4 h, the polyamide 6 component is basically completely depolymerized, and the non-polyamide 6 component that cannot be depolymerized is discharged from the reaction system to become solid waste. The flow direction of the steam is opposite to that of the material, which can strengthen the collision contact between polyamide 6 and steam molecules and improve the depolymerization efficiency. Multiple reaction kettles can reduce the backmixing phenomenon and narrow the residence time distribution.

[0030] The intermittent production process has simpler equipment than the continuous production process and is suitable for the depolymerization of small batches of waste polyamide 6 containing impurities; the continuous production process has a simpler process flow than the intermittent production process and is suitable for the depolymerization of large batches of waste polyamide 6 containing impurities.

[0031] The following catalysts are all of analytical purity.

[0032] Example 1 of the present invention is as follows:

[0033] A method for extracting caprolactam from waste polyamide 6 containing impurities, comprising the following steps:

[0034] 400 g of polyamide 6 chips with a relative viscosity of 2.4 and 2.0 wt% of stannous oxide are added into a reaction kettle with a heating and stirring system (previously purged with nitrogen 3 times), and then stirring is started and the temperature is raised. When the temperature is raised to 280 °C, superheated steam at 280 °C (flow rate of 20 g / min) is introduced from the bottom of the reaction kettle, and timing is started. The depolymerization is carried out under normal pressure. The steam introduced from the bottom will pass through the polymer melt and carry the caprolactam generated by depolymerization out of the upper outlet of the reaction kettle together. After being condensed by a condensing device, an aqueous solution of caprolactam can be obtained. The content of caprolactam in the condensate collected within a certain reaction time is measured, and the measurement method refers to GB / T 23296.20—2009, and the caprolactam yield is calculated accordingly.

[0035] Example 2 of the present invention is as follows:

[0036] The difference between Example 2 and Example 1 is that the catalyst is 4.0 wt% NaOH.

[0037] Example 3 of the present invention is as follows:

[0038] The difference between Example 3 and Example 1 is that the catalyst is 2.0 wt% phosphoric acid.

[0039] Comparative Example 1 of the present invention is as follows:

[0040] The difference between Comparative Example 1 and Example 1 is that no catalyst is added.

[0041] The caprolactam yields of Examples 1 - 3 and Comparative Example 1 are calculated, and the calculation results are shown in Table 1.

[0042] Table 1

[0043]

[0044]

[0045] As can be seen from the data in Table 1, stannous oxide has excellent catalytic activity. For the steam depolymerization of pure polyamide 6, the catalytic activity of 2.0 wt% stannous oxide is close to that of 2.0 wt% phosphoric acid and higher than that of 4.0 wt% NaOH.

[0046] Example 4 of the present invention is as follows:

[0047] The difference between Example 4 and Example 1 is that the waste heteropolyamide 6 comes from waste carpets, in which the content of polyamide 6 is 61.3 wt%, and the remaining components are PP, SBR and CaCO 3 .

[0048] Example 5 of the present invention is as follows:

[0049] The difference between Example 5 and Example 4 is that the catalyst is 4.0 wt% NaOH.

[0050] Example 6 of the present invention is as follows:

[0051] The difference between Example 6 and Example 4 is that the catalyst is 2.0 wt% phosphoric acid.

[0052] Comparative Example 2 of the present invention is as follows:

[0053] The difference between Comparative Example 2 and Example 4 is that no catalyst is added.

[0054] Calculate the caprolactam yields of Examples 4 to 6 and Comparative Example 2, and the calculation results are shown in Table 2.

[0055] Table 2

[0056]

[0057] As can be seen from Table 2, for the waste heteropolyamide 6 from waste carpets, the phosphoric acid catalyst (Example 6) shows serious deactivation. The calcium carbonate filler in the waste carpets will react with phosphoric acid, resulting in deactivation. Stannous oxide (Example 4) and sodium hydroxide (Example 5) still show catalytic activity and no deactivation occurs.

[0058] Example 7 of the present invention is as follows:

[0059] The difference between Example 7 and Example 1 is that the waste heteropolyamide 6 comes from polyamide 6 / PET hybrid fabrics, in which the content of polyamide 6 is 69%.

[0060] Example 8 of the present invention is as follows:

[0061] Example 8 is different from Example 7 in that the catalyst is 4.0 wt% NaOH.

[0062] Example 9 of the present invention is as follows:

[0063] Example 9 is different from Example 7 in that the catalyst is 2.0 wt% phosphoric acid.

[0064] Comparative Example 3 of the present invention is as follows:

[0065] Comparative Example 3 is different from Example 7 in that no catalyst is added.

[0066] The caprolactam yields of Examples 7 to 9 and Comparative Example 3 were calculated, and the calculation results are shown in Table 3.

[0067] Table 3

[0068]

[0069] As can be seen from Table 3, for the waste polyamide 6 containing impurities derived from polyamide 6 / PET hybrid fabrics, the NaOH catalyst (Example 8) showed serious deactivation. The acidic substances generated by PET degradation reacted with NaOH, resulting in its deactivation. Stannous oxide (Example 7) and phosphoric acid (Example 9) still showed catalytic activity and no deactivation occurred.

[0070] According to Tables 1 to 3, stannous oxide has excellent catalytic activity in the steam depolymerization of polyamide 6 containing impurities, and there is no deactivation problem that easily occurs in traditional catalysts such as phosphoric acid and NaOH.

[0071] Example 10 of the present invention is as follows:

[0072] 400 g of polyamide 6 chips with a relative viscosity of 2.4, 2.0 wt% stannous oxide and 1 wt% acid black BNG dye were added into a reaction kettle with a heating and stirring system (purged with nitrogen 3 times in advance), and then stirring was started and the temperature was raised. When the temperature reached 280 °C, superheated steam at 280 °C (flow rate: 20 g / min) was introduced from the bottom of the reaction kettle and timing was started. The caprolactam generated by depolymerization would be discharged from the top outlet of the reaction kettle together with the steam, and an aqueous solution of caprolactam was obtained after condensation by a condensing device. When the depolymerization was carried out for 5 h, the condensate collected from the top of the reaction kettle no longer contained caprolactam, indicating that the depolymerization was complete. After cooling, when the kettle was opened, coking substances could be seen on the stirring paddle and the inner wall of the reaction kettle. By comparing the mass of the reaction kettle before and after depolymerization (the mass of the reaction kettle body + the stirring paddle), the mass of the residual coking substances in the kettle could be calculated.

[0073] Example 11 of the present invention is as follows:

[0074] Example 11 is different from Example 10 in that the catalyst is 4.0 wt% NaOH.

[0075] Example 12 of the present invention is as follows:

[0076] The difference between Example 12 and Example 10 is that the catalyst is 2.0 wt% phosphoric acid.

[0077] The weight of the residue in the reactor of Test Examples 10 to 12 was measured, and the test results are shown in Table 4.

[0078] Table 4

[0079] Group number Weight of residue in the reactor (g) Example 10 10.82 Example 11 22.93 Example 12 17.58

[0080] As can be seen from Table 4, when stannous oxide is used as the catalyst in steam depolymerization, the occurrence of side reactions can be inhibited, and the formation of coke in the reactor is greatly reduced.

[0081] Example 13 of the present invention is as follows:

[0082] A method for extracting caprolactam from waste polyamide 6 containing impurities (batch production mode), comprising the following steps:

[0083] 400 g of polyamide 6 chips with a relative viscosity of 2.4 in a molten state are mixed with 0.3 wt% of stannous chloride by an extruder screw and added to a reaction kettle, and the reaction kettle is heated. When the temperature is raised to 280 °C, superheated steam at 280 °C (flow rate: 20 g / min) is introduced from the bottom of the reaction kettle. The caprolactam produced by depolymerization will be discharged from the upper part of the reaction kettle together with the steam, and an aqueous solution of caprolactam is obtained after condensation by a condensing device. When the caprolactam concentration in the discharged condensate is lower than 1%, the next batch of polyamide 6 chips is added to continue steam depolymerization.

[0084] Please refer to Figure 1 As shown, Example 14 of the present invention is as follows:

[0085] A method for extracting caprolactam from waste polyamide 6 containing impurities (continuous production mode), comprising the following steps:

[0086] 10 Kg of polyamide 6 chips with a relative viscosity of 2.4 in a molten state are mixed with 5 wt% of stannous hydroxide by an extruder screw and continuously fed into a reactor. The reactor is a continuous stirred reactor composed of three vertical reaction kettles in series with a heating and stirring system; the residence time of the material in the reactor is controlled at 3 h; the flow direction of the steam and the material is countercurrent, the steam flow rate is 10 g / min, the steam temperature is 270 °C, the depolymerization temperature is controlled at 270 °C, and the caprolactam produced by depolymerization will be discharged from the top outlet of the reactor together with the steam, and an aqueous solution of caprolactam is obtained after condensation by a condensing device.

[0087] Example 15 of the present invention is as follows:

[0088] Example 15 is different from Example 14 in that: the tin-based metal catalyst is tin oxide, the number of reaction kettles is 5, the superheated steam temperature is 350 °C, the internal temperature of the reactor is controlled at 350 °C, and the residence time of the material in the reaction system is controlled at 4 h.

[0089] Example 16 of the present invention is as follows:

[0090] Example 16 is different from Example 14 in that: the number of reaction kettles is 2, and the residence time of the material in the reaction system is controlled at 1 h.

[0091] In summary, the method for extracting caprolactam from waste polyamide 6 containing impurities provided by the present invention uses a tin-based metal catalyst in steam depolymerization, avoiding the problem of catalyst deactivation caused by foreign components in the polyamide 6 waste during the depolymerization process, eliminating the pretreatment and impurity removal process for waste polyamide 6 containing impurities, and effectively extracting caprolactam from waste polyamide 6 containing impurities; in addition, it can effectively inhibit the formation of coke deposits during depolymerization, ensuring the stable and long-term continuous operation of the reactor.

[0092] The above are only examples of the present invention, and do not limit the patent scope of the present invention. Any equivalent transformation made using the content of the specification and drawings of the present invention, or directly or indirectly applied in related technical fields, shall be equally included in the patent protection scope of the present invention.

Claims

1. A method for extracting caprolactam from waste polyamide 6 containing impurities, characterized in that, it comprises the following steps: Mix the waste polyamide 6 containing impurities with a tin-based metal catalyst and then introduce superheated steam for depolymerization. The caprolactam generated by depolymerization is discharged with the superheated steam and then condensed to obtain an aqueous caprolactam solution. The waste polyamide 6 containing impurities includes basic inorganic fillers and PET polyester; The tin-based metal catalyst is at least one of stannous oxide, stannous hydroxide, stannous chloride, and tin oxide.

2. The method for extracting caprolactam from waste polyamide 6 containing impurities according to claim 1, characterized in that, the addition amount of the tin-based metal catalyst is 0.3 - 5.0 wt% of the mass of polyamide 6.

3. The method for extracting caprolactam from waste polyamide 6 containing impurities according to claim 1, characterized in that, the addition amount of the tin-based metal catalyst is 1.0 - 3.0 wt% of the mass of polyamide 6.

4. The method for extracting caprolactam from waste polyamide 6 containing impurities according to claim 1, characterized in that, in the waste polyamide 6 containing impurities, the mass fraction of polyamide 6 > 50%.

5. The method for extracting caprolactam from waste polyamide 6 containing impurities according to claim 1, characterized in that, the depolymerization temperature during depolymerization is 250 - 350 °C.

6. The method for extracting caprolactam from waste polyamide 6 containing impurities according to claim 1, characterized in that, an intermittent production process is adopted for production: after the polyamide 6 is completely depolymerized or when the caprolactam concentration in the condensate is lower than 1%, the residue is discharged, and the next batch of waste polyamide 6 containing impurities is added to continue steam depolymerization.

7. The method for extracting caprolactam from waste polyamide 6 containing impurities according to claim 1, characterized in that, a continuous production process is adopted for production: the residence time of all materials in the reaction system is 1 - 4 h, and the flow direction of the steam is opposite to the flow direction of the materials.

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