Purification method for recycling and pretreating aqueous solution containing caprolactam monomer

Abstract

The invention relates to a purification method for recovery pretreatment of a caprolactam monomer-containing aqueous solution, and relates to the technical field of caprolactam monomer recovery treatment.The purification method comprises the following steps: adding a flocculating agent into a to-be-recovered caprolactam monomer aqueous solution for physicochemical treatment, and then performing settling separation, a first caprolactam monomer aqueous solution is obtained; filtering the first caprolactam monomer aqueous solution to obtain a second caprolactam monomer aqueous solution; and carrying out anion exchange treatment on the second caprolactam monomer aqueous solution, and then carrying out cation exchange treatment to obtain the purified and recycled caprolactam monomer aqueous solution. The invention provides a purification method for recycling and pretreating the aqueous solution containing the caprolactam monomer, which not only is used for solving the long-standing problems of pipeline blockage, difficult conveying and storage and the like, but also effectively solves the problems of high production cost and environment cost of the recycled monomer and the like.

Description

Technical Field

[0001] The invention relates to the technical field of caprolactam monomer recovery and treatment, in particular to a purification method for recovering and pre-treating an aqueous solution containing caprolactam monomer. Background Art

[0002] Polyamide 6 chips, commonly known as nylon 6 or nylon 6 chips, are made of caprolactam as raw material, with certain additives added, polymerized into high molecular weight polymers under certain process conditions, and produced into white crystalline columnar particles through processes such as pelletizing, extraction and drying. Nylon 6 chips themselves have excellent electrical insulation, good acid and alkali corrosion resistance, high mechanical strength and impact strength, excellent toughness and heat resistance, and have the best wear resistance among synthetic fibers.

[0003] The application fields of nylon 6 chips are very rich. According to the purpose, they can be divided into: 1. Fiber-grade chips, used in civilian silk, made into underwear, socks, shirts, etc., used in industrial silk, made into cords, parachutes, seat belts, etc.; 2. Engineering plastic-grade chips, used in gears, housings, cable sheaths, and equipment parts of precision machines; 3. Film-drawing-grade chips, used in the packaging industry, made into food packaging, medical packaging materials, etc.; 4. Special composite materials, including impact-resistant nylon, reinforced high-temperature resistant nylon, etc., are used in fields with special requirements, such as the manufacture of impact drills, lawn mowers, etc. With the rapid changes in market demand, the use of nylon 6 chips will be more extensive.

[0004] In the process of large-scale and continuous production of nylon 6, the hydrolysis and ring-opening polymerization of caprolactam is mainly used according to the reaction mechanism. The main reactions are the hydrolysis and ring-opening of caprolactam monomer, addition, polycondensation and other steps. Since the entire reaction process is reversible, when the reaction is balanced, the final conversion rate of caprolactam monomer is generally 90%. Therefore, there will be 10% other components in the nylon 6 polymer, mainly including about 8% of caprolactam monomer and about 2% of oligomers (low molecular cyclic and linear caprolactam polymers). Usually, the presence of these monomers and oligomers will seriously affect the downstream processing and product quality. Therefore, these components in the nylon 6 polymer must be removed to meet the quality requirements of the slices.

[0005] During the polymerization process of nylon 6, some auxiliaries or additives are usually added, including organic acids, inorganic alkali auxiliaries, and additives such as matting agent titanium dioxide. This type of matting agent usually contains a mineral coating layer (containing silicon and trace impurities such as metal compounds). During production, these coating layers are often worn and fall off, thus entering the extraction process.

[0006] In the production of nylon 6 chips, in order to remove monomers and oligomers from nylon 6 polymers, single-stage or multi-stage solid-liquid two-phase countercurrent extraction is mainly used, and the extractant is generally hot water at 80-120°C. The extraction water produced in this process usually contains monomer solutions with impurities such as caprolactam, oligomers, a small amount of suspended solids, silicon ions, and trace amounts of metal ions, with a concentration between 6.5% and 10.5%. Then, the extraction water is evaporated and concentrated to a monomer content of 75-85%. Most of these recovered high-concentration monomer concentrates use direct reuse technology. One part of the process is to directly enter the monomer concentrate into the polymerization; the other part of the process is to first depolymerize the oligomers and then enter the polymerization. Since this direct recovery technology cannot remove the oligomers (including dimers, trimers and cyclic oligomers) and other impurities contained in the extraction water, these impurities will accumulate in the entire production system, resulting in low quality and low added value of the polymerization product. At the same time, the presence of these impurities also causes the monomer concentrate to easily precipitate crystals during production, increasing the difficulty of concentration, often blocking pipelines, causing difficulties in transportation and storage, and seriously affecting continuous production.

[0007] In patent CN202010548123.4 "A method for recovering and refining caprolactam monomer", the pre-treatment method of pre-filtration and resin adsorption is used for the monomer aqueous solution, which cannot effectively remove the oligomers and other impurities contained in the aqueous solution, and the subsequent refining process is relatively long. At the same time, the pre-filter in this treatment scheme is frequently cleaned, and the regeneration cycle of the resin exchange bed is relatively short, which adds additional energy and water consumption to the recovery system. In order to solve the problems existing in the monomer solution recovery process, improve the monomer recovery rate and the quality of the polymer product, and reduce the production cost and environmental cost of the recovered monomer, a more effective production method is needed, and the existing technical route is updated. Summary of the invention

[0008] To solve the above problems, the present invention provides a purification method for recovering and pre-treating an aqueous solution containing caprolactam monomer. The present invention not only solves the long-standing problems of pipeline blockage, transportation and storage difficulties, but also effectively solves the problems of high production cost and environmental cost of recovered monomers.

[0009] The present invention provides a purification method for recovering and pretreating an aqueous solution containing caprolactam monomer, the purification method comprising the following steps:

[0010] Adding a flocculant to the caprolactam monomer aqueous solution to be recovered for physicochemical treatment, followed by sedimentation separation to obtain a first caprolactam monomer aqueous solution;

[0011] filtering the first aqueous caprolactam monomer solution to obtain a second aqueous caprolactam monomer solution;

[0012] The second caprolactam monomer aqueous solution is subjected to anion exchange treatment and then to cation exchange treatment to obtain a purified and recovered caprolactam monomer aqueous solution.

[0013] Furthermore, the purification method further comprises the following steps:

[0014] The extraction water generated after the nylon PA6 slices are washed with water and the aqueous solution containing caprolactam monomer generated in other processes are collected in a monomer aqueous solution collection tank to obtain the caprolactam monomer aqueous solution to be recovered; the other processes include filament pelletizing, slice drying and cation resin purification.

[0015] Furthermore, the main components of the caprolactam monomer aqueous solution to be recovered include: caprolactam, oligomers, suspended solids, silicon-containing ions and trace metal ions; wherein the oligomers include dimers, trimers and cyclic oligomers;

[0016] In terms of weight percentage, the total initial concentration of the caprolactam monomer and the oligomer in the aqueous caprolactam monomer solution to be recovered is 6.5% to 10.5%, wherein the weight percentage of the oligomer is 15 to 25%.

[0017] Furthermore, the flocculant is a cationic polyacrylamide flocculant aqueous solution, the weight percentage concentration of the cationic polyacrylamide flocculant aqueous solution is 1-5%; the added weight of the cationic polyacrylamide flocculant aqueous solution accounts for 0.01-0.05% of the weight of the caprolactam monomer aqueous solution to be recovered.

[0018] Furthermore, the sedimentation separation adopts a vertical sedimentation tank to perform multi-stage sedimentation separation with a separation accuracy of more than 1.0 μm;

[0019] The vertical sedimentation tank includes a primary sedimentation unit, a secondary sedimentation unit and a tertiary sedimentation unit. The primary sedimentation unit includes: a feed pipe, a diverter, a primary sedimentation area, a primary sludge chamber and a distributor; the secondary sedimentation unit includes: a diffuser, a secondary sedimentation area, a secondary sludge chamber, a water distribution chamber, a separation chamber, a water collecting trough, a water outlet chamber and a mud discharge pipe; the tertiary sedimentation unit includes: an inclined tube filling chamber and an inclined tube filling. The inclined tube filling is a hexagonal honeycomb ethylene-propylene copolymer pipe with an adjustable horizontal inclination angle of 30 to 60°.

[0020] Furthermore, before filtering, the turbidity value of the aqueous solution of caprolactam monomer is 1.0 to 1.5 NTU; after filtering, the turbidity value of the aqueous solution of caprolactam monomer is 0.1 to 0.5 NTU;

[0021] The filtration treatment adopts a multi-media bed filter, which includes a first filter material layer, a second filter material layer and a third filter material layer from top to bottom. The property parameters of the filter material in the first filter material layer include: PP plastic, height 1000mm, particle size 2.5-3.5mm; the property parameters of the filter material in the second filter material layer include: quartz sand, height 800mm, particle size 0.7-1.25mm; the property parameters of the filter material in the third filter material layer include: gravel, height 200mm, particle size 2.0-3.15mm.

[0022] Furthermore, the operating and regeneration conditions of the caprolactam monomer-containing aqueous solution entering the multi-media bed filter are a temperature of 80-100° C. and a pressure of 3-5 bar.

[0023] Furthermore, the initial impurity content of the caprolactam monomer aqueous solution is: 15-35 ppm silicon element ions, 35-65 ppm total trace metal ions, and a conductivity of 50-130 μs / cm; after ion exchange, the impurity content of the caprolactam monomer aqueous solution is: 0.2-1.5 ppm silicon element ions, 2-5 ppm total trace metal ions, and a conductivity of 1-20 μs / cm.

[0024] Furthermore, the working condition parameters of the anion exchange treatment and the cation exchange treatment include: an operating temperature of 80-100° C. and an operating pressure of 3-5 bar; a regeneration temperature of the anion and cation exchange resins of 80-100° C. and a regeneration pressure of 3-5 bar.

[0025] The above technical solution provided by the embodiment of the present invention has at least the following advantages compared with the prior art:

[0026] The embodiment of the present invention provides a purification method for recycling and pre-treating an aqueous solution containing caprolactam monomers, which removes different types of impurities in the monomer aqueous solution through vertical sedimentation, multi-stage bed filtration and anion and cation exchange resins, thereby recovering the purified monomer aqueous solution. The method is not only used to solve the long-standing problems of blocked pipelines, transportation and storage difficulties, but also effectively solves the problems of high production costs and environmental costs of recycled monomers. Specifically, the salient features of the present invention are:

[0027] (1) Effectively extend the service life of the deep filter and reduce the cleaning frequency. Under the same production capacity conditions, the service life of the filter unit is extended by more than 6 times;

[0028] (2) The exchange rate of anion and cation exchange resins was significantly improved, the exchange rate per cycle was increased by more than 1.5 times, and the resin life was increased by more than 1 times;

[0029] (3) The problems of transportation and storage difficulties caused by the presence of impurities such as oligomers, suspended solids, silicon-containing compounds and trace amounts of metal ions in the monomer recovery solution, as well as high production and environmental costs, are completely solved;

[0030] (4) It avoids the accumulation of oligomers, suspended solids, silicon-containing compounds and trace metal ions during the polymerization process, thereby improving the quality of recycled nylon 6 chips;

[0031] (5) An integrated automatic control system is used, in which the filtration system determines whether to backwash and regenerate based on the online turbidity meter, and the anion and cation exchange system determines whether acid-base regeneration is needed based on the conductivity of the effluent, thereby reducing human error in operation, achieving overall layout, unified management, and fully automated operation, reducing the cost of monomer recovery, improving the recovery rate and quality of monomers, and meeting the requirements of energy conservation, emission reduction, and consumption reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0034] Figure 1 A flow chart of a purification method for recovering and pretreating an aqueous solution containing caprolactam monomer provided by the present invention;

[0035] Figure 2 This is a schematic diagram of a process for recovering and pretreating an aqueous solution containing caprolactam monomer in an embodiment of the present invention;

[0036] Figure 3 It is a schematic diagram of a process for recovering and pretreating an aqueous solution containing caprolactam monomer in the prior art;

[0037] Figure 4 This is a schematic diagram of the actual operation process system for recovering and pretreating the aqueous solution containing caprolactam monomer in an embodiment of the present invention;

[0038] Figure 5 It is a structural schematic diagram of the vertical sedimentation tank in the present invention;

[0039] Figure 4In: 1-slice extraction water, 2-other monomer aqueous solutions, 3-monomer aqueous solution collection tank, 4-flocculant, 5-mixer, 6-vertical sedimentation tank, 7-multi-media bed filter, 8-turbidity detector, 9-cation and cation resin mixed bed, 10-conductivity detector, 11-purification liquid collection tank, 12-purification liquid evaporation and concentration;

[0040] Figure 5 Middle: 13-feed pipe, 14-diverter, 15-primary sedimentation zone, 16-primary sludge chamber, 17-distributor, 18-diffuser, 19-secondary sedimentation zone, 20-secondary sludge chamber, 21-water distribution chamber, 22-inclined tube filling chamber, 23-inclined tube filling, 24-separation chamber, 25-water collecting trough, 26-water outlet chamber, 27-sludge discharge pipe. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0043] The present invention provides a purification method for recovering and pretreating an aqueous solution containing caprolactam monomer, such as Figure 1 As shown, the purification method comprises the following steps:

[0044] Adding a flocculant to the caprolactam monomer aqueous solution to be recovered for physicochemical treatment, followed by sedimentation separation to obtain a first caprolactam monomer aqueous solution;

[0045] filtering the first aqueous caprolactam monomer solution to obtain a second aqueous caprolactam monomer solution;

[0046] The second caprolactam monomer aqueous solution is subjected to anion exchange treatment and then to cation exchange treatment to obtain a purified and recovered caprolactam monomer aqueous solution.

[0047] The embodiment of the present invention provides a purification method for recycling and pre-treating an aqueous solution containing caprolactam monomers, which removes different types of impurities in the monomer aqueous solution through vertical sedimentation, multi-stage bed filtration and anion and cation exchange resins, thereby recovering the purified monomer aqueous solution. The method is not only used to solve the long-standing problems of blocked pipelines, transportation and storage difficulties, but also effectively solves the problems of high production costs and environmental costs of recycled monomers. Specifically, the salient features of the present invention are:

[0048] (1) Effectively extend the service life of the deep filter and reduce the cleaning frequency. Under the same production capacity conditions, the service life of the filter unit is extended by more than 6 times;

[0049] (2) The exchange rate of anion and cation exchange resins was significantly improved, the exchange rate per cycle was increased by more than 1.5 times, and the resin life was increased by more than 1 times;

[0050] (3) The problems of transportation and storage difficulties caused by the presence of impurities such as oligomers, suspended solids, silicon-containing compounds and trace amounts of metal ions in the monomer recovery solution, as well as high production and environmental costs, are completely solved;

[0051] (4) It avoids the accumulation of oligomers, suspended solids, silicon-containing compounds and trace metal ions during the polymerization process, thereby improving the quality of recycled nylon 6 chips;

[0052] (5) An integrated automatic control system is used, in which the filtration system determines whether to backwash and regenerate based on the online turbidity meter, and the anion and cation exchange system determines whether acid-base regeneration is needed based on the conductivity of the effluent, thereby reducing human error in operation, achieving overall layout, unified management, and fully automated operation, reducing the cost of monomer recovery, improving the recovery rate and quality of monomers, and meeting the requirements of energy conservation, emission reduction, and consumption reduction.

[0053] In some specific embodiments, compared to Figure 3 The prior art shown in FIG. 1 is a method for recovering and pretreating an aqueous solution containing caprolactam monomers. The present invention is a method for purifying an aqueous solution containing caprolactam monomers by recovering and pretreating the aqueous solution, such as Figure 2 As shown, the following steps are included:

[0054] a) Collection of monomer aqueous solution: The extraction water produced after the nylon PA6 slices are eluted and the aqueous solution containing caprolactam monomer produced in other processes are collected into the monomer aqueous solution collection tank for storage and mixing. The monomer aqueous solution contains caprolactam, oligomers (dimers, trimers and cyclic oligomers), a small amount of suspended solids, silicon-containing ions and trace metal ions and other impurities. Among them, the initial concentration of caprolactam and oligomers in the monomer aqueous solution after mixing is 6.5-10.5%, of which the weight of oligomers accounts for 6.5-10.5%. 15-25%; b) Removal of some oligomers and a small amount of suspended solid particles: The monomer aqueous solution is transported to a vertical sedimentation tank for multi-stage sedimentation separation with a separation accuracy of more than 1.0 μm. Before entering the sedimentation device, a flocculant (type: cationic polyacrylamide) is added to the monomer aqueous solution through a pipeline mixer for physical and chemical treatment to remove some oligomers and a small amount of suspended solid particles and other impurities, wherein the mass of the added flocculant accounts for 0.01-0.05% of the monomer aqueous solution; c) A small amount of oligomers and all suspended solid particles are removed. Removal of floating solid particles: The monomer aqueous solution obtained in step b is passed into a multi-media bed filter for deep filtration to remove a small amount of oligomers, all suspended solid particles and other impurities. The effluent from the multi-media bed filter is judged by an online turbidity meter to determine whether it is qualified and in backwash condition; wherein, the initial turbidity value of the monomer aqueous solution is 1.0-1.5 NTU, and the turbidity value after filtration is 0.1-0.5 NTU; d) Removal of a very small amount of soluble oligomers, silicon-containing ions and trace metal ions: The monomer aqueous solution obtained in step c is first passed into a multi-media bed filter for deep filtration to remove a small amount of oligomers, all suspended solid particles and other impurities. Anion exchange is performed, followed by cation exchange, to remove a very small amount of impurities such as oligomers, silicon-containing ions and trace metal ions, and the effluent from the anion and cation resin exchange bed is judged by an online conductivity meter to be qualified and in regeneration condition; wherein the anion exchange resin is a macroporous anion resin, model D201, and the cation exchange resin is a macroporous cation resin, model D001; e) Collection and storage of purified solution: the monomer aqueous solution obtained in step d is stored in a monomer purified liquid collection tank to ensure continuous and smooth operation of subsequent process.

[0055] In some specific embodiments, Figure 5 As shown, Figure 5In step b, a vertical sedimentation tank is provided with three-stage sedimentation units; wherein the first-stage unit comprises: a feed pipe, a flow divider, a first-stage sedimentation zone, a first-stage mud chamber and a distributor; the second-stage unit comprises: a diffuser, a second-stage sedimentation zone, a second-stage mud chamber, a water distribution chamber, a separation chamber, a water collection tank, a water outlet chamber and a mud discharge pipe; the third-stage unit comprises: an inclined tube packing chamber and an inclined tube packing, wherein the inclined tube packing is a hexagonal honeycomb ethylene-propylene copolymer pipe, and the horizontal inclination angle is adjustable from 30 to 60 degrees. Process description: The monomer-containing aqueous solution containing flocculants enters the primary sedimentation unit and enters the primary sedimentation area 15 through the feed pipe 13 and the diverter 14 at the bottom thereof. The suspended matter settles to the primary sludge chamber 16. The clear liquid flows upward from the bottom through the uniform distributor 17 and enters the secondary sedimentation unit, and then enters the secondary sedimentation area 19 through the diffuser 18. The suspended matter settles from the secondary sedimentation area to the secondary sludge chamber 20. The clear liquid enters the tertiary sedimentation unit through the water distribution chamber 21. After the flocculated suspended solid particles are separated from the solution through the inclined tube filler 23 in the tertiary sedimentation chamber 22, the solid particles enter the secondary sedimentation area downward through the inclined tube, and the clear liquid enters the separation chamber 24 of the secondary sedimentation unit upward, and then overflows to the water outlet chamber 26 through the sump 25. The clear liquid flows out of the sedimentation device and enters the multi-media filtration unit through the pump. The solid residues in the primary and secondary sludge chambers are discharged through the mud discharge pipe 27.

[0056] In some specific embodiments, in step b, the flocculant is first prepared into an aqueous solution with a concentration of 0.1-0.5%, and then added to the monomer aqueous solution.

[0057] In some specific embodiments, in step c, the operating and regeneration conditions of the monomer aqueous solution entering the multimedia bed filter are a temperature of 80-100° C. and a pressure of 3-5 bar.

[0058] In some specific embodiments, in step c, the multi-media bed filter comprises three types of filter materials: upper, middle and lower; wherein, the upper filter material: PP plastic, height 1000mm, particle size 2.5-3.5mm; the middle filter material: quartz sand, height 800mm, particle size 0.7-1.25mm; the lower filter material: gravel, height 200mm, particle size 2.0-3.15mm.

[0059] In some specific embodiments, in step d, the initial impurity content of the monomer aqueous solution is: 15-35 ppm silicon element ions, 35-65 ppm total trace metal ions, and a conductivity of 50-130 μs / cm; after ion exchange, the impurity content of the caprolactam monomer aqueous solution is: 0.2-1.5 ppm silicon element ions, 2-5 ppm total trace metal ions, and a conductivity of 1-20 μs / cm.

[0060] In some specific embodiments, in step d, the operating temperature of the anion and cation exchange resin is 80-100° C. and the operating pressure is 3-5 bar; the regeneration temperature of the anion and cation exchange resin is 80-100° C. and the regeneration pressure is 3-5 bar.

[0061] In some specific embodiments, in the embodiments of the present invention, the aqueous solution containing caprolactam monomer is recovered and pretreated. The actual operation process system is as follows: Figure 4 As shown; Figure 4 In the pretreatment process, 1. The extraction water produced by the nylon PA6 slices after water elution and the aqueous solution containing caprolactam monomer produced in other processes are collected in the monomer aqueous solution collection tank for storage and mixing. The initial concentration of the monomer aqueous solution is 6.5-10.5%, of which the weight proportion of the polymer is 15-25%; 2. The monomer aqueous solution and the flocculant with a mass ratio of 0.01-0.05% of the monomer aqueous solution enter the vertical sedimentation tank through the mixer to separate impurities such as suspended solid particles with a particle size of more than 1.0μm; 3. After passing through the vertical sedimentation tank, the monomer aqueous solution is pumped to the multi-media bed filter for Deep filtration to remove a small amount of polymers and all suspended solid particles and other impurities, and use an online turbidity meter to determine whether the effluent from the multi-media bed filter is qualified and meets the backwash conditions (turbidity value ≥ 0.5NTU); 4. After passing through the multi-media bed filter, the monomer aqueous solution undergoes anion exchange and cation exchange to remove a very small amount of soluble polymers, silicon-containing ions and trace metal ions and other impurities, and uses an online conductivity meter to determine whether the effluent from the cation and anion resin exchange bed is qualified and meets the regeneration conditions (conductivity ≥ 20μs / cm); 5. After the previous filtration and purification, the monomer aqueous solution is temporarily stored in the purification liquid collection tank to facilitate the continuous and smooth operation of the subsequent evaporation and concentration process.

[0062] The present invention will be further described below in conjunction with specific examples. It should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples that do not specify specific conditions are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0063] Example 1

[0064] This example provides a purification method for recovering and pretreating an aqueous solution containing caprolactam monomer, the purification method comprising the following steps:

[0065] (1) Monomer solution collection

[0066] The extracted water produced by washing nylon 6 slices with circulating water during the production of bright nylon 6 slices, as well as the dilute monomer solution produced in other production processes, are collected together in the monomer solution collection tank for storage and mixing. After the mixing is stabilized, the concentration of monomers and oligomers is about 6.9%, of which the proportion of oligomers is 17%.

[0067] (2) Vertical settlement

[0068] The monomer aqueous solution obtained in step (1) is transported to a vertical sedimentation tank for multi-stage sedimentation separation. Before entering the sedimentation tank, a flocculant (preferably a cationic polyacrylamide flocculant aqueous solution with a concentration of 2% and a weight ratio of 0.03 to the recovered caprolactam monomer aqueous solution) is added through a pipeline mixer for physical and chemical treatment to remove impurities from part of the oligomers and a small amount of suspended solid particles. The solid particle size is 1.0 μm and the solid particle removal rate is above 96%;

[0069] (3) Multi-media bed filtration

[0070] The monomer aqueous solution obtained in step (2) is transported to a multi-media bed filter for deep filtration to remove a small amount of oligomers, all suspended solid particles and other impurities, and the effluent from the multi-media bed filter is judged by an online turbidity meter to be qualified for backwashing and regeneration; wherein, preferably, the initial turbidity value of the monomer aqueous solution is 1.3NTU, the turbidity value after filtration is 0.2NTU, and the condition value of backwashing and regeneration judged by the online turbidity meter is 0.5NTU;

[0071] (4) Anion exchange resin

[0072] The monomer aqueous solution obtained in step (3) is first subjected to anion exchange and then to cation exchange to remove a very small amount of impurities such as oligomers, silicon-containing compounds and trace metal ions, and an online conductivity meter is used to determine whether the effluent from the anion and cation resin exchange bed is qualified and whether acid-base regeneration is performed; wherein the anion exchange resin is a macroporous anion resin, the model of which is D201, and the cation exchange resin is a macroporous cation resin, the model of which is D001. The initial impurity content of the monomer aqueous solution is 22 ppm of silicon ions, 44 ppm of total trace metal ions, and 66 μs / cm of conductivity. After ion exchange, the impurity content of the monomer aqueous solution is: 0.7 ppm of silicon ions, 2 ppm of total trace metal ions, and 2 μs / cm of conductivity. The condition value of acid-base regeneration determined by the online conductivity meter is 20 μs / cm;

[0073] (5) Collection of purified solution

[0074] The monomer aqueous solution obtained in step (4) is collected and stored in a purified liquid collection tank to facilitate the continuous and smooth operation of the subsequent evaporation and concentration process.

[0075] According to the test, in this embodiment, after the monomer solution is purified, the total concentration of monomers and oligomers is about 6.0%, of which the oligomers account for 4.0%.

[0076] Example 2

[0077] This example provides a purification method for recovering and pretreating an aqueous solution containing caprolactam monomer, the purification method comprising the following steps:

[0078] (1) Monomer solution collection

[0079] The extracted water produced by washing nylon 6 slices with circulating water during the preparation process of semi-dull nylon 6 slices, as well as the dilute monomer solution produced in other production processes, are collected together in a monomer solution collection tank for storage and mixing. After the mixing is stabilized, the concentration of monomers and oligomers is about 9.4%, of which the oligomers account for 21%;

[0080] (2) Vertical settlement

[0081] The monomer aqueous solution obtained in step (1) is transported to a vertical sedimentation tank for multi-stage sedimentation separation. Before entering the sedimentation tank, a flocculant (preferably a cationic polyacrylamide flocculant aqueous solution with a concentration of 2% and a weight ratio of 0.03 to the recovered caprolactam monomer aqueous solution) is added through a pipeline mixer for physical and chemical treatment to remove impurities from part of the oligomers and a small amount of suspended solid particles, with a solid particle size of more than 1.0 μm and a solid particle removal rate of more than 93%;

[0082] (3) Multi-media bed filtration

[0083] The monomer aqueous solution obtained in step (2) is transported to a multi-media bed filter for deep filtration to remove a small amount of oligomers, all suspended solid particles and other impurities, and the effluent from the multi-media bed filter is judged by an online turbidity meter to be qualified for backwashing and regeneration; wherein, preferably, the initial turbidity value of the monomer aqueous solution is 1.3NTU, the turbidity value after filtration is 0.2NTU, and the condition value of backwashing and regeneration judged by the online turbidity meter is 0.5NTU;

[0084] (4) Anion exchange resin

[0085] The monomer aqueous solution obtained in step (3) is first subjected to anion exchange and then to cation exchange to remove a very small amount of impurities such as oligomers, silicon-containing compounds and trace metal ions, and an online conductivity meter is used to determine whether the effluent from the anion and cation resin exchange bed is qualified and whether acid-base regeneration is performed; wherein the anion exchange resin is a macroporous anion resin, the model of which is D201, and the cation exchange resin is a macroporous cation resin, the model of which is D001. The initial impurity content of the monomer aqueous solution is 26 ppm of silicon ions, 51 ppm of total trace metal ions, and 82 μs / cm of conductivity. After ion exchange, the impurity content of the monomer aqueous solution is: 0.9 ppm of silicon ions, 3 ppm of total trace metal ions, and 4 μs / cm of conductivity. The condition value of acid-base regeneration determined by the online conductivity meter is 20 μs / cm.

[0086] (5) Collection of purified solution

[0087] The monomer aqueous solution obtained in step (4) is collected and stored in a purified liquid collection tank to facilitate the continuous and smooth operation of the subsequent evaporation and concentration process.

[0088] According to the test, in this embodiment, after the monomer solution is purified, the total concentration of monomers and oligomers is about 8.0%, of which the oligomers account for 6.8%.

[0089] Example 3

[0090] This example provides a purification method for recovering and pretreating an aqueous solution containing caprolactam monomer, the purification method comprising the following steps:

[0091] (1) Monomer solution collection

[0092] The extracted water produced by washing nylon 6 slices with circulating water during the production of full-dull nylon 6 slices, as well as the dilute monomer solution produced in other production processes, are collected together in the monomer solution collection tank for storage and mixing. After the mixing is stabilized, the concentration of monomers and oligomers is about 10.3%, of which the proportion of oligomers is 24%;

[0093] (2) Vertical settlement

[0094] The monomer aqueous solution obtained in step (1) is transported to a vertical sedimentation tank for multi-stage sedimentation separation. Before entering the sedimentation tank, a flocculant (preferably a cationic polyacrylamide flocculant aqueous solution with a concentration of 2% and a weight ratio of 0.03 to the recovered caprolactam monomer aqueous solution) is added through a pipeline mixer for physical and chemical treatment to remove impurities from part of the oligomers and a small amount of suspended solid particles, with a solid particle size of more than 1.0 μm and a solid particle removal rate of more than 85%;

[0095] (3) Multi-media bed filtration

[0096] The monomer aqueous solution obtained in step (2) is transported to a multi-media bed filter for deep filtration to remove a small amount of oligomers, all suspended solid particles and other impurities, and the effluent from the multi-media bed filter is judged by an online turbidity meter to be qualified for backwashing and regeneration; wherein, preferably, the initial turbidity value of the monomer aqueous solution is 1.3NTU, the turbidity value after filtration is 0.2NTU, and the condition value of backwashing and regeneration judged by the online turbidity meter is 0.5NTU;

[0097] (4) Anion exchange resin

[0098] The monomer aqueous solution obtained in step (3) is first subjected to anion exchange and then to cation exchange to remove a very small amount of impurities such as oligomers, silicon-containing compounds and trace metal ions, and an online conductivity meter is used to determine whether the effluent from the anion and cation resin exchange bed is qualified and whether acid-base regeneration is performed; wherein the anion exchange resin is a macroporous anion resin, the model of which is D201, and the cation exchange resin is a macroporous cation resin, the model of which is D001. The initial impurity content of the monomer aqueous solution is 32 ppm of silicon ions, 60 ppm of total trace metal ions, and a conductivity of 122 μs / cm. After the ion exchange, the impurity content of the monomer aqueous solution is: 1.2 ppm of silicon ions, 4 ppm of total trace metal ions, and a conductivity of 7 μs / cm. The condition value of acid-base regeneration determined by the online conductivity meter is 20 μs / cm.

[0099] (5) Collection of purified solution

[0100] The monomer aqueous solution obtained in step (4) is collected and stored in a purified liquid collection tank to facilitate the continuous and smooth operation of the subsequent evaporation and concentration process.

[0101] According to the test, in this embodiment, after the monomer solution is purified, the total concentration of monomers and oligomers is about 8.7%, of which the oligomers account for 9.3%.

[0102] In summary, the embodiments of the present invention provide a purification method for recovering and pretreating an aqueous solution containing caprolactam monomer, which removes different types of impurities in the monomer aqueous solution through vertical sedimentation, multi-stage bed filtration and anion and cation exchange resins, thereby recovering the purified monomer aqueous solution. The method is not only used to solve the long-standing problems of pipeline blockage, transportation and storage difficulties, but also effectively solves the problems of high production cost and environmental cost of recovered monomers.

[0103] Various embodiments of the present invention may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present invention; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0104] The foregoing is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A purification method for recovering and pretreating an aqueous solution containing caprolactam monomer, characterized in that: The purification method comprises the following steps: Adding a flocculant to the caprolactam monomer aqueous solution to be recovered for physicochemical treatment, followed by sedimentation separation to obtain a first caprolactam monomer aqueous solution; filtering the first aqueous caprolactam monomer solution to obtain a second aqueous caprolactam monomer solution; The second caprolactam monomer aqueous solution is subjected to anion exchange treatment and then to cation exchange treatment to obtain a purified and recovered caprolactam monomer aqueous solution.

2. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 1, characterized in that: The purification method further comprises the following steps: The extraction water generated after the nylon PA6 slices are washed with water and the aqueous solution containing caprolactam monomer generated in other processes are collected in a monomer aqueous solution collection tank to obtain the caprolactam monomer aqueous solution to be recovered; the other processes include filament pelletizing, slice drying and cation resin purification.

3. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 1, characterized in that: The main components of the caprolactam monomer aqueous solution to be recovered include: caprolactam, oligomers, suspended solids, silicon-containing ions and trace metal ions; wherein the oligomers include dimers, trimers and cyclic oligomers; In terms of weight percentage, the total initial concentration of the caprolactam monomer and the oligomer in the aqueous caprolactam monomer solution to be recovered is 6.5% to 10.5%, wherein the weight percentage of the oligomer is 15 to 25%.

4. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 1, characterized in that: The flocculant is a cationic polyacrylamide flocculant aqueous solution, the weight percentage concentration of the cationic polyacrylamide flocculant aqueous solution is 1-5%; the added weight of the cationic polyacrylamide flocculant aqueous solution accounts for 0.01-0.05% of the weight of the caprolactam monomer aqueous solution to be recovered.

5. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 1, characterized in that: The sedimentation separation adopts a vertical sedimentation tank to perform multi-stage sedimentation separation, and the separation accuracy is above 1.0 μm; The vertical sedimentation tank includes a primary sedimentation unit, a secondary sedimentation unit and a tertiary sedimentation unit. The primary sedimentation unit includes: a feed pipe, a diverter, a primary sedimentation area, a primary sludge chamber and a distributor; the secondary sedimentation unit includes: a diffuser, a secondary sedimentation area, a secondary sludge chamber, a water distribution chamber, a separation chamber, a water collecting trough, a water outlet chamber and a mud discharge pipe; the tertiary sedimentation unit includes: an inclined tube filling chamber and an inclined tube filling. The inclined tube filling is a hexagonal honeycomb ethylene-propylene copolymer pipe with an adjustable horizontal inclination angle of 30 to 60°.

6. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 1, characterized in that: Before filtering, the turbidity value of the aqueous solution of caprolactam monomer is 1.0-1.5 NTU; after filtering, the turbidity value of the aqueous solution of caprolactam monomer is 0.1-0.5 NTU; The filtration treatment adopts a multi-media bed filter, which includes a first filter material layer, a second filter material layer and a third filter material layer from top to bottom. The property parameters of the filter material in the first filter material layer include: PP plastic, height 1000mm, particle size 2.5-3.5mm; the property parameters of the filter material in the second filter material layer include: quartz sand, height 800mm, particle size 0.7-1.25mm; the property parameters of the filter material in the third filter material layer include: gravel, height 200mm, particle size 2.0-3.15mm.

7. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 6, characterized in that: The operating and regeneration conditions for the aqueous solution containing caprolactam monomer entering the multi-media bed filter are a temperature of 80-100° C. and a pressure of 3-5 bar.

8. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 1, characterized in that: The initial impurity content of the caprolactam monomer aqueous solution is: 15-35 ppm silicon element ions, 35-65 ppm total trace metal ions, and a conductivity of 50-130 μs / cm; after ion exchange, the impurity content of the caprolactam monomer aqueous solution is: 0.2-1.5 ppm silicon element ions, 2-5 ppm total trace metal ions, and a conductivity of 1-20 μs / cm.

9. The purification method for recovering and pretreating an aqueous solution containing caprolactam monomer according to claim 1, characterized in that: The working condition parameters of the anion exchange treatment and the cation exchange treatment include: an operating temperature of 80-100° C. and an operating pressure of 3-5 bar; a regeneration temperature of the anion and cation exchange resins of 80-100° C. and a regeneration pressure of 3-5 bar.

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