Integrated and improved process for the production of acrylonitrile fibers

Abstract

A method of integrated and improved production of acrylonitrile fibers is described, in particular from the comonomer start until the spinning step to obtain the final fiber.

Description

[0001] This invention relates to an integrated and improved method for producing acrylonitrile fibers, specifically a method from the comonomer to the spinning step to obtain the final fiber.

[0002] More specifically, this invention belongs to the field of acrylonitrile fiber production, which provides a method for preparing polymers from acrylonitrile or copolymers mainly composed of acrylonitrile (90-99 wt% of the total polymer weight) and one or more other comonomers in constant amounts ranging from 1-10 wt% of the total polymer weight.

[0003] Preferred comonomers are neutral vinyl molecules such as methyl acrylate, methyl methacrylate, vinyl acetate, acrylamide and the like, or molecules with one or more acid groups such as acrylic acid, itaconic acid, sulfonated styrene and the like, or other comonomers that can impart different chemical and physical properties to the obtained polymer, such as vinylpyridine that enables the material to be dyed with anionic dyes.

[0004] The polymers and copolymers thus prepared are then spun to produce fibers collected in bundles, which are suitable for later processing into products for textile and technical applications.

[0005] Specific acrylic fiber precursors are the fiber "precursors" for carbon fibers; they are high molecular weight copolymers of acrylonitrile and one or more comonomers selected from the above, typically in amounts ranging from 1 to 5 wt%. These polyacrylonitrile-based fiber "precursors" are then subjected to appropriate heat treatment to obtain carbon fibers.

[0006] There are various industrial methods for preparing acrylonitrile fibers, which employ different polymerization and spinning methods.

[0007] The existing technologies can be divided and summarized as follows:

[0008] A. Intermittent method (two steps) .

[0009] In a two-step batch process, the polymer is typically produced in an aqueous suspension, separated, and, in the case of carbon fibers, subsequently dissolved in a suitable solvent for spinning and conversion into fibers or fiber precursors. The most commonly used solvents for preparing spinning solutions are: dimethylacetamide (DMAc), dimethylformamide (DMF), an aqueous solution of sodium thiocyanate (NaSCN), and finally, a mixture of dimethyl sulfoxide (DMSO) with varying amounts of water, as recently described in patent EP2894243B1.

[0010] B. Continuous method (one-step) .

[0011] On the other hand, in continuous processes, the polymerization reaction takes place in a solvent, and the resulting solution is used directly for spinning without intermediate polymer separation steps. The most commonly used solvents in these methods are: dimethylformamide (DMF), dimethyl sulfoxide (DMSO), aqueous zinc chloride (ZnCl2), and aqueous sodium thiocyanate (NaSCN).

[0012] From a management perspective, the batch process has significant advantages. The polymerization and spinning steps are actually independent. Before the spinning step, trace impurities and unreacted monomers can be easily separated from the polymer through washing and filtration.

[0013] Therefore, this type of method has been more widely used in the industrial practice of producing acrylonitrile fibers and accounts for a considerable share in the production process of carbon fiber precursors.

[0014] Industrially, the batch process involves drying the polymer obtained from water suspension polymerization using a belt or fluidized bed dryer. The polymer, in powder form, is then transferred to a silo and stored until use. To prepare the spinning solution (spinning slurry), the powdered polymer is thoroughly mixed with a solvent to obtain a solution free of lumps and gels. After filtration, the spinning slurry is finally fed into the spinning machine. Figure 1 The above method according to the prior art is illustrated.

[0015] However, overall, the fiber preparation process has various shortcomings, and there is room for improvement in optimizing the performance of the resulting products and production costs.

[0016] The first drawback of the method described in the prior art is the consumption of large amounts of water during the polymerization, polymer washing and drying steps, and during the spinning step in the case of producing fiber precursors. In fact, in the latter case, the final washing step of the fiber is carried out with softened water to completely remove residual solvent. The wash water is typically sent to wastewater treatment plants because it is uneconomical to send it to solvent recovery plants due to its very low solvent content (below 0.5%).

[0017] The second drawback, which generally involves the polymerization step, is that the choice of comonomer is mainly limited to liquid or water-soluble comonomers, while solid and insoluble or low-water-soluble comonomers, even reactive ones, are difficult to feed.

[0018] Furthermore, the separation, drying, and conveying steps of powdered polymers, as well as the preparation of spinning solutions, involve complex and expensive equipment, requiring special attention to safety due to the presence of fine, potentially explosive powders in the process. In addition, these process steps are also at a significant energy disadvantage from a practical standpoint due to the presence of drying units and powdered polymer conveying units typically operated with hot air or nitrogen.

[0019] EP3375915 recently described a simplified method that uses aqueous suspension polymerization followed by spinning using DMSO containing reduced water as a solvent. This method eliminates the polymer drying step, the transport of the polymer to a storage silo, and subsequent steps of preparing the spinning solution from powder.

[0020] Another limitation of the traditional two-step process, particularly in the production of carbon fiber precursors, lies in the difficulty of adding ammonia or primary or secondary amines, additives known to significantly contribute to improving precursor production processes and obtaining high-performance carbon fibers. In conventional techniques, this difficulty is overcome by treating the spinning slurry with gaseous ammonia or by adding an additional step in a spinning machine that allows the addition of amines or ammonia under stretching conditions. This step is followed by a relaxation stage, and then a new stretching stage at a higher temperature.

[0021] Italian patent application No. 102019000014880 recently described the addition of ammonia during the preparation of spinning slurry; however, this solution is only feasible if the solvent used is DMSO containing a small amount of water.

[0022] Therefore, the object of the present invention is to provide a method for preparing acrylonitrile fibers that overcomes the above-mentioned limitations and disadvantages of the known technology and has significant advantages in terms of product quality and production cost. Invention Details

[0023] This invention relates to a method comprising the following steps:

[0024] i) The comonomer is polymerized in an aqueous suspension in the presence of a solvent, wherein the solvent accounts for 3-25% by weight of the total weight of the aqueous suspension, preferably 5-10%, wherein the solvent is the same as the solvent used in steps ii)-vi), and wherein the water constituting the aqueous suspension includes recycled water from the final washing step of the spinning step, in an amount ranging from 10-40 wt% by weight of the total weight of water in the aqueous suspension, preferably 20-30 wt%. The polymerization reaction is catalyzed by a redox pair of ammonium persulfate / ammonium bisulfite in the presence of a catalytic amount of ferric sulfate. Unreacted monomers are then removed, and the aqueous suspension is filtered and washed to obtain a filter cake comprising polymer and water containing trace amounts of solvent, in a weight ratio between 40 / 60 and 60 / 40, preferably equal to or greater than 1:1.

[0025] ii) Disperse the first filter cake obtained in step i) in a solvent / water mixture or pure solvent at a weight ratio of 2-10 times the weight of the filter cake, preferably with stirring at a temperature below or equal to 7°C for 5-15 minutes.

[0026] iii) The solid / liquid phase of the polymer dispersion is separated by filtration or centrifugation to obtain a second filter cake comprising a polymer and a solvent / water mixture, wherein the weight ratio of solvent to water is 60 / 40 to 85 / 15;

[0027] iv) The second filter cake obtained in step iii) is dispersed again in the same solvent or water / solvent mixture as in step ii), wherein the weight ratio of the solvent or water / solvent mixture is 2-10 times the weight of the polymer in the cake, and carried out at room temperature or below room temperature for 5-20 minutes to obtain a uniform dispersion or "slurry", wherein the weight ratio of the solvent / polymer ranges from 90 / 10 to 70 / 30.

[0028] v) Heat the uniform dispersion or slurry obtained in step iv) until the polymer is completely dissolved, preferably by passing the slurry through a heat exchanger to obtain a uniform spinning solution.

[0029] vi) Feed the uniform spinning solution obtained at the end of step v) into the spinning step.

[0030] The spinning step is carried out using a wet spinning process or a dry-jet wet spinning process. After a coagulation stage in a coagulation bath composed of a mixture of water and solvent, the resulting fiber bundle is continuously stretched and washed to a length approximately 8-12 times, preferably 9-10 times, of the initial length. A final washing step with water is then performed to remove any remaining trace amounts of solvent. The washing water is then recycled back to step i).

[0031] The spinning solution obtained by the method according to the invention is advantageously free of gels and undissolved residues, and therefore can be directly fed into the spinning production line.

[0032] In fact, the present invention can obtain acrylonitrile homopolymer or copolymer solutions that are gel-free and do not form insoluble aggregates, improving upon the advantages associated with aqueous suspension polymerization, but eliminating the dangerous and costly steps of transporting dried polymer, powdered polymer to storage silos, and subsequently resolving it in a solvent for spinning. Therefore, the method according to the present invention can integrate the polymerization and spinning steps in a simplified and economical manner, but the possibility of sending the homogeneous spinning solution to an intermediate storage tank, if advantageous, cannot be excluded.

[0033] In aqueous suspension polymerization methods using the same solvent as that used in subsequent spinning slurry preparation and spinning steps ii)-vi), the polymerization kinetics are improved due to the increased solubility of monomers in the reaction medium. This reduces the time required to achieve the same conversion rate by approximately 10-15% compared to existing methods that use only water.

[0034] Furthermore, step iv) of mixing the pure solvent with the polymer is particularly easy to implement, especially when the solvent is DMSO, because the powder has been soaked in a solvent containing trace amounts of water, without the dispersion difficulties described in U.S. Patent 4,403,055.

[0035] As disclosed in US 9296889 B2, the close absorption and swelling of wet powder with solvent prevents the formation of agglomerates that are difficult to disperse and dissolve, while optimizing the formation of a fine and uniform suspension.

[0036] In this specification, the term polymer generally refers to homopolymers obtained from acrylonitrile and copolymers obtained from acrylonitrile with one or more other comonomers.

[0037] Specifically, the polymer is a high molecular weight polymer with a molecular weight range of 80,000-200,000 Da.

[0038] Polymerization step i) is carried out in an aqueous suspension using a redox catalyst such as ammonium persulfate / ammonium bisulfite pair, in the presence of ferric sulfate in a catalytic amount that acts as a "promoter" responsible for polymerizing itself to form free radical species, and in the presence of a solvent in a content varying from 3 to 25 wt%, preferably 5 to 10 wt%, of the total weight of the aqueous suspension, wherein the solvent is the same as that used in the preparation of the spinning slurry and in spinning steps ii)-vi).

[0039] Preferred solvents are dimethylformamide (DMF), dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO).

[0040] Any possible solid comonomer (e.g., itaconic acid, acrylic acid, etc.) can also be conveniently fed into the polymerization reactor in solution form in the solvent.

[0041] The water constituting the aqueous suspension includes recirculated water from the final washing stage of the spinning production line, in an amount ranging from 10-40 wt%, preferably 20-30 wt%, of the total water weight in the aqueous suspension. This water from the final washing step of the spinning process contains a small amount (<0.5 wt%) of the same solvent, which is particularly convenient for use in the polymerization reaction and reduces the total consumption of softened water.

[0042] The polymerization step is terminated by adding an iron chelating agent such as EDTA in the form of sodium or ammonium salts. The aqueous suspension leaving the polymerization reactor then undergoes subsequent steps of removing unreacted monomers, filtration, and washing the aqueous suspension.

[0043] More specifically, the aqueous suspension leaving the reactor is fed into a suitable stripping tower to separate excess acrylonitrile and unreacted volatile comonomers from the aqueous suspension, thereby recycling the acrylonitrile and unreacted comonomers to the polymerization step.

[0044] The resulting aqueous suspension, comprising the polymer, residual salts of the catalytic system, trace amounts of solvent, and reaction byproducts, is fed into a rotary vacuum filter using known techniques to separate the liquid phase from the solid phase, with the solid forming a filter cake on the filter surface. This filter cake is washed with warm water to remove any inorganic salts present. At the end of the washing process, the resulting filter cake comprises the polymer and water containing trace amounts of solvent in a weight ratio ranging from 40 / 60 to 60 / 40, preferably from 45 / 55 to 55 / 45, and even more preferably equal to or greater than 1:1.

[0045] In step ii) of the method according to the invention, a cold, i.e., typically below or equal to 7°C, pure solvent or water / solvent mixture is used. This temperature may also be below 0°C if the freezing point of the solvent allows.

[0046] The amount of water in the solvent / water mixture in step ii) can vary between 0 and 20 wt% of the total weight of the mixture.

[0047] The solvents applicable in each step of the method according to the invention, namely in steps ii) and iv), are common solvents used to form polymer solutions for acrylonitrile fiber spinning, such as dimethylacetamide (DMAc), dimethylformamide (DMF), or dimethyl sulfoxide (DMSO).

[0048] The conditions for obtaining the dispersion in step ii) are that the polymer particles must not dissolve or swell. Therefore, the water concentration in the solvent / water mixture, the operating temperature for obtaining the dispersion, and the residence time in the stirred tank where the dispersion is performed must be properly defined.

[0049] In the dispersion operation of step ii), the filter cake is dispersed in a solvent / water or pure solvent mixture in an amount ranging from 2 to 10 times the weight of the filter cake.

[0050] Under these conditions, i.e. at low temperatures, using a specific solvent, in the case of a solvent / water mixture, providing a precise amount of water, the dispersion medium in step ii) does not have solvent capacity for the polymer. More specifically, in step ii), the solid particles of the filter cake, as well as the interior of the polymer particles themselves, come into contact with the dispersion medium, are soaked in a solvent or solvent / water mixture, thereby achieving an exchange with the water present in the filter cake fed into step ii) and replacing that water with pure solvent or solvent / water mixture (as already indicated, containing a small percentage of water).

[0051] Therefore, the residence time is sufficient for the water / solvent concentration in the dispersion medium to reach equilibrium, i.e., the time during which the water soaking the polymer obtained in step i) is replaced with pure solvent or a mixture of water and solvent in the dispersion medium of step ii). This condition is typically achieved by keeping the dispersion stirred for 5-15 minutes to obtain a suitable polymer dispersion.

[0052] In the subsequent step iii), the two solid / liquid phases are separated by filtration or centrifugation to form a second polymer filter cake, which is then immersed in an aqueous solution that is rich in solvent but has no ability to dissolve polymers.

[0053] The water content remaining in the second filter cake at the end of filtration / centrifugation in step iii) will be related to the following: the water / solvent ratio used to redisperse the polymer in step ii), the amount of water / solvent used to redisperse the first filter cake, and the final liquid / solid ratio of the second filter cake obtained from step iii) by filtration or centrifugation.

[0054] The liquid phase obtained from filtration or centrifugation in step iii) is usually sent to a solvent recovery system by distillation, or recycled to dispersion step ii after possible concentration correction with pure solvent.

[0055] The second filter cake from step iii), with a known polymer / solvent / water ratio, is transferred to step iv) via a screw or belt conveyor system, where it is added to a solvent in a stirred tank to form a slurry, which is then converted into a spinning slurry via a downstream heat exchanger.

[0056] In step iv), the second filter cake obtained in step iii) is actually dispersed again in the same solvent or water / solvent mixture as in step ii), wherein the weight ratio of the solvent or water / solvent mixture is 2-10 times the weight of the polymer in the cake, and carried out at room temperature or below room temperature for 5-20 minutes to obtain a homogeneous dispersion or "slurry". When using DMSO, the temperature below room temperature also refers to approximately 7°C, and when using solvents such as DMAc and DMF, it refers to -5 / -10°C.

[0057] In step v), the resulting slurry is heated to a temperature of 50-100°C, preferably 60-90°C, under atmospheric pressure via a heat exchanger for 3-60 minutes. This heating step yields a homogeneous spinning solution containing 12-22 wt% polymer, 1-10 wt% water, and 68-85 wt% solvent.

[0058] Figure 2 A specific embodiment of the method of the present invention is illustrated.

[0059] Another advantage of the method according to the invention is determined by the specific amount of water contained in the spinning solution or spinning slurry fed into the spinning step: the percentage of water remaining in the homogeneous solution for producing acrylonitrile fibers obtained according to the method of the invention is practically compatible with acrylonitrile fiber spinning technology, whether based on dry or wet spinning technology or DJWS technology (dry-jet wet spinning or air gap): therefore, it is not necessary to completely remove the water from the solution sent to the spinning step.

[0060] Furthermore, as claimed in US 3932577, the presence of a small percentage of water in the acrylonitrile fiber spinning solution promotes compatibility between the solution and the coagulation bath, resulting in fibers free of voids and cracks. These properties are particularly advantageous for producing precursors for carbon fibers or textile fibers with good luster and a dense structure.

[0061] Another advantage associated with the presence of such a small amount of water in the spinning slurry is that it makes it possible to use water-soluble additives as polymers in the redispersion step ii) and the slurry preparation step iv), and thus to provide special properties to the final fiber.

[0062] Non-limiting examples of additives that can impart specific properties to polymers and thus to the final fibers are ammonia, primary amines, secondary amines, quaternary ammonium salts, salts of metal ions such as copper or silver that can salt polymer ionic terminal groups, water-soluble polymers that alter the rheology of polymer solutions, and so on.

[0063] In particular, for carbon fiber precursors, it should be remembered that adding appropriate amounts of ammonia or amine to the polymer solution, as described in European patent application EP 3783132, further improves fiber extrusion in the coagulation bath, provides denser and vacuolated fibers, and enhances the reaction kinetics of the oxidation / carbonization stage.

[0064] The resulting spinning solution or spinning slurry can be immediately used to feed into a suitable spinning production line or stored in a heating tank.

[0065] As mentioned above, in order to illustrate the method according to the prior art, refer to Figure 1 The apparatus described herein continuously feeds acrylonitrile (21), a possible liquid comonomer (22), an aqueous solution of a catalyst system (23), a possible aqueous solution of a solid comonomer (24), and water (25) into polymerization reactor 1. The polymer from polymerization reactor 1, in slurry form in water, is treated in stripping tower 2 to remove unreacted monomers, and then washed and filtered under vacuum on a rotary filter 3. The powdered polymer is conveyed to storage silo 14 via drying unit 12, typically operated with hot air or nitrogen, and subsequently via pipeline 13, the polymer is fed to mixer element 15 via screw or other conveying means, while fresh solvent is delivered via pipeline 16 from storage tank.

[0066] In mixer 15, the powdered polymer is dispersed in a solvent, and the resulting polymer slurry is conveyed to storage tank 17 and converted into a spinning solution via exchanger 18. The solution is then conveyed to a set of filter presses 19, where selective filter cloths of 40 μm–5 μm are used to remove any possible particles, and then through pipeline 20 to the spinning line or storage tank. Figure 1 (Not shown in the image).

[0067] To illustrate an embodiment of the method according to the present invention, reference is made below. Figure 2 The apparatus diagram shown indicates that the method is preferably performed continuously.

[0068] Acrylonitrile (21), possible liquid comonomers (22), an aqueous solution of the catalyst system (23), a possible solid comonomer solution (24) in the same solvent used for spinning, a solvent (25), and water (26) are continuously fed into polymerization reactor 1, the water (26) also including the required amount of recirculated water from the final washing section of the spinning production line.

[0069] The polymer from polymerization reactor 1, in the form of an aqueous slurry, is treated in stripping tower 2 to remove unreacted monomers. It is then washed and filtered under vacuum on rotary filter 3 to obtain a filter cake composed of polymer and water, transitioning from step i) to step ii) according to the method of the invention. The filter cake from rotary filter 3 is then redispersed in stirred tank 4, where a solvent / water mixture, possibly containing ammonia, or pure solvent is fed at low temperature via line 27. In stirred tank 4, the suspension is maintained at or below 7°C. The resulting suspension is held under stirring for several minutes and then sent to a second rotary filter or centrifuge 5, implementing step iii) according to the method of the invention. The material immersed in water / solvent is fed into stirred tank 17 via turbine or other conveying device 6, and fresh solvent from a storage tank is also fed to this tank via line 20. The entire mixture is then converted into a spinning solution via exchanger 18.

[0070] The solution is then fed to a set of filter presses 19, with 40μm-5μm selective filter cloth to remove any possible particles, and then through pipeline 20 to the spinning pipeline or spinning slurry storage tank. Figure 2 (Not shown in the image).

[0071] The spinning production line used can be a wet spinning type, in which the spinneret is immersed in a coagulation bath consisting of a mixture of water and solvent. After coagulation, the fiber bundle is continuously stretched and washed according to known techniques to produce filaments, which are then collected in a spool or box and sent to a carbonization production line to produce carbon fibers. The solvent used in the spinning step described here is the same as the solvent used in steps ii)-vi).

[0072] Alternatively, the spinning production line used can be a dry-jet wet-spinning type (air gap spinning), where the spinneret is kept in the air at a short distance from the surface of a coagulation bath consisting of a mixture of water and solvent. After coagulation, the fiber bundle is continuously stretched and washed according to known techniques to produce filaments, which are then collected in bobbins or boxes and sent to a carbonization production line to produce carbon fibers. The solvent used in the spinning step described here is the same as the solvent used in steps ii)-vi).

[0073] In both of these spinning technologies, softened water is used in the final washing step of the fiber to remove trace amounts of solvent. This water, containing trace amounts of solvent, is not sent to a solvent recovery unit (which is uneconomical and inconvenient) or a wastewater treatment unit (wastewater), but is instead returned to polymerization reactor 1. This helps reduce total water consumption and saves energy. Example

[0074] As non-limiting embodiments of the present invention, some examples of implementation schemes according to the method of the present invention are provided below.

[0075] Based on Comparative Example 1 of the prior art, refer to Figure 1

[0076] 100 kg / h of acrylonitrile, 1 kg / h of methyl acrylate, 2 kg / h of itaconic acid in a 5 wt% aqueous solution, 0.4 kg / h of ammonium persulfate in an aqueous solution, 0.5 kg / h of ammonium bisulfite in an aqueous solution, 2 g / h of ferric sulfate in an aqueous solution, and 250 kg / h of water, along with sulfuric acid sufficient to maintain the reaction pH in the range of 2.0-3.5, were continuously added to an aluminum reactor equipped with a stirrer and an overflow pipe at 62°C. The feed was introduced at a rate allowing a 90-minute residence time. After 90 minutes, the reaction was terminated by adding an aqueous EDTA solution through the overflow pipe, and the slurry was sent to a stripping tower to remove unreacted acrylonitrile and methyl acrylate, resulting in a polymer-in-water slurry at the bottom. The conversion of acrylonitrile to copolymer was 78% of the reactor feed weight. Figure 1 As shown, the polymer is filtered, washed, and dried, and the resulting powder is stored in silo 14. Subsequently, it is processed... Figure 1 The static mixer 15 and heat exchanger 18 shown dissolve this polymer in DMAc at -10°C. The resulting solution is filtered through a filter press with a series of selective filter cloths gradually decreasing from 40 μm to 5 μm and then fed into a wet spinning production line with a 24,000-hole spinneret. At the end of the stretching and washing section with recycled water from the solvent recovery unit, a final washing step is performed using softened water, which is then sent to wastewater treatment.

[0077] At the end of the spinning process, a 24K precursor spool with the following properties is obtained:

[0078] • Fineness: 1.25 dtex;

[0079] • Strength: 61.1 cN / tex;

[0080] • Elongation: 15.2%

[0081] Suitable for the production of carbon fiber.

[0082] Example 2, Reference Figure 2

[0083] 100 kg / h of acrylonitrile, 1 kg / h of methyl acrylate, 2 kg / h of itaconic acid dissolved in DMAc, 0.4 kg / h of ammonium persulfate dissolved in water, 0.5 kg / h of ammonium bisulfite dissolved in water, 2 g / h of ferric sulfate dissolved in water, 200 kg / h of water, along with sulfuric acid sufficient to maintain the reaction pH in the range of 2.0-3.5, 40 kg / h of water from the final washing step of the spinning section, and 15 kg / h of DMAc, were continuously added at 62°C to an aluminum reactor equipped with a stirrer and an overflow pipe. The feed was introduced at a rate allowing a 90-minute residence time. After 90 minutes, the reaction was terminated by adding an aqueous EDTA solution through the overflow pipe, and the slurry was sent to a stripping tower to remove unreacted acrylonitrile and methyl acrylate, resulting in a polymer-in-water slurry at the bottom. The conversion of acrylonitrile to copolymer was 84% ​​of the reactor feed weight.

[0084] The polymer from the polymerization reactor was in the form of a slurry in water. After being treated in a stripping tower to remove unreacted monomers, it was washed and filtered under vacuum on a rotary filter to obtain a filter cake consisting of polymer (53 wt%) and water (47 wt%).

[0085] 100 kg of filter cake was transferred to a stirred tank and 255 kg of pure dimethylacetamide maintained at -10 °C was added. The resulting suspension was cooled at this temperature for 5 minutes with stirring in the tank, and then fed into a rotary vacuum filter. After filtration, a material containing 40 wt% DMAc, 9 wt% water and 51 wt% polymer was obtained.

[0086] The filter cake discharged from the filter is transferred to a stirred tank containing 148 kg of DMAc, maintained at -5°C and stirred for 10 minutes to produce a slurry containing 21 wt% polymer, 75 wt% DMAc and 4 wt% water.

[0087] This slurry is then transferred via a gear pump to a step of converting it into a spinning slurry, which is carried out using the following equipment:

[0088] Tube bundle heat exchangers;

[0089] • Static mixer for homogenization;

[0090] • A set of filter presses with selective filter cloths that vary in size from 40μm to 5μm.

[0091] The spinning slurry thus produced is fed into a wet spinning production line, where 24,000-hole spinnerets are immersed in a coagulation bath containing 60% DMAc and 40% water and maintained at 55°C. The resulting fiber bundles are continuously stretched to 10 times their initial length and washed. At the end of the stretching and washing section with recycled water from the solvent recovery unit, a final washing step is performed using softened water, which is then sent to… Figure 2 The polymerization reactor 1. At the end of the stretching and washing section, the filaments are collected onto a spool at a speed of 70 m / min to obtain a 24K precursor spool with the following properties:

[0092] • Fineness: 1.22 dtex;

[0093] • Strength: 59.5 cN / tex

[0094] • Elongation: 14.5%;

[0095] Suitable for the production of carbon fiber.

[0096] Example 3, Reference Figure 2

[0097] 100 kg / h of acrylonitrile, 2 kg / h of methyl acrylate, 2 kg / h of itaconic acid dissolved in DMSO, 0.4 kg / h of ammonium persulfate dissolved in water, 0.5 kg / h of ammonium bisulfite dissolved in water, 2 g / h of ferric sulfate dissolved in water, 200 kg / h of water, along with sulfuric acid sufficient to maintain the reaction pH in the range of 2.0-3.5, 40 kg / h of water from the final washing step of the spinning section, and 25 kg / h of DMSO, were continuously added at 62°C to an aluminum reactor equipped with a stirrer and an overflow pipe.

[0098] The feedstock was fed at a rate allowing a 90-minute residence time. After 90 minutes, the reaction was terminated by adding an aqueous EDTA solution to the overflow pipe, and the slurry was fed into a stripper to remove unreacted acrylonitrile and methyl acrylate, resulting in a polymer-in-water slurry at the bottom. The conversion of acrylonitrile to copolymer was 86% of the reactor feed weight.

[0099] The polymer from the polymerization reactor is in the form of a slurry in water. After being treated in a stripping tower to remove unreacted monomers, it is washed and filtered on a rotary vacuum filter to obtain a filter cake consisting of polymer (55 wt%) and water (45 wt%).

[0100] 100 kg of filter cake was transferred to a stirred tank, and 570 kg of a mixture consisting of 80% DMSO and 20% water, maintained at 7°C, was added. The resulting suspension was cooled at this temperature for 5 minutes with stirring in the tank, and then fed into a rotary vacuum filter. After filtration, a material containing 42 wt% DMSO, 10 wt% water, and 48 wt% polymer was obtained.

[0101] The filter cake discharged from the filter is transferred to a stirred tank containing 180 kg of DMSO, producing a slurry containing 19 wt% polymer, 77 wt% DMSO, and 4 wt% water. This slurry is then transferred by a gear pump to a step for converting it into a spinning slurry, which is carried out using the following equipment:

[0102] Tube bundle heat exchangers;

[0103] • Static mixer for homogenization;

[0104] • A set of filter presses with selective filter cloths that vary in size from 40μm to 5μm.

[0105] The spinning slurry produced in this manner is fed into a dry-jet wet-spinning production line. The spinneret has 3000 orifices and is located 4 mm from the surface of the coagulation bath, which contains 35% DMSO and 65% water at a temperature of 5°C. The resulting fiber bundles are stretched in water, then stretched in steam (steam stretching) to nine times their initial length, and finally washed to remove residual solvent. At the end of the stretching and washing section using recycled water from the solvent recovery unit, a final washing step is performed using softened water, which is then sent to… Figure 2 The polymerization reactor 1. At the end of the spinning process, a 12K precursor spool is obtained by stacking four 3K filament bundles from a single spinneret. The obtained fibers are collected on a bobbin at a speed of 240 m / min, with a perfectly circular cross-section, dense, crack-free, and possessing the following properties:

[0106] • Fineness: 1.0 dtex;

[0107] • Strength: 65.3 cN / tex;

[0108] • Elongation: 14.1%

[0109] Suitable for the production of carbon fiber.

Claims

1. An integrated and improved method for producing acrylonitrile fibers, comprising the following steps: i) The comonomer is polymerized in an aqueous suspension in the presence of a certain amount of solvent, the weight of which accounts for 3-25% of the total weight of the aqueous suspension, wherein the solvent is the same as that used in steps ii)-vi), and wherein the water constituting the aqueous suspension includes recycled water from the final washing step of the spinning step, the amount of which is in the range of 10-40 wt% of the total weight of the water in the aqueous suspension, the polymerization is catalyzed by a redox pair of ammonium persulfate / ammonium bisulfite in the presence of a catalytic amount of ferric sulfate, then unreacted monomers are removed, the aqueous suspension is filtered and washed to obtain a first filter cake comprising polymer and water containing trace amounts of solvent, the polymer to water weight ratio being in the range of 40 / 60-60 / 40; ii) Disperse the first filter cake obtained in step i) in a solvent / water mixture or pure solvent with a weight ratio of 2-10 times the weight of the filter cake under stirring; iii) The solid / liquid phases of the polymer dispersion are separated by filtration or centrifugation to obtain a second filter cake comprising a polymer and a solvent / water mixture, wherein the weight ratio of solvent to water is in the range of 60 / 40 to 85 / 15; iv) The second filter cake obtained in step iii) is dispersed again in the same solvent or water / solvent mixture as in step ii), wherein the weight of the solvent or water / solvent mixture is in the range of 2 to 10 times the weight of the polymer in the second filter cake, and the dispersion is carried out at room temperature or below room temperature for 5 to 20 minutes to obtain a uniform dispersion or "slurry", wherein the weight ratio of solvent to polymer is in the range of 90 / 10 to 70 / 30. v) Heat the uniform dispersion or slurry obtained in step iv) until the polymer is completely dissolved to obtain a uniform spinning solution; vi) Feed the homogeneous spinning solution obtained at the end of step v) into the spinning step. In step ii), the first filter cake obtained in step i) is dispersed at a temperature of 7°C or below for a period of 5-15 minutes.

2. The method of claim 1, wherein in step i), the polymerization of the comonomer is carried out in an aqueous suspension in the presence of a solvent in an amount ranging from 5 to 10 wt% relative to the total weight of the aqueous suspension, and wherein the water constituting the aqueous suspension includes recycled water from the final washing step of the spinning step, in an amount ranging from 20 to 30 wt% of the total weight of the water in the aqueous suspension.

3. The method of claim 1, wherein in step v), the uniform dispersion or slurry obtained in step iv) is heated by passing it through a heat exchanger.

4. The method according to claim 1, wherein the spinning step is performed using a wet spinning process or a dry-jet wet spinning process, wherein, After the coagulation step is carried out in a coagulation bath consisting of a mixture of water and solvent, the resulting fiber bundle is continuously stretched and washed to a length of 8-12 times the initial length, and then a final washing step with water is performed to remove the last trace amount of solvent. The wash water containing less than 0.5% by weight of solvent is then fed back to step i) as recirculated water.

5. The method of claim 4, wherein the fiber bundle is continuously stretched and washed to a length of 9-10 times the initial length.

6. The method of claim 1, wherein polymerization step i) is terminated by adding an iron chelating agent, and the aqueous suspension undergoes subsequent steps of removing unreacted monomers, filtering the aqueous suspension, and washing it to obtain a first filter cake comprising polymer and water containing trace amounts of solvent, wherein the polymer to water weight ratio in the first filter cake is in the range of 40 / 60 to 60 / 40.

7. The method of claim 6, wherein the weight ratio of polymer to water in the first filter cake is in the range of 45 / 55 to 55 / 45.

8. The method of claim 6, wherein the weight ratio of polymer to water in the first filter cake is equal to or greater than 1:

1.

9. The method of claim 6, wherein the iron chelating agent is selected from EDTA in the form of sodium or ammonium salts.

10. The method of claim 1, wherein the same solvent is used in steps i)-vi) and the spinning step.

11. The method of claim 1, wherein the same solvent is used in steps i)-vi) and the spinning step, the solvent being selected from dimethylacetamide, dimethylformamide and dimethyl sulfoxide.

12. The method of claim 1, wherein in step ii), the temperature is below 0°C.

13. The method of claim 1, wherein the amount of water in the solvent / water mixture of step ii) varies between 0 and 20 wt% relative to the total weight of the mixture.

14. The method of claim 1, wherein in step iv), the temperature is equal to 7°C.

15. The method of claim 1, wherein the uniform spinning solution contains 12-22 wt% of polymer relative to the total weight of the spinning solution.

16. The method of claim 1, wherein a water-soluble additive is added in steps iii) and iv), the additive being selected from ammonia, primary amine, secondary amine, quaternary ammonium salt, salt of a metal ion capable of salting the polymer ionic terminal groups, or a water-soluble polymer that alters the rheological properties of the polymer solution.

17. The method of claim 1, wherein the polymer is selected from high molecular weight polymers, the high molecular weight range being 80,000-200,000 Da, or the polymer is selected from medium molecular weight polymers, the medium molecular weight range being 40,000-55,000 Da.

18. The method of claim 1, wherein the polymer is an acrylonitrile copolymer comprising 90-99 wt% of acrylonitrile and one or more comonomers comprising 1-10 wt% of the total polymer weight.

19. The method of claim 18, wherein the comonomer other than acrylonitrile in the acrylonitrile copolymer is selected from neutral vinyl compounds, compounds with one or more acid groups, and compounds capable of imparting different chemophysical properties to the polymer.

20. The method of claim 19, wherein the neutral vinyl compound is selected from methyl acrylate, methyl methacrylate, vinyl acetate, and acrylamide, the compound having one or more acid groups is selected from acrylic acid, itaconic acid, and sulfonated styrene, and the compound capable of imparting different chemical and physical properties to the polymer is vinylpyridine.

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