Method for recovering starting materials from mixed textile waste

Abstract

The present invention provides a method for recovering raw materials from mixed textile waste, comprising the following steps in the stated order: a) providing a mixed textile waste containing at least a cellulose component and at least a polyester component; b) treating the mixed textile waste in an aqueous treatment solution to depolymerize the polyester component and dissolve it in the treatment solution; c) separating the cellulose component from the treatment solution and recovering a cellulose raw material; d) filtering the treatment solution to remove extraneous matter, particularly dyes and metal ions, from the treatment solution; and e) precipitating terephthalic acid from the treatment solution, separating the precipitated terephthalic acid, and recovering a polyester raw material containing terephthalic acid. Within the scope of the method, it is proposed that the filtration of the treatment solution in step d) comprises at least filtration through an adsorbent filter medium, in order to enable the recovery of raw materials with an increased level of purity.

Description

Technical Field

[0001] The present invention relates to a method for recovering raw materials from blended textile waste.

Background Art

[0002] In recent years, the recovery or reuse of raw materials from textile waste has become increasingly important for the textile industry, especially in reducing the environmental impact of such textile waste.

[0003] The simple mechanical recycling of textile waste has been well known for quite some time. In that case, the textile waste is shredded into fine pieces from which directly recycled end products such as cleaning wipes, fillers, or thermal insulation materials are produced. When spinning yarn from such recycled textile fibers, yarns are typically obtained that are of low quality and suitable for the production of new textiles only to a limited extent.

[0004] Chemical recycling methods are suitable for overcoming the above problems. For example, cellulose-based fibers can be spun again as regenerated cellulose-based fibers after undergoing chemical pretreatment. However, the production process of such regenerated cellulose-based molded bodies is very sensitive to impurities in the cellulose raw materials, and for this reason, the cellulose raw materials recycled in this way are generally unsuitable for spinning into fibers.

[0005] WO 2015 / 077807 A1 shows a pretreatment method for recycled cotton fibers from textile waste, where first the metal is removed from the recycled cotton fibers and then they are exposed to an oxidative bleaching agent. The cotton fibers thus recycled can then be used for the production of molded bodies from regenerated cellulose.

[0006] International Publication No. 2018 / 115428 A1 discloses a method of treating cotton-based raw materials under alkaline conditions in combination with a gaseous oxidizing agent.

[0007] International Publication No. 2018 / 073177 A1 similarly shows a method of recycling cellulose raw materials from cellulose-based fiber waste. In this case, the cellulose-based fibers in the fiber waste are swollen, and thus the fiber waste is treated under alkaline conditions in the presence of a reducing agent to promote the removal of foreign substances. Following the alkali treatment, the cellulose raw material is bleached with oxygen and / or ozone.

[0008] In such methods, generally, pure cellulose-based fiber waste is used as the starting material. However, in practice, fiber waste from clothing and fabrics is mixed fiber waste, that is, a mixture of cellulose-based and synthetic fibers. Here, the predominant fraction is mixed fiber waste containing polyester and cellulose-based fibers. Also, fiber waste from cotton fiber products often contains polyester from sewing threads, labels, etc. However, the above types of methods cannot remove significant contamination of synthetic polymer fibers, so usually, mixed fiber waste cannot be treated.

[0009] On the other hand, International Publication No. 2014 / 045062 A1 describes a method of extracting polyester from fiber products with an extraction solvent. This enables the recovery of the polyester component. However, the cellulose component is considerably contaminated due to each of the residue of the extraction solvent, significant degradation of the molecular chain, and the residue of the remaining polyester, and is not suitable for use in a method for producing a regenerated cellulose-based molded article. Furthermore, the polyester component is contaminated with foreign substances, dyes, dulling agents, etc., and as a result of the decomposition process, the length and properties of its molecules have changed significantly, making an additional treatment process for the polyester component inevitable. SUMMARY OF THE INVENTION

[0010] Accordingly, one object of the present invention is to provide a method for recovering raw materials from mixed fiber waste of the type described at the beginning, which enables the recovery of raw materials at a higher purity level.

[0011] According to the present invention, the above object is achieved by the method described in claim 1.

[0012] Providing mixed fiber waste containing at least a cellulose component and at least a polyester component, treating the mixed fiber waste in an aqueous treatment solution to depolymerize the polyester component and dissolve it in the treatment solution, separating the cellulose component from the treatment solution, and recovering the cellulose raw material, by which a high-quality cellulose raw material can be recovered from the mixed fiber waste by a process-technologically simple method, and as a result, it can be reused as a cellulose-based starting material. Further, by filtering the treatment solution to remove foreign substances from the treatment solution, precipitating terephthalic acid from the treatment solution, separating the precipitated terephthalic acid, and recovering the polyester raw material containing terephthalic acid, particularly effective utilization and recovery of the polyester component can be achieved, and in the process, the obtained polyester raw material can be supplied as a starting material for polyester production.

[0013] For the purposes of the present invention, "polyester" mainly refers to polyethylene terephthalate (PET) consisting of the monomers terephthalic acid and ethylene glycol. However, the present invention also works very well with other widely used polyesters, such as polypropylene terephthalate (PPT), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), or mixtures of these polyesters. In each case, it is important that the alcoholic component, i.e., ethylene glycol, butanediol, propylene glycol, trimethylene glycol, etc., can be easily dissolved in the treatment solution and does not precipitate with terephthalic acid.

[0014] If the filtration of the treatment solution in step d) includes at least filtration with an adsorbent filter medium, especially for removing dyes and metal ions from the treatment solution, it is possible to ensure that the precipitate of terephthalic acid obtained after precipitation of the treatment solution in step e) is of high purity. Otherwise, metal ions and dyes each exhibit a high affinity for binding to terephthalic acid molecules, thereby contaminating the terephthalic acid precipitate during precipitation and significantly impairing the quality of the recovered terephthalic acid-containing starting material. According to the filtration of the present invention with an adsorbent filter medium, the main contaminants are likely to bind to the particles of the adsorbent filter medium, so that the main contaminants can be efficiently removed from the treatment solution prior to precipitation.

[0015] If the adsorbent filter medium also includes activated carbon and / or zeolite, due to the adsorptive and reductive properties of activated carbon, impurities such as dyes and decomposition products of fiber auxiliaries can be adsorbed with particularly high reliability and, in part, selectively, and can be removed from the treatment solution. This selectivity can be improved by further coating the adsorbent filter medium.

[0016] For the purposes of the present invention, "recycled cellulose raw material" generally refers to recycled pulp, textile pulp, cotton pulp, rag pulp, etc., or combinations thereof. More specifically, such cellulose raw materials can be suitable as starting materials for the production of regenerated cellulose fibers such as lyocell, viscose, modal, or cupro fiber. Alternatively, the recycled cellulose raw material can also be a starting material for the production of paper, paper-like materials, or pulp-based nonwovens.

[0017] Generally, for the purposes of the present invention, "mixed fiber waste" may be a mixture containing any cellulose fibers forming the cellulose component of the mixed fiber waste and any polyester fibers forming the polyester component. Suitable cellulose fibers include, for example, natural cellulose fibers such as cotton, flax, hemp, ramie, kapok, or regenerated cellulose fibers such as rayon, viscose, lyocell, cupro, or modal. The synthetic polymer component can include, for example, polyamide or polyester fibers, or other synthetic fibers that can be decomposed by hydrolysis. The above fibers can have various diameters and lengths, can be continuous fibers (filaments) or staple fibers, and can also be present in the form of nonwovens. Such mixed fiber waste contains each of the cellulose component and the polyester component in an amount of 1% by weight or more, preferably 2% by weight or more, and more preferably 3% by weight or more.

[0018] Furthermore, when the mixed fiber waste is pre-consumer and / or post-consumer fiber waste, a particularly economical and reliable recycling method can be provided. Post-consumer fiber waste refers to fiber products that have already reached the end consumer and, as a result of being used, may contain foreign substances, and in some cases, may contain a significant amount. Post-consumer fiber waste includes one or several of the following: used clothing such as shirts, jeans, skirts, dresses, suits, coveralls, pants, underwear, sweaters, etc.; used home textiles such as bed linen, towels, curtains, cloths, tablecloths, sheet covers, upholstery fabrics, etc.; non-woven products such as wipes, diapers, filters, etc. Pre-consumer fiber waste refers to fiber materials that have not yet reached the end consumer and have become waste during the manufacturing process. This may include cuttings or waste generated during the manufacture of clothing, home textiles, non-wovens, etc., or waste generated during the manufacture of yarn, fiber products or regenerated cellulose fibers.

[0019] This method can be further improved when the aqueous treatment medium contains one or more hydrolyzing agents. Further, in such a case, the amount of the one or more hydrolyzing agents can be adjusted according to the amount of the polyester component in the mixed fiber waste so that the polyester component is substantially completely depolymerized during the treatment of step b), that is, more than the amount at which substantially all of the hydrolyzable bonds in the polyester component are cleaved during the treatment of step b), the hydrolyzing agent is added. Here, the hydrolyzing agent can promote the hydrolysis reaction during the cleavage of the hydrolyzable bonds in the polyester component. The hydrolyzable bonds in the polyester component are substantially ester bonds formed between the monomers of terephthalic acid and ethylene glycol. The amount of the hydrolyzing agent required in relation to the content of the polyester component in the mixed fiber product is added to the treatment solution by an appropriate method before and / or after the treatment of step b). In particular here, the addition is carried out so that no excess hydrolyzing agent remains in the treatment solution after the complete depolymerization of the polyester component, and therefore, an increase in the degradation of the cellulose component by the excess hydrolyzing agent does not occur, and the quality of the cellulose raw material recovered in step c) can be improved. Further, after the complete depolymerization of the polyester component, the addition may be carried out so as to intentionally leave a certain amount of the hydrolyzing agent in the treatment solution, whereby the molecular chain length in the cellulose component is degraded to a desired degree, and therefore the viscosity of the recovered cellulose raw material can be adjusted. In this way, a method can be provided that gives better control over the behavior of the degradation reactions of the polyester component and the cellulose component.

[0020] When the aqueous treatment solution is an aqueous alkaline treatment solution, the controllability of the present method is further improved. Here, the hydrolysis of the polyester component in the treatment solution occurs mainly as alkaline hydrolysis or saponification, which is characterized by a faster reaction rate compared to aqueous hydrolysis. When the hydrolyzing agent is a base, a particularly simple and cost-effective method can be further obtained. The base can act as an effective accelerator in the alkaline hydrolysis that cleaves the ester bond. During this alkaline hydrolysis, the base is consumed in the reaction, for example, by forming salts with the monomer components of the cleaved polyester component. Furthermore, the formed salt of terephthalic acid (disodium terephthalate) advantageously exhibits high solubility in an aqueous solution. Thus, after the complete decomposition of the synthetic polymer component, only a small amount of excess free base remains in the aqueous alkaline treatment solution, and the decomposition of the cellulose component is effectively suppressed. Furthermore, the treatment of the cellulose component in the aqueous alkaline treatment solution is suitable for removing dyes, foreign substances, impurities, or fiber auxiliaries (e.g., crosslinking agents) bound to the cellulose component. Thus, a purified high-quality recycled cellulose raw material can be obtained.

[0021] Sodium hydroxide (NaOH) is widely used in the treatment of pulp and cellulose raw materials. When the base used as the hydrolyzing agent is sodium hydroxide (NaOH), a reliable and cost-effective method is provided. In such a case, the total content of NaOH in the aqueous alkaline treatment medium is 10 to 300 g, for example, 20 to 250 g, per 1 kg of mixed fiber waste, depending on the proportion of the polyester component in the mixed fiber waste.

[0022] When treating the mixed fibrous waste in step b), if the temperature of the treatment solution is higher than 100 °C, especially higher than 110 °C, highly reliable depolymerization of the polyester component into monomer components can occur. Furthermore, when the temperature of the treatment solution is lower than 200 °C, sufficiently mild conditions can be guaranteed to avoid any excessive decomposition of the cellulose component. In a further embodiment of the present invention, the temperature may be especially 110 °C to 190 °C, preferably 120 °C to 180 °C, more preferably 125 °C to 175 °C, and most preferably 130 °C to 170 °C.

[0023] Furthermore, when the separation of the cellulose component in step c) includes at least screening, pressing, or centrifugation of the treatment solution, a regenerated cellulose raw material can be obtained from the treatment solution by a technically simple method. In this case, the cellulose component that is not substantially decomposed or dissolved can be simply separated from the treatment solution. According to the above decomposition process, a part of the cellulose component may exist dissolved in the treatment solution and thus remain in the treatment solution during step c). This is also the case for the monomer components of the dissolved polyester component and foreign impurities.

[0024] The precipitation formation of terephthalic acid in step e) can be promoted when step e) includes at least acidification of the treatment solution. The acidification may especially include adding an acid to the treatment solution, and the addition of the acid can be carried out, for example, until complete precipitation of terephthalic acid or until the pH value is lower than a specific value. The acid used is preferably sulfuric acid (H2SO4) which is commonly used in the pulping process of cellulose.

[0025] The reliability of the method can be further improved if the mixed fiber waste is comminuted and / or singularized before the treatment in step b). Comminuting and / or singularizing the mixed fiber waste can be used to mechanically separate the cellulose part from the polyester part, thereby enabling a more reliable decomposition (dissolution) of the polyester component in the aqueous treatment medium.

[0026] The reliability of the method can be further improved if at least partially, non-fibrous solids are removed from the mixed fiber waste before the treatment in step b). Non-fibrous solids can include, for example, buttons, zippers, decorative elements, prints, labels, and / or dirt, and parts thereof.

[0027] According to the present invention, the cellulose raw material recovered by the method is also suitable for the production of regenerated cellulose fibers, in particular, the production of regenerated cellulose fibers by the viscose, modal, cupro, or lyocell process.

Embodiments for Carrying Out the Invention

[0028] In the following, the present invention is illustrated based on a first embodiment variant. Further embodiment variants result from the modifications described in the specification, and they can be combined with each other in any way.

[0029] According to the first exemplary embodiment, a method for recovering raw materials from mixed fiber waste according to the present invention provides, in a first step, mixed fiber waste comprising at least a cellulose component and at least a polyester component. In this case, such mixed fiber waste comprises a mixture of any cellulose fibers forming the cellulose component and any polyester fibers forming the polyester component. For example, in one exemplary embodiment, the mixed fiber waste comprises a mixture of cotton and polyester fibers (in particular PET), which may be mixed at the yarn level in the mixed fiber.

[0030] In a further step, the mixed fiber waste is then treated in an aqueous treatment solution in order to depolymerize the polyester component and dissolve it in the treatment liquid. In the first exemplary embodiment, the aqueous treatment solution is an aqueous alkaline treatment solution, in particular a diluted caustic soda solution containing NaOH as a hydrolyzing agent. The treatment is carried out at a temperature higher than 100 °C, preferably higher than 110 °C. During the depolymerization of the polyester component, the molecular weight and the length of the molecular chains of the polyester molecules are intentionally reduced by hydrolysis occurring in the presence of the aqueous treatment solution. Thus, the degraded molecules of the polyester component gradually reduce their molecular chain length and finally split into monomeric starting materials, namely terephthalic acid and ethylene glycol (C2H6O2), which is an alcohol. In this process, terephthalic acid has two Na +Ions are consumed to form terephthalate, namely disodium terephthalate (C8H4O4Na2). As a result of hydrolysis, the readily soluble disodium terephthalate and ethylene glycol are present in dissolved form in the aqueous treatment solution. Subsequently, this enables the process-technologically simple separation of the polyester component depolymerized from the cellulose component, thereby making it possible to recover a high-level purity cellulose raw material from the mixed fiber waste. In fact, because the conditions are generally mild, the decomposition of the cellulose polymer in the cellulose component is very slight and occurs only to a relatively insignificant extent. More specifically, the decomposition is so slight that there is no or only a minimal amount of glucose monomers separated from the cellulose polymer. However, at the same time, the cellulose component can advantageously be partially pulped by the treatment solution and can be made free of impurities such as bound dyes or crosslinking agents, which in turn benefits the quality and purity of the recovered cellulose raw material.

[0031] In another embodiment of the method, the mixed fiber waste contains other polyesters such as PTT and PBT as the polyester component, and as a result, other alcohols are produced as monomeric starting materials in the depolymerization. Even in such cases, the above method is equally applicable.

[0032] In a further step, the cellulose component is then separated from the treatment solution, and in this process, the cellulose raw material is recovered. Since the aqueous treatment solution contains the depolymerized polyester component dissolved together with the dyes and foreign substances eluted from the cellulose component, the insoluble cellulose component can be separated from the liquid portion, i.e., the treatment solution, by simple solid / liquid separation such as screening, pressing, or centrifugation. Thus, the purified and conditioned cellulose component is obtained as the cellulose raw material. Thereafter, it is also possible to wash and / or dry this cellulose raw material for further conditioning for further use. The aqueous alkaline treatment solution remaining as a liquid in the separation still contains the depolymerized polyester component (disodium terephthalate and ethylene glycol) and possible contamination by foreign substances.

[0033] The remaining treatment solution is then filtered in the next step to separate unwanted substances from the depolymerized polyester component. According to the present invention, in this process, the treatment solution is filtered through an adsorbent filter medium. In particular, this filtration can be carried out in the form of a fixed bed filter, but it is also possible to disperse the filter medium in the treatment solution and then separate the added (loaded) filter medium together with the solid matter to be separated by simple solid / liquid separation again. In a first embodiment, the adsorbent filter medium includes activated carbon and / or zeolite. However, in a further embodiment, the filter medium can also include other adsorbent filter media suitable for the adsorption of metal ions / dyes, etc. In fact, activated carbon enables the particularly reliable and even selective adsorption of fiber auxiliaries such as dyes, metal ions, or crosslinking agents, and these substances are preferably adsorbed by the reducing action of activated carbon. This selectivity can also be further improved in another embodiment, for example, by applying an additional coating to the adsorbent filter medium, i.e., by coating each of activated carbon and zeolite with a suitable substance.

[0034] In the final step, terephthalic acid is precipitated as a precipitate from the treatment solution in order to recover the recyclable and purified polyester raw material. By filtering beforehand, it can be ensured that no foreign contaminants are incorporated into the terephthalic acid precipitate during the precipitation step. If foreign contaminants are incorporated into the terephthalic acid precipitate, in part, it will result in substantial contamination such that the terephthalic acid can only be made available for subsequent use, for example for repolymerization, through complex and costly purification steps. For precipitation formation, a suitable acid, such as sulfuric acid, is added to the treatment solution or the treatment solution is acidified thereby until the terephthalic acid is separated from the treatment solution in the form of a precipitate. The anion of the acid is neutralized by the Na + cation of disodium terephthalate, so that during acidification, terephthalic acid, which has a very low solubility and precipitates immediately from the solution, is formed. After the precipitation formation of terephthalic acid is complete, it is separated again from the liquid by simple solid / liquid separation by well-known operating steps, washed again if necessary, and finally terephthalic acid is obtained as the starting material for polyester.

Examples

[0035] Example 1: Post-consumer fiber waste (a mixture of cotton and polyester, 80:20 wt%) was heated with caustic soda (15% NaOH based on the mass of the fiber waste) at a bath ratio of 1:7 (mass of fiber waste: lye) at 150 °C for 120 minutes. Due to the depolymerization of the polyester fiber occurring under such conditions, disodium terephthalate was formed, which was water-soluble under such conditions and dissolved out into the heating bath and was finally separated from the remaining solid material (cotton fiber) by a screen.

[0036] The separated lye was stirred with an excess of activated carbon, whereby impurities such as metal ions, dyes and / or their decomposition products were selectively adsorbed and removed from the lye. Approximately 100 g of activated carbon was used per 2000 ml of lye and stirred for 1 hour at room temperature. Then, the activated carbon was separated and precipitated by filtration and subsequent centrifugation, and the supernatant was aspirated using filter paper. And the filtrate free of activated carbon was acidified to pH 2 using sulfuric acid. Thereby, terephthalic acid precipitated as a precipitate, which was subsequently aspirated using a glass frit and dried in a drying chamber.

[0037] As a result of the above activated carbon filtration, the obtained terephthalic acid contained almost no contaminants, which is shown based on, for example, the analyzed metal content (Table 1), and thus was recyclable without undergoing further complex purification steps.

[0038] The metal content was calculated as follows: Approximately 20 g of the sample was ashed, and subsequently the ash was dissolved and digested with sodium tetraborate, and the residue was dissolved using 1.6 M nitric acid. To calculate the iron content by photometry, potassium thiocyanate was added to the sample, and the red color of iron thiocyanate was measured by a calibration curve. To calculate the silicon content by photometry, ammonium molybdate was added to the sample, and the blue color of silicon molybdate was measured by a calibration curve. To mask the phosphate, the phosphate was complexed with oxalic acid.

[0039] Example 2 (Comparative Example): The method described in Example 1 was repeated, except that the purification stage with activated carbon was not performed.

[0040]

Table 1

Claims

1. a) providing a mixed fiber waste comprising at least a cellulose component and at least a polyester component; b) treating the mixed fiber waste in the treatment solution to depolymerize the polyester component and dissolve it in an aqueous treatment solution; c) separating the cellulose component from the treatment solution and recovering a cellulose raw material; d) filtering the treatment solution to remove foreign impurities from the treatment solution; and e) precipitating terephthalic acid from the treatment solution, separating the precipitated terephthalic acid, and further recovering a polyester raw material containing terephthalic acid; comprising in this order, wherein the filtration of the treatment solution in step d) comprises at least filtration through an adsorbent filter medium; wherein the aqueous treatment solution is an aqueous alkaline treatment solution and contains a hydrolyzing agent, A method for recovering raw materials from mixed fiber waste.

2. The method according to claim 1, wherein the adsorbent filter medium comprises activated carbon and / or zeolite.

3. The method according to claim 1 or 2, wherein the temperature of the treatment solution during the treatment of the mixed fiber waste in step b) is higher than 100 °C.

4. The method according to any one of claims 1 to 3, wherein the separation of the cellulose component in step c) comprises at least screening, pressing or centrifugation.

5. The method according to any one of claims 1 to 4, wherein the precipitation of the terephthalic acid in step e) comprises at least acidifying the treatment solution.

6. The method according to any one of claims 1 to 5, wherein the foreign impurities in step d) are dyes and metal ions.

7. Use of a cellulose raw material recovered by the method according to any one of claims 1 to 6 for the production of regenerated cellulose fibers.

8. Use of the cellulose raw material according to claim 7, wherein the regenerated cellulose fiber is a regenerated cellulose fiber by viscose, modal, cupro, or lyocell process.

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