Process for obtaining cellulose pulp from textiles

Abstract

The present invention relates to a process for obtaining cellulose pulp from textiles, to the cellulose pulp obtained by the disclosed process, and to the further processing and use thereof.

Description

[0001] Process for obtaining pulp from textiles

[0002] The present invention relates to a process for obtaining cellulose from textiles as well as to the cellulose obtained by the process according to the invention and to its further processing and use.

[0003] The recycling of valuable raw materials such as textiles has a long tradition, beginning with so-called ragpickers who earned their living by collecting old textiles. Textile recycling is now largely industrialized and can be divided into material recycling, chemical recycling, and thermal recycling. The raw materials recovered from textiles find widespread use, including as raw materials for the textile and paper industries.

[0004] Despite a high recyclable content, a large portion of the waste textiles generated continues to be landfilled or incinerated. This poses the problem that high-quality recycling of textile fibers is difficult and costly. Therefore, every recycling process begins with sorting. The actual processing then depends on whether the materials are to be reused in the textile industry ("closed-loop recycling") or used in other industries ("open-loop recycling"). The challenge here lies primarily in increasing the proportion of "closed-loop recycling," i.e., processing the textiles so that they can be reused in the textile industry.

[0005] Although textiles generally contain a high amount of cotton, or cellulose, this is usually dyed and mixed with synthetic fibers, requiring complex purification before the cellulose is of a quality that allows further processing. Another problem is that the chain lengths of cellulose shorten with each reprocessing, making the fibers unsuitable for further processing after several recycling cycles.

[0006] A number of processes for processing textiles are known in the prior art, focusing primarily on "open-loop recycling," in which the fibers are processed for further use in the paper industry. Against this background, WO 2020 / 245053 proposes a process for producing a cellulosic paper stock in which used textiles are provided as the starting material. The used textiles comprise cellulose and non-cellulosic foreign substances, in particular synthetic plastic and / or metal oxides; the non-cellulosic foreign fibers are at least partially depleted from the cellulose to provide a depleted starting material, and a cellulosic paper stock is formed from the depleted starting material.

[0007] WO 2020 / 245058 describes a method for continuously providing a processed cellulose-containing starting material, in particular a starting material for producing a cellulosic molded body, the method comprising: feeding a cellulose-containing starting material having a predefined composition to a reactor device; continuously processing the cellulose-containing starting material in the reactor device to obtain the processed cellulose-containing starting material; and discharging the processed cellulose-containing starting material from the reactor device.

[0008] WO 2019 / 140245 discloses a process for producing cellulose and / or terephthalic acid from a textile waste material containing cotton and / or a blend of cotton and polyester. The textile waste material is treated with subcritical water at a temperature of 105 to 190 °C and a pressure of 40 to 300 psi for 0 to 90 minutes. The cellulose thus produced has a degree of polymerization of 150-2500. In addition to the cellulose, dissolved terephthalic acid and ethylene glycol are obtained.

[0009] WO 2010 / 104458 relates to a process for producing shaped cellulosic material from lignocellulose by a sequence of separation, dissolution and cellulose shaping steps.

[0010] US 2023 / 0124761 describes a process for treating a starting material comprising cellulosic and non-cellulosic material. The process comprises, among other things, a pretreatment of the starting material intended to alter the viscosity and average molecular weight of the cellulosic material. The pretreatment may be, for example, a non-alkaline wash, an enzyme treatment, an amorphous phase water treatment, a swelling agent treatment, a supercritical CO2 treatment, a bleaching agent treatment, or an organic solvent treatment.

[0011] Cellulose from textiles is usually recovered in the form of pulp, which is then subjected to further processing, for example, into longer fibers or yarns. Despite existing proposals in the state of the art, there is still a need for efficient and sustainable recycling processes for textiles, particularly closed-loop recycling, so that the recovered pulp can be processed back into textiles. In particular, there is a need for a holistic approach that not only focuses on the recovery of the material but also enables this through a resource- and environmentally friendly process.

[0012] It is the object of the present invention to meet this need and to provide a resource- and environmentally friendly process for obtaining cellulose from textiles.

[0013] A first aspect of the present invention is therefore a process for obtaining pulp from textiles, comprising the steps of: a) providing a starting textile fiber mixture comprising cellulose fibers and foreign fibers; b) separating the cellulose fibers and the foreign fibers by treatment with a methanolic solvent; and c) carrying out a hydrothermal treatment of the cellulose fibers to obtain the pulp.

[0014] In the context of the present invention, the terms "cellulose" and "dissolving pulp" are used synonymously and refer to cellulose in the form of short fibers or particles. These terms generally refer to cellulose, which serves as a starting material for the production of cellulose regenerates from which fibers such as viscose or films can be produced. In the context of the present invention, "cellulose" refers to a polysaccharide composed of ß-D-glucose units linked together by ß-1,4-glycosidic bonds.

[0015] In the context of the present invention, it has surprisingly been found that high-quality pulp is obtained by means of the process according to the invention, which can be further processed into new products in a variety of applications.

[0016] Within the scope of the present invention, both used and unused textiles, such as those obtained from used clothing collections or from leftovers from textile and clothing production, are suitable as sources for the starting textile fiber mixture. Thus, there are no limits to the process according to the invention with regard to the starting textile fiber mixture. In a preferred embodiment, the starting textile fiber mixture contains at least 10 wt.% of natural fibers, in particular cotton fibers, based on the total weight of the starting textile fiber mixture.

[0017] Although textiles continue to be largely composed of cellulose fibers, the proportion of foreign fibers continues to increase. In a preferred embodiment, these foreign fibers are selected from the group consisting of polyester fibers, in particular polyethylene terephthalate (PET) fibers, polyether fibers, polyurethane fibers, polyamide fibers, and mixtures thereof.

[0018] In a first step, the process according to the invention provides for a separation of the starting textile fiber mixture to deplete the proportion of foreign fibers. For this purpose, the starting textile fiber mixture is subjected to a treatment with a methanolic solvent. In the context of the present invention, a methanol-containing solvent is understood to mean a solvent or solvent mixture whose main constituent is methanol. The proportion of methanol in the methanol-containing solvent is preferably at least 50 vol.%, more preferably at least 60 vol.%, and in particular at least 80 vol.%, based on the total volume of the solvent. This treatment is preferably carried out at a temperature of 160 to 240 °C and / or at a pressure of 1.8 to 8 MPa, more preferably 170 to 220 °C and / or at a pressure of 2.2 to 5.8 MPa, in particular 180 to 200 °C and / or a pressure of 2.7 to 4 MPa.The preferred treatment duration has been found to be 1 to 120 minutes, preferably 1 to 60 minutes or 15 to 120 minutes, particularly preferably 10 to 90 minutes.

[0019] By treating the starting textile fiber mixture with a methanolic solvent, polyester fibers contained in the starting textile fiber mixture can be separated, with PET fibers being the most common fiber type. Treating the starting textile fiber mixture under the conditions mentioned above allows the PET to be gently converted back into its monomers, which can then be used to produce new PET. Within the scope of the present invention, by treating the PET with a methanolic solvent, it is advantageously converted into dimethyl terephthalate as a monomer, which can then be polymerized again or used for other applications.

[0020] The treatment in step b) of the process according to the invention can be carried out in the presence or absence of a catalyst. Examples of catalysts that can be used are Zn(OAc)2 or sodium methoxide. In an alternative preferred embodiment, step b) of the process according to the invention can be carried out in a basic medium, for example by adding NaOH or KOH.

[0021] Preferably, step b) of the process according to the invention is carried out in the absence of a catalyst; in particular, no active addition of a catalyst is carried out. Eliminating the catalyst offers the advantage that, on the one hand, contamination of the fibers by the catalyst is avoided, and, on the other hand, no additional salt load is generated, as would otherwise occur upon catalyst removal.

[0022] When treating the starting textile fiber mixture with the methanolic solvent, the solvent can be in either liquid or vapor form. Treatment with a vaporous methanolic solvent has the advantage that a larger quantity of textile fibers can be brought into contact with the solvent, thus enabling better mixing even with long fiber material. Accordingly, an embodiment of the process according to the invention is preferred in which the treatment is carried out with a vaporous methanolic solvent. Alternatively, the starting textile fiber mixture can also be suspended in the solvent, thereby achieving comprehensive wetting of the fibers. In a particularly preferred embodiment, the methanolic solvent is in equilibrium between vapor and liquid.

[0023] Other solvents can be added to the methanolic solvent. This additional solvent is preferably selected from the group consisting of water, dichloromethane, and chloroform, as well as mixtures thereof. If the methanol-containing solvent contains other solvents, their proportion is preferably no more than 20 vol.%, more preferably no more than 10 vol.%, based on the total volume of the solvent. In a particularly preferred embodiment, a methanol-water mixture is used, with the water proportion preferably being a maximum of 10 vol.%, based on the total volume.

[0024] The separation of the cellulose fibers from the starting textile fiber mixture in step b) of the process according to the invention can comprise further purification steps in addition to the removal of foreign fibers. Thus, in a preferred embodiment, oxidizing agents and / or metal complexing agents can be added to the starting textile fiber mixture. In this way, a bleaching effect of the cellulose fibers can be achieved or any metallic impurities present in the starting textile fiber mixture can be removed. The oxidizing agent is preferably selected from the group consisting of oxygen, ozone, and H2O2, while the metal complexing agents are preferably different from any catalysts used. Examples of suitable metal complexing agents include EDTA, DTPA, MGDA, EDDS, NTA, and IDS.

[0025] In an alternative embodiment, the treatment in step b) is carried out in the absence of oxygen. This minimizes the risk of undesirable degradation of the cellulose fibers contained in the textile fibers.

[0026] In a preferred embodiment, the process according to the invention comprises a step in which the starting textile fiber mixture is washed. This washing step preferably follows step b) of the process according to the invention. The washing medium is preferably selected from the group consisting of methanol, ethanol, chloroform, dichloromethane, ethyl acetate, water, and mixtures thereof.

[0027] As a further step, the process according to the invention provides for the performance of a hydrothermal treatment. In a preferred embodiment, this treatment is carried out at a temperature of 130 to 200 °C, preferably 160 to 190 °C, and / or a pressure of 0.3 to 1.6 MPa, preferably 0.6 to 1.3 MPa. The preferred treatment duration is 1 to 120 minutes. During the treatment, the water can be in the liquid phase or vapor state, or as a mixture thereof.

[0028] Like step b) of the process according to the invention, step c) can also be carried out in the absence or presence of a catalyst, with hydrothermal treatment in the absence of a catalyst being preferred. While conventional processes generally use water and a catalyst such as NaOH or KOH, it has surprisingly been found within the scope of the present invention that the process is economically efficient even without a catalyst, thus having the advantage of eliminating the need for downstream neutralization and the associated disadvantages such as additional salt loading or the use of additional chemicals.

[0029] In a preferred embodiment of the process according to the invention, steam pressure digestion can be carried out before and / or after step c). This allows the surface area of ​​the fibers to be increased, allowing for more efficient treatment.

[0030] Within the scope of the process according to the invention, it was surprisingly found that a comparatively large amount of cellulose fibers can be processed, allowing optimal use of the energy used. In a preferred embodiment, the ratio of fibers to solvent in step b) of the process according to the invention is 1:1 to 1:20.

[0031] In a further preferred embodiment, the ratio of fibers to water in step c) of the process according to the invention is 1:1 to 1:20. Textiles are often dyed, which means that for recovery purposes, a decolorization step must be carried out during the recycling process in which the dyes bound to the cellulose fibers are removed or rendered colorless. Two methods are available for decolorization: oxidative and reductive decolorization. In a preferred embodiment of the process according to the invention, step c) of the process according to the invention is therefore further preceded by a reductive and / or oxidative treatment of the cellulose fibers. Within the scope of the process according to the invention, decolorization under basic conditions and with the addition of oxidizing agents has proven to be the gentler and therefore preferred method.Carrying out the decolorization under basic conditions with an oxidizing agent, for example, hydrogen peroxide or ozone, offers the advantage that the cellulose fiber can be decolorized with less damage while maintaining a good decolorization result. The decolorization step is preferably carried out at a temperature of 60 to 180 °C in a basic environment. In a preferred embodiment, a suitable metal complexing agent such as EDTA, DTPA, MGDA, EDDS, NTA, and / or IDS and / or a suitable stabilizing agent in the form of an antioxidant such as propyl gallate or MgSO4 can be added to further protect the cellulose and reduce the metal content.

[0032] Used textiles are usually shredded, so their fibers are present in a variety of different fiber lengths. The process according to the invention has a high tolerance for long fibers. Nevertheless, in some applications, it may be preferable to limit the fibers to a certain length, for example, to prevent spinning or knotting of the fibers during mechanical stirring. Therefore, an embodiment of the process according to the invention is preferred in which the starting textile fiber mixture is shredded before the treatment in step b).

[0033] In order for the recovered pulp to be processed back into textiles, a certain cellulose chain length is required. The intrinsic viscosity is usually given as a measure of the chain length, with the value of the intrinsic viscosity decreasing with increasing degradation of the polymer chain, with the intrinsic viscosity being given as an average value. Although the process according to the invention allows recovery without significant degradation of the polymer chains, it may nevertheless be useful for certain applications to adjust the intrinsic viscosity. Therefore, in a preferred embodiment, the process according to the invention further comprises a step in which the pulp is mixed with further cellulose fibers such that the resulting mixture has a target value for the average intrinsic viscosity.For example, in a preferred embodiment, the pulp obtained by the process according to the invention can be mixed with cellulose fibers having a lower intrinsic viscosity in order to achieve a desired target value of the average intrinsic viscosity.

[0034] The process according to the invention produces high-quality pulp that can be further processed in a range of applications. Therefore, an embodiment in which the obtained pulp is subjected to further processing, for example, further processing into textiles or paper, in particular further processing into textile fibers, is preferred. In the field of textile production, the regenerated pulp is further processed into spinning dope, which serves as the starting material for the production of cellulose fibers and thus textiles. Particularly with regard to storage and transport, it has proven advantageous to provide the pulp in the form of boards. Therefore, an embodiment of the process according to the invention in which the obtained pulp is pressed into boards is preferred.

[0035] Another object of the present invention is pulp obtained according to the process according to the invention.

[0036] The pulp obtained by the process according to the invention is intended in particular for further processing into new textiles. For this purpose, it has proven advantageous if the cellulose has a specific chain length. Therefore, an embodiment is preferred in which the pulp has an intrinsic viscosity of at least 400 mL / g and / or a maximum of 700 mb / g, preferably from 450 to 650 mL / g, more preferably from 450 to 580 mb / g, determined according to ISO 5351:2010.

[0037] The pulp obtained by the process according to the invention is characterized by high purity. Surprisingly, it has been shown that a significant increase in whiteness (io) can be achieved even without decolorizing the fibers. Therefore, an embodiment in which the pulp has a whiteness index according to ISO 11475 of at least 90% is preferred.

[0038] In addition to a favorable degree of whiteness, the pulp according to the invention is further characterized by a low content of foreign matter. Thus, an embodiment in which the pulp has an iron (Fe) content of 10 mg / kg or less is preferred. Preferably, the pulp according to the invention contains less than 80 mg / kg SiO2 and / or less than 80 mg / kg calcium (Ca). The ash content, determined at 650 °C + / - 10 °C, is preferably 0.2% or less.

[0039] The present invention further relates to the use of the pulp obtained by the process according to the invention, in particular for the production of textiles and / or paper. For this use, the pulp can be further processed, for example, into fibers or pulp. In an alternatively preferred embodiment, the pulp serves as a starting material for the production of cellulose acetate, which can be further processed, for example, into films.

[0040] A further object of the present invention are cellulose fibers obtained from the pulp according to the invention.

[0041] The present invention is explained in more detail with reference to the following examples and figures, which, however, are in no way to be understood as a limitation of the inventive concept.

[0042] Example 1 :

[0043] An undyed starting textile fiber mixture comprising cellulose and PET fibers in a ratio of 80:20 was suspended in methanol (2.5 g solid in 100 mL methanol). The suspension was heated to a temperature of 170 to 220 °C to depolymerize the PET. The cellulose fibers were then filtered off and dried. The resulting cellulose fibers were suspended in water and heated to a temperature of 160 to 190 °C. The resulting pulp was then filtered off, washed, and dried. FTIR spectroscopy subsequently revealed no further PET, indicating a residual content of < 2%. This was confirmed by gravimetric analysis, which showed complete degradation of the PET within the limits of the measurement accuracy.The resulting pulp had an average intrinsic viscosity of 415 mg / L and was subsequently blended with further cellulose with a higher intrinsic viscosity to obtain a final product with an average intrinsic viscosity of 550 mg / g.

[0044] The workup was carried out without the active addition of a catalyst.

[0045] Example 2:

[0046] A dyed starting textile fiber mixture comprising cellulose fibers and PET fibers in a 50:50 ratio was treated with vaporized methanol at a temperature of 170 to 220 °C to depolymerize the PET. The dyed cellulose fibers were then separated and dried. The cellulose fibers were decolorized by alkaline hydrolysis in an aqueous 1M KOH solution at 90 to 140 °C, then washed and dried. FTIR spectroscopy subsequently revealed no further PET, indicating a residual content of < 2%. This was confirmed by gravimetric analysis, which showed complete degradation of the PET within the limits of measurement accuracy. The decolorized cellulose fibers were suspended in water at a fiber concentration of 2% and heated to a temperature of 160 to 190 °C. The resulting pulp was then filtered off, washed, and dried.The obtained pulp had an average intrinsic viscosity of 418 mg / L.

[0047] The workup was carried out without the active addition of a catalyst.

[0048] The examples show that with the help of the process according to the invention, textile waste can be processed into high-quality pulp in an economical and sustainable manner.

[0049] Figure 1 shows an exemplary schematic sequence of the process according to the invention. The textile waste to be processed can originate from collected and sorted used textiles, such as those generated during used clothing collection. This textile waste is shredded or otherwise reduced to a starting textile fiber mixture. To separate foreign fibers, the starting textile fiber mixture is treated with a methanolic solvent (S1). The separated cellulose fibers can optionally be decolorized before being subjected to a hydrothermal treatment (S2). The pulp thus obtained can then be further processed, for example, into cellulose fibers.

Claims

Patent claims:

1. A process for obtaining pulp from textiles, comprising the steps of: a) providing a starting textile fiber mixture comprising cellulose fibers and foreign fibers; b) separating the cellulose fibers and the foreign fibers by treatment with a methanol-containing solvent; and c) performing a hydrothermal treatment of the cellulose fibers to obtain the pulp.

2. The process according to claim 1, characterized in that the treatment in step b) is carried out at a temperature of 160 to 240 °C, preferably 170 to 220 °C, particularly preferably 180 to 200 °C, and / or a pressure of 1.8 to 8 MPa, preferably 2.2 to 5.8 MPa, particularly preferably 2.7 to 4 MPa, preferably for a period of 1 to 120 minutes, preferably 1 to 60 minutes or 15 to 120 minutes, particularly preferably 10 to 90 minutes.

3. Process according to at least one of the preceding claims, characterized in that the treatment in step b) is carried out in the presence or absence of a catalyst.

4. Process according to at least one of the preceding claims, characterized in that the methanolic solvent is liquid or vaporous or a mixture thereof.

5. Process according to at least one of the preceding claims, characterized in that the starting textile fiber mixture has a proportion of natural fibers, in particular cotton fibers, of at least 10% by weight, based on the total weight of the starting textile fiber mixture.

6. Process according to at least one of the preceding claims, characterized in that the textile fibers originate from textile waste.

7. Process according to at least one of the preceding claims, characterized in that the proportion of methanol in the methanol-containing solvent in step b) is at least 50 vol.%, particularly preferably at least 60 vol.%, and in particular at least 80 vol.%, based on the total volume of the solvent.

8. Process according to at least one of the preceding claims, characterized in that the treatment in step c) is carried out at a temperature of 130 to 200 °C, preferably 160 to 190 °C, and / or a pressure of 0.3 to 1.6 MPa, preferably 0.6 to 1.3 MPa, and preferably for a duration of 1 to 120 minutes.

9. Process according to at least one of the preceding claims, characterized in that the treatment in step c) is carried out in the presence or absence of a catalyst.

10. The method according to at least one of the preceding claims, characterized in that the treatment in step c) is carried out by means of liquid water or steam or a mixture thereof.

11. Process according to at least one of the preceding claims, characterized in that in step b) the ratio of fibers to solvent is 1:1 to 1:20 and / or characterized in that the ratio of fibers to water in step c) is 1:1 to 1:

20.

12. Process according to at least one of the preceding claims, characterized in that step c) of the process is further preceded by a reductive and / or oxidative treatment.

13. A process according to at least one of the preceding claims, characterized in that the process further comprises a step of further processing the pulp obtained.

14. Pulp obtained by a process according to at least one of claims 1 to 13.

15. Pulp according to claim 14, characterized in that the pulp has an intrinsic viscosity of at least 400 and / or at most 700 mL / g preferably from 450 to 600 mL / g, particularly preferably 450 to 580 mL / g, determined according to ISO 5351:2010.

16. Pulp according to at least one of claims 14 or 15, characterized in that the pulp has a brightness of 90%, determined according to ISO 11475.

17. Use of pulp according to at least one of claims 14 to 16 for the production of textiles and / or paper and / or cellulose acetate.

18. Cellulose fibers produced from a pulp according to at least one of claims 14 to 16.

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