Process for recycling ingredients from mixed textile waste

technical field

The present invention relates to a method for the treatment and utilization of waste liquor contaminated with degradation products from an alkaline digestion process for obtaining a cellulose raw material from mixed textile waste.

State of the art

The recovery or recycling of raw materials from textile waste has become increasingly important for the textile industry in recent years, not least in order to reduce the burden of this textile waste on the environment.

The purely mechanical recycling of textile waste has been well known for a long time, with the textile waste being crushed and directly recycled end products such as cleaning cloths, filling materials or insulating materials being produced from it. Spinning yarns from such recycled textile fibers usually results in low quality yarns which are not suitable for the production of new textiles.

[0004] Chemical recycling processes can be used to overcome the above problems. For example, cellulosic fibers can be spun into regenerated cellulosic fibers after chemical pretreatment. However, such processes for the production of regenerated cellulosic shaped bodies are very sensitive to impurities in the cellulose raw material, which means that such recycled cellulose raw material is generally unsuitable for spinning fibers from it.

[0005] WO 2015/077807 A1 shows a method for the pretreatment of recycled cotton fibers from textile waste, in which metals are first removed from the recycled cotton fibers and these are then subjected to oxidative bleaching. The cotton fibers recycled in this way can then be used to produce regenerated cellulosic molded articles.

WO 2018/115428 A1 discloses a method for treating a raw material based on cotton under alkaline conditions in conjunction with gaseous oxidizing agents.

WO 2018/073177 A1 in turn describes a method for recycling a cellulose raw material from cellulose textile waste. The textile waste is treated under alkaline conditions in the presence of a reducing agent in order to swell the cellulosic fibers in the textile waste and thus better remove foreign matter. After the alkaline treatment, the cellulose raw material is bleached with either oxygen and/or ozone.

Such methods use pure cellulosic textile waste as the starting material. In practice, however, the textile waste from clothing and fabrics is mixed textile waste, i.e. mixtures of cellulosic and synthetic fibers. The predominant fraction is mixed textile waste, which contains polyester and cellulosic fibers. Textile waste from cotton textiles is also usually contaminated with polyester from sewing threads, labels or the like. However, methods of the aforementioned type cannot usually process mixed textile waste, since significant contamination with synthetic polymer fibers, in particular polyesters, can no longer be removed.

However, the present method relates to processes for the chemical treatment of mixed textile waste, which also contain significant amounts of a polyester component in addition to a cellulose component.

US 2019/0218362 A1 describes systems and methods involving subcritical aqueous treatment to recycle the cellulosic and polyester component of cotton scraps and cotton-polyester blended fabric scraps that would otherwise be discarded.

The work of Anna Peterson: "Towards Recycling of Textile Fibers Separation and Characterization of Textile Fibers and Blends Master's Thesis in Materials Chemistry and Nanotechnology", CHALMERS UNIVERSITY OF

TECHNOLOGY, Gothenburg, Sweden, January 1, 2015, discloses a process for the complete hydrolysis of polyester from blended textiles using the phase transfer catalyst benzyltributylammonium chloride under mild conditions.

In the alkaline-chemical digestion of such mixed textile waste at the appropriate pressure and temperature, in particular for the recovery of a cellulose raw material, however, considerable amounts of waste caustic are produced, which contain various degradation products, including a large amount of polyester degradation products. Such waste liquors cannot be passed on to conventional wastewater treatment without prior treatment, as they usually have too high a chemical oxygen demand (COD) and metal content.

Disclosure of Invention

It is therefore an object of the invention to provide a method of the type mentioned for the treatment and recycling of spent liquor, which contains the corresponding degradation products mentioned above and thereby ae Allows the spent caustic to be recycled and reduces the burden on wastewater treatment processes.

The stated object is achieved according to the invention by a method according to claim 1.

[0015] If, in the process according to the invention for the treatment and recycling of waste liquor contaminated with degradation products from an alkaline digestion process for obtaining a recycled cellulose raw material from mixed textile waste, water is evaporated from the waste liquor in order to precipitate the polyester degradation product from the waste liquor and a two-phase mixture with an aqueous To obtain phase and the polyester degradation product having solid phase, a reproducible separation of the polyester degradation products from the contaminated waste liquor can be made possible. If the solid phase is further separated from the aqueous phase, then the aqueous phase of the waste caustic can be fed to conventional waste water treatment, as a result of which the load and the chemical oxygen demand of the waste water treatment can be minimized. Finally, the solid phase can be recycled independently of the aqueous phase, which means that the process in question can be made more environmentally friendly.

For the purposes of the present invention, “polyester” is mainly understood to mean polyethylene terephthalate (PET), which consists of the monomers terephthalic acid and ethylene glycol. However, the invention also works very well with other widespread polyesters such as polypropylene terephthalate (PPT), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT) and the like, or also mixtures of these polyesters. It is important in each case that the alcoholic component, i.e. ethylene glycol, butanediol, propylenediol, trimethylene glycol, etc., is readily soluble in the treatment solution and is not co-precipitated when the terephthalic acid is precipitated.

Under a spent caustic in the sense of the present invention is generally understood as a used aqueous alkaline treatment solution, wherein the treatment solution is used to treat the mixed textile waste to obtain a recycled cellulose raw material from the cellulose component of the mixed textile waste. Since the cellulose component in the mixed textile waste is usually contaminated by dyes and other foreign matter and is mixed with the polyester component at the fiber level, the foreign matter and the polyester component are dissolved out by the alkaline treatment solution and broken down into degradation products, in particular hydrolyzed. The waste caustic thus contains at least polyester degradation products and optionally dye or other foreign matter degradation products as degradation products.

[0018] During the alkaline pulping process, the molecular weight and the molecular chain length of the polyester molecules in the polyester component are reduced in a targeted manner by hydrolysis, which takes place in the presence of the aqueous alkaline treatment solution. In this way, the molecules of the polyester component are continuously reduced in their molecular chain length and finally split into their monomeric building blocks, terephthalic acid and ethylene glycol. The type of terephthalate formed in the alkaline treatment solution depends essentially on the alkaline hydrolysis agent contained in the treatment solution. For example, when NaOH is used, the polyester degradation product will primarily contain disodium terephthalate, which is readily soluble in the treatment solution. This subsequently enables a simple process-technical separation of the polyester degradation products from the cellulose component, as a result of which the cellulose raw material can be recovered with high purity from the mixed textile waste. Because of the generally mild process conditions, only an insignificant degradation of the cellulose polymers in the cellulose component takes place. At the same time, the

However, cellulose components are advantageously partially broken down by the treatment solution and freed from impurities such as dyes or crosslinking agents, which in turn is beneficial to the quality of the recovered cellulose raw material.

The disodium terephthalate contained in the waste liquor as a polyester degradation product precipitates when water evaporates from the waste liquor, forming a two-phase mixture with a solid phase and an aqueous phase, the solid phase having the disodium terephthalate and the aqueous phase is formed from the aqueous supernatant containing residues of the alkaline treatment solution and optionally other degradation products. This aqueous phase can either be fed directly to a waste water treatment with little environmental pollution or alternatively used to recover the alkaline treatment solution. In a further embodiment, the aqueous phase is again evaporated, during which further terephthalate can be precipitated. With this, on the one hand, The total calorific value of the substances obtained according to the invention is increased and, on the other hand, the COD of the aqueous phase fed to the waste water treatment is further reduced. The solid phase can therefore be recycled independently of the aqueous phase and does not have to be processed together in a resource-intensive and less environmentally friendly way.

For the purposes of this invention, recycled cellulose raw material is understood as meaning recycled pulp, textile pulp, cotton pulp, rag pulp or the like or combinations thereof. This cellulose raw material can in turn be particularly suitable as a starting material for the production of regenerated cellulose fibers such as lyocell, viscose, modal or cupro fibers. Alternatively, the recycled cellulosic raw material can be used as a starting material for the production of paper, paper-like materials or nonwovens from chemical pulp.

In general, it is also mentioned that the mixed textile waste in the context of the present invention can be a mixture containing any cellulosic fibers that form the cellulosic component of the mixed textile waste and any polyester fibers that form the polyester component. Suitable cellulosic fibers include, for example, natural cellulosic fibers such as cotton, flax,

hemp, ramie, kapok, etc., or regenerated cellulosic fibers such as rayon, viscose, lyocell, cupro, or modal. The fibers specified above can vary in diameter and length and can be continuous fibers (filaments) or staple fibers or can also be in non-woven form. Such mixed textile waste has at least 1% by weight, preferably at least 2% by weight, particularly preferably at least 3% by weight, of the cellulose and polyester components.

Furthermore, a particularly economical and reliable recycling process can be provided if the mixed textile waste is pre- and/or post-consumer textile waste. Post-consumer textile waste refers to textiles that have already reached the end consumer and can therefore contain foreign substances, sometimes to a considerable extent, as a result of their use. Post-consumer textile waste may include one or more of the following items: worn-out garments such as shirts, jeans, skirts, dresses, suits, overalls, pants, underwear, sweaters, and the like; used home textiles such as bed linen, towels, curtains, cloths, tablecloths, seat covers, curtains, upholstery fabrics or the like; Non-woven articles such as towels, diapers, filters or the like. Pre-consumer textile waste is understood to mean textile materials that have not yet reached the end consumer, but have been generated as waste during production. This can be leftovers from cuttings or waste from the manufacture of clothing, home textiles, nonwovens, etc. or production waste from the manufacture of yarns, textiles or regenerated cellulose fibers.

Further preferred embodiments of the invention result from the dependent claims 2 to 11.

Ways to carry out the invention

[0024] In the following, the invention is exemplified by means of a first embodiment variant. Further design variants result from the modifications mentioned in the description, which can be combined with one another in any desired way.

According to a first embodiment variant, the waste liquor containing degradation products is processed and utilized in the method according to the invention, which from an alkaline pulping process for obtaining a recycled

Cellulose raw material comes from a mixed textile waste. The mixed textile waste has at least one cellulose component and one polyester component, which can also be contaminated with dyes, crosslinking agents or the like. The alkaline digestion process is preferably an alkaline boiling of the mixed textile waste in an aqueous alkaline treatment solution in order to hydrolyze the polyester component to form polyester degradation products and to dissolve them in the treatment solution. The cellulose component is minimally leached out or degraded in the pulping process and can therefore be easily separated from the aqueous treatment solution and thus separated from the polyester degradation products to recover the recycled cellulose raw material. The remaining, spent treatment solution is then treated as the waste liquor containing the degradation products according to the present method.

[0026] Since the spent liquor contains the degradation products from an alkaline digestion process of mixed textile waste, its composition differs fundamentally from spent liquors which occur in classic pulp production processes (e.g. through sulfate or sulfite digestion). The main goal of the chemical digestion in the context of the process in question is also the leaching of polyester (especially PET) and other non-cellulose-containing components, as well as various dyes and other substances used in the textile industry (plasticizers, etc.) from the mixed textile waste in order to obtain a cleaned cellulose component. As previously described, the spent liquor therefore contains significant polyester degradation products and is essentially free of lignins and other wood-specific impurities, degradation products or ingredients. In the context of the present invention, “essentially” means that none of the substances occurring in the process according to the invention usually contain any lignin. Traces of lignin could at most get into the process according to the invention accidentally and unintentionally.

According to the first embodiment, NaOH is used as the hydrolysis agent for the alkaline digestion. Accordingly, the polyester degradation products mainly consist of disodium terephthalate (Cs^CLNaa) and monoethylene glycol (C2H6O2). From 1 mol of PET and 2 mol of NaOH, 1 mol of disodium terephthalate and 1 mol of monoethylene glycol are produced stoichiometrically as polyester degradation products. In terms of weight, the ratio is then as follows: For the conversion of 1 kg of PET, about 0.42 kg of NaOH are consumed stoichiometrically and about 1.1 kg of disodium terephthalate and 0.33 kg of monoethylene glycol are formed. Since the alkaline digestion process usually works with super-stoichiometric ratios of NaOH, the spent digester liquor also contains corresponding amounts of effective alkali metal/NaOH that has not yet been used. An essential property of the spent liquor is its content of the polyester degradation products disodium terephthalate and monoethylene glycol, which make it impossible to treat the spent liquor with the recovery processes customary in the pulp industry. Furthermore, compared to conventional digester liquors from pulping, the spent liquor lacks any lignin and also any hemicelluloses, since mixed textile waste is being digested and not wood. Disodium terephthalate, an organic salt, poses a challenge for the process because, due to its solubility during the evaporation of the spent liquor, which is necessary for caloric utilization of the spent liquor in order to adjust the calorific value of the spent liquor for incineration, from a solids content I from about 20% by mass of disodium terephthalate begins to precipitate (depending on the residual content of other solids).

In a further embodiment variant, other suitable hydrolysis agents can also be used for the alkaline digestion, but this does not have to be explained in more detail here.

The spent liquor coming from the alkaline pulping process has a solids content of 3-10%, depending on the cooking conditions and raw material properties. First, the liquor passes through a slot screen (fiber filter) customary in the pulp industry to largely remove the remaining fibrous components, which would have a disadvantageous or disruptive effect in later evaporation.

In a first step of the process for the treatment and utilization of the waste liquor containing the degradation products, water is evaporated from the waste liquor in order to precipitate the polyester degradation product from the waste liquor and a two-phase mixture with an aqueous phase and a solid phase containing the polyester degradation product to get phase. The waste liquor, freed from fiber residues, is fed to an evaporation unit. In one embodiment, this unit can be a falling-film evaporator (e.g. a thin-film evaporator) that is heated with low-pressure steam. Other suitable aggregates and heating means can also be used. Here, attention must be paid to the solubility of the main components in the lye. If this is taken into account, the disodium terephthalate begins to precipitate as a polyester degradation product at a solids content of approx. 20% and the increase to approx. 30% then takes place correspondingly quickly. From a solids content of about 30%, the evaporation process should be interrupted in order to avoid adverse effects of the solid phase on the evaporation unit (caking, declining heat transfer). This is favored above all by the strong tendency of the disodium terephthalate particles to form larger agglomerates.

patent claims

1. A process for processing and recycling waste liquor containing at least polyester degradation products from an alkaline pulping process to obtain a cellulose raw material from mixed textile waste, which contains at least one cellulose component and one polyester component, the process comprising the following steps:

a) evaporating water from the spent liquor in order to precipitate the polyester degradation product from the spent liquor and to obtain a two-phase mixture with an aqueous phase and a solid phase containing the polyester degradation product,

b) separating the solid phase from the aqueous phase, and

c) utilization of the solid phase.

2. The method according to claim 1, characterized in that after step b) the aqueous phase of a waste water treatment or other use in the pulp production process is fed.

3. The method according to any one of claims 1 to 2, characterized in that the solid phase in step c) is thermally utilized to generate energy in the form of steam and / or electricity.

4. The method according to claim 3, characterized in that after step c) the solid phase is fed to a boiler with melting chamber firing.

5. The method according to any one of claims 1 to 4, characterized in that in step a) water is evaporated from the spent caustic until it has a solids content of between 20 and 40% by weight.

6. The method according to any one of claims 1 to 5, characterized in that the spent liquor comprises NaOH and the polyester degradation product is a sodium salt of

is terephthalic acid.

7. The method according to any one of claims 1 to 6, characterized in that the contaminated spent liquor is essentially free of lignins.

8. The method according to any one of claims 1 to 7, characterized in that the alkaline pulping process of the mixed textile waste comprises an alkaline hydrolysis of the polyester component to polyester degradation products.

9. The method according to claim 8, characterized in that the alkaline pulping process further comprises separating the cellulose component from the polyester degradation products in order to obtain the cellulose raw material.

10. The method according to claim 1, characterized in that the evaporation of water from the spent liquor takes place in two or more stages, preferably in two to ten stages, particularly preferably in two to six stages.

11. The method according to claim 10, wherein at least one stage is followed by a mechanical separator for separating the solid and liquid phase.

12. The method according to claim 10, characterized in that so many passes are carried out until the final filtrate no longer contains any residue of disodium terephthalate.