METHOD FOR THE RECYCLING OF INGREDIENTS FROM MIXED TEXTILE WASTE

Description

Technical field

[0001] The present invention relates to a process for the treatment and utilization of waste liquor contaminated with degradation products from an alkaline pulping process for obtaining a cellulose raw material from mixed textile waste. State of the art

[0002] 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 environmental impact of these textile wastes.

[0003] The purely mechanical recycling of textile waste has long been common knowledge. This involves shredding the textile waste and directly producing recycled end products such as cleaning cloths, filling materials, or insulation materials. Spinning yarns from such recycled textile fibers typically results in low-quality yarns that are unsuitable for the production of new textiles.

[0004] Chemical recycling processes offer a suitable solution to overcome the aforementioned problems. For example, cellulosic fibers can be re-spun into regenerated cellulosic fibers after chemical pretreatment. However, such processes for producing regenerated cellulosic molded articles are highly sensitive to impurities in the cellulose raw material, which generally makes such recycled cellulose raw material unsuitable for spinning fibers.

[0005] WO 2015 / 077807 A1 describes a process for the pretreatment of recycled cotton fibers from textile waste. Metals are first removed from the recycled cotton fibers, which are then subjected to oxidative bleaching. The recycled cotton fibers can then be used to produce regenerated cellulosic molded articles.

[0006] WO 2018 / 115428 A1 discloses a process for treating a cotton-based raw material under alkaline conditions in conjunction with gaseous oxidizing agents.

[0007] WO 2018 / 073177 A1, in turn, describes a process for recycling a cellulose raw material from cellulosic textile waste. The textile waste is treated under alkaline conditions in the presence of a reducing agent to swell the cellulosic fibers in the textile waste and thus better remove contaminants. After the alkaline treatment, the cellulose raw material is bleached with either oxygen and / or ozone.

[0008] Such processes use pure cellulosic textile waste as the starting material. In practice, however, 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 containing polyester and cellulosic fibers. Likewise, textile waste from cotton textiles is usually contaminated with polyester from sewing threads, labels, or the like. However, processes of the aforementioned type are generally unable to process mixed textile waste, as significant contamination with synthetic polymer fibers, especially polyester, cannot be removed.

[0009] However, the present process concerns processes for the chemical treatment of mixed textile waste which, in addition to a cellulose component, also contain significant amounts of a polyester component.

[0010] US 2019 / 0218362 A1 describes systems and methods involving subcritical aqueous treatment to recycle the cellulosic and polyester components of cotton waste and cotton-polyester blended textile waste that would otherwise be discarded.

[0011] Anna Peterson's thesis "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, 1 January 2015, discloses a process for the complete hydrolysis of polyester from blended textiles using the phase transfer catalyst benzyltributylammonium chloride under mild conditions.

[0012] However, during the alkaline-chemical digestion of such mixed textile waste at appropriate pressure and temperature, especially for the recovery of a cellulose raw material, considerable quantities of waste liquor are generated, which contain various degradation products, including a large amount of polyester degradation products. Such waste liquors cannot be transferred to conventional wastewater treatment without prior treatment, as they generally have excessively high chemical oxygen demand (COD) and metal content. Disclosure of the invention

[0013] It is therefore an object of the invention to provide a process of the type mentioned at the outset for the treatment and utilization of waste liquor, which process contains the degradation products mentioned above and thereby enables the utilization of the waste liquor and reduces the burden on wastewater treatment processes.

[0014] The object is achieved according to the invention by a method according to claim 1.

[0015] In the process according to the invention for the treatment and utilization of waste liquor contaminated with degradation products from an alkaline pulping process to obtain a recycled cellulose raw material from mixed textile waste, water is evaporated from the waste liquor to precipitate the polyester degradation product from the waste liquor and obtain a two-phase mixture with an aqueous phase and a solid phase containing the polyester degradation product. This enables a reproducible separation of the polyester degradation products from the contaminated waste liquor. If the solid phase is further separated from the aqueous phase, the aqueous phase of the waste liquor can be fed to a conventional wastewater treatment plant, thereby minimizing the pollution and chemical oxygen demand of the wastewater treatment process.Finally, the solid phase can be utilized independently of the aqueous phase, making the process in question more environmentally friendly.

[0016] For the purposes of the present invention, "polyester" primarily refers to polyethylene terephthalate (PET), which consists of the monomers terephthalic acid and ethylene glycol. However, the invention also works very well with other widely used polyesters such as polypropylene terephthalate (PPT), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), and similar materials, or even mixtures of these polyesters. It is important that the alcoholic component, i.e., ethylene glycol, butanediol, propylenediol, trimethylene glycol, etc., is readily soluble in the treatment solution and is not precipitated during the precipitation of the terephthalic acid.

[0017] A waste liquor within the meaning of the present invention is generally understood to mean a spent 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 generally contaminated by dyes and other foreign substances and mixed with the polyester component at the fiber level, the foreign substances and the polyester component are dissolved out by the alkaline treatment solution and degraded, in particular hydrolyzed, to form degradation products. The waste liquor thus contains, as degradation products, at least polyester degradation products and, optionally, dye or other foreign substance degradation products.

[0018] During the alkaline decomposition process, the molecular weight and molecular chain length of the polyester molecules in the polyester component are deliberately reduced by hydrolysis, which takes place in the presence of the aqueous alkaline treatment solution. In this way, the molecular chain length of the polyester component molecules is continuously reduced and ultimately 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. If NaOH is used, for example, the polyester decomposition product predominantly contains disodium terephthalate, which is readily soluble in the treatment solution.This subsequently enables a simple separation of the polyester degradation products from the cellulose component, allowing the cellulose raw material to be recovered from the mixed textile waste with high purity. Due to the generally mild process conditions, only insignificant degradation of the cellulose polymers in the cellulose component occurs. At the same time, however, the cellulose component can advantageously be partially digested by the treatment solution and freed from impurities such as dyes or crosslinkers, which in turn benefits the quality of the recovered cellulose raw material.

[0019] The disodium terephthalate contained in the waste liquor as a polyester degradation product precipitates when water is evaporated from the waste liquor, forming a two-phase mixture with a solid phase and an aqueous phase. The solid phase comprises the disodium terephthalate, and the aqueous phase is formed from the aqueous supernatant, which contains residues of the alkaline treatment solution and possibly other degradation products. This aqueous phase can be fed directly to wastewater treatment with minimal environmental impact or, alternatively, used to recover the alkaline treatment solution. In a further embodiment, the aqueous phase is evaporated again, whereby further terephthalate can be precipitated. This increases the total calorific value of the substances obtained according to the invention and further reduces the COD of the aqueous phase fed to the wastewater treatment.The solid phase can therefore be recycled independently of the aqueous phase and does not have to undergo a resource-intensive and environmentally unfriendly joint processing process.

[0020] For the purposes of this invention, recycled cellulose raw material is understood to mean recycled pulp, textile pulp, cotton pulp, rag pulp, or the like, or combinations thereof. This cellulose raw material can, in particular, be suitable as a starting material for the production of regenerated cellulose fibers such as lyocell, viscose, modal, or cupro fibers. Alternatively, the recycled cellulose raw material can serve as a starting material for the production of paper, paper-like materials, or nonwovens from cellulose.

[0021] In general, it is also mentioned that the mixed textile waste within the meaning of the present invention can be a mixture containing any cellulose fibers, which form the cellulose component of the mixed textile waste, and any polyester fibers, which form the polyester component. Suitable cellulose fibers include, for example, natural cellulose fibers, such as cotton, flax, hemp, ramie, kapok, etc., or regenerated cellulose 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 even be in nonwoven form. Such a mixed textile waste comprises at least 1 wt. %, preferably at least 2 wt. %, particularly preferably at least 3 wt. %, of the cellulose and polyester components.

[0022] 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 final consumer and may therefore contain foreign substances, sometimes in significant quantities, due to their use. Post-consumer textile waste can include one or more of the following elements: worn garments such as shirts, jeans, skirts, dresses, suits, overalls, trousers, underwear, sweaters, and the like; used home textiles such as bed linen, towels, curtains, cloths, tablecloths, seat covers, drapes, upholstery fabrics, or the like; nonwoven items such as wipes, diapers, filters, or similar. Pre-consumer textile waste refers to textile materials that have not yet reached the final consumer but have accrued as waste during production.These can be cutting scraps or waste from the production of garments, home textiles, nonwovens, etc. or production waste from the manufacture of yarns, textiles or regenerated cellulose fibres.

[0023] Further preferred embodiments of the invention emerge from the dependent claims 2 to 11. Ways to implement the invention

[0024] The invention is illustrated below using a first exemplary embodiment. Further embodiments result from the modifications mentioned in the description, which can be combined with one another in any desired way.

[0025] According to a first embodiment, the process according to the invention involves processing and utilizing the waste liquor containing degradation products, which originates from an alkaline digestion process for obtaining a recycled cellulose raw material from mixed textile waste. The mixed textile waste comprises at least one cellulose component and one polyester component, which may additionally be contaminated with dyes, crosslinkers, 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 polyester degradation products and to dissolve them in the treatment solution. The cellulose component is only minimally dissolved or dissolved in the digestion process.degraded, and can therefore be easily separated from the aqueous treatment solution, and thus from the polyester degradation products, to obtain the recycled cellulose feedstock. The remaining, spent treatment solution is then processed as waste liquor containing the degradation products according to the present process.

[0026] Since the waste liquor contains the degradation products from an alkaline pulping process of mixed textile waste, its composition differs fundamentally from waste liquors generated in conventional pulp production processes (e.g., through sulfate or sulfite pulping). The main objective of the chemical pulping process in this process is to remove 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 purified cellulose component. Therefore, as previously described, the waste liquor contains a significant amount of polyester degradation products and is essentially free of lignins and other wood-specific contaminants, degradation products, or ingredients.In the context of the present invention, "essentially" means that all substances used in the process according to the invention typically do not contain lignin. Traces of lignin could only enter the process according to the invention accidentally and unintentionally.

[0027] For alkaline digestion, according to the first variant, NaOH is used as the hydrolysis agent. The polyester degradation products therefore predominantly comprise disodium terephthalate (C 8 H 4 O 4 Na 2 ) and monoethylene glycol (C 2 H 6 O 2 ). From 1 mol PET and 2 mol NaOH, 1 mol disodium terephthalate and 1 mol monoethylene glycol are formed stoichiometrically as polyester degradation products. In terms of weight, the ratio is then as follows: For the conversion of 1 kg PET, approximately 0.42 kg NaOH is stoichiometrically consumed, and approximately 1.1 kg disodium terephthalate and 0.33 kg monoethylene glycol are formed. Since the alkaline digestion process usually works with superstoichiometric ratios of NaOH, the digester waste liquor also contains corresponding amounts of unused effective potassium / NaOH.A key property of the waste liquor is its content of the polyester degradation products disodium terephthalate and monoethylene glycol, which make it impossible to treat using recovery processes commonly used in the pulp industry. Furthermore, compared to conventional digester waste liquors from pulping, the waste liquor lacks any lignin or hemicelluloses, as it is a pulping of mixed textile waste and not wood. Disodium terephthalate, an organic salt, poses a challenge for the process because, due to its solubility, it begins to precipitate at a solids content of approximately 20% by mass of disodium terephthalate during evaporation of the waste liquor, which is necessary for calorific utilization of the waste liquor to adjust the calorific value of the waste liquor for combustion (depending on the residual content of other solids).

[0028] In a further variant, other suitable hydrolysis agents can also be used for alkaline digestion, but this need not be explained in detail here.

[0029] The waste liquor 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 slotted screen (fiber filter) commonly used in the pulp industry to largely remove residual fibrous components that would have a detrimental or disruptive effect on subsequent evaporation.

[0030] In a first step of the process for treating and utilizing the waste liquor containing the degradation products, water is evaporated from the waste liquor to precipitate the polyester degradation product from the waste liquor and obtain a two-phase mixture with an aqueous phase and a solid phase containing the polyester degradation product. The waste liquor, freed of 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) heated with low-pressure steam. Other suitable units and heating media can also be used. Attention must be paid to the solubility of the main components in the liquor. Taking this into account, disodium terephthalate, a polyester degradation product, begins to precipitate at a solids content of approximately 20%, and the increase to approximately 30% then occurs correspondingly rapidly.At a solids content of approximately 30%, the evaporation process should be interrupted to avoid adverse effects of the solid phase on the evaporation unit (caking, reduced heat transfer). This is primarily due to the strong tendency of disodium terephthalate particles to form larger agglomerates.

[0031] In a further step of the process, the solid phase is separated from the aqueous phase. The spent liquor, which has been evaporated to a solids content of between 20 and 30%, is then fed to a mechanical separator. Various types of centrifuges and decanters are preferably used here. The solids content and particle properties are very disadvantageous for cake-forming filtration, as this can only be achieved with the use of large quantities of filtration aids. In this separation unit, a large part of the solid phase is separated from the aqueous phase, the latter consisting mainly of water, residual alkali and monoethylene glycol or its other degradation products. The supernatant liquid is separated from the solid phase in this process. The solid phase then has a solids content of approx. 60%, as the inherent moisture cannot be completely separated. However, heating orA calorific value sufficient for the combustion of the solid in a combustion boiler can be achieved, since the residual moisture also contains monoethylene glycol, which increases the calorific value in addition to that of the solid phase, whereby in any case the highest possible solid content is desirable.

[0032] In preferred embodiments of the process according to the invention, water is evaporated from the waste liquor in two or more stages, preferably in two to ten stages, particularly preferably in two to six stages. The more stages used, the more energy-efficient the evaporation operation, but the higher the investment expenditure. In particular, the evaporation should be carried out in as many stages until the final filtrate no longer contains any residue of disodium terephthalate. Each stage can be followed by a mechanical separator (e.g. a centrifuge) to separate the solid phase and utilize it in a combustion boiler, or the evaporation stage itself can be designed accordingly for the evaporation of suspensions (e.g. thin-film evaporator). In a preferred embodiment of the present invention, at least one stage is followed by a mechanical separator to separate the solid and liquid phases.However, several stages can also be followed by a mechanical separator to separate the solid and liquid phases.

[0033] In one design variant, the thus separated solid phase is fed to a combustion boiler – in this case with a slag-chamber furnace – via a conveyor screw, where it is burned for thermal and / or energy recovery. Steam can be generated through combustion by steam generation (steam drum with natural or forced circulation). If this steam is generated at least partially as high-pressure steam, it can also be used to generate electricity by feeding the high-pressure steam or a partial flow of the high-pressure steam to a steam turbine, which drives a generator.

[0034] In the combustion boiler, which preferably includes a smelting chamber furnace (so-called soda or power boiler), a soda (Na 2 CO 3 ) melt and ash, which is largely composed of soda (Na 2 CO 3 ), are formed. The flue gas produced during combustion is dedusted using an electrostatic precipitator or bag filter, further processed if necessary (desulphurization, denitrification), and then fed into a stack. The separated dust, which consists largely of soda, can then be collected, compacted, and potentially sold to various industries.

[0035] The soda melt withdrawn from the boiler is fed to a melt dissolving tank, where it is cooled and redissolved. The resulting aqueous stream containing dissolved sodium carbonate can then either be used to neutralize acidic wastewater from pulp production or—if there is sufficient residual effective potash content—be fed into a conventional kraft process chemical recovery system to recover NaOH.

[0036] Alternatively, in another variant, the amount of disodium terephthalate present in the digester waste liquor can be mixed with an appropriate amount of acid for further recycling (e.g., sulfuric acid, which, on a molar basis, corresponds to the amount of disodium terephthalate present). The reaction with sulfuric acid then produces terephthalic acid, which, due to its solubility, immediately precipitates, and sodium sulfate, which remains in solution. The terephthalic acid could then be incinerated, and the filtrate, with a certain sulfate content, would have to be sent to wastewater treatment, or the sodium sulfate could be recovered from the filtrate by evaporation or cooling crystallization.

[0037] Depending on the residual solids content of the aqueous phase from the separator, this aqueous phase can be fed directly to a wastewater treatment plant in one embodiment, or alternatively, it can be evaporated again for further utilization. The vapors from the first evaporation step can be used for evaporation after vapor compression. Fresh steam (low-pressure steam) must be added if necessary. The evaporated liquor is then separated into a solid and a liquid phase using a filter or separator (depending on the solids content). Depending on the starting raw material and separation efficiency, the filtrate can either be returned to the pulp process for washing steps or for neutralizing acidic wastewater, or it can be fed to a wastewater treatment plant. The resulting solids are fed to the combustion boiler. Example:

[0038] Post-consumer used textiles (a blend of cotton and polyester, 80 to 20 wt.%) were boiled with caustic soda (15 wt.% NaOH, based on the mass of used textiles) at a liquor ratio of 1:7 (mass of used textile:lye). The temperature was 150°C for a boiling time of 120 min. Due to the depolymerization of the polyester fibers that took place under these conditions, the disodium terephthalate formed, which is water-soluble under these conditions, entered the boiling liquor, which was finally separated from the remaining solid (the cotton fibers) through a sieve. The boiling of used textiles produces a lye quantity of approximately 1.75 t per hour, with a dry matter content of approximately 10% and a temperature just below boiling point (approx. 95°C). This waste liquor is thickened to a dry matter content of 65% in a multi-stage series of evaporation and filtration, whereby the liquor has a calorific value of 7,300 kJ / kg.The mass flow after evaporation is 0.27 t / h. This occurs in three sequential evaporation stages, whereby the recycling of vapors is initially neglected and each stage is supplied with fresh steam. This entire evaporation requires approximately 1.52 t / h of steam at a temperature level of 115°C and 1.677 bar (absolute), which corresponds to a low-pressure steam quantity of approximately 1.47 t / h (4.9 bar (absolute), 155°C). After each evaporation, the solids are separated by a filtration unit and ultimately remixed with the filtrate to prevent caking on the heating surfaces and transport problems. The total condensate quantity is 1.48 t / h. This corresponds to a heat output of approximately 3.4 GJ / hr. The thickened liquor stream is then fed into a liquor combustion boiler (natural circulation). At a combustion chamber temperature above the ash melting temperature (approx.870°C) the following process streams can be generated assuming complete combustion with a fresh water supply of approx. 330 kg / hr: . Flue gas: approx. 1000 Nm 3 < / hr at a temperature of 130°C and pressure of 1.00 bar (absolute) with the following composition ∘ CO 2 : 12.3 vol% ∘ N 2 : 66.18 vol% ∘ O 2 : 1.27 vol% ∘ H 2 O: 20.24 vol% ∘ SO 2 : 0.0033 vol% Ash: approx. 8.5 kg / hr with an ash temperature of 210°C and an ash composition of 99.1% sodium carbonate (rest sodium sulfate) Melt: approx. 77 kg / hr with a temperature of 850°C and the following composition (Na 2 CO 3 : 96.45 wt%, Na 2 SO 4 : 0.18 wt%, Na 2 S: 0.9 wt%, remainder carbon and inert materials) Energy in the form of high pressure steam (500°C, 50 bar): approx. 300 kg / hr Required additional air for combustion: approx. 900 m 3 / hr (regulated to 9% excess air) Boiler blowdown: approx. 6.5 kg / hr

Claims

1. A method for the treatment and utilization of waste liquor containing at least polyester degradation products and originating from an alkaline pulping process for the extraction of cellulose raw material from blended textile waste which includes at least a cellulose component and a polyester component, the method comprising the following steps: a) evaporating of water from the waste liquor in order to precipitate the polyester degradation product from the waste liquor and to obtain a two-phase mixture having an aqueous phase and a solid phase including the polyester degradation product, b) separating the solid phase from the aqueous phase, and c) utilizing the solid phase.

2. The method as claimed in claim 1, characterized in that, following step b), the aqueous phase is fed to a waste water treatment process or put to another use in the pulp production process.

3. The method as claimed in any of claims 1 to 2, characterized in that the solid phase is utilized thermally in step c) for the generation of energy in the form of steam and / or electric power.

4. The method as claimed in claim 3, characterized in that, following step c), the solid phase is fed to a boiler provided with slag tap firing.

5. The method as claimed in any of claims 1 to 4, characterized in that, in step a), water is evaporated from the waste liquor until it has a solids content between 20 and 40 wt%.

6. The method as claimed in any of claims 1 to 5, characterized in that the waste liquor includes NaOH and that the polyester degradation product is a sodium salt of terephthalic acid.

7. The method as claimed in any of claims 1 to 6, characterized in that the contaminated waste liquor is substantially devoid of lignins.

8. The method as claimed in any of claims 1 to 7, characterized in that the alkaline pulping method of the blended textile waste comprises an alkaline hydrolysis of the polyester component into polyester degradation products.

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

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

11. The method as claimed in claim 10, wherein a mechanical separator for separating the solid and the liquid phase is provided downstream of at least one stage.

12. The method as claimed in claim 10, characterized in that as many cycles are carried out as are necessary until the final filtrate no longer contains any disodium terephthalate residue.