Separation and purification

Abstract

Method for the separation and purification of cellulosic fibers from textile waste comprising the following steps: a) providing textile waste which comprises cellulosic and non-cellulosic fibers and contaminants, as raw material, b) mechanical treatment of the raw material, c) mixing the mechanically treated raw material with a liquid medium in a mixer to produce a pulp suspension, d) screening of raw material particles in the pulp suspension to produce a reject fraction stream and an accept fraction stream based on the size of the raw material particles, e) weight-based purification of the accept fraction stream to produce a purified cellulose fraction.

Description

[0001] The present invention relates to a method for the separation and purification of cellulosic fibers from textile waste. Furthermore, the invention relates to an apparatus for the separation and purification of cellulosic fibers from textile waste.BACKGROUND OF THE INVENTION

[0002] Global fabric production has risen steadily in recent decades, resulting in an annual volume of 100 million tons of fabrics. With such a quantity of fabrics, there is also a lot of post-consumer waste, but less than 1% of the global fiber market came from pre- and post-consumer recycled textiles. As a result, the fabric industry is facing enormous challenges in terms of sustainability, resource efficiency and the need for effective recycling solutions. Accordingly, there is a gaining interest in various commercial and industrial applications in fabric recycling to address this problem.

[0003] One important component in the production of fabrics are cellulosic fibers, which can either be plant fibers or man-made cellulose fibers (MMCF), such as viscose or lyocell. In the production of wood-based cellulosic fibers, a high purity of cellulose is extremely important, especially in the production of lyocell. As a result, there is a strong interest in recycling processes that can separate and purify cellulosic fibers from textile waste.

[0004] In principle, the separation and purification of the cellulosic fibers from different synthetic fibers and other impurities present in textile waste is challenging. Both procedures, separation and purification, require the application of different processes, equipment and techniques based on morphological and surface differences between cellulosic and synthetic fibers, e.g. fiber length, thickness, stiffness, specific surface area, wettability and so on.

[0005] The prior art describes several methods for separation of fibers and one of these methods is disclosed in document WO 2020 / 127453 A1, which provides a method for the separation of fibers comprising the steps of providing a mixture of cellulose and non-cellulose fibers, reducing the cellulose chain length, subjecting the mixture of fibers to mechanical treatment to break agglomerates of fibers, adjusting the concentration of fibers in the mixture and subjecting the mixture to flotation to remove the non-cellulose fibers. In the step of reducing the cellulose chain length, the viscosity of cellulose fibers is lowered to between 200 and 900 ml / g. This chemical pre-treatment is performed before the flotation process. In the field of separation techniques, the term "flotation" denotes a separation process wherein hydrophobic materials are selectively separated from hydrophilic materials. A slurry of hydrophobic particles and hydrophilic particles is introduced into flotation cells that are aerated to produce bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is skimmed from the cell.

[0006] Another method and device for the separation of mixed fibers by using the triboelectric effect is disclosed in WO 2022 / 156775 A1. This method includes different steps, starting with extraction of single fibers from the mixed fibers, wherein the mixed fibers are consisting of protein-based fibers, cellulose-based fibers and / or synthetic fibers. This is followed by the usage of an air flow to carry the single fibers to a triboelectric device, which charges the single fibers. Afterwards another air flow is used to carry the charged fibers to a separation device, which separates the positively charged fibers from the negatively charged fibers. The separated fibres are then transported again via an air flow to a collecting device, which collects the charged fibers with different polarities. However, this method is more suitable for separation of fibers with having opposite electrical charges. Cellulosic fibers carry a low electrical charge and can therefore hardly separated from the other synthetic fibers by triboelectric effect.SHORT DESCRIPTION OF THE INVENTION

[0007] In view of the above description of the background of the invention, the objective task of the invention is to provide a method for efficient separation and purification of cellulosic fibers from synthetic fibers and other impurities. More specifically, cellulosic fibers are separated and purified from textile waste, wherein the separation is based on the morphological and surface differences between those two types of fibers.

[0008] A solution is provided by a method for the separation and purification of cellulosic fibers from textile waste comprising the following steps: a) providing textile waste which comprises cellulosic and non-cellulosic fibers and contaminants, as raw material, b) mechanical treatment, c) mixing the mechanically treated raw material with a liquid medium in a mixer to produce a pulp suspension, d) screening of raw material particles in the pulp suspension to produce a reject fraction stream and an accept fraction stream based on the size of the raw material particles, e) weight-based purification of the accept fraction stream to produce a purified cellulose fraction.

[0009] The method according to the invention provides an efficient process, which enables obtaining as pure as possible cellulosic fibers from textile waste comprising synthetic fibers. Furthermore, the screening step followed by the weight-based purification increase the energy efficiency and reduce CO 2 emissions. Such an efficient method simplifies the production of high-purity pulp or high-purity cellulosic fibers, which is required for the lyocell process and for the viscose process. Even if the viscose process allows higher tolerances for impurities, the purity of the pulp or cellulosic fibers should be as high as possible.

[0010] In the first step a) of the process, the raw material, which may consist of collected and pre-sorted textile waste or used textiles, is provided. The textile waste or used textiles comprise cellulosic fibers, non-cellulosic fibers and non-cellulosic contaminants. Moreover, the textile waste or used textiles can originate from used clothes, home textiles, recycled textiles, recycled textile fibers, used textiles from textile production or similar, for example. The textile waste may be a pre- and / or post-consumer textile waste. A post-consumer textile waste may comprise one or multiples of the follows: worn garments such as shirts, jeans, skirts, dresses, gowns, suits, overalls, trousers, pants, underwear, sweaters, pullovers, etc.; used home-textiles such as bed linen, towels, curtains, cloths, tablecloths, seat covers, drapery, upholstery fabrics or the like; nonwoven articles such as wipes, diapers, filters or similar. Pre-consumer textile waste may comprise cuttings or trimmings from the production of garments, home-textiles, nonwovens, etc., or production waste from the manufacturing of yarns, textiles or regenerated cellulosic fibers.

[0011] Afterwards in step b) mechanical treatment of the raw material is performed, wherein the mechanical treatment is preferably carried out in a dry state. The mechanical treatment can comprise a comminution (size reducing treatment) of the raw material. Preferably, the comminution selected from the list of cutting, shredding, grinding, milling, fiberizing or a combination thereof. Comminution refers to a process of cutting, shredding, grinding, milling, fiberizing, comminution or other conventional techniques for reducing the size, particle size or fiber length of raw material into smaller textile pieces, particles or fiber lengths. Accordingly, any comminutor such as a shredder, cutter, grinder, mill, fiberizer or combination thereof that is suitable for this task can be used for this mechanical treatment. The term "fiber length" refers to the length of cellulosic and non-cellulosic fibers. As a result, energy can be saved by dry disintegration compared to wet disintegrating methods.

[0012] In step c), the mechanically treated raw material is mixed with a liquid medium in a mixer to produce a pulp suspension. The liquid medium is preferably water or water based. A water-based liquid medium refers to a cooking liquor or other alkaline effluents, for example. These water-based liquid mediums can come from viscose production, paper production or pulp processing. Black liquor or white liquor are examples for cooking liquors. Black liquor is a byproduct of kraft process for conversion of wood into wood pulp and contains most of the original inorganic components and a high concentration of dissolved organics. It is usually used to recover cooking chemicals and produce high-pressure steam used in the pulp making process. In contrast, white liquor is a strongly alkaline solution mainly of sodium hydroxide and sodium sulphide, which is typically used in the first stage of the kraft process.

[0013] In one embodiment the pulp suspension is a low-consistency pulp suspension, preferably comprising from 0.5 % w / w to 5% w / w solids.

[0014] The term "consistency", as it is used herein refers to the contents of solids in a suspension. The consistency is usually given as a weight percentage (% w / w). The term "low-consistency", as it is used herein, refers to a consistency of 8% w / w or less.

[0015] The steps after mixing the size reduced raw material with a liquid medium, which can preferably be done in any suitable type of industrial mixer or in a pulper, are carried out under wet conditions.

[0016] After the mixing, the screening step (step d) and the weight-based purification step (step e) are performed. It should be noted that step d) does not necessarily be immediately followed by step e), as further process steps can be arranged in-between those two steps. The screening step produces the reject fraction stream and the accept fraction stream based on the size of the raw material particles, while the weight-based purification step purifies the accept fraction stream to produce a purified cellulose fraction. For example, a screener can be used to perform the screening step, and the reject fraction stream preferably comprises non-cellulosic fibers such as for example polypropylene and polyethene and / or other non-cellulosic contaminants, which can be removed by the screening.

[0017] The term "screen", as it is used herein, refers to any device that separates material depending on particle sizes. The screen poses a barrier to larger particles and lets smaller particles pass. There are many different screen designs and they can usually be categorized by their specific slots size or holes size.

[0018] The purified cellulose fraction or purified material resulting from the method according to the invention can be used as a starting material for manufacturing regenerated cellulosic molded bodies. In general, it is stated that a regenerated cellulosic shaped body is understood in particular to mean a fiber, a filament (continuous fiber), a film, a powder or a microsphere (microbead) which have been produced by a xanthate process (such as the viscose or modal process) or a direct solution process (such as the lyocell process / amine oxide process). These cellulosic molded bodies can be used to produce new textile materials, such as clothing, woven fabric, non-woven fabric, a microsphere, beads and a sponge. Furthermore, the cellulosic molded bodies comprise mainly cellulosic components due to the method according to the invention.

[0019] In a preferred embodiment, the method comprises a step of breaking down fiber bundles in the pulp suspension and separating fibers into singular fibers in the pulp suspension using a low consistency refining and / or deflaking. By using low consistency refining (LC refining) and / or deflaking the specific energy demand is reduced, especially compared to high consistency refining.

[0020] The low consistency refining and / or deflaking can be applied to the low-consistency pulp suspension between the step of mixing with liquid media and the step of screening. Due to the low consistency refining and / or deflaking, fiber bundles are broken down and separated into fibers in the singular state. Consequently, the rejection of useful or recyclable cellulosic fibers is reduced. Accordingly, the efficiency of the screening process is enhanced.

[0021] In another embodiment, the low consistency refining and / or deflaking is applied to a reject fraction stream from the screening step, wherein the refined and / or deflaked reject fraction stream is fed back to the screening step. As a result, the screen rejects need to be treated by the low consistency refining and / or the deflaker, which results in an increase in efficiency and quality as well as a reduction of the amount of rejects. The screen throughput may then be mixed with the screen rejects treated by LC refining and / or the deflaker for the next step. The increased efficiency and quality are achieved by treating only those fibers, fiber bundles and particles that require further processing. Furthermore, this enables the recycling of more cellulosic fibers.

[0022] In some embodiments the screening step or steps correspond to a multiple stage screening, preferably two- to four-stage screening, which is preferably performed in a cascade. The utilisation of multiple stage screening reduces the reject rate by weight. Furthermore, long and stiff fibers, as well as large size fiber bundles and dirt particles can be removed by the screening stage or multiple screening stage. Large and long fiber bundles can cause roping that can lead to production problems It is advantageous to use a two- to four-stage screening, as single stage screening can lead to significant fiber loss compared to that for multiple stage screening.

[0023] The method can comprise a washing process, which is performed before the weight-based purification process. In this washing process, the liquid medium could be pressed and / or filtered off and the remaining fibers can be washed in one or more washing steps. After washing, the fibers can be again suspended in a different liquid medium to be suitable for the weight-based purification. The washing process prevents that some liquid media, such as cooking liquor, lead to extensive foaming in the weight-based purification. Extensive foaming would be unfavourable for weight-based purification processes.

[0024] In a preferred embodiment the weight-based purification of the screening accept fraction stream comprises a two-stage purification process. These two stages of weight-based purification need to be performed consecutively but can be performed in a different order. This splitting into two stages leads to a process with increased efficiency and optimized separation. This is due to the possibility of highly specialized systems being optimized for specific tasks.

[0025] A first weight-based purification stage can remove non-cellulosic contaminants and impurities, which possess a higher density and / or lower specific surface area compared to cellulosic fibers. This first stage removes especially heavy-weight non-cellulosic contaminants and impurities such as metal particles, dirt or leather. In a preferred embodiment a heavyweight cleaner, preferably a heavyweight hydrocyclone stage or heavyweight hydrocyclone stages, is used to perform the first purification stage.

[0026] A second weight-based purification stage can remove non-cellulosic contaminants and impurities, which possess a lower density and / or higher specific surface area compared to cellulosic fiber. Such non-cellulosic contaminants and impurities are particularly synthetic material such as rubber or synthetic fibers such as elastane, polyester, polyamide, polypropylene. In a preferred embodiment a lightweight cleaner, preferably a lightweight hydrocyclone or lightweight hydrocyclones, is used to perform the second purification stage. With such lightweight hydrocyclones it is possible to remove impurities, fiber fragments or foreign fine impurities such as rubber, elastane, polyester, polyamide, polypropylene.

[0027] The step of weight-based purification of the accepts fraction stream can comprise a CombiCleaner, which can perform heavyweight and lightweight hydrocycloning. A CombiCleaner, which can perform heavyweight and lightweight hydrocycloning at the same time, can be used to remove non-cellulosic contaminants and impurities. The CombiCleaner is a specially designed hydrocyclone, that can combine heavyweight and lightweight cycloning in one stage, so that only or mainly cellulosic fibers remain as purified cellulose fraction or purified material. Thus, it is possible to perform the purification process in a single step.

[0028] The term hydorcycloning refers to the process of separating the remaining cellulosic fiber, non-cellulosic fibers and non-cellulosic contaminants by means of centrifugal force.

[0029] In another embodiment, the step of weight-based purification of the screen accepts fraction stream comprises a combination from the selection of heavyweight hydrocyclone, lightweight hydrocyclone and CombiCleaner. Hydrocyclones usually generate movement in liquid mixtures to separate heavy-weight and lightweight components. The liquid flows (from the screen accepts tank) tangentially into a conical device, creating a rotating movement. Heavy-weight particles are forced to discharge and transported downwards in a spiral, while lightweight particles can escape upwards. Through a respective downward outlet or upward outlet, it is possible to remove the respective heavy-weight or light-weight non-cellulosic fibers or contaminants.

[0030] A solution of the problem to be solved is also provided by an apparatus for the separation and purification of cellulosic fibers from textile waste, the apparatus comprising (i) a comminutor for textile waste comprising cellulosic and non-cellulosic fibers and contaminants, (ii) a mixer for mixing a mixer for mixing comminuted textile waste with a liquid medium to produce a pulp suspension, (iii) a screen for screening of particles in the pulp suspension to produce a reject fraction stream and an accepts fraction stream based on the size of the raw material particles, and (iv) a purifier for weight-based purification of the accepts fraction stream to produce a purified cellulose fraction.

[0031] The comminutor for textile waste comprises means for disintegration or size reduction of the raw material.

[0032] The apparatus can comprise a refiner and / or deflaker performing fiber bundles or flocks separation to singular fibers at low consistency of about 0.5% w / w to 5% w / w.DETAILED DESCRIPTION OF THE INVENTION

[0033] The foregoing and other objects, features and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. Fig. 1shows a schematic diagram of a method according to the invention. Fig. 2illustrates a conceptual diagram of an alternative embodiment of a method according to the invention.

[0034] In Fig. 1 a schematic diagram of a method according to the invention is illustrated. Textile waste is fed to a comminuter in which the coherent parts of the textile waste are disintegrated and / or reduced in its particle size. The comminuter can, for example, be any shredder, cutter or grinder that suitable for this task. The disintegrated and / or size reduced textile waste is then fed to a mixing device, such as an industrial mixer or a pulper, where the textile waste is dispersed in a liquid medium, preferably water or a water based liquid medium, to produce a pulp suspension, preferably a low-consistency pulp. Also, other liquid media could be used, such as cooking liquor or other liquid media known in the prior art. If necessary, effluents from other processes, such as the viscose production process, the paper production process or and chemical or paper pulp treatment or production process can be used. The pulp fluid 1 from the pulper is then fed to a refiner or deflaker which performs the respective low consistency refining 11 or deflaking 12 process. Through the low consistency refining 11 or deflaking 12, the fiber bundles in the pulp fluid are broken down and separated. Subsequently, long and stiff fibers, large size fiber bundles and dirt particles are well dispersed in this pulp and can be more efficiently from fibers fraction.

[0035] A screen is used in a screening 20 stage to remove long and stiff fibers, fiber bundles, and relatively large size contaminants. The screen produces a reject fraction stream and an accept fraction stream based on the size of the material particles in the pulp suspension. Depending on the nature and quality of the raw material textile waste and the production parameters and the processing quality, which determine the nature of the rejects fraction, the rejects can be used as a raw material for other processes or fed to another cleaning or recycling step.

[0036] The screen accept fraction stream typically still contains contaminants and impurities with lower or higher specific surface area or weight compared to cellulosic fiber. Such contaminants and impurities include metal particles, leather, hard plastics, rubber or synthetic fibers such as elastane, polyester, polyamide, polypropylene or plastic fibers, for example. A set of hydrocyclones, particularly heavyweight and lightweight hydrocyclones are used for weight-based purification of the accept fraction stream to produce a purified cellulose fraction. Therefore, the weight-based purification of the accept fraction stream can comprise a two-stage purification process, which is shown in Fig. 1 and 2. A first weight-based purification stage 31 removes non-cellulosic contaminants and impurities, which possess a higher density and / or lower specific surface area compared to cellulosic fibers, while a second weight-based purification stage 32 removes non-cellulosic contaminants and impurities, which possess a lower density and / or higher specific surface area compared to cellulosic fiber. Accordingly, a heavyweight hydrocyclone stage or heavyweight hydrocyclone stages, is or are used to perform the first weight-based purification stage 31, while a lightweight cleaner, preferably a lightweight hydrocyclone or lightweight hydrocyclones, is or are used to perform the second purification stage 32. As an alternative a CombiCleaner (or any reasonable combination of such devices) could be used to perform the weight-based purification of the accept fraction stream to produce a purified cellulose fraction. The CombiCleaner can perform heavyweight and lightweight hydrocycloning. After the second purification stage, purified pulp 2 is obtained.

[0037] Some media, such as cooking liquor, can lead to extensive foaming which would be unfavourable for weight-based purification processes. Therefore, it could be preferable to add a washing step before the weight-based purification process. In this washing step, the liquid medium could be pressed and / or filtered off and the remaining fibers can be washed in one or more washing steps. After washing, the fibers can be again suspended in a different liquid medium to be suitable for the weight-based purification.

[0038] In an alternative configuration, as shown in Fig. 2, the low consistency refining 11 and / or deflaking 12 is applied to a reject fraction stream from the screening 20 step, wherein the refined and / or deflaked reject fraction stream is fed back to the screening 20 step. As a result, only the screen rejects need to be treated by the low consistency refiner and / or the deflaker. The screen throughput may then be mixed with the screen retentions treated by low consistency refining 11 and / or deflaking 12 for the next step. The following steps can be carried out as described in connection with Fig. 1 to obtain purified pulp 2.

[0039] The removal efficiency of the foreign / synthetic material from cellulosic fibers for such inventive systems can be well above 96-98%, as was shown in multiple trials.

Claims

1. Method for the separation and purification of cellulosic fibers from textile waste comprising the following steps: a) providing textile waste which comprises cellulosic and non-cellulosic fibers and contaminants, as raw material, b) mechanical treatment of the raw material, c) mixing the mechanically treated raw material with a liquid medium in a mixer to produce a pulp suspension, d) screening (20) of raw material particles in the pulp suspension to produce a reject fraction stream and an accept fraction stream based on the size of the raw material particles, e) weight-based purification of the accept fraction stream to produce a purified cellulose fraction.

2. Method according to claim 1, wherein the mechanical treatment comprises comminution of the raw material preferably by at least one of cutting, shredding, grinding, milling, or fiberizing.

3. Method according to claim 1 or 2, wherein the pulp suspension is a low-consistency pulp suspension, preferably comprising from 0.5 % w / w to 5% w / w solids.

4. Method according to any of the claims 1 to 3, wherein the method comprises a step of breaking down fiber bundles in the pulp suspension and separating fibers into singular fibers in the pulp suspension using low consistency refining (11), deflaking (12) or both low consistency refining (11) and deflaking (12).

5. Method according to claim 4, wherein the low consistency refining (11), deflaking (12) or both low consistency refining (11) and deflaking (12) is applied to the low-consistency pulp fluid between the step of mixing and the step of screening.

6. Method according to claim 4, wherein the low consistency refining (11), deflaking (12) or both low consistency refining (11) and deflaking (12) is applied to a reject fraction stream from the screening step, wherein the refined and / or deflaked reject fraction stream is fed back to the screening (20) step.

7. Method according to any of the claims 1 to 6, wherein the screening (20) step or steps comprise a multiple stage screening (20), preferably two- to four-stage screening (20), which is preferably performed in a cascade.

8. Method according to any of the claims 1 to 7, wherein the method comprises a washing process, which is performed before the weight-based purification process.

9. Method according to any of the claims 1 to 8, wherein the weight-based purification of the accept fraction stream comprises a two-stage purification process.

10. Method according to claim 9, wherein a first weight-based purification stage (31) removes non-cellulosic contaminants and impurities, which possess a higher density and / or lower specific surface area compared to cellulosic fibers.

11. Method according to claim 10, wherein a heavyweight cleaner, preferably a heavyweight hydrocyclone stage or heavyweight hydrocyclone stages, is used to perform the first weight-based purification stage (31).

12. Method according to claim 9, wherein a second weight-based purification stage (32) removes non-cellulosic contaminants and impurities, which possess a lower density and / or higher specific surface area compared to cellulosic fiber.

13. Method according to claim 12, wherein a lightweight cleaner, preferably a lightweight hydrocyclone or lightweight hydrocyclones, is used to perform the second purification stage (32).

14. Method according to any of the claims 1 to 13, wherein the weight-based purification of the accepts fraction stream comprises a CombiCleaner, which can perform heavyweight and lightweight hydrocycloning.

15. Apparatus for the separation and purification of cellulosic fibers from textile waste comprising • comminutor for textile waste comprising cellulosic and non-cellulosic fibers and contaminants, • a mixer for mixing comminuted textile waste with a liquid medium to produce a pulp suspension, • a screen for screening (20) of particles in the pulp suspension to produce a reject fraction stream and an accepts fraction stream based on the size of the raw material particles, and • a purifier for weight-based purification of the accepts fraction stream to produce a purified cellulose fraction.

16. Apparatus according to claim 15, wherein the apparatus comprises a refiner and / or deflaker performing fiber bundles or flocks separation to singular fibers at low consistency of about 0.5% w / w to 5% w / w.

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