HK40052981B - Separation of fibers - Google Patents

Description

Fiber separation

Technical Field

This invention relates to methods for purifying cellulose fibers by removing non-cellulose fibers, such as synthetic fibers containing man-made polymers, particularly methods related to textile recycling.

Background Technology

Cellulose is an important component of plants and contains anhydrous glucose units. Cellulose is used, for example, to manufacture synthetic fibers through spinning or yarn production. Recycled cellulose is renewable and can be used in spinning, yarn, fibers, etc.

Several methods are known to dissolve cellulose for a variety of applications, including the manufacture of regenerated cellulose fibers. Expensive chemicals are sometimes used in such processes. (Ohno H and Fukaya Y (2009), "Task-specific ionic liquids for cellulose technology," Chemistry Letters V38)

WO 2013/124265 discloses a method for regenerating cellulose. It discloses treating cellulose with oxygen in an alkaline step. It mentions using oxygen to reduce viscosity. It mentions the recycling of fabrics and the potential need for pretreatment to reduce the degree of polymerization. It also mentions that cellulose can be used to manufacture new fibers, such as viscose fiber.

WO 2010/1124944 discloses a method for hydrolyzing cellulose, comprising the following sequential steps: a) mixing cellulose with a viscosity of less than 900 ml/g with an aqueous solution to obtain a liquid, wherein the liquid contains cellulose particles with a maximum diameter of 200 nm, wherein the temperature of the aqueous solution is less than 35°C, and wherein the pH of the aqueous solution is greater than 12; b) subjecting the liquid to at least one of the following steps: i) lowering the pH of the liquid by at least 1 pH unit; ii) raising the temperature by at least 20°C; and c) hydrolyzing the cellulose.

CN 102747622 discloses a method for removing indigo from jeans. The method involves adding water to the fabric at a weight ratio of 1:20-30 and heating at 85-95°C. Then, 2-3 g/L sodium hydroxide, 4-5 g/L stripping agent, 3-5 g/L fatty alcohol polyoxyethylene ether O-25, and 4-5 g/L sodium dithionite are added. The mixture is then subjected to ultrasonic vibration, drained, and the fabric is washed with water 2-3 times.

WO 2014/045062 discloses a method for extracting polyester fibers using solvents.

WO 2018/104330 discloses a method for treating cellulose fibers, wherein the cellulose fibers are treated with a reducing additive to cause the fibers to swell, and then bleached under alkaline conditions with oxygen and/or under acidic conditions with ozone. Cellulose-based fibers can then be manufactured from viscose fibers produced using the lyocell fiber process.

WO 2018/073177 discloses a method for recycling textiles containing cellulose, comprising the steps of: optionally breaking down the textiles, swelling the cellulose under reducing conditions, wherein at least one reducing agent is present during at least a portion of the swelling, and then performing at least one of the following two bleaching steps in any order: i) bleaching the material with oxygen under alkaline conditions in the pH range of 9-13.5, and ii) bleaching the material with ozone under acidic conditions at a pH below 6.

WO 2015/077807 discloses a method for pretreating recycled cotton fibers used to produce molded articles from recycled cellulose, wherein the pretreating of the recycled cotton fibers includes a metal removal stage and an oxidative bleaching stage.

There is still a need for improved methods for recycling textiles containing cellulose fibers and other fibers in mixtures. Fibers other than cellulose fibers need to be removed. In textiles primarily containing cellulose fibers, fibers made of polyester, elastic fibers, acrylic fibers, and other fibers that need to be removed may be present.

Other fibers besides cellulose fibers need to be removed because their effects during the later use of cellulose fibers are unknown. Cellulose can be used in, for example, subsequent viscose fiber processes, subsequent lyocell fiber processes, subsequent CarbaCell processes, or similar processes.

Removing synthetic polymers from recycled paper using flotation is known in the art. However, such synthetic polymers do not exist in the form of fibers like textiles. Other existing methods, such as sieve plates, drum screens, vibrating screens, and pressure screens, generally remove synthetic polymers from recycled paper quite well.

One problem with removing non-cellulose fibers from cellulose fibers is that cellulose and other materials exist in fibrous form. This introduces considerable entanglement and difficulty to fiber separation. This is not the case with impurities in recycled pulp, where non-cellulose materials such as synthetic polymers do not exist in fibrous form. Compared to cellulose fibers in recycled paper, cellulose in textiles has longer and helical cellulose fibers with different surface properties.

Another challenge in recycling textiles when the intention is to reuse cellulose is that the cellulose fibers cannot become too short during the process. Otherwise, they become unusable for subsequent processes, or their use in subsequent processes becomes uneconomical. Therefore, it is important to maintain a high proportion of the desired cellulose structure when recycling textiles.

Summary of the Invention

One object of the present invention is to eliminate at least some of the disadvantages of the prior art and to provide an improved method for removing non-cellulose fibers from a mixture comprising cellulose fibers and non-cellulose fibers.

Although separating entangled fibers while maintaining the desired structure seems impossible, the inventors have discovered a window through which flotation can be used to remove non-cellulose fibers. For flotation to work, the cellulose chain length must be reduced to a certain level. However, considering the intended use of cellulose, the chain length cannot be too short. The inventors unexpectedly discovered that by adjusting the chain length within certain intervals, it is possible to use flotation to remove non-cellulose fibers. Flotation is not feasible for cellulose fibers entangled with synthetic polymer fibers without degradation.

Surprisingly, flotation can be used in this situation due to the entanglement of cellulose and non-cellulose fibers.

In a first aspect, a method for separating fibers is provided, comprising the following steps:

a) Provide a mixture comprising cellulose fibers and non-cellulose fibers,

b) Reduce the cellulose chain length of the cellulose fibers so that the limiting viscosity, as determined by ISO 5351, is within the range of 200-900 ml/g.

c) subjecting the fiber mixture to mechanical treatment to break the fiber aggregates into smaller fragments.

d) Adjust the fiber concentration in the mixture to a range of 0.1-4 wt% fiber by weight, and

e) subject the mixture to flotation to remove non-cellulose fibers.

In the second aspect, a cellulose-containing material obtained by the above method is provided.

In a third aspect, the use of the recycled cellulose materials described above in the process of producing viscose fibers is provided.

The advantages of this invention include the ability to remove non-cellulose fibers, such as synthetic fibers, in a very specific manner without or substantially without removing cellulose fibers from the mixture. This provides a very high degree of removal while maintaining high yields due to the removal of little or no cellulose fibers. Cellulose fibers are preserved to a degree suitable for manufacturing viscose fibers and other purposes. It also provides the possibility of recycling the removed non-cellulose fibers.

Detailed Implementation

Before disclosing and describing the invention in detail, it should be understood that the invention is not limited to the specific compounds, configurations, method steps, substrates, and materials disclosed herein, as such compounds, configurations, method steps, substrates, and materials may vary. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention, as the scope of the invention is limited only by the appended claims and their equivalents.

It should be noted that the singular forms “a/an” and “the” used in this specification and the appended claims include the plural forms, unless otherwise expressly stated above.

Unless otherwise defined, any terms and scientific terms used herein are intended to have the meaning commonly understood by one of ordinary skill in the art to which this invention pertains.

As used herein, the term "cellulose" refers to all forms of natural cellulose (cotton, flax, jute, etc.) and all forms of regenerated cellulose such as rayon. Specifically, it encompasses all textiles containing cellulose, including those containing treated and modified cellulose. The term "non-cellulose" refers to materials and fibers that are not composed of or made from cellulose.

"Dissolving pulp" (sometimes also called dissolved cellulose) refers to bleached pulp or cotton linters with a high cellulose content (>90 wt%). It has special properties, including high brightness and a uniform molecular weight distribution. Dissolving pulp is so named because it is not made into paper, but is dissolved in a solvent or derivatized into a homogeneous solution, which makes it completely chemically accessible and removes any residual fibrous structure.

"Limiting viscosity value" refers to the limiting viscosity value determined according to ISO 5351. The term intrinsic viscosity is sometimes used synonymously. The cellulose chain length is shortened during this process; however, it is difficult to perform direct measurements of the cellulose chain length quickly and efficiently. The limiting viscosity value is related to the cellulose chain length and can therefore be used as an indirect measure of the cellulose chain length, especially in cases where chain length reduction is desired. It is generally accepted that the limiting viscosity value is related to the cellulose chain length. While a direct correlation between chain length and limiting viscosity value may not always be present, the existing relationship is sufficient for the purposes of this invention.

In the context of this method, "mixture" refers to the subject of the process. It is understood that the mixture will change during the process. Therefore, the starting mixture differs from the final mixture, although the term "mixture" is used throughout the process. After chain length reduction, a mixture with shortened cellulose chains exists. After mechanical treatment, the fiber aggregates in the mixture decrease. After adjusting the fiber concentration, a dilution mixture generally exists. After flotation, a purified mixture exists.

In a first aspect, a method for separating fibers is provided, comprising the following steps:

a) Provide a mixture comprising cellulose fibers and non-cellulose fibers,

b) Reduce the cellulose chain length of the cellulose fibers so that the limiting viscosity, as determined according to ISO 5351, is within the range of 200-900 ml/g.

c) subjecting the fiber mixture to mechanical treatment to break the fiber aggregates into smaller fragments.

d) Adjust the fiber concentration in the mixture to a range of 0.1-4 wt% fiber by weight, and

e) subject the mixture to flotation to remove non-cellulose fibers.

The mixture comprises both cellulose fibers and non-cellulose fibers, and it is desirable to separate these fibers. In one embodiment, the mixture comprises a high proportion of cellulose fibers and a small amount of non-cellulose fibers containing man-made polymers. Recycled textiles have many different components, and for some components, there is a high content of cellulose fibers, as well as additional contents of various other fibers, including, for example, polyester fibers, elastic fibers, etc. In one embodiment, the mixture is a recycled textile comprising 95-99 wt% cellulose fibers and about 1-2 wt% polyester fibers.

During the step of reducing the cellulose chain length of the cellulose fibers to achieve an limiting viscosity value in the range of 200-900 ml/g as determined according to ISO 5351, the mixture is typically aqueous, i.e., the fibers are mixed with water. Water is by far the most practical solvent for large-scale processes, although other solvents could theoretically be used. In one embodiment, the cellulose fiber chain length is reduced such that the limiting viscosity value as determined according to ISO 5351 is in the range of 300-900 ml/g. In another embodiment, the cellulose fiber chain length is reduced such that the limiting viscosity value as determined according to ISO 5351 is in the range of 400-600 ml/g. In yet another embodiment, the cellulose fiber chain length is reduced such that the limiting viscosity value as determined according to ISO 5351 is at least 550 ml/g. In one embodiment, the cellulose fiber chain length is reduced such that the limiting viscosity value as determined according to ISO 5351 is at least 400 ml/g. In yet another embodiment, the cellulose fiber chain length is reduced such that the limiting viscosity value as determined according to ISO 5351 is at least 500 ml/g. In one embodiment, the chain length of the cellulose fibers is reduced such that the limiting viscosity value, as determined according to ISO 5351, is at least 250 ml/g. In another embodiment, the chain length of the cellulose fibers is reduced such that the limiting viscosity value, as determined according to ISO 5351, does not exceed 700 ml/g. In yet another embodiment, the chain length of the cellulose fibers is reduced such that the limiting viscosity value, as determined according to ISO 5351, does not exceed 600 ml/g. The limiting viscosity value of cellulose in textiles is always above the range of 200-900 ml/g, and therefore generally needs to be reduced.

If the cellulose chain length is reduced, it is unexpectedly found that flotation may be possible. However, due to the entanglement of different fibers, flotation is not expected to be usable. Nevertheless, the chain length cannot be reduced so much that the cellulose becomes unusable, making it less attractive for subsequent steps in cellulose regeneration. For example, for the lyocell process, a limiting viscosity of approximately 400 ml/g or higher is desired. For the CarbaCell process, 250 ml/g or higher is sufficient.

In one embodiment, the chain length of cellulose fibers is reduced by ozone treatment under acidic conditions below pH 6. In another embodiment, the chain length of cellulose fibers is reduced by oxygen treatment under alkaline conditions above pH 10. These two reductions in chain length can be combined sequentially. One advantage of using ozone treatment is that the cellulose chains are weakened during the treatment. If step b) of reducing the cellulose fiber chain length includes ozone treatment, and if step c) is performed after step b), then the cellulose fibers will be weakened during step b), but the non-cellulose fibers will generally not be weakened, at least not to the same extent as the cellulose fibers. During the subsequent step c), mechanical treatment will destroy the weakened cellulose fibers to a greater extent, but destroy the non-cellulose fibers to a lesser extent. The altered tendency to destroy during the mechanical treatment in step c) will result in shorter cellulose fibers and substantially unchanged non-cellulose fiber lengths. The shortened cellulose fibers and the somewhat retained non-cellulose fibers make it easier to separate the non-cellulose fibers by flotation. Therefore, in one embodiment, step c) is performed after step b), and step b) includes ozone treatment and optional other steps.

In one implementation, step c) is performed before step b). Step b) of reducing cellulose chain length and step c) of mechanical treatment can be performed in any order. Step b) can be divided into several steps performed at different points in the method.

Furthermore, step d) can be performed in any order related to steps b) and c). It can be performed before, between, or after steps b) and c). Step d) is dilution in most cases because it is more economical to maintain a higher fiber concentration during steps b) and c), therefore, in most cases, step d) is performed after steps b) and c) and before flotation in step e).

In one implementation, step c) is performed by grinding. Other mechanical treatment methods may also be used to mechanically separate fiber fragments into smaller fragments. Any fiber aggregates are ground into smaller fragments. This step can be omitted if the fiber mixture does not contain fiber aggregates. For most fiber mixtures, especially for recycled textiles, aggregates are present, and therefore mechanical treatment step c) cannot be omitted. Aggregates contain entangled fibers. Different types of fiber aggregates make it difficult to separate fibers in flotation, or will reduce flotation yield because more fibers can be screened out from the aggregates.

In one embodiment, the fiber concentration in the mixture is adjusted during step d) to be in the range of 0.2-1.5 wt% fiber by weight. In another embodiment, the fiber concentration in the mixture is adjusted during step d) to be in the range of 0.7-1.5 wt% fiber by weight. In yet another embodiment, the fiber concentration in the mixture is adjusted during step d) to be in the range of 0.7-2 wt% fiber by weight. In most embodiments, the concentration adjustment is a dilution.

In one embodiment, the mixture is aqueous. At the start of the method, water is mixed with the fibers.

In one embodiment, flotation is performed in several chambers connected in series. In another embodiment, flotation is performed using more than one air diffusion nozzle that provides different bubble sizes. In yet another embodiment, several chambers are used, each supplied with bubbles of a different size. This facilitates the removal of fibers of different sizes from the different chambers.

In one embodiment, the mixture is filtered prior to flotation in step e). This optional filtration step ensures that any remaining large aggregates of entangled fibers are not subjected to flotation. In one embodiment, the slag from the filter is ground and filtered again prior to flotation. Any removed large aggregates are ground and fed back to the filter.

In one implementation, the mixture is made from textiles to be recycled. Generally, this process is intended for recycling textiles, but other applications are also envisioned.

In one embodiment, the textile is milled in a dry state prior to step a). This additional drying step may include cutting and chopping the textile into small pieces.

In one embodiment, prior to step b), the mixture is treated in water under reducing conditions and at a pH above 10. This optional step is performed before the reduction of cellulose chain length in step b). In one embodiment, the reducing conditions are achieved by adding dithionite anions. In one embodiment, sodium dithionite is added. The inventors believe that this treatment, carried out under reducing conditions at high pH, achieves swelling of the cellulose fibers, which in turn facilitates the separation of non-cellulose fibers from cellulose fibers during subsequent flotation.

In one embodiment, when treated in water under reducing conditions and at a pH above 10, the additional grinding is performed in a wet state. In another embodiment, this additional grinding is performed in the presence of reducing conditions.

In one embodiment, the non-cellulose fiber is a synthetic fiber comprising a man-made polymer. Examples of synthetic fibers comprising man-made polymers include, but are not limited to, nylon-based fibers, polyester-based fibers, acrylic-based fibers, modified acrylic-based fibers, polyurethane-based fibers, and polyolefin-based fibers. Such fibers are well known in the art and are widely used today. In one embodiment, the non-cellulose fiber comprises polyester fibers. In one embodiment, the non-cellulose fiber comprises elastic fibers. In one embodiment, the non-cellulose fiber comprises polyacrylonitrile. In commercially available textiles, cellulose-based fibers are typically blended with a smaller amount of non-cellulose fibers. For example, cellulose fibers made from cotton are often blended with a few percent of polyester-based fibers.

In one embodiment, the mixture following step e) is dehydrated and dried. Dehydration and drying are performed using known techniques in the pulping field, including, for example, vacuum dehydration, pressing, and drying.

The product obtained by this method can be used in a variety of different ways. It can be used alone or mixed with other materials as a raw material for regenerated cellulose. In some cases, this material can replace cotton linters.

In one embodiment, the mixture following step e) is used as a raw material in the viscose fiber manufacturing process.

In one embodiment, when the resulting mixture is used in a viscose fiber process, the limiting viscosity value determined according to ISO 5351 is reduced to a value above 550 ml/g in step b).

In one embodiment, the mixture following step e) is used as a raw material in the lyocell fiber manufacturing process. In another embodiment, when the resulting mixture is used in the lyocell fiber process, the limiting viscosity value determined according to ISO 5351 is reduced to a value above 400 ml/g in step b).

In one embodiment, the non-cellulose fibers are recovered and optionally further purified. In another embodiment, the non-cellulose fibers are collected and used for various purposes.

In one embodiment, at least one surfactant is added prior to flotation in step e). The surfactant is any surfactant. Examples include compounds having both hydrophilic and hydrophobic portions. In one embodiment, saponified fatty acids are used as the surfactant. Surfactants having a variety of polar and nonpolar groups can be used for flotation. Surfactants whose primary function is to make solid surfaces hydrophobic are called trapping agents. Surfactants whose primary function is to provide the required stability to the top foam layer in the flotation cell and to influence the kinetics of particle-bubble adhesion are called foaming agents. These are typically nonionic surfactants, which enhance the rate of film thinning and contribute to the stability of particle-bubble aggregates. In another embodiment, other additives are added prior to or during flotation in step e). Examples include, but are not limited to, defoamers, foam stabilizers, and substances for increasing or decreasing water hardness. In one embodiment, additives commonly used in flotation are added.

In one embodiment, the additional flotation step is performed before at least one of steps c) and d). This serves as additional pre-flotation and removes, for example, larger aggregates and other impurities. In one embodiment, the aggregates from this pre-flotation are mechanically treated to break them up, for example, by grinding. In one embodiment, the aggregates separated in the additional flotation are subjected to mechanical treatment to break them down. The additional flotation is performed before adjusting the concentration, which is typically high in fiber.

In one implementation, the flotation in step e) is repeated to improve the removal of unwanted fibers.

In one embodiment, the mixture provided in step a) is a textile comprising at least one of cotton and regenerated cellulose fibers.

In one embodiment, the temperature during flotation in step e) is in the range of 30-90°C for at least a portion of the flotation. In another embodiment, the temperature during flotation in step e) is in the range of 50-80°C for at least a portion of the flotation. In one embodiment, the temperature is within the aforementioned range throughout the entire flotation period. In an alternative embodiment, the temperature is within the aforementioned range for a portion of the flotation. The latter embodiment may relate to several flotation steps performed in series. Temperatures in the range of 30-90°C, preferably 50-80°C, produce better results and lower aqueous solvent viscosity. In an alternative embodiment, flotation is carried out at ambient temperature.

In a second aspect, a cellulose-containing material obtained by the above method is provided.

In the third aspect, it provides the use of recycled cellulose materials for the production of viscose fibers.

Those skilled in the art will understand other features and uses of the invention and their related advantages after reading this specification and the accompanying embodiments.

It should be understood that the present invention is not limited to the specific embodiments shown herein. The following examples are provided for illustrative purposes and are not intended to limit the scope of the invention, which is limited only by the appended claims and their equivalents.

Example

Example 1

Fiber materials:

Fiber: Cotton containing a small amount of non-cellulose fibers from textile waste. The fiber is broken down and bleached. First, the fiber is broken down from any woven and spun structure and refined into shorter fiber lengths. Second, the fiber is treated with dithionite in a redox reaction, and further treated with ozone bleaching (acidic conditions) and oxygen bleaching (alkaline conditions). Finally, the fiber is refined again to remove any knots and clumps.

The experiment used a 1-liter graduated cylinder (diameter: 90 mm; H: 210 mm) and a circular porous sand filter stone (diameter: 80 mm; H: 20 mm). The graduated cylinder contained 500 ml of fiber suspension, 0.5 wt%. The porous sand filter stone was placed at the bottom of the graduated cylinder, which created a uniform bubble flow in the fiber suspension. 1-2 drops of flotation agent were added to obtain foam as a carrier for the non-cellulose fibers. The non-cellulose fibers adhered to the surface of the bubbles and were removed when the foam reached the top of the graduated cylinder.

The flotation experiment was repeated three times using the same fiber mixture. The residual non-cellulose fiber content obtained for samples 1-3 from different experiments can be seen in the table below. The original non-cellulose fiber content was measured twice, and the results are shown in the table. One flotation step reduced the non-cellulose fiber content by an average of 89.1%.

Non-cellulose fiber content Sample 1 0.18wt% Sample 2 0.12wt% Sample 3 0.20wt% Reference 1 1.76wt% Reference 2 1.35wt% Average reduction of non-cellulose fibers 89.1%

Example 2

Use Voith Laboratory flotation cell Delta 25.

Different blends of various fibers are used.

•FM: White-cut cotton fibers. The fibers contain a smaller amount of non-cellulose fibers, such as polyester fibers. The fibers are broken down and bleached. First, the fibers are broken down from any woven and spun structure and refined into shorter fiber lengths. Second, the fibers are treated with dithionite in a redox reaction, and further treated with ozone bleaching (acidic conditions) and oxygen bleaching (alkaline conditions). Finally, the fibers are refined again to remove any knots and clumps.

• HM+ Polyester Fiber: Raw material: white sheared material. Pulp is refined using a Walley pulper with added green polyester fiber.

• HM/FM+ Polyester Fiber: The same fiber as FM above, but with the addition of green polyester fiber and additional refining using a Walley beater.

The SR in the table indicates the degree of abrasion of different fibers. Higher numbers indicate smaller fibers.

The fiber mixtures described above were operated in a flotation cell at a pulp concentration of 1 wt%, a flotation volume of 24 liters, a temperature of 21°C, an air flow rate of 11 liters/minute, and a pH of 9-10. The flotation times for the different mixtures were 6, 9, and 12 minutes, respectively.

The flotation samples were run with and without the addition of saponified olefinic fatty acids. 150 g of saponified fatty acids were added to the flotation cell. The saponified fatty acids were prepared by reacting 0.8 wt% fatty acids with 2 wt% NaOH.

The aforementioned FM fibers were also run during the mechanical filtration process to compare the results. This test is known as FM slidepac.

*High cellulose concentration: 1.35wt%

*Unable to produce foam

As can be seen, flotation is an effective method for separating cellulose fibers (such as cotton fibers) from other fibers (such as polyester fibers). The screening concentration of polyester fibers is as high as 60.4%. This means that the loss of cellulose fibers is relatively low.

Claims (31)

1. A method for separating fibers, comprising the following steps in sequence: a) Provide a mixture comprising cellulose fibers and non-cellulose fibers, b) Reduce the cellulose chain length of the cellulose fibers so that the limiting viscosity value, as determined according to ISO 5351, is in the range of 200-900 ml/g. c) subjecting the mixture of fibers to mechanical treatment to break the fiber aggregates into smaller fragments. d) Adjust the fiber concentration in the mixture to a range of 0.1-4 wt% fiber by weight, and e) subject the mixture to flotation to remove the non-cellulose fibers. 2. The method according to claim 1, wherein the chain length of the cellulose fiber is reduced such that the limiting viscosity value, as determined according to ISO 5351, is in the range of 300-900 ml/g. 3. The method according to any one of claims 1-2, wherein the chain length of the cellulose fiber is reduced such that the limiting viscosity value determined according to ISO 5351 is in the range of 400-600 ml/g. 4. The method according to any one of claims 1-2, wherein the chain length of the cellulose fiber is reduced such that the limiting viscosity value determined according to ISO 5351 is at least 550 ml/g. 5. The method according to any one of claims 1-2, wherein the chain length of the cellulose fibers is reduced by ozone treatment under acidic conditions below pH 6. 6. The method according to any one of claims 1-2, wherein the chain length of the cellulose fibers is reduced by treatment with oxygen under alkaline conditions above pH 10. 7. The method according to any one of claims 1-2, wherein step c) is performed by grinding. 8. The method according to any one of claims 1-2, wherein during step d), the concentration of fibers in the mixture is adjusted such that it is in the range of 0.2-1.5 wt% fibers by weight. 9. The method according to any one of claims 1-2, wherein during step d), the concentration of fibers in the mixture is adjusted such that it is in the range of 0.7-1.5 wt% fibers by weight. 10. The method according to any one of claims 1-2, wherein the mixture is aqueous. 11. The method according to any one of claims 1-2, wherein the flotation is performed in a plurality of chambers connected in series. 12. The method according to any one of claims 1-2, wherein the flotation is performed by using more than one air diffusion nozzle that provides different bubble sizes. 13. The method according to any one of claims 1-2, wherein the mixture is filtered prior to the flotation in step e). 14. The method of claim 13, wherein the slag from the filter is ground and filtered again prior to the flotation. 15. The method according to any one of claims 1-2, wherein the mixture is made from textiles to be recycled. 16. The method of claim 15, wherein the textile is milled in a dry state prior to step a). 17. The method of claim 15, wherein the mixture is treated in water under reducing conditions and at a pH above 10 prior to step b). 18. The method of claim 17, wherein the reduction conditions are achieved by adding dithionite anions. 19. The method of claim 17, wherein the additional grinding is performed in a wet condition. 20. The method according to any one of claims 1-2, wherein the non-cellulose fiber is a synthetic fiber comprising a man-made polymer. 21. The method according to any one of claims 1-2, wherein the non-cellulose fiber comprises polyester fiber. 22. The method according to any one of claims 1-2, wherein the non-cellulose fiber comprises an elastic fiber. 23. The method according to any one of claims 1-2, wherein the non-cellulose fiber comprises polyacrylonitrile. 24. The method according to any one of claims 1-2, wherein the mixture after step e) is dehydrated and dried. 25. The method according to any one of claims 1-2, wherein the non-cellulose fibers are recovered and optionally further purified. 26. The method according to any one of claims 1-2, wherein at least one surfactant is added prior to the flotation in step e). 27. The method according to any one of claims 1-2, wherein the additional flotation step is performed before at least one of steps c) and d). 28. The method of claim 27, wherein the aggregates separated in the flotation are subjected to mechanical treatment to decompose the aggregates. 29. The method according to any one of claims 1-2, wherein the mixture provided in step a) is a textile comprising at least one of cotton and regenerated cellulose fibers. 30. The method according to any one of claims 1-2, wherein the temperature during the flotation in step e) is in the range of 30-90°C during at least a portion of the flotation. 31. The method according to any one of claims 1-2, wherein the temperature during the flotation in step e) is in the range of 50-80°C during at least a portion of the flotation.