Fiber separation

Abstract

For example, a method for separating fibers in recycled textile products is provided, starting with a mixture containing cellulosic and non-cellulosic fibers, reducing the cellulose chain length of the cellulosic fibers so that the intrinsic viscosity, as determined in accordance with ISO 5351, is in the range of 200 to 900 ml / g, mechanically breaking down fiber aggregates into small fragments, adjusting the fiber concentration to 0.1 to 4 wt.%, and subjecting the mixture to flotation to remove non-cellulosic fibers. Non-cellulosic fibers, such as synthetic fibers, can be removed very specifically without removing or substantially removing cellulosic fibers from the mixture. This allows for a very high degree of removal while maintaining a high yield due to little or no removal of cellulosic fibers.

Description

[Technical field]

[0001] The present invention relates to a process for refining cellulosic fibers by removing non-cellulosic fibers, such as synthetic fibers containing man-made polymers, particularly in the context of textile recycling. [Background technology]

[0002] Cellulose is an important component of plants and contains anhydroglucose units. Cellulose is utilized to make synthetic fibers, for example, by spinning into yarns or threads. Recycled cellulose can be regenerated and used to spin yarns, threads, fibers, and the like.

[0003] Several methods are known to dissolve cellulose for various applications, including the production of regenerated cellulosic fibers. Such processes can involve the use of expensive chemicals (Ohno H and Fukaya Y (2009) Task specific ionic liquids for cellulose technology Chemistry Letters V38).

[0004] WO 2013 / 124265 discloses a process for the regeneration of cellulose. It discloses treating cellulose with oxygen in an alkaline process. A reduction in viscosity with oxygen is mentioned. Recycling of textiles is mentioned, and pretreatment may be desirable to reduce the degree of polymerization. It is also stated that cellulose can be used to make new fibers such as viscose.

[0005] WO 2010 / 1124944 discloses a process for the hydrolysis of cellulose comprising the successive steps of a) mixing cellulose having a viscosity of less than 900 ml / g with an aqueous solution to obtain a liquid, in which cellulose-containing particles have a diameter of at most 200 nm, the temperature of said aqueous solution being less than 35° C. and the pH of said aqueous solution being greater than 12, b) subjecting said liquid to at least one of: i) decreasing the pH of the liquid by at least 1 pH unit; ii) increasing the temperature by at least 20° C.; and c) hydrolyzing the cellulose.

[0006] China Patent Publication No. 102747622 discloses a process for removing indigo color from jeans. The fabric is put into water in a weight ratio of 1:20-30, heated at 85-95℃, 2-3g / l sodium hydroxide, 4-5g / l stripping agent, 3-5g / l peregal O-25, and 4-5g / l sodium hyposulfite are added, ultrasonic vibration is performed, the mixed solution is discharged, and the fabric is washed with water for 2-3 times.

[0007] WO 2014 / 045062 discloses a process for extracting polyester with the aid of a solvent.

[0008] WO 2018 / 104330 discloses a method of treating cellulose fibres, in which the cellulose fibres are treated to swell them with a reducing additive and then bleached with oxygen under alkaline conditions and / or with ozone under acidic conditions, after which cellulosic fibres can be produced in the Viscose or Lyocell process.

[0009] WO 2018 / 073177 discloses a method for recycling textile products containing cellulose, optionally comprising the steps of disintegrating the textile product, swelling the cellulose under reducing conditions, in which at least one reducing agent is present during at least a part of the swelling, and then carrying out, in any order, at least one of two bleaching steps: i) bleaching the material with oxygen under alkaline conditions at a pH in the range of 9 to 13.5, ii) bleaching the material with ozone under acidic conditions at a pH below 6.

[0010] WO 2015 / 077807 discloses a process for pre-treating regenerated cotton fibres for use in producing moulded bodies from regenerated cellulose, the pre-treatment of the regenerated cotton fibres comprising a metal removal step and an oxidative bleaching step.

[0011] Nevertheless, there is a need for improved methods for recycling textile products that contain not only cellulose fibers but also other fibers, and there is a need to remove the non-cellulose fibers. Some cellulose-based textiles may contain polyester, elastane, and acrylic fibers, as well as other fibers that one wishes to remove.

[0012] It is desirable to remove non-cellulose fibers because the effect of subsequent use of the fibers is unknown: cellulose can be used, for example, in a subsequent viscose process, a subsequent lyocell process, a subsequent CarbaCell process, etc.

[0013] It is known in the art to remove man-made polymers from waste paper using flotation. However, such man-made polymers are not in the form of fibers as in textile products. Furthermore, existing methods such as screen plates, drum screens, vibrating screens, pressure screening, etc. are generally effective in removing man-made polymers from waste paper.

[0014] One problem with removing non-cellulosic fibers from cellulose fibers is that both cellulose and other materials are present in fibrous form, which can lead to significant entanglement of the fibers and make separation difficult. This is not the case with impurities in wastepaper pulp, as the non-cellulosic materials, such as synthetic polymers, are not in fibrous form. Cellulose in textile products has longer fibers, helical cellulose fibers, and different surface properties compared to cellulose fibers in wastepaper.

[0015] Another difficulty with textile recycling for the purpose of reusing the cellulose is the need to avoid shortening the cellulose fibers too much during the process, otherwise they would be unusable or uneconomical to use in the next step. It is therefore important to maintain a high percentage of the desired cellulose structure when recycling textiles. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] International Publication No. 2013 / 124265 Brochure [Patent Document 2] International Publication No. 2010 / 1124944 Brochure [Patent Document 3] China Patent Application Publication No. 102747622 [Patent Document 4] International Publication No. 2014 / 045062 Brochure [Patent Document 5] International Publication No. 2018 / 104330 Brochure [Patent Document 6] International Publication No. 2018 / 073177 Brochure [Patent Document 7] International Publication No. 2015 / 077807 Brochure [Non-patent literature]

[0017] [Non-Patent Document 1] Ohno H and Fukaya Y (2009) Task specific ionic liquids for cellulose technology Chemistry Letters V38 Summary of the Invention [Means for solving the problem]

[0018] It is an object of the present invention to obviate at least some of the shortcomings of the prior art and to provide an improved process for removing non-cellulosic fibers in a mixture containing cellulosic and non-cellulosic fibers.

[0019] Although it may seem impossible to separate entangled fibers while maintaining the desired structure, the inventors have found a window in which flotation can be used to remove non-cellulosic fibers. For flotation to work, the chain length of cellulose needs to be below a certain level. However, given the intended use of cellulose, the chain length cannot be too short. The inventors have unexpectedly found that by adjusting the chain length within a certain range, flotation can be used to remove non-cellulosic fibers. Cellulosic fibers that are entangled with synthetic polymer fibers cannot be used for flotation unless they are broken down.

[0020] It was unexpected that flotation could be used in this context, since cellulosic fibers are intertwined with non-cellulosic fibers.

[0021] In a first aspect, there is provided a method for separating fibers, comprising the steps of: a) providing a mixture comprising cellulosic and non-cellulosic fibers; b) reducing the cellulose chain length of said cellulose fibers such that the intrinsic viscosity, determined according to ISO 5351, is in the range of 200 to 900 ml / g; c) subjecting the mixture of fibers to a mechanical treatment to break down fiber aggregates into smaller fragments; d) adjusting the concentration of fibers in the mixture so that the concentration of fibers in the mixture is in the range of 0.1 to 4% by weight of fibers; e) subjecting the mixture to flotation to remove the non-cellulosic fibers. A method is provided that includes:

[0022] In a second aspect, there is provided a cellulose containing material obtained using the above method.

[0023] In a third aspect, there is provided the use of recycled cellulosic material (regenerated cellulosic material) as described above in a viscose manufacturing process.

[0024] The advantage of the present invention is that it allows for very specific removal of non-cellulosic fibers, such as synthetic fibers, without removing or substantially removing the cellulose fibers in the mixture, thereby providing a very high degree of removal while at the same time keeping the yield high, since no or only little cellulose fibers are removed, which are preserved to such an extent that they can be used, for example in the manufacture of viscose, and there is also the possibility of recovering the removed non-cellulosic fibers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Before the present invention is disclosed and described in detail, it is to be understood that the present invention is not limited to the specific compounds, compositions, method steps, substrates, and materials disclosed herein, as such compounds, compositions, method steps, substrates, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used only for the purpose of describing particular embodiments, and is not intended to be limiting, as the scope of the present invention is limited only by the appended claims and equivalents thereof.

[0026] It must be noted that as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0027] Unless otherwise defined, all 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.

[0028] The term "cellulosic" as used herein refers to all natural cellulosic forms (cotton, hemp, jute, etc.) and all regenerated cellulosic forms such as rayon. In particular, all textiles containing cellulose are included, including textiles containing treated and modified cellulose. In contrast, the term "non-cellulosic" refers to materials and fibers not composed of or made from cellulose.

[0029] "Dissolving pulp" (sometimes called dissolving cellulose) means bleached wood pulp or cotton linters with a high cellulose content (>90% by weight). Dissolving pulp has characteristics such as high brightness and uniform molecular weight distribution. Dissolving pulp is so named because it does not form into paper, but is dissolved in a solvent or derivatized into a homogenous solution that makes it completely chemically accessible and removes any remaining fibrous structure.

[0030] "Intrinsic viscosity number" means the intrinsic viscosity number determined according to ISO 5351. The term intrinsic viscosity is sometimes used as a synonym. The cellulose chain length shortens during the process, however, direct measurement of the cellulose chain length is difficult to perform quickly and efficiently. The intrinsic viscosity number is related to the cellulose chain length and can therefore be used as an indirect measurement of the cellulose chain length, especially when, for example, shortening of the chain length is desired. It is generally accepted that the intrinsic viscosity number is related to the cellulose chain length. Although there may not always be a direct correlation between the chain length and the intrinsic viscosity number, for the purposes of the present invention, the existing relationship is sufficient.

[0031] By "mixture" in the context of the method of the invention is meant the object of the process. It is understood that the mixture changes during the process. Thus, although the term "mixture" is used throughout the process, the starting mixture is not the same as the final mixture. After chain shortening, you have a mixture with reduced cellulose chain length. After mechanical treatment, you have a mixture with reduced fiber aggregates. After adjustment of fiber concentration, you generally have a diluted mixture. After flotation, you have a refined mixture.

[0032] In a first aspect, there is provided a method for separating fibers, comprising the steps of: a) providing a mixture comprising cellulosic and non-cellulosic fibers; b) reducing the cellulose chain length of said cellulose fibers such that the intrinsic viscosity, determined according to ISO 5351, is in the range of 200 to 900 ml / g; c) subjecting the mixture of fibers to a mechanical treatment to break down fiber aggregates into smaller fragments; d) adjusting the concentration of fibers in the mixture so that the concentration of fibers in the mixture is in the range of 0.1 to 4% by weight of fibers; e) subjecting the mixture to flotation to remove the non-cellulosic fibers. A method is provided that includes:

[0033] The mixture contains cellulosic as well as non-cellulosic fibers and it is desired to separate the fibers. In one embodiment, the mixture contains a high percentage of cellulosic fibers with a small amount of non-cellulosic fibers including man-made polymers. There are various fractions of recycled textile products, some of which have a high content of cellulosic fibers, to which is added a content of various other fibers, e.g. polyester, elastane, etc. In one embodiment, the mixture is a recycled textile product containing 95-99% by weight of cellulosic fibers and about 1-2% by weight of polyester.

[0034] During the step of reducing the cellulose chain length of the cellulose fibers, such that the intrinsic viscosity, determined according to ISO 5351, is in the range of 200-900 ml / g, the mixture is typically aqueous, i.e. the fibers are mixed with water. Water is the most practical solvent for large-scale processes, but in theory it is possible to use other solvents. In one embodiment, the chain length of the cellulose fibers is reduced such that the intrinsic viscosity, determined according to ISO 5351, is in the range of 300-900 ml / g. In one embodiment, the chain length of the cellulose fibers is reduced such that the intrinsic viscosity, determined according to ISO 5351, is in the range of 400-600 ml / g. In one embodiment, the chain length of the cellulose fibers is reduced such that the intrinsic viscosity, determined according to ISO 5351, is at least 550 ml / g. In one embodiment, the chain length of the cellulose fibers is reduced such that the intrinsic viscosity, determined according to ISO 5351, is at least 400 ml / g. In one embodiment, the chain length of the cellulose fibers is reduced such that the intrinsic viscosity 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 intrinsic viscosity determined according to ISO 5351 is at least 250 ml / g. In one embodiment, the chain length of the cellulose fibers is reduced such that the intrinsic viscosity determined according to ISO 5351 is no greater than 700 ml / g. In one embodiment, the chain length of the cellulose fibers is reduced such that the intrinsic viscosity determined according to ISO 5351 is no greater than 600 ml / g. A reduction is generally necessary since the intrinsic viscosity of cellulose in textile products is always higher than the range of 200-900 ml / g.

[0035] The unexpected finding is that flotation can be exploited by reducing the chain length of the cellulose. It was expected that flotation would not be possible due to the intertwining of different fibres. However, shortening the chain length too much can make the cellulose unusable or unattractive for use in the downstream process of cellulose regeneration. For example, for the Lyocell process, it is desirable to have an intrinsic viscosity of about 400 ml / g or more. For the Carvacel process, 250 ml / g or more may be sufficient.

[0036] In one embodiment, the chain length of the cellulose fibers is reduced by treatment with ozone under acidic conditions, pH less than 6. In one embodiment, the chain length of the cellulose fibers is reduced by treatment with oxygen under basic conditions, pH greater than 10. These two chain length reductions can be combined after each other. One advantage of using treatment with ozone is that the cellulose chains are weakened during the treatment. If step b) of reducing the chain length of the cellulose fibers comprises a treatment with ozone and step c) is performed after step b), the cellulose fibers are weakened during step b), while the non-cellulose fibers are generally not weakened, at least not to the same extent as the cellulose fibers. During the subsequent step c), mechanical treatment promotes the destruction of the weakened cellulose fibers, but only to a lesser extent the non-cellulose fibers. The modification of the tendency to break leads to shortening of the cellulose fibers during the mechanical treatment of step c), while the length of the non-cellulose fibers remains substantially unchanged. The shortened cellulose fibers and to some extent the preserved non-cellulose fibers facilitate the separation of the non-cellulose fibers by flotation. Thus, in one embodiment, step c) is carried out after step b), which comprises treatment with ozone and optionally other steps.

[0037] In one embodiment, step c) is carried out before step b). Step b) of cellulose chain length reduction and step c) of mechanical treatment can be carried out in any order. Step b) can be divided into several steps carried out at different points of the method.

[0038] In addition, step d) can be performed in any order with respect to steps b) and c). Step d) can be performed before, between or after steps b) and c). Since it is more economical to keep the fiber concentration high during steps b) and c), step d) is most often a dilution, and therefore step d) is most often performed after steps b) and c) and before the flotation in step e).

[0039] In one embodiment, step c) is carried out by grinding. Other mechanical treatment methods may be used to mechanically separate the fiber fragments into fine fragments. Any aggregates of fibers are ground to break them into smaller fragments. If the fiber mixture is such that no fiber aggregates are present, this step may be omitted. In most fiber mixtures, especially in the case of recycled fiber products, aggregates are present and the mechanical treatment step c) cannot be omitted. Agglomerates contain entangled fibers. Agglomerates of different fiber types make it difficult to separate the fibers in the flotation or reduce the yield of the flotation since more fibers may be excluded in the aggregates.

[0040] In one embodiment, the concentration of fiber in the mixture is adjusted during step d) to be in the range of 0.2-1.5% fiber by weight. In one embodiment, the concentration of fiber in the mixture is adjusted during step d) to be in the range of 0.7-1.5% fiber by weight. In another embodiment, the concentration of fiber in the mixture is adjusted during step d) to be in the range of 0.7-2% fiber by weight. This concentration adjustment is in most embodiments a dilution.

[0041] In one embodiment, the mixture is aqueous. Water is mixed with the fibers at the start of the process.

[0042] In one embodiment, the flotation is performed in multiple chambers connected in series. In one embodiment, the flotation is performed using multiple air diffusion nozzles that provide different bubble sizes. In one embodiment, multiple chambers are used, with each chamber being fed with different sized bubbles. This helps to remove different sized fibers in different chambers.

[0043] In one embodiment, the mixture is filtered prior to flotation in step e). Such an optional filtering step can ensure that any remaining large aggregates of entangled fibers are not exposed to flotation. In one embodiment, the material removed from the filter prior to flotation is ground and filtered again. The removed larger aggregates are ground and fed back into the filter.

[0044] In one embodiment, the mixture is made of textiles that are to be recycled. Generally, the process is directed to textiles that are to be recycled, although other applications are envisaged.

[0045] In one embodiment, the textile product is dry ground prior to step a), this additional drying step may include cutting and chopping the textile product into small pieces.

[0046] In one embodiment, the mixture is treated in water under reducing conditions and a pH above 10 prior to step b). This optional step is performed prior to reducing the chain length of the cellulose in step b). In one embodiment, the reducing conditions are achieved by the addition of dithionite anion. In one embodiment, sodium dithionite is added. The inventors believe that this treatment under reducing conditions at high pH achieves swelling of the cellulose fibers, which helps to separate the non-cellulosic fibers from the cellulose fibers during subsequent flotation.

[0047] In one embodiment, treatment in water under reducing conditions and a pH greater than 10 involves additional grinding in the wet state, which in one embodiment occurs when reducing conditions are present.

[0048] In one embodiment, the non-cellulosic fiber is a synthetic fiber that includes a man-made polymer. Examples of synthetic fibers that include a man-made polymer include, but are not limited to, nylon-based fibers, polyester-based fibers, acrylic-based fibers, modacrylic-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-cellulosic fiber includes polyester. In one embodiment, the non-cellulosic fiber includes elastane. In one embodiment, the non-cellulosic fiber includes polyacrylonitrile. Cellulosic-based fibers are generally mixed with smaller amounts of non-cellulosic fibers in commercially available textile products. For example, cellulose fibers made from cotton are often mixed with a few percent of polyester-based fibers.

[0049] In one embodiment, the mixture after step e) is dewatered and dried using techniques known in the art of pulping, including, for example, vacuum dewatering, pressing and drying.

[0050] The product obtained from this process can be used in many different ways. It can be used alone or mixed with other materials as a raw material for regenerating cellulose. In certain cases, this material can replace cotton linters.

[0051] In one embodiment the mixture after step e) is used as a raw material in a viscose manufacturing process. In one embodiment, if the resulting mixture is to be used in a viscose process, the intrinsic viscosity, determined according to ISO 5351, is reduced in step b) to a value higher than 550 ml / g.

[0052] In one embodiment, the mixture after step e) is used as a raw material in a lyocell manufacturing process. In one embodiment, if the resulting mixture is to be used in a lyocell process, the intrinsic viscosity, determined according to ISO 5351, is reduced in step b) to a value higher than 400 ml / g.

[0053] In one embodiment, the non-cellulosic fibers are recovered and optionally further refined. The non-cellulosic fibers, in one embodiment, are recovered and used for a variety of purposes.

[0054] In one embodiment, at least one surfactant is added before the flotation in step e). The surfactant is any surface active agent. Examples include compounds with hydrophilic and hydrophobic parts. In one embodiment, saponified fatty acids are used as surfactants. Surfactants with a wide variety of polar and non-polar groups can be used for flotation. Surfactants whose main role is to make the solid surface hydrophobic are called collectors. Surfactants whose main role is to provide the necessary stability to the top froth layer in the flotation cell and to affect the rate of particle and bubble attachment are called frothers. These are usually non-ionic surfactants and can increase the rate of film thinning and contribute to the stability of particle-bubble aggregates. In a further embodiment, other additives are added before or during the flotation in step e). Examples include, but are not limited to, defoamers, foam stabilizers, and substances for increasing or decreasing the hardness of the water. Additives commonly used in flotation are added in one embodiment.

[0055] In one embodiment, an additional flotation step is carried out before at least one of steps c) and d), which acts as an additional pre-flotation, for example to remove larger agglomerates and other impurities. In one embodiment, the agglomerates from this pre-flotation are mechanically treated to break them, for example by grinding. In one embodiment, the agglomerates separated in the additional flotation are subjected to mechanical treatment to break them down. The additional flotation is carried out before adjusting the concentration, which is usually a high fiber concentration.

[0056] In one embodiment, the flotation of step e) is repeated to improve removal of unwanted fibers.

[0057] In one embodiment, the mixture provided in step a) is a textile product comprising at least one of cotton and regenerated cellulose fibers.

[0058] In one embodiment, the temperature during the floating in step e) is in the range of 30-90°C during at least a part of the floating. In one embodiment, the temperature during the floating in step e) is in the range of 50-80°C during at least a part of the floating. In one embodiment, the temperature is in the aforementioned range throughout the entire floating. In another embodiment, the temperature is in the aforementioned range during a part of the floating. The latter embodiment may be relevant when several floating steps are carried out in series. A temperature in the range of 30-90°C, preferably 50-80°C, results in a lower viscosity of the aqueous medium and thus a better effect. In an alternative embodiment, the floating is carried out at ambient temperature.

[0059] In a second aspect, there is provided a cellulose containing material obtained by the above method.

[0060] In a third aspect, there is provided the use of recycled cellulosic material for the manufacture of viscose.

[0061] Other features and applications of the present invention and their associated advantages will become apparent to those skilled in the art from a reading of this specification and the accompanying examples.

[0062] 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 present invention, which is limited only by the appended claims and their equivalents. EXAMPLES

[0063] Example 1 Fiber Material: Fiber: Cotton with lower amounts of non-cellulosic fibers from textile fabric waste. The fibers were disassembled and bleached. First, the fibers were disassembled from woven and spun structures and refined to shorter fiber lengths. Next, the fibers were treated with a redox reaction using dithionite, followed by ozone bleaching (acidic conditions) and oxygen bleaching (alkaline conditions). Finally, the fibers were refined again to remove knots and clumps.

[0064] A 1-liter graduated cylinder (diameter 90 mm, height 210 mm) and a round air stone (diameter 80 mm, height 20 mm) were used for the experiment. 500 ml of a 0.5 wt.% fiber suspension was placed in the graduated cylinder. The air stone was placed at the bottom of the cylinder, which ensured a uniform flow of air bubbles in the fiber suspension. One to two drops of a flotation agent were added to obtain a foam that acted as a carrier for the non-cellulosic fibers. The non-cellulosic fibers adhered to the surface of the foam and were removed when the foam reached the top of the cylinder.

[0065] This flotation experiment was repeated three times with the same fiber mixture. The resulting remaining non-cellulosic fiber content can be seen in the table below for samples 1-3 from the different experiments. Two measurements of the original non-cellulosic fiber content were made, the results of which can be seen in the table. After one flotation step, the non-cellulosic fiber content was reduced by an average of 89.1%.

[0066] [Table 1]

[0067] Example 2 A Voith laboratory flotation cell Delta 25 was used.

[0068] Different mixtures of various fibers were used.

[0069] FM: Cotton fibre from white clipping. This fibre contains lesser amount of non-cellulosic fibres such as polyester. This fibre was disassembled and bleached. Firstly, the fibre was disassembled from the woven or spun structure and refined to shorter fibre lengths. Then, this fibre was treated with redox reaction using dithionite, followed by ozone bleaching (acidic conditions) and oxygen bleaching (alkaline conditions). Finally, the fibre was refined again to remove knots and clumps.

[0070] ·HM+Polyester: Made from white clippings. The pulp is refined using a valley beater and then polyester, which is environmentally friendly, is added.

[0071] · HM / FM+Polyester: The same fiber as FM above, with environmentally friendly polyester added and further refined in a valley beater.

[0072] The SR in the table indicates the grinding degree of different fibers, the higher the number means the finer the fibers.

[0073] The above fiber mixtures were run in a flotation cell with a pulp concentration of 1 wt%, flotation volume of 24 liters, temperature of 21°C, air flow rate of 11 L / min, and pH of 9 to 10. The above different mixtures were run for flotation times of 6, 9, and 12 minutes, respectively.

[0074] Flotation samples were performed with and without the addition of saponified olefinic fatty acid. 150 g of saponified fatty acid was added to the flotation cell. The saponified fatty acid was made by reacting 0.8 wt% fatty acid with 2 wt% NaOH.

[0075] The above FM fibers were also subjected to a mechanical filtration process and the results were compared. This test is called FM Slide Pack.

[0076] [Table 2]

[0077] Thus, flotation is an efficient method for separating cellulosic fibers, such as cotton fibers, from other fibers, such as polyester fibers. The excluded concentration of polyester is high, at 60.4% by weight, which means that the loss of cellulosic fibers is small.

Claims

1. A method for separating fibers, comprising the steps of: a) providing a mixture comprising cellulosic and non-cellulosic fibers; b) reducing the cellulose chain length of said cellulose fibers such that the intrinsic viscosity, determined according to ISO 5351, is in the range of 200 to 900 ml / g; c) subjecting the mixture of fibers to a mechanical treatment to break down fiber aggregates into smaller fragments; d) adjusting the concentration of fibers in the mixture so that the concentration of fibers in the mixture is in the range of 0.1 to 4% by weight of fibers; e) subjecting said mixture to flotation to remove said non-cellulosic fibers; Including, The method wherein the step e) is carried out after the steps a) to d).

2. 2. The method of claim 1, wherein the chain length of the cellulose fibers is reduced by treatment with ozone under acidic conditions of pH less than 6.

3. 3. The method of claim 1 or claim 2, wherein the chain length of the cellulose fibers is reduced by treatment with oxygen under basic conditions at a pH above 10.

4. 4. The method according to any one of claims 1 to 3, wherein the flotation is carried out using a number of air diffusion nozzles providing different air bubble sizes.

5. 5. The method according to claim 1, wherein the mixture is filtered prior to the flotation in step e).

6. 6. The method of claim 5, wherein the filtered material prior to flotation is ground and refiltered.

7. 7. The method according to any one of claims 1 to 6, wherein the mixture is made of textile products to be recycled.

8. 8. The method of claim 7, wherein the mixture is treated in water under reducing conditions and a pH greater than 10 prior to step b).

9. 9. The method according to any one of claims 1 to 8, wherein the mixture after step e) is dewatered and dried.

10. 10. The method according to any one of claims 1 to 9, wherein the mixture after step e) is used as a raw material in a viscose production process.

11. 11. The method according to any one of claims 1 to 10, wherein the mixture after step e) is used as a raw material in a lyocell manufacturing process.

12. 12. The method according to any one of claims 1 to 11, wherein at least one surfactant is added prior to the flotation in step e).

13. 13. The method according to any one of claims 1 to 12, wherein an additional flotation step is carried out before at least one of steps c) and d).

14. 14. The method of claim 13, wherein the agglomerates separated by flotation are subjected to a mechanical treatment to break up the agglomerates.

15. 15. The method according to any one of claims 1 to 14, wherein the temperature during said flotation in step e) is in the range of 30 to 90°C during at least a part of said flotation.

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