Method for recovering cellulose from a dyed cellulose containing textile raw material

Abstract

A method for recovering cellulose from a cellulose containing textile raw material containing dyed textiles, the method comprising subjecting a disintegrated textile raw material to an oxidative pretreatment before alkaline cooking and any further processing of the cellulose pulp obtained after the alkaline cooking. After cooking, spent alkaline cooking solution containing dye and dye residues is withdrawn from the textile derived cellulose pulp and is transferred to a recovery boiler in a Kraft pulp mill. An apparatus for carrying out the method is also disclosed.

Description

[0001] METHOD FOR RECOVERING CELLULOSE FROM A DYED CELLULOSE CONTAINING

[0002] TEXTILE RAW MATERIAL

[0003] TECHNICAL FIELD

[0004] The disclosure pertains to a method for recovering cellulose from a dyed cellulose containing textile raw material. In particular, the disclosure pertains to a method for recovering cellulose from a dyed cellulose containing textile raw material in a pulp mill, such as a Kraft pulp mill, having a chemical recovery system including a recovery boiler.

[0005] The disclosure further pertains to an apparatus on which the method may be carried out.

[0006] The method and apparatus may be used for producing dissolving pulp which may be further used in the production of manmade cellulosic fibres (MMCF) such as viscose and lyocell fibres.

[0007] Furthermore, the invention may be applicable to other chemical pulping processes where recovery of cooking chemicals is possible.

[0008] Examples of such chemical pulping processes are, but not limited to, soda pulping and sulphite cooking processes.

[0009] BACKGROUND

[0010] There is an increasing interest in recovering cellulose from textile waste containing cotton and using the recovered cellulose in the production of regenerated cellulose fibres, such as viscose and lyocell fibres. However, textile waste containing mixed materials require removal of unwanted textile components such as polyester, as well as components such as buttons and zippers, impurities, and various types of dyes. A method for removing polyester from a mixed cotton / polyester textile material is disclosed in WO 2020 / 013755 A1. The method in WO 2020 / 013755 A1 also results in a certain removal of dyes from the recycled textile material. However, depending on the type of dye present in the textile raw material, the amount of dye that is removed will vary. To obtain further decolourization it is generally necessary to treat the pulp obtained from the process in WO 2020 / 013755 A1 by subjecting it to one or more bleaching steps. Before the pulp can be bleached with conventional bleaching chemicals, the pulp must be carefully washed to obtain a good effect of the bleaching chemicals that are used.

[0011] A problem with bleaching of dyed textiles is that dyes and degradation products from dyes end up in the aqueous phase of bleaching and washing steps and must be handled and removed from the aqueous phase. This means that in a pulp mill producing pulp from both textile waste and lignocellulosic raw materials, the wastewater treatment system of the mill must be equipped to handle complicated chemical compounds which are foreign to a conventional pulp production process.

[0012] An object of the present invention is to provide a way of efficiently and economically removing dyes from a dyed cellulose containing textile material. A further object of the present invention may be to reduce water consumption when removing dyes from a cellulose containing textile material.

[0013] SUMMARY

[0014] At least one of the above objects may be achieved with a method for recovering cellulose according to claim 1 and / or an apparatus according to claim 12. Variations of the invention are set out in the dependent claims, and in the following disclosure.

[0015] Disclosed herein is a method for recovering cellulose from a cellulose containing textile raw material containing dyed textiles, the method comprising in the following order: a) subjecting the textile raw material to mechanical disintegration; b) subjecting the disintegrated textile raw material to an oxidative pretreatment; c) subjecting the pretreated textile raw material to cooking in an alkaline solution for a time period of at least 10 minutes and at a temperature in the range of from 100°C to 180°C to produce a textile derived cellulose pulp; d) withdrawing spent alkaline solution containing dyes and dye residues from the textile derived cellulose pulp and transferring the withdrawn spent alkaline solution to a recovery boiler in a pulp mill, such as a Kraft pulp mill; e) washing the textile derived cellulose pulp; f) recovering the textile derived cellulose pulp. The disintegration step a) may be carried out by milling or grinding, aiming at freeing, or substantially freeing the textile fibres from the textile raw material. The textile raw material is preferably disintegrated into individualized fibres.

[0016] The process parameters in step b), such as the duration of the treatment, temperature, pH, etc., are governed by the particular chemical used in the treatment, as set out in the examples.

[0017] It is further to be understood that the temperature during step c) may vary. By way of example, the cooking step c) may include an initial heating stage and / or a final cooling stage.

[0018] In the method as disclosed herein, it has surprisingly been found that there is no need for performing a washing step between step b) and step c). After the treatment in step b), the disintegrated textile raw material may optionally be dewatered before being subjected to the cooking step c).

[0019] After step d), all or a part of the spent alkaline solution containing dyes and dye residues is transferred to a recovery boiler, the recovery boiler being part of a chemical recovery system in a pulp mill, such as a Kraft pulp mill.

[0020] When subjecting the dyed cellulose containing textile raw material to an oxidative pretreatment before alkaline cooking, a large part of the dyes in the textile material is released into the cooking liquor and can be removed from the cellulose pulp with the spent alkaline liquor which is withdrawn after the cooking step c).

[0021] Thereby, the amount of dye which remains in the pulp after cooking and which may end up in the process liquor of any subsequent washing, bleaching and purification steps is considerably reduced. In the production of dissolving pulp for use in the manufacture of MMCF, one or more bleaching steps are usually performed to achieve a high final brightness in the final dissolving pulp which is recovered in step f). Thus, by employing oxidative pretreatment before an alkaline cooking step, in accordance with the present invention, increased purification of the textile material with a minimized generation of spent liquors and filtrates may be achieved.

[0022] Depending on the chemical structure of the dye, the oxidative chemical used in pretreatment, and the type of dye (e.g., disperse, direct, vat, reactive), the dyes may be released in intact form. It is also possible that steps b) and c) cause the destruction of the original dye structure in such a manner that coloured or uncoloured degradation products are released and removed in subsequent steps. In the case of reactive dyes which are covalently bound to the cellulose, a degradation product may remain bonded to the cellulose whereas the main degradation products will be transferred to the removed liquor.

[0023] The dyes and dye degradation products which are released into the cooking liquor and which are removed from the pulp in the spent cooking liquor are subsequently burned in the recovery boiler. Disposing of dyes which are released in the process liquors during bleaching and washing steps following on the cooking step by means of burning is unfeasible. The large liquid volumes used in these process steps would entail a greatly increased need for evaporation and energy consumption. Hence, the method as disclosed herein offers a way of efficiently and economically removing dyes from a textile raw material by means of recovery equipment which is already available at a pulp mill, such as a Kraft pulp mill.

[0024] “Dyeing” as referred to herein, is typically carried out with dyes that have affinity for the fibres and that can be made soluble in the dyeing media. Classification of dyes can be made both depending on dyeing method (e.g., reactive dyes, direct dyes, or vat-dyes) and based on the key structure of the dye (e.g., azo dyes, phthalocyanine dyes and anthraquinone dyes). Depending on the fibre type, different dyeing methods are used, as there must be an interaction between the fibres and the dye / dyeing system.

[0025] Colouring by “pigmenting”, as referred to herein, typically involves printing systems based on pigments that generally are very stable and resistant to chemical degradation and where the pigments are essentially water insoluble and are applied as suspensions. Pigments generally exhibit a low affinity to textile fibres, requiring the use of a polymer binder to protect the pri nt / textile from abrasion and wear. Pigmenting of whole textiles and garments are sometimes also performed with insoluble pigments in a manner similar to pigment printing.

[0026] With the method as disclosed herein, good decolourization of a dyed textile material can be achieved at the same time as the dye residues largely follow the spent alkaline cooking liquor and are burned in the recovery boiler. The decolourization method as disclosed herein has a limited impact on the need for evaporation and thereby on the energy consumption of the process. Furthermore, the fraction of dyes, dye residues and other components that reach the wastewater treatment plant of the pulp mill is minimized.

[0027] It is well known that chlorine free oxidative treatments such as treatment with one or more of ozone, oxygen, and peroxides, such as hydrogen peroxide, are relatively effective treatments for decolouring dyed fabrics. By performing such treatment before cooking instead of after, dyes and dye residues can be removed with the spent alkaline cooking liquor and be burned in the recovery boiler instead of being handled in the wastewater treatment plant of the pulp mill.

[0028] In a method as disclosed herein, all process liquors from the cooking step as well as wash liquors from any optional dewatering step preceding the cooking step are sent to the black liquor handling system of the pulp mill, where they are burnt in the recovery boiler. Thus, by subjecting the textile raw material to oxidative pretreatment before the cooking step, the need for removing dye and dye residues in the effluents from the bleaching, following the cooking, may be eliminated or at least considerably reduced.

[0029] As disclosed herein, in addition to steps a) to f), the method may comprise one or more further steps for treatment of the cellulose containing textile material, such as bleaching, washing, purification, viscosity adjustment, drying, etc.

[0030] Accordingly, one or more bleaching steps may be performed on the cellulose pulp after the alkaline cooking step. Bleaching is normally performed after the washing step e) and may be carried out using bleaching agents well known to the skilled person. Useful bleaching agents can be both oxidative and reducing bleaching agents. Examples of such are chlorine dioxide, oxygen, ozone, hydrogen peroxide, dithionite and formamidine sulfinic acid. In a method as disclosed herein, further purification steps may be performed after step e). Such further purification steps may include further washing as well as other types of purification, such as screening and floatation.

[0031] As set out herein, the oxidative pretreatment is preferably a chlorine free oxidative treatment, such as treatment with one or more of ozone, oxygen, and peroxides, such as hydrogen peroxide, peracetic acid, performic acid and oxone (potassium peroxomonosulfate) or other persulphuric acid-based compounds.

[0032] The alkaline solution in the cooking step may comprise or consist of an industrial white liquor containing sodium sulphide (Na2S), such as an industrial white liquor derived from a wood pulping process, such as a Kraft pulping process.

[0033] In the method as disclosed herein, step f) may comprise a step of physically separating the cellulose pulp from a liquid phase, such as by filtering. Filtering is the most commonly used separation method in pulp production. However, it is to be understood that other separation methods such as drying (evaporation), centrifugation, sedimentation or any combination thereof may be used, as found suitable.

[0034] The method as disclosed herein may be a continuous process, a batch process, or any combination of one or more continuous process step(s) and batch process step(s).

[0035] The method for recovering cellulose from a dyed cellulose containing textile raw material, as disclosed herein, may be performed in a Kraft pulp mill, and may be linked to or integrated with the Kraft pulping process. The method for recovering cellulose from a dyed cellulose containing textile raw material may be linked to the Kraft pulping process by utilizing process liquors from the Kraft pulping process and / or by being performed using equipment and process systems which are present in the Kraft pulp mill. The method as disclosed herein may suitably be integrated with the production of Kraft type dissolving pulp or paper pulp from lignocellulosic material. The method for recovering cellulose from a dyed cellulose containing textile raw material may involve mixing the textile derived pulp obtained after step d) with pulp derived from lignocellulosic material in a Kraft pulping process. Thus, the method as disclosed herein may be used for producing a cellulose pulp, such as a dissolving pulp, comprising cellulose material derived both from textile material and from lignocellulosic material.

[0036] In a Kraft pulping process, also referred to as a sulphate process, wood chips are converted into wood pulp, consisting of cellulose fibres.

[0037] The wood chips are treated in a digester with an aqueous cooking liquor, commonly referred to as “white liquor” and containing sodium hydroxide (NaOH) and sodium sulphide (Na2S) which when dissolved give the active ions OH- and HS“, that react with lignin in the wood material. The white liquor is a strongly alkaline solution of mainly sodium hydroxide, sodium sulphide and sodium carbonate, which is produced at the mill.

[0038] The cooking process breaks the bonds that link lignin, hemicellulose, and cellulose. After cooking, the cellulose fibres are further treated in a pulp production process involving washing, bleaching, and drying to produce cellulose pulp.

[0039] In a Kraft pulping process, such as a paper pulp or dissolving pulp process, spent alkaline liquor, commonly referred to as “black liquor”, is derived from the process, following on cooking in a digester. The black liquor is subjected to evaporation in an evaporator where the black liquor from the pulping process is concentrated. The concentrated black liquor is then transferred to a recovery boiler.

[0040] The recovery boiler is part of the recovery system of the pulp mill. In the recovery boiler, concentrated black liquor from the pulping process is burnt to recover chemicals and thermal energy generated by the burning process.

[0041] The smelt of inorganic material which is formed at the bottom of the recovery boiler is then used for the preparation of fresh white liquor, also referred to herein as “industrial white liquor”.

[0042] The recycled cellulose containing textile raw material used in the cellulose recovery method as disclosed herein may contain cellulose material of any kind as used in textiles, including but not limited to: cotton, regenerated cellulosic fibres (viscose, modal, lyocell, etc.), linen, sisal, jute, etc. The textile raw material comprises dyed textiles and may also comprise pigmented textiles. An “alkaline solution” as used herein, refers to a solution containing hydroxide ions. The hydroxide ion may be derived from compounds such as, but not limited to, NaOH and KOH. The effective alkali (EA) concentration is commonly reported as sodium hydroxide (NaOH) equivalents regardless of counter ion and may be determined by titration according to SCAN N 30:85 or SCAN N 33:94. The terms “effective alkali” and “alkali” are used interchangeably herein. The measuring method according to SCAN N 30:85 is suitable for use when the alkali charge originates from white or green liquor. The measuring method according to SCAN N 33:94 is suitable for use when the alkali charge originates from black liquor.

[0043] The alkaline solution in the cooking step c) of the method as disclosed herein may have an initial effective alkali concentration (NaOH equivalents) in the range of from 5 to 150 g / l, such as in the range of from 10 to 140 g / l, or from 15 to 130 g / l, or from 20 to 125 g / l, or from 25 to 120 g / l, or from 30 to 115 g / l, or from 35 to 110 g / l, or from 40 to 105 g / l. A particularly useful range may be the range of from 25 to 120 g / l.

[0044] The ratio between the volume in dm3of the alkaline solution and the weight in kg of textile raw material in step c) may be from 1.5:1 to 25:1, i.e., from 1.5 dm3 / kg to 25 dm3 / kg. The amount of liquid in relation to the amount of textile raw material may vary in the process. Without limitation, the textile raw material may first be suspended in an amount of liquid to reach a ratio between the volume in dm3of the alkaline solution and the weight in kg of textile raw material of e.g., 4:1. After such initial wetting of the textile raw material, the suspension may be diluted to a ratio of e.g., 9:1 or more. The ratio between the volume in dm3of the alkaline solution and the weight in kg of textile raw material may suitably be adjusted after the cooking step to achieve processability of the textile pulp in a conventional waste-water treatment plant of a pulp mill, such as a Kraft pulp mill.

[0045] The alkaline solution used in the method as disclosed herein may contain components obtained from a Kraft pulping process. For instance, the alkaline solution may contain or substantially consist of white liquor derived from the Kraft pulping process. It may also contain black liquor generated during digestion and / or green liquor obtained from the chemical recovery process of the Kraft pulp mill. Disclosed herein is also an apparatus for treating cellulose from a cellulose containing raw material, such as a textile cellulose containing raw material, the apparatus comprising the following parts: a) a mechanical disintegration unit; b) an oxidative pretreatment unit; c) an alkaline cooking device; d) a pulp washing arrangement; e) a pulp recovery arrangement; and f) a connection from the alkaline cooking device to a recovery boiler in a pulp mill, such as a Kraft pulp mill; and wherein the parts a) to e) are connected in series.

[0046] A meaning of that the parts a) to e) are connected in series is that said parts are connected in series in alphabetical order from a) to e). The mechanical disintegration unit of part a) may be a textile disintegration unit. Optionally, the alkaline cooking device of part c) is not a Kraft pulping digester. As an alternative option, the cooking device of part c) is not a Kraft pulping digester or a sulphite pulping digester.

[0047] The apparatus for treating cellulose is suitable for use in the method for recovering cellulose from a cellulose containing textile raw material containing dyed textiles, as disclosed herein.

[0048] BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The process as disclosed herein will be further explained hereinafter with reference to the appended drawings wherein:

[0050] Figure 1 shows a schematic illustration of a method for recovering cellulose from a dyed cellulose containing textile raw material;

[0051] Figure 2 is a diagram showing the amount of dye remaining in dyed samples after cooking;

[0052] Figure 3 is a diagram showing the ratio k / s calculated from reflectance as described using D65 / 10 for Direct Red 81 , Direct Blue 1, Remazol Navy Blue and Remazol Orange, after cooking with and without a preceding O3 step; Figure 4 is a diagram showing the ratio k / s calculated from reflectance as described herein using D65 / 10 for Vat Blue 1 (Indigo) after cooking with and without a preceding O3 step;

[0053] Figure 5 is a diagram showing the effect of white liquor treatment on colour removal from post-consumer textile material using D65 / 10; and

[0054] Figure 6 is a diagram showing the effect of three different oxidative pretreatments on remaining colour after a white liquor pretreatment of post-consumer textile material using D65 / 10.

[0055] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0056] Different aspects of the present disclosure will be described more fully hereinafter with reference to the enclosed schematic drawing.

[0057] Fig. 1 shows the order of method steps in a method for recovering cellulose from a cellulose containing textile raw material containing dyed textiles. The method comprises subjecting a disintegrated textile raw material to an oxidative pretreatment before alkaline cooking and any further processing of the cellulose pulp obtained after the alkaline cooking. Spent alkaline cooking solution containing dye and dye residues is withdrawn from the textile derived cellulose pulp and transferred to a recovery boiler in a Kraft pulp mill. The textile raw material may be any dyed cellulose containing textile raw material such as mixed used textiles containing old clothes and other consumer textiles, industrial textile waste, etc. The textile raw material may contain other fibrous components in addition to cellulose fibres, such as polyester fibres.

[0058] As set out herein, the invention may be applicable to other chemical pulping processes where cooking chemicals are being recovered. Examples of such chemical pulping processes are, but not limited to, soda pulping and sulphite cooking processes.

[0059] The textile raw material is first subjected to mechanical disintegration in a disintegration step a). The disintegration step may be performed by grinding, milling, or the like, to produce individualized fibres which can easily be mixed with a process liquid to form a suspension of the fibres in the process liquid.

[0060] The disintegrated textile raw material is then subjected to an oxidative pretreatment step b). The pretreatment may be carried out at high consistency levels of about 25-50%. Using high consistency levels is manageable in processes for bleaching of wood-based pulp with ozone and oxygen and can also be implemented in oxidative pretreatment of disintegrated cellulose containing textiles. Such high consistencies may be employed also in oxidative pretreatment using oxidative chemicals other than ozone and oxygen, such as peroxides.

[0061] A further possibility is to use a consistency being lower than 25%. In such cases, a certain degree of dewatering of the pretreated cellulose containing material before it is mixed with the alkaline cooking liquor may be appropriate. At least part of the filtrate from the dewatering step may be used to adjust the consistency of the suspension of cellulose containing textile material in the oxidative pretreatment. However, a high degree of recirculation of filtrate may lead to accumulation of organic material and metal ions in the process liquid which may negatively influence the pretreatment step. Therefore, a suitable degree of recirculation will vary depending on the textile raw material and the chemicals added in the oxidative pretreatment.

[0062] After the oxidative pretreatment, the textile raw material is subjected to a cooking step c). Cooking is carried out in an alkaline solution for a time period of at least 10 minutes and at a temperature in the range of from 100°C to 180°C to produce a textile derived cellulose pulp. As set out herein, longer reaction times are generally required when operating at lower reaction temperatures. The alkaline cooking liquor may comprise or consist of liquor derived from a Kraft pulping process in which wood is converted to wood derived pulp. Accordingly, the alkaline cooking liquor may comprise industrial white liquor, optionally mixed with industrial black liquor and / or green liquor. By industrial black liquor is implied spent alkaline cooking liquor from a wood pulping process, typically a Kraft pulping process in a Kraft pulp mill.

[0063] After cooking the cellulose containing textile raw material, the spent alkaline solution containing dye residues, which have been released into the alkaline solution from the textile derived cellulose pulp, is withdrawn from the treated textile derived cellulose pulp. The textile derived cellulose pulp is then subjected to washing and optional subsequent bleaching and the resulting cellulose pulp is recovered. Before washing and optional bleaching, the textile derived cellulose pulp may be blended with wood derived cellulose pulp, such that the final recovered pulp contains textile derived cellulose pulp and wood derived cellulose pulp.

[0064] The alkaline solution which is withdrawn after step c) is preferably transferred to a recovery boiler in a chemical recovery system in a pulp mill, such as a Kraft pulp mill.

[0065] EXAMPLES

[0066] Example 1

[0067] Cotton textile was dyed using five different dyes, of which two were direct dyes (Direct Blue 1, Direct Red 81), two were reactive dyes (Remazol Orange and Remazol Navy Blue) and one vat dye (Indigo).

[0068] Ozone treatment (pretreatment) was performed in medium consistency (12%) in room temperature in a plastic flow-through reactor. Prior to feeding the dyed and grinded textile material into the reactor, the textile was mixed with water and sulfuric acid used for pH adjustment. Ozone charge was determined from titration using potassium iodide solution and sodium thiosulfate. An ozone charge of 0.5 % (based on the textile material) was used at a pH of 2. After the ozone treatment, the textile materials were carefully with deionized water.

[0069] After the ozone pretreatment, cooking was performed in autoclaves in a hot-air oven according to the following procedure: liquid:textile = 9:1 (dm3 / kg), effective alkali addition 14 % based on textile weight (industrial white liquor; EA=38,1 g / l, sulfidity = 34%, measured according to SCAN N 30:85) with temperature ramping from 25 °C to 70 °C being performed at a constant rate. After 15 minutes stabilization of the temperature at 70 °C, the temperature was raised with 0.75 °C / min to a final temperature of 140 °C and holding time at maximum temperature was 60 minutes.

[0070] After cooling the autoclaves by spraying with cold water, the samples were washed thoroughly with deionized water.

[0071] The optical properties of the samples were evaluated using a Minolta Spectrophotometer CM- 3630 using D65 / 10. The sheets used for optical evaluation were prepared according to ISO 3688:2022. The reflectance values over the measured wavelengths (360-740 nm with spatial resolution of 10 nm) were converted to the ratio between the light absorption coefficient (k) and the light scattering coefficient (s) using the Kubelka-Munk relation k / s=(1-R )<2> / 2 / R. The benefit of recalculating to k / s is that this ratio is linearly correlated to the chromophore concentration (at the same s) which the reflectance is not.

[0072] The amount of remaining dye was calculated from the k / s ratio at the absorption maximum before the treatment (Remazol Orange 500 nm, Remazol Navy Blue 610 nm, Vat Blue 660 nm, Direct Blue 1 650 nm and Direct Red 81 520 nm) compared to the k / s ratio at the same wavelength after the treatment.

[0073] As seen in Figure 2 it was found that between 98 % and 40% more dye was removed when an ozone treatment was performed before the cooking. The k / s graphs after cooking with and without a preceding ozone treatment are shown in Figures 3 and 4. It was found that much more dye was removed if an ozone step was performed before the cooking stage.

[0074] Example 2

[0075] A batch of post-consumer waste textiles was disintegrated and subjected to different oxidative pretreatments prior to white liquor treatment.

[0076] The batch of post-consumer textiles was obtained from a sorter of second-hand clothing. No sorting according to colour was made. The textiles mainly comprised T-shirts and other knitted goods and the textiles contained mainly cotton and polyester. The content of non-cellulosic fibres in the batch of textiles was estimated to be 10 % by assuming that the solid residue obtained after acid hydrolysis of textile material according to ISO 21437:2020 reflects the content of non cellulosics (mainly polyester).

[0077] The effect of three different oxidizing chemicals in a pretreatment step was compared with the case where the pretreatment was omitted. The oxidizing chemicals used were: peracetic acid (38-40% from Supelco), oxone (potassium peroxomonosulfate from Thermoscientific) and oxygen.

[0078] The treatments with peracetic acid (Paa) and Oxone were performed at 80 °C and a pulp consistency of 10 % for 60 minutes in plastic bags submerged in a preheated water bath. The charge of the peracids corresponded to 10 kg H2O2-equivalents / ton dry textile. Also alkali was added, and in the case of Paa, 15 kg NaOH / ton dry textile was added and in the case of Oxone the charge was 10 kg NaOH / ton.

[0079] The oxygen pretreatment was performed in autoclaves at 10 % consistency with addition of 40 kg NaOH / ton dry textile and 3 kg MgSO^ton dry textile. An oxygen pressure of 5 bar was applied and the autoclaves were rotated in a hot-air oven, increasing the temperature from room temperature to 105 °C during 50 minutes and holding the temperature at this level for 90 minutes. Then the autoclaves were cooled by spraying with cold water and the oxygen pressure was released.

[0080] After the oxidative pretreatments, the pretreated textile samples were dewatered on a Buchner funnel to a dry content corresponding to 25% based on original dry textile. The pH of the filtrate from the different pretreatments was 5,3 in the case of Paa, 4,0 in the case of Oxone and 12,6 after Oxygen.

[0081] No washing of the partly dewatered samples was performed before they were subjected to the white liquor treatment. The white liquor treatment was performed as in Example 1 , but in this case the charge of effective alkali was increased to 32 % (based on dry original weight of textile material) in order to ensure an effective hydrolysis of the polyester part of the textile material. The extensive removal of polyester in the white liquor treatment was confirmed by measuring the solid residue obtained after acid hydrolysis of the white liquor treated material. Also, here the acid hydrolysis was carried out according to ISO 21437:2020. The measured solid residue was shown to be on a very low level (<1 %) for all four samples.

[0082] The optical properties of the samples were evaluated using the same instrument and standard as in Example 1. Figure 5 shows that there was a substantial removal of dye when the textile material was treated with white liquor and in Figure 6 it is shown that the introduction of an oxidative treatment prior to the white liquor substantially enhances the dye removal.

Claims

1. CLAIMS1. A method for recovering cellulose from a cellulose containing textile raw material containing dyed textiles, the method comprising in the following order: a) subjecting the textile raw material to mechanical disintegration; b) subjecting the disintegrated textile raw material to an oxidative pretreatment; c) subjecting the pretreated textile raw material to cooking in an alkaline solution for a time period of at least 10 minutes and at a temperature in the range of from 100°C to 180°C to produce a textile derived cellulose pulp; d) withdrawing spent alkaline solution containing dye and dye residues from the textile derived cellulose pulp and transferring the spent alkaline solution to a recovery boiler in a pulp mill, such as a Kraft pulp mill; e) washing the textile derived cellulose pulp; f) recovering the textile derived cellulose pulp.

2. A method according to claim 1, wherein no washing step is performed between step b) and step c).

3. A method according to claim 1 or 2, wherein the oxidative pretreatment is a chlorine free oxidative treatment, such as treatment with one or more of ozone, oxygen, and peroxides, such as hydrogen peroxide, peracetic acid, performic acid and oxone (potassium peroxomonosulfate) or other persulphuric acid based compounds.

4. A method according to any one of the preceding claims, wherein the alkaline solution in the cooking step comprises an industrial white liquor containing sodium sulphide (Na2S), such as industrial white liquor in a Kraft pulp mill.

5. A method according to any one of the preceding claims, wherein the alkaline solution in the cooking step comprises an industrial black liquor, such as industrial black liquor in a Kraft pulp mill.

6. A method according to any one of the preceding claims, wherein the textile derived cellulose pulp obtained after step d) is blended with a wood derived cellulose pulp.

7. A method according to any one of the preceding claims, wherein one or more bleaching steps are performed on the cellulose pulp after the cooking step c) and withdrawal ofspent alkaline solution in step d), such as between the washing step e) and the recovering step f).

8. A method according to any one of the preceding claims, wherein step f) comprises a step of physically separating the cellulose pulp from a liquid phase, such as by filtering, evaporation, centrifugation, sedimentation, or any combination thereof.

9. A method according to any one of the preceding claims, wherein further purification steps are performed after step e).

10. A method according to any one of the preceding claims, wherein the method is performed in a Kraft pulp mill and is linked to or integrated with the Kraft pulping process.

11. A method according to any one of the preceding claims, wherein the textile derived cellulose pulp recovered in step f) is a dissolving pulp.

12. An apparatus for treating cellulose from a cellulose containing raw material, such as a textile cellulose containing raw material, the apparatus comprising the following parts: a) a mechanical disintegration unit; b) an oxidative pretreatment unit; c) an alkaline cooking device; d) a pulp washing arrangement; e) a pulp recovery arrangement; and f) a connection from the alkaline cooking device to a recovery boiler in a pulp mill, such as a Kraft pulp mill; and wherein the parts a) to e) are connected in series.

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