Metal removal in recycling of cellulosic textiles

Abstract

A method for removal of metal ions during recycling of textiles comprising cellulose, said method comprising the sequential steps of providing at least one textile material comprising cellulose, treating the material to swell the cellulose, under alkaline and reducing conditions, wherein at least one reducing agent is present at least during a part of the swelling, and treating the material according to at least one of the following steps: i) decreasing the pH to 5.5 or below, and ii) adding a chelating agent. The metal ion content of the material is thereby reduced. The process gives a more efficient metal removal during textile recycling.

Description

[0001] METAL REMOVAL IN RECYCLING OF CELLULOS IC TEXTILES

[0002] Technical field

[0003] The present invention relates to a method for removing metal ions in the process of recycling of cellulose from textile comprising cellulose .

[0004] Background

[0005] Despite its environmental impact , the textile industry is growing fast . The textile industry is a contributor to pollution, including CO2 emissions . It is desirable to reduce the environmental impact of the textile industry by increasing the recycling of textiles .

[0006] Textiles can be reused, chemically recycled or mechanically recycled . Chemical recycling of textiles can be done in various ways , producing multiple products depending on the textile source . One example of a chemical recycling is using recycled pulp material in the viscose process , where fibers of regenerated cellulose are produced . The viscose process is a well-known multistep process to produce a regenerated textile fiber . The exact partition and labeling of the process step varies , however, the chemistry remains the same .

[0007] There are several known ways to dissolve cellulose for various applications including manufacture of regenerated cellulosic fiber .

[0008] WO 2022 / 139651 discloses a method for ozone bleaching of pulp from recycled textile . In fig . 2 and in the text there is disclosed an embodiment , which have the initial step of pulping the recycled textile material to form cellulose pulp, for instance by swelling the material under reducing conditions as described in WO2018 / 073177 . The pulping may comprise washing the formed pulp prior to an optional oxygen bleaching stage in which gaseous oxygen is introduced into the liquid phase of the (high consistency) pulp . The oxygen bleaching optionally comprises dewatering the pulp and / or adding an alkaline such that the oxygen bleaching is carried out at alkaline conditions . The oxygen bleaching 11 may comprise washing the formed pulp prior to pretreating 12 . The pulp is pretreated by adding sulphuric acid to reduce the pH of the pulp to within a range from 2 to 3 , and adding additives together with, or simultaneously with the sulphuric acid . The additives comprise oxalic acid and / or EDTA and / or magnesium sul fate . In connection with another embodiment there is disclosed pretreating the pulp prior to the ozone bleaching, wherein the pretreating comprises adding at least one acid to reduce the pH of the pulp, and adding one or more additives . The one or more additives may be added together with, or simultaneously with the at least one acid . Alternatively, the one or more additives may be added prior to , or after the at least one acid . The at least one acid may comprise sulphuric acid . The at least one acid may be added at a charge such that the pH is reduced to within a range from 2 to 3 , preferably from 2 to 2 . 5 or from 2 . 5 to 3 . The one or more additives may comprise one or more chelating agents and / or radical scavengers . Chelating agents provide the ef fect that selectivity in the ozone bleaching step is improved by reducing the content of metal ions in the pulp, which metal ions catalyze the degradation of ozone to free radicals , which in turn cause cellulose degradation . Radical scavengers have a similar ef fect by scavenging the metal ions . The use of chelating agents and radical scavengers has proven particularly advantageous together with the unusually high ozone charges used in the present invention, which otherwise may cause excessive cellulose degradation and consequently too low viscosity of the resulting pulp. It is not disclosed that swelling the material under alkaline conditions and under reducing conditions can improve metal removal .

[0009] WO 2018 / 073177 discloses a process for recycling of cellulose from textile, where textile material is swelled under reducing conditions, before a bleaching step is performed. The pH is not restricted for the swelling step, however in one embodiment the pH during the swelling step is 9-13.5 and in another embodiment the pH during the swelling step is Ills.3.

[0010] WO 2013 / 124265 discloses a process for the regeneration of cellulose. It discloses treatment of cellulose in an alkaline step and with oxygen. Reduction of the viscosity with oxygen is mentioned. Recycling of cloth is mentioned and a pretreatment may be desirable to lower the degree of polymerization. It is also mentioned that the cellulose can be used to make a new fiber such as viscose.

[0011] Metal ions such at calcium ions are present in cotton-based textile to a large extent. Calcium can cause problem in processes making man-made fibers since calcium precipitations can clog nozzles when fibers are spun. Other metal ions could cause problems of different sorts. E.g. iron-ions catalyzes the ageing process. It is therefore desirable to include a step of removing at least some of the metal ions present in the textile material at some point in a process of chemical recycling of textiles. Different methods for metal removal in these processes are known. WO 2021 / 185704 discloses a one-stage step in a process for regenerating reclaimed cellulose, where acid metal removal is combined together with acid oxidative bleach in one single stage .

[0012] Although there are working methods for recycling of textiles, it is nevertheless desirable to provide an improved metal removal method that can be used as a part of a process for chemical recycling of textiles comprising cellulose.

[0013] Summary

[0014] It is an object of the present invention to obviate at least some of the disadvantages in the prior art and provide an improved process for the removal of metal ions in a process of chemical recycling of textiles comprising cellulose.

[0015] In a first aspect there is provided a method for removal of metal ions during recycling of textiles comprising cellulose, said method comprising the sequential steps of: a) providing at least one textile material comprising cellulose, b) treating the material to swell the cellulose, under alkaline and reducing conditions, wherein at least one reducing agent is present at least during a part of the swelling, c) treating the material according to at least one of the following steps: i) decreasing the pH to 5.5 or below, and ii) adding a chelating agent.

[0016] In one embodiment, step c) comprises treating the material according to only step i) i.e. decreasing the pH to 5.5 or below. In one embodiment, step c) comprises treating the material according to only step ii) i.e. adding a chelating agent. In one embodiment, step c) comprises treating the material according to both steps i) and ii) . In the latter case the steps i) and ii) can be performed in any order. The acid treatment or chelating treatment lowers the content of calcium and other metal ions . It has been found that combining a swelling treatment in alkaline and reducing conditions with a subsequent acid or chelating treatment step results in a reduced content of calcium and other metal ions compared to when only a moderate alkali swelling is performed followed by an acid or chelating treatment . Although only alkali conditions are able to degrade impurities in for instance cotton to some extent and make the metal ions available for removal to some extent , it has been found that the combination of swelling treatment in alkaline and reducing conditions is more ef ficient for metal removal for recycled textiles .

[0017] Without wishing to be bound by any particular scienti fic theory the inventors believe that the alkaline and reducing conditions results in increased permeability of the cellulose and metal ions are more easily released from the material in upcoming treatment steps , such that the subsequent metal removing step is more ef fective . Again without wishing to be bound by any particular scienti fic theory the inventors believe that the treatment under alkaline and reducing conditions degrade at least to some extent impurities in the cellulose in the textiles so that the metal ions are more accessible . The new combination of the reducing and alkaline swelling step followed by an acid or chelating treatment step has been found to decrease the content of calcium and other metal ions in recycled textile pulp, as shown in the appended experimental results .

[0018] Metal ions are present in recycled cellulose and it is desired to reduce the content of such metal ions . Again without wishing to be bound by any speci fic scienti fic theory the inventors believe that the removal of metal ions reduces the propensity of the cellulose chains to form clusters and cause clogging and other problems . The method for recycling of a textile may thus be used to treat cellulosic material that can be used in a process for production of viscose without , or at least with a signi ficant lowered tendency of causing clogging of noz zles in the viscose production process .

[0019] In a second aspect there is provided a recycled cellulose material obtained by the method .

[0020] Detailed description

[0021] Before the invention is disclosed and described in detail , it is to be understood that this invention is not limited to particular compounds , configurations , method steps , substrates , and materials disclosed herein as such compounds , configurations , method steps , substrates , and materials may vary somewhat . It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof .

[0022] It must be noted that , as used in this speci fication and the appended claims , the singular forms "a" , "an" and "the" include plural referents unless the context clearly dictates otherwise .

[0023] I f nothing else is defined, any terms and scienti fic terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains .

[0024] "Solution" is used herein to denote a homogeneous mixture comprising at least one substance dissolved in a solvent . The term " alkaline condition" should be understood to mean conditions with a pH of 8 . 5 or higher for this particular invention .

[0025] The term "reducing conditions" refers generally to chemical conditions that promote reduction reactions , in which electrons are gained by a molecule , ion, or atom . These conditions are characteri zed by a certain concentration of a reducing agent in a system . Reducing agents are substances that readily donate electrons to other molecules or ions . In this particular case , the cellulose is to be subj ected to reducing conditions so that the cellulose is at least partially reduced . Thus reducing conditions in this particular case are conditions where the cellulose is at least partially reduced .

[0026] The term "chelating agent" refers to a chemical compound that binds metal ions and form stable complexes called chelates .

[0027] The term "cellulosic material" as used herein refers to all natural cellulosic forms ( cotton, linen, j ute , etc . ) and all regenerated cellulosic forms such as rayon . In particular, all textiles comprising cellulose are encompassed including textiles comprising treated and modi fied cellulose .

[0028] The term "textile" refers to a material that is made up of fibers or yarn that are woven, knitted, or otherwise processed into a fabric for use in clothing, home furnishings , and other applications . Examples of textiles include but are not limited to textiles comprising cotton, wool , silk, linen, polyester, nylon, and rayon .

[0029] Textile recycling refers to the process of recovering material ( s ) from textiles , often used or discarded textiles , and then mechanically or chemically ( or both) processing them to create new textile products or other new products . This involves breaking down the textiles , and then using the material to make new fabrics or other products . One example of chemical recycling is using recycled pulp material in the viscose process , where fibers of regenerated cellulose are produced . Textile reuse , which should not be confused with recycling, involves finding new uses for textiles without breaking them down into raw materials . This can include donating used clothing or fabrics to charity for further use , repurposing old clothes into new items such as bags or quilts , or using fabric scraps for other crafts or proj ects . The main di f ference between recycling and reuse of textiles is that recycling involves breaking down the textiles to create new products , while reuse involves finding new uses for the textiles without breaking them down . The present invention relates to recycling of textiles .

[0030] It is known that an acid treatment or chelating treatment may be used to lower the content of calcium and other metal ions in recycled textile pulp made for producing man-made fibers . However, it has surprisingly been found that combining a treatment in alkaline and reducing conditions with a subsequent acid treatment or chelating treatment reduces the content of calcium and other metal ions more than i f only moderate alkali swelling is performed followed by an acid treatment or chelating treatment . Hence , such a combination improves the pulp quality .

[0031] In a first aspect there is provided a method for removal of metal ions during recycling of textiles comprising cellulose , said method comprising the sequential steps of : a . providing at least one textile material comprising cellulose , b. treating the material to swell the cellulose, under alkaline and reducing conditions, wherein at least one reducing agent is present at least during a part of the swelling, and c. treating the material according to at least one of the following steps: i. decreasing the pH to 5.5 or below, and ii. adding a chelating agent.

[0032] In one embodiment, the at least one textile is at least one selected from the group consisting of cotton, lyocell, rayon, and viscose. In one embodiment, the at least one textile is cotton. It has been shown that textiles comprising cellulose such as cotton can suitably be recycled using the present method. Mixtures of different types of textiles are also encompassed as long as at least a part of the textiles comprise cellulose.

[0033] In one embodiment, the textile is mechanically disintegrated before step b) so that the textile is at least partially disintegrated in order to improve the subsequent steps. In one embodiment, the textile is mechanically disintegrated in a shredder before step b) . Other methods of disintegrating the textiles are also encompassed. The shredding and disintegration creates a larger surface of the material so that the subsequent steps are facilitated. In some embodiments the textile to be recycled is provided as finely divided pieces so that the need for a mechanical disintegration is less or not necessary. In other embodiments, where larger pieces of textile are provided a disintegration step is suitable before step b) .

[0034] The pH during step b) is above 8.5. In one embodiment, the pH during step b) is above 9. In one embodiment, the pH during step b) is in the range of 9-13.5. In another embodiment, the pH during step b) is in the range of 11-13.3. The swelling is thus in these embodiments carried out under alkaline pH.

[0035] In order to provide the alkaline conditions of step b) , an alkaline substance may be added to the material. Alkaline substances are also known as bases or basic substances. Examples of alkaline substances include sodium hydroxide (NaOH) , calcium carbonate (CaCOs) , and ammonia (NH3) . In one embodiment, NaOH is present during step b) in a concentration from 0.01 to 0.5 mol / 1. NaOH has the advantage of being a cost efficient way of raising the pH.

[0036] Reducing conditions not only reduces the cellulose at least partially, the reducing conditions may also promote the reduction of metal ions to form metals or metal ions of other oxidation numbers, at least to some extent. Examples of reducing agents, which may be used to provide the reducing conditions, include the alkali metals, formic acid, oxalic acid, and sulfite compounds. In one embodiment, the at least one reducing agent comprises sodium dithionite, Na2S2<34. In other embodiments, other reducing agents are used. In one embodiment, sodium dithionite, Na2S2O4is present during step b) in a concentration from 0.01 to 0.25 mol / 1. In one embodiment, the reducing conditions are such that the redox potential is in the interval -1400 to -100 mV. The redox potential is measured according to ASTM D1498-14 (2022 ) .

[0037] In one embodiment, the temperature during step b) is in the range 50-100 °C.

[0038] In one embodiment, step b) is carried out during 20 minutes - 24 hours. For the acid treatment of step c i) , the pH is decreased by addition of an acid. Decreasing the pH value to 5.5 or below means that the pH is decreased to a value < 5.5, i.e. equal to or below 5.5. In one embodiment, the pH of the material is decreased to below a value selected from 5.5, 5, 4.5, 4, 3, 2 and 1. In one embodiment, the pH during step c i) is decreased to be within an interval selected from 1.5-5, 0- 5.5, 0-5, 1-5, 0-4, 2-3, and 1-3. Any suitable acid or mixture of acids can be used in order to obtain an acidic pH. In one embodiment, the acid used is sulphuric acid. In one embodiment, the acid is an acid selected from hydrochloric acid, formic acid and citric acid. In an alternative embodiment the pH is decreased in step c i) by addition of CO2 • H2CO3 may also be used, particularly in combination with CO2 • This is advantageous for use in an industrial scale, where CO2 can be recycled in the process.

[0039] In one embodiment, the acid treatment step c i) is carried out at a temperature in the interval 30-120 °C, such as 40-60 °C or 30-75 °C. In one embodiment, the acid treatment step c i) is carried out during 20 minutes-24 hours, such as 20-120 minutes. The wording that the step is carried out means that the pH is below the specified limit.

[0040] Especially strong effect of the combination of the treatments is seen involving the acid treatment (see table 2) . This is an advantageous effect of the invention, since the chelating agents frequently used typically have an impact on the environment due to low biodegradability. The mechanism behind the particularly good effect in connection with acidic metal removal is not completely clear but the inventors believe that the presumed increased permeability of the cellulose and the presumed increased accessibility of metal ions caused by the alkaline an reducing conditions will be particularly advantageous when combined with acidic metal removal. Thus in one embodiment the step c) of treating the material is carried out by decreasing the pH to 5.5 or below. This corresponds to that only step c) i) is carried out.

[0041] Alternatively a chelating agent can be used alone or in combination with an acid. In one embodiment, of the chelating treatment step c ii) , the chelating agent added is EDTA. In one embodiment, the chelating agent is DTPA. In another embodiment, the chelating agent is a citrate or a gluconate. In one embodiment, the amount of the chelating agent is in the interval 1-5 kg / ton pulp. In one embodiment, the amount of a chelating agent selected from the group consisting of EDTA and DTPA is in the interval 1-5 kg / ton pulp. In one embodiment, the amount of EDTA is in the interval 1-5 kg / ton pulp. In one embodiment, the amount of DTPA is in the interval 1-5 kg / ton pulp.

[0042] In one embodiment, step c ii) is carried out at a temperature in the interval 60-120 °C. In one embodiment, step c ii) is carried out at a temperature in the interval 40-100 °C.

[0043] If the temperature is above the boiling point during any step then it is intended that the step should be carried out under pressure so that the mixture still is liquid.

[0044] In one embodiment, the step c ii) is carried out during 10- 100 min 30 min-120 min. In one embodiment, the step c ii) is carried out during 10 - 100 min.

[0045] In one embodiment, a wash is carried out before step c) . Since step c i) is carried out at low pH and the preceding step is carried out at high pH it is an advantage to wash after step b) , before the pH is lowered in step c i) . A dewatering step is in one embodiment carried out between steps b ) and c ) . In one embodiment , the material is dewatered to a water content of less than 66 wt% before step c ) , after step b ) . In one embodiment , the material is dewatered to a water content of less than 75 wt% before step c ) , after step b) . A water content of 75 wt% means a content of all other substances of 25 wt% . This step also serves to minimi ze the carry-over of liquid with high pH to the step with low pH . Both a washing and / or a dewatering step can be carried out between step b ) and step c ) regardless i f step c i ) and / or step c ii ) is carried out .

[0046] The cellulose is recovered after step c ) . The recovered cellulose can be used for making new textiles or for any other purpose where cellulose is used .

[0047] In a second aspect there is provided recycled cellulose material obtained by a method as described above . The recycled cellulosic material may be used in a process for production of viscose .

[0048] It is conceived that the steps according to the invention is to be carried out together with additional steps and stages in textile recycling and regeneration of reclaimed cellulose . Additional stages are known in the art and can easily be combined with the steps according to the invention by a skilled person . A number of additional stages are suitably performed in the regeneration of reclaimed cellulose as described in the prior art . Textiles may be collected for recycling . Buttons , zips and other obj ects of solid metal are In one embodiment , removed before step b ) . A mechanical treatment may be carried out before step b ) , such as for instance shredding and milling . A step of removing non- cellulosic fibers may be performed before step b ) , in particular i f the textile material is a mixture of cellulose fibers and other fibers .

[0049] It should be noted that a number of additional steps may be performed between step b ) and step c ) . Such additional steps may be additional steps related to various aspects of textile recycling and known in the art .

[0050] It is to be understood that this invention is not limited to the particular embodiments shown here . The following examples are provided for illustrative purposes and are not intended to limit the scope of the invention since the scope of the present invention is limited only by the appended claims .

[0051] Examples

[0052] Denim material was mechanically disintegrated by cutting the material in approximately 15 x 15 mm pieces and using a kitchen blender to trans fer it into a pulp material . Subsequently, the material was subj ected to four di f ferent treatment combinations . An alkaline and reducing swelling treatment step (Y) followed by washing and either by a subsequent acid (A) , using sulphuric acid, or a chelating ( Q) treatment step was compared to a reference method with a slightly alkaline swelling treatment step (E ) without reducing conditions followed by washing and either by a subsequent acid (A) or chelating ( Q) treatment step . In both the Y and E stages the pH was adj usted by using sodium hydroxide . The parameters of the process conditions are shown in Table 1 .

[0053] Table 1 . Process conditions for the tests performed

[0054] The consistence is measured as gravimetric percentage of denim (weight) to liquid (weight) .

[0055] The results (Table 2) show that the calcium content and the iron content of the material decreases more with an alkaline and reducing treatment followed by an acid or chelating treatment compared to the reference method.

[0056] Table 2. Metal ions, analyzed according to SCAN-CM 38, in the raw material and treated material in mg / kg

Claims

Claims1. A method for removal of metal ions during recycling of textiles comprising cellulose, said method comprising the sequential steps of: a. providing at least one textile material comprising cellulose, b. treating the material to swell the cellulose, under alkaline and reducing conditions, wherein at least one reducing agent is present at least during a part of the swelling, and c. treating the material according to at least one of the following steps: i. decreasing the pH to 5.5 or below, and ii. adding a chelating agent.

2. The method according to claim 1, wherein the at least one textile material comprises at least one selected from the group consisting of cotton, lyocell, rayon, and viscose.

3. The method according to claim 1, wherein the at least one textile material comprises cotton.

4. The method according to any one of claims 1-3, wherein the at least one textile material is mechanically disintegrated before step b) .

5. The method according to any one of claims 1-4, wherein the pH during step b) is above 8.5.

6. The method according to any one of claims 1-5, wherein the pH during step b) is in the range of 9-13.5.

7. The method according to any one of claims 1-6, wherein NaOH is present during step b) in a concentration from 0.01 to 0.5 mol / 1.

8. The method according to any one of claims 1-7, wherein the temperature during step b) is in the range 50-100°C.

9. The method according to any one of claims 1-8, wherein the at least one reducing agent comprises sodium dithionite, Na2S2O4.

10. The method according to any one of claims 1-9, wherein step c i) is performed and the pH is decreased in step c i) to below 5.

11. The method according to any one of claims 1-10, wherein step c i) is performed and the pH is decreased by addition of at least one of CO2and H2CO3.

12. The method according to any one of claims 1-11, wherein step c i) is carried out at a temperature in the interval 60-120 °C.

13. The method according to any one of claims 1-12, wherein step c i) is carried out during 20 minutes-24 hours.

14. The method according to any one of claims 1-13, wherein step c ii) is performed and the chelating agent is EDTA.

15. The method according to any one of claims 1-14, wherein step c ii) is carried out at a temperature in the interval 40-100 °C.

16. The method according to any one of claims 1-15, wherein step c ii) is carried out during 20 minutes-24 hours.

17. The method according to any one of claims 1-16, wherein a wash is carried out before step c) .

18. The method according to any one of claims 1-17, wherein the material is dewatered to a water content of less than 66 wt% before step c) .

19. Recycled cellulose material obtained by the method according to any one of claims 1-18.

20. Use of recycled cellulosic material according to claim 19 in a process for production of viscose.

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