SE545758C2

SE545758C2 - Method for bleaching pulp from recycled textile material

Info

Publication number: SE545758C2
Application number: SE2250746A
Authority: SE
Inventors: Maria Wennerström; Sofia Bylund
Original assignee: Valmet Oy
Priority date: 2022-06-20
Filing date: 2022-06-20
Publication date: 2024-01-02
Also published as: SE2250746A1; WO2023249527A1

Abstract

A method for bleaching pulp formed from recycled textile material comprising cellulose. The method comprises subjecting the pulp to a first bleaching stage (4; 14), washing (5; 15) the bleached pulp from the first bleaching stage, and subjecting the washed pulp from the washing to a second bleaching stage (8; 18), wherein the first bleaching stage comprises ozone bleaching and/or chlorine dioxide bleaching, and wherein said second bleaching stage comprises ozone bleaching. A corresponding system is also disclosed.

Description

TECHNICAL FIELD The invention relates to a method for bleaching pulp formed from recycled textile material comprising cellulose, which method comprises ozone bleaching. The invention also relates to a corresponding system.

BACKGROUND Over the recent years, recycling of textile material has gained interest. The company Renewcell has developed a process for recycling cellulose from textile material comprising cellulose, see for instance WO2018/073177A1. Recycled cellulose can be used for different purposes, for example for producing viscose. ln the above-mentioned publication, the need for de-colorization/bleaching of the recycled textile material is identified, and it is suggested to use a combination of oxygen and ozone bleaching.

Ozone bleaching of cellulose pulp from wood material is well known in the art, where ozone reacts with and de-composes lignin originating from the wood material. When it comes to bleaching of pulp from textile material, the purpose of the ozone bleaching is somewhat different - mainly to bleach or decompose color pigments contained in the pulp, which pulp mainly consists of cellulose and liquid (water). Despite the use of ozone, it has however proven challenging to achieve sufficient brightness of the resulting pulp. Further, the oxidative properties of ozone will also attack the cellulose fibers and may reduce the viscosity of the resulting pulp to an unsatisfactorily low level.

SE 544236C2 proposes ozone bleaching of pulp formed from recycled textile material by means of adding ozone at a significantly higher charges (at least 12 kg/odt) than what is normally used.

SUMMARY An object of the invention is to provide a method for bleaching pulp formed from recycled textile material with further improved performance in terms of brightness and/or ozone consumption and/or viscosity.

These and other objects are achieved by the present invention by means of a method according to the independent claim.

According to a first aspect of the invention, there is provided a method for bleaching pulp formed from recycled textile material comprising cellulose. The method comprises subjecting the pulp to a first bleaching stage to obtain bleached pulp, washing the bleached pulp from the first bleaching stage to obtain washed and bleached pulp, and subjecting the washed and bleached pulp from the washing to a second bleaching stage, wherein said second bleaching stage comprises ozone bleaching. The first bleaching stage comprises chlorine dioxide bleaching. The first bleaching stage may additionally comprise hydrogen peroxide bleaching and/or oxygen bleaching. ln other words, the method comprises two bleaching stages with an intermediate washing stage, wherein the second bleaching stage comprises ozone bleaching. lt is understood that the method is a method for bleaching pulp (previously) formed from recycled textile material comprising cellulose, i.e. the forming of the pulp (the pulping stage) is not necessarily part of the method according to the first aspect of the invention. Put differently, the method comprises providing pulp formed from recycled textile material comprising cellulose, which pulp is subjected to two bleaching stages with an intermediate washing stage. lt is furthermore understood that although the second bleaching stage is described as comprising ozone bleaching, it may comprise additional steps, as will be explained in the embodiments below. lt is furthermore understood that the method may comprise further stages prior or after the first bleaching stage and/or prior or after the second bleaching stage.

The pulp may be provided to the first bleaching stage at high consistency (HC), for example at a consistency (by weight) of at least 30%, preferably within a range from 35-42%. Alternatively, the pulp may be provided to the first bleaching stage at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12%. Furthermore, the pulp may be provided to the second bleaching stage also at high consistency (HC), for example at a consistency (by weight) of at least 30%, preferably within a range from 35-42%. Such high consistency to the second bleaching stage may be achieved by providing pulp at high consistency to the first bleaching stage followed by the washing stage and an optional dewatering stage before the second bleaching stage. Alternatively, the pulp may be provided to the second bleaching stage at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12%. Such medium consistency to the second bleaching stage may be achieved by providing pulp at high consistency to the first bleaching stage followed by the washing and an optional diluting stage (or pretreating stage which comprises dilution) before the second bleaching stage. Alternatively, the pulp may be provided to the first bleaching stage at medium consistency (MC), for example at a consistency (by weight) of 8- 16%, preferably 10-12%. ln this case, the pulp may be provided to the second bleaching stage also at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12%. Alternatively, the pulp may be provided to the second bleaching stage at high consistency (HC), for example at a consistency (by weight) of at least 30%, preferably within a range from 35-42%. Such high consistency to the second bleaching stage may be achieved by providing pulp at medium consistency to the first bleaching stage followed by the washing and a dewatering stage before the second bleaching stage.

Ozone may be added as a gas mixture, for example with 8-14 % ozone by weight in an ozone- air mixture. The ozone bleaching of a HC pulp may be performed for a duration of 10 seconds - 5 minutes, such as 30-60 seconds, for example at a temperature of 40-70°C.

The invention is based on the insight that by providing an intermediate washing stage between the two washing stages, dissolved material resulting from the first bleaching stage will be removed which othenNise will consume ozone in the second bleaching stage without being useful for brightness increase of the pulp. ln other words, a larger portion of the ozone charge provided to the second bleaching stage contributes to bleaching of the pulp. Thus, for a given desired brightness of the pulp, ozone consumption may be reduced compared with using an ozone bleaching stage without a preceding washing stage. The total ozone charge of the second bleaching stage may be less than 12 kg/odt, preferably less than 10 kg/odt, or less than 8 kg/odt. Thus, a lower overall ozone charge is possible compared to SE544236C2. lt is understood that odt is an abbreviation for oven dry ton, where 1 ton = 1000kg, i.e. odt = oven dry metric ton (odmt). ln embodiments of the method according to the first aspect of the invention, the second bleaching stage comprises ozone bleaching followed by chlorine dioxide bleaching. Thus, the second bleaching stage may be a ZD-stage. The chlorine dioxide bleaching may follow directly after the ozone bleaching, i.e. without intermediate washing, and may thus be referred to as a (ZD) stage. This may be advantageous since a combination of chlorine dioxide and ozone results in a synergistic effect regarding brightness level. Therefore, an addition of chlorine dioxide directly after the ozone stage without intermediate washing will reinforce the bleaching effect. Furthermore, advantageous effects in terms of viscosity control may be achieved since ozone impacts viscosity while chlorine dioxide does not. The chlorine dioxide charge may, if only one of the first and second bleaching stages comprises chlorine dioxide bleaching, correspond to 30 kg active chlorine /odt or less. lf both the first and the second bleaching stages comprise chlorine dioxide bleaching, the total chlorine dioxide charge may correspond to 40 kg active chlorine /odt or less. ln embodiments of the method according to the first aspect of the invention, the second bleaching stage comprises ozone bleaching followed by hydrogen peroxide bleaching. Thus, the second bleaching stage may be a ZP-stage. The hydrogen peroxide bleaching may follow directly after the ozone bleaching, i.e. without intermediate washing, and may thus be referred to as a (ZP) stage. Hydrogen peroxide is a secondary reactions product in ozone bleaching, therefore an addition of hydrogen peroxide directly after the ozone stage without intermediate washing will reinforce the ozone bleaching stage. The total hydrogen peroxide charge of the first and/or second bleaching stages may be less than 40 kg hydrogen peroxide /odt. ln embodiments of the method according to the first aspect of the invention, fresh ozone gas is supplied to, and counter currently charged in, the ozone bleaching second bleaching stage. This may provide advantageous bleaching results, in particular when the pulp subjected to bleaching has a high consistency (typically 30% or more). Such counter currently charging is most advantageous when the pulp fed to the ozone bleaching of the second bleaching stage is at HC (as defined above). ln embodiments of the method according to thefirst aspect of the invention, the method further comprises pretreating the pulp prior to said first bleaching stage and/or prior to the second bleaching stage (between the washing stage and the second bleaching stage), wherein said pretreating comprises adding at least one acid to reduce the pH of the pulp and/or 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. The one or more additives may be chosen from a group comprising oxalic acid, acetic acid, ethylenediaminetetraacetic acid, EDTA, diethylenetriaminepentaacetic acid, DTPA, magnesium sulfate or calcium chloride. The additives may be added at charge of 1-20 kg/odt, or more specifically if the additive is EDTA or DTPA at charge of 1-2 kg/odt. ln embodiments of the method according to the first aspect of the invention, the second bleaching stage comprises adding one or more additives, the additives comprising at least one chelating agent and/or at least one radical scavenger. Thus, the second bleaching stage may be referred to as a Zq-stage. The one or more additives may be added directly after the ozone bleaching in the second bleaching stage, i.e. without any intermediate washing or other step (a (Zq)-stage). The second bleaching stage may comprise a dilution step (using a dilution screw for example) directly after the ozone bleaching step in which dilution liquid and the at least one chelating agent and/or at least one radical scavenger is added. The one or more additives may alternatively be added before the ozone bleaching of the second bleaching stage. Chelating agents, which may also be referred to as complexing agents, provide the effect 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 effect by scavenging the metal ions. The one or more additives may be chosen from a group comprising oxalic acid, acetic acid, ethylenediaminetetraacetic acid, EDTA, diethylenetriaminepentaacetic acid, DTPA, magnesium sulfate or calcium chloride. The additives may be added at charge of 1-20 kg/odt, or more specifically if the additive is EDTA or DTPA at charge of 1-2 kg/odt. ln embodiments of the method according to the first aspect of the invention, the second bleaching stage comprises alkaline extraction after the ozone bleaching. Thus, the second bleaching stage may be a Ze-stage.

According to a second aspect of the invention, there is provided a method for producing bleached cellulose pulp from recycled textile material comprising cellulose. The method comprises pulping the recycled textile material to form cellulose pulp and bleaching the cellulose pulp using a method according to the first aspect of the invention or embodiments thereof.

The features of the embodiments described above are combinable in any practically realizable way to form embodiments having combinations of these features. Further, all features and advantages of embodiments described above with reference to the first aspect of the invention may be applied in corresponding embodiments of the second aspect of the invention and vice VGFSQ.

BRIEF DESCRIPTION OF THE DRAWINGS Above discussed and other aspects of the present invention will now be described in more detail using the appended drawings, which show presently preferred embodiments of the invention, wherein: fig. 1 schematically illustrates embodiments of the methods according to the first and second aspects of the invention, and fig. 2 schematically illustrates other embodiments of the methods according to the first and second aspects of the invention.

Fig. 1-2 also illustrate embodiments ofthe system according to the third aspect of the invention.

DETAILED DESCRIPTION Fig. 1 schematically illustrates an embodiment of the method according to the second aspect of the invention. The method comprises pulping 1 the recycled textile material to form cellulose pulp, for instance by swelling the material under reducing conditions as described in WO2018/073177A1. The pulping 1 may comprise washing the formed pulp. Thereafter, the pulp is pretreated 2 by adding sulphuric acid to reduce the pH of the pulp to within a range from 2 to 3. The pretreating also comprises adding one or more additives, the additives comprising at least one of oxalic acid, EDTA and magnesium sulfate. Thus, the pretreating stage may be referred to as an aq-stage. The additive(s) are added together with, or simultaneously with the at least one acid. The pretreatment may comprise diluting the pulp to low consistency prior to adding acid. Depending on the desired consistency, the acidified pulp is subjected to an optional dewatering/thickening stage 3 such that the pulp is provided to the first bleaching stage 4 at high consistency (at least 30%, preferably within a range from 35- 42%). The first bleaching stage 4 comprises ozone bleaching, i.e. bleaching by means of adding ozone to the pulp followed directly by chlorine dioxide bleaching. Thus, the first bleaching stage is a (ZD)-stage. After the first bleaching stage 4, the pulp is washed 5 to remove dissolved material from the first bleaching stage, whereafter it is pretreated 6 once again in a corresponding manner as described above with reference to stage 2. Depending on the desired consistency, the pretreated pulp is subjected to an optional dewatering/thickening stage 7 such that the pulp is provided to the second bleaching stage 8 at a HC consistency of at least 30%, preferably within a range from 35-42%. The second bleaching stage 8 comprises ozone bleaching followed directly by chlorine dioxide bleaching. Thus, the second bleaching stage is also a (ZD)-stage. The retention time of the ozone bleaching in the first bleaching stage is shorter than in the ozone bleaching of the second bleaching stage, however. ln the ozone bleaching of the first and second bleaching stages, fresh ozone gas is supplied to, and counter currently charged.

Other embodiments correspond to the embodiment in fig. 1 except in that the first bleaching stage 4 is replaced with a Z-stage or a (Zq)-stage. Yet other embodiments correspond to the embodiment in fig. 1 except in that the first and/or second bleaching stages are (ZP)-stages.

Yet other embodiments correspond to the embodiment in fig. 1 except in that one or more of the dewatering stages 3, 7 is/are omitted or replaced with diluting stages such that the pulp is supplied to the bleaching stages 4, 8 at medium consistency (MC). ln such embodiments, the ozone gas is not counter currently charged.

Steps 2-8 of fig. 1 also illustrate an embodiment of the method according to the first aspect of the invention.

Fig. 2 schematically illustrates another embodiment of the method according to the second aspect of the invention. The method comprises a pulping/washing stage 11, optional dewatering stages 13/17, washing stage 15 and a pretreating stage 16 corresponding to stages 1, 3/7, 5 and 6, respectively in fig. 1. The embodiment in fig. 2 however differs in that the first bleaching stage 14 solely comprises chlorine dioxide bleaching, and in that the second bleaching stage 18 solely comprises ozone bleaching. Thus, the first bleaching stage is a D- stage and the second bleaching stage is a Z-stage. The embodiment in fig. 2 further differs from fig. 1 in that the no pretreating stage before the first bleaching stage is present.

Steps 13-18 of fig. 2 also illustrate an embodiment of the method according to the first aspect of the invention. ln the above-described embodiments, ozone is added as a gas mixture, for example with 8- 16% ozone by Weight in an ozone-air mixture, preferably 10-12% ozone by weight. The ozone bleaching of each of the first and second bleaching stages may be performed for a duration of 10-60 seconds at a temperature of 40-70°C. The overall ozone charge of the first and second bleaching stages combined amounts to less than 8g/odt, typically 5-6 kg/odt.

Fig. 1-2 could also be said to illustrate embodiments of the system according to the third aspect of the invention, which embodiments comprises a pulping/washing arrangement 1/11, pretreating arrangements 2, 6/16, dewatering/thickening arrangements 3/13, 6/16, ozone bleaching apparatuses 4, 8/18, chlorine dioxide bleaching apparatus 14, and washing apparatuses 5/15. Such arrangements and apparatuses are all well-known in the art and will not be described in further detail here.

The description above and the appended drawings are to be considered as non-limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. For example, depending on the required brightness of the resulting pulp, one or more of the bleaching stage(s) prior to or after the first and second bleaching step(s) may be added, or part(s) of the first and second bleaching stages (for example the chlorine dioxide bleaching in stages 4 and/or 8) may be omitted.

Claims

Claims A method for bleaching pulp formed from recycled textile material comprising cellulose, said method comprising: - subjecting the pulp to a first bleaching stage (4; 14); - washing (5; 15) the pulp from the first bleaching stage, and - subjecting the pulp from the washing to a second bleaching stage (8; 18), wherein said first bleaching stage comprises chlorine dioxide bleaching, and wherein said second bleaching stage comprises ozone bleaching.
Method according to claim 1, wherein in said second bleaching stage (8; 18), said ozone bleaching is followed directly by chlorine dioxide bleaching.
Method according to any of the preceding claims, wherein the ozone charge of the ozone bleaching of the second bleaching stage (8; 18) is less than 8 kg/odt, preferably less than 6 kg/odt.
Method according to claim 2, wherein the total chlorine dioxide charge of the first and second bleaching stages (4, 8; 14, 18) corresponds to 40 kg active chlorine /odt or less.
Method according to any of the preceding claims, wherein the first bleaching stage (14) comprises said chlorine dioxide bleaching with a chlorine dioxide charge corresponding to 30 kg active chlorine /odt or less, and wherein the second bleaching stage (18) does not comprise chlorine dioxide bleaching.
Method according any of the preceding claims, wherein the pulp is provided to the first bleaching stage (4; 14) at a consistency of at least 30%, preferably within a range from 35-42%.
Method according to any of claims 1-5, wherein the pulp is provided to the first bleaching stage (4; 14) at a consistency of 8-16%, preferably 10-12%.
Method according to any of the preceding claims, wherein the pulp is provided to the second bleaching stage (8; 18) at a consistency of at least 30%, preferably within a range from 35-42%.
Method according to any of claims 1-7, wherein the pulp is provided to the second bleaching stage (8; 18) at a consistency of 8-16%, preferably 10-12%.
Method according to claim 8, wherein fresh ozone gas is supplied to, and counter currently charged in, the ozone bleaching of the second bleaching stage (8; 18).
Method according to claim any of the preceding claims, further comprising pretreating (2, 6; 16) the pulp between the washing and the second bleaching stage, wherein said pretreating comprises adding at least one acid to reduce the pH of the pulp and/or adding one or more additives, said one or more additives comprising at least one chelating agent and/or at least one radical scavenger.
12.A method for producing bleached cellulose pulp from recycled textile material comprising cellulose, said method comprising: - pulping (1; 11)the recycled textile material to form cellulose pulp, and - bleaching the cellulose pulp using the method according to any of the preceding claims.