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. In 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 and a system 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 and a system according to the independent claims. 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 may comprise hydrogen peroxide bleaching and/or ozone bleaching. Alternatively, or additionally, the first bleaching stage may comprise chlorine dioxide and/or oxygen bleaching.

In other words, the method comprises two bleaching stages with an intermediate washing stage, wherein at least the second bleaching stage comprises ozone bleaching. It 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. It 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. It 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%. In 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 ozoneair 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 otherwise will consume ozone in the second bleaching stage without being useful for brightness increase of the pulp. In 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, and of the optional ozone bleaching of the first bleaching stage) may be less than 12 kg/odt, preferably less than 10 kg/odt, or less than 8 kg/odt. In other words, if the first bleaching stage comprises ozone bleaching, the total ozone charge of the first and second bleaching stages is 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. It is understood that odt is an abbreviation for oven dry ton, where 1 ton = 1000kg, i.e. odt = oven dry metric ton (odmt).

In embodiments of the method according to the first aspect of the invention, the first and/or second bleaching stages comprises ozone bleaching followed by hydrogen peroxide bleaching. Thus, the first and/or 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. In embodiments of the method according to the first aspect of the invention, the first and/or second bleaching stage comprises ozone bleaching followed by chlorine dioxide bleaching. Thus, the first and/or 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. If 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.

It is understood that the two embodiments described above may be combined, for example such that the first bleaching stage is a (ZD)-stage and the second bleaching stage is a (ZP)- stage.

In embodiments of the method according to the first aspect of the invention in which the first bleaching stage comprises ozone bleaching, the ozone bleaching in the first bleaching stage has a shorter retention time than the ozone bleaching in the second bleaching stage. In order for the retention times to be compared in this embodiment, it is understood that the consistency of the pulp fed to the (ozone bleaching of the) first and second bleaching stages must be substantially the same, i.e. the pulp is at high consistency (HC), for example at a consistency (by weight) of at least 30%, preferably within a range from 35-42% at both the first and second bleaching stages, or the pulp is at medium consistency (MC), for example at a consistency (by weight) of 8-16%, preferably 10-12% at both the first and second bleaching stages. Having a shorter retention time in the ozone bleaching of the first bleaching stage may be advantageous since some chemical compounds require rather short retention time while the byproduct require longer retention time. For example, it has been shown that a rather short time is required for ozone to remove reactive black through the, i.e, N=N- chromphoric double bond breaking. Consequently, it may be advantageous to use a shorter retention time in the ozone bleaching of the first bleaching stage to remove dyes such as reactive black which decompose quickly, whereas the remaining dyes which decompose more slowly can, after dissolved material from the first bleaching stage has been removed by means of washing, be efficiently removed in the second bleaching stage (having a longer retention time). In 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 (and/or in the ozone bleaching of the first bleaching stage, if existing). 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 first and/or second bleaching stage is at HC (as defined above).

In embodiments of the method according to the first aspect of the invention, the method further comprises pretreating the pulp prior to the second bleaching stage (such as between the washing stage and the second bleaching stage) and/or, if the first bleaching stage comprises ozone bleaching, prior to the first 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.

In embodiments of the method according to the first aspect of the invention, the first bleaching stage does not comprise ozone bleaching. For example, the first bleaching stage may solely comprise hydrogen peroxide bleaching.

In embodiments of the method according to the first aspect of the invention, the second bleaching stage (and/or the first bleaching stage if it comprises ozone bleaching) 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 (and/or the first 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 (and/or in the first bleaching stage), i.e. without any intermediate washing or other step (a (Zq)-stage). The second bleaching stage (and/or in the first 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 (and/or in the first 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. The embodiments described above may be combined, such that for example the first bleaching stage is a (Zq)-stage while the second bleaching stage is a (ZD) or (ZP) stage.

In embodiments of the method according to the first aspect of the invention, the second bleaching stage (and/or the first bleaching stage if it comprises ozone bleaching) comprises alkaline extraction after the ozone bleaching. Thus, the second bleaching stage (and/or the first 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.

According to a third aspect of the invention, there is provided a system for bleaching pulp formed from recycled textile material comprising cellulose. The system comprises a first bleaching arrangement arranged to bleach the pulp, a washing arrangement connected to the first bleaching arrangement and arranged to wash bleached pulp received from the first bleaching arrangement and a second bleaching arrangement connected to the washing arrangement and arranged to bleach washed pulp received from the washing arrangement. The first bleaching arrangement may comprise at least one of an ozone bleaching apparatus, a chlorine dioxide bleaching apparatus, a peroxide bleaching apparatus and an oxygen bleaching apparatus. In other words, the first bleaching arrangement may comprise for example an ozone bleaching apparatus and a hydrogen peroxide bleaching apparatus, or solely a hydrogen peroxide bleaching apparatus, or an ozone bleaching apparatus and a chlorine dioxide bleaching apparatus. The second bleaching arrangement comprises an ozone bleaching apparatus. It is understood that the second bleaching arrangement is physically separate from the first bleaching arrangement, i.e. the ozone bleaching apparatus and optional additional apparatuses of the second bleaching arrangement is/are separate from the bleaching apparatus(es) and optional additional apparatuses of the first bleaching arrangement.

In embodiments of the system, the system comprises a pulping arrangement arranged to receive the recycled textile material to produce pulp therefrom, wherein the first bleaching arrangement is arranged to receive pulp from the pulping arrangement. Such an embodiment may be referred to as a system for producing bleached cellulose pulp from recycled textile material comprising cellulose.

In embodiments of the system, the first bleaching arrangement comprises an ozone bleaching apparatus and a hydrogen peroxide bleaching apparatus arranged to receive bleached pulp directly from the ozone bleaching apparatus in the sense that there are no intermediate stages such as diluting or washing.

In embodiments of the system, the second bleaching arrangement further comprises a hydrogen peroxide bleaching apparatus, wherein the hydrogen peroxide bleaching apparatus is arranged to receive bleached pulp directly from the ozone bleaching apparatus in the sense that there are no intermediate stages such as diluting or washing.

In embodiments of the system, the system further comprises at least one pretreating arrangement arranged upstream of the first bleaching arrangement and/or upstream of the second bleaching arrangement (for example between the washing arrangement and the second bleaching arrangement), the pretreating arrangement being configured to add at least one acid to reduce the pH of the pulp and/or one or more additives, said one or more additives comprising at least one chelating agent and/or at least one radical scavenger.

In embodiments of the system, the ozone bleaching apparatus of the second bleaching arrangement (and/or of the first bleaching arrangement if it comprises an ozone bleaching apparatus) comprises means for supplying fresh ozone gas and counter currently charging the ozone gas in said ozone bleaching apparatus.

In embodiments of the system, the first and/or second bleaching arrangement comprises means for adding one or more additives. The additives may comprise at least one chelating agent and/or at least one radical scavenger. The means for adding may be arranged downstream of the ozone bleaching apparatus, if present. For example, the means for adding may comprise a dilution screw directly after the ozone bleaching step in which dilution liquid and the one or more additives are added. Alternatively, the means for adding may be arranged upstream of the ozone bleaching apparatus.

In embodiments of the system, the first and/or second bleaching arrangements comprises an alkaline extraction apparatus downstream of the ozone bleaching apparatus.

In embodiments of the system, the first and/or second bleaching arrangements comprises a chlorine dioxide bleaching apparatus. The chlorine dioxide bleaching apparatus may be arranged downstream of the respective ozone bleaching apparatus (if present). The hydrogen chlorine dioxide apparatus may be arranged directly after the ozone bleaching apparatus in the sense that there are no intermediate stages such as diluting or washing.

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 and third aspects of the invention and vice versa.

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 of the 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 hydrogen peroxide bleaching. Thus, the first bleaching stage is a (ZP)-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 hydrogen peroxide bleaching. Thus, the second bleaching stage is also a (ZP)-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. In 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 (ZD)-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). In 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 hydrogen peroxide bleaching, and in that the second bleaching stage 18 solely comprises ozone bleaching. Thus, the first bleaching stage is a P-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.

Other embodiments correspond to the embodiment in fig. 2 except in that the second bleaching stage 18 is a (ZP)-stage and/or in that the pretreating stage 16 is omitted.

In 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 seconds - 5 minutes, such as 30-60 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 stages 4/14 8/18 comprising ozone bleaching apparatuses and hydrogen peroxide bleaching apparatuses 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 hydrogen peroxide bleaching in stages 4 and/or 8) may be omitted.