Abstract
The invention relates to a method for bleaching pulp formed from recycled textile material comprising cellulose, said method comprising ozone bleaching (3; 13a, 13b) of said pulp by means of adding ozone at a charge of at least 12 kg/odt, wherein said ozone bleaching (13a, 13b) is performed as two subsequent steps, each comprising adding ozone at a charge such that the overall charge is at least 12 kg/odt.
Claims
1. A method for bleaching pulp formed from recycled textile material comprising cellulose, said method comprising: ozone bleaching said pulp, which comprises two steps of adding ozone, the two steps being performed at a charge such that the overall charge is at least 12 kg/odt.
2. The method according to claim 1, wherein: the two steps of adding ozone are performed at a charge such that an overall charge is at least 15 kg/odt.
3. The method according to claim 1, further comprising: pretreating the pulp prior to said ozone bleaching, wherein said pretreating comprises: adding at least one acid to reduce the pH of the pulp, and adding one or more additives.
4. The method according to claim 3, wherein: said one or more additives comprises one or more chelating agents.
5. The method according to claim 3, wherein: said one or more additives comprises at least one of oxalic acid, acetic acid, ethylenediaminetetraacetic acid, EDTA, diethylenetriaminepentaacetic acid, DTPA, magnesium sulfate or calcium chloride.
6. The method according to claim 1, further comprising: after said ozone bleaching, peroxide bleaching the pulp.
7. The method according to claim 6, further comprising: after said peroxide bleaching, chlorine dioxide bleaching the pulp.
8. The method according to claim 7, further comprising: prior to or during said peroxide bleaching, adding a radical scavenger.
9. The method according to claim 1, wherein: prior to the ozone bleaching, the pulp is provided at a consistency of at least 30%.
10. The method according to claim 3, wherein: said one or more additives comprises one or more radical scavengers.
11. The method according to claim 8, wherein: the radical scavenger is magnesium sulfate.
12. The method according to claim 1, wherein: prior to the ozone bleaching, the pulp is provided at a consistency within a range from 35-42%.
13. The method according to claim 1, wherein: the two steps of adding ozone are consecutive.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] 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:
[0017] FIG. 1 shows a flowchart illustrating an embodiment of the method according to the invention, and
[0018] FIG. 2 schematically illustrates another embodiment of a method according to the invention.
DETAILED DESCRIPTION
[0019] FIG. 1 shows a flowchart illustrating an embodiment of the method according to the first aspect of the invention. The method comprises providing 1 pulp at a consistency, which in embodiments of the invention is at least 30%, preferably within a range from 35-42%. Thereafter, the pulp is pretreated 2, which comprises the sub-steps of 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 EDTA. The pretreated pulp is thereafter subjected to ozone bleaching 3 by means of adding ozone in two subsequent steps at an overall charge of at least 12 kg/odt. In an embodiment of the invention, the overall charge is at least 15 kg/odt. The ozone bleached pulp is thereafter subjected to an optional hydrogen peroxide bleaching step 5. Concurrently with, or immediately before the peroxide bleaching step, adding 4 a radical scavenger such as magnesium sulfate. After the peroxide bleaching, the pulp is optionally subjected to chlorine dioxide bleaching 6. Although not shown in the flowchart in FIG. 1, it is understood that the method further comprises at least one washing step; washing steps may be comprised in the providing step 1, between the ozone bleaching 3 and peroxide bleaching 5 and/or between the peroxide bleaching 5 and chlorine dioxide bleaching 6. Suitable devices for carrying out the above-mentioned steps are well known in the art and will not be described herein.
[0020] FIG. 2 schematically illustrates another embodiment of a method according to the first aspect of the invention. The method comprises pulping 10 the recycled textile material to form cellulose pulp, for instance by swelling the material under reducing conditions as described in WO2018/073177A1. The pulping 10 may comprise washing the formed pulp prior to an optional oxygen bleaching stage 11 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 12 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 sulfate. The pretreatment may comprise diluting the pulp to low consistency prior to adding acid and additives. The additives may be added at charge of 1-2 kg/odt. The pulp is preferably dewatered/thickened prior to the first ozone bleaching 13a such that the pulp is provided thereto at a consistency of at least 30%, preferably within a range from 35-42%. After the first ozone bleaching stage, the pulp is subjected to a second ozone bleaching stage 13b. 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 step may be performed for a duration of 1-10 minutes at a temperature of 40-70° C. The overall ozone charge of the two ozone bleaching stages combined amounts to at least 12 kg/odt, or 15 kg/odt. The ozone bleached pulp is thereafter sent to a hydrogen peroxide bleaching stage 15, optionally via a washing stage. A radical scavenger such as magnesium sulfate is added in the peroxide bleaching stage 15. After the peroxide bleaching, the pulp is subjected to a chlorine dioxide bleaching stage 16. Optionally, the pulp is subjected to a washing stage between the peroxide and chlorine dioxide bleaching stages.
[0021] 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 step(s) prior to or after the ozone bleaching step(s) may be omitted, for example the chlorine dioxide bleaching step (along with the related addition of a radical scavenger step) may be omitted, or the peroxide bleaching step may be omitted. Furthermore, the ozone bleaching may be performed using fewer or further sub steps.
