CA2052758A1

CA2052758A1 - Process for the low chlorine bleaching and delignification of cellulose

Info

Publication number: CA2052758A1
Application number: CA002052758A
Authority: CA
Inventors: Werner Sturm; Ludwig Eibl
Original assignee: Individual
Current assignee: Evonik Operations GmbH
Priority date: 1990-10-05
Filing date: 1991-10-03
Publication date: 1992-04-06
Also published as: NO913848D0; NO913848L; FI914694A0; DE4031597A1; EP0479319A1; ATE120815T1; ES2070388T3; DE59105105D1; FI914694A; EP0479319B1

Abstract

Abstract Process for the low chlorine bleaching and delignif-ication of cellulose The present invention provides a process for the low chlorine bleaching and delignification of cellulose, wherein a cellulose which has been pre-bleached with an at least two-step bleaching sequence is mixed separately in an extractive end bleach with an aqueous solution of cyanamide and an alkaline solution of hydrogen peroxide. In this way, cellulose can be produced with a high degree of whiteness in a chlorine-free end bleach.

Description

20~27~8 Lhe present invention is concerned with the low chlorine bleaching and delignification of pre-bleached cellulose in several steps.
~ or the production of white-stable and/or high strength paper products, substantially lignin-free cellulose is needed. For this purpose, the celluLose whi.ch is obtained from vegetable raw materials, for example wood, cane, straw, bagasse and the like, by a chemical digestion (sulphate, sulphite or organo-solve digestion) is subaected to a delignifying bLeaching process. In the case of this usually multi-step pleaching process, there were previousl~ essent-ially emplo~ed elementary chLoride (C) and hyp~chlorite (H) as treating agents, in which case, for sulphite ceLluLose, the following bleaching sequence was conventional:
(C) - (E) - (~) - (H) in which ~C) - chlorine bLeac~ng, (E) = extraction of the chLorolignins with an agueous solution of ~sodium hydroxide and (H) = hypochlorite treatment.
For the reduction of the environmentally stressing use of elementary chloride, the extraction step was increasingLg carried out with peroxide, in which case a distinct reduction of the use of chlorine could be achieved.
Further reductions of the use of elementary chlorine were achieved by the use of chlor ne dioxide and/or the previous introduction of an oxygen step (EO), " 20~27~8 : -3-the following prebleach~ng sequences having been developed over the course of time:
C - EP
:EO(P) - C - ~P
C/D - EP
EO(P) - D - EP
EOP - H - EP
in which EOP ~ extraction with the assistance of oxygen/hydrogen peroxide, EP =extraction with hydrogen peroxide and sodium hy ~ xide, C = treatment with chlorine, CjD = treatment with chlorine/chlorine dioxide, D = treatment with chlorine dioxide, E~ = extraction with the assistance of oxygen and H = treatment with ~ hypochlorite~
However, celluloses which have been treated with the abo~e-mentioned bleaching sequences as a rule display a degree of whiteness which is too low for high requirements. Since, however, an end bleaching with chlorine-containing bleaching agents, such as ; ~ hypochlorite and chlorine dioxide, involves a contamination of waste water and exhaust air, as well as possibly organo-chlorine residues in the cellulose, an absolutely chlorine-lree solution o~
the problem is increasingly aimed for. ~he replace-ment of a ~nal bleach with hypochlorite or chlorine dioxide by a further bl~ching step ~Jith hydrogen peroxide does not, in many cases, lead to the desired degree of whiteness.

.

20~27~8 ~ herefore, it is an object of the present invention to provide a process for a chlorine-free end bleaching o~ celluloses pre-treated with chlorine-free and/or chlorine-containing chemicals or elementary chlorine which does not suffer from the mentioned disadvanta~es of the prior art but rather, with low expense and i~
possible without the help of chlorine-containing treating agents, makes possible the production of celluloses with a high de~ree of whiteness.
l'hus, according to the present invention, there is provided a process for the low chlorine bLeaching and delignification of cellulose, wherein a cellulose pre-bleached wiJh an at least two-step bleaching seguence is mixed in an extractive end bleach separately with an aqueous cyanamide solution and an alkaline hydrogen peroxide solution.
Surprisingly, we have found that, in this way, the use of chlorine-containinDchemicals can be marke~ly reduced without having to make any compromise in the achievement of a high de~,ree of whiteness.
~ he activation of peroxide by means of cyanamide derivatives is admittedly known, for example, from published Federal Republic of Germany Patent Specification D~-OS ~5 44 ~98. ~here, however, in contradistinction to the present inven'ion, is only described the trea~m~n~o~unbleached material or of material only pre- trea~ed.

205~7~

In contradistinction thereto, according to the present invention, for the achievement of a higher end de~ree of whiteness, instead of the usual extractive step with an aqueous solution of sodium hydroxide, the chlorine-containing pre-bleach is followed by an extractive step stren~thened by the hydrogen peroxide for the removal of the components disturbing the following activat~d bleaching step.
The ?re-bleached ceLlulose can be a material which has been treated with chlorine-containing or chlorine-free bleaching agents or a sequence of chlorine-containing and chlorine-free bleaching agents, for example oxygen-chLorine-hydrogen peroxide, oxygen/
peroxide-chlorine dioxide-peroxide or the like.
The process according to the present invention comprises an extractive bleaching step with two treatment agents which are added in any desired sequence but separate.
For this extractive bleaching step, a celluLose pre-bleached with an at least two-step bleachinG
sequence, which usually has a de~ree of whiteness of about 70 to 80,oS~ kappa values of 1.5 to 3.0, as well as materialconsistenciesof 5 to 30,CG b~ weight and preferably of 10 to 15~ by weight, is treated with the first treatment agent, namely, an aqueous solution of cyanamide. ~he amount of cyanamide to be used ~`` 20~2~8 depends upon the weight of the cellulose and amounts to 0.01 to L~O~o by weight, preferably 0.1 to 0.5~ by weight, referred to the dry weight of tbe cellulose.
~ he concentration of the cyanamide used in the aqueous solution can vary within wide limits and amounts to 0.1 to 600,~ by weight and preferably to 1 to 107v' by weight.
hs second treatment agent, there is added an alkaline hydrogen peroxide solution to the cellulose.
Suchtreatment agents are already known from the prior art and usually contain, as alkalisation agent, sodium hydroxide, sodium carbonate or magnesium oxideO ~he amount of alkali compound, for example sodium hydroxide, depends upon the celLulose used and preferably amounts to 0.1 to 2.5~' by weight, especially 0.5 to 1.5~' by weight, referred to the dry weight of t~e cellulose.
~ he amount of hydrogen peroxide used is also dependent upon the dry weight of the cellulose and is usually from 0.1 to 3.0ch by weight and especially 0.5 to 1~5YL by weight.
~ he hydrogen peroxide addition strengthens the action of the alkaline solution of extracting the components which dist~ in the case of the bleaching.
It is ~mportant for the present invention that both treatment agents used according to the present invention are not added simultaneously to the cellulose but rather separately, whereby, as already stated, the seguenc~ of the addition is not decisive~ ~owever, ` 20527~8 in most cases, it has proved to be especially advantageous first to incorporate the agueous cyanamide solution in the cellulose and subseguently thereto the alkaline hydrogen peroxide solution. ~he space of time between the separate mixing in of cyanamide and alkaline hydrogen peroxide solution can be relatively short, i.e. 10 to 20 seconds.
~urthermore, a simultaneous but spatially separated addition of the treatment agents can take place when an intensively-acting mixing aggregate is present at the point of addition.
~ he reaction temperatures for the extractive bleaching step can also be varied within wide limits.
~or reasons of economy, the bleaching is carried out at 20 to 90C and preferably at 40 to 80C.
As a rule, there hereby result bleaching times of 15 to 420 minutes and preferably of 60 to 240 minutes.
After the usual working up of the cellulose, such as dilution withprocess water, adjustment of the pH
value to the neutral range and re -washing, there is obtained a ceLlulose with a degree of whiteness of about 85 to 89,~
~ he bleaching process according to the present invention is, in general, carried out at a pH range of 8 to 13 a~d preferably of L0 to 12.
n the basis of this high degre~ of whiteness and of the low lignin content of the cellulose produced, the process according to the present invention is -`` 20~27~8 especially well suited technically since, at the same time, it manages with very little chlorine-containing treatment agents.
~ he following Examples ~re given for the purpose of ilLustrating the present invention.
Examples.
All bleaching steps were carried out in polg-ethylene bags.
Determinations:
Degree of whiteness:
~ rom the washed cellulose were formed standard white sheets on a sheet former (according to Rapid Kothen) and, with a measurement apparatus of the type Elrepho 2000 (Datacolor), the degree of whiteness was determined in percent according to iso-norm 2470, Kappa number:
~ he determination of this paramet~,which is a measure for the lignin content in the cellulose, took place according to iso-norm 302.
xample 1 (comparison).
lnto 67.11 g of spruce magnesium bisulphite cellulose (20 g otro), after a C-EP pre-treatment, with 77.o% degree of whiteness and kappa 2.3, were intensively kneaded 132.89 g of an aqueous solution which contained 3.0 g of 1070 sodium hydroxide solution (iOe. 0.3 g of sodium hydroxide) and 0.67 g of 300,~' hydrogen peroxide solution (i.e. 0,20 g of hydrogen peroxide). ~or reacting out, the 10,~ cellulose slurry `" 20~27~
g was placed in a water-bath with a temperature of about 50C. After a bleaching period of 240 minutes, the suspension was diluted with process water, the pH value was adjusted to 6.0, followed by filtering off uith suction and re-washing. ~here was obtained a degree of whiteness of 81.9 and a kappa value of 1.6.
Example 2.
Into 67.11 g of spruce magnesium bisulphite cellulose (20 g otro), after a C-~P pre-treatment, ~Jith 77.0% degree of whiteness and kappa 2,3, were incorporated 51.89 g of an aqueous solution which contained 0.34 g of a 10/o cyanamide solution (i.e.
0,034 g of cyanamide), After 2 to 5 minutes, there were added 80.99 g of an aqueous solution which cDntained 3.0 g. of 10Vh sodium hydroxide solution and 0.67 g of 30~J hydrogen peroxide soLution and again intensively mixed in. ~he bleaching conditions and working up corresponded to 3xample 1. ~here was obtained a degree of whiteness of 85~7~0 and a kappa vaLue of 1.3.
Example 3 ~
Into 100 g. of spruce magnesium bisulphite cellulose (20 g otro), after an ~OP-D-~P treatment sequence, with a degree of whiteness of 80.2% and kappa 1.8, were intensively incorporated 100 g of an aqueous solution ~hich contained 1 g LO,' sodium hydroxide solution and 0~33 g of 30,o~ hydrogen peroxide solu~ion. For reacting out, the 10c- material suspension 20~27~
-LG-was placed in a water-bath with a temperature of about 70C. After a bleaching period of 120 minutes, the fibre suspension was diluted with process water, adjusted to a p~ value of 6.0, filtered off ~Jith suction and re -washed. The degree of wbiteness was 86.0 Exam~le 4.
Into 100 g of spruce magnesium bisulphite cellulose (20 g otro), after an EOP-D-~P treatment sequence, with a degree of whiteness of 80.20,~ and kappa 1,8, were intensively incorporated 39.57 g of an agueous solution which contained 0.18 g of 10 cyanamide solution in the course of 2 to 5 minutes.
Subsequently, there were added thereto 60.43 g of an aqueous solution which contained 1 g of 10%
sodium hydroxide solution and 0.33 g of a 300~ hydrogen peroxide solution and knea~ed in. ~he bleaching conditions and the working up corresponded to Example 3. ~he degree of whiteness was 89.3%.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-l. A process for the low chlorine bleaching and delignification of cellulose, wherein a cellulose which has been pre-bleached at least with a two-step bleaching sequence is mixed separately in an extractive end bleach with an aqueous solution of cyanamide, and an alkaline solution of hydrogen peroxide.
2. A process according to claim 1, wherein said cyanamide is present in an amount of 0.01 to 1% by weight, referred to the dry weight of the cellulose.
3. A process according to claim l or 2, wherein the amount of cyanamide is 0.1 to 0.5% by weight, referred to the dry weight of the cellulose.
4. A process according to claim l, wherein the concentration of the cyanamide in the aqueous solution is adjusted to 0.1 to 60% by weight.
5. A process according to claim 1, 2 or 4, wherein the concentration of the cyanamide in the aqueous solution is adjusted to l to 10% by weight.
6. A process according to claim l, wherein the hydrogen peroxide solution is adjusted alkaline with sodium hydroxide.
7. A process according to claim 6, wherein said sodium hydroxide is used in an amount of 0.1 to 2.5% by weight, referred to the dry weight of the cellulose.
8. A process according to claim 7, wherein said sodium hydroxide is used in the amount of 0.5 to 1.5%
by weight, referred to the dry weight of the cellulose.
9. A process according to claim 1, wherein the amount of hydrogen peroxide amounts to 0.1 to 3.0% by weight, referred to the dry weight of the cellulose.
10. A process according to claim 1, 2, 4, 6, 7, 8 or 9, wherein said hydrogen peroxide is present in an amount of 0.5 to 1.5% by weight, referred to the dry weight of the cellulose.
11. A process according to claim 1, 2, 4, 6, 7, 8 or 9, wherein said aqueous cyanamide solution is added first to the cellulose followed by the alkaline hydrogen peroxide solution.
12. A process according to claim 1, wherein the extractive end bleach step is carried out at a temperature of 20 to 90°C.
13. A process according to claim 1, 2, 4, 6, 7, 8, 9 or 12, wherein the extractive end bleach step is carried out at a temperature of 40 to 80°C.
14. A process according to claim 1, wherein the reaction is carried out at a pH value of 8 to 13.
15. A process according to claim 1, 2, 4, 6, 7, 8, 9, 12 or 14, wherein the reaction is carried out at a pH value of 10 to 12.
16. Cellulose, whenever bleached with low chlorine and delignified by the process according to any of claims 1, 2, 4, 6, 7, 8, 9, 12 or 14.