CA2036173C

CA2036173C - Process for the chlorine-free bleaching and delignification of cellulose

Info

Publication number: CA2036173C
Application number: CA002036173A
Authority: CA
Inventors: Werner Sturm
Original assignee: SKW Trostberg AG
Current assignee: Evonik Operations GmbH
Priority date: 1990-02-13
Filing date: 1991-02-12
Publication date: 1996-01-23
Anticipated expiration: 2011-02-12
Also published as: NO910458D0; NO176329C; CA2036173A1; ES2051531T3; DE4004364A1; EP0442425A1; DE59101249D1; DE4004364C2; EP0442425B1; NO910458L; FI910703A; FI910703A0; ATE103649T1; NO176329B

Abstract

The present invention provides a process for the chlorine-free bleaching and delignification of cellulose pre-bleached with oxygen and peroxide or oxygen and or ozone, wherein the pre-bleached cellulose is treated twice with peroxide activated by cyanamide and/or a cyanamide salt.

Description

_ 2 The present invention is concerned with a process for the chlorine-free bleaching and delignification of cellulose which has already been pre-bleached with oxygen and peroxide or oxygen and or ozone.
For the production of white-stabilised and/or high strength paper products, substantially lignin-free cellulose is needed. For this purpose, cellulose which is obtained from vegetable raw materials, for example wood, cane, straw, bagasse and the like, by a chemical digestion (sulphate or sulphite digestion) is subjected to a delignifying bleaching.
In the case of this usually multi-step bleaching process, there were previously essentially employed elementary chlorine (C) and hypochlorite (H) as treating agents, in which case, for example, for sulphite cellulose the following bleaching sequence was conventional:
(C) - (E) - (H) - (H) in which (C) = chlorine bleaching, (E) = extraction of the chlorolignins with an aqueous solution of sodium hydroxide and (H) = hypochlorite treatment.
For the reduction of the environmentally stressing use of elementary chlorine, the extraction step was increasingly carried out with peroxide, in which case a distinct reduction of the use of chlorine could be achieved.
In order to be able completely to avoid the use of elementary chlorine, in recent years the chlorine step has been replaced by an oxygen step (EO) or by an extraction step (EOP) `- 2036173 assisted with oxygen and peroxide. However, with the help of these bleaching steps, only semi-bleached celluloses are obtainable which have a degree of whiteness of 75 to 80~.
Three quarter-bleached celluloses with a degree of whiteness of 80 to 87~ are admittedly possible without the use of elementary chlorine but, instead thereof, it is necessary to make use of a treatment step with hypochlorite (H) or chlorine dioxide (D), in which case the following sequences are then usual:
EOP - D - EP
EOP - H - EP
EOP = extraction with oxygen/peroxide EP = extraction with peroxide D = treatment with chlorine dioxide H = treatment with hypochlorite.
The same also applies in the case of fully bleached celluloses with degrees of whiteness of 88 to 90 ~ which are obtainable, for example, with the following bleaching sequence:
EOP - D - EP - D - EP
However, as before, not only the use of hypochlorite but also of chlorine dioxide represents a loading of the waste water and of the exhaust air.
Therefore, it is an object of the present disclosure to develop a process for the chlorine-free bleaching and delignification of cellulose which has been pre-bleached with oxygen and peroxide or oxygen and or ozone, which process does not suffer from the disadvantages of the prior art but rather, without the help of chlorine-containing treatment agents and i ~r .~-_ 4 with a small technical expense, makes possible the production of celluloses with a high degree of whiteness.
Thus, according to the present invention, there is provided a process for the chlorine-free bleaching and delignification of cellulose which has been pre-bleached with oxygen and peroxide or oxygen and or ozone, wherein the pre-bleached cellulose is treated twice with peroxide activated by cyanamide and/or a cyanamide salt.
The activation of peroxide by means of cyanamide derivatives is admittedly known, for example, from published Federal Republic of Germany Patent Application No. 35 44 398 but, according to this process, essentially unbleached cellulose is used as starting material.
Furthermore, from our own investigations, it is known that the action of these activators decreases very quickly in the case of celluloses pre-bleached with chlorine or hypochlorite, as well as in the case of relatively high temperatures and long reaction times. Therefore, it was completely surprising that, in the case of cellulose pre-bleached with oxygen and peroxide, the action of the cyanamide derivatives was also so good in the case of comparatively high temperatures that, in this way, a very good degree of whiteness could be achieved.
In this case of the new process here described, as starting material there is used pre-bleached cellulose which has been pre-treated by a conventional extraction step with oxygen and peroxide or oxygen and or ozone and which, according to the nature of the cellulose, displays greatly differing degrees of whiteness. Whereas, for example, pinewood cellulose .
.,p .~

_ 5 pre-bleached in this way has a degree of whiteness of 65 to 75~, the corresponding degree of whiteness for beechwood cellulose is already, in some cases, above 80~. The kappa values for the degree of delignification are normally from 2 to 10 and especially from 3 to 8.
The two treatment steps include the bleaching and delignification of cellulose with peroxide and cyanamide or cyanamide salts, which are preferably used in the form of alkaline-reacting salts, for example sodium hydrogen cyanamide, calcium cyanamide or magnesium cyanamide. The amounts of peroxide or of cyanamide derivatives differ only slightly in the two treatment steps. As peroxide, there can be used all water-soluble, bleach-active peroxy compounds, the use of hydrogen peroxide or sodium peroxide being preferred.
Whereas in the first step, the cyanamide content is preferably 0.05 to 1.0~ by weight and especially 0.07 to 0.33 by weight, in the second step it is preferable to work with a cyanamide content of 0.01 to 0.7~ by weight and especially of 0.03 to 0.23~ by weight, in each case referred to the dry weight of the cellulose.
During the first treatment step, the peroxide content can be varied from 0.1 to 3.0 by weight and during the second treatment step from 0.1 to 2.0~ by weight, referred to the dry weight of the cellulose.
The other process conditions for the two treatment steps are substantially identical. The pH value is, as is usual in the case of peroxide bleachings, in each case adjusted alkaline and, as a rule, is from 9 to 12. The adjustment of the pH value ' ~

.~ 6 takes place with known alkaline-reacting substances, in which case sodium hydroxide is preferred for reasons of better solubility. For this purpose, 0.3 to 1.5% by weight of sodium hydroxide normally suffice in order, in each case, to adjust the desired pH value.
In both treatment steps, the solids densities can be varied within wide limits, in which case solids densities of 5 to 30% by weight, especially those of 7 to 20% by weight are especially preferred.
Depending upon the nature of the cellulose and the desired degree of whiteness, the treatment steps here described are preferably carried out at a temperature of from 30 to 95C. and especially preferably of from 55 to 80C., treatment times of from 1 to 8 hours thereby usually being necessary in order to achieve the desired degree of whiteness. However, by means of the addition of cyanamide and/or of cyanamide salts, it is, in general, possible to shorten the reaction time and/or to lower the temperature in comparison with the peroxide treatment without this activator if, for any reason, this is desired.
Thus, with the help of the new process herein, three-quarter to completely bleached cellulose can be produced in a technically simple manner and without the help of chlorine-containing chemicals, said celluloses having a degree of whiteness of >80%.
The following Examples are given for the purpose of illustrating the present invention:
Examples.

,~

All the bleaching steps were carried out in polyethylene bags.
Determinations Degree of whiteness: from washed cellulose were formed standardised white sheets on a sheet-making apparatus (according to Rapid Kothen) and the degree of whiteness determined in percentages according to the iso-standard 2470 with a measurement apparatus of the type Elrepho 2000*
(Datacolor).
Kappa number: the determination of this parameter, which is a measure for the lignin content in cellulose, takes place according to iso-standard 302.
Bleaching sequences: these are abbreviated as follows:
EOP =extraction assisted with oxygen and hydrogen peroxide EP =extraction assisted with hydrogen peroxide (peroxide bleaching steps).
Exam~le 1 (comparison).
1st treatment step.
Into 175.82 g. of pinewood-magnesium bisulphite cellulose (40 g. otro), after EOP pre-treatment, with 73.6~ whiteness and kappa 4.8, were intensively kneaded 224.18 g. of an aqueous solution which contained 3.60 g. of sodium hydroxide (10~) and 0.69 g. hydrogen peroxide (35~). For the complete reaction, the 10~ cellulose slurry was introduced into a water-bath with a temperature of about 74C. After a * Trademark X

8 2o36l73 bleaching period of 120 minutes, the suspension was diluted with tap water and the pH adjusted to 6.0, followed by filtering off with suction and subsequent washing. The cellulose obtained had a degree of whiteness of 79.4.
2nd treatment step.
The cellulose so pre-treated was subjected to a further peroxide treatment.
amount of material: 140.85 g.
bleaching solution: 259.12 g. with 2.00 g. sodium hydroxide (10~) and 0.34 g. hydrogen peroxide (35~) bleaching temperature: 74C.
bleaching period: 120 minutes degree of whiteness: 81.1 Example 2.
1st treatment step.
Into 175.82 g. pinewood-magnesium bisulphite cellulose (40 g. otro), after EOP pre-treatment, with 73.6~ whiteness and kappa 4.8, were incorporated 88.44 g. of an aqueous solution which contained 0.80 g. of the 10~ cyanamide solution. After 2 to 5 minutes, a further 134.74 g. of an aqueous solution, which contained 3.60 g. sodium hydroxide (10~) and 0.69 g. hydrogen peroxide (35~), were added thereto and also intensively kneaded in. For the complete reaction, the 10~ cellulose suspension was introduced into a water-bath with a temperature of about 74C.
After a bleaching period of 120 minutes, the suspension was diluted with tap water, adjusted to pH 6.0, followed by ~r g filtering off with suction and subsequent washing. The degree of whiteness of the cellulose obtained was 82.2.
2nd treatment step.
The cellulose so pre-treated was subjected to a further peroxide treatment.
amount of material: 140.85 g.
aqueous cyanamide solution: 102.96 g. with 0.40 g.
cyanamide (10~) bleaching solution: 156.19 g. with 2.00 g. sodium hydroxide (10%) and 0.34 g. hydrogen peroxide (35%) bleaching temperatures: 74C.
bleaching period: 120 minutes degree of whiteness: 85.5~.
Example 3 (comparison).
Beechwood-magnesium bisulphite cellulose was, after an EOP
pre-treatment, with a degree of whiteness of 82.6~, further treated analogously to Example 1 under the following conditions:
1st treatment step.
amount of material: 357.14 g. (100 g. otro) bleaching solution: 642.86 g. with 5.00 g. sodium hydroxide (10~)and 1.00 g. hydrogen peroxide (35%) bleaching temperature: 55C.
bleaching period: 120 minutes degree of whiteness: 86.0%.
2nd treatment ste~.
amount of material: 178.59 g. (50 g. otro) 2036173 leaching solution: 321.43 with 2.50 g. sodium hydroxide (10%)and 0.50 g. hydrogen peroxide (35%) bleaching temperature: 55C.
bleaching period: 120 minutes degree of whiteness: 86.7%.
Example 4.
Beechwood-magnesium bisulphite cellulose corresponding to Example 2 with a degree of whiteness of 82.6~ was post-treated under the following conditions:
1st treatment step.
amount of material: 357.14 g. (100 g. otro) aqueous cyanamide solution: 255.46 g. with 1.20 g.
cyanamide (10%) bleaching solution: 387.39 g. with 5.00 g. sodium hydroxide (10%) and 1.00 g. hydrogen peroxide (35%) bleaching temperature: 55C.
bleaching period: 120 minutes degree of whiteness: 88.3%
2nd treatment step.
amount of material: 178.57 g. (100 g. otro) aqueous cyanamide solution: 127.73 g. with 0.60 g.
cyanamide (10%) bleaching solution: 193.70 g. with 2.50 g. sodium hydroxide (10%) and 0.50 g. hydrogen peroxide (35%) bleaching temperature: 55C.
bleaching period: 120 minutes degree of whiteness: 89.5%.

, .

Claims

1. Process for the chlorine-free bleaching and delignification of cellulose pre-bleached with oxygen and peroxide or oxygen and/or ozone, wherein the pre-bleached cellulose is treated twice with peroxide activated by cyanamide and/or a cyanamide salt.

2. Process according to claim 1, wherein, in the first treatment step, 0.05 to 1.0% by weight of cyanamide and/or cyanamide salt, referred to the dry weight of the cellulose, is added.

3. Process according to claim 2, wherein, in the first treatment step, 0.07 to 0.33% by weight of cyanamide and/or cyanamide salt, referred to the dry weight of the cellulose, is added.

4. Process according to claims 1, 2 or 3, wherein, in the first treatment step, the peroxide content is adjusted to 0.1 to 3.0% by weight, referred to the dry weight of the cellulose.

5. Process according to claims 1, 2 or 3, wherein, in the second treatment step, there are added 0.01 to 0.7% by weight, referred to the dry weight of the cellulose, of cyanamide and/or cyanamide salt.

6. Process according to claim 5, wherein, in the second treatment step, there are added 0.03 to 0.23% by weight, referred to the dry weight of the cellulose, of cyanamide and/or cyanamide salt.

7. Process according to claim 5, wherein, in the second treatment step, the peroxide content is adjusted to 0.1 to 2.0% by weight, referred to the dry weight of the cellulose.

8. Process according to claims 1, 2, 3, 6 or 7, wherein, the pH
value in both treatment steps is adjusted to 9 to 12.

9. Process according to claims 1, 2, 3, 6 or 7, wherein, in both treatment steps, the solids material density is adjusted to 5 to 30%
by weight.

10. Process according to claim 9, wherein, in both treatment steps, the solids material density is adjusted to 7 to 20% by weight.

11. Process according to claims 1, 2, 3, 6, 7 or 10, wherein both treatment steps are carried out at a temperature of 30 to 95°C.

12. Process according to claim 11, wherein both treatment steps are carried out at a temperature of 55 to 80°C.

13. Process according to claims 1, 2, 3, 6, 7, 10 or 12, wherein the period of treatment per step is 1 to 8 hours.

14. Process according to claim 1 for the chlorine-free bleaching and delignification of cellulose, substantially as hereinbefore described and exemplified.

15. Bleached and delignified cellulose, whenever produced by the process according to claims 1, 2, 3, 6, 7, 10, 12 or 14.