EP0442425B1 - Process for the chlorine-free bleaching and delignification of pulp - Google Patents

Abstract

Cl-free bleaching and delignification of cellulose pre-bleached with O2 and peroxide, or O2 and O3, involves a 2-stage treatment with peroxide activated with cyanamide and/or cyanamide salts. - In the first stage, the content of cyanamide and/or cyanamide salts is 0.05-0.5 (0.07-0.33) wt.%, and the peroxide content is 0.1-1 wt.%, both w.r.t. dry wt. of cellulose. In the second stage, the content of cyanamide is 0.01-0.3 (0.03-0.23) wt.%, and the peroxide content is 0.1-0.7 wt.%, w.r.t. dry wt. of cellulose. Both stages are at pH 10-12, solids content of 5-30 (7-20) wt.%, and 40-90 (55-80) deg.C, for 1-6h for each stage.

Description

The present invention relates to a process for chlorine-free bleaching and delignification of cellulose which has already been pre-bleached with oxygen and peroxide or oxygen and ozone.

Lignin-free cellulose is largely required to produce white-stable and / or high-strength paper products. For this purpose, the pulp, which is made from vegetable raw materials such as wood, reeds, straw, bagasse or similar. is obtained by chemical digestion (sulfate or sulfite digestion), subjected to a delignifying bleach.

(C) = Chlorbleiche, (E) = Extraktion der Chlorlignine mit wäßriger Natronlauge, (H) = Hypochloritbehandlung.In these usually multistage bleaching processes, elemental chlorine (C) and hypochlorite (H) were previously used as treatment agents, the following bleaching sequence being common for sulfite pulp, for example: $$\text{(C) - (E) - (H) - (H)}$$

(C) = chlorine bleach, (E) = extraction of the chlorine lignins with aqueous sodium hydroxide solution, (H) = hypochlorite treatment.

In order to reduce the environmentally harmful use of elemental chlorine, the extraction stage was increasingly carried out with peroxide, with a significant reduction in the use of chlorine being achieved.

$$\text{EOP - H - EP}$$

EOP =

Extraktion mit Sauerstoff/Peroxid

EP =

Extraktion mit Peroxid

D =

Behandlung mit Chlordioxid

H =

Behandlung mit Hypochlorit.

In order to be able to completely dispense with elemental chlorine, the chlorine level has been replaced by an oxygen level (EO) or an extraction level supported by oxygen and peroxide (EOP) in recent years. EP-A-0 325 731 describes a process for chlorine-free bleaching and delignification of kraft pulp, in which the kraft pulp is washed with acid, then treated with oxygen and then with peroxide. With the help of these bleaching stages, however, only semi-bleached pulps are available that have a whiteness of 75 to 80%. Three-quarter bleached pulps with whiteness levels of 80 to 87% are possible without using elemental chlorine, but you have to resort to a treatment with hypochlorite (H) or chlorine dioxide (D) instead, whereby the following sequences are common: $$\text{EOP - D - EP}$$

$$\text{EOP-H-EP}$$

EOP =

Extraction with oxygen / peroxide

EP =

Peroxide extraction

D =

Treatment with chlorine dioxide

H =

Treatment with hypochlorite.

Sowohl der Einsatz von Hypochlorit als auch von Chlordioxid stellen jedoch nach wie vor eine Belastung für das Abwasser und die Abluft dar.The same applies to fully bleached pulps with whitenesses of 88 to 90%, which are accessible, for example, with the following bleaching sequence: $$\text{EOP - D - EP - D - EP}$$

However, both the use of hypochlorite and chlorine dioxide continue to be a burden on wastewater and exhaust air.

The present invention is therefore based on the object of developing a process for chlorine-free bleaching and delignification of pulp pre-bleached with oxygen and peroxide or oxygen and ozone, which does not have the disadvantages of the prior art mentioned, but rather with little technical effort and without assistance of chlorine-containing treatment agents enables the production of pulps with high whiteness.

This object has been achieved according to the invention by subjecting the pre-bleached pulp to a double treatment with peroxide which has been activated with cyanamide and / or cyanamide salts.

The activation of peroxide by means of cyanamide derivatives is known, for example, from DE-A-35 44 398, but essentially unbleached pulp is used as the starting product according to this process. Finally, it was also known from our own experiments that the activity of these activators wore off very quickly in the case of pulp bleached with chlorine or hypochlorite, and at relatively high temperatures and long reaction times. It was therefore completely surprising that with pulp pre-bleached with oxygen and peroxide, the effect of the cyanamide derivatives was so good even at comparatively high temperatures that very good degrees of whiteness could be achieved in this way.

In the process of the invention, pre-bleached pulp is used as the starting material, which has been pretreated with oxygen and peroxide or oxygen and ozone by a customary extraction step and has quite different degrees of whiteness depending on the type of pulp. For example, while pre-bleached spruce pulps have whiteness levels of 65% to 75%, the corresponding whiteness levels of beech pulp are already over 80% in some cases. The kappa values for the degree of delignification are normally between 2 and 10, in particular between 3 and 8.

The two treatment stages according to the invention include the bleaching and delignification of peroxide with cyanamide or cyanamide salts, which are preferably in the form of the alkaline salts such as e.g. Sodium hydrogen cyanamide, calcium cyanamide or magnesium cyanamide can be used. The amounts of peroxide and cyanamide derivatives differ only slightly in the two treatment stages. All water-soluble bleach-active peroxy compounds can be used as the peroxide. Hydrogen peroxide or sodium peroxide are preferred.

While in the first stage the cyanamide content is preferably 0.05 to 1.0% by weight, in particular 0.07 to 0.33% by weight, the second stage is preferably carried out at a cyanamide content of 0.01 to 0.7% by weight, in particular 0.03 to 0.23% by weight, in each case based on the dry weight of the pulp.

The peroxide content can preferably be varied between 0.1 and 3.0% by weight during the first treatment stage and between 0.1 and 2.0% by weight based on the dry weight of the pulp during the second treatment stage.

The other process conditions are largely identical for both treatment stages. As is customary for peroxide bleaching, the pH is made alkaline and can generally be from 9 to 12. The pH is adjusted with the known alkaline substances, sodium hydroxide being preferred for reasons of better solubility. Normally, 0.3 to 1.5% by weight of sodium hydroxide, based on the dry weight of the pulp, is sufficient to set the desired pH.

The solid densities can be varied within wide limits in both treatment stages, solid densities of 5 to 30% by weight, in particular those of 7 to 20% by weight, being particularly preferred.

Depending on the type of pulp and the desired degree of whiteness, the treatment stages according to the invention are preferably carried out at temperatures from 30 ° to 95 ° C., particularly preferably from 55 ° to 80 ° C., wherein treatment times of 1 to 8 hours are usually required in order to achieve the desired degree of whiteness to reach. However, the addition of cyanamide and / or cyanamide salts generally makes it possible to shorten the reaction time and / or to lower the temperature compared to peroxide treatment without this activator, if this is desired for any reason.

With the aid of the method according to the invention, three-quarter or fully bleached cellulose can be produced in a technically simple manner and without the aid of chlorine-containing chemicals, which have whiteness levels of> 80%.

The following examples are intended to illustrate the invention in more detail.

Examples

All bleaching stages were carried out in polyethylene bags.

Provisions

• Whiteness: From the washed cellulose, standard white sheets were formed on the sheet former (according to Rapid Köthen) and the whiteness in% according to the ISO 2470 standard was determined using a measuring device from Elrepho 2000 (Datacolor).
• Kappa number: This parameter, which is a measure of the lignin content in the pulp, was determined in accordance with ISO standard 302.
• Bleaching sequences:
  were abbreviated as follows:

  EOP:

  extraction assisted with oxygen and hydrogen peroxide

  EP:

  extraction supported with hydrogen peroxide (peroxide bleaching stage)

Example 1 (comparison) 1st stage of treatment

In 175.82 g of spruce magnesium bisulfite pulp (40 g otro) after EOP pretreatment with 73.6% whites and kappa 4.8 224.18 g of an aqueous solution which contained 3.60 g of NaOH (10%) and 0.69 g of hydrogen peroxide (35%), kneaded intensively. The 10% pulp slurry was placed in a water bath at approximately 74 ° C. to react completely. After 120 minutes of bleaching, the suspension was diluted with process water, adjusted to pH 6.0, suction filtered and washed. Whiteness 79.4.

2nd stage of treatment

Stoffmenge:

140,85 g

Bleichlösung:

259,12 g mit 2,00 g NaOH (10 %ig) und 0,34 g Wasserstoffperoxid (35 %ig)

Bleichtemperatur:

74°C

Bleichdauer:

120 Minuten

Weißgrad:

81,1 %

The pulp pretreated in this way was subjected to a further peroxide treatment.

Amount of fabric:

140.85 g

Bleaching solution:

259.12 g with 2.00 g NaOH (10%) and 0.34 g hydrogen peroxide (35%)

Bleaching temperature:

74 ° C

Bleaching time:

120 minutes

Whiteness:

81.1%

Example 2 1st stage of treatment

In 175.82 g of spruce magnesium bisulfite pulp (40 g otro) after EOP pretreatment with 73.6% whiteness and kappa 4.8, 88.44 g of an aqueous solution containing 0.80 g of a 10% cyanamide Contained solution, incorporated. After 2 to 5 minutes, a further 134.74 g of an aqueous solution which contained 3.60 g of NaOH (10% strength) and 0.69 g of hydrogen peroxide (35% strength) were added and likewise kneaded intensively. To react completely, the 10% cellulose suspension was placed in a water bath at about 74 ° C. After 120 minutes of bleaching, the suspension was diluted with process water, adjusted to pH 6.0, suction filtered and washed. Whiteness 82.2.

2nd stage of treatment

Stoffmenge:

140,85 g

Wäßrige Cyanamidlösung:

102,96 g mit 0,40 g Cyanamid (10 %ig)

Bleichlösung:

156,19 g mit 2,00 g NaOH (10 %ig) und 0,34 g Wasserstoffperoxid (35 %ig)

Bleichtemperatur:

74°C

Bleichdauer:

120 Minuten

Weißgrad:

85,5 %

The pulp pre-treated in this way was subjected to a further peroxide treatment subject.

Amount of fabric:

140.85 g

Aqueous cyanamide solution:

102.96 g with 0.40 g cyanamide (10%)

Bleaching solution:

156.19 g with 2.00 g NaOH (10%) and 0.34 g hydrogen peroxide (35%)

Bleaching temperature:

74 ° C

Bleaching time:

120 minutes

Whiteness:

85.5%

Example 3 (comparison)

Beech magnesium bisulfite pulp was further treated after an EOP pretreatment with 82.6% whiteness analogously to Example 1 under the following conditions:

1st stage of treatment

Amount of fabric:

357.14 g (100 g otro)

Bleaching solution:

642.86 g with 5.00 g NaOH (10%) and 1.00 g hydrogen peroxide (35%)

Bleaching temperature:

55 ° C

Bleaching time:

120 minutes

Whiteness:

86.0%

2nd stage of treatment

Amount of fabric:

178.59 g (50 g otro)

Bleaching solution:

321.43 g with 2.50 g NaOH (10%) and 0.50 g hydrogen peroxide (35%)

Bleaching temperature:

55 ° C

Bleaching time:

120 minutes

Whiteness:

86.7%

Example 4

Beech magnesium bisulfite pulp according to Example 3 with 82.6% whiteness was analogous to Example 2 under the following conditions aftertreated:

1st stage of treatment

Amount of fabric:

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 NaOH (10%) and 1.00 g hydrogen peroxide (35%)

Bleaching temperature:

55 ° C

Bleaching time:

120 minutes

Whiteness:

88.3%

2nd stage of treatment

Amount of fabric:

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 NaOH (10%) and 0.50 g hydrogen peroxide (35%)

Bleaching temperature:

55 ° C

Bleaching time:

120 minutes

Whiteness:

89.5%

Claims (9)

1. Process for the chlorine-free bleaching and delignification of cellulose pre-bleached with oxygen and peroxide or oxygen and ozone, characterised in that one treats the pre-bleached cellulose twice with peroxide activated by cyanamide and/or cyanamide salts.
2. Process according to claim 1, characterised in that, in the first treatment step, one adds 0.05 to 1.0 wt.%, especially 0.07 to 0.33 wt.% of cyanamide and/or cyanamide salt, referred to the dry weight of the cellulose.
3. Process according to claim 1 or 2, characterised in that, in the first treatment step, one adjusts the peroxide content to 0.1 to 3.0 wt.%, referred to the dry weight of the cellulose.
4. Process according to one of claims 1 to 3, characterised in that, in the second treatment step, one adds 0.01 to 0.7 wt.%, especially 0.03 to 0.23 wt.% of cyanamide and/or cyanamide salt, referred to the dry weight of the cellulose.
5. Process according to one of claims 1 to 4, characterised in that, in the second treatment step, one adjusts the peroxide to between 0.1 to 2.0 wt.%, referred to the dry weight of the cellulose.
6. Process according to one of claims 1 to 5, characterised in that one adjusts the pH value in both treatment steps to 9 to 12.
7. Process according to one of claims 1 to 6, characterised in that, in both treatment steps, one adjusts solids material density of 5 to 30 wt.%, especially 7 to 20 wt.%.
8. Process according to one of claims 1 to 7, characterised in that one carries out both treatment steps at a temperature of 30 to 95°C, preferably of 55 to 80°C.
9. Process according to one of claims 1 to 8, characterised in that the treatment period per step amounts to 1 to 8 hours.