EP0442425A1 - Process for the chlorine-free bleaching and delignification of pulp - Google Patents

Abstract

For chlorine-free bleaching and delignification of pulp, which has been pretreated with oxygen and peroxide or oxygen and ozone, two treatment stages are carried out with peroxide which has been activated by cyanamide and/or cyanamide salts. In this way, pulps of high whiteness can be produced at low engineering expense and without the aid of chlorine-containing treatment agents.

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 pulp is largely required to produce white-stable and / or high-strength paper products. For this purpose, the pulp made from vegetable raw materials such as wood, reeds, straw, bagasse or the like. 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 multi-stage bleaching processes, essentially 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In order to be able to completely dispense with elemental chlorine, the chlorine level has been replaced by an oxygen level (EO) or an extraction level supported by oxygen and peroxide (EOP) in recent years. With the aid 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 from 80 to 87% are possible without using elemental chlorine, but you have to resort to a treatment level 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 with the following bleaching sequence, for example:

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

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 using cyanamide derivatives is known, for example, from DE-OS 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 with pulp bleached with chlorine or hypochlorite, as well as with 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 conventional extraction step and which 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 with 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 pulps 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 standard 2470 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 IsoNorm 302.
• Bleaching sequences: were abbreviated as follows:

  EOP:

  extraction assisted with oxygen and hydrogen peroxide

  EP:

  extraction supported by 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% whiteness and 4.8 kappa 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. To react, the 10% pulp slurry was placed in a water bath at about 74 ° C. After a bleaching time of 120 minutes, 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 Solution contained, 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, the 10% cellulose suspension was placed in a water bath at about 74 ° C. After a bleaching time of 120 minutes, 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)

Process for chlorine-free bleaching and delignification of pulp pre-bleached with oxygen and peroxide or oxygen and ozone,
characterized by
that the pre-bleached pulp is treated twice with peroxide activated by cyanamide and / or cyanamide salts. Method according to claim 1,
characterized,
that 0.05 to 1.0% by weight, in particular 0.07 to 0.33% by weight, of cyanamide and / or cyanamide salt, based on the dry weight of the pulp, is added in the first treatment stage. Method according to claim 1 or 2,
characterized,
that in the first treatment stage the peroxide content is adjusted to 0.1 to 3.0% by weight, based on the dry weight of the pulp. Method according to one of claims 1 to 3,
characterized,
that in the second treatment stage 0.01 to 0.7% by weight, in particular 0.03 to 0.23% by weight, of cyanamide or / and cyanamide salt, based on the dry weight of the pulp, is added. Method according to one of claims 1 to 4,
characterized,
that the peroxide content in the second treatment stage is between 0.1 and 2.0% by weight, based on the dry weight of the pulp. Method according to one of claims 1 to 5,
characterized,
that the pH is adjusted to 9 to 12 in both treatment stages. Method according to one of claims 1 to 6,
characterized,
that solid densities of 5 to 30% by weight, in particular 7 to 20% by weight, are set in both treatment stages. Method according to one of claims 1 to 7,
characterized,
that one carries out both treatment stages at a temperature of 30 to 95 ° C, preferably 55 to 80 ° C. Method according to one of claims 1 to 8,
characterized,
that the treatment time per stage is 1 to 8 hours.