NO176329B - Process for chlorine-free bleaching and delignification of chemical 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 method for chlorine-free bleaching and delignification of chemical pulp, which has been pre-bleached with oxygen and peroxide, respectively oxygen and ozone.

For the production of whiteness-stable and/or high-strength paper products, lignin-free chemical pulp is largely needed. For this purpose, the chemical pulp, which is extracted from vegetable raw materials such as wood, reeds, straw, bagasse etc., is subjected to a chemical digestion (sulphate or sulphite digestion), a delignifying bleaching.

In this, usually multi-stage, bleaching process, elemental chlorine (C) and hypochlorite (E) were used in the past as treatment agents, whereby, for example, the following bleaching sequence was common for sulphite chemical pulp: (C) = chlorine bleaching, (E) = extraction of chlorlignins with aqueous caustic soda, (H) = hypochlorite treatment.

To reduce the environmentally harmful use of elemental chlorine, the extraction step has increasingly been carried out with peroxide, whereby a significant reduction in the use of chlorine could be achieved.

In order to completely dispense with elemental chlorine, the chlorine stage has been replaced in recent years with an oxygen stage (RO), respectively with an extraction stage supported by oxygen and peroxide (EPO). By means of these bleaching steps, however, only semi-bleached chemical pulps are obtainable, exhibiting a degree of whiteness of 75 to 80%. Three-quarter bleached chemical pulps with degrees of whiteness of 80 to 87% are indeed obtainable without the use of elemental chlorine, but one must instead use a treatment step with hypochlorite (H) or chlorine dioxide (D), whereby the following sequences are common:

EPO = extraction with oxygen/peroxide

EP = extraction with peroxide

D = treatment with chlorine dioxide

H = treatment with hypochlorite.

The same also applies to completely bleached chemical pulps with degrees of whiteness of 88 to 90%, which are, for example, available with the following bleaching sequence:

Both the use of hypochlorite and chlorine dioxide constitute, as before, a burden on the waste water and waste air.

The present invention is therefore based on the task of developing a method for chlorine-free bleaching and delignification of chemical pulp that has been bleached with oxygen and peroxide, respectively oxygen and ozone, which does not exhibit the aforementioned disadvantages of the state of the art, but which, on the other hand, is technically simple manner and without the use of chlorine-containing treatment agents enables the production of chemical pulps with high levels of whiteness.

According to the invention, this task was solved by a method of the kind mentioned at the outset, characterized by treating the bleached chemical mass twice 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-OS 3544398, but according to this method, essentially unbleached chemical pulp is used as the starting product. Finally, it was also known from our own experiments that the effect of these activators on chemical masses bleached with chlorine or hypochlorite, as well as at relatively high temperatures and long reaction times, decreased very quickly. Consequently, it was completely surprising that the effect of the cyanamide derivatives on chemical masses bleached with oxygen and peroxide, even at relatively high temperatures, was so good that very good degrees of whiteness could be achieved in this way.

In the method according to the invention, pre-bleached chemical masses are used as starting materials, which have been pre-treated in a normal extraction step with oxygen and peroxide, respectively oxygen and ozone, and which, depending on the type of chemical mass, exhibit very different degrees of whiteness. While, for example, chemical pulps of spruce bleached in this way have degrees of whiteness of 65% to 75%, the corresponding degrees of whiteness for chemical pulps of beech are already partly above 80%>. The kappa values for the degree of delignification are normally between 2 and 10, especially between 3 and 8.

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

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

The peroxide content can preferably be varied during the first treatment step between 0.1 and 3.0 wt-# and during the second treatment step between 0.1 and 2.0 wt-#, on the basis of the dry weight of the chemical mass.

The other process conditions are largely identical for the two treatment steps. The pH value is, as usual with peroxide bleaching, set alkaline and can usually be from 9 to 12. The pH value is set with the known alkaline reacting substances, whereby sodium hydroxide is preferred due to better solubility. Normally, for this purpose, 0.3 to 1.5 wt.-# of sodium hydroxide, based on the dry weight of the chemical mass, is sufficient to set the pH value desired in any case.

The solids densities in both treatment steps can be varied within wide limits, whereby solids densities of 5 to 30 wt-#, especially 7 to 20 wt-# are particularly preferred.

The treatment steps according to the invention are carried out depending on the type of chemical mass and desired degree of whiteness, preferably at temperatures from 30° to 95° C, particularly preferred 55° to 80° C, whereby usually treatment times of 1 to 8 hours are required to achieve the desired degrees of whiteness. With the addition of cyanamide and/or cyanamide salts, however, it is generally possible, compared to peroxide treatment without this activator, to shorten the reaction time and/or lower the temperature, if this is desired for one reason or another.

With the help of the method according to the invention, three-quarters or fully bleached chemical pulps can be produced in a technically simple manner and without the use of chlorine-containing chemicals, which exhibit degrees of whiteness of > 80%.

The following examples shall illustrate the invention more closely.

Examples

All bleaching steps were carried out in polyethylene bags.

Provisions

Degree of whiteness: Washed chemical pulp remained on

leaf former (Rapid Kothen) formed whiteness-standardized leaves and with a measuring device of the brand "Elrepho 2000"

(Datacolor) the degree of whiteness in % was determined according to iso-norm 2470.

Kappa number: The determination of this characteristic size, which is a measure of the 1 ignin content in the chemical mass, took place according to iso-norm 302.

Bleaching sequences: were shortened as follows:

EOP: with oxygen and hydrogen peroxide assisted extraction.

EP: with hydrogen peroxide supported extraction (peroxide bleaching step).

Example 1 (Comparison)

1. Processing step

Into 175.82 g of magnesium bisulphite chemical pulp (40 g otro) after EOP pre-treatment with 73.6% degree of whiteness and kappa 4.8, an aqueous solution containing 3.60 g of NaOH (10%) and 0.69 g of hydrogen peroxide (35%). For reaction, the 10# slurry of chemical pulp was placed in an approx. 74° C hot water bath. After 120 minutes of bleaching time, the suspension was diluted with ordinary water, adjusted to pH 6.0, sucked off and washed.

Degree of whiteness 79.4%.

2. Processing step

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

Amount of material: 140.85 g

Bleach solution: 259.12 g with 2.00 g of NaOE (10%) and 0.34 g of hydrogen peroxide solution (35%>)

Bleaching temperature: 74°C

Bleaching duration: 120 minutes

Whiteness: 81.1%.

Example 2

1. Processing step

Into 175.82 g of magnesium bisulphite chemical pulp (40 g otro) after EOP pretreatment with 73.6% whiteness and kappa 4.8 was incorporated 88.44 g of an aqueous solution containing 0.80 g of a 10 # cyanamide solution. After 2 to 5 minutes, an additional 134.74 g of an aqueous solution containing 3.60 g of NaOH (10 <&) and 0.69 g of hydrogen peroxide (35 $ >) was added and this was also vigorously kneaded. For reaction, the 10% suspension of chemical pulp was held in an approx. 74 "C hot water bath. After 120 minutes of bleaching time, the suspension was diluted with water, adjusted to pH 6.0, aspirated and rewashed. Whiteness degree 82.2 56.

2. Processing step

The chemical mass pretreated in this way 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%)

Bleach solution: 156.19 g with 2.00 g NaOH (10 56) and 0.34 g hydrogen peroxide (35

Bleaching temperature: 74"C

Bleaching duration: 120 minutes

Degree of whiteness: 85.5%.

Example 3 (Comparison)

Beech magnesium bisulphite chemical pulp was, after an EOP pre-treatment with 82.6% whiteness, further processed analogously to example 1 under the following conditions:

1. Processing step

Material quantity: 357.14 g (100 g otro)

Bleach solution: 642.86 g with 5.00 g NaOH (10 K>) and 1.00 g hydrogen peroxide (35%)

Bleaching temperature: 55<0>C

Bleaching duration: 120 minutes

Degree of whiteness: 86.0%.

2. Processing step

Material quantity: 178.59 g (50 g otro)

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

Bleaching temperature: 55°C

Bleaching duration: 120 minutes

Degree of whiteness: 86.7

Example 4

Beech magnesium bisulphite chemical pulp corresponding to example 3 with 82.6% whiteness was post-treated analogously to example 2 under the following conditions:

1. Processing step

Material quantity: 357.14 g (100 g otro)

Aqueous cyanamide solution: 255.46 g with 1.20 g cyanamide

(10 56)

Bleach solution: 387.39 g with 5.00 g NaOH (10 56) and 1.00 g hydrogen peroxide (35 56)

Bleaching temperature: 55"C Bleaching duration: 120 minutes

Degree of whiteness: 88.3 56.

2. Processing step

Material quantity: 178.57 g (100 g otro)

Aqueous cyanamide solution: 127.73 g with 0.60 g cyanamide

( 10 56 )

Bleach solution: 193.70 g with 2.50 g NaOH (10 56 ) and 0.50 g hydrogen peroxide (35 56)

Bleaching temperature: 55°C

Bleaching duration: 120 minutes

Degree of whiteness: 89.5 56.

Claims (9)

1. Process for chlorine-free bleaching and delignification of chemical pulp that has been bleached with oxygen and peroxide, respectively oxygen and ozone, characterized by treating the bleached chemical pulp twice with peroxide that has been activated with cyanamide and/or cyanamide salts. 2. Method according to claim 1, characterized in that in the first treatment step, 0.05 to 1.0 wt-Sé, in particular 0.07 to 0.33 wt-# cyanamide and/or cyanamide salt, is added, calculated on the basis of the dry weight of the chemical the mass. 3. Method according to claim 1 or 2, characterized in that in the first treatment step the peroxide content is set at 0.1 to 3.0 wt-#, calculated on the basis of the dry weight of the chemical mass. 4. Process according to one of claims 1 to 3, characterized in that in the second treatment step 0.01 to 0.7 weight-Sé, in particular 0.03 to 0.23 weight-5é cyanamide and/or cyanamide salt, calculated on the basis of of the dry weight of the chemical mass. 5. Method according to one of claims 1 to 4, characterized in that the peroxide content in the second treatment step is set between 0.1 and 2.0 wt-#, calculated on the basis of the dry weight of the chemical mass. 6. Method according to one of claims 1 to 5, characterized in that the pH value at both treatment stages is set to 9 to 12. 7. Method according to one of the claims 1 to 6, characterized in that in both treatment stages, solids densities of 5 to 30 wt-#, especially 7 to 20 wt-56 are set. 8. Method according to one of claims 1 to 7, characterized in that both treatment steps are carried out at a temperature of 30 to 95°C, preferably 55 to 80°C. 9. Method according to one of claims 1 to 8, characterized in that the treatment duration per steps amount to 1 to 8 hours.