DE4004364C2 - - 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 pulp, that already with oxygen and peroxide or oxygen and pre-bleached ozone.

For the production of white stable and / or high strength Paper products become largely lignin-free pulp needed. To this end, the pulp that is made vegetable raw materials such as wood, reeds, straw, bagasse or similar by chemical digestion (sulfate or Sulfite digestion) is obtained, a delignifying Subjected to bleaching.

In these usually multi-stage bleaching processes were previously essentially elemental chlorine (C) and Hypochlorite (H) is used as a treatment agent, whereby For example, the following bleaching sequence is common for sulfite pulp was:

(C) - (E) - (H) - (H)

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

To reduce the environmentally harmful use of Elemental chlorine became the extraction stage increasingly performed with peroxide, with a clear Reduction in the use of chlorine could be achieved.

In order to be able to completely do without elemental chlorine, In recent years, the chlorine level has been Oxygen level (EO) or through one with oxygen and Peroxide assisted extraction stage (EOP) developed. With the help of these bleaching steps, however, are only  semi-bleached pulps available that have a whiteness have from 75 to 80%. Three-quarter bleached Pulps with whiteness levels of 80 to 87% are without The use of elemental chlorine is possible, but you have to instead, on a hypochlorite treatment level (H) or chlorine dioxide (D) fall back, then the following sequences are common:

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 applies to fully bleached cellulose with whitenesses from 88 to 90%, for example with following bleaching sequence are accessible:

EOP-D-EP-D-EP

Both the use of hypochlorite and However, chlorine dioxide remains a burden for wastewater and exhaust air.

Another option for chlorine-free bleaching is in the US-PS 41 96 043 described, the pulp subjected to at least two treatment stages with ozone becomes. Because of the toxicity of ozone, the extensive Security measures required, and the difficult incorporation of this treatment agent into The pulp suspension has ozone as a bleach cannot be implemented industrially.

The object of the present invention was therefore based on a process for chlorine - free bleaching and Delignification of with oxygen and peroxide or  Oxygen and ozone to pre-bleached pulp develop, which the disadvantages of the state the technology does not have, but with little technical effort and without the help of chlorine-containing treatment agents the production of Allows pulp with high degrees of whiteness.  

This object was achieved in that one the pre-bleached pulp two times Subjects treatment with peroxide, which with cyanamide and / or cyanamide salts has been activated.

The activation of peroxide using cyanamide derivatives is Although known for example from DE-OS 35 44 398, but will substantially unbleached according to this procedure Pulp used as the starting product. Finally was also known from own experiments that the effect of these activators with chlorine or hypochlorite pre-bleached pulps and at relatively high Temperatures and long reaction times very quickly subsided. It was therefore completely surprising that with Oxygen and peroxide pre-bleached pulp Effect of the cyanamide derivatives also in comparison high temperatures was so good that way good degrees of whiteness could be achieved.

According to DE-OS 37 14 107, residual Hydrogen peroxide using aqueous cyanamide Solutions destroyed or deactivated, but this occurs Effect in the presence of another substrate, namely Pulp, not on.

In the method according to the present invention is pre-bleached pulp as a raw material used by using a conventional extraction step Oxygen and peroxide or oxygen and ozone was pretreated and quite depending on the type of pulp has different degrees of whiteness. During, for example this way pre-bleached spruce pulp whiteness of 65 to 75%, the corresponding degrees of whiteness are of beech pulp already over 80% in some cases. The Kappa values for the degree of delignification are normally between 1 and 10, especially between 3 and 8.  

The two treatment stages according to the invention include the bleaching and delignification of peroxide (Hydrogen peroxide or sodium peroxide) with cyanamide or cyanamide salts, which are preferably in the form of alkaline salts such as. B.  Sodium hydrogen cyanamide, calcium cyanamide or Magnesium cyanamide can be used. The amounts of Peroxide and cyanamide derivatives differ in the two Treatment levels just a little.

While in the first stage the cyanamide content preferably 0.05 to 0.5% by weight, in particular 0.07 to 0.33 wt .-% is in the second stage preferably with a cyanamide content of 0.01 to 0.3% 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 be increased during the first Treatment stage preferably between 0.1 and 1% by weight and during the second treatment stage between 0.1 and 0.7% by weight based on the dry weight of the pulp can be varied.

The other procedural conditions are for both Treatment levels largely identical. The pH will as usual with peroxide bleaches, each alkaline and is usually 10 to 12. The pH adjustment is done with the known alkaline reacting substances, taking sodium hydroxide Considered preferred for reasons of better solubility is. Usually 0.3 to 1.5% by weight is sufficient for this. Sodium hydroxide based on the dry weight of the Out of the pulp to get the pH you want adjust.

The solid densities can be at both treatment levels can be varied within wide limits Solid densities of 5 to 30% by weight, preferably 7 to 20 wt .-%, have proven to be particularly advantageous.  

The treatment stages according to the invention depend on Kind of pulp and desired whiteness preferred at temperatures from 40 ° to 90 ° C, in particular 55 ° to 80 ° C, usually with treatment times from 1 to 6 hours are required to get the desired To achieve whiteness. By adding cyanamide and / or cyanamide salts, however, it is generally possible to Comparison to peroxide treatment without activator Shorten response time and / or temperature too lower if desired for any reason becomes.

With the help of the method according to the invention can thus in a technically simple manner and without the aid of chlorine-containing chemicals three-quarters or fully bleached pulps are produced which Have whiteness levels of <80%.

The following examples are intended to illustrate the invention illustrate.

Examples

All bleaching stages were in polyethylene bags carried out.

Provisions

• - Whiteness: The washed pulp was Sheet formers (according to Rapid Köthen) in accordance with standards White leaves formed and with a measuring device of the brand Elrepho 2000 (Datacolor) the white content in% after the ISO standard 2470 determined.
• - Kappa number: The determination of this parameter, which a The measure for the lignin content in the pulp is done according to ISO standard 302.
• - bleaching sequences: were abbreviated as follows:
  EOP: extraction supported by 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% whiteness and kappa 4.8 was 224.18 g of an aqueous solution, the 3.60 g NaOH (10%) and 0.69 g hydrogen peroxide (35%) contained, kneaded intensely. Was to react the 10% pulp slurry into a about 74 ° C given a warm water bath. After 120 minutes of bleaching the suspension was diluted with process water pH 6.0 adjusted, suction filtered and washed. Whiteness 79.4.  

2nd stage of treatment

The pulp pretreated in this way became another Subjected to peroxide treatment.

Quantity 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 were 88.44 g of an aqueous solution, the 0.80 g a 10% cyanamide solution, incorporated. After 2 to 5 minutes another 134.74 kg became one aqueous solution containing 3.60 g NaOH (10%) and 0.69 g Contained hydrogen peroxide (35%), added and also kneaded intensively. Was to react the 10% pulp suspension in a 74 ° C warm Given water bath. After 120 minutes of bleaching, the Suspension diluted with process water, to pH 6.0 adjusted, vacuumed and washed. Whiteness 82.2.

2nd stage of treatment

The pulp pretreated in this way became another Subjected to peroxide treatment.

Quantity 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 made after an EOP Pretreatment with 82.6% whiteness as in Example 1 below the following conditions:

1st stage of treatment

Quantity of substance: 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

Quantity 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 accordingly Example 3 with 82.6% whiteness was analogous to Example 2 under aftertreated following conditions:

1st stage of treatment

Quantity of substance: 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

Quantity of substance: 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. A process for chlorine-free bleaching and delignification of pulp pre-bleached with oxygen and peroxide or oxygen and ozone, characterized in that two treatment stages are carried out with peroxide activated by cyanamide and / or cyanamide salts. 2. The method according to claim 1, characterized in that the amount of the cyanamide and / or the cyanamide salts in the first stage 0.05 to 0.5 wt .-%, in particular 0.07 to 0.33 wt .-%, cyanamide based on that The dry weight of the pulp is. 3. The method according to claims 1 and 2, characterized characterized in that the peroxide content in the first stage to 0.1 to 1 wt .-% based on that Adjusts the dry weight of the pulp. 4. The method according to claims 1 to 3, characterized characterized in that the second stage with a Cyanamide content of 0.01 to 0.3 wt .-%, in particular 0.03 to 0.23 wt .-%, based on the dry weight of the pulp. 5. The method according to claims 1 to 4, characterized characterized in that the peroxide content in the second Level between 0.1 and 0.7 wt .-% based on the The dry weight of the pulp is. 6. The method according to claims 1 to 5, characterized characterized in that the pH at both levels 10 to 12 is set.   7. The method according to claims 1 to 6, characterized characterized in that with both stages with Solid densities of 5 to 30% by weight, especially 7 up to 20% by weight. 8. The method according to claims 1 to 7, characterized characterized in that both treatment stages at a temperature of 40 to 90 ° C, preferably 55 to 80 ° C. 9. The method according to claims 1 to 8, characterized characterized in that the duration of treatment per stage Is 1 to 6 hours.