CH641515A5 - Multi-stage process for complete bleaching of pulp - Google Patents

Description

641 515

2nd

PATENT CLAIMS

1. Multi-stage process for full bleaching of pulp, characterized in that it comprises at least three stages, and that the pulp is bleached in aqueous solution in the first stage with a per compound, in the second stage with chlorite and in the third stage with a per compound .

2. The method according to claim 1, characterized in that further bleaching stages are connected.

3. The method according to claim 1, characterized in that between the first and second bleaching stage is washed with water.

4. The method according to claim 2 or 3, characterized in that between the further bleaching stages is washed with water.

5. The method according to any one of the preceding claims, characterized in that in the first stage as per compound hydrogen peroxide in a concentration of 0.5-5 wt .-%, preferably 0.8-12 wt .-% and also NaOH in one Concentration of 1.5-10% by weight, preferably 2.0-4.0% by weight, both calculated as 100% substance, is used and at a consistency of 5-20% by weight, preferably 8- 12 wt .-%, all three based on dry cellulose, at 40-95 ° C, preferably 50-75 ° C, bleached for 30-240 min, preferably 60-180 min.

6. "Method according to one of the preceding claims, characterized in that in the second stage as chlorite sodium chlorite in a concentration of 0.05 to 1.0 wt .-%, preferably 0.1-0.3 wt .-% , calculated as 100% substance, with a consistency of 5-15% by weight, preferably 8-12% by weight, both based on dry cellulose, with an initial pH of 2.5-5.0 , preferably 3.5 to 4, at 20-80 ° C, preferably 25-60 ° C, for 30-240 min, preferably 60-180 min, bleaches.

7. The method according to any one of the preceding claims, characterized in that in the third stage as the per compound hydrogen peroxide in a concentration of 0.2-2.0 wt .-%, preferably 0.4-0.8 wt .-%, and also NaOH in a concentration of 0.5-3.0% by weight, preferably 0.8-1.2% by weight, both calculated as a 100% substance, and with a consistency of 5-15% .-%, preferably 8-12 wt .-%, all three based on dry cellulose, at 40-95 ° C, preferably 50-65 ° C, bleached for 30-240 min, preferably 60-120 min.

8. Application of the method according to claim 1 to pulp which has been subjected to a pretreatment with magnesium salts and / or organic complexing agents.

9. Use according to claim 8, characterized in that the pretreatment with a Mg salt, corresponding to a concentration of 0.05-0.6 wt .-%, preferably 0.1-0.4 wt .-% Mg, and 0.05-0.6% by weight, preferably 0.1-0.3 to 0.3% by weight of a complexing agent, with a consistency of 2-15% by weight, preferably 3-5% by weight %, both based on dry cellulose, at 5-80 ° C, preferably 5-20 ° C for 5-60 min, preferably 5-10 min.

From DE-OS 2327900 and DE-PS 2219504 it is known to bleach pulp using peroxides.

In the process according to DE-PS 2219 504, which consists of a step sequence P-D-P, considerable corrosion problems can occur.

It is an object of the present invention to provide a method in which there are no or substantially reduced corrosion problems.

According to the invention, this is achieved in that the method comprises at least three stages, and in that the pulp, optionally after pretreatment with magnesium salts and / or organic complexing agents, in the first stage with a per compound, in the second stage with chlorite and in the third stage is bleached with a per connection.

In the case of pulp which is difficult to bleach or to set a desired degree of polymerization, further known bleaching stages can be connected.

Such bleaching stages are described in Ullmann's Encyclopedia of Industrial Chemistry, 3rd edition, 18th volume, page 778 (1967). The process according to the invention can be carried out, for example, in bleaching devices as described in Ullmann's Encyclopedia of Industrial Chemistry, 3rd edition, 18th volume, page 755.

A washing with water can be carried out in a known manner between the individual bleaching stages. In a preferred embodiment of the invention, however, washing can be omitted between the second and third stages without a disadvantage for the quality of the product being ascertained.

The method according to the invention can be used for all known types of cellulose. Such pulps can be any softwood or hardwood sulfite pulps, e.g. Spruce sulfite pulp or beech sulfite pulp. The process according to the invention can be used with particular advantage for bleaching spruce paper and viscose sulfite pulp or beech sulfite rayon pulp. But it can also be used for bleaching all long and short-fiber pulps, such as those of poplar or birch.

The pretreatment of the pulp can be carried out in aqueous solution at a consistency of 2 to 15% by weight (based on dry cellulose), preferably 3 to 5% by weight. The temperature can be between 5 and 80 ° C, preferably between 5 and 20 ° C. The duration of the pretreatment can be between 5 and 60 minutes, preferably between 5 and 10 minutes. The concentration of magnesium salt can be 0.05 to 0.6% by weight, preferably 0.1 to 0.4% by weight (based on dry cellulose and MgSO-t).

Magnesium salts which can be used are: MgSCU, MgCh, Mg (N03>, Mg EDTA (ethylenediaminetetracetic acid) or magnesium salts of polyoxycarboxylic acid. Such polyoxycarboxylic acids are described, for example, in DE-OS 1904940,

DE-OS 1904941 and DE-OS 1942556 are described.

The concentration of the complexing agents in the pretreatment liquor can be 0.005 to 0.6% by weight, preferably 0.1 to 0.3% by weight (based on dry cellulose). The following can be used as complexing agents: ethylenediamine acetic acid (EDTA), NasDTPA, polyoxycarboxylic acids, such as those e.g. can be described in DE-OS 1904940, DE-OS 1904941 or DE-OS 1942556.

The first bleaching stage is carried out with a per compound, in particular at a consistency of 5 to 50% by weight, preferably 8 to 12% by weight (based on dry cellulose), in aqueous solution. An organic or inorganic peroxide such as hydrogen peroxide, sodium peroxide or t-butyl hydroperoxide can be used as the per compound. In a preferred embodiment, hydrogen peroxide is used. The temperature in the first bleaching stage can be between 40 and 95 ° C, preferably between 50 and 75 ° C.

The duration of the first stage can be between 30 and 240 minutes, preferably between 60 and 180 minutes.

The concentration of per compound can be from 0.5 to 5

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% By weight, preferably from 0.8 to 1.2% by weight (based on dry cellulose and 100% H2O2).

The concentration of alkali, preferably sodium hydroxide, can be 1.5 to 10% by weight, preferably 2.0 to 4.0% by weight (based on dry cellulose and 100% NaOH).

In the second stage, an alkali chlorite, preferably sodium chlorite, can be used as chlorite in the aqueous solution. The consistency of the second stage can be 5 to 15% by weight, preferably 8 to 12% by weight (based on dry cellulose). The temperature can be 20 to 80 ° C, preferably 25 to 60 ° C. The duration of the second stage can be between 30 and 240 minutes, preferably between 60 and 180 minutes. The concentration of the alkali chlorite can be 0.05 to 1.0% by weight, preferably 0.1 to 0.3% by weight (based on dry cellulose and 100% sodium chlorite). The pH of the second stage can be 2.5 to 5.0, preferably 3.0 to 4. The pH can be adjusted by adding an organic acid, e.g. Formic acid or acetic acid, or a mineral acid can be adjusted, in a preferred embodiment nitric acid (HNO3) is used.

The third bleaching stage is carried out with a per compound in aqueous solution, in particular at a consistency of 5 to 15% by weight, preferably 8 to 12% by weight (based on dry cellulose). Inorganic or organic peroxides such as e.g. Hydrogen peroxide, sodium peroxide or t-butyl hydroperoxide can be used. Hydrogen peroxide can preferably be used as the per compound. The temperature can be 40 to 95 ° C, preferably 50 to 65 ° C. The duration of the third bleaching stage can be 30 to 240 minutes, preferably 60 to 120 minutes. The concentration of per compound, preferably hydrogen peroxide, can be 0.2 to 2.0% by weight, preferably 0.4 to 0.8% by weight (based on dry cellulose and 100% H2O2). The concentration of alkali can be 0.5 to 3.0% by weight, preferably 0.8 to 1.2% by weight (based on dry cellulose and 100% NaOH). Sodium hydroxide can preferably be used as the alkali.

The process according to the invention has the advantage that the combination and sequence of the bleaching stages per compound, chlorite and per compound as well as the management of the chlorite bleaching stage, in particular at a pH between 3 and 5, make it possible to use chlorite in this way to keep it low so that no significant amounts of chlorine get into the sewage system. That means that the method according to the invention works almost without chlorine.

The whiteness content of the end product is improved compared to the end products of the known processes, and the yield can also be increased by 2 to 3%.

Furthermore, the washing between the second and third bleaching stages can be dispensed with, as a result of which an improved heat economy is achieved.

The temperature in the second stage can be lowered to 20 ° C, which causes a significant reduction in the corrosion problems which e.g. can occur in the process according to DE-PS 2219504 in the chlorine dioxide stage (D).

The process according to the invention is explained and described in more detail with reference to the following examples. Percentage concentration data are by weight.

Comparative experiment a)

Bleaching of a beech sulfite rayon pulp according to the prior art

641 515

1st stage 2.2% Ch 1 hour 3% substance 150 C wash

2nd stage 3.6% NaOH 3 hours 10% substance. 75 ° C wash

3rd level 0.4% act. Chlorine 4 hours 10% substance 40 wash (Ca hypochlorite)

example 1

Bleaching of a beech sulfite rayon pulp according to the invention

1st stage 1.0% H2O2 3 hours 10% substance 75 ° C

3.0% HaOH

Ablution

2nd stage 0.15% NaC102 12 hours 10% substance. 30 ° C

0.4% HNO3 no washing

3rd stage 0.4% H2O2 2 hours 10% substance 65 ° C

1.0% NaOH

Results

Comparative example 1

try a)%

%

White content bleached 92.0 94.6 («Elrepho»)

Yield 93.3 96.6

a-cellulose 90.6 90.4

R19 94.2 93.8

Viscosity (SKV) 178 214

For a beech sulfite rayon pulp, the two experiments show that better bleaching is achieved with the method according to the invention. The 3.3% higher pulp yield is obtained with the a-cellulose content almost unchanged. The only slightly reduced R19 value indicates that the yield gain in the mash alkalization and viscose production is largely retained. The higher viscosity indicates that the process is extremely gentle on the fibers.

The total chlorine use in the known bleaching method according to comparative experiment a) was 2.6%, based on cellulose, in the bleaching according to Example 1 0.06%.

Comparative experiment b)

Bleaching of a spruce sulfite pulp according to the prior art

1st stage

4% Ch

1 hour

3%

Fabric

15 ° C

Ablution

2nd stage

3.5% NaOH

1 hour

10%

Fabric

70 ° C

Ablution

3rd stage

1.0% act. chlorine

3 hours

10%

Fabric

40 ° C

Ablution

4th stage

1.0% act. chlorine

3 hours

10%

Fabric

40 ° C

Example 2

Bleaching a spruce sulfite pulp for paper according to the invention

1st stage 1.5% H2O2 1 hour 10% substance 70 ° C

3.0% NaOH

Ablution

2nd stage 0.4% NaC102 3 hours 10% substance. 50 ° C

0.5% ENT no washing

3rd

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4th

3rd stage 1.0% H2O2 3 hours 10% substance 600C

1.5% NaOH

Ablution

Results

Comparative

Example 2

try b)

%

%

White content bleached

90.3

90.7

yield

92.5

93.1

If the whiteness is slightly higher and the yield is somewhat better, the chlorine content in the wastewater is reduced by the process according to the invention from 6%, based on the pulp used, to 0.16%.

Example 3

Bleaching of a spruce sulfite pulp according to the invention

Pretreatment level 5 0.3% MgS04 10 min

Thickening to 10% consistency 1st stage 1.3% H2O2 1 hour

3.0% NaOH

Ablution

10 2nd stage 0.4% NaClOî 3 hours 3rd stage 1.0% H2O2 3 hours

1.5% NaOH

The bleached fabric reached a whiteness of 91.0. The 15 pretreatment with MgS04 reduced the use of peroxide in the 1st stage to 1.3% H2O2.

5% substance 15 ° C 10% substance 70 ° C

10% substance 30 ° C 10% substance 60 ° C

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