DE2903014A1 - ANTI-POLLUTANT Bleaching Process For Paper Pulp - Google Patents

Description

29030U

GROUPEMENT D'INTERET ECONOMIQUE DIT: GROUPEAdENT EUROPEEN.DE LA CELLULOSE

5, Rue Jean Mermoz 75008 Paris / France

Anti-pollutant bleaching process for paper pulp

909831 / 077Ö

The invention relates to an anti-pollutant bleaching process with a high degree of whiteness, for everyone in a conventional manner, for example with a peroxide or with pressurized oxygen chemical pulps pre-oxidized in an alkaline medium can be used.

The manufacture of bleached paper pulps involves two main ones Phases.

The first phase consists of cooking the mnocellulosic material with With the help of chemical reagents designed to dissolve a large part of the lignin and free the cellulose fibers. The residual liquids from this cooking process are generally recovered and burned.

The second phase is to eliminate the residual lignin and the to bleach fibers obtained by boiling.

This phase is carried out in several stages with the help of various reagents: Chlorine, Soda, Chlorine Dioxide, Hypochlorite, etc. are carried out.

At present, the waste water coming from the bleaching process cannot be recovered because of the presence of corrosive C hloridver bonds, which explains that the greater part of the aqueous Contamination of a paper pulp plant comes from the bleaching plant.

It has recently been discovered that it is possible to increase the number of phases to reduce this bleaching process and thus the corresponding Contamination, thanks to a pre-oxidation of the raw paper pulp or unbleached paper pulp. This oxidation occurs in an alkaline Environment with the help of hydrogen peroxide or oxygen

909831 / 071Ö

are lfer and

Printing carried out, i.e. with products that are free of Chl consequently are compatible with the recovery of residual liquids from the cooking process.

The companies that use this type of pulp pre-oxidation, are therefore able to complete the lignin removal achieved during the cooking process by means of an oxidizing lignin removal, by creating wastewater that can be recovered and burned. This makes it possible for some companies to reduce their contamination by bleaching to a considerable extent. The factories that produce paper pulps with a very high degree of whiteness (whiteness ISO> 88) have meanwhile had to maintain a long but contaminating bleaching process after the oxidation, which in the generally includes 5 levels, which are symbolized by CEDED:

C: chlorination E: alkaline extraction D: chlorine dioxide.

The invention allows the number of bleaching such Preoxidized paper pulp to a high degree of whiteness necessary to reduce considerably. In addition, it is very under certain precise conditions possible to realize a fully oxidizing bleaching process, which on a large scale the contamination reduced without therefore reducing the quality of the paper pulp.

It is also possible to take advantage of the effluents from each of the stages and recycle that effluent accordingly allows one quality pulp in spite of reducing the number of the procedural steps.

909831/0770

The invention thus has an anti-pollution bleaching process high degree of whiteness to the object, which for everyone in a conventional way, for example chemical pulps pre-oxidized with a peroxide or with oxygen under pressure in an alkaline medium and which is characterized in that the pre-oxidized paper pulp is treated essentially in three oxidizing phases:

a) A first phase is mixed chlorination by introduction by gaseous chlorine followed by the addition of chlorine dioxide without intermediate washing, the gaseous chlorine used corresponds to an amount that is less than the chlorine requirement of the paper pulp and where the complement (the missing remainder) is supplied by chlorine dioxide;

b) The second phase consists in a treatment in an alkaline medium at a pH value between 8 and 12, preferably 10.5 to 11.5, at a low temperature between 30 and 60 C, preferably between 40 and 50 C in the presence of ₈ an oxidizing agent such as hypochlorite or peroxide;

c) The third phase consists of a treatment with chlorine dioxide a temperature between 60 and 90 C.

The three necessary stages of the bleaching process using this technique can be symbolized, for example, by (C + D) HD or (C + D) PD:

C + D = mixed chlorination: gaseous chlorine fed to first and then chlorine dioxide second.

P = peroxide phase

H = hypochlorite phase

D = chlorine dioxide phase.

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The originality of this bleaching process lies mainly in the first Mixed chlorination phase. It was known that chlorination, which is carried out in the heat, is expressed by a deterioration in the paper pulp. To avoid this deterioration, Various inventors have recommended the use of protective agents, particularly chlorine dioxide; however, this is chlorine dioxide either in a mixture with chlorine or sequentially «with the chlorine dioxide being added first. It has now proven to be special pointed out interesting, according to the invention a reverse mixed stage, i.e. that the gaseous chlorine of the paper pulp comes first and then the chlorine dioxide only afterwards is fed as part of the bleaching process of the pre-oxidized paper pulp. In addition, under these conditions an increased Temperature to advantage because the chlorine is consumed practically in its entirety and in a very rapid manner, which is usually the case avoids deterioration observed in the warmth. It is known that this is because the unused chlorine is in contact remains with the warm pulp during the entire conventional chlorination, which is carried out at elevated temperature.

According to a characteristic feature of the invention, the first phase of mixed chlorination is carried out at a temperature between 30

O 'O

and 80 ° C., preferably between 30 and 50 ° C.

It should also be noted that according to the process of the invention it is necessary to introduce an amount of chlorine which is less than the total chlorine requirement of the paper pulp (i.e. below its ROE index). The complement is then in the form of chlorine dioxide added.

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According to a further feature of the invention, the gaseous chlorine used corresponds to 60 to 90 % of the total amount of chlorine plus some =

led chlorine dioxide, calculated as chlorine equivalent.

The addition of the chlorine is in the rest un advantageously = indirectly after the supply of gaseous chlorine and prior to the feed of the chlorine dioxide with the aid of a sensor monitors _s using a known analytical principle (potentiometry, polarography, optics, etc.) is used.

According to a further feature of the invention, the introduction of the gaseous chlorine into the suspension of the paper pulp corresponds to a short retention time of 10 seconds to 5 minutes, preferably 1 to 2 minutes, and the addition of the chlorine dioxide solution is at the end of this time and correspondingly one Retention time from 3 min to 180 min _s, preferably from 6 to 40 min, carried out »

According to a further feature of the invention, the Einführungsseit and the retention time of the gaseous chlorine are associated with a lowering of the pH of the pulp to a value below 3, and preferably to a value between Z and Z _1, 5 and as soon as this pH is reached, one introduces chlorine dioxide.

In fact, the conventional bleaching processes include a chlorination step, which is carried out with a low concentration of the paper pulp ȴ / as the need to dilute the paper pulp at the beginning of the bleaching process and therefore at the time of thickening entails the need to discharge a substantial throughput of wastewater. The bleaching process reduced to 3 levels has the advantage "that it can be used without prior dilution, i.e. at an increased concentration of the paper pulp can.

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According to a further feature of the invention, the mixed chlorination phase is carried out at a concentration of the paper pulp between 3 and 20%, preferably between 9 and 11 % .

However, it must be underlined that this is not a necessity and that a bleaching process with a low concentration of the paper pulp always remains possible with the process under consideration here. However shows the possibility of working with a concentrated paper pulp a considerable advantage. Indeed, it allows implementation of bleaching according to the disclosed process under these conditions not only reduces the amount of contamination, but also a considerable reduction in volume.

It is even possible to use the waste water from the second stage when washing the unbleached paper pulp (and consequently in the waste heat boiler) due, especially if the latter is carried out with a peroxide, because of the very greatly reduced chlorine content.

It is also easily possible to use a washing process in the bleaching process to be carried out in direct countercurrent. It is found that in the examples given below it will be possible to use the waste water of bleaching in the order of 85% of the volume.

Finally, this procedure allows because of the returns effected in the three stage bleaching process, one of which, the final stage, is carried out at elevated temperature, a considerable reduction the thermal energy consumption by reducing the steam consumption (consequently of the heating oil or the heating gas that is necessary is to generate this steam).

909831/0778

After the first phase of mixed chlorination by gaseous chlorine and then by chlorine dioxide; after washing, in a second phase the pulp is treated in an alkaline medium in the presence of an oxidizing agent of the hypochlorite or peroxide type. This stage _s is applied to a pre-oxidized with oxygen (or another oxidizing reagent) pulp ,, whereupon in accordance with the method described above is chlorinated · ,, deteriorates the paper value _s if certain conditions are not respected. In particular, it is necessary to work at a relatively low temperature between 30 and 60 C _0, preferably between 40 and 50 C _0, and at a set pH between 8 and 12, preferably between 10 ^ 5 and H ₀ 5.

For this it is advantageous ^ alkaline waste water of the second. Phase of To use bleaching process for washing the paper pulp coming from the first phase (chlorination), which on the one hand results in a reduction of the externally supplied alkali, which is used for the setting the pH of the extraction phase is necessary, and on the other hand, a reduction in the throughput of the discharged waste water, the reoxidation of part of this waste water returned to the oxidizing extraction phase and, consequently, ultimately the reduction the pollution generated by bleaching.

After the second treatment phase, after washing, the paper pulp is subjected to a treatment step with chlorine dioxide at an elevated temperature between 60 and 90 ° C. “ preferably with reuse of the waste water from this step, which are reused as chemical reagents.

This time, the acidic waste water from the chlorine dioxide treatment is used to remove the alkali residue from the second phase of bleaching to reduce; the recirculation of this wastewater is said to be in the manner

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take place that one has a precisely defined pH value between 5 and 11, preferably between 7 and 9 »at the entrance of the third phase of the Bleaching.

The second and third phases of the bleaching process can either in conventional towers or using particularly advanced techniques, such as bleaching through preferential storage be performed.

The following examples are not intended to limit the invention carrying out the bleaching process described above for explain different types of pre-oxidized (with oxygen or with peroxide) paper pulps.

The invention is not limited to the specifically described embodiments limited, because various changes can be made here without departing from the scope of the invention.

example 1

Hardwood kraft pulp (oak, beech, various), pre-oxidized with pressurized oxygen, with a kappa value of 11.4 and comprising approximately 3% dissolved materials on the paper pulp (imperfect washing process) is carried out on the one hand by the conventional CEDED method and on the other hand by the bleached process according to the invention.

The main parameters of the bleaching process and the results are below specified.

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Conventional bleaching process CEDED

First stage: 40% C, 20 min

Concentration of the pulp: 3.5%% of the introduced Cl _9: 4.59

La

Second stage: 60 min, 40 ° C, 0.9% NaOH Third stage: 150 min, 70 ° C, 0.8% ClO ₉ Fourth stage: 60 min, 50 ° C, 0.4% NaOH Fifth stage: 180 min, 80 ° C, 0.4 % ClO

Whiteness: 90.1

Yield: 96.0%

Tear length at 40 ° SR (Schopper-Riegler)

(Jokro): 6.3 km
Tearing (tearing away) at 40 ° SR

(Jokro): 62
Burst at 40 ° SR

(Jokro): 36

Bleaching according to the invention (C + D) PD

First stage: C -fi D

Paper pulp concentration: 9 %

Temperature: 40 C

Duration of introduction of chlorine: C = 1 min

pH value at the end of the introduction of chlorine: 2.1

Total duration of introduction of chlorine and introduction

of chlorine dioxide C + D: 5 min

% of the Cl _{9 introduced} in relation to the paper pulp: 4.19 % of the ClO _{9 introduced} in relation to the paper pulp: 0.15 (or 0.4% calculated on the active chlorine)

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Second stage: P

Temperature: 50 C

Duration: 1 h

PH value:

"Jo vonH_O_ in relation to the paper pulp: 0.75

° fo of NaOH in relation to the weight of the paper: 0.9

Third stage: D

Temperature: 80 ° C Duration: 3 h

"Jo ClO" in relation to the pulp: 0.8 Initial pH: Whiteness: 90.1
Yield: 96.1 % tear length at 40 ° SR

(Jokro): 6.7 km tearing (tearing away) at 40 ° SR

(Jokro): 69 bursts at 40 ° SR (Jokro): 38

A result is obtained which is better than that according to the conventional method, namely with three stages as opposed to five stages in the known method, using a smaller amount of reagents, with a higher concentration of the paper pulp and with one Much shorter retention time for chlorination: 5 minutes in contrast to 20 minutes for the known method with only chlorine alone.

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Example 2

Hardwood - kraft pulp (oak, beech, various). pretreated with oxygen in such a way that the kappa value is reduced to 10.4 is bleached in three stages. The concentration of the paper pulp is 3.5%.

The paper pulp comprises approximately 1% dissolved materials, which from result from an imperfect washing process (industrial laundry).

The results given below emphasize the importance of the mix Level (C + D) in relation to the quality of the paper pulp compared to a blind test with chlorination only with chlorine.

C uring process CHD Bleaching process (C + D) HD

First level: level C conventional Mixed level 40 40 Temperature (C) 40 (C + D)
40 2 60 Duration C alone
(min) 30 ' 2 28 1.2 C + D (min) 28 1.81 0.9 % Ci ₂ 3.12 2.66 1.0 11.5 % ClO ₂ - 0.35 2.2 pH value (after the Ein
guidance of chlorine) 2.2 Second step Temperature (C) 40 40 Duration (min) 60 60 % Hypochlorite NaOCl 1.8 1.4 % NaOH 1.0 0.9 PH value 11.6 11.4

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Third step
temperature Level C conventional
80 Mixed
(C ₊ D)
80 Step .
80 Duration (min) 180 180 180 % ci / ₂ 0.8 0.8 0.7 initial pH value 8.3 8.3 White ISO 88.8 89.1 88.8 Degree of polymerization
(DP) 650 820 875

In the blank test, in which the chlorination was carried out only with chlorine and without chlorine dioxide, the paper pulp obtained is of poor quality (low degree of polymerization). To obtain the same whiteness or the same White _s it is therefore necessary to provide a great use of soda and hypochlorite.

Example 3

'Hardwood - kraft pulp (oak "beech) with a kappa value 18 is pre-oxidized with hydrogen peroxide under the following conditions:

Preoxidation temperature: 70 ° C. Preoxidation duration: 2 h Reagents: NaOH = 0.8% / paper pulp H_O = 0.25%. / Paper pulp

This pre-oxidized paper pulp is then bleached according to the disclosed method. The results obtained are as follows:

I am in progress (C + D) HD first stage

Temperature (C) 40

Duration C (min) 2

C + D (min) 30

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909831/0770 ORIGINAL INSPECTED

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% ClO ₂ O _O 25 PH value 2.2 Second step Temperature (C) 40 Duration (min) 60 % NaOCl 1.5 % NaOH 0.9 PH value 10 ₀ 8 Third step temperature 80 Duration (min) 180 % ClO ₂ O ₀ 8 pH starting value 8th White 89 _a 6 D. P. 1010

The method of the invention allows, _s to obtain a bleaching process for the pre-oxidized with peroxides pulps comparable to those pulps which are pre-oxidized with pressurized oxygen having a polymerization degree which is slightly better.

Example 4

Kraft pulp from various deciduous trees, before oxidized with oxygen, a kappa value of 11 and comprising a further 3% dissolved materials, which were not removed in the previous washing , is processed in the usual way and according to the previously described method with two variants (C + D ) HD and (C + D) PD bleached.

The results shown below that the results _s (C + D) and HD (C + D) PD due to the inventive processes B Ie I obtained are better than those obtained by the conventional method.

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(C + D) PD

Sequence

C EDED

(C-I-B) HD

40 ° C

C = 2 min% Cl, remainder

C + D = 30 min

PH value

3.2 0.35

O ₅ 58

2.2

3.2
0 "35

0.5

2.15

3.2

0.35

0.5

2.2

E or

P or

1 h 15 min

40 ° C

% NaOH

% Hypochlorite

% H ₂ O ₂

% Leftovers

PH value

I ₅ O

11.8 0.9
1.5
0

O ₉ 04
11.7

0.9

0.6

Lanes 11.6

2 h 30 min

70 ° C

% ₂

% Cl ₂ react

PH value

0.8 0.49

2.9

1 h 50 min

50 ° C

% NaOH pH

0.4 11.5

D.
3 h
80 ° C

% Cl, remainder

Lt

pH start value pH end value

0.4 0.39

4.1 0.8

0.53

7.8

3.7

0.8 0.52 8.1 3.8

White

Yield %

D. P. (Polymer is ations gr ad)

90.2 96.6 740 89.5
97.8
750

90.0 97.7 740

uC (mixture of Cl and ClO)

This example proves that the hypochlorite oxidizing agent and peroxides are interchangeable. According to the invention, a better yield is obtained with a smaller amount of reagents.

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Example 5

Calcium bisuir-dtaeilstoff obtained from resinous HOIS (Tann, spruce), pre-oxidized with oxygen under pressure and do it a kappa value of 6.2 by the. ⁵ billion oxidation by the process of the bleached He »invention. You get:

First stage 2.1 % ci ₂ 0.15 % ClO 30 ° C Temperature (C) 2 Duration C (min) 25th Duration C + D (rain) 2.4 PH value Second step 1.0 % NaOCl 1.0 % NaOH 11.6 PH value 50 Temperature (C) 40 Duration (min) Third step 0.6 % ClO ₂ 80 Temperature (C) 210 Duration (min) 8.8 initial pH value 92 ₅ 5 White D. P. Polymerization 954 Degree

0.15 (or 0.4 % " based on chlorine)

This example shows that the present method can also be used for calcium BisulfdtssllstofT obtained from resinous woods, is applicable.

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Example 6

Hardwood kraft pulp (oak, beech, various), pre-oxidized with oxygen and a kappa value of 10 is carried out according to a method according to the invention and according to a classical method bleached. The results given below were obtained by also considering the effectiveness of the Washing process of the paper pulp before bleaching has varied. The results particularly show the advantage of Bleaching process according to the invention with regard to the contamination, regardless of the effectiveness of the washing process Paper pulp.

909831/0770

• sequence conditions - dissolved residual material (C + D) HD 0.5 1 2 4.5 CEDED 0.5 1 I. 4.5 40 ° C at the beginning of the bleaching process
ganges (% / paper mass) 2.5 2.7 3.2 4.3 2.8 3.0 I.
\ 2 4.6 (C + D) HD duration % Cl_ introduced 0.25 0.25 0.25 0.25 0 0 3.5 0 C = 2 min
C + D = 30 min ⁰ Jo ClO ₂ introduced / ^ 0.43 0.47 0.49 0.40 0.37 0.35 0 0.47 CEDED
Duration C = 30 min / 0 win XV.C9L 2.2 2.2 2.2 2.2 2.1 2.1 0.44 2 ₆ 1 1 h PH value 0.9 0.9 0.9 0.9 0.8 0.8 2.1 0.8 40 ° C % NaOH 1.5 1.5 1.5 1.5 0 0 0.8 0 % Hypochlorite 0.44 0.45 0.46 0.46 0 % Cl ₂ remainder 11.6 11.6 11.7 11.6 11.5 11.5 11.5 2 h 30 min PH value 0.8 0.8 11.4 0.8- CO 70 ° C % ClO ₂ introduced 0 _s 25 0.23 0.8 0.23 O
CS % Cl remainder 3.5 3.5 0.24 3.4 OO 1 h PH value 0.4 0.4 3 _S 4 0.4 50 ° C % NaOH 11.5 11.4 0 _s 4 11.4 σ 3 h PH value 0.8 0.8 0.8 0.8 0.4 0.4 11 _S 4 0.4! -j: 80 ° C % ClO ₂ introduced 0.36 0.38 0.38 0.37 0.29 0.31 0.4 0.31 I. ο % Cl ₂ remainder 3.5 3.6 3.6 3.6 3.1 3.2 0.30 3p 2 9 pH final value 7.7 8.1 8.1 8.1 3.2 initial pH value 88.9 88.5 88.5 88.5 88.8 88.7 88 _S 9 I. white 96.5 96.3 96.6 96.0 96.4 96.0 88.8 95.5 Yield % 754 763 736 736 673 645 96.3 700 I. D.P. (degree of polymerization) 673 Index points blackness 0.7 0.6 0.7 0.85 0.7 0.9 0.6 (Purity) 0.8

CD O GO

co

co

approx

sequence 0.5 (c +: D) HD 4.5 0.5 CEDED 2 4.5 dissolved residual material
Conditions at the beginning of the bleaching process
ganges (% / paper mass) 1 2 1 Waste water pollution colour 387.5 nm 3.2 13.1 14.1 16.1 22.6 kg Pt / t pb AD (kg platinum per t bleached, air
dried paper pulp) 5.0 4.3 6.4 27.9 28.7 15.1 35.7 51.3 465 nm
kg Pt / t pb AD! 8.8 13.4 31.5 DCO (chemical oxygen demand) / 20.2 35.5 27.9 36 48.8 kgO "/ t pb AD 23 27.2 32.4 DBO_ (biological oxygen demand) 7.2 12.6 9.2 11.6 14.5 kg O_ / t pb AD 7.0 9.5 9.6

/% \ introduced 2 min after the chlorine (stage C + D)

This example shows that the method of the invention eliminates pollution allowed to decrease (color, DCO and DBO decreased with respect to the conventional Procedure CEDED).

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Example 7

Hardwood - kraft pulp (oak, beech)., Beforehand with Oxygen is pretreated to a kappa number of 11 accordingly bleached according to the method of the invention, but the reuse of the laundry after the second stage varies will. The results given below demonstrate the importance of employing recirculation of the waste water from the third stage for washing the pulp at the exit of the second stage.

First stage

(C + D): retention time Cl = 2 min

Retention time ClO ₂ = 28 min. Temperature: 40 _{° C% Cl 2} -4.1. % ClO ₂ : 0.4

Second step

H: 40 ° C, 1 h, pH 11.4% NaOCl: 1.5 % NaOH: 0.9

Third step

D: chlorine dioxide, 80 ° C, 3 h% ClO ₂ : 0.8

with a wash after the second stage: water / filtrates of the third

Stage (variable throughput for regulating the remaining alkali that is fed into the subsequent stage of the Bleaching process occurs).

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Regulation of repatriation

Residual alkali at the entrance to the

3rd stage (% NaOH / paper pulp) 0 0.3 0.6 1.0

10 11.5

88.7 85

pH initial worth
3rd stage) (at the entry the 4th 8th White in the end 88 89.1 example 8th

The bleaching process of hardwood - kraft pulp (oak, beech, various), pre-oxidized with oxygen, is continuous with a hourly throughput of 20 t of the paper pulp in a conventional Bleaching operation carried out. The properties of the paper pulp are:

Degree of whiteness = 89 D.P. = 828

The system is changed in such a way that a method according to the invention is used, where in stage (C + D) = 2.0% Cl- with a retention time of 2 minutes, 0.4% ClO ₂ with a retention time of 28 min, is used, the temperature being adjusted to 37 ° C. and the pH value to 2.5. Stage H is carried out at a pH of 10.5 with 1.5% sodium hypochlorite for 1 hour at 40.degree. Stage D is carried out at 80 ° C. for 3 hours with 0.8% ClO and an initial pH of 7.9. The wastewaters are used as reagents

* used again.

The paper pulp produced has a whiteness of 89 and a degree of polymerization D.P. of 827. The characteristic impurities of the waste water during the bleaching process are now the following in the classical method and in the method according to the invention.

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Paper pulp: hardwood kraft pulp (beech, oak, etc.) conventional (C + D) HD Preoxidation: oxygen under pressure, 115 ° C, 7.5 bar, 45 min
Final kappa value: 11 4.5
37.5
13th
40 1.2
9
5
7th Bleaching process Dissolved materials at the beginning
of the bleaching process,% / paper -
Dimensions
Wastewater pollution
From the BI I pass:
Color: 387.5 nm
kg / t of the paper pulp
DBO_ kg / t of the paper pulp
DCO kg / t of the paper pulp

Example 9

Comparison attempts

Hardwood kraft pulp (oak, beech, various), pretreated with oxygen in such a way that the kappa value is reduced to 10.4 is bleached in 3 stages. The paper pulp also comprises 1% of dissolved residual materials from an incomplete wash (industrial laundry).

The results given below illustrate the importance of the mixed stage and recirculations. It should be mentioned in particular that the sequence in which chlorine and chlorine dioxide are added has only a limited influence on the degree of whiteness or the degree of polymerization which is achieved at the end of the bleaching process, but this sequence is particularly important for the quantities of chemical products necessary to achieve these results (reduced consumption of reagents in the case of the present invention). This is expressed in the fact that adding chlorine in the first place to an industrial Kraft paper pulp or to an industrial Kraft pulp that is incompletely washed (i.e. alkaline) _s a reduction in the pH value below 3 (2 to 2, 5), which enables the chlorine dioxide to react particularly effectively at the beginning of the bleaching process. At the end of the bleaching process (third stage) it is weeendes

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In contrast, the low residual lignin content is necessary to raise the pH value again (pH 7-9) for a better effect of the chlorine dioxide, which the recirculations reveal.

Hardwood kraft pulp (paper pulp) (oak, beech, various ), pretreated

1st stage

Temperature (C)
% Cl- introduced

Lt

% ClO- introduced

Reagent supplied
on 1. Job

Retention time of the 1st reagent alone
total retention time

pH before adding the

2. reagent

CHD

40 3.12

^C1 2

30 min 30 min

(D + C) HD

xx)

40 2.66 0.35

ClO ₂

2 min 30 min

4.3

(C + D) HD with recirculation (method according to the invention

40 2.66 0.35

^C1 2

2 min 30 min 2.1

2nd stage

Temperature (C)
duration

% Sodium hypochlorite
(in introduced Cl ")

% NaOH
PH value
White

40

60 min

1.8

1.3 11.6 78

40

60 min

1.7

1.2 11.5 78

40

60 min

1.4

0.9 11.4 78

3rd stage

pH initial worth

Temperature (° C)

duration

% ClO introduced

9.2 80 180 min

0.8 88.8 650

9.4 80 180 min

0.9 88.7

780

7.7 80 min

0.8 89.1 82 * 0

Comparative experiment only with chlorine without chlorine dioxide in the 1st Step.

Comparative Experiment with chlorine dioxide fed in the first place and then chlorine in the first stage.

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In summary, it is stated that according to the method, the bleaching of the pre-oxidized paper pulp (paper pulp) in three Phases is carried out: A first phase of mixed chlorination by introducing chlorine followed by the addition of chlorine dioxide, a second treatment phase in an alkaline environment if present an oxidizing agent and a third phase with chlorine dioxide.

Such a process allows the number of bleaching stages required for bleaching the pre-oxidized paper pulp to a high degree of whiteness; reduce pollution and to reduce the amount of reagents consumed and thereby to obtain a good quality paper pulp.

909831/0770

Claims (9)

Patent claims 1. High whiteness anti-pollutant bleaching process that works for everyone pre-oxidized in a conventional manner, for example with a peroxide or with pressurized oxygen in an alkaline medium Chemical pulping is applicable, characterized in that it essentially comprises the three treatment phases: a) a first bleaching phase consists of a mixed chlorination of the pre-oxidized paper mass by introducing gaseous material Chlorine followed by the addition of chlorine dioxide without intermediate washing, the gaseous chlorine used in an amount which is lower than the chlorine requirement of the paper pulp (pulp) and where the complement (the missing remainder) comes through Chlorine dioxide is supplied; b) the second phase of bleaching consists of an alkaline treatment Environment at a pH between 8 and 12, preferably between 10.5 and 11.5, at a low temperature between 30 and 60 C, preferably between 40 and 50 C, in the presence of an oxidizing agent of the hypochlorite or peroxide type; c) a third phase of the bleaching process is carried out with chlorine dioxide at a temperature between 60 and 90.degree. 2. The method according to claim 1, dadurchgekennzeich.net that the first The mixed chlorination phase is carried out at a temperature between and 80.degree. C., preferably between 30.degree. And 50.degree will. 909831/0770 3. The method according to claim 1 or 2, characterized in that for the first phase of chlorination the introduction of the gaseous chlorine into the suspension of the paper pulp to be bleached a short retention time of 10 seconds to 5 minutes, preferably 1 to 2 minutes, and the addition of the chlorine dioxide solution a retention time of 3 to 180 minutes, preferably of 6 to 40 min. 4. The method according to claim 1, characterized in that the retention time of gaseous chlorine, a reduction in the pH of the paper pulp to a value below 3, preferably between 2 and 2, 5, corresponds to what as soon as this is reached pH value the chlorine dioxide solution is supplied. 5. The method according to any one of claims 1 to 4, characterized in that that the gaseous chlorine used in the first bleaching phase corresponds to 60 to 90% of the total amount of chlorine plus introduced Chlorine dioxide, calculated as chlorine equivalent. 6. The method according to one of claims 1 to 5, characterized in that that the first phase (chlorination) is carried out at a concentration of the paper pulp (pulp) of 3 to 20%, preferably 9 to 11% will. 7. The method according to any one of claims 1 to 6, characterized in that that the waste water from the second bleaching phase is used to neutralize the paper pulp coming from the first bleaching phase will. 909831/0770 8. The method according to claim 7, characterized in that in the event a second bleaching phase with peroxide, the excess wastewater is returned to the washing of the raw paper pulp, in such a way, that this can be burned with the residual liquid from the cooking process. 9. The method according to claim 1, characterized in that the third Bleaching phase is carried out with chlorine dioxide, at a temperature between 60 and 90 C and an initial pH value from 5 to 11, preferably from 7 to 9, obtained by recycling waste water from this third bleaching phase to the "pulp laundering" (Pulp) is regulated, which comes from the second phase. 909831/0776