DE2202662A1 - Process for reducing the colored waste water from a multi-stage bleaching process - Google Patents

Description

Dipl. H.g. H. Wsickrnans, Pipl. Phys. Or. K. Fincke

HOOKER CHEMICAL CORPORATION ⁰¹ P ¹ ■ ^ln 9 Ρ.Λ Weickmsrn, DipUiwm. B. Huber _Iiia: ara Falls, NY 14302 8 KfincHen 27 Möhlstr. 22 2202662 .St.A. 2683

Process for reducing: the colored waste water of a multi- stage bleaching process.

The industrial wastewater poses a purification problem represents which of the problems of urban sewage treatment is completely different. The sewage streams of each one technical processes are different depending on the type of process and require special work-up. In general is the neutralization of acidic or alkaline wastewater up to a pH of about 6-9, which is necessary for control The water contamination is particularly important, easily caused by direct chemical treatment. The distance, dissolution or passivation of specific impurities is a more difficult problem to solve, which is often a combination of physical, requires biochemical and chemical treatments. The problem of paint contamination of the surface water is particularly agonizing to prevent into which the industrial effluents are drained. The paint pollution of surface water poses both poses a visible aesthetic problem as well as a problem with regard to the changed light penetration of surface water.

In papermaking, the brown color of the wastewater essentially results from extracted tannins, lignins and their derivatives, which are removed from the cellulose pulp by bags after various special chemical treatment stages. The mass of the colored material is removed during cellulose pulp production in the alkaline extraction stage, the previously dissolved non-cellulose constituents of the pulp being separated from the cellulose material. Lignin and its chlorinated and oxidized derivatives are very resistant to microbiological degradation; the such lignin extracts own color happens the worth en biological treatment? - "tp.tioron to 7 :: r :. ^ bl. ^ nsen in ei- · - Obori'l ^ honvrisser. Lie caoaisc ¹ " · treatment of the dyes It is true that in the wastewater from paper manufacture is a very effective method of reducing the dye content; economically, however, these systems are not very efficient

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and therefore impractical in modern technology. Therefore, chemical removal of colored materials is usually not applied to paper-making wastewater. The use of active sludge can be used to remove around 10-15 % of the color in the wastewater from a paper mill and its bleaching equipment. In this way, about 85-90 % of the dyed material from a paper mill or from its bleaching plants run through the biochemical degradation stage until it is drained into the surface water,

The purification of wastewater from paper mills is a special one The problem of wastewater treatment is because this wastewater uses the usual known wastewater treatment methods as they are nowadays used to purify urban sewage are less accessible. As the wastewater from paper mills traces of metals and chemical compounds that are resistant to biological degradation, one tries To reduce water pollution primarily by chemically treating the pulp in order to obtain the desired product. Any change in the implementation of the pulp treatment that results in wastewater with fewer impurities is reduced the requirements for wastewater treatment before discharging the wastewater into the surface water. With this chemical Bleaching method, various problems arise in combination, namely the economy, the effectiveness of the chemical treatment; furthermore, the bleached product must have the desired properties which are required by the paper industry. Therefore, this procedure is critical to any active water pollution control program. Changes to the Chemical treatment of the porridge, though, cannot be viewed as a panacea for lessening water pollution will; however, substantial improvements in the paper mill bleaching process are important to achieve the desired goals.

A common bleaching process, which is called a multi-stage bleaching process is used for softwood pulp, consists in that the cellulose pulp is first treated with chlorine, in front of the second stage is alkaline extraction and the third stage is kypochlorite bleaching and the fourth stage is one Chlorine dioxide bleach follows. This multi-step bleaching process

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_ 3 -

Commonly referred to as CEETD, these letters mean chlorination, extraction, hypochlorite treatment, and chlorine dioxide treatment. The extraction stage of this bleaching process is responsible for around 88 % of the total color produced by this multi-stage bleaching process. If the CEHD bleaching process is used on a laboratory scale for softwood pulp, it can be seen that the amount of dyes in the bleaching liquid after each treatment stage is as follows (based on the volume): about 332 after the chlorination, 2,953 after the extraction, 23 after the hypochlorite treatment and 39 after the chlorine dioxide application; the total amount of dyes in the effluent from the bleaching process is 3.34-7 ppm.

The wastewater from a softwood bleaching process is true stained much more strongly than the wastewater when bleaching hardwood by the same process; but the wastewater is in both cases to complain. It has been found that using the CEHD bleaching process on a hardwood pulp after each treatment step the following dye impurities found in wastewater (based on volume): about 227 after chlorination, 809 after Extraction, 27 after the hypochlorite treatment and 76 after the Chlorine dioxide application; the total content of colorants in the wastewater is 1.141 ppm in this bleaching process.

State and state efforts to control the coloration of paper mill effluents have become a standard the water quality of wastewater, with about 50 ppm APHA (American Public Health Association) being envisaged as a short-term target became. Therefore, any contribution that supports the achievement of this goal is an essential advance in the control of Impurities.

The present invention relates to a multi-stage bleaching process for cellulose pulp, wherein in the first stage the non-cellulosic material contained in the pulp is solubilized: then the usual second stage of alkaline extraction can be omitted or with the following hypochlorite bleaching; can be combined, and you get a waste water, the dye content of which is about 50-90 % less than the usual alkaline S>: tral: l-ion

of solubilized non-cellulosic materials.

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The process according to the invention for bleaching a cellulose pulp consists in treating this pulp in aqueous suspension with chlorine or chlorine dioxide in order to make the non-cellulose materials contained in the cellulose pulp soluble, after which the pulp (corresponding to a 3-15 wt .- # igen hypochlorination) 15-180 minutes with about 0.5-3.0 % hypochlorite (based on the dry weight of the pulp) at a temperature of about 24-71 ⁰ C at a pH of about 7-9 .

It has surprisingly been found that not only the color of the waste water in the second stage hypochlorite bleaching of a multistage bleaching process of CsLulosebrei is pH-dependent, but also the amount of colored material formed depends on the pH. Accordingly, the color of the effluents from a conventional CEHD bleaching process can be reduced by about 50-90 % if the alkaline extraction step is omitted and the pH of the hypochlorite step is kept at about 7-9. The pH-control in the hypochlorite bleach can be obtained by addition of alkali or by use of a buffer can be effected, wherein the pH value to 7-9, preferably 7.5-8.5, in particular 7 »8-8.2 set will. The use of a buffer system to control the pH during hypochlorite bleaching is preferred over the direct addition of alkali to adjust the pH because the reduction in dye content is much greater with the use of a buffer than with the pH control with one alkaline reagent.

A recent innovation in the field of multi-stage cellulose pulp bleaching is based on the discovery of Jack et al. (U.S. Patent 3,433,702); Here, in the first stage, the bleaching is carried out using chlorine dioxide and then with Chlorine carried out, whereupon in the second stage the alkaline extraction, in the 3rd stage the hypochlorite bleaching and in the 4th stage the chlorine dioxide treatment follows. This bleaching process is referred to as D EHD. V / ird according to the process according to the invention In the D EHD method, the alkaline extraction of the - ?. step omitted and the hypochlorite bleaching carried out at controlled pH levels, the amount of dyes in the wastewater is reduced

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reduced in the same way as with the CEHD method.

To control the pH value in the hypochlorite bleach of the Any alkaline material can be used in the process of the invention use which does not interfere with the hypochlorite bleach, e.g. the alkaline earth hydroxides and alkali hydroxides. Sodium hydroxide is preferred, because it is easily available and cheap.

Similarly, any buffer system can be used which is inert to the reaction components present in hypochlorite bleaching and which sets a pH of about 7-9, e.g. buffer systems made of acid potassium phosphate / disodium phosphate, acid potassium phosphate / sodium hydroxide, boric acid / borax, borax , Boric acid / sodium hydroxide, furthermore water-soluble alkali and alkaline earth silicates. Preferred buffers are the water-soluble alkali silicates of the formula Na ₂ O (SiO ₂ ). ^ Where x = 2-4-. The amount of water-soluble silicate added during hypochlorite bleaching is about 0.5-5 # »with smaller amounts of water-soluble silicate at lower final pH values in the hypochlorite bleaching stage and lower amounts of dye (in ppm) in the waste water of the hypochlorite -Bleach lead. . Preferably, the sodium silicate used as a buffer in an amount of about 0.5-1 wt -% (based on the dry weight of the treated slurry} A buffer needs not only during the hypochlorite bleach used to be, but rather can be together with the Hypochlorite introduce a small amount of sodium hydroxide during the bleaching reaction. However, adding sodium hydroxide together with the sodium silicate buffer in the hypochlorite bleaching stage will raise the final pH of the pulp, thus increasing the amount of colored materials in the wastewater from the hypochlorination stage If during the hypochlorination an increasing amount of sodium hydroxide is present together with the silicate buffer, the brightness of the end product increases slightly and the reversion decreases after 18 hours as the concentration of sodium hydroxide increases a silicate buffer is made up of sodium hydroxide concentration rations of 0.5-2 ° / o (based on weight of the treated pulp aas) is not affected.

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The greatest reduction in the dye content of wastewater the alkaline extraction stage of a multi-stage bleaching process results from the treatment carried out in the 1st stage with chlorine dioxide / chlorine, followed by a hypochlorite bleach according to the invention under controlled pH conditions. To reduce the production of colored materials in the wastewater of a Papielmühlen bleaching in the 1st stage two consecutive bleaching steps should be carried out, while the second step is hypochlorite bleaching the qoptimal ratio of chlorine dioxide to chlorine in the 1st stage about 1: 2 to 2: 1, expediently 1: 1.

The temperatures during the multi-stage bleaching process according to the invention are as usual. Thus, the temperature range of 31eichbehandlung the 1st stage may be between about 24-'and 38 ° C, but no substantial change in effect is observed at temperatures of up to 49-60 ⁰ C during treatment of the cellulose pulp with chlorine dioxide and chlorine. Similarly, the hypochlorite bleaching to be carried out in the 2nd stage can be carried out at temperatures of about 24-4-9 ° C, but without significant changes occurring at temperatures up to 71 ° C.

The application time for each stage of the multistage process according to the invention is also as usual. The optimal time for hypochlorite treatment under controlled pH conditions is approximately 60 minutes. However, the hypochlorite treatment can carried out within a period of 15-180 minutes, the results depending on the type of paste treated and change a little depending on the conditions used in the 1st stage. Generally a treatment time of 30-90 minutes provides satisfactory results.

The buffered hypochlorite bleaching of the 2nd stage of the multi-stage The bleaching process is applicable to both hardwood and softwood pulps. The greatest reduction in dyes joins when using buffered hypochlorite bleach Softwood pulp, which after the sulfate (Kraft) or SuIfit-

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Process was established. This dye reduction is particularly noticeable because soft pulp has the largest amount of it. colored sewage supplies. When using the buffered hypochlorite bleach according to the invention, the reduction in the dyes in a Kraft pulp compared to a conventional alkaline extraction stage is 70-90 % \ the reduction in the colored waste water in a Harthol-B3. see under analogous conditions is about 50-70 %.

Another advantage of using the buffered hypochlorite bleach according to the invention instead of the usual alkaline extraction is an increased slurry yield. If one compares the multi-stage bleaching processes CEHI) and D _rt HD, one finds that the pulp yield in the latter case has increased by 0.5 % for softwood pulp and by 2.56 % for hardwood pulp. This increase in yield represents the average of 5 independent tests with different softwood and hardwood pulp samples.

substantial and means a decisive advantage over the usual CEHD procedure.

The invention is explained in more detail in the following examples. Here, the dye content of the wastewater from a customary multi-stage CEHD bleaching process under the precise conditions of a paper mill is compared with the wastewater from the process according to the invention. All laboratory bleaching methods are carried out in the usual way. The physical and chemical properties of the unbleached and bleached pulp are determined according to the following! Standard methods determined:
ΐΑΡΡΙ standards and other methods

Formation of handsheets for optical test with the pulp - T218-m59 Conditioning of paper and cardboard for examination - T4-O2-m4-9 Brightness of Porridge - Elrepho Instrument - T217-m48 Cupric Ethylenediamine Dispersed Viscosity of Porridge - T230-mu63 Hyponummer-McLean, J.D. Pulp and Paper Magazine of Canada, 66, no. T1O5-T1O6 (1965)

Abwaeser staining Hach colorimeter.

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The color of the wastewater obtained from the multi-stage bleaching process is determined by the following methods:

1. After completing the special stage, the porridge will be under • Using a Buchner funnel from the aqueous solution

severed.

2. The volume of the undiluted waste water is determined. 3 · The pH is adjusted to 7 »O.

4. The sample is placed in a Hach colorimeter and set determines whether one can peel off directly or whether it has to be thinned.

5- The color is read from the Hach colorimeter, it is indicated directly in ppm.

6. If necessary, the dilution is corrected.

7 · The color is given in ppm for the specific stage.

In the following tables, the color of the wastewater is given in ppm as a direct color value for each example, corrected depending on the consistency of the pulp treated and depending on the Volume differences between the various stages. The total additive color number (corrected for volume) is also given. The values for total chlorine, COD (chemical oxygen demand) and solids were determined in the effluents of each stage and the total number was determined in a manner similar to that Number for the total color. The usual analytical methods are used to determine the chlorides, COD and solids.

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example 1

A sample of pitch pine and spruce pulp with a hypo number of 5 »6 and a viscosity of 30.7 centipoises becomes the usual Subjected to CEHD bleaching processes under the following conditions:

Symbol level> chemical at- time tempera- porridge consi-

sentence (# related (min.) door (C) stenz (#) on pulp weight)

C chlorination 6.5 60 38 3

E extracts 2.0 60 71 10

H hypochlorination 1.15 60 46 10

D chlorine dioxide 0.45 180 71 3 ⁺

I t

⁺ Consistency 3 has been chosen for ease of use in the laboratory

, chosen. The results obtained are those with a consistency; of 10-12 % equivalent.

I The volume and the color of the wastewater not diluted with washing water I of each stage are determined, also the brightness, reversion (1 and 18 hours at 105 ° C) and the viscosity. This ver! drive with a sample of spruce pulp is used as a control.

\ The obtained with this control results are given in the \ Table 1 below.

The wastewater from the CEHD process also contains 719 ppm Chlorides, 1854 ppm solids; COD 813 ppm.

ι table

Bleaching effect and waste water color in softwood pulp according to the CEHD method

step

E.
H
D.

Temperature C Time, minutes Consistency % 38 60 3 71 60 10 46 60 10 71 180 3

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-JO-

Bleach level

Cheraikalzusatζ (weight color (ppm)

Colorimeter reading

Corrected consistency. Corrected volume

Brightness (#)
Final value
reversion

Λ-1 hour
Δ -18 hours

Viscosity (centipoise)

C.
6.5 E.
2.0 H / NaOH
1.15 / 0.5 2202662
D.
0.45 Total Vol. 850
850
332 27,000
7,560
2.953 210
59
23 100
100
39 Corr. 3.347

90.5

4.2 8.5

9.9.

example

In this comparative example, a sample of the pulp used in Example 1 is used, but which is subjected to the _{CH 3 D bleaching process according to the invention.} It should be noted that the reference to the multi-stage bleaching process CH_D relates to the following bleaching stages: chlorination, hypochlorite treatment with silicate buffered, and finally chlorine dioxide bleaching.

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Tabel

S. level 8th D. S. D. D. step Bleaching effect and paint color on softwood pulp after this # pH 7 60 volume 3 - • 86.6 6.6 1 Viscose eat NaOH C. NaOH σ σ CH D method with sodium silicate buffer in the hypochlorination 60 corrected 10 Brightness % 4-18 hours activity
Centipoise ο H H
S. H step 180 450 3 Reversior. OO WaOH D ^s 1.025 Color ppm 404 Final value 4-I St. co
| v "V σ Chem. Temperature ⁰ C time, minutes, consistency % 3.290 consistency 88.0 8.0, ·, VN /
'ν. H additive 38 corrected -Jk / 1-3 1.025 35 TOTAL VOL, 14.2 15.2 O 71 80 921 450 OO 6.5 1,025 336 84.6 7.3 2.0 2.740 0.5 80 . CORR. 889 1.) 1,025 31 TOTAL VOL. 14.3 15. q 0.5 . 70 767 450 6.5 1,025 103 2.5 840 ¹ IO 0.5 ' 70 118 TOTAL VOL. , KbRR. 817 ", 14.0 CP3.0 1.1 UO. 1,025 N) ' 0.5 235 O> 6.5 • NJ 3-0.
0.5 UO- CORR. 601 0.9 ORK 0.5 GI ϊ- 1 m
O

example

In this example, a multi-step bleaching process according to the present invention (D "H_D) is described. The bleaching stages consist of a first stage in which bleaching is carried out with chlorine dioxide and chlorine in succession second hypochlorite A stage with buffering with silicate and one third chlorine dioxide bleaching stage. A sample of the pulp according to Example 1 is used. That obtained from this bleaching process Wastewater contains a total of 613 ppm chlorides, 1430 ppm solids and 768 ppm COD.

b e 1 1 e 3 49 Ä
⁰ C 91 2 2 Time, minutes consistency % T a Bleaching effect and wastewater color with softwood pulp after 49 5 3 D ^ H "D ⁺ -Process with sodium silicate buffer in the Hypochlorierunsrs- 46 5, 1 25th step. 71 10, 7th 60 10 ^{D /} c 17, 9 180 3 Level temperature ^{H /} s / Na0H ^D
, 0 / 0.5 / 0.68 0.5 D. 1.24 / 3.25 C. 600 100 950 168 100 D. 950 73 44 Total Vol. Corr. 5? 4 Bleach level 417 Chemicals
additive (wt .- #) Color ppm Colorimeter
Reading consistency
corrected volume
corrected Brightness (#) Viscosity (centipoise) Final value reversion A -1 hour Δ-18 hours

⁺ Dc ratio 50/50

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The data presented in the above three examples can be summarized as follows for comparison purposes:

Weic hhoL · (spruce) process

sewage ^D c ^H s ^D a • 3.25 CH ₃ D CEHD Total 91.2 0.98 Color (ppm) 534 2.0 601 3347 Chlorides (ppm) 613 5.1 1.74 - 719 COD (ppm) 768 10.7 0.5 813 Pesticides (ppm) 1430 17.9 - 1854 Bleached porridge Brightness (#) Final value 8.8.0 90.5 reversion Δ -1 hour 8.0 4.2 Δ - 18 hours 14.0 8.5 Viscosity (cp) 13.0 9.9 Chemical additive (#) Ci ₂ 6.5 6.5 NaOH 1.2 2.8 NaOCl 3.0 1.15 ClO ₂ 0.5 0.45 Na ₀ SiO * 0.5 -

As can be seen, the total amount of dyes in the wastewater in the various bleaching processes is substantially reduced if the alkaline extraction step is omitted and the hypochlorite bleaching step is carried out under controlled acidity conditions. The reduction in the dye content in the waste water is around 70-90 % compared to a bleaching process with an alkaline extraction stage. Other obvious advantages of using a hypochlorite bleaching stage under controlled acidity conditions rather than an alkaline extraction stage are that the wastewater contains fewer chloride ions than in the CEHD process. A.ußerden are the erfindungsgeraäßen multi-stage bleaching process 17 /> less fresh water required as the extraction stage of OEHD process and the associated wash cycles eliminated. As a result, one obtains

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17 % less wastewater using the improved process of the present invention. A further advantage is based on the fact that 30 % less steam is required in the three-stage bleaching process using a buffered hypochlorite bleaching stage compared to the 4-stage CEHD process; This is because not only is the number of stages reduced, but the temperature requirements for the buffered hypochlorite bleaching process are lower than for the alkaline extraction. Obviously, the reduction in the number of stages required to achieve the desired bleaching effect (3 stages in the present invention, 4 stages in the CEHD process) is a decisive advantage; the process according to the invention also gives a better quality of pulp, as can be seen from the viscosity data in the table above. .

The finding that instead of an alkaline extraction stage, hypochlorite bleaching under conditions is more controlled Can use acidity is all the more surprising because the usual hypochlorite bleaching processes at very high pH values (generally 10-12) can be carried out; this is mostly seen in the literature as a very desirable, if not even critical Reaction parameters for the hypochlorite treatment of cellulose pulp specified.

Example 4

The effect of the sodium hydroxide concentration on it wildly pH control in the hypochlorite bleaching of the D H D process determined at constant silicate content. It turns out that the amount of dyes formed during the hypochlorination through Control of the pH can be decreased. The viscosity also increases slightly with increasing pH. The change in brightness in the 18 hour reversion, the pH value decreases. The final brightness is essentially equivalent.

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Tabel

OO Is?

Level D

c H

s D

NaOH

0.5 1.0 2.0

Effect of the NaOH concentration in the hypochlorination of a DHD process (softwood pulp, constant silicate content;

concentration

D _c ratio

1.24 ClO ₂ 50/50 587 3.25 Cl ₂ 673 2.0 NaOCl 744 0.5 Na ₂ SiO ₅ 0.5 cio ₂ Final pH Total paraffin (ppm) 7.0 9.1 11.0

Temperature C Time i Minutes consistency % 49 VJl 3 49 25th 46 60 10

180

Brightness $ final value Δ -18 hours rev.

88.3 12.4

89.8 9.6

90.8 7.8

viscosity
(Centipcfee)

17.8

19.7
20.0

CD

CD CO

ro

example

The effect of the silicate concentration with pH control in the hypochlorite bleaching of a D HD process is determined, namely with softwood pulp and with a constant sodium hydroxide content, both with the gradual addition of sodium hydroxide. Si} icate and hypochlorite, as well as when adding a previously prepared mixture of these reaction components to the hypochlorite bleach. As the final pH value rises, the color of the wastewater increases. The viscosity also increases slightly. The data show that there is little or no difference in the manner in which the reactant coraponents are added in the hypochlorite stage. Comparing the data in the following table with those in Table 4, it can be seen that the buffering of the hypochlorite stage with sodium silicate in the DH _e D bleaching process results in an overall coloration of the waste water which is about 100-200 ppm less than when adjusting the pH with sodium hydroxide.

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!Tabel

N » level
«Ο D '

ο _£

Effect of the silicate concentration (constant NaOH content) in hypochlorination of a D ^ H, D process

concentration
1.24-ClO ₂
3.25 Cl ₂

2.0 NaOCl
0.68 NaOH

0.5 ClO ₂

D ratio 50/50

Temperature ⁰ C 49 49 4-6

71

Time, minutes Consistency % 5 3 25th 60 10

180

Na ₉ SiO-. $

Gradual addition
in the H level

0.0

0.5 .

1.0

2.0

. 3.0
• 4.0

Adding one
Mix in the
H stage e ;

0.0

0.5
1.0
2.0
3.0
4.0

Final pH total dye (ppm)

7.2 497

7.8 515

8.2 525

7.8 550

8.5 536

8.7 578

7.5
7.7

8.1
8.3
8.6
8.6

541 537

551 544 544 577

Brightness % Δ-18 hours rev. viscosity H
S] Final value 11.3 (Centipoise) I. 90.6 11.4 9.8 90.6 10.7 12.2 90.5 10.5 13.9 90.0 10.7 14.4 90.3 10.4 15.0 89.9 16.0

89.5 12.0 89.6 12.1 90.3 12.0 90.6 11.0 90.6 \\ Λ SO.2 11.2

12.4 13.8

13.2

15.0 15.9 16.4 *

CD KJ

cn

CD NJ

It can be seen from the data of Examples 1-4 that the greatest reduction in waste water coloration is obtained in a DHD process under the following conditions: D ratio 50:50, temperature 4-9 ° C., reaction time 30 minutes, hypochlorite bleach buffered with 0.5-1.0 % sodium silicate (35 ° Be) to pH 7.8-8.2. The brightness is the same as or greater than with the CEHD method (91.2 compared to 90.5). The viscosity (17.9 compared to 9.9j is much better than that of the control. The change in brightness during the 1-hour reversion (5.1 im

Compared to 4.2) is _{slightly higher in the D ft} H D bleaching process.

it

Example 6

This example is carried out essentially as in Example 1; however, the bleaching process is being held on CEHD Softwood pulp applied to hardwood pulp. Table 6 summarizes the data obtained for this control experiment.

Tabel

Bleaching effect and paint color 38 • ⁰ C E. with hardwood pulp after this D. 3 t
) Corr. CEHD process 71 2.0 0.5 10 46 Time, minutes, consistency % 10 Level temperature 71 , 400 60 200 3 C. C. , 072 60 200 E. 4-, O 809 60 78 H 180 D. 580 H / NaOH Bleach level 580 7th 1, VO, 5 Chemicals
additive (wt .- #) 227 2 Total VoI, Color (ppm) 250 1.141 Colorimeter
Reading 70 consistency
corrected 27 volume
corrected

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Continuation of table 6 Brightness (ff)

Final value

reversion

Δ-1 hour _>

Δ-18 hours

Viscosity (centipoise)

86.1

3.1 5.4

Example 7

A sample of the same hardwood pulp as in Example 6 is bleached using the CEHD method according to the invention. the Conditions and results are summarized in Table 7,

Table 7

Bleaching effect and waste water color in hardwood pulp after C. 49 Time to go out Consistency % D. D _Ä H "D procedure H ₈ 49 5 3 0.5 D. 46 25th Level temperature ⁰ G Bleach level 71 60 10 100 D. Chemicals to
rate (wt .- ^) * U 180 3 100 Color ppm 0.76 / 2.0 ^{H /} s / NaOH 44 Total vol. Colorineter
Reading 2.0 / 0.5 / 0.68 consistency
corrected 600 volume
corrected 600 1000 263 280 123

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Continuation of Table 7

brightness
Final value
reversion

hour
hours

85.8

3.4
6.4

Viscosity (centipoise) 15.5

The comparison of the results obtained according to Examples 6 and 7 is best possible with the aid of the following compilation. From this it can be seen that the dye reduction of the wastewater is considerable in the D H D process of the present invention; at the same time there is a reduction in the chloride content, Solids and COD. The brightness of the end product is only slightly lower, the viscosity is higher.

Hardwood

procedure

sewage
Total

Color (ppm)
Chlorides (ppm)
COD (ppm)
Solids (ppm)

Bleached porridge

Brightness (#)

Final value
reversion
Λ - 1 hour
Δ - 18 hours
Viscosity centipoise

Chemical additive ( ⁿ y)

NaOH

NaOCl

ClO ₂

^D - ^H -s? -

209832 /

430 484

363 1096

85.8

15.5

CEHD

1141
558

507
1290

86.1

3.1 14.7

4.0

2.8 1.4 0.5

The percentage reduction in impurities in the Waste watering of the bleaching process DH D compared to the known CEHD method for hardwood and VJeichholz pulp results from following compilation:

Percentage reduction in wastewater contamination

DH D method compared to GEFID method. —C — s ~~

^ -Reduction Weichho]
pollution

Dye 84

¹ chlorides 15

COD 6

Solids 23

⁺ The wastewater in the DH D process is 17 % compared to the

O S

CEHD procedure reduced. This percentage reduction was calculated on the basis of an equivalent volume.

For the person skilled in the art it is evident that the process according to the invention, in which instead of the usual alkaline extraction of softwood and hardwood pulp a buffered hypochlorination bleach stage is used, has wide applicability for contaminant control; it can be used as a three step bleaching process used for softwood and hardwood pulp, which were obtained by the Kraft or SuIfit process.

Softwood 62 Hardwood 13th at equr with equivalent 28 Volume ⁴ " Volume " ¹ " 15th 66 86 23 ♦ 24- 36 16 24 31

209832/1081 ORIGIN «->» - ^{f £ CTED}

Claims (13)

Claim e> ^r ■ ', J.. / Γ. rl./ A method for bleaching a cellulose pulp, characterized in that that he a) this pulp treated in aqueous suspension with chlorine or chlorine dioxide in order to make the ^non -cellulose materials contained in the cellulose pulp soluble, whereupon one b) the slurry (corresponding to 3-15 parts by weight Faigen hypochlorination) 15-180 minutes with approximately 0.5-3.0% hypochlorite (based on the dry weight of the pulp) at a temperature of about 71 ⁰ C treated at a pH of about 7-9. 2. The method according to claim 1, characterized in that the pH value during the hypochlorination stage is about 7.5-8.5. 3. The method according to claim 1, characterized in that the pH value during the hypochlorination stage is about 7 »8-8.2. M, A method according to claim 1, characterized in that the pH of the cellulose pulp is set before the treatment with hypochlorite by the addition of alkali metal hydroxide. 5. The method according to claim 1, characterized in that the pH of the cellulose pulp is controlled during the hypochlorination by adding 0.5-1% of a water-soluble silicate (based on the dry weight of the pulp). 6. The method according to claim 5, characterized in that a water-soluble silicate of the formula Na ₂ O (SiO ₂ ) is used, in which χ is 2-4, 7. The method according to claim 5 »characterized in that a water-soluble silicate of the formula Na2SiO- is used. 8. The method according to claim 1, characterized in that the temperature during hypochlorination is around 38-71 ° C. 209832/1081 9. The method according to claim 1, characterized in that the treatment of the cellulose pulp with hypochlorite is carried out within 30-90 minutes at a temperature of 38-71 ° C. 10. Multi-stage bleaching process for cellulose pulp, characterized in that an aqueous pulp suspension at a Consistency of 3-15 wt .- # in order of the following bleaching stages subject: a) Reaction of the cellulose pulp with chlorine dioxide and then with chlorine, about 2-90 % of the total chlorine in this chlorine dioxide / chlorine mixture being added in the form of chlorine dioxide, b) Reaction of the cellulose pulp with an alkali hypochlorite at a pH of about 7-9 »and c) Reaction of the cellulose pulp with chlorine dioxide. 11. Multi-stage bleaching process for cellulose pulp, characterized in that cellulose pulp at a consistency of 3-15 wt .- ^ subjected in sequence to the following bleaching stages : a) conversion of the wood pulp with chlorine, b) reaction of the cellulose pulp with an alkali hypochlorite at a pH of about 7-9, and c) Reaction of the cellulose pulp with chlorine dioxide. 12. A method for reducing the dye content of waste water from an alkaline extraction in a multi-stage bleaching process for cellulose pulp, characterized in that instead of this alkaline extraction stage, a hypochlorite bleaching is carried out, the pulp consistency being 3-15 % and 0.5-3 % Hypochlorite (based on the dry weight of the pulp) can be used at a pH of about 7-9, which is maintained by adding a water-soluble silicate buffer, at a temperature of about 38-71 ° C in a time of about 30-90 minutes. 13. Multi-stage bleaching process for vr.Bri ^ e cellulose pulp ..-, .. suspensions, consisting of the following, to be carried out in sequence Treatment levels: 209832/1081 a) Dissolution of the accompanying step contained in the egg made of non-cellulose material, b) extraction of the dissolved non-cellulosic material, and c) hypochlorination of the pulp with an alkali hypochlorite, followed by one or more treatment levels, characterized in that the extraction and hypochlorination stage are combined in a single stage, using about 0.5-5 % alkali hypochlorite (based on the dry weight of the pulp) at a pulp consistency of about 3-15 # at a temperature of about 24-71 ⁰ C and a pH of about 7-9 within a time of 15-180. 209832/1081