FI62361C - BLEKNING AV CELLULOSA MED EN AVSEVAERT REDUCERAD KLORMAENGD - Google Patents

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Federal Republic of Germany-Förbundsrepubliken lyskland (DE) P 2327900.7 Proven-Styrkt (71) Degussa Aktiengesellschaft, Weissfrauenstrasse 9 »6000 Frankfurt 1,

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) (72) Horst Kruger, Darmstadt, Helmut Schröter, Grossauheim, Werner Traser, Darmstadt-Arheilgen, Herbert Pfand, Rodenbach, Helmut Rath, Langen,

Federal Republic of Germany-Förbundsrepubliken lyskland (DE) (7 * 0 Oy Kolster Ab (5 * 0 Pulp bleaching with significantly reduced chlorine content -Blekning av Cellulosa med en avsevärt reducerad klormängd

The invention relates to a multi-stage process for the complete bleaching of pulp with a small amount of chlorine at a temperature between 55 ° C and the boiling point of the bleaching mixture, at atmospheric pressure and with a consistency of the mixture of 10 to 20%.

Pulps, as obtained by several temporal or acidic pulping processes, still contain significant amounts of lignin, hemicellulose, and other by-products that must be removed by bleaching to obtain products with improved whiteness, whiteness stability, and specific processability, such as the ability to form a scatter. Today, bleaching chemicals such as chlorine (C), chlorine dioxide (D), sodium or calcium hypochlorite (H) are generally used in several stages, and finally extracted with alkali (E). In addition, it is known to use hydrogen peroxide and organic per-compounds for pulp bleaching, whether this is a single-stage bleaching or a final stage of multi-stage bleaching, in particular to achieve good whiteness stability (yellowing resistance).

2 62361

Full bleaching of the pulps (degree of whiteness 88% MgO, or 90% MgO of bone marrow) requires one or more chlorination steps, as well as hypochlorite and optionally chlorine dioxide steps. These processes release significant amounts of hydrochloric acid, or neutralizing sodium chloride, into the effluent and thus into the water intake basin and water bodies into which the effluent is discharged. Examples of currently used bleaching methods are the series C / E / H, C / E / H / H, C / E / D / E / D, C / E / H / D / P, C / E / H / D / E / D and C / E / H / D / H. So all these processes start with chlorination.

In connection with the above-mentioned wastewater problems and the loading of salts and dissolved organic substances in the water intake basin, bleaching series such as PDPDP or also P / PES / P / PES / P have recently been proposed, which allow the closure of -45 and P 22 19 505.7-45). Thus, in these methods, the treatment with chlorine in the sulfate pulping process has been replaced by the treatment with molecular ha-Pella. In this case, however, you must work under pressure. It is still uncertain whether the strength losses due to alkaline oxygen pressure treatment can be prevented by additives known from the literature, such as MgCO 2 or MgO, or by others (German Public Expression 21 09 542; Rowlandson, Tappi 54, 962-967 (1971), No. 6).

However, this new technology has so far only been used on an industrial experimental scale, at which point nothing is yet known about the uses for sulphite pulp.

The object of the invention is a multi-stage process for the complete bleaching of pulps, in which case it is possible to operate with substantially lower amounts of chlorine than with the technical equipment already existing in a pulp bleaching plant.

It has now been found that the pulps can be fully bleached in several stages, preferably without pressure, when the bleaching series, possibly combined with a slight partial chlorination, begins with strongly alkaline peroxide bleaching.

This strongly alkaline peroxide bleaching step (P), which at the same time also has a chemical defibering effect, 3 62361 replaces the hitherto ordinary acid chlorination followed by the alkaline extraction step (i.e. steps C to E in the above-mentioned methods). Using this replacement, it is possible to work with substantially lower amounts of chlorine than before.

The bleaching result here - as in known bleaching methods - depends on the species of wood, i.e. conifers or deciduous trees or annual plants, as well as on the manner in which the cellulose process is carried out and thus on the degree of hardness.

Possible partial chlorination is usually only necessary when it is necessary to bleach hard, i.e. lignin- and hemicellulose-rich pulps to more than 90% MgO whiteness. In this case, partial chlorination can be carried out either before the actual bleaching, i.e. the strongly alkaline peroxide step, or it is combined directly with the strong alkaline peroxide step as the partial chlorination and extraction step. This consumes about one-fifth of the amount of chlorine used so far and about half of the amount of alkali used so far.

The amounts of chlorine used in these chlorination steps are only a small fraction of the amounts normally required for chlorination, as indicated above. In the peroxide step, as mentioned above, a large excess of alkali is used. This partial chlorination can also be omitted if the oxidative bleaching steps following the usual initial bleaching, such as the hypochlorite-chlorine dioxide or peroxide steps, are confirmed. Naturally, this partial chlorination is omitted when no more than 90% MgO whites are required, but whites between 85 and 90% MgO are sufficient.

Bleaching with chlorine and alkali (steps C to E) brings oxygen-consuming substances and highly chlorine-containing compounds into the pulp mill effluent as a result of the dissolution of organic matter, which must no longer be discharged into the water intake basin.

Chlorine-containing or chloride-ion-containing bleach effluents are known to lead to strong corrosions in regeneration equipment. The strongly alkaline bleaching step described allows the evaporation and concentration of solutes, their incineration and alkali regeneration in conventional equipment. Strongly alkaline peroxide treatment has further surprisingly shown that 85-90% of the pollutants in the wastewater are soluble at this stage and therefore do not enter the water intake basin.

Suitable per compounds are hydrogen peroxide, sodium peroxide and other inorganic or organic peroxides or hydroperoxides, preferably hydrogen peroxide, sodium peroxide or t-butyl hydroperoxide or mixtures of these components in aqueous solutions. Hydrogen peroxide and sodium peroxide are particularly suitable. The amounts used can range from 0.2 to 10% by weight, preferably from 0.6 to 6% by weight, calculated as 100% hydrogen peroxide and absolutely dry mass.

As the alkalis required for the strongly alkaline peroxide bleaching step described, i.e. alkali or alkaline earth metal hydroxides, sodium hydroxide or also potassium hydroxide or ammonium hydroxide (NH3 or NH4OH) is preferably used and then 2-15% by weight, preferably 4-8% by weight. %, calculated as 100% on a volatile matter-free basis.

The alkaline peroxide bleaching step is carried out in a manner known per se between temperatures of 20 ° C and the boiling point of the bleaching solution. As such, it would of course be possible to bleach under pressure, but the advantage of using per-compounds and said series is precisely the non-pressure of the process, i.e. without the use of additional pressure and equipment.

The bleaching process is carried out, as mentioned above, in already existing bleaching plants, in which case pulp densities of preferably between 10 and 20% are common. By pulp consistency is meant the amount of pulp in the pulp stock as a percentage by weight. However, it is also possible to carry out the alkaline peroxide step at higher dry matter contents, using suitable technical aids, such as pumps for thick masses, mixers and high-pressure filters up to 35%.

An additional advantage is that water glass is not used as otherwise in the peroxide bleaching steps.

Conventional inorganic complexing agents or stabilizers such as magnesium sulfate or organic N- or β-containing complexing agents such as ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid and nitrile triacetic acid can be added to the bleaching solution.

5 62361

It is also possible to use N- and P-free complexing agents, such as polyoxycarboxylic acids, respectively (patent applications P 19 04 940.2, P 19 04 941.3 and P 19 42 556.0).

The technical development of the method according to the invention is that it is possible to bleach the pulps completely, i.e. also hard, and even with pulp bleaching equipment already existing. There is a further substantial advantage in obtaining wastewater from which the substances loading the water intake basin are dissolved out already in the first stage. The resulting effluents, including effluents from full bleaching of hard pulps, can be evaporated and regenerated without corrosion.

To illustrate the invention, the following examples are given in which the percentages given always refer to percentages by weight based on the unbleached dry mass. In all cases, tap water with a hardness of 5 ° dH in emulsified tanks of 100-500 g pulp (dry weight) was used, the pH values were measured at the beginning of bleaching. During each step, the pulp was washed with tap water and acidified with SO 4 after the last step. The degrees of whiteness (remission degrees) given were determined according to German standard methods (Zell-cheming-Merkbeätter) with a Zeiss-Elrepho degree of whiteness, filter R 46.

Example 1

The beech-silk sulphite pulp (chapter number 9.6, whiteness unbleached 51.0% MgO) prepared according to the calcium sulphite method was bleached in a four-stage series under the following conditions: Step 1 1.3% H2 ° 2 + 8% NaOH 12% consistency 65 ° C 2 h Step 2 0.36% NaOCl 12% consistency 50 ° C 1.5 h Step 3 0.13% C102 14% consistency 65 ° C 1.5 h Step 4 0.57% NaOCl 12% consistency at 55 ° C for 2 h

The whiteness of the pulp after the fourth stage was 93.3%

MgO. An even slightly higher whiteness, 94.4% MgO, could be achieved with the following series: Step 1 1.3% "8% NaOH 12% consistency 65 ° C 2 h Step 2 0.13% C102 14% consistency 65 ° C 1.5 h Step 3 0.36% NaOCl 12% consistency 50 ° C 1.5 h Step 4 0.57% NaOCl 12 3> consistency 55 ° C 2 h 6 62361

The number of pieces in the unbleached pulp could be reduced from about 9.6 to less than 1. The lignin content then decreased from 1.2% to 0.18%.

Similarly, other chemical parameters correspond to previously used procedures, such as hemicellulose, O-cellulose, R 18, ash, and raethanol-benzene extraction; in particular, the pulp can be excellently processed into lust. The material loss by this bleaching method (P-H-D-H) is about 7.5% based on unbleached beech-artificial silk cellulose.

Example 2

The beech-paper sulphite pulp prepared according to the calcium bisulphite method (number 22.3, whiteness unbleached 56.7% MgO) was bleached in a four-step series under the following conditions: Step 1 1.3% ^ 2 ^ 2 “®% NaOH 12% consistency 65 ° C 2 h Step 2 0.5% Na OCl 12% consistency 40 ° C 1.5 h Step 3 0.22% CIO2 14% consistency 65 ° C 2 h Step 4 0.17% NaOCl 12% consistency at 50 ° C for 2 h

The final whiteness here was 88.0% MgO.

If the amount of peroxide used in the first step is increased to 2.6%, it is generally already three steps (P-D-H) to reach a degree of whiteness of 91.4% MgO.

Step 1 2.6% 1 ^ 02 “8% NaOH 12% consistency 65 ° C 2 h Step 2 0.22% CIO 2 12% consistency 65 ° C 2 h Step 3 0.5% NaOCl 14% consistency at 45 ° C for 2 h

Example 3

In order to achieve the greatest bleaching effect in the beech-calcium sulphite pulp mentioned in Example 2, a weak chlorination step is used before the strongly alkaline peroxide bleaching step.

Step 1 0.5% Cl 2 4% cold consistency 0.25 h Step 2 1.3% 1 ^ 02 - 6% NaOH 12% consistency 65 ° C 2 h Step 3 0.66% NaOCl 12% consistency 50 ° C 2 h Step 4 0.23% CIO 2 14% consistency 65 ° C 1.5 h Step 5 0.27% NaOCl - 12% consistency 52 ° C 2 h

0.2% NaOH

62361 By these steps a whiteness of 95.4% MgO was achieved, with a pulp yield of 93.2% based on the unbleached pulp. In both first stages (C-P), 87.1% of the wastewater-polluting substances are dissolved, which can then be led to the regeneration process.

Example 4

The six-pulp (number 19.55, unbleached 50.2% MgO) prepared according to the calcium bisulphite process is bleached in a four-step process according to the following series: Step 1 2.6% H2 ° 2 ~ ®% NaOH 12% consistency 65 ° C 2 h Step 2 0.2% C102 12% consistency 45 - 50 ° C 1 h Step 3 0.3% C102 14% consistency 57 ° C 1 h Step 4 0.63% NaOCl 12%: consistency 43 ° C 2 h This bleaching method gives a whiteness of 87,4%

MgO.

This example clearly proves that the chlorination step can be dispensed with in order to achieve whitenesses between 85 and 90% MgO, even with harder six-pulps. To achieve whitenesses above 90%, the following steps using sub-chlorination are suitable: Step 1 1.3% H2®2 “®% NaOH 12% consistency 65 ° C 2 h Step 2 1.3% Cl2 4% cold consistency 0.5 h Step 3 1.2% NaOH 12% consistency 60 ° C 1 h Step 4 0.62% NaOCl 12% consistency 50 ° C 2 h Step 5 0.34 % C102 14% consistency 70 ° C 2 h Step 6 0.30% NaOCl 12% consistency 50 ° C 2 h

The whiteness here was 92.5% MgO.

Pre-partial chlorination gives a whiteness of 94.1% MgO.

Step 1 1.0% Cl2 4% cold consistency 0.25 h Step 2 1.3% H 2 O 2 - 6% NaOH 12% consistency 65 ° C 2 h Step 3 0.66% NaOCl 12% Consistency 50 ° C 2 h Step 4 0.30% C102 14% Consistency 65 ° C 1.5 h Step 5 0.66% NaCl - 0.2% NaOH 12% Consistency 53 ° C 2 h

The above examples show that the new bleaching process achieves the degree of whiteness required for full bleaching of pulps. The yield corresponded to the order of magnitude known for strongly chlorine-containing multistage bleaching processes. Since a further 7/8 part of the soluble organic matter in the alkaline peroxide bleaching stage is no longer transported to the water intake basin but is sent to regeneration plants, the load on the wastewater is decisively reduced.

Claims (3)

9 62361 A multi-stage process for the complete bleaching of pulp with a small amount of chlorine at a temperature between 55 ° C and the boiling point of the bleaching mixture, at atmospheric pressure and a mixture consistency of 10-20%, characterized in that the bleaching series, possibly combined with low chlorination, begins with a strong alkaline peroxide treatment. , wherein the amount of peroxide is 0.2 to 10% by weight, based on 100% of hydrogen peroxide and the amount of Atro pulp, and the amount of alkali is 2 to 15% by weight, preferably 4 to 8% by weight, based on 100% of the substance and the amount of pulp. Process according to Claim 1, characterized in that, in the case of hard pulps, the strongly alkaline peroxide bleaching step is preceded by partial chlorination with about one-fifth of the chlorine used hitherto, which is 1.5 kg / kg of lignin, based on the first chlorination step. Process according to Claim 1, characterized in that, in the case of hard pulps, the strongly alkaline peroxide bleaching step is combined with a weak partial chlorination and extraction step, using only one-fifth of the 1.5 kg / kg lignin used hitherto and half 1.5 to 2.5 kg of sodium hydroxide / 100 kg of pulp, calculated from the chlorination-extraction step.