NO141317B - MULTI-STEP PROCEDURE FOR CHLORINE-LOW CLEAR BLEACHING OF CELLULOSE - Google Patents

Description

Celluloses as they are produced by various digestion methods of the alkaline or acid type still contain considerable amounts of lignin, hemicelluloses and other accessory components, which must be removed by subsequent bleaching processes in order to obtain products of higher whiteness, whiteness stability and specific further processing capability, such as . viscosification ability. Today, bleaching chemicals such as chlorine (C), chlorine dioxide (D), sodium or calcium hypochlorite (H) are usually used in a variety of steps with subsequent alkaline extractions (E). Furthermore, it is known to use ^2°2' Na2°2' organic compounds for bleaching cellulose, whether it is in the form of single-stage bleaching or as the final step in a multi-stage bleaching in order to especially achieve good whiteness stability (yellowing resistance).

For complete bleaching of celluloses (whiteness degree 88% MgO, even better 90% MgO) one or more chlorination steps are required as well as hypochlorite and optionally also chlorine dioxide steps. During these processes, considerable amounts of hydrochloric acid, or when it is neutralized, sodium chloride are released into the wastewater and thus into streams and waters that have to absorb the wastewater. As examples of the bleaching methods used today, the sequence of steps C/E/H should be given,

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. Everyone

these processes thus begin with a chlorination and subsequent extraction step.

In connection with the above-mentioned wastewater problems and the loading of streams by salts and dissolved organic substances, bleaching sequences such as PDPDP or also P/PES/P/PES/P have recently been proposed, which make it possible to include the bleaching wastewater for a predominantly part of the boiling water regeneration procedure. Here P means peroxide and PES peracid (see German official documents 22.19.504 and 22.19.505).

In this context, the attempts must be seen, in the sulphate cellulose process, to replace the treatment with chlorine with a treatment with molecular oxygen. Admittedly, work must be done under pressure. Furthermore, it is also unclear whether the firmness losses caused by the alkaline oxygen pressure treatment can always

is prevented by the additions of MgC03 suggested in the literature

or MgO or others (DOS 2,109,542, Rowlandson, Tappi 54, 962-

967 (1971), No. 6).

However, this new technology has so far only been introduced

1 large-scale technical test scale, as a further limitation was that nothing was known about the possibility of the application for sulphite cellulose.

The aim of the invention is to provide a multi-stage process for fully bleaching cellulose, which can be carried out with substantially smaller amounts of chlorine or completely without chlorine in the present technical devices of a cellulose bleaching plant. in open vessels.

The invention thus relates to a multi-stage process for chlorine-free complete bleaching of cellulose at temperatures from 55°C up to the boiling point of the bleaching bath at atmospheric pressure and in fabric densities from 10 to 20%, the process being characterized by the bleaching sequence beginning, possibly combined with a small partial chlorination, with a strong alkaline peroxide digestion, where the amount of peroxide is 0.2 to 10% by weight, calculated on 100% hydrogen peroxide and referred to atro cellulose, and the amount of alkalis 2 to 15% by weight, preferably 4 to 8% by weight, calculated on 100% -ig substance and referred to atro cellulose.

This strongly alkaline peroxide bleaching step (P), which at the same time also acts as an absorbent, replaces the previously common acid chlorination with a subsequent alkaline extraction step,

i.e. step C-E in the above procedure. With this replacement, work can be done with significantly smaller amounts of chlorine than before,

as a chlorine quantity of 1.5 kg/kg lignin was previously used.

The bleaching result is here the same as with the known bleaching methods of wood types, i.e. coniferous or hardwood or one-year-old plants both depending on the completion of the cooking process and therewith the degree of absorption.

The possible partial chlorination is only then absolutely necessary, when hard, i.e. lignin- and hemicellulose-rich celluloses are to be bleached to a whiteness content of over 90% MgO. In this way, partial chlorination can either take place before the actual bleaching, i.e. before the strongly alkaline peroxide step, or a partial chlorination and extraction step can be connected directly to the strongly alkaline peroxide step. Thereby, approx. a fifth-

part of the previous amount of chlorine and approx. half of the previous amount of alkali, as previously 1.5-2.5 kg NaOH/100 kg cellulose was used.

The amounts of chlorine used in these chlorination steps are only a small part of the amounts normally used in chlorination as shown above. The peroxide step is carried out - as mentioned above - with a large excess of alkali. This partial chlorination can also be carried out when the usual oxidative bleaching steps following digestion bleaching, such as hypochlorite, chlorine dioxide or peroxide, are enhanced. This partial chlorination is omitted when a whiteness content above 90% MgO is not required, but a whiteness content between 85 and 90% MgO is sufficient.

Bleaching with chlorine and alkali (C-E stage) releases oxygen-consuming substances and strongly chlorine-containing compounds in the waste water of the cellulose factories, which should no longer be discharged into rivers.

As is well known, chlorine-containing or chloride-ion-containing bleach water leads to strong corrosion in the regeneration devices. The highly alkaline bleaching digestion referred to enables evaporation and concentration of the dissolved substances, their combustion and alkali regeneration in ordinary apparatus. The highly alkaline peroxide treatment has also surprisingly shown that 85-90% of the substances that pollute the waste water are released in this step and are thus not fed into rivers.

Percompounds include hydrogen peroxide, sodium peroxide and other inorganic or organic peroxides or hydroperoxides, preferably hydrogen peroxide, sodium peroxide or t-butyl hydroperoxide resp. mixtures of the aforementioned components in aqueous solutions. Particularly suitable are hydrogen peroxide and sodium peroxide. The quantities used can be between 0.2 and 10% by weight, preferably 0.6-6% by weight are used, calculated as 100% hydrogen peroxide and referred to atro cellulose (atro = absolutely dry).

As for the mentioned strongly alkaline, peroxidic bleach solutions, necessary alkalis, i.e. alkali or alkaline earth hydroxides, are preferably used sodium hydroxide or also

calcium hydroxide or ammonium hydroxide (NH^ or NH^OH),

namely between 2-15% by weight, preferably 4-8% by weight, calculated as 100% substance and referred to atro cellulose.

The alkaline-peroxidic bleaching process is carried out in a known manner at temperatures of 20°C to the boiling point of the bleaching bath. In and of itself, it could of course be bleached under pressure, but the advantage of using percompounds and the aforementioned sequences lies precisely in the pressurelessness of the method, i.e. without the use of extra pressure and equipment.

The bleaching process is carried out as mentioned above in existing bleaching plants, as fabric densities between 10 and 20% are preferably common. Fabric density is understood as the weight % of cellulose in the fabric suspension. However, it is also possible to carry out the strongly alkaline peroxide step at a higher dry matter content, namely using suitable technical aids, up to 35%, such as e.g. condensate pumps, kneaders, high-pressure filters.

It is a further advantage that water glass is not used as is otherwise the case with peroxidic bleaching steps.

In the bleaching bath, common inorganic complex formers or stabilizers, such as magnesium sulfate or organic N- or O-containing complex formers, such as e.g. ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, nitrolotriacetic acid. It is also possible to use N- and P-free complex formers such as e.g. polyoxycarboxylic acids corresponding to German official documents 1904 940, 1904 941 and 1942 556.

The technical progress of the method according to the invention lies, firstly, in carrying out a complete bleaching of cellulose, and of hard celluloses in the existing apparatus

in cellulose bleaching plants. A further significant advantage lies in the extraction of waste water, where the substances polluting the river have already been removed in the first step. The resulting waste water can be evaporated and regenerated corrosion-free, including the waste water that results from the complete bleaching of hard celluloses.

The following examples shall serve to explain the invention, in which the percentages are always to be understood as % by weight, referred to unbleached dry cellulose. In all experiments, tap water of 5°dH was used with mixtures of 100 g-500 g cellulose (dry weight) in enamelled containers, the pH value was measured at the beginning of the bleaching. After each step, the cellulose was washed with tap water, after the last step it was deacidified with SC^- The specified whiteness degrees (Remission degrees) were determined according to the German unit methods (Zellcheming-Merkblatter) with a Zeiss-Elrepho whiteness meter, filter R 46.

Example 1

A beech artificial silt sulphite cellulose (Kappa number 9.6 degree of whiteness unbleached 51.0% MgO) produced according to the calcium bisulphite process was bleached in a four-step sequence under the following conditions:

After the fourth step, the cellulose had a whiteness content of 93.3% MgO. A somewhat higher whiteness content of 94.4% MgO could also be achieved by the following sequence:

The kappa number could be reduced from approx. 9.6 in unbleached fabric to below 1. The lignin content thereby decreased from 1.2% to 0.18%.

Likewise, the other chemical characteristics such as hemicellulose, α-cellulose, R 18 value, ash and methanol-benzene extraction correspond to usual working methods. In particular, the fabric can be pre-worked excellently into viscose. The material loss in this bleaching process (P-H-D-H) amounts to approx. 7.5%, referred to unbleached beech rayon cellulose.

Example 2

A beech-paper-sulphite cellulose (kappa number 22.3, degree of whiteness unbleached 56.7% MgO) produced by the calcium bisulphite process was bleached in a four-step sequence under the following conditions:

The final whiteness content was here at 88.0% MgO.

If the use of peroxide is increased in the first step to 2.6^ H2°2', a whiteness degree of 91.4% MgO can be achieved with only a total of three steps (P-D-H).

Example 3

To achieve the highest bleaching effect on a beech-calcium bisulphite paper cellulose mentioned in example 2, a weak chlorination step is connected to the strongly alkaline-peroxidic bleach solution.

Following this sequence of steps, a whiteness content of 95.4% MgO was obtained, at a material yield of 93.2%, referred to unbleached material. In the first two stages (CP), 87.1% of the substances that pollute the wastewater are released and which can then be added to a regeneration process.

Example A

Spruce paper cellulose (kappa number 19.55, degree of whiteness unbleached 50.2% MgO) produced by the calcium bisulphite method is bleached in a four-step process according to the following sequence:

After this bleaching process, a whiteness content of 87.4% MgO is achieved.

This example shows unilaterally that to achieve a whiteness content of 85 and 90%"~MgO even with harder spruce paper cellulose, a chlorination step can be disregarded. For a whiteness content above 90%, the following sequences with subsequent partial chlorination are suitable:

The whiteness content was 92.5% MgO.

With pre-connected partial chlorination, a whiteness content of 94.1% MgO is achieved.

The example shows that with the help of the new bleaching solution, the degree of whiteness required for high bleaching of cellulose is achieved. The yield thereby corresponded to the order of magnitude, as is known from conventional highly chlorine-containing multi-stage bleaching methods. Furthermore, since 7/8 of the organic substances that can be released by alkaline-peroxide bleaching are no longer supplied to the rivers, but to regeneration facilities, the waste water transport is decisively reduced.

Claims (3)

1. Multi-stage process for chlorine-free complete bleaching of cellulose at temperatures from 55°C up to the boiling point of the bleaching bath, at atmospheric pressure and in fabric densities from 10 to 20%, characterized by the bleaching sequence beginning, possibly combined with a small partial chlorination, with a strong alkaline peroxide digestion, where the amount of peroxide amounts to 0.2 to 10% by weight, calculated on 100% hydrogen peroxide and referred to atro cellulose, and the amount of alkalis 2 to 15% by weight, preferably 4 to 8% by weight, calculated on 100% substance and referred to atro cellulose. 2. Method according to claim 1, characterized in that in the case of hard celluloses, the strongly alkaline-peroxidic bleach digestion is preceded by a partial chlorination with approximately one fifth of the hitherto used chlorine amount of 1.5 kg/kg lignin, referred to the first chlorination step. 3. Method according to claim 1, characterized in that in the case of hard cellulose, a weak partial chlorination and extraction step is associated with the strongly alkaline-peroxidic bleaching solution, where only approx. one fifth of the hitherto used chlorine amount of 1.5 kg/kg lignin and half of the hitherto used alkali amount of 1.5-2.5 kg NaOH/100 kg cellulose, referred to the chlorination extraction step.