NO754077L - - Google Patents

Description

Bleaching of cellulose with

significantly reduced chlorine use.

From Norwegian application no. 7^0.888, a multi-stage process for the complete bleaching of cellulose is known, which can be carried out with significantly smaller quantities of chlorine than previously in the existing technical devices in a cellulose bleaching plant.

Thereby, the cellulose can be fully bleached in several stages, preferably without pressure, when the bleaching sequence, optionally combined with a small partial chlorination, begins with a strong alkaline peroxide bleaching solution.

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.

The bleaching result is here - as with the known bleaching methods - dependent on the type of wood, e.g. coniferous or hardwood resp. annual plants, as well as the completion of the cooking process and thus the degree of digestibility.

Any partial chlorination is only absolutely necessary when hard, i.e. lignin- and hemi-cellulose-rich celluloses above a whiteness content of 90% MgO are to be bleached. Hereby, 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, approximately one fifth of the previous amount of chlorine and approximately half of the previous amount of alkali is used.

In a further development of the method according to Norwegian patent no. 7^0,888, it has now surprisingly been found that by observing certain reaction conditions at the peroxide stage, a considerably further amount of alkali can be detected.

The object of the invention is a multi-stage process for chlorine-free complete bleaching of cellulose, in which the bleaching sequence is optionally combined with a small partial chlorination, beginning with an alkaline peroxide digestion, preferably pressure-free, the process being characterized by bleaching the cellulose in the peroxide stage at a content of alkali from 0 ,5

to 3% by weight, calculated on 100% fabric and referred to atro cellulose, and at a temperature of the bleaching bath of at least 65°C up to the boiling point.

Temperatures of 65~85°C are preferably used. The amount of chlorine used in the chlorination step is only small

part of the quantity that was used in the hitherto known methods of chlorination. The peroxide step is carried out with excess alkali. The partial chlorination step can also be avoided when the known oxidation bleaching steps that follow digestion bleaching such as hypochlorite, chlorine dioxide and peroxide are enhanced. Of course, 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) brings oxygen-consuming substances and strongly chlorine-containing compounds into the wastewater from cellulose factories, which can no longer be emptied into rivers.

Bleaching water containing chlorine and chloride ions

as is known, leads to strong corrosion in regeneration devices. The mentioned alkaline bleach digestion enables evaporation and concentration of the released substances, their combustion and alkali regeneration in ordinary equipment. The alkaline •peroxide treatment has also shown that 95 - 90% of the contaminated substances in the waste water are released in this step and are thus not supplied to the pre-flooder.

Per compounds 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 µg of hydrogen peroxide and referred to atro cellulose (atro = absolutely dry).

The alkalis required for the mentioned alkaline, peroxidic bleaching solution, i.e. alkali or alkaline earth hydroxides, are preferably sodium hydroxide or; also calcium hydroxide or ammonium hydroxide (NH^ or NH^OH), namely between 0.5 - 3% by weight, preferably 1-3% by weight, calculated as 100% substance and referred to atro cellulose.

The alkaline-peroxidic bleach digestion is carried out in a known manner at temperatures from 65°C to the boiling point of the bleaching bath. In and of itself, it can of course be bleached under pressure, but the advantage of using percompounds and the mentioned sequence lies precisely in the pressurelessness of the process, i.e. without the use of additional pressure and equipment.

The bleaching process is carried out as mentioned above in existing bleaching plants, the usual fabric densities being preferably between 10 and 20%. Fabric density is then understood to mean the weight of cellulose in the fabric suspension. However, it is also possible to carry out the alkaline peroxide step at a higher dry content, namely using suitable technical aids up to 35%3 such as e.g. thickener pumps, kneaders, high-pressure filters.

A further advantage is that no glass of water is used as is otherwise the case with peroxidic bleaching steps.

In the bleaching baths, common inorganic complex formers or stabilizers, such as magnesium sulfate or organic N- or P-containing complex formers, such as e.g. ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, nitrilotriacetic acid. It is also possible to use N- and P-free complex formers, such as e.g. polyoxycarboxylic acids.

In addition to the technical progress according to application no. 7^0.888, namely a complete bleaching of cellulose, also of hard celluloses, namely in existing apparatus in cellulose bleaching plants as well as extraction of waste water, where the substances that pollute the pre-water have already been released in the first step, whereby corrosion-free wastewater is produced and can be easily regenerated - the method according to the invention entails yet another saving on alkali.

The following examples will explain the invention in more detail, where the percentages are always to be understood as % by weight, referring to unbleached dry cellulose. In all experiments, the work was done with tap water of 5° dH in enamelled containers and after each step the cellulose is washed with tap water and acidified after the last step with SOp. The indicated degree of whiteness (degree of remission) is determined according to the German unit methods (Zellcheming-Merk sheet) with a Zeiss-Elrepho degree of whiteness meter, filter R 46. Example 1.

A spruce-paper cellulose produced according to the calcium-bisulphite method with a kappa number of 22, which in the unbleached state has a whiteness content of 5-1$ MgO, is bleached in a 5-stage bleaching process to a whiteness above 92$ MgO:

As this example clearly shows, wood can also

harder spruce-paper celluloses, the content of chlorine is substantially reduced, namely from the previous 5.5 - 6.5$ Cff^tll 1.1 - 1.7$ Cl2. The. achieved firmness properties are in the order of magnitude of the cellulose, which was bleached according to the classic method.

Furthermore, the purity of the cellulose was particularly high and the cellulose had a lower content of bark, bast and bark parts than before.

Over 90% of the organic matter in the dewatered water forms from the first two bleaching sequences and can thus easily be added to a thickening and incineration.

Example 2.

A beech rayon cellulose produced by the calcium bisulphite process with a kappa number of 5.5 and a whiteness content of 60.7% MgO in the unbleached state was bleached in a 3-step sequence on an operational scale under the following conditions:

The procedural width in the amount of addition of H2O2 and NaOCl emerges from the technical process for setting viscosity values for artificial silk cellulose.

The achieved whiteness content is 95.8% MgO. Corresponding grades of the alkaline-peroxidic bleaching solution, the hemicellulose content drops from 13$ to 8.6$. The desired α-cellulose content of 91 - 92% is achieved. The cellulose has an excellent filtration value and a small gel particle volume. The rate of emergence and the thickness swelling factor were within the framework of the usual values for beech cellulose.

Example 3-

A beech paper cellulose produced according to the calcium bisulphite process, with a kappa number of 14 and a whiteness content of 53$ MgO unbleached, was bleached on an operational scale in a 4-stage process to a final whiteness of 95 - 96$ MgO.

The firmness properties of the beech paper cellulose bleached following this sequence were higher than the values obtained in normal company bleaching. This is due to the fact that the hypochlorite step is disregarded. In addition, of course, the 4-stage method entails an economic advantage compared to the classic 5-stage bleaching.

Claims (2)

1. Multi-stage process for chlorine-free complete bleaching of cellulose, where the bleaching sequences, possibly combined with a minor partial chlorination, begin with an alkaline peroxide digestion, preferably pressure-free, characterized by bleaching the cellulose in the peroxide stage at a content of alkali from 0.5 to 3% by weight, calculated on 100$ fabric and referred to atro cellulose, and at a temperature of the bleaching bath of at least 65°C. 2. Method according to claim 1, characterized in that it is bleached with alkali amounts of 1 - 3% by weight, calculated for 100% - i.g of substance, - referred to atro cellulose and at temperatures of 65 - 85°C.