FI62869C - FLAT CONTAINER FOR CHLORFATTED HELBLEKNING AV CELLULOSA - Google Patents

Description

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Federal Republic of Germany-Förbundsrepubliken Tyskland (nE) P 2538673.6 (71) Degussa Aktiengesellschaft, Weissfrauenstrasse 9, 6000 Frankfurt 1,

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) (72) Horst Kruger, Darmstadt, Werner Traser, Darmstadt-Arheilgen,

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) (7U) Oy Kolster Ab (5M Multi-stage low-chlorine process for complete bleaching of pulp -Förfarande i flera steg för chlororfattig helblekning av Cellulosa

The invention relates to a multi-stage, low-chlorine process for the complete bleaching of pulp, in which the bleaching stages begin, optionally in combination with a minor partial chlorination, with an alkaline peroxide kititol which takes place mainly without pressure.

The process according to the invention is characterized in that the pulp is bleached using an alkali of 0.5 to 2% by weight, based on 100% of an absolutely dry pulp, at a temperature above 65 ° C.

The subject of Finnish patent application 1326/74 is a method for the complete bleaching of pulp, which can be carried out in the technical equipment available for the pulp bleacher with much smaller amounts of chlorine than before. With the known method, full-bleaching can be carried out in several stages, mainly without pressure, if the bleaching cycle, which may involve a slight partial chlorination, 2 62869 begins with a strongly alkaline peroxide bleaching process.

This strongly alkaline peroxide bleaching step (KP), which also has a fibrous effect at the same time, replaces the acid chlorination used hitherto, followed by the alkaline extraction step.

The bleaching result then depends - as in known bleaching methods - on the quality of the wood, such as conifers or deciduous trees or annual plants, and on the performance of the cooking process and thus on the degree of defibering.

Possible partial chlorination is usually only necessary when it is necessary to bleach hard pulp, i.e. high in lignin and semi-cellulose, to a degree of difficulty of more than 90% MgO. The partial chlorination can then take place either before the actual bleaching begins, i.e. before the strong alkaline peroxide step, or the partial chlorination and extraction step is combined directly with the strongly alkaline peroxide step. In this case, approximately one-fifth of the amount of chlorine to date and approximately half of the amount of alkali to date is used.

It has now been found that, following well-defined reaction conditions in the peroxide step, an even lower alkali content is sufficient.

Temperatures of 65 to 85 ° C are preferably used in the process of the invention. The amount of chlorine used in the chlorination step is only a small part of the amounts used in the chlorination methods known so far. There is an excess of alkali in the peroxide step. The partial chlorination step can also be omitted altogether if the known oxidation steps following bleaching cooking, such as hypochlorite, chlorine dioxide or peroxide, are made stronger. Partial chlorination is, of course, omitted unless a degree of difficulty of more than 90% MgO is required, but degrees of difficulty of 85-90% MgO are sufficient.

Chlorine and alkali bleaching (C - E stage) introduces organic substances by dissolving oxygen-consuming substances and highly chlorine-containing compounds in the pulp mill wastewater, and such wastewater can no longer be discharged into the water intake basin. Wastewater from bleaching containing chlorine or chloride ions is known to cause severe corrosion in regeneration equipment. The alkaline whitening soup of the invention 3 628: 69 allows the evaporation and concentration of solutes, their incineration and the recovery of alkali in conventional equipment. In connection with the alkaline peroxide treatment, it has also been found that 85-90% of the substances which load the wastewater dissolve at this stage and are therefore not discharged into the water intake basin.

Of the per compounds, hydrogen peroxide and sodium peroxide are particularly suitable. The amounts used can be from 0.2 to 10% by weight, in particular from 0.6 to 6% by weight, calculated as 100% hydrogen peroxide from absolutely dry pulp.

The alkalis used in the described alkaline peroxide bleaching soup, i.e. alkali and alkaline earth hydroxides, are mainly sodium hydroxide or also calcium hydroxide (NH 4 or NH 4 OH) and are used in an amount of 0.5 to 3% by weight, preferably 1% by weight, based on 100% of an absolutely dry pulp.

The alkali peroxide bleaching cooking is carried out in a known manner at a temperature in the range from 65 ° C to the boiling point of the bleaching solution. As such, it would of course be possible to bleach using pressure, but the advantage of using per-compounds and of said stepwise process is precisely that it can be carried out without pressure, i.e. without accessories.

The bleaching process is carried out, as mentioned above, in bleaching equipment already in use, and the fiber stock concentrations are usually 10 to 20%. By this is meant the concentration of pulp in the pulp suspension. But it is also possible to carry out the alkaline peroxide step at higher dry matter contents, namely by using suitable technical aids, such as thick pulp pumps, mixers, high-pressure filters, whereby the dry matter content can rise to 35%.

An additional advantage is that no water glass is used at all as is usually the case in peroxide bleaching steps.

Common inorganic complexing agents or stabilizers, such as magnesium sulfate or organic nitrogen- or phosphorus-containing complexing agents, such as ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, nitrile acetic acid, can be added to the bleaching broth. It is also possible to use nitrogen- and phosphorus-free complexing agents such as polyoxycarboxylic acids.

The technical advance of the main method is the complete bleaching of pulp, including hard pulp, in the existing equipment of the bleaching plant and the generation of effluents from which the substances in the abstraction tank are dissolved in the first stage, which does not cause corrosion the method also provides alkali savings.

The following example illustrates the invention. The percentages given are percentages by weight of unbleached dry pulp. The example was worked using tap water in 5 ° dH enamelled tanks and the pulp was washed after each step with tap water and after the last step it was acidified with swamps. The reported degrees of difficulty (degrees of remission) were determined according to German standard methods (Zellcheming manuals) with a Zeiss-Elhepro brightness meter, filter R 46.

Example

Calcium bisulfite beech pulp having a kappa number of 14 and a weight of 53% MgO unbleached was bleached on a factory scale by a four-step process to a final grade of 95-96% MgO.

Step 1 1 to 1.6% chlorine, fiber stock concentration 4%, 1 h Step 2 1.3 to 1.6% H 2 O 2, 1.3 to 1.5% NaOH, fiber stock concentration 12%, 2 h 3. step 0.2% C102, fiber stock concentration 14%, 70 ° C, 4 h Step 4 0.1% C102, fiber stock concentration 12%, 65 ° C, 2 h According to this batch process, the strength properties of the bleached beech-paper pulp were better than the values obtained in standard factory-r bleaching. This is because the hypochlorite step was omitted. In addition, the 4-step process naturally represents an economic advantage over classical five-step bleaching.