FI68684C - FOERFARANDE FOER FAERGFOERBAETTRING OCH LUKTBORTTAGNING AV SULFATMASSA - Google Patents

Abstract

1. A process for lightening and deodorizing kraft or sulphate pulp, characterized in that from 0.05 to 3 % by weight, based on completely dry pulp, of a peroxide (based on 100 % peroxide) are added after sulphate cooking during pulp preparation in the region of the diffusion wash using residual alkalinity of the pulp suspension which is still present in the pulp as a result of the alkaline cooking.

Description

[7 ^ 71 Γοΐ ANNOUNCEMENT to to Λ «tSÄMP ^ UTLÄGG NIN G SSKRI FT ΟΟΟθ4 C (45) * aUr.Lil l? lj 1 :; 1935

Patani niuddel. '. T (51) Kv.lk. * / Lnt.CI.4 0 21 C 9/16 FINLAND —FINLAND (21) Patent application - Patentansökning 793720 (22) Application date - Ansökningsdag 27 11 79 (FI) (23) ) Starting date - Giltighetsdag 27.11 .79

(41) Made public - Blivit off en tl ig gg gQ

National Board of Patents and Registration Date of publication and publication. - 2g Q, Or

Patent- och registerstyrelsen '' Ansökan utlagd och utl.skriften publicerad ^ O.UD.Op (32) (33) (31) Requested privilege - Begärd priority 1 6.02.79

Federal Republic of Germany Förbundsrepubliken Tyskland (DE) P 2905936.7 Proven-Styrkt (71) Degussa Aktiengesellschaft, Weissfrauenstrasse 9, 6000 Frankfurt 1,

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) (72) Horst Kruger, Darmstadt, Wilhelm Berndt, Frankfurt,

Jakob Wink, Gelnhausen-Hailer, Federal Republic of GermanyEast-Förbundsrepubl i ken Tyskland (DE) (74) Oy Kolster Ab (5 * 0 Method for improving the color and odor removal of sulphate pulp -Förfarande för färgförbättring och 1uktborttagning av sulfatmassa

Chemical pulps prepared according to the kraft or sulphate method have excellent technical properties. They are used e.g. after bleaching in large quantities for papermaking.

In the unbleached state, considerable amounts of kraft or sulphate pulp are used for the production of packaging materials, such as cartons. Due to its very good strength properties, kraft or sulphate pulp is suitable for packaging where high mechanical stress must be taken into account.

However, unbleached kraft or sulphate pulp has significant disadvantages which limit its use in the manufacture of packaging materials undesirably.

Thus, for example, the packaging materials obtained have a low-value and ugly appearance due to the brown color of the pulp. In addition, sulphate digestion produces highly odorous sulfur-containing compounds, such as ethyl and methyl mercaptan, which give the kraft or sulphate pulp a characteristic odor which cannot be removed with sufficient certainty during washing and drying. These properties of kraft or sulphate pulp in food packaging are particularly detrimental, so that it has not been possible to use hitherto unbleached kraft or sulphate pulp for this purpose, e.g. milk packaging.

Thus, there is a huge need to economically tolerate the conventional equipment available in pulp mills to lighten and deodorize kraft and sulfate pulp and thus make it a useful substance for the manufacture of food packaging.

When unbleached kraft or sulphate pulp is obtained with a brightness value of about 25-26, the brightness of the pulp for food packaging should be 30-32 - without losing its good strength properties.

In order to give a pleasant color to the packaging material, it is sufficient to improve the brightness of the kraft or sulphate mass by about 5-10 units.

The invention relates to a process for bleaching and deodorising kraft or sulphate pulp, the process being characterized in that peroxide is added in an amount of 0.05 to 3% by weight after sulphate cooking during post-treatment of the pulp in a diffusion washing area using residual alkalinity still present in the pulp after alkaline cooking. (calculated as 100% peroxide), calculated on absolutely dry pulp.

In this case, an amount of peroxide bond of between 0.5 and 2.5% by weight (100% of peroxide), calculated on absolutely dry pulp, may be necessary for the desired improvement in brightness, while an amount of between 0.05 and 0% may be required for deodorization alone, 3% by weight of peroxide, based on absolutely dry pulp, may be sufficient.

As the peroxide, hydrogen peroxide in aqueous solution is preferably used. The residual alkalinity of the cellulose slurry at the beginning of the peroxide addition may correspond to a leaching rate of 0.2 to 1.8% by weight of Ν32504, preferably 1.2 to 1.5% by weight of Na 2 SO 4. In the process according to the invention, it is primarily advantageous that in the step used in the process for producing kraft or sulphate pulp, a pulp slurry having a consistency of 3 68684 10-15% by weight, based on absolute dry pulp, is obtained at high temperatures between 60-80 ° C and stored for several hours, preferably 7-20 hours.

In addition, the process according to the invention can be carried out at consistencies between 3 and 20% by weight and at temperatures between 40 and 90 ° C. A temperature between 60-70 ° C is particularly preferred.

In addition, the storage time in the cell may be between 1 and 25 hours and in a preferred embodiment 7 to 15 hours.

Normally, in the production of sulphate pulp, a degree of leaching in the area of diffusion washing, which is optimal for peroxide treatment, is achieved. If this is not the case, it is necessary to correct the residual alkalinity (degree of leaching).

Too high alkalinity of the pulp causes too rapid peroxide wear and thus a poor result. In such cases, it is preferable to provide the peroxide used with a suitable amount of acid sufficient to bring the alkalinity of the pulp to the optimum range. As the acid, inorganic or organic acids which do not contain reducing agents can be used. Preferably sulfuric acid can be used.

The process according to the invention is surprisingly possible at the described stage of the kraft or sulphate pulp production process, although the known instructions for pulp bleaching with peroxide do not achieve sufficient results at this stage of the pulp production process. Therefore, there is a widespread perception that the non-rinsable oxidizable parts still present in the cooking would lead to premature peroxide wear and make bleaching impossible.

Surprisingly, it has now been shown in the process according to the invention that peroxide bleaching is possible at the described stage of the kraft or sulphate pulp production process, when the addition of soda liquor, which is otherwise absolutely necessary for peroxide bleaching, is dispensed with. The use of known bleach stabilizers can also be dispensed with.

The kraft or sulphate pulp treated by the process according to the invention can easily be used as a material for food packaging, e.g. milk packaging.

The method according to the invention is illustrated and described in more detail by the following examples.

68684

Example 1

The sulphate pulp prepared by the bed is passed, after passing through the usual sorting and washing steps, over the last washing filter, which has a leaching rate of 1.2-1.5% ^ 230 ^. The pulp is thickened as a filter to a pulp consistency of 15%, it falls at 70 ° C into a screw conveyor and is pumped into a thick pulp tower where it is stored for 7-20 hours (diffusion tower).

Upon entering the tower, the brightness of the mass is 26.5. In a screw conveyor, 1500 1 / h of a 10% aqueous hydrogen peroxide solution is evenly distributed by means of a drip grinder.

Corresponding to the production of 10,000 kg / h of sulphate pulp, this corresponds to the use of 1.5% H 2 O 2. After a reaction time of 7 hours, the brightness of the mass continuously pumped out of the tower is 32 while the color tone has changed from grayish brown to brownish yellow. It is virtually odorless. The strength properties are the same before and after treatment.

Example 2

In the unit described in Example 1, 150 l / h of a 10% hydrogen peroxide solution are metered in at the same addition point, which corresponds to 0.15% by weight. After a reaction and storage time of 10 hours, the brightness of the pulp taken from the storage tower was virtually unchanged at 27.5. The previously memorable odor had disappeared so that the pulp could be used to make milk carton.