JPH02289194A - Method for bleaching lignocellulosic substance with oxygen - Google Patents

Abstract

PURPOSE:To improve delignifcation ratio and reduce the Kappa number by washing a black liquor discharged from a digestion process until the solid content therein attains a specific value or below and then bleaching a lignocellulosic substance with oxygen. CONSTITUTION:Pulp after digesting is bleached with oxygen. In the process, a black liquor discharged from a digestion process is washed until the solid content therein attains <=10kg/ton bone-dry pulp, preferably <=5kg/ton bone-dry pulp by COD expression and bleaching with oxygen is then carried out afford pulp, improved in delignifcation ratio and having a low Kappa number at reduced waste water load.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for oxygen bleaching lignocellulosic materials.

(Prior art) Oxygen bleaching using oxygen and alkali is widely known as a method for bleaching lignocellulosic materials, and many of its devices are currently in operation in major pulp and paper producing countries around the world, including Japan. .

In the oxygen bleaching method, alkali and oxygen are added to the pulp in a reaction vessel under heating and pressure to oxidize and decompose lignin, resin, etc. in the pulp, thereby bleaching the pulp. Oxygen bleaching is generally carried out on the pulp after cooking, but the pulp after cooking is contaminated with cooking waste called black liquor, which contains lignin, carbohydrates, organic acids, etc. The amount of solids contained in this black liquor depends on the degree of digestion in the cooking process, but in general, COD
(Chemical oxygen demand) is approximately 800 to 900 kg per ton of bone-dried pulp. The pulp after cooking is washed in a washing step before oxygen bleaching, and the solid content in the black liquor that accompanies the pulp entering the oxygen bleaching step is usually about 20 kg or more per ton of bone-dry pulp in terms of COD.

(Problem to be solved by the invention) In recent years, especially in response to environmental problems, it has become important to increase the delignification rate using oxygen bleaching and obtain pulp with a low kappa number in order to reduce the amount of wastewater load. Ta.

However, the oxygen bleaching method has the problem that delignification is inhibited by the solid content in the black liquor that accompanies the pulp, making it impossible to obtain pulp with a low kappa number. As a result of extensive research to solve the above-mentioned problems, the present inventors understood the relationship between the solid content in the black liquor accompanying the pulp and the delignification rate, and based on this knowledge, completed the present invention. I ended up doing it. An object of the present invention is to provide an oxygen bleaching method that solves the above-mentioned problems in the oxygen bleaching method and obtains pulp with a low kappa number.

(Means for Solving the Problems) When lignocellulosic material is oxygen bleached, the present invention converts the solid content brought in from the cooking process in the solution accompanying the lignocellulosic material into bone dry lignocellulosic material 1 in COD. This is an oxygen bleaching method for lignocellulosic materials, which is characterized by oxygen bleaching at a concentration of 10 kg or less per ton.

Oxygen bleaching in the present invention reduces the solid content of the black liquor brought in from the cooking process along with the pulp to 10K in COD.
g/1 ton of bone dry pulp or less, preferably 5 kg/1 ton of bone dry pulp or less.

The solids in the black liquor that accompanies the pulp entering the oxygen bleaching process are organic dissolved substances such as lignin, carbohydrates, and organic acids generated during the cooking process. This solid content inhibits delignification during oxygen bleaching. The relationship between solid content and delignification efficiency is shown in Figure 1 (examples and comparative examples described later).
When the solid content reaches 10 kg, the delignification efficiency improves rapidly and a pulp with a low kappa number can be obtained.

The amount of solids in the black liquor that accompanies the pulp after the cooking process is about 800 to 900 kg/1 ton of bone-dry pulp, so in the next washing process, it is washed so that the solid content is less than 10 kg/1 ton of bone-dry pulp. do. Cleaning methods include displacement cleaning, dilution and cleaning.
There are dehydration cleaning and press cleaning, etc., and equipment that applies these methods include rotary vacuum cleaners, diffusion cleaners, rotary pressure cleaners, horizontal belt cleaners, cleaning press equipment, dilution/extraction cleaners, etc. There is. By combining one or more of these washing machines in multiple stages, it is possible to achieve a solid content in the black liquor of 10 kg or less per ton of bone-dried pulp.

(Examples) The present invention will be explained in more detail below with reference to Examples, but the present invention is not limited by these Examples.

Example 1 Using domestic hardwood chips, kraft cooking was carried out at a liquid ratio of 3, an active alkali addition rate of 16% (N!□O based on the weight of bone dry chips), a maximum temperature of 170°C, and a holding time of 45 minutes, resulting in a kappa number of 18. .5 unbleached pulp was obtained. The COD of the black liquor accompanying the pulp after cooking was measured and the solid content was calculated to be 844 kg/1 ton of bone dry pulp.

The unbleached pulp was washed by a displacement washing method by controlling the amount of dilution water so that the solid content in the black liquor accompanying the pulp was 2.5 and 10 kg/1 ton of bone-dried pulp. After washing, the pulp was oxygenated at a rate of 2.0% (per weight of bone-dry pulp), an alium addition rate of 1.7% (as NaOH per weight of bone-dry pulp), a pulp concentration of 11%, and a reaction temperature of 10%.
0°C, reaction pressure 6Kg/ad.

Oxygen bleaching was performed under conditions of a residence time of 50 minutes. The kappa numbers after oxygen bleaching were 9.0, 9.9, and 10.5, respectively.

Comparative Example 1 Using the unbleached pulp obtained by cooking in Example 1, the solid content in the black liquor accompanying the pulp after washing was 15.2
The same washing as in Example 1 was carried out, except that the amount of washing water was adjusted to give 0 and 40 kg/1 ton of bone dry pulp, and after each washing, the pulp was oxygen bleached under the same conditions as in Example 1. . The kappa number after oxygen bleaching was 10.9.
It was 11.1.11.4.

The relationship between solid content and kappa number in Examples and Comparative Examples is
As shown in the figure, it is clear from the figure that by setting the solid content of the present invention to 10 kg/1 ton of bone-dry bulb or less, the delignification efficiency improves and pulp with a low kappa number can be obtained.

(Effects of the Invention) The oxygen bleaching method of the present invention has the advantage that pulp with a lower kappa number can be obtained compared to the conventional oxygen bleaching method, and in particular, the amount of wastewater load can be reduced.

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[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the solid content in the black liquor accompanying the washed pulp and the kappa number. Fig. 1 4゜D mouthparts ← Now outside / 80c ref 6t

Claims (1)

[Claims] When bleaching lignocellulosic material with oxygen, the solid content brought in from the cooking process in the solution accompanying the lignocellulosic material is reduced to 10 kg or less per ton of bone-dry lignocellulosic material in terms of COD. Oxygen bleaching method for lignocellulosic materials.