JP2000220088A - Bleaching of pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for reducing the COD value of bleaching effluent in no need of large alteration of existing bleaching installations and the pulp quality. SOLUTION: The pulp is treated with oxygen or ozone, then preliminarily bleached with hypochlorous acid and/or hypochlorite salt, followed by, without intermediate washing process, the main bleaching treatment with chlorine and/or chlorine dioxide. In this case, the available chlorine for the pulp to be used in the preliminarily treatment is set to 3-40 wt.% based on the pulp available chlorine in the main treatment to reduce the COD in the bleaching effluent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a method for reducing COD of bleaching effluents prior to chlorine and / or chlorine dioxide treatment.
The present invention relates to a method for bleaching pulp, wherein a pretreatment is performed by adding hypochlorous acid and / or hypochlorite.

[0002]

2. Description of the Related Art Generally, chemical pulp is required to have high whiteness. Therefore, chlorine bleaching is performed for the purpose of removing lignin as an impurity, followed by alkali extraction to remove chlorinated and oxidatively decomposed lignin. To improve the efficiency of extraction as a variant of alkali extraction, or to reduce the load of subsequent bleaching,
It is also known to add an oxidizing agent such as hydrogen peroxide or oxygen in the former alkali extraction process. It is also common to add chlorine dioxide to the chlorine stage in order to maintain pulp strength or reduce organic chlorine compounds in wastewater.

[0003] As chlorine bleaching chemicals, chlorine, hypochlorous acid, hypochlorite and the like are generally known. These compounds change their form depending on the pH, and a mixture of chlorine and hypochlorous acid at pH 3.5 or lower, hypochlorous acid near pH 3.5-4.5, and hypochlorous acid at pH 4.5 to 8. When the mixture of acid and hypochlorite is at pH 8 or higher, hypochlorite is mainly present. Among them, hypochlorous acid is known to be oxidized not only with lignin but also with cellulose, so that the pulp viscosity is reduced. Usually, pulp bleaching is performed in the pH range where the proportion of hypochlorous acid is low in consideration of the strength of pulp, and generally chlorine, which has relatively low oxidation reactivity to cellulose fibers,
The treatment is performed in a pH range in which hypochlorite is a main component.

The waste water of bleaching with chlorine bleach contains chlorine ions and is therefore corrosive, making it difficult to recover by a recovery boiler. A method of removing chlorine ions from chlorine dioxide bleaching wastewater is also known, but chlorine bleaching increases the chlorine ion concentration in the bleaching wastewater and it is difficult to sufficiently remove it.
Therefore, chlorine-based bleaching wastewater other than those used for dilution and washing is usually discharged out of the system.

[0005] Pulp mill wastewater including chlorine-based bleaching wastewater is subject to stricter COD regulations year by year due to environmental impact, and wastewater treatment such as coagulation sedimentation and activated sludge is being carried out. Although it is possible to reduce wastewater COD by collecting wastewater using ozone and hydrogen peroxide, which are non-chlorine bleaching chemicals, the cost of equipment and chemicals increases, and binding fibers remain in pulp. There are various problems, such as the ease with which the bleaching effluent COD can be improved.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to provide a method for reducing the COD of bleaching effluent without greatly changing the existing bleaching equipment and the quality of the bleach.

[0007]

The inventor of the present invention has made intensive studies on the pulp bleaching method. As a result, the pulp treated with oxygen or ozone was treated with hypochlorous acid and / or hypochlorite. A pre-bleaching process, and then performing a main treatment with chlorine and / or chlorine dioxide without intermediate washing, wherein an available chlorine amount relative to the pulp used in the pre-treatment is determined based on an available chlorine amount relative to the pulp used in the main treatment. It has been found that the bleaching effluent COD can be reduced by setting the amount to 3 to 40% by weight, and the present invention has been accomplished based on this finding.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The amount of hypochlorous acid and / or hypochlorite to be used in the pretreatment of the present invention is calculated as effective chlorine in terms of chlorine and / or chlorine dioxide added in the present treatment. In an amount corresponding to 3 to 40% by weight of the total available chlorine amount of When the effective chlorine addition amount in the pretreatment of the present invention is less than 3% by weight, the effect of reducing wastewater COD is reduced. If the addition is performed at a ratio exceeding 40% by weight, the consumption of the added chemicals does not proceed sufficiently in a short time, so that the addition efficiency is poor and the pulp strength may be reduced due to the progress of the oxidation reaction of cellulose. In addition, it is necessary to add an acid such as sulfuric acid in order to control the pulp pH when mixed with chlorine and / or chlorine dioxide added in the present treatment, and preferably 5 to 20% by weight.

Usually, the pH of the pulp after the oxygen treatment is alkaline, so that it is not necessary to adjust the pH of the pulp slurry by adding an alkali particularly in the above pretreatment. Since ozone-treated pulp usually shows acidity, an alkali is separately added during pre-treatment from the viewpoint of protection of cellulose fibers to adjust the pH of the pulp slurry to be alkaline. The pH is preferably 9 to 11.

[0010] The higher the temperature and pulp concentration in the pretreatment, the faster the reaction speed in the pretreatment and the shorter the pretreatment time, but the bleaching temperature and pulp used in the present treatment of the present invention can be shortened. There is no problem if the concentration conditions are satisfied, and a processing effect can be obtained.

There is no particular limitation on the time for the pretreatment of the pulp. However, it is possible to omit the necessity of newly installing a reaction tower which requires particularly high equipment costs, and to shorten the time from 10 seconds to 10 minutes in the sense that the effect of the treatment can be obtained. It is desirable that

As an example of a bleaching sequence incorporating the bleaching method of the present invention, the pretreatment (hereinafter abbreviated as h) of the present invention is performed after oxygen bleaching (hereinafter abbreviated as O), and the bleaching sequence has been widely adopted in Japan. Chlorine and chlorine dioxide bleaching (hereinafter abbreviated as C / D), followed by alkali extraction (hereinafter abbreviated as E), hydrogen peroxide bleaching (hereinafter abbreviated as P), and chlorine dioxide bleaching (hereinafter abbreviated as D). O- which bleaches to a whiteness of 86% or more.
The pulp from which lignin which is an impurity in the pulp is removed to some extent by h / C / DEPD bleaching and ozone bleaching (hereinafter abbreviated as Z) after oxygen bleaching is subjected to a pretreatment. OZh / D combined with chlorine dioxide bleaching followed by alkali extraction (hereinafter abbreviated as E / P) to which hydrogen peroxide is added as a second stage bleaching, and chlorine dioxide bleaching
A bleaching sequence such as -E / PD-bleaching is conceivable.
In the production of high brightness bleached pulp incorporating the bleaching method of the present invention, it is not necessary to increase the amount of bleaching chemicals used after the bleaching, and the pulp quality is inferior to the case where the method of the present invention is not incorporated. Not something.

As the hypochlorite source, besides the purchased hypochlorite solution, by-product hypochlorite at the time of preparing chlorine dioxide can also be effectively used. There is no difference in the respective effects of sodium salt and calcium salt as hypochlorite.

[0014]

The hypochlorous acid and hypochlorite added to the pulp as the pretreatment of the present invention effectively act as an agent for oxidizing and decomposing organic compounds such as lignin which are dissolved in the pulp. Subsequent addition of chlorine and / or chlorine dioxide to the pulp without washing adjusts the pulp to acidic. Here, unreacted hypochlorite is mainly converted to chlorine, and partly to hypochlorous acid. Oxidation reaction caused by chemicals remaining in the pretreatment simultaneously with chlorine and / or chlorine dioxide bleaching as is generally known. Progresses.

By performing the preliminary treatment for a short period of time with hypochlorite having a strong oxidizing action as described above, the dissolved components of the wastewater are easily oxidized in the main treatment, and the COD amount of the wastewater after this process is reduced. Seem.

[0016]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Evaluation of pulp and drainage was carried out by the following method. Whiteness The pulp whiteness was measured using Hunter reflectance in accordance with JISP8123. COD The COD of the bleaching wastewater was measured according to JISK0102.

<Example 1> Pretreatment with sodium hypochlorite using hardwood kraft pulp (K value: 7.6, whiteness: 46.0%) subjected to oxygen bleaching (hereinafter abbreviated as h)
, Followed by a main treatment by chlorine / chlorine dioxide treatment (hereinafter abbreviated as C / D). Hardwood kraft pulp slurry subjected to oxygen bleaching adjusted to pulp concentration of 3% using bleaching wastewater for dilution (washing wastewater of factory chlorine / chlorine dioxide bleached pulp) usually used in factories in a plastic bag in a thermostat After heating to a temperature of 60 ° C. and adding 0.2% by weight of sodium hypochlorite as available chlorine (vs. absolutely dry pulp) and reacting for 3 minutes to carry out a pretreatment, chlorine without washing The addition amount is 1.5% by weight (vs. absolutely dry pulp), and chlorine dioxide is 0.11% by weight (vs. absolutely dry pulp) as chlorine dioxide (this is 0.29% by weight as available chlorine (vs. absolutely dry pulp)) ), Followed by a reaction at a temperature of 60 ° C. for 17 minutes in a thermostatic oven to carry out this treatment. The total amount of available chlorine, including the pretreatment, is 1.
99% by weight (vs. dry pulp). Here, the pH adjustment in the pretreatment stage was not performed because the oxygen bleached pulp was alkaline and the hypochlorite aqueous solution was also alkaline. After this treatment, tap water was added to the pulp to dilute and wash the pulp to a pulp concentration of 1.5%. The obtained pulp was subjected to whiteness measurement and COD measurement of wastewater.

The obtained pulp was subsequently subjected to an alkali extraction treatment to which hydrogen peroxide was added (hereinafter abbreviated as E / P). The pulp obtained was adjusted to a pulp concentration of 15% using the diluted washing wastewater after the main treatment in a plastic bag, and then tap water and sodium hydroxide were added in an amount of 1.1% by weight (vs. absolutely dry pulp). Hydrogen was added in an amount of 0.3% by weight (vs. absolutely dry pulp) to adjust the pulp concentration to 10%.
Reaction was performed at 90 ° C. for 90 minutes. After E / P treatment, tap water was added to the pulp to dilute and wash the pulp to a pulp concentration of 1.5%. The obtained pulp was subjected to whiteness measurement and COD measurement of wastewater.

The obtained pulp was subsequently treated with chlorine dioxide (hereinafter abbreviated as D). The obtained pulp is adjusted to a pulp concentration of 15% using diluted washing wastewater after E / P treatment in a plastic bag, and then 0.4% by weight of tap water and chlorine dioxide as chlorine dioxide (vs. absolutely dry pulp) is added. Then, the pulp concentration was adjusted to 10%, and the reaction was carried out at 75 ° C. for 180 minutes in a thermostat. D. After treatment, tap water is added to the pulp to dilute and wash to a pulp concentration of 1.5%.
Dehydration was performed to 0%. The whiteness of the obtained pulp was measured. Table 1 shows the measurement results.

<Example 2> Compared to Example 1, the amount of chlorine added was 1.7% by weight (based on absolute dry pulp), and the effective chlorine amount including pretreatment was 2.19% by weight (based on absolute dry pulp). ) Was carried out in the same manner as in Example 1 except that the above was changed.

<Comparative Example 1> Same as Example 1 except that the pretreatment with sodium hypochlorite was not performed as in Example 1, but the amount of chlorine added was 1.7% by weight (vs. absolutely dry pulp). Was performed in the same manner as in Example 1 except that the total available chlorine amount was 1.99% by weight (based on absolutely dry pulp).

<Comparative Example 2> Same as Example 2 except that the pretreatment with sodium hypochlorite was not carried out as compared with Example 2, but the amount of chlorine added was 1.9% by weight (vs. absolutely dry pulp). Was performed in the same manner as in Example 2 except that the total available chlorine amount was 2.19% by weight (based on absolutely dry pulp).

[0024]

[Table 1] Table 1

[0025]

As shown in Table 1, it was confirmed that the pretreatment with sodium hypochlorite was effective in reducing the amount of COD in the waste water and the bleaching process after the treatment according to the present invention.
It was also confirmed that the pulp brightness after the final bleaching treatment was improved as another effect. In addition, liquid chlorine is often purchased in Japan as a chlorine source, and the present invention can reduce the amount of purchased chlorine, which is economically advantageous.

Claims (2)

[Claims] 1. An oxygen- or ozone-treated pulp is pretreated with hypochlorous acid and / or hypochlorite and then treated with chlorine and / or chlorine dioxide without intermediate washing. A bleaching method for carrying out the present treatment, wherein the amount of available chlorine to pulp used in the preliminary treatment is 3 to 40% by weight of the amount of available chlorine to pulp used in the main treatment. 2. The bleaching method according to claim 1, wherein the time of the pretreatment is 10 seconds to 10 minutes.