JP3275271B2 - Bleaching method of chemical pulp - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the treatment of chemical pulp for papermaking, and more particularly to an improvement in the delignification and bleaching treatment of chemical pulp.

[0002]

2. Description of the Prior Art Bleaching of chemical pulp is carried out by a multi-stage bleaching process. Conventionally, chlorine bleaching chemicals have been used as bleaching agents in this multi-stage bleaching. Specifically, a combination of chlorine (C), hypochlorite (H), and chlorine dioxide (D) provides, for example, CEHD, C
Bleaching by a sequence such as / DEHED (C / D is a combined bleaching stage using chlorine and chlorine dioxide, and E is an alkali extraction stage) has been performed.

[0003] However, these chlorine-based bleaching chemicals produce by-products of organic chlorine compounds that are harmful to the environment during use, and the environmental pollution of bleaching wastewater containing these organic chlorine compounds has become a problem. Organochlorine compounds are generally analyzed and evaluated by the AOX method, for example, US Environmental Agency Act (EPA - METHOD) 9020.

[0004] As a method for reducing the amount of chlorine-based bleaching chemicals used, an oxygen bleaching method in which oxygen is allowed to act under high temperature and pressure in an alkaline medium at the first stage after the digestion treatment has been developed and is now widely used. According to this oxygen bleaching method,
Since 40-50% of the lignin remaining after the digestion is removed, it is possible not only to reduce the amount of the bleaching chemical used in the subsequent stage, but also to recycle the waste liquid of the oxygen bleaching process to the digesting process, and Energy can be recovered,
In addition, the load of wastewater treatment can be reduced.

[0005] However, in order to obtain high whiteness by the introduction of the oxygen bleaching method, it is still necessary to use a considerably large amount of chlorine-based bleaching chemicals in the subsequent stage, and as a result, a considerable amount of organochlorine compounds is added. Living is inevitable.
As an improvement of the oxygen bleaching method, a method of bleaching pulp after digestion using oxygen and a peroxide is disclosed in JP-A-62-53497 and JP-A-3-14687. However, as a result of investigations by the present inventors, delignification and whitening were not sufficient, and a considerable amount of chlorine bleaching chemicals had to be used.

Further, Japanese Patent Publication No. 2-15671 discloses that a system of oxygen, peroxide, a cyclic keto compound and / or a cyclic amino compound enables high whiteness without deterioration of pulp. However, this method makes it difficult to recover and reuse expensive cyclic keto compounds and / or cyclic amino compounds, so that the chemical cost becomes considerably high,
Economically disadvantaged.

Further, as a method of subjecting a chemical pulp after the digestion treatment to an acid treatment and then performing bleaching with hydrogen peroxide (Ep), JP-A-51-102103, JP-A-56-85489 and JP-B-58-85489. No. -17679 has been proposed, in which, after acid treatment of pulp, delignification is increased by performing hydrogen peroxide bleaching at 100 ° C. or less without adding oxygen. . However, as a result of investigations by the present inventors, none of these methods necessarily provide a sufficient effect in terms of delignification and high whiteness.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an economical method for bleaching chemical pulp to promote delignification and whitening without deterioration of the pulp and to reduce the load on the subsequent bleaching stage. Is to provide a specific means. Thus, the amount of chlorine-based bleaching chemicals used in the subsequent bleaching stage is reduced, and the environmental pollution of the bleaching wastewater is reduced.

[0009]

DISCLOSURE OF THE INVENTION The present inventors have worked on a method for delignifying and bleaching pulp economically, and disclosed in Japanese Patent Application Laid-Open No. 3-64589 at 50-90 ° C. after acid treatment. An oxygen bleaching method using peroxide at atmospheric pressure to 3 kg / cm ² was announced. The present inventors have further researched and obtained high-brightness and high-quality pulp without the need for drastic changes in the commonly used bleaching equipment, and significantly reduced the amount of chlorine-based bleaching chemicals used. As a result of intensive studies on the method to be carried out, after the pulp after the digestion is acid-treated, by performing oxygen bleaching using peroxide in combination, it is possible to effectively obtain delignified and highly whitened pulp. Find out,
The present invention has been completed.

That is, the present invention is characterized in that a digested chemical pulp is subjected to an acid treatment step and then to a delignification / bleaching step using a peroxide and pressurized oxygen in an alkaline medium. This is a method of bleaching chemical pulp.
INDUSTRIAL APPLICABILITY The present invention is suitable for bleaching chemical pulp for papermaking, and particularly suitable for delignification and bleaching of kraft pulp derived from hardwood and softwood.

In the present invention, the digested chemical pulp is first treated with an acid solution (hereinafter, this acid treatment may be referred to as A stage). The acid treatment of the present invention depends on the state of the pulp.
30%, preferably 2-15%, most preferably 3-10%
At a temperature of 30 to 95 ° C., preferably 40 to 80 ° C., for a treatment time of 5 to 120 minutes, preferably 15 to 60 minutes.

The pH of the acid solution in the acid treatment step is preferably
It is preferably 1 to 5, more preferably 1.5 to 3. Used
The acid used is preferably an inorganic acid, such as sulfuric acid, nitric acid, sulfurous acid, and nitrous acid.
Acids and mixtures of these acids are preferably used. What
However, if sulfuric acid and sulfurous acid are only available at low cost,
Low corrosion resistance to the material
You. Especially after kraft cooking, after acid treatment
The acid solution of the above as a chemical supplement in kraft cooking
Produces the convenience of being available.

[0013] The pulp after the acid treatment is treated with an acid solution by a dehydrator.
And then preferably washed. Separated
The acid solution is reused in the acid treatment and / or the digestion process
Can be used for subsequent countercurrent washing of pulp, and in some cases cooking
It can also be recycled into the process. Acid treatment of the present invention
The dehydration and washing steps are included in the process.

The pulp after the acid treatment step is subsequently subjected to a delignification / bleaching step using peroxide and pressurized oxygen.
(Hereinafter, this bleaching step may be referred to as a peroxide combined oxygen bleaching step or an Op stage.)
Pulp is subjected to delignification and bleaching action by peroxide and pressurized oxygen in an alkaline medium. The peroxide and pressurized oxygen effectively act on the pulp simultaneously.

As the alkaline agent in the Op stage, caustic soda, caustic potash, lime, soda ash and the like can be used. Among them, caustic soda is inexpensive and can be preferably used because it can be recycled to the digestion step to reduce the replenishment of chemicals in the digestion step. The amount of alkaline agent used is
It is preferably 0.5 to 6 % per absolute dry pulp in terms of NaOH,
3 % is more preferred. Delignification, bleaching effect and the amount of the alkali agent is less than 0.5% is low, the viscosity of the pulp is significantly decreased with more than 6%.

As an oxygen source in the Op stage, oxygen gas and air can be used, but oxygen gas is preferable. The amount of oxygen used is preferably 0.5 to 5.0% per bone dry pulp. O
operating pressure of p stage is preferably 2.5~10Kg / cm ^2, 3.5~8Kg /
cm ² is more preferred.

Examples of the peroxide include hydrogen peroxide, an adduct of hydrogen peroxide and an inorganic salt, sodium peroxide,
Over ants acid, peroxides, inorganic and organic, such as peracetic acid can be used, generally hydrogen peroxide is used. The amount of peroxide used is 0.1 to 6 per absolutely dry pulp in terms of 100% hydrogen peroxide.
%, More preferably 0.2 to 2.5%. If the amount of peroxide used is less than 0.1% , the delignification / bleaching effect is low, and if it is more than 6% , the efficiency of peroxide decreases.

The order of adding the chemicals to the pulp in the Op stage of the present invention is preferably an alkali agent and then oxygen, and the peroxide is preferably added after the addition of the alkali agent and immediately before, simultaneously with and immediately after the addition of oxygen. It is preferred to be performed in.

The pulp concentration in the Op stage of the present invention is preferably from 7 to 30%, more preferably from 10 to 20%. Temperature is 60-130
C is preferable, and 90 to 110 C is more preferable. The processing time is preferably from 20 to 150 minutes, more preferably from 30 to 90 minutes.

In addition, we have unexpectedly discovered that acid
It has been found that pulp pretreated with an acid using a mixture of sulfuric acid and sulfurous acid in the treatment is delignified and bleached very effectively in the subsequent peroxide-oxidized bleaching step.

In this case, the ratio of sulfuric acid to sulfuric acid in the mixture is preferably 0.01 to 2, more preferably 0.02 to 1, and most preferably 0.05 to 0.5 in terms of sulfuric acid / sulfuric acid (weight ratio). The pH of the solution in the acid treatment and other treatment conditions, the method of operating the waste liquid, and the like are the same as the above-described acid treatment conditions.

Further, in the Op stage, it is preferable to use a magnesium compound in order to prevent the viscosity of the pulp from decreasing and to stabilize the peroxide. As magnesium compounds, magnesium sulfate, magnesium hydroxide,
Although magnesium oxide, magnesium carbonate, magnesium nitrate and the like can be used, magnesium sulfate is generally used. The amount of the magnesium compound used is preferably 0.01 to 0.4%, more preferably 0.02 to 0.1%, based on the absolutely dried pulp as magnesium ions. Addition of the magnesium compound, an alkali agent, to be made in addition by <br/> Ri previous oxygen and peroxide preferred. In particular,
After neutralizing the acid-treated pulp with an alkaline agent,
The nesium compound is added, followed by the required amount of
A method of adding a luka agent, a peroxide and oxygen, or
Or, add magnesium compound to pulp after acid treatment
Then, the necessary amount of alkali agent (neutralized acid
), And peroxide and oxygen are preferably added.
Appropriately adopted.

The bleached pulp obtained by the A-Op of the present invention has a considerably high whiteness as it is, but a pulp having a higher whiteness can be obtained by full bleaching which is continuously added with multiple bleaching steps. It is possible to get. In that case, a high degree of delignification has already been performed in the Op stage, and since the whiteness is high, the subsequent bleaching can be carried out with little or no use of chlorine and hypochlorous acid.

For example, after the A-Op step, chlorine dioxide bleaching (D) and then peroxide bleaching (P) are carried out (A-Op
Op-DP), without the use of chlorine or hypochlorite, the current representative full-bleach sequence OC-
A pulp product having a high viscosity and high whiteness equivalent to or higher than / DEH-D can be obtained.

In this case, the condition of the chlorine dioxide bleaching (D) stage is the same as the condition of the D stage which is performed in a normal full bleaching sequence. For example, pulp concentration 7-30%, temperature
40-90 ° C, time 1-4Hr, chlorine dioxide usage 0.1-2.0
%.

The peroxide bleaching (P) stage of the present invention is carried out under the conditions of the P stage, which is carried out in a normal full-bleach sequence. That is, the pulp concentration is 7 to 30%, the temperature is 40 to 100 ° C., and the time is 1 to 4 H.
Performed in the range r. Examples of the alkaline agent include caustic soda, caustic potash, lime, soda ash, and the like. Generally, caustic soda is used, and the amount of the alkaline agent used is in the range of 0.1 to 2% in terms of caustic soda.

[0027] As the peroxide used in the P stage, hydrogen peroxide, an adduct of hydrogen peroxide and inorganic salts, peroxides soda, peroxide ants acids, inorganic and organic, such as peracetic acid peroxide In general, hydrogen peroxide is used, and the amount of peroxide used is in the range of 0.1 to 2.0% based on 100% hydrogen peroxide, based on absolutely dried pulp.

It is considered that the action of the acid treatment in the present invention is to remove heavy metals having a peroxide decomposing activity from pulp and to change lignin. That is, first, in the pulp after the acid treatment, there is almost no heavy metal, so that unnecessary decomposition of peroxide is reduced, and the amount of peroxide involved in bleaching is maintained. Next, in the acid treatment, delignification does not progress in itself, but delignification further progresses in the oxygen bleaching after the acid treatment and the peroxide combined oxygen bleaching after the acid treatment. Is shown. The delignification effect becomes more remarkable in peroxide combined oxygen bleaching. The above shows that the acid treatment has an effect of changing lignin so as to be easily subjected to decomposition by oxygen and peroxide.

It is considered that due to these effects of the acid treatment, delignification and high whiteness proceed efficiently and high quality pulp can be obtained. In carrying out the A-Op of the present invention, an acid supply line and a tank (or tower) for acid treatment are provided for an oxygen bleaching facility that is currently in widespread use.
In addition, conventional equipment such as dehydration / washing equipment and a peroxide supply line can be provided, and no major equipment change is required. The materials of these attached facilities are SUS304 and SUS30 commonly used in chemical plants.
4L, SUS316 or SUS316L is preferably used. Therefore, the present invention can be implemented without requiring a large capital investment.

In carrying out the A-Op-DP full breach of the present invention, the following D-stage and P-stage can use the existing D-stage device, E-stage or P-stage device, respectively. , Without capital expenditures.

[0031]

Next, the present invention will be described specifically with reference to examples. The amount of each chemical used is indicated by weight% per absolutely dry pulp,
The amount of hydrogen peroxide used is calculated as 100% hydrogen peroxide. The pulp used was unbleached L-pulp after kraft cooking (Hunter whiteness 32.7%, K value 12.9, viscosity 36.0 cP
). The analysis and evaluation were performed by the following method.

Whiteness: JIS-P8123 (Hunter whiteness method) K value: TAPPI K value method Viscosity: J.TAPPI No.44 method AOX: Based on the EPA METHOD 9020 method, Mitsubishi Kasei Co., Ltd.
Measured with TSX-10

Example 1 Sulfuric acid was added to pulp that had not been exposed to L wood after kraft cooking,
2. The pulp concentration was 5%, and an acid treatment was performed at 45 ° C for 60 minutes. Next, water was added to a pulp concentration of 2.5%, followed by dehydration to a pulp concentration of just over 20%. Subsequently, the mixture was neutralized with an aqueous NaOH solution to obtain an acid-treated pulp having a pH of 7.5 and a pulp concentration of 20%.
The K value of the pulp after the acid treatment was 12.9, which was the same value as the pulp before the acid treatment.

The acid-treated pulp was mixed with magnesium sulfate for 0.1 minute.
After adding 25%, put it in a pressurized pulp bleaching device,
After heating to 6 ° C, NaOH 1.55%, hydrogen peroxide 0.5%,
2% oxygen was added, pressurized and mixed. The pulp concentration in this state was 10.5%. 7 kg / cm ^{2 in} pulp mixture first
After applying pressure, gradually reduce the pressure at a constant speed,
It was adjusted to 4 kg / cm ² after 60 minutes. During this time, the temperature of the pulp mixture was maintained at 106 ° C. After 60 minutes, cool and depressurize,
The pulp mixture was poured into cold water to form a pulp slurry having a pulp concentration of 2.5%, which was dewatered to obtain bleached pulp.

Example 2 Using an acid mixture of sulfurous acid and sulfuric acid (weight ratio = 0.2),
The same acid treatment and bleaching as in Example 1 were carried out except that the acid treatment was carried out at 30 ° C. for 30 minutes.

Example 3 The acid treatment was carried out at a temperature of 80 ° C. for 15 minutes, and the amount of hydrogen peroxide used was 1.0%
Acid treatment and bleaching were carried out in the same manner as in Example 1 except for the above.

Comparative Example 1 Similar to the current oxygen bleaching method, bleaching was carried out in the same manner as in Example 1 except that no acid treatment step was performed, and no hydrogen peroxide was used, and only oxygen was used.

Comparative Example 2 The same treatment and bleaching as in Example 1 were carried out except that neutral water was used in place of the acid in the acid treatment step of Example 1.

Comparative Example 3 The same treatment and bleaching as in Example 1 were carried out except that in the acid treatment step of Example 1, the pH was adjusted to 6 with sulfuric acid and 1% of EDTA was used instead of pH 2.

Comparative Example 4 The same acid treatment and bleaching as in Example 1 were carried out except that only oxygen was used without using hydrogen peroxide in the bleaching step.

Comparative Example 5 In the bleaching step, the same acid treatment and bleaching as in Example 3 were performed except that oxygen was not supplied at a temperature of 90 ° C. and normal pressure. Table 1 shows the results of Examples 1 to 3 and Comparative Examples 1 to 5.

[0042]

[Table 1]

Example 4 The same acid as in Example 1 was used except that the temperature of the acid treatment was 55 ° C. for 30 minutes, the amount of NaOH used in the bleaching step was 2.0%, and the amount of hydrogen peroxide used was 2.0%. After treatment and bleaching, the resulting pulp is bleached with chlorine dioxide under the following conditions (D)
Next, hydrogen peroxide bleaching (P) was performed. After bleaching, the waste liquids of the D stage and the P stage are combined to form an organic chlorine compound (AOX).
The amount was measured.

[0044]

Comparative Example 6 As the current oxygen bleaching method, bleaching was carried out in the same manner as in Comparative Example 1, and the resulting pulp was subjected to C / D (effective chlorine substitution rate of 10% by D) in accordance with generally carried out conditions. %)-
A subsequent bleaching stage of EHD was performed. After bleaching, the waste liquids of all the bleaching stages were combined and the amount of the organic chlorine compound (AOX) was measured.

Table 2 shows the results of Example 4 and Comparative Example 6.

[0047]

[Table 2]

[0048]

According to the present invention, delignification which is superior to the current oxygen bleaching method can be performed without a large decrease in viscosity, and a bleached pulp having a considerably high whiteness can be obtained. . This bleached pulp has a high degree of delignification and whiteness, so that the use of chlorine bleaching chemicals in the subsequent bleaching stage can be greatly reduced. In addition, without using any chlorine and hypochlorite in the subsequent bleaching stage, A
Full bleaching with extremely low OX generation is possible.
As a result, by-products of organic chlorine compounds can be significantly reduced, and the environmental pollution of the bleaching wastewater can be significantly reduced. Furthermore, the resulting pulp has a high viscosity.

In carrying out the method of the present invention, the oxygen bleaching equipment currently in widespread use can be used without any change. In facilities that already have bleaching sequences, including oxygen bleaching, full bleaching can be performed without significant additional investment.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-64589 (JP, A) WO 91/11554 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) D21C 9/10-9/16

Claims (1)

(57) [Claims] 1. A digested chemical pulp is subjected to an acid treatment step using a mixture of sulfuric acid and sulfurous acid, and then subjected to delignification by peroxide and pressurized oxygen in an alkaline medium.
A method for bleaching chemical pulp, which comprises performing a bleaching step.