JPH04222286A - Method for bleaching lignocellulosic substance - Google Patents

Abstract

PURPOSE:To obtain a pulp, having high whiteness and hardly fading by specifying the amount of an added alkali and the treating temperature in a bleaching stage for pulp obtained from a lignocellulosic substance with a hypochlorite. CONSTITUTION:An alkali such as caustic soda in an amount of 1.0-3.0%, preferably 1.5-2.0% based on the weight of bone-dry pulp obtained from a lignocellulosic substance is added to the pulp and treatment is carried out at 60-90 deg.C, preferably 70-75 deg.C in a bleaching stage for the aforementioned pulp with a hypochlorite.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for bleaching lignocellulosic materials.

More specifically, the present invention relates to a multi-stage pulp bleaching process that includes an improved hypochlorite bleaching step that causes less discoloration of the pulp.

[Conventional technology]

In order to use lignocellulosic materials in many paper product applications, it is necessary to bleach the resulting pulp by chemical or mechanical action.

For example, kraft pulp made from wood is generally manufactured using bleaching chemicals such as chlorine, hypochlorite (hypo), chlorine dioxide, oxygen, hydrogen peroxide, and caustic soda, except for use in packaging materials such as bags and adhesive tape. And lignin, which is the cause of coloring of unbleached pulp, is removed by bleaching with a bleaching aid.

When bleaching chemical pulp that requires pulp strength,
In order to maintain the strength of the pulp fibers themselves and to minimize the decomposition of carbohydrates during bleaching, a multi-stage bleaching sequence was adopted in which the bleaching conditions were kept as mild as possible, avoiding excessive bleaching in a single stage. It is normal to have one.

In the multi-stage bleaching process, the pulp is first treated with chlorine to chlorinate the lignin contained in the pulp and make it soluble in water. When the pulp is then treated with an alkali such as caustic soda, the chlorinated lignin is leached from the pulp to the liquid side. The pulp is then further treated with bleaching chemicals such as hypochlorite, chlorine dioxide, and caustic soda, alternating with bleaching aids to decompose and remove the small amount of lignin remaining in the pulp, and finally undergoes a dust removal process. As a result, pulp with high whiteness is obtained.

Chlorine treatment in this multi-stage bleaching is C, and alkali treatment is E.
, hypochlorite treatment as H, chlorine dioxide treatment as D, and hydrogen peroxide treatment as P. Typical bleaching sequences are C-E-H-E-D, C-E-D-E. -D, C-E-
It is expressed as H-P-D.

Normally, five to six bleaching stages were required to obtain completely bleached pulp, but in recent years, in the alkaline extraction stage following the chlorination stage, oxygen (EO), hydrogen peroxide (EP)
The development of technologies that use chlorine or hypochlorite (EH) or several of these simultaneously (e.g. EPO) has made it possible to shorten the bleaching sequence to 3 or 4 steps, which It is called short sequence.

Furthermore, as a method for significantly reducing the amount of residual lignin in the pulp entering the bleaching stage, there is an oxygen bleaching method (or O2 bleaching method) which is carried out between the cooking step and the bleaching step.

In this oxygen bleaching method, alkali and oxygen are mixed with the cooked pulp, and while the pulp is passed through a reaction vessel under pressure, the lignin in the pulp is oxidized and decomposed and solubilized in the alkali.
This is a method of delignification. In this method, since the waste liquid does not substantially contain chlorine ions, it is recycled as a washing liquid for the pulp that has already been cooked, and is finally recovered together with the cooking waste liquid (Japanese Patent Laid-Open No. 47-5202 and Showa 4
(Refer to Publication No. 9-7503), since organic matter is used as fuel in the recovery boiler, the pollution load from bleached wastewater can be significantly reduced, leading to a reduction in wastewater treatment costs. It has been adopted in many major countries.

This pulp oxygen bleaching method is applied in combination with the conventional bleaching method using chlorine-based chemicals to produce not only fully bleached pulp but also semi-bleached pulp.

For example, O2-C/D-E (Japanese Patent Publication No. 47-7202), C/D-O2-D-E-D (Japanese Patent Publication No. 47-102)
41 Publication), O2-Dc-O2-E (Special Publication No. 47-4)
4441 Publication), O2-D/C-E-D (Special Publication No. 1983)
Bleaching sequences such as JP-A-17605) are well known.

Furthermore, regarding the combination of oxygen bleaching and bleaching with chlorine chemicals, see Paper Trade Journal,
August/5, 49 (1968) and Tappi
54(11):1893 (1971), various bleaching sequences have been proposed.

The whiteness of fully bleached pulp obtained by these bleaching sequences as measured by a Hunter brightness meter is usually in the range of 80 to 95%, but it is possible to
The degree of fading (J, TAPPI No. 21), which is determined from the difference between the whiteness measurements before and after the whiteness measurement, is significantly influenced by the bleaching sequence or bleaching conditions.

For example, in a short sequence, the number of bleaching stages is less than that of a normal multistage bleaching, so if the bleaching conditions are not severe, the level of pulp brightness will not be the same as that of pulp from a multistage bleaching sequence, whereas the bleaching conditions As a result, the whiteness of pulps from short sequences, especially softwood pulps, is highly dependent on the pulp quality, especially the strength, since increasing the severity will cause damage to the pulp fibers and reduce the pulp viscosity, which in turn is accompanied by a loss of mechanical strength of the pulp. Since the balance has to be taken into account, the standards cannot be maintained high and the color fading becomes a problem.

For this reason, conventional multi-stage bleaching sequences are still used in mills producing and selling BKP (bleached kraft pulp).

Discoloration can be improved by using chlorine dioxide or hydrogen peroxide as bleaching chemicals or by enhancing extraction with alkali as a bleaching aid. In particular, the higher the bleaching temperature and chemical addition rate and the longer the bleaching time, the greater the improvement effect (The
Bleaching of Pulp, 382, 384
, TAPPI PRESS (1978) and N. Har
tler, TAPPI 43 (11) 903 (1960
)reference).

On the other hand, hypochlorite (hypo) has lower chemical costs than chlorine dioxide and hydrogen peroxide, and has excellent chemical handling and pulp bleaching properties, but it has a negative effect on the discoloration of bleached pulp. However, the higher the addition rate of hypo, the higher the bleaching temperature, and the longer the bleaching time, the worse the discoloration of the pulp (The Bleaching
of Pulp383, TAPPI PRESS (19
79))).

The reason for this is said to be that carbonyl groups are introduced at the ends of the cellulose chains of bleached pulp by hypobleaching (WH. Rapson, C. B. Ander-so).
n,G. F. King, TAPPI 41(8)442
(1958)).

Furthermore, the decrease in pulp viscosity in the hypo stage was more pronounced than in other bleaching chemicals, and improvements were made in adjusting the temperature, pH before and after bleaching, shortening the reaction time, and using viscosity reduction inhibitors such as urea and sulfamic acid. It's elaborate.

The conditions of the usual hypo bleaching stage are to use calcium hypochlorite or sodium hypochlorite as the hypochlorite, the addition rate to be 0.1 to 3% per bone-dry pulp weight, and the pulp concentration to be 8 to 3%. 15%, reaction temperature 50°C or less, reaction time 12
It is around 0 minutes.

To prevent pulp viscosity loss in the hypo stage, caustic soda is commonly used as an alkali source to reduce the final p of the hypo stage.
H is adjusted to 7 to 8, and the caustic soda addition rate for this is 0.5% or less based on the weight of bone-dry pulp (The Bleach-ing of Pulp,
TAPPI PRESS 101-112 (1979)
reference).

In recent years, high-temperature hypobleaching has been developed, and this method is carried out at a high temperature of 60 to 80°C for a short reaction time of 5 to 20 minutes; However, this method focused on the fact that if the reaction time was shortened, the decrease in pulp viscosity was small, but it still had the same problem with the conventional hypobleaching method in terms of discoloration.

As described above, although hypobleaching has the advantage of low bleaching cost, its biggest drawback is the discoloration of pulp.

[Problem to be solved by the invention]

In view of the current situation, the present inventors conducted intensive research on hypo-bleaching, and found that in hypo-bleaching, alkali is added in excess of that in normal hypo-bleaching, and bleaching is carried out at a temperature higher than the normal hypo-bleaching temperature. The present invention was completed based on the discovery that not only the whiteness of the pulp can be increased while maintaining the pulp strength at a certain level, but also the fading resistance can be significantly improved.

Therefore, the present invention utilizes the advantages of hypobleaching,
The purpose of this invention is to provide a pulp bleaching method that can obtain stable pulp quality with significantly less discoloration while maintaining pulp strength and whiteness at a high level. purpose.

[Means to solve the problem]

The present invention provides a bleaching method comprising the step of treating pulp obtained from lignocellulosic material with hypochlorite,
1.0% per weight of bone dry pulp by drifting with hypochlorite.
This method of bleaching lignocellulosic materials is characterized by adding 0% or more alkali and treating pulp at a temperature of 60° C. or higher.

The chlorine compounds used in the chlorine stage (C) of the bleaching method of the present invention are known chlorine and chlorine dioxide, and these are used alone or in a mixture in any proportion.

As for the method of adding chlorine and chlorine dioxide, both may be mixed, or chlorine may be added within several seconds after the addition of chlorine dioxide. Alternatively, a portion of chlorine dioxide may be added first, and then the remaining chlorine dioxide and chlorine may be mixed and added.

The addition rate of chlorine compounds per weight of bone-dry pulp, expressed as the amount of available chlorine, is proportional to the lignin content of unbleached pulp.
It ranges from 1 to 5% of lignin.

The reaction temperature in stage C is 20 to 60°C, the pulp concentration during the reaction is 1 to 10%, the reaction time is 5 to 60 minutes, and the pH of the liquid at the end of the reaction is in the range of 2 to 5.

After the bleaching step of Stage C, a known pulp washing or dewatering step follows.

The alkali used in the alkaline stage (E) of the method of the present invention is selected from alkaline compounds represented by caustic soda, and is 0.5 to 3% (NaOH
The reaction temperature is 40 to 70°C, the pulp concentration during the reaction is 5 to 20%, the reaction time is 30 to 120 minutes, and the pH of the liquid at the end of the reaction is in the range of 8 to 12. Even after the extraction step in stage E, the washing or dehydration process of the pulp is subsequently performed as in the case of stage C.

The hypochlorite used in the hypochlorite bleaching stage (H) of the method of the present invention is calcium hypochlorite or sodium hypochlorite, but precipitation and separation of calcium compounds and generation of scale may occur. Considering workability such as prevention, sodium hypochlorite is preferable, although the chemical cost is high.

The addition rate of Hypo per bone dry pulp weight is 0.1 to 3
0%, preferably 0.3 to 1.0%, and the alkali used as an auxiliary chemical, most typically caustic soda, is 1.0 to 3.0% per bone dry pulp weight.
% (in terms of NaOH), preferably in the range of 1.5 to 2.0%.

In the present invention, it is important to adjust the pH using an alkali in the H stage, and the pH of the liquid at the end of the reaction is adjusted to be in the range of 8 to 12.

If the alkali addition rate is less than 1%, the alkalinity will be insufficient and the improvement in fading will be unsatisfactory;
The above is not preferable because the effect reaches a plateau and leads to wasteful use of chemicals.

The reaction temperature in the H stage is 60-90°C, preferably 70-70°C.
The temperature is 5°C, the pulp concentration during the reaction is 5 to 20%, and the reaction time is in the range of 30 to 150 minutes.

The improvement in discoloration of the pulp in the H stage according to the present invention is more pronounced at higher temperatures.

If the temperature is below 60°C, there will be little improvement effect, and if it is above 90°C, the effect will reach a plateau, leading to a loss of thermal energy, so it is better not to raise the temperature above 90°C.

The reason why the discoloration property of the pulp is improved in the H stage of the present invention is thought to be that the terminal carbonyl group undergoes air oxidation and loses its discoloration property under high alkali and high temperature conditions.

In the H stage as well as the other stages, the pulp after the reaction is subsequently washed or dehydrated.

The ClO2 used in the chlorine dioxide stage (D) is obtained from a known chlorine dioxide generation method.

The addition rate of chlorine dioxide per weight of bone dry pulp is ClO
Range of 0.2 to 3% in terms of 2, reaction temperature 40 to 90℃
pulp concentration is in the range of 5 to 15%, reaction time is in the range of 15 to 300 minutes, and pH at the end of the reaction is in the range of 2 to 6.
is within the range of

The present invention provides C(C/D or D/C)-E-H-D-E
-D, C-E-H-D, C-E-H-P-D, etc., are used in multi-stage bleaching incorporating H stages, and may be combined with oxygen bleaching.

That is, the present invention can be used independently for bleaching chemical pulps such as ordinary kraft pulp, modified kraft pulp, alkali pulp, and sulfite pulp, oxygen bleached pulps of these pulps, and other wood pulps and non-wood pulps. Alternatively, it can be used alternatively as part of a conventional bleaching sequence.

〔Example〕

The present invention will be described in more detail with reference to Examples below, but the present invention is of course not limited to these.

In the Examples and Comparative Examples shown below, the bleaching conditions for the lignocellulosic material and the measurement of the physical properties of the resulting pulp were carried out by the following procedures unless otherwise specified.

Oxygen bleaching (O stage) 50 g of bone-dry hardwood unbleached kraft pulp (mixed material of domestically produced hardwood, kappa number 16.6) was placed in a 1 liter electric heating autoclave, and a predetermined amount of NaOH proportional to the kappa number was added. (kappa number x 0.1%), and then added a predetermined amount of water to make the pulp concentration 15%.Then, the temperature was raised while stirring with a stirrer, and after reaching 100℃, oxygen gas (purity 99 .99%) was added until the gauge pressure reached 5 kg/cm2, and then held for 60 minutes.

After the reaction was completed, the sample was taken out from the autoclave, washed with water, and centrifugally dehydrated.

Chlorine bleaching (C stage) Put 50 g of extremely dry hardwood unbleached kraft pulp (mixed material of domestic hardwood, kappa number 16.6) or oxygen bleached pulp obtained above (kappa number -10.4) into a plastic bag. A predetermined amount of chlorine gas is added in proportion to the kappa number (kappa number x 0.2%), and the pulp concentration is further increased to 5%.
A predetermined amount of water was added to the mixture, and the mixture was thoroughly mixed by hand stirring, and the mixture was immersed in a constant temperature bath at 50°C for 30 minutes.

After the reaction was completed, the sample was taken out from the bag, washed with water, and dehydrated by centrifugation.

Alkaline extraction (E stage) Take the centrifugally dehydrated C stage treated pulp into a plastic bag,
A predetermined amount of alkali (caustic soda) proportional to the kappa number
(Kappa number x 0.1%), and further added a predetermined amount of water to make the pulp concentration 10%, mixed thoroughly by hand stirring, immersed in a constant temperature bath at 60 ° C., and kept for 60 minutes. .

After the reaction was completed, the sample was taken out from the bag, washed with water, and dehydrated by centrifugation.

Hypo bleaching (H stage) The centrifugally dehydrated E stage treated pulp is collected in a plastic bag.
Add a predetermined amount of alkali and Hypo (sodium hypochlorite), add a predetermined amount of water to make the pulp concentration 10%, mix thoroughly by hand stirring, and immerse in a constant temperature bath at the desired temperature. , and held for 90 minutes.

After the reaction was completed, the sample was taken out from the bag, washed with water, and dehydrated by centrifugation.

Chlorine dioxide bleaching (D stage) The centrifugally dehydrated H stage treated pulp is placed in a plastic bag.
Add a predetermined amount of chlorine dioxide (Cl per weight of bone dry pulp)
0.5% (calculated as O2) and a predetermined amount of water to make the pulp concentration 10%, thoroughly mixed by hand stirring, immersed in a constant temperature bath at 70°C, and held for 180 minutes.

After the reaction was completed, the sample was taken out from the bag, washed with water, and dehydrated by centrifugation.

Measurement of physical properties of pulp (1) Brightness After disintegrating the centrifugally dehydrated pulp, Tappi test method T2
A sheet with a basis weight of 60 g/m2 was prepared according to 05 os-71 (JIS P 8209), and JIS P 812
Measured according to 3.

(2) Pulp viscosity After the centrifugally dehydrated pulp is finely kneaded by hand, Ta
Measured according to ppi test method T230 os-82.

(3) Tearing length and specific tearing strength Centrifugally dehydrated pulp was disintegrated and 400 ml C.I. S. F. (Canadian Standard Freeness) to create a sheet with a basis weight of 60 g/m2 according to JISP 8209, and JISP 8113.
and P8116.

(4) Fading The centrifugally dehydrated pulp of the bleaching agent was disintegrated to create a sheet with a basis weight of 60 g/m2, and the sheet was subjected to fading treatment for 24 hours in a hot air circulation constant temperature bath at 105°C, and the whiteness was measured according to J1S P 8123. .

Example 1 and Comparative Example 1 Hardwood unbleached kraft pulp (domestic mixed material, kappa number 16.6) was bleached in a C-E-H-D sequence.

In Example 1, the temperature at the H stage was 70°C, the alkali (caustic soda) addition rate was 2.0% per bone dry pulp weight, and the hypo (sodium hypochlorite) addition rate was 0 per bone dry pulp weight.
．． 8%, and in Comparative Example 1, the temperature at the H stage was 45°C, and the alkali (caustic soda) addition rate was 0.2% per bone dry pulp weight.
, Hypo (sodium hypochlorite) addition rate is 0.8%.

The results obtained are shown in Table 1.

Example 2 and Comparative Example 2 Hardwood unbleached kraft pulp (domestic mixed material, kappa number 16.6) was bleached in an O2-C-E-D sequence.

In Example 2, the temperature in the H stage was 70°C, the alkali (caustic soda) addition rate was 1.5% per bone dry pulp weight, and the hypo (sodium hypochlorite) addition rate was 0 per bone dry pulp weight.
．． 6%, and in Comparative Example 2, the temperature at stage H was 45°C, and the alkali (caustic soda) addition rate was 0.2% per bone dry pulp weight.
, Hypo (sodium hypochlorite) addition rate is 0.6% per bone dry pulp weight.

The results obtained are shown in Table 2.

As can be seen from Tables 1 and 2, the pulp produced by the method of the present invention bleached in the H stage at a higher temperature and a higher alkali addition rate than normal hypobleaching has poor strengths such as tearing length and specific tear strength. Although there is almost no difference, the whiteness is high and the fading resistance is significantly improved.

The fact that the pulp is resistant to discoloration makes it possible to lower the whiteness target, and if the same level of whiteness as conventional pulp is achieved, the chemical addition rate can be reduced, and the pollution load is reduced accordingly, making it possible to reduce the target whiteness. Processing costs are reduced.

〔Effect of the invention〕

The present invention has the effect of being able to produce pulp that has higher whiteness and is less prone to discoloration than conventional methods.

When producing pulp with the same level of brightness as the conventional method, the addition rate of expensive chemicals can be greatly reduced without requiring major equipment changes, so a reduction in bleaching chemical costs can be expected.

Claims (1)

[Claims] Claim 1. A bleaching method comprising the step of treating pulp obtained from lignocellulosic material with hypochlorite, in which the bleaching stage with hypochlorite contains 1.0% or more of the pulp obtained by weight of bone-dry pulp. A method for bleaching lignocellulosic materials, which comprises adding an alkali and treating the pulp at a temperature of 60° C. or higher.