JPH076147B2 - Bleaching method for lignocellulosic material - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for bleaching lignocellulosic material.

(Prior Art) In order to use a lignocellulosic material in many applications, it is necessary to bleach the pulp obtained by chemical or mechanical action. Unless you use kraft pulp for applications that do not require whiteness, such as packaging materials,
It is necessary to bleach with chlorine, hypochlorite (hypo), chlorine dioxide, oxygen, hydrogen peroxide, bleaching agents such as caustic soda and bleaching aids to remove residual lignin, which is a color-causing substance of unbleached pulp. There is.

In the bleaching of chemical pulp, which requires high strength, in order to keep the strength of pulp fiber itself high, avoid extreme static static bleaching so as to minimize the influence on the decomposition of carbohydrates (such as cellulose). Generally, a multi-stage bleaching process (sequence) is used in which the bleaching agent and the bleaching conditions are gently changed.

Usually, lignin is first chlorinated by chlorination to add solubility, and then lignin is dissolved and extracted with alkali. After that, a small amount of residual lignin is decomposed and removed by using hypochlorite, chlorine dioxide or the like to obtain a pulp having high whiteness.

When the chlorine treatment is represented by (C), the alkali treatment is represented by (E), the hypozinc salt treatment is represented by (H), the chlorine dioxide treatment is represented by (D), and the hydrogen peroxide treatment is represented by (P), this bleaching sequence is
According to the order of the bleaching agent and / or the bleaching aid used, C-E-H-E-D, C-E-D-E-D, C-E-H-
It is carried out in a plurality of bleaching stages such as PD.

Usually, 5 to 6 bleaching stages were required to obtain completely bleached pulp, but in recent years, in the alkali extraction stage following the chlorination stage, oxygen (Eo) or hydrogen peroxide (Ep) or hypozinc chloride The bleaching stage can be shortened to 3 to 4 stages due to the development of a technique of using an acid salt (E _H ) or using them together (for example, Epo). These are commonly referred to as short sequence, and in 1986
According to Pulping Coference proceedings, P481, the short sequence is a softwood KP pulp with a whiteness of 88% ISO.
It is defined as a sequence that can be bleached in 4 steps or less and in the case of hardwood KP and SP in 3 steps or less.

For example, 1986 Pulping Coference proceedings, P
153, P167, P489 and P491 have C / D-Epo-D, C / D-Epo
-D-P, to over reset sequence, such as _{O 2 -C / D-Eo-} D and C _D -Eo-D is, 1987Pulping Coference proceedi
The ngsP15 C-Eo-H-P , C D -Eo-H-D, C-Eo-
A short sequence such as H-H has been proposed, and some sequences are operating on an actual machine.

C / D, C _D or D _C indicates a chlorination method in which chlorine and chlorine dioxide are used in combination.

In addition to this, as described in U.S. Pat. No. 4,238,281, Simplified Bleachin is used for bleaching in a relatively short period of time except for a wash before the final chlorine dioxide stage.
g) is also bleached with a similar idea, although it is different from the short sequence. As the sequence, the example 1987Pulping Coference proceedings, P487 C _D -
_{E- (HD), C D -Eo-} (HD), C D -E- (DED), C D -Eo-
_{(DED), C D -E- (} HDED), C D -Eo- (HDED) have been proposed.

Incidentally, () indicates that there is no washing during this period.

In these factories with existing bleaching equipment, these short sequences can be expected to reduce energy usage, reduce bleaching wastewater load, or increase production (for example, 6-stage bleaching is divided into 3 stages and 2 series). In the case of a new factory, it has the advantage of being able to significantly reduce investment costs, and is attracting the attention of each country.

Further, as a method of simplifying the short sequence, a method of greatly reducing the amount of residual lignin in the pulp entering the bleaching stage can be considered. As this method, a method of combining a modified continuous cooking method and an oxygen bleaching method (O ₂ stage) described later can be considered. For example, 1987 Pulping Coference proceedi
For ngs, P249, bleached pulp with 90% whiteness ISO is C / D-Eo-
It is stated that it can be obtained in three stages of D. In the foreign plant running actually modified continuous cooking method, the hardwood pulp without oxygen stage, by a sequence of C _D -Ep- (HD), whiteness manufactures bleached pulp 90% ISO (1987 Pulping Conference proceedings, P453). Another advantage of this combination is that it can significantly reduce bleaching wastewater loads, especially chlorine wastewater loads (Oxgen De
lignification Symposium Proceedings, p88 (198
Five)).

In the oxygen bleaching method developed in recent years, the bleaching effluent can be circulated and used as a washing liquid for pulp after digestion, and can be finally burned and recovered together with the cooking effluent. (Corresponding to Japanese Patent No. 3,759,783), JP-A-49-7
503 publication (corresponding to U.S. Pat. No. 3,843,473) ",
Currently, many oxygen bleaching equipments are operating in major pulp and paper countries in the world including Japan.

In the oxygen bleaching method, pulp after digestion is dipped in a relatively low concentration alkaline solution, the pulp concentration is increased by squeezing it, and after heating, oxygen and pulp are mixed with a mixer etc. in a reaction vessel under pressure. The lignin remaining in the pulp is oxidatively decomposed and solubilized with alkali to bleach the pulp (Th
e Bleaching of Pulp, P159-209, TAPPI PRESS1979).

This oxygen bleaching method (O ₂ stage) is combined with the conventional chlorine-based bleaching method and is applied not only to the completely bleached pulp but also to the semi-bleached pulp. For example bleaching sequence of Japanese Patent Publication 47-7202 Patent Publication (US Patent corresponds to the 3,652,388 Pat) O ₂ -C / D-E is, 47-10241 JP (German Published Pat. No. 1,947,931 JP C / D-O ₂ -D- to correspond) to
The bleaching sequence of E-D is described in Japanese Patent Publication No. 47-44441 (corresponding to US Pat. No. 1,342,580) as O ₂ -Dc-O.
_The bleaching sequence consisting of _2- E is
No. 05 publication (corresponding to Canadian Patent No. 992,265) proposes a bleaching sequence composed of O ₂ -D / C-E-D. Paper Trade Journal, August /
5, P49 (1968) and Tappi54 (11): 1893 (1971), proposed various bleaching sequences with oxygen bleaching and chlorine bleaching agents, and these methods are effective in reducing chlorine wastewater and environmental protection. Have contributed.

In addition, the recently developed Modified Continuous
ous cooking) can remove lignin from wood as much as possible while suppressing deterioration of pulp quality at the stage of producing pulp from wood chips, and reduce the amount of residual lignin in the pulp brought into the bleaching stage. As a result, the bleaching wastewater load can be significantly reduced (EUCEPA Symposium p
reprints p16 (1980), Paperija Puu65 (4): 227 (198
3), Tappi J. 66 (9): 97 (1983), Svensk Papperstid
n.87 (10): 30 (1984)). Currently, only a few modified continuous digesters are in operation in the world, but this method has a great effect of reducing chlorine drainage, and is expected to be adopted in many plants including Japan in the future.

The modified continuous cooking method is used when the chips are cooked by the craft method.
Make the alkali concentration relatively uniform during cooking. Especially in the early stages of delignification is most advanced, hydrosulfide ions (SH ^-) as high as possible a concentration. Especially in the latter stage of cooking, the concentration of dissolved lignin existing in the cooking liquor is lowered. Lower the temperature in the early and late stages of cooking.
Therefore, the pulp viscosity, which is an indicator of pulp quality, is the same level as ordinary kraft cooking, and produces unbleached pulp with a lignin content of about 30% (1987Pulping Coferenc
e proceedings, P249).

However, due to the bleaching method using a short sequence, it is possible to obtain bleached pulp in a short time, but since the number of bleaching stages in the sequence is smaller than that in the normal sequence, a similar whiteness is obtained unless the bleaching conditions are strict. Difficult to do. As a result, there are problems such as an increase in bleaching chemicals, a decrease in pulp strength, and easy fading.

The present inventor, as a result of multi-faceted examination of this point,
In the C-E-D ₁ -D ₂ short sequence, by adding an alkali in the latter stage of the reaction in the D ₁ stage, performing alkali extraction for a short time, and then treating with chlorine dioxide (D ₂ stage), Even in the sequence, not only can the chemical addition rate be reduced to the level of the normal bleaching sequence to obtain the whiteness level of the normal bleaching, but also the fading of the pulp can be improved and the strength of the pulp can be further improved. They have found the present invention and completed the present invention. Further, when the modified lignocellulosic material obtained by the modified continuous cooking method, the combination of the ordinary kraft method and the oxygen bleaching method, and the modified continuous cooking method and the oxygen bleaching method is bleached according to the present invention, the whiteness It is expected that not only the improvement of water quality and the reduction of chemical addition rate, but also the wastewater load will be significantly reduced and the quality of wastewater will be greatly improved.

(Object of the invention) The present invention has been made to take advantage of the above-mentioned bleaching method of Syoto Sequen and to solve the drawbacks thereof. Therefore, the object thereof is to obtain a pulp having a high whiteness in a short sequence. It is to provide a bleaching method.

Still another object is to provide a bleaching method for reducing the chemical addition rate, yet another object is to provide a bleaching method for producing a strong bleached pulp, and yet another object is to prevent fading. It is to provide a bleaching method for producing a bleached pulp, and yet another object is to provide a bleaching method for producing a bleached pulp with a low drainage load, and still another object will be apparent from the following description. Ah

DETAILED DESCRIPTION OF THE INVENTION In general, the present invention provides pulp obtained from a lignocellulosic material with chlorine and / or chlorine dioxide (C and / or D) stages-alkali / oxygen (Eo).
Stage - Chlorine dioxide (D ₁₎ stage - in the bleaching sequence to be processed by chlorine dioxide (D ₂₎ stage, adding an alkali to the late chlorine dioxide treatment in the _first stage D, and the _first stage and the D ₂ stage D dioxide It relates to a bleaching method, characterized in that the proportion of chlorine is between 40/60 and 70/30.

That is, the present invention makes it possible to produce a pulp having a high whiteness at a low cost in a short time by combining a chlorine stage, an alkali / oxygen stage, and a chlorine dioxide stage as a bleaching sequence of cellulose. Since the ratio is low, it is possible to produce a bleached pulp having a high strength, and further it is possible to produce a pulp having a markedly improved discoloration property due to alkali extraction in the D ₁ stage.

Next, the elements constituting the present invention will be described in detail. The agents used in the first bleaching stage of the present invention are chlorine alone (C stage),
Mixture of chlorine and chlorine dioxide in any proportion (C / D stage)
Or, it is used with chlorine dioxide turbidity (stage D). Chlorine and chlorine dioxide may be added as a mixture of both.
Chlorine may be added within a few seconds after the addition of chlorine dioxide. Alternatively, a part of chlorine dioxide may be added, and then the rest of chlorine dioxide and chlorine may be mixed and added.

The addition ratio of chlorine compounds to the dry pulp, which is represented by the amount of active chlorine, is proportional to the amount of lignin in the unbleached pulp, and is preferably between 1 and 5% by weight of lignin (the same applies hereinafter). The reaction temperature is preferably between 20 ° C and 60 ° C, the pulp concentration during the reaction is between 1% and 10%, and the reaction time is from 5 minutes.
It is between 60 minutes and the pH at the end of the first bleaching stage is preferably between 2 and 5. After the end of the first bleaching stage, a washing or compacting step follows, as is known in the art.

The alkali used in the alkaline oxygen stage (Eo stage) of the bleaching system of the present invention can be selected from many alkaline compounds known to those skilled in the art. The alkali addition rate per pulp is preferably 0.5% to 3% (versus dry pulp) in terms of NaOH. The oxygen used in the E stage may be either oxygen or oxygen-enriched air, and may be used under pressure or under atmospheric pressure. The addition rate is preferably 0.5% to 3% (versus dry pulp). The reaction temperature is preferably between 40 ° C and 70 ° C, the pulp concentration during the reaction is preferably between 5% and 20%, the reaction time is between 30 and 120 minutes, and the final pH is between 8 and 12. Is preferred. After the E stage, a washing or compression step is carried out in the same manner as after the first bleaching stage.

The chlorine dioxide used in the D ₁ stage of the bleaching system of the present invention may be chlorine dioxide obtained from many chlorine dioxide generation methods known to those skilled in the art.

Chlorine dioxide addition rate is 0.2% to 3% (vs. dry pulp)
And preferably between 0.4% and 1%. The reaction temperature is between 40 ° C and 90 ° C, preferably 50 ° C
Between 80 ° C. The pulp concentration during the reaction is between 3% and 20%, preferably between 5% and 15%. Reaction time is 15
It is between minutes and 300 minutes, preferably between 30 and 240 minutes. The final pH is between 2 and 6, preferably between 3 and 5. The flow may be upflow or downflow, and an upflow having a short residence time may be added before the downflow.

D In the case of a downflow tower, a method of adding alkali to the _1st stage is to install a ring tube in the lower part of the tower, and to add it concentrically from there, and the bottom circulating diluent line usually attached to the lower part of the tower May be added to. It may also be added to the suction of a pump that sends pulp to the latter stage. In the case of an upflow tower, a ring pipe may be provided at the upper part of the upflow tower and added concentrically from there, or may be added to the suction of a pump for sending pulp to the latter stage. The alkali addition place may be a place where the alkali extraction time is 1 to 10 minutes, or a place where the chlorine dioxide consumption amount is 80% to 90% of the added chlorine dioxide amount.

The alkali to be used can be selected from those known to those skilled in the art such as caustic soda and oxidized white liquor, but is preferably caustic soda, and the addition rate is between 0.1% and 1.5% (versus dry pulp), It is preferably between 0.1% and 1.0%. After D ₁ , a washing or compression step follows, as in the first bleaching stage.

Types of chlorine dioxide used in the D ₂ stage of the bleaching system of the present invention,
The bleaching conditions are the same as those for D stage ₁ .

Also, the ratio D ₁ / D ₂ of chlorine dioxide in the D ₁ stage and D ₂ stage is from 40/60
Between 70/30.

The method for bleaching the lignocellulosic material of the present invention is not limited to chemical pulp such as Kraft pulse (KP), alkaline pulp (AP), and sulfite pulp (SP), as well as the modified continuous cooking method (Mo
It can also be applied to pulp obtained by dified continuous cooking), pulp that has been digested by a normal kraft method and then oxygen bleached, and chemical pulp that has been digested by a modified continuous cooking method and then oxygen bleached. Needless to say, it can be applied to both wood pulp and non-wood pulp. The bleaching method of the present invention can be carried out as a single bleaching step, or may be replaced as a part of the steps in the conventional bleaching sequence.

(Examples) Next, examples of the present invention will be described, but the present invention is not limited thereto.

In the examples given below, the bleaching conditions of the lignocellulosic material and the physical properties of the pulp were measured by the following operating means, unless otherwise indicated.

First bleaching stage Softwood pulp (mixed material of Douglas fir and pine, mixing ratio
70/30) Take 50 g of absolutely dry in a plastic bag, add an amount of effective chlorine (total of chlorine amount and chlorine dioxide amount) proportional to the Kupper number (Kupper number x 0.2%), and further increase the pulp concentration to 5
%, Water was added, the mixture was stirred well, and the mixture was placed in a constant temperature bath at 50 ° C. and kept for 30 minutes. After completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

Alkali / Oxygen stage (E ₀ stage) Centrifuge dehydration washed C stage pulp is put in a plastic bag,
Add an amount of alkali proportional to the Kupper number (Kupper number x 0.1%), add water so that the pulp concentration becomes 10%, stir well and put the pulp into the vertical autoclave 2 and 60 ℃. After warming up, oxygen was injected until the pressure reached 2 kg / cm ² . After stirring under pressure for 10 minutes, the pressure was reduced to atmospheric pressure, and the reaction was further continued for 50 minutes. After completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

Take chlorine dioxide bleaching / (abbreviated hereinafter D ₁ / e stage) alkaline extraction stage E ₀ stage pulp spun dry cleaned plastic bag,
Add a predetermined amount of alkali so that the initial pH is 6.5-6.0, add the amount of chlorine dioxide (based on chlorine dioxide) shown in Tables 1, 2, and 3 and stir at 70 ° C for 3 Held for hours. Immediately without washing after the reaction, add 0.5% (vs. dry pulp) of alkali (NaOH), and further increase the pulp concentration to 5%.
Hot water of 70 ° C. was added so that the mixture became alkaline, and alkali extraction was performed at 70 ° C. for 3 minutes. After completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

For comparison, treatment with chlorine dioxide alone was performed under the same conditions except that alkali extraction was not performed (D ₁
Stage) Chlorine dioxide bleaching (D ₂ stage) experiment Centrifugal dehydration washed D ₁ and / or D ₁ / e stage pulp is put in a plastic bag, 0.5% (chlorine dioxide standard) chlorine dioxide is added, and after stirring at 70 ° C Held for 3 hours. After completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

Measurement of physical properties of pulp Regarding whiteness, the pulp dehydrated and washed by centrifugal separation was defibrated, and then a sheet having a basis weight of 60 g / m ² was prepared by hand according to Tappi test method T205os-71 (JIS P 8209). It was measured according to JIS P 8123.

Viscosity was measured according to Tappi test method T230, after manually centrifuging pulp that had been dewatered by centrifugation.

Regarding the breaking length and specific tear strength, pulp dehydrated and washed by centrifugation was beaten to 550 ml canadian freeness with a PFI mill, and a sheet with a basis weight of 60 g / m ² was prepared by the same method as above, and used for papermaking. The strength was measured according to the pulp strength test method (JIS P 8123).

In the discoloration test, the bleached pulp was discolored for 2 hours in an air circulation type constant temperature bath at 105 ° C., and then measured in accordance with JIS P 8123 as in the case of measuring the whiteness.

Example 1 and Comparative Example 1 Example 1 and Comparative Example 1 show an experiment in which a mixed chip of Douglas-fir and pine was cooked by an ordinary kraft method and the unbleached pulp was used.

Example 1 is the unbleached Douglas fir and pine with a Kupper value of 31.9.
This is an example of bleaching KP in the sequence of C-Eo-D ₁ / e-D ₂ .

Comparative Example 1 is an example of bleaching the unbleached KP a sequence of _{C-Eo-D 1 -D 2} .

The results are shown in Table 1 below.

Example 2 and Comparative Example 2 Example 2 and Comparative Example 2 show an experiment in which a mixed chip of Douglas-fir and pine was cooked by a conventional kraft method, and a pulp which was bleached with medium concentration oxygen was used.

Example 2 is an example of bleaching oxygen bleached pulp of Douglas fir and pine Katsupa value 15.8 in a sequence of _{C-Eo-D 1 / e} -D 2.

Comparative Example 2 is an example of bleaching the oxygen bleached pulp in a sequence of _{C-Eo-D 1 -D 2} .

The results are shown in Table 2 below.

Example 3 and Comparative Example 3 In Example 3 and Comparative Example 3, a mixed chip of Douglas-fir and pine was cooked by a modified continuous cooking method (abbreviated as MCC),
An experiment using the unbleached pulp is shown.

Example 3 is MCC of Douglas-Fur and pine with a Kupper value of 25.2.
This is an example of bleaching pulp with a sequence of C-Eo-D ₁ / e-D ₂ .

Comparative Example 3 is an example in which bleaching of the same MCC pulp in a sequence of _{C-Eo-D 1 -D 2} .

The results are shown in Table 3 below.

Example 4 and Comparative Example 4 In Example 4 and Comparative Example 4, a mixed chip of Douglas-fir and pine was cooked by a modified continuous cooking method (abbreviated as MCC),
Further, an experiment using a pulp bleached with medium concentration oxygen is shown.

Example 4 is MCC of Douglas-Fur and pine with a Kupper value of 13.6.
Digestion and is an example of bleaching oxygen bleached pulp in a sequence of _{C-Eo-D 1 / e} -D 2.

In Comparative Example 4, MCC-cooked and oxygen-bleached pulp was treated with C-Eo.
This is an example of bleaching in the sequence of -D ₁ -D ₂ .

The results are shown in Table 4 below.

As seen from Tables 1 to 4, regardless of the type of pulp used in the pre-bleaching stage, the pulp bleached by the C-Eo-D ₁ / e-D ₂ sequence had a C-Eo-D ₁ -D ₂ It has higher whiteness and higher strength such as breaking length and specific tear strength than pulp bleached by sequence, and is less likely to fade. Further, since the whiteness of the bleached pulp is high, it is clear that the chemical addition rate can be reduced when obtaining the same whiteness.

(Effects of the Invention) As is clear from the above description, the novel bleaching sequence of the method of the present invention can provide a pulp which is superior in pulp whiteness and strength and is more resistant to fading as compared with the conventional method. It is said.

Further, the method of the present invention is not only advantageous in terms of equipment in that it does not require a large change in equipment, but is also economically advantageous in that the chemical addition rate can be greatly reduced.

Claims (1)

[Claims] 1. When bleaching a pulp obtained from a lignocellulosic material in a chlorine and / or chlorine dioxide stage-alkali / oxygen stage-chlorine dioxide stage-chlorine dioxide stage bleaching sequence, A method for bleaching a lignocellulosic material, characterized in that alkali is added in the latter stage of the reaction in the first step and the addition ratio of chlorine dioxide in the first step and the second step is 40/60 to 70/30.