JPH10298886A - Production of bleached pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remarkably remove lignin while minimally keeping lowering of pulp viscosity and remarkably reduce an amount of bleaching chemical necessary to bleach so as to have the desired whiteness by bleaching unbleached pulp with alkali oxygen and then, subjecting the bleached pulp to acid treatment under pressure. SOLUTION: Unbleached pulp obtained by digesting wood is charged into an autoclave and an aqueous solution of caustic soda is poured thereinto and the pulp is treated under pressure by compressed oxygen gas to subject the pulp to bleaching with alkali oxygen and the pulp is subjected to acid treatment in 5-40 wt.% pulp concentration at pH 2-6 of an acid solution at 50-150 deg.C for 5-120 min under 0.5-9.0 kg/cm<2> G pressure by using a nitrogen-containing gas produced as a by-product in a process concentrating oxygen gas used for the above alkali acid bleaching from air. Then, the treated pulp is subjected to bleaching by one stage or several stages containing at least one alkali extraction stage to which oxidation-based chemical selected from oxygen and hydroperoxide is added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing bleached pulp from wood. More specifically, the present invention provides an unbleached pulp obtained by digesting wood, which is subjected to alkali oxygen bleaching and then treated with an aqueous acid solution under a pressure of a nitrogen-containing gas, thereby removing the pulp without reducing the pulp viscosity. The present invention relates to a method for producing bleached pulp capable of advancing lignin and greatly reducing the amount of bleaching chemicals required to bleach the pulp to a desired whiteness.

In order to use wood as a raw material for papermaking in many applications, wood is digested with a cooking liquor and pulped by chemical action of chemicals, or in the case of wood, wood chips are machined using a refiner or the like. After pulping by a mechanical action, the resulting pulp must be bleached with a bleaching chemical to increase whiteness. For example, kraft pulp obtained by digesting wood chips with a kraft cooking liquor is usually subjected to alkali oxygen bleaching or without alkali oxygen bleaching, except when used for applications requiring strength, such as packaging materials. , Bleached in a single-stage or multi-stage sequence with a bleaching agent consisting of bleaching agents and bleaching aids such as atomic chlorine, hypochlorite, chlorine dioxide, oxygen, ozone, hydrogen peroxide, caustic soda, etc. Lignin and the like, which are coloring agents contained therein, are removed, and Hunter whiteness is reduced to 70.
It is generally used as semi-bleached kraft pulp of about 90% or completely bleached kraft pulp.

[0003] Semi-bleached pulp is obtained by bleaching unbleached pulp after digestion in one stage with hypochlorite to obtain a pulp having a Hunter whiteness of less than 70%, with the yellow color being emphasized, as packaging paper, envelopes, and office bags. Except when used, usually bleaching is performed without alkali oxygen bleaching or after two or three stages of alkali oxygen bleaching and bleaching to a Hunter brightness of about 70 to 80%,
For example, it is used as a connecting pulp for producing newsprint paper. However, when producing completely bleached pulp having a Hunter whiteness of about 80 to 90% from unbleached pulp, it is necessary to maintain the strength of the pulp fiber itself to a certain extent in bleaching. Avoid extreme single-stage bleaching to minimize the degradation of carbohydrates such as cellulose and hemicellulose,
It is general to employ a three- to six-stage multi-stage bleaching method in which an alkaline oxygen bleaching is first performed, and then a bleaching chemical and bleaching conditions are variously combined and mildly bleached.

Conventionally, in a multi-stage bleaching method, pulp is first treated with atomic chlorine, lignin contained in the pulp is chlorinated, the lignin is made soluble, and then the chlorinated lignin is dissolved with alkali. Extract and remove lignin from pulp, and further use hypochlorite, chlorine dioxide, etc. to decompose and remove a small amount of residual lignin,
A method for obtaining pulp having high whiteness has been adopted. However, in recent years, there has been concern about the effects of organic chlorine compounds contained in the bleaching effluent from the pulp chlorination stage on the environment, and there has been active research on bleaching sequences that do not use atomic chlorine for pulp bleaching. Also, when hypochlorite is used, chloroform is generated during bleaching of pulp, which may adversely affect the environment. Therefore, a bleaching sequence not used for pulp bleaching is being sought.

At present, as a substitute for atomic chlorine and hypochlorite, ozone, oxygen, and oxygen-based bleaching chemicals such as peracids such as peracetic acid and persulfuric acid are attracting attention. However, these chemicals, except for oxygen and hydrogen peroxide, are expensive and explosive and difficult to handle,
At the moment it has not reached the general public,
It is a promising bleaching method in the future. At present, compared to chlorine and hypochlorite, the oxygen-based bleaching chemicals have low selectivity for delignification, so that if the reaction is excessively advanced, the strength of the pulp fiber decreases. There is an ongoing search for optimal bleaching sequences and conditions.

On the other hand, as a method for reducing the amount of bleaching chemicals used while maintaining the whiteness of bleached pulp at a constant level, a method of promoting delignification as much as possible during cooking to reduce the kappa number of unbleached pulp ( For example, JE Jiang et al., Appita, 45 (1), 19 (19)
92)), the unbleached pulp is further alkali oxygen bleached,
A method for lowering the kappa number, or a method for further reducing the kappa number by further promoting delignification in alkaline oxygen bleaching using a pretreatment chemical such as nitrous acid (for example, JP-A-4-316690), bleaching A method of treating pulp with a xylan-degrading enzyme before the treatment step (for example, JP-A-2-264087), and a method of treating pulp with a lignin-degrading enzyme before the bleaching treatment step (for example, 4-316689) and the like.

[0007] However, there is still room for improvement in the above-mentioned method. For example, a method of proceeding delignification at a level higher than a usual level during cooking to obtain a pulp having a low kappa number often has a low pulp yield. There is a risk that the strength of the pulp fiber may be reduced. A method of further reducing the pulp kappa number by further proceeding delignification in the alkali oxygen bleaching stage often involves a risk of reducing the strength of the pulp fiber. In the method of pretreating pulp by enzymatic treatment prior to bleaching, the reaction conditions are relatively mild, so that the decrease in pulp strength and the decrease in pulp yield are small, but the reaction speed is slow and the processing time is long. In short, there is a problem that the amount of reduction in the kappa number is extremely small.

Japanese Unexamined Patent Publication (Kokai) No. 6-158573 discloses that unbleached pulp obtained after digesting wood chips is bleached with alkali oxygen, and the pulp after bleaching the alkali oxygen with a pulp concentration of 1 to 30% at a temperature of 30 to 95 ° C. , Processing time 5-120
A method is disclosed in which after an acid treatment step consisting of an acid solution having a pH of 1 to 5 is performed, delignification and bleaching with peroxide or peroxide and oxygen are performed in an alkaline medium.
However, the acid treatment step proposed in the above publication aims to remove heavy metals from pulp and prevent depolymerization of cellulose in bleaching with peroxide or peroxide and oxygen. There was almost no delignification effect, and the effect of reducing bleaching chemicals in the bleaching process was not sufficient.

Japanese Unexamined Patent Publication (Kokai) No. 6-280177 discloses that unbleached pulp after digestion can be prepared by using an inorganic acid having a pH of 1.0 to 1.0.
6. Temperature 80 ° C-boiling temperature of acid treatment liquid, pulp concentration 5
Disclosed is a method of bleaching pulp by subjecting it to an acid treatment under a condition of about 20% for a time of 1 to 3 hours, and then performing an alkali extraction treatment of lignin.

[0010] However, the acid treatment conditions proposed in the above publication are harsh, with a pH of 1.0 to 1.6, and the pulp viscosity may be significantly reduced due to acid hydrolysis of the polysaccharide, and the strength may be impaired. In addition, such a pH itself requires a large amount of chemicals, and is economically problematic. In addition, this method has a potential problem that, by lowering the pH at the time of the acid treatment, delignification occurs, but decomposed lignin is condensed and re-adsorbed to pulp fibers, thereby deteriorating bleachability. Exists.

Japanese Patent Application Laid-Open No. 8-158284 discloses (1) a step of subjecting a cooked chemical pulp to an oxygen treatment under high temperature and pressure under alkaline conditions, (2) tungsten, molybdenum, vanadium, selenium, titanium and the like. A step of mixing a reaction catalyst composed of an oxyacid of a group IV, V, or VI element or a salt thereof and hydrogen peroxide and treating at a pH of 3 or less and at a temperature of 75 to 110 ° C, (3) alkaline There is disclosed a method for producing a chemical pulp for papermaking, which comprises treating the medium in the order of treatment with a peroxide. In this method, when a chemical pulp is treated with a combination of peroxide and oxygen in an alkaline medium and delignified, a step of adding a catalyst together with the peroxide under acidic and high temperature conditions and adding a step of pretreatment is added. The pulp viscosity is hardly reduced, and the delignification effect is significantly improved.

However, in the method proposed in the above-mentioned publication, delignification can be remarkably promoted without substantially reducing the pulp viscosity by the presence of the oxyacid of heavy metal or a salt thereof in the aqueous acid solution. However, in order to add the heavy metal, when delignification with an oxidizing chemical in the bleaching process, there is a problem that a large amount of a new protective agent must be added and used to prevent depolymerization of cellulose. ing. In addition, in the acid treatment of pulp, performing the delignification by adding the acid of the metal or a salt thereof means that a part of the heavy metal is brought into the pulp manufacturing process or the papermaking process together with the pulp or waste liquid, In addition to being a cause of sticky substances, pitch troubles, scale troubles and the like, there is also a problem that even trace amounts may be mixed into paper products, which is not preferable in terms of hygiene.

The unbleached pulp obtained by digesting the wood chips is bleached with alkaline oxygen, and then pH 3.5, 11
A method of selectively removing hexeneuronic acid, which is a xylan side chain, from pulp by performing acid treatment at 0 ° C. for 1 hour to reduce the amount of bleaching chemicals used in the subsequent bleaching of pulp with bleaching chemicals. Are also known (for example,
Tapauni Vuorinen et al., 1996 International Pulp Bleach
ing Conference Proceeding 43-51). However, when the acid treatment under the above conditions, hexeneuronic acid in the pulp is removed, but delignification itself hardly occurs, so that the bleachability of the pulp is not significantly improved,
Therefore, the effect of reducing the amount of the bleaching chemical used in the subsequent stage was not sufficient.

On the other hand, in many cases, an alkaline oxygen bleaching step is introduced for the above-mentioned reason. However, when producing oxygen gas used for alkaline oxygen bleaching, a nitrogen-containing gas is produced as a by-product, and its effectiveness is reduced. Searching for an appropriate usage method was also one of the major issues.

[0015]

In view of this situation, the present inventors have made various studies on a method for reducing the amount of bleaching chemicals used in the bleaching process by acid-treating pulp after alkaline oxygen bleaching. As a result of stacking, it is found that delignification in the acid treatment step proceeds by applying pressure when the pulp is subjected to acid treatment, and it is found that a remarkable decrease in the Kappa number occurs, and furthermore, a nitrogen-containing gas is used as the gas to be pressurized Thus, the present inventors have found that a decrease in pulp viscosity can be suppressed to a minimum, and have completed the present invention.

[0016] It is an object of the present invention to minimize the decrease in pulp viscosity by subjecting unbleached pulp obtained by cooking wood to alkaline oxygen bleaching and then acid-treating under pressure with a nitrogen-containing gas. It is an object of the present invention to provide a method for producing bleached pulp which can significantly remove lignin and greatly reduce the amount of bleaching chemicals used in a bleaching step required for bleaching to a desired whiteness.

[0017]

SUMMARY OF THE INVENTION The present invention relates to a method for producing bleached pulp by subjecting unbleached pulp obtained by digesting wood to alkaline oxygen bleaching, followed by one-stage or multi-stage bleaching in a bleaching process. A method for producing bleached pulp, comprising an acid treatment step under pressure with a nitrogen-containing gas immediately before the bleaching treatment step.

In the present invention, the acid treatment of the pulp after the alkaline oxygen bleaching may be carried out in a pulp concentration of 5 to 40% by weight,
H2-6, temperature 50-150 ° C, time 5-120 minutes, pressurization pressure by nitrogen-containing gas 0.5-9.0 kg / cm ²
(Gauge pressure) is a method for producing bleached pulp.

The present invention is a process for producing bleached pulp, wherein a nitrogen-containing gas produced as a by-product in the step of concentrating oxygen gas used for alkaline oxygen bleaching from air is used as the nitrogen-containing gas.

The present invention is a method for producing bleached pulp, wherein waste water from the acid treatment step and the bleaching treatment step is recovered.

The present invention is a method for producing bleached pulp, wherein the bleaching step includes at least one alkali extraction stage to which an oxidizing chemical selected from oxygen and hydrogen peroxide is added.

[0022]

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an unbleached pulp obtained by digesting wood by alkali oxygen bleaching, and then subjecting the pulp after the alkali oxygen bleaching to acid treatment under pressure with a nitrogen-containing gas. Thereafter, bleaching is performed by a known one-stage or multi-stage bleaching method using a bleaching chemical containing or not containing atomic chlorine or chlorine-based bleaching chemicals, and the wastewater discharged from the acid treatment is collected or the atomic chlorine is bleached. In the case of multi-stage bleaching without using a varnish, it is a method of producing bleached pulp having a Hunter brightness of 70 to 90% while collecting wastewater discharged from acid treatment and multi-stage bleaching.

The pulp used in the present invention is unbleached chemical pulp from softwood and hardwood. As a digestion method for obtaining the pulp used in the present invention,
Known cooking methods such as kraft cooking, polysulfide cooking, soda cooking, and alkali sulphite cooking can be used, but the kraft cooking method is preferably used in consideration of pulp quality, energy efficiency and the like. For example, when wood is kraft cooked, the sulphidity of the kraft cooking liquor is 5
７５75%, preferably 15-45%, and the effective alkali addition rate is 5-30% by weight, preferably 1
0 to 25% by weight, cooking temperature is 140 to 170 ° C, and the cooking method may be either continuous cooking or batch cooking. When using a continuous cooking furnace, a modified cooking method in which cooking liquor is added at multiple points. However, the method is not particularly limited.

In the cooking, a known cyclic keto compound such as benzoquinone,
Naphthoquinone, anthraquinone, anthrone, phenanthroquinone and alkyl-substituted products such as alkyl and amino of the quinone-based compound, or hydroquinone-based compounds such as anthrahydroquinone which is a reduced form of the quinone-based compound, and further anthraquinone by the Diels-Alder method A stable compound obtained as an intermediate in the synthesis method,
One or more selected from 10-diketohydroanthracene compounds and the like may be added, and the addition rate is 0.001 to 1.0% by weight based on the absolute dry weight of the wood chips.
It is.

In the present invention, the unbleached chemical pulp obtained by the known digestion method is subjected to washing, roughing and selective steps, and then delignified by the known alkaline oxygen bleaching method. As the alkali oxygen bleaching method used in the present invention, a known medium concentration method or a high concentration method can be applied as it is, but the medium concentration method currently used generally at a pulp concentration of 8 to 15% by weight is used. preferable.

The alkali used in the alkaline oxygen bleaching method by the above-mentioned medium concentration method is caustic soda or oxidized kraft white liquor, and oxygen gas and alkali are added to a medium concentration pulp slurry in a medium concentration mixer and mixed sufficiently. After that, the mixture is sent to a reaction tower capable of holding a mixture of pulp, oxygen and alkali under pressure for a certain period of time and delignified. The medium-concentration mixer has a rotor that rotates at a high speed of 500 to 1000 rpm depending on a manufacturer, and applies a high shearing force to the medium-concentration pulp slurry to change the pulp slurry into a fluid such as water. , Oxygen and alkali can be sufficiently mixed. The addition rate of oxygen gas is 0.5 to 3% by weight per absolutely dry pulp weight, and the alkali addition rate is 0.1%.
5-4% by weight, the reaction temperature is 80-120 ° C, the reaction time is 15-100 minutes, the pulp concentration is 8-15% by weight,
Known conditions can be applied to the other conditions. The pulp subjected to the alkali oxygen bleaching is washed, and then sent to an acid treatment step to be acid-treated.

In the acid treatment step of the present invention, the pulp is immersed in an aqueous acid solution having a pH in the range of 2 to 6, or the aqueous solution is contained and pressurized in the presence of a nitrogen-containing gas to maintain a specific time and temperature. Is defined as The acid used in the acid treatment step of the present invention may be any of an inorganic acid and an organic acid as long as the pH at the time of the acid treatment can be adjusted to 2 to 6,
Inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, sulfurous acid and nitrous acid, among which sulfuric acid are preferred because they are easily available and easy to handle. In the acid treatment step of the present invention, an auxiliary agent such as a peroxide, a nitrite, a nitrate, or methanol can be added. In particular, the addition of peroxide has the effect of accelerating the bleachability after acid treatment, but on the other hand causes a decrease in pulp viscosity,
The addition rate must not exceed 1% per pulp weight.

The pulp concentration in the acid treatment step of the present invention is in the range of 5 to 40% by weight, preferably 8 to 35% by weight, more preferably 10 to 25% by weight. Pulp consistency of 5
If the amount is less than% by weight, a large-capacity facility is required for the treatment, which is not suitable. When the pulp concentration exceeds 40% by weight, it is difficult to uniformly mix the pulp and the acid, and the effect of the acid treatment cannot be sufficiently obtained, which is not suitable. In the acid treatment step of the present invention, it is necessary to uniformly mix the pulp and the acid in order to improve the treatment effect. In order to obtain a uniform mixture, a mixer can be appropriately selected from a low-concentration mixer, a medium-concentration mixer, a static mixer, a high-concentration mixer, and the like according to the type and concentration of the pulp at the time of processing. The treatment can be performed in the same manner as in the case of the oxygen bleaching method.

That is, if the pulp concentration in the acid treatment step is medium or high, it becomes difficult to stir the pulp. Therefore, a medium concentration mixer capable of imparting a high shearing force with or without a disperser. Or pulp with a high concentration mixer,
It is better to mix the nitrogen-containing gas and the acid instantaneously, introduce them into a reaction column that can be immediately maintained under pressure, and maintain a certain time and temperature. When the acid treatment of the pulp is performed at a medium concentration, a continuous downflow or upflow type with or without a distributor and a discharger can be used. The reaction tower when performing acid treatment of pulp at a high concentration is equipped with a shelf with a hole in one place in a vertical downflow type, and a horizontally rotating blade for transferring pulp to a lower shelf, One that can drop pulp from shelf to shelf, or one that has multiple stages of pressure-resistant horizontal tubes, transfers the pulp horizontally with a screw for transferring pulp that rotates slowly, and transfers the next tube from the end of the tube to the next tube A Pandia type which can transfer and discharge hepulp is convenient.

In the acid treatment step of the present invention, the pulp concentration is 5 to 40% by weight and the pH is 2 to 6, preferably 2.
5-5, temperature 50-150 ° C, preferably 90-120
° C, retention time 5 to 120 minutes, preferably 20 to 90 minutes,
0.5-9.0 kg / cm ² pressurization with nitrogen-containing gas,
Preferably 1.5 to 7.0 kg / cm ² (gauge pressure)
Is performed under the following conditions. If the pH at the time of the acid treatment step is less than 2, the decomposition of polysaccharides becomes more remarkable than the decomposition of lignin, and the pulp strength and yield are reduced, which is not suitable. If the pH exceeds 6, delignification hardly occurs even under pressure, which is not suitable. In the present invention, since the acid treatment step is performed by pressurizing with a nitrogen-containing gas, unlike the known acid treatment step, sufficient delignification can be obtained even when the pH exceeds 3 and is high. When the treatment temperature is lower than 50 ° C., delignification by acid is insufficient, and the temperature is lower than 150 ° C.
If the temperature exceeds ℃, the decomposition of the polysaccharide becomes remarkable, and the strength of the pulp is greatly reduced. A treatment time of 5 minutes or more in the acid treatment step of the present invention is sufficient.
Even if the length is longer than a minute, the effect of delignification reaches the upper limit, and a longer processing time causes deterioration in quality, which is not suitable.

The nitrogen-containing gas for pressurization used in the acid treatment step of the present invention may be any gas as long as the nitrogen gas content is 95% or more. However, from an economic point of view, oxygen from the cryogenic separation method used for alkaline oxygen bleaching, oxygen from PSA (Pressure Swing Adsorption),
Nitrogen-containing gas by-produced when producing oxygen gas such as oxygen from VSA (Vacuum Swing Adsorption) is preferably used. If the pressurizing pressure in the acid treatment step is less than 0.5 kg / cm ² (gauge pressure), the effect of the pressurization becomes small, so that it is not suitable. If the pressurization pressure is higher than 9.0 kg / cm ² , the effect of pressurization reaches its limit, and furthermore, the production cost of the reaction vessel itself that can withstand such conditions increases, and it is not economical. Absent.

The pulp that has undergone the above-mentioned acid treatment step is sent to the bleaching step as it is or after a washing step. The bleaching chemicals used in the bleaching process of the present invention include atomic chlorine (C), caustic soda (E), hypochlorite compound (H), chlorine dioxide (D), oxygen (O), and hydrogen peroxide. Bleaching agents comprising known bleaching agents such as (P), ozone (Z) and organic peracids and bleaching aids can be mentioned, and these are appropriately selected and used as bleaching agents.

As the one-stage bleaching treatment in the bleaching treatment step in the present invention, treatment using a single bleaching chemical such as H, E, P, D and Z, or EO / P, D
A single-stage bleaching treatment combining a plurality of bleaching chemicals, such as Z, can also be used. As a multi-stage bleaching sequence in the bleaching process in the present invention, for example, CE / OH
A bleaching sequence containing atomic chlorine and a chlorine bleaching chemical, such as -D, C / DE / O-HD, can be used, D-ED, D-E / OD, E / OD, E
Bleaching sequences that do not contain atomic chlorine, such as -OD, ZD, can also be used. Also, Z-E-P,
A bleaching sequence such as ZE / OP, E / OP-PO, which does not use any atomic chlorine and chlorine-based bleaching chemicals can also be used. Needless to say, in order to further enhance the effects of the present invention, an enzyme treatment step such as a xylan-decomposing enzyme or a lignin-degrading enzyme may be provided before the bleaching treatment step.

In the present invention, in the bleaching step, an oxidizing agent selected from oxygen and hydrogen peroxide is added to a known alkali extraction stage to perform a sequence such as E / O, E / P or E / OP. When bleaching is performed in a bleaching sequence using at least one stage, the whiteness after bleaching is further improved. The alkali extraction to which oxygen and / or hydrogen peroxide has been added may be performed under pressure in accordance with the alkali oxygen bleaching method. It may be a method.

In the present invention, the wastewater after the acid treatment step is recovered and sent to a recovery boiler together with the waste cooking liquor from the cooking step.
Organic matter dissolved or contained in the wastewater by combustion can be used as thermal energy. At the same time, in the bleaching step following the acid treatment step in the present invention, bleached pulp is produced in a bleaching sequence that does not use atomic chlorine as a bleaching chemical, and further in a bleaching sequence that does not use both atomic chlorine and chlorine-based bleaching chemicals. Because you can
The wastewater from the bleaching can also be collected to close the bleaching process, and there is virtually no discharge of organic halogenated compounds (AOX) into the environment due to the bleaching wastewater.

In the present invention, the pulp after alkaline oxygen bleaching is subjected to an acid treatment under a pressure of a nitrogen-containing gas, whereby delignification proceeds remarkably while the decrease in pulp viscosity is minimized, and the kappa number is reduced. Can be done. Therefore, the amount of the bleaching chemical used in the bleaching process can be significantly reduced due to the reduced kappa number, and the pulp strength and the pulp yield can be maintained.

The reduction rate of the kappa number (relative to the kappa number before the treatment) in the acid treatment step of the present invention is very remarkable for hardwood pulp, and is the case for hardwood pulp (pulp kappa number after alkaline oxygen bleaching of 7 to 11). Is 20-70%
However, in the case of softwood pulp (pulp kappa number of 12 to 15 after alkaline oxygen bleaching), it is slightly lower, and decreases in the range of 10 to 30%.

In the present invention, the reason why the delignification progresses greatly while suppressing the decrease in the pulp viscosity by minimizing the acid treatment under pressure with a nitrogen-containing gas is clearly not deferred to future studies. Although it cannot be determined, the pressurization accelerates the delignification reaction by the acid, and the acid solution easily penetrates into the pulp fiber by the pressurization, and the fiber, as well as the lignin exposed on the pulp fiber surface, While the lignin present inside is also decomposed by acid, delignification proceeds, but the reaction system is filled with inert nitrogen gas, which suppresses the decomposition of cellulose in the pulp fiber by radicals. It is inferred that

[0039]

EXAMPLES The present invention will be described more specifically with reference to examples and comparative examples below, but of course the present invention is not limited to these examples. In Example 1 and Comparative Examples 1 to 3 shown below, unbleached pulp obtained by digesting wood chips was bleached with alkali oxygen and then bleached in the D1-ED2 sequence after or without acid treatment. In Example 2 and Comparative Example 4, D1
-E / O-D2 bleaching sequence,
In Example 3 and Comparative Example 5, bleaching was performed in an E / O-PO sequence after acid treatment, and Example 4 and Comparative Example 6 were used.
Is obtained by one-stage bleaching with PO after acid treatment. Unless otherwise indicated, the reduction rate of the Kappa number due to the acid treatment, the reduction rate of the chemicals in the D1 and D2 stages, and the E stage, the pulp yield in the acid treatment, and the total pulp yield in the acid treatment and bleaching are as follows, respectively. It was calculated as follows. In addition, the addition rates of the chemicals in Examples and Comparative Examples are shown by weight% based on the weight of the absolutely dried pulp.

1. Rate of decrease in kappa number due to acid treatment The rate of decrease in kappa number due to acid treatment was determined by measuring the kappa number of pulp before and after acid treatment according to JIS P 8211,
It was calculated by equation (1). 1. Reduction rate of kappa number due to acid treatment, (%) = {(kappa number before acid treatment−kappa number after acid treatment) / kappa number before acid treatment} × 100 (1) Reduction rate of total chemicals in D1 and D2 stages The chemical reduction ratio was calculated from the total chemical ratio of chlorine dioxide in D1 and D2 stages used for bleaching with and without acid treatment of the pulp by equation (2). Reduction rate of total chemicals in D1 and D2 stages, (%) = ｛(total chemical addition ratio in D1 and D2 stages without acid treatment−total chemical addition ratio in D1 and D2 stages with acid treatment) / Total chemical addition rate in D1 and D2 stages without acid treatment 処理 × 100 (2)

3. Reduction rate of chemicals in stage E The reduction ratio of chemicals was calculated from the chemical addition ratio of caustic soda in stage E used for bleaching with and without acid treatment of the pulp by equation (3). Reduction rate of chemicals in stage E, (%) = ｛(chemical addition ratio in stage E without acid treatment-chemical addition ratio in stage E with acid treatment) / in stage E without acid treatment 3. Chemical addition rate x 100 ... (3) Pulp Yield in Acid Treatment The pulp yield in acid treatment was calculated from the absolute dry weight of the pulp before and after the acid treatment according to equation (4). 4. Pulp in acid treatment {yield, (%) = {(absolute dry weight of pulp after acid treatment) / absolute dry weight of pulp before acid treatment} × 100 (4) Total pulp yield from acid treatment to bleaching The total pulp yield from acid treatment to bleaching was determined by multiplying the pulp yield in acid treatment by the pulp yield in multistage bleaching.

Example 1 500 g of hardwood mixed wood chips consisting of 70% domestic hardwood material and 30% eucalyptus wood were sampled in an absolute dry state, and the liquid ratio was 4, effective alkali was 18% per absolute dry chip weight, and the sulphidity of the cooking liquor was 25. %, A cooking temperature of 160 ° C, and a cooking time of 120 minutes, using a laboratory indirect heating autoclave for kraft cooking, then separating the waste liquid and pulp, and selecting the pulp using a flat screen equipped with a 10-cut screen plate. Hunter whiteness 45.2%, kappa number 20.
1. Pulp viscosity 23.3 mPa · s (J. TAPPI
229 g of hardwood unbleached kraft pulp (measured by No. 44) was obtained in an absolute dry weight (selective pulp yield: 45.8%).

90.0 g of the absolutely dry weight of the hardwood unbleached kraft pulp was sampled, 1.5% of caustic soda was added to the absolutely dried pulp weight, and then diluted with ion exchanged water to adjust the pulp concentration to 10%. Into an indirectly heated autoclave and pressurized with commercially available compressed oxygen gas having a purity of 99.9% so that the gauge pressure becomes 5 kg / cm ² ,
The mixture was heated at 0 ° C. for 60 minutes and subjected to alkaline oxygen bleaching by a medium concentration method. The obtained pulp was washed and dehydrated using ion-exchanged water, and the absolute dry weight of 88.8 g having a whiteness of 51.1 was obtained.
%, Kappa number 10.2, pulp viscosity 18.8 mPa
・ S pulp was obtained.

80.0 g of this pulp was collected in absolute dry weight, concentrated sulfuric acid was added to ion-exchanged water and diluted with a sulfuric acid aqueous solution having a pH of 2.6 to adjust the pulp concentration to 10%. The autoclave was placed in a 2-liter indirectly heated autoclave, and the inside of the autoclave was replaced with nitrogen gas using 99.9% purity nitrogen gas. Thereafter, nitrogen gas was further introduced to make the gauge pressure 5 kg / cm ^2. Pressurized and heated at a temperature of 95 ° C. for 60 minutes while maintaining the pressure,
The pulp was acid-treated. After cooling the autoclave, the pulp obtained by the acid treatment was washed with ion-exchanged water,
Dehydrated. 51.4% whiteness of absolutely dry weight of 79.3g,
An acid-treated pulp having a kappa number of 4.5 and a pulp viscosity of 17.2 mPa · s was obtained. The reduction rate of the pulp copper value was 55.9%, and the pulp yield was 99.1%.

The absolute dry weight of the pulp after the acid treatment is 70.0 g
Is put into a plastic bag, the pulp concentration is adjusted to 10% using ion-exchanged water, 0.41% of chlorine dioxide is added per weight of the absolutely dried pulp, and immersed in a constant temperature water bath at a temperature of 70 ° C. for 30 minutes. D1 bleaching was performed. The obtained pulp was washed with deionized water and dehydrated. The pulp after the D1 stage is put into a plastic bag, and the pulp concentration is adjusted to 10% using ion-exchanged water. Then, caustic soda １ is added 1.7 times the D1 addition ratio, and the temperature is 70 ° C. in the same manner as the D1 stage. At 12
The mixture was treated for 0 minutes, and E-stage extraction was performed. The obtained pulp was washed and dehydrated using ion exchanged water.

Subsequently, the pulp after the E-stage was put into a plastic bag, and the pulp concentration was adjusted to 10% using ion-exchanged water. In the same manner, the mixture was treated at a temperature of 70 ° C. for 180 minutes to carry out bleaching in D2 stage. The obtained pulp was washed with deionized water and dehydrated to obtain a bleached pulp having an absolute dry weight of 69.2 g. Pulp yield in multi-stage bleaching is 98.9%
And the total pulp yield from acid treatment to bleaching treatment is 9
8.0%. D After the pulp after the second stage is disintegrated,
appi test method T205os-71 (JISP 820)
9) A sheet having a basis weight of 60 g / m ² was prepared according to JIS.
Result 8 of measuring whiteness of pulp according to P 8123
It was 5.4%. Table 1 shows the kappa number of the pulp after the acid treatment, the reduction rate of the kappa number due to the acid treatment, and the pulp yield.
Table 2 shows the chemical addition rates of the D1 and D2 stages, the reduction ratio of all the chemicals in the D (D1 + D2) stage, the reduction ratio of the E stage chemicals, the pulp yield in the bleaching treatment, and the total pulp yield from the acid treatment to the bleaching treatment. .

Example 2 Pulp after acid treatment obtained in Example 1 (pulp kappa number after acid treatment 4.5, pulp viscosity 17.2 mPa · s)
Using 70 g of the same pulp, D1 stage treatment was carried out in the same manner as in Example 1, washed and dehydrated with ion-exchanged water, and then 0.69% of caustic soda was added to the pulp based on the absolute dry pulp weight.
And then diluted with deionized water to reduce the pulp concentration to 1
It was adjusted to 0%, placed in an indirectly heated autoclave, and a commercially available compressed oxygen gas having a purity of 99.9% and a gauge pressure of 1.
After pressurizing to 5 kg / cm ² and heating at a temperature of 70 ° C. for 15 minutes, the pressure was reduced, the temperature was maintained at 70 ° C. at atmospheric pressure (gauge pressure: 0) for 105 minutes, and alkali extraction (E) added with oxygen was performed. / O) treatment was performed. The resulting pulp was washed and dehydrated using ion-exchanged water, and the D2 stage was carried out in the same manner as in Example 1 except that the addition ratio of chlorine dioxide was changed to 0.18% per pulp weight. A pulp having a pulp brightness of 85.4% was obtained. The pulp yield after multi-stage bleaching was 69.3 g, the yield was 99.0%, and the total pulp yield from acid treatment to bleaching treatment was 98.1%.

Table 1 shows the kappa number of the pulp after the acid treatment, the reduction rate of the kappa number by the acid treatment, and the pulp yield.
Table 2 shows the addition rates of the D2 stage chemicals, the reduction ratio of all the chemicals in the D (D1 + D2) stage, the reduction ratio of the E stage chemicals, the pulp yield in the bleaching treatment, and the total pulp yield from the acid treatment to the bleaching treatment.

Example 3 Pulp after acid treatment obtained in Example 1 (pulp kappa number after acid treatment 4.5, pulp viscosity 17.2 mPa · s)
70 g of the same pulp as above was collected, 1.0% of caustic soda and 0.5% of hydrogen peroxide were added to the absolutely dry pulp weight, and then diluted with ion-exchanged water to adjust the pulp concentration to 10%. Put in autoclave, purity 99.9
% Commercially available compressed oxygen gas and a gauge pressure of 1.5 kg / c
m ² and heated at a temperature of 70 ° C. for 15 minutes, then reduced in pressure, held at atmospheric pressure at a temperature of 70 ° C. for 105 minutes, and extracted with oxygen and hydrogen peroxide by alkali extraction (E / O).
P) Processing was performed. After cooling the autoclave, the pulp obtained by the alkali extraction (E / OP) treatment was washed and dehydrated using ion-exchanged water.

Subsequently, 1.5% caustic soda and 3.0% hydrogen peroxide per absolute dry pulp weight were added to the pulp after the E / OP stage.
Was added, and then diluted with ion-exchanged water to adjust the pulp concentration to 10%. The mixture was placed in an indirectly heated autoclave, and commercially available compressed oxygen gas having a purity of 99.9% and a gauge pressure of 5% was added.
It was pressurized to a pressure of kg / cm ² , heated at a temperature of 100 ° C. for 60 minutes, and subjected to hydrogen peroxide (PO) treatment by pressurizing oxygen. The obtained pulp was washed with deionized water and dehydrated to obtain bleached pulp having a pulp brightness of 84.2%. The absolute weight of the obtained pulp was 68.8 g and the yield was 98.3.
% And pulp viscosity was 14.8 mPa · s. Table 3 shows the pulp brightness after the bleaching and the total pulp yield from the acid treatment to the bleaching treatment.

Example 4 Pulp after acid treatment obtained in Example 1 (pulp kappa number after acid treatment 4.5, pulp viscosity 17.2 mPa · s)
70 g of the same pulp as above was collected, 1.5% of caustic soda and 2.0% of hydrogen peroxide were added to the absolutely dried pulp weight, and then diluted with ion-exchanged water to adjust the pulp concentration to 10%. Put in autoclave, purity 99.9
% Of commercially available compressed oxygen gas and a gauge pressure of 5 kg / cm ²
, And heated at a temperature of 100 ° C. for 60 minutes to perform a hydrogen peroxide (PO) treatment by pressurizing oxygen. The obtained pulp was washed with deionized water and dehydrated to obtain a semi-bleached pulp having a pulp brightness of 72.8%. The absolute dry weight of the obtained pulp was 69.5 g, the yield was 99.3%, and the pulp viscosity was 15.7 mPa · s. Table 3 shows the pulp brightness after the bleaching and the total pulp yield from the acid treatment to the bleaching treatment.

Comparative Example 1 Except that the pulp was not subjected to the acid treatment and the chemical addition ratio in the D1 stage was 0.92% based on the weight of the absolutely dried pulp.
Multistage bleaching (D1-ED2) was performed in the same manner as in Example 1 to obtain bleached pulp having a pulp brightness of 85.3% and an absolute dry weight of 68.7 g (pulp yield of 98.1%). Obtained. Table 1 shows the kappa number of the pulp after oxygen bleaching.
Table 2 shows the two-stage chemical addition rate and the pulp yield (total pulp yield) in the bleaching treatment.

Comparative Example 2 The acid treatment was performed in a nitrogen gas atmosphere (without pressurization).
And D1 stage chemical addition rate is 0.6 per absolute pulp weight
Acid treatment and multi-stage bleaching (D1-ED2) were performed in the same manner as in Example 1 except that the pulp brightness was 8%.
A bleached pulp of 5.3% in absolute dry weight of 69.1 g was obtained. The kappa number after the acid treatment was 6.5, the reduction rate of the kappa number in the acid treatment was 36.3%, and the pulp viscosity was 17.2.
mPa · s, pulp yield is 79.5 g in absolute dry weight, pulp yield is 99.4%, pulp yield in multistage bleaching is 69.1 g, pulp yield is 98.7%, from acid treatment to bleaching Had a total pulp yield of 98.1%. Table 1 shows the kappa number of the pulp after the acid treatment, the reduction rate of the kappa number due to the acid treatment, and the pulp yield. Table 1 shows the D1 and D2 stage chemical addition rates, D (D1 + D2) stage total chemical reduction ratio, and E stage chemical reduction. Table 2 shows the ratio, the pulp yield in the bleaching treatment, and the total pulp yield from the acid treatment to the bleaching treatment.

Comparative Example 3 Acid treatment and multi-stage bleaching (D1-D1) were carried out in the same manner as in Comparative Example 2 except that no nitrogen gas was used during the acid treatment and no pressure was applied.
E-D2) was performed to obtain a bleached pulp having a pulp brightness of 85.3% and an absolute dry weight of 69.1 g. The kappa number after the acid treatment was 6.5, the reduction rate of the kappa number in the acid treatment was 36.3%, the pulp viscosity was 16.8 mPa · s, the pulp yield was 79.4 g in absolute dry weight, and the pulp yield was low. The rate is 99.
3%, the pulp yield in multistage bleaching was 69.1 g, the pulp yield was 98.7%, and the total pulp yield from acid treatment to bleaching was 98.0%. Table 1 shows the kappa number of the pulp after the acid treatment, the reduction rate of the kappa number due to the acid treatment, and the pulp yield, and the D1 and D2 stage chemical addition rates, D (D1 + D
2) Table 2 shows the reduction ratio of the whole stage chemicals, the reduction ratio of the E stage chemicals, the pulp yield in the bleaching treatment, and the total pulp yield from the acid treatment to the bleaching treatment.

COMPARATIVE EXAMPLE 4 70 g of the same pulp as the pulp after the acid treatment (kappa number 6.5 after the acid treatment, pulp viscosity 16.8 mPa · s) obtained in Comparative Example 3 was sampled. Was subjected to a D1 stage treatment, washed with deionized water, and dehydrated.
D1 stage treatment in the same manner as above, washing with ion exchanged water,
After dehydration, 1.11% of caustic soda was added to the pulp based on the weight of the absolutely dried pulp, and the pulp was diluted with ion-exchanged water to adjust the pulp concentration to 10%. % Of commercially available compressed oxygen gas until the gauge pressure becomes 1.5 kg / cm ² , heated at a temperature of 70 ° C. for 15 minutes, then reduced in pressure, and at atmospheric pressure (gauge pressure of 0) at a temperature of 70 ° C. The mixture was held for 105 minutes, and an alkali extraction (E / O) treatment to which oxygen was added was performed. The obtained pulp was washed with deionized water, dehydrated, and D2 stage was performed in the same manner as in Comparative Example 3 except that the chlorine dioxide addition rate was changed to 0.28% per pulp weight. A pulp having a pulp brightness of 85.4% was obtained.

The pulp yield after multi-stage bleaching was 69.1 g.
The yield was 98.7%, and the total pulp yield from acid treatment to bleaching was 98.0%. Table 1 shows the kappa number of the pulp after the acid treatment, the reduction rate of the kappa number due to the acid treatment, and the pulp yield.
Table 2 shows the reduction rate of + D2) stage chemicals, the reduction ratio of E stage chemicals, the pulp yield in the bleaching treatment, and the total pulp yield from the acid treatment to the bleaching treatment.

Comparative Example 5 In the same manner as in Example 5, 70 g of the pulp obtained in Comparative Example 3 after the acid treatment (the kappa number after the acid treatment was 6.5 and the pulp viscosity was 16.8 mPa · s) was used. And multi-stage bleaching (E / OP-PO) to obtain pulp whiteness of 81.4%
A bleached pulp was obtained. Pulp yield after multi-stage bleaching is 68.
9g, pulp yield 98.4%, pulp viscosity 14.1
mPa · s. Table 3 shows the pulp brightness after the bleaching and the total pulp yield from the acid treatment to the bleaching treatment.

Comparative Example 6 In the same manner as in Example 6, the same pulp as the pulp after the acid treatment obtained in Comparative Example 3 (the kappa number after the acid treatment was 6.5 and the pulp viscosity was 16.8 mPa · s) was used. Tandem bleaching (PO) to obtain a bleached pulp having a pulp brightness of 69.8%. The pulp yield after one-stage bleaching is 69.5 g, the pulp yield is 99.3%, and the pulp viscosity is 15.1 mPa · s.
Met. Table 3 shows the pulp brightness after the bleaching and the total pulp yield from the acid treatment to the bleaching treatment.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

[Table 3]

As is clear from the table, the unbleached pulp is bleached with alkali oxygen (Kappa number 10.2) and then subjected to an acid treatment by pressurizing with nitrogen gas to minimize the decrease in pulp viscosity. In the subsequent bleaching process, a pulp having a desired whiteness can be obtained with a very small amount of a bleaching chemical while maintaining the pulp yield within a practical range. (Examples 1-4). The pulp treated with acid by pressurizing with nitrogen gas of the present invention is subjected to an alkali extraction stage (E / O) to which oxygen is added in a subsequent bleaching process.
The amount of the bleaching chemical required for bleaching to the desired whiteness can be significantly reduced (Example 2) because the delignification property of the dye is improved.

Further, by performing the acid treatment of the present invention,
In the subsequent bleaching with bleaching chemicals, the pulp can be bleached to the desired whiteness (84.2%) by using E / PO-PO without using any atomic chlorine and chlorine chemicals (Example 3). In the acid treatment without pressurization with nitrogen gas, the whiteness of the pulp cannot reach the desired level in the same sequence (pulp whiteness 81.4, Comparative Example 5). On the other hand, when the pulp was not subjected to the pressurization even if the pulp was subjected to the acid treatment under the nitrogen atmosphere, or when the pulp was subjected to the acid treatment without using the nitrogen gas itself (Comparative Examples 2 to 6), the pressurization with the nitrogen gas was performed. As compared with the case, the reduction rate of the Kappa number in the acid treatment was lower (36%), the reduction rate of the bleaching chemical in the subsequent bleaching process was lower (Comparative Examples 2 and 3), and
Even when the alkali extraction stage (E / O stage) to which oxygen is added is used, a remarkable decrease in the reduction rate of the bleaching chemicals cannot be achieved (Comparative Example 4). The acid-treated pulp of the present invention had a pulp brightness of 72.8 due to hydrogen peroxide bleaching with oxygen.
% Bleached pulp (Example 4), but with acid treatment without pressurization with nitrogen gas, the level of pulp brightness is low (pulp brightness 69.%) when bleached in the same bleaching sequence. 8%, Comparative Example 6).

[0064]

By providing the acid treatment step under the pressurization of nitrogen-containing gas according to the present invention immediately before the bleaching treatment step, the total pulp yield and bleaching from the alkali oxygen bleaching of the unbleached pulp to after the bleaching treatment step are obtained. It has become possible to provide a method for producing bleached pulp that can significantly reduce the amount of bleaching chemicals used without reducing the pulp viscosity of the pulp.

Claims (5)

[Claims] 1. A method for producing bleached pulp by subjecting unbleached pulp obtained by digesting wood to alkaline oxygen bleaching, followed by one-stage or multi-stage bleaching in a bleaching step, wherein nitrogen bleaching is performed immediately before the bleaching step. A method for producing bleached pulp, comprising an acid treatment step under pressure with a contained gas. 2. The acid treatment step of the pulp after the alkaline oxygen bleaching, wherein the pulp concentration is 5 to 40% by weight and the pH of the aqueous acid solution is 2 to 40% by weight.
6. The bleached pulp according to claim 1, wherein the bleaching pulp is performed at a temperature of 50 to 150 [deg.] C., for a time of 5 to 120 minutes, and at a pressure of 0.5 to 9.0 kg / cm < ² > (gauge pressure) by a nitrogen-containing gas. Manufacturing method. 3. The production of bleached pulp according to claim 1, wherein a nitrogen-containing gas produced as a by-product in a step of concentrating oxygen gas used for alkaline oxygen bleaching from air is used as the nitrogen-containing gas. Method. 4. The method for producing bleached pulp according to claim 1, wherein wastewater from the acid treatment step and the bleaching treatment step is recovered. 5. The method for producing bleached pulp according to claim 1, wherein the bleaching step includes at least one alkali extraction stage to which an oxidizing chemical selected from oxygen and hydrogen peroxide is added.