JPH1025686A - Bleaching of paper-making pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for bleaching paper-making pulp that carries out, in the pulp bleaching process, delignification by feeding a compressed gas into the pulp bleaching process and replacing the used gas in the bleaching reaction with a fresh gas, and increases whiteness of the pulp while controlling reduction of the pulp strength. SOLUTION: This bleaching method for paper-making pulp by hydrogen peroxide compressed using a gas which contain 0-100vol.% of oxygen comprises passing the gas continuously through pulp fibers in a pressurized reactor tower at a total pressure of 0.5-9kg/cm<2> (in gauge) while replacing the used gas with a fresh gas and also while degassing at least once in the process of the bleaching reaction to lower the pressure and recovering the pressure by feeding fresh gas to maintain the bleaching pressure, thus carrying out the bleaching at temperature of 70 to 120 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for bleaching pulp comprising a pressurized hydrogen peroxide stage by pressurizing a hydrogen peroxide stage with a gas and replacing used gas with fresh gas during the reaction. The present invention relates to a pulp bleaching method that increases delignification and increases whiteness while suppressing a decrease in pulp strength.

[0002]

2. Description of the Prior Art In order to use lignocellulosic substances of wood and vegetable fibers as pulp fibers for papermaking, the lignocellulosic substances must be digested and pulped with a chemical such as caustic soda or caustic soda and sodium sulfide, or refiner or grinder. After pulping by a mechanical action using pulp or the like, the obtained pulp is bleached to increase whiteness. For example, softwood kraft pulp, except for applications where strength is required for paper, such as wrapping paper and bags,
Usually, bleaching is performed in multiple stages with bleaching agents such as chlorine, hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide, and caustic soda, and bleaching aids, and bleaching is performed after lignin, a coloring cause substance contained in pulp, is removed. It is commonly used as kraft pulp. Further, since the fiber strength of hardwood kraft pulp is not as large as that of softwood pulp, most of the pulp is bleached by multi-stage bleaching and is used in fields where high strength is not required, such as high quality paper and printing coated base paper.

[0003] However, also in the case of bleached pulp, many techniques for bleaching without impairing the fiber strength of the pulp fiber itself have been proposed. Fiber damage that occurs during bleaching of pulp is caused by the decomposition of cellulose constituting the pulp fiber by the bleaching chemical when lignin contained in the pulp fiber is made water-soluble with a bleaching chemical and removed. . In order to keep the decomposition of the carbohydrates constituting cellulose to a minimum, it is common to employ a multi-stage bleaching process in which mild bleaching is performed by avoiding extreme single-stage bleaching and changing bleaching agents and bleaching conditions appropriately. is there. Normally, in the multi-stage bleaching process, first, lignin contained in pulp is chlorinated with chlorine (stage C), and the lignin is rendered soluble, and then chlorinated lignin is extracted with alkali (stage E). , Lignin is separated from the pulp fiber, and the remaining small amount of lignin is further decomposed and removed using hypochlorite (H stage), chlorine dioxide (D stage), etc. The method of obtaining is adopted.

[0004] In recent years, there has been an increase in demand for so-called chlorine-free paper bleached without using chlorine gas or the regulation of the amount of organic chlorine compounds (AOX) contained in the bleaching wastewater discharged from the multi-stage bleaching process. Attention has been paid to a bleaching method using a bleaching sequence using a non-chlorine bleaching chemical, and ozone, oxygen, hydrogen peroxide, peroxy acid and the like are known as the non-chlorine bleaching chemical. Of these, the bleaching method using ozone gas requires the installation of new equipment,
Due to the high investment cost, it has not yet become widely used. On the other hand, the bleaching method using hydrogen peroxide has advantages such as low investment cost and easy handling of chemicals, but disadvantages include a long reaction time with pulp and a large decrease in strength. And the like. Also, they have only been used for auxiliary bleaching of chemical pulp due to the relatively high chemical costs.

[0005] According to the Pulp and Paper Technology Association, edited by the Japan Pulp and Paper Institute, Vol. 5, "Pulping and Bleaching" (Revised, supplemented in 1986), pages 246 to 266, hydrogen peroxide is represented by the formula (1). Dissociation is disclosed.

Embedded image That is, it is considered that the peroxy anion (HOO-) generated by this dissociation performs bleaching action.
On the other hand, equation (2) shows a self-decomposition reaction of hydrogen peroxide, and when this reaction occurs, wasteful consumption of hydrogen peroxide occurs.
It has been recognized that nascent oxygen generated by decomposition damages cellulose and the like, leading to a decrease in strength. H ₂ O ₂ → H ₂ O + 1 / 2O ₂ +23.4 Cal (2)

In order to solve the above problems in bleaching with hydrogen peroxide, for example, INTERNATIO in 1994
NAL PULP BLEACHING CONFERENCE-PROCEEDINGS, 53 ~
Page 58 lists three methods of increasing the whiteness in the hydrogen peroxide stage: "pressurizing with oxygen,""increase the temperature," and "increase the pulp concentration." Similarly, 19
1994 INTERNATIONAL PULP BLEACHING CONFERENCE-PROC
EEDINGS, pp. 199-209, "Pressurize with Oxygen"
As an effect, when the pressurized oxygen pressure in the hydrogen peroxide stage is 5 bar, 85% (ISO) to 8
Although it is reported to reach a brightness of 7.5%, it is disclosed that, even in this case, the disadvantage that the pulp viscosity is greatly reduced still exists.

[0007] Japanese Patent Application Laid-Open No. 6-101186 discloses that
A method for bleaching chemical pulp is disclosed in which a digested chemical pulp is subjected to an acid treatment step and then subjected to a delignification / bleaching step using peroxide and pressurized oxygen in an alkaline medium. Further, JP-A-6-158573 discloses that a chemical pulp that has been digested is subjected to a high-temperature and high-pressure oxygen bleaching treatment, and then subjected to an acid treatment or a chelating agent treatment, and then treated with peroxide in an alkaline medium. A method for bleaching chemical pulp for papermaking is disclosed in which delignification and bleaching are performed with oxygen. These methods produce high-brightness paper pulp without chlorine and hypochlorite in a subsequent multi-stage bleaching sequence, but use conventional chlorine and hypochlorous acid. Compared to pulp bleached in a multi-stage bleaching sequence with salt, the resulting pulp does not yet show a satisfactory level of quality in terms of brightness and fiber strength.

Japanese Patent Publication No. 7-686 discloses a method for delignifying and bleaching digested chemical pulp with alkali and oxygen.
No. 75 discloses that, when a medium-concentration cellulose pulp slurry is delignified and bleached with alkali and oxygen, a delignification is performed by using a unit having a combination of a high-shear mixer and an oxygen delignification device in a plurality of stages in series. ,
A method for performing bleaching is disclosed. In this method, a plurality of high-shear mixers are provided to promote the dissolution of oxygen taken in the reaction system or newly added oxygen into an alkali solution, and furthermore, the contact of oxygen and alkali with cellulose pulp. In order to promote delignification. In addition, Japanese Patent Publication No. Hei 6-72386 discloses that a cellulose pulp slurry having a medium concentration is prepared by using an apparatus in which a combination unit of a high-shear mixer and a tower-shaped pressurized oxygen delignification apparatus is provided in a plurality of stages in series. In delignification and bleaching with alkali and oxygen, the entire amount of alkali is added at the portion between the unbleached raw material chest and the pump provided in front of the first high shear mixer, while oxygen is supplied in multiple stages of high shear. A method of delignifying and bleaching cellulose pulp with oxygen, which is added to a mixer, is disclosed. In this method, an alkali is added to a certain place, and oxygen is added in a plurality of times, and the purpose is to promote delignification by oxygen in the same manner as the above method.

[0009]

SUMMARY OF THE INVENTION In view of this background, the present inventors have found that when bleaching digested pulp by a multi-stage bleaching sequence including a water-peroxide stage pressurized with gas, gas-pressurized peroxide is used. As a result of intensive research on a method for obtaining high brightness pulp without damaging the pulp fiber in the hydrogen stage, the decomposition of hydrogen peroxide during the delignification of pulp with hydrogen peroxide by pressurizing with gas was performed. Harmful substances caused by lignin and delignification are generated and may damage the cellulose fiber, and by focusing on this, while performing delignification with hydrogen peroxide under pressure, such harmful substances are removed from the reaction system. It has been found that pulp having high whiteness and viscosity can be obtained while suppressing damage to the pulp fiber to a minimum by discharging the pulp to a minimum, thereby completing the present invention.
It is an object of the present invention to pressurize pulp with gas when bleaching pulp with pressurized hydrogen peroxide, release waste gas during or during the bleaching reaction, and then replenish fresh gas. By bleaching with accompanying pressurized hydrogen peroxide,
An object of the present invention is to provide a method for bleaching pulp for papermaking, which can impart high whiteness and viscosity to pulp and can further suppress a decrease in pulp strength.

[0010]

A first aspect of the present invention is a method for bleaching papermaking pulp with pressurized hydrogen peroxide using a gas, wherein the gas is continuously passed through a pulp fiber of a reaction tower pressurized. And displacing used gas and fresh gas while maintaining the pressure in the range of 0.5 to 9 kg / cm ² (gauge pressure). A second aspect of the present invention is to degas at least once during the bleaching reaction in the bleaching with pressurized hydrogen peroxide, reduce the pressure, and then restore the pressure by adding a new gas to maintain the pressure. The method for bleaching paper pulp according to claim 1, characterized in that: The third aspect of the present invention is that the gas for pressurization in the bleaching with the pressurized hydrogen peroxide contains oxygen of 0 to 1
The method for bleaching paper pulp according to the first or second aspect of the present invention, wherein the bleaching pulp contains 100% by volume. The fourth aspect of the present invention is:
The bleaching method for papermaking pulp according to any one of the first to third aspects of the present invention, wherein the bleaching with the pressurized hydrogen peroxide is performed at a temperature of 70 to 120 ° C.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The pulp for papermaking used in the present invention is wood or non-wood from polysulfide pulping (PS), kraft pulping (KP), sulphite pulping (SP), alkali pulp. Chemical method (A
P) and other pulps obtained by chemical pulping methods, and also from wood, ground pulping method (GP), refiner ground pulping method (RGP), thermomechanical pulping method (TMP), chemi-thermomechanical pulping Law (C
Pulp obtained by a mechanical pulping method such as TMP) and newspapers, flyers, high-quality paper, waste paper pulp from information paper used in OA equipment, and the like, and preferably obtained by the kraft pulping method. It is pulp. Wood used for pulping includes pine, spruce, western hemlock, softwood such as Douglas fir, hardwood such as aspen, beech, acacia, eucalyptus, mangrove, and non-wood such as bagasse, kenaf, esparto grass,
Examples include rice, but are not necessarily limited thereto.

The pulp treated by bleaching with pressurized hydrogen peroxide of the present invention may be used without being exposed, or may be subjected to bleaching treatment such as oxygen delignification, ozone treatment, chlorine dioxide treatment or the like as pretreatment. What performed acid cleaning processing may be used. The type of gas used for bleaching the pressurized hydrogen peroxide of the present invention can be used irrespective of the type of active gas, inert gas, etc., and nitrogen substantially free of oxygen, a mixture of nitrogen and oxygen, Pure oxygen is preferably used. However, air (oxygen content 21% by volume) or oxygen-enriched gas is preferred from the viewpoint of gas production cost and the effect on pulp quality. Such a gas may have a concentration higher than the oxygen concentration in the atmosphere (21% by volume), but the higher the oxygen concentration, the higher the bleaching effect. From the viewpoint of the bleaching effect, the preferred oxygen concentration is 80% by volume or more. The oxygen used is PSA (Pressure Swing A).
dsorption oxygen, VSA (Vacum S)
The production method is not particularly limited as long as it can be used on an industrial scale, such as wing adsorption (oxygen) or cryogenic oxygen. The oxygen-enriched gas used for pressurization is
The oxygen gas obtained by these methods may be diluted with air before use.

In the bleaching stage of papermaking pulp with pressurized hydrogen peroxide according to the present invention, the above-mentioned fresh (unused) gas may be added to the pressure system and the used gas may be discharged out of the system. The bleaching apparatus used for this purpose is a known type of vertical or horizontal bleaching tower or tube having pressure resistance, a pump for feeding raw pulp, a mixer using high-speed shearing force, a stirrer, and the like. Are used alone or in combination. The pulp concentration in the bleaching stage with pressurized hydrogen peroxide of the present invention is 5 to 15 except for pulp fibers having a short fiber length such as mechanical pulp and waste paper pulp.
%, Preferably in the range of 8 to 12% by weight. If the pulp concentration is less than 5% by weight, the reaction between the pulp fiber and hydrogen peroxide will be poor, and if the pulp concentration exceeds 15% by weight, a uniform reaction will be difficult. On the other hand, when processing mechanical pulp or waste paper pulp, the pulp concentration can be in the range of 10 to 30% by weight. The temperature of the bleaching stage with pressurized hydrogen peroxide is in the range of 70 to 120 ° C, preferably 100 to 1
It is in the range of 10 ° C. If the bleaching temperature is lower than 70 ° C., the reaction rate is low even if pressure is applied, and a sufficient bleaching effect cannot be obtained. On the other hand, when the temperature exceeds 120 ° C., the decomposition reaction of hydrogen peroxide becomes large, so that the effective amount of hydrogen peroxide with respect to the pulp decreases, and since the generated radicals attack carbohydrates, the degree of polymerization of carbohydrates decreases. Is not suitable because pulp strength is reduced.

The addition of hydrogen peroxide and alkali in the bleaching with pressurized hydrogen peroxide of the present invention is performed in the same manner as in the case of bleaching with known pressurized hydrogen peroxide. 0.5 to 6.0% by weight per dry pulp weight
, Preferably in the range of 2.0 to 4.0, and the alkali is usually added with caustic soda.
In the range of 0.5 to 4.0% by weight, preferably 0.7
It is appropriately selected and used from the range of ~ 3.0% by weight. The residence time of the pulp in the bleaching stage ranges from 5 to 120 minutes, preferably from 10 to 90 minutes. If the time is less than 5 minutes, the increase in whiteness is small, and if it exceeds 120 minutes, the whiteness reaches equilibrium and reaches a plateau, which is not economical. In the case where bleaching with pressurized hydrogen peroxide is performed in a plurality of bleaching towers or tubes, it is effective to provide a large number of short residence times so that a large amount of gas can be replaced, so that effective bleaching can be performed. In the actual plant, the total time required from the injection of the pressurized gas to the point where the pressure is released is defined as the pulp residence time. Also, for the sequence of increasing the whiteness of the pulp with hydrogen peroxide remaining after the pressure treatment, the amount of hydrogen peroxide added was slightly increased, and after the pressure treatment,
Subsequently, a holding tower for holding the pulp at a normal pressure for a specific time may be provided. Care must be taken when holding the pulp in the absence of residual hydrogen peroxide, as color reversion may occur.

The addition of the above-mentioned gas to the bleaching stage by pressurized hydrogen peroxide and the discharge of the used gas out of the system include, for example, two bleaching reactors and a pulp feed pump attached thereto, a high shear mixer. In an up-flow type apparatus equipped with a gas, oxygen or other gas is fed into the pulp slurry or pulp to which hydrogen peroxide and alkali have been added from the lower part of the first bleaching tower, and the pulp and gas are separated by a high shear mixer. Mix well and send to the high pressure reaction tower with medium or high concentration pump, bleaching is performed while pulp rises inside the tower and reaches the top of the reaction tower, and used at the top The gas is released out of the pressure system, the pressure in the first bleaching tower is reduced, and the pulp sent to the second reaction tower is blown with fresh gas, and the mixing is sufficiently performed in the same manner as described above. Second while doing The bleach tank was introduced again into the lower part of the tower to increase the pressure, and then bleached while the pulp was rising, releasing the pressure from the top of the second bleaching tower to a blow tank that also held the pulp. After discharging and after a certain residence time, the pulp is sent to known washing equipment. on the other hand,
Using the same up-flow type device, while maintaining the pressure difference between the lower part and the top part of the reaction tower at about the head difference,
Bleaching can be efficiently carried out by continuously introducing fresh gas into the lower part of the column, flowing through the column, and continuously discharging spent gas from the top of the column. or,
In the bleaching with the pressurized hydrogen peroxide of the present invention, when a horizontal tubular reaction vessel is used, the pressure at the inlet and the outlet of the first bleaching tube is maintained at substantially the same pressure, and the introduction of fresh gas, The bleaching can also be performed while circulating into the column and discharging the used gas to the outside of the pressure system, and thus continuously replacing fresh gas and used gas. In this case, it is desirable that the rotation of the pulp be performed also with stirring using a rotating screw.

In the present invention, the introduction of gas is carried out using raw pulp.
Pump or high-concentration pump for introducing
Pump suction or medium or high density mixer
Pressurized by directly injecting gas in the
The bleaching reaction was carried out for a certain time in the reaction tower
After that, release the used gas while keeping the pressure constant
And release the used gas to lower the system pressure.
By lowering and then increasing the pressure
A method is also acceptable. In the present invention, a constant pressure is set in the reaction tower.
With displacement or pressure drop due to gas flow under force
The reason for the substitution is that the bleaching reaction with hydrogen peroxide
Harms the radicals and carbohydrates of the pulp
The purpose is to discharge gas out of the reaction system.
If used gas can be discharged outside the system,
The amount of gas to be released, the degree of pressure drop,
What is necessary is just to determine while watching, and it does not specifically limit. Pressurized peroxide
The pressure when pressurizing with gas in the bleaching stage with hydrogen is as follows:
On top of upflow and downflow type bleach towers
9kg / cm^Two (Gauge pressure) within the range
It is preferable to release spent gas.
For example, in the upflow type, the pressure at the top
Reducing the pressure below the sum vapor pressure will
The contained chemicals boil, hindering the operation of the device, and
It is dangerous, and it can suppress the decomposition reaction of hydrogen peroxide
No bleaching, except when pulp is blown
Should be avoided. Therefore, used as described above
Pot for lowering pressure by releasing gas
The top pressure is in addition to the saturated vapor pressure at the bleaching temperature.
0.1kg / m ^TwoAbove, preferably the total pressure is 0.5 kg
/ M^Two(Gauge pressure) or more.

As described above, in the present invention, pulp and gas containing hydrogen peroxide and alkali are introduced into a vertical bleaching reaction tower or a horizontal tubular reaction tower and pressurized, and bleaching with hydrogen peroxide is performed freshly. A method in which the reaction pressure is maintained in the range of 0.5 to 9 kg / cm ² while replacing the used gas with the used gas, or the used gas is released during the bleaching reaction, and the pressure in the bleaching stage is increased. After decreasing the temperature within the range, the reaction may be performed by adding fresh gas and increasing the pressure, or the bleaching reaction may be performed in a single reaction vessel depending on the degree of delignification and the combination of the bleaching sequences. Whether to perform the reaction or using a plurality of reaction vessels is appropriately selected. In the present invention, the rate of addition of hydrogen peroxide may be appropriately selected depending on the amount of residual lignin in the pulp to be bleached, the combination with other bleaching sequences, the degree of whiteness of the target final pulp, and the like.

The heavy metal brought into the bleaching stage by pressurized hydrogen peroxide promotes self-decomposition of hydrogen peroxide. To prevent this, it is preferable to use a chelating agent in addition. Such a chelating agent may be added in a step before the bleaching step with pressurized hydrogen peroxide, or may be added in the bleaching step with pressurized hydrogen peroxide. As the chelating agent used in the present invention, a known diethylenetriaminepentaacetic acid (DTP) usually used as a chelating agent may be used.
A), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentamethylene sulfate (DTMPA),
Nitrilotriacetic acid (NTA) and the like are used, and one or a plurality of them are selected and used. The amount of the chelating agent to be added depends on the amount of metal ions brought into the bleaching stage with pressurized hydrogen peroxide, but is in the range of 0.05 to 0.5% by weight based on the weight of the absolutely dried pulp. When a chelating step is provided before the bleaching step with pressurized hydrogen peroxide, the temperature is 40 to 95 ° C., preferably 60 to 80 ° C., and the reaction time is 5 to 40 minutes. Adjusted to things. For example, when the treatment is performed using DTPA, the treatment is preferably performed in a weakly acidic region having a pH of 4 to 6.5.

The reason why the whiteness of the pulp can be increased and the decrease in the strength of the pulp can be reduced in the bleaching stage using the pressurized hydrogen peroxide of the present invention can be reduced. Preventing decomposition,
By increasing the pressure, the oxygen bubbles are reduced and the oxygen partial pressure can be increased, so that the dissolved oxygen amount also increases, the frequency of the reaction between pulp and oxygen increases, and the pressure increases the hydrogen peroxide into the pulp fiber. Promotes infiltration, releases spent waste gas during or during the bleaching reaction,
It is presumed that the replacement of the gas by adding a fresh gas results in a synergistic effect such as removal of generated radicals, harmful components, and the like that damage the pulp fiber. In particular, the replacement of gas, which introduces fresh gas during the bleaching reaction and releases waste gas, suppresses or reduces these attacks on carbohydrates in the pulp fiber by removing radicals and harmful components. The most important feature of the present invention is that the pulp fiber is not damaged even if the brightness is finally increased, so that the pulp strength is also improved.

The bleaching stage with pressurized hydrogen peroxide of the present invention comprises:
Depending on factors such as the type of pulp, the degree of bleaching, the final brightness of the pulp, and the use of the pulp, it may be used in various bleaching sequences. For example, bleaching with pressurized hydrogen peroxide according to the present invention is PO, O is oxygen delignification (or oxygen bleaching), C is chlorine, C / D is chlorine-enriched chlorine, E is alkaline extraction, and oxygen-enriched. Eo for alkali extraction, Eop for alkali extraction enriched with oxygen and hydrogen peroxide, H for hypochlorite, D for chlorine dioxide, pressurized hydrogen peroxide other than the bleaching of the present invention enriched with oxygen To P
o, Q for chelate treatment, Z for ozone, A for acid washing step,
When the washing step with water or hot water is indicated by-, the bleaching sequence including bleaching with pressurized hydrogen peroxide of the present invention includes the following.

In order to obtain a pulp having a final whiteness of 80% or more, CEH-PO, C-Eo-D-PO, C / D-
Eop-PO, OCH-PO, OC-Eo-P
O, D-Eop-PO, O-D-Eop-PO, OP
OQ-POQ, OQ-PO-Q-PO, OQ-Po
-Q-PO, O-ZQ-POQ, etc. Further, in order to make mechanical pulp or waste paper pulp into pulp having a whiteness of 50 to 75%, Q-PO, AZ-PO, Z-PO, QP
O, D-PO, etc., are selected as appropriate, and an optimal sequence is used. Further, an enzymatic treatment stage such as xylanase may be provided in a part of the bleaching step for the purpose of adding chemicals and improving pulp quality in the above sequence.

[0022]

The present invention will be described in more detail with reference to the following Examples, but it should be understood that the present invention is by no means restricted thereto. In addition, the density | concentration and addition rate which are shown by% in an Example and a comparative example show weight% with respect to absolutely dry pulp weight. The Examples and Comparative Examples are oxygen delignified softwood kraft pulp (kappa number 11.1, pulp whiteness 34.5%,
Q ₁ -Po- using pulp viscosity of 15.7 mPa · s)
The bleaching was performed according to the Q ₂ -PO sequence, and the bleaching conditions of the PO stage were variously changed.

Example 1 Q ₁ : A pulp slurry in which 80 g of absolutely dry weight of the above-mentioned softwood kraft pulp which had been oxygen delignified was diluted with ion-exchanged water to a concentration of 4.0% and uniformly dispersed with a disintegrator was obtained. Next, sulfuric acid was added to this slurry to make the pH slightly acidic, and DTPA (diethylenetriaminepentaacetic acid) was added at 0.1% in terms of solid content. The addition of sulfuric acid was adjusted so that the pH after the reaction was 5.5. Subsequently, the concentration of the pulp slurry was adjusted to 3.5% by diluting with ion-exchanged water, and maintained at a temperature of 65 ° C. for 30 minutes with gentle stirring. The pulp was transferred to a wooden box on which a 150 mesh stainless steel wire mesh was attached, and after deliquoring, it was sufficiently washed with ion-exchanged water.

The pulp concentration after the Po: Q ₁ treatment was adjusted to 13%, and the pulp was placed in a 2-liter closed pressure vessel (stirred autoclave) equipped with a heating heater and a stirrer, and then peroxided. The pulp concentration was adjusted to 10% by adding 3% of hydrogen, 1.3% of sodium hydroxide, and 0.1% of magnesium sulfate heptahydrate in terms of magnesium sulfate. Thereafter, the content was heated while being stirred at 300 rpm, pure oxygen having a purity of 99.5% by volume was introduced into the container at a temperature of 100 ° C., and the pressure was increased to 3.0 kg / cm ^2.
(Gauge pressure) and kept for 15 minutes. After 15 minutes, the pressure was released and maintained at 90 ° C. for 90 minutes at atmospheric pressure. The obtained pulp was washed with ion-exchanged water, centrifugally dehydrated, and the pulp concentration was set to 30%. The whiteness of the obtained pulp was 65.3% (ISO), and the pulp viscosity was 18.5 mPa · s. Q _2: they were treated pulp by the same operation as the Q _1.

The pulp concentration after the PO: Q ₂ treatment was adjusted to 13%, and the pulp was placed in the above-mentioned pressure vessel equipped with a heating heater and a stirrer. Then, 4% of hydrogen peroxide and sodium hydroxide were added. 1.3% and magnesium sulfate heptahydrate were added at 0.1% in terms of magnesium sulfate, and ion-exchanged water was added to adjust the pulp concentration to 10%. Thereafter, the content was heated with stirring at 300 rpm, and the temperature was set to 110 ° C.
In the above, pure oxygen gas having a purity of 99.5% by volume was introduced into the container, and the pressure was adjusted to 7.0 kg / cm ² (gauge pressure). It was kept flowing for 60 minutes. Thereafter, the pulp taken out of the container was washed with ion-exchanged water to obtain bleached pulp.

The following test methods were used to evaluate pulp quality and strength. (1) Whiteness: ISO whiteness was determined using Ellejo 2000 (manufactured by Data Color). (2) Pulp viscosity: TAPPI STD T230
The measurement was performed according to om-89. (3) Tensile strength—measured using JIS P8113 and shown as the breaking length. (4) Tear strength: Measured using JIS P8116 and shown as specific tear strength.

Example 2 In PO, oxygen gas was introduced into the vessel and the pressure was increased to 3.0.
Bleaching was carried out in the same manner as in Example 1 except that the pressure was maintained at kg / cm ² (gauge pressure), a bleached pulp was obtained, and the pulp was tested in the same manner as in Example 1.

Example 3 Bleaching was performed in the same manner as in Example 1 except that the flow rate of oxygen gas in PO was changed to 0.5 liter per minute to obtain bleached pulp. The pulp was tested.

Example 4 PO instead of pure oxygen having a purity of 99.999% by volume
Bleaching was performed in the same manner as in Example 1 to obtain bleached pulp, and bleaching was performed in the same manner as in Example 1, except that nitrogen gas was used. The pulp was tested.

Example 5 99.5% by volume of purity at 110 ° C. in PO
Is introduced into the container and the pressure is increased to 7.0 kg / cm.^Two 
(Gauge pressure), and 1
After maintaining the pressure for 5 minutes, the pressure is increased to 1 kg / cm^TwoWhen
Release waste gas and reduce pressure until
Is introduced into the container and the pressure is 7 kg / cm. ^TwoAnd overall
Drift in the same manner as in Example 1 except for holding for 60 minutes.
Perform whitening to obtain bleached pulp, and further in the same manner as in Example 1.
The pulp was tested.

COMPARATIVE EXAMPLE 1 After introducing oxygen gas into the container in PO, the pressure was maintained without flowing oxygen gas and without releasing waste gas and re-introducing oxygen gas during the bleaching reaction. Except for this, bleaching was performed in the same manner as in Example 1 (Po) to obtain bleached pulp, and the pulp was tested in the same manner as in Example 1.

Comparative Example 2 After introducing oxygen gas into the container in PO, the pressure was maintained without flowing oxygen gas and without releasing waste gas and re-introducing oxygen gas during the bleaching reaction. Except for this, bleaching was performed in the same manner as in Example 2 (Po) to obtain bleached pulp, and the pulp was tested in the same manner as in Example 1.

Comparative Example 3 After the nitrogen gas was introduced into the container in the PO, the pressure was maintained without flowing the nitrogen gas and without releasing the waste gas and re-introducing the nitrogen gas during the bleaching reaction. Except for this, bleaching was performed in the same manner as in Example 1 to obtain bleached pulp, and the pulp was tested in the same manner as in Example 1.

COMPARATIVE EXAMPLE 4 In PO, oxygen gas was not introduced into the container, and therefore, the saturated vapor pressure (0.46 kg /
cm ² gauge pressure) except that carried out the bleaching reaction in the below,
Bleaching was performed in the same manner as in Example 1 (P), and bleached pulp was obtained. Further, the pulp was tested in the same manner as in Example 1.

Comparative Example 5 Example 1 was repeated except that the temperature was 60 ° C. in PO.
The bleached pulp was obtained in the same manner as described above, and the pulp was further tested in the same manner as in Example 1.

Table 1 shows the results obtained in Examples and Comparative Examples.

[0037]

[Table 1]

As can be seen from Table 1, the bleached pulp according to the present invention has excellent strength with high whiteness and viscosity (Examples 1 to 5). The higher the pressure in the bleaching stage, the more bleaching proceeds and there is a slight difference in the brightness of the pulp (Example 1).
Comparison between Examples 1 and 2), but the amount of the oxygen gas flow rate hardly affects the results (Comparison between Examples 1 and 3). On the other hand, the pulp obtained from the bleaching method without gas replacement has a low pulp viscosity even with high whiteness, and particularly low tensile strength (Comparative Examples 1 and 2). Also, when nitrogen is used as the gas for pressurizing in the PO stage, the whiteness and viscosity of the pulp are slightly lower than when oxygen is used (comparison between Example 1 and Example 4). If the gas is not replaced, the pulp quality is further deteriorated (comparison between Example 4 and Comparative Example 3). Further, when there is no pressurization by gas in the PO stage (Comparative Example 4), the whiteness of the pulp is low. Pulp whiteness is much lower.

[0039]

The present invention has the effect of providing a bleaching method which is used in combination with other bleaching methods, has high pulp whiteness and viscosity, and suppresses a decrease in pulp strength.

Claims (4)

[Claims] 1. A method for bleaching papermaking pulp with pressurized hydrogen peroxide using a gas, wherein the gas is continuously passed through pulp fibers of a pressurized reaction tower, and used gas and fresh gas 0.5 to 9 kg / c while replacing
A method for bleaching paper pulp, characterized in that the pulp is maintained in the range of m ² (gauge pressure). 2. The method according to claim 1, wherein the gas is released at least once during the bleaching reaction in the bleaching with the pressurized hydrogen peroxide, the pressure is reduced, and then the pressure is restored by adding a new gas to maintain the pressure. The method for bleaching pulp for papermaking according to claim 1, wherein 3. The method for bleaching pulp for papermaking according to claim 1, wherein the gas for pressurization in bleaching with pressurized hydrogen peroxide contains 0 to 100% by volume of oxygen. 4. The bleaching with pressurized hydrogen peroxide at a temperature of 7
The method for bleaching pulp for papermaking according to any one of claims 1 to 3, wherein the method is performed in a temperature range of 0 to 120 ° C.