JPH08260370A - Bleaching method for lignocellulosic material - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a bleaching method for suppressing the consumption of hydrogen peroxide during the multi-stage bleaching process in pulp, increasing the effect of increasing whiteness in a short time, and suppressing the decrease in pulp strength. [Structure] In a multi-stage bleaching process of pulp, at least two hydrogen peroxide stages pressurized with oxygen are included, and the treatment pressure in the first pressurized hydrogen peroxide stage is 0.1 to 5.0 kg /
cm ² · G, and the processing pressure of the pressurized hydrogen peroxide stage arranged in the bleaching step after that is 5.0 to 9.0 kg.
/ Cm ² · G, and the difference between the processing pressure of the first pressurized hydrogen peroxide stage and the processing pressure of the pressurized hydrogen peroxide stage arranged after that is 2 kg / cm ² · G or more, 9 kg /
A bleaching method characterized by being in the range of less than cm ² · G.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a process for bleaching chemical and mechanical pulps, and more particularly to pressurizing the hydrogen peroxide stage during the bleaching sequence with oxygen to reduce the self-decomposition of hydrogen peroxide. The whiteness can be increased in a short time, and the whiteness can be increased by performing the pressurized hydrogen peroxide stage at least twice under different pressure conditions.
The present invention also relates to a method for bleaching a lignocellulosic material, which makes it possible to minimize a decrease in pulp strength.

[0002]

In order to use a lignocellulosic material for many purposes, it is bleached after being pulped by a chemical action such as digestion or by a mechanical action using a refiner or the like. It is necessary to increase the whiteness. For example, kraft pulp is normally used as a bleaching agent and bleaching aid such as chlorine, hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide, caustic soda, etc., except when it is used for applications requiring strength such as packaging materials. It is generally used as a bleached kraft pulp after being bleached with an agent to remove lignin, which is a color-causing substance contained in the pulp, and the like.

Also in bleached pulp, it is necessary to maintain the strength of the pulp fiber itself to some extent, and for this reason, the radical 1 is used so as to minimize the decomposition of carbohydrates (cellulose etc.) constituting the pulp fiber. It is common to avoid a bleaching step and change the bleaching agent and the bleaching conditions to perform a mild bleaching step.

Usually, in a multi-stage bleaching process, first, chlorine is used to chlorinate the lignin contained in the pulp (stage C),
After adding solubility to lignin, the lignin is then dissolved and extracted with alkali (E stage) to separate lignin from the pulp, and hypochlorite (H stage), chlorine dioxide (D stage), etc. Used to further decompose and remove a small amount of residual lignin,
The method of obtaining pulp with high whiteness is adopted.

In recent years, due to the regulation of organic chlorine compounds contained in bleaching wastewater and the demand for chlorine-free paper, a bleaching method using a non-chlorine bleaching chemical has attracted attention. Hydrogen oxide, peracid, etc. are known.
Of these, the bleaching method using ozone gas requires new installation of equipment, and has not reached widespread use due to the large investment cost.

The advantages of the bleaching method using oxygen and hydrogen peroxide include low investment cost and easy handling of chemicals. On the other hand, disadvantages include long reaction time with pulp and a large decrease in strength. Especially in the case of hydrogen peroxide, it has only been used for auxiliary bleaching of chemical pulp or bleaching of relatively low whiteness pulp such as mechanical pulp because of high chemical cost.

According to the Paper and Pulp Technology Association, "Complete Paper Pulp Manufacturing Techniques," Vol. 5, "Pulp Treatment and Bleaching" (Revised 1986 Supplement), pages 246-266, hydrogen peroxide has the formula (1 ) Dissociate. It is believed that the peroxyanion (HOO ⁻ ) generated by this has a bleaching action. On the other hand, the formula (2) shows the self-decomposition reaction of hydrogen peroxide, and when this reaction occurs, the hydrogen peroxide is wasted, and the nascent oxygen generated by the decomposition damages the cellulose and the like, and It is recognized that it will cause a decline.

[0008]

Embedded image

In order to solve the above-mentioned problems in bleaching using oxygen and hydrogen peroxide, for example, in 1994, "INTERNATIONA
L PULP BLEACHING CONFERENCE-PROCEEDINGS ", No. 53-5
On page 8, there are three methods of increasing whiteness in the hydrogen peroxide stage: "pressurize with oxygen", "increase temperature" and "increase pulp concentration". Among them, as the effect of “pressurizing with oxygen”, in the oxygen treatment-chelate treatment-pressurized hydrogen peroxide treatment, when the oxygen pressure of the pressurized hydrogen peroxide stage is 0.4 MPa, the whiteness degree is about 10% compared to the non-pressurized treatment. It is reported to increase by 1.8 points and increase by about 2.1 points at 0.8 MPa.

[0010] Also in 1994, "INTERNATIONAL PULP BLEAC
HING CONFERENCE-PROCEEDINGS ", pp. 199-209,
It is reported that when the pressurized oxygen pressure of the hydrogen peroxide stage is 5 bar, the whiteness of 85% (ISO) to 87.5% can be reached in the first stage of pressurized hydrogen peroxide treatment. However, in this case, the disadvantage is that the pulp viscosity is greatly reduced. The use of these disclosed methods has not yet shown satisfactory qualities in terms of whiteness and strength as compared to bleached pulp using conventional chlorine-based chemicals.

[0011]

In order to solve the above problems, the present invention provides a pulp bleaching method having high whiteness and high strength by bleaching chemical and mechanical pulps using a chlorine-free chemical. Is to provide.

[0012]

The present invention comprises at least two hydrogen peroxide stages pressurized with oxygen in a multi-stage bleaching process for pulp, the treatment pressure in the first pressurized hydrogen peroxide stage being a gauge pressure. 0.1-5.0 kg / c in measured value
m ² and the processing pressure of the pressurized hydrogen peroxide stage arranged in the bleaching step after that is 5.0 to 9.0 k in gauge pressure.
g / cm ² , and compared with the processing pressure of the first pressurized hydrogen peroxide stage, the processing pressure difference in the pressurized hydrogen peroxide stage disposed after that is 2.0 kg / cm ^{2 in} gauge pressure. As described above, the bleaching method is characterized by being in the range of less than 9.0 kg / cm ² .

The processing pressure of the first pressurized hydrogen peroxide stage in the above-mentioned multi-stage bleaching step is 3.0 to 5.
0 kg / cm ² , the processing pressure of the pressurized hydrogen peroxide stage arranged in the bleaching step thereafter is 6.0 to 8.0 in gauge pressure.
The bleaching method according to claim 1, wherein the bleaching method is kg / cm ² . Furthermore, in a multi-stage bleaching process of pulp including a plurality of pressurized hydrogen peroxide treatment stages, a treatment stage using a chelating agent is performed before the first pressurized hydrogen peroxide treatment stage or simultaneously with the pressurized hydrogen peroxide treatment, Or a bleaching method characterized in that it is carried out at least once in any stage between two or more hydrogen peroxide stages.

The lignocellulosic material to be used for pulp production in the present invention comprises a polysulfide-containing kraft pulping method, a conventional kraft pulping method (KP), a sulfite pulping method (SP), an alkaline pulping method. A pulp obtained by a chemical pulping method such as a method (AP) and a mechanical pulping method (GP, TMP, CTMP) or a pulp derived from waste paper is preferable, and a pulp obtained by a kraft pulping method is particularly preferable. .

Lignocellulosic materials derived from woody plants used for pulping include softwood materials such as pine, cedar, spruce and Douglas fir, aspen, beech, acacia,
Examples of hardwood materials such as eucalyptus and mangrove, and lignocellose materials derived from herbaceous plants include, but are not limited to, bagasse, kenaf, esparto grass, rice and the like.

As the pulp to be treated in the pressurized hydrogen peroxide stage of the present invention, pulp pulped by the above method may be used as it is, or oxygen pre-treatment such as oxygen delignification or ozone may be used. It may be a bleaching treatment such as a treatment or a chlorine dioxide treatment, and is not particularly limited.

The oxygen used in the bleaching of the present invention may be any oxygen concentration higher than the atmospheric oxygen concentration, but the higher the oxygen concentration, the higher the effect. A particularly preferable oxygen concentration is 80% or more. The oxygen used is P
SA (Pressure Swing Adapt)
ion) oxygen, VSA (Vacum Swing Ad)
The production method is not particularly limited as long as it can be used on an industrial scale, such as oxygen) or oxygen in the deep-cooling method.

The first pressurized hydrogen peroxide stage (hereinafter (Po) L
Abbreviated) oxygen pressure is 0.1 kg / cm ^{2 to} 5.0 kg
The range up to / cm ² (gauge pressure) is preferable. Furthermore, the oxygen pressure in the pressurized hydrogen peroxide stage (hereinafter abbreviated as (Po) H) arranged after this (Po) L stage is 5.0 to 9.0 kg /
The range of cm ² (gauge pressure) is preferable. Also, (Po) L
And (Po) H pressure difference is + 9kg / cm ² > in gauge pressure.
The value satisfies [(Po) H- (Po) L] ≧ + 2 kg / cm ² .

In the (Po) L stage, when the oxygen pressure is less than 0.1 kg / cm ^{2 in} gauge pressure, the oxygen concentration during the treatment is low,
The decomposition reaction of hydrogen peroxide occurs. Also, 5 kg / cm ²
When it exceeds, the pulp strength is greatly reduced because the reaction proceeds rapidly. The oxygen pressure in the (Po) H stage is higher than that in the (Po) L stage, but if it is less than 5 kg / cm ² , the increase in pulp whiteness after bleaching is small, while it is 9 kg / cm.
Even if it exceeds ² , the increase in whiteness is in equilibrium. In addition,
When the difference between the (Po) L stage and the (Po) H stage is less than 2 kg / cm ² , the effect of pressurization in the (Po) H stage becomes small and high whiteness cannot be obtained. When the present invention is applied to an existing plant, it depends on the pressure near the point where the pressurizing pressure is released.For example, in the case of an upflow reactor, the column top pressure is used, and when the retention tube is used as a reactor. Refers to the end of the retention tube.

The pressure treatment time in each pressurized hydrogen peroxide stage is preferably 5 to 120 minutes, particularly preferably 10 to 90, since the whiteness decreases when the hydrogen peroxide is completely consumed during the treatment. Minutes. This is because the increase in whiteness is small when the pressure treatment time is less than 5 minutes, and the increase in whiteness reaches equilibrium when the treatment time exceeds 120 minutes.
In an actual plant, the time required from the injection of oxygen to the point where the pressure is released is calculated as the pressure holding time. Further, in order to increase the whiteness by the hydrogen peroxide remaining after each pressure treatment, it is preferable to provide a holding column at atmospheric pressure after the pressure treatment, and the atmospheric pressure holding time there is Since no reversion occurs, it takes a long time to complete the reaction before the hydrogen peroxide is completely consumed, and the time is not particularly limited and is appropriately selected from the range of 30 to 240 minutes depending on the operating conditions.

The processing temperature of each pressurized hydrogen peroxide stage is 70 to 1
20 ° C. is preferable, and 90 ° C. to 110 ° C. is particularly preferable. If the temperature is lower than 70 ° C, the reaction rate is low even if pressure is applied,
When the temperature exceeds 120 ° C, the decomposition reaction of hydrogen peroxide becomes large, so the amount of effective hydrogen peroxide for pulp decreases, and the radicals produced attack carbohydrates, resulting in a decrease in the degree of polymerization of the carbohydrates and further a decrease in the strength. .

The treated pulp concentration (absolute dry weight of pulp / g of total weight including water, hereinafter abbreviated as pulp concentration) is 5 to 15
%, More preferably 8 to 12% in the middle concentration range. If it is less than 5%, the reaction between the pulp and hydrogen peroxide will be poor, while if it exceeds 15%, stirring will be poor and uniform reaction will be difficult.

In the present invention, the addition rate of hydrogen peroxide is not particularly limited and may be appropriately selected depending on the amount of lignin remaining and the desired whiteness of the final pulp.

In order to prevent the self-decomposition of hydrogen peroxide promoted by heavy metals carried into the pressurized hydrogen peroxide stage,
The treatment stage with the chelating agent is carried out before the first pressurized hydrogen peroxide stage, or simultaneously with the pressurized hydrogen peroxide stage,
Alternatively, at least once between any two or more hydrogen peroxide stages, preferably a chelate stage is provided before each pressurized hydrogen peroxide stage.

As the chelating agent, diethylenetriaminepentaacetic acid (DTP) which is usually used as a chelating agent is used.
A), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentamethylenephosphoric acid (DTMP
A), nitrilotriacetic acid (NTA), and the like, and one or more of them are used. The addition amount of the chelating agent depends on the amount of metal ions brought in, but is preferably 0.05% to 0.5% per dry pulp, and the reaction temperature is 40 when it is used alone as a chelating stage.
To 95 ° C, particularly preferably 60 to 80 ° C.
The reaction time may be any time as long as the metal ions are blocked, and the time is not particularly limited, but is preferably 5 to 40 minutes. Since the optimum reaction pH depends on the metal ion to be blocked and the chelating agent used, the pH may be adjusted according to the purpose. For example, when treating with DTPA, p
It is good to carry out in the weakly acidic region of H4 to 6.5.

As described above, in the conventional multi-stage bleaching sequence including the pressurized hydrogen peroxide stage, bleaching is performed under severe conditions in order to increase the whiteness of the pulp by the single-stage pressurized hydrogen peroxide treatment. On the other hand, in the present invention, the pressurized hydrogen peroxide treatment is performed at least twice, and the pressurized hydrogen peroxide treatment under mild conditions {(Po) L stage} is performed, and then the pressure applied after that is applied. In the hydrogen peroxide stage {(Po) H stage}, the gauge pressure is 2 kg / cm ² or more than the first (Po) L stage, 9 k
It is characterized in that it is carried out at a high processing pressure of less than g / cm ² .

The reason why the whiteness can be remarkably increased and the strength reduction can be reduced is that the mechanism thereof has not yet been fully clarified, but the above decomposition of hydrogen peroxide is prevented, and the oxygen gas itself is involved in bleaching. It is presumed that, by applying pressure, the bubbles of oxygen become smaller and the area that reacts with the pulp becomes larger, and by applying pressure, the permeation of hydrogen peroxide into the pulp was promoted. In addition, the first pressurized hydrogen peroxide stage is processed under relatively mild pressure conditions, and washing is performed to selectively remove lignin and rapidly increase the whiteness without damaging the carbohydrate in the fiber, It is considered that by further applying a higher pressure in the (Po) H stage, it is possible to efficiently attack the strong residual lignin and the chromophore that could not be removed under mild conditions.

Examples of the bleaching sequence to be bleached in this way include O-Q- (Po) L-Q- (Po) H (Examples 1 to 5) and O-Q- (Po) L- (Po. ) H (Example 6), O-Q- (Po) L-Q- (Po) L- (Po) H, O
-Q- (Po) L-Z- (Po) H, O-ZQ- (Po) L-
(Po) H, OD-Q- (Po) L- (Po) H, OD-
(Po) L-Q- (Po) H, O-Q- (Po) L- (Po) L
-(Po) H, Q-((Po) LQ)-(Po) H, Q- (P
o) L-((Po) HQ) and the like.

Here, (O) is an oxygen stage, (Q) is a chelate treatment, (P) is a hydrogen peroxide stage, and (Po) L is 0.1 to 5 k.
a pressurized hydrogen peroxide stage pressurized with oxygen to g / cm ² (P
o) H is higher pressure than (Po) L stage (5-9kg / cm ² )
Shows the pressurized hydrogen peroxide stage pressurized with oxygen. The pressure applied at each stage is the gauge pressure (Po) H minus (Po) L is 2kg / cm ² or more, 9kg / cm ² or more. ^The pressure is less than ² . In addition, (Z) shows an ozone stage and (D) shows a chlorine dioxide stage. Further, (−) between each stage represents a washing process. Further, an enzyme treatment stage may be provided in a part of the bleaching step in the above sequence for the purpose of reducing chemicals and improving pulp quality.

[0030]

According to the present invention, by pressurizing the hydrogen peroxide stage in the bleaching sequence with oxygen, consumption of hydrogen peroxide due to self-decomposition can be suppressed, and whiteness can be increased in a short time. By performing the pressurized hydrogen peroxide stage at least twice under different pressure conditions, it is possible to bleach chemical and mechanical pulps while increasing the whiteness and minimizing the decrease in pulp strength.

[0031]

EXAMPLES The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples. In addition, the addition ratios in the examples and comparative examples are shown by weight% based on absolute dry pulp.

Example 1 Bleaching by Q1- (Po) L-Q2- (Po) H sequence: (1) Pretreatment: Whiteness 37.6%, Kappa number 11.
2, spruce, pine, and fur-based softwood oxygen delignification kraft pulp (hereinafter referred to as NOKP) having a pulp viscosity of 20.4 cP (hereinafter referred to as NOKP) is diluted to 80% by absolute dry weight and ion-exchanged water to adjust the pulp concentration to 4.0%. did. (2) Q1 treatment: DTP is added to this NOKP pulp slurry.
Before the reaction, A was added to the absolutely dry pulp in an amount of 0.1% in terms of solid content (hereinafter, the addition rate represents the weight% based on the absolutely dry pulp) so that the pH after the reaction becomes 5.5. The pH was adjusted by adding sulfuric acid to the pulp. Further, the pulp concentration was adjusted to 3.5% with ion-exchanged water, and the pulp was treated at a treatment temperature of 65 ° C. for 30 minutes. The obtained pulp was washed with ion-exchanged water.

(3) (Po) L stage: The pulp concentration after the Q1 treatment was adjusted to 13%, and the pulp was put into a self-made 2 liter capacity agitator with a closed pressure resistant heating heater (agitator autoclave). Then, 3% of hydrogen peroxide, 1.3% of sodium hydroxide and 0.1% of magnesium sulfate heptahydrate in terms of magnesium sulfate were added to adjust the pulp concentration to 10%. Warm while stirring at 300 rpm to 100
15 at ℃, pressurized oxygen pressure 3.0 kg / cm ² (gauge pressure)
Hold for minutes, then return to normal pressure and hold at 90 ° C. for 90 minutes. The obtained pulp was washed with ion-exchanged water and dehydrated to a pulp concentration of 30%. (4) Q2 treatment: 0.1% of DTPA in terms of solid content was added to pulp whose pulp concentration was adjusted to 4%, and p was added after the reaction.
Sulfuric acid was added so that H became 5.5, and the total pulp concentration was adjusted to 3.5%, followed by chelating treatment at 65 ° C. for 30 minutes. The pulp obtained after the reaction was washed with ion-exchanged water.

(5) (Po) H stage: Pulp adjusted to a pulp concentration of 13% was placed in the agitator equipped with the heating heater described above, hydrogen peroxide was 5%, sodium hydroxide was 1.3%,
Magnesium sulfate heptahydrate was added in an amount of 0.1% in terms of magnesium sulfate, and ion-exchanged water was added to adjust the pulp concentration to 1
Adjusted to 0%. The mixture was heated with stirring, kept at 110 ° C. and a pressurized oxygen pressure of 7.0 kg / cm ² (gauge pressure) for 90 minutes, then returned to normal pressure and kept at 90 ° C. for 90 minutes. The obtained pulp was washed with ion-exchanged water to obtain a finished bleached pulp.

Example 2 A finished bleached pulp was obtained in the same manner as in Example 1 except that the oxygen blowing pressure in the (Po) L stage was changed to 0.5 kg / cm ² . Example 3 A finished bleached pulp was obtained in the same manner as in Example 1 except that the oxygen blowing pressure in the (Po) H stage was set to 5.5 kg / cm ² .

Example 4 (Po) L stage oxygen blowing pressure was 5.0 kg / cm ² ,
A finished bleached pulp was obtained in the same manner as in Example 1 except that the oxygen blowing pressure in the (Po) H stage was set to 9.0 kg / cm ² . Example 5 A finished bleached pulp was obtained in the same manner as in Example 1 except that the hydrogen peroxide addition rate in the (Po) H stage was 3% and the pressure holding time was 60 minutes. Example 6 A finished bleached pulp was obtained in the same manner as in Example 1 except that the chelate stage between the (Po) L stage and the (Po) H stage was omitted.

Comparative Example 1 Oxygen pressurization was not performed in the (Po) L stage and the (Po) H stage,
A finished bleached pulp was obtained in the same manner as in Example 1 except that the reaction temperature was 95 ° C and the reaction time in the hydrogen peroxide stage 1 and stage 2 was 240 minutes. Comparative Example 2 A finished bleached pulp was obtained in the same manner as in Example 1 except that the oxygen blowing pressure in the (Po) H stage was set to 4.0 kg / cm ² .

Comparative Example 3 A finished bleached pulp was obtained in the same manner as in Example 2 except that the oxygen blowing pressure in the (Po) H stage was set to 4.0 kg / cm ² .

Comparative Example 4 (Po) L stage oxygen blowing pressure was 7.0 kg / cm ² , hydrogen peroxide addition rate was 5.0%, and (Po) H stage oxygen blowing pressure was 3.0 kg / cm ^2. A finished bleached pulp was obtained in the same manner as in Example 1 except that the hydrogen peroxide addition rate was 3.0%. Comparative Example 5 A finished bleached pulp was obtained in the same manner as in Example 1 except that the chelating treatment was not performed.

Comparative Example 6 C (chlorine stage) -Eo-H (hypo) -D sequence bleaching: (1) Stage C: oxygen delignified pulp after pulp concentration 5%
Then, 2.1% of chlorine and water were added to adjust the pulp concentration to 3.5%, and the mixture was reacted at 40 ° C. for 40 minutes and then washed with ion-exchanged water. (2) Eo stage: 0.3% of oxygen and 1.3% of sodium hydroxide were added, pulp concentration was 10%, oxygen-enriched alkali extraction was performed at 60 ° C. for 90 minutes, and then ion-exchanged water Washed with. (3) Stage H: 0.5% of sodium hypochlorite was added, and pulp concentration was 10%, 60 ° C., 12 under weak alkaline.
After reacting for 0 minute, it was diluted with ion-exchanged water and washed. (4) Stage D: Chlorine dioxide (0.35%) was added, pulp concentration was 10%, reaction was carried out at 70 ° C. for 180 minutes, followed by washing with ion-exchanged water to obtain a finished bleached pulp.

The whiteness and pulp viscosity of the finished pulps obtained in Examples and Comparative Examples, and the strength of the handmade sheet of pulp beaten to a freeness of 500 ml were measured, and the results are shown in Table 1.

[0042]

[Table 1] Table 1 Pulp quality and strength measurement results ───────────────────────────────── Whiteness Pulp viscosity ratio Tear strength Tear length (ISO)% cP km ────────────────────────────────── Example 1 87.3 11.3 135 7.7 Example 2 86.9 11.1 129 7.3 Example 3 83.5 12.9 139 7.4 Example 4 87.9 10.2 123 8.3 Example 5 83.6 13.3 142 7.9 Example 6 86.7 10.5 124 7.3 ────────────── ──────────────────── Comparative Example 1 78.9 15.3 164 8.2 Comparative Example 2 82.9 9.2 110 7.5 Comparative Example 3 80.7 12.3 138 7.6 Comparative Example 4 85.7 8.1 114 7.1 Comparative Example 5 85.8 6.4 98 6.2 Comparative example 6 85.3 14.6 144 8.4 ──────────────────────────────────

Regarding the method of measuring the pulp quality and strength, the ISO whiteness was determined using Erlejo 2000 for the whiteness. Pulp viscosity is TAPPI STD T230o
The measurement was performed according to m-89. Tensile strength is JIS P8
It was shown as the breaking length using 113. Tear strength is JIS
It was shown as specific tear strength using P8116.

As is clear from Table 1, the bleached pulp according to the present invention maintains high whiteness and high viscosity and strength (Examples 1 to 6). On the other hand, the bleached pulp not subjected to the pressurized hydrogen peroxide treatment has a low whiteness even if the reaction time is extended (Comparative Example 1). Further, when the pressure at the (Po) H stage in the example is low, the whiteness is low (Comparative examples 2 and 3). On the other hand, when the pressure of the (Po) L stage in the example is increased, the whiteness is increased, but the viscosity and the strength are decreased (Comparative Example 4).
Whiteness will be higher if chelation is not applied,
The viscosity and strength are reduced (Comparative Example 5). Even when compared with the pulp obtained by the conventional bleaching method based on chlorine-based chemicals (Comparative Example 6), the pulps of Examples 1 to 6 maintain the same strength.

[0045]

As is apparent from the above description, the bleaching method of the present invention enables bleaching with a high degree of whiteness and a reduction in pulp strength as compared with the conventional bleaching method using hydrogen peroxide.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Iwasaki 1-10-6 Shinonome, Koto-ku, Tokyo Shinoji Paper Mills Central Research Laboratory

Claims (3)

[Claims] 1. A multi-stage bleaching process for pulp, comprising at least two hydrogen peroxide stages pressurized with oxygen, wherein the treatment pressure in the first pressurized hydrogen peroxide stage is 0.1 as measured by gauge pressure. ˜5.0 kg / cm ² , and the processing pressure of the pressurized hydrogen peroxide stage arranged in the bleaching step after that is 5.0 to 9.0 kg / cm ² in gauge pressure, and Compared with the processing pressure of the pressurized hydrogen peroxide stage, the processing pressure difference in the pressurized hydrogen peroxide stage disposed after that is 2.0 kg / cm ² or more in gauge pressure, 9.0 kg / cm ²
A method of bleaching a lignocellulosic material, characterized in that it is in the range below. 2. In the multi-stage bleaching step, the first pressurized hydrogen peroxide stage has a gauge pressure of 3.0 to 5.0 kg / cm.sup.2.
^2. The lignocellulosic material according to claim 1, wherein the pressurized hydrogen peroxide stage arranged in the bleaching step after that has a processing pressure of 6.0 to 8.0 kg / cm ^{2 in} gauge pressure. Bleaching method. 3. A multi-stage bleaching process for pulp, which comprises two or more pressurized hydrogen peroxide stages, and a treatment with a chelating agent is carried out at the same time as the first pressurized hydrogen peroxide stage or in a step prior thereto. The method for bleaching a lignocellulosic material according to any one of claims 1 and 2, characterized in that.