JP5471050B2 - TCF bleaching method - Google Patents

Description

  The present invention relates to a method for bleaching unbleached pulp obtained by digesting lignocellulosic material. More specifically, the present invention relates to a TCF bleaching method in which bleaching of bleached pulp is improved and bleaching is performed without using any chlorinated chemicals.

  Until now, chlorine bleaching chemicals have been used for bleaching paper pulp. However, since chlorine-based bleaching chemicals produce organic chlorine compounds that are harmful to the environment at the time of bleaching, conversion is proceeding to bleaching methods that do not use molecular chlorine, and furthermore, bleaching methods that do not use any chlorine-based chemicals. The method of bleaching without using molecular chlorine is called ECF (elementary chlorin free) bleaching, and the method of bleaching without using any chlorine-based chemicals is called TCF (totally chlorin free) bleaching.

As TCF bleaching, a method using ozone, peroxide, enzyme, and oxygen bleaching in combination is known (see Patent Documents 1 and 2).
Ozone has a lower ability to remove hexeneuronic acid, which causes deterioration in bleaching of bleached pulp than conventional chlorine-based chemicals. The acid needs to be removed. However, an increase in ozone may decompose cellulose excessively and cause a significant decrease in viscosity.

  As another method, a method using an inorganic peroxy acid without using ozone bleaching is also known. In Patent Document 3, as an alternative to chlorine bleaching or delignification treatment with a combination of chlorine and chlorine dioxide, unbleached pulp is subjected to oxygen bleaching, chelating agent treatment, alkaline hydrogen peroxide treatment, and monopersulfuric acid treatment. Is disclosed. Further, Patent Document 4 discloses a method of performing monopersulfuric acid treatment after chelating agent treatment and then alkaline hydrogen peroxide treatment as delignification method. However, there is no description of a treatment method in which pulp-entrained water is added to a treatment stage using an inorganic peroxyacid, hexeneuronic acid removal, and discoloration improvement.

JP 7-48792 A Japanese National Patent Publication No. 8-504005 Japanese National Patent Publication No. 8-507332 Japanese National Patent Publication No. 10-500188

  An object of the present invention is to provide a bleaching method in which the bleaching properties of bleached pulp are improved by improving the hexeneuronic acid removal ability of inorganic peroxyacids in TCF bleaching using inorganic peroxyacids.

  As a result of diligent study on the above problems, the present inventors have conducted a multistage bleaching treatment including a treatment stage using an inorganic peroxyacid on pulp after bleaching and alkaline oxygen bleaching. It was found that the amount of hexeneuronic acid in the pulp after bleaching was reduced and the discoloration property of the bleached pulp was improved by adding the water accompanied by the pulp with the monopersulfuric acid alone.

That is, the present invention is a TCF bleaching method of unbleached pulp obtained by digesting lignocellulosic material,
(1) pulp-entrained water separated from at least one delignified pulp slurry selected from the unbleached pulp of the same or different bleaching sequence after alkaline oxygen bleaching and after each bleaching of multi-stage bleaching;
(2) The present invention relates to a TCF bleaching method for pulp, which is added to a treatment stage using an inorganic peroxyacid in a multistage bleaching treatment including a treatment stage using an inorganic peroxyacid.

  According to the present invention, in the complete chlorine-free bleaching in which bleaching is performed without using any chlorinated chemicals, the ability to remove hexeneuronic acid from inorganic peroxyacids is improved, and bleached pulp having good thermo-fading properties can be obtained, and the bleaching cost can be reduced. It can be kept low. Moreover, it becomes possible to shorten the processing time of inorganic peroxyacid, and can suppress cost low.

  The lignocellulosic material used in the present invention is preferably a hardwood material containing a large amount of methylglucuronic acid that generates hexeneuronic acid, but it may be a softwood material, a non-wood material such as bamboo or hemp, or a mixture thereof. Well, not particularly limited.

  As the cooking method for obtaining the pulp used in the present invention, known cooking methods such as kraft cooking, soda cooking, polysulfide cooking, alkali sulfite cooking, etc. can be used, but considering pulp quality, energy efficiency, etc. Kraft cooking or polysulfide cooking is preferably used.

  For example, when kraft cooking 100% hardwood wood lignocellulose, the kraft cooking liquor has a sulfidity of 5 to 75% by mass, preferably 15 to 45% by mass, and an effective alkali addition rate of 5 to 30% by mass of absolutely dry wood. %, Preferably 10 to 25% by mass. The cooking temperature is 130 to 170 ° C., and the cooking method may be either a continuous cooking method or a batch cooking method. When a continuous cooking kettle is used, a modified cooking method in which a cooking solution is added at multiple points may be used. It doesn't matter.

  In cooking, as a cooking aid for cooking liquor used, known cyclic keto compounds, for example, benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone, and quinone-based compounds such as alkyls and aminos, or quinone-based compounds. Hydroquinone compounds such as anthrahydroquinone, which is a reduced form of the compound, can be used. Furthermore, 1 type, or 2 or more types selected from the 9,10-diketohydroanthracene compound etc. which are the stable compounds obtained as an intermediate of the anthraquinone synthesis method by Diels Alder method may be added. The addition rate of these cooking aids is a normal addition rate, and is, for example, 0.001 to 1.0% by mass per the absolute dry mass of the wood chips.

  The pulp used in the present invention is cooked by a known cooking method, passed through washing and roughing steps, and delignified by a known alkaline oxygen bleaching method. As the alkaline oxygen bleaching method used in the present invention, a known medium concentration method or high concentration method can be applied as it is, but a medium concentration method in which the pulp concentration currently used for general purposes is 8 to 15% by mass is used. preferable.

In the alkali oxygen bleaching method by the medium concentration method, caustic soda or oxidized kraft white liquor can be used as the alkali. As the oxygen gas, oxygen from a cryogenic separation method, PSA (Pressure Swing Adsorption) Oxygen, oxygen from VSA (Vacuum Swing Adsorption), etc. can be used.
The oxygen gas and alkali are added to a medium-concentration pulp slurry in a medium-concentration mixer and mixed sufficiently, and then sent to a reaction tower capable of holding a mixture of pulp, oxygen, and alkali for a certain period of time under pressure, and delignified. Is done. The oxygen gas addition rate is 0.5 to 3% by mass per bone dry (BD) pulp mass, the alkali addition rate is 0.5 to 4% by mass, the reaction temperature is 80 to 120 ° C., and the reaction time is 15%. It is preferable that the pulp concentration is 8 to 15% by mass for ˜100 minutes, but the embodiment is not particularly limited. In the alkali bleaching treatment step, it is more preferable to carry out the treatment by the alkali oxygen bleaching method a plurality of times continuously, proceed with delignification as much as possible, and reduce the heavy metal content. The pulp subjected to the alkaline oxygen bleaching treatment is usually subjected to a washing step, but the pulp after the washing step is sent to a multistage bleaching treatment step.

The multistage bleaching treatment for separating the pulp-entrained water of the present invention is preferably a multistage bleaching process in which processing stages used in general TCF bleaching processes are appropriately combined, or a multistage bleaching process including a process stage using an inorganic peroxyacid. is there.
Examples of the multi-stage bleaching process in which processing stages used in general TCF bleaching processes are appropriately combined include, for example, A (acid treatment stage), Z (ozone treatment stage), Eop (alkali treatment using oxygen and hydrogen peroxide in combination) Eo (alkaline treatment stage combined with oxygen), Ep (alkali treatment stage combined with hydrogen peroxide), P (hydrogen peroxide treatment) Stage), Pa (peracetic acid treatment stage) and the like. Examples of the multistage bleaching sequence comprising these processing stages include Z-Eo-P, Z-Eop-P, Z-Ep-PP, A-Eop-Z, and A-Ep-Z.

In addition, as the multi-stage bleaching process including a process stage using an inorganic peroxy acid, the types of bleaching stages other than the Px process stage are not limited, and bleaching of A, Z, Eop, Eo, Ep, P, Pa, etc. The treatment can be exemplified, and a multi-stage bleaching process step including a treatment stage using an inorganic peroxy acid can be appropriately combined. Here, the Px treatment represents treatment using an inorganic peroxy acid.
Examples of the multistage bleaching sequence comprising these processing stages include Z-Eop-P-Px, Z-Eo-P-Px, Px-Z-Ep-PP, A-Eop-Z-Px, Z- Examples include bleaching by a sequence such as Eop-Px and Px-Z-Eop-P.
Moreover, you may use the pulp accompanying water after the process step using inorganic peroxy acid.

  In the present invention, it is possible to use pulp-entrained water separated from at least one pulp slurry after the treatment selected from the alkali oxygen bleaching treatment and after each of the multistage bleaching treatments. Pulp entrained water separated from the slurry is desirable. Usually, washing and dewatering are performed after the alkaline oxygen bleaching treatment and after each of the multistage bleaching treatments, but the pulp-entrained water used in the present invention is separated from any pulp slurry before, during and after washing. However, entrained water separated from the pulp slurry before washing is particularly preferable. Moreover, when separating pulp accompanying water from the pulp slurry after each bleaching treatment in the multistage bleaching treatment, the accompanying water in any bleaching step may be used, or the accompanying water after the Px step may be used.

  As a method for separating the entrained water from the pulp slurry, a part or the whole of the entrained water can be separated using a known method such as filtration or centrifugation. In addition, before separating the entrained water from the pulp slurry by filtration or the like, water can be added to the pulp slurry to improve the separation efficiency of the entrained water from the pulp. As an apparatus for separating entrained water from pulp, a cylinder thickener, a pressure filter, a vacuum filter, a diffuser washer and the like can be used.

  Further, the pulp-entrained water may be separated from the processing stage used for the ECF bleaching process in addition to the processing stage used for the TCF bleaching process. However, since the separated pulp entrained water is used in a TCF bleaching sequence including a treatment stage using an inorganic peroxy acid, it is necessary to separate the entrained water not containing chlorine dioxide. For example, when the ECF bleaching treatment sequence is Z-Eop-D or Z-Eo-D-D, the entrained water may be separated from Z, Eop, and Eo. However, if the chlorine compound is completely removed from the separated entrained water, the entrained water separated from the chlorine dioxide stage can be used.

The separation of the pulp-entrained water of the present invention may be a different sequence or the same sequence as a bleaching sequence including a treatment stage using an inorganic peroxy acid. For example, the pulp-entrained water to be added to a multi-stage bleaching sequence (A-Eop-Z-Px) including a processing stage using an inorganic peroxy acid after alkaline oxygen bleaching of unbleached pulp is used in a different bleaching sequence. After unbleached unbleached pulp after alkaline oxygen bleaching and after multi-stage bleaching (Z-Eo-P, Z-Eop-P, Z-Ep-P-P, Z-Eop-P-Px, etc.) Using pulp-entrained water separated from at least one treated pulp slurry selected from the above, or after alkali-oxygen bleaching and multi-stage bleaching (A-Eop-Z) -Px) pulp-accompanying water separated from at least one treated pulp slurry selected after each bleaching treatment.
Among these, the form in which the treatment stage using the inorganic peroxyacid to which the pulp-entrained water is added is included in the bleaching sequence of the multistage bleaching process for separating the pulp-entrained water is the most preferable form. By separating the pulp-entrained water from the same sequence, operations from separation to addition can be performed efficiently, and costs for manufacturing equipment such as a line can be reduced.

  Next, the separated pulp-entrained water is added to a treatment stage using an inorganic peroxyacid for multistage bleaching. In the multi-stage bleaching process including a process stage using an inorganic peroxyacid, the types of bleaching stages other than the Px process stage are not limited, and A, Z, Eop, Eo, Ep that can be generally used for the TCF bleaching process. , P, Pa, etc. are used. The separated pulp-entrained water may be added as it is, or may be added after concentration or dilution with water. The amount of pulp-accompanying water added to the treatment stage using an inorganic peroxy acid is not particularly limited. A larger effect can be obtained as the addition amount is larger, and the amount is preferably 1 to 90% by mass per the dry pulp.

  The order of addition of inorganic peroxyacid, pulp entrained water, pH adjuster, etc. in the treatment stage using inorganic peroxyacid is not particularly limited. For example, an inorganic peroxyacid, pulp-entrained water, and a pH adjuster may be separately added to the pulp, or the inorganic peroxyacid and pulp-entrained water may be mixed and then added to the pulp to add a pH adjuster. . In addition, it is also preferable to use a chelating agent or a polyvalent carboxylic acid in combination with a treatment stage using an inorganic peroxy acid because it can suppress a decrease in viscosity of the pulp.

  Further, the separated pulp-entrained water used in the present invention can be added to a treatment stage using an inorganic peroxyacid after being treated with an oxidizing agent. It does not specifically limit about the oxidizing agent and processing method in the oxidation process of pulp accompanying water. For example, as the oxidizing agent, potassium permanganate, peracetic acid, hydrogen peroxide, nitric acid, etc. can be used, but hydrogen peroxide is used from the viewpoint of simplicity of treatment after treatment, effect and cost. Particularly preferred. As the treatment method, for example, when hydrogen peroxide is used as the oxidizing agent, the separated pulp-entrained water and 1 to 40% by mass hydrogen peroxide water are mixed, and the pH is adjusted appropriately using a pH adjuster. By adjusting to 13 and heating at 40 to 60 ° C., pulp-entrained water treated with an oxidizing agent can be obtained.

  Further, an unreacted oxidant may remain in the pulp-entrained aqueous solution after the oxidant treatment. The pulp-entrained aqueous solution containing the unreacted oxidant may be added to the treatment stage using the inorganic peroxyacid, but the unreacted oxidant may inhibit the action of the inorganic peroxyacid. It is desirable to use after removing the oxidizing agent. For example, when unreacted hydrogen peroxide remains, it is desirable to remove the unreacted oxidant using a metal such as platinum or an enzyme such as catalase, and then subject to a treatment stage using an inorganic peroxy acid.

Examples of the inorganic peroxyacid used in the present invention include monopersulfuric acid, peroxydisulfuric acid, monoperphosphoric acid, perboric acid, percarbonate and peroxopolyacid, and salts thereof, and the removal effect and economy of hexeneuronic acid. It is preferable to use monopersulfuric acid from a practical aspect.
Monopersulfuric acid used in the present invention is also called peroxymonosulfuric acid, and can be produced by hydrolyzing peroxydisulfuric acid, or hydrogen peroxide and sulfuric acid can be mixed in any proportion. However, the manufacturing method is not particularly limited. It is also possible to use something like oxone is a monopersulfate double salt _{(2KHSO 5 · KHSO 4 · K} 2 SO 4). However, in consideration of economy, it is a preferred embodiment to produce monopersulfuric acid at low cost by mixing inexpensive high concentration hydrogen peroxide and inexpensive high concentration sulfuric acid.

A method for producing monopersulfuric acid by mixing high concentration hydrogen peroxide and high concentration sulfuric acid is 20 to 70% by mass, preferably 80 to 98% by mass in 35 to 70% by mass hydrogen peroxide water. A method of dropping and mixing concentrated sulfuric acid having a concentration of preferably 93 to 98% by mass is suitable. The mixing molar ratio of the sulfuric acid and hydrogen peroxide is preferably 1: 1 to 5: 1, more preferably 2: 1 to 4: 1. If both hydrogen peroxide solution and sulfuric acid have low concentrations, the production efficiency of monopersulfuric acid decreases, which is not suitable. Further, if these concentrations are too high, the risk of ignition and the like increases, which is not suitable. Furthermore, when the mixing molar ratio of sulfuric acid and hydrogen peroxide deviates from 1: 1 to 5: 1, it is not preferable because the production efficiency of monopersulfuric acid is lowered.
Further, for example, it is preferable to produce on-site using hydrogen peroxide and sulfuric acid as raw materials by a method as shown in International Publication No. 2008/047864.

  The addition rate of the inorganic peroxyacid used in the present invention is not particularly limited, and the greater the amount added, the greater the amount of hexeneuronic acid removed. It is 2 mass%, More preferably, it is 0.1-1.5 mass%. Moreover, suitable pH is 1-7, More preferably, it is 2-5. The treatment time is 1 minute to 5 hours, preferably 10 to 200 minutes, and the treatment temperature is 20 to 90 ° C, more preferably 40 to 80 ° C. Although a pulp density | concentration is not specifically limited, Usually, it is 5-30 mass%, Preferably it is 8-15 mass% from the point of operativity.

  Usually, the hexeneuronic acid concentration in the pulp decreases as the multi-stage bleaching process proceeds, and it becomes difficult to further remove hexeneuronic acid. Therefore, the treatment stage using the inorganic peroxyacid of the present invention is the final stage of the multi-stage bleaching treatment, such as Z-Eop-P-Px, Z-Eo-P-Px, Z-Eo-P-P-Px, A-Eop. It is particularly effective to add the pulp-entrained water of the present invention to the Px stage of the sequence exemplified by -Z-Px, Z-Eop-Px and the like.

  As processing conditions of the ozone bleaching stage (Z) of the present invention, general conditions can be used, but the ozone addition rate is 0.1 to 1% by mass per mass of absolutely dry pulp, and the reaction temperature is 20 to 80 ° C. The reaction time is preferably 0.5 to 60 minutes, and the reaction pH is preferably 2.0 to 6.0, and known alkalis and acids can be used for pH adjustment. Although it does not specifically limit regarding a pulp density | concentration, Preferably it is 3-40 mass%.

  As the processing conditions of the processing stage using hydrogen peroxide (Eop, Ep, P, etc.), general conditions can be used. For example, the alkali amount is 0.5 to 3.0% by mass. The amount of oxygen is 0.05 to 0.3% by mass, and the amount of hydrogen peroxide is 0.05 to 1.0% by mass. The treatment pH is preferably 10 to 12, more preferably 11.0 to 11.7 as the pH after bleaching. The treatment temperature is 50 to 90 ° C., and the treatment time is preferably 15 minutes to 5 hours, more preferably 30 minutes to 3 hours. The alkali-treated pulp combined with oxygen and hydrogen peroxide undergoes a washing step. When separating the pulp-entrained water of the present invention, the pulp-entrained water is separated from the pulp slurry before being subjected to the washing step. It is preferable to do. In the washing step, any washing machine can be used as long as the residual chemicals in the pulp, COD components, and the like can be washed efficiently.

  As processing conditions for the bleaching stage other than those described above, general processing conditions that are usually performed may be used.

  As the acid for adjusting the pH used in the present invention, inorganic and organic acids such as sulfuric acid, nitric acid, formic acid and oxalic acid can be used, but sulfuric acid is preferred. As the alkali for pH adjustment, inorganic and organic alkalis such as caustic soda, caustic potassium, sodium carbonate, calcium carbonate, ammonia and amines can be used, but caustic soda is preferred.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to these examples. Unless specifically limited in the examples and comparative examples shown below, the measurement of the amount of hexeneuronic acid of the pulp, the measurement of whiteness, the evaluation of fading, and the production of monopersulfuric acid were carried out by the following methods, respectively.

1. Measurement of amount of hexeneuronic acid (HexA) of pulp 0.16 g of pulp was taken in an absolutely dry mass, and distilled water was added so that the total amount became 80 g. 0.02 g of formic acid was added thereto and stirred well. The whole amount was transferred to a pressure vessel and heated at 120 ° C. for 4 hours to perform acid hydrolysis of hexeneuronic acid. After heating, 2-furancarboxylic acid and 5-formyl-2-furancarboxylic acid, which are acid hydrolysates of hexeneuronic acid in the filtered solution, were quantified by HPLC, and hexeneuronic acid was calculated from the total molar ratio. The amount was determined.

2. Measurement of Pulp Whiteness After bleaching the bleached pulp, a sheet having a basis weight of 60 g / m ² was prepared according to JIS P8209, and the whiteness of the pulp was measured according to JIS P8148.

3. Pulp fading evaluation (calculation of PC value)
After bleaching the bleached pulp, aluminum sulfate was added to adjust the pH to 5.5, and then a sheet having a basis weight of 60 g / m ² was produced. The produced sheet was allowed to stand for 24 hours under the conditions of 80 ° C. and relative humidity 65%, and the PC value was calculated according to the following formula from the whiteness before and after the treatment.
PC value = {(1−whiteness after fading) ² / (2 × whiteness after fading) − (1−whiteness before fading) ² / (2 × whiteness before fading)} × 100

4). Manufacturing conditions for monopersulfuric acid (MPS) Monopersulfuric acid was synthesized from 75.6 g (1 mol) of industrial grade 45% hydrogen peroxide solution manufactured by Mitsubishi Gas Chemical Co., Ltd., and industrial grade 95% concentrated sulfuric acid with an iron concentration of 10 ppm or less. 310 g (3 mol) was added dropwise at a rate such that the mixed solution did not exceed 70 ° C., and diluted with dechlorinated water to obtain 3500 g of a 2.1% aqueous solution of monopersulfuric acid. The monopersulfuric acid concentration after the synthesis was obtained by subtracting the hydrogen peroxide concentration obtained by oxidation-reduction titration using sulfuric acid (IV) cerium from the total peroxide concentration obtained by iodine titration.

Example 1 (Px-Z-Eop-P sequence)
The unbleached L-pulp (the amount of hexeneuronic acid 41.1 μmol / g pulp, whiteness 54.8%) after the eucalyptus material was cooked and bleached with alkali oxygen was treated under the following conditions.

<Method for separating pulp-accompanying water>
The pulp slurry after alkaline oxygen bleaching with a pulp concentration of 18.5% was filtered until the pulp concentration became 23%, and the resulting filtrate was used as pulp-entrained water.

<Px processing>
86.4 g of pulp slurry having a pulp concentration of 18.5% after alkaline oxygen bleaching was weighed and placed in a polyethylene bag. Thereto, 8.0% of 2% monopersulfuric acid and 1.4 g of 10% sodium hydroxide and 64.2 g of pulp-entrained water were added to prepare a sample having a pulp concentration of 10%. It was warmed at 60 ° C. for 60 minutes. Table 1 shows the amount of each substance used during the Px treatment and the hexeneuronic acid concentration in the pulp after the Px treatment.

The pulp after the Px treatment was treated under the following conditions.
<Z-Eop-P treatment>
・ Z: pulp concentration 10%, ozone addition amount 0.6 mass%, pH 3 after reaction, temperature 60 ° C., time 3 minutes Eop: pulp concentration 10%, hydrogen peroxide addition amount 0.25 mass%, oxygen addition amount 0.15%, treatment pH 11-12, temperature 60 ° C., time 70 minutes
・ P: Pulp concentration 10%, hydrogen peroxide addition amount 1.0 mass%, treatment pH 11-12, temperature 80 ° C., time 120 minutes ・ Washing conditions at each stage: washing rate 90%, pulp concentration 20% after washing It dehydrated and used as the pulp after bleaching of each stage.
Table 1 shows the hexeneuronic acid concentration, whiteness, and PC value in the pulp after P treatment.

Example 2
<Oxidant treatment of pulp-accompanying water>
200 g of a 35% hydrogen peroxide solution was added to 165 g of the pulp-entrained water separated in Example 1, and 40 g of 10% sodium hydroxide was added to prepare a solution having a hydrogen peroxide concentration of 17 mass% and a pH of 12. The solution was heated at 55 ° C. for 3 hours, and then the hydrogen peroxide was removed using catalase so that the residual hydrogen peroxide concentration was 10 ppm or less.

<Px processing>
The pulp-entrained water of Example 1 was replaced with the pulp-entrained water after the oxidizing agent treatment, and the same treatment was performed.
Table 1 shows the amount of each substance used during the Px treatment and the hexeneuronic acid concentration in the pulp after the Px treatment.

  The pulp after Px treatment was subjected to pulp bleaching in the same manner as in Example 1. Table 1 shows the hexeneuronic acid concentration, whiteness, and PC value in the pulp after P treatment.

Comparative Example 1
<Px processing>
A similar treatment was performed by replacing the pulp-entrained water of Example 1 with dechlorinated water. Table 1 shows the amount of each substance used during the Px treatment and the hexeneuronic acid concentration in the pulp after the Px treatment.

  The pulp after Px treatment was subjected to pulp bleaching in the same manner as in Example 1. Table 1 shows the hexeneuronic acid concentration, whiteness, and PC value in the pulp after P treatment.

  As shown in Table 1, hexene uronic acid can be effectively removed and fading can be improved by adding pulp-entrained water or pulp-entrained water after oxidation treatment when the pulp after alkaline oxygen bleaching is Px-treated. It was confirmed that

Claims (5)

A method for TCF bleaching of unbleached pulp obtained by cooking lignocellulosic material,
(1) pulp-entrained water separated from at least one delignified pulp slurry selected from the unbleached pulp of the same or different bleaching sequence after alkaline oxygen bleaching and after each bleaching of multi-stage bleaching;
(2) A TCF bleaching method for pulp, which is added to a treatment stage using an inorganic peroxyacid in a multistage bleaching treatment including a treatment stage using an inorganic peroxyacid. The TCF bleaching method according to claim 1, wherein the treatment stage using an inorganic peroxyacid is included in a bleaching sequence of a multistage bleaching process for separating pulp-entrained water. 3. The TCF bleaching method according to claim 1, wherein the pulp-entrained water is treated with an oxidizing agent and then added to a treatment stage using an inorganic peroxyacid. The TCF bleaching method according to claim 3, wherein the oxidizing agent is hydrogen peroxide. The TCF bleaching method according to claim 3, wherein the inorganic peroxyacid is monopersulfuric acid.