Method of bleaching pulp

Abstract

To provide a method capable of preventing deterioration of the whiteness (brightness) of bleached pulp resulting from the passage of time, when bleaching pulp using hypochlorite.SOLUTION: One aspect of the present invention relates to a method of bleaching pulp using hypochlorite, the method including adding an aqueous solution at least including one of combined chlorine and combined bromine to pulp slurry to be subjected to a bleaching treatment using hypochlorite to carry out a pre-bleaching treatment, and adding an aqueous solution including hypochlorite to the pulp slurry having been subjected to the pre-bleaching treatment to carry out a bleaching treatment.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a method for bleaching pulp using hypochlorite, a method for improving the color fastness of bleached pulp, and a method for producing raw pulp material. [Background technology]

[0002] The papermaking process generally includes a pulping process for obtaining pulp raw material from raw materials such as wood, a stock preparation process for mechanically treating the pulp raw material obtained in the pulping process and adding chemicals, and a papermaking process for forming the pulp into sheets.

[0003] There are three main types of pulping processes (pulp manufacturing processes): chemical pulp, which uses chemicals to extract pulp; mechanical pulp, which is made by mechanically grinding wood to obtain pulp; and recycled paper pulp, which is made from recycled paper. Of these, chemical pulp and recycled paper pulp are the most widely used in Japan.

[0004] Kraft pulp, a type of chemical pulp, is generally prepared through a cooking process in which wood chips are cooked with a mixture of caustic soda and sodium sulfide, an unbleached pulp washing process in which black liquor containing pulp and lignin is separated, an oxygen delignification process in which lignin is removed from the pulp using oxygen and alkali, and a bleaching process.Waste paper pulp is generally prepared through a disintegration process in which waste paper is mechanically disintegrated while being mixed with water, a dust removal process in which foreign matter contained in the raw material waste paper is removed, a deinking process in which ink components are removed by adding a deinking agent, a dehydration process in which pulp is dehydrated, and a bleaching process in which the dehydrated pulp is bleached.

[0005] There are various methods for bleaching kraft pulp and waste paper pulp, such as a method using chlorine, a method using hydrogen peroxide, a method using chlorine dioxide, and a method using hypochlorite (for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2005-240188 A [Patent Document 2] JP 2007-162148 A [Patent Document 3] Special Publication No. 2009-544857 Summary of the Invention [Problem to be solved by the invention]

[0007] Due to the ease of management and the high required whiteness (brightness), bleaching using hypochlorite is performed as a method for bleaching pulp in the pulping process (pulp manufacturing process). The pulp raw material produced in the pulping process may be processed into a sheet shape and temporarily stored as a pulp sheet, but the present inventors have found a problem that the whiteness (brightness) of bleached pulp (pulp raw material) that has been bleached with hypochlorite decreases over time. Therefore, the present disclosure provides a method that can suppress the decrease in whiteness (brightness) of bleached pulp over time in pulp bleaching using hypochlorite. [Means for solving the problem]

[0008] The present disclosure relates, as one aspect, to a method for bleaching pulp using hypochlorite, the bleaching method including: performing a bleaching pretreatment by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with hypochlorite; and performing a bleaching treatment by adding an aqueous solution containing hypochlorite to the pulp slurry that has been bleached pretreated.

[0009] The present disclosure relates, as one aspect, to a method for improving the color fastness of bleached pulp, the method including: performing a bleaching pretreatment by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with a hypochlorite; and performing a bleaching treatment by adding an aqueous solution containing a hypochlorite to the pulp slurry that has been bleached pretreated.

[0010] The present disclosure relates, as one aspect, to a method for producing a pulp raw material, the method including: performing a bleaching pretreatment by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with a hypochlorite; and performing a bleaching treatment by adding an aqueous solution containing a hypochlorite to the pulp slurry that has been bleached pretreated. Effect of the Invention

[0011] According to the present disclosure, it is possible to suppress the decrease in whiteness (brightness) over time of bleached pulp that has been bleached with hypochlorite. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is an example of a graph showing the change over time in ISO brightness of bleached pulp obtained in the examples and comparative examples. [Diagram 2] FIG. 2 is an example of a graph showing the degree of discoloration (ΔBr) of bleached pulp obtained in the examples and comparative examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present inventors have found that by pretreating the pulp slurry with combined chlorine and / or combined bromine prior to bleaching the pulp with hypochlorite, the decrease in whiteness (brightness) of the resulting bleached pulp over time can be suppressed. Furthermore, they have found that by pretreating the pulp with combined chlorine and / or combined bromine, the whiteness of the resulting bleached pulp can be improved and the amount of hypochlorite used for bleaching can be reduced.

[0014] The whiteness index in this disclosure refers to ISO whiteness index, which can be measured by the method described in the Examples.

[0015] In one embodiment, "suppression of decrease in whiteness over time" in the present disclosure means that the difference (fading degree: ΔBr=whiteness(1)-whiteness(2)) between the whiteness of a pulp sheet prepared immediately after bleaching treatment with hypochlorite (whiteness(1)) and the whiteness 14 days after bleaching treatment with hypochlorite (contact with hypochlorite) is 0.6 or less, 0.55 or less, or 0.5 or less. In addition, in one embodiment, "suppression of decrease in whiteness over time" in the present disclosure means that the above-mentioned fading degree (ΔBr = whiteness (1) - whiteness (2)) is 1.5% or less, 1.4% or less, 1.3% or less, 1.2% or less, or 1.1% or less of the whiteness of the pulp sheet immediately after the bleaching treatment (whiteness A).

[0016] In one or more embodiments, the pulp in the present disclosure may include wood pulp, non-wood pulp, deionized paper pulp (DIP), broken pulp, etc. In one or more embodiments, the wood pulp may include chemical pulp such as kraft pulp (KP) and sulfite pulp (SP), and mechanical pulp such as groundwood pulp (GP) and thermomechanical pulp (TMP).

[0017] In the present disclosure, "pulp slurry to be bleached with hypochlorite" refers to a pulp slurry containing pulp before being bleached with hypochlorite. In one or more embodiments, the pulp slurry to be bleached with hypochlorite may contain deinked pulp and / or unbleached pulp after delignification, or may contain pulp that has been bleached with an agent other than hypochlorite. In one or more embodiments, the unbleached (unbleached) pulp may be unbleached chemical pulp, unbleached mechanical pulp, or the like. In one or more embodiments, the pulp slurry to be bleached with hypochlorite may be pulp slurries that have been treated with a digestion process in which wood chips are digested with a mixture of caustic soda and sodium sulfide or an aqueous solution of sulfite (sulfite solution), an unbleached pulp washing process in which black liquor containing pulp and lignin is separated, and an oxygen delignification process in which lignin is removed from the pulp with oxygen and alkali. In one or more embodiments, the pulp slurry to be bleached with hypochlorite may be pulp slurries that have been treated with a digestion process in which waste paper raw material is mixed with water and digested by mechanical force, a dust removal process in which foreign matter contained in the waste paper raw material is removed, a deinking process in which ink components are removed by adding a deinking agent, and a dehydration process in which pulp is dehydrated.

[0018] In the present disclosure, "bleached pulp" refers to pulp that has been subjected to a bleaching treatment using hypochlorite. In one or more embodiments, the bleached pulp refers to deinked pulp that has been subjected to a deinking treatment, and unbleached (unbleached) pulp such as unbleached chemical pulp and unbleached mechanical pulp, and refers to pulp that has been subjected to a bleaching treatment with hypochlorite. In one or more embodiments, the bleached pulp in the present disclosure may also include pulp that has been subjected to a bleaching treatment using an agent other than hypochlorite.

[0019] In the present disclosure, the term "pulp raw material" refers to waste paper pulp and / or bleached pulp obtained in the pulping process. In one or more embodiments, examples of bleached pulp include bleached chemical pulps such as bleached kraft pulp and bleached sulfite pulp, and bleached mechanical pulps such as bleached thermomechanical pulp and bleached chemothermomechanical pulp. In one or more embodiments, the pulp raw material may contain agents such as paper strength agents, retention aids, drainage aids, internal sizing agents, auxiliary agents, dyes, and fluorescent whitening agents. In one or more embodiments, the pulp raw material may be paper pulp or dissolving pulp. In one or more embodiments, the pulp raw material may be in a sheet form or a cotton form. In one or more embodiments, the pulp raw material is not particularly limited, and may be a paper pulp sheet or the like. In one or more embodiments, the waste paper pulp raw material obtained from a raw material containing waste paper may be used as a raw material for paperboard or gypsum board.

[0020] [Combined chlorine and combined bromine] In this disclosure, "combined chlorine" is also referred to as stabilized chlorine, and includes monochloramine, dichloramine, trichloramine, and N-chlorosulfamate. In the present disclosure, "bound bromine" is also referred to as stabilized bromine, and examples thereof include monobromamine, dibromamine, tribromamine, and N-bromosulfamate.

[0021] In the present disclosure, "monochloramine" refers to NH 2 In this disclosure, the term "monobromamine" refers to a compound represented by the formula NH 2 Br (a compound in which one of the hydrogen atoms of ammonia is replaced by a bromine atom). Monochloramine and monobromamine are OCl - (Br - )+NH 4 + →NH 2 Cl(Br)+H 2 In one or more embodiments, monochloramine can be produced by mixing hypochlorite with an ammonium compound. In one or more embodiments, the hypochlorite may include sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, and magnesium hypochlorite. In one or more embodiments, the ammonium compound may include ammonium sulfate, ammonium bromide, ammonium chloride, ammonium sulfamate, ammonium bromide, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium nitrate. These may be used alone or in combination of two or more. In one or more embodiments, the molar ratio of the hypochlorite to the ammonium compound is 1:1 to 1:2, 1:1.1 to 1:2, 1:1.2 to 1:2, or 1:1.2 to 1:1.6, as the molar ratio of the total residual chlorine amount to nitrogen.

[0022] [Bleaching method] In one aspect, the present disclosure relates to a method for bleaching pulp using hypochlorite, the method comprising: pretreating a pulp slurry to be bleached with hypochlorite by adding an aqueous solution containing at least one of combined chlorine and combined bromine; and pretreating the pulp slurry by adding an aqueous solution containing hypochlorite. In one or more embodiments, the bleaching method of the present disclosure can suppress a decrease in the whiteness (brightness) of the resulting bleached pulp over time. In one or more embodiments, the bleaching method of the present disclosure can improve the whiteness of the resulting bleached pulp, and can reduce the amount of hypochlorite used for bleaching.

[0023] In one or more embodiments, the solid content (bone dry pulp weight) of the pulp slurry to be bleached is 0.5% by weight to 30% by weight. In one or more embodiments, the solid content (bone dry pulp weight) of the pulp slurry is 0.8% by weight or more, 1% by weight or more, 3% by weight or more, 5% by weight or more, 10% by weight or more, 11% by weight or more, 12% by weight or more, 13% by weight or more, 14% by weight or more, or 15% by weight or more. In one or more embodiments, the solid content (bone dry pulp weight) of the pulp slurry is 29% by weight or less, 28% by weight or less, 27% by weight or less, 26% by weight or less, or 25% by weight or less.

[0024] The bleaching method of the present disclosure includes a bleaching pretreatment of the pulp slurry with combined chlorine and / or combined bromine. The addition rate of combined chlorine and / or combined bromine (when a plurality of agents are used, the total amount thereof, the same applies below) is 0.075 kg / T to 1.5 kg / T per solid content (bone dry pulp weight) of the pulp slurry in one or more embodiments, as an effective chlorine concentration equivalent value. In one or more embodiments, the addition rate of combined chlorine and / or combined bromine is 0.15 kg / T or more, 0.2 kg / T or more, or 0.3 kg / T or more, from the viewpoint of further suppressing the decrease in whiteness over time and further improving the whiteness of the obtained bleached pulp. In the present disclosure, the unit "T" means bone dry ton, and refers to the actual weight calculated based on the bone dry solid concentration.

[0025] The pretreatment time with combined chlorine and / or combined bromine, that is, the time from adding combined chlorine and / or combined bromine to the pulp slurry before the bleaching treatment with hypochlorite to performing the bleaching treatment with hypochlorite, is 1 minute or more, 1.5 minutes or more, or 2 minutes or more in one or more embodiments. The upper limit of the pretreatment time is not particularly limited, and in one or more embodiments, it is 60 minutes or less. Therefore, in one or more embodiments, the pretreatment time is 1 minute to 60 minutes, and the lower limit can be selected from the above.

[0026] In one or more embodiments, the temperature of the pretreatment with combined chlorine and / or combined bromine is 30° C. to 80° C. In one or more embodiments, the treatment temperature is 30° C. or more, 35° C. or more, or 40° C. or more, from the viewpoint of further suppressing the decrease in whiteness over time and further improving the whiteness of the resulting bleached pulp. In one or more embodiments, the treatment temperature is 80° C. or less, 70° C. or less, or 60° C. or less, from the same viewpoint.

[0027] In one or more embodiments, the pH of the pulp slurry when or after the combined chlorine and / or combined bromine are present, or when or after the combined chlorine and / or combined bromine are added, is pH 8 to 11. In one or more embodiments, the pH is preferably 8.5 or higher, pH 9 or higher, or pH 9.5 or higher, from the viewpoints of further suppressing the decrease in brightness over time and further improving the brightness of the resulting bleached pulp. The pH can be measured by the method described in the Examples.

[0028] The bleaching method of the present disclosure includes adding an aqueous solution containing hypochlorite to a pulp slurry that has been subjected to a bleaching pretreatment, and performing a bleaching treatment with the hypochlorite. The pulp slurry that has been subjected to the bleaching pretreatment contains combined chlorine and / or combined bromine. In one or more embodiments, the pulp slurry that has been subjected to the bleaching pretreatment contains combined chlorine and / or combined bromine (when a plurality of agents are used, the total amount) in an amount of 0.075 kg / T to 1.5 kg / T per solid content (bone dry pulp weight) of the pulp slurry. In one or more embodiments, the content of combined chlorine and / or combined bromine in the pulp slurry that has been subjected to the bleaching pretreatment is 0.15 kg / T or more, 0.2 kg / T or more, or 0.3 kg / T or more, from the viewpoint of further suppressing a decrease in brightness over time and further improving the brightness of the resulting bleached pulp.

[0029] The hypochlorite addition rate is 4 kg / T to 30 kg / T per solid content (bone dry pulp weight) of the pulp slurry in one or more embodiments, as an effective chlorine concentration conversion value. The hypochlorite addition rate is 10 kg / T or more, 15 kg / T or more, or 20 kg / T or more in one or more embodiments, from the viewpoint of further suppressing the decrease in whiteness over time and further improving the whiteness of the obtained bleached pulp. From the same viewpoint, in one or more embodiments, it is 29 kg / T or less, 27 kg / T or less, 26 kg / T or less, 25 kg / T or less, or 24 kg / T or less.

[0030] In one or more embodiments, the pH of the pulp slurry during the bleaching treatment is from pH 8 to 11, and from the viewpoint of further improving the brightness of the obtained bleached pulp, the pH is preferably 8.5 or higher, pH 9 or higher, or pH 9.5 or higher. In one or more embodiments, the treatment temperature during the bleaching treatment is from 30° C. to 80° C., and preferably from 40° C. to 70° C. In one or more embodiments, the bleaching treatment time is from 30 minutes to 360 minutes, and preferably from 60 minutes to 180 minutes.

[0031] In one or more embodiments, the bleaching method of the present disclosure may further include adjusting the pH by adding an alkaline agent to the pulp slurry that has been subjected to a bleaching pretreatment prior to the bleaching treatment with hypochlorite, in order to further suppress the decrease in whiteness over time and to further improve the whiteness of the resulting bleached pulp. In one or more embodiments, the pH is 8 to 11, and in order to further improve the whiteness of the resulting bleached pulp, the pH is preferably 8.5 or higher, 9 or higher, or 9.5 or higher. In one or more embodiments, examples of the alkaline agent include sodium hydroxide, potassium hydroxide, and sodium carbonate.

[0032] [How to improve color fastness] In another aspect, the present disclosure relates to a method for improving color fastness of bleached pulp, the method comprising: pretreating a pulp slurry to be bleached with hypochlorite by adding an aqueous solution containing at least one of combined chlorine and combined bromine; and pretreating the pulp slurry by adding an aqueous solution containing hypochlorite. According to the method for improving color fastness of the present disclosure, in one or more embodiments, it is possible to suppress a decrease in whiteness (brightness) over time of the obtained bleached pulp, improve the fading resistance of the bleached pulp, and improve the color fastness.

[0033] The bleaching pretreatment and bleaching treatment in the color fastness improving method of the present disclosure can be carried out in the same manner as in the bleaching method of the present disclosure.

[0034] [Manufacturing method of pulp raw materials] In yet another aspect, the present disclosure relates to a method for producing a pulp raw material, the method comprising: adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with a hypochlorite to perform a bleaching pretreatment; and adding an aqueous solution containing a hypochlorite to the pulp slurry that has been bleached pretreated to perform a bleaching treatment. According to the method for producing a pulp raw material, in one or more embodiments, it is possible to produce a pulp raw material in which a decrease in whiteness (brightness) over time is suppressed.

[0035] The bleaching pretreatment and bleaching treatment in the method for producing pulp raw material of the present disclosure can be carried out in the same manner as in the bleaching method of the present disclosure.

[0036] In one or more embodiments, the method for producing pulp raw material of the present disclosure may include, prior to the bleaching pretreatment, a cooking step in which wood chips are cooked with a mixed solution of caustic soda and sodium sulfide or an aqueous solution of sulfite (sulfite solution), an unbleached pulp washing step in which black liquor containing pulp and lignin is separated from the pulp, and an oxygen delignification step in which lignin is removed from the pulp using oxygen and alkali.In one or more embodiments, the method for producing pulp raw material of the present disclosure may include, prior to the bleaching pretreatment, a disintegration step in which the waste paper raw material is mechanically disintegrated while being mixed with water, a dust removal step in which foreign matter contained in the waste paper raw material is removed, a deinking step in which ink components are removed by adding a deinking agent, and a dewatering step in which the pulp is dehydrated.

[0037] In one or more embodiments, the method for producing a pulp raw material of the present disclosure may further include a preparation step in which the bleached pulp is subjected to physical processing such as addition of chemicals and / or beating, etc. In one or more embodiments, examples of the chemicals to be added include paper strength agents, retention aids, drainage aids, internal sizing agents, auxiliary agents, dyes, and fluorescent whitening agents.

[0038] The present disclosure further relates to one or more of the following embodiments. [1] A method for bleaching pulp using hypochlorite, comprising the steps of: A pre-bleaching treatment is performed by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with hypochlorite; and and adding an aqueous solution containing hypochlorite to the pulp slurry that has been subjected to the bleaching pretreatment to perform a bleaching treatment. [2] The bleaching method according to [1], further comprising, prior to the bleaching treatment, adding an alkaline agent to the pulp slurry that has been pretreated for bleaching to adjust the pH of the pulp slurry. [3] The bleaching method according to [1] or [2], wherein the pulp slurry contains waste paper pulp and / or unbleached pulp after delignification. [4] The bleaching method according to any one of [1] to [3], wherein the combined chlorine and combined bromine are selected from the group consisting of monochloramine, dichloramine, trichloramine, monobromamine, dibromamine, tribromamine, N-chlorosulfamate and N-bromosulfamate. [5] A method for improving colorfastness of bleached pulp, comprising the steps of: A pre-bleaching treatment is performed by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with hypochlorite; and The improvement method includes adding an aqueous solution containing hypochlorite to the pulp slurry that has been subjected to the bleaching pretreatment, thereby carrying out a bleaching treatment. [6] A method for producing pulp raw materials, comprising: A pre-bleaching treatment is performed by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with hypochlorite; and and adding an aqueous solution containing hypochlorite to the pulp slurry that has been subjected to the bleaching pretreatment to perform a bleaching treatment. [7] The manufacturing method according to [6], wherein the pulp raw material is a pulp raw material for papermaking.

[0039] The present disclosure will be described in more detail below with reference to examples, but these are merely illustrative and the present disclosure is not limited to these examples. EXAMPLES

[0040] [Preparation of raw materials] Pulp samples were prepared by disintegrating old newspapers. Specifically, commercially available newspapers were cut into 3 x 3 cm pieces, and tap water from Osaka City was added to the pieces so that the solid content concentration was 1%, and disintegration treatment was carried out for 10 minutes at 2000 rpm using a tabletop disintegrator (product name: Standard Pulp Disintegrator, manufactured by Kumagai Riki Kogyo Co., Ltd.). The water from the pulp slurry after disintegration treatment was removed using a 100 mesh sieve, and a pulp sample was prepared. Note that no chemicals were added during the disintegration treatment.

[0041] [NH 2 Preparation of Cl reaction solution] A sodium hypochlorite solution (residual chlorine content: 120 g / L (Kishida Chemical Co., Ltd.)) was diluted with deionized water so that the residual chlorine content was 5 g / L, and then a 30% aqueous ammonium sulfate solution (30 g of ammonium sulfate (Kishida Chemical Co., Ltd.) was dissolved in deionized water to make the total amount 100 g) was added and mixed so that the molar ratio of residual chlorine content to nitrogen was 1:1.2.

[0042] [Method of measuring available chlorine] The available chlorine was measured using a DP residual chlorine meter DP-3F (manufactured by Kasahara Chemical Industry Co., Ltd.) by spectrophotometric measurement using the DPD reagent color reaction, and the total chlorine concentration was measured.

[0043] [pH measurement method] The pH was measured by the glass electrode method using a tabletop pH meter (Navi: manufactured by HORIBA) in auto mode at about 25°C.

[0044] [Example 1] (1) The pulp sample was divided into small portions of 2.5 g bone dry weight. (2) White water was added to prepare a pulp slurry so that the pulp concentration (solids concentration) was 15%. (3) Reduce NH so that the concentration (effective chlorine equivalent concentration) is as shown in Table 1 below. 2 The Cl reaction solution was added and mixed for 2 minutes at 25°C. 2 The pH of the pulp slurry at the time of (or after) the addition of the Cl reaction solution was 7.1. (4) Sodium hydroxide solution was added to adjust the pH to 10. (5) Sodium hypochlorite solution was added to the mixture so as to reach the concentration (converted into available chlorine concentration) shown in Table 1 below, and the mixture was mixed for 2 minutes. (6) The mixture was allowed to stand at 40°C for 6 hours or for 14 days. (7) After washing with 2 L of tap water, hand sheets were made using a circular sheet machine (basis weight 100 g / m 2 ). (8) The handsheet was conditioned overnight in a constant temperature and humidity room, and then the ISO brightness was measured. The results are shown in Table 1 below.

[0045] [Comparative Example 1] A hand sheet was prepared in the same manner as in Example 1 above, except that (3) was not carried out, and the ISO brightness index was measured.

[0046] [Comparative Example 2] (3) is not carried out and (5) is NH 2 A hand sheet was prepared in the same manner as in Example 1 above, except that the Cl reaction solution and the sodium hypochlorite solution were added simultaneously, and the ISO brightness was measured.

[0047] [Comparative Example 3] A hand sheet was prepared in the same manner as in Example 1 above, except that (3) was not performed and the concentration of the sodium hypochlorite solution added in (5) was 24 kg / T (effective chlorine equivalent concentration), and the ISO whiteness index was measured.

[0048] [Comparative Example 4] A hand sheet was prepared in the same manner as in Example 1 above, except that (3) was not performed and the concentration of the sodium hypochlorite solution added in (5) was 26.4 kg / T (effective chlorine equivalent concentration), and the ISO whiteness index was measured.

[0049] [Comparative Example 4] A hand sheet was prepared in the same manner as in Example 1 above, except that no chemicals were added in (3) and (5), and the ISO brightness was measured.

[0050] [ISO whiteness measurement] The ISO whiteness was measured using a spectrophotometer / color difference meter PF7000 (Nippon Denshoku Industries Co., Ltd.). The degree of fading (ΔBr) after 14 days was calculated based on the following formula, and the results are shown in Table 1 and Figures 1 and 2. (ΔBr) = [ISO whiteness (after 6 hours)] - [ISO whiteness (after 14 days)]

[0051] [Table 1]

[0052] As shown in Table 1 and Figure 2, NH 2 According to Example 1, in which pretreatment with Cl reaction solution (monochloramine) was performed, the degree of discoloration (ΔBr) was suppressed by about half or more compared to Comparative Examples 1-4. 2 It can be said that pretreatment with Cl-reactive solution (monochloramine) significantly suppressed the decrease in whiteness over time caused by hypochlorite bleaching. Furthermore, the ISO brightness of the handsheets prepared immediately after treatment (6 hours later) was compared. 2 In Example 1, which was pretreated with Cl reaction solution (monochloramine), NH 2 It was higher than Comparative Example 2 in which the Cl reaction liquid (monochloramine) and sodium hypochlorite were added simultaneously. Specifically, in Example 1, the whiteness index improved by 1.59 points compared to the blank, and was 30% higher than Comparative Example 2. In addition, the ISO brightness of Example 1 was comparable to that of Comparative Examples 3 and 4, which had sodium hypochlorite concentrations 10% or 20% higher than those of Example 1. 2 It was confirmed that the amount of sodium hypochlorite used could be reduced by performing pretreatment with Cl-reactive solution (monochloramine).

Claims

1. 1. A method for bleaching pulp with hypochlorite, comprising the steps of: A pre-bleaching treatment is performed by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with hypochlorite; and and adding an aqueous solution containing hypochlorite to the pulp slurry that has been subjected to the bleaching pretreatment to perform a bleaching treatment.

2. 2. The bleaching method according to claim 1, further comprising, prior to said bleaching treatment, adjusting the pH of the pulp slurry that has been subjected to said pre-bleaching treatment by adding an alkaline agent to said pulp slurry.

3. The bleaching method according to claim 1 , wherein the pulp slurry contains recycled paper pulp and / or unbleached pulp after delignification.

4. 4. The bleaching method according to claim 1, wherein the combined chlorine and combined bromine are selected from the group consisting of monochloramine, dichloramine, trichloramine, monobromamine, dibromamine, tribromamine, N-chlorosulfamate and N-bromosulfamate.

5. 1. A method for improving colorfastness of bleached pulp, comprising: A pre-bleaching treatment is performed by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with hypochlorite; and The improvement method includes adding an aqueous solution containing hypochlorite to the pulp slurry that has been subjected to the bleaching pretreatment, thereby carrying out a bleaching treatment.

6. A method for producing a pulp raw material, comprising: A pre-bleaching treatment is performed by adding an aqueous solution containing at least one of combined chlorine and combined bromine to a pulp slurry to be bleached with hypochlorite; and and adding an aqueous solution containing hypochlorite to the pulp slurry that has been subjected to the bleaching pretreatment to perform a bleaching treatment.

7. The method according to claim 6 , wherein the pulp stock is a paper pulp stock.

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