JP3768986B2 - Process for producing bleached kraft pulp with improved thermal fading - Google Patents

Description

In the present invention, pulp obtained by digesting lignocellulosic material in a batch or continuous digester by kraft method is subjected to alkaline oxygen delignification and washing and selection, and further generally known acid treatment is processed. This invention relates to a method for producing chlorine-free bleached kraft pulp with improved thermal fading, which is obtained by bleaching in a multi-stage chlorine-free bleaching sequence with ozone or chlorine dioxide as the first stage after removing transition metal ions from the pulp. is there.

  Currently, pulp, which is a raw material for papermaking, is mainly produced by the kraft cooking method. Kraft pulp is manufactured in a batch or continuous digester, and then environmental impact COD (chemical oxygen demand from the bleaching process). Amount) and BOD (biological oxygen demand) are reduced by alkaline oxygen delignification, the kappa number of unbleached pulp is further lowered, and the process proceeds to the bleaching step. There are mainly 3 bleaching methods for pulp, which are the conventional chlorine bleaching method, chlorine-free bleaching method (Elementary Chlorine Free = ECF) and complete chlorine-free bleaching method (Totally Chlorine Free = TCF). In the chlorine bleaching method, pulp is bleached with a combination of chemicals such as elemental chlorine, hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide, and caustic soda. The use of chlorine has been avoided due to the recent increase in environmental protection, and there is also a concern that chloroform generated by the use of hypochlorite may adversely affect the human body. ECF and TCF bleaching, which does not contain chlorine and hypochlorite, has become the main method. In that case, common chemicals used are chlorine dioxide and peroxidation by the ECF bleaching method. Hydrogen, ozone, oxygen, caustic soda, etc. are chelating agents, hydrogen peroxide, ozone, oxygen, caustic soda and peracetic acid in the TCF bleaching method, but ECF pulp is whiter than TCF pulp. Degree is high, because the cost is also inexpensive, ECF bleaching method has now become mainstream.

The multistage chlorine-free bleaching process, the first stage is generally chlorine dioxide stage (D ₀ stage substantially) or ozone stage (Z stage substantially), according to operational experience paper pulp industry, washing machines after these first stage The scale of calcium oxalate is frequently occurring, which is a serious problem. Hexenuronic acid (abbreviated as HexA) in the pulp reacts with chlorine dioxide or ozone to produce oxalic acid, which then reacts with calcium in the liquor to produce calcium oxalate. In order to prevent the calcium oxalate problem, the presently preferred method is to treat the pulp in an acidic aqueous solution under acid treatment conditions before entering the bleaching step (abbreviated as stage A). There are several stage A conditions described in the public literature. For example, at the UPM-Kimeni Visa Forest factory in Finland, pH 3.5, temperature 90 ° C, residence time up to 7.5 hours from July 1996. Stage A (Sitala, M., Winberg, K., Alenius, M., Henricson, K., Lonnberg, B., 1998 International Pulp Bleaching Conference, Book 1, page 279), followed by Rio Bcel, Brazil From September 1999, the A stage (Ratnieks, E., Ventura, JW, Mensch, MR, Zanchin, RA, Pulp Paper Can., 102) (pH 3.5, temperature 90 ° C, residence time 100-150 minutes) ): 93 (2001)), and further, Ahlstrom (currently changed its name to Andritz) is the condition (pH: 3-4, temperature: 90-110) of its own AHL ^TM stage (similar to the A stage). (Henricson, K., Paperi ja Puu, 79 (8): 546 (199) 7)).

  The chlorine-free bleaching method uses chlorine dioxide, hydrogen peroxide, ozone, etc., so the finished bleached pulp is more fading than in the case of chlorine-bleached during distribution and storage, and the fading is generally due to heat rather than light. In order to improve the thermal fading (80 ° C.) of chlorine dioxide-based ECF bleached pulp according to JP 2002-266271, 2003-96680, the potassium permanganate value according to JIS P8206 is 1.5. Hereinafter, it is necessary that the PC value is 10.0 or less and the hexeneuronic acid (abbreviated as HexA) is 10 mmol / kg BD pulp or less.

Moreover, in JP-A-2003-105684, in the chlorine-free bleaching of kraft pulp after alkaline oxygen delignification, the P stage that is carried out on the acidic side (pH: 1.5 to 4.5) is used as an alternative to the D0 stage, And when HexA of the pulp after the acid P stage satisfies the condition of 15 mmol / kg BD pulp or less, the thermal fading property of the bleached kraft pulp was improved.
Furthermore, in Japanese Patent Application No. 2003-334843, an international CIE before and after thermal bleaching was performed on a chlorine-free bleached kraft pulp having a final whiteness of 82 to 90% under conditions of a hot air dryer temperature of 105 ° C. and a residence time of 12 hours. The difference in b ^* value (Δb ^* ) of the L ^* a ^* b ^* chromaticity system is 2.20 or less, and the thermal fading is improved to be equal to or less than that of conventional chlorine bleached pulp. It was found that the value was 2.20 or less and the carboxyl group was 22.0 meq / 100 g BD pulp or less.

JP 2002-266271 A JP2003-96680 JP2003-105684A

  In the above-mentioned publicly known Japanese Patent Laid-Open Publications, residual lignin (expressed in terms of potassium permanganate value or kappa number), HexA, carboxylic acid, etc. are described as factors affecting the thermal fading of chlorine-free bleached kraft pulp, of which residual lignin And HexA are the most notable factors. However, the factors that affect thermal fading have not been fully elucidated due to complicated problems, and it is estimated that the lignocellulosic materials hemicellulose, cellulose and extract are also the cause. The component transition metal ions are not often touched. Transition metal ions have the ability to initiate or maintain a redox reaction in the bleaching process, even at very low concentrations, and to generate color-developing substances that cause thermal bleaching of the bleached pulp.

It is said that transition metal ions in pulp generally decompose bleaching agents such as ozone, oxygen, and hydrogen peroxide, and generated hydroxyl radicals degrade pulp quality. If there was an alkaline hydrogen peroxide stage in the multi-stage bleaching sequence, i.e., at the middle or final stage of the sequence, diiron (Fe2 ⁺ ) and manganese (Mn2 ⁺ ) in the action of the transition metal ions of the pulp ) Decomposes hydrogen peroxide, but tri-iron (Fe ³⁺ ) and copper were found to have a higher rate of residual lignin decomposition than that of hydrogen peroxide (Smith, PK, McDonough, TJ , Svensk Papperstidn., 88 (12): R106 (1985)). On the other hand, transition metal ions increase the decomposition rate of ozone by catalysis and decrease the bleaching efficiency. Metal ions are adsorbed on the fiber when alkaline, but when they become acidic they elute into the liquid, so the ozone stage becomes a “metal trap” and transition metal ions are likely to accumulate (Takanori Miyanishi, “Pulp washing / selection / bleaching”). Pulp and Paper Technology Association 2000, p. 248).

  The positions of transition metal ions in the pulp fiber are two places, hemicellulose and lignin, and the transition metal ions bind to uronic acid of hemicellulose and also form a complex with lignin. Uronic acid includes HexA, 4-O-methylglucuronic acid, galacturonic acid, glucuronic acid and the like. HexA, 4-O-methylglucuronic acid has galactouronic acid and glucuronic acid in the side chain of xylan. Tungsten to the side chain of galactoglucomannan, transition metal ions bind to the carboxyl groups of these uronic acids (Vuorinen, T., Teleman, A., Fagerstrom, P., Buchert, J., Tenkanen, M., 1996 International Pulp Bleaching Conference, Book 1, p. 43; Forsskahl, I., In “Forest Products Chemistry”, Eds., Stenius, P., Fapet Oy, Helsinki, Finland, 2000, p. 303). Furthermore, recently, copper, which is a transition metal ion, has been found to be complexed with lignin, and it is presumed that such a complex also exists in natural lignocellulosic materials (Yoon, BH., Wang, LJ., Kim, GS., J. Pulp Paper Sci., 25: 289 (1999); Ghosh, A., Ni, Y., J. Pulp Paper Sci., 24 (1): 26 (1998) ).

  The present invention has been made in order to improve the above-described problems in chlorine-free bleached kraft pulp, and comprises the following means.

In the present invention, in improving the thermal fading of chlorine-free bleached kraft pulp, pulp components obtained by digesting lignocellulosic material in a batch or continuous digester by kraft digestion are washed with alkaline oxygen delignin and washed and selected. The process is the same as in the conventional method for producing unbleached kraft pulp until the point is further processed in an acidic aqueous solution. In the chlorine-free bleaching process, in the case of ozone bleaching, the acid treatment is followed by a high-concentration ozone bleaching stage without a washing stage, and the pulp is not washed after the ozone treatment, and the low-pH and short residence time weak alkaline extraction stage is used. In the case of chlorine dioxide bleaching, the pulp is washed after the acid treatment, the first stage chlorine dioxide treatment is performed, and then the conventional high pH alkali extraction stage is carried out. In either case, a finished bleached pulp having a final whiteness of 82 to 90% is produced by treatment with chlorine dioxide and hydrogen peroxide in the latter stage, and under the conditions of a hot air dryer temperature of 105 ° C. and a residence time of 12 hours. Difference in b ^* value (Δb ^* ) of L ^* a ^* b ^* chromaticity system of international CIE before and after thermal fading is 2.20 or less, and conventional chlorine bleached pulp (Δb ^* = 2.20) Thermal fading is improved below or equal. In these chlorine-free bleached kraft pulps, it is recommended that the iron content measured by molecular absorption spectrophotometry is 25 ppm or less, the copper content is 1 ppm or less, and the manganese content is 3 ppm or less. The content of these transition metal ions is ppm by weight of the dry pulp.

  Hereinafter, the present invention will be described in more detail.

  Under the acid treatment (A stage) conditions in Japanese Patent Application No. 2003-308517 described above, that is, pH 2.8 to 3.2, temperature 85 to 90 ° C., reaction time 150 to 180 minutes, pulp concentration 8 to 13% (preferably 10-12%), the acid content of unbleached pulp reduces the transition metal ion content in the pulp. On the other hand, for the purpose of adjusting the acidic pH for ozone bleaching (abbreviated as a-stage), the pulp temperature is 30-60 ° C (preferably 50 ° C or less), and the residence time is 0-30 minutes (preferably 5-10 minutes). ), PH 2 to 3 (in sulfuric acid), and pulp concentration of 8 to 13% (preferably 10 to 12%), the transition metal ion content of the pulp also decreases. In the embodiment of the present invention, in stage A, pH 3.1, temperature 90 ° C., reaction time 150 minutes, pulp concentration 10%, stage a pH 2.7, temperature 50 ° C., reaction time 10 minutes, pulp concentration Unbleached hardwood pulp (abbreviated as LUKP) was treated under the condition of 10%, and the transition metal ion measurement results are shown in Table 1. The removal rate of the remaining transition metal ions copper and manganese is clearly higher in the A stage than in the a stage.

  Therefore, in spite of the first ozone stage or chlorine dioxide stage of the multi-stage chlorine-free bleaching sequence, it is recommended that the transition metal ion content of the finished bleached pulp is further reduced and the thermal fading is improved when the stage A is performed first. The

  According to the latest ASTM (American Society for Testing and Materials) recommended method for thermal fading, thermal fading is performed at a temperature of 100 ° C. and a residence time of 5 days, or at a temperature of 90 ° C. and a residence time of 14 days, and a glass tube. The results are equivalent to natural fading (ASTM's Paper Aging Research Program, 2002, ASTM International, PA), but there is no standard test method for thermal fading until this method exists. The thermal fading test was carried out at a temperature of 105 ° C. and a residence time of 12 hours.

Whiteness difference (ΔR ₄₅₇ ) or PC (Post color) value before and after fading of paper and pulp is a commonly known index of fading, but strength and whiteness are important quality factors in paper and color. It is also an important quality factor. Furthermore, bleached pulp and paper have a yellowish appearance when subjected to thermal fading, and chemicals at the first stage of the bleaching sequence (eg: chlorine dioxide, ozone, chlorine, etc.) (For example: chlorine dioxide, hydrogen peroxide, etc.), the final whiteness is the same, but the b ^{* of the} finished bleached pulp may be different. The b ^* difference (Δb ^* ) before and after thermal fading is used, where b ^* is from the International CIE L ^* a ^* b ^* chromaticity system. In the present invention, correlation between the chromaticity b value of b ^* values and Hunter CIE L ^* a ^* b ^* chromaticity system is as follows. The CIE L ^* a ^* b ^* chromaticity and the Hunter Lab chromaticity were measured with a Macbeth Color-eye meter.
CIE system chromaticity b ^* value = Hunter chromaticity b value × 0.9429 + 0.0541
Correlation coefficient R ² = 0.9997
According to this correlation, the b value measured by the Hunter chromaticity system is slightly higher than the b ^* value obtained from the CIE L ^* a ^* b ^* chromaticity system, and the b value of the Hunter chromaticity system is used. In this case, it is found that when Δb before and after thermal fading becomes 2.30 or less, the thermal fading is improved by the same or lower than that of conventional chlorine bleached pulp.

Regardless of the type of lignocellulosic material, kraft pulp cooking method and conditions, unbleached pulp kappa, chlorine-free bleaching sequence, etc. The Δb ^* after thermal bleaching at 105 ° C. for 12 hours in the dryer is 2.20 or less, which is equal to or less than that of conventional chlorine bleached pulp, and the thermal bleaching property is improved. In that case, it is recommended that the residual iron content is 25 ppm or less, the residual copper content is 1 ppm or less, and the residual manganese content is 3 ppm or less.

  In the present invention, pulp is ashed at 550 ° C. in an electric furnace, 5 mg ash is dissolved with 5 ml concentrated hydrochloric acid (35 to 36%), diluted to 100 ml with distilled water, and this solution is used as a solar absorption spectrometer, Solara. Iron, copper and manganese were measured with an M Series AA Spectrometer.

As the chlorine-free bleaching method of the present invention, when ozone (Z stage) is used in the first stage and chlorine dioxide or hydrogen peroxide is used in the last stage, a finished bleached pulp having a whiteness of 82 to 90% is a conventional chlorine bleached pulp ( Compared to Δb ^* value = 2.20), the Δb ^* value is equal to or less than that, and it is found that the thermal fading is improved.

  As the lignocellulosic material used in the present invention, in addition to wood materials (hardwood, conifers, etc.), non-wood materials (for example, kenaf, bagasse, bamboo, firewood, etc.) can also be used. There are conventional conventional or improved kraft cooking methods, but the cooking solution is white liquor (main components: caustic soda and sodium sulfide) or orange liquor (polysulfide liquor) alone, or orange liquor at the first stage of cooking, and after the cooking stage. White liquor may be used.

  In the process of alkaline oxygen delignification and cleaning / selection described in the specification of the present invention, conventionally known equipment and operation methods can be used. Alkaline oxygen delignification may be a conventionally known one-stage system or two-stage system (for example, Dualox, OxyTrac two-stage system, etc.).

  The chlorine-free bleached kraft pulp with improved thermal fading described in the present invention has the following effects.

(1) In the chlorine-free bleached kraft pulp with improved thermal fading produced by the production method of the present invention, the temperature of the hot air dryer is 105 ° C. and the residence time is 12 hours in the range of finished whiteness of 82 to 90%. The difference in chromaticity b ^* values before and after thermal bleaching under the conditions (Δb ^* ) is 2.20 or less, which is equivalent to or less than that of conventional chlorine bleached pulp. It does not occur, and the pulp does not need to be rebleached, the paper quality in the next papermaking process can be protected, and the cost can be reduced.

  (2) In addition, the finished chlorine-free bleached kraft pulp has a residual iron content of 25 ppm or less, a residual copper content of 1 ppm or less, and a residual manganese content of 3 ppm or less. It can be stored in a warehouse, running costs can be reduced, and the amount of natural raw material lignocellulosic substances (for example, hardwood, coniferous and non-wood chips) can be reduced, which has the effect of protecting the global environment. .

In the chlorine-free bleached kraft pulp with a final whiteness of 82-90% described in Japanese Patent Application No. 2003-334443 according to the prior application of the present applicant, heat fading under conditions of a hot air dryer temperature of 105 ° C. and a residence time of 12 hours The difference in the b ^* value (Δb ^* ) of the international CIE L ^* a ^* b ^* chromaticity system before and after the test was 2.20 or less, the same or less than the conventional chlorine bleached pulp, and improved thermal fading In these chlorine-free bleached pulps, it was recommended that the copper number be 2.20 or less and the carboxyl group be 22.0 meq / 100 g BD pulp or less. In the present invention, the residual transition metal of these pulps Since the influence on the thermal fading property of ions was found, some examples and comparative examples are shown below. The E stage in these examples and comparative examples includes the Eo stage, the Ep stage and the Eop stage (oxygen alone, hydrogen peroxide alone or a neutral or alkaline extraction stage including the joint use of oxygen and hydrogen peroxide). It is included.

  The chemical addition rate in each of the following bleaching sequences is% by weight per dry pulp, and a horizontal bar (-) between each stage indicates a washing stage. Other than the high concentration Z stage, the other bleaching stages were carried out in polyethylene bags.

(Examples 1 to 3 and Reference Example 1)
Bleaching temperature, pulp concentration, reaction time for the sequence of (AZE) -D ₁ -D ₂ , (AZE) -DP, (AZE) -PD, AD ₀ -ED ₁ -D ₂ Details of the chemical addition rate are as follows.

Example 1
(AZE) -D ₁ -D ₂ bleaching (1) Pulp used: Sampling of mixed eucalyptus hardwood kraft pulp (abbreviated as LUKP) with an alkaline oxygen delignification and washing of the actual production line with a copper number of 11.7 Stored at 5 ° C. while using.

(2) Stage A (acid treatment stage): LUKP having a pulp concentration of 10.5% was prepared, and then the pH of the pulp slurry was adjusted to about 3.1 with a 4N sulfuric acid solution to adjust the final pulp concentration to 10.0%. . (Sulfuric acid addition rate: 1.10%, pH: 3.10) This pulp was treated in a constant temperature bath at 90 ° C. for 150 minutes, and then washed to a pulp concentration of 38.7% in a centrifuge without washing. Dehydrated. (Copper number: 8.0)
(3) Stage Z (high-concentration ozone treatment stage): The pulp with a concentration of 38.7% after stage A was placed in a 2 liter vertical flask at 60 g BD / batch and attached to a laboratory evaporator. 10% ozone gas was injected into the flask to react ozone and pulp (ozone addition rate: 0.46%). The high-concentration ozone treatment was performed in a constant temperature bath at 50 ° C. Ozone bleached pulp was collected and mixed without washing.

  (4) Stage E (weakly alkaline extraction stage): 0.9% of caustic soda was added to the 38.7% concentrated pulp after the Z stage, and the pulp was held at 70% for 30 minutes at a pulp concentration of 14%. The extracted pulp was washed with tap water (2 liters / 200 g absolute dry pulp) and dehydrated and concentrated to a pulp concentration of 18%.

(5) D _1st stage (chlorine dioxide bleaching stage): 0.2% of chlorine dioxide was added to the pulp after stage E, and chlorine dioxide treatment was performed at a pulp concentration of 14% and 75 ° C. for 117 minutes. The pulp filtrate pH after the reaction was 5.5. The obtained pulp was washed with tap water (2 liters / 200 g absolute dry pulp) and dehydrated to a pulp concentration of 18%.

(6) D ₂ stage (chlorine dioxide bleaching stage): D ₂ stage pulp concentration, treatment temperature, reaction time, filtrate pH after reaction, and pulp washing method are all the same as D ₁ stage, but chlorine dioxide Only the addition rate of 0.1% was low. As a result, a finished pulp by the (AZE) -D ₁ -D ₂ sequence was obtained.

(Example 2)
(AZE) bleaching -D-P sequence (1) (AZE) stage: Using been (AZE) -D ₁ -D ₂ sequence (AZE) stage pulp described above.

(2) D stage (chlorine dioxide bleaching stage): Pulp after D ₁ stage of the above (AZE) -D ₁ -D ₂ sequence was used.

  (3) P stage (hydrogen peroxide bleaching stage): 0.15% of hydrogen peroxide was added to the pulp after stage D, and after performing hydrogen peroxide treatment at a pulp concentration of 14% and 70 ° C. for 120 minutes, tap water The pulp was washed and dehydrated with water (2 liters / 200 g absolute dry pulp) to obtain a finished bleached pulp. The pulp filtrate pH after stage P was 9.5.

Example 3
(AZE) bleaching with -P-D sequence (1) (AZE) stage: Using been (AZE) -D ₁ -D ₂ sequence (AZE) stage pulp described above.

  (2) P stage: It implemented on the conditions of P stage of said (AZE) -DP sequence, and the hydrogen peroxide addition rate was 0.55%.

(3) D stage: The conditions of this bleaching stage are the same as those of D ₂ stage of the (Aze) -D ₁ -D ₂ sequence, but the chlorine dioxide addition rate is 0.18%, and the obtained pulp Was washed and dehydrated with tap water to obtain a finished bleached pulp.

(Reference Example 1)
Bleaching with _{A-D 0 -E-D 1} -D 2 Sequence (1) A stage: take the pulp after A stage of the described (AZE) -D ₁ -D ₂ sequence, were washed with tap water, Dehydrated to a pulp concentration of 18%.

(2) D ₀ stage: 0.366% chlorine dioxide is added to the pulp after stage A, the pH is adjusted to 3.5 with 4% pulp concentration, 4N dilute sulfuric acid, temperature 58 ° C., time 25 minutes After the treatment, the pulp was washed and dehydrated with tap water (2 liters / 200 g absolutely dry pulp).

(3) E stage (alkali extraction stage): 0.72% of caustic soda was added to the pulp after D ₀ stage, and the pulp concentration was 14% and kept at 70 ° C. for 66 minutes. The extracted pulp was washed with tap water and concentrated to a pulp concentration of 18%.

(4) D ₁ stage and D ₂ stage (chlorine dioxide bleaching stage): Pulp after E stage is the same as the conditions of D ₁ stage and D ₂ stage of (AZE) -D ₁ -D ₂ sequence described above was carried out, were each chlorine dioxide addition rate 0.06% (D ₁ stage) and 0.15% (D ₂ stages). The obtained pulp was washed and dehydrated with tap water to obtain a finished bleached pulp.

(Examples 4 to 6)
The bleaching temperature, pulp concentration, and reaction time of the sequences (aZE) -D ₁ -D ₂ , (aZE) -DP, and (aZE) -P-D are the same as those in Examples 1 to 3 above, and The processing stage a is described in paragraph [0013] above. Table 2 shows the chemical addition ratios of Examples 1 to 6.

(Reference Example 2)
A-D ₀ bleaching -E-D ₁ -P Sequence (1) A stage (acid stage): Creates a pulp concentration of 10.5 percent LUKP, then the pH of the pulp slurry with 4N sulfuric acid solution of about 3. 1 and the final pulp concentration was adjusted to 10.0% (sulfuric acid addition rate: 1.10%, pH: 3.2). This pulp was treated in a constant temperature bath at 90 ° C. for a reaction time of 150 minutes, washed with tap water, and dehydrated to a pulp concentration of 18%. (Kappa number: 8.1; original pulp copper number: 11.3).

(2) D ₀ stage: 0.335% of chlorine dioxide is added to the pulp after stage A, the pH is adjusted to 3.5 with a pulp concentration of 4.5% and 4N dilute sulfuric acid, and the temperature is 58 ° C. for 25 hours. After a minute treatment, the pulp was washed and dehydrated with tap water (2 liters / 200 g absolute dry pulp).

(3) E stage: caustic soda was added 0.741% on D ₀ stage after concentration of 18% of the pulp, pulp consistency 14%, was held 65 minutes at 70 ° C.. The extracted pulp was washed with tap water (2 liters / 200 g absolute dry pulp) and dehydrated and concentrated to a pulp concentration of 18%.

(4) D stage ₁ : 0.07% of chlorine dioxide was added to the pulp after stage E, and chlorine dioxide treatment was performed at a pulp concentration of 14% and 75 ° C. for 65 minutes. The pulp filtrate pH after the reaction was 5.5. The obtained pulp was washed with tap water (2 liters / 200 g absolute dry pulp) and dehydrated to a pulp concentration of 18%.

(5) P stage: 0.25% of hydrogen peroxide is added to the pulp after the _first stage D, adjusted with caustic soda so that the pulp concentration is 14% and the initial pH is 10.6, and then 60 ° C. in a thermostatic bath at 75 ° C. Hydrogen peroxide treatment was performed for a minute. The filtrate pH after the reaction was 9.3, and the treated pulp was washed with tap water and concentrated. Thus, to obtain a finished pulp by _{A-D 0 -E-D 1} -P sequence.

(Comparative Example 1)
Conventional chlorine bleaching C stage by C-E-H-D sequence, E stage, D stage of the processing temperature, reaction time and pulp consistency, D of _{A-D 0 -E-D 1} -P sequence of the reference example 2 _Although it is equivalent to ₀ stage (equivalent to C stage), E stage, and D ₁ stage (equivalent to D stage), the chemical addition rate is as follows. Chlorine in stage C: 1.13%; Caustic soda in stage E: 0.865%; Chlorine dioxide in stage D: 0.35%. For stage H, hypo treatment was performed under the conditions of treatment temperature: 75 ° C .; reaction time: 35 minutes; pulp concentration: 14%; hypo addition rate: 0.25%. The pulp after each treatment stage was washed with tap water.

(Comparative Example 2)
Bleaching with aD ₀ -ED ₁ -D ₂ Sequence The a treatment stage is described in paragraph [0013] above. The pulp concentration, reaction temperature, and reaction time of the D ₀ stage, E stage, D ₁ stage, and D ₂ stage are the same as the conditions of the corresponding stages of the A-D ₀ -ED-D ₁ -D ₂ sequence of Reference Example 1 described above. Similarly, the chemical addition rate is as follows. Chlorine dioxide in D ₀ stage: 0.961%; sodium hydroxide in E stage: 1.52%: chlorine dioxide by D ₁ stage: 0.06%; chlorine dioxide in D ₂ stage: 0.15%. The pulp after each treatment stage was washed and concentrated with tap water.

(Comparative Example 3)
Bleaching A processing stage by _{A-D 0 -E-P-} D 1 sequence described in the preceding paragraph "0013". The pulp concentration, reaction temperature, and reaction time of the other D ₀ stages, E stages, P stages, and D ₁ stages are the same as the conditions of the corresponding stages of the A-D ₀ -ED ₁ -P sequence in Example 8 above. The chemical addition rate is as follows. Chlorine dioxide in D ₀ stage: 0.335%; sodium hydroxide in E stage: 0.741% hydrogen peroxide in P stages: 0.45%; chlorine dioxide by D ₁ stage: 0.238%. The initial pH and final pH of the P stage were 10.8 and 10.4, respectively. The pulp after each treatment stage was washed and concentrated with tap water.

<Method for measuring pulp quality>
Each finished pulp was subjected to whiteness measurement and a thermal fading test after a handsheet was prepared by the method described in “JIS P8222” and “JIS P8223”.

  Whiteness: Hunter whiteness was determined with a Hunter whiteness meter based on the method described in “JIS P8123”.

  Thermal fading test: The handsheet for whiteness measurement was subjected to a thermal fading test under the conditions of a temperature of 105 ° C. and a treatment time of 12 hours in a commercial hot air dryer in the vertical direction.

In the present invention, in the chlorine-free bleached kraft pulp having a final whiteness of 82 to 90%, Examples 1 to 6 have a hot air dryer temperature of 105 ° C. and a residence time of 12 hours as compared with the conventional chlorine bleached pulp of Comparative Example 1. The Δb ^* values before and after thermal bleaching under the conditions were the same or lower, and the results of improving the thermal fading of these chlorine-free bleached pulps were obtained. It was also found that the residual iron content was 25 ppm or less, the residual copper content was 1 ppm or less, and the residual manganese content was 3 ppm or less.

Claims (4)

The pulp component obtained by digesting lignocellulosic material by kraft cooking method is subjected to alkaline oxygen delignification and washing selection sequentially, and the first of the bleaching sequence is an acid treatment stage without a washing stage between the following treatment stages, The bleaching pulp is a bleached pulp that is bleached by a chlorine-free bleaching method that does not contain elemental chlorine and hypochlorous acid. The chromaticity b ^* value of the international CIE system before and after the bleaching pulp has a final hunter whiteness of 82 to 90% and is subjected to thermal fading under conditions of a hot air dryer temperature of 105 ° C. and a residence time of 12 hours. A method for producing chlorine-free bleached kraft pulp with improved thermal fading, wherein the difference (Δb ^* ) is 2.20 or less and the residual iron content is 25 ppm or less. 2. The method for producing chlorine-free bleached kraft pulp with improved thermal fading according to claim 1, wherein the bleached pulp has a residual copper content of 1 ppm or less. The method for producing chlorine-free bleached kraft pulp with improved thermal fading according to claim 1 or 2, wherein the bleached pulp has a residual manganese content of 3 ppm or less.   The method for producing chlorine-free bleached kraft pulp with improved thermal fading according to any one of claims 1 to 3, wherein the lignocellulosic material comprises hardwood chips, softwood chips or non-wood.