JP2003278088A - Process for producing ECF bleached pulp - Google Patents

Abstract

(57) [Problem] To improve the whiteness of unbleached pulp without reducing the yield of unbleached pulp, and to reduce the total amount of bleaching chemicals used in the ECF bleaching method mainly using chlorine dioxide. Provided is a method for producing bleached pulp. The method for producing bleached ECF pulp comprises the following steps (A) to (C). (A) In the kraft cooking process using a lignocellulosic substance of hardwood, the effective alkali concentration in the black liquor at the end of the cooking is 5 g / liter or more, and the H factor is 700.
Kraft cooking is performed as follows, and the unbleached pulp after the cooking is adjusted to have a kappa number of 15 to 18 and a whiteness of 25% or more. (B) In the oxygen bleaching step, an oxygen bleaching treatment is performed. (C) In a multi-stage bleaching process in an ECF bleaching process mainly comprising chlorine dioxide, the whiteness of the ECF bleached pulp after bleaching is 84- The amount of the total bleaching chemicals required to reach 88%, expressed as effective chlorine, is 30 to 40 kg / p.
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Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an ECF bleached pulp which is subjected to a bleaching treatment by an ECF bleaching method mainly using chlorine dioxide. For more information,
The present invention relates to a method for producing an ECF bleached pulp which aims to improve the whiteness of the unbleached pulp without reducing the yield of the unbleached pulp, and to reduce the total amount of bleaching chemicals used in the ECF bleaching method mainly comprising chlorine dioxide. Things. 2. Description of the Related Art Among the methods for producing pulp for papermaking, the method for producing pulp by the kraft method has a high unbleached pulp yield and strength, and is a method for recovering calories from cooking waste liquor and alkali content as cooking chemical. Established Moreover, it is applicable to a wide range of lignocellulosic materials such as raw wood, and has advantages such as being able to speed up by continuous digestion. [0003] In the cooking step in the conventional kraft pulp manufacturing method, a lignocellulosic material such as selected wood chips is transported from a silo to the top of a digester tower by a belt conveyor, and a predetermined amount is supplied to the digester from the top. The cooking liquor is
The black liquor, which is the waste liquor, is burned, and white liquor, which is a mixed solution of sodium hydroxide and sodium sulfide produced by causticization, is used. [0004] The air present in the digester facilitates the disintegration of the cellulose of the lignocellulosic material during kraft cooking,
In addition, the air in the lignocellulose is desirably removed as much as possible, because the air impedes the digestion liquid. [0005] Although it differs depending on the type of lignocellulosic substance and pulp, the cooking chemical is fed to the digester with an appropriate chemical concentration and an alkali equivalent with respect to the absolute dryness of the lignocellulose substance. A lignocellulosic material such as wood chips and an alkaline cooking liquor are treated in a digester while controlling the cooking conditions such as temperature, pressure and time to remove the inter-fiber binding material such as the wood chips. Dissolve in the black liquor to leave pulp as a residue. [0007] After the digestion, the pulp and black liquor are blown into a blow tank. The pulp and black liquor in the blow tank are diluted, large unpulverized matter is removed by a knotter, sent to a subsequent washing step, and separated into black liquor and pulp. The pulp is further sent to a fine selection step for removing undisintegrated bound fibers and foreign matters other than fibers, an oxygen bleaching step, and further to a multi-stage bleaching step. The black liquor is burned in a recovery boiler through a concentration step, causticized and regenerated into white liquor. [0008] The digester of the kraft digestion method includes a batch type and a continuous type. The batch method is a method in which a kettle is filled with the lignocellulose substance, and then an appropriate amount of white liquor is charged and digested. At the end of the digestion, it is an intermittent type in which the entire amount of pulp and black liquor is discharged from the bottom of the pot and the next digestion is performed. The continuous method is a method in which the lignocellulose and the white liquor are continuously fed from the top of a cylindrical kettle, while heating and digesting at a predetermined temperature and time while moving in the kettle, and pulping from the other end. And cooking waste liquid are continuously blown. [0009] From the conditions of temperature and time, the digestion process is mainly divided into three periods: a heating period, a cooking temperature maintenance period, and a blowing period. The heating phase is very important in penetrating the cooking liquor into the lignocellulosic material. Since the penetration of the alkali solution into the cellulose material is easier than the acid solution,
In conventional kraft cooking, the temperature is quickly raised to the cooking temperature. However, when the temperature is raised to a very high temperature, unpulped matter increases due to poor permeation. The cooking temperature in kraft cooking is generally in the range of 160-180 ° C., and the total cooking time is 2-3 hours for unbleached pulp. Conventionally, as an index in the digestion process,
The "H factor" in which the effect of temperature and reaction time on cooking is used as one variable is generally used. For example, 1
The reaction rate at 00 ° C. is 1, and the H-factor becomes 1 when reacted at this temperature for 1 hour. The higher the temperature, the higher the relative reaction rate, and the area surrounded by the reaction time and relative reaction rate curves represents the H factor. According to this, when the time and the temperature at which the H factor becomes the same are combined, pulp of almost the same quality can be obtained. When the effective alkali concentration in the black liquor during the cooking temperature maintenance period is low, if the effective alkali concentration in the black liquor at the later stage of the cooking is consumed, the lignocellulose component is condensed, and the conventional lignocellulose component is condensed. It is a well-known fact that the kappa number, a general indicator of the degree of digestion, rises, resulting in high consumption of bleaching chemicals. It is natural that the temperature and time during the cooking temperature maintenance period greatly affect the pulp yield and the viscosity, but in the range of 150 to 170 ° C., there is almost no effect on the pulp strength. If the temperature exceeds 180 ° C., there is a risk of dissolution and elution of carbohydrates, pulp yield, viscosity,
The pulp strength is reduced. In addition, when the temperature becomes high, heating and infiltration become non-uniform, unpulverized matter increases, and pulp yield decreases. In the method for producing kraft pulp, a high-strength pulp is obtained without disintegrating and releasing cellulose and hemicellulose as much as possible, and the kappa number is reduced to reduce the amount of bleaching chemicals used in the subsequent bleaching step. This is very important. Various improved methods have been proposed for the kraft cooking method so far, such as a super-batch method for a batch method, an MCC method, an ITC method, and a Lo-solid method for a continuous method. These improvement methods increase the pulp yield and improve the pulp quality, but have major problems such as the necessity of equipment modification. On the other hand, improvements in cooking liquor, delignification aids, and the like for improving pulp yield have been proposed, such as polysulfide cooking and addition of quinone compounds. The polysulfide cooking method is a method in which white liquor is oxidized to form an oxidized white liquor obtained by converting sodium sulfide to polysulfide sulfur, and this is used as a cooking liquor. Since conventional kraft pulp is difficult to bleach, in the bleaching step, a method of performing a multi-stage bleaching step with chlorine, alkali, hypochlorite, chlorine dioxide or the like has been adopted. In addition, oxygen bleaching for the purpose of improving the pulp yield and reducing the bleaching load due to the reduction of the kappa number of the unbleached pulp has been incorporated in the preceding step of the bleaching step. In such a multi-stage bleaching process, operations such as dilution, addition and mixing of a chemical solution, heating, and washing with a filter need to be repeated many times, and there are many problems in terms of energy and environment. The merit of reducing the amount of addition and shortening the sequence is very large. In recent years, there has been a movement toward many regulations on organic chlorine compounds generated by chlorine bleaching of chemical pulp from the viewpoint of environmental measures. From these, in the multistage bleaching method of kraft pulp, instead of the conventional bleaching method using elemental chlorine, an ECF bleaching method using no elemental chlorine, or a TCF method using no chlorine-based chemical is used.
A bleaching method and the like have been proposed, and at present, the ECF bleaching method has been widely used, and a great effect has been brought about in terms of environment. However, in the ECF bleaching method, oxidizing chemicals such as chlorine dioxide, hydrogen peroxide, and ozone are mainly used as bleaching chemicals.
In the bleaching method, the cost of the bleaching chemical is generally higher than that of the conventional bleaching method using elemental chlorine. In addition, for example, chlorine dioxide is mainly produced on its own, and if a large amount of chlorine dioxide is used, a larger capacity of a carbon dioxide production facility is required to satisfy the amount. SUMMARY OF THE INVENTION The present invention relates to an ECF for performing bleaching by an ECF bleaching method mainly comprising chlorine dioxide.
A method for producing bleached pulp that aims to improve the whiteness of unbleached pulp without reducing the yield of unbleached pulp, and to reduce the total amount of bleaching chemicals used in the ECF bleaching method mainly based on chlorine dioxide. It is. The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, the relationship between the amount of bleaching chemical used in the ECF bleaching method mainly containing chlorine dioxide and the amount of the bleaching chemical used has been reduced. Compared to the commonly used control index, kappa number of pulp at the bleaching process entrance (here, pulp after oxygen bleaching), the whiteness shows a higher correlation. I found it. That is, if the whiteness of the pulp at the bleaching process inlet is high, the amount of the bleaching chemical used is reduced.
Furthermore, in the kraft cooking method for lignocellulosic material in hardwood, the copper value obtained by performing kraft cooking with the effective alkali concentration in the black liquor at the end of the cooking at 5 g / liter or more and the H factor at 700 or less at 15-18. The unbleached pulp after the digestion of the unbleached pulp having a kappa number of 15 to 18 obtained by a kraft cooking method in which the effective alkali concentration in the black liquor is less than 5 g / liter and the H factor is greater than 700 at the end of the cooking In comparison, it was found that the whiteness was higher at the same kappa number. The relationship between the two whitenesses is that the oxygen bleaching process, which is the next process, is carried out under the same conditions, and that the bleaching process inlet pulp having a kappa number of 9 to 11 after the oxygen bleaching process is maintained. 5g / liter or more effective alkali concentration in black liquor at the time, H factor 7
It was found that the pulp subjected to the kraft digestion with the bleeding rate of not more than 00 had higher whiteness at the same kappa number as the pulp at the bleaching inlet after the oxygen bleaching treatment, and completed the present invention. That is, the method for producing an ECF bleached pulp of the present invention comprises the following steps (A) to (C). (A) In the kraft digestion process using lignocellulosic material of hardwood, the effective alkali concentration in the black liquor at the end of the digestion is 5 g / liter or more, and the H factor is 700.
Kraft cooking is performed as follows, and a kappa number of the unbleached pulp after the cooking is adjusted to 15 to 18 and whiteness to 25% or more (B) In the oxygen bleaching step, an oxygen bleaching process is performed, and the kraft after the bleaching process is performed. (C) In a multistage bleaching process in an ECF bleaching process mainly comprising chlorine dioxide, the bleaching process of the ECF bleached pulp has a whiteness of 84 to 88 in which the kappa number of the pulp is 9 to 11 and the whiteness is 45% or more. % Of the total amount of bleaching chemicals required for conversion to effective chlorine and expressed as 30-40 kg / p
DESCRIPTION OF THE PREFERRED EMBODIMENTS The method for producing ECF bleached pulp of the present invention has the effect of reducing the amount of bleaching chemicals used in the ECF bleaching step, and does not affect pulp quality. Things. Hereinafter, the present invention will be described in detail. In the present invention, the lignocellulosic material used as a pulp raw material is mainly eucalyptus wood,
The present invention is also applied to hardwood materials generally used for kraft pulp other than eucalyptus wood, and is not limited to eucalyptus wood. The kraft cooking of the lignocellulosic material of the present invention uses an alkaline cooking liquor containing sodium hydroxide and sodium sulfide. In the present invention,
Use of the polysulfide cooking liquor, addition of a delignification aid, and cooking method are not limited. The concentration of sodium hydroxide in the kraft cooking step of the lignocellulosic material of the present invention is Na ₂ O
50 to 150 g / l in conversion, preferably 90 to 1
20 g / liter and sodium sulfide concentration of Na ₂
2 to 40 g / liter, preferably 10 to 6 in terms of O
0 g / liter. In the cooking step, the cooking time during the heating period is as follows:
Although it depends on the type of lignocellulosic material, the cooking method and the cooking temperature, it is usually 20 to 90 minutes, preferably 30 to 6 minutes.
0 minutes. The cooking temperature in the cooking temperature maintenance period after the heating period is 140 to 180 ° C, preferably 150 to 170 ° C.
° C, and the holding time is 40 to 200 minutes, preferably 50 to 150 minutes. In the present invention, setting the H factor at the end of the cooking to 700 or less does not change the cooking time in the heating period and the cooking temperature maintaining period, and lowers the H factor in the cooking temperature maintaining period to 700 or less. It is to adjust to. In the present invention, the same kappa number as in the case where the effective alkali concentration at the end of the digestion is 5 g / l or more, the H factor is 700 or less, and the kappa number of the unbleached pulp after the digestion is 15 to 18 is used. And the whiteness of the bleached inlet pulp after oxygen bleaching can be improved, and the amount of bleaching chemicals added in the ECF bleaching process mainly containing chlorine dioxide can be reduced. . In the practice of the present invention, the sequence of the ECF bleaching process mainly using chlorine dioxide is as follows:
For example, D (E / O) PD, D (E / O) DP, DED
A combination of a chlorine dioxide treatment stage (D) and an alkali extraction treatment stage (E) or a treatment stage using oxygen for the alkali extraction stage (E / O) as in a sequence such as ED, and a hydrogen peroxide stage (P) , But multi-stage bleaching of 3 to 5 stages, but there is no particular limitation except for the use of chlorine dioxide. The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples. In the kraft cooking process using lignocellulosic material of hardwood, the effective alkali concentration at the end of the cooking is 5 g / liter or more, and the H factor is 70.
The unbleached pulp having a kappa number of 15 to 18 and a whiteness of 25% or more after the digestion is subjected to kraft cooking with the kraft pulp having been subjected to the oxygen bleaching treatment in the next oxygen bleaching step. The kappa number is 9-11 and the whiteness is 45% or more.
A table experiment was conducted to prove that the amount of bleaching chemical used in the ECF bleaching process mainly composed of chlorine dioxide required to be 4 to 88% was 30 to 40 kg / pt in terms of the effective chlorine addition. Examples 1 to 3 and Comparative Examples 1 to 3 Wood chips used for kraft cooking were tested after removing the irregularly shaped chips and dust from the eucalyptus chips used in the factory, and after sufficiently air-drying to determine the water content. The water content was calculated from the weight after air drying and the weight after drying all day and night with a dryer at 115 ° C. As the white liquor used for the kraft cooking, a commercially available reagent was prepared, and the following numerical values were used for the concentration of each component. NaOH: 79.3 g / l terms of Na ₂ O Na ₂ S: 38.2g / l terms of Na ₂ O Na ₂ CO _3: 19.5g / l terms of Na ₂ O effective alkali: 97.4 g / l terms of Na ₂ O For the kraft cooking, a fixed pot with a capacity of 4 liters capable of controlling the heating time, the cooking temperature and the cooking time was used. The wood chips having an absolute dry weight of 500 g and the white liquor in a predetermined effective alkali addition amount were added to a fixed pot, and kraft digestion was performed. The H factor was calculated from the heating time, cooking temperature, and cooking time. After the completion of the kraft cooking, degas the kettle and
Return the internal pressure to the vapor pressure, take out the entire contents, collect black liquor from it, analyze the effective alkali concentration in the black liquor,
This was defined as the effective alkali concentration at the end of cooking. The digested pulp was washed with water to remove the adhering cooking waste liquid. Next, the uncooked chips were separated by a flat screen having a slit width of 0.2 mm, and the pulp content and the absolute dry weight of the uncooked chips were weighed to determine the unbleached pulp yield. In addition, using a part of the pulp, the kappa number and the unbleached whiteness, which are indicators of the degree of cooking, were measured. Next, the pulp was subjected to oxygen bleaching. After dehydration of the pulp with a centrifugal separator, the water content was determined, and a sample having an absolute dry weight of 100 g was used as a sample. The alkali used for oxygen bleaching was sodium hydroxide. Sodium hydroxide was added as a solution in which solid sodium hydroxide was dissolved in pure water, and pure water was used as the dilution water when adjusting the pulp concentration. The processing conditions for oxygen bleaching were adjusted so that the kappa number after oxygen bleaching became 9.5 to 10.0 in the range of sodium hydroxide addition ratio of 0.8% by mass to 1.2% by mass, and pulp concentration 10% by mass, processing temperature 110 ° C, processing time 60
The treatment was performed in minutes. First, the sample, a sodium hydroxide solution and pure water corresponding to the above conditions were added to a polyvinyl bag, and the mixture was rubbed and mixed well. Next, the sample was placed in a stainless steel net basket, and was placed in a pot in which steam and oxygen gas could be sealed and the temperature in the pot could be controlled by a heater. Thereafter, oxygen gas was sealed until the internal pressure of the kettle became 6 kg / cm ² . After the oxygen bleaching, the pulp was washed with water, and a part of the pulp was used to measure the Kappa number and the unbleached whiteness after the oxygen bleaching. In the multi-stage bleaching after oxygen bleaching, the first stage was a chlorine dioxide stage, the second stage was an alkali extraction stage, the third stage was a hydrogen peroxide stage, and the fourth stage was a chlorine dioxide stage, ie, DEPD. A column of ECF bleaching was performed. In the multi-stage bleaching, the bleaching chemical used was an actual product. Pure water was used as a dilution water for obtaining a predetermined pulp concentration. The mixing of the bleaching chemical and the pulp sample was carried out in a polyvinyl bag and rubbed well. Thereafter, the mixture of the pulp sample and the bleaching chemical was submerged in a thermostatic water bath adjusted to the processing temperature, and allowed to stand for a predetermined processing time. After completion of each bleaching stage, the contents in the plastic bag were thoroughly washed with pure water, and after the pulp for each measurement was separated, the next bleaching stage was performed. The processing conditions of the chlorine dioxide stage, which is the first stage of the multistage bleaching, are as follows. First, the initial pH is set to 4.9 by adding dilute sulfuric acid.
Adjusted to the range of ~ 5.1, chlorine dioxide addition rate is 0.6 ~
The whiteness after the first stage is set to be about 68 to 72%, preferably 70% in a range of 1.3% by mass (based on absolute dry pulp), and the pulp concentration is 10% by mass and the processing temperature is 55%.
The treatment was performed at 30 ° C. for a treatment time of 30 minutes. The processing conditions of the second stage, the alkali extraction stage, were such that the sodium hydroxide addition rate was 0.1%.
The addition rate at which sufficient extraction was performed was set in the range of 7 to 1.2% by mass (based on the absolute dry pulp mass), and the treatment was performed at a pulp concentration of 10% by mass, a treatment temperature of 55 ° C, and a treatment time of 75 minutes.
The processing conditions in the third hydrogen peroxide stage were as follows: a hydrogen peroxide addition rate of 0.4% by mass (based on absolute dry pulp), a pulp concentration of 10% by mass, a treatment temperature of 75 ° C, and a treatment time of 75 minutes. The processing conditions of the fourth chlorine dioxide stage were such that the chlorine dioxide addition rate was 86.2% to 86.5% final whiteness, pulp concentration 10% by mass, processing temperature 75 ° C., and processing time 140 minutes. And The amount of available chlorine used in all bleaching stages is
The sum of the value obtained by multiplying the sum of the chlorine dioxide addition ratios of the third and fourth stages by a coefficient of 2.63 and the value obtained by multiplying the hydrogen peroxide addition ratio of the third stage by a coefficient of 2.09 was used. [Table 1] [Table 2] Examples 1 to 3 in Tables 1 and 2 above correspond to each other, and have an effective alkali concentration of 5 to 8 g / l and an H factor of 600 to 680 at the end of the digestion.
In this example, the effective alkali addition rate and the cooking temperature were set so that the kappa number after the cooking was in the range of 16.0 to 17.0 in the range described above, and the cooking was performed. Comparative Examples 1 to 3 in Tables 1 and 2 correspond to each other, and Comparative Example 1 has an effective alkali concentration of less than 5 g / l at the end of the digestion and an H factor of 70.
When 0 or more, Comparative Example 2 has an effective alkali concentration at the end of the digestion of 5 g / L or more, but when the H factor is more than 700, Comparative Example 3 has an effective alkali concentration at the end of the digestion of less than 5 g / L, H factor 70
It is the case of 0 or less. In each of the examples, the effective alkali addition rate and the cooking temperature were set so that the kappa number after the cooking was in the range of 16.0 to 17.0 as in Examples 1 to 3, and the cooking was performed. As a result, from Table 1, Examples 1 to 3 have whiteness approximately 2 to 4 points higher than those of Comparative Examples 1 to 3, though the Kappa number after digestion is not much different. You can see that. Similarly, the relationship between the whiteness after oxygen bleaching and the Kappa number is found to be about 2 to 4 points higher in Examples 1 to 3 than in Comparative Examples 1 to 3. In addition, the unbleached pulp yield after digestion was as in Examples 1 to
3 and Comparative Examples 1 to 3 were almost equivalent. still,
Among Examples 1 to 3, the higher the effective alkali concentration at the end of digestion and the lower the H factor, the higher the whiteness after digestion and after oxygen bleaching. From Table 2, Examples 1 to 3 have an effective chlorine usage of about 8 in ECF bleaching as compared with Comparative Examples 1 to 3.
-12 kg / pt lower. In spite of the fact that the kappa numbers after oxygen bleaching were almost the same,
It is apparent that the higher whiteness after oxygen bleaching had an effect. According to the present invention, the ECF mainly containing chlorine dioxide is used.
The method for producing ECF bleached pulp in which bleaching is performed by a bleaching method does not reduce the yield of unbleached pulp, improves the whiteness of unbleached pulp, and uses all bleaching chemicals in the ECF bleaching method mainly using chlorine dioxide. The amount can be greatly reduced.

Claims (1)

Claims 1. A method for producing an ECF bleached pulp,
A method for producing bleached ECF pulp, comprising the following steps (A) to (C). (A) In the kraft cooking process using a lignocellulosic substance of hardwood, the effective alkali concentration in the black liquor at the end of the cooking is 5 g / liter or more, and the H factor is 700.
Kraft cooking is performed as follows, and the unbleached pulp after the cooking is adjusted to have a kappa number of 15 to 18 and a whiteness of 25% or more. (B) In the oxygen bleaching step, an oxygen bleaching process is performed. (C) In a multi-stage bleaching process in an ECF bleaching process mainly comprising chlorine dioxide in which the pulp has a kappa number of 9 to 11 and a whiteness of 45% or more, the whiteness of the bleached ECF bleached pulp is 84 to 88. % Of the total amount of the bleaching chemicals required for conversion to effective chlorine and expressed as 30 to 40 kg / p.
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