JP2002227083A - Method for bleaching cellulosic fiber material pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for bleaching cellulosic fiber material pulp, capable of decreasing the amount of a chlorine-based chemical to be used, more efficient than conventional breaching sequences and giving improved bleaching capability. SOLUTION: This method for bleaching cellulosic fiber material pulp in producing a chemical pulp is characterized by improving bleaching capability in the latter steps by washing a loaded digested pulp or oxygen-bleached pulp, adding diluting water when continuously fed into a bleaching line and performing heating extraction treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for bleaching pulp of cellulosic fibrous material in the production of chemical pulp, wherein the introduced pulp is continuously fed to a bleaching line and subjected to a heat extraction process. The present invention relates to a method for bleaching pulp of a cellulosic fiber material.

[0002]

2. Description of the Related Art Chlorine dioxide is an important reagent for pulp bleaching due to its high oxidizing power and reaction selectivity. However, as the bleaching wastewater of chlorine dioxide contains chlorine ions, there is increasing interest in methods of reducing the amount of chlorine dioxide used, while concerns are being placed on the environmental impact of wastewater from pulp and paper mills.

[0003] From such a flow, ozone bleaching, which is an oxygen-based chemical, has been receiving particular attention. However, while ozone is a very good reagent in the recovery of bleaching effluent, the amount used for bleaching is limited in order to maintain pulp strength because of low reaction selectivity. Therefore, chlorine dioxide is also commonly used in a sequence including ozone bleaching, and at present, the recovery of bleaching wastewater is restricted in the entire sequence.

Against this background, there are many patent applications and reports on acid treatment, alkali extraction treatment and enzyme treatment as known pretreatment methods capable of reducing chlorine-based chemicals such as chlorine dioxide. . Further, since the wastewater of the pretreatment does not contain harmful substances such as chlorine ions, it is easy to reuse the washing water, and it can be collected by AC washing in the unbleached washing step. This indicates that introducing a pretreatment and reducing the chlorine-based chemicals reduces the toxicity level of the overall bleaching process.

A method of reducing bleaching chemicals by alkali extraction, which is one of the pretreatment methods (Norn Liebergott, 1994 Pul
Ping Conference, 357) is a known fact. But,
In the alkali extraction stage, a method is generally used in which pulp whose pH has been lowered by digestion or washing after the completion of oxygen bleaching is treated with a high pH by newly adding an alkali.
However, in such a method, the hemicellulose re-adsorbed in the latter stage of the digestion is dissolved again, and the bleaching yield is reduced, so that it is not a good method from the viewpoint of maintaining biomass resources. Also, the addition of a new alkali affects the bleaching conditions in the step following the pretreatment,
For example, there has been a problem that chlorine dioxide bleaching does not fall within the optimum pH range.

[0006]

SUMMARY OF THE INVENTION It is a first object of the present invention to provide a pulp of cellulosic fibrous material that reduces chlorine-based chemicals and is more efficient than conventional bleaching sequences. It is to provide improved bleachability.

[0007] It is another object of the present invention to provide a more closed bleaching process by reducing the use of chlorine and chlorine dioxide.

It is yet another object of the present invention to provide an improved process for bleaching pulp of cellulosic fibres, which results in a bleached pulp having typical brightness and strength on the market with less chemical usage. It is in.

Yet another object of the present invention is to enable bleaching under more favorable conditions in the bleaching step subsequent to the present invention.

[0010]

SUMMARY OF THE INVENTION According to the present invention, these objects are attained in a method for bleaching pulp of cellulosic fibrous material by adding dilution water after washing the pulp after cooking or after cooking. After bleaching and washing the pulp with oxygen, and before continuous feeding to a bleaching line having a pre-stage and a post-stage, the pulp is subjected to a heat extraction treatment to substantially improve the bleachability of the post-stage. This is achieved by a method for bleaching pulp of a cellulosic fibrous material.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical pulp used in the present invention includes those manufactured by a soda method or a kraft method. Further, as a material, in addition to conifers and hardwoods, so-called non-wood materials such as kenaf and straw can be considered. As for the craft method,
MCC, EMCC, ITC, Lo-solid method and the like are known, but are not limited to those methods, and may be either those subjected to oxygen delignification treatment or those not subjected to oxygen delignification treatment.

The pulp fed and treated in the bleaching step preferably has a concentration of about 0.5 to 15%. If the pulp concentration is less than 0.5%, the tower required for the processing step becomes large or the processing time becomes long, so that it is not suitable for more efficient and economical bleaching. On the other hand, if the pulp concentration is 15% or more, it is not appropriate because free water for eluting soluble lignin is insufficient.

In a preferred embodiment of the invention, the dilution water is added at one or more points and is heated in a processing tower. At each of these points, a mixer is used to homogenize the pulp slurry. The pH of the heat treatment in the present invention depends on the washing condition of the pulp after cooking or after cooking-oxygen bleaching.
5 is preferred. If it is less than 6.5, the degree of improvement of the bleaching effect in the latter stage is small, and if it exceeds 12.5, the pulp reduction is large. The heating time is 10 to 30 hours.
0 minutes is preferred. If the time is less than 10 minutes, the degree of improvement of the bleaching effect in the subsequent stage is small. On the other hand, if the time exceeds 300 minutes, a significant improvement in the bleaching effect in the latter stage is not recognized, and the economic disadvantage such as an increase in the size of the equipment is undesirably increased.

The heat extraction of the pulp is performed at a temperature of 40 to 95 ° C. If the temperature is lower than 40 ° C., the diffusion rate of the extract becomes low, which is not suitable. On the other hand, if the temperature exceeds 95 ° C., the pulp quality is adversely affected, which is not appropriate.

The addition of the diluting water can be arbitrarily set at one or more locations from the time of washing after the digestion or oxygen bleaching to the entrance of the heating extraction tower. The dilution water is preferably added immediately after washing. That is, by having a longer retention time, a higher extraction effect can be obtained. Further, as the dilution water, the washing water of the latter alkali extraction stage and hydrogen peroxide bleaching can be used.

[0016]

[Action of the Invention] Chlorine dioxide has high reaction selectivity and is excellent as a bleaching reagent, but it releases chlorine ions into the bleaching wastewater. Need to be restricted. However, in order to use a smaller number of bleaching steps to obtain bleached pulp with the brightness and strength typical of the market, delignification can be performed efficiently while minimizing the use of chlorine dioxide. Must promote.

The digestion or oxygen bleaching is carried out under relatively strong alkaline conditions. However, in the latter stage of the process, the pH becomes low, and hemicellulose is re-adsorbed on the pulp surface, and at the same time, some of the solubilized lignin is adsorbed again I do. On the other hand, it is considered that there is also lignin inside the fiber which has been reduced in molecular weight and solubilized but cannot be eluted due to the steric structural hindrance of the pulp. Such soluble lignin has many phenolic structures, and therefore, in chlorine dioxide bleaching that selectively reacts with phenolic lignin, soluble lignin on the fiber surface or inside the fiber is more selective than polymer lignin inside the fiber. It is thought to react to. However, since such soluble lignin can be sufficiently eluted without reacting with the bleaching chemical, it is a wasteful reaction in terms of efficient bleaching with less bleaching chemical. Therefore, in order to perform more efficient bleaching, diluting water is added to the pulp after digestion or oxygen bleaching, the mixture is heated well, and the lignin on the fiber surface and inside the fiber is dissolved in the aqueous solution. It can be diffused and removed, and the bleaching chemicals added in the subsequent bleaching stage react with the high molecular weight lignin remaining inside the fiber, improving delignification selectivity.

By performing such heat extraction under alkaline conditions after digestion or oxygen bleaching, efficient and effective treatment can be performed without increasing the metal ion concentration in the entire bleaching step by adding caustic soda. Can be done. Furthermore, performing the treatment under milder conditions without newly adding an alkali can reduce a polysaccharide-derived decrease in the progress of alkali extraction or the like. Also, because the amount of alkali brought into the next bleaching process decreases,
For example, it is possible to optimize the final pH in chlorine dioxide bleaching and to reduce the amount of sulfuric acid added in ozone bleaching. This fact is particularly advantageous when obtaining bleached pulp having a typical brightness and strength on the market with less chemical use.

Another advantage of the heat extraction process is the effective use of wastewater as viewed throughout the bleaching process. As an example, in the case where chlorine dioxide bleaching is performed after oxygen bleaching followed by heat extraction and then alkali / hydrogen peroxide bleaching, alkali / hydrogen peroxide bleached wastewater can be used as the dilution water in the heat extraction stage. Further, wastewater from the heating extraction stage can be used as dilution water in the oxygen bleaching stage. That is, a more effective bleaching sequence can be established with a smaller amount of bleaching wastewater.

The heat extraction treatment is preferably carried out at a temperature as high as possible within a range that does not adversely affect the pulp quality, since the higher the temperature, the higher the diffusion rate and the higher the extraction effect. From this point of view, the pulp after digestion or after oxygen bleaching is considered to be effective also in terms of effective use of heat because the pulp after washing is relatively hot.

FIG. 1 shows an example of the bleaching system of the present invention. After washing, the pulp after oxygen bleaching is washed with diluting water 19 in a well-stirred state with the mixer 4, and then sent out to the heating extraction stage 5 by the pump 3. After the treatment, it is washed by the washing machine of No. 6 and then introduced into the subsequent bleaching by the pump of No. 7.

[0022]

EXAMPLES Comparative examples and examples are shown in Tables 1, 2 and 3. The pulp concentration in the specification indicates% by weight.

Example 1 Softwood chips were digested in a continuous digester and subjected to oxygen delignification, and pulp having a kappa number of 14.4 obtained by adding diluent water to a pulp concentration of 10%.
%, A reaction temperature of 70 ° C. and a reaction time of 60 minutes were subjected to heat extraction treatment, followed by washing. Pulp slurry during heating extraction
pH was 10.2. Further, the subsequent bleaching was performed under the conditions shown in Table 1.

Comparative Example 1 The pulp used in Example 1 was
Example 1 except that heating extraction and subsequent washing were not performed
Bleached under exactly the same conditions.

"Comparison of whiteness after full bleaching between Example 1 and Comparative Example 1" The whiteness after full bleaching when the heat extraction treatment of Example 1 was performed was 79.7%.
75.3% when the heat washing process was not performed,
A clear difference in bleaching properties was confirmed.

Example 2 After softwood chips were digested in a continuous digester and subjected to oxygen delignification, pulp having a kappa number of 16.3 was added to the pulp having a pulp concentration of 10 by adding dilution water.
%, A reaction temperature of 60 ° C. and a reaction time of 120 minutes were subjected to a heat extraction treatment, followed by washing. Pulp slurry during heating extraction
pH was 9.1. Further, the subsequent bleaching was performed under the conditions shown in Table 2.

Comparative Example 2 The pulp used in Example 2 was
Bleaching was carried out under exactly the same conditions as in Example 2 except that no heat extraction and subsequent washing were performed, and the amount of chlorine dioxide added in the D1 stage was increased.

"Comparison of whiteness after full bleaching between Example 2 and Comparative Example 2"
The whiteness after full bleaching when bleaching with 10.4 kg / ADT of chlorine dioxide at the stage was 81.7%. It was 81.5% when bleached at 14.0 kg / ADT, and a clear chlorine dioxide reduction effect was confirmed.

"Comparison of pulp viscosity after full bleaching between Example 2 and Comparative Example 2" The case where the heat extraction treatment of Example 2 was performed and the chlorine dioxide addition amount in D1 stage was bleached at 10.4 kg / ADT. The pulp viscosity after full bleaching was 14.3 cP, which was 13.1 cP when the chlorine dioxide addition amount in the D1 stage was bleached at 14.0 kg / ADT without performing the heat washing treatment of Comparative Example 2. Quality improvement effect was confirmed.

Example 3 After hardwood chips were digested in a continuous digester and subjected to oxygen delignification, pulp having a kappa number of 12.3 obtained by adding dilution water to a pulp concentration of 10% was obtained.
%, A reaction temperature of 60 ° C. and a reaction time of 120 minutes were subjected to a heat extraction treatment, followed by washing. Pulp slurry during heating extraction
pH was 6.9. Further, the subsequent bleaching was performed under the conditions shown in Table 3.

Comparative Example 3 The pulp used in Example 3 was
Bleaching was carried out under exactly the same conditions as in Example 3 except that no heat extraction and subsequent washing were performed, and the amount of chlorine added in the C stage was increased.

"Comparison of whiteness after full bleaching between Example 3 and Comparative Example 3"
The whiteness after full bleaching when the chlorine addition amount in the stage was 22.2 kg / ADT was 86.8%, and the chlorine addition amount in the stage C was 26.7 kg without performing the heat washing treatment in Comparative Example 3. / ADT
The bleaching rate was 85.8%, indicating a clear chlorine reduction effect.

Example 4 Hardwood chips were digested in a continuous digester to obtain pulp having a kappa number of 15.5 and a whiteness of 24.2%. Dilution water was added to the pulp to obtain a pulp concentration of 10% and a reaction temperature of 80 ° C. A heating extraction treatment was performed for a reaction time of 60 minutes, followed by washing. The pH of the pulp slurry was 10.4 at the start of heat extraction and 8.7 at the end of heat extraction. As shown in Table 4, the Kappa number after washing decreased to 13.3. Also, the whiteness was improved to 26.5%. Further, the subsequent bleaching was performed under the conditions shown in Table 4. Table 4 also shows the obtained bleaching results.

Comparative Example 4 The same digested pulp as in Example 4 was used, except that no heat extraction and subsequent washing were performed, and the amount of chemicals added during the subsequent bleaching was increased.
Bleached under the same conditions. Table 4 shows the bleaching conditions and the obtained bleaching results.

"Comparison of whiteness after full bleaching between Example 4 and Comparative Example 4" The heat extraction of Example 4 was performed, and the chlorine addition amount in the C stage was 21.5 kg / ADT, and the alkali extraction stage was used. The whiteness after full bleaching when the amount of NaOH added was bleached at 8.1 kg / ADT was 86.7%. On the other hand, the amount of chlorine added in the C stage was 25.1 kg / A
DT, the amount of NaOH added in the alkali extraction stage was 10.8 k
The whiteness after full bleaching when bleached with g / ADT was 86.3%, confirming a clear chemical reduction effect.

Example 5 Hardwood chips were digested in a continuous digester to obtain a pulp having a kappa number of 19.2 and a whiteness of 27.5%. A heating extraction treatment was performed for a reaction time of 60 minutes, followed by washing. The pH of the pulp slurry was 10.2 at the start of heat extraction and 8.3 at the end of heat extraction. As shown in Table 4, the Kappa number after washing was reduced to 18.1. Also, the whiteness was improved to 28.2%. Although full bleaching as in Examples 1 to 4 was not performed here, lignin removal similar to Examples 1 to 4 was obtained from the effect of lowering the kappa number and the effect of improving whiteness, as in Example 4. It was predicted that the amount of bleaching chemicals in the latter stage would be reduced.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[Brief description of the drawings]

FIG. 1 shows an example of the bleaching system of the present invention.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Takanori Miyanishi 5-21-1, Oji, Kita-ku, Tokyo Nippon Paper Engineering Co., Ltd. F-term (reference) 4L055 AA02 AA03 AA05 AC06 AC08 BB15 BB17 BB20 BB30 FA05 FA20 FA21 FA30

Claims (6)

[Claims] 1. A method for bleaching pulp of a cellulosic fibrous material when producing chemical pulp, wherein dilution water is added after washing the digested pulp or oxygen bleaching the digested pulp. After, and before continuous feeding to the bleaching line having the former and the latter,
A bleaching method for pulp of a cellulosic fibrous material, wherein the bleaching property in the subsequent stage is improved by performing a heat extraction treatment. 2. The pulp treated by the heat extraction stage is about
The method of claim 1 having a pulp concentration in the range of 0.5 to 15%. 3. The method according to claim 1, wherein the pH in the heating extraction stage is in the range of 6.5 to 12.5. 4. The method according to claim 1, wherein the processing time in the heating extraction stage is in the range of 10 to 300 minutes. 5. The method according to claim 1, wherein the processing temperature in the heating extraction stage is in the range of 40 to 95 ° C. 6. The method according to claim 1, wherein the dilution water is added at a point before the heating extraction stage.