JP4321273B2 - Method for bleaching pulp for papermaking - Google Patents

Description

  The present invention relates to a method for bleaching paper pulp, and more particularly to a method for producing bleached pulp that improves bleaching efficiency when bleaching with ozone and chlorine dioxide and does not reduce pulp strength. is there.

  Conventionally, as a method for bleaching paper pulp, there is a multi-stage bleaching method such as C (elemental chlorine) -E (alkali extraction) -H (sodium hypochlorite) -D (chlorine dioxide), mainly chlorine-based. Although chemicals have been used, organic chlorine compounds such as dioxins that are harmful to the environment produced by the reaction of chlorine with organic matter in the pulp become a problem, and the bleaching stage using oxygen is used in the first stage, chlorine bleaching chemicals A method has been adopted to reduce this. In recent years, ECF (Elemental Chlorine Free) bleaching that does not use elemental chlorine and TCF (Totally Chlorine Free) bleaching that does not use any chlorinated chemicals have been put to practical use. Oxygen such as oxygen, ozone, and hydrogen peroxide Series chemicals are effective bleaching chemicals.

Compared with chlorinated chemicals, bleaching of pulp with ozone has the advantage of not producing organochlorine compounds in the bleaching process, unlike chlorinated bleaching, in addition to the high reactivity of ozone with lignin.
For this reason, ozone bleaching has been attracting attention for a long time, but ozone is expensive to manufacture and has disadvantages such as its tendency to damage pulp fibers due to its strong oxidizing power, and these are major factors that hinder the practical application of ozone bleaching. It was. However, in recent years, the ozone production cost has been greatly reduced by the improvement in the performance of the ozone generator, and the bleaching method has also been improved, so that the intermediate concentration ozone bleaching method in which ozone bleaching is performed at a pulp concentration of 6 to 15% by mass, or pulp A high-concentration ozone bleaching method in which ozone bleaching is performed at a concentration of 38 to 42% by mass has been attracting attention.

The bleaching conditions of the medium-concentration ozone bleaching method have been studied for a long time. For example, in JP-A-3-152286 (see Patent Document 1), the pulp concentration is 7 to 15% by mass, the temperature is 40 ° C to 70 ° C, and the pH is 2-3. The conditions for ozone bleaching at a pressure of 1-15 bar are disclosed. However, as described above, ozone is likely to damage the pulp fiber, so that the addition rate is substantially limited, and when high quality, especially whiteness and strength are required for the pulp, there is a technique for combining with chlorine dioxide. Widely used. For example, JP-A-3-263687 (Patent Document 2) discloses a method of adding chlorine dioxide and then adding ozone. However, pulp that has been bleached with chlorine dioxide before ozone has a problem that lignin is reduced and the protective effect of cellulose is reduced, so that cellulose is easily attacked by ozone and the strength is greatly reduced. JP-A-4-289288 (Patent Document 3) discloses a bleaching method in which pulp is bleached with chlorine dioxide and ozone in any order in the same step, and no washing is performed between the addition points. It is disclosed. However, this method assumes that ozone bleaching and chlorine dioxide bleaching are bleached under the same conditions. In order to make the bleaching temperature suitable for chlorine dioxide bleaching, it must be heated before ozone bleaching. There was a problem that bleaching was performed at high temperature and ozone bleaching was not performed efficiently.

In JP 2001-316993 A (Patent Document 4), the pressure of pulp sent to chlorine dioxide bleaching following ozone bleaching is increased by a pump to prevent chlorine dioxide from flowing back into the ozone bleaching process. A method for reducing the amount of active chlorine is disclosed. However, the bleaching efficiency of ozone and chlorine dioxide bleaching cannot be improved under these conditions.

In JP 2002-069879 (Patent Document 5), after the intermediate concentration ozone bleaching, diluted water is added to lower the pulp concentration to 6% by mass or less, and then chlorine dioxide bleaching is performed. Disclosed is a method for liberating ozone-reduced lignin in an aqueous solution to facilitate the reaction of chlorine-dioxide with unreacted lignin remaining in the fiber, thereby improving the delignification selectivity of chlorine dioxide. However, when chlorine dioxide bleaching is performed at a low concentration, the reactivity of chlorine dioxide is remarkably lowered, and the added chlorine dioxide is not consumed, so that the addition rate of chlorine dioxide is limited. If high-temperature water is used as the dilution water, the temperature at the time of chlorine dioxide bleaching increases and the reactivity becomes high, but it is offset from the negative effect due to dilution and is disadvantageous in terms of energy.

Japanese Patent Laid-Open No. 3-152286 JP-A-4-263687 JP-A-4-289288 JP 2001-316993 A JP 2002-069879 A

  An object of the present invention is to provide a paper pulp pulp bleaching method that reduces chlorine chemicals and is more efficient than the prior art.

In the same process, the present inventors first added ozone, and then added chlorine dioxide without intermediate washing to perform bleaching (ZD bleaching), and the pulp concentration was a medium concentration of 6 to 15% by mass. It was found that the efficiency of ZD bleaching was greatly improved by adding steam after ozone bleaching in the region and performing subsequent chlorine dioxide bleaching at a temperature higher than that of ozone bleaching.

The present invention includes the following inventions.
(1) When manufacturing chemical pulp, the pulp of cellulosic fiber material is bleached in the order of ozone and chlorine dioxide at a medium concentration of 6 to 15% by weight, and the pulp is warmed before ozone bleaching. After soaking in an acidic solution and treating for 1 to 120 minutes, washing is performed, ozone bleaching is performed at 20 ° C. to less than 45 ° C. without heating , chlorine dioxide is added without washing, and the chlorine dioxide is added. Steam for papermaking is added immediately before, simultaneously with, or immediately after the addition, and the pulp slurry temperature at the time of chlorine dioxide bleaching is set to 40 ° C. to 80 ° C., and the chlorine dioxide bleaching temperature is at least 5 degrees higher than that at the time of ozone bleaching. Bleaching method.

(2) The method for bleaching paper pulp according to (1) , wherein the ozone addition rate in ozone bleaching is 0.2 to 0.7% by mass per pulp.

(3) The method for bleaching paper pulp according to (1) or (2), wherein the chlorine dioxide addition rate in chlorine dioxide bleaching is 0.1 to 2.0 mass% per pulp.

(4) After ozone bleaching, alkali is added immediately before, simultaneously with, or immediately after adding chlorine dioxide, and the pH after chlorine dioxide bleaching is set to a range of 3.0 to 5.5. Any of (1) to (3) 2. A method for bleaching a paper pulp according to claim 1.

As raw material pulp used in the present invention, wood chips are modified continuous cooking method (Modified Continuous Cooking), extended modified continuous cooking method (Extended Modified Continuous Cooking), all-can isothermal cooking method (Isothermal Cooking), Lo-Solids cooking A chemical pulp obtained by a kraft cooking (including polysulfide method) such as a cracking method, a superbatch cooking method, a Compact cooking method, a KOBUDO ^MARI cooking method, or a cooking method such as sulfite cooking can be used. Furthermore, the wood chip used in the present invention is not particularly limited, and hardwood materials such as aspen, beech, acacia, eucalyptus, mangroves, and softwood materials such as pine, spruce, Douglas fir are preferable, but bagasse, Needless to say, the present invention can also be applied to lignocellose materials derived from herbaceous plants such as kenaf, esparto grass, rice and reed.

  In the present invention, unbleached pulp obtained by a known cooking method is preferably oxygen-bleached so as to have a kappa number of 6 to 16, preferably 8 to 14. If the kappa number of the pulp in this treatment step exceeds 16, the chemicals are unnecessarily consumed in the subsequent bleaching step. On the other hand, if it is less than 6, the strength and yield of the pulp are significantly reduced before bleaching. Therefore, it is not preferable.

The pulp subjected to oxygen bleaching is preferably acid-treated at pH 2.5 or lower for 1 to 120 minutes before ozone bleaching. The acid treatment may be heated if necessary. However, the lower the ozone bleaching temperature, the better the reactivity between ozone and pulp. Therefore, considering the pulp slurry temperature brought into ozone bleaching, the acid treatment can be performed without heating. It is preferable to perform processing. In order to remove heavy metals eluted in the acid treatment step, the acid-treated pulp is washed with a known washing machine and supplied to the bleaching step.

In the present invention, ozone is bleached in the middle concentration region of 6 to 15% by mass when adding ozone and then performing chlorine bleaching and bleaching without performing intermediate washing (ZD bleaching). Steam is then added and warmed, followed by chlorine dioxide bleaching at a higher temperature than ozone bleaching. If the pulp concentration during ozone bleaching is less than 6%, ozone tends to react with impurities in water and bleaching efficiency is lowered, which is not appropriate. Further, when the pulp concentration exceeds 15% by mass, it is necessary to introduce a press washer before ozone bleaching in order to increase the pulp concentration, which is disadvantageous in terms of cost. When the chlorine dioxide bleaching concentration is less than 6%, the reactivity of chlorine dioxide is remarkably lowered, the added chlorine dioxide is not consumed, and the addition rate of chlorine dioxide is limited. By heating without reducing the pulp concentration using steam, chlorine dioxide can react even at a relatively low temperature, and the amount of heat taken away by the accompanying water can be reduced, and the energy load can be greatly reduced. When high temperature water is used instead of steam, the pulp concentration is lowered, the effect of temperature increase is offset by dilution, and a large amount of heat is required to raise the temperature of the accompanying water, which is not appropriate.

The pulp slurry temperature during ozone bleaching is preferably 20 ° C or higher and lower than 45 ° C. The bleaching efficiency of ozone and pulp becomes better as the temperature is lower. However, in order to make the temperature less than 20 ° C., it takes a lot of energy for cooling, and the amount of steam necessary to make the subsequent chlorine dioxide bleaching suitable temperature. When the temperature is 45 ° C. or higher, the self-decomposition of ozone is promoted and the bleaching efficiency is lowered, which is not preferable. In the present invention, it is not particularly necessary to warm the pulp slurry during ozone bleaching. In order to increase the reaction efficiency of ozone, the pulp slurry can be cooled with a cooler or the like.

An appropriate pulp slurry temperature during chlorine dioxide bleaching varies depending on the reaction time, and is preferably 40 to 80 ° C. The reactivity between chlorine dioxide and pulp is lower than that of ozone, and it is necessary to adjust the temperature to an appropriate temperature. Chlorine dioxide bleaching is preferably performed at a temperature 5 ° C. or more higher than ozone bleaching. If the temperature is lower than 40 ° C., chlorine dioxide remains in the solution without completely reacting with the pulp, and the bleaching efficiency is lowered.

In the present invention, chlorine dioxide bleaching is performed by adding chlorine dioxide and steam after 10 to 99% of ozone added by ozone bleaching has reacted. The ozone addition rate during ozone bleaching is preferably 0.2 to 0.7% by mass per pulp. When the ozone addition rate is less than 0.2% by mass, a sufficient bleaching effect cannot be obtained, and when it exceeds 0.7% by mass, the pulp strength is significantly lowered, which is not preferable. The chlorine dioxide addition rate during chlorine dioxide bleaching is preferably from 0.1 to 2.0% by mass. If the chlorine dioxide addition rate is less than 0.1% by mass, a sufficient bleaching effect cannot be obtained, and if it exceeds 2.0% by mass, the bleaching efficiency decreases, which is not preferable. The steam to be added varies depending on the amount of heat applied to the pulp slurry, but low pressure steam of 0.1 to 0.5 MPa is preferable in terms of cost, and medium pressure steam of 0.6 to 1.6 MPa may be used.

The pH during ozone bleaching is generally in the range of pH 2.0 to 4.0, preferably pH 2.0 to 3.0. If the pH is less than 2.0, a large amount of acid for pH adjustment is required, which is disadvantageous in terms of cost. If the pH exceeds 3.0, ozone self-decomposition occurs and bleaching efficiency is lowered, which is not preferable. The pH during chlorine dioxide bleaching is preferably in the range of pH 3.0 to 5.5, and it is preferable to adjust to this pH by adding an alkali when chlorine dioxide is added. When the pH after chlorine dioxide bleaching is less than 3.0 or exceeds 5.5, it is not preferable because the pulp whiteness after chlorine dioxide is lowered.

The bleaching sequence after ozone / chlorine dioxide bleaching (hereinafter abbreviated as ZD) of the present invention is not particularly limited as long as it is ECF bleaching. For example, ZD-ED, ZD-Eo-D, ZD-ED Sequences such as -P, ZD-Eo-DP, ZD-EDD, ZD-Eo-DD, ZD-E-PD are listed. Furthermore, oxygen delignification (O) and acid treatment (A) can be combined in each preceding stage. (E) is an alkali extraction stage and (Eo) is an alkali extraction stage reinforced with oxygen. In addition, (D) shows a chlorine dioxide stage, and (P) shows a hydrogen peroxide stage. Moreover, (-) between each stage represents a washing | cleaning process. Further, an enzyme treatment (Ez) stage may be provided in a part of the bleaching process for the purpose of chemical reduction and pulp quality improvement in the above sequence, and the D stage chlorine dioxide stage is dividedly added (dnD, DND, dED).

The pulp kappa number measuring method, pulp whiteness measuring method, and pulp viscosity measuring method used in the present invention are as follows.

Method for measuring kappa number The kappa number of the pulp was measured according to JIS P 8211.

Method for measuring pulp whiteness The whiteness of the pulp was measured according to JIS P 8212.

Pulp viscosity measurement method The viscosity of the pulp was measured according to JIS P 8215.

Next, examples of the present invention will be described, but the present invention is not limited thereto. In addition, unless otherwise indicated, the chemical | medical agent addition rate in an Example and a comparative example represents the mass% with respect to a pulp.

Example 1
Unbleached pulp obtained by kraft cooking of softwood was washed and oxygen bleached to obtain a pulp having a copper number of 12.4. This pulp was diluted to a pulp concentration of 3.5% by mass, adjusted to pH 2.5 by adding sulfuric acid, then subjected to acid treatment at 40 ° C. for 10 minutes and washed. This pulp was used in the following examples and comparative examples.
This pulp was subjected to ozone bleaching at a pulp concentration of 10% by mass, an ozone addition rate of 0.3%, a reaction temperature of 30 ° C., a reaction time of 2 minutes, and a final pH of 2.8, and then alkali, chlorine dioxide, Low-pressure steam (0.25 MPa, about 127 ° C.) was added in that order, and chlorine dioxide bleaching was performed under the conditions of 0.6% chlorine dioxide addition, reaction temperature 60 ° C., reaction time 30 minutes, and final pH 4.0. The bleaching conditions and results at this time are shown in Table 1.

Example 2
Bleaching was performed under the same conditions except that the reaction temperature during ozone bleaching in Example 1 was 40 ° C., the reaction temperature during chlorine dioxide bleaching was 50 ° C., and the reaction time was 60 minutes. The bleaching conditions and results at this time are shown in Table 1.

Example 3
Bleaching was performed under the same conditions except that the order of chemical addition during chlorine dioxide bleaching in Example 1 was low-pressure steam, alkali, and chlorine dioxide. The bleaching conditions and results at this time are shown in Table 1.

Comparative Example 1
Bleaching was performed under the same conditions except that the reaction temperature during bleaching of chlorine dioxide in Example 1 was 30 ° C. The bleaching conditions and results at this time are shown in Table 1.

Comparative Example 2
Bleaching was performed under the same conditions except that the reaction temperature at the time of ozone bleaching in Example 1 was 60 ° C. The bleaching conditions and results at this time are shown in Table 1.

Comparative Example 3
Bleaching was performed under the same conditions except that the temperature during ozone bleaching in Example 1 was 60 ° C and the temperature during chlorine dioxide bleaching was 30 ° C. The bleaching conditions and results at this time are shown in Table 1.

Comparative Example 4
Dilution water was added at the time of bleaching chlorine dioxide in Example 1, and bleaching was performed under the same conditions except that the pulp concentration was 3.5%. The bleaching conditions and results at this time are shown in Table 1.

Comparative Example 5
Bleaching was performed under the same conditions except that ozone and chlorine dioxide were added simultaneously in Example 1. The bleaching conditions and results at this time are shown in Table 1.

From Examples 1 and 2 and Comparative Examples 1 to 3 in Table 1, the bleaching efficiency is greatly improved without lowering the viscosity by increasing the temperature during chlorine dioxide bleaching than the temperature during ozone bleaching. I understand. From Example 1 and Comparative Example 4, it is also understood that good bleaching efficiency can be obtained by keeping the pulp concentration at the time of chlorine dioxide bleaching at a medium concentration. From Example 1 and Comparative Example 5, it can also be seen that good bleaching can be achieved by sequentially performing ozone bleaching and chlorine dioxide bleaching. As described above, in the present invention, a high whiteness pulp can be obtained while maintaining strength, which is suitable for efficient ozone bleaching of papermaking pulp.

Claims (4)

In the method of bleaching cellulosic fiber material pulp in the order of ozone and chlorine dioxide at a pulp concentration of 6 to 15% by mass when producing chemical pulp, an acidic solution without heating the pulp before ozone bleaching And after washing for 1 to 120 minutes, washing is performed, and then ozone bleaching is performed at a purple slurry temperature of 20 ° C. or more and less than 45 ° C. without heating, and immediately before or simultaneously with adding chlorine dioxide without washing , or Steam is added immediately, the pulp slurry temperature at the time of chlorine dioxide bleaching is 40 ° C to 80 ° C, and the chlorine dioxide bleaching temperature is 5 ° C higher than that at the time of ozone bleaching. Method. Process for bleaching paper pulp according to claim 1, wherein the ozone addition rate in the ozone bleaching is 0.2 to 0.7 wt% per pulp. The method for bleaching pulp for papermaking according to claim 1 or 2, wherein the chlorine dioxide addition rate in chlorine dioxide bleaching is 0.1 to 2.0 mass% per pulp. After ozone bleaching, by adding an alkali to the immediately before or simultaneously with or immediately after the addition of chlorine dioxide, according to claim 1, characterized in that in the range of 3.0 to 5.5 and the pH after chlorine dioxide bleaching The bleaching method of the pulp for paper manufacture of any one.