JP2008019533A - Method for ecf bleaching of kraft pulp - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for ECF bleaching of kraft pulp. <P>SOLUTION: The method for the ECF bleaching of kraft pulp comprises the addition of an acid after the addition of chlorine dioxide in the final chlorine dioxide stage in ECF bleaching. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for ECF bleaching of kraft pulp. More specifically, the present invention relates to ECF bleaching of kraft pulp characterized by adding acid after adding chlorine dioxide in the final chlorine dioxide stage in ECF bleaching. It is about the method.

  A method for bleaching kraft pulp that does not use elemental chlorine is called Elemental Chlorine Free (ECF) bleaching, and is currently the mainstream of kraft pulp bleaching methods in Japan and overseas.

  In ECF bleaching, the chlorine dioxide stage is often used in place of the conventional chlorine stage. By using chlorine dioxide as an alternative to chlorine, the chromaticity of the bleaching wastewater is reduced, and the generation of organochlorine compounds is further achieved. Can also be suppressed.

  Since chlorine dioxide has a high reaction selectivity with lignin and hardly reacts with carbohydrates, it is possible to obtain a high whiteness pulp with almost no loss of pulp strength. Thus, chlorine dioxide bleaching is very important from the viewpoint of environment, pulp strength and bleaching efficiency.

  The ECF bleaching sequence including the chlorine dioxide bleaching stage is diverse, but examples include D-Eop-D, D-Ep-D, D-Eop-DD, D-Eop-D-ED, etc. it can. Here, D: chlorine dioxide stage, Eop: alkali / oxygen / peroxide stage, Ep: alkali / peroxide stage, E: alkali stage. These sequences are characterized in that the first and last stages of ECF bleaching are chlorine dioxide stages. The first chlorine dioxide stage is added mainly for the purpose of delignification and whiteness improvement, and the final chlorine dioxide stage is added mainly for the purpose of whiteness improvement.

  Factors affecting chlorine dioxide bleaching include temperature in the bleaching system, bleaching time, pulp concentration, chlorine dioxide addition amount, pH in the bleaching system, and the like. Among these, the pH in the bleaching system is a particularly important factor.

  Chlorine dioxide produces chlorate and chlorite ions in water. Although chlorite ions themselves are inactive, chlorite is produced from chlorite ions when the pH is 6 or less. Chlorous acid contributes to bleaching because of its bleaching activity. Chlorate ions are inactive when the pH is higher than 1. For this reason, if the pH in the bleaching system is higher than 6, the efficiency of chlorine dioxide bleaching decreases.

  The stage before the chlorine dioxide stage in the final stage of ECF bleaching is often a stage using alkali. In the chlorine dioxide stage immediately after the alkali stage, there is a problem that the pH in the bleaching system is increased due to carry-over of the remaining alkali in the previous stage and the bleaching efficiency is lowered. In order to prevent this problem, there is an example in which a heating stage without chemical treatment is provided immediately before the chlorine dioxide stage (see, for example, Patent Document 1).

Thus, the importance of pH in chlorine dioxide bleaching is widely recognized, and a method for improving the bleaching efficiency by adjusting the pH to keep the pH after completion of chlorine dioxide bleaching within a certain range has been proposed (for example, (See Patent Documents 2 to 4). In conventional common sense, these pH adjustments are based on the premise that an acid or alkali is added before adding chlorine dioxide, and there has been no idea of adding an acid or alkali after adding chlorine dioxide.
JP 2004-232164 A JP 2003-253582 A JP 2003-247185 A JP 2003-268688 A

  The present invention has been made for the purpose of providing a method for efficiently ECF bleaching kraft pulp.

  As a result of intensive studies on a method for efficiently ECF bleaching kraft pulp, the present inventors have added kraft pulp by adding acid after adding chlorine dioxide in the final chlorine dioxide stage in ECF bleaching. Was found to be capable of ECF bleaching efficiently, leading to the present invention.

  According to the ECF bleaching method for kraft pulp of the present invention, kraft pulp with high whiteness can be obtained with a smaller amount of added chlorine dioxide.

  The present invention is the ECF bleaching method for kraft pulp, characterized in that in the final chlorine dioxide stage in ECF bleaching, acid is added after chlorine dioxide is added.

  Specific examples of the present invention are shown below, but the present invention is not limited to the following specific examples.

  Although there is no restriction | limiting in particular in the plant used as the raw material of the kraft pulp used for this invention, A conifer or a hardwood is preferable.

  Although there is no restriction | limiting in particular in the kappa number and whiteness of the kraft pulp before ECF bleaching in this invention, in order to improve the efficiency of ECF bleaching, a kappa number is 7.5-9.5 and whiteness is 49.5% or more. It is preferable that From the viewpoint of chemical recovery and delignification efficiency, it is more preferable that oxygen delignification is carried out after kraft cooking and the pulp having reached the kappa number and whiteness is subjected to ECF bleaching.

  There is no particular limitation on the ECF bleaching sequence up to the preceding stage of the heating stage (hereinafter referred to as L stage) without chemical treatment in the present invention, but from the viewpoint of bleaching efficiency, whiteness of 80% or more is reached by the L stage. A sequence consisting of a chlorine dioxide stage-alkali / peroxide stage is preferred because it is preferred.

  Although there are no particular limitations on the treatment conditions of the L stage in the present invention, it is necessary to minimize the carryover of the bleaching chemical from the previous stage of the L stage and increase the bleaching efficiency of the chlorine dioxide stage of the final stage. Treatment conditions of 110 ° C. and 15 to 180 minutes are preferred.

  The amount of chlorine dioxide added in the final chlorine dioxide stage in the present invention is not particularly limited, but if it is less than 0.05% per pulp dry mass, a sufficient bleaching effect cannot be obtained, and more than 0.40%. Even if it is added, no further increase in whiteness can be expected, so the addition amount is preferably 0.05 to 0.40% per pulp dry mass.

  Although there is no restriction | limiting in particular regarding the kind of acid used for this invention, A sulfuric acid is preferable from the point of cost and operativity. Moreover, there is no restriction | limiting in particular also about an addition amount, As long as the completion | finish pH of the chlorine dioxide stage of the last stage is the addition amount which is settled in the range of 4-6, you may change an addition amount timely.

  Although there is no restriction | limiting in particular regarding the addition timing of the acid used for this invention, It is preferable to add in the mixing condition immediately after chlorine dioxide addition, More preferably, it exists in a mixing condition at the timing within 1 minute after chlorine dioxide addition. It is preferable to add to the pulp slurry.

  There is no restriction | limiting in particular regarding the addition method of the acid used for this invention, A required amount may be added at once and may be added in a divided manner. Also, there is no particular restriction on the place of addition, and it may be added anywhere as long as it is between the chlorine dioxide addition place and the bleaching tower where bleaching is performed, and it may be added at one place or at multiple places. You may do it.

  The treatment conditions of the final chlorine dioxide stage in the present invention are not particularly limited. However, when the temperature is lower than 65 ° C., the reactivity with lignin decreases, and when it is higher than 80 ° C., the reaction with carbohydrate occurs and the pulp yield. Is unfavorable because of lowering. Accordingly, the treatment temperature is preferably 65 to 80 ° C. The treatment time is preferably 60 to 240 minutes.

Examples are given below, but the present invention is not limited to these examples. ECF bleaching by a sequence of D0-Ep-L-D1 was carried out using hardwood kraft pulp (kappa number 8.8, whiteness 50.5%) subjected to oxygen delignification after kraft cooking under the conditions shown below. Here, D0 is the first chlorine dioxide stage, Ep is the sodium hydroxide / hydrogen peroxide stage, L is the heating stage without chemical treatment, and D1 is the last chlorine dioxide stage. The pulp concentration and chemical addition rate are all% per mass of air-dried pulp.
D0: Pulp concentration 10%, treatment temperature 60 ° C, treatment time 30 minutes, chlorine dioxide addition rate 0.7%
Ep: Pulp concentration 10%, treatment temperature 70 ° C., treatment time 75 minutes, sodium hydroxide addition rate 0.7%, hydrogen peroxide addition rate 0.4%
L: Pulp concentration 10%, treatment temperature 70 ° C, treatment time 75 minutes D1: Pulp concentration 10%, treatment temperature 75 ° C, treatment time 140 minutes, chlorine dioxide addition rate 0.15%
After adding chlorine dioxide in stage D1, sulfuric acid was added to adjust the pH, and then bleaching was performed. Handsheets were prepared from the ECF bleached pulp thus obtained, and the whiteness was measured based on JIS P8148.

  ECF bleaching and whiteness measurement were performed in the same manner as in Example 1 except that the chlorine dioxide addition amount in stage D1 was 0.20%.

  ECF bleaching and whiteness measurement were performed in the same manner as in Example 1 except that the chlorine dioxide addition amount in stage D1 was 0.25%.

  ECF bleaching and whiteness measurement were performed in the same manner as in Example 1 except that the chlorine dioxide addition amount in the D1 stage was 0.30%.

(Comparative Example 1)
ECF bleaching and whiteness measurement were carried out in the same manner as in Example 1 except that sulfuric acid was added and pH was adjusted before adding chlorine dioxide in stage D1.

(Comparative Example 2)
ECF bleaching and whiteness measurement were carried out in the same manner as in Example 2 except that sulfuric acid was added and pH was adjusted before adding chlorine dioxide in stage D1.

(Comparative Example 3)
ECF bleaching and whiteness measurement were carried out in the same manner as in Example 3, except that sulfuric acid was added and pH was adjusted before adding chlorine dioxide in stage D1.

(Comparative Example 4)
ECF bleaching and whiteness measurement were performed in the same manner as in Example 4 except that sulfuric acid was added and pH was adjusted before adding chlorine dioxide in stage D1.

(Comparative Example 5)
ECF bleaching and whiteness measurement were performed in the same manner as in Example 4 except that pH adjustment by acid addition was not performed in stage D1.

(Comparative Example 6)
ECF bleaching and whiteness measurement were performed in the same manner as in Example 4 except that the pH was adjusted by adding sulfuric acid after the completion of bleaching in stage D1.

  As is clear from the examples and comparative examples in Table 1 above, in the final chlorine dioxide stage, the pulp whose pH was adjusted by adding acid after adding chlorine dioxide was more white at the same chlorine dioxide addition amount. Showed the degree.

  In the final chlorine dioxide stage, by adding an acid and adjusting the pH after adding chlorine dioxide, a high whiteness kraft pulp could be obtained with a smaller chlorine dioxide addition amount.

Claims (2)

  A method for ECF bleaching of kraft pulp, characterized in that, in the final chlorine dioxide stage in ECF bleaching, acid is added after adding chlorine dioxide.   2. The ECF bleaching method for kraft pulp according to claim 1, wherein the last stage of the chlorine dioxide stage in the ECF bleaching is a heating stage without chemical treatment.