JP2006045736A - Method for bleaching kraft pulp - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide suitable acid treating conditions in a method for bleaching kraft pulp in a chlorine-free or perfectly chlorine-free bleaching style. <P>SOLUTION: This method for bleaching the kraft pulp in the chlorine-free or perfectly chlorine-free bleaching style is characterized by treating the kraft pulp with an acid under conditions comprising pH 1.5 to 2.5, a reaction temperature of 75 to 80°C and a reaction time of 300 to 500 min. A mineral acid for the acid treatment is preferably a pure mineral acid and/or a waste mineral acid obtained in any other process. They are recommended that the hexenuronic acid content of the pulp obtained after the acid treatment is lower by ≥40% than that before the acid treatment, and that the kappa value of the pulp obtained after the acid treatment is lower by ≥30% than that before the acid treatment. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for bleaching kraft pulp which is carried out in a chlorine-free or completely chlorine-free bleaching method, and more particularly, after the alkali oxygen delignification step of kraft pulp and in the first stage chlorine dioxide and / or ozone chlorine-free or The present invention relates to an acid treatment method for pulp performed before a complete chlorine-free bleaching step.

  Pulp obtained by digesting wood and non-wood chips as raw materials for paper and pulp by the kraft method is sequentially subjected to alkaline oxygen delignification and selective cleaning steps, and then transferred to a bleaching step. Currently, there are three bleaching methods: chlorine bleaching, chlorine-free bleaching (Elemental Chlorine Free (ECF)), and total chlorine-free bleaching (Totally Chlorine Free (TCF)). Hazardous organochlorine compound loads in the discharged bleached wastewater due to the use of chlorate, etc. may adversely affect the environment, and the use of elemental chlorine, hypochlorite, etc. can be avoided due to the recent increase in environmental protection. At present, ECF and TCF bleaching are the main methods. In that case, as general bleaching chemicals used, chlorine dioxide, hydrogen peroxide, ozone, oxygen, caustic soda, etc. are used in the ECF bleaching method, and chelating agents are used in the TCF bleaching method. Hydrogen peroxide, ozone, oxygen, caustic soda, peracetic acid, etc. However, since ECF pulp is less expensive than TCF pulp, the ECF bleaching method is now becoming the mainstream.

In general, when converting from chlorine bleaching to ECF bleaching, it is an effective way for companies to use the old equipment of the chlorine stage and hypochlorite stage in order to reduce equipment costs. The alternative to the chlorine dioxide stage (D ₀ stage) and the hypochlorite stage is the hydrogen peroxide stage (P stage). Recently, ozone bleaching has been introduced, and the installation position is the first in the bleaching sequence. Or in the middle.

  However, the pulp obtained by the ECF bleaching method in which the first stage is a chlorine dioxide stage is inferior in thermal fading at the time of distribution and storage as compared with the conventional chlorine bleaching method, and deteriorates the quality of the paper as the final product (the present invention). Japanese Patent Application No. 2003-334843 concerning the prior invention of the former. It is found that there is hexeneuronic acid (hereinafter referred to as HexA) in bleached pulp as one cause of the deterioration of thermal fading.

The side chain of xylan, which is a hemicellulose of lignocellulosic material (wood, non-wood), contains 4-O-methylglucuronic acid residues, undergoes an alkaline hydrolysis reaction during the kraft pulping process, and further undergoes a demethoxylation reaction. To generate HexA (Buchert, J. et al. , Tappi J. 78 (11): 125 (1995)).

In ECF bleaching using chlorine dioxide, HexA generates dicarboxylic acid-free chlorinated and chlorinated react with chlorine dioxide D ₀ stage, exacerbating thermal fading of the bleached pulp. When acid treatment is performed prior to ECF bleaching, HexA in the pulp is hydrolyzed, and byproducts of 2-furonic acid and 5-carboxy-2-furaldehyde are produced, thereby improving the thermal fading of the bleached pulp ( Gellerstedt, G., Li, J. Carbohydrate Research 294 : 41-54 (1996)).

On the other hand, in ECF bleaching using ozone, HexA consumes ozone and produces oxalic acid, which also reacts with the calcium of the pulp system liquor, generating scaled calcium oxalate to reduce pulp production. causative (Vuorinen, T. et al, 9 th International Symposium on Wood and Pulping Chemistry (ISWPC), Montreal, Quebec, Canada, 6 September 1997 -. 12 days, the second volume, 901 pages). That is, if the acid treatment is performed before ozone bleaching, the ozone bleaching efficiency can be maintained, and the scale problem of calcium oxalate can be avoided.

  On the other hand, according to the patent documents mentioned later, ECF pulp and TCF pulp have higher bleaching chemical costs than chlorine bleached pulp, and as a cause of this, HexA is greatly involved in substances that consume pulp bleaching chemicals in addition to conventional lignin. It is pointed out that, and therefore, if HexA can be removed, the efficiency of the bleaching chemical can be improved.

  As a specific means, it has been pointed out that if a predetermined acid treatment is performed on the pulp before the chlorine-free or complete chlorine-free bleaching step of the kraft pulp, it is effective in reducing HexA.

  In patent document 1, the following means is disclosed in claim 1 as a specific method of the acid treatment.

Claim 1 of Patent Document 1
The cellulose pulp is heated and treated at a temperature of about 85-150 ° C. at a pH of about 2-5 to remove at least about 50% of HexA in the cellulose pulp and reduce the pulp kappa number by 2-9 units. .

  Further, as another conventional example referring to the acid treatment in the bleaching method of kraft pulp performed in a chlorine-free bleaching method, there is a patent document 2 described later, and in the patent document 2, the claims 1 to 2 are provided. Discloses the following means.

Claim 1 of Patent Document 2
After the lignocellulosic material is digested with an alkaline chemical, the oxygen delignification step treatment is performed to make the pulp kappa number 8-15, and the obtained pulp is treated in an acidic aqueous solution, compared with the oxygen delignification step of the previous step. The pulp kappa number is reduced by 30% or more to make the pulp kappa number 3-10.

Claim 2 of Patent Document 2
The treatment in the acidic aqueous solution is performed under the conditions of pH 2.5 to 4.0 and reaction temperature of 80 ° C to 130 ° C.

  As yet another conventional example that refers to the reduction of HexA in the bleaching method of kraft pulp performed in a chlorine-free bleaching system, there is the following Patent Document 3, and in Patent Document 3, in the scope of the claims, The following means are disclosed.

Claim 1 of Patent Document 3
In the bleaching step of hardwood pulp after oxygen delignification, the pulp is acid-treated under the conditions of pH = 2.0 to 3.0, reaction temperature of 80 to 85 ° C., and reaction time of 0.5 to 5.0 hours.

JP 10-508346 A JP 2000-290887 A JP 2003-342885 A

  The inventor of the present application has studied various acid treatment methods (kappa number and HexA reduction method) described in each of the above patent documents. In addition to these known acid treatment methods, useful acid treatments different from these methods are useful. Knowing that there is a method and having arrived at the present invention, the specific contents are as follows.

  The present invention is a method for bleaching kraft pulp (mainly hardwood kraft pulp) which is first carried out in a chlorine-free or complete chlorine-free bleaching system, and has a pH of 1 after the alkali oxygen delignification step and before the chlorine-free or complete chlorine-free bleaching step. .5 to 2.5, reaction temperature of 75 to 80 ° C., and reaction time of 300 to 500 minutes. The same effect as the processing method can be obtained.

  As described above, in the present invention, pulp (mainly hardwood kraft pulp) is bleached by chlorine-free or completely chlorine-free bleaching treatment, so that it is compared with conventional bleaching methods using chlorine or chlorine-based chemicals. In addition, the use of a novel acid treatment method provides a reduction effect of bleaching chemicals, and the bleached pulp produced by the conventional method is conventionally used. As compared with the pulp by chlorine bleaching, there is an effect that the degree of improvement in thermal fading is equal or higher.

  In addition, the pulp produced by acid treatment by the method of the present invention has a slightly reduced pulp viscosity as compared with pulp produced without such acid treatment, but the strength (breaking length, burst strength) , Tear strength, bending strength, etc.) can be improved to the same extent or more.

The pulp applied to the present invention may be wood or non-wood:
1) Soda / anthraquinone (AQ) method,
2) Conventional craft method (KP),
3) Polysulfide method (PS), PS / AQ method or thiourea method, thiourea / AQ method (Japanese Patent Application No. 2002-310672 relating to the prior invention of the present inventor), which is a craft method for improving yield.
4) MCC, EMCC, ITC, Lo-Solids, Kobudo ^Mari , Compact method, which are commercial craft improvement methods,
The digester may be either a continuous digester or a batch digester, and the cooking conditions (cooking temperature, H factor, activated alkali or effective alkali percentage, sulfidity of white liquor, etc.) The kappa number before the alkaline oxygen delignification treatment step is preferably in the range of 14-20, and then the alkaline oxygen delignification treatment is performed, and this alkaline oxygen delignification treatment is performed at a low concentration ( Pulp concentration: 3% -8%), medium concentration (pulp concentration: 8% -15%), and high concentration (pulp concentration: 20% -30%) are not particularly limited, but after the alkaline oxygen delignification treatment step That is, the kappa number before the acid treatment step is preferably in the range of 8-14.

  As the acid used for the acid treatment, mineral acids such as sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid can be used, but in order to reduce chemical costs, it is possible to use waste acid generated in other processes. Chemical pulp production plants generally have a bleaching process, in which chlorine dioxide is produced on-site, and the production methods are R2, R8 / SVP, etc., both of which contain waste sulfuric acid as a waste material. The purity of waste sulfuric acid is 4-10N, which is lower than 36N of pure sulfuric acid.

  According to Patent Documents 2 and 3, when the pH during acid treatment is less than 2, the removal of HexA is sufficient, but the acid hydrolysis of the pulp proceeds, and the decrease in pulp viscosity and pulp strength is undesirably large. ing. However, as described below, in the present invention, the lower the pH during acid treatment, the lower the pulp viscosity. However, at pH 1.5 or higher, the pulp strength after acid treatment is low before the acid treatment even though the pulp viscosity is low. It can be confirmed that there was no adverse effect on the physical properties of the paper product made of pulp after acid treatment, that is, the acid treatment did not decompose the pulp fiber and depolymerized. By setting the acid treatment pH to 1.5 to 2.5, the temperature of the acid treatment can be lowered, the amount of steam used for the acid treatment can be reduced, and the cost can be reduced.

  According to Patent Document 3, if the temperature during acid treatment is less than 80 ° C., there is no HexA removal effect. However, as described below, in the present invention, the acid treatment temperature range of 75-80 ° C. at pH 1.5-2.5 is suitable and has the effect of removing HexA. Since the acid treatment temperature is lower than 80-150 ° C., the amount of steam used is small and the energy cost is low.

  The reaction time of the acid treatment in the method of the present invention is in the range of 300 to 500 minutes. When the reaction time is less than 300 minutes, the decrease in the kappa number is small, and the effect on the bleaching property in the subsequent step is small, which is not preferable. On the other hand, if the reaction time exceeds 500 minutes, the acid-treated pulp has a lower whiteness than the acid-treated pulp, and even in this case, the effect on the bleaching property of the pulp is small, which is not preferable.

  The pulp value of the pulp treated with acid under the typical acid-treated acid conditions described in Patent Documents 1 and 2 (temperature: 90 ° C .; pH: 3.0; reaction time: 180 minutes) For the difference of 41%, the kappa number of the pulp obtained after the acid treatment in the case of carrying out the present invention is 41-42% lower than that before the acid treatment, which is almost the same as the results of Patent Documents 1 and 2. . When the kappa number after the acid treatment is lowered by 45% or more, the whiteness of the pulp is decreased, and the effect on the pulp bleaching property in the subsequent process is small, which is not preferable. On the other hand, if the kappa number after acid treatment is reduced by less than 30%, the residual lignin and HexA contents in the pulp after acid treatment are high, and the effect on the bleaching property in the subsequent step is small, which is not preferable.

Furthermore, the HexA content of the pulp obtained under the acid-treated acid conditions described in Patent Documents 1 and 2 (pH: 3.0) is 52.5% lower than that before the acid treatment. On the other hand, in carrying out the present invention, the HexA content of the pulp obtained after the acid treatment is 48.7% in the range of the acid treatment pH 2.0-2.5 than that before the acid treatment, and the acid treatment pH 1.5. It has been confirmed that it decreases by 45.4% in the range of −2.0, and the removal of HexA is sufficient. According to these results, the lower the acid treatment pH, the lower the removal rate of HexA, in other words It can be seen that the removal rate of HexA depends on the acid treatment pH. Moreover, since the kappa number of the pulp which acid-treated at these pH is equivalent, when the acid treatment pH is low, it turns out that the residual lignin in a pulp becomes easier to remove than HexA.
In carrying out the present invention, the acid-treated pulp is not subjected to a washing step, and only dehydration is sufficient, and the pulp is then bleached so that the final whiteness is 82 to 90% ISO by multi-stage chlorine-free bleaching. It is desirable to do. In this case, it is recommended that the first stage of the multi-stage chlorine-free bleaching is a chlorine dioxide stage or a high-concentration ozone stage. (AZE) (D ₁ ) (D ₂ ); (AZE) (D) (P); (AZE) (P) (D); (AD ₀ ) (Eo) (D ₁ ) (D ₂ ) (AD ₀ ) (Eop) (D ₁ ) (D ₂ ); (AD ₀ ) (Eo) (D ₁ ) (P); (AD ₀ ) (Eo) (P) (D ₁ ) . However, between () is a washing stage, inside () is a single or no washing stage, A is acid treatment, Z is high or medium ozone bleaching, D is chlorine dioxide bleaching, P is excess Hydrogen oxide bleaching, E for alkali extraction, Eo for alkali extraction with addition of oxygen, Eop for alkali extraction with addition of oxygen and hydrogen peroxide, D ₀ for first stage chlorine dioxide bleaching, D ₁ and D ₂ for subsequent stage chlorine dioxide Means bleaching.

  Furthermore, Japanese Patent Application No. 2003-308517 relating to the prior invention of the inventor of the present application includes a high concentration ozone stage in the multi-stage chlorine-free bleaching process, and further, without performing a washing stage after the high-concentration ozone stage, an initial pH of 7-8. And a technique including performing an alkali extraction stage under conditions of a final pH of 6 to 7 and a treatment time of 20 to 40 minutes.

  Hereinafter, the present invention and its examples will be described more specifically with reference to some test examples, but the present invention is of course not limited to these examples. Pulp quality was measured using the following experimental method.

<Pulp quality measurement method>
Measurement of kappa number: The kappa number was measured by the method described in “JIS P8211”.

Measurement of HexA: HexA in the pulp was measured by the method described in detail in the following document. Gellerstedt, G. , Li, J. “A HLPC Method for the Quantitative Determination of Hexeneuronic Acid Groups in Chemical Pulps”, Carbohydrate Research 294 : 41-54 (1996). HexA was measured with an ultraviolet spectrophotometer UV-265FS manufactured by Shimadzu Corporation.

Measurement of b ^* : b ^* of the CIE system was measured with a Macbeth Color-eye tester.

  Measurement of whiteness: Whiteness was measured by the method described in “JIS P8123”.

  Viscosity measurement: Viscosity was measured by the method described in “Tappi Standards T230 om-89” in the United States.

  The pulp with and without acid treatment was beaten with a PFI mill described in “JIS P8221-2”, and after measuring the freeness with a Canadian standard freeness described in “JIS P8121”, “JIS P222” and A handsheet at each freeness level was prepared by the method described in “JIS P8223” and subjected to a paper quality test. The measurement items are described below.

  Breaking length: The breaking length was determined based on the method described in “JIS P8113”.

  Specific rupture degree: indicated as a specific rupture degree using the method described in “JIS P8112”.

  Specific tear degree: The specific tear degree was measured by the method described in “JIS P8116”.

  Folding resistance: Folding resistance was measured based on the method described in “JIS P8115”.

(Test Example 1)
Sulfuric acid was added to hardwood kraft pulp after alkaline oxygen delignification (kappa number 11.11, viscosity 21.4 mPa.s) so that the pH was 1.85, and the acid treatment was performed by changing the reaction time at 7 levels. went. The pulp after acid treatment was measured for kappa number, HexA, viscosity, whiteness, b ^* , tear length, burst strength, tear strength, and folding resistance.

Acid treatment conditions: Pulp concentration 10%; pH 1.85; Temperature 77 ° C; Reaction time 2,3,4,5,6,7 hours (Test Example 2)
Sulfuric acid was added to hardwood kraft pulp after alkaline oxygen delignification (kappa number 11.11, viscosity 21.4 mPa.s) so that the pH was 2.20, and acid treatment was performed by changing the reaction time at 7 levels. went. The pulp after acid treatment was measured for kappa number, HexA, viscosity, whiteness, b ^* , tear length, burst strength, tear strength, and folding resistance.

Acid treatment conditions: Pulp concentration 10%; pH 2.20; Temperature 77 ° C; Reaction time 2,3,4,5,6,7 hours (Test Example 3)
Sulfuric acid was added to hardwood kraft pulp after alkaline oxygen delignification (kappa number 11.11, viscosity 21.4 mPa.s) so that the pH was 3.15, and the acid treatment was performed by changing the reaction time at three levels. went. The pulp after acid treatment was measured for kappa number, HexA, viscosity, whiteness, b ^* , tear length, burst strength, tear strength, and folding resistance.

Acid treatment conditions: Pulp concentration 10%; pH 3.15; Temperature 90 ° C; Reaction time 1, 2, 3 hours (Test Example 4)
Sulfuric acid was added to hardwood kraft pulp after alkaline oxygen delignification (kappa number 12.24, viscosity 22.6 mPa.s) so that the pH was 1.78, and the reaction time of 7 levels was changed to perform acid treatment. went. The pulp after acid treatment was measured for kappa number, viscosity, whiteness, and b ^* .

Acid treatment conditions: Pulp concentration 10%; pH 1.78; Temperature 77 ° C; Reaction time 2,3,4,5,6,7 hours (Test Example 5)
Sulfuric acid was added to hardwood kraft pulp after alkaline oxygen delignification (kappa number 12.24, viscosity 22.6 mPa.s) so that the pH was 3.08, and the acid treatment was performed by changing the reaction time at 7 levels. went. The pulp after acid treatment was measured for kappa number, whiteness, and b ^* .

Acid treatment conditions: Pulp concentration 10%; pH 3.08; Temperature 90 ° C .; Reaction time 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 hours These test examples 1 to 5 Table 1 shows the results of acid treatment conditions (pH, acid treatment temperature, acid treatment time) indicated by these test results, and various physical properties of the pulp (kappa number, HexA content, viscosity, whiteness, b) ^* , FIG. 1 to FIG. 7 show the tendency of changes in tear length, burst strength, tear strength, folding resistance, etc.

  Next, knowledge obtained from these test examples will be described.

  According to Table 1 and FIGS. 1 and 2, the low pH and the low reaction temperature (for example, pH 1.85 and temperature 77 ° C. in the example of Test Example 1, pH 2.20, temperature 77 ° C. and Test in the example of Test Example 2) In the example of Example 4, acid treatment at a pH of 1.78 and a temperature of 77 ° C. is performed at a general high pH and high reaction temperature (for example, in the example of Test Example 3, the pH is 3.15 and the temperature is 90 ° C.). The reaction time required to reach a kappa number equivalent to the final kappa number (6.54 in the example of test example 3) was about twice as long (in the example of test example 1, the kappa number was 6.46). About 6 hours until the kappa number of 6.71 was reached in the example of test example 2, and about 6 hours until the kappa number of 6.51 was reached in the example of test example 4. In this example, it takes about 3 hours for the copper number to reach 6.54). However, as can be seen from Test Examples 1, 2, and 4 above, in order to obtain a kappa number equivalent to that in the case of general acid treatment, if the acid treatment time is increased (300 to 500 minutes), the present application A low temperature (75 to 80 ° C.) as in the method of the present invention is also possible, and the pH range for this purpose is preferably 1.5 to 2.5 as described above.

  Also, regarding the amount of HexA reduction, in the case of Test Example 1, the acid treatment time is 6 hours, and the HexA reduction rate is about 54% (84.4 → 39.0 mmol / g absolutely dry pulp). Similarly, the acid treatment time is 6 hours, the HexA reduction rate is about 57% (84.4 → 35.7 mmol / g absolutely dry pulp), and a general acid treatment method (for example, acid treatment time 3 hours in Test Example 3) In comparison with the HexA reduction rate in the case (about 62%: 84.4 → 31.9 mmol / g absolutely dry pulp), there is no practical problem.

  Further, regarding other physical properties of other pulps, those within the scope of the present invention have a slightly lower viscosity, but other physical properties (whiteness, b *, break length, burst strength, tear strength, As for the degree of folding, etc., it can be seen that almost the same numerical values are obtained as compared with those of the general acid treatment method (see FIGS. 3 to 7).

  From the above knowledge, the inventor of the present application has a range in which a practically problematic numerical value can be obtained as the kappa number after acid treatment (those that are reduced by 30% or more from the kappa number before acid treatment), That is, in Test Examples 1, 2, and 4, those having an acid treatment time of 5 hours (300 minutes) or more are defined as examples of the present invention.

It is a graph which shows the relationship between the acid treatment time and the kappa number in Test Example 1 and Test Example 3. It is a graph which shows the relationship between the acid treatment time and the kappa number in Test Example 2 and Test Example 3. 4 is a graph showing the relationship between acid treatment time and whiteness in Test Examples 1, 2, and 3. It is a graph which shows the relationship between acid treatment time and b * in Test Examples 1, 2, and 3. It is a graph which shows the relationship between the fracture length and burst strength in Test Examples 1, 2, and 3. It is a graph which shows the relationship between the tear length and tear strength in Test Examples 1, 2, and 3. It is a graph which shows the relationship between the fracture length in Example 1, 2, and 3 and bending strength.

Claims (3)

  A method for bleaching kraft pulp which is carried out in a chlorine-free or completely chlorine-free bleaching system, having a pH of 1.5 to 2.5 and a reaction temperature of 75 after the alkali oxygen delignification step and before the chlorine-free or complete chlorine-free bleaching step. A bleaching method for kraft pulp, characterized by performing acid treatment of kraft pulp under conditions of -80 ° C and a reaction time of 300-500 minutes.   The method for bleaching kraft pulp according to claim 1, wherein the mineral acid for acid treatment is a pure mineral acid and / or a waste mineral acid obtained in another step. The hexeneuronic acid content of pulp obtained after acid treatment is reduced by 40% or more than that before acid treatment, and the kappa number of pulp obtained after acid treatment is further reduced by 30% or more than that before acid treatment. The method for bleaching kraft pulp according to claim 1 or 2.

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