JP2013181258A - Method for producing pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a kraft cooking method capable of producing unbleached pulp that contains a small amount of residue and is easily bleached, with a high yield by using hardwood as a raw material.SOLUTION: The unbleached pulp that has a Kappa number of 16-25 and contains 50-70 mmol/kg of hexeneuronic acid is obtained by subjecting the hardwood to kraft cooking treatment under a condition of a cooking temperature of 130°C or higher and lower than 150°C and a cooking period of time of 240 min or longer, and preferably 480 min or shorter. Bleached pulp that has high whiteness and contains a reduced amount of hexeneuronic acid is obtained by processing the obtained unbleached pulp with oxygen delignification treatment, acid treatment and further multistage bleaching processing.

Description

  The present invention relates to a method for producing pulp by digesting hardwood. In particular, the present invention relates to a method for producing a papermaking pulp that is easily bleached and useful as a raw material for paper in a high yield.

  In order to use a lignocellulosic material as a papermaking raw material in many applications, it is necessary to digest it into chemical pulp or mechanically treat it with a refiner or the like to obtain mechanical pulp. These pulps are bleached as necessary, adjusted to a desired whiteness, and then used as a papermaking raw material. At present, the chemical pulping method is mainly used because it is easy to adjust to the desired whiteness and pulp characteristics. Especially, the cooking method called kraft method can regenerate chemicals and there are few restrictions on raw materials used. From the mainstream of chemical pulping.

  However, the kraft method has a low pulp yield, and a cooking method in which a quinone compound, which is a cyclic keto compound such as anthraquinone sulfonate, anthraquinone or tetrahydroanthraquinone, is added to the cooking solution as a cooking aid to improve the yield (for example, Patent Document 1 (Japanese Patent Publication No. 55-1398), Patent Document 2 (Japanese Patent Publication No. 57-19239), Patent Document 3 (Japanese Patent Publication No. 53-45404), Patent Document 4 (Japanese Patent Publication No. 52-37803) Publication))) has been used. The quinone compound improves the selectivity of delignification and contributes to the reduction of the kappa number of the digested pulp, that is, the reduction of chemicals and the improvement of the pulp yield.

  Patent Document 5 (Japanese Patent Laid-Open No. 7-189153) discloses cooking using a quinone compound and an alkaline cooking solution containing polysulfide in combination.

  By the way, from the end of the 1970s to the beginning of the 1980s, a technology called “leveling” of alkali transitions by the pioneering work of STFI Institute in Sweden (Non-Patent Document 1 (Svensk Paperstinging, 87 (10): 30 (1984)). This method, characterized by “white liquor split addition” and countercurrent treatment, known as “modified kraft cooking”, was widely adopted in the pulp industry in the 1980s. The equipment is known as MCC, and this counter-current method was later extended to the addition of white liquor to the counter-current washing zone known as the high heat washing zone, “enlarged modified kraft cooking” (EMCC) Known as.

  Furthermore, in the 1990s, the Lo-Solids cooking process and related equipment were introduced, which became the next dramatic improvement of the kraft cooking process. In this method, the waste cooking liquor is selectively withdrawn at the first stage of the pulp production process and is replenished with cooking liquor and diluent, for example, a washer filtrate containing only a low concentration of lysate. It became possible to produce clean cellulose pulp.

  Patent Document 6 (Japanese Patent Laid-Open No. 2000-336586) and Patent Document 7 (Japanese Patent Laid-Open No. 2000-336587) propose a pulp yield improving technique corresponding to the above-mentioned new cooking method. These technologies use hardwood or softwood chips, contain polysulfide sulfur at a concentration of 3 to 20 g / L as sulfur, and have a total cooking active sulfur content and total alkali contained in an alkaline cooking solution introduced into the cooking system. An alkaline cooking liquid containing 45 to 100% by mass of sulfur and 45 to 79% by mass of effective alkali is added to the top of the digester and further contains 0.01 to 1.5% by mass of a quinone compound per absolutely dry chip. There is provided a method for cooking lignocellulosic material, characterized in that an alkaline cooking solution is supplied to the digester.

  However, further improvements in pulp yield and reduction in chemical intensity are required.

  On the other hand, while interest in the environmental impact of substances discharged from the bleaching process of pulp and paper mills is increasing, ECF that does not use elemental chlorine from conventional bleaching methods mainly using chlorine and / or chlorinated chemicals. Bleaching and TCF bleaching, which does not use any chlorinated chemicals at all, are becoming mainstream all over the world.

  However, most of chemicals such as chlorine dioxide, hydrogen peroxide, oxygen, and ozone, which are mainly used in ECF bleaching and TCF bleaching, are more expensive than conventional bleaching chemicals such as chlorine and sodium hypochlorite. There is a problem that the processing cost of ECF bleaching and TCF bleaching becomes high.

  Under these circumstances, there is a demand for the development of a bleaching method for pulp that can reduce the amount of bleaching chemicals used in ECF bleaching and TCF bleaching and can reduce bleaching costs.

  It is known that hexeneuronic acid is greatly involved in addition to lignin as a substance that consumes pulp bleaching chemicals. This hexeneuronic acid is produced by demethylation from methylglucuronic acid in hemicellulose in the cooking process and has a double bond, so it reacts with potassium permanganate and is counted as a kappa number, and at the same time, bleaching chemicals Reacts and consumes bleaching chemicals. Therefore, if the hexeneuronic acid can be removed, the efficiency of the bleaching chemical can be improved.

  As one method for removing hexeneuronic acid, a relatively high-temperature acid treatment technique has been proposed. In this method, the hexeneuronic acid is hydrolyzed and removed by treating the unbleached pulp under high temperature and acidity, thereby reducing the kappa number and bleaching the pulp to the desired whiteness. It is known that the bleaching chemicals required for this can be reduced. For example, in Patent Document 8 (Japanese Patent Publication No. 10-508346), treatment is performed at a pH of about 2 to 5 at about 85 to 150 ° C., and at least about 50% of hexeneuronic acid in the pulp is removed. A technique for reducing the kappa number by 2 to 9 units is disclosed. Among them, preferred pH is described as 2.5 to 3.5 for softwood pulp and 3 to 5 for hardwood pulp. Examples of the acid to be used include mineral acids such as sulfuric acid, nitric acid and hydrochloric acid, and organic acids such as formic acid and acetic acid.

  However, in this technique, the pulp must be processed at a high temperature of 85 to 150 ° C or higher, and a large amount of steam is required to raise the temperature and maintain the reaction temperature. Since the pressure rises in the inside, high pressure equipment is necessary, and there is a practical problem such as requiring a large amount of investment. However, when this warm acid treatment method is used to decompose and remove hexeneuronic acid to a level equivalent to that obtained by chlorine bleaching (for example, 1 mmol / kg absolutely dry pulp), the pulp is kept at low pH and high temperature. It is necessary to treat for a long time, and there is a problem that not only hexeneuronic acid but also cellulose is partially hydrolyzed and the pulp fiber is damaged.

  Patent Document 9 (Japanese Patent Application Laid-Open No. 2003-96680) discloses unbleached pulp having a kappa number of 18 to 23 and a hexeneuronic acid content of 35 to 45 mmol / kg of absolutely dry pulp by digesting hardwood. A method for producing paper pulp having improved fading property by further bleaching the hexeneuronic acid amount to less than 10 mmol / kg of absolutely dry pulp is disclosed.

  As described above, in ECF bleaching and TCF bleaching, pulp bleaching technology that minimizes bleaching chemical consumption and improves fading by efficiently removing hexeneuronic acid, which consumes bleaching chemicals, from the pulp. Development is desired.

Japanese Patent Publication No.55-1398 Japanese Patent Publication No.57-19239 Japanese Patent Publication No.53-45404 JP-A-52-37803 Japanese Unexamined Patent Publication No. 7-189153 JP 2000-336586 JP 2000-336587 Japanese National Patent Publication No. 10-508346 JP2003-96680

Svensk Papersting, 87 (10): 30 (1984)

  However, further improvements in pulp yield and reduction in the amount of bleaching chemicals are required. An object of the present invention is to provide a kraft cooking method capable of obtaining unbleached pulp which is easy to bleach while having a high yield from a hardwood material.

  As a result of earnestly examining the kraft cooking method of unbleached pulp that is easy to bleach while being high in yield, the present inventors have determined that hardwood wood has a cooking temperature of 130 ° C. or more and less than 150 ° C. and a cooking time of 240 minutes or more. It has been found that the above problem can be solved by performing kraft cooking and obtaining unbleached pulp having a copper number of 16 to 25 and a hexeneuronic acid content of 50 to 70 mmol / kg.

  Although the hexeneuronic acid content of the unbleached pulp of the present invention is relatively high, by performing the acid treatment after the oxygen delignification treatment, the hexeneuronic acid content can be reduced to a level where there is no problem.

  According to the present invention, it is possible to obtain an unbleached pulp which can be easily bleached while using a hardwood as a raw material in a high yield.

  In the present invention, the hardwood material kraft-cooked in the cooking process is not particularly limited, and any material can be used.

  Kraft cooking in the present invention includes kraft-anthraquinone cooking, kraft-polysulfide cooking, and kraft-polysulfide-anthraquinone cooking in addition to ordinary kraft cooking. Furthermore, as for the kraft cooking method, MCC, EMCC, ITC, Lo-solid, Compact Cooking method and the like are known as correction methods, and the present invention can be applied to these methods. For example, when kraft cooking wood, the kraft cooking white liquor has a sulfidity of 5 to 75%, preferably 20 to 35%, and an effective alkali addition rate of 5 to 30%, preferably 10 ~ 25%. Moreover, you may add a cooking aid as needed.

  The cooking aid is a quinone compound, a hydroquinone compound or a precursor thereof, and at least one compound selected from these can be used. Examples of these compounds include anthraquinone, dihydroanthraquinone (for example, 1,4-dihydroanthraquinone), tetrahydroanthraquinone (for example, 1,4,4a, 9a-tetrahydroanthraquinone, 1,2,3,4-tetrahydroanthraquinone). Methyl anthraquinone (eg 1-methyl anthraquinone, 2-methyl anthraquinone), methyl dihydroanthraquinone (eg 2-methyl-1,4-dihydroanthraquinone), methyl tetrahydroanthraquinone (eg 1-methyl-1,4,4a) , 9a-tetrahydroanthraquinone, 2-methyl-1,4,4a, 9a-tetrahydroanthraquinone) and the like, anthrahydroquinone (generally 9,10-dihydroxyanthracene), Tyranthrahydroquinone (for example, 2-methylanthrahydroquinone), dihydroanthrahydroanthraquinone (for example, 1,4-dihydro-9,10-dihydroxyanthracene) or an alkali metal salt thereof (for example, disodium salt of anthrahydroquinone, 1 , 4-dihydro-9,10-dihydroxyanthracene disodium salt), and precursors such as anthrone, anthranol, methylanthrone, and methylanthranol.

  The cooking temperature must be 130 ° C. or higher and lower than 150 ° C., preferably 135 ° C. or higher and 145 ° C. or lower. When cooking so that the kappa number is in the range of 16 to 25, if the cooking temperature is less than 130 ° C, the pulping is insufficient and the amount of soot is increased, and if it exceeds 150 ° C, the pulp yield is reduced, which is preferable. Absent.

  The cooking time in the present invention is the time from when the cooking temperature reaches the maximum temperature until the temperature starts to decrease. The cooking time needs to be 240 minutes or more, preferably 240 minutes or more and 480 minutes or less. If the cooking time is less than 240 minutes, pulping does not proceed, and if it exceeds 480 minutes, the pulp production efficiency deteriorates, which is not preferable. The liquid ratio is preferably 2.5 to 10. If the liquid ratio is less than 2.5, the amount of cooking chemical liquid is too small relative to the wood chips, so that cooking is insufficient, and if the liquid ratio exceeds 10, the production efficiency is lowered, which is not preferable.

  In the present invention, kraft cooking is performed so that the unbleached pulp obtained has a kappa number of 16 to 25 and a hexeneuronic acid content of 50 to 70 mmol / kg. When cooking is continued until the kappa number is less than 16, the yield decreases greatly. Further, when the cooking is continued until the hexeneuronic acid content is less than 50 mmol / kg, the decrease in yield increases, and when it exceeds 70 mmol / kg, the decrease in whiteness during long-term storage increases. The content of hexeneuronic acid in the pulp was acid hydrolysis treatment at pH 1.8 and 95 ° C. for 4 hours, the pulp slurry was filtered, and 2-furan carboxylic acid (2-furan carboxylic acid in the filtrate) was filtered. acid) and 5-formyl-2-furan carboxylic acid (both are acid hydrolysates of hexeneuronic acid) measured by HPLC (high performance liquid chromatography). And the hexeneuronic acid content in a pulp can be calculated | required from the sum total of the molar amount.

  Kraft cooking can set cooking temperature and cooking time using H-factor (HF) as an index. The H-factor is a standard representing the total amount of heat given to the reaction system during the cooking process, and is represented by the following formula in the present invention. The H-factor is calculated by time-integrating the term on the right side of the integration symbol from the time when the chip and the alkaline cooking liquid are mixed to the time when cooking ends.

HF = ∫ln ⁻¹ (43.20−16113 / T) dt
(In the formula, HF represents an H-factor, T represents an absolute temperature at a certain time point, and dt is a function of time changing with time according to a temperature profile in the digester).

  Conventionally, it was considered that if the H-factor is about the same in the cooking chemical solution having the same composition, a pulp having the same kappa number and yield can be obtained even if the cooking temperature and cooking time are different. It has been found that in the range of the cooking temperature of 130 ° C. or higher and lower than 150 ° C. of the invention, compared to the case where the cooking temperature is 150 ° C. or higher, a kappa number is low and a high yield pulp can be obtained.

  In the present invention, the unbleached pulp obtained after cooking is subjected to a known oxygen delignification treatment as necessary. As for the oxygen delignification used in the present invention, a known medium concentration method or high concentration method can be applied as it is, but currently, a medium concentration method in which the pulp concentration used for general purposes is 8 to 15% by mass is used. preferable. Sodium hydroxide and potassium hydroxide can be used as the alkali in oxygen delignification, and oxygen gas includes oxygen from a cryogenic separation method, oxygen from PSA (Pressure Swing Adsorption), VSA (Vacuum Swing Adsorption). Oxygen etc. from) can be used.

The oxygen gas and alkali are added to a medium-concentration pulp slurry in a medium-concentration mixer. After sufficient mixing, the oxygen gas and alkali are sent to a reaction tower capable of holding a mixture of pulp, oxygen, and alkali for a certain period of time under pressure. Ligned. The reaction conditions for the oxygen delignification treatment are not particularly limited as long as they are conventionally performed. The oxygen pressure is 3 to 9 kg / cm ² , more preferably 4 to 7 kg / cm ² , and the alkali addition rate is 0.5-4 mass%, temperature is 80-120 ° C, more preferably 85-105 ° C, treatment time is 30-180 minutes, more preferably 50-90 minutes, pulp concentration is 8-15 mass%, Known conditions can be applied. In the present invention, the oxygen delignification treatment may be performed a plurality of times.

  The pulp subjected to the oxygen delignification treatment is then sent to the washing step, and after washing, it is sent to the multistage bleaching step, where it can also be subjected to the multistage bleaching treatment. The multi-stage bleaching treatment of the present invention is not particularly limited, but acid (A), chlorine dioxide (D), alkali (E), oxygen (O), hydrogen peroxide (P), ozone (Z), It is preferable to combine a known bleaching agent such as peracid and a bleaching aid. For example, the first stage of the multistage bleaching process uses a chlorine dioxide bleaching stage (D) or an ozone bleaching stage (Z), the second stage is an alkali extraction stage (E), the hydrogen peroxide stage (P), the third stage or later. A bleaching sequence using chlorine dioxide or hydrogen peroxide is preferably used. The number of stages after the third stage is not particularly limited, but considering energy efficiency, productivity, etc., it is preferable to finish in three or four stages in total. Further, a chelating agent treatment stage with ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) or the like may be inserted during the multistage bleaching treatment.

  In the present invention, it is preferable to perform the acid treatment after the oxygen delignification treatment because the amount of bleaching chemicals used in multistage bleaching can be reduced. The amount of hexeneuronic acid can be effectively reduced by acid treatment. As the type of acid to be used, mineral acids such as sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, nitrous acid, and residual acid of chlorine dioxide generator can be used. Sulfuric acid is preferred.

  The pH during the acid treatment is preferably 2.0 to 4.0. When the pH is less than 2.0, the removal of hexeneuronic acid and harmful metals is sufficient, but the acid is excessive and the pulp viscosity is greatly reduced. On the other hand, when the pH exceeds 4.0, the acid concentration is low, and the removal of hexeneuronic acid and harmful metals becomes insufficient. By setting the pH during acid treatment to 2.0 to 3.0, the temperature during acid treatment can be lowered, the amount of steam used can be reduced, and the acid treatment cost can be reduced.

  The reaction temperature of the acid treatment is in the range of 80 to 95 ° C. If the temperature is lower than 90 ° C., the metal removal effect is effective, but the removal effect of hexeneuronic acid is low. On the other hand, when the temperature exceeds 95 ° C., the effect of removing metal and hexeneuronic acid is enhanced, but a large amount of steam is required to raise and maintain the reaction temperature, and the processing cost increases. Further, if the temperature is 100 ° C. or higher, the pressure in the reaction vessel becomes high, and a high-pressure vessel for oxygen treatment is necessary.

  The reaction time is 30 minutes to 5 hours, preferably 30 minutes to 4 hours, more preferably 1 to 3 hours. If it is less than 30 minutes, the reaction by the acid is not sufficient, and even if the treatment is performed for more than 5 hours, the increase in the acid treatment effect is small.

Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited thereto.
Pulp copper number: Measured according to JIS P 8221.
Yield: calculated from the absolute dry weight of wood chips before cooking and the absolute dry weight of unbleached pulp obtained after cooking.
・ Fraction rate: Unbleached pulp obtained after cooking was carefully selected with a 6-cut flat screen, and the absolute dry weight of the material that did not pass through the screen was calculated as the drought.
・ Quantitative determination method of hexeneuronic acid in pulp: pH 1.8 (adjusted with dilute sulfuric acid) prepared from fully washed pulp, 50 g of 10% by weight pulp slurry in a heat-resistant plastic bag, Acid hydrolysis treatment was performed for a time. After the treatment, the pulp slurry is filtered, and 2-furan carboxylic acid and 5-formyl-2-furan carboxylic acid (both hexene uron in the filtrate). The amount of acid hydrolyzate) was measured by HPLC (high performance liquid chromatography, manufactured by Shimadzu Corporation, LC-10 series), and the original amount of hexeneuronic acid was determined from the total molar amount. . Moreover, the removal rate of hexeneuronic acid was calculated | required from the following formula.
100- (Amount of hexeneuronic acid after reaction / Amount of hexeneuronic acid before reaction) × 100 (%)
Pulp whiteness measurement: Pulp was disaggregated, a sheet having a basis weight of 60 g / m ² was prepared according to Tappi test method T205os-71 (JIS P 8209), and a whiteness meter (Color Co., Ltd.) The ISO whiteness of the pulp was measured with a technical laboratory product, trade name: CMS-35SPX.

[Example 1]
Mixed hardwood chips made of eucalyptus and domestic hardwood (mixing ratio 85:15), active alkali addition rate 18%, sulfidity 25%, liquid ratio 2.5 (L / kg), cooking time 417 minutes, maximum temperature 140 ℃ , Cooking with a rotary autoclave at H-factor (HF) = 478. The resulting pulp was measured for kappa number, yield, percentage, hexeneuronic acid amount, and whiteness, and the results are shown in Table 1.

[Example 2]
Cooking was performed in the same manner as in Example 1 except that the cooking time was 262 minutes and the maximum temperature was 145 ° C.

[Example 3]
Cooking was conducted in the same manner as in Example 1 except that the cooking time was 704 minutes, the maximum temperature was 135 ° C., and HF = 478.

[Comparative Example 1]
Cooking was carried out in the same manner as in Example 1 except that the cooking time was 156 minutes and the maximum temperature was 150 ° C.

[Comparative Example 2]
Cooking was carried out in the same manner as in Example 1 except that the cooking time was 186 minutes, the maximum temperature was 145 ° C., and HF = 346.

[Comparative Example 3]
Cooking was carried out in the same manner as in Example 1 except that the cooking time was 400 minutes, the maximum temperature was 150 ° C., and HF = 1148.

  As shown in Table 1, when cooking was performed such that the kappa number was 16 to 25 and the hexeneuronic acid content was in the range of 50 to 70 mmol / kg, in Examples 1 to 3, the yield was high. An unbleached pulp with a low percentage of wrinkles was obtained. However, in Example 3 where the cooking time was lengthened, the whiteness decreased. On the other hand, in Comparative Example 1 in which the cooking temperature was 150 ° C. or higher, the yield was low and the soot rate was high. Moreover, in Comparative Example 2 where the cooking time was less than 240 minutes, the rate of koji and the kappa number were high. If the cooking temperature is 150 ° C. or higher and the cooking time is lengthened, the rate of soot decreases but the yield decreases greatly.

[Example 4]
The unbleached pulp produced in Example 1 was subjected to multistage bleaching treatment of oxygen delignification treatment-acid treatment-D ₀ stage treatment-E / P-D ₁ stage treatment under the following conditions to obtain bleached pulp. The whiteness and hexeneuronic acid content of the obtained bleached pulp were measured, and the results are shown in Table 2.
Oxygen delignification treatment: Pulp concentration 10%, oxygen pressure 6 kg / cm ² , sodium hydroxide addition amount 2% (per dry pulp mass), temperature 98 ° C., treatment time 60 minutes.
Acid treatment conditions: Pulp concentration 10%, sulfuric acid addition 10% (per dry pulp mass), temperature 88 ° C., treatment time 120 minutes, starting pH 3.0.
D ₀ stage treatment conditions: pulp concentration 10%, chlorine dioxide addition amount 0.85% (per dry pulp mass), temperature 58 ° C., treatment time 40 minutes.
E / P treatment conditions: Pulp concentration 10%, sodium hydroxide addition 1.1% (per dry pulp mass), hydrogen peroxide addition 0.4% (per dry pulp mass), temperature 60 ° C., treatment time 60 minutes.
D _1- stage treatment conditions: pulp concentration 10%, chlorine dioxide addition amount 0.25% (per dry pulp mass), temperature 70 ° C., treatment time 180 minutes.

[Example 5]
The unbleached pulp produced in Example 2 was subjected to multistage bleaching in the same manner as in Example 3 to obtain bleached pulp.

[Example 6]
The unbleached pulp produced in Example 3 was subjected to multistage bleaching in the same manner as in Example 3 to obtain bleached pulp.

[Comparative Example 4]
The unbleached pulp produced in Comparative Example 1 was bleached in the same manner as in Example 3 to obtain bleached pulp.

[Comparative Example 5]
The unbleached pulp produced in Comparative Example 2 was bleached in the same manner as in Example 3 to obtain bleached pulp.

[Comparative Example 6]
The unbleached pulp produced in Comparative Example 3 was bleached in the same manner as in Example 3 to obtain bleached pulp.

  As shown in Table 2, in Examples 4 to 6, bleached pulp having high whiteness and a reduced hexeneuronic acid content to a level at which there was no problem was obtained.

Claims (3)

  A kraft cooked hardwood material with a cooking temperature of 130 ° C or higher and lower than 150 ° C and a cooking time of 240 minutes or longer to obtain an unbleached pulp having a copper number of 16 to 25 and a hexeneuronic acid content of 50 to 70 mmol / kg Production method.   The method for producing pulp according to claim 1, wherein the unbleached pulp is treated by oxygen delignification treatment, acid treatment, and further in a multi-stage bleaching step to obtain bleached pulp.   Unbleached pulp with a kappa number of 16 to 25 and a hexeneuronic acid content of 50 to 70 mmol / kg, obtained by kraft cooking of hardwood wood under a cooking temperature of 130 ° C or higher and lower than 150 ° C and a cooking time of 240 minutes or longer.