JP4513959B2 - Semi-bleaching bleaching process for chemical pulp for papermaking - Google Patents

Description

  The present invention relates to semi-bleach bleaching of chemical pulp for papermaking, and more particularly to a method for improving the bleaching effect of peroxide bleaching.

  Semi-bleached chemical pulp (abbreviated as SBKP) is a chemical pulp with a whiteness of about 50% to 70%, and its use is a pulp that is blended about 10% to 20% mainly to give the strength of newspapers. is there.

  The bleaching of SBKP is carried out by single-stage or multi-stage bleaching. Conventionally, in this single-stage or multi-stage bleaching treatment, a chlorine chemical is used as a bleaching agent. Specifically, hypochlorite (H stage) is used in single stage bleaching. In multi-stage bleaching, as a bleaching agent, a combination of chlorine (C stage), alkali extraction (E stage), hypochlorite (H stage), chlorine dioxide treatment (D stage), for example, C-E-H, Semi-bleach bleaching is performed by a bleaching sequence such as C / D-E-H. Here, C / D is a combined treatment of chlorine and chlorine dioxide. However, these chlorine bleaching chemicals produce organic chlorine compounds that are harmful to the environment during bleaching, and environmental pollution of bleaching wastewater containing these organic chlorine compounds has become a problem.

  It is most effective to reduce or prevent the use of chlorinated chemicals in order to reduce or prevent organochlorine by-products. Especially, it is most effective not to use molecular chlorine in the first stage. Is the method. Pulp produced by this method is called ECF (elementary chlorin free) pulp, and pulp produced without using any chlorinated chemicals is called TCF (totally chlorin free).

  Oxygen, hydrogen peroxide, chlorine dioxide, ozone, and organic peracids are examples of bleaching chemicals for ECF and TCF that produce little or no organochlorine compound. However, oxygen has the merit that the chemical cost is low, but has a problem that the reactivity with pulp is low, and sufficient whiteness cannot be obtained even if treated under high temperature and high pressure reaction conditions. Moreover, there exists a problem that a pulp viscosity will fall, if it processes on high temperature and a high pressure condition.

  Hydrogen peroxide is an excellent oxidizing agent from the environmental aspect because it decomposes into water and oxygen and does not produce any organic chlorine compounds. However, there is a problem that decomposition occurs when heavy metals such as manganese, copper, and iron are present in the pulp. In order to remove heavy metals in the pulp, acid treatment and chelating agent treatment of the pulp before hydrogen peroxide bleaching can remove the heavy metals to increase the bleaching efficiency of hydrogen peroxide and bleach to high whiteness. As bleaching conditions, a high temperature of about 90 ° C. and a high addition amount of hydrogen peroxide are required, and there is a problem that the cost is significantly increased as compared with the current chlorine bleach.

  Chlorine dioxide is an excellent oxidizing agent that can be bleached to high whiteness without reducing the viscosity of the pulp and produces less organic chlorine compounds. However, there is a problem that the chemical cost is high. Although SBKP can be bleached with chlorine dioxide to achieve high whiteness, a large amount of chlorine dioxide is required, and as a result, there is a problem that the cost is significantly increased compared with the current chlorine bleach.

  Ozone is an excellent oxidizing agent that can bleach SBKP to a high degree of whiteness, but has a problem that the viscosity of the pulp is greatly reduced due to its high reactivity. In addition, ozone generation requires expensive equipment such as an ozone generator that requires a large amount of electricity and a reactor that reacts ozone with pulp, which greatly increases the cost compared to the current chlorine bleach. Then there is a problem.

  Organic peracids such as peracetic acid and performic acid are bleaching agents that can bleach SBKP with high whiteness. However, the cost is high and there is a problem that the cost is significantly increased as compared with the current chlorine bleaching agent.

    An object of the present invention is to provide a bleaching method in which the discharge of organochlorine compounds, which is an environmental problem, is very low in a method of semi-bleaching bleaching chemical pulp for papermaking. Specifically, it is to improve the bleaching efficiency of the peroxide and provide an economical method that does not increase the cost as compared with the method performed with a conventional chlorine bleach.

As a result of intensive studies on chemicals for improving the bleaching efficiency of peroxide in the method of semi-bleaching and bleaching chemical pulp for papermaking, the present inventors have found that a small amount of chlorine dioxide (d In the second stage), it was found that the bleaching efficiency of the peroxide was greatly improved by performing the treatment for a short time, and the present invention was completed. That is, the present invention relates to the following method.

1. Semi-bleaching bleaching of chemical pulp for papermaking, characterized in that the chlorine dioxide content (wt%) is in the following range in the method of bleaching chemical pulp after oxygen bleaching by chlorine dioxide bleaching and then by bleach bleaching. Method.
(1) When chemical pulp is from hardwood pulp, within 0.016 times of unbleached Kappa value (2) When chemical pulp is from softwood pulp, within 0.04 times of unbleached Kappa value The method for semi-bleaching bleaching of chemical pulp for papermaking as described in 1 above, wherein after bleaching with chlorine dioxide, peroxide bleaching is performed without washing.

  In bleaching semi-bleached chemical pulp that is mainly used to maintain the strength of newspapers, the bleaching efficiency of peroxide bleaching can be improved by carrying out a small amount of chlorine dioxide treatment followed by peroxide bleaching. As a result of significant improvements, it is possible to produce pulp with very low organochlorine compounds. In addition, since a small amount of chlorine dioxide can be used to improve the bleaching efficiency of the peroxide, it can be carried out simply by adding a compact chlorine dioxide treatment facility to the current bleaching tower, so it can be done at low cost. In addition, as a result of greatly improving the peroxide bleaching efficiency, the bleaching cost can be implemented at a lower cost than the conventional chlorine bleaching-alkali extraction-hypochlorination bleaching or hypochlorite bleaching.

  The pulp used in the present invention is preferably a pulp containing polysulfide or derived from a chemical pulping method such as an ordinary kraft pulping method (KP), a sulfide pulping method (SP), an alkali pulping method (AP), More preferred is a pulp obtained by a kraft pulping method. Moreover, it does not specifically limit about the woody plant and herbaceous plant used for pulping.

  In the present invention, the pulp to be treated in ECF bleaching has been subjected to a known oxygen delignification treatment so as to have a Kappa value of 30 or less as a pretreatment, and preferably a pulp having a Kappa value of 20 or less.

  The unbleached pulp is then treated with chlorine dioxide (d). As for the bleaching conditions in the d stage, the chlorine dioxide usage (%) is within 0.016 times the unbleached Kappa value in the case of hardwood pulp (L material), and the unbleached Kappa value in the case of softwood pulp (N material). Within 0.04 times, preferably 0.015% by weight for L material, 0.05% by weight for N material, and 3-20% by weight, preferably 5-15% by weight. The treatment temperature is 30 to 95 ° C, preferably 40 to 60 ° C, and the treatment time is 5 to 15 minutes. The final pH after the bleaching reaction is 1-7, preferably 2-6, most preferably 3-5. The pulp after the d treatment is then subjected to an alkali treatment (Ep stage) using a peroxide together or an alkali treatment (Eop stage) using oxygen and a peroxide together. In addition, between the d process and the next Eop process, cleaning / dehydration may be performed or may not be performed. In the treatment by washing / dehydrating, a diffuser or a round net dehydrating / washing machine is used.

  In the Ep stage and the Eop stage, alkali is first added and mixed. As the alkaline agent, for example, caustic soda, caustic potash, lime, soda ash and the like can be used. Among these, caustic soda is preferably used because it is inexpensive. Moreover, the usage-amount of an alkaline agent is 0.1 to 5 weight% in conversion of NaOH, Preferably it is 0.5 to 3 weight%. As an addition place, any place may be used as long as it is before addition of peroxide. In the case of a diffuser type washer, it is added to the outlet of the diffuser, the inlet of the medium concentration pump, the inlet of the intermediate concentration mixer, and the like. In the case of a round net dewatering and washing machine, it is added to the pulp sheet, transport screw section, medium concentration pump inlet, medium concentration mixer inlet, and the like. In any method, the alkali is preferably added and mixed with the pulp before the peroxide.

  In the case of carrying out peroxide bleaching without washing after the treatment d, a magnesium salt is added and mixed before the addition of alkali as a stabilizer. As the magnesium salt, magnesium sulfate, magnesium nitrate, magnesium acetate, and magnesium chloride can be used. Among these, magnesium sulfate is preferably used because it is inexpensive. The amount of magnesium sulfate used is 0.01 to 1% by weight, preferably 0.05 to 0.5% by weight, based on the absolutely dry pulp. When sodium silicate is used as a stabilizer, the order of addition may be sodium silicate, alkali, or alkali, sodium silicate, or simultaneously. The amount of sodium silicate used is 1 to 15% by weight, preferably 2 to 10% by weight, based on the dry pulp. When using a chelating agent as a stabilizer, it must be mixed with the pulp prior to the peroxide. As the chelating agent, an aminocarboxylic acid chelating agent or an aminophosphonic acid chelating agent is used. The amount of chelating agent used is 0.05 to 0.5% by weight, preferably 0.1 to 0.3% by weight, based on the absolutely dry pulp.

As oxygen in the Eop stage, oxygen gas and air can be used, but oxygen is preferable. The amount of oxygen is preferably 0.1 to 1.0 wt% per absolute dry pulp, it is preferred operating pressure is 0.0~9.3 × 10 ⁵ Pa · s. The oxygen is added after the alkali, and may be either before or after hydrogen peroxide.

  As the peroxide in the Ep stage or the Eop stage, inorganic and organic peroxides such as hydrogen peroxide, adducts of peroxides and inorganic salts, sodium peroxide, performic acid, peracetic acid can be used, for example. In general, hydrogen peroxide is preferably used. The amount of peroxide used is 0.05 to 2% by weight, preferably 0.1 to 1% by weight, based on 100% hydrogen peroxide and per dry pulp.

  The order of addition of hydrogen peroxide and oxygen in the Eop stage may be hydrogen peroxide, then oxygen, or oxygen, then hydrogen peroxide, or simultaneous addition, as long as alkali is added before hydrogen peroxide addition. Any method may be used.

  In the semi-bleach-free chlorine bleaching method for chemical pulp for papermaking in the present invention, after treatment with a small amount of chlorine dioxide, hydrogen peroxide bleaching is carried out, whereby conventional chlorine-alkali / oxygen-hypochlorite treatment, Or, compared to the hypochlorite single-stage treatment, the first effect is that the hydrogen peroxide bleaching efficiency in the next stage is greatly improved by a small amount of chlorine dioxide treatment, resulting in improved economic efficiency compared to the conventional method. That is. The second effect is that since only a small amount of chlorine dioxide is used, the amount of organic chlorine compounds harmful to the environment is greatly suppressed.

  The third effect is that in the case of the conventional chlor-alkali / oxygen-hypochlorite treatment, since this equipment can be used, no equipment cost is required. In addition, when conventional hypochlorite single-stage treatment is performed, since a small amount of chlorine dioxide is used in the method of the present invention, the reaction time may be short, and as a result, the chlorine dioxide treatment apparatus is an apparatus of the order of a tube. But you can. In addition, if cleaning is not performed between the small amount of chlorine dioxide treatment and the subsequent hydrogen peroxide treatment, the cost of installing a washing machine can be saved.

  Next, the present invention will be specifically described with reference to an example in which hydrogen peroxide bleaching was performed after a small amount of chlorine dioxide treatment. However, the present invention is not limited to this embodiment.

  The amount of each chemical used is indicated by weight percent per dry pulp, and the amounts of chlorine dioxide and hydrogen peroxide used are in 100% conversion. The pulps used were hardwood pulp A and softwood pulp B subjected to kraft cooking-oxygen bleaching. Moreover, the analysis evaluation was based on the following method.

Pulp type A: Hunter whiteness 53.5%, Kappa value 9.4
B: Hunter whiteness 32.2%, Kappa value 12.7
・ Whiteness: JIS-P8123 (Hunter whiteness method)
・ Kappa value: JIS P8211 (Kappa value measurement method)

Reference Examples 1-6
Kraft cooking-Oxygen bleached pulp A required for chlorine dioxide 0.025%, 0.05%, 0.075%, 0.10%, 0.125%, 0.15%, and final pH 3.0 Sulfuric acid was added, and the mixture was treated for 60 minutes under the conditions of 10% pulp concentration and 60 ° C. After completion of the reaction, cold water was added and diluted so that the washing rate was 90%, followed by dehydration to a pulp concentration of 20% to obtain first-stage D-treated pulp. Further, 0.25% hydrogen peroxide and 0.5% NaOH were added to each pulp, the initial bleaching pH was set to about 11.5, the pulp concentration was 10%, and the temperature was 60 ° C. for 90 minutes. After completion of the reaction, it was diluted with cold water to a pulp concentration of 2.5% and dehydrated to a pulp concentration of 20% to obtain an alkali extracted / peroxide bleached pulp. 15 g of this pulp absolutely dried was diluted with 2 L of water, adjusted to pH 5.5, paper-made, and air-dried overnight to measure the whiteness. The results are shown in Table 1.

Reference Examples 7-12
Reference Example 1 except that the chlorine dioxide addition amount during the first stage D treatment of pulp A is 0.0%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6% The same was done. The results are shown in Table 1.

  As shown in Table 1, in the case of hardwood pulp, bleaching of hydrogen peroxide by treatment with 0.15% chlorine dioxide, that is, 0.016 times the unbleached Kappa number, before hydrogen peroxide treatment Efficiency has been improved most.

Examples 1-4
Kraft cooking-The same procedure as in Reference Example 1 was conducted except that bleached pulp B was bleached at a chlorine dioxide addition amount of 0.2%, 0.3%, 0.4%, and 0.5%. The results are shown in Table 2.

Comparative Examples 1-9
The amount of chlorine dioxide added during the first stage D treatment of pulp B is 0.05%, 0.10%, 0.15%, 0.0%, 0.7%, 0.9%, 1.1%, and 1. The same procedure as in Reference Example 1 was performed except that bleaching was performed at 3% and 1.5%. The results are shown in Table 2.

  As shown in Table 2, in the case of softwood pulp, bleaching of hydrogen peroxide by treating with 0.5% chlorine dioxide, that is, 0.04 times the unbleached Kappa value, before the hydrogen peroxide treatment Efficiency has been improved most.

Examples 5-9
The same procedure as in Example 2 was performed except that the chlorine dioxide treatment was performed for 2.5 minutes, 5 minutes, 7.5 minutes, 10 minutes, and 15 minutes.

Comparative Examples 10-13
The same procedure as in Example 5 was performed except that the chlorine dioxide treatment was performed at 0 minutes, 20 minutes, 30 minutes, and 40 minutes. The results of Examples 5-9 and Comparative Examples 10-13 are shown in Table 3.

  As shown in Table 3, in the case of the method of the present invention, the time for the small amount of chlorine dioxide treatment (d stage) was sufficient within 15 minutes. In the treatment for 15 minutes or longer, residual chlorine dioxide disappeared, and thus a decrease in whiteness due to maintaining an acidic state was observed.

Examples 10 and 11
Same manner as in Example 8, in the Example 10 was washed between the minor chlorine dioxide treatment and peroxide treatment, it did not washed is Example 11. The results are shown in Table 4.

As shown in Table 4, when no washing is performed between the small amount of chlorine dioxide treatment and the hydrogen peroxide treatment, the whiteness is slightly reduced as compared with the washing, but the small amount of chlorine dioxide treatment is not performed (Comparative Example 4 ). In comparison, the bleaching property of hydrogen peroxide was greatly improved.

Example 12
The same procedure as in Example 11 was performed except that sodium silicate 5% was used as a stabilizer during hydrogen peroxide bleaching.

Example 13
The same procedure as in Example 11 was performed except that 0.25% magnesium sulfate was used as a stabilizer during bleaching with hydrogen peroxide.

Example 14
At the time of bleaching hydrogen peroxide, the same procedure as in Example 11 was performed except that 0.25% of diethylenetriaminepentaacetic acid Na salt (DTPA) was used as a stabilizer.

Example 15
At the time of bleaching hydrogen peroxide, the same procedure as in Example 11 was carried out except that 0.25% magnesium sulfate and 0.25% diethylenetriaminepentaacetic acid Na salt (DTPA) were used as stabilizers.

Example 16
At the time of hydrogen peroxide bleaching, the same procedure as in Example 11 was carried out except that sodium silicate 5% and diethylene triamine pentaacetic acid Na salt (DTPA) 0.25% were used as stabilizers.

Example 17
Similar to Example 11 except that 5% sodium silicate, 0.25% magnesium sulfate and 0.25% diethylenetriaminepentaacetic acid Na salt (DTPA) were used as stabilizers during bleaching with hydrogen peroxide. went. The results of Examples 12 to 17 are shown in Table 5.

  As shown in Table 4, when no washing was performed between the small amount of chlorine dioxide treatment and the hydrogen peroxide treatment, the bleaching property of hydrogen peroxide was slightly lowered as compared with the washing. As shown in Table 5, without washing, when using a combination of sodium silicate, magnesium salt, chelating agent and these stabilizers, washing is performed between a small amount of chlorine dioxide treatment and hydrogen peroxide treatment. Similar bleaching efficiency could be obtained.

Claims (6)

In a method in which softwood wood pulp is digested and then oxygen bleached chemical pulp is bleached with chlorine dioxide and then half bleached to a whiteness range of 50 to 70% by peroxide bleaching. A bleaching method for chemical pulp for papermaking, characterized in that it is in the range of 0.016 to 0.04 times the unbleached Kappa value after oxygen bleaching .   The method according to claim 1, wherein after bleaching with chlorine dioxide, peroxide bleaching is carried out without washing. The method according to claim 1 or 2, wherein the chlorine dioxide is used in an amount of 0.20 wt% or more and 0.5 wt% or less per pulp.   The method according to claim 1 or 2, wherein the chlorine dioxide treatment time is within 15 minutes.   The method according to claim 2, wherein one or more of sodium silicate, magnesium salt and chelating agent is used as a stabilizer for peroxide bleaching.   The bleaching method according to claim 1 or 2, wherein the peroxide is hydrogen peroxide.