JPH0672385B2 - Bleaching method for lignocellulosic material - Google Patents

Abstract

The present invention relates to a process for bleaching of ligno-cellulosic pulp digested by a chemical process for reduced formation of chloro-organic compounds, while preserving the final brightness and strength properties of the pulp, by treating the pulp in a first stage with only chlorine dioxide at a charge factor of up to 2.0, whereupon the pulp is treated in an alkaline extraction stage reinforced with from 3.0 kg of hydrogen peroxide/ton of pulp. After the two-stage treatment the pulp can be finally bleached to the desired brightness in one or more chlorine dioxide stages.

Description

The present invention relates to a method for bleaching lignocellulosic pulp,
In the first treatment step, chlorine dioxide alone has a low addition coefficient (charge f
Then, in the second step, treatment with hydrogen peroxide was performed under alkaline conditions while increasing the amount of peroxide added, and the chlorophyll was maintained while maintaining the final whiteness and strength characteristics of the pulp. It relates to a method for bleaching lignocellulosic pulp which reduces the production of organic substances. After the two-step treatment described above, the resulting pulp can be finally bleached to the desired brightness with one or more treatment steps with chlorine dioxide.

The term lignocellulosic pulp described above means a chemical pulp derived from softwood and / or hardwood that has been delignified by the sulfite method, the sulfate method, the soda method or the organic solvent method or a modified method and / or combination thereof. Prior to the bleaching sequence with chlorine containing chemicals according to the present invention, the pulp may be delignified in an oxygen step.

BACKGROUND ART When producing a high-whiteness chemical pulp, first, wood chips are digested to separate cellulose fibers. During the cooking, a part of the lignin that holds the cellulose fibers is decomposed and modified and can be removed in the next washing step. However, in order to obtain sufficient whiteness, further lignin must be removed together with the whiteness-damaging group (chromophore). This treatment is often carried out by a delignification step with oxygen and several bleaching steps that follow this step.

Bleaching of chemical pulp is mainly carried out with chlorine-containing bleaching agents, such as chlorine, chlorine dioxide and hypochlorite, in addition to alkali and hydrogen peroxide and / or
Alternatively, it is performed in an intermediate extraction step with oxygen. The chlorine-containing substance reacts with lignin to make it soluble in the same process as above or in the subsequent alkali extraction process.

The reaction products produced in the bleaching process with chlorine-containing chemicals lead to environmentally harmful emissions. These chloroorganic compounds are, among others, AOX (= absorbable amount o
f organic halogen: absorption amount of organic halogen), and its level can be calculated by the following formula (1).

AOX = k ₁ × (C + D / k ₂ ) (1) where C = total amount of added Cl ₂ D = total amount of added ClO ₂ calculated as active chlorine k ₁ = about 0.1, k ₂ = about 5 chlorine content The amount of active chlorine is shown so that the bleaching action of substances can be compared, and this amount is the value obtained by multiplying the oxidation potential by the ratio of chlorine in the molecule. Therefore, 1 kg of chlorine dioxide corresponds to 2.63 kg of active chlorine.

Obviously, the effect of chlorine dioxide on AOX formation is only one fifth of molecular chlorine. Changes to higher proportions of chlorine dioxide have occurred over the last few years as environmental legislation has become increasingly stringent. The level of approved emissions is expected to be further reduced, and there is a need to find a chlorine-free bleaching method that meets the customer's desire for final product whiteness.

The normal bleaching sequence for softwood treated by the sulfate method is as follows.

(C + D) E1 D E2 D where (C + D) = addition of chlorine (C) and chlorine dioxide (D) to the same step.

D = chlorine dioxide process.

E1 = alkali extraction step, optionally peroxide (EP) and /
Or add oxygen (EPO and EO, respectively).

E2 = alkaline extraction step, optionally adding peroxide (EP).

The (C + D) and E1 steps are defined as pre-bleached sequesins. Step D E2 D is called final bleaching. In this application,
This final bleaching can be limited to one or two D steps. The aforementioned delignification with oxygen is usually regarded as part of the cooking process.

The amount of active chlorine used / necessary in the (C + D) step is defined as the addition coefficient CF represented by the following formula (2).

CF = total active chlorine amount (kg / tonn of pulp / (C + D) Kappa number before step) (2) The amount of chloro organic compound produced is calculated from molecular chlorine according to the formulas (1) and (2), respectively. It is reduced by a combination of switching to chlorine dioxide and / or a reduction in the addition coefficient. As mentioned above, the emission of environmentally harmful substances can be reduced by reducing the Kappa number, which is a measure of the amount of lignin, ie the organic substances to be eluted. Therefore, efforts are being made to further reduce the amount of lignin that must be removed by chlorine bleaching with different pretreatment and prebleaching steps.

Modifying the pre-bleaching, for example to use predominantly chlorine dioxide in the (C + D) step, usually results in a loss of whiteness of the ready-bleached pulp, resulting in a loss of quality. Lowering the addition factor makes it increasingly difficult to obtain sufficient final whiteness. E1 and
It is possible to reduce the consumption of active chlorine while maintaining the final whiteness (90% ISO) by using several kg of hydrogen peroxide per ton of pulp in the process of E2, for example, Germgard et al. By Nordic Pulp an Paper Research Journal
d Paper Research Journal, 3 (4), 166-171 (1988). The bleaching sequence (D90 C10) E1 D E2 D was used in birch sulphate pulp with an addition factor of 2.4 (a normal level where AOX production cannot be greatly restricted).

U.S. Pat. No. 3,720,577 and West German Patent No. 2,754,949 relate to a method using 100% chlorine dioxide in the first bleaching step.
In the former patent, an addition factor of 2.7 is used in Example 1, which is a much higher value than the above value. Step D is followed by a step of peracetic acid and optionally another step D. This peracetic acid reduces the viscosity of the cellulose and produces acetic acid as a by-product, which significantly increases the COD value (chemical oxygen demand), another important measure of emissions to the environment. In West German Patent No. 2,754,949, a low addition factor is adopted in the D step, but there is no indication that the subsequent extraction step must be fortified with hydrogen peroxide in order to reach an acceptable concentration of AOX. Furthermore, these documents do not indicate the importance of using a fairly high proportion of chlorine dioxide in the final bleaching step to produce pulp with maximum bleaching capacity and low environmental impact. Not not.

Svensk Papperstidning by Alfthan et al., 88 (13)
Vol. 24, page 27 (1985), a test is made in which the first step is the bleaching sequence E1 step and optionally the E2 step is hydrogen peroxide enhanced. According to the experimental quality tests and factory tests carried out on birch sulfate pulp with a Kappa number of 18, the total chlorine dioxide consumption during final bleaching was calculated as active chlorine up to 20 kg per tonne of pulp. Can be reduced. The addition of more than 2.5 kg of H ₂ O ₂ per tonne of pulp in the first extraction step caused problems with the purity of ready-bleached pulp. Furthermore, laboratory experiments carried out on sulphate pulp of conifers show that the action of H ₂ O ₂ is considerably better during the second extraction step (during final bleaching) than during prebleaching. It was However, such changes in final bleaching have a much smaller effect on AOX emissions than the corresponding changes in prebleaching.

In addition, the pulp should already have 3.5% after the first extraction step in order to obtain an acceptable final whiteness.
To reach the following Kappa price means that Jiri Basta et al.'S TAPPI Proceedings, 1989 Pulp Conf.
erence), Volume 2, pages 427-436. This limitation reduces the possibility of using high proportions of chlorine dioxide and indeed low addition factors, as shown in FIG.

TECHNICAL PROBLEM It is already known to reduce the emission of chloroorganic compounds from chemical pulp bleaching plants by using chlorine dioxide instead of chlorine. This method is already being used on a large scale in the final bleaching step. Therefore, the normal bleaching sequence of chemically digested lignocellulosic pulp is currently composed of (C + D) E D E D, the extraction process of which is optionally enhanced. Prior art processing techniques that further reduce AOX (= organohalogen compound uptake) emissions have limited success in obtaining acceptable final whiteness with process modifications,
It is mainly composed of expensive external processes.

AO is also used in the preliminary bleaching process by using chlorine dioxide instead of chlorine.
It would of course be desirable to be able to reduce the amount of X and at the same time reduce emissions. However, it has hitherto not been possible to obtain a process that can obtain an acceptable final whiteness of the pulp by modifying the subsequent extraction and final bleaching steps.

DISCLOSURE OF THE INVENTION The present invention relates to a treatment method in which the AOX emissions are greatly reduced and the whiteness and strength properties after the final bleaching are simultaneously maintained. This process is performed in two steps.
That is, the pulp is first bleached with chlorine dioxide at a low addition factor, then treated with hydrogen peroxide fortification, and optionally with oxygen treatment in the second step. This two-step process provides a bleaching process that is much less harmful to the environment in that the amount of chlorine-containing chemicals in the bleaching process is reduced.

The present invention therefore relates to a method for treating lignocellulosic pulp as disclosed in the claims.

The present invention relates to a process for prebleaching pulp which is able to achieve the desired reduction of AOX by combining a low addition factor, a high proportion of chlorine dioxide and intensification of the extraction process with hydrogen peroxide.

The main difference with the prior art methods is that the delignification treatment in the chlorine-containing step of the pre-bleaching is not performed as previously thought necessary. By reducing the amount of chlorine dioxide per ton of pulp at the beginning of the bleaching sequence, it is possible to reduce the total AOX emissions to about 0.5 Kg per ton of pulp, which compared to prior art methods. This is a significant improvement. But this AO
A prerequisite for obtaining X levels is the presence of more hydrogen peroxide than previously used in order to obtain the proper brightness.

Chlorine dioxide as the only chlorine-containing bleach in step 1
It is chlorine dioxide produced in the usual way without the external addition of chlorine, i.e. so-called industrial chlorine dioxide. In other words, this chlorine dioxide contains chlorine generated during the production and dissolved in the absorption water. One example of an industrial method for producing a certain amount of chlorine is a method of reducing chlorate with chloride. Other chlorate reducing agents such as sulfur dioxide and methanol give small amounts of chlorine. Chlorine dioxide water obtained from such essentially chlorine-free processes is particularly preferred.

In the two-step method according to the present invention, the first step is carried out by treatment with industrial chlorine dioxide with an addition coefficient of 2.0 or less, preferably in the range of 0.6 to 1.8. The most preferable range of the addition coefficient is 0.75 to 1.25. PH at the end of the first step
The value should be in the range of 1-4. The time and temperature are adjusted so that the concentration of residual chlorine is 0.1 to 0.5 kg / ton pulp. The pulp concentration should be 1 to 40% by weight,
It is preferably 5 to 15% by weight.

In the second step, the pulp has a pH of 7 or higher, preferably 7-
Treated with hydrogen peroxide under a pH range of 13. The most preferred pH range is 8-12. The second step is pulp 1
It can be fortified with up to 10 kg of oxygen per ton, but if no oxygen is used at all, more peroxide will remain unreacted after extraction. This residual peroxide continues to react with the pulp, improving its properties and economics. Hydrogen peroxide is added in an amount of 3.0 kg or more per ton of dry pulp. The upper limit is not critical, but for economic reasons it can be set up to 20 kg per ton of dry pulp. Hydrogen peroxide is added per ton of dry pulp
The range of 40 to 10 kg is preferred. The pH at the end of the second step is
It may be in the range of 8.5 to 13, and preferably 10 to 12. The residence time may be from about 30 minutes to about 6 hours at a temperature of about 50-130 ° C, preferably 80-100 ° C. The pulp concentration may be 1 to 40% by weight, preferably 5 to 15% by weight.

Lignocellulosic pulp is a chemical pulp derived from softwood and / or hardwood, which has been digested by the sulfite method, the sulfate method, the soda method or the organic solvent method, or a modified method and / or combination thereof. The method according to the invention is
It is also applicable to the bleaching of treated pulp, where the bleaching sequence is preceded by a delignification treatment by a step with oxygen.

After the above two-step treatment, the resulting pulp is fortified with the sequence D E D, D (EP) D, D D, D, or the chlorine dioxide process (D) and optionally hydrogen peroxide (P). In another combination with the extraction step (E), it can be finally bleached with chlorine dioxide to the desired whiteness in a known manner. A preferred embodiment of the present invention is the first step using chlorine dioxide with an addition coefficient of 2.0 or less, the second step using hydrogen peroxide having a pH of more than 7 and an addition amount of 3.0 kg or more per ton of dry pulp, And at least including a third step with chlorine dioxide, optionally with an intermediate extraction step,
It is a bleaching sequence. Most preferred is a final bleaching sequence consisting of two chlorine dioxide steps and an intermediate extraction step optionally enriched with hydrogen peroxide.

According to the invention, the ratio of the amount of chlorine dioxide added in the pre-bleaching sequence to the amount of chlorine dioxide added in the final bleaching sequence is 1: 1 to 1: 5, preferably 1: 1 to 1: 1. It was also found that AOX reduction was improved when it was within the range of 3.

Examination of the quality of the pulp produced by the prior art method and the pulp produced according to the invention showed that the viscosity number of the pulp, which is a measure of the chain length of the carbohydrate, does not change. High viscosity means long chain length and thus strong pulp. Furthermore, the finally bleached pulp has a Kappa number as low as the reference pulp, which means that it has a low amount of undissolved lignin. Whiteness after final bleaching can be up to about 89.5% ISO. This whiteness is sufficient and acceptable for this type of pulp. Whiteness is% IS
Measured as O, it is a well-accepted standardized technique in the cellulose industry.

The invention and its advantages are explained in more detail in the following examples, which are intended only to illustrate the invention and not to limit it.

In the following examples, the Kappa number after the oxygen treatment step is 17.
0, viscosity is 1040dm ³ / Kg and whiteness is 33.3% ISO,
Sulfate pulp of coniferous wood which was subjected to delignification treatment with oxygen was used. The Kappa number, viscosity and whiteness of the pulp, and the amount of AOX were all measured by the Scandinavian Pulp Standards method. The amount and ratio of chlorine dioxide was calculated as active chlorine.

Example 1 The purpose of this example is (C + D) for chemical pulp.
It is to show the effect of the proportion of chlorine dioxide (D) in the process on the amount of AOX produced in the (C + D) process, the next E1 process and the final bleaching sequence D E2 D. The addition factors were all 1.8.

Processing conditions: (C + D) step: Final pH is about 1. at 15% and 50% D.
5, 100% D 2.5 ± 0.3; time and temperature were adjusted so that the content of residual chlorine was 0.1 to 0.5 kg per ton of pulp.

E1 step; final pH 11.0 ± 0.3; 90 ° C .; 60 minutes; 2 kg H ₂ O _{2 /} ton pulp; 5 kg oxygen / ton pulp.

Step D: final pH 2.5 ± 0.3; time and temperature were adjusted so that the residual chlorine content was 0.1 to 0.5 kg per ton of pulp.

Step E2: final pH 11.0 ± 0.3; 60 ° C .; 60 minutes; 2 kg H ₂ O _{2 /} ton pulp.

Step D: final pH is 3.4 ± 0.2; time and temperature were adjusted so that the content of residual chlorine was 0.1 to 0.5 kg per ton of pulp.

As can be seen from Table I above, the majority of AOX emissions occur during the pre-bleaching step. Therefore, D in the final bleaching process
It seems more weight to improve the chlorine-containing process rather than the process. According to equation (1), the amount of AOX produced decreases as the proportion of chlorine dioxide in the first step increases. Unexpectedly, the effect on AOX production is much greater in the E1 step than in the (C + D) step. When 100% chlorine dioxide is added, the amount of AOX in the first extraction step is at a level equivalent to the emission from the entire final bleaching step. Total 1.0 Kg AOX for the entire bleaching sequence
To make it less than (per ton of pulp) the change to 100% chlorine dioxide is not sufficient and further measures are needed as shown in Example 2.

Example 2 The purpose of this example is to determine the addition factor (CF) according to equation (2), the addition of hydrogen peroxide in the E1 step, the entire bleaching sequence for pulps which have been delignified with oxygen.
To show the relationship between AOX emissions and quality characteristics. The bleaching sequence was D E1 D E2 D with two first steps involved in the pre-bleaching. As shown in the table, varying amounts of 100% chlorine dioxide were used in the prebleaching step D. A total of 32 kg of chlorine dioxide (per ton of pulp), calculated as active chlorine, was used in the final bleaching step.

Treatment conditions: Step D: final pH 2.5 ± 0.3; time and temperature were adjusted so that the content of residual chlorine was 0.1 to 0.5 kg per ton of pulp.

E1 step: Final pH 11.0 ± 0.3; 90 ° C .; 60 minutes; 5 kg oxygen per ton pulp.

Step D: final pH 2.5 ± 0.3; time and temperature were adjusted so that the residual chlorine content was 0.1 to 0.5 kg per ton of pulp.

Step E2: final pH 11.0 ± 0.3; 60 ° C .; 60 minutes; 2 kg H ₂ O _{2 /} ton pulp.

Step D: final pH is 3.4 ± 0.2; time and temperature were adjusted so that the content of residual chlorine was 0.1 to 0.5 kg per ton of pulp.

As can be seen from tests 1 and 3, the total AOX emission is significantly reduced when the addition factor is reduced from 2 to 1, but the whiteness is unacceptable when the next extraction step is only the oxygen-enhanced alkaline step. A bleaching method was designed according to the invention (test 4) and the following E1 step was significantly enhanced with an addition of 6 kg hydrogen peroxide per tonne of pulp, the final whiteness of the resulting pulp being on the market. The whiteness was acceptable. This result was obtained despite the fact that the Kappa number after this extraction step was as high as 5.7,
This Kappa number is a value corresponding to insufficient delignification treatment as reported by Basta et al. Therefore, the above results were unexpected, proving that delignification in the pure D step can be reduced while preserving the quality of the final product. At the same time, the AOX production obtained was clearly the most favorable value.

The final viscosity of the pulp varied from 916 to 920 dm ³ / Kg throughout the test. This shows that the method according to the invention does not adversely affect the strength properties (measured as viscosity).

In test 2 where CF = 1.5 and the addition of hydrogen peroxide in the E1 step was 2 kg / ton pulp, the final whiteness was acceptable but the AOX production was still high, which is an undesirable value.

Example 3 This example is a method according to the present invention having a chlorine coefficient of 1.0, and AOX in pre-bleaching and final bleaching when hydrogen peroxide was added to the second step (E1) in various addition amounts.
The effect on the final production of pulp and final whiteness of pulp was shown. The treated pulp is composed of softwood non-oxygen delignified sulphate pulp, which has a Kappa value of 26.8, a viscosity of 1329 Kg / dm ³ and a whiteness of 30.0% ISO before treatment.
Met. The amount of chlorine dioxide is calculated as active chlorine.

Bleach sequence: D0 E1 D1 E2 D2 [where D0 E1
Constitutes the preliminary bleaching sequence and D1 E2 D2 constitutes the final bleaching sequence. The processing conditions for each step of D0 and the final bleaching sequence were kept constant throughout tests 1-5.
The processing conditions were as follows.

D0: 100% chlorine dioxide; addition factor 1.0 corresponding to 27 kg of active chlorine per ton of dry pulp.

E1: 0 to 20 kg hydrogen peroxide per ton of dry pulp; fortified with oxygen.

D1: 25 kg of active chlorine per ton of dry pulp.

E2: 2 kg hydrogen peroxide per ton of dry pulp.

D2: 7 kg of active chlorine per ton of dry pulp.

The ratio between the amount of chlorine dioxide added to each of the final bleaching sequence as well as the pre-bleaching sequence is 1.19.

The results obtained are shown in the table below.

As is clear from Table III above, increasing the amount of hydrogen peroxide added in the extraction step reduces the amount of AOX produced in the final bleaching and increases pulp brightness.

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-51-7201 (JP, A) JP-A-1-168985 (JP, A) JP-A 61-138793 (JP, A) JP-A 54- 116404 (JP, A) JP-A-52-49305 (JP, A) JP-B-53-28523 (JP, B2) US Patent 3655505 (US, A)

Claims (10)

[Claims] 1. A method for reducing the amount of chloro-organic substances produced during delignification and bleaching of chemically digested lignocellulosic pulp, the first step comprising: Bleaching using chlorine dioxide as chlorine-containing bleach with an addition factor of 2.0 or less and a pulp concentration of 15% by weight or less, and in a second step,
A process as described above, characterized in that the pulp is treated with hydrogen peroxide at a pH of 7 or higher and using hydrogen peroxide in an addition amount of at least 3.0 kg per tonne of dry pulp. 2. The pulp in the first step, 1 ~
A process according to claim 1, characterized in that bleaching is carried out at a pulp concentration of 15% by weight. 3. The treated pulp is bleached to the desired brightness with chlorine dioxide in one or more steps, optionally with an intermediate extraction step. Method. 4. The treated pulp is finally bleached to the desired brightness with chlorine dioxide in two steps, optionally with hydrogen peroxide, with an intermediate extraction step. The method according to claim 3, wherein 5. The ratio of the amount of chlorine dioxide added in the pre-bleaching sequence to the amount of chlorine dioxide added in the final bleaching sequence is in the range of 1: 1 to 1: 5. The method according to claim 1 or 3, wherein 6. The method according to claim 1, wherein the addition coefficient is in the range of 0.6 to 1.8. 7. The method according to claim 1, wherein the addition coefficient is in the range of 0.75 to 1.25. 8. The amount of hydrogen peroxide is 4. per ton of dry pulp.
A method according to claim 1, characterized in that it is in the range of 0 to 10 kg of hydrogen peroxide. 9. The method of claim 1, wherein the second treatment step is oxygen enhanced. 10. Process according to claim 1, characterized in that the lignocellulosic pulp is treated with an oxygen delignification step after cooking, but before the treatment according to claim 1.