FI117392B - Multi-step bleaching process for bleaching chemical pulp - Google Patents

Description

117392 i

Multi-step bleaching process for chemical pulp bleaching - Flerstegsblekningsforfarande for blekning av chemisk in cellulose

The present invention relates to a multi-step bleaching process for bleaching a chemical pulp of cellulose.

The purpose of chemical pulping is to complete the removal of residual lignin from the pulp after cooking. Nowadays bleaching is often started with oxygen delignification, after which bleaching can be carried out by various methods. In completely chlorine-free bleaching (TCF), totally delignification can be continued, for example, with ozone-10, peracetic acid or hydrogen peroxide under acidic or alkaline conditions. For Al *, elemental chlorine-free bleaching (ECF) uses the chlorine dioxide steps and the alkaline steps between them. ECF bleaching has increasingly begun to use oxygen chemicals such as oxygen, ozone, hydrogen peroxide and peracids to enhance bleaching. For example, the use of hydrogen peroxide in ECF bleaching sequence 15 can save chlorine dioxide. On the other hand, for environmental reasons, lower doses of chlorine dioxide in bleaching have begun. In addition, methods have been developed using chlorine dioxide and peracetic acid in the same step.

Peracetic acid (PAA) is a compound formed by acetic acid and hydrogen peroxide. reacting in the presence of a catalyst. Various products are commercially available, for example, 38% distilled peracetic acid (dPAA) and an equilibrium mixture of peracetic acid (ePAA), typically containing about 20% by weight of peracetic acid.

'* · I • · · · · m · ;; : ...; U.S. Patent 3,865,685 discloses a bleaching process comprising a plurality of * 1: T: alternating steps employing chlorine dioxide and the parent compound as active::: T: bleaching agents. In particular, the process uses hydrogen peroxide as the parent compound.

»·» * »M 25 Applicant's Unpublished FI Patent Application 974 221 describes an enhanced kerni- • · · ·. 1 · [/ ali pulp bleaching process using optionally .e v for final bleaching: chlorine dioxide (ClOj) or a combination of chlorine dioxide and peracetic acid (C102-PAA).

»· ·. "· *** The disadvantage of normal concomitant use of chlorine dioxide and peracetic acid is: a relatively high amount of peracetic acid (the optimum ratio of C102-PAA is 1: 1). If: f: the amount of peracetic acid is reduced, the combined amount of chlorine dioxide and peracetic acid: use in final bleaching does not significantly improve the brightness of the final mass compared to final bleaching using chlorine dioxide alone.

117392 2

Normally, pulp can be bleached relatively easily to lightness levels of 88-90% ISO by standard ECF bleaching methods. Depending on the type of wood and how the pulp is cooked, etc., further raising the brightness from this level can significantly increase the cost of chemicals. These last units of brightness are normally the most expensive in terms of the increase in brightness in terms of chemical costs.

The object of the invention is to improve the final bleaching as a multi-stage bleaching process and in particular to provide an improvement over normal chlorine dioxide final bleaching. By means of the invention it is possible to achieve an increase in brightness in final bleaching efficiently and economically inexpensively.

The main features of the invention are apparent from the appended claims.

Thus, according to the invention, there is provided an improved multi-step bleaching process for the production of a chemical pulp, which in the final bleaching step comprises post-bleaching wherein the delignified and bleached cellulose pulp is bleached with percarboxylic acid.

Said percarboxylic acid may be permic acid, peracetic acid or per-propionic acid. The preferred percarboxylic acid is peracetic acid which may be an equilibrium solution of peracetic acid in water or distilled peracetic acid. Distilled peracetic acid is particularly preferred. It is also possible to use percarboxylic acid in the form of a salt, for example an alkali metal salt, such as a sodium salt.

* * * * * V .... 20 Percarboxylic acid is preferably used in the range of about 0.1-5 kg / ton dry mass, particularly preferably about 0.5-3 kg / ton dry mass expressed as 100% * J * * * * as percarboxylic acid.

··:

According to the invention, it has been found that peracetic acid can be used as a post-whitening • • ♦ * agent to increase the brightness of already bleached pulps, so that only a few ... 25 PAA kilos can be used to increase pulp brightness by 1.5-2% ISO from a lightness level of: -91% ISO.

The pH of said final PAA bleaching step is preferably about 4-8.

• · • *.

According to a preferred embodiment of the invention, said last 1: bleaching step is preceded by a chlorine dioxide step (D step).

• ·.

* '· * ·' 30 Chlorine dioxide is preferably used in an amount of about 5 to 30 kg / tonne dry weight calculated as active chlorine.

117392 3 The pH value in D stage is preferably about 2-5.5.

If necessary, it is possible to remove the chlorine dioxide residue from the reaction medium after Step D by acidification, e.g. with sulfur dioxide or a bisulfite such as sodium bisulfite.

Preferably, according to the invention, there is no pulp washing between said last bleaching step and the preceding D step.

Thus, the process of the invention comprising the D stage and the subsequent percarboxylic acid step, preferably the PAA stage, is suitable for use as post bleaching for ECF pulps. In normal ECF bleaching, the last step is often the D step. According to the invention, it is possible to dispense peracetic acid in bleaching plants to the pulp after step D without intermediate washing. In this case, the chlorine dioxide has already run out, after which the pH of the pulp can be slightly increased before the administration of peracetic acid, if necessary. The process described above has the advantage of combining chlorine dioxide and peracetic acid (meaning in the same step) 15 that both chemicals can be used as accurately as possible within their optimum pH range. In addition, the process results in buffering and stabilizing the pH of the pulp, which may not be as successful with the combined use of chlorine dioxide and peracetic acid.

According to the invention, it has surprisingly been found that a synergistic bleaching effect of peracetic acid and chlorine dioxide is obtained, which is additionally achieved at low doses of * ·· ** '1 peracetic acid, e.g., about 1-3 kg / ton dry mass, expressed as 100-; peracetic acid.

·. *::: IM ·· * - * · *: According to another embodiment of the invention, said final bleaching step - »·· ·.:; preceded by an alkaline peroxide step (P step).

The pH in the alkaline peroxide step may be from about 8 to about 12, preferably from about 10 to about 12.

Preferably, the process is carried out without pulping between said final bleaching step and the preceding alkaline peroxide step.

► »* •« *

The latter method can be applied to TCF bleaching and ECF bleaching, · *: ending in an alkaline peroxide step, which may be pressurized peroxide 30 or depressurized. In this case, the final pH of the peroxide step is alkaline, whereby peracetic acid can be used as a final acidification chemical after the alkaline step.

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The process of the invention is particularly suitable for chemical pulp, which is a sulphate pulp obtained from hardwood or softwood. Furthermore, the process according to the invention is well suited for use in the production of market pulp for bleaching pulp going to a drying machine. The process of the invention is also suitable for use in integrated pulp and paper mills.

In the process of the invention, the consistency of the pulp is preferably from about 3% to about 30%. particularly preferably about 5-15%. According to the invention, the consistency can be reduced before the PAA step, e.g. after the D step, but without a separate washing step. Reducing or increasing the consistency by adding or removing water does not affect the operation of the invention.

In the final solution, the pulp is in most cases quite free from various interferences which are harmful to peracids, such as heavy metals (eg Fe and Mn). However, it is possible, if necessary, to add stabilizing agents, such as complexing agents, alkaline earth metal salts, or other known stabilizing agents for the coating compounds to the final stage. The use of possible stabilizers does not affect the operation of the invention as they are merely intended to stabilize the bleach so that it is not lost through possible degradation reactions and that the radicals generated in the degradation reaction do not cleave the pulp carbohydrates and damage the pulp strength.

The brightness of the pulp to be bleached before the percarboxylic acid whitening step prior to the invention in accordance with the invention is preferably 70% ISO or more, the pulp *:: *: * being preferably less than 5.

I M ·· *** · ···

The brightness of the bleached pulp before percarboxylic acid bleaching is preferably above ··. "f 85% ISO, particularly preferably 88% ISO or higher. The mass kappa number is preferably 0 '.

The temperature in the bleaching step or steps may be from about 40 ° C to about 100 ° C, preferably from about 50 ° C to about 90 ° C. The reaction time in the D stage can be about 60-300 min and in the percarboxylic acid step it can be about 1-300 min.

• · · · · »·»! *: * · *: Some chlorine dioxide reactors produce significant amounts of chlorine dioxide water. a lot of chlorine. Such chlorine dioxide water can be used in the *: 30 process of the invention. Thus, the chlorine used in the process according to the invention * · · ·. * · The water content of the dioxide water may vary from 0% to 20% of the total active chlorine / 5 ....

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The bleaching steps D-PAA and P-PAA of the invention described above may be preceded by bleaching steps known per se, of which E (alkaline extraction), H (hypochlorite), D (chlorine dioxide), O (oxygen), P (peroxide), Z (ozone) may be mentioned. ), PAA (per acetic acid) and their various combinations.

The invention will now be explained in more detail by means of Examples 1-5. In the examples, bleaching experiments were performed on delignified and bleaching pulp sulfate taken from the ECF bleacher prior to the final chlorine dioxide step. The mass is referred to herein as the base mass. The brightness of the base mass was 82% ISO and kappa 1.8.

In the examples, distilled peracetic acid containing about 38% by weight of per-10 acetic acid was used.

Example 1

The base pulp was first bleached with 5 kg / tonne pulp with chlorine dioxide (based on active chlorine). The reaction time was 120 minutes and the reaction temperature was 70 ° C. The consistency of the pulp was 10%. The initial pH of the base mass was 5 and the pH of the blend after D bleaching step was 4.7. After step D, the pulp had a brightness of 87.8% ISO.

The pulp was then transferred without intermediate washing to a post-bleaching stage where the pulp was bleached with 2 kg / tonne of peracetic acid (calculated as 100% peracetic acid). The reaction time was 60 minutes and the reaction temperature was 60 ° C. The consistency of the pulp was still 10%. The initial pH of the pulp was 6 and after the PAA phase the pH was 5.5 and the weight of the pulp was * V? ··· 89.9% ISO. Thus, the brightness of the pulp increased by 2.1% at ISO PAA and totaled •• f »mlV at 7.9% ISO basis weight.

«·· ·· * ·· :; s' **: Example 2> ·· ·:! /: ** ·. * * The base mass was first bleached with 7 kg / tonne of chlorine dioxide (based on active chlorine). The reaction time was 120 minutes and the reaction temperature was 70 ° C. The weight of the mass: T: 25 was 10%. The initial pH of the stock was 5 and the pH of the mixture after D was 4.7.

::: T: After the D stage, the pulp had a brightness of 88.2% ISO.

! * · · '* «· T: The pulp was then transferred without intermediate washing to a post-bleaching stage where the pulp was bleached; * :: 2 kg / tonne mass of peracetic acid (calculated as 100% peracetic acid). The reaction time for the second post-bleaching step was 60 minutes and the reaction temperature, ···, 30-60 ° C. The consistency of the pulp was still 10%. The initial pH of the pulp was 6 and after the PAA

*; *** the pH of the flask was 5.5 and the lightness of the pulp was 90.0% ISO. The lightness of the mass then increased by 1.8% ISO

PAA phase and a total of 8.0% ISO based on base mass.

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Example 3

The base pulp was first bleached with 5 kg / tonne pulp with chlorine dioxide (based on active chlorine). The reaction time was 120 minutes and the reaction temperature was 70 ° C. The consistency of the pulp was 10%. The initial pH of the stock was 5 and the pH of the mixture after D was 4.7. After step D, the pulp had a brightness of 88.0% ISO.

After Step D, the pulp was subjected to sulfur dioxide acidification to remove residual chlorine dioxide.

The pulp was then transferred without intermediate washing to a post-bleaching stage where the pulp was bleached with 2 kg / tonne of peracetic acid (calculated as 100% peracetic acid / 10). The reaction time was 60 minutes and the reaction temperature was 60 ° C. The consistency of the pulp was above 10%. The initial pH of the pulp was 6 and after the PAA phase the pH was 5.5 and the pulp was 89.3% ISO. The lightness of the pulp increased by 1.3% at the ISO PAA stage and by a total of 7.3% relative to the ISO base mass.

Comparative Example 4

The base mass was bleached with a mixture of 5 kg / tonne chlorine dioxide (based on active chlorine) and 2 kg / tonne peracetic acid (calculated as 100% peracetic acid). The reaction time was 120 minutes and the reaction ·, Temperature 70 ° C. The consistency of the pulp was 10%. The initial pH of the base mass was 5.5 and the pH of the mixture

φ '· · ** V after the D / PAA phase was 4.7. The lightness of the mass after the D / PAA step was 88.2% · * '! 20 ISO and brightness increase over base weight 6.2% ISO. Lightness of the final mass · ;;; was significantly lower than the final brightness (89.9% ISO) of the pulp bleached by the method of Example 1. ·· *. Bleaching with a mixture of chlorine dioxide and peracetic acid v: thus produced a pulp which was only slightly higher in brightness than that of the bleached pulp with chloride only.

Comparative Example 5 i * t • · ♦ 2 The base mass was bleached with a mixture of 7 kg / tonne chlorine dioxide (based on active chlorine) and 2 kg / tonne peracetic acid (calculated. ·· *. 100% peracetic acid). The reaction time was 120 minutes and the reaction temperature was

y temperature 70 ° C. The consistency of the pulp was 10%. The initial pH of the base mass was 5.5 and the pH of the mixture

The 30 after the D / PAA step was 4.7. The lightness of the pulp after the D / PAA phase was 88.3% ISO and the increase in brightness compared to the base mass was 6.3% ISO. The brightness of the final pulp was clearly lower than the final brightness of the pulp bleached by the method of Example 2 (90.0% ISO). Furthermore, bleaching with a mixture of chloride-117392 7 oxide and peracetic acid produced a pulp of only slightly higher brightness than that of the pulp bleached with chlorine dioxide alone.

The results of the examples are summarized in Table 1.

Table 1 __First, 1 Ex. 2 Ex. 3 Cf. 4 Cf.

Step__D__D__D D / PAA D / PAA

Time (min) __ 120__120__120 120 120 Temperature (° C) __ 70__70__70 70__70

Consistency (%) __ 10__10__10 10__10 C102 (kg / tonne; 5 7 5 5 7 act Cl) ______ PAH (kg / tonne) __-__-__-__ 2__2

Initial pH__5__5__5__5.5 5.5

End pH__4.7 4.7 4.7 4.7 4.7

Brightness (% ISO) __ 87.8 88.2 88.0 88.2 88.3 ~ Intermediate Do Not Wash No Wash S02 Hand. / ___No washing

Vaihe__PAA__PAA__PAA

; Time (min) __ 60__60__60 Temperature (° C) __ 60__60__60

Consistency (%) __ H) __ 10__10 [·· '. PAH (kg / tonne mass) __ 2__2__2

Initial pH__6__6__6

End pH__5 ^ 5__5.5__5.5 1 Brightness (% ISO) 89.9 90.0 1 89.3

Ml • · * • «• · · B I * * · · M« * · · i * * *

Ml «» • · I ·· m m ♦ 9 9 ψ 9 9 9 Ψ «• ·· ♦ · 9 φ 99 9

Claims (14)

A multi-step bleaching process for bleaching a chemical pulp, characterized in that it comprises as a final bleaching step a bleaching with delignified and bleached cellulose pulp having a brightness of at least about 85 Process according to claim 1, characterized in that said final bleaching step is preceded by a chlorine dioxide step. Process according to claim 2, characterized in that the chlorine dioxide is used in an amount of about 5 to 30 kg / tonne dry weight, calculated as active chlorine. Process according to claim 2 or 3, characterized in that the pH value in the chlorine dioxide step is about 2-5.5. 5 ISO. Process according to one of Claims 2 to 4, characterized in that the chlorine dioxide residue is removed from the reaction medium after the chlorine dioxide step. 5. ISO, bleached with percarboxylic acid. Process according to one of Claims 2 to 5, characterized in that no pulping is carried out between said last bleaching step and the preceding chlorine dioxide step. . . A process according to claim 1, characterized in that said final bleaching step is preceded by an alkaline peroxide step. ··· «•« i The process according to claim 7, characterized in that no pulping is carried out between said late bleaching step and the preceding alkaline peroxide step. Process according to any one of the preceding claims, characterized in that the percarboxylic acid is permic acid, peracetic acid or perpropionic acid. • · · ··· ·. ···. Process according to any one of the preceding claims, characterized in that * · [25 percarboxylic acid is equilibrium peracetic acid or distilled peracetic acid, * · ♦ · * pre-distilled peracetic acid. ··· • · • · »» A process according to any one of the preceding claims, characterized in that about 0.1 to 5 kg / ton dry pulp, preferably about 1 to 0.5 kg / ton dry pulp, percent percarboxylic acid. 117392 Process according to any one of the preceding claims, characterized in that the pH in said final bleaching step is about 4-8. Process according to any one of the preceding claims, characterized in that the pulp has a brightness of at least about 88% before said final bleaching step. Process according to any one of the preceding claims, characterized in that the chemical cellulose pulp is sulphate pulp.