FI107545B - Process for processing mechanical masses - Google Patents

Abstract

The invention relates to a method for treating mechanical pulp so that aliphatic percarboxylic acid in the amount of 0.5 - 5 kg/ton of pulp is added to the pulp to improve the opacity.

Description

107545

A method for treating mechanical pulps

The invention relates to a method for treating mechanical pulps which can influence the opacity of the pulp.

5

The opacity of paper is illustrated by opacity, which is an important property of pulp in paper making, in addition to lightness. Almost without exception, however, the opacity of the pulp decreases with increasing brightness. Nowadays, mechanical pulps 10 (masses, grinds and chemimechanical pulps) are increasingly bleached with hydrogen peroxide. Also, dithionite bleaching is used, either alone or in combination with peroxide bleaching, whereby dithionite is used either as refiner bleaching or after bleaching. In peroxy bond bleaching of pulps, particularly mechanical pulps, a decrease in opacity is clearly observed, whereas dithionite bleaching does not necessarily reduce opacity. Usually the opacity of the pulp decreases the lighter the pulp is bleached (Figure 1). For certain paper grades, opacity is an important feature. When it comes to promoting peroxide bleaching at the expense of dithionite bleaching, it would be important to be able to optimize peroxide bleaching so that opacity remains as high as lightness increases.

The chemicals used in the peroxide bleaching of mechanical pulps are more generally hydrogen peroxide, lye (alkaline) and water glass. The purpose of the base is to raise the pH high enough for hydrogen peroxide to dissociate to produce perhydroxyl anions. The purpose of the water glass is to stabilize peroxide-30 bleaching.

It is known that peracetic acid may be formed in situ from, for example, acetic anhydride or TAED (tetraacetyl ethylene diamine) or other similar activator. The disadvantage of TAED is its high price and the fact that it is a solid. TAED would need to be dispersed in water before being added to the pulp, which makes it difficult to use. In addition, TAED contains nitrogen, which can be a problem for environmental protection. Acetic anhydride would be relatively inexpensive, but it would cause odor nuisance and would be a difficult substance to protect. In addition, when fed to an alkaline bleaching grade (NaOH + H 2 O 2 + water glass), it would easily cause silicate precipitates and consume lye.

10

In the manufacture of papers, the aim is to achieve higher levels of lightness. The brightness of the paper can be affected, for example, by treating the paper with coatings containing e.g. pigments, binders and plasticizers (JP application 2 15 84598). However, the use of a number of coatings in the final paper production process further increases the manufacturing cost.

A technically useful pulp opacifying agent should be liquid, stable and preferably of pH value to avoid silicate precipitation in bleaching. An additional benefit would be the non-nitrogenous material for environmental reasons. Bleaching aids should also be advantageous to papermakers. Thus, for the bleaching of pulps, it is necessary to find a substance which, in order to meet the above conditions, is a readily available commercially available chemical and can be added already at the pulp bleaching stage. In addition, attention must also be paid to other effects of the substance, such as its usefulness under factory conditions.

It is an object of the present invention to find a useful opacifying agent for pulp bleaching which meets the above requirements.

3 107545

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

It has now surprisingly been found that the addition of a small dose of 5 peracids to either peroxide bleaching or post-bleaching of peroxide bleaching improves the opacity of the pulp compared to the equivalent brightness level bleached with hydrogen peroxide alone. At the same time it has been observed that peracids do not increase the brightness much.

10

The use of peracetic acid for bleaching chemical pulps is known, for example, from Japanese Patent Application JP 57-21591. However, such bleaching processes aim to remove the lignin from the pulp, and in this case there is no purpose in controlling the opacity of the pulp.

Tappi, Vol. 48, no.2 (2/1965), W.H. Rapson et al. Paper Bleaching - A New Process, pp. 65-72 discloses a method of bleaching paper using e.g. peretikkahap-acid. However, this method does not aim at improving the opacity of the mass but at increasing the brightness. In the presentation. the process also uses peracetic acid in considerable amounts in the range of about 1.2-2.5% (i.e. 12-25 kg / tonne of pulp). Such a dose can significantly dissolve lignin, etc., in the pulp, which can severely disrupt the papermaking process. It can also be seen from the publication that the technical implementation of the method differs from our invention.

The use of peracids as a biocide is also known, for example, from Chem. 3 (1995), Jyri Maunuksela, Control of Microbes with Peracetic Acid, pp. 242-244. Such a method is only aimed at eradicating microbial growths from the paper machine. It is self-evident that peracetic acid, known as an effective biocide, inhibits microbial activity in the water cycle if it is present in sufficient quantities. The method disclosed in the publication uses the so-called. equilibrium peracetic acid, which contains a significant amount of free acetic acid and hydrogen peroxide. Acetic acid and hydrogen peroxide that come with peracid can interfere with the papermaking process.

Also disclosed are peracid based bleaching methods, with the aim of improving pulp brightness 10 compared to normal bleaching methods. Such a method is disclosed, for example, in Pulp and Paper Magazine of Canada, Convention issue, 1972, pp. 123-131, and in the same issue 3/1968 on pages 51-60. The doses of peracid used in these processes are very high and thus significantly dissolve lignin from the fiber surfaces. In addition, these methods were aimed at increasing the brightness of the pulp, i.e., bleaching.

The difference between our invention and the above-mentioned treatments is that the peracid treatments carried out in the manner described therein failed to increase the opacity of the pulp, nor did any of the aforementioned publications attempt to achieve a significant increase in opacity.

The invention relates to a process in which peracid is added either to peroxide bleaching, or preferably after peroxide bleaching, either to a bleached pulp storage tower or, for example, to a papermaking machine among the pulp. · Typically about 1-3 kg / tonne of mass. Due to the low dose, the 30 peracid does not dissolve the lignin or the like in the pulp, thus it does not interfere with the papermaking process. It is particularly advantageous to use peracid solutions which have been purified so as to contain essentially only the said acid. peracid and water. In this case, the addition of peracid does not significantly affect the pH of the pulp 107545 and no additional COD is introduced into the process. The invention differs from the known methods based on peracids in that it does not aim at increasing the brightness but at increasing the opacity. In particular, it should be noted that the process differs from the known bleaching treatments also in that the doses of peracid used are very low and the process seeks to avoid dissolving organic material from the fibers.

The process is not really a bleaching process, and the chemicals used in the actual bleaching of the pulp have no effect on the performance of the invention. The pulp may be bleached with dithionite, peroxide, dithionite and peroxide or even be completely unbleached. The chelating or other auxiliaries used in the preparation of pulp 15 also have no effect on the performance of the invention.

The consistency of the pulp at the point of addition of the peracid may be 1-40%, the temperature may be 20-100 ° C. The pH of the pulp stock may vary from 3 to 11, preferably 4 to 8, depending on the process. However, if peracid is added to alkaline peroxide bleaching, a preferred pH range is 9-11. A suitable reaction time with peracid is 1-300. minutes depending on the process.

The method is suitable for use in the treatment of mechanical pulps, such as pulp (SGW, PGW) and pulp (TMP) or chemi-mechanical pulps (CTMP). The type of wood used to make the pulp is irrelevant to the practice of the invention. in terms of ease.

30

A suitable dose of peracid has been found to be 0.5-5 kg / tonne, typically 1-3 kg / tonne. The peracid used can be any peracid that is reasonably soluble in water. Peracetic acid and perpropionic acid 6 107545 have been found to be preferred, and peroxide-free distillates prepared from their equilibrium solutions are particularly preferred. The method of making peracid has no effect on the operation of the invention. The peracid can be either purified by distillation or otherwise purified peracid solutions or peracid equilibrium solutions. Various mixtures of peracids are also useful. In one variation of the invention, a mixture of peracid and Caronha p may be used.

10

Example 1.

The CTMP pulp was bleached as usual with peroxide. Peracids were added to the bleaching solution, the results are shown in Table 1 and Figure 2.

15

Table 1 .CTMP, Bleaching solution: NaOH 24 kg + Water glass 20 kg + H 2 O 2 30 kg 70 ° C, consistency 30%, 120 min, chelated pulp peracid added to bleach tm% ISO * 0pas, teettl -El -: - 7S7 - ST5 PAA 2 79.1 67 · 2 PPA 2.2 78.3 70.7

Figure 2.; 71 t

s ^^^! I

2 2.2 No PAA PPA

Dose in kg / tm

From the results it was found that peracids produce significantly higher opacity at the same brightness level as peroxide bleaching alone.

Example 2.

7 107545

The bleached CTMP pulp was treated with peracids, the results are shown in Table 2 and Figure 3.

Table Bleached CTMP

Aftertreatment _t = 50 ° C, 30min, pH 7, 5% consistency

Dose, Light9 Opacity __kg / tm% ISO * __

No - 65.9 71.5 PAA 2 66.6 74.1 PPA 2.2 65.8 74.3

Figure 3. 75 T

74.5 "« Nam 74 - gpHi ΗΒΗ s73.5 "im MaSSm '1 72.5- lii ftiffSIwm 72 Warn yQ --- μ —- ^ - 2 2,2! | No Dosage kg / tm PAA PPA i | _! 5

From the results, it can be seen that post-treatment has a clear "effect on opacity.

Example 3.

. machine pulp containing 16% softwood pulp, 64% TMP pulp and 20% coated wreck (base mass: mixed softwood: TMP 20:80) was treated with peracetic acid. Coniferous pulp was normal ECF sulfate pulp. TMP was bleached with dithionite. The results are shown in Table 3.

15 8 107545

Taulukko3.

Furnish:

Konema pulp Coniferous pulp 16% TMP 64% 5 _ Coated wreck 20% _ 30 min, t = 70 ° C, consistency 3.2% (tested with factory water) Head, "TOC, Brightness, I Opacity, __kg / tm__kg / tm% ISO __% __ 0 0 5.0 10.4 71.9 91.3 Untreated 10 0 4.9 9.6 71.6 91.8 30 min at 70C

2 1 4.6 8.9 71.7 96.5 Distilled PAA

10 3 2 4.6 9.6 71.9 95.8 Distilled PAA

4 4 4.2 10.2 71.3 96.9 Distilled PAA

5 2 4.5 11.0 70.7 97.1 Equilibrium solution

6 1 5.0 9.5 71.6 93.7 Distilled PAA

7 2 4.9 10.3 71.5 94.4 Distilled PAA

8 __2__5j0_10J__68,8__93.4 Equilibrium solution

Distilled PAA = 38% PAA, 1% H 2 O 2. Equilibrium solution = PAA 13%, acetic acid 22%, H2O2 15% 15

The results show that the opacity was significantly increased due to PAA treatment. It is also observed that the equilibrium solution adversely affects the brightness. Total organic carbon (TOC) values indicate that the treatment did not dissolve significant amounts of organic material from the fibers.

«· 9 107545

Figure 1.

Figure 1. Change in the opacity of Six-TMP when bleaching pulp with peroxide to different light levels.

5 1% ISO - -0.5% Opacity

96 T

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92 * - · - ^.

S 90, I \

S 88 - X

Ω. y ·,

Oh X NS

x x ^ X X

v; * x J * 84 -% '' -ϊ * X *

82 - X

80 -1-1-1-1 60 65 70 75 80

Brightness,% IS0

Claims (9)

107545 A process for treating mechanical pulp, characterized in that aliphatic percarboxylic acid is added to the pulp as an active ingredient in an amount of 0.5 to 5 kg / tonne of pulp. Process according to Claim 1, characterized in that the aliphatic percarboxylic acid is added in an amount of 1-3 to 10 kg / tonne mass. Process according to claim 1 or 2, characterized in that the aliphatic percarboxylic acid is selected from the group consisting of formic, peracetic or perpropionic acid. 15 Process according to one of the preceding claims, characterized in that the aliphatic percarboxylic acid is distilled or balanced peracetic acid. Process according to Claim 1, characterized in that the aliphatic percarboxylic acid is added to the pulp during or after bleaching. Process according to claim 5, characterized in that the pulp is bleached with hydrogen peroxide and / or dithionite. Process according to claim 1 or 5, characterized in that the aliphatic percarboxylic acid is typically added at a pulp consistency of 1-40%. 8. A process according to claim 1, characterized in that a mixture of aliphatic percarboxylic acid and Caronic acid is added to the pulp. 107545 9. Paper characterized in that it is made from pulp treated according to claim 1. tl * * * 107545