[0001] The invention relates to a process for treating mechanical pulps, which can be used
to affect the opacity of the pulp.
[0002] The nontransparent aspect of paper is described by opacity, which, along with brightness,
is an important property of pulp in paper manufacture. Almost invariably, however,
the opacity of the pulp decreases when the brightness increases. At present, mechanical
pulps (refiner mechanical pulp, groundwood pulp, and chemi-mechanical pulp) are more
and more often bleached with hydrogen peroxide. Dithionite bleaching is also used
either alone or together with peroxide bleaching, whereupon dithionite is either used
as refiner bleaching or after-bleaching. In the peroxide bleaching of pulp, mechanical
pulp in particular, the decrease of opacity is clearly detectable, while the dithionite
bleaching does not necessarily decrease the opacity. Generally, the lighter the level
of bleaching the pulp, the lower the opacity of the pulp. The appended Fig. 1 that
shows a variation in the opacity of spruce TMP, when peroxide is used to bleach pulp
to various degrees of brightness manifests this. In certain paper grades, opacity
is an important property. If we want to advance peroxide bleaching at the expense
of dithionite bleaching, it would be important to be able to optimize peroxide bleaching
so that the opacity remains as high as possible while the brightness grows.
[0003] Generally, the chemicals used in the peroxide bleaching of mechanical pulps are hydrogen
peroxide, lye (alkali), and waterglass. The purpose of the base is to increase the
pH to a sufficiently high level, so that the hydrogen peroxide is dissociated producing
perhydroxyl anions. The purpose of the waterglass is to stabilize the hydrogen peroxide
bleaching.
[0004] It is well known that peracetic acid can be produced in situ, for example, from acetanhydride
or TAED (tetra acetyl ethylene diamine) or some other corresponding activator. One
disadvantage of TAED is its high price and that it is a solid substance. It would
be necessary to disperse the TAED in water before adding it to the pulp, which makes
it difficult to use. Furthermore, TAED contains nitrogen, which might constitute a
problem for environmental protection. Acetanhydride is relatively cheap, but it would
cause odour nuisance and be an inconvenient substance from the point of view of industrial
safety. In addition, when fed into an alkaline bleaching solution (NaOH + H
2O
2 + waterglass), it would readily cause silicate precipitate and consume the lye.
[0005] Paper manufacture aims at ever-higher brightness levels. The brightness of paper
can be affected, for example, by treating the paper with coating agents containing,
among other things, pigments, binding agents, and plasticizing agents (JP application
284598).
[0006] However, the use of several coating agents at the final stage of paper manufacture
adds to the manufacturing costs.
[0007] The agent that affects the opacity of a technically useful pulp should be liquid
and stable, and it should preferably have a suitable pH value, so that no silicate
precipitate would form in the bleaching. Because of environmental matters, a nitrogen-free
substance would provide an additional benefit. The additive of the bleaching should
also be cost-effective for the paper manufacturers. Consequently, a substance should
be found for pulp bleaching, which, to fulfil the conditions mentioned above, is a
reasonable, commercial chemical that is easy to get and can be added to the pulp as
early as at the bleaching stage. Furthermore, attention should also be paid to the
other effects of the substance, such as applicability in plant conditions.
[0008] The purpose of this invention is to find a useful substance that is used in pulp
bleaching and that affects the opacity, fulfilling the conditions mentioned above.
[0009] The main features of the invention are disclosed by the appended Claims.
[0010] Surprisingly, we have now observed that, when a small portion of peracid is either
added to the peroxide bleaching or with the purpose of finishing the peroxide bleaching,
the opacity of pulp improves compared with pulp of a corresponding brightness level
that is bleached with hydrogen peroxide alone. At the same time, we have observed
that peracids scarcely increase the brightness.
[0011] The use of peracetic acid in bleaching chemical pulps is disclosed by the published
Japanese application JP 57-21591, for example. However, bleaching processes like this
aim at removing the lignin from the pulp and, in this case, the purpose is in no way
to adjust the opacity of the pulp.
[0012] WO-A-97/22749 relates to a process of pre-treatment of lignocellulosic materials
with (among others) a mixture of Caro's acid and peracetic acid so as to render the
fibers flexible and conformable with low refining energy while improving the strength
and brightness of the resulting pulp and subsequently subjecting the fibers to refining.
WO-A-97/22749 does not teach how to adjust the opacity of the pulp.
[0013] Tappi publication, Vol. 48, No. 2 (2/1965), W.H. Rapson et al. Paper Bleaching -
A New Process, pp. 65-72, discloses a method for bleaching paper by using peracetic
acid, among others. However, this method does not aim at improving the opacity of
pulp, but increasing the brightness. Furthermore, the disclosed method uses considerable
amounts of peracetic acid, about 1.2 - 2.5% (i.e., 12-25kg/ton of pulp). Such a portion
can dissolve a considerable amount of lignin etc. from the pulp, whereupon the paper
manufacturing process can be severely disturbed. The publication also reveals that
the technical performance of the method deviates from our invention.
[0014] The use of peracids as a biocide is also disclosed by publication Kemia, No. 3 (1995),
Jyri Maunuksela, Mikrobien torjunta peretikkahapolla (Microbe Prevention with Peracetic
Acid), pp. 242-244. Such a method aims at destroying microbe populations in the paper
machine only. It is self-evident that peracetic acid, which is known to be an effective
biocide, prevents the functioning of microbes in the water circulation, if a sufficient
amount is present. The method disclosed by the publication uses a so-called equilibrium
peracetic acid that contains a considerable amount of free acetic acid and hydrogen
peroxide. The acetic acid and hydrogen peroxide that come with the peracid can disturb
the paper manufacturing process.
[0015] Bleaching methods based on peracetic acid have also been presented, aiming at improving
the brightness of the pulp compared with normal bleaching methods. Such a method is
presented, for example, by publication Pulp and Paper Magazine of Canada, Convention
Issue, 1972, pp. 123-131, and by number 3/1968 of the same magazine in pages 51-60.
The doses of peracid used by these methods are very high and, consequently, dissolve
a considerable amount of lignin from the surfaces of the fibres. In addition, these
methods aim at increasing the brightness of the pulp, i.e. at bleaching.
[0016] One difference between the treatments mentioned above and our invention in particular
is that the peracid treatments implemented by the means disclosed by them fail to
improve the opacity of the pulp; none of the publications mentioned above tried to
achieve a considerable increase in the opacity either.
[0017] The invention relates to a method, in which the peracid is either added to the peroxide
bleaching or, preferably, after the peroxide bleaching, either to the storage tower
of the bleached pulp or, for example, among the machine pulp in the paper machine.
[0018] Typically, the amount of peracid added to the pulp is about 1-3 kg/ton of pulp. Because
of its small amount, the peracid does not dissolve the lignin or the like from the
pulp, whereupon it does not disturb the paper manufacturing process. It is especially
advantageous to use peracid solutions, which have been purified, so that they mainly
contain the peracid in question and water only. In that case, the addition of peracid
does not have a considerable effect on the pH value of the stock, and no extra COD
gets into the process. The invention differs from the known methods based on peracid
namely in that it does not aim at increasing the brightness but growing the opacity.
A matter worth noticing in particular is that the method differs from the known bleaching
treatments also in that the peracid doses used are very small and the method tries
to avoid the dissolution of organic matter from the fibres.
[0019] The method is not actually a bleaching method, and the chemicals used in the actual
bleaching of the pulp have no effect whatsoever on the functioning of the invention.
[0020] The pulp can be bleached with dithionite, peroxide, dithionite and peroxide, or it
can even be completely unbleached. Any chelation agents or other additives used in
pulp manufacture have no effect on the functioning of the invention either.
[0021] The consistency of the pulp at the moment the peracid is added can be 1-40%; the
temperature can be 20-100°C. Depending on the process, the pH of the stock can be
3-11, preferably 4-8. If the peracid is added to alkaline peroxide bleaching; an advantageous
pH value is 9-11, however. A suitable reaction time for the peracid is 1-300 minutes
depending on the process.
[0022] The method is well suited to the treatment of mechanical pulp, such as groundwood
pulp (SGW, PGW) and refiner mechanical pulp (TMP), or to treat chemi-mechanical pulps
(CTMP). The sort of wood used to manufacture the pulp has no importance to the functioning
of the invention.
[0023] A suitable dose of peracid has been found to be 0.5-5 kg/ton of pulp, typically 1-3
kg/ton of pulp. The peracid used can be any peracid that reasonably dissolves in water.
We have noticed that peracetic acid and perpropionic acid are preferable peracids,
and especially preferable are the peroxide-free distillates that are prepared from
the equilibrium solutions of these. The manufacturing method of peracid has no effect
on the functioning of the invention. Peracid solutions that are either purified by
distillation or some other method, or the equilibrium solutions of peracids can be
used as peracid. Various mixtures of peracids are also usable. One modification of
the invention can use a mixture of peracid and Caro's acid.
Example 1
[0024] CTMP pulp was bleached with peroxide in a normal manner. Peracids were added to the
bleaching solution, the results are shown in Table 1.
Table 1
CTMP, bleaching solution: NaOH 24 kg +waterglass 20 kg + H2O2 30 kg/ton of pulp, 70°C, consistency 30%, 120 min, chelated pulp, peracid added to
the bleaching, PAA = peracetic, PPA = perpropionic acid |
|
Dosage, kg/ton of pulp |
Brightness, % ISO |
Opacity |
None |
- |
78.4 |
64.9 |
PAA |
2 |
79.1 |
67.2 |
PPA |
2.2 |
78.3 |
70.7 |
[0025] The results show that, by using peracids, a distinctly higher opacity with the same
brightness level is achieved than by using peroxide bleaching alone.
Example 2
[0026] Bleached CTMP pulp was treated with peracids, the results are shown in Table 2.
Table 1
Bleached CTMP |
After-treatment |
t = 50°C, 30 min, pH 7, consistency 5% |
|
Dosage, kg/ton of pulp |
Brightness, % ISO |
Opacity |
None |
- |
65.9 |
71.5 |
PAA |
2 |
66.6 |
74.1 |
PPA |
2.2 |
65.8 |
74.3 |
[0027] The results show that an after-treatment has an obvious effect on the opacity.
Example 3
[0028] So-called machine pulp taken from the plant and containing 16% of softwood pulp,
64% of TMP pulp, and 20% of coated broke (the basic pulp mixture was softwood: TMP
20:80) was treated with peracetic acid. The softwood pulp was normal ECF sulphate
pulp. The TMP had been bleached with dithionite. The results are shown in Table 3.
Table 3
|
Pulp mixture: |
|
Machine pulp |
Softwood pulp |
16% |
|
TMP |
64% |
|
Coated broke |
20% |
30 min, 70°C, Cs 3.2%, (the test was conducted by using the water of the plant) |
No. |
PAA, kg/ton of pulp |
pH |
TOC, kg/ton of pulp |
Brightness, % ISO |
Opacity, % |
Note! |
0 |
0 |
5.0 |
10.4 |
71.9 |
91.3 |
Untreated |
1 |
0 |
4.9 |
9.6 |
71.6 |
91.8 |
30 min, at 70°C |
2 |
1 |
4.6 |
8.9 |
71.7 |
96.5 |
Distilled PAA |
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 |
5.0 |
10.1 |
68.8 |
93.4 |
Equilibrium solution |
Distilled PAA = 38% of PAA, 1% of H2O2 |
Equilibrium solution = PAA 13%, acetic acid 22%, H2O2 15% |
[0029] The results show that the opacity grew considerably because of the treatment with
PAA. They also show that the equilibrium solution has an adverse effect on the brightness.
The TOC values (total organic carbon) show that the treatment did not dissolve considerable
amounts of organic matter from the fibres.
1. A method for treating mechanical pulp, characterized in that an aliphatic percarboxylic acid is. added to the pulp, in the amount of 0.5 - 5 kg/ton
of pulp, to affect the opacity.
2. A method according to Claim 1, characterized in that the amount of aliphatic percarboxylic acid that is added is 1 - 3 kg/ton of pulp.
3. A method according to Claim 1 or 2, characterized in that the aliphatic percarboxylic acid is selected from among performic acid, peracetic
acid or perpropionic acid.
4. A method according to any of the preceding Claims, characterized in that the aliphatic percarboxylic acid is distilled or equilibrated peracetic acid.
5. A method according to Claim 1, characterized in that the aliphatic percarboxylic acid is added to the pulp in connection with or after
bleaching.
6. A method according to Claim 5, characterized in that the pulp has been bleached with hydrogen peroxide and/or dithionite.
7. A method 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 method according to Claim 1, characterized in that a mixture of aliphatic percarboxylic acid and Caro's acid is added to the pulp.
9. The use of aliphatic percarboxylic acid to improve the opacity of mechanical pulp,
in which the amount of percarboxylic acid is 0.5 - 5 kg/ton of pulp.
1. Verfahren zur Behandlung von Holzstoff, dadurch gekennzeichnet, daß eine aliphatische Percarbonsäure zu der Pulpe in einer Menge von 0,5-5 kg/t Pulpe
zugegeben wird, um die Opazität zu beeinflussen.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die zugegebene Menge der aliphatischen Percarbonsäure 1-3 kg/t Pulpe beträgt.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die aliphatische Percarbonsäure unter Peroxyameisensäure, Peroxyessigsäure oder Peroxypropionsäure
ausgewählt ist.
4. Verfahren nach einem der vorausgehenden Ansprüche, dadurch gekennzeichnet, daß die aliphatische Percarbonsäure destillierte oder äquilibrierte Peroxyessigsäure
ist.
5. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die aliphatische Percarbonsäure zu der Pulpe in Verbindung mit oder nach dem Bleichen
gegeben wird.
6. Verfahren nach Anspruch 5, dadurch gekennzeichnet, daß die Pulpe mit Wasserstoffperoxid und/oder Dithionit gebleicht wurde.
7. Verfahren nach Anspruch 1 oder 5, dadurch gekennzeichnet, daß die aliphatische Percarbonsäure typischerweise zu einer Pulpe mit einer Konsistenz
von 1-40% gegeben wird.
8. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß eine Mischung von aliphatischer Percarbonsäure und Caroscher Säure zu der Pulpe gegeben
wird.
9. Verwendung von aliphatischer Percarbonsäure, um die Opazität von Holzstoff zu verbessern,
wobei die Menge von Percarbonsäure 0,5-5 kg/t Pulpe beträgt.
1. Procédé pour traiter une pâte mécanique, caractérisé en ce qu'un acide percarboxylique aliphatique est ajouté à la pâte, en la proportion de 0,5
à 5 kg/tonne de pâte, pour affecter l'opacité.
2. Procédé selon la revendication 1, caractérisé en ce que la proportion d'acide percarboxylique aliphatique qui est ajouté est de 1 à 3 kg/tonne
de pâte.
3. Procédé selon la revendication 1 ou 2, caractérisé en ce que l'acide percarboxylique aliphatique est choisi parmi l'acide performique, l'acide
peracétique ou l'acide perpropionique.
4. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'acide percarboxylique aliphatique est de l'acide peracétique distillé ou équilibré.
5. Procédé selon la revendication 1, caractérisé en ce que l'acide percarboxylique aliphatique est ajouté à la pâte pendant ou après le blanchiment.
6. Procédé selon la revendication 5, caractérisé en ce que la pâte est blanchie avec du peroxyde d'hydrogène et/ou du dithionite.
7. Procédé selon la revendication 1 ou 5, caractérisé en ce que l'acide percarboxylique aliphatique est généralement ajouté à une consistance de
pâte de 1 à 40 %.
8. Procédé selon la revendication 1, caractérisé en ce qu'un mélange d'acide percarboxylique aliphatique et d'acide de Caro est ajouté à la
pâte.
9. Utilisation d'acide percarboxylique aliphatique pour améliorer l'opacité d'une pâte
mécanique, dans laquelle la proportion d'acide percarboxylique est de 0,5 à 5 kg/tonne
de pâte.