Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1
Me#~hc~ f~~ ~n~azowe the onac ity of mer~hani_ca3 pulp by using aliphatic
peroxyacids and use of peroxyacids to prove opacity.
The invention relates to a process for treating mechanical pulps, which can be
used
to affect the opacity of the pulp.
The nontransparent aspect of paper is described by opacity, which, along with
brightness, is an important property of pulp in paper manufacture. Almost
invaria-
bly, 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. Di-
thionite 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
mani-
fests 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
im-
portant to be able to optimize peroxide bleaching so that the opacity remains
as high
as possible while the brightness grows.
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
in-
crease the pH to a sufficiently high level, so that the hydrogen peroxide is
dissoci-
ated producing perhydroxyl anions. The purpose of the waterglass is to
stabilize the
hydrogen peroxide bleaching.
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
sub-
stance. 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 rela-
tively 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
3 5 bleaching solution (NaOH + H202 + waterglass), it would readily cause
silicate pre-
cipitate and consume the lye.
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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
applica-
tion 284598).
However, the use of several coating agents at the final stage of paper
manufacture
adds to the manufacturing costs.
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
precipi-
tate would form in the bleaching. Because of environmental matters, a nitrogen-
free
l0 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.
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.
The main features of the invention are disclosed by the appended Claims.
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.
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 propose
is in no
way to adjust the opacity of the pulp.
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
consid-
erable 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,
where-
upon the paper manufacturing process can be severely disturbed. The
publication
also reveals that the technical performance of the method deviates from our
inven-
lion.
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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
popu-
lations 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
publica-
tion 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.
Bleaching methods based on peracetic acid have also been presented, aiming at
im-
proving 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.
One difference between the treatments mentioned above and our invention in par
ticular 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.
The invention relates to a method, in which the peracid is either added to the
perox-
ide 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.
Typically, the amount of peracid added to the pulp is about 1-3 kg/ton of
pulp. Be-
cause 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
espe-
cially 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
3 5 small and the method tries to avoid the dissolution of organic matter from
the fibres.
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.
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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.
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 ad-
vantageous pH value is 9-11, however. A suitable reaction time for the peracid
is 1-
300 minutes depending on the process.
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.
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 per-
acids, 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 puri-
fied 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
modifica-
tion of the invention can use a mixture of peracid and Caro's acid.
Example 1
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 + HzOz 30 kg/ton of
pulp,
70°C, consistency 30%, 120 min, chelated pulp, peracid added to the
bleaching, PAA =
peracetic, PPA = perpropionic acid
Dosa eg , kg/ton Bri htness, % O aci
of u1 ISO
None 78.4 64.9
PAA 2 79.1 67.2
PPA 2.2 78.3 70.7
The results show that, by using peracids, a distinctly higher opacity with the
same
brightness level is achieved than by using peroxide bleaching alone.
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Example 2
Bleached CTMP pulp was treated with peracids, the results are shown in Table
2.
Table 1
Bleached CTMP
5 After-treatment
t = 50°C, 30 min, pH 7, consistency 5%
Dosa e, ton of u1 Bri htness, % O aci
ISO
None 65.9 71.5
P~ 2 - 66.6 74.1
PPA 2.2 65.8 74.3
The results show that an after-treatment has an obvious effect on the opacity.
Example 3
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,
70C,
Cs
3.2%,
the
test
was
conducted
b
usin
the
water
of
the
lant
No. PAA, kg/tonpH TOC, kg/tonBrightness,Opacity, Note!
of u1 of u1 % ISO %
0 0 5.0 10.4 71.9 91.3 Untreated
1 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
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 E uilibrium
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 E uilibrium
solution
Distilled PAA = 3 8% of PAA, 1 % of HZOZ
Equilibrium solution = PAA 13%, acetic acid 22%, H202 15%
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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.
