Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method of bleaching
~ cellulosic containing material with an oxidizing gas, par-
,; ticularly oxygen gas, in an alkaline envirament, particularly
in connection with production of pulp according to the
sulphate method.
Up until the present teachings, oxygen gas bleaching
of pulp was usually carried out in order that after washing
~ and pressing to remove the liquid from the fibers of the pulp,- 10 the iber concentration of the pulp was approximately 30%.
, Subsequently, the amount of alkali (NaOH solution) which
-~ is requiréd for bleaching is added and the pulp stirred
'' which results in a concentration decrease down to approximately
~! 25%. The pulp i~ then fed into a reactor and disintegrated
by a shredder which is located at the upper part of
the reactor in accordance with the method as outlined in
Swedish patent 333,498.
The earlier proposed method has certain drawbacks which
the present invention will eliminate as described in more
detail as follows.
In accordance with the present teachings, oxygen is
used in the bleaching in place of chlorine in order to
~- act on the lignin in the wood fibers so that the lignin may
be eliminated. As the bleaching agent also effects the
cellulose of the material, but at a slower rate, it is
; desirable that the reaction take place as selectively as
, possible, that is, so that the lignin is eliminated as
.~ quickly as possible and the cellulose is effected as little
as possible.
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The oxygen gas bleaching involves a process employing
alkali, water, heat and oxygen in order to delignify cellulose
fibers. The amount of reagents (2~ NaOH) which is transferred
to the fiber is directly dependent on the size of the phase
surface, which at a maximum can be the outer surface of the
fiber~ This presupposes that the fibers are detached from
each other. If on the other hand the fibers are flocculated
together, the total contact surface of the fibers becomes
smaller and the required reaction time for release of the
lignin from the fibers located in the interior of the
10ck thereby becomes longer. This may result in over
bleaching of the fibers taking place at the outer surface of the
flocks where even the cellulose in the fiber may be effected
and the bleaching becomes uneven. When the fibers are
detached from each other, a homogeneous bleaching result is
attained.
Pulp with a high fiber concentration is difficult to
process and problems are encountered in mixing the required
amount of alkali into the pulp and because of this the alkali
becomes unevenly distributed. Pulp with high fiber concen-
trations are also difficult to transport to the reaction
vessel. With the presently known operations, it was
previously impossible to feed the pulp into the reaction
vessel in forms other than flocks.
The known methods for oxygen bleaching till now
required, due to the presence of flocks, longer reaction
times which results in the reactor having to be of larger size.
-~ The ,nnermost and outermost fibers in the fiber flock have
- different strengths and viscosities and as the alkali
may become unevenly distributed, the desired bleaching is
not completed in some areas.
The amount of oxygen needed for removal of lignin from
cellulose fibers is about 1 to I.S~ by weight of the pulp.
A reactor which has a volume of 1 m3 contains approximately
100 kg of wet fluffy pulp which is approximately 25% fiber
concentration or 25 kg dry cellulose. If the reaction takes
'~ place under a pressure of 10 kp/cm2, the amount of oxygen
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required is approximately 10 kg, i.e. more than 30 times
the quantity required thus the reaction requires a very
large excess of oxygen.
In contrast, and in accordance with the present method,
the amount of oxygen may now be revised so that it corresponds
- to the quality which is actually required. According to
prior methods, it was not possible to mix into the pulp as
much gas as would be needed as the gas forms a continuous
phase andthe pulp-alkali-water a dispersed phase. In accordance
with the present teachings, the proportion is in the reverse,
with the gas forming the dispersed phase and the pulp-alkali-
water the continuous phase.
The purpose of the present invention is to eliminate
the drawbacks of the prior art processes whereby bleaching
will now take place in the presence of surface active
agents which act to make small oxygen gas bubbles to form
a stable foam whereby the fibers are dispersed.
In accordance with a preferred embodiment of the present
teachings, the foaming agent is added to the pulp suspension
before it is ~rought into contact with the oxygen gas thus
the oxygen gas feed may be utilized to effect the foaming .
I In accordance with the present procedure, reduction of
pressure may be effected before bleaching whereby the bubbles
of oxygen gas in the stable foam are expanded.
At times, no foaming agent is required to be added
particularly if the pulp suspension after the digestion is
permitted to retain, after washing, a sufficient quantity
of lye which will act as foaming agent. A sufficient
quantity of foaming agent is 1 kg/ton pulp.
Thus, in accordance with the present teachings, an
improvement is provided in a method of bleaching cellulosic
material with an oxidizing gas in an alkaline enviroment in
[:
~ which a pulp suspension which has a maximum consistency of
;:~
15% is treated with an alkali solution and the oxidizing
gas for effecting removal of lignin from the cellulosic
material such treatment being effected at elevated temperature
and pressure, wherein the improvement aomprises transforming
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the pulp suspension into a stable foam in the presence of
21 foaming agent.
In the following, the present invention is described in
~ore detail with reference to the drawings wherein Figures 1
and 2 show alternative embodiments for application of the
present process in accordance with the teachings.
With respect to Figure 1, reference numeral 1 indicates
a digester which is provided with a washing zone, wherein the
lye, after diges.ion is removed from the pulp with the
pulp being conducted through a blow pipe 2 to a mixer 3.
The mixer may be a defibrator. Alkali is added through
pipe 4 to the pulp with oxygen being introduced through 5 and
a foaming agent via 6. From the mixer 3, the pulp flows
through pipe 7 into the bleaching reactor 8. A portion of
lS the oxygen gas as well as the foaming agent is added before
or into the mixture with the remaining oxygen and alkali solution
being added into the reactor, preferably so that percolation
; and circulation may take place. From the reactor 8, the pulp
, passes through line 9 to a washing diffuser 10 wherein the
pulp may be washed with additional alkali. Between the
bleaching and the washing, the pressure is reduced and the
; pulp degassified in apparatus 11. Between the mixer and
the reactor, a pressure reducing valve may be installed
whereby the pressure in the reactor may be regulated to
be at a lower value than that of the digester. Because of
the reduced pressure, the bubbles expand in the foamy pulp.
If necessary, steam may also be added to the pulp in order
to raise the temperature. To bring about foam formation,
the lye remaining in the pulp after digestion and washing
may be utilized and/or synthetic foaming agents may be added
, such as polyoxyalkylene-condensates, dodecylbenzenesulphonate
and octylphenoxypolyethoxyethanol.
In accordance with prior art teachings, foam formation
in pulp had been attempted to be avoided as causing an
inconvenience and because of this any lye which was present
was washed away and removed as much as possible. In accordance
with the present teachings, foam is employed for dispersion
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of the fibers and the foam has been found to facilitate
the transport of the pulp. Inhibitors may be fed in before
or after the mixer.
In place of oxygen gas, air or other gases as for
example ozone C12, C12 + C102 or gaseous mixtures may be
used. In accordance with the present teachings, the
reaction time is considerably shortened and requires about
10 to 20 minutes in comparison to the prior methods which
require 30 to 60 minutes. It is thus seen that the volume
of the bleaching reactor may be considerably smaller and
for example may even be a pipe which has a suitable
length.
The tests which were carried out show that the method
may be employed at fiber concentrations of approximately
up to 15~. In cellulose production, in a continuous digester,
the pulp may have a concentration amounting to approximately
10% which is a value suitable for application of the present
concept, The method is illustrated by the following examples.
Example 1
~ A test reactor was filled with pulp which had been
; impregnated with oxygen gas and heated with steam. A
NaOH solution was allowed to percolate through the pulp
during circulation. Bleaching tests were performed under
the same conditions with pulp containing 1% TEEPOL (Shell's
trade mark), a synthetic washing detergent, which was foamed
in a mixture, as well as ordinary pulp.
~ Before the tests, the kappa-figure and viscosity of
- the pulp where:
K = 29.3
;~ n = 1086
After percolation with NaOH solution with the following
prerequisites:
- alkali charge 6~
pressure 4 ato
temperature 110C
:
pulp consistency 8%
time 15 min.
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the kappa-figure and viscosity of the pulp were
Test 1: with foam Test 2: without foam
= 16.5 K = 22.4
n =985
It is difficult to obtain a lower kappa-figure than 20
without foam and without increasing the consistency to over
10%. When the pulp consistency is increased, the bleaching
results were uneven.
In tests wherein no foam was employed and with an
alkali charge of over 10% with the same prerequisites as in
previous tests, the following results were obtained:
Test 3: without foam
K = 21.2
n = soo
A comparison between the results of test 1 and test 3
clearly indicates that with the same viscosity, a lower
kappa-figure was obtained when foam was employed. Treatment
without foam requires higher alkali charge.
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Sulphate cellulose pulp which has a fiber concentration
; of 10% was treated in an apparatus in accordance with that
~; shown in Figure 2. The pulp suspension was blown out of
the digester 1, at a pressure of 15 kp/cm2 and at a temper-
ature of 90C. In the blow pipe 2, and before the mixer,
via pipe 4 there was added to the pulp about 30 kg of NaOH/ton
~; ce~lulose and via pipe 6 a foaming agent containing polyoxyalkylene-
condensate as well as octylphenoxypolyethoxyethanol to
approximately 0.5 kg/ton cellulose and through pipe 5 was
~ added approximately 10 kg oxygen/ton cellulose. The mixture
-~ 30 was than mixed in mixer 3 at a temperature of 120C whereby
-- stable foam is obtained which was fed via pipe 7 into reactor 8. The
pressure had been reduced to a pressure of 3 kp/cm through
i valve 12 in pipe 7. Apparatus 11 acts to decrease the pressure
-~ to 1 kp/cm2. After removal of the liquid containing unused
chemical, such liquid may be recycled through return pipes
~i 13 and 14 to the reactor and to the digester respectfully.
The pulp is finally washed in diffuser 10 whereby a pulp having
~;i a viscosity of 900 cm3/g and a kappa-figure equal to 14 was
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obtained while the corresponding value after the digester l
were 1080 cm /g and 29. It is thus readily seen that the
viscosity of the pulp had not decreased to any appreciable
extent however on the other hand considerable decrease in the
kappa-figure of the pulp was experienced.
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