Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present inven-tion relates to a proces~ Por producing
mechanical pulp having high strength and brlghtne~s.
Mechanical pulp is not an unambiguous concept~ but ;n
the present context it is lntended to mean refiner mechanical
pulp~ thermomechanical pulp, chemicao-mechanical pulp and
semichemic~l pulp, i.e. pulps produced with a yield higher
than 75% based on the wood raw material.
Due to the fact that mechanical pulp can be produced
with a very high yield based on the wood raw material it is
inexpensive and therefore quite attractive and much effort has
been invested in improving its quality so as to permt its use
in place of the-lmore expensive chemical pulp in many applica-
tions. It has become possible to bleach mechanical pulp with,
for example, dithionite or peroxides to brightnesses of 82-84%
SCAN for hardwood and 74-76% SCAN for s~twood.
The greatest drawback for mechanical pulp, however, is -
th~t its strength properties are poorer than those of chemical
pulp~ Its absorbency and softness are also poorer, but this is
of less significance in most cases.
The reason for the poorer strength of the mechanical
pulp is essentially the same as the reason for the higher
yield, namely, the higher wood ligning content of the fibres,
which means that the flexibility and binding strength are
relatively poor. Another factor which contributes to lowering
of strength is that the fibres are, to a large extent,
shortened by cutting and beating into meal-like fragments in
the defibration of the wood. Certain lipaphilic substances
such as resins and fatty acids and other so-called extractable
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substances mayl in certain instances, remain in the pulp and
influence the strength properties of a finished paper in a
negative direction.
In order to come to grips with these problems various
methods are employed for softening the wood and thereby the
bonds which join the individual fibres, primarily the inter-
cellular lignin, to thereby obtain a gentler defibration with
accordingly longer and softer fibers.
In the thermomechanical process the wood is thus
heated with steam and is softened prior to defibration. This
may also be combined with various concurrent chemicals such
as sulfite solutions having various pH values or peroxides.
Another method for softening the wood is to employ
chemicals and a heating restricted to that which is required
to obtain a reasonable reaction time. These chemicals
include sulfite solutions having various pH values, alkali
metal carbonates and/or hydroxi:des, wherein alkali metal
hydroxides, in particular, are very effective in terms of
softening. Depending on the reaction time, the amount of
chemicals and the temperature, various pulp properties and
yields are obtained. Such pulps include chemico-mechanical
and semi-chemical pulps, including cold soda pulp.
In practice this chemical treatment can be carried
out in a variety of ways, the simplest being to spray the
wood, normally in the form of chips, with the chemicals
immediately prior to the defibration. Even this simple
method produces good results with respect to strength prop-
erties. This is described, for examp~e, in Swedish Patent
Application 1850/72 filed by Elec~rokemiska Aktiebolager,
laid open to public inspection on August 16, 1973.
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Another method is to treat the wood in a separate step
prior to defibration. The penetra-tion of the softening
chemicals into the wood can -thereby the improved and the time,
temperature and pressure can be varied within wider limits.
Examples of such processes would include the cold soda process
and the procedures described in Swedish Patent Specifications
303 088 and 226 593 and U.S. Patent Specifications 3 069 309
and 3 023 140.
Common to both thermal and chemical softening of the
wood prior to defibration is a pronounced darkening o the
resulting pulp. If the pulp is to subsequently be bleached by
known processes after the softening, the consumption of
bleaching agent will be appreciable.
It is possible, and in certain cases even advantageous,
to introduce bleaching agent during the thermal and/or
chemical treatment, and the above-cited patent specifications
describe precisely this procedure. The difficulty with a
combined thermal or chemical softening and bleaching is~lthat
the optimal condit~ons for so~tening raxely or never coincide
with the optimal conditlons for bleaching, Suitable pH values
for the impregnat;ng liquid are 11.0 - 18.5 for softening,
while the most suitable pH for peroxide bleaching lies between
8.5 and 11. If the process is directed to maximal softening,
there is a pronounced peroxide decomposition because of the
high hydroxide ion content, and this must be compensated by
increased peroxide if the intended brightness ig to be
obtained. If, on the other hand, the same softening of the
wood is achieved without concurrent introduction of bleaching
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agent and at the same high pE~ value, the brightne~s o~ ~he
resulting pUlp will be so low that, as a rule, even largér
amounts of peroxide must be used than in the combined
instance to produce the same brightness.
The object of the present invention is to achieve an
alkali softening of the wood under optimal conditions for
softening without a reduction of the pulp brightness. It
has been ~ound, quite surprisingly, that even very small
charges of peroxide in the form of hydrogen peroxide,
organic peroxide or sodium peroxide are sufficient to
inhibit, or in any case sharply reduce the darkening of the
pulp within the pH internal which is optimal for softening.
In summary it can thus be stated that softening in a
first step with strong alkali produces a strong pulp, but
without softening the fibres are broken and the pulp becomes
weaker. If a small amount of hydrogen peroxide is added to
the alkali the darkening is prevented and the subsequent
bleaching at a lower pH is facilitated.
Thus, according to the invention, there is provided a
process for continuously producing bleached mechanical pulp
from lignocellulosic material which comprises: (a) softening
lignocellulosic material in a separate impregnation step
with a peroxide-containing solution of alkaline hydroxides
in an amount such that the pH is above ll but not more than
about 13.5; (b) passing the thus-softened material into a
bleaching stage where a peroxide-containing solution is
added and the pH is maintained below ll but not less than
about 8.5; (c) adjusting the amount of peroxides added to
the system .so that from lO to 30 per cent Oe that totally
required is added in said softening stage and the balance
in said bleaching stage; and (d) pereorming mechanical
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disintegration of the material to pulp in at lea~t one
defibrating apparatus after the softening treatment.
It has, of course, been previously established that
a high pH results in peroxide decomposition and a logical
measure has therefore been to divide up the peroxide charge
between the softening pretreatment step and defibration.
Such attempts have been made and are described in the
above-cited patent specifications and in Pulp and Paper
Magazine of Canada, Vol. 73, 1972, p. 80, but therein at
least 75% of the total peroxide was added in the pre-
treatment step and the remainder to a separate bleaching
step or to the refiner after the softening step. In all
instances the wholly differing pH
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optima of the par~ial processes have been ignored.
What is thus surprising in our pre8ent invention is not
that the charged p~roxide i5 better u~ilized if it is divided
up into several steps, but rather that the peroxide charge in
the pretreatment step can be held so low. The best result is
namely achieved at a charge as low as even 103~0% of the total
required peroxide to the softening step, and in no case has it
been found necessary to go as shigh as 75%.
Pretreatment is carried out to the point where the
falling pH value permits a true bleaching to take place, and
it is therefore advantageous to place a bleaching tower
directly after the refiner for final bleaching of the pulp.
The required pH reduction from the softening step to the
bleaching step can only be achieved, in most cases, by
controlling the residence time and the temperature during the
pretreatment so that the correct pH for the bleaching is
obtain~d. It is, of course, p~ssible to lower the pH when
required by adding an acid such as sulf~te solution, sulfuric
acid or acidic white water from another fabrication step. In
order to achieve an ef~ective mixture of the bleaching agent,
the agent can be introduced before the refiner since the
refiner acts as a very effective mixer.
Because the peroxide charge to the pretreatment step is
so low, the residu~l peroxides from the shbsequent bleaching
step are often sufficient for charging to the pretreatment
step. The reason why such a small amount of hydrogen peroxide
is required in the pretreatment is not fully understood, but
it may depend on the fact that released carbohydrate acids in
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the pretreatment step, and if recycl~ng i5 used, al50 from the
bleaching step act as complexing or chela~ing agents for heavy
metals in the wood and thereby stabilize the peroxide. A
recycling of waste liquor from the bleaching step to the pre-
treatment step also sharply reduces the amount of released
oxygen consuming substance.
With this point of departure attempts have also been
made to additionally introduce chelating agents to the hydrogen
peroxide in the pretreatment step. Chelating agents from the
detergent fi.eld, NTA, EDTA, DTPA, i.e. nitrilotriacetic acid,
ethylene diaminetetraacetic acid and diethylenetriaminoperlta-
: acetic acid, and tripolyphosphates, i.a. have been investigated.
According to the invention optimal strength is thusobtained for the pulp with a pretreatment step which is
carried out without loss of brightness and with a minimal
amount of peroxide. The subsequent bleaching is pe~formed by
` Xnowm means in a refiner and/or in a bleaching tower, and
since the pulp prior to the actual bleaching step already has
ah~igh brightness and since the bleachlng step can, in
addition~ be carried out under optimal conditions, a high
brightness is bobtained at maximal strength for the pulp with
a minimal consumption of peroxide.
The possi~ ty of controlling the charges of alkali
and hydrogen peroxide independently allows all reaction
conditions to be held at optimal levels. Prior processes have
utilized alkali and hydrogen peroxide in a constant ratio and
a charge, for example, of sodium peroxide alone produces a
constant ratio of l/2 mol hydrogen peroxide per mol sodium
peroxide.
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The invention is illustrated by the following examples
wherein the same test procedure is used in all tests. The
different variables are presented in the table below in which
tests 1-10 represent spruce and 11-14 hardwood. Tes~s 1, 2, 5,
7, 13 and 14 are compara~lve examples according to the prior
art while the remaining tests are performed according to the
invention.
In all tests match chips having a dimension of 25 x 3 x
3 mm are placed in a steel vessel which is then evacuated. The
chemicals together with water are drawn into the wood and a
hydraulic overpressure of 6 bar is thereafter applied. Unless
otherwise indicated the impregnation period is 1 hour and the :
impregnation temperature is 45C. 41 Be water glass in an
amount corresponding to 4.5~ based on the wood is introduced
in all cases in connection with the alkali charging.
After the pretreatment the chisp are defibrated to ~0
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! ml freeness Canadian Standard in a laboratory defibrator.
The bleaching step is made so that additional chemicals
are added, in part, to the defibrator during defibration, and ,
in part, separately after defibration, The pulp concentration
during bleaching is 15%, both in the defibrator bleaching and
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`~; the separate bleaching. In the separate bleaching after
defibration the residence time ls 2 hours and the temperature
7QC. When the bleaching is done as defibrator or refiner
` bleaching the pulp is stored at 70C after defibration so that
the total residence time is 2 hours.
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Table I
Pre-treatment Defibration Bleachin~ Paper Qual_ies
~cp.NaOH H202 NaOH H22 NaOH H22 R-H22 ~ight TIndrexLength g
No. % % % % % % % % SCAN km
.
1 1.0 4.3 20 75.1 34 1.57
2 1.0 0.1+) 4.3 18 74.3 35 1.56
3 1.0 1.0 0.2 ) 3.3 34 76.2 35 1.61
4 4.5 4.5 10 72.9 38 3.2
4.5 4.5 17 69.8 40 3.1
6 4.5 1.0 3.5 35 74.2 41 3.1
7 4.5 3.5 1.0 12 72.3 40 3.1
8 4.5 0.8 ) 3.7 38 74.1 41 3.0
9 4.5 0.8++) 3.7 3~ 74.0 40 3.2
4.5 1.0 ~.5 40 73.5 35 2.9
11 +++) +~+~ 1 o 1.1 2.5 35 81.5 - -
. .,
12 2.0 1.0 2.5 32 83.1 - -
13 2.0 4.5 19 78.1
14 2.0 4.5 20 78.0
: :
)To obtain the correct initial pH in the bleaching step.
++)Recycled residual peroxides from Example 6.
++ )Pretreatment at 110C and 0~5% S032 charge. Tirne about 5 min.
Depending on the wood~ -the type of wood and the desired
type of pulp the chemical charges can vary within wide limits
and the examples show brightness variations for comparable
bleaching agent charges. The streng-th values obtained apply -to
samples prepared in a laboratory reFiner and must thus be seen
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as illus-trations of the impressive improvernent in streng-th ~hen
the alkalinity in the softening s-tep increasesO
The test series clearly shows the appreciable increase
in brightness obtained according to -the invention in spite of
the high alkali chargesO Tests 8 and 9 deserve special mentionS
The peroxide demand to -the softening step is completely
covered by the residual peroxides in the waste spent liquor
" from the final bleaching step.
Test 11 shows ~ha~ the invention can also be applied to
` 10 thermomechanical pulp.
The present inven-tion is not limited to embodiments
with a special pretreatment apparatus. Pretreatment apparatus
is herein intended to mean any wood impregna-ti.on equipment
with or without heating~ such as cellulose digesters 7 pressure ::
vessels with or without screw feed, chip washing with a closed
washing fluid system~ vibrating chip processers such as "live :
~ bottom bins"~ etc. The application can as well take place with
:; direct softening in the first defibrati.on step and further
bleaching in a second refi.ner step or a separate bleaching
step on a combination of other refiner and bleaching steps. :
,.~ This is illustrated by tests 9, 10 and 11. What is essen-tial
is that the softening takes place at a pH higher than 11 in
the presence of relatively small amounts of peroxide and that
the actual bleaching thereafter occurs a-t pH values less than
11 with charging of additional peroxide.
The invention also has a beneficial effec-t on the
release of oxygen consuming substance~ BS7~ from the process.
In bleaching two samples of refiner pulp, half o~ the white
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water from -the first sample was used as ~ilution w~ter in the
second sample. 4% hydrogen pe-roxide~ 4% water glass and 1.4%
: caus-tic soda were used in -the -tes-tO The bleaching temperature
`: was 60C~ the bleaching -time 2 hours and the concentration 10%.
The values obtained for oxygen consumption are seen in Table 2
~` below.
: Table 2
Ex. No. BS7 in waste liquor Recycled BS7 New BS7
_ _ kg/t _ _ _ kg/-t _ kg/t
1 15.6 0 15.6
2 16.5 7.6 8~9
Erom the values it may be seen that -the application of
the invention reduces the new formation of oxygen consuming
.~ 15 substances to about half.
Even lignocellulose material other than sof-twood and
hardwood such as grass., bamboo~ bagasse, etc. can be used for
pulp p~oduction acoording to the invention.
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