Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02673175 2009-06-18
RPV13437 WO
Process for producing tissue paper
The invention relates to a process for producing a
tissue web, which is produced from a stock suspension
comprising fibers.
The invention also relates to a process for producing a
stock suspension for use in particular for the
production of tissue webs.
Tissue products are currently mainly produced from
fully cellulosic materials, in particular kraft pulps.
Mechanically produced fibrous materials find only
limited use, since here the tendency to yellowing and
the poor strength properties of the stocks prevent
widespread use.
Common mixture ratios between long fiber and short
fiber stocks lie in the region of 50:50.
The porosity and the permeability of the tissue paper
are determined critically by the freeness of the fibers
in the stock suspension from which the tissue paper is
produced.
Here, a high freeness necessitates a high content of
fines in the suspension, which leads to lower porosity
and permeability.
Furthermore, a high freeness causes a high water
retention value for the fibers of the stock suspension,
which means that the tissue paper is difficult to
dewater during its production.
At high machine speeds, the poor dewatering ability
often results in too low a dryness during production.
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For instance, before the Yankee drying cylinder a
certain dryness is needed in order to prevent lifting
of the tissue paper web as a result of its contact with
the hot circumferential surface of the tissue drying
cylinder.
In addition, the tissue must be tear-resistant.
The tearing strength is determined both by the
production process and by the freeness of the fibers.
In order to increase the tearing strength, the tissue
paper must be consolidated during its production. In
order to obtain a high tearing strength, the proportion
of fines must also be high.
The requirements on the tearing strength thus
contradict the requirements on the water absorption
capacity, the absorbency and the dewatering ability.
The object of the invention is therefore the production
of tissue paper with a high specific volume, as high a
tearing length as possible, with the lowest possible
freeness.
According to the invention, the object is achieved in
that the stock suspension contains lignocellulosic
fibrous material made of wood or annual plants which
has a tearing length of more than 6. 0 km at 12 SR or a
tearing length of more than 7.5 km at 15 SR and a
lignin content of at least 15%, based on the oven-dry
fibrous material, for coniferous wood in the unbleached
state, or a tearing length of more than 4.5 km at 20 SR
and a lignin content of at least 12%, based on the
oven-dry fibrous material, for deciduous wood in the
unbleached state, or a tearing length of more than 3.5
km at 20 SR and a lignin content of at least 10%, based
on the oven-dry fibrous material, for annual plants in
the unbleached state.
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The fibers already exhibit high strength values at a
freeness which is far lower as compared with fibers
used hitherto. The fibrous material according to the
invention is already capable of forming good bonds with
neighboring fibers at a lower freeness and therefore
also with a lower expenditure of refining energy.
The lignin content of the unbleached fibrous material
in the case of coniferous wood can advantageously
comprise at least 15%, preferably at least 18%, in
particular at least 21%, of the oven-dry fibrous
material, in the case of deciduous wood at least 12%,
preferably at least 14%, in particular at least 16%, of
the oven-dry fibrous material and, in the case of
annual plants, at least 10%, preferably at least 12 %
and in particular at least 19%, of the oven-dry fibrous
material.
The higher the lignin content of the fibrous material,
the lower are the losses of woody substance during
production of the fibrous material.
In this case, it is entirely possible to achieve even
higher strength values. Therefore, the tearing length
for coniferous wood fiber stock at 12 SR should be
greater than 7 km, preferably greater than 7.5 km and
in particular greater than 8 km. The tearing length
for coniferous wood fiber stock at 15 SR should be
greater than 9 km, preferably greater than 9.5 km and
in particular greater than 10 km.
The tearing length for deciduous wood fiber stock at a
lignin content of at least 12% and a freeness of 20 SR
should be greater than 6 km, preferably greater than
7 km and in particular greater than 7.5 km.
The tearing length for annual plant fiber stock at
20 SR should be greater than 3.5 km, preferably greater
than 4 km and in particular greater than 4.5 km.
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However, the fibrous material according to the
invention is not just distinguished by high tearing
lengths. Instead, the strength level overall is high.
If the fibrous material according to the invention is
subjected to a bleaching treatment, the fiber
properties are enhanced considerably. The bleaching
treatment is required for many applications with higher
requirements on the whiteness. However, it is also
aimed at the setting and improvement of the fiber
properties. With the bleaching treatment, the tearing
lengths increase.
Thus, the stock suspension should contain
lignocellulosic fibrous material made of wood or annual
plants which has a tearing length of more than 7.5 km
at 15 SR and a lignin content of at least 13%, based on
the oven-dry fibrous material, for coniferous wood in
the bleached state, or a tearing length of more than
5.0 km at 20 SR and a lignin content of at least 10%,
based on the oven-dry fibrous material, for deciduous
wood in the bleached state, or a tearing length of more
than 5.5 km at 20 SR and a lignin content of at least
10%, based on the oven-dry fibrous material, for annual
plants in the bleached state.
Here, too, higher tearing lengths are advantageous.
Thus, the tearing length for coniferous wood fiber
stock at 15 SR should be greater than 9 km, preferably
greater than 10 km.
The tearing length for deciduous wood fiber stock at
20 SR should be greater than 5.5 and the tearing length
for annual plant fiber stock at 25 SR should be greater
than 5 km, preferably greater than 5.5 km and in
particular greater than 6 km.
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In order to be able to make optimal use of the
advantages with respect to a high specific volume and
high strength at the lowest possible freeness, the
stock suspension should exclusively contain
lignocellolosic fibrous material according to the above
description.
For many applications, however, it is sufficient if the
stock suspension is only partly formed from such
lignocellulosic fibrous material. In this case it is
advantageous if between 20 and 80%, preferably between
30 and 50%, of the fibrous material of the stock
suspension is formed from lignocellulosic fibrous
material according to the above description.
Following the formation of a tissue web, this is
preferably led between an upper structured and
permeable belt and a lower permeable belt in a
dewatering step, pressure being exerted on the upper
belt, the tissue web and the lower belt along a
dewatering section.
The pressure exerted on the arrangement comprising the
upper belt, tissue web and lower belt can be effected
by a gas flow and/or by a mechanical pressing force.
Preferably, during a dewatering step, a gas flows
firstly through the upper belt, then the tissue web and
then the lower belt. In this case, the dewatering
takes place in the direction of the lower belt.
Additionally or alternatively to the through flow of
gas, it may be advantageous if, during the dewatering
step, the arrangement comprising the upper belt, tissue
web and lower belt is led in at least some sections
between a press belt under tension and a smooth
surface, the press belt acting on the upper belt and
the lower belt being supported on the smooth surface.
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Preferably, the gas flow flows through the arrangement
comprising the upper belt, tissue web and lower belt,
at least in some sections in the region of the
dewatering section, so that the dewatering is carried
out simultaneously by the pressing force of the press
belt and the through flow of the gas.
Trials have shown that the gas flow through the tissue
web should amount to about 150 m3 per minute and meter
length along the dewatering section.
In the interests of adequate dewatering of the tissue
web, the press belt should be under a tension of at
least 30 kN/m, preferably at least 60 kN/m and in
particular 80 kN/m.
In order to be able to achieve good dewatering of the
tissue web by means of the mechanical tension of the
press belt and also on account of the gas flow through
the press belt, the press belt should have an open area
of more than 50% and a contact area of at least 15%.
The smooth surface is preferably formed by the
circumferential surface of a roll. The gas flow can
advantageously be produced via a suction zone in the
roll and/or a positive pressure hood arranged above the
upper belt.
During the production of the lignocellulosic fibrous
material according to the invention, it is important
that at least a proportion of the stock suspension is
produced from wood or annual plants having a lignin
content of at least 15% for coniferous wood and 12% for
deciduous wood and 10% for annual plants, in each case
based on the oven-dry fiber mass, by the following
steps:
- producing a chemical solution with more than 5% of
chemicals (calculated as NaOH) for coniferous wood
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or with more than 3.5% of chemicals (calculated as
NaOH) for deciduous wood or with more than 2.5% of
chemicals (calculated as NaOH), in each case based
on the oven-dry quantity of the wood used,
- mixing the chemical solution with the wood or
annual plants in a prescribed liquor ratio,
- heating the chemical solution and the wood or
annual plants to a temperature above room
temperature and then either (lst alternative)
- removing free-flowing chemical solution and
- digesting the wood or annual plants in the vapor
phase or (2nd alternative)
- digesting the wood or annual plants in the
presence of the chemical solution in liquid phase
and
- separating the free-flowing chemical solution and
the wood or annual plants.
The process according to the invention is based on the
fact that, in order to produce high-yield fibrous
materials, higher quantities of chemicals are used than
were previously usual. More than 5% of chemicals for
coniferous wood is considerably above the quantities of
chemicals previously usual for industrial fibrous
material production, likewise more than 3.5% of
chemicals for deciduous wood and 2.5% for annual
plants. This high use of chemicals produces fibrous
materials with good yield and excellent strength
properties. Thus, for coniferous wood at freenesses of
only 12 SR to 15 SR, tearing lengths of more than 8 km
but also tearing lengths of more than 9 km and more
than 10 km are measured. For deciduous woods at only
20 SR, values of more than 5 km but also tearing
lengths of more than 6 km and more than 7 km are
measured. The desired high strength level is therefore
achieved.
It is to be viewed as an extraordinary advantage of the
orocess accordinq to the invention that the strength
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values are already achieved at extremely low
freenesses, such as were not available hitherto for
high-yield fibrous materials. Fibrous materials
according to the prior art do not exhibit an acceptable
strength level at freenesses of 12 SR to 15 SR for
coniferous wood fibrous materials or of 20 SR for
deciduous wood. Known fibrous materials at these low
freenesses have until now resulted in fibers which have
not demonstrated adequate strength properties for
economic use of such fibers.
Suitable annual plants are in particular bamboo, hemp,
rice straw, bagasse, wheat, miscanthus and the like.
On the other hand, at freenesses in the range from
12 SR to 15 SR, the fibrous materials produced by the
process of the invention already have tearing lengths
of more than 8 km up to 11 km and tear propagation
resistances of more than 70 cN up to more than 110 cN,
based on a sheet weight of 100 g/m2. These low
freenesses are moreover achieved with a low specific
requirement for refining energy, which is less than
500 kWh/t of fibrous material for coniferous wood; in
the case of deciduous wood the need for refining energy
can even be less than 300 kWh/t of fibrous material.
The finding that the high strength level is already
reached at low freenesses of 12 SR to 15 SR for
coniferous wood and at 20 SR for deciduous wood and
less is a substantial part of the invention.
These high strength values in combination with low
freenesses for fibrous materials with a lignin content
of more than 15% for coniferous wood fibrous materials,
of more than 12% for deciduous wood fibrous materials
or of more than 10% for annual plants, have hitherto
not been known. The high strength level can, however,
also be maintained for fibrous materials having an even
higher lignin content. The process according to the
invention is even suitable for producing coniferous
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wood fibrous materials having a lignin content of more
than 18%, preferably more than 21%, advantageously more
than 24%, based on the oven-dry fiber mass. Deciduous
wood fibrous materials having a lignin content of more
than 14%, preferably more than 16%, particularly
preferably more than 18%, and also annual plants having
a lignin content of more than 10%, preferably more than
12%, in particular more than 19%, can likewise be
produced with the process according to the invention
and exhibit a high strength level.
The composition of the chemical solution used for the
digestion can be defined in accordance with the wood or
annual plants used for the digestion and the desired
fibrous material properties. As a rule, only a sulfite
component is used. Alternatively or as a supplement, a
sulfide component can also be added. Digestion with a
sulfite component is not disrupted by the presence of
sulfide components. Industrially, sodium sulfite is
normally used but the use of ammonium or potassium
sulfite or of magnesium bisulfite is also possible. In
particular if high quantities of sulfite are used, it
is possible to dispense with the use of an alkaline
component since a high pH, which encourages digestion,
is established even without the addition of alkaline
components.
In order to adjust the pH and to assist the
delignification, an acid and/or an alkaline component
can also be metered in. Industrially, the alkaline
component used is normally sodium hydroxide (NaOH).
However, the use of carbonates is also possible, in
particular sodium carbonate. All statements relating
to quantities of chemicals in the digestion process in
this document, for example to total chemical used or to
the subdivision of the sulfite component and the
alkaline component are, if not otherwise specified, in
each case calculated and stated as sodium hydroxide
(NaOH).
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Acids can be metered in as acid components in order to
set the desired pH. However, preference is given to
the addition of SO2r if appropriate in aqueous solution.
It is inexpensive and easily available, in particular
when the used chemical solution, for example based on
sodium sulfite, is conditioned for further use
following the digestion.
It is seen as an independent achievement of the
invention to have recognized the advantages of the use
of a quinone component for the high-yield digestion
according to the invention. Quinone components, in
particular anthraquinone, have until now been used in
the production of pulps with a minimal lignin content,
in order to prevent undesired action on the
carbohydrate towards the end of the digestion. By
adding quinone components it becomes possible to
continue the digestion of wood further until the
approximately complete breakdown of the lignin. It has
emerged as a previously unknown, unexpected property of
quinone components that these raise the rate of the
lignin breakdown significantly during the production of
high-yield pulps. The duration of the digestion, for
example during the production of coniferous wood
fibrous materials, can be shortened by more than a
half, depending on the digestion conditions by more
than three-quarters. This noticeable effect is
achieved with minimal use of quinone, for example. A
use of, for example, anthraquinone which is between
0.005% and 0.5% is optimal. A use of anthraquinone of
up to 1% also produces the desired effect. A use of
more than 3% anthraquinone is normally uneconomic.
A chemical solution is produced from an individual
chemical or a plurality of the aforementioned
chemicals. An aqueous solution is normally added. As
an option, the use or the addition of organic solvents
can also be provided. Alcohol, in particular methanol
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and ethanol, in a mixture with water gives particularly
effective chemical solutions for the production of
high-quality high-yield fibrous materials. The mixture
ratio of water and alcohol can be optimized for the
respective raw material in a few trials.
The quantity of chemicals to be used according to the
invention for producing a fibrous material with a yield
of at least 70% is at least 5% for coniferous wood, at
least 3.5% for deciduous wood and at least 2.5% for
annual plants, in each case based on the oven-dry wood
or annual plant mass to be digested. The quality of
the fibrous material produced exhibits the best results
with a chemical usage of up to 15% for coniferous wood,
of up to 10% for deciduous wood and up to 10% for
annual plants. Preferably, between 9% and 11% of
chemicals, based on the oven-dry wood used, is added in
the case of coniferous wood. For deciduous wood, the
use of the chemicals is somewhat lower, preferably
between 4% and 10%, particularly preferably between 6%
and 9%, and between 3% and 10o in the case of annual
plants.
As already explained above, the setting of a specific
pH is in no way required. Only when, for example,
particular properties of the pulp (particularly high
whiteness, a specific ratio of tearing length and tear
propagation resistance) are to be achieved with the
digestion may it be expedient to add acid or an
alkaline component before or during the digestion.
According to an advantageous refinement of the
invention, irrespective of the chosen use of chemicals
overall, a ratio between an alkaline component and
sulfur dioxide (SiO2) can be set over a wide range.
Here, SOZ is named as representing the acid component
mentioned above. It is therefore also possible to use
an acid instead of SO2. Since the quinone component
possibly added is used only in minimal quantities,
normally considerably below 1%, it can be disregarded
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in setting this ratio. A ratio of alkaline
component:S02 in a range from 5:1 to 1.6:1 is well
suited to carrying out the process of the invention and
to achieving fibrous materials with high strength
properties. A usual, particularly suitable range is
lies between 2:1 and 1.6:1. The proportional
components are coordinated on the basis of the raw
material to be digested and the respectively chosen
process management (digestion temperature, digestion
time, impregnation).
The process according to the invention can be carried
out in a wide pH range. The ratio of alkaline
component to acid component and the use of an acid or
alkaline component can be set in such a way that at the
start of the process a pH between 6 and 11, preferably
between 7 and 11, particularly preferably between 7.5
and 10, is set. The rather alkaline pH values between
8 and 11, which are advantageous for the process
according to the invention, also encourage the action
of the quinone component. The process according to the
invention is tolerant with respect to the pH; few
chemicals are needed for pH adjustment. This has a
beneficial effect on the costs for chemicals.
Without any further addition of acid or alkaline
component, a pH between 5 and 9, normally between 6.5
and 9, for example for coniferous wood, is established
in the free-flowing chemical solution at the end of the
digestion and also in the organic components dissolved
therein, which are liquefied by the digestion. The
dissolved organic substances primarily include
lignosulfates.
The liquor ratio, i.e. ratio of the quantity of oven-
dry wood or annual plants to the chemical solution, is
set between 1:1.5 and 1:6. A liquor ratio of 1:2 to
1:4 is preferred. In this range, good and simple
mixinq and impregnation of the material to be digested
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is ensured. For coniferous wood, a liquor ratio of
1:3.5 is preferred. For wood chips with a large
surface, the liquor ratio can also be considerably
higher, in order to permit rapid wetting and
impregnation. At the same time, the concentration of
the chemical solution can be kept so high that the
quantities of liquid to be circulated do not become too
large.
The mixing or impregnation of the wood or annual plant
material to be digested is preferably carried out at
elevated temperatures. Heating the chips and the
chemical solution to up to 110 C, preferably to up to
120 C, particularly preferably to up to 130 C, leads
to rapid and uniform digestion of the wood. For the
mixing or impregnation of the chips, a time period of
up to 30 minutes, preferably of up to 60 minutes,
particularly preferably of up to 90 minutes; is
advantageous. The respectively optimal time period
depends, amongst other things, on the quantity of
chemicals, the liquor ratio, the chosen temperature and
the type of digestion (liquid or vapor phase).
The digestion of the lignocellulosic material mixed or
impregnated with the chemical solution is preferably
carried out at temperatures between 120 C and 190 C,
preferably between 140 C and 180 C. For most woods,
digestion temperatures between 150 C and 170 C are set.
Higher or lower temperatures can be set but in this
temperature range the expenditure of energy for the
heating and the acceleration of the digestion are in an
economic relationship with each other. Higher
temperatures can additionally have a detrimental effect
on the strengths and the whiteness of the fibrous
materials. The pressure generated by the high
temperatures can readily be absorbed by appropriate
design of the digester. The duration of the heating is
normally only a few minutes, normally up to 30 minutes,
advantaqeously up to 10 minutes, in particular when
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steam heating is used. The duration of the heating can
be up to 120 minutes, preferably up to 60 minutes, for
example when digestion in the liquid phase is carried
out and the chemical solution has to be heated together
with the chips.
The duration of the digestion is primarily chosen on
the basis of the desired fibrous material properties.
The duration of the digestion can be shortened to up to
2 minutes, for example for the case of vapor-phase
digestion of deciduous wood having a low lignin
content. However, it can also be up to 180 minutes, if
for example the digestion temperature is low and the
natural lignin content of the wood to be digested is
high. Even if the initial pH of the digestion is in
the neutral range, a long digestion time can be
necessary. In particular, the digestion time is up to
90 minutes, particularly in the case of coniferous
wood. The digestion time is particularly preferably up
to 60 minutes, advantageously up to 30 minutes. A
digestion time of 60 minutes is suitable in particular
in the case of deciduous woods.
In the case of annual plants, the digestion time is up
to 90 minutes. The use of a quinone component, in
particular anthraquinone, permits a reduction in the
digestion time of up to 25% of the time required
without the addition of anthraquinone. If the use of
quinone components is omitted, the digestion time for
comparable digestion results is lengthened by more than
an hour, for example from 45 minutes to 180 minutes.
According to an advantageous embodiment of the process
according to the invention, the duration of the
digestion is set as a function of the chosen liquor
ratio. The lower the liquor ratio, the shorter the
process duration can be set.
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The production of high-yield fibrous material with high
chemical use of more than 5% for coniferous wood, of
more than 3.5% for deciduous wood and at least 2.5% for
annual plants initially appears uneconomic. However,
trials have shown that only part of the chemicals is
consumed during the partial digestion of the
lignocellulosic material. The predominant part of the
chemicals is removed unused, either before the
digestion (vapor-phase digestion) or after the
digestion (digestion in the liquid phase). The actual
consumption of chemicals is below the quantities used
in the digestion solution.
The chemical consumption is registered as the quantity
of chemicals which - based on the quantity of chemicals
originally used - is measured after the removal or
separation of the chemical solution and, if
appropriate, the capture of chemical solution which is
measured after the difibering or in conjunction with
capture of the chemical solution. The chemical
consumption depends on the absolute quantity of
chemicals used for the digestion, based on the oven-dry
mass of wood to be digested. The higher the use of
digestion chemicals, the lower the direct conversion of
chemicals. Given a use of 27.5% of chemicals, based on
oven-dry mass of wood, for example only about 30% of
the chemicals used are consumed. Given the use of 15%
of chemicals, based on oven-dry mass of wood, 60% of
the chemicals used are consumed, however, as could be
verified in laboratory trials. The chemical
consumption of the process according to the invention
according to a preferred embodiment of the process
during the digestion is up to 80%, preferably up to
60%, particularly preferably up to 40%, advantageously
up to 20%, particularly advantageously up to 10%, of
the chemical input at the start of the digestion.
The chemical consumption for producing a tonne of
fibrous material is around 6% to 14% sulfite and/or
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sulfide component and also alkaline and/or acid
component and also, if appropriate, quinone component,
based on oven-dry fibrous material (deciduous and
coniferous wood or annual plants) . According to the
invention, this quantity of chemicals is enough to
produce a fibrous material having the prescribed
properties. In order however to ensure a uniform
process result and possibly to obtain particular,
desired fiber properties, it may prove to be expedient
to use higher quantities of chemicals for the
digestion, for example the aforementioned up to 30% of
chemicals based on oven-dry wood or annual plant mass.
The use of these quantities of chemicals at the start
of the digestion exhibits an advantageous effect, since
the fibrous materials obtained in this way have
previously unavailable properties, in particular high
strength properties and high whitenesses. In
particular, no digestion process which produces fibrous
materials with high strength values over a wide pH
range from neutral as far as the alkaline range has
hitherto been available. It has been shown to be
economically particularly attractive that the fibrous
materials produced in accordance with the invention can
be refined to prescribed freenesses with an energy
demand far lower than known fibrous materials. In
addition, they already develop the high strengths at
unusually low freenesses of 12 SR to 15 SR for
coniferous wood and of 20 SR for deciduous wood.
After the mixing and impregnation of the wood with the
chemical solution or after the digestion, there is an
excess of chemicals in the free-flowing liquid. This
excess is drawn off before the digestion (lst
alternative) or after the digestion (2nd alternative).
According to an advantageous development of the
process, the composition of the chemical solution
removed is captured and subsequently adjusted to a
prescribed composition for renewed use for the
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production of fibers. The chemical solution which is
removed before or after the digestion of the wood or
the annual plants no longer has the composition set at
the beginning. At least part of the chemicals used for
the digestion has - as described above - penetrated
into the material to be digested and/or has been
consumed in the digestion. The unused chemicals can
readily be used again for the next digestion. However,
the invention proposes firstly determining the
composition of the chemicals removed and then
supplementing the used proportions of, for example,
sulfite, alkaline component, quinone component or else
water or alcohol, in order once more to produce the
prescribed composition of the next digestion. This
supplementation step is also designated strengthening.
It is to be viewed as a considerable advantage of this
measure that the chemical solution, in the case of
removal before the digestion but also in the case of
removal after the digestion, really contains no
substances at all or very few substances which prove to
be disruptive during renewed use of the strengthened
chemical solution for the next digestion. The process
according to the invention, which is based on making a
surplus of digestion chemicals available during the
impregnation, is also able to operate extremely
economically, despite the procedure of the high
chemical use, initially appearing uneconomic, for the
removal or the separation and the strengthening of the
chemical solution can be carried out simply and cost-
effectively.
The process according to the invention is controlled
specifically in such a way that only as little as
possible of the starting material used is broken down
or dissolved. The aim is to produce a fibrous material
which, for coniferous wood, has a lignin content of at
least 15%, based on the oven-dry fiber mass, preferably
a lignin content of at least 180, particularly
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preferably of 21%, advantageously of at least 24%. For
deciduous wood, the aim is to achieve a lignin content
of at least 12%, based on the oven-dry fiber mass,
preferably of at least 14%, particularly preferably of
at least 16%, advantageously of at least 18%. In the
case of annual plants, the preferred lignin content is
between 10 and 28%, in particular between 12 and 26%.
The yield of the process according to the invention is
at least 70%, preferably more than 75%, advantageously
more than 80%, in each case based on the wood used.
This yield correlates with the lignin content of the
fibrous material specified above. The original lignin
content of wood is specific to the type. The loss of
yield in the present process is predominantly
represented as a loss of lignin. In the case of non-
specific digestion processes, the proportion of
hydrocarbons is increased considerably, for example
because digestion chemicals also put cellulose or
hemicelluloses into solution in a manner that is
undesired per se.
A further, advantageous measure, after the defibering
and possibly the refining of the lignocellulosic
material, is to remove the chemical solution still
remaining and to supply it to further use. In a
preferred refinement, this further use can comprise two
aspects. Firstly, the organic material broken down or
put into solution during the partial digestion,
predominantly lignin, can be used further. For
example, it is burned in order to obtain process
energy. Or it is prepared in order to be used in a
different manner. Secondly, the used and unused
chemicals are reconditioned, so that they can the used
for a renewed partial digestion of lignocellulosic
material. This includes the preparation of consumed
chemicals.
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According to a particularly preferred variant of the
process according to the invention, the chemical
solution employed is used extraordinarily efficiently.
After the defibering and possibly the refining, the
fibrous material is washed, in order to displace the
chemical solution as far as possible my means of water.
The filtrate arising during this washing and
displacement operation contains considerable quantities
of chemical solution and organic material. According
to the invention, this filtrate is supplied to 'the
removed or separated chemical solution before the
chemical solution is strengthened and fed to the next
digestion. The chemicals contained in the filtrate and
organic constituents do not disrupt the digestion. To
the extent that they make a contribution to the
delignification during the next digestion, their
content of chemicals is registered and taken into
account during the determination of the quantity of
chemicals needed for this digestion. The chemicals
further contained in the filtrate behave inertly during
the impending digestion; they do not interfere. The
organic constituents contained in the filtrate likewise
behave inertly. They are used further during the
conditioning of the chemical solution after the next
digestion, either to produce process energy or in
another way.
It is viewed as particularly advantageous that, as a
result of this management of the filtrate, less fresh
water and fewer chemicals are used for the digestion.
At the same time, a maximum of dissolved organic
material is captured. This improved utilization of the
organic materials that have gone into solution also
improves the economy of, the process according to the
invention.
In the following text, the invention is to be explained
in more detail in a number of exemplary embodiments.
In the appended drawinq:
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Figure 1 shows an apparatus for carrying out the
inventive method; and
figure 2 shows a second apparatus.
Firstly, however, the details of the method according
to the invention for producing the stock suspension
will be explained in more detail below in exemplary
embodiments.
The following trials were evaluated in accordance with
the following instructions:
- The yield was calculated by weighing the raw
material put in and the pulp obtained after the
digestion, in each case dried to constant weight
at 105 C (absolutely dry).
- The lignin content was determined as Klason lignin
in accordance with TAPPT T 222 om-98.
The acid-soluble lignin was determined in
accordance with TAPPI UM 250.
- The paper technological properties were determined
on test sheets which were produced in accordance
with Zellcheming Note Sheet V/8/76.
- The freeness was registered as per Zellcheming
Note Sheet V/3/62.
- The bulk was determined as per Zellcheming
Instruction V/11/57.
- The tearing length was determined as per
Zellcheming Instruction V/12/57.
- The tear propagation resistance was determined as
per DIN 53 128 Elmendorf.
- The determination of tensile, tear and burst index
was carried out in accordance with TAPPI 220
sp-96.
- The whiteness was determined by producing the test
sheets as per Zellcheming Note Sheet V/19/63,
measured as per SCAN C 11:75 with a Datacolor
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elrepho 450 x photometer; the whiteness is
specified in percent as per ISO Standard 2470.
- The viscosity was determined as per Note Sheet
IV/36/61 of the German Association of Pulp and
Paper Chemists and Engineers (Zelicheming).
- All the % statements in this document are to be
read as percent by weight if not otherwise
individually indicated.
- The statement "o.d." in this document refers to
"oven-dry" material, which has been dried to
constant weight at 105 C.
- The chemicals for the digestion are specified in
percent by weight as sodium hydroxide if not
otherwise explained.
Example 1 - Coniferous wood digestion in the liquid
phase
A mixture of birch wood and Douglas fir chips, after
steaming (30 minutes in saturated steam at 105 C), was
dosed with a sodium sulfite digestion solution with a
liquor ratio of wood:digestion solution of 1:3. The
total use of chemicals was less than 15%, based on o.d.
wood. The pH at the start of the digestion was
adjusted to pH 8.5 - 9 by adding SOZ.
The birch wood/chips mixture impregnated with chemical
solution was heated to 170 C over a time period of 90
minutes and digested at this maximum temperature over
60 minutes.
The free-flowing liquid was then removed by
centrifuging, collected and analyzed and strengthened
in an arrangement for feeding back unused liquid and in
this way conditioned for the next digestion.
The digested chips were defibered. Partial quantities
of the fibrous material produced in this way were
refined for different times in order to determine the
strenath at different freenesses. The expenditure of
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energy for defibering the partly digested chips was
less than 300 kWh/t of fibrous material.
The yield in this trial was around 77%, based on the
wood mass used.
This corresponds to a fibrous material having a lignin
content of far above 20%. The average lignin content
for birch wood is given as 28%, based on the o.d. wood
mass (Wagenfuhr, Anatomie des Holzes [Anatomy of Wood],
VEB Fachbuchverlag, Leipzig, 1980). The actual lignin
content of the fibrous material is higher than 20%
since, during the digestion, it is predominantly but
not exclusively lignin which is broken down.
Carbohydrates (cellulose and hemicelluloses) are also
dissolved in small quantities. The values specified
show that the digestion exhibits good selectivity with
regard to the breakdown of lignin and carbohydrates.
The whiteness is unexpectedly high with values over 55%
ISO and thus offers a good starting basis for possible
subsequent bleaching, in which whitenesses of 75% ISO
can be achieved.
With an initial freeness of 12 SR, these materials
already have a 6 km tearing length at a specific weight
of 1.87 cm3/g.
In order to refine the fibrous materials to a freeness
of 15 SR, a refining time of 20 to 30 minutes is
needed. Up to a refining time of 20 minutes (freeness
12 SR - 15 SR), the freeness develops within a narrow
range irrespective of the pH at the start of the
digestion (pH 6 to pH 9.4).
Likewise irrespective of the initial pH of the
digestion and the refining time needed to reach the
freeness, a high strength level is reached at a
freeness of 15 SR.
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Example 2
The fibrous material was produced from birch chips, the
pH at the start of the digestion being 9.4.
In addition to the 15% total chemicals (sulfite and
NaOH in the prescribed ratio), 0.1 anthraquinone, based
on the quantity of wood used, was added.
The digestion time was 60 minutes.
The following values resulted:
Yield (o): 81.1
Lignin content: 22.7
Whiteness (% ISO): 53.7
Tearing length (km): 9.6
Tear propagation resistance
(cN; 100 g/m2): 75.0
As a result of the addition of 0.1% anthraquinone, the
digestion time can be reduced from about 180 minutes to
60 minutes under otherwise unchanged digestion
conditions. This time gain is valuable, above all
because the fibrous material production plants can be
dimensioned smaller. Further potential savings reside
in the fact that the temperature needed for the
digestion has to be maintained over only a very much
shorter time period.
Furthermore, it was determined that, with a decreasing
use of overall chemicals to values between 5 and 15% in
the case of coniferous wood, fibrous material with
largely equally good properties is produced. The
results do not depend on the use of the anthraquinone.
The anthraquinone has the effect of accelerating the
digestion but the desired fibrous material can also be
digested without the addition of anthraquinone.
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Example 3: Deciduous wood digestion in the liquid phase
Eucalyptus chips, after steaming, have a sodium sulfite
digestion solution added at a liquor ratio of
wood:digestion solution of 1:3. The use of chemicals
was 10.5% here (as NaOH) on o.d. chips.
Over a time period of 90 minutes, the material to be
digested was impregnated and the digestion material was
heated to the maximum digestion temperature of 170 C.
The digestion time was 50 minutes.
Digestions with eucalyptus wood show that these
materials cam be produced with a specific energy input
for defibering of less than 250 kWh/t.
The yield in these trials was around 77%, based on the
wood mass used. Given an initial freeness of 14 SR,
these materials already have a 3.5 km tearing length
with a specific ... of 2.05 cm3/g. In the subsequent
bleaching these materials could be bleached to
whitenesses of 79.9% ISO.
Trials have shown that the digestions in the vapor
phase exhibit a lower overall time requirement. As
compared with digestion in the liquid phase, the
heating to the maximum digestion temperature is carried
out very much faster. The actual digestion then needs
the same amount of time as a digestion in the liquid
phase. During the vapor phase digestion there is no
free-flowing chemical solution; this is drawn off after
the impregnation and before the digestion. It
therefore has less organic material added than the
chemical solution which is drawn off after the
digestion in the liquid phase. However, this has no
significant influence on the quality of the fibrous
material produced.
Whereas in the case of vapor phase digestions similar
values in terms of yield can be achieved, the whiteness
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of the fibrous materials produced in vapor-phase
digestion are considerably lower, however. A
significant effect is achieved by reducing the maximum
digestion temperature from 170 C to 150 C; the
whiteness rises.
The fibrous materials produced in the vapor phase
exhibit excellent strengths. The tearing length was
measured as 10 km, for example, and as 11 km at 15 SR.
The tear propagation resistance was measured as 82.8 cN
and 91.0 cN, for example. These values correspond to
the best values for fibrous materials with a high
lignin content which have been achieved for digestions
in the liquid phase, or are even higher. Comparable
strength values are not known from the prior art for
fibrous materials with a high lignin content.
From the examples it can be gathered particularly
clearly that the fibrous materials according to the
invention need only little expenditure of energy during
refining in order to build up high tearing lengths,
without the tear propagation resistance being reduced.
A freeness of 12 SR was in each case reached in 0-10
minutes; a freeness of 13 SR in 5-30 minutes, normally
10-20 minutes. In order to reach a freeness of 14 SR,
the Jokro mill had to operate for 30-40 minutes and for
a freeness of 15 SR between 35 and 40 minutes were
needed. It is obvious that refining to freenesses
around 40 SR would require enormous expenditure on
refining energy. A particular advantage of the process
according to the invention is therefore to be seen in
the fact that fibrous materials with high strengths can
be refined with little expenditure of energy.
The apparatus for providing a stock suspension which is
used below in the process according to the invention
for producing a tissue web, comprises a pulper, in
which the dry raw and semifinished materials and waste
paper are slushed in water and transformed into a state
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that can be pumped. The stock formed in this way is
then fed to a mixing chest.
During the subsequent refining operation, the stock
suspension is refined to a freeness of 12 SR or more.
After the machine chest, the stock suspension is
diluted very highly with white water and fed to a
headbox 13.
Irrespective of how the stock suspension is obtained,
it is important for the production of tissue paper that
the stock suspension emerging from the headbox 13 has a
freeness of less than 20 SR and a tearing length of
more than 4.5 km.
A stock suspension 1 having the abovementioned
properties emerges from the headbox 13 in such a way
that this is injected into the ingoing gap between a
forming fabric 14 and a structured, in particular
3-dimensionally structured, belt 3, by which means a
tissue web 1 is formed.
The forming fabric 14 has a side oriented toward the
tissue web 1 which is smooth relative to that of the
structured belt 3.
Here, the side of the structured belt 3 pointing toward
the tissue web 1 has deepened regions and regions
elevated with respect to the deepened regions, so that
the tissue web 1 is formed in the deepened regions and
the elevated regions of the structured belt 3. The
difference in height between the deepened regions and
the elevated regions is preferably 0.07 mm and 0.6 mm.
The area formed by the elevated regions is preferably
10% or more, particularly preferably 20% or more and
particularly preferably 25% to 30%.
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In the exemplary embodiments illustrated, the
arrangement comprising upper belt 3, tissue web 1 and
forming fabric 14 is deflected around a forming roll 15
and the tissue web 1 is dewatered substantially by the
forming fabric 14, before the forming fabric 14 is
taken off the tissue web 1 and the tissue web 1 is
transported onward on the belt 4.
The voluminous sections of the tissue web 1 formed in
the deepened regions of the belt 3 have a higher volume
and a higher grammage than the sections of the tissue
web 1 formed in the elevated regions of the belt 3.
Consequently, on account of its formation on the
structured belt 3, the tissue web 1 already has a
3-dimensional structure.
However, the sheet formation can also take place
between two smooth forming fabrics 14, so that a
substantially smooth tissue web 1 without a
3-dimensional structure is formed.
During a dewatering step following the formation of the
tissue web 1, the tissue web 1 is led between the
structured belt 3, which is arranged on the top, and a
lower, permeable belt 2 formed as a felt, pressure
being exerted on the structured belt 3, the tissue web
1 and the belt 2 along a dewatering section during the
dewatering step, in such a way that the tissue web 1 is
dewatered in the direction of the belt 2, as indicated
by the arrows in the two figures.
During the dewatering, the tissue web 1, together with
the belts 2, 3, wraps around a roll 5.
Because the tissue web 1 is dewatered in the direction
of the belt 2 during this dewatering step and because
the tissue web 1 is dewatered on the structured belt 3
on which it has already been formed, the voluminous
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sections are compressed less intensely than the other
sections, so that as a result the voluminous structure
of these sections is maintained.
The pressure for dewatering the tissue web 1 during the
dewatering step according to figure 1 is produced
simultaneously, at least in some sections, by a gas
flow and by a mechanical pressing force.
In this case, the gas flow flows first through the
structured belt 3, then the tissue web 1 and then the
lower belt 2 formed as a felt. The gas flow through
the tissue web 1 is about 150 m3 per minute and meter
web length.
In the present case, the gas flow is produced by a
suction zone 10 in the roll 5, the suction zone 10
having a length in the range between 200 mm and 2500
mm, preferably between 800 mm and 1800 mm, particularly
preferably between 1200 mm and 1600 mm.
The vacuum in the suction zone 10 is between -0.2 bar
and -0.8 bar, preferably between -0.4 bar and -0.6 bar.
With regard to the performance of the dewatering step
carried out by means of mechanical pressing force and
optionally or additionally by means of a gas flow, and
also to the various configurations of apparatuses for
carrying out such a dewatering step, the entire extent
of PCT/EP2005/050198 is also to be incorporated into
the disclosure content of the present application.
According to figure 1, the mechanical pressing force is
produced in that, during the dewatering step, the
arrangement comprising structured belt 3, tissue web 1
and belt 2 is guided ... a dewatering section 11 between
a press belt 4 under tension and a smooth surface, the
press belt 4 acting on the structured belt 3 and the
belt 2 being supported on the smooth surface.
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Here, the smooth surface is formed by the
circumferential surface of the roll 5.
The dewatering section 11 is defined substantially by
the wrap region of the press belt 4 around the
circumferential surface of the roll 5, the wrap region
being defined by the spacing of the two deflection
rollers 12.
The press belt 4 is under a tension of at least
30 kN/m, preferably at least 60 kN/m or 80 kN/m, and
has an open area of at least 25% and a contact area of
at least 10% of its total area pointing toward the
upper belt 3.
In a specific case the press belt 4, embodied as a
spiral link fabric, has an open area of between 51% and
62% and a contact area of between 38% and 49% of its
total area pointing toward the upper belt 3.
With regard to the structure of the press belt, the
entire extent of PCT/EP2005/050198 is to be
incorporated into the disclosure content of the present
application.
The tissue web 1 leaves the dewatering section 11 with
a dryness of between 25% and 55%.
In a further dewatering step following the dewatering
step, the tissue web 1, together with the structured
belt 3, is then led through a press nip, the tissue web
1 in the press nip being arranged between the
structured belt 3 and a smooth roll surface of a Yankee
drying cylinder 7. Here, the press nip is an extended
press nip formed by the Yankee drying cylinder 7 and a
shoe press roll 8.
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The tissue web 1 rests on one side with a relatively
great area on the circumferential surface of the Yankee
drying cylinder 7, the tissue web 1 resting on the
structured belt 3 on the other side.
The deepened regions and the regions elevated relative
thereto of the structured belt 3 are here formed and
arranged in relation to one another in such a way that
the voluminous sections are substantially not pressed
in the press nip. On the other hand, the other
sections are pressed, by which means the strength of
the tissue web 1 is increased further.
Between the two dewatering steps described, a further
dewatering step can be provided, which can be carried
out by means of an apparatus 9.
Optionally, provision can be made for the tissue web 1,
before it runs through the press nip, to be led
together with the structured belt 3 around an evacuated
deflection roll, the structured belt 3 being arranged
between the tissue web 1 and the evacuated deflection
roll (not illustrated).
From figure 2 it can be seen that the gas flow can
additionally be produced by a positive pressure hood 6
arranged above the structured belt 3, the dewatering
step in this case being carried out without any
mechanical pressing force, i.e., as opposed to figure
1, no press belt 4 which wraps around some section of
the roll 5 being provided.
