Note: Descriptions are shown in the official language in which they were submitted.
`_- 215788~
Production of chemomechanical and/or
chemothermomechanical wood pulps
The invention relates to a process according to the
preamble of Claim 1 for producing chemomechanical and/or
chemothermomechanical wood pulps from lignocellulosic raw
material, such as wood chips, wood shavings, previously
defibered wood or sawdust.
A considerable disadvantage of wood pulps is the low
binding capacity of lignin-containing fibers. Lignin as a
hydrophobic substance scarcely has any points of attachment
for hydrogen bonds. Sulfonation of lignin increases its
hydrophilicity and thus the binding potential of lignin-
containing fibers. However, sulfonation is also the first
step for the solubilization of the lignin, for which,
however, a minimum degree of sulfonation must be achieved.
If it is desired to produce wood pulps in high yield
with a high surface binding potential and thus good paper
strength, for the sulfonation, a solution of sodium sulfite
is used which can only sulfonate the Ax and Ay groups of the
lignin, of which there are only between 50 and 30 mol~
present in the lignin (S.A. Rydholm, Pulping Processes,
Intersciences Publishers, New York, London, Sydney, 1965).
This restricts the sulfonation and also the solubi-
lization of the lignin. Furthermore, the remaining process
parameters such as temperature, time and amount of chemicals
" 21S7885
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used are selected so that only small amounts of wood sub-
stance are solubilized.
The lignin is sulfonated by bisulfite ions. In an SO2-
water system, the following equilibrium is formed: S2 + H2O
<===, H2SO3 ~===> H+ + HSO3. If the temperature of such a
solution is increased, the equilibrium is shifted wholly
towards the left, as a result of which sulfonation is no
longer possible. On the other hand, sulfonation of the lignin
requires a defined temperature level. However, by using a
base, bisulfite ions form even at elevated temperatures,
various bases being used:
MHSO3 ~ M+ + HSO3
(M = base = l/2Ca2+, l/2Mg2+, Na+, NH4+).
The sulfonation rate increases with decreasing pH.
Lignin condensations occur as a competing reaction which not
only prevents the sulfonation but also effects a darkening of
the lignin. Therefore, in acidic sulfite processes, the
reaction temperatures must be restricted (to a maximum of
140C) or else the pH and the bisulfite ion concentration
must be increased by increased use of base.
These two routes are also followed for the production of
CTMP and high-yield pulps. Thus, PCT-WO 91/19040 teaches that
wood can be sulfonated in the gas phase, the sulfonation
occurring with SO2 below 100C. The wood chips are then
treated with solutions of bases and the temperature is
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increased to a maximum of 130C so that no degradation of
carbohydrate occurs in the wood. Jayme et al., in contrast,
use higher temperatures, up to 180C, in the vapor-phase
digestion. However, they must considerably increase the usage
of base in this process and employ bisulfite solutions having
a pH around 4 (G. Jayme, L. Broschinsky and W. Matzke, Das
Papier 18, 7, 308-314, 1964. G. Jayme and W. Matzke, Wbl.
Papierfabrik. 11/12, 311-314, 1964).
It is now known that metallic bases in sulfite solution
can be replaced by methanol (Schorning, Faserforschung und
Textiltechnik 12, 487, 494, 1957). DE 39 32 347 A1 describes
how such solutions are used for producing CTMP pulps with low
energy consumption. Of course, it is also possible to replace
only some of the metallic base by methanol and to employ a
mixture.
However, the use of methanol in an industrial process is
associated with safety and health hazards.
The object of the present invention is to develop a CTMP
production process which uses acidic sulfite solutions and
succeeds without methanol and with minimum amounts of
metallic bases.
The object is achieved by the characterizing part of
Claim 1. Further advantageous developments are specified in
the subclaims.
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In the solution according to the invention, it
surprisingly appears that, in contrast to the prior art to
date, sulfonation can proceed at high temperatures under the
conditions chosen without lignin condensation reactions
having dark or black reaction products as a result occurring,
in particular for the whiteness of the lignocellulosic raw
material to be treated.
According to the process, lignocellulosic raw materials,
in particular wood chips, are first impregnated with a
solution up to a content of 0.2 - 4.0~ by weight of base and
1 - 21~ by weight of SO2, based on oven dry wood and the
excess solution is taken off. The wood chips saturated with
the impregnation solution are thereupon heated very rapidly
to a reaction temperature of 130 to 180C and kept at this
temperature for a time of 2 to 15 minutes and at a low pH in
the gas phase. The wood chips are withdrawn from the reaction
compartment, the excess sulfur dioxide gas being taken off
from the reaction material. Using dilution water to set the
pulp density, the wood chips are fed to a defibering device
known per se, when using a preselected specific beating
energy of 1200 to 1900 kWh/t of fibrous material, the
reaction material is defibered to a preset degree of
fineness.
A substantial advantage of a pretreatment with acidic
sulfite solution is the easy recoverability of the SO2. Since
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only small amounts of base are required, the loss of
chemicals is small: after pretreatment of the wood, the S2~
to the extent that it is not chemically bound to the wood or
irreversibly bound to the base, can be recovered in a
degassing stage, if necessary also with reinforcement by a
vacuum.
Because of the low pH and the high temperature, the wood
cell walls are specifically weakened and wood pulps are
produced at high yield with low energy input.
However, a precondition for this is the use only for a
short time of temperatures above 100C in the case of wood
impregnated by acidic sulfite solutions. This absolutely
necessary precondition can only be complied with by a gas-
phase process.
The novel process can reduce the specific beating energy
by 40~ in comparison with conventional pretreatment, without
elevated yield losses occurring.
The wood chips are impregnated until saturation of the
same at a temperature ~ 100C. This has the advantage that
when solutions having a high sulfur dioxide content are used,
the partial pressure of the S02 gas is still relatively low,
so that no great requirements with respect to pressure
resistance are placed on the impregnation vessel.
The impregnation solution itself contains 1 - 34 g/l of
base and 20 - 145 g/l of SO2, depending on the impregnation
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conditions.
To achieve the higher SO2 concentration, if appropriate
gaseous SO2 is additionally forced into the closed
impregnation or reaction vessel. A further measure to obtain
the desired degree of impregnation is the choice of the dry
matter content of the wood chips before impregnation which
can be set, for example, by treatment with a screw press to
expel water.
The use of SO2 at high concentration is fundamental in
this process. It is important in this case also to use a high
concentration of free SO2 in comparison to the amount of base
used.
Particularly advantageous physical properties are
achieved if more than approximately 70~ of the total SO2 used
is used as free SO2. However, a proportion larger than
approximately 85~ leads to decreased wood pulp quality. A
lower SO2 proportion leads to higher beating energy require-
ment. The pH of the digestion solution is as below pH2.
The base used is advantageously MgO. The use of other
bases conventional in sulfite technology based on sodium,
calcium or ammonium is also possible. However, MgO has the
advantage of simple handling in the process and low price.
Since the base and the SO2 bound as monosulfite cannot
be recovered in such a process, the production costs increase
when large amounts of base are used. A great advantage of the
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process employed here is therefore that only a small amount
of base is necessary.
The necessary reaction period is not only a function of
the amounts of chemicals used, but also of the temperature.
The wood chips, after the impregnation, are transferred
directly into the gas phase of the reactor where the reaction
takes place. The sought-after sulfonation rates are only
achieved in this process if the temperature is sufficiently
high in the reaction region.
Example 1:
Spruce wood chips ~industrial wood residue) are
impregnated at room temperature with an acidic magnesium
bisulfite solution. 1000 g of oven dry spruce wood chips are
used. The solution contains 73.3 g of SO2/l and 10 g of
MgO/l. The liquor ratio is 1:6. After the wood chips are
withdrawn, these contain 1.7~ of MgO/oven dry wood and 14~ of
SO2/oven dry wood. The dryness content is 37~. The chips are
then placed in a reactor. The reactor is then heated to 169C
in the course of 60 s with steam. This temperature is
maintained for a further 330 s. The gas phase is formed over
a chemical liquid phase which contains 7~ of SO2 and 1~ of
MgO in 2300 ml of solution, or by introducing 124 g of SO2
and steam into the reactor. The gas phase volume is 35
liters. The chips are withdrawn from the reactor, suspended
21S788~
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in water at 80C, charged into a defibrator and defibered
under a steam atmosphere at 130C and 20~ pulp density. The
defibrator coarse material is then beaten under atmospheric
pressure in a laboratory refiner. To produce a pulp having
75SR, 1730 kWh/t of beating energy are required, while in
the case of spruce wood chips which were pretreated with 5~
of Na2SO3/oven dry wood at 130C, 2660 kWh/t are required to
achieve this degree of beating. Whereas the conventional CTMP
reaches a breaking length of 6060 m, the acid pretreatment
achieves a breaking length of 6380 m. In order to achieve a
breaking length of 5000 m, 1880 kWh/t must be employed in the
conventional pretreatment, while in the acid treatment, only
1880 kWh/t of beating energy must be used. Thus 42~ of the
beating energy to achieve the target strength is saved.
Example 2:
Spruce wood chips (industrial-wood residue) are
impregnated as described in Example 1. The solution contains
68 g of SO2/l and 10 g of MgO/l. The chips in this case
adsorb 1.7~ of MgO/oven dry wood and 13~ of SO2/oven dry
wood. A treatment as described in Example 1 is then
performed. The reaction temperature in this case is 157C for
a period of 6 minutes. In the subsequent beating stage,
2040 kWh/t are employed in order to achieve 75SR. The
breaking length is then 6540 m. In order to achieve the
- 2157886
breaking length of 5000 m, 1150 kWh/t of beating energy are
employed. The energy saving in comparison to conventional
CTMP is thus 39 ~ .
Example 3:
Spruce wood chips (industrial wood residue) are
impregnated as described in Example 1. The solution contains
30 g of SO2/l and 1. 7 g of MgO/l. The wood chips adsorb in
this case 0. 4~ of MgO/oven dry wood and 4.1~ of SO2/oven dry
wood. Further treatment is performed as in Example 1. The
specific beating energy in order to achieve 50SR is
1189 kWh/t. A breaking length of 4800 m is achieved. After a
pretreatment with 5~ of Na2SO3/oven dry wood, 1750 kWh/t of
beating energy are necessary in order to achieve this degree
of beating. The breaking length is then 4720 m. In order to
achieve a breaking length of 4000 m with the acid pretreat-
ment of the chips, 850 kWh/t are required, while after the
pretreatment with Na2SO3, 1380 kWh/t of beating energy need
to be employed in order to achieve this breaking length. The
saving in beating energy in comparison to conventional CTMP
is therefore 38~.
Example 4:
Spruce wood chips (industrial wood residue) are
impregnated as described in Example 1. The solution contains
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50 g of SO2/1 and 16.7 g of MgO/l. The chips in this case
adsorb 3.1~ of MgO/oven dry wood and 11~ of SO2/oven dry
wood. The further treatment is performed as in Example 1.
1650 kWh/t are employed for beating to 75SR. In order
to achieve a breaking length of 8000 m, 3640 kWh/t of beating
energy are required. Using the same beating energy, after
treatment with 5~ of Na2SO3/oven dry wood, a breaking length
of 7170 m is achieved.
Example 5:
Spruce wood chips (industrial wood residue) are
impregnated as described in Example 1. The solution contains
68 g of SO2/l and 14 g of CaO/l. A treatment as described in
Example 1 is then performed. The reaction temperature in this
case is 157C for a period of 6 minutes. In the subsequent
beating stage, 1380 kWh/t are employed in order to achieve
75SR. The breaking length is then 5190 m. In order to
achieve a breaking length of 5000 m, 1330 kWh/t of beating
energy are employed. The energy saving in comparison to
conventional CTMP is thus 29~.
Example 6:
Spruce wood chips (industrial wood residue) are
impregnated as described in Example l. The solution contains
68 g of SO2/l and 20 g of NaOH/l. A treatment as described in
2~788~
Example 1 is then performed. The reaction temperature in this
case is 157C for a period of 6 minutes. In the subsequent
beating stage, 1770 kWh/t are employed in order to achieve
75SR. The breaking length is then 5810 m. In order to
achieve a breaking length of 5000 m, 1350 kWh/t of beating
energy are employed. The energy saving in comparison to
conventional CTMP is thus 28~.
The physical properties of the fibrous materials
obtained are summarized in Table 1. The whitenesses of the
acidic magnesium bisulfite materials are higher by 5 to 10~
than with the use of Na2SO3, so that when these materials are
used in printing papers an additional bleaching can either be
wholly dispensed with or else can be carried out with
considerably reduced consumption of chemicals. The strength
potential achieved of the fibrous materials produced by the
process according to the invention is close to the conven-
tional pulps, so that these can be at least partially
replaced.
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Table 1
Trial 1 2 3 4 5 6
Degree of beating SR 75 75 50 92 75 75
(ZMV/7/61)
Breaking length (m) 6380 6540 480080005190 5810
~DIN 53 112)
Apparent density 0.57 0.55 0.460.680.52 0.52
g/cm3 (DIN 53 12)
Tearing strength J/m 1.09 1.12 1.22 0.68 1.10 1.08
(DIN 53 112)
Light scattering 37.1 43.3 47.546.143.7 43.5
coefficient m2/kg
(DIN 54 500)
Whiteness ~ ISO 59.1 60.1 55.464.461.3 64.1
Yield ~ 97 98 98 98 98 98
