Note: Descriptions are shown in the official language in which they were submitted.
The inven-tion relates to a method of producing cellu-
loses with low lignin contents by extracting lignin sub-
s-tances from cellulose-containing vege-table material a-t an
elevated temperature, in particular at 170 C to 210 C,
as well as at an elevated pressure by means of aqueous
organic solven-ts and, if desired, bleaching the cellul.ose.
It is known that cellulose-containing vegetable ma-
terial can be digested by organic solvents.
Thus, a method is described in U.S. patent No.
3,585,104 in which fibrous vegetable material is treatedin a continuous countercurrent extraction with a mixture
oE water and an organic solvent, for instance ethanol, at
a temperature of about 150 to 200 C at an elevated pres-
sure. This method, which is known as organosolv process,
was advanced according to German patent No. 26 44 155 to
the effect that the divided plant fiber raw material is
immersed in the pulping agent used at a pulping tempera-
ture, is further treated in the countercurrent, and is fi-
nally washed with a fresh pulping agent at a lower tempe-
rature, the latter having a pH of more than 8, due to theaddition of basic substances.
In German Offenlegungsschrift No. 28 03 465 it is dis-
closed that a cellulose may be gained during solvent-ex-
traction digestion procedures, which is rich in low-mole-
cular hemicelluloses. Since the presence of large amounts
of hemicelluloses in a chemical conversion pulp is unde-
sired, it is proposed according to the German Offenlegungs-
schrift to separate these escort substances by a mild hy-
drolysis prior to the solvent extraction proper. The hy-
drolysis is effected by wood acids forming during the
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~ i
treatment of the wood chips with steam at 130 C.
Canadian patent No. 238,294 relates to a solvent
pulping process with an aqueous solution oE a lower alka-
nol being used as extraction liquor. The extraction is
carried out in a plurality of vessels, extraction liquor
with an increasingly reduced lignin content being admitted
to the individual vessels. Subsequently, fresh extraction
liquor is guided through the individual vessels.
From the published European patent application No.
10 0 012 960 a digestion process is known in which the vege-
table fiber material is impregnated with an organic sol-
vent and charged into the head of a reactor. The extrac-
tion liquor is supplied in the center of the reactor at a
temperature of between 130 and 210 C. There is a forced
guidance of the fiber material from the head of the reac-
tor downwardly in countercurrent direction to the extrac-
tion liquor and to the water fed at the bottom of the re-
actor as washing liquor and drained in the center of the
same.
The products resulting from the known processes are
cellulosesthat are well suited as raw materials for the
paper industry. These organosolv substances, as they are
called, however, are not suited to further processing into
other products, such as regenerated cellulose products.
From organosolv substances, for instance, a viscose is
thus always obtained which contains fiber and gel portions
and is unfilterable and thus unsuited for possible spinning
into fibers. Not even does a very intensive bleaching of
such organosolv substances wi-th a high consumption of
chemicals yield a cellulose that is processable into an
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easily filterable viscose.
The poor suitability of the organosolv cellulose in
view oE its Eurther processing into viscose is due to the
high residual content of lignin.
The lignin con-tent oE the cellulose is indicated by
the socalled kappa number. The kappa number represents the
consumption of 10 KMnO4 solution/gram of cellulose (abs.
dry) after a treatment time of 10 minutes at 20 C. ~y
multiplying the kappa number by the factor 0.18, the lig
nin content is obtained in percent, based on the cellulose,
according to Hagglund.
With a cellulose that is to be upgraded into a chemi-
cal conversion pulp by subsequent bleaching, the kappa num-
ber must not exceed 8 - 10. Otherwise, it is impossible to
lower this value to about 1 in an economical manner by
industrially common bleaching with soda lye, hydrogen per-
oxide, calcium hypochlorite or chlorine dioxide, as is re-
quired for chemical conversion pulp.
Lignin is the more difficult to extract by the various
digestion liquors, the higher its molecular weight. More-
over, part of the lignin is incrusted in the cell wall of
the plant and thus difficult to accede. Finally, part of
the lignin enters into condensation reactions during heat-
ing, thereby becoming insoluble.
With a conventional solvent extraction process the
kappa numbers of the celluloses obtained always have
values of substantially above 8 - 10. Departing from green
wood, the kappa number after the extraction is about 25,
with aged wood it is even 35 - 40, unless a mean weight-
average degree of polymerization (DP) of the cellulose of
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1,500 is fallen short of.
~ ontrary to the conventional digestion processes forobtaining celluloses wi-th low lignin contents that work
w:ith sulphur compounds, no air and water pollution is in~
volved with the application o:E the extraction method, since
the solvents are guided in a closed circuit.
The invention has as its object to eliminate the dif-
ficulties described and to provide an extraction method by
means of which a cellulose is obtained by the exclusive
utilization of solvents that are recoverable by distilla-
tion, which cellulose may be processed further into an
easily filterable viscose by usual bleaching, i.e. when
using green wood as starting material such a chemical con-
version pulp is obtained whose kappa number can be lowered
to 1 - 2 by subsequent bleaching under industrially applied
conditions. In case aged wood is used as starting material,
a kappa number must be attained that enables the upgrading
into a chemical conversion pulp under slightly aggravated
bleaching conditions.
The set object is achieved with a method of the initi-
ally defined kind in that the extraction is carried out in
at least two steps, wherein the crude cellulose resulting
from the first extraction step is post-extracted in a fur-
ther extraction step at a temperature lower than in the
first extraction step with a medium having a swelling ef-
fect on the cellulose and dissolving lignin or lignin
particles, which medium is selected from the group con-
sisting of dimethylformamide (DMF), dimethylsulfoxide
(DMS0), phenol, dioxan, methanol/benzene, ethanol/benzene,
ethanol/ammonia, phenol/water and mixtures thereof.
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8~
In this manner, a cellulose is obtained that can be
processed into an easily filterable viscose after techni-
cally common bleaching or upgrading.
The system methanol/benzene or ethanol/benzene are
particularly used as azeotropic mixtures of these solvents.
All the media mentioned reduce the kappa number of the
crude cellulose. Due to the swelling effect of the media
on cellulose, the structure of the crude cellulose is
loosened and thus the accessibility to the incrusted lig-
10 nin residues facilitated, whereupon lignin and lignin par-
ticles are easier to extract. The swelling of the cellu-
lose is also connected with aremarkably positive heat
tone if DMF and DMS0 are used.
Furthermore, the swelling has a certain activation
effect on the cellulose, favoring the subsequent bleaching.
Dioxan is a good solvent for lignin, but has a remarkably
sliyhter swelling effect.
Suitably, the post-extraction takes place for 10 to
30 minutes at temperatures of between room temperature and
20 110 C.
According to a preferred embodiment~ the post-extrac-
tion takes place at room temperature with DMF. In this case
a particular advantage consists in the extremely short ex-
traction time required.
According to another preferred embodiment, the ex-
traction is effected with DMS0 at about 80 C.
Preferably, the post-extraction step(s) is (are) car-
ried out at a bath ratio of 1 : 7 to 1 : 20.
It is of a particular advantage if the cellulose is
30 subjected to a washing treatment after each extraction step.
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If, for ins-tance, a thoroughly washed organosolv cel-
lulose is treated with DMF for 15 minutes at room tempera-
ture at a bath ratio of 1 : 10, the kappa number, in all
examined cases, has decreased by ~0 to 70 ~, based on the
starting value. When using green wood a kappa number of
10 - 12 is attained. With a combined ex-traction of aged
wood, this value amounts to 20 - 25. DMF may be used sev-
eral times for the extraction. If the lignin concentration
and the water and ethanol contents in DMF have increased
such that the extractability of lignin from the organosolv
substance is reduced, the DMF is freed from lignin by a
simple distillation, concentrated and used anew.
As is illustrated by Example 1, a cellulose having
been post-extracted in this way can be upgraded by usual
bleaching conditions so as to have a kappa number of 1.2,
and processed into an easily filterable viscose.
With respect to its functioning mechanism, the post-
extraction basically is to be differentiated from the
high-temperature extraction initially carried out, pref-
erably with an alcohol-water mixture. During the high-tem-
perature extraction the hemicellulose is decomposed hydro-
lytically and the lignin is decomposed radically, the
fragments being dissolved. On the other hand, the post-ex-
traction is a pure dissolution and swelling procedure in
which the lignin residues that are particularly difficult
to accede and highly molecular are extracted and the cel-
lulose is activated for subsequent bleaching.
As already mentioned, one aspect of the invention is
to be seen in that the digestion process is feasible only
with chemicals that are recoverable by distillation. With
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the usual chemical conversion pulp production by means of
sulphur-containing digestion liquors, -the crude cellulose
obtained after digestion is treated with hot diluted soda
lye. The applica-ti.on of this treating method to an organo-
solv substance, after a treatment time oE 30 minutes, cau-
sed a decrease in the kappa number to merely 75 ~ of its
initial valueO Moreover, hot soda lye has a number of un-
desired side effects, i.e. part of the ~-cellulose is de-
composed into hemicellulose. In addition, the soda lye can
be recovered only with a great energy consumption by eva-
poration and burning of the organic extract. The extraction
is carried out at 85 C for 30 minutes and in itself con-
sumes accordingly high energy amounts.
The method according to the invention, when compared
to other methods for the production of cellulose with a
low lignin content, thus constitutes an effective, energy-
saving and environmentally neutral method, by means of
which the kappa number of an organosolv cellulose is lower-
ed to such low values that its further processing into
chemical conversion pulp is feasible. Any extraction agent
possibly remaining in the cellulose is innoxious, because,
if this is DMF, for instance, it is hydrolyzed into formic
acid and dimethylamine in the subsequent alkaline bleach-
ing steps and is drained with the bleaching lyes.
When testing the bleached chemical conversion pulp
for a suitable quality, the processability into viscose
according to a method standardized by E. Treiber (E. Trei-
ber, C. Rehnstrom, Améen, F., Kolos, Das Papier, Vol.16
(1962), p. 85) was applied. In this case, the measure for
the quality of the viscose is its filterability. The fil-
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tering values were determined according to S. Zauner, ~I.
Hupfl, Das Papier, Vol.20, Number 3 (1966), p. 125. The
filtering value indicates the amount of viscose that
flows through a certain filter medium un-t:il its obstruc-
tion. The higher the filtering value, the better the qual-
ity of the chemical conversion pulp processed into viscose.
The method of the invention will now be explained in
more detail by way of the following examples. The compa-
rative examples serve to demonstrate the progress achieved
when obtaining an organosolv cellulose according to the in-
vention as opposed to a known method.
Comparative Example 1:
Aged beechwood chips were digested under pressure in
an autoclave for 4.5 hours at 195 C in a mixture of 55
ethanol and 45 % water, and extracted. Subsequently, the
digestion liquor was let off and replaced by a fresh al
cohol-water mixture several times, the cellulose thus be-
ing freed from adhering lignin residues.
The purified substance was beaten up in water and
freed from undigested wood splinters. The crude cellulose
obtained in this manner had a kappa number of 34.
The upgrading into a chemical conversion pulp suited
for further processing into viscose was attempted in a
multi-step bleaching cycle under aggravated conditions as
follows:
1) Delignification with a mixture of 3 % soda lye and 3 %
hydrogen peroxide at 85 C.
2) Delignification with a mixture of 3 % soda lye and 1.0
% hydrogen peroxide at 85 C.
3) Bleaching with calcium hypochlorite with an active chlo-
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rine content of 0.5 %.
4) Bleaching with 0.3 % soda lye and 0.3 % hydrogen per-
oxide.
All percentages are based on the crude cellulose
uti]ized.
After this treatment the kappa number oE the cellulose
obtained was 2.4. From the substance a viscose was pro-
duced according to the standardized method of E. Treiber,
which viscose was, however, difficult to filter and thus
only poorly suited for spinning into viscose fibers (fil-
tering values of two batches: 18 and 31).
Example 1:
Chips of aged beechwood were extracted with an etha-
nol-water mixture under the conditions described in Compa-
rative Example 1. The extracted substance had a kappa num-
ber of 34.5. Subse~uently, this substance was extracted
with DMF of room temperature at a bath ratio of 1 : 15 for
15 minutes. Swelling and a slight heat development were
observed. After this treatment the kappa number was still
20 20.5.
Upgrading into a chemical conversion pulp was effect-
ed in the following steps:
1) Delignification with a mixture of 2 % soda lye and 2.5
% hydrogen peroxide at 85 C.
2) Delignification with 1 % soda lye and 1 % hydrogen per-
oxide at 85 C.
3) Bleaching with 0.5 % active chlorine as Ca(OCl)2.
4) Bleaching with 0.3 % soda lye and 0.3 % hydrogen per-
oxide.
The bleached substance had a kappà number of 1.2. It
g
was turned into viscose according to the standardized
method of E. Treiber, yielding a viscose having a filter-
ing value oE 290, which is to be considered as good.
Comparative Example 2:
Chips of old-aged beechwood were digested according
to the method described in Comparative Example 1. The
crude cellulose had a kappa number of 41.6 and was divided
into two parts:
One part was treated with 3 % soda lye at 85 C and
10 % consistency for 30 minutes. Thereafter, the kappa num-
ber of the cellulose was determined to be 30.4.
Example 2:
The other part of the crude cellulose obtained ac-
cording to Comparative Example 2 was extracted with DMF at
20 C for 15 minutes. After this treatment a kappa number
of 26.5 was measured.
Comparative Example 3:
Chips of green beechwood were extracted according to
the method described in Comparative Example 1. The crude
material has a kappa number of 20.8 and was divided into
two parts.
One part was extracted with soda lye according to
Comparative Example 2, a kappa number of 15.9 resulting
for the cellulose obtained.
Example 3:
The other part of the crude cellulose obtained ac-
cording to Comparative Example 3 was treated with DMF ac-
cording to Example 2 and then had a kappa number of 12.1.
Example 4:
A crude cellulose having a kappa number of 41.5 was
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post-extracted with the following media:
1)Azeotropic mixture of methanol and benzene:
Extraction in the Soxhlet apparatus yielded a kappa
number of 30.
2) DMS0:
Ex-traction in A beaker glass at 80 C and a bath ratio
of 1 : 20 reduced the kappa number to 29.
3) DMF:
Extraction in a beaker glass at 20 C and a bat~ ratio
10 of 1 : 15 yielded a kappa number of 27.
The extraction of the organosolv substance in the
Soxhlet apparatus with acetone merely reduced the kappa
number to 3~.
Example 5
An organosolv crude substance with a kappa number of
37.4 was post-extracted with the following media:
Extraction medium Temperature Extraction time Kappa
_ / C7 /min7 number
20 Phenol/water 80 10 21.6
Phenol 110 15 22.1
Ethanol with 75 g
ammonia/l 20 10 23.8
Dioxan 20 10 19.2
DMF 20 10 19.2
Ethanol 20 10 27.0
Example 6:
An organosolv substance having a kappa number of 24.5
was extracted with DMF for a total of 2 hours at 80 C.
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After 15 minutes the kappa number was 17.5, after 30 minu-
tes it was 17.6, after an hour 17.7, and after 2 hours it
was 17.2.
It is to be seen that the value of the kappa number
attained already after 15 minutes cannot be further re-
duced by a longer lasting extraction. A reduction oE -the
kappa number to the same extent is also achieved by ex-
traction with DMF at room temperature.
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