Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1295~95
A process for the production of a hemicellulose hydro-
lysate and special pulp
The invention relates to a process for the pro-
duction of a hemicellulose hydrolysate and special
pulp from a material containing lignocellulose through
two steps, the first step comprising the hydrolysis of
hemicelluloses into simple sugars and the second step
the dissolving of lignin for liberating cellulose
fibres.
Traditionally, there are two processes for the
production of special pulps having a high content of
alpha cellulose, such as dissolving pulp: the far-
advanced acidic bisulphite cooking and the prehydroly-
sis-sulphate cooking. The former was developed at the
beginning o~ khe 20th century and the latter in the
1930's, see e.g. Rydholm, S.E., Pulping Processes,
p. 649 to 672, Interscience Publishers, New York,
1968. The basic idea in both processes is to remove as
much hemicellulose as possible from cellulose fibres
in connection with the delignification so as to obtain
a high content of alpha cellulose. This is essential
because the various uses of dissolving pulp, for in-
stance, do not tolerate short-chained hemicellulose
molecules with indefinite structure~ In the sulphite
process, the removal of hemicellulose takes place
during the cooking simultaneously with the dissolving
of lignin. The cooking conditions are highly acidic
and the temperature varies from 140 to 150C, whereby
the hydrolysis is strong. The result, however r is
always a compromise with delignification, and no high
content of alpha cellulose is obtained. Another draw-
back is the decrease in the degree of polymerization
of cellulose and the yield losses, which also limit
the hydrolysis ~possibilities. Various improvements
1295~95
have been suggested in traditional sulphite cooking,
the use of additional chemicals, for instance. Such
additional chemicals, used in addition to the basic
chemicals of sulphite cooking, include sulphide, white
liquor, and anthraquinone, see e.g. Finnish Patent
Specification 67 104 and U.S. Patent Specification
4 213 821. These sulphite cooking variations do not,
however, imply hydrolytic conditions.
A separate prehydrolysis step is interesting in
the view of the fact that it enables the adjustment of
the hydrolysis of hemicelluloses as desired by varying
the hydrolysis conditions. In the prehydrolysis-sul-
phate process the delignification is not carried out
until in a separate second cooking step. The prehydro-
lygis i9 aarried out either as a water prehydrolysis
or in the presence of a catalyst. Organic acids libe-
rated from wood in the water prehydrolysis perform a
major part of the process, whereas small amounts of
mineral acid or sulphur dioxide, in some cases even
sulphite waste liquor, are added to the digester in
"assisted" prehydrolysis. It has previously been ne-
aessary to effect the lignin dissolving step after the
prehydrolysis as sulphate cooking which has several
drawbacks. The prehydrolysis-sulphate process has e.g.
the following drawbacks:
- The yield is low because of the strong alka-
line reaction conditions which cause splitting of cel-
lulose. Thus the wood consumption per one ton of ael-
lulose is high.
- The content of residual lignin is rather high
because the step for the removal of residual lignin in
the sulphate cooking process is extremely non-select-
ive. Thus there is a great need of bleaching for com-
~plete removal of lignin, and the consumption of chemi-
cals is high; further, at least five bleaching steps
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are required.
- Industrial realization of sulphate cooking is
complicated, and the cost of investment very high.
Previously the use of sulphite cooking has not
been possible, because it is not possible to dissolve
from wood material lignin deactivated in the pre-
hydrolysis by means of traditional sulphite cooking
processes. It has been regarded as impossible to use a
sulphite cooking step (cf. Rydholm above) even though
it would have advantages over sulphate cooking.
It has now been found out unexpectedly that ex-
cellent results can be ob~ained by effecting the
lignin dissolving after the prehydrolysis by an alka-
line neukral ~ulphite cooking with anthraquinone or a
derivative thereof as a catalyst. Such a cooking is
known per se ~rom the prior art (see e.g. U.S. Patent
Specification 4 213 821); on the contrary, a com-
bination of prehydrolysis and such a cooking has not
been set forth previously.
The invention relates to a process for the pro-
duction of hemicellulose hydrolysate and special pulp
from a material containing lignocellulose through two
steps, the first step comprising the prehydrolysis of
r the material and the second step dissolving of the
lignin contained in the prehydrolyzed material. The
process is characterized in that the dissolving of
lignin is carried out by means of neutral sulphite
cooking with anthraquinone or a derivative thereof as
a catalyst, the pH of the cooking liquor being in-
; itially at least 10.
Suitable prehydrolyzing agents include e.g.
water, mineral acid, sulphur dioxide, sulphite cooking
acid, and sulphite waste liquor. Preferred prehydro-
lyzing agents include sulphur oxide, sulphuric acid,
and~water.
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12~5CP95
A suitable prehydrolyzing temperakure is 100 to
180C, preferably 155 to 170C, ancl a suitable hydro-
lyzing time is 10 to 200 minutes, p:referably 90 to 170
minutes.
The material containing lignocellulose prefer-
ably consists of softwood or hardwood.
The cooking step is suitably carried out with a
cooking liquor comprising 100 to 400 g of sodium sul-
phite/kg of dry wood; lO to 100 g of sodium carbon-
ate/one kg of dry wood; sodium hydroxide for rising
the pH of the cooking liquor to a value between 10 and
13; and 0.01 to 0.2%, calculated on dry wood, of
anthraquinone or a derivative thereof.
The cooking temperature preferably ranges from
160 to 180C, and the cooking time is suitably 100 to
200 minutes after the temperature has risen 0.1 to
2C/min from a temperature varying between room tem-
perature and 100C.
It is typical of the prehydrolysis-neutral sul-
phite-anthraquinone process (PH-NS-AQ process) that
delignification to a low content of residual lignin is
easy to carry out while the yield of cellulose fibre,
however, remains on an exceptionally high level. Thus
it is possible to use strong prehydrolysis conditions
(e.g. strong acids, such as H2SO4), whereby the hydro-
lysis of hemicelluloses into simple sugars is effi-
cient; on the other hand, the alpha cellulose content
representing the content of residual hemicellulose in
cellulose fibre is high and the content of residual
pentosan is;low. Due to these properties the process
is particularly suitable for the production of high-
qua~lity dissolving pulp, for instance! whereby mono-
saccharides are obtained simultaneously.
As to the ~new~process, it was found out that
the use of the so called neutral sulphite anthraqui-
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lZ95~35
none cooking process effects a partial ionization of
the lignin inactivated in the prehydrolysis, the in-
itial pH being at least 10, e.g. 11 to 12, and that
anthraquinone as an additive in the cooking catalyzes
the breaking of nucleophilic beta aryl ether bonds,
which at the end results in the liberation of fibres,
i.e. a successful cooking. It was further found out
that sulphite ions in neutral sulphite cooking react
simultaneously and participate in the decomposing of
the structure of lignin and above all sulphonate the
lignin material and fragments which thus become more
hydrophilic and dissolve more easily in the cooking
liquor, thus contributing to the formation of a
successful cooking and to the continuation thereof to
a very low content of residual lignin. In short, the
prehydrolysis-neutral sulphite anthraquinone process
according to the invention not only gives a result as
successful as that of the sulphate process but also
provides all the advantages typical of sulphite
cooking.
~ The increased yield of the procass according to
; the invention is due to the fact that there does not
occur splitting of cellulose to any greater degree
r during the neutral sulphite cooking step. In sulphate
cooking, on the contrary, the high alkalinity causes
alkaline hydrolysis, and the peeling-off reaction in
particular results irrevocably in a yield loss. The
process according to the invention enables the recov-
~ery of~ nearly all of the high molecular weight cellu-
lose material originally contained in the wood ma-
terial.
In the process-chemical sense, another advan-
tage is that pulp which has undergone neutral sulphite
anthraquinone cooking is easy to bleach, i.e. the re-
sLdual lignin remaining in the fibre after t~e cooking
;
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lzssass
is easy to remove. This is due to the fact that the
delignification resembles sulphite cooking; the con-
densation of the structure of lignin is insignificant;
and the sulphonation makes lignin more hydrophilic.
Contrary to this, the residual lignin in sulphate
cooking is strongly condensated and the content there-
of is on a higher level. The xemoval of this kind of
residual lignin in bleaching requires five to six
bleaching steps and plenty of expensive chlorine di-
oxide. The bleaching of pulp obtained by means of the
process according to the invention can be carried out
by three steps only and the demand of chemicals, too,
is lower.
~he process according to the invention has the
following advantages:
- The yield of the special pulp to be produced
.in connection with the production of sugars is in-
creased, which improves the production economy.
- The process after the prehydrolysis is sim-
plified, which decreases the cost of investment.
- The easier delignification in the cooking
step decreases the need of bleaching, thus improving
the production economy and reducing the emission of
r chlorinated compounds from the bleaching.
- The oxygen or peroxide step after the cook-
ing is extremely efficient as compared with that of
the prehydrolysis-sulphate process, whereby the re-
covery and economy are improved.
- Small-scale production is economically more
interesting because it is possible to operate in con-
nection with an existing sodium-based sulphite pulp
mill without any appreciable additional investments.
The following examples are illustrative of the
invention.
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~295~95
The following abbreviations are used in the
examples:
Steps of the bleachinq Processes
O = Oxygen step
D = Chlorine dioxide step
E = Alkali extraction
P = Peroxide step -
H = Hypochlorite step
C = Chlorination
Standards
SCAN = Scandinavian standard
TAPPI = U.S. standard
Example 1
Production of a birch hydrolvsate and special
Pulp by means of the PH-NS-AO Process from birch chips
Chips and a prehydrolyzing liquor were metered
into a chip basket positioned in a 20-litre forced
circulation digester. The cover of the digester was
closed and the prehydrolysis was carried out according
to the temperature program by heating the digsster
circulation indirectly by means of steam. After the
hydrolysis time had passed, the hydrolysate was re-
moved from the digester and recovered. The prehydro-
s lyzed chip material contained in the digester was
washed in the digester for 5 minutes with warm water,
the cover was opened, and the chips were passed into a
centrifuge in which excess water was removed. The cen-
trifugalized material was weighed and a dry substance
sample was taken~for determining the hydrolysis loss.
The prehydrolyzed chip material was returned to
the digester, cooking liquor and anthraquinone were
added, the cover was closed, and the ~ooking was
carried out according to the temperature program. At
the end o~ the cooking the cooking liquor was removed
rapidly and the~ digester was filled with cold water,
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~LZ95('J~5
whereafter water was allowed to flow for 10 hours for
washing the cooked chip material. After the wash the
pulp was disintegrated by means of a wet disintegrator
for one minute and assorted with a flat screen plate
of 0.35 mm. Shives were recovered and weighed dry for
determining the shive content. The accepted fraction
was passed into the centrifuge for dewatering, homo-
genized, and weighed. Laboratory analyses were carried
out on this pulp and the pulp was further used in
bleaching tests.
Prehydrolyzinq step
Wood amount, g of abs. dry chips 2000
Prehydrolyzing agent S2
Amount o~ prehydrolyzing agent,
% on dry wood 0.25
Liquor ratio 6:1
Temperature rising time, min 40
Prehydrolysis temperature, C 155
Prehydrolysis time, min 170
Prehydrolysis loss, ~ on wood 26.6
Cookinq step
Na2SO3, % on wood as NaOH 22
r Na2CO3, % on wood as NaOH 5
Anthraquinone, % on wood 0.1
Liquor ratio 4.5:1
pH of the cooking liquor 11.3
Rising of the temperature Cimin
Cooking temperature, C 175
Cooking time, min 170
Yield, % on wood 39.3
Kappa number 17.2
Shive content, % on wood 0.1
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1295Cg5
Properties of O D-E-D bleached pulp
Final yield, % of wood 36.7
ISO brightness 87.1
Alpha cellulose % 94.2
Viscosity, SCAN dm3/kg 764
Example 2 -.
Production of a birch hydrolysate and special
pulp by the PH-NS-AQ process from birch chips
The test was carried out as disloced in
Example 1.
PrehYdrolYzina step_
Wood amount, g of abs. dry chips 2500
Prehydrolyzing agent S2
~mount of prehydrolyzing agent,
% on dry wood (SO2) 0 25
Liquor ratio 3.5:1
Temperature rising time, min 40
: Prehydrolysis temperature, ~ 155
: PrehydroIysis time, min 170
Cookinq step
Na2SO3, % on wood as NaOE 20
Na2CO3, % on wood as NaOH 6
~ Anthraquinone, % on wood 0.1
: Liquor ratio 4.5:1
~: : pH of the cooking liquor 11.3
Risîng of the temperature C/min
::Cooking temperature, C 175
Cooking:~time~, min ~ : 170
Yield: ~%;~on~wood~ ~5.7
Kap~a:number 48.1
Shive content, %::on wood ~ 1.35
lz~sass
Properties of O-P-H bleached pulp
Final yield, % on wood 39.7
ISO brightness 87.1
Alpha cellulose % 91.7
Viscosity, SCAN dm3/kg 530
Example 3
Production of a birch hYdrolysate and s~ecial
pulp by the PH-NS-AQ Process from birch chips_
The test was carried out as disclosed in
Example 1.
Prehydrolyzinq step
Wood amount, g o~ abs. dry chi.ps 2500
Prehydrolyzing agent H2SO~
~mount o~ prehydrolyzing agent,
% on dry wood 1.0
Liquor ratio 3.5:1
Temperature rising time, min 40
Prehydrolysis temperature, C 155
Prehydrolysis time, min 90
Prehydrolysis loss, % on wood 25.4
Cookinq step
Na2SO3, % on wood as NaOH 22
Na2CO3, % on wood as NaOH 5
Anthraquinone, % on wood 0.1
~iquor ratio 4.5:I
pH of the cooking liquor 11.3
Rising of the temperature C/min
Cooking temperature, C 175
~ : :Cooking time, min 170
: ~ : : Yield, % on wood 37.0
Kappa number : 24.9
:Shive content, % on wood 0.6
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1;~95~95
Properties of C-E-D bleached pulp
Final yield, % on wood 34.2
ISO brightness 90.0
Alpha cellulose % 94.6
Viscosity, SCAN dm3/kg 730
Properties of O-P-D bleached pulp
Final yield, % on wood 34.7
ISO brightness 84.4
Alpha cellulose % 94.5
Viscosity, SCAN dm3/kg 720
Example 4
Production of a pine hydrolysate and special
pulp bv the PH-NS-AQ process ~rom pine chiPs
The test was carried out as disclosed in
Example 1.
Prehydrolyzinq step
Wood amount, g of abs. dry wood 2000
Prehydrolyzing agent H2O
Liquor ratio 6~
Temperature rising time, min 45
Prehydrolysis temperature, C 170
Prehydrolysis time, min 15
PrehYdrolYsis loss, % on wood 13.2
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Cookinq step
Na2SO3, ~ on wood as NaOH 22
Na2CO3, %~ on:wood as NaOH 5
Anthraquinone,~% on wood : 0.2:
Liquor ratio : ~ 4.5:1
pH of the cooking liquor 11.3
Rising of~the~temperature C/min
Cooking temperature, C 175
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Yield, % on wood 40.3
Xappa number 16.5
Shive content, % on wood 0.4
Properties of O-D-E-D bleached pulp
Final yield, % on wood 37.2
ISO brightness 84.2
Viscosity, SCAN dm3/kg 890
Reference examPle
It was studied how lignin dissolves in cooking
: processes generally in use as compared with the cook-
ing step of the process according to the invention
when the chips are prehydrolyzed according to the
prior art. Sulphate cooking and various modifications
o sulphite cooking are processes in general u9e.
In the tests the prehydrolysis/cooking was car-
ried out as follows:
Test 1
Sulphur dioxide water prehydrolysis, normal
Normal acidic Ca bisulphite cooking step
Kappa number 150
Test 2
Sulphur dioxide water prehydrolysis, noxmal
: ~ ~ r Normal acidic Ca bisulphite cooking step
Kappa number 126
Test 3
Water prehydrolysis, weak
~:~ : : Normal acidic Ca bisulphite cooking step
: : : Rappa number 118 ~ :
Test 4 ~ :
: Sulphur dioxide wa~er prehydrolysis, weak
: Neutralizing lime milk treatment
Acidic Ca bisulphite cooking step with an :ex-
tremely~hlgh bound 52
: Kappa number 106
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Test 5
Sulphur dioxide prehydrolysis
Cooking step 1: ammonium neutral sulphite
cooking
Cooking step 2: sulphur dioxide water acidic
sulphite cooking
Kappa number 141
Test 6
Sulphur dioxide water prehydrolysis, normal
Neutral sulphite-anthraquinone cooking step
Kappa number 48
Test 7
Sulphur dioxide water-prehydrolysis, normal
Sulphate cooking step, normal
Rappa number 14
Lignin concentrations measured from the digest-
er during the cooking step by means of a cooking
liquor analyzer as a function of the cooking time re-
duced to the same scale appear from the attached
figure 1. The curves thus illustrate the dîssolving of
lignin as measured as an increase in the lignin con-
tent of the cooking li~uor. The results show that the
cooking step after the prehydrolysis in Tests 1 to 4
does not dissolve lignin:efficiently even though at-
tempts have been made to improve these sulphite pro-
cesses as much as possible. The dissolving obtained in
Test 5 was;better because the prehydrolysis is excep-
tional and;not technically reasonable. The content of
residual lignin in Test 5`~tthe kappa number exceeding
100) is, however,~technically impossible, the reason-
able level being:the kappa number of about 50 (= about
lO~ of lignin in cooked pulp).~ In Tests 6 and 7,
lignin starts to~ dissoIve:rapidly in the relative
cooking time~: of 100, the subsequent step being the
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main delignification of a successful cooking which is
completed by a slow residual delignification towards
the end of the cooking. In this way, the kappa level
of 40 in Test 5 and the kappa level of 15 in Test 7
were achieved. Accordingly, it is obvious that an
efficient removal of lignin from prehydrolyzed chip
material takes place in the cooking step of the pro-
cess according to the invention such as disclosed in
Test 6; thus, it can replace the sulphate cooking used
in Test 7.
The tests carried out show that normal tech-
nical prehydrolysis conditions inactivate lignin to
such an extent that no cooking modification within an
acidic or neutral cooking pH range is able to dissolve
lignin even though the chip material would be neutral-
ized between the prehydrolysis and the cooking. The
sulphite cooking step used in the process according to
the invention is operative only when the cooking con-
ditions and the cooking catalyst are chosen appro-
priately.
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