Note: Descriptions are shown in the official language in which they were submitted.
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process for_producing special high alpha-cellulose pulps by
means of a multistage cook including an acid prehydrolysis
When producing high-quality celluIose pulps~ a two-stage
cooking process is o~ten used, the first stage of which is
a prehydrolysis carried out with water or steam at the tem-
perature of 170 to 175 C. After the prehydrolysis and an
eventual remo~al of t~e hydrolysate, white liquor is added to
the cooker and a normal sulfate cook is carried out, ~hereby
high-quality pulp is obtained, the alpha-cellulose contant of
which is over 90 % before bleacing. The high alpha number is
achieved due to the prehydrolysis stage, as there the hemi-
cellulose which can be easily hydrolyzed, is almost comple-
tely dissolvedr and the only task which remains for the
sulfate cook is to remove lignin.
The hydrolysis and the dissolving of the hemicellulose of
wood requires acidic conditions. When the hydrolysis is
carried out by means of water or steam, the acidity is
achieved by organic acids, such as acetic, formic or propionic
acid, which are derived from the wood itself. As the above
mentioned organic acids are weak acids, the pH cannot go very
low in the pxehydrolysis react1on. The low acidity of the
reaction conditions must thus be compensated by quite a high
temperature, in order to achieve a sufficiently rapid hydro-
lysis.
When producing high alpha-cellulose pulps for chemical pro
cessing, the most common way is water prehydrolysis at a
temperature of 170 to 175 C with a reaction time of from
40 to ~20 minutes at the maximum temperature. The prehydro-
lysis loss is then, depending on the species of wood, from
18 to 25 %. When softwood is used, lignin may condensate
noticeably, whereby the kappa number after the cook increases
much and the bleaching consumes disproportionate amounts of
p~
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c~lorine or chlorine oxide.
Steam prehydrolysis principally resembles water prehydrolysis
and requires the same reaction conditions. As in steam pre-
hydrolysis the liq~lid-to-wood ratio is considerably smaller
than in the latter, less thermal energy is needed. ~lso-
the required cooker volume and investing costs are smaller.
The biggest risk factor when using steam pre~ydrolysis is
the quality of the pulp where there ~an be irreguIarities
due to the small liquid-to-wood ratio.
The dry matter of wood dissolving in the prehydrolysis is
thus primarily hemicellulose and it consists of pentoses or
hexose~ ost of the hemicellulose is decomposed in the
prehydrolysis and 50 to 60 ~ of the dry matter of the hydro-
lysate is monosaccharides. Part of these are, however, still
in the form of oligo- or polysaccharides. In addition to
these, a considerable amount of organic acids as well as
furfural are formed in the process from the pentoses of
the wood. ~rhe formation of acetic acid and furfural in
particular increases rapidly as the temperature oE the
hydrolysis rises in the range above 150C~
The hydrolysate formed in the prehydrolysis stage including,
in addition to sugars and polysaccharides, also organic acids
and furfural, is valuable and its utilization improves the
profitability of pulping.
In addition to what has been mentioned, the utilization of the
hydrolysate is important also because when it is discharged
unused, it causes sewage problems which are difficult to
handle. Factories in which the utilization of the hydrolysate
as a raw material for chemical products or in the production
of energy has not been completely solved, have unreasonably
high sewage purification costs.
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~s ~ consider~ble part of the hemicellulose in watex or steam
pxehy~rol~sis re~ain'~ in the oli~o- or pol~s~ccharide stage
and simu~taneously large ~mounts of 'fur'fural, w~ich is partly
converted into 'fur'~uran resins, is formed, it is difficult to
handle the formed hydrolysate. It is characterized by a so
called caramellizing tendency which is caused by the poly-
saccharides and the 'fur'furan resins that are being formed.
Already as a we~k'so'lution, the hydrolysate tends to form
on the equipment deposits and layers which are difficult to
remoVe; these disadvantages are emphasi~ed when hydrolysate
is evaporated. The difficulties in the evaporation plant are
often reflected as an increased pollution load and as
increased handling costs. In case hydrolysate is used as a
raw material for biochemical processes, evaporation is often
not necessary.
If the prehydrolysis is carried out under very acidic
conditions, the hydrolysis temperature can be lowered. The
formation of furfural decreases as does natural'ly the
formation of furfuran resins as well. Due to the low pH, the
polysaccharides are decomposed almost completely to mono-
and disaccharides.
The use of strong mineral acids as the hydrolyzing agent has
been known for a long tim~. The Swedish paten-t No. 11357~
discloses e.g. the production of viscose pulp from straw by
using prehydrolysis with less than 1 % of sulfuric acid at
a temperature of 100 to 130 C followed by a sulfate cook.
The Swedish patent No. 113580 discloses the production of
viscose pulp from softwood chips under similar conditions.
The use of a strong S02 so'lution in the prehydrolysis has
been disclosed in several U.S. patents.
The hydrolysate thus formed does not have the tendency to
caramellize, neither does it have any other troublesome
properties; it can be stored without any problems for long
periods of time, p~nped and in particular, it can be evaporat~d
O
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to ~ hig~ dry ~atter eo~tent, whexe~Y i~$ us~bility and its
proee~sin~ possih'llities arç imp~o~ed decisively.
Thç' use of ~tron~ aeids is not, however,'uneomplieated, e.g.
due to dif'fieuIt eoxrosion eonditions and the faet that the
variations in the yield and quality of puIp are at low pH-
Yalues steeper than u'sual'ly. As far as is known, the above
mentioned patents are nowhere appliçd to in pulp produetion.
It is extremely diffieult to get the aeid to absorb into the
ehips so evenly that the entire wood material is prehydrolyzed
in thç same way, whieh is neeessary in consideration of the
uniform quality of the pulp. The uniformity of absorption ean
be improved by using smaller ehips than usually, but then the
circulation of the eooking liquid in the eooker beeomes more
difficult partieularly 'during the alkaline stage following
the hydrolysis.
The present invention relates to a process for produeing high-
quality puIps suitable for ehemical processing by means of a
multi-stage cook which is characterized in that the prehydro-
lysis is carried out under strictly eontrolled, comparatively
acidie eonditions. In order to ensure the even absorbance of
the prehydrolysis agent into the chips, they are steamed for
5 to 10 minutes at a temperature of 110 to 120 C. The
optimum length of the chips does not have to be shorter than
20 to 21 mm which does not yet have an effeet on the eireula-
tion of the cooking liquid. Excessively eoarse, over 25 mm
long chips should, however, be earefully screened away. E.g.
sulfuric acid is used from 1 to 2 % of the weight of the
wood, whereby the sulfuric aeid content of the prehydrolysis
liquid is 0,3 to 0,6 ~. When sulfur dioxide is used, the most
suitable amount is 0,3 ~o 1,0 ~ based on the weight of wood.
The prehydrolysis temperature is 140 to 150 C and the hydro~
lysis time 30 to 60 minutes.
When sulfuric aeid i5 used a~ a hydrolysis agent, sugar acids
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a,nd li~nosulpho~ cids are formed, Which together with SO2
are effec~i~e ~u~ ost of the xe~ction time. This is how
an efficie~t hydrolysis is carried out, in which the actual
cellulose is not da~aged, part of the lignin is dissolved
and it is possible to obtain high-quality pulp in a sulfate
cook following the ~ydrolysis'under mild conditions. The
lignosulph,onic acids which are formed in the process assist
in obtaining a sufficiently low p~. It is known that sulfuric
acid also decomposes sugars more than other mineral acids. At
the same time, ligno'sulphonic acids cause the hydrolysate not
to form deposits in the after tre~tment.
By intensifying either one or more of the above mentioned
conditions within given limits, the prehydrolysis loss can
be increased, this in turn resulting in chips including less
hemicellulose than before. In the alkaline cook following
this stage, these chips will yield pulp which is cleaner and
has an extremely high alpha number.
A counter-current wash with a recovery grade of 70 to 80 %
of the prehydrolyzed chips is an essential feature of the
method. Then the conditions of the alkaline cook can be
controlled with su~fient accuracy, as substantially no alkali
is spent for the neutralization of the hydrolysate. Also
hydrolysate from a previous hydrolysis with a dry matter
content of e.g. 7 to 8 % can be added to the hydrolysis
staye. The dry matter content of the hydrolysate can thus
be increased by recirculation, whereby the evaporation costs
of the hydrolysate are decreased essentially.
When using hardwood, e.g. birch, for the processing of which
this method is particularly suitable, by adjusting the above
mentioned hydrolysis conditions within the given limits, with
a prehydrolysis loss o~ 20 to 30 % and with.a sulfate cook
chemical dosage of 14 to 15 % Na2O, a high-quality pulp is
obtained, the pentosan content of which is 2 to 8 % as un-
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bleached, ~app~ numbe~ 8 t~ 12 ~nd ~ie~d o~ w~od 32 t~ 36 %.When the ~u~p is blç~ched otherwise no~ma~ but by using
at lçast 75 % ~cti~e c~lorine dioxide in the ~irst stage,
high-alpha pulps (alpha number 90 to 97'% depending on the
residu~l pentosans) suitable for che~ic~l Processing are
produced without an intensi~ied ho~ or cold alkali treatment.
Due to the strict'ly controlled conditions and the eVen absorp-
tion of the hydrolysis liquid,' unadvanta~eous condensation of
li~nin in t~e chips can be avoided also when ~ydrolyzing soft-
wood, and consequently, the bleaching is relatively easy.
.
The hydrolysate which has been carefully separated from the
chips includes the dissolved hemicel'luIose, most of which is
decomposed to monosaccharidesl the mineral acid used as agent
and organic acids.
When using birch, most of the monosaccharides are xyloses
and when'using softwood, hexoses. The dry matter content of
the hydrolysate is 6 to 8 % and due to the special hydrolysis
conditions - the hemicel'luIose is primarily in the form of
monosaccharides and the amount of furfurai formed is minimal
its further treatment, e.g. evaporation, is easy and
caramellizing does not occur.
Prehydrolysis carried out in this way rnakes the production of
high~quality pulp easier, as the condensation of lignin is
insignificant at the low hydrolysis temperature and therefore
the sulfate cook is easily carried out after the prehydro-
lysis. A small dosage of alkali is sufficient and a low
cooling temperature is employed, wherefore the energy
consumption of the process decreases also in the sulfate
cook.
The amount of alkali used is small also bec`ause of the
efficient intermediate wash which consumes only a littie
alkali for the neutralization of the hydrolysate.
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It is also within the scope of the invention to
carry out a method wherein the hydrolys,ate formed as described
above and containing mostly mono- and disaccharides and having
a low furfural content, is neutralized in order to bind the
organic acids, and concentrated by means of evaporation until
it contains 60 to 70~ dry matter, whereafter it can be economic-
ally used as cattle feed or raw material for chemical processes.
