Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~p ~y,~G~T~Y~
~t~ 3.
-1-
PROCESS FOR PREPARING KRAFT PULP
FIELD OF THE INVENTION
The present invention relates to a process for
preparing kraft pulp. More particularly, the present
invention relates to processes for preparing kraft pulp
in which cellulosic material is treated with white or
fresh liquor for dissolving the lignin therein. The
present invention specifically relates to the
pretreatment of the lignin-containing material before
the lignin digestion step.
BACKGROUND OF THE INVENTION
In the various kraft pulp processes cellulosic
material or chips are generally treated at elevated
temperatures with alkaline cooking liquor containing
sodium hydroxide and sodium hydrogen sulfide. In these
processes, fresh cooking liquor is generally referred to
as white liquor, and spent liquor is generally referred
to as black liquor.
On a chemical basis, the kraft pulp process
used industrially is the same toclay as was the case one
hundred years ago, while it is true that many different
chemical means have been proposed for the purpose of
improving factors such as-the yield and selectivity of
the processes, none of these proposals has led to
acceptable practical solutions to these problems because
each of them has entailed complicated equipment,
additional process steps or the use of expensive
chemicals.
In addition, different chemical methods for
the pretreating of chips have also been proposed. Many
of these proposed chemical pretreatment methods have
been based upon the use of hydrogen sulfide or
bisulfide. For example, Finnish Patent No. 28611
describes a pretreatment process utilizing hydrogen
sulfide under elevated pressure. Also, Swedish Patent
No. 309530 relates to a pretreatment process utilizing
liquid hydrogen ~sulf.ide at a pH of between 4 and 10.
CA 02049322 2001-10-15
-2-
Polysulfide treatrnent has also been proposed as a second
pretreatment step,
The kraft process, however, has been developed
by means of different technical processing means. In
particular, the need to save energy has led to new
solutions, the most important of which have been
continuous cooking processes (see, e.g., Finnish Patent
No. 54155). The equipment used in such continuous
cooking processes can include the use of several ca- and
countercurrent circulations, as well as separate
impregnation vessels.
Batch processes have also been developed for
the purpose of saving energy. In many of the processes
which have thus been developed, hot black liquor is
displaced from the digester prior to discharge. This
displaced liquor is then used for preheating the chips,
or as cooking liquor in subsequent batches (see, e.g.,
U.S. Patent No. 4,578,199 and Finnish Laid Open
Publication No. 71:176).
It has been proposed to improve the quality
of the pulp being produced by avoiding digester discharge
which utilizes hard hot blow techniques. This can be
accomplished by using the cold blow method (see, e.g.,
Finnish Patent Application N0. 791205 published October
z"5 12, 1980), or by means of pump discharge (see, e.g.,
U.S. Patent No. 4,814,042).
SU ARY_OF THE INVENTION
In accordance with the present invention the
objects of this inu~ention and improvements in the kraft
:30 pulp process have now been provided by means of a
process for the preparation of kraft pulps from lignin-
containing cellulosic materials, which comprise
impregnating the cellulosic material with spent alkaline
cooking liquor at a temperature of between about 20 and
35 100°C, heating the impregnated cellulosic material at a
temperature of between about 120 and 180'C, and
delignifying the heated cellulosic material with fresh
alkaline cooking liquor.
-3-
laa accordance faith one embodir~eat of the
process of the presetat invention, impregnating
of the
sellulosic material with spent. alkaline soaking
liquor
employs lic~uor having a pH of between abcr~t 11.5
and
13.x, andl preferably between ak~aut 12.5 and 1~.5.
In acaarda:nce with a preferred embadimen~ of
the px-cacess of the presewt irav~!ntian, heating
of the
impregnz~ted cellulosfc ~nateri~al is car~ciad out
for a.
period of from about 1 fm 30 minutes, whereby the
pH c~f
1.0 the apent albtalins soaking liguar impregnated into
tree
cellulosic material is decreased to between about
9 and
11, and preferably to between about. 9.5 and 10.x.
,~, In accordance with another c~mboditnent of the
' process of the g~res~eat inrrerstion, the wpan't
and tresfi~
1~ al3caline cooking litluor comprises st~diur~ hydroxide.
rre=srdc~.~y, the spent alkaline cooking liquor
has a a
residual sodiuaa hydroxide .content of betwemn akrout
4 and
2~ qratn~ of sodium hydroxide per liter, and more
preferably between about s and l~ grams of sodium
20 hydra~cide per liter.
In accordance with ~x~ath~r emt~adiment of the
p~racess of the gresent inventia~n the $tep of heating
the
impregnated ce11ea1osi~ matr~~rial is carriec9 o~av
at a
temperature of be'~ween about l;r~ and sa3'~. ~~.~t~a,t~k~ly
25 this step is carried out far a g~eriod ~f between
abat~t
and ~0 minutes.
In asaarr3ance ~rith on~~ preferred embodiment of
the prooess of the present invaratian the cellulosic
material is hardwood and the step of delignifyinci
the
30 heated aellulosic material a.w carried out using
an
H-factor o~ between aa~out ~uu as~d 1800, i vivar
to
produce a readily fiberizeci papez~ pulp.
In accardancg with another preferred
. embodamgnt of the ~araCess of 'they present. invention
the
~5 callulosfa material is eaft,wood arid the sxep cat
delignityfng the heated cellu7.osic material is
carried
~ur. using an tt-factor of between a~rout X00 and
700, ire
order t-.o produce a .readily fik~erized paper pulp.
In accordance with another embodiment of the
process of the present invention the step of
delignifying the heated cellulosic material is carried
out at a temperature of between about 180 and 1g0C.
The principal advantage of the process of the
present invention is that digestion of the lignin
with
white liquor is greatly facilitated by means of
this
process.
DETAINED DESCRIPTION
It is essential that an accordance with the
present invention the chips are pretreated with
spent
cooking liquor, or so-called black liquor. This
pretreatment takes place in two steps. In the first
step the chips are im~iregnated with the spent liquor,
and in the second step they are reacted with it.
In the impregnation step the chips are
essentially filled with the spent liquor. The
temperature of this impregnation step must be below
100C in order to avoid reaction therewith on the
surface of the chips. In practice temperatures of
from
about 20 to 100C can be utilized. The time of this
impregnation step should be from at least about
minutes, and preferably between about 15 and 20
minutes.
Impregnation times of more than about 30 minutes
are
25 unnecessary.
' The pH of the spent liquor is between about
12.5 and 13.5, and the residual alkali content is
from
about ~ to 20 g NaOH/1, and preferably between about
6
and 15 g NaOH/1.
30 The pretreatment reaction or heating step
which follows the impregnation step is carried out
at an
elevated temperature of from about 120 to 180C.
The
reaction time depends on the temperature which is
utilized, and is generally from about 1 to 30 minutes.
35 Preferab~.y, a reaction temperature of from about
135 to
155C, and a reaction time of from about 10 to 30
minutes is utilized. In this heating step the residual
chemicals in the black liquor react with the wood
°
5-
material, and alkali is consumed. The pH within the
chips is thus decreased to from about 9 to 10. It is
believed that in this altered chemical environment
sulfur compounds react with the lignin, and thereby
render it more reactive in the digestian step which
follows thereafter. It is also assumed that hydrogen
sulfide reacts with the end groups of carbohydrates in
the wood, thus protecting them against alkaline
decomposing reactions.
Pretreatment of the chips in this manner
renders the subsequent digestion step substantially
easier. The severity of the ;ligestion conditions which
are required (i,.e., reaction temperature and time:) is
generally determined by-the so-called H-factor. In a
normal kraft process of, 2.g., Scandinavian softwood,
H-factors of from about 1600 to 1800 are required. In
the present process, H-factors can be diminished by
about 400 to 1000. This means that the overall.
digestion time can be significantly shortened. On the
other hand, it has also been observed that exceptionally
high digestion temperatures, such as from about 180 to
190°O, can be employed in the present process. This can
lead to further shortening of the digestion time. In
conventional kraft processes, the digestion step
generally takes about one hour. In accordance with the
present invention, however, digestion times of about
one-half hour are now possible.
An additional advantage of the present process
is the increased selectivity of the delignification
reaction. This, in turn, leads to higher yields and
superior pulp quality, or to a lower consumption of
cooking chemicals.
Because of the increased selectivity of the
digestion step, and of the quality and yield of pulp,
the digestion reaction can now also be run for a longer
period of time, and a lower lignin concentration can
thus. be achieved than is the case in conventional
processes. The pulp which i.s obtained thereby thus
a
requires less bleaching, which, in turn, decreases the
amount of harmful compounds which are discharged from
the bleach plant into the waste waters therefrom.
Accordingly, by utilization of the present
process there are a number of advantages which can be
achieved, depending upon one's specific individual
requirements.
It is essential in understanding the role of
the present invention that it be appreciated that it
constitutes an intermediate process stage before the
reaction environment is rendered strongly alkaline by
the addition of fresh or white liquor. Accordingly,
that stage can ,be incorporated with virtually any type
of cooking process which utilizes kraft delignification.
In batch cooking techniques, all of the steps
can be carried out in the same reactor, i.e., the
digester. After the black liquor impregnation step, the
contents of the digester are heated to a temperature in
the range of the reaction temperature in the case of (j)
conventional batch processes, by means of the digester
circulation being equipped with a heat exchanger, or by
direct steam injection, and (ia) in case of low energy
batch cooking, using the displacement technique, by
displacing the colder impregnation black liquor with
hotter black liquor for the purpose of carrying the
process heat back to the digester.
Another embodiment of this invention utilizing
batch digesters is to impregnate the chaps with the
black liquor in the context of chip filling in separate
equipment. The reaction stage would thus appear as the
first step in the digester after chip filling, and could
be very effectively carried out by the use of direct
steam subsequent to the draining of the impregnation
black liquor, or by displacing the impregnation/filling
media black liquor by hotter black liquor. In this case
continuous impregnation is carried out while charging
the digester and is combined with batch cooking
techniques, thus resulting in (.a) compensation for the
~.~,~R, afiTitefi
~ _ :'fin,.>s~,r,s
extra time spent with the black liquor stage, and (ii)
reduction of the total cooking cycle time due to the
greater speed of the cooking step.
The present invention can also be carried out
in connection with continuous cooking processes. The
continuous digester equipment presently being used,
including separate impregnation vessels and various co
and countercurrent circulations, effectively segregate
the cooking process into several steps, in which the
present invention can include starting the process with
black liquor and without white liquor. Accordingly, the
chips are fed into the digester or impregnation vessel
""~; along with the black liquor, the temperature is elevated
to the reaction range by heating with the aid of liquor
circulation-heat exchanger. After a process delay which
corresponds to the time required for the black liquor
and waod to interact, the white liquor is then fed into
the digester, displacing the black liquor, the
temperature is again increased by means of a
circulation-heat exchanger and the rest of the process
is carried out in the conventional manner. An
alternative continuous process is to carry out the black
liquor treatment stage as a countercurrent operation.
In continuous cooking processes, application
of the present invention can lead to remarkable results.
Utilizing the present conventional processes, continuous
cooking to kappa numbers of about 30 generally requires
a reaction time of from 60 to 90 minutes in the cooking
temperature range. If extended cooking to lower kappa
~0 numbers of between about 23 and 25 are required, an
extra cooking stage, and an additional 60 minutes of
cooking time is generally required, thus totalia~g at
least two haurs of cooking time. By utilizing the
acceleration of the delignification step of this
invention, however, the cooking time, and the size of
the cooking zone in the continuous digester, can be cut
in half, therefore also rendering the equipment cheaper,
and its operation far simpler.
_g-
Example 1
A forced circulation 20 liter digester was
charged with pine chips in an amount corresponding 'to
3kg of absolutely dry wood, and 15 liters of spent black
liquar was added (pH 13.2, residual alkali concentration
7 g NaOH/1 as effective alkali), so that the liquid
ratio was 5:1. The digester was then closed, and
pressurized with nitrogen in order to permit the taking
of samples and the equalization of impregnation.
The circulation was initiated, and the
temperature of the digester was elevated from 20°C to
70 ° C in five minutes by means of a heat exchanger, and
it was then held at that temperature for 55 minutes.
Samples were then taken from the circulation, cooled
down to 25°C, and their pH measured. The procedure and
development of the pH in the Cook are shown in FIG. 1.
The procedure was then repeated using a
different temperature profile, as follows:
- 70°C 5 mln.
20 70°C 10 min.
70 - 140°C 10 min.
140°C 20 min.
This procedure, and development of the pH of
this Cook, are shown in FIG. 2
25 It can be seen in FIGS. 1 and 2 that the black
liquor treatment at 70°C consumed the residual alkali by
only a small amount, and the pH fell rapidly when the.
temperature was elevated. i~hen the temperature had been
elevated to 140°C in 10 minutes, the pH had thus already
fallen to 11.5, and when the treatment was continued at
140°C, in 20 minutes the pH further fell to 10.2.
This Example demonstrates that when the system
is heated above 100°C a new reaction phase is initiated
in which the residual alkali is rapidly consumed. Since
the final pH's were 11.8 and 10.2, it can be seen that,
in the latter experiment the H+-ion concentration is
almost one hundred times greater than is the case in the
former case. Since the pH could only be measured from
-9-
~'Ti 7 ~z~r-~,v-D
a~~L~~a~~,kr~'.ss~s
the circulating cooking liquid, it is thus clear that in
the latter experiment within the chips themselves the
consumption of alkali would actually be even greater.
Example 2
An industrial batch digester having a capacity
of 140 m3 was filled with pine chips and spent black
liquor (pH 13.4) from previous cookings. The
temperature was elevated to 140°C, and maintained at
that temperature for 15 minutes. The pH thus decreased
to 11. White liquor was then added so that the alkali
dosage was 18.2 of effective alkali, given as Na20.
The temperature was then raised to 1'70°C, and digestion
continued to the desired level of delignification
reduction, by altering the digestion time. The digester
was then discharged, H-factor utilized registered, and
the pulp was analyzed.
This digestion procedure was carried out six
times by changing the strength of the black liquor
pretreatment, but at the same time keeping the alkali
dosage and the overall procedure constant. The
following results were obtained:
Experimental Cook 1
Hlack liquor impregnation at 85°C for 20
minutes. White liquar was added directly
~ after filling with black licguor.
H-factor 1420
Kappa number 27.0
viscosity loco
Experimental Cook 2
Black liquor impregnation at 90°C for 20
minutes. White liquor was added directly
after filling with black liquor.
H-factor 1110
Kappa number 38.3
Viscosity 1135
,~~.c, ~ ~ ~-r~;;~
_ tCe .: ~...9~i!~.,m
Experimental Cook 3
Black liquor impregnation at 90°C far 20
minutes, and black liquor treatment at
125°C for 10 minutes.
5 H-factor 1214
Kappa number 29.6
Viscosity 1115
Experimental Cook 4
Black liquor impregnation at 90°C for 20
10 minwtes, and black liquor pretreatment at
145°C for 20 minutes.
H-factor 860
Kappa number 36
Viscosity 1160
Experimental Cook 5
(Like Cook No. 4)
H-factor 1077
Kappa number 25.3
Viscosity 1065
Experimental Cook 6
(Like Cook No. 4)
H-factor 1089
Kappa number 25.4
Viscosity 1045
These results are~also presented in F1G. 3,
which shows the H-factor in each digestion as a function
of the kappa number of the pulp obtained therein.
The effect of black liquor pretreatment on the
acceleration of digestion can be seen by observing the
H-factor required, or the digestion time at constant
temperature. rn order 'to achieve a kappa number of 30,
1325 H-factor units are required if the impregnated
chips are net heated, but digestion is carried out
immeditely after the impregnation step (see line-through
points 1 and 2). When mild heating was utilized {125°C
for 10 minutes), 1220 H-factor units were required (see
point 3). When.strong pretreatment was utilized {145°C
for 20 minutes), a kappa number of 30 was achieved with
--11-
980 H-factor units (see line-through points 4, 5 and 6).
With conventional batch digesting techniques about 1600
to 1800 H-factor units are required in order to achieve
a kappa number of 30.
The effect upon the duality' of the pulp was
examined by combining 'the pulp samples from Cook Nos. 1
and 2, so as to represent cooking without black liquor
treatment, and by combining the pulp samples from Cook
Nos. 4, 5 and 6, so as to represent cooking with black
liquor treatment. In FIG. 4 the duality of these pulps
is compared by setting forth the tear index as a
function of the tensile strength. It can thus be seen
that, e.g., at a tensile strength of 70, the tear index
of the pulp thus obtained employing the treatment (see
curve A) is 1 to 2 units higher than that of pulps
produced without utilizing this treatment.
Exa~le 3
In this example two experimental Cooks were
carried out to far greater degrees of delignification.
Cook SB
This Cook was carried out in the manner
of Experimental Cook 23os. 4, 5 and 6 in
' Example 2 with the following exceptions:
An alkali charge of 20% effective alkali
as Na20 per wood
H-factor 1850
Fulp kappa number 15.2
Pulp viscosity 905
Cook C
This Cook was carried out in the manner
of a conventional batch Cook, without
black liquor impregnation and treatment
stages:
The alkali charge was 21% effective
alkali as Na20 per wood
H-factor 2000
Pulp kappa number 71.1
Pulp viscosity . 9G5
~YJ~~ ~.., ~T~
~s~ ~'~ i_.Ae~ ~'7 U ~l M IIVp
-12°
The pulps were analyzed in terms of strength
by tear-tensile comparison, as is illustrated in FIG. 5.
It is clear therefrom that, when the tensile index is
increased to the useful range for paper making by
beating (i.2., a tensile index of from 70 to 80), the
conventionally cooked pulp loses its tear strength
(curve "C"), while the pulp cooked with the treatment
stage of the present invention still maintains its tear
strength (curve °'SB°') . The advantage for pulp
°'SB°' is
three tear index units, or from 20 to 25~ higher.
At present, cooked Scandinavian market pulps,
at a kappa number of 30, demonstrate a tear index of
from 13 to 15 at a tensile index of 70. In terms of
present-day pulping technology, those few mills which
apply cooking to lower than normal kappa numbers
generally regard a kappa number of from 23 to 25 as
representative of "extended cooking,' Results of a
nature of those shown above, which were obtained by
using the beneficial black liquor-temperature treatment
hereof, have only been achievable in the past after a
post-digester oxygen delignification process.
Exampl a 4
This example demonstrates a unique-way to take
advantage of the black liquor-temperature treatment
stage of this invention. It is generally known, both in
mill practice and textbooks, that the maximum sulphate
cooking temperature should not exceed 175°C due to the
severe pulp strength losses which result therefrom, as
well as the lower yield which will then be realized.
An experimental cook was carried out as in
Example 2, Cooks 5 and 6, except that the cooking
temperature was not limited to 170°C (curve "NTP" in
FIG. 6j, but instead the cook was heated up as far as
was possible with the available steam and heat
exchangers (curve "DTP" in FIG. 6). The end temperature
was i81°C. All other cooking conditions were equal.
Temperature of the black liquor
treatment was 145°C. The time of
!? ~I-. y
' ~~~~~~~~i;~
o~~..
b~.~ck lieluor tr~at~nsnt was ~0
m1n11'tL~. '1'ri~ dlK~li ulacaa~a s~a5
18.2 effective alkali as ~7a~~7 par
wood
F3~fact~o~~ 1050
(~xampl.s 2 A ~:Of7lC~ $, 5: 1080,
~lp 3cappa numher 28.1
~~'~t~~~~s 2, ~~3S~k~ S' ~ 0 2~. ~?
The tsar-kensils r~la~t3onsta~.p of the ~ul~ ~t~as
1~ ~naly~sd in order ts~ evaluates the ~r~tlp strength. ~t a
useful tensile ir~dsx six 7b, the tsar index was 1b, which
eq~ais the va3.ue found an curvy ~,~~ i1~ F'I~. 4 in example
aPPlYing a . nc~~n~7. cooking t~rnperat~ui°~ and bl~cl~
3i,c~uor treat~uant. ~°h.i~ sl3.gtatl~ exceeded ttaa'~ of a
noa~nal oooking '~em~eaature with no blac3~ liguoac
treatment.
This retention of pulp strength can bs of
considsrabi~ ~ignificat~c~ ~rhen c~x'eat~r produat.ion p~
diges'ce~' volua~~ unit is requis~d4 ~I~. 8 eats forth
2a compaa°ison between ooo9cing t~~nper~ture and time pl°mf3l~~s
f~~' t.h~ ee~olk in thj~ '~aeampl~, end ~h~t~ of Coo3c 3~os. 5
and ~ in ~xampi~ 2, raPresenting ns~~ma1 cooking
temperatures. '
lend t~mperatugs~ 1~1'C
fin~sl ~I-tact~r 10~0
Time to end from 19~°~ 6o minuta$
End temperature 17~"~
~~ final 11-faCto~' 100
Time tc~ end from 14~'~ 10o minutes
It is evident from these results that the
cooking time after 44 mi.nutss bf heat.ing was cu'G dawn to
20 minutes by t'h~e ~Zigh tempora~tur~ profile, instead of
~0 minutes with constant 170°C oooking tempsratur~.
minute ~avings~ in cook.znc~ time easily r~prsssnts a 15
to 20~ lrawer total cys~le time, with the c~r~espondi~as~
oppox'tunity to incrc~s~a production without oQ~npro~aisi.ng
-14 ° f:'c>..»~~~lr,;r':a
pulp quality. In terms of yield it appears that the
yield of the very fast cooking method of this invention
is then 1 to 2~ higher.
Example 5
The results of this Example demonstrate that
the pulps inside the digester prepared in accordance
with the present invention are in extremely good
condition to resist the physical damage during the
discharge which arises by various blow methods, as
compared to pulps cooked without the use of such a black
liquor treatment stage.
The pulp conditions prior to the blow were
determined by hanging baskets filled with the same chip
material inside the digester. After the blow, pulp
which had not been blown could thus be recovered from
these baskets, and compared to samples of the blown
pulp.
Tn this case, the analysis carried out was in
terms of a so-called strength delivery, which is the
percentage of the pulp strength as tear index at a
tensile index of 70 measured in the blown pulp as
compared to that of non-blown pulp ira the basket.
The Cooks were carried out with a black liquor
treatment stage as described in Example 2, Cook Nos.
4-6, discharged by: hot blow, directly from full
cooking temperature; cold blow, after cooling
displacement to under 100°c; and pump discharge after
cooling displacement.
Reference data is given from U.S. Patent No.
4,814,042, which represents the effect of the blow
method subsequent to conventionally cooked sulphate
batch cooks.
The following table summarizes these results.
Table 1 (Pulp quality given as strength delivery
~ percentages of blown pulp compared to that of
non-blown pulp strength.
_15_
K~ Via. wJ~
Sulphate Cooking Conventional
with Treatment of Batch
Discharge Method This Invention Cooking
Hot Blown Pulp 95 77
Cold Blown Pulp 99 85
Pump Discharged
Cold Pulp 99 90
It is evident from Table 1 that pulp cooked by
a method comprising the black liquor treatment of this
invention does not require any improvement in terms of
strength delivery, and the pulp is in optimum condition.
Although the invention herein has been
described with reference to particular embodiments, it
is to be understood that these embodiments are merely
illustrative of the principles and applications of the
present invention. It is therefore to be understood
that numerous modifications may be made to the.
illustrative embodiments and that other arrangements may
be devised without departing from the spirit and scope
of the present invention as defined by the appended
claims.
