Note: Descriptions are shown in the official language in which they were submitted.
~036758
AS is generally known, an example of applying
oxidative delignification with oxygen in a pulp and paper
industry has been oxygen bleaching in which delignification
is carried out at a pulp consistency of more than about
18~ by weight based on the mixture of pulp and an alkaline
aqueous medium, i.e. in the vapor phase. The reason why so
high a pulp consistency is necessary is that the solubility
degree and the dissolution rate of oxygen into an alkaline
aqu~ous medium are small. For this reason, pulp should be
fluffed in a reaction vessel at a high pulp consistency to
enlarge the surface area of pulp and to let the alkaline
aqueous medium exist around the pulp fiber surface only as
a thin layer. By contacting oxygen gas with such fluffed
pulp through the thin layer of the alkaline aqueous medium,
delignification proceeds effectively.
,
10367S~ .
In s~ch`delignification at the high pulp con-
sistency as descri~ed above, it is possible, in the case
where residual lignin i~n the pulp is small, to operate the
system safely withbut risk of explosion due to the heat
generation during lignin degra~ation if careful attention
is paid to the control of reaction, since the exothermic
reaction of lignin degradation by oxygen is not violent
in this case. On the other hand, in case of delignification `
of high yield pulp having high lignin content, there is a
big risk of explosion so far as the controlling method
in commercial use is applied, due to the great heat
generation caused by lignin degradation. It is now essential
that some fundamental solutions for this problem be obtained.
On the other hand, many studies have also been
done as to oxidative delignification at relatively low
pulp consistency, i.e in the liquid
phase. In general, oxidative delignification is considered
to be the heterogeneoussurface reaction in which lignin
degradation proceeds by the contact of cellulosic materials,
alkaline aqueous medium and oxygen. Therefore, conditions
of their contact and reaction conditions such as reaction
temperature, pressure, time and type of alkali used have
significant effects upon delignification, especially in
case of oxidative delignification in the liquid phase.
For this reason, the choice of optimum conditions for
delignification is so difficult that oxidative delignification
in the liquid phase has never yet been put to complete
practical application.
It is therefore an object of the present-invention
to provide a new method of delignifying cellulosic material`s
,.
103675~
in which disadvantages of prior art methods as described
above are overcome and high lignin content pulp and further
wood chips, as well as chemical pulp, can effectively
be delignified.
It is another object of the present invention
to provide a new method o delignifying cellulosic materials
in which a risk of e~plosion due to the heat generation
during lignin degradation is substantially eliminated.
It is further object of the present invention
to provide a new method of delignifying cellulosic materials
in which the contact of cellulosic materials, alkaline
aqueous medium and oxygen is effectively made, to conduct
effective delignification and to significantly shorten
the reaction time.
It is further object of the present invention
to provide a new method of delignification of cellulosic
materials wherein high quality pulp can be produced.
In accordance with the invention, there is provided,
in brief, a me~hod of delignifying cellulosic materials
with oxygen in the presence of an alkaline aqueous medium
at an elevated temperature in a pressurized reaction
vessel which comprises previously dissolving said oxygen
into said alkaline aqueous medium, contacting said alkaline
medium containing dissolved oxygen with said cellulosic
materials in said reaction vessel to conduct oxidative
delignification of said cell~losic materials, the consistency
of said cellulosic materials being kept so as to form an
i agglomeration of the cellulosic materials and the alkaline
aqueous medium not having any gaseous space therein~, dis-
charging a part of or substantial part of the waste liquor,
_ ~ _
,! .
103675~
formed during delignification, from the vessel, while
supplementarily charging the fresh alkaline aqueous medium
containing dissolved oxygen into the vessel, whereby the
displacement of the waste liquor with the fresh alkaline
aqueous medi`um containing dissolved oxygen is carried out
continuously throughou~t the delig~ification.
~ The aforementioned and other objects and features
of the invention will be apparent from~the following detailed
d~scription, when read in conjunction with the accompanying
drawings, in which
Fig. 1 is a schematic diagram illustrating
equipment used in a preferred embodiment of the invention;
and
Fig. 2 is a graph showing a relationship between
reaction times and Kappa numb~rs of resulting pulp obtained
by the present invention and by a conventional method.
. We~have investigated various methods of oxidative
delignification of cellulosic materials with oxygen in
liquid phase at comparatively low cellulosic materials
consistency of between ab`out 5 to about 20% by weight based
on the mi~ture of cellulosic materials and the alkaline
! ~ aqueous medium, in order to provide a new and improved
¦ method wherein pulp of any lignin content can be delignified
and the problem in vapor phase treatment as mentioned above
can be solved. As the results of the investigation,
following information has been obtained: in the case
~, where cellùlosic materials are subject to delignification
with oxygen in liquid phase,
` 1) The oxidative degreadation of lignin proceeds
I very quickly in the alkaline aquoues medium under certain
` '
~0367S8
conditio~ of temperature and pressure, while the dissolution
and transfer rate of oxygen gas in the alkaline aqueous
m~dium are low~ For this reason, the` ambient zone of the
reaction is considered to continuously suffer from a
`'shortage of dissolved oxygen". Due to "oxygen starvation",
the reaction to be eff~ected is~delayed. Therefore, in order
to keep the reaction proceeding rapidly, it is necessary
to maintain constant contac~ between the alkaline aqueous
medium containing dissolved oxygen and the cellulosic
materials.
2) On the other hand, soluble salts of organic
acids which arè produced by oxidative degradation of lignin
tend to accumulate inthe interface between the alkaline
aqueous medium and cellulosic materials. This not only
decreases the solubility of oxygen into the alkaline
medium, but also adversely affects the contactive reaction
of oxygen wi~h cellulosic materials due to a type of
Donnan's membrane equilibrium. Therefore, it is preferred
that such salts of organic acids by-produced be removed
from the interface for effective delignification with
oxygen.
The present invention is based on the above-
mentioned discoveries as to delignification of cellulosic
materials with oxygen in the alkaline aqueous medium.
According to the present invention, oxygen is
previously dissolved into the alkaline aqueous medium and
the waste alkaline medium is continuously displaced with
fresh medium containing dissolved oxygen, thereby keeping
constant contact of the alkaline agueous medium co-ntaining-
dissolved oxygen with cellulosic materials.
. . -
- 5 -
10367S8
Furthermore, in order to remove salts of organic
acids from the interface, the oxygen dissolved alkaline
medium is passed through the pulp or chips suspènsion, mat,
layer or the like to thereby break down the so-called
Donnan's membrane'equilibrium.
The consistency of cellulosic materials to be
employed in the present invention should be determined within
the range in which displacement of alkaline medium can
easily occur. That is to say, it is necessary to choose
the consistency range in which the agglomeration of cellulosic
materials in the alkaline medium retains its form as if it
were a mat or layer, ~r the like without any
gaseous space therein. ~owever, because pulp fibers have good
affinity to water compared with undefibrated cellulosic
materials such as wood chips, the consis'tency of pulp
like materials to form the agglomeration of cellulosic
materials and alkaline medium as a fiber mat, iayer or bed
becomes iower than that for wood chips. Thus, the pre-
ferable consistency varies according to the cellulosic
materials being employed. In general, the consistency of
cellulosic materials of between about 5 to about 15% by
weight based on the mixture of cellulosic materials and
the alkaline aqueous medium is preferred for chemical and
semi-chemical grade pulp, and about 10 to about 20% for
wood chips.
In the process of the present invention, cellulosic
materials are treated at relatively low consistency, so
~that there is much medium which absorbs generated heat of
reaction. The absorbed heat is therefore always carried
away together with'the medium which is discharged from the
- 6 -
. ' . .
103675~
reaction vessel continuously throughout delignification,
by medium displacement. ~ccor~ingly, there may be no risk
of explosion or overheating in the reaction vessel as in
the case of delignification at high pulp consistency.
Alkaline chemicals such as sodium hydroxide,
carbonate, and bicarbonate may be used as the alkaline medium
in the invention. It is preferred, however, to use sodium
carbonate solution, from the recovery point of view, in
case of delignification o defibrated materials wherein the
rate of alkali penetration into materials is minor factor
On the other hand, sodium hydroxide is well employed on
wood chips wherein rapid and effective penetration of alkaline
medium into chips is needed.
It is advantageous that the reaction time is
remarkably reduced by.the process of the present invention,
so when the process of` the invention is applied in bleaching
of che~ical pulp, bleaching is accomplished within about
10 minutes, whereas applied in delignification of semi-
chemical grade pulp or chip materials, delignification is
attained within about 20 to about 40 minutes, which is
quite short when compared with processes of prior art.
An incidental advantage of theinvéntion is that good
quality pulp can be obtained, because the shorter the
reaction time required, the less oxidative degradation of
cellulosic fiber there is~
As far as reaction temperature is çon~erned, it
may be generally about 80 to 160C, preferably about 100
to 150C, depending on thè kind o cellulosic materials
and the degree of delignification. Partial oxygen-pressure
under which oxygen is dissoived in the alkaline aqueous
- 7 -
10367S~
medium previous to the introduction into the reaction vessel
maybe at least about 4 kg/cm2, and preferably more than
about 7 kg/cm2 when a high degree of delignification is
required.
Therefore, the total reaction pressure at the
delignification may be required to be more than the partial
oxygen pressure which has been previously employed to
dissolve oxygen in the alkaline aqueous medium.
In practising th~ process of the present invention,
either continuous or batch system may be employed. Fig. 1
shows a schematic device usable for practising a preferred
embodiment of the process of the invention in which delignification
is conducted continuously. Defibrated or non-defibrated
cellulosic materials are continuously fed into a pressurized
react~ion vessel (1) from the top thereof. The alkaline
aqueous medium in which oxygen has previously been dissolved
in an oxygen dissolving tank ~2) is also continuously fed
into the vessel, which pressure is maintained at the same
as or more than the partial oxygen pressure in the oxygen
dissolving ta~nk, through a center pipe (3) having a large
number of holes or slits arranged lengthwisely on the surface
thereof. The alkaline aqueous medium containing dissolved
oxygen is then radially spread out from the center pipe,
across cellulosic materials fed into ~he vessel. Thus,
the contact of cellulosic materials with the alkaline
aqueous medium containing oxygen is performed and deli-
gnification proceeds during such contact. Waste liquor
ormed during delignification is continuously discharged
or extracted through suction pipes -(4) and (5), and, at
the same time, supplemental fresh alkaline aqueous medium
,- 8 -
1036758
containin~ dissolved oxygen is continuously charged or fed
into the vessel from the tank t2) through the center pipe (3).
The consistency of cel~ulosic materials inthe reaction
vessel i~ kept constant by controlling the volume of input
medium fed through pipe ~3) and output waste liquor ex-
tracted thxough pipos ~4), ~5~ The lignin content of
cellulosic materials is reduced by degrees, as cellulosic
mat~rials moVe downwardly in the reaction vessel, and thus
obtained delignified pulp is removed from the bottom of
the vessel through a blow valve ~6).
Some part of the waste liquor, which is discharged
from the relatively early stage of delignification through
theupper suction pipe (4), may be transferred to a chemical
recovery system (not`shown). The alkaline chemical obtained
by the recovery system may be fed to the tank (2) for reuse.
Another part of the liquor, which is discharged from the
lr
latter stage of delignification through the lower suction
pipe (5), may be re~cycled to the tank (2), where the liquor
is mixed with the~ alkaline aqueous solution recovered by
the recovery system and, if resuired, make-up solution and
oxygen is thoroughly dissolved under pressure into the mixed
solution. The regenerated orretreated solution thus
obtained is recharged into the reaction vessel and reused as
the supplemental fresh alkaline aqueous medium containing
dissolved oxygen.
In this embodiment, two stages of delignification
are considered, i.e., the early stage and the latter stage.
However, it may be possible in an other embodiment tO consider
more multiple stages of dellgnification and to assume more
.. ~
j, multiple layers corresponding to different degrees of
.j . . .
'I _ g _
,1
.1 ` .
10367Sff
delignification of cellulosic materials in the reaction
vessel by separately discharging the waste liquor from each
of layers in the reaction vessel. By employing such multi-
layered system as described above, it becomes possible to
use different k~nds of alkali and different pH values
of alkaline aqueous medium at the same time in a single
reaction vessel.
For instance, when wood chips are used as cellulosic
materials in the process of this invention, it is pre-
ferred, in order to obtain good quality of pulp, to treat
chips with sodium hydroxide solution of relatively high
pH at the relatively early stages or layers in the vessel,
andthen with sodium carbonate solution of low pH at the
latter stages or layers w`nere delignification has proceeded
to some extent Furthermore, it may be possible to carry
out both delignification of cellulosic materials and
washing of the resultantpulp in one vessel by treating the
resultantpulp with hot water instead of the alkaline
aqueous solution at the final stage or layer in the vessel.
When the liquor extracted from ~he latter stages of deli-
gnification is recirc~ulated, after oxygen is thoroughly
dissolved thereinto, in the relatively early stages in
the vessel, on that occasion; the process may be carried
out in a counter-current system~ By employing such a
counter-current system, the total volume of the alkaline
aqueous medium in use is mada small and the waste liquor
having a higher concentration of organic substances is
obtained consequently. This is preferable from the
chemical recovery point of view. -
The following examples are given by way ofillustration only.
-- 10 --
,
~0367S8,
~ EXAMP~E 1
Unbleached k~aft pulp made from Douglas fir of
a Kappa number 32.8 was treated for delignification according
to the prèsent invention.
100 g of the pulp was placed in the center of a 8Q
autoclave ` by sandwiching the pulp with two wire mesh
plates so as to provide a pulp consistency between two
wire mesh plates of about 7% by weight when immersed in
an alkaline`aqueous medium.
The alkaline solution which contained sodium
carbonate and sodium hydroxide (~a2C03/NaOH=4/1) and had
the alkaline concentratio,n of 5 g/~ as sodium oxide, was
used as the alkaline medium for cooking.
The alkaline medium in which oxygen had previously
been dissolved under a partial oxygen pressure of 8 kg/cm2
was quickly fed into and filled up the autoclave at a
temperature of l10C. Then, into the thus filled-up
vassel, fresh alkaline medium containing dissolved oxygen
was additionally and continuously pumped from the bottom
thereof at the rate of 1 ~/minute for 5 minutes, while the
overflow surplus liquor was taken out from the top of the
vessel. By this procedure, a part of the alkaline cooking
medium was continuously displaced during delignification by
the fresh alkaline aqueous medium containing dissolved
oxygen.
The delignified pulp obtained had Kap~a number
of 6.8 and b~ightness of 46.7 (GE),
From the above results, it is evident t~at the
treatment,described above is well applied to the first
stage of à conventional bleaching process in bleaching kraft
pulp .
.
~ 11 --
~036758
EXAMPLE :2
Eucalyptus chips were cooked at a temperature
of 180C for one ho`ur in an alkaline solution of sodium
carbonate at an amount of, as sodium oxide, 15% by weight
based on wood, and then we~e~defibrated by a disk.refiner. The
crude pulp obtained had a Kappa number of 132 in a yield
of 68~
The crude p~lp was delignified in the same manner
andunder the sam~ conditions as in Example 1 except for
employing the following: `
Partial Oxygen Pressure = 12kg/cm2
Temperature = 140C
Reactlon time = 10 minutes
Magnesium carbonate was additionally mixed in the
alkaline cooking medium at the concentration of
0.1 g/Q.
For comparison, the same chips were delignified
by a conventional Xraft process under the following
conditions:
Alkali charge = 22% as Na2O based on woods
Sulphidity = 31.6%
Time to maximum temperature = 70 minutes
Time at maximum temperature = 60 minutes
Wood to liquor ratio = 1:5
The comparative data obtained by the process of
the invention and by the kraft process are given in the
following Table.
.
.
- 12 -
10367S8
Screened Kappa Brightness Breaking* Burst* Tear*
Pulp ~ield ~mber - _ Length,Km Facter Factor
Pulp by the !
process of ~ 47.77.6 50.2 6~53 5.46 116
invention
Kra~t Pulp ¦ ~2.6~ 10,~ 29~5 5.74 5.57 115
~* PFI mill 400 cc freeness)
As is appaxent from the table, the quality of the
pulp made by the process of the invention is superior t~
the kraft pulp in yield and brightness, and equivalent in
physical parameters such as breaking length, burst and tear
factors.
EXAMPLE 3
Crude pulp having a Kappa number of 126 was pre-
pared from hardwood chips by cooking the chips with sodium
carbonate solution and then defibrating the softened chips
at the yield of 69.~%.
The thus obtained crude pulp was delignified
in the.same manner and under the samè conditions as in
Example 1 except for employing the following:
Partial oxygen pressure = ;2 kg/cm2
Temperature - 140C
The relationship between Kappa number of the thus
delignified pulp and reaction time is shown by a curv.e A
in Fig. 2.
As a comparative experiment, delignification was
carried out in the same manner and under the same-conditions
as above-described except for employing an alkaline aqueous
. - 13 -
103Y~7S8
medium into which oxygen was not previously dissolved, and
blowing oxyaen gas directly~into the vessel under a partial
oxygen pressure of 12 kg/cm . The relationship obtained
is also shown by a curve B in Fig. 2.
From these two curves, it is evidently understood
that more than 80~ delignification was attained within 5
minutes in case "A" wherein oxygen had previously been
dissolved. On the other hand,~same delignification as in "A"
was not attained even after 30 minutes in case "B" wherein
oxygen gas was put in~to the vessel directly.
EXAMPLE 4
~ Shredded Douglas fir chips were cooked at a
temperature of 170C for 30 ~inutes in an alkaline solution
of sodium ~ydroxide at an amount of, as sodium oxide, 15%
by weight based on wood, to thereby obtain undefibrated
materials having a ~appa number of 128 in a yield of 62.8~.
The thus prepared undefibrated materials were then
delignified in the same manner and under the same conditions
as in Example 1 except for employing following:
Consistency of cellulosic materials = 10%
Partial oxy~en pressure = 12 kg/cm2
Tempèrature - 150C
Alkaline a~ueous medium;
at early period of 10 minutes = sodium hydroxide
solution
at latter period of 10 minutes = mixture solution
of sodium carbonate
and hydroxide
(Na2C03/NaOH=4/1)
The results obtained were as follows:
~03675~
Screened pulp yield 44.2%
Rejects O
Kappa number 18.2
Brightness 38.2 ~GE)
As can be seen from the above described process
and examples, the present invention provides an effective
method for rapid delignification of any type of cellulosic
materials, such as chemical pulp, semi-chemical grade pulp,
and wood chips. It is understood that various changes in
the details, arrangements, materials, and process s~eps
which are herein dèscribed and illustrated to better
explàin the nature of the invention may be made by those
skilled in the art without departing from the scope of
the invention.
- 15.-
.
