Note: Descriptions are shown in the official language in which they were submitted.
Zl~O
SPECIFICATION
Most cellulose pulp produced today in commerce is prepared
by the sulfate or kra-ft proce3s, in which wood is digested or pulped with
aLl{ali and sodium sulfide, and sodium sulfate is used as the make-up
5 chemical to cover losses in the recovery cycle. The greater part of
the remainder of the cellulose pulp is produced by the three variants
of the sulfite process, in which the active digestion or pulping chemi-
cals comprise acid sulfite, bisulfite, or neutral sulfite.
.. . ............ . ..
Some pulp is still produced by the so-called soda process,
10 in which sodium carbonate is used instead of sodium sulfate as the make-
up chemical, but this process has a number of serious disadvantages,
compared to the kraft process, particularly a low pulp yield and a
poor pulp quality, and consequently this process is no longer used to
any considerable extent.
The sulfate process also has a number of disadvantages,
however, the most serious one, from the standpoint of pollution of
the environment, being the discharge of sulfur dioxide, hydrogen sul-
fide and other waste gases, as well as the black liquor efnuent, which
must be captured, processed for recovery, and recycled to maintain
20 an economical operation. Even with maximum recovery and reutili-
zation of waste chemicals, contamination of the atmosphere and of
- waterways adjacent a sulfate pulp mill remains a serious problem
which has notbeen entirely overcome, and as a result, manykraft
pulp mills today are faced with the necessity of developing a work-
25 able and practical alternative to the sulfate process. This alterna-
tive is not provided by the sulfite or soda processes.
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It h~s b~en suggested that the part played by the sulfide in
freeing the cellulose pulp fibers from the wood can be taken by
oxygen .
It has been known for several years that wood can be
digested to form cellulose pulp by using oxygen gas in the presence
of aqueous alkali. The oxygen gas serves as an oxidant which in the
presence of the aqueous alkaline liquid phase attacks the lignin o the
wood, and converts it into soluble degradation products which dissolve
in the aL~aline digestion liquor. However, although the process has
many attractive features, it has not received commercial accept-
ance because the reaction is quite difficult to control. A non-
uniform degradation of the wood produces cellulose pulp which
. . . . . .
contains excessively degraded carbohydrate materials. Moreover,
access to the interior of the wood is difficult, due to slow mass
kansfer, and it is almost impossible to obtain a thorough pulping of
the wood, and at the conclusion of the digestion a considerable
proportion of the wood remains in the form of slivers, which have to
be separated from the pulp.
Harris U. S. patent No . 2, 673, 148, dated March 23, 1954,
proposed an oxygen digestion process using quite high oxygen pressùres,
of the order of at least 800 psi. This was thought necessary in order
to obtain and maintain a sufficiently high oxygen concentration in the
digestion liquor. This is one of the serious problems in oxygen diges-
tion processes due to the fact that oxygen is a gas which has a low
solubility in the digestion liquor. The results obtained in this process
were not satisfactory.
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Granga~rcl ancl Saunders, U.S. patent No. 2, 926,114, dated
February 23, l9G0, stated that oxygen prior to 1957 had been tried both
at low and at high oxygen pressures. Grangaard et al. proposed a
digestion at pH 7 to 9 over at least a m~jor portion of the cooking
time,under oxygen pressures of 40 to 250 psi, using conventional batch
digesters. The pH is maintained within the desired range by a buffer
such as sodium bicarbonate, or by continuous addition of aLkali such as
sodium hydroxide or sodium carbonate, to neutralIze free acids formed
throughout the digestion. However, the process gives the pulp
unacceptable properties. The main improvement is in the spent liquor.
Other disadvantages are:
(1) An extremely high consumption of sodium bicarbonate.
(2) An extremely high oxygen consumpt.on.
(3) It is difficult to control the process, and a nonuniform
pulp is obtained.
As the Grangaard et al~ patent illustrates, it has not been
possible to develop a practical pulping process using oxygen in place of
sulfide. A recent investigation o~ the oxygen digestion process by J. CO
Lescot, "Essais de delignification de bois feuillus par l'oxygene en milieu
alcalin" (Ph.D. Thesis, Univ. of Grenoble, France, Octo 27, 1967),
resulted in the conclusion that aLkaline oxygen digestion was not feasible
commercially, since the difficulties of impregnation were important,
even when using magnesium oxide as a protectorO
: . ... . ~. . ..
Nonetheless, work has continued, with the objective of
2~ developing the aL~cali-oxygen gas digestion process to a stage where
it is practical commercially.
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U.S. patent No. 3,764,464, patented Octo~er 9, 1973, proposed that
celiulose pulp be prepared from wood by alkaline oxygen digestion in
the presence of alkali metal carbonate or bicarb~nate. A gas phase
including unreacted oxygen gas and carbon dioxide is formed during
the digestion, and the carbon dioxide is separated from the gas phase
by absorption while oxygen gas from which the carbon dioxide has
been separated is recycled for subsequent digestion of wood.
Samuelson stated that in this way the alkaline oxygen
digestion of wood can be controlled so as to inhibit formation slivers,
as well as to prevent undue degradation, thereby to increase the
- uniformity of the cellulose pulp, and improve its color and strength ~
3 properties. In the process, the alkaline digestion liquor comprises ~ ~ -
aL~ali metal bicarbonate or carbonate, or both, oxygen gas is pro- ~ -
vided under pressure to the reaction system, and carbon dioxide that
3 15 is formed and enters the oxygen phase during the digestion process
is separated at least once during the digestion, and preferably either
continuously, or from time to time, so as to maintain a high partial
pressure of oxygen in the gas phase. The carbon dioxide that is
separated can be recovered and used to form alkali metal carbonate,
or bicarbonate, or both, and recycled, and since the resulting process
i~ more efficient, this results in greater economy both of aLkali and .;
of oxygen.
Samuelson and Noreus, U.S. patent No. 3,769,152, :
patented October 3~, 1973, accordingly proposed to digest wood with a
mixture of alkali and oxygen by limiting the amount of alkali at the
beginning of the digestion to at most 75%, and preferably from about
5 to about 20%, of the total molar quantity of alkali required for the
digestion, and to add the alkali progressively, either continuously or
in increments, during the digestion, while maintaining the pH of the
.
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lQ`~;~160
digestion liquor in the course of the digestion within the range from
about 9 . 5 to al)out 13, and preferably from about 9 . 5 to about 11. 5.
Under these conditions, if the amount of allcali is restricted
(unlike prior processes in which all of the alkali required for the
5 digestion is added ab_ itio), condensation of the lignin in the cellulose
is avoided, and thus a darkening due to this condensation is prevented,
while at the same time the cellulose ~s protected against excess
degradation. Moreover, at the lower alkali concentration, the
deleterious effect of oxygen in the production of an oxidative de-
10 gradation of the cellulose in the wood may also be inhibited to someextent and the method can be used for the production of low-viscosity
pulps e.g. rayon pulp.
Attempts also have been made to circumvent the difficulties
encountered with aL~ali oxygen gas digestion processes, by first -
15 predigesting the wood material, then mec~anically defibrating the
wood to provide a semichemical pulp, and then delignifying the pulp with
aL~ali and oxygen gas. Such processes have been designated oxygen gas
digestion processes, but they are properly designated processes for
bleaching semichemical or mechanical pulp. These methods are ex-
20 tremely expensive, and the pulp yield is low.
In accordance with the invention, it has been determined
that the all~aline oxygen gas digestion of wood in coarse particulate
form such as wood chips is facilitated and the properties of the
resulting cellulose pulp improved, if during the digestion process, the
r 25 wood is treated, either continuously or intermittently, in the
presence of the aqueous digestion liquid, so as to detach fibers
from the surface of the wood, the detached fibers suspended in
the digestion li~uor, and removed from the digestion zone; and then
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lV42160
- continuing the cligestion of the remaining wood with oxygen gas and alkali
-; in aqueous solutioll to form cellulose pulp.
The process according to the present in~rention can be applied
both to the continuous digestion of wood and to the batchwise digestion
- 5 of wood. The treatment proceæs for detachment of the fibers can be
applied only once, or it can be continued throughout the digestlon process,
continuously or intermittently, or from time to time. Normally,
it is more advantageous to begin the treatment process only after
the surface of the wood material has been digested, e. g., after
the digestion process has conlinued for a period of time within
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the range from about 1/2 to about ten ho .rs, corresponding to la
propo~tion witbin the range from about 20% to about 70% of the total
digestio~ time requirèd. -
A variety of known physical te~hniques can be applied
15 to the surface of the wood to detach fibers therefrom. Liquids ormechanical tools can be used. The treatment is applied to the sur-
face of the wood only; it is not intended to penetrate deeply into the
wood particles or to defibrate or disintegrate the particles in their
entirety. A depth of from a fraction of a millimeter, corresponding
20 to one or a few fiber diameters, up to ~ f~w millimeters, e. g. 2 mm
is quite sufficient. The treated particles retain their particulate
identity, and do not in any way correspond to semichemical pulp.
One way of using a liquid to tear away the surface of
the particles is to direct jets or streams of liquid against the wood
25 with such a force that surface fibers are detached. As digestion
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1(~42160
proceeds by attack on the wood particle surface, fibers are fully or
partially exposed. Pressurized high velocity streams of fluid
forced under pressure through a mass or bed of such wood particles
will detach SUCIl fibers. For this purpose, the digestion liquor itself
or any liquor obtained from any oxygen gas-aLkali treatment can be
used.
It is also possible to dètach surface fibers by directing a
gaseous stream,blast,or jet ,preferably of oxygen gas,at elevated
- pressure against the wood material or through a bed thereof.
Application of gaseous streams is suitably effected in the presence
` of digestion liquor.
It has been found to be particularly advantageous to -;
effect a simultaneous treatment of the wood material with both
liquid and gas. This treament is suitably applied to partially digested
wood material in a mass or bed. Irrespective of whether the process
is effected solely with liquid, solely withgas, or with both, the height
of the mass or bed may, to advantage, be between 0. 5 and 5 meters. -
Larger or smaller heights can be used, however. The liquid or
gas should be applied with such force that the mass bed is fluffed up or
stirred, and the wood particles tumbled or otherwise moved about
slowly or rapidly. ;
Another way to detach fibers is to apply mechanical
1 cutting, tearing, abrading or gouging to the wood surface. A
.J mechanical tool is applied with insufficient force to disintegrate the
-~ 25 particles. The mechanical tool can be applied together with or insequence totreatment with gas and/or liquid, as described above.
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fibers in the liquor. 10421~0
It is impol~tant that the detached fibers l~e separated from
the remaining wood material in the digestion zone The term
"digestion zone" refers to the zone or apparatus in which the digestion
of the wood is carried out, and includes the actual digester apparatus
in which the digestion of wood with o~ygen and alkali takes place, or
the zone or zones of an apparatus system in which such digestion ta}~es
place. The separated fibers are not subjected to unnecessary de-
' composition in the digestion liquor, and can, if desired, be treat.~d separately
with aLl~ali and oxygen gas under conditions more appropriate to
- fibers. Unexpectedly, and possibly owing to several interrelated
-' factors, the continued digestion of the remaining wood material in
the digestion zone is greatly facilitated, and the selectivity in the
J process (defined as the quotient between the rate of delignification
and the rate of cellulose decomposition) noticeably increases. This
appears to be the direct result of detaching fibers from the wood
material.
After the fiber suspension has been removed from the
digestion zone, the fibers are separated from the digestion'liquor.
This can be effected with or without cooling of the suspension. The
fibers can be separated from the liquor in one or more stages,
using known devices, or combinations thereof. It is particularly
advantageous to fractionate the fiber suspension into two or more
fractions having different fiber contents, suitably, into a completeb
or substantially completely fiber-free liquid fraction, and a
fraction which is rich in fibers. In accordance with a preferred
' embodiment, this fractionation is effected by centrifugal separation
.
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Satisractory tools include agitators whicll work the
partially digested woocl ma~erial to release the fibers, and/or
pump means, such as centrifugal pumps or so-called high-con-
centration pumps for pulp pumping, ~d other known devices for
5 detaching exposed fibers from non-digested or poorly digested
particulate wood material.
Abrasion methods can àlso be used for detaching fibers
in accordance with the invention. The wood material is preferably
in the form of chips, and by suitable means the chip pieces- are caused
10 to rub against each other, and thus abrade their surfaces. Abrasion
can be effected, for example, by using a rotary digester. This latter
method is particularly suitable for application in conjunction with
batch digestion processes. Methods based on rubbing or other
abrasion techniques to detach the fibers can be, to advantage, com-
lS bined with one or more of the previously mentioned treatment methods.
Vibration is a particularly effective method for detaching -
fibers. One or more vibrators can be placed at appropriate positions
in the digester apparatus. Another suitable vibration method is to pass
the partially digested material over a vibrating table or a vibrating
20 screen. The vibration methods can also be combined advantageously
with the other previously mentioned methods for detaching fibers
during the digestion process from the partially digested wood material.
The detachment of the fibers from the surface of the wood
particles is preferably though not necessarily carried out in the
25 presence of the alkali-oxygen gas digestion liquor. The liquor then
serves as a suspending medium for the detached fibers. The de-
taching technique can aid in dispersing and suspending the detached
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1042160
processes, suital~ly by using hydrocyclones and/or separatore, or
other continuously oper.lted centrifuges.
Screens or filters can also be used to remove the fibers
or a fiber concentrate from the suspension.
For thermal technology reactions and also recovery of
the fibers from the suspension as soon as possible, it is often suitable
to maintain the fiber suspension at~a high temperature during the
separation,su~h as at at most 10 lower than the temperature of the
suspension when it was removed from the digestion zone.
Flotation and sedimentation techniques ma~ also be used,
in a manner known per se, to effect the fiber separation or as a
step therein.
3 It has been found particularly suitable to fractionate the
suspension into a fiber-rich suspension and a fiber-lean or fiber-
free suspension in an array of hydrocyclones operating at a temperature
which approximates the temperature of the digestion liquor. The ~
fiber-rich suspension is then subjected to a further separation in one
or more steps, to separate the fibers therefrom, in a manner kno~vn
per se, for-example by screening and filtering. At least the last
step can be effected to advantage subsequent to lowering the temperature.
This lowe~ing of the temperature can be continued down to a
temperature, forexample, of 60-90~C., sothatsaidseparation
process can be effected without disadvantage at atmospheric pressure.
The separated liquid fraction, which is completely or sub-
stantially free of fibers, can be recycled for use as a digestion liquor in
oxygen gas-alkali digestion processes. Thus, the liquor can be re-
cycled to the same digestion process as that from which it was re-
moYed, or it can be used in a different digestion charge, or it can be
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charged to another cligestion zone. It can also be used as treatment
liquid in the hydraulic treatment of wood material Ior releasing
fibers in accordance with the invention.
Before recycling, the alkali content and the content of
any digestion adjuncts, such as degradation inhibitors, stabilizers,
and buffering agents, can be replenished or modified in any way,
and the pH adjusted, if necessary, to within the desired range.
Subsequent to separating the fibers, but prior to returning
the liquid suspending medium to the oxygen gas digestion process, it
may be suitable to heat the liquor, preferably to a temperature which
is higher than that prevailing during the digestion process. In con-
junction therewith, it is convenient to maintain the liquor in intimate
contact with oxygen gas or air, optionally in the presence of a catalyst
which expedites the destruction of readily oxidized substances. I
1i 15 The oxygen digestion process of the invention is applicable
, to any kind of wood. In general, hardwood such as asp~n and birch can
be pulped more easily than softwood, such as spruce and pine, but both
types of wood can be pulped satisfactorily using this process. Exemp-
lary hardwoods which can be pulped include birch, beech, poplar,
` 20 cherry, sycamore, hickory, ash, oak, chestnut, aspen, maple, alder
' and eucalyptus. Exemplary softwoods include spruce, fir, pine, cedar,
~uniper and hemlock.
In the case of softwood, the processing conditions, including
the particle size of the wood fragments, the digestion temperaiNre, the
aL~ali concentration, and the oxygen pressure, should be carefully de-
termined and controlled during the digestion.
. 11
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The wood should be in particulate form. Wood chips having
dimensions that are conventionally employed in the sulfate process can
bn used. However, appreciable advantages with respect to uniformity
of the digestion process under all kinds of reaction conditions within
the stated ranges can be obtained if the wood is in the form of nonuni-
form fragments of the type of wood shavings or chips having an aver-
age thickness of at most 3 mm., e.g. about 0.2 to about 2 mm.
Other dimensions are not critical. Wood slivers and splinters,
wood granules, and wood chunks, and other types of wood fragments
can also be used.
The process of the invention can be eEfected to advantage
on wood which, prior to being digested with oxygen gas and aL~ali,
. ~ - . . , . - . .
- has been subjected to a chemical pretreatment process with acid,
~ . . . . . . . .
- neutral and ~ or alkaline aqueous solutions . The pretreatment process5 - i& suitably effected at elevated temperatures, for example, within
the range from about 100 C to about 200 C., so that at least partial
deacetylatlon of the wood is obtained. Treatment in an acid en-
vironment with acid-additives or solely water can-be effected- to ad-
vantage if pulps are desired which have a low content of hemi-
cellulose, that is to say, pulps with relatively low yield.
When producing paper pulps having a relatively high yield,
for example, from 50 to 70 kg unbleached pulp per 100 kg dry wood,
it is particularly suitable to pretreat the wood material with a liquid
containing basic neutralizing agents at a temperature within the range
12
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1~14;~160
from about 12U to al:out 2ûO~C., preferably from 130 to 1803C.
The basic neutralizing agent used may be the came alkali used in
the oxygen gas-alkali digestion process. It has been found particularly
suitable to use sodium bicarbonate, although sodium carl)onate and/or
5 mixtures thereof with sodium bicarbonate have been found to give
good results.
It has been found suitable to de-gas carbon dioxide fbrmed
during the pretreatment process.
The reaction time during the pretreatment process is
10 suitably adapted so that the alkali added to said process is not totally
consumed, so that the solution has a certain buffer effect, so as to
avoid acid hydrolysis, unless otherwise desired. An addition of from
, 5 to 30% by weight sodium bicarbonate, calculated on the dry weight
~ of the wood, and a tre~tment time of from 0. 5 to ~ hours at from
- 15 120 to 180 C is particularly preferred.
As a result of the pretreatment process, the chips are often
dark brown or black in color, although this color rapidly disappears
during the subsequent oxygen gas aLkali digestion process. In
combination with the method of the present invention, the pretreatment
20 process affords surprising advantages with respect to the selectivity
of delignification, and, additionally, results in an appreciable
shortening of the time required for the oxygen gas-alkali digestion
process.
When preparing the pretreatment liquid, it is an advantage
25 to use digestion liquor from the oxygen gas-alkali digestion process,
said liquor being removed at the end of a cooking sequence or during
the course of such a sequence.
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The total amount of alkali that is required for the digestion
is determined by the quality and type of the pulp to be produced, and
is within the range from about l to about 10 kilomoles per 1000 kg.
of dry wood. It is well known that certain types of pulp are more
5 digested than others. This is entirely conventional, and does not
form a part of the instant invention. Cellulose pulps intended to be
used in the production of regenerated cellulose fibers, such as vis-
cose, acetate and cuprammonium pulps, are quite fully digested,
and should have a low content of lignin and hemicellulose. In the
10 production of such pulps, in accordance with the process of the
invention, the amount of aLkali can be within the range from about
6 to about 8 kilomoles calculated as NaOH per 1000 kg. of dry wood.
For the production of bright paper pulp, which is readily defibered when
the digester is blown, the amount of alXali used in the process of the
15 invention can be within the range from about 2 . 5 to about 5 kilomoles.
Generally, for most of the types of pulps given an intermediate degree
of digestion, such as pulps for fine paper~ plastic fillers, and soft
paper or tissue paper, the amount of alkali in the process of the
invention is within the range from about 2 to a~out 6 kilomoles per
20 1000 kg. of dry wood.
Any alkali metal hydroxide or a3kali metal carbonate can
be employed, such as sodium hydroxide, potassium hydroxide, ~
. .
lithium hydroxide, sodium carbonate, potassium carbonate and lithium
carbonate. The sodium carbonate o~tained in the burning of cellulose
25 digestion waste liquors can be used for this purpose. The use of alkali
..... ..
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metal car~onate~ alld/or bicarbonates may be more advanta~eous than the
use of alkali metal hydroxides in maintaining the pH of the digestion liquor
within the staged range, because of the buffering properties of the
carbonate or bicarbonate present or formed in situ. Consequently,
5 mixtures of aL~ali metal hydroxides and a~ali metal carbonates are
particularly satisfactory to obtain the advantages of each, and dilute their
disadvantages .
It is also possible to use mixtures with all~ali metal hydroxides or
carbonates with aLl~ali metal bicarbonates such as sodium bicarbonate and
10 potassium bicarbonate. The aLkali metal bicarbonate in this case serves
as a buffer. Other bu~fering agents, compounds of alkali metals with
nondeleterious acidic anions, can be employed, such as a~ali metal acid
phosphates.
A buffering agent, particularly a bicarbonate, is especially
15 desirable when it is desired to operate at a relatively low pH, for example,
-`~ from about 6. 5 to about 10. In this case, bicarbonate or other buffering
- agent can be added to advantage even if a~ali metal carbonate is present.
For economic reasons, the sodium compounds are preferred as
the alkali metal hydroxide, a~ali metal carbonate and aLkali metal
20 bicarbonate.
All of the aL'cali required for the digestion can be added to the
digestion liquor initially. Limiting the amount of alkali metal hydroxide
and /or aLkali metal carbonate in the initial stages of the process
improves the quality of the cellulose pulp, both if the digestion i8 at a
25 high pN, in excess of 1(1or at a low pH.
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.. . .
If a mixture o~a~ka;~l me~)al hydroxide and aLlcali metal
carbonate is used, it is particularly suitable if the initial charge
comprises sodium carbonate, optionally with an addition of sodium
bicarbonate as described above, the remainder of the a~kali added
as the digestion proceeds being sodium hydroxide. If the aLkali
-
charge initially is al~ali metal hydroxide, it is usually important in
producing pulps having a low lignin content that the initial charge be
low, within the range from about 2 to about l 0%, of the total molar
quantity of aLkali.
Whether or not the digestion process is carried out contin-
uously or as a batch process, the alkali metal hydroxide and/or alkali
metal carbonate can be charged continuously or in increments-to the
digestion liquor. In a continuous digestion, the wood is caused to
' move through the digester from one end to the other which thereby
constitutes a reaction zone In a batch process, the wood, usually in
the form of chips, is retained in the reaction vessel throughout the
digestion.
The oxygen gas digestion process can be effected, for
example, with sodium hydroxide as an active allcali at a pH exceeding
10, wherewith it is advantageous to add the active alkali incrementally
as it iS consumed. For the majority of wood types and pulp qualities,
s it is more advantageous however to maintain the pH during the major
~ . ....... _ _ .. . .......................... . . . ..
portion of the digestion process with oxygen gas within the range from
3 about 6. 5 to about 10. When producing paper pulp from hardwood, it
25 has been found that much higher mechanical strength properties are
obtained if the pH lies within the range from about 7 to about 9.
.
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~()4;~160
All pH-values refer to determination at 20 C on a sample withdrawn
from the cligester through a cooler. Even in this case it is suitable to
- add the active alkali successively as it is consumed. The active aLkali
used is pre~erably sodium carbonate and/ or sodium bicarbonate and
mixtures therof. The developed carbon dioxide retards the reaction,
since it contributes to a decrease in pH. Carbon dioxide should
normally be removed, e.g. by bleeding together with oxygen gas, by
absorption, cooling or in accordance with other known methods.
Since the oxygen that is employed as an essential compon-
10 ent in the digestion process of the invention is a gas, the so-called
gas phase digestion procedure can be used to advantage. In this case,
- the wood and the film of digestion liquor present on the wood are kept
in continuous contact with the oxygen-containing gas. E the wood is
completely or substantially immersed in the digestion liquor, it is
s 15 important to agritate the wood and/or the gas and/or atomize the gas3 or the liquor. The oxygen should be dissolved or dispersed in the
digestion liquor to the greatest extent possible. Dissolution or dis- -
? persion of the oxygen in the liquor can take place within the digestion
vesseI and/or externally of the same, such as in nozzles, containers
or other known devices used for dissolving or dispersing gases in ;~
liquids.
Transfer of oxygen to the wood material impregnated with
.
digestion liquor is important in the process, and is controlled by ; ~ -
adjusting the oxygen pressure? the digestion tempsrature, and/or the
. ~
17
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propurtion of gas-liquid contact surfaces, includ~ng the wood impreg-
nated with digec;tion liquor.
The oxygen is preferably employed as pure oxygen? but
mixtures of oxygen with other inert gases c~n be used, such as, for
5 example, mixtures of oxygen with nitrogen and with carbon dioxide
and with both, as well as air. Compressed air can also be used,
although this complicates the devices for dissolving or dispersing
the oxygen in the reaction mixture.
Prior to contact with the oxygen, the wood suitably in the
10 form of chips can be impregnated with an aqueous digestion liquor
containing the desired chemicals. The chips are impregnated under
vacuum, or under atmospheric pressure or superatmospheric pres-
sure, or by other methods conventional in wood digestion processes.
The wood may also be treated with steam before being brought to the
1~ digestion zone.
The temperature employed during the impregnation can be
w~thin the range from about 20 to about 120 C., although temperatures
within the range from 90 to 120C. would not normally be used except
under special circumstances. ~ -
The digestion can be carried out at a temperature within
the range from about 100 to about 180 C. Usually, it is advantageous
if the digestion temperature is permitted to rise during the digestion
process from an initial temperature of the order of from 100 to 120 C.
.
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to the maYimum digestion~;emperature, of the order of from 150
or 175~C.
At a maximum temperature of 100 C., the digestion pro-
cess proceeds slowly, hut on the other hand, moderate oxygen pressure
s 5 and simple technical apparatus can be used.
If a maximum digestion temperature of from 150 to 180 C
is used, the digestion will proceed rapidly. On the other hand,
selectivity is relatively poor. At these temperatures, an exceedingly
effective transfer of oxygen to the wood from the gas phase is re-
quired. This requires intimate contact and high oxygen pressure. By
effective control methods, however, all of which are conventional, it
is possible to control the digestion within this temperature range,
particularly when producing cellulose pulp of moderate yield.
Normally, a digestion temperature during the major part ;~
of the digestion process withinthe range from-120 to 16ûC. is preferred,
at which temperature the digestion can take place in a reasonable time
using relatively simple apparatus and under moderate oxygen pressure,
with good control of pulp quality, and good selectivity. A relatively
rapid reaction and a very high degree of selectivity is obtained within ;the temperature range of from- 135 to 150 C. For the majority of
wood materials and pulp types, this latter temperature range is con-
sidered the optimum range, when the pH is within the range 6. 5 - 10.
A wide range of partial pres~ures of oxygen can be used.
Normally, it is possible at pressures to work within the range oi
from about 1 to about 300 bars, although in practice the range of
from 5 to 100 bars is more feasible. The optimum pressure range
lies in many instances at from 10 to about 50 bars, and very often
at from 20 to about 40 bars.
19 :
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. .. . - . . . .
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Tt has been found that an improved degree of selectivity is
obtained when using a high partial pressure with respect to oxygen
gas. Economic and safety reasons place an upper limit, the position
- of which depends, for example, on the raw material and test~ of materials.
Because of the consumption of oxygen in the course of the
digestion, and the higher rate at wpich the digestion proceeds at high
reaction temperatures, it follows that the higher the reaction temper-
ature, the higher the pressure that should be applied during the reac-
tion. The optimum temperature and pressure conditions for a given
pulp can be determined by digestion sampling procedures, as is well
J known. Such trial-and-error experimentation is conventional, and is
not a part of this invention.
Pulps for a certain field of use, for example, for use in
the production of most types of paper, should have a high strength. In
such cases, it is suitable to carry out the digestion in the presence
of an inhibitor or mixture of inhibitors which protect the cellulose
and hemicellulose molecules against uncontrolled degradation. The
effect of the inhibitors is reflected by the viscosity of the pulp, and
the degree of polymerization of the cellulose.
The inhibitors can to advantage be charged to the digestion
liquor during an early stage of the digestion,prefarably, at the
beginning, before the digestion heating is begun. Thus, they can be
added to the digestion liguor before combination with the wood, or
shortly thereafter. Suitable inhibitors are water-insoluble magnesium
-compounds such as magnesium carbonate, waterffoluble magnesiumcompounas, such as magnesium sulphate,and complex magnesium
compounds.
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~ 4;~160
It is oEtcn suitable during the digestion to withdraw a portion
of the di~ estion liquor, SUCil as by draining, pressing, displacement or
f-ltering. This liquor can be returned to the digestion process at a
later stage, or to a subsequent batch, and in this event it is advan~age-
O~lS to heat the liquid or a part thereof under pressure to an elevated
temp~rature of the order of from 110 to about 200C. in intimate con-
tact with an o~Tgen-containing gas such as air in order to oxidize
organic substances in the liquor. The liquor can be fortified by
adding alkali metal hydroxide and/or all~ali metal carbonate and/or
inhlbitor before or after pressure-heating.
It may be desirable to wash the wood with water between
s the pretreatment stage and the oxygen digestion process. Thi~ wash-
ing step may be desirable in the case of any of the pretreatment proc~
esses described above. The washing, however, increases the cost of
the processing, and a~o increases the risk of water contamination of
the pulp with metal ions and metal compounds, and consequently it may
.
often be more practical to omit the washing step
A surface-active agent can be added to the digestion liquor,
and contributes to a reduction in the resin content of the wood cellu-
lose produced from the wood. This also surprisingly contributeæ to
a reduction in the lignin content, and a more uniform delignification.
The æurface-active agent is suitably added at the beginning of the
digestion process, or during an early stage of the digestion, and may
J be present during all or only a part of the digestion. Cationic, anionic,
21
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...
.. .
iO4~ 0
and nonionic surf~ce-active agents and mixtures thereof can be used.
If liquor is circulated during the digestion proces;s, it is suitable to
use agcnts which do not produce foam. Examples of suitable surface-
active agents are polyaL~cylene glycol ethers of fatty alcohols and alkyl
5 phenol polyoxyallcylene glycol ethers. Sulfonated anionic surface-
active agents such as the aL'cylbenzene sulfonates can also be used.
The unbleached pulp has a high brightness. With birch pulps
a brightness of 52~C according to SCAN is obtained, with a
Kappa number of 15 and a total yield of 58%. With a Kappa number
10 of ~, a yield of 56~G and a brightness of 61~/c according to SCAN has
been obtained. With spruce, there was obtained a SCAN brightness ;
of 50~ at a Kappa number of 19 and a yield of 54(3;~G by weight.
The pulps can be used directly with no bleaching and are
comparable with semi bleached sulphate pulp and unbleached
1 ~i sulphite pulp, and can be used in place of these for different fields
of use.
For example, they can be used to advantage directly for
producing tissue paper, light cardboard and magazine paper. When
a higher degree of brightness is desired, as for fine paper, rayon
20 and cellulose derivatives, the pulp can easily be bleached in accord-
ance with known methods by treatment with chlorine, chlorine di-
oxide, chlorite, hypochlorite, peroxide, peracetate, oxygen or any
combinations of these bleaching agents in one or more bleaching
sequences as described in Canadian patent No. 901, 220 issued
22
~ . , .
1~)4Z160
May 30, 1972. Chlorine dioxide has been found to be a particularly
suita~le bleaching agent for the o~ygen digested cellulose pulp obtained
in accordance with this invention. The consumption of bleaching
chemicals is markedly lower in bleaching oxygen digested
pulps of the invention than when bleaching sulfate cellulose.
The chemicals used for the digestion process can be
recovered after the waste liquor is burned and subsequent to optionally
causticizing all or part of the carbonate obtained when burning the
liquor.
- ïo The method is particuiarly suitable for application in the
continuous manufacture of wood cellulose pulp, since, owing to the
fact that detached fibers are removed, the whole volume of the
digestion zone is available for use. To enable the apparatus to be
used in the most effective manner, with batchwise digestion processes,
15 ~ it is suitable to interrupt the oxygen gas digestion process whilst a
certain quantity of partially digested chips (so-called shives) still
remain (for example 5 to lO~o by weight of the wood material), and
~, to separately convert the shives to pulp with exposed fibers, e. g.
by a separate oxygen gas digestion process, optionally subsequent
to mechanically treating the shives.
A preferred embodiment of the digestion process of the
invention and of the cellulose pulp of the invention is shown in the
following Example:
23
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,. ,: . . - .. . , . .. ~ . . . .
104~ ;0
Cornmercially available birch chips about 5 mm thick and Z5 mm
long were suhjected to a pretreatment process with 20% by weight
aqueous sodium bicarbonate at 160 C for 2 hours at a ratio of wood:
liquid of 1:5 (kg/dm~3). The treatment process was carried out in an
5 autoclave. The pressure in the autoclave was lowered every half-hour
to almost atmospheric pressure, to remove expelled carbon dioxide.
During the pretreatment process readily soluble hemicellulose was re-
moved and the wood deacetylated. Subsequent to this treatment process,
the yield was 85%, calculated on the dry weight of the wood, and the chips
` 10 wereblack in color.
The chips were subjected to an aLkali oxygen gas digestion in an
autocIave digester in which a circulating aqueous digestion liquor pre- -
pared from sodium bicarbonate was sprayed over the chips. The wood:
liquor ratio was 1:14. The oxygen gas pressure was 9 bars and the
15 temperature 140C. During the digestion process, saturated aqueous
sodium bicarbonate solution was added, and carbon dioxide was removed
continuously, so that the pH measured on cold (20 C) digestion liquid
withdrawn from the digester-under cooling was maintained between 7. 5 and
and 8. The amount of sodium bicarbonate charged was 1. 8~o by weight
20 during the course of the digestion, calculated on the dry weight of the
ingoin~ wood.
After 3 hours digestion, digestion liquor was forced up through
the bed of partialb digested chips at a speed such as to cause the chips in
the bed to tumble about. In this way, fibers which were liberated during
25 the digestion were detached from the chips, and passed into suspension in
the digestion liquor. ~ ~ -
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,
24
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--" 104;~160
T}l~ liqu~r was contilluously withdrawn from the digester and
fractionated in ~n array Or hydrocyclones which separated the
suspension into a fiber-lean and fiber-rich fraction. The input
suspension had a pulp concentration of 0. 5% by weight, and the
fiber-rich suspension had a pulp concentration of 1. 5% by weight.
The fiber lean fraction was recycled by means of a
centrifugal pump to the digester and circulated through the bed up-
wardly from the bottom of the bed. This hydraulic tumbling of the
chip bed was continued for 10 minutes, during which time the digestion
apparatus was under 9 bars oxygen gas pressure. ~ ~ -
The fiber-rich fraction was passed through a pressurized
dewaterer. The filtrate obtained was recycled to the digester. The
pulp was washed. The process of forcing the digestion liquor from
below through the bed of partially digested chips and the fiber
15 separation steps in accordance with the foregoing were repeated at
intervals of 30 minutes, until a total digestion time of 5. 5 hours had
elapsed. The pressure was then lowered, and the remaining pulp
and partially digested wood were blown out. The pulp was screened
and washed. The shives were subjected to continuous oxygen gas
20 digestion.
Laboaratory tests on the resulting birch chips showed that ~ -
after an elapsed digestion time of 3 hours, 26. 5% by weight of
the original wood was obtained as pulp in the suspension, while
42. ~% was present as incompletely digested chips. After 3. 5 hours,
lV42160
the corresponding figures were 38. 9% and 30. 0%. The viscosity
of the pulp recovered from the suspension was 910 cm3/g,
g according to SCAN. The quantity of alkali-resistant pulp
(determined in 5% NaOH according to SCAN (R5 value) was 68. 9%
by weight, and the viscosity of the alkali-resistant plllp was
1268 cm /g.
At the end of the digestion time (i. e. after a total digestion
time of 5. 5 hours) the total yield of screened pulp was 54. 9~o. The
viscosity of the pulp was 880 cm /g. The R5 value was 70. 0 and the
viscosity of the aLt~ali-resistant pulp was 1200 cm /g. In addition,
there was ob~ained 6. 2% shives calculated on the original dry wood.
After the shives had been digested for 2 hours, a further
3% of screened pulp was recovered.
;~ The total pulp-yield was thus 57. 9% by weight, calculated
.
on the dry, commercially available birch chips. The results from
determinations on the viscosity of the pulp extracted with 5% NaO~
æhowed that the method results in but slight attach on the cellulose
molecules, as compared wlth other methods of oxygen gas
digestion.
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26
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