Note: Descriptions are shown in the official language in which they were submitted.
~057007
FIELD OF THE INVENTION
The present invention relates to the impregnation
of wood particle~ or like lignocellulosic material. It
relates more particularly to the impregnating of small
particles of lignocellulose material for subsequent con-
~ersion to pulp.
DESCRIPTION OF THE PRIOR ART
AR a first stage in the production of pulp, wood
has been conventionally reduced to particles referred to
as chips. These chip~ are produced by the action of knives
which cut the log at an angle and reduce it to lengths that
may ~e set in the range of 5/8~ to 1-~ but are normally
set to about 3/4~. The wedge action of the knife results
in splitting of the wood with the grain, and the thickness
and width of these chips are somewhat random, the thick-
ness being in the range of about 1~ mm to as much as 8 mm.
In addition, splintexs and 'chipper dust' are formed in
variable amounts depending on sharpness of knive~, wood
moisturQ and other factors.
The dige~tion stage consi3ts of treating the
chips at elevated temperature and pressure with a solution
of a chemical capable of dissolving the lignin and other
materials which bind the fibres together in the wood struc-
ture. In the krat process, the digestion chemical consists
of a solution of sodium hydroxide and sodium sulfide. This
digQstion liquor is obtained by a recycling process which
involves evaporation and combustion of the residual liquor
follo~ed by treatment of a solution of the inorganic com-
bustion re~idue with lime. With batch digestion ~nd using
chips produced from spruce approximately 16 grams of 'effec-
tive alkali' expres~ed as Na20 are required per 100 grams
dry weight of wood. The concentration of effective alkali
1~57(~)7
(i.e. ~he sum of NaOH + ~ Na2S) expres~ed as Na20 in the
cooking liquor i8 nonmally about 90 grams per litre, and in
this casQ 178 cc of liquor are therefore required per 100
gram~ wood.
S If the wood contains 50% moisture, the total
volume of liquid, including the water in the wood, ig 178 -
100 278 cc per 100 g wood. Because of the porous nature
of wood, about 200 cc can be held within 100 grams of the
wood structure. When the digestion liquor is added to the
chips, the alkali is ~trongly absorbea by the chips first
contacted and it has been found that in order to at least
partially equalize the di~tribution of chemical throughout
the chip ma~8~ good circulation is required. It has been
found that in order to establish adequate circulation for
this purpose, a total volume of approximately 400 cc total
liquid per 100 grams chip~ is necessary. Since the reaqent
plus initial moisture in the chips is 278 cc, an additional
122 cc o dilution liquid is necessary and resldual liquor
from a previou~ cook i~ normally used. Digestion is
carried out at elevated temperature, i.e. approximately
175C. About one half of the digestion chemical i~ inside
the chip~ with the balance as 'free' liquor outside. A~
the alkali in~ide the chlps is depleted through react~on
with lignln and other wood substances, it i~ replenished in
part by diffusion of alkali from the surrounding liquor.
The rate of pulping by thi8 method is controlled by the rate
of diffu~ion of chemical into the chip~ during digestlon.
The total time for batch digestion for the production of
bleachable kraft pulp i~ approximately 1 hour for bringing
to temperature and a further hour and 30 minutes to complete
the digestionO
Conventional continuous pulping system~ such as a
Xymar system are essentially adaptations of the batch
system.
- 3 -
.
^: ~0S7007
An improved method of pulping has been described
by Tomlinson (Canada Patent 721,960 issued November 23,
1965) whereby the chips are fir~t continuously impregnated
by recycling an impregnation liquor through a mass of chips
in an impregnating zone at a temperature below the cooking
temperature ~i.e. at 150C), such that the chips are im-
pregnated with an adequate amount of chemical to complete
the pulping. The chip~ are then lifted from the impregnating
liquor and are dropped into a cooking zone where the diges-
tion i8 carried out in the absence of 'free' liquor at a
temperature of 180C in a time of 15 to 25 minute~. This
rapid digest$on time is possible because all of the chemical
i8 in place in the chips before the dige~tion commences,
with the result that the 910w diffusion from the surrounding
liquor as the che~ical i~ depleted, which i8 characteristic
of conventional pulping, i8 not a factor. In addition, le~s
effective alkali is normally required, i.e. approximately
13~ to 14% versus 15% to 16~ for conventional production of
chemical pulp. The low liquor to wood ratio in the diges-
tion, a~ compared to conventional pulping, results in alower ~team requirement for cooking, and the resultant
higher concentration of di~solved solids in the residual
liquor re~ults in a decreased steam requirement for evapora-
tion in the chemical plant. The lower digestion chemical
requirement give~ a higher organic to inorganic ratio in the
residual liquor which is an advantage in the combustion
process involved in recovery.
Spec$al cond$tions must be maintained in the im- -
pregnation zone in order to place sufficient chemical in
the chips in order to carry out the digestion. This is
accomplished by passing the chips continuously through an
impregnation vessel at a controlled rate and contacting them
with a liquor which i~ passed through the vessel at a sub-
~0 57 ~ 0 7
~tantially higher rate, e.g. at a rate 5 to 20 times grea~erthan that of the chips. To accomplish this, impregnation
liquor i~ extracted from the impregnator through a screen
and then returned to the impregnator after it has baen re-
inforced with the correct quantity of chemical, e.g. 14effectlve Na20 on wood, and its liquid content has
been adjusted so that the net imput of liquid to the
impregnation sy~tem equals that required to completely
saturate the chips. When using wood having normal moi~ture
content, the total liquid imput from the chips plu~ that
of reinforcing chemical liquor is normally greater than
that which can be ta~en up by the chips and the resultant
recycled liquor must be partially evaporated as de~crlbed
in the aforesaid patent in order to maintain the correct
water balance. Because of the di~tinct separation of
impregnation and digestion 6teps, only the liquid satura-
ting the chips carries into the digestion stage and there-
fore the volume of recycled impregnation liquor will
increase unle~s controlled by evaporation. In contrast
to this, with the convention~l system 'free' impregnation
liquor carries directly from the impregnation stage into the
dige~tion stage, and the digestion liquor must be diluted
with black liquor in order to have sufficient volume to get
adequate circulation for redistribution of alkali during the
impregnation stage.
The h~gh rate of liquor circulation i.e. S to 20
time~ greater than the chips results in a relatively ~mall
variation in chemical concentration of the liquor entering
and leaving the impregnator 80 that the driving force
tending to drive chem~cal ~nto the chip remains substantially
constant throughout the impregnation stage. Moreover the
chemical concentration in the impregnator i8 ~ubstantially
lower tha~ the incoming white liquor in as much as it i8
- 5 -
-~` 1057007
diluted by the relatively large volume ratio of the depleted
recyeling liquor.
A disadvantage of said Tomlinson digestion pro-
cess i8 that thick chips may not be completely impregnated
and these can result in reject material such a~ knotter re-
~ects and ~hive~ amounting to 2~ to 5% depending on the
distribution of chip thicknes~. With small particle~, such
as sawdust or normal chips which have been broken down to
smaller particle~ (by passing through an attrition mill or
the like) these rejects may be reduced to a low level of
le88 than 1%. Unfortunately, it has not been found
possible on an industrial scale to maintain circulation
through ~uch wood subdivision because of an increased
plugging of the screens after start-up which cannot be re-
opened on a practical continuous basi~ by periodic 'baek-
washing' or other means found effec~ive when dealing with
eonventlonal chip~.
When wood chips are contacted with an alkaline liquor
the alkali is very rapidly absorbed by the chips. In the ease
of Kraft liquors this results in selective ab~orption of alkali
into and on the chips and the accumulation of sulfide in the
impregnation zone. Uniform impregnation of the chips is
diffieult. The chips which first eontact the alkali absorb
lt avidly causing rapid depletion of the alkali in the
liquor and reduced availability of alkali for chips further
down along the path of the liquor. Thus some chips will have
ab~orbed too much chemical and some too little and in subse-
quent cooking ~ome chips will be overcooked snd some will not
delignify suffieiently. It i~ for this reason that with
eonventional pulping of chip~ the liquor is diluted by the
addition of black liquor to permit relative circulation of
the liquor through the chip mass and thereby obtain better
equal$zation of chemical. The problem is, or course, ag-
- 6 -
''
57007
gravated when the chips are small particles of wood, ~uch
as sawdust or shavings or chip~ specially prepared for
ea~y penetration of cooking liquor, such as thin wafers,
crushed chips, pin chips, wood chips have been broken down
in a disc refiner and the like material. Generally such
small chips will have their minimum dimension no greater
than 3mm. Chips of thi~ type, which will be referred to
generally herein as small chips, have a large ~urface in rela-
tion to volume and because of the above-mentioned affinity of
wood to alkali, will deplete the impregnation liquor particu-
larly rapidly and the difference in concentration of chemical
w~ll be very pronounced between one end and the other of the
impregnation zone.
In sawdust digesters recirculation of liquor ha~
been found to be impossible due to the fine wood particles t
plugging the screens and the resulting uneven distribution
of chemical in the wood results in the formation of ~bird
seed~ i.e. incompletely digested particles of wood. In
attempting to compensate for the problem normally more
chemical is used in sawdust digestion than in chip digestion
with a resultant decrease in pulp yield.
aRIEE DESCR~PTION OP THE INVENTION
The present invention provides a method for more
uniform impregnation of small chips with sufficient chemical
for subsequent cooking preferably in a steam atmosphere.
The present invention eliminates the need for
screen~ for the ~xtraction of recycled impregnation liquor
from a pressurized vessel and yet has all the proce~sing
advantages found for the aforesaid patent. This allows
the use of such materials a~ sawdust and shavings, pin-
chips, thin wafers or chips which have been broken down
in thickness by passing through an attrition mil1. It
- 7 -
~L~57~07
also allows the production of ~uality pulp from such
materials as board end~, cuttings from furniture, hockey
sticks, etc., ~hich cannot be fed through conventional
chippers. This later material can then be 'hogged' and then
further broken down to small particles by means of a
disc mill or the like.
~ he present invention i8 particularly ~uited for
small chips and it is contemplated that regular chips may
be crushed or otherwise reduced or specially prepared for
easy penetration of liquor for use with the present inven-
tion.
The fines, pin chips etc. often contained in con-
ventional chip~ give problems in extraction of the liquor
from convQntional digesters, particularly continuous di-
lS ge~ter~. It will be seen that with the pre~ent inventionsuch problems (screen plugging) are eliminated and a ~ub-
stantially uniform pulp obtained by ad~usting the-
time for proper impregnation.
Broadly the present invention comprises continuous-
ly feeding small chip~ and a cooking chemical containing imrpregnation liquor at controlled rates into an impregnation
zone and maintaining a consistency of no more than 10%
~i.e. liquor to wood ratio of no less than 9 to 1 in ~aid
zone at a concentration to insure a sub~tantial exce~ of
cooking chemical is readily available to ~aid small chips,
passing said small chips and liquor co-currently through
thQ said impregnation zone, while agitating the chip~ and
liquor to improve the contact between the liquor and the
ch~ps, maintaining the residence time of the individual
chips in the impregnation zone substantially the same,
coordinating the temperature and pressure conditions and
residence time of said chips thereby to impregnate said
~mall chips without cooking to substantially soften said
105~7007
ch$ps, ~eparating said impregnated small chips from the
port$on of said impregnation liquor not sorbed by said
chips, returning at least some of said portion to said
impregnation zone, introducing said impregnated small chips
into a cooking zone, and digesting the said small chips.
Preferably the consistency of the chip liquor
mixture will be maintained ~ubstantially uniform through-
out the impregnation zone by the agitation or mixing
action in the zone.
Preferably a water balance will be maintained
in the system by regulating the amount of water entering
the ~ystem.
BRIE:F DESCR~PTION OF DRAWINGS
Further features, ob~ect~ and advantages will be
ev$dent from the following detailed description of a preferred
embodiment of the present invention taken in con~unction with
the accompanying drawing.
Figure 1 iY a schematic illu~tration of the process
of the instant invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1 the basic elements of the pre-
sent invention comprise a chip bin 10 from which small chip~
are fed into a pre-steamer 12 which is operated at a control-
led ~peed via metering drive 9 thereby to meter the chips flow
to the system. The small chips are steamed at substantially
atmospheric pre~sure in the presteamer 12 and pa~s from the
pre-steamer into the impregnator 14 via down pipe 16.
~he impregnator i8 provided with at least one and
prefcrably with a plurality of stirrers; in the illustrated
-~ arrangement three stirrer~ 18, 20 and 22 have been provided,
but the number i8 dependent on the type of stirrer and size
. . ' ''' ' . - '' '- :' ~
, .
57~07
and ~hape of the impregnation zone 14. The stirrers 18, 20
and 22 are mounted on a shaft 24 which i8 driven by a varia-
ble speed drive 26. Suitable baffles as indicated at 19
and 21 project in from the walls of the ve~el between the
S ~tirrer to aid in controlling the flow of small chip~ and
liquor. The baffle~ 19 and 21 divide the impregnation
vessel into a plurality of separate interconnected mixing
zone~ wherein the 7 iquor and chips are intmately mixed
thereby improving con~act between the liquor and chips and
facilitating the transfer of chemical from the liquor to
the ch~ p8 . ~he baffles 19 and 21 and the stirrers or
i~pellers 18, 20 and 22 are designed to obtain lateral
mixing of the liquor and chips while insuring that each
chip has substantially the same residence time in the
lmpregnator (i.e. the impregnator operates substantially
on a first in, first out basis). Suitable equipment to
obtain this flow and mixing action is well known for
example in the chemical lndustry i.e. as a plug flow tubular
type reactor.
Under stable operating conditions a substantially
constant liquid level L is maintained in the impregnator
14.
In the illustrated arrangement a pump 28 draws a
mixture of liquor and small chips from the impregnator 14
via Line 30 and pump~ this mixture via Line 32 into the
drainer 34.
The drainer 34 may take any convenient form, for
example, as illustrated it may comprise an inclined screw 36
contained within a houRing. The screw 36 lifts the impreg-
nated small chips from the excess impregnation liquor whichdrains into the zone 38. Alternatively the drainer may take
the fonm of any conventional press adapted to squeeze excess
liquor from the small chips.
-- 10 --
~057007
~ xcess liquor from the drainer 34 pa~ses via line 40
back into thc impregnator 14. If required, a suitable pump
~not shown) may be provided for deliverinq the liquor from the
drainer 34 to the impregnator 14. Minor amounts of the fibre
may be carried back to the impregnator with the excess liquor
but it is preferred to keep these amounts as low as possible.
Presh chemical is added to the impregnation system
via line 56 in an amount sufficient to replace the chemical
carried w$th the small chips into the Digestor 60. To
operate the system generally the amount of chemical added via
line 56 is co-ordinated with the amount of incoming chips to
maintain the required concentration of chemical in the impreg-
nation zone and to regulate the amount of chèmical sorbed by
the chips.
In operating a continuous system it is important to
maintain a water balance in the sy~tem. A preferred method of
maintaining such a water balance is by pas~ing a portion of
the liquor from the drainer 34 via line 47 through a heat
exchanger 48 and then bac~ via line 49 to the line 40. The
heat applied to the liquor passing through the heat exchanger
48 i~ controlled by level sensing mechanism 42 sensing the
Level L and controlling the valve 44 in the steam line 46 to
the heat exchanger 48. Valve 44 i9 controlled to apply suf-
ficient heat to the liquor passing through the heat exchanger
48 to evaporate a controlled amount of water from the li~uor to
maintain level ~ substantially constant a~ described in the
~sid Canadian patent. The water evaporated from the liquor
leaves ~he system as processed steam via line S0~
It is preferred that only a portion of the liquor
from the drainer pa88 through the heat exchanger 48 as it i3
desired to maintain the temperature in the impregnator 14
below 100C.
-- 11 --
1057007
In some ca~es, e.g. when the small chips are very
dry, the liquor level L may tend to decrea e as insuffici2nt
water is entering the ~ys~em. Under these circumstance~ the
~team valve 44 i~ closed and water i~ added via Line 52 under
control of the level SenQer 42 operating valve 54 to maintain
the level L constant.
The impregnated wood particle~ ~arrying sufficient
chemical for ~ubsequent digestion leave the drainer 34 and
pass through a suitable mechanism such as a valve 58 into the
dige~ter 60 where they are contacted with direct steam and
brought to cooking temperature. The direct ~team enters the
dige~tor 60 via line 62 and the cooked pulp leaves via line
64 and is directed to a blow tank or other suitable processing
~teps.
When it i9 de~ired to make a ~ubstantial change in
productton rate it i8 preferred to temporarily deactivate the
automatic water balance controls 80 that too much water is
not added or evaporated to maintain the level L con~tant.
When such major changes in production rate are in~-
tituted, liquor i9 directed to or from the surge tank 68. For
ex~mple if the flow rate of chips and the flow rate of fresh
chem~cal are increased to increase production the level ~
n~turally tends to r~se. Under these condition~ the Valve 70
in the Line 72 leading to the tank 68 ln ~ine 40 is opened to
divert somQ of the flow of liquor into tank 68. This diverted
flow continues until feed rates stabilize and then the Valve 70
is closed and level control 42 functions normally to control
the steam in the heat exchanger 48 or the additional water via
Valve 54. On the other hand if the production rate is decreased
the le~el in the tank tends to fall since less chips and che-
mical are bein~ introduced. Impregnation liquor i8 then pumped
via pump 76 from the tank 68 through Valve 74 which i8 opened
and line 78 into impregnator 14 to maintain the level L. Pump
-- 12 --
1057Q07 t
76 and the Valve 74 are in operative position until ~t~bl~
feed rates are atta~ned.
The operation of the sy3tem of the instant invention
is as follows:
The small chips are fed from the chip bin 10 into
the pre-~teamer 12 where they are steamed at atmospheric pres-
sure to remo~e air. The small chips drop through the tube
16 into the impregnator 14 and travel with the liquor through
the impregnator. The ~lows of liquor and the small chips
through the impregnator are preferably regulated ~o that
both flow together through the Lmpregnator at sub~tantially
the same ~peed i.e. the residence times for the liquor and
small chips are sub~tantially the ~ame. The~e flows may
be regulated by the rotation of the paddles 18, 20 and 22
preferably in cooperation with the baffles 19, 21, etc.,
which limit vert~cle mixing.
; The mixture of impregnation liquor and ch$ps or wood
particles i8 withd rawn from the impregnator 14 and $8 pumped
~y pump 28 through line 32 into the drainer 34. Liquor i~ se-
parated from the chips in the drainer 34 and this separated
liquor is returned via line 40 to the impregnator 14. A por-
tlon o~ this ~eparated liquor may be d$rected through the heat
exchanger 48 and a controlled amount of this l$quor i~ evapo-
rated to maintain the water balance as above decribed. Alter-
natively water may be added via line 52 to maintain the water
balance.
Fresh chemical is added to the sy~tem via line 56 in
an amount coordinated with the amount of small chips fed by the
pre-steamer 12 to provide the desired chemical concentration in
the i~pregnat~on liquor and thereby en~ure that the small chips
may ~orb the required chemical for sub~equent digestion and
carry this required amount of chemical from drainer 34 into the
digestor 60.
- 13 -
- 1057007
The impregnated small chips pass from the drainer 34
via valve 58 into the digestor 60 where they are cooked using
the chemical ~orbed on the small chips and the resulting pulp
is directea to a blow tank or other proce~ing equipment via
S line 64.
The temperature pressure and time cond~tions main-
tained in the impregnator are preferably set 80 that the
small chips are not significantly digested and retain at
least a major portion of their rigidity when they leave the
drainer so that they will drain more ea~ily i.e. they are
not softened to the extent to make ~eparating excess liquor
too difficult. This may be obtained, for example, by
malntaining the impregnator at atmospheric pressure and a
temperature below 100C. Retention time in the impregnator
wlll be less than 60 minutes preferably betwean 10 and 40
minutes and will in part be dependent upon the thickness
of the small chips being processea.
Th~ consistency in the impregnator is extremely
important as this permits operation with a substantial exces~
of chemical while maintaining a moderate chemical concen-
trat~on in the liquor and permits relative motion between
the liquor and the chips to insure good equalization of
chemlcal on the chip~. The maximum consistency in the
impregnator should be less than about 10~ i.e. a liquor to
wood ratio of no less than 9 to 1 and it i~ generally
preferred that the consistency be within the range of about
5 - 7%.
The moderate chemical concentration in the im-
pregnation liquor of from about 20 to 50 gram~ per liter
depending on the pulping process usea in~ureq that the chip~
are not damaged by contact with highly concentrated liquor.
Ev~n wlth such a low concentration for example with ~ Kraft
~ystem operating for example at 5~ consistency and a
- 14 -
1(~57007
chemical concentration in the impregnation liquor of 36.2
grms per liter the percent alkali to wood in the impregna-
tor i~ about 100~. Bearing in mind that the alkali con-
~umption for such a cook would be in the order of 14%, it
will be apparent tha~ about seven (7) times the amount of
alkali necessary for the cook is available in the im-
pregnator.
As above indicated, the conditions in the impregna-
tion zone, namely, the temperature, pressure, concentration
and consistency are important to the operation of the instant
invention. Particularly it is important to make the chemical
in the impregnator liquor available in substantially the ~ame
proportions to each unit of chips. The differences in specific
gravity between the saturated small wood chips and the impreg-
nation liquor and the non-uniformity of the specific gravity
of the saturated ~mall chips make it neces4ary to r~gulate
the flows of chips and liquor through the impregnation zone
14 and insure that the chemical i~ uniformely available to the
chips. 9tirrer~ 18, 20 and 22 ~three have been illu~trated
but fewer or more ~tirrers may be used depending on the size of
the impregnation vessel) add turbulence to the flows of small
chips and liquor to insure ~ub~tantially uniform availability
o~ chemical to the chip~. The stirring regulate~ the flows
80 that the residence time of the small chips and liquor in
the impregnation zone are substantially the same, i.e. the
avQrage velocities of impregnation liquor and of the small
chips through the lmpregnation zone will be substantially
th~ same. A~ above indicated the rotation of the shaft 24
18 controlled by the ~ariable speed drive 26 which is
manipulated a~ required to obtain the desired flow conditions ~ -
or turbulence in the impregnator 14.
A~ above indicated the high liquor to wood ratio
(no lower than 9 to 1%) in the impregnation zone i5 also
- 15 -
.. . .
1057007
important to permit proper mixing of the liquor and chips in
the zone. If the liquor ratio i8 too low, i.e. the con-
sistency i5 too high, it will be sub~tantially impo~sible for
the impellor~ to circulate the liquor relative to the chips
and proper ~o~ption of chemical uniformly by the chip mas~
will be inhibited.
It i8 preferred to mix the liquor with the chips
and to maintàin a substantially uniform con~entration
throughout the impregnation zone, however, this i~ not
ab~olutely essential. Bearing in mind that within the fir~t
few minutes of contact with the impregnation liquor the
chip~ sorb about 80% of the chemical it will be apparent
that mixing during this initial period is more critical.
Mixing and maintaining a uniform con6istency throughout the
impregnation zone facilitates control of the system and the
removal of the impregnated chip and liquor from the im-
pregnator at the same rate as they enter~ the dwell time is
controllable by the pumping rate of pump 28.
The amount of chemical carried into the dige~ter 60
i~ parti~lly dependent upon the amount of liquor carried ~ith
the wood particles when they leav~ the drainer 34 and pass
into the digester 60 but in any event the particles will carry
sorbed thereon sufficient chemical for digestion. It is pre-
ferred that the consistency of the mixture entering the di-
gestor be in the range of about 26 to 38~. Signif$cantly
higher consi~tencies may result in difficulty obta~ning
proper cooking while ~ignificantly lower con~istencies may
result in significant waste of chemical.
Typical dige~tion conditions are a temperature
of 185OC under pres~ure of 150 p.s.i. for approximately
20 mlnutesO
EXAMPLE 1
; The foll~wing i8 a specific example of a coo~ that
- 16 -
1057007
wa~ carried out in the lab to illustrate the condition~ con-
te~plated for use in the present invention.
Soft wood sawdust and shaving~ w~re presteamed at
102C. under a pressure of 2 psig. for 5 minutes and then
impregnated at 90C for 15 minutes at a con~istency of 5%. The
composition of the impregnation li~uor wa~ 48 grams per litre
efective alkal~ and 38% ~ulphidity. The thus impregnated
small chip~ were drained to a consistency o about 30~ and
cooked for 20 minute~ at 150 psig.
The resultant pulp had a total yield of 46.3%7 an
accepted yield of 45.5~S and a rejects of 0.8~. The Kappa numr
ber wa~ 33.7% v~scosity 187 and brightne~s ~CPPA absolute
~8 filter) 32.8.
EXAMPLE 2
Pulp wa~ produced from ~pruce and bal~am ~awdust
in a continuous experimental digester operating at a daily
production rate of approx~mately 1.5 ton~. The sawdust
wa~ slurried in a large volume of fortified recycled Kraft
liquor and was evenly impregnated with the cooking chemical
and after the wood was separated from the exce~s chemical
it was fed to the digester. The resulting pulp was tested
and evaluated and the following are deta~ls of the conditions
for the specific trial.
IMPREGNATION
Sawdust having the following particle size di~-
tribution,
Retained on a 6 me~h 21.8~
Retained on a 10 mesh 3002%
Retained on a 20 mRsh 36.4%
Past a 20 mesh 11.6~
was fed by a metering screw at a controlled rate
to a pre-steamer whera is was steamed for 2 minutes at at-
- 17 -
~ 7~07
mosph~ric pre~sure to remove air. The conditions for ~m-
pregnation were as follows:
The feed rate of wood was 4 oven dried pound~ per
minute. The impregnation liquor had a strength of 36.2
S gram~ per litre effective alkali and was composed of flow
of 30 U.S. gallon~ per minute recycled impregnation liquor at
90C. with a chemical concentration of 35 grams per litre
effective alkali with a sulphidity of 30% together with
.85 U.S. gallons per minute of white liquor having a chemical
concentration of 78.7 grams per litre effective alkali and
a ~ulphidity of 29~.
The chemical pick up on the wood was approximately
14~.
Approx$mately 2% con~istency was maintained in
the impregnator which represents a total percent chemical
on wood of 233~ in the impregnation tower.
The wood and ~mpregnation liquor were mixed as a
~lurry ~at 2~ consi~tency) for 30 minute~ at 90C. in the
lmpregnation tower.
The impregnation tower was 4 feet in diameter,
12 f~et high and had 4 mixing stages in series and was
open to the atmo~phere.
DE~ATERING
The ~mpregnated sawdust was separated from the
impregnation liquor in a device consisting of a settling
tank with an inclined screw bottom. Slurry from the im-
pregnation tower was pumped into the settling tank and
the wood ~ettled into thè screw while the clear liquor over-
flowed. The wood was conveyed up the screw and above the
liquid level in the tank the exces~ liquor was returned to
the impregnation tower while the 3eparated impregnated
sawdust was delivered to the digestion zoneO
The screw for the dewatering device is 10 inche~
-- 18 --
057007
in diameter 20 feet long and was inclined at 30 degree~.
The screw turned at 8 rpm. so that the wood was in the
screw for about 3 minutes and drained to approximately 30%
solid content of wood.
S The impregnated wood was damp to the touch and
was reddish brown in colour however in 8i ze and shape and
firmnes~ it was almost identical to the sawdust fed to the
impregnator.
PULPING
~igester used cons~st~ of two Bauer M & D tubes
in serie3. Each tube was 24 inches diameter by 12 feet
6 inche~ long. Feeding and discharge of the digQster was
through a 6 inch Bauer rotary valve~. The digester was
maintained at a pressure to 150 psi and a temperature in
the range of 180 to 186C. ~otal pulping time was 22
mlnutes. Liquid ~mainly stea~ and condensate) was con-
tinuously removed from the tubes to maintain vapour phase
pulping conditions. Liquid withdrawn from the second
tube contained 3 to 7 grams per litre effective alkali.
The following table gives the results of the te~ts
Table
Fibre Fractionation
(Clark Classifier)
Mesh Size
Passed Retained %
Cooking Proce~ -
Kraft Vapour Pha~e 10 .4
Wood - Softwood Sawdust 10 28 5.8
Beating Equipment - PFI Mill 28 48 25.6
Percent Yield - 44.5 48 100 13.2
Kappa No. - 30.0 100 7.0
CED Viscosity 13,3
Brightness CPPA 30
-- lg -- .
. :
1057007
____ ____________________ ____ _________ _______________
Canadian Standard Freeness550. 450. 300.
Bulk (CC/G) 1.59 1.49 1.42
Burst Factor 42. 49. 55.
Tear Factor 91. 84. 78.
Tensile (BL in KM) 7.8 8.6 9.3
Percent ~longation 2.0 2.2 2.4
M I T Folds 200. 500. 900.
Bausch & Lomb Opacity 94. 92. 90.
Reject level of the pulp produced wa~ les~ than 1%
and it has been found with Kappa numbers in the range 30
to 60 reject~ level will be maintained le~s than about 1~.
In the preceding di~cussion, the proposed proce~s
has been described primarily in terms of its application to
the pulping of ~awdust and shaving~ with Xraft liquors. In
fact, however, thi~ process could be applied to the pulping
of wood from different sources with a variety of cooking
chQmicals.
For example, liquors such as sodium hydroxide,
~odium carbonate, magnesium or sodium bisulfite, or mixtures
of sodium carbonate and ~odium sulfite may be used as the
pulping reagant. A high yield sodium hydroxide or sodium
carbonate pulp may be produced by the pre~ent invention and
followed by a second stage oxygen treatment. The cooking
time to achieve these yield levels would be substantially
less than those described above (e.g., 5-15 minute~ at 150
psig.).
Pulp produced at the 75 - 80~ yield level by the
described process with either sodium carbonate alone or with
mixtures of sodium carbonate and sodium sulfite as cooking
chemicals would make a good pulp for corrugating medium
manufacture because of the good liquor impregnation that
would be achieved.
- 20 -
)57007
The proposed proces~ i8 al~o attractive for pulping
by bisulfite proces~e~ ~e.g. Magnefite) because the low
temperature in the impregnation stage would minimize the
formation of thiosulfate and the 1098 of ~ulphur dioxide
vapours from the liquor. For the~e proces~es, a chemical
charge of 8 - 10% combined S02 on wood and the digestion
times of 2 - 3 hours at 166C are conventionally required
to produce chemical pulp. High yield pulp8, such as tho~e
used in new~print could be produced with lower chemical
charges and shorter time~. In these bisulfite methods,
it is of particular value to use a vapour pha~e digestion,
in which there is no liquor surrounding the chip, ~ince this
liquor i8 particularly prone to decomposition at digestion
temper~tures.
Modiflcations will be evident to those skilled in the ~ -
art without departing from the spirit of the invention as de-
fined in the appended claims. For example, while the instant
invention has been described a~ a continuous process it will
be evident that the impregnation and draining portions of the
instant invention may be continuous and the resultant drained
and chemically impregnated small chips may be delivered to one
or more batch digestors as desired for cooking.
- 21 -
