CA1057007A - Impregnation of wood particles - Google Patents

Abstract

IMPREGNATION OF WOOD PARTICLES
ABSTRACT OF DISCLOSURE:

A method particularly useful in the treatment of small chips to produce wood pulp is disclosed. The me-thod includes impregnating small chips in an impregna-tion zone using a large quantity of impregnation liquor having a moderate concentration of cooking chemical which permits more uniform impregnation and the formation of a more uniform pulp. The liquor and small chips at a liquor to wood ratio of no more than 9 to 1 i.e. at a consistency of less than 10% pass through the impregnation zone. Im-pregnated small chips are separated from the impregnation liquor by means of a drainer or a press and the impregnated small chips are cooked at a low liquor to wood ratio pre-ferably in the vapour phase.

Description

~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.

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^: ~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 -~` 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-'' 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 ~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.

~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.

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 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.

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 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 .. . .

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

; The foll~wing i8 a specific example of a coo~ that 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.

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-~ 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~

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 -- .

. :

____ ____________________ ____ _________ _______________ 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.

)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.

Claims (9)

1. A method of digesting small chips to produce a chemical wood pulp comprising: presteaming said chips in a presteamer, continuously introducing said presteamed chips and a chemical-containing impregnation liquor into an impregnation zone in an impregnator to form a mixture con-sisting essentially of all of said chips and impregnation liquor in said impregnator, said mixture having a consistency of less than 10%, said liquor in said mixture containing a substantial excess of said chemical over that required for subsequent digestion of the chips, continuously passing said mixture through said impregnation zone, agitating said mixture substantially throughout said zone during passage of said mixture through said zone, said agitation being sufficient to maintain said consistency substantially con-stant and to mix said liquor with said chips and facilitate transfer of chemical from said liquor to said chips, main-taining the residence time of the individual chips in said impregnation zone substantially the same, coordinating the temperature, pressure and chemical concentration conditions and said residence time thereby to impregnate said chips with the required amount of chemical for subsequent digestion without cooking to substantially soften said chips, continu-ously withdrawing said mixture from said impregnator after passage through said zone, removing said chips impregnated with sufficient chemical for subsequent digestion from excess impregnation liquor, returning at least a portion of said excess liquor to said impregnation zone together with a fortifying amount of cooking chemical, introducing said separated impregnated chips into a cooking zone and digesting said impregnated chips in said cooking zone.

2. A method as defined in claim 1 wherein water enters the impregnator with the chips and with the impregn-ation liquor, said method further comprising maintaining a water balance by controlling the amount of said water entering the impregnator.

3. A method as defined in claim 1 wherein the concent-ration of said chemical in said impregnation liquor entering said impregnation zone is in the range of 20 to 50 grams/
litre.

4. A method as defined in claim 1 wherein said con-sistency is between 5% and 7% in said impregnation zone.

5. A method as defined in claim 1 wherein said impreg-nation zone is at atmospheric pressure and at a temperature of below 100°C.

6. A method as defined in claim 4 wherein said impreg-nation zone is at atmospheric pressure and at a temperature of below 100°C.

7. A method as defined in claim 1 wherein said cooking of said chips is in a vapour phase.

8. A method as defined in claim 4 wherein said cooking of said chips is in a vapour phase.

9. A method as defined in claims 1, 3 or 4 wherein said impregnation zone is at atmospheric pressure and a temperature of less than 100°C and the concentration of said chemical in said impregnation liquor entering said impregnator is in the range of 40-45 grams per litre.