US2592300A

US2592300A - Method of removing hemicellulose from wood pulp

Info

Publication number: US2592300A
Application number: US696009A
Authority: US
Inventors: Limerick Jack Mck
Original assignee: Individual
Current assignee: Individual
Priority date: 1946-09-10
Filing date: 1946-09-10
Publication date: 1952-04-08
Anticipated expiration: 1969-04-08
Also published as: USRE23868E

Description

April 8, 1952 J. MOK. LIMERICK 2,592,300

METHOD OF REMOVING HEMICELLULOSE FROM WOOD PULP Filed Sept. 10, 1946 3 Sheets-Sheet l IN VEN TOR.

Jack M Limerick April 8, 1952 c UMERICK 2,592,300

METHOD OF REMOVING HEMICELLULOSE FROM WOOD PULP Filed Sept. 10, 1946 3 Sheets-Sheet 2 r O Q .5 N 2; z 3 Q IN VEN TOR.

Jack K Limerick Ap 1952 J. MCK. LIMERICK METHOD OF REMOVING HEMICELLULOSE FROM woon PULP Filed Sept. 10, 1946 s Sheets-Sheet 3 IN V EN TOR.

Jack 114 K- Limerick 'sidualJignin' and hemicellulose.

, Patented Apr. 8 1952 UNHT SAT 1' METHOD OF REMOVING HEMICEIJLULOSE FROM WOOD PULP J aclr McK. Limerick, Bathurst, New Brunswick, Canada Application September 10,1946, seriaiNo'r'eeacos 4 Claims.

Fig. l is an isometric diagram-illustrating a.

preferred arrangement of equipment'for carry- 'ing out theimprovedprocess;

"Fig; 2 is an enlarged view of the left-hand side of Fig. 1; and

Fig. 3 is an'enlargedview ofthe'right-hand 'side of'Fig'. 1.

i Ordinarily wood pulp as it comes from the digester; and; almost regardless of the process by which the pulpihasbeenproduced; contains be- '.tween 85 and 89% pure alpha cellulose-the bal- 1 'ancebeing made: up'of a small amount-of re- It is clear, IIJthGI'BfOIE; that purification would result in a n'yield'of'between 85'and 89% alpha cellulose based z'on'the'original weight 'of the pulp. 'If' the treated "i pulp istobe usedifor the manufacture of'rayon, 1192" or 93% alpha content is sufiicient but if the irpulp isto'be used for "the formation of clear "cellulose acetate, the'minimum permissible quali ity is 96% "alpha; ,Under the practice ofthe prior r-sart: the -law'of'diminishing returns operates sewerely upon any attempt to raise'quality above 92% alpha cellulose and the yield drops off :Iabruptly.- For example: in one instance of cur '5? rent practice; there isan 80% yield with an alpha cellulose content of 92% while when the quality is raised to 96% the" yield drops to 67%. The :"process of the'instant invention however, has -regularly produced qualities between 96 and 97% alpha cellulose with yields between 83 and 84%.

This is an enormous economic diiference.

Referring now-to Fig. 1 there is illustrated in the upper left-hand corner a largely convenctionalinstallationA for-vaporizing liquid chlolrinefrom tank carsand bringing it into the system. A control panel is illustrated at B, the ini- -tial chlorination stage atC, a fresh water washfollowing, chlorination at D, the first dilu- "tions'tep at E,the first'steep at F, the second .dilution at G, the second steep at H, thethird Ij 'dilutionlat I, thethird steep atJ, the fourth dilution at'K; thefourthsteep at L, the thickening "step at M, three consecutive washings at N. N andv N", the soaking stageat 0, the washing following" the soak' at P, the first hypochlorite stage at Q and Q, the wash following the first hypochlorite stage at R, the' second hypochlorite 'stage' at S, S ancsfi 'the wash following the second hypochlorite stage at T, the sulphur dioxide stage at U, the screening followingthe'sulphur dioxide stage at V; wash-following the screening at W, the' final .stock chest: at 'X. Y represents the sulphur dioxide supply; and' Z the hypochlorite supply.

-Referringnow"'to*Fig. 2: washed, :unblea'ched sulphite I u1p' is stored in a stock chest I O at a consistency between 8 and 11%. A; pump l2,

through a line' l, sen'dsthe pulp to a mixing box-l6 where, through a, line l8, it is diluted-with fresh water 1' to ab'out 3-4%' consistency. The

-mixing box IE- is conventional and has ar-return line -20-to take excess pulp back to the steel;

chest 10.; -Dilute pulp leaves the box'l 6 through a line 22 and passes' throu'gh a mixer zd which also receives chlorine-from'plantA (shown in Fig; 1) through a line '26 from control panel B.

The pulpand' chlorine then passin" sequence through towers 28-and 30. which constitutestation C. The chlorine used preferably 'amounts to" 65% of all chlorine to'beused. "It isareal bleaching step. A pump 32 takes the pulp-from tank through a line 34xto a washer' and a thickener D. Onentering the washerD-the-pulp is diluted'to 1% consistency by means sofa-line 38 supplied' by a' pump 40 which receives 'water fromthejsuctionlinel'42 'ofrthe washer D. A freshwater shower '44" washes. the pulp as it "passes over the drum 46 of thewasherD; the

shower "44' being supplied by a'freshwater line 38. The pulp preferablyatabout 12.5% consistency leaves the washer D through a-'line 50 'to a washer E. p

' Depending on the type of pulp it may be desirable not only to give the pulp a directchlorine bleach in towers 213 and 30 but to follow this by a hypochlorite bleach. In such-case the first hy- -pochlorite stage QQ' will be placed betweenthe -washer D and the washer E. It will alsqbe'desirable to add a washer. similar to washerDjbetween the relocated hypochlorite stagef'Q.Q ttr'idlthe washer E.- The latter (E) as will appear hereinafter, functions not. only to .wash the pulp "but primarily to replace the liquor which is contai'nedin' the pulp as it approachesthe washer with treatment liquor' and thus to avoid dilution or "contamination of the treatment liquor.

. A wash step such as'D permits operation ofthe washer E to bepconcentrated on the replacement as, distinct from the l washing function.

-pn enteringthe washer E the pulp'is diluted to about 1% consistency by a' 'linefi l supplied by a pump 56 from'the suction-line"58' of the drum 60 of the washer E. A shower 62 plays on the drum 60. The washer E first thickens to 6-7% consistency and then dilutes the pulp to about 35% consistency. The pulp leaves the washer E through a line 64 which feeds a pump 66 which passes the pulp through a steeping tower F.

Since the actual removal of hemi-cellulose begins as the pulp leaves washer E, it is essential A that the treating liquor supplied by line 82 and shower 62 be not diluted by liquor carried over from washer D. Accordingly the volume of shower 62 is adjusted relative to the suction on the drum 62 so that shower liquor is drawn completely through the pulp mat on the drum, thereby displacing the liquor in the pulp ahead of the shower. In order to maintain control, a pH controller 6| is placed in the line 58. The controller 6| acts, through any conventional line 63 to control a valve 65 in the supply line of the shower 62. The volume of shower 62, therefore, is regulated to maintain a pH in the line 58 sufficiently high to guarantee that liquor from shower 62 is drawn through the pulp mat.

The line 64 contains a mixer 65 which receives dilute caustic (NaOH) from the control panel B through a line 61, and for pitch control, sodium phosphate maybe supplied through a line 61. This caustic supply is used as a control to effect adjustment of the liquor concentration. As will later appear, when the process is seen as a whole, an adjustment of concentration in the fresh liquor added at the last steep would require considerable time to affect all of the steeps. The mixer 65 greatly shortens this lag. The vital factor to be controlled is hemi-cellulose concentration. Control is used early at the first steep for the reason that only there can an excessive concentration develop.

From the tower F a pump 68 sends the pulp through a line 10 to a washer G. It is to be noted that the suction on the drum 12 of washer G draws liquor through a line 14 and that this liquor by means of a pump 16 is used, first, by means of a line I8, to dilute the stock going to washer G to a consistency of 1%. The remaining portion of the liquor goes through a line 80 to the shower 62 of washer E and also, through a line 82, to dilute the pulp leaving the washer E.

Since the total liquor removed from washer G through line I4 is greater than the total demand of lines 18 and 80 the excess liquor is bled off through a line 84 and is disposed of in a manner to be described hereafter.

Washer G first thickens the pulp to a consistency of 1213% and then dilutes it back to 3-5%. The dilute stock leaves washer G through a line 88 through which, by means of a pump 90, it is circulated through a second steeping tower H. The pulp leaves the tower H through a line 92 and by means of a pump 84 is delivered through a line 96 to a washer I. The suction drum 98 of the washer I delivers extracted liquor through a line I00 and this liquor is distributed by a pump I02 through a line I04 to dilute the pulp in line 86 to 1% consistency and by means of a line I06 the balance of the liquor extracted is delivered to a shower I08 on washer G and through a line IIO the liquor in line I06 also dilutes the pulp leaving the washer G.

The drum 98 of washer I thickens the pulp to '7-8% consistency. The pulp is then re-diluted to a 3-5% consistency and leaves the washer I through a line I I2. The line II2 supplies a pump I I4 which feeds the pulp through a third steeping tower J. The pulp leaves the steeping tower J through a line I I6 and by means of a pump H8 is supplied by a line I20 to a washer K.

The liquor extracted by the drum I22 of washer K leaves the washer through a line I24 and by means of a pump I26 is delivered through a line I28 to intercept the pulp line I20 and to dilute the pulp to a consistency of 1%; the balance of the liquor through a line I30 serves a shower I32 of the washer I and through a line I34 dilutes the pulp going to the steeping tower J.

The drum I22 of the washer K thickens the pulp to a consistency of from 10-12%. The pulp is then diluted to 4-5% consistency and leaves the washer K through a line I36 which feeds a mixer I38. The mixer I38 also receives, via the control panel B (Fig. 1), a line I40 which supplies sufficient fresh caustic liquor to dilute the pulp to 3-5% consistency. For a purpose to be described later the line I40 is passed through a refrigeration unit I42.

The pulp leaves the mixer I38 through a line I44 and is delivered by pump I46 to the fourth steeping tower L and leaves the tower L through a line I48. The line I48 feeds a pump I50 which sends the pulp to a thickener M which brings the pulp to 22-24% consistency. The pulp entering the thickener M is diluted to 1% consistency by a line I52 which is su plied by a line I54 which in turn receives its liquor from the suction line I56 of the drum I58 of the thickener M. The line I54 supplies a shower I60 and, through a line I62, supplies dilution liquor to the washer K. The sole function of the thickener M is to remove as much liquor as possible from the pulp with no dilution. It will be noted that there is no shower on the face of the drum and it may be desirable to mount press rolls over the drum to increase the liquor extraction.

The showers on Washers G, I and K are operated in a manner similar to the shower on washer E, that is, the volume of these showers is sufliciently relative to the vacuum on the drums to assure shower water being drawn through the pulp mat. This effectively displaces the previous liquor and provides sharp demarcation between the several steeping stages. To attain the same demarcation by increased thickening between stages would involve costly apparatus and high power consumption. It is not necessary, at washers G, I and K to provide a pH controller as at washer E. Excessive flow of the shower 62 on washer E ultimately would mean a loss of chemicals to the sewer, as is clear in Fig. 2. Excessive flow in the showers on washers G, I and K is not diluted as in line 58 of washer E and is recovered in line 84 as hereafter described.

Agitation is provided in each of the steeping towers, F, H, J and L. The most important agitating effect, however, occurs in the intermediate washing steps where the extreme dilution, followed by thickening, followed by extremely agitated redilution serves thoroughly to break up any zones of hemi-cellulose saturated liquor around any pulp fibers. This assures maximum effectiveness of each steeping treatment. It also adds to the yield by minimizing the necessity for mechanical agitation in the steeping towers. Such agitation in a cold caustic suspension of substantial consistency (3-5%) increases the hydration and solution of alpha cellulose. Any alpha cellulose which is dissolved is lost, to the detriment of yield.

Analysis of samples indicates that, on the average, each steeping treatment accomplishes most"of the hemi-cellulose-dissolving 'ofwhich it "-is capableinfrom '-l0-'to l'minutes. Uniformity of product-however,- demands the safeguard of "longer--exposure;--hence;in practice; the steep is -continued for-from 30 to 60 minutes.

'The pulp -leaving -the thickener 'M- is'diluted 'back' to-1%consistency then goes" through aline fi HiGto a'washerm'thence'to a second'washe'r' N, thence'toathirdwasher N". These washers are conventional and are arranged for regular counter-currentwashing. A portion of the liquor output 'of washer- Nis bled-oil through'line I86 for a purpose 'tobe described" later.

At the washer N" a shower168 is'supplied with fresh water from the'1ine1l10 andstock leaving the washer N"; is diluted to about4% consistency by freshwater supplied by. a line I12. The line i H! .is :connected through an injector by 'means of' a line H4 with a'steam line I16. A similar line and injector I18 connects the line I12 with "thesteam line I16.

The refrigeration unitl42 on the caustic line Mil 'has'already been mentioned. A similar refrigeration unit 48 is placed in the line 38 which brings fresh shower water to the shower of the washer and thickener'D: Still another refrigeration unit 48' placed' around the line. 80 going from washer E-to the first steeping tower F.

" "It is essential that the steeping in towers F, H,

flJy-and L be carried out at a relativelylow temperature preferably between 15 and degrees Centigrade. The caustic liquor entering the system through line I40 will usually be' quite warm "and the refrigeration unit I42 will operate 'to reduce-temperature to the desired range. The fresh water supply at'most mills, particularly northern mills; during most of the year will be within the desired range. Therefore, the refrigeration unit 48' in the line 48 will only occasionallybe used, and most of 'the'cooling'will be done by heat exch'angers using fresh water. The refrigeration unit '48 in line 8! will take care of any temper- "ature rise in" the treating liquoroccurring during its passage from one steeping tower to the next. It greatly reduces the time lag of a temperature change effected by unit 48. Indeed, fora good 1 part ofeachyear, particularlyat northern mills "unit"48' willdo the whole job'and unit'48 need not'be operated.

The-pulp leaving thelast washer N" is diluted to 34% consistency with-hot-fresh water heated by the injectors H6 and H8. It goes'through a line I-"to the soaking tower 0 shown in Fig. 3. This tower is similar in proportions to the steeping-towers; F; H, J and L and, on the average, pulp will take halfan hour to go through the soaking -*-tank.- The pulp leavesthe tankthrough aline I82 1 "sand goes to a washer Pwhere it'receives a' shower 484 of fresh water.- Stock leaves the washer P through a line I86 which takes it to amixer I88 -"-'where it is treated with hypochlorite solution i'brought tothemixer I 88 through a line I 90 from "thehypochlorite supply" Z (Fig. '1) through the control panel B also shown in Fig. 1. If desired, caustic sufficient to maintain alkalinity-during --the hypochlorite bleach may also be supplied to the-mixer I88. Stock plus hypochlorite leaves the mixer I88 and through a line I92 goes to a hypochlorite tower Q thence through a line I94 to a second. hypochlorite tower Q The stock leaves the tower Q through a linelSS which takes it to a washer R where it receives a fresh water shower "198." From the washer R the stock leaves through atlineiflfi to a stock chest 292 where it is stored and agitated by conventional agitators 2%.

A pump 206- removes from the chest zoz and,

- through a line 208 delivers the stock to mixer 2 l0.- The mixer 21 Il -receiveshypochloritesolution through a line 212 which, through-control" panel B, receives hypochlorite solutionifromthe-bleach supply Z. A line 2|3 supplies fresh water to the mixer 210- a a means of controllingconsistency. From the mixer m the stock proceedsYthrough

parallel lines

214, 2 I6 and M8 to parallel bleaching towers S, S and S" respectively.

It will be noted that pulp is delivered to the top of towers S; S" and S". '-When "each' tower is filled; the pulpis' circulated in the towereand "samples are periodically taken. 'when-thew-sampling ind-icatesthat the desiredqualities have'been attained in'any tower, the tower is dumped into i a chest'220 where it-is'agitated'by conventional .a'gitators222. ,ThiSfiS essentially a batch operation'of each towerand, throughthisopportunity for close control, extreme uniformity of -the 'end a 1% solution of the pulp in a'standardized-cup- -ramm'onium solution. For ordinary-discose work, the solution viscosity should befifrom 20 30 centipoise; for acetates or nitrates the viscosity should be 30 .40 centipoise. The oxidationoccurring in the hypochlorite stage tends'to shorten'the alpha cellulose molecule and thus to lower the solution viscosity .of the pulp. This should" be done on the alkalineside, with "pH' of 8 or..9,preferably9. Carried too far, this process produces cxy-cellulose to the detriment of yield. .Such degradation of the pulp varies'directly' with'c'onsistency which, accordingly, must be controlled. A pump 224 takes stock'from'the' chest 220*and through a line.226 delivers it to a washer'Twh'ere it receives a fresh water sh0we'r228. Stockleaves the washer Tthrough a line 233 to a mixe'ri1232. The mixer also receives sulphur dioxideithrough alinei234 which is supplied via the control panel B from the SOzsupply Y. "From the mixer 232 stock goes through ailine 235 to a treatment tower .U. It leaves the tower U through a line 238 to a consistency regulator 2M] where it is diluted with fresh water through a line 242 and also inpart with the 'efiluent from a washer W through a line 2 14., From the consistency regulator 240s'tock goes thru' lines 246 to a set of conventional'fiat screens V.

.ates substantially as a thickener to remover-the extreme dilution (0.28% consistency) encountered at the'fiat screens.

Accepted stock from the-fiat screens' 'V-t-goes through a line 248-to a chest-250 from which a pump 252 carries the stock through a line 254 to hot this process is a juxta-position of kraft and sulphite pulp mills 'since the system is demon- Percent Sodium hydroxide 7.00 Sodium sulphide 4.25 Sodium carbonate Sodium sulphate With such an installation the bleed-off through lines 84 and I66 may be evaporated back to the concentration of the standard cooking liquor and may be used in the digestor. It is also permissible to use this liquor directly in the digestor. When this is done, the amount of black liquor normally used to dilute the digestor liquor is reduced. The presence of between 2 and 5% hemi-cellulose in the reclaimed white liquor, when used to make up a portion 'of the digestor charge, provides a species of protective colloid exercising a buffer effect against too drastic an action on the caustic of the wood. Moreover the addition of hemi-cellulose adds to the Mullen test of paper made from pulp thus digested.

It will, of course, be noted that the liquor from the last steeping stage in tower L goes from thickener M back to washer K and, therefore, recirculates in large part through the tower L. The portion of this liquor going to the shower ISO is delivered as shower and dilution water to the washer I which supplies the third steeping tower J and fresh caustic is added only at the fourth steep L.

Liquor containing the largest percentage of hemi-cellulose leaves the system through line 84 from the washer G. The rate of bleeding is adjusted proportionally to the rate of addition of fresh liquor so as to limit the maximum concentration of hemi-cellulose in the steeping liquor to not more than 5% and preferably to 3% in first steeping tower F and to 2% or less in the other steeping towers. This limitation, combined with the relatively low temperature, appears to be critical in securing the combination of high yield and. high quality. As the hemi-cellulose content of the steeping liquor increases it becomes progres sively more difficult to dissolve hemi-cellulose in the liquor. Also, there is a degradation both of caustic soda and of sodium sulphide with an increase in the percent of sodium carbonate. There is also a definite increase in viscosity which in turn tends to slow the process of dissolving hemi-cellulose. While 3% hemi-cellulose content in the liquor is believed to be optimum from all standpoints, it is still feasible to permit the concentration to rise to 5%. Beyond 5%, however, the ability of the liquor to dissolve hemi-cellulose drops rapidly, viscosity increases sharply and the degradation of caustic and of sodium sulphide is so marked that the liquor would not be useable in the kraft process.

The soaking step carried out in tower O is of particular advantage. It affords an opportunity for virtually complete removal, not only of caustic but also of the sodium sulphide in the kraft white liquor. If any substantial quantity of sodium sulphide were permitted to remain it would negative a corresponding portion of the hypochlorite and greatly increase bleaching costs. In addition, in practice, the use of this soaking step definitely increases quality by removing a small amount of hemi-cellulose.

The caustic steepings carried out by this process proceed on a counter-current principle, that is,

fresh incoming pulp with the greatest proportion of available hemi-cellulose. is subjected to the action of liquor which contains the heaviest permissible concentration of dissolved hemi-cellulose while the most potent liquor with the lowest concentration of hemi-cellulose is used to treat the pulp with the least proportion available of hemi-cellulose. This is infinitely more eflicient than a single prolonged steeping. The repeated steps are also important because of the opportunity thus afforded in dilution, thickening and re-dilution for breaking up localized concentrations of hemi-cellulose adjacent the individual fibers and thus permitting the great mass of liquor to be more fully effective.

As used in the claims the term stage is intended to include the complete cycle of each event. That is, with reference to a steeping stage, the term stage includes dilution, thickening with concomitant displacement of the previous liquor, redilution and then steeping. The term step is used to designate any particular operation such as dilution or thickening or redilution or steeping.

I claim:

1. A multi-stage continuous process for purifying pulp each stage of which consists of the consecutive steps of providing a slurry of pulp and alkaline treating liquor in which the pulp,

by weight, is from approximately 3 to approximately 5%, diluting the slurry to a pulp consistency of approximately 1% by the addition of alkaline treating liquor, then thickening the slurry to a consistency of approximately 6 to approximately 12% while simultaneously displacing the first alkaline treating liquor with a second, stronger alkaline treating liquor, then diluting the slurry to approximately 3 to approximately 5% pulp consistency by the addition of more of the second, stronger alkaline treating liquor, then advancing the thus diluted slurry continuously along a closed, predetermined path for a period of approximately fifteen to approximately sixty minutes, then repeating all of the above steps in three to four stages using consecutively stronger liquors for the final dilution step in each stage, the final dilution liquor in each stage coming from the thickening step of the next succeeding stage, continuously removing a predetermined proportion of liquor from one of the early stages, and continuously adding an equivalent amount of fresh liquor at the final dilution step of the last stage and regulating the rates of addition of fresh liquor and the removal of spent liquor to maintain the concentration of hemi-cellulose in the liquors below 5%, the several diluting steps being carried out with agitation to break up localized concentrations of dissolved hemi-cellulose adjacent the individual pulp fibers.

2. The process set forth in claim 1 in which the pulp following the several stages therein set forth is washed and then steeped approximately fifteen to approximately sixty minutes in hot fresh water.

3. The process set forth in claim 1 in which the stages therein set forth are preceded by a bleaching treatment with a fresh water washing stage intermediate the bleaching treatment and the said stages set forth in claim 1, said bleaching treatment using an agent selected from the group of chlorine and hypochlorites.

4. The process set forth in claim 3 in which the pulp following the completion of the process of claim 3 is given a fresh water wash and then 9 is steeped for approximately fifteen to approximately sixty minutes in hot ireshja'veter.

JACK McKi LIMERICK. REFERENCES omen I The following references are of {record in the file of this patent: A

UNITED STATES PA IEENTS Number Name I Date 1,632,802 Richter June 21, 1927 1,742,218 Richter Jan. 7, 1930 1,798,987 Rue 1Ma1-. 31, 1931 1,801,782 Richter Apr. 21, 1931 1,829,378 Thiriet Oct. 27, 1931 15 v 10 Number Name Date 1,906,885 Richter May 2, 1933 1,933,609 Wagner Nov. 7, 1933 2,041,666 Richter May 19, 1936 2,219,174 Richter July 15, 1941 23 24,230 Olsen July 13, 1943 2,363,684 Richter Aug. 28, 1945 2,385,259 Collings Sept. 18, 1945 1 OTHER REFERENCES Cellulose and Cellulose Derivatives by Ott, publish'ed by Interscience Publishers. Inc., New York (1943), pages 274, 511 to 515, 601' to 803, 813 and 814.

Claims

1. A MULTI-STAGE CONTINUOUS POROCESS FOR PURIFYING PULP EACH STAGE OF WHICH CONSISTS OF THE CONSECUTIVE STEPS OF PROVIDING A SLURRY A PULP AND ALKALINE TREATING LIQUID IN WHICH THE PULP, BY WEIGHT, IS FROM APPROXIMATELY 3 TO APPROXIMATELY 5%, DILUTING THE SLURRY TO A PULP CONSISTENCY OF APPROMIMATELY 1% BY THE ADDITION OF ALKALINE TREATING LIQUOR, THEN THICKENING THE SLURRY TO A CONSISTENCY OF APPROXIMATELY 6 TO APPROXIMATELY 12% WHILE SIMULTANEOUSLY DISPLACING THE FIRST ALKALINE TREATING LIQUOR WITH A SECOND, STRONGER ALKALINE TREATING LIQUOR, THEN DILUTING THE SLURRY TO APPROXIMATELY 3 TO APPROMIMATELY 5% PULP CONSISTENCY BY THE ADDITION OF MORE OF THE SECOND, STRONGER ALKALINE TREATING LIQUOR, THEN ADVANCING THE THUS DILUTED SLURRY CONTINUOSLY ALONG A CLOSED, PREDETERMINED PATH FOR A PERIOD OF APPROXIMATELY FIFTEEN TO APPROXIMATELY SIXTY MINUTES, THEN REPEATING ALL OF THE ABOVE STEPS IN THREE TO FOUR STAGES USING CONSECUTIVELY STRONGER LIQUORS FOR THE FINAL DILUTION STEP IN EACH STAGE, THE FINAL DILUTION LIQUOR IN EACH STAGE COMING FROM THE THICKENING STEP OF THE NEXT SUCCEEDING STAGE, CONTINUOUSLY REMOVING A PREDETERMINED PROPORTION OF LIQUOR FROM ONE OF THE EARLY STAGES, AND CONTINUOUSLY ADDING AN EQUIVALENT AMOUNT OF FRESH LIQUOR FROM ONE OF THE STEP OF THE LAST STAGE AND REGULATING THE RATES OF ADDITION OF FRESH LIQUOR AND THE REMOVAL OF SPENT LIQUOR TO MAINTAIN THE CONCENTRATION OF HEMI-CELLULOSE IN THE LIQUORS BELOW 5%, THE SEVERAL DILUTING STEPS BEING CARRIED OUT WITH AGITATION TO BREAK UP LOCALIZED CONCENTRATIONS OF DISSOLVED HEMI-CELLULOSE ADJACENT THE INDIVIDUAL PULP FIBERS.