BACKGROUND AND SUMMARY OF THE INVENTION

According to conventional knowledge in the art of kraft pulping of cellulose, the level of dissolved organic materials (DOM) ~ which mainly comprise dissolved hemi-cellulose, and lignin, but also dissolved cellulose, extractives, and other materials extracted from wood by the cooking process — is known to have a detrimental affect in the later stages of the cooking process by impeding the delignification process due to consumption of active cooking chemical in the liquor before it can react with the residual or native lignin in wood. The effect of DOM concentration at other parts of cooking, besides the later stages, is according to conventional knowledge believed insignificant. The impeding action of DOM during the later stages of the cook is minimized in some state- of-the-art continuous cooking processes, particularly utilizing an EMCC ^® digester from Kamyr, Inc. of Glens Falls, New York, since the counter-current flow of liquor (including white liquor) at the end of the cook reduces the concentration of DOM both at the end of d e "bulk delignification" phase, and throughout the so-called "residual delignification" phase.

According to the present invention, it has been found tiiat not only does DOM have an adverse affect on cooking at the end of die cooking phase, but that the presence of DOM adversely affects the strength of the pulp produced during any part of the cooking process, that is at the beginning, middle, or end of the bulk delignification stage. The mechanism by which DOM affects pulp fibers and thereby adversely affects pulp strength has not been positively identified, but it is hypothesized diat it is due to a reduced mass transfer rate of alkali extractable organics through fiber walls induced by DOM surrounding me fibers, and differential extractability of crystalline regions in the fibers compared to amorphous regions (i.e. nodes). In any event, it has been

demonstrated according to the invention that if the DOM level (concentration) is minimized throughout the cook, pulp strength is increased significantly. It has been found, according to the present invention, that if the level of DOM is close to zero throughout a kraft cook, tear strength of the pulp is greatly increased, i.e. increased up to about 25 % (e.g. 27%) at 11 km tensile compared to conventionally produced kraft pulp. Even reductions of the DOM level to one-half or one-quarter of their normal levels also significantly increase pulp strength. In state-of-the-art kraft cooks, it is not unusual for the DOM concentration at some points during me kraft cook to be 130 grams per liter (g/1) or more, and at 100 g/1 or more at numerous points during the kraft cook (for example in the bottom circulation, trim circulation, upper and main extractions and MC circulation in Kamyr, Inc. MCC ^® continuous digesters), even if the DOM level is maintained between about 30-90 g/1 in the wash circulation (at later cook stages, according to conventional wisdom). In such conventional situations it is also not unusual for the lignin component of the DOM level to be over 60 g/1 and in fact even over 100 g/1, and for the hemi-cellulose component of the DOM level to be well over 20 g/1. It is not known if the dissolved hemi- cellulose component has a stronger adverse affect on pulp strength (e.g. by adversely affecting mass transfer of organics out of the fibers) than lignin, or vice versa, or if the effect is synergistic, although the dissolved hemi-celluloses are suspected to have a significant influence.

According to d e present invention it has been recognized for the first time that the DOM concentration throughout a kraft cook should be minimized in order to positively affect bleachability of the pulp, reduce chemical consumption, and perhaps most significantly increase pulp strength. By minimizing DOM levels, one may be able to design smaller continuous digesters while obtaining the same throughput, and may be able to obtain some benefits of continuous

digesters with batch systems. A number of these beneficial results can be anticipated by keeping the DOM concentration at 100 g/1 or less throughout substantially the entire kraft cook (i.e., beginning, middle and end of bulk delignification), and preferably about 50 g/1 or less (the closer to zero DOM one goes, the more positive the results). It is particularly desirable to keep the lignin component at 50 g/1 or less (preferably about 25 g/1 or less), and the hemi-cellulose level at 15 g/1 or less (preferably about 10 g/1 or less). According to the present invention it has also been found that it is possible to passivate the adverse affects on pulp strength of the DOM concentration, at least to a large extent. According to d is aspect of the invention it has been found that if black liquor is removed and subjected to pressure heat treatment according to U.S. patent 4,929,307 (me disclosure of which is hereby incorporated by reference herein), e.g. at a temperature of about 170-350 °C (preferably 240°C) for about 5-90 minutes (preferably about 30-60 minutes) and men reintroduced, an increase in tear strength of up to about 15% can be effected. The mechanism by which passivation of the DOM by heat treatment occurs also is not fully understood, but is consistent with the hypothesis described above, and its results are real and dramatic on pulp strength. According to the present invention various methods are provided for increasing kraft pulp strength taking into account the adverse affects of DOM thereon, as set forth above, for both continuous and batch systems. Also according to the present invention increased strength kraft pulp is also provided, as well as apparatus for achieving the, desired results according to the invention. Further, according to die invention, the H factor can be significantly reduced, e.g., at least about a 5 % drop in H factor to achieve a given Kappa number. Also, me amount of effective alkali consumed can be significantly reduced, e.g., by at least about 0.5 % on wood (e.g. about 4%) to achieve a particular Kappa number. Still further, enhanced bleachability can be achieved, for example,

increasing ISO brightness at least one unit at a particular full sequence Kappa factor.

According to one aspect of the present invention, a method of producing kraft pulp by cooking comminuted cellulosic fibrous material is provided. The method comprises the steps of continuously, at a plurality of different stages during kraft cooking of the material to produce pulp: (a) Extracting liquor containing a level of DOM substantial enough to adversely affect pulp strength. And, (b) replacing some or all of the extracted liquor with liquor containing a substantially lower effective DOM level than the extracted liquor, so as to positively affect pulp strength. Step (b) is typically practiced by replacing the withdrawn liquor witii liquor selected from the group consisting essentially of water, substantially DOM free white liquor, pressure-heat treated black liquor, washer filtrate, cold blow filtrate, and combinations thereof. For example for at least one stage during cooking, black liquor may be withdrawn, and treated under pressure and temperature conditions (e.g. superatmospheric pressure at a temperature of about 170-350 °C for about 5-90 minutes, and at least 20 °C over the cooking temperature) to significantly passivate the adverse affects of DOM. The term "effective DOM" as used in d e specification and claims means that portion of the DOM that affects pulp strength, H factor, effective alkali consumption and/or bleachability. A low effective DOM may be obtained by passivation (except for effect on bleachability), or by an originally low DOM concentration.

The method according to the invention can be practiced in a continuous vertical digester, in which case steps (a) and (b) may be practiced at at least two different levels of the digester. There is also typically the further step (c) of heating the replacement liquor from step (b) to substantially the same temperature as the withdrawn liquor prior to the replacement liquor being introduced into contact with the material being cooked. Steps (a) and (b) can

be practiced during impregnation, near the start of me cook, during the middle of me cook, and near the end of die cook, i. e. , during substantially the entire bulk delignification stage.

According to another aspect of d e present invention, a method of kraft cooking is provided comprising the steps of, near the beginning of the kraft cook: (a) Extracting liquor containing a level of DOM substantial enough to adversely affect pulp strength. And, (b) replacing some or all of the extracted liquor wid liquor containing a substantially lower effective DOM level than the extracted liquor, so as to positively affect pulp strength. According to anodier aspect of the present invention a method of kraft cooking is provided comprising me steps of, during impregnation of cellulosic fibrous material: (a) Extracting liquor containing a level of DOM substantial enough to adversely affect pulp strength. And, (b) replacing some or all of the extracted liquor with liquor containing a substantially lower effective DOM level than the extracted liquor, so as to positively affect pulp strength. According to still anodier aspect of the present invention a method of kraft cooking pulp is provided comprising d e following steps: (a) Extracting black liquor from contact with the pulp at a given cooking stage, (b) Pressure- heating the black liquor to a temperature sufficient to significantly passivate the adverse effects on pulp strength of DOM therein. And, (c) re-introducing the passivated-DOM black liquor back into contact with the pulp at the given stage.

The invention also comprises the kraft pulp produced by the mediods set forth above. This kraft pulp is different than kraft pulps previously produced, having a tear strength as much as 25 % greater at a specified tensile for fully refined pulp (e.g. at 9 km tensile, or at 11 km tensile) (and at least about 15% greater) compared to kraft pulp produced under identical conditions without the DOM maintenance or removal steps according to die invention, or as much as

15 % greater (e.g. at least about 10% greater) where passified black liquor is utilized.

The invention is also applicable to kraft batch cooking of cellulosic fibrous material utilizing a vessel containing black liquor and a batch digester containing the material. In such a method of kraft batch cooking according to me invention there are the steps of: (a) Pressure-heating the black liquor in the vessel to a temperature sufficient to passivate the adverse effects on pulp strength of DOM therein. And, (b) feeding the black liquor to the digester to contact the cellulosic fibrous material therein. Step (a) is practiced to heat me black liquor at superatmospheric pressure at a temperature of about 170-350 °C for about 5-90 minutes (typically at least about 190°C for about 30-60 minutes, and at least 20 °C over cooking temperature), and step (b) may be practiced to simultaneously feed black liquor and white liquor to me digester to effect cooking of the cellulosic fibrous material. According to another aspect of the present invention an apparatus for kraft cooking cellulose pulp is provided. The apparatus comprises the following elements: An upright continuous digester. At least two withdrawal/extraction screens provided at different levels, and different cook stages, of the digester. A recirculation line and an extraction line associated wi each of the screens. And, means for providing replacement liquor to the recirculation line to make up for die liquor extracted in the extraction line, for each of the recirculation lines. Each recirculatory loop typically includes a heater, and die digester may be associated with a separate impregnation vessel in which removal of high DOM concentration liquor and replacement with lower DOM concentration liquor also takes place (including in a return line communicating between die top of the impregnation vessel and the high pressure feeder). The invention also relates to a commercial method of kraft cooking comminuted cellulose fibrous material by the step (a) of continuously passing

substantially DOM-free cooking liquor into and out of contact with the material until completion of the kraft cook thereof, at a rate of at least 100 tons of pulp per day. This method is preferably practiced utilizing a batch digester having a capacity of at least 8 tons/day (e.g. 8-20), and by me further step (b), prior to step (a), of filling the digester with cellulose material, and me further step (c), after step (a) of discharging kraft pulp from the digester. The invention also relates to a batch digester system for practicing this aspect of me invention, each batch digester having a capacity of at least 8 tons per day (i.e. of commercial size as compared to laboratory size). The invention also relates to a modification of a number of different types of continuous digesters, conventional MCC ^® Kamyr, Inc. digesters or EMCC ^® Kamyr, Inc. digesters, to achieve significant dilution of the effective DOM of the cooking liquor during at least one early or intermediate stage of the cook. By arranging the extraction and recirculation screens in a particular way, the advantageous results according to d e invention can be achieved in existing digesters merely by re-routing various fluid flows and introducing low DOM dilution liquor and/or white liquor at various points, in all conventional types of continuous digesters including single vessel hydraulic, two vessel hydraulic, etc. It is die primary object of the invention to produce increased strength kraft pulp, and/or also typically reducing H factor and alkali consumption, and increasing bleachability. This and omer objects of the invention will become clear from an inspection of the detailed description of d e invention and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic illustration of one exemplary embodiment of continuous kraft cooking equipment according to the invention, for practicing exemplary methods according to the present invention; FIGURES 2 and 3 are graphical representations of the strength of pulp produced according to d e present invention compared with kraft pulp produced under identical conditions only not practicing the invention;

FIGURE 4 is a schematic view of exemplary equipment for the improved method of batch kraft cooking according to the invention; FIGURE 5 is a schematic side view of another embodiment of exemplary batch digester according to die present invention;

FIGURE 6 is a graphical representation of die H factor for producing pulp according to die invention compared widi kraft pulp produced under identical conditions not practicing the invention; FIGURE 7 is a graphical representation of the consumed effective alkali during die production of pulp according to the present invention compared widi the production of pulp under identical conditions only not practicing the invention;

FIGURE 8 is a graphical representation of the effective alkali consumed vs. a percentage of mill liquor compared to DOM-free liquor; FIGURE 9 is a graphical representation comparing brightness response for pulps produced according to die present invention compared widi kraft pulp produced under identical conditions not practicing die invention;

FIGURES 10 dirough 14B are further graphical representations of various strength aspects of pulp produced according to die present invention, in FIGURES 12A-B being compared with kraft pulp produced under identical conditions only not practicing the invention;

FIGURE 15 is a graphical representation of DOM concentrations based upon actual liquor analysis for lab cooks with three different sources of liquor at various stages during cooking;

FIGURE 16 is a schematic illustration of an exemplary digester of a two vessel hydraulic cooking system which practices die present invention; FIGURE 17 is a graphical representation of a theoretical investigation comparing DOM concentration in a conventional MCC ^® digester compared widi the digester of FIGURE 16;

FIGURES 18 through 20 are schematic illustrations of other exemplary digesters according to the present invention; and

FIGURES 21 dirough 25 are graphical representations of theoretical investigations of varying dilution and extraction parameters using me digester of FIGURE 19.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGURE 1 illustrates a two vessel hydraulic kraft digester system, such as mat sold by Kamyr, Inc. of Glens Falls, New York modified to practice exemplary methods according to the present invention. Of course any other existing continuous digester systems also can be modified to practice the invention, including single vessel hydraulic, single vessel vapor phase, and double vessel vapor phase digesters.

In the exemplary embodiment illustrated in FIGURE 1, a conventional impregnation vessel (IV) 10 is connected to a conventional vertical continuous digester 11. Comminuted cellulosic fibrous material entrained in water and cooking liquor is transported from a conventional high pressure feeder via line 12 to the top of die IV 10, and some of die liquor is withdrawn in line 13 as is conventional and returned to the high pressure feeder. According to die present invention, in order to reduce the concentration of DOM (as used in this

specification and claims, dissolved organic materials, primarily dissolved hemi- cellulose and lignin, but also dissolved cellulose, extractives, and odier materials extracted from wood by die kraft cooking process) liquor is withdrawn by pump 14 in line 15 (or from me top of vessel 10) and treated at stage 16 to remove or passivate DOM, or selected constituents diereof. The stage 16 may be a precipitation stage (e.g. by lowering pH below 9), an absorption stage (e.g. a cellulose fiber column, or activated carbon), or devices for practicing filtration (e.g. ultrafiltration, microfiltration, nanofiltration, etc.) solvent extraction, destruction (e.g. by bombardment with radiation), supercritical extraction, gravity separation, or evaporation (followed by condensation).

Replacement liquor (e.g. after stage 16) may or may not be is added to d e line 13 by pump 14 in line 17, depending upon whether impregnation is practiced co-currently or counter-currently. The replacement liquor added in line 17, instead of extracted liquor treated in stage 16, may be dilution liquor, e.g. fresh (i.e. substantially DOM-free) white liquor, water, washer filtrate (e.g. brownstock washer filtrate), cold blow filtrate, or combinations thereof. If it is desired to enhance the sulfidity of the liquor being circulated in the lines 12, 13, black liquor may be added in line 17, but the black liquor must be treated so as to effect passivation of die DOM dierein, as will be described hereafter.

In any event, the liquor withdrawn at 15 has a relatively high DOM concentration, while that added in 17 has a much lower effective DOM level, so that pulp strength is positively affected. In the impregnation vessel 10 itself the DOM is also controlled preferably utilizing a conventional screen 18, pump 19, and reintroduction conduit 20. To the liquid recirculated in conduit 20 is added - as indicated by line 21 — dilution liquid, to dilute the concentration of the DOM. Also the dilution

liquid includes at least some white liquor. That is the liquor reintroduced in conduit 20 will have a substantially lower effective DOM level than the liquor wididrawn through the screen 18, and will include at least some white liquor. A treatment stage 16' — like stage 16 — also may be provided in conduit 20 as shown in dotted line in FIGURE 1.

From die bottom of the IV 10, die slurry of comminuted cellulosic fibrous material passes through line 22 to die top of the digester 11, and as is known, some of me liquid of the slurry is wididrawn in line 23, white liquor is added thereto at 24, and passes through a heater (typically an indirect heater) 25, and then is reintroduced to die bottom of the IV 10 via line 26 and/or introduced close to the start of the conduit 22 as indicated at 27 in FIGURE 1. In existing continuous digesters, usually liquid is withdrawn at various levels of the digester, heated, and dien reintroduced at die same level as withdrawn, however under normal circumstances liquor is not extracted from me system and replaced widi fresh reduced-DOM liquor. In existing continuous digesters, black liquor is extracted at a central location in the digester, and the black liquor is not reintroduced, but rather it is sent to flash tanks, and tiien ultimately passed to a recovery boiler or the like. In contra-distinction to existing continuous digester, die continuous digester 11 according to die present invention actually extracts liquor at a number of different stages and heights and replaces the extracted liquor widi liquor having a lower DOM concentration. This is done near the beginning of the cook, in me middle of die cook, and near the end of die cook. By utilizing die digester 11 illustrated in FIGURE 1 , and practicing the method according to die invention, die pulp discharged in line 28 has increased strength compared to conventional kraft pulp treated under otherwise identical conditions in an existing continuous digester. The digester 11 includes a first set of wimdrawal screens 30 adjacent die top

thereof, near the beginning of the cook, a second set of screens 31 near the middle of the cook and third and fourdi sets of screens 32, 33 near the end of die cook. The screens 30-33 are connected to pumps 34-37, respectively, which pass through recirculation lines 38-41 , respectively, optionally including heaters 42-45, respectively, these recirculation loops per se being conventional. However according to the present invention part of the wididrawn liquid is extracted, in die lines 46-49, respectively, as by passing the line 46 to a series of flash tanks 50, as shown in association with the first set of screens 30 in FIG. 1. To make up for the extracted liquor, which has a relatively high DOM concentration, and to lower the DOM level, replacement (dilution) liquor is added, as indicated by lines 51 dirough 54, respectively, the liquor added in the lines 51 dirough 54 having a significantly lower effective DOM concentration than die liquor extracted in lines 46-49, so as to positively affect pulp strength. The liquor added in lines 51 dirough 54 may be the same as the dilution liquors described above widi respect to line 17. The heaters 42-45 heat the replacement liquor, as well as any recirculated liquor, to substantially the same temperature as (typically slightly above) die wididrawn liquor. Any number of screens 30-33 may be provided in digester 11. Prior to transporting the extracted liquor to a remote site and replacing it with replacement liquor, the extracted liquor and die replacement liquor can be passed into heat exchange relationship with each other, as indicated schematically by reference numeral 56 in FIGURE 1. Further, the extracted liquor can be treated to remove or passify the DOM dierein, and dien be immediately reintroduced as die replacement liquor (with odier, dilution, liquor added thereto if desired). This is schematically illustrated by reference numeral 57 in FIGURE 1 wherein the extracted liquor in line 48 is treated at station 57 (like stage 16) to remove DOM, and dien reintroduced at 53. White

liquor is also added diereto as indicated in FIGURE 1 , as a matter of fact at each of the stages associated widi die screens 30-33 in FIGURE 1 white liquor can be added (to lines 51-54, respectively).

Another option for the treatment block 57 — schematically illustrated in 5 FIGURE 1 -- is black liquor pressure heating. From the screens 32 liquor that may be considered "black liquor" is withdrawn, and a portion extracted in line 48. The pressure heating in stage 57 may take place according to U.S. patent 4,929,307, die disclosure of which is hereby incorporated by reference herein. Typically, in stage 57 the black liquor would be heated to between about 170-

10 350°C (preferably above 190°C, e.g. at about 240°C) at superatmospheric pressure for about 5-90 minutes (preferably about 30-60 minutes), at least 20 °C over cooking temperature. This results in signification passivation of the DOM, and die black liquor may then be returned as indicated by line 53. The treatment stage illustrated schematically at 58 in FIGURE 1, associated

15 widi die last set of withdrawal/extraction screens 33, is like stage 16. A stage like 58 may be provided, or omitted, at any level of the digester 11 where there is extraction instead of adding dilution liquor. White liquor may be added at 58 too, and d en the now DOM-depleted liquor is returned in line 54.

0 Whether treated extracted liquor or dilution liquor is utilized, according to die invention it is desirable to keep the total DOM concentration of the cooking liquor at 100 g/1 or below during substantially the entire kraft cook (bulk delignification), preferably below about 50 g/1; and also to keep die lignin concentration at 50 g/1 or below (preferably about 25 g/1 or less), and the 5 hemi-cellulose concentration at 15 g/1 or less (preferably about 10 g/1 or below). The exact commercially optimum concentration is not yet known, and may differ depending upon wood species being cooked.

FIGURES 2 and 3 illustrate the results of actual laboratory testing pursuant to

the present invention. FIGURE 2 shows tear-tensile curves for three different laboratory kraft cooks all prepared from the same wood furnish. The tear factor is a measure of die inherent fiber and pulp strength. In FIGURE 2 curve A is pulp prepared utilizing conventional pulp mill liquor samples (from an MCC ^® commercial full scale pulping process) as die cooking liquor. Curve B is obtained from a cook where the cooking liquor is the same as in curve A except that die liquor samples were heated at about 190°C for one hour, at superatmospheric pressure, prior to use in the cook. Curve C is a cook which used syndietic white liquor as the cooking liquor, which synthetic white liquor was essentially DOM-free, (i.e. less tiian 50 g/1). The cooks for curves A and B were performed such that me alkali, temperature (about 160°C), and DOM profiles were identical to tiiose of the full-scale pulping process from which the liquor samples were obtained. For curve C the alkali and temperature profiles were identical to tiiose in curves A and B, but no DOM was present.

FIGURE 2 clearly illustrates mat as a result of low DOM liquor contacting the chips during die entire kraft cook, there is approximately a 27% increase in tear strength at 11 km tensile. Passivation of the DOM utilizing pressure heating of black liquor, pursuant to curve B according to die invention, also resulted in a substantial strengtii increase compared to die standard curve A, in this case approximately a 15 % increase in tear strength at 11 km tensile. FIGURE 3 illustrates further laboratory work comparing conventional kraft cooks with cooks according to die invention. The cooks represented by curves D through G were prepared utilizing identical alkali and temperature profiles, for the same wood furnish, but widi varying concentrations of DOM for the entire kraft cook. The DOM concentration for curve D, which was a standard MCC ^® kraft cook (mill liquor) was the highest, and die DOM concentration for curve G was the lowest (essentially DOM-free). The DOM concentration

for curve E was about 25% lower than the DOM concentration for curve D, while the DOM concentration for curve F was about 50% lower than the DOM concentration for curve D. As can be seen, there was a substantial increase in tear strength inversely proportional to the amount of DOM present during the complete cook.

Cooking according to die invention is preferably practiced to achieve a pulp strength (e.g. tear strength at a specified tensile for fully refined pulp, e.g. 9 or 11 km) increase of at least about 10%, and preferably at least about 15% , compared to otherwise identical conditions but where DOM is not specially handled.

While widi respect to FIGURE 1 the invention was described primarily with respect to continuous kraft cooking, the principles according to die invention are also applicable to batch kraft cooking. FIGURE 4 schematically illustrates conventional equipment mat may be used in die practice of the Beloit RDH™ batch cooking process, or for the Sunds Super Batch™ process. The system is illustrated schematically in FIGURE 4 includes a batch digester 60 having withdrawal screen 61, a source of chips 62, first, second and third accumulators 63, 64, 65, respectively, a source of white liquor 66, a filtrate tank 67, a blow tank 68, and a number of valving mechanisms, the primary valving mechanism illustrated schematically at 69. In a typical conventional operating cycle for the Beloit RDH™ process, me digester 60 is filled widi chips from source 62 and steamed as required. Warm black liquor is then fed to die digester 60. The warm black liquor typically has high sulfidity and low alkalinity, and a temperature of about 110-125°C, and is provided by one of the accumulators (e.g. 63). Any excess warm black liquor may pass to a liquor tank and ultimately to evaporators, and dien to be passed to chemical recovery. After impregnation, me warm black liquor in digester 60 is returned to accumulator 63, and dien the digester 60 is filled

widi hot black and white liquor. The hot black liquor may be from accumulator 65, and die hot white liquor from accumulator 63, ultimately from source 66. Typically the white liquor is at a temperature of about 155 °C, while the hot black liquor is at a temperature of about 150-165 °C. The chips in the digester 60 are then cooked for die predetermined time at temperature to achieve die desired H factor, and then die hot liquor is displaced with filtrate direct to d e accumulator 65, the filtrate being provided from tank 67. The chips are cold blown by compressed air, or by pumping, from the vessel 60 to the blow tank 68. During the typical RDH™ process, white liquor is continuously preheated wid liquor from the hot black liquor accumulator and dien is stored in die hot white liquor accumulator 64. The black liquor passes to the warm weak black liquor accumulator 63, and d e warm black liquor passes through a heat exchanger to make hot water and is stored in an atmospheric tank before being pumped to die evaporators.

With regard to FIGURE 4, die only significant difference between the invention and die process described above is die heating of d e black liquor, which may take place directly in accumulator 65, in such as way as to effect significant passivation of the DOM tiierein. For example this is accomplished by heating the black liquor to at least 20°C above cooking temperature, e.g. under superatmospheric pressure to at least 170°C for about 5-90 minutes, and preferably at or above 190°C (e.g. 240°C) for about 5-90 minutes. FIGURE 4 schematically illustrates this additional heat being applied at 71; d e heat may be from any desired source. During tiiis pressure heating of the black liquor, off-gases rich in organic sulfur compounds are produced and wididrawn as indicated at 72. Typically, as known per se, the DMS (dimethyl sulfide) produced in line 72 is converted to methane and hydrogen sulfide, and the methane can be used as a fuel supplement (for example to provide die heat in

line 71) while the hydrogen sulfide can be used to pre-impregnate the chips at source 62 prior to pulping, can be converted to elementary sulfur and removed or used to form polysulfide, can be absorbed into white liquor to produce a high sulfidity liquor, etc. If the heat treatment in accumulator 65 is to about 5 20-40°C above cooking temperature, black liquor can be utilized to facilitate impregnation during kraft cooking.

Alternatively, according to me invention, in the FIGURE 4 embodiment, the valving mechanism 69 may be associated widi a treatment stage, like stage 16 in FIG. 1, to remove DOM from cooking liquor being withdrawn from screen

10 61 and recirculated to the digester 60 during batch cooking.

FIGURE 5 schematically illustrates an exemplary commercial (i.e. producing at least 8, e.g. 8-20, tons of pulp per day) batch digester system 74 according to die present invention. A laboratory size version of die solid line embodiment of system 74 as seen in FIGURE 5 was used to obtain plot C

15 from FIGURE 2, and has been in use for many years. The system 74 includes a batch digester 75 having a top 76 and bottom 77, with a chips inlet 78 at the top and outlet 79 at die bottom, with a chips column 80 established tiierein during cooking. A screen 81 is provided at one level tiierein (e.g. adjacent die bottom 77) connected to a witiidrawal line 82 and pump 83, leading to a heater 0 84. From the heater 84 the heated liquid is recirculated through line 85 back to the digester 75, introduced at a level therein different than the level of screen 81 (e.g. near the top 76).

Prior to the heater 84, a significant portion (e.g. to provide about three turnovers of liquid per hour) of the withdrawn lignin in line 82 is extracted at 5 line 86. This relatively high DOM concentration liquor is replaced by substantially DOM free (at least greatly reduced DOM concentration compared to that in line 86) liquor at 87. The substantially DOM-free liquor added at 87 may have an alkali concentration that is varied as desired to effect an

appropriate kraft cook. A varying alkali concentration may be used to simulate a continuous kraft cook in the batch vessel 75. Valves 88, 89 may be provided to shut down or initiate liquor flows, and/or to substitute or supplement the desired treatment using die system shown in dotted line in FIGURE 5. In accordance with the invention, instead of, or supplemental to, the extraction and dilution lines 86, 87, die desired level of DOM and its components (e.g. <50 g/1 DOM, <25 g/1 lignin, and < 10 g/1 hemi-cellulose) may be achieved by treating the extracted liquor for DOM, for example by passing me high DOM level liquor in line 90 to a treatment stage 91 - like the stage 16 in FIGURE 1 -- where DOM, or selected constituents thereof, are removed to greatly reduce their concentrations in the liquor. Makeup white liquor (not shown) can be added too, die liquor reheated in heater 92, and dien returned via line 93 to the digester 75 instead of using lines 90 and 93, lines 86 and 87 can be connected up to treatment unit 91, as schematically illustrated by dotted lines 95, 96 in FIGURE 5.

Other laboratory test data showing advantageous results that can be achieved according to die present invention are illustrated in FIGURES 6 through 15. In this laboratory test data, procedures were utilized which simulate continuous digester operation by sequentially circulating heated pulping liquor through a vessel containing a stationary volume of wood chips. Different stages of a continuous digester were simulated by varying the time, temperature and chemical concentrations used in die circulations. The simulations used actual mill liquor when the corresponding stage of a continuous digester was reached in die lab cook. The effect of minimizing DOM in pulping liquors upon required pulping conditions (tiiat is, time and temperature) is illustrated in FIGURE 6. FIGURE 6 compares the relationship between Kappa number and H factor for laboratory cooks using mill black liquor and substantially DOM-free white

liquor. The wood furnished for the cooks represented in FIGURE 6 was a typical north-western United States soft wood composed of a mixture of cedar, spruce, pine and fir. The H factor is a standard parameter which characterizes the cooking time and temperature as a single variable and is described, for example, in Rydholm Pulping Processes, 1965, page 618.

Line 98 in FIGURE 6 shows the relationship of Kappa number to H factor for a lab cook using mill liquor (collected at a mill and then used in a laboratory batch digester). A lower line, 99, indicates the relationship of Kappa number to H factor for a lab cook using substantially DOM-free white liquor manufactured in the lab. Lines 98, 99 indicate tiiat for a given Kappa number, the H factor is substantially lower when the DOM is lower, for example, for Kappa number 30 in FIGURE 6, there being approximately a 100 H factor units difference. This means mat for the same furnish with the same chemical charge if lower DOM cooking liquor is utilized, a less severe cook (that is, less time and lower temperature) than for a conventional kraft cook is required. For example, by extracting liquor containing a level of DOM substantial enough to adversely affect the H factor, and replacing some or all of the extracted liquor with liquor containing a substantially lower effective DOM level than the extracted liquor so as to significantly reduce die H factor; preferably the steps are practiced to decrease the H factor at least about 5 % to achieve a given Kappa number, and the steps are practiced to keep die effective DOM concentration at about 50 g/1 or less during die majority of the kraft cook. As illustrated in FIGURE 7, when utilizing reduced DOM concentration according to the present invention, the effective alkali (EA) consumed is reduced. EA is an indication of die amount of cooking chemicals, particularly NaOH and Na ₂ S used in a cook. The results obtained in FIGURE 7 were obtained utilizing die same furnish as in FIGURE 6, and die two graph lines

100, 101 were obtained at the same conditions. Line 100 indicates the results when die cooking liquor was conventional mill liquor, while line 101 shows the results when the cooking liquor was substantially DOM-free white liquor. At a Kappa number of 30, the DOM-free cook consumed approximately 30% less alkali (i.e. 5 % less EA on wood) than tiie conventional mill liquor cook. Thus, by extracting liquor containing a level of DOM substantial enough to adversely affect the amount of effective alkali consumed to reach a particular Kappa number, and replacing some or all of the extracted liquor with a liquor containing a substantially lower effective DOM level, the amount of effective alkali consumed to reach a particular Kappa number may be significantly reduced, e.g., die amount of alkali consumed may be decreased by at least about 0.5 % on wood (e.g. about 4% on wood) to achieve a particular Kappa number. Both the beneficial H factor and EA consumption results illustrated in FIGURES 6 and 7 may be achieved by replacing extracted relatively-high DOM liquor with water, substantially DOM-free white liquor, pressure heat- treated black liquor, filtrate, and combinations thereof. FIGURE 8 provides a further graphical representation of effective alkali consumption compared to the percentage of mill liquor to substantially DOM- free white liquor. Plot 101 indicates tiiat for the same relative Kappa number, the effective alkali consumed decreases widi decreasing percent mill liquor (that is, increasing percent substantially DOM-free white liquor). Table 1 below shows the actual lab results which were used to make the plot 101 of FIGURE 8.

Table 1

Effective Alkali Consumption

Cook Number A3208 A3219 A3216 A3239 A3217 Description Mill Liq 75% mill 50% mill 25% mill Lab Liq

Total EA 15.8 16.5 14.9 15.7 14.0 consumed, %

Kappa, 30.7 30.6 28.0 29.8 30.8 screened

Reduction or elimination of DOM in pulping liquor also improves the ease with which the resulting pulp is bleached, tiiat is, its bleachability.

FIGURE 9 illustrates actual laboratory test results showing how the brightness of a bleached cedar-spruce-pine-fir pulp increases with d e increase of bleaching chemical dosage. The parameter plotted on die X-axis of d e graph of FIGURE 9, the "full sequence Kappa factor", is a ratio of equivalent chlorine dosage to die incoming Kappa number of the pulp. That is, it is a somewhat normalized ratio of chlorine used to initial lignin content of die brownstock pulp. FIGURE 9 tiius shows how pulp brightness responds to die amount of bleaching chemical used.

The curves 102, 103, 104 and 105 of FIGURE 9 are, respectively, substantially DOM-free white liquor (102), conventional mill liquor (103), a mill-cooked pulp (not a laboratory pulp using mill liquor) (104), and mill heat treated black liquor which was heat-treated (105). These graphical representations clearly indicate tiiat the best bleachability is achieved when substantially DOM-free liquor is used for the cooking liquor. Thus, by extracting liquor containing a level of DOM substantial enough to adversely effect die bleachability of the pulp, and replacing some or all of the extracted liquor widi liquor containing a substantially lower effective DOM, the

bleachability of the pulp produced may be significantly increased, for example, at least one ISO brightness unit at a particular full sequence Kappa factor. Alternatively, this data indicates tiiat a specific ISO brightness can be achieved while using a reduced bleaching chemical charge. However, graph line 105 indicates tiiat while heat treated black liquor may improve delignification (see FIG. 2), die residual lignin may not be as easily removed. Thus, die treated black liquor may not be desirable for use as a dilution liquor where increased bleachability is desired, but rather water, substantially DOM-free white liquor, and filtrate (as well as combinations thereof) would be more suitable as dilution liquors. However, die heat-treated liquor may be used for pulp that is not bleached, i.e., unbleached grades.

As earlier discussed, reducing d e DOM concentration of pulping liquors appears to have the most dramatic effect upon pulp strength. This is further supported by data graphically illustrated in FIGURES 10 dirough 14B. All of this data is for the same cedar-spruce-pine-fir furnish as discussed above widi respect to FIGURES 6 through 9, and this data indicates tiiat under the same cooking conditions the tear strength significantly increases as the amount of DOM increases. For example, FIGURE 10 indicates that the tear strength at 11 km increases (see line 106) as the amount of mill liquor decreases (and tiius the amount of substantially DOM-free white liquor increases) for the laboratory cooks illustrated tiiere. FIGURE 11 indicates die same basic relationship by graph line 107, which plots percentage mill liquor versus tear at 600 CSF. Table 2 below shows die tear strength at two tensile strengths for lab cooks performed with various liquors, with a tear for a mill-produced pulp shown for comparison. The data from cooks 2 and 3 in Table 2 indicate a twenty percent (20%) increase for tear at 10 km tensile for the lab cook with substantially DOM-free white liquor compared with a lab cook using mill liquor, and a

twelve percent (12%) increase is indicated for tear at 11 km tensile. Lab cooks 4, 5 and 6 in Table 2 show the result of replacing DOM-free liquor in specific parts of the cook with corresponding mill liquor. For example, in cook 4 the liquor from the bottom circulation, BC, line replaced die lab-made liquor in the BC stage of the lab cook. Similarly, in cook 5 BC and modified cook, MC, mill liquor was used in die lab cook in die BC and MC stages, while substantially DOM-free liquor was used in die otiier stages. The data in Table 2 indicate tiiat minimization of DOM is critical throughout the cook, not simply in later stages, and fully supports the analysis provided above witii respect to FIGURES 2 and 3.

Table 2 Effect of Dissolved Organics on Pulp Tear Strength for Hemlock Furnish

Cooking Conditions Tear @ 10 km Tear @ 11 km

1) Mill Cook 123 N/A

2) Lab Cook w/Mill Liquor . (A) 174 156 (S\ 173 150 Average 173.5 153

3) Lab Cook (A) 207 174 w/Lab Liαuor (B. 206 170 Average 206.5 172

4) Lab Cook 183 159 w/Mill BC Liquor

5) Lab Cook 181 157 w/Mill BC and MC Liquor

6) Lab Cook 187 N/A w/Mill Wash Circulation Liquor

FIGURES 12A - 14B illustrate the effect of DOM upon bleached pulp strength. FIGURE 12A shows the tear and tensile strength for unbleached

pulp, line 108 showing pulp produced by substantially DOM-free lab liquor, line 109 from pressure-heat treated black liquor, and line 110 from conventional mill liquor. FIGURE 12B shows the tear versus tensile relationship after the pulps graphically illustrated in FIGURE 12A were 5 bleached utilizing die laboratory bleach sequence of DE ₀ D(nD). Line 111 shows die substantially DOM-free-white-liquor-produced, bleached pulp; line 112, the pressure-heat-treated-mill-liquor-produced pulp; and line 113, a conventional mill-liquor-produced, bleached pulp, while, for comparison, line 114 shows the strength of the mill pulp taken from the decker, after bleaching.

10 FIGURE 12B shows mat not only is the substantially DOM-free cooked pulp stronger than die mill liquor pulp, but this relative strength is maintained after bleaching. The heat treated liquor cooked pulp also maintains higher strength than die mill liquor cooked pulp after bleaching, but the difference in strength after bleaching is minimal.

15 FIGURES 13A and 13B plot the results of testing of the same cooks/bleaches as FIGURES 12A and 12B only tear factor is plotted against Canadian standard freeness (CSF). Line 115 is substantially DOM-free pulp; line 116; pressure-heat-treated-mill-liquor-produced pulp; line 117, mill-liquor- produced pulp; line 118, bleached, substantially DOM-free-produced pulp; line

20 119, pressure-heat-treated-liquor-produced, bleached pulp; line 120, bleached, mill-liquor-produced pulp; and line 121, taken at die mill decker.

FIGURES 14A and 14B are plots of same cooks/bleaches as in FIGURES 12A and 12B only plotting tensile vs. freeness. Line 122 is for mill-liquor-produced pulp; line 123, for pressure-heat-treated-mill-liquor-

25 produced pulp; line 124, for substantially DOM-free produced pulp; line 125, for mill-liquor-produced, bleached pulp; line 126, for substantially DOM-free- liquor-cooked, bleached pulp; line 127, at the decker; and line 128, for pressure-heat-treated-mill-liquor-cooked, bleached pulp. FIGURES 14A and

14B show that tensile declines for both heat-treated-liquor-cooked pulp and substantially DOM-free-liquor-cooked pulp, however FIGURE 14B shows that the bleaching reduces the relative tensile strength of the heat-treated liquor pulp below that of the DOM-free liquor cooked pulp. Again, as noted above, die heat-treated-liquor process may be suitable for unbleached pulps. The laboratory cooks discussed above all simulated the pulping sequence of a Kamyr, Inc. MCC ^® continuous digester. Each lab cook has a corresponding impregnation stage, co-current cooking stage, counter-current MCC ^® cooking stage, and a counter-current wash stage. Typical DOM concentrations based upon actual liquor analysis are shown in FIGURE 15 for lab cooks with three sources of liquor. The line 130 is for mill liquor; line 131, for 50% mill liquor and 50% substantially DOM-free lab white liquor; and the X's 132, for 100% substantially DOM-free lab white liquor. In FIGURE 15, note that at time = 0, the beginning of impregnation, all lab liquors used were DOM-free. This was done because tiiere was no reliable method of sampling die liquor at this stage of the cook in the mill. Thus, the DOM concentrations of the mill and 50/50 liquor cooks at die end of impregnation are lower than expected for tiiis set of data, and more representative concentrations are extrapolated and shown in parentiiesis in FIGURE 15. FIGURE 15 does show how each of die concentrations follow a consistent trend tiiroughout the cook, the concentrations gradually increasing until the extraction stage and dien gradually decreasing during the counter- current MCC ^® and wash stages. Even widi a substantially DOM-free source of liquor, of course, DOM is released into die liquor as cooking proceeds. FIGURE 16 illustrates an exemplary continuous digester system 133 that utilizes the teachings of the present invention to produce pulp of increased strength. System 133 comprises a conventional two-vessel Kamyr, Inc. continuous hydraulic digester with MCC ^® cooking, the impregnation vessel not

being shown in FIGURE 16, but the continuous digester 134 being illustrated. FIGURE 16 illustrates a retrofit of the conventional MCC ^® digester 134 in order to practice the lower DOM cooking techniques according to the present invention. The digester 134 includes an inlet 137 at die top tiiereof and an outlet

136 at die bottom thereof for produced pulp. A slurry of comminuted cellulose fibrous material (wood chips) is supplied from the impregnation vessel in line 137 to the inlet 138. A top screen assembly 138 withdraws some liquor from the introduced slurry in line 139 which is fed back to die BC heaters and die impregnation vessel. Below me top screen assembly 138 is an extraction screen assembly 140 including a line 141 therefrom leading to a first flash tank 142, typically of a series of flash tanks. Below the extraction screen assembly 140 is a cooking screen assembly 143 which has two lines extending therefrom, one line 144 providing extraction (merging widi die line 141), and die otiier line 145 leading to a pump 145' . A valve 146 may be provided at die junction between die lines 144, 145 to vary the amount of liquor passing in each line. The liquor in line 145 passes through a heater 147 and a line 148 to return to the interior of the digester 134 via pipe 151 opening up at about die level of the cooking screen assembly 143. A branch line 149 also may introduce recirculated liquid in pipe 150 at about the level of the extraction screens 140. Below the cooking screen assembly 143 is the wash screen assembly 152, with a withdrawal line 153 leading to the pump 154, passing liquor through heater 155 to line 156 to be returned to die interior of die digester 134 via pipe 157 at about the level of the screen 152. For the system 133, the mill has presently increased die digester's production rate beyond die production rate it was designed for, and production is presently limited by the volume of liquor that can be extracted. This limitation can be circumvented by utilizing the techniques according to d e

invention, as specifically illustrated in FIGURE 16. Since die amount of extraction in line 141 is limited, this will be augmented according to the present invention by supplying extraction also from line 144. For example, the rate of extraction will be, utilizing the invention, typically about 2 tons of 5 liquor per ton of pulp. In effect, 1 ton of liquor per ton of pulp extracted at line 144 is replaced with dilution liquor (wash liquor) from the source 158. This is accomplished in FIGURE 16 by passing die wash liquor from source 158 (e.g. filtrate water) through a pump 159, and valve 160, die majority of the wash liquor (e.g. 1.5 tons liquor per ton of pulp) being introduced in line

10 161 to the bottom of the digester, while the rest (e.g. 1 ton of liquor per ton of pulp) passing in line 162 into the line 145 to provide die dilution liquor. Also, substantially DOM-free white liquor from source 163 may be added in line 164 to the line 145 prior to heater 147, and recirculation back to the digester dirough pipes 150 and/or 151. Of course, white liquor may also be added to

15 die wash circulation in line 153 (see line 165) to effect EMCC ^® cooking. The flow arrows 166 illustrate the co-current zone in digester 134. As a result of the modifications illustrated in FIGURE 16, die counter-current flow in the MCC ^® cooking zone 167 will contain cleaner, DOM -reduced, liquor with improved results in pulp strength, and in this case also an increase in the 0 digester 134 production rate.

The effect of the modifications illustrated in FIGURE 16 upon DOM concentration has been investigated using a dynamic computer model of a Kamyr, Inc. continuous digester. Preliminary results of tiiis theoretical investigation are illustrated schematically in FIGURE 17. FIGURE 17 5 compares variation in DOM concentration in a conventional MCC ^® digester with the digester illustrated in FIGURE 16, die conventional MCC ^® digester results being illustrated by line 168, and the digester of FIGURE 16 results by line 169. As can be seen in FIGURE 17, the DOM concentration at the screen

assembly 143 drops dramatically with the addition of DOM-reduced dilution, also reducing d e DOM in the counter-current flow back up to the extraction screen assembly 140. Furthermore, the downstream, counter-current wash liquor contains less DOM since less DOM is being carried forward widi the pulp. Graph lines 170, 171, part of the lines 168, 169, indicate that in the counter-current cooking zone the DOM always increases in tiie direction of liquor flow. That is, the counter-current flow is cooking and accumulating DOM as it passes through tiie down-flowing chip mass.

FIGURES 16 and 17 tiius illustrate the dramatic impact of only a single extraction-dilution upon the DOM profile in a continuous digester, which DOM reduction may have a corresponding dramatic effect upon resulting pulp strength.

FIGURE 18 illustrates another mill variation implementing techniques according to the invention. This also indicates a digester 134 tiiat is part of a two-vessel hydraulic digester. Since many of the components illustrated in FIGURES 16 and 18 are the same, they are indicated by die same reference numerals. Only the modifications from one to the other will be described in detail.

In die FIGURE 18 embodiment, an even more dramatic DOM reduction will occur. In tiiis embodiment, the screens 140, 143 are reversed compared to die FIGURE 16 embodiment, and also anodier screen assembly 173 is provided between the screen assemblies 138, 143. The screen assembly 173 is a trim screen assembly; according to the invention the withdrawal conduit 174 tiierefrom provides extraction to the flash tank 142. In the embodiment of FIGURE 18, as one particular operational example, two tons of liquor per ton of pulp will be extracted in line 174, and four tons of liquor per ton of pulp in line 141. Dilution liquor will be added in line 162 and substantially DOM-free white liquor in line 164. This will

result in the flows 176, 177 illustrated in FIGURE 18, the digester 134 thus being characterized as co-current, counter-current, co-current, counter-current flow (which may be called alternate-flow continuous cooking).

FIGURE 19 illustrates another digester system 179 according to die 5 present invention. In this two-vessel system, the impregnation vessel 180 is illustrated, having an inlet 181 at the top thereof and an outlet 182 at d e bottom. Liquid wididrawn at 183 is recirculated to die conventional high pressure feeder, while white liquor is added at 184. Liquor withdrawn at 185 may be passed to an introduction point between the first flash tank 186 and

10 second flash tank 187. The slurry from the line 182 is introduced at 188 into die top of the digester 189, having a "stilling well" arrangement 190, from which liquor is withdrawn at 191 and recirculated to die bottom of the impregnation vessel 180. The liquor is heated in heater 192 when recirculated. Digester 189 also has a trim screen assembly 194 with the withdrawal

15 195 tiierefrom in this case merging with the recirculating liquid in line 191. Cooking screen assembly 196 is provided below the trim screen assembly 184, with liquid wididrawn in line 197 passing dirough valve 198 into a line 199, and optionally some of the liquid passing from valve 198 being directed in line 200 to the flash tank 186. The liquid in line 199 is diluted with lower DOM 0 liquor, such as the substantially DOM-free white liquor 201 and die filtrate 202, before passing through heater 203 and being reintroduced into die digester 189 by the conduit 204 at about die level of the screen assembly 196. The extraction screen assembly 206 has a withdrawal line 207 therefrom which leads to the flash tank 186. The wash screen assembly 208 includes 5 recirculation line 209 to which white liquor at 210 may be added before the liquor passes through heater 211, and dien is reintroduced by a conduit 212 at about the level of the wash screen assembly 208. Filtrate providing wash liquor is added at 213, while die produced pulp is wididrawn in line 193.

Note that the system 179 has die potential to extract from line 197, through valve 198 into conduit 200. The dilution liquid in d e form of filtrate also is preferably added at 214 to die line 182, while substantially DOM-free white liquor is added at 214' . 5 FIGURE 20 illustrates a one vessel hydraulic digester that is modified according to the teachings of the present invention, this modification also including two sets of cooking screens, as is conventional. This increases the potential for the introduction of extraction/dilution at two more locations. The single vessel hydraulic digester system 215 includes the

10 conventional components of chips bin 216, steaming vessel 217, high pressure transfer device (feeder) 218, line 219 for adding cellulose fibrous material slurry to die top 220 of die continuous digester 221, and a withdrawal 222 for produced pulp at the bottom of the digester 221. Some of die liquid has been withdrawn in line 223 and recirculated back to the high-pressure feeder 218.

15 The cooking screens are below the line 223, e.g. me first cooking screen assembly 224 and d e second cooking screen assembly 225.

Associated witii the first cooking screen assembly 224 is a first means for recirculating the first portion of liquid wididrawn from the cooking screen assembly 224 into the interior of the digester 221, including line 226, pump

20 227, and heater 228, with reintroduction conduit 229 at about die level of die screen assembly 224. A valve 230 may be provided for extraction prior to die heater 228, into line 231, while dilution liquid, such as white liquor (e.g. 10% of the total white liquor utilized) is added by a conduit 232 just prior to the heater 228.

25 Second means for recirculating some withdrawn liquor, and extracting other withdrawn liquor, is provided for the second cooking screen assembly 225. This second system comprises the conduit 235, pump 236, heater 237, valve 238, and reintroduction conduit 239. One portion of the liquid is

augmented with dilution liquid in conduit 242 while dilution liquid in the form of white liquor is added in line 241, and while some liquor is extracted in line 240. In this way, die DOM concentration is greatly reduced in the cooking zone adjacent die screen assemblies 224, 225. 5 Located below the second cooking screen assembly 225 is extraction screen assembly 245 having a conduit 246 extending tiierefrom to a valve 247. From the valve 247 one branch 248 goes to the first flash tank 249 of a recovery system which typically includes a second flash tank 250. Some of the liquor in line 246 may be recirculated by directing valve 247 into line 251.

10 The digester 221 furtiier comprises a third screen assembly 253 located below die extraction screen assembly 245, and including a valve 254 branching out into a wimdrawal conduit 255 and an extraction conduit 256. That is, depending upon d e positions of die valves 247, 254, liquid may flow from line 246 to line 255, or from line 256 to line 248.

15 The line 255 is connected by pump 257 to heater 260 and return conduit 261 at about d e level of d e tiiird screen assembly 253. Dilution liquor is added to the line 255 before the heater 260, white liquor (e.g. about 15 % of the white liquor used for cooking) being added via line 258, and dilution liquid, such as wash filtrate, from source 243 being added via line

20 259.

The digester 221 also includes a wash screen assembly 263 including a wididrawal conduit 264 to which white liquor from source 233 may be added (e.g. 15 % of die total white liquor for the process) via line 265. A pump 266, heater 267, and return conduit 268 for re-introducing wididrawn liquid at about

25 die level of the screen assembly 263, are also provided. Wash filtrate is also added below die screen assembly 263 by conduit 269 connected to wash filtrate source 243.

In one exemplary operation according to die invention, 55 % of die white liquor used for treatment of the pulp is added in line 271 to impregnate the chips as they are handled by die high pressure transfer device 218 and sluiced into the line 219, 5% is added to die high pressure feeder 218 via line 272, 10% is added, collectively, in lines 232, 241 (e.g. 5% each), and 15% is added in each of die lines 258, 265.

Utilizing the single vessel hydraulic continuous digester assembly 215 of FIGURE 20, a low level of DOM will be maintained, and additionally, tiiere are numerous modes of operation. For example, at least each of the following three modes of operation may be provided:

(A) Extended modified continuous cooking with extraction/dilution at die lower cooking screens: In this mode, die digester 221 operates with conventional extraction in line 246, and witii extended modified continuous cooking, white liquor being added in 232, 258, 265. Extraction also occurs in line 240 widi a corresponding dilution liquor added at 242 from the wash filtrate 243, resulting in a DOM-reduced liquor flow either counter-current or co-current between the extraction screen assembly 245 and die lower cooking screen assembly 225. Whether the flow is counter-current or co-current depends upon die values of the extractions at 240, 246.

(B) Extended modified continuous cooking witii extraction/dilution at modified continuous cooking circulation: In mis mode, all of die flows just described witii respect to (A) are utilized and in addition an extraction occurs in line 256, valves 247, 254 being controlled to allow a portion of the liquid from the third screen assembly 253 (the modified continuous cooking screen assembly) to pass to line 248. Dilution liquid to make up for this extraction is added at 259, resulting in yet

another reduced DOM, counter-current liquid flow between d e screen assemblies 245, 253.

(C) Displacement impregnation and extraction dilution in upper cooking screens: This mode may be used alone or with a conventional modified continuous cooking process, or in addition to die modes (A) and (B) above. This mode includes extraction at the upper screen assembly 224, as indicated by a line 231, under die control of valve 230, and dilution witii white liquor in line 232. Additional dilution can be provided from line 259 (not shown in FIG. 20). This results in displacement impregnation, which occurs when a counter-current flow at the inlet to me digester is induced not by an extraction, but by the liquor content of the incoming chips. Low liquor content of the chips will cause the hydraulically-filled digester 221 to force dilution flow back up into die inlet 220 which results in a counter-current flow of reduced DOM liquor.

The system 215 illustrated in FIGURE 20 is not limited to the modes A-C described above, but tiiose modes are only exemplary of the numerous modified forms the flow can take to utilize the low DOM principles according to the present invention to produce a pulp of increased strength. Note that all of the embodiments of FIGURES 16 and 18 through 20 may be retrofit to existing mills, and exact details of how die various equipment is utilized will depend upon die particular mill in which tiie technology is employed. All will result in the benefits of reduced DOM described above, e.g. enhanced strength, enhanced bleachability, reduced effective alkali consumption, and/or lower H factor. This is best demonstrated for the configuration of FIG. 19 with respect to FIGURES 21-25.

In FIGURE 19, 185 is considered the first extraction, 200 the second extraction, 207 the third extraction, 214 the first dilution, 202 d e second dilution, and 213 the third dilution.

FIGURE 21 shows a computer simulation comparison of the DOM profiles for a standard EMCC ^® cook and a similar cook according to die invention using me system of FIGURE 19 with extended co-current cooking. In a standard EMCC ^® cook, extraction is from conventional extraction screens and white liquor is added to die conventional cooking circulation and wash circulation, with the liquor flow from the top of the digester to die conventional extraction screens being co-current, while the flow for the remainder of die digester is counter-current. According to die extended co- current mode of FIGURE 21, the third extraction 207 is the primary extraction so that co-current cooking takes place all the way to screen assembly 206. FIGURE 21 shows the conventional EMCC ^® cook by graph line 275, and die cook according to the extended co-current cooking mode by graph line 276. In the computer model generating FIGURE 21, die tonnage rate was 1200 , ADMT/D and die distribution of white liquor was 60% in the impregnation 184, 5 % in the BC line 214', 15% in the MCC ^® circulation 201, and 20% in the wash circulation 210. At 213 1.5 tons of liquor per ton of pulp washer filtrate was added as counter-current was liquid.

As can be seen from FIGURE 21, although the DOM concentration is initially reduced in die cooking zone, tiie DOM concentration is greater in the counter-current stage. Therefore, little improvement in DOM concentration is provided with tiiis form of extended co-current cooking (276). While d e computer model does have some limitations, FIGURE 21 does show tiiat DOM concentration can be varied throughout the cook.

FIGURE 22 illustrates the theoretical effect of adding white liquor at 201 and low DOM dilution liquor at 202 in FIGURE 19. In FIGURE 22, 1.0

tons of liquor per ton of pulp washer filtrate is added at 202, along widi 0.6 t/tp white liquor. A corresponding liquor flow of 1.6 t/tp is extracted at 200. As seen by graph line 277, compared to graph line 276 of FIG. 21, d e resulting DOM concentration drops dramatically between the screens 196, 206. FIGURE 23 shows the effect of varying the distribution of washer filtrate to dilution at 202 and 213. In tiiis case the total washer filtrate of 1.5 + 1.0 = 2.5 t/tp is distributed at 213 and at 202. Graph line 278 shows a simulation for 1/3 of the dilution liquor being added at 202; 279, 1/2 at 202; and 280, 2/3 at 202 (the rest at 213 in each case). Thus, it is clear that DOM profile varies significantly with varying dilution flow, and die more dilution is added to the cooking zone, die more the DOM decreases there (though increasing in the wash zone).

FIGURE 24 illustrates the tiieoretical effect of varying the extraction at 200. Graph line 281 predicts die DOM profile where the extraction at 200 is 1.35 t/tp; line 282, where the extraction at 200 is 1.85 t/tp; and line 283, where the extraction at 200 is 2.6 t/tp. In each case tiie total 2.5 t/tp dilution is split evenly between 202 and 213, and an additional 0.6 t/tp white liquor is added at 201. FIGURE 24 clearly shows that the theoretical DOM concentration in the cooking zone decrease with increased extraction at 200, and is essentially unchanged throughout the counter-current zone. Therefore, this extraction can be varied to accommodate extraction-screen pressure drop witiiout affecting the DOM profile very much.

FIGURE 25 shows the effect of extracting from 185 (the top of the impregnation vessel 180) to create a zone of counter-current impregnation while employing extended co-current cooking with dilution. In tiiis case the reference co-current impregnation vessel data are identical to tiiose shown in FIGURE 22. The extraction flow 185 is 1.1 t/tp; the extracted liquor is not replaced by washer filtrate, but by white liquor at 184. In the previous models

of FIGURES 21-24, 60% of the white liquor added was added at 184 and 5 % at 214'; in FIGURE 25, these are reversed, 5% at 184, and 60% at 214'. Graph line 284 shows the results for co-current impregnation vessel flow, while line 285 shows the results for counter-current flow (60% white liquor at 214'). Thus, this demonstrates tiiat the theoretical DOM concentration decreases botii in the vessel 180 and in the cooking zone, and is comparable in the counter-current cooking zone. Thus, lower DOM concentrations are possible due to extraction in the vessel 180 in addition to extraction and dilution in die digester 189. It will tiius be seen that according to the present invention, a method and apparatus have been provided which enhances the strengtii of kraft pulp by removing, minimizing (e. g. by dilution), or passifying DOM during any part of a kraft cook and/or enhancing otiier pulp or process parameters. While the invention has been herein shown and described in what is presently conceived to be die most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made tiiereof within tiie scope of the invention, which scope is to be accorded die broadest interpretation of the appended claims so as to encompass all equivalent structures, methods, and products.