CA2049322C

CA2049322C - Process for preparing kraft pulp

Info

Publication number: CA2049322C
Application number: CA002049322A
Authority: CA
Inventors: Panu Tikka
Original assignee: Metso Chemical Pulping Oy
Current assignee: GL&V Management Hungary Kft
Priority date: 1990-02-09
Filing date: 1991-02-11
Publication date: 2002-08-20
Anticipated expiration: 2011-02-11
Also published as: JP2900091B2; ES2091319T3; NO913873D0; FI93866B; EP0468016B1; CA2049322A1; FI900663L; ZA91958B; NZ236941A; BR9104333A; NO178935C; WO1991012368A1; ATE141351T1; DE69121332T2; DE69121332D1; NO913873L; FI900663A0; PT96713B; AU639304B2; NO178935B

Abstract

Processes for preparing kraft pulp are disclosed. The processes include pretreating cellulosic material or chips with spent cooking liquor at the temperature of about 20 to 100°C, followed by heating the impregnated chips at the temperature of from about 120 to 180°C, followed by digestion of the lignin with white liquor, which is facilitated by using this pretreatment process.

Description

~p ~y,~G~T~Y~
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PROCESS FOR PREPARING KRAFT PULP
FIELD OF THE INVENTION
The present invention relates to a process for preparing kraft pulp. More particularly, the present invention relates to processes for preparing kraft pulp in which cellulosic material is treated with white or fresh liquor for dissolving the lignin therein. The present invention specifically relates to the pretreatment of the lignin-containing material before the lignin digestion step.
BACKGROUND OF THE INVENTION
In the various kraft pulp processes cellulosic material or chips are generally treated at elevated temperatures with alkaline cooking liquor containing sodium hydroxide and sodium hydrogen sulfide. In these processes, fresh cooking liquor is generally referred to as white liquor, and spent liquor is generally referred to as black liquor.
On a chemical basis, the kraft pulp process used industrially is the same toclay as was the case one hundred years ago, while it is true that many different chemical means have been proposed for the purpose of improving factors such as-the yield and selectivity of the processes, none of these proposals has led to acceptable practical solutions to these problems because each of them has entailed complicated equipment, additional process steps or the use of expensive chemicals.
In addition, different chemical methods for the pretreating of chips have also been proposed. Many of these proposed chemical pretreatment methods have been based upon the use of hydrogen sulfide or bisulfide. For example, Finnish Patent No. 28611 describes a pretreatment process utilizing hydrogen sulfide under elevated pressure. Also, Swedish Patent No. 309530 relates to a pretreatment process utilizing liquid hydrogen ~sulf.ide at a pH of between 4 and 10.

Polysulfide treatrnent has also been proposed as a second pretreatment step, The kraft process, however, has been developed by means of different technical processing means. In particular, the need to save energy has led to new solutions, the most important of which have been continuous cooking processes (see, e.g., Finnish Patent No. 54155). The equipment used in such continuous cooking processes can include the use of several ca- and countercurrent circulations, as well as separate impregnation vessels.
Batch processes have also been developed for the purpose of saving energy. In many of the processes which have thus been developed, hot black liquor is displaced from the digester prior to discharge. This displaced liquor is then used for preheating the chips, or as cooking liquor in subsequent batches (see, e.g., U.S. Patent No. 4,578,199 and Finnish Laid Open Publication No. 71:176).
It has been proposed to improve the quality of the pulp being produced by avoiding digester discharge which utilizes hard hot blow techniques. This can be accomplished by using the cold blow method (see, e.g., Finnish Patent Application N0. 791205 published October z"5 12, 1980), or by means of pump discharge (see, e.g., U.S. Patent No. 4,814,042).
SU ARY_OF THE INVENTION
In accordance with the present invention the objects of this inu~ention and improvements in the kraft :30 pulp process have now been provided by means of a process for the preparation of kraft pulps from lignin-containing cellulosic materials, which comprise impregnating the cellulosic material with spent alkaline cooking liquor at a temperature of between about 20 and 35 100°C, heating the impregnated cellulosic material at a temperature of between about 120 and 180'C, and delignifying the heated cellulosic material with fresh alkaline cooking liquor.

laa accordance faith one embodir~eat of the process of the presetat invention, impregnating of the sellulosic material with spent. alkaline soaking liquor employs lic~uor having a pH of between abcr~t 11.5 and 13.x, andl preferably between ak~aut 12.5 and 1~.5.

In acaarda:nce with a preferred embadimen~ of the px-cacess of the presewt irav~!ntian, heating of the impregnz~ted cellulosfc ~nateri~al is car~ciad out for a.

period of from about 1 fm 30 minutes, whereby the pH c~f 1.0 the apent albtalins soaking liguar impregnated into tree cellulosic material is decreased to between about 9 and 11, and preferably to between about. 9.5 and 10.x.

,~, In accordance with another c~mboditnent of the ' process of the g~res~eat inrrerstion, the wpan't and tresfi~

1~ al3caline cooking litluor comprises st~diur~ hydroxide.

rre=srdc~.~y, the spent alkaline cooking liquor has a a residual sodiuaa hydroxide .content of betwemn akrout 4 and 2~ qratn~ of sodium hydroxide per liter, and more preferably between about s and l~ grams of sodium 20 hydra~cide per liter.

In accordance with ~x~ath~r emt~adiment of the p~racess of the gresent inventia~n the $tep of heating the impregnated ce11ea1osi~ matr~~rial is carriec9 o~av at a temperature of be'~ween about l;r~ and sa3'~. ~~.~t~a,t~k~ly 25 this step is carried out far a g~eriod ~f between abat~t and ~0 minutes.

In asaarr3ance ~rith on~~ preferred embodiment of the prooess of the present invaratian the cellulosic material is hardwood and the step of delignifyinci the 30 heated aellulosic material a.w carried out using an H-factor o~ between aa~out ~uu as~d 1800, i vivar to produce a readily fiberizeci papez~ pulp.

In accardancg with another preferred . embodamgnt of the ~araCess of 'they present. invention the ~5 callulosfa material is eaft,wood arid the sxep cat delignityfng the heated cellu7.osic material is carried ~ur. using an tt-factor of between a~rout X00 and 700, ire order t-.o produce a .readily fik~erized paper pulp.

In accordance with another embodiment of the process of the present invention the step of delignifying the heated cellulosic material is carried out at a temperature of between about 180 and 1g0C.

The principal advantage of the process of the present invention is that digestion of the lignin with white liquor is greatly facilitated by means of this process.

DETAINED DESCRIPTION

It is essential that an accordance with the present invention the chips are pretreated with spent cooking liquor, or so-called black liquor. This pretreatment takes place in two steps. In the first step the chips are im~iregnated with the spent liquor, and in the second step they are reacted with it.

In the impregnation step the chips are essentially filled with the spent liquor. The temperature of this impregnation step must be below 100C in order to avoid reaction therewith on the surface of the chips. In practice temperatures of from about 20 to 100C can be utilized. The time of this impregnation step should be from at least about minutes, and preferably between about 15 and 20 minutes.

Impregnation times of more than about 30 minutes are 25 unnecessary.

' The pH of the spent liquor is between about 12.5 and 13.5, and the residual alkali content is from about ~ to 20 g NaOH/1, and preferably between about and 15 g NaOH/1.

30 The pretreatment reaction or heating step which follows the impregnation step is carried out at an elevated temperature of from about 120 to 180C.
The reaction time depends on the temperature which is utilized, and is generally from about 1 to 30 minutes.

35 Preferab~.y, a reaction temperature of from about 135 to 155C, and a reaction time of from about 10 to 30 minutes is utilized. In this heating step the residual chemicals in the black liquor react with the wood °

material, and alkali is consumed. The pH within the chips is thus decreased to from about 9 to 10. It is believed that in this altered chemical environment sulfur compounds react with the lignin, and thereby render it more reactive in the digestian step which follows thereafter. It is also assumed that hydrogen sulfide reacts with the end groups of carbohydrates in the wood, thus protecting them against alkaline decomposing reactions.
Pretreatment of the chips in this manner renders the subsequent digestion step substantially easier. The severity of the ;ligestion conditions which are required (i,.e., reaction temperature and time:) is generally determined by-the so-called H-factor. In a normal kraft process of, 2.g., Scandinavian softwood, H-factors of from about 1600 to 1800 are required. In the present process, H-factors can be diminished by about 400 to 1000. This means that the overall.
digestion time can be significantly shortened. On the other hand, it has also been observed that exceptionally high digestion temperatures, such as from about 180 to 190°O, can be employed in the present process. This can lead to further shortening of the digestion time. In conventional kraft processes, the digestion step generally takes about one hour. In accordance with the present invention, however, digestion times of about one-half hour are now possible.
An additional advantage of the present process is the increased selectivity of the delignification reaction. This, in turn, leads to higher yields and superior pulp quality, or to a lower consumption of cooking chemicals.
Because of the increased selectivity of the digestion step, and of the quality and yield of pulp, the digestion reaction can now also be run for a longer period of time, and a lower lignin concentration can thus. be achieved than is the case in conventional processes. The pulp which i.s obtained thereby thus a requires less bleaching, which, in turn, decreases the amount of harmful compounds which are discharged from the bleach plant into the waste waters therefrom.
Accordingly, by utilization of the present process there are a number of advantages which can be achieved, depending upon one's specific individual requirements.
It is essential in understanding the role of the present invention that it be appreciated that it constitutes an intermediate process stage before the reaction environment is rendered strongly alkaline by the addition of fresh or white liquor. Accordingly, that stage can ,be incorporated with virtually any type of cooking process which utilizes kraft delignification.
In batch cooking techniques, all of the steps can be carried out in the same reactor, i.e., the digester. After the black liquor impregnation step, the contents of the digester are heated to a temperature in the range of the reaction temperature in the case of (j) conventional batch processes, by means of the digester circulation being equipped with a heat exchanger, or by direct steam injection, and (ia) in case of low energy batch cooking, using the displacement technique, by displacing the colder impregnation black liquor with hotter black liquor for the purpose of carrying the process heat back to the digester.
Another embodiment of this invention utilizing batch digesters is to impregnate the chaps with the black liquor in the context of chip filling in separate equipment. The reaction stage would thus appear as the first step in the digester after chip filling, and could be very effectively carried out by the use of direct steam subsequent to the draining of the impregnation black liquor, or by displacing the impregnation/filling media black liquor by hotter black liquor. In this case continuous impregnation is carried out while charging the digester and is combined with batch cooking techniques, thus resulting in (.a) compensation for the ~.~,~R, afiTitefi ~ _ :'fin,.>s~,r,s extra time spent with the black liquor stage, and (ii) reduction of the total cooking cycle time due to the greater speed of the cooking step.
The present invention can also be carried out in connection with continuous cooking processes. The continuous digester equipment presently being used, including separate impregnation vessels and various co and countercurrent circulations, effectively segregate the cooking process into several steps, in which the present invention can include starting the process with black liquor and without white liquor. Accordingly, the chips are fed into the digester or impregnation vessel ""~; along with the black liquor, the temperature is elevated to the reaction range by heating with the aid of liquor circulation-heat exchanger. After a process delay which corresponds to the time required for the black liquor and waod to interact, the white liquor is then fed into the digester, displacing the black liquor, the temperature is again increased by means of a circulation-heat exchanger and the rest of the process is carried out in the conventional manner. An alternative continuous process is to carry out the black liquor treatment stage as a countercurrent operation.
In continuous cooking processes, application of the present invention can lead to remarkable results.
Utilizing the present conventional processes, continuous cooking to kappa numbers of about 30 generally requires a reaction time of from 60 to 90 minutes in the cooking temperature range. If extended cooking to lower kappa ~0 numbers of between about 23 and 25 are required, an extra cooking stage, and an additional 60 minutes of cooking time is generally required, thus totalia~g at least two haurs of cooking time. By utilizing the acceleration of the delignification step of this invention, however, the cooking time, and the size of the cooking zone in the continuous digester, can be cut in half, therefore also rendering the equipment cheaper, and its operation far simpler.

_g-Example 1 A forced circulation 20 liter digester was charged with pine chips in an amount corresponding 'to 3kg of absolutely dry wood, and 15 liters of spent black liquar was added (pH 13.2, residual alkali concentration 7 g NaOH/1 as effective alkali), so that the liquid ratio was 5:1. The digester was then closed, and pressurized with nitrogen in order to permit the taking of samples and the equalization of impregnation.
The circulation was initiated, and the temperature of the digester was elevated from 20°C to 70 ° C in five minutes by means of a heat exchanger, and it was then held at that temperature for 55 minutes.
Samples were then taken from the circulation, cooled down to 25°C, and their pH measured. The procedure and development of the pH in the Cook are shown in FIG. 1.
The procedure was then repeated using a different temperature profile, as follows:
- 70°C 5 mln.
20 70°C 10 min.
70 - 140°C 10 min.
140°C 20 min.
This procedure, and development of the pH of this Cook, are shown in FIG. 2 25 It can be seen in FIGS. 1 and 2 that the black liquor treatment at 70°C consumed the residual alkali by only a small amount, and the pH fell rapidly when the.
temperature was elevated. i~hen the temperature had been elevated to 140°C in 10 minutes, the pH had thus already fallen to 11.5, and when the treatment was continued at 140°C, in 20 minutes the pH further fell to 10.2.
This Example demonstrates that when the system is heated above 100°C a new reaction phase is initiated in which the residual alkali is rapidly consumed. Since the final pH's were 11.8 and 10.2, it can be seen that, in the latter experiment the H+-ion concentration is almost one hundred times greater than is the case in the former case. Since the pH could only be measured from ~'Ti 7 ~z~r-~,v-D
a~~L~~a~~,kr~'.ss~s the circulating cooking liquid, it is thus clear that in the latter experiment within the chips themselves the consumption of alkali would actually be even greater.
Example 2 An industrial batch digester having a capacity of 140 m3 was filled with pine chips and spent black liquor (pH 13.4) from previous cookings. The temperature was elevated to 140°C, and maintained at that temperature for 15 minutes. The pH thus decreased to 11. White liquor was then added so that the alkali dosage was 18.2 of effective alkali, given as Na20.
The temperature was then raised to 1'70°C, and digestion continued to the desired level of delignification reduction, by altering the digestion time. The digester was then discharged, H-factor utilized registered, and the pulp was analyzed.
This digestion procedure was carried out six times by changing the strength of the black liquor pretreatment, but at the same time keeping the alkali dosage and the overall procedure constant. The following results were obtained:
Experimental Cook 1 Hlack liquor impregnation at 85°C for 20 minutes. White liquar was added directly ~ after filling with black licguor.
H-factor 1420 Kappa number 27.0 viscosity loco Experimental Cook 2 Black liquor impregnation at 90°C for 20 minutes. White liquor was added directly after filling with black liquor.
H-factor 1110 Kappa number 38.3 Viscosity 1135 ,~~.c, ~ ~ ~-r~;;~
_ tCe .: ~...9~i!~.,m Experimental Cook 3 Black liquor impregnation at 90°C far 20 minutes, and black liquor treatment at 125°C for 10 minutes.
5 H-factor 1214 Kappa number 29.6 Viscosity 1115 Experimental Cook 4 Black liquor impregnation at 90°C for 20 10 minwtes, and black liquor pretreatment at 145°C for 20 minutes.
H-factor 860 Kappa number 36 Viscosity 1160 Experimental Cook 5 (Like Cook No. 4) H-factor 1077 Kappa number 25.3 Viscosity 1065 Experimental Cook 6 (Like Cook No. 4) H-factor 1089 Kappa number 25.4 Viscosity 1045 These results are~also presented in F1G. 3, which shows the H-factor in each digestion as a function of the kappa number of the pulp obtained therein.
The effect of black liquor pretreatment on the acceleration of digestion can be seen by observing the H-factor required, or the digestion time at constant temperature. rn order 'to achieve a kappa number of 30, 1325 H-factor units are required if the impregnated chips are net heated, but digestion is carried out immeditely after the impregnation step (see line-through points 1 and 2). When mild heating was utilized {125°C
for 10 minutes), 1220 H-factor units were required (see point 3). When.strong pretreatment was utilized {145°C
for 20 minutes), a kappa number of 30 was achieved with 980 H-factor units (see line-through points 4, 5 and 6).
With conventional batch digesting techniques about 1600 to 1800 H-factor units are required in order to achieve a kappa number of 30.
The effect upon the duality' of the pulp was examined by combining 'the pulp samples from Cook Nos. 1 and 2, so as to represent cooking without black liquor treatment, and by combining the pulp samples from Cook Nos. 4, 5 and 6, so as to represent cooking with black liquor treatment. In FIG. 4 the duality of these pulps is compared by setting forth the tear index as a function of the tensile strength. It can thus be seen that, e.g., at a tensile strength of 70, the tear index of the pulp thus obtained employing the treatment (see curve A) is 1 to 2 units higher than that of pulps produced without utilizing this treatment.
Exa~le 3 In this example two experimental Cooks were carried out to far greater degrees of delignification.
Cook SB
This Cook was carried out in the manner of Experimental Cook 23os. 4, 5 and 6 in ' Example 2 with the following exceptions:
An alkali charge of 20% effective alkali as Na20 per wood H-factor 1850 Fulp kappa number 15.2 Pulp viscosity 905 Cook C
This Cook was carried out in the manner of a conventional batch Cook, without black liquor impregnation and treatment stages:
The alkali charge was 21% effective alkali as Na20 per wood H-factor 2000 Pulp kappa number 71.1 Pulp viscosity . 9G5 ~YJ~~ ~.., ~T~
~s~ ~'~ i_.Ae~ ~'7 U ~l M IIVp -12°
The pulps were analyzed in terms of strength by tear-tensile comparison, as is illustrated in FIG. 5.
It is clear therefrom that, when the tensile index is increased to the useful range for paper making by beating (i.2., a tensile index of from 70 to 80), the conventionally cooked pulp loses its tear strength (curve "C"), while the pulp cooked with the treatment stage of the present invention still maintains its tear strength (curve °'SB°') . The advantage for pulp °'SB°' is three tear index units, or from 20 to 25~ higher.
At present, cooked Scandinavian market pulps, at a kappa number of 30, demonstrate a tear index of from 13 to 15 at a tensile index of 70. In terms of present-day pulping technology, those few mills which apply cooking to lower than normal kappa numbers generally regard a kappa number of from 23 to 25 as representative of "extended cooking,' Results of a nature of those shown above, which were obtained by using the beneficial black liquor-temperature treatment hereof, have only been achievable in the past after a post-digester oxygen delignification process.
Exampl a 4 This example demonstrates a unique-way to take advantage of the black liquor-temperature treatment stage of this invention. It is generally known, both in mill practice and textbooks, that the maximum sulphate cooking temperature should not exceed 175°C due to the severe pulp strength losses which result therefrom, as well as the lower yield which will then be realized.
An experimental cook was carried out as in Example 2, Cooks 5 and 6, except that the cooking temperature was not limited to 170°C (curve "NTP" in FIG. 6j, but instead the cook was heated up as far as was possible with the available steam and heat exchangers (curve "DTP" in FIG. 6). The end temperature was i81°C. All other cooking conditions were equal.
Temperature of the black liquor treatment was 145°C. The time of !? ~I-. y ' ~~~~~~~~i;~
o~~..
b~.~ck lieluor tr~at~nsnt was ~0 m1n11'tL~. '1'ri~ dlK~li ulacaa~a s~a5 18.2 effective alkali as ~7a~~7 par wood F3~fact~o~~ 1050 (~xampl.s 2 A ~:Of7lC~ $, 5: 1080, ~lp 3cappa numher 28.1 ~~'~t~~~~s 2, ~~3S~k~ S' ~ 0 2~. ~?
The tsar-kensils r~la~t3onsta~.p of the ~ul~ ~t~as 1~ ~naly~sd in order ts~ evaluates the ~r~tlp strength. ~t a useful tensile ir~dsx six 7b, the tsar index was 1b, which eq~ais the va3.ue found an curvy ~,~~ i1~ F'I~. 4 in example aPPlYing a . nc~~n~7. cooking t~rnperat~ui°~ and bl~cl~
3i,c~uor treat~uant. ~°h.i~ sl3.gtatl~ exceeded ttaa'~ of a noa~nal oooking '~em~eaature with no blac3~ liguoac treatment.
This retention of pulp strength can bs of considsrabi~ ~ignificat~c~ ~rhen c~x'eat~r produat.ion p~
diges'ce~' volua~~ unit is requis~d4 ~I~. 8 eats forth 2a compaa°ison between ooo9cing t~~nper~ture and time pl°mf3l~~s f~~' t.h~ ee~olk in thj~ '~aeampl~, end ~h~t~ of Coo3c 3~os. 5 and ~ in ~xampi~ 2, raPresenting ns~~ma1 cooking temperatures. ' lend t~mperatugs~ 1~1'C
fin~sl ~I-tact~r 10~0 Time to end from 19~°~ 6o minuta$
End temperature 17~"~
~~ final 11-faCto~' 100 Time tc~ end from 14~'~ 10o minutes It is evident from these results that the cooking time after 44 mi.nutss bf heat.ing was cu'G dawn to 20 minutes by t'h~e ~Zigh tempora~tur~ profile, instead of ~0 minutes with constant 170°C oooking tempsratur~.
minute ~avings~ in cook.znc~ time easily r~prsssnts a 15 to 20~ lrawer total cys~le time, with the c~r~espondi~as~
oppox'tunity to incrc~s~a production without oQ~npro~aisi.ng -14 ° f:'c>..»~~~lr,;r':a pulp quality. In terms of yield it appears that the yield of the very fast cooking method of this invention is then 1 to 2~ higher.
Example 5 The results of this Example demonstrate that the pulps inside the digester prepared in accordance with the present invention are in extremely good condition to resist the physical damage during the discharge which arises by various blow methods, as compared to pulps cooked without the use of such a black liquor treatment stage.
The pulp conditions prior to the blow were determined by hanging baskets filled with the same chip material inside the digester. After the blow, pulp which had not been blown could thus be recovered from these baskets, and compared to samples of the blown pulp.
Tn this case, the analysis carried out was in terms of a so-called strength delivery, which is the percentage of the pulp strength as tear index at a tensile index of 70 measured in the blown pulp as compared to that of non-blown pulp ira the basket.
The Cooks were carried out with a black liquor treatment stage as described in Example 2, Cook Nos.
4-6, discharged by: hot blow, directly from full cooking temperature; cold blow, after cooling displacement to under 100°c; and pump discharge after cooling displacement.
Reference data is given from U.S. Patent No.
4,814,042, which represents the effect of the blow method subsequent to conventionally cooked sulphate batch cooks.
The following table summarizes these results.
Table 1 (Pulp quality given as strength delivery ~ percentages of blown pulp compared to that of non-blown pulp strength.

_15_ K~ Via. wJ~
Sulphate Cooking Conventional with Treatment of Batch Discharge Method This Invention Cooking Hot Blown Pulp 95 77 Cold Blown Pulp 99 85 Pump Discharged Cold Pulp 99 90 It is evident from Table 1 that pulp cooked by a method comprising the black liquor treatment of this invention does not require any improvement in terms of strength delivery, and the pulp is in optimum condition.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the.
illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of kraft pulp from lignin-containing cellulosic material, comprising (a) pretreating the cellulosic material. in two steps, whereby said cellulosic material is firstly impregnated with spent alkaline cooking liquor having a pH of between about 12.5 and 13.5 at a temperature of between about 20 and 100°C, and then reacted with spent alkaline cooking liquor at a temperature of between about 120 and 180°C, whereby the pH of said spent alkaline cooking liquor impregnated into said cellulosic material is decreased to between about 9 and 11, and (b) delignifying said heated cellulosic material with fresh alkaline cooking liquor.

2. The process of claim 1, wherein said spent and fresh alkaline cooking liquor comprises sodium hydroxide and said spent alkaline cooking liquor has a residual sodium hydroxide content of between about 4 and 20 grams of sodium hydroxide per liter.

3. The process of claim 1 or 2, wherein said impregnated cellulosic material is heated so that the pH of said spent cooking liquor is decreased to between about 9.5 and 10.5

4. The process of any one of claims 1 to 3, wherein said spent and fresh alkaline cooking liquor comprises sodium hydroxide.

5. The process of claim 4, wherein said spent alkaline cooking liquor has a residual sodium hydroxide content of between about 4 and 20 grams of sodium hydroxide per liter.

6. The process of claim 5, wherein said spent alkaline cooking liquor has a residual sodium hydroxide content of between about 6 and 15 grams of sodium hydroxide per liter.

7. The process of any one of claims 1 to 6, wherein said step of impregnating said cellulosic material is carried out for a period of between about 10 and 30 minutes.

8. The process of claim 7, wherein said step of impregnating said cellulosic material is carried out for a period of between about 15 and 20 minutes.

9. The process of any one of claims 1 to 8, wherein said impregnated cellulosic material is reacted with spent alkaline cooking liquor at a temperature of between about 135 and 155°C.

10. The process of claim 9, wherein said impregnated cellulosic material is reacted with spent alkaline cooking liquor for a period of between about 10 and 30 minutes.

11. The process of any one of claims 1 to 10, wherein said cellulosic material comprises hardwood and said step of delignifying said heated cellulosic material is carried out using an H-factor of between about 900 and 1000.

12. The process of any one of claims 1 to 10, wherein said cellulosic material comprises softwood and said step of delignifying said heated cellulosic material is carried out using an H-factor of between about 400 and 700.

13. The process of any one of claims 1 to 12, wherein said step of delignifying said heated cellulosic material is carried out at a temperature of between about 180 and 190°C.