NZ236941A - Preparation of kraft pulp: impregnation with spent alkaline cooking liquor, heating, and subsequent delignification with fresh alkaline cooking liquor - Google Patents

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 DEG C, followed by heating the impregnated chips at the temperature of from about 120 to 180 DEG C, followed by digestion of the lignin with white liquor, which is facilitated by using this pretreatment process.

Description

2 3 6 9 4 1 r p-> cl. .Cc.h JWlv. ! | 1.1 .ijjiolv | Ch^: . P2-J.C.I.j.0b, .DXiCSjO^ Publication C-;-- .. ? £ P^I ®.. P.O. Journal, Mo: ...|3rf3. i Patents Form No. 5 Patents Act 1953 COMPLETE SPECIFICATION PROCESS FOR PREPARING KRAFT PULP We, SUNDS DEFIBRATOR RAUMA OY, of PL 34, 28101 Pori, Finland, a Finnish Corporation hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 (followed by la) 50 236 94 1 V.

PROCESS... FOR. JPErARI.NG >'PAF';.. PUL? FIELD. OFHE. INVENTION The present invention relates to a process for preparing kraft pulp. More particularly, the present 5 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 pre treatment of the 1ignin-containing material before 10 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 1iquor containing 15 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 20 used industrially is the same today 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 25 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 30 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. 29611 describes a pretreatment process utilising hydrogen 35 sulfide under elevated pressure. Also, Swedish Patent No. 3095 30 relates to a pretreatment process utilizing liquid hydrogen sulfide at a pH of between 4 and 10. 236 94 1 ■2- Pclysulride treatment has also c.= en proposed as a second pretre at.jent seep.

The kraft process, however, has been developed by means of different technical processing means. In 5 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 cocking processes can include the use of several co- and 10 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 15 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, 149 and Finnish Laid Open Publication No. 71176).

It has also 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 No. 791205), or by 25 means of pump discharge (see, e.g., U.S. Patent No. 4,314,042).

Sl'MKARY OF THE INVENTION In accordance with the present invention the objects of this invention 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 cocking 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 130°C, and delignifying the heated cellulosic material with fresh a 1 >;a 1 ine cooking 1 .iquor. 236 94 1 In accordance with one embodiment or" the process of the present invention, impregnating of the cellulosic material with scent alkaline cocking liquor employs liquor having a pH of between about 11.5 and 5 13.5, and preferably between about 12.5 and 13.5.

In accordance with a preferred embodiment of the process of the present invention, heating of the impregnated cellulosic material is carried out for a period of from about 1 to 30 minutes, whereby the pH of 10 the spent alkaline cooking liquor impregnated into the cellulosic material is decreased to between about 9 and 11, and preferably to between about 9.5 and 10.5.

In accordance with another embodiment of the process of the present invention, the spent and fresh 15 alkaline cooking liquor comprises sodium hydroxide.

Preferably, the spent alkaline cooking liquor has a residual sodium hydroxide content of between about 4 and 20 grams of sodium hydroxide per liter, and more preferably between about 6 and 15 grams of sodium 2 0 hydroxide per liter.

In accordance with another embodiment of the process of the present invention the step of heating the impregnated cellulosic material is "carried out at a temperature of between about 135 and 155°C. Preferably 25 this step is carried out for a period of between about and 30 minutes.

In accordance with one preferred embodiment of the process of the present invention the cellulosic •material is hardwood and the step of delignifying the 30 heated cellulosic material is carried out using an H-factor of between about 300 and 1000, in order to produce a readily fiberized paper pulp.

In accordance with another preferred embodiment of the process of the present invention the 35 cellulosic material is softwood and the step of delignifying the heated cellulosic material is carried out using an H-factor of between about 4 00 and 7 00, in order to produce a readily fiberized paper pulp. 2 3 6 9 4 In accordance with arother :: od i .-.enZ of ■; h e process of the present invention the step of delignifyi.ng the heated cellulosic -r.eria 1 is carried out at a temperature of between about 130 and 190=C.

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.

DETA J LED DESCRIPTION 10 It is essential that in accordance with the present invention the chips are pretreated with spent cocking liquor, or so-called black liquor. This pretreatment takes place in two steps. In the first step the chips are impregnated with the spent liquor, 15 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 100'C in order to avoid reaction therewith on the 20 surface of the chips. In practice temperatures of from about 20 to 100'C can be utilized. The time of this impregnation step should be from at least about 10 minutes, and preferably between about 15 and 20 minutes. Impregnation times of more than about 30 minutes are 2 5 unnecessary.

The pH of the scent liquor is between about 12.5 and 13.5, and the residual alkali content is from about 4 to 20 g NaOH/1, and preferably between about 6 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 130 :C. The reaction time depends on the temperature which is utilized, and is generally from about 1 to 30 minutes. 3 5 Preferably, a reaction temperature of from about 135 to 155 5 C, 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 236 94 1 material, and alkali is ccr.:? d . The cK within the chips is thus decreased to from .jccu: S to 10. It is believed that in this altered chemical er.v i ronmsnt sulfur compounds react with the lianin, and thereby render it more reactive in the digestion 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 decompcs ing react ions .

Fretreatment of the chips in this manner renders the subsequent digestion step substantially easier. The severity of the digestion 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, e.g., Scandinavian softwood, H-factors of from about .1600 to 13 00 are required. In the present process, H-factors can be diminished by about 4 00 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 130 to 190'C, 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 delignificat ion reaction. This, .in turn, leads to higher yields and superior pulp quality, or to a .lower consumption of cock ing chemica .1 s.

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 is obtained thereby thus 23 6 94 1 r-:-qu; res less bleaching, which, in turn, decreases the i.TiO'jr." of harmful corccunds which are discharged fron the clench plant into the waste waters therefrom.

Accordingly, by utilization of the present 5 process there are a nunber of advantages vhich can be achieved, depending upon one's specific individual requ.i rerr.ent s.

It is essential in understanding the role of the present invention that it be appreciated that it 10 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. 15 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 (i) 20 conventional batch processes, by means of the digester circulation being equipped with a heat exchanger, or by direct steam injection, and (ii) in case of low energy batch cooking, using the displacement technique, by displacing the colder . impregnation black liquor with 25 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 chips with the black liquor in the context of chip filling in separate 3 0 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 35 rcc-dia 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 (i) compensation for the 236 94 1 e"J tine spent vith the black liquor stage, and (ii) reduction of Che total cooking cycle tine due to the greater speed o: the cooking step.

The present invention can also be carried out 5 in connection with continuous cooking processes. The continuous digester equipment presently being used, including separate impregnation vessels and various cc-and countercurrent circulations, effectively segregate the cooking process into several steps, in which the 10 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 15 circulation-heat exchanger. After a process delay which corresponds to the time required for the black liquor and wood to interact, the white liquor is then fed into the digester, displacing the black liquor, the temperature is again increased by means of a 20 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 25 of the present invention can lead to remarkable results.

Utilizing the present conventional processes, continuous cocking to kappa numbers of about 30 generally requires a reaction tine of from 60 to 30 minutes in the cooking temperature range. If extended cooking to lower kappa 30 nu-.;:ers of between about 23 and 25 are required, an extra cocking stage, and an additional 60 minutes of cooking tune is generally required, thus totaling at least two hours of cooking time. By utilizing the acceleration of the del ignification step of this 35 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. 236 94 1 Ex -rel e __1 A forced circulation 20 liter disaster was charged with pir.e chips in an amount corresponding to 3kg of absolutely dry wood, and 15 liters of spent black liquor vas 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 SC to 7 0 '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 255C, 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 min. 70°C 10 min. 70 - 140°C 10 min. 140a C 20 min.

'This procedure, and development of the pH of this Cook, are shown in FIG. 2 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 r-apidly 'when the temperature was elevated. When the temperature had been elevated to 140 5 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 co_nsurr,ed. Since the final pH's were 11.3 and 10.2, it can be seen that, in the latter experiment the II+-ion concentration is almost one hundred times greater than is the case in the former case. Since the pH could only be measured from 236 94 1 — j — the circulating cocking liquid, it is th. the latter experiment within the chips themselves the ccr.su-pt i on of alkali vould actually be even greater. Exanole_.2 An industrial batch digester having a capacity of 140 m"5 was filled with pir.e chips and spent black liquor (pH 13.4) from previous cookings. The temperature was elevated to 140sC, and maintained at that temperature for 15 minutes. The pH thus decreased 10 to 11. White liquor vas then added so that the alkali dosage vas 13.2% of effective alkali, given as Na20. The temperature vas then raised to 170'C, and digestion continued to the desired level of delignification reduction, by altering the digestion time. The digester 15 vas then discharged, H-factor utilized registered, and the pulp vas analysed.

This digestion procedure vas carried out six tines by changing the strength of the black liquor pretreatment, but at the same time keeping the alkali 2 0 dcsage and the overall procedure constant. The folloving results were obtained: Ext>er.inental Cook 1 Black liquor impregnation at 35°C for 20 minutes. White liquor vas added directly 25 after filling with black liquor.

H-factor 14 2 0 Kappa nu."bar 27.0 Viscosity 1030 Expe/rjjriental Cook 2 30 Black liquor impregnation at 90'C for 20 minutes. White liquor vas added directly after filling with black liquor.

H-factor 1110 Kappa number 3 3.3 Viscosity 1135 2 3 6 9 4 -1C- Exp_e_r irer ~ I Cook._3 Slack liquor i-r. regr.ac icn a: 90'C for 20 nil notes, and black liquor treatment at 125:C for 10 minutes.

H-factor 1214 Kappa number 29.6 Viscosity 1115 Exjo er.i:r. e n t a 1 _„Coo k ..4.

Slack liquor .impregnation at SO°C for 20 minutes, and black liquor pretreatment at 145'C for 2 0 minutes.

H-factor 3 60 Kappa number 3 6 Viscosity 1160 Experimental .Cook 5 (Like Cook No. 4) H-factor 1077 Kappa number 2 5.3 Viscosity 1065 Exoerirnenta 1 Cook 6 (Like Cook No. 4) H-factor 1039 Kappa number 2 5.4 Viscosity 104 5 These results are also presented in FIG. 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. In order to achieve a kappa number of 30, 1325 H-factor units are required if the impregnated chips are not heated, but digestion is carried out imrr.editely after the impregnat ion step (see line-through points 1 and 2). When mild heating was utilised (125:C for 10 minutes), 1220 H-factor units were required (see point 3). When strong pretreatment was utilized ( 14 5 5 C for 20 minutes), a kappa number of 30 was achieved with 23 6 9 ■?oO H-factor units (see line-through points 4, 5 and 6). With conventional batch digesting techniques about 1600 to 1c 00 H-factor units are required in order to achieve ■a kappa nurbar of 30.

The effect upon the quality of the pulp vas examined by combining the pulp samples from Cook Nos. 1 ajvd 2, so as to represent cooking without black liquor treat/rent, and by combining the pulp samples from Cook Nos. 4, 5 and 6, so as to represent cocking with black 10 liquor treatment. In FIG. 4 the quality 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 15 curve A) is 1 to 2 units higher than that of pulps produced without utilizing this treatment.

Example 3 In this example two experimental Cooks were carried out to far greater degrees of delignification. 20 Cook S3 This Cock was carried out in the manner of Experimental Cook Nos. 4, 5 and 6 in Example 2 with the following exceptions: An alkali charge of 20% effective alkali 2 5 as Na20 per wood H-factor 1S50 Pulp kappa number 15.2 Pulp viscosity 905 Cook,C This Cock was carried out in the manner of a conventional batch Cook, without black liquor impregnation and treatment stages: The alkali ch_arge was 211 effective 35 alkali as per wood H-factor 2 000 Pulp kappa number 71.1 Pulp viscosity 905 2 3 6 9 4 1 The pulps »ers analyzed in ter.ts o: strength by tea r-ter.si ] e comparison, as is illustrated in FIG. 5. It is clear therefrom that, when the tensile index is in or 3 a sod to the useful range for paper making by 5 beating (i.e., a tensile index of from 70 to 30), the conventionally cooked pulp loses its tear strength (curve "C"), while the pulp cooked with the treatment st=i'je of the present invention still maintains its tear strength (curve "SB"). The advantage for pulp "SB" is 10 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 15 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 20 hereof,, have only been achievable in the past after a post-digester oxygen delignification process.

Example 4 This example demonstrates a unique" way to take advantage of the black liquor-temperature treatment 25 stage of this invention. It is generally known, both in mill practice and textbooks, that the maximum sulphate cooking temperature should not exceed 17 5"C due to the severe pulp strength losses which result therefrom, as well as the lover yield which w.ill then be realized. 30 An experimental cook vas carried out as in Example 2, Cooks 5 and 6, except that the cooking temperature vas not limited to 170"'C (curve "NT?" in FIG. 6), but instead the cook vas heated up as far as vas possible with the available steam and heat 35 exchangers (curve "DTP" in FIG. 6). The end temperature vas 131:C. All other cooking conditions vere equal.

Temperature of the black liquor treatment was 145'C. The time of 2 3 6 9 13' i_ r o i - rr. 5 i"" . r.-: cna r q e • JS OiiO/. ii'TJCr minutes. The 13.2% effective alkali as per VOOd H-factor 1000 (Example 2, Cooks 5, 6: 1030) Pulp kappa number 23.1 (Example 2 , Cooks 5,6: 25.4) The tear-tensile relationship of the pulp was analyzed in order to evaluate the pulp strength. At a useful tensile index of 70, the tear index vas 16, which equals the value found on curve "A,T in FIG. 4 in Example 2, applying a nor-mal cooking temperature and black liquor treatment. This slightly exceeded that of a normal cooking temperature with no black liquor treatment.

This retention of pulp strength can be of considerable significance when greater production per digester volume unit is required. FIG. 6 sets forth a comparison between cooking temperature and time profiles for the Cook in this Example, and that of Cook Nos. 5 and 6 in Example 2, representing normal cooking temperatures.

Curve "DTP" End temperature Final H-factor Time to end from 140°C 131 ° C 1000 60 minutes curve NIP" 170*C 1030 100 minutes End temperature Final K-factor Time to end from 14 0'C It is evident from those results that the cockir.g ti-e after 50 minutes of heating was cut down to 20 inutes by the hi•-jh temperature profile, .instead of 60 inutcs with constant 170 = C cooking temperature. A 50 nlnute savings in cooking time easily represents a 15 to 20% lover total cycle time, with the corresponding opportunity to increase production without compromising — 1 * - A. *1 2 3 6 9 4 pulp quality. In terms of yield it appears that the yield of the very fast cooking method of this invention is then 1 to 2k higher.

E x a .T' o 1 e 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 10 compared to pulps cooked without the use of such a black liquor treatment stage.

The oulp conditions prior to the blow were determined by hanging baskets filled with the same chip material inside the digester. After the blow, pulp 15 which had not been blown could thus be recovered from these baskets, and compared to samples of the blown pulp.

In this case, the analysis carried out was in terms of a so-called strength delivery, which is the 20 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 in the basket.

The Cooks were carried out with a black liquor treatment stage as described in Example 2, Cook Nos. 2 5 4-6, discharged by: hot blow, directly from full cocking temperature; cold blow, after coding displacement to under 100°C; and pump discharge after cooling displacement.

Reference data is given from U.S. Patent No. 3 0 4,3 14,042, which represents the effect of the blow method subsequent to conventionally cooked sulphate batch cooks.

The following table summarizes these results. Ta b 1 e _ 1 (Pulp quality given as strength delivery 3 5 percentages of blown pulp compared to that of non-blown pulp strength. 23694 1 Sulphate Cocking Conventional with Tr2":t.7.isnt of Batch P. .1 ;i'3-.tr;pd Th_i s . Tp.vfn.ti on Cpck.i.rig Hot 51own ?uip 5 5 7 7 Cold Blown Pulp 99 3 5 Pump Discharged Cold Pulp 93 9 0 It is evident from Table 1 that pulp cooked by a method comprising the black liquor treatment of this 10 invention doss 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 15 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 2 0 of the present invention as defined by the appended cla ims. 16

Claims (11)

WHAT WE CLAIM IS:-

1. A process for the preparation of kraft pulp from 1ignin-containing cellulosic material comprising the steps of: filling a vessel containing cellulosic material with an alkaline cooking liquor having a pH of between 11.5 and 13.5 and consisting essentially of spent kraft cooking liquor; impregnating said cellulosic material with said alkaline cooking liquor at a temperature between 20° and 100° for a period of time sufficient to provide an equalization of impregnation of said cellulosic material with said alkaline cooking liquor and to essentially fill said cellulosic material with said alkaline cooking liquor, said alkaline cooking liquor being maintained at an impregnation temperature which will avoid reaction between said cooking liquor and said cellulosic material; subjecting said essentially filled, impregnated cellulosic material to a pretreatment reaction at a temperature between 120° and 180° and for between 1 to 30 minutes wherein the impregnated alkaline cooking liquor reacts with said cellulosic material under conditions sufficient to lower the pH of said alkaline cooking liquor to between 9 and 10; and delignifying said pre-treated cellulosic material with fresh alkaline cooking liquor.

2. The process for the preparation of kraft pulp as claimed in claim 1 wherein said impregnation step is conducted for between 10 and 3 0 minutes. ^r,\7Z,-y;'orF!C£ 20 AUG 1993 .J ^' t I w }#4 1 17

3. The process for the preparation of kraft pulp as claimed in claim 2 wherein said impregnation step is conducted for between 15 and 20 minutes.

4. The process for the preparation of kraft pulp as claimed in any one of claims 1-3 wherein said impregnation temperature is between 7 0°C to 100°C.

5. The process for the preparation of kraft pulp as claimed in any one of claims 1-3, wherein said impregnation temperature range is between 50'C and 70 C.

6. The process for the preparation of kraft pulp as claimed in claim 5 wherein said pretreatment reaction is conducted over a time between 15 and 20 minutes.

7. The process for the preparation of kraft pulp as claimed in any one of claims 1-4 wherein said pretreatment reaction includes elevating the temperature of said impregnated alkaline cooking liquor and said cellulosic material to between 135°C and 155°C by directly heating said alkaline cooking liquor.

8. The process for the preparation of kraft pulp as claimed in claim 1 wherein said pretreatment reaction includes elevating the temperature of said impregnated alkaline cooking liquor and said cellulosic material to 140°C by displacing the said non-impregnated alkaline cooking liquor with hotter said liquor.

9. The process for the preparation of kraft pulp as claimed in any one of claims 1-4 wherein said pretreatment reaction includes elevating the tempereture^pp^ 'feaid 20 AUG 1993 4 f 18 impregnated alkaline cooking liquor and said cellulosic material by displacing the non-impregnated alkaline cooking liquor from the vessel with a liquor having a temperature between 135;C and 155°C.

10. The process for the preparation of kraft pulp as claimed in any one of claims 1-9 wherein said delignifying is conducted at a temperature between 180°C and 190CC.

11. A process for the preparation of kraft pulp substantially as hereinbefore described with reference to anyone of examples 1-5. SUNDS DEFIBRATOR RAUMA OY by their authorised agents P.L. BERRY & ASSOCIATES PAVEW7 CFf-iCE 20 AUG 1993