NZ193228A - Preparation of paper pulp:at least two parallel intermeshed screws rotating in sheath - Google Patents

Abstract

1. Process for grinding and washing a cellulosic material for the preparation of a paper pulp by passing the material, reduced in size to chips and impregnated with a reagent for removing lignin in a machine comprising at least two parallel screws driven in rotation inside a casing and provided with helical flights the pitches of which vary so as to determine several successive treatment zones, each zone including a section for conveyance and progressive compression and a braking section constituted by flights of inverted pitch provided with apertures allowing downstream passage of the material, having a size which is adjusted to allow selective advancement in the downstream direction of the material as a function of the degree of grinding achieved upstream of said apertures, characterised by the fact that the grinding and the washing of the chips is carried out simultaneously in the successive treatment zones of the machine, the casing of which is, for this purpose, constituted by an alternating series of filled regions and filtering regions, covering, respectively, the upstream portion and the downstream portion of each section for conveyance, each filled region being extended into the preceding braking section, the material being subjected as it passes from upstream to downstream in the machine whilst passing through each treatment zone, successively to a dilution by carrying out mixing with an actual washing liquid introduced upstream of the treatment zone then, into the flights of the screws to alternation between relaxation phases and phases of increase in pressure and finally to a drainage operation carried out at the same time as the grinding downstream of the section for conveyance and in the braking section, the used liquid being evacuated through the filtering regions of the casing.

Description

w t 93228 Jgfc-iority Date(s): .Vh Zk \ Xomplete Specification Filed: Cass: Publication Date: .TTV. ...

P.O. Journal, No: ..

NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION "PREPARATION OF PAPER PULP" M / We, CREUSOT-LOIRE, a company incorporated under the laws of France, of 15, rue Pasquier, 75383 Paris Cedex 08, France hereby declare the invention for which }fx/ we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- 193228 The invention concerns improvements in processes for preparation of a paper pulp starting from a lignocellulosic material in the form of wood chips and an improved apparatus for carrying out the new process.

It is known that paper pulp is generally produced starting from lignocellulosic material and, most usually, from wood. In most of the processes used the wood is first converted into pieces called chips and subjected to a treatment for softening or eliminating the lignin in such a way as to 10 permit the following separation of fibres by a mechanical stimulation treatment.

Usually the softening of the lignin is achieved simply by steaming the chips so as to obtain a so-called thermomechanical pulp. But one generally uses chemical reagents and the 15 importance of the mechanical treatment necessary for achieving the separation of fibres varies inversely in relation to the extent of the chemical treatment used. If the mechanical fibrillation treatment is relatively extensive, so-called mechanical-chemical or semi-chemical pulps are obtained. 20 Generally, the fibrillation treatment is carried out in apparatus revolving at high speed, called disc defibrillators.

However, the present applicant has developed a new fibrillation process which is described, together with a Affwr 2 ten)i t/f machine for carrying out the process, in/franch patenfe 3P&<.t£uartoef iStsS'i. * / application no. 75 23911 filed 31sL July;—1975'.

Such a machine comprises two parallel screws rotating t 19 3228 inside a sheath which encases them, the screws being provided with threads of pitch which varies along their length and defines several successive treatment zones; each zone includes a sec'tion for conveying the entrained material downstream by the rotation of the screws, followed by a braking section; the starting material, usually wood chips, is introduced through an orifice situated upstream of the sheath and is entrained by the rotation of the screws until reaching a downstream orifice; each braking section causing the progressive compression of the material towards the end of the preceding conveying section and forming, by accumulation of material, a continuous plug which allows the distinguishing of one of the other two succeeding zones.

T^e apparatus described in the above-mentioned patent J w 15 ^appl-ioatiofi- aims to carry out a fibrillation treatment by using the combined effects of forced compression and shearing obtained by the entrainment of the material by the screws. . > In the fibrillation apparatus described in this patent sp&ttz ie Arto*s 4"^^ ^application, each braking section is made up of threads having reverse pitch which would normally drive the material upstream. Thus, at the end of the conveying section a progressive compression of the material results for overcoming the effects of the braking section. On the other hand, the reverse threads are provided with windows permitting the .. 25 passage downstream of a certain quantity of material and in consequence of controlling the build up of pressure \\\ gPtfci/Sirdf'oV According to an essential characteristic of this/applioatdon, -f- 1932 2 8 by the choice of window size one provides for a selective advancement of the material. In effect, material not having the required degree of fibrillation for passage through the windows is kept in the most compressed zone where it undergoes 5 an intense kneading, notably as a result of the rotation of the screws in the same direction, which combined wth the effect of the pressure results in fibrillation without significant reduction in the length of the fibres.

First, by choice of the window size and in general by 10 choice of the parameters controlling the advance of the material through the apparatus (e.g. mechanical characteristics of the threads, diameter of the screws, speed of rotation, etc.) one is able to carry out in each zone of the apparatus a fibrillation of greater or lesser intensity. 15 However, whatever fibrillation process is used, in every case in which one uses a reagent for softening or dissolving the lignin, it is necessary to subject the pulp after the fibrillation treatment to a washing step for removing the residues from the reaction of the reagent on 20 the wood, which appear principally in the form of a kind of sludge mixed in with the wood. This sludge must be eliminated so as to provide clean fibres for the production of paper.

Up to the present time, it has always been necessary to carry out successively the operations of preparation of the .25 paper pulp, first of all the softening of the chips by impregnation with a reagent, then the isolation of the fibres, and finally the washing step. 1 932 2 It was quite natural to use special materials for carrying out different operations but this resulted in cumbersome installations and expenses which were only economical for productions of considerable scale.

The improvements according to the invention however allow for a considerable economony of material.

Thus, according to the invention, the material impregnated with a reagent is submitted to washing carried out at the same time as the fibrillation in at least one of the treatment zones, the washing liquid being injected at the beginning of the conveying section for effecting a dilution of the material, this being then subjected to drying by filtration off of a part of the liquid phase across cleaning orifices in the sheath upstream of the two braking sections surrounding the treatment zone.

In a preferred mode of operation, the quantity of washing liquid injected after each drying upstream of the succeeding treatment zone is regulated in accordance with the conditions of transport of the material in the braking zone situated downstream so that the degree of drying achieved is limited to 50% to 55% so as to allow the advancement of the material downstream in the following braking section.

The invention will now be described with reference to a particular embodiment, given by way of example and shown in the accompanying drawings.

Figure 1 represents a longitudinal cross sectional view of an apparatus for treatment according to the invention. 9322S fcr 2* Figure 2 is a view from above representing a treatment zone to a larger scale.

Figure 3 is a view along the line III-III of Figure 2.

Figure 4 is a view along the line IV-IV of Figure 2.

In Figure 1 is represented in longitudinal section a treatment apparatus of the type described in the abovementioned SiPtUnZuar HHS9Q. patent/applioation no.—75 23011. » 25 T The apparatus comprises a sheath 1 constituted by two cylindrical lobes enveloping two screws 2 provided with intermeshing threads, as may be seen from Figure 2. The threads are identical and the screws are rotated in the same sense. The sheath is provided at one end with an orifice 10 for introducing the material which is entrained by the rotation of the screws up to the exit orifice 11 situated at the extreme downstream end.

The screws are provided with threads of variable pitch which define a series of successive zones. In the example shown, the apparatus includes four succeeding zones, I, II, III, IV followed by a zone V for removal of the treated material.

According to a characteristic already described in the £P£iirit#riot>/ above-mentioned patent^applieatiui'i, each treatment zone comprises a conveying section A provided with threads of direct pitch for advancing downstream the material in one sense of rotation of the screws, for example in the sense of the arrows shown in Figures 3 and.4, and a braking section B preferably provided with reverse threads tending to return -6- , 9322 the material upstream and thus creating a braking effect on the material and in consequence a compression downstream of the section A.

In addition to the entry orifice 10 for the material, the sheath 1 is provided in its upper part and in its vertical plane of symmetry with a plurality of orifices 12, 13, 14 each placed downstream of a braking section, respectively Bl, B2, B3. .*>^4, te&cap,«c, ^.4" Furthermore, the part 3 of the sheath^ pro go coding each . braking section Bl, B2, B3, B4 is provided with slits 31 allowing the filtration of the material entrained by the rotation of the screws inside the sheath. The sheath is thus constituted by a series of continuous portions 4 and filter portions 3, each filter portion 3 being situated upstream of a braking section B by a length of the order of the remainder of the preceding conveying section A, the ,A& \ continuous portion 4 extending over the preceding braking section and extending into the upstream part of the conveying section.

When the apparatus is intended to carry out a mechanical fibrillation treatment of chips, the reverse threads constituting the braking zones Bl, B2, B3, B4 are provided, as described S P£cii-itAfto*S in the above-mentioned^applioa fa-ion, with windows allowing, the passage downstream of a certain quantity of material. 25 As shown in Figure J>, the windows 24 are the apertures provided in the reverse threads 23 and radially directed from the central shaft 21 towards the periphery of the t 932 2 thread. The windows 24 are regularly spaced apart and the threads are disposed such that preferably two windows simultaneousl; pass in the horizontal plane of symmetry through the screw axes.

The fibrillation process has already been described in 94Tt£*r seecietoiriotf t9tFfi+ '^24" the above-mentioned^application 75-33911. The material, constituted by wood chips, is introduced into the interior >5 of the apparatus through the orifice 10 and is entrained down-stream by the first conveying section A1. On arrival 10 in the zone Bl, the material is slowed down by the effect of the reverse pitch and thus accumulates downstream of the zone Al. There is thus produced in the zone Al in the downstream direction, a compression of the material to a pressure corresponding to the effect of the braking before passing 15 into the following zone situated downstream.

In the zones where the material is under little pressure, its displacement is produced essentially by translation parallel to the screw axis. Gradually as the material becomes compressed, the effect of translation diminishes and 20 the material tends to be entrained by friction around the shafts 21, passing from one screw to the other.

Inside the braking zone, the passage of the material downstream essentially occurs through the windows in particular \\\ ;''n t*3ie centra^ zone where the openings are superimposed. 6 , a* spaci&c'tno#' v 25 Thus, as described in the above-mentioned^applioatio'n, the braking section creates a selective passage for the material downstream. In effect, those chips which are insufficiently 1 *32 2 fibrillated cannot pass through the windows by reason of their large size and they thus remain upstream where they are submitted to an intense kneading by a combination of the effects of compression and shearing and eventually torsion of the fibres, notably due to passage from one screw to the other, this working-up continuing until portions which are sufficiently fibrillated pass downstream.

It is thus possible to achieve progressive fibrillation of the material, by passage through the successive treatment 10 zones each comprising a conveying section and progressive ^ compression section followed by a braking section constituted by reverse threads provided with windows whose size diminishes, preferably, from one braking section to the following braking section in such a way as to obtain progressively the desired 15 degree of fibrillation. prfr&A/r specifiieiQTtoM' l9t S8% The process described in^tho application■of—31afe July' 1975 allows the production of so-called mechanical pulps, thermomechanical or mechanical-chemical pulps, the sheaths being provided with orifices which allow the introduction into 20 the treatment zones of reagents for delignification.

However, it is equally possible to treat in an apparatus of this type chips which have already been subjected to a softening treatment for lignin either by steam or by chemical reaction. It is kftown that in this case, the material £5 should be subjected to a mechanical fibrillation whose pj?iiaSDi*/ct extent depends on the degree of / firoooodingf chemical treatment l-Y>* * P m 34* which has been carried out. This mechanical treatment may \ 1 9327 f: advantageously be carried out in an apparatus of the type described above, since it is only necessary to alter the mechanical characteristics of the screws, and particularly the pitch of the threads, the size of the windows and the speed of rotation of the threads, in order to create transport conditions in the different zones which allow the desired intensity of fibrillation to be obtained.

Such a fibrillation apparatus may also be fed with chips which have been subjected to a preliminary delignification treatment, or impregnation with a reagent may be carried out in the first treatment zones.

It is known that the reagent and the residues must be eliminated by washing and the novelty of the present invention resides principally in the process of washing which is carried out in the interior of the apparatus at the same time as fibrillation and which occurs in the following manner; The material introduced through the orifice 10 which has been cooked is infused with a delignification reagent and may have, for example, a degree of dryness of 20%.

As indicated above, the material is entrained in the first conveying section Al and the braking in the' section Bl provokes a progressive compression in the last threads of the conveying section.

In the second part of the conveying section, the sheath is provided with slits 31 whose size is sufficient for allowing passage of the liquid phase and the dissolved i m ~z' pp residues whilst retaining the solid phase constituted by the fibres. As a result of the progressive compression, there is produced a filtration of the material upstream of the braking section and in a direction perpendicular to the 5 direction of advancement of the material.

The pressure is at its maximum during passage through the braking section Bl, which provokes a significant wringing-out of the material to a degree of dryness of about 50%. The liquid phase expelled by the compression in the beginning 10 of the braking section returns upstream into the less pressurised zones and exits through the filter portion 3 of the sleeve.

Thus, the material arriving in the subsequent conveying section A2 has a degree of dryness of about 50%. On the other hand, if the braking section Bl is provided with 15 windows of adequate size, the material may equally well be subjected to a first fibrillation treatment.

In effect, according to the process described in the S AT to*/ above-mentioned/a-pplioatiow, the windows in the braking T section Bl only allow the passage downstream of portions of the material which have achieved the required degree of fibrillation. Those portions which are insufficiently fibrillated remain upstream of. the zone Bl - that is to say at the end of zone Al. The compression at this position is even higher and thus produces a significant wringing-out of the material. On the other hand, at this position, as SA«Seufi(c Afiot/ indicated in the above-mentioned patent^application, the \ > 2^ essential defibrillation work is carried out. However this 1* ff22 occurs by a combination of the effects of compression and shearing with in addition a certain amount of torsion of the fibres.

This working-up facilitates the removal of liquor which has penetrated into the pores of the wood and the removal of the residues by the washing liquid. However, in carrying out the wringing of the pulp during fibrillation, the effectiveness of the washing is increased.

However, the wringing-out should not be carried on to such a degree that it hinders the advancement of the material in the braking section. For this reason, one regulates the parameters of the apparatus and notably the pitch of the threads and the size of the windows so as to achieve a degree of drying which allows the downstream passage of a sufficient quantity of material. In practice, the transport conditions in the braking section are regulated such that the material is dried there to a degree of dryness of about 40%.

The material admitted upstream of the section A2 thus has two characteristics. On the one hand it has been fibrillated to a predetermined degree by the braking section Bl and on the other hand it has lost a significant quantity, in practice around half, of the liquor which it contained.

Then the material is diluted with a washing liquid, normally clean water, introduced through the orifice 12 and this dilution is facilitated by the kneading effect produced by the screws. Thus, one obtains a homogenous mixture 1 S 72 2 p' having a degree of dryness around 20% to 25% which is once again compressed gradually as it advances through the conveying section A2. As described above, the braking section B2 allows material not having the required degree of fibrillation to be held back at the end of the section A2 and to achieve on the other hand a wringing-out of this material, the liquid phase returning through the less compressed zones upstream where it is eventually removed through the slits 31.

As above, the material arriving in the zone A3 has a supplementary degree of fibrillation and has been once again wrung out up to a degree of dryness of around 50%. Then it is once again diluted with a washing liquid introduced through the orifice 13.

Thus, one carries out a series of washing operations which produce at the apparatus outlet a pulp which has been fibrillated and washed at the same time.

It can be seen that in the process according to the invention one carries out the same series of operations as in the known washing processes, that is to say successive dilutions followed each time by filtration. However, according to the present invention, the dilution and filtration are carried out under very particular conditions. According to the classical processes, in order to achieve a good distribution (' of fibres in the washing liquid one must generally dilute the material with a very large quantity of liquid. This leads to the inconvenience of increasing the consumption of 1 9 3.2 2 9 water and equally the amount of energy necessary to eliminate this water in the subsequent filtration step.

On the contrary, in the process according to the invention one works on very much less diluted pulps since, in the upstream portion of the conveyor sections where the material is brought into contact with the washing liquid, the effects of kneading produced by the screws allows one to obtain a homogenous mixture without it being necessary to utilise large quantities of dilution liquid.

It has already been proposed, for reducing the amounts of water used, to carry out the washing in the successive zones of a screw press.

However the process and apparatus according to the present invention provides very important advantages in comparison to known processes.

First of all, as indicated above, carrying out the washing at the same time as the fibrillation greatly increases the effectiveness of the washing liquid and thus allows a reduction in consumption. However, in the known process, the washing is carried out in successive zones where one recovers the filtered liquid in one zone in order to inject it into the preceding zone. This recycling is not used in the process according to the present invention since the filter liquid, by reason of the effectiveness of the washing process, is extremely concentrated so that it can no longer be used as washing liquid.

However the essential economic advantage resides in the 1 932 2 removal of the necessity to have a washing installation, the modifications that one must bring about on the fibrillation machine being much less costly.

Furthermore, the efficiency of washing is also due to 5 the cleaning properties of the apparatus used and to the quite particular process of advancement of the material in the interior thereof.

In effect, as indicated above, in a machine according to the present invention where the material is entrained by 10 two screws having identical threads penetrating one into the other and rotating in the same sense, the material circulates in the interior of the screws according to two methods. In the zones where little compression takes place, the material is entrained essentially by translation parallel to the axes 15 of the screws. However, in the more compressed zones, a more significant quantity of the material has a tendency to be entrained by friction around the spindle in the interior of the screw thread. In this way, a certain amount of ft material passes from one screw to the other and tends to 20 accumulate in the part of the thread preceding the central zone of penetration 25, as schematically represented in V*' Figure 3. Thus although the material starts to become compressed, in each screw there is a sector 26 of reduced pressure where the material has a tendency to be entrained 25 by translation and a compressed sector 27 where the material accumulates upstream of the zone 25 where the threads penetrate one into the other and after being put under pressure, pass onto the adjacent thread of the other screw. 1 9322 First, there is produced in the interior of each thread in the compressed zone, an alternation of phases of relaxation and pressurisation which favourises washing since the material accumulated in the pressurised zone 27 expels the liquor 5 which returns towards the zone 26 then, in passing through the relaxed zone 26 of the adjacent screw becomes diluted once again to a certain extent in the liquid, before returning into a pressurised zone 270 where the procedure starts all over again.

The importance of the pressurised zones 27 and the relaxed zones 26 varies progressively as one advances downstream, the compressed sectors 27 increasing in size until they occupy all the thread when this becomes completely filled upstream of the braking section B. It is in this region 15 where the threads are completely filled that the wringing- out process occurs, but the presence in the upstream threads of relaxed sectors favours the return upstream of the liquid phase which is then filtered through the slits 31. the same side of a plane passing through the axes, for example, on the figures, below the plane of the axis for the However there is necessarily a space between the periphery of the screw and the sheath through which the liquid phase 25 may pass from one thread to another towards the less compressed All the relaxed zones of the same screw are located on UP#><r# IflWlf I? zones i.e. upstream. Thus, there occurs in the interior of the conveying zone a movement of the liquid phase which 9322 ..3* tr» facilitates replenishment.

^« H ' First of all the material coming from the upstream braking zone dilutes itself in the injected liquid. Then a portion of liquid is entrained downstream with the material 5 which becomes progressively compressed, the other portion filling the relaxed zones. In each thread the alternation of phases of compression and relaxation favours the expulsion of used liquor and the absorption of clean liquid. As the significance of the compressed portion increases, the expelled 10 liquor returns upstream along the sheath and is removed through the filter portions 31. However, there is always a filtration of used liquid through the periphery and a regeneration of cleaner liquid entrained downstream with the material inside the threads.

This continual liquid regeneration process occurs without the necessity of injecting it under a great pressure since the use of several screws engaged one in the other provides a pumping effect which advances the material even when the threads are not filled, which cannot happen in 20 machines having only one screw. The result is that for the same quantity of injected liquid, the present process allows one to have a very much more effective washing.

Thus in the apparatus according to the invention there is an alternation of phases of dilution, compression with 25 filtration 25 and wringing-out; not just in the longitudinal direction of advancement from one washing zone to the following one but also in the transverse direction from one 193228 thread to the other in the interior of the corresponding zone, and this is one of the important causes of the efficiency of the present process.

However, the manner in which the slits 31 are formed 5 also has an influence on the washing efficiency.

The slits are constituted by the spaces left between the profiles 3 in the form of sectors of circumference arranged parallel in a direction transverse to the axis of the screws.

This disposition, represented according to a plan view in Figure 2 is adopted in order to achieve good filtraton. In effect as one can see in the plan view of Figure 2, the slits perpendicular to the axis make a small angle with the threads which are themselves slightly inclined. In consequence, 15 each screw thread groove corresponds at every instant to one or several slits around almost all its periphery and the expelled liquid phase may be immediately removed, the filter presenting a very low loss of load.

Furthermore, due to the translational effect of the 20 threads due to their rotation, there is produced periodically a scraping of each slit which at each instant reintroduces in the process of advancement material deposited on the filter. Thus there is never any risk of blockage since it is permanently being scraped as filtration gradually progresses. 25 Furthermore, the profiles have a trapezoidal section having a wide base towards the interior so as to present an 193228 internal concave cylindrical face. Therefore the particles of sludge which deposit in a slit are immediately scraped off and discharged but there is no risk of blocking the slit since the shape of the profiles deviate one from another and facilitate expulsion.

Thus, due to all the characteristics described the process and apparatus developed allow one to achieve very efficient and rapid washing.

As indicated above, it is particularly interesting to be able to produce inside the apparatus the mechanical work of fibrillation at the same time as the washing. The intensity of these operations depends on the conditions of transport of the material in the braking sections which as indicated above are determined largely by the choice of pitch of the threads and their depth, the number of threads in each zone, the size of the windows and the direction of rotation of the screws. For preference one should produce in each braking section a maximum wringing limited depending on the degree of dryness which allows a good advancement of the material from one zone to another.

The conditions in which are achieved progressive compression and wringing do not depend only on the size of the windows. First, as well known, one may increase the compression and render it more rapid by reducing the pitch of the threads towards the end of the conveying zone. It is thus possible by adjusting the different^poramoters of the apparatus to control the quality of the pulp. Thus, a 193228 starting material having been cooked for a short time necessitates, in order to produce a usable mechanical-chemical pulp, a fairly significant amount of mechanical fibrillation and a fairly low degree of washing. Conversely, if the chips 5 have been subjected to prolonged cooking, they must be subjected to an intense washing in order to eliminate the used liquor and the residues but the necessary mechanical working is fairly restricted.

The process which has just been described and the 10 apparatus for carrying it out presents many advantages.

Thus, the washing is more rapid and more effective, it i produces a more concentrated pulp and allows the reduction in consumption of water for washing and in consequence reduces the amount of liquor discarded.

Thus, it is known that the efficiency of washing may be measured generally by the relationship of the difference between the quantity of liquor introduced with the chips and the quantity of liquor removed, divided by the quantity of liquor introduces. In the apparatus according to the 20 invention, this relationship is of the order of 60% and may in certain conditions achieve 80%.

On the other hand, there is no need of a subsequent washing stage since washing is carried out concurrently and the machine utilised is very much less cumbersome than known 25 installations for preparing pulp.

However, although we have shown by way of example an apparatus having four wringing sections, which is generally 1 932 sufficient, it is clear that one may vary the number of washing stages as desired.

Thus, the invention is not limited to the details just described since it may be carried out in numerous various ways employing equivalent means for obtaining the results sought.

It is thus that the apparatus allows one to carry out very easily, if desired, a countercurrent washing which recovers the filtered liquid for recycling in a zone situated further upstream. However recycling is less justified than in the known installations due to the fact that one uses much less washing water and that the filtered liquors are more concentrated. }

Claims (8)

193228

1. Apparatus for preparation of a paper pulp from a cellulosic starting material in the form of chips, which comprises P,*/ 4 /OiCtS-i- s. a ^ at least twcyparailed intenaeshed screws rotatable inside a sheath and iA- ■■ each provided with helical threads whose pitch varies so as to define one or j^) more successive treatment zones, each zone comprising a conveying section and a braking section constituted by a reverse thread provided with windows to allow downstream passage of material having achieved a predetermined degree of fibrillation; at least one zone having an inlet for injecting washing liquid into the beginning of the conveying section of the zone, and orifices in the sheath upstream of the braking section for allowing the expulsion of liquid wrung out of the pulp by compression in the braking section.

2. Apparatus according to claim 1 wherein the orifices are in the form of circumferentially extending slits.

3- Apparatus according to claim 2, wherein each slit is trapezoidal in cross-section, the narrow end being towards the inner surface of the sheath. -22- 193228

4. Apparatus according to any preceding claim, wherein the. orifices are present around the whole peripheral area of the sheath directly upstream of the braking section.

5. Process for the preparation of a paper pulp from a cellulosic material preliminarily impregnated with a delignification agent in an apparatus according to claim 1, which comprises injecting washing liquid into the beginning of the conveying section of at least one of the treatment zones, and allowing used washing liquid containing delignification agent to be expelled through the orifices in the sheath.

6. Process according to claim 5 wherein the quantity of washing liquid injected is determined. so as to provide an optimal degree of wringing in the braking section commensurate with allowing continued advancement of material downstream through the-braking section.

7. Process according to claim 5 or 6, wherein the amount of liquid injected is controlled such that the material is diluted to a degree of dryness of substantially 20% and that the conditions of passage through the braking section are determined such that the material there is dried to a degree of dryness of substantially 55%.

8- Process or apparatus for the preparation of paper pulp substantially as described with reference to and as shown in the drawings. By <fefe/their authorised Agents