CA1100268A - Continuous pulp digesting method and a digester with associated devices for applying the method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention provides an improvement in a continuous sulphite or bisulphite cooking method for pulp when the pulp is subjected to a digesting step. The improvement relates to waste liquor being withdrawn from the digester and subjected to one or more expansion steps and characterized in that a low pressure secondary vapour withdrawn from the expansion step is the only material recirculated. The invention also provides an apparatus for carrying out the above.

Description

` 1:100Z68 This application is a divisional application of co-pending Canadian Patent Application Serial Number 254,340 filed June 8,1976.
The subject of the`present invention is a continuous pulp-cooking method and a digester the purpose of which is, with a high yield, within the range of-yields of 55 to 80 per cent of wood, to produce pulp with good paper-technical properties, e.g.
tear 2.5 m2 brightness 60 to 70 SCAN, the yield being 65 per cent of wood, the pulp moreover having a sufficiently low content of fibre bunches and screening residue in order that the running quality of the pulp both in the paper machine and in particular in the printing machine should be sufficiently good so that it could replace the sulphate pulp, which has been used so far primarily, as a component of groundwood newsprint or printing paper, either fully or in part.
The methods so far known have usually been batch cooking methods, in which one of the greatest drawbacks has been the in-sufficiently precise control of the variables in this in itself difficult process. The quality of the pulp to be manufactured must be sufficiently uniform and good in order that undisturbed functioning of the modern high-speed paper and printing machines should be possible. Moreover, the heat consumption in batch cook-ing is less advantageous than in continuous cooking. On the other hand, in the continuous digesters so far constructed, a drawback has consisted in uneven impregnation, which is seen in the quantity of the screening fraction and in the paper-technical properties of the pulp. Said uneven, or more appropriately, insufficient impregnation has made it uneconomical to use, for example, chips mainly consisting of pine as the ~awmaterial.
In the continuous processes so far known, another draw-back has consisted in the uncertain feeding of the chips into the r~
';~

``` 1100268 cooking process especially when the pH of the cooking liquor be-comes so low that free SO2 has been separated from the cooking liquor, for example, in the case of sodium base at the level of pH = 3.5 to 4.0, which would, on the other hand, be otherwise suitable for the process. Moreover, in the continuous digesters known at present, it is impossible to restore the secondary vapour expanded from the waste liquor separated from the digester back to the preheating of the chips, because, owing to insuffi-cient impregnation, it has been necessary to keep the concentra-tion of the cooking liquor high right to the end of the cooking.
Thus, the expansion vapours have, among other things, contained free SO2, which, as passed to the chips before impregnation, as is known, causes deterioration in the brightness of the pulp.
This has meant an unnecessarily poor heat economy in the process, on one hand because the expansion vapours have been used for secondary purposes and, on the other hand, as an un-necessarily large quantity of chemicals in the recovery cycle of chemicals.
A further drawback in the present digesters has been the insufficient degree of fibration of the pulp after the digester, which is seen as an unnecessarily high power consump-tion of the further fibration, on one hand, and in the strength qualities of the pulp, on the other hand. In other words, the best obtainable properties have not been obtained from the raw material.
The present invention attempts to eliminate the draw-backs listed above.
The invention is mainly characterized by what is stated in claim 1 as well as in the sub-claims.
An embodiment of the invention will be described in the following specification with reference to the attached figures `` 1~00268 . , 1 and 2.
Figure 1 shows a digester in accordance with the invention as a schematical presentation. According to an embodiment, the chips are blown pneumatically through the pipe 41 into a chips cyclone 11. From the cyclone, the chips fall into a chips hopper 12, and further onto a measuring device 13, which, like in figure 1, may be a twin-screw feeder and has the duty to maintain the chips flow as constant. From the twin-screw feeder the chips fall onto a pocket-type feeder 14, which has the function of constituting a pressure lock between the pre-steaming vessel 15 and the pressure of the open air. From the low-pressure feeder the chips fall into said pre-steaming vessel 15, which is provided with a conveyor screw for the purpose of feeding the chips further in the direction of the arrow a to the high-pressure feeder 16 and from there further through a bellows equalizer 17 and a pipe 18 to the impregnation device l9. In the embodiment` shown this device is a cylindrical, diagonally positioned vessel 19, which is provided with a conveyor screw 61 so as to move the chips in the direction of the arrow d. The height of the impregnation solution in the impregnation vessel is, for example, in agreement with the levels b and c, of which b assumes the level of the exhaus~ opening of the impregnation vessel and the position of c is determined by the difference in pressures between the vapour spaces A and B.
From the impregnation vessel l9 the chips as well as a quantity of impregnation solution that corresponds the quantity of soluiion that has been passed to the impregnation vessel falls through the bellows equalizer 20 of the digester and through the normally open closing valve 21 into the digester.
In the digester, the chips constitute a chips colu~n whose upper surface is denoted by the broken line e. The liquid ~a l~U(~268 assumes such a position that the upper surface constitutes the level f. The level of the chips surface is measured by a radioactive measuring device 60, and according to a reading given by this device, the quantity of pulp to be removed from the digester is controlled so that the level of the surface e remains constant. The liquid level f,which can also be lower than the level shown, e.g. f'(shown as broken line) is made constant by controlling the exhaust flow through the pipe 52.

Of course, the quantity of chips and/or cooking liquor fed into the digester 22 can also be adjusted. The pulp is removed through the bottom of the digester either through the pipe 42 or through the pipe 44. In order to facilitate the removal of the pulp, a rotary bottom plough is arranged at the bottom of the digester, the arms 24 of which plough move the pulp to the middle of the digester and from there further-into the blow line 42 or 44. Into the blow line 42 or 44, respectively, a thickener 27 or 28, respectively, is arranged which has the function of removing liquid out of the pulp and thereby to increase the consistency. As to its construction, the thickener may be, for example, a tubular or cylindrical object 27, 28. Inside this outer mantle is placed another tube, i.e. i.nner mantle, made of screen plate and placed at a certain distance from the inner sur-: face of the outer mantle, into which inner mantle the pulp is passed from the blow line 42, 44 and from there further into the defibrator 29, 30. As the screen ~be and the thickener are otherwise closed at their ends, the liquid that has passedthrough the slits in the walls of the screen tube can be removed through pipe connections 62, 63. Said inner mantle may be ad-vantageously made of slit-type screen plate whose lengthy slits are placed in the direction of movement of the pulp.
In the thickener, the space between the outer ~i and the inner mantle may be divided into two parts or chambers, for example, by means of a partition wall parallel to the radius of the tube 28, 29. Then, by means of timer etcO equipment(not shown in the drawing), it is possible to take the escaping liquid at certain periods of time only from one of the chambers, for example, in accordance with the drawing, from the left -~chamber and through the left pipe connection 63. The right chamber is then at the rest stage and the slits of the inner mantle therein can become clean. At the next stage, on the other hand, the left chamber is at the rest stage and liquid is - taken from the right chamber and from the right pipe connection -62, 63. In this way the stages of work and rest can alternate further. Moreover, a defibrator 29 or 30, respectively, is - arranged into the blow line,through which defibra~tor the pulp is passed 15 as defibrated into the blow line 43 or 45, respectively, and further into the blow tank (not shown in the drawing). As the ; pulp flows in the blow line continuously, then the defibrator - can be considered as, from the point of view of the processing of the pulp, being practically immediately after the digester, 20 in spite of the rather short blow line 42 or 44, respectively, because there is no blow tank between the digester and the defibrator~ Then, the defibrator can be, for example, of the so-called disc-refiner type, which is provided with means for adjusting the distance of the discs/or the disc clearance.
In accordance with figure 2, the impregnation tank may also be provided with a screw 31 for the purpose of sinking the chips in the impregnation solution if the chips are so dry that they do not sink otherwise by their own weight.
When the blow line 42, 43, the thickener 27, and - 30 the defibrator 27 are in use, the blow lines 44, 45, the thick-ener 28, and the defibrator 30 may be out of use and they may -~,;,;

~a~oz6g be taken into use, for example, if a disturbance of operation occurs in the blow lines 42,43, in the thickener 27 and/or in the defibrator 29 and maintained in operation until the dis-turbance has been eliminated. In this way, the operation of equipment in pairs can be alternated if required, or they can also be used at the same time.
In figure 1 the cooking liquor, which is in the case of the present invention an aqueous solution of some soluble base ~ammonium, sodium or magnesium) and sulphurdioxide, is passed from the recovery and chemical preparation or regenera-tion plant (not shown in the drawing)through the pipe 46 into the cooking liquor tank 32, which may also be a pressurized tank, depending on the properties of the cooking liquor at the temperature concerned. From the tank 32 the cooking liquor is passed by means of a pump (not shown in the drawing) through pipe 47, heat exchanger 33, and further through pipe 48 into the impregnation tan]c 19, below the surface c. The cooking liquor is hereby heated up to the impregnation temperature, suitably 90C to 130C and preferably 100C to 120C, immediately or practically immediately before it enters the impregnation space, while the solution flows continuously through the heat exchanger 33 and from there through the pipe 48 into the vessel 19. In this case, the cooking liquor does not have to stay a longer time as hot in the pipe system or in some intermediate tank, which would have harmful effects on the composition of the cook-ing liquor, which, when hot, easily tends to be changed becauseof chemical reactions or corresponding phenomena then taking place in the liquor.
The used cooking liquor, the so-called waste liquor, - 30 is removed from the digester through screen devices 23 provided with screen surfaces into the pipe 52 and further into the llUl~Z68 expansion tank 34 where it is allowed to expand. The produced vapour flows through the pipes 53 and 54 or 53 and 55 into the pre-steaming vessel 15, where it is used for heating the chips.
Of the pipe 55, the portion at the feeding device 14 has not been shown in the drawing in order to clarify the drawing.
The separated waste liquor can, if required, be passed through the pipe 56 to the next expansion tank 35, from which --the expanded vapour flows through the pipes 57 and 57a, for example, to the heating of the washing water (not shown in the drawing). Alternatively, the vapour can also be passed to the heating of the chips through the pipes 57 and 57b. The separated waste liquor is passed through the pipe 58 to the evaporation plant (not shown in the drawin~) for further processing.
The vapours necessary for maintaining the pressures and temperatures in the digester can be passed into the equip-ment through the points indicated by the arrows 49, 50, and 51through pipes and valves (all of which have not been shown in the drawing). In order to cool the pulp down to an appropriate blow and defibrating temperature, cooled waste liquor is passed to the bottom of the digester from the washing plant (not shown in the drawing) through the pipe 59, which is connected to the arms of the bottom plough 24. From nozzles placed in these arms, the liqour is spread to the bottom part of the digester.
The equipment and components denoted above and below by refer-ence numerals can be in themselves previously known equipment and components in use in pulp industry. Therefore, all ofthem are not described more in detail here.
In addition to what was described above, a great number of closing and controlling valves, controlling equipment, meters, and other auxiliary equipment and pipe systems, pumps, etc. are required in the system, which are, however, not essential from .- .~

- ~luaz6s the point of view of the invention and which are therefore not described in the present specification. ~ --By means of the method and equipment in accordance with the invention, pulp has been produced on the industrial scale, which pulp is mainly used as the chemical pulp for news-print and printing papers, in which use it replaces the sulpha e pulp, which has been used until now. ~mong the results obtained, the following should be mentioned by way of example:

rawmaterial: factory chips, 60 % pine, 40 ~ spruce cooking liquor: SO2-concentration 4 %, pH 4.3 Pulp: Kappa-number 95 yield 66 %
brightness 61.5 SCAN

extract 0.4 dichloromethane tear 2.3 m2 degree of grinding 26 / SR
When the pulp meant above is used as an admixture of paper instead o-f sulphate pulp, it has been noticed that the paper machines and the printing machines function normally, and no disturbances have been noticed. This involves a remarkable saving in the c~ns-umption of wood when, for example, a sulphate pulp produced with a yield of 45 per cent and a bisulphite pulp with corresponding properties and produced with a yield of 65 per cent are compared with each other.
The good properties of the pulp mentioned above result~

among other things, from the following factors. From the high-pressure pre-steaming taking place before impregnation, at which pre-steaming the air is removed from the chips as completely as possible, from resulting proper impreynation before cooking, which guarantees a low content of reject, from sufficiently uniform cooking circumstances, as well as from proper defibrating ~- .
; ~, .

'` l~OOZ68 that takes place under correct circumstances.
When the defibrator is placed immediately or practi-cally immediately after the digester, the pulp can advantage-ously be fed into the defibrator by means of the pressure of ` 5 the digester. Then the pulp comes immediately after the cook-ing in a more favourable form to the defibrator which requires ~ less energy at the defibrating stage. sy thickening the pulp - into an appropriate consistency, it is possible to keep the disc clearance of the defibrator higher than otherwise. Thls also results in the fact that a larger part of the fibres become defibrated when they are just ground against each other.
Then the disc of the defibrator meets fewer fibres from the pulp that has passed through the defibrator and cannot damage them.
The properties of the pulp, for example its brightness, are also improved by the fact that the sulphurdioxide that is gasified from the cooking zone cannot enter through the blocking liquid in the impregnation zone 19 and come into contact with unimpregnated chips.
The decomposition of the cooking liquor is promoted by organic substances brought into same, which deteriorates the properties of the pulp to be cooked. Consequently, it has usually not been considered as desirable to re-circulate any waste products obtained from cooking back to the cooking. The waste vapour obtained in accordance with the present invention is, however,to such an extent pure, especially rather free from detrimental sulphurdioxide, that it can be re-circulated back to the process. This means a considerable economy in the heat-ing expenses. Out of the above reasons, according to the inven-tion, however, only vapour is re-circulated. The waste liquor etc. products are hereby not re-circulated.

-` ~lOOZ68 Of course, the example given above is only one embodiment of the invention, which can be varied extensively within the scope of the patent claims. For example, one is not restricted to the indicated pH range of 3.5 to 4 alone, but the p~ may also be lower.
The equipment putting the invention into effect can also be of a different construction~ for example, the feeder 13 ~ay be of the pocket type instead of twin-screw, the feeders 14 and 16 of the pocket type may be plug-screws instead of the pocket feeder, the screen device 23 may be a mere screen plate - that is arranged in the wall of the digester instead of a self-cleaning screen, etc.
The invention described in the above specification can be used either as such or as applied in one or some of the following methods 1, 2, 3, 4, and 5, in their processes, in equipment or sets of equipment connected with them, and~or in connection with one or some of them.

Method 1 A continuous cooking method, i.e. flow cooking, for pulp, in which method pulp-containing fibrous material is contin-uously fed into a digester under elevated pressure and in which between the digester itself 22 and the block feeder 16, a zone appropriately an impregnation zone 19, is arranged which zone contains blocking solution that prevents the sulphurdioxide gasified from the cooking zone 22 from entering onto the feeder 1~ of the digester and from thereby coming into contact with unimpregnated chips.
In this method the blocking solution may consist of the cooking liquor~ which is passed to the blocking zone of the ,. ~

110~268 digester, appropriately to the impregnation zone, at a tempera-ture lower than the cooking temperature, appropriately at 90C
to 130C and preferably to 100 to 120 C. The blocking solution may at the same time also function as the impregnation solution, S and its chemical content is then adjusted so that the quantity of chemicals absorbed into the pulp-containing fibrous material is suitable for performing the cooking in liquid or vapour phase.
The difference in the levels of the surfaces b, c of the blocking solution can be controlled to the desired value either by means of the screw feeder 61 feeding the pulp-contain-ing fibrous material, such as chips, or on the basis of the difference in pressure between the vapour space of the digester ` 22 and the vapour space 18 before the blocking-liquid zone.
In order to put the method into effect, it is possible to construct a continuous digester 22 with related equip-ment, which comprises a vapour space 15 under elevated pressure and mainly containing water vapour for the purpose of passing the pulp-containing fibrous material into same and for passing the material from there further into the space 19 provided with a screw feeder and partly filled with blocking liquid and from this space further into an appropriately vertical digester 22, whereby said blocking liquid separates the vapour spaces before and after the digester from each other.
In this case, the space 19 partly filled with block-ing liquid may consist of an oblong vessel that is positioned diagonally and provided with a conveyor screw 61 and that may be connected to a space A projecting from it for the purpose of guiding the pulp-containing material, and this space may possibly involve a separate conveyor screw 31 for sinking the pulp-containing material into the blocking or impregnation liquid,whereby the latter conveyor screw 31 is positioned perpendi-llnoz6s cularly or substantially perpendicularly to the former conveyor screw 61.

Method 2 A continuous cooking method for pulp, in which method chips of any conifer tree of mixture or conifer trees are dissol-ved in a bisulphite cooking liquor or the manufacturing of pulp with high yield and high quality of paper-technical properties and in which method the chips are passed into an impregnation solution under the pressure of the digester, which solution is appropriately said bisulphite cooking liquor, through a vapour space 15 and/or 18 under the pressure of the digester, in which vapour space the chips remain from a few seconds up to 3 minutes, however, appropriately no more than 2 minutes, and whereby the chips can be continuously passed from the impregnation solution further into the digester 22.
Hereby, the vapour in the space 15 and/or 18 ahead of the impregnation solution may be water vapour or maximum degree of saturation. The chips can be fed from the vapour space 18 into the impregnation solution by their own weight or by means of a screw 62.
In order to perform the method, it is possible to construct a continuous digester 22 with related equipment, where-by said vapour space and impregnation space are connected to a substantially vertical tube 18 and to a cylindrical space 19 positioned diagonally and placed after said tube, a conveyor screw 61 being placed inside said cylindrical space. In connec-tion with said diagonal conveyor or so-called impregnation screw 61, a screw 31 directed substantially perpendicularly to the axis of said former screw 61 may be arranged for the purpose of feeding the chips into the impregnation solution. Into the ~lO(~Z68 vapour space 15 pro~ided ahead of the impregnation space and : being substantially under the pressure of the digester, the chips can be fed by means of an, in itself known, block feeder - 13 of the pocket type or by means of an, in itself known, plug - 5 conYeyor screw 14.

Method 3 A continuous sulphite or bisulphite cooking method for pulp, in which the cooking liguor is an aqueous solution of one or some so-called soluble bases, such as ammonium, sodium, and magnesium, and of sulphurdioxide and in which, before the cooking proper, impregnation is performed substantially under the pressure of the digester and at a temperature lower than the cooking temperature, and in which the pulp-containing mat-erial is any conifer tree or any mixture of conifer trees, withthe purpose of manufacturing pulp with a high yield and with h-igh-~uality paper-technical properties, and whereby the cooking liquor is heated to the impregnation temperature immedi-ately before it is passed into the impregnation space. The cooking liquor used as the impregnation solution can be passed further, together with the chips, into the digester 22, and when falling into the digester, it is heated by means of direct vapour to the cooking temperature.
In order to perform the method, it is possible to construct a continuous digester 22 with related equipment, in which the chips are fed into the impregnation space 19 of the continuous digester 22 through a vapour space 18 substantially : under the pressure of the digester and in which the chips are passed into the impregnation space and from there further into the digester 22, and whereby a heat-exchanger 33 is placed in -the feeding pipe of the cooking liquor immediately or practically ^" 11C10268 :`

immediately before the impregnation space. The impregnation space may also be arranged so that a quantity of cooking liqu~r corresponding the quantity of cooking liquor passed into the impregnation space 19 is passed further, together with the chips into the vapour space B placed at the top portion of the digester 22, where the temperature of the cooking liquor is raised to the cooking temperature by means of direct vapour.
From the impregnation space 19 the impregnation solution may flow into the digester 22 as overflow.

Method 4 A continuous sulphite or bisulphite cooking method for pulp, in which the cooking liquor is an aqueous solution of one or some so-called soluble bases, such as ammonium, sodium, and magnesium, and of sulphurdioxide and in which the fibrous rawmaterial is a mixture of one or some conifer trees, with the purpose of manufacturing pulp with a high yield and with high-quality paper-technical properties, and in which the pulp is removed from the digester through a defibrating stage, where-by the defibrating takes place primarily under the pressure of 20 the digester and within the temperature range of 90 to 120 C and fully or mainly without washing. The defibrating can then take place at each occurring consistency while using a disc clearance of the defibrator 29, 30 in which the degree of grinding of the pulp is not changed essentially. The consistency cf the pulp during defibrating can also be maintained as high as possible, for example by thickening the pulp.

In order to perform the method, it is possible to construct a continuous digester 22 with related equipment, where-by the defibrator 29,30 is placed in the pulp ccnduit leaving the digester, i.e. in the so-called blow line 42,44, immediately . .

` `" llU(?Z68 or practically immediately after the digester.
The defibrator 29,30 may be a defibrator of the so-; called disc refiner type, which is provided with means for adjusting the disc clearance.
A thickener 27, 28 may be placed between the bottom of the digester 22 and the defibrator 29, 30 for the purpose of - thickening the pulp to the desired consistency. As to its construction, the thickener 27, 28 may be a tube, inside which an inner mantle is placed, which inner mantle is made of screen plate and in which tube necessary pipe connections 62, 63 are ~- arranged for draining the isolated liquid.~
The inner mantle may be manufactured of slit-type screen plate whose lengthy slits are positioned in the direction of movement of the pulp.

Method 5 A continuous sulphite or bisulphite cooking method for pulp, in which the cooking liquor is an aqueous solution of one or some so-called soluble bases, such as ammonium, sodium, and magnesium, and of sulphurdioxide and in which the fibrous rawmaterial consists of a conifer tree or of a mixture of conifer trees, with the purpose of manufacturing pulp with a high yield and with high-quality paper-technical properties, and in which the only substance that is re-circulated back to the cooking is the low-pressure secondary vapour expanded or isolated from the waste liquor separated from the digester,~
the S02-content of which vapour has been made so low, by means of impregnation that has taken place before cooking, substan-tially at the pressure of the digester but at a temperature lower than the cooking temperature and by means of possible pre-steaming at a lower pressure, so that such a vapour can be fed -` l~O(JZ68 into the chips in order to steam and/or heat them, and, besides that or instead of that, the vapour is also used for other purposes at each particular time concerned in connection with the pulp manufacturing.
Then, said expansion vapour can be used for steaming the chips to be fed into the digester.
The waste liqour separated from the digester can be expanded at two stages and the produced secondary vapour be used for steaming the chips to be fed into the digester and~or for heating the washing water to be used at the washing plant.
The secondary Yapour under ele~ated pressure, obtained from the first expansion, can be used for steaming the chips and the secondary vapour obtained from the second expansion can be used for heating the chips before said steaming. The waste liquor to be separated from the digester can be separated at two or ~ore stages and the vapours of different pressures obtained in this way can be used for steaming the chips and/or -for heating the chips and/or for heating the pulp washing water and or for heating the waste liquor to be passed to the evaporation plant.
In order to perform the method, it is possible to construct a continuous digester 22 with related equipment, the bottom part of which digester is provided with liquid-separators 23,appropriately with a ccreen zone or with screen faces separate from each other, whereby one or more expansion cyclones 34, 35 are placed in the exhaust pipe 52 of the screen zone for the purpose of expanding the liquid under elevated pressure - and whereby the cyclone or cyclones, respectively, are provided with pipe connections 53, 57, 58 for passing the vapour liberat-ed and the liquid separated in the expansion further to the desired object.
If the number of expansion cyclones 34,35 is higher .

~lno26s than or equal to two, they can be connected in series so that the liquid separated from the first cyclone 34 is passed to the second cyclone 35 for expansion and the liquid separated from the second cyclone is passed to the third cyclone for expansion, etc., whereby the separated secondary Yapours are passed to separate objects of use of their own. Both for heating the washing water of pulp and for heating the liquid for the evapor-ation plant or for the heating of either one alone (by means of separated vapour~ it is possible to use an indirect heat ~ -exchanger.
Re-circulating of the low-pressure vapour expanded from the waste liquor separated from the digester back to a suitable stage of the cooking, such as for steaming and~or heat-ing the chips to be fed, is, besides from the viewpoint of heat control, even otherwise advantageous in view of correct balancing of the circumstances of the cooking according to the present application.
`.

Claims (8)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In a continuous sulphite or bisulphite cooking method for pulp when the pulp is subjected to a digesting step, the improvement wherein waste liquor is withdrawn from the digester, subjected to one or more expansion steps, characterized in that a low pressure secondary vapour withdrawn from said expansion step is the only material recirculated.

2. The improvement of claim 1 characterized in that said low pressure secondary vapour contains a low sulfur-dioxide content is employed for steaming or heating wood chips.

3. The improvement of claim 2 characterized in that the chips are impregnated at a temperature of between 90°C to 130°C.

4. The improvement of claim 3 wherein the chips are impregnated at a temperature of between 100°C to 120°C.

5. The improvement of claim 1 characterized in that said waste liquor is subjected to a two stage expansion.

6. The improvement of claim 5 characterized in that said vapour under elevated pressure obtained from the first expansion is used for steaming the chips and said secondary vapour obtained from the second expansion is used for heating the chips before said steaming.

7. In a pulping apparatus including the continuous digester having means located at the bottom portion thereof for withdrawing waste liquor, the improvement wherein there is provided at least one expansion tank placed in an exhaust outlet for receiving the spent liquor and expanding the liquid under elevated pressure, and means for communicating at least part of the vapour produced to a pre-treatment portion of the digesting apparatus.

8. The apparatus of claim 7 wherein there are provided first and second expansion tanks connected in series characterized in that said expansion tanks are connected in series whereby liquid separated from the first tank is passed through the second tank expansion, and that separated secondary vapours are employed for pre-treatment of the chips.