FIELD OF THE INVENTION

The present invention relates to a method and system for cooking of chemical pulp, and relates in particular to a process for dealing with rejects in conjunction with the cooking of chemical pulp.

BACKGROUND OF THE INVENTION

When cooking pulp using a well defined wood chip size, the majority of the raw wood which is cooked will have been delignified to a sufficient extent for defibration to be achieved. However, a relatively small proportion of the raw wood consists of knots which differ in character from the rest of the raw wood insofar as they are more difficult to cook, i.e., they require a longer dwell time in order for sufficient delignification to be achieved. Some digesters is also fed with chips with a wide range of chip sizes, i.e. not that well defined in size by chip sorting procedures, that will introduce a large amount of oversized chips that are more difficult to impregnate and thus will not be sufficiently delignified in the cooking process, forming a volume of rejects in the screening room. In the subsequent description is the term rejects used for both knots and such rejects resulting from oversized chips.

This problem is usually solved by separating these rejects off, after cooking, and returning them to the digester for repeated treatment. In this respect, the rejects are normally separated off by screening either directly after the digester or after the brown wash after the digester, and conveyed to a reject vessel. The rejects are then fed/blown from the reject vessel into the chip bin and thereby commence their second cooking run. The disadvantage of such a process is that a relatively large amount of energy is required to transport the rejects right up to the top of the relatively high chip bin. If the rejects are blown into the chip bin is also the gas volumes in the chip bin ventilation system increased. If instead the rejects are pumped in a liquid flow to the chip bin could the steaming process be hampered. Moreover, fairly expensive equipment, such as a sluice feeder, etc., is needed for transporting the rejects. In addition, it is of course a disadvantage that the chip bin has to be constructed in such a way as to take into account the extra load which is exerted by the equipment for transporting and, if appropriate, for dewatering. One solution has been presented by Metso (former Kvaerner Pulping) in US5.672.245 for solving this problem of adding the rejects back into the chip bin, where instead the rejects are pumped with a centrifugal pump, here a vortex pump, into the chute of the high pressure feeder. However, no distinct defibration effect is imposed upon the rejects being forwarded.

The older versions of the high pressure feeder had some limitations in capacity, and a typical filling degree of chips in the pockets of the high pressure feeder could reach some 50-60%. This was improved by the screen less design in the first Compact Feed systems, as shown in US6.120.646, making it possible to increase filling degree some 10-20%. Thus, there is a capacity problem when using especially high pressure feeders, where one want to have as high concentration of chips as possible being fed into the pockets of the high pressure feeder, and the contradicting effects of adding a diluted flow of rejects into the chute, which per se would increase liquid volumes in the chute and thus reduce feeding capacity of the high pressure feeder.

OBJECTS OF THE INVENTION

It is an object of the invention to achieve an improved method and system for recycling of rejects in a process for cooking chemical pulp not having the disadvantages with prior art solutions.

Still another object of this invention is to provide an improved method and system for recycling of rejects wherein the pumping of the rejects from the reject vessel is made with a first chopper pump having a disintegrating effect upon the rejects being pumped, thus improving the subsequent delignification of these recycled rejects in the cooking process. While using a first chopper pump and a second high pressure pump could an ideal pumping solution be obtained, using the first chopper pump for making the rejects smaller while not being optimized for a greater pressure increase but still sufficient for pressurizing the slurry in the inlet of a second high pressure pump, and using an optimized centrifugal pump for a far higher pressure increase in said second high pressure pump. Such a system could easily cope with the extended pumping lines from the screen room and back to the digester and associated pressure losses in piping. By diluting the reject flow to a dry matter concentration in the range of less than 10-20%, typically in the range 2-7%, and after a thorough disintegration of rejects in the chopper pump, could also an improved availability of the system be obtained, less vulnerable for plugging or other risks combined with pumping a flow with cellulose material.

As the rejects must be pumped in a liquid slurry a further objective is to slurry rejects in a treatment liquor that is anyway needed to be supplied to the digester in concern for establishment of the correct cooking conditions. As such treatment liquors anyway is needed in the cooking process of the digester is no parasitic pumping losses established by the need to reintroduce the rejects back to the digester, and no additional liquid or gas volumes are obtained which must be handled in the system. A complementary effect is that the rejects will also be exposed to a prolonged impregnation or soaking effect in the treatment liquors, thus improving the complete delignification in next cooking passage through the digester.

SUMMARY OF THE INVENTION

The objectives of the invention are accomplished in accordance with the invention in that

the method for the cooking of chemical pulp, comprises the steps of:

feeding comminuted cellulosic material to a digester;

adding a treatment liquor to the cellulosic material forming a slurry

pressurizing said slurry;

heating the slurry to an appropriate cooking temperature during a time period sufficient for forming a delignified cellulosic material;

subjecting the delignified cellulosic material to washing;

screening said washed cellulosic material, such that an accepted flow of delignified cellulosic material and a rejected flow of not completely delignified cellulosic material are obtained;

collecting the rejects in a reject vessel and pumping the rejects with a chopper pump in a recirculation loop from an end of the vessel to the other end of the vessel, said chopper pump having at least one cutting edge on the impeller and a cooperating anvil in the pump housing,

splitting the flow in the recirculation loop and leading a first part back to the reject vessel and a second part to a high pressure pump; pressurizing said second part of the flow from the recirculation loop in said high pressure pump and adding the pressurized second part of the flow from the recirculation loop with a treatment liquor being fed to or circulated within the digester. This method will enable an optimized handling of rejects, where the chopper pump will perform all the tasks of homogenization of the content in the reject vessel with a treatment liquor that is to be introduced into the cooking system anyhow, defibrating the rejects, and pressurizing the reject flow to a sufficient extent avoiding cavitations in the high pressure centrifugal pump needed in order to supply the reject flow into the pressurized cooking system.

In a further embodiment of the invention the method is further characterized by, wherein at least a part of the treatment liquor needed for the digester is added first to the reject vessel, forming a reject slurry of rejects and treatment liquor, thus forming this reject slurry in the recirculation loop. The rejects will thus have time for a thorough impregnation of the treatment liquors needed for the cooking process, thus preparing the rejects for a full fiber liberation in subsequent passage through the cooking process.

In yet a further embodiment of the invention the method is further characterized by wherein said treatment liquor added to the reject vessel comprises at least one liquor out of a group of liquors comprising white liquor, black liquor, green liquor, cooking additives or spent washing liquor from a washing stage downstream of the digester system. These are all examples of treatment liquors that are needed for and added to the cooking process.

In yet another further embodiment of the invention the method is further characterized by wherein said first part of the flow in the recirculation loop is equal or larger in volume than the second part of the flow splitted from the total flow in the recirculation loop. This will subject the rejects to at least one defibration of larger particles in the rejects and for a major part of the reject flow also a repeted passage of the chopper pump and improved fiber liberation, i.e. defibration, effect.

In yet a practical implementation of the inventive method said first part of the flow in the recirculation loop in volume flow exceeds 10 l/s per 1000 ton of cellulosic material being treated in the digester system per day, and preferably exceeds 20 l/s. A high volumetric flow rate will keep the volume in the reject vessel under circulation and will improve mixing of new rejects being added into the reject vessel. From a system point of view, the inventive system for the cooking of chemical pulp comprises,

a digester vessel for cooking comminuted cellulosic material at elevated temperature and pressure;

a feeding system connected to the digester vessel for feeding cellulosic material and a treatment liquor to the digester vessel, forming a slurry of treatment liquor and cellulosic material in the digester;

a pressurizing system connected to the digester vessel for pressurizing the slurry in the digester;

a heating system connected to digester vessel for heating the slurry in the digester whereby the comminuted cellulosic material undergoes a delignification process forming a delignified cellulosic material;

a washing system connected to or after the digester wherein the delignified cellulosic material is subjected to washing;

a screening system connected to the washing system wherein said washed cellulosic material is divided into at least one accept flow of delignified cellulosic material and a reject flow of not completely delignified cellulosic material, ;

a reject vessel connected to the screening system for collecting the rejects in the reject vessel;

a treatment liquor supply line connected to the reject vessel for adding a treatment liquor intended to be used in the digester into the reject vessel;

a chopper pump connected to the reject vessel for pumping the rejects and added treatment liquor in a recirculation loop from an end of the vessel to the other end of the vessel, said chopper pump having at least one cutting edge on the impeller and a cooperating anvil in the pump housing,

a flow splitter in the recirculation loop for splitting the flow in the into a first part back to the reject vessel and a second part to a high pressure pump;

a first branch line for leading the first part back to the reject vessel and a second branch line for leading the second part away from the reject vessel; a high pressure pump connected to the second branch line and pressurizing said second part of the flow from the recirculation loop in said high pressure pump and mixing the pressurized second part of the flow from the recirculation loop with the treatment liquor being fed to the digester by said feeding system. This system will enable an optimized system for handling rejects, where the chopper pump will perform all the tasks of homogenization of the content in the reject vessel with a treatment liquor that is to be introduced into the cooking system anyhow, defibrating the rejects, and pressurizing the reject flow to a sufficient extent avoiding cavitations in the high pressure centrifugal pump needed in order to supply the reject flow into the pressurized cooking system.

In a further embodiment of the invention the system is further characterized by, said chopper pump being equipped with a pump impeller having at least one cutting edge on the impeller vanes facing the inlet flow. This will introduce a distinct cutting effect on the pumped rejects enabling a disintegration of knots and rejects that are difficult to break up in the cooking process.

In yet a further embodiment of the invention the system is further characterized by, said chopper pump being equipped with at least one stationary anvil arranged essentially transversely over the inlet of the pump, forming a shear action between the cutting edge of the impeller vanes and said stationary anvil. This will introduce a defibration effect on the pumped rejects already in the initial inlet of the chopper pump. In yet another further embodiment of the invention the system is further characterized by, said high pressure pump connected to the second branch line being a centrifugal pump. This will enable a sufficient pressurization of the reject flow to the inlet of the centrifugal pump by said chopper pump, avoiding cavitations in said high pressure centrifugal pump.

In a preferred embodiment of the invention the system said flow splitter is preferably designed as a vertical up flow pipe connected between the outlet of the chopper pump and the upper part of the vessel, said vertical up flow pipe having a height in the range of at least 5-8 meter, and with the high pressure pump connected to this vertical up flow pipe. This will establish sufficient pressure head in inlet of high pressure pump and in the put let of the chopper pump, avoiding cavitations and improved run ability of the pumps in system.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of our invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which:

FIG. 1 is a flow-diagram showing the implementation of the invention in a process for the cooking of chemical pulp using a continuous digester; and

FIG. 2 shows an example of a chopper pump used in the inventive system; and FIG. 3 is a block-diagram showing the implementation of the invention in a process for the cooking of chemical pulp using a batch digester.

SPECIFIC DESCRI PTION

FIG. 1 thus shows the "front part" of a fibre line. Chips CH are fed into a chip bin, 2. From the chip bin 2 the chips are fed via a low pressure feeder 3 into a steaming vessel 4, and from the end of the steaming vessel the chips are fed down into a chip chute 5. The pressure in the steaming vessel is approximately 1 .5 bars. The chip chute 5 is mounted on top of a high pressure feeder 6, the purpose of which is to pass the chips into the top of a digester 1 which is at a considerably higher pressure (typically in the range 4-8 bar) and located at least 20 meter above ground level. The chips are thus fed with the aid of the high pressure feeder 6 in a slurry flow F _c to the top of the digester 5, where some of the transport liquor is separated off and recirculated in flow LIQRET to the chute of the high pressure feeder. In figure 1 is shown an improved feeding system for a high pressure feeder according to the Metso Compact Feed G2 system. The principles of this feeding system are also disclosed in US 7.422.657.

The cooked pulp, which has been delignified in the digester 1 using any available cooking process, is conveyed via a blow valve BV to any appropriate brown stock washer, here a pressure diffuser 9. Typically also a washing step is performed in the bottom of the digester 1 . A significant pressure drop in the order of 0,5-2 bar at least is induced over the blow valve BV, thus subjecting the cooked pulp for a defibration effect. After washing in the pressure diffuser 9 the washed pulp is conveyed to the screen room 10 where reject material, partially in the form of knots, is separated off and is diverted in a flow F _RE j to a reject vessel preferably in form of a container or chute 1 1 . A treatment liquor supply line is connected to the reject vessel for adding a treatment liquor WL into the reject vessel, thus forming a slurry of rejects and treatment liquor.

The treatment liquor added is in this context preferably a treatment liquor that is needed to be added into the digester in order to establish the correct chemical constitution of the treatment liquors in the digester. Alternatively, instead of white liquor being added as the sole treatment liquor to the reject vessel 1 1 , it is possible if implemented in a kraft cooking process, to use any combinations with;

• alkaline filtrate, preferably from one of the washing stages downstream of the knot screen, or

· black liquor,

• green liquor

• cooking additives, for example antraquinone or polysulfide.

According to the invention is a chopper pump 12 connected to the reject vessel 1 1 for pumping the slurry with rejects and the treatment liquor in a recirculation loop Q ₂ from one end to the other end of the vessel 1 1 . The chopper pump will thus subject the rejects for a repeated defibration effect that will break them up into smaller bits and pieces, and will also mix the treatment liquor and rejects for an improved impregnation and soaking effect. A flow splitter of any suitable design is arranged in the recirculation loop for splitting the total flow Q1 from the chopper pump into a first part Q ₂ back to the reject vessel and a second part Q ₃ away from the reject vessel 1 1 to a high pressure pump 13.

In the shown embodiment is the flow splitter designed with a vertical up flow pipe with a height H that establishes a predetermined minimum pressure on the outlet from the chopper pump 12. This height H is defined between the outlet of the chopper pump 12 and the highest position of the vertical upflow pipe before being connected to the upper part of the vessel 1 1 , and the volume contained in this vertical pipe section establish a static hydraulic pressure corresponding to this height. In a test installation it has proven sufficient to have a height H of about 6 meter, and for normal applications the height should preferably be in the range of at least 5-8 meter. As the high pressure pump 13 is controlled by level control LC in the vessel 1 1 , the entire system becomes self regulating, as the pump 13 will draw the flow Q ₃ from the total flow Qi in the lower part of the vertical pipe, as indicated by level control LC if set to control the level in vessel 1 1 at a predetermined height, and all the remaining flow Q ₂ is recirculated back to the vessel 1 1 . If the amount of treatment liquor WL added to the vessel is adjusted to the current demand in the cooking process, then also the equivalent amount of liquid will be bled out in flow Q ₃ , together with a volume of defibrated rejects corresponding to the inflow volume of rejects F _RE j.

The flow splitter could alternatively in a simple embodiment be implemented by flow control valves, V ₂ and V ₃ respectively, in the branch lines for each respective flow Q ₂ and Q ₃ . The high pressure pump 13 is connected to the second branch line forwarding the flow Q ₃ and pressurizes said second part of the flow from the recirculation loop in said high pressure pump. The pump speed and thus the volume Q ₃ being pumped is controlled by a conventional level control LC.

Typically the flow Q ₂ is within the range of 50-75% of the total flow Qi from the chopper pump, thus establishing a repeated passage of the rejects trough the pump. Q ₃ is typically within the range of 10-25% of the flow exiting the low pressure outlet of the high pressure feeder 6, in figure the lowermost outlet, thus establishing the necessary dilution factor for complete emptying of the pocket in the high pressure feeder in the high pressure position.

The pressurized second part of the flow, i.e. Q ₃ , from the recirculation loop, is then added into the pressurized part of the chip slurry being fed to the digester.

The chopper pump 12 being equipped with a pump impeller having at least one cutting edge on the impeller vanes facing the inlet flow which is in contrast to conventional centrifugal pumps that are optimized for pressure build up and minimum internal flow losses. As will be explained later in connection with figure 2 said chopper pump being equipped with at least one stationary anvil arranged essentially transversely over the inlet of the pump, forming a shear action between the cutting edge of the impeller vanes and said stationary anvil.

The high pressure pump 13 connected to the second branch line is a conventional centrifugal pump optimized for pressure build up and less internal flow losses. An example of the cooperating chopping components of the chopper pump 12 in accordance with the present invention are best seen in figure 2 showing one type of chopper pump sold by Vaughan CO. INC and also disclosed as such in WO96/04467. For internal chopping of solid matter in the liquid being pumped, the impeller 17 is of open design having two or more pumping vanes or blades 21 spiraled rearward relative to the direction of rotation of the impeller. Each blade is of substantially constant width, measured in a direction parallel to the axis of rotation, from its root to its outer tip and has a cupped leading face 22 such that both the intake edge 23 and opposite edge 24 of each blade are sharpened. The sharpened intake edge 23 of each impeller blade cooperates with the arcuate intake apertures 20 for a slicing and cutting action.

At the closed side of the pump bowl, a chopper plate or disk 30 is provided with at least one, preferably two, inward-projecting anvil ribs 31 . Such ribs extend linearly outward from the central bore 32 which closely receives the hub 33. Preferably, ribs 31 extend almost radially such that the cutting edges 24 of the impeller blades which are swept rearward in the direction of impeller rotation pass closely across the ribs gradually from the radially inner portion of each blade toward its radially outer portion for an outward-directed slicing action at the closed side of the pump bowl. Again, the tendency is to force solid matter outward and circumferentially for passage to the pump outlet.

In addition to the cutting action achieved by cooperation of the intake edges 23 of the impeller with the sides of the intake apertures 20 and the slicing action achieved at the closed side of the bowl by cooperation of the sharpened edges 24 of the impeller blades with the abrupt anvil ribs 31 , preferably the pump in accordance with the present invention also has at least one external cutter-chopper 34 coupled to and rotated with the impeller. In the embodiment shown the intake plate 16 has a cylindrical outward-opening depression or recess 35 in which the external cutter 34 rotates. Such cutter can have an externally threaded stud 36 for reception in an internally threaded end bore 37 of the impeller drive shaft 12 and a circular stepped hub 38 journaled in the central aperture 39 of the intake end plate 16. Cutter 34 has preferably two blades 40 extending oppositely from the hub. Narrow anvil ribs 41 extend radially outward and project axially from the otherwise planar outer face 42 of the base of the end plate recess 35. Such ribs include inner portions extending across the joining sections 43 between the arcuate intake apertures 20 and outer portions extending across the unapertured annular portion 44 of the end plate surrounding the apertures. Such outer portions of the base ribs 41 lead to side ribs 45 extending axially along the peripheral wall 46 of the end plate depression and projecting abruptly radially inward. Preferably, additional side ribs 47 are spaced along such wall.

The external cutter blades 40 include linear sharpened leading edges 48 in close cutting relationship to the base ribs 41 as the cutter is rotated. In addition, each blade has an outturned tip or fin 49 extending generally perpendicularly from the radial portion of the blade and sharpened for cutting cooperation with the anvil ribs 45 and 47 spaced around the circumference of the end plate recess. The external cutter 34 also includes an outer circular hub 57 of a diameter approximately equal to the inner diameter of the arcuate intake apertures 20. Two axially extending teeth 55 are formed by cutting a wide groove 56 through the hub at an angle of approximately 45 ^Q relative to the direction of projection of each of the external cutter blades 40. The resulting teeth have sharp arcuate cutting edges.

The combined effect is to chew and grind solid matter by the external teeth 55, chop and slice such matter externally of the pump casing by the cutter 34 in cooperation with the base and side ribs 41 , 45 and 47, and continue to slice and chop such matter inside the pump at both the intake side and the closed side of the pump bowl.

In figure 3 is shown an implementation of the invention in a batch cooking system. It is understood, that a batch cooking vessel 1 a is used, and chips is fed to one end of vessel and cooked pulp fed out from the other end, while liquor is charged at some location of the vessel 1 a and displaced liquor leave from another location of the vessel. A number of sequential treatment steps, here 8 steps, are performed in this vessel as shown from Seq.1 to Seq. 8. It is to be noted that the number of steps performed could be different and the actual steps could be modified.

The kraft batch cook as exemplified in figure 3 is started in the first sequence, Seq.1 , by charging the digester with wood chips, or with another lignocellulosic material, and steaming of said material.

After the lignocellulosic material charge, the digester is filled hydralically in a second sequence, Seq.2, with any appropriate impregnation liquor from tank LQ1 topping the digester full of liquor and subsequently pressurizing the vessel during an appropriate period of time. The cooking process is then continued in a third sequence, Seq.3, by pumping in a volume of hot black liquor from hot black liquor accumulator LQ2, while displacing the used first impregnation liquor. Preferably a first part of the cooler and used impregnation liquor, which is less contaminated with dissolved organic material, that is displaced by hot black liquor, is conducted to filtrate tank FT1 to be re-used in subsequent batches, while a second part of the used impregnation liquor displaced by hot black liquor, is conducted to recovery REC. Pumping a volume of white liquor WL from tank 1 1 a via a heater 15 into the digester then continues the cooking sequence in a fourth sequence, Seq.4. The liquor displaced by hot white liquor is preferably conducted to recovery, REC. At the end of the fourth sequence described above, the digester is close to the final cooking temperature.

The final heating-up is carried out using direct or indirect steam heating and digester re-circulation in a fifth sequence, Seq.5.

The actual cooking phase is then activated in a sixth sequence, Seq.6, while circulating the liquor trough the digester.

After the desired cooking time when the delignification has proceeded to the desired final reaction degree, the spent and hot cooking liquor is ready to be displaced with wash filtrate from tank FT1 in a seventh sequence, Seq.7, and the hot displaced volume is conducted to hot black liquor accumulator LQ2.

In a final eight sequence, Seq.8, the remaining liquor is displaced with wash filtrate from tank FT2, which preferably is an alkaline wash filtrate from any subsequent brown stock wash. The remaining liquor displaced is conducted to the tank LQ1 . After completed final displacement, the digester content is discharged for further processing of the pulp. As shown in figure 3 is a screen room 10a located directly after the digester.

Similarly to what is described in relation to figure 1 , are reject material, partially in the form of knots, separated off and is diverted in a flow F _RE j to a reject vessel preferably in form of a container or chute 1 1 a. A treatment liquor supply line is connected to the reject vessel for adding a treatment liquor WL into the reject vessel, thus forming a slurry of rejects and treatment liquor. The treatment liquor added is in this context a treatment liquor that is needed to be added into the digester in order to establish the correct chemical constitution of the treatment liquors in the digester. Alternatively, instead of white liquor being added as the sole treatment liquor to the reject vessel 1 1 , it is possible if implemented in a kraft cooking process, to use any combinations with;

• alkaline filtrate, preferably from one of the washing stages downstream of the knot screen, or

· black liquor,

• green liquor

• cooking additives, for example antraquinone or polysulfide.

According to the invention is a chopper pump 12a connected to the reject vessel 1 1 a for pumping the slurry with rejects and the treatment liquor in a recirculation loop Q ₂ from one end to the other end of the vessel 1 1 a. The chopper pump will thus subject the rejects for a at least one defibration effect by passage of said chopper pump, and a major part of the flow will be subjected to repeated defibration effect that will break them up into smaller bits and pieces, and will also mix the treatment liquor and rejects for an improved impregnation and soaking effect. A flow splitter of any suitable design is arranged in the recirculation loop for splitting the total flow Qi from the chopper pump into a first part Q ₂ back to the reject vessel and a second part Q ₃ away from the reject vessel 1 1 a to a high pressure pump 13a. The flow splitter could be of the same design as described in association with the embodiment shown in figure 1 . The high pressure pump 13a is connected to the second branch line forwarding the flow Q ₃ and pressurizes said second part of the flow from the recirculation loop in said high pressure pump.

Typically the recirculated flow Q ₂ is within the range of 50-75% of the total flow Qi from the chopper pump, thus establishing a repeated passage of the rejects trough the pump. Q ₃ is typically corresponding to the total amount of treatment liquor that is added in step Seq.4 and added with the reject flow F _RE j,, thus establishing the necessary alkaline supply for the cooking phase in step Seq.6. .

The pressurized second part of the flow, i.e. Q ₃ , from the recirculation loop, is then added into the pressurized digester. Preferably is the container or chute 1 1 a of considerable size capable of storing the total charge of treatment liquor being added to the digester, while storing the rejects in this vessel 1 1 a at a dry matter concentration less than 10-20%, typically in the range 2-7%.

The chopper pump 12a being equipped with a pump impeller having at least one cutting edge on the impeller vanes facing the inlet flow which is in contrast to conventional centrifugal pumps that are optimized for pressure build up and minimum internal flow losses. The high pressure pump 13a connected to the second branch line is a conventional centrifugal pump optimized for pressure build up and less internal flow losses.

The invention is not limited by what has been shown hereinabove, but instead can be varied within the scope of the patent claims which follow. Thus, the person skilled in the art will realize that it is possible, for example, to use another batch cooking process than that disclosed in Figure 3. A part of the reject flow Q ₃ could also be added earlier or later in the batch cooking process, for example during the impregnation or hot liquor fill phase, i.e. Seq.2 or Seq.3, or during the cooking phase, i.e. Seq.6. When implemented in a continuous cooking process as that disclosed in figure 1 , could the reject flow Q ₃ at least in part also be added to the approach flow Fc to the digester, and/or to any cooking circulation flow in the digester.