FI20215861A1

FI20215861A1 - A method, use of the same, a pulp composition, and a system

Info

Publication number: FI20215861A1
Application number: FI20215861A
Authority: FI
Inventors: Mikko Nikamaa
Original assignee: Metsae Board Oyj
Priority date: 2021-08-17
Filing date: 2021-08-17
Publication date: 2023-02-18
Also published as: WO2023021243A1; CN117795155A; SE2430124A1; CA3228121A1; DE112022003992T5

Abstract

According to an example aspect of the present invention, there is provided a method comprising: providing wood chips; impregnating the wood chips with an impregnating composition to obtain impregnated wood chips; refining the impregnated wood chips in at least one refiner to obtain a refined pulp composition; screening the refined pulp composition to obtain an accept fraction and a reject fraction; and directing at least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.

Description

TITLE

A method, use of the same, a pulp composition, and a system

FIELD

[0001] The present invention relates to processes used in manufacturing of pulp.

BACKGROUND

[0002] In the known pulping methods, chipped wood may first be impregnated with chemicals. The impregnated chips are then directed to primary refining, which may consist — of several consecutive primary refiners. After the refining, the consistency of the pulp is typically decreased and latency removed. After the latency removal, the pulp is screened to an accept and a reject fraction. The accept fraction may be directed to a disc filter, wherein the consistency of the pulp is increased, and then further to bleaching and washing steps.

The reject fraction is typically stored in reject storage silos and treated in a separate reject treatment line involving one or more reject refiners.

[0003] The present invention is aimed at improving the known pulping methods.

SUMMARY OF THE INVENTION

— [0004] The invention is defined by the features of the independent claims. Some

N

N 20 — specific embodiments are defined in the dependent claims. 3

W [0005] According to a first aspect of the present invention, there is provided a > method comprising: providing wood chips; impregnating the wood chips with an

Ao + impregnating composition to obtain impregnated wood chips; refining the impregnated 2 wood chips in at least one refiner to obtain a refined pulp composition; screening the

N 25 — refined pulp composition to obtain an accept fraction and a reject fraction; and directing at

O

N least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.

[0006] Various embodiments of the first aspect may comprise one or more features from the following bulleted list: e The impregnating composition is an aqueous composition comprising one of more of the following: NaOH, Na2SOs3, white liquor, such as oxidised white liquor, green liquor, such as oxidised green liquor, and H202. e In said refining, the refining energy consumption is in the range 0.2 to 1.5 MWh/t. e After the refining step, the pulp composition has a Pulmac shive content of less than 10%, such as less than 5%. e After the refining step, the pulp composition has Canadian Standard Freeness (CSF) in the range 285 to 750, for example 285 to 470, or 350 to 750. e In the screening step at least 80 wt-%, such as at least 95 wt-% of the refined pulp composition becomes directed to the accept fraction. e The Pulmac shive content of the accept fraction is not more than 10 percentage points lower, such as not more than 5 percentage points lower than the Pulmac shive content of the incoming refined pulp composition fed to the screening. e TheCSF of the accept fraction differs from the CSF of the incoming refined pulp composition fed to the screening by not more than +20%. e The method further comprises dewatering the accept fraction output from the screening.

N 20 e Said dewatering is carried out by a screw press and/or by a disc filter.

N

S e The dewatering comprises dewatering carried out by a screw press.

MN

- e The pulp composition, such as the accept fraction, that is fed to the screw press has a = a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably 3 o to 4%.

LO

O 25 e The dewatering comprises dewatering carried out by a disc filter. e The pulp composition, such as the accept fraction, that is fed to the disc filter has a consistency of at least 0.5%, such as 1.0 to 1.5%.

e In the dewatering, the pulp composition is dewatered to a consistency of at least 20%, such as at least 30%. e The dewatered pulp composition is fed to a bleaching step, optionally after decreasing the consistency of the pulp composition to less than 15%, such as to less than 10%. e The consistency of the accept fraction is first increased to above 20% and then decreased to about 8 to 12%, preferably to about 10%, for example to clean the pulp. e The reject fraction is fed to the impregnating step at a consistency in the range 4 to 20%, such as 4 to 10%.

[0007] According to a second aspect of the present invention, there is provided use of the method according to the first aspect in the manufacturing of bleached or unbleached thermomechanical pulp.

[0008] According to a third aspect of the present invention, there is provided a pulp composition, obtained by the method according to the first aspect. Preferably, the pulp composition comprises BCTMP.

[0009] According to a fourth aspect of the present invention, there is provided a system configured to execute the method according to the first aspect.

[0010] According to a fifth aspect of the present invention, there is provided a — method comprising: providing a pulp composition; screening the pulp composition to

N obtain an accept fraction and a reject fraction; and dewatering the accept fraction output

O

N from the screening by a screw press. 8

K [0011] Various embodiments of the fifth aspect may comprise one or more features

I from the following bulleted list: a o 25 e The method comprises before the screening step: providing wood chips; refining co

D the wood chips in at least one refiner to obtain a refined pulp composition; directing

N

N the refined pulp composition to said screening.

e The method comprises before the screening step: providing a pulp composition comprising mechanical pulp, such as groundwood pulp; directing the pulp composition comprising mechanical pulp to said screening. e The pulp composition, such as the accept fraction, that is fed to the screw press has a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably 3 to 4%. e The dewatering is carried out both during the screening and in the subsequent

SCrew press. e In the dewatering, the pulp composition is dewatered to a consistency of at least 20%, such as at least 30%. e The dewatered pulp composition is fed to a bleaching step, optionally after decreasing the consistency of the pulp composition to less than 15%, such as to less than 10%. e The consistency of the accept fraction is first increased to above 20% and then decreased to about 8 to 12%, preferably to about 10%, for example to clean the pulp composition. e The screening comprises screening carried out by a first screening device and screening carried out by a subseguent second screening device. e The consistency of the pulp composition in the output of the first screening device is lower than the consistency of the pulp composition in the output of the second

N screening device.

N

2 e In the screening step at least 50 wt-%, such as at least 80 wt-% of the refined pulp = composition becomes directed to the accept fraction.

Tr - e The method comprises directing the reject fraction from the screening step to a o 25 reject treatment line.

LO

O e The reject treatment line comprises at least a step of refining the reject fraction by a reject refiner.

e The method comprises directing an output of the reject treatment line to said screw press, and preferably said output has a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably 3 to 4%. e The method further comprises, before the screening step, impregnating the wood 5 chips with an impregnating composition to obtain impregnated wood chips, and refining the impregnated wood chips in at least one refiner. e The impregnating composition is an aqueous composition comprising one of more of the following: NaOH, Na?SO3, white liquor, such as oxidised white liquor, green liquor, such as oxidised green liquor, and H202. e The method comprises directing at least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.

[0012] According to a sixth aspect of the present invention, there is provided use of the method according to the fifth aspect in the manufacturing of chemithermomechanical — pulp or bleached chemithermomechanical pulp.

[0013] According to a seventh aspect of the present invention, there is provided use of the method according to the fifth aspect in the manufacturing of mechanical pulp, such as groundwood pulp, pressure groundwood pulp, stone groundwood pulp, or thermomechanical pulp. — [0014] According to an eighth aspect of the present invention, there is provided a _ pulp composition, obtained by the method according to the fifth aspect. Preferably, the

O pulp composition comprises BCTMP. 3 [0015] According to a ninth aspect of the present invention, there is provided a = system configured to execute the method according to the fifth aspect.

T

= _ 25 — [0016] Advantages of the invention >

O [0017] An advantage of the present invention is that the amount of reject generated

S in the screening step may be decreased.

[0018] A further advantage of the present invention is that relatively high CSF levels may be used throughout the process.

[0019] A further advantage of the present invention is that a disc filter may be dispensed with.

[0020] A further advantage of the present invention is that tailoring of the pulping process for the purpose of preparing various pulp end compositions may be facilitated.

[0021] An advantage of the present invention is that energy savings may be attained as the process involves less eguipment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIGURES 1 and 2 illustrate the method in accordance with at least some embodiments of the present invention; and

[0023] FIGURE 3 illustrates the screening step in the method in accordance with at least some embodiments of the present invention.

EMBODIMENTS

— [0024] As used herein, the term “accept” refers to the fraction that has been accepted to be fed to the succeeding step in the process, such as to the succeeding step in the main line process.

[0025] As used herein, the term “reject” refers to the fraction that has not been accepted to be fed to the succeeding step in the process, such as to the fraction that has

N 20 — been rejected from continuing in the main line process.

N

3 [0026] In the present context, Canadian Standard Freeness (CSF) is measured = according to ISO 5267-2. = > [0027] In the present context, brightness is measured according to brightness 457 © nm, ISO 2470.

LO

S 25 — [0028] In the present context, shive content refers to shive content *Pulmac (0.1 mm)”.

[0029] In some embodiments of the present invention it has been surprisingly observed that in a pulping process it may be possible to entirely dispense with a separate reject treatment line.

[0030] In some embodiments of the present invention it has been surprisingly observed that a disc filter may be replaced by a screw press in a dewatering step of a pulping process.

[0031] In one embodiment, the present method comprises: providing wood chips; impregnating the wood chips with an impregnating composition to obtain impregnated wood chips; refining the impregnated wood chips in at least one refiner to obtain a refined — pulp composition; screening the refined pulp composition to obtain an accept fraction and a reject fraction; and directing at least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.

[0032] In another embodiment, the present method comprises: providing wood chips; refining the wood chips in at least one refiner to obtain a refined pulp composition; — screening the refined pulp composition to obtain an accept fraction and a reject fraction, and dewatering the accept fraction by a screw press.

[0033] The wood to be chipped may comprise softwood, such as spruce or pine or mixtures thereof, or hardwood, such as birch, poplar, aspen, alder, maple, eucalypt tropical hardwood, or mixtures thereof, or it may comprise a mixture of softwood and hardwood.

[0034] The impregnating composition may be an aqueous composition comprising one of more of the following: NaOH, Na2SOs3, white liquor, such as oxidised white liquor, = green liquor, such as oxidised green liquor, and H202. & ob [0035] During the refining, the wood fibres become released from the wood material 7 that has been softened in the impregnating step. As a result of the mechanical forces in refining, the fibres are shortened, the fibre diameter is decreased, and the fibres become a fibrillated. The fibrillating (external fibrillating) involves breaking of the smooth fibre o surface to make it hairy by introduction of microfibrils on the fibre surface. Also internal = fibrillating typically occurs, which increases flexibility of the fibres. As to the quality of

N the obtained refined mass, a low shive content, high strength, and a sufficient fibre length — may be aimed at.

[0036] The refiner that is used for refining the wood chips may be a high- consistency refiner or a low-consistency refiner. In the case of a high-consistency refiner, the consistency of the pulp to be refined is typically 20 to 60%. In the case of a low- consistency refiner, the consistency of the pulp to be refined is typically 3 to 5%.

[0037] The refiner may be a flat disc refiner or a conical disc refiner.

[0038] The refiner may be a one-disc or a two-disc refiner. Preferably the refiner is a one-disc refiner comprising a stator and a rotor.

[0039] The distance between the blades of the refiner may be in the range 0.1 to 1.0 mm. — [0040] In said refining, the refining energy consumption may be in the range 0.2 to 1.5 MWh/t.

[0041] After the refining step, the pulp may have a Pulmac shive content of less than 20%, such as less than 10%, for example less than 5%.

[0042] After the refining step, i.e. in the output of the last refiner, the refined pulp — may have Canadian Standard Freeness (CSF) of at least 285, such as at least 350, for example at least 400. In some embodiments, the refined pulp may have Canadian Standard

Freeness of less than 750, such as less than 470.

[0043] In one embodiment, the refined pulp may have Canadian Standard Freeness (CSF) in the range 285 to 470. Such pulp may be suitable for the manufacturing of CTMP that is usable as a material of lightweight coated printing paper.

N [0044] In one embodiment, the refined pulp may have Canadian Standard Freeness

N

& (CSF) in the range 350 to 750. Such pulp may be suitable for the manufacturing of ~ BCTMP that is usable as a material of folding boxboard.

E [0045] Before directing the refined pulp to the screening, a latency removal step is 5 25 typically present. During the latency removal, curling of the fibres is reduced and the fibres e become straightened. a

N [0046] In one embodiment, the refined pulp composition that is fed to the screening step may have a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably 3 to 4%.

[0047] During the screening step or at least in the accept output of the screening step the pulp has a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably 3 to 4%. An advantage of having such a high consistency at least in the accept output of the screening is that the accept fraction may be directly feedable to a screw press after the screening. In lower consistencies, the mass flow rate might be too high to be handled by a screw press.

[0048] In the screening step, at least 50 wt-%, for example at least 80 wt-%, such as at least 90 wt-%, preferably at least 95 wt-% of the refined pulp composition becomes directed to the accept fraction.

[0049] Preferably, the composition and properties of the refined pulp are not substantially altered as a result of the screening. In other words, the amount of reject separated from the pulp in the screening step is preferably minimised.

[0050] The Pulmac shive content of the accept fraction is preferably not more than 50 percentage points lower, such as not more than 10 percentage points lower, for example — not more than 5 percentage points lower than the Pulmac shive content of the incoming refined pulp fed to the screening.

[0051] For example, the Pulmac shive content of the accept fraction may be less than 10%, such as less than 5%, preferably less than 1%.

[0052] The CSF of the accept fraction may differ from the CSF of the incoming refined pulp fed to the screening by not more than +20%,

[0053] In some embodiments, in steps succeeding the screening step, no incoming

N pulp feeds are combined to the accept fraction that is output from the screening step. Thus,

A the steps downstream of the screening step, typically dewatering and bleaching, receive as 7 an input only the accept fraction from the screening. Therefore, preferably the properties — and composition of only this sole accept fraction need to be adjusted if the input of post- a screening steps needs adjustment. >

L [0054] In one embodiment, the dewatering may be carried out in a screw press. In

O this embodiment, the fraction that is fed from the screening to the dewatering step preferably has a consistency of at least 2%, such as at least 3%, for example 2 to 5%,

preferably 3 to 4%. Preferably said fraction consists of the accept fraction from the screening.

[0055] In one embodiment, the dewatering may be carried out by a disc filter. In this embodiment, the fraction that is fed from the screening to the dewatering step preferably has a consistency of 0.5 to 2%, such as 1.0 to 1.5%. Preferably said fraction consists of the accept fraction from the screening.

[0056] In one embodiment, the dewatering may be carried out both during the screening and in a subsequent screw press or disc filter. Thus, part of water removal may take place already in the screening step, for example by using at least two screening — devices arranged in series and by configuring the latter screening device to partly dewater the pulp composition.

[0057] In a preferred embodiment, a disc filter is not used for dewatering the accept fraction in steps succeeding the screening step. An advantage of not using a disc filter is that CSF of the pulp may be adjusted to higher levels throughout the process, as cake- forming problems may be avoided. Such cake-forming problems might be encountered in methods that use a disc filter.

[0058] An advantage of using a screw press for the dewatering is that the consistency may be increased to high levels, such as to 15 to 40%, for example to at least 20%, preferably to at least 30%. The ability to effectively remove water from the screened — pulp, if needed, may be advantageous.

[0059] The accept fraction from the screening may be dewatered to a consistency of = at least 20%, such as at least 30%. & ob [0060] The dewatered accept fraction may be fed to a bleaching step, optionally after 7 decreasing the consistency of the accept fraction to less than 15%, such as to less than = 25 10%, a 5 [0061] In some embodiments, the consistency of the accept fraction is first increased

D to above 20% and then decreased to about 8 to 12%, preferably to about 10%, for example

O to clean the pulp.

[0062] The reject fraction from the screening may be fed back to the impregnating step or to a step preceding the impregnating step. Preferably the reject fraction is fed to the impregnating step or to a step preceding the impregnating step at a consistency in the range 2 to 45%. In one example, the pulp composition, such as the reject fraction, fed to the impregnating step has a consistency in the range 4 to 10%.

[0063] Advantageously, the reject is not fed directly to the refining step but instead to a step preceding the refining step. In that way problems related to maintaining an appropriate consistency in the refiner may be avoided. In this embodiment, the consistency of the reject may be allowed to vary in a broader range.

[0064] An advantage related to feeding the reject to the impregnating step or to a step preceding the impregnating step is that the reject fraction may receive a further impregnation before continuing to the refining.

[0065] In preferred embodiments, the reject fraction is fed to the impregnating step or to a step preceding the impregnating step without separate dewatering of the reject fraction and/or without separate refining of the reject fraction. For example, the reject fraction may be fed directly from the screening reject output to the impregnating step or to a step preceding the impregnating step.

[0066] In some embodiments, at least part of the reject fraction, such as substantially all of the reject fraction, may be directed from the screening to a reject treatment line that is separate from the main line, i.e. does not form part of the main line.

[0067] The reject treatment line may include a reject refiner.

[0068] The reject treatment may comprise one or more of the following steps: dewatering the reject, refining the reject by the reject refiner, and screening the reject to an

N accept and a reject of the reject treatment line.

N

3 [0069] The reject treatment line may comprise one or more screening devices, such

MN . . — as at least two screening devices.

Tr - 25 — [0070] The pulp composition input to the screening step of the reject treatment line

O may have a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably

LO

N 3 to 4%.

O

N

[0071] An output, such as an accept of the reject treatment line may be fed back to the main line, for example to the main line dewatering step, thus to become combined with the accept output from the main line screening.

[0072] In one embodiment, the accept that is output from the reject treatment line may have a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably 3 to 4%. An advantage is that the accept of the reject treatment line can then be fed directly to a screw press in the mainline.

[0073] In one embodiment, an accept from a reject treatment line is directed to the main line dewatering step, such as to the screw press for example at a consistency of at — least 2%.

[0074] It is preferred to adjust the consistency of the accept that is output from the reject treatment line to the range 2 to 5%.

[0075] In one embodiment, a part of the reject from the screening step is directed back to the impregnating step and another part of the reject from the screening step is — directed to the reject treatment line.

[0076] The present invention may be applicable for the manufacturing of pulp selected from the following group: mechanical pulp, thermomechanical pulp (TMP) chemithermomechanical pulp (CTMP), bleached chemithermomechanical pulp (BCTMP), semichemical pulp, groundwood pulp (GW), pressure groundwood pulp (PGW), stone groundwood pulp (SGW), and combinations thereof.

[0077] The pulp may be bleached or unbleached.

N

N [0078] Preferably, the method is for the manufacturing of chemithermomechanical 3 pulp or bleached chemithermomechanical pulp.

MN

I [0079] In some embodiments, the method is for the manufacturing of mechanical a - 25 — pulp, groundwood pulp (GW), pressure groundwood pulp (PGW), stone groundwood pulp < (SGW) or thermomechanical pulp (TMP).

O [0080] BCTMP obtained by some embodiments of the present method may be applicable as a material of one or more layers of paperboard, such as folding boxboard, or of paper, such as printing paper or writing paper or tissue paper.

[0081] In one embodiment, the method is for preparation of such pulp that is adapted for the manufacturing of paperboard with the following characteristics:

[0082] Preferably the density of the paperboard is in the range 0.5 — 1.3 g/cm’.

Preferably the brightness of the paperboard is larger than 84%.

[0083] In another embodiment, the method is for preparation of such pulp that is adapted for the manufacturing of paper with the following characteristics:

[0084] Preferably the density of the paper is in the range 0.7 — 1.4 g/cm’. Preferably the brightness of the paper is larger than 77%.

[0085] The present invention also concerns a system configured to execute the — method according to any of the embodiments described above or their any combinations.

The system may form part of a pulp mill. The system may comprise one or more of the following devices: a wood chipping device, a wood chip impregnating system, one or more primary refiners, one or more pulp screening devices, one or more reject refiners, a reject screening device, a pulp dewatering device, such as a disc filter or a screw press, a pulp bleaching system.

[0086] Turning now to the drawings:

[0087] FIGURE 1 illustrates process steps in a method in accordance with at least some embodiments of the present invention. The process comprises the following steps: chipping 10, impregnating 11, refining 12, screening 13, dewatering 14, optional bleaching 15, and optional washing 16. The screening produces as outputs an accept fraction and a reject fraction. The accept fraction continues in the main line and is directed from the

N screening 13 to the dewatering 14. The reject fraction, if existing, or at least a part of the

N ob reject fraction, is fed from the screening 13 back to a step preceding the refining step, for

O

~ example to the impregnating step 11, as shown by an arrow in FIG. 1. Here, the dewatering

I 25 — 14 may comprise a screw press or a disc filter. a a 5 [0088] FIGURE 2 illustrates process steps in a method in accordance with at least

D some embodiments of the present invention. Here, the main line comprises the steps of

N . . .

S chipping 20, optional impregnating 21, refining 22, screening 23, dewatering 24, optional bleaching 25, and optional washing 26. The method further includes a reject treatment line = 27 which is separate from the main line. The reject fraction obtained in the screening 23 is directed to the reject treatment line 27. The reject treatment line may comprise a reject refining step and/or a reject screening step. An output, such as an accept fraction from the reject treatment line 27 may be directed back to the main line, such as to the dewatering 24. Here, the dewatering 24 preferably comprises a screw press.

[0089] FIGURE 3 illustrates the screening step in the method in accordance with at least some embodiments of the present invention. The incoming pulp composition P is fed to the screening step 33. The screening produces as outputs an accept fraction A and a reject fraction R. The accept fraction A is accepted to continue in the mainline process to the next step, the dewatering by a screw press 34. The reject fraction R is rejected from continuing in the mainline process and instead directed either to a reject treatment line or backwards in the mainline, for example to an impregnating step (not shown here).

[0090] The consistency of the incoming pulp P, which may be refined pulp, is preferably at least 3.5%, for example 3.5 to 4.5%.

[0091] The consistency of the accept fraction A is preferably at least 3%, such as 3.5 to 4%.

[0092] The consistency of the reject fraction is preferably at least 4%, such as at least 4.5%.

[0093] EXAMPLE

[0094] A pulping process comprising direct feeding of the reject from the screening — to the impregnating was tested. The process main line included chipping, impregnating, refining, screening, dewatering by a disc filter, and bleaching. The consistency of the pulp

N composition input to the bleaching was about 10%. The accept from the screening was

N ob directed to the disc filter. The reject from the screening was directed either to a

O

K conventional reject treatment line or alternatively back to the impregnating step.

E 25 — [0095] In Test 1 and Test 2 processes, the conventional reject treatment line was 5 bypassed and the reject from the screening was fed back to the impregnating step. Tests 1

D and 2 were conducted on different days.

O

N [0096] The Reference process did not include any back-feeding of the reject but instead the reject from the screening was directed to the conventional reject treatment line comprising a reject refiner.

[0097] We measured the Canadian Standard Freeness (CSF) of the pulp composition that was output from the disc filter (i.e. at a point between the disc filter and the bleaching).

The results are presented in Table 1. The results show that the CSF of the pulp composition in Test 1 and 2 processes were successfully maintained at the same level as the CSF of the — pulp composition in the Reference process.

I

I A |"e000ne?

[0098] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the — relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0099] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment”

N in various places throughout this specification are not necessarily all referring to the same

N

AN embodiment. <Q

J [00100] As used herein, a plurality of items, structural elements, compositional = elements, and/or materials may be presented in a common list for convenience. However, a - 20 — these lists should be construed as though each member of the list is individually identified

O . koe .

Q as a separate and unique member. Thus, no individual member of such list should be

N construed as a de facto eguivalent of any other member of the same list solely based on

N their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[00101] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[00102] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[00103] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The — features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.

S s INDUSTRIAL APPLICABILITY = 25 — [00104] The present invention is industrially applicable at least in the manufacturing

E: of pulp, such as CTMP or BCTMP. 2

ES ACRONYMS LIST

TMP thermomechanical pulp

CTMP chemithermomechanical pulp

BCTMP bleached chemithermomechanical pulp

GW groundwood pulp

PGW pressure groundwood pulp

SGW stone groundwood pulp

CSF Canadian Standard Freeness

REFERENCE SIGNS LIST

10, 20 chipping 11,21 impregnating 12, 22 refining 13,23,33 — screening 14, 24,34 — dewatering 15,25 bleaching 16,26 washing 27 reject treatment

N

O

N

MN

I a a © co

LO

N

O

N

Claims

CLAIMS:

1. A method comprising: — providing wood chips; — impregnating the wood chips with an impregnating composition to obtain impregnated wood chips; — refining the impregnated wood chips in at least one refiner to obtain a refined pulp composition; — screening the refined pulp composition to obtain an accept fraction and a reject fraction; and — directing at least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.

2. The method according to any of the preceding claims, wherein the impregnating composition is an aqueous composition comprising one of more of the following: NaOH, Na2SOs, white liquor, such as oxidised white liquor, green liquor, such as oxidised green liquor, and H202.

3. The method according to any of the preceding claims, wherein in said refining, the refining energy consumption is in the range 0.2 to 1.5 MWh/t.

4. The method according to any of the preceding claims, wherein after the refining step, the — pulp composition has a Pulmac shive content of less than 10%, such as less than 5%. O 25 s

5. The method according to any of the preceding claims, wherein after the refining step, the pulp composition has Canadian Standard Freeness (CSF) in the range 285 to 750, for E example 285 to 470, or 350 to 750. < 30 —

6. The method according to any of the preceding claims, wherein in the screening step at N least 80 wt-%, such as at least 95 wt-% of the refined pulp composition becomes directed - to the accept fraction.

7. The method according to any of the preceding claims, wherein the Pulmac shive content of the accept fraction is not more than 10 percentage points lower, such as not more than 5 percentage points lower than the Pulmac shive content of the incoming refined pulp composition fed to the screening.

8. The method according to any of the preceding claims, wherein the CSF of the accept fraction differs from the CSF of the incoming refined pulp composition fed to the screening by not more than +20%.

9. The method according to any of the preceding claims, further comprising dewatering the accept fraction output from the screening.

10. The method according to any of the preceding claims, wherein said dewatering is carried out by a screw press and/or by a disc filter.

11. The method according to any of the preceding claims, wherein the dewatering comprises dewatering carried out by a screw press, and preferably the pulp composition, such as the accept fraction, that is fed to the screw press has a consistency of at least 2%, such as at least 3%, for example 2 to 5%, preferably 3 to

4%.

12. The method according to any of the preceding claims, wherein the dewatering comprises dewatering carried out by a disc filter, and preferably the pulp composition, such as the accept fraction, that is fed to the disc filter has S 25 —aconsistency of at least 0.5%, such as 1.0 to 1.5%. 3 m

13. The method according to any of the preceding claims, wherein in the dewatering, the z pulp composition is dewatered to a consistency of at least 20%, such as at least 30%. a 2 30 —

14. The method according to any of the preceding claims, wherein the dewatered pulp N composition is fed to a bleaching step, optionally after decreasing the consistency of the N pulp composition to less than 15%, such as to less than 10%.

15. The method according to any of the preceding claims, wherein the consistency of the accept fraction is first increased to above 20% and then decreased to about 8 to 12%, preferably to about 10%, for example to clean the pulp.

16. The method according to any of the preceding claims, wherein the reject fraction is fed to the impregnating step at a consistency in the range 4 to 10%.

17. Use of the method according to any of the preceding claims in the manufacturing of bleached or unbleached thermomechanical pulp.

18. A pulp composition, obtained by the method according to any of claims 1 to 16.

19. The pulp composition according to claim 18, wherein the pulp composition comprises BCTMP.