Method, Use of Method, Pulp Composition, and System

Abstract

According to an exemplary embodiment of the present invention, there is provided a method including providing wood chips, impregnating the wood chips with an impregnation 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 returning at least a portion of the reject fraction from the screening step to the impregnation step or to a step preceding the impregnation step.

Description

[Technical field]

[0001] The present invention relates to a process used in the manufacture of pulp. [Background technology]

[0002] In known pulping processes, chipped wood may first be impregnated with chemicals. The impregnated chips are then transferred to a primary refinement, which may consist of several successive primary refiners. After refining, the consistency of the pulp is typically reduced and latency is removed. After latency removal, the pulp is screened into an accept fraction and a reject fraction. The accept fraction is transferred to a disc filter, where the consistency of the pulp is increased and may subsequently be transferred to further bleaching and washing steps. The reject fraction is typically stored in a reject storage silo and processed in a separate reject processing line with one or more reject refiners.

[0003] The present invention aims to improve known pulping processes. Summary of the Invention

[0004] The invention is defined by the features of the independent claims. Some particular embodiments are defined in the dependent claims.

[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 impregnation 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 returning at least a portion of the reject fraction from the screening step to the impregnation step or to a step preceding the impregnation step.

[0006] Various embodiments of the first aspect may include one or more features of the following bulleted list: The impregnation composition is an aqueous composition comprising one or more of NaOH, Na2SO3, white liquor such as oxidized white liquor, green liquor such as oxidized green liquor, and H2O2. - In refining, the refining energy consumption is in the range of 0.2~1.5MWh / t. After the refining step, the pulp composition has a Pulmac shive content of less than 10%, such as less than 5%. After the refining step, the pulp composition has a Canadian Standard Freeness (CSF) in the range of 285 to 750, such as 285 to 470, or 350 to 750. In the screening step, at least 80% by weight, such as at least 95% by weight, of the refined pulp composition is directed to the accept fraction. · The Pulmac fragment content of the accept fraction is up to 10 percentage points lower, such as up to 5 percentage points lower, than the Pulmac fragment content of the influent refined pulp composition sent to screening. · The CSF of the accept fraction differs by up to ±20% from the CSF of the influent refined pulp composition sent to screening. The method further comprises dehydrating the accept fraction output from the screening. Dewatering is carried out by means of a screw press and / or by means of a disc filter. Dewatering includes dewatering carried out by a screw press. The pulp composition, such as the accept fraction, fed to the screw press has a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%. Dewatering includes dewatering performed by disc filters. The pulp composition, such as the accept fraction, sent to the disc filter has a consistency of at least 0.5%, such as 1.0-1.5%. In dewatering, the pulp composition is dewatered to a consistency of at least 20%, such as at least 30%. The dewatered pulp composition is passed to a bleaching step, optionally after reducing the consistency of the pulp composition to less than 15%, such as less than 10%. The consistency of the accept fraction is first increased, for example to more than 20%, to clean the pulp, and then decreased to about 8-12%, preferably about 10%. · The reject fraction is sent to the impregnation step with a consistency ranging from 4-20%, such as 4-10%.

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

[0008] According to a third aspect of the present invention there is provided a pulp composition obtainable 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 carry out 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 obtain an accept fraction and a reject fraction, and dewatering the accept fraction output from the screening by a screw press.

[0011] Various embodiments of the fifth aspect may include one or more features from the following bulleted list: The method includes, prior to the screening step, providing wood chips, refining the wood chips in at least one refiner to obtain a refined pulp composition, and directing the refined pulp composition to screening. The method includes, prior to the screening step, providing a pulp composition comprising a mechanical pulp, such as groundwood pulp, and directing the pulp composition comprising the mechanical pulp to screening. The pulp composition, such as the accept fraction, fed to the screw press has a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%. Dewatering is carried out both during screening and subsequently in a screw press. In dewatering, the pulp composition is dewatered to a consistency of at least 20%, such as at least 30%. The dewatered pulp composition is passed to a bleaching step, optionally after reducing the consistency of the pulp composition to less than 15%, such as less than 10%. The consistency of the accept fraction is first increased, for example to above 20%, to clean the pulp composition, and then decreased to about 8-12%, preferably about 10%. The screening includes screening performed by a first screening device and subsequent screening performed by a second screening device. The consistency of the pulp composition at the output of the first screening device is lower than the consistency of the pulp composition at the output of the second screening device. In the screening step, at least 50% by weight, such as at least 80% by weight, of the refined pulp composition is directed to the accept fraction. The method includes directing a reject fraction from the screening step to a reject processing line. The reject processing line includes at least a step of refining the reject fraction by a reject refiner. The method includes directing the output of the reject processing line to a screw press, preferably the output has a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%. The method further comprises impregnating the wood chips with the impregnation composition to obtain impregnated wood chips, and refining the impregnated wood chips in at least one refiner prior to the screening step. The impregnation composition is an aqueous composition comprising one or more of NaOH, Na2SO3, white liquor such as oxidized white liquor, green liquor such as oxidized green liquor, and H2O2. The method includes returning at least a portion of the reject fraction from the screening step to the impregnation step or to a step preceding the impregnation step.

[0012] According to a sixth aspect of the present invention there is provided the use of the method according to the fifth aspect in the manufacture of chemi-thermomechanical pulp or bleached chemi-thermomechanical pulp.

[0013] According to a seventh aspect of the present invention there is provided the use of the method according to the fifth aspect in the manufacture of mechanical pulp, such as groundwood, pressure groundwood, stone groundwood or thermomechanical pulp.

[0014] According to an eighth aspect of the present invention there is provided a pulp composition obtainable by the method according to the fifth aspect. Preferably the pulp composition comprises BCTMP.

[0015] According to a ninth aspect of the present invention there is provided a system configured to carry out a method according to the fifth aspect. [Advantages of the present invention]

[0016] An advantage of the present invention is that the amount of rejects generated in the screening step may be reduced.

[0017] A further advantage of the present invention is that relatively high CSF levels can be used throughout the process.

[0018] A further advantage of the present invention is that the disc filter may be omitted.

[0019] A further advantage of the present invention is that it can facilitate tailoring of the pulping process to produce a variety of pulp end compositions.

[0020] An advantage of the present invention is that less equipment is involved in the process, so energy savings may be realized. [Brief description of the drawings]

[0021] [Figure 1] 1 illustrates a method in accordance with at least some embodiments of the present invention. [Diagram 2] 1 illustrates a method in accordance with at least some embodiments of the present invention. [Diagram 3] 1 illustrates a screening step in a method according to at least some embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] As used herein, the term "pulp" typically refers to wood pulp.

[0023] As used herein, the term "accept" refers to a fraction that is approved to be sent to the next subsequent step in a process, such as the next subsequent step in a mainline process.

[0024] As used herein, the term "reject" refers to a fraction that is not approved to be sent to the next step in a process, such as a fraction that is refused to continue into the mainline process.

[0025] In the present context, the term "reject fraction" typically refers to the reject fraction from the screening step.

[0026] In the present context, Canadian Standard Freeness (CSF) is measured according to ISO 5267-2.

[0027] In the present context, luminance is measured according to ISO 2470, luminance 457 nm.

[0028] In the present context, debris content refers to the debris content "Pulmac (0.1 mm)".

[0029] In some embodiments of the present invention, it has surprisingly been found that it may be possible to completely omit a separate reject processing line in the pulping process.

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

[0031] Typically, the method may be integrated into a pulp manufacturing process.

[0032] In one embodiment, the method includes providing wood chips, impregnating the wood chips with an impregnation 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 returning at least a portion of the reject fraction from the screening step to the impregnation step or to a step preceding the impregnation step.

[0033] In another embodiment, the method includes 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 with a screw press.

[0034] The wood to be chipped may include softwoods such as spruce or pine or mixtures thereof, or hardwoods such as birch, poplar, aspen, alder, maple, eucalyptus, tropical hardwoods, or mixtures thereof, or the wood to be chipped may include a mixture of softwoods and hardwoods.

[0035] The impregnation composition may be an aqueous composition including one or more of NaOH, Na2SO3, white liquor such as oxidized white liquor, green liquor such as oxidized green liquor, and H2O2.

[0036] During refining, wood fibers are released from the wood material softened in the impregnation step. The mechanical forces in refining result in shortening of the fibers, a decrease in fiber diameter, and fiber fibrillation. Fibrillation (external fibrillation) involves breaking up the smooth fiber surface by covering it with hairs through the introduction of microfibrils to the fiber surface. Internal fibrillation, which increases the flexibility of the fibers, also typically occurs. With regard to the quality of the resulting refined mass, low debris content, high strength, and sufficient fiber length can be aimed for.

[0037] The refiner used to refine 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 refined pulp is typically 20-60%. In the case of a low consistency refiner, the consistency of the refined pulp is typically 3-5%.

[0038] The refiner may be a flat disc refiner or a conical disc refiner.

[0039] The refiner may be a one-disc refiner or a two-disc refiner. Preferably, the refiner is a one-disc refiner comprising a stator and a rotor.

[0040] The distance between the blades of the refiner may be in the range of 0.1 to 1.0 mm.

[0041] In the refinery, the refinery energy consumption may be in the range of 0.2 to 1.5 MWh / t.

[0042] After the refining step, the pulp may have a palmic fragment content of less than 20%, such as less than 10%, for example less than 5%.

[0043] After the refining step, i.e., at the output of the final refiner, the refined pulp may have a Canadian Standard Freeness (CSF) of at least 285, such as at least 350, such as at least 400. In some embodiments, the refined pulp may have a Canadian Standard Freeness of less than 750, such as less than 470.

[0044] In one embodiment, the refined pulp may have a Canadian Standard Freeness (CSF) in the range of 285 to 470. Such pulp may be suitable for the manufacture of CTMP, which can be used as a source of lightweight coated printing paper.

[0045] In one embodiment, the refined pulp may have a Canadian Standard Freeness (CSF) in the range of 350 to 750. Such pulp may be suitable for the manufacture of BCTMP, which can be used as a material for folding boxboard.

[0046] Before the refined pulp is directed to screening, there is typically a latency removal step. During latency removal, fiber curling is reduced and the fibers straighten out.

[0047] In one embodiment, the refined pulp composition passed to the screening step may have a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%.

[0048] During the screening step or at least at the accept output of the screening step, the pulp has a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%. The advantage of having such a high consistency, at least at the accept output of the screening, is that the accept fraction can be sent directly to the screw press after screening. At low consistency the mass flow rate may be too high to be processed by the screw press.

[0049] In the screening step, at least 90% by weight of the refined pulp composition is directed towards the accept fraction, preferably at least 50% by weight, such as at least 80% by weight, such as at least 95% by weight.

[0050] Preferably, the composition and properties of the refined pulp are not substantially changed as a result of screening, in other words, the amount of rejects separated from the pulp in the screening step is preferably kept to a minimum.

[0051] The palmic fragment content of the accept fraction is preferably at most 10 percentage points lower, such as at most 50 percentage points lower, such as at most 5 percentage points lower, than the palmic fragment content of the influent refined pulp sent to screening.

[0052] For example, the palmic debris content of the accept fraction may be less than 10%, such as less than 5%, preferably less than 1%.

[0053] The CSF of the accept fraction may differ by up to ±20% from the CSF of the influent refined pulp sent to screening.

[0054] In some embodiments, the steps following the screening step have no influent pulp feed associated with the accept fraction output from the screening step, and thus the steps downstream of the screening step, typically dewatering and bleaching, receive only the accept fraction from the screening as input, and thus preferably, if adjustments are required to the input of the steps following the screening, they need to adjust the properties and composition of only this single accept fraction.

[0055] In one embodiment, dewatering may be carried out in a screw press. In this embodiment, the fraction sent to the dewatering step from screening preferably has a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%. Preferably, the fraction consists of the accept fraction from screening.

[0056] In one embodiment, dewatering may be performed by a disc filter. In this embodiment, the fraction sent to the dewatering step from screening preferably has a consistency of 0.5-2%, such as 1.0-1.5%. Preferably, the fraction consists of the accept fraction from screening.

[0057] In one embodiment, dewatering may be performed both during screening and subsequently in a screw press or disc filter, so that part of the water removal may be performed beforehand in the screening step, for example by using at least two screening devices arranged in series and configuring the rear screening device to dewater the pulp composition to some extent.

[0058] In a preferred embodiment, no disc filter is used to dewater the accept fraction following the screening step. The advantage of not using a disc filter is that the CSF of the pulp can be adjusted to a high level throughout the process, avoiding the problem of cake formation, which may be encountered in processes using disc filters.

[0059] An advantage of using a screw press for dewatering is that the consistency can be increased to a high level, such as 15-40%, for example to at least 20%, preferably to at least 30%. The ability to effectively remove water from the screened pulp can be an advantage, if desired.

[0060] The accept fraction from the screen may be dehydrated to a consistency of at least 20%, such as at least 30%.

[0061] The method may include providing wood chips, impregnating the wood chips with an impregnation 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 dewatering the accept fraction output from the screening with a screw press, wherein the accept fraction sent to the screw press has a consistency of at least 3%, and during dewatering, the pulp composition is typically dewatered to a consistency of at least 20%.

[0062] The dewatered accept fraction may be sent to a bleaching step, optionally after reducing the consistency of the accept fraction to less than 15%, such as less than 10%.

[0063] In some embodiments, the consistency of the accept fraction is first increased to greater than 20% to, for example, clean the pulp, and then decreased to about 8-12%, preferably about 10%.

[0064] The reject fraction from the screening may be returned to the impregnation step or to the step preceding the impregnation step. Preferably, the reject fraction is sent to the impregnation step or to the step preceding the impregnation step at a consistency in the range of 2-45%. In one example, the pulp composition, such as the reject fraction, sent to the impregnation step has a consistency in the range of 4-10%.

[0065] Advantageously, the rejects are not sent directly to the refining step, but instead to a step preceding the refining step, so that problems with maintaining the proper consistency in the refiner can be avoided. In this embodiment, the consistency of the rejects may be allowed to vary over a wider range.

[0066] An advantage of sending the rejects to an impregnation step or to a step preceding the impregnation step is that the reject fraction can be further impregnated before continuing on to purification.

[0067] In a preferred embodiment, the reject fraction is sent to the impregnation step or to a step preceding the impregnation step without being separately dehydrated and / or separately purified. For example, the reject fraction may be sent directly from the reject output of the screening to the impregnation step or to a step preceding the impregnation step.

[0068] In some embodiments, the reject fraction is sent to the impregnation step or the step immediately preceding the impregnation step, typically without any purification.

[0069] The impregnation step may be preceded by a step of washing the wood chips.

[0070] The method may comprise returning at least a portion of the reject fraction, preferably substantially all of the reject fraction, directly to the impregnation step or to the step immediately preceding the impregnation step.

[0071] The method may include returning substantially all of the reject fraction directly to the impregnation step, preferably without any purification.

[0072] In some embodiments, at least a portion of the reject fraction, such as substantially all of the reject fraction, may be directed from the screening to a reject processing line that is separate from the main line, i.e., does not form part of the main line.

[0073] The reject processing line may include a reject refiner.

[0074] The reject processing may include one or more of dewatering the rejects, refining the rejects through a reject refiner, and screening the rejects into accept and reject in a reject processing line.

[0075] The reject processing line may comprise one or more screening devices, such as at least two screening devices.

[0076] The pulp composition input to the screening step of the reject processing line may have a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%.

[0077] The output, such as the accept, of the reject processing line may be returned to the mainline, for example to a dewatering step of the mainline, thereby combining with the accept output from the mainline screening.

[0078] In one embodiment, the accepts output from the reject processing line may have a consistency of at least 2%, such as at least 3%, for example 2-5%, preferably 3-4%. An advantage is that the reject processing line accepts can then be sent directly to the mainline screw press.

[0079] In one embodiment, the accept from the reject processing line, for example at a consistency of at least 2%, is directed to a mainline dewatering step, such as a screw press.

[0080] It is preferable to adjust the consistency of the accepts output from the reject processing line to a range of 2 to 5%.

[0081] In one embodiment, a portion of the rejects from the screening step are returned to the impregnation step and another portion of the rejects from the screening step are directed to a reject processing line.

[0082] The present invention may be applicable to the production of pulps selected from the group of mechanical pulp, thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), bleached chemi-thermomechanical pulp (BCTMP), semi-chemical pulp, groundwood (GW), pressure groundwood (PGW), stone groundwood (SGW), and combinations thereof.

[0083] The pulp may be bleached or unbleached.

[0084] Preferably the process is for the production of chemi-thermomechanical pulp or bleached chemi-thermomechanical pulp.

[0085] In some embodiments, the method is for the production of mechanical pulp, groundwood (GW), pressure groundwood (PGW), stone groundwood (SGW), or thermomechanical pulp (TMP).

[0086] The BCTMP obtained by some embodiments of the present process may be applicable as one or more plies of paperboard, such as folding boxboard, or as a material for paper, such as printing or writing paper or tissue paper.

[0087] In one embodiment, the method is for the production of pulp suitable for the manufacture of paperboard having the following characteristics:

[0088] Preferably, the density of the paperboard is between 0.5 and 1.3 g / cm 3 Preferably, the brightness of the paperboard is greater than 84%.

[0089] In another embodiment, the method is for the production of pulp suitable for the manufacture of paper having the following characteristics:

[0090] Preferably, the density of the paper is 0.7 to 1.4 g / cm 3Preferably, the brightness of the paper is greater than 77%.

[0091] The present invention also relates to a system configured to carry out the method according to any of the above embodiments or any combination thereof. The system may form part of a pulp mill. The system may comprise one or more of a wood chipping device, a wood chip impregnation 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.

[0092] With reference to the following drawings:

[0093] Figure 1 shows process steps of a method according to at least some embodiments of the present invention. The process includes the steps of chipping 10, impregnation 11, purification 12, screening 13, dewatering 14, optional bleaching 15, and optional washing 16. Screening produces an accept fraction and a reject fraction as output. The accept fraction continues in the main line and is directed from screening 13 to dewatering 14. If present, the reject fraction or at least a portion of the reject fraction is returned from screening 13 to a step preceding the purification step, such as impregnation step 11, as indicated by the arrow in Figure 1. Here, dewatering 14 may comprise a screw press or a disc filter.

[0094] 2 shows process steps of a method according to at least some embodiments of the present invention, where the main line includes the steps of chipping 20, optional impregnation 21, purification 22, screening 23, dewatering 24, optional bleaching 25, and optional washing 26. The method further includes a reject processing line 27 separated from the main line. The reject fraction obtained in screening 23 is directed to the reject processing line 27. The reject processing line may include a reject purification step and / or a reject screening step. An output such as an accept fraction from the reject processing line 27 may be returned to the main line, such as dewatering 24, where dewatering 24 preferably comprises a screw press.

[0095] 3 illustrates the screening step of a method according to at least some embodiments of the present invention. The influent pulp composition P is fed to a screening step 33. The screening produces as outputs an accept fraction A and a reject fraction R. The accept fraction A is approved to continue in the mainline process and passes to the next step, dewatering by a screw press 34. The reject fraction R is rejected from continuing in the mainline process and is instead directed to a reject processing line or downstream in the mainline, for example to an impregnation step (not shown here).

[0096] The consistency of the influent pulp P, which may be refined pulp, is preferably at least 3.5%, for example between 3.5 and 4.5%.

[0097] The consistency of the accept fraction A is preferably at least 3%, such as 3.5-4%.

[0098] The consistency of the reject fraction is preferably at least 4%, such as at least 4.5%. EXAMPLES

[0099] A pulping process was tested that included feeding rejects directly from screening to impregnation. The main lines of the process included chipping, impregnation, refining, screening, dewatering through a disc filter, and bleaching. The consistency of the pulp composition input to bleaching was approximately 10%. The accepts from the screening were directed to a disc filter. The rejects from the screening were directed to a conventional reject processing line or alternatively returned to the impregnation step.

[0100] In the processes of Test 1 and Test 2, the conventional reject processing line was bypassed and the rejects from the screening were returned to the impregnation step. Tests 1 and 2 were carried out in different ways.

[0101] The reference process did not include a backfeed of rejects, but instead the rejects from the screening were directed to a conventional reject processing line equipped with a reject refiner.

[0102] The Canadian Standard Freeness (CSF) of the pulp composition output from the disc filter (i.e., at a point between the disc filter and bleaching) was measured. The results are shown in Table 1. The results show that the CSF of the pulp composition of the Test 1 and 2 processes was successfully maintained at the same level as the CSF of the pulp composition of the reference process. [Table 1]

[0103] It is to be understood that the disclosed embodiments of the invention are not limited to the particular structures, process steps, or materials disclosed herein, but extend to equivalents thereof as would be recognized by one skilled in the relevant art. It is also to be understood that the terminology used herein is used for the purpose of describing particular embodiments only, and is not intended to be limiting.

[0104] Throughout this specification, reference 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 invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.

[0105] As used herein, a plurality of items, structural elements, components, and / or materials may be presented in a common list for convenience. However, these lists should be construed such that each element of the list is individually recognized as a separate and unique element. Thus, any individual element of such a list should not be construed as a de facto equivalent of other elements of the same list solely based on presentation in a common group, unless otherwise indicated. In addition, various embodiments and examples of the invention may be referred to herein, along with various alternatives of its components. It should be understood that such embodiments, examples, and alternatives should not be construed as de facto equivalents of each other, but should be considered as separate and independent representations of the invention.

[0106] 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. However, one skilled in the relevant art will recognize 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 have not been shown or described in detail to avoid obscuring aspects of the invention.

[0107] While the foregoing examples are illustrative of the principles of the present invention in one or more specific applications, it will be apparent to those skilled in the art that numerous modifications in form, use, and details of implementation are possible without the exercise of inventive faculty and without departing from the principles and concepts of the present invention. Accordingly, it is not intended that the present invention be limited except as by the scope of the claims which follow.

[0108] The verbs "to comprise" and "to include" are used in this document as open limitations which neither exclude nor require the presence of unrecited features. Features recited in the dependent claims are mutually freely combinable, unless otherwise expressly stated. Furthermore, it is to be understood that throughout this document the use of "a" or "an", i.e. the singular, does not exclude a plurality. [Industrial Applicability]

[0109] INDUSTRIAL APPLICABILITY The present invention is industrially applicable at least in the production of pulp such as CTMP or BCTMP. [List of acronyms]

[0110] TMP Thermomechanical Pulp CTMP Chemi-thermomechanical pulp BCTMP Bleached Chemi-Thermo-Mechanical Pulp GW Groundwood Pulp PGW Pressurized Groundwood SGW Stone Ground Wood Pulp CSF Canadian Standard Freeness [Explanation of symbols]

[0111] 10, 20 chips 11, 21 Impregnation 12, 22 Purification 13, 23, 33 Screening 14, 24, 34 Dehydration 15, 25 bleaching 16, 26 Cleaning 27 Rejection Processing

Claims

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

2. The method according to claim 1, wherein the impregnating composition is an aqueous composition comprising one or more of NaOH, Na2SO3, white liquor such as oxidized white liquor, green liquor such as oxidized green liquor, and H2O2.

3. The method according to claim 1, wherein in the refining, the refining energy consumption is in the range of 0.2 to 1.5 MWh / t.

4. The method according to claim 1, wherein after the refining step, the pulp composition has a pulp mac fragment content of less than 10%, such as less than 5%.

5. The method according to claim 1, wherein after the refining step, the pulp composition has a Canadian Standard Freeness (CSF) in the range of 285 to 750, such as 285 to 470, or 350 to 750.

6. The method according to claim 1, wherein in the screening step, at least 80 wt%, such as at least 95 wt% of the refined pulp composition is directed to the accept fraction.

7. The method according to claim 1, wherein the pulp mac fragment content of the accept fraction is at most 10 percentage points lower, such as at most 5 percentage points lower than the pulp mac fragment content of the incoming refined pulp composition sent to the screening.

8. The method according to claim 1, wherein the CSF of the accept fraction differs from the CSF of the incoming refined pulp composition sent to the screening by at most ±20%.

9. The method according to claim 1, further comprising dewatering the accept fraction output from the screening.

10. The method according to claim 9, wherein the dewatering is performed by a screw press and / or a disk filter.

11. The dewatering includes dewatering performed by a screw press, Preferably, the method according to claim 9, wherein the pulp composition such as the accept fraction sent to the screw press has a consistency of at least 3%, for example 2 - 5%, preferably 3 - 4%, at least 2%.

12. The dewatering includes dewatering performed by a disk filter, Preferably, the method according to claim 9, wherein the pulp composition such as the accept fraction sent to the disk filter has a consistency of 1.0 - 1.5%, at least 0.5%.

13. In the dewatering, the method according to claim 9, wherein the pulp composition is dewatered to a consistency of at least 20%, such as at least 30%.

14. The method according to claim 1, wherein the dewatered pulp composition is optionally sent to a bleaching step after reducing the consistency of the pulp composition to less than 15%, such as less than 10%.

15. The method according to claim 1, wherein the consistency of the accept fraction is increased, for example, to exceed 20% first to clean the pulp, and then decreased to about 8 - 12%, preferably about 10%.

16. The method according to claim 1, wherein the reject fraction is sent to the impregnation step with a consistency in the range of 4 - 10%.

17. The method according to claim 1, including returning at least a part of the reject fraction, preferably substantially all of the reject fraction, directly to the impregnation step or the step immediately before the impregnation step.

18. The method according to claim 1, including returning substantially all of the reject fraction, preferably without any purification, directly to the impregnation step.

19. The method according to claim 1, further including dewatering the accept fraction to obtain a pulp composition.

20. Use of the method according to any one of claims 1 - 19 in the production of bleached thermomechanical pulp or unbleached thermomechanical pulp.

21. A pulp composition obtained by the method according to any one of claims 1 - 19.

22. The pulp composition according to claim 21, including BCTMP.

23. A pulp composition obtained by a pulp manufacturing process involving the method according to any one of claims 1 - 19.

24. The method according to claim 23, wherein the pulp composition is CTMP such as BCTMP.

25. A system configured to perform the method according to any one of claims 1 to 19.

26. The system according to claim 25, forming part of a pulp mill and comprising a wood chipping device, a wood chip impregnation system, one or more primary refiners, one or more pulp screening devices, a pulp dewatering device, and optionally a pulp bleaching system.

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