A process for washing and / or fractionating a wood-based material

Abstract

A process for washing and / or fractionating a wood-based material comprising fines is disclosed. The process may comprise providing the wood-based material as a suspension; and subjecting the suspension to centrifugal forces, thereby separating the suspension into at least two fractions based on at least their densities.

Description

[0001] A PROCESS FOR WASHING AND / OR FRACTIONATING A WOOD-BASED

[0002] MATERIAL

[0003] TECHNICAL FIELD

[0004] The present disclosure relates to a process for washing and / or fractionating a wood-based material .

[0005] BACKGROUND

[0006] Fine and ultra- fine cellulose materials , for example microcrystalline cellulose (MCC ) and other fine cellulose grades , may be used in various end applications , including pharmaceutical , medical , and food purposes , and technical applications such as fluid management , structure formation, gelation, thickening, plastic substitution, filter aiding, and so on .

[0007] However, large-scale production of such materials may involve relatively high costs . The production of such materials may often also require large amounts of energy .

[0008] Relatively fine wood-based materials , such as cellulose and lignin, may also be quite challenging to handle in large quantities . Traditional dewatering and washing devices have several limitations . They commonly include wires , felts or screens of a speci fic mesh si ze . These machinery dimensions define the yield or loss of the machine . The bigger the mesh si ze , the more losses incurred, and vice versa . Secondly, the dewatering capacity of such machines may be limited due to the fact that pressing forces are limited by the strength of the wires or felts in use .

[0009] For example , washing and dewatering such fine materials may be a compromise between the loss of fines and dewatering rate . Controlling the particle si ze distribution of the fine and ultra- fine cellulose and other wood-based materials may be challenging . Clogging may also be an issue with various devices . Further, using conventional processes , it may be challenging to control the particle si ze distribution of fine and ultra- fine wood-based materials . For example , in order to control the si ze distribution using a filter employing a fabric, the fabric will need to be changed in order to change the separation degree . This may require a full shutdown of the process , and there may be a separation threshold, above or below which fiber fractions may not be ef fectively separated .

[0010] SUMMARY

[0011] A process for washing and / or fractionating a wood-based material comprising fines is disclosed . The process may comprise providing the wood-based material as a suspension; and subj ecting the suspension to centri fugal forces , thereby separating the suspension into at least two fractions based on at least their densities .

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings , which are included to provide a further understanding of the embodiments and constitute a part of this speci fication, illustrate various embodiments . In the drawings :

[0014] Fig . 1 illustrates an embodiment of a process for washing and / or fractionating a wood-based material comprising fines ;

[0015] Fig . 2 illustrates another embodiment of a process for washing and / or fractionating a wood-based material comprising fines ;

[0016] Fig . 3 shows another embodiment of a process for washing and / or fractionating a wood-based material comprising fines ; and

[0017] Fig . 4A and Fig . 4B illustrate di f ferential distribution and the cumulative distribution for cellulose particles obtained in a lighter fraction . DETAILED DESCRIPTION

[0018] A process for washing and / or fractionating a wood-based material comprising fines is disclosed . The process may comprise providing the wood-based material as a suspension; and subj ecting the suspension to centri fugal forces , thereby separating the suspension into at least two fractions based on at least their densities .

[0019] The suspension may comprise the wood-based material comprising fines and a l iquid . The liquid may comprise e . g . water and / or an aqueous solution .

[0020] The at least two fractions may include at least a lighter fraction and a heavier fraction . The heavier fraction may comprise particles of the wood-based material having a larger particle si ze than the woodbased material in the lighter fraction, and / or the maj ority of the wood-based material . The lighter fraction may include liquid derived from at least a part of the liquid included in the suspension and optionally particles of the wood-based material having a smaller particle si ze than the wood-based material in the heavier fraction .

[0021] With the process , it is possible to produce large quantities of wood-based materials of di f ferent particle si zes and particle si ze distributions in a relatively cost-ef fective manner . The wood-based materials may be washed and dewatered . It is also possible to separate the wood-based material into one or more fine and / or ultrafine fractions having desired properties .

[0022] By selecting suitable conditions , it is possible to adj ust and even tailor the properties of the wood-based materials or of the fractions . In addition to the good dewatering properties combined with high yield, the present process may enable a relatively low water consumption and the recovery of used water soluble chemicals, for example acid or similar chemicals. These recovered chemicals can be reused in the process. Further, reduced water consumption may reduce the total costs involved in the washing / dewatering process. Sustainability may also be improved .

[0023] The process may be integrated into existing processes that produce wood-based material, including the wood-based material comprising fines, such as a pulp mill .

[0024] The wood-based material may originate e.g. from wood, cotton or agricultural material, or any mixture or combination thereof.

[0025] The wood-based material may comprise or be cellulose material; lignin; hemicellulose; nanof ibrillated cellulose; microf ibrillated cellulose; or any mixture or combination thereof.

[0026] The wood-based material may comprise or be cellulose material.

[0027] The wood-based material, such as cellulose material, may be obtainable e.g. from pulp. The woodbased material, such as cellulose material, may be obtainable from a chemical pulping process, such as a kraft (sulphate) process, sulphite process, soda process, and / or organosolv process.

[0028] The cellulose material may comprise or be e.g. partially hydrolysed cellulose material.

[0029] Such cellulose materials comprise cellulose of different particle sizes. The cellulose in the cellulose material may thus have a particle size distribution that differs e.g. from the particle size distribution of pulp. The cellulose in the cellulose material may also have a degree of polymerisation that differs e.g. from the degree of polymerisation of pulp. Cellulose having a small particle size and / or low degree of polymerization (DP) is particularly challenging to handle, wash, dewater, and fractionate further. Likewise, lignin comprising fines may be challenging to wash, dewater, and fractionate further.

[0030] Degree of polymerization (DP) of the cellulose material may be estimated by measuring its CED viscosity. The cellulose material in the suspension may have e.g. a CED viscosity in the range of about 10 - 150 ml / g. The cellulose material in the suspension may have e.g. a CED viscosity in the range of about 100 - 200 ml / g, or for some applications, in the range of about 150 - 170 ml / g.

[0031] The term "CED viscosity" may be understood as referring to limiting viscosity number in cupri- ethylenediamine (CED) solution. The CED viscosity may be measured e.g. according to the standard ISO 5351:2010. CED viscosity is related to the degree of polymerization. The degree of polymerization may be measured e.g. using the standard ISO 5351:2010. The degree of polymerization (DP) may be estimated from the CED viscosity value obtained according to the standard according to:

[0032] DP = O.75CIJ.]1 / 0'905, where [p] is the CED viscosity value .

[0033] The cellulose material may have e.g. a CED viscosity e.g. in the range of 10 - 150 ml / g, or in the range of 100 - 200 ml / g, or, for some applications, 150 - 170 ml / g; and an average fiber length in the range of 0.15 - 1 mm. The average fiber length may be determined using e.g. a Valmet Fiber Image Analyzer FS5. The average fiber length may be a weighted average of the length of fibers as measured using an optical measurement. The measurement using the FS5 device may automatically measure the length of fibers and produce a weighted average for these. The average fiber length may be a number average fiber length. The lower the DP and the CED viscosity, the more fine and / or ultrafine cellulose there is in a particular cellulose material or fraction .

[0034] The fine and / or ultrafine fractions may be understood as referring to particles having a diameter in the nanometer and / or micrometer range . The fine fraction may be understood as re ferring to particles having a diameter of about 10 nm or greater, typically in the micrometer range . The ultrafine fraction may be understood as referring to particles having a diameter of about 5 - 10 nm .

[0035] Fine cellulose may be understood as referring to cellulose particles having a diameter of about 10 nm or greater, typically in the micrometer range . Fine cellulose may also be referred to as fines . It may, in some embodiments , contain residual lignin and / or hemicellulose in addition to cellulose .

[0036] Ultrafine cellulose may be understood as referring to cellulose particles having a diameter of about 5 - 10 nm. Ultrafine cellulose may, in some embodiments , contain residual hemicellulose in addition to cellulose . The ultrafine cellulose may thus be understood as cellulose particles smaller than fine cellulose .

[0037] The wood-based material in the suspension may be washed and optionally dewatered by subj ecting the suspension to the centri fugal forces . In other words , the centri fugal forces may be used to wash and optionally dewater the wood-based material . For example , when the wood-based material is partially hydrolysed cellulose material obtainable by acidic hydrolysis of cellulose , such as pulp, acid present in the partially hydrolysed cellulose material or in the suspension may need to be removed at least partially by washing and optionally dewatering the cellulose material . Acid remaining in the partially hydrolysed cellulose material may reduce the quality of the cellulose material . In at least some embodiments, the suspension may be subjected to the centrifugal forces, thereby separating the wood-based material in the suspension into one or more fine and / or ultrafine fractions based on at least their densities. In other words, the centrifugal forces may be used to separate the woodbased material of different particle sizes into the one or more fine and / or ultrafine fractions. When the woodbased material is cellulose material, it may be separated into one or more fine and / or ultrafine cellulose fractions based on at least their densities. The centrifugal forces may be used to separate the woodbased material, such as cellulose into the one or more fine and / or ultrafine fractions, for example one or more fine and / or ultrafine cellulose fractions, having a more narrow particle size distribution than the particle size distribution of the wood-based material.

[0038] The process may be a continuous process.

[0039] The suspension may be subjected to the centrifugal forces in a decanter centrifuge. Decanter centrifuges are particularly well suited for the present process, as the process may be arranged as a continuous process and adjusted in a continuous manner. This is because the decanter centrifuge may be operated and / or controlled continuously. Material losses may also be minimized.

[0040] Various types of decanter centrifuges are known and available; the type or structure of the decanter centrifuge is not particularly limited. For example, the decanter centrifuge may be a horizontal decanter centrifuge .

[0041] A decanter centrifuge may be considered to increase the rate of settling through the use of continuous rotation. Thus mixtures containing cellulose materials of different particle sizes can be settled in a matter of seconds using a decanter centrifuge. This form of separation enables more rapid and controllable results .

[0042] The decanter centri fuge may comprise a bowl rotatable about its longitudinal axis for separating the suspension into two fractions , i . e . a lighter fraction and a heavier fraction, based on at least their densities due to centri fugal force , wherein the bowl includes a cylindrical portion and a frusto- conical portion at one end; a heavier fraction discharge at one end of the bowl and a lighter fraction discharge at the other end of the bowl ; and a screw conveyor coaxially mounted for rotation within the bowl , the screw conveyor adapted to rotate at a speed relative with respect to the bowl for conveying the heavier fraction towards the heavier fraction discharge .

[0043] The speed at which the screw conveyor may be adapted to rotate relative with respect to the bowl may be adj usted such that the amount of the suspension that is fed into the decanter centri fuge is such that it will not clog the decanter centri fuge .

[0044] One or more of the at least two fractions and / or the one or more fine and / or ultrafine fractions may be further subj ected to the centri fugal forces in a second decanter centri fuge . For example , the heavier fraction or the lighter fraction may be conveyed to the second decanter centri fuge and washed and optionally dewatered in the second decanter centri fuge .

[0045] In other words , the one or more of the at least two fractions and / or the one or more fine and / or ultrafine fractions may be further subj ected to the centri fugal forces in at least two decanter centri fuges connected in series . The at least two decanter centri fuges may be similar or dif ferent in structure . For example , they may be similar to any embodiment of the decanter centri fuge described in this speci fication . Other embodiments of the decanter centrifuge ( s ) may also be contemplated.

[0046] For example, the wood-based material in the suspension may be washed and dewatered by subjecting the suspension to the centrifugal forces in a first decanter centrifuge, and subsequently further subjected to the centrifugal forces in a second decanter centrifuge, wherein the wood-based material in the suspension is separated into one or more fine and / or ultrafine fractions based on at least their densities.

[0047] With the present process, in particular when employing a decanter centrifuge, it may be possible to recover / dewater very low consistency wood-based material. The consistency of the wood-based material in the suspension may be e.g. at least 0.5 % (w / w) , or in the range of 0.5 - 15 % (w / w) .

[0048] Relatively high consistencies of the woodbased material or of the one or more fine and / or ultrafine fractions may be achieved. The consistency of the wood-based material in one or more of the at least two fractions, or in the one or more fine and / or ultrafine fractions, may be at least 20 % (w / w) , or in the range of 20 - 35 % (w / w) , preferably in the range of 20 - 27 % (w / w) .

[0049] The consistencies may be calculated on the basis of the total weight of the suspension or of the total weight of the fraction (s) .

[0050] The decanter centrifuge ( s ) may further comprise a dam plate (i.e. one or more dam plates) for adjusting the consistency of the wood-based material in the suspension and / or retention time in the decanter centrifuge. The dam plate (s) may improve the consistency and increase retention time of the suspension in the decanter centrifuge.

[0051] In embodiments involving a second decanter centrifuge and optionally further decanter centrifuge ( s ) , the decanter centrifuge ( s ) may further comprise a dam plate for adjusting the consistency of the wood-based material in the one or more of the at least two fractions and / or in the one or more fine and / or ultrafine fractions.

[0052] The one or more fine and / or ultrafine fractions, such as the one or more fine and / or ultrafine cellulose fractions, may comprise or be one or more of microcrystalline cellulose; nanocrystalline cellulose; colloidal cellulose; fines; hemicellulose; or partially hydrolysed hemicellulose.

[0053] In the context of this specification, microcrystalline cellulose (MCC) may be understood as referring to partially depolymerized cellulose derived from a a-cellulose precursor, from which mainly crystalline regions of the cellulose have been recovered. MCC may be defined as a purified, partly depolymerized cellulose prepared by treating a- cellulose, obtained as a pulp from fibrous plant material, with mineral acids. MCC may have the appearance of a fine or granular powder. MCC may typically have a degree of polymerization of about 400 or lower, or about 350 or lower. MCC may typically not contain more than 10 % of cellulose particles with a particle size smaller than 5 pm. MCC may be considered to include different MCC types of e.g. different average particle sizes.

[0054] In the context of this specification, nanocellulose (NC) may be understood as encompassing cellulose nanocrystals (CNC) , nanocrystalline cellulose (NCC) and cellulose nanofibers (CNF) . CNC / NCC is in the form of particles, ultrafine for instance, and CNF is in fibril form. NCC may be in the form of particles shorter than cellulose nanofibrils (CNF) , with lengths ranging e.g. from hundreds to thousands of nanometers. Nanocrystalline cellulose (NCC) may be understood as referring to crystalline regions of cellulose derived from cellulose microcrystals, mainly by strong acid hydrolysis at an elevated temperature. NCC may have a low aspect ratio. NCC may have e.g. a typical diameter of 2-20 nm and a length of 100-600 nm.

[0055] In the context of this specification, colloidal cellulose may be understood as referring to cellulose in a colloidal state, in which it is dispersed in a medium in the form of nanocellulose or microcrystalline cellulose, with particles having a size of e.g. approximately 1 nm - 1 pm. In this state, cellulose can exhibit properties of both a solid and a liquid, making it highly adaptable for various uses. It may be used in industrial processes where its ability to form stable suspensions is required. Examples include emulsions for food and cosmetics, biomedical applications and pharmaceuticals, and barriers.

[0056] In the context of this specification, cellulose fines may be understood as referring to a cellulose pulp fraction (particles detached from cellulose fibres) that passes through a screen with a diameter of the round holes of 76 pm (e.g. T261 cm-94 TAPPI Standard method) or through a 200 mesh screen. Cellulose fines may have a diameter of 0.2 mm or smaller.

[0057] In general, in the context of this specification, fines may be understood as referring to a fraction (particles detached e.g. from cellulose fibres) that passes through a screen with a diameter of the round holes of 76 pm (e.g. T261 cm-94 TAPPI Standard method) or through a 200 mesh screen. Fines may have a diameter of 0.2 mm or smaller.

[0058] Partially hydrolysed hemicellulose may be obtainable from partial hydrolysis of hemicellulose, for example by acidic hydrolysis of pulp containing the hemicellulose .

[0059] The process may further comprise comminuting the wood-based material in one or more of the at least two fractions and / or in the one or more fine and / or ultrafine fractions. The comminuting may be done e.g. by milling.

[0060] The centrifugal forces and / or one or more parameters of the decanter centrifuge may be adjusted, thereby adjusting properties of the wood-based material in one or more of the at least two fractions and / or of the one or more fine and / or ultrafine fractions.

[0061] The one or more parameters may include at least one of consistency of the wood-based material in the suspension; g-force and / or rotation speed of the decanter centrifuge; torque of the decanter centrifuge; or feeding rate of the suspension. For example, when the centrifugal forces are reduced, more fine and / or ultrafine wood-based material, such as cellulose, may be included in the lighter fraction. When the consistency and / or feeding rate of the suspension are increased, more fine and / or ultrafine wood-based material, such as cellulose, may be included in the lighter fraction. When the rotation speed and / or torque are reduced, more fine and / or ultrafine wood-based material, such as cellulose, may be included in the lighter fraction. The rotation speed of the decanter centrifuge may, at least in some embodiments, refer to differential rotation speed of the decanter centrifuge.

[0062] The properties of the wood-based material, such as cellulose material, may comprise e.g. at least one of degree of polymerization, particle size, particle size distribution, or fiber length.

[0063] The properties of the wood-based material, such as cellulose material, may comprise e.g. particle size and / or particle size distribution.

[0064] The properties of the cellulose material may comprise e.g. degree of polymerization, particle size, particle size distribution, and / or fiber length.

[0065] The cellulose material in one or more of the at least two fractions and / or in the one or more fine and / or ultrafine cellulose fractions may have e.g. a CED viscosity e.g. in the range of 10 - 150 ml / g, or in the range of 100 - 200 ml / g, or, for some applications, 150 - 170 ml / g; and an average fiber length in the range of 0.15 - 1 mm. The average fiber length may be determined using e.g. a Valmet Fiber Image Analyzer FS5. The average fiber length may be a weighted average of the length of fibers as measured using an optical measurement. The measurement using the FS5 device may automatically measure the length of fibers and produce a weighted average for these. The average fiber length may be a number average fiber length. Any references to fiber length in this specification may be understood, at least in some embodiments, as referring to number average fiber length. In some embodiments, the average fiber length may be measured according to the standard ISO 16065-2.

[0066] The fine and / or ultrafine fractions, e.g. fine and / or ultrafine cellulose fractions, may be utilized in several applications, for example in textile production, films, packaging materials, biomedical applications, composite materials, barrier materials, and / or 3D printing materials.

[0067] EXAMPLES

[0068] Reference will now be made in detail to various embodiments, an example of which is illustrated in the accompanying drawing.

[0069] The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

[0070] Figure 1 illustrates an embodiment of a process for washing and / or fractionating a wood-based material 1 comprising fines. In this exemplary embodiment, the wood-based material 1 may be cellulose material comprising cellulose of different particle sizes. However, as a skilled person will understand, the woodbased material 1 could be any wood-based material described in this specification, or any mixture of combination thereof. The cellulose material 1 is provided as a suspension. The cellulose material 1 may comprise e.g. partially hydrolysed cellulose material obtainable e.g. from acidic hydrolysis of cellulose, such as pulp. The cellulose material 1 is fed into a decanter centrifuge 2, which is herein illustrated schematically. In this exemplary embodiment, the decanter centrifuge 2 is a horizontal-type centrifuge; however, other types could be contemplated instead. The decanter centrifuge 2 comprises a feed inlet 3, and the cellulose material 1 is fed as the suspension into the decanter centrifuge 2 via the feed inlet 3. The feeding rate of the suspension comprising cellulose material 1 may be selected e.g. such that a desired separation of a lighter fraction 5 and a heavier fraction 6 and / or a desired throughput of the process may be achieved.

[0071] The decanter centrifuge 2 comprises a bowl 4 rotatable about its longitudinal axis for separating the suspension into two fractions, i.e. the lighter fraction 5 and the heavier fraction 6, based on at least their densities due to centrifugal force. The bowl 4 includes a cylindrical portion 7 and a f rusto-conical portion 8 at one end. A heavier fraction discharge 9 is provided in the restricted end of the f rusto-conical portion 8 of the bowl 4. A screw conveyor 10 is coaxially mounted for rotation within the bowl 4. The screw conveyor 10 has a spiralled conveyor flight 11 extending radially outwardly to a position adjacent the inside wall of the bowl 4. The screw conveyor 10 may be adapted to rotate at a relative speed with respect to the bowl 4 for conveying the heavier fraction 6 towards the heavier fraction discharge 9. The conveyor flight 11 is adapted to push the separated heavier fraction 6 along the inside wall of the bowl 4 towards the f rusto-conical end of the bowl and the heavier fraction discharge 9.

[0072] The suspension containing the cellulose material 1 is thus subjected to centrifugal forces in the decanter centrifuge 2, thereby separating the suspension into the two fractions 5, 6 based on at least their densities. Cellulose particles with a higher density are collected and compacted on the inside wall of the bowl 4. Separation of the heavier fraction 6 from the cellulose material 1 fed as the suspension is a function of e.g. the residence time of the suspension in the bowl 4, the feeding rate, the difference in specific gravity of the heavier fraction 6 from the lighter fraction 5, differential rotation speed of the decanter centrifuge 2 (i.e. the difference between the rotational speed of the bowl 4 and the rotational speed of the screw conveyor 10) , and the ability of the decanter centrifuge 2 to separately discharge the heavier fraction 6 and the lighter fraction 5.

[0073] A lighter fraction discharge 12 is provided at the other end of the bowl 4. The lighter fraction 5 is discharged by flowing from the cylindrical portion 7 through the lighter fraction discharge 12. The lighter fraction discharge 12 may be located, typically, at the opposite end of the bowl 4 from the heavier fraction discharge 9.

[0074] The lighter fraction 5 may contain smaller cellulose particles as well as the majority of the liquid present in the suspension containing the cellulose material 1. If e.g. the cellulose material 1 has been obtained by acidic hydrolysis of cellulose, such as pulp, the lighter fraction 5 may contain an acidic solution derived from the liquid present in the suspension containing the cellulose material 1. The acid present in the lighter fraction 5 may be removed at least partially e.g. by separating the liquid in the lighter fraction 5 at least partially from the cellulose material present in the lighter fraction 5, for example by screening or by e.g. subjecting the lighter fraction 5 further to centrifugal forces. Thus the acid may be recovered from the suspension. Components of the lighter fraction 5, such as the acid present in the lighter fraction 5 and removed therefrom, may be reused and / or recycled, for example in the acidic hydrolysis of the cellulose .

[0075] The decanter centrifuge 2 may further comprise e.g. a casing 13 enclosing the bowl 4. The decanter centrifuge 2 may further comprise a dam plate 14. The bowl 4 may have one or more weirs 15. The dam plate 14 may be a disc-shaped dam plate, or the decanter centrifuge 2 may comprise a plurality of dam plates 14 arranged at weirs 15 of the bowl 4. The dam plate (s) 14 may be adapted to restrict the flow of the lighter fraction 5 from the bowl 4 to the lighter fraction discharge 12. It / they may thereby be adapted to adjust the consistency of the cellulose material 1 in the suspension and / or retention time in the decanter centrifuge 2.

[0076] In some embodiments, the cellulose material 1 in the suspension is washed and optionally dewatered by subjecting the suspension to the centrifugal forces in the decanter centrifuge 2. In such embodiments, the lighter fraction 5 may comprise or be water or mainly water (or, if the liquid in the suspension is an aqueous solution, the aqueous solution or mainly the aqueous solution) , and the cellulose material 1 may at least partially or completely end up in the heavier fraction 6. Thus water may be removed from the suspension comprising the cellulose material 1.

[0077] In some embodiments, the cellulose material 1 in the suspension is separated in the decanter centrifuge 2 into two fine and / or ultrafine cellulose fractions 5, 6 based on at least their densities. In such embodiments, the lighter fraction 5 contains a less dense cellulose fraction, and the heavier fraction 6 contains a more dense cellulose fraction. The centrifugal forces and / or one or more parameters of the decanter centrifuge 2 may be adjusted to thereby adjust properties of the cellulose material in the lighter fraction 5 and in the heavier fraction 6, such as their particle size distributions.

[0078] Figure 2 illustrates another embodiment of a process for washing and / or fractionating a wood-based material 1 comprising fines. In this exemplary embodiment, the wood-based material 1 may be cellulose material comprising cellulose of different particle sizes. However, as a skilled person will understand, the wood-based material 1 could be any wood-based material described in this specification, or any mixture or combination thereof. The cellulose material 1 is provided as a suspension. The cellulose material 1 may comprise e.g. partially hydrolysed cellulose material obtainable e.g. from acidic hydrolysis of cellulose, such as pulp.

[0079] The cellulose material 1 in the suspension is fed into a first decanter centrifuge 2 and separated in the first decanter centrifuge 2 into two fine and / or ultrafine cellulose fractions 5, 6 based on at least their densities. The lighter fraction 5 contains a less dense cellulose fraction, and the heavier fraction 6 contains a more dense cellulose fraction. The centrifugal forces and / or one or more parameters of the decanter centrifuge 2 may be adjusted to thereby adjust properties of the cellulose material in the lighter fraction 5 and in the heavier fraction 6.

[0080] The heavier, i.e. coarser, fraction 6 may be further subjected to centrifugal forces in a second decanter centrifuge 2' . It may assist in washing and dewatering the heavier fraction 6 further, such that a higher solids content is achieved for the heavier fraction 6. The first decanter centrifuge 2 and the second decanter centrifuge 2' are thus connected in series. The heavier fraction 6 may be dispersed e.g. using a suitable disperger, mixer or any other device 15 suitable for dispersing cellulose material. The dispersed heavier fraction 16 thus obtainable may then be fed into the second decanter centrifuge 2' . A heavier fraction 6' may thus be obtained from the second decanter centrifuge 2' . A lighter fraction 5' is also obtained from the second decanter centrifuge 2' . It may be directed back to 15 for dispersing the heavier fraction 6 obtained from the first decanter centrifuge 2.

[0081] A third decanter centrifuge 2" may be further employed in the process. It may assist in washing and dewatering the lighter fraction 5 further, such that a higher solids content is achieved for the lighter fraction 5. The lighter fraction 5 obtained from the first decanter centrifuge 2 may be further subjected to centrifugal forces in the third decanter centrifuge 2". The first decanter centrifuge 2 and the third decanter centrifuge 2" are thus connected in series. The lighter fraction 5 may be dispersed e.g. using a suitable disperger, mixer or any other device 15' suitable for dispersing cellulose material. The dispersed lighter fraction 17 thus obtainable may then be fed into the third decanter centrifuge 2".

[0082] A heavier fraction 6" may thus be obtained from the third decanter centrifuge 2" - however, from the view of the entire process, the heavier fraction 6" may be considered as a fine fraction product. A lighter fraction 5" is also obtained from the third decanter centrifuge 2". It may be directed at least partially back to 15 as back wash water return for dispersing the heavier fraction 6 obtained from the first decanter centrifuge 2. Either the second decanter centri fuge 2 ' or the third decanter centri fuge 2" may be omitted, i f not necessary, for example i f a required purity for the fraction ( s ) is reached without it / them .

[0083] In the embodiment illustrated in Fig . 2 , water may be recycled ef ficiently .

[0084] Figure 3 illustrates a further embodiment of a process for washing and / or fractionating a wood-based material 1 comprising fines .

[0085] Figure 3 and the process illustrated therein may be otherwise similar to the one illustrated in Fig . 2 , except that fresh water 18 may be added to the heavier fraction 6 and / or to the disperger, mixer or any other device 15 for dispersing the cellulose material . Additionally or alternatively, fresh water 18 may be added to the lighter fraction 5 and / or to the disperger, mixer or any other device 15 ' for dispersing cellulose material .

[0086] The lighter fraction 5" obtained from the third decanter centri fuge 2" may contain acidic water, e . g . i f the cellulose material 1 has been obtained from acidic hydrolysis of cellulose . The lighter fraction 5" may be directed at least partially back to the heavier fraction 6 and / or to the the disperger, mixer or any other device 15 as back wash water return for dispersing the heavier fraction 6 obtained from the first decanter centri fuge 2 . Additionally or alternatively, it may be directed at least partially to the lighter fraction 5 and / or to the disperger, mixer or any other device 15 ' for dispersing cellulose material . Further, the lighter fraction 5" may be at least partially directed to a process 19 from which the cellulose material 1 is obtained, for example a hydrolysi s process , such as an acidic hydrolysis process . The water flows in the process illustrated in Fig . 3 are such that the washing of the cellulose material 1 is highly ef ficient , and water consumption may be reduced signi ficantly . Further, acidic water contained e.g. in the lighter fraction 5" may be returned back to the process 19 from which the cellulose material 1 is obtained.

[0087] EXAMPLE 1

[0088] Two acid hydrolysed cellulose samples PIO and P14 were obtained. PIO and P14 were hydrolysed to a different extent, such that P14 had been subjected to a more extensive hydrolysis, as indicated by the larger proportion of fine fiber fractions (Table 1) .

[0089] The samples ("feed") were washed in a decanter centrifuge setup in three runs (KPI, KP2, KP4 for sample PIO) or five runs (KPI, KP2, KP3, KP6, KP4 for P14) . The feed in each of the runs was first fed into a first decanter centrifuge; the heavier fraction obtained (GEA1) from the first decanter centrifuge was collected and dispersed. The dispersed heavier fraction was then fed into a second decanter centrifuge; the heavier fraction obtained (GEA2) from the second decanter centrifuge was then collected.

[0090] The conditions in the different runs were slightly different; however, as the intention was mainly to wash the hydrolysed cellulose, the exact parameters were not essential. The decanter centrifuges were run at full speed using dam plates.

[0091] Table 1 shows various parameters measured from the feed and samples of the heavier fractions (GEA1 and GEA2 ) in each run. The length weighted fiber length and proportion of fines were determined using a Valmet Fiber Image Analyzer FS5. Length weighted fiber length was measured according to the standard ISO 16065-2.

[0092] The pH of the samples after the second decanter centrifuge (GEA2 samples) was near neutral, indicating that acid present in the feed was removed to a significant extent by the washing in the decanter centrifuges. The dry solids contents of the samples after the first and second decanter centri fuge ( GEA1 and GEA2 samples ) were also higher than of the feed, indicating that the hydrolysed cellulose could be simultaneously concentrated to a relatively high solids content . No signi ficant amounts of the hydrolysed cellulose was lost in the runs .

[0093] It was also observed that the length weighted fiber length of the GEA1 and GEA2 samples was smaller than that of the feed, and that the proportion of fines increased in GEA1 and GEA2 samples as compared to the feed . This indicates that the hydrolysed cellulose was further broken into smaller fragments in the decanter centri fuges .

[0094] The lighter fraction obtained from GEA1 (KPI from P14 ) was further collected and analysed using a laser di f fraction device in three parallel measurements (measurement 1 , 2 and 3 ) to determine the distribution of cellulose particles contained therein . The lighter fraction mainly contained water . However, as shown in Figs . 4A and 4B illustrating the di f ferential distribution and the cumulative distribution, respectively, the cellulose particles had a particle si ze mainly below 100 nm . This indicates that a fraction containing very fine cellulose particles could be separated and collected using the decanter centri fuge . It could be further concentrated and / or dewatered using a further decanter centri fuge , i f desired .

[0095] It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways . The embodiments are thus not limited to the examples described above ; instead they may vary within the scope of the claims .

[0096] The embodiments described hereinbefore may be used in any combination with each other . Several of the embodiments may be combined together to form a further embodiment . A process , a product , or a use , disclosed herein, may comprise at least one of the embodiments described hereinbefore . It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments . The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages . It will further be understood that reference to ' an ' item refers to one or more of those items . The term "comprising" is used in this speci fication to mean including the feature ( s ) or act ( s ) followed thereafter, without excluding the presence of one or more additional features or acts .

Claims

CLAIMS1. A process for washing and / or fractionating a wood-based material comprising fines, wherein the process comprises providing the wood-based material as a suspension; and subjecting the suspension to centrifugal forces, thereby separating the suspension into at least two fractions based on at least their densities.

2. The process according to claim 1, wherein the wood-based material in the suspension is washed and optionally dewatered by subjecting the suspension to the centrifugal forces.

3. The process according to claim 1 or 2, wherein the suspension is subjected to the centrifugal forces, thereby separating the wood-based material in the suspension into one or more fine and / or ultrafine fractions based on at least their densities.

4. The process according to any one of claims 1 - 3, wherein the suspension is subjected to the centrifugal forces in a decanter centrifuge.

5. The process according to claim 4, wherein one or more of the at least two fractions and / or the one or more fine and / or ultrafine fractions is / are further subjected to the centrifugal forces in a second decanter centrifuge .

6. The process according to any one of claims 1 - 5, wherein the wood-based material comprises cellulose material, wherein the cellulose material optionally comprises or is partially hydrolysed cellulose material.

7. The process according to any one of claims 1 - 6, wherein the consistency of the wood-based material in the suspension is at least 0.5 % (w / w) .

8. The process according to any one of claims 1 - 7, wherein the consistency of the wood-based material in one or more of the at least two fractionsis at least about 20 % (w / w) , or in the range of about 20 - 35 % (w / w) .

9. The process according to any one of claims 4 - 8, wherein the decanter centrifuge ( s ) comprise (s) a dam plate for adjusting the consistency of the woodbased material in the suspension and / or retention time in the decanter centrifuge.

10. The process according to any one of claims 3 - 9, wherein the one or more fine and / or ultrafine fractions comprise or are one or more of microcrystalline cellulose; nanocrystalline cellulose; colloidal cellulose; fines; hemicellulose; or partially hydrolysed hemicellulose.

11. The process according to any one of claims 1 - 10, wherein the process further comprises comminuting the wood-based material in one or more of the at least two fractions and / or in the one or more fine and / or ultrafine fractions, optionally by milling.

12. The process according to any one of claims 1 - 11, wherein the centrifugal forces and / or one or more parameters of the decanter centrifuge ( s ) are adjusted, thereby adjusting properties of the wood-based material in one or more of the at least two fractions and / or of the one or more fine and / or ultrafine fractions .

13. The process according to claim 12, wherein the one or more parameters include at least one of consistency of the wood-based material in the suspension; g-force and / or rotation speed of the decanter centrifuge ( s ) ; torque of the decanter centrifuge ( s ) ; or feeding rate of the suspension.

14. The process according to claim 12 or 13, wherein the properties of the wood-based material comprise at least one of degree or polymerization, particle size, particle size distribution, or fiber length .

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