Method of dry defibrating pulp sheets and system therefore

Abstract

The present invention relates to a method (100) of dry defibrating pulp sheets (2) into defibrated pulp (5) using a defibration system (1) comprising a first mill (10) and a second mill (20) arranged downstream from the first mill (10). The first mill (10) is operated at maximum 50% of the number of revolutions compared to the second mill (20). The method (100) comprises acts of continuously feeding (200) the pulp sheets (2) as individual stacks (3) into the first mill (10), deconstructing (300) the pulp sheets (2) into an intermediate product (4), feeding (400) the intermediate product (4) into the second mill (20) and defibrating (500) the intermediate product (4) into defibrated pulp (5). The invention further relates to an intermediate product (4) obtained by the method (100) and a dry defibration system (1) adapted to perform the method (100).

Description

[0001] Method of dry defibrating pulp sheets and system therefore

[0002] Field of the Invention

[0003] The present invention relates to a method of dry defibrating pulp sheets into defibrated pulp using a defibration system comprising a first mill and a second mill arranged downstream from the first mill. The first mill is operated at no more than 50% of the number of revolutions compared to the second mill.

[0004] The method comprises acts of continuously feeding the pulp sheets into the first mill and deconstructing the pulp sheets into an intermediate product. Then feeding the intermediate product into the second mill and defibrating the intermediate product into a defibrated pulp.

[0005] The pulp sheets are fed to the first mill as individual stacks of pulp sheets with fibres having an initial fibre-length.

[0006] The invention further relates to an intermediate product obtained by the method and a dry defibration system adapted to perform the method.

[0007] Background of the Invention

[0008] In current dry defibration systems, milling machines are often fed with rolls of wood pulp sheets, individual pulp sheets, or an array of partially overlapping pulp sheets. Feeding with rolls of wood pulp sheets achieves the most uniform fluff pulp for use in air laid systems, wet laid systems, and / or hygienic product production. The only feed interruption occurs in the transition between emptying the current roll of pulp and the start of feeding from a new roll. Using rolls of wood pulp sheets, compared to baled sheeted pulp, is more expensive, more difficult to handle, and bulky, thereby increasing the cost of production as well as the cost of storage and transportation.

[0009] Using baled sheeted pulp, instead of rolls of wood pulp sheets, reduces the cost of materials by about 20%, however, dry defibrating pulp sheets using the current systems and methods have other disadvantages.

[0010] Current systems, using baled sheeted pulp, require either single sheets to be fed into the dry defibrating system or the sheets being shingled to form a continuous array of pulp sheets. When feeding single sheets, the interruption between two consecutive sheets disturbs the uniformity of the defibrated fluff pulp resulting in an end-product with hard nits and flocks as well as a substantial degradation of the fibre length in the fluff pulp compared to the initial baled sheeted pulp.

[0011] Using current systems to defibrate baled sheeted pulp reduces the quality of the defibrated pulp and requires pre-processing such as pre-cutting, sorting, re-arranging, etc.

[0012] JP2020507195A discloses a system and method of pulping one or more of a CNT sheet, to form a CNT pulp. The method includes a step of pulping by an apparatus. The method includes milling CNT pulp in a first mill and further disaggregating the CNT pulp in a second mill, the mills can be of different types. JP2020507195A does not describe feeding pulp sheets to the first mill as individual stacks or using a difference between the number of revolutions from the first to the second mill, or that the intermediate product obtained by the method is flakes with frayed edges to prevent hard nits and flocks in the final product.

[0013] Object of the Invention

[0014] One objective of the present disclosure is to achieve a method of dry defibrating pulp sheets into a uniform defibrated pulp with minimal to no hard nits and flocks while maintaining the fibre-length, and thereby quality, from the initial pulp sheets to the defibrated pulp.

[0015] Another objective of the present disclosure is to achieve an intermediate product being flakes with frayed edges obtained by the method and a dry defibration system adapted to perform the method.

[0016] Description of the Invention

[0017] One objective of the invention is achieved by a method of dry defibrating pulp sheets into defibrated pulp using a defibration system. The dry defibration system comprises a first mill and a second mill arranged downstream from the first mill.

[0018] The method comprises acts of:

[0019] - continuously feeding the pulp sheets into the first mill;

[0020] - deconstructing the pulp sheets into an intermediate product in the first mill; - feeding the intermediate product into the second mill; and

[0021] - defibrating the intermediate product into defibrated pulp in the second mill.

[0022] The pulp sheets fed to the first mill are fed as individual stacks of pulp sheets each stack comprising at least one pulp sheet with fibres having an initial fibre-length.

[0023] The first mill is operated at no more than 50% of the number of revolutions compared to the second mill. Preferably, the first mill is operated at no more than 30% of the number of revolutions compared to the second mill. More preferably, the first mill is operated at no more than 20% of the number of revolutions compared to the second mill.

[0024] An advantage of the first mill being operated at a lower number of revolutions compared to the number of revolutions the second mill is operated at, is deconstructing the inputted stacks of pulp sheets to achieve the intermediate product. The intermediate product may comprise flakes or ‘stamps’ of pulp instead of shredded pulp known in the art.

[0025] Feeding the intermediate product into the second mill achieves a more uniform defibrated pulp with minimal to no hard nits and flocks. Furthermore, feeding intermediate product to the second mill maintains the fibre-length from the initial pulp sheets, further providing optimal fibre properties such as fibre-length distribution, a low percentage of fines, and a low percentage of knots.

[0026] Disadvantages of defibrating shredded pulp include achieving a defibrated pulp comprising hard nits and flocks. These hard nits and flocks remain when further processing the defibrated pulp to achieve an air laid product, a wet laid product and / or in hygienic product production.

[0027] The sheet pulp may comprise fibre types such as, but not limited to, wood pulp, synthetic, or natural fibres, including SBSK pulp, NBSK pulp, CTMP pulp, or eucalyptus pulp.

[0028] Continuously feeding the pulp sheets into the first mill is to be understood as continuously feeding stacks of sheet pulp after one another. As the pulp sheets are uniformly ripped into flakes, compared to being inconsistently shredded and mashed by milling with a higher number of revolutions, the small interruptions occurring when feeding consecutive stacks of sheet pulp become insignificant.

[0029] The intermediate product may be fed into the second mill directly from the first mill. Alternatively, the intermediate product may be fed into the second mill with a delay. The intermediate product may be automatically transferred between the first and second mill by means of a conveyor. Alternatively, the intermediate product may be collected and subsequently transferred to the second mill.

[0030] The first and second mill may be the same type of mill, variations of the same type of mill, or different types of mills.

[0031] The method may further comprise controlling the quality of the intermediate product. If the intermediate product is deemed to be too large, the faulty intermediate product may be fed into the first mill again.

[0032] The intermediate product is defibrated into a defibrated pulp in the second mill. The defibrated pulp may be used as a replacement for fluff pulp in systems achieving an air laid product, a wet laid product, and / or in hygienic product production.

[0033] The individual stacks of pulp sheets may comprise pulp sheets arranged in substantial vertical layers on top of one another. Shingling the pulp sheets is known in the art as a mean to overcome the interruptions caused by feeding single pulp sheets to a milling machine. However, the dry defibrating system of the present invention makes shingling the sheets redundant as the intermediate product is more forgiving in terms of creating a continuous flow of intermediate material being supplied to the second mill. The stacks of sheet pulp may consequently be fed directly from pulp sheet bales to the first mill without pre-cutting, sorting, or re-arranging the pulp sheets.

[0034] An advantage of feeding stacks of pulp sheets is increasing the feed volume, compared to single sheet feeding, and decreasing the amount of pre-processing, such as shingling, usually needed when feeding multiple pulp sheets at the same time. The quality of the fibres, such as minimal to no hard nits and flocks as well as minimal degradation of fibre-length and fibre distribution, can thus be maintained at a high throughput rate.

[0035] In an embodiment, the individual stacks of pulp sheets may each comprise a single pulp sheet. The defibrated pulp achieved by the present invention is especially advantageous for use in air laid nonwovens. Air laid nonwovens are a type of nonwoven fabric that is made by using air to disperse fibres and then bonding them together, e.g. by thermal bonding or mechanical processes.

[0036] The process involves opening and blending fibres of different fibre types, such as wood pulp, synthetic fibres, natural fibres, or blends of those with the use of air to form randomly oriented fibres.

[0037] Air laid nonwovens are used for personal hygiene products such as diapers or feminine care products, for medical supplies such as drapes or surgical gowns, for filtration products, food packaging for the food industry but also for household and personal hygiene such as facial wipes or cleaning cloths.

[0038] Achieving a uniform defibrated pulp is especially important when used in absorbent articles such as diapers or hygiene pads, as the absorbency would vary across otherwise identical products.

[0039] Compared to other nonwoven technologies, air laid has the unique ability to lay down short fibres, either 100% pulp fibres, or mixtures of pulp and short cut synthetic fibres, to form a homogeneous and continuous web. Consequently, it is very important to avoid hard nits and flocks in the defibrated pulp, as they negatively affect the homogeneousness in the formed continuous web.

[0040] The defibrated pulp achieved by the present invention may be especially advantageous for use in dry moulding fibre (DMF) machines and process in which the quality of fibre’s opening is very critical for final moulded product’s quality and performances.

[0041] In one embodiment, the method may further comprise an act of collecting the intermediate product from one or more first mills in a buffer arranged between the first mill and the second mill.

[0042] The buffer may control the flow of intermediate product being supplied to the second mill. The continuous flow may ensure that the interruption between stacks of sheet pulp supplied to the first mill does not disrupt the flow of the intermediate product being supplied to the second mill, thereby ensuring a uniform defibrated pulp without nits and flocks.

[0043] In one embodiment of the method, the second mill may comprise an output screen comprising holes.

[0044] The holes may be cylindrically and / or conically shaped.

[0045] The cylindrical shaped holes may have a diameter of 2-30 mm, 2-25 mm, 2-15 m, 2- 10, 3-20 mm, 3-15 mm, 3-10 mm, 4-20 mm, 4-15 mm, 4-10 mm, 5-20 mm, 5-15 mm, or 5-10 mm, preferably with the holes being 3, 4, 5, 6, 7, 8, 9 or 10 mm or in between.

[0046] An advantage of the second mill comprising an output screen with holes is the prevention of larger chunks, comprising not entirely defibrated pulp, from being mixed in with the defibrated pulp in the desired quality. Consequently, the output screen acts as a quality control to ensure that the outputted defibrated pulp meets a predetermined quality criterion.

[0047] In one embodiment of the method comprising an output screen with holes, the holes may be conically shaped with a first opening having a first diameter and a second opening having a second diameter being larger than the first diameter.

[0048] The second opening may be arranged downstream from the first opening such that the defibrated pulp is outputted by entering the output screen through the first opening and exiting through the second opening. An advantage of the entry point having a smaller diameter than the exit point is achieving better release of the fibres in the defibrated pulp. Consequently, the conical holes prevent defibrated pulp from accumulating and thus help maintain a high throughput rate.

[0049] In one embodiment of the method comprising an output screen with conically shaped holes, the first diameter may be 1 -10 mm, 2-9 mm, 2-7 mm, 2-5 mm, 3-4 mm, preferably 3.2-4 mm.

[0050] In one embodiment of the method, the first mill may be a hammermill. More specifically, the hammermill may be a hammermill with swinging hammers. The hammermill may comprise at least one rotor blade or disc with 4-10, preferably 9, hammer tips which pass an input opening per revolution.

[0051] The hammermill may comprise an array of rotor blades or discs comprising hammer tips.

[0052] An advantage of the first mill being a lower speed hammermill with swinging hammers, is preventing degradation of the initial fibre-length compared to the length of fibres in the defibrated pulp. Deconstructing the pulp sheet at a higher speed causes shredding of the pulp sheets, achieving inconsistent shreds and mashed pulp. The higher speed further causes heat to be generated which causes additionally mashed and clumped shreds.

[0053] In one embodiment of the method, the second mill may be a sawmill. More specifically, the sawmill may be a sawmill defibrator. An advantage of the second mill being a sawmill is achieving a defibrated pulp from the intermediate product.

[0054] The sawmill may comprise at least one rotor blade or disc with 20-30, often 22 or 23, saw tips passing an input opening per revolution.

[0055] The sawmill may comprise an array of rotor blades or discs comprising saw tips.

[0056] As the first mill is operated at no more than 50% of the revolutions compared to the second mill, especially if the first mill is a hammermill, an intermediate product such as flakes are achieved. An advantage of the second mill being a sawmill is achieving a uniformly defibrated pulp, as the saw rotor ensures that each flake of the intermediate product is treated substantially the same, as the saw rotor comprises more saw tips compared to for instance a hammermill.

[0057] In one embodiment of the method, the dry defibration system may comprise more first mills than second mills.

[0058] As the first mill is operated at no more than 50% of the number of revolutions compared to the second mill, the infeed of the intermediate product needed by the second mill to sustain a continuous infeed flow, to achieve a uniform and continuous output flow of defibrated pulp, may be more than what a single first mill can supply. Consequently, it may be advantageous for the system to comprise more first mills than second mills. For dry defibrating systems comprising a buffer, the buffer may be arranged between the first mills and the second mills. The dry defibration system may comprise groups of first mills, one or more buffers, and second mills, such as two first mills, one buffer, and one second mill, or four first mills, two buffers, and three second mills.

[0059] Each buffer may comprise an intermediate product of a specific fibre type, and thus more buffers supplying an intermediate product to one second mill may result in a defibrated pulp comprising a mix of fibre types.

[0060] In an aspect, the second mill may run delayed or separately from the first mill such that the intermediate product is collected in the buffer and supplied continuously to the second mill in batches.

[0061] In one embodiment of the method, the act of deconstructing the pulp sheets into an intermediate product in the first mill, may be performed at a rotational speed of the first mill in the range of: 500-1500 RPM, 500-1000 RPM, or 500-800 RPM, preferably 500- 700 RPM.

[0062] The act of defibrating the intermediate product into defibrated pulp in the second mill, may be performed at a rotational speed of the second mill in the range of: 2000-4000 RPM, 2500-3500 RPM, or 2700-3200 RPM, preferably about 3000 RPM.

[0063] Mills in current defibration systems are known to be operated at about 3000 RPM. However, some mills operate up to 5000 RPM, depending on the diameter, to achieve a tip speed of about 100 m / s.

[0064] Using a lower rotational speed in the first mill minimizes heat generation and thus minimizes the need for cooling to prevent the pulp from spontaneously igniting.

[0065] The preferred mill rotor diameter range is from 300 mm up to 700 mm.

[0066] Additionally, as shredding of the pulp sheets is prevented, there is almost no fibre length degradation compared to the initial fibre-length, and consequently a defibrated pulp is achieved having fibres with substantially the same quality as the initial sheet pulp. In one embodiment of the method, the degradation from the initial fibre-length, of the fibres in the pulp sheets, to the defibrated pulp may be 0-15%, 0-10%, 0-7%, or 0-5%, and preferably <10% and more preferably <5%.

[0067] Fibre-length tests have shown that sequentially milling the pulp sheets in the first mill and milling the intermediate product in the second mill maintain between 90-100% of the fibre-length. Under optimal process conditions, it is possible to maintain 100% of the initial fibre-length.

[0068] The test further shows that for dry defibration systems of the prior art, 75-80% of the fibre-length is maintained.

[0069] The percentage of fines in the defibrated pulp is further shown to be significantly lower when using the method of dry defibration according to the invention.

[0070] Some of the test results are listed in the table below. For these specific tests, the first mill was a hammermill, and the second mill was a sawmill. The prior art system comprised a single hammermill.

[0071] Using the method achieves fibre-length and fibre-length distribution comparable to wet laid disintegration.

[0072] The defibrated pulp achieved by the present invention may have a similar quality as defibrated pulp used in commercial air and wet laid processes.

[0073] As longer fibres are especially suitable for producing nonwovens, an advantage of preventing and / or minimizing the degradation of fibre length is achieving defibrated pulp suitable for use in the production of nonwovens. Longer fibres further increase the tensile strength and increase the absorbency capacity while decreasing the absorbency time. Longer fibres further influence web cohesion, fibre breakage, and web uniformity after processing.

[0074] In one embodiment of the method, the act of continuously feeding the pulp sheets into the first mill may be performed by applying a vacuum to an upper surface of a top pulp sheet in the stack.

[0075] An advantage of applying a vacuum to the upper surface is achieving an effective in- feed of sheet pulp stacks into the first mill. Vacuum is fast and easy to operate, requires minimal supervision, and can easily be automated. The vacuum may be adjusted to pick up a predetermined number of pulp sheets depending on the pressure used to achieve the vacuum seal.

[0076] The vacuum may be applied by suction and use of an automatically operated arm. The automatically operated arm may advantageously be a robot with an automatically operated arm.

[0077] Another objective of the invention is achieved by an intermediate product, obtained by the method, being flakes with frayed edges.

[0078] The flakes with frayed edges may be substantially rectangular or square, having sides with lengths of 5-40 mm, 5-30 mm, 5-20 mm, 7-40 mm, 7-30 mm, 7-20 mm, 10-40 mm, 10-30 mm, 10-20 mm, 15-40 mm, 15-30 mm, 15-20 mm, preferably between 10 and 20 mm.

[0079] The term ‘substantially rectangular or square’ is used to indicate a number of shapes appearing substantially rectangular or square. The term does not exclude flakes with uneven edges and / or flakes that do not have 90-degree angles.

[0080] The flakes comprise frayed edges as the first mill deconstruct, for instance by ripping, the pulp sheet instead of cutting or shredding the fibres. Cutting the pulp sheets and thereby cutting the fibres causes blunt edges and degradation of the fibre-length. Shredding the pulp sheets causes inconsistent clumps of shredded pulp as well as nits and flocks in the defibrated pulp both of which are avoided by deconstructing the pulp sheet into the intermediate product. Yet another objective of the invention is achieved by a dry defibration system for defibrating pulp sheets.

[0081] The dry defibration system comprises:

[0082] - a first mill configured for being continuously fed pulp sheets to be processed and outputting an intermediate product; and

[0083] - a second mill arranged downstream from the first mill, the second mill is configured for being fed the intermediate product to be processed and outputting a defibrated pulp through an output screen comprising holes.

[0084] The dry defibrating system is adapted to perform the method.

[0085] In an aspect, the advantages achieved by the dry defibration system are substantially similar to the advantages achieved by the intermediate product and the method of dry defibrating pulp sheets of the invention.

[0086] In one embodiment of the dry defibration system for defibrating pulp sheets or parts thereof, the intermediate product may be flakes with frayed edges.

[0087] Description of the Drawings

[0088] Various examples are described hereinafter with reference to the figures. Like reference numerals refer to like elements throughout. Like elements will, thus, not be described in detail with respect to the description of each figure. It should also be noted that the figures are only intended to facilitate the description of the examples. They are not intended as an exhaustive description of the claimed invention or as a limitation on the scope of the claimed invention. In addition, an illustrated example need not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular example is not necessarily limited to that example and can be practiced in any other examples even if not so illustrated, or if not so explicitly described.

[0089] Exemplary embodiments of the invention are described in the figures, wherein:

[0090] Fig. 1 illustrates the dry defibration system according to the invention.

[0091] Fig. 2 illustrates the output screen according to the invention.

[0092] Fig. 3 illustrates the defibrated pulp achieved by the invention and by the prior art.

[0093] Fig. 4 illustrates the method of dry defibrating pulp sheets according to the invention. Detailed Description of the Invention

[0094] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings. In this regard, the present examples may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the examples are merely described below, by referring to the figures, to explain aspects.

[0095] Throughout the specification, when an element is referred to as being “connected” to another element, the element is “directly connected” to the other element, “electrically connected”, “fluidically connected” or “communicatively connected” to the other element with one or more intervening elements interposed therebetween.

[0096] The terminology used herein is for the purpose of describing particular examples only and is not intended to be limiting. As used herein, the terms “comprises", "comprising", "includes" and / or "including" when used in this specification specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0097] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by those skilled in the art to which this invention pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined in the present specification.

[0098] No Item

[0099] 1 Dry defibration system

[0100] 2 Pulp sheet

[0101] 2’ Top pulp sheet

[0102] 3 Stack

[0103] 4 Intermediate product

[0104] 5 Defibrated pulp

[0105] 6 Fibre

[0106] 7 Fibre-length 10 First mill

[0107] 20 Second mill

[0108] 21 Output screen

[0109] 22 Hole

[0110] 23 First opening

[0111] 24 Second opening

[0112] 30 Buffer

[0113] 100 Method of dry defibration

[0114] 200 Continuously feeding (first mill)

[0115] 300 Deconstructing

[0116] 350 Collecting

[0117] 400 Feeding (second mill)

[0118] 500 Defibrating

[0119] Figure 1 illustrates an embodiment of the dry defibration system 1 for performing the acts of the method 100 of dry defibrating pulp sheets 2 into defibrated pulp 5.

[0120] The dry defibration system comprises a first mill 10 and a second mill 20 arranged downstream from the first mill 10. In the illustrated embodiment, the first mill 10 is a hammermill and the second mill 20 is a sawmill, however, other types of mills and / or combination of mills may be used instead.

[0121] The first mill 10 is configured for being continuously fed 200 pulp sheets 2 to be deconstructed 300 and outputting an intermediate product 4.

[0122] The pulp sheets 2 fed to the first mill 10 are fed as individual stacks 3 of pulp sheets 2. Each stack comprises at least one pulp sheet 2 with fibres 6 having an initial fibrelength 7. The pulp sheets are arranged in substantial vertical layers on top of one another in each individual stack that comprises more than one pulp sheet. Figure 1 illustrates an embodiment of a stack 3 comprising four pulp sheets.

[0123] Stacks of sheet pulp may be fed 200 directly from pulp sheet bales (not illustrated) to the first mill 10 without pre-cutting, sorting, or re-arranging the pulp sheets 2.

[0124] The stacks of sheet pulp may be fed 200 directly from pulp sheet bales to the first mill 10 by applying a vacuum to an upper surface of a top pulp sheet 2’ in the stack 3. The vacuum may be applied by suction and use of an automatically operated arm. The vacuum may be adjusted to pick up a predetermined number of pulp sheets 2 depending on the pressure used to achieve the vacuum seal.

[0125] The second mill 20 is configured for being fed 400 the intermediate product 4 to be defibrated 500 and outputting a defibrated pulp 5. As illustrated in figure 1 and in detail in figure 2, the second mill 20 comprises an output screen 21 comprising holes 22.

[0126] The first mill 10 is operated at no more than 50% of the number of revolutions compared to the second mill 20 to achieve the intermediate product 4. As an example, the deconstructing 300 of pulp sheets 2 in the first mill 10 is performed at a rotational speed of 700 RPM, while the rotational speed of the defibration 500 of the intermediate product 4 in the second mill 20 is performed at a rotational speed of 3000 RPM.

[0127] In the illustrated embodiment, the intermediate product 4 is collected 350 from the first mill 10 in a buffer 30 arranged between the first mill 10 and the second mill 20.

[0128] As illustrated, the dry defibration system 1 comprises one first mill 10, one buffer 30, and one second mill 20, however, the dry defibration system 1 may comprise more first mills 10 than second mills 20.

[0129] For dry defibrating systems 1 that comprise more first mills 10 than second mills 20 and that comprise one or more buffers 30, the buffer(s) 30 may be arranged between the first mills 10 and the second mills 10.

[0130] The dry defibration system 1 may comprise groups of: first mills 10, one or more buffers 30, and second mills 20, for instance two first mills 10, one buffer 30, and one second mill 20. Another example could be four first mills 10, two buffers 30, and three second mills 20.

[0131] Each buffer 30 may comprise an intermediate product 4 of a specific fibre 6 type, and thus more buffers 30 supplying an intermediate product 4 to one second mill 20 may result in a defibrated pulp 5 comprising a mix of fibre 6 types.

[0132] Figure 2A illustrates a detailed view of the output screen 21 comprising holes 22 from figure 1. The holes may be cylindrically shaped as illustrated in figure 2B or conically shaped as illustrated in figure 2C. In some embodiments, the output screen 21 may comprise a mix of cylindrically shaped holes 22 and conically shaped holes 22. The conically shaped hole 22 as illustrated in figure 2C comprises a first opening 23 having a first diameter and a second opening 24 having a second diameter being larger than the first diameter. Preferably, the first diameter is 3.2-4 mm.

[0133] The degradation from the initial fibre-length 7 of the fibres 6 in the pulp sheets 2, to the defibrated pulp 5 may be 0-15%, 0-10%, 0-7%, or 0-5%, and preferably <5%.

[0134] Figure 3A illustrates the intermediate product 4, the defibrated pulp 5 achieved by dry defibrating the intermediate product 4, and a wet laid sheet achieved by further processing the defibrated pulp 5. Although a wet laid sheet is illustrated, the defibrated pulp 5 may also be used for achieving air laid products, other wet laid products, and / or hygiene products.

[0135] The prior art is illustrated in figure 3B, showing (in the direction of the arrows): shredded pulp, the defibrated pulp 5 achieved by dry defibrating the shredded pulp, and a wet laid sheet achieved by further processing the defibrated pulp from shredded pulp. As indicated by the circles in figure 3B, the wet laid sheet from shredded pulp comprises a lot of hard nits and flocks, whereas the wet laid sheet in figure 3A is uniform with no hard nits or flocks present.

[0136] As illustrated in figure 3A and 3B the quality of the defibrated pulp varies greatly depending on whether the intermediate product 4 or a shredded pulp is dry defibrated.

[0137] The intermediate product 4 obtained by the method is flakes with frayed edges. As illustrated, the flakes with frayed edges may be substantially rectangular or square. Preferably, the length of each side is 10-20 mm.

[0138] The flakes achieve the frayed edges as the first mill 10 deconstructs 300, by ripping, the pulp sheet 2 instead of cutting or shredding the pulp sheet 2. As illustrated, shredding the pulp sheets causes inconsistent clumps of shredded pulp as well as nits and flocks in the defibrated pulp and the wet laid sheet. If the pulp sheets 2 were cut instead of shredded, the fibres in the pulp sheet 2 would also be cut, causing both degradation of the fibre-length and blunt edges which further cause hard nits. The disadvantages by dry defibrating shredded pulp or cut pulp are avoided by deconstructing, by ripping, the pulp sheet 2 into the intermediate product 4 to be defibrated 500. Figure 4 illustrates a method of dry defibrating 100 pulp sheets 2 into defibrated pulp 5 using a defibration system 1. The defibration system 1 comprises a first mill 10 and a second mill 20 arranged downstream from the first mill 10.

[0139] The method 100 comprises an act of continuously feeding 200 the pulp sheets 2 into the first mill 10. The pulp sheets 2 fed to the first mill 10 are fed as individual stacks 3 of pulp sheets 2. Each stack comprises at least one pulp sheet 2 with fibres 6 having an initial fibre-length 7.

[0140] The act of continuously feeding 200 the pulp sheets 2 into the first mill 10 may be performed by applying a vacuum to an upper surface of a top pulp sheet 2’ in the stack 3.

[0141] The method 100 comprises an act of deconstructing 300 the pulp sheets 2 into an intermediate product 4 in the first mill 10. The first mill 10 is operated at no more than 50% of the number of revolutions compared to the second mill 20.

[0142] The act of deconstructing 300 the pulp sheets 2 into an intermediate product 4 in the first mill 10 may be performed at a rotational speed of the first mill 10 in the range of: 500-1500 RPM, 500-1000 RPM, or 500-800 RPM, preferably 500-700 RPM.

[0143] The method 100 comprises an act of feeding 400 the intermediate product 4 into the second mill 20.

[0144] The method 100 comprises an act of defibrating 500 the intermediate product 4 into defibrated pulp 5 in the second mill 20.

[0145] The method 100 further comprises an act of collecting 350 the intermediate product 4 from one or more first mills 10 in a buffer 30 arranged between the first mill 10 and the second mill 20.

Claims

CLAIMS1. A method (100) of dry defibrating pulp sheets (2) into defibrated pulp (5) using a defibration system (1 ) comprising a first mill (10) and a second mill (20) arranged downstream from the first mill (10), wherein the method (100) comprises acts of:- continuously feeding (200) the pulp sheets (2) into the first mill (10);- deconstructing (300) the pulp sheets (2) into an intermediate product (4) in the first mill (10);- feeding (400) the intermediate product (4) into the second mill (20); and- defibrating (500) the intermediate product (4) into defibrated pulp (5) in the second mill (20), wherein the pulp sheets (2) fed to the first mill (10) are fed as individual stacks (3) of pulp sheets (2) each stack comprising at least one pulp sheet (2) with fibres (6) having an initial fibre-length (7); and wherein the first mill (10) is operated at no more than 50% of the number of revolutions compared to the second mill (20).

2. The method (100) according to claim 1 , comprising an act of collecting (350) the intermediate product (4) from one or more first mills (10) in a buffer (30) arranged between the first mill (10) and the second mill (20).

3. The method (100) according to claim 1 or 2, wherein the second mill (20) comprises an output screen (21 ) comprising holes (22).

4. The method (100) according to claim 3, wherein the holes (22) are conically shaped with a first opening (23) having a first diameter and a second opening (24) having a second diameter being larger than the first diameter.

5. The method (100) according to claim 4, wherein the first diameter is 1 -10 mm, 2-9 mm, 2-7 mm, 2-5, 3-4 mm, preferably 3.2-4 mm.

6. The method (100) according to any of the preceding claims, wherein the first mill (10) is a hammermill and wherein the second mill (20) is a sawmill.

7. The method (100) according to any of the preceding claims, wherein the dry defibration system (1 ) comprises more first mills (10) than second mills (20).

8. The method (100) according to any of the preceding claims, wherein the act of deconstructing (300) the pulp sheets (2) into an intermediate product (4) in the first mill (10) is performed at a rotational speed of the first mill in the range of: 500-1500 RPM, 500-1000 RPM, or 500-800 RPM, preferably 500-700 RPM.

9. The method (100) according to any of the preceding claims, wherein the degradation from the initial fibre-length (7), of the fibres (6) in the pulp sheets (2), to the defibrated pulp (5) is 0-15%, 0-10%, 0-7%, or 0-5%, and preferably <5%.

10. The method (100) according to any of the preceding claims, wherein the act of continuously feeding (200) the pulp sheets (2) into the first mill (10) is performed by applying a vacuum to an upper surface of a top pulp sheet (2’) in the stack (3).1 1 . An intermediate product (4) obtained by the method according to any of the preceding claims is flakes with frayed edges.

12. A dry defibration system (1 ) for defibrating pulp sheets (2) comprising- a first mill (10) configured for being continuously fed pulp sheets (2) to be processed and outputting an intermediate product (4); and- a second mill (20) arranged downstream from the first mill (10), the second mill (20) is configured for being fed the intermediate product (4) to be processed and outputting a defibrated pulp (5) through an output screen (21 ) comprising holes (22), and wherein the dry defibrating system (1 ) is adapted to perform the method according to any of claims 1 -10.

13. A dry defibration system (1 ) for defibrating pulp sheets (2) according to claim 12 or parts thereof wherein the intermediate product (4) is flakes with frayed edges.

Images