Method and apparatus for the production of a fiberboard

Abstract

A process for producing a fiberboard from wood fibers and / or other lignocellulosic fibers using a wet process, in which water (9) is added to the fibers (7) to form a suspension (10) with a fiber content of 10 wt.% or less, in which the suspension (10) is pumped onto a long screen (11) and dewatered by gravity to form a fiber fleece cake (13), characterized in that the fiber fleece cake (13) is fed to a squeezer (14) and mechanically dewatered to a residual moisture content of less than 20 wt.% to form a fiber fleece (16), and the fiber fleece (16) is then transferred to a continuously operating hot press (18) and pressed there to form a fiberboard (19) under heat input.

Description

[0001] The invention relates to a wet process for producing a fiberboard from wood fibers and / or other lignocellulosic fibers, in which water is added to the fibers to form a suspension with a fiber content of 10 wt.% or less, and in which the suspension is pumped onto a long screen and dewatered by gravity to form a fiber web cake. The invention further relates to an apparatus for producing a fiberboard, in particular an apparatus for carrying out the aforementioned process.

[0002] Methods for producing fiberboard from wood fibers and / or other lignocellulosic fibers in general, and specifically using the wet process, are well known in the prior art, so a separate printed reference is not required here. Reference is therefore only made by way of example to DE 25 16 121 A1, which discloses a previously known wet process.

[0003] In the wet process, the previously processed fibers are mixed with water. This results in a suspension with a fiber content of 10 wt.% or less. DE 25 16 121 A1 discloses a fiber content of 1 to 2 wt.% in this context.

[0004] In the next process step, the suspension is transferred to a long screen. This is typically done by pumping, although other transfer methods are also known. On the long screen, mechanical dewatering occurs due to gravity. This partial dewatering produces a fiber fleece cake, which is then fed into a hot press. Further dewatering, including evaporation, and pressing take place in the hot press.

[0005] Hot pressing involves the use of discontinuous presses, or molds into which a fiber fleece cake slurry is transferred and then pressed.

[0006] A particular advantage of the wet process is that no additional binders are required to produce finished fiberboards. Instead, the lignin already present in the lignocellulosic fibers acts as the sole binder. This lignin is activated by introducing steam into the fiber mat or its slurry.

[0007] A disadvantage of the wet process, however, is that continuous operation is not possible due to the discontinuous pressing process. Furthermore, lignin activation requires the introduction of steam into the fiber web cake and thus a vapor-permeable support material for the fiber web cake, which is typically a screen. This leaves an unfavorable imprint pattern on the corresponding side of the finished fiberboard, which contradicts the desired smooth surface finish.

[0008] In order to overcome the aforementioned disadvantages of the wet process, the so-called dry process was developed.

[0009] In the dry process, the fiber fleece cake is formed from dry fibers, which subsequently enables the use of continuous hot presses. Unlike the wet process, the dry process can therefore be operated continuously. Furthermore, the dry process allows for the production of smooth fiberboard surfaces, as a vapor-permeable screen as a substrate material is no longer required.

[0010] A disadvantage of the dry process, however, is the necessary use of additional binders, which are typically applied in liquid form via a blow-line spray onto the previously dried fibers. Urea resins are typically used as binders, but these release formaldehyde during normal board production, which is detrimental. The use of other synthetic resin adhesives as binders is also known in the art, but these are generally natural gas-based and therefore not very environmentally friendly. Furthermore, the dry process requires a significant amount of energy, which also has a negative impact on the climate.

[0011] Based on what has been described, the task is to constructively develop a generic process in such a way that climate-improved production of fiberboards of the desired shape is made possible.

[0012] To solve this problem, the invention proposes a generic method characterized in that the fiber fleece cake is fed to a squeezer and mechanically dewatered to a residual moisture content of less than 20 wt.% while forming a fiber fleece, and then the fiber fleece is transferred to a continuously operating hot press and pressed there into a fiberboard under heat input.

[0013] According to the invention, the fiber fleece cake, previously formed by gravity dewatering of the suspension, is fed to a squeezer. Using this squeezer, the fiber fleece cake is mechanically dewatered a second time. This dewatering process results in a fiber fleece that can then be further processed.

[0014] The fiber fleece cake is squeezed, i.e., compressed, using the squeezer, resulting in mechanical water removal. This process dewaters the fiber fleece cake to a residual moisture content of less than 20 wt.%. Preferably, dewatering is carried out to a residual moisture content of between 8 and 15 wt.%, and more preferably between 10 and 12 wt.%.

[0015] As a result of the water being driven out by squeezing, a fiber fleece is formed which, unlike a fiber fleece cake, already possesses sufficient inherent stability to allow transfer to a hot press without any further aids. This advantageously enables the use of a continuously operating hot press in the subsequent process step. In this hot press, the fiber fleece is then pressed into the finished fiberboard under heat input.

[0016] As a result of the process according to the invention, a wet cake without binders can be produced in a manner known per se, wherein the physical bonding of the individual fibers in the hot press is achieved in particular by activating the naturally occurring lignin. Furthermore, the continuously operating hot press allows for variable dimensions in terms of length and width, which was previously not possible with wet processes, since the wet cake, due to its lack of inherent stability, must be transferred into a geometrically defined shape.

[0017] The method according to the invention proves to be advantageous compared to the previously known wet method as well as to the aforementioned dry method, particularly for three reasons.

[0018] Unlike the dry process, binders can essentially be dispensed with. The wood's own binding forces serve to bond the fibers; in particular, the lignin already present in the fibers plays a key role, which is activated by heat input in the continuously operating hot press. In this respect, the process according to the invention proves to be significantly more environmentally friendly, especially compared to the previously known dry process.

[0019] In the context of the invention, the complete absence of binders means that the process can be carried out without the addition of any further binder, i.e., the intended formation of a fiberboard is achieved solely through the use of the wood's own binding properties. However, binder-free production in this sense is not strictly necessary. Smaller quantities of binder can also be used, for example, on the order of < 2 wt.%, preferably < 1.5 wt.%. In any case, significantly less binder by weight is used than in the dry process, where the amount of binder used is typically around 9 to 14 wt.%. According to the inventive process, therefore, either no binders are used or they are used in such a small quantity that a significant reduction is achieved compared to the amount of binder used in a dry process.

[0020] A second advantage is that continuous production is possible despite the wet process, achieved through the intermediate step of the press. Unlike pre-made molds, the continuously operating hot press has the advantage that no fixed dimensions are specified; instead, size variability is possible in both width and length.

[0021] A third advantage is that a smooth surface finish on both sides of the board is possible. Unlike the previously known wet process, no screen is required for steam removal. In the previously known wet process, a screen is necessary because water vapor is generated when the fiber web is heated, and this vapor must be removed. This is achieved by the screen, i.e., the long screen, through whose mesh the water vapor can escape. Because of this necessary steam removal, a screen cannot be omitted in the previously known wet process, which results in a screen imprint pattern, and thus a non-smooth surface, on at least one side of the finished fiberboard. In contrast, the invention allows the use of a hot press with two smooth press belts.Unwanted screen printing patterns are thus avoided, so that, similar to the dry process, a smooth surface can be achieved on both sides of the finished fiberboard.

[0022] The process according to the invention enables a combined realization of properties that are inherently contradictory according to the previously known wet process on the one hand and the previously known dry process on the other. With the process according to the invention, the production of a fiberboard in a continuous process is made possible for the first time, essentially without the use of binders. Furthermore, the formation of smooth board surfaces on both sides is permitted.

[0023] If binders are used, an amount of less than 2% by weight, and in particular less than 1.5% by weight, is preferred. Suitable binders include, in particular, aminoplast resins such as urea-formaldehyde resins.

[0024] According to a further feature of the invention, it is provided that additional fibers are added to the fibers. Lignocellulosic fibers, such as synthetic fibers, are also used as additional fibers. The use of bicomponent fibers is particularly preferred. "Bicomponent fiber" here refers to a fiber formed from two materials, the materials used preferably having different melting points. The use of fibers comprising a fiber core made from a first material component and a sheathing layer encasing the fiber core made from a second material component is particularly preferred.

[0025] According to a further feature of the invention, it is provided that the fiber fleece originating from the crusher is transferred to a thermally resistant intermediate belt and from there fed to the continuously operating hot press.

[0026] The optional intermediate belt, like the long screen, is designed as a circulating conveyor belt, with the intermediate belt-side deflection roller of the long screen and the long screen-side deflection roller of the intermediate belt being arranged as close together as possible, so that only a minimal gap between the fiber fleece has to be bridged at the moment of transfer from the long screen to the intermediate belt.

[0027] Unlike the long screen, the intermediate belt is temperature-resistant. This is necessary because the fiber web is transferred from the intermediate belt to the hot press. While it's conceivable to transfer the fiber web directly from the long screen to the hot press without an intermediate belt, the hot press generates such intense heat that the transferring belt also heats up. Therefore, a temperature-insensitive belt is required, making the use of the temperature-resistant intermediate belt an option. Alternatively, the long screen could be made heat-resistant, but this is generally avoided for cost reasons.

[0028] According to a further feature of the invention, the thickness of the fiber fleece cake is selected depending on the desired thickness of the finished fiberboard. For example, if the final board thickness is 3 mm, a fiber fleece cake of approximately 50 mm is required. The thickness of the fiber fleece cake thus directly influences the final board thickness.

[0029] According to a further feature of the invention, a temperature between 200°C and 280°C, preferably between 220°C and 260°C, is selected in the hot press. As a result of this temperature input, residual moisture still contained in the fiber fleece is driven off. Thus, further drying occurs due to the heat input. Furthermore, the intended heat input in the hot press activates the lignin, which serves as a natural binder, leading to the bonding of the individual fibers.

[0030] Lignocellulosic fibers, especially wood fibers, can be used. However, the use of other lignocellulosic fibers is also conceivable, for example, bamboo / grass and / or recycled paper fibers.

[0031] The invention further proposes a device for producing a fiberboard from wood fibers and / or other lignocellulosic fibers using a wet process, in particular for carrying out the process described above, comprising a long screen, a continuously operating hot press and a squeezer upstream of the hot press, which serves to mechanically dewater a fiber web cake provided by the long screen, forming a fiber web with a residual moisture content of less than 20 wt.%.

[0032] According to the invention, the device includes a crusher. This crusher is positioned downstream of the long screen in the transport direction and upstream of the continuously operating hot press. The crusher serves to mechanically dewater the fiber fleece cake provided by the long screen, thereby forming a fiber fleece. Dewatering is achieved down to a residual moisture content of less than 20% by weight, preferably less than 15% by weight.

[0033] The fiber fleece produced by squeezing the fiber cake has such a low residual moisture content that, due to its inherent stability, it can be further processed in a continuously operating press. Unlike previously known wet processes, the use of indexed presses is therefore unnecessary. As a result, a completely continuous process can be achieved.

[0034] The relatively low residual moisture content of the fiber fleece also makes it possible to transfer the fleece from one conveyor belt to the next. This is possible because the fiber fleece already possesses sufficient inherent stability to even bridge gaps between two adjacent conveyor belts.

[0035] As a result, the advantages already explained above with reference to the method according to the invention are achieved.

[0036] According to a further feature of the invention, the crusher comprises at least one pair of rollers. With the intermediate arrangement of the long screen, one of the two rollers is positioned above the fiber web cake provided by the long screen, and the other of the two rollers is positioned below the long screen. The fiber web cake located on the long screen is crushed, i.e., compressed, by means of the two rollers of the roller pair, resulting in water discharge. The rollers are spaced apart vertically such that the fiber web cake is compacted, leading to water discharge, so that the fiber web exiting the crusher has a residual moisture content of less than 20% by weight.

[0037] Preferably, several pairs of rollers are provided. These are arranged one behind the other in the transport direction. The distance between the rollers of a roller pair decreases in the transport direction, resulting in an overall essentially wedge-shaped gap shape.

[0038] The rollers of a roller pair are not unheated, i.e., cool. However, according to an alternative embodiment, the rollers can also be heated. This ensures not only mechanical but also thermal dewatering. Any pattern that may form on the screen side is smoothed out during subsequent pressing in the hot press. The rollers of the roller pair—whether heated or unheated—preferably have a diameter of at least 280 mm. Larger diameters, for example 300 mm, are also conceivable.

[0039] According to a further feature of the invention, the hot press comprises two circumferential steel belts, each with a smooth working surface. These steel belts are heated in the intended process and have a temperature of, for example, between 200°C and 280°C. Heat is thus transferred to the fiber fleece via the steel belts. The working surface of the steel belts is preferably smooth, so that the process results in a finished fiberboard with smooth surfaces.

[0040] According to a further feature of the invention, the steel strips are arranged at intervals with respect to their respective effective surfaces, forming a press gap that tapers in the transport direction. The design of the press gap determines the final thickness of the finished fiberboard. The press gap is preferably adjustable, allowing for the production of different board thicknesses.

[0041] Further advantages and features of the invention will become apparent from the following description with reference to the figures. These show: Fig. 1 in schematic representation a process according to the invention and Fig. 2 in schematic representation a device according to the invention.

[0042] Fig. Figure 1 shows an exemplary schematic representation of a process according to the invention.

[0043] In a first process step, water vapor 2 and wood 3 are fed into a defibrator 1, whereby the wood 3 is broken down into fibers. The resulting fiber vapor mixture 4 then passes to a separator 5, in which the excess vapor 6 is separated.

[0044] The fibers 7 leaving the separator 5 are fed to a vat unit 8, where a suspension 10 with a fiber content of 10 wt.% or less is produced using water 9. This suspension 10 is then applied to a conveyor belt designed as a long screen 11, for example by pumping.

[0045] The suspension 10 applied to the long screen 11 is partially dewatered by releasing water 12, specifically by gravity dewatering. This partial dewatering forms a fiber fleece cake 13.

[0046] In the next step of the process, the fiber fleece cake 13 is fed to a squeezer 14. Here, the water-based fleece cake 13 is squeezed, i.e., compressed, resulting in mechanical dewatering and the release of water 15. The squeeze produces a fiber fleece 16 with a residual moisture content of less than 20% by weight. Thus, the fiber fleece cake 13 is fed to a squeezer 14 and mechanically dewatered to a residual moisture content of less than 20% by weight, forming a fiber fleece 16.

[0047] The long screen 11 is designed as an endless circulating conveyor belt, from which the fiber fleece 16 is transferred to an intermediate belt 17. The intermediate belt 17 is also designed as a circulating conveyor belt, but unlike the long screen, it is thermally resistant.

[0048] The fiber fleece 16 transferred to the intermediate belt 17 is then transferred to a continuously operating hot press 18 and pressed there into a fiberboard 19 under heat input.

[0049] The process described above is advantageously carried out continuously, although the fiberboard 19 is produced using a wet process. Fiberboard 19 can thus be produced without the addition of any further binders, and with smooth surfaces on both sides.

[0050] A device 29 according to the invention, in particular for carrying out the method according to the invention, is shown in a schematic representation Fig. 2.

[0051] The device 29 according to the invention has an endlessly rotating long screen 11. In the transport direction 27, the endlessly rotating long screen 11 is followed by an equally endlessly rotating intermediate belt 17. A continuously operating hot press 18 is connected downstream of this in the transport direction 27.

[0052] The continuously operating hot press 18 has two rotating steel bands 24 and 25, which are heated during intended use and are spaced apart from each other, forming a pressing gap 28 between them. Both steel bands 24 and 25 have a smooth working surface 26.

[0053] The continuously operating hot press 18 is followed in the transport direction 27 by another endlessly circulating conveyor belt 23, which serves to transport a fiberboard 19 leaving the hot press 18.

[0054] In its intended use, a fiber suspension 10 with a fiber content of 10 wt.% or less is fed into the long screen 11 from a vat unit 8. The suspension 10 is dewatered by gravity on the long screen 11, forming a fiber fleece cake 13.

[0055] In the illustrated embodiment, the fiber fleece cake 13 is passed below a limiter 20 arranged in the vertical direction above the long screen 11, so that a uniform fiber fleece cake thickness D1 of, for example, 50 mm is formed.

[0056] The fiber web cake 13 is then fed to a squeezer 14. In the illustrated embodiment, this squeezer 14 has a total of six rollers, comprising three pairs of rollers. Each pair of rollers consists of an upper roller 21 and a lower roller 22. The lower rollers 22 are arranged below the screen belt 11, while the upper rollers 21 are in direct contact with the fiber web cake 13.

[0057] The rollers 21 and 22 of the roller pairs are arranged at different distances from each other, so that an overall squish gap is created that tapers to a point in the transport direction 27.

[0058] As the fiber fleece cake 13 passes through the squeeze 14, it is dewatered. This results in a fiber fleece 16 with a residual moisture content of less than 20 wt.%, preferably less than 15 wt.%. The fiber fleece 16 has a thickness D2, which is, for example, 10 mm.

[0059] The fiber fleece 16, which is the end product of the compression process, is then transferred to the intermediate belt 17 and from there into the hot press 18. In the hot press 18, further compression takes place with the simultaneous application of heat, resulting in the finished fiberboard 19. This has a thickness D3 of, for example, 3 mm. Reference sign 1 defibrillator 2 Water vapor 3 wood 4 Fiber vapor mixture 5 separators 6 Steam 7 fibers 8 deckle units 9 Water 10 Suspension 11 Long sieve 12 Water 13 Fiber cakes 14 Squeezers 15 Water 16 fiber fleece 17 Intermediate Volume 18 Hot press 19 Fiberboard 20 limiters 21 roller 22 rollers 23 Conveyor belt 24 steel band 25 steel band 26 Effective surface 27 Transport equipment 28 Press gap 29 Device

Claims

[1] A wet process for producing a fiberboard from wood fibers and / or other lignocellulosic fibers, wherein water (9) is added to the fibers (7) to form a suspension (10) with a fiber content of 10 wt.% or less, wherein the suspension (10) is pumped onto a long screen (11) and dewatered by gravity to form a fiber web cake (13), characterized by , that the fiber fleece cake (13) is fed to a squeezer (14) and mechanically dewatered to a residual moisture content of less than 20 wt.% by forming a fiber fleece (16) and then the fiber fleece (16) is transferred to a continuously operating hot press (18) and pressed there into a fiberboard (19) by the input of heat. [2] Method according to claim 1, characterized by , that further fibers, in particular bicomponent fibers, are added to the fibers (7). [3] Method according to claim 1 or 2, characterized by, that the fiber fleece (16) taken from the squeeze (14) is transferred to a thermally resistant intermediate belt (17) and from there fed to the continuously operating hot press (18). [4] Method according to any one of the preceding claims, characterized by , that the thickness of the fiber fleece cake (13) is chosen depending on the desired thickness of the fiberboard (19). [5] Method according to any one of the preceding claims, characterized by , that in the hot press (18) a temperature of 200°C to 280°C, preferably of 220°C to 260°C, is selected. [6] Apparatus for producing a fiberboard from wood fibers and / or other lignocellulosic fibers using a wet process, in particular for carrying out the process according to any one of the preceding claims 1 to 5, comprising a long screen (11), a continuously operating hot press (18) and a squeezer (14) upstream of the hot press (18), which serves to mechanically dewater a fiber fleece cake (13) provided by the long screen (11) by forming a fiber fleece (16) with a residual moisture content of less than 20 wt.%. [7] Device according to claim 6, characterized by , that the squeezer (14) has at least one pair of rollers, wherein, with intermediate arrangement of the long screen (11), one of the two rollers (21) is arranged above the fiber nonwoven cake (13) provided by the long screen (11) and the other of the two rollers (22) is arranged below the long screen (11). [8] Device according to claim 7, characterized by, that a roller (21, 22) has a diameter of at least 280 mm. [9] Device according to any one of the preceding claims 6 to 8, characterized by , that the hot press (18) has two circumferentially formed steel bands (24, 25) each with a smooth working surface (26). [10] Device according to claim 9, characterized by , that the steel bands (24, 25) are arranged apart from each other with respect to their respective effective surface (26) forming a press gap (28) that tapers in the transport direction (27).

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