Method of processing of cellulose fibers by mechanical compression, particularly for paper pulp

Abstract

The present invention relates to a method for mechanically processing cellulose fibres in aqueous suspension by compression in a mixer (1) comprising a body (10) defining a mixing chamber (11), the wall of which forms a fixed compression surface (12), and at least one mixing arbor (20) positioned in the mixing chamber (11) and comprising a rotationally mobile shaft (21) and a plurality of mixing elements (22) fixed to the shaft (21), projecting radially from the shaft, and each equipped with a rotationally mobile compression surface (23). In said method, the compression surfaces (23) of the mixing elements (22) face the compression surface (12) of the mixing chamber (11) so as to define between them compression zones (ZC) for the suspension of fibres (2) circulating between the body (10) and the mixing arbor (20). The compression surfaces (23) of the mixing elements (22) have a compression profile of which the radius relative to the shaft (21) varies according to the angular position of said shaft, so as to compress the suspension of fibres (2) in said compression zones (ZC) as said mixing arbor (12) rotates.

Description

DOMAINE TECHNIQUE DE L'INVENTION

[0001] The present invention relates to a mechanical treatment process for cellulosic fibers in aqueous suspension, particularly to increase the binding potential of the fibers used in the composition of paper, the generic term "paper" covering paper, paperboard and cardboard. ETAT ANTERIEUR DE LA TECHNIQUE

[0002] Fiber refining is one of the most important processes in the pulp and paper industry. This operation is the key to adjusting not only the strength properties but also the quality of the paper, such as its mechanical, optical, and printability properties. Depending on the level of refining, the range of paper types varies from blotting paper (unrefined) to tracing papers requiring intensive refining. It also allows, when the pulp is enriched with mineral fillers, for an improvement in the retention rate of these fillers in the paper, and compensates for the reductions in mechanical properties caused by the fillers, such as tensile strength and / or tear resistance. Refining is therefore an essential operation.

[0003] Paper pulp, also called "pulp," is a suspension of cellulose fibers dispersed in water. It is generally introduced into the air gap of a rotating machine such as a refiner. The air gap is the space between two opposing grooved surfaces arranged on discs or cones in relative motion. The speed differential at the periphery between the rotor (rotating part) and the stator (stationary part) is generally on the order of 20 to 25 m / s, generating a shearing effect. When the paper pulp passes through the air gap, which is approximately 100 µm to 300 µm (compared to the fiber dimensions of approximately 1 to 3 mm in length and 15 to 40 µm in diameter), the fibers are subjected to a succession of compressive and shearing forces, which induce morphological changes in their ultrastructure.Three primary effects are generally associated with these morphological transformations: hydration, fibrillation, and cutting.

[0004] The compressive forces will compress the fibers, resulting in their hydration, delamination of their cell walls, softening them, and making them more flexible. This increased fiber flexibility significantly increases the bonding surface area between the fibers, which is beneficial for the subsequent formation of the paper sheet and its mechanical properties.

[0005] Shear forces generate surface fibrillation by tearing away fragments from the outer part of the fiber wall. They also cause fiber shearing and the removal of surface fibrils, which increases the fine particle content of the paper pulp.

[0006] However, the presence of fine cellulosic particles in paper pulp is detrimental to the subsequent paper sheet formation process. These particles clog the screens and impair the pulp's drainage, leading to reduced speeds in paper production machines, thus lowering productivity, and increasing steam consumption during drying, resulting in a significant increase in the energy required for paper drying. Furthermore, the fine particles recovered in process water, when not recycled, generate non-recoverable waste.

[0007] US documents 4,614,304, DE 102 56 856 and DE 102 36 962 describe methods of mechanical treatment of cellulosic fibers. EXPOSE DE L'INVENTION

[0008] The present invention aims to overcome these drawbacks (clogging of the screens, impaired drainage, increased energy consumption associated with vacuum generation, limited speed of production machines, reduced yield, increased steam consumption and therefore energy required for paper drying, non-recycled fine particles, and non-recoverable waste) by proposing an alternative fiber treatment process (advantageously for cellulosic fibers) that does not generate shearing, or at least so little as to be negligible, thus reducing or even eliminating the generation of fine particles and cut fibers. The invention also aims to provide such a process that is less energy-intensive, while simultaneously improving fiber properties and therefore the intrinsic properties of the paper, and potentially even leading to the development of new papers. The process according to the invention increases the fiber bonding potential.

[0009] More specifically, for a given mechanical resistance of a sheet of paper or cardboard, the present invention makes it possible to refine the cellulosic fiber, limiting, or even avoiding, the alteration of the drainability properties (°SR).

[0010] To this end, the invention relates to a method for the mechanical treatment of cellulosic fibers by mechanical compression of an aqueous suspension of cellulosic fibers comprising the following steps preparation of an aqueous suspension of cellulosic fibers having a concentration between 5% and 20% by weight of cellulosic fibers, mechanical compression of the aqueous suspension of cellulosic fibers in a mixer, the mixer comprising: a body defining a mixing chamber delimited by a wall forming a fixed compression surface suitable for contacting the fiber suspension, at least one mixing shaft positioned in the mixing chamber and comprising: a rotating movable shaft, a plurality of mixing elements (or paddles) fixed to the shaft, projecting radially from the shaft, and each provided with a rotating movable compression surface suitable for contacting the fiber suspension, a method in which the compression surfaces of the mixing elements face the compression surface of the mixing chamber, so as to define, between said respective compression surfaces of the mixing elements and the mixing chamber, compression zones of the fiber suspension circulating between the body and the mixing shaft,and wherein the compression surfaces of the mixing elements have a compression profile whose radius relative to the shaft varies according to the angular position of said shaft, so as to compress the fiber suspension in said compression zones during the rotation of said mixing shaft (in other words, the compression profile, at a defined point of the mixing chamber, varies during each complete rotation of the mixing element), the process having a velocity differential between at least one mixing shaft and the mixing chamber less than or equal to 15 m / s, the mixer having a minimum distance separating at least one mixing shaft from the mixing chamber of between 0.1 mm and 1 mm, the aqueous suspension of cellulosic fibers having a residence time in the mixer of between 15 seconds and 15 minutes.

[0011] According to a particular embodiment, the mixing elements are configured to provide an increasing compression profile in the mixer.

[0012] Advantageously, the mixer comprises at least two mixing axes positioned in the mixing chamber, parallel to each other, with the compression surfaces of the mixing elements of said mixing axes facing each other, so as to define between them additional compression zones of the fiber suspension circulating between the mixing axes.

[0013] The mixer can be easily integrated into a continuous process, for example between the pulper and the paper or board production machine, after a pulp thickening step (aqueous suspension of cellulose fibers). The presence of a thickening step is particularly well-suited to processes using recycled cellulose fiber pulp; in this case, a thickener can 1) separate the water from the pulp preparation circuit from the water from the paper machine and 2) feed the mixer.

[0014] Advantageously, the mixing chamber comprises at least an inlet zone, a mixing zone and an outlet zone, the fiber suspension being introduced into the inlet zone and being extracted from the outlet zone, each mixing shaft having at least one worm screw fixed to the shaft, disposed in the inlet zone and arranged to move the fiber suspension from the inlet zone to the outlet zone through the mixing zone, said plurality of mixing elements being disposed in the mixing zone and the outlet zone being or not integrated into the mixing zone.

[0015] Advantageously, the mixing zone comprises several successive mixing sectors. the mixing elements of each mixing axis being distributed in series, one series corresponding to a mixing sector, the mixing elements of the same series being identical, and the mixing elements from one series to another being different in their shape and / or their thickness and / or their radius and / or their compression profile (the compression profile including the orientation of the mixing element with respect to the axis).

[0016] Advantageously, the mixing elements consist of flat or complex parts, having a cross-section selected from the group comprising an ovoid cross-section with one lobe, an oblong cross-section with two lobes, a triangular cross-section with three lobes, a polygonal cross-section with a number of lobes corresponding to the number of angles, and a compression profile selected from the group comprising a straight (or flat) profile, a helical profile, a curved profile, or a serrated profile. The lobe(s) may have a flattened end.

[0017] According to a particular embodiment, the mixer comprises an alternation of mixing elements having a straight (or flat) compression profile and a helical profile.

[0018] Advantageously, the minimum distance between the mixing shaft(s) and the mixing chamber is between 0.1 mm and 0.6 mm, preferably between 0.15 mm and 0.6 mm, and in particular between 0.3 mm and 0.6 mm or between 0.3 mm and 0.5 mm. This minimum distance may also be between 0.15 mm and 0.4 mm. Beyond 1 mm, the mixer fails to make the cellulosic fibers more flexible, which is the desired effect for improving their properties.

[0019] Advantageously, the minimum distance separating two mixing axes is identical to the minimum distance separating the mixing axis(es) from the mixing enclosure.

[0020] Advantageously, when the mixer includes at least two mixing shafts positioned in the mixing chamber, the mixing shafts are co-rotating, in that they rotate in the same direction of rotation.

[0021] Advantageously, when the mixer includes at least two mixing shafts positioned in the mixing chamber, the mixing elements of two mixing shafts whose compression surfaces face each other have an identical compression profile.

[0022] Advantageously, the velocity differential between the periphery of the paddles (mixing elements) and the mixing chamber is less than or equal to 15 m / s. This differential can be at least 0.1 m / s, for example at least 2 m / s, particularly between 0.1 and 15 m / s. It can also be between 0.5 and 10 m / s, particularly between 0.5 and 5 m / s, advantageously between 0.5 and 1.5 m / s. Above 15 m / s, the cellulosic fibers undergo significant shearing and are therefore shortened.

[0023] Advantageously, the residence time of the fiber suspension in the mixer is between 15 seconds and 15 minutes, for example between 15 seconds and 10 minutes, preferably between 15 seconds and 6 minutes, preferably between 15 seconds and 5 minutes, and more between 15 seconds and 3 minutes.

[0024] Advantageously, the residence time of the fiber suspension in the mixer and the velocity differential between the mixing shaft(s) and the mixing chamber allow a number of compressions of at least 500, advantageously between 500 and 5000, more advantageously between 3000 and 5000.

[0025] Advantageously, the aqueous suspension of cellulosic fibers is prepared from a mixture of cellulosic fibers and water, and optionally mineral fillers, with the proportion of dry cellulosic fibers being between 5 and 20% by weight, advantageously between 5 and 15% by weight, and preferably between 8 and 15% by weight of the aqueous suspension. A concentration exceeding 20% ​​by weight, for example, between 25 and 45% by weight, damages the cellulosic fibers and does not improve their mechanical properties, such as flexibility, which increases the bonding surface area (binding effect). Indeed, above 20% by weight, cellulosic fibers tend to be shortened and to kink and curl.Below 5% by weight, the aqueous suspension of cellulosic fibers undergoes a phase separation phenomenon in the mixer, which prevents homogeneous treatment in particular.

[0026] Advantageously, the process according to the invention further comprises, prior to the mechanical compression of the aqueous suspension of cellulosic fibers, a step of thickening said aqueous suspension of cellulosic fibers to form a paste. When necessary, this step allows the concentration of the aqueous suspension of cellulosic fibers to be increased to between 5 and 20% by dry weight of cellulosic fibers.

[0027] In addition to the significant reduction of fine elements in the suspension during treatment, the treatment process of the invention has the advantage of minimizing waste, reducing production costs, improving the life cycle of paper, and entering a virtuous circle favorable to the environment.

[0028] Furthermore, because the fibers are compressed by kneading, rather than refined by shearing, the long softwood fibers are cut less frequently thanks to the process of the invention. Thus, to adjust tear resistance in particular, it becomes possible to reduce the proportion of softwood in the hardwood / softwood mixture used to form the paper sheet. This results in savings on the cost of raw materials.

[0029] Another advantage of the process of the invention is that it leads to an improvement in paper recycling, insofar as the latter contains fewer fine elements, generally eliminated by the recycling process (flotation, washing in particular), further promoting the life cycle of paper. FIGURES

[0030] The present invention and its advantages will become more apparent from the following description of several embodiments given by way of non-limiting examples, with reference to the figures. There figure 1 is a plan view representing a mixer inserted between a pulper and a papermaking machine. figure 2 is a radial section of the mixer of the figure 1 . There figure 3 is a perspective view of two mixing axes of the mixer of the figure 2 . There figure 4 is a perspective view of another form of realization of the mixing axes of the figure 3 . There figure 5 is a graph comparing the ratio of drainage to mechanical properties of the paper between the kneading process according to the invention and a conventional refining process. figure 6 is a graph comparing the permeability / tensile strength ratio of the paper between the kneading treatment process according to the invention and a conventional refining process. figure 7 This is a graph illustrating the influence, on tear resistance, of the concentration of the cellulose fiber suspension (eucalyptus) during mixing as a function of rotation speed and compression profile. figure 8 This is a graph illustrating the influence, on tensile strength, of the concentration of the cellulosic fiber suspension (eucalyptus) during mixing as a function of rotation speed and compression profile. figure 9 is a graph illustrating the influence, on drainability (°SR), of the concentration of the cellulose fiber suspension (eucalyptus) during mixing as a function of rotation speed and compression profile. figure 10 This is a graph illustrating the influence, on tear resistance, of the concentration of the cellulose (resinous) fiber suspension during mixing as a function of rotation speed and compression profile. figure 11 This is a graph illustrating the influence, on tensile strength, of the concentration of the cellulosic (resinous) fiber suspension during mixing as a function of rotation speed and compression profile. figure 12 is a graph illustrating the influence, on drainability (°SR), of the concentration of the cellulose fiber suspension (resinous) during mixing as a function of the rotation speed and the compression profile.

[0031] THE figures 1 à 6 They adopt the following nomenclature: 1: Mixer 2: Fiber suspension 3: Pulper 4: Papermaking machine 5: Sheet of paper 10: Body 11: Mixing chamber 12: Compression surface (fixed) 13: Inlet orifice 14: Outlet orifice 15 and 16: Inlet ports for injecting products, for example chemical reagents 20: Mixing shaft 20': Mixing shaft ( fig 4 ) 21: Tree 22: Mixing elements 22': Mixing elements ( fig 4 ) 23: Compression surface (mobile) 24: Lobes 24' Lobes ( fig 4 ) 25: Worm screw X: Rotation axis ZC: Compression zones DM: Minimum distance ZE: Inlet zone ZM: Mixing zone ZS: Outlet zone

[0032] Even if the figures 1 à 4 show two mixing axes, the description of these figures can be generalized to mixers having one or more than two mixing axes. DESCRIPTION DES MODES DE REALISATION

[0033] In the illustrated embodiments, identical elements or parts bear the same reference numbers. Furthermore, terms with a relative meaning, such as vertical, horizontal, right, left, front, back, above, below, etc., should be interpreted under normal conditions of use of the invention, as shown in the figures. The X, Y, and Z axes are defined by an orthonormal coordinate system illustrated in the figure. figure 1 Furthermore, the geometric positions indicated in the description and claims, such as "perpendicular," "parallel," and "symmetrical," are not limited to the strict geometric definition, but extend to geometric positions that are close to these terms, meaning they allow a certain tolerance within the technical field considered, without affecting the result obtained. This tolerance is notably introduced by the adverb "substantially," without this term necessarily being repeated before each adjective.

[0034] The mechanical treatment process according to the invention is applicable to any aqueous suspension of fibers, preferably cellulosic fibers for the paper industry, covering paper, paperboard, and cardboard. The suspension may comprise a mixture of cellulosic fibers and water, and optionally mineral fillers, depending on the specifications. The proportion of dry cellulosic fibers can be significant, for example, between 5 and 20% by weight, and preferably between 8 and 15% by weight of the aqueous suspension, without these values ​​being limiting.

[0035] As explained below, the process of the invention has the advantage of making cellulosic fibers more flexible by compression-kneading, that is, of softening them, while generating few or no fine particles or broken fibers. The resulting improvement in mechanical properties makes it possible to reduce the paper's basis weight according to the desired quality / price ratio.

[0036] Furthermore, the process of the invention also makes it possible to improve certain properties of cellulosic fibers. This advantage allows for the use of blends of fibers from, for example, hardwoods, softwoods, and other sources, and for varying the ratios from 0 to 100%, depending on the desired paper quality / price ratio. Since softwood fibers have a significantly higher production cost than hardwood fibers, they can thus represent a smaller proportion of the paper without compromising its properties, particularly its tear resistance.

[0037] Since the process according to the invention generates little or no shearing, it is possible to use fibers from annual plants, which are generally sensitive to shearing.

[0038] With reference to the figure 1 The mechanical treatment process according to the invention consists of a step of compressing the aqueous suspension of cellulosic fibers in a mixer, instead of the conventional compression and shearing step between two rotating blade surfaces.

[0039] The mixer used in this process is known in fields quite different from papermaking. It is primarily used to homogenize viscous materials, such as polymers, compounds, composites, or similar substances, used in cosmetics, adhesives, plastics processing, chemicals, etc. It may, in particular, be a UCP-type mixer from the HASLER company.

[0040] In the present invention, the mixer 1 as shown in the figure 1 is fed by an aqueous suspension of cellulosic fibers 2 from a pulper 3 or any other equivalent machine. Its output feeds a papermaking machine 4 to form a sheet or a strip of paper 5. The mixer 1 can be easily integrated into a continuous process, between the pulper and the machine after a pulp thickening step, allowing for increased productivity and reduced production costs.

[0041] Mixer 1 mainly comprises: a hermetically sealed body 10, elongated along an axis X, defining a mixing chamber 11, delimited by a peripheral wall forming a fixed compression surface 12, said body 10 being provided with at least one inlet orifice 13 upstream, for the admission of the fiber suspension to be treated, and an outlet orifice 14 downstream of the mixing chamber 11, for the reception of at least a fraction of the treated suspension, and at least one mixing axis 20, elongated along the axis X, positioned in the mixing chamber 10.

[0042] The mixing shaft 20 includes: A shaft 21, rotatable about said axis X, and a plurality of mixing elements 22, also called "paddles," fixed to the shaft 21, projecting radially from the shaft, each delimited by a peripheral wall forming a compression surface 23 that rotates rigidly with the shaft. The compression surface 23 is located at a non-constant radius from the axis of rotation X, which varies between a maximum and a minimum radius depending on the point considered on said wall. As will be described in more detail later in this text, the irregular profile of the compression surface of the mixing elements relative to the axis of the shaft ensures, in cooperation with another corresponding compression surface, optimal compression of the fibrous suspension, thus leading to satisfactory mixing.

[0043] The mixing elements 22 arranged along the shaft 21 are preferably alternated (as opposed to eclipsed, along the X axis) in that they are oriented relative to each other in different angular positions to create a variable compression profile along the mixing axis 20.

[0044] According to said method, the compression surfaces 23 of the mixing elements 22 facing the compression surface 12 of the mixing chamber 11 on the one hand, and said compression surface 12 of the mixing chamber 11 on the other hand, define between them compression zones ZC of the fiber suspension 2 circulating between the body 10 and the mixing axis 20. The compression zones ZC thus correspond to the remaining volume between the mixing axis 20 and the mixing chamber 11, which contains the fiber suspension 2. They are very roughly identified by ovals. figure 2 Thanks to the variable compression profile of the mixing elements 22, the shape, location and volume of the compression zones ZC are not constant and vary continuously with the angular position of the shaft 21, resulting in the generation of a large number of compression forces on the fiber suspension 2 during the rotation of the mixing shaft 20 during the mixing operation. Thus, the cellulosic fibers are kneaded, crushed, compressed, compressed, mixed, kneaded, but are not or very little cut or sheared because they do not undergo or very little shearing stress (the rotation speed of the paddles is low, typically in the order of 20 to 600, in particular between 20 and 350 revolutions per minute for example - rotation per minute or the acronym "rpm" in English - for a paddle diameter of about 25 mm, and therefore the shear at the wall is low, i.e. < 1 m / s for a laboratory mixer.For an industrial mixer, the diameter of the paddles can, for example, be approximately 300 mm, with a wall shear rate of about 6 m / s, which remains low compared to the size of the mixer and the quantity of suspension processed (approximately 2 tonnes per hour). In other words, unlike conventional refining, the process of the invention induces negligible fiber shear compared to their compression.

[0045] The mixer 1 may have more than one mixing shaft 20, and for example two or more mixing shafts 20, positioned in the mixing chamber 11, parallel to each other. Thus, the compression surfaces 23 of the facing mixing elements 22 define additional compression zones ZC for the suspension of fibers 2 circulating between the mixing shafts 20. In this case, mixing shafts 20 of identical or nearly identical structure are preferably chosen, positioned so that the compression profile of one of the mixing shafts matches the compression profile of the other mixing shaft, and rotating at the same speed. In this preferred configuration, it is advantageous to ensure that two facing mixing elements 22 have the same shape. The mixing shafts 20 can be co-rotating and thus rotate in the same direction of rotation, as shown in the figure 2 by the arrows in the counter-clockwise direction, or counter-rotating and thus turning in opposite directions of rotation.

[0046] There figure 2 This illustrates a radial section of a mixer 1 comprising two parallel mixing shafts 20 within a mixing chamber 11. Each mixing shaft 20 comprises oblong mixing elements 22. The mixing shafts 20 are angularly offset from each other, such that the two corresponding mixing elements 22 are positioned perpendicular to each other, although this relative position is not mandatory or identical for the other mixing elements 22 not shown. This means, for example, that for a given position of the mixing shafts, a first pair of facing mixing elements may be offset by 90° from each other, while a second pair of facing mixing elements may be offset by 45° from each other. The mixing shafts 20 rotate in the same direction of rotation and at the same speed.The center distance between the two shafts 21 of the mixing axes 20, as well as the profile of the compression surface 12 of the mixing chamber 11 are defined according to the geometry of the mixing elements 22, to introduce a respective minimum distance DM between the mixing axes 20 and the mixing chamber 11, and between the mixing axes 20 themselves.

[0047] The minimum distance DM, which is represented at the figure 2 The minimum distance DM, defined by circles, is preferably between 0.1 mm and 0.6 mm, preferably between 0.15 mm and 0.6 mm, and in particular between 0.3 mm and 0.6 mm or between 0.3 mm and 0.5 mm. This minimum distance may also be between 0.15 mm and 0.4 mm. This minimum distance DM allows, in particular, the rotation of the mixing shafts 20 within the mixing chamber 11 without conflict, while permitting, in a very limited way, the movement of the fiber suspension 2 within these restricted spaces. The boundary zones located between the mixing shafts 20 on the one hand, and between each mixing shaft 20 and the mixing chamber 11 on the other hand, which are defined by the minimum distance DM, therefore correspond to areas of high mechanical stress for the fibrous composition.During the rotation of the mixing axes 20, these boundary zones move, so as to alternately constrain the composition in the volume of the enclosure, which ensures optimal compression of the fibrous composition in the compression zones ZC.

[0048] The mixing elements 22 can have different geometries, thicknesses, and compression profiles depending on the type of treatment and the degree of flexibility to be achieved, the nature and composition of the fiber suspension to be treated, and the paper manufacturer's specifications. The mixing elements 22 illustrated in the figure 2 are flat pieces, oblong in shape, symmetrical with respect to two perpendicular planes passing through the axis of rotation X, and have two diametrically opposed lobes 24.

[0049] This example is by no means exhaustive. The mixing elements 22 can have an ovoid shape, symmetrical with respect to a plane passing through the axis of rotation X and comprising a single lobe (not shown). They can have a triangular shape, symmetrical with respect to a plane passing through the axis of rotation X and one of the vertices, and comprising three lobes (shown in the figure). figure 4 They can also have a polygonal cross-section, symmetrical or not with respect to a plane passing through the rotation axis X, and whose number of lobes corresponds to the number of angles of the polygon (not shown). Finally, they can have a complex shape, symmetrical or not with respect to a plane passing through the rotation axis X (not shown).

[0050] Similarly, the mixing elements 22 may include parts whose peripheral wall, i.e., the compression surface, is flat or not, and whose compression profile through the thickness of the part is variable, such as a straight (or flat) profile, a helical profile, a curved profile, a serrated profile, or a complex profile. For example, a succession along the shaft of parts whose thickness has a helical profile leads to a helical mixing shaft forming a complete helix, each part constituting a fraction of the helix.

[0051] There figure 3 illustrates two parallel, identical mixing axes 20, offset from each other by an angle of 90°, each equipped with flat, oblong, two-lobed mixing elements 22 24, with a straight compression profile through the thickness, substantially identical to those shown in the figure 2 The mixing elements 22 of the same axis are angularly offset from each other by 90°, and the mixing elements 22 of the two mixing axes 20 are axially alternated to allow their radial interlocking.

[0052] There figure 4 This illustrates two parallel, identical mixing axes 20', without angular offset, each equipped with flat, triangular mixing elements 22' with three equidistant lobes 24', and a straight compression profile through the thickness. The mixing elements 22' on the same axis are angularly offset from each other by 30°, and the mixing elements 22' of the two mixing axes 20' are radially interlocked.

[0053] The mechanical processing method according to the invention offers a flexible, modular, adaptable, and scalable compression mixing solution, allowing for the variation, modification, and refinement of fiber properties depending on the paper to be manufactured. This method also supports research and development of new papers. To this end, the structure of the mixer 1 and its operating parameters can be easily selected, modified, and / or combined.

[0054] As seen above, the structure of the mixer 1 is determined by the number of mixing shafts 20, as well as by the arrangement and geometry of the mixing elements 22, but not only that. The mechanical processing method can also involve several steps to achieve different degrees of flexibility. These different steps can be carried out in various ways: by a mixer 1 comprising a mixing chamber 11 and at least one mixing shaft 20 on which are mounted one or more series of mixing elements 22 of different and suitable structures, in which the fiber suspension 2 is rotated in a loop; by an extended mixer 1 comprising a mixing chamber 11 and at least one mixing shaft 20 on which are mounted several series of mixing elements 22 of different and suitable structures; by a mixer (not shown) comprising several successive mixing chambers, connected in series, each comprising one or more mixing shafts of different and suitable structures; or by several successive mixers (not shown), connected in series, of different and suitable structures.

[0055] In these different scenarios, the treated fiber suspension 2 can be extracted at each stage of the process and / or at the end of the process stages as needed.

[0056] The operating parameters of mixer 1 can also be selected, modified, and / or combined to vary the mixing characteristics. Examples of modifiable parameters include, but are not limited to, the following: the feed rate of the mixer 1 in suspension of fibers 2, defining the mixing time;Mixing temperature: The mixer operates at ambient temperature, but can also operate at a temperature higher or lower than ambient temperature thanks to the mixer body 10, which can consist of a double jacket (not shown) allowing the circulation of a heat transfer fluid. More specifically, the mixer is kept at ambient temperature by cooling to compensate for the temperature rise due to mixing, in order to better control the viscosity of the paste. In practice, a temperature rise of approximately +10°C to +30°C is observed, depending on the operating concentration, the rotation speed, and the residence time. Alternatively, it is possible to operate at high temperature, particularly to carry out oxidation reactions in conjunction with mixing (process intensification), or at low temperature, for example in the presence of sodium hydroxide to dissolve the cellulose.Mixing pressure: preferably, the mixing operation is carried out in a mixing chamber 11 at atmospheric pressure, but depending on the requirements, it could be carried out at a pressure higher or lower than atmospheric pressure. For example, it may be advantageous to work at a pressure higher than atmospheric pressure to adjust the filling speed of the feed screw (for example, of the auger type as described later in this text); the rotational speed of the mixing shaft(s) 20: this speed is preferably relatively low, for example, between 20 and 600 rpm, preferably between 20 and 400 rpm, or between 20 and 350 rpm; the speed differential between the mixing shaft(s) 20 (rotating part) and the mixing chamber 11 (fixed part) is less than or equal to 15 m / s, thus producing little or no shearing.the direction of rotation of the mixing axes 20: co-rotating or counter-rotating; the mixing power transmitted to the fiber suspension 2 by measuring the resisting torque of the mixing axes 20; the mixing time: the residence time of the fiber suspension 2 in the mixing chamber 11 can be between 15 seconds and 15 minutes, preferably between 15 seconds and 10 minutes, and more preferably between 15 seconds and 5 minutes; the minimum distance DM between the mixing axes 20 and the mixing chamber 11 by modifying the mixing elements 22; the thickness of the mixing elements 22 to increase or decrease the compression surfaces.

[0057] The mixing tests carried out with the process of the invention revealed an unexpected and counterintuitive physical phenomenon for those skilled in the art of papermaking: the higher the rotational speed of the mixing shafts 20 (20 to 600 rpm), the shorter the duration of the mechanical compression action on the cellulosic fibers, and the better the mixing result. A high speed limits the shearing and fibrillation of the cellulosic fibers, and therefore the formation of fine particles. These conditions favor short compressions, but a high number of compressions, advantageously at least 500, more advantageously between 500 and 5000, and even more advantageously between 3000 and 5000.

[0058] In summary, the rotational speed of the mixing shafts (20 to 600 rpm) and the mixing conditions (speed differential ≤ 20 m / s and residence time between 15 seconds and 15 minutes) allow for short, frequent mechanical compression, which limits the damage to the cellulosic fibers. Thus, the cellulosic fibers are less damaged, or even preserved, compared to long and frequent or infrequent mechanical compressions.

[0059] This phenomenon, which is neither detectable nor conceivable before implementing the process of the invention, makes it possible to further increase the performance of the mixing and the advantages of the process that result from it, such as better fiber flexibility, improved fiber content, new possible paper pulp formulations, reduced power consumption allowing significant energy savings, very little wear of moving parts allowing reduced maintenance of the mixer 1.

[0060] In the example of mixer 1 shown in the figure 1 The mixing chamber 11 comprises an inlet zone ZE, a mixing zone ZM, and an outlet zone ZS, which are axially arranged in succession. The inlet zone ZE is coupled to the inlet port 13 and preferably includes a screw 25 fixed to the corresponding shaft 21 of each mixing axis 20, for moving the fiber suspension 2 axially towards the mixing zone ZM and then towards the outlet zone ZS. The mixing zone ZM includes the mixing elements 20 described previously. The outlet zone ZS may coincide with the end of the mixing zone ZM and also includes mixing elements 20, which may or may not be different from those of the mixing zone ZM. The outlet zone ZS communicates with the outlet port 14, which may be gravity-fed or combined with any other extraction method.The mixer 1 may have several inlet ports 15, 16 that can be used to introduce additives, fillers, and other materials into the fiber suspension 2 during mixing. It may also have several outlet ports (not shown) that can be used to extract all or part of the fiber suspension 2 during mixing, as well as gases and other condensates generated during processing. This is particularly advantageous because the operator can recover several fractions of mixed fibers with different degrees of compression and therefore different properties, depending on their respective outlet port, during the same mixing operation.

[0061] THE figures 5 And 6These are graphs from tests carried out on paper made from cellulosic fibers treated according to the compression kneading process of the invention (points on the graphs) and according to a conventional refining process by compression and shear (curve on the graphs). They illustrate the improvement in the mechanical properties and drainability of the paper pulp obtained with the process of the invention.

[0062] The present invention is of course not limited to the examples of embodiment described but extends to any modification and variant obvious to a person skilled in the art within the limits of the annexed claims. EXEMPLES

[0063] There figure 5 The graph represents the evolution of the draining index (°SR) on the y-axis as a function of the burst index (or "burst index") on the x-axis. The draining index, or Schopper-Riegler index, is defined according to ISO 5267-1 as the number of centiliters of water that drain through a cake of dough flowing from the overflow of a tank. It represents a measure of the rate at which water can be extracted from a dilute dough suspension.

[0064] The bursting index, on the other hand, corresponds to the mechanical resistance of the fibers. Simply put, the higher this index, the stronger and better quality the paper.

[0065] It is observed that with a suspension of fibers (eucalyptus fibers at 5% by weight) refined by a standard refining process (disc refiner at 25 m / s), the drainage speed reaches a maximum for a bursting index close to 4, which is relatively average.

[0066] In contrast, a fibrous suspension (eucalyptus fibers at 8-15% by weight in water + compression with a velocity differential of 0.5 to 1 m / s) treated by kneading according to the process of the invention makes it possible to achieve high drainage velocities with a high bursting index. This results in a good quality paper whose production rate is improved through the high drainage speed of the fibrous suspension from which it is derived.

[0067] This leads to a very strong improvement in the compromise between drainage and the mechanical properties of the fibers, resulting in a reduction of energy consumption at the paper machine level.

[0068] There figure 5 shows the improvement in the drainage / mechanical properties ratio of the paper when the cellulosic fibers are processed in a mixer according to the invention compared to conventional refining in a disc refiner (points connected by the dashed curve on the figure 5 ). The points not connected by a line correspond to the treatment according to the invention, at different speeds and concentrations of cellulosic fibers (8 to 15% by weight).

[0069] On the figure 5 The points connected by the discontinuous curve correspond to: no refining (bursting index = 1 kPa.m 2< / g) 50 kW.h per tonne of cellulosic fibres (bursting index close to 2.1 kPa.m 2< / g), 100 kW.h per tonne of cellulosic fibres (bursting index close to 3 kPa.m 2< / g), 200 kW.h per tonne of cellulosic fibres (bursting index close to 4 kPa.m 2< / g), 400 kW.h per tonne of cellulosic fibres (bursting index close to 4.1 kPa.m 2< / g).

[0070] More specifically, at equivalent drainability, treatment in a mixer makes it possible to increase the burst index, for example to triple or almost quadruple it for a draining of 20°SR.

[0071] Generally speaking, the lowest possible drainage index is desired. figure 5 shows that values ​​greater than 65° SR are achieved for conventional refining (200 and 400 kW.h per tonne of cellulosic fibers) whereas it is systematically less than 52° SR for the treatment according to the invention.

[0072] There figure 6 represents the evolution of the paper's air permeability as a function of its tensile strength index. The air permeability of paper is defined by the ISO 5636 series of standards as the average air flow rate through a unit area under a unit pressure difference in a unit time, under specified conditions.

[0073] It is observed that with a fiber suspension (5% by weight softwood fibers) refined by a standard refining process (disc refiner at 25 m / s), air permeability drops with tensile strength, even at 35 Nm / g. In contrast, a fibrous suspension (8-15% by weight eucalyptus fibers in water + compression with a velocity differential of 0.5 to 1 m / s) treated by kneading according to the process of the invention allows the decrease in air permeability to occur at higher tensile strength values. This makes it possible to obtain a paper with good mechanical strength while maintaining relatively high air permeability.

[0074] This leads to a significant improvement in the permeability / tensile strength trade-off, making it possible to consider the development of new papers.

[0075] There figure 6 shows the improvement in the ratio between the air permeability and the tensile strength of the paper when the cellulosic fibers are processed in a mixer according to the invention compared to conventional refining in a disc refiner (points connected by the discontinuous curve on the figure 6 ). The points not connected by a line (above 28 Nm / g) correspond to the treatment according to the invention, at different speeds and concentrations of cellulosic fibers (8 to 15% by weight).

[0076] On the figure 6 The points connected by the discontinuous curve correspond to: no refining (tensile strength index close to 28 Nm / g) 50 kWh per tonne of cellulosic fibers (tensile strength index close to 33 Nm / g), 100 kWh per tonne of cellulosic fibers (tensile strength index close to 42 Nm / g), 200 kWh per tonne of cellulosic fibers (tensile strength index close to 55 Nm / g), 400 kWh per tonne of cellulosic fibers (tensile strength index close to 62 Nm / g).

[0077] More specifically, at equivalent permeability, treatment in a mixer increases the tensile strength index, for example a gain of 50% (+20 Nm / g) for an air permeability of approximately 1800 mL / min.

[0078] Different suspensions of cellulosic fibers (suspension at 8% by weight) were processed in a mixer under the following conditions, with a velocity differential between the mixing axis and the mixing chamber of between 0.5 and 1 m / s ( figures 7-12 ) : mixing shaft rotation speed: 50 to 350 rpm (or revolutions per minute) mixing profile "std": alternating helical bilobed paddles and flat bilobed paddles at 45° to each other, mixing profile "ls": helical bilobed paddles at 45° to each other, mixing profile "hs": flat bilobed paddles at 90° to each other.

[0079] Eucalyptus fiber suspensions ( figures 7-9 ) or coniferous ( figures 10-12 ) were used.

[0080] THE figures 7 à 12 show that the best compromise between tear resistance / tensile strength / drainage is obtained for a cellulosic fiber concentration between 5 and 20% by weight in water. Drainage values ​​are consistently below 60° SR ( figures 9 And 12 ) whereas they can reach over 85° SR in the case of conventional refining ( figure 5 ).

Claims

1. A method for mechanical processing cellulose fibres by mechanical compression of an aqueous suspension of cellulose fibres, comprising the following steps: - preparation of an aqueous suspension of cellulosic fibres (2) having a concentration of between 5 and 20% by weight of cellulosic fibres, - mechanical compression of the aqueous suspension of cellulose fibres (2) in a kneader (1), the kneader (1) comprising: - a body (10) defining a kneading chamber (11) delimited by a wall forming a fixed compression surface (12) capable of coming into contact with the fibre suspension (2), - at least one kneading axis (20) positioned in the kneading chamber (11) and comprising: • a rotatable shaft (21), • a plurality of kneading elements (22) fixed to the shaft (21), projecting radially from the shaft (21), and each provided with a rotationally mobile compression surface (23) capable of coming into contact with the fibre suspension (2), wherein the compression surfaces (23) of the kneading elements (22) face the compression surface (12) of the kneading chamber (11), so as to define between said respective compression surfaces (23, 12) of the kneading elements (22) and of the kneading chamber (11) compression zones (ZC) of the fibre suspension (2) flowing between the body (10) and the kneading axis (20), and wherein the compression surfaces (23) of the kneading elements (22) have a compression profile the radius of which with respect to the shaft (21) varies according to the angular position of said shaft, so as to compress the fibre suspension (2) in said compression zones (ZC) during the rotation of said kneading axis (20), - the method having a speed differential between the at least one kneading axis (20) and the kneading chamber (11) of less than or equal to 15 m / s, - the kneader having a minimum distance (DM) separating the at least one kneading axis (20) from the kneading chamber (11) of between 0.1 mm and 1 mm, - the aqueous suspension of cellulosic fibres has a residence time in the kneader of between 15 seconds and 15 minutes.

2. Processing method according to claim 1, wherein the kneader (1) comprises at least two kneading axes (20) positioned in the kneading chamber (11), parallel to each other, and wherein the compression surfaces (23) of the kneading elements (22) of said kneading axes (20) face each other, so as to define between them additional compression zones (ZC) of the fibre suspension (2) flowing between the kneading axes (20).

3. Processing method according to any one of claims 1 and 2, wherein the kneading chamber (11) comprises at least one inlet zone (ZE), one kneading zone (ZM) and one outlet zone (ZS), wherein the fibre suspension (2) is introduced into the inlet zone (ZE) and is extracted from the outlet zone (ZS), and wherein each kneading axis (20) comprises at least one worm (25) fixed to the shaft (21), arranged in the inlet zone (ZE) and arranged to move the fibre suspension (2) from the inlet zone (ZE) to the outlet zone (ZS) through the kneading zone (ZM), said plurality of kneading elements (22) being disposed in the kneading zone (ZM) and the outlet zone (ZS) being integrated or not into the kneading zone (ZM).

4. Processing method according to claim 3, wherein the kneading zone (ZM) comprises a plurality of successive kneading sectors, wherein the kneading elements (22, 22') of each kneading axis (20, 20') are distributed in series, a series corresponding to a kneading sector, and wherein the kneading elements (22, 22') of a same series are identical, and the kneading elements (22, 22') from one series to another are different in shape and / or thickness and / or compression profile.

5. Processing method according to any one of claims 1 to 4, wherein the kneading elements (22, 22') consist of flat or complex parts, which have a section selected from the group comprising an ovoid section comprising a lobe, an oblong section comprising two lobes (24), a triangular section comprising three lobes (24'), a polygonal section comprising a number of lobes corresponding to the number of angles, and a compression profile selected from the group comprising a straight profile, a helical profile, a curved profile, a notched profile.

6. Processing method according to any one of claims 3 to 5, wherein the body (10) of the kneader (1) comprises several kneading chambers (11), connected in series, each comprising at least one kneading axis (20) of different structure from one chamber to another to generate a different mechanical processing.

7. Processing method according to any one of claims 1 to 6, wherein the minimum distance (DM) separating the kneading axes (20) from the kneading chamber (11), and the minimum distance (DM) separating the kneading axes (20) from each other when there are at least two of them, is of between 0.15 mm and 0.4 mm.

8. Processing method according to any one of the preceding claims in combination with claim 2, wherein the kneading axes (20) are co-rotating, in that they rotate in the same direction of rotation.

9. Processing method according to any one of the preceding claims in combination with claim 2, wherein the kneading elements (22, 22') of two kneading axes (20) the compression surfaces (23) of which face each other have an identical compression profile.

10. Processing method according to any one of claims 1 to 9, wherein the method has a speed differential between the kneading axis or the kneading axes (20) and the kneading chamber (11) of at least 0.1 m / s and less than or equal to 15 m / s, advantageously of between 0.5 m / s and 10 m / s.

11. Processing method according to any one of claims 1 to 10, wherein the residence time of the fibre suspension (2) in the kneader (1) is of between 15 seconds and 10 minutes.

12. Processing method according to any one of claims 1 to 11, wherein the aqueous suspension of cellulosic fibres (2) is prepared from a mixture of cellulosic fibres and water, and optionally mineral fillers, the proportion of dry cellulosic fibres being of between 8 and 15% by weight of the aqueous suspension.

13. Processing method according to any one of claims 1 to 12, wherein the aqueous suspension of cellulose fibres undergoes a number of compressions in the kneader of at least 500, advantageously from 500 to 5000.

14. Processing method according to any one of claims 1 to 13, wherein the aqueous suspension of cellulosic fibres comprises mineral fillers.

15. Processing method according to any one of claims 1 to 14, wherein the at least one kneading axis (20) has a rotational speed of between 20 and 600 rpm, the kneader comprising an alternation of kneading elements having a straight compression profile and a helical profile.

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