Method for processing cellulose fibres by mechanical compression, notably 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

[0001] PROCESS FOR TREATING CELLULOSIC FIBERS BY MECHANICAL COMPRESSION, PARTICULARLY FOR PAPER PULP

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a method for mechanical treatment of cellulose fibers in aqueous suspension, in particular to increase the binding potential of the fibers used in the composition of paper, the generic term "paper" covering paper, paperboard and cardboard.

[0004] PRIOR STATE OF THE ART

[0005] Fiber refining is one of the most important processes in the pulp industry. This operation is the key process for adjusting the strength properties but also the quality of the paper, such as the mechanical, optical, and printability properties of the paper. Depending on the level of refining, the range of paper types varies from blotting paper (without refining) to tracing papers requiring intense refining. It also allows, when the paper pulp is enriched with mineral fillers, to improve the retention rate of these fillers in the paper, and to compensate for the drops in mechanical properties of the paper induced by the fillers, such as its tensile strength and / or tear strength. Refining is therefore an essential operation.

[0006] 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 refiner-type machine. The air gap is the space between two facing 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 of the order of 20 to 25 m / s, and generates a shearing phenomenon. When the paper pulp passes through the air gap, which is approximately 100 µm to 300 µm (compared to the dimensions of the fibers of the order of 1 to 3 mm by 15 to 40 µm in diameter), the fibers are subjected to a succession of compression and shearing forces, which induce morphological changes in their ultrastructure.Three primary effects are generally associated with these morphological transformations: hydration, fibrillation, and cutting.

[0007] The compressive forces will compress the fibers, having the technical effect of hydrating the fibers, delaminating their walls, softening them and making them more flexible. Increasing the flexibility of the fibers has the advantage of significantly increasing the bonding surface between the fibers, which is subsequently favorable for the formation of the paper sheet and its mechanical properties.

[0008] Shear forces will generate surface fibrillation by tearing off pieces on the outer part of the fiber wall. They will also generate the cutting of the fibers, but also the tearing of the surface fibrils, which will increase the fine element content of the paper pulp.

[0009] However, the presence of fine cellulosic elements in the paper pulp is detrimental to the subsequent paper sheet formation process. They will cause clogging of the grids and alter the "drainability" of the paper pulp, leading to a limitation of the speed of the paper production machines, therefore a reduction in productivity, and an increase in steam consumption during drying, therefore a significant increase in the energy consumption required for drying the paper. In addition, the fine elements, recovered in the process water, when not recycled, generate non-recoverable waste.

[0010] US 4,614,304, DE 102 56 856 and DE 102 36 962 describe methods for mechanical treatment of cellulosic fibers.

[0011] STATEMENT OF THE INVENTION

[0012] The present invention aims to overcome these drawbacks (clogging of the fabrics, alteration of drainability, increase in the energy associated with the generation of vacuum, limitation of the speed of the production machines, reduction in yield, increase in the consumption of steam and therefore of energy necessary for drying the paper, fine elements not recycled, non-recoverable waste) by proposing an alternative method for treating fibers (advantageously cellulosic), which does not generate shear, or at least so little that it becomes negligible, therefore reduces or even eliminates the generation of fine elements and cut fibers. The invention also aims to provide such a method which is less energy-intensive, while making it possible to improve the properties of the fibers and therefore intrinsically the properties of the paper, or even to develop new papers. The method according to the invention makes it possible to increase the binding potential of the fibers.

[0013] More precisely, for a given mechanical resistance of a sheet of paper or cardboard, the present invention makes it possible to refine the cellulose fiber, while limiting, or even avoiding, the alteration of the drainability properties (°SR).

[0014] For this purpose, the invention relates to a method for mechanically treating 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 of 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 capable of coming into contact with the fiber suspension, at least one mixing axis positioned in the mixing chamber and comprising: a rotating shaft, a plurality of mixing elements (or paddles) fixed to the shaft, projecting radially from the shaft, and each provided with a rotating compression surface capable of coming into contact with the fiber suspension,method in which the compression surfaces of the mixing elements face the compression surface of the mixing enclosure, so as to define between said respective compression surfaces of the mixing elements and of the mixing enclosure compression zones of the suspension of fibers circulating between the body and the mixing axis, and in which 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 suspension of fibers in said compression zones during the rotation of said mixing axis (in other words, the compression profile, at a defined point of the mixing enclosure, varies during each complete rotation of the mixing element), the method having a speed differential between the at least one mixing axis and the mixing enclosure less than or equal to 15 m / s,the mixer having a minimum distance separating the at least one mixing axis from the mixing enclosure of between 0.1 mm and 1 mm, the aqueous suspension of cellulose fibers has a residence time in the mixer of between 15 seconds and 15 minutes.,

[0015] According to a particular embodiment, the mixing elements are configured so as to provide an increasing compression profile in the mixer.

[0016] Advantageously, the mixer comprises at least two mixing axes positioned in the mixing enclosure, parallel to each other, 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 suspension of fibers circulating between the mixing axes.

[0017] The mixer can be easily integrated into a continuous process, for example between the pulper and the paper or cardboard production machine, after a thickening step (or thickening) of the pulp (aqueous suspension of cellulose fibers). The presence of a thickening step is particularly suitable for processes using recycled cellulose fiber pulp; in this case, a thickener can 1) separate the water from the paper pulp preparation circuit from the water from the paper machine and 2) feed the mixer.Advantageously, the mixing enclosure comprises at least one 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 axis comprising at least one endless screw fixed on the shaft, arranged 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 arranged in the mixing zone and the outlet zone being or not integrated into the mixing zone.

[0018] Advantageously, the mixing zone comprises several successive mixing sectors, the mixing elements of each mixing axis being distributed in series, one series corresponding to one 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 relative to the axis).

[0019] Advantageously, the mixing elements consist of flat or complex parts, which have a section chosen from the group comprising an ovoid section comprising one lobe, an oblong section comprising two lobes, a triangular section comprising three lobes, a polygonal section comprising a number of lobes corresponding to the number of angles, and a compression profile chosen from the group comprising a straight (or flat) profile, a helical profile, a curved profile, a notched profile. The lobe(s) may have a flattened end.

[0020] According to a particular embodiment, the mixer comprises an alternation of mixing elements having a straight (or flat) compression profile and a helical profile. Advantageously, the minimum distance which separates the mixing axis(es) from the mixing enclosure is between 0.1 mm and 0.6 mm, preferably between 0.15 mm and 0.6 mm, in particular between 0.3 mm and 0.6 mm or between 0.3 mm and 0.5 mm. This minimum distance may in particular be between 0.15 mm and 0.4 mm. Beyond 1 mm, the mixer does not allow the cellulose fibers to be made flexible, whereas this is the desired effect in order to improve the properties of the cellulose fibers.

[0021] Advantageously, the minimum distance separating two mixing axes is identical to the minimum distance separating the mixing axis(es) from the mixing enclosure.

[0022] Advantageously, when the mixer comprises at least two mixing axes positioned in the mixing enclosure, the mixing axes are co-rotating, in that they rotate in the same direction of rotation.

[0023] Advantageously, when the mixer comprises at least two mixing shafts positioned in the mixing enclosure, the mixing elements of two mixing shafts whose compression surfaces face each other have an identical compression profile.

[0024] Advantageously, the speed differential between the periphery of the paddles (mixing elements) and the mixing enclosure is less than or equal to 15 m / s. This differential may be at least 0.1 m / s, for example at least 2 m / s, in particular between 0.1 and 15 m / s. It may in particular be between 0.5 and 10 m / s, in particular between 0.5 and 5 m / s, advantageously between 0.5 and 1.5 m / s. Beyond 15 m / s, the cellulose fibers undergo significant shear and are therefore shortened.

[0025] 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 preferably between 15 seconds and 3 minutes. Advantageously, the residence time of the fiber suspension in the mixer and the speed differential between the mixing axis(es) and the mixing enclosure allow a number of compressions of at least 500, advantageously between 500 and 5000, more advantageously between 3000 and 5000.

[0026] Advantageously, the aqueous suspension of cellulosic fibers is prepared from a mixture of cellulosic fibers and water, and optionally mineral fillers, the proportion of dry cellulosic fibers being between 5 and 20% by weight, advantageously between 5 and 15% by weight, preferably between 8 and 15% by weight of the aqueous suspension. A concentration greater than 20% by weight, for example between 25 and 45% by weight, damages the cellulosic fibers and does not improve the mechanical properties, for example the flexibility allowing the bonding surface (binding effect) to be increased, of the cellulosic fibers. Indeed, above 20% by weight, the cellulosic fibers tend to be shortened and to be "kinked" and curled.Below 5% by weight, the aqueous suspension of cellulose fibers undergoes a phase separation phenomenon in the mixer, which prevents homogeneous treatment in particular.

[0027] Advantageously, the method according to the invention further comprises, prior to the mechanical compression of the aqueous suspension of cellulose fibers, a step of thickening said aqueous suspension of cellulose fibers to form a paste. When necessary, this step makes it possible to increase the concentration of the aqueous suspension of cellulose fibers, between 5 and 20% by dry weight of cellulose fibers.

[0028] In addition to the significant reduction of fine elements in the suspension during treatment, the treatment method of the invention has the advantage of minimizing waste, reducing production costs, improving the life cycle of the paper, and entering into a virtuous circle favorable to the environment. Furthermore, since the fibers are compressed by kneading, and not refined by shearing, the long softwood fibers are cut less thanks to the method of the invention. Thus, to adjust the tear resistance in particular, it becomes possible to reduce the proportion of softwood in the hardwood / softwood mixture intended to form the sheet of paper. 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 the recycling of paper, insofar as the latter contains fewer fine elements, generally eliminated by the recycling process (flotation, washing in particular), further promoting the life cycle of the paper.

[0030] FIGURES

[0031] The present invention and its advantages will appear better in the following description of several embodiments given as non-limiting examples, with reference to the figures.

[0032] Figure 1 is a plan view showing a mixer interposed between a pulper and a papermaking machine.

[0033] Figure 2 is a radial section of the mixer of Figure 1.

[0034] Figure 3 is a perspective view of two mixing shafts of the mixer of Figure 2.

[0035] Figure 4 is a perspective view of another embodiment of the mixing shafts of Figure 3.

[0036] Figure 5 is a graph comparing the drainage / mechanical properties ratio of paper between the kneading treatment process according to the invention and a conventional refining process.

[0037] Figure 6 is a graph comparing the permeability / tensile strength ratio of paper between the kneading treatment process according to the invention and a conventional refining process.

[0038] Figure 7 is a graph illustrating the influence on the tear strength of the concentration of the cellulose fiber suspension (eucalyptus) during mixing as a function of the rotation speed and the compression profile. Figure 8 is a graph illustrating the influence on the tensile strength of the concentration of the cellulose fiber suspension (eucalyptus) during mixing as a function of the rotation speed and the compression profile.

[0039] Figure 9 is a graph illustrating the influence, on the drainability (°SR), of the concentration of the cellulose fiber suspension (eucalyptus) during mixing as a function of the rotation speed and the compression profile.

[0040] Figure 10 is a graph illustrating the influence on tear strength of the concentration of the cellulose (softwood) fiber suspension during mixing as a function of rotation speed and compression profile.

[0041] Figure 11 is a graph illustrating the influence on tensile strength of the concentration of the cellulose (softwood) fiber suspension during mixing as a function of rotation speed and compression profile.

[0042] 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.

[0043] Figures 1 to 6 show the following nomenclature:

[0044] 1: Mixer

[0045] 2: Suspension of fibers

[0046] 3: Pulper

[0047] 4: Paper making machine

[0048] 5: Sheet of paper

[0049] 10: Body

[0050] 11: Mixing chamber

[0051] 12: Compression surface (fixed)

[0052] 13: Inlet orifice

[0053] 14: Exit orifice

[0054] 15 and 16: Inlet connections for injecting products, for example chemical reagents

[0055] 20: Mixing axis

[0056] 20': Mixing axis (fig 4)

[0057] 21: Shaft 22: Mixing elements

[0058] 22': Mixing elements (fig. 4)

[0059] 23: Compression surface (mobile)

[0060] 24: Lobes

[0061] 24' Lobes (fig 4)

[0062] 25: Worm screw

[0063] X: Axis of rotation

[0064] ZC: Compression Zones

[0065] DM: Minimum Distance

[0066] ZE: Entrance Zone

[0067] ZM: Mixing zone

[0068] ZS: Exit Zone

[0069] Although Figures 1 to 4 show two mixing axes, the description of these figures can be generalized to mixers having a single mixing axis or more than two.

[0070] DESCRIPTION OF EMBODIMENTS

[0071] In the illustrated embodiments, identical elements or parts bear the same reference numbers. Furthermore, terms that have a relative meaning, such as vertical, horizontal, right, left, front, rear, above, below, etc., must be interpreted under normal conditions of use of the invention, and as shown in the figures. The X, Y and Z axes are defined by an orthonormal reference system illustrated in Figure 1. Furthermore, the geometric positions indicated in the description and the claims, such as “perpendicular”, “parallel”, “symmetrical” are not limited to the strict sense defined in geometry, but extend to geometric positions that are close, that is to say which accept a certain tolerance in the technical field considered, without influence on the result obtained.This tolerance is notably introduced by the adverb "substantially", without this term necessarily being repeated before each adjective. The mechanical treatment method according to the invention applies 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 may be significant and 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.

[0072] As explained below, the process of the invention has the advantage of making the cellulosic fibers flexible by compression-kneading, that is to say of making them supple, while generating few or no fine elements and cut fibers. The resulting improvement in mechanical properties makes it possible to reduce the grammage of the paper according to the objectives sought in terms of the quality / price ratio of the paper.

[0073] In addition, the method of the invention also makes it possible to improve certain properties of cellulosic fibers. This advantage makes it possible to consider mixtures of fibers from, for example, hardwoods, softwoods and others, and to vary the ratios from 0 to 100%, depending on the objectives sought in terms of the quality / price ratio of the paper. Fibers from softwoods having a cost price much higher than that of fibers from hardwoods can thus represent a lower share than that of other fibers, without harming the properties of the paper, and in particular tearing.

[0074] Since the process according to the invention generates little or no shear, it is possible to use fibers from annual plants, even though these are generally sensitive to shear.

[0075] With reference to Figure 1, the mechanical treatment method according to the invention consists of a step of compressing the aqueous suspension of cellulose fibers in a mixer, instead of the conventional compression and shearing step between two rotating blade surfaces. The mixer used in said method is known in fields very different from papermaking. It is mainly used to homogenize viscous materials, such as polymers, compositions, composites or the like, used in cosmetics, adhesives, plastics, chemistry, etc. It may in particular be a UCP type mixer from the company HASLER.

[0076] In the present invention, the mixer 1 as shown in Figure 1 is fed with an aqueous suspension of cellulosic fibers 2 from a pulper 3 or any other equivalent machine. It feeds at the output a papermaking machine 4 to form a sheet or a web of paper 5. The mixer 1 can be easily integrated into a continuous process, between the pulper and the machine after a step of thickening the pulp, allowing a gain in productivity and a reduced production cost.

[0077] The 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 suspension of fibers 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.

[0078] The mixing axis 20 comprises: a shaft 21 rotatable about said axis X, and a plurality of mixing elements 22, also called “paddles”, fixed on the shaft 21, projecting from the shaft in a radial direction, each delimited by a peripheral wall forming a compression surface 23 integral in rotation with the shaft. The compression surface 23 is distant from the axis of rotation X by a non-constant radius, which varies between a maximum radius 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 makes it possible to ensure, in cooperation with another corresponding compression surface, optimal compression of the fibrous suspension, thus leading to satisfactory mixing.

[0079] 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.

[0080] According to said method, the compression surfaces 23 of the mixing elements 22 which face the compression surface 12 of the mixing enclosure 11 on the one hand, and said compression surface 12 of the mixing enclosure 11 on the other hand, define between them compression zones ZC of the suspension of fibers 2 circulating between the body 10 and the mixing axis 20. The compression zones ZC thus correspond to the volume remaining between the mixing axis 20 and the mixing enclosure 11, which contains the suspension of fibers 2. They are identified very roughly by ovals in Figure 2.Due 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, having the effect of generating a large number of compression forces on the fiber suspension 2 during the rotation of the mixing axis 20 during the mixing operation. Thus, the cellulose 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 forces (the rotation speed of the paddles is low, typically of 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 approximately 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 blades can for example be around 300 mm, for a wall shear of around 6 m / s, which remains low compared to the size of the mixer and the quantity of suspension treated of around 2 tonnes per hour). In other words, unlike conventional refining, the process of the invention induces a shear of the fibres which is negligible compared to their compression.

[0081] The mixer 1 may comprise more than one mixing axis 20, and for example two mixing axes 20 or more than two mixing axes, positioned in the mixing enclosure 11, parallel to each other. Thus, the compression surfaces 23 of the mixing elements 22 which face each other respectively define between them additional compression zones ZC for the suspension of fibers 2 circulating between the mixing axes 20. In this case, mixing axes 20 of identical or almost identical structure will preferably be chosen, positioned between them in such a way that the compression profile of one of the mixing axes matches the compression profile of the other mixing axis, and rotating at the same speed. In this preferred embodiment, it will be advantageous to ensure that two mixing elements 22 which face each other have the same shape.The mixing axes 20 can be co-rotating and thus rotate in the same direction of rotation, as shown in Figure 2 by the counterclockwise arrows, or counter-rotating and thus rotate in opposite directions of rotation.

[0082] Figure 2 illustrates a radial section of a mixer 1 comprising two mixing shafts 20 parallel to each other in a mixing chamber 11. The mixing shafts 20 each comprise mixing elements 22 of oblong geometry. The mixing shafts 20 are angularly offset relative to each other, so that the two corresponding mixing elements 22 are positioned perpendicular to each other, without this relative position being obligatory and 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 mixing elements which face each other can be offset by 90° relative to each other, while a second pair of mixing elements which face each other can be offset by 45° relative to each other. The mixing axes 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 enclosure 11 are defined as a function of the geometry of the mixing elements 22, to introduce a respective minimum distance DM between the mixing axes 20 and the mixing enclosure 11, and between the mixing axes 20 themselves.

[0083] The minimum distance DM, which is represented in Figure 2 by circles, is preferably between 0.1 mm and 0.6 mm, preferably between 0.15 mm and 0.6 mm, in particular between 0.3 mm and 0.6 mm or between 0.3 mm and 0.5 mm. This minimum distance may in particular be between 0.15 mm and 0.4 mm. This minimum distance DM allows in particular the rotation of the mixing axes 20 in the mixing enclosure 11 without conflict, while allowing in a very limited manner the circulation of the suspension of fibers 2 in these restricted spaces. The limit zones located between the mixing axes 20 on the one hand, and between each mixing axis 20 and the mixing enclosure 11 on the other hand, which are defined by the minimum distance DM, therefore correspond to zones of high mechanical stress for the fiber composition.When the mixing axes 20 rotate, 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.

[0084] The kneading elements 22 may 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 specifications of the papermaker. The kneading elements 22 illustrated in Figure 2 are flat parts, of long ob shape, symmetrical with respect to two perpendicular planes passing through the axis of rotation X, and comprise two diametrically opposed lobes 24.

[0085] This example is of course not limiting. The mixing elements 22 may 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 may 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 Figure 4). They may also have a polygonal section, symmetrical or not with respect to a plane passing through the axis of rotation X, and the number of lobes of which corresponds to the number of angles of the polygon (not shown). Finally, they may have a complex shape, symmetrical or not with respect to a plane passing through the axis of rotation X (not shown).

[0086] Likewise, the mixing elements 22 may comprise parts whose peripheral wall, that is to say the compression surface, is flat or not, and whose compression profile in the thickness of the part is variable, such as a straight (or flat) profile, a helical profile, a curved profile, a notched profile, a complex profile. For example, a succession along the shaft of parts whose thickness has a helical profile leads to a helical mixing axis forming a complete helix, each part constituting a fraction of the helix.

[0087] Figure 3 illustrates two parallel, identical mixing axes 20, offset from each other by an angle of 90°, each provided with flat, oblong mixing elements 22 with two lobes 24, with a straight compression profile in the thickness, substantially identical to those shown in 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.

[0088] Figure 4 illustrates two parallel, identical mixing axes 20', without angular offset, each provided with flat, triangular mixing elements 22' with three equidistant lobes 24', with a straight compression profile in the thickness. The mixing elements 22' of the same axis are angularly offset from each other by 30°, and the mixing elements 22' of the two mixing axes 20' fit together radially.

[0089] The mechanical treatment method according to the invention offers a flexible, modular, adaptable, scalable compression kneading solution, making it possible to vary, modify, and refine the properties of the fibers depending on the paper to be manufactured. This method is also part of a research and development approach for new papers. For this purpose, the structure of the kneader 1 and its operating parameters can be easily chosen, modified, and / or combined. As seen above, the structure of the kneader 1 is determined by the number of kneading axes 20 as well as by the arrangement and choice of geometry of the kneading elements 22, but not only. The mechanical treatment method can also comprise several steps to achieve different degrees of flexibility.These different steps can be carried out in different 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 appropriate 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 appropriate structures; by a mixer (not shown) comprising several successive mixing chambers, connected in series, each comprising one or more mixing shafts of different and appropriate structures; or by several successive mixers (not shown), connected in series, of different and appropriate structures.

[0090] In these different cases, the treated fiber suspension 2 can be extracted at each stage of the process and / or at the end of the process stages as required.

[0091] The operating parameters of the mixer 1 can also be chosen, modified and / or combined to vary the mixing characteristics. Among the modifiable parameters, the following examples may be cited, without this list being exhaustive: the feed rate of the mixer 1 in fiber suspension 2, defining the mixing duration;the mixing temperature: the mixer operates at room temperature, but can also operate at a temperature above or below room temperature thanks to the body 10 of the mixer 1 which can be made up of a double jacket (not shown) allowing the circulation of a heat transfer fluid. In more detail, the mixer is kept at room 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 work hot, in particular to carry out oxidation reactions in conjunction with mixing (intensification of the process) or cold, for example in the presence of soda to dissolve the cellulose;the mixing pressure: preferably the mixing operation is carried out in a mixing chamber 11 at atmospheric pressure, but depending on the needs could be carried out at a pressure higher or lower than atmospheric pressure. For example, it may be interesting to work at a pressure higher than atmospheric pressure to adjust the filling speed of the feed screw (for example of the endless screw type as described in the remainder of this text); the rotation speed of the mixing axis(es) 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 axes 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 phenomenon;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 resistive torque of the mixing axes 20; the mixing time: the residence time of the fiber suspension 2 in the mixing enclosure 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 enclosure 11 by modifying the mixing elements 22;the thickness of the kneading elements 22 to increase or decrease the compression surfaces. The kneading tests carried out with the method of the invention revealed an unexpected and counter-intuitive physical phenomenon for those skilled in the art of papermaking: the higher the rotation speed of the kneading axes 20 (20 to 600 rpm), the shorter the duration of application of the mechanical compression action on the cellulose fibers, and the better the kneading result. A high speed makes it possible to limit the shearing and fibrillation of the cellulose fibers and therefore the formation of fine elements. These conditions favor brief 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.;

[0092] In summary, the rotation 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 and frequent mechanical compression, which has the effect of limiting the deterioration of the cellulose fibers. Thus, the cellulose fibers are less damaged, or even preserved, compared to long and frequent or infrequent mechanical compressions.

[0093] This phenomenon, which cannot be detected or envisaged before implementing the method of the invention, makes it possible to further increase the mixing performance and the advantages of the method which result from it, such as better flexibilization of the fibers, improved fiber content, new possible paper pulp formulations, reduced power consumption allowing significant energy savings, very little wear of the moving parts allowing reduced maintenance of the mixer 1.

[0094] In the example of the mixer 1 shown in Figure 1, the mixing enclosure 11 comprises an inlet zone ZE, a mixing zone ZM and an outlet zone ZS, which follow one another axially. The inlet zone ZE is coupled to the inlet orifice 13 and preferably comprises a worm screw 25 fixed on the corresponding shaft 21 of each mixing axis 20, to move the fiber suspension 2 axially towards the mixing zone ZM and then the outlet zone ZS. The mixing zone ZM comprises the mixing elements 20 described previously. The outlet zone ZS can be confused with the end of the mixing zone ZM and also comprises 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 orifice 14 which can be gravity-driven or combined with any other means of extraction.The mixer 1 may comprise several inlet taps 15, 16 which can be used to introduce additives, fillers and other materials into the fibre suspension 2 during mixing. It may also comprise several outlet taps (not shown) which can be used to extract all or part of the fibre suspension 2 during mixing, gases and other condensates generated during treatment. This is particularly advantageous because the operator can recover several fractions of mixed fibres having different degrees of compression and therefore different properties depending on their respective outlet tap, and this during the same mixing operation.

[0095] Figures 5 and 6 are graphs from tests carried out on paper made from cellulose fibers treated using the compression kneading process of the invention (points on the graphs) and using a conventional compression and shear refining process (curve on the graphs). They illustrate the improvement in the mechanical and drainability properties of the paper pulp obtained using the process of the invention.

[0096] The present invention is of course not limited to the embodiments described but extends to any modification and variant obvious to a person skilled in the art within the limits of the appended claims. Furthermore, the technical characteristics of the different embodiments and variants mentioned above may be, in whole or in some cases, combined with each other.

[0097] EXAMPLES

[0098] Figure 5 shows the evolution of the freeness index (°SR) on the ordinate as a function of the burst index on the abscissa. The freeness index, or Schopper-Riegler index, is defined according to ISO 5267-1 as the number of centilitres of water drained through a dough cake flowing through the overflow of a tank. It represents a measure of the speed at which water can be extracted from a dilute dough suspension.

[0099] The burst index corresponds to the mechanical strength of the fibers. In simple terms, the higher this index, the stronger and better the quality of the paper.

[0100] It is observed that with a suspension of fibers (5% eucalyptus fibers by weight) refined by a standard refining process (disc refiner at 25 m / s), the drainage speed reaches a maximum for a burst index close to 4, which is relatively average.

[0101] In contrast, a fibrous suspension (eucalyptus fibers at 8-15% by weight in water + compression with a speed differential of 0.5 to 1 m / s) treated by kneading in accordance with the process of the invention makes it possible to achieve high drainage speeds with a high burst index. This produces a good quality paper whose production rate is improved through a high drainage speed of the fibrous suspension from which it is derived.

[0102] This leads to a very significant improvement in the compromise between drainage and mechanical properties of the fibers, with the effect of reducing energy consumption at the paper machine level.

[0103] Figure 5 shows the improvement in the drainage / mechanical properties ratio of paper when the cellulosic fibers are treated in a mixer according to the invention compared to conventional refining in a disc refiner (points connected by the broken curve in 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). In Figure 5, the points connected by the broken curve correspond to: no refining (burst index = 1 kPa.m 2 / g)

[0104] 50 kW.h per tonne of cellulose fibres (bursting index close to 2.1 kPa.m 2 / g),

[0105] 100 kW.h per tonne of cellulose fibres (bursting index close to 3 kPa.m 2 / g),

[0106] 200 kW.h per tonne of cellulose fibres (bursting index close to 4 kPa.m 2 / g),

[0107] 400 kW.h per tonne of cellulose fibres (bursting index close to 4.1 kPa.m 2 / g).

[0108] More precisely, with 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.

[0109] Generally speaking, the lowest possible drainage index is sought. Figure 5 shows that values ​​above 65° SR are achieved for conventional refining (200 and 400 kW.h per tonne of cellulosic fibres) whereas they are systematically below 52° SR for the treatment according to the invention.

[0110] Figure 6 shows the evolution of the air permeability of paper 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 that passes through a unit area under a unit pressure difference in a unit time, under specified conditions.

[0111] 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), the air permeability drops with the tensile strength, starting from 35 Nm / g. In contrast, a fiber suspension (8-15% by weight eucalyptus fibers in water + compression with a speed differential of 0.5 to 1 m / s) treated by kneading in accordance with the process of the invention makes it possible to shift the drop in air permeability towards high tensile strength values. This makes it possible to obtain a paper with good mechanical strength while maintaining a relatively high air permeability.

[0112] This leads to a significant improvement in the permeability / tensile strength compromise, making it possible to consider the development of new papers.

[0113] Figure 6 shows the improvement in the ratio between air permeability and tensile strength of paper when the cellulosic fibers are treated in a kneader according to the invention compared to conventional refining in a disc refiner (points connected by the broken curve in 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).

[0114] In Figure 6, the points connected by the discontinuous curve correspond to: no refining (tensile strength index close to 28 Nm / g)

[0115] 50 kW.h per tonne of cellulose fibres (tensile strength index close to 33 Nm / g),

[0116] 100 kW.h per tonne of cellulose fibres (tensile strength index close to 42 Nm / g),

[0117] 200 kW.h per tonne of cellulose fibres (tensile strength index close to 55 Nm / g),

[0118] 400 kW.h per tonne of cellulosic fibres (tensile strength index close to 62 Nm / g).

[0119] More precisely, at equivalent permeability, treatment in a mixer makes it possible to increase the tensile strength index, for example a gain of 50% (+20 Nm / g) for an air permeability of approximately 1800 mL / min.

[0120] Different suspensions of cellulose fibers (8% suspension by weight) were treated in a mixer under the following conditions, with a speed differential between the mixing shaft and the mixing chamber of between 0.5 and 1 m / s (Figures 7-12): rotation speed of the mixing shaft: 50 to 350 rpm (or revolutions per minute) mixing profile “std”: alternating bilobe helical paddles and bilobe flat paddles at 45° to each other, mixing profile “1s”: bilobe helical paddles at 45° to each other, mixing profile “hs”: bilobe flat paddles at 90° to each other.

[0121] Suspensions of eucalyptus fibers (Figures 7-9) or softwood fibers (Figures 10-12) were used.

[0122] Figures 7 to 12 show that the best compromise between tear strength / tensile strength / drainage is obtained for a cellulosic fiber concentration between 5 and 20% by weight in water. Drainage values ​​are systematically lower than 60° SR (Figures 9 and 12) whereas they can reach more than 85° SR in the case of conventional refining (Figure 5).

Claims

CLAIMS Method for mechanical treatment of cellulose fibers by mechanical compression of an aqueous suspension of cellulose fibers comprising the following steps: preparation of an aqueous suspension of cellulose fibers (2) having a concentration of between 5 and 20% by weight of cellulose fibers, mechanical compression of the aqueous suspension of cellulose fibers (2) in a mixer (1), the mixer (1) comprising: a body (10) defining a mixing chamber (11) delimited by a wall forming a fixed compression surface (12) capable of coming into contact with the suspension of fibers (2), at least one mixing axis (20) positioned in the mixing chamber (11) and comprising: • a rotating 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 movable compression surface (23) capable of coming into contact with the fiber suspension (2), a method in which the compression surfaces (23) of the kneading elements (22) face the compression surface (12) of the kneading enclosure (11), so as to define between said respective compression surfaces (23, 12) of the kneading elements (22) and of the kneading enclosure (11) compression zones (ZC) of the fiber suspension (2) circulating between the body (10) and the kneading axis (20), and in which the compression surfaces (23) of the kneading elements (22) have a compression profile whose radius relative to the shaft (21) varies according to the angular position of said shaft, so as to compressing the fiber suspension (2) in said compression zones (ZC) during rotation of said mixing axis (20),the method having a speed differential between the at least one mixing axis (20) and the mixing enclosure (11) less than or equal to 15 m / s, the mixer having a minimum distance (DM) separating the at least one mixing axis (20) from the mixing enclosure (11) of between 0.1 mm and 1 mm, the aqueous suspension of cellulose fibers has a residence time in the mixer of between 15 seconds and 15 minutes. Treatment method according to claim 1, wherein the mixer (1) comprises at least two mixing axes (20) positioned in the mixing enclosure (11), parallel to each other, and wherein the compression surfaces (23) of the mixing elements (22) of said mixing axes (20) face each other, so as to define between them additional compression zones (ZC) of the fiber suspension (2) circulating between the mixing axes (20).Treatment method according to any one of claims 1 and 2, wherein the mixing enclosure (11) comprises at least one inlet zone (ZE), a mixing zone (ZM) and an outlet zone (ZS), wherein the fiber suspension (2) is introduced into the inlet zone (ZE) and is extracted from the outlet zone (ZS), and wherein each mixing axis (20) comprises at least one worm screw (25) fixed on the shaft (21), arranged in the inlet zone (ZE) and arranged to move the fiber suspension (2) from the inlet zone (ZE) to the outlet zone (ZS) through the mixing zone (ZM), said plurality of mixing elements (22) being arranged in the mixing zone (ZM) and the outlet zone (ZS) being or not integrated into the mixing zone (ZM).Treatment method according to claim 3, in which the mixing zone (ZM) comprises several successive mixing sectors, in which the mixing elements (22, 22') of each mixing axis (20, 20') are distributed in series, one series corresponding to one mixing sector. and wherein the mixing elements (22, 22') of the same series are identical, and the mixing elements (22, 22') from one series to another are different in their shape and / or their thickness and / or their compression profile. Treatment method according to any one of claims 1 to 4, wherein the mixing elements (22, 22') are made up of flat or complex parts, which have a section chosen from the group comprising an ovoid section comprising one 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 chosen from the group comprising a straight profile, a helical profile, a curved profile, a notched profile.Treatment method according to any one of claims 3 to 5, in which the body (10) of the mixer (1) comprises several mixing chambers (11), connected in series, each comprising at least one mixing axis (20) of different structure from one chamber to another to generate a different mechanical treatment. Treatment method according to any one of claims 1 to 6, in which the minimum distance (DM) which separates the mixing axes (20) from the mixing chamber (11), and the minimum distance (DM) which separates the mixing axes (20) from each other when there are at least two of them, is between 0.15 mm and 0.4 mm. Treatment method according to any one of the preceding claims in combination with claim 2, in which the mixing axes (20) are co-rotating, in that they rotate in the same direction of rotation.Treatment method according to any one of the preceding claims in combination with claim 2, in which the mixing elements (22, 22') of two mixing axes (20) whose compression surfaces (23) face each other have an identical compression profile. Treatment method according to any one of claims 1 to 9, wherein the method has a speed differential between the mixing axis(es) (20) and the mixing enclosure (11) of at least 0.1 m / s and less than or equal to 15 m / s, advantageously between 0.5 m / s and 10 m / s. Treatment method according to any one of claims 1 to 10, wherein the residence time of the suspension of fibers (2) in the mixer (1) is between 15 seconds and 10 minutes. Treatment method according to any one of claims 1 to 11, wherein the aqueous suspension of cellulosic fibers (2) is prepared from a mixture of cellulosic fibers and water, and optionally mineral fillers, the proportion of dry cellulosic fibers being between 8 and 15% by weight of the aqueous suspension.Treatment method according to any one of claims 1 to 12, in which the aqueous suspension of cellulosic fibers undergoes a number of compressions in the mixer of at least 500, advantageously from 500 to 5000. Treatment method according to any one of claims 1 to 13, in which the aqueous suspension of cellulosic fibers comprises mineral fillers. Treatment method according to any one of claims 1 to 14, in which the at least one mixing shaft (20) has a rotation speed of between 20 and 600 revolutions / minute, the mixer comprising an alternation of mixing elements having a straight compression profile and a helical profile.

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