Method and installation for preparing a fibrous pulp from the stems of an annual plant, as well as the use of such a fibrous pulp
The described process for preparing fibrous pulp from annual plant stems addresses the issue of inhomogeneity by grinding and sorting whole stems to maintain consistent mechanical properties, enabling effective use in various industries.
Patent Information
- Application Number
- FR2023012703
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-11-20
- Publication Date
- 2025-11-21
- Estimated Expiration
- 2043-11-20
AI Technical Summary
Existing methods for preparing fibrous pulp from annual plants, such as hemp, often result in inhomogeneous materials due to the separation of long and short fibers, leading to inconsistent mechanical properties and potential degradation, particularly when whole hemp stalks are not processed uniformly.
A process and installation that grind whole annual plant stems to produce a homogeneous fibrous mass by interlacing long and short fibers, followed by a sorting step to remove particulate residues, and a thermomechanical treatment in an extruder to maintain controlled mechanical properties.
The process ensures a homogeneous fibrous pulp with consistent mechanical properties, allowing for efficient use in paper, packaging, and construction materials, while minimizing energy consumption and material loss.
Smart Images
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Abstract
Description
Title of the invention: Method and installation for preparing a fibrous pulp from the stems of an annual plant, as well as the use of such a fibrous pulp
[0001] The present invention relates to a method for preparing a fibrous pulp from the stems of an annual plant. It also relates to a use of a fibrous pulp prepared by this method. It also relates to an installation for preparing a fibrous pulp from the stems of an annual plant.
[0002] Annual plants are those whose life cycle is on the order of one year. Examples of annual plants are hemp, flax, etc.
[0003] Industrial hemp is easy to cultivate, particularly in Europe. Its natural fibers can replace synthetic fibers in the composition of certain materials. The life cycle of these materials is then generally less impactful on the environment: indeed, hemp fibers are biodegradable at the end of these materials' life. Moreover, the use of hemp does not generate a "transfer of impact" because its cultivation is ecologically sound in that it does not deplete the soil.
[0004] In its mature state, hemp comprises a stem from which leaves, flowers, and fruits extend. The hemp stem consists of a central core, called the hurd, which is rich in short fibers, typically less than 2 mm in length, and an outer sheath, which is rich in long fibers, typically greater than 5 mm in length. The long fibers, which represent 30 to 45% of the stem's dry weight, are strong and rich in cellulose; they are commonly used as a base material for high-strength paper pulp, as a base material in rope making, as a sealant in plumbing / heating under the name "fiberglass," as an insulation material for buildings, etc.The hurd, which represents between 40 and 55% of the stem's dry weight, is also of interest, particularly because of its moisture-absorbing and insulating properties; thus, the hurd is commonly used as animal bedding, a component of building bricks, agricultural mulch, etc.
[0005] Traditionally, the outer long fibers and the hurd of the hemp stalk are used separately. In practice, following the harvest of hemp stalks and a more or less lengthy retting process, and after removing the leaves, flowers, and / or fruits they bear, the stalks are usually subjected to a decortication operation that breaks down the stalks to separate the hurd and the outer long fibers. Such a decortication operation is typically carried out by crushing the hemp stalks by beating, for example, using a mill. hammer. At the end of the decortication operation, the long outer fibers and the hurd are recovered separately, as well as dust, which represents less than 30% by dry weight of the stem and which can be used in various ways, for example to produce energy.
[0006] FR 3015529, for its part, considered preparing raw paper pulp using essentially the entire hemp stalk. To do this, FR 3015529 proposes cutting the whole hemp stalks into chips, without a prior step of separating the long and short fibers contained within the hemp fibers. The chips, which measure between 3 and 5 cm, are then directly fed into an extruder where they are successively mixed with water, steam, and an alkali hydroxide, while being compressed and sheared by the extruder's interpenetrating screws. FR 3015529 provides no instructions regarding the equipment used to cut the whole hemp stalks into chips.On the contrary, in the examples given in FR 3015529, it is explicitly stated that what is introduced into the extruder is a composition of lignocellulosic material, which has been reconstituted by combining several classes of hemp straw that has already been crushed and cleaned. This composition of lignocellulosic material, which is therefore not directly obtained from whole hemp stalks, is a reconstituted mixture of long hemp fibers, resulting from prior defibration and contained in the outer sheath of the hemp stalk, and short hemp fibers, also resulting from prior defibration and contained in hurd straw, the respective proportions of long and short hemp fibers in this mixture being adjusted to be similar to those of a whole hemp stalk.
[0007] The object of the present invention is to propose a process and a preparation installation which are truly integrated from the whole stems of annual plants to the obtaining of a fibrous paste having controlled mechanical properties.
[0008] To this end, the invention relates to a process for preparing a fibrous paste from the stems of an annual plant, the process comprising:
[0009] - a grinding step in which whole stems of annual plants are ground to obtain a pulverized material containing mainly unfibrated stem fragments with a maximum dimension of less than 80 mm,
[0010] - a sorting step in which the crushed material is divided into two fractions, namely:
[0011] - fibrous elements which are linked together by interlacing so as to form a homogeneous fibrous mass, and
[0012] - particulate residues that are free with respect to the homogeneous fibrous mass, the particulate residues being removed, and
[0013] - a thermomechanical treatment step in which the fibrous mass homogeneous is introduced and processed continuously in an extruder from the outlet of which an extrudate is recovered forming said fibrous paste.
[0014] The invention also relates to an installation for preparing a fibrous pulp from the stems of an annual plant, the installation comprising:
[0015] - a grinder that is suitable for grinding whole stems of annual plants in order to obtain a mulch containing mainly unfibrated stem fragments with a maximum dimension of less than 80 mm,
[0016] - a sorter which is adapted to divide the crushed material into two fractions, namely:
[0017] - fibrous elements which are linked together by interlacing so as to form a homogeneous fibrous mass, and
[0018] - particulate residues that are free with respect to the homogeneous fibrous mass, the particulate residues can be removed by the sorter, and
[0019] - an extruder, which is adapted to continuously process the homogeneous fibrous mass introduced into the extruder, and at the outlet of which an extrudate forming said fibrous paste can be recovered.
[0020] One of the ideas underlying the invention is to introduce into an extruder a raw material that is directly obtained from whole annual plant stems. To this end, the invention provides that the whole annual plant stems are first crushed, avoiding being predominantly defibrated, before the resulting crushed material is sorted so that only a homogeneous fibrous mass consisting of fibrous elements that are linked together by interlacing fibers is sent to the extruder.These interwoven fibrous elements contain both long and short plant fibers, which, although a priori less interesting from a mechanical point of view than long plant fibers, are acceptable in the raw material entering the extruder as long as these short plant fibers are not free with respect to the long plant fibers but, on the contrary, are interwoven with the long plant fibers and are thus found to be distributed homogeneously with these long plant fibers.Conversely, the short plant fibers, which are free from the long plant fibers in the aforementioned ground material, do not end up in the homogeneous fibrous mass and are, along with the remaining free particulate residues, removed to prevent them from entering the extruder. This avoids the inhomogeneity that these various free particulate residues would induce in the raw material entering the extruder, which could lead to inhomogeneity in the extrudate continuously exiting the extruder and thus to a localized but consistent degradation of its mechanical quality. The mechanical properties of the fibrous pulp formed from the extrudate continuously exiting the extruder are therefore controlled, and in particular, remain homogeneous over time. and within the extrudate, while allowing work to be done directly from whole annual plant stems.
[0021] In practice, the process and installation according to the invention are preferably applied to hemp in which the long fibers of the outer sheath of whole stems are found, after the crushing and sorting steps, almost entirely within the homogeneous fibrous mass, while the short fibers of the hurd from the whole hemp stems are largely found within the homogeneous fibrous mass, the remainder of the hurd being removed with the particulate residue resulting from the sorting step. That being said, the process and installation according to the invention are applicable to other annual plants, such as flax.
[0022] It should also be noted that the fibrous pulp prepared by the invention can be used for various purposes, preferably in the paper, packaging, or construction materials industries. Thus, the invention relates to a use of a fibrous pulp prepared by the process as defined above, in which the fibrous pulp is used as:
[0023] - paper pulp,
[0024] - component for packaging material, or
[0025] - component for building material.
[0026] According to additional advantageous features of the process and installation according to the invention, taken individually or in all technically possible combinations:
[0027] - Whole annual plant stems include whole hemp stems so that : - the homogeneous fibrous mass contains long hemp fibers, as well as short hemp hurd fibers, held together with the long hemp fibers by interlacing, and
[0028] the particulate residues contain dust and particles of hemp shives, not retained by the long hemp fibers contained in the homogeneous fibrous mass.
[0029] - The grinding step includes a shredding operation, in which each whole annual plant stems are fractionated by cutting, and this is continued until the whole annual plant stems are cut into said unfibrated stem fragments.
[0030] - At the end of the grinding step, the maximum size of the stem fragments non-fibrated is less than 70 mm, preferably less than 60 mm, more preferably less than 50 mm.
[0031] - The sorting step includes a sieving operation in which the crushed material is sieved in order to separate the particulate residues and the fibrous elements forming the homogeneous fibrous mass.
[0032] - During the thermomechanical treatment step, an alkaline chemical agent is introduced into the extruder to chemically treat the homogeneous fibrous mass.
[0033] - The crushing and sorting stages are carried out continuously with the thermomechanical treatment stage.
[0034] - The crusher comprises: - a rotor equipped with knives adapted for splitting whole annual plant stems by cutting, and
[0035] - a grid, which surrounds the rotor and through which the crushed material exits the crusher.
[0036] - The grid is provided with openings with a substantially square profile, the sides of which have a dimension between 20 and 80 mm, preferably between 30 and 70 mm, more preferably between 35 and 60 mm, even more preferably between 40 and 50 mm.
[0037] - The separator includes a sieve which allows particulate residues to pass through it, without being crossed by the fibrous elements forming the homogeneous fibrous mass so that these fibrous elements are recovered from an inlet face of the sieve to form the homogeneous fibrous mass.
[0038] The invention will be better understood upon reading the following description, given solely by way of example and made with reference to the drawings on which [Fig.1] the [Fig.1] is a diagram of an installation according to the invention, implementing a process according to the invention.
[0039] An example of a process for preparing, preferably continuously, a fibrous pulp from the stems of an annual plant is described in detail below. This process is advantageously implemented by an installation 1 shown schematically in [Fig. 1]. The process will be described progressively, detailing its steps successively and describing in parallel the parts of the installation 1 that enable these steps to be carried out.
[0040] The process makes it possible to prepare a fibrous pulp, referenced 2 in [Fig. 1], from annual plant stems, referenced 3. The annual plant stems 3 are made available after these stems have been harvested and, where applicable, stripped of their leaves, flowers, and / or fruits. In all cases, the annual plant stems 3 used as input to the process and the installation 1 are whole, that is to say, each of these stems consists of the part of the annual plant which, just before harvesting, extends lengthwise upwards from the ground. This part of the annual plant is cut at its base, that is to say, just above the ground, during harvesting, without any other significant shortening or, more generally, any operation that compromises the integrity of the stem as such, independently of any operations not affecting the stem as such but the leaves, flowers and / or fruits that grew on the stem before harvest, in order to rid the stem of these leaves, flowers and / or fruits.
[0041] Preferably, the annual plant stems 3 are hemp stems. These hemp stems 3 are, at the input to the process and the installation 1, whole, as explained above. In particular, these hemp stems 3 comprise, in the form of an integral elongated body, an outer sheath rich in long fibers, typically longer than 5 mm, and a central core, called hurd, which is enclosed within the outer sheath and is rich in short fibers, typically shorter than 2 mm. For convenience, the remainder of the description applies to the case where the annual plant stems 3 are hemp stems.
[0042] The process includes a grinding step in which the whole hemp stalks 3 are preferably continuously ground to obtain a ground material 4 containing unfibrated stalk fragments with a maximum dimension of less than 80 mm, preferably less than 70 mm, more preferably less than 60 mm, and even more preferably less than 50 mm. For this purpose, the installation 1 includes a grinder 10.
[0043] In the example considered here, the installation 1 includes a conveying means 20, such as a conveyor belt, which allows the crusher 10 to be continuously fed with whole hemp stalks 3 from a stock of the latter, made available at the input of the installation 1, in particular of the crusher 10.
[0044] In all cases, the grinding step shortens the length of the whole hemp stalks 3, which, at the input of the grinding step and the grinder 10, is well over 80 mm, typically being on the order of 1 m, while, at the output of the grinding step and the grinder 10, each of the whole hemp stalks 3 is in the form of the aforementioned stalk fragments. During the grinding step, each hemp stalk 3 goes from its state as a whole stalk to that of the aforementioned stalk fragments by means of its subdivision at several points distributed along the hemp stalk 3. In other words, the hemp stalk 3 is progressively shortened by cuts transverse to the longitudinal direction of the hemp stalk 3, which are repeated and distributed along the hemp stalk.Thus, the aforementioned stem fragments are cut from each of the 3 whole hemp stems without defibration, that is to say without separating, within each of these stem fragments, the outer sheath and the hurd from each other, which thus retain their natural reciprocal arrangement intact.
[0045] In line with the preceding considerations, the grinding step includes, according to a preferred embodiment, a shredding operation, by which each of the whole hemp stalks 3 is fractionated by cutting, this operation the shredding process continues until the whole hemp stalks 3 are cut into the aforementioned unfibrated stalk fragments. For this purpose, the shredder 10 is advantageously a so-called "knife shredder", which comprises a rotor 11 that can be driven to rotate about itself around an axis XI1, typically arranged horizontally, relative to a fixed casing 12 of the shredder 10. This rotor 11 is equipped with knives 13, which are distributed around the axis XI1 and which are each adapted to break down the hemp stalks 3 by cutting. In addition, the crusher 10 includes a grid 14, which, in operation, is fixed relative to the fixed casing 12 and which is arranged around the rotor 11, surrounding the latter, typically coaxially to the axis XI1, while providing, at the inlet of the crusher 10, a passage 15 large enough for the admission of whole hemp stems 3 into the grid 14.The screen 14 is provided with through-holes, each connecting the inside and outside of the screen 14, through which the aforementioned stem fragments pass from the inside to the outside of the screen 14. In other words, due to the size of the through-holes in the screen 14, the screen retains the hemp stems 3 inside, where they are subjected to the cutting action performed by the knives 13 as soon as these are driven by the rotor 11, and this continues until the hemp stems 3 are successively shortened into the aforementioned stem fragments. At the same time, once the hemp stems 3 are in the form of the aforementioned stem fragments, the resulting shredded material 4 exits the grinder 10 through the screen 14.
[0046] According to a practical and efficient embodiment, the through openings of the grid 14 have a square profile with sides having a dimension between 20 and 80 mm, preferably between 30 and 70 mm, more preferably between 35 and 60 mm, even more preferably between 40 and 50 mm.
[0047] It is understood that the shredded material 4 does not contain only the aforementioned unfibrated stem fragments, but also fibrated residues and offcuts resulting from the side effects of the fractionation, particularly by cutting, of the whole hemp stems 3. That being said, the aforementioned unfibrated stem fragments represent a major part of the shredded material 4, that is to say, at least 50% by dry weight of the shredded material 4 that comes from the grinding stage, in other words, that exits the grinder 10. Advantageously, these unfibrated stem fragments represent at least 60%, preferably at least 70%, or even at least 80% by dry weight of this shredded material 4, particularly thanks to the use of the knife grinder described above. In all cases, it is understood that the hurd initially contained in the whole hemp stems 3 remains, within the shredded material 4, largely bound to the long fibers of the outer sheath of the aforementioned stem fragments.
[0048] In practice, to implement the grinding step, the grinder 10 comprises either a single grinding unit, such as that schematically illustrated in [Fig. 1] and detailed above, or several grinding units, which are individually such as that schematically illustrated in [Fig. 1] and detailed above. In the second case, the grinding units of the grinder 10 are arranged in parallel, so that the feed flow to the grinder 10, consisting of whole annual plant stems 3, is distributed between the parallel grinding units, and / or are arranged in series, so that the respective actions of the series grinding units on the annual plant stems 3 are cascaded and thus progressive.
[0049] Following the grinding step, the process includes a sorting step during which the ground material 4, resulting from the grinding step and exiting the grinder 10, is preferably continuously divided into two fractions, namely:
[0050] - fibrous elements which are linked together by interlacing so as to form a homogeneous fibrous mass 5, these fibrous elements being here made up of both long hemp fibers and short hemp hurd fibers which are held to the long hemp fibers by interlacing, and
[0051] - particulate residues 6 which are free with respect to the homogeneous fibrous mass 5, these particulate residues 6 containing here dust and particles of hemp shives, not retained by the long hemp fibers contained in the homogeneous fibrous mass 5.
[0052] The homogeneous fibrous mass 5 constitutes the fraction of the ground material 4, which is subjected to the rest of the process in order to obtain the fibrous pulp 2, while the particulate residues 6 are rejected, being evacuated from the installation 1, without being used to obtain the fibrous pulp 2.
[0053] For this purpose, the installation 1 includes a separator 30 which treats the shredded material 4 entering it, so as to separate the fibrous mass 5 from the particulate residues 6. In practice, various embodiments are conceivable for the separator 30.
[0054] In a practical and efficient embodiment, the separator 30 comprises a sieve 31 which allows the particulate residues 6 to pass through it, without being penetrated by the fibrous elements of the homogeneous fibrous mass 5. The sieve 31 thus retains, on its inlet face, which is advantageously oriented upwards when the sieve 31 is arranged horizontally as schematically shown in [Fig. 1], the homogeneous fibrous mass 5, which is removed from this inlet face of the sieve 31 to be subjected to the rest of the process and the installation 1, while the particulate residues 6 pass through the sieve 31 and are collected on the side of an outlet face of the sieve 31, in particular under the sieve 31, in order to remove these particulate residues 6, as schematically illustrated in [Fig. 1]. In practice, the sieve 31 takes the form of various designs, such as a flat sieve, as schematically illustrated in [Fig.1], or a drum sieve.
[0055] According to an optional arrangement, which improves the efficiency of the sieve 31, the sorter 30 includes a vibrating device 32 which vibrates the sieve 31.
[0056] Whatever the specifics of the sieve 31, the latter generally allows, during the sorting stage, a sieving operation to be implemented in which the ground material 4 from the grinding stage, in particular from the shredding operation, is sieved so as to separate the particulate residues 6 and the homogeneous fibrous mass 5.
[0057] In all cases, it is understood that the hemp fibers, both long and originating from the outer sheath of the hemp stems 3, and short and originating from the hurd of the hemp stems 3, are, within the homogeneous fibrous mass 5, distributed homogeneously by the interlacing of these long and short fibers with each other, while being free of particulate residues 6, partially fibrous but which the homogeneous fibrous mass 5 has been cleared during the sorting stage, under the action of the sorter 30.
[0058] In practice, to implement the sorting step, the sorter 30 comprises either a single sorting unit, such as that schematically illustrated in [Fig. 1] and detailed above, or several sorting units, which are individually such as that schematically illustrated in [Fig. 1] and detailed above. In the second case, the sorting units of the sorter 30 are arranged in parallel, so that the ground material 4 from the grinding step is distributed between the parallel sorting units, and / or are arranged in series, so that the respective actions of the series sorting units on the ground material 4 are cascading and thus progressive.
[0059] The process also includes a thermomechanical treatment step, in which the homogeneous fibrous mass 5, from the sorting step and exiting the sorter 30, is introduced and continuously treated in an extruder 40 of the installation 1. At the end of the thermomechanical treatment step, an extrudate exiting the extruder 40 is recovered to form the fibrous paste 2.
[0060] According to an advantageous embodiment, the extruder 40 comprises two interpenetrating and self-cleaning screws, mounted for rotation in a barrel of the extruder 40, in particular in a co-rotating manner. The homogeneous fibrous mass 5 is introduced between the two aforementioned screws from an inlet of the extruder 40, and is then progressively processed by these two screws as it travels along them, until it exits the aforementioned barrel to form the extrudate. In practice, the introduction of the homogeneous fibrous mass 5 into the extruder is advantageously carried out by a suitable dosing system, not illustrated in [Fig. 1].
[0061] In all cases, the structural and operational specifications of the extruder 40 are not limiting and are adjustable by a person skilled in the art, so that the thermomechanical treatment applied by the extruder 40 to the homogeneous fibrous mass 5 allows the separation of the different fibers of the homogeneous fibrous mass 5 and the continuation of the cutting of the long fibers of the latter, while softening the lignin associated with these different fibers, and this in order to obtain, at the outlet of the extruder 40, the fibrous paste 2 having suitable properties, in particular mechanical.
[0062] According to an optional arrangement not illustrated in [Fig.1], an alkaline chemical agent, such as sodium hydroxide or potassium hydroxide, is introduced into the extruder 40 during the thermomechanical treatment step to chemically treat the homogeneous fibrous mass 5, in addition to the thermomechanical treatment applied to the latter by the extruder 40. In this way, the aforementioned lignin reacts chemically with this chemical agent in the extruder, with a view to the partial dissolution of this lignin.
[0063] According to another optional arrangement, which can be combined with the previous one, water vapor is introduced into the extruder 40 during the thermomechanical treatment step to thermally treat the homogeneous fibrous mass 5, in addition to the thermomechanical treatment applied to the latter by the extruder 40.
[0064] In all cases, it is understood that the fibrous homogeneity of what is introduced into the extruder 40, i.e., of the homogeneous fibrous mass 5, makes it possible to control the properties, particularly the mechanical properties, of the fibrous paste 2 along the extrudate exiting the extruder 40, by avoiding any local inhomogeneity in this extrudate. Thus, the presence of a significant proportion of hemp shives in the homogeneous fibrous mass 5, which minimizes the loss of material used to obtain the fibrous paste 2, is not a problem for giving the fibrous paste 2 satisfactory properties, provided that this hemp shives are distributed homogeneously throughout the homogeneous fibrous mass 5.Advantageously, this also reduces the energy consumption of the thermomechanical treatment stage, since this stage can rely solely on the natural heating generated by the mechanical action of the extruder screws on the material being processed. This limits, or even eliminates, the heating of the extruder barrel 40 and / or limits, or even eliminates, the introduction of water vapor into the barrel. Similarly, this also advantageously limits, or even eliminates, the need to introduce any alkaline chemical agents into the extruder barrel 40.
[0065] Optionally, the process further comprises a post-treatment step in which the fibrous pulp 2 is subjected to one or more additional treatments for its final use, for example, refining. The specifics of this The post-processing stage and the equipment required to implement it are not exhaustive and fall within the usual knowledge of the field.
[0066] In all cases, the fibrous paste 2, where applicable after it has undergone the post-treatment step, can be used for various purposes, in particular as:
[0067] - paper pulp,
[0068] - component for packaging material, particularly in the form of products molded and / or thermoformed cellulosic materials, or in the form of packaging reinforcements, or
[0069] - component for building material.
[0070] Various modifications and variations to the process and installation 1, having been described so far, are conceivable:
[0071] - rather than, as described above by way of preference, the grinding steps and sorting are implemented continuously with the thermomechanical treatment step, one and / or both of these grinding and sorting steps are operated sequentially; and / or
[0072] - as mentioned above, in addition to or as a replacement for hemp, other Annual plants are suitable for preparing fibrous pulp 2 from whole stems, such as flax, kenaf, jute, abaca.
Claims
Demands
1. Use of a fibrous pulp (2) as: - paper pulp, - component for packaging material, or - component for construction material, the fibrous pulp (2) being prepared by a preparation process from annual plant stems (3), the process comprising: - a grinding step in which whole annual plant stems (3) are ground to obtain a ground material (4) containing predominantly unfibrated stem fragments having a maximum dimension of less than 80 mm, - a sorting step in which the ground material (4) is separated into two fractions, namely: - fibrous elements which are bound together by interlacing to form a homogeneous fibrous mass (5), and - particulate residues (6) which are free from the homogeneous fibrous mass, the particulate residues being removed,- a thermomechanical processing step in which the homogeneous fibrous mass (5) is directly introduced and continuously processed in an extruder (40) from the outlet of which an extrudate forming said fibrous paste (2) is recovered, and - optionally, a refining step of the fibrous paste (2).
2. Use according to claim 1, wherein the whole annual plant stems (3) include whole hemp stems such that: - the homogeneous fibrous mass (5) contains long hemp fibers, as well as short hurd fibers, retained to the long hemp fibers by interlacing, and - the particulate residues (6) contain dust and hurd particles, not retained by the fibers long hemp contained in the homogeneous fibrous mass (5).
3. Use according to any one of claims 1 or 2, wherein the grinding step includes a shredding operation, wherein each of the whole annual plant stems (3) is fractionated by cutting, and which is continued until the whole annual plant stems are cut into said unfibered stem fragments.
4. Use according to any one of the preceding claims, wherein at the end of the grinding step, the maximum dimension of the unfibrated stem fragments is less than 70 mm, preferably less than 60 mm, more preferably less than 50 mm.
5. Use according to any one of the preceding claims, wherein the sorting step includes a sieving operation in which the ground material (4) is sieved so as to separate the particulate residues (6) and the fibrous elements forming the homogeneous fibrous mass (5).
6. Use according to any one of the preceding claims, wherein during the thermomechanical treatment step, an alkaline chemical agent is introduced into the extruder (40) to chemically treat the homogeneous fibrous mass (5).
7. Use according to any one of the preceding claims, wherein the grinding step and the sorting step are carried out continuously with the thermomechanical treatment step.
8. Installation for preparing a fibrous pulp (2) from annual plant stems (3), the installation (1) comprising: - a crusher (10) adapted to crush whole annual plant stems (3) to obtain a ground material (4) containing mainly unfibrated stem fragments with a maximum dimension of less than 80 mm, - a separator (30) adapted to divide the ground material (4) into two fractions, namely: - fibrous elements which are linked together by interlacing so as to form a homogeneous fibrous mass (5), and - particulate residues (6) which are free with respect to the homogeneous fibrous mass, the particulate residues being evacuable by the sorter (30), and - an extruder (40), which is adapted to continuously process the homogeneous fibrous mass (5) introduced into the extruder, and at the outlet of which an extrudate forming said fibrous pulp (2) can be recovered, said fibrous pulp (2) being usable as paper pulp, component for packaging material, or component for construction material.
9. Installation according to claim 8, wherein the crusher (10) comprises: - a rotor (11) provided with knives (13) adapted to split whole annual plant stems (3) by cutting, and - a grid (14), which surrounds the rotor (11) and through which the crushed material (4) exits the crusher (10).
10. Installation according to claim 9, wherein the grid (14) is provided with substantially square profile openings whose sides have a dimension between 20 and 80 mm, preferably between 30 and 70 mm, more preferably between 35 and 60 mm, even more preferably between 40 and 50 mm.
11. An installation according to any one of claims 8 to 10, wherein the sorter (30) comprises a sieve (31) which allows the particulate residues (6) to pass through it, without being crossed by the fibrous elements forming the homogeneous fibrous mass (5) so that these fibrous elements are recovered from an inlet face of the sieve to form the homogeneous fibrous mass (5).