Method and installation for preparing a fibrous pulp from annual plant stems, as well as use of such a fibrous pulp

Abstract

Method and installation for preparing a fibrous pulp from annual plant stems, as well as use of such a fibrous pulp. This method comprises a grinding step in which whole annual plant stems (3) are ground so as to obtain a ground material (4) containing mainly non-fibrated stem fragments which have a maximum dimension of less than 80 mm. The method also comprises a sorting step in which the ground material is divided 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 removed.The method also comprises a thermomechanical treatment step in which the homogeneous fibrous mass is introduced and continuously treated in an extruder (40) at the outlet of which an extrudate forming the fibrous paste is recovered. Figure for abstract: Figure 1.

Description

Title of the invention: Method and installation for preparing a fibrous paste from annual plant stems, as well as use of such a fibrous paste

[0001] The present invention relates to a method for preparing a fibrous pulp from annual plant stems. 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 annual plant stems.

[0002] So-called annual plants are plants whose life cycle is of 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 therefore generally less impactful on the environment: in fact, hemp fibers are biodegradable at the end of the life of these materials. In addition, the use of hemp does not generate an "impact transfer" because its cultivation is ecologically virtuous in the sense that it does not deplete the soil.

[0004] In its mature state, hemp comprises a stem from which extend leaves, flowers and fruits. The hemp stem is composed of a central core, which is called hemp shiv and 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 dry weight of the stem, are strong and rich in cellulose; they are commonly used as a base material for high-strength paper pulps, as a base material in the manufacture of rope, as a seal in plumbing / heating under the name "tow", as an insulation material for buildings, etc.Hemp shiv, which represents between 40 and 55% of the dry weight of the stem, is also of interest, particularly due to its moisture absorption and insulation properties; thus, hemp shiv is commonly used as animal bedding, a component of building bricks, agricultural mulch, etc.

[0005] Traditionally, the long outer fibers and the shiv of the hemp stem are used separately. In practice, following the harvesting of hemp stems followed by a more or less long retting, and after having removed the latter from the leaves, flowers and / or fruits that they bear, the stems are usually subjected to a decortication operation which defibrates the stems so as to separate the shiv and the long outer fibers. Such a decortication operation is typically carried out by crushing the hemp stems by beating, for example using a grinder. hammer. At the end of the decortication operation, the long external fibers and the hemp shiv are recovered separately, as well as dust, which represents less than 30% of the dry weight of the stem and which can be used in various ways, for example to produce energy.

[0006] For its part, FR 3015529 considered preparing a 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 in the hemp fibers. The chips, which measure between 3 and 5 cm, are then directly introduced into an extruder in which the chips are successively mixed with water, with water vapor and with an alkali hydroxide, while being compressed and sheared by interpenetrating screws of the extruder. FR 3015529 does not provide any information 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 already crushed and cleaned hemp straw. This composition of lignocellulosic material, which is therefore not directly obtained from whole hemp stalks, is a reconstituted mixture between long hemp fibers, resulting from prior defibering and contained in straw from the outer sheathing of the hemp stalk, and short hemp fibers, also resulting from prior defibering and contained in hemp shiv straw, the respective shares of long and short hemp fibers in this mixture being adjusted to be similar to those of a whole hemp stalk.

[0007] The aim of the present invention is to propose a preparation method and installation which are truly integrated from the whole annual plant stems to obtaining a fibrous paste having controlled mechanical properties.

[0008] To this end, the invention relates to a process for preparing a fibrous paste from annual plant stems, the process comprising:

[0009] - a grinding step in which whole annual plant stems are crushed so as to obtain a ground material containing mainly non-fibrated stem fragments which have a maximum dimension of less than 80 mm,

[0010] - a sorting step in which the ground 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 which are free with respect to the homogeneous fibrous mass, the particulate residues being evacuated, and

[0013] - a thermomechanical treatment step in which the fibrous mass homogeneous is introduced and processed continuously in an extruder at the outlet of which an extrudate forming said fibrous paste is recovered.

[0014] The invention also relates to an installation for preparing a fibrous paste from annual plant stems, the installation comprising:

[0015] - a grinder which is adapted to grind whole annual plant stems of in order to obtain a ground material containing mainly non-fibrated stem fragments which have a maximum dimension of less than 80 mm,

[0016] - a sorter which is adapted to divide the ground 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 which are free with respect to the homogeneous fibrous mass, the particulate residues being evacuable 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 seek to introduce into an extruder a raw material which is directly obtained from whole annual plant stems. To do this, the invention provides that the whole annual plant stems are first ground while avoiding being mainly defibrated, before the ground material thus obtained is sorted to send to the extruder only a homogeneous fibrous mass consisting of fibrous elements which are linked together by interlacing of fibers.These fibrous elements linked together by interlacing contain both long plant fibres and short plant fibres which, although a priori less interesting from a mechanical point of view than the long plant fibres, are acceptable in the raw material entering the extruder since these short plant fibres are not free with respect to the long plant fibres but, on the contrary, are linked by interlacing to the long plant fibres and are thus distributed homogeneously with these long plant fibres.Conversely, the short plant fibers, which are free with respect to the long plant fibers in the aforementioned ground material, are not found in the homogeneous fibrous mass and are, together with the rest of the free particulate residues, removed so as not to be introduced into the extruder and thus to prevent the inhomogeneity that these various free particulate residues would induce in the raw material entering the extruder from causing inhomogeneity in the extrudate continuously exiting the extruder and therefore from occasionally but regularly degrading the mechanical quality of this extrudate. The mechanical properties of the fibrous paste consisting of the extrudate continuously exiting the extruder are thus controlled, in particular homogeneous over time and within the extrudate, while making it possible to work directly from stems of . whole annual plants.

[0021] In practice, the method and installation according to the invention are preferably applied to hemp, the long fibers of the outer sheathing of whole stems of which are found, at the end of the grinding and sorting steps, almost entirely in the homogeneous fibrous mass, while the short fibers of the hemp shiv of the whole stems of the hemp are found, in large part, in the homogeneous fibrous mass, the rest of the hemp shiv being removed within the particulate residues resulting from the sorting step. This being the case, the method and installation according to the invention are applicable to other annual plants, such as flax.

[0022] It will also be noted that the fibrous pulp prepared by the invention can be used for various purposes, preferably in the papermaking, packaging or building materials industries. Thus, the subject of the invention is 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 characteristics of the method and the installation in accordance with the invention, taken in isolation or in all technically possible combinations:

[0027] - Whole annual plant stems include whole hemp stems such that that : - the homogeneous fibrous mass contains long hemp fibres, as well as short shiv fibres, held to the long hemp fibres by interlacing, and

[0028] the particulate residues contain dust and particles of hemp shiv, not retained by the long hemp fibers contained in the homogeneous fibrous mass.

[0029] - The grinding step includes a shredding operation, in which each of the whole annual plant stems are split by cutting, and are continued until the whole annual plant stems are cut into said unfibrated stem fragments.

[0030] - At the end of the grinding step, the maximum dimension of the non-rod fragments defibrated is less than 70 mm, preferably less than 60 mm, more preferably less than 50 mm.

[0031] - The sorting step includes a screening operation in which the ground material is screened so as 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 grinding step and the sorting step are carried out continuously. with the thermomechanical treatment step.

[0034] - The crusher comprises: - a rotor equipped with blades suitable for splitting whole annual plant stems by cutting, and

[0035] - a grid, which surrounds the rotor and through which the ground material exits the grinder.

[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 sorter has 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 on reading the description which follows, given solely by way of example and with reference to the drawings in which [Fig.l] [Fig.l] is a diagram of an installation in accordance with the invention, implementing a method in accordance with the invention.

[0039] An example of a process for preparing, preferably continuously, a fibrous paste from annual plant stems is described below in detail. Here, this process is advantageously implemented by an installation 1 shown schematically in [Fig.l]. The process will be described progressively, detailing its steps in succession and describing in parallel the parts of the installation 1, which make it possible to implement these steps.

[0040] The method makes it possible to prepare a fibrous paste, referenced 2 in [Fig.l], from annual plant stems, referenced 3. The annual plant stems 3 are made available after these stems have been harvested and, where appropriate, stripped of their leaves, flowers and / or fruits.In all cases, the stems of the annual plant 3 used as input to the process and the installation 1 are whole, that is to say that each of these stems is made up of the part of the annual plant, which, just before harvesting, extends in length upwards from the ground, this part of the annual plant being cut at its base, that is to say just above the ground, during harvesting, without any other significant shortening, nor, more generally, any operation affecting the integrity of the stem as such, independently of any operations not relating to the stem as such but to the leaves, flowers and / or fruits having grown on the stem before harvesting, 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 of the method and the installation 1, whole, as explained above. In particular, these hemp stems 3 include, in the form of an integral elongated body, an external sheath, rich in long fibers, typically of a length greater than 5 mm, and a central core, called hemp shiv, which is enclosed inside the external sheath and which is rich in short fibers, typically of a length less than 2 mm. For convenience, the remainder of the description is applied to the case where the annual plant stems 3 are hemp stems.

[0042] The method comprises a grinding step during which the whole hemp stalks 3 are, preferably continuously, ground so as to obtain a ground material 4 containing non-fibrated stalk fragments which have a maximum dimension of less than 80 mm, preferably less than 70 mm, more preferably less than 60 mm, even more preferably less than 50 mm. For this purpose, the installation 1 comprises a grinder 10.

[0043] In the example considered here, the installation 1 comprises a conveying means 20, such as a belt, which makes it possible to continuously feed the crusher 10 with the whole hemp stalks 3 from a stock of the latter, made available at the entrance to the installation 1, in particular to the crusher 10.

[0044] In all cases, the grinding step makes it possible to shorten the length of the whole hemp stalks 3, which, at the inlet of the grinding step and the grinder 10, is much greater than 80 mm, typically being of the order of 1 m, while, at the outlet of the grinding step and the grinder 10, each of the whole hemp stalks 3 is in the form of the aforementioned stem fragments. During the grinding step, each hemp stalk 3 passes from its state of whole stalk to that of the aforementioned stem fragments by means of its subdivision into several points which are distributed along the hemp stalk 3. In other words, the hemp stalk 3 is progressively shortened by transverse cuts to the longitudinal direction of the hemp stalk 3, which are repeated while being distributed along the hemp stalk.Thus, the aforementioned stem fragments are cut from each of the whole hemp stems 3 without defibration, that is to say without separating, within each of these stem fragments, the external sheathing and the hemp shiv from each other, which thus keep their natural reciprocal arrangement intact.

[0045] In continuation of the above considerations, the grinding step includes, according to a preferred implementation, a shredding operation, by which each of the whole hemp stalks 3 is fractionated by cutting, this shredding operation being continued until the whole hemp stalks 3 are cut into the aforementioned non-fiberized stalk fragments. For this purpose, the grinder 10 is advantageously a so-called “knife” grinder, which comprises a rotor 11 which can be driven in rotation on itself. even around an axis XI1, typically arranged horizontally, relative to a fixed casing 12 of the crusher 10. This rotor 11 is provided with knives 13, which are distributed on the rotor 11 around the axis XI1 and which are each adapted to split the hemp stalks 3 by cutting. In addition, the grinder 10 comprises a grid 14, which, in use, is fixed relative to the fixed casing 12 and which is arranged around the rotor 11, surrounding the latter, typically coaxially with the axis XI1, while providing, at the inlet of the grinder 10, a passage 15 large enough for the admission of the whole hemp stalks 3 inside the grid 14. The grid 14 is provided with through openings, which each connect the inside and the outside of the grid 14 to each other and through which the aforementioned stem fragments pass from the inside to the outside of the grid 14.In other words, due to the dimensioning of the through openings of the grid 14, the latter retains the hemp stems 3 inside it, where the latter are subjected to the cutting action carried out by the knives 13 as soon as the latter are driven by the rotor 11, and this as long as the hemp stems 3 do not end up, by successive shortening, in the form of the aforementioned stem fragments. At the same time, once the hemp stems 3 end up in the form of the aforementioned stem fragments, the ground material 4 formed by the latter leaves the grinder 10 through the grid 14.

[0046] According to a practical and effective embodiment, the through openings of the grid 14 have a square profile whose sides have a dimension of 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] Of course, it is understood that the ground material 4 does not only contain the aforementioned non-fibrated stem fragments, but also defibrated residues and offcuts, which result from the edge effects of the fractionation, in particular by cutting, of the whole hemp stems 3. This being the case, the aforementioned non-fibrated stem fragments represent a majority share of the ground material 4, that is to say at least 50% by dry weight of the ground material 4 which comes from the grinding step, in other words which leaves the grinder 10. Advantageously, these non-fibrated stem fragments represent at least 60%, preferably at least 70%, or even at least 80% by dry weight of this ground material 4, in particular thanks to the use of the knife grinder presented above. In any case, it is understood that the hemp shiv initially contained in the whole hemp stems 3 remains, within the ground material 4, largely linked to the long fibres of the external sheathing 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 illustrated schematically in [Fig.l] and detailed above, or several grinding units, which are individually such as that illustrated schematically in [Fig.l] and detailed above. In the second case, the grinding units of the grinder 10 are arranged in parallel, so that the feed flow of the grinder 10, consisting of the whole annual plant stems 3, is distributed between the grinding units in parallel, and / or are arranged in series, so that the respective actions of the grinding units in series on the annual plant stems 3 are in cascade and thus progressive.

[0049] Following the grinding step, the method comprises a sorting step during which the ground material 4, resulting from the grinding step and leaving 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 shiv fibers which are retained 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 here containing dust and particles of hemp shiv, 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 paste 2, while the particulate residues 6 are rejected, being removed from the installation 1, without being used to obtain the fibrous paste 2.

[0053] For this purpose, the installation 1 comprises a sorter 30 which treats the ground material 4 entering the latter, so as to separate the fibrous mass 5 from the particulate residues 6. In practice, various embodiments are possible for the sorter 30.

[0054] According to a practical and effective embodiment, the sorter 30 comprises a sieve 31 which allows the particulate residues 6 to pass through it, without being crossed by the fibrous elements of the homogeneous fibrous mass 5. The sieve 31 thus makes it possible to retain, on its inlet face which is advantageously turned upwards when the sieve 31 is arranged horizontally as envisaged schematically in [Fig.l], the homogeneous fibrous mass 5 which is removed from this inlet face of the sieve 31 to be subjected to the rest of the method 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, with a view to evacuating these particulate residues 6, as illustrated schematically in [Fig.l]. In practice, the sieve 31 takes various embodiments, such as a flat sieve, as illustrated schematically in [Fig.l], or a drum sieve.

[0055] According to an optional arrangement, which improves the efficiency of the sieve 31, the sorter 30 comprises a vibrating device 32 which sets the sieve 31 into vibration.

[0056] Whatever the specificities of the sieve 31, the latter allows, more generally, to implement, during the sorting step, a sieving operation in which the ground material 4 from the grinding step, 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 coming from the external sheathing of the hemp stems 3, and short and coming from the shiv of the hemp stems 3, are, within the homogeneous fibrous mass 5, distributed homogeneously due to the interlacing of these long and short fibers between them, while being free of the particulate residues 6, partially fibrous but from which the homogeneous fibrous mass 5 has been freed during the sorting step, 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 illustrated schematically in [Fig.l] and detailed above, or several sorting units, which are individually such as that illustrated schematically in [Fig.l] 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 sorting units in parallel, and / or are arranged in series, so that the respective actions of the sorting units in series on the ground material 4 are in cascade and thus progressive.

[0059] The method also comprises a thermomechanical treatment step, in which the homogeneous fibrous mass 5, resulting from the sorting step and leaving the sorter 30, is introduced and treated continuously in an extruder 40 of the installation 1. At the end of the thermomechanical treatment step, an extrudate leaving 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 in 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, then is progressively treated by these two screws by traveling along the latter, until it leaves 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 an ad hoc metering system, not illustrated in [Fig.l].

[0061] In all cases, the structural and operational specificities of the extruder 40 are not limiting and are adjustable by those skilled in the art, so that the thermomechanical treatment applied by the extruder 40 to the homogeneous fibrous mass 5 makes it possible to separate the different fibers from the homogeneous fibrous mass 5 and to continue cutting the long fibers of the latter, while softening the lignin associated with these different fibers, and this to obtain, at the outlet of the extruder 40, the paste fibrous 2 having adequate properties, particularly mechanical.

[0062] According to an optional arrangement not illustrated in [Fig.l], an alkaline chemical agent, such as sodium hydroxide or potassium hydroxide, is, during the thermomechanical treatment step, introduced into the extruder 40 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, during the thermomechanical treatment step, introduced into the extruder 40 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, that is to say of the homogeneous fibrous mass 5, makes it possible to control the properties, in particular mechanical properties, of the fibrous paste 2 along the extrudate leaving the extruder 40, while avoiding any local inhomogeneity in this extrudate. Thus, the presence of a significant portion of hemp shiv in the homogeneous fibrous mass 5, which makes it possible to minimize the loss of material used to obtain the fibrous paste 2, is not a problem for giving the fibrous paste 2 satisfactory properties, since this hemp shiv is distributed homogeneously in the homogeneous fibrous mass 5.Advantageously, this also makes it possible to reduce the energy consumption of the thermomechanical treatment step since the latter can be satisfied with heating naturally generated by the mechanical work of the screws of the extruder on the material treated by the latter, by limiting, or even avoiding, the heating of the barrel of the extruder 40 and / or by limiting, or even avoiding the introduction of water vapor into the barrel. Similarly, this also makes it possible, advantageously, to limit, or even avoid introducing any alkaline chemical agent into the barrel of the extruder 40.

[0065] Optionally, the method further comprises a post-treatment step in which the fibrous pulp 2 is subjected to one or more additional treatments with a view to its final use, for example refining. The specificities of this post-treatment step and the equipment enabling the implementation thereof are not limiting and fall within the usual knowledge of the field.

[0066] In any case, the fibrous pulp 2, where appropriate after it has been subjected to the post-treatment step, can be used for various purposes, in particular as:

[0067] - paper pulp,

[0068] - component for packaging material, in particular in the form of cellular products molded and / or thermoformed losiques or in the form of packaging reinforcements, or

[0069] - component for building material.

[0070] Various arrangements and variants to the method and installation 1, having been described so far, are conceivable:

[0071] - rather than, as described above preferentially, the grinding and sorting are implemented continuously with the thermomechanical treatment step, one and / or the other of these crushing 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 from whole stems 2, such as flax, kenaf, jute, abaca.

Claims

Claims

1. A method for preparing a fibrous pulp (2) from annual plant stems (3), the method comprising: - a grinding step in which whole annual plant stems (3) are ground so as to obtain a ground material (4) containing predominantly non-fibrated stem fragments which have a maximum dimension of less than 80 mm, - a sorting step in which the ground material (4) is divided into two fractions, namely: - fibrous elements which are bound 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 removed, and - a thermomechanical treatment step in which the homogeneous fibrous mass (5) is introduced and continuously treated in an extruder (40) at the outlet of which an extrudate forming said fibrous pulp (2) is recovered.

2. A method 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 hemp shiv fibers, retained to the long hemp fibers by interlacing, and - the particulate residues (6) contain dust and shiv particles, not retained by the long hemp fibers contained in the homogeneous fibrous mass (5).

3. A method according to one of claims 1 or 2, wherein the crushing step includes a shredding operation, in which 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 non-fibered stem fragments.

4. A method according to any preceding claim, wherein which at the end of the grinding step, the maximum dimension of the non-fibrated stem fragments is less than 70 mm, preferably less than 60 mm, more preferably less than 50 mm.

5. A method according to any one of the preceding claims, wherein the sorting step includes a screening operation in which the ground material (4) is screened so as to separate the particulate residues (6) and the fibrous elements forming the homogeneous fibrous mass (5).

6. A method 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) therein.

7. A method according to any preceding claim, wherein the grinding step and the sorting step are carried out continuously with the thermomechanical treatment step.

8. Use of a fibrous pulp (2) prepared by the method according to any one of the preceding claims, wherein the fibrous pulp (2) is used as: - paper pulp, - component for packaging material, or - component for construction material.

9. Installation for preparing a fibrous paste (2) from annual plant stems (3), the installation (1) comprising: - a grinder (10) which is adapted to grind whole annual plant stems (3) so as to obtain a ground material (4) containing mainly non-fibred stem fragments which have a maximum dimension of less than 80 mm, - a sorter (30) which is 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 treat the homogeneous fibrous mass (5) introduced in the extruder, and at the outlet of which an extrudate forming said fibrous paste (2) can be recovered.

10. Installation according to claim 9, in which the crusher (10) comprises: - a rotor (11) provided with knives (13) adapted to split the whole annual plant stems (3) by cutting, and - a grid (14), which surrounds the rotor (11) and through which the crushed material (4) leaves the crusher (10).

11. Installation according to claim 10, in which the grid (14) is provided with openings with a substantially square profile, the sides of which have a dimension of 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.

12. Installation according to any one of claims 9 to 11, in which 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).

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