A method for obtaining at least two cellulosic pulps with different characteristics

Abstract

Method for obtaining at least two cellulosic pulps having different characteristics The invention relates to a method for obtaining cellulosic pulps which comprises: a step of mixing plant materials comprising at least 60% of lignocellulosic material having a lignin content greater than or equal to 18% and at least 2% of fibrous material having a lignin content less than or equal to 9%, a step of cooking the mixture of plant materials, a step of drawing off the liquid phase in order to isolate a cellulosic pulp, a step of fractionating the cellulosic pulp into at least two cellulosic pulps each having at least one different fiber characteristic.Thus, unlike the prior art which selects a part of a specific plant material to obtain a given cellulosic pulp, the invention aims to transform a mixture of less selective plant materials to obtain a cellulosic pulp and then to fractionate it in order to obtain two distinct cellulosic pulps.

Description

Title of the invention: Method for obtaining at least two cellulosic pulps having different characteristics

[0001] The present application relates to a process for obtaining at least two cellulosic pulps having different characteristics.

[0002] A hemp-based textile can be manufactured from fibers extracted from the hemp bark. To promote the quality of the bast fibers, hemp is cultivated according to a textile-specific agricultural protocol that involves harvesting the hemp before the seeds mature to prevent the fibers from thickening and stiffening. This textile-specific agricultural protocol is characterized by a high sowing density, on the order of 80 kg / ha.

[0003] According to a procedure, the cut plant material undergoes a first retting stage, consisting of soaking it in a humid environment or leaving it to retting in the field after cutting, to facilitate the separation of the fibers from the heartwood of the stem (also called hurd), followed by a second scutching stage consisting of separating the fibers from the heartwood of the stems by crushing and beating. The resulting fiber is relatively long, on the order of 70 to 80 cm, and must be spun on specific tools to obtain yarn for making fabric.

[0004] Alternatively, the hemp fibers obtained from defibration are shortened and blended with short fibers (such as cotton) so that the resulting fibers can be spun on cotton spinning tools, widely used in the textile industry. However, according to this second method, the resulting textile material comprises at most 70% hemp and is limited in fineness, thus reducing its range of applications.

[0005] Regardless of the processing method, hemp textiles are generally obtained from fibers harvested from plants before the seeds have matured, meaning that the hemp is not used optimally, depriving farmers of income from seed harvesting. Furthermore, retting, scutching, or defibration are necessary to process these fibers, and the retting stage degrades the hurd, which can limit its use to low-value mulching.

[0006] To overcome these drawbacks, according to another agricultural protocol known as mixed farming, hemp is cultivated until the seeds mature. In this case, the entire plant can be utilized. However, hemp fibers from the mixed farming protocol are generally not used to obtain textile fibers for clothing due to the low yields of sufficiently fine bast fibers and the difficulty in exploiting them with tools from the textile industry or with the specific tools for hemp from the agricultural textile protocol.

[0007] Documents FR3106600 and WO2021151904 describe a process for obtaining a hemp-based cellulosic pulp which, after dilution and / or dissolution steps followed by regeneration, yields a textile product such as a textile filament. This process uses plant material comprising, by weight, at least 50% hemp stalks harvested after flowering, said hemp stalks having a hurd content of at least 30% and a length of less than 80 cm. This process includes a cooking step of the plant material as well as at least one refining step aimed at reducing the length of the fibers and / or pieces contained in the cooked plant material.

[0008] According to another application, a cellulosic pulp can be used for the production of paper pulp. In a non-limiting method, this cellulosic pulp is prepared from bast fibers obtained from plant material produced according to a specific agricultural protocol. This plant material is subjected to pretreatment steps such as defibration, aimed at selecting a fraction of the plant material, namely the bast fibers, by removing, in particular, the heartwood (or woody core) of the plant material. These bast fibers are then transformed into cellulosic pulp by a processing method comprising at least one cooking step.

[0009] According to one drawback, for bast fiber plants such as flax, hemp, kenaf, and nettle, producing plant material according to specific, restrictive specifications for farmers generally leads to resource limitations and / or reduced biodiversity. According to another drawback, the pretreatment stages (defibration, scutching, etc.) of the plant material prior to pulp production, aimed at fractionating the plant material to extract, in particular, the bast fibers, are generally energy-intensive and demanding in terms of product cleanliness, as these pretreatment stages are specific to agricultural protocols and / or the plant.

[0010] Finally, the transformation steps of the selected fraction of the selected plant material into cellulosic pulp are specific to that fraction and result in a single type of cellulosic pulp. Thus, the same production of cellulosic pulp from a specific fraction of plant material produced according to specific specifications yields a single cellulosic pulp with specific characteristics.

[0011] The present invention aims to remedy all or part of the drawbacks of the prior art.

[0012] To this end, the invention relates to a process for obtaining cellulosic pulps characterized in that it comprises: a. At least one mixing step of plant materials comprising at least 60% of a first material, called lignocellulosic, having a lignin content greater than or equal to 18% and at least 2% of a second material, called fibrous, having a lignin content less than or equal to 9%, b. at least one cooking step of the mixture of plant materials with a cooking liquid, at a temperature greater than or equal to 80°C, with, at least for part of the cooking time, at least one delignifying agent, c. at least one step of drawing off the liquid phase from the cooked mixture of plant materials in order to isolate a cellulosic pulp, d. at least one step of fractionating the cellulosic pulp into at least two cellulosic pulps, each having at least one different fiber characteristic.

[0013] According to the invention, a less selective mixture of plant materials is used to obtain a cellulosic pulp which is then fractionated to isolate cellulosic pulps, each having at least one given characteristic different from one pulp to another.

[0014] Consequently, unlike the prior art, the specifications of agricultural protocols are less restrictive, which helps to increase the resource and promote biodiversity. The invention allows for greater versatility in the supply of agricultural materials.

[0015] According to another advantage, certain steps of fractionating plant material in order to isolate a portion of the raw plant material can be eliminated, thereby helping to reduce energy consumption and facilitating the adoption of these plants as raw material for the production of cellulosic pulps.

[0016] Finally, the process of the invention makes it possible to obtain, from the same production, at least two distinct cellulosic pulps.

[0017] Other features and advantages will become apparent from the detailed description of the invention which will follow.

[0018] According to one method, a process for obtaining cellulosic pulps includes at least one step of mixing plant materials comprising at least 60% of a first material, called lignocellulosic, having a lignin content greater than or equal to 18% and at least 2% of a second material, called fibrous, having a lignin content less than or equal to 9%.

[0019] For the present application, the percentages correspond to percentages by weight of dry matter.

[0020] According to one configuration, the mixture of plant materials comprises at most 98% of lignocellulosic material. Preferably, the mixture of plant materials comprises between 70 and 90% lignocellulosic material.

[0021] By lignocellulosic material is meant a lignocellulosic material or a mixture of lignocellulosic materials. The lignocellulosic material is selected from annual plants or parts of annual plants such as flax hurds, hemp shives, kenaf woody kernels, nettle woody kernels, miscanthus, reeds, bamboo, bagasse, certain cereal straws, pericarps or flower stems. This list is not exhaustive.

[0022] The lignocellulosic material is in the form of chips which have a length of less than 8 cm, preferably less than 4 cm.

[0023] According to one configuration, the mixture of plant materials comprises at most 40% fibrous material. Preferably, the mixture of plant materials comprises between 10 and 30% fibrous material.

[0024] Fibrous material means a fibrous material or a mixture of fibrous materials. The fibrous material is selected from bast fibers such as hemp fibers, flax fibers, jute fibers, kenaf fibers, nettle fibers, or ramie fibers, for example; pseudostem fibers such as banana fibers, for example; leaf fibers such as abaca fibers or sisal fibers, for example; cotton fibers; or kapok fibers. This list is not exhaustive. Fibrous material is more flexible as opposed to a lignocellulosic material, which is more rigid and / or brittle.

[0025] The fibers of fibrous material have a length of less than 80 cm, preferably less than or equal to 8 cm.

[0026] The mixture of plant materials comprises several different plant species and / or different fractions of the same plant species containing bast fibers.

[0027] The process for obtaining cellulosic pulps includes at least one step of cooking the mixture of plant materials with a cooking liquid, at a temperature greater than or equal to 80°C, preferably greater than 140°C. According to one operating method, the cooking temperature is less than or equal to 190°C.

[0028] In one configuration, the cooking liquid is water in liquid and / or gaseous form. The liquid / solid ratio is greater than or equal to 2, preferably greater than 3.5.

[0029] A cooking step is carried out for a duration, at maximum temperature, less than or equal to 480 min, preferably less than 300 min.

[0030] According to one method, the cooking step comprises at least two successive first and second phases, the first phase consisting of cooking the mixture of plant materials without a delignifying agent, at a temperature greater than or equal to at 80°C, followed by the removal of at least part of the liquid phase of the cooked vegetable material mixture at the end of the first phase of the cooking step, the second phase of the cooking step consisting of cooking the remainder of the mixture in the presence of at least one delignifying agent, at a temperature greater than or equal to 80°C.

[0031] During the first phase of the cooking step, the mixture gradually becomes acidic. As soon as the pH of the mixture falls below a given threshold value corresponding to an acidic pH, preferably in the range of 3 to 5, the first phase is stopped. Thus, the first phase is stopped when the liquid phase contains at least 10% of the hemicelluloses present in the starting material. These hemicelluloses generally have a xylan content of at least 50%, with the quantity of xylans being greater than that of mannans.

[0032] At the end of the first phase of the cooking step, the liquid phase, containing in particular hemicelluloses, polysaccharides or any other valuable by-product such as furfural and acetic acid, is separated from the solid phase of the mixture.

[0033] The second phase of the cooking step is carried out at a temperature greater than or equal to 80°C, the solid phase of the mixture from the first phase of the cooking step being remixed with water and at least one delignifying agent.

[0034] According to a simplified operating method, the cooking comprises only one phase, the cooking step being carried out from the beginning with at least one delignifying agent.

[0035] However, carrying out the cooking step in two phases promotes delignification and makes it possible to obtain a purer cellulosic pulp.

[0036] By delignifying agent, we mean a delignifying agent or a mixture of delignifying agents.

[0037] By way of example, the delignifying agent is sodium hydroxide, a sulfite or a deep eutectic solvent.

[0038] After the cooking step, the production process includes at least one step of drawing off the liquid phase of the cooked mixture of plant materials, also called liquor, which notably contains lignins, in order to isolate a cellulosic pulp. This lignin-containing by-product can be used, for example, to generate energy. These lignins can also be extracted from the liquor to obtain products in the fields of cosmetics, adhesives, resins, or carbon fibers. They have a mineral content of less than 10%, generally less than 5%; an EC50 value (antioxidant capacity) generally between 0.15 and 0.7 mg / mL; and a carboxyl group content generally between 0.7 and 1.6 mmol / g.

[0039] At the end of this drawing-off step, generally at least 80% by weight of the The lignins contained in the initial mixture of plant materials were removed.

[0040] Finally, the process for obtaining cellulosic pulps includes at least one step of fractionating the cellulosic pulp into at least two cellulosic pulps, each having at least one different fiber characteristic.

[0041] Preferably, the fractionation step allows the cellulosic pulp obtained from cooking to be separated into at least two cellulosic pulps, each having at least two different fiber characteristics from each other, including fiber length, degree of polymerization, kappa index, cellulose purity, whiteness and ash content, the cellulosic pulps being used for differentiated applications.

[0042] Thus, unlike the prior art, which involves fractionating a specific plant material prior to obtaining cellulosic pulp to produce a cellulosic pulp with fibers possessing given characteristics, the invention uses a less selective mixture of plant materials to obtain a cellulosic pulp that is then fractionated to isolate cellulosic pulps, each exhibiting at least one given characteristic that differs from one pulp to another. Consequently, the specifications of agricultural protocols are less restrictive, which helps to increase the resource and promote biodiversity.

[0043] Being able to avoid pretreatment steps such as retting, defibration, scutching, ..., which aim to select a part of the plant material before obtaining a cellulosic pulp, helps to facilitate the integration of agricultural biomass into cellulosic pulps and reduce the energy consumption of the process of obtaining cellulosic pulps.

[0044] Finally, the fact of selecting less of the plant material subjected to the cooking stage makes it possible to remove larger volumes of by-products, promoting their valorization.

[0045] According to one method, the cellulose pulp is fractionated so as to split it into a first cellulose pulp with short fibers, of average length less than a given value, and a second cellulose pulp with long fibers of average length greater than or equal to the given value. Depending on one configuration, the given value is in the range of 300 to 800 µm, preferably in the range of 400 to 600 µm.

[0046] By way of example, to carry out this fractionation step, the cellulosic pulp is separated by sieving through a sieve with openings between 0.2 mm and 0.5 mm. The passage rate of bast fibers, according to their length, is closely linked to the dimensions of the sieve openings. The yield per type of cellulosic pulp depends in particular on the dimensions of the sieve openings as well as the initial fibrous material content.

[0047] Of course, the invention is not limited to this method for the fractionation step. Thus, the cellulosic pulp from the cooking step can be fractionated using a centrifugation process.

[0048] The first pulp consists of fibers that have passed through the sieve, which ensures a homogeneous quality of cellulosic pulp in terms of fiber length (and width) and a lower degree of polymerization. This first cellulosic pulp can be used as is or undergo further processing.

[0049] According to one application, the first cellulosic pulp can be used as a dissolving paste. In this case, the cellulosic pulp is generally subjected to a bleaching and / or purification step in order, in particular, to reduce the level of residual lignin, adjust the degree of polymerization and / or reduce the ash content.

[0050] The second pulp consists of fibers with dimensions larger than the sieve slots. This second cellulosic pulp comprises fibers with a greater average fiber length than the first cellulosic pulp, and is more heterogeneous in terms of fiber length. This fiber mixture exhibits greater strength, and the fibers are more delignified. This second cellulosic pulp is particularly suitable for packaging or molding applications of objects requiring good strength, especially due to the cohesion of its fibers.

[0051] Of course, the invention is not limited to fiber length as a characteristic for the cellulosic pulp fractionation step. Other characteristics are conceivable, such as fiber distribution, degree of polymerization, whiteness, kappa number, cellulose purity, or ash content. This list is not exhaustive.

[0052] Furthermore, the process of obtaining cellulosic pulps may include several successive fractionation steps.

[0053] Each cellulosic pulp obtained from the process of the invention can be mixed with at least one other cellulosic pulp.

[0054] At least one of the cellulosic pulps obtained from the process of the invention can be used to obtain products, such as a dissolving paste, a textile product (such as filaments obtained by dissolution and then spinning, for example), a membrane, a molded object, a package or any other product obtained from the cellulosic pulp obtained alone according to the process of the invention alone or mixed with at least one other cellulosic pulp.

[0055] The invention is now described by way of an example.

[0056] The mixture of plant materials comprises 50% fibrous hemp shives (in which (20 to 40% bast fibers remain), 40% miscanthus and 10% flax shives.

[0057] Regarding hemp, it generally comprises 50% hurd, 27% bast fibers, and 23% dust. The hurd has the following chemical composition: approximately 46% cellulose, 24% lignin, and 18% hemicellulose. The bast fibers have the following chemical composition: more than 70% cellulose, approximately 5% lignin, and 10 to 15% hemicellulose.

[0058] In the present case, the hemp straw is dusted and shortened to a length of less than 10 cm and preferably less than 5 cm. The hemp does not undergo any defibration or scutching step.

[0059] Regarding miscanthus, it has the following chemical composition: approximately 40 to 45% cellulose, 25% lignin and 25 to 28% hemicellulose.

[0060] Regarding flax shives, they have the following chemical composition: approximately 43% cellulose, 25% lignin, and 27% hemicellulose. Flax shives can be obtained from flax scutching.

[0061] This mixture of plant material is introduced into a reactor in which water is added so as to obtain a liquid / solid ratio of the order of 3.5 to 7.5.

[0062] Next, this mixture of plant matter and water is subjected to a two-phase cooking step.

[0063] During the first phase, the mixture is heated to a temperature of approximately 120°C to 200°C for a maximum of 150 minutes. When the pH becomes acidic, the filtrate is extracted. This filtrate contains, in particular, a portion of the hemicelluloses (more than 10% by weight).

[0064] In the second phase, the hydrolyzed material is reintroduced into the reactor. Subsequently, a delignifying agent and water are added. For example, the delignifying agent is Kraft liquor or a deep eutectic solvent. The liquid / solid ratio is in the range of 3.5 to 7.5. The mixture is heated to a temperature of approximately 100°C to 170°C for a maximum duration of 60 to 300 minutes.

[0065] At the end of the second phase of the cooking step, the cooking liquor is drawn off. This liquor includes, in particular, a portion of the lignins (more than 60% by weight).

[0066] After the extraction step, the cellulosic pulp is rinsed, resuspended, and then passed through a sieve to perform the fractionation step. For example, the sieve has slits measuring from 0.2 to 0.5 mm.

[0067] The cellulose pulp passing through the sieve corresponds to the first cellulose pulp, and that remaining above the sieve corresponds to the second cellulose pulp. The second cellulose pulp represents 10 to 25% of the total cellulose pulp before the fractionation step.

[0068] The first cellulosic pulp is homogeneous and has a kappa index of approximately 12 to 25 and a degree of polymerization between 800 and 1300. This The initial cellulose pulp may undergo at least one additional step, such as bleaching, to adjust its degree of polymerization and / or increase its purity. This initial cellulose pulp can then be used as a dissolving paste.

[0069] The second cellulose pulp is more heterogeneous in terms of fiber length and has a kappa coefficient at least 2 points lower than that of the first pulp. This second cellulose pulp can be used as molding paste and / or specialty papers.

Claims

Claims

1. Process for obtaining cellulosic pulps characterized in that it comprises: a. At least one step of mixing plant materials comprising at least 60% of a first material, called lignocellulosic, having a lignin content greater than or equal to 18% as well as at least 2% of a second material, called fibrous, having a lignin content less than or equal to 9%, b. At least one step of cooking the mixture of plant materials with a cooking liquid, at a temperature greater than or equal to 80°C with, at least for part of the cooking time, at least one delignifying agent, c. At least one step of drawing off the liquid phase of the cooked mixture of plant materials in order to isolate a cellulosic pulp, d. At least one step of fractionating the cellulosic pulp into at least two cellulosic pulps each having at least one fiber characteristic different from each other.

2. Method according to claim 1, characterized in that the cooking step comprises first and second successive phases, the first phase consisting of cooking the mixture of plant materials without delignifying agent after removal of at least part of the liquid phase of the mixture of plant materials cooked at the end of the first phase of the cooking step, the second phase of the cooking step consisting of cooking the remainder of the mixture in the presence of at least one delignifying agent.

3. Method according to the preceding claim, characterized in that the first phase of the cooking step is stopped as soon as the pH of the mixture is lower than a threshold value corresponding to an acid pH, preferably of the order of 3 to 5.

4. Method according to one of the preceding claims, characterized in that the cooking temperature is between 140 and 190°C.

5. Method according to one of the preceding claims, characterized in that the mixture of plant materials comprises between 70 and 90% lignocellulosic material and between 10 and 30% fibrous material.

6. Method according to one of the preceding claims, characterized in that the lignocellulosic material is in the form of chips which have a length of less than 8 cm, preferably less than 4 cm.

7. Method according to one of the preceding claims, characterized in that the fibers of the fibrous material have a length of less than 80 cm, preferably less than 8 cm.

8. Method according to one of the preceding claims, characterized in that the fractionation step is configured to allow the cellulosic pulp resulting from cooking to be separated into at least two cellulosic pulps each having at least two fiber characteristics different from each other, including in particular the fiber length, the fiber distribution, the degree of polymerization, the kappa index, the cellulose purity, the whiteness and the ash content.

9. Method according to one of the preceding claims, characterized in that the cellulosic pulp is fractionated so as to split it into a first cellulosic pulp having short fibers, of average length less than a given value, and a second cellulosic pulp having long fibers, of average length greater than or equal to the given value.

10. Method according to one of the preceding claims, characterized in that the cellulosic pulps are separated by sieving or centrifugation.

11. Method according to the preceding claim, characterized in that the cellulosic pulp is fractionated by passing it through a sieve having slots between 0.2 mm and 0.5 mm, the cellulosic pulp passing through the sieve corresponding to the first cellulosic pulp, that remaining above the sieve corresponding to the second cellulosic pulp.