ES2806475T3 - A continuous process for the production of cellulose pulp - Google Patents

Abstract

A continuous process for the production of cellulose pulp of very high whiteness from grass-type vegetable raw material, wherein said process comprises the stages of: (i) preparing the grass-type vegetable raw material by grinding to produce a raw material with a longitudinal size distributed from 5 to 30 mm and a diameter of 0.1 to 2 mm, and with fine dust particles removed by dedusting said raw material with a fan; (ii) continuous digestion of a vegetable raw material without grass type dust prepared in step (i) in a continuous digester (1) formed as a longitudinal column internally equipped with an endless screw conveyor and a heating unit; wherein the grass-like vegetable raw material is continuously fed directly into the upper part of said digester by the conveyor (4); characterized in that in parallel to said raw material feed, the chemical products for digestion: NaOH, NaCl, and fresh water and / or the regenerated white liquor; they are continuously introduced into the upper part of said digester (1); maintaining the digestion temperature at 70 to 120 ° C and the average composition of the suspension thus formed during said continuous digestion is kept within the following ranges: (a) 0.50 to 2.00% w / w NaOH; (b) 0.50 to 1.50% w / w NaCl; (c) 10 to 15% w / w of grass-type vegetable raw material; and (d) process water; up to 100% of the suspension; wherein the weight percentages of the ingredients are calculated on the weight of the total suspension; and wherein a dissolution of non-cellulosic substances of the grass-like vegetable raw material occurs during mass transfer from one side to the other of said longitudinal digester lasting 1.5 to 3 hours; (iii) where the cooked pulp is discharged continuously, at the same rate as at which the raw material is fed into the digester (1), from said digester (1), by the conveyor, directly into the disperser ( 5), wherein the suspended cellulose pulp is further subjected to main dispersion, producing finely dispersed cellulose fibers; (iv) wherein the fine suspension of the dispersed cellulose pulp from step (iii) is further subjected to bleaching: (a) without any removal of the chemicals from the digestion phase (ii); and (b) without the use of additional NaOH for the purpose of bleaching; wherein the dispersed cellulose pulp is continuously fed into the continuous bleaching reactor (6) which is formed as a longitudinal column equipped with an endless screw conveyor and a heating unit; and wherein the bleaching chemicals are: (a) hydrogen peroxide / H2O2 / aqueous solution with 20 to 40% w / w H2O2; and (b) sodium silicate / xNa2O-ySiO2 / aqueous solution with 30 to 45% w / w Na2O + SiO2 combined, with Na2O: SiO2 molar ratio = 1: 1 to 3.3: 1; and wherein said chemical products are introduced through the collector (7) into the bleaching reactor (6), maintaining the bleaching temperature from 70 to 100 ºC, for 45 minutes to 1.5 h, and the average composition of the suspension thus formed during the complete bleaching process is maintained within the following ranges: (a) 0.50 to 2.00% w / w of said H2O2 solution; (b) 0.50 to 2.00% w / w of said xNa2O-ySiO2 solution; (c) 10 to 15% w / w cellulose; and (d) process water; up to 100% w / w of the suspension; wherein the weight percentages of the ingredients are calculated on the weight of the total suspension; (v) the suspension of the bleached cellulose pulp thus obtained, discharged from the bleaching stage (iv), is optionally further processed through the disperser (8); and then it is transferred to the dewatering device (9); (vi) where the bleached and finally dispersed cellulose pulp from stage (v) is drained, it is separated from the black liquor producing: (a) bleached cellulose pulp of 10 to 15% w / w of dry matter; and (b) black liquor containing lignin and other cooking and bleaching by-products; and (vii) the bleached cellulose pulp is further washed in the wash container (12) with additional fresh water, which is introduced through the collector (13) giving: (a) bleached cellulose pulp and final wash of very high whiteness, greater than 90%, of 10% w / w of dry matter, and lignin content <5% w / w, which is transported out of the complete process through the collector (14); and (b) additional amount of water from the washing process that is transferred by the collector (15), together with the black liquor from the drain stage (vi), which is transported by the collector (10) to the electrolytic reactor (19 ); (viii) where the black liquor and the water from the washing process are also subjected to electrolysis with direct electrical current between two electrodes, at an electrical potential of 3 to 30 V, and an electrical current density of 1 to 10 A / dm2, at 10 to 95 ° C, where the lignin and by-products are separated on top of the electrolyte solution in the anode compartment, and are continuously withdrawn from the electrolytic reactor; and where the resulting electrolyte solution with regenerated NaOH and NaCl solution, which represents the white liquor, is regenerated from the cathode compartment back to the cooking process, through the collector (21), to the continuous digester (1), closing thus the complete pulp manufacturing process.

Description

DESCRIPTION

A continuous process for the production of cellulose pulp

Technical field

The invention relates to an improved continuous process for the production of cellulose pulp of very high whiteness,> 90%, and lignin content <5% w / w, from raw material type ground grass such as miscanthus (Miscanthus xgiganteus , Anderson).

Technical problem

The main technical problem, solved by the present invention, is the formation of the novel process for the production of cellulose pulp having high whiteness, from a grass-like raw material in an efficient way. This efficient way is achieved by using as little cheap environmentally acceptable chemicals as possible and under conditions of digestion and bleaching as mild as possible. Furthermore, such a process should ensure the maximum preservation of the natural cellulose fibers of the starting raw material.

Conventional cellulose pulping uses high energy and water consuming sulfur based chemicals or milder but energy consuming sodium hydroxide based processes in intense pulp disperging, all together has overall a great negative impact on the environment.

The most important features of any successful pulp production from grass-type raw material are to ensure:

(i) digestion as gentle as possible of the ground plant material to remove the unwanted non-cellulosic fraction, mainly lignin;

(ii) the use of minimal mechanical shear force including smooth dispersion and further processing of a pulp suspension;

(iii) careful and effective bleaching; to preserve natural cellulose fibers which is the key basis for high quality paper;

(iv) efficient removal of lignin and other non-cellulosic by-products to the acceptable level, preferably <5% w / w; and with

(v) efficient regeneration of digestion chemicals and black liquor processing water to ensure the closed cycle of process material balance and therefore minimal environmental impact.

The first technical problem solved with the described invention is the use of more gentle and optimized chemical reagents during the digestion and bleaching stages and very high whiteness, greater than 90%, thus avoiding harsh manufacturing conditions and high energy consumption. sulfur-based and hazardous chemicals, and environmental contamination with wastewater containing residues from all of these processing chemicals.

The technical problem solved with the described invention is to find a solution within the framework of "green" chemical technology, characterized by:

(a) very mild digestion and bleaching conditions with a novel bleaching system based on hydrogen peroxide (H ² O ² ) without the use of sodium hydroxide;

(b) high concentration of the cellulose suspension during digestion; therefore high pulp production by volume (productivity);

(c) high preservation of the natural fibers of the raw material using mild conditions of the digestion and bleaching reactions, as well as minimal mechanical stress during dispersion and optional screening; and (d) the complete regeneration of the digestion chemicals and the process waters practically without affecting the environment, in an integrated way, for the elimination of the lignin and other non-cellulosic by-products of the spent liquor (black) and the residual water is it accompanies the regeneration of the solution of the digestion chemicals - called “white liquor”.

To the best of our knowledge, this is the first ecological process for the manufacture of cellulose pulp that is carried out under such mild digestion conditions, with very low water consumption, low energy consumption, which can produce cellulose of very high whiteness, higher than 90%, with <5% lignin, which can produce superior quality paper with excellent mechanical properties. The process according to this invention works well only when grass-type raw material is used.

Prior state of the art

The production of cellulose pulp for papermaking from renewable, easy-growing and cheaper plant raw material is of increasing importance in the modern paper industry. In this way, the classical wood-based processes come to be replaced by grass-like raw material such as miscanthus (Miscanthus x giganteus, Anderson), rod grass (Panicum virgatum, Linne), sorghum (Sorghum sp., Linne), reed (Phragmites) australis, Cav.), cane (Arundo donax, Linne), straw of various cereals, etc.

For example, miscanthus (Miscanthus x giganteus, Anderson) is one of the most suitable grass-type raw materials for such use; see references 1 and 2:

1) G. Wegener: “Pulping innovations in Germany”, Ind. Crops. Prod. 1 (1992) 113-117;

2) C. Cappelletto, F. Mongardini, B. Barberi, M. Sannibale, M. Brizzi, V. Pignatelli: "Papermaking pulps from the fibrous fraction of Miscanthus x giganteus", Ind. Crops. Prod. 11 (2000) 205-210.

Within the pulping process, the most important phase is digestion. This means cooking the crushed lignocellulosic material in the aqueous solution of suitable digestion chemicals. There are various pulping processes in relation to the chemicals used. The best known technologies are based on digestion with solutions of:

(i) Sulfur-containing chemicals: sodium carbonate (Na2CO3) and sodium sulfite (Na2SO3), magnesium hydroxide [Mg (OH) ² ] and magnesium sulfite (MgSO3), ammonium hydroxide (NH ⁴ OH) and ammonium sulfite [(NH4) 2SO3], calcium hydrogen sulfite [Ca (HSO ³ ) ² ], magnesium hydrogen ^{sulfite [Mg (HSO 3} ) ² ], sodium hydroxide (NaOH), sodium sulfide (Na ² S) and sodium sulfate (Na2SO4);

(ii) chemicals that do not contain sulfur: sodium carbonate (Na2CO3), sodium hydroxide (NaOH); and (iii) acids such as nitric acid (HNO ³ ).

Such a solution of the digesting chemicals is also known as "white liquor", which represents either fresh or regenerated solution of the digesting chemicals. The white liquor helps to remove non-cellulosic materials, which thus dissolve in the solution, leaving relatively pure cellulose fibers suspended in this liquid phase.

The raw material cooked with the non-cellulosic materials removed, that is, a "pulp", is, at the end of digestion, suspended in the used solution, which contains various chemical forms of non-cellulosic plant ingredients and preserves the chemicals of digestion. This aqueous phase is called the "black liquor". Therefore, the pulp after digestion is a suspension of basically pure cellulose fibers in the black liquor. Regarding the type of digestion, as one of the most important technological aspects of pulping, processes that are based on the decreased use of sulfur-based chemicals are of significant advantage. The most prominent reason is ecological. The use of sulfur-free processes are of the highest importance in preserving the environment, also avoiding corrosion problems in production equipment, as well as toxicological issues.

One of the most environmentally friendly sulfur-free processes uses sodium hydroxide-based technology. The use of sodium hydroxide (NaOH) as the sole digestion chemical is known in the art and also specifically in processing based on grass-type raw materials such as rice straw, esparto, rush, jute and others. One of the processes is carried out with a 5% aqueous solution of sodium hydroxide (NaOH) at 90 ° C for several hours; see reference 3:

3) GB 770,687; "Method of producing cellulose"; Applicant Aschaffenburger Zellstoffwerke (DE).

The digestion steps can be performed with microwave assisted heating (MO). Therefore, Zhu et al. Described the process of pretreatment of miscanthus (Miscanthus x giganteus, Anderson) with sodium hydroxide solution (NaOH) at very high temperatures (130 to 200 ° C) at elevated pressures for 20 minutes under heating by MO . Such pretreated miscanth also gave much better performance in sulfuric acid (H ² SO ⁴ ) catalyzed hydrolysis to glucose as raw material for fermentation in bioethanol; see reference 4:

4) Z. Zhu, D. J. Macquarrie, R. Simister, L. D. Gomez, S. J. McQueen-Mason: "Microwave assisted chemical pretreatment of Miscanthus under different temperature regimes", Sustain. Chem. Process 3 (2015) DOI: 10.1186 / s40508-015-0041-6.

Although this process focuses on the production of glucose from miscanthus, such pretreatment suggests some potential in the use of OM for the digestion of miscanthus. Of course, the reaction conditions described in reference 4 are very harsh and clearly not compatible with the production of high quality cellulose fibers with high whiteness.

In addition to digestion, bleaching is another important key stage in the manufacture of high-whiteness cellulose is the bleaching stage. The most common systems for cellulose bleaching are those based on chlorine-containing systems, for example sodium hypochlorite (NaOCl), or systems based on hydrogen peroxide (H ² O ² ), the latter being preferred.

Therefore, patent GB681661 describes a process for bleaching cellulose pulp by using the following bleaching system:

(a) 0.30 to 1.75% hydrogen peroxide (H ² O ² );

(b) 0.75 to 3.25% sodium hydroxide (NaOH);

(c) 20 to 65% cellulose pulp (dry matter); Y

(d) process water up to 100%;

at temperatures below 54.4 ° C (130 ° F). Optionally, the sodium silicate solution (xNa ² O ^ ySiO ² ) is used as a hydrogen peroxide stabilizer; see reference 5:

5) Document GB681661A; "Treatment of Chemical Pulp"; Applicant: Buffalo Electro-Chemical Co., Inc. (US).

This document suggests the use of a combination of hydrogen peroxide (H ² O ² ) and sodium hydroxide (NaOH) as the bleaching system for cellulosic materials at relatively mild reaction conditions, below 54.4 ° C. The use of sodium silicate solution is mentioned only in the context of its stabilizing action on H ² O ² .

The present invention is also based on hydrogen peroxide (H ² O ² ) bleaching, but the main crucial and advantageous difference is in the use of a combination of H ² O ² solution and sodium silicate (xNa ² O ^ ySiO ² ), basically without the use of sodium hydroxide (NaOH) as a co-reactant which, in reaction with H ² O ² , generates an equilibrium concentration of hydroperoxide anions (HOO-) as a real oxidizing species.

In addition to conventional heating in the cellulose pulp bleaching process, microwave assisted heating (MO) has also been employed. In this way, Law described the method and apparatus for the MO-assisted bleaching of cellulose pulp; see reference 6:

6) Document CA2038651A1; K. N. Law: "Method and apparatus for bleaching pulps"; Applicants: K. N. Law, J. L. Valade (US).

Furthermore, Law et al. Described the method for cellulose pulp bleaching using OM heating and a combination of bleaching chemicals consisting of hydrogen peroxide (H ² O ² ) and sodium hydroxide (NaOH). In this process, the sodium silicate solution can also optionally be used as the stabilizer for H ² O ² ; see reference 7:

7) K. N. Law, S. G. Luo, J. L. Valade: "Characteristics of Peroxide Bleaching of Microwave-Heated Thermomechanical Pulps", J. Pulp Paper Sci. 19 (1993) J181-J-186.

Both documents teach that, in general, MO can be used as an alternative heating medium in cellulose pulp bleaching processes. However, these papers remain silent on the effect of the hydrogen peroxide (H ² O ² ) -sodium silicate (xNa ² O ^ ySiO ² ) system combined, without the use of highly alkaline sodium hydroxide (NaOH), on the efficacy (yield, quality and whiteness) of cellulose pulp bleaching from grass-like raw material.

Furthermore, a feature of the process for making cellulose pulp of very high whiteness is certainly whether the cooking chemicals from the digestion stage are removed from the pulp or can be retained in the pulp prior to the bleaching stage.

Ancient processes used to remove the black liquid that contains digestion chemicals, lignin, and other non-cellulosic by-products.

Abu et al. Described a process for the production of fibrous cellulose material that includes: (1) crushing the plant material, (2) mixing it with a 0.5 to 1% aqueous sodium hydroxide (NaOH) solution, (3 ) sieve, (4) pass the material thus obtained from the extruder where the material is mixed with hydrogen peroxide (H ² O ² ) at 120 to 150 ° C at 2x106 Pa (20 bar), and (5) discharge it outside from the extruder and wash it with water; see reference 8:

8) Document DE19603491A1; "Production of fibrous cellulose material free from other plant components"; Applicants: SI Abu, D. Kistmacher, R. Berg (DE).

This document suggests the possibility of continuous processing of plant material containing cellulose with digestion chemicals such as sodium hydroxide (NaOH) and additional bleaching with hydrogen peroxide (H ² O ² ), without the use of intermediate washing of the products. digestion chemicals (NaOH) before bleaching.

The present invention is also characterized by one of the key details that it does not involve washing the remaining cooking chemicals after the digestion phase, but directly undergoes dispersion and bleaching steps.

However, the key difference between the process of the present invention and DE19603491A1 is in the facts that the above process uses:

(i) digestion system based on specific optimized concentrations of the digestion chemicals NaOH and NaCl; Y

(ii) bleaching system based on also specific and optimized concentrations of H ² O ² and sodium silicate, basically without additional NaOH.

Mikulic described a continuous process for the production of cellulose pulp from grass-like raw material such as miscanthus (Miscanthus x giganteus, Andersson), using a vertical digester with smooth internal walls without any sieving or mixing element inside, by using of very dilute cooking chemicals, sodium hydroxide (NaOH) and sodium chloride (NaCl) or sodium sulfite (Na2SO3), with the following average digestion suspension composition:

(a) 0.90% to 1.50% w / w NaOH;

(b) 0.15% to 0.40% w / w NaCl or Na2SO3;

(c) 15 to 18% w / w of raw material type ground grass; Y

(d) process water up to 100%;

wherein the digestion process is heated by conventional means, either through direct heating of the digester or with heating means; see reference 9.

In addition to a new type of simplified vertical digester, this process also relies on the subsequent processing of the cooked pulp within the screening and fractionating device that isolates a good cellulose pulp fraction and separates the inappropriate one. The latter is further subjected to pulp milling in one or more suitable pulp mills producing the appropriate cellulose pulp. This can also optionally be bleached with conventional bleaching chemicals to produce high quality cellulose suitable for paper production.

To the best of our knowledge, this is the most detailed prior art document for the present invention; see reference 9:

9) WO2015 / 150841A1; M. Mikulic: "A continuous process for production of cellulose pulp from grass-like feedstock"; applicant: M. Mikulic.

Unlike WO2015 / 150841A1, the process of the present invention employs a combination of:

(a) specific and optimized concentration of the digestion chemicals, sodium hydroxide (NaOH) and sodium chloride (NaCl), and at a lower content of raw material in the digestion suspension, 10 to 15% w / w matter dry;

(b) dispersion; basically without removing the remaining cooking chemicals, subsequently (c) modified bleaching process based on hydrogen peroxide solutions (0.50 to 2.00% w / w H ² O ² ) and sodium silicate (0.50 to 2.00% w / w xNa ² O ^ ySiO ² ) of specific and optimized concentrations, basically without the use of additional sodium hydroxide (NaOH) for the purpose of bleaching; as well as

(d) integrated electrolytic regeneration of cooking chemicals and isolation of lignin and other by-products, enabling fully closed-loop, high-efficiency cellulose manufacturing of process materials.

The digestion (a) and bleaching (c) processes can be carried out by microwave-assisted heating (MO) with good comparable results within the significantly shorter period of time for each stage.

The combination of steps (a) - (c) results in very high quality cellulose pulp of very high whiteness,> 90%, with <5% w / w lignin, which cannot be produced by the process described in reference 9.

The removal of lignin and other by-products from the black liquor solution through the use of an electrolytic reactor (cell) is known in the art. Typical example of such technology was described by Edel et al; see reference 10:

10) US4584076A; E. Edel, J. Feckl, C. Grambov, A. Huber, D. Wabner: "Process for obtaining lignin from alkaline solutions thereof"; Applicant: MD Organocell Zellst Umwelttec (DE).

In summary, during the electrolysis of the black liquor solution by using direct current (DC) within the appropriate electrodes, the lignin and other by-products are separated in the anode compartment, while the regeneration of sodium hydroxide (solution of cooking or white liquor) is regenerated in the cathode compartment. The process of this invention broadly uses a similar electrolytic reactor for the removal of lignin and other by-products and for the parallel regeneration of cooking chemicals, NaOH NaCl solution (white liquor), but the main and crucial difference is in the fact that the electrolytic process in this invention works with the black liquor solution comprising dilute sodium lignin and sodium chloride (NaCl). In this way, the black liquor solution of this invention has a better conductance due to the presence of sodium chloride, while the process of reference 10 is based on the processing of sodium lignin alone from Organosolv technology for the manufacture of cellulose.

The process of the present invention provides very high quality cellulose of very high whiteness,> 90%, at high yield, at very low consumption of water and energy.

The technology for the production of cellulose pulp from grass-like raw material according to this invention represents a novel and inventive technology, as described in the detailed description of the invention. Compendium of the invention

The present invention describes a continuous process for the production of cellulose pulp of very high whiteness from grass-like vegetable raw material. Understanding the process the stages of:

(i) preparation of the grass-type vegetable raw material by grinding to produce a raw material with a longitudinal size distributed from 5 to 30 mm and a diameter of 0.1 to 2 mm, and with fine dust particles removed by dedusting of said raw material with fan;

(ii) continuous digestion of a grass-type dustless vegetable raw material prepared in step (i) in a continuous digester (1) formed as a longitudinal column internally equipped with an endless screw conveyor and a heating unit; wherein the grass-like vegetable raw material is continuously fed directly into the upper part of said digester by the conveyor (4);

where in parallel to said raw material feed, the chemical products for digestion, sodium hydroxide (NaOH) and sodium chloride (NaCl), and fresh water and / or the regenerated white liquor, are continuously introduced into the part top of said digester (1); maintaining the digestion temperature at 70 to 120 ° C and the average composition of the suspension thus formed during said continuous digestion is kept within the following ranges:

(a) 0.50 to 2.00% w / w NaOH;

(b) 0.50 to 1.50% w / w NaCl;

(c) 10 to 15% w / w of grass-type vegetable raw material; Y

(d) process water; up to 100% w / w of the suspension;

wherein the weight percentages of the ingredients are calculated on the weight of the total suspension;

wherein a dissolution of the non-cellulosic substances of the grass-like vegetable raw material occurs during the mass transfer from one side to the other of said longitudinal digester lasting 1.5 to 3 hours;

(iii) where the cooked pulp is discharged continuously, at the same rate as at which the raw material is fed into the digester (1), from said digester (1), by the conveyor, directly into the disperser ( "Disperger") (5), where the suspended cellulose pulp is subjected to main dispersion, producing finely dispersed cellulose fibers;

(iv) wherein the fine suspension thus obtained from the dispersed cellulose pulp is further subjected to bleaching: (a) without any removal of the chemicals from the digestion phase (ii); Y

(b) without the use of additional NaOH for the purpose of bleaching; Y

wherein the dispersed cellulose pulp is continuously fed into the continuous bleaching reactor (6) which, analogously to the digester (1), is formed as a longitudinal column internally equipped with an endless screw conveyor and a heating unit;

where the bleaching chemicals are:

(a) 20 to 40% w / w aqueous hydrogen peroxide (H ² O ^{2) solution;} Y

(b) 30 to 45% aqueous sodium silicate (xNa ² O ^ ySiO ² ) solution with the molar ratio Na2O: SiO2 = 1: 1 to 3.3: 1; which are introduced through the collector (7) into the bleaching reactor (6), maintaining the bleaching temperature from 70 to 100 ° C, for 45 minutes to 1.5 h, and the average composition of the suspension thus formed during the complete bleaching process it remains within the following ranges:

(a) 0.50 to 2.00% w / w of said H ² O ² solution;

(b) 0.50 to 2.00% w / w of said xNa ² O ^ ySiO ² solution;

(c) 10 to 15% w / w cellulose; Y

(d) process water; up to 100% w / w of the suspension;

wherein the weight percentages of the ingredients are calculated on the weight of the total suspension;

(v) the suspension of the bleached cellulose pulp thus obtained, discharged from the bleaching stage (iv), is optionally further processed through the disperser (8); and then it is transferred to the dewatering device (9); (vi) where the bleached and finally dispersed cellulose pulp is drained, it is separated from the black liquid producing:

(a) cellulose pulp bleached from 10 to 15% w / w in dry matter; Y

(b) black liquor containing lignin and other cooking and bleaching by-products; Y

(vii) The bleached cellulose pulp is further washed in the washing container (12) with additional fresh water, which is introduced through the collector (13), giving:

(a) bleached and final washed cellulose pulp, of very high whiteness, greater than 90%, of 10% w / w in dry matter, and lignin content> 5% w / w, which is transported out of the complete process to through the manifold (14); Y

(b) Additional amount of water from the washing process, which is transferred through the collector (15), together with the black liquor from the drainage stage (vi), which is transported by the collector (10), to the electrolytic reactor ( 19); (viii) where the black liquor and the water from the washing process are subjected to electrolysis with the direct electric current (DC) between two electrodes, at an electric potential of 3 to 30 V, and an electric current density of 1 to 10 A / dm2, at 10 to 95 ° C, where the lignin and by-products are separated on top of the electrolyte solution in the anode compartment, and are continuously removed from the electrolytic reactor; and where the resulting electrolyte solution with regenerated NaOH and NaCl solution, which represents the white liquor, is regenerated from the cathode compartment back to the cooking process, through the collector (21), to the continuous digester (1), closing thus the entire cellulose manufacturing process.

The continuous process for the production of cellulose pulp according to the present invention further optionally involves the following manufacturing steps:

(ix) screening and fractionation, wherein the diluted suspension from step (vii) is processed through a screening and fractionating device (22) equipped with a 0.1 to 0.5 mm sieve, producing two fractions;

(a) the first fraction that does not pass through the 0.1 to 0.5 mm sieve; Y

(b) the second fraction that passes through the 0.1 to 0.5 mm sieve, which is considered a good cellulose pulp of very high whiteness, greater than 90%, suitable for making higher quality paper or cellulose sheets, which is transported out of the complete process through the collector (23); and (x) milling the cellulose pulp; where the (a) fraction of the cellulose pulp from stage (ix), undergoes one or two subsequent processes through the pulp mills (24,25) producing the cellulose pulp, which is transferred, by the collector (26), to the screening and fractionating device (22).

In another embodiment of this invention, the cooking chemicals for the digestion step (ii) are introduced as a mixture of concentrated aqueous solutions of 20 to 50% w / w NaOH and 10 to 30% w / w NaCl.

The continuous digestion phase (ii) is preferably carried out at 90 to 100 ° C.

Furthermore, in another embodiment of the present invention, the bleaching chemicals for bleaching stage (iii) are introduced to the continuous bleaching reactor (6) in the form of concentrated aqueous solutions of 30 to 45% w / w of sodium silicate (xNa ² O ^ ySiO ² ); and 20 to 40% w / w hydrogen peroxide (H ² O ² ).

The continuous bleaching process is preferably carried out at 85 to 100 ° C.

The continuous process for the production of cellulose pulp according to the present invention can alternatively be carried out by using microwave-assisted heating (MO). In this case, the heating units in the continuous digester (1) and / or the continuous bleaching reactor (6) are microwave generating magnetrons (MO).

The grass-like raw material that can be processed by the process of this invention includes the stems of selected plant species from the group of: wheat (Triticum vulgare, Linne); rice (Oryza sativa, Linne); barley (Horedum vulgare, Linne); oats (Avena sativa, Linne); flax (Linum usitatissimum, Linne); corn (Zea mays, Linne); millets: millet (Panicum miliaceum, Linne), pearl millet (Pennisetum glaucum, Linne); "Browntop millet" (Panicum ramosum, Linne), and Japanese millet (Echinochloa frumentaceae, Linne); triticale (xTriticosecale, Wittm. ex A. Camus); buckwheat (Fagopyrum esculentum, Moench); miscanthus (Miscanthus x giganteus, Andersson); Rod grass (Panicum virgatum, Linne); sorghum (Sorghum sp, Linne); reed (Phragmites australis, Cav.), cane (Arundo donax, Linne), silk cane (Neyraudia reynaudiana, Kunth.), cattail (Typha sp., Linne), papyrus (Cyperus papyrus, Linne), esparganio (Sparganium sp. , Linne), thatched-roof reed (Thamnochortus insignis, Linne); esparto (Stipa tenacissima, Linne and Lygeum spartum, Linne); jute (Corchorus olitorius, Linne); bamboo (Bambusoideae sp .; Linne); bagasse; or a mixture of them.

In a specific case, the grass-like raw material is miscanthus (Miscanthus x giganteus, Andersson).

Brief description of the drawings

Figure 1 - shows a block diagram of the process for the production of cellulose pulp of very high whiteness from raw material such as ground grass according to the invention; the key stages are: continuous digestion, dispersion, continuous bleaching, optional dispersion, drain that removes black liquor, and washing of the resulting pulp producing white cellulose pulp.

The black liquor is optionally partially evaporated and further processed by electrolysis, isolating the lignin and other non-cellulosic by-products, and to regenerate the solution from the cooking chemicals.

In the digestion and bleaching stages, microwave heating (MO) can optionally be employed; marked with dashed line.

Figure 2 - shows a block diagram of the process for the production of cellulose pulp of very high whiteness according to the invention; with emphasis on the additional stages: screening and fractionation and milling of the cellulose pulp, which are carried out optionally.

In the digestion and bleaching stages, microwave heating (MO) can optionally be employed; marked with dashed line.

Detailed description of the invention

The invention relates to an improved continuous process for the production of cellulose pulp of very high whiteness, greater than 90%, of 10% w / w of dry matter, and lignin content <5% w / w, from ground grass type raw material such as miscanthus (Miscanthus xgiganteus, Andersson).

Such grass-like raw material typically contains about 30 to 45% w / w cellulose with 15 to 32% w / w lignin; see bibliography reference 11:

11) C. Ververis, K. Georghiou, N. Christodoulakis, P. Santas, R. Santas: “Fiber dimensions, lignin and cellulose content of various plant materials and their suitability for paper production”, Industrial Crops Prod. 19 (2004) 245-254. Much of this lignin has to be removed in a very gentle manner to ensure maximum preservation of the cellulose fibers for very high whiteness premium grade cellulose pulp. According to the present invention, the process is carried out in a continuous manner through several manufacturing phases, as shown in Figures 1 and 2:

(i) Preparation of the grass-type vegetable raw material

The raw material is prepared by grinding, to produce a material with distributed longitudinal size of 5 to 30 mm and diameter of 0.1 to 2 mm. The crushing of the grass-type starting material is carried out by conventional crushing devices or supplied directly from the fields, if the crops were harvested by a suitable harvester equipped with a suitable cutting device producing the plant material of the above particle size. indicated.

Mainly, the grinding should be carried out in a gentle manner producing fibrous material predominantly grinding along the fibers, to preserve them.

This is the reason why crushing is not shown in Figure 1, because it represents either conventional pretreatment or can even be carried out during harvest in the field.

The shredded material is then dedusted by removing fine, dusty, non-fibrous plant material which would otherwise reduce the quality of the resulting cellulose pulp. This is done by a suitable fan that produces a strong air circulation that allows fine light particles to blow away.

This fine powder does not enter the process at all, thus saving significant amounts of digestion chemicals that would otherwise be wasted through the reaction of this material with sodium hydroxide (NaOH). Furthermore, the effluents are not contaminated with such a level of organic matter, which would significantly negatively affect the environment. This fine non-fibrous material mainly comes from the central part of the plant's stems. In the case of miscanth, the percentage of this fraction is approximately 8 to 9% w / w. Thus, the collected fine non-fibrous dust can be used in the process as fuel in energy production or as a starting material in the manufacture of xylan.

(ii) Continuous digestion of grass-type vegetable raw material

The continuous digestion or cooking of a vegetable raw material without grass type dust prepared in step (i) is carried out in a continuous digester (1) formed as a longitudinal column, internally equipped with an endless screw conveyor and a heating unit. , where the grass-type vegetable raw material is fed continuously directly in the upper part of said digester by the conveyor (4).

Parallel to said raw material feeding, the chemical products for digestion, sodium hydroxide (NaOH) and sodium chloride (NaCl), and fresh water and / or the regenerated white liquor, are continuously introduced into the upper part of said digester (1); maintaining the digestion temperature at 70 to 120 ° C and the average composition of the suspension thus formed during said continuous digestion is kept within the following ranges:

(a) 0.50 to 2.00% w / w NaOH;

(b) 0.50 to 1.50% w / w NaCl;

(c) 10 to 15% w / w of grass-type vegetable raw material; Y

(d) process water; up to 100% w / w of the suspension;

wherein the weight percentages of the ingredients are calculated on the weight of the total suspension.

Under this digestion (cooking) of the grass-like raw material, the dissolution of the non-cellulosic substances occurs during the mass transfer from one side of said longitudinal digester to the other. The digestion or cooking phase lasts 1.5 to 3 hours.

Preferably, the continuous digestion process according to this invention is carried out by introducing cooking chemicals such as concentrated aqueous solutions of 20 to 50% w / w sodium hydroxide (NaOH) and 10 to 30% w / w sodium chloride. sodium (NaCl). Alternatively the sodium hydroxide and sodium chloride solutions can be added as a previously prepared mixture.

In relation to the temperature of the digestion process, the preferred range is 90 to 100 ° C.

(iii) Continuous scattering

The cooked cellulose pulp is discharged continuously, at the same rate as at which the raw material is fed into the digester (1), from said digester (1), by the conveyor, directly into the disperser (5) to carry out the dispersion phase (iii), where the suspended cellulose pulp is subjected to main dispersion, producing finely dispersed / separated cellulose fibers.

The use of disperser (5) in this phase of the process is absolutely essential for high quality cellulose pulp. (iv) Continuous bleaching

The fine suspension thus obtained of dispersed cellulose pulp, after disperser (5), is further subjected to the bleaching phase (iv):

(a) without any removal of the chemicals from the digestion phase (ii); Y

(b) without the use of additional NaOH for the purpose of bleaching.

In the bleaching phase (iv), the previously dispersed cellulose pulp is continuously fed into the continuous bleaching reactor (6) which is,

(b) without the use of additional NaOH for the purpose of bleaching.

In the bleaching phase (iv), the previously dispersed cellulose pulp is continuously fed into the continuous bleaching reactor (6) which is formed, analogously to the digester (1), as a longitudinal column internally equipped with a screw conveyor. endless and a heating unit.

Bleaching is carried out with a solution of the bleaching chemicals that are:

(a) 20 to 40% w / w aqueous hydrogen peroxide (H ² O ^{2) solution;}

(b) solution of "waterglass" or sodium silicate (xNa ² O ^ ySiO ² ) aqueous at 30 to 45% w / w with the molar ratio Na2O: SiO2 = 1: 1 to 3.3: 1; Y

(c) basically without the use of additional sodium hydroxide (NaOH).

These are introduced through the collector (7) into the bleaching reactor (6), maintaining the bleaching temperature at 70 to 100 ° C, for 45 minutes to 1.5 h.

The average composition of the suspension thus formed during the complete bleaching process is kept within the following ranges:

(a) 0.50 to 2.00% w / w of said H ² O ² solution;

(b) 0.50 to 2.00% w / w of said xNa ² O ^ ySiO ² solution;

(c) 10 to 15% w / w cellulose; Y

(d) process water; up to 100% w / w of the suspension;

wherein the weight percentages of the ingredients are calculated on the weight of the total suspension. Preferably, the continuous bleaching process according to the present invention is carried out at temperatures of 85 to 100 ° C.

Furthermore, the bleaching chemicals are preferably introduced into the continuous bleaching reactor (6) separately in the manner that is well known in the art. A typical example of such an optical brightener is liquid Tinopal ABP-A (from CIBA Specialty Chemicals Corporation), which contains 22 to 24% dry matter of triazinyl stilbene type fluorescent brightener. Such products are generally added at a dosage of 0.25 to 0.50% to the dry matter content of cellulose pulp.

This treatment can be done to further increase the whiteness of the final cellulose when manufacturing the higher quality products, as much as possible.

(v) Continuous dispersion (optional)

Subsequently, the suspension of the bleached cellulose pulp thus obtained, discharged from the bleaching stage (iv), is optionally further subjected to further dispersion (phase v), through the disperser (8).

(vi) Continuous drainage

In the additional phase (vi), the cellulose pulp is transferred to the dewatering device (9), where the bleached and finally dispersed cellulose pulp is drained, separated from the black liquor producing:

(a) cellulose pulp bleached from 10 to 15% w / w dry matter; Y

(b) black liquor containing lignin and other cooking and bleaching by-products.

The black liquor from the drain stage (vi) is transported by the collector (10) to the electrolytic reactor (19) for the regeneration of the solution of the cooking chemicals and the isolation of lignin and other non-cellulosic by-products.

In the present invention, the term "black liquor" includes the process effluent from both digestion (ii) and bleaching (iv) phases; It contains residual sodium hydroxide (NaOH), sodium chloride (NaCl), solubilized lignin, other non-cellulosic by-products, and finally traces of hydroxide peroxide remaining.

Alternatively, the black liquor can be partially concentrated in the evaporator (16) to produce regenerated water that is transported back to the washing stage (vii) and the concentrated black liquor that is taken to the electrolytic reactor (19) by the collector ( 18).

(vii) Continuous washing process

The bleached cellulose pulp is further washed (phase vii) in the washing container (12) with additional fresh water, which is introduced through the collector (13), giving:

(a) bleached and final washed cellulose pulp of very high whiteness, greater than 90%, of 10% w / w dry matter, and lignin content> 5% w / w, which is transported out of the complete process through the collector (14); and (b) additional amount of water from the washing process, which is transferred through the collector (15), together with the black liquor from the drainage stage (vi), which is transported by the collector (10), to the electrolytic reactor. (19).

In the present invention, the term "scrubbing process water" includes the process effluent from the continuous scrubbing process (phase vii); it contains black liquor residues that always remain adsorbed on the cellulose fibers within the pulp that comes out of the drain phase (vi). Therefore, the water from the washing process is more likely as highly dilute black liquor.

(viii) Continuous electrolytic regeneration of the solution of the cooking chemicals and insulation of lignin and other non-cellulosic by-products

Continuous electrolytic regeneration of the solution of the cooking chemicals (white liquor) and the isolation of lignin and other non-cellulosic by-products is carried out either:

(a) directly with combined black liquor that arrives through the collector (10) and the water from the washing process that arrives through the collector (15), without prior concentration in the evaporator (16); or

(b) optionally, the combined black liquor and the water from the washing process are particularly concentrated in the evaporator (16) producing concentrated black liquor which is transferred through the manifold (18) to the electrolytic reactor (19).

In the case of the optional concentration of the black liquor, the optional degree of the concentration of black liquor is preferably up to 0.9 to 1.05 w / w sodium (as Na +) in the concentrated black liquor entering the electrolytic reactor (19).

The term "evaporator" implies not only the classical evaporation devices which are based on either distillation or vacuum distillation, but also all other means of removing water from aqueous solutions, for example, by reverse osmosis, exchange process ionic, electrodeionization, etc.

In the electrolytic reactor (19), either the original combined black liquor ("as is") (a) or the concentrated black liquor (b) is subjected to electrolysis with direct electric current (DC) between two electrodes, at potential electrical current of 3 to 30 V, and electrical current density of 1 to 10 A / dm2, at 10 to 95 ° C, where the lignin and by-products are separated on top of the electrolyte solution in the anode compartment , and is continuously removed from the electrolytic reactor.

The electrolytic removal of lignin is carried out in the electrolytic cell similar or identical to those described in reference 10 of the literature, where the cathode and anode compartments are separated by a suitable semi-permeable membrane or diagram.

The electrodes, the cathode and the anode, are made of suitable electro-conductive materials resistant to highly reactive chemicals formed in their compartments, eg high pH NaOH. Such suitable materials are: metals such as carbon steel or stainless steels, graphite, magnetite, etc. Preferably, the cathode is made of carbon steel, for example A36 type, or stainless steels, for example AISI 304, 316, 321 types, while the anode is formed of graphite or magnetite.

The electrodes can be formed in various shapes, of which plates and wire mesh are preferred.

The diaphragm is made of materials selected from the group comprising: Asbestos, Rock Wool (Stone Wool), Portland Cement, Aluminum Oxide (Al2O3), Titanium Dioxide (TiO ² ), Zirconium Dioxide (ZrO ² ), Polyethylene (PE), polyethersulfone (PES), polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE; Teflon®), fluoride polyvinylidene (PVDF), sulfonated polytetrafluoroethylene (Nafion®), clay and sodium silicate, combinations of these materials, or other suitable chemically resistant materials.

Preferably, the electrolytic removal of lignin and the regeneration of the white liquor is conducted at an electrical potential of 3 to 10 V and an electrical current density of 3 to 7 A / dm2.

The alkaline black liquor from the cellulose manufacturing process is introduced into the anode compartment.

During the electrolysis of the black liquor solution the lignin and other by-products are separated in the upper part of the electrolyte solution from the anode compartment (+), and are removed from the process by conventional mechanical means, by the collector or the like.

The separation of lignin within the anode compartment (+) is facilitated by the evolution of oxygen gas (O ² ) as a by-product of the electrolysis of the accompanying water.

At the same time, in the cathode compartment (-) the main reaction is evolution of hydrogen (H ² ), which is accompanied by an increasing concentration of sodium hydroxide (NaOH). The sodium hydroxide solution thus regenerated from the cathode compartment, together with sodium chloride (NaCl) remaining in the solution, is transported back to the digester (1) by the collector (21).

The resulting electrolyte solution with regenerated solution of sodium hydroxide (NaOH) and sodium chloride (NaCl), which represents the white liquor, is regenerated back to the digestion / cooking process, by the collector (21), to the continuous digester (1), closing the entire pulp manufacturing process.

In the present invention, the term "white liquor" includes the process effluent from phase (viii) of the continuous electrolytic regeneration of the solution of the cooking chemicals, NaOH and NaCl. It contains primarily dilute sodium hydroxide and sodium chloride, with trace amounts of lignin and other non-cellulosic products originating from black liquor.

The lignin is separating on top of the electrolyte solution in the cathode compartment and is being continuously removed by the conventional conveyor type collector (20).

The presence of these trace amounts of residual ingredients of plant origin does not influence the performance of the entire process and the quality of the final cellulose pulp after the use of this regenerated white liquor as the only process water during repeated process cycles.

The term "electrolytic reactor" in the present invention includes alone or a battery (series) of several or a large number of combined electrolytic cells. This depends on the capacity or scale of the manufacture of the cellulose pulp using the process according to this invention. For example, the larger manufacturing facility may comprise a series of 10 to 100 electrolytic cells. In the case of a high tonnage manufacturing facility, such an "electrolytic reactor" may comprise several hundred such electrolytic cells.

Optionally, the process for the production of very high whiteness cellulose pulp according to the present invention further comprises the manufacturing steps of screening, fractionation, and milling.

The complete cellulose manufacturing process according to the present invention through phases (i) - (vii) is presented in Scheme 1.

(ix) Screening and fractionation of cellulose pulp

In these operations, the diluted suspension from stage (viii) is further processed through a sieving and fractionating device (22) equipped with a 0.1 to 0.5 mm sieve, producing two fractions;

(a) the first fraction that does not pass through the 0.1 to 0.5 mm sieve; Y

(b) the second fraction that passes through the 0.1 to 0.5 mm sieve, which is considered a good cellulose pulp suitable for making higher quality paper or cellulose sheets, which is transported out of the process through the manifold (23).

(x) Pulp milling

This is required to process the (a) fraction of the cellulose pulp from step (ix) through one or two subsequent milling stages using the pulp mills (24,25), producing cellulose pulp that is transferred, through the collector (26), back to the screening and fractionating device (22). In this way, the total amount of finally unsuitable cellulose pulp fraction is processed to the final desired product that comes out of the complete process through the collector (23).

The complete cellulose manufacturing process according to the present invention through phases (i) - (vii) including optional phases (ix) and (x) is presented in Scheme 2.

The use of alternative means of heating

A continuous process for the production of cellulose pulp, of very high whiteness, according to the present invention can be carried out using microwave heating (MO) in the continuous digester (1) and / or the continuous bleaching reactor (6) of the manner known in the art. In this case, the heating units in the continuous digester (1) and / or the continuous bleaching reactor (6) are microwave generating magnetrons (MO), or a series of the magnetrons.

Grass type raw material

The grass-type raw material that can be used as the starting raw material for the process of the present invention includes stems of plant species selected from the group of: wheat (Triticum vulgare, Linne); rice (Oryza sativa, Linne); barley (Horedum vulgare, Linne); oats (Avena sativa, Linne); flax (Linum usitatissimum, Linne); corn (Zea mays, Linne); millets: millet (Panicum miliaceum, Linne), pearl millet (Pennisetum glaucum, Linne); "Browntop millet" (Panicum ramosum, Linne), and Japanese millet (Echinochloa frumentaceae, Linne); triticale (xTriticosecale, Wittm. ex A. Camus); buckwheat (Fagopyrum esculentum, Moench); miscanthus (Miscanthus x giganteus, Andersson); Rod grass (Panicum virgatum, Linne); sorghum (Sorghum sp, Linne); reed (Phragmites australis, Cav.), cane (Arundo donax, Linne), silk cane (Neyraudia reynaudiana, Kunth.), cattail (Typha sp., Linne), papyrus (Cyperus papyrus, Linne), esparganio (Sparganium sp. , Linne), thatched-roof reed (Thamnochortus insignis, Linne); esparto (Stipa tenacissima, Linne and Lygeum spartum, Linne); jute (Corchorus olitorius, Linne); bamboo (Bambusoideae sp .; Linne); bagasse; or a mixture of them. The preferred herb-type raw material is miscanthus (Miscanthus x giganteus, Andersson).

Results of the analysis of cellulose manufactured by the process of the present invention

The superior characteristics of the cellulose made by the present invention can be clearly seen when the results of the analysis of the resulting paper samples are compared with also good cellulose made by the similar process described in the more detailed prior art; see bibliography reference 9.

The results of the cellulose analysis and the comparison with the same parameters for the cellulose obtained by the more detailed prior art process are presented in Table 1.

Table 1. Comparison of the properties of the paper prepared from the cellulose obtained by the process of the present invention and the cellulose obtained by the process of the more detailed prior art, reference 9 (as a control).

a The paper sample of the control cellulose pulp was made by the more detailed prior art process (see reference 9); in the bleaching stage, H ² O ² + NaOH was used as bleaching system. The H ² O ² concentration was kept the same as in the present invention.

b The paper sample of the cellulose pulp prepared by the process of the present invention.

c The lignin content in cellulose pulp is generally <5% w / w, calculated on the dry matter, as this ensures good mechanical properties of the resulting paper.

d Based on previous experiments.

na = not analyzed.

The experimental results showed significantly better mechanical properties (tensile and burst strengths) of the cellulose and the paper thus resulting at the same (comparable) grammage (basis weight around 100 g / m2). The tensile strength was 5.410 versus 4.800, which is 13% improvement, and the burst strength was 3.25 versus 2.95, which is a 10% increase. These results clearly suggest better mechanical properties of the cellulose of the present invention which probably ensures the improved preservation of the original cellulose fibers during processing.

In addition, the brightness (whiteness) of the cellulose was significantly improved to over 90% (97.94%) versus the maximum 80% that could be achieved by using the more detailed prior art process (reference 9 of the bibliography).

Final changes to the process parameters may result in some further improvement of the key quality parameters of the resulting cellulose, but such changes are considered to be within the scope of this invention.

Industrial applicability

The present invention is obviously industrially applicable.

Furthermore, the process of the present invention is characterized by the following key features:

(i) very mild digestion and bleaching process for grass-type raw material resulting in highly preserved cellulose pulp with improved mechanical properties, lignin content of <5% w / w, and whiteness> 90%, making it suitable for the production of superior quality papers;

(ii) smooth dispersion between the digestion and bleaching phases, basically without any removal of the black liquid after the digestion phase;

(iii) where both digestion and bleaching processes are technologically optimized to ensure the minimum consumption of chemical products;

(iv) where the bleaching process performed by the novel hydrogen peroxide (H ² O ² ) -sodium silicate (xNa ² O ^ yS¡O ² ) system, basically without any use of sodium hydroxide (NaOH) ; Y

(v) integrated and efficient electrolytic removal of lignin from the black liquid solution with accompanying regeneration of sodium hydroxide solution (+ NaCl), which was reused again in the cooking process as white liquor, thus closing the entire manufacturing cycle without any significant residue or environmental impact.

List of references

1 - continuous digester

2 - collector to introduce fresh water

3 - manifold to introduce digestion chemicals

4 - conveyor for feeding the raw material type crushed grass

5 - disperser

6 - continuous bleaching reactor; basically the same as continuous digester

7 - collector to introduce the bleaching chemicals

8 - disperser; this is optional; marked with dashed line

9 - drain

10 - collector for the transport of the black liquor from the dewatering device to the electrolytic reactor for the removal of lignin, or optionally, to the evaporator 16

11 - collector for transporting the drained cellulose pulp to the washing container 12

12 - cellulose pulp washing container

13 - collector for the introduction of fresh water

14 - collector for the outlet of the final white cellulose pulp

15 - collector for transporting spent water from the pulp pulp washing process to the electrolytic reactor for regeneration, or optionally, to the evaporator 16

16 - optional evaporator; for the partial concentration of the black liquor water from the combined washing process 17 - optional manifold for transporting clean reclaimed water from the evaporator back to the cellulose washing vessel

18 - collector for the transport of the black liquor to the electrolytic reactor 16, or optionally, the concentrated black liquor if the evaporator is used

19 - electrolytic reactor for the removal of lignin and other by-product

20 - collector for the outlet of lignin and other non-cellulosic by-products

21 - manifold to transport regenerated cooking chemicals back to digester 1

22 - screening and fractionation device

23 - collector for the outlet of white cellulose pulp when screening and optional fractionation is carried out 24 - pulp mill

25 - pulp mill

26 - collector for transporting the milled white cellulose pulp back to the screening and fractionating device

Claims (10)

1. A continuous process for the production of cellulose pulp of very high whiteness from grass-like vegetable raw material, wherein said process comprises the stages of: (i) preparation of the grass-type vegetable raw material by grinding to produce a raw material with a longitudinal size distributed from 5 to 30 mm and a diameter of 0.1 to 2 mm, and with fine dust particles removed by dedusting of said raw material with fan; (ii) continuous digestion of a vegetable raw material without grass type dust prepared in step (i) in a continuous digester (1) formed as a longitudinal column internally equipped with an endless screw conveyor and a heating unit; wherein the grass-like vegetable raw material is continuously fed directly into the upper part of said digester by the conveyor (4); characterized by parallel to said raw material feed, the chemical products for digestion: NaOH, NaCl, and fresh water and / or the regenerated white liquor; they are continuously introduced into the upper part of said digester (1); maintaining the digestion temperature at 70 to 120 ° C and the average composition of the suspension thus formed during said continuous digestion is kept within the following ranges: (a) 0.50 to 2.00% w / w NaOH; (b) 0.50 to 1.50% w / w NaCl; (c) 10 to 15% w / w of grass-type vegetable raw material; Y (d) process water; up to 100% of the suspension; wherein the weight percentages of the ingredients are calculated on the weight of the total suspension; and wherein a dissolution of non-cellulosic substances of the grass-like vegetable raw material occurs during mass transfer from one side to the other of said longitudinal digester lasting 1.5 to 3 hours; (iii) where the cooked pulp is discharged continuously, at the same rate as at which the raw material is fed into the digester (1), from said digester (1), by the conveyor, directly into the disperser ( 5), wherein the suspended cellulose pulp is further subjected to main dispersion, producing finely dispersed cellulose fibers; (iv) wherein the fine suspension of the dispersed cellulose pulp from step (iii) is further subjected to bleaching: (a) without any removal of the chemicals from the digestion phase (ii); Y (b) without the use of additional NaOH for the purpose of bleaching; wherein the dispersed cellulose pulp is continuously fed into the continuous bleaching reactor (6) which is formed as a longitudinal column equipped with an endless screw conveyor and a heating unit; Y where the bleaching chemicals are: (a) hydrogen peroxide / H ² O ² / aqueous solution with 20 to 40% w / w H ² O ² ; Y (b) aqueous sodium / xNa ² O ^ ySiO ² / silicate solution with 30 to 45% w / w Na ² O + SiO ² combined, with Na2O: SiO2 molar ratio = 1: 1 to 3.3: 1; and wherein said chemical products are introduced through the collector (7) into the bleaching reactor (6), maintaining the bleaching temperature from 70 to 100 ° C, for 45 minutes to 1.5 h, and the average composition of the suspension thus formed during the complete bleaching process is maintained within the following ranges: (a) 0.50 to 2.00% w / w of said H ² O ² solution; (b) 0.50 to 2.00% w / w of said xNa ² O ^ ySiO ² solution; (c) 10 to 15% w / w cellulose; Y (d) process water; up to 100% w / w of the suspension; wherein the weight percentages of the ingredients are calculated on the weight of the total suspension; (v) the suspension of the bleached cellulose pulp thus obtained, discharged from the bleaching stage (iv), is optionally further processed through the disperser (8); and then it is transferred to the dewatering device (9); (vi) where the bleached and finally dispersed cellulose pulp from stage (v) is drained, it is separated from the black liquor producing: (a) cellulose pulp bleached from 10 to 15% w / w dry matter; Y (b) black liquor containing lignin and other cooking and bleaching by-products; Y (vii) The bleached cellulose pulp is further washed in the washing container (12) with additional fresh water, which is introduced through the collector (13) giving: (a) bleached and final washed cellulose pulp of very high whiteness, greater than 90%, of 10% w / w dry matter, and lignin content <5% w / w, which is transported out of the complete process through the collector (14); Y (b) additional amount of water from the washing process that is transferred through the collector (15), together with the black liquor from the drain stage (vi), which is transported by the collector (10) to the electrolytic reactor (19) ; (viii) where the black liquor and the water from the washing process are also subjected to electrolysis with direct electrical current between two electrodes, at an electrical potential of 3 to 30 V, and an electrical current density of 1 to 10 A / dm2, at 10 to 95 ° C, where the lignin and by-products are separated on top of the electrolyte solution in the anode compartment, and are continuously withdrawn from the electrolytic reactor; and where the resulting electrolyte solution with regenerated NaOH and NaCl solution, which represents the white liquor, is regenerated from the cathode compartment back to the cooking process, through the collector (21), to the continuous digester (1), closing thus the complete pulp manufacturing process. 2. A continuous process for the production of cellulose pulp according to claim 1, characterized in that it further comprises the steps of: (ix) screening and fractionation, wherein the diluted suspension of step (viii) in claim 1 is processed through a screening and fractionation device (22) equipped with a 0.1 to 0.5 mm sieve, producing two fractions; (a) the first fraction that does not pass through the 0.1 to 0.5 mm sieve; Y (b) the second fraction that passes through the 0.1 to 0.5 mm sieve, which is considered a good cellulose pulp of very high whiteness, greater than 90%, suitable for making higher quality paper or cellulose sheets, which is transported out of the complete process through the collector (23); Y (x) where fraction (a) of the cellulose pulp from step (ix) undergoes one or two subsequent processing through the pulp mills (24, 25) producing the cellulose pulp, which is transferred , through the collector (26), back to the screening and fractionating device (22). 3. A continuous process for the production of cellulose pulp according to any of the preceding claims characterized in that the chemicals for digestion in step (ii) of claim 1 are introduced as a mixture of concentrated aqueous solutions of 20 to 50 % w / w NaOH and 10 to 30% w / w NaCl. 4. A continuous process for the production of cellulose pulp according to any of the preceding claims characterized in that the digestion temperature is kept in the range of 90 to 100 ° C. 5. A continuous process for the production of cellulose pulp according to any of the preceding claims characterized in that the bleaching temperature is kept in the range of 85 to 100 ° C. A continuous process for the production of cellulose pulp according to any of the preceding claims characterized in that the heating units in the continuous digester (1) and the continuous bleaching reactor (6) are microwave generating magnetrons (MO). A continuous process for the production of cellulose pulp according to any of the preceding claims characterized in that the black liquor and the waters of the washing process combined leaving the process stages (vi) and (vii) in claim 1 by the collectors (10) and (15) they are optionally concentrated to the total sodium content 0.90 to 1.05% w / w in the evaporator (16) before further processing in the electrolytic reactor (19). 8. A continuous process for the production of cellulose pulp according to any of the preceding claims characterized in that the electrolytic removal of lignin and the regeneration of white liquor in stage (viii) of claim 1 within the electrolytic reactor (19) is conducts with graphite anode and carbon steel or stainless steel cathode, at electric potential of 3 to 10 V, electric current density of 3 to 7 A / dm2. 9. A continuous process for the production of cellulose pulp according to any of the preceding claims, characterized in that the grass-type raw material includes stems of plant species selected from the group of: wheat (Triticum vulgare, Linne); rice (Oryza sativa, Linne); barley (Horedum vulgare, Linne); oats (Avena sativa, Linne); flax (Linum usitatissimum, Linne); corn (Zea mays, Linne); millets: millet (Panicum miliaceum, Linne), pearl millet (Pennisetum glaucum, Linne); "Browntop millet" (Panicum ramosum, Linne), and Japanese millet (Echinochloa frumentaceae, Linne); triticale (xTriticosecale, Wittm. ex A. Camus); buckwheat (Fagopyrum esculentum, Moench); miscanthus (Miscanthus xgiganteus, Andersson); Rod grass (Panicum virgatum, Linne); sorghum (Sorghum sp, Linne); reed (Phragmites australis, Cav.), cane (Arundo donax, Linne), silk cane (Neyraudia reynaudiana, Kunth.), cattail (Typha sp., Linne), papyrus (Cyperus papyrus, Linne), esparganio (Sparganium sp. , Linne), thatched-roof reed (Thamnochortus insignis, Linne); esparto (Stipa tenacissima, Linne and Lygeum spartum, Linne); jute (Corchorus olitorius, Linne); bamboo (Bambusoideae sp .; Linne); bagasse; or a mixture of them. A continuous process for the production of cellulose pulp according to claim 8, characterized in that the grass-like raw material is miscanthus (Miscanthus xgiganteus, Andersson).