RS60519B1 - A continuous process for production of cellulose pulp - Google Patents

Description

Description

Field of Technique

[0001] This invention relates to an improved continuous process for the production of cellulose pulp of very high whiteness, >90%, with a lignin content <5wt.%, from shredded grass-like raw material such as miscanthus (Miscanthus x giganteus, Andersson).

Technical Problem

[0002] The main technical problem, solved by this invention, is the development of a new process for the production of cellulosic pulp that has high whiteness, from grass-like raw materials in an efficient way. This efficient method is achieved by using as few as possible cheap chemicals that are environmentally acceptable and under as mild digestion and bleaching conditions as possible. Such a procedure should also ensure maximum preservation of the natural cellulose fibers from the initial raw material.

[0003] The usual production of cellulose pulp uses sulfur-based chemicals with a high consumption of water and energy or milder procedures based on sodium hydroxide, but with a high energy consumption for intensive dispersing of the pulp, which all together have a large overall negative impact on the environment.

[0004] The most important characteristics of any successful production of pulp from grass-like raw materials should ensure:

(i) the mildest possible digestion of the chopped plant material in order to remove the unwanted non-cellulosic fraction, mainly lignin;

(ii) use of minimal mechanical shear force including gentle dispersing and further processing of the pulp suspension;

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

(iv) effective removal of lignin and other non-cellulosic by-products to an acceptable level, preferably to <5wt.%; and

(v) efficient regeneration of digestion chemicals and technical water from black liquor to ensure a closed loop process material balance and therefore minimal impact on the environment.

[0005] The first technical problem solved by the described invention is the use of milder and optimized chemical reagents during the stages of digestion and bleaching and very high whiteness, greater than 90%, which avoids difficult production conditions with high energy consumption, dangerous and sulfur-based chemicals, and environmental pollution with wastewater containing residues of all these process chemicals.

[0006] The technical problem solved by the described invention is finding a solution within the "green" chemical technology, characterized by:

(a) very mild digestion and bleaching conditions with a new bleaching system based on hydrogen peroxide (H2O2) without the use of sodium hydroxide;

(b) high concentration of cellulose suspension during digestion; therefore, a high yield of cellulose per volume (productivity);

(c) high preservation of natural fibers from the raw material by applying mild conditions in digestion and bleaching reactions, as well as minimal mechanical stress during dispersing and optional sieving; and

(d) complete regeneration of digestion chemicals and technical waters practically without affecting the environment, in an integrated manner, so that the removal of lignin and other non-cellulosic by-products from spent (black) liquor and waste water is accompanied by the regeneration of the solution of digestion chemicals - the so-called "white liquor".

[0007] According to our knowledge, this is the first ecological process for the production of cellulose pulp that works on such mild digestion conditions, with very low water consumption, low energy consumption, which can produce cellulose of very high whiteness, greater than 90%, with <5 wt.% lignin, which can give high quality paper with excellent mechanical properties. The process of the present invention only works properly when a grass-like feedstock is used.

State of the Art

[0008] The production of cellulose pulp for papermaking from renewable and more economical plant raw material, which is growing rapidly, is of increased importance in the modern paper industry. Therefore, the usual wood-based processes are replaced by grass-like raw materials such as miscanthus (Miscanthus x giganteus, Andersson), prairie millet (Panicum virgatum, Linne), sorghum (Sorghum species, Linne), reed (Phragmites australis, Cav.), sedge (Arundo donax, Linne), straw of various cereals, etc.

[0009] For example, miscanthus (Miscanthus x giganteus, Andersson) is one of the most suitable grass-like raw materials for such an application; 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.

[0010] Digestion is the most important stage within this procedure for pulp production. It represents the cooking of shredded lignocellulosic material in an aqueous solution of suitable chemicals for digestion. There are several pulping processes depending on which chemicals are used. The most well-known technologies are based on solution digestion:

(i) sulphur-containing chemicals: sodium carbonate (Na2CO3) and sodium sulphite (Na2SO3), magnesium hydroxide [Mg(OH)2] and magnesium sulphite (MgSO3), ammonium hydroxide (NH4OH) and ammonium sulphite (NH4)2SO3], calcium hydrogen sulphite [Ca(HSO3)2], magnesium hydrogen sulphite [Mg(HSO3)2], sodium hydroxide (NaOH), sodium sulphite (Na2S) and sodium sulfate (Na2SO4); (ii) chemicals that do not contain sulfur: sodium carbonate (Na2CO3), sodium hydroxide (NaOH); and (iii) an acid such as nitric acid (HNO3).

[0011] Such a solution of digestion chemicals is also known as "white liquor", which is either a fresh or regenerated solution of digestion chemicals. White liquor helps remove non-cellulosic materials, which are thus dissolved in the solution, leaving relatively pure cellulosic fibers suspended in this liquid phase.

[0012] Cooked raw material with non-cellulosic materials removed, ie. "pulp," at the end of digestion, is suspended in the used solution, which contains various chemical forms of non-cellulosic plant ingredients and residues of digestion chemicals. This aqueous phase is called "black liquor". Therefore, the pulp after digestion is a suspension of essentially pure cellulosic fibers in black liquor.

[0013] Considering the type of digestion, as one of the most important technological aspects of pulp production, procedures based on reduced use of sulfur-based chemicals have significant advantages. The most important reason is ecology. The use of sulfur-free processes is of utmost importance in preserving the environment, and thus also avoids corrosion problems on production equipment, as well as toxicological problems.

[0014] One of the most environmentally friendly sulfur-free processes uses technology based on sodium hydroxide. The use of sodium hydroxide (NaOH) as the only digestion chemical is known in this field, and also specifically in processing based on grass-like raw materials such as rice straw, esparto, cane, jute, and others. One of the procedures is performed 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).

[0015] These stages of digestion can be performed by microwave (MW) assisted heating. Therefore, Zhu et al. described a procedure for pretreatment of miscanthus /Miscanthus x giganteus, Andersson/ with sodium hydroxide (NaOH) solution at very high temperatures (130-200 °C) at elevated pressures for 20 minutes under MW heating. Such pretreated miscanthus also gives a much better yield in hydrolysis catalyzed by sulfuric acid in relation to glucose as raw material in fermentation to 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.

[0016] Although this procedure focuses on the production of glucose from miscanthus, such a pretreatment hints at some potential in the use of MW for miscanthus digestion. Of course, the reaction conditions described in reference 4 are very severe and are clearly incompatible with the production of high-quality, high-whiteness cellulosic fibers.

[0017] Bleaching is, in addition to digestion, another important key stage in the production of high white pulp is the bleaching stage. The most common pulp bleaching systems are those based on chlorine-containing systems, e.g. sodium hypochlorite (NaOCl), or systems based on hydrogen peroxide (H2O2), the latter being preferred.

[0018] Therefore, patent GB681661 describes a process for bleaching cellulosic pulp using the following bleaching system:

(a) 0.30-1.75% hydrogen peroxide (H2O2);

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

(c) 20-65% cellulose pulp (dry matter); and

(d) up to 100% technical water;

at temperatures below 54.4 °C (130 °F). Sodium silicate solution (xNa2O•ySiO2) is optionally used as a hydrogen peroxide stabilizer; see reference 5:

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

[0019] This document suggests the use of a mixture of hydrogen peroxide (H2O2) and sodium hydroxide (NaOH) as a 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 effect on H2O2.

[0020] This invention is also based on bleaching with hydrogen peroxide (H2O2), but the main, crucial, and desirable difference is in the use of a mixture of H2O2 and a solution of sodium silicate (xNa2O•ySiO2), essentially without the use of sodium hydroxide (NaOH) as a co-reagent which, in reaction with H2O2, creates a balance of the concentration of hydroperoxide anions (HOO-) as a true oxidizing species.

[0021] In addition to conventional heating in the pulp bleaching process, microwave (MW) assisted heating is also used. Therefore, Law describes a process and apparatus for MW-assisted bleaching of cellulosic pulp; see reference 6:

6) CA2038651A1; K.-N. Law: Method and apparatus for bleaching pulps; applicants: K.-N. Law, J.L.

Valade (US).

[0022] Furthermore, Law et al describe a process for bleaching cellulosic pulp using MW heating and a bleaching chemical mixture consisting of hydrogen peroxide (H 2 O 2 ) and sodium hydroxide (NaOH). In this process, sodium silicate solution can also optionally be used as a stabilizer for H2O2; 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.

[0023] Both documents explain that MW can generally be used as an alternative heating agent in cellulosic pulp bleaching processes. However, these documents do not discuss the effect of the combined hydrogen peroxide (H2O2) - sodium silicate (xNa2O•ySiO2) system, without the use of highly alkaline sodium hydroxide (NaOH), on the bleaching efficiency (yield, quality and whiteness) of cellulosic pulp from grass-like feedstock.

[0024] In addition, one characteristic of this process for the production of very high whiteness cellulosic pulp is certainly whether the cooking chemicals from the digestion stage are removed from the pulp or can be retained in the pulp after the bleaching stage.

[0025] Older processes performed the removal of black liquor containing digestion chemicals, lignin, and other non-cellulosic by-products.

[0026] Abu et al. describe a process for the production of fibrous cellulosic material that includes: (1) shredding the plant material, (2) mixing it with a 0.5-1% aqueous solution of sodium hydroxide (NaOH), (3) sieving, (4) extruding the thus obtained material from an extruder in which the material is mixed with hydrogen peroxide (H2O2) at 120-150 °C at 20 bar, and (5) ejecting from the extruder and washing with water; see reference 8:

8) DE19603491A1; Production of fibrous cellulose material free from other plant components; applicants: S.I. Abu, D. Kistmacher, R. Berg (DE).

[0027] This document suggests the possibility of continuous processing of plant material containing cellulose with digestion chemicals such as sodium hydroxide (NaOH) and further bleaching with hydrogen peroxide (H2O2), without intermediate washing of digestion chemicals (NaOH) before bleaching.

[0028] This invention is also characterized by one of the key details that it does not involve washing off the remaining cooking chemicals after the digestion stage, but is directly subjected to the dispersing and bleaching stages.

[0029] However, the key difference between the method according to this invention and DE19603491A1 is in the fact that the previous method uses:

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

(ii) a bleaching system based on also specific and optimized concentrations of H2O2 and sodium silicate, essentially without any additional NaOH.

[0030] Mikulić describes a continuous process for the production of cellulosic pulp from a grass-like raw material such as Miscanthus (Miscanthus x giganteus, Andersson), using a vertical autoclave with smooth internal walls without any sieve or mixing element inside, using very diluted cooking chemicals, sodium hydroxide (NaOH) and sodium chloride (NaCl) or sodium sulphite (Na2SO3), with the following average composition of the digestive suspension:

(a) 0.90-1.50 wt.% NaOH;

(b) 0.15-0.40 wt.% NaCl or Na2SO3;

(c) 15-18% by mass of chopped grass-like raw materials; and

(d) up to 100% technical water;

in which the digestion process is heated by conventional means, either by direct heating of the autoclave or by a heating medium; see reference 9.

[0031] In addition to the new type of simplified vertical autoclave, the process is also based on the subsequent processing of the cooked pulp inside a device for sieving and dividing into fractions that isolates the good fraction of cellulose pulp and separates it from the unsuitable. The latter is further subjected to pulping in one or more suitable pulp mills to obtain a suitable cellulosic pulp. It can optionally be further bleached with conventional bleaching chemicals to obtain a high quality pulp suitable for paper production.

[0032] To our knowledge, this prior art document is the closest to this invention; see reference 9:

9) WO2015/150841A1; M. Mikulic: A Continuous Process for Production of Cellulose Pulp from Grasslike Feedstock; applicant: M. Mikulic.

[0033] Contrary to document WO2015/150841A1, the method according to the present invention uses a combination of:

(a) specific and optimized concentrations of digestion chemicals, sodium hydroxide (NaOH) and sodium chloride (NaCl), with a lower content of raw material in the digestive suspension, 10-15% by mass of dry matter;

(b) subsequent dispersion; substantially without removal of residual cooking chemicals, (c) modified bleaching procedure based on specific and optimized concentrations of hydrogen peroxide solution (0.50-2.00 wt.% H2O2) and sodium silicate (0.502.00 wt.% xNa2O•ySiO2), substantially without additional sodium hydroxide (NaOH) used for bleaching purposes; as well as

(d) integrated electrolytic regeneration of cooking chemicals and isolation of lignin and other by-products, which enables high efficiency pulp production with a completely closed cycle of process materials.

[0034] Digestion (a) and bleaching (c) processes can be performed by microwave (MW) assisted heating with comparably good results within a significantly shorter time period for each stage.

[0035] The combination of phases (a)-(c) results in a very high quality cellulosic pulp of very high whiteness, >90%, with <5wt% lignin, which cannot be produced by the process described in reference 9.

[0036] The removal of lignin and other side products from black liquor solutions using an electrolytic reactor (cell) is known in this field. Edel et al describe a typical example of such technology; 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).

[0037] Briefly, during direct current (DC) electrolysis of black liquor solution within suitable electrodes, lignin and other by-products are separated in the anode section, while sodium hydroxide (cooking solution or white liquor) regeneration is regenerated in the cathode section.

[0038] The process according to this invention uses a roughly similar electrolytic reactor for the removal of lignin and other side products and for the parallel regeneration of cooking chemicals, NaOH NaCl solution (white liquor), but the main and crucial difference is the fact that the electrolysis process in this invention works with a black liquor solution that includes diluted sodium lignin and sodium chloride (NaCl). In this way, the black liquor solution according to the present invention has a better conductivity due to the presence of sodium chloride, while the process of reference 10 is based on the treatment of only sodium lignin from the organosolv technology for the production of cellulose.

[0039] The process according to the present invention provides a very high-quality cellulose of very high whiteness, >90%, with a high yield, at a very low consumption of water and energy.

[0040] The technology for the production of cellulosic pulp from grass-like raw materials according to the present invention represents a new and inventive technology, as described in the detailed description of the present invention.

Brief Summary of the Invention

[0041] This invention describes a continuous process for the production of very high whiteness cellulosic pulp from grass-like plant material. This procedure includes the following stages:

(i) preparation of grass-like plant raw material by shredding to produce raw material with a longitudinal size distribution of 5-30 mm and a diameter of 0.1-2 mm, and with fine dust particles removed by dusting said raw material with a fan;

(ii) continuous digestion of the dust-free grass-like plant material prepared in phase (i) in a continuous autoclave (1) designed as a longitudinal column, the interior of which is equipped with a screw conveyor and a heating unit; where the grass-like plant material is continuously introduced directly to the top of said autoclave via a transport conveyor (4);

where digestion chemicals, sodium hydroxide (NaOH) and sodium chloride (NaCl), and fresh water and/or regenerated white lye are continuously introduced to the top of said autoclave (1) in parallel with said raw material being introduced; maintaining a digestion temperature of 70-120 °C, and the average composition of the thus derived suspension during said continuous digestion is maintained in the following ranges:

(a) 0.50-2.00 wt.% NaOH;

(b) 0.50-1.50 wt.% NaCl;

(c) 10-15% by mass of grass-like plant material; and

(d) technical water; up to 100 wt.% suspension;

whereby the mass percentages of the ingredients are calculated in relation to the mass of the entire suspension; where the dissolution of non-cellulosic substances from the grass-like plant material occurs during mass transfer from one side to the other side of said longitudinal autoclave, which lasts 1.5-3 hours;

(iii) in which the cooked pulp, at a speed equal to the speed at which the raw material is introduced into the autoclave (1), is continuously discharged from the said autoclave (1), via a transport conveyor, directly into the disperser (5), where the suspended cellulose pulp is further subjected to the main dispersion, thereby obtaining finely dispersed cellulose fibers;

(iv) in which the fine suspension of dispersed cellulosic pulp thus obtained is further subjected to bleaching:

(a) without any removal of chemicals from stage (ii) digestion; and

(b) without using any additional NaOH for bleaching purposes; and

whereby the dispersed cellulose pulp is continuously introduced into the continuous bleaching reactor (6) which, analogously to the autoclave (1), is formed as a longitudinal column, the interior of which is equipped with a screw conveyor and a heating unit;

where are the bleaching chemicals:

(a) 20-40 wt.% aqueous solution of hydrogen peroxide (H2O2); and

(b) 30-45% aqueous solution of sodium silicate (xNa2O•YSiO2) with a molar ratio of Na2O:SiO2= 1:1 to 3.3:1; which are introduced into the bleaching reactor (6) through the pipeline (7), maintaining the bleaching temperature of 70-100 °C, for 45 minutes to 1.5 h, and the average composition of the thus derived suspension during the entire bleaching procedure is maintained in the following ranges:

(a) 0.50-2.00 wt.% of said H2O2 solution;

(b) 0.50-2.00 wt.% of said xNa2O•ySiO2 solution;

(c) 10-15% by weight of cellulose; and

(d) technical water; up to 100 wt.% suspension;

whereby the mass percentages of the ingredients are calculated in relation to the mass of the entire suspension;

(v) the suspension of bleached cellulosic pulp thus obtained, which comes out of stage (iv) of bleaching, is further optionally processed through a disperser (8); and then transferred to the dewatering device (9); (vi) in which the bleached and optionally additionally dispersed cellulosic pulp is dewatered and separated from the black liquor to obtain:

(a) bleached cellulose pulp of 10-15 wt.% dry matter; and

(b) lye containing lignin and other by-products of cooking and bleaching; and

(vii) the bleached cellulose pulp is additionally washed in the vessel (12) for washing with additional fresh water, which is introduced through the pipeline (13), which results in:

(a) finally bleached and washed cellulosic pulp, very high whiteness, greater than 90%, of 10 wt.% dry matter, with a lignin content >5 wt.%, which exits the whole process through the pipeline (14); and

(b) an additional amount of technical washing water, which is transferred through the pipeline (15), together with black liquor from phase (vi) dewatering, which is transported through the pipeline (10), into the electrolytic reactor (19);

(viii) in which black liquor and technical washing water are subjected to direct current (DC) electrolysis between two electrodes, at an electric potential of 3-30 V, and a current density of 1-

1

10 A/dm<2>, at 10-95 °C, whereby lignin and side products are separated at the top of the electrolytic solution in the anode part, and are continuously removed from the electrolytic reactor; and

whereby the resulting electrolytic solution with the regenerated solution of NaOH and NaCl, which represents white lye, is regenerated from the cathodic part back to the cooking process, through the pipeline (21), to the continuous autoclave (1), thus closing the entire pulp production process.

[0042] The continuous process for the production of cellulosic pulp according to the present invention further optionally includes the production stages:

(ix) sieving and fractionation, whereby the diluted suspension from step (vii) is processed through a sieving and fractionation device (22) equipped with a 0.1-0.5 mm screen, thereby obtaining two fractions;

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

(b) the second fraction that passes through a 0.1-0.5 mm sieve, and which is considered a good cellulose pulp of very high whiteness, greater than 90%, suitable for the production of high quality paper or pulp sheets, which exits the entire process through a pipeline (23); and

(x) pulp mills; in which fraction (a) of the cellulosic pulp from stage (ix) is subjected to one or two successive processes through pulp mills (24,25) to obtain cellulosic pulp, which is transferred, via pipeline (26), to a device (22) for screening and fractionation.

[0043] In another way of realizing this invention, cooking chemicals for phase (ii) digestion are introduced into a mixture of concentrated aqueous solutions of 20-50 wt.% NaOH and 10-30 wt.% NaCl.

[0044] The continuous phase (ii) of the digestion is preferably carried out at 90-100 °C.

[0045] In addition, in a different way of implementing this invention, bleaching chemicals for phase (iii) of bleaching are introduced into the continuous bleaching reactor (6) in the form of concentrated aqueous solutions of 30-45 wt.% sodium silicate (xNa2O•ySiO2); and 20-40 wt.% hydrogen peroxide (H2O2).

[0046] The continuous bleaching process is preferably carried out at 85-100 °C.

[0047] The continuous process for the production of cellulosic pulp according to the present invention can alternatively be carried out by microwave (MW) assisted heating. In this case, the heating units in the continuous autoclave (1) and/or the continuous bleaching reactor (6) are magnetrons that produce microwaves (MW).

[0048] The grass-like raw material that can be processed by the process according to the present invention includes stems of plant species selected from the group consisting 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); millet: cultivated millet (Panicum miliaceum, Linne), pearl millet (Pennisetum glaucum, Linne), common millet (Panicum ramosum, Linne), and Japanese millet (Echinochloa frumentaceae, Linne); triticale (x Triticosecale, Wittm. ex A. Camus); buckwheat (Fagopyrum esculentum, Moench); miscanthus (Miscanthus x giganteus, Andersson); prairie millet (Panicum virgatum, Linne); sorghum (Sorghum species, Linne); reed (Phragmites australis, Cav.), shevar (Arundo donax, Linne), Burmese reed (Neyraudia reynaudiana, Kunth.), rush (Typha spp., Linne), papyrus (Cyperus papyrus, Linne), sea urchin (Sparganium spp., Linne), South African reed (Thamnochortus insignis, Linne); esparto grass (Stipa tenacissima, Linne and Lygeum spartum, Linne); jute (Corchorus olitorius, Linne); bamboo (Bambusoideae spp.; Linne); bagasse; or mixtures thereof.

[0049] In a certain case, the grass-like raw material is miscanthus (Miscanthus x

giganteus, Andersson).

Brief Description of the Drawing

[0050]

Fig. 1 - shows a block diagram of a process for the production of very high whiteness cellulosic pulp from chopped grass-like raw material according to the present invention; the key stages are: continuous digestion, dispersing, continuous bleaching, optional dispersing, dewatering which removes black liquor, and washing the resulting pulp to obtain a white cellulosic pulp.

Black liquor is optionally partially evaporated and further processed by electrolysis, which isolates lignin and other non-cellulosic by-products, and regenerates the cooking chemical solution. In the stages of digestion and bleaching, microwave (MW) heating can optionally be used; indicated by a dashed line.

Fig. 2 - shows a block diagram of a process for the production of very high whiteness cellulose pulp according to the present invention; with an emphasis on additional stages: sieving and dividing into fractions and pulp pulp grinding, which are optionally performed.

[0051] In the stages of digestion and bleaching, microwave (MW) heating can optionally be used; indicated by a dashed line.

Detailed Description of the Invention

[0052] This invention relates to an improved continuous process for the production of cellulose pulp of very high whiteness, greater than 90%, of 10 wt.% dry matter, with a lignin content of <5 wt.%, from shredded grass-like raw material such as miscanthus (Miscanthus x giganteus, Andersson).

[0053] Such grass-like feedstock typically contains approximately 30-45 wt.% cellulose with 15-32 wt.% lignin; see reference 11 in the literature:

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.

[0054] Much of this lignin must be removed in a very gentle manner to ensure maximum preservation of cellulosic fibers in a quality cellulosic pulp for high quality very high whiteness paper.

[0055] According to the present invention, this process is carried out continuously through several production stages, as shown in Fig. 1 and 2.

(i) Preparation of grass-like plant material

[0056] The raw material is prepared by crushing, in order to produce a material with a longitudinal size distribution of 5-30 mm and a diameter of 0.1-2 mm. Shredding of the initial grass-like material is carried out by conventional shredding devices or delivered directly from the field, if the crops are harvested with a combine equipped with a suitable shredding device, which results in plant material of the above-mentioned part dimensions.

[0057] Primarily, shredding should be carried out in a mild manner, which results in fibrous material predominantly shredded along the fibers, in order to preserve them.

[0058] This is the reason why shredding is not shown in Fig.1, because it is either a common pretreatment or it can be performed during harvest in the field.

[0059] The shredded material is then subjected to dedusting by removing fine, dusty, non-fibrous plant material, which would otherwise reduce the quality of the resulting cellulosic pulp. This is achieved by a suitable fan that produces a strong air circulation that allows fine light particles to be blown out.

[0060] The latter fine dust does not enter the process at all, thus saving significant amounts of digestion chemicals that would otherwise be consumed in the reaction of this material with sodium hydroxide (NaOH). In addition, the effluents are not contaminated with such a level of organic matter, which would have a significant negative impact on the environment. This non-fibrous fine material generally comes from the central part of plant stems. In the case of miscanthus, the percentage of this part is approximately 8-9% by mass.

[0061] The fine non-fibrous dust collected in this way can be used in the process as fuel in the production of energy or raw material in the production of xylan.

(ii) Continuous digestion of grass-like plant material

[0062] The continuous digestion or cooking of the dust-free grass-like plant material prepared in phase (i) is carried out in a continuous autoclave (1) designed as a longitudinal column, the interior of which is equipped with

1

a screw conveyor and a heating unit, where the grass-like plant material is continuously introduced directly to the top of the mentioned autoclave via the transport conveyor (4).

[0063] Digestion chemicals, sodium hydroxide (NaOH) and sodium chloride (NaCl), and fresh water and/or regenerated white lye are continuously introduced at the top of said autoclave (1) in parallel with said raw material being introduced; maintaining a digestion temperature of 70-120 °C, and the average composition of the thus derived suspension during said continuous digestion is maintained in the following ranges:

(a) 0.50-2.00 wt.% NaOH;

(b) 0.50-1.50 wt.% NaCl;

(c) 10-15% by mass of grass-like plant material; and

(d) technical water; up to 100 wt.% suspension;

whereby the mass percentages of the ingredients are calculated in relation to the mass of the entire suspension.

[0064] During this digestion (cooking) of the grass-like raw material, the dissolution of non-cellulosic substances occurs during the transfer of the mass from one side to the other side of the mentioned longitudinal autoclave. The digestion or cooking phase lasts 1.5-3 hours.

[0065] The process for continuous digestion according to this invention is preferably performed by introducing cooking chemicals as concentrated aqueous solutions of 20-50 wt.% sodium hydroxide (NaOH) and 10-30 wt.% sodium chloride (NaCl). Sodium hydroxide and sodium chloride solutions can alternatively be added as a premix.

[0066] Regarding the temperature of the digestion process, the preferred range is 90-100 °C.

(iii) Continuous dispersion

[0067] Cooked cellulose pulp is, at a speed equal to the speed with which the raw material is introduced into the autoclave (1), continuously discharged from the said autoclave (1), via the transport conveyor, directly into the disperser (5) in order to carry out phase (iii) of dispersing, where the suspended cellulose pulp is subjected to the main dispersion, which results in finely dispersed/separated cellulose fibers.

[0068] In this phase of the procedure, the use of a dispersant (5) is absolutely necessary for a high quality cellulose pulp.

(iv) Continuous bleaching

[0069] The thus obtained fine suspension of dispersed cellulose pulp is, after the disperser (5), further subjected to phase (iv) of bleaching:

(a) without any removal of chemicals from stage (ii) digestion; and

(b) without using any additional NaOH for bleaching purposes.

[0070] In phase (iv) of bleaching, the pre-dispersed cellulose pulp is continuously introduced into the continuous bleaching reactor (6) in which,

(b) does not use any additional NaOH for bleaching purposes.

[0071] In phase (iv) of bleaching, the pre-dispersed cellulose pulp is continuously introduced into the continuous bleaching reactor (6) which, analogously to the autoclave (1), is designed as a longitudinal column, the interior of which is equipped with a screw conveyor and a heating unit.

[0072] Bleaching is carried out with a solution of bleaching chemicals that make up:

(a) 20-40 wt.% aqueous solution of hydrogen peroxide (H2O2);

(b) 30-45 wt.% aqueous solution of water glass or sodium silicate (xNa2O•ySiO2) with molar ratio Na2O:SiO2= 1:1 to 3.3:1; and

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

[0073] They are introduced through the pipeline (7) into the bleaching reactor (6), maintaining the bleaching temperature of 70-100 °C, for 45 minutes to 1.5 hours.

[0074] The average composition of the thus formed suspension during the entire bleaching procedure is maintained in the following ranges:

(a) 0.50-2.00 wt.% of said H2O2 solution;

(b) 0.50-2.00 wt.% of said xNa2O•ySiO2 solution;

(c) 10-15% by weight of cellulose; and

(d) technical water; up to 100 wt.% suspension;

whereby the mass percentages of the ingredients are calculated in relation to the mass of the entire suspension. The continuous bleaching process according to the present invention is preferably carried out at temperatures of 85-100 °C.

[0075] In addition, the bleaching chemicals are preferably introduced into the continuous bleaching reactor (6) separately in a manner well known in the field. A typical example of such an optical brightener is Tinopal ABP-A liquid (from CIBA Specialty Chemicals Corporation), which contains 22-24% dry matter of triazinylstilbene type fluorescent brightener. Such products are typically added in a dosage of 0.25-0.50 wt.% to the dry matter content of the cellulose pulp.

[0076] This treatment can be performed to increase the whiteness of the final pulp when producing the highest possible, high quality products.

1

(v) Continuous dispersing (optional)

[0077] Then, the suspension of bleached cellulose pulp thus obtained, which comes out of stage (iv) of bleaching, is further optionally subjected to additional dispersing (stage v), through the disperser (8).

(vi) Continuous dewatering

[0078] In the next phase (vi), the cellulose pulp is transferred to a dewatering device (9), where the bleached and optionally additionally dispersed cellulose pulp is dewatered, and separated from the black liquor to obtain:

(a) bleached cellulose pulp of 10-15 wt.% dry matter; and

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

[0079] The black liquor from phase (vi) of dewatering is transferred through the pipeline (10) to the electrolytic reactor (19) for the regeneration of the solution of cooking chemicals and the isolation of lignin and other non-cellulosic products.

[0080] The term "black liquor" in this invention includes process effluent from both digestion stage (ii) and bleaching stage (iv), it contains residual sodium hydroxide (NaOH), sodium chloride (NaCl), dissolved lignin, and other non-cellulosic by-products, and finally traces of residual hydrogen peroxide.

[0081] Alternatively, the black liquor can be partially concentrated in the evaporator (16) in order to obtain regenerated water which is transferred back to the washing stage (vii) and the concentrated black liquor which goes to the electrolytic reactor (19) through the pipeline (18).

(vii) Continuous washing procedure

[0082] The bleached cellulose pulp is additionally washed (phase vii) in the vessel (12) for washing with additional fresh water, which is introduced through the pipeline (13), which results in:

(a) finally bleached and washed cellulose pulp of very high whiteness, greater than 90%, of 10 wt.% dry matter, with a lignin content >5 wt.%, which exits the whole process through the pipeline (14); and (b) an additional amount of technical washing water, which is transferred through the pipeline (15), together with the black liquor from the dewatering stage (vi), which is transported through the pipeline (10), into the electrolytic reactor (19).

[0083] The term "technical washing water" in this invention includes the process effluent from the continuous washing process (vii phase); it contains black liquor residues that always remain absorbed on the cellulosic

1

fibers inside the pulp coming out of stage (vi) dewatering. Therefore, technical washing water is like highly diluted black liquor.

(viii) Continuous electrolytic regeneration of cooking chemical solution and isolation of lignin and other non-cellulosic by-products

[0084] Continuous electrolytic regeneration of the solution of cooking chemicals (white liquor) and isolation of lignin and other non-cellulosic by-products is carried out either:

(a) directly with the combined black liquor coming through the pipeline (10) and technical washing water coming through the pipeline (15), without prior concentration in the evaporator (16); or (b) the optionally combined black liquor and technical washing water are partially concentrated in the evaporator (16), thereby obtaining a concentrated black liquor which is transferred to the electrolytic reactor (19) through the pipeline (18).

[0085] In the case of optional black liquor concentration, the optimal level of black liquor concentration is preferably up to 0.9-1.05 wt.% sodium (as Na<+>) in the concentrated black liquor entering the electrolytic reactor (19).

[0086] The term "evaporator" includes not only ordinary evaporation devices based on either distillation or vacuum distillation, but also all other means for removing water from aqueous solutions, eg reverse osmosis, ion exchange process, electrodeionization, etc.

[0087] In the electrolytic reactor (19), either the original ("as is") combined black liquor (a) or concentrated black liquor (b) is subjected to electrolysis with direct current (DC) between two electrodes, at an electric potential of 3-30 V, and a current density of 1-10 A/dm<2>, at 10-95 °C, whereby lignin and by-products are separated at the top of the electrolytic solution in anode part, and are continuously removed from the electrolytic reactor.

[0088] The electrolytic removal of lignin is carried out in an electrolytic cell similar or identical to that described in reference 10 of the literature, in which the cathodic and anodic sections are separated by a suitable semi-permeable membrane or partition.

[0089] Electrodes, cathode and anode, are made of suitable electroconductive materials resistant to highly reactive chemicals derived from their parts, e.g. to high pH NaOH. Such suitable materials are: metals such as carbon steel or stainless steel, graphite, magnetite, etc.

[0090] The cathode is preferably made of carbon steel, e.g. type A36, or stainless steel, e.g. types AISI 304, 316, 321, while the anode is made of graphite or magnetite.

[0091] Electrodes can be made in various forms, of which plates and wire mesh are preferred.

[0092] The partition is made of materials selected from the group that includes: asbestos, mineral wool

1

(rock wool), Portland cement, aluminum oxide (Al2O3), titanium dioxide (TiO2), zirconium dioxide (ZrO2), polyethylene (PE), polyethersulfone (PES), polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE; Teflon<R>), polyvinylidene fluoride (PVDF), sulfonated polytetrafluoroethylene (Nafion<R>), clay and sodium silicate, from combinations of the mentioned materials, or other suitable chemically resistant materials.

[0093] Electrolytic removal of lignin and regeneration of white liquor is preferably performed at an electric potential of 3-10 V and a current density of 3-7 A/dm<2>.

[0094] Alkaline lye from the pulp production process is introduced into the anode part.

[0095] During the electrolysis of the black liquor solution, lignin and other side products are separated at the top of the electrolytic solution from the anodic (+) part, and are removed from the process by the usual mechanical means, via a transport conveyor or the like. The separation of lignin within the anodic (+) section is facilitated by the evolution of gaseous oxygen (O2) as a by-product of the associated aqueous electrolysis.

[0096] At the same time, the main reaction in the cathode (-) part is the evolution of hydrogen (H2), which is accompanied by an increase in the concentration of sodium hydroxide (NaOH). Therefore, the regenerated sodium hydroxide solution from the cathode part is transferred, together with the sodium chloride (NaCl) that remains in the solution, through the pipeline (21) back to the autoclave (1).

[0097] The resulting electrolytic solution with the regenerated solution of sodium hydroxide (NaOH) and sodium chloride (NaCl), which represents white lye, is regenerated back to the digestion/cooking process, through the pipeline (21), to the continuous autoclave (1), closing the entire pulp production process.

[0098] The term "white liquor" in this process includes the process effluent from stage (viii) of the continuous electrolytic regeneration of a solution of cooking chemicals, NaOH and NaCl. It contains mostly dilute sodium hydroxide and sodium chloride, with very small amounts of lignin and other non-cellulosic materials derived from black liquor.

[0099] The lignin is separated at the top of the electrolytic solution in the cathodic section and is continuously removed via a conveyor (20) of the usual type of transport conveyor.

[0100] The presence of these small amounts of remaining ingredients that occur in plants does not affect the performance of the entire procedure nor the quality of the final cellulose pulp after using this regenerated white lye as the only technical water during repeated cycles of the procedure.

[0101] The term "electrolytic reactor" in the present invention includes one or a set (array) of several or a large number of combined electrolytic cells. This depends on the capacity or scale of production of cellulose pulp by the process according to the present invention. For example, larger production facilities may include an array of 10-100 electrolytic cells. In the case of a large tonnage production facility, such an "electrolytic reactor" may include several hundred such electrolytic cells.

[0102] The process for the production of cellulose pulp, very high whiteness, according to the present invention further optionally includes the production stages of screening, fractionation, and grinding.

[0103] The entire process for the production of cellulose according to the present invention is presented through stages (i)-(vii) in Scheme 1.

1

(ix) Screening and fractionation of cellulose pulp

[0104] In these actions, the diluted suspension from phase (viii) is further processed through a device (22) for sieving and dividing into fractions equipped with a 0.1-0.5 mm sieve, which results in two fractions;

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

(b) another fraction that passes through a 0.1-0.5 mm sieve, and which is considered a good cellulose pulp suitable for the production of high-quality paper or pulp sheets, which exits the entire process through a pipeline (23).

(x) Pulp grinding

[0105] This is necessary in order to process the fraction (a) of cellulose pulp from phase (ix) through one or two successive stages of grinding with pulp mills (24, 25), which obtains cellulose pulp which is transferred via pipeline (26) back to the device (22) for sieving and dividing into fractions. In this way, the entire amount of potentially unsuitable cellulose pulp is processed into the desired final product, which exits the entire process through the pipeline (23).

[0106] The entire process for the production of cellulose according to the present invention is represented by steps (i)-(vii) including optional steps (ix) and (x) in Scheme 2.

Use of an alternative means of heating

[0107] The continuous process for the production of cellulose pulp, very high whiteness, according to the present invention can be carried out using microwave (MW) heating in a continuous autoclave (1) and/or a continuous bleaching reactor (6) in a manner known in the field. In this case, the heating units in the continuous autoclave (1) and/or the continuous bleaching reactor (6) are magnetrons that produce microwaves (MW), or a series of magnetrons.

Grass-like raw material

[0108] Grass-like raw material that can be used as starting raw material for the process according to the present invention includes stems of plant species selected from the group consisting 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); millet: cultivated millet (Panicum

miliaceum, Linne), pearl millet (Pennisetum glaucum, Linne), common millet (Panicum ramosum, Linne), and

1

Japanese millet (Echinochloa frumentaceae, Linne); triticale (x Triticosecale, Wittm. ex A. Camus); buckwheat (Fagopyrum esculentum, Moench); miscanthus (Miscanthus x giganteus, Andersson); prairie millet (Panicum virgatum, Linne); sorghum (Sorghum species, Linne); reed (Phragmites australis, Cav.), shevar (Arundo donax, Linne), Burmese reed (Neyraudia reynaudiana, Kunth.), rush (Typha spp., Linne), papyrus (Cyperus papyrus, Linne), sea urchin (Sparganium spp., Linne), South African reed (Thamnochortus insignis, Linne); esparto grass (Stipa tenacissima, Linne and Lygeum spartum, Linne); jute (Corchorus olitorius, Linne); bamboo (Bambusoideae spp., Linne); bagasse; or mixtures thereof. The preferred grass-like raw material is miscanthus (Miscanthus x giganteus, Andersson).

Results of analyzes of cellulose produced by the process according to the present invention

[0109] The superior properties of the pulp produced according to the present invention can be clearly seen when the results of analyzes of the resulting paper samples are compared with the equally good pulp produced by a similar process described in the prior art; see reference 9.

[0110] The results of cellulose analysis and comparison with the same parameters for cellulose obtained by the process according to the closest state of the art are presented in Table 1.

Table 1. Comparison of the properties of paper prepared from the pulp obtained by the process according to the present invention and from the pulp obtained by the process according to the closest prior art, reference 9 (as a control).

2

[0111] The test results showed significantly better mechanical properties (tensile strength and resistance to tearing) of the cellulose stack and the resulting paper at the same (comparable) grammage (surface mass about 100 g/m2). Tensile strength was 5,410 vs. 4,800, a 13% improvement, and tear resistance was 3.25 vs. 2.95, a 10% improvement. These results clearly indicate better mechanical properties of the cellulose according to the present invention which is probably provided by the improved preservation of the original cellulose fibers during processing.

[0112] In addition, the brightness (whiteness) of the pulp was significantly improved to over 90% (97.94%) against the maximum 80% that could be achieved by the process according to the closest state of the art (reference 9 in the literature).

[0113] Possible changes in process parameters may result in some further improvements in the key quality parameters of the resulting pulp, but such changes are considered to be covered by the present invention.

Industrial Applicability

[0114] This invention is obviously industrially applicable.

[0115] The method according to the present invention is further characterized by the following key features:

(i) very mild processes for digestion and bleaching of the grass-like raw material resulting in highly preserved cellulosic pulp, with improved mechanical properties, a lignin content of <5 wt.%, and a whiteness >90%, which makes it suitable for high quality paper production;

(ii) by slightly dispersing between the digestion and bleaching stages, essentially without any removal of the black liquor after the digestion stage;

(iii) in which the processes for both digestion and bleaching are technologically optimized to ensure minimal consumption of chemicals;

(iv) where the bleaching process is carried out with a new hydrogen peroxide (H2O2)-sodium silicate (xNa2O•ySiO2) system, essentially without any use of sodium hydroxide (NaOH); and (v) integrated and efficient electrolytic removal of lignin from the black liquor solution with associated regeneration of the sodium hydroxide solution (+ NaCl), which is reused in the cooking process as white liquor, thereby closing the entire production cycle without any significant waste and environmental impact.

Callsign list

[0116]

1 - continuous autoclave

2 - pipeline for the introduction of fresh water

3 - pipeline for introduction of digestion chemicals

4 - transport conveyor for introduction of chopped grass-like raw material

5 - disperser

6 - continuous bleaching reactor; essentially the same as a continuous autoclave

7 - pipeline for introduction of bleaching chemicals

8 - disperser; this one is optional; indicated by a dotted line

9 - dewatering device

10 - pipeline for transferring black liquor from the dewatering device to the electrolytic reactor for lignin removal, or optionally, to the evaporator 16

11 - pipeline for transferring the dehydrated cellulose pulp to the vessel 12 for washing

12 - vessel for washing cellulose pulp

13 - pipeline for the introduction of fresh water

14 - pipeline for the output of the final cellulose pulp

15 - pipeline for transferring used technical water from cellulose pulp washing to the electrolytic reactor for regeneration, or optionally, to the evaporator 16

16 - optional evaporator; for partially concentrating the combined black liquor of technical washing water

17 - optional piping for transferring regenerated clean water from the evaporator to the pulp washing vessel

18 - pipeline for transferring black liquor to the electrolytic reactor 16, or optionally, concentrated black liquor if the evaporator is in use

19 - electrolytic reactor for removing lignin and other side products

20 - pipeline for the output of lignin and other non-cellulosic side products

Claims (10)

21 - pipeline for transferring regenerated cooking chemicals back to autoclave 1 22 - device for sieving and dividing into fractions 23 - pipeline for the output of white cellulose pulp when optional screening and fractionation is carried out 24 - pulp mill 25 - pulp mill 26 - pipeline for transferring the milled white cellulose pulp back to the device for sieving and dividing into fractions Patent claims 1. A continuous process for the production of very high whiteness cellulosic pulp from grass-like plant material, said process comprising the steps of: (i) preparation of grass-like plant raw material by shredding to produce raw material with a longitudinal size distribution of 5-30 mm and a diameter of 0.1-2 mm, and with fine dust particles removed by dusting said raw material with a fan; (ii) continuous digestion of the dust-free grass-like plant material prepared in phase (i) in a continuous autoclave (1) designed as a longitudinal column, the interior of which is equipped with a screw conveyor and a heating unit; where the grass-like plant material is continuously introduced directly to the top of said autoclave via a transport conveyor (4); indicated by, which chemicals for digestion: NaOH, NaCl, and fresh water and/or regenerated white lye are continuously introduced to the top of the mentioned autoclave (1) in parallel with the mentioned raw material being introduced; maintaining a digestion temperature of 70-120 °C, and the average composition of the thus derived suspension during said continuous digestion is maintained in the following ranges: (a) 0.50-2.00 wt.% NaOH; (b) 0.50-1.50 wt.% NaCl; (c) 10-15% by mass of grass-like plant material; and (d) technical water; up to 100 wt.% suspension; whereby the mass percentages of the ingredients are calculated in relation to the mass of the entire suspension; and where the dissolution of non-cellulosic substances from grass-like plant material occurs 2 during the transfer of the mass from one side to the other side of the aforementioned longitudinal autoclave, which lasts 1.5-3 hours; (iii) in which the cooked pulp, at a speed equal to the speed at which the raw material is introduced into the autoclave (1), is continuously discharged from the said autoclave (1), via a transport conveyor, directly into the disperser (5), where the suspended cellulose pulp is further subjected to the main dispersion, thereby obtaining finely dispersed cellulose fibers; (iv) in which the fine suspension of dispersed cellulosic pulp from step (iii) is further subjected to bleaching: (a) without any removal of chemicals from stage (ii) digestion; and (b) without using any additional NaOH for bleaching purposes; whereby the dispersed cellulose pulp is continuously introduced into the continuous bleaching reactor (6), which is formed as a longitudinal column equipped with a screw conveyor and a heating unit; and where are the bleaching chemicals: (a) aqueous solution of hydrogen peroxide /H2O2/ with 20-40 wt.% H2O2; and (b) aqueous solution of sodium silicate /xNa2O•ySiO2/ with 30-45% by mass of combined Na2O+SiO2, with a molar ratio of Na2O:SiO2= 1:1 to 3.3:1; and wherein the mentioned chemicals are introduced through the pipeline (7) into the bleaching reactor (6), maintaining the bleaching temperature of 70-100 °C, for 45 minutes to 1.5 h, and the average composition of the thus derived suspension during the entire bleaching process is maintained in the following ranges: (a) 0.50-2.00 wt.% of said H2O2 solution; (b) 0.50-2.00 wt.% of said xNa2O•ySiO2 solution; (c) 10-15% by weight of cellulose; and (d) technical water; up to 100 wt.% suspension; whereby the mass percentages of the ingredients are calculated in relation to the mass of the entire suspension; (v) the suspension of bleached cellulosic pulp thus obtained, which comes out of stage (iv) of bleaching, is further optionally processed through a disperser (8); and then transferred to the dewatering device (9); (vi) in which the bleached and optionally additionally dispersed cellulosic pulp from stage (v) is dewatered and separated from the black liquor to obtain: (a) bleached cellulose pulp of 10-15 wt.% dry matter; and (b) lye containing lignin and other by-products of cooking and bleaching; and (vii) the bleached cellulose pulp is additionally washed in the vessel (12) for washing with additional fresh water, which is introduced through the pipeline (13), thereby obtaining: (a) finally bleached and washed cellulosic pulp of very high whiteness, greater than 90%, of 10 wt.% dry matter, with a lignin content <5 wt.%, which exits the whole process through the pipeline (14); and (b) an additional amount of technical washing water that is transferred through the pipeline (15), together with the black liquor from phase (vi) dewatering, which is transported through the pipeline (10) to the electrolytic reactor (19); (viii) in which black liquor and technical washing water are further subjected to electrolysis with direct current between two electrodes, at an electric potential of 3-30 V, and a current density of 1-10 A/dm<2>, at 10-95 °C, whereby lignin and by-products are separated at the top of the electrolytic solution in the anodic part, and are continuously removed from the electrolytic reactor; and whereby the resulting electrolytic solution with the regenerated solution of NaOH and NaCl, which represents white lye, is regenerated from the cathodic part back to the cooking process, through the pipeline (21), to the continuous autoclave (1), thus closing the entire pulp production process. 2. A continuous process for the production of cellulose pulp according to claim 1, characterized in that it further includes the stages: (ix) sieving and fractionation, whereby the diluted suspension from stage (viii) according to claim 1 is processed through a device (22) for sieving and fractionation equipped with a 0.1-0.5 mm screen, thereby obtaining two fractions; (a) the first fraction that does not pass through the 0.1-0.5 mm sieve; and (b) the second fraction that passes through a 0.1-0.5 mm sieve, and which is considered a good cellulose pulp of very high whiteness, greater than 90%, suitable for the production of high quality paper or pulp sheets, which exits the entire process through a pipeline (23); and 2 (x) in which fraction (a) of the cellulosic pulp from step (ix) is subjected to one or two successive processings through pulp mills (24, 25) to obtain cellulosic pulp, which is transferred, via pipeline (26), back to the device (22) for screening and fractionation. 3. A continuous process for the production of cellulosic pulp according to any of the preceding claim characterized in that the digestion chemicals in phase (ii) of claim 1 are introduced into a mixture of concentrated aqueous solutions of 20-50 wt.% NaOH and 10-30 wt.% NaCl. 4. A continuous process for the production of cellulosic pulp according to any of the preceding requires that the digestion temperature is maintained in the range of 90-100 °C. 5. A continuous process for the production of cellulosic pulp according to any of the preceding requires that the bleaching temperature be maintained in the range of 85-100 °C. 6. A continuous process for the production of cellulosic pulp according to any of the preceding requires that the heating units in the continuous autoclave (1) and the continuous bleaching reactor (6) are magnetrons that produce microwaves (MW). 7. A continuous process for the production of cellulosic pulp according to any of the preceding requirement indicated by the fact that with the combined black liquor and technical washing waters that exit from the process of phases (vi) and (vii) via pipelines (10) and (15) according to requirement 1, the concentrations in the evaporator (16) are optionally raised to a total sodium content of 0.90-1.05% by mass before further processing in the electrolytic reactor (19). 8. A continuous process for the production of cellulosic pulp according to any of the preceding requirements characterized by the fact that the electrolytic removal of lignin and the regeneration of white liquor in phase (viii) according to claim 1 inside the electrolytic reactor (19) is performed with a graphite anode and a cathode made of carbon steel or stainless steel, at an electric potential of 3-10 V, and a current density of 3-7 A/dm<2>. 9. A continuous process for the production of cellulosic pulp according to any of the preceding requires, indicated by the fact that the grass-like raw material includes stems of plant species selected from the group consisting 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/; millet: cultivated millet /Panicum miliaceum, Linne/, pearl millet /Pennisetum glaucum, Linne/, common millet /Panicum ramosum, Linne/, and Japanese millet /Echinochloa frumentaceae, Linne/; triticale /x Triticosecale, Wittm. ex A. Camus/; buckwheat /Fagopyrum esculentum, Moench/; miscanthus /Miscanthus x 2 giganteus, Andersson/; prairie millet /Panicum virgatum, Linne/; sorghum /Sorghum species, Linne/; reed /Phragmites australis, Cav./, sedge /Arundo donax, Linne/, Burmese reed /Neyraudia reynaudiana, Kunth./, rush /Typha spp., Linne/, papyrus /Cyperus papyrus, Linne/, sea urchin /Sparganium spp., Linne/, South African reed /Thamnochortus insignis, Linne/; esparto grass /Stipa tenacissima, Linne and Lygeum spartum, Linne/; jute /Corchorus olitorius, Linne/, bamboo /Bambusoideae spp., Linne/, bagasse, or their mixtures. 10. Continuous process for the production of cellulose pulp according to claim 8, characterized in that the grass-like raw material is miscanthus /Miscanthus x giganteus, Andersson/. 2