FI108945B - Method for production of pulp - Google Patents

Description

108945

METHOD FOR PREPARING PULP

The present invention is directed to a novel process for producing chemical pulp. The object of the invention is to improve the previous solvent cooking methods, e.g., 5 organic solvent cooking methods, which are generally known from, for example, U.S. Patent 3,585,104.

The wood material consists of cellulose-containing fibers and lignin connecting them, both within and between the fibers. In order to separate the fibers without damaging them, the lignin must be removed between the fibers. This is usually accomplished by dissolving the lignin. Usually the soups consist of sodium hydroxide (soda soup), sodium hydroxide containing sodium sulphide | (sulfate or kraft soup, also known as alkaline soup) or, for example, sulfite ions (sulfite soup, also known as acidic sulfite soup). Some organic solvents, the best known of which are ethanol and methanol, can also be used to remove lignin (known under the trade name organosolv process). Formic acid has also been proposed for use as an organic solvent.

Methanol and ethanol can be used as solvents in both alkaline and acidic soups. The advantage of acidic cooking is the ease with which chemicals are recovered, since the wood '. The acids in 20 make the soup acidic when methanol or ethanol alone are used as solvents. The disadvantage of acidic cooking is the poor quality of the pulp produced, as the cellulosic fibers are partially degraded by acidic treatment. Alkaline * ·; the quality of the pulp produced in the soup is good, but the problem with basic soups is the recovery of chemicals. One base, usually a sodium base, must first be added to the soup from which it must be recovered and then reused. . The known basic organic solvent cooking method is the so-called. Organocell method,,; ·, Currently in use in Germany. The soup used in this process is soda, methanol soup.

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The present invention is based on the observation that during solvent cooking, acids are formed at the very beginning of the cooking. The pH of the soup can be maintained in the desired range (generally neutral) by removing the acids from the beginning of the soup, i.e. almost immediately after their formation. Keeping the pH of the soup generally neutral improves the quality of the pulp in both stream and batch cooking processes. Generally this is achieved in counter cooking with countercurrent cooking, whereas in batch cooking this is achieved by removing solvent from the batch digester for a certain time after the start of the cooking (and possibly periodically thereafter) as the cooking continues after solvent removal and addition of new solvent.

A process for making cellulosic pulp i from pulverized cellulosic fibrous material in organic solvent based cooking according to an embodiment of the invention comprises the step of a) adjusting the pH of the material to be generally neutral by removing the acids initially formed in the cooking with To keep the pH at the desired level. The process can be utilized by a treatment vessel and may include the following additional steps: b) causing the cellulosic fibrous material to flow in the treatment vessel in the first direction and removing it from the treatment vessel in the same, first direction, adding the required amount of base to the material before treatment; and c) conducting a liquid stream containing an organic solvent '·' · * 20 to dissolve the lignin from the cellulosic fibrous material »* * in a second direction opposite to the first direction. Step a) is carried out by removing the acids formed during cooking together with the organic solvent "" "I used from the treatment vessel.

According to another embodiment of the invention, the process for producing cellulosic pulp from comminuted cellulosic fibrous material in solvent cooking comprises: ': ·, From the following steps: .1, causing the chopped cellulosic fibrous material to flow t' '\ as a first stream in a first direction; 1 Λ r ·. 3, the stream of organic solvent is made to flow in functional contact with the chopped cellulosic fibrous material upstream of the first stream, i.e. in a second direction opposite to the first direction during the entire functional connection of the solvent and the fibrous material to form organic acids as the organic solvent dissolves the lignin from the cellulosic fibrous material; and removing the organic acids formed in the solvent cooking from the intimate contact with the cellulosic fibrous material together with the organic solvent flowing upstream of the fibrous material.

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It is a primary object of the present invention to provide an efficient solvent cooking with a stable pH. This and other objects of the invention will be apparent from the detailed description of the invention and the appended claims.

The present invention will now be described with reference to the accompanying drawings, in which Figure 1 is a schematic representation of an exemplary cooking / extraction process according to the invention; Figure 2 is a schematic representation of another exemplary cooking / extraction process according to the invention; FIG. 3 is a schematic representation of the washing step following the cooking / extraction process; FIG. 4 is a schematic representation of an exemplary embodiment of the present invention. the solvent recovery process; Fig. 5 is a schematic representation of yet another exemplary cooking / extraction process according to the invention; Figures 6 and 7 show the change in pH as in Figure 1. ·: ·. soup / extraction process; . Fig. 8 is a diagram of the method f of the present invention; the equipment used for its implementation; Fig. 9 is a schematic representation of the solvent and base recovery process of the invention; Figure 10 is a possible process option for an embodiment of the invention; and Fig. 11 is a possible industrial scale 5 process variant of an embodiment of the invention.

Figure 1 is a schematic representation of the principle of a countercurrent cooking / extraction vessel / step 20 according to the invention. Reference numeral 10 denotes comminuted cellulosic fibrous material, e.g. wood, which is fed to the process, while reference numeral 11 denotes woody cellulosic fibers produced by dissolving and extracting lignin from wood. The number 12 refers to the organic solvent to be added to the process, while the number 13 refers to the used organic solvent to be removed, which also contains lignin. The process shown in Figure 1 was investigated under laboratory conditions to give the surprising results shown in Figure 6.

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In normal softwood organosolv cooking, the pH changes were in the 100 curve. As the acids in the wood dissolve, the pH drops to 3.5 - 3.6 in about 15 minutes to maintain it for the rest of the cooking. In deciduous organosolv cooking, the pH drops to 3.8 - 4.0. This indicates that all acids either form or dissolve in the early stages of cooking. 20 The poor strength of the fiber produced is due to these acidic conditions. Low pH in coniferous cooking may also cause lignin * · · · 'to prevent lignin dissolution.

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In the countercurrent organosolvent digestion of the present invention, the pH changes were as shown in curve 101. During the initial extraction, the pH was found to decrease rapidly. but after that it quickly rose back to its almost original level of 5.5.

This is due to the fact that the organic acids in the wood initially dissolve in the solvent, and as the process proceeds upstream, the acids are washed away with the solvent in subsequent stages of cooking. the pH can be kept almost neutral throughout the cooking process 5 i ο ε? 4 5 without addition of base. The quality of the pulp produced is improved due to the reduced acidity of the subsequent stages of cooking.

Quinde lectured at the Solvent Pulping Conference in 1989 on pine extractives in 5 methanol soups (80% methanol, 205 ° C). He concluded that 90% of the resin acids and about a quarter of the fatty acids are soluble in the first five minutes. This supports our surprising finding that a decrease in pH can be prevented by countercurrent cooking, as shown in Figure 6. (Or - during batch cooking - removing the acids formed as soon as they are formed.) Obviously, the remaining 10 resin and fatty acids are leached out due to the countercurrent nature of the cooking. Thus, according to the present invention, the pH of the extraction process can be controlled both by removing the organic acids formed during the process and by the reverse flow of the cooking solvent to the fibrous material.

The experiments depicted in Figure 6 can be further elucidated with reference to Figure 7.

In the region of Figure 7, the pH of C is less than 4.5, whereby the quality of the pulp deteriorates. During normal ethanol cooking, the pH is below this level and the quality of the pulp produced is poor. This is illustrated by curve 104 in Figure 7. The upstream current * * '| of the invention With the 20 advanced cooking method, however, the pH remains below the lowest desired pH (line. *; 105) for only a very short time. Since the pH value is low for only a short time, the quality of the pulp produced is * · »* V. This is illustrated by the curve 103 of Figure 7.

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The pulp quality can be further improved by the addition of a base in the initial stage of countercurrent cooking. In Figure 7, this is illustrated by curve 102. At the beginning of the process, the added base raises the pH value, whereupon the acids formed or dissolved neutralize the cooking and: ':': the pH becomes almost neutral during cooking. Also, the highest desirable pH · · line shown on line 106 should not be exceeded (area A in Figure 7), as this will render the amount of * * base to be recovered unnecessarily high, thereby increasing the recovery cost. 30 Thus, good quality pulp is obtained by maintaining the pH in the range B.

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It can be said that most preferably the pH of the solvent cooking process is maintained in the range of 4.5 to 12, but it may momentarily drop below 4.5 or rise above 12. Minimum is achieved without base and maximum when used. The base may also be used in such a way that the pH of the soup remains between the upper and lower limits.

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The countercurrent extraction of Figure 2 ensures that the pH does not drop at the beginning of the cooking / extraction 20. Addition of base 14, e.g., CaO, Na 2 Cl 2, or NaOH, or a combination thereof, to the wood feed 10 will neutralize the initial stage of cooking and thus prevent a decrease in pH. Thus, the pH cannot fall to about 3-4, where the hydrolysis would damage the cellulose fibers. The amount of acid formed during the initial stages of cooking is 8-20 kg / tonne of mass, and the amount of base corresponding to this amount of acid must be increased if the pH is to be kept out of the acidic range (i.e. generally neutral, e.g. about 6.5-7.5). Due to the completely countercurrent nature of the soup, the soup 20 contains some base in the initial stages, but is leached off and removed together with the dissolved lignin in stream 13.

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In countercurrent cooking experiments under laboratory conditions, it was also found that a high ethanol or methanol content (e.g., greater than 70%) produces a better lignin removal result. It was generally assumed that the maximum ethanol content would be 50% because of the water in the wood. By countercurrent cooking, the ethanol content can be substantially increased so that its proportion of the organic solvent is greater than 50% by keeping the feed streams

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12 (Figures 1 and 2) high ethanol content. According to the invention, the water is washed off at the initial stage of cooking and is removed with streams 13. So only with power soup can ''! both to regulate the pH and to increase the concentration of the organic solvent.

After extraction 20, the residual solvent in the fiber stream must be washed off as shown in Figure 3. . The fiber stream 11 consists of organic solvents and wood fibers. Because, ':'. soup / extraction is countercurrent, it can be assumed that the fiber suspension consists of fibers in addition to .1. almost pure solvent. In the washing step 21, the solvent is washed off with washing liquid 16, preferably water. This gives a fiber-water suspension 15 and a solution 17 of water and solvent 30, mainly a mixture of water and ethanol or methanol.

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The solvent is recovered from stream 17 e.g. as shown in Figure 4. The water-solvent solution 17 is fed to a distillation apparatus 22 where the water 18 is separated from the solvent 12. The solvent 12 is supplied to the soup 20. As shown in Figure 4, it is possible to carry out the washing 21 with water while maintaining the high ethanol / methanol the pulp still chipped is defibrated before washing to optimize washing. Preferably, the organic solvent 12 has an ethanol / methanol content of substantially greater than 30%.

Figure 5 illustrates a more advanced method than that just described. The wood 10 30 fed to the process is heated at the beginning of the cooking process by feeding steam 31 to the wood in the pre-evaporator 23.

At the same time, a base 32 is introduced into the wood to raise the pH in the initial stage of cooking. Catalyst 33 may be added either during pretreatment 23 or at the beginning of cooking 20 to enhance lignin dissolution. One catalyst commonly used in acidic processes is CaCl2. Ca 2+ ions are advantageous for the process, but on the other hand, the amount of unwanted acid increases in the reaction 2CL + 2H + → 2HCl. Organic bases such as amine can be used as catalysts in both acidic, neutral and basic processes. Ethanol or methanol 34 may be added to cover solvent losses.

The process is further enhanced by treating the lignin-containing liquid to be removed 36.

Of course, the removable liquids 13 of the previous embodiments can be treated with a · · *. *. * Corresponding manner. The stream 36 of solvent to be removed from the stove 20 contains mostly solvent, polysaccharides, lignin and water. This blend 36 is passed to a distillation / evaporation step 24 in which the solvent is predominantly ethanol and / or methanol 37; is separated from water 38 and lignin and polysaccharide material 39. Solvent 37 may be recycled to boiling 20 together with solvent 12 from distillation step 22.

_. t Water and lignin are fed to the incinerator 25 or other processing unit. Lignin, ·; *, can also be separated and produced e.g. vanillin. The added base 32 is fed to the burner 25, after which it can be reused or, if desired, removed.

<»" '. Typically, the base is removed if its amount is small, e.g. 10-50 kg / tonne of pulp.

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Under laboratory conditions, a good quality pulp was produced when the amount of catalyst (CaCl 2) used was 5-50 g / L (i.e. 5-50 g of catalyst per liter of material to be treated). The cooking temperature was 195 ° C and the cooking time was 3 hours. Other possible catalysts besides Ca ++ include Mg ++ and Na +. For lignin extraction, it is advantageous to impregnate the wood with 5 bases (14, 32) as this will swell the wood and thus facilitate lignin extraction.

Ethanol is probably a better solvent than methanol in carrying out the process of the invention, but deacetylation in wood yields some methanol. Thus, some methanol is always present in the process, even if the only fresh solvent 34 is ethanol.

The invention can be applied to a variety of soups, both batch and rental. The invention! can be used, for example, in diggers with a pretreatment zone or in diggers with a separate pretreatment vessel. In applying the invention to batch digesters 15, acids formed at the beginning of cooking can be removed by removing the solvent from the digester after a predetermined time (e.g., 5-10 minutes, however, depending on the material to be treated, the exact composition of the solvent used, etc.) at least some fresh solvent. If desired, removing the solvent and feeding a new '·' · * 20 may be repeated several times, although this is usually not necessary * · · as most acids are formed during the first five minutes. Alternatively, or in addition to the recent method, the pH may be maintained generally during batch cooking ”! neutral in the early stages of cooking by adding a base to the material to be treated before or into the digester.

25,:. The invention may be further illustrated by the following examples.

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Example:

Laboratory tests were carried out on coniferous wood to study the method. The process flow was as follows:

Step 1: The chips were pretreated with a pretreatment liquor consisting of sodium hydroxide and water. The pretreatment took about 30 minutes at 120 ° C. The purpose of the pretreatment step was to make the fiber swell to facilitate the next 10 ethanol extraction steps. The chips were also pre-treated to add a base to maintain the pH sufficiently high for the rest of the cooking.

During the pretreatment step, the amount of base ranged from 0.25 to 1.50 mol NaOH / L. In the experiment, it was found that when the amount of base was less than 0.25 mol NaOH / l, the delignification was not sufficient and the amount of non-fibrous wood was high. When the amount of base was greater than 1.0 mol NaOH / 1, the loss of yield during cooking became too great. The amount of base needed will probably depend on the type of wood used. For the Nordic spruce tested, the optimal amount of base was 0.5 to 1.0 mol NaOH / L.

* * * 20 Step 2: The chips were removed from the pretreatment liquor by lifting it out of the container. It was found that 30-70 kg NaOH / tonne pulp was bound to the r «* • *« chip.

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The base was also consumed in the pretreatment vessel. The consumption of sodium hydroxide was 50 - 300 • »* !!! kb / ton of pulp.

«· * U 25,:. Step 3: After Steps 1 and 2, the chips were subjected to the countercurrent ethanol cooking step.

t. ; ·. This solvent extraction took about 120 minutes at 185 ° C. Anthraquinone was added to the solvent to improve delignification. The amount of anthraquinone added was 0-1.0 mmol.

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The concentration of the ethanol solvent was varied between 25 and 100%. The lowest lignin residues were reached when the solution concentration was 40-70% in the liquid phase. Both pulp properties and bleachability were found to be similar to kraft pulp in terms of both bleachability and strength.

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Figure 8 illustrates a device for implementing the process of Example 1. The apparatus includes a conventional chips silo 201 connected via a low pressure feeder 202 to a conventional horizontal steaming vessel 203 which in turn is connected via a feed hopper 203 to a conventional high pressure feeder 204. The wood chips 10 are steamed and preheated, in basic solution. The alkaline solution is introduced into the system via the unit 207. At the top of the vessel is a conventional liquid / dry separator, from which the separated liquid is recycled along line 208 to the high pressure inlet of feeder 204. The base may be obtained from one or more of the following sources: - wastewater from basic bleaching stages, e.g., E or P stages; - sodium hydroxide, whether fresh or factory-causticised from sodium carbonate; and / or * '' '' '- 20 - sodium carbonate if the required alkalinity is so low that only; »1 t» «« '·' · 1 part of the base must be sodium hydroxide.

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From the pretreatment vessel 206, the chips are led to the extraction zone 215 of the second pressure vessel 213, where the pressure is much higher than that of the pressure vessel 206. The chips are fed through the line 211 25 to the upper portion of the vessel 213 via a high pressure feeder 209. A conventional liquid / solid separator is disposed at the top of the vessel 213, from which the discharged liquid is recycled together 212 to the high pressure inlet of the feeder 209. In order to adjust the temperature of the liquid to be recycled f f _ '' '. to minimize any inconvenience to high pressure feeder 209, fluid 30 is passed through heat exchanger 210.

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Preferably, two zones 213 are formed in the second vessel 213: 1) zone 215 where the chips are extracted with ethanol / methanol; and 2) zone 216 where the chips are washed before being removed from the container.

5 The filtrate from the washing or bleaching step after this step is used as the washing liquid. The washing liquid is led to the bottom of the vessel 213 through the 218. The mass at the bottom of vessel 213 is washed and discharged to port 219.

Ethanol and / or methanol is added along compound 217 above wash zone 216. The strength and amount of ethanol to be added are adjusted so that optimum conditions for extraction are achieved in extraction zone 215. If necessary, water may be separated from the circuit 220 by distillation to adjust the ethanol / methanol concentration in the extraction zone 215.

The extraction broth is removed from the extraction / cooking vessel 213 to the outlet 214. The spent base broth, used ethanol / methanol and dissolved lignin are recycled.

Figure 9 shows a simplified solvent and base recovery system. The extraction broth 214 in Fig. 8 is treated in the following steps: a) Separation of ethanol / methanol • ♦ • · * 20 250. The solvent is then reused in extraction vessel 213. b) 4 1 «

Evaporation 252 to remove water, (c) Combustion of lignin and polysaccharides 254.

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During combustion, molten 255 is formed, consisting mainly of sodium carbonate. The molten • i · is dissolved in water and either used as a Na2CO2 containing liquid in the pretreatment vessel * »· 25 206 or causticized to sodium hydroxide prior to use in the pretreatment. If the amount of sodium carbonate is low, there is no need to re-use it.

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If chlorine-free pulp is desired, the pulp may be bleached with oxygen, ozone t * ', and peroxide. Figure 10 shows a diagram of the process whereby initial bleaching is carried out with oxygen and the final final bleaching with ozone and peroxide.

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In the embodiment shown in Fig. 10, wood chips 300 which are introduced into the process are first heated by supplying steam 301 to the wood material in the pretreatment vessel 323. The base 302 is introduced into the same Aaa wood chips so that during processing the wood chips have a pH of 11-12. The base is obtained from bleach wastewater containing 40-120 kg NaOH / adt. If the amount of waste water is too large, it must be evaporated to reduce the volume. If more base is needed, additional sodium hydroxide or sodium carbonate can be added to the process.

After pretreatment 323, wood chips 310 are led to an extraction step 320 where some 10 ethanol and / or methanol 312 'may be added initially to adjust the treatment liquid / wood ratio or to increase the amount of methanol / ethanol at the beginning of the extraction 320.

After extraction, chips 311 are washed with bleach wastewater 302 ', 303, 304, which may be acidic or alkaline depending on how the bleaching is performed. Where possible, acidic filtrates 303 and 304 are used in washing 321 and alkaline filtrate 302 in wood chips pretreatment 323. The solvent-containing washing liquid 317 may need to be concentrated by distillation 322 prior to the methanol / ethanol addition 312 of the extraction step 320.

The pulp from the scrubbing step 315 is led to the bleaching plant 330 where the pulp is bleached * *! 20 in OZP.

! r, * t i% \ ^ t * I / | | After the extraction step 320, the spent liquor 306 is recovered, evaporated and incinerated. If the amount of t t base is small, the Na2CO3 formed in the recovery is removed from the factory and »t t fresh sodium hydroxide is replaced. Part of the sodium carbonate can be used in the * »25 pretreatment step. If the amount of base contained in the effluent stream is large, it is probably more practical to causticize the formed sodium carbonate to sodium hydroxide and r! I », ':' thus produces at the factory fresh sodium hydroxide for bleaching and pretreatment.

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In a successful experiment, the wood was pretreated with a mixture of 75% sodium carbonate and 25% sodium hydroxide in an amount equivalent to about 200 kg Na / ADt, calculated as sodium hydroxide. Thus, the amount of chemicals used was about 50 kg / adt NaOH, the remainder being Na 2 CO 3.

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The amount of sodium hydroxide required in the bleaching sequence OZEP is also about 50 kg / adt. Thus, all the bases needed for wood chips pre-treatment are obtained in the form of sodium hydroxide from the bleach wastewater. The remainder of the base can be used as sodium carbonate, in the form of which sodium is obtained by incineration of the waste liquors in a 10 recovery boiler.

Thus, the mill does not need a causticisation plant, but the sodium carbonate used there is obtained from the waste liquor incineration and the sodium hydroxide from the bleach wastewater.

Bleaching filtrates can also be used to dissolve sodium carbonate from a recovery boiler, further reducing the volume of wastewater.

Figure 11 is a flowchart illustrating the process of performing initial bleaching with oxygen and actual final bleaching with ozone and peroxide.

: Y: 20 * 1 t ':: Although the invention has been described above in the form of its most practical embodiment * 1,; · ·, the invention should not be limited to the embodiment described above, but the scope of the invention as defined in the appended claims solutions.

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Claims (18)

A process for producing cellulosic pulp from comminuted cellulosic fibrous material in an organic solvent cooking, characterized in that: b) treating the pre-treated cellulosic fibrous material with an organic solvent-containing liquid to extract the lignin contained therein; and that e) washing the treated cellulosic fibrous material to remove and extract the dissolved lignin and to remove unwanted chemicals from the treated material. Method according to claim 1, characterized in that steps b) and c) are carried out in countercurrent extraction and in countercurrent washing. Process according to claim 1, characterized in that steps a), b) and c) are carried out ·, ·. essentially continuously. * * * ·: 20 Process according to claim 2 or 3, characterized in that steps b) and c) are carried out. :. utilizing a treatment vessel, and wherein • · · - causing the cellulosic fibrous material to flow in a first direction from the treatment vessel, from where it is also removed in the same first direction, adding the necessary base to the material to be treated before being introduced into the treatment vessel; '; · ·: 25 - introducing an organic solvent-containing liquid stream into the treatment vessel in a second direction opposite to the first direction to dissolve the lignin in the cellulosic fibrous material; and the acids formed during cooking are countercurrently removed; ''; together with the organic solvent used from the treatment vessel. »* ·: 30 A process according to claim 1, characterized in that step b) removes the spent organic solvent which is treated to separate the organic solvent and the dissolved material contained therein. Process according to claim 5, characterized in that the organic solvent separated in the separation step 5 is introduced as an organic solvent into step b). A process according to claim 5, characterized in that the material separated from the organic material in the separation step is incinerated to produce a sodium carbonate-containing melt and the melt is dissolved in water to produce either a caustic or non-caustic base 10 liquid which is added to the cellulosic fibrous material in step a). A process according to claim 1, characterized in that some of the chemicals removed in the wash are an organic solvent which is separated from the wash liquid and passed to step 15 b). Process according to claim 8, characterized in that the organic solvent is separated off. . by distillation of the washing liquid. : 20 Process according to Claim 1, characterized in that it contains cellulose. the fibrous material is steamed before or during step a). • · · # • t · A process according to claim 1, characterized in that a catalyst is added to the cellulosic fibrous material i before step b) in order to enhance the solubility of lignin '': 25. »·: 'I '; Process according to claim 11, characterized in that the catalyst is selected from the group consisting essentially of salts of alkaline earth metals, anthraquinone and organic: ·. bases, and that about 5 to 50 g / liter of material to be treated is added to said catalyst. : 30 • · 1,08945 Process according to claim 1, characterized in that the treatment is carried out in a batch digester such that the spent solvent containing the acids formed is removed from the digester soon after the majority of the acids have formed at the beginning of the digestion. Process according to Claim 1, characterized in that the solvent contained in the organic solvent-containing liquid belongs to the group consisting essentially of methanol, ethanol and mixtures of methanol and ethanol. 14 1 0 8 94 5 REQUIREMENTS: Process according to claim 14, characterized in that the amount of methanol, ethanol and mixtures of methanol and ethanol is substantially greater than 50% of the total amount of the liquid containing the solvent. Process according to Claim 1, characterized in that sodium hydroxide is used as the base in step a). 15 Process according to Claim 1, characterized in that sodium carbonate is also used as the base of step a). 18. The method of claim 1, wherein in step c): the cellulosic fibrous material is washed with water. a • · · a * i · »· · a • ·» · »» »a a i 17 1 0 8 9 4 5