US2749241A - Process for producing semi-chemical pulp - Google Patents

Description

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Paolo Marpillero, Milan, Italy, assignor to Development Tecnique de la Pate A Papier Anonyme fiuisse (denommee ci-apres Detepa), Bale (Basel), Elwitzerland, a corporation of Switzerland No Drawing. Application March 8, 1952, Serial No. 275,671

Claims priority, application Italy May 29, 1950 4 Claims. (Cl. 92-6) This invention relates to a new process for producing semi-chemical pulp from a wide variety of cellulose-containing raw materials including all kinds of soft woods (many of which are not now used in semi-chemical pulp processes), hard woods, annual and other plants, stem plants, and the like. More particularly, the invention relates to a novel acid bisulfite process for producing chemical pulps and especially relates to the production of semi chemical pulps in exceptionally high yields for given degrees of delignification, ofttimes from raw materials heretofore considered unsuited for the production of such pulps such as unbarked woods.

A brief consideration of the chemical and physical structure of trees, plants, and other cellulose containing materials is considered advantageous for the understanding of the process of this invention. As typified by wood and apart from various components such as fats and fatty acids, resin and resin acids, small amounts of mineral matter, and the like, all known cellulose containing plants and trees can be considered to be composed of three main components, namely, (A) cellulose, which is present substantially wholly in the form of cellulose fibers, (B) lignin and other similarly acting substances, including pectins, serving primarily as binders between the cellulosic fibers, and (C) the non-cellulosic carbohydrates, generally referred to as hemicellulose, and sometimes referred to herein as non-cellulosic hydrocarbons. In woods, the ingredients, other than A, B and C, are usually designated as extraneous components or extractives and are not a part of the cell walls. Such components designated as D seldom exceed 6% of the dry weight of the wood and may be as little as 1% and are not the concern of this invention. Very little is known about the real chemical and physical nature of these three main components A, B and C. In woods and like plants it is known that the fibrous cellulose A is by far the most important among the major components and is present in a greater proportion than either lignin B or hemicellulose and the like C, usually being present in a quantity of the order of 40 to 55% of the woody material. The hemicellulose C is usually present in a lesser quantity than the lignin B and is intimately associated with the fibrous cellulose. The cellulose A and the hemicellulose C and the like are together designated as the holocellulose content. The lignin B is the ingredient of wood and like cellulosic fibrous material which primarily serves to bind the fibers together, and although it is present as an ingredient of the fibrous proportion of the cellulosic material, it is primarily present between the fibrous ingredients in the so-called middle lamella.

When it is desired to use the cellulosic content of cellulose-containing raw materials, it is customary to convert the raw materials to a pulp. Generally speaking,

2,74%,241 Patented dune 5, 1956 there are two broad classes of pulp, mechanical pulp and chemical pulp. The mechanical pulp is the type of pulp which is obtained from the cellulose-containing materials by mechanical means. Chemical pulp is the type of pulp which is obtained from the cellulose-containing materials by chemical means, such as, by chemical extraction or conversion. When substantially all of the non-cellulosic content of the cellulose-containing raw material is removed by chemical means, leaving substantially only pure cellulose, the resulting pulp is designated as a chemical pulp. A semi-chemical pulp is produced when substantial quantities of the non-cellulosic hydrocarbon content of the cellulose-containing raw material is left unextracted in the chemical treatment. In producing a semi-chemical pulp the treated raw material is sufiiciently delignified to permit a relatively easy defibration of the raw material in the form of wood chips or parts of plants as contrasted with the relatively severe treatment of mechanical pulping processes characterized by grinding or other severe abrading techniques used on logs, the whole stem of plants, or the like, by which most of the fibers are broken or damaged. ln making rayon textiles, resins, varnishes, and other applications where the pulp is to be dissolved, it is usually advantageous to remove substantially all of the lignin B and the non-cellulosic hydrocarbon content C. 0n the other hand, where the pulp is to be used in making paper, it is often desirable to retain a substantial part of the hemicellulose C while removing as much lignin B as possible to facilitate the defibration of the pulp. The retention of certain proportions of hemicellulose is not only permissible in the paper making art but is ofttimes advantageous, serving to improve the fiber-to-fiber bonding strength of the paper and some of the mechanical properties of the final product, in particular, the folding endurance of the paper. in other words, in producing pulp for the paper industry, it is often desirable to obtain the optimum degree or" delignification to facilitate defibration of the pulp with an optimum retention of the holocellulose (A and B) content of the raw material to improve the quality of the paper made from the pulp.

A water solution of S02 when employed at a high temperature, is one of the best known and most effective reactants to sulfonate and thereafter dissolve out the lignin and form chemical pulps. Such a pulping process is called a bisulfite or sulfite process and may take place under acid or neutral conditions, but it is most widely used under acid conditions at pHs below 2. an acid bisulfite process. Generally speaking, acid bisulfite processes consist of the digestion of wood or other cellulose-containing fibrous raw materials in the form of chips or other small pieces, customarily designated as a comminuted material, at temperatures of to C. or even as high as C., in an aqueous solution containing alkali bisulfites, usually alkaline earth bisulfites (such as calcium bisulfite or a mixture of calcium and magnesium bisulfites, although ammonia and sodium bisulfites are also used) and an excess of sulfur dioxide. In the conventional acid bisulfite process, the bath or liquor is prepared by introducing the desired amount of S02 and then adding an alkali, usually an alkaline earth alkali, such as MgO or preferably CaO, to adjust the pH from about 1.1 of pure S02 solutions to the desired pH of 1.5 or 1.7. In such a process the S02 is always used in a quantity in excess of the amount consumed during the process of the digestion and delignification of the raw material to form the pulp.

For example, in treating a metric ton of wood, the amount of S02 really consumed in a conventional bi- This is known as 3 sulfite process, i. e., the amount of SO: chemically combined with the components of the raw materials when the digestion of the wood is carried as far as possible without undesirable burning or attack by the reactants or degradation of the cellulose A, is of the order of 120 to 130 kg. Yet the amount of S02 contained in the pulping bath or liquor is usually 180' to 200 kg. or higher, say 250 kg. It will be understood, therefore, that the conv'e'ritiohzil bisulfite process, when referred to in this appiica'tion, refers to a process wherein the cooking step lattes place at temperatures of the order of 130 to 150 C.- and wherein digestion baths are used which have an pH of the order of 1.5 to 1.7 and contain S02 substantially in excess of the amount consumed or absorbed during the delignification process. In speaking generally of the amount of S02 consumed in a conventional bisulfite process, the amount referred to is the amount consumed when dclignification is carried a far as practical without undesirable degradation of the cellulose A content of the cellulosic raw material. It will be understod also that the weights of the raw materials used, yields of pulp, and the like are always expressed herein in metric units and on an oven-dry basis.

In bisulfite processes including the present process, the delignification or the digestion takes place in two steps oi stages, namely, the soaking or impregnation step and the cooking step. In the soaking step, it is necessary to obtain as thorough a penetration of the cellulosic raw material a possible before raising the temperature to the cooking temperature, such as previously described, for otherwise a so-called burning reaction will take place impairing the digestion of the wood. It is desirable to obtain complete penetration as quickly as possible without burning or other degrading or carbonizing reactions taking place. Thus, as penetration proceeds, the temperature is raised to cooking temperatures as rapidly as possi ble while avoiding burning reactions at the operating pH. During the soaking period it is generally believed that the formed lignosulfonic acid combines with the HSOs ions but there is no cleavage of the lignin B from the woody substance by hydrolysis which only takes place during the cooking step.

After thorough penetration has taken place, the cellulosic material is subjected to cooking temperatures within the previously stated range, the particular temperature being adjusted to the nature of the raw material and the pH used. In known acid bisulfite processes, the time consumed in achieving a predetermined degree of delignification is determined by the pH and temperature used. As applied to woods, a pulp yield of the order of 40 to 50% having a lignin content of about 2 to 5% is obtained.

Efiorts have been made to obtain improved results in known acid bisulfite processes by varying the amount of excess SO: used, but such variations have proven of little significance as compared to variations in temperature or pH. Due to the fact that known bisulfite processes take place in the presence of a large excess of sulfurous acid, it has long been known that the process approximately obeys the laws of a first order reaction and that the reaction is at least apparently pseudomonomolecular. Thus it is not surprising that variations in the amount of excess S02 are of little significance as compared to the two major factors, temperatures and pH, in affecting the delignification of wood in known acid bisulfite processes.

It is an object of this invention to provide a semi-chemical sulfite pulp in desire yields with a higher degree of delignification than heretofore obtained.

It is another object of this invention to obtain semichemical sulfite pulps in desired yields which may be more easily defibrated than sulfite pulps produced in the same yields by known sulfite processes.

It is a further object of this invention to produce desired yields of semi-chemical sulfite pulps with a drastic reduction in the consumption of chemicals as compared to the consumption of chemicals in known pulping processes.

It is an improtant object of this invention to provide a novel acid sulfite process permitting the utilization of many cellulosecontaining raw materials not heretofore successfully used in such a process.

Yet another object of this invention is to obtain sulfite pulp of a quality superior to that produced in like yields by known sulfite processes.

Other objects, features and advantages of this invention will be apparent from the more detailed discussion which follows.

The foregoing and other objectives of this invention are obtained by the delignification of wood and other cellulosic fibrous materials in an acid sulfite bath containing an amount of S02 less than the amount consumed in a conventional bisulfite process and at a pH in excess of that used in the conventional bisulfite process, namely at a pH of at least 2.1 to 2.2, and preferably at a pH of 2.5 to 5.0. In view of the large excess of S02 used in known acid bisulfite processes, the amount of S02 used in this process would be considered absolutely insutficient in any known acid bisulfite process and is much less than the amount used in a conventional bisulfite process. The temperature used in this invention is correlated with the pH and the nature of the raw material so as to prevent the burning or degradation or carbonization of the cellulosic material but it is usually lower than the temperature used in known acid bisulfite processes, thereby resulting in a considerable saving in heat energy.

It is possible, or even probable, that the small quantity of S02 used in this invention has converted the delignification into a diiierent order of reaction. Regardless of the explanation, the greater degrees of delignification obtained in practicing this invention for a given yield of pulp proves that the order or priority of reaction with lignin B, or the speed of reaction therewith, or both, have been greatly modified so as to orient the sulfonation and hydrolysis reactions in the direction of an increased reaction with lignin B in comparison with the extent of the reaction with the hcmicellulose C. Thus the processes of this invention provide pulps in any desired yields which pulps, due to their greater degree of delignification, are superior in quality and are much more easily difibrated than known sulfite pulps produced in kindred yields. It has been surprisingly found that the process of this invention in producing a given yield of pulp consumes considerably less, not more, chemicals than does the conventional bisulfite process while at the same time providing the stated significantly greater degree of delignification.

In a special feature of the invention, the amount of S02 used in the digestion bath is of the order of 60 to kg. per ton of raw materials being delignified and normally does not exceed kg. per ton.

For a given yield of pulp product, the pulps of this invention are much more easily defibrated, and they may be bleached with a much smaller quantity of chlorine. Under such circumstances, the process of this invention provides a pulp with a very large percentage of its hemicellulose C content retained, while the conventional bisulfite process has removed substantially all of the hemicellulose C. Such pulps differ markedly in their properties from analogous known sulfite pulps and may have entirely different utilities due to their greater strength and other properties.

The great saving of chemicals in the instant process will be further illustrated by comparing the chemicals consumed in this process in the production of pulps from woods with the chemicals consumed in the bisulfite process. In a preferred practice of this invention not more than 100 kg. of SO: (a part of which is recovered at the end of the reaction) and only a few kg. of lime or other alkali are introduced into the digester. Moreover, the instant process uses lower temperatures and generally lower pressures in the digestcr, thereby consuming substantially less heat. The conventional bisulfite process, on the other hand, uses not less than 180 to 200 kg. of S02 and at least 120 to 130 kg. are actually consumed in the process. Thus it will be seen that the largest amount of S02 used per ton of soft wood in the present process is 100 kg. when a very high, nearly approaching the maximum degree of possible delignification giving a yield of about 65% of pulp is to be obtained. Lesser amounts of S02 are used when higher yields of pulp and therefore lesser degrees of delignification are desired.

It has been found also that when unbarked wood and yearly plants containing cortical layers, such as cotton stalks, are submitted to digestion with liquors having the range of pH and the amount of S02 specified in this invention and the times and temperatures specified herein, these cortical layers can be dissolved and the residual content of cellulose, if any, will be left.

Therefore sulfite pulps can be produced by the process of this invention from unbarked soft or hard woods also and from unbarked woods of very small diameter, such as unbarked branches, by cooking them in their natural state apart from chipping in the conventional way. Similarly, stems comprising an outer layer of bark and an inside woody part, such as cotton stalks and similar materials may be treated by the process of this invention in order to obtain unbleached or pure white bleached pulps. Similarly, pure cortical layers, such as ramie (China grass) may be cooked by the process of this invention, and, in this case, the fibers obtained may be used bleached or unbleached for textile or paper purposes.

in all the cases Where a raw material comprising bark is treated the amount of S02 to be employed is determined for the woody part of the material under treatment in the exposed way and for the bark or cortical layer by determining the S02 necessary to dissolve it experimentally by means of cooking the pure bark.

Obviously the S02 consumption of pure bark may be completely diiferent from that of woody parts and the result to be obtained in this case is not only that of obtaining the cellulosic fibers contained in the bark if any but also of removing and dissolving the undesired parts of the bark or of transforming some parts into forms removable by further treatments with chlorine and alkalies. In some cases these undesired parts of the bark may substantially represent the totality of the bark as for instance in the case of soft woods and also in this case this bark is removable under the conditions of treatment corresponding to this process.

The following examples further illustrate the inven- I tion but are not to be considered limitations thereupon.

Example 1 is 88 kg. S02 per ton of wood and the amount of lignin to be removed is about 220 kg.

The cooking time will be about 7 hours. When operating for instance, with a pH of 2.70 in the presence of 88 kg. of S02 per ton of oven-dry wood, such a speed of lignin removal requires a temperature of 135 C. for soft wood.

In conformity with the foregoing analysis, 1000 kg. of soft wood (oven-dry basis) are charged into the digester with a sufiicient amount of water to cover the wood. Normally about 5000 kg. of water are needed. The required 88 kg. of S02 provides an aqueous bath containing 1.76% of S02 in about 5000 kg. of water. In order to bring the pH of this solution to 2.7 about 34 ,kg. of Ca0 are dissolved in it (or a corresponding amount 6 temperature is then maintained for about 7 hours, but not for a longer period of time than when the S02 con-' tent of the liquor is reduced from 1.7% to about 0.3%. The digester is then blown out, recovering in known Ways the still available amounts of a. The pulp is then unloaded, washed and defibrated in known ways. The yield of pulp is 75%. Chemicals used per ton of oven-dry wood are 88 kg., 502, less about 10% recovery at the end of cooking, or 80 kg. Thus 40 kg. of sulfur is used per 750 kg. of oven-dry pulp or 54 kg.

of sulfur per ton of oven-dry pulp, or about 49 kg. of

sulfur per ton of air-dry pulp based upon the assumption that the pulp content of air-dry pulp is about 90% of the weight of oven-dry pulp. This pulp may be bleached with 14 to 15% of chlorine, giving a yield in bleached pulp of about 60%. The physical properties of such pulps are excellent and are generally superior to those of a conventional bisulfite process giving a pulp yield of about 40 to 50%.

Example 2 A beech wood is employed. The desired yield of pulp is about 85%. The amount of S02 to be employed is 72.2 kg. per ton of treated wood. In accordance with the process of this invention, the cooking time should be 5.8 hours and the amount of lignin removed should be about 130 kg. of the about 240 kg. in a ton of Wood. When the pH is adjusted, for instance, to a value of 3, a temperature of C. is required. The digester is therefore charged with 1000 kg. of wood (oven-dry basis) and with about 3500 kg. of liquor containing 72.0 kg. of S02. This liquor is usually sufiicient to cover the 1000 kg. of wood and gives a liquor with about 2.5% of S02. In order to obtain a pH of about 3 about 28 kg. of Mg0 are added to the digester and the temperature is then gradually raised during a soaking period to 120 C., and this temperature is maintained for 5.8 hours but not beyond a period of time at which the S02 content of the liquor is reduced to 0.3%. The digester is blown out and any remaining S02 is recovered in known ways. The pulp is washed and defibrated in known ways. Chemicals used per ton of pulp yield about 40 kg. of sulfur and 33 kg. of MgO.

Example 3 Same wood is used as the one used in Example 2. A 65% yield is to be obtained. The amount of S02 to be employed is 118 kg. per ton of wood. The cooking time will be 8.2 hours and the amount of lignin to be removed is about 215 kg. of the about 240 kg. in a ton of wood. It is found that such a speed of lignin removal at a pH of 3 requires a temperature of C. Other operations and results are analogous to those of Example 2.

Example 4 Cotton stalks containing in total 25% lignin and about 25% bark are treated in order to obtain any easy bleachable pulp of mixed cortical and inside fibers with a yield of about 65%.

The amount of S02 required is 120 kg. The cooking time is determined to be 7.6 hours. As it seems preferable by experience to use a high pH for such a peculiar raw material this is fixed at pH 4.

Temperature at this pH for the removal speed of lignin with about the same amounts of S02 employed is known to be C. for hard woods. An experiment is made at this temperature and at 130 C. :5 C. temperatures to determine the better speed of removal in order to approach the desired results. This temperature is found to be 133l35 C.

1000 kg. stalks are loaded in a digester with 8000 kg. of water containing 120 kg. S02 thereby making a concentration of 1.5% S02. An amount of about 40 kg.

MgO is added in order to bring the pH to a value of 4. i

7 The digester is brought to a temperature of 133135 C. in a known manner. This temperature is maintained for 7.6 hours, but not after the S02 has reached a value of 0.3%.

Other operations were followed, as in the previous case, the result being 65% yield pulp bleachable in known ways with 8% chlorine.

Example Unbarked soft wood: The wood is the same as in Example 1 but is treated with full bark, the bark repre senting 12% by weight of the treated wood. A yield of 65% is desired for the woody part of the material. With a yield of 65%, S02 needed is 90 kg.

Experimentally it is determined that 1000 kg. of pure bark when treated at temperatures of 125-145 C. at pH between 2.5-5 for a length of time from 6 to 10 hours will be dissolved or transformed into products which can be dissolved with chlorine when about 170 kg. S02 are employed and used. Therefore the amount of S02 needed corresponding to the amount of 120 kg. bark will be 0.12X170=20.5 kg. S02. The amount of S0: to be employed is therefore 90+20.5=110.5 kg. 80:.

The cooking time equals 6 hours. When operating with pH=3.00 and with an amount of 110.5 kg. S02 this speed corresponds to a temperature of 137 C.

Cooking will therefore be conducted as follows:

1000 kg. of chips (oven-dry basis) of unbarked wood containing 12% bark are charged in a digester together with 500 kg. of water containing 110.5 kg. S02; therefore at 2.20% S02 concentration, and a sufiicient amount of alkali to bring the pH to the value of 3.00. Temperature is raised gradually to 137 C. and this temperature is maintained for about 6 hours.

The pulp obtained with a yield of about 60% calculated on the unbarked wood may be bleached to pure white employing about 14% chlorine of which about 6% is used to dissolve and remove the residual parts of bark.

Example 6 Ramie (China grass) is to be treated in order to obtain a textile fiber. It is known by analysis that this material contains 78% of cellulose fibers. With a yield of 75% or kg. S02 per ton are employed. The pH, is determined to be pH 3.3. With this material and with the determined amount of S02 a removal speed of 28 kg./hour at pH 3.3 the temperature is 120 C.

Cooking therefore is so performed as follows:

1000 kg. China Grass (oven-dry basis) are charged in a digester with a sufficient amount of water to cover it, namely 11,000 kg. water. This water contains 45 kg. S02; therefore the cooking liquor is an S02 solution of the strength of This liquor contains a suflicient amount of alkali to bring its pH to 3.3. The temperature is raised slowly to 120 and then maintained thereof for 3.2 hours.

While details are not always given in the preceding examples, it will be understood that conventional techniques are used for forming wood chips and other comminuted forms before introducing the cellulosic raw materials into the digester. It will also be understood that conventional techniques and apparatus are used to determine the extent of defibration of the pulp and there by determine when defibration is to be terminated.

In addition to the illustrative examples, this invention is particularly suitable for all annual plants typified by cotton stalks having non-homogeneous stems composed of lignified fibers (inner fibers of shive) and also composed of nearly delignified fibers (bast fibers) such as the straw of seed flax, the whole stem of hemp, bamboo, bagasse, kenaf, elephantine grass, ramie, and the like.

Sutermeister in his work, Chemistry of Pulp and Paper Making, 2nd edition, page 95, in discussing the delignification of such materials says: No way has yet been discovered of cooking the two together without overcooking the bast or undercooking the shive, either of which is fatal to the production of high grade stock." Such a negative conclusion was repeated after 10 years by the same author in a third edition of this work. Yet the process which is the subject of this invention has solved these problems with the most striking and as tonishing yield and a saving in chemicals. The practical importance of this invention will be apparent when considering the fact that very large quantities of hemp sternps, bamboo, bagasse, cotton stalks, wheat straw, and flax straw are available at a very low cost in many parts of the World.

This application is a continuation-in-part of my application, Serial Number 166,966, filed June 8, 1950.

While I have described certain preferred methods of practicing my invention, it will be understood that various modifications and changes can be made in the illustrative embodiments without departing from the scope or spirit of the invention which is intended to be defined in the appended claims.

What is claimed is:

l. A process for the production of a semi-chemical pulp from comminuted cellulosic raw material in the form of wood chips and small pieces of plants, comprising digesting said raw material in an acid bisulfite liquor having an initial pH of 2.2 to 5.0 and containing per ton of dry raw material an amount of 502 between about 60 to 120 kg. per ton of dry raw material which is insufficient to remove all of the lignin in the raw material, said digesting taking place at cooking temperatures not substantially exceeding 137 C. and until at least a major part of the weight but not all of the lignin is removed, said liquor containing about 3.5 to 11 times as much liquid weight as dry raw material, and mechanically defibrating the digested raw material to form said pulp.

2. A process for the production of a semi-chemical pulp from wood chips comprising digesting said wood chips in an acid bisulfite liquor having an initial pH of to 5.0 and containing per ton of dry chips an amount of S02 between about 60 to kg. per ton of dry chips which is insufiicient to remove all of the lignin in the chips, said digesting taking place at cooking temperatures not substantially exceeding 137 C. and until at least a major part of the weight but not all of the lignin is removed, said liquor containing about 3.5 to 11 times as much liquid by weight as dry wood, and mechanically defibrating the digested chips to form said pulp.

3. A process for the production of a semi-chemical lp from wood chips comprising digesting said wood chips in an acid bisulfite liquor having an initial pH of 2.7 to 3.00 and containing per ton of dry chips an amount of S02 between about 60 to 100 kg. per ton of dry chips which is insufiicient to remove all of the lignin in the chips, said digesting taking place at cooking temperatures not substantially exceeding 137 C. and until at least a major part of the weight but not all of the lignin is removed, said liquor containing about 3.5 to 11 times as much liquid by weight as dry wood, and mechanically defibrating the digested chips to form said pulp.

4. A process for the production of semi-chemical pulp from small pieces of yearly plants, comprising digesting said yearly plants in an acid bisulfite liquor having an initial pH of about 3 to 4 and containing per ton of dry yearly plants an amount of S02 between about 60 and kg. per ton of dry yearly plants which is insulficient to remove all of the lignin in the plants, said digesting taking place at a cooking temperature not substantially exceeding 137 C. and until at least a major part of the weight but not all of the lignin is removed, said liquor containing 9 about 3.5 to 11 times as much liquid by weight as dry OTHER REFERENCES yearly plants, and mechanically defibratmg the digested Importance of Bisulphite Ion Concentration in the yearly P1ants to form Sald P P- Sulphite Cooking Process, by Prof. Dr. Erik Hagglund References Cited in the file of this patent 5 grnzmsilgbe Paper Industry of July 1931, pages 511-515 UNITED STATES PATENTS Role of Base in Sulphite Pulping, by C. R. Mitchell 1,507,559 Alfthan Sept 9, 1924 and F. H. Yorston, from Pulp and Paper Magazine of 1,880,042 Richter Sept 27 1932 Canada for March 1936, pages 195-198 and 209. 1,931,575 D 1 R O L 24 1933 Yorston: Canada Dept. of Mines and Resources, e a m 10 D. F. s. Bull. 97, pp. 16, 17, 32, 3441, 44, 46, 5s, FOREIGN PATENTS 54 (1942) 279,411 Germany Jan. 11, 1914

Claims (1)

1. A PROCESS FOR THE PRODUCTION OF A SEMI-CHEMICAL PULP FROM COMMINUTED CELLULOSIC RAW MATERIAL IN THE FORM OF WOOD CHIPS AND SMALL PIECES OF PLANTS, COMPRISING DIGESTING SAID RAW MATERIAL IN AN ACID BISULFITE LIQUOR HAVING AN INITIAL PH OF 2.2 TO 5.0 AND CONTAINING PER TON OF DRY RAW MATERIAL AN AMOUNT OF SO2 BETWEEN ABOUT 60 TO 120KG. PER TON OF DRY RAW MATERIAL WHICH IS INSUFFICIENT TO REMOVE ALL OF THE LIGNIN IN THE RAW MATERIAL, SAID DIFGESTING TAKING PLACE AT COOKING TEMPERATURES NOT SUBSTANTIALLY EXCEEDING 137* C. AND UNTIL AT LEAST A MAJOR PART OF THE WEIGHT BUT NOT ALL OF THE LIGNIN IS REMOVED, SAID LIQUOR CONTAINING ABOUT 3.5 TO 11 TIMES AS MUCH LIQUID BY WEIGHT AS DRY RAW MATERIAL, AND MECHANICALLY DEFIBRATING THE DIGESTED RAW MATERIAL TO FORM SAID PULP.