US2851355A

US2851355A - Process for the preparation of semi-cellulose

Info

Publication number: US2851355A
Application number: US256593A
Authority: US
Inventors: Battenberg Ernst; Haas Heinz
Original assignee: Zellsloff Fabrik Waldhof AG
Current assignee: Zellsloff Fabrik Waldhof AG
Priority date: 1951-02-28
Filing date: 1951-11-15
Publication date: 1958-09-09
Anticipated expiration: 1975-09-09
Also published as: CH300957A; GB736300A; FR1062445A; NL90306C; BE504423A

Description

United States Patent 2,851,355 PROCESS FGR THE PREPARATION OF SEMLCELLULOSE Ernst Battenberg, Mannheim-Waldhof, and Heinz Haas,

Weinheirn, Bergstrasse, Germany, assignors to l felistoftfahrilr Waldhof, Mannheim-Waldlrof, Germany, a German joint-stock company No Drawing. Application November 15, 119 51 Serial No. 256,593 Claims priority, application Germany February 28, 1951 -3 Claims. (Cl. 92-41) The invention relates to a process for the preparation of semi-chemical cellulose pulp by the treatment of cellulose containing raw materials with an acidified bisulfite cooking solution at a pH of about 3.5-6, containing from l.43.0% of S at cooking temperatures between 115 and 140 C., to effect thereby a mild decomposition of the cellulose starting material, which only slightly attacks the cellulose and hemi-cellulose and which removes to a great extent the lignin binding the cellulose fibers in the middle lamellae of the starting cellulose material to produce cellulose fiber bundles which adhere so solidly that they require additional mechanical defibrillation to provide separate individual fibers in the product.

As starting materials cellulose containing plant prod-- sets may be employed which are obtained from any convenient source such as deciduous or foliaceous trees, coniferous trees, soft woods, hard woods, annual plants, and plant wastes such as bagassee, palm oil fibers and alfalfa fibers.

The hard WOOdS may be hardwoods such as: Aspen (Populus tremuloides and delt0z'des)Large-toothed aspen (Populus grandidemata)-Beech (Fagus grandiyolis Antropuncea and .s'ylvatl'ca) Red alder (Alnus rubrflfisycamore (Platanus occidenlnlis)'fupelo gum (Nyssa aquatica)- Black gum (Nyssa sylvatica) Red gum (Liquidamber styraciflua) Red maple (Acer rubrum) White maple (Acer saccharinum) Buckeye (Aesculus glabra)-Cucumber magnolia (Magnolia acumirmta)-Yellow poplar (Liriodendron tulipifera)- American elm (Ulmus americana)--Easswood (Tilia americana)- Paper birch (Betula papyrifera)-Yellow birch (Betula lutea) Chestnut (Costa/tea dentata).

Yearly plants such as: Esparto (Sn-pa terzaci.sima)-- Corn stalks (zea mays) -Sorgho (sorghym Lum)-Ampelodesma Tenax-Papyrus (Cyperus papyrus)-Straw (rice, barley, wheat)--Datlodil (Asphodelus ramousus) are useful.

Stem-plants such as: Hemp stem (Cannabis saziva), manila stem (Musa textilis), jute stem (Corchorus capsularz's) paper mulberry (Broussonetia papyrz'yera), Agave (Agave rigida), sisal, flax straw (Linum usitatissimum), cane (arlmdo donax and Plzragmites), Ghinda Calotropis procera), cotton stems (Gossypium), sugar cane bagasse, bamboo (Bambusa arwzdinacea) are useful.

Plant wastes of special plants as: palm oil (Elaeis guineensis) fibers, and other fibers having same physicochemical composition, Cecropia may be used.

Wood Waste such as saw mill waste, ply-wood waste,

matches waste may be used.

The semi-chemical pulp in the process of the application is obtained in yields of approximately 70-80% relative to the cellulose content of the starting material as contrasted with the yields of approximately 45% realized in the normal pulping process.

In addition to the advantage of obtaining higher yields f of cellulose by the process of the invention, the improved strength characteristics of the semi-chemical pulp product, the lower liguin content of the product and the very high Numerous semi-chemical processes have heretofore.

become known, among which particular attention has been paid to an alkaline process, preferably with caustic soda, a neutral sulfite process with Na SO in presence of sodium bicarbonate, developed more especially in the United States, and an acid process with bisulfite-cooking acids.

The alkaline semi-chemical decomposition processes consist in using the cooking liquors which are conventional for the sulfite or soda processes, in some cases with addition of accumulated waste liquors, which may be of lesser concentration, and conducting the cooking in such manner at temperatures of l60-170 C. and at a pH above 7 that the obtained semi-chemical fiber bundles are capable of mechanical defibrillation.

The alkaline decomposition process for the preparation of semi-chemical cellulose pulp is required to conduct the cooking of the lignin containing cellulose starting material to a point where the fibers are easily separated. Such cooking under the alkaline conditions present in the cooking liquor and at the temperature employed causes a far greater loss of hemi-cellulose in the semi-chemical pulp product and at the same time provides lower yields as increasing amounts of lignin are removed. This results in poorer strength characteristics in the product.

The neutral sulfite process is carried out with concentrations of 3-5%, all the sulfurous acid being bound to alkali so that generally an excess of alkali is present, this process being therefore carried out at pH- values above 7.0 and at temperatures of 160-170".

In the neutral sulfate process of alkali sulfate semichemical cellulose pulp decomposition the cellulose decomposition is greater, but there is a smaller dissolvingout of hemi-celluloses and extracted substances, and more lignin than obtained in the alkaline process.

in the alkaline sulfate process, the cellulose decomposition, as compared to the other processes, is lower, the fraction of dissolved-out hemi-celluloses and extracted substances is higher, and the lignin solution is smaller.

In the acid semi-chemical cellulose pulp process, e. g. With sodium bisulfite, which because of its 'high requirement for chemical reagents is less in vogue today, use is made of a decomposition liquid with 3.5-6% and more of S0 at temperatures of the conventional bisulfite process between -l40. 50% of the sodium-bound sulfurous acid is in the form of NaHSO so that there is a pH- value of about 2.0. A comparison of the constituents in alpha-celluloses, hemi-celluloses, lignin and extracted substances, removed in these decomposition processes shows, for equal yields, the following:

In the acid process with bisulfite-cooking acids, the cellulose decomposition is somewhat less than in the neutral sulfite process, more hemi-celluloses are retained and the fraction of dissolved lignins is higher. In addi- Fat-exited Sept. 9, 1958,

plants, Aronowsky et al., Agricultural Residue Pulps Comparison of Pulping Processes Paper Trade Journal, vol. 126, N. 26 (1949) concluded that the acid pulping processes produced relatively weak and brittle pulps from straw. It was apparent that these processes were unsuitable for producing strong and satisfactory pulps from agricultural residues of high silica content. These workers using conventional bisulfite liquor S0 at 140 obtained a yield of 40% raw pulp and which corresponded to a consumption of 200 kg. of sulphur per ton of airdried pulp.

The neutral sulfite process, which is more expensive than the process of the invention because of its greater consumption of chemicals, has nevertheless been widely adopted in the United States because it was not heretofore possible according to known acid bisulfite processes to obtain semi-chemical cellulose pulp with sufficient strength properties, as compared to those from the neutral sulfite process. This neutral sulfite process represents a compromise between the high yield of the acid bisulfite process and the good strength properties of the alkaline process.

"The following'Table I sets forth yield data with the usual sulfite cellulose processes obtained from deciduous wood, and in which according to Professor Chidester, the deciduous starting wood contains approximately 50% cellulose, approximately 25% hemi-cellulose, approximately 21% lignin and approximately 4% residue in the form of extract substances.

uring Methods: Alpha-cellulose T201 m-4-6, hemi-cellulose T223 m-48. Lignin was determined according to D. M. Halse (Paper Journal 10, 121, 1926).

Table 1 indicates that the acid semi-chemical pulps have the lowest lignin contents and at the same time the highest semi-cellulose contents.

Table 11, below, summarizes the operating conditions of the alkali semi-chemical pulp process, the neutral sulfite process, the acid sulfite process of the prior art and of the process of the invention.

The values indicated in Table 11 below encompass deciduous woods and coniferous woods of the most valuable type, the individual data corresponding to the particular wood being used. The yield in the normal bisulfite process usually amounts to about -50% and for all semi-chemical cellulose pulp decomposition processes to about 70-85%. These yields are not determined by the process itself, but are dependent upon particular selected conditions.

TABLE II Process of manufacturing paper-pulps Propor- Tempera- H1=Irnprcgnati0n Process Amount of SO; in Liquor, tion, wood: pH by cooking, beginning ature by time in hours: percent liquor cooking, Hg=cooking time C. in hours Conventional bisul- 46% 1:4.5 1.2-2.0 over 125 H1=2-5 or 0-8;

'phltc-ptoccss. H2=58 or 16. Acid process of semi- 3.5-6% 2.0-3.0 120-140 Hz=ca. 6-7.

chemical pulps. Neutral sulphite 35% 114-5 6.0-8.0 140-170 H =12;l.lz=4l0.

process of semichernical pulps. Application process 1.43.0%(withleafwood1.4 1:4-4.5 3.5-6.0 (with Calcium pH= 115-140 H =3-7; IIz=3-5.

for semi-chemical 1.8% coniferous trees to 4.5, Temp. 125-130"; with pulps. 2.8% straw ca. 1.4%). 11%?) pH=5-6, Temp. ca.

If deciduous wood, e. g. aspen wood, is decomposed In the following Table III the manufacturing costs, callnto semi-chemical cellulose pulp according to the known culated from the costs of the chemicals and the total proprocess under alkaline, neutral and acld conditions, and 50 duction costs such as steam, water, labor, including colthen according to the process of the application, in each case to a 75% wood yield, there obtain the aforementioned total ingredients in the amounts as set forth in Table I.

It is to be noted that, of these values, the cellulose and lateral costs of unbleached semi-chemical cellulose pulp prepared according to the known process are compared with the corresponding costs for semi-chemical cellulose pulp prepared according to the process of the present invention, and that for deciduous woods (beech). The values relate to one ton of absolutely dry (atro) semichemical cellulose pulp, at a 75% yield.

TABLE III For known semi-cellulose processes For the processes according to the (neutral sulfite processes) in the invention United States and other countries Material consumption, consumption mipower, labor, incidental Required Costs Required Costs costs etc. (Production costs) quantities quantities in kg. per in kg. per ton of semiton of semicellulose, DM (DM 4.20: cellulose, DM (DM 4.20:

Wood (Beech) 1, 335 142. 09 33. 83 1, 335 142.09 33. 83 Sul hur- 70 9. 15 2.18 43 5. 1. 35 S0 350 42. 00 10. 00 NaOFl' 25 2. 0. 64 Polyphosphate 0. 3 0. 0. 21 Lime- 68 1. l0 0. 26 Productmn Costs 114. 55 27. 27 102. 53 24. 41

it can be seen from Table III that the costs of the new process, relative to the known process, are approximately 17.5% less than the known processes. This represents a very substantial decrease in cost of production.

An object of the invention is the decomposition of cellulose containing starting materials with a bisulfite cooking liquor containing from about 1.4-3% of S at a pH of from 3.5-6 and a reaction temperature of about 115- 140 C. to obtain thereby a removal of the lignin from the cellulose fibers in the middle lamellae of the starting material and to provide a product which can be readily mechanically defibn'llated.

A further object of the invention is the preparation of semi-chemical cellulose pulp by the treatment of cellulose containing raw material with a bisulfite solution containing 1.4-3% S0 in the bisulfite liquor, at a pH of 3.5-6, and a reaction temperature of about 1l5-140 C., Where- 111 the cooking liquor is buttered by polyphosphates, metaphosphates, hexainetaphosphates, imidoacetic acid, nitrilotriacetic acid, etc. together with a base such as sodium hydroxide, sodium carbonate, potassium hydroxide, calcium hydroxide, magnesium oxide, aluminum hydroxide, ammonia and alkaline organic amines. Polyphosphates of general formula (Na,, )P,,(O are used, especially those wherein n=3 or 4. Also suitable are metaphosphates of the general formula (KPOQ hexametaphosphates of the foregoing formula with x=6, imino-diacetic acid of the formula HN(CH CO H) nitrilotriacetic acid of the formula N(CH CO H) ethylene diamine tetraacetic acid of the formula C H N (CH CO H) lower aliphatic amines, etc.

Other and further objects of the present invention will appear from the more detailed description set forth below, it being understood that such more detailed description is given by way of illustration and explanation only and not by way of limitation.

According to the invention, the semi-chemical cellulose pulp decomposition takes place in aqueous solution of S0 and of a base, such as bases derived from calcium, magnesium, sodium, ammonia, aluminum, etc., under reaction conditions which may broadly be considered as between those of the known acid bisulfite process and the neutral sulfite process. This requires that the pH-value of the cooking acid, as compared with the pH-values of the acid process, be higher and that the SO -content as well as the reaction temperatures be lower, relative to the neutral sulfite process. The new reaction conditions are explained in greater detail in the following.

The increase in pH-value of an alkali-, e. g. calcium bisulfite-cooking acid, means that the quantity of free S0 is greatly reduced and the possibility of a hydrolytic decomposition of the cellulose decreased. The reaction conditions prevailing in the neutral sulfite process are thus more nearly approached. With the individual cations, this is possible only to a certain extent, since a certain amount of free S0 has to be present if the bisulfite is to remain in solution. According to the invention, the increase in pH-value can take place after buffering e. g. with NaOH, KOI-I, NH amines, etc.

In order not to exceed the solubility of the sulfite, e. g. the solubility of the CaSO with calcium as the base, the butfering agent has to be added in the presence of a com plex builder. Complex builders are e. g. polyphosphates, such 'a Calgon (a water-softening agent consisting essentially of sodium hexametaphosphate), tripolyphosphates, sodium polyphosphate, sodium pyrophosphate, acid sodium pyrophosphate, etc., nitrilotriacetic acid or imidodiacetic acid.

It is necessary to raise the pH-value to 3.5-6. When using bisulfites of other cations, e. g. of magnesium, it is possible by the addition of NaOH only to prepare a substantially neutral cooking liquid. It has been found that the acidity during the cooking exerts an important influence on the strength properties and gives best values with the above-indicated starting valve.

Since, in the semi-chemical cellulose pulp decomposition of the invention, only a partial removal of the lignin, specifically, that of the middle lamellae, is achieved, the process employs a smaller quantity of S0 relative to the normal bisulfite decomposition. About 45% of the lignin present is sulfonated and removed as lignin sulfonic acid or its salts in the process of the invention.

In order to realize a suflicient sulfonation and hydrolytic splitting of the sulfonic acid, when the buffered acid has a pH=3.5-6, use is made according to the invention of a cooking acid with a 1.4-3% total SO content, depending upon the starting material. With foliaceous trees, especially beech trees, which strongly reduce the pH-value during the cooking by the splitting oif of organic acids, the use of a cooking acid with 1.4- 1.8% S0 is favorable, while with coniferous trees, which contain more lignin, higher sO -concentrations up to 2.8% and higher are necessary. With annual plants, e. g. straw, sO -concentrations of about 1.4% suffice.

In the acid decomposition process of the invention the reaction temperature exerts an essential influence on the hydrolysis of the hemi-celluloses and of the cellulose present. Although, in an alkaline decomposition process as well as in a neutral sulfite semi-chemical cellulose pulp process, an increase in temperature appreciably infiuences the reaction speed, in the acid process a temperature increase is prejudicial to the fiber strength properties because of an increase in the hydrolysis. Temperatures can be adopted for various degrees of acidity, below which temperatures, no appreciable impairment of fiber strength takes place. For a process carried out according to the invention, e. g. with calcium as the base, at a phi-4.5, the temperatures may amount to about 125- l30; for a magnesium bisulfite solution with a pH=5-6, temperatures of about 140 are indicated.

The ratio of wood to liquor is, according to the invention, about 1 :4-5. The time of impregnation, i. e. of the first part of the cooking, amounts to about 3-7 hours, at temperatures which, compared to the end temperatures of the cooking proper, are about 20-30 C. lower than the end temperatures. The cooking proper lasts 3-5 hours, the temperature being preferably raised in one stage.

According to the new semi-chemical cellulose pulp decomposition process With a cooking acid at a pH=4-6 and a total SO content of 1.4-3%, fibrous materials are obtained in a yield of 70-85% (with certainty) after mechanical treatment without bleaching. The amount of cellulose and hemi-celluloses obtained fluctuates only slightly, so that the yield depends essentially upon the amount of removed lignin.

Notwithstanding the high lignin content of 10-15% the product is Well bleachable by the usual bleaching processes employing a total quantity of about 10-14% chlorine, the achieved degree of whiteness being between and units on the GE scale. The bleachability is essentially better than that obtained according to the sulfite process, for which the bleaching degree is at most 85% on GE scale.

The yield in cellulose product rich in bleached semichemical cellulose pulp is 60-65%, relative to the wood, which yield also is greater than the yield of cellulose according to the neutral sulfite process.

The strength properties and bleachability of the semichemical cellulose pulp are considerably improved compared With the corresponding cellulose of the usual processes. Semi-chemical cellulose pulps which are cooked to obtain yields of 75-80%, have the best strength values, while with higher yields, i. e., with increasing lignin content, an increasing brittleness and a decrease in strength properties, especially in folding number, becomes perceptible. It has been found that the strength properties of the semi-chemical cellulose pulp prepared according to the process of the present invention are equal to those obtained from the material decomposed according to the neutral-sulfite process; a particular advantage relative to this process of the invention is the lower consumption of chemicals and thus the lower manufacturing costs. A further improvement of the properties of the material is realized by bleaching, the strength properties being enhanced due to the removal of the lignin.

Compared with the conventional acid bisulfite decomposition using liquors containing the usual amount of free S at a highly acid pH (ca. 2-3), semi-chemical cellulose pulp with even better strength properties is obtained when the acid is buffered to a higher initial pH=about 4-6 and a temperature of about 125-130" is employed. In carrying out the new process according to the invention, any suitable acid, e. g. bisulfite liquor, can be used in making up the cooking acid. The acid should be diluted and gassed-up to such extent that by addition of the calculated amount of dilute buffer solu tion, e. g. caustic soda solution, the particular SO -concentration desired is obtained. The requisite caustic soda or the like for raising the pH value can not be calculated stoichiometrically but must be determined in each instance. The character of the complex builder is of essential infiuence on the amount of buffer solution which has to be added. It appears that the addition of a certain amount of caustic soda solution for buffering reaches a maximum pH value in the presence of the complex builder and that, upon further additions, a drop in pH takes place which is accompanied by a strong clouding of the solution.

After adjustment of the desired pH value, the cooking liquid is heated up to the temperature of 105-120", depending upon the starting material, necessary for the sulfonation. The per se known impregnation, and formation of the lignin sulfonic acid, takes place at these temperatures within about 3-7 hours, depending upon the starting material. An increase in temperature of 10-20", above the foregoing temperature causes a hydrolytic splitting of the lignin sulfonic acid of the lignin sulfonic acid-cellulose compounds to take place in 3-5 hours, so

that a loosening up of the fiber bundles takes place. The SO -content of the cooking acid drops during the impregnation and cooking.

By degasification of the cooking lignin upon termination of the cooking, a part of the unconsumed 80; can

be recovered from the spent liquor.

The semi-chemical cellulose pulp shreds obtained by the decomposition can be defibrillated into individual fibers in a known manner with the usual defibrillating devices, such as disk mills or the like, to obtain the resultant fibers which are of light color, somewhat like that of unbleached conventional sulfite cellulose. The waste liquors can be recovered and used for the production of yeast.

With the same process of the invention, the strength properties depend on the pH as shown by the following two examples for which two beech semi-chemical pulps were employed and having the same cellulose yield:

tion, use is made of a cooking acid with a pH of 4.3, having a total SO -content of 1.57%, 46% of the S0 being bound to CaO, and 27% being buffered by NH The pH value of 4.3 is realized by the addition of 50 mg./ liter of Calgon, Na P O a water softening agent containing sodium hexametaphosphate as the main constituent.

The cooking temperature is raised to C. in 1 hour, maintained at this temperature for 6 hours, heated 1 hour at 1l5 C. and 3 additional hours at C. After raising the temperature to 125 for 4 hours, as is necessary for the decomposition of the lignosulfonic acid, the cooking is ended.

The pH-value drops during the cooking to about 2.9-

15 3.0. Compared to an unbuffered cooking acid, this represents a terminal pH value which is higher by one pH unit. This difference of one unit shows up greatly at elevated temperature so that, due to weaker hydrolytic treatment' much better yields and strength properties are obtained.

The obtained semi-chemical cellulose pulp shreds are defibrillated in known manner in a disk mill.

The yields amount to 82% relative to the starting wood.

Analytical date of the unbleached material follows:

After a grinding in the Jokro mill, standard strength values are obtained:

Degree of Grlnding-Degrees Breaking Folding Breaking. Schopper-Riegler Length, Number Pressure,

The measurements in Example 1 and all of the remaining examples were obtained as follows:

The degree of whiteness in percent GE was measured with an American General Electric Brightness Tester, which is described in TAPPI T217 m-48.

The degree of grinding fineness was determined with a Schopper-Riegler (SR) degree of fineness tester.

Tearing length, fold number and breaking pressure were determined by the method of Verein der Zellstoffund Papier-Chemiker und Ingenieure, a technical publication for cellulose strength testing, pages 101-103, Otto Elsner Verlag, Berlin.

The degree of decomposition and the copper viscosity were carried out according to the book by Dr. Rud. Sieber, Die Chemisch-Technischen Untersuchungs-Methoden TABLE IV Yield 70% A 68% 79% 81% 70% 68% 79% 81% Starting pH 4. 3 5. O 4. 4 5. 7 4. 3 6. 0 4. 4 5. 7

Grinding Degree- Degrees Sehopperbreaking length in m. rubbing test Riegler EXAMPLE 1 50 kg. of birch wood chips with a moisture content of 11% are charged into a rotating cooker with 240 liters of decomposing liquor. In accordance with the invender Zellstoitund Papierindustrie, Springer-Verlag, Berlin, 1943, which is conventionally used in Germany, and the degree of decomposition being carried out according 75 to the Sieber book, page 340, determination of the 9 chlorine consumption number according to Sieber (Sieber number);

Copper viscosity was determined according to the Kiing-Modified Method of the Technical Association of the Pulp and Paper Industry, Sieber book, page 514 et seq.;

Alpha cellulose was determined according to TAPPI In a jacketed cellulose digester, a cooking acid which contains 1.6% total SO and 0.6% CaO is added to beechwood chips. The pH value is adjusted to 4.6 by buifering with 0.5 NaOH in the presence of 50 mg./liter of nitrilotriacetic acid N(CH CO H) After impregnation for 6 hours at 105", final cooking is carried out in 4 hours at 128 C. The SO -content of the cooking liquid is about 0.5% at the end of the cooking.

The Waste liquor contains 11% reducing substance.

The semi-chemical cellulose pulp, obtained in a yield of 76%, determined according to standard methods, has the following strength properties:

The strength properties of the product, which is prepared with a cooking acid of the same sO -concentration but without buffering according to the invention, are essentially lower. The maximum tear length at 85 S. R. is only 6.5 km., the fold numbers run about 350 and the breaking pressure is 3.5 kg./cm.

By stage bleaching, with an amount of chlorine of 14%, a bleached cellulose with a degree of whiteness of 86% G. E. is obtained in a yield of 64% calculated relative to the wood. The strength properties are further improved by the removal of lignin and have the following values:

Pine wood chips are covered in a cooker with a cooking liquid which contains 2.2% total S 0.56% CaO and 0.5% NaOH. The pH value is adjusted to 4.2 by the addition of caustic soda solution in the presence of a tripolyphosphate of the formula, H P O In the usual way, the initial temperature is maintained constant for 4 hours below 105. After a 3-hour intermediate temperature of 115, decomposition of the lignin sulfonic acid is achieved by a 6-hour cooking at 135. The SO -content of the cooking acid drops, during the cooking, to 0.4%. Upon completion of the cooking, the pH value is 3.4. A 77% yield is obtained of a light-colored pine semi-chemical cellulose pulp, which is easily defibered, and which has a degree of decomposition (according to Sieber) of 97 and a Cu-viscosity of 250 cp.

The strength properties determined according to standard methods are the following:

The achieved strengths are very good; they are not equaled even by the usual good kraft materials.

The bleachability of the semi-chemical cellulose pulp is good. A yield of 59% relative to the wood is obtained. There are a degree of whiteness of 86%, a maximum tear length of 9000 m. and a folding number of 6000 in the product.

EXAMPLE 4 A ten-fold quantity of cooking acid, which contains 1.3% S0 0.64% CaO, 0.3% NaOH and 0.005% of a sodium polyphosphate of the formula, Na P O is added to an unsorted mixture of comminuted Wheat straw in a cooker provided with a circulating pump. After a precooking period of 3 hours and a cooking period of 7 hours at 123, a readily defibered product is obtained in a yield of- 81%. Of the added S0 12% can be recovered by degasification following the cooking.

EXAMPLE 5 In a jacketed cellulose cooker provided with a recycler, beechwood chips are treated with a magnesium bisulfite acid. The amount of S0 used is 88 kg. per ton of absolutely dry Wood. The pH value of the cooking acid, employed in the proportion of 5 cubic meters per ton of Wood, was raised to a pH of 6.2 by the addition of dilute caustic soda solution in the presence of 50 mg. per liter of sodium h'exametaphosphate of the formula (NaPO After an impregnation stage at C., the cooking takes place for a period of 4 hours at 152 C. maximum temperature. There is produced a semi-chemical cellulose pulp in a yield of 72%, which semi-chemical cellulose pulp can be defibered in normal manner in a disk mill.

The thus obtained cellulose is distinguished by an especially high pliability for beechwood as is shown by the very high folding number of 2000 at a degree of grinding of 83 Schopper-Riegler. The tear length, at this degree of grinding, is 8000 meters. The semi-chemical cellulose pulp can be bleached in a four-stage bleach with a total of 12.5% chlorine, relative to the material, to a whiteness degree of 83% GE.

EXAMPLE 6 EXAMPLE 7 Beechwood chips are treated in a cooker with a sodium bisulfite cooking acid having an SO -content of 1.6%. The quantity of cooking acid is adjusted that 4-5 cubic meters of acid per ton of wood is used. The NaOH- bound amount of S0 is of such magnitude that a pH value of 5.8 prevails. After a precooking and impregnating period of 8 hours and a following cooking period of 8 hours at a 73% yield of easily defibrillated prodnot is obtained.

a rotary cooker with a ammonium bisulfite cooking total S0 0.88% S0 being The pH value of this solution is EXAMPLE 8 Pine Wood chips are covered in a cooker with a cooking liquid which contains 2.2% total SO 0.5 C210 and 0.5% NaOH. The pH value is adjusted to 4.2 by the addition of caustic soda solution in the presence of 0.4% methylamine in solution. In the usual way, the initial temperature is maintained constant for 4 hours below 105. After a 3-hour intermediate temperature of -11 115 decomposition of the lignin sulfonic acid is achieved by a 6-hour cooking at 135. The SO -contcnt of the cooking acid drops, during the cooking, to 0.4%. Upon completion of the cooking, the pH value is 3.4. A 77% yield is obtained of a light-colored pine semi-chemical cellulose pulp, which is easily defibered, and which has a degree of decomposition (according to Sieber) of 97 and a Cu-viscosity of 250 op.

The bleachability of the semi-chemical cellulose pulp is good.

In each of the examples above, any of the polyphosphates mentioned may be used interchangeably in appropriate equivalent quantities with the pyrophosphates, imidoacetic acid and nitrilotriacetic acid without any change in the process conducted and product obtained.

Likewise the imidoacetic acid, nitrilotriacetic acid and pyrophosphates may be used interchangeably with each other and with the polyphosphates, without change in L the process conducted and product obtained.

Having thus disclosed the invention, what is claimed is:

1. Process for the preparation of semi-chemical cellulose which comprises treating cellulose-containing raw materials obtained from foliaceous trees, coniferous trees and annual plants with buffered bisulfite cooking solutions consisting of aqueous sulfur dioxide containing about 1.4-3.0% sulfur dioxide and a base selected from the group consisting of the hydroxides of calcium, magnesium, sodium, and ammonium, in an amount, depending upon the base and cellulose raw material employed, to provide an initial pH value of 3.5-6, cooking with said solution in the presence of a buffer selected from the group consisting of alkali metal hexametaphosphates, tripolyphosphates, polyphosphates and pyrophosphates at a 12 reaction temperature of 115-140" C., to repress the excessive hydrolytic decomposition of the cellulose and to remove a substantial amount of the lignin'binding the cellulose fibers in the middle lamallae, and thereafter mechanically subdividing the product into individual fibers.

2. A process as in claim 1 wherein the base is calcium hydroxide and the initial pH is 4.5 at a reaction temperature of l25-130 C.

3. A process as in claim 1 wherein the base is magnesium hydroxide and the initial pH is 5-6.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Richter, Chemical Abstracts 35, 3439.

Benjamin, Corn. of Australia, Council of Sci. and Ind. Res. Bull. 37, page 76 (1928), Cook 156.

Manufacture of Pulp and Paper, 3d ed., vol. III, Sec. 4, pp. 1, 2, 59 (1937), published by McGraw-Hill, N. Y.

Yorston, Canada Dept. of Mines and Resources DFS- d Bull. 97, pp. 21, 33, 34, 53 and 80 (1942).

Claims

1. PROCESS FOR THE PREPARATION OF SEMI-CHEMICAL CELLULOSE WHICH COMPRISES TREATING CELLULOSE-CONTAINING RAW MATERIALS OBTAINED FROM FOLIACEOUS TREES, CONIFEORUS TREE AND ANNUAL PLANTS WITH BUFFERED BISULFITE COOKING SOLUTIONS CONSISTING OF AQUEOUS SULFUR DIOXIDE CONTAINING ABOUT 1.4-3% SULFUR DIOXIDE AND A BASE SELECTED FROM THE GROUP CONSISTING OF THE HYDROXIES OF CALCIUM, MAGNESIUM, SODIUM, AND AMMONIUM,IN AN AMOUNT, DEPENDING UPON THE BASE AND CELLULOSE RAW MATERIAL EMPLOYED, TO PROVIDE AN INITIAL PH VALUE OF 3.5-6, COOKING WITH SAID SOLUTION IN THE PRESENCE OF A BUFFER SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HAXAMETAPHOSPHATES, TRIPOLYPHOSPHATES, POLYPHOSPHATES AND PYROPHOSPHATES AT A REACTION TEMPERATURE OF 115-140*C., TO REPRESS THE EXCESSIVE HYDROLYTIC DECOMPOSITION OF THE CELLULOSE AND TO REMOVE A SUBSTANTIAL AMOUNT OF THE LIGNIN BINDING THE CELLULOSE FIBERS IN THE MIDDLE LAMALLAE, AND THEREAFTER MECHANICALLY SUBDIVIDING THE PRODUCT INTO INDIVIDUAL FIBERS.