INDUSTRIAL AND ENGIiVEERIhlG CHEMISTRY
March, 1929
10 per cent of the pulp during the bleaching process or about 3 to 3.5 per cent of the original weight of stalk, giving an overall yield of good paper-grade pulp amounting to 30 to 32 per cent of the original weight of stalks with an alpha-cellulose content of 70 to 75 per cent. T a b l e V-Cellulose CHSYICALS
USED
275
The sodium hydroxide, soda, Kraft, and halogen processes make pulps suitable for magazines, books, and writing paper. The lime-soda and sulfite processes give a weaker paper, suitable perhaps for toilet paper and similar grades, while the lime pulp seems suitable for box board and other articles, where an unbleached pulp can be used.
Yields w i t h Various Cooks
$~~~~~ Tm8 PRG&ASRE
YIELD OF
ALPHACELLULOSE
PULP
I N PULP
Per cent Hours Lbs./sq. i n . Per cent Per cent Sodium hydroxide 10 to I 5 2 50 t o 7 5 35 70 t o 7 5 Sodium Kraft hydroxide 9 5 50 43 t o 4 8 60 Sodium sulfide 1 Sodium carbonate 15 50 33 70 5 Magnesium chloride 4 to 5 35 Unsatisfactory pulps Sodium hydroxide Sodium sulfate 25 50 Unsatisfactory pulps 5 Sodium hydroxide 10 1 50 5 38 65 Lime 51 Sodium silicate Up t o 70 50 4 No pulping effect Sodium carbonate 10 I Sodium hydroxide sulfite 50 40 65 Sodium (sulfiteJ 1 Sodium silicate 34 Poor results Sodium hydroxide 312' 30 Sodium carbonate ~ : ~ ] 3 t o 1 6 25 Poor results Sodium sulfide Lime 30 40 65 t o 7 0 6:7d?5 Sulfur Lime 20 4 50 80 Pulp hard t o bleach D e Vain's procedure as spedfied in patents 33 65 t o 7 0 Pomilio'sl(gaseous chlorine) procedure as speafied in patents 30 60 t o 6 5 Bromine water (saturated) Z/S None 28to30 60
i
'8 1
5"t '21
1
Conclusion The experimental work done in this laboratory and elsewhere indicates that cornstalks are more easily reduced to cellulose than are woods. On the other hand, cornstalks are more bulky, give a lower yield of cellulose, and the pulp requires slightly more bleach. At the present time the production of wall board from stalks offers the most attractive outlet, and a factory for the manufacture of wall board from stalks will soon start operations in Dubuque, Iowa. Another promising field is the manufacture of cellulose for rayon and paper pulp, and the progress of the factory a t Tilton, Ill., is being observed with great interest. I n the near future it is believed that, owing to the increasing cost of wood pulp for paper-making, this industry will offer a large outlet for cornstalk cellulose. Still another field is the manufacture of nitrocellulose from cornstalk pulp. It has been determined that both bleached and unbleached pulps nitrate readily and satisfactorily with a high degree of stability. Important statistics on the economics of harvesting will be available within a year or two.
Cellulose from Cereal Straws' Sidney D. Wells PAP8R MILL LABORATORIES, INC., QUINCY,ILL.
Cereal straws constitute a n enormous source for cellulose which is readily gathered and capable of purification by very mild processes. Caustic soda in dilute solution will remove more than half the incrusting substances a t room temperature and more than 70 per cent a t 100" C. Higher temperatures are even more effective. Sodium carbonate has considerable action a t 100' C. and a t higher temperatures is a very effective agent, especially in the presence of sulfur compounds of a reducing nature. Milk of lime is the most commonly used agent in technical operations, but it is useful only where the purity of the pulp is of minor importance or where it is to be further purified with chlorine. Chlorine is a n effective agent used alternatively with 1 per cent caustic soda, and yields of 55 per cent of white fiber may be obtained. Yields
of 66 per cent may be obtained by cooking with sodium carbonate and sulfur and bleaching with calcium hypochlorite in several steps. Yields of 43 per cent of pentosan, lignin, and ash-free cellulose, comparable with alpha cellulose from cotton or wood pulp, may be obtained by alternate chlorine and caustic soda treatments. Xylan may be extracted from straw with caustic soda to the extent of almost 15 per cent. Another 9 per cent of furfural-yielding components remains which can be largely removed after chlorination by further extraction with caustic soda. The spent cooking liquors from digestion with sodium carbonate and sulfur are capable of hydrolysis and fermentation with the production of lactic acid, acetic acid, and other valuable products in promising quantities.
..............
T
HE straw from the cultivation of cereals is one of the largest potential sources for cellulose. Under this classification is usually included the straw of wheat (triticum), rye (secali), oats (avena), and barley (hordeurn). Rice straw is not usually included in general discussions of the subject, although enormous quantities are produced and it lends itself readily to the production of cellulose. The results of studies of the cellulose of the cereal straws have usually been reported on the whole material. Straw, however, is a mixed product, being composed of cells of widely different characteristics and functions. There are Presented before the Division of Cellulose Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 to 14. 1928.
variations in the cell structure not only of separate portions of the stem but also of the leaf sheaf and the various elements of the head, such as the chaff, consisting largely of the glumes and palets, dwarf cells, etc. The stem is made of widely different structures, of which the internode with a welldeveloped fistula is most characteristic. There are also the nodes and false nodes or motile organs and the rachis from the head, which give considerable trouble to the paper maker as well as to the student endeavoring to obtain typical samples. A cross section of the stem shows on the outside the epidermis, made up of thin and broad cells with serrated edges which dovetail into each other and form a compact shell. Below these we have the chlorenchyma spaces, which give color to the plant and take an active part in the con-
INDUSTRIAL AND ENGINEERING CHEMISTRY
276
version of sunshine into cellulose. We next have the bast fibers, which give the material its value to the paper maker. Then come the fibro-vascular bundles of wood cells, in which are the large wood vessels or xylem instrumental in the transfer of water and food up and down the stem. Surrounding these bundles are the parenchyma or pith cells. I n the internode a large proportion of the pith cells have disappeared in the development of the fistula. Composition of Straws
The proportion of the different elements varies somewhat in the straws of the different cereals, but from the standpoint of the paper maker the variation is not sufficient to preclude the use of any of them in his art. The great bulk of the straw used, however, comes from wheat, and on account of the outstanding importance of this crop, both agriculturally and technically, all the work reported herein has been conducted on wheat straw. Silica occurs to the greatest extent in the cuticle and more or less in the other cells beneath. The other constituents probably vary in amount in the different types of cells occurring in the plant and in the different portions of the plant. Studies like those of Ritter2 on wood are greatly needed and would undoubtedly add greatly to our knowledge of this great agricultural product largely wasted in most of the grain-growing areas of the world. Chemical investigators usually report the constituents of cereal straw as ash, fats, protein, pentosans, cellulose, and lignin. The amount of ash varies from 3.8 to 9 per cent of which from one-half to three-fourths is silica. This wide variation is probably due to differences in location and soils. From 2 to 2.5 per cent of fats and from 2.5 to 3.5 per cent of protein are usually present. The main constituents are the pentosans, 25 to 30 per cent; cellulose, 38 to 45 per cent, and lignin, 24 to 28 per cent. The considerable range reported for each constituent is probably due partly to the approximate nature of the methods available and slight variations in the steps taken and partly to small differences in composition. Action of Water and Various Pulping Agents on Straw
I n studying the chemical characteristics of straw, the action of water and various pulping agents is of technical importance because it is by such studies that advance in technic is possible and progress made in the production of new products. WATER-Ten per cent of the straw is soluble in boiling hot water. The substance so separated consists largely of pectose, carbohydrates, and albuminous substances evident from the ease with which the extract purifies. The amount soluble is not enough to justify a preliminary boil before treatment with pulping agents either from the standpoint of the value of the matter dissolved or for the sake of economy in cooking chemical. CAUSTICSODASoLuTIoNs-Considerable matter in straw is soluble in cold caustic soda solution. Magnus3 reports that a solution containing 8 per cent of the weight of the straw in caustic soda was capable of dissolving 12 per cent in 15 minutes, 15 per cent in 2 hours, and 20 per cent in 4 hours. I n a solution containing 12 per cent of the straw in caustic soda a loss of 27 per cent is reported after 3 days. This loss consisted of the following components based on the weight of the straw: Ritter, IND.END.CHBM.,17,1194 (1925); 20, 941 (1928). Magnus, "Theoris und Praxis der Strohaufschliessung," p. 8, Paul Parey, Berlin, 1919.
Ash Protein Fats
Per cent 1.1 0.8 1.1
Vol. 21, No. 3 Pentosan Cellulose Lignin
Per cenl 3.7 1.6 13.8
By raising the temperature to between 50" and 55" C. a solution containing 8 per cent of the weight of the straw in caustic soda was capable of dissolving 28 per cent of the straw and 29.5 per cent if stirred ten times during a period of 4 hours. The lignin dissolved was 12.4 per cent and ash 2.2 per cent. By boiling 5l/* hours in the open, 42 per cent of the straw was dissolved. The constituents dissolved as follows: Ash Protein Pentosan
Per cent 2.0 2.9 8.4
CeI1u1ose Lignin
Per cent 8.5 16.9
By cooking 4 hours a t 154" C., 47 per cent was dissolved consisting of the following constituents: Ash
Protein Pentosan
Per cent 1.6 2.2 9.7
CeI1u1ose Lignin
Per cent 7.6 18.7
At temperatures of 100" C. or above we find the solution of approximately two units of lignin for each unit of the pentosans. The same is true a t room temperature for periods of 8 hours or less. When extended to 3 days, however, lipain continued to show loss without any increase in the loss of pentosan. SODIUMCARBONATE SOLUTIONS-Asolution containing 8 per cent of sodium carbonate based on the weight of the straw was reported by Magnus4 as being capable of dissolving 26 per cent of the straw when boiled in the open. The components removed were as follows: Ash
Protein Pentosan
Per cent 2.7 1.4 4.3
Cellulose Lignin
Per cent 0.5 10.0
Longer treatment or more chemical does not materially alter the results. The writer found that a solution containing 7.5 per cent of the weight of the straw heated to 145' C. for 6 hours dissolved 45.4 per cent. The composition of the material removed was not determined. Considerable hydrolysis must have taken place to account for the large amount of material removed. Caustic soda was but little more drastic. A solution of 7.5 per cent sodium carbonate and 1.5 per cent sodium sulfite5 when boiled a t 129" C. for 10 hours dissolved 33 per cent of the straw. Of this were the following constituents: Ash
Pentosan
Per cenl 2.9 10.0
Cellulose Lignin
Per cent 3.9 11.6
The loss of cellulose by this method is half of what it is with caustic soda when used to accomplish equal results in solution of incrusting matter and separation of fibers. The high removal of ash is due to the fact that the straw was wheat from the Mississippi valley with an ash content of 8.8 per cent, while the other values were obtained on German straw with ash contents of 3.8 and 4.9 per cent. SODIUM CARBONATE SOLUTION IN PRESENCE OF SULFUF~-A solution containing 7.5 per cent of the weight of the straw as soda ash and 1 per cent elemental sulfur dissolved 26.6 per cent after boiling 6 hours a t 145" C. The sulfur gradually went into solution, so that none was left at the end of the cook and the gases given off from the autoclave on opening
2
I
4 6
Magnus, 09. cit., p. 27. Rue and Monsson, Paper Trade J . , 81,No. 15, 52, No. 20, 49 (1925).
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1929
were similar to those obtained when sodium sulfide is one of the cooking chemicals. Sulfur digested in a solution of sodium carbonate a t above 170” C. goes into solution to the extent of 1.5 parts per 100 parts sodium carbonate. Hydrogen sulfide is given off on testing with sulfuric acid. Equilibrium isthus established at extremely low concentrations of sulfide. I n the presence of straw, however, the sulfides act on the straw as soon as formed and are removed from solution, thereby disturbing the equilibrium and causing more sulfur to be dissolved. There is an admirable buffer action and the results are much better than where soda ash alone is used. The sulfides prevent formation of oxycellulose and the solution of only 26.6 per cent of straw as against 45.4 per ceflt with the same amount of sodium carbonate under similar conditions. With the higher yield, however, the pulp was separated into its ultimate cells quite as readily. MILK OF LIME-A liquor containing milk of lime equivalent to 9 per cent of the straw dissolved 27 per cent in 5 hours’ boiling in the open. The material dissolved was as follows: Ash Protein Pentosan
Per cent 0 0 1 4 5 8
Per cent Cellulose Lignin
4 0
10.0
A liquor containing milk of lime equivalent to 7.3 per cent of the straw dissolved 37 per cent in 41/2 hours’ cooking a t 154” C. The material dissolved was as follows:
277
product with four chlorinations. A yield of 55 per cent was obtainable. The cellulose obtained yielded 13.3 per cent furfural corresponding to 20.7 per cent xylan. The yield of glucose by hydrolysis of the straw amounted to 99.4 per cent of the theoretical based on the pentosans, lignin, and ash-free cellulose. The yield of xylan-free cellulose thus amounted to 43 per cent of the original straw. The formation of bleached pulp from straw can be accomplished by remarkably mild pulping action. Straw boiled with caustic soda solution in the open can be bleached to a white product with calcium hypochlorite solution in two or three steps. Pulp obtained by digestion with 5 per cent of the weight of the straw in sodium carbonate and 0.5 per cent sulfur at 145’ C. with a yield of 75 per cent can be bleached with 80 pounds of standard bleaching powder containing 35 per cent available chlorine per 100 pounds of pulp. The loss of pulp substance amounted to 12 per cent, giving an ultimate yield of 66 per cent of white pulp based on the original weight of stra,w. Four bleaching treatments were necessary, with a water wash after each. Substitution of chlorine water for the first three treatments gave less satisfactory results arid indicat.ed that the alkaline conditions of the bleaching powder solution were better for purifying straw. Much of the coloring matter seemed to be removed by simple solution in the bleach solution along with the products of oxidation. Chemical studies of the producbs formed at the several steps of t,his operation would give ‘extremely interesting results and undoubtedly lead to valuable technical applications. Xylan in Straw
The low ash removal is due to the fact that the lime may add as much as is taken out and it was only by extremely effective washing that the ash content did not show an increase. Usually the ash content of commercial strawboard is double the content of the original straw. Lime is not a very effective agent for the removal of lignin. It serves to neutralize the acids formed by the hydrolysis of the straw at high temperatures in the presence of water and maintains alkaline conditions. For a crude product it is a satisfactory cooking agent and deserves consideration also where further purification with chlorine water or gas is contemplated. The lime process is the most destructive to the proteins of any discussed herein. The analyses indicate this and it is also apparent in the considerable quantities of ammonia formed in the digestion. This action was decidedly unfavorable to the nutritive properties of cooked straw and prevented its adoption in Germany during the World War, when large quantities of straw were treated with caustic soda for the production of fodder for horses and cattle. It is a remarkable fact that straw thus digested with a “crude fiber” content 41 per cent greater than the untreated straw was assimilated much more readily by livestock. Feeding experiments by Kellner in 1900 indicated that straw pulp similarly produced had a feeding value of 63 per cent against 40 per cent for meadow hay and only 18 per cent for wheat straw. CHLORINE-The action of chlorine on straw is of great interest on account, of its use in the laboratory as an agent for the determination of cellulose and technically in the manufacture of bleached pulp. Haugs showed that repeated chlorinations followed by hoilings in 2 per cent sodium sulfite were capable of yielding white fiber to the extent of 53 per cent of the weight of the straw. As many as twelve chlorinations were necessary. The substitution of 1 per cent caustic soda solution for washing the chlorinated product facilitated the operation and made possible the production of a white 6
Haug, 00. ti!., pp, 18 and 23.
The most characteristic feature of cereal straw is the xylan content. Some recent work by Haug’ showed it to amount to 14.85 per cent of the straw extractable with 6 per cent sodium hydroxide solution, with 9.01 per cent of pentosan remaining in the straw after the most complete extraction possible. Cross and Bevan8 suggest that the pentosan removable with alkaline extraction is xylan while that left in the fiber is furfuroids. The amount left in the fiber after chlorination, however, varied from 8 to 12 per cent, depending on the degree of alkaline extraction before the chlorination. The chlorinated fiber can be further purified so that the pentosan content is no more than that in cotton or purified wood cellulose. Haugg considers all the pentosan content as xylan. The extreme resistance of a large portion of the furfural-yielding constituents to alkaline hydrolysis or chlorination would indicate the need of further investigation. It is interesting to note that large quantities of xylan, lignin, and albuminoids are present in the liquor resulting from the digestion of straw in sodium carbonate and sodium sulfite or sodium carbonate and sulfur. Acidification will coagulate the solutions so that the flasks containing them can be inverted without the contents flowing out. Boiling precipitates the lignin and albuminoids and converts the xylan to xylose with a return of the liquors to fluidity. They readily support bacterial growth and fungi with the formation of various products, notably lactic acid, acetic acid, and alcohol. Preliminary work a t the University of Wisconsin by Fred and Peterson’O has indicated that the products obtainable from the 30 per cent of the straw dissolved in the waste liquors is of greater gross value than the strawboard made from the pulp portion amounting to 70 per cent.
’
Haug, “uber die Natur der Cellulose aus Getreidestroh,” p. 56, Hofmann, Berlin, 1925. 8 Cross and Bevan, “Cellulose,” p. 163, Longmans, Green & Co., 1903; “Researches on Cellulose, 189>1900,” p. 101; Ibid., “1900-1905,” p. 145, Longmans, Green 81 Co. 9 Haug, o p . cit., p. 75. ‘0 Fred and Peterson, unpublished report.
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Characteristics of S t r a w Pulps
The characteristics of straw pulps are quite different from wood pulps. The pulp obtained by the lime process is soft and spongy when wet, but paper made therefrom dries and can be calendered to a hard, stiff sheet noted for its ability to hold corrugations or indentations better than any other paper. Wet lime-cooked straw pulp gives up its water more readily than any other paper stock and will yield 7 to 10 per cent more water on pressing. On the other hand, straw pulps produced by caustic soda or sodium carbonate cooking give up water very slowly and dry correspondingly hard. They make a stronger and stiffer product, however. The pulps
VOl. 21, No. 3
prepared by sodium carbonate cooking and bleached in several steps with bleaching powder to give a high yield give up water more readily on pressing than the unbleached product but not so readily as the lime cooked product. The white pulp obtained undoubtedly contains lignin as well as pentosan and is far from being a pure cellulose. Its physical properties are favorable for paper manufacture, however, and it will make most of the common grades of white paper. The commercial bleached soda and sulfate straw pulps are slower in giving up their water and produce a stiff paper. They are seldom used alone, in the United States, however, but are mixed with softer and stronger pulps to impart snap to the ptoduct.
Pulping Flax Straw VI-Properties of Flax Straw Cellulose and Its Value in the Cellulose Industries‘ Earl R. Schafer a n d M a r k W. Bray U. S. FOREST PRODUCTS LABORATORY, MADISON,WIS.
HE increasing demands of industry for raw materials for the production of cellulose products is prompting the inquiry intoathe possible utilization of other fibrous materials in addition to wood. Industry has a t times gone far afield for its raw material and a consideration of the luxuriant growth of the wild plants in tropical re,’cwns as a source of cellulose should not be ignored. The importance, however, of first utilizing the agricultural wastes occurring a t our very doors is apparent. One phase of this agricultural waste problem is the utilization of flax straw. This paper deals with some of the properties of flax straw and pulps produced therefrom which are of interest to the cellulose industries. Other papers from the United States Forest Products Laboratory on flax straw are:
T
“A Study of Flax Straw for Paper Making,” by John D. Rue, Sidney D. Wells, and Earl R. Schafer.2 “Chemical Constituents of Flax Straw,” by Sidney D. Wells and Earl R . S ~ h a f e r . ~ “Pulping Flax Straw: I-Hydrolysis with Sodium Sulfite,” by Mark W. Bray and Clifford E. Peterson;4 “11-Chemical Studies with Chlorine as a Pulping Agent,” by Earl R. Schafer, “111-Hydrolysis Mark W. Bray, and Clifford E. P e t e r ~ o n ; ~ and Delignification with Sodium Hydroxide and with a Mixture of Sodium Hydroxide and Sodium Sulfite,” by Mark W. Bray 1 Presented under the title “Cellulose from Flax Straw” before the Division of Cellulose Chemistry a t the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 to 14, 1928. 2 Rue, Wells, and Schafer, Paper Trade J., 79, No. 13, 45 (1924); Pulp Paper Mag. Can., 22, 1068, 1087 (1924). 8 Wells and Schafer, Paper Trade J . , 80, No. 17, 47 (1925); Paper Znd., 7, 239 (1925). 4 Brav and Peterson. IND. ENG.CHEM.,19, 371 (1927); Paper Trade J . , 84,-No. 23, 317 (1927). 6 Schafer, Bray, and Peterson, Paper Trade J., 84, No. 8 , 207 (1927);
and Clifford E. Peterson$ “IV-Further Studies on Hydrolysis and Delignification with Alkaline Reagents,” by Earl R. Schafer and Clifford E. Peterson;’ “V-Production of Pulp by the Chlorine Process,” by Earl R. Schafer and Clifford E. Peterson;S “VII-Strawboard and Insulating Board,” by Earl R. Schafer and Clifford E. Peterson.9
Flax straw is obtained as a by-product or waste in the production of flax seed. The principal areas under cultivation in this country lie in the northwestern states, 97 per cent of the seed production being from Minnesota, the Dakotas, and Montana. I n 1927, 2,654,000 acres were devoted to flax culture and the production of seed was approximately 24,400,000 bushels. The amount of straw produced is estimated to vary from one to one-sixth of a ton per acre depending on the locality and prevailing weather conditions. Threequarters of a ton per acre is considered a good average. Nearly all of this annual production of 2 million tons of straw is burned. Only about 200,000 tons are used in the manufacture of upholstery tow and of insulating materials. The varieties of flax grown for seed purposes are those which produce seed of high oil content. The seed is thinly sown for the maximum production of branches and consequently of seed. The seed variety of flax is botanically the same as that grown for fiber. The difference between the two lies only in the strains selected and the methods of cultivation. 6 Bray and Peterson, Paper Trade J., 86, No. 3, 48 (1828); Pulp Paper Mag. Can., 26, 37 (1928). 7 Schafer and Peterson, Paper Trade J., 86, No. 3, 51 (1928); Pulp Paper Mag. Can., 26, 477 (1928). 8 Schafer and Peterson, Paper Trade J . , 87, KO.16, 41 (1928); Pulp PaDer Mag. C a n , 26, 1588 (1928). 8 Schafer and Peterson, to be published.
PENTOSANS
SOLUBILITY MATERIAL
~~
In alcoholbenzene
In hot water
In 1%
IN
CELLULOSE
TOTAL
CauStlC
Total
soh
CELLULOSE
Based on cellulose content
COPPER NUMBER
ASH
0
~
Seed flax straw Bast fiber of seed flax Sbives from seed flax
%
%
%
%
3.3 3.7 2.3 6.5
12.1 9.9 11.1 5.1
33.7 32.1 29.2 24.2
23.3 24.2 10.1 27.9
%
7%
%
%
%
70
53.8
17.1 19.4 6.0 25.6
13.1 16.0 3.4 20.1
7.0 8.2 2.4 11.4
8.0
3.6 6.0 4.7 3.5
51.3 71.9 57.0
...
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