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THE UTILIZATION OF THE NONCARBOHYDRATES IN PULPING SPENT LIQUORS1 W. M. HEARON Chemical Products Division, Crown Zellerbach Corporation, Camas, Washington
ONE
of the most challenging problem facing the pulp and paper industry today is the utilization of its pulping spent liquors. While this problem is by no means solved, sufficient progress has been made t o give an inkling of the tremendous potential which is waiting t o be tapped. A major break-through in the utilization of pulping spent liquors could well start a new, large type of chemical operations having profound effects on the paper industry and the industry in the United States at large. Of the 22,000,000 tons of wood pulp produced in the United States during 1956, about 12,000,000 tons were manufactured by the sulfate or kraft process and 2,800,000 tons by the sulfite process (1) These two processes represent the largest source of pulping spent liquors. An additional approximately half-million tons of soda pulp were also produced and represent an additional small source of pulping spent liquors. The remainder of the pulp production was either in groundwood or miscellaneous processes which produce spent liquors of very low solids and are normally discarded. In the krsft, soda, or sulfite process a little less than half of the wood used becomes pulp. The remaining wood is dissolved in the pulping spent liquor. Thus Presented as part of the Symposium on Wood as a Chemical
Raw Material before the Divisions of Chemical Education and Cellulose Chemistry a t the 133rd Meeting of the American Chemical Society, San Francisco, April, 1958.
the amount of material in these liquors is roughly equal to the amount of pulp produced. Perhaps a brief review of the principal pulping operations will help in understanding the problems concerned with the utilization of the spent pulping liquors. CHEMICAL PULPING PROCESSES
Historically, the first chemical pulping process is the sulfite. A mixture of sulfurous acid and a metal bisulfite is used as the pulping chemicals. Wood chips are treated with a mixture of these chemicals in solution for many hours at about 135°C. during which time the lignin is dissolved as lignin sulfonic acid and is neutralized by the metal ion present. Eimultaneously, some of the carbohydrates are hydrolyzed and dissolved as simple sugars. The residue of wood consists mainly of cellulose fibers which are separated as pulp. The liquid that is drained off the pulp, containing principally the lignin sulfonic acids as a metal salt, the sugars, and free sulfurous acid, constitutes the spent pulping liquor. The majority of sulfite operations use calcium as the metal ion. There are, however, other operations using ammonium as the cation and a few which use magnesium. Table 1 gives a typical analysis of calcium base sulfite spent liquor (8). I n the kraft operations, the cooking liquor consists of a solution of sodium hydroxide and sodium sulfide. These chemicals are able t o dissolve the lignin as a
JOURNAL OF CHEMICAL EDUCATION
TABLE 1 Typical Analysis of Calcium Bese Sulfite Spent Liquor Total solids Residue on ignition (as sulfates) Formio acid Acetic acid Celcium ss CaO . Total S aas S Sulfate as SO8 Free SO2 Loosely combined SO1 Sulfone sulfur, as SOz Methyl alcohol Ethyl alcohol Acetone Furfural Lignin Total mgars Pentoses Hexoses
(G7ams per liter) 114.8 16.35 0.65 4.28 6.47 10.85 1.736 3.43 5.5 11.37 1.01 0.166 0.152
Source: A. M. PARTANSKY AND H. K. BENSON, Paper Trade J., 102, No. 7, 81-7 (1936).
phenolic substance leaving the cellulosic pulp. Simultaneously, with the solution of the lignin a substantial amount of the carbohydrates is degraded into saccharinic acids or similar substances. Kraft spent liquor, called black liquor, then contains lignin as a phenolic sodium salt and various degraded carbohydrates as well as inorganic chemicals. Table 2 gives a typical analysis of kraft spent liquors (3). TABLE 2 Typical Analysis of Kraft Black Liquor
% of total orga?~ies Inorganic d t t s Organic material Fats and resins Lignin Oxyacids Acetlc acid Volatile8
Yo of total
5.9 47.4 34.7 Trace 12.0
solids
33.6 66.4
Source: F J. CINWS,Papw Trade J . , 91, No. 19, 20C-2 (1930).
% of total solids Carbon 34.4 Hydrogen 4.7 Sulfur 3.3 Methaxyl 5.7 Ash 60.7 Source: Crown Zellerbach Carportttion.
The black liquor is recovered by washing the pulp and is concentrated t o around 65% solids and burned in a special-type furnace. The heat produced is recovered as steam for the pulping operations and the inorganic salts form a smelt which is redissolved and treated with l i e to produce new cooking liquor. Therefore, in the kraft operations the black liquor is utilized t o produce heat and to regenerate the cooking chemicals. The soda process is similar to the kraft except that no sodium sulfite is used. The black liquor is recovered from the pulp, concentrated, burned, and causticized to produce fresh cooking liquor. Again, the inorganic chemicals are utilized and the organic materials are burned to produce heat. SPENT LIQUOR UTILIZATION PROBLEMS
There are several problems connected with the utilization of any pulping spent liquors. First, the liquor is a t a relatively low concentration of solids; VOLUME 35, NO. 10, OCTOBER, 1958
therefore, considerable work must be put into the liquors before they are in a usable form. Second, the pulping operations are carried out under relatively vigorous conditions so that many reactions occur and the resulting liquors contain a wide variety of chemicals which are difficult t o separate. Third, the structure of most of these chemicals is unknown. For example the structure of lignin is still undetermined despite almost a hundred years of effort t o solve this problem. Also, the structure of many of the carbohydrate degradation products is obscure. Fourth, the most reactive centers in the chemicals in the spent liquors have already been reacted upon during the pulping operation. This makes it difficult t o carry out further reactions on the chemicals which might lead t o new compounds. We should perhaps differentiate hetween utilization of pulping spent liquors and disposal of them. The kraft and soda black liquors are utilized, in a sense, as mentioned above, by the recovery of heat and inorganic chemicals. This utilization, howevcr, is of a rather low order, and potentially a utilization of the alkaline liquors t o produce organic chemicals for saleshould result in a better return. Most of the sulfite waste liquor is still put into the sewer and has no utilization a t all. A small part is converted into chemicals for sale and this will be discussed later. There is, however, a great deal of effort being put into disposal methods for sulfite waste liquor since the liquor presents a contamination problem in the water-ways in which it is discharged. These disposal processes are of diverse types. Some of the disposal systems do produce byproducts for sale, thus combining a disposal system with some utilization. Other disposal systems are designed t o burn the sulfite spent liquor and recover part of the value in the form of heat. Still other disposal systems have no utilization whatsoever, but are merely means to get rid of the liquor in such a fashion that it will not cause a pollution problem. Ideally, a utilization process for sulfite spent liquor would eliminate its contamination of streams and at the same time produce chemicals which could be sold profitably. Such a solution, which can be applied universally, however, has not been reached. What products have been produced from sulfite spent liquor have not found a sufficient market t o allow the entire sulfite pulping industry t o produce them. CHEMICALS PRODUCED
Now let us look at the processes which are being used for the utilization of pulping spent liquors. Two approaches have been tried. The first and most common one is to produce some mixture of unknown composition, or relatively unknown composition, which has utility and can be sold. The second approach is t o produce directly from the spent pulping liquors a pure organic chemical. This is a much more difficult job but has the advantage of giving a product for sale whose chemical and physical properties can be determined or predicted based on classical organic chemistry. Tall Oil. One product coming from kraft black liquor which has become a resounding success is tall oil. This material started out as a waste item and has now become a product in high demand.
Tall oil consists of the resinous parts of the wood. Its production, therefore, is a function of the type of wood used in pulping and practically all of the tall oil installations are in the southeastern part of the United States where pine is t,he principal pulp wood. During the concentration of the black liquor, a soap-like scum is formed which is collected from settling basins. These so-called skimmings are actually the sodium salt of a number of organic compounds. The skimmings are treated with sulfuric acid to liberate the free acidic materials which constitute tall oil. The tall oil consists principally of rosin, long-chain fatty acids, and sterols. I n some cases the pulp mill collects only the skimmings and sells them t o a processor while in other cases the pulp mill produces the crude tall oil and may even have a purification unit for the tall oil. Interestingly enough, the rosin finds its way back t o the paper mill as a sizing agent. The tall oil, as such, finds some uses, for example as a reagent in the beneficiation of ores, but a great deal of it is separated into its components and the rosin and fatty acids are used for a variety of purposes. I n 1956 about 297,900 tons of crude tall were produced (4). The crude tall oil sells for about $50 per ton; therefore, this represents a $15,000,000per year industry. Turpentine. While the collection of sulfate turpentine does not represent a true utilization of pulping spent liquors, it is almost always obtained concurrently with tall oil. The turpentine constituents occur principally in pine wood. I n the southern kraft mills the vapor relief from t h e digesters is run through large surface condensers and the turpentine along with considerable water is collected. The crude turpentine is then skimmed from the water and sold to chemical manufacturers who either purify it for solvent purposes or separate it into its terpene constituents for further chemical utilization. During 1956, 15,300,000 gallons of sulfate turpentine were produced (4). The crude turpentine sells for about 29# per gallon so that this represents nearly a $5,000,000per year industry. Absorbent Charcoals. Another profitable utilization of kraft spent liquors is the production of decolorizing charcoal. One large pulp and paper manufacturer has been producing such charcoal for many years and has sold it under the trade name of Nuchar. The charcoal is produced by a controlled pyrolysis of the total black liquor solids (5). It is one of the most effective of the absorbent charcoals available. Lignin. The Same pulp and paper company has also produced lignin from kraft spent liquors and sold it under the trade name of Indulin. This product finds a wide variety of uses, principally as a dispersing agent (6), reinforcing agent for rubber (7), and as an ingredient and extender for resins (8). Recently, the company has produced a new line of dispersing agents under the trade name of Polyfon (9) which are prepared by sulfonating the kraft lignin t o give a lignin snlfonic acid. The total utilization of kraft spent liquors including charcoal and lignin production has been reported as grossing $7,000,000 per year. The soda process produces a lignin which is somewhat similar t o that of the kraft operations. One paper company for many years isolated some of its soda lignin and sold it under the trade name of Meadol. Recently, however, this product has been withdrawn
from the market apparently due t o the lack of sufficiently large outlets. Dimethyl SulJirZe. An entirely different utilization of kraft spent liquors is the production of dimethyl sulfide. One large pulp and paper compauy about a year ago announced the availability in commercial quantities of this pure organic chemical made from kraft lignin. The dimethyl sulfide is produced by heating kraft black liquor with an excess of sulfide sulfur (10). The first reaction involves the splitting off of methoxyl groups t o form methyl mercaptau which reacts with additional methoxyl groups t o form dimethyl sulfide. The yield of dimethyl sulfide is approximately 2.5% of the kraft spent liquor solids. Thus a 100 ton per day kraft mill could produce over 3,000,000 pounds per year of dimethyl sulfide. The entire kraft industry in the United States could produce about 350,000 tons of this material. The dimethyl sulfide production is still in its infancy but already this material is finding use as an ingredient in odorant formulations for natural and artificial gas, as an odor intensifier in perfumes, as an ingredient in counterodorant formulations, and as a raw material for further synthesis. Dimethyl sulfide represents a chemurgic product which can be produced basically a t less cost than by synthetic means. Mercaptans. As stated above, the dimethyl sulfide process produces methyl mercaptan as an intermediate. This material can be isolated and by changing the conditions of the process it is possible t o obtain methyl mercaptan as the predominant product. Therefore, this process is capable of producing two materials and they in turn open up possibilities for producing a wide variety of additional sulfur compounds. Already, this pulp and paper company has advertised availability of dimethyl disulfide and dimethyl sulfone in sample quantities. While the present largest market for methyl mercaptan is as an ingredient in making methianine for animal feeds, other markets are expected to develop. One of these is for the production of a new type of rubber having unusual heat-, chemical-, and oxidationresistant properties (11). Considerably more effort has been spent on the utilization of sulfite pulping liquors since these represent a stream pollution problem. There have been some 2000 patents issued on the utilization of sulfite spent liquor but unfortunately few of these have proved t o be of practical use. Nevertheless, a number of utilization processes are now in operation. Lignin Sulfonic Acids. Several pulp and paper companies are putting out products which are essentially their sulfite spent liquor concentrated and spray dried. Some of them have further modified their sulfite spent liquor products for specific applications. I n some cases, the products have been purified t o remove the sugars present leaving only the lignin sulfonic acids. All of these products arc sold principally as binders or as dispersing agents. As binders, they find use in the manufacture of ceramic articles (121, linoleum paste (I$), and foundry cores ( 1 4 , and as pelletizing and briquetting materials (15). Considerable quantities of sulfite waste liquor products are also used as binders t o control dust on roadways (16). As dispersing agents, they are used in the JOURNAL OF CHEMICAL EDUCATION
manufacture of rewettable insecticide powders (17), asphalt emulsions ( I S ) , gypsum wallboard (19), industrial cleaning compounds (2O), carbon black dispersions (21), oil well drilling muds (22), concrete ( M ) ,and as ore flotation reagents (24). According to the 1956 Tariff Commissiou report, there are about 69,000 tons of such products sold each year valued a t .W5,053,000. Vanillin. One of the chemical uses for lignin sulfonic acid occurring in sulfite spent liquor is as a raw material for producing vanillin. Essentially all of the vanillin produced in the United States, amounting to about 1,500,000 pounds per year, is made from the alkaline degradation of lignin sulfonic acid. Vanillin is the only pure organic chemical being produced commercially from sulfite spent liquor. Sugars. The sugars in sulfite spent liquor represent a raw material for making other products. In the Upited States there is one plant producing ethyl alcohol a t the rate of about 10,000 gallons per day by the fermentation of sulfite spent liquor. This plant was built during World War I1 to increase the then short supply of alcohol. Under peace-time conditions, however, paper companies have not felt the economics sufficiently attractive to build similar plants. Another process for the utilization of sugars is the manufacture of yeast. Two paper mills in the Midwest are currently producing yeast which is being sold mainly as an ingredient in animal feeds. The yeast sells a t around 10b per pound and represents an industry which may gross over a million dollars a year. The lignin sulfonic acid residues from the above fermentation processes are also being sold as dispersing agents for a variety of uses. They are particularly effective for applications where the presence of sugars is deleterious, such as concrete and oil-well drilling mud. Metal Complexing Agats. One property of sulfite spent liquor which has not been exploited until recently is its ability to complex metals. Metals such as iron, zinc, copper, nickel, tin, cobalt, silver, and manganese are readily complexed by sulfite spent liquor. One paper company is now marketing a complex made with iron under the trade name of Greenz (25). This product is being sold as an agricultural spray to combat iron deficiency in plants. Another paper company is selling a complex of iron and chromium with sulfite spent liquor under the trade name of Q-Broxin as an additive to oil-well drilling mud. This additive seems to be particularly effective in the high lime muds found in the Gulf region. POSSIBLE NEW DEVELOPMENTS
While the above products represent commercial utilization of sulfite spent liquor, there are several interesting developments which could lead to new commercial products. During the pulping of western hemlock wood by the sulfite process, a minor wood extractive called conidendrin is dissolved in the sulfite spent liquor. This material can be readily removed from the sulfite spent liquor by a simple precipitation process (26). Conidendrin is a complex phenol which might well have use as a modifier for phenolic type resins. I t also is a raw material for producing a powerful rubber VOLUME 35, NO. 10, OCTOBER, 1958
antioxidant, called conideudrol (27). Although conideudrin is only a minor constituent in the wood, nevertheless a 400 ton per day sulfite mill could produce ahout 2 tons per day of conideudrin.
LJocH, OH
Conidendrin
KH Conidendrol
Another process which has been worked on recently is the isolation of pure crystalline sugars from spent sulfite liquor (28). The solids from spruce wood spent sulfite liquor were treated with acetone and acid to produce the isopropylidene derivatives. From these, pure D-mannose, D-xylose, L-arabinose, and D-galactose were isolated. This process could be the basis for a practical method to produce at least mannore which is the predominant sugar in sulfite spent liquor. One possible market for these pure sugars would be for reduction to the corresponding alcohols which have found wide use as humectants and plasticizers. Recently work has been reported on the isolation of pure saccharinic acids from kraft black liquor (29). Some seven different saccharinic acids were identified by paper chromatography and two of them, alphaD-isosaccharinic acid and beta-D-glucometasaccharinic acid, were isolated as their anilides by fractionation on a cellulose column. While this work is definitely of a preliminary nature it is possible that it could lead to a process for obtaining some of these acids by a commercial process. There are no known markets for the saccharinic acids but they might find use as humectants or raw materials for the manufacture of pharmaceuticals and other fine chemicals. While the uses to date for the spent pulping liquors have resulted in only a limited utilization of the tremendous volume of liquors available, nevertheless the work shows that some profitable utilization is possible. It is almost certain that new uses will be found for sulfite spent liquor and for some of the components in it. In addition, there is no question but that lignin occurring in either kraft or sulfite spent liquors represents an abundant source of raw material which is continually being replenished. Should a way be found to break down lignin to simple aromatic
chemicals, a vast new area in the chemical industry would be created. It is the possible achievement of this goal which makes it mandatory that research should continue on the utilization of spent pulping liquors. LITERATURE CITED
(7) (8) (9) (10) (11) . . (12) (13)
ANONYMOUS, Pulp and Paper, 31, 176 (1957). PARTANSKY, A. M., AND H. K. BENSON,Papw Trade J., 102, No. 7,81-7 (1936). CINVES,F. J., Paper Trade J., 91, No. 19, 200-2 (1930). ANONYMOUS, Chem. Eng. N m s , 35, No. 3 6 , 1 6 2 0 (1957). STEELY,J. E., R. E. SWART., AND D. S. TROUBS,U. S. Patent 2,300,600 (Nov. 3, 1943); C.A., 37, 2164 (1953). For example, LAWSON, L.R., A N D J. J. K E ~ E NAm. , Ceram. Sac. Bull.. 30. 143-7 (1951): W. A.. Znd. Eno. . , MCINTOSH. Chem.. 44: 1656-9 (19521. KEILEN,'J. W. K. DDUGHTERY, AND W. R. COOK,Znd. Eng. Chem., 44,163-7 (1952). For example, FARBER, E., U. S. Patent 2,443,576 (June 15, 1948); C.A.,42,6136 (1948). KEILEN,J. J., Chernu~gieDig., 14, No. 3, 13-14 (1955). HEARON, W. M., Forest P~oductsJ . , 7 , 4 3 2 4 (1957). METER.G. E.. ETAL..Rubberworld., 136.529-36 (1957) . . . and 136,695-7oi (1957j. BURGESS, G. D., Am. Ceram. Soc. Bull., 36, 168-71 (1957). PHELPS,G. W.,Cwam. Age, 53, 11-12, 40 (1949).
i,
(14) MOSER,F. R., U. S. Patent 2,556,335 (June 12, 1951); C.A.. 45.7509 119511. ,-.,..-. ~ -..-,. (15) HARTMANN, W., U. S. Patent 2,778,718 (Jan. 22, 1957); C.A., 51,6981 (1957). J. E., Can. Chem. P~ocessing,37, No. 2,58,60-1 (16) HWRTUBI~E, (1953). W. D., U. S. Patent 2,568,035 (Sept. 18, 1951); (17) STEWART, C.A., 46, 1212 (1952). (18) JESSEPH.D. C.. U. S. Patent 2.494.708 (Jan. 17. 1950): , ~~,~ ~~,
~~
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(19) BUDNIKOV, P. P., AND Y. M. BUTT.,Compt. rend. aead. sei. U.R.S.S., 56, 503-5 (1947); C.A., 42, 9176 (1948). (20) BRAUN,K., Seifensieder-Ztg., 64, 207-8 (1937); C.A., 31, 1772 I,A"". l U ? ? ),. (21) SALVESEN, J. R., A N D W. C. BROWNING, Chem. Znd., 61, 2 3 2 4 (1947). W. C., U. S. Patent 2,771,421 (Nov. 20, 1956); (22) BROWNING, C.A., 51,3978 (1957). E. W.. Enc. News-Reco~d.127. 8 0 5 7 11941). (231 SCRIPTURE. U. ~ . ' ~ a t e n2,485;083 t i24j BOOTH,R.'B., ANDR."A. PICKENS, (Oct. 18, 1949); C.A., 44, 998 (1950). A,, A N D R. T. ASHCROFT, Soil Sei., 82, No. 3,233(25) WALLACE, 6 (1956). H. B., W. W. MOYER,AND W. M. HEARON, Tappi, (26) LACKEY, 32,469-71 (1949). (27) HEARON, W. M., H. B. LACKEY, A N D W. W. MOYER,J. Am. Chem. Sac.. 73.4005 (1951).
