Chemical Recovery from
Pulp-MiII Waste Liquor A Staff-Industry Collaborafive Report D. GRAY WEAVER, Associate Editor in collaboraiion with
W. A. BIGGS, Jr., Sonoco Products Co., Research and Development, Hartsville, S. C.
Today’s trend in North American papermaking is to hurdwood.
The most
important process yet developed to meet this trend, Sonoco Products’ new Neutral Sulfite Semi-Chemical
(NSSC) pulping, permits use of black liquor to produce valuable chemicals and thus avoids stream pollution from this high -B. OD.waste
STREAM
POLLUTION, from the discharge of both pulping liquor and white water, has always been a problem for paper producers. In recent years, this problem has become increasingly acute. Paper mills consume vast quantities of water in their operations. They liberate high tonnages of organic and inorganic pollutants from pulp and pulping chemicals. Disposing of pulp liquors today is a major problem particularly for sulfite mills. Governmental action severely restricts dumping such efHuent into natural waterways. And getting rid of the dissolved and suspended contaminants, so the discharge water can be dumped, is expensive. I t would be very helpful if saIstble or usable materiaIs could be recovered economically in the process of making the liquors innocuous. Sonoco Products Co. has a practical answer to this big ‘‘if”. The sulfate chemical recovery process, which completely recovers inorganic chemicals, is as old as sulfate pulping and a vital economic factor in the success of the pulping method. Black liquor, concentrated and burned in a furnace, produces a smelt composed of sodium sulfide and sodium carbonate. These components, dissolved in water and causticized with lime, yield sodium hydroxide and sodium hydrosulfide which is used for kraft pulping. The lime is calcined and re-used. This is a straightforward, uncomplicated recovery operation. The only significant drawback is bad odors. Nothing so straightforward has ever been developed for the sulfite processes. For example, when soda-based sulfite
VOL. 53.
NO. 10
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OCTOBER 1961
773
Sonoco is the world's largest producer of paper cones for t h e textile industry. The firm also manufactures paperboard, spools, tubes and cores (both plain and impregnated), and bituminized fiber pipe. Besides serving such conventional applications as containers, paper-mill cores, air ducts, concrete construction forms, and sewer and drain pipes, the firm finds a big market opening up for its tubes a s storage bins. Tubes, 1 foot or more in diameter, and stacked horizontally, make excellent nests for storing all sorts of things :
b
Rolled-up rugs, carpets and fabrics (manufacturers, distributors, retailers)
b
Extruded or machined lengths of plastics, metal, and wood (warehouses, shops)
b
Golf bags (pro shops or sporting goods stores)
Sonoco also manufactures specialty synthetic resins, paints, varnishes, lacquers, and adhesives. black liquors are burned, the resulting smelt contains the same chemicals (sulfide and carbonate) as from the kraft recovery. This presents the problem of oxidizing the sulfide to sulfite without producing either the sulfate or thiosulfate. For calcium-based sulfite, the problem is considerably greater. A number of total recovery operations for sulfite pulping have been developed in recent years (since 1950). These operations and processes include the Institute of Paper Chemistry, Mead-Babcock & Wilcox, Sivola, Western Precipitation, Stora, and AST. (See Recovery Processes for Sulfite, below). At least one plant of each type is now in operation except Western Precipitation and .4ST. These are not considered by the industry generally to be either adequate economically or technically satisfactory for high yield sulfite pulping operations. T h e biggest success story economically has been the Marathon-Howard process for producing vanillin and other chemi-
cals from calcium bisulfite black liquor. This process does not, however, deal completely with the pollution problem, of itself. Other sulfite partial chemical recovery successes are the production of Torula yeast by fermentation of low-yield black liquor and ethyl alcohol production by fermentation of the hexose sugars in low-yield liquor. Sulfite Integration with Kraft Pulping
The most satisfactory sulfite chemical recovery operation is that practiced in integration with kraft pulping. I n this case, all the sulfite black liquor is combined with kraft black liquor to sustain flartially the chemical requirements of the kraft operation; hence, all sulfite chemical and organic matter is disposed of economically in the kraft recovery system. T o effect this integration with total sulfite chemical recovery and black liquor disposal, the two mills must be close together. Sonoco operates one of the first
hardwood neutral-sulfite mills ever commercialized. The firm, a major hardwood papermaker for unbleached paper, finds itself with some 1400 tons a day of black liquor from its Keutral Sulfite Semi-chemical (NSSC) process. The company has been fighting this pollution problem since shortly after World War 11. Two of Sonoco's major efforts to avoid stream pollution involved lagooning of the liquor (unsatisfactory because of fermentation and resulting foul odor) and biochemical oxidation (impractical because of cost). Most recent development is a chemical recovery plant based on solvent extraction with methyl ethyl ketone (MEK). This produces commercial organic acids formic (907') and glacial acetic (99.5y0 minimum)-from black liquor. Added credits arise from sale of the whole raffinate to kraft mills as fuel and salt cake make-up. High B.O.D. Boosts Pollution The greatest pollution hazard from this black-liquor arises from its high biochemical oxygen demand (B.O.D.) resulting from the large quantities of sodium acetate and sodium formate in the effluent. Primary goal of Sonoco management in deciding to t i y chemical recovery was to separate these cornponents; this in itself would eliminate a considerable public nuisance. If sales of recovered materials could make the operation self-supporting, so much the better. For a quarter century since 1933, the Hartsville mill produced nine-point corrugating board on a Fourdrinier machine a t a production rate of 150 tons per day. Besides this, eight cylinder machines have been making about 400 tons per day of board from waste papers. The B.O.D. of just the white water from this operation-together with normal plant waste-water-was all that the
=Recovery Processes for Sulfite Pulping Institute of Paper Chemistry Brown, R. W., Jackson, D. T., Tongren, J. C., Pulp & Paper 33, NO. 6, 66-9 (1959). Whitney, R. P., Han, S. T., Davis, J. L., T a p p i 40, 587-94 (1957). FVhitney, R. P., Han, S. T., Davis, J. L. (to the Institute of Paper Chemistry), U. S. Patent 2,802,791 (Aug. 13, 1957). Mead-Babcock & ~ V i l c o x Campbell, J., Jr., Shick, P. E., Paper Trade J . 140, No. 51, 22-3 (Dec. 24, 1956). Markant, H. P., Tappi43, 699-702 (1960). Scheid, L. J., Careaga, R., Pulp @ Paper 33, No. 3, 59-62 (1 959). Shick, P. E. (to Mead Corp.), U. S. Patent 2,849,292 (Aug. 26, 1958). Stcola Kennedy, E. H., T a p p i 43,683-8 (1960) Sivola, George, U. S. Patent 2,730,445 (Jan. 10, 1956). Western Preczpitation Boyer, R . Q., Tappi 43, 688-98 (1960). Bradley, Linn, Boyer, R. Q. (to Wrestern Precipitation Corp.), U. S. Patent 2,792,350 (May 14, 1957); Zbid., U. S. Patent 2,862,887 (Dec. 2, 1958).
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Stora-Kopparsbergs Ahlborg, N. K. G., Cederquist, K. N. (to Stora-Kopparbergs Bergslags Aktiebolags), Swed. 158,363 (March 26, 1957) ; Zbid.. Can. Patent 567,659 (Dec. 16, 1958); Zbid., U. S. Patent 2,864,669 (nec. 16,1958). Cederquist, K. N., Ahlborg, N. K. G., Lunden, B. O., Wentworth, T. O., Tappi 43,702-6 (1960). Marathon-How ard Howard, G. C. (to Guy C. Howard Co.), U. S. Patent re 18,268 (Dec. 1, 1931), (Reissue of U. S. Patent 1,699,845, Jan. 22, 1929). Howard, G. C., U. S. Patent 1,856,558 (May 3, 1932). Sanborn, L. T. (to Marathon Paper Mills Co. and Guy C. Howard Co.), U. S. Patent 2,104,701 (Jan. 4, 1938). Sandborn, L. T., Salvesen, J. R., Howard, G. C. (to h4arathon Paper Mills Co. and Guy C. Howard Co.). U. S. Patent 2,057,117 (Oct. 13, 1936). Sonoco Sonoco Products Co., Hartsvilie, S. C., U. S. Patent 2,714,118 (July 26, 1955). Ibid., 2,744,927 (May 8 , 1956). Zbid., 2,974,081 (March 7, 1961).
Basic steps in the waste liquor recovery system are shown here 3.
1.
Triple-effect, forced-circulation evaporator. 2. Agitated in-line acidifier. covery column. 6. Flash drum. 7. Acetic azeatropic column. 8. Decanter. fining column
_IT1
9.
Extractor column. 4. Raffinate 'stripping column 5. MEK re. Formic 'azeotropic 'column. 10. Decanter. 11. Acetic acid re-
SODIUM SULFATE
- -
EVAPORATORS
HARDWooD
NSSC PULPING
t
4--!
KRAFT PULPING
FURNACE
>
GREEN LIQUOR
SOFTWOO0
* CHps
I
I
KRAFT LIQUOR
+
i
NSSC PULP
KRAFT PULP 4 TONS
1 TON Conventional integration of NSSC and kraft pulping would follow this pattern
EVAPORATORS PULPING
-
WATER
PULPING
CHIPS A
.GREEN T
i
T
ORGANIC ACIDS RECOVERY r
I
*
-N a A 0
4,
NaSOi-
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OCTOBER 1961
775
local watercourse, Black Creek (average stream flow is 200 million gallons per day), could tolerate and still sustain fish life. This meant that all of the black liquor must be disposed of some other way. Sonoco’s NSSC pulping involves processing a variety of trees-chiefly black gum, sweet gum, and tupelo, as well as minor amounts of such hardwoods as elm. oak, and poplar. Pulp yield from the cooking runs around 80% through a two-compartment Sprout-Waldron digester (the first one of its kind to get into operation in the industry) and from spherical digesters. This yields dissolved solids in black liquor with a total B.O.D. of 400,000 p.p.m. (on a basis of total solids)--equivalent to about 300 pounds of B.O.D. per ton of pulp. Half of this B.O.D. results from the presence of sodium acetate and formate (alphamethoxy and alpha-ethoxy groups are knocked from the lignin and hemicelluIose structure during cooking). Wood sugars, fatty-acid components, and resins account for the rest of the B.O.D. Lignin sulfonates do not seem to add to this problem. Acetate-Formate Content High
The acetate and formate content of the liquor typically adds u p to about 18 pounds of sodium acetate and 2 pounds of sodium formate per 100 pounds of black liquor solids. As 740 pounds of dissolved solids result for every ton of pulp produced, a ton of pulp gives about 134 pounds of acetic acid and 15 pounds of formic acid. By removing these chemicals economically, the company would reduce its black liquor B.O.D. problem by 5070-or, alternatively, could double pulp production without increasing the B.O.D. situation. Further, Sonoco reasoned that by using sulfuric acid to liberate these low-organic acids from their sodium salts, the residual black liquor might be sold for its salt cake content and fuel value to nearby kraft mills. Market studies on formic and acetic acids, and cost studies on acid-separation processes, showed that a 150-ton-per-day mill would not provide enough black liquor to make the acid recovery economical. But a larger mill might. Consequently, Sonoco decided to boost its hardwood pulping to 350 tons per day and to build the world’s first formicacetic acid recovery plant, processing paper-mill waste. This size plant could produce about 47,000 pounds per day of acetic acid-over 17 million pounds annually. I t would also yield nearly 2 million pounds per year of formic acid. Sonoco’s central southeastern location, near both kraft and textile industries, puts it in a position to exploit blackliquor recovery as a potential source of profit even though the recovery is not
776
integrated either for total chemical recovery (re-using the inorganic in sulfite pulping) or with local kraft production (getting heat credit, in addition to salt-cake, from burning the residual black liquor for kraft pulping). Possible Recovery Methods
Several existing methods of recovering formic and acetic acids offered possibilities : 0 Esterification-simplest recovery route from black liquor-requires only concentration and acidification of the liquor, then reaction with an alcohol and distillation. Washing and redistilling can purify the mixed esters produced this way, and they can then be separated by fractional distillation. Typically, for acetic acid, esterification is straightforward :
ROH
+ CHaCOOH+CH3COOR + HzO
Pilot-plant studies have produced ethyl and butyl acetates and formates
INDUSTRIAL AND ENGINEERING CHEMISTRY
this way. But ttvo unfavorable market situations exist for practical commercial operation. For one thing, the alcohols used cost virtually as much as the products (up to amyl esters) would sell for. For another, lacquer and coatings people are switching, to a considerable extent, from acetate esters to synthetic ketone solvents. 0 Direct distillation-impractical, because acids concentration is too dilute. Also vapor-liquid equilibria provide almost the same acid content in the vapor as in the liquid, and the market for dilute acids is quite limited. e Azeotropic distillation-impractical, because water removed from system leaves acids in very large volume of solids. Besides, steam demand is excessive. 0 Solvent extraction-most economical of these processes in steam costs. Widely used in cellulose acetate production and in acetic acid production from pyroligneous acid in wood distillation, this is closest to what is needed for recovery from black liquor.
Tallest column in Sonoco’s NSSC recovery operation is the 100-foot-tall countercurrent extractor which processes 100 gallons per minute of a mixture of 40 parts acidified black liquor with 60 parts MEK-H20 azeotrope and feeds to six other columns in series, located on other side of structure. Aluminum storage tanks at right contain finished HOAc product
4
b Raffinmate product, as well as finished acids, leaives recolvery plaint by tank car
Nerve center of NSSC recovery process is the control room with iis typical simplified display panel
4
However, Sonoco chemists found that the usual extraction solvents-ethyl ether, isopropyl ether, ethyl acetate, butyl acetate, and methyl isobutyl ketone-emulsify badly in the sulfuricacidified black liquor. This occurs at all temperatures up to the solvent boiling point and at all solids-content levels above 10% in the liquor.
MEK Extraction Best While rigorous centrifuging or long standing produces some phase separation, an efficient solvent should separate rapidly and completely under normal gravity conditions. Two possibilities seemed to fit these needs: 2-butanone (methyl ethyl ketone, MEK) and a mixed
Chemical recovery-to help reduce pollution from wood pulping operations-attracts wide interest from softwood as well as hardwood pulpers, from users of sulfite as well as sulfate processes. A n u m b e r of total recovery techniques for sulfite pulping have been developed over the past decade. But none of t h e m seems to be as promising, technically o r economically, as the Sonoco process integrated with kraft pulping. However, the latest process to take a bow boasts a t least the novelty of a n unusual approach-it involves use of ion exchange resins to recover soluble bases from spent sulfite liquors. Its C a n a d i a n developers,
solvent consisting of a 3 to 1 mixture of isopropyl acetate and isopropyl alcohol. MEK apparently had never been considered seriously for HOAc extraction because of its high reciprocal solubility with water (at 23O C., M E K is 26.3y0 soluble in HzO; HzO is 12.6% soluble in MEK). But M E K unexpectedly proved to be highly efficient for Sonoco’s
Abitibi Power & P a p e r Co., Ltd., consider it to be a feasible m e t h o d of base recovery from soluble-base cooking in low-yield acid-sulfite a n d bisulfite systems. I n pilot-plant operations, the new Abiperm process utilizes a sulfonic-type cation exchange resin “in hydrogen form,” m a d e by I o n a c Chemical Division of Pfaudler Permutit. I t claims significant savings, between $1 a n d $4 per ton of pulp, in chemical costs a n d additional p u l p recovery. T h e Rayonier C o r p . holds patents o n similar ion exchange processes for p u l p waste-liquor chemical recovery, b u t none has been reduced to commercial practice in the field as yct.
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Useful Extraction Solvent Requirements for NSSC Black liquor
A practical extraction should show b l o w reciprocal water
solubility
with
,High partition coefficient for acetic acid in water ,High azeotropic properties with water PNonazeotropic properties acetic and formic acids FNonreactivity components
with
with
black-liquor
b l o w dissolving action on liquor solids bNonemulsifying action with acidifled liquor b l o w corrosive effects on equipment
b Commercial
availability
economy b l o w toxicity
Methyl ethyl ketone fits this pattern best.
and
extraction process. Apparently the sodium sulfate, formed upon adding HzSO, to the black liquor, and the sulfonated lignin, salt out most of the M E K from the water. Concentrating the liquor to 40y0 solids before acidification saturates the system with NazSOd, and M E K then extracts the formic and acetic acids very efficiently. The isopropyl acetate-alcohol combination is a useful system but does not measure up to MEK. Sonoco Acid-Recovery Process
Sonoco's NSSC process oxidizes sulfur in conventional burners to SOZ; then the gas reacts with soda ash solution in three countercurrent contact towers to make NazS03. This digests hardwood chips in the pulpwood digesters at 160' to 180' C. and 100 p.s.i.g. Sonoco gets about an 807, yield from its pulping process, producing the 1400 tons or SO per day of black liquor containing 8 to 107, dissolved solids. This dilute liquor passes through triple-effect. forced-circulation evaporators lined with stainless steel. These boost total solids to 407,. and the concentrated liquor is pumped to wooden surge tanks holding 100,000 gallons each. From here: it goes through an acidifier where metered-in concentrated sulfuric acid reacts with residual sodium acetate and formate in the liquor to liberate the organic acids. The acidified black liquor then flows countercurrently through a binary azeotrope composition of methyl ethyl ketone. and water, in a specially designed extractor, which removes the acetic and
An intriguing resin-and-wax by-product of NSSC hardwood pulping by the Sonoco Process, which might be considered the hardwood counterpart o f tall oil, contains a wide variefy of organic Components including: Vanillic acid Syringic acid p-Hydroxybenzoic acid Lactic acid Palmitic acid Lauric acid C-18 to C-29 acids Phenol glucosides Ascorbic acid Glycollic acid Tartaric acid p-Coumaric acid Stearic acid Linoleic acid 2,6-Dimethoxyphenol
778
Vanillin Syringaldehyde Succinic acid Oleic acid Myristic acid Capric acid Sterols Galactose Glucose Arabinose Xylose Rhamnose
INDUSTRIAL AND ENGINEERING CHEMISTRY
Acetovanillone Acetosyringone
formic acids from the liquor. (Initially a gelatinous sludge of pulp fines and calcium sulfate fouled the extractor; the special extractor design overcame this problem). The binary MEKwater mixture, distilled in a column from the extract, condenses for recycle to the extractor; bottoms contain crude mixed acetic-formic acid plus dissolved oils, waxes, and resins. Pumped through a heat exchanger and into a flash drum, the mixed acids and water vaporize from the dissolved solids. Passing then into another azeotrope column, the flash stream contacts ethylene dichloride (EDC). From here, the overheads (water, formic acid, EDC azeotropes) condense into a decanter. This separates the aqueous formic acid from the solvent, in two layers. The solvent layer returns as reflux to the column; the formic acid solution then undergoes another similar azeotropic distillation, where the bottoms from this column consist of 9070 formic acid in water. This, vaporized and condensed again, goes to storage as product. Bottoms from the second azeotropic column are essentially glacial acetic acid plus traces of higher-boiling organic acids and furfural. A refining column gives glacial acetic acid as an overhead product. Bottoms from the flash drum (concentrated solution of fats and resins in a small amount of mixed acids) are distilled to remove the acids for recycle to the extractor. Raffinate liquor from the extractor goes to a stripper column which returns the MEK-water binary for re-use. The solvent-stripped raffinate? after concentration to 50 to 557, solids in a forced-circulation evaporator, goes to by-product storage. Specially equipped tank cars take the concentrated raffinate to kraft mills for use as cheap fuel and a source of sodium sulfate. The most serious problem Sonoco has experienced in developing its scaled-up chemical-recovery operation is corrosion, despite the use of normally corrosionresistant stainless steel throughout the entire plant. Mixtures of formic and acetic acids seem to be much more corrosive than each alone. And: in the presence of higher-molecular-weight organic acids, this effect is even worse. But plant tests have now turned up newly available alloys satisfactory for this severe use. And as components need replacement, these alloys will be employed. The most serious implication of this corrosion has been mechanical failure of the formic acid column. This made it difficult to produce specification formic acid. As soon as the improved alloys get into all critical parts of the installation, water-white formic acid will be available for sale--probably later this year.
