Ma?;, 1934
INDUSTRIAL AND
ENG I N E E R I N G CHEMISTRY
The condensable vapors from the digester will require about 2.1 kg. of chlorine per metric ton (4.2 pounds per ton) of pulp. The vapors from the evaporation of the black liquor require much larger quantities of chlorine. The mercaptan odor can be removed but other odors remain. It is believed impractical to attempt to destroy all of the mercaptans by means of the chlorine treatment. Liquors containing dissolved or condensed mercaptans should be used as first wash mater for the pulp or for making up fresh cooking liquor. They should not be discharged from the mill. Gases containing mercaptans should be led into the furnace used for calcining the Mack liquor, where they can be consumed.
567
The black liquors should be concentrated to 75 per cent solids in vacuum pans and sprayed as a liquid fuel into a very hot calcining furnace or boiler combustion chamber to burn the organic sulfur compounds completely. A spray chamber in which gas may be treated with hypochlorite solution should be provided for use in emergencies where the furnace is not working properly.
ACKNOWLEDGMENT The authors wish to express their appreciation of the cooperation received from members of the staff of the University of Alabama and of the Gulf States Paper Company which operates a sulfate mill near by. RECEIVBD November 1. 1933.
Purification of Sweet and White Potato Starches F. H. THURBER AND H. S. PAINE, Carbohydrate Division, Bureau of Chemistry and Soils, Washington, D. C .
T
Data on the tabling of sweet potato starch and commercial uses were investiHE specifk g r a v i t y of starch is approximately on the hydroseparation of sweet and white potato gated. Laundries and t e x t i l e mills reported the starch to be one and one-half times as starches demonstrate that they can be produced s a t i s f a c t o r y f o r t h e i r work great as that of water; it accordingly settles in water and in (’7, 8 ) . Other phases of the subin a high degree of p u r i f y and indicate that its manufacture i s s e p a r a t e d hydroseparators are worthy of further testing in ject discussed in bulletins from this station are the sweet potato a commercial potato starch factory. from lighter impurities by varias a starch producer ( l a ) , the Dilute alkaline sulJite solutions are llsed with ous modifications of the settling process. White p o t a t o starch determination of starch in the satisfactory results in the purification of white slveet potato ( l s ) ,and the granules are r e l a t i v e l y large (10) a n d s e t t l e r a p i d l y i n Potato starch. of potash upon the starch conwater. This property, together tent of sweet potatoes (14). with the fact that white potatoes contain very little coloring Attempts to establish a sweet potato starch industry in the matter, made the production of starch from white potatoes United States have not been successful in the past, owing, a comparatively simple process and resulted in the commer- among other reasons, to the low extraction and poor quality cia1 introduction of the starch at an early date. The German of the starch manufactured (3). A satisfactory method for potato starch industry, dating from 1765, was fostered by the production of high-grade starch, in which an alkaline Frederick the Great ( 5 ) , and in the United States a patent sulfite is added to aid in the removal of colored substances was granted for the manufacture of potato starch in 1802 (4). and other impurities, has nom been developed (3, 17). LaboSweet potatoes contain coloring matter which is difficult ratory studies have shown that this purified starch has some to remove from the starch. The granules are relatively very desirable qualities and may find a place in the textile small (11) and settle slowly in water; the starch is thus dif- and other industries (16). Tields by this process are comficult to purify by methods used in the manufacture of white parable to the average yields obtained by the German white potato starch, with the result that the starch which has been potato starch industry. Attempts are being made to inplaced on the market is generally gray and of poor quality crease further the percentage of starch extracted and to debecause of the colored compounds and other rionstarch sub- velop still further an industrial method of purification. stances which have not been removed from it. The experimental part of this paper deals with various This starch has been produced under the name “Brazilian phases of the settling process referred to above. Figure 1 Arrowroot” (6, 9) and was also manufactured in Japan a t an is reproduced here from a previous article (17‘) and illustrates early date (1). The province of Chiba-ken in Japan pro- the flow diagram for sweet potato starch manufacture. I n duced 294,000 pounds of sweet potato starch in 1884 and the extraction of white potato starch this process is simpli1,477,000 pounds in 1885. The production in this province fied to some extent, since the chemical treatment for the regradually increased to 29,968,000 pounds in 1899 (1). Thirty moval of color may be omitted. HonTever, in the experimillion pounds were produced in Japan in 1926 (15). The mental runs very dilute alkaline sulfite solutions were used extraction process used is similar to that employed in small- with excellent results. White potato starch is often purified by allowing the starch scale, white potato starch manufacturing plants ( 2 ) . This starch is chiefly used in Japan in the making of laundry starch, milk from the screens (Figure 1) to settle in large tanks. paper stock, textile stock, and tooth powder, and for foods After the removal of dirty water and refuse, the starch is again taken up in clear water and allowed to settle a second (1). Pioneer experimental work on sweet potato starch in the time. This cycle may be repeated until practically all of the United States was begun a t the South Carolina Agricultural pulp and other foreign material has been removed. This Experiment Station in 1895 (8). The starch was extracted modification was not fully successful when applied to sweet by the usual white potato starch process, and some of its potato starch (3) since the starch settled so slowly that it
INDUSTRIAL AND ENGINEERlNG CHEMISTRY
568
Vol. 26, No. 5
of such separators for this purpose. A tube 17 feet high and 1 foot in diameter was set up. The tube has an overflow outlet a t the top and a valve a t the bottom for the removal of starch. There are testing outlets a t regular intervals WASHED S W E E T POTATOES from the top to the bottom of the tube. I n some tests a 20c ALKALINE SULPHITE 4 G R I N DER inch tube and in others a n 8-inch tube was used. Since i I S 1 SCREEN there was very little friction on the wall of the tube, it would DULP ------XRCH appear that the diameter of the separator would have little c CENTRIFUGE 2 N D SCREEN or no effect upon the results obtained. Starch milk was inPULP IS' M I X I N G TANK ~ ~ p ~ \ ~ : troduced a t a point approximately 1 foot from the bottom of ). the separator, and the feed rate was adjusted so that the upTAILINGSIS'TABLE NEUTRALIZER i t ward flow in the tube was slightly less than the downward CONCENTRATER L N D MIXING TANK+ i settling rate of the major portion of the starch granules. DRIER TAILINGS-2 N D TABLE i Under these conditions, a large proportion of the waste WAREMOUSE SETTLING TANK WASHING TANK t 4 material, together with some starch, passed off in the over2 N D STARCH FILTER flow, and relatively clean starch settled to the bottom of the i c NEUTUALIZLR NEVTWALIZER separator and was drawn off a t regular intervals. The inflow 4 ). FILTET1 FILTER from the feed line agitates the rather concentrated starch t. ). DRIER STARCH mixture a t the bottom of the tank and aids in the separation f t DRIER WAREHOUSE of the lighter upward-flowing waste material from the lower t WAREHOUSE downward-flowing starch granules.' FIGURE1. FLOWDIAGRAM FOR SWEET Preliminary runs demonstrated that only a very slow rate POTATO STARCH MANUFACTURE of rise in the tube could be maintained with highly concencovery of the starch in a solid basket centrifuge combined trated sweet potato starch suspensions; for this reason the with the alkaline sulfite treatment and tabling process was concentration of the feed was reduced to approximately 8 per cent in the major portion of the tests conducted. The found to yield a high-grade product (17). results of a typical run with the 20-inch tube, in which the TABLING OF SWEET POTATO STARCH overflow was a t the 1Cfoot level, are given in Table I1 (run Commercial starch tables that are widely used have a 1) and in Figure 2A. To insure equilibrium in the separator slope of 1/32 inch per foot and are approximately 110 feet long. As the starch milk flows down the table, the heavy starch settles out and the lighter waste material flows off with the water. Data are given in Table I on the initial tabling rate of starch which was prepared from Nancy Hall sweet potatoes. Alkaline sulfite of the same concentration recommended in a previous article (Id)-that is, 0.005 N sodium hydroxide and 0.0025 N sulfur dioxide-was used in making up the starch suspension for the table tests and for all of the hydroseparator tests which follow. was contaminated somewhat by the colored compounds from the pulp and also contained an excessive amount of dirt, which settled a t the same rate of speed as the starch. Re-
c ;
-hJ-;
-
TABLEI. SWEETPOTATO STARCH TABLING FLOW 1-FT.
RUN
LENQTH OF TABLB Feet 45.0 65.0 65.0 90.0
90.0 90.0
90.0 90.0 90.0
CONCN. OF FEED
% 12 12 12 17 12 12 10 10 2
WIDTHOF
TABL PER ~ MIN. Gallons 0.5
0.6 0.7 0.95 0.9 1.15 1.0 1.1 2.1
STARCH IN OVERFLOW Trace None Trace
0.74%
Trace Excessive
None Trace Trace
The trace of starch in the overflow reported was negligible; that is, the capacity of the table had just been reached. A table 110 feet long was used in the tests and observations were made when only a trace of starch could be collected from the water which flowed beyond the length of table indicated. The figures reported represent initial rates of flow.
HYDROSEPARATION OF SWEETPOTATO STARCH As may be seen from Table I, a very large table area would be required in a sweet potato starch plant; also a considerable amount of labor would be required during the deposition of the starch and in its removal from the tables. If equipment could be devised to take the place of the tables, the manufacturing process would be greatly simplified. Since the starch would set,tle in water, it would appear that a continuous-operating settling tank or hydroseparator might have possibilities in the purification of potato starches. Accordingly, test runs were made to determine the efficiency
FIGURE 2. HYDROSEPARATION TESTSON SWEET POTATO STARCH A . Data from Table 11, run 1 E . Data from Table 11, run 2
a t the time of taking samples, the tube was filled approximately half full with the material to be tested. The rate of inflow was then adjusted to the required value and maintained there for a length of time sufficient to fill the tube a t least one and one-half times before samples were taken. Samples were taken a t the heights on the tube which are indicated in the table in order to determine the progress of separation in the tube. The average overflow in this test contained 1.82 per cent starch, and, since the concentration of the feed was 8.8 per cent, the approximate loss in the overflow was 20.8 per cent. I n other test runs a tube 12 inches in diameter and 17 feet in height was used. The concentration of the feed was reduced, and the rate of rise in the tube was increased with the hope that the increased rate of rise would produce a purer starch. The starch recovered, however, appeared to contain more waste material than in test 1in which a higher concentration and lower rate of rise were used. Results of a typical run under these conditions are given in Table I1 (run 2 ) and in Figure 2B. I n this run approximately 32.7 per cent of the starch was lost in the overflow. Data covering 1 The Dorr Company, Inc., New York, has developed commercial equipment of this kind.
INDUSTRIAL AND ENGINEERING
May, 1934
other tests which were made are given in Table 11, runs 3 and 4. TESTSO N SWEET POTATO STARCH TABLE11. HYDROSEPARATION DISTANCE FROM
BOTTOM OF TUBE Feet
STARCH CONTENT
AIR-DRY
WEIQHT
STARCH Qrams (ounces)
%
RUN 1: DIAM. OB T U B E 20 IN.; OVERFLOW AT &FT. LEVEL; CONCN. OF RATE OF RISE I N T U B E . 0 . 6 F T . / H R . ; STARCH LOST I N OVERFLO--, F E E D 8.8%; 20.8%; V O L . OF SAMPLE, 1000 c c . (33.8 FLUID OUNCES)
1.5 3.5 7.5 10.5 1 4 . 0 (overflow)
569
basket centrifuge, followed by tabling of the more concentrated suspension. A more economical plan would be to use this overflow as wash water for freshly ground pulp on the first screen. Some of the refuse would be screened out in its second passage over the screen, and the starch after collection in the solid basket centrifuge would be returned to the hydroseparator. To prevent an accumulation of refuse
19.2 l?.?
(6.72) (5.32) (1.92) (1.54) (0.637)
192 152 55 44 18.2
CHEMISTRY
J.D
4.4 1.82
R U N 2: D I h b l E T E R O F T U B E , 12 I N . ; OVERFLOW AT 17-FT. L E Y E L ; CONCN. OF RATE OF RISE I N T U B E 0.67 F T . / H R . ; STARCH LOST IN OVERFLOW, F E E D , 5.8%; 3 2 . 7.7 _ 0 : VOL. OF SAMPLE 2W CC. (6.8 F L U I D OUNCES)
13.0 11.4 10.1 6.9 3.8
2 6 8 14 17
RONS
CONCN. OF FEED
RUN 3 4
(0.455) (0.399) (0.353) (0.241) (0.133)
a
6.5 5.7 5.05 3.45 1.9
AND 4
STARCH LOST
CONCN. OF OVERFLON
%
RATEOF RISE IN TUBE F t . (cm.)/hr.
%
%
6.6 6.6
0.54 (16.5) 0 . 8 1 (24.7)
1.2.5 2.53
18.94 39.24
IN OVERFLOW
These tests indicated that a rate of rise in the separator of 0.5 to 0.6 foot per hour could be used without a n excessive loss, and that, under the conditions used, less dirt settled in the starch when the suspension was more concentrated. At least three zones are formed in the tube, a clear zone a t the top, a cloudy middle zone, and a semi-solid zone a t the bottom. I n the tests conducted, it would appear that the best results were obtained, in so far as dirt removal is concerned, when a fairly concentrated starch suspension was used and the feed rate was maintained a t a value sufficient to keep the overflow as near the bottom of the second zone as was possible without incurring too great a loss of starch. The color was removed to a high degree in all of the tests, presumably as a result of the longer contact of the starch with the alkaline sulfite solution. For some grades of starch it would appear that the hydroseparator alone might give a sufficient degree of purity, whereas for other grades one passage through the hydroseparator, followed by tabling, might be required.
0
2
4
6
8
FIGURE3. HYDROSEPARATION TESTSON WHITE POTATO STARCH A. B.
Data from Table 111. run 1 Data from Table 111, run 3
in the separator, it probably would be necessary to divide the overflow, sending a small part of i t directly to the tables. I n this manner a direct outlet for waste from the hydroseparator would be maintained.
RESULTSOF HYDROSEPARATOR TESTS The operation of the hydroseparator would be continuous and practically automatic. It would also occupy only approximately one-third of the floor space which is required for the tables, The tests conducted were encouraging and indicate that the use of hydroseparators in the purification of potato starches may prove to be practicable and is worthy of further testing in a commercial potato starch manufacturing plant.
ACKNOWLEDGMENT The authors are indebted to R. M. Kingsbury of this HYDROSEPARATION TESTSWITH WHITE POTATO STARCH division for analyses of the hydroseparator samples. They also wish to express their appreciation of the assistance renAn %inch tube was used in these tests. With a 6.95 per dered by A. J. Fischer and B. J. Larpenteur of the Dorr Comcent suspension and a 2.5-foot rise per hour [Table I11 (run pany in conducting the hydroseparation tests on sweet I), Figure 3 A ] the loss was excessive and, moreover, a con- potato starch. siderable amount of dirt settled with the starch. A higher LITERATURE CITED concentration, a lower feed rate, and a shorter tube gave ex(1) Anonymous, J . A g r . SOC.Japan,z 255, 40-2 (1902). cellent results [Table I11 (run 3), Figure 3 B ] . The starch (2) -4sakawa. Ihid.,2 317, 55-7 (1907). from this run was finished without tabling and was excellent (3) Balch and Paine, 1x11. ENQ.CHEM.,23, 1205-13 (1931). in appearance. (4) Biddis, John, U. S.Patent issued in March, 1802. TABLE111. HYDROSEPARATION TESTS ON WHITE POTATO (5) Eynon and Lane, “Starch: I t s Chemistry, Technology and Uses,” p. 2, W. Heffer & Sons, Ltd., Cambridge, England, STARCH 1928. [Diameter of tube, S inches; volume of sample 200 c,-. (6.8 fluid o u n c e s ) ) DISTANCE FROM WEIQHTAIR-DRY STARCH BOTTOU OF TUBE STARCH COXTENT Feet Grams (ounces) % R O N 1: OVERFLOW AT 14-FT. L E V E L ; CONCN. OF F E E D , 6.9570; RATE OF R I S E STARCH LOST I N OYEI
