LANTS Flake Feeding Device for Solvent Extraction of Oil-Bearing Materials H. K. GARDNER', E. L. D'AQUIN, J. S. PARKER2, AND E. A. GASTROCK Southern Regional Research laboratory, New Orleans, La.
N OPERATIONS of the continuous solvent-extraction pilot plant ( 3 ) at the Southern Regional Research Laboratory, it was demonstrated that fluctuations in the flow of material into the extractor disturbed the equilibrium within it and had an adverse effect on the mechanical operation and extraction efficiency. Another fault was the escape of solvent vapor a t the point of material entry, which resulted in a fire hazard and excessive solvent losses. A continuous feeder was needed which would handle a variety of flaked oilseeds a t a uniform positive rate, maintain a seal to prevent the escape of solvent vapor to the atmosphere and a t the same time not break up the material excessively. A feeding device was constructed which fulfills these requirements and which is relatively inexpensive in first cost and maintenance. This feeder has been installed for some time as a permanent unit of the extraction pilot plant. It has operated satisfactorily with flakes prepared from cottonseed, peanuts, okra seed, and commercial rice bran over a wide range of conditioning treatments. The construction and principles of operation of this feeder are described and some data reported to show its operating characteristics in pilot plant runs with these oil-bearing materials. The feeder (Figures 1 and 2) is a screw-type, self-contained portable unit scale in size to serve the extractor which has a top capacity of about 200 pounds per hour of flakes (cottonseed). As shown in Figure 1, the unit comprises, essentially, a hopper ( A ) , a feed screw ( B ) equipped with a variable speed drive (H, I),a tubular vapor seal screw housing ( D ) , and an interdriven feed distributor screw (C), all mounted on a sturdy supporting framework of angle iron and plate. The feeder is driven by a a/,hp., 3-phase, 1750-r.p.m. motor ( G ) . Motor and controls are Class 1, Group D, explosionproof to conform to the requirements of the National Electric Code for use in flammable atmospheres. All Vbelts are of the static-conducting type. The main feed screw is a 4-inch diameter, standard pitch, righthand conveyor which extends out of the hopper and through an 18-inch section of standard 4inch pipe, which, when filled with the feed material, serves as a vapor seal. The variable speed mechanism is of the vari-pulley, sliding motor base type, and will permit a speed variation of about 2.5/1. The main feed screw maintains a continuous flow of flaked material from the bottom or throat of the hopper and through the tubular seal section for discharge into the extractor. In operation with materials of high oil content and relatively thin flakes, such as those of cottonseed and peanuts, this arrangement provides an adequate seal against the maximum pressure (0.40 inch of water) of the extractor vent system and thus prevents the escape of solvent vapor. But with
I
1 Present
address, Ft. Bliss, El Paso, Tex.
* Present address, Wesco Water Paints, Ino., Goodhope, La.
relatively coarse material or material of high hull content such as flaked okra seed, or with thick oilseed flakes, the screw can readily be adjusted to form a plug of from 0 to 6 inches in depth by simply sliding the screw and its shaft back longitudinally through the two self-aligning outboard bearings. The end portion of the tubular section, being then no longer occupied by the screw, fills with the material and continues to run full, as feeding by the screw continues. This compacted mass forms a moving plug that is relatively impervious to air and solvent vapor a t the 10%existing system pressure. The plug section would, of course, pravide the vapor seal, should the feed supply to the hopper be cut off or reduced sharply. As a safeguard against compacting, when very moist or sticky material is to be handled, it may be advisable to taper the interior of the tubular section approximately inch per foot opposite to the direction of the flow of the material. Rotational Speeds of the Feed and Distributor Screws A r e Calculated to M o v e Material in Opposite Directions at Equal Rates Located immediately above the main feed screw and with a clearance of only inch is the feed distributor screw, a stnndard 6-inch diameter, left-hand screw conveyor. The distributor screw serves to keep the main feed screw evenly filled with material and to prevent bridging. The flighting extends only within 12 inches of the rear wall of the hopper to avoid grinding of the flakes. This screw moves the material in the upper section of the hopper counter to the direction of its movement by the feed screw. It is chain-driven from the drive shaft of the main feed screw. The relative speeds of rotation of the two screws vere calculated to move the feed material toward opposite ends of the hopper a t exactly equal rates and thus to prevent the mnterinl from piling a t the rear of the hopper. The hopper is of 18-gage sheet iron and is built in the shape of s truncated prism to reduce the tendency for the +ked material to bridge the feed screw. It is approximately 17 inches tall and measures 12 X 36 inches a t the top. Its volumetric capacity IQ about 2.8 cubic feet, equivalent to 70 to 85 pounds of cottonseed flakes. This size was ample to take care of rate variations (surge) of flaked material delivered from the preparation plant. The arrangement requires hand loading of the hopper, but antomatic operation can easily be obtained by providing an extension of the distributor screw to permit a material run-around. The volumetric displacement rate of the feeder is controlled by varying the speed of the main feed screw within the limits of the speed variation range (2.5/1). Since the discharge rate in weight per unit time of a screw conveyor running full is theoretically directly proportional t o both the screw speed and the average bulk
2261
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
2262
Vol. 44, No. 9
ture content, thickness, hull content, etc., to determine to what extent each type of flake was disintegrated by the feed screw and by the action of the plug seal and to detect if any leakage of solvent dieve Percentageb of Material through Screens 1B 1C 2A 2B 2C 3A 3R 3C No.a 1A vapor through the material occurred in either case. 95.98 98,32 99.17 94.94 99.29 5 8 6 , 8 4 93.64 97.88 99.44 I n the three cottonseed runs, the method8 of ‘71.57 92.78 74.28 94.35 70.83 56.84 80.61 88.20 92.28 8 preparation used were: ( a ) whole cottonseed 47.86 71.70 44.56 62.91 50.11 30.80 54.99 71.50 14 68.26 27.04 23.82 40.56 29.59 41.03 34.11 15.80 29.94 39.75 20 meats, practically hull-free, of 8.6% moisture con15.21 13.15 13.23 10.53 16.06 9.30 14.02 40 16.58 7.17 8.32 4.60 5.40 5.87 7.00 5.84 8.16 60 4.06 8.29 tent (optimum, 5)prepared from wetted and equili3.15 5.57 3.74 3.59 5.62 4.46 4.75 5.76 3.00 80 brated delintered prime seed, were flaked t o 0.0112.72 4.59 3.16 2.86 4.65 3.50 3.61 4.66 2.42 100 3.53 2.61 2.33 2.92 3.09 2.31 3.85 2.36 3.80 120 inch thicknees; ( b ) typical mill-run meats of 8.6% 2.78 1.90 1.82 2.15 2.02 1.68 2.86 2.06 2.73 170 2.63 1.79 1.74 1.98 1.57 2.55 moisture content, prepared from the same cotton1.78 2.48 1.99 200 2.14 1.62 1.27 2.16 1.60 1.68 2.20 1.93 1.53 230 seed and consisting of the normal mixture of whole 1.93 1.51 2.03 1 23 1.55 1,50 1.39 1.88 2.OG 300 meats, fine meats, and fine hulls were flaked to 2 U. S. Sieve Series, with t h e exception of KO. 300 (aOO-mesh, 0.0018-inch opening). 0.013-inch thickness; (c) whole meats prepared *C Identification Calculated o n a n approximate oil-free basis. as in ( a ) mere flaked to 0.013-inch thickness and Whole cottonseed meats, 8.6% moisture content flaked of samples: 1 t o a p roximately 0.011 i n c h , 2 = whole a n d fine cottonseed meats, 8.6% moisture obntent, oven-dried a t 120’ F. to about 5.2% moisture flake$ t o approximately O.Oi3 i n c h , a n d 3 = whole cottonseed meats 8.6% moisture content, flaked to approximately 0.013’ inch, oven-dried t o approximatel; 5.2% moisture concontent. t h e material after passage through the feed screw alone: tent. 4 = original flakes: B The two runs with peanuts were made with e a f t e r passage through the feed screw a n d t h e 6-inch deep ~ a p oseal r plug. flakes prepared as follows: ( a ) Cracked kernels, prepared from U. S. No. 1 Farmers’ Stock peadensity of the material, then t o obtain any desired discharge rate nuts, were adjusted to 10.0% moisture content and flaked to 0.012-inch thickness; ( b ) cracked kernels from the same lot of the feeder, the corresponding approximate feed screw speed can be cspressed by the relationship as (a)were moistened to lO.O%, flaked to 0.012-inch thickness, and oven-dried a t 120’ F. t o 4.2% moisture content. All flakDischarge rate (lb./min.) ing was done on a set of 1-high horizontally opposed emooth R.p.m. of the feed screw = K X material bulk density (Ib./cu. ft.) rolls, 12 inches in diameter and 10 inches long, In a typical test run, 400 pounds of flakes were prepared; 200 The constant, IC, represents the average displacement in cubic pounds of these were passed through the feeder without the plug icet per revolution. The value for K of 0.0175 for this particular seal, and the remainder was passed through with the plug seal s r c m was determined by actual calibration with various feed maadjusted to a 6-inch maximum depth. To permit comparisons, terials having a wide range of bulk densities. The calculated the feed ficrew in each run was operated a t an identical speed (5 :crew speed may be Bet by regulating the crank of the variable r.p.m.), selected because it corresponded to the maximum feed specd device, or when more accurate setting is required, by using rate of the continuous extractor with cottonseed. the crank to regulate the speed of the jackshaft. For either purThe leakage of solvent vapor through the material was measpose, a calibration curve is provided which correlates the feed ured by means of a recording combustible gas analyzer, whose >crew speed against both the motor-base scale units and the jacksample inlet was located in the hopper precisely above the feed shaft speed. material. The instrument was adjusted t o sound an alarm a t a It must be emphasized that where the bulk density of the mahexane concentration in air equal to 60Yo of the lower explosive telial has not been determined in advance, preliminary checking limit. I n addition, the operator made routine observations to of the actual discharge rate is a requisite. This is done at the detect the characteristic odor o€the solvent. >tart by dumping a definite weight (20 or 30 pounds) of the material into the hopper and noting the time taken to discharge it. One or two such determinations n d l usually indicate whatever Special Wet-Screen Analyses Predict the Processminor setting adjustment is required t o establish the rate accuability of Materials in Continuous Solvent Extraction rately (&3yo).However, the actual leveling off rate should be The wet-screen analyses were determined by a method which checked periodically as a safeguard against possible fluctuations was developed a t this laboratory for evaluating and comparing in the material bulk density. the stability of oilseed flakes. This method has given consistent The top capacity of the feeder is about 200 parts per hundred and reasonably duplicative results and has proved to be one of (p.p.h.) of cottonseed flakes, having a bulk density of 26.0 the useful criteria for predicting the processability during continupounds per cubic foot. This corresponds to a feed screw displaceous solvent extraction of variously prepared flakes. I n this proment of 7.7 cubic feet a t 7.3 r.p.m. The lower capacity is about cedure, a 100-gram sample of the material to be tested is defatted 80 p.p.h,, corresponding to 3.1 cubic feet of material delivered a t in four consecutive washings with commercial hexane and then 4 feed screw speed of 2.9 r.p.m. The capacity can be varied above screened through a series of 13 successively finer U. S. Standard or belom these figures by the proper selection of sheaves and 8-inch diameter screens while immersed in the solvent. For fiprockets. convenience, the term “mesh” is used to designate the corresponding U. S. Sieve series numbers. Test Runs Were M a d e with Wet-Screened The sample is defatted by spreading it on a 300-mesh screen Cottonseed, Peanut, and Rice Bran Flakes and panning gently for 10 minutes in 2000 ml. of solvent contained in a 12-inch diameter evaporating dish. The screen is then lifted Since the particle size distribution or screen analysis of the feed out of the solvent until drainage is fairly complete. The wash mateiial is of the utmost importance in continuous solvent exsolvent or filtrate is poured off and retained. The washed and traction, webscreen analyses (that is, with the material immersed drained sample on the screen is given a second washing by panin solvent) were made of the flakes fed to and discharged from the ning again for 10 minutes in 2000 ml. of fresh solvent using the feeder in six test operations. Three of the runs were made with same evaporating dish and retaining the filtrate. The third and cottonseed flakes, two with peanut flakes, and one with cornmerfourth washings are done with 5 minutes of panning. The comcia1 rice bran, substantially free of polish. Wet-screening was sebined filtrate from the four washings contains practically all of lected in preference to dry-screening since it more closely approxithe through-300-mesh size solids or fines contained in the original mated conditions within the continuous extracLor. sample. This fraction is separated by filtering through a BuchThe flakes had been piepared under various conditions of moisTable 1.
P
Wet-Screen Analyses of Cottonseed Flakes before and after Passage through Feeder
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
September 1952 Table II. Sieve N0.O
Particle Size Data on Cottonseed Flakes before and after Passage through the Feeder Calculated from Wet-Screen Analyses (Table I) by the method of BrPwnell and Katz (2) 1A 1B 1c 2A 2B 2c 3A 94.60 94.12 93.01 91.68 94.16 95.40 95.94 0.0429 0,0483 0.0460 0.0411 0.0384 0.0355 0.0610 98.77 98.46 98.48 98.48 98.62 98 0 8 98.13 0.0357 0.0259 0.0331 0.0392 0.0314 0.0286 0.0505 100.00 100.00 100.00 100.00 100.00 100.00 100.00 0.0282 0.0276 0,0255 0.0239 0.0301 0.0209 0.0341 I
Particle sire e Bulk density, lb./ cu. ft. a
2263
26.00
27.80
27.75
27.10
28.75
29.00
27.21
3B 91.84 0.0348 97.96 0.0259 100.00 0.0206
3c 91.71 0.0358 97.94 0.0266 100.00 0.0210
29.35
30.25
U.S. Sieve Series, with exqeption of N o . 300 (300-mesh, 0.0018-inch opening). Particle siee expressed in inahes of diameter.
nal sample. The weights of these two fractions are combined to obtain the total weight of the through-300-mesh solids. The combined weight (approximately solvent- and oil-free) of all the individual solid fractions, including that of the fine solids washed out in the defatting operation, is obtained, and the percentage of the total is calculated for each fraction and expressed in terms of percentage of the material through the respective screens. The average particle size in each case was calculated from the wet-screen analysis by method No. 1 of Brownell and Katz ( a ) ZM using the formula D = -, where D is the average particle diam-
M
Zz
A B
---
Figure I. Isometric Sketch of Feeding Device
Feed hopper Feed screw C = Distributor screw D Tubular vapor seal section E = Extraclor feed chute F Observation port G Motor
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H Varispeed pulley I = Varispeed motor base J = Jackshaft assembly K = Fixed gear reducer L-L V-belts M-M = Roller chains N = Sprocketr(st~ur)
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ner funnel using a tared filter paper-Schleicher and Schull Co. No. 589 ( 4 ) or equivalent. A slight vacuum is applied to the filter flask to speed up the filtration. The separated fraction is then air-dried to remove the solvent and is weighed. The wet-screening analysis of the defatted sample is carried out as follows. The first screen, 5-mesh, is placed in a 12-inch diameter evaporating dish containing sufficient fresh solvent (approximately 2000 ml.) to cover the sieve to a depth of 1 inch. The sample is transferred to this screen, using a wash bottle to remove, the particles adhering to the screen. The screen is gently panned in and out of the solvent, until the amount of solids remaining on it does not appear to be further reduced. Then the screen plus solids are removed, placed in a forced-draft laboratory hood until the particles are free of solvent, and the solids are removed and weighed. The entire through-screen or liquid portion remaining in the evaporating dish is poured over the next size screen (8mesh) using a separate evaporating dish. The transfer is completed by washing the first dish with a small amount of hexane. Panning is again carried out until the amount of one-screen solids remains approximately constant. The screen is removed, as before, to dry and weigh the solids, and the through-portion is similarly processed through the remaining screens, which include eleven sizes ranging up to 300-mesh (Tables I and 111). Finally, the last liquid portion, the one passing the 300-mesh screen, is filtered in a Biichner funnel through a tared filter paper, and the weight of the ultrafine solids is determined after air-drying. This fraction represents the fine solids generated in the wetscreening procedure. It is normally only a small percentage of the fine solids separated in the preliminary defatting of the origi-
eter in inches, M is the weight fraction of a given particle size, d, and d is the average size of the given weight fraction, M , taken as the arithmetical average of the screen openings that pass and that retain the particle. Average particle-size figures were calculated separately for the solids retained on the 60-mesh screen, on the 300-mesh screen, and on the total sample basis (on- and through-300-mesh). The results of the wet-screen analyses on the cottonseed flakes before and after passage through the feeder are given in Table I, and the data on average particle size and bulk density in Table 11. Tables I11 and I V give similar data for the peanut flakes.
Table 111. Sieve NO.0
5 8 14 20 40 GO 80 100 120 170 200 230 300 a
U. S.
Wet-Screen Analyses of Peanut Flakes before and after Passage through the Feeder
Percentage6 of Materiala through Screens 1A 1B 1c 2A 2B 2c 93.96 93.84 89.06 86.89 82.96 89.02 67.11 76.42 75.40 74.11 80.33 80.49 51.90 44.41 55.83 57.75 66.55 55.40 31.41 32.86 31.14 34,58 40.42 35.33 17.40 22.86 23.83 17.55 21.43 24.43 12.91 18.33 19.60 12.10 15.33 18.78 13.76 17.39 11.65 18.25 10.87 17.61 11.29 10.56 17.32 17.14 13.21 18.02 16.92 10.94 12.89 17.73 10.15 17.06 16.69 9.76 16.77 10.65 12.49 17.54 16.67 9.71 16.73 10.63 12.44 17.50 16.63 10.61 9.66 16.69 12.40 17.48 12.35 17.45 16.56 10.59 9.57 16.56 Sieve Series, with exception of No. 300 (aOO-mesh, 0.0018 inch
opening). b Calculated on an approximate oil-free basis. Identification of samples: 1 = Cracked peanut meats 10.0% moisture content flaked to approximately 0.012 inch. and 2 = crrtcded peanut meats 10.0% ’moisture content, flaked to approkirnatey 0 . 0 1 2 inch, and oven: dried to approximately 4.2% moisture content. original flakes: B = the material after passage through the feed screw alone; C = after passage through the feed screw and the 6-inch deep vapor seal plug.
The through-300-mesh fraction represents the percentage of exceedingly fine or powderlike solids. This fraction has been shown in a previous publication of this laboratory ( 4 ) to be a fairly reliable measure of the amount of so-called fines encountered in continuous solvent extraction. Since the control of fines is an important problem in commercial processing ( I ) , the amount of this fraction generated by the feeder is of primary significance in evaluating the unit for this application. It is apparent from the tables that the mechanical action of the feed screw and that within the seal plug broke up the original flakes into smaller flakes as well as very small particles t h a t ap-
2264
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No.
9
13.0% on the original bran to about 15.3y0 after passage through the feeder using the vapor seal plug.
Feeder Is SelCContained, Portable, and Economical The feeder has been used in 20 experimental continuous ,mivent-ext,raction runs of from 8 to 30 hours each t o feed 21,000 pounds of cottonseed flakes, 22,800 pounds of peanut flakes, 3200 pounds of okra seed flakes, and 6600 pounds of rice bran. Thr successful performance of the device with these materials is evidenced by the relatively small amount of fines produced, the absence of any detectable leakage of solvent vapor through the vapor seal mechanism, the eliminating of bridging of material acro.;j the feed screw, and the minimization of lumping or packing of material in t,he vapor seal section. The unit, is self-contained, port,able, relatively inexpensive in Figure 9. Pilot Plant Flake Feeder Used at Southern Regional first cost, and is economical to operate from a standpoint of power Research Laboratory in Solvent Extraction of Cottonseed and requirement and maintenance. The design provides improvcPeanuts ments over conventional scren- conveyor-type feeders in that it utilizes the material to form a positive variable length plug type proach the size of fines (through-300-mesh), as n-ould be expected. But with none of the feed materials was an excessive amount of seal without comminuting the material excessively and eliminate; the bridging of material above the metering screw. It is suffines generated. Even with the vapor seal plug the percentage ficiently versatile for use in a wide variety of feeding applicationi increase in the fines content after passage of the material through the feeder was not excessive. The random variations in the reother than with oilseed materials. A single complete set of shop drawings covering this unit can br sults reported in the tables for per cent fines are sufficiently large to obscure the exact quantity of fines actually produced. HOW- made available on request to anyone interested in duplicating it> construction or scaling up for a larger capacit'y application. ever, the authors feel that the determinations are accurate enough Operating data obtained wit'h the pilot plant model indicate to justify the conclusion that the percentage of fines generated in that wit,h minor revisions as noted, scaling up the unit to semi- or all instances would not be considered excessive. full-scale commercial size should be entirely feasible, with little or no sacrifire in the operational features claimed. To Table IV. Particle Size Data on Peanut Flakes before and after Passage provide automatic feeding of the hopper. through the Feeder the distributor screw would have to bc Calculated from aet-screen analyses (Table 111) b y method of Brownell a n d K a t s (a) ext,ended through the side of the hopper Sieve No.a 1A 1B 1c 2A 2B 2c so as to permit' a material run-around. In BO 70 On-screen 80.41 81.67 81.22 87.09 87.90 84.68 regard to the seal mechanism, as a safeParticle size 0.0550 0.0403 0,0453 0,0465 0.0441 0.0399 300 Yo On-screen 82.67 83.45 83.44 89.42 90.43 87.66 guard against compacting where sticky or Particle size * 0,0460 0.0423 0.0385 0.0340 0.0372 0.0337 moist materials are to be handled, it 300 Yo On-, t+o;gh 100 . O O 100,O 100.00 100.00 100.00 100.00 Particle size 0,0091 0,0094 0,0092 0,0128 0.0134 0,0110 would be advisable to t,aper tjhe interior of Bulk density, lb./cu. it. 30.50 33.30 36.30 23.50 22.50 22.50 the tubular section approximately inch a U.S. Sieve Series, with,exception o f , S o . 300 (3OO-mesh, 0.0018 inch opening). per foot opposite to the direction of t,he * Particle size expressed in inches of diameter. material flow. I n any case, the depth of the plug for adeauate scaling ~- required n-ould be varied to suit the material ant1 the static pressure of the system against which the plug is to Lumping of the material in the seal section was not a problem since when it occurred the lumps broke up directly on emergence seal. In particular applicat,ions of the feeder, nrhere it is newsary or desirable to reduce and control the size of the dischargfrom the feeder or on contact with the solvent in the extractor. ing material, such as in processing soybean flakes in percolation Some breakage of the large cottonseed flakes occurred in passtype solvent extractors, t,he screws would be fitted with breakw age through the feeder in each run and was consistently somepaddles as required. what greater where the plug seal was used. Without the plug, practically no fines mere produced in feeding the flakes prepared Acknowledgment under the conditions designated as Nos. 1 and 2. X i t h the plug, only with flakes KO. 2 was a small percentage (0.40%) of fines The authors gratefully acknowledge the assistance of Lcali generated. The heated (dry) flakes, No. 3, produced the largest Katz in the prepara.t'ion of the isometric drawing and of the Meamount of fines (about 0.80%) with or nithout the plug. chanical Service Division for t,he construction of the feecicr With peanut flakes some breakage of the large flakes occurred in passage through the feed screw alone, but practically no fines Literature Cited were produced. Khere the seal plug wa5 used, the production of (1) Bonotto, Michele, Oil & Soap, 23, No. 9, 297-9 (1946). fines from flakes KO. 1 was negligible, but with flakes No. 2 (2) Brownell, L. E., a n d Katz, D. L.. C'hem. &no. Piogress, 43,S o . 10.546 (1947). (heated) it was appreciable, even though the heated flakes were (3) Gastrock, E. A., and D'ilquin, E. L., Oil M i l l Gar., 53, No. 4 , definitely more stable (more resistant to disintegration into par13-21 (1948). ticles smaller than 40-mesh) than the unheated (columns 1A and (4) Graci, A . V., Crovetto, A . J., P a r k e r , J. S., and Reuther, C. G.. 2A, Table 111). This increase, however, is relatively small when J.Am. Oil Chemists' Soc., 29, No. 2, 71-3 (1962). (5) Reuther, C. G., Jr., Westbrook, R. D., Hoffman, Wr. H., Jr., Vix, expressed as a percentage of the fines contained in the original H. L. E., and Gastrock, E. A., Ibid., 28, 146-9 (1951). flakes. R E C E I VED for review M a r c h 10, 1952. ACCEPTEDJ u l y 1, 1922. I n the test with the commercial rice bran, which had a moisture One of t h e laboratories of the Bureau of Agricultural a n d Indiistrial content of 11.0% and a bulk density of 25.0 pounds per rubic Chemistry, Agricultiiral Research Administration, U. S. Department o f foot, the fines content (through-300-mesh) increased from about -4griculture.
