ANALYTICAL EDITION
212 GROUP1 Add borax, 4% Locust kernel gels Others neg.
GROUP2 Add KOH Tragacanth, bright yellow Quince seed, stringy ppt.
GROUP3 GROUP4 Add borax, 4% Add phosphoric acid Galagum gels Hull emul. aissolves Schweitzer’s reagent
K~~~~~ turns pink Arabic dissolves in excess of Millon’s reagent
To 3 cc. of test soh. add KOH:
Vol. 3, No. 2
Locust bean may be distinguished from locust kernel by the addition of iodine solution. The former is colored purplish. Galagum may be distinguished from both of these by the stringy precipitate With Millon’s reagent. Each gum has its own peculiar method of going into solution in water. This peculiarity may be sufficient, if one often works with gums, to give an index as to the gum.
Irish moss gels, Agar s o h . clarifies
Acknowledgment
Notes Indian gum will give a pink coloration when boiled with either phosphoric or hydrochloric acid. When allowed to stand in either solid or solution form, karaya develops an acetic acid odor. Quince seed will give a stringy precipitate, which rises to the top with Schiff’s reagent. Hull emulsifier is rendered completely soluble by boiling with potassium hydroxide. Agar-agar solution is clarified by boiling with concentrated sulfuric acid. Irish moss smells like seaweed and is jelled by. potassium hydroxide. Tragacanth gives a bright yellow solution and stringy precipitate when boiled with potassium hydroxide solution.
The authors wish to thank Walter Weinberger for the use of some of his data in the above work. This work was done in the laboratories of the Department of Health of New York City. Literature Cited (1) Congdon, J. IND. END.CHEM.,7, 606 (1915). (2) Cook and Woodman, Ibid., 10, 630 (1918). (3) Patrick, U.S.Dept. Agr., Bur. Chem., BuZZ. 116, 26 (1914). (4) Revis and Bolton, “Allen’s Commercial Organic Analysis,” Vol. VIII, p. 193, Blakiston, 1914. (6) Weinberger and Jacobs, J . Am. Pharrn. Assocn., 18,34 (1929).
Accurate Air Separator for Fine Powders‘ Paul S. Roller NONMETALLIC MINERALS EXPERIMENT STATION, U. S. BUREAUOF MINES,N s w BRUNSWICK, N. J.
An apparatus is described for separating quantitaPortland cement, considerHE properties of microtively a 1-kg. charge of fine powder into a series of able quantities of each of the scopic powders2 are fractions beginning 0-2.5 microns in size. Except for vaTious fractions were regreatly dependent on t h e finest fractions, at or below 5 microns, where atquired. It was decided to retheir mean particle size and construct the air a n a l y z e r trition by the air current takes place in t h e case of soft distribution of sizes. An air powders, the particle sizes separated are very homopreviously described so that analyzer has been described a large amount of maherial geneous within the limits given by Stokes’ law. (6)for determining the perSeveral causes affect the rate of separation, b u t the could be started with, thus centage in a microscopic most important is that of t h e rate of air flow. Under conveniently p e r m i t t i n g a powder of the successivefracsimilar conditions t h e rate of separation is proportional large air flow and consequent tionsS 0-5, 5-10, 10-20, 20rapid rate of separation. t o t h e air flow. Depending on the latter, initial rates of 40,40-60, and > 60 microns. separation of the particle-size fractions have been made This apparatus operated on a Description of Apparatus up to 135 grams per hour at a flow of 140 liters per minsample of between 25 and 30 ute. grams. I n basing the design of the Continuous separation can be effected by t h e use of For rapidity and accuracy s e p a r a t o r on the air anaan offset separator tube with separate collection of the in analyzing a microscopic lyzer; it was p a r t i c u l a r l y oversize. At a 30-micron particle size cut of a Portland powder for its distribution desired to duplicate the selfcement powder, with an air flow of 500 liters per minute, of p a r t i c l e sizes, such a c i r c u l a t i o n of the powder t h e equivalent rate of feed was 5.4 kg. per hour, while small sample is d e s i r a b l e . charge as illustrated in Figt h e rate of recovery was 0.92 kg. per hour. The efficiency H o w e v e r , n u m e r o u s inure 1. This figure shows that of recovery, based on the maximum possible recovery s t a n c e s a r i s e when large the particles composing the in a run, was not in excess of 28 per cent. q u a n t i t i e s of each of the Dowder charge circulate in a Darticle-size f r a c t i o n s are clockwise dGection; a t the wanted in order to determine their particular physical and same time, the whole mass builds up in a direction opchemical properties; one may also require large quantities posite that of the incoming air. Thus the powder charge is below a minimum particle size of, for example, 30 microns. adequately prepared for the air separation. It was soon found In order to study further the properties of fine particles with the larger apparatus that the condition of self-circulation of natural anhydrite in connection with the retardation of of the powder charge was easily attained. Figure 2 is a photograph and Figure 3 a drawing of the air * Received January 20, 1931. Published by permission of the Director, separator which has been successfully operated over a long U. S. Bureau of Mines, and not subject to copyright. * A microscopic powder is defined as that portion passing through the period of time. Container C holds about 1 kg. of powder charge. It is a 200-mesh sieve. In making an analysis into six fractions, it has subsequently been found standard 7.6-cm. (3-inch) iron U-bend, with a radius of that there is no loss of accuracy, but rather that the time required may be 11.4 cm. (41/2 inches). At the bottom is a plug for emptying very considerably reduced by using the same separator tube for the 10this container conveniently. The hard rubber cam shown micron as for the 5-micron separation. The smaller separator tubes are in the figure rotates a t about 300 r. p. m. and operates against then employed at the higher rate of air flow for the successive fractions beginning 20 microns. Opportunity is also taken to remark on the microa ball-bearing bicycle hub that is fastened to container C. scopic examination of the fractions. It has been found time-saving and The rotation of the cam causes C to oscillate about the bearing more e5cient to conclude the dry dispersion of the grains with a grounded B. The lug L is riveted to C, and on the upward stroke rises platinum wire as described by a similar wet dispersion after the addition about 1 cm. The downward stroke is accelerated by spring of a drop of liquid.
T
April 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
S , via the arm R, acting on lug L. The complete oscillation of container C is abruptly halted in a horizontal plane by impact of lug L against the abutment A . The combined set of forces and reactions causes a circulation of the powder charge as illustrated in Figure 1. The air enters a t I by way of a rubber hose through a brass tube bent concentric with container C and into the end of which screws any one of a set of nozzles, 1 to 14 mm. in diameter. As already noted, the air flow is opposite in direction to that in which the powder charge builds up for selfcirculation.
Figure 1-Self-circulation ofiPowder Charge
Above bearing B of container C, a piece of rubber inner tube connects by a set of flanges with gasket to separator tube T, made of sheet monel and carefully grounded electrically. I n these experiments a set of separator tubes, 7.6, 15.2, 30.4, and 60.8 cm. in diameter, was used. These tubes revolve about a central post and can be successively brought into connection with container C. The cylindrical portion of separator tube T in these experiments was about 31 cm. tall and tapered over the same length (except for the 60.8-cm. tube that was tapered over 45 em.) to the 7.6-cm. (3-inch) nipple and flange leading to container C. G is a filter which is connected t o separator T by a brass gooseneck with flanged screw cap.
213
Taking 7 = 1.82 X a t 20" C. for air ( I ) , a table like Table I may be constructed for the velocity of fall in air against particle size. Table I-Velocity DIAMETER
Micvons
of Fall in Still Air of Spherical Particle Calculated from Stokes' Law RATIOVELOCITYOF FALL VELOCITYOF FALL TO DEKSITY FOR CEMENT ( p = 3.1) Cm./sec./p Cm./ sec
.
Table I has been constructed from Equation 1. By employing an air flow such that the velocity in the separator tube corresponds to a given particle diameter (Equation 1 or Table I), all particle sizes below the particle diameter in question will be separated. A series of separations may thus be effected so that a set of particle-size fractions is obtained. Experimentally, it was found on both soft and hard materials that the separations effected with the 1-kg. air separator described above were quite as homogeneous and accurate as with the air analyzer ( 2 ) . Figure 4 is a set of photomicrographs of a series of Portland cement fractions previously reported (2) and represents the results obtained on Portland cement with the new air separator. Figure 5 is a set of photomicrographs of a series of fractions, beginning 0-2.5 microns, obtained with the air separator on a finely ground anhydrite. Except for a small amount of undersize within a few microns, due principally to insufficient time for complete separation, the fractions are seen to be pure in the particle-size range delimited by Stokes' law. However, one fraction, 2.5-5 microns, of the anhydrite is contaminated with 0-2.5 micron particles, owing to attrition of the soft powder by-the-highvelocity air at the inlet.
Size of Particle Separated I n conducting a series of separations, the size of separated particle is controlled by the terminal velocity of the air in the separator tubes. Three other factors, the length of separator tube, the inlet nozzle size, and incomplete separation, may affect the accuracy of a fractionation. If the separator tube is too short, the desired equilibrium velocity of the air will not be attained therein, causing the separation of oversize; if the nozzle is too small, the grains may be disrupted into fine particles that are carried over by the air; finally, incomplete separation will show up as undersize in the next fraction. Defining the diameter of a grain as the arithmetic mean of the length, breadth, and depth (or more conveniently but less accurately, of the length and breadth), it was found by experiment (2) that the size of particle separated is related closely to the velocity of the air, in accordance with Stokes' law. Consequently, this law is used as a guide in effecting the various particle-size separations. Stokes' law; which was derived for small falling spheres, is: Figure 2-Aesembly of Air Separator
where
e,
= terminal velocityof fall in cm.per sec. in stationary
g = p = q = d =
fluid constant of gravitation in c. g. s. units density of particles in gram per cc. viscosity of fluid in c. g. s. units diameter of sphere in microns (1micron
=
Rate of Separation
Principally it isj(1) the rate of air flow that controls tho rate of separation. Other factors are (2) the size of inlet nozzle, (3) the concentration in the powder of the fraction separated, 10 -4 cm.) (4) nature of the powder, and (5) size of particle separated.
ANALYTICAL EDITION
214
'
Owing to the operation of factors (2) to (5), the relation between the initial rate of separation and the rate of air flow is somewhat variable. Numerous experiments indicate, however, that under similar conditions the two rates are directly proportional. Approximately the same proportionality applies to different powders and different particle sizes a t the same concentration.
Figure 3-Air
Separator, Vertical Section
Since the air velocity in the separator tube controls the size of particle separated, it is clear that the rate of separation is chiefly determined by the size of separator tube, and, indeed, roughly as the square of the diameter. The use of a series of tubes of diameters in the ratio 1:2:4:8 permits the separation of a corresponding series of particle-size fractions a t a convenient constant rate of air flow. With regard to the other factors that affect the rate of separation, it was found that a t a given rate of air flow there exists an optimum size of inlet nozzle for securing the maximum rate of separation of each of the fractions. For separations above 5 microns, a ratio of approximately 0.7 to 1 of liters of air per minute to nozzle diameter squared in sq. mm. seems to be best. At and below 5 microns, a suitable fine jet is necessary to overcome the high electrostatic forces between the grains. Corresponding t o the effect of decrease of concentration in the powder of a given fraction, the rate of separation falls off exponentially with time, rapidly a t first, then slowly approaching a zero rate, except, in the case of soft powders, for fractions a t or below 5 microns where the end rate coincides with a constant rate of attrition. Operation of Air Separator
Fine powders have been accurately separated following Stokes' law into all or some of the fractions 0-2.5, 2.5-5, 5-10, 10-20, 20-40, and 40-100 microns.
VOl. 3, No. 2
For separation of the 0-2.5 micron fraction of a fine anhydrite powder (density 3.0), a 60.8-em. separator tube and a 1-mm. inlet nozzle were used. At the required flow of 8.6 liters per minute there was a back pressure of 40 cm. of mercury. The initial rate of separation was 5.5 grams per hour, and the collected partiFles were less than 1 micron in size. Subsequently, as is true in general, the collected particle sizes approached the upper limit of the fraction. The end rate was constant a t about 3 grams per hour and corresponded to attrition of the powder charge by the high-velocity air jet, the product consisting of 0-2.5 micron particles. As already observed, a suitable fine jet is necessary a t particle sizes a t and below 5 microns in order to disperse the clusters of electrostatically charged particles. The same 60.8-em. tube, but with a 2-mm. instead of a 1-mm. inlet nozzle, was employed in effecting the 2.5-5 micron separation of the same powder. The rate of air flow was now four times that in the 0-2.5 micron separation. Correspondingly, the initial rate of separation was higher, 30 grams per hour; after a few hours there was a constant attrition rate of 17 grams per hour, the collected particles ranging from 0 to 5 microns in size. As in the 0-2.5 micron separation, the attrition products presented a markedly different physical aspect from the average product, appearing gritty instead of fluffed up. At a 0-5 micron separation of a Portland cement powder under conditions similar to the 2.5-5 micron separation of anhydrite, the initial rate of separation was the same, but the end rate approached zero and the collected particles were largely of the order of 5 microns in size. Thus there was practically no attrition of the cement powder. This difference in result with the cement powder as against the anhydrite, which is true in general, is due to the greater hardness of the former, 4 to 5 as against 3 to 3.5. Separations at 10 microns of the 5-10 micron fraction were made with both the 30.4- and the 60.8-om. tube, the optimum nozzle sizes being about 7 mm. and 14 mm., respectively. In the former case the initial rate of separation, a t an air flow of 35 liters per minute, was 35 grams per hour; with the larger tube, necessitating an air flow of 140 liters per minute, the initial rate of separation was correspondingly about four times as great, 135 grams per hour. The 10-micron and subsequent separations were terminated at an end rate of about 3 grams per hour as a matter of convenience, owing to the slow approach to a zero rate and to the fact that, indicating the complete absence of attrition, the particle sizes were almost wholly in the upper boundary of the delimited fractions. The 10-20 and 20-40 micron separations were made with a 15.2- and 7.6-em. tube, respectively, with a nozzle size, as for the 5-10 micron fraction of 7 mm.; also, a t an air flow four times as great, with a 30.4- and a 15.2-em. tube, respectively, with a nozzle size of 14 mm. Tke initial rates of separation of these fractions were approximately the same as that of the 5-10 micron fraction a t the same rate of air flow. Collection of Powder Fractions
Filtering was adopted as the most convenient means of removing theseparated fractions from the air stream. Closely woven cloth and canvas were tried but were found,unsuitable because of the high loss of powder through the comparatively large pores. A so-called white piano felt was quite satisfactory, except for a tendency to shed hairs, for collecting the finest fractions. As the quantity of powder fraction accumulated, the resistance to the air flow increased rapidly. To avoid frequent removal of the collected powder in order to relieve the back pressure, a large bag was required. A capacity of about 50
April 15, 1931
IhTDUSTRIAL AND ENGINEERING CHEMISTRY
0-5 Microns, 310 X
5-10 Microns, 310 X
20-40 Microns, 7 0 X
40-60 Microns, 70 X Figure 4-Portland Cement
0-2.5 Microns, 190 X
10-20 Microns, 190 X
2.5-5 Microns, 190 X
20-40 Microns, 45 X Figure 5-Anhydrite
216
10-20 Microns, 310 X
80-100 Microns, 7 0 X
5-10 Microns, 190 X
40-100 Microns, 45 X
ANALYTICAL EDITION
216
liters was found suitable. The bag was conveniently wired to the rim of the circular top of a commercial tin funnel which was soldered at the other end to the exit gooseneck of the separator tube (Figure 3). By cutting the end of the bag and closing with a removable clip, a convenient outlet for the collected fractions is provided.
The latter was analyzed (6) and found to contain 54.8 per cent by weight in the range 0 to 30 microns. Four separations were made on the successive residues from 1 kg. of powder with results as shown in Table 11. Table 11-Continuous NUMBER
u FILTER
Figure 6-Assembly for Continuous Separation
As shown by tests made with this material, a paper extraction thimble, could it be had of sufficiently large capacity, would also be satisfactory for collecting the powder fractions. Continuous Separation
Where large quantities of a powder are to be separated a t a given particle size, continuous separation is evidently desirable. A suitable arrangement for effecting continuous separation with the apparatus described above is shown in Figure 6. A 7.6-cm. sheet-metal pipe connects the 7.6-om. U-shaped powder container with an offset separator tube, The separated fraction passes up with the air to a bag filter, while the oversize falls into a hopper a t the bottom of the separator. Continuous feed, though not employed in these experiments, could be maintained by means of a flexible connection at point F of the container. Depending on the length of the 7.6-cm. connector tube and the angle 0 a t which it entered the separator tube, the time required to blow out the major portion of a 1-kg. charge varied considerably. Although the time required was increased thereby, it was found that the most efficient recovery of fine product wa6 secured when the angle 6 was n i l 4 e., when the connecting tube entered directly upwards into the separator tube. Consequently, this arrangement was used in the separations described below. With a 38.1-cm. separator tube, an inlet nozzle 25 mm. in diameter, and an air flow of 500 liters per minute, corresponding to separation a t a particle size of 30 microns, 905 grams of a 1-kg. charge of Portland cement powder were blown out of the container in 10 minutes; in other words, a rate of feed could be maintained of 5.4 kg. per hour. The separated fraction was analyzed (2) and microscopically examined. It was found to consist entirely of particles below 30 microns. The oversize, however, was badly contaminated with particles in the range 0 to 30 microns, indicating that only a fraction of this material in the original powder had been separated.
Vol. 3, No. 2
Separation a t 30 Microns of 1 kg. of Cement Powder
AMOUNT SEPARATED Grams 155 100 63 40
1 2
3 4
0-30 MICRONMATERIAL SEPARATED Residual Original
% 28 3 25.4 21.5
17.4
% 28 3 18 2
11 5 7.3
It is seen from column 3 of Table I1 that the efficiency of the successive separations based on the actual 0-30 micron content of the residues decreases with each successive separation. From column 4 it is seen that the first separation nets 28.3 per cent of the total 0-30 micron material in the powder; in successive separations the figure decreases until it is only 7.3 per cent for the fourth separation. I n a total of four Separations, 65.3 per cent of the original 0-30 micron material has been recovered from the powder. This result is perhaps to be expected in view of the large surface exposed to fines in the above 30-micron oversize in the cement powder. Since 155 grams of 0-30 micron powder were collected in 10 minutes, it follows that the rate of recovery during the first cut is 0.93 kg. per hour. I n what may be called the batch separation with set-up as in Figure 3, but otherwise under the same conditions of air flow and nozzle size, but with a 30.4-cm. separator tube, the initial rate was 0.73 kg. per hour. Consequently, it may be concluded that the rate of recovery of fine fraction is greater in the arrangement for continuous separation than in the batch process. I n another experiment on continuous separation of Portland cement powder, the conditions were the same as before but the inlet nozzle was much finer, 9 mm. instead of 25 mm. Eight hundred grams of powder were blown out in 11minutes, or the equivalent rate of feed was 4.4 kg. per hour. This is only about 0.8 of the rate with the larger inlet nozzle. The efficiency was also lower, as will be seen from Table 111, which is constructed in the same way as Table 11. Table 111-Continuous NUMBER 1
2 3 4
Separation Powder a t 30 Microns of 1 kg. of Cement
AMOUNT SEPARATED 0-30 MICRON MATERIAL SEPARATED Residual Original Grams ?6 % 110 2O:Q 20:o 17.1 13.7 75 51 14.0 9.3 29 9.3 5.3
From column 3 of Table 111it is seen that, as before, the efficiency decreases with each successive separation. The maximum efficiency is 20.0 per cent as against 28.3 per cent with the larger inlet nozzle. Since 110 grams were separated in 11 minutes, the rate of recovery of fine product is 0.60 kg. per hour compared to 0.93 kg. per hour with the larger inlet nozzle. I n contrast to the above result with a fine nozzle, tests with a 30-mm. inlet indicated a slightly higher rate of separation and efficiency of recovery. I n accordance with previous conclusions on the optimum ratio of air flow to nozzle size, an inlet approaching this diameter would seem most suitable a t the air flow in question, 500 liters per minute. Acknowledgment is made of the cooperation of C. M. Davis, mechanic, in the required constructions. Literature Cited (1) Millikan, Ann. Physik, 41, 763 (1913). (2) Roller, U. S. Bur. Mines, Tech. Paper 490 (1930).
