Figure 1.
Spin mill process
HERBERT G. TANNER Fort Detrick, Md.
New Type of MiII for Refined Chemicals
A
MACHINE for pulverizing fine chemicals has been developed (5’). which is novel in construction and probably in principle of operation. Penicillin. dried egg albumin, bismuth, mol\ bdenum sulfide, color lakes, silica gel. and many
salts have been disintegrated to powders in which half or more of the particles have diameters of 1 to 5 microns. The machine is compact, the power consumption is low (2 kw. under maximum operating conditions), and the production
Editor’s Note. Many of the comparative data which were established in the development of this equipment are classified. Since some of them were almost in the realm of the spectacular, the Chemical Corps engaged a well known group of engineers thoroughly familiar with the milling field, to make an exhaustive survey of all known mills adapted to fine grinding and to make comparative performance tests with the spin mill of those mills that appeared pertinent. The editors have been advised by the Chemical Corps Research and Development Command that the data confirm the author’s findings and the spin mill was rated as an outstanding tool to assist investigation of the finely divided state.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
capacity ranges from 0.5 to 3 pounds per hour, depending upon the properties of the material being ground. Fine grinding is known to improve the rate of solubility-of penicillin, for example-the strength of color lakes, the effectiveness of dusting powders, and the thermal emission of electrons from oxide coatings. These properties, however, can be impaired if the grinding causes degradation by heating or oxidation, or by conraminants such as bearing grease or foreign matter eroded from balls or hammer blades. A distinguishing feature of the mill described is the ability to pulverize without disturbing the intrinsic properties of the processed material. Thus far contamination due to erosion has been too small for measurement. Figure 1 presents schematically the general assembly of the mill and ancil-
lary equipment. Gas is bled a t a rate between 0.5 and 1.5 cu. feet per minute, and mixes with 40-mesh granular material or smaller, delivered slowly by a feeder. The mixture of solids and gas falls into the mill on one side of a rotor, which is simply a disk, about 6 inches in diameter, rotated rapidly within a housing of inside diameter 0.040 inch larger than that of the rotor. The speed of the rotor is adjustable, and a cooling jacket, not shown, surrounds the housing. The mixture of solids and gas is in turbulent flow when passing through the small annular space between rotor and housing. The effluent may be conducted to a small settling trap, and thence to a bag filter. Although particles have diameters of a few microns, their concentration in the gas is so high that the trap collects about SOY0 of the material. A vacuum cleaner paper bag is useful for the end filter. For small test samples, a paper Soxhlet extraction thimble may be used to collect the solids. The pressure of the discharged gas is approximately 1 inch of water. The chief function of the gas, which is mixed with the input solids, is to transport the solid particles through the grinder and into the collector. Helium, air, nitrogen, carbon dioxide, Freon, and sulfur hexafluoride have been tested ; the pulverizing effect was not changed significantly by the choice of gas used. The gases of lower density consumed less power and caused less heating. A tank of compressed nitrogen was a convenient all-purpose source of gas, although clean, dry air was satisfactory for substances insensitive to oxidation. Figures 2 and 3 show a mill engineered and built under contract by the Specialized Instrument Co. (Spinco Division, Beckman Instruments, Inc., Palo Alto, Calif.). The rotor in this machine is a n anodized aluminum disk 1 inch thick
Figure 3.
Figure 2.
and 6 inches in diameter, which may be driven by three 1.5-hp. motors to a maximum speed of 42,000 r.p.m. (Three motors were used instead of one to meet a special volume requirement for the machine.) Critical speeds were subdued almost to extinction. Figure 4 is an end view, with cover plate removed to show the narrow annular space in which the grinding occurs. The interior surface of the housing is smooth, and is chromium-plated. The inlet and outlet ducts of the grinding compartment are tangentially mounted near the periphery of the rotor to obtain pumping action during operation. This action minimizes holdup in the machine, and reduces the pressure
Spin mill opened for cleaning
Figure 4.
Spin mill
within the machine to slightly below atmospheric. No dust, therefore, escapes from the machine during operation. The average time a given particle remains in the machine depends upon the feed rate, but is only several milliseconds. The material to be ground is fed into the mill with about 1 cubic foot of gas per minute, through a very narrow annulus, 1 inch long. Holdup is negligible. The linear velocity of this transverse flow of gas is calculable and, as the velocity is known, the time required for the average particle in the gas to travel 1 inch is readily calculated. If the housing is cooled by a refrigerant, the temperature of the effluent may be as low as -36' C.
End view, showing grinding zone VOL. 49, NO. 2
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Experimental Data During early development of the mill, rotors made of steel and laminated fiber glass were tried. One steel rotor was lapped and polished; the peripheral surface of another was intentionally roughened by tool marks and coarse grit. Except for smoothness, all rotors had the same dimensions. As both rotors gave substantially the same results, the action was not predominantly that of abrasion. The highly polished rotor lost some of its luster, however, when a quantity of Iceland spar was ground. The Spinco machine was designed to allow tests on rotors differing slightly in diameter, and ranging in thickness from 1 to 3 inches. Thickness made very little difference in size reduction; the factor of clearance between rotor and housing was more significant. A decrease in clearance from 0.060 to 0.020 inch (measured with the rotor stationary) shifted the particle size spectrum to lower values-at a constant rotor speed of 700 feet per second, a clearance of 0.060 inch gave for one material a size spectrum ranging from 2 to 30 microns; a t a clearance of 0.020 inch, the size spectrum ranged from 0.5 to 15 microns. Change of rate of solids feed in one instance from 4 ounces to 2 pounds per hour did not alter the size spectrum of the product appreciably, and no difference in performance was observed when the transport gas rate was varied between 0.5 and 2 cubic feet per minute. The predominant factor altering the size spectrum of the product was rotor speed. The higher the rotor speed, the lower the mass median particle diameter of the product, but speeds above 500 feet per second caused only a small percentage change.
Theory of Pulverizing Action
As the width and the smoothness of rotors were minor factors with respect t o particle size reduction, impact and abrasion were considered to be secondary effects. If laminar flow existed in the annular space between rotor and housing, as some authorities claim, particles would be broken by shear. However, the easily calculated shear force on a particle under such a circumstance is insignificant compared to the known shear strength of many materials pulverized by this mill. Breakage of the particles by collision with each other and with the surfaces of the grinder is one explanation ( 3 ) for the grinding action of air-jet mills, where steep velocity gradients exist in the gas phase. One would expect grinding action to be closely related to the Reynolds number, and to be especially pronounced a t the transition from lam-
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inar to turbulent flow. Data, presented below, do not support the collision theory as applied to this machine. I t was assumed that turbulence produced by a rotor moving close to a stationary wall might be of different character from the turbulence existing when a gas flows through a tube. Radial and transverse forces exist in the mill. but are absent in a tube. The bold assumption was made that innumerable cyclonic microeddies characterize the turbulence, and cause entrapped particles to spin. These eddies would be accentuated by the transverse flow of gas and the high tangential velocity of parcels of gas evolved from the rotor. This hypothesis allows quantitative exploration. The eddy spins, and, therefore, the particle spins should be related to the peripheral speed of the rotor; the spin speed required to rupture a given particle should be predictable from the physical properties of the solid. Spin would develop stress in tension in a particle. If the rate of spin of a particle were gradually increased, the internal stress would eventually become equal to the tensile strength of the material. At this critical speed the particle would rupture-an effect comparable to the “explosion” of a flywheel rotated too rapidly. The stress in tension developed within any rotating homogeneous solid can be calculated if the density, shape, axis, and rate of rotation be known ( 7 ) . For a spherical, homogeneous solid, the internal stress is related to the rate of spin by the equation:
where S is the stress on the supporting cross-sectional plane containing the axis of spin, p is the density, and Vis the rate of rotation expressed as the peripheral or surface speed, the terms being expressed in consistent units. For the following mixed units, where SI is in pounds per square inch, p l is in grams per cc., and VI is in feet per second Equation 1 becomes :
When SIis assigned a definite valuein this case the tensile strength (ultimate) of the material concerned-and the shape is spherical, the surface speed, VI, a t which rupture will occur becomes a predictable value. Operationally, a test of the spin hypothesis involves the simple procedure of passing a suitable material through the mill, removing samples at various levels of rotor speed, and analyzing the samples for particle-size distribution. The ideal test material would be comprised of spherical particles to guarantee but one axis of spin, be homogeneous, stress-
INDUSTRIAL AND ENGINEERING CHEMISTRY
free initially, and of known density, and should have a known tensile strength independent of size. Such a material has not been found, but sodium chloride offered a reasonable approximation. Sodium chloride forms cubic crl-stals whose predominant planes of fracture include the three axes of symmetry. The fragments formed on fracture are by no means spherical, but they are not laminae or needles. The tensile strength of single-crystal sodium chloride has been the subject of numerous publications. Although its tensile strength is not a single-valued property: fair agreement exists for the most probable breaking strength. Data published by Schonfeld ( 4 ) have an average value of 227 grams per sq. mm. (323 pounds per square inch). If the latter value be substituted in Equation 2, with 2.16 grams per cc. as the density of salt, the predicted spin speed (surface velocity) required to break a spherical particle of salt of any size is 211 feet per second. This must be regarded as a n approximation, because Equation 2 is not strictly applicable to the experimental limitations. However, the calculated value predicts an approximate minimum rotor speed at which sodium chloride may be expected to undergo disintegration according to the spin hypothesis. With this information available, a quantity of dried, reagent-grade sodium chloride was crushed in a mortar until it passed through a 40-mesh sieve. The mass median particle diameter was 250 microns. I t was fed into the mill at a uniform rate along with dry nitrogen. The rotor speed was changed from time to time, and product samples were collected during periods when the rotor speed was kept constant. The time of operation of the mill during collection of each sample was approximately 10 minutes, furnishing samples of about 50 grams each. The holdup in the machine never exceeded 0.5 gram. The “circulating load” in the machine was, therefore, practically nil at all times. The feed rate was 5.0 grams per minute; the transport gas rate, 1 cu. foot per minute. The exhaust gas temperatures of the mill were between 3’ and 6’ C. for nitrogen. The difference in power with and without solids fed into the mill could not be detected at any rotor speed. Each sample was carefully analyzed for particle size frequency by the optical count method. These frequencies were converted to weight percentages for calculation of the mass median diameter. The weight percentages for the various size particles in each sample were subtotaled into three groups representing coarse, medium, and fine fractions, The coarse fraction included the total weight percentages of all particles having diameters above 60 microns; the medium, below 60 and above 10 microns; and the fine, 10 to 0.5 microns. T h e
Table 1.
Particle Size Reduction of Sodium Chloride as a Function of Rotor h e e d
Sample No.
Rotor Speed (Peripheral), Ft. /Sec.
Wt. % Fine Fraction
Wt. % Coarse Fraction
Reynolds No.
Nitrogen as Transport Gas 2 3 4 5 6 7 8 9 10 11
0 31 94 157 189 220 251 283 314 393 471
1 2 3 4 5 6 7 8 9
31 94 157 251 314 393 471 550 628
1
98 92 65 59 43 51 0
0.0 0.3
0.7 0.5 1.1 1.2 31.0 85.0 67.0 77.0 90.0
0
1790 2154 2508 286 1 3226
0 0 0
SF6 as Transport Gas 0 0 0 0.2 0.2 17.0 25.0 41.0 83.0
weight percentages for the coarse and fine fractions of each sample are presented in Table I, with corresponding rotor speed data. The mass of particles smaller than 0.5 micron in diameter was not directly measured; it was so small that it was combined with the mass percentage of the 1-micron particles. The coarse fraction diminished gradually up to a rotor speed of 251 feet per second, where it dropped suddenly to zero. The mass in the fine fraction rose slightly during this same interval, but rose suddenly at 251 feet per second. The gradual decrease of the coarse fraction for rotor speeds up to 251 feet per second, unaccompanied by a corresponding increase in the fine fraction, is interpreted as normal fracture of coarse particles resulting from tumbling, collision, and abrasion while passing through the machine. Sharply, a t a rotor speed of 251 feet per second, all particles in the coarse fraction disappeared coincidentally with a sudden rise in the weight percentage of the fine fraction. These data force the conclusion that a fundamental change in the mechanism of disintegration occurred a t or near a rotor speed of 251 feet per second. The rotor speed of 251 feet per second is only 20% above the calculated speed of 211 feet per second. Considering the assumptions that have been made, and that particle spin speed lagged behind the rotor speed, the closeness of agreement of observed and predicted values may be only fortuitous. By coincidence, in this experiment the Reynolds number for the nitrogen,
95 93 90 88 96 0 0
3,000 26,000 33,000 41,000 49,000
0 0
Direct application of his equations to a compressible fluid of relatively small thickness, between a high-speed rotor and a stationary surface, does not appear warranted at this time. However, his concept that ordinary eddy turbulence may be only one of various kinds of turbulence may explain why the observed pulverizing action of the mill was not proportional to the Reynolds number. Data, which may be published later, show considerable differences in the properties of materials pulverized by various means. One interpretation is that fracture of particles by tension yields a cleaner surface than fracture by compressive or shear forces. The spin hypothesis presented above is tentative. Additional experiments, with other substances of known particle strength, are indicated, along with fundamental exploration of turbulence theory. Currently, the spin hypothesis offers the best explanation for the data available. Accordinelv. the machine is called the spin mill. A deduction from the spin hypothesis is that a particle, having, for example, a diameter of 60 microns and a peripheral speed of 211 feet per second, must rotate approximately 20,000,000 r.p.m. The mill can fracture particles a t least as small as 10 microns in diameter. At the maximum available rotor speed, a 10-micron particle of adequate tensile strength may acquire a rotational speed exceeding 500,000,000 r.p.m. “
moved by the rotor a t 251 feet per second, is close enough to the region of transition from laminar to turbulent flow to arouse suspicion that the transition to turbulent flow caused the grinding action. T o check this possibility, the salt experiment was repeated with sulfur hexafluoride as the transport gas. The clearance between rotor and housing was increased from 0.020 to 0.030 inch. Because sulfur hexafluoride has a higher density and lower viscosity (0.0145 centipoise, 2) than nitrogen, the transition from laminar to turbulent flow would occur at a lower rotor speed than with nitrogen. The data for the sulfur hexachloride experiment are presented in Table I with calculated Reynolds numbers. I t is apparent that the comminution started suddenly, as before, but a t a Reynolds number remote from the region of transition from laminar to turbulent flow. The sudden onset of disintegration occurred between 314 and 393 feet per second, which is even farther from the flow-transition point than formerly. The greater clearance between rotor and housing probably prevented closer agreement with the data for the nitrogen experiment. However, it is clear that the pulverizing effect is not closely related to the Reynolds number, and the fact that breakup occurred near the predicted rotor speed lends additional support to the particle-spin hypothesis. Experiments with incompressible fluids between two coaxial glass tubes, the inner one being rotated, led Taylor (6, 7) to conclude that there may be more than one type of turbulence.
I
,
Acknowledgment
The author is appreciative of the valuable suggestions contributed by Edward Pickels and staff members of the Specialized Instrument Co., and acknowledges the very generous and helpful assistance of numerous colleagues a t Fort Detrick. literature Cited
(1) Biscoe, J., Pickels, E. G., Wyckoff, R. W. G., Rev. Sei. Instr. 7, 296 (1936). ( 2 ) General Chemical Division, Allied Chemical & Dye Corp., 40 Rector St., New York, N. Y . , private communication, 1955. ( 3 ) Perry, J. H., “Chemical Engineers’ Handbook,” 2nd ed., p. 1931, McGraw-Hill, New York, 1941. (4) Schonfeld, H., 2. Phys. 75, 422 (1932). ( 5 ) Tanner, H. G., U. S. Patent 2,552,603 (May 15, 1921). (6) Ta lor, G. I., A o c . Roy. Sod. A102, 541 6922-23); 164, 15 (1938). ( 7 ) Taylor, G. I., Green, A. E., Ibid., 158, 499 (1937). RECEIVED for review April 9, 1956 ACCEPTED August 17, 1956 Division of Industrial and Engineering Chemistry, 129th Meeting, ACS, Dallas Tex., April 1956. VOL. 49, NO. 2
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