J.
m.
D.OTSON
EXTENDING T H E RANGE OF JET MILLS
Uses: Reduction to submicron particle sizes of hard-to-grind substances such as insecticides, pharmaceuticals, resins, and ores
Advantages:
-No
moving ports in grinding area -Classification and air conveyance of product -Combination of grinding with chemical reactions --Isolation of grinding operations -Ease of operation under pressure
mills are generally regarded as FIuid-energy specialty ' millsor jetwhich require excessively high
1 4
APPROXIM
amounts of power; therefore they are used mostly where extremely fine particle sizes are required. However, the work reported here shows that efficiency of jet mills is influenced substantially by size, type, and operating variables and that it is reasonable to consider some types for uses other than fine grinding. For example, one mill showed an efficiency comparable to that of a ball mill. On the other hand, another proved impractical except for very mild grinding. The literature contains insufficient information for comparing jet mills with conventional mills, on the basis of new surface generated per unit of energy ex*ended. Where comparisons are made, different reductions in particle size are not taken into account (7, 6). Consequently energy requirements listed, especially far small throughputs, are sometimes 10 to 50 times more than that required by conventional mills.
BALL Mlll Mlll BALL
TFCT 4 d TEST
Experimental Scheme
Material ground was silicon metal which has a Mohs hardness of 7 and a specific gravity of 2.4. Grinding fluid was room temperature air, inert gas, or nitrogen. Two types of commercially available mills were tested, one with a circular race and one with an elliptical raceand using a test apparatus, a high velocity gas stream was injected into a fluidized bed. The ball mill used for comparison was a Hardmge conical type, size 4 feet by 16 inches, which was operated in an air-swept closed circuit grinding system with a recycle to feed ratio of about 0.5 to 1. For the mills having an elliptical race, two sizes were tested-the 0405 size has nominal internal race diameters of 4 and 5 inches for down-stack and up-stack, respectively, and the 0202 size has a race inside diameter of 2 inches (Figure 1). The mill with the circular race has a flat cylindrical grinding chamber with a peripheral diameter of 8 inches and a volume somewhat comparable to the 0202 size (2). Product and spent grinding fluid are discharged centrally. In the fluidized bed equipment, the column consisted of glass pipe, 4 inches in inside diameter and 10 feet long, which was equipped with a conical perforated platetype gas distributor through which was fed 20% of the total gas to the column. The remaining 80% of the gas was fed at sonic velocity through a concentrically located nozzle (Figure 3). Specific surface areas were calculated (Figure 2) using simplifying assumptions: particles were spheres having a shape factor of 6, and diameter was equivalent to the average sieve opening. For the -400 mesh fraction, average used was 17 microns. For fine grinding done in the 0202 mill, the -400 fraction was further divided into 4 subfractions as analyzed by a Bahco centrifugal-type classifier. Based on these simplifying assumptions, surface areas reported are minimum values. However, they are on an equivalent basis and therefore valid for comparison purposes. Energy requirements were calculated from the adiabatic expansion energy dissipated in reducing the volume of gas from the initial pressure shown to that of the mill. Pressure of the mill was essentially atmospheric. Power for the ball mill was calculated from actual electrical measurements on the motor; therefore mxiliary equipment such as blower and screener, about iO% of the mill power, was not included.
0.01 0.1
1
IO 20 50 80 CUMULATIVE % UNDERSIZE
99.5
7999 79.99
Figure 2. Particle-size distribution datafor tha 0405 mill, wing 705mirronfced at a rate of 675poundrpcr hour and room-tempnotwe air asgrinding gas. In tert 3, gnsprcssure kept at 60p.s.i.g. resulted inn pow rote of 63 cubic feet pn minute at sonic velocity. In tcsf 4, by reducing gas prcssure to 32.5 p.s.i.g., with a corrcsponding reduction in ratc to 34 cubicfeet per minute, product w a coarser. In test 5, wing 60p.s.i.g. gas and n rerdtiq rate of 44 cubic feet pcr minute, product sizc was intemdiote between thc two previous tests
Figwe 3. Tcst apparatus for fheJuidized bed techniquc. Available Emtic energy is sprmd out o w too many partiiler so that only a few nftnin mflciently high veIociQforfrmturc VOL. 5 4
NO. 2 F E B R U A R Y 1 9 6 2
63
t6 l !u5
L
YI
0
zn
z 8
SOLIDS FEED RATE At a Comparable Degree of Particle Size Reduction, Efficiency of the 0405 Mill Compares with That of a Ball Mill
-
Figure 4. For each miZZ size and gas throughput, 6 h m ir on optimum solidsfecd rate which gives maximum eflcicncy
NCW
Solids Feed Rate,
Test NO. Lb./Hr.
Surface
Grindins Gar Cu.Ft./ Press. mm. psig.
Gcnnnted PMt. size, # $4. Ft.1 Feed Prod. Hp.-Hr.
0405 Elliptical Mill
285 780 615 615 615
1 2 3 4 5
80 80 63 34 44
60 60 60 30 60
115 87 105 105 105
30 43 42 90 57
5540
9040 1 I870 7650 12700 9360
Average
0202 Elliptical Mill
11.8 8.3 8.0 10.3 8.6 10.0
4
5 6
55 55
73 72 72 70 53 51
15.5
15.0 12.5 12.0
86 86 86 86 86 86
IO IO 17 26 27 32
1880 I 230
2140
Average
%In. Circular Mill
3
360 350 420
60 60 60
115 85
60
.
60 60
85
96 63 59
1780 2950 -
_...
Average
Fluidized Bed
1
2 3
2oa 20" 20" 20'
2.8 2.5 2.8 3.1
30 60 90
88 88
500
84
88
80 77 75 71
58 71 49 x) -
50
Average
Ball Mill 1
2
710 1035
1/ac 1/8"
22.9 22, 5b
Average
* Lb. total charge. 64
b
Mill hp.
0
60 67
8970 12960
10965
Inches.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Factors Affecting Efficiency
Operating variables affect efficiency considerably (Figure 4). Data from manufacturers (7) indicate that for each mill and gas throughput, there is an optimum solids feed rate which gives maximum efficiency, and also that efficiency increases with sonic velocity of the gas. Sonic velocity is determined by molecular weight and temperature of the gas using an equation obtainable in the literature ( 3 ) . In this work, efficiency of the 0405 mill was increased (from 5540 to 9040 sq. ft. per horsepower per hour) by increasing solids feed rate from 285 to 780 pounds per hour. For the 0202 and the 8-inch mill similar effects occurred hut to a lesser degree. Efficiency could also he increased by decreasing the grinding gas rate at the same pressure drop, hut to a lesser degree than the solids feed rate. For the fluidized bed, increasing pressure drop from 30 to 500 p.s.i. had no significant effect. Energy efficiency depends strongly on size and type of the mill. The extremes are represented by the 0405 mill and the fluidized bed: Efficiency for the mill was some 200 times greater than that of the bed. Further, even though design of the 0405 mill is identical to that of the 0202 mill, its efficiency is about four times greater. This ratio represents approximately the difference in cross-sectional area of the grinding races. Shape of the race, however, seems to have little effect-efficiency of the 8-inch circular mill was intermediate between the two elliptical mills. Low efficiency of the fluidized bed is probably caused by an excessively high concentration of particles near the jet. Grinding Mechanism
The grinding mechanism illustrated in Figure 5 explains most of the results. Particle-particle collision explains the initial increase in grinding efficiency with an
-
PARTICLE COLLISION A
Figure 5. A simpIc mechanism txfilains pnfwmanca of jet mills Particles, entrained by the gas and accelerated to high velocities, fracture by collidins with either the ihambn wall or each other
-
increase in solids feed rate. Higher particle population in the mill thus increases the chances for particles to collide. Mill size may affect grindiqg efficiency in two ways; with increased size, chamber volume increases faster than wall area and the larger vohxne affords more chances for particle-particle collision before particles are stopped by hitting. the wall. Also increased.mil1 size provides a greater distance for more -particles to accelerate before impact with slower moving particles. The centrifugal force, acting on a particlewith a given peripheralvelocity, is inversely proportional to the radius of curvature of the particle’s path. Thus, a larger mill with its greater radius of curvature of the mill race provides a longer distance for acceleration of a particle before hitting the outer race wall. Higher sonic gas velocity also increases the chances for a particle to accelerate to a sufficiently high speed for fracture. But even with a high velocity gas, the James M. Dotson is Engineerinp Leader, Organosilanes Process Dcuelopement, with the General Electric Co. in Waterford, N . Y . Hispreuious publications have been onfluidization and reaction kinetics.
AUTHOR
particle must first he entrained, and second it must have a sufficient distance for acceleration before colliding with another particle. Here again particle population is important. Above the optimum solids feed rate, excessively high concentration keeps the particles from accelerating enough to fracture on impact. Available kinetic energy is spread out over too many particles so that only a few attain sufficiently high velocity. The dense phase fluidized bed, where the concentration is much too high, is an extreme case. Thus, except for very mild grinding, direct gas injection into a fluidized bed is not recommended.
SUGGESTED READING ( 1 ) Albus, F., Chm. Eng. Propr. 57,90-4 (1961). ( 2 ) Berry, C. E., IND.END.CHEM. 38, 672-8 (1946). ( 3 ) “Chemical Engineers’ Handbook” (J. H. Perry, ed.), 3rd ed., p. 375, McGraw-Hill, New York, 1950. ( 4 ) Ibtd., p. 1134. ( 5 ) Fluid Enerqy Processing & Equipment Co., Philadelphia, Pa., Data Sheet, “Jet-0-Mizer Mills.” ( 6 ) Power92,753 (1948). (7) Stephenoff, N. N., Fluid Energy Processing & Equipment Co., Philadelphia, Pa.: private communication. VOL. 5 4
NO. 2
FEBRUARY 1962
65
