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II/E~
Unit Operations Review
Size Reduction by Lincoln T. Work, Consulting Engineer, New York, N . Y.
b Developments in fluid energy and vibrating pebble mills mark progress in size reduction b With new techniques for automatic counting, measurement of size distribution will be more extensive and a valuable aid to fundamentals
IN
THE period between the 1959 Chemical Exposition and Dec. 1, 1960, the new aspects of this broad field have been largely development and consolidation from the status a t the show. T h e previous review brought together a wide variety of contributions to size measurement. size reduction. size enlargement. classification. and collection. Basic improvement in data is shoiving. and this should help to resolve the differences in theories of size reduction with respect to size distribution and energy requirements. Even now. generalizations are made all too often on inadequate or erroneous data. The applications areas of fluidization and of dust control are showing definite growth. while there are a few aspects of grinding which have come into the limelight.
Particle Size Measurement T h e problem of measurement is ever present. To know the nature of the material to be ground, with respect to its response to deformation under stress, and to know the full distribution of resultant particle size after grinding has been all too time-consuming. The rapid techniques which have come into use, such as using an over-all turbidimeter value to estimate surface or a n adsorption or permeability value for the same result or for a surface mean diameter, fulfill a purpose; but they still leave much g to be desired in interpreting grin data. This is a precaution against ready acceptance of inadequate d a t a ; for some work, carefully done, has been useful over the years. T h e danger in extrapolating distribution curves to large or small particles, because of the skewness of such curves, has been noted by Phelps ( 2 3 B ) ; he might also have noted the multimodal distributions so common to ground
products. Michaels and others (79B) have observed that there are significant differences in size distribution depending upon the dispersion technique. While this is striking for fine tungsten powder on which they worked, it is important in all measurements in the low micron sizes. T h e fundamental consideration is the taking of a sample, particularly from gaseous media, where it is difficult to ensure that it is respiesentative. Sampling devices, simple in structure and operating on sound principles, help. A light weight sampler, jet actuated by canned gas, has been announced by Union Industrial Equipment Corp. Other sampling (9B, 22B) devices or direct counting of aerosols (5B) also help. LVork progresses on test methods and their interpretation. Analysis of sieve test data (27B). automatic decankition (77B). krypton adsorption (24B), tracers ( 8 B ) . chromatography ( I B ) , higher order Tyndall spectra (73B),single tube sedimentation (74B),improved Werner sedimentation equipment (25B), and radioactivity (7B) are aspects of current development. T h e significant approach to measurement, which seems to meet the basic requirements of adequacy and speed, appears to be coming in the use of electronic counting. Even mill manufacturers are beginning to employ such tests. A report on the Coulter counter ( 2 B ) shows continued improvement and extension of applications. Irani (72B) used electrical resistivity changes, which he calibrated in a novel fashion, and he compared this with sedimentation and electronic size counting. Brooks (4B), Kubitschek (17B), and Morgan and Meyer (20B) have contributed to this aspect of size estimation. Tools for measurement studies include a ballistic particle size separater (78B), uniform metal and alloy powders of
20-micron and smaller size (Linde Division of Carbide), particle size data on pigments (6B). and a discussion (3B) on emulsion particle size. With respect to the behavior of particulates, H a m (7OB) reported on the diffusion-limited growth of precipitate particles. I n a study of phosphors, Kremheller and others (75B, 76B) made microscope observations before and after ball milling and found improvement when milling before final heat treatment. I n a study of particle growth, they found that size varies exponentially with reciprocal T and directly as the square root of firing time. These findings are not unrelated to effects in pigment calcination.
Size Reduction T h e various reviews ( 7 A , 4’1)including the 1959 review (February 1960) in this journal, present a cross-section of the developments in size reduction. Performance, operation, maintenance, controls, and other aspects are important; yet, in addition, there are some new areas in which substantial breakthroughs could bring greater benefits. I n the past year, the opposed jet fluid energy mills and the vibratory ball mills stand out. They are basically fine grinders for which energy considerations are of major importance and even a little contamination is often to be avoided. During the year, there was some sound work in theory, some special purpose mills have been offered. and there were several interesting points in normal milling. As to theory, Bond (3C) has amplified his discussion of the three principles of comminution. Schuhmann (7dC) has correlated energy input and size distribution. Kremheller and others (72C) have studied the effect of ball milling on lattice chloride in zinc sulfide. Cadwell and Duwell (5C) used a ball shaking test VOL. 53, NO. 3
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MARCH 1961
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Unit Operations Review
:OUR-ESY'
A L L I S - C H A L M E R S MFO. CO
Dual drive vibrating b a l l mill
rather than tumbling to evaluate resistance of abrasive grits to breakage. Culbertson (6C)evaluated wear resisting material. Special mills range widely. Jeffrey hlfg. Co. announced a disintegrator for slabs of \\'et synthetic rubber. Sprour\Valdron has a Ivater-tight saw-toothed crusher, available in stainless steel. T h e Herles disposal mill is for crushing cans and breaking bottles, thus making The Jetscrap easier to handle. htogenizer (Buschman Products, Inc.. New York. N. Y.) employs an out-ofregister alig-nment to give pulses, and claims to homogenize with ultrasonics. T h e Tornado mill (F. J. Stokes Corp.), with its full periphery screen? has been used for dispersing tungsten carbide after it has been reduced by ball milling. A new approach in cement grinding has been proposed by Zacher (75C). Zieman (76C) reported on a forum on ball and rod mill liners. Bennett and Wild (2C) discussed control in the Quemont grinding circuit. Power measurement (7C)>possible flow control by turbidity (B-I-F Industries, Inc.), and controls for peak grinding (7OC) mark the trend toward automation. Boszhardt and H a h n (4C) have pointed out the value of preventive maintenance for crushers and screens. Performance of mills has been described-Maybell on uranium ore (7C)> gyratories on trap rock (9C), a portable gyratory (73C), high production rates and low power claimed for the No11 mill, and pebble mills a t gold mines (77C). I n
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the process industries. the grinding of plastics is a problem now often solved by refrigeration; but Strong Scott reports that their Pulvocron now grinds polystyrene to 200 mesh without cooling. I n lieu of tumbling barrels or ball and pebble mills. the use of frequent short impacts. and related attrition in vibratory mills is a new development (8C).They are circular in shape. generally have a center tube, are springmounted. and activated by motor driven
INDUSTRIAL AND ENGINEERING CHEMISTRY
eccentric shafts whereby the vibratory energy is transmitted from inner and outer shells to the mass between. These mills are used for deburring and, with pebbles? for wet or dry grinding. For many applications, they appear to be from five to 30 times more effective in capacity per unit of po\ver than pebble mills. T h e Vibratom, marketed here by Schutz-O'Seill of Minneapolis. ' is a horizontal shaft device, lvhile the Sweco (Southlvestern Engineering Co.), described in the 1959 revieiv, is vertical. .4llis-Chalmers have a vibrating ball mill of the horizontal type (left) employing dual eccentric mechanisms running horizontally on each side of the grinding chamber. This is available in four sizes from 1 .?-inch at 15-hp. to 42 inch at 225 hp. Capacities are said to be 15 to 30 times those of tumbling mills. Fluid energ!' mills have found a modest but significant place in fine grinding, dispersion. and coating-largely because jet impact grinding, though known for years. needed to associate with classification effective a t fine sizes. Afore recent developments in opposed jet grinding associated with classification have introduced what appears to be a more effective use of energy. T h e Majac mill with a mechanical classifier and the Trost mill with static classifier are examples. T h e ne\v Trost mill (belolv) features a shallow circular chamber lvith peripheral return of coarse material and central discharge of fluid and fines. Continued improvement in energy economy and the use of selected liners for minimal contamination may enlarge the field for fluid energy mills.
Input-
Replaceable L her3 COURTESY:
Diagram of jet mill
TROST JET M I L L CO.
an Classification and Collection Those aspects of particle separation and collection which a r e closely associated with size reduction show a number of interesting facets. I n screening. topics for consideration include Entoleter’s Cirlyptic screen \vith its circularelliptical motion, the article by Eck and \Valter [ I D ) o n specifying industrial sifters. the thin profile, wedge \\.ire screen (National-Standard Co.), and the freerollinq loose rods o n screens across the flo\\- 11)- .4llis-Chalmers. Cartridge filters (American Felt Co.) IO control particle size in filtration. the case of control of Buell classifiers. miniature Hydroclone classifiers (2D).the separation of particles in suspension by nonuniform clectric fields ( 3 0 . J D ) , and the Bendix micron filter cover separations for size. I n collection. striking developrrents are the built-in cyclone in a filter of Dustex Corp.. the large dust trap (Dust Suppression Pr Engineering C o . ) . the successful use b)- Dracco of sonics to dust off collector bags. electrithe Fiberglas Type fied air filters (5D). 004-1 2 glass filter. Research Cottrell’s scrubbrr. and the modular units of M i k1.o - Pul sa ir e .
Applications Several examples may be selected to illustrate applications related to particulates. Contributions to processing. technology were made by \$-est and others (22E) o n the use of a colloid mill to ensure dispersion of ball clays; by Jonas a n d Roberson ( 6 E ) on particle size as a factor influencing expansion of clay; o n pulverizing ceramics with no mo\-ing parts ( 3 E ) ; by TVen and Haskinger ( Z I E ) in the elutriation of solid particles from a dense-phase fluid bed ; by Shinnar a n d Church ( 7 6 E ) on statistical theories of turbulence in predicting particle size in agitated dispersions; by Lehmann ( 9 E )o n voltage dependence on particle size in phosphors; by Torobin and Gauvin (79E) o n motion of a particle accelerated in gas flow; a n d by Jessen a n d h l u n g a n (.5E) o n the influence of particle size and exchange cation o n gel properties of bentonite. Some applications were discussed by Luborsky ( 77E) o n magnet properties; by Gessler ( 4 E ) on carbon blacks; o n mechanical plating systems (74E); by Simons a n d Jeffery (77E) o n pulverized fuel ash; by Borg a n d Leet (7E) o n particles in lubricating grease; by Takemoto and others (18E)on low grinding temperature for cement clinker; a n d by the Lagallys (8E) o n paper strength. Brief mention m a y be m a d e of glass flakes 0.00008-inch thick which are finding a place in lieu of mica flake.
O n aerosols and dusts, there a r e reports by \Vise (23E) o n a power station, Caplan ( ? E ) on the reverse jet filter. o n cement (75E). on electostatic clearing of oil mists bv Liebrecht (7OE). glass fiber filters ( 7 2 E ) . and on high-efficiency air filtration ( 7 3 5 ) . Size enlargement w a s discussed b) K u r t z a n d Barduhn ( 7 E ) and T o u r v (2OE).
Bibliography Books a n d General (1.4) Can. Cheni. Piocessing 44, 78-9 (Februarv 1960). (2.4)’ Leiva.’ Max. “Fluidization.” McGrawHill. SeLv York. 1959. (3.4) Soderberq. H . E.. others, Eng. .\fining J. 151, 195-205 (June 1960). (4.4) Sollrnberger, C. I>.. .tfining Eng. 12, 123-4 (February 1960). (5.4) Zenz, F. X.. Othmer. D. F., “Fluidization and Fluid Particle Systems,” Reinhold, Nrwk’ork. 1960. Properties a n d Measurements of Particulates (1B) Benes.. H . .4.,.4na/. Cherrz. 32, 1410-11 (1960). (2B) Berg.. R. H . . Kovac. G. M.. Food En
