A kv to further growth in the use ofJuid energy mills, against the competition developing from other. devices, lies in majoy energy impyouement
SIZE REDUCTION GETS
NEW STATURE L I N C O L N T. W O R K
ize reduction, with its related operations of dispersize enlargement, size measurement, size classification, and collection, is maturing to a sound scientific stature as a unit operation for the chemical process industries. In spite of the long history of size reduction, time and refinement of data were needed to make this possible. The recent Chemical Show in IXew York gave evidence of the wide variet>-of devices, many of which are directed toward specific applications. In broad aspects, significant items in this field currently are : the First European Symposium on Size Reduction of the European Federation of Chemical Engineering to be held in Frankfurt (Main), April 10-13, 1962; the Chemical Engineering Symposium of the American Chemical Society's Division of Industrial and Engineering Chemistry, December 28-9, 1961 at Newark, Del.; the Journal of Stanford Research InstLtute (3rd quarter 1961) on particles and applications; thr forthcoming monograph of the Electrochemical Society on fine particles; George L. Clark's Encyclopedia of Microscopy (Reinhold 1961); and Arthur Stern's treatise on air pollution (Academic Press, January 1962). In addition, there are a number of current ne\v aspects to be considered.
S sion,
In equipment, some manufacturers are supplying devices for specific functions-e.g., machines for chopping scrap tile for reuse in plastic mixtures, for breaking bottles to have compact disposal, and for reducing garbage to get smooth flow in waste sluicing. Large mill systems are steadily under study for economy in energy and maintenance. Particular note may be made of comparing pebbles and steel bails and in the use of Xi-hard as a wear-resistant low cost alloy for balls. The recent change made by U. S. Stoneware Co., .4kron, Ohio, in batch pebble mill construction gives a real advantage of plate fabrication over casting (Figure 1). The actual structure forms jacketed ends which may be used to introduce cooling media. I n high speed impact mills, The Rauer Brothers Co., Springfield, Ohio, offer
Pulverization
Significant contributions have been made to the basic understanding of the size reduction operation. Kenny and Piret [Chem. Eng. A o g r . 7 , 199-202 (1961)l have shown the effect of slow compression crushing of single particles of glass, Some methods are effective and others are not. thus giving mill producers food for thought in how best to create the force application for size reduction. A. M. Gaudin at M I T has been workinq on resultant size distribution in repeated fracture. This may have an important bearing on size distribution of product as well as on energy requirements; also It may sharpen the treatment of closed circuit grinding on a fundamental basis, largely empirical today. 52
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
COURTESY U S
Figure 7.
STONEWARE 00
Ball mill with plate construction and end jacket
the Hurricane pulverizer (Figure 2). This is a vertical type with hammers, above which is a multiplate classifier. Pulverized material suspended about the periphery is carried upward and then inward between the plates by the air flow, while oversize material, unable to travel inward against the rotational forces, is recycled to the grinding section for further reduction. Pulverizing Machinery Co., Summit, N. J., has introduced a new Mi!uo-Monitor to control feed in terms of mill loading and power demand. Such automatic schemes make for operation at greatest effectiveness. Vibratory pebble mills are beiig well received and are extending their applications. They appear to have good capacity with low power required. But data are presently Scant as to the precise energy-surface relationships in this type of milling. The Vibro energy mill of Southwestern Engineering Co., Los Angeles, Calif., is now being applied in paint formulation, involving both grinding and dispersion. In these mills, smaller grinding media are being used for some operations. The regular pebble size is the '/Anch cylinder, with I/,and P/sinch also being available. Fluid energy mills are used for finegrinding,dispersion, reactions, and coating. For 20 or so years, these mills occupied a distinctive place in dry grinding to fine sizes, but they are being crowded by other mills with good classification. The relatively large expenditure of energy that was characteristic of these mills is becoming
MATEFJAL FLOW
AYTHOR Lincoln T . Work is a Consultmng Chemical Engineer in New York, N . Y . He has authored IGlECs Size
Reduction Annual Reuiew since 7947.
There Is Now No Excuse for loose Claims Based on Inadequate Meoruremenl
Though there are few basic departures from established methods, much has been done toward improving and perfecting present ones. The new Handbook 53 of W. S. Tyler Co., Cleveland, Ohio, is a welcome addition on testing sieves and their uses. Though the effective limit of wire-mesh sieves is being made finer, electromesh sieves in the 5- to 40-micron range help greatly to increase the usefulness of sieves in testing. Hain and Coslett present the state of the art in electron miuoscopy (Interscience 1961), and one investigator offers a simple powder dispersion technique for electron microscopy. An approach to an absolute technique in turbidimetry and a continuous recording sedi-
mentation device appear to enhance accuracy and speed in testing. Orifice counting, as embodied in the Coulter counter, is now implemented by use of a criterion of conductivity. Electrostatic separation of air-borne particles may become useful as a test. Gas chromatography has been used to determine surface area. Comparison of analytical methods shows improved correlation, recognizing that large differences often exist between values of external and total surface. Irani and Kaelble [Awl. Chcm. 33, 1168-70 (August 1961)] have compared centrifugal separation, electron microscopy, and electron counting with good correlation. The subject of measurement is
complex, and progress over the years has been slow because of the diffi-. culty in securing accurate and comparable measurements. Speed and economy of testing are also essential. Today, shape and surface quality are under serious study. Although little can yet be called standard, this study is proving revealing. The general concept of surface stresses in particles is being demonstrated. Particulates are bemg studied with respect to lattice defects and other structural imperfections, after the current fashion in physical metallurgy. With applications of these concepts in ceramics and pigments, there is now opened a basic new area for study in the properties and behavior of particulates. VOL 5 4
NO. 3 M A R C H 1 9 6 2
53
too costly. These mills should face a new drive for economy of energy. A mill of this type probably can be designed to operate in a range of energy economy comparable with that of the pebble mill, while still maintaining excellent control over a fine particle size. Such an achievement could substantially increase the use of fluid energy grinding. Last month I&EC published an article by J. M. Dotson showing how the effective range of jet mills could be extended. For high temperature reactions and reduction, the fuel-air injection system of Balasa might offer added merit. Disintegration and Dispersion
Besides the typical agitators and colloid mills for dispersion and disintegration, some special devices are lindmg application. The Turbuliuer of Strong-Scott Manufacturing Co., Minneapolis, Minn., is proving effective in some medium energy applications. Sonic Engineering Co., Stamford, COM., uses a vibrating blade activated by the stream movement to achieve dispersion and homogenizing. Then there is a variant of typical turbine-type agitators in the Dispersator of Premier Mill Corp., New York, N. Y. (Figure 3). Liquid-liquid reactions are achieved by intimate mixing, and also reactions to produce fine particulates are carried out with such equipment. Size Enlargement
Agglomeration, granulation, and pelleting continue to be of interest. The techniques employed for ores
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often include sintering. Other materials, including encapsulates, are processed by spray drying. Mechanical techniques of roll bonding and tumbling are gaining in interest with applications to chemicals. Materials such as catalysts and feeds are handled by extrusion or pelleting mills. Classification and Collection
Separation schemes for both liquid and gaseous suspensions are becoming increasingly better for the lowmicron cuts needed to prepare fine particles free of oversize. Liquid systems including centrifugal separators have been generally better than air systems because of easier dispersion and better control of the higher viscosity media. A comparatively new entry in liquidsolid separation is the cyclone which is being used more extensively for separations. Applications in coal washing are currently noted. In the gaseous field, electrostatic classification of air-borne particles is active. For example, this technique is employed in a new separator of Separations Engineering Corp., New York, N. Y., whereby particles falling in air are arranged for separation under electrostatic forces. Such techniques can be of real help in composition and size control related to grinding. Recovery of pulverized solids from liquids or gases involves various unit operations otherwise covered. The filtration study by Whitby for Torit Mfg. Co., St. Paul, Mmn., is informative. Torit has also announced an automatic filter.
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