GRINDING
C
Lincoln T. Work,
I K C E particle size measurement is essential to fundamental. and practical developments in this field, particularly in finer sizes, the tremendous extension of such measurement during t,he thirties has tended toward more exact work in crushing and grinding. Old methods were perfected and made more econoinical to use, while new principles were being tested. The war period has seen a continuation of this trend in the special applications to war industries. There has thus d.eveloped a more extensive “knowhow”, and simple, well established methods of particle size measurement are available for use. The potentialities are still only partially realized, but good progress has been made. The prewar period was marked by the successful application of a tluid-jet mill and by a sudden general interest in the new aspects of. this principle which show today in developments in varying stages of completeness and effect>iveness. Fluid energy grinding is an old idea which apparently had little application until modern knowledge of jet performance and unique combination of grinding and classifying elements made possible the practical production of fine particle size. While these developments were in progress before the forties, the war period created an entirely new set of problems. The mining field was faced with the need for efficiency because only thus could the needed volumes of production be attained, and this had to be done with mill types then available. There was little manpower or time for research on the current problems or on the uses of these operations in the chemical induetries. Some specific and exacting requirements for the armed services demanded closely controlled ranges of particle size, and manufacturers were faced with studies of procedure to meet thesc needs with an economic utilization of materials. These problems also necessitated consideration of alternatives, as when spraying replaced grinding. Throughout this period there were those who thought about the trends. Some contributed to the immediate problems, while others looked into the future, so that now the field stands to profit from its war experience, and new developments are beginning to take their form. MEABUREMENT. In order that product control may be effectively carried out or that the fundamental principles in size reduction may be established, rapid and economicnl methods of size measurement, particularly in the subsieve range, are essential. Considerable progress has been made in recent years, arid the Twelfth Annual Chemical Engineering Symposium of the Division of Industrial and Engineering Chemistry, AMERICANCHEMICAL SOCIETY, gives evidence of this. (This symposium, Measurement m d Creation of Particle Size, was held in December 1945.) A brief review of developments follows. Just before tJhe war, standardization of sieves &-as effected in the American Standards Association through a committee sponsored by the American Society for Testing Materials and the Kational Bureau of Standards (1). These two organizations have adopted this standard. Procedures for sieving and the performance testing of sieves are not yet fully standardized. In the subsieve range the electron microscope has opened new vistas for photographic examination, with the result that magnifications with good resolution are now many fold greater than with the light microscope. Sedimentation methods have been checked with others (bd), and the usefulness of the hydrometer, Andreasen pipet, and Wagner turbidimeter has been defined. Centrifugal sedimentation has been developed (16), and indications are that
METAL AND THERMIT CORPORATION,RAHWAY, N. J.
there will be further progress. The air analyzer, one form of which was defined by Roller (19), is finding a place in the range 5 to 40 microns for those materials which are not altered by the vigor of the operation. Spectral transmission studies using a range of wave lengths finds a place from 0.2 up to 2 or 3 microns (4). Gas permeability methods for surface have been developed and are being extended to fine sizes by the use of higher gas pressures and compressiori of pellets (IO). Gas adsorption for total surface, both internal and external, is finding more extended use (If). Methods of dispersion appear solved for many materials, but there is no standard and the problem still requires study by new procedures. Many of the methods of measurement use different criterisfor example, the sieve aperture, Stokes law diameter, or surface evaluation by gas adsorption. There have been many serious problems of correlation, but the literature is now beginning t o show the relation of these methods to one another; the exception is that the gas adsorption method measures total surface, whereas the other methods depend largely on the external surface characteristics of the material. There is extensive literature on the correlation of shape factors (21). PRODUCTS OF SIZE REDUCTION.Recent work on the laws of grinding as they affect the product has given a more fundamental approach through action even on individual particles. Bond (7‘) compared compression tesL3 on cores with impact tests, the latter more closely resembling the grinding action. Gaudin and associates (12) studied surface developed in relation to size, using controlled fracture of quartz, glass, galena, and other materials. They found that size distribution of broken fragments made by a single fracture is such that the new surface on each grade is the same, and that this generalization may be carried into multiple fractures. Such information may ultimately lead to scientific design of equipment for a predictable product. ENERGY. Theoretical calculations are based on the energy necessary to create new surface without unusual distortion of the particle, whereas the actual behavior represents distortion of the particle to the point of rupture, and the restoration of its form may return energy as heat to the system. It appears, therefore, that there is opportunity to develop grinding methods where tbe energy would be applied in such a way as t o produce fracture without too serious a distortion of tbe total particle. GRINDABILITY.The old hardness scale was never more than % rough criterion of the ease or difficulty of grinding a material, although it would serve to give Yome measure of wear on the grinding media. Furthermore, many materials contained a variety of particles of widely differing hardness-for example, coal containing silica-and the hardness method would have no applicability in giving a measure in such a case. Grindability methods were developed through many experimental phascs during the thirties, and some information reached the literature a t that time. More recently methods have been standardized, at least tentatively, as the ball mill method (8) and the Hardgrove machine method (3). Hardgrove grindability depended on sieve determinations, and there the method has been improved by using the air permeability test for measuring surface (20). Grindability studies have been further extended to the mesh sizes to which an ore is to be ground in practice so that the capacity of a mill can be calculated directly (9). Wilcoxson (26) reported, relative to Hardgrove grindability, “there are indications that the higher the grindability, the higher the specific (Continued on page Sf)
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January 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
CRUSHING AND GRINDING CONTINUED FROM PAGE
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surface produced a t a given mesh screen analysis”. The progressive development of data on grindability is proving t o be of material help in choosing the size of mill for a grinding operation and in estimating capacity for different feed materials under given nlill conditions (18). EQUIPMENT. In 1941 Taggart ($4) presented a one-page “vignettteof the future” with an accompanying set of notations “in retrospect”. The latter revealed the comparatively slow progress which has taken place in the field of grinding. It showed the beginnings of the gravity stamp in the sixteenth century, and the development of rolls, jaw, and gyratory crusher and of the ball mill during the nineteenth century. He evaluated the future in terms of the early war problems, and found these problems were essentially mechanical, structural, and operational. Pressure of wartime efficiency demanded a greater consideration of production on all equipment. These questions involved such matters as larger size bins in order to enable steady and uniform feeding of the crushing device, better and more economical structural design of mill in order to minimize weight without impairing strength in the operating structure, careful attention to lubrication schedules and the location of bearings in unexposed positions, and the removal of tramp material which might cause breakdown. Lack of availability of pebbles and liners for ball mills led to many interesting results. Hatch (18) reported a liner made of steel rails embedded in concrete. Howes (14) described a double-step liner to improve ball mill operation. Berry (6) reported on replacements for Belgian silex as pebble mill linings, and found quartzite and granite satisfactory. Ball size and grading and ball mill control by the “electric ear” have been of help in improving operations. Major developments of the recent period include high-speed mills and classifiers, jet-energy mills and classifiers, and grinding aids. Berry (5) reported in considerable detail on the first two. Furthermore, hammer mill progress has been made by increased speeds and by improved design. This has usually involved an improvement in classification so that finer oversize is returned, and the resulting product is more closely cut as to “top size”. The development of jet-energy mills, of which the Micronizer was first to become fully established, has led to further effort in this field. The Micronizer embodies an interesting combination of the application of high fluid energy and fine classification controlled by the jets and without mechanical parts. Other work in process of development includes fracture in the expansion zone of the jet as Yellott and Singh (36) are doing with coal and the use of the older principle of the jet and anvil type ( I ? ) . With respect to grinding aids, some progress has been made in improving performance of the dry grinding operation. This is a comparatively old art for wet grinding, but the use of sulfonated compounds or ethanolamine salts or even of coal and graphite in improving production and the degree of grinding which may be obtained have been the subject of considerable study (8, 16, 28). LITERATURE CITED
(1) Am. Soc. for Testing Materials, Standards, Pt. 111,pp. 1048-54, E-11-39 (1944). Ibid., Pt. 111,pp. 1170-3, D-408-37T (1944). Ibid., Pt. 111,pp. 1174-7, D-409-37T (1944). Bailey, E. D., IND.ENG.CHEM.,ANAL.ED., 18, 365-70 (1946). Berry, C. E., IND.ENG.CHEM.,38, 672-8 (1946). Berry, C. E., Mining Tech., 10, Tech. Pub. 1948 (1946). Bond, F. C., Ibid., 10, Tech. Pub. 1895 (1946). Bond and Agthe, Ibid., 4, Tech. Pub. 1160 (1940). Bond and Maxson, Ibid., 7, Tech. Pub. 1579 (1943). (10) Carman, P. C., Trans. Inst. Chem. Engrs. (London), 15, 150-66 (1937); Lea and Nurse, J . SOC.Chem. Ind., 58, 277-83 (1939); (2) (3) (4) (5) (6) (7) (8) (9)
31
Gooden and Smith. IND.ENG.CHEM..ANAL. ED.. 12, 479-82 (1940); Pechukas and Gage, Ibid., 18, 370-3 (1946); Keyes, W. F., Ibid., 18, 33-4 (1946). (11) Emmett and Brunauer, J. Am. Chem. SOC.,59, 1553-64 (1937). (12) Gaudin and Hukki, Mining Tech., 8, Tech. Pub. 1779 (1944); Gaudin and Yavasca. Ibid.. 9, Tech. Pub. 1819 (1945). (13) Hatch, R., Mining Congr. J., 67 (Feb. 1942). (14) Howes, W. L., Eng. Mining J.,143, No. 6, 57 (1942). (15) Jacobsen and Sullivan, IND.ENQ.CHEM.,ANAL.ED., 18, 360-4 (1946). (16) Kennedy and Mardulier, Rock Products, 44, 78-9 (1941). (17) Kiselhof, M. L., Eng. Digest, 3, 394-6 (1946). (18) Michaelson, 6.D., Mining Tech., 9, Tech. Pub. 1844 (1945). (19) Roller, P. S., U. S. Bur. Mines, Tech. Pub. 490 (1931). (20) Romer, J. B., Proc. A m . SOC.Testing Materials, 41, 1152-72 (1941). (21) Schweyer, H. E., Chem. Revs., 31, 295-317 (1942). (22) Schweyer, H. E., IND.E m . CREM.,ANAL. ED., 14, 622-32 (1942). (23) Sweitzer and Craig, IND.ENG.CHEM.,32, 751-6 (1940). (24) Taggart, A. F., Eng. Mining J., 142, No. 8, 116 (1941). (25) Wilcoxson, L. S., Iron Steel Engr., 21, No. 6, 74-85, 88 (1944). (26) Yellott and Singh, Power Plant Eng., 49, 82-6 (1945).
FILTRAT10N CONTINUED F R O M PAGF,
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FILTER MEDIA Akin, R. B., Chem. Industries, 52, 456-60, 599-603 (1943). Alfthan, J., U. S. Patent 2,400,091 (May 14, 1946). Anonymous, Modern Plastics, 22, No. 1, 91-7, 184-92 (1944). Anonymous, Plastics News Letter, 4, No. 36, 4 (1944). Anonymous, Rayon Teztils Monthly, 25, 78 (1944). Anonymous, Teztile World, 93, No. 9, 108ff (Sept. 1943). Bateson, S., Chemistry &Industry, 1941, 341. Behan, F. D., Public Works, 76, No. 5, 26-9 (1945). Bentley, W. B., J . Junior Insts. Engrs. (London), 56, 25-32 (1945). Black, R. F., Intern. Sugar J.,48, 207-8 (1946). Bogaty, H., and Carson, F. T., J . Research Natl. Bur. Standards, 33, 353-62 (1944). Bogtstra, J. F., Arch. Suikerind. Ncderland en Ned. India, 2, 178-81 (1941). Bray, U. B., Gas Oil Power, 37, 219-20 (1942). Cruickshank, G. A., U. S. Patent 2,327,250 (Aug. 17, 1943). Daumas, M., Chimie & industrie, 52, 10-15 (1944). Desmond, J., M f g . Chemist, 13, 216-17 (1942). Diclcey, G. D., and Bryden, C. L., “Theory and Practice of Filtration”, Chap. 5 (1946). Dittmar, J. H., and Harvey, F. L., Chem. & Met. Eng., 50, No. 9, 117-18 (1943). Foulon, A,, Wochbl. Papierfabr., 73, 362-3 (1942). Gardner, G. M., Water & Sewage Works, 93, 31-4 (1946). Garriga, J. M., U. S. Patent 2,387,726 (Oat. 30, 1945). Herfurth, 0. R., Deut. Wollen-Gewerbe, 74, 461-6 (1942). (49j Herfurth. 0. R.. Zellwolle. Kunstseide. Seide. 47, 2-8 (1942). (50) Jones, C.~S.,Indian Teztile J.,56, 741-6 (1946). (51) Kocatopcu, S. S., Bull. A m . Ceram. SOC.,25, 53-5 (1946). (52) Koehring, R. P., U. S. Patent 2,300,048 (Oct. 27, 1942). (53) Loasby, G., Teztile Mfr., 69, 230 (1943). (54) Luttge, W. G., Chem. & Met. Eng., 48, No. 6, 98 (1941). (55) Micro Metallic Co., “Announcing a New Filter Medium”, Forest Hills, N. Y., 1946. (56) Reck, W. H., Deco Trefoil, 9, No. 9, 4 (1945). (57) Robertson, A. M., Chemistry & Industry, 1946, 138-9, 146-7. (58) Robitschek, J. M., Ceram. Age, 42, 8-11 (1943). (59) Rudolph, €I., Kolloid-Z., 103, 164-6 (1943). (60) Rugeley, E. W., U. S. Patent 2,355,822 (Aug. 15, 1944). (61) Bandera, K., Z . Zuckerind. BBhmen-Mtihren, 67, 1-6 (1943). (62) Schwarzkopf, P., Product Eng., 17, 268-72 (1946). (G3) Scribner, R. IT7.,and Wilson, W. K., J . Research Natl. Bur. Standards, 34, 453-8 (1945). (64) Shearer, H. E., Ragon Teztile Monthly, 25, 431-2 (1944). (65) Sieminski, M. A., and Hotte, G. H., Ibid., 25, 608-10 (1944). (66) Tavasci, B., Chimica e industria (Italy), 24, 119-22 (1942). (67) Tech. -4ssoc. of Pulp h Paper Ind., Paper Trade J., 122, NO. 12, 57-8 (1946). (68) Turner, H. G., IND.ENQ.CHEM.,35, 145-7 (1943). (69) Van Antwerpen, F. J., Ibid., 32, 1580-4 (1940).
