US 3022017 A
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Feb. 20, 1962 P. M. MCKENNA METHOD OF AND APPARATUS FOR COMMINUTING HARD MATERIALS Filed Dec. 14. 1956 INVENTOR. PHILIP M. M KENNA R E 5 E K 0a N O S N B O R s N D m W3 5 E D HIS ATTOR N EYS United States Patent 3,022,017 NLETHOD OF AND APPARATUS FOR COP/ MINUT- ING HARD MATERIALS Philip M. McKenna, Greensburg, Pa., assignor to Kennametal Inc., Latrobe, Pa., a corporation of Pennsylvania Filed Dec. 14, 1956, Ser. No. 628,375
4 Claims. (Cl. 241183) This invention relates to a method of and apparatus for comminuting hard, solid material such as minerals and inorganic substances. More particularly, it relates to a method of obtaining, in a rapid and efiicient manner, a preferential dispersion of sizes in the cornminuted product, and in efiecting the com minution in such a way that the resulting particles have certain physical properties, which render them especially desirable for use in powder metallurgy techniques. I have found my new comminuting process to be particularly useful in connection with the preparation of hard carbides of refractory metals, such a tungsten, titanium, tantalum and columbium, and hard substances containing those carbides, by the process of sintering with an admixed auxiliary metal such as cobalt or nickel. For example, I find my invention useful in making sintered tungsten carbide alloys from macrocrystalline tungsten carbide and a minor proportion of cobalt. More broadly, the invention relates to a comminuting process which is more eficient, both as to the time and the energy required to effect the disintegration of the material, than any other known process with which I am familiar.
The apparatus which I have devised for carrying out the process is new and unique in several important respects. Furthermore, its usefulness is not limited to the comminution and preparation of hard carbide powders but it is also adapted for the milling of ores to permit the separation of the constituent minerals therein in a known manner, for the grinding of abrasive mineral substances both natural and artificial, and forthe preparation of clays and ceramic materials. it is also a more efiicient device than the ball mills or rod mills of previous constructions.
Accordingly, it is an object of the present invention to provide a new process for comminuting hard, solid materials, so as to obtain finer comminution and greater uniformity of particle size, than has heretofore been possible by the use of conventional methods.
Another object of my invention is to provide a milling process in which small diameter rods, having a modulus of elasticity exceeding 60,000,000 p.s.i., are used as the grinding elements, and in which the mill is rotated at reduced speeds so as to avoid cascading of the rods.
Another object of my invention is to provide a new method of comminuting hard carbides which consists in charging the coarse carbide powder together with powdered cobalt into a mill containing small diameter rods of hard carbide alloy and thereafter rotating the mill at a speed less than which will cause cascading of the rods.
Another object of my invention is to provide a novel method of lapping hard carbide alloy rods to render them straight and true.
Another object of my invention is to provide an improved form of rod mill in which small diameter rods, having a modulus of elasticity exceeding 60,000,000 p.s.i., are employed as the grinding elements.
Another object of my invention is to provide a rod mill in which small diameter rods of hard carbide alloy are employed as the grinding elements, and in which a plurality of spaced, annular rings are provided therein for supporting the rods for rolling movement out of contact with the interior surface of the drum.
. With these and other objects in view, which will be- 3,022,01? Patented Feb. 20, 1052 come apparent from the following description, the invention includes certain novel features of construction and methods of procedure the essential features of which are set forth in the appended claims and a preferred form or embodiment of which will hereinafter be described with reference to the drawings, which form a part of this specification.
in the drawings: a
FIG. 1 is a perspective View of a rod mill with one of the jars or drums shown in its open or loading position.
FIG. 2 is an end view of one of the drums showing it as it appears when closed.
FIG. 3 is a cross-sectional view taken along the line 33 in PEG. 2.
FIG. 4 is a view similar to FIG. 3 showing a'modified form of construction.
FIG. 5 is a cross-sectional view taken along the line 5-5 in FIG. 2. 7
'FIG. 6 is a perspective view of one of the grinding rods.
In the past, ball mills have been used exclusively in the co mminution of the materials used in the preparation of hard carbide compositions. Rod mills have not been used for this purpose because of their inutility in eliecting a reduction in particle size below mesh, i.e., 149 microns. The powder produced by ball mills, however, is characterized by a large variation in particle size which is undesirable for the production of cemented hard carbides since the fine particles tend to dissolve more rapidly in the auxiliary, or matrix, metal and recrystallize on the coarser particles thereby increasing their size. resulting hard carbide composition accordinglyIwill contain a high percentage of large crystals which are more easily torn from the matrix than are the smaller crystals thereby weakening the material. Hence, it is desirable to provide a method of comrnlnuting the hard carbide material which will yield a preferential distribution of particle sizes, i.e., one in which an increased proportion of the particles will be of approximately the same size.
According to my process, I start with a coarse powder,
for example a macro-crystalline carbide powder of 40 to 300 mesh, average size, and mill it down by the use of small diameter rods of hard carbide alloy instead of balls. The mill is rotated at reduced speed so as to. cause the rods to grind the powder without percussion or cascading. in both ball mills and rod mills it has been customary to revolve the mills at speeds calculated to produce cascading with percussive effect of the balls or rods. That speed, in revolutions per minute, is defined by the formula r.p.m.=200/ /D, where D is the inside diameter of the mill in inches. By operating my rod mill at such reduced speed, the shattering effect on the particles, which is present in ball mills, is avoided. In a ball mill, when two balls hit against each other they meet at a pointand there is a shattering of any particle caught at that point regardless of its size or whether it is already fine enough for the use intended. In my grinding process, however, the small, rotating rods make line contact with one another, so that the whole energy of the rods is concentrated upon the larger particles caught therebetween and not upon the smaller ones which are shielded from the force of the rods by the larger particles. Hence, a greater uniformity of particle size is obtained by the use of this method of grinding than when a .ball mill is utilized. Experiments using steel rods instead of hard carbide alloy rods showed no improvement over the ball mill, with'regard to uniformity of particle size.
lmproved efficiency of the comminution process is obtained through the use of cemented hard carbide rods as the grinding elements. This results from the vast numbers of hard points and edges provided by the sharp carbide crystals of from 1 to 10 microns insize which are held in a matrix of a softer metal such as cobalt or nickel( A suitable carbide for this purpose is the wella lid or cover 22 (see also FlGS. 2 and 3) provided with a pair of handles 23 to facilitate removal and reknown cemented tungsten carbide which may contain a unit pressures on the particles being ground, which tends to crack the particles along their lines of weakness, rather .thanto crush them, into a fine powder. Hence, notv v distances which relieve pressure on particles of this small size. I have found that rods of A to inch in diameter, of material having a modulus of elasticity above 60,000,-
a 000 psi, are suitable. for reducing macro-crystalline powder of 40 to 300 mesh to particles approximately 1 /2 microns in size. The rods need not all be of the same diameter and, in fact, it may be found advantageous to make one-third of the'rods, by weight, three times the diameter of the remaining rods.
I have also found in connection with the comminution' of macro-crystalline hard carbide that it is desirable to introduce a minor proportion of cobalt into the mill along 7 with the carbide material, where cobalt is to be the matrix metal employed in forming the cemented hard carbide, and then to grind the two metals together in the mill. The cobalt may be introduced either as a powder or in the form of rods similar to the grinding rods. Not over one-half of the rods should be of the cobalt material however. .Since every metal has a characteristic thickness, i.e., the thickness to which it may be beaten untilthe film breaks, the cobalt metal thus introduced will be rolled out into folia of predetermined thickness. These folia will act as minimum gauges between the rods and thereby limit the size of the material ground by the rods. Also, the thin films of cobalt are well adapted to cover and coat each carbide particle thereby forming a pellicule of cobalt around each carbide grain.
To illustrate the manner in which my novel milling process may be carried out, the following example is given: 7 V
500 gram mix consisting of 92% macro-crystalline tungsten carbide of 40 to 300 mesh and 8% cobalt powder was-charged into a 4 inch inside diameter rod mill containing 3000 grams of tungsten carbide rods, /4
inch in diameter and 5% inches long. The charge was milled at 50 to 60 r.p.m. for 8 days. When removed placement thereof on the drum. The cover is adapted to be held in place on the end of the drum by a clamping bar 24 the ends of which are received in apertures provided in plates 25 weldedor otherwise secured to the sides of the'drum. With the bar in place, as shown in FIG. 3, the cover is clamped tightly against the end of the drum by turning down the clamping bolts 26 received in tapped holes provided in the bar 24, To remove the cover, the bolts as are loosened and the bar 24 slid out of the apertured plates 25 after which the cover 22 may then be pulled ofi of the drum. 7
Each drum 20is swiveled on the frame 11 by means of a pair of trunnion bolts 30 which are mounted in pads 31 secured to the sides of the drum. The bolts 39 are pivotally received in holes provided therefor in the side bars of frame 11 and are fitted with nuts 32 (FIG. 5) to prevent displacement of the bolts. Each pad 1s also provided with an aperture 33 adapted to receive the. end of a thumb screw 34 whenthe drum is in its rotating position as shown in FIG. '5. By loosening the thumb screw 34, the drum may be swung to the position shown in FIG. 1 to facilitate removal of the cover 22 and emptying or loadingof the drum. I
It is not essential that the axes of the drums coincide exactly with the axis of rotation of the frame 11. In fact it may be desirable to mount the drums slightly offcenter so as to produce a cyclical acceleration of the rods and thereby increase the pressure of the rods against one another during a portion of each revolution of the drum.
As mentioned earlier herein, each rod 21 (FIG. 6) is cylindrical in shape having a large length/diameter ratio and constructed of material having a modulus of elasticity exceeding 60,000,000 p.s.i. ,I prefer to make the rods of cemented hard carbide; I have found that rods constructed of this material'make excellent grinding elements of the matrix metal in which the crystals are bonded and thereby tend to fracture the particles along their lines 7 of weakness.
7 Rod mills employing the cemented hard carbide rods for comminution of the material charged into the mill,
a are particularly suited for the grinding of macro-crystalfrom the mill, the particle size of. the milled powder was found to be 1.9 microns, average. The rod mill incorporating my invention is shown in the accompanying drawings where there is shown a con-- axis. Power for rotating the frame is provided by an electric motor 12 which drives a pulley 13 through a worm gear drive 14; The pulley 13 is connected by a belt 15 with a pulley lfivmounted on one of the trunnions on which the frame 11 revolves.
Supported on the frame 11 is a pair of jars or drums 20 each adapted to receive a plurality of grinding elements in the form of rods 21. Each drum is fitted with line hard carbides into fine powders 'of the desired particle size. In grinding this material, it is essential to make sure that the final powder is not contaminated with iron removed from the interior surface of the drum 20 during the grinding operation. One way of avoiding this would be to line the drum with a cemented hard carbide material, so that any small. particles removed from the surface of the drum during the milling-operation will not contaminate the powder being reduced therein. It has been found extremely difficult and costly, however, to manufacture such linings. To eliminate this difliculty I have discovered that it is possible to provide, instead, a pair of cemented hard carbide rings 40 and 41 (FIG. 3), which are held in place within the drum by spacers 42, 43 and 44 of somewhat thinner material, and which may be constructed of steel although cobalt or nickel may be used, if desired. As shown in 'FIG. 3, the rings 40 and 41 are spaced at'different distances from the end of the drum so as to allow the rods to be turned end for end and thereby prevent excessive grooving thereof by the rings.
The cover 22 isshown faced with a disk 45' of hard carbide material while/the end of the drum is covered by a similar disk as of the hard material. The disks 46 will prevent any contamination of the powder in the mill by attrition of the end plates by the ends of the rods 21.
For best results, the drum 20 is filled approximately one-half full of rods 21 and is rotated at reduced speed so as to avoid cascading of the rods within'the mill. Fast and efficient grinding is thereby effected, while, at the same time, obtaining a high degree of uniformity in the size of the particles in the final, powdered product. More specifically, the driving connections from the motor 12 to the rotating frame 11 should be such as to cause the rod mill to rotate at a speed of from /3 to of the speed given by the formula r.p.m.=200/ /TJ where r.p.m.=revolutions per minute, and D=inside diameter of the rod mill in inches.
In the grinding of hard carbide materials for the making of cemented carbides, it may be found expeditious to include a number of cobalt or nickel rods with the grinding rods 21 for the purpose of providing nickel or cobalt particles which will provide the minimum thickness gauges between the rods 21 as heretofore mentioned. As also earlier mentioned, these particles of cobalt or nickel tend to coat the harder carbide particles which is beneficial in the forming of cemented hard carbides. In this connection, it is recommended that not over onehalf of the total number of rods be of the cobalt or nickel metal.
I have also found it to be advantageous to make certain of the rods 21 of larger diameter than the others, and, specifically, to make approximately /3 of the rods, by weight, of a diameter equal to three times that of the rods 21 which are preferably of small diameter as compared to their length. In the case of rods 21 which are approximately 6 inches in length, their diameter will be in the neighborhood of inch. Hence, the oversize rods, which are comparatively few in number, should be approximately /1 inch in diameter.
I have also found it to be advantageous to impart a reciprocable motion to the rods, in addition to the rotary motion given thereto by the rings 40 and 41 as the drum 2) revolves. In FIG. 4 of the drawings, is shown a rod mill of modified construction in which the additional endwise shifting movement of the rods will be provided as the drum rotates. As therein shown, the mill includes a drum 50 closed at its left-hand end by a bottom plate 51 and at its right-hand end by a removable cover 52. Both the plate 51 and cover 52 are constructed so as to vary in thickness from one side to the other, so as to present an inclined surface to rods supported within the mill on the cemented hard carbide rings 53 and 54. The plate 51 and cover 52 are each faced with a disk of cemented hard carbide material 55 and 56, respectively, to prevent contamination of the powder being grounded by the iron of the plate 51 and cover 52. The inside surfaces of the plate and cover shown in FIG. 4 are inclined at 80 with respect to the axis of the drum, but this angle may be varied somewhat while still retaining the advantages of this form of my invention.
To prevent the powder being ground in the mill, or at least a part thereof, from collecting in the space between the rods and the inner surface of the drum, a series of cleats 57 made of cobalt or nickel metal may be secured to the inner surface of the drum or of the liner provided therein, such cleats being of suitable thickness to lie approximately flush with the rings 53 and 54. The cleats will carry any of the powder collecting between the rings upwardly as the mill rotates and cause it to fall down among the grinding rods 21 where it will be ground in the desired manner.
While I have described my invention in connection with one possible form or embodiment thereof and have used, therefore, certain specific terms and language herein, it is to be understood that the present disclosure is illustrative rather than restrictive and that changes and modifications may be resorted to without departing from the spirit of the invention or the scope of the claims, which follow:
1. A rod mill comprising the combination of a drum mounted to rotate about an axis and having an inner surface, a plurality of rods each having a diameter not greater than five percent of the length 'of the rod, said rods being loosely housed within said drum and lying substantially longitudinally thereof, a pair of annular members longitudinally spaced apart within said drum and connected thereto, said longitudinal spacing being greater than the spacing between each of said members and the adjacent ends of the drum, said rods and said annular members being of hard material having a modulus of elasticity greater than twice that of steel, each of said members having an inner wall defining a substantially circular opening therein with said inner walls cooperatively engaging said rods and preventing their contact with the inner surface of said drum, and means for rotating the drum about said axis without cascading of said rods at a speed substantially less than that given by the formula r.p.rn.=200/ /D, where rpm. is the revolutions per minute of the mill, and D is the internal diameter of the mill in inches.
2. A rod mill in accordance with claim 1 having a longitudinally extending cleat member of hard material selected from the group consisting of cobalt and nickel and disposed between said annular members and secured to the inner surface of the drum, said cleat extending radially inward from said inner surface by an amount not greater than flush with the inner walls of said annular members.
3. A rod mill comprising the combination of a hollow drum mounted to rotate about an axis and having an inner surface defining a longitudinally extending central opening and having an open end and a closed end portion, a plurality of rods each having a diameter not greater than five percent (5%) of the length of the rod, said rods being loosely housed within the drum and lying substantially longitudinally thereof, a cover member, means for detachably securing the cover member to the open end of the drum, two annular members longitudinally spaced apart within the drum and connected thereto, said longitudinal spacing being greater than the spacing between each of said members and the adjacent ends of the drum, said rods and said annular members and the inner faces of said closed end portion and said cover all being of hard material having a modulus of elasticity greater than twice that of steel, each of said annular members having an inner wall defining a substantially circular opening therein with said inner walls cooperatively engaging said rods and preventing their contact with said inner surface, and means for rotating the drum about said axis.
4. A rod mill in accordance with claim 3 wherein the inner faces of said closed end portion and said cover member are substantially planar and parallel and angularly disposed by approximately eighty degrees relative to said axis.
References Cited in the fiie of this patent UNITED STATES PATENTS 280,520 Sammons July 3, 1883 1,017,268 Parsons Feb. 13, 1912 1,068,289 White et al. July 22, 1913 1,295,726 Garrow Feb. 25, 1919 1,414,768 Behneman et al. May 2, 1922 1,449,592 Eggert Mar. 27, 1923 1,460,008 Willis June 26, 1923 (Other references on following page) 7 560,745- Germany Oct. 6, 1932 7 8 UNITED STATES PATENTS OTHER REFERENCES:
1,395,354" Taylor r V Miner-a1' Dressing by'Gaudin; McGraw-Hill BookCorm- 1,913,100 7 Bake June pan-y, 1939, pages 115-117. Comswck V- P 1 1939' 5 Handbook of Mineral Dressing by Taggart, published 211v6:161 Balke Jam 1938 by John H. Wiley and Sons, 1954, pages 5-04; 5 05, 5-08 2,680,568 Weston June 8, 1954 and 5 2,680,570 Wastcn June 1954' Handbook of Mineral Dressing, Taggart; tgublished by FOREIGN PATENTS 7 John Wiley'and' $9115 1945;, section 5, pages 3 1, 3-2. I 5594 Netherlands nunknm 111131111921 10 Information CII'CDIQI No. 6658, Department of Com merce, Bureau of Mines, October 1932-, pages 1-36', pages- 1194,800 Germany p 23, 1991 12, 13 W114 only cited I 634,939 Great Britain Mar. 29, 1950'