US2510574A - Process of forming spherical pellets - Google Patents

Description

June 6, 1950 c. w. GREENHALGH 2,510,574

PROCESS OF FORMING SPHERICAL PELLETS Filed June 7, 1947 um mm magma 24 /6 I H WIS I", INVENTOR 06905 W. GRffA/HA LGH ATTORN EY Patented June 6, 1950 PROCESS OF FORMING SPHERICAL PELLETS Cyrus W. Greenhalgh, Westport, Conn, assignor to Remington Arms Company, Inc., Bridgeport, Conn., a corporation of Delaware Application June 7, 1947, Serial No. 753,245

9 Claims.

This invention relates to a method of forming solid pellets of uniform size characteristics from a material in the liquid state and has particular application to the formation of spherical shot from molten alloys of such metals as lead.

For a period somewhat in excess of 100 years, shot has been produced in substantially the same manner by the dropping process. This process involves pouring the molten lead into a dropping pan at the top of a high shot tower. The bottom of the dropping pan is perforated and the hydrostatic head of molten lead in the pan is so related to the size of the perforations that the small globules of molten lead separate from the pan as discrete particles to assume spherical form, solidify, and fall into water. This traditional method fails to produce shot of uniform size and shape whenever a continuous fiow exists through the perforations. In producing the large sizes of shot, it has been necessary to use a dross pan in which the dross has a function not unlike that of a filter and reduces the effective hydrostatic head at the separating point to a degree which permits relatively large globules to separate as discrete particles. The dross process has several disadvantages, of which some of the most important are the relatively low rate of fiow through the dross, the added labor in maintaining the dross in the pan in proper condition, and the difficulty of controlling the variables which deter.- mine the size of the product.

The mechanics of globule separation in discrete particles are well illustrated in Figs. 2, 3 and 4 of U. S. Patent No. 2,113,279 and an improved dross pan is the subject of-U. S. Patent No. 2,383,315. These patents furnish good dis- 'cllssion of what has been heretofore regarded as the best practice by the two largest producers in the industry. Notwithstanding anything which may have been said in either of these patents, it is extremely difficult to produce consistently spherical shot of the desired size and substantially impossible to achieve both sphericity and size uniformity when any other than pure lead alloyed with optimum percentages of arsenic and antimony is used.

It is the object of this invention to present a shot dropping process capable of producing shot of great uniformity and to do so at a greatly increased rate.

It is an additional object to produce a shot dropping process which functions uniformly with any size of shot and one in which the only practical limitation on shot size is that imposed by the height of the shot tower. The height is a 30 product.

limitation in that time of fall must be adequate to permit the shot to have solidified to an extent which will enable them to resist deformation on impact with the water at the bottom. Naturally, the extremely large shot have such a capacity for the storage of heat as to require impractically high shot towers. I a

A further object is to present a shot dropping process in which shot of uniform size and sphericity may be produced with virtual independence of the previous requirements for alloys of particular composition.

The invention accomplishes these objects by a radical departure from previous practice, in that the molten material flows from the dropping pan in continuous streams rather than dripping from it in discrete globules, and in that means are provided to cause the continuous streams to separate into individual globules of predictable size characteristics. Specifically, the invention resides in the discovery that the application of vibration of predetermined frequency will cause predictable separation of the continuous stream and in the discovery that the combined use of vibration and a non-oxidizing atmosphere in at least the upper part of the tower permits the shotting of secondary lead with consistent results as to size, uniformity, and sphericity of the The exact nature of the invention as well as other objects and advantages thereof will more clearly appear from consideration of the following detailed specification referring to the appended drawing, in which:

Figure 1 shows diagrammatically the separation into individual particles of a vibrated stream of molten material.

Figure 2 is a vertical sectional view illustrating one method of applying vibration to the stream.

Figure 3 is a vertical sectional view of an arrangement suitable for working with secondary lead. A modified vibratory system is shown.

The form of a liquid flowing vertically through a circular orifice is at first nearly cylindrical though its diameter gradually diminishes from the orifice downwards because of the increasing velocity of the liquid. After it leaves the orifice the liquid is subject to no forces except gravity, the pressure of the air, and its own surface tension. Of these, gravity has no eifect on the form of the stream except in drawing asunder its parts in a vertical direction, because the lower parts are moving faster than the upper parts. The resistance of the air producesv little disturbance until the velocity becomes very great. But the of an external vibrator thatis adjusted in quency and amplitudeto set up a wave train,

surface tension begins to produce enlargements and contractions in the stream as soon as the liquid has left the orifice, and these inequalities in the form of the column go on increasing until it is broken up into elongated fragments. These fragments as they are falling through the air continue to be acted on by surface tension. They therefore shorten themselves and, after a series of oscillations in which they become alternately elongated and flattened, settle down into the form of spherical drops.

Since the principal problem is the determination of the mode of disintegration of an infinite cylinder and since a true cylinder is a stable form, the unstable equilibrium which is required for the breaking up of the column necessarily depends upon the peculiarities of the small displacements or irregularities to which the system is subjected. Surface tension endeavors "to contract the surface of the liquid so that the stability, or instability, of the cylindrical form of equilibrium depends upon whether the surface enclosing a given volume is greater or less respectivel after the displacement than before.

The propogation of deformations through a deforma'lo'le medium such as a stream of liquid is accomplished by wave motion; the distance between two successive corresponding points on the wave being the wave length of the disturbance. It has been shown by the classical researches of Plateau, Lord Rayleigh and others, that the equilibrium of a jet is stable if the wave length of the disturbance is less than the circumference of the cylindrical stream, but unstable if greater than the circumference of the stream. Disturbances of the former kind lead to vibrations of harmonic type whose amplitudes always remain small; but disturbances whose wave length exceeds the circumference result in a greater and greater departure from a cylindrical form, resulting in disintegration of the stream. The joint operation of surface tension and inertia in fixing the wave length of maximum instability has been analyzed. On both theoretical and experimental bases it has been shown that the maximum instability of the stream is effected where the wave length of any displacement or disturbance present is approximately four and a half times the average diameter of the stream in the region where the incipient separation is developing.

The relative importance of two harmonic disturbances depends upon their initial magnitudes and upon the rates at which they grow. When the initial values are very small the latter consideration is much the more important and that one is ultimately preponderant for which the measure .of instability is the greatest. The

smaller the causes by which the original equilibrium. is upset the more will the cylindrical mass tend to divide itself regularly into portions ,whose length is equal to 4.5 times the diameter.

But a disturbance of less favorable wave length may gain preponderance in case its magnitude is suflicient to produce distintegration in a lesser time than that required by the other disturbances present.

No great difficulty is recognized in the applieationof these results to the shotting of lead. The most certain method of obtaining complete regularity of resolution is to bring the reservoir and hence the liquid stream under the influence the distance from trough to trough or rarefacor nearly so as at 9.

tion to rarefaction, as the case may be, corresponding to the wave length of maximum instabilit of the stream.

Referring to the drawing by characters of reference, Figure 1 illustrates in rudimentary form the factors involved in the practice of the invention. There is provided a pan l in which there may be maintained by any suitable means a supply of the molten material 2 adequate to insure flow from the orifice 3 in the form of a continuous stream 4. By means of a loudspeaker 5, driven by an amplifier 6, excited by an oscillator 1 or other suitable means, a pressure wave may be applied to the surface of the molten material 2 which is communicated hydrodynamical'l to the stream 4. As pointed out in the theoretical discussion preceding, the most favorable condition for separating the column will be one in which the frequency of the applied energy is such as to tend to produce a vibratory node at intervals approximately 4.5 times the diameter of the column. Obviously, the velocity of flow through the orifice will be the determining factor in arriving at a determination of the frequency best adapted to cause separation. As shown diagrammatically, the vibratory nodes will tend to cause separation such as at 8. Once started, the separating action proceeds rapidly and at the next vibratory node may be complete Separation is shown complete at l8 and particles ll, l2 and 13 show the conversion to spherical form under the effect of surface tension. Although a particular frequency is, as noted above, best adapted to cause separation, the frequency may be varied over fairly wide limits without affecting the consistency of separation. Ina typical example, it was found possible to vary the frequency of vibration from to about 2'70 cycles per second cor responding to wave length variations be mean about 3% and 5% times the diameter of the stream without causing irregular or spurious separations. Obviously, the frequency governs the separation between adjacent vibratory nodes and hence the volume of material going into a given pellet. Thus, with the same size of aperture and the same hydrostatic head, a high frequenc causes short intervals and small pellets while a frequency on the lower end of the range of allowable frequencies produces larger pellets. When the applied frequency is much below that tending to produce a condition of maximum instability of the column, an unstable condition of separation may exist in which one or more fine particles will separate themajor particles, or in which one or more fine particles may merge with a maior particle to produce an oversized particle. Above the optimum frequency the range of permissible variation is much greater. In fact, it may often be desirable to employ a higher frequency than the calculated optimum, for within a very short distance of the pan the stream velocity will be much greater than that upon which the calculations were based and the stream diameter will be materially less. The best theoretical condition for uniformity is that in. which the applied frequency considered in conjunction with stream velocity, which is, in turn, dependent upon hydrostatic head in the pan, is such as to tend to cause separation in lengths corresponding to the maximum instability of the stream, although, as noted, variations over a fairly wide range above the calculated optimum frequency maintain adequate control.

A desirable minimum frequency may be approximated by determining the velocity of flow from an orifice in accordance with Torricellis theorem v=vat .035" diameter orifice 0.71 coefiicient .043" diameter orifice 0.84 coefficient .055" diameter orifice 0.90 coefiicient .070" diameter orifice 0.88 coefficient The value on the smaller orifice probably indicates partial obstruction by dross which may be inhibited by fluXing discussed hereinafter. Since it has already been pointed out that the condition of maximum instability of the liquid column exists when 1=4.5d, we may readily obtain the length in feet (1) of the section of maximum instability. Then, using an orifice coefficient of .85 as a suitable working value,

A m 2Z2 l,

f 45d cycles per .ecoad I where h is hydrostatic head in feet and d is orifice diameter in feet. As previously pointed out, this value of frequency is an approximation since orifice coefiicients may vary somewhat under different conditions. It is, however, sufficiently close to arrive at a value of frequency well within the operative limits for any particular orifice and hydrostatic head, although it should be noted that departures from it should generally be in the direction of a higher frequency. Since the factors of hydrostatic head and frequency are readily controllable, experimental runs, particularly with the aid of a stroboscopically flashing lamp, synchronized with the frequency applied, will serve to determine the end values.

Although both frequency and hydrostatic head are, as noted above, controllable factors, the securing of optimum results is dependent upon the maintenance of diametral uniformity in the various streams flowing from the orifices. Molten lead exposed to the air in the shower pan naturally oxidizes and lead oxide or dross is likely to cause partial obstruction of one or more orifices, One method which has been employed to good advantage has been the application .of an oxidation preventing flux to the alloy in the shot pan. Zinc chloride has been shown to be an effective flux and others are also well known.

The exact method of applying the vibratory,

impulse to the stream or the axis of the applied vibration do not appear to be particularly critical so long as the vibration carries through to appear as a vibration of the flowing stream per se in which substantially the frequency of maximum instability is predominant. Experimentally, the column has been vibrated by means of a pressure wave applied to the liquid in the shower pan, by vibration of a plate immersed therein, by vertical or transverse vibrations of the pan itself and less efficiently by a pressure wave directed upon the flowing stream. Although by reason of flexibility in control electrical vibration is deemed preferable, mechanical vibratory techniques have been utilized effectively with suitable control of amplitude and frequency and with precautions taken to insure freedom from undesirable spurious frequencies.

Figure 2 illustrates an installation consisting of the guard or stack 20 surrounding the falling shot in the tower and the water filled tank or cistern 2| which receives them. Suspended at the upper end of the tower is the shower pan 22 which may be provided with an inner annular partition 23 terminating a suitable distance above the perforated bottom of the pan. A ring burner 24 may be used to assist in maintaining in molten condition lead supplied from a furnace (not shown) through a conventional tube 25. The outer annular chamber provides a reservoir of molten lead 26 which flows uniformly over the partition 23 to maintain the desired hydrostatic head in the central chamber 21. The flow through the tube 25 may be conveniently adjusted to equal the rate of flow through the perforations. The use of the annular partition is desirable in that it provides for isolating the liquid directly above the apertures from turbulence or spurious vibrations incident to flow through the supply tube. Vibration of the streams may convern'ently be excited by means of a plate 28 immersed in the reservoir 21 and connected by means of a drive rod 29 to a drive unit 30, Conveniently the drive unit may consist of a coil movably supported in an intense magnetic field. The coil can be excited by passing therethrough current from an amplifier 3| which may be excited by an oscillator 32 or other convenient source of the desired, and preferably adjustable, frequency.

The operation of the setup shown in Figure 2 is believed to be obvious since the plate 28 causes the molten liquid in the reservoir 27 to transmit vibration through the perforations to each of the streams 33 which separate as shown in Figure 1 into discrete particles 34 of regular size. As in the conventional process, surface tension draws the molten shot into spherical form and they solidify as they fall through the tower. The water in the cistern 2| breaks the fall of the pellets and they are removed therefrom by suitable conveying means for drying, graphiting, inspection, and exact size grading.

In prior research on shot dropping problems, it has been established that the formation on the pellets of oxides which have a melting point sensibly higher than that of the lead alloy is detrimental. For this reason, it has been common practice to add to relatively pure lead small quantities of arsenic which has been shown to result in the formation of a complex oxide which has a melting point lower than that of lead. This lowered melting point is important, since, in the absence of arsenic, a globule of lead passing through an orifice in the shot dropping pan is otherwise immediately coated with a solid sheath of lead oxide which prevents the forces of surface tension from causing the globule to assume a spherical form. Shot on which such solid oxides have formed show a characteristic teardrop or tailed form. When an attempt is made to use secondary lead or any alloy of lead containing minute quantities of tin, magnesium, zinc, cadmium, and other impurities, it is found that no amount of arsenic will result in the elimination of tails from the shot. Prior work has demonstrated that the use of a neutral or reducing atmosphere beneath the pan, and far enough therebelow to insure that the globules have become spherical, is partially succesful in eliminating tails. However, for some reason, not adequately understood, the use of artificial atmospheres, irrespective of the alloy tested, adversely affected the size uniformity of the resulting shot.

When, however, highly purified inert 'atmos p-heres are used and vibration of the proper frequency is induced in the continuous streams flowing from the shower pan, spherical shot of uni- :form size are producedifrom a wide variety of different lead alloys. There are no tails such as are ordinarily characteristic of secondary lead and the size is quite uniform in contrast to the random sizing otherwise characteristic of the controlled atmospheres with any type of alloy.

Figure 3 shows a typical installation for dealing with secondary lead in the manner just described. The installation includes the usual tower 46 and cistern 4! to receive the solidified shot. Suspended above the tower is a shower pan 52 which may be similar to that shown in Figure 2. As shown, the pan is provided with a partition 53 separating a reservoir of molten metal 56 from the metal ll above the apertures. A burner 44 may be provided to assist in maintaining the pan at the proper temperature, although in regular production suflicient heat is absorbed from the lead flowing through the system to permit outside heating to be dispensed with. The shower pan may be supplied with molten lead in conventional fashion through a tube 45 from a furnace not shown. Vibration is imparted to the flowing streams by means of drive bars ts con nccting the shower pan to a drive unit 5!) preferably, although not necessarily, of an electrical type. The drive unit may be conveniently energized by means of intermittentl applied direct current or by an alternating current of suitable, and preferably adjustable, frequency. Since, in this case, the entire pan assembly is vibrated, the drive unit must be selected to provide sufiicient power. Care should be taken to design the system in such a way that spurious frequencies, perhaps resulting from resonance phenomena in some portion of the apparatus, do not predominate the desired frequency.

Secured in fluid tight relation to the bottom of the shower pan is a tubular member 53 several feet in length to insure that it extends over the distance required for the globules to have assumed spherical form. Just below the shower pan there is provided a circular manifold M having a number of approximately uniformly spaced gas inlets. The manifold is connected by suitable tubing 55 to a gas cylinder 56 containing a highly purified inert or reducing gas substantially free from oxygen or water vapor.

Oil pumped nitrogen having a purity of 99.8 was not satisfactory apparently because of the oxygen and water vapor which were the major impurities present. Pro-purified, oil pumped nitrogen, which is the most highly purified nitrogen commercially available, was, however, entirely satisfactory.

Although the remaining percentage of impurities in such a grade of nitrogen is not known, it is understood that the purification process involves passing relatively pure nitrogen such as the 99.8% purity discussed above, through a mass of heated copper shavings which serve to remove most, if not all of the oxygen and water vapor present. Hydrogen of similar purity is also useful.

In operation, the tube 53 is purged by flowing about thirty times its volume of gas therethrough prior to commencement of pouring the shot and thereafter relatively small quantities of gas may be supplied to produce enough flow to prevent substantial diffusion into the tube through the open bottom. When the molten lead streams through the orifices, the stream is vibrated as a result of the vibration of the pan. The separation of each individual stream into discrete globules takes place in substantially the same fashion as discussed in connection with Figure 1. The product collected from the cistern is consistently spherical and quite uniform in size.

Although my preferred process has been described herein byreference to fairly Specific examples, it should be understood that I do not con' sider my invention to be limited thereto. For example, the same principles govern the production of discrete particles from other metals and from non-metallic materials such as certain plastics which may be reduced to a liquid form and which it is desirable to separate into particles of 1. A process of producing discrete particles of solid material which comprises the steps of flowing a continuous stream of the material in liquid form through an aperture, applying to said stream vibration of a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between three and onequarter and five and three-quarters times the diameter of said continuous stream to insure separation of the stream into discrete globules of uniform size, and maintaining said globules separated until they have returned to solid condition.

2. A process of producing discrete particles of solid material which comprises the steps of reducing the solid material to a liquid condition, flowing a continuous stream of the liquid material through an aperture, applying to said stream periodic vibratory impulses having a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between 3% and 5% times the diameter of the stream whereby 'the stream is caused to separate into discrete globules of uniform size, and maintaining said globules separated until they have returned to solid condition.

3. A process of producing discrete particles of solid material which comprises the steps of flowing a continuous stream of the material in liquid form through an aperture, applying to said stream vibration of a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between three and one-quarter an: five and three-quarters times the diameter of said continuous stream to insure separation of the stream into discrete globules of uniform size, and allowing said globules to fall until they have returned to solid condition.

4. A process of producing discrete particles of a meltable solid material which comprises the steps of melting the material to liquid form, fiowing a continuous stream of the liquid material through an aperture, applying to said stream periodic vibratory impulses having a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between 3 and 5% times the diameter of said stream whereby the stream is caused to separate into discrete globules of uniform size, and maintaining said globules separated until they have cooled to a solid condition.

5. A process of producing shot from molten material which comprises flowing the molten material in continuous stream form from at least one aperture, applying to said stream vibration of a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between three and one-quarter and five and three-quarters times the diameter of said continuous stream to insure separation of the stream into discrete globules of uniform size, and allowing said globules to fall until they have cooled below the freezing point of the material.

6. A process of producing shot from molten material which comprises flowing continuous streams of the molten material from apertures of uniform size, applying to said streams vibration of a frequency determined by the ratio of the velocity of said continuous stream to adiuisor of between three and one-quarter and five and three-quarters times the diameter of said continuous stream to insure separation of each stream into discrete globules of uniform size, allowing said globules to fall freely until they have assumed spherical form and have solidified to an extent sufiicient to prevent distortion, and collecting said globules in a bath of cooling fluid.

'7. A process of producing shot from molten lead alloys which comprises applying an oxidation inhibiting flux to said molten alloy, flowing a continuous stream of the molten alloy from an aperture, applying to said stream vibration of a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between three and one-quarter and five and three-quarters times the diameter of said continuous stream to insure separation of the stream into discrete globules of uniform size, and allowing said globules to fall freely until they have solidified sufficiently to resist distortion on impact with water.

8. A process of producing shot from molten lead alloys which comprises supplying said molten alloy to a shower pan provided with apertures of uniform flow characteristics, applying to the alloy in said shower pan an oxidation inhibiting flux, flowing continuous streams of the alloy from said apertures, and applying to said streams vibration of a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between three and onequarter and five and three-quarters time the diameter of said continuous stream to insure separation of the stream into discrete globules to fall freely until they have solidified sufliciently to resist distortion on impact with water.

9. A process of producing shot from impure lead alloys which comprises flowing the molten lead alloy in continuous streams from apertures in a shot pan into an atmosphere inert as to the molten lead alloy and substantially free from oxygen or water vapor, applying to said streams vibration of a frequency determined by the ratio of the velocity of said continuous stream to a divisor of between three and one-quarter and five and three-quarters times the diameter of said continuous stream to insure separation of said streams into discrete globules of uniform size, allowing said globules to fall through said atmosphere until they have assumed spherical form, and allowing said globules to further fall until they have cooled below the freezing point of the alloy.

0 CYRUS W. GREENHALGI-I.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 228,481 Shiver June 8, 1880 260,976 Guild July 11, 1882 194,271 Shiver Aug. 14, 1887 2,287,029 Dowdell June 23, 1942

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