US2787534A

US2787534A - Metal powder

Info

Publication number: US2787534A
Application number: US294425A
Authority: US
Inventors: Henry A Golwynne
Original assignee: Rufert Chemical Co
Current assignee: Rufert Chemical Co
Priority date: 1952-06-19
Filing date: 1952-06-19
Publication date: 1957-04-02
Anticipated expiration: 1974-04-02

Description

April 2, 1957 Flled June 19, 1952 E N f 526,5 w N 1 ...M 1 11 #l wax wm R W (/ms 5, m m@ E JIuI/U.. y m n 7 m Y f mw hw MUSE m, //J. n. S Vla. A T a a u?. I i ,A m M S c 6 I P mm um a NN \n IIN j Y. d QN B, E BQ N 55E e N Dn W w W L w mommm O .MM l; G m Q A. m H.

ESI l o ma N@ mocmm April 2, 1957 H. A. GOLWYNNE 2,787,534

METAL POWDER 6 Sheets-Sheet 2 Filed June 19, 1952 (gy JNVENTOR.

HENRY A GOLWYNNE -1 J ATTORNE S April 2, 1957 H. A. GOLWYNNE METAL. POWDER Filed June 19, 1952 6 Shets-Sheet 3 ,11:13: 1;', l: if f INVENTOR.

April 2, 1957 H. A. GOLWYNNE METAL. POWDER 6 Sheets-Sheet 4 Filed June 19, 1952 April 2, 1957 H. A. GoLwYNNl-I METAL. POWDER FiledY June 19. 1952 6 Sheets-Sheet 5 INVENTOR.

HENRY A GOLWYNNE 4% AT TOR N Yo April 2, 1957 H. A. GoLwYNNl-z 2,787,534

METAL POWDER Filed June 19, 1952 6 Sheets-Sheet 6 FIG.|2

FIG. I3

BY @MW/f ATTO RN 5 United States Patent O NIETAL POWDER Henry A. Golwynne, New York, N. Y., Chemical Company, Essex, Conn., Delaware assigner to Rufert a corporation of This invention relates to metal powders and `has for its object improvements in metal powders as 'a product or article of manufacture. i

Metal powders are produced in various ways: mechanically, in which larger pieces of metal are ground, beaten or broken into smaller pieces; electrolytically, in which finely divided particles are electrodeposited out of solution onto a cathode; chemically, in which metal particles are precipitated out of solution; and metallurgically, 1n which a body of molten metal is converted into finely divided particles. The latter operation is conducted 1n various ways, one of them being to atomize a fine stream of the metal with gas in a container filled with the same gas.

The manner in which the metal powder is produced affects its physical and chemical characteristics, and hence its usefulness. Among the physical characteristics are particle size and shape, density, surface area and smoothness, cleanliness, etc. Thus the particles may be variously' shaped: fiat, round, oval, elliptical, spherical, spheroidal, etc., with smooth or rough surfaces. Among the chemical characteristics are the presence or absence in or on the powder of metal compounds, such as oxides, nitrides, carbonates, sulfates, etc., of the metal or metals employed.

As is to be expected, therefore, a metal powder of given physical and chemical characteristics is better adapted for certain uses than for other use-s. In the case of magnesium powder, for example, its usefulness for flash purposes, such as in illumination shells and incendiary bombs, iiash bulbs, tracer bullets, and the like, much used in modern warfare, is greatly impaired when the powder particles are coated with oxide, nitride, carbonate, sulfate, etc. This is for the reason that such coated particles are not in condition to ignite satisfactorily, too little bare metal surface area being available for the purpose. That surface area, moreover, is external, being governed by the size of the powder particles, the smaller the particles the larger' being the total surface area of `a given amount of the powder.

My investigations have led to the discovery of greatly improved metal powders which may be produced in such a manner as to increase and improve substantially its effective surface area. That is, a particle of metal powder of given size has substantially greater surface area than a particle of metal powder of the same size made by conventional methods, and more especially from molten metal.

These and other features of the invention will be better understood by referring to the accompanying drawings, taken in conjunction with the following description, in which:

Fig. l is a diagrammatic flow sheet showing the syst-ern as a whole;

Fig. 2 is a side elevation of the purifier shown in Fig. l;

Fig. 3 is a plan view of one of the trays in the purifier;

Fig. 4 is a cross-section on the line 4 4 of Fig. 3;

Fig. 5 is a cross-sectional view of one of the melting furnaces shown in Fig. 1;

atented Apr. 2, 1957 ICC Fig. 6 is an enlarged cross-sectional view of the atomizer shown in Fig. 5;

Fig. 7 is a longitudinal section of the chamber shown in Fig. l for atomizing molten metal and forming metal powder;

Fig. 8 is a transverse section on the line 8-8 of Fig. 7;

Fig. 9 is an enlarged side view of one of the powder-gas separators shown in Fig. 1;

Fig. 10 is a similar view, partly in section, of the lower end thereof;

Fig. l1 is a front elevation of the same.

Fi g. l2 is a diagrammatic exterior representation of particles from a sample of metal powder illustrative of a practice of the invention; and

Fig. 13 is a diagrammatic interior representation of other particles from the same sample of metal powder.

Reference is made to Fig. 1 for an overall picture of the apparatus. Since purification of the gas in the system is of initial importance, the main features of its system or circuit will first be described.

This includes a gasometer 1, a conduit 2, a main conduit 3, a branch conduit 4, a dust filter 5, a conduit 6, a branch conduit 7, a filter 8, a branch conduit 9, a branch conduit 10, a filter 11, a branch conduit 12, a compressor 13 connected on its inlet or suction side with conduit 6, a conduit 14 connecting the outlet or pressure side of the compressor, a filter 15, a Conduit 16, a branch conduit 17 connecting the inlet of a purifier 18 surrounded by a heating furnace or chamber 19 having an inlet 20 for the introduction of heating gases and an outlet 21 for the escape of spent gases, and a conduit 22 connecting the outlet of the purifier with the gasometer. The latter conduit may, of course, connect the system at any other suitable point. While some valves or closure members are indicated, it will be clear that any suitable number may be employed at other points in the circuit. ln general, it will be seen that gas may pass from the main system by way of thc gasometer and main conduit 3 to and through dust filter 5, one or both filters 8 and 11, to and through the compressor, through filter 15, purifier 1S and back into the system. This cyclic operation may be continued until all of the gas in the system reaches the desired purity. What has been described may be regarded as part of the main system, or indeed a circulating system within the main circulating system. When all of the gas is purified, it is unnecessary to continue with the purification operation and it may be shut-off until again needed.

The main system or circuit for producing the metal powder will now be traced. It includes a branch conduit 3i) connecting main conduit 3 with a bonnet 31 at one end of a molten metal atomizing and powder forming chamber 32 for providing cool inert gas with which to freeze the atomized metal to powder. Another branch conduit 33 also connects main conduit 3 with the lower portion of the chamber in order to supply additional gas for keeping the newly formed metal powder in suspension, as will be described in more detail below. A branch conduit 34 similar to branch conduit 30, connects main conduit 3 with a bonnet 35 at the other end of the chamber. Conduit 36 connects `the same or discharge end of the chamber with a blower 37, a conduit 38, a powdergas separator 40, a conduit 41, another powder-gas separator 42, a conduit 43, and a filter 44 which in turn connects main conduit 3. A branch conduit 45 connects the main conduit with a filter 46 and a conduit 47. Similarly a branch conduit 50 connects the main conduit with a filter 51 and conduit 52. In practice it is customary to alternate use of

filters

46 and 51, so that while one is being cleaned or otherwise serviced, the other is used.

The next system or circuit to be traced is the one having to do with the use of gas to force a fine stream of molten metal to the atomizer and the use of gas to atomize the stream of molten metal. 1t includes conduit 16, from the outlet or pressure side of compressor 13 and a branch conduit 6d terminating in branch conduits 61 and 62. Branch 61 connects with a conduit 63 one end of which connects with the interior of a melting furnace 64 and the other end of which connects t-:ith the interior of chamber 32. A bonnet 65 connects the furnace to bonnet 31 ofthe atomizing and powder forming chamber.

A similar arrangement is provided for the other end of the chamber and includes a branch conduit con necting conduit 16 and terminating in

branch conduits

71 and 72. Branch 71 connects with a conduit 73 one end of which connects with the interior of a melting furnace 74 and the other end of which connects with the interior of chamber 32. A bonnet 75 connects the furnace to bonnet 35 of the atomizing and powder forming chamber.

Figs. 2, 3 and 4 show the purifier in detail. It is cylindrical in form with aclosed bottom (see Fig. 2) resting on supports 81 on the bottom of the interior of the heating chamber. Superposed trays or baskets 32 rest on an inner tiange support 83 integrally secured to the side wall of the purifier. thc flange being located directly above inlet 17. An imperforate cover 85 is securable to the top of the purifier by means of a plurality of bolts 86 through an outer flange 87. All joints of the purifier are gas tight so that heating chamber gases cannot enter the purifier and so that gases inside the purifier cannot escape into the heating chamber.

rThe trays (see Figs. 3 and 4) are generally cylindrical in shape. being formed of a band of sheet metal 90 with a turned in bottom flange 91 and a similar turned in top flange 92. A circular piece of coarse mesh circular screen 94 rests on and is secured to the bottom flange. A circular piece of fine mesh screen 95 in turn rests on the coarse mesh screen. the latter serving as a firm support for the former. A relatively thin layer of finely divided metal. particles 96, such as magnesium powder if magnesium powder is to be produced, rests on the screen. A handle or bar 97 is secured to the underside of upper flange 92.

In preparing the purifier `for use, the top of heating chamber 19 is removed, cover 85 of purifier 18 is also removed and loaded trays or baskets 82 are placed therein, one superposer! on the other. In the case of the bottom tray. its bottom flange 91 rests on inner flange support $3. Lower flange 91 of the second tray Will rcs! on upper flange 92 of the first tray, etc. The purifier is provided with :1 sufficient number of the loaded trays. Cover 85 is then returned and bolted securely to outer flange 87 with bolts 86: after which the top of the purifier is returned.

Melting furnace 64 is shown in detail in Fig. 5. It rests on a dolly 90 so that it may be wheeled into and away from operating position with respect to chamber 32. The furnace is formed of a rectangular heating chamber 91 surrounded by a metal casing 92 lined with refractory brick 93. The chamber is surmounted by a removable top 95. A plurality of spaced openings 97 are provided through the side walls near the bottom of the chamber into cach of which is fitted an oil or gas burner 98. Three such burners are at present employed.

A stack 99 is mounted on the top for the escape of spent heating gases. A melting pot 100 rests on supports 101 at thc bottom of the heating chamber. The top of the melting pot is provided with a removable cover 102 securable to an outer flange 103 by means of a plurality of bolts 134 to provide a nonleaking joint. A metal charging conduit 106 extends through the top of the heating chamber and communicates with the interior of the melting pot. Tt is secured to cover 102 by means of `plurality of bolts 107. A removable cover 110 fits over the top of the charging conduit and can be screwed thereon to malte a leak-proof joint. Branch conduit 63 for inert gas connects with the charging conduit above furnace top 9S. A pyrometer 112 extends through the top and cover 102 well into the interior of the melting pot. Branch conduit 62 `for inert gas extends into heating chamber 91 and is spirally wound around the melting pot. The discharging end of the conduit extends into the interior passageway 114 of brick-lined furnace bonhet 65. A conduit 115 for the passage of molten metal extends from near the bottom of the melting pot upwardly through its cover 102 and into passageway 114 of furnace bonnet 65 where it connects with an atomizer 15.6 extending through endcover 117 into chamber bonnet 31. The two bonnets are connectible with bolts 119. The furnace bonnet is provided with an opening 120 in close proximity to the atomizcr and is fitted with a burner 122 adapted to supply heating gases at and around the atomizer.

Atomizcr 116 is shown in more detail in Fig. 6. It is in the form of a main body portion 124 hollowed out at one end to receive the discharge end of conduit 115 from thc melting pot and hollowed out at the other end tu receive an atomizing nozzle 125 and to form a gas distributing chamber 126 around the nozzle. The tip end of the nozzle, with discharge outlet 12S, extends through cap 129 screwed onto the other end of the main body portion. Conduit 62 for inert gas connects with a nipple 13d communicating with the gas distributing chamber. Cap 129 is provided with a plurality of inclined circumfercntially spaced discharge ports 132 adapted to blast small streams of preheated inert gas against a streamv of molten metal discharged through nozzle outlet 128 at a point or area 134 a suitable distance forward of the nozzle tip.

Figs. 7 and S show chamber 32 for atomizing molten metal and forming powder therefrom in more detail. lt is in the form of a cylindrical tank divided essentially into an upper compartment 140, which is in the form of a trough, a lower compartment 141, which is essentially in the form of a duct; and two

side compartments

142 and 143, which are separated from each other as wel! as from the other two compartments. All of the cornpartments extend longitudinally of the chamber. The upper and lower compartments are tapered in reverse order. Thus, upper compartment has its smallest cross-section at the right end of the tank, and its largest cross-section at the left end. The lower compartment or gas distributing duct 141, on thc other hand. has its largest crosssection at the right end, where the gas is shown to enter through conduit 33. and its smallest crosssccticn at the left end.

The compartments are obtained by the use of opposed pairs of overlapping plates 151 to 158, top plates 161 to 168 and a pair of spaced side plates 171 and 1'72. They are suitably welded to the cylindrical wall and to each other to provide an integral structure. Plates 151 to 158 are pitched or slanted at an angle, toward the bottom of the trough,y so that powder settling thereon will tend to slide down into the bottom of the trough. Referring for a moment to Fig. 7, it will be seen that an overlying lip 174 is welded to the head oi' the cylindrical tank at the right end to provide a slot 175 between inlet conduit 33 and upper compartment 140, Similar underlying lips 176 to 182 provide slots 185 to 191. Upper plate 161 of the duct is spaced above the bottom of the tank to provide a similar slot 192 at the left end of the tank, in direct communication with outlet conduit 36. The overlying and underlying lips are sul'liciently long to cause gas passing therethrough to sweep along the bottom of the upper compartment. in other words. gas going through slot 175 sweeps along the top of. plate 16S, gas passing through slot 135 sweeps across the top of plate 167, gas passing through alot 186 sweeps across the top of. plate 166, etc.

A gas blow-off 195 is provided alongA the top of' the tank. ln case of an explosion, the blow-off is opened to release explosive forces, thus saving the chamber from damage. An inlet 19S connects each side compartment with inlet conduit 33. An outlet 199 connects each side compartment with outlet 36. While the side compartments are normally sealed from each other and the other compartments, it is possible for leaks to develop at the welded joints. lt is therefore desirable to have an arrangement which will permit the side compartments to be cleared of air and filled with inert gas, and this can be done with the inlets and outlets referred to.

Figs. 9, and 1l show powder-gas separator 40 in some detail. Powder-gas separator 42 is advantageously of the same general construction.

Referring first to Fig. 9, the apparatus shown is in the form of a combination powder-gas separator and collector, being specifically a conventional cyclone 200, with an inlet 201 for powder-gas and an outlet 202 for gas, and a hopper 203 integrally secured to and depending from the cyclone. The lower end of the hopper terminates in a laterally extending discharge conduit 205 at or near the outlet end of which is an upright relief chamber 206. A closure member 207 is pivotally supported at the outlet.

Referring next to Fig. l0, it will be seen that relief chamber 206 provides a substantial amount of free space 210 at its upper end which extends a suitable distance above the place Where the chamber joins the conduit. The top of the relief chamber is fitted with a screw cap 211 provided with a plurality of circumferentially spaced handles 212.

In the specific construction shown the discharge outlet is at a convenient angle to cooperate with closure member 207, which is in the form of an enclosed chute pivotally or hingedly connected at 213 to a fixed support 214. The chute has a bottom 215, back 216,

side walls

217 and 218, a top 219 and a discharge opening 220. A piece of resilient gasket material 222, such as rubber, is integrally secured to the bottom of the chute so that it may be brought into sealing contact with the discharge outlet.

The discharge outlet is in the form of a pipe welded at its side to the main part of the conduit, the upper end of the pipe being an extension which forms the relief chamber. The chute is integrally secured at its top 219 to an outer collar 224 fitting loosely around the lower end of the discharge conduit. The upper end of the collar is integrally secured to the lower end of a flexible sleeve 225 fitting loosely around the discharge conduit. This sleeve may be of rubber and is adapted to act like a bellows. The upper end of the flexible sleeve is in turn integrally secured to the discharge conduit. When the chute is moved upwardly and downwardly, the exible sleeve yields sufficiently to permit the discharge outlet to be closed and opened.

A handle 228 is secured to the top of the chute at its discharge end so that the operator may readily lower the chute to open the discharge outlet and raise the chute to close the outlet. This is a manual operation which may remain wholly in control of the operator.

If the operator, however, should instinctively let go of the handle in case of lire, or for any other reason, or should leave the scene, self-closing means associated with the chute at once take care of such a contingency. The particular self-closing means disclosed include a pair of

spring tensioning devices

230 and 231 Secured at their lower ends to the chute and at their upper end to a fixed support, in the instant construction at the upper portion of the relief chamber. A turnbuckle is attached to each spring so that the springs may be placed under the proper amount of tension. When downward pressure is applied to the chute, such as by bearing down on handle 228, the springs yield sufficiently to permit the chute to drop away angularly from the discharge outlet. To make certain that the chute is not lowered unduly, for example, by the operator when excited, a chain 234 of predetermined length is fastened at its lower end to the chute and at its upper end to a fixed support. The chain fixes and limits the amount `of drop for the chute. As a further precaution, a holding device 236 with a turnbuckle 238 is hooked at its lower end to the chute and at its upper end to a fixed support (not shown). When the chute is in its closed position, the turnbuckle is tightened. Pressure on handle 228 will, therefore, not open the chute. This feature is particularly desirable if children or non-trained operators have access to the apparatus.

When, therefore, it is desired to discharge metal powder from the apparatus, the operator must deliberately loosen turnbuckle 238 in order to release holding device 236. He then applies downward pressure on handle 228, which causes chute 207 to move or open downwardly. Metal powder then moves by gravity from hopper 203 through conduit 205 and its discharge outlet into the chute, and downwardly through the chute out of its discharge opening 220 into a container 240.

lf all goes well the handle is kept down until the desired amount of powder runs into the container. The handle is then lifted by the operator to close the discharge outlet and the holding device is again replaced. With the self-closing means in use, however, the operator need merely release the handle and spring-tensioning

devices

230 and 231 will lift the chute into its closed position. As already indicated this feature is particularly desirable in case of fire and if the operator should let go of the handle or leave the scene in fright. In case of re this is precisely what he should do for safety.

The apparatus may be operated as follows:

Since the circulatory system is initially filled with air, it is important that it be replaced with a suitable inert gas, such as helium. Helium for this purpose is obtained commercially in cylinders. With an adequate supply of cylinders on hand, the helium is fed into the system at one or more high points and air is removed therefrom at one or more low points; for example, referring to Fig. 1, the helium may be fed into gasometer 1 and air may be withdrawn from conduit 36 in advance of blower 37. Whatever practice is followed a sufficient amount of helium is fed into the system to flush out most of the air. Enough helium is thus used to place the system under substantial positive pressure greater than atmospheric to keep outside air from seeping into the systern. A manometer pressure of 2" water has worked Well.

Since helium and air are readily miscible, no matter what precautions are taken an appreciable amount of air will be in the system after the flushing operation; and since this residue of air and the helium contain objectionable amounts of oxygen and nitrogen, purifier 18 is placed in operation. As shown in Fig. l circulation of the helium through the system may be effected by the use of compressor 13, as well as by blower 37, or both. In any event helium from the system is passed continuously and cyclically through the purifier. Heating gases, such as provided by oil or `gas burners, are fed through inlet 20 into heating chamber 19, while spent heating gases escape from the heating chamber through outlet 21 to the open atmosphere. impure helium from the system is forced by compressor 13 through conduit 14, filter 15, conduit 16 and branch conduit 17 into the purifier 18. The purilied helium rises upwardly through conduit 22 and is re turned to the main system.

As more particularly shown in Figs. 2, 3 and 4, the impure helium passes upwardly through layers 9S of finely divided metal particles in superposed perforated trays 82. The heating gases applied externally to the purifier are adapted to heat the metal particles to a temperature sufficiently high for the oxygen and nitrogen impurities to react therewith to form metal oxide and metal nitride. Assuming that magnesium powder is to be produced, the layers are formed preferably of magnesium powder, about 1A" to V2" deep; the depth being such that the helium may pass readily therethrough. The oxygen and nitrogen impurities react with the magnesium to form magnesium oxide and magnesium nitride which are retained in the 7 layerson `the trays. In the particular arrangement shown, the helium thus purified (see Fig. l) passes through conduit 22 into gasometer 1 where it mingles with helium still to be purified.

Since the circulatory system is open, helium passes continuously through gasometer 1.

conduits

2, 3 and 4. tiiter 5. conduit 6, branch conduit 7, filter 8, branch conduit 9, branch conduit 10, filter 11, branch conduit I2, compressor 13. conduit 14, filter 15, conduit 16, branch conduit 17, purifier 18, conduit 22, and again gasometer 1. As this cyclic operation continues, helium is also forced by blower 37 through conduit 38, powder-gas separator 40, conduit 41, powder-gas separator'42, conduit 43, filter 44, main conduit 3, branch conduit 45, filter 46, conduit 47, as well as branch conduit 50, filter 51, conduit 52, main conduit 3, branch conduits 3i) and 33 into and through chamber 32, inlets 198, side compartments 142 and 143, outlets 199. conduit 36, and back to blower 37.

ln a similar .manner compressor 13 also pulls some of the helium from main conduit 3 through branch conduit 4, dust filter 5, conduit 6, branch conduit 7, lter 8, branch conduit 9, as well as branch conduit l0, filter 11, branch conduit 12, conduit 6, the compressor itself, conduit 14, lilter 15, conduit 16 past branch conduit 17 to and through branch conduit 60, branch conduits 61 and 62 to furnace 64. As more clearly shown in Figs. 5 and 6, helium passing through conduit 6l goes into melting pot 100. This helium may be passed through conduit 63 into chamber 32. The helium passing through branch conduit 62 moves through the coils surrounding the melting pot and is passed into and through atomizer 116, nfier which it mingles with helium passing through chamber 32.

lf furnace 74 is also placed in operating position with respect to chamber 32, its melting pot maybe flushed of air and lled with helium undergoing purification in a manner similar to furnace 64. Helium -is forced by the compressor through

conduits

16 and 70, and

branch conduits

71 and 72` Helium passing through conduit 71 into the melting pot may be passed through conduit 73 into chamber 32. Helium passing through branch conduit "72 is passed through the coils around the melting pot and finally through the atomizcr into chamber 32.

It will be clear from what has been said ythat if the irnpure helium in the system is circulated for a sufficiently long time, it will continue to pass through the purifier until substantially all of the objectionable oxygen and nitrogen are removed. Periodic tests are made, with an Orset tester, to determine the oxygen content of the helium. When it is sufficiently pure the valves in inlet 17 and outlet 22 are closed to cut-out the purifier circuit. It may be cut-in from time to time as needed. If all goes well. the purified helium may bc retained in the system and used over a prolonged period of time.

With the circulatory system full of purified helium, it is ready for the actual metal powder producing operation. Referring for the moment to Fig. 5, which shows melt ing furnace 64 and its auxiliary equipment in more detail. cover 110 is removed from charging conduit 106. lngots of metal to be converted into metal powder are dropped into melting pot G. After a suitable amount is placed therein the cover is returned. The apparatus is at present being used to produce magnesium powder and also magnesium-aluminum alloy powder. The inside dimensions of the pot are 24" in diameter and 42" high. A charge ol about 18() pounds of magnesium is thus introduced. Oil burners 9S are operated to iheat the pot externally until its magnesium content reaches a suitable molten temperature, from 1300 to l350 F. The burners are kept in operation to maintain this tcmperature.

Pressure-gas conduit 6l is opened to admit-helium vinto the charge conduit and top yportion of the pot ubovethe level of the molten magnesium. Since the pressureegas conduticummunicates zwith the compressor the molten magnesium is placed Lunder isuiicient gas pressure to forceva stream ofzmagnesium .upwardly through moltenmetalconduit to and through atomizer 116 into atomizing and lpowder forming chamber 32. A pressure ol" 12-15 pounds per square inch is applied initially to the top of the body of Vmolten magnesium to start its passage through `the conduit and the atomizer. After atomization of the metal is underway `the pressure is reduced to about 5 pounds per square inch. Helium under ad equate pressure by the compressor is passed through atomizing-gas conduit 62 where it is heated in the coils around the melting pot. The preheated helium is passed through nipple (see Fig. 6) into gas distributing chamber 126 of'the atomizer, from which it issues through discharge ports 132 of cap 129 in a plurality of streams and blasts or atomizes the tine stream of molten magnesium discharged through nozzle outlet 123 at point or area 134 a suitable distance lforward of the nozzle tip. The pressure of the helium at the nr,... varies somewhat according to its construction. A pressure in the neighborhood of 60 pounds per square inch works satisfactorily with the nozzles now employed.

Returning for a moment to Fig. l, it will be recalled that blower 37 causes the helium to circulate continuously through the system. Since chamber 32 is on the suction side of the blower, the helium is in effect sucked through and from the chamber only to be forced cr pushed from the pressure side of the blower around the system. Relatively cool helium enters the chamber by way of

branch inlet conduits

30 and 33 at the same end, adjacentmelting furnace 64. The atomized magnesium is promptly enveloped and frozen into powder particles in the cool helium sweeping through upper compartment `(see Figs. 7 and 8).

While the newly formed particles of magnesium powder tend to remain in suspension in the helium :is the helium passes through the chamber to the blower, :vom: of it also tends to settle by gravity onto plates 151 to 153 and toward the bottom or base of the trough of the upper compartment. Additional amounts of relatively cool helium are, therefore, introduced into the upper Compartment by way of inlet conduit 33. According to a present practice, the amount ol helium entering the upper compartment by way of inlet 33 and lower compartment or duct 141 substantially exceeds that entering by way of inlet conduit 30. Some of the helium from inlet 33 issues as a relatively small stream through slot 175, the stream being directed along the top of plate 168 in the bottom of the trough. Any magnesium powder settling or tending to settle on that platz: is.. therefore, swept up and placed in suspension in the main and large current of gas of helium passing through thc upper compartment.

In a similar manner some helium in lower compartment or duct 141 passes as a stream through slot 185 along the top of plate 167 at the bottom of the trough of the upper compartment. Similar streams of helium pass through slots 186 to 191, so that plates 166, 165'. 164, 163, 162 and 161 at the bottom of the trough of the upper compartment are continuously swept with streams of helium under sutiicient velocity to 'keep the powder from settling and to keep it in suspension. An additional stream of helium passes through duct 192 into .the discharge end of the upper compartment. Some of the helium thus introduced through the slots spreads to the side and sweeps across plates 151 to titl to keep them clean of powder. The mixture of gas and powder is under a great deal ol turbulence which inhibits Settling of the powder. The powder laden gas is in effect sucked from the upper compartment through outlet 36 and blown through conduit 38.

If melting furnace 64 must be shut down for some reason, such asvfor repairs, melting furnace '74 (see Fig. l) is put in operation at the other end of the chamber.

In this case it is advisable to close inlet conduit 30 and to open inlet conduit 34. As before, aline stream of molten magnesium issuing from the nozzle of the atomizer is blasted with helium and the atomized magnesium is promptly enveloped in the relatively cool helium gas entering the upper compartment by way of inlet conduit 34 as well as by helium catering the chamber through inlet conduit 33.

The force behind the fine stream of molten metal issuing from the nozzle of the atomizer at either end of the chamber and the force of the helium used to atomize the stream of molten magnesium is suliicient to throw the atomized magnesium substantially `across the length of the chamber. When melting furnace 64 is in operation the direction of the stream of molten metal is in general concurrent with the streams of helium passed into and through the chamber. On the other hand, when melting furnace 74 is in operation the direction of the stream of molten metal issuing from the nozzle is in general countercurrent to the helium entering the upper compartment by way of inlet conduit 33 but concurrent with the helium entering the chamber through inlet conduit 34. As a result of these contrary movements, at least initially, the helium is kept in a turbulent state. ln any event the suction force of the blower is sufficient to draw the resulting powder-gas mixture from the chamber.

Still referring to Fig. l, the magnesium powder laden helium is forced by blower 37 through conduit 38 to powder-gas separator 40. As shown in Figs. 9, l and ll and as described in some detail above, the larger powder particles are separated selectively from the helium and smaller particles in cyclone 200 and fall into hopper 293. After a suliicient supply of powder has collected in the hopper, some of it is withdrawn from time to time; care being taken not to break the powder seal in discharge conduit 205 so that air cannot enter the system.

Again returning to Fig. l, the helium containing the smaller particles of magnesium powder is `forced from powder-gas separator 40 through conduit 41 gas separator 42. Since the separator operates like the other, the same procedure is followed in withdrawing powder. As already indicated one or more additional powder-gas separators may be employed.

From the chamber to and through the powder-gas Separators the helium undergoes a substantial drop in temperature. As it leaves the last powder`gas separator in the series it still contains some lines which should be removed before it reaches the compressor. To this end the helium leaving powder-gas separator 42 is passed through conduit 43 into dust-collector 44. In the present practice this is in the form of a plurality of filter bags, the specific device employed being a Dracco dust collector. While a substantial amount of the dust is thus removed from the helium, some remains.

Helium leaving dust-collector 44 passes into main conduit 3 and is then diverted through branch conduit 4S, filter 46 and conduit 47 back to main conduit 3. This is a wet filter, of the oil type, which needs to be cleaned from time to time. When cut-out of the circuit for that purpose, helium is diverted from main conduit 3 through branch conduit 50, a similar lilter 51 and conduit S2 back to main conduit 3.

A substantial amount of the helium thus treated passed continuously through main conduit 3 and branch conduits 3l), 33 and 34 back to chamber 32. Some of the thus treated helium is, however, diverted from main com duit 3 to the compressor. Additional steps are taken to remove further amounts of dust from this helium before it reaches the compressor. To this end the helium is diverted through conduit 4 into and through filter 5, when may be an oil filter similar to

filters

46 and 51. Although the helium thus treated is substantially dust free, it usually contains some suspended oil and moisture, both of which contribute to the tire hazard. Helium leaving the to powderlter enters conduit 6 and is diverted through branch conduit 7, filter S and conduit 9 back to conduit 6 which connects the inlet or suction side of compressor 13. When lter 8 is cut-out for cleaning, helium leaving lilter 5 is diverted from conduit 6 through branch conduit 10, filter 11, and conduit 12 back to conduit 6. Filters 8 and 1l are advantageously of the pot type containing a filter layer of steel wool or other suitable material, through which the helium is passed to abstract the suspended oil and moisture.

Since the helium may pick up some oil and moisture in the compressor, it is again filtered. Helium leaving the outlet or pressure side of the compressor is passed through conduit 14 and filter 15. This iilter may contain a filter layer of felt, for example, such as a Cuno filter, which abstracts the oil and moisture as the helium passes therethrough. The helium thus treated for the removal of powder, lines, oil and moisture is passed through the remainder of the system, including the purilier when cut-in, the melting pots, and the atomizcrs, as already described.

Some losses of helium from the system are unavoidable, not only from minor leaks but also when charging the melting furnaces, etc. Replacement helium must, therefore. be added to the system. However` by keeping the system under positive pressure, ingress of air is for the most part avoided. Such oxygen and nitrogen as enter by replacement helium is so small compared with the total volume of helium in the system that their effeet is of little consequence. They react with the atomized magnesium or highly heated powder in the chamber and are quickly eliminated. That is, after the system is in metal powder producing operation, it acts to purify itself so far as the extremely small amounts of oxygen and nitrogen are concerned.

It will thus be seen that a confined system may be provided in which an inert gas of high purity under positive pressure is continuously circulated while metal to be converted into powder is melted and atomized therein. Although the gas initially placed in the system is not satisfactory for the purpose, it may be so treated as to remove hearmful impurities. After the gas is purilied, it may be used over a long period of time to produce metal powder. While various inert or non-reactive gases may be employed, helium is especially suitable because of its availability. Metal powder may be produced from various metals and their alloys, such as magnesium, aluminum, zinc, cadmium, lead, etc. The principal limitation is the ability of the materials in the system economically to withstand the necessary wear and tear.

So much for producing the metal powder in accordance with the particular method and apparatus disclosed. The physical structure of particles of the metal powder will now be considered in brief detail, more especially as illustrated in Figs. l2 and 13, which were made with the aid of photomicrographs. Three main samples of magnesium powder were examined. Sample l was minus 50 plus 10D mesh. Sample 2 was minus 100 plus 200 mesh. Sample 3 was minus 200 mesh. While similar photomici'ographs were taken of each sample, those of sample l are diagrammatically shown because they are generally illustrative as well of the other two samples and the larger shadowgraph size of the particles permitted closer duplication in the figures.

Fig. l2 is an exterior representation of a number of the particles. The photomicrograph was taken at a magnication of 10U and the ligure represents an enlarged portion of the photomicrograph. It will be noted that th particles generally are spherical or spheroidal in shape: and that a fairly large proportion of them have marked protuberances or projections, usually one or more relatively large and a plurality of small ones. I can only speculate as to why such projections are obtained, but will defer doing so until Fig. i3 is considered.

Referring again to Fig. l2.

particles

250, 252, 254, 256,

258, 260, 262, 264, 266, 268, 270, 272, 274, 276, 278, 280 and 282 are detailed. It will be noted at once that

particles

252, 260, 264, 276, 280 and 282 show fairly

large protuberances

286, 288, 290, 292, 294 and 296, respectively; that all of the particles show one or more smaller protuberances 300; and that a number of particles appear to have fused protuberances 304 of the larger type, which may in fact only overlie each other. Some of the particles show numerous small projections. It will be appreciated that other large and small projections may exist but are not shown because not silhouetted. Thus, they may be in the foreground or background of each particle and, therefore, are not revealed in the photomicrograph.

Fig. 13 is an interior representation of a number of the particles. The actual photomicrograph is that of a cross-section of the particles after being polished and etched, in the usual manner for such purposes. As before, the magnilication was 100. The figure, as before, represents a portion of the photomicrograph. It will be noted that a large proportion of the particles show fairly large and small hollow cavities, pockets or voids; and that they cause the particles to be non-solid, discontinuous, or non-homogenous.

Particles

310, 312, 314, 316, 318, 320, 322, 324, 326, and 328 are shown in cross-section. Since Figs. l2 and 13 are derived from separate specimens of sample 1, a direct comparison of particles as between the two figures is not possible, or necessary. It will be noted that

particles

310, 312, 314, 316, 318, 322, and 324 show fairly large

hollow cavities

330, 332, 334, 336, 338, 342 and 344, respectively; that all of the particles indicate the presence of somewhat smaller hollow cavities 350; and the particles also contain smaller hollow cavities 354; that the particles also have still smaller cavities which may be regarded as intergranular channels, or pores, indicated by heavy lines 356; and that each particle contains innumerable grain boundaries indicated by light lines 358.

Since the particles are shown in section, having been polished or ground down for the taking of the photomicrograph, some of the original cavities were no doubt obliterated; other cavities undoubtedly remained in the remaining portions of the particles but were not revealed when the photomicrograph was taken. This is truc also of the many grain boundary lines. In any event, the particles appear to be honeycombed with cavities, thus making the solid body structures of the particles porous.

It will be noted that the body portions of the particles appear to be sponge-like, particularly around the larger cavities or pockets and to a lesser extent generally around the smaller cavities. While very small cavities or channels 356 show up as microscopic pores, it is believed that sub-microscopic pores of the same nature exist; and `that for the most part the cavities, large and small, microscopic and sub-microscopic, interconnect; thus giving the particle as a whole a porous sponge-like structure.

Also, as is to be expected, Fig. 13 shows some protuberances extending from the exteriors of the particles. Thus

particles

322 and 324 show

large protuberances

360 and 362, respectively. As in Fig. 12, some of the protuberances undoubtedly are concealed, because not silhouetted when the photomicrogarph was taken.

As in Fig. l2, projections on the particles are in evi dence in Fig. 13. l am inclined to believe that the cause of their formation is the same, in general, as that of the porosity of the interiors. It will be recalled (Fig. 5) that the body of molten magnesium in melting pot 100 is placed under very substantial positive pressure by the inert gas passed therein through conduit 61 from compressor 13 (Fig. 1). A certain amount of the gas is dissolved or occluded by the molten magnesium and remains therein so long as the elevated pressure and ternperature continue. In other Words, the molten magnesium is saturated with helium at whatever elevated pressure 12 and temperature are maintained on and in the body of molten magnesium by the gas and applied heat. That gas remains in the molten magnesium that passes from the melting pot through conduit to atomizer 116, because pressure and temperature conditions in that part of the system are substantially stable.

Such pressure and temperature stability, however, is short-lived. As soon as the tine stream of molten magnesium issues from the atomizer, there is a sudden drop in pressure and temperature. Some lof the gas in solution rushes with eruptive violence from each atomized molten particle. As this occurs, metal at the surface is spewed in spots, while the chilling or freezing action continues. That is, as the gas in solution in the interior forces its way through the exterior of a particle, a small portion of the molten metal is pushed aside to permit escape of gas. Since the newly formed particles are chilled simultaneously, the portion thus pushed aside is frozen into a pimple-like proturbance; somewhat like the formation of a volcano. In any event, the net result is to increase the exterior surface area of the particle. Since many of the particles may have a plurality of protuberances, their exteriors are highly reactive, from a surface exposure viewpoint.

The presence of the hollow cavities in the particles appears to bear out my theory Ion how the protuberances are formed. Such escape of dissolved gas creates a void and since the molten particle is quickly solidified, there is no opportunity, in many instances, for the surrounding shell portion of the particle to contract and close the hollow cavities. Hence the numerous hollow interiors. They add, of course, lto the available surface area of thc particle.

The hot gas under elevated pressure in solution in the interior of the particle may be considered as in a race with the cold gas under reduced pressure at the exterior of the particle in chamber 32. The hot gas rushes to get out of lthe molten particle while the cold gas in the chamber rushes to freeze and thus to solidify the molten particle. As a result of these opposed actions, the physical structure of the particle necessarily is dis turbed. Hollow cavities of various sizes result in many, if not at all, of the particles.

Some of the gas in solution may be precipitated and occluded in the interiors of the particles. As a particle is chilled, shrinkage forces are set up. These occur initially in the outer portion as a hard shell tends to develop. As the outer portion solidifies, gas is trapped in the molten interior, the pressure of the gas being increased by the opposed shrinkage forces moving inwardly. Gas, therefore, separates, forces molten metal outwardly, and collects in hollow cavities as the remainder of the particle solidifies.

When, as must often be the case, the` amount of gas thus occluded and its pressure are not sufficient to explode the particle, and hence to release the gas, a powder particle is formed at the end of the operation with hollow cavities, pockets or channels lilled with gas. As will be explained below, such occluded gas may serve a very useful purpose.

In any event, it will be evident that the many hollowcavity surface areas of the interiors of the particles, taken in conjunction with the extended exterior surface areas, add enormously to the over-all or sum-total surface area, thus providing a product that is highly rear tive from an exposed or exposable surface viewpoint.

Even though the particles have voids in their interiors, they withstand normal handling without apparent disintegration. In other words, the porous nature of the product does not appear to impair its mechanical stability. The particles also have a relatively high density, particularly compared with the mechanically broken-up type of product.

The product also has a high surface stability. That is to say, it may be stored for long intervals without substantial loss in chemical reactivity. Metal powder, as

customarily made, suffers substantial loss in surface stability. This is due to the formation of a coating of a compound of the metal, such as an oxide, nitride, carbonate, sulfate, etc., which insulates the metal interiors of the particles.

When made in accordance with the invention herein disclosed, however, the metal powder particles are substantially free of such metal compound coating. They have clean metallic surfaces. Such surfaces are assured initially because the atomizing and powder-forming steps are conducted in an atmosphere really inert to the metal. Furthermore, the newly formed particles are chilled to ordinary temperatures while suspended in the same inert atmosphere, thus having no opportunity to react while heated with deleterious gases to form such coatings.

The metallic surfaces of the cooled particles remain substantially free of a metal compound coating, even when stored for extended periods. l am inclined to believe that this very desirable result is due to several important factors. First, the particles do not lose all of their dissolved or occluded gas during the powder-forming step. Some of the inert gas remains in the particles, in cavities, pockets or channels, for example, and tends to diffuse very slowly to the surface, thus helping to pro tect the surface from reaction with noxious gaseous elements. Second, the tree space in the container in which the powder is stored tends to be filled with such diffused inert gas. And, third. the particles tend to be enveloped in n mono-molecular lm of the inert gas, thus providl ing additional protection for the particles against the elements. Between the three factors, and perhaps 0thers, the metal powder is well protected and is able to withstand storage in containers without substantial loss of surface stability.

It will be clear to those skilled in this art that the practice of the invention lends itself readily to a number of useful modifications. While the production of magnesium powder of the specified structure is described specifically, magnesium alloys, such as an alloy of magnesium and aluminum, aluminum alone, other metals and other metal alloys may be produced in powder form in the practice of the invention.

l claim:

l. ln atomized metal powder generally spherodal in shape, the improvement comprising particles of the metal having internal hollow cavities to provide interior surface areas in addition to their normal exterior surface areas, the particles being substantially free of a compound of the metal.

2. Metal powder according to claim 1, in which the particles have protuberances of the metal on their exleriors to provide additional surface areas.

3. Metal powder according to claim 1, in which the particles contain occluded gas inert to and soluble in the metal to inhibit the formation of a coating of a compound of the metal on the exterior surfaces of the particles during storage and the like.

4. Metal powder according to claim 1, in which the particles contain occluded helium to inhibit the formation of a coating of a compound of the metal on the 14 exterior surfaces of the particles during storage and the like.

5. In atomized magnesium bearing metal powder generally spherodal in shape, the improvement comprising particles ol the magnesium bearing metal having internal hollow cavities to provide interior surface areas in addition to their normal exterior surface areas, the particles being substantially free of a compound of the metal.

6. Magnesium bearing metal powder according to claim 5, in which the particles have protuberances of the metal on their exterior-s to provide additional surface areas.

7. Magnesium bearing metal powder according to claim 5, in which the particles contain occluded gas inert to and soluble in the metal to inhibit the formation of a coating of a compound of the metal on the exterior surfaces ot the particles during storage and the like.

tl. Magnesium bearing metal powder according to claim 5, in which the particles contain occluded helium to inhibit the formation of a coating of a compound of the metal on the exterior surfaces of the particles during storage and the like.

9. ln atomized aluminum bearing metal powder generally spherodal in shape, the improvement comprising particles of the aluminum bearing metal having internal hollow cavities to provide interior surface areas in addition to their normal exterior surface areas, the particles being substantially free of a compound of the metal.

l0. Aluminum bearing metal powder according to claim 9, in which the particles have protuberances of the metal on their exteriors to provide additional surface areas.

l1. Aluminum bearing metal powder according to claim 9, in which the particles contain occluded gas inert to and soluble in the metal to inhibit the formation of a coating of a compound of the metal on the exterior surfaces of the particles during storage and the like.

12. Aluminum bearing metal powder according to claim 9, in which the particles contain occluded helium to inhibit the formation of a coating of a compound of the metal on the exterior surfaces of the particles during storage and the like.

References Cited in the le of this patent UNITED STATES PATENTS Paddle June 18, 1946 Golwynne Mar. 4, 1952 OTHER REFERENCES

Claims

1. IN ATOMIZED METAL POWDER GENERALLY SPHEROIDAL IN SHAPE, THE IMPROVEMENT COMPRISING PARTICLES OF THE METAL HAVING INTERNAL HOLLOW CAVITIES TO PROVIDE INTERIOR SURL FACE AREAS IN ADDITION TO THEIR NORMAL EXTERIOR SURFACE AREAS, THE PARTICLES BEING SUBSTANTIALLY FREE OF A COMPOUND OF THE METAL.