US2699576A - Atomizing magnesium - Google Patents

Description

Jan. 18, 1955 N. R. COLBRY ET AL 2,

ATOMIZING MAGNESIUM Filed March 18, 1953 28heets-Sheet l IN V EN TORS. Norman R. Co/bry Gordon FT Hers/7 ey ATTORNEYS Jan. 18 1955 COLBRY ETAL 2,699,576

ATOMIZING MAGNESIUM Filed March 18, 1955 2 Sheets-Sheet 2 y INVENTORS.

Norman R. Co/b/y BY Gar-don E He/ s/oey A T TORA/EKS United States Patent ()fiice 2,699,576 Patented Jan. 18, 19 55 1 2,699,576 ATOMIZING MAGNESIUM Norman R. Colin-y, Breckenridge, and Gordon F. Hershey,

Midland, Mich, assignors to The Dow Chemical Company, Midi-and, Mich, a corporation of Delaware Appiication March 18, 1953, Serial No. 343,158 3 Claims. (Cl. 1847.2)

The invention relates to methods of atomizing metal. It more particularly concerns an improved method of converting molten magnesium into fine substantially spherical particles with a minimum of off-size particles.

The two types of methods heretofore proposed of atomizing magnesium are beset with many dimculties. One of these types of methods involves impinging upon a falling stream of the molten magnesium a jet of gas which upon striking the molten metal breaks it up into droplets solidifying as they become cooled in the atmosphere of the jetting gas. A process of this kind is disclosed in U. S. Patents 1,351,865 and 2,371,105. The characteristic disadvantages of the method of atomizing magnesium with a jet of gas are that the atomized particles are not uniform in size; it is difficult, if not impossible, to produce relatively small adequately dust-free particles without screening the product many times and reworking the coarser particles; there is always a small proportion of dust-like fines (less than 600 mesh) which are detrimental in that they are easily flammable, have low corrosion resistance, and such particles cannot be readily separated from the rest of the product due to their tendency to cling to the larger particles. Another difficulty in using a jet of gas for atomizing magnesium is that the method requires large volumes of an inactive gas which must be filtered and cleaned before recirculation through the process.

The other of the two types of atomizing processes involves impinging the molten magnesium onto a rapidly revolving disc. A method of this type is disclosed in British patent specification No. 510,320. According to the British patent specification, the molten metal to be atomized is impinged upon a rapidly revolving cooled disc and cooled at the same time by directing a cooling agent, such as a liquid or gas, against the metal at the point of impingement of the melt upon the disc.

In attempts to atomize molten magnesium by impinging the metal upon a cool rapidly revolving steel disc strong enough to withstand the stresses of high speed rotation fail to produce atomized metal. Instead, the metal in part solidifies on and clings to the disc in thick masses from which pieces break off from time to time and in part splashes off the metal already solidified on the disc in relatively large masses without atomizing. Cooling the molten metal during the impingement on the disc accentuates these diificulties. If the disc is heated to a temperature above the melting point of the magnesium so that the metal does not solidify on the disc, the disc becomes rapidly eroded. At the same time, the molten metal leaving the disc as it spins does not atomize into a uniform sized product. Instead, the particles progressively become larger and are interspersed with unatomized metal as the operation proceeds and the disc erodes.

Insofar as we are aware, there is no satisfactory method commercially available by which molten magnesium can be atomized into uniformly sized fine spherical particles. Accordingly, it is the principal object of the invention to provide a method which fulfills the foregoing need.

The present invention is predicated upon the discovery that by including in the magnesium to be atomized from 0.025 to 1.0 per cent of zirconium and at least 0.25 per cent of zinc, the resulting metal may be atomized into fine substantially uniformly sized particles by allowing the molten metal to fall freely in a thin stream onto a disc of steel (preferably tool steel) rapidly rotating in a nonreactive gas, the disc being maintained at a temperature above the melting point of the molten metal. Under the stated conditions, atomization is effected without splashing of the molten metal and without significant attack upon the disc. The invention then consists of the improved atomizing method herein fully described and particularly pointed out in the claims.

In carrying out the invention, the magnesium to be atomized is melted and the requisite amounts of the two metals, zinc and zirconium, are dissolved in the melt. In the case of the zirconium enough is added to produce in solution in the magnesium a concentration of 0.025 to 1 per cent of zirconium by weight. A preferred concentration of zirconium is about 0.05 to 0.6 per cent. In the case of the zinc enough is added to the magnesium melt to produce therein a dissolved zinc concentration of at least 0.25 per cent. A zinc concentration as high as 7 per cent may be used. In general, a desirable proportion for the zinc is between about 0.5 and 1.5 per cent of the melt by weight.

The magnesium melt containing the requisite concentrations of zinc and zirconium is brought to a temperature between about 680 and 800 C. and is then allowed to fall in a thin stream (e. g. 4; x in diameter) a distance of about 2 to 10 inches onto a spinning steel disc, either flat or concave, the axis of rotation of the disc being substantially vertical. The point of impingement of the molten metal, which falls onto the disc, is preferably at or close to the center of rotation of the disc.

A concave disc having a spherical concavity is preferable. It is oriented so that the concave surface faces the falling stream of the molten zincand zirconium-containing magnesium. Discs having diameters of 2 to 6 inches may be used. A preferable diameter of the disc is 3% inches. Running speeds of 2,000 to 100,000 R. P. M. or more may be used depending upon the diameter and the strength of the steel.

The space in which the disc operates is charged with an inactive gas, such as natural gas or with one or more of the principal hydrocarbon constituents thereof, e. g. methane, ethane, propane, butane, preferably at room temperature, although temperatures up to about 240 C. may be used. The so-called inert gases may be used, c. g. helium, argon, and these gases may be used at higher temperatures. Other gases unreactive to magnesium may be used, e. g. hydrogen.

The temperature of the disc is important and is brought up to and maintained at working temperature by the molten metal to be atomized. This is accomplished by suitably heating the molten metal before impingement on the disc and allowing the so-heated metal to fall onto the disc while it is rotating at working speed and continuing the impingement until the disc thereby becomes heated. The disc is at least partially insulated against heat loss as by a backing of thermal insulation to allow the working face to reach operating temperature. The proper tem perature for the disc is readily ascertained by either ob-- serving the Working surface or sampling the product thrown off as the disc spins. When the disc is at a proper working temperature, it becomes wet with a fluid film of the molten metal which may be seen on visual inspection during operation. At the same time, when the disc is wet, the molten metal, which is thrown oil the disc and allowed to cool, is found to be in fine uniformly sized spherical particles. The temperature then of the molten magnesium, which is deposited upon the spinning disc and there atomized in accordance with the invention, is made sufficiently hot to heat the disc to and maintain it at a temperature above the melting point of the molten metal, so that by virtue of the presence in solution in the magnesium of the aforesaid amounts of zinc and zirconium, the disc becomes wetted by the molten metal. With a suitable amount of thermal insulation at the back of the disc, e. g. about inch thickness of asbestos paper, heating the molten metal to between about 680 and 800 C. suifices to produce the desired molten metal film on the disc. In this mode of operation, the disc remains smooth, splashing is virtually eliminated with the consequent elimination of the formation of irregularly shaped particles. The sieve analysis of the atomized product remains substantially constant during operation. The amount of extremely fine or dust-like particles formed, if any, is negligible. Instead, the molten metal is atomized into a mass of spherical particles conforming to a relatively narrow range of desirable particle sizes.

The invention may be further explained and illustrated by reference to the accompanying drawing showing an apparatus with which the method invention may be practiced.

In the said drawing wherein like numerals designate like arts: p Fig. 1 is a diagrammatic view of the atomizing plant showing the elements thereof and their relationship.

Fig. 2 is a detailed view largely in section of a portion of the apparatus of Fig. 1.

Fig. 3 is an enlarged sectional view on the line 3-3 of Fig. 2.

Referring to the drawing and more particularly Fig. 1, there is shown an atomizing tank 1 in which atomization takes place. The tank has a conical bottom 2 having an outlet 3 which is connected by pipe line 4 to a settling tank 5. The settling tank 5 has an outlet 6 at the bottom the opening through which is subject to control by a valve 7. The settling tank is connected by a pipe 8 to the cyclone separator 9. The separator is provided with a bottom outlet 10 controlled by a valve 11. The top outlet 12 of the separator 9 is connected by a pipe 13 to a gas holder 14 which holds a supply of inert gas which may be introduced through the inlet pipe 15 connected to a source not shown. Inert gas is withdrawn from the gas holder 14 through pipe 16 by means of the gas compressor 17. The gas compressed by the compressor 17 is delivered through pipe 18 to the cooler 19 in which the compressed gas is cooled. The cooled compressed gas passes from the cooler through pipe 20 to the turbine 21 shown in Fig. 2 in tank 1.

Referring more particularly to Fig. 2, turbine 21 is supported on legs 22 the lower ends of which (not shown) are secured to the inside of tank 1. The turbine 21 drives the vertical spindle 23 to the upper end of which is welded face plate 24, as shown in detail in Fig. 3. As shown in the periphery of the face plate is cut a thread 25 which engages the internally threaded recess 26 of the atomizing disc 27. The atomizing disc 27 has a concave upper face 28. Clamped between the face plate 24 and the back 29 of the disc 27 is a layer 30 of thermal insulation, such as for example asbestos paper about Vs inch thick.

As shown in Fig. 2, the top 31 of tank 1 has a centrally disposed concave portion 32 on which is mounted the furnace setting, indicated generally by numeral 33. This is comprised of a relatively smaller diameter lower portion 34 and an upper portion 35 having a larger diameter so as to accommodate a melting pot 36. Attached to the bottom of the melting pot and forming an outlet therefor is the tubular spigot 37 having an outlet 38 of smaller diameter than the bore 39 of the spigot. Between the outlet 38 and the bore 39 is a shoulder 40 which forms a seat for a valve 41. As shown, the valve 41 is formed as a shoulder on the lower end of the push rod 42 which enters the bore 39 of the spigot. The push rod also carries a smaller diameter portion 43 as a prod capable of being pushed through the outlet 38 to clean it as the valve 41 engages the shoulder 40 in closing the outlet 38.

The spigot has an external taper 44, the taper being designed to seat in the internally tapered nipple 45, as shown, the bottom of which is joined to the concave portion 32 of the tank 1 around the central opening 46. The outlet 38 is directly above the center of the disc 27, a distance of about 2 to 10 inches. The furnace setting 33 is provided with openings 47 and 48 in the lower and upper portions, respectively, through which the flame of heating burners (not shown) is projected to heat the tapered nipple 45, spigot 37, and melting pot 36 to a suitable operating temperature. Telescope 49 is provided for viewing the atomizing disc.

In operation, when starting up, the opening 38 is closed by lowering the push rod 42 so as to seat the valve 41 on shoulder 40 and a charge of the metal to be atomized is introduced into the melting pot 36. The pot is maintained sufficiently hot to maintain the charge at about 680 to 800 C. The compressor 17 is started up and inert gas is thereby withdrawn from the gas holder and compressed. The compressed gas is discharged from the compressor through pipe 18 into the cooler 19 which removes more or less of the heat of compression. The so-cooled compressed gas is delivered by pipe 20 to the turbine 21, thereby spinning the atomizing disc 27 The exhaust from the turbine enters the tank 1 by way of the turbine exhaust pipe 50 and maintains in the tank an inert gas atmosphere. When the disc 27 reaches a suitable speed of rotation, e. g. 20,000 R. P. M., the push rod 42 is raised enough to allow the molten metal from pot 36 to fall in a thin stream through the opening 38 onto the concave surface 28 of the disc 27 while it spins. After a time, the disc 27 acquires a suitable operating temperature, viz. a temperature above the melting point of the metal to be atomized, the disc 27 on being so-heated becomes wetted with the magnesium containing the zinc and zicronium so that the surface 28 is coated with a thin film 51 of the molten metal. Atomization then begins as the molten metal released from the pot on falling onto the molten metal coated disc is flung off toward the sides of the tank 1 in tiny uniformly sized spherical drops which solidify into spherical particles in the inert gas atmosphere of the tank 1. The atomized particles thus produced fall onto the conical bottom 2 and are carried by the inert gas, exhausting from the turbine, through the outlet 3 into pipe 4 and thence into the settling tank 5. Most of the particles settle out of the inert gas in the tank 5 and may be withdrawn from time to time through the outlet 6 by opening valve 7. The relatively small amount of fines which do not settle out of the inert gas in tank 5 are carried into the cyclone separator 9 by the inert gas which flows from tank 5 into the cyclone through pipe 8. The inert gas separated from the fines in the cyclone separator is discharged through pipe 13 into the gas holder 14 for recirculation through the system as described. From time to time, the fines may be withdrawn from the cyclone separator through outlet 10 by opening valve 11.

As the operation proceeds, the tank 1 heats up to an extent depending upon the temperature of the molten metal and rate of input as well as the heat dissipating characteristics of the tank and the temperature of the incoming inert gas as well as the cooling eflect of the expansion of the inert gas in driving the turbine 21. In general, it is desirable to keep the inert gas temperature in the atomizing zone, i. e. the area adjacent to the spinning disc 27, below the temperature capable of being withstood by the gas used without decomposition. Temperatures in the range of 100 to 240 F. are generally satisfactory for hydrocarbon gases.

The following data of atomizing runs are illustrative of the effect of zinc and zirconium on the quality of the atomized product and their effect on the steel atomizing disc. In these runs, the disc was maintained above the melting point of the metal by the conditions of operation, the temperature of the molten metal being between about 680 and 750 C. so that the disc was coated with a film 51 of molten metal except in the runs tabulated as blanks. In these runs, the disc was not wetted with molten metal. The atmosphere was natural gas for all the tabulated runs.

Table l.--At0mizing runs Metal Composition Disc Pounds Run N0. jgg of Condition Percent Percent Percent Product Speed, Diameter, Zn Zr Mg R. P. M. Inches Before After 1 6 0. 6 Bal. 5 3, 083 Smooth. Smooth 9, 000-9, 600 2. 2 3 0 6 Bai. 1. 75 (1 ....d0 12, 300-12, 500 2. 75 3 0.67 0. 064 Ba]. 5. 5 ..d0 7, 200-7, 500 3. 75 4 1. 53 0.25 Bal. 0.73 .do.. 7, 000 2.75 5. 0.85 0. 27 132.1. 1.0 (10.. 7, 000 2. 75 6. 1.36 0. 17 Bal. 0.9 (10.. 7, 000 2. 75 7. 1.29 0. 09 B211. 0. 45 d0 7, 000 2. 75 8. 0. 07 0. 05 Bal. 0. 75 d0 7, 000 2. 75 A Blank 4. 76 Eroded to depth 10, 350-10, 450 4. 0

of 0.062 inch.

B Blank 0.76 0.011 Ba-l 0.78 hed 7,000 2. 75 0 Blank 0. 025 0. 28 Hal 1. 2 48, 000-54, 000 2. 75 D Blank 0.92 Bal 1. 0 7, 000 3. 75

Table lL-Screen analysis of atomized product f i 'gg Total Screen Analysis Percent Run No. Start P @139 of duced MilllltGS +20 20/35 35/65 55/100 ---1O[) 2 20. 5 9. 43. 40. U 6. 0 1. 5 1 90 925 10. 0 45. 0 36- 75 5. 75 2. 5 240 2, 467 8. 5 43. 0 39. 0 6. 0 3. 5 300 3, 083 8. 5 41. U 40. 3 6. 5 3. 5 2 101 0. 25. 25 52. 25 15. 5 6. 75 105 1, 010 0. 5 20. 0 53. 5 14. U 7. 0 90 900 0. 4 2G. 8 61. 9 8. 1 2. 8 3 125 1, 250 1. 0 25. 8 60. 2 9. 0 4. 0 240 2, 400 0. 4 26. 2 52. 2 8. 0 3. 2 330 3, 320 1. 0 21. 6 64. 7 9. 3 3. 4

+35 10 39 G. 2 21.? 54. g g 24 2 7 5. 0 2. 5. 0 Blank e 45 445 7. 5 39. 5 23. 4 29.6 55 544 9. 5 36. 8 20. 8 32. 9 72 692 21. 4 43. 9 18. 8 15. 9

1 Corresponds to runs of Table I.

Referring to Table I, runs 1 to 8 inclusive, were carried out in accordance with the invention by including 25 in the magnesium sufiicient amounts of both zinc and zirconium to prevent erosion or attack by the molten metal on the atomizing steel disc, the condition of which before and after the runs is tabulated. For comparison,

blanks were run with commercial electrolytic magnesium zirconium as in run C, for example. The lack of uniformity of the particle size of the product of run C is shown in the screen analysis where the proportion of the product retained upon a No. 35 standard sieve progressively increased during the run from 6.2 per cent to 21.4 per cent, while the proportion passing through a No. sieve and retained on a No. sieve decreased from 24 per cent to 18.8 per cent during the run. At the same time, a large and varying amount of excessively fine material was produced as indicated by the figures in the -100 column of run C. screen analyses for runs 1, 2, and 3, in which the content of zinc and zirconium was controlled in accordance In sharp contrast, the 50 with the invention, show a uniformity of particle size throughout the runs and a negligible amount of fines below a No. 100 sieve size.

We claim:

1. In a method of atomizing magnesium the steps which consist in dissolving in the molten magnesium from 0.025 to 1.0 per cent of zirconium and from 0.25 to 7 per cent of zinc, letting fall the so-prepared molten metal in a thin stream upon a spinning disc of steel having a temperature above the melting point of the soprepared molten metal whereby the disc becomes wetted with a film of the molten metal and the molten metal is flung off the disc in fine uniformly sized particles, and solidifying and collecting the resulting particulated metal.

2. In a method according to claim 1 maintaining the disc in an atmosphere of hydrocarbon gas at a temperature below 240 C.

3. In a method according to claim 2 in which the molten metal is let fall at a temperature between 680 and 800 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,040,168 DeBats May 12, 1936 2,061,696 DeBats Nov. 24, 1936 FOREIGN PATENTS 510,320 Great Britain July 26, 1939

Claims (1)

1. IN A METHOD OF ATOMIZING MAGNESIUM THE STEPS WHICH CONSIST IN DISSOLVING IN THE MOLTEN MAGNESIUM FROM 0.025 TO 10 PER CENT OF ZIRCONIUM AND FROM 0.25 TO 7 PER CENT OF ZINC, LETTING FALL THE SO-PREPARED MOLTEN METAL IN A THIN STREAM UPON A SPINNING DISC OF STEEL HAVING A TEMPERATURE ABOVE THE MELTING POINT OF THE SOPREPARED MOLTEN METAL WHEREBY THE DISC BECOMES WETTED WITH A FILM OF THE MOLTEN METAL AND THE MOLTEN METAL IS FLUNG OFF THE DISC IN FINE UNIFORMLY SIZED PARTICLES, AND SOLIDIFYING AND COLLECTING THE RESULTING PARTICULATED METAL.

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