US3551532A - Method of directly converting molten metal to powder having low oxygen content - Google Patents

Description

Dec. 29, 1970 w. a. LAIRD 3,551,532 METHOD OF DIRECTLY CONVERTING MOLTEN METAL TO POWDER HAVING LOW OXYGEN CONTENT Filed May ,25, 1967 2 Sheets-Sheet 1 I/VVENTOR W/LL/AM B. LA/RD ATTORNEY w. B. LAIRD 3,551,532 METHOD OF DIRECTLY CONVERTING MOLTEN METAL TQ Dec. 29; 1970 Filed May 25} 1967 POWDER HAVING LOW OXYGEN CONTENT 2 Sheets-Sheet z INVENTOR WILL/AM B4 LA/RD ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE Molten iron is stream from a tundish and quenched and comminuted by high velocity jets of water as the melt enters an atomizing chamber. The melt is entrained Within a confluence of the jets that jointly define a closedend trough for uniform and predictable quenching and comminution.

'1 Claim The atomizing chamber is open at its lower end and H mounted Within a tank containing the powder-collecting water, the level of which is maintained above the lower edge of the chamber. The steam generated incident to quenching completely fills the chamber for excluding air and maintaining therein a complete steam ambient through which the iron powder passes from the area of comminution to the collecting water.

This invention relates to conversion by continuous process, of molten metal to metal powder by quenching and comminuting a stream of the molten metal by jets of quenching liquid, and has for its principal object an improved method of and apparatus for directly converting molten metal to powder having a low oxygen content and generally predictable particle size.

Prior to this invention, known methods of molten metal-to-powder conversion using liquid jets for quenching and comminution involved material disadvantages, such as comparatively high rate of oxidation of the powder immediately following comminution, and unpredictable size and density of the metal particles so formed. Where, for example, the metal is iron and the quenching medium water, the hot metal particles upon comminution are subject to a high rate of oxidation unless the ambient medium is inert in this respect. Ordinarily, a considerable amount of air is included in the ambient due to induction effect of the quenching jets, etc.

For minimizing this oxidation and thereby avoiding a subsequent reducing operation on the metal powder, it has, for example, been proposed that an inert gas such as argon or nitrogen be continuously supplied for providing an inert ambient for the metal powder directly after comminution when it is most subject to rapid oxidation. This method however, requires precise regulation for properly adjusting the flow of the inert medium through the equipment, and moreover, is expensive.

In accordance with this invention, the vapor of the quenching liquid itself automatically provides in a simple and inexpensive manner a substantially constant and complete inert ambient for the metal powder during comminution and subsequent passage of the powder into a collecting medium.

The invention will be more fully set forth in the following description referring to the accompanying drawings, and the features of novelty will be pointed out with particularity in the claim annexed to and forming a part of this specification.

Referring specifically to the drawings:

FIG. 1 is an elevational view in section illustrating an embodiment of the invention for converting molten metal to powder;

FIG. 2 is a partial plan View taken along the line 22 of FIG. 1 showing the fluid jets and associated quenching bafile relationship;

FIG. 3 is a partial elevational view of the main and auxiliary jets arrangement of FIG. 1 taken along the line 3-6 thereof, and

FIG. 4 shows a modified form of chamber venting.

Metal powder made in accordance with this invention has numerous uses in industrial metallurgy, as for example in the making of coatings of welding electrodes. For this purpose, powdered iron of high purity, i.e., having very low content of carbon, silicon, etc., with oxygen content not exceeding approximately 0.75%, is required and is produced directly from refined molten iron in accordance with and by the method and apparatus herein described.

Referring to FIG. 1, a tundish 1 is supplied with molten iron 2 that is produced in conventional manner according to commercial requirements for high purity. The tundish at its bottom has a pour nozzle 3 designed for metering the flow of melt to be comminuted, and for directing the flow in an even and coherent stream 4 into a main processing housing 5.

The housing has in vertical alignment directly beneath the tundish a spray compartment 6 for quenching equipment, and an atomizing chamber 7 within which the melt is quenched and comminuted as hereinafter described.

As the viscosity or fluidity of iron decreases with purity, compensating higher pour temperatures are necessary for preventing freezing during pouring. High purity melt in the tundish must therefor be kept at super temperatures, which in turn require the use of heavy duty, high refractory materials resistant to abrasion and deterioration, for the tundish lining and pour nozzle.

As shown in FIG. 1, the tundish comprises a metal casing 8 generally in the form of a hopper of rectangular cross-section and inverted flare. The casing is mounted on and above the main housing 5 and has at its upper flared end, an insulating cover 9 with an opening 10 through which the iron melt is poured into the tundish as required, and another opening 11 within which a burner nozzle 12 is mounted for maintaining the tundish melt at sufficiently high temperatures.

The tundish lining in the present instance consists of a layer of rammed zircon 13 (or high alumina brick) that is thermally spaced from the tundish casing by insulating brick 14. The pour nozzle 3 is made of zircon and is mounted at the converging end of the tundish within a collar 15 of a high alumina refractory ramming mix known to the trade as Coralite, that forms with the zircon layer the lower refractory lining of the tundish. The pour nozzle has an annular shoulder 3' that seats on a refractory collar 16 of alusite tile, the open center of which forms an extension of the nozzle exhaust passage. The lower end of the tundish casing has beyond the pour nozzle a rectangular extension 17 that is lined with fire brick 18 for defining an expanded insulating exhaust passage 19 for the flow of the highly heated melt as it leaves the tundish.

The spray compartment 6 into which the melt is first directed has at its upper part a funnel-shaped splash baffle 25 with a central opening 26 for the melt stream, and has an insulating lining 25' of asbestos pulp. The compartment contains a plurality of jet nozzles 20, 21, 22 and 23 of fan-spray type, such as are used for scaling steel in rolling mills, that extend in converging relation from a closed loop or ring header 24. The header surrounds and is supported by the splash bafile 25 and is connected at 27 to a high pressure source of cooling liquid, specifically, water.

The nozzles are located above a second baflie 28 that in effect separates the spray compartment from the atomizing chamber 7 and partially functions to confine a vapor ambient within the chamber. The baflie 28 has a centrally positioned, somewhat elongated opening 29, FIG. 2, through which liquid jets from the nozzles are directed at high velocity for confluence with the melt at the opposite side of the baflle, i.e., within the chamber 7, for quenching, comminuting and cooling the metal therein.

Referring more specifically to FIGS. 1 to 3, the header 24 is conveniently of rectangular form and has at two opposite sides, connections at 27 for the high pressure source of water (not shown) that preferably is in excess of 2000 p.s.i. In the present form, four nozzles are connected respectively to the four sides of the header, FIG. 2, and are directed downward and toward the baffle opening 29, FIGS. 2 and'3. The nozzles 20 and 22, termed main nozzles, are connected to the header at the sides of high pressure supply and are equally inclined so that the jets therefrom make angles of approximately 30 with the longitudinal axis of the melt stream below the battle opening 29. The joint fan-spray pattern of the main jets accordingly form an acute angle trough 30 of high velocity liquid spray that intercepts the melt only after the melt has entered the atomizing chamber. The trough apex (jets confluence) indicated at 31, FIG. 2, is approximately in vertical alignment with the central major axis of baflle opening 29.

A trough so formed by the confluence of two spray jets has heretofore been used for comminuting a stream of molten metal. However, I have found that due to a considerable amount of metal splashing and bouncing from the trough, comminution i unpredictable and incomplete. This appears to be due to material amounts of the melt splashing and bouncing from the ends of the trough before it is properly quenched.

In accordance with the invention, quenching and comminution are greatly improved by the use of auxiliary nozzles such as at 21 and 23 for directing the respective jets into opposite ends of the trough, making therewith blocking spray screens 21' and 23, respectively, tending to close off the trough ends, FIGS. 2 and 3. The melt is thereby entrapped within curtains of the quenching liquid and uniformly comminuted. Specifically, the plane of an auxiliary fan-spray extends transversely of the trough apex, and preferably is inclined with respect to the longitudinal axis of the melt stream so that the auxiliary jets tend to converge beneath the confluence of the main jets. The angles that the auxiliary jets make with the trough apex are equal and can be varied within material limits.

I have found that in practice, this aspect of the invention provides for greatly improved comminution and cooling of the metal. At the high jet pressures used, the spray patterns tend to persist even after confluence, thereby promoting further cooling of the metal powder immediately following comminution, especially at the lower confiuence of the auxiliary jets. The main jets have practically direct connection with the pressure source, and supply a major part of the high velocity quenching liquid; the auxiliary jets are located closer to the trough so that the flow intensity thereof at the trough is substantially equal to that of the main jets at normal source pressure. This ensures substantially thorough and consistent comminution.

For increasing the production of metal powder, two or more streams of melt can be comminuted at one trough simply by increasing the size, i.e., length, thereof. This is readily done by using two or more main jets at each side of the trough so that the respective fan-spray patterns overlap and form a longer trough apex, or line of confluence; also, where it is desired to obtain higher jet intensities at the trough for obtaining finer comminution, the nozzle fan-spray angle of each jet can be decreased for more intense concentration of quenching liquid on the melt without necessarily increasing the source pressure.

Where multiple auxiliary jets of smaller fan-angle are also used in similar manner for the trough end-screens, the auxiliary nozzles can be spaced further from the trough, thereby minimizing any damage thereto by splashed metal. In this instance, the baflle 28 can be completely fiat, rather than centrally depressed as at 28', FIGS. 1 and 3. 1

Hot iron particles produced by comminution as described above, are ordinarily subject to such high rate of oxidation that within the brief time required for the particles to fall within a collecting liquid comparative large amounts of oxide are formed. Such oxidation of the iron powder is effectively avoided by providing automatically in the atomizing chamber a substantially complete and inert ambient of steam that is generated incident to quenching and by maintaining the collecting liquid and ambient in direct liquid-vapor phase relationship. To this end, the atomizing chamber is conveniently formed as a cylinder of limited diameter joined to the underside of the bafile 28 in concentric relation to the longitudinal axis of the melt stream 4, FIGS. 1 and 2. The lower end of the cylinder extends somewhat beneath the level 36 of the collecting liquid 37 that partially fills a tank 38, for making a liquid seal with that end of the atomizing chamber. The tank which forms the lower part of the housing 5 has a weir or the like 39 for maintaining at constant level the collecting liquid which is continuously supplemented by the quenching liquid. For even flow at the weir, an inverted weir or baflle 40 is located opposite the weir overflow. Atmospheric pressure is maintained within the tank by vent openings 38' in lateral extensions of the splash baffle communicating with the spray compartments.

As the atomizing chamber 7 fills with steam incident to Water quenching of the iron melt at the trough 30, the vapor pressure within the chamber increases tending to develop disruptive pressures therein. By suitable venting, this vapor pressure is automatically regulated for maintaining it slightly above atmospheric pressure, while expelling extraneous air from the chamber so that a complete and inert ambient of steam only remains. This small positive pressure depresses somewhat the level of the liquid in the chamber as shown in FIG. 1. Once the converting apparatus is in operation the atomizing chamber remains filled with steam at positive pressure; comparatively small or negligible amounts of air enter the chamber from the baffle opening 29 as the counter-effect of steam pressure generated at the trough area apparently blocks the inflow of air ordinarily induced by the quenching jets.

The excess pressure venting means for the atomizing chamber can assume various forms; for example, the chamber can be vented directly into the tank above the water level 36 by side openings 41 in the chamber cylinder. The openings are proportioned according to a standard rate of powder production for limiting the chamber pressure to a substantially constant positive pressure at that rate. The area of the openings 41 can readily be adjusted by obvious means for variable venting if so required An alternative method of venting is shown in FIG. 4 wherein the lower edge of the chamber cylinder is scalloped at 42 for example, for releasing excess pressure at the level of the collecting liquid. In this arrangement, the venting area is automatically increased as the chamber pressure is increased, by corresponding depression of the level of the chamber liquid; this action tends to maintain a balance between the venting area and chamber pressure. Where the level of the tank liquid is comparatively stable and turbulence not excessive, the atomizing chamber may be pressure-vented simply by decreasing the distance between the lower edge of the cylinder 35 and the constant level 36. Here, the excess chamber pressure is relieved when the chamber liquid is depressed sufficiently to vent the vapor around the lower edge of the cylinder directly into the collecting liquid or tank atmosphere, as the case may be.

In the arrangements described above, the atomizing chamber provides a complete inert ambient for the iron powder as it falls from the area of comminution through the chamber to the collecting liquid. A removable tray 43 for transferring accumulated powder from the tank housing is shown only by way of example; the transfer of the powder which may be by gravity, magnetic separation, etc., and the subsequent processing thereof are not within the scope of this invention.

Iron powder produced as described above is found to be of unusualy uniform texture and predictable average mesh size. It also has very low oxygen content, i.e., for practical purposes, an insignificant amount of oxide, and in this respect is equal in purity to powder produced by methods using comparatively expensive inert gases. The iron particles are characterized by absence of voids and have considerably higher density than powder made by sponge iron process.

The typical particle shapes are spherical, distortions of spheres and accretions of spheres, all with comparatively smooth surfaces, as contrasted with the angularity of particles made by other methods. The particles, due to their general smoothness, accordingly lend themselves, inter alia, to extrusion methods.

The size of the particles can be readily regulated by varying the flow rate of the melt, or quenching liquid, or both. The relationship can be expressed generally as r1=f M L) where P is average particle diameter, V is rate of metal flow, and V is rate of quenching liquid flow.

Size variation of the powder can be obtained within wide limits; for example, iron powder used in extruded coatings for welding electrodes is generally specified within certain mesh limits, as between 30 and 200 mesh for example, with certain proportions of the powder being within the upper and lower mesh sizes and an intermediate, say 100, mesh respectively. Such powder can be predictably produced by the method described above.

It should be t nderstogd that this invention is not lim- 6 ited to specific details of construction and arrangement thereof herein illustrated, and that changes and modifications may occur to one skilled in the art without departing from the spirit of the invention.

I claim:

1. The method of directly converting by continuous process, molten metal to a powder having a low oxygen content which comprises:

(a) directing a stream of molten metal through an upper chamber maintained at atmospheric pressure, into a substantially closed chamber through a bafile having an opening therein,

(b) directing a pair of main spray jets emanating from within the upper chamber through the opening at high velyocity to intercept at a V-shaped configuration within the substantially closed chamber,

(c) directing a pair of auxiliary spray jets emanating from within the upper chamber through the opening at high velocity to block the opposite ends of the V-shaped trough,

(d) positioning the main spray jets and the auxiliary jets to intercept the stream of molten metal within the substantially closed chamber, for quenching and comminuting the molten metal,

(e) confining steam generated incident to the quenching and comminuting of the metal to create and maintain a substantially constant and complete inert steam vapor atmosphere within the chamber at slightly above atmospheric pressure, and

(f) collecting the powder in a cooling medium immediately after it has passed through the vapor atmosphere.

References Cited UNITED STATES PATENTS 2,787,534 4/1957 Golwynne 75-.5

3,281,893 11/1966 Ayers 75-.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner

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