Fluid sprinkling – spraying – and diffusing – Processes – Including mixing or combining with air – gas or steam
Reexamination Certificate
2000-06-16
2002-11-19
Morris, Lesley D. (Department: 3752)
Fluid sprinkling, spraying, and diffusing
Processes
Including mixing or combining with air, gas or steam
C239S294000, C239S418000, C239S424000, C239S597000
Reexamination Certificate
active
06481638
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and nozzle for producing fine powder, preferably having a spherical physical appearance, by atomizing molten material with gases such as known for EP-A-0444-767.
To produce metal powders, gas atomization techniques are known throughout the industry. Different nozzle constructions are utilized, all of which have in common that a pressurized atomization gas escapes from one or more gas nozzles and, as a turbulent stream, approaches at an angle molten material flowing out of a molten material nozzle and atomizes such molten material. An overview of various nozzle constructions is provided, for example, by A. J. Yule and J. J. Dunkley “Atomization of Melts”, Oxford, 1994, pages 165 to 189. On its way to the molten material, the gas loses a large portion of its energy. With atomization gas pressures up to about 35 bar, relatively coarse metal powder having average granular diameters d
50
in the atomization state of about 50 &mgr;m and greater result. The thus produced powders generally have a broad granular size distribution because the atomization pulse is subjected to great deviations due to the turbulence. J. Ting, et al., “A novel high pressure gas atomizing nozzle for liquid metal atomization”, Adv. Powder Metallurgy and Particulate Materials, 1996, pages 97 to 108, discloses special high pressure nozzles having operating pressures of up to 100 bar, which at a very high gas consumption can produce average granular sizes of about 20 &mgr;m. All known methods having turbulent gas flow are unsuitable for the direct production of fine powders having average granular diameters d
50
of about 10 &mgr;m.
DE 33 11 343 A1 discloses a method of producing fine metal powders as well as a device for carrying out the method, and proposes the use of laminar gas streams in a concentric Laval nozzle having preheated atomizing gas. The molten material nozzle is positioned in such a way that it is disposed in the converging portion of the Laval nozzle, i.e., that the molten material nozzle extends into the Laval nozzle. The flow in the upper portion of the Laval nozzle is laminar. In contrast to methods having turbulent gas flows, finer powder with a narrower granular size distribution, accompanied by relatively low specific gas consumption, result, as illustrated, for example, in
FIG. 2
of the publication of G. Schulz, “Laminar sonic and supersonic gas flow atomization” PM
2
TEC '96, World Congress On Powder Metallurgy And Particulate Materials, U.S.A., 1996, pages 1 to 12. The specific gas consumption for the production of a steel powder having an average granular diameter of 10 &mgr;m is approximately 7 to 8 Nm
3
Ar/kg corresponding to about 12.5 kg to 14.2 kgAr/kg steel.
DE 35 33 964 C1 discloses a method and an apparatus for producing very fine powders in spherical form, according to which the atomizing gas is introduced via a radially symmetrical, heatable gas hopper into the Laval nozzle, whereby the metal exiting the molten material nozzle, which is placed within this gas hopper, is overheated or heated by heat transfer via radiation, which originates from the heated gas hopper.
DE 37 37 130 A1 similarly discloses a method and an apparatus for producing very fine powders, according to which the underpressure resulting from the gas flowing in the Laval nozzle is utilized to draw in molten material from a separate molten material device. Here also a radially symmetrical nozzle system having a molten material nozzle placed within the Laval nozzle is involved.
From the publication of G. Schulz, “Laminar sonic and supersonic gas flow atomization—The NANOVAL—Process”, Adv. Powder Metall. & Particulate Matter. (1996), 1, pages 43-54, it is furthermore known that for the production of fine metal powder it is necessary to keep the mass flow exiting the radially symmetrical nozzle small if fine powder is to be produced. Indicated here are 12 to 30 kg/h and nozzles with molten material nozzle diameters of 1 mm or less.
Common to all of the previously known methods is that these have serious technical and economical drawbacks. For example, the heretofore utilized concentric or radially symmetrical nozzle systems having molten material nozzle diameters of 1 mm or less, are, due to the type of construction, particularly susceptible to mechanical clogging due to foreign particles or gas bubbles that are carried along. In addition, due to a given unfavorable ratio of outer molten material nozzle surfaces to the molten material volume, great heat losses occur that can effect an undesired congealing of the molten material nozzles and then, as is also the case with the mechanical clogging, result in a termination of the atomization and longer down times. Furthermore, the production capacity that up to now could be achieved is low, and the specific gas consumption is high. During the production of fine powders, the production capacity and the specific gas consumption are very decisive in determining the manufacturing costs. There is therefore a need for an atomizing method that is characterized by low gas consumption and high production capacity.
Taking into account this state of the art, the object of the invention is to improve a method of the aforementioned general type, while avoiding the described drawbacks, in such a way that an economical production of fine, gas-atomized powder is possible. Furthermore, down times due to clogging from impure molten material, and from congealing due to heat losses, are to be avoided. In particular, it should be possible to finely and uniformly atomize metallic, metallic alloy, salt, salt mixture, or also polymeric molten material on a large scale, in an economical manner, and in particular, however, with a low gas consumption and a high molten material throughput. Furthermore, the molten material nozzle should be as stable as possible relative to mechanical clogging from impure molten material as well as relative to congealing.
SUMMARY OF THE INVENTION
The object is inventively realized in that the molten material flows out of a molten material nozzle having an essentially rectangular cross-sectional area in the form of a film, and subsequently, together with an atomizing gas, issues through an initially converging and then diverging gas nozzle that is in the form of a linear Laval nozzle, has an essentially rectangular cross-sectional area, and through which flow is laminar, whereby the laminar accelerated gas flow stabilizes and simultaneously stretches the film of molten material in the converging portion of the Laval nozzle until the film of molten material, after passing the narrowest cross-sectional area, is uniformly atomized over its entire length.
Surprisingly, it is possible to stabilize the film of molten material, which is primarily issuing from the essentially rectangular molten material nozzle, and that would be unstable due to its large surface area by virtue of free discharge, by the introduction into the accelerated gas stream in the converging portion of the similarly essentially rectangular Laval nozzle. In so doing, an extremely favorable relationship of powder molten material nozzle surface to the molten material volume is achieved, so that clogging due to congealing is precluded. Furthermore, individual foreign particles in impure molten material can in the most unfavorable situation affect only a small portion of the cross-sectional area of the molten material nozzle, so that the atomizing process is also not terminated under such conditions. Below the narrowest cross-sectional area of the Laval nozzle, the film of molten material is uniformly atomized with high specific pulse to a fine powder that preferably has a spherical physical appearance.
Pursuant to a further advantageous proposal of the invention, the ratio of the pressure above the Laval nozzle and below the Laval nozzle corresponds at least to the critical pressure ratio of the atomizing gas that is utilized, so that the gas reaches the speed of sound in the narrowest cross-sectional area of the Laval nozzl
Becker R W
Morris Lesley D.
Nguyen Dinh Q.
R W Becker & Associates
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