US3520718A - Method of atomizing molten magnesium - Google Patents

Description

United States Patent 3,520,718 METHOD OF ATOMIZING MOLTEN MAGNESIUM John N. Reding, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation f Delaware No Drawing. Filed Aug. 3, 1967, Ser. No. 658,037 Int. Cl. B22d 23/08; C23c 1/00 US. Cl. 117131 5 Claims ABSTRACT OF THE DISCLOSURE In producing alloy pellets by the atomizing method disclosed in US. Pat. No. 2,699,576 by allowing molten metal to fall freely in a thin stream onto a steel disc or wheel rapidly rotating in a non-reactive gas atmosphere, heretofore, only magnesium base alloys containing from 0.025 to 1.0 percent by weight zirconium and from 0.25 to 7.0 percent by weight zinc could be so atomized.

As disclosed in the above patent, magnesium base alloys containing no zirconium or zinc, and magnesium base alloys containing less than the minimum amount of either zirconium or zinc, or both, as required by US. Pat. 2,699,576 cannot for the most part be successfully atomized using the method of said patent to obtain a uniform, quality, pelletized product. The disc in these cases is substantially unwetted by the molten metal resulting in a non-uniform pellet product mixed with large irregular agglomerates which are flung off the rotating disc.

A principal object of the present invention, therefore, is to provide an improved process whereby magnesium and essentially all magnesium base alloys can be atomized by the method as disclosed in US. Pat. 2,699,576.

An improved method of atomizing has now been found wherein the above and other objects and advantages have been found obtainable by adherently coating a clean ferous metal atomizing disc, whose surface to be coated is essentially free of oxides, with certain metals as hereinafter described.

In practicing the present invention, therefore, a ferrous metal disc, for example, as used in U.S. Pat. No. 2,699,576 for atomizing is initially adherently coated, using essentially any coating means or techniques, with a thin layer of a metal selected from the group consisting of cadmium, copper, nickel, tin and preferably zinc. Thereafter, the so-coated disc is used for atomizing magnesium and/or magnesium base alloys, without regard for the limitation with respect to observing minimum amounts of zinc and zirconium as was heretofore required and set forth in US. Pat. 2,699,576.

The so-coated wheel is initially wetter by the magnesium base alloy being atomized, with continued wetting by said alloy of the underlying ferrous metal surface now exposed by dissolution of the coating metal. Dissolution of the :superstrate coating off the atomizing disc as it rotates by the molten metal intended to be atomized usually takes from about 5 to about 20 minutes.

By adherently coating the atomizing disc initially with a thin layer of at least one metal of the group cadmium, copper, nickel, tin, or zinc, essentially all magnesium ice base alloys as well as primary magnesium itself can be atomized to provide a visibly uniform pelletized product having a particle size substantially no coarser than about 20 mesh (US. Standard Sieve Series) and an essentially spherical shape.

Essentially any ferrous metal atomizing disc described in the aforesaid patent can be treated as set forth herein to accomplish the objects of the invention and to prepare a resulting uniform pellet product therefrom. The surface or surfaces of the disc intended to be employed in atomizing prior to coating with one or more of the coating metals should be relatively clean in that it should be essentially free of grease, dirt and the like and free of the various oxides usually appearing on ferrous surfaces due to corrosion, etc. The method of coating the ferrous disc surface is not limited other than by the requirement that the coating so-applied should reasonably be tenacious and adherent. Any conventional or other method of coating or deposition may be employed in the improvement of the present invention such as galvanizing, electroplating or chemical plating, and the like. As aforesaid, essentially all the iron oxides and other foreign material that may be present on ferrous metal surfaces should be removed therefrom either before or during the coating operation, as in galvanizing, such that no intermediate layer of oxide or other foreign material exists between the coating metal and ferrous metal.

The thickness of the metal coating applied on the ferrous disc surface is not critical, except that a continuous coating should be obtained. Normally said coating will be thin and only a few mils (thousands of an inch) in thickness. No advantage results in applying a thick coating of metal and only results in a disadvantage of requiring a longer time to be dissolved away by the molten coating metal during the first minutes of the atomizing operation.

The following examples serve to further illustrate the application of the present invention.

The following magnesium base alloys having the composition indicated, the balance of each composition being essentialy magnesium, were prepared.

(1) 0.17% zinc+0.17% calcium and 0.4% zirconium, (2) 3.0% aluminum+l.0% zinc and 0.2% manganese, (3) 1.1% zinc+0.2% Misch metal.

In the molten state each melt was atomized by dropping a thin continuous stream thereof onto the center of a zinc galvanized, slightly concaved surface of a 4 inch diameter steel disc, the major axis of which was horizontally disposed. The disc was rotating at about 10,000 r.p.m. in a non-reactive gas atmosphere. The underside of the disc was partially thermally insulated so that the upper surface could be maintained at a temperature slightly above the melting point of the particular alloy being atomized.

The disc was initially wetted by the molten metal. During continuation of the atomizing operation the initial zinc coating on the disc was subsequently dissolved away. Essentially no problems were encountered as continued wetting of the disc by the molten alloy and atomizing of the alloy into uniform solidified pellets resulted by contact of the metal being atomized with the exposed clean disc surface. The molten alloy was flung off the rotating disc whereupon the small droplets quickly solidified into uniform spherical pellets with a minimum of fines. Essentially all the pellets so-produced were no coarser than about 20 mesh (US. Standard Sieve). After the atomizing of each melt was completed and the disc cooled, a wettable solidified coating of the alloy re- 1 A commercial mixture of rare earth metals containing predominant amounts of cerium, neodymium. the balance comprising smaller amounts of other rare earth metals.

mained on the atomizing surface of the disc such that the disc could be subsequently used again without an initial coating operation as hereinbefore described.

COMPARATIVE EXAMPLES In order to further illustrate the beneficial results obtained by the improvement of the present invention, additional melts of the above alloys were prepared and atomized similarly as described above except that the ferrous atomizing disc was not coated by any coating metal. The molten metal atomized only slightly and irregularly wetted the disc surface. The pellets produced were generally non-uniform and irregular in shape. Moreover, a large portion of the molten magnesium base alloys was flung off the rotating disc in relatively large masses or blobs which did not solidify prior to impact with a shield surrounding the atomizing disc. Rather, the material became virtually plastered into large accumulations on said shield which would periodically fall off to render the pellet product collection excessively non-uniform.

The results of employing an atomizing disc not coated in accordance with the invention when compared to that obtained as illustrated in Examples 1, 2 and 3 above, clearly show the great benefit and utility of the improvement in atomizing of the present invention.

The present invention may be modified and changed without departing from the spirit and scope thereof and the invention is only limited as defined in the appended claims.

I claim:

1. In a method of atomizing molten metal in which a thin stream of molten magnesium or magnesium base alloy falls upon the upwardly facing surface of a rapidly rotating, heated, steel disc to fiingoff said metal as molten droplets which solidify upon being expelled from the disc to produce atomized pellets therefrom; the improvement comprising coating said disc surface intended to contact the molten metal to be atomized with a thin adherent continuous coating of a coating metal selected from the group cadmium, copper, nickel, tin or zinc prior to contacting said disc with said molten metal to be atomized.

2. The improvement of claim 1 wherein the disc is coated with a thin adherent coating of zinc.

v3. The improvement of claim 1 wherein the disc is coated with a thin adherent coating of cadmium.

4. The improvement of claim 1 wherein the disc is coated with a thin adherent coating of tin.

5. The improvement of claim 2 wherein the disc is coated with zinc by galvanizing.

References Cited UNITED STATES PATENTS 2,699,576 1/1955 Colbry et a1. 264-8 2,912,346 11/1959 Kanter l17131 X 3,300,298 1/1967 Reding 117--131 X ALFRED L. LEAVITI, Primary Examiner J. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.