Specialized metallurgical processes – compositions for use therei – Processes – Producing solid particulate free metal directly from liquid...
Reexamination Certificate
1999-07-02
2001-06-26
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing solid particulate free metal directly from liquid...
C075S332000, C075S366000, C065S019000
Reexamination Certificate
active
06251158
ABSTRACT:
TECHNICAL FIELD
This invention relates to the production of solid metallic granules from molten metal and, in particular, to the production of granules of a reactive metal such as magnesium or a magnesium alloy.
BACKGROUND ART
There is a need in industry for reactive metal granules and, in particular, for granules of Mg or Mg alloy for the treatment of steel, aluminum or other metals and for other purposed such as thixotropic injection moulding. These applications require granules of at least 1 mm in size and the granules should be substantially free of surface oxides. For some uses, granules coated with a layer protecting them from oxidation may be advantageously used and various salts, for example, have provided this advantage.
There are few commercial processes which directly produce reactive metal granules. For many applications, such granules are produced by cutting or shearing material from larger pieces of metal.
U.S. Pat. No. 4,457,775 issued on Jul. 3, 1984 to Legge et. al. discloses a method for producing Mg granules by mixing Mg into a salt bath of specific composition with agitation, then partially separating the product from the bath to obtain a salt/granule mixture. Because of the production method, the composition is somewhat variable.
Metal granules or shot from less reactive metals (iron, steel, copper, etc.) have been produced by injection from a nozzle into liquid baths or into counter-current gas streams. The former process is a difficult operation for a reactive metal and the latter process requires a spray tower of substantial height, and is limited in practice to granules of small diameter because of cooling considerations.
Furthermore, in order to be adapted to reactive metals, substantial quantities of inert gas would be required.
PCT publication WO-A-86 06013 (and equivalent U.S. Pat. No. 4,915,729) disclose a process in which a molten metal is contacted with a bed of moving beads. The molten metal breaks up into fine particles which are rapidly cooled in contact with the beads. However, the mechanical agitation produces particles of metal in the form of angular flakes rather than spherical granules. The method is not well suited to the formation of particles of reactive metals, since the large surface area tends to encourage oxide formation and reaction.
There is accordingly a need for an improved method of producing granules of reactive metals.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a method for producing acceptably uniform metal granules, preferably of a reactive metal, with substantially no surface contamination
Another object of the invention is to provide a method for producing acceptably uniform metal granules, preferably of a reactive metal, of a size range suitable for alloying with metals, for example steel or aluminum.
Another object of the invention is to provide a method for producing metal granules, preferably of a reactive metal, which avoids the use of molten salt baths, liquid coolants and excessive quantities of gas.
Yet another object of the invention is to provide a method for producing reactive metal granules, preferably magnesium or magnesium alloy granules, that can be coated or doped in a controlled manner to reduce oxidation of the granules or to provide other chemical additives (such as fluoride or chloride salts) to the granule product.
Still another object of the invention is to provide a novel magnesium granule product for use in metal alloying applications.
According to one aspect of this invention, there is provided a process of producing granules of a metal, comprising providing a source of molten metal having a solidus temperature; forming discrete droplets of said molten metal from said source; fluidizing a bed of particles by means of a gas, and maintaining said bed at a temperature substantially below the solidus temperature of the metal, said particles being of a size substantially smaller than granules produced by freezing said droplets; immersing said droplets while still substantially molten in said fluidized bed of particles to freeze said droplets as discrete granules of metal in said bed; and removing said granules from said fluidized bed.
The invention is particularly suited for the production of reactive metal granules but may, if desired, be used for producing granules of other metals, e.g. non-reactive metals of many different kinds.
According to another aspect of the invention, there is provided apparatus for producing granules of a metal, comprising a source of molten metal having a solidus temperature; a droplet forming device for forming discrete droplets of molten metal from said source; a bed of particles for receiving droplets of molten metal from said droplet forming device while said droplets are still substantially molten; means for introducing a gas for fluidizing the bed; cooling equipment for maintaining said fluidized bed at a temperature substantially below the solidus temperature of the metal; and a separator for separating solidified granules of said metal from particles of said fluidized bed.
According to yet another aspect of the invention, there is provided magnesium-containing alloying additive for use in aluminum alloying, comprising granules of a magnesium-containing metal having a solidus temperature, said granules being at least partially coated with a chloride salt and having a granule size in the range of 1 to 10 millimeters, said chloride salt being attached to said granules, at least in part, by physical embedding of said salt into surface of said granules. The magnesium containing metal may be either magnesium or a magnesium alloy.
The reactive metals to which the present invention preferably relates are characterized as being sufficiently reactive with air or water such that the use of water or large quantities or air to quench and cool the metal granules would give rise to substantial oxidation of the product. Many metals in Group Ia, IIa or IIIa are of this type, e.g. lithium, sodium, potassium, cesium, magnesium, calcium, beryllium, aluminum, and strontium, and most importantly aluminum and magnesium and their alloys.
Discrete droplets of the metal can be formed in a number of ways, e.g. by the use of a vibrating nozzle, or by the use of a fixed nozzle or array of fixed nozzles. It is particularly preferred because of cheapness and reliability to use an array of fixed nozzles. When using fixed nozzles, the droplet size may be controlled not only by the nozzle diameter but also by the differential pressure of the molten metal applied to the upstream side of the nozzle, and by the nozzle geometry.
The fluidized bed of particles may consist of a wide range of particulate materials, for example, metals (e.g. as metal shot), carbon or graphite, refractory materials or salts. The particle sizes are selected to be substantially smaller than the desired product granule size, and of a size that can be readily fluidized. Suitable particle sizes are typically in the range 30 to 200 Tyler mesh (74 to 500 microns). Particles of refractory materials and salts, and mixtures of the two are particularly useful.
Fluidized bed particles may be selected to have a composition and size such that they react at a slow rate with the metal granules to form surface coatings. Particles may alternatively be selected to be non-reactive with the metal granules. In this case, bed particles can be chosen that adhere to the metal granule surface as it solidifies within the bed to form a full or partial coating of non-reactive particles, at least partially embedded in the surface of the granule. If non-reactive bed particles have a melting point near or below the temperature of the metal used to form the granules, partial melting of the particles can occur as the metal granules contact the particles, further improving the coating quality.
The fluidized bed is operated at an average temperature below the solidus temperature of the metal and preferably at least 100° C. below the solidus temperature of the metal and most preferably at least 200° C. below the solidus temp
Dube Ghyslain
Dupuis Claude
Langlais Joseph
Lavoie Serge
Rompre Stephane
Alcan International Limited
Cooper & Dunham LLP
Wyszomierski George
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