Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
1998-12-29
2002-04-23
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S682000, C075S683000, C075S684000, C075S685000
Reexamination Certificate
active
06375712
ABSTRACT:
FIELD OF THE INVENTION
When aluminum is extracted electrolytically from alumina dissolved in a cryolite-based molten bath (Na
3
AlF
6
+other stable fluorides), sodium is codeposited with the aluminum. Small proportions of other fluorides are employed, such as lithium fluoride or magnesium fluoride, which can confer advantageous properties on the bath leading to improved economics, resulting in traces of lithium and magnesium in the pool of molten aluminum. The concentrations of these light alkali and alkaline earth metals are dependent upon physical and chemical conditions within the electrolytic reduction cell and the manner in which it is operated. Typical concentrations of sodium, lithium, magnesium, and calcium in freshly tapped molten aluminum metal are in the ranges of from 40 to 180 PPM, 5 to 25 PPM, 5 to 150 PPM, and 4 to 10 PPM respectively and are contaminants which must be removed or lowered to below specifications for the aluminum metal to be marketed for many uses. For many primary aluminum products of casting operations, the specifications require levels of Na and Li to be less than 2 PPM and in some cases as low as 0.5 PPM.
BACKGROUND OF THE INVENTION
The prior art teaches treatment of primary aluminum metal for the removal of traces of alkali metals by passing through a packed bed of carbonaceous material mixed with solid, granular aluminum fluoride, such as disclosed in U.S. Pat. Nos. 3,305,351 and 3,528,801. Although a bed of alternating layers from 1 to 5 feet deep containing from 5 to 90 wt. % AlF
3
can treat from 50,000 to 100,000 pounds of primary aluminum metal, there is difficulty in keeping the bed open, porous, and combusting uniformly, promoting the dispersion of aluminum fluoride. The residue from the spent bed presents environmental problems for disposal because it contains potentially hazardous waste material.
U.S. Pat. No. 4,277,280 discloses flowing molten aluminum through a bed of coarse, granular AlF
3
-containing material wherein the reactive aluminum fluoride can combine with the alkali metals to form a liquid phase and cause clogging of the initial granular bed. Bath components unavoidably tapped with the liquid aluminum from the cells can also react with the AlF
3
to form either unwanted liquid or channels in the bed, thus decreasing its effectiveness.
U.S. Pat. No. 4,470,846 discloses removal of contaminating alkali and alkaline earth metals by reaction with aluminum fluoride-yielding material dispersed into a stable vortex in the molten aluminum surface. A rotating impeller with the pitch of the blades designed to force the liquid metal downwardly creates a vortex in the surface of the metal around the shaft connected to a motor. Controlled quantities of granular AlF
3
were added into the vortex from a hopper.
U.S. Pat. No. 4,138,246 discloses a filter bed method for lowering the concentration of sodium in molten aluminum, by gravitationally driven flow through a loosely packed filter bed of granular material, which in part, comprises carbon. It suggests that carbon preferentially absorbed a portion of the sodium.
Another form of bed filter for removing sodium, lithium and calcium was described by Achim and Dubé (Light Metals, 1982, pp. 903-916) wherein molten aluminum containing approximately 100 PPM Na and 30 PPM Li flowed through a filter bed of egg-shaped granules of a mixture of cryolite 3%, alumina 9%, and aluminum fluoride 86%. While this application did remove up to 90% of the light metals, the briquettes forming the bed were consumed and the vessel containing the bed for in-line metal cleaning had to be taken down, cleaned and repacked with AlF
3
-containing 15-50 mm diameter pellets after processing 1000 to 1500 tons of liquid aluminum.
The Mixal process for treating aluminum in potline crucibles (Archard and Leroy, Light Metals 1990 pp. 765-768) is a prior art method wherein a mixture of gases, chlorine-argon or chlorine-nitrogen is injected into molten aluminum through a spinning nozzle or rotor. After about 11 minutes of treatment the concentrations of sodium and lithium is reduced to 5 and 3 ppm respectively. The products of the reaction are volatile and the effluents are drawn off through a covering lid to a lime injected bag filter capture system. Lack of liquid slag or dross from the reactions depleting the alkali metals is an advantage of this process. However, the risk of recycling chlorides mixed with the fluorides to the reduction cells presents the possibility that untenable corrosion will occur in the electrical connections to the buswork and in the fume capture system of the potrooms.
Primary aluminum is obtained by electrolytic extraction from alumina dissolved in a cryolite-based molten bath (Na
3
AlF
6
+other stable fluorides). Sodium is codeposited with the aluminum during this process along with lithium and magnesium fluorides, which can confer advantageous properties when they are used, resulting in not only sodium but also traces of lithium and magnesium being present in the pool of molten aluminum. The concentrations of these light alkali and alkaline earth metals are dependent upon physical and chemical conditions within the electrolytic reduction cell and the manner in which it is operated. Typical concentrations of sodium, lithium, magnesium, and calcium in freshly tapped molten aluminum metal are in the ranges of from 40 to 180 PPM, 5 to 25 PPM, 5 to 150 PPM, and 4 to 10 PPM respectively.
These metal traces are contaminants which must be removed or lowered for many commercial products. Primarily aluminum many products of casting operations include specifications requiring levels of Na and Li to be less than 2 PPM and sometimes as low as 0.5 PPM.
Although fluxing with gas mixtures such as N
2
—Cl
2
, N
2
—CO—Cl
2
, Ar—Cl
2
, and single gases such as argon and nitrogen is routinely practiced in holding furnaces and in launders to remove particles, hydrogen and light metals, this is not a preferred method for decreasing the amounts of Na, Li, Ca, and Mg. These metals form halides, promote the formation of extra dross, react with the refractory lining of the holding/melting furnace and require extended time of fluxing to reduce their concentration in typical virgin primary aluminum to lower levels required in semi-finished aluminum products.
The methods presently practiced to remove or reduce the concentrations of light metals include charcoal filtering in which a tapping crucible of primary aluminum is poured into another crucible containing a bed of burning charcoal and AlF
3
. The combined effects of the agitation of the metal, reaction with AlF
3
, oxidation from entrained air, and the propensity for sodium to react with carbon in the presence of fluoride, depress the concentration of Na and Li to about 10 and 5 ppm respectively. In both Alcan and Hycast “Treatment in Crucible” methods, a crucible of virgin molten metal is taken to a station where an impeller is lowered onto the open crucible. In the Alcan method, the rotating impeller creates a vortex into which granular AlF
3
is added for a period of 5 to 15 minutes. In the Hycast method granular AlF
3
is injected into the metal with an inert carrier gas through a hollow axle and into the metal through a rotating impeller for a period of 5 to 20 minutes. As in the previous methods described, there is enough latent heat for the molten aluminum to remain liquid throughout these treatments. Aluminum fluoride reacts according to the equations:
AlF
3
+3NaF=Na
3
AlF
6
AlF
3
+3LiF=Li
3
AlF
6
The slags/drosses are skimmed off the surface of the aluminum metal after the rotor is stopped. The treated aluminum metal is thereafter transported to the casthouse holding furnace.
SUMMARY OF THE INVENTION
This novel innovative process for removal of alkali and alkaline earth metals from virgin or primary molten aluminum comprises the apparatus and method for injecting a powdered, vaporizable fluoride carried in a gas stream through a rotating impeller, thus creating bubbles and differential shear forces. This normally is accomp
Forberg Helge O.
Richards Nolan Earle
Andrews Melvyn
Hanegan Herbert M.
Hanson Eric J.
Lischer Dale
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