Note: Descriptions are shown in the official language in which they were submitted.
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DISPERSING GAS INTO MOLTEN METAL
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Background of the Invention
1. Field of the Invention
The invention relates to dispersing gas into molten
metal and, more particularly, to techniques for causing finely
divided gas bubbles to be dispersed uniformly throughout the
mol ten met al .
2. Description of the Prior Art
In the course of processing molten metals, it sometimes
is necessary to treat the metals with gas. For example, it is
customary to introduce gases such as nitrogen and argon into
molten aluminum and molten aluminum alloys in order to remove
undesirable constituents such as hydrogen gas, non-metallic
inclusions, and alkali metals. The gases added to the molten
metal chemically react with the undesired constituents to convert
them to a form (such as a precipitate or a dross) that can be
separated readily from the remainder of the molten metal. In
order to obtain the best possible results, it is necessary that
the gas be combined with the undesirable constituents efficiently.
Such a result requires that the gas be dispersed in bubbles as
small as possible and that the bubbles be distributed uniformly
throughout the molten metal.
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As used herein, reference to "molten metal" will be
understood to mean any metal such as aluminum, copper, iron, and
alloys thereof, which are amenable to gas purification. Further,
the term "gas" will be understood to mean any gas or combination
of gases, including argon, nitrogen, chlorine, freon, and the
like, that have a purifying effect upon molten metals with which
they are mixed.
Heretofore, gases have been mixed with molten metals by
injection through stationary members such as lances, or through
porous diffusers. Such techniques suffer from the drawback that
inadequate dispersion of the gas throughout the molten metal can
occur. In order to improve the dispersion of the gas throughout
the molten metal, it is known to stir the molten metal or
otherwise convey it past the source of gas in~eetion. Devices
also are known that accomplish both of these functions, that is,
the devices stir the molten metal while simultaneously injecting
gas into the molten metal.
Despite the existence of combined stirring/in,jecting
devices, certain problems remain. Combined stirring/in~eating
devices often exhibit poor stirring action. Sometimes cavitation
occurs or a vortex is established that moves around the inside of
the vessel within which the molten metal is contained.
Frequently these devices dispensa bubbles that are too large or
which are not uniformly distributed throughout the molten metal.
A problem with one known prior device is that it utilizes an
impeller having passageways that can be clogged with dross or
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foreign objects. Most of the prior devices are expensive,
omplex, and usable with only one type of molten metal-handling
system. Other problems frequently encountered are poor longevity
of the devices due to oxidation, erosion, or lack of mechanical
strength. These latter concerns are particularly troublesome in
the case of aluminum because the stirring/in~ecting devices
usually are made of graphite, and graphite is rapidly oxidized
and eroded by molten aluminum. Accordingly, devices that
initially perform adequately often become quickly oxidized and
eroded so that their mixing and gas dispersing effectiveness
diminishes rapidly; in severe cases, complete mechanical failure
can occur.
Summary of the Invention
The present invention provides a new and improved
technique for dispersing gas within molten metal that overcomes
the foregoing problems. Apparatus according to the invention
includes an impeller in the form of a rectangular prism having
upper and lower faces, a width (A), a depth (B), and a height
(C), with (A) preferably being equal to (B). The impeller has a
gas discharge outlet opening through the lower face of the prism.
An elongate, rotatable shaft is rigidly connected to the impeller
and pro~eets from the upper face of the impeller. The apparatus
also includes means for conveying gas to the gas discharge
outlet, whereby gas to be dispersed into molten metal can be
pumped along the lower face of the impeller.
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In the preferred embodiment, the gas discharge outlet
defined by an opening extending through the upper and lower
faces of the impeller, and the means for conveying gas to the gas
discharge outlet is a longitudinally extending bore formed in the
shaft, the shaft being connected to the impeller such that the
bore in the shaft and the opening in the impeller are in fluid
communication with each other. Desirably, the outer surface of
the shaft and the inner surface of the opening in the impeller
are threaded, and the shaft is connected to the impeller by
threading the shaft into the opening.
The invention also includes a method for dispersing gas
into molten metal that comprises the steps of providing an
impeller, a shaft, and means for conveying gas as described
previously, immersing the impeller into molten metal contained
within a vessel, rotating the shaft about its longitudinal axis,
2nd pumping gas through the gas discharge outlet while rotating
the shaft so as to discharge gas along the lower face of the
impeller. Large gas bubbles are sheared into finely divided
bubbles by impact with the corners of the impeller. If the
molten metal is contained within a vessel having an inner
diameter D, the impeller is centered within the vessel and the
ratio of A to D should be within the range of 1:6 to 1:8.
Further, for an impeller and vessel having the foregoing
dimensional relationships, the shaft should be rotated within the
range of 200-400 revolutions per minute in order to obtain
optimum mixing action.
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By use of the present invention, the problems
_~sociated with prior devices are overcome. The apparatus
according to the invention is inexpensive, easy to manufacture,
and it has excellent longevity due to its inherently reliable,
strong design. The device cannot be clogged with dross or
foreign objects. It is usable with all types of molten metal
handling and transport systems, and it has an excellent stirring
and gas dispersal action that avoids problems such as cavitation
and the creation of vortices. The gas is dispersed by way of
Finely divided bubbles that ,are uniformly mixed throughout the
molten metal.
The foregoing and other features and advantages of the
invention are illustrated in the accompanying drawings and are
described in more detail in the specification and claims that
follow.
Brief Description of the Drawings
Figure 1 is a cross-sectional view of a vessel
containing molten metal into which gas dispersing apparatus has
been immersed;
Fig ure 2 is an enlarged view of the dispersing
apparatus of Figure 1, with an impeller and a shaft being
illustrated in spaced relationship; and
Figure 3 is a bottom plan view of the impeller of
Figuro 2.
Description of the Preferred Embodiment
Referring to Figures 1-3, a gas in~eetion device
according to the invention is indicated generally by the
reference numeral 10. The device 10 is adapted to be immersed in
molten metal 12 contained within a vessel 14. The vessel 14 is
provided with a removable cover 16 in order to prevent excessive
heat loss from the upper surface of the molten metal 12. The
vessel 14 can be provided in a variety of configurations, such as
cubic or cylindrical. For purposes of the present description,
the vessel 14 will be described as cylindrical, with an inner
diameter indicated by the letter D in Figure 1. For
non-cylindrical applications, the letter D will identify that
dimension defining the average inner diameter of the vessel 14.
The apparatus 10 includes an impeller 20 and a shaft 40.
1'he impeller 20 and the shaft 40 usually will be made of
graphite, particularly if the molten metal being treated is
aluminum. If graphite is used, it preferably should be coated or
otherwise treated to resist oxidation and erosion. Oxidation and
erosion treatments for graphite parts are practiced commercially,
and can be obtained from sources such as Metaullies Systems,
31935 Aurora Road, Solon, Ohio 44139.
As is illustrated in Figure 1, the shaft 40 is an
elongate member that is rigidly connected to the impeller 20 and
Which extends out of the vessel 14 through an opening 22 provided
in the cover 16. The impeller 20 is in the form of a rectangular
prism having an upper face 24, a lower face 26, and side walls
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28, 30, 32. 34. The impeller 20 includes a gas discharge outlet
36 opening through the lower face 26. In the preferred
embodiment, the gas discharge outlet 36 constitutes a portion of
a threaded cpening 38 that extends through the impeller 20 and
which opens through the upper and lower faces 24, 26. The faces
24, 26 are parallel with each other as are the side walls 28, 32
and the side walls 30, 34. The faces 24, 26 and the side walls
28, 30, 32, 34 are planar surfaces which define sharp,
right-angled corners 39.
As shown in Figures 2 andl3, the side walls 30, 34 have
a width identified by the letter A, while the side walls 28, 32
have a depth indicated by the letter B. The height of the
impeller 20, that is, the distance between the upper and lower
faces 24, 26, is indicated by the letter C. Preferably,
dimension A is equal to dimension B, and dimension C is equal to
1/3 dimension A. Deviations from the foregoing dimensions are
possible, but best performance will be attained if dimensions A
and H are equal to each other (the impeller 20 is square in plan
view), and if the corners 39 are sharp and right-angled. Also,
the corners 39 should extend perpendicular to the lower face 26
at least for a short distance above the lower face 26. As
illustrated, corners 39 are perpendicular to the lower face 26
completely to their intersection with the upper face 24. It is
possible, although not desirable, that the upper face 24 could be
larger or smaller than the lower face 26 or that the upper face
24 could be skewed relative to the lower face 26; in either of
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these cases, the corners 3g would not be perpendicular to the
:wer face 26. The best performance is attained when the corners
39 are exactly perpendicular to the lower face 26. It also i s
possible that the impeller 20 could be triangular, pentagonal, or
otherwise polygonal in plan view, but any configuration other
than a rectangular, square prism exhibits reduced bubble-shearing
and bubble-mixing performance.
The dimensions A, B, and C also should be related to
the dimensions of the vessel 14, if possible. In particular, the
impeller 20 has been found t.o perform best when the impeller 20
is centered within the vessel 14 and the ratio of dimensions A
and D is within the range of 1:6 to i:8. Although the impeller
20 will function adequately in a vessel 14 of virtually any size
or shape, the foregoing relationships are preferred.
The shaft 40 includes an elongate, cylindrical center
portion 42 from which threaded upper and lower ends 44, 46
pro~eet. The shaft 40 includes a longitudinally extending bore
48 that opens through the ends of the threaded portions 44, 46.
The shaft 40 can be fabricated from a commercially available flux
tube, or gas in~eetion tube, merely by machining threads at each
end of the tube. A typical flux tube suitable for use with the
present invention has an outer diameter of 2.875 inches, a bore
diameter of 0.75 inch, and a length dependent upon the depth of
the vessel. As is illustrated in the Figures, the lower end 46
is threaded into the opening 38 until a shoulder defined by the
cylindrical portion 42 engages the upper face 24. If desired,
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_the shaft 40 could be rigidly connected to the impeller 20 by
techniques other than a threaded connection, as by being cemented
or pinned. A threaded connection is preferred due to its
strength and ease of manufacture. The use of coarse threads
(4-1/2 inch pitch, UNC) facilitates manufacture and assembly.
The threaded end ~4 is connected to a rotary drive
mechanism (not shown) and the bore 48 is connected to a gas
source (not shown). Upon immersing the impeller 20 in molten
metal and pumping gas th rough the bore 48, the gas will be
discharged through the opening 36 in the form of large bubbles
that flow outwardly along the lower face 26. Upon rotation of
the shaft 40, the impeller 20 will be rotated. Assuming that the
gas has a lower specific gravity than the molten metal, the gas
bubbles will rise as they clear the lower edges of the side walls
28, 30, 32, 34. Eventually, the gas bubbles will be contacted by
the sharp corners 39. The bubbles will be sheared into finely
divided bubbles which will be thrown outwardly and thoroughly
mixed with the molten metal 12 which is being churned within the
vessel 14. In the particular case of the molten metal 12 being
aluminum and the treating gas being nitrogen or argon, the shaft
40 should be rotated within the range of 200-400 revolutions per
minute. Because there are four corners 39, there will be
800-1600 shearing edge revolutions per minute.
By using the apparatus according to the invention, high
volumes of gas in the form of finely divided bubbles can be
pumped through the mol ten metal 12, and the gas so pumped w ill
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have a long residence time. The apparatus 10 can pump gas at
iominal flow rates of 1 to 2 cubic feet per minute (efm), and
flow rates as high as 4 to 5 cfm can be attained without choking.
The apparatus 10 is very effective at dispersing gas and mixing
it with the molten metal 12. The invention is exceedingly
inexpensive and easy to manufacture, while being adaptable to all
types of molten metal s';,orage and transport systems. The '
apparatus 10 does not require accurately machined, intricate
parts, and it thereby has greater resistance to oxidation and
erosion, as well as enhanced mechanical strength. Because the
impeller 20 and the shaft u0 present solid surfaces to the molten
metal 12, there a re no orifices or channels that can be clogged
by dross or foreign ob~eets.
Although the invention has been described in its
preferred form with a certain degree of particularity, it will be
understood that the present disclosure of the preferred
embodiment has been made only by way of example and that various
changes may be resorted to without departing from the true spirit
and scope of the invention as hereinafter claimed. It is
intended that the patent shall cover, by suitable expression in
the appended claims, whatever features of patentable novelty
exist in the invention disclosed.
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