Patent Application
20140008849
1. Field of the Invention
The invention relates to dispersing gas into molten metal. More particularly, the invention relates to a device, such as a rotary degasser, having an impeller that efficiently mixes gas into molten metal and efficiently displaces the molten metal/gas mixture. This application is a continuation of, and claims priority to U.S. patent application Ser. No. 12/853,255, filed Aug. 9, 2010, by Paul V. Cooper which claims priority to U.S. Provisional Application No. 61/232,384, filed Aug. 7, 2009, by Paul V. Cooper.
2. Description of the Related Art
As used herein, the term “molten metal” means any metal in liquid form, such as aluminum, copper, iron, zinc and alloys thereof, which is amenable to gas purification or that otherwise has gas mixed with it. The term “gas” means any gas or combination of gases, including argon, nitrogen, chlorine, fluorine, freon, and helium, that are mixed with molten metal.
In the course of processing molten metals it is sometimes necessary to treat the molten metal 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 and non-metallic inclusions. Chlorine gas is introduced into molten aluminum and molten aluminum alloys to remove alkali metals, such as magnesium. The gases added to the molten metal chemically react with the undesired constituents to convert them to a form (such as a precipitate or dross) that separates or can be separated from the molten metal. In order to improve efficiency the gas should be dispersed (or mixed) throughout the molten metal as thoroughly as possible. The more thorough the mixing the greater the number of gas molecules contacting the undesirable constituents contained in the molten metal. Efficiency is related to, among other things, (1) the size and quantity of the gas bubbles, and (2) how thoroughly the bubbles are mixed with the molten metal throughout the vessel containing the molten metal.
It is known to introduce gases into molten metal by injection through stationary members such as lances or porous diffusers. Such techniques suffer from the drawback that there is often inadequate dispersion of the gas throughout the molten metal. It is also known to inject degassing flux through an opening into the molten metal, which again, results in the flux mixing with only the molten metal near where it is released. In order to improve the dispersion of the gas throughout the molten metal, it is known to stir the molten metal while simultaneously introducing gas, or to convey the molten metal past the source of gas injection. Some devices that stir the molten metal while simultaneously introducing gas are called rotary degassers. Examples of rotary degassers are shown in U.S. Pat. No. 4,898,367 entitled “Dispersing Gas into Molten Metal” and U.S. Pat. No. 5,678,807 entitled “Rotary Degassers,” the disclosures of which are incorporated herein by reference.
Devices that convey molten metal past a gas source while simultaneously injecting gas into the molten metal include pumps having a gas-injection, or gas-release, device. Such a pump generates a molten metal stream through a confined space such as a pump discharge or a metal-transfer conduit connected to the discharge. Gas is then released into the molten metal stream while (1) the stream is in the confined space, or (2) as the stream leaves the confined space.
Many known devices do not efficiently disperse gas into the molten metal bath. Therefore, the impurities in the molten metal are not adequately removed and/or an inordinate amount of gas is used to remove the impurities. This inefficiency is a function of, among other things, (1) an inability to create small gas bubbles to mix with the molten metal, and (2) an inability to displace the gas bubbles and/or the molten metal/gas mixture throughout the vessel containing the molten metal. With conventional devices (other than the previously-described pumps), gas released into the bath tends to rise vertically through the bath to the surface, and the gas has little or no interaction with the molten metal in the vessel relatively distant from the gas-release device. The molten metal/gas mixture is not sufficiently displaced throughout the entire bath. Therefore, to the extent gas is mixed with the molten metal, it is generally mixed only with the molten metal immediately surrounding the device.
In accordance with the invention, an improved impeller for use with a rotary degasser is disclosed. The impeller (also referred to as a rotor) has a connector, a first (or top) portion, a second (or lower) portion, a top surface, a side surface, a bottom surface, a gas-release opening, and a plurality of cavities formed in the side surface of the second portion, and open to the lower surface. The impeller is driven by a drive source that rotates a drive shaft connected to the impeller. The first end of the drive shaft is connected to the drive source, which is typically a pneumatic motor but can be any suitable drive source, and the second end of the drive shaft is connected to the connector of the impeller.
The impeller is designed to displace molten metal, thereby efficiently circulating the molten metal within a vessel while simultaneously mixing the molten meal with gas. The impeller's top portion is preferably rectangular (and most preferably square) in plan view, has four sides, a top surface, a side surface, and a lower surface. The top portion may, however, be of any suitable size and shape to help prevent gas released from the gas release opening from escaping to the surface of the molten metal bath without mixing with the molten metal by the rotation of the second portion of the impeller.
The second portion of the impeller includes a plurality of cavities, wherein the cavities are open to the lower surface of the impeller. Preferably, there are eight cavities, equally, radially spaced about the circumference of the second portion, although any suitable number could be utilized. The connector is preferably located in the first portion and connects the impeller to the second end of the shaft. Most preferably the connector is a threaded bore extending into the impeller. The bore threadingly receives the second end of the shaft. The gas-release opening may be, and is preferably, the opening in the lower surface of the impeller formed by the bore that accepts the second end of the drive shaft. The second end of the shaft preferably terminates at or before the gas-release opening, and gas passing through the shaft can escape through the gas release opening at the bottom of the impeller, where it rises and at least some enters the cavities.
The drive source rotates the shaft and the impeller. A gas source is preferably connected to the first end of the shaft and releases gas into the passage. The gas travels through the passage and is released through one or more gas-release openings in the bottom surface of the impeller. At least part of the gas enters the cavities, where it is mixed with the molten metal as the impeller rotates, and the top portion helps prevent the gas from rising to the surface in order to facilitate better mixing. The molten metal/gas mixture is displaced radially by the impeller as it rotates.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and together with the description, serve to explain principles of the invention.
Device 10, which is preferably a rotary degasser, includes a shaft 100, an impeller 200 and a drive source (not shown). Device 10 preferably also includes a drive shaft 5 and a coupling 20. Shaft 100, impeller 200, and each of the impellers used in the practice of the invention, are preferably made of graphite impregnated with oxidation-resistant solution, although any material capable of being used in a molten metal bath B, such as ceramic, could be used. Oxidation and erosion treatments for graphite parts are practiced commercially, and graphite so treated can be obtained from sources known to those skilled in the art.
The drive source can be any apparatus capable of rotating shaft 100 and impeller 200 and is preferably a pneumatic motor or electric motor, the respective structures of which are known to those skilled in the art. The drive source can be connected to shaft 100 by any suitable means, but is preferably connected by drive shaft 5 and coupling 20. Drive shaft 5 is preferably comprised of steel, has an inner passage 6 for the transfer of gas, and preferably extends from the drive source to which it is connected by means of a rotary union 7. Drive shaft 5 is coupled to impeller shaft 100 by coupling 20. The preferred coupling 20 for use in the invention is described in U.S. Pat. No. 5,678,807, the disclosure of which is incorporated herein by reference.
As is illustrated in
First end 102 is connected to the drive source, preferably by shaft 5 and coupling 20, as previously mentioned. In this regard, first end 102 is preferably connected to coupling 20, which in turn is connected to motor drive shaft 5. Shaft 5 is connected to rotary union 7. A typical rotary union 7 is a rotary union of the type described in U.S. Pat. No. 6,123,523 to Cooper, the disclosure of which is incorporated herein by reference. Side 106 is preferably cylindrical and may be threaded, tapered, or both, at end 102. In the embodiment shown, end 102 (which is received in coupling 20) is smooth and is not tapered. Side 106 is preferably threaded at end 104 for connecting to impeller 200. Passage 108 is connected to a gas source (not shown), preferably by connecting the gas source to nozzle 9 of rotary union 7, and transferring gas through a passage in rotary union 7, through inner passage 6 in shaft 5 and into passage 108.
Turning now to
By operating impeller 200 at a lower speed, less stress is transmitted to the moving components, which leads to longer component life, less maintenance and less maintenance downtime. Another advantage that may be realized by operating the impeller at slower speeds is the elimination of a vortex. Some conventional devices must be operated at high speeds to achieve a desired efficiency. This can create a vortex that draws air into the molten metal from the surface of bath B. The air can become trapped in the molten metal and lead to metal ingots and finished parts that have air pockets, which is undesirable.
Referring to
In one embodiment, protrusions 224 are preferably equally spaced (e.g., preferably at 45 degree angles) around the center of the impeller 200. However, one or more of the protrusions 224 could be formed at varied angle increments from each other. In one embodiment, the center of the outward face of the protrusion 224 is approximately 22.5 degrees from a line formed from the extension of corner 218 to the center of the gas-release opening 223. Each protrusion 224 preferably has identical dimensions and configuration. The protrusions 224 need not, however, be identical in configuration or dimension, as long as a portion of the gas released through the gas-release opening 223 is capable of entering the spaces (or cavities) between protrusions 224, so it is mixed with the molten metal entering the space. Further, an impeller according to the invention could function with fewer than, or more than, eight protrusions 224 and fewer than, or more than, eight cavities. Additionally, the length of each protrusion 224 may be greater or smaller than shown.
An impeller 200 may have one or more protrusions 224 formed in top portion 202 of impeller 200, and the lower surface 220 of the impeller 200 may or may not also include one or more protrusions 224. Impeller 200 can be used conjunction with a device that directed molten metal downward towards the spaces (or cavities) between the protrusions 224 in top portion 202. Such a device could be an additional vane on impeller 200 above top portion 202, wherein the additional vane directs molten metal downward towards the one or more spaces (or cavities) between the protrusions 224. The spaces (or cavities) between the protrusions 224 in top portion 202 may have the same shape, number and relative locations with respect to the spaces (or cavities) between the protrusions 224 in lower surface 220.
Second end 104 of shaft 100 is preferably connected to impeller 200 by threading end 104 into connector 222. If desired, shaft 100 could be connected to impeller 200 by techniques other than a threaded connection, such 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 pitch, UNC) facilitates manufacture and assembly. The threads may be tapered (not shown).
As with the described embodiments of impellers 200 and 400, top portion 202 of impeller 500 is preferably rectangular and most preferably square in plan view, with four corners 212, 214, 216 and 218, and sides 204, 206, 208, and 210, being preferably equal in length. It also is possible that top portion 202 could be triangular, circular, pentagonal, or otherwise polygonal in plan view. Though top portion 202 may be any suitable dimension, top portion 202 extends from the center of the gas-release opening 223 beyond the length of the protrusion 224 from the center of the gas-release opening 223.
Any of the impellers described herein may be used with components or devices formed or placed above and/or below the impeller. Such device or devices could either direct molten metal upward from the bottom of the bath or downward from the top of the bath. Such device(s) may be attached to the shaft and/or attached to the impeller. For example, any of the impellers described herein may have an additional vane or projection beneath the lower surface to direct molten metal upward, or an additional vane or projection above the upper surface to direct molten metal downward. Unless specifically disclaimed, all such embodiments are intended to be covered by the claims.
Upon placing impeller 200 in molten metal bath B and releasing gas through passage 108, the gas will be released through gas-release opening 223 and flow outwardly along lower surface 220. Gas-release opening 223 is preferably located in the center of the bottom surface 220 of the impeller 200. Alternatively, there may one or more gas-release openings 223 in each of spaces (or cavities) between the protrusions 224, at location 232, in which case opening 223 would be preferably sealed. Further, end 104 could extend beyond lower surface 220 in which case the opening in end 104 would be the gas-release opening.
As shaft 100 and impeller 200 rotate, the gas bubbles rise and at least some of the gas enters spaces (or cavities) between the protrusions 224. The released bubbles are sheared into smaller bubbles as they move past a respective edge 240 of lower surface 220 before they enter the space (or cavity) between the protrusions 224. As impeller 200 turns, the gas in each of spaces (or cavities) between the protrusions 224 mixes with the molten metal entering the spaces between the protrusions 224. This mixture is pushed outward from impeller 200 at least partially by the top portion 202. The molten metal/gas mixture is thus efficiently displaced within vessel 1. When the molten metal is aluminum and the treating gas is nitrogen or argon, shaft 100 and impeller 200 preferably rotate within the range of 200-400 revolutions per minute.
The present invention allows high volumes of gas to be thoroughly mixed with molten metal at relatively low impeller speeds. Unlike some conventional devices that do not have spaces (or cavities) between the protrusions 224, the gas cannot simply rise past the side of the impeller. Thus, impeller 200 can operate at slower speeds than conventional impellers, yet provide the same or better results. Some impellers operate at high speeds in an effort to mix the gas quickly before it rises past the side of the impeller. Device 10 can pump a gas/molten metal mixture at nominal displacement rates of 1 to 2 cubic feet per minute (cfm), and flow rates as high as 4 to 5 cfm can be attained.
Having thus described different embodiments of the invention, other variations and embodiments that do not depart from the spirit of the invention will become apparent to those skilled in the art. The scope of the present invention is thus not limited to any particular embodiment, but is instead set forth in the appended claims and the legal equivalents thereof. Unless expressly stated in the written description or claims, the steps of any method recited in the claims may be performed in any order capable of yielding the desired product.
| Number | Date | Country | |
|---|---|---|---|
| 61232384 | Aug 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12853255 | Aug 2010 | US |
| Child | 14027237 | US |
