Abstract:
A device for dispersing gas into molten metal includes an impeller, a drive shaft having a gas-transfer passage therein, and a first end and a second end, and a drive source. The second end of the drive shaft is connected to the impeller and the first end is connected to the drive source. The impeller includes a first portion and a second portion with a plurality of cavities. The first portion covers the second portion to help prevent gas from escaping to the surface without entering the cavities and being mixed with molten metal as the impeller rotates. When gas is transferred through the gas-transfer passage, it exits through the gas-release opening(s) in the bottom of the impeller. At least some of the gas enters the cavities where it is mixed with the molten metal being displaced by the impeller. Also disclosed are impellers that can be used to practice the invention.

Description:
This application is a continuation of, and claims priority to U.S. patent application Ser. No. 12/853,255 (Now U.S. Pat. No. 8,535,603), 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. 
    
    
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     SUMMARY OF THE INVENTION 
     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&#39;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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a side view of a gas-release device according to the invention positioned in a vessel containing a molten metal bath. 
         FIG. 2  is a partial perspective view of the device of  FIG. 1  showing the degasser shaft and impeller. 
         FIG. 3A  is a perspective view of the underside of the impeller shown in  FIGS. 1 and 2 . 
         FIG. 3B  is a top view of the impeller shown in  FIGS. 1, 2, and 3A . 
         FIG. 3C  is a side view of the impeller shown in  FIGS. 1, 2, 3A, and 3B . 
         FIG. 4A  is a top view of another impeller according to an embodiment of the invention. 
         FIG. 4B  is a side view of the impeller shown in  FIG. 4A . 
         FIG. 5A  is a top view of another impeller according to an embodiment of the invention. 
         FIG. 5B  is a side view of the impeller shown in  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows an exemplary gas-release device  10  according to the invention. Device  10  is adapted to operate in a molten metal bath B contained within a vessel  1 . Vessel  1  is provided with a lower wall  2  and side wall  3 . Vessel  1  can be provided in a variety of configurations, such as rectangular or cylindrical. In this exemplary embodiment, vessel  1  includes a cylindrical side wall  3  and has an inner diameter D. 
     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  FIGS. 1 and 2 , shaft  100  has a first end  102 , a second end  104 , a side  106  and an inner passage  108  for transferring gas. Shaft  100  may be a unitary structure or may be a plurality of pieces connected together. The purpose of shaft  100  is to connect to an impeller to (1) rotate the impeller, and (2) transfer gas. Any structure capable of performing these functions can be used. 
     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  FIG. 3A , an impeller  200  according to one embodiment of the invention is shown. Impeller  200  is designed to displace a relatively large quantity of molten metal in order to improve the efficiency of mixing the gas and molten metal within bath B. Therefore, impeller  200  can, at a slower speed (i.e., lower revolutions per minute (rpm)), mix the same amount of gas with molten metal as conventional devices operating at higher speeds. Impeller  200  can also operate at a higher speed, thereby mixing more gas and molten metal than conventional devices operating at the same speed. 
     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. 
       FIG. 3A  depicts the underside of impeller  200 . Impeller  200  has a top surface  201  of top portion  202 , a side surface  203 , and a lower surface  220 . Top portion  202  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. Top portion  202  could also be triangular, circular, pentagonal, or otherwise polygonal in plan view. Though it 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 . Top portion  202  assists in the capture of gas, mixing of gas and molten metal, and dispersal of mixed molten metal. 
     Referring to  FIG. 2 , connector  222  is formed in top portion  202 . Connector  222  is preferably a threaded bore that extends from top portion  202  to lower surface  220  and terminates in gas-release opening  223 . Top portion  202  may comprise any other suitable structure for connecting the top portion  202  and the shaft  100 . 
     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 . 
       FIGS. 3B and 3C  depict top and side views, respectively, of the impeller  200 . The spaces (or cavities) between the protrusions  224  formed in the side surface  203  are open to lower surface  220 . Protrusion  224  has two radiused sides  226  and  228 . Though it may be any suitable shape, a convex radiused center  233  connects sides  226  and  228 . This convex shape assists in the smooth rotation of the lower portion of impeller  200  through the molten metal. Additionally, though it may be any suitable shape, a concave radiused center  232  in each cavity connects sides  226 ,  228  of adjoining protrusions  224 . This preferred, concave shape (or cavity) assists in the capture of gas exiting the gas-release opening  223 . The space (or cavity) between the protrusions  224  is partially formed between adjoining sides  226 ,  228 , connected by the concave radiused center  232  and underneath a top wall  230  (bottom surface of top portion  202 ). A lip  234  is formed between top wall  230  and the top surface  201  of top portion  202 . Lip  234  may have an approximate width of 1 inch. Lower surface  220  has edges  240  between each of the spaces (or cavities) between the protrusions  224 . 
     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). 
       FIGS. 4A and 4B  depict top and side views, respectively, of another embodiment of the present invention. In this embodiment, an upper impeller portion  403  of impeller  400  is located between an lower impeller portion  203  and top portion  202 . This lower impeller portion  203  is coupled to, and may be offset from, the upper impeller portion  403 . Additional impeller portions may be added and oriented as desired to further direct, mix, and distribute gas and molten metal. Lower impeller portion  203  and upper impeller portion  403  may be integral to each other, the top portion  202  and/or the device or they may be separate components. 
       FIGS. 5A and 5B  depict top and side views, respectively, of another embodiment of the present invention. In this embodiment, impeller  500  has a lower surface  220  with edges  240  adjacent to the gas-release opening  223 . This orientation allows for efficient transfer of gas into the spaces (or cavities) between the protrusions  224 . The cavities and protrusions  224  of impeller  500  are oriented to direct the flow of gas from the gas-release opening  223  into the cavities  223 . In the embodiment depicted in  FIGS. 5A and 5B , the protrusions  224  are sloped. The protrusions  224  can have any suitable slope to aid in the dispersal and mixing of gas with molten metal, including vertical (i.e., perpendicular with the top surface  201 ). In an embodiment with vertically sloped protrusions  224 , the space (or cavity) between the protrusions  224  may comprise channels along surface  230  for the gas to travel within. These channels may extend from the lip of the gas-release opening  223  to the end of the protrusion  224 . Impeller  500  may have fewer or more than eight protrusions  224  and more or fewer than eight cavities for directing the flow of gas. 
     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.