Method of gas fluxing molten aluminium with impellers located one above the other and mounted on a common shaft

Abstract: A method of gas fluxing molten aluminum with at least two, relatively small diameter upper and lower rotatable dispersers located in the molten aluminum and mounted on a shaft. Fluxing gas is added to the molten aluminum beneath each of the rotatable dispersers at a substantial rate of gas flow while rotating the dispersers at a substantial rpm in the molten aluminum. The dispersers directly shear gas bubbles that form in the molten aluminum as the fluxing gas is directed into the molten aluminum beneath each of the dispersers. The direct shearing of the gas bubbles maintains a high surface area between the bubbles and molten aluminum to effect efficient removal of impurities in the molten aluminum.

Description: The present invention relates generally to fluxing practices that remove impurities from molten aluminum, and particularly to the use of at least two mechanical stirrers and the addition of fluxing gas introduced into the molten aluminum beneath each of the mechanical stirrers. U.S. Patent 5,342,429 to Ho Yu et al, which issued August 30, 1994, discusses the problems with impurities in molten aluminum, such impurities including oxide particles, dissolved gas and chemical impurities such as calcium, sodium, magnesium and lithium. Mr. Yu is one of the inventors of the present disclosure and application. U.S. Patent 5,364,450 to Eckert et al discloses a method for dispersing a treatment media in molten aluminum using two rotating impellers mounted on a common shaft. The treatment media, which may be nitrogen containing gases, inert gases such as argon, or chlorinaceous gases, is introduced below one or more impellers to be dispersed in the molten aluminum. Standard processes for fluxing molten aluminum generally employ fluxing gas rates of 0.0004 X 10 -4 to 0.004 X 10 -4 standard cubic meters per second ((0.005 to 0.05 SCFH) (standard cubic feet per hour)) per 0.45 kg (pound) of metal using a single impeller having a 30 cm (twelve-inch) diameter, such as shown in U.S. Patent 3,839,019 to Bruno et al. The rate of rotation of the impeller is at a relatively low rpm, i.e., about 200 rpm. In the case of the above Yu et al patent, purging gas is introduced into a body of molten aluminum on the order of 0.0004 X 10 -4 standard cubic meters per second (0.005 SCFH) per 0.45 kg (pound) of aluminum beneath the lowermost of two rotors mounted on a single shaft. The invention is directed to downsizing a vessel or box containing a body of molten aluminum, and increasing substantially the efficiency of the process of removing impurities from molten aluminum. This is accomplished by using multiple disperser rotors and multiple feeds of fluxing gas into the molten aluminum beneath each of the rotors. For example, the invention uses six-inch diameter rotors (mounted on a hollow shaft) in place of the standard twelve-inch diameter rotors. The rotors are rotated in the range of 400 to 900 rpm, depending upon the size of the fluxing system and the impurities to be removed. A fluxing gas rate of 13.8 X 10 -4 to 19.7 X 10 -4 standard cubic meters per second (170 to 250 SCFH) is employed, with a typical gas flow being on the order of 0.034 X 10 -4 standard cubic meters per second (0.43 SCFH) of gas per 0.45 kg (pound) of metal. Such a gas loading is 50% greater than the processes of the prior art. The "50%" here is in comparison to the disclosure of the above Patent 5,342,429 (6.29 x 10 -4 to 15.7 X 10 -4 standard cubic meters per second (80 to 200 SCFH)) and is about eight times that of dispersed gas loading per 0.45 kg (pound) of metal of the prior art, i.e., eight times the above 0.004 X 10 -4 standard cubic meters per second (0.05 SCFH) per 0.45 kg (pound) of metal. In accordance with the invention, there is provided a method of gas fluxing molten aluminum, comprising adding fluxing gas to said molten aluminum at locations directly beneath each disperser of a plurality of dispersers located one above the other in a generally vertical direction in said molten aluminum, said fluxing gas comprising a reactive or halogenous and/or a nonreactive gas selected from the group consisting of argon gas, nitrogen gas, or mixtures thereof, said fluxing gas being added to said molten aluminum at a rate of greater than 0.004 x 10 -4 and typically 0.034 X 10 -4 standard cubic meters per second (0.05 and 0.43 SCFH) of gas per 0.45 kg (pound) of metal, said fluxing gas when entering the molten aluminum beneath each disperser providing an initial interfacial area between the gas and the molten aluminum, and rotating the plurality of dispersers at a high rate of speed, directly shearing bubbles of the gas that form in the molten aluminum beneath each of the dispersers to create an interfacial area between the fluxing gas and molten aluminum, using said interfacial area to remove impurities from the molten aluminum. Also in accordance with the invention there is provided a method of gas fluxing molten aluminum, said method comprising providing a body of molten aluminum, locating a gas dispersing unit in the body of molten aluminum, said unit having at least two impellers mounted on a common shaft extending into said body of molten aluminum, rotating said unit at a high rate of speed, simultaneously with said rotation, adding a fluxing gas to the body of molten aluminum at a rate of greater than 0.004 X 10 -4 and typically 0.034 X 10 -4 standard cubic meters per second (0.05 and 0.43 SCFH) of gas per 0.45 kg (pound) of molten aluminum, said fluxing gas being added directly beneath each impeller, said fluxing gas comprising a reactive or halogenous and/or a nonreactive gas selected from the group of argon, nitrogen or mixtures thereof, using said impellers to directly shear gas bubbles that form in the molten aluminum beneath each impeller when the fluxing gas is added to provide finely divided bubbles in the molten aluminum without substantial splashing of the molten aluminum, and redispersing coalesced fluxing gas bubbles with said impellers as the fluxing gas rises toward the surface of the body of molten aluminum. The invention, along with its advantages and objectives, will be better understood from consideration of the following detailed description and the accompanying drawings in which: Figure 1 is a diagrammatic representation of a three-rotor fluxing system for removing impurities from a body of molten metal, and Figure 2 is a chart that compares single rotor and multiple rotor systems in regard to calcium removal rate from a body of molten aluminum. Referring now to the drawings, Figure 1 thereof shows schematically a process box and vessel 10 containing molten aluminum 12. The vessel comprises a system for purifying the aluminum, which enters the vessel through a conduit or pipe 14 and exits the vessel via an outlet 16. Before exiting the vessel, the molten metal travels beneath a baffle 18 to reduce the amount of oxide, salt particles and fluxing gas entering the exit stream. Gas bubbles generally rise and substantially leave the metal bath before exiting the box. Extending vertically into vessel 10 is a shaft 20 suitably connected to a motor 22 for rotating the shaft and a plurality (three in Figure 1) of impellers 24 mounted and vertically displaced on the shaft. Preferably the shaft is hollow for conducting a fluxing gas, such as chlorine and/or a nonreactive gas selected from the group consisting of argon and nitrogen or mixtures thereof, into the vessel and thus into the molten aluminum. The gas can enter shaft 20 above motor 22 from a source of the gas (not shown) or enter a coupling 25 that permits stationary input to the shaft while the shaft itself rotates. Openings 26 are provided in shaft 20 immediately beneath the upper two impellers in Figure 1 for directing the fluxing gases from the hollow shaft and into the molten aluminum. Fluxing gas is directed from the lower end of the shaft and thus beneath the lowermost impeller, which lower end is open. Gas bubbles 28 form beneath the impellers and rise toward the upper surface of the molten metal, as seen in Figure 1. The flow of gas through openings 26 and the lower end of shaft 20 is self-regulating. The back pressure of the molten metal is the highest in the lowermost regions of the molten metal such that gas enters the molten metal more readily from the uppermost opening(s) in the shaft. The next capability of gas admission to the molten metal is the next intermediate opening(s) in the shaft. The amount of gas leaving the lower end of the shaft will be somewhat less than that of the intermediate opening(s) assuming the amount of gas entering the shaft from the gas source is sufficient to supply all exits of the shaft. Shaft openings 26 and the lower open end of shaft 20 allow a substantial flow of gas into the molten metal such that the efficiency of the fluxing system of the invention is substantially improved over the disclosure of above U.S. Patent 5,342,429. This will be discussed below in terms of the data presented in Figure 2 of the drawings. This efficiency has permitted downsizing of the box 10 (containing the molten metal) including reducing in half the diameters of the impeller, such that 15 cm (six-inch) diameter impellers (24) can be used and can be rotated by motor 22 at a substantial rpm, up to 900 rpm, for example. In addition, since gas bubbles 28 form in the molten metal beneath each rotating impeller and rise past the edges of the rotating impellers, the impellers directly shear the gas bubbles. The shearing of the bubbles reduces their tendency to coalesce, as they rise, such that the number of small size bubbles remains large to provide large surface areas for contacting impurities in the molten metal, such as dissolved hydrogen, inclusions and elements such as calcium, sodium, magnesium and lithium. The contact with impurities strips the molten metal of the impurities, i.e., dissolved gases combine with the fluxing gases and rise to the surface of the molten metal and escape from the vessel with the fluxing gases. The vessel has a lid (not shown) equipped with an exhaust to allow the gas to leave. The gases, in addition, strip unwanted elements and particulates from the molten metal by reacting with reactive gas, e.g. chlorine, to form salt, which are then removed from the vessel as skim on the surface of the bath or as a vapor which escapes through the exhaust. The fluxing gas enters the molten metal at a high rate, i.e., on the order of 19.7 X 10 -4 standard cubic meters per second (250 SCFH) for the three impeller disperser system of Figure 1, such that the gas loading provided by the present invention is about fifty percent greater than the prior practices of about 13.8 X 10 -4 (170 SCFH). A typical flow rate per 0.45 kg (pound) of molten metal for the gas is 0.034 X 10 -4 standard cubic meters per second (0.43 SCFH), which is eight times the 0.004 X 10 -4 standard cubic meters per second (0.05 SCFH) of current practices. Such a rate, in combination with 15 cm (six-inch) diameter impellers 24 rotating at the rpm's of the Figure 2 chart provided the high removal rates of calcium from a body of molten aluminum, in comparison to the single, 30 cm (twelve-inch) diameter impeller of the prior art. The removal rate of calcium in Figure 2 is expressed in terms of percent of calcium per hour (hr) per pound (lb) of metal. As shown, the removal rates effected by the double and triple high speed, small diameter impellers or dispersers far exceeded the capabilities of the single (both 15 cm and 30 cm (six- and twelve-inch) diameter) impellers or dispersers tested. Certain operating parameters of the fluxing process were employed to correlate data presented in Figure 2. These are listed as follows: rotor rpm impeller or disperser diameter mass of the metal in box 10 gas flow rate into the box, and upper surface area 30 of the metal bath. Because dispersers 24 have a relatively small diameter, the high speed of rotation of the rotors does not generate substantial turbulence in the body of molten metal 12 such that undue splashing of the metal in box 10 does not occur. This reduces the tendency of the metal to acquire oxygen and water vapor from the atmosphere within the box and the resulting formation of aluminum oxide and hydrogen gas impurities.

Claims: A method of gas fluxing molten aluminum, comprising: adding fluxing gas to said molten aluminum at locations directly beneath each disperser of a plurality of dispersers located one above the other in a generally vertical direction in said molten aluminum, said fluxing gas comprising a reactive or halogenous and/or a nonreactive gas selected from the group consisting of argon gas, nitrogen gas, or mixtures thereof, said fluxing gas being added to said molten aluminum at a rate of greater than 0.004 X 10 -4 and typically 0.034 X 10 -4 standard cubic meters per second (0.05 and 0.43 SCFH) of gas per 0.45 kg (pound) of metal, said fluxing gas when entering the molten aluminum beneath each disperser providing an initial interfacial area between the gas and the molten aluminum, and rotating the plurality of dispersers at a high rate of speed, directly shearing bubbles of the gas that form in the molten aluminum beneath each of the dispersers to create an interfacial area between the fluxing gas and molten aluminum, using said interfacial area to remove impurities from the molten aluminum. The method of claim 1, in which the rate of gas flow into the molten aluminum lies in the range of 13.8 X 10 -4 to 19.7 X 10 -4 standard cubic meters per second (170 to 250 SCFH). The method of claim 1 or 2, in which the dispersers are rotated in the range of 400 to 900 rpm. A method of gas fluxing molten aluminum, said method comprising: providing a body of molten aluminum, locating a gas dispersing unit in the body of molten aluminum, said unit having at least two impellers mounted on a common shaft extending into said body of molten aluminum, rotating said unit at a high rate of speed, simultaneously with said rotation, adding a fluxing gas to the body of molten aluminum at a rate of greater than 0.004 X 10 -4 and typically 0.034 X 10 -4 standard cubic meters per second (0.05 and 0.43 SCFH) of gas per 0.45 kg (pound) of molten aluminum, said fluxing gas being added directly beneath each impeller, said fluxing gas comprising a reactive or halogenous and/or a nonreactive gas selected from the group of argon, nitrogen or mixtures thereof, using said impellers to directly shear gas bubbles that form in the molten aluminum beneath each impeller when the fluxing gas is added to provide finely divided bubbles in the molten aluminum without substantial splashing of the molten aluminum, and redispersing coalesced fluxing gas bubbles with said impellers as the fluxing gas rises toward the surface of the body of molten aluminum. The method of any one of the preceding claims, wherein the molten aluminum comprises calcium impurities and is contained in a container, the method being characterized in that the diameters of the dispersers, the mass of the molten aluminum in the container, the rate at which said fluxing gas is added to the molten aluminum in the container and an upper surface area of the molten aluminum in the container are such that the calcium impurities are removed from the molten aluminum at a rate above 0.10%/hr/0.45 kg aluminum.

Inventors: SCHERBAK MICHAEL, YU HO, SCHERBAK, MICHAEL, YU, HO, Scherbak, Michael, Alcoa Technical Center, Yu, Ho, Alcoa Technical Center
Applicants: ALUMINUM CO OF AMERICA, ALUMINUM COMPANY OF AMERICA
Application year: 1995
Main classification: C22B21
IPC codes: C22B21