Abstract:
A molten metal handling device comprising an outer shell defined by a bottom and two side walls, an insulating layer partially filling the outer shell and a thermally conductive castable refractory body for carrying molten metal, the refractory body being within the insulating layer. The device further includes at least one heating element positioned in the insulating layer, adjacent to the refractory body. The refractory body is preferably fabricated from a castable alumina or castable silicon carbide material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to handling devices used in the transfer of molten metals such as aluminum and more specifically to heated such devices designed to maintain the temperature of the molten metal during handling. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 6,973,955 B2 issued Dec. 13, 2005 describes a heated trough for the transfer of molten metal. This patent broadly describes a trough comprising an outer shell defined by a bottom wall and two side walls, an insulating layer filling the outer shell and a conductive U-shaped refractory trough body for carrying molten metal, the trough body being embedded in the insulating layer. The device of this patent further includes at least one heating element positioned in the insulating layer, adjacent to but spaced apart from the trough body, to provide an air gap between the heating element and the trough body. The trough is described as being fabricated from a material that is generally highly conductive and resistant to corrosion from molten metal. The only examples of suitable such materials provided in this patent are dense refractories such as silicon carbide and graphite. No definition of the terminology “highly conductive” is presented in this patent. Presumably this term as used in this patent is meant to refer to thermal conductivity since the structure described is utilized to impart heat to molten metal being transferred in the trough. From the literature, it is known that silicon carbide has a thermal conductivity on the order of about 100-300 W/m-K°, and graphite is reported to have a thermal conductivity on the order of above 100 W/m-K° (see www.matweb.com that provides materials properties data of this type). Thus, to the skilled artisan, these materials would be recognized as “highly thermal conductive”. 
     While the foregoing device provides a useful tool for the maintenance of molten metals temperature during transfer in a trough, it possesses several inherent commercial and technical weaknesses. Most notably, silicon carbide is a very expensive material that, while providing excellent thermal conductivity as noted hereinabove, and very high resistance to erosion and attack by, for example, molten aluminum, it is very difficult to form into shapes such as troughs. As a general observation, a trough fabricated from such a material would be very expensive to produce, and would in all probability not be commercially viable. As to a graphite trough, again while the thermal conductivity of such a device would be very desirable form the point of view of heat transfer, graphite is generally considered much too prone to oxidation and is furthermore generally quite friable (not particularly wear resistant), thus not providing a trough material of suitable commercial value. 
     Accordingly there exists a continuing need for a device such as that described in U.S. Pat. No. 6,973,955 B2 that is equally useful, but significantly less expensive to produce while providing heat transfer and wear characteristics adequate for use in most metal transfer applications. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the present invention to provide a commercially viable heated molten metal handling device that provides both adequate wear properties and heat transfer characteristics at a commercially acceptable cost. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a molten metal handling device comprising an outer shell defined by a bottom and two side walls, an insulating layer partially filling the outer shell and a conductive refractory body comprising a trough or metal containing body for carrying molten metal, the refractory body being within the boundaries of the insulating layer. The device further includes at least one heating element positioned within the insulating layer, in contact with the refractory body. The refractory body is fabricated from a castable alumina or silicon carbide refractory as described hereinafter. Quite unexpectedly, the use of castable refractories as substitutes for the much higher thermal conductivity more conventional silicon carbide and graphite materials described in the prior art in these applications has proven highly successful. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a heated trough in accordance with one preferred embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of an alternative embodiment of the heated trough of the present invention incorporating a metallic fence external to the castable trough lining. 
         FIG. 3  is a cross-sectional view of another alternative embodiment of the heated trough of the present invention incorporating a metallic fence internal to the castable trough lining. 
         FIG. 4  is yet a perspective view of another alternative embodiment of the heated trough of the present invention wherein the external surface of the castable portion of the trough liner that contacts the heaters has is rippled or defines a wavy pattern. 
         FIG. 5  is a top plan view of the trough liner of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1  which shows a cross-sectional view of a heated trough that represents one preferred embodiment of the present invention, trough  10  comprises an outer shell  12  having a bottom  14  and side walls  16 , an insulating layer  18 , a calcined castable refractory trough body  20  within the insulating layer and a heating element(s)  22  positioned between outer shell  12  and calcined castable refractory trough body  20  and in contact with thermally conductive refractory trough body  20 . One or more heating elements  22  may be used and these may be positioned as shown in  FIG. 1 , i.e. between sides  16  and refractory trough body  20 . Element  24  in the area of bottom  14  may comprise suitable thermal insulation or, alternatively, an additional heater  22  for imparting heat to calcined castable refractory trough body  20 . According to the preferred embodiment shown in  FIG. 1 , heaters  22  are maintained in position below and along side refractory trough  20  through the use of insulating material supports  18  that comprise an insulating material such as Wollite. Heaters  22  drive heat into refractory trough  20  as shown by the arrows in  FIG. 1 . 
     As used herein, the term “castable refractory” is readily understood by those skilled in the refractory shape forming arts and is meant to refer to a composition that can be shaped or molded in the “green state” and then subsequently fired or calcined at a suitable elevated temperature to produce a hard, tough ceramic-like structure having the shape of the shaped or molded “green state” product. 
     According to the present invention, preferred, but not limiting, castable refractories include: low moisture alumina based refractories such as ArmorKast 65AL commercially available from ANH Refractories Co., Cherrington Corporate Center, 400 Fairway Drive, Moon Township, Pa. 15108 and Pyrocast SC-2600 a high purity silicon carbide based refractory commercially available from Pyrotek, Inc., E. 9503 Montgomery Ave., Spokane, Wash. 99206. The preferred alumina based refractory comprises from about 60 to about 65 weight percent alumina and from about 25 to about 35 weight percent silica in the calcined state. The preferred silicon carbide based refractory comprises from about 80 to about 85 weight percent silicon carbide and from about 10 to about 15 weight percent alumina in the calcined state. While these specific materials are preferred in the successful practice of the present invention, it will be readily understood by the skilled artisan and that many other similar castable refractories demonstrating thermal conductivities similar to these materials may be substituted therefor. 
     The thermal conductivities of the preferred castable refractory materials in the calcined state are: for the alumina based refractory from about 1.5 to about 1.9 W/m-K°; and for the silicon carbide based refractory from about 9 to about 11 W/m-K°. These thermal conductivities while significantly below those of silicon carbide and graphite as discussed above (and certainly not considered “highly conductive” in the thermal arts) have surprisingly proven highly useful and successful in application to the structure described above while being much more cost effective due to the availability of common commercial production processes and the relatively lower cost of the starting materials when compared to, for example, silicon carbide. Additionally they are significantly “tougher’ and more resistant to erosion, abuse and attack by molten metal such as aluminum than graphite. 
     Referring now to  FIGS. 2 and 3  that depict alternative preferred embodiments of the present invention that incorporate a fence  30  either about the periphery of refractory trough  20  ( FIG. 2 ) or cast internally to refractory trough  20  ( FIG. 3 ). The purpose of fence  30  is to provide protection for heater(s) in the case of a perforation of refractory trough  20  through cracking or otherwise. The presence of fence  30  prevents the infiltration of molten metal contained in refractory trough  20  from contacting and destroying heater(s)  22 / 24 . 
     Fence  30  may is typically between about 1 and 5 mm in thickness, preferably from about 1.5 and about 2.5 mm in thickness and may comprise any number of materials including, but not limited to, a non-oxidizing metallic sheet, a composite ceramic or a glass fabric whose pores have been sealed with a ceramic slurry. 
     Depicted in  FIGS. 4 and 5  is yet another additional alternative preferred embodiment of the heated trough of the present invention. According to this embodiment, external surfaces  32  of refractory trough  20  are cast in a waved or rippled pattern comprising waves  34 . Such a pattern maximizes the surface area exposed to heater(s)  22  thus increasing the effectiveness of heat transfer from heaters  22  to refractory trough  20  all while not weakening sides  36  of refractory trough  20  and also providing a stable surface for heater panels  22  to bear against without being damaged. It is important to note that flat surfaces  38  are provided between waves  34  and flat surfaces  40  are provided to supply support for heater panels  22  without inducing compressive damage. 
     A further preferred embodiment of the heated trough  10  of the present invention includes a coating  42  (see  FIG. 1 ) of a high emissivity material on outer surface  44  of refractory trough  20 . Such a coating can be sprayed or brushed on to surface  44  at a thickness of typically less than about 1 mm and may comprise materials having the ability to absorb and radiate thermal energy. Typical such materials are zirconium and chromium. While as depicted in the accompanying Figures, such a coating is shown specifically in connection with the embodiment of the invention depicted in  FIG. 1 , it will be readily understood that such a coating could be equally well applied in the various alternative preferred embodiments depicted in  FIGS. 2-5 . 
     While the invention has been described largely in connection with its use as a metal transfer trough, it will be readily understood by the skilled artisan that the principles of design and the physical configuration of the device is readily applicable to other molten metal handling devices such as holders, crucibles, and filters. 
     As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.