1. Field of the Invention
The invention relates to the filtration of molten metal and, more particularly, to a relatively inexpensive molten metal filter that provides high flow rates while attaining excellent filtration capabilities.
2. Description of the Prior Art
In the course of casting molten metal, the metal first is reduced to a molten state at an elevated temperature by means of a furnace. After being liquified in the furnace, the molten metal is directed into a mold for casting, or it is directed into a secondary receiver such as a holding furnace from which it later is directed into a mold for casting.
One of the major detriments that an occur during the casting process is for impurities in the form of solids to be included in the molten metal as it is poured into the casting molds. These impurities can originate from metal being melted itself, the furnace in which the metal is melted, transport elements through which the metal is conveyed, or external sources such as oxidation from the atmosphere. The inclusion of such impurities in the metal being cast can result in castings of poor quality that will require that the resultant castings be scrapped. In addition to problems associated with solid impurities, it sometimes happens that the molten metal contains dissolved gases, such as hydrogen, oxygen, or nitrogen. During solidification of the molten metal in the casting molds, the gases may precipitate out of solution and form gas bubbles or pockets that also may negatively affect the quality of the finished metal casting.
In view of the problems that can arise from the presence of solid or gaseous impurities in molten metal being cast, it is desirable to remove those impurities before solidification commences. A variety of filters have beer proposed for that purpose. In general, these filters employ a porous, refractory filter medium that will withstand the temperatures of the molten metal being filtered while, at the same time, trapping impurities that are being passed through the filter element. Such filters have been provided in the form of horizontally oriented plates, vertically oriented plates, and disks that are disposed within conduits through which the molten metal is flowed.
A particularly effective molten metal filter is the MULTICAST filter system that is commercially available from the Metaullics Systems Division of The Carborundum Company, 31935 Aurora Road, Solon, Ohio 44139. In the MULTICAST filter system, a filter assembly is disposed within a holding furnace/filter box. The filter assembly includes a horizontally oriented sealing plate and several vertically oriented filter elements that are attached to the sealing plate. The filter elements ar&lt;hollow and surround openings in the sealing plate through which molten metal can flow. The filter elements are made of a porous ceramic material in the form of bonded silicon carbide or alumina particles that create numerous tortuous flowpaths that provide effective filtration capability. Reference is made to U.S. Patent Application Ser. No. 06/661,543, filed Oct. 16, 1984, by Edwin P. Stankiewicz, entitled "Multiple-Use Molten Metal Filters,", the disclosure of which is incorporated herein by reference.
Another type of molten metal filter is a so-called cartridge filter. In a cartridge filter, a plurality of horizontally oriented, refractory tubes are disposed within a holding furnace/filter box. The ends of the tubes are attached to, and supported by, graphite end plates. One of the end plates is solid, while the other end plate includes openings that are in fluid communication with the hollow interiors of the refractory tubes. Molten metal is filtered by disposing the cartridge filter within the holding furnace/filter box and by bringing molten metal into contact with the exterior surfaces of the tubes. The molten metal flows through the tubes (where it is filtered) and is discharged through the openings in the end plate for conveyance into the casting molds.
Despite the filtration capabilities of prior art molten metal filters, particularly the MULTICAST molten metal filter and cartridge filters, certain problems remain. While the MULTICAST filter provides excellent filtration capability, its flow rate is less than desired for modern casting requirements. If molten aluminum is the metal being processed, the MULTICAST filter assembly can handle approximately 1,000-1,200 pounds per minute (with 6-grit particle size). Current industry trends demand that higher flow rates, i.e., 2,000-2,400 pounds per minute, be available. Desirably, an effective molten metal filter would be available that would have the filtration capability of the MULTICAST filter assembly, while providing higher flow rates.
Although a cartridge filter provides acceptable flow rates, certain problems have not been addressed by conventional cartridge filters. In particular, a cartridge filter is large and expensive. The configuration of the filter elements is such that a great deal of wasted space is taken up by the filter. Also, the use of two large graphite end plates unduly increases the expense of the filter assembly. Desirably, a molten metal filter would be available that would (1) have a higher flow rate capability than the MULTICAST filter assembly, (2) have excellent filtration capability (as with the MULTICAST filter assembly), (3) occupy much less space than a cartridge filter, and (4) be much less expensive than a cartridge filter.