Patent Number: 051200295
Section: summary

TECHNICAL FIELD The present invention relates to linings for crucible furnaces and transfer vessels used to repetitively handle molten metal and provides an improved composite lining. BACKGROUND ART In a crucible furnace, the metal to be heated is placed in a silicon carbide crucible supported on a pedestal within a furnace which is commonly heated by gas-fired or oil-fired burners acting on the crucible. The furnace has a steel shell with an internal lining. The working inner face (hotface) of the shell lining is subjected to the maximum temperature in the furnace and is spaced from the crucible a predetermined distance. Hence, since the maximum size of crucible for a given furnace is limited by the thickness of the lining, it is desirable to minimize the lining thickness. In the handling of molten metal, it is advantageous to have a durable lining with the physical ability to withstand the conditions at the hotface for long life while also insulating against heat loss from the vessel. Unfortunately, strong refractory materials generally do not have the heat resistivity required to meet efficient thermal requirements. In the past, it has been common in the crucible furnace art to use ceramic fiber liner material offering reactively fast installation with no cure-out and good insulation quality. The disadvantages are relatively poor heat retention and resistance to burner erosion, and short life due to poor resistance to metal spills. Various castable refractories have been used having the advantage of being inexpensive but having as disadvantages poor resistance to erosion and metal attack if formulated for a good insulation characteristic and poor heat retention if formulated for strength. Also, the castable refractories have been criticized as being messy and unduly time-consuming to apply and as requiring long cure-outs. Various plastics have also been used to give strong mechanical strength to resist erosion and good resistance to metal wetting, but these too have had relatively poor heat retention and insulation characteristics, have been overly time-consuming to install, and have involved extensive cure-out schedules. DISCLOSURE OF THE INVENTION Since there is not a single ideal liner layer, the present invention aims to provide a composite liner which is relatively thin overall to maximize capacity and which has improved thermal efficiency to the extent of substantially decreasing fuel consumption and shortening the melting period in a crucible furnace. The invention also aims to provide a liner which is easy and quick to apply, has a long life, and is advantageous for lining transfer vessels, such as ladles, as well as crucible furnaces. The composite lining of the present invention has an outer insulating liner with an unusually good insulating ability and a dense inner working liner with a durable hotface. The working liner has a high heat-retention characteristic and low insulating value as compared to the insulating liner, and is cast in position. The insulating liner is applied in board-like form and has adequate structural strength to support the working liner at high temperatures and to serve as an outer form complementing a portable inner form when the working liner is cast. The present invention permits the working liner to be selected in terms of density, volume (thickness), K-factor (thermal conductivity), and specific heat to define a heat-retention capability maximizing thermal efficiency. It has been found that, ideally, the heat-retention capability should approximate the heat required to melt the metal being heated in the crucible and raise its temperature to casting temperature after taking into account exterior heat losses via the furnace shell. The composite lining of this invention, by providing a relatively thin, outer insulating liner (layer) with a high insulating characteristic between the furnace shell and an inner working liner (layer) of high heat-retention ability, makes it possible to obtain the desired heat-retention capability for the working liner without increasing the overall liner thickness. In fact, it has been possible in some crucible furnace operations to decrease the liner thickness, and therefore increase the furnace capacity, while at the same time decreasing fuel consumption and decreasing the heating period for each cycle of bringing a charge (aluminum, for example) from room temperature to a casting state. In carrying out the present invention, it is preferred to use a readily castable, water-free refractory material for the working liner, such as the "DRI-VIBE".RTM. refractories made by Allied Mineral Products, Inc., Columbus, Ohio. Such includes "DV60A".RTM., which has been used in the practice of this invention for aluminum-melting crucible furnaces. This liner material is 60% Al.sub.2 O.sub.3, 38% SiO.sub.2, and 2% TiO.sub.2, in addition to containing heat-setting sintering mechanisms, and has a density of about 145 pounds per cubic foot. It develops adequate strength after one hour at 800.degree. F. to initiate use. For the insulating liner of the composite lining, it is preferred to use a product such as "BARNESBOARD,".RTM. sold by R. A. Barnes, Inc., Seattle, Washington, which is a fiberboard-type product containing silica (73%-91%), mineral wool or other suitable inorganic fibers (1%-6%), organic fibers (1%-3%), calcium silicate (3%-5%), diatomaceous earth (2%-5%), and binder (2%-8%), such as a suitable phenolic resin. This product has a density in the range of about 40 pounds per cubic foot and a K-factor of about 0.25 at room temperature, compared to a K-factor of about 10.0 for the "DV60A".RTM. refractory material. Typically, the insulating liner is one inch thick and is in sheet form, with parallel V-grooves extending at regular intervals along its length so that the sheet may be readily bent in a circle and with the sections between grooves forming chords of the circle. The above-identified fiberboard product, preferred for the insulating liner, has an unusually low K-factor through a wide temperature range and also maintains adequate crushing strength through a wide temperature range to support the working liner. For example, a thermal conductivity test, ASTM C-201, indicates that when the hotface of the insulating liner board is 1993.degree. F., the coldface is only 202.degree. F.; and a hot crushing test on two-inch cubes of the liner board indicates an average crushing pressure of 27 psi at 1000.degree. F. and 11 psi at 2000.degree. F. to reach 20% deformation. The cold crushing strength is 225 psi.