Patent Number: 051200295
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION A typical crucible furnace has a cylindrical shell 10 with a flat, closed bottom 10a. The burner(s) extends through a side port 10b in the shell and lining. After the used lining to be replaced is removed and the burner(s) is shifted out of the way, in the practice of the present invention, sections of insulating board are fitted around the inside of the cylindrical side wall and over the round bottom wall of the furnace shell to provide an insulating liner 12. Dry refractory material 14 for the working liner is then poured into the shell to a desired height from the bottom circular insulating liner portion and hand-tamped and de-aired to get a smooth, compact, level surface. Preferably, a vent hole is provided in the bottom by using a ceramic tube, wooden dowel or other suitable core. A plug 16 is also provided for each burner port. Then a cylindrical steel form 18 is lowered into the shell, onto the bottom layer of refractory material, and its outer face engages the end of the burner port plug(s) 16, which is correspondingly shaped. Refractory material 14 is then poured between the steel form and the surrounding insulating liner 12, which then also functions as a form. The next step is to vibrate the steel form 18 as by mounting a portable vibrating machine 20 on a rod 22 extending diametrically across the top of the form. Following this step, the refractory 14 is heated for at least one hour at 800.degree. F. as by a heating torch set in the steel form, whereupon the form is cooled and lifted out of the furnace. Removal of the burner plug 16 completes the lining operation. The theoretical heat requirements for a well-designed 450-lb, gas-fired, aluminum-melt crucible furnace having a crucible and pedestal weight of 250 lbs and a steel shell weight of 500 lbs is as follows using the composite liner materials previously discussed and assuming a one-inch thick insulating layer and a five-inch thick working layer in a furnace having an inside diameter of 42 inches and an inside height of 39 inches: ______________________________________ K-factor for insulating liner .33 K-factor for working liner 9.80 ##STR1## ##STR2## Heat loss at 220.degree. F. = 377 BTUH/ft.sup.3 Coldface area = 45.4 ft.sup.2 Heat loss through lining = 17,124 BTUH Volume of working liner = 14.14 ft.sup.3 Density of DV60A = 145 lb/ft.sup.3 Weight of working liner = 2050 lbs Mean temperature of working liner 1020.degree. F at casting temperature of Al = (Coldface of working liner = 530.degree. F.) Specific heat of working liner = 0.258 Heat retention (working liner) 539,478 BTUH (205)(1020)(.258) ______________________________________ A typical, current, 6-inch thick lining system for such a furnace would have the top 20 inches of the 39-inch height as a ceramic fiber working layer to a thickness of 3 inches, and would have the remaining 3 inches outside of the ceramic fiber and the 6 inches of lining below the ceramic fiber as a castable, dense refractory, having, for example, a specific weight of 130 lb/ft.sup.3 and a K-factor of 4.0 at 1000.degree. F. mean temperature. Assuming ceramic fiber typical values of 9 lb/ft and a K-value of 1.0 at 1000.degree. F. mean temperature, a calculation will show a heat loss of 7600 BTUH and heat retention of only 128,466 BTUH as compared to the 539,478 BTUH heat retention of the composite lining of this invention for the same furnace. A typical start-up of a crucible furnace with the composite liner of the present invention to achieve a first heat will typically take about twice as long as for subsequent heats. By use of the composite lining of the present invention, the heat-retention ability of the lining can be selected to shorten the melt period. Since there is a maximum rate at which the metal charge being melted in the crucible will absorb heat, there is little advantage in having a surplus of heat-retention ability in the working liner. In fact, too much surplus can result in breakage of the crucible. Accordingly, it has been discovered that it is preferable to make the heat-retention ability of the working lining approximately equal to the heat required to melt the charge and raise it to casting temperature. This heat requirement can be readily calculated for a given weight of metal being melted since the specific heat, melting temperature, heat of fusion, and casting temperature will be known. Hence, the composite lining of the present invention can be readily engineered to provide the preferred heat retention in the lining. Although the foregoing discussion has been directed to crucible furnaces, the composite lining of the invention is of value for molten metal transfer vessels such as ladles. Because of the increased heat retention by the lining, the tap temperature of the molten metal at the melt furnace can be lower at the start of the transfer operation than otherwise, thus saving energy and wear on the furnace, reducing oxidation of the molten metal, and allowing for more consistent temperatures in the casting operations. Furthermore, the lining in the ladle will last significantly longer than before. In this regard, a thin, third liner layer can be applied to the working liner as a wear surface to be replenished to increase the life of the rest of the liner. From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.