Simple Solutions That Work! Issue 8

furnace time, unless the wash heat can be salvaged. An additional unattractive feature is the high cost of the metal component that is used as the form. In summary, consumable inductor forms represent an inefficient, expensive practice that invites innovation. The inductor of a channel furnace represents a major refractory challenge of the system. Because of the rapid movement of metal through the loop, erosion potential is highest. Because this area, as the heat source, sees the highest temperatures in the system, chemical attack is maximized. Failures may occur for these reasons: • Erosion – from rough surface and chemical attack • Metal penetration – proportional to available surface area • Cracking – from metal penetration and thermal shock There is room for improvement. IMPROVED INDUCTOR LINING PRACTICE Recent development has resulted in a ceramic product that addresses some limitations of current inductor installation practice. Blasch Precision Ceramics has introduced pre-fired ceramic shapes that become the surface of the inductor loop. The proprietary process yields a ceramic with an average pore size of five microns, an order of magnitude less than conventional rammed or cast materials. Pore size is critical in control of metal penetration, a major cause of refractory failure. Metal penetration of refractory materials exacerbates chemical attack by greatly increasing the surface area available for attack. The denser, smoother surface of the pre-fired shape also contributes to reduced erosion and metal adhesion. There is no curing requirement for the surface exposed to molten metal, thus reducing installation downtime, a real cost of operation. The potential need for wash heats is eliminated, as no potential for contamination exists. LOOP FORM MATERIAL OPTIONS As the ceramic inductor loop form is an innovation, in any existing furnace it becomes an addition to current practice. Hopefully, the current furnace lining refractory is optimum. It then appears obvious to match, as closely as possible, the loop form chemistry to that of the existing refractory. This will yield the best resistance to chemical attack and erosion. It has the additional advantage of closely matching thermal expansion characteristics, a desired result. The proprietary technology is relatively independent of chemical variations, so such matches are usually easily achieved. For most iron applications, a high alumina, low silica material may be optimum. For nonferrous applications, a high alumina and possibly some addition of silicon carbide can improve performance. Continued on page 30 In order to deal with the high temperature and chemical activity of the molten metal contained, both upper case and inductor channels must be lined with refractory materials. These may vary from bricks to castables to ram materials, depending on chemical or physical requirements, or simply on practice preferences. The inductors, mainly because of the complex shape design, limit these choices for that part of the furnace. Current practice is almost exclusively to burn or melt out a preformed shape to create the tubular channel loop. LIMITATIONS OF CURRENT PRACTICE As mentioned, there are two types of consumable channel loop forms. The so-called burnout forms are normally wooden. These are positioned in the lower shell, and the refractory is rammed or cast around them. During the sintering or curing of the refractory, the heat causes combustion of the forms, leaving a void that becomes the channel loop. In the case of a melt out form, the positioning is the same, with a form usually of the same metal to be processed in the furnace. After ramming or casting of the inductor, the sintering is followed by a wash heat, during which the form melts and becomes part of the heat itself. Because of the relatively soft nature of the burnout forms, it is difficult to achieve a dense surface in the inductor loop. The result is a somewhat porous interface with the molten metal, creating the opportunity for more rapid erosion and metal penetration. In addition, the burnout process is costly, unproductive use of the furnace. Melt out forms provide a somewhat more dense surface in the loop, but still not optimum, and it again represents unproductive use of 29 Figure 3. Micropore Distribution in Ceramics