Simple Solutions That Work! Issue 9

BACK-2-BASICS slag formed during the melting operation. Because these oxides and nonmetallics are not soluble in the molten metal, they float in the liquid metal as an emulsion. This emulsion of slag particles remains stable if the molten metal is continuously agitated, the result of the magnetic stirring inherent in coreless induction melting. Until the particle size of the nonmetallic increases to the point where buoyancy effects countervail the stirring action, the particle will remain suspended. When flotation effects become great enough, nonmetallics rise to the surface of the molten metal and agglomerate as a slag. Once the nonmetallics coalesce into a floating mass on the liquid metal they can be removed. The use of a suitable flux greatly accelerates this floatation process. When slag makes contact with an area of the refractory wall that is colder than the melting point of the slag, the cooling slag will adhere to the lining. That adhering material is called buildup. High melting point slags are especially prone to promoting buildup. If not prevented from forming or not removed early during formation, buildup will reduce the overall efficiency and capacity of the furnace. The mineral composition of the refractory lining utilized for melting iron will almost invariably be silica. Silica constitutes a compromise between good thermal insulation, adequate mechanical impact strength to protect the coil, and good thermal shock resistance during a batch melting process. Decreasing refractory wall thickness in a coreless furnace improves the coil efficiency and increases the effective power input. Studies have shown a substantial reduction in power consumption with decreasing thickness of the refractory lining. (2) With increasing furnace operating time and progressive refractory wear, power consumption decreased by 9% three weeks after a new lining was installed on a 3 metric ton coreless furnace. Conversely, the accumulation of insoluble slag buildup on the refractory wall will have the exact opposite effect. Not only will buildup increase the effective refractory wall thickness, but coil efficiency will decrease as shown in Figure 2.(3) As the effective refractory thickness increases from slag buildup, coil efficiency decreases and the amount of electrical energy required to melt increases (shown as the approximated percentage of rated power). The coil efficiency at the optimum lining thickness is 82% and the percentage of rated power in kW's is 100%. As the buildup thickness approaches 2.5 inches, it is estimated that an additional 25% increase in kW's will be required to melt. A thicker effective refractory lining equates to the metal bath being further away from the coil. This results in a lower coil-power factor and lower coil efficiency that produces higher current and greater electrical losses. Insoluble slag buildup has the same effect as increasing refractory thickness. Since there are more electrical losses in the coil, there is less energy available to melt metal, so every melt will take somewhat longer than it would with a standard refractory thickness. This causes increased conductive and radiated heat losses, increasing the amount of energy consumed even further. Adding to this scenario is the overall capacity of the furnace will decrease, resulting in reduced production. (4) Controlling buildup allows for more continuous furnace operation. Buildup can be controlled or eliminated with the addition of fluxes. It should be noted that the use of fluxes in ferrous foundries has been widely discouraged by refractory companies in the past. However, new developments in flux chemistry (Redux U.S. Patent 7,68,473) allow use in furnaces lined with even silica refractories without refractory attack. Generally, adding fluxes ensures that slags have a melting point below the coldest temperature in the system. Fluxes can help prevent slags and other insolubles from freezing on the cooler refractory surfaces. The use of a flux allows for the flotation of the emulsified oxides; it also reduces the melting point of the slag to below the lowest temperature encountered in the melting furnace and associated liquid metal handling system. An example of severe buildup in a coreless furnace is shown in Figure 3. Continued on page 54 53