This invention relates to a composition or mix and method for insulating and inducing slag formation in contact with molten metal, particularly in the manufacture of iron and steel and, more particularly, to an insulating and slag inducing mix which is cast or spread over the surface of the molten metal to form a layer of slag that acts as a refractory thermal insulation and prevents oxidation of the metal by the surrounding atmosphere.
In steelmaking and other molten metal processes, a layer of slag on the upper surface of the molten metal prevents both oxidation and excessive heat loss. The slag is formed by the selective oxidation and combination of various impurities (for example, in a charge of blast-furnace iron, scrap iron and other added materials) and causes separation of the material components into desired compositions of steel and slag.
Various materials have been used or proposed as additives to form an insulating slag cover for molten metal in steelmaking furnaces. In steelmaking processes, ferrous blast furnace slag has been used as an inexpensive slag inducer to form a layer over the molten metal in basic oxygen or electric arc furnaces. Ferrous slag, however, releases sulfur compounds which cause pollution of the atmosphere. Ferrous slag addition, moreover, creates a thicker than usual layer of slag that increases refractory wear and, in addition, promotes the formation of undesirable sticky or viscous slag. Viscous slag, as has been long known, hinders the rapid transfer of oxygen to the molten metal mass and causes the undesirable build-up of oxygen in the slag in the form of iron oxides.
Mixes of sand and silica have been used as inexpensive fillers but silica can create excessive amounts of slag as well as sticky slag formations and can contaminate the molten metal. Pitch or tar has also been blended with such mixes to provide a less expensive slag inducer. However, the carbon content of pitch and tar gives rise to fire safety and contamination problems. In order to reduce slag formation, aluminum oxide or alimina can be added to the mix. Free or nonfused aluminum oxide, however, has a tendency to plug the furnace orifices and causes undesirable bridging.
Thus, it is apparent that satisfactory slag and slag inducer compositions must have good chemical and thermal stability while in contact with the molten metal and must neither be subject to overly rapid breakdown or excessive slag formation, if contamination of the molten steel as well as air polluting gas formations are to be avoided. Furthermore, as in most industrial processes, it is important that materials selected for inducing slag formation be chosen on the basis of cost effectiveness.