Physicochemical properties of the slags intended for steel refining in a ladle-furnace unit

Abstract

For the refining of steel in a ladle‐furnace unit, highly basic slags are used. In this case, for steel deoxidized with aluminum, the following main composition of refinery slag is commonly recommended (wt %): 56‐62 CaO, 6‐10 SiO 2 , 20‐25 Al 2 O 3 , 6‐8 MgO, and ≤ 1.0 (FeO + MnO). For steel not deoxidized with aluminum, the base slag should have the same contents of CaO, MgO, and (FeO + MnO), a higher content of SiO 2 (15‐20%), and a lower content of Al 2 O 3 (5‐8%); the slag fluidity is sufficient at (5‐10%) CaF 2 [1]. However, in practice, a slag containing CaF 2 is also formed for refining steel deoxidized with aluminum. Searching for additions other than fluorite to decrease the viscosity of high-basic refinery slags is topical, in particular, from an ecological standpoint (decomposition of CaF 2 in the zone of electric arcs with the formation of volatile hydrogen fluoride). To form free-running refinery slags without calcium fluoride, sodium oxide Na 2 O can be used; its sources are sodium carbonate and various production wastes containing Na 2 O. Na 2 O not only decreases the slag viscosity and surface tension but also significantly increases its sulfide capacity [2]. However, the efficiency of replacing of CaF 2 with sodium oxide should be first tested by studying the effect of Na 2 O on the physical properties of refinery slags, such as the density, surface tension, viscosity, and electrical conductivity. These properties to a significant degree determine the physicochemical processes of refining of steel from detrimental impurities. In this work, we calculated the sulfide capacity and studied the surface tension and density of refinery slags having a base composition with an Na 2 O content up to 12 wt % for a ladle‐furnace unit. As the charge materials, powders of chemically pure calcium, aluminum, silicon, and magnium oxides and calcium fluoride were used. Sodium oxide was introduced using sodium hydrocarbonate (baking soda), which rapidly decomposes into Na 2 O, CO 2 , and water at a temperature above 1500 ° C. The density and surface tension were measured in one experiment using the method of the maximum pressure in a gas (Ar) bubble with an MMN-240 manometer. We described the procedures of charge preparation and measurements in [3]. Table 1 lists the chemical compositions (in charge) of the slags under study, which were divided into two groups. The first slag composition corresponds to the mean composition of the refinery slag which is generally recommended for treatment of steel deoxidized with aluminum in a ladle-furnace unit and is character