PbO solubility in lead-blast furnace slags
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I.
INTRODUCTION
THEfactors which determine lead and lead oxide solubility in blast-furnace slags have been a subject of thermodynamic investigation for some time. Such studies have importance not only in limiting production losses of Pb bullion, but also in minimizing the potentially hazardous waste content of a normally-discarded material. Current controversy over the latter issue ~has increased interest in controlling Pb presence in slags, and hence in determining what slag parameters have the greatest effect on this equilibrium. Lead blast-furnace slags are complex mixtures of various oxides, with some sulfur content as well. 2 Unfortunately, much of the previously reported research on the thermodynamic properties of these slags has been limited to the three-component system PbO-MO-SiO2, where M is a basic cation such as Ba, Ca, Co, Fe, Ni, or Zn. 3-9 More recent work, such as that reported by Taskinen et al. 10 and by Vaskevich et al. 1L has dealt with more complex systems, but limitations are associated with both studies. In the former, equilibrium conditions were created to simulate direct smelting of lead sulfide ore concentrate, possibly reducing applicability with regard to blast-furnace conditions; in the latter, the common slag constituent CaO was largely omitted from the system under study. For these reasons, a new study of PbO solubility in slag was undertaken to identify the M.E. SCHLESINGER, Graduate Student, and D.C. LYNCH. Associate Professor, are with the Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721. Manuscript submitted January 13, 1986.
METALLURGICALTRANSACTIONS B
parameters which must be controlled to yield slags of known PbO content for subsequent leaching studies. The system chosen for study in this investigation was PbO-CaO-FeO-Fe203-SiO2. This system is similar to that studied by Taskinen et al. at 1573 K, ~~with the exception that by lowering the partial oxygen pressure the authors have attempted to simulate better the reducing conditions of the blast furnace. In addition, further experiments were conducted wherein slags were equilibrated in contact with liquid Pb-bearing alloys, rather than pure molten lead, in order to reduce further the PbO content of the slag without altering the oxygen potential or other chemical parameters.
II.
EXPERIMENTAL PROCEDURES
Several methods are available for creating a reduced Po2 over a molten oxide/metal system. However, the volatility of Pb at temperatures of interest in this study generally requires either a large reservoir of molten metal or alloy, or the use of a closed system. As the use of smaller systems improves the likelihood of quenching the molten slag to a single-phase solid while keeping the oxide and metal phases separate, the latter alternative was employed. Figure 1 illustrates the method used to seal the admixture of oxides and metal in a fused-silica ampule under a rotary pump vacuum. Two elements of this procedure are of special significance. First of these was the use of support tubes
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