Lead solubility in FeO x -CaO-SiO 2 slags at iron saturation
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I. INTRODUCTION
THE primary production of lead metal is currently undertaken using technologies such as the lead blast furnace, ISP (Imperial Smelting Process), Kivcet, QSL (QueneauSchurmann-Lurgi), and Sirosmelt (Isasmelt and Ausmelt) processes. These processes operate in a wide range of temperatures and oxygen partial pressures. In the ISP process, the reduction of zinc oxide to zinc vapor requires very low oxygen partial pressures, which produce slags with less than 1 wt pct lead. Slags produced from the lead blast furnace, Kivcet, and QSL processes normally contain between 2 and 5 wt pct lead. In contrast, the first stage of direct oxidation smelting of sulfide minerals produces slags containing 30 to 60 wt pct lead oxide. For example, the oxidation stage of the Isasmelt-process slags can contain about 60 wt pct lead oxide.[1] Recent environmental restrictions on the disposal of solid waste have resulted in increasing pressure to lower the lead content of the slag from these smelters. Data on the lead solubilities at iron saturation conditions are, therefore, important to enable the limiting lead contents of reduced iron–containing cleaning slags to be predicted. Only limited data on lead solubilities and activity coefficients of PbO in the FeOx-CaO-SiO2 slags at metallic iron saturation have been previously reported.[2,3] In this system, the oxygen potential is minimum and the lead solubility is the lowest in the FeOx-CaO-SiO2 slag when it is in equilibrium with metallic iron. Several articles have been published on the lead solubilities and activity coefficients in the iron oxide–containing slag.[2–10] However, no work has been done on Pb-containing slags in equilibrium with metallic iron. Essential components in lead-containing smelting slags are silica, iron, calcium, zinc, and aluminium oxide. As the
first step toward complete characterization of phase equilibria and thermodynamics of these complex industrial slags, an experimental investigation of the FeOx-CaO-SiO2 system has been selected. The new experimental data obtained have been incorporated into a thermodynamic slag model using the software package FACT,[12] which contains thermodynamic databases with over 5000 compounds as well as evaluated solution models. The FACT package is used for the calculation of slag/solid/gas/matte/metal/salt equilibria in multicomponent systems of industrial interest. II. EXPERIMENTAL Fifteen grams of lead metal and 10 g of a powder mixture, whose composition simulated the slag composition, were loaded into an iron crucible with an i.d. of 20 mm and a height of 40 mm. Reagent-grade (991 pct purity) powders of CaO, SiO2, Fe2O3, and Fe were used to prepare the mixture (the main impurities in the initial oxide powders H2O and CO2 were eliminated by preliminary calcination). The experiments were carried out using a silicon carbide resistance tube furnace. The temperature was controlled to within 38C. At the start of the experiment, the crucible was suspended at the top of a reaction tube. A continuous flow of argon gas at
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