Abstract
As a fundamental study to control compositions of the slag in the oxidation zone of a QSL lead smelting furnace, phase equilibrium between the metallic lead and the PbO-FeOx-CaO-SiO2 or PbO-FeOx-CaO-SiO2-ZnO slag was investigated at 1,323∼1,523 K and a composition region of the homogeneous liquid slag phase and the composition and chemical formula of solid phases in the slag coexisting with the liquid phase were determined. The PbO-FeOx-CaO-SiO2 slag with Q (= mass % CaO/(mass % CaO + mass %SiO2)) of 0.4 is completely fusible at 1,423 K when the mole fraction ratio of FeOx , R, defined as R = NFeOx /(NFeOx NCaO + NSiO2 ), is around 0.4, and departure from this value gives rise to a solid-liquid two phase region in the range of the low PbO content. This result indicates that the QSL process, which is operated practically with R of about 0.4, is quite reasonable. Solubility of ZnO in the PbO-FeOx-CaO-SiO2-ZnO slag with Q = 0.4 and R = 0.4 at 1,423 K decreases with increasing PbO content in the composition range between 0 and 18 mol % PbO, then increases in the range between 18 and 33 mol % PbO, and finally becomes stagnant at about 18 mol % ZnO in the composition range above 33 mol % PbO. This result has an important meaning for the QSL operation using a charge with a high content of zinc. Hence, it is considered that precautious control of the PbO content in the slag is required for avoiding formation of a solid phase in the slag.
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