Experimental Study of the Phase Equilibria in the PbO-Rich Region of the Pb-As-O System
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UCTION
LEAD softening is one of the stages in the pyrometallurgical refining of lead during which the minor element impurities of arsenic, antimony, and tin are partially or completely removed from lead bullion by oxidation and transfer to a skimmable oxide slag phase. Previous attempts to construct a thermodynamic model for lead softening or predict the equilibrium conditions based on the available thermodynamic data suffered from a lack of sufficient and reliable data on the Pb-AsO system.[1–4] Davey[5] emphasized the lack of sound activity data in lead softening oxide systems and stated that the physical chemistry of softening has not been fully examined to some extent because of the complexity of the system. The present work follows a qualitative laboratory scale investigation of oxide formation during lead softening.[6,7] This previous work established the need to further determine the thermodynamics of the system in order to explain the phases that were seen to form during exposure of lead bullion to oxygen gas during softening. A combination of experimental measurements and thermodynamic modeling[8,9] using FACTSAGE*[10] *FACTSAGE is a trademark of CRCT - ThermFact Inc., Montreal, PQ, Canada and GTT - Technologies, Aachen, Germany.
SADEGH FIROOZI, formerly Student, Department of Mining and Metallurgical Engineering, is with the Amir Kabir University of Technology, Tehran 15875-4413, Iran. RALPH HARRIS, Professor, is with the Department of Mining, Metals and Materials Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Contact e-mail: sadegh.fi[email protected] Manuscript submitted May 17, 2006. Article published online May 23, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
thermochemical software were carried out to characterize the phase equilibria and determine the thermodynamic properties of the Pb-PbO-As2O3 system as part of a study of the wider Pb-PbO-As2O3-Sb2O3[8,9] system. This article presents the experimental work that was required to establish a better understanding of the oxide phases and their structure in the Pb-As-O system as well as to produce thermodynamic data necessary for a consistent description of the system by optimization of a thermodynamic model.
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PREVIOUS STUDIES
Pelzel[11] conducted the first significant study on the PbO-As2O3 phase diagram by thermal analysis. He measured a substantial decrease in PbO liquidus temperature with the addition of As2O3 up to 0.25 mol fraction As2O3. Pelzel suggested the possibility of the existence of 3PbOÆAs2O3 (or Pb3(AsO3)2) and 2PbOÆAs2O3 (or Pb2As2O5) compounds in the PbOAs2O3 phase diagram, even though he was not able to detect them. Zunkel and Larson[12] studied the PbO-As2O3 system by constructing a phase diagram of the PbO-As2O3 system up to 0.22 mol fraction As2O3 via thermal analysis and equilibration techniques. The liquidus temperatures measured by Zunkel and Larson were in fair agreement with those measured by Pelzel. Zunkel and Larson reported a solubility of about 5 mol pct As2O3 in solid PbO at 700 C based on a graphic
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