Visualization and quantitative oxidation kinetic experiments on 100 g samples of Pb-As at 600°C; thermal analysis and phase-equilibrium measurements of Pb-PbO-As2O3 samples under argon over the temperature range of 420°C to 875°C; computational thermodynamic solution modeling; and phase diagram and equilibrium calculations using FACTSage(TM) were performed to elaborate the poorly documented thermodynamics of the slags in the lead softening stage in the pyrometallurgical refining of lead. In the softening stage, the minor element impurities: arsenic, antimony and tin are removed from lead bullion by oxidation and are transferred to a skimmable oxide slag phase. / It was found that optimizing an ionic molten oxide solution model that was conceptualized to contain Pb2+ and O2- with AsO3-4 and AsO3-3 ions, or with SbO3-4 and SbO3-3 ions in the respective PbO rich regions of the Pb-As-O and the Pb-Sb-O systems, was able to accurately reproduce the measured and published thermodynamic data. It was also found that the subsystems in the PbO-As2O 3-As2O5 and PbO-Sb2O3-Sb 2O5 systems showed small deviation from the ideal ionic solution model and small magnitude excess Gibbs energy parameters were sufficient to fit the predicted liquidus curves to the experimental measurements. / Arsenic in the +3 and +5 oxidation states was measured in the PbO rich region of the Pb-As-O liquid solution in the temperature range of 420°C to 875°C. The variability in the ratio of trivalent arsenic to the total arsenic content, as well as the complex variation of arsenic distribution between metal and oxide phases found strong interaction between the lead, arsenic and oxygen atoms at the 3PbO to 1AS2O3 molar ratio thus suggesting a short range ordering corresponding to the formation of AsO3-3 groupings, and indicating that the Pb3(AsO3) 2(l) species was likely to be present in the PbO rich region of the Pb-As-O system and contributing to an understanding of the Pb-As-O liquid oxide structure. Also, two new compounds (Pb3(AsO3) 2(s), Pb2AsO4(s)) were identified in the Pb-PbO-As 2O3 quenched samples via wavelength-dispersive spectrometry using the electron microprobe. The present work has application in commercial oxygen partial lead softening (OPLS), as uniquely practiced at Teck Cominco Ltd., British Columbia. There, pure oxygen gas is injected into the bath of impure bullion through a number of submerged lances in order to oxidize only part of the arsenic, antimony and tin into a slag phase. For such an operating practice, it was concluded from the visualization and quantitative oxidation experiments that the formation of solid oxides as the product of oxidation produced a physical barrier to the progress of oxidation and resulted in the commercially observed, highly-problematic, process initiation issues. When the product was liquid, there was much less of a barrier to rapid oxygen mass transfer to the minor element impurities and the softening reactions were easy to initiate. Such a change in the physical state of the products of oxidation was correlated to the optimized ternary Pb-As-O and Pb-Sb-O phase diagrams. / A current point of interest in partial lead softening is to increase the arsenic content of the slag phase. Arsenic distribution between lead bullion and slag calculated by the optimized solution model of the Pb-As-O system suggests that this can be achieved in a counter-current contacting of the slag and bullion.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.100362 |
| Date | January 2005 |
| Creators | Firoozi, Sadegh. |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Mining, Metals and Materials Engineering.) |
| Rights | © Sadegh Firoozi, 2005 |
| Relation | alephsysno: 002494778, proquestno: AAINR25144, Theses scanned by UMI/ProQuest. |
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