Part I – January 1969 - Papers - Activity of Sb2O3 in PbO-Sb2O3 and PbO-SiO2-Sb2O3 Slags

The activity of Sb,03 in PbO-Sb,03 slags containing less than 50 mol pct Sb,03 was determined by the inert-gas saturation method at 700°C. In this composition range, the activity gf SbzO3 shows a strong negative deviation from ideality. The activity of PbO in these slags was calculated by application of the Gibbs-Duhem iniegration to the Sb203 activity data. The calculated activity of PbO in slags containing more than 63 mol pct PbO was found to deviate in a positive direction from ideality uthile a negative deviation was found for slags containing less PbO. The standard Gibbs free energies of formation of Sb,03 and PbO. Sb203 have been calculated and compared with existing data in the literature. The activity of Sb203 in PbO-Si0,-Sb203 (PbO/SiO, = 2) slags containing less than 25 mol pct Sb,03 was also determined by the inert-gas saturation method at 700°C. In this composition range, the activity of Sbz03 shows a very large negative deviation from ideality. VERY little experimental work has been published in the past to determine equilibrium data in the oxide systems connected with the refining of lead. These data are of value since impurities such as antimony, arsenic, and tin must be removed from lead and recovered for further treatment. Equilibrium studies on antimony and arsenic systems are also of interest for the design of new processes for lead refining and lead dross treatment. Maier and ~incke' first determined the liquidus curves for the PbO-Sb203 system and identified the compound PbO . Sb203. They found the phase diagram for this system to be two simple eutectics located on either side of the congruent melting compound. They also determined a very limited amount of vapor pressure data for Sb406 abbve PbO-Sba3 melts at 697"~. A second phase diagram investigation on this system was reported by Hennig and Kohlmeyer' who confirmed the existence of the compound PbO . Sb203 as well as the form of the diagram. A disagreement was noticed, however, in that their liquidus temperatures over nearly the entire composition range were higher than those reported by Maier and Hincke. Barthel~ and pelze14 redetermined the liquidus curve at the PbO-rich end of the PbO-Sb@, system and agreed very closely with the results of Maier and Hincke. None of the investigators mentioned above reported any mutual solid solubility in the PbO-SbD3 system. Zunkel and Larson5 have determined the phase diagram for the PbO-rich end of the PbO-Sb203 system by slag-metal equilibrium studies in the Pb-PbO-Sb203 system and by thermal analysis studies in the PbO-Sbz03 system. A maximum solid solubility of 5.6 mol pct Sb203 in PbO was observed at the eutectic temperature of 604°C. Their results for the phase diagram agree favorably with those of Maier and Hincke. The vapor pressure of Sb2O3 in the temperature range from 470" to 800°C has been determined by Hincke, using a modification of the transportation meth~d.~ His results for temperatures below the melting point of Sba3 are the only data reported in the published literature. The predominant vapor species has been shown to be Sb,06 by Norman and staley.? Myzenkov and Klushin,8 using the boiling-point method, have determined the pressure of Sb406 above liquid SbD3 in the temperature range from 715" to 1025°C. The agreement between these two studies is not very close. A portion of the discrepancy lies in the fact that Hincke used silica crucibles, which were attacked by the liquid Sbz03 at high temperatures. This fact does not account, however. for the large difference observed at the melting point. ~aier' gives a brief summary of vapor pressure data for Sb,O, above pure liquid Sbz03 which agree quite well with the data of Myzenkov and Klushin at temperatures near the melting point. This paper describes the determination of Sb2O3 activity data in the PbO-Sb203 and Pb0-Si02-Sb203 (PbO/SiOz = 2) systems by the inert-gas saturation method. These activity data are compared with the data calculated by Zunkel and arson. EXPERIMENTAL Materials. The materials used in this investigation were analytical reagent-grade PbO (99.8 pct PbO, 0.14 pct insoluble in CHsCOOH, 0.02 pct not precipitated by HB, 0.1 pct CaO, and 0.08 pct SiOz), Sbf13 (99.6 pct Sb203, 0.004 PC~ C1-, 0.005 PC~ SO;-, 0.15 p~t AS, 0.001 pct Fe, and 0.03 pct other heavy metals such as Pb), and SiOz (chromatographic grade). Apparatus for Vapor Pressure Determinations. The apparatus used in this investigation consisted of a transportation reaction system with two separate gas trains. The argon transporting gas was first mixed with a small amount of hydrogen, metered, and dried by passage through silica gel and anhydrone drying tubes arranged in series. After this preliminary drying, the argon was passed through copper wool at 500°C to convert the residual oxygen to water vapor which was removed by three anhydrone drying tubes. A second stream of argon was metered and dried and then passed around the outside of the alumina reaction tube to flush away the volatile species to pre-