A comprehensive characterization of silver and bismuth refining crusts produced during the pyrometallurgical refining of lead was undertaken. Parameters such as intermetallic crystal size and composition, and degree of metal entrainment were determined. Silver crust was found to be composed mostly of the $ epsilon$-phase (AgZn$ sb3$) with an average particle size of 50 $ mu$m. Liquid lead entrainment of 80 volume percent was measured. The intermetallic phase in bismuth crust was composed entirely of CaMg$ rm sb2Bi sb2$ crystals with an average diameter of 50 $ mu$m. Lead entrainment in bismuth crust was approximately 90 volume percent. The high degree of metal entrainment observed in both silver and bismuth crusts was not adequately explained through a capillary model of liquid metal entrainment in a static bed of solid particles. The models used to predict swelling in colloidal particle systems (such as clay soils) were found to better approximate the high degree of liquid entrainment. / Experiments were performed in which particle beds of $ gamma$-phase ($ rm Ag sb5Zn sb8$) intermetallic compound were infiltrated with liquid lead under controlled conditions and examined under the optical microscope. Image analysis revealed pronounced swelling of fine particle beds relative to coarse. The effect was most noticeable for average particle diameters less than 100 $ mu$m where the solids volume fraction decreased from 50 to 30 vol.%. Data showed samples LO exhibit a constant increase in inter-particle spacing of approximately 50 $ mu$m. The increase in spacing explained the degree of swelling at low particle diameters and was also used to develop an expression to predict height of capillary rise. / Based on an osmotic pressure analogy in colloidal particle systems, a swelling pressure term was defined to explain the increase in inter-particle spacing. The swelling pressure was defined as equal and opposite to the capillary pressure tending to force particles to random packing. It was concluded that swelling was related to mass transfer at the solid-liquid interface of the intermetallic crystals giving a reduced static pressure due to a localized minimization of Gibbs energy. The static pressure change (swelling pressure) was attributed to overlap of mass transfer boundary layers where mid-point static liquid metal pressure is less than the pressure in the bulk liquid. For the system studied, the mass transfer boundary layer was estimated to be approximately 100 $ mu$m in thickness. / Further experiments, using the sessile drop technique, were used to support the study findings by showing rapid spreading of liquid lead on Ag-Zn intermetallic substrates. The rapid spreading was attributed to mass transfer at the solid-liquid interface giving a reduced interfacial energy, $ gamma sb{ rm s1}$ due to a localized reduction in Gibbs energy. / The analysis of experimental results led to the development of two new industrial processes in the lead industry aimed at reducing lead loss to silver and bismuth crusts. Tests employing continuous agitation during bismuth removal from lead promoted the growth of large $ rm CaMg sb2Bi sb2$ crystals. The larger crystals minimized the effect of mass transfer boundary layer overlap and reduced lead entrainment from 90 to 80 volume percent. The addition of a low melting point KCl-ZnCl$ sb2$ salt during silver crust formation was found to remove up to 90% of entrained lead by the salt preferentially wetting Ag-Zn intermetallic particles. This was found to eliminate surface forces which retained liquid in the system of solid particles.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.28772 |
| Date | January 1994 |
| Creators | Hancock, Peter |
| Contributors | Harris, R. (advisor) |
| 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 and Metallurgical Engineering.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001448196, proquestno: NN05718, Theses scanned by UMI/ProQuest. |
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