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Vacuum Dezincing of Silver Crusts

1991; Taylor & Francis; Volume: 30; Issue: 1 Linguagem: Inglês

10.1179/cmq.1991.30.1.7

1879-1395

Peter Hancock, Z. Wang, Phillip G. Evans, R. Harris,

Metal Extraction and Bioleaching

AbstractCurrent practice for the recovery of zinc from lead desilverizing crusts at Brunswick Mining and Smelting Corporation Limited's Belledune lead smelter consists of zinc distillation at 1200°C under atmospheric pressure in conventional Faber du Faur retorts. Efforts to improve plant hygiene and reduce the recirculation of silver associated with this practice have led to investigation of an alternate process in which the crusts are vacuum dezinced at a temperature of 600°C. A calculation of the thermodynamic properties of the ternary lead-silver-zinc system, from the three binary systems lead-zinc, silver-zinc, leadsilver was carried out using the Alcock and Richardson equation (ACTA Metall. 6, 358, 1958). The calculations were also carried out using the “Facility for the Analysis of Chemical Thermodynamics” (F*A*C*T) in which the properties of the ternary are evaluated from the binaries via the Kohler method (Monatsh Chemie 91, 738, 1960). The calculations showed that in a typical Parkes desilverizing crust, 75 wt% Pb, 7 wt% Ag, 18 wt% Zn, the activity coefficients of the components were calculated to be 6.0, 0.59 and 2.1, respectively. In dezinced Parkes crust, the activity coefficients were calculated to be 1.9, 2.0 and 3.5, respectively. Small scale batch vacuum distillation experiments at 600°C and 1 mm Hg on commercial Parkes crust found that the zinc content decreased from 15 to 1.5 wt% in 20 min and to 0.1 wt% in a further 20 min. The rate constant for the initial period was 0.35 min−1 and for the slower period, it was 0.08 min−1.Additional trials on a synthetic non-oxidized crust, found zinc elimination rates of 0.08 min−1. It was concluded that the high rates were due to the porous nature of the commercial crust which tended to liquate as zinc was removed as the experiment progressed. The lower rates which were similar for the liquated commercial crust and the synthetic crust were representative of vacuum refining from a quiescent liquid metal pool.