SILVER CRUST PROCESSING BY VACUUM

When cleaned from impurities lead is treated with zinc reacting with precious metals to form refractory intermetallics that are not soluble in the base metal – a silver crust (SС). The laboratory studies for processing of SС containing, %: 78–80 Pb; 15–17 Zn; 3–5 Ag; 0,0001–0,002 Au by vacuum distillation (t = 800÷1200 °C, Р = 10^–1÷10^–3 mm Hg; τ = 2÷42 h) to separate zinc, lead and precious metals. At t ~ 800 °C, Р = 10–1 mm Hg, zinc condensate was obtained containing, %: 99,85 Zn, 0,14 Pb. At t ~1000 °C, Р = 9·10^–3 mm Hg, zinc-lead concentrate (15,7 Zn; 83,6 Pb; 0,02 Ag), lead concentrate (86–87 Pb; 0,4–2,8 Zn; 0,04–0,12 Ag), silver concentrate (50–67 Ag; 0,1–5,3 Pb, 0,04–0,2 Zn). At t ~1200 °C, Р = 6,5·10^–3 mm Hg, lead concentrate (91–97 Pb, 0,6–1,7 Zn; 0,01–1,2 Ag) and silver concentrate (92 Ag, 1,4 Pb, 0,1 Zn) were obtained. It is shown that the degree of vacuum in the system increased up to over 0,1 mm Hg causes no significant changes in the indicators of silver crust vacuum distillation. Quantitative sublimation of zinc takes place at a temperature not exceeding 800 °C for 1 h. The end of the process is indicated by a pressure surge in the system up to Р = 1,5÷2,0 mm Hg. SC liquation producing crude lead (~42 % of the original quantity of Pb) and liquated silver crust (LSC) containing, %: 76,39 Pb; 16,56 Zn; 6,254 Ag, is possible for 2 h in an inert gas atmosphere (Ar) at t = 700±10 °C without vacuum. Temperature of lead draining is 380±10 °C. Quantitative sublimation of lead and zinc from LSC is optimal (>99 %) at t £ 1000 °C for 12 h, At the same time ~20 % silver of the original quantity in SC is extracted from the composition. Velocities of zinc (800 °C), lead and silver (1000 °C) sublimation are identified (calculation/experience) and equal to v·10^–4 g/(cm²·s): 19,13/24,05 Zn; 6,25/8,6 Pb; 0,0068/0,0065 Ag. These v values can be used to design the equipment for vacuum distillation of silver crust.

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