Surface Depletion in the Vacuum Distillation of Metals from Bismuth

Surface depletion was investigated in laboratory- and plant-scale distillation units with mixing by natural convection or by mechanical surface agitation. During the distillation of ²¹⁰Po, Zn, Cd, Te, and Mg from bismuth at 650 to 750°C, surface depletion was significant for the more volatile components: ²¹⁰Po, Zn, and Cd. The extent of surface depletion was decreased by relatively intense agitation of the entire liquid-metal surface. A model was developed for predicting the degree of surface depletion during the distillation of metals from bismuth as a function of temperature, still pot dimensions, and degree of agitation.