Kinetics of oxygen transfer through molten silver cupellation slag

Oxygen transfer rates through 0.03 to 0.14 kg samples of pure PbO and binar y PbO/Fe3O4 melts and through molten samples of commercial silver cupellati on slag were measured at a temperature of 1050 degrees C. The oxygen transf er rate was calculated from the rate of oxidation of a molten lead pool tha t was beneath the molten oxide or slag. The molten lead and oxide or slag w ere contained in high purity alumina crucibles. The source of the oxygen wa s air that was flowed onto the top surface of the melts from a lance separa ted from the melt at distances ranging from 0.02 to 0.22 m. Air flow rate w as 8.3 x 10(-5) m(3)/s (5 l/min @ 298 K, 1 ATM) and was equivalent to a pip e Reynold's Number of 900 at the lance tip and at the melt temperature. It was found that the rate of oxygen transfer through the molten oxide or slag layer was roughly constant for lance separations greater than 0.15 m. The oxygen transfer rate increased with decreasing lance separation for separat ions less than 0.15 m. Gas phase mass transfer resistance needed to be take n into account when analyzing the kinetic data for lance separations greate r than 0.15 m. It was concluded that oxygen transfer through pure PbO melts occurred by molecular diffusion and that the measured permeability of oxyg en was 1.33 x 10(-8) kmol O-2/m/s. The diffusion coefficient of molecular o xygen in pure molten PbO was calculated as 3 x 10(-8) m(2)/s. When small am ounts of Fe3O4 were added to the pure PbO melts, the rate of oxygen transfe r increased by at least a factor of ten and the oxygen transfer mechanism c hanged to ionic diffusion. The rate of oxygen transfer was seen to vary wit h the amount of addition of Fe3O4 to the PbO melt and exhibited a maximum a t 6 wt% Fe3O4. The rate of oxygen transfer through samples of commercial si lver cupellation slag was not increased by adding Fe3O4 because the effect of Fe3O4 was overshadowed by impurities such as Fe, As, Sb and Bi naturally present in the commercial slag. (C) 1999 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.