Ionic mass transfer of zinc in alkaline solutions with simultaneous hydrogen evolution

Sherwood numbers were determined for mass transfer of Zn in alkaline solutions with coevolution of hydrogen (0.15-0.9 mmol/cm exp 3 ZnO + 8.4 mmol/cm exp 3 KOH, 300K). The separation between the Zn and the H current was performed using polarization curves. The separation between Sherwood numbers due to natural and bubble convection was obtained by an additive law since H evolution took place only at current densities higher than the Zn limiting current density. It is shown that the contribution to mass transfer by bubbles, relative to natural convection, increases with both Zn concentration and H current density. Under industrial conditions of high Zn concentration (0.9 mmol/cm exp 3 ZnO) and high current densities (--0.24 A/cm exp 2 ), the effect of the H bubbles is predominant even if the H current efficiency is low (3.3%), the Sherwood numbers being, respectively, 424 and 115. The data obtained in this work and others give support to the dimensionless correlation StSc exp 0.5 = 0.035(FrRe) exp --0.25 .