Mechanism of the High Removal Rate of Cobalt in Conventional Cementation by Zinc Dust with Arsenic Oxide in the Presence of Cupuric Ions

摘要

The roles of As_2O_3 and Cu~(2+) in the cementation of Co by Zn dust were studied in synthetic Zn leach solutions by polarization measurements, scanning micro-electrode technique, ESCA and EPMA analyses. It was confirmed that Co had an inherent property of not beginning to deposit at its equilibrium potential and that the potential at which Co began to deposit was polarized in Zn sulfate solution almost to reach the equilibrium potential of Zn. These properties peculiar to Co were directly responsible for the extremely small removal rate of Co in the cementation using Zn dust alone. The partial polarization curves of Co were significantly shifted to more noble direction in the presence of As_2O_3 and Cu~(2+) in Zn sulfate solution, indicating that these additives acted as a catalyst which promoted the kinetically suppressed deposition rate of Co in the presence of large amounts of Zn~(2+) Considering the fact that Cu, which began to electrodeposit at its equilibrium potential, was reduced to metallic state by Zn dust immediately, the deposited Cu on the initial Zn dust surface would offer active cathode area on which Co can deposit easily. On the other hand, As_2O_3 was also reduced by Zn dust in preference to Co and deposited in the forms of metal and oxide. The metallic As acted as an inhibitor for hydrogen evolution on the deposited Co, while As oxide worked as a catalyst to reduce the inherent deposition overpotential of Co and promoted the cementation rate of Co.