APPLICATIONS OF POROUS FLOW-THROUGH ELECTRODES .5. ELECTROWINNING OF ZINC FROM FLOWING ALKALINE ZINCATES AT PACKED-BED ELECTRODES

The electrowinning of Zn was achieved from flowing alkaline zincate so lutions at packed-bed electrodes. The current efficiency of the proces s is affected by the simultaneous hydrogen evolution, the zincate conc entration, flow rate, conductivity, and temperature of the electrolyte . The process is mass-transfer controlled. The limiting current is dir ectly proportional to the concentration and flow rate of the zincate s olution. The maximum current efficiency coincides with the limiting cu rrent efficiency, similar increases in the temperature and/or electrol yte conductivity had adverse effects. This result is interpreted in th e light of existing electrochemical theory. While the current efficien cy indicates the percentage of the total current consumed in electrowi nning, we also defined the conversion efficiency which is a measure of the ability of the packed bed electrode to electrodeposit the zincate ions as they flow through the electrode. This conversion efficiency, as well as the limiting current, depend on the mechanism of mass trans fer within the packed bed. Although we measured Zn limiting currents o f a few hundred milliampere per square centimeter at only modest elect rolyte flow rates, the conversion efficiency was low in view of the re latively large size of the particles of the bed. The results agree wit h predictions based on a dimensionless group which combines the struct ural and transport properties of the system. Conditions are defined wh ere one can achieve higher conversion efficiencies.