Applications of Porous Flow‐Through Electrodes: V . Electrowinning of Zinc from Flowing Alkaline Zincates at Packed‐Bed Electrodes

Abstract

The electrowinning of Zn was achieved from flowing alkaline zincate solutions at packed‐bed electrodes. The current efficiency of the process is affected by the simultaneous hydrogen evolution, the zincate concentration, flow rate, conductivity, and temperature of the electrolyte. The process is mass‐transfer controlled. The limiting current is directly proportional to the concentration and flow rate of the zincate solution. The maximum current efficiency coincides with the limiting current of the Zn electrowinning reaction. While increases in the zincate concentration and/or flow rate increased the current efficiency, similar increases in the temperature and/or electrolyte conductivity had adverse effects. This result is interpreted in the light of existing electrochemical theory. While the current efficiency indicates the percentage of the total current consumed in electrowinning, 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 transfer within the packed bed. Although we measured Zn limiting currents of a few hundred milliampere per square centimeter at only modest electrolyte flow rates, the conversion efficiency was low in view of the relatively large size of the particles of the bed. The results agree with predictions based on a dimensionless group which combines the structural and transport properties of the system. Conditions are defined where one can achieve higher conversion efficiencies.