Effects of gelatin and polyoxyethylene ether on zinc electrowinning in a Zn(II)-NH4Cl-H2O system

Abstract

Zinc-electrowinning in Zn(II)-NH4Cl-H2O systems has many advantages, such as a high current efficiency and insensitivity to F−, Cl− and metallic impurities. Additives in electrowinning are important because they influence the growth and structure of the resulting deposits and current efficiency. The effect of gelatin and polyoxyethylene ether (PEO) on the nucleation, growth mechanism, and morphology of samples are investigated through cathodic polarization, chronoamperometry, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses during the electrowinning of Zn from an ammonium solution. The fraction-temperature diagram shows that the dominant species is ZnNH3Cl3− in the 5 M NH4Cl electrolyte when the temperature is between 298 K and 373 K. The average current efficiency reaches 95.48% in samples containing both gelatin and PEO. The surface morphology of samples with both gelatin and PEO is basically free of dendrites and shows a dense, layer-by-layer, stacked structure. The preferred growth orientation of zinc is enhanced by the presence of PEO and gelatin. The reduction kinetics of zinc are inhibited by the addition of gelatin. In the presence of gelatin and PEO, Zn nucleation and growth can be explained by the 3D growth characteristics of zinc nuclei under diffusion-limited nucleation. The arrangement and size of zinc grains are characterized through front and cross-section SEM.